Frontiers in the Science and Technology of Polymer Recycling

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Series E: Applied Sciences - Vol. 351

Frontiers in the Science and Technology of Polymer Recycling

edited by

Guneri Akovali Middle East Technical University, Ankara, Turkey

Carlos A. Bernardo University of Minho, Guimares, Portugal

Jacob Leidner Ortec Corp., Missisagua, Ontario, Canada

Leszek A. Utracki National Research Council of Canada, Boucherville, Quebec, Canada

and

Marino Xanthos Polymer Processing Institute at Stevens Institute of Technology, Hoboken, U.S.A.

Springer-Science+Business Media, B.v.

Proceedings of the NATO Advanced Study Institute on Frontiers in the Science and Technology of Polymer Recycling Antalya, Turkey 16 -27 June 1997

A C.I.P. Catalogue record for this book is available from the Library of Congress.

ISBN 978-90-481-5074-8 ISBN 978-94-017-1626-0 (eBook) DOI 10.1007/978-94-017-1626-0

Printed on acid-free paper

All Rights Reserved © 1998 Springer Science+Business Media Oordrecht Originally published by Kluwer Academic Publishers in 1998 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photo­copying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

PREFACE LIST OF PARTICIPANTS GROUP PICTURE

TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION

Introduction to Recycling J. Leidner

Regulations and Practices of Polymer Recycling in NATO Countries (A). European Countries D. Curto and Y. Ba~ar

Regulations and Practices of Polymer Recycling in NATO Countries (B). Canada and United States of America F. H. C. Edgecombe

Economic Aspects of Plastics Recycling M. J. Bevis

Polymer Recycling for Energy Recovery An Application of Life Cycle Analysis Principles M. Xanthos and A. L. Bisio

CHAPTER 2. FUNDAMENTAL ISSUES PERTINENT TO POLYMER RECYCLING

Upgrading of Recyclates - the Solution for High Value Applications: Restabilization and Repair H. Herbst, K. Hoffmann, R. Pfaendner and H. Zweifel

IX

Xlli

xxii

3

17

29

41

57

73

VI

Biodegradable Materials: State of Art and Future Perspectives C. Bastioli

Polymer Blends' Technology for Plastics Recycling L. A. Utracki

Compatibilization of Heterogeneous Polymer Mixtures from the Plastics Waste Streams H. J. Radusch, J. Ding and G. Akovali

Morphology Development During Processing of Recycled Polymers H. J. Radusch

CHAPTER 3. REPROCESSING OF SINGLE TYPE POLYMERS

Derivation and Validation of Models to Predict the Properties of Mixtures of Virgin and Recycled Polymers C. A. Bernardo

Reprocessing of Poly (Vinyl chloride), Polycarbonate and Polyethyleneterepthalate F. P. La Mantia

Reprocessing of Polyolefins: Changes in Rheology and Reprocessing Case Studies A. T. P. Zahavich and J. Vlachopoulos

CHAPTER 4. REPROCESSING OF MIXTURE OF POLYMERS

Separation Technologies J. Leidner and G. Boden

Reprocessing of Commingled Polymers and Recycling of Polymer Blends L. A. Utracki

Non-Convential Processing Techniques for Polymer Recycling M. J.Bevis

103

123

153

191

215

249

271

301

333

355

Reprocessing and Properties of Homopolymer Blends of Virgin and Recycled Polymers F. P. La Mantia

CHAPTER 5. RECOVERY OF CHEMICALS AND ENERGY

PVC Recycling with Chlorine Recovery G.Menges

Thermolytic Processes M. Xanthos and J. Leidner

Solvolysis M. Xanthos and S. H. Patel

Fluidized Bed Incinerator with Energy Recovery System as a Means of Plastic Recycling S. Suzuki and T. Minoura

CHAPTER 6. THE WAY FORWARD

Future Perspectives and Strategies of Polymer Recycling H. J. Radusch

General Discussion - the Participants' view

INDEX

vii

371

389

407

425

437

451

469

473

PREFACE The main source of energy on earth is the Sun. From the energy and the basic raw materials

we make products necessary for the daily life. Then, after a service span, the used-up products

are discarded, usually returning to earth as waste. This has been going on for thousands of

years.

Through out the centuries that has passed, raw-materials were considered as boundless and

their availability was only restricted by the capacity to collect and transform them. So was

also considered the capacity of the earth to absorb and transform the waste. With time this has

changed. On the one hand, world population kept growing and, on the other, the materials

became more sophisticated. Not only the volume of the waste started to cumulate

exponentially, but also its character has changed. The nature had to assimilate not only the

classical materials like wood, but progressively different ones, with extended life-span, viz.

ceramics, glasses, synthetics and advanced alloys.

In the beginning of the 19th century there was a dramatic change in the availability of

structural materials. First the rubber industry, then (about half a century later) the plastic

industry was born. There was nothing in the history of humanity resembling the rapid

expansion of the plastics technology. By the year 1900, global annual production of plastics

was 30,000 tons, whereas 100 years later it will reach 151 million tons. Furthermore, in

analogy to metallic alloys, polymers are progressively formulated to be more performing and

less prone to the natural degradation processes, for example, during the last 25 years the UV

stability of polypropylene fibers increased by a factor of II! Evidently, ancient methods of

disposal by relying on the forces of nature are incapable to cope with the volume and quality

of modem wastes.

Only during the last two decades, the conscience of the natural limits, and the perception of

the waste disposal problems become wide spread. We see ourselves interacting with the earth,

a part of a cycle of materials and energy, and worry about the optimum way of managing it.

The concept of recycling and recovery emerged mainly from this worry. So did ultimately this

book.

The twentieth century has been known as the century of plastics. On the volume basis, the

plastics production is about three times as large as that of steel. In the industrialized countries,

the annual consumption of plastics varies from 50 to 200 kg per person. It is amazing that this

prominence was reached in less than two generations.

IX

x

The notoriety of plastics is enhanced by their dominant role in disposable items, e.g., in

packaging. Light in weight, high in rigidity and transparency, inexpensive plastic packaging is

designed to be discarded after a single use. For many people this is unreasonable waste of

high quality materials that clearly contributes to the growing problem of the global waste

disposal. Few realize that the plastics content in the municipal waste amounts to 4-6 %.

Plastics are visible, long lasting pollutants, that float on rivers, lakes and oceans. They are

considered the eye-sore of the environment, or the so-called "visual pollution." To blame

plastics for pollution is equivalent to blaming trees for the forest fires. However, in spite of

the poor logic, the vision of plastics as being the main culprit for pollution, permeates the

legislative bodies that impose targets for plastics recycling.

Environmentalists have given the highest priority to reuse and recycle as means to handle

plastics, or any other material, after their projected service time. However, these activities are

only a part of the broader concept of recovery, that includes mechanical and chemical

feedstock recycling as well as energy recuperation. Thus, recycling includes the concept that

burning plastics can be used to save natural resources. Recycling also includes reprocessing of

the waste generated by the plastics industry directly in-plant.

Many fields of expertise are necessary for the understanding of problems involved in the

value recovery from plastics waste, either as materials, chemicals or energy. For these reasons

contributions are required from chemists, physicists, engineers as well as polymer and

materials scientists. Due to the dimension of the subject, the participation of the

environmental, legal and economic experts is also of paramount importance.

It was therefore opportune for the workers in the forefront of these different areas to meet, to

discuss and to exchange ideas. This, indeed, was achieved in the NATO Advanced Study

Institute held at Antalya, Turkey, from June 16 to 27,1997.

The lectures and discussions held during the Institute covered the totality of problems

associated with plastics recycling. Regulations and practices of recycling in NATO countries,

economics and energy criteria for assessing recycling provided the necessary framework for

the discussion of these themes. Various educational, legal and economic aspects of the waste

collection, as well as availability of raw materials for the recycling plants were considered in

depth. While one of the principal emphasis was on the mechanical recycling (reprocessing) of

plastics, other options, viz. thermolysis, solvolysis and energy recovery; have also been

discussed in depth.

Within the scope of the mechanical recycling, three basic areas were discussed: (I) the

fundamentals of recycling, (2) the technology of recycling, and (3) the pertinent aspects of

recycling specific polymers or their mixtures. Thus, basic properties of polymers found most

Xl

commonly in the waste stream were identified. The mechanical recycling can be performed

using either a single-type of decontaminated resin, a clean mixture of polymers, or their

contaminated varieties. For these reasons, diverse aspects of waste separation, segregation and

cleaning were hotly debated. Fundamental and applied aspects of blends, their

compatibilization and methods of suitable morphology development were presented. This

constituted a basis for understanding the processing and performance of parts made from

plastics waste. Since the materials were already exposed to melt processing and weathering,

their stabilization is also of a paramount importance.

One of the alternative of recycling is the use of polymers that can be easily decomposed by

natural processes, namely by UV - and bio-degradation. The biodegradability of polymers is

gaining acceptance in several domains of modern life, viz. compo sting bags, agricultural

mulching films, loose-filler (to replace EPS), etc. The present world market is about 12

kton/y. Thus, the aspects of weatherability of polymers and their biodegradability were also

discussed.

Most of the lectures and discussions are presented in this volume. It is hoped that the book

meets the ambitious goals of the Institute, and will become a reference for those interested in

helping the society to attain sustainable development with plastics.

The meeting was made possible by the supports of Nato Scientific Commitee and Advanced

Study Institute and we want to acknowledge the kind cooperation provided to us at all times.

Finally we want to thank to Prof. N. Uyanik (Istanbul Technical University), Miss T. Demir

and Mr. S. Gokcesular (both from Middle East Technical University, Ankara) for their

invaluable helps during the preparation and duration of the meeting.

G. Akovali

C. A. Bernardo

J. Leidner

L. A. Utracki

M. Xanthos

LIST OF P ARTICIP ANTS (By Countries in Alphabetical Order)

(a) SCIENTIFIC COMMITTEE

G. Akovali, (Director, lecturer) Departments of Chern. and Polymer Sci .& Technology Middle East Technical University, Ankara- 06531.TURKiYE

C. A. Bernardo, (Co-Director, lecturer) Department of Polymer Engineering University ofMinho, 4800 - Guimares. PORTUGAL

J.Leidner, (lecturer) Polymer Tech., Ortec Corp. 2395 Speakman Drive, Missisagua Ontario L5K IB3 . CANADA

L. A. Utracki, (lecturer) National Research Council of Canada (NRCC) Industrial Materials Institute 75 de Mortagne. Boucherville Quebec J4B 6Y4. CANADA

M. Xanthos, (lecturer) Polymer Processing Institute at Stevens Institute of Technology Castle Point on Hudson, Hoboken. NJ. 07030. USA

(b) OTHER LECTURERS AND PARTICIPANTS

A.Akar Dept.of Chemistry; Technical University ofIstanbul 80626 Maslak-Istanbul TURKIYE

$. Altun P.K.557 -16373 Ulucami, Bursa-TURKIYE

s. Basan Dept.of Chemical Engineering Cumhuriyet University, SlVas- TURKIYE

xiii

XIV

y. Ba~ar Petkim Petrochemicals Co. R&D Center, P.O.B 9; 41740 K6rfez, Kocaeli- TURKIYE

C. Bastioli, (lecturer) Novamant S.p.a. Via Fauser 8, 28100 Novara- ITALY

C. Baykam SASA- P.O.B. 371 Adana- TURKIYE

H. Betchev 8 Kl. Ohridsky Blvd. 1756. Sofia- BULGARIA

M. J. Bevis, (lecturer) Wolfson Centre, BruneI University Uxbridge. Middlesex UB8 3PH-UK

G. Boden School of Applied Sciences University ofWolverhampton Wulfruna St., Wolverhampton.WV1 1S8- UK

M. Brebu P.Poni Inst. Of Macromolecular Chemistry 41 A Grigore Ghica- Voda Alley RO 6600 Iasi- ROMANIA

J. A. Covas Dept.of Polymer Engineering University ofMinho. 4800Guimares- PORTUGAL

A. Cunha Dept.of Polymer Engineering University ofMinho. 4800Guimares- PORTUGAL

T. Demir, (Conf. Techn. Assistant) Departments of Chern. and Polymer Sci .& Technology Middle East Technical University, Ankara- 0653 1. TURKEY

C. A. Diogo Instituto Superior Tecnico Dept. of Eng. Materials Av. Rovisco Pais; 1096 Lisbon- PORTUGAL

Aiaz Efendiev Inst. of Polymeric Materials, Academy of Sciences. Samed Vurgun Str., 124. Sumgait. 373204. Azerbaijan

H.Y.Erbil TUBITAK-Marmara Res. Center., Dept.of Chern. Eng. P. O. Box. 21. Gebze. 41470. TURKIYE

Ruya Eskimergen Techn. Dev. Foundation of Turkey (TTGV) Atatiirk Bulvarl 221 6100. K. Dere. Ankara. TURKIYE

s. Fakirov Instituto de Estrucctura de la Materia Consejo Superior de Investtigaciones Cientificas Serrano. 119- 28006. Madrid. SPAIN

s. Gok~esular, (ConI Techn. Assistant) Departments of Chern. and Polymer Sci .& Technology Middle East Technical University, Ankara- 06531.TURKiYE

G.Giirdag Dept. of Chemical Engineering University of Istanbul, 34850 AvcIlar-Istanbul. TURKIYE

s. Giiven Pilsa A.~. Ceyhan Yolu Ozeri 7. km.- P. K. 87 1321. Adana. TURKIYE

xv

XVI

K. M.Harth 3M Co., 935 Bush Avenue P. O. Box. 33331; 2- 3E- 09 St.Paul. MN. 55133- 3331. U.S.A

s. irgiide1 Brisa Bridgestone Sabancl Lastik San T.A.~. P.O.B.250 izrnit. Kocaeli. TURKiYE

R. Iltcheva University of Chern. Technology and Metallurgy 8. Kl. Ohridski Blvd. 1756. Sofia. BULGARIA

F.H.Inci Ye~il Plastik, Atalar Mah. Y.Erkan Sok. 6 P. K. 244. Yarlrnca. Kocaeli. TURKIYE

V. Khunova Dept.of Plastics and Rubber Faculty of Chern. Technology, Slovak Techn.Univ. Bratislava. SLOVAK REPUBLIC

P.P.Klemchuk, ((lecturer) Institute of Materials Science University of Connecticut. Storrs. Conn.- USA

N. KlTan Pure & Appl. Chern. Dept. Univ.of Stratchclyde Lab.C-63A-295 Cathedral Street- T.Graham Bldg. Glasgow. G 1 1XL. U.K.

s. Koseva University of St. Cyril & Methodius of Skopje, Faculty of Technology and Metallurgy Rujder Bascovic. 51000. Skopje. MACEDONIA

D.Kotzias, ((lecturer) EC Directorate General X/I- JRC Science Joint Res. And Dev. Res. Center /-21020 Ispra. ITALY

A. Kozlowska Technical University; Wybrzeze Wyspianskiego 27 50-370. Wroclaw. POLAND

M. Kozlowski Foundation for Development, Technical University; Wybrzeze Wyspianskiego 27 50-370. Wroclaw. POLAND

A. Krzan Kemijski Institut, Hasdrihova 19 1000. Ljubljana. SLOVENIA

S. Kudaibergenov Dept.of High Molecular Compounds Faculty of Chemistry; Kazakh Nat. '1 State University Vinogradov Str. 95. Almaty. 4800123- KAZAKHISTAN

H.-SikLee CET Umwelttechnik,Weidmann Strasse 21 60596. Frankfurt-am Main. GERMANY

S. Suzuki Alton House. 1 74-177. High Holborn London. WCCIV 7AA. U.K.

G. Lewis Eng.Division, School of Engineering Univ.ofWolverhampton. Wolverhampton. WVIISB. U.K.

D. Lostar Brisa A. ~. - P. K. 2500. Izmit- Kocaeli. TURKIYE

G. Manos Dept.of Chemical Engineering. South Bank University 103 Borough Road. London. SEI OAA., U.K

F. P. La Mantia, (lecturer) Universita di Palermo Dipartimento di Engegneria Chimica del Processi e dei Materiali. 90126 Palermo, ITALY

xvii

XV 111

O. Mecit Sentapol, Synthetic Resins and Coatings Co. R&DGroup Dilovasl Mevkii, Gebze. 41470 Kocaeli. TURKiYE

G. Menges, ( lecturer) Institut fUr Kunstoffverarbeitung (IKV) an der RWTH- Aachen Aachen. 62068. GERMANY

A. Mehammma Institute of Industrial Chemistry University of Setif. Setif. ALGERIA

B. Mihai Petru Poni Inst. of Macromol. Chemistry Iasi. ROMANIA

E. Mlecknik Vrije Universiteit- Victor Driessenstr. 12 D. 2018- Antwerp. BELGIUM

M.Ozdemir Dept.of Chemistry; Technical University ofIstanbul 80626 Maslak-Istanbul TURKIYE

C. Oztiirkcan Petkim Petrokimya Holding A. ~. Kalite Kontrol ve Teknik Servis Md.liigu Aliaga. izmir. TURKiYE

P. O. Petkov Research Inst. for Irrigation, Drainage and Hydraulic Eng., 136 Tzar Boris III Blvd. 110. Sofia. BULGARIA

S. D.Petrenko Dept. of Equipment and Polymer Compositions and Waste Processing, R&D Institute UkrNTplastmash 1. Shevtsova Str. 252121 Kiev. UKRAINE

A. Pontes Institute of Materials, Campus Azurem, University of Minho, 4800. Guimares. PORTUGAL

H. J. Radusch, (lecturer) Martin Luther Universitat, Fachbereich Werkstoffwissenschaflen Kunstofftechnik. Halle- Wittenberg D.06099 Halle. GERMANY

R. L.Reis Dep. Engenharia Metallurgica, Faculdade de Engenharia Universidade do Porto Rua dos Bragas. 4099. Porto Codex. PORTUGAL

L Sanduluscu Faculty of Chemistry, Univ. of Bucharest P. O. Box 12-24l. Bucharest. ROMANIA

T. Sava~r;l TUBITAK-Marmara Res. Center., Dept.of Chern. Eng. P. O. Box. 2l. Gebze. 41470. TURKIYE

S.Sugumar Central Inst of Plastics Eng. and Technology Guindy. Madras. 600 032. INDIA

W. Sulkowski Dept.of Chern. Technology, Silesian University. Szkolna. 9 400-006. Katowice. POLAND

H.Smuda CET Umwelttechnik, Weidmann Strasse 21 60596. Frankfurt-am Main. GERMANY

L Siiylemez SASA Co. P. O. Box 371 Adana- TURKiYE

XIX

xx

A. $i1lllek Atalar Mah. Y ~ar Erken Sok. 6 P. K. 24 Yanmca. Kocaeli- TURKiYE

Y. Ulcay Dept. of Textile Eng., Uludag University Goriikle. Bursa. 16059. TURKIYE

N. Uyanik, (Advisor, Con! Techn. Secretary) Dept. of Chemistry; Technical University oflstanbul 80626. Maslak- Istanbul. TURKIYE

S.E. (jziimkesici Pure & Appl. Chern. Dept. Univ.of Stratchclyde. Lab. C-63A -295 Cathedral Street- T. Graham Bldg. Glasgow. GI1XL. U.K.

J. Vlachopoulos, (lecturer) Dept. of Chemical Engineering Mc Master University, CAPPA-D 128800 Main Street West. Hamilton. Ontario. L8S 4L7 - CANADA

A .. Wasiak Inst.ofFundamental Techn. Research Polish Academy of Science Svietokrzyskaya 21. PL. 00-040. Warszawa. POLAND

J. Yamk Dept. of Chemistry Ege University. izmir- TURKiYE

T. Ya1rmyuva Istanbul University, Faculty of Engineering Dept. of Chern. Eng. Avcllar. 34850. Istanbul. TURKIYE

T. Zaharescu Faculty of Chemistry, Univ. of Bucharest P. O. Box 12- 241. Bucharest. ROMANIA

A. Zahavich Uniplast Ind.Co.- 301.Forest Avenue., P.O.Box 2000 - Orilla. Ontario. L3V 6R9. CANADA

A. Zimmermann Engler Bunte Institut an der Universitat Karlsruhe; Bereich, Gas und Kohle Richard- Willstatter- Alee. 5. 761331. Karlsruhe. GERMANY

H. Zweifel Ciba Specialitatchemie AG. R-I002.11.62 CH-4002 Basel- SWITZERLAND

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Chapter.1 INTRODUCTION

INTRODUCTION TO RECYCLING

JACOB LEIDNER ORTECH Corp. 2395 Speakman Dr. Mississsauga, Ontario -CANADA

Although the level of recycling activities fluctuates over time, the underlying driving forces point to the overall increase of these activities. Recycling is both an economic as well as environmental activity. As an economic activity, recycling represents recovery of residual value from waste product. As an environmental activity, recycling is neither inherently positive nor negative. Life cycle assessment methodology can be applied to the recycling process just like to any other process to assess the overall impact. The environmental impact can be assessed in terms of local, regional and global impacts. Ecoprofile is a form of life cycle assessment but with the application of weighing factors which allow for comparison and rating of impacts.

l.Introduction

Recently we have witnessed decline of the interest in the environmental issues and, therefore, also a decline in recycling activities. With the election of conservative governments in the industrialized countries and the focus on economic activities, the zeal and emotions caused by environmental issues seem to be a distant memory. Examining the recent history of plastics recycling might help in putting this situation in a proper perspective. Figure 1.1 schematically illustrates the intensity of our inte-rest in recycling of plastics. Plastics have been recycled to some extent throughout their existence but the major focus on large scale recycling did not occur until mid seventies. The major driving force at the time was the si;.')rtage of resin caused by the oil embargo and inadequate resin manufacturing capacity - all that at the time when plastics were displacing other materials and the demand was growing. It seemed that the only way in which the shortages of resin can be reduced was through recycling.

High prices of resins caused by the shortages were an inducement for the development of recycling technologies.

All that changed in the early eighties. Oil embargo was lifted and new manufacturing capacities went on stream - at the time when the major world economies entered recession. Most of the technologies developed in the seventies were abandoned.

3

G. Akovali et al. (eds.). Frontiers in the Science and Technology of Polymer Recycling, 3-16. © 1998 Kluwer Academic Publishers.

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The next major explosion of interest in plastics recycling occurred in the early nineties. This time it was caused by the general concern about the environment. Plastics had an image of being environmentally undesirable and the pressure was applied by the consumer as well as the governments to recycle as a way of making plastic products environmentally friendlier. In some cases the consumer was willing to pay premium for the products containing recycled material. This new situation resulted in a tremendous amount of plastics recycling activities both at the research as well as commercial level. By the mid nineties the world has changed once more. Another recession caused adaptation of a more conservative political agenda - fighting deficits became the main priority. Environmental issues lost some of their appeal. One has to ask - what next? These rather short term fluctuations of interest and activities are likely to continue - while the underlying trend will be towards increased recycling activities. This trend will be caused by the rapid population increase straining availability of raw materials and affecting the environment, increasing standard of living and per capita consumption of goods as well as environmental awareness propagated by the education system,[l].

2. Availability of Plastics Waste

The total world production of plastics in 1994 was 107.9 mIn metric tons and is expected to grow at 3.8% per year per year to reach 135 mIn metric tons by the year 2000. The annual growth of plastics consumption is expected to be of the order of 3.4% in North America, similar in Western Europe, only 2.5% in Eastern Europe but 5.2% in Asia/Oceania and 4.5% in Latin America. The growth of plastics consumption is caused by two main factors

- expansion of the end use markets such as automotive, packaging and construction, and

- substitution for the more traditional materials (especially in the less developed countries),[2].

It is very difficult to compare the amounts of waste plastics generated in different countries . The methodologies used to obtain the statistics are different and dates of publications vary . The Organization for Economic Cooperation and Development (OECD) consists of Austria, Australia, Belgium, Canada, Denmark, Finland, France, Germany, Italy, Japan, the Netherlands, New Zealand, Norway, Spain, Sweden, Switzerland, Turkiye and the United States. OECD countries produced a total of 420 millions tons of municipal waste per year ( late 80s ),[3]. If one assumes that 8% of that waste is plastic the total amount of plastics in the municipal solid waste is 34 million tons. If that material was worth only $0.10 per kilogram, the total value would be around $3.4 billion.

The quantity of the municipal solid waste in the US has grown from 88 million tons in 1960 to 152 million tons in 1980 and 209 million tons in 1994 and is further expected to increase to 223 million tons by the year 2000. The amount of plastics in the municipal

6

solid waste has increased from 0.4% in 1960 to 5.1% in 1980 to 9.5% in 1994 and is expected to further increase to 10.5% is expected by the year 2000,[6].

Plastics are disposed of in municipal solid waste as well as through other means (for example industrial waste) . Table 1 gives disposal patterns for various types of plastics.

Table I Disposal patterns of different plastic types (US, 1988 ),[4].

Resin MSW Non MSW disposal disposal % %

ABS 65 35 HDPE 85 15 LDPE 93 8 PET, PBT 88 12 PP 81 19 PS 89 11

AVERAGE 86 14 Acrylic 3 97 Nylon 29 71 Phenolic 4 96 PUR 46 54 PVC 23 77 Unsat. Polyester 10 90 Urea & melamine 8 92 AVG 21 79

Total Average 61 39

The plastics in MSW are present in the following products (US, 1994): Durable goods 28% Non - durable goods 24% Containers and packaging 48% [6]

The approximate composition of plastics waste in the MSW is PET 6% HDPE 20% PVC 7% LDPE 28% PP 13% PS 13% Other 13% [6]

In addition to the plastics in MSW, other large potential streams for plastics recycling are;

7

-carpet and textile waste ( 75% nylon): 10 million tons -automotive shredder waste ( 25% PU ): 0.9 million tons -wire and cable ( 62% polyolefins) 0.2 million tons

In 1970, recycling of plastics from the solid municipal waste was virtually non -existent. By 1980 only 0.3% of plastics in the municipal solid waste was recovered increasing to 2.2% by 1990 and 4.7% by 1994. That still compares poorly with the recycling rates of 35% for paper 23% for glass and 36% for metals, [6]. Recovery of plastics from the municipal solid waste generates approximately 1 million tons of new materials which - even at $0.10 per kilogram - is worth $100 million. These numbers indicate two things:

-Recycling of plastics is a medium size - but a growing industry -There is still potential for its further growth

3. Recycling As an Economic Activity.

3.1. CLASSIFICATION OF PLASTICS RECYCLING

Recycling can be seen in one of two ways - as an economic or environmental activity. Recycling - seen as an economic activity - is a recovery of economic value present in the product which has already served the purpose for which it has been intended. Depending on the fraction of the value which is recovered recycling of plastics can be classified into four categories :

-primary recycling - most of the value is recycled. Typical example is recycling of sprues and runners in injection molding. These are usually ground and returned into the process together with the virgin resin. Although somewhat degraded, the resin is used as a substitute for virgin material in the same application ..

-secondary recycling -recycled material is used in a less demanding application as compared to the original use. Use of mixed or contaminated plastics waste to produce plastic lumber is a good example. Recycled plastics often compete with other materials such as concrete or lumber.

-tertiary recycling - plastic waste is converted into a raw material state and recovered as such. The original value added to the raw material to convert it into plastic resin is lost. Typical example might be recovery of styrene by pyrolysis of polystyrene.

- quaternary recycling - only the energy is recovered through incineration of plastics waste,[5].

3. 2. RESIDUAL VALUE

As plastic product moves through various stages of its useful life, its useful as well as residual values change. The useful value represents its ability to serve its intended

8

purpose while its residual value represents the value which can be recovered from the product which has already fulfilled the purpose for which it has been intended.

Figure 2 illustrates the change of both useful as well as residual values of a plastic product - in this case a margarine container. We have assigned Vo - the original value to the value of the resin delivered to the molder. As the container gets molded, decorated, delivered to the packager filled with margarine, closed and delivered to the store its useful value increases reaching its maximum when the products is on a store shell. Its useful value declines when the product is purchased and then declines further when the margarine is consumed. At that time the container can be cleaned and prepared for separate collection or disposed of combined with other domestic waste. The residual value ( V r ) will be higher in the former then latter case.

As the container goes through these stages in its useful life and its useful value increases its residual value declines. It becomes less and less valuable for other then intended purpose. There are following reasons for that decline of value: • contamination with other materials (ink, margarine, lid made of different material) • dispersion (in order to recover the value as a plastic, the containers have to be collected and transported back to the molding facility ).

After the product is consumed, the residual value of the plastic product somewhat increases. Eventually the residual/useful value of the product become the same, [1].

The residual value of a plastic product can be described by:

V reV 0-V dept+ V cd+Cdisp (1)

where (V r) is residual value, (VO) is original value, (V dept) is depleted value and (Cdisp) is disposal cost.

The value of consumer demand can have both positive as well as negative value, and: lfPositive;

*Consumer will pay premium for product containing recycled material, *Given a choice, consumer will purchase a product containing recycled material, * Consumer favours the products manufactured by a company known to have a

positive environmental image, *Manufacturer has to use recycled material to conform with the legislation, *Regulations, or the threat of regulations require the company to manage its own

waste. If Negative;

*Consumer expects to pay less for the product containing recycled material, *Given a choice, consumer will purchase a product made of virgin material, Disposal cost represents a liability associated with the waste if recycling option is not

exercised. Depletion of residual value occurs during production as well as distribution and usage of the product:

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V prod=V degr-V form+ V cont (3)

Value lost due to contamination is equal to the cost of cleaning and separation and value lost due to new form is equal to the cost of conversion to the proper form (container into pellets ).

V conFCcl+Csep (4)

V form=Cconv (5)

Value depleted during distribution is due to dispersion of the product away from the processing centers and further contamination with other wastes. Value lost due to dispersion is equal to the cost of collection and the value lost due to contamination is. equal to the cost of cleaning and separation.

V distr=Ccoll+C' sep+C' cl (6)

The overall residual value of plastic waste can be expressed as follows:

Vr=VO-«Vdeg+ Ccl+ Csep+ Cconv)+( Ccoll+C'sep+C'cJ))+Vcd+Cdisp (7)

For economically successful recycling, a high residual value of plastic waste is required.

4. Recycling of Plastics As an Environmental Activity.

4.1 LIFE CYCLE ASSESSMENT

Recycling is an industrial activity with its own environmental impacts - neither inherently positive nor inherently negative. Life cycle assessment gives us means to compare environmental and resource impacts for specific plastic products with and without recycling as well as to compare environmental impacts of various types of recycling. Life cycle assessment is an objective process to evaluate environmental and resource impacts associated with a product, process, or activity by identifying and quantifying energy and material usage and environmental releases. The objectives of life cycle assessment are to assess the impact of energy usage and environmental releases on the environment, to compare environmental impacts of alternative processes or products and to assess and implement opportunities for environmental improvement. The assessment includes the

11

INPUT OUTPUT

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Fig.3. Model for life cycle assessment .[7]

12

whole life cycle of the product from the acquisition of raw materials to the disposal of the waste. Useful output from one step in the process serves as an input into the next step. The overall input is raw materials and energy - the output is products, water effluents, airborne emissions, solid wastes and other releases (Fig 3), [7]. The reliability of the results of life cycle assessment depend on the reliability of input data. This data are not always universal - for example different sources of energy (coal, natural gas, etc.) might be used to manufacture the same product in different areas and the environmental impacts might therefore be different. Different processes can be compared in terms of individual effluents ( for example water effluent or solid wastes ) but it is not possible to assign a single environmental ranking to a process.

4.2. ECOPROFILES

Ecoprofile is based on the same principle as a life cycle assessment but with the application of weighing factors which allow for comparison and rating of impacts from different manufacturing sites.

Emissions can be classified into three categories depending on the area they affect: * global ~ global warming, ozone layer depletion, * regional - acidification, nutrient enrichment, low level ozone.formation; * local - toxicity, area degradation,[8].

In order to compare the effect of different emissions the concept of equivalents has been cre ated. For example global warming is caused by both carbon dioxide as well as methane. It is accepted that the molecule of methane creates 25 times global warming of the molecule of carbon dioxide and 69 times more on a weight basis. The global warming effects can be described interI1Js of the equivalent weight of C02. If the process releases methane the emission has to be multiplied by the factor of 69, [9]. Other emissions can be represented in similar fashion. In order to further normalize the equivalents, we can divide them by the average annual emission per inhabitant affected by that emission. The units of normalized emissions are called person equivalents and are calculated as:

PEx=eWxe IX (8)

where (x) is one of the environmental effects, (Pex) is person equivalent of effect x, (e) is emission with environmental effect x, (wxe) is weighting factor which standardizes the emission e, and, (X) is average annual emission of substances with an environmental effect x per inhabitant, [10];[11].

A similar approach can be applied to the calculation of consumption of resources. These are already expressed in units of weight so that the weighing factor w is equal 1 and X represents world consumption of that resource divided by the total world population. The consumption is also divided by two factors - representing the available reserves of that resource and the importance of that resource. Figure 4 shows a

13

schematic which can be used for calculation of ecoprofiles. In the figure, each of the boxes represents environmental and resource impacts associated with individual activities. Environmental and resource impacts saved by recycling represent impacts of manufacturing of resin or other material saved because of recycling of plastic ( for example if plastic is incinerated and energy recovered the environmental impact of production of the equivalent amount of coal is saved).

The environmental and resource impacts expressed in person equivalents are calculated as follows:

PEx,o=Mx(1-a)+Xx+Ux +Dx (l-b)+Rx b-Sx (b-a) (9)

If the above equation is used for comparison of various recycling processes we can subtract constant terms (Mx+ Xx+Ux) leaving us with a simplified form,

PEx=Dx (l-b)+Rx b-Sx (b-a)- Mxa (10)

Molgaard [9] calculated ecoprofiles for the following methods of recycling and disposal of plastics. I.-manual separation, washing, drying, melting and pelletizing 2.- recycling with solvent separation 3.-recycling without separation by molding comingled plastics into lumber replacement. 4.-landfill 5-incineration with heat recovery 6-pyrolysis

The outcome of these calculations is shown below for both environmental effects expressed in micro person equivalents (j..lPE ) per kg of plastic waste recycled and the resource consumption expressed in the same units, [1 0].

Negative numbers in the table.2 below indicate that the environmental effects saved are greater than the effect of the process.

Success of recycling activities is largely dependent on the favourable economics of the process. For the recycling process to fulfill its environmental objective at the same time both the ecoprofile of the process as well as economics have to be favourable. That can be achieved by both the educated consumer demanding recycled products when recycling makes environmental sense and educated voter demanding that governments create an environment where recycling makes economic sense.

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Table 2. Environmental and resource impacts of various disposal and recycling processes, [10] (Reprinted from Claus Molgaard, Environmental impacts by disposal of plastic from municipal solid waste, Resource Conservation and Recycling 15 (1995) pp 51-63, with kind permission of Elsevier Science - NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Nederlands)

Environmental Process effect

1 2 3 4 5 6 global warming -40 -67 52 0 -12 145 ozone layer 0 0 0 0 0 0 depletion acidification -120 -128 23 0 -94 22 low level ozone - - 7 0 1

2 3 6 6 7 0 1 0 2 )

solid waste 3 3 2 8 8 7 6 1 8 7 6 9 ) 7

~ )

Resource Process consumption

1 2 3 4 5 6 Crude oil -1420 -1417 -43 0 -125 -516 Natural gas -2077 -1360 59 0 -2083 -433 Coal 390 231 299 0 -406 371

16

5. References

1. J. Leidner, Concept of residual value and economics of recycling, AutoRecycle '95, Dearborn, Michigan, Nov 15-16, 1995, pp151-160 2. L. Young, World Plastics Outlook in Modem Plastics Encyclopedia, 1996, McGraw­Hill Inc. New York, NY. 3. J . E . McCarthy, Recycling and reducing packaging waste: How the United States compares to other countries, Resources, Conservation and Recycling, 8 (1993) pp 293-360 4 . Franklin Associates, Characterization of plastics products in municipal solid waste, Final Report, (1990). Prepared for Council for Solid Waste Solutions. 5.1. Leidner, Plastics Waste, Recovery of Economic Value,(1981), Marcel Dekker, New York. 6. Environmental Protection Agency,(March 1996). Characterization of municipal solid waste in the United States; 1994 update, EPA 530-R-96-001. Washington, DC. 7. 1. Fava et aI., A Technical framework for life - cycle assessments, Workshop Report, (August 18 - 23, 1990),Society of Environmental Toxicology and Chemistry, Smugglers Notch, Vermont. 8. M. Z. Hauschild, B. A. Ntim, Selected Topics in Environmental Management, (1993). UNESCO Series of Leaming Materials in Engineering Sciences. 9. R. G. Hunt, LCA considerations of solid waste management alternatives for paper and plastics, Resources, Conservation and recycling, 14 (1995)., pp 225-231 10. C. Molgaard, Environmental impacts by disposal of plastics from municipal solid waste, Resources, Conservation and Recycling, 15 (1995) pp 51-63 11. C. Molgaard, L. Alting, Ecoprofiles for disposal processes Annual Technical Conference of the SPE, (1994), pp 3021-3025

REGULATIONS AND PRACTICES OF POLYMER RECYCLING IN NATO

COUNTRIES

A - EUROPEAN COUNTRIES

DONATO CURTO Dipartimento di Ingegneria Chimica dei Processi e dei Materiali Universita di Palermo, viale della Scienze, Palermo - ITALY, and Y ASEMIN BASAR Petkim Petrochemicals Co. Research and Development Center P. o Box 9; 41740 KorJez / Kocaeli- TURKEY.

1. General Outlook

Environmental policy in all European NATO countries, with reference to legal regulations about recycling, are nearly the same as the environmental policy in European Community, EC.

From 1973 EC has started to arrange particular measures and sketched out preliminary forms of intervention to apply in member States. However, environmental policy finds an official place inside EC policy in the "Single European Act" drawn up at Luxembourg (17 of February 1986) and at Aja (28 of February 1986) which pursue to following aims: - safeguarding, protection, improvement of environmental quality and protection of human health; wise and rational use of natural resources; - international promotion of measures assigned for the resolution of environmental problems at a regional and world level.

Fundamental principles for the whole environmental Community are the prevention (correction of environment damages from the starting point) and the well-known criterion of "who makes a mistake has to pay". Even before the codification of concepts of EC action plans, the important directive 75 / 442 was provided for adoption of measures for recycling and transformation of waste. Unfortunately this directive has not been adopted for many years (Italy assimilated it after seven years, in 1982). And hence, for a long period of time the recycling sector had an uncertain juridical system, articulated in the interface between waste discharge action and process

17

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 17-28. © 1998 Kluwer Academic Publishers.

18

of raw materials transformation. The next following directive of 1991 confirmed an approach similar to that of 1975, with some additional terminological clarifications. Obviously not only general directives but specific interventions were needed at that time. To achieve this purpose, EC adopted a "Community directive" in 1985 for food packaging, showing that legislative attention was focusing mainly on packaging. The aim of this directive was to define a series of actions concerning " the production, commercialisation, use, recycling and re-use of food-liquid packaging; and, removal of used packaging materials". These actions were expected to facilitate ''the re-filling of food-liquid packaging", and, "for packaging of non refillables" They were also expected to promote a "selected assemblage", "effective processes for collecting packaging materials from domestic wastes", and "provide outlets for materials made from used packaging materials". However, these regulations brought some important arguments as well (such as, more concern for the production pace of recyclates is necessary and, moreover, it is necessary to define and boost outlets for materials made from recycled products).

Some European States have elaborated a body of legislation detached from Community approach, obliging the EC to enlarge its horizons again and to provide for the use of a directive dedicated to all kinds of packaging materials (and not only for the food-liquid containers). With this more general characteristic - altough still it is limited to a specific category of products -, the Community directive about packaging and waste-packaging set some principles about recovery and waste management in the Community territory. It also made some explicit expectations about recycling, by imposing minimum objectives to which all European Unity States will have to adapt themselves.

The last body of legislation about packaging and waste-packaging (European Parliament and European Council directive 94/62/CEE of20 December 1994) provides particular recovery objectives of 50% and 65% by weight (in relation to the amount of materials contained in waste-packaging), with a minimum of 15% for each material ( i.e, paper, glass, plastics, metals, etc.). Community regulations are based on two pillars:

- national regulations regarding packaging wastes; removal of obstacles for a free circulation of goods and elimination of distortion or restriction of competition;

- prevention of packaging wastes production, and reduction in the quantity of wastes assigned to be dumped, employing re-packaging and other forms of recovery. Particularly, each member State has to:

• undertake actions to reduce the quantity of waste, promoting the use of "clean" technologies and products; and elaborate a system for packaging wastes with 50 to 65% recovery within 5 years of the directive (that is by 2001); to recycle packaging materials of between 25% and 45%, with basic specific objectives for materials of at least 15% and encourage, in some cases, the use of recycleates evolving from packaging materials and their recovery for production of new packaging or other products;

19

• ensure that, by January 1998, the packaging materials introduced into market will correspond to specific requirements shown by this directive, including their weight and volume minimisation, re-use, recycling and recovery;

• limit contents of heavy metals in packaging materials and encourage packaging re­use systems, if they are pro-environment and are included in the Treaty; • introduce economic instruments to achieve community objectives according to

community environmental policy principles; introduce superior recovery and recycling objectives which do not obstruct interior market and objectives achievement in member States and notify committee of passed or pending measures.

Directive establishes that a packaging material is "recyclable" if it contains a certain percent (limits are not yet defined) by weight of permitted recyclable materials. "Recycling" is, juridically, re-introduction of materials in production cycles, and "re_ use" is re-introduction of the products themselves. The Community directive about packaging and packaging wastes, to day, represents a juridical instrument which mainly contributes to recycling markets and represents an important instrument for all types of polymeric recyclables besides packaging. i.e, polymeric wastes that can evolve from construction, transportation, electric, electronic and even medical applications. There is a new study in progress now concerning directive proposal projects specific for each sector, inspired by similar principles as those established for packaging.

2- Consumption of Plastics

Let us examine plastics consumption in European Nato Countries mainly by using the data presented by the Environmental report of the European Centre for Plastics (Environmental Unit of the Association of Plastics Manufacturers in Europe), and Turkish DIE Statistics; both for 1994, which are summarized in Tables 1, .2 and 3 in the following page. As it is seen from these tables, the realistic plastics consumption figure in Western Europe for 1994 is above 27 million tons in which polyolefins have the biggest share in packaging applications, as expected.

Table I - Plastics consumption by type - Western Europe (1994) Plastics type Tons.xlOOO Plastics type

LDPEILLDPE 6,095 EPS HDPE 4,066 PMMA PP 3,959 Acetals PVC 5,688 Polycarbonates Thermosets 2,806 Polyamides PS 1,815 Acrylics PET 800 Others ABS/SAN 550

Tons.xlO00 603 234 104 20

395 11

291

20

Table 2 - Total plastics consumption by country - Western Europe (1994) Country Consumption %

BelgiumlLux. 1,145 4.1 Denmark 473 1.7 France 3,728 13.7 Germany 7,016 25.7 Greece 322 * 1.2 Ireland 157 * 0.6 Italy 4,422 16.2 Netherlands 1,085 4.0 Portugal 403 1.5 Spain 2,019 7.4 Turkiye (Turkey) 1,000 3.7 United Kingdom 3,188 11.7 Austria 568 2.1 Finland 383 1.4 Norway 197 0.7 Sweden 584 2.1 Switzerland 570 2.1 Western Europe 27,260 100.0

* Estimates Unit: x 1000 tons

Table 3- Plastics Consumption by industrial sector - Western Europe (1994),average-Packaging 42% Automotive 8% Building/Construction 20% Agriculture 4% ElectricallElectronic 11 % Other 15%

3 - Current Regulations and Practices in Some European Nato Countries

In European Nato Countries, usually each State makes a choice of her own regulations to interpret general applications of EC directive, and thus, many and sometimes deep differences may occur among the legislative plans of European Nato States on recycling of plastics.. In the following, a brief summary will be presented on the interpretation of Community directives made by some European member States and their incorporation into the existing webs.

21

AUSTRIA Existing regulations of polymer recycling in Austria are very similar to Germany and producers, distributors and importers are held responsible for the packaging materials used; which involves an obligation to find ways to re-use them, alternatively, to have an open system of gathering and recovery. Only small producers and distributors are excluded from this, namely; those that introduce less than 300 kg of paper and cardboard, 800 kg of glass, 100 kg of metal, 100 kg of plastics, 100 kg of wood and 50 kg of other packaging materials, into market; annually. Organizations that join to this recovery system must adhere to these obligations. The regulation considers energy recovery as well and utilizes the "green point" system, but the "mark" is applied only on primary packaging.

BELGIUM In Belgium, responsibilities and regulations for wastes management are given to regional authorities. However, the Belgian State has shown the necessity for harmonized regulations, rather than different regional laws, to avoid discrepancies between economic operators and consumers in different regions. As a result, a federal law draft has been prepared regarding environmental protection and public safety while supporting production and consumption methods. This regulation will also absorb various aspects of Community directives connected with the essential packaging requirements, for those containing heavy metals and with rules relating to their prevention. These already have promoted re-use and recovery (particularly recycling) substantiallly to prevent or reduce incineration.

Responsibilities for packaging-wastes utilization certainly should fall on users and importers. Overall objectives of recovery and recycling set are: from 35% of recycling during 1996 and it should reach to 50% by 1999. Objectives have to be achieved for both domestic and industrial packaging materials. Specific directive objectives regarding materials are 15%. Societies and organizations that do not obey these objectives will have a sanction of 20,000 Belgian francs for every ton below recovery objectives, and of 30,000 FB for every ton below recycling objectives. In Belgium, societies that introduce more than 10 tons of packaging material each year into the market are expected to prepare a three-year prevention program. Industrial packaging users can decide alternatively ifthey choose to return used packaging materials either to the purveyors, to municipality or to national organizations appropriately constituted, or choose to provide their own recycling or utilization systems. And during 1995, in Belgium; recycling of domestic packaging gave some interesting results: recycling of 47% of glass, 24% of metals, 10% of paper and cardboard, 60% of plastic bottles from wastes (with a targeted objective of 172,000 tons equivalent to 23% of all recycling) was achieved. Belgium adopted the "green point" marking system for packaging materials.

22

DENMARK In Denmark, "the Environmental Protection Act", adopted in 1993, requires producers and importers to increase both lifetime cycle and recycling of their products and to assure that their dumping does not involve any damage to the environment. Users and consumers are expected to contribute disposal problems and promote recycling.

There are certain requirements for certain product types: the use of PET re-fillable bottles is obligatory nationally in Denmark for bottling beer and all carbonated beverages; whereas, there is an exemption for plain beverages. Latter condition has provoked some protest inside the European Community and it appears likely that this regulation will be modified soon. According to a voluntary agreement of 1988 (which is strongly criticized as well), Danish industry has asked to reduce PVC use in packaging, which already caused an appreciable decrease of the use of this material.

Denmark already has about 40 incinerators used for energy recovery and hence can achieve a very high level of recovery by combustion, which is 90% . Certainy this results to some compatibility problems with the Community objectives provided by directive 94/62/CE, where the target set for recycling of domestic wastes is 13 %.

FRANCE The decree of "January '93, (Lalonde Decree)" forces packaged goods producers and importers to contract for packaging-waste recovery with a private national system (that has to be recognized and approved by Government) or with an independent recovery system. This regulation aims to reduce the amount of packaging wastes destined to be dumped.

The most important difference between the French and German regulation lies in the fact that the first does not discriminate between different types of packaging wastes "utilization", by considering re-filling, recycling and energy recovery in the same way. In July '94, France adopted an another decree regarding industrial and commercial packaging materials. This regulation came into force in September of the same year for some packaging categories (especially paper and cardboard), and by July '95 for other packaging categories.

"Eco-Emballages" is the first organization that works according to French standards regarding management of packaging wastes derived from the collection of city solid wastes. It finances the gathering efforts made by local authorities, to assure recycling of 75% of packaging present in domestic wastes, by 2003. The term of the contract with Eco-Emballages is for three-years. Societies that join the system can use the "green mark" trademark on their packaging. Since the end of 1994, all main societies working in the packaging sector have supported this organization. Ninety four percent of the packaging introduced into French markets is indicated by the green point, and Eco­Emballages has issued so far more than 10,000 licences.

23

GERMANY The German Federal Order of April, 1991, the "Topfer Regulations", represents the first example of national legislation where packaging recovery problems are considered in their totality. According to this regulation, packaging producers and distributors are considered as being responsible for their packaging-wastes recovery and recycling. Packaging-wastes management has to be done outside of a public system of wastes gathering; distributors must remove secondary and tertiary packaging before their products sale, and they can give used packaging to their purveyors, in order to guarantee recycling of packaging materials. Consumers can also return used packaging materials (in this case it will be primary packaging) to locations where they were purchased. Producers and distributors who are a part of the "Duales System" do not have to guarantee packaging re-use and recycling, because this organization picks up, selects and returns sale-packaging for recycling.

The gathering and selection systems are founded under a trade-mark - the "green mark" - given to firms that are a part of this system. The amount due, in this system, is such that 80% of used packaging material has to be collected. Within this total percentage, 90% of glass, tin-plated band and aluminium, and 80% of plastics, paper and cardboard compounds have to be assigned to recycling. Combustion for energy recovery is not a part of these objectives. Regulations put forward by this order are (probably) so strict that the same European Committee began violation proceedings in December 1995 against the German Government, for non-observance of "art. 30 of the Rome Treaty". Stating costs for packaging materials recovery and the general preference given to re-usable packaging are possible causes of distortions of free property circulation in the European Market. The German Government prepared an Order to modify and amend previous regulation versions and introduce necessary changes to comply with Community directive 94/62/CE. New regulation objectives seem to agree more with Community provisions, even if they are in some cases outside of the EC targets. These regulations should result in energetic recovery, but only for transport and sale packaging, "directly" made with renewable materials (wood, cotton, jute, etc.). This expectation seems non-applicable to paper and cardboard, because they are made "indirectly" with renewable materials. Packaging-wastes management is assigned to the private system.

HOLLAND Dutch packaging industries and the government had an agreement in 1991 to reduce the levels of dumping or energy recovery by combustion to zero level by the year 2000. Same agreement also provides weight reduction of new packaging materials that can be introduced into the market (where target is set for a reduction of 10% by 2000 as compared to 1996), removal of over and multiple packaging, and substitution of materials for those combinations that cannot be recycled. Reduction in the number of different polymer types used for packaging, replacement of heavy metal containing

24

printing inks and solvents with their more environment friendly counterparts are also provided by the agreement. There certainly is a preference given for re-usable forms of packaging materials rather than for mono-usables. The obligation of 1995 to recycle 45% of used packaging materials is still valid.

Holland objected adoption of Community directive because it was found less effective than the current national regulations set. A proposal to accomodate directive 94/62/CE, in fact, provided objectives much greater than that established by EC (65% for recovery and 45% for recycling). These objectives, for primary, secondary and tertiary packaging, were to be achieved by October 1997. Local authorities will be responsible for the restatement and financial support of the differentiated collections of used packaging materials present in municipal solid wastes. This prescription is obligatory for glass, paper and cardboard, textiles and organics, whereas it is desirable for plastic bottles and metal containers.

Groups which do not want to sign the agreement, are given certain obligations to fulfill. Users and distributors can satisfy their own obligations by organizing a personal recovery system (approved by the Environmental Ministry every 5 years), pruticipating in a system that obliges every economic operator working in a defined industrial sector. Since January 1996, disposal of used packaging materials by dumping is forbidden in Holland.

ITALY In Italy, recently a new recycling law has been approved. It is "the legislative decree n.22 of 5 February 1997", known as "Decree Ronchi" from the name of the Minister of Environmental Policy who promulgated it. It has filled the gaps existed in this area since 1993 even after the promulgation of a number of decrees which contradicted themselves. New decree needs a number of integrative prescriptive points to be really operative, among which, there are several technical regulations. The decree is aimed at re-utilization, recycling and recovery of waste: dicharge is a marginal activity valid only for fundamental technical reasons and not for economical ones. In the hierarchical scale of waste management, there is; first of all, their prevention followed by recovery, and only in particular cases do we find dicharge. There is also a new important concept in the decree: waste management has to adapt responsible and cooperative principles regarding all areas of production, distribution and consumption of goods that produce wastes. The concept is: "whoever produce and distribute goods that lead to wastes is responsible for their correct recovery and/or discharge as well". Hence there is "the principle of widespread and shared responsibility" which means the responsibility for choices and costs of the entire products management, and also, of waste discharge for the producers and distributors. Hence, both producers and users would contribute to the activity of "Co.Na.l", the "national consortium for packaging". The obligatory overall consortium would deal with all packaging waste coming from families or from municipal solid waste. Co.Na.l will abide by a National Observatory on waste that will

25

oversee the management of packaging waste. There are following possible ways to achieve this aim: - an autonomic organization of collection, reuse, recycling and recovery of packaging wastes; - creation of joint consortium for typology of packaging material and use of caution payments. The most relevant aspects of the law can be summarized as follows: • the term "residual", to indicate waste for recycling or recovery, completely disappears. Waste remains as waste, even if it is sold and recovered in other processing cycles, hence the words such as "secondary materials", "residuals" or "recoverable waste" does not mean much; • a system for managing packaging waste, indicated in the directive CEE 94/62 introduced an obligatory consortium for polyethylene goods.

As for packaging, the European directive n.62 (1994) set following goal to be achieved within the next 5 years: recovery of all packaging materials by 50- 65% (weight); their recycling by 25 - 45% (weight), with a minimum of 15% for each type of material used. Both of these goals superpose nicely with those of the separate collection systems: the law provides that the cities will collect 15% , 25% and 35% of the urban waste within next two, four and six years; respectively.

In Italy, currently 14 million tons of packaging materials are produced (plastics represent 17% of it which is about 2.4 millions tons) with a turnover of about 14,000 milliards of Itl, where there are 30,000 employed people and 2,000 producting units. At the present, there are two different tendencies in the production of plastics packaging in Italy: decrease of the consumption of raw materials per unit of product, and increase of use of recycled plastics whenever possible (mainly for bags, bottles etc); the latter of which represent about 13% of total plastics used.

When Co.Na.I will be instituted, all other obligatory consortiums for recycling instituted by a law of 1988, (among these there is also Replastic), will be out. It is useful to remember that in 1996 Replastic doubled the collection overcoming the 76,000 tons of collected material destined for recycling and recovery (Fig. 1). About one half of the Italian population is involved in this separate collection ..

26

80000~----------------------------~

60000

40000 D Materials recycling

• Energy recovery

20000

Figure 1 - Amount (tons) of separate plastics collection in Italy

PORTUGAL With the law decree n.322/95, Portugal has carried out its first step assumption of the Community directive "Packaging and Packaging Waste". This regulation shows two packaging management systems: a caution money system and an "integrated system". Local authorities are responsible for collection of solid city wastes. Each packaging line must pay additional costs associated with differentiated collections and with packaging­wastes selection. This can be done with contracts or voluntary agreements that deal in gathering and selection. Moreover economic operators must guarantee resumption and successive destination to utilization of gathered used packaging.

In Portugal, too, there would be adopted a system based on "green point". Raw materials producers and developers are responsible for utilization of the packaging wastes fraction content in solid city-wastes. Commercial and industrial packaging distributors and users are responsible for utilization of these products, which must be done in specific places, with cautional money systems, or with an integrated system.

The new Portuguese body of law quotes faithfully many Community rules, including objectives, definitions and packaging essential requirements. The law decree came into force in June '96. As a summary of DL 322/95 another decree was issued the 5 June 1996 for assumption of Community directive 94/62/CE. This regulation applies very severe measures to some packaging categories: in particular it provides that, by 1999, containers for soft-drinks, beers and restorative mineral waters have to be packaged in re-fillable containers. Retailers who introduce products in non re-fillable containers must obligatorily offer the same product categories in re-fillable packaging. For the non re-usable packaging, responsibility for post-consumer management is on

27

producers, transformers and users. The decree confirms that economic operators can create their own packaging resumption system, or join an integrated system (that concerns also industrial packaging as in Austria and United Kingdom, but contrary to Franch regulations).

SPAIN Since 1993 Spanish related industry and local authorities had been looking for a convenient plan of voluntary agreement for packaging wastes management nationwide, an agreement similar to the French regulation, that is a system based on a "green point" paid by compounders. After a number of attempts, effords to reach to an agreement finally have failed and a new text has been prepared recently which is awaiting for official adoption. According to this draft, producers and distributors are given certain obligations for recovery of their products. Government, after consultations with regional authorities and industry will establish the limits of recovery, annually; except for re-fillable containers. Producers and distributors would be exempt from these obligations if they join to an integrated recovery system. This system (or organism) would then be approved by autonomous regions and the approval would be valid for 5 years. The system mentioned would guarantee collection of packaging at consumer level, and it would also assure the achievement of recovery and recycling objectives in accordance weith the art.6 of Community directive 94/62/CE (after 5 years from its commencement). Several objectives to decrease the use of certain types of packaging materials are provided in the system, in particular for a reduction by 20% in the use of PVC for food-packaging If the system is not able to achieve these objectives, that type of packaging will be subjected to certain "eco-taxes". Local authorities, as well, will be obliged to participate in the integrated system. Additional costs (that is the difference between differentiated and undifferentiated collection) will be paid by each industry with a system based on the "green point". A committee is working on to define a financing system of local authorities and the correspondent contribution by operators (which should vary according to the types of packaging materials).

TURKIYE Although the total plastics consumption is currently more than one million tons (Table. 1 ) with a high demand, in Turkiye; yet it is not enough to carry the consumption per capita figure (17.5 kg.) to the level of Western countries. About 60% of plastics consumed are thermoplastic with highest contributions from LDPE, PVC, PP and PET. From the recycling point of view; this consumption pattern indicates high possibility of recycling especially in the case of LDPE and PET. Although it is not possible to give the exact figure of plastics recycling in Turkiye, at least; the existance of an organised collection system indicates a high degree of recycling. A survey has been carried out by the Ministry of Environment about recycling capacities of different facilities, but the capacities available has been repeated for each type of plastic in the

28

survey hence it is far from giving the correct recycling capacity. Several sources indicate recycling rates as 40% which mainly consists of polyethylene.

The plastic waste generated during production as scraps are either sold or used directly by the manufacturer. Post-consumer wastes, on the other hand; are either collected by big consumers (i.e, greenhouse films and fertilizer bags) by a rather well organized system and sold afterwards; or, in the case of small consumers (like supermarkets, restaurants and families), they are collected from the garbage bins by the people who carry out this as a profession. Nearly 60% of the plastic scrap in the garbage is collected by this way, and rest is transferred to waste fields of the municipal Government. The waste in waste fields are sold on bidding base and second re­separation of waste plastics and other recyclables are done here by the buyer. So far, no physical or chemical separation technique is applied for separation and only visual classification is carried out manually. From time to time, producers of PET and PVC bottles organize campaigns for the collection during which a refund is paid to the collector. Also to cope up with the quota application (to be given in the next paragraph), deposit is an another way of collection.

There are various methods applied for the reprocessing of collected scrap in Turkiye, varying from almost completely automatic systems to primitive recycling facilities. Although physical recycling is carried out mainly, there is also one chemical PET recycling unit already on-stream.

Establishment of waste management policies for the Country is assigned to the Ministry of Environment. "Control of Solid Waste Act", put in power after its publication in the Official Gazette on March,14;1991 deals also with the plastics waste, and in fact, it is more compherensive than the legislations existing in some European Countries.

REGULATIONS AND PRACTICES OF RECYCLING IN NATO COUNTRIES B - CANADA AND UNITED STATES OF AMERICA

F. H. C. EDGECOMBE Canadian Plastics Industry Association 5925 Airport Road, Suite 500 Mississauga, Ontario, L4V I WI-CANADA

CANADA

1. Plastics Production and Consumption

1.1. Plastics Production

By virtue of the abundance of oil and especially natural gas, Canada is a major producer of polymer resins. World scale plants located in the Western Canadian province of Alberta, the province of Ontario and to a lesser degree in the province of Quebec, compete in the domestic arid world markets for sales of the commodity thermoplastics LDPE, LLDPE, HDPE, PP, PS and PVC. Table 1 illustrates 1995 production levels.

Table. 1. Canadian Resin Production, 1995 (Thousands of Metric Tonnes)

Low Density/ L. L. D. Polyethylene High Density Polyethylene Polyvinyl Chloride Polystyrene & Copolymers Polypropylene ABS Polyester, Unsaturated Other Total

1308 765 434 155 330 34 58 93

3177

Domestic producers compete in the Canadian market with other North American polymer manufacturers as well as resin producers located in other parts of the world. By

29

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 29-39. © 1998 Kluwer Academic Publishers.

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the same token, Canadian manufacturers enjoy substantial export sales particularly in Pacific rim countries.

1.2. Plastics Consumption

Table 2 shows Canadian domestic consumption of plastic resins.(allowance made for Imports & Exports in 1995).

Table. 2. Canadian Consumption of Plastics Resins (Thousands Of Metric Tonnes)

L. D.IL.L. D. Polyethylene High Density Polyethylene Polyvinyl Chloride Polystyrene & Copolymers Polypropylene ABS Polyester, Unsaturated Other Total

640 345 357 148 359 76 61

419 2505

Canada possesses a sophisticated plastics processing industry which competes on a North American basis under the auspices of the North American Free Trade Agreement in many sectors of the economy of which the transportation sector (automobile parts) is a major example. The Canadian processing industry has capability in all of the important applications demanded by a highly developed consumer society. Table 3 illustrates the Canadian pattern of consumption by market application.

Table. 3. Canadian Plastics Consumption by Application Packaging 34% Construction 26% Transportation 18% ElectricallElectronics 5% Furniture Other

2. Plastics Recycling

5% 12%

The recycling of plastics has existed in Canada since the establishment of the first processor of plastic resin. The manufacturers of plastics products have always recycled

31

where possible their own production wastes. This was either done in house by merely regrinding the scrap and feeding it back to the process or in some instances having an outside reprocessor handle the scrap material for them. This type of reprocessing is generally quite simple with the scrap for reprocessing being a single variety of polymer and generally quite clean.

During the last fifteen years the reprocessing, recycling industry has expanded and become more sophisticated in its technology as the need to handle post consumer plastic products has grown.

The reprocessor of post consumer plastic products today must have capability to sort, grind, wash, dry and repelletize post consumer plastics products such as milkjugs and detergent bottles and produce a plastic resin which can compete in the marketplace with virgin resins to produce new plastic articles.

3. Post Consumer Plastics Recycling

Post consumer plastics recycling in Canada has emphasized the recycling of used plastic packaging. Although plastics make up a small proportion (7% by weight) of Canada's municipal solid waste stream, their visibility to the householder and the regulator has attracted substantial attention. Indeed all packaging which is about 35% by weight of municipal solid waste has been singled out for attention.

In 1989, Canada promulgated its National Packaging Protocol, which among other things, created a target for the diversion of packaging waste from landfill or incineration. This target was a 50% diversion of the quantity of packaging sent for disposal in 1988 by the year 2000. The target made no allowance for growth in popUlation and thereby consumption during the 12 year interval and it required that the diversion be achieved by source reduction, reuse and recycling. Recovery of energy was excluded as a valid means of diversion.

The protocol applied to all packaging; that is packaging used in the transportation of goods as well as the packaging used to contain consumer products. Although the protocol was voluntary it was subscribed to by all ten Canadian Provinces and the two territories as a keystone in their waste reduction programmes. The protocol included a provision to measure progress at the end of 1992 and again in 1996. Failure to achieve the interim targets set for those two years could result in the voluntary nature of the programme being revoked and a regulatory regime being substituted in its place. Progress was measured in a statistically sound manner by Statistics Canada. A relatively large sampling of Canadian businesses assured confidence in the aggregate result.

The results of the 1996 survey were analyzed in late 1997 and they indicated that Canadians had achieved a diversion of packaging from waste which exceeded the 50% target for the year 2000. The achievement was most noticeable in the reduction of industrial packaging used for the transportation of goods to market. None the less,

32

significant progress had been made in the diversion of post consumer packaging as well. Table 4 lists some of the results tabulated during the most recent survey.

Table. 4. Packaging Disposed From Waste ( 1996 Figures Compared With 1988 ) Packaging disposed (1988) 5.41 million tonnes Packaging disposed (1996) 2.64 million tonnes Percent reduction 51.2% Percent reduction (per capita basis) 56%

During the course of the 1996 survey for the packaging protocol, a study of post consumer household packaging recycling was also carried out. The results are shown in Table 5.

Table. 5. Post Consumer Household Packaging Recycling '(1996)- tonnes-From Deposit Systems 111,464 From CurbsidelDepot Systems 452,428 Total 563,892

The Canadian plastics industry conducts on an annual basis a survey of post consumer plastics recycling carried out in Canada. Since the inception of this survey, a steady growth in post consumer plastics recycling has been recorded.

3.1.Collection and Separation of Post Consumer Plastics

Non hazardous waste management in Canada is a provincial responsibility carried out by municipal bodies. The provincial and territorial governments influence waste management by means such as legislating deposit return systems for beverage containers or by mandatory curbside and depot collection systems for post consumer recyclables. Each of these systems affects the collection of plastics for recycling. In most Canadian Provinces, the ubiquitous PET soft drink bottle is collected through a deposit return system. Return may be to point of purchase or to a network of "ecodepots" established throughout a province. One advantage of the deposit return system is that it permits an easy sorting of containers, albeit at a high cost in manpower. Most PET beverage containers collected in Canada are baled and exported to the United States for recycling. Apart from one or two washing, grinding operations, little reprocessing of PET is carried out in Canada. A small operation which produced PET sheet from soft drink containers existed in Western Canada for a number of years. Canada's most populated province Ontario has promoted curbside collection of post consumer recyclables including the PET beverage container. Over a period of about 10 years, industry, the provincial government and municipalities have established a curbside collection infrastructure which reaches more than 95% of Ontario's single family dwellings. This curbside system collects PET bottles, other plastic bottles

33

(mainly HDPE) and in some communities plastic tubs, low density polyethylene film and polystyrene in addition to a wide range of nonplastic materials such as paper, aluminium, glass and steel containers. This mixture of materials is taken to a local materials recovery facility (MRF) where the products are sorted and baled. Most sorting is done by hand with some mechanical assistance. A fully automated plastics sorting line does not exist in Canada since most material recovery facilities lack sufficient volume of plastic containers to justify the high capital cost associated with an automated sorting line. Baled products from a MRF are sold to local plastics reprocessors for recycling into plastic pellets.

3.2. Plastics Reprocessing

Most plastics recovered from the waste stream are reprocessed as resin specific streams, e.g., pure HDPE. Very little is reprocessed as a comingled stream into products such as plastic lumber. Plastics reprocessing is carried out in modem large scale plants strategically located near centres of population density where reasonable quantities of feedstock exist. As a result, one finds plastics reprocessors in Vancouver British Columbia, near Calgary, Alberta, the Toronto- Ontario area and in proximity to Montreal Quebec. Facilities are lacking in the prairie provinces and in Atlantic Canada. Atlantic Canada however does have some material reprocessed in the Eastern United States.

Over the years the existing plastics reprocessing industry in Canada has become increasingly financially stable however the vagaries of the world virgin resin prices with which they have to compete can send shock waves through the industry. The Canadian reprocessing industry apart from servicing Canadian materials recovery facilities also imports for reprocessing certain commodities from nearby United States communities.

4. Industrial Plastics Recycling

Apart from the reprocessors of post consumer plastics, other companies have continued to specialize in waste plastics from industrial and commercial operations. The recycling of plastic pallet wrap is expanding across the country. Used electrical wire and cable is being reprocessed primarily for the recovery of metals however a number of specialized operations recycle the cable insulators, polyethylene and polyvinyl chloride. Similarly a few facilities exist to recycle waste automotive trim and foam polystyrene packaging. A considerable effort is being expended to find applications for plastic auto shredder residue which is the by product of the automobile recycling industry which currently recovers metals from end of life automobiles. At present viable applications are still understudy and current residues continue to be landfilled.

34

5. Recycling Rate

An accurate plastics recycling rate for Canada has resisted calculation. The large quantity of empty and filled packaging which enters Canada from the United States and abroad is known only by its dollar value. It is impossible to separate the product from the packaging containing it. Probably more important in terms of the conservation of resources than recycling is source reduction. The ability of plastics to be transformed into light weight efficient articles and packages is its greatest contribution to eco-efficiency. Once Canadian jurisdictions permit and recognize energy recovery as an ecologically sound mechanism the contribution of plastics to resource conservation will be increased markedly.

6. Regulations

As previously stated it is the Provinces and Territories which are responsible for the management of non hazardous waste. This distribution of power to 12 legislative bodies has created a patch work ofregulations across the country. However, at the present time, there are fewregulations which impact uniquely or specifically on plastics and their recycling.Most of the current regulations which affect plastics are the broad based deposit return systems imposed on beverage containers. With the exception of Ontario(which regulates only refillable glass soft drink containers and beer containers) all of the other jurisdictions have some form of deposit return system which ranges from full return of the deposit paid to a return of half of the deposit on containers which cannot be filled. This includes plastics.

The province of Ontario requires that all communities with popUlations greater than 5000 have curbside or depot collections for post consumer PET bottles, aluminium soft drink cans, steel cans, newspapers and glass plus 2 other materials selected from a list. No province in Canada requires recycled content in plastics packaging or other products. No province stipulates a recovery rate for plastic products although all. Provinces have committed to waste diversion targets such as those mentioned in the previous discussion of the National Packaging Protocol. Although not embodied in law, all Provinces support an existing industry sponsored programme to recover used agricultural herbicide and pesticide containers. One province, Nova Scotia, has promulgated a regulation which will come into force in 1998 to ban high density and low density polyethylenes (among other non plastic materials) from its landfills. The plastics industry is working with the government of Nova Scotia to modify the situation which if taken to its extreme would ban plastic garbage bags from landfill which is not the government's intent.

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UNITED STATES OF AMERICA

1. Plastics Production and Consumption

1.1. Plastics Production

The United States with a population exceeding 250 million people and a highly developed consumer economy is the worlds largest producer of plastic resins. Production tends to be concentrated in States bordering the Gulf of Mexico which have been for some time the sites of major petrochemical complexes. Some production facilities are located in other parts of the country where special circumstances warrant their situations. Approximately 11 % of U.S. resin production is exported and by virtue of a world market in the commodity resins the United States imports a quantity of material equal to about 5% of production. Table 1 illustrates 1995 production levels of the high volume resins.

TABLE 1. United States Resin Production 1995 (Thousands of Metric Tonnes)

Low DensitylL. L. D.Polyethylene High Density Polyethylene Polyvinyl Chloride Polystyrene & Copolymers Polypropylene ABS Thermo plastic Polyester Polyester, Unsaturated Phenolics Other Total Source: Society of Plastics Industry, Inc.

5846 5086 5578 2566 4941 661

1717 715

1453 1709

30272

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1.2. Plastics Consumption

In 1995, the United States imported 1592 k tonnes of plastic resin and exported 3459 ktonnes. This trade being concentrated primarily among the polyolefines and pvc.

The apparent consumption of plastic resins in the United States totalled 28405 ktonnes in 1995.

Table 2 illustrates the United State's pattern of consumption by market application.

Table. 2. United States Plastic Consumption by Application Packaging 30% Construction 22% Transportation 6% ElectricallElectronic 5% Furniture 5% Consumer & Institutional 14% Other 1 8% Source: Society of Plastics Industry, Inc.

2. Plastics Recycling

The plastics processing industry where possible has always endeavoured to recycle its own production wastes. Regrinding in house for immediate re-extrusion or reprocessing through an exte. nal operator was generally a simple operation which provided clean streams of material for reuse. The development of multilayer materials, alloys, blends and composites has complicated "in house" recycling in some applications and necessitated the development of new uses for these "more sophisticated" waste resources.

The advent of the recycling of post consumer materials notably used packaging has necessitated the creation of a new industry capable of dealing with mixed streams of products which are contaminated with non plastic residues.

Most recycling in the United States may be termed mechanical recycling. A very limited quantity of material primarily polyester bottle resin is reprocessed through chemical means such as methanolysis.

Limited attempts have been made to employ pyrolysis as a recovery technology but these generally have failed due to lack of public acceptance of the technique as a means of recycling. The same public attitude has restricted the use of energy recovery processes to capture the inherent energy content of most plastic polymers. As a result, the recycling industry in the United States is based in large part on mechanical recycling. Comingled processing of plastic wastes is employed to produce products such as plastic lumber. In early comingled processing streams of mixed resins were

37

as plastic lumber. In early comingled processing streams of mixed resins were reprocessed with little sorting into different resin types. Unfortunately, many of the products resulting from comingled processes were of poor quality and performance due primarily to the variable nature of the mixed feedstock.Recent developments in comingled processing and the resulting products has required feedstocks which are polyolefine rich and consistent in quality. As a result, preprocessing or sorting of materials has become necessary. Today most mechanical recycling requires streams of specifc resins which may be reprocessed into products (pellets) which resemble their virgin resin counterparts. As a result, waste plastics destined for recycling must be sorted into specific resin types, sorted by colour in some instances, washed and re­extruded into pellets. A process which in its entirety is not cheap to operate. Sortation of the simplest plastic waste stream, rigid containers, is generally carried out by hand. Technologies do exist to sort rigid containers mechanically using sophisticated equipment however the high capital cost of these units requires large volumes of material to be processed in order to justify their installation. Only a few operations have a supply of feedstock which meets these requirements.

The North American markets for recycled resins are in no way subsidized by industry, government or the public and as a result, recycled resins must compete in price amongst themselves as well as compete with virgin resins in an open market. As will be discussed later; several local attempts have been made by state governments to promote the use of recycled content in products by legislation but these have had little effect outside of the unique jurisdiction.

3. Post Consumer Plastics Recycling

Although as previously stated the general public can be critical of certain recycling, recovery technologies, the public nonetheless wants many materials including plastics to be recycled.

Most states in the United States have established waste diversion/reduction goals for themselves. Both diversion targets and dates to reach them vary from state to state. Most programmes are voluntary in nature.

The combination of public pressure for recycling and the states' desire to divert waste from landfill or incineration has led to a major increase in curbside collection programmes for recyclables over the last decade. In 1996, approximately 135 million or 51 % of the U.S. population had access to a curbside recycling programme. In addition, depot collection and deposit return systems for beverage containers augment the public's ability to recycle.

Curbside collection programmes collect a variety of materials ranging from paper to metals to plastics. In addition, some jurisdictions handle household organics (kitchen wastes) as well as compostable yard waste (grass and brush). By far the most collected plastic containers are PET soft drink bottles and polyethylene milk jugs. To a lesser extent, other plastic bottles and some other rigid containers are collected in some cities

38

and towns. A recent survey published in the journal Biocycle indicates that 706 k tonnes of plastics were recovered in 1996 by the 23 states reporting to the survey.

A study carried out for the American Plastics Council (APC) reported in May of 1997 that there existed in the United States 1824 facilities which processed U.S. generated post consumer and/or industrial plastics. Of this number only 120 process nothing but industrial scrap plastics.

4. Regulations

In the United States regulations affecting plastics recycling are promulgated primarily by individual states. In some instances, a local municipal ordinance may be in place. Regulations may take a variety of forms.

4.1 Forced Deposit Laws

Ten jurisdictions impose deposits on containers which carry various consumer beverages such as beer, wine, carbonated soft drinks, mineral water, etc. The deposits which range from 5 to 10 cents are paid by the consumer who is encouraged to redeem his deposit through return of the empty container generally to an established redemption centre. Built into the price of the beverage is a handling fee which supports the redemption centres and the programme as do the unredeemed deposits. The plastic most collected for recycling through deposit laws is PET (soda bottles).

4.2 Disposal Bans

Many states have imposed disposal bans on selected waste materials. Motor vehicle batteries and tires are examples of such bans. One state bans non-degradable grocery bags from its landfills and one other state bans "single polymer plastics".

4.3 Restrictions on Rigid Plastics Containers

The states of California and Oregon have specific regulations which deal with rigid plastic containers. Wisconsin also has a recycled content law and regulations.

4.3.1 California

The law requires all 8 ounce to 5 gallon rigid plastic packaging containers (RPPC's) sold in the state to meet one of the following compliance criteria. 1.Be made with at least 25 percent post consumer material;. 2.Be recycled at one the following rates:

39

a) All RPPC's in the aggregate 25% b) RPPC's composed ofpETE 55% c) All product-associated RPPC's 45% d) All particular-type RPPC's 45%

3.Be reusable or refillable 4.Be "source reduced" 10%

The details of the legislation are too complex to be elaborated further in this review.

4.3.2 Oregon

Beginning January 1,1998, all non-exempt rigid plastic containers sold in Oregon must comply with the law through one of the following options. 1. Be manufactured with a plastic resin containing at least 25 percent post consumer recycled material; 2. Be reusable or refilled at least 5 times, or; 3. Meet a 25 percent recycling rate. The regulations provide extensive explanations of how recycling rates will be calculated.

4.3.3 Wisconsin

The law requires that plastic containers of 8 ounces or more to contain 10 percent recycled or remanufactured material by weight. The allowance for remanufactured material means that plant scrap may be used.

4.4. Mandatory Coding of Plastic Bottles

For many years the Society of the Plastics Industry, Inc. (SPI) in the United States has romoted a voluntary coding system for the identification of the resins used in rigid plastic containers. The system comprising a number 1 to 7 and several letters is generally moulded into the bottom of a container. For example, the number 1 and the letters PETE indicates that the container is made of polyethylene terephthalate while the number 2 and the letters HDPE signify that the container is comprised of high density polyethylene. Thirty-nine U.S. states have mandated that plastic bottles 16 ounces or more and other rigid plastic containers of 8 ounces or more must carry the SPI resin code.

4.5. Other Government Criteria to Encourage Recycling

A large number of states have had programmes of financial aid to municipalities and local public initiatives which were designed to encourage and promote recycling and waste diversion.

ECONOMIC ASPECTS OF PLASTICS RECYCLING

M. J. BEVIS Consultant Director Wolfson Centre for Materials Processing BruneI University Uxbridge, Middlesex UB8 3PH - U.K.

1. Introduction

Putting waste plastics in landfill is safe though it is wasteful of their potential value as secondary raw materials or as energy. The debate regarding the best way to realise this potential is complex. Plastics offer a choice; mechanical recycling into second life plastics products, or conversion back into feedstock for re-use in either the chemicals or oil industries, or ultimately as replacement for traditional fuels for power generation. Finding the best balance between these in terms of environmental benefit and overall cost to society is currently proving to be a key issue, and largely influenced by legislation introduced to promote recycling practice. Recycling involves major procurement, quality and technical problems. Factors which relate to the development of plastics recovery and in particular associated economic aspects are presented below, and should be considered in relation to the overall waste management strategy within the European Union represented by the ladder of waste disposal options in Figure 1. IThe strategy is to push waste disposal practises as far up the ladder as possible. Figure 2 represents the environmental setting for an industrial company involved in the manufacture or application of plastics materials.

2. Plastics recovery options

The expected increase in consumption of plastics materials over the next decade will intensify the influences referred to in Figure 2, and tend to encourage the development of:­- primary recycling (recycling on a comparable quality level) - secondary recycling (recycling on lower quality level, or downcycling) - tertiary recycling (decomposition of the plastic into ex. monomers or feedstock products) - quaternary recycling (incineration with energy retrieval)

41

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 41-55. © 1998 Kluwer Academic Publishers.

42

Figure 1

Figure 2

Central planning by the state

- conservation of resources - self dependence in

supplies of strategic commodities

- legislation to control Government composition of and ~

T extent of packaging materials (ex. imposing levies)

- compulsory utilisation of recycled plastics

Economic Activity ----~

Fluctuating prices of energy and oil derivatives - promote or hinder

utilisation of recycled materials

< ~-...~ ...... - ......

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Pressure groups - emphasis on

~----~pollution control and recycling

Social Influences

- conservation of fossil fuels

- Speed of technological advance - development of new primary,secondary, tertiary and quaternary recycling processes. Rapid obsolescence of plastics grades, with respect to conversion efficiency and physical properties.

44

Secondary or mechanical recycling processes differ from tertiary or feedstock recycling and quaternary or energy recovery processes principally in the potential to retain some of the energy used from plastics production, and in general terms provide for financially advantageous options. There is a wide range of plastics waste disposal and recovery practices, differing degrees of national legislation and extremes of public opinion on the acceptability of each option. Developments in the practice of plastics recycling was the subject of a Delphi Survey on the future of the Plastics Industry, with particular reference to Germany [1], and published in 1979, in a time of intense interest and activity in plastics recycling, and fuelled by a plastics feedstock supply crisis in the early 1970s. The Commission of European Communities at about that time [2] published the results of an examination of the Economics of Materials Reclamation, which highlight the key issues of the economics of recycling and puts plastics in context with respect to other materials, and remains largely relevant today. According to the Delphi Study [1], 75% of those experts from all areas of the plastics industry taking part thought that the use of mixed plastics waste without separation would be possible; the following diverse possibilities were envisaged:

monomerisation by pyrolysis ..................... 26% for low value products ................................. 18% combustion for recovery of energy............... 8% as a filler for the building sector.................. 23%

Recycling of mixed plastic wastes - is hardly possible.......................................... 23% - can only be possible with govemment support..... 1 % - depends on the rise in price of crude oil............ 1 %

With the introduction of a separation process, the expectations were as follows: -In future, recycling will be economically possible: 60%

The following divisions of opinion were also noted: -No restrictions in recycling operations .............. 39% -Subject to introduction of new technology... ....... 13 %

-Only practical for expensive plastics ................. 8 % -Little chance as this would be uneconomic ......... 40%

The expectations on the use of sorted plastics waste, were "possible without restrictions,

45

especially for recycling within processing plants: 55%", "possible with restrictions, in particular for low value products: 34%" and "for Uneconomical or problematical: 11 % Recycled plastics were expected to take a greater share of the market, though it was questionable as to whether recycled plastics would be used to as great an extent as scrap ]lletals in the metal industry. 50% of the experts who participated in the Delphi Survey believed that this would not be possible in the foreseeable future, with the precondition that raw material prices did not drastically increase; 37% of the experts believed that towards 1995 recycled plastics would assume the same importance as reprocessed metals in the metal industry; 13 % believed that this was unrealistic as the use of the greater proportion of the reprocessed plastic was not possible on the grounds of quality . This wide diversity of opinion was also reflected in the overall results of a very recent Delphi Study [3], where the forecast is that each of primary, secondary, tertiary and quaternary recycling was almost equally promising as the dominant recycling process in the year 2010, in contrast to the opinion of each panel member separately. If any ranking would be derived based on the results [3], then secondary recycling would be ranked fIrst, followed by tertiary recycling and quaternary recycling; primary recycling would be ranked last. Confronted with these outcomes, it was decided in round two of the Delphi Study [3] to present panel members with the following thesis for comment: "For plastics as one category of materials, no single optimal type of recycling (primary, secondary, tertiary or quaternary) can or should be designated, but the different plastics themselves are each best recycled using one, maybe two, types of recycling. In other words - for each plastic there are one or two recycling types that are clearly the best suited ones" . Of the panel 65 per cent agreed with the above thesis and 35 per cent disagreed. The reasons for disagreeing proved to be more numerous than the reasons for agreeing, and are summarised in reference [3].

3. Legislation to promote recovery of plastics

In broad terms, the fmdings of the Delphi Study [1], in relation to the diversity of recycling processes, are reflected in practice in Europe today. Encouraged by the German Packaging Decree which came into force in June 1991, three years before the European Packaging and Packaging Waste Directive (p and PWD) which came into force in December 1994, there has been a signifIcant increase in the volume of plastics

46

recycling in recent years. A stated aim of the P & PWD is to reduce the overall impact of packaging on the environment. It seeks to achieve this by reducing packaging at source, eliminating harmful materials in packaging waste, maximising the recovery of packaging for reuse and recycling, and minimising the quantity of packaging waste going to landfill. The implementation of the Decree in Germany had a marked effect on sales of some packaging materials which began to disappear from the market. A detailed consideration of the German recycling experiment and its lesson for the United States is given by Steven P Reynolds [4], and contains an extensive bibliography. The introduction of legislation relating to the recycling and recovery of packaging materials which represent more than 50 per cent of the tonnage consumption of the whole of plastics, is likely to be a forerunner to legislation concerned with other classes of scrap materials, ex large tonnage arisings from brown goods (ex. TV, audio and video equipment), white goods (ex. freezers and washing machines), and automobiles. Future disassembly and recycling technology for the electronics and automotive industry were considered in a recent Delphi Study [3]. The Delphi Study panel was asked to respond to react to the following question: "For the coming decades, will recycling be primarily a market-driven activity, an activity carried out because of legislation, or an activity carried out because of increased environmental awareness?" The results for consumer electronic goods and automobiles predict that recycling will be primarily a legislation-driven activity, with important differences between the waste categories, environmental awareness and time of transition. The speed of technological advance coupled with the introduction of legislation can lead to very rapid development of recycling industries. The remarkable changes which have occurred in the level of utilisation of recycled steel and glass, and the relative attractiveness of different materials are classic examples, and also serve as indicators of the important consequences which follow the introduction of collection schemes, such as bottle banks and other forms of pre-segregation at municipal and industrial levels. In Europe, according to the Association of Plastics Manufacturers (APME) four million tonnes of plastics, equivalent to 26 per cent of total plastics waste, were recovered during 1995. Materials recycling including feedstock and mechanical recycling, contributed 9.2 per cent to plastics waste recovery and energy recovery from waste a further 16.8 per cent. Total plastics waste across all sectors was 16 million tonnes, with feedstock recycling beginning to feature in statistics, see Figure 3. The activity is fairly evenly distributed throughout the region but the sources of recyclable plastics are overwhelmingly from thick LDPE films used in distribution and agriculture, and rigid polyolefm transport packaging. Recycling plastics from both these sources is driven by economics, and other areas such as automotive and electrical are improving.

Filrnre 3

Total post-use plastics waste management in Europe Association of Plastics Manufactures in Europe (APME)

Feedstock recycling 51 99

Mechanical recycling 915 1057 1222

Energy recovery 2425 2348 2698

Total (K tonnes) 3340 3456 4019

Polypropylene Grade Characteristics

Typically 50-100 Grades per producer.

Polymer Parameters

Molecular weight Molecular weight distribution Homopolymer Copolymer Additives/Modifiers

Performance Criteria

Rheology/Toughness Rheology Rigidity/Clarity Impact/Rigidity balance Suitability for food contact UV stability Flame retardance Surface properties (slip/antiblocklvisual appearance) Fabrication requirements Process consistency

47

48

4. The targets set for recycling of packaging

From December 1997, only packaging that complies with the essential requirements of the Directive can be placed on the market in European Union. Targets to be met by Member States are:

• recovery of between 50 and 65 per cent by weight of packaging waste by June 2001; recovery includes a number of processes that result in a net benefit being derived from used packaging; one particularly important process that falls into this category is incineration with energy recovery

• recycling of between 25 and 45 per cent by weight of packaging waste by 2001, with a minimum of 15 per cent by weight for individual materials (paper, plastic, metal and wood); recycling describes an operation in which used packaging is reprocessed to form material that can be reused for its original or another purpose

• the combined concentration levels of lead, cadmium, mercury and hexavelant chromium in packaging must not exceed 600 parts per million (ppm) by June 1998,250 ppm by June 1999 and 100 ppm by June 1001.

To meet their obligations, member States are required to set up return, collection, reuse and recovery systems for packaging waste. These systems must take into account environmental and consumer health, safety and hygiene requirements as well as the technical and quality considerations of the recovered materials. The introduction of the German Packaging Decree resulted in the rapid accumulation of large quantities of waste plastics. The utilisation and disposal of this material had repercussions on plastics recycled throughout Europe, especially through the effects of the sale of low cost scrap plastics on the economic viability of existing recycling operations. The true cost of recycling in terms of environmental gain, the real fmancial implications and the potential for distortion of material pricing is being seriously questioned. To avoid creating mountains of useless rubbish, there is a requirement to rapidly embrace an integrated policy for conversion of waste plastic by primary, secondary, tertiary and quaternary recycling processes. Provision for some quaternary recycling is incorporated in the targets that are summarised above, and is to be applauded, providing for great

49

flexibility for implementation as represented by the data given in Figure 3, whilst recognising the technical limitations of mechanical recycling. It should be noted that the Japanese rejected landfill and mechanical recycling in favour of waste to energy schemes, while also supporting the exploration of separation and mechanical recycling processes. Today Japan has 800 modern waste to energy plants. Recent research from the Association of Plastics Manufacturers in Europe shows that for each tonne of waste plastics used as fuel, 1.4 tonnes of coal can be saved in industrial processes. Mixed plastic waste was shown to give a lower average heavy metal concentration than coal, and with no solid or ash residues created and no increase in air emissions. Within Europe (6), Switzerland, Luxembourg and Denmark already recover energy from 70 per cent of their waste. Of the four largest EU nations, Germany has the highest waste to energy position, at 35 per cent. Like many European countries, the USA relies heavily on landfill, whilst Japan only landfills about 40 per cent of waste, much of the rest being disposed of by incineration with and without energy recovery.

5. The longer term prospect for plastics recycling

The P & PWD type of legislation as proposed for a wider range of products should ensure that the material intended for mechanical recycling provides for:

a market value thereby satisfying demand identification, separation and collection as at reasonable cost significant supply of materials to enable the recycling infrastructure to be developed

There are substantial technical barriers in relation to identifying outlets for the recycled product in competition with virgin resins. An indication of the challenge is provided in Figure 4, which shows the complexity and consistency now being demanded by customers from their resin suppliers. The example featured [5] is for polypropylene where a typical manufacturer's range covers 50-100 different grade types, each being supplied to tight specification ranges for a range of different molecular, copolymer or additive variants. Reclaimers have to extract quality out of increasingly diverse feedstocks, and tailor the recyclate for specific end-uses. When quality and technical barriers were considered, only 22 per cent of the original waste was assessed as mechanically recyclable. In terms [5] of environmental benefit, while recycling is undoubtedly the preferred option, it is also the lowest cost alternative The implementation of the European Packaging and Packaging Waste Directive will stimulate the more widespread accumulation of large quantities of plastics scrap, with the economics of plastics recycling being very substantially influenced by the large

50

quantities arising and by the introduction of levies on the initial sales of packaging materials. The levies provide fInance or part fInance to cover the collection, recycling or energy recovery from packaging materials. The full effects of these influences on the economics of plastics recycling have yet to stabilise. The demand for secondary materials may be limited by the attitudes of consumers to items made from reclaimed material. In this context, consumers include both private individuals and industrial users that may impose standards of quality or performance that put products containing reclaimed material at a disadvantage compared with those made from virgin feedstock. In some cases, there are clearly justifIed technical reasons for doubt about the ability of the product made from reclaimed material to perform consistently as well as that using virgin feedstock. In other cases, the standards imposed may be less fIrmly based on the technical requirements but may reflect more the qualities of the virgin material conventionally used than the real needs of the consumer. Even where the reclaimed material may have a considerable price advantage to the user over the substitute raw material, there still may be limited demand for the reclaimed material: because the actual or perceived quality of the secondary material (or product or material incorporating a proportion of the secondary material) is inferior and this attitude may be traded on by the primary material producers; or because the total use of this type of product or material may be limited; or because the user does not know of the availability of the substitute material for conversion into products using contemporary machinery. Automated disassembly of electronic and automotive products would be of considerable benefIt to the mechanical recycling of plastics fractions. The results of a Delphi Study [3] provided an indication of the full or partial technical feasibility and economic viability of automated disassembly. The cost-effective dismantling of brown goods, white goods, automotive products and other large arisings of products that have expended their useful life, such as cables, plastics pipeline systems, building products, would provide for effective recycling. However, the relatively long lives of these products in use, as compared to packaging, would tend to promote tertiary and quaternary recycling. During the service lives of long lasting products, the polymer grades used in original manufacture are under continual development, that provides for enhanced performance, giving longer service lives and/or downgauging. In addition, with new developments, in conversion and fabrication technology, new grades also offer substantial potential economic benefIts. The secondary recycling of plastics arising in the industries referred to above, and being recovered for conversion into products in ten, twenty or thirty or

51

more years hence, would present the converter with a problem. The polymer grade would be out dated and probably not optimal for conversion into products using contemporary machinery. The alternative route of standardising plastics grades in the long term as an aid to identification of used plastics, would have the disadvantage of stifling continuing development of new grades and the benefits to be realised from conversion process efficiency and final product quality. Recent announced statistics by APME indicate that standardising plastics is not a necessary option for the maximisation of recovery, in that resources may be conserved effectively by an integrated management approach involving feedstock, mechanical recycling and energy recovery.

6. An economic model for recovery of plastics

With respect to current technology, there are significant technical barriers associated with every process link in the commercial infrastructure, from the source of scrap polymer to utilisation of the recovered material. Most importantly, the economic driving force today is insufficient (or not adequately defmed) in relation to potentially large arisings of scrap from brown, white or automotive products, to attract entrepreneurs or to encourage those in related businesses to extend their operations to include used plastics. An economic model has been provided by the American Plastics Council [6] of the current and potential commercial infrastructure that recovers value from a portion of the ten million vehicles disposed of annually in the United States. The model provides for identification of transactions, costs, values and other factors that strongly affect decisions regarding plastics disposal, and could serve as the basis for modelling the recovery of plastics from a range of industries producing large arisings. Today's end points considered include recycling of the plastic themselves; feedstock recycling to reusable monomers, oils and gases; conversion to energy (electricity and steam); and landfill disposal for material not suitable for more preferred alternatives. Study of the infrastructure, assisted by the economic model, assists identification of those costs that drive the choice of disposal, and provides additional information that will gradually divert an increasingly large amount of used plastic parts from landfill disposal to resource recovery. An economic model has been prepared, documented and is available for use [6]. A "Base Case" was constructed using the then best available input information. All that is claimed to be required for individual use is a personal computer, a limited knowledge of

52

Microsoft (R) Excel and a brief period of self-instruction. Comparison of the base case with several test cases has shown that the model can provide useful output regarding economic driving forces favouring one disposal option over alternatives, and shows that the dismantling business is economically sound, but shredding profits are thin, and in accord with information gathered in field interviews. In addition, cases were studied to examine the relative economic attractiveness of (i) waste-to-energy plants, (ii) conventional recycling of thermoplastics and thermosets, polypropylene in one case and nylon in another, and (iii) recycling of a thermoset plastic. Additionally in-put values were changed to allow examination of two hypothetical cases: (iv) plastic recovery from automotive shredder residue, or fluff, and (v) a case involving depolymerisation of scrap polyolefins by as yet undefined process technology. The objective of the work was to develop an economic model for the near- and longer­term recovery and recycling of automotive plastic components and materials. The model begins with removal of plastics from vehicles and ends with saleable parts, granulate or pelletised resin and the disposal of residuals. It allows examination of the cost of specific recovery operations, based on assumed inputs, and is intended to facilitate initiation of commercial operations by individual companies. The economic model is intended for application to real life situations. It allows analysis of the relative economic attractiveness of different forms of recovery of used automotive plastics. It allows determination of how changes in cost, in one part of the overall vehicle disposal reprocessing infrastructure, affects profitability in other parts of the infrastructure. In particular, the model is stated to be [6] capable of the following: • focusing on plants and specific unit operations; • accepting input data in such a way that the model is user-friendly,

balancing simplicity of use against complexity and detail of output; • identifying those cost elements where improvements will benefit overall

economics; • projecting the economics and profitability of proposed recycling businesses; and • providing insight into start-up support useful to new recycling

businesses The model tracks the flow of hulks from cars and other vehicles through the various industries deriving value and economic returns from them. These industries consist of dismantlers, shredders, ferrous and non-ferrous metal recovery operations, waste-to­energy operations, depolymerisation facilities and ultimately landfills. The model allows exploration of how changes in relative economic attractiveness of certain

53

transactions causes perturbations in the entire infrastructure and the types of changes required to reduce landfill disposal by making other options relatively more attractive. The intent is to show how variations in costs (material cost, processing costs, etc.) affect the choices of mechanical recycling, feedstock recycling, energy recovery and landfill disposal. It is understood that the economics developed are imperfect. The model however, should be qualitatively correct and capable of ready upgrading as more information becomes available. It should also be capable of easy "What if?" types of analysis, merely by changing input data.

7. The optimum economic options for the recovery or re-use of plastics

A very comprehensive and matter of fact consideration of the best way to realise the potential value of scrap plastics as secondary raw materials or as energy has been presented by M T Dennison in a series of papers, see for ex. References [5] and [7]. The paper [5], titled 'Plastics Recycling: Product, Feedstock or Energy? - a Future View', considers the finding of the best balance in terms of environmental benefit and overall cost to society between mechanical recycling into second life plastics products, conversion back into feedstock for re-use in either the chemicals or oil industries, or ultimately as replacement for traditional fuels for power generation. Overall, mechanically recycling into second life plastics products represents the most economically attractive route for the recovery of a relatively small proportion of plastics waste arisings, with the high cost engineering plastics offering the greatest financial benefit. The extent to which reclamation of plastics from a specific arising(s) proves financially advantageous, will depend on local factors such as the cost of transport of the material to the potential user, the quality arising in the area and the scale of the processing plants. Successful mechanical recycling operations tend to be based on niche businesses where there are effective linkages between the waste generators and the users of the plastics waste arisings. However, in general terms the fluctuations of overall demand in the economy have a major impact on the demand and pricing for secondary materials. At times when overall demand is high, secondary materials are also in high demand and command high prices because of a general material scarcity. But often during periods of recession, demand for certain of the secondary materials is negligible and recyclable value is low. Waste producers and waste handlers often view the cyclical nature of demand for reclamation with something less than enthusiasm for this reason.

54

In the paper 'The Future of Plastics Recycling in Europe I, Dennison [7] considers political, technological and economic factors likely to influence development, and attempts to forecast where Europe will be post-2000 in terms of levels of recycling and recovery, the likely funding requirements, and possible impact on virgin polymer demand. Dennison concludes significant and rapid progress will be required in developing plastics recycling and recovery systems to meet the challenging targets set by the European Union. With adequate funding and market development it is possible that 15 per cent material recycling could be achieved by 2000, and if technological progress continues at its current pace, feedstock recycling could account for five to ten per cent by the early twenty-first century. Based on the above forecast of likely recycling and recovery levels and, assuming waste to energy is priced competitively with landfill, it is estimated that European funding needs will be moderate. An indication of the magnitude of the costs involved was given, and for packaging, for example, when expressed as a proportion of the price of packaged goods sold by the retailer, it would be well below one per cent, and when expressed as a proportion of the plastics raw material price, it would be around ten per cent, although dependent upon future virgin price levels. Dennison and Mennicken [8] provide an authorative update on plastics recycling in Europe, and summarise mechanical and feedstock recycling, and various types of energy recovery. Most importantly the gate fee subsidies needed to make each process a stand alone viable business are summarised. It is proposed that only a limited quantity (7%) of plastics waste can be recycled economically, and thereafter increasing percentages will require increasing subsidy though with increasing environmental benefits. For society, the lowest overall costs for the bulk of plastics waste will be in Municipal Solid Waste incineration plants, which provide for both electricity generation and heat recovery, and with increasing cost of subsidy and environmental benefit, the recovery of waste plastics by feedstock recycling and mechanical recycling apart from the limited quality of plastics waste that can be recycled economically.

REFERENCES

1. Ruchmann, H., Milcke, W., Frieger, A., and Burghoff, G. (1979) in G. Menges, H. Potente and R. Schulze-Kadelbach (Eds)., Results of the IKV Delphi Survey. Institut fuer Kunstoffuerarbeibung an der RWTH.

2. Environmental Resources Ltd (1978). The Economics of Recycling (ISBN 0860101231). Graham and Trotman (London).

3. Boks, C.B. and Tempelman, E. (1997) Delphi Study on future disassembly and recycling technology for the electronics and automotive Industries. Internal document code K370, Faculty of Industrial Design Engineering, Delft University of Technology.

4. Reynolds, S.P. (1996) The German recycling experiment and its lessons for United States Policy. The Villanova Environmental Law Journal Q, Issue 1.

5. Dennison, M.T. (1993) Plastics recycling: product, feedstock or energy? - A future view. Maack Conference 'Recycle '93', Davos, Switzerland.

6. Economics of Recovery and Recycling (1994). American Plastics Council, Automotive Report Series P12.

7. Dennison, M.T. and Lovell, 1.S. The future of plastics recycling in Europe. Paper presented to the (1994) De Witt Petrochemical Review, Houston, USA.

8. Dennison, M.T. and Mennicken, T. Plastics recycling in Europe. PACIA 96 Convention, Brisbane.

55

POLYMER RECYCLING FOR ENERGY RECOVERY

An Application of Life Cycle Analysis Principles

M.XANTHOS Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA and Polymer Processing Institute, Castle Point, Hoboken, NJ 07030, USA and A.L. BISIO ATROAssociates, P.D. Box 1367, Mountainside, NJ, 07092, USA

ABSTRACT

For plastics containing waste streams disposal/recycling alternatives to landfilling or incineration include: a) reuse of parts, b) reclamation/melt reprocessing of contaminated single plastics or commingled streams, c) reclamation followed by chemical modification of single plastics or compatibilization of commingled streams, d) thermolysis to fuels/chemicals, and e) thermolysis/solvolysis to monomers. Life cycle analysis principles can be applied to these alternatives to establish an approximate hierarchy of energy recovery potential. In this article, a methodology to rank the various recovery options in terms of potential energy savings by defining their respective sub-systems and estimating energy requirements is presented. By establishing the proper boundary conditions that place the alternatives considered on a comparable basis, it is shown that the overall energy requirements for most alternatives are well below those of landfilling (highest energy consumption), and for some alternatives closer to those of reuse (lowest energy consumption).

1. Life Cycle Assessment (LCA) - General

There is increasing recognition that the impact on the environment of plastic and composite products cannot be considered in isolation from how these products were designed, manufactured, used, and discarded. Since plastic products can and do affect the environment at many points in their lifetime, there is a growing interest on the part of both government agencies and industry in life cycle assessments, or, as they are often abbreviated, LeAs.

LeA is a rapidly evolving procedure for evaluating, ("from cradle to grave"), the natural resource requirements and environmental releases to air, water, and land associated with both manufacturing processes and resulting products. The term LeA refers to a set of tools used to evaluate the environmental consequences of either a product or a set of activities. Performing LeAs requires the acquisition and analysis of a significant amount of complex data, some private or proprietary, and some significantly uncertain; the established inventory serves, then, for impact assesssment and improvement.

57

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 57-69. © 1998 Kluwer Academic Publishers.

58

The interactions involved in developing an LeA are shown in Fig. 1.4.1, [1]. The diagram defmes material and energy inputs and outputs at various stages in the life of the product within defined system boundaries. Beyond a given boundary, the environment is the source of materials and energy as well as the sink for the emissions and waste. A specific life cycle diagram i,ncorporating tertiary recycling, (thermolysis, solvolysis, depolymerization), as the waste management options for a plastic product is shown in Fig. 1.4.2, [2]. Methods and approaches for the evaluation of ecobalances including case studies have been recently discussed in detail [3]. There is also a growing and increasing number of software packages available that facilitate the preparation of LeAs. All the packages are essentially similar in their aim, i.e. the preparation of appropriate energy and material balances. A review of the packages that appeared in the market place as of late 1995 is available [4]. Note the availability of the Boustead model [5] that contains extensive data of direct relevance to the manufacture of plastic resins and plastic products.

We are, at least, a decade away from being able to incorporate a formal LeA methodology into the initial design of plastic and composite products or even the selection of the "best" or "most environmentally efficient" plastic and composite materials to produce a part. However, in the interim, we possess the tools to significantly improve the environmental management of plastics and composites, through a combination of conceptual LeA studies and an in-depth knowledge of design, manufacturing, and waste management techniques.

2. Energy Flows in LeA of Plastic Products (Embodied Energy)

An analysis of the flows of energy involved in the production of any product is only one aspect of life cycle assessment.; often one may wish to calculate the emissions and energy burdens associated with a specific product, e.g., the production of low density polyethylene resins, so that the potential and actual environmental and health effects associated with the use of the necessary resources and environmemental releases can be calculated. However, the focus of this article is the determination of the total, (both direct and indirect), energy required for the production of the product of interest, (energy flow analysis).

Energy flow analyses such as those done for the plastics industry in the U.S.A by Franklin Associates [1] consider three broad categories of energy, feedstock, process, and transportation, as being associated with the production of a product. The total energy consumption begins at the point of raw material extraction from the earth followed by processing, materials manufacturing, product fabrication and transportation to market as shown in Fig. 1.4.1.

The feedstocks used for the manufacture of resins, the precursors to the plastics, are gas and petroleum; coal is today used only to an almost insignificant degree. Since these feedstocks are principally used as fuels, the heat of combustion of the consumed feedstock must be considered as is part of the total energy required to manufacture a plastic product.

Total energy consumption values termed embodied energy developed by Franklin Associates [1] are given in Table 1.4.1. The energy embodied in plastic resins has been calculated by numerous investigators; unfortunately, the reported numbers for the same resin often differ significantly. For example, Gaines and Shen [Ref. 2, Table 8] calculated them from the sum of the heat of combustion of the feed and the net process energy (total fuel required to complete all steps of the manufacturing process). As shown in Table 1.4.1, there are significant differences for reasons that are not well understood.

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Table 1.4.1. The Embodied Energy of Plastic Products

"Parts" (Products)

ABS HOPE LOPE Melamine Nylon 6, 6 Polycarbonate Polyethylene terephthalate Phenolic Polypropylene Polystyrene Polyurethane Polyvinyl Chloride Polyester (25% Glass) Urea-Formaldehyde

BTUsilb

47,700 42,200 (36,500) 44,400 (38,500) 48,500 63,500 68,200 45,800 (48,700) 38,400 41,000 (34,200) 50,400 (34,300) 31,700 34,000 (25,600) 37,200 33,600

61

Note: The majority of data are from Franklin Associates, "A Comparison of Energy Consumption by the Plastics Industry to Total Energy Consumption in the United States", a study for the Society of Plastics Industry, 1990 [Ref. 1]; data in parentheses derived by Gaines and Shen [see Table 8, Ref. 2]

62

For most plastics, however, the embodied energy values are in the range of 40,000 BTUllb (1 BTUllb=2.34 kJ/kg). It should be noted that the term embodied energy is a misnomer that leaves the impression that a value of energy consumption is a thermodynamic quantity; it is not! Embodied energy for plastic products reflect both historic manufacturing practices and markets. At best, embodied energy values in the literature should be considered as biased approximations, (perhaps as much as 20% higher), to the energy consumption, (replacement energy), required for the production of new plastic products.

3. Applicability of Life Cycle Analysis to Plastics Recycling

Life cycle analysis can be applied to the recovery and reprocessing of discarded plastics from waste streams to establish an approximate hierarchy of energy savings. Plastics disposal/recycling options include: o Landfillling o Combustion in Waste - to -Energy Units o Reuse o Melt Reprocessing into New Finished Products oChemical Modification / Compatibilization During Melt Reproceessing oThermolysis / Solvolysis into Liquid / Gaseous Fuels, Monomers, Chemicals (tertiary recycling)

When the discarded plastic objects and parts are landfilled or combusted in a waste-to­energy unit, separation of the plastic items from the waste streams is not a requirement. However, if other options are to be utilized then, some degree of separation and processing as shown in Table 1.4.2 will be required.

Reuse and reprocessing of plastic objects and parts, (regardless of the specific set of technologies used), will never be absolute, i.e. not all of the discarded plastics in a waste stream can be recovered or reused. Therefore, if the identical quantity of plastic objects or parts, e.g. one (1) pound, is to be produced, as has been discarded, some fraction will have to be made from virgin resins. Ideally, one would want to know the minimum quantity of energy required to produce the needed objects or parts, i.e. the replacement energy. Unfortunately, estimates of replacement energy are not available, nor can they be calculated from available published information. Therefore, estimates of embodied energy may be used as a surrogate for the replacement energy.

4. Energy Flows and Calculations in Plastics Recycling

The savings in energy that might be achieved by the various melt reprocessing or chemical recycling options of various plastics waste streams, (contaminated or mixed), are not inherently obvious, particularly as related to the reprocessing into new finished articles. The description of the energy requirements for the performance of a given recycling/disposal/recovery option, (termed a system in Table 1.4.2), requires that the overall system be divided into a series of subsystems linked to each other by balanced flows of materials and energy. Each system of interest may be broken down to a level where each subsystem corresponds to a set of physical operations for which the energy requirements are approximately known. To carry out a life cycle analysis requires that the boundaries of the global system, e.g. a set of subsystems, must be defined precisely. The analysis that follows is a simplification of the methodology presented in Refs. 1 and 6 where more details on the assumptions used in the calculations can be found. The overall objectives of the analysis are: a) The application of life-cycle assessment principles to plastics recovery /recycling

63

Table 1.4.2. Waste Management of Discarded Plastic Objects and Parts

Waste to Reclamation! Reclaim! Pyrolysis Pyrolysis/ Landfill ~ ~ RellroC,"SS Comlla- To Fuels Hydrolysis

tibilize To Monomers

Collect Collect Source Collect Collect Collect Collect Separate

~ ~ ~ ~ ~ ~ ~ Handle Handle Collect Sort Sort Sort Sort

~ ~ ~ ~ ~ ~ ~ Combust Inspect

Bury (Recover (Accept! Transport Transport Transport Transport Energy) Reject)

~ ~ ~ ~ ~ Reclaim Reclaim

Wasb/ (Flake/Pellet) (Flake/Pellet)Pyrolyze Reclaim Repair

~ ~ ~ ~ ~ Inspect Fabricate Pyrolyze/ (Accept! Products Transport Fuels Hydrolyze Repair)

~ ~ ~ ~ Compatibilize/ Purify

Package Package Modify Monomer

~ ~ ~ ~ Transport Transport Fabricate to User to User Products Transport

~ ~ Package Polymerize

~ ~ Transport Fabricate

to User Products

~ Package

~ Transport

to User

64

options in order to establish a hierarchy of energy recovery potential b) Ranking of recycling/recovery options in terms of their potential energy savings and compare with incineration, landfilling or reuse.

It should be noted that for a process to have a higher technological merit it needs to result in an overall energy savings relative to incineration. Furthermore, the point of view in the present analysis is not one of the participants in the recovery/recycling of plastics but the economy as a whole. The participants are concerned only with the productivity of their specific facilities. Our goal, which is the replacement of a unit weight of discarded plastic objects with a unit weight of new objects (made from a combination of virgin and recycled plastics) is not directly of concern to them.

Fig. 1.4.3 shows the procedure used to rank the various recycling/recovery options. It is assumed throughout, that a given option will produce material Ml, or energy El corresponding to material MI. Since the objective is to produce a unit weight of material M3 having embodied (replacement) energy E3, then additional new material M2, equivalent to energy E2 needs to be added. The replacement energy and the heat of combustion in our example was taken to be approximately that of one lb of polysyrene (47,250 and 18,000 BTU respectively).

For the various subsystems shown in Table 1.4.2, one can estimate the amount of additional energy, (or the equivalent material), that needs to be added for a given option. Table 1.4.3 lists options with relatively high energy requirements. Landfilling has, of course, the highest energy requirements, (equivalent to 100% of the replacement energy), whereas incineration and thermolysis to gases/fuels result in some modest energy savings. The highest energy savings are obtained in the case of reuse where the only energy consumption, (approx. 5,000 BTU), would be related to sorting, cleaning, inspection and return to user (Table 1.4.4). Processes with intermediate energy requirements (10,500-27,000 BTU) are shown in Table 1.4.5; melt reprocessing options for either single polymers or commingled streams involve energy consumption mostly associated with reclamation and processing, whereas the energy consumed in the thermolysis to monomers option is highly dependent on the selectivity of the particular system to monomer(s).

Estimates of the energy flows (consumption), associated with alternatives for the disposal or recovery/recycling of plastics in waste streams, are summarized in Table 1.4.6, [1]. For convenience in analysis, the estimates of energy flows have been converted to a figure of merit. The figures of merit have been keyed to waste-to-energy incineration, which is given a value of 100. Values higher than 100 are less efficient (from an energy consumption point of view) whereas alternatives with a figure of merit less than 100 are more efficient than waste-to-energy incineration.

In all the alternatives, some fraction of the new plastics products will have to be made from virgin resin since not all the plastics in the waste streams can be recovered or recycled. As shown earlier, landfilling has the highest figure of merit (highest consumption of energy) and reuse of a product or object the lowest (the lowest consumption of energy).

The results of our calculations are in general agreement with data reported by Curlee and Das [2]. In their calculations the highest energy savings are obtained with "secondary" recycling (equivalent to our melt reprocessing options); "tertiary" recycling comprising thermolytic and solvolytic processes result in intermediate savings, whereas "quaternary" recycling, (incineration), results in the lowest energy savings. Of course, all the embodied energy would be lost if plastics were landfilled. On an aggregate basis, Curlee and Das calculate the total energy content of plastics in the U.S. Municipal Solid Waste stream to about 1,443 billion BTUs in 1993. Secondary recycling would have retrieved an estimated 1,227 billion BTUs, tertiary recycling about 1,000 billion BTUs,

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66

Table 1.4.3 .. Recovery/RecyclinglDisposal Options with High Energy Requirements

Landfilling El ::: 0 E2::: 47,250 BTU E3::: 47,250 BTU

Incineration El ::: 18,000 E2::: 29,250 BTU E3::: 47,250 BTU

Thermolysis to GaseslFuels El ::: x18,000 BTU E2::: 31,050 BTU E3::: 47,250 BTU

x @ 0.9 includes: endothermic reaction energy (900 BTU) separation energy (500 - 900 BTU)

Table 1.4.4. Recovery/RecyclinglDisposal Option with the Lowest Energy Requirements

Reuse El ::: x47,250 BTU E2 ::: 5,000 BTU E3::: 47,250 BTU

x @ 0.9 includes: sortationlcleaning energy (3,000 BTU) recovery efficiency (2,000 BTU)

Table 1.4.5. Recovery/RecyclinglDisposal Options with Intermediate Energy Requirements

ReclamationlMelt reprocessing single polymer

El = x47,250 BTU E2 =12,000 BTU E3 = 47,250 BTU

x @ 0.75 includes: - reclamation/separation efficiency (5,000 BTU) - flakes/pellet production (5,000 BTU) - fabrication fmished product (2,000 BTU)

ModijicationiCompatibilization mixed polymers

E1 = x47,250 BTU E2 = 10,500 BTU E3 = 47,250 BTU

x @ 0.78 includes: - reclamation/separation efficiency (2,000 BTU) - modifier embodied energy (1,500 BTU) - reactive compounding and

pellet production ( 5,000 BTU) - fabrication finished product (2,000 BTU)

Thermolysis to Monomers

E1 = x47250 BTU E2 = 21,000-27,000 BTU E3 = 47,250 BTU

x @ 0.43-0.55 includes: - polymerization/fabrication (6,000 BTU)

67

- efficiency (selectivity to monomer, monomer purity, reclamation) {15,000-21,000 BTU}

68

Table 1.4.6. Life Cycle Energy Flow Index of Merit

DisposaU Fraction of Plastic Recycling Content of Waste Energy Flow Index

Option Stream Recovered of Merit

LandfIll 1.0 138

Thermolysis to Fuel Products 1.0 102

Waste to Energy 1.0 100

Thermolysis to 0.6 91-96 Monomers 0.9 69-78

Melt Reprocessing! 0.6 90 ModificationlCompatibilization 0.9 62

Reuse of Product 0.6 84 or Object 0.9 54

Notes: - The Index of Merit has been developed relative to waste-to-energy incineration; the lower

the merit number, the lower the energy consumption. - Range in thermolysis to monomers reflects differences in selectivity to monomers.

69

whereas the energy savings by incineration would have only been about 693 billion BTUs.

The above discussion has stressed energy consumption, not economics. However, the relative economics of alternatives are the controlling elements. For example, reclaimed commodity thermoplastics have been and continue to be more expensive than crude oil. It follows that thermolytic processing to fuels is not likely to occur short term, unless subsidized.

Authors Note:

Some of the information included in this paper is based on a report titled "Waste Plastics Recycling - A Research Needs Assessment" prepared in 1994 by the Polymer Processing Institute for the U.S. Department of Energy under contract DE-AC02-91ER30168. The authors were principal investigators in this study.

References

1. AL. Bisio and N.C. Merriam, "Technologies for Polymer RecoverylRecycling and Potential for Energy Savings", Chapter 3, pp. 15-31 in AL. Bisio and M. Xanthos, Eds., "How to Manage Plastics Waste: Technology and Market Opportunities", Carl Hanser Verlag, Munich, New York (1994). 2. T. R. Curlee and S. Das, "Back to Basics? The Viability of Recycling Plastics by Tertiary Processes", Working Paper #5, Yale Program on Solid Waste Policy, Yale University, New Haven, CT, Sept. 1996 3. I. Boustead, P. Fink et al., "Ecological Balancing", Section 2, pp. 71-192 in J. Brandrup, M. Bittner, G. Menges and W. Michaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 4. LCA Software Review, Center for Environmental Strategy, University of Surrey, UK (1996). 5. Boustead Consulting, Boustead Model (Version 2), West Grindstead, Horsham, UK. 6. M. Xanthos and AL. Bisio, "Energy Savings from Plastics Recovery Technologies", The Intersociety Polymer Conference, Baltimore, MD, October 7-10,1995.

Chapter.2 FUNDAMENTAL ISSUES PERTINENT TO POLYMER RECYCLING

UPGRADING OF RECYCLATES - THE SOLUTION FOR HIGH VALUE APPLICATIONS: RESTABILIZATION AND REPAIR

Abstract

H. Herbst aJ, K. Hoffmann a), R. Pfaendner a)and H. Zweifel b)

a)Ciba Specialty Chemicals GmbH, Additives Division, D686i9

Lampertheim, Germany b)Ciba Specialty Chemicals inc., Additives Division,

CH-4002 Basel, Switzerland

For the plastic waste management the material recycling is of particular interest. However, it is important that the recyclate can compete with the virgin resins in higher value added applications. The remaining stabilizer residues in post consumer recyclates are often insufficient for new and often changed end-use applications. This paper discusses importance of the recyclates stabilization, considering their processing and end-use, e.g., in such demanding applications as in the automotive, construction or packaging industry.

1. Introduction

Recycled plastics are considered to be useful only for low value applications. However in recent years, the mechanical recycling of used plastics has received a growing attention. The level of plastics recycling is expected to increase during the next 10 years from the present level of about 10% to about 15-25% of all plastic waste. Recycling of plastic waste by other techniques (such as hydrogenation, feedstock recycling or energy recovery) will also increase during this period, while the amount of plastic waste that is disposed as landfill will drastically decrease n -4] Figure 1 gives an overview of the situation concerning the plastic waste management in Western Europe for 1995 [51.

Recycling of the industrial scrap, e.g., from extrusion or injection molding, has been a common practice in the converting industry. Currently, the demand and consequently the market for post consumer recyclates is steadily growing. The driving forces are several, viz. recycle content legislations, procurement policies, expanding waste collection network, improvements in recycling technology, etc. Figure 2. illustrates the situation in Western Europe concerning mechanical recycling of the post-user plastic waste by resin in 1995 [51.

73

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 73-10l. © 1998 Kluwer Academic Publishers.

74

Total recovery activity (Western Europe. '995)

Post Ua.er last;c waste Hous&holds

'0'139 Tolal: '6'056 Indusltill !--"""'-'i><!'I"!'_E ___

Landf,lI

11'354 70%

Inclneratlon

517 3.1%

IRaw malerial.

I 99 I 0.5%

~ L-__ ....J...---=:5'9.::.':.:.7---J ...,.,.., ~,%

............. ....,. .. - f--'':'==-; ,=====,86%

ProcO$SQ($ pI .. 1lc waste I ' "818 r--Mtd\Ii!Mealr~1ng ----.

Ul'IIl W100 tDftl lye_

Figure 1. Plastic waste management

Figure 2. Mechanical recycling of post-user plastic waste Total mechanical recycling: 1'222'000 tons

Mochonlcol recycling by , .. In (Western Europe, 1995)

PA ,'"

lAOI'E

Mechanical recycling aimed at the use of plastic parts in high value applications requires that properties of the recycled material are tailor-made for the targeted product and end-use [6 -91. The role of additives in the polymer technology is well - recognized. They enable the manufacturer to produce suitable materials for the demanding applications in, e.g., the electronic, automotive, building, and consumer goods areas. Stabilizers, pigments, lubricants, impact modifiers and a variety of other additives make it possible to safely process the recycled polymers. The additives also guarantee suitable mechanical properties and appearance during the service life time of the plastic part under exposure. The concepts presented here comprise the development of appropriate formulations and techniques for the mechanical recycling of post-consumer plastic waste in high-end applications.

75

2. Mechanical Recycling of Post-consumer Plastic Waste

Mechanical recycling of post-consumer plastic waste makes economical, ecological and social political sense only if an efficient system of collecting and sorting the waste has been well conceived and implemented. • Close-loop applications from clearly defined sources, e.g., bottle crates, trash bins,

automotive parts (such as bumpers or battery cases), PET bottles, and from variety of other origins, including the industrial scrap.

• Recyclates from similar plastic materials, e.g., packaging plastics that might comprise different polyolefins, such as polyethylenes (HDPE, LDPE, LLDPE), polypropylene (PP) and/or copolymers, PS, PET and PET -copolymers, different polyamides (PA), rigid and plasticized PVC, etc.

• Mixed plastic waste, e.g., from household, consisting of a variety of compositions comprising polyolefins, PVC, PS, polycondensates (viz. PA, PC, PET), etc. In terms of the economical and ecological performance the single-component plastics

and mixed plastics having well-defined composition and known history are best suited for the mechanical recycling. These recyclates can be used to substitute the virgin resins, i.e., they can be reprocessed to have the same functional properties as the comparable virgin materials. Structural heterogeneity's and residual impurities, e.g., different pigments, fillers, flame retardants may be present even in the carefully sorted and cleaned recyclates. The influence of different additives on mechanical recycling is summarized in Table 1.

Plastics usually contain stabilizers that must protect the polymer during melt processing. They also contribute to the service life time of the finished article upon exposure (especially for out-door applications) to light and heat. The stabilizers are partially consumed during the processing and exposure of the plastic parts, thus they must be replaced. The amount of required stabilizer depends on the reprocessing conditions as well as on the envisaged use of recycled polymers. It must be noted that the stabilizers' transformation products remain in the plastics.

Plastics undergo an oxidative degradation during processing and end-use. The molecular weight and molecular weight distribution of the initial polymer can change by the scission of macromolecular chains or by the cross-linking reactions. Both of these lead to irreversible changes in mechanical properties, e.g., changing from the ductile to brittle behavior (especially critical for the partially crystalline polymers). For such polymers as polyesters and polyamides that are sensitive to hydrolysis, the average molecular weight may decrease during the service life time. The appearance of the article may significantly change, e.g., showing surface cracks and discoloration. However, the most important effects are invisible - the presence of the precursors and functional moieties that contribute to further, enhanced oxidative degradation of the polymer, viz. acid-, ester-, peracid-, perester-, hydroperoxide- and ketone groups.

Appropriate recycling must take into account the oxidative damage introduced to a polymer during its lifetime. Providing for the necessary processing and long-term stability of recyclates include appropriate deactivation of moieties with pro-degrading properties, especially peroxides and hydroperoxides. A balanced combination of suitable stabilizers and co-stabilizers contribute to processing stability and to service life time of a recyclate during its foreseen applications.

76

Table 1. Relevance of the nature of additives to mechanical recycling

ADDITIVE RELEVANCE TO MECHANICAL RECYCLING Antioxidants Further addition of antioxidants depends on the foreseen Hydro- application, e.g., on processing conditions, required peroxide service time under given exposure conditions. If decomposers possible, the level of residual stabilizers should be processing determined prior to addition of the new ones. No stabilizers antagonistic effects are expected here. Heat Usually, an ample reserve of heat stabilizers is present in stabilizers products to be recycled. PVC usually does not require for PVC further addition of additives. However, mixing

recyclates that contain different classes of stabilizers may cause a problem, e.g.,discoloration by mixing lead and tin-type stabilizers.

Light Depending on the foreseen applications and service life stabilizers time, an addition of light stabilizers may be required.

Antagonistic effects are possible when mineral acids are present in the recyclate.

Flame Owing to the wide range of used flame retardants , retardants mechanical recycling may not always be possible, since

required standards must be meet. Fillers and Fillers and reinforcements do not present any particular reinforce- problem for mechanical recycling. Extra wear on ments processing equipment should be considered. The

mechanical properties of end-materials might be affected.

Plasticizers Addition of plasticized PVC to unplasticized PVC will result in loss of rigidity. Blending of finely ground unplasticized PVC is a possible solution, provided that the plasticizer content is > 10 % to avoid embrittlement.

Lubricants Generally, no relevance to mechanical recycling. Pigments Mixing different classes of pigments may cause

discoloration.

2.1. Reprocessing, General Considerations

The manufacture of a plastic part involves several processing steps in the melt. Because of high shear stresses (see Figure 3), particularly in the region of entangled polymer molecules the C-C -bond scission can lead to a decrease of molecular weight, Mw. The degradation caused by shear stress is referred to as thermo-mechanical degradation. Furthermore, the hydroperoxide groups present in the polymer decompose at the processing tmlperatures and also lead (through ~-scission) to a decrease of molecular weight. For example, processing of PP and PS leads to decrease of molecular weight,

77

Mw, whereas processing of polyethylene at first leads to degradation, then to chain branching and crosslinking nO,II1. Depending on the processing conditions, PE viscosity and molecular weight may either decrease or increase (see Figures 4 and 5).

104rM.~"~~~OC~O'~~~(P~.J~I __________________________ -.

f .h.llt rI.el_j

Fig. 3. Relation between shear rate and melt viscosity and the influence of molecular weight distribution in typical processing conditions

Numbe, of extrusIons

Fig. 4. Changes in melt viscosity upon processing of PP and PE

78

MF o

I chain sd

I chnln branching

.~ ____________________ -J

m ~ m m ~ m m ~ m m Extruslen tom,.'C

Fig 5. Relationship between extrusion temperature and MFR of the extruded product

Changes in molecular weight do influence the mechanical properties of the polymer involved. Figure 6 shows the relationships between Mw and mechanical properties of a PP. A sharp decrease in the performance takes place when the molar mass falls below a critical value, Me' a value which depends on the polymer [121.

100 000

o.

I 6 ..

j -; .:; m E ". !! 60 i ~ .. P 12 Ii .....

~40 11 m

!3 i 11

0 if E ,.

3 iii ... . 2 20

...... 200000 300000 ...... ...... M.(OPe}

Figure 6. Relation between molar mass and mechanical properties ofPP

Since chain branching and crosslinking change the polymers' mechanical properties one should preserve the molecular weight and molecular weight distribution during processing by adding suitable stabilizers. During melt processing of virgin polymers these stabilizers are partially consumed. Thus, recycling must include "restabilization" - a step where further processing stabilizers are added. Furthermore, the recyclates have been exposed during their "first life" to various levels of heat and light. For a given plastic part, a suitable stabilizer package must protect the article during foreseen service life times. After the stabilizers are consumed, upon further exposure the polymer degradation starts. The period during which the stabilizers efficiently protect polymer against degradation as is known as the "induction period," shown in Figure 7.

(') :r 0> ::I

"" " :;. "0

o "0

" ;:l n' on

Stabilizer conium plion Sm all increase in ROOH Slow o:':Yltn up-luke

I Degradatloll

Mechanical (ailuro ChllDge in Molecuhu weichl Molecular welShl distribulion Carbonyl buill-up Rapid in crean in ROOR Fasl oxygen upalAke

Time

Figure 7. Aging of polymers in relation to changes in properties

79

The rheological and mechanical properties of the plastic recyclates should also be known. Analytical determination of the type and the content of remaining stabilizers facilitate the task of restabilization. The choice of the stabilizer package may result in synergistic stabilization. It is important to avoid possible antagonisms between different classes of unsuitable stabilizers. Best results for demanding applications of a recyclate will be obtained using a close loop system of known polymers (or sorted plastic post­consumer waste of known composition and degree of degradation). However, one has to accept that an oxidized, damaged polymer formed after the induction period, may not be result in a recyclate with similar properties as virgin material.

2.2. Restabilization - Reprocessing

2.2.1. Polyolefins

2.2.1.1. Polypropylene Even in the case of virgin PP, the melt processing is possible only in the presence of a processing stabilizer or a blend of processing stabilizers with different, synergistic mode of action. Best processing stability of polyolefins, polystyrenes and most other polymers (excepting PVC) is achieved by using a combination of sterically hindered phenols with either a phosphite or phosphonite. In Figure 8 effects of PP stabilization by a combination of Irganox 1010 and Irgafos 168 are shown. In addition to these stabilizers, acid acceptors, e.g., Ca-stearate, contribute to improved process stability n 11. The stabilizers are partially consumed during melt processing. Conversion of the phosphite by a stoichiometric reaction with hydroperoxides, peroxy- and alkoxy radicals into the phosphate takes place. Thus, after processing of a virgin polymer to any end-use plastic parts, the total concentration of the remaining processing stabilizer is low. It further decreases during exposure of the plastic to light and heat. Therefore, replacement of the

80

AO : .... noauno PS : qal04111

o 10 20 30 40

\e 111 EJdrutIon • 15111 EJdrutIon

50 60

IIRI 1111 1<o/%lO'C. dgImIn))

> >70

70 80

Figure 8. Processing stability of a PP homopolymer as function of phenol Iphosphite ratio

processing stabilizer and the hindered phenol serves as the basis for assuring the be~ possible reprocessing and application stability for recycled materials.

To simulate the recycling conditions at different stresses existing during re processing in different machines, the following experiments were performed re extruding the formulation several times, while examining the material performance afte each. The effect of addition of 0.05 wt% of a phosphite, Irgafos 168, during variou stages of degradation (induced by multiple extrusions) is shown in Figure 9. All sample initially contained 0.05 wt% of a sterically hindered phenol, Irganox 1010, whil additional phosphite was incorporated after each successive extrusion [} 31. The uppe curve represents PP with hindered phenol alone. It shows a slow, but steady increase 0

the melt flow rate (MFR) with each successive extrusion. A portion of the material originally containing only the hindered phenol, was taken out after each extrusion restabilized by adding 0.05 wt% of the phosphite and then reprocessed. All polymer had undergone a total of five extrusions. The data show that addition of phosphit, protected PP during processing. However, it is impossible to "heal" already damaged PI chains. Reprocessing a badly damaged polymers, such as polyolefins or PS, does no lead to high-value recycled plastic parts. PP-battery cases recyclates, processing stability.PP from battery cases is commerciall: reclaimed and re-used. In the 1994, in U.S.A. about 100,000 ton, and in Western Europ' around 43,000 recovery. Figure 10 shows the processing behavior ofPP recyclate fron the battery cases.

20 MFR 111110 mIni

15

to

5 • o+-____ -, ______ ,-____ -, ______ ,-____ -, ______ ~

o 2 J 4 5 6

AIm ("OOC/l.1O ka) Nu mbet af Elcru loIU .t 16Q11'C Bult $1.billuUDn: 0.05 % lrellnos 1010

Figure 9. Influence of phosphite addition on the processing stability of a PP homopolymer

11 MFR 11i .... lo)

1. __ _ I 'tF10WCf\opotlt

II

.6 MechAnieal .. PropcniCi

• 1 "BIllf1u,' CIJe,j

•• 0 2 J 4 5 6

NurabnorEl:tru ioaJ

[.slnuto.: TWIn .CftW. 1611"C. 100 rpm AlPR: lSOllll.ua-cJ1.".,

Figure 10. Processing stability ofPP scrap from battery cases

81

Adding a stabilizer package such as Irganox B 215 (a synergistic mixture of a hindered phenol and a phosphite), Irganox 1010, and Irgafos 168 (a phosphite), improves the processing stability. However, a system specially developed for the use with recyclates,

e.g. , Recyclostab@, is the best preserving the molecular weight. The Recyclostab@ systems are based on hindered phenols, phosphites and other co-additives, e.g., acid scavengers. An increase of MFR with the number of extrusions is significantly reduced.

2.2.2. Reprocessing HDPE HDPE recycling very much depends on the quality of the post-consumer plastic recyclate (PCR), and the product value for the targeted application. Good PCR quality and high value added applications (such as for non-pressurized pipes, cable ducts, trash bins, crates, bottles, drums, pallets, playground material, plastic lumber, etc.) ascertain good economics. The stabilization system (processing, thermal, light and co-stabilizer) has to be formulated and adjusted considering presence of the residual stabilizers n 4, 15).

82

The processing behavior of different grades and types depend on the catalyst type used in resin production. Improved processing stability must be achieved by restabilization the HOPE recyclates that have a tendency to reduce the molecular weight during processing (Ziegler-type catalysts), or that of chain branching and crosslinking (Phillips­type catalysts).

0.8 MFR (230'C/10 kg)

~===:::"_""3_...::0:;,:.2~0.;:·1c;:.. ~Re~fYClostab 411

0.20 % Irganox 8225 0.6

0.4 - - - - - --

without restabllizatlon

0.2 -J----,.--....,..-----,,------,---,------,----.J o

RNWjuat StAbihllf can.,nt 0069" "ganoJll010 0015% Irglllo. , 68

ExtnnlCn TWII'l Scr8W', 250-C. 100 rpm

2 3 4 5 6

Extrusion passes

Figure 11. Processing stability ofHDPE (Phillips-type catalysts with and without restabilization

The stabilization effects are illustrated in Figure 11 for a Phillips-type resin. Without restabilization the crosslinking dramatically reduces the MFR. Furthermore, possible gel formation and changing processing conditions will disturb the production. With 0.2 wt% of Recyclostab 411 the MFR becomes nearly constant over the five extrusions at 250°C. Smooth production, high output and, therefore, improved economics are a result. Again, a specific stabilizer system leads to superior behavior, as compared to conventional system such as Irganox B 225. Thus, a variety of applications, e.g., non­pressurized pipes, bottle crates, waste bins and others, are accessible for restabilized HOPE. However, one should take into account a possibility that other polyolefins (e.g., LOPE or PP), PVC, and others, may be present in the PCR. These may negatively influence the processing and long-term behavior ofthe recycled HOPE products.

2.2.3. Reprocessing of Polystyrene Polystyrene (PS) is an important fraction of post-consumer plastic waste. PS and expanded polystyrene (EPS) are extensively used in packaging, hence PS recycling is important. The processing stability of EPS packaging material is shown in Fig. 12. Addition of a sterically hindered phenol, Irganox 245, to compacted EPS allows to maintain the average molecular weight of the polymer at a level sufficient for EPS­processes [} 6, 171.

80 Viscosity Number [mVgI

75 - - - - _ - - - - - - - - - - - _

70 - - - - - - - -

~;-;::;-"'7"::--:--:-:::--::-"" 65 - Without Restablllzation

- -- 0.10 % trganox 245

60 +-----~----~----_r----~

EaIru./On: Twtn _ . 220 ·C. 'OOrpm """"""~orIng" 1h. 'w·e

o 2 3 4 Number of extrusions

Figure 12. Processing stability of EPS packaging materials at 220°C

2.2.4. Reprocessing of Polyethyieneterephthaiate

83

PET recycling is economically viable because of the significant price difference between the virgin PET and PCR resins, and because of the high quality and availability of the PET bottle waste stream. The recycling options for PET are either mechanical or chemical (glycolysis, methanolysis, alkaline cleavage). From the economic point of view the mechanical recycling is the method of choice [\41. The resin is sensitive to hydrolytic chain scission, as well as to thermomechanical and radiation-induced degradation. These processes are catalyzed by impurities such as acids, hydroperoxides, metals, etc. Melt processing alone, even under the optimum conditions, leads to a loss of Mw and to discoloration. Some applications tolerate a loss of Mw or the intrinsic viscosity (IV), e.g., fibers and fiberfill. Again, a specifically designed stabilizer system based on phenol, phosphite and suitable co-stabilizers, Recyclostab 411, ascertains improved processing stability of PET recyclate from bottles (see Figure 13).

08

07

06

O.S

Oinolorwtion 1\'11 .-r-------------.,---------------=-..:, 8

C Bottle flakes 0 IMlhoot restab.l!zatJon Cl 0 25% Rocyclostab 44 t

ExtruSIOn, TWIn screw, 280·C. 50 rpm YI : measured en molded pfaques

Figure 13. Processing stability of PET recyclate from bottle scrap at 280°C

84

2.2.5. Reprocessing PET by the solid state post-condensation Sometimes, Mw or IV of PET recyclate is too low. Therefore, mechanical and melt strength properties are not adequate for the targeted process and/or applications. The usual solution of this problem (for both virgin and recycled PET), is the solid state polycondensation [}8-211. However, the process is time consuming with high energy consumption and high costs (e.g., around 20 h at 230 QC). Accelerating the condensation would lead to significant cost savings. An experimental product, CGX 195, when added in an amount of 0.25 wt% to the solid state post-condensation leads to a PET with higher IV and accelerated solid state polycondensation (see Figure 14). New applications, where high IVs are required, are therefore within reach.

2.5 Inlttnsk: Viscosity (dUg)

2

1.5

alnddM P!T e .... I!*uIIDfr """ KnW . 210 ~. IO Ipd • • ~.o.,16fc111r1

20 40 60 ao 100 120 1.0 160

TIme (hI

Figure 14. PET solid state post-condensation, continuous processing

2.2.6. Reprocessing polyamides - using the molecular repair concept Engineering thermoplastics, such as polyamides and polyesters, are produced by polycondensation. These products are sensitive to hydrolysis, that reduces the molecular weight and leads to a loss of physical and mechanical properties. For more demanding applications the losses have to be compensated for. One such method is the mentioned above solid state polycondensation. However, the reactive extrusion that builds-up the molecular weight of the damaged polymer provides a better cost-to-performance relation. A variety of compounds suitable not only for the polycondensates, but also for the repairing polyolefins have been described in the literature. By adding a combination of reactive additives, hydrolytically damaged polyamides can attain their initial molecular weight and, consequently match the mechanical properties of the virgin resins [}41. The reactive additives are usually based on bi-functional oxirane compounds. A catalytic additive accelerates the reaction making the process feasible during an extrusion. Moreover, the additive system is effective in providing processing stability while maintaining good long-term heat stability. Careful concentration adjustment of various components provides the basis for manufacturing tailor-made products. Figure 15 illustrates how polyamide-66 (PA-66) production waste (from injection molding) can be transformed into material with adequate mechanical properties for re-injection molding applications. In this case PA-66 was reactively extruded with an experimental product EB 36-50.

250 ~~--~~----------------------------,

200

150

100

50

o DIN ,,'55 Elong_UI)n

Ill)

DIN ... 57 DIN "'55 ISO 180/1A E-Modulus Tensile Strength ltod.lmpad SlrRngth

[Nlmm1 [NJmm1 (1<J1m')

I(""' PrOdUctio n Waste . 0.75 % EB 36-501

Figure 15. Polyamide 6.6, molecular weight built-up, mechanical properties

Conclusions -1

85

Reprocessing of PCR gives best results using sorted waste in a "closed loop" strategy. Restabilization of the recyc1ate leads to end-use articles with adequate quality for high value applications. A severely damaged polymer, however, is not suitable for such use. In the case of polycondensates, viz. polyesters or polyamides, a solid state post­condensation makes it possible to rebuilt the molecular weight of PCR. A molecular repair concept involving reactive oxirane derivatives has shown promising results in view of high-end applications of PCR polyamides. New developments will satisfy further high value applications for the plastic parts indemallcling areas.

2.3. Restabilization: Selected Applications A variety of plastic articles based on diverse PCR resin types have been developed during the past 5 years. Few examples are given in Table 2. In the following part, a detailed analysis of selected cases will show to what extent the set targets (regarding the long term use under given exposure of heat, weathering and light) can be achieved. Again, one has to take into account that stability under long term exposure conditions needs appropriate stabilizer systems, based on sterically hindered phenols, hindered amines and the light stabilizers. Basic knowledge of the use of such stabilizers is needed to obtain the best results. To achieve these, the additive producers must work together with the universities, converters, third party organizations and other customers, especially during the initial set-up period.

86

Table 2. Selected examples of value-added restabilization ofPCR:

Automotive Packaging! Electro/ Distribution Construction Eledroni ••

Polyolefins ~OPE_.

HOPEllOPEp",", bumpora HOPE "",101M,. PPIEPOM Hor5M1.r'''1IS

HOPEpipm LDPEILLDPE piPh PPhousingo

PO Blends pp battery ca.sa. P? conllllJnera PPcarpel POpollab PO pla,l1c lumbo< PO bin.

Polystyrene ~:!:'!~ ~~'P' bolll •• ,oor..g_

PVC agriaJltural mml windOwproftl .. cable hi', ",",,1.1 po1U"

Engineering PBTIPC ",""pors PETbottlos PAhubQJPI PET 11_ PAhousqa Plastics -:'s=:~ PETf.,., ABS houIlngs

Mixed nolto protoaUon walls Plastics 1>11101.1 plUllc lumbor

2.3.1. PP Battery cases recyclate, long term thermal stability It was demonstrated that scrap from battery cases can be reprocessed for the production of "new" battery cases through injection molding, provided that a suitable stabilizer system is used. Figure 16 shows the long term thermal stability of PCR polypropylene, in the presence of different stabilizers. By adding twice as much of Irganox B 215 (a synergistic mixture of a hindered phenol and a phosphite) as normally used for stabilization of "virgin" PP, the life-time specifications for the material based on PCR are not meet. However, using the specifically designed stabilizer package, Recyclostab 451 (based on a high performance phenolic antioxidant and co-stabilizers), the required stability can be obtained. Extensive research has proved that processing stability and stability during end-use of all the recyclates from battery cases, regardless of the processing techniques and the origin of the battery cases, can be achieved.

38 (0.10 % _ .... noJ: 0 l iS)

Rdtn.n«

0.1-0 % It'!ADO\ 0115 22

.10 ·'" RH)'d 4KIa b 451

>95

o 20 40 60 80 100 120

Day 10 ~mbrillltP1tnt a t 135 0

Ell,...., T.I ... ,""" lMI"C,I_,.. TeII....,.as: 2 .... -}tdJwt ~,a...

Figure 16. Long-term thermal stability of recycled PP from battery cases

87

2.3.2. Car bumpers from PPIEPDM recyclate, light stability Automotive bumpers are interesting materials for plastics recycling because of their size (around 3 kg). Furthermore, it is easy to identify the plastics type and to dismantle the part. Artificial weathering of plaques made with PPIEPDM recyclates reveals that without restabilization there is a considerable loss of gloss after only 2000 hours of weathering (see Figure 17). Restabilization with a combination of a hindered phenol/phosphite blend, Irgafos B 561, and a high and low molecular weight hindered amine stabilizer system shows that approximately 50 % of the initial gloss is retained even after 4000 hours.

GIou

BQ

GO

40

20

o o 1000 2000

HoonllllOM

JOOO 4000

Figure 17. Artificial weathering ofPP/EPDM recyclate, injection molded plaques, loss of gloss

2.3.3. Bottle crates from HDPE recyclates, light stability Recycling of bottle crates provides a classical example of the restabilization effects for closed loop applications. Figure 18 shows the results of artificial weathering of 100 % recycled HDPE crates that formerly had experienced five years of service. Time to crack formation and impact resistance is shown as function of stabilizer contents. The use of 100 % recyclate without additional stabilization resulted in a dramatic loss of weathering resistance, as compared to virgin material. By adding 0.1 % of a hindered amine, the same weathering resistance was achieved for the recyclate as that for the virgin material. Addition of suitable processing stabilizer system to maintain molecular properties during processing is required. However, this does not contribute to the materials' light stability. A combination of low and high molecular weight hindered amines would probably give the best performance, since the high molecular weight amine contributes to long term thermal stability. The addition of a UV-absorber should increase the service life of the recycled crates, depending on the pigments used.

88

10000 rndlin~ .11U1_1ft" ~I"nllh 100

10

60

~o

10 2000

l.IoT_rnr.1l'II 0.10%,... ....... ". • O.fU 't("lrp1IU IDIO

Wd11wma: WOM • t." % (lvr ... " Ttfl u.,a. Cd· ",...... pil~ltd l ldrW Shbill.nu... I. •• '" 1,. .... 1111'10 loLl. uv.~

Figure 18. Artificial weathering ofHDPE bottle crates recyclate (5 years use-time)

The beneficial contribution of HALS is clearly demonstrated by chemiluminescence. The decrease of time to onset of chemiluminescence is directly related to the oxidative damage of the material. Thus, recycling of bottle crates after 5 years of service without adding new HALS leads to severely damage recycled bottle crates, prone to early breakdown upon exposure to outdoor weathering. Adding of only 0.1 % new HALS to the recyclate dramatically improved the light stability (see Figure 19).

~r------------------------------'

I· Vi'gin, wlh HALS ] I 0 Vi'gin. no HALS

1 )C Recyclllte, no HALS, no AotPS,

I- Recyclate, no HALS I

I_ Ftecyclllte, wah HA.LS

Outdoor .. po ..... time In month.

,----------------------------------------~

Figure 19. Influence ofrestabilization on outdoor exposure of recycled HDPE bottle crates

(Exposure: Florida, 450 South, - 1700 kWhlm2 / year)

2.3.4. PO-Blends (HDPEIPP) recyclate for non-pressurized pipe applications, thermal and light stability

Switzerland has established a Swiss pipe project. The targets are sewage pipes and cable ducts. The aging behavior of recyclates was carefully investigated. The recyclates originated from HDPE bottles and LDPE films. Both materials were typically used for a short term application, thus only low level of processing stabilizer (phosphite) and

89

phenolic antioxidant was required. However, the projected use of the recyclate as sewage pipes is typically long-term - many years of service are to be expected.

Figure 20 depicts oven aging at 110°C of one of the preferred blends, LDPEIHDPE (70/30) recyclate. The blend without restabilization showed crack formation after 52 days. Restabilization with a combination of 0.1 wt% Irganox B 215 and a 0.1 wt% of a hindered amine stabilizer, Chimassorb 944, lead to a significant improvement. The aging resistance could further be enhanced to 106 days, and 116 days by the adding 0.1 wt% of Recyclostab 411 or Recyclostab 421, respectively, or to 125 days by using the mixture of 0.2 wt% Recyclostab 421 and 0.1 wt% Chimassorb 944. These Recyclostab systems are again based on high performance phenolic antioxidants and co-stabilizers. Further studies are being carried out, involving the water storage at 80°C, and natural weathering in South Africa .

• 0 1 % Re<ycloslab 411 o W. Chun .. ""b 9-14

.01% RoC)clo".b 421+ o 1% Chun ... ..,'b 944

. 02% Re<yclo.lIIb 421+ o 1% Chlm.""orb 94-1

o 20 40 60 80 100 120 140

cmr.k rnnt'Uuinn Itlnv("1

Figure 20. Long-term thermal aging ofLDPEIHDPE (70:30) recyclate, injection molded plaques, aging at 110°C

Tensile impact strength (kJ/m'l 1100 ..,.-_____________________ -,

1000

100

... • 00

100

Oven aging at 120 DC Ihl (in Tausendl

Rocydate C. .. """d ... , 64% PE-HD. l1% '!:oLD. 1.% PP T_I .. pm ......... , Dl/'lM'"

Figure 21. Influencliof carbon black on the long-term thermal stability of PO recyclate

90

Generally, non-pressurized pipes contain carbon black (CB) as a pigment. However, CB may have a negative effect on the thermo-oxidation of polyolefins. The influence of CB on the long term thermal behavior of irrigation pipes (for vineyards in Cap Province, South Africa) based on LLDPEILDPE recyciate is shown in Figure 21.

During the first year of the outdoor exposure, crack formation and breakage were observed for the recycied pipes. This did not occur for pipes made of virgin material. Even 2.5 % carbon black alone did not impart sufficient protection against the light degradation. Maintenance of the physical properties during 9000 hr of artificial weathering, that corresponds to 2 - 3 years of natural weathering in South Africa (approx. 160 kLy/year), requires the use ofHALS (see Figure 22).

.""" ''''''' IDOOO

Arlindal weatbering Ihl

Vlratn : '" "-"rpll •• tOIO 1'Q,QJ 'M. l l'J,dftlA d.r .. + CD

rqtlAt.a .. CD +O.J ": .. Tiluni. 6U

Figure 22. Light stability of PE-LLDIPE-LD recyciate, irrigation pipes

2.3.5. PO-Blends (PE I PP) recyclate for trash bins Trash bins are other good examples for utilizing PCR polyolefin blends. The investigated blend (HDPEIPP = 73/27 wt%) represents a typical HDPE bottle fraction of post-consumer plastic waste in domestic waste stream in Germany. Again a long-term , partially outdoor application of trash bins was targeted, starting with the used packaging materials. Therefore, processing-, long-term- and UV light stability were of importance. Figure 23 shows the influence of UV-light on the elongation properties of this blend during artificial weathering. Without restabilization the flexural strength at break declined below 40% after 1000 hours. With 0.4% Recyciossorb 550 the elongation at break > 60 % was observed after 4000 hours of weathering. [Recyciossorb 550 is a newly developed stabilizer system based on hindered amine light stabilizers for polyolefin peR. It improves weatherability and imparts processing and long-term heat stability as well.]

100 -~~ __ -.------~~~----------------------~

Recyclate:

90 80 70 60 50 40 30 20 10 o

Test samples: Exposure:

o 1000 2000 3000

Hours

I_ Reference 0 0.40% Recyclossorb 550

PE-HD 73%. PP 27% 2mm injection molded plaques Weather-D-Meler CI65 A. BPT 53'C. RH 60%

4000

Figure 23, Artificial weathering of mixed polyolefin recyclate (HDPE/PP, 73 :27)

91

Although relatively good segregation of HDPE blow molded plastic parts from the waste stream is possible, PP closures and bottles as well as other materials, e.g., PVC or PET, can not be completely excluded. With the increasing PP content, the time to embrittlement during oven aging of test specimens was dramatically reduced. A relatively small amount of PVC (0.2 wt%) further reduced oven aging time. Moreover, the processing was affected. These experiments indicate that, within certain limits, constant composition of the recyclate is an important quality criterion. However, adding specifically developed stabilizer systems, containing phenolic antioxidants, processing­and light stabilizers as well as other co-additives improved the stability behavior of such PCR blends when used for outdoor applications. Good example is provided by sound (noise) reduction walls -- a demanding task since they must last for many years. Further developments in using mixed plastic waste may involve specifically designed compatibilizers that improve mechanical properties of these materials.

Conclusions -2 Although there are many points that have to be carefully considered when using recyclates, there are, as shown, many examples of successful recyclate applications. Recycling of plastics in a closed loop system is a reality. It embraces products like battery cases, bottle crates, waste bins, containers, all kind of plastic films and bottles. Furthermore, there are numerous examples of the high grade use of recyclates made from packaging materials, e.g., for non-pressurized drainage and cable ducts, as well as pallets. Recyclates can be used in many applications to replace virgin resins. Restabilized recyclates, not only match the virgin material in terms of performance, but they also have ecological advantages.

The growth and future economic success of the PCR industry are fundamentally possible. This will be driven by an optimized collecting- and recycling process, and by targeting high value products and applications. The technical literature and our own findings (as outlined above) lead to the conclusion that use of recyclates is possible even for demanding applications, provided that sufficient re-stabilization is provided. This

92

observation is equally valid for a single component as well as for mixed plastics peR. is often assumed that recyclates originate from well-stabilized virgin materials that w{ not excessively damaged. Restabilization has to take into account previous damagt

subsequent application and residual stabilizer content.

93

Appendix I Note: Mw is the molecular weight (g/mol), Mp is the melting point (0C), and CAS stands for Chemical Abstract Service number.

Irganox Antioxidants

IRGANOX 1010

Mw = 1178, Mp = 110-125°C CAS: 6683-19-8

IRGANOX245

"O~C"'C,J-o-c"c"o-c,,-c--2

Mw = 587, Mp = 76-79°C CAS: 36443-68-2

Irgafos Processing Stabilizer

IRGAFOS 168

~t 3

Mw = 647, Mp = 180-186°C CAS: 31570-04-4

94

IRGANOX B - Blends

IRGANOX B 225 (1 :1) IRGANOX B 215 (1:2) IRGANOX B 561

IRGANOX 1010 IRGAFOS 168

IRGANOX 1010 IRGAFOS 168

IRGANOX 1010 IRGAFOS 168 (1:4)

Hindered Amine Stabilizers (lIAS)

TINUVIN 770 - -

t>-J c H2

- -8 Mw = 481, Mp = 81-85°C CAS: 52829-07-9

TINUVIN 622

Mw - >2500, Mp = 55-70°C CAS: 65447-77-0

L-¢

n

CHIMASSORB 944

-tj [ r~srN~N~

NH,' An ~ R

Mw = 2600 - 3000, Mp = 100 - 135°C CAS: 71878-19-8

UV -Absorbers

CHIMASSORB 81

Mw = 326, Mp = 48°C CAS: 1843-05-6

TINUVIN 327

Mw = 358, Mp = 154-157°C CAS: 3864-99-1

95

96

RecycJate Stabilizer Systems

RECYCLOST AB 411 RECYCLOSTAB 421 RECYCLOSTAB 451 (contains Antioxidants and co stabilizer)

RECYCLOSSORB 550 (contains antioxidants, costabilizers and HAS-light stabilizers

RECYCLOBLEND EB 36-50 (oxirane compound) CGX-195 (organic phosphorous compound)

Sorted Plastic Waste

Bottle Crates ( PE-HD )

Pipes ( PE-HD I PE-LD )

97

98

Sorted Plastic Waste

Battery Cases PP)

Bumper ( PP / EPDM)

99

Mixed Solid Plastic Waste

Sound Reduction Walls

100

References

1. Murphy, J. (1994) Recycling Plastics- Guidelines for Designers. Techline Industrial Data Services Limited,

2. 2. Dennison, M. T., and Lovell, J. S. (1994) The Future of Plastics Recycling in Europe. Presentation De Witt Petrochemical Review, Houston.

3. Maak, H. (1992) Plastics Recycle Acceptance Problems - Maximum Content Levels - West Europe Year 2000. Presentation: Recycle'92 Conference, Davos.

4. Desarnauts, J. (1995) Plastic Waste Recovery - Objectives for the year 2002. Presentation: Recycle'95 Conference, Davos.

5. Anonymous (1997) Information system on plastic waste management in Western Europe - European Overview,1995 Data, SOFRES CONSEIL and APME.

6. Poschet, G. (1992) "Beurteilung und Priifung der Eigenschaften von Kunststoffprodukten aus KunststofJ Restgut". in: Dolfen; E., Breuer, H .. , Kunststoff-Recycling-Handbuch, Hiithig Verlag, Heidelberg.

7. Dietz, S. (1990) The Use and Market Economics of Phosphite Stabilizers in Post Consumer Recycle. Presentation, Recycle '90 Conference, Davos.

8. Sadrmohaghegh, C., Scott, G., and Setudeh, E. (1985) Recycling of Mixed Plastics. Polym. Plast. Techno!. Eng., 24,149 - 185.

9. Pospisil, J., Nespurek, S., Pfaendner R and Zweifel, H., (1997) Material Recycling of Plastics Waste for Demanding Applications: Upgrading by Restabilization and Compatibilization, Trends Polym. Sci., 5(9), 194.

10. Moss, S., and Zweifel, H. (1989) Degradation and Stabilization of High Density Polyethylene during Multiple Extrusions. Polym. Degradat. Stabil., 25,

11. Zweifel, H. (1997) in: Stabilization of Organic Polymers, Springer -Verlag, Heidelberg.

12. Kramer E., and Zweifel, H., to be published. 13. Drake, W.O.; Franz, T; Hofmann, P.; and Sitek, F. (1991) The Role of

Processing Stabilizer in Recycling of Polyolefins; Presentation, Davos Recycle '91, Davos.

14. Pfaendner, R, Herbst, H. and Hoffmann, K., (1996) Recent Developments in Restabilization of Recyclates, Eng. Plast., 217 - 251

15. Herbst, H.; Hoffmann, K.; Pfaendner, R; and Sitek, F. (1995) QualiUitsverbesserung von Recyclaten durch Additive (Stabilisatoren), In: Bittner, Brandrup, Menges, and Michaeli: Wiederverwertung von KunststofJen, Miinchen, Hanser, 289 - 308

16. Sitek, F., Herbst, H., Hoffmann, K., and Pfaendner, R (1994) Upgrading of Used Plastics: Why and How? Presentation, Davos Recycle '94, Davos.

17. Pfaendner, R., Herbst, H., Hoffmann, K., and Sitek, F. (1994)

101

Applications - An Overview. Presentation, Sixteenth Annual International Conference on Advances in the Stabilization and Degradation of Polymers, Luzern.

18. LaMantia, F.P., and Vinci, M. (1994) Recycling of Polyesterterephthalate, Polym. Degrad. Stab., 45, 121 - 125

19. Giannotta, G; Po, R.; Cardi, N.; Tampellini, E.; Occhiello, E.; and Garbassi, F. (1994) Processing Effects on Polyethyleneterephthalate from Bottle Scrap, Polym. Eng. Sci., 34,1219 - 1223

20. Karayannidis, G.P.; Kokkolas, D.E.; and Bikiaris, D.N. (1993) Solid State Polycondensation of Polyethyleneterephthalate Recycled from Post Consumer Soft Drink Bottles, J Appl. Polym. Sci., 50, 2135 -2142.

21. Hoffmann K., Herbst H. and Pfaendner R., (1996) Tailor Made Stabilizer Systems Boost Recycled Plastics, Addcon '96, Proceedings.

BIODEGRADABLE MATERIALS: STATE OF ART AND FUTURE PERSPECTIVES

CA TIA BASTIOLI Novamont S.p.A., Via G. Fauser, 8,1-28100 Novara, Italy

ABSTRACT

Biodegradable polymers constitute a loosely defined family of polymers that are designed to degrade through the action of living organisms. They offer a possible alternative to traditional non-biodegradable polymers when recycling is not practical or not economical. The main driving force behind this technology is the solid waste problem, particularly with regard to the decreasing availability of landfills, the litter problem and the pollution of marine environment by non-biodegradable plastics. Technologies like composting used for the disposal of food and yard waste, are the most suitable for the disposal of biodegradable materials. Thermoplastic starch-based polymers and aliphatic polyesters are the two classes of biodegradable materials with the greatest near-term potential.

This paper revises the wide variety of properties, structures and biodegradation behavior of thermoplastic starch in combination with polymers of vinyl alcohol and with poly-c-caprolactone and of some aliphatic polyesters like poly-hydroxybutyrate­valerate, poly-lactic acid, poIY-E-caprolactone and polybutylene succinate.

INTRODUCTION

The management of solid waste disposal with regard to the decreasing availability of landfills, the litter problem and the pollution of marine environment is becoming urgent in the industrialized countries and risks to quickly extend to the developing countries. Obliged approach is the valorization of waste as a resource through its separation into specific fractions, to be transformed in new products with a certain market value.

Technological aspects such as the development of safer and more efficient recycling technologies and the development of materials easy to reuse or recycle, or of biodegradable materials, can significantly contribute to the solution of the problem.

103

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 103-121. © 1998 Kluwer Academic Publishers.

104

Biodegradable polymers constitute a loosely defined family of polymers that are designed to degrade through the action of living organisms. They offer a possible alternative to traditional non-biodegradable polymers when recycling is not practical or not economical. Technologies like composting used for the disposal of food and yard waste, accounting for 25-30% of total municipal solid waste, are the most suitable for the disposal of biodegradable materials together with soiled or food-contaminated paper.

International organizations such as the American Society for Testing and Materials (ASTM) in connection with the Institute for Standards Research (ISR), the European Standardization Committee (CEN), the International Standardization Organization (ISO), the German Institute for Standardization (DIN), the Italian Standardization Agency (UNI), the Organic Reclamation and Composting Association (ORCA), are all actively involved in developing definitions and tests for biodegradability in different environments and compostability [1,2].

Although a standard worldwide definition for biodegradable plastics has not been established, the definitions already in place (ASTM, CEN, ISO) correlate the degradability of a material to a specific disposal environment and to a specific standard test method. The method must simulate such an environment for a time-span that determines the material classification.

According to this approach CEN, ORCA, UNI and DIN have defined, at draft level, the basic requirements for a product to be declared compostable based on: • Complete biodegradability of the product, measured through respirometric tests like

ASTM D5338-92, ISO/CDI4855 and CEN proposal xxI or the modified Sturm test ASTM 5209, in a time period compatible with the composting technology (few months).

• Disintegration of the material during the fermentation phase. • No negative effects on compost quality and in particular no toxic effects of the

compost and leachates to the terrestrial and aquatic organisms. • Control of laboratory scale results on pilot/full scale compo sting plants.

These requirements set forth a common base for a universal marking system to readily identifY products to be composted.

An important driving force for the development of biodegradable materials and the related compostability label is the recently adopted European Directive 94/621EC on packaging waste where composting of packaging waste is considered a form of material recycling.

Cost is a major obstacle to wide-scale use of biodegradable polymers. Main classes of biodegradable materials under development are starch-based, polyester, and natural

105

products like proteins, chitin, chitostane, etc. The sector of natural products other than starch is at a very early stage of development.

It follows a short description of starch-based materials (Table 1) and polyesters (Table 2) That are either already on the market or at an advanced development stage.

106

BIODEGRADABLE MATERIALS UNDER DEVELOPMENT

2.1 Starch-Based Materials

Starch is an inexpensive product available annually from com and other crops, produced in excess of current market needs in the United States and Europe. It is totally biodegradable in a wide variety of environments and can permit the development of totally degradable products for specific market demands. Degradation or incineration of starch products recycles atmospheric CO2 trapped by starch producing plants and does not increase potential global warming. All these reasons excited a renewed interest for starch based plastics in the last years.

Starch is constituted of two major components: amylose, a mostly linear u-D-(l-4)­glucan and amylopectin, an u-D-(1-4) glucan that has u-D-(1-6) linkages at the branch point. The linear amylose molecules of starch have a molecular weight of 200 to 2,000 kg/mol, while the branched amylopectin molecules have molecular weights as high as 100,000 to 400,000 kg/mol (Fig. 1). In nature, starch is found as crystalline beads of about 15 to 100 !lm in diameter, in three crystalline modifications designed A (cereal), B (tuber), and C (smooth pea and various beans), all characterized by double helices: almost perfect left-handed, six-fold structures.

Table 1: Starch-based materials on the market

Material Technology Patent Company Installed Market Descrietion situation caeacitl': aeelication

Mater-Bi Thennoplastic, blended Dominating Novamont 4000 Composting compatibilized, complex teclUlology bags, paper of starch with lamination, polyesters, vinyl alcohol paper wrapping, copolymers, cellulose cutlery, foams, acetate and other cotton swabs, t11ernlOplastic polymers dog bones

Novon Thennoplastic, blended Teclmology Novon Int. Plant just ? starch acquired from (Ecostar; installed

Warner- before- of about Lamber in Warner- 3000 1995 Lan1ber) ton/year

Envirofil Thennoplastic starch License from Enpac 5000 Loose-fillers Warner- (DuPont- (through only Lamber Conagra customers

107

limitcd to joint- and sub-loosc-fillers venture) licencees)

Ecofoam ThemlOplastic high Nich National 5000 ? Loose-fillers amylose starch technology Starch and other

with W-L expanded license materials

Chisso As for Novon limited to W-L license Chisso ? Composting Japan bags of old

technology

Biopac Wafer technology Dominating Haas 500 ? Food trays niche teclUlology

Biotec Themloplastic starch Acquisition Melitta Pilot Composting ofFluntera (1000?) bags since 1995 patents

Table 2: Biodegradable polyesters under development

Material Trade- Company "Installed Market Development mal'k capacity applications stage

T/year

Poly-hydroxy Biopol Monsanto 600 Ext. coating, Development butyrate bottles, cutlery valerate

PoIY-E- Tone Union > 5000 Blcnds with starch Industrial caprolactone Carbide

Placel Daicel Capa Intcrox

Poly-lactic Eco-Pla Cargill 2000 Thermofomled Development acid Neste items, bottles,

Chronopol fibers, film Mitsui Toatsu

Poly-butylene Bionolle Showa 3000 Fibers, film, Development succinate Skygrecn Denko bottles, cutlery

Sunkyong

Poly ester- Bak Bayer ? Film, fibers Development amide

Polyethylene Eastman Film Research terephthalate BASF A.-G. adipate

108

CH,OH 01,011 ClI,OH

~Jfo H V~~~ ~H O"H o H H O---.J~J-O OH H O ..

I J

H OH H OH H OH H OH

"4",°" HO~;~ c II 1 o I

Amy/ose

CH,OH CH, CH,OH CH,OH

H)--=-O .11 II J-~o, H If )-:"0 H H~r-:-( O,.H l:' ~LI LI •. ~ ~(~u u'~ ~/ ~LI LI '~ "

o--'~'--O-.~'-o--'~'-o~ OH H 0.· ! I ,: I

II all H Oil II 011 H OH Amv/opecrill

Figure 1: Chemical structure of amylose and amylopectin

2.2 Crystalline starch

Crystalline starch beads can be used as a natural filler in traditional plastics [3]. They have been particularly used in polyolefins. When blended with starch beads, polyethylene films biodeteriorate upon exposure to a soil environment. The microbial consumption of the starch component leads to increased porosity, void formation, and loss of integrity of the plastic matrix. Generally, starch is added at fairly low concentrations (6-15 wt%). The total disintegration of these materials is obtained using transition metal compounds, soluble in the thermoplastic matrix, used as pro-oxidant additives to catalyze the photo and thermo-oxidative processes [4]. These products belong to the first generation of degradable polymers that biodeteriorate more than mineralize to CO2 and H20 in a time

109

period of some months. Such starch filled polyethylenes containing pro-oxidants are commonly used in agricultural mulch film, in bags, and in six-pack yoke package.

2.3 Thermoplastic Starch

Starch can be made thermoplastic according to extrusion cooking technology. Extrusion cooking and forming is characterized by sufficient work and heat being applied to a cereal based product to cook or gelatinize completely all the ingredients. Equipment used for high pressure extrusion heat materials during processing, and continually compress them. Gelatinized materials with different starch viscosity, water solubility and water absorption have been prepared by altering the moisture content of the raw product and the temperature or the pressure in the extruder. An extrusion cooked starch can be solubilized without any formation of maltodextrins, and the extent of solubilization depends on the extrusion temperature, moisture content of starch before extrusion and the amylose to amylopectin ratio [5-9] (Fig. 2).

Thermoplastic starch alone can be processed as a traditional plastic. Its sensitivity to humidity, however, makes it unsuitable for most of the applications (Fig. 3). Main use of neat thermoplastic starch is in soluble compostable foams (loose-filler, etc.) as a replacement for polystyrene.

110

2.4 Thermoplastic starch-composites

Starch can be destructurized in combination with different synthetic polymers to satisfY a broad spectrum of market needs. Thermoplastic starch composites can reach starch contents higher than 50%.

For example, the starch/vinyl alcohol copolymer systems can generate a wide variety of morphologies and properties [IO]. These depend on the processing conditions, starch type, and on copolymer composition, that affect the extent of a complex formation between amylose and the synthetic molecules

30 -

l- I-

r?0 ..200

r

... - ....

5000 l-

I-

.. - .. -I I I

65 90 129

I I

I I

I

)1

.. '

I

XI I

I I

I I

I

}.

I

I I

!

X Ql "0 .~

6 c o

2 + -

e.10 :il

.D

'"

.... - 2 '

I I I 170 220 225 250

Extrusion temperature (Oe)

Figure 2: Effects of extrusion temperature on expansion ( .), breaking strength (+), viscosity at 50°C ( ..... ), water absorption index n, and water-solubility index (x) of extruded products from corn grits. Initial moisture content before extrusion was 18.2 wt%.

0.6

resorption desorption temp. (K) A t::. 291 Rakowski (1911)

293 Farrow & Swan (1923) 298 Anonymous (1947) 298 Hellman & Melvin (1950) 293 Schierbaum (1960) 293 Hofer (1962)

0.5

" o a 298 Dupra' (1975) -0-.- 293 van den Berg et a!. (1975) ---.--- 298 van Krevelen & Hotijzer (1976)

0.4

... 0.1 ...

o 0.2 0.4 0.6 0.8

Water activity

, , , , ,

10

111

Figure 3: Some sorption isotherms of water vapor on native potato starch as reported in the literature

A model has been proposed considering large individual amylopectin molecules, hydrogen bonded at several points per molecule and co-entangled by chains of amylose/vinyl alcohol copolymer V complexes. This structure has been labeled as "interpenetrated. "

The biodegradation rate of starch in these materials is inversely proportional to the content of amylose/vinyl alcohol copolymer complex. As an example, the products based on starchlpoly(ethylene-co-vinyl alcohol) (EV AI) show mechanical properties good enough to meet the needs of specific industrial applications. Their moldability in film blowing, injection molding, blow molding, thermoforming, foaming, etc., is comparable with that of traditional plastics such as PS, ABS, LDPE [11].

112

Main limitation of these materials is their high sensitivity to low humidity, with consequent embrittlement. The biodegradation of these composites has been demonstrated in different environments [12]. The degradation rate of2-3 years in watery environments is to slow to consider these materials as compostable (Fig. 4).

Starch can also be destructurized in the presence of more hydrophobic polymers such as aliphatic polyesters [13].

It is known that aliphatic polyesters having low melting point are difficult to be processed by conventional techniques for thermoplastic materials, such as film blowing and blow molding. It has been found that the blending of starch with aliphatic polyesters makes it possible to improve their processability and biodegradability. The particularly suitable polyesters are poly-e-caprolactone and its copolymers, or polymers at higher melting point formed by the reaction of glycols (such as 1,4 butanediol) with succinic, sebacic, adipic, azelaic, decanoic or brassilic acids. The presence of compatibilizers between starch and aliphatic polyesters such as amyloselEVAI V-type complexes [10], starch grafted polyesters, chain extenders like diisocyanates, epoxies, etc., can improve the properties of starch-composites [13]. These types of materials are characterized by excellent compostability, excellent mechanical properties and reduced sensitivity to water.

Thermoplastic starch can also be blended with polyolefins, possibly in the presence of a compatibilizer. Starch/cellulose derivative systems are also reported in the literature [12].

The combination of starch with a soluble polymer such as polyvinyl alcohol (PV AI) and/or polyalkylene glycols was widely considered since 1970. In the last years the system thermoplastic starchIPV AI has been used for production of starch-based loose fillers as a replacement for expanded polystyrene (Fig. 5).

The results obtained in the field of thermoplastic starch in combination with polymers or copolymers of vinyl alcohol with aliphatic polyesters and co polyesters in terms of biodegradation kinetics, mechanical properties and reduced sensitivity to humidity make these materials ready for a real industrial development starting from film and foam applications. The present global market is around 12000 tons/year. Main producers are Novamont with Mater-Bi trade-mark, ENPAC and National Starch. The tensile properties of films made of two Novamont's Mater-Bi grades are reported in Table 3, in comparison to these of low density polyethylene (LDPE). Figs. 6-7 show applications of Mater-Bi starch-based materials now on the market.

~YS 0 .... , O$!\ (1)

In v; !If; I~O '[';1 3110

JS 71 J6 ~7 III 50

SCAS lIST .t 2'i·C wrlGHI los.~ nr MilO P~R'~ I" IlAnK 81 AIO~ ('HI{IHU'" SOD ,IJ")

113

i ~17 7r.S

~~ 'l1

Figure 4: Weight loss (WI) of Mater-Bi AI05H (A class) under Semi-continuous Activated Sludge (SCAS) test conditions as a function of time (I).

Figure 5: Loose-fillers made ofMater-Bi of class V. Content of starch 385 wt%.

114

Figure 6: Compo sting bag in Mater-Bi of class Z. Main components thermoplastic starch and poly-s-caprolactone

Figure 7: Pens made ofMater-Bi of class Y. Main components: thermoplastic starch and cellulose derivatives

100

90

80 I: .2_ 70 'lU'" "0 0 60 ."u ~g 50 "00 .2u

40 In -

-.4-Pu re cellulose

~ 0 30 -.-ZF03U/A

20

10 -t- ZIOlU

0

0 10 20 30 "'0 50 60

Time (Days)

Figure 8: Aerobic biodegradation under controlled composting conditions ASTM D 5338-92

115

Fig. 8 shows the biodegradability in composting environment of two Mater-Bi grades for bags containing thermoplastic starch and poly-e-caprolactone.

116

POLYESTERS

3.1 Polyhydroxyalkanoates

The bacterial poly-hydroxyalkanoates are aliphatic polyesters, homo-or copolymers of [R]-f3-hydroxyalkanois acids produced by microorganisms [14]. Prokaryotic organisms, such as bacteria and cyanobacteria, accumulate poly(3-hydroxy butyrate), as inclusions in the cytoplasmic fluid, amounting to 30-80% of the cellular dry weight, when their growth is limited by the depletion of an essential nutrient such as nitrogen, oxygen, phosphorus, sulfur or magnesium. These microbial polyesters can be defined as intracellular storage products, and yield, composition, and molecular weight are influenced by carbon source and nutrients.

Long branches, novel chain polymers have also been produced by the addition of appropriate substrates in the culture medium [15]. The different polyesters obtained with this technology can be thermoplastic or elastomeric and that containing valeric and butyric monomers are mineralized.

The physical properties (Table 4) and biodegradability of microbial polyesters may be regulated by blending with synthetic or natural polymers. Their rates of hydrolytic degradation are dramatically affected by the presence of polysaccharides. The mechanism of biodegradation of poly(3-hydroxybutyrate) involves the enzyme poly(3-hydroxybutyrate depolymerase), which depolymerizes the polymer to the corresponding acid [16]. Recent efforts to produce the copolymer in Escherichia coli and in plants [17] is drawing interest due to the possibility of reducing production costs and simplifying purification procedures.

High molecular weight poly(3-hydroxy-butyrate) and its copolymers with poly(3-hydroxy-valerate) can also be produced synthetically from racemic 13-butyrolactone and f3-valerolactone, using an oligomeric alumoxane catalyst.

Due to the difference in structure, the synthetic polyesters are less susceptible to enzymatic degradation than the bacterial polyester. The bacterial copolyesters are mainly isotactic with random atactic stereo sequences, whereas the synthetic polyesters have blocks of only partial stereoregularity. In the latter case the [S]-stereo block hinders the enzymatic degradation, making difficult for depolymerase to penetrate into the surface and access the available [R]-stereo blocks.

Polyhydroxybutyrate-valerate is produced under BIOPOL trade mark by Monsanto, with a production capacity of about 500 ton/year.

117

3.2 PoJy(Jactic acid)/PoJy(gJycolic acid).

The linear aliphatic polyester, poly(lactic acid), is a thermoplastic polymer, chemically synthesized by polycondensation of the free acid or by catalytic ring-opening polymerization of the lactide (dilactone oflactic acid) [18]. The enantiomeric monomers for the synthesis, L-Iactic acid (naturally occurring) and D-Iactic acid, can be produced via biological or chemical methods. The ester linkages in the polymer are sensitive to both enzymatic and chemical hydrolysis. Poly(lactic acid) is hydrolyzed by many enzymes including pronase, proteinase K, bromelain, ficin, esterase, and trypsin [19]. Copolymers of glycine and D,L-Iactic acid have been synthesized and are biodegradable.

Poly(lactic acid) in the past has been primarily considered for medical implants and drug delivery, but broader applications in packaging and consumer goods are also targeted [18]. An attractive feature of this material is the potentially relatively low cost of the monomer, lactic acid, which can be derived from biomass (fermentation), coal, petroleum or natural gas.

Poly(ether-ester) block copolymers with polyethylene oxide (molecular weight MW =

600 to 6,000 g/mol) and poly(lactic acid) have also been synthesized - they are highly hydrophilic.

Copolymers of glycolide L( -)lactide have been commercialized for biomedical applications and are high strength biodegradable thermoplastic materials. Poly(glycolic acid) and copolymers with D,L-lactides are presumed to be biodegradable, although the role of chemical hydrolysis vs. enzymatic depolymerization in this process remains open to debate.

Other materials with a possible future commercial interest are the copolymers of lactic acid and-e-caprolactone.

The market for high polymers from lactic acid is still in its early stages. A large market growth for the polymer requires additional sources of lactic acid as raw material. Cargill started-up a 4.5 kton/year lactic acid/poly-Iactide plant in 1994 and announced a large plant with a capacity as high as 114 kton/year in the next years. Other producers of poly­lactic acid are Mitsui Toatsu Chemical (which has developed a polycondensation process of lactic acid), Shimatsu, Dainippon Ink, and Neste.

The mechanical properties of polylactic acid are similar to those of polystyrene (Table 4). Problems still unsolved are the too low glass transition temperature, the limited stability to hydrolysis and the slow crystallization rate. Main applications at development stage are in the sectors of bottles, thermoformed containers for food, films and fibers.

118

3.3 Poly-e-caprolactone.

Poly-e-caprolactone is a biodegradable, aliphatic polyester that is made by ring-opening polymerization of e-caprolactone. The polymer (having the molecular weight up to several thousand), is either a waxy solid or viscous liquid that has been used either as polyurethane intermediate, a reactive diluent for high solid coatings. or plasticizer for vinyl resins. By contrast, poly-e-caprolactone having MW > 20 kg/mol, is used as a thermoplastic polymer with mechanical properties similar to these of polyethylene (see Table 4).

Table 3: Selected properties of starch/poly-&-caprolactone composites

Property Method Unit Mater-Bi Mater-Bi LDPE ZF03U1A ZlOlU

Tensile strength at ASTM MPa LD * 30 15 16 break D682 (+) CD 24

** Tensile strain at ASTMD % LD 800 850 873 break (+) CD 720 Tensile modulus ASTM MPa LD 296 150 191

D852 (+) CD 251 Tear resistance ASTM N/nun LD 68 58 63

DI938 P 68 58 63 (+) CD 84 65

P 84 65

Elmendorf tear TAPPI N/mm LC 20 18 22 strength T414 CD 37 30

Biodegradation in ASTM % 100 100 0 controlled 05338 composting for 60 days Notes: * Longitudinal direction; ** Cross direction; I = Primer; P =

Propagation

119

Table 4: Selected properties of some polyesters under development

Propet'ty PHB PHB-V Tone Ecopla Bionolle Bionolle BAK (Biopol) (Biopol) 787 1000 3000 1095

Melting point 177 135 60 177-180 96 114 125

Tensile stress at 40 25 4 45 40 60 25 break(MPa)

Elongation at break 6 25 800 to 3 600 800 400 (%)

Tensile modulus 1000

(MPa) 4000 1000 386 2800 300 500 180

Density (g/mL) 1.25 1.25 1.145 1.21 1.3 1.2 1.07

Biodegradability in 100 100 100 100 20 90 100 controlled composting; 60 days (%)

(ASTM 5338)

Notes: Bionolle is poly alkylcn-succinates; Tone is polY-E-caprolactone; Ecopla is poly-lactic acid; BAK is polyester amides; * 16% V (hydroxy valerate)

A number of studies have clearly stated the biodegradability of polY-E-caprolactone [20]. The rate of hydrolysis depends on the molecular weight and degree of crystallinity. The E-hydroxy caproic acid was detected as an intermediate during the degradation process by Penicillium sp. [21]. Molecular weight and degree of crystallinity of polY-E­caprolactone influence its biodegradation rate.

Chemical hydrolysis rate is slow for the homopolymer, particularly when compared with poly(glycolic acid) and poly(glycolic acid)-co-(lactic acid). The rate can be increased by copolymerization. Cross-linked polY-E-caprolactone films with lower degree of crystallinity than the uncross-linked material were also found to be biodegradable.

Union Carbide, Daicel and Interox are the three producers of polY-E-caprolactone world wide. High molecular weight polY-E-caprolactone may be processed by a variety of techniques, including film blowing and slot casting. Main application in the field of biodegradable plastics is in combination with thermoplastic starch in films, sheets and injection molded parts.

120

POLYAMIDES

Recently, the bulk low cost synthesis of polyaminoacids such as polyaspartic acid in 95% yield was demonstrated [22] The polymers are biodegradable and could replace polyacrylic acid as dispersants in paints, detergents, scaling in piping, and absorbents in diapers and medical products.

Bayer has recently launched on the market a polyester-amide constituted by poly butylene-adipate and &-caprolactam. Its properties are reported in Table 4.

Other approaches to the synthesis of polyaminoacids are possible, some based on the use oftri-functional aminoacids as starting materials. New biodegradable functional polymers will be forthcoming that may prove useful as fibers for rigid composites or compatibilizers. The costs associated with some of these processes will also have to be addressed.

CONCLUSION

Starch-composites are biodegradable materials ready for a real industrial development starting from the applications of biodegradable films and foams. Polyesters are at an advanced level of application development and are promising in the sectors of biodegradable fibers, thermoformed items, rigid film etc.

The future role of biodegradable polymers in the world's solid waste management strategy is still being developed. The rate of market growth for biodegradable polymers depends primarily on: • The speed at which legislation requiring degradables is enacted at international,

national and regional level. • How quickly governments/industry can develop infrastructures for collection and

composting. • Consumer attitudes toward protecting the environment. • The availability of truly biodegradable materials able to degrade in time-frames

compatible with specific disposal infrastructures, and to work in in-use conditions.

121

REFERENCES

1. ASTM Standards on Environmentally Degradable Plastics, ASTM Publication Code Number (DCN): 03-420093-19 (1993)

2. CEN TC 261 SC4 W62 draft "Requirements for packaging recoverable in the form of compo sting and biodegradation. Test scheme for the final acceptance of packaging", March 11, (I996)

3. G. J. L. Griffin, U.S. Patent 4016117, (1977)

4. G. Scott, UK Pat. 1,356,107, (1971)

5. C. Mercier, P. Feillet, Cereal ChemistlY 52(3),283-297, (1975)

6. J. W. Donovan, Biopolymers 18, 263, (I 979)

7. P. Colonna, C. Mercier, PhytochemistlY, 24 (8), 1667-1674 (1985)

8. J. Silbiger, J. P. Sacchetto, and D. J. Lentz, Ellr. Pat. Appl. 0404 728, (1990)

9. C. Bastioli, V. Bellotti, G. F. Del Tredici, Eur. Pat. Appl. WO 901EP1286, (1990)

10. C. Bastioli, V. Bellotti, M. Camia, L. Del Giudice, A. Rallis November 9-11, 1993 to be published - Proceedings III International Scientific Workshop on Biodegradable Plastics and Polymers - Osaka, Japan

11. C. Bastioli, V. Bellotti, A. Rallis, "Microstructure and melt flow behaviour of a starch-based polymer", Rheologica Acta 33:307-316 (1994)

12. C. Bastioli, V. Bellotti, L. Del Giudice, G. Gilli J.Environ. Polym. Degradation 1(3), 181-191, (1993)

13. C. Bastioli, V. Bellotti, G. F. Del Tredici, R. Lombi, A. Montino, R. Ponti, Int. Pat. Appl. WO 92/19680, (1992)

14. A. Steinbuchel, "Polyhydroxalkanoic Acids", III Biomaterials: Novel Materials from Biological Sources, New York: Stockton Press, pp. 123-124, (1991)

15. B. A. Ramsay, I. Saracova, J. A. Ramsay and R. H. Marchessault, Applied Environ. Microbiol., 57 625-629, (1991)

16. Y. Doi, Y. Kensawa, M. Kunioka and T. Saito, Macromolecules, 23, 26-31, (I 990)

17. Y.Poirier, D.E.Dennis, K. Klomparens and C.Somerville, Science, 256, 20-522, (1992)

18. E. S. Lipinsky, and R. G.Sinclair, Chem. Eng. Progress, 82 26-32, (1986)

19. D. F. Williams, Eng. in Medicine, 105-7, (1981)

20. D. Jarrett, C. Benedict, P. Bell, J. A. Cameron and S. J. Huang, Polym. Pre prints, American Chemical SOCiety Division (?f Polymer Chemistry, 2432-33 (1983)

21. Y. Tokiwa, T. Ando and T. Suzuki, J.Fermelltation Technol., 54 603-608, (1976)

22. L. Koskan, Industrial Bioprocessing (May) 1-2 (1992)

POLYMER BLENDS' TECHNOLOGY FOR PLASTICS RECYCLING

L. A. U tracki National Research Council Canada, Industrial Materials Institute, 75 de Mortagne, Boucherville, QC, Canada J4B 6Y4

ABSTRACT

Out of the three methods of plastic waste recycling, that is, polymer recycling, feedstock recycling, and energy recovery, the former is the most desirable. The polymer recycling usually involves: segregation, washing, shredding and extruding. During melt-extrusion, the material undergoes devolatilization, stabilization, compatibilization, alloying, filtering and pelletization. The alloying, blending, and compounding are the basic ABC processing steps of the technology for polymer recycling. Hence, this chapter summarizes these elements of blending technology that are pertinent for polymer recycling in three parts: (i) thermodynamics of polymer blends, (ii) flow behavior of polymers and their blends, and (iii) compounding and processing of polymer blends.

The first part provides a brief outline of the principal aspects of the thermodynamics (miscibility and interfacial properties) and compatibilization (either by addition of a compatibilizer, by reactive or mechanical compatibilization). The second part focuses on melt rheology - the flow behavior of multi component, polymeric systems (including the fundamental principles of the morphology modeling). The third part summarizes the basics of mixing, blending, alloying, and compounding methods, as well as it outlines the principles of morphology modeling. Since compatibilization and development of blends' morphology are topics of other chapters in this book, these topics are only outlines to provide proper image of the technology.

1. INTRODUCTION

Polymer blends are mixtures of at least two macromolecular species, polymers and/or copolymers. They are either miscible or immiscible - the latter prevailing. The polymer/polymer miscibility is limited to a "miscibility window," a range of independent variables, such as composition, molecular weight, temperature, pressure, etc. More than 1600 of these "miscibility windows" have been identified [1].

The miscibility stretches from highly miscible pairs (e.g., low MW blends of PS with PVME showing high negative value of the binary thermodynamic interaction parameter, X12 « 0), to antagonistically immiscible mixtures, viz. PE/PA, where X12 » O. Performance of the materials at either end of the miscibility scale is poor. The sing1e­phase miscible blends are usually brittle and must be compounded either with aUt

123

G. Akovali et al. (eds.). Frontiers in the Science and Techoology of Polymer Recycling, 123-152. © 1998 Kluwer Academic Publishers.

124

e~astomer or reinforced. Moreover, since this type of blends is extremely rare (it nearly kllied the PPE production, since the early PPEIPS blends were very brittle - luckily, an addition of HIPS saved the day).

By contrast, the antagonistically immiscible blends are quite common and they must be properly processed to maximize their excellent potential performance characteristics. Two steps are required: compatibilization and development of the most suitable morphology for the envisaged application. There are three aims of compatibilization: 1. Reduction of the interfacial tension to facilitate dispersion, 2. Protection of the generated morphology against possible modification during the subsequent processing steps, and 3 . Enhancements of adhesion between the phases, which would facilitate the stress transfer, hence improve the mechanical properties of the product.

The goal of blending is generation of optimum morphology for specific applications. The morphology depends on both fundamental blends, properties: thermodynamics, and flow. Flow affects morphology in several ways: by changing the size and shape of individual domains, and by imposing a gradient of concentration and/or morphology across the part. Since flow introduces energy into the system, it may also affect the miscibility. Knowledge of these mechanisms can be used to commercial advantage, viz. to generate lamellar structures during blow molding, to lubricate flow through dies, or to generate a protective layer on the surface of an extrudate.

2. THERMODYNAMICS OF POLYMER BLENDS

2.1. DEFINITIONS

The following basic terminology for polymer blends has been adopted [1]:

Miscible polymer blend- Blends, homogenous to the molecular level, or having the domain size comparable to the macromolecular dimension, negative value of the free energy of mixing, AGm ~ ARm ~ 0, and rf LiG ",10;2 > o.

Immiscible blends- Blends whose free energy of mixing; AGm ~ ARm> O.

Polymer alloy - Immiscible, compatibilized polymer blend with the modified interface and morphology.

Interphase - Third phase in binary polymer alloys, engendered by interdiffusion or compatibilization. Its thickness Al = 2 to 60 nm depends on polymers' miscibility and compatibilization.

Compatibilization - Process of modification of the interphase in immiscible polymer blends, resulting in reduction of the interfacial energy, development and stabilization of the desired morphology, leading to the creation of a polymer alloys with enhanced performance.

2.2. MISCIBILITY

To determine miscibility, usually isobaric phase diagrams are constructed. An example of a system with the lower critical solution temperature (Tc = LeST) is shown in Fig. 1. The

125

binodals indicate limits of miscibility. They are computed by equating the chemical potential of each component across the interface [2]:

(1)

Here, ~Gm is the excess Gibbs free energy of mixing, and Xi is the molar fraction of component i. The spinodals in Fig. 1 indicate the boundary condition between the meta­stable and spinodal regions. They and the critical point are computed from the second and third derivatives:

(2)

The inequality in Eq 2 specifies the condition for stability of the system.

Schematic phase diagram for binary system

,....., 255 U 0 '-'

region

~ r. = 253 .... eIiI r. ~ CI.

/ bloodal

e ~

Eo< 251

2490 0.4 0.8 Volume fraction of component 2

Figure 1. Binary phase diagram at constant pressure. with the lower critical solution temperature. Tc = LeST. The solid and broken lines indicate binodal and spinodal curves. respectively. The single phase. two meta-stable regions. and a spinodal region are shown. Majority of polymer blends (whose miscibility depends on specific interactions) show this type of behavior.

The oldest and the best known expression for ~Gm = ~Gm (~i> P, T) is that published in 1941, first by Huggins and then by Flory:

(3)

where ~i is volume fraction and Vi is molar volume of component i, R is the gas constant, )(12 is the binary thermodynamic interaction function, which depends on: ~i' T, P,

126

molecular weight, MW, molecular weight distribution, MWD, etc. It can be shown that the necessary condition for phase separation in binary polymer blends is: XI2 ~ XI2.cr = o.

The most successful statistical theory ofliquids is that derived by Simha and Somcynsky. The model considers liquids to be mixtures of voids dispersed in solid matter, i.e., a lattice of unoccupied and occupied sites. The occupied volume fraction, y (or its counterpart: the free volume fraction: f= 1 - y), is the principal variable: y = y(ep" P, T). From the configurational partition function the configurational contribution to the Helmholtz molar free energy ofliquid i was expressed as [3]:

Fi IRT=ln(Yi Is,}+s,[(l-y,)/y,]ln(l-y,}+(si -l)[l-ln(z-l)]

- Ci {In[v~ (1- r1;)l 1 Q,] + (3 1 2)ln[21tMoiRT(N Ahf2

-YiQ~(AQ~ -2B)1 (2T,}}

(4)

In the dependence the following terminology was used: Yj = Sj Nj I( Sj N j + NhJ is a fraction of occupied lattice sites; Sj is the number of segments per chain of molar mass ~; Nj and Nhj are the number of occupied and vacant sites, respectively; 3cj is the number of external degrees of freedom per chain; Moj is the segmental molar mass ~/sj; v\ and E\ are respectively the intermolecular repulsion volume and maximum intermolecular attraction energy between segments of liquid i and liquid j; A, B, and z are numerical constants equal to 1.011, 1.2045, and 12, respectively; NA and h are Avogadro's and Planck's numbers, respectively; qjZ = sj(z-2) + 2 is a number of intermolecular contacts. The quantities with tilde represent dimensionless (scaled) variables of state for:

Pressure: Pi=Pi/P;; Temperature: Ti=Ti/T;; Volume: "i=Vi/V; while Qi = 11 (Yi"i); and 'Ili = 2-Ii6 YiQ:/l are dimensionless expressions.

From Eq 4, the molar Helmholtz free energy of a binary mixture is:

F m 1 RT = XI lnxi + x2lnx2 +« s > Iy)(l-y)ln(l-y)-« s > -l)ln[(z-l)1 e] (5)

- < c > {In[ < v* > (I-'Il)l I Q] - (yQ2 I 2T)(AQ2 - 2B)}

- (3 1 2){xlcl In[21t < MOl> RT(N Ahf2] + x2c2ln[21t < M02 > RT(N Ahf2])

The values in the angle brackets, < >, are compositional averages:

< s >= XISI + X2S2; < C >= XICI + X2C2; < Mo >= (xlslMol + X2s2MoJI < s >

< E* >< v* >k= X~E>:~ + X~E;2V;~ + 2XIX2E:2 v:~; k = 2,4

where XI =1-X2=xl[sl(z-2)+2]/<qz>

The two cross-interaction parameters are expressed as: Interaction energy: E:2 = S,(E;IE;2)1/2

Repulsion volumes: v:2 = [(V:: /3 +v;~13)/2]l

(6)

For several investigated systems, good agreement was obtained assuming S£ = I. By contrast with the complex functional dependence of X12 of Eq 3, the two cross-interaction parameters ofEqs 5-6 are indeed constant.

127

2.3. INTERFACE

2.3.1. Theories of the interphase. For a molten, immiscible, binary blend of polymers A and B, Helfand et al. developed a quantitative lattice theory of the interphase [4-6]. The model assumed that: interactions between statistical segments of polymers A and B are determined by X12' the isothermal compressibility is negligible, and there is no volume change upon blending. The segmental density profile, Pi where i = 1 or 2, was solved for infinitely long macromolecules, Mw ~ 00 :

(7)

where b is a lattice parameter. In Figure 2 the dependence predicted by Eq 7 as well as the definition of the interphase thickness, ~l, are shown [7]. Typical values of ~l are given in Table 1.

Density profile across the interface 0.0 ---

....... a.

>-- 0.8

!:: til Z w 0.6 0 r/ Po-yz/(l + /}; I-z y = exp[(6 x} I/Z (x/b)] W ::::E 0.4 l!) w til 0 W u 0.2 ::J 0 W ~

0 ----10 -6 -2 2 6 10

REDUCED DISTANCE ACROSS THE INTERFACE, x/b

Figure 2. Theoretical representation of the interface, with the definition of the interphase thickness, ,1/, [7].

The interphase thickness, Ll1oo' and tension coefficient, voo ' were derived as:

(8)

The Helfand-Tagami lattice theory predicts that: 1. Product, ~loo V 12,oo = const.; 2. Surface free energy is proportional to rx;;; 3, Polymeric chain-ends concentrate at the interface;

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4. Low molecular weight components migrate to the interface; and 5. Interfacial tension coefficient increases with molecular weight to an asymptotic value, v",:

Table 1. Interphase Thickness [8]

Type of blend Immiscible Block copolymer Polymer/Copolymer Reactive Compatibilization

Radius of gyration, < r! >1/2

Thickness (nm) 2

4 to 6 30

30 to 60 5 to 35

(9)

Blends of polymers A and B with a block copolymer, either A-b-B or X-b-Y, have been described by Noolandi and his coworkers [9-10]. The derived system of equations can be solved numerically for the interfacial composition profiles, interfacial tension, etc. The theory makes it possible to compute 61"" v"" and the concentration profiles of all components.

2.3.2. Concentration o/copolymer-critical micelle concentration, CMe. As predicted by thermodynamics, there is an equilibrium between copolymer concentration at the interface and that in both polymeric phases. Thus, copolymer addition reduces v x only to the limit determined by CMC. CMC in B-rich phase is given by [11]:

(10)

where the square bracket represents the chemical potential on copolymer at CMC, ZAc and ZBc are respectively the number of A and B segments, Zc = ZAc + ZBc is the total number of segments of the copolymer, and / is a fraction of polymer A in copolymer A-B. Theoretically, the most efficient copolymer composition is: fopt = 112.

A semi-empirical relation was derived for V12 and diameter of the dispersed drops, d [12]:

(11)

where K is considered an adjustable parameter, K - XZc. As shown in Figure 3, this dependence well represents the experimental data.

Di-block copolymers have a higher interfacial activity than triblock or graft copolymers. They more readily interact with two homopolymers, forming appropriate entanglements that result in reduction of the interfacial tension coefficient, and enhanced interphasial adhesion in the solid state. The amount of the interfacial agent required to saturate the interface, Wcr. depends on its Mw, the total surface area, and the assumed geometry of the copolymer arrangement on the interface. For perpendicular penetration across the interface and for a random coil configuration at the interface the values are, respectively:

129

perpendicular: w cr = 6~Mw / dAN A

coil: Wcr = 6~Mw / d« r2 > /9)NA

Here ¢ is volume fraction of the dispersed phase, and Mw is the weight average MW of copolymer. Experimental values are expected to fall in between these two limits [13].

PS/PB/SB BLENDS 0.65

v. v + (v -v I exp(-K,l "" , CIIC -~ 0.55

:c: .s. Value Error

v 0.29005 0.017594 "" v . 0.6078 0.015059

K 1.0522 0.16582

'" 0.45

h

Chlsq 0.00070078 NA R 0.99444 NA

>

0.35

0.25

0 ¢ (58 wt 96) 3

Figure 3. Interfacial tension coefficient as a function of diblock copolymer concentration ~. System: matrix - BR, dispersed phase - PS, compatibilizer - styrene-b-butadiene di-block copolymer. Line was computed from Eq 11, points are experimental {14}.

2.4. COMPATIBILIZATION OF POLYMERS

2.4.1. By addition of a compatibilizer Efficiency of a compatibilizer to lower v 12 depends on the preferential deposition on the interface. This is accompanied by its dissolution, as well as possible formation of micelles in the two polymeric and meso-phases. Compatibilization reduces V12 and parallel with it, the size of the dispersion (viz. Fig. 3). Since the process is dynamic, the amount of compatibilizer to saturate the interface depends on the mixing time and equipment, the affinity of the emulsifier to the dispersed phase, the size of the dispersion, the orientation of the emulsifier at the interface and its ability to stabilize the interface against flocculation and coalescence [13].

Two main methods of v12-measurements are the so-called "filament breakup," and the retraction of deformed drop. The advantages of the latter are: (i) simplicity and rapidity of measurements, (ii) possibility of the interfacial tension coefficient measurements in both directions: 1 -+ 2 and 2 -+ I, and (iii) ability to study the time dependence of V12 [15].

Compatibilization also affects the interphase thickness. Unfortunately, for the same blend values of V l2 and Al were not measured. For binary PEIPP blends the interfacial thickness was reported as: Al = 1.5 to 2.8 nm. In PSIPMMA blends, with and without block

130

copolymer P(S-b-MMA), the values L11 = 2 - 6 nm were found using diverse techniques. Addition of 2-5 wt% of P(HB-b-I-S) to PS/PE, increased the interphase thickness from L11 = 2.0 to L11 = 10-12 nm [16]. The effect on the interphasial thickness is one of several effects on blends' morphology. Different types of compatibilizer introduce different morphological effects. Three types should be distinguished: 1. Co-solvents, e.g., Phenoxy in PBTIPMMA; 2. Copolymers with inherent mOI;phology, e.g., SEBS in PPIPC; and 3. Copolymers with irregular morphology that, depending on concentration and flow conditions, can help forming either dispersed or co-continuous structures.

Compatibilizer frequently affects blends' crystallinity by either increasing or decreasing the nucleation rates and by modifying the rate of crystallization [1, 17, 18]. The maximum of the latter process takes place mid-way between the melting point, T m, and the glass transition temperature, Tg, i.e., at Tc = (Tm + Tg}12. Since in blends Tg is a function of composition, the kinetic of crystallization depends on the local concentration. Thus, blends' crystallinity is a complex function of many variables: configuration and MW of macromolecules, composition, compatibilization method, processing parameters, post­processing treatments, type and degree of dispersion, time the blend was at T > T m,

crystallization conditions (Te, cooling rates, annealing, nucleating impurities), etc.

2.4.2. Reactive compatibilization Here, the compatibilizing block and/or graft copolymers are produced in situ. From the economic and the performance points of view, this method is more important than that by addition of a compatibilizer.

The basic requirements for efficient reactive compatibilization are: • Sufficient mixing to achieve the desired dispersion.

Presence of reactive functionality for covalent or ionic bond formation. • Ability to react across the phase boundary within time constraints of an extruder. • Stability of the formed bonds during the processing steps that follow.

The interchain copolymer formation involves reactive groups of both polymers that form block or graft copolymers, with MW equal to the sum of the two homopolymers. Because of the short residence time in an extruder, either high concentration of reactive groups, or highly reactive functional groups are required [13].

The reactive compatibilization leads to thick interphase, L11 = 10-60 nm (see Table 1). The thickness originates either in non-uniformity of the copolymer concentration and MW along the interface, or in its undulatory shape. Experimentally, the thickness increases with annealing time up to a plateau, whose value depends on the temperature and net concentration of reactive sites [8]. The increase may simply indicate reduction of the interfacial area.

Several types of reactive processing have been identified [13]. Here only two more important will be mentioned (l) Reaction between functional, highly reactive groups, e.g., to compatibilize PPEIPA blends. (2) The trans-reactions, e.g., transesterification during blending of polyesters [19-21], or trans-reactions between amide and ester groups, leading to formation ofPA-PEST copolymers [7, 22, 23].

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3. RHEOLOGY

3.1. INTRODUCTION

Rheology is a part of continuum mechanics that assumes continuity, homogeneity and isotropy. In multiphase systems, there is a discontinuity of material properties across the interface, a concentration gradient, and inter-dependence between the flow field and morphology. The flow behavior of blends is complex, caused by viscoelasticity of the phases, the viscosity ratio, A, (that varies over a wide range), as well as diverse and variable morphology. To understand the flow behavior of polymer blends, it is beneficial to refer to simpler models - for miscible blends to solutions and mixtures of fractions, while for immiscible systems to emulsions, block copolymers, and suspensions [1,24].

3.2. CONCENTRATION DEPENDENCE OF VISCOSITY

At the low concentration of the dispersed phase, the "equilibrium" morphology can be visualized as polymeric emulsion. Increasing the concentrations above the percolation threshold volume fraction, $e = 0.16, progressively changes the blends' structure into co­continuous [25]. By analogy to colloidal emulsions, the model of polymer blend as an emulsion suggests higher viscosity than expected from the log-additivity of the components' viscosities, log 1] = r Wj log 1]j. In short, for blends the emulsion model predicts positive deviation from the log-additivity rule, PDB (identified as curve #1 in Fig. 4). While PDB has been found in about 60% of blends, there are four other types of the log 1] vs. $ dependencies: #2. negatively deviating blends, NDB, #3. the log­additivity, #4. PNDB, and #5. NPDB [26]. The negative deviations mean that besides the emulsion effect, there is at least one other mechanism contributing to the flow behavior. This second mechanism is known as the interlayer slip [27]. The interlayer slip originates in the theoretically predicted by Helfand et al. [4-6] low entanglement density in the interphase, caused by the preponderance of the chain ends and the low molecular weight species. Both factors lead to low viscosity of the interphase. For example, in PSIPMMA 1]jnteljlh = 90 Pas was three orders of magnitude smaller than the corresponding viscosity of the polymers [28].

2 Schematic representation of the five types of PAB viscosity-concentration dependence

1.5

~-

';:."'1

~ 0.5

_ -2'-

, \

-0.5 '--__ --'--__ ---'-___ -1..-__ -'-__ --'

o 0.2 0.4 0.6 0.8

rP2 Figure 4. Five types of the relation between shear viscosity and concentration for immiscible polymer blends: 1. PDB, 2. NDB, 3. additivity, 4. PNDB, and 5. NPDB [26].

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The morphology of blends flowing through channels, dies, or capillaries changes, what in tum modifies the rheological properties. For example, the interlayer slip is responsible for reduction of the apparent viscosity of blends, and parallel with it delamination during the injection molding. To generate the interlayer slip the interphases must coalesce. This can take place during flows with large strains, at stresses sufficient to break the interphase. The interlayer slip, first observed for mixtures of low molecular weight liquids, was empirically described by the fluidity-additivity equation (published by Bingham in 1922):

(12)

where Wi represents either the volume or weight fraction of component i. Recently, for multilayer flow with interlayer slip, the following NDB-type dependence was derived [27]:

(13)

were the viscosites are to be taken at constant stress, and k is a characteristic material parameter for given system that controls the NDB behavior.

The PDB-type relation can be derived assuming an emulsion model, and assuming that at the phase inversion volume fraction, f1l = I - f21' the viscosity of blends: polymer-I dispersed in polymer-2 and polymer-2 in polymer-I, are equal [26]:

In 11PDB = }: «Pi in 11i + 11 .... {1- [( IPI - «pS / (<<PIIP~I + «P2«P:,)]} (14)

Here 11max is the characteristic material parameter. Combining these two effects, i.e., Eqs 13 and 14, was found to well describe all dependencies of Fig. 4.

3.3. ApPARENT YIELD STRESS

The fundamental assumption of the classical rheological theories is that liquid structure is either stable (Newtonian behavior) or changes in a well-defined manner (non-Newtonian behavior). This is not always the case for flow of multiphase systems. For example, orientation effects in sheared layers may be responsible for either dilatant or pseudoplastic behavior while strong interparticle interactions may lead to yield stress or to transient behaviors. Aggregation, agglomeration and flocculation range from transient rotating doublets (within the dilute region) to a solid-like behavior of flocculated suspensions with yield stress originating either in thermodynamic interparticle interactions, chemical bonding, or geometric croWding.

There are several methods for determining the yield stress, oy Among these is the modified Casson equation [I, 29]:

..!f =..JF: + a..JF: (15) where F may be any rheological function, Fy indicates its yield value, Fm is the F-value of the matrix at the same deformation stress as F, and a is a constant

133

In many multiphase systems (e.g., emulsions, suspensions, or blends), the value of yield stress depends on the time-scale of the measurements. It is advantageous to consider that in these systems there are aggregates of different size, characterized by the dynamic interparticle interactions. For a given material these interactions, have specific strength, u oo, while the aggregates have a characteristic relaxation time, 'r • This leads to: y y

ay(O) = a;[l-exp{-T/il}r (16) where u = 0.2-1.0 characterizes polydispersity of the aggregates.

3.4. MICRORHEOLOGY

When a neutrally buoyant, initially spherical drop is suspended in another liquid and subjected to shear or extensional stress, it deforms and then breaks up into smaller droplets. At low stress, in a steady, uniform shearing flow, the deformation can be expressed by means of three dimensionless parameters: the viscosity ratio, the capillarity number, and the reduced time [1, 24]:

A == 'Ildispersed / 'Il rnatrix ; (17)

where l1dispersed and l1matrix are the dispersed phase and matrix viscosities, respectively, O"lj

is the local stress, y is the deformation rate, and d is the droplet diameter. The equilibrium deformation is reached at the reduced time t~ ~ 25 [30].

The deformabi1ity D is defined in terms of the major and minor prolate ellipsoid diameters, at and a2, respectively. For Newtonian systems undergoing small, linear deformation (smaller than that which would lead to breakup), D of a sheared drop is:

It is convenient to express the capillarity number in its reduced form K' == K / K .. , where Kef' represents the minimum value of K sufficient to cause breakup of the deformed

drop. Depending on the magnitude of K', drops do not deform, K' < 0.1, deform but do not break, 0.1 < K' < 1, deform then split into two primary droplets, 1 <K' < 2, or deform into stable filaments, K' > 2.

Similar to K .. , the critical time for drop breakup, t~, also varies with A - in shear flow the reduced time is 100::;t~ ::;160.

When values of the capillarity number and the reduced time are within the region of drop breakup, the mechanism of breakup depends on the viscosity ratio, A. In shear, four regions have been identified: for A« 0.1 tip spinning, for 0.1 < A < 1 drop breaks into two principal and an odd number of satellite droplets; for 1 < A < 3.8 a fiber breaking into small droplets is the preferred mechanism, whereas for A >3.8 in shear drops do not break, but they do in elongation.

During mixing, the dispersed phase progressively breaks down until an equilibrium drop diameter is reached. The coalescence, controlled by equilibrium thermodynamics and/or

134

flow, may occur at cjl ~ 0.005. For the shear coagulation, the equilibrium drop diameter of dispersed polymer (at volume fraction, cjl) can be expressed as [24]:

deq = d~ + ~6ClCcrt:4>813 (19)

where C is the coalescence constant, and d~ is the drop diameter value within the low concentration region (Taylor's limit). The only unknown in Eq 19 is C - since its value is characteristic of the system, it can be determined from data in any mixer.

3.5. DYNAMIC FLOW

The rheological responses measured at low values of strain better reflect the effects of the blend structure. For multiphase systems, there are serious disagreements between the predictions of continuum-based theories and experiments, that is, between the small and large deformation behavior. For example, the identity of zero-deformation rate dynamic and steady state viscosity is seldom found, and so is the Trouton rule. Similarly, the derived by Cogswell, relationship between the extensional viscosity and the capillary entrance pressure drop, and derived by Tanner equation for calculating the fIrst normal stress difference from the extrudate swell, are rarely valid.

The dynamic testing of polymer blends at small amplitude is simple and reliable, but the storage and loss shear moduli, G' and G", respectively, should be corrected for yield stress behavior. Two types of rheological phenomena can be used for detection of a blend's miscibility: influence of polydispersity on rheological functions, and the inherent nature of the two-phase flows. The fIrst principle makes it possible to draw conclusions about miscibility from, e.g.,

• G' and G" can be used to compute the frequency relaxation spectrum, HG. The coordinates of its maximum are related to miscibility.

• Cross-point coordinates: (G", co,,), where Gx == G' (cox) = G"(cox)' • Free volume gradient of viscosity: a l " olOTjI 0(1 If) ,

• Initial slope of the stress growth function: S=oloTj; 18(lot),

• The power-law exponent: 0=0100'12/8(1oy)",S, etc. The second principle involves evaluation of, for example:

• Extrudate swell parameter: B = DIDo • Strain (form) recovery • Yield stress, etc.

Palierne [31] described the rheological behavior of a liquid mixture from that of neat ingredients, their content and the interfacial energy expressed by: VI2/d. The theory is based on the assumptions that: (i) the system consists of two viscoelastic liquids, (ii) the concentration of the dispersed phase is moderate, (iii) the drops are spherical, polydisperse and deformable, (iv) the drop deformation is small, so the blend behavior is linear viscoelastic, and (v) V12 = constant. The model was found to well describe the dynamic behavior of several diluted blends, supporting the idea that the long relaxation times originate from the geometrical relaxation of droplets [24]. Replacing the sums in Palierne's relation by integrals, writing the appropriate expressions for G' and G", then applying the Tikhonov regularization method made it possible to determine a fIne structure of the relaxation spectra, as well as the distribution of dispersed PS drops [32].

135

3.6. COMPATIBILIZATION EFFECTS

Compatibilization, making the interface more rigid causes the constant stress viscosity to increase. Similarly, an increase of the apparent volume of the dispersed phase causes the relative viscosity to increase. Furthermore, increased interactions between the phases reduce the possibility of the interlayer slip and increase formation of an associative network, resulting in systems with increased yield stress. Thus, compatibilization is expected to increase melt viscosity, elasticity and the yield stress. These effects are especially large at low frequencies, but may not be significant at high deformation rates. However, other mechanisms that may reverse this tendency: 1. Preferential micellization of compatibilizer, 2. Increase of the free volume in the blend, etc.

3.7. BLEND ELASTICITY

Four measures of melt elasticity have been used: the first normal stress difference, N I, the storage modulus, G', and the two indirect ones, the entrance-exit pressure drop, Pe, and the extrudate swell, B. In homogeneous melts, the four measures are in qualitative agreement. In multiphase systems containing difficult to deform dispersed phase, Pe and B are small. By contrast, in blends with deformable dispersed phase, the deformation-and­recovery provides mechanism for energy storage, leading to large elastic response -neither Pe nor B can be used to measure elasticity. In both cases, the form deformation dominates the observed behavior.

3.8. ELONGATIONAL FLOWS

Most work on the extensional flow of immiscible polymer blends was performed on PO systems. Film blowing conceptually involves two engineering operations: extrusion and blowing, with the latter operation limiting the throughput. For LOPE, the strain hardening, SH, provides a self-healing mechanism, permitting for high line speeds. However, HOPE and LLOPE resins have negligible (if any) SH, hence they should be blended with either LOPE, elastomers, copolymers, or other types of LLOPE resins. SH was also found to be an important resin characteristic in wire coating [1, 24].

The flow behavior ofPP, PA-6, and their compatibilized blend was studied at 225-250°C in a steady shear, dynamic shear, and extensional flow fields [33]. The large strain capillary flow was found to be insensitive to temperature, suggesting a major modification of morphology during testing. The dynamic flow curves for the blend were higher than predicted from the components' flow behavior. However, they were regular, pseudoplastic, without apparent yield stress. Similarly, the extensional viscosity of the blends was significantly higher than could be expected from the component polymers' behavior.

The measured elongational viscosity, 17E, for two homopolymers agreed quite well with the value calculated from the entrance-exit pressure drop in capillary flow, Pe [34]:

(20)

where n is the power-law exponent. However, for the blend, the value calculated from Eq 20 was one order of magnitude higher than measured. In another work it was

136

observed that compatibilization of PPIP A-6 blends resulted in a dramatic change of the viscosity-concentration dependence, from NDB to PDB [35].

Delaby et al. [36] investigated the relative deformation of the dispersed phase to that of the matrix. From Palieme's theory the dependence:

(21)

Here, Y d and Y m are, respectively, strain of the dispersed phase (defined as a ratio of the long axis to the original drop diameter) and that of the matrix, and AE is the extensional viscosity ratio. The relation was experimentally verified for A[ < 1.

3.9. TIME-TEMPERA TORE SUPERPOSITION

The time-temperature superposition principle, t-T, is not valid even in miscible blends, e.g., in PSIPVME, where the deviation was evident in tan6 VS. 0) plot. It was postulated that the number of couplings between the macromolecules varies with concentration and temperature. Thus, even in miscible blends, as either cjl or T changes, the chain mobility is differently affected. Thus, the relaxation spectra of the polymeric components have different temperature dependencies, what make the t-T principle invalid. In immiscible blends, the t-T principle does not hold. Two processes must be taken into account: the t-T superposition, and the aging time - at test temperatures, the polymeric components are at different distances from their respective Tg'S, T - Tgl ~ T - Tg2.

4. COMPOUNDING AND PROCESSING OF POLYMER BLENDS

4.1. MIXING AND COMPOUNDING PRINCIPLES

In the plastics nomenclature, mixing is a general term indicating the physical act of homogenization (e.g., mixing of fractions), the term blending is used to indicate the processes that lead to formation of polymer blends and alloys, while compounding refers to preparation of a compound, i.e., incorporation of diverse additives into a polymeric matrix, viz. antioxidants, lubricants, pigments, fillers, or reinforcements.

4.1.1. THE REASONS FOR MIXING Mixing is the most important step in polymer processing. Homogenization of MW, temperature, composition, entanglement density, etc., is the keys to performance. Owing to high viscosity, polymers flow is laminar. Two types of mixing flow are distinguished: dispersive and distributive.

* Dispersive mixing involves the application of ,stresses that break dispersed domains to the desired size. The dispersed phase may be liquid, gel particles, aggregates, etc. This type is described by microrheology.

* Distributive mixing involves homogenization of a fluid, accomplished by the application of strain. Homogenization may involve a single-phase fluid (for example, homogenization of temperature), a miscible system (homogenization of composition), or a multiphase system, blend or composite (homogenization of dispersion).

137

4.1.2. LAMINAR MIXING The model of laminar flow usually assumes that the flow is unaffected by the components' distribution with a "passive," or invisible interface-the effects of flow and morphology are "de-coupled." Thus, the laminar mixing provides the first approximation for mixing. The distributive mixing is considered - there is no need for dispersive when there is no interface and rheological properties of both fluids are the same.

Laminar mixing depends on strain. Upon imposition of strain, the interfacial area, Ao'

grows according to the relation [37]:

where \'s are the principal elongation ratios (i = x, y, z), while a and ~ are the orientation angles. Depending on the type of deformation, the ratios take on values listed in Table 2.

Table 2. Values of A, for Different types of deformation

Deformation Elongation ratios A, Comment Plane elongation Ax = 1..0; Ay = II 1..0 ; Az = 1 1..0 regular, flow between rolls

Plane elongation Averaging isotropic mixture 1..0 /2 random input orientation

Pure elongation Ax = 1..0; Ay = A, = 1..0.112 1..01/2 uniaxial stretching

Pure elongation Ax = 1..0; Ay = Az = 1..0.112 11 1..0 biaxial stretching

Simple shear expressed by shear strain, y y cos ~ Couette or Poiseuille flow

The energy required to generate the same degree of mixing in different flow fields can be expressed as [37]:

E onL ,,' •• ,. = (1211 / t)[ In( SA, / 4) r; E'lmPI.'hea, = (411 / t )A~ (23)

Predictions of Eq 23 are shown in Fig. 5. The simple shear is inefficient for generating large interfacial area. The inefficiency can be reduced by employing a stretch-and-fold strategy, viz. Ar = (y/2t (n is the number of steps), but the same strategy is applicable to mixing in extensional flow field. Thus, the Figure represents true relative efficiencies of the two types of mixing.

The rates of deformation during laminar flow, as well as the time-dependent local strains were computed by Poitou [38] (see Fig. 6). The elongational deformation provides more efficient and rapid mixing. On all three accounts: the magnitude of interfacial area increase, the energy consumption, and the rate of spatial separation, a significantly better mixing is expected in the extensional than in the shear flow. In laminar mixing there is no coalescence, thus the materials always are stretched and deformed, hence the degree of mixedness continuously improves. The model describes homogenization of idealized mixtures, that provides but a guidance for mixing real polymers.

138

Mixing energy vs. interface growth ratio

.. .......

.. ....... simple shear .... .......

." ..

uniaxial extension

.......... . .. l~ :.::.: :.:" iW :.: :J

. :.: :.: :.. -, plan extension

...... ...

Figure 5. Energy consumption as a function of the change of the interface area in extensional (uniaxial, biaxial and plane strain) and in simple shear flows. The most efficient is the biaxial stretch, the worst (by a factor of 500,000!) is shear (after [37]).

1040~ ____ ~ ____ ~ ____ ~ ____ ~ __ ~

" , , ,

, , uniaxial el.(ggation

" " ,

, "

" simple shear

1.~-"==~======~::::I:::::JC::::j 10°1j! o 36 72

Time, t(s) Figure 6. Kinematics of separation of two material points during laminar flow uniaxial elongation and simple shear mode.

4.1.3. DISTRIBUTIVE MIXING

in the

The motionless mixers, MM, operate on the principle of splitting a flow stream into ne channels, reorienting them by 90°, and dividing again. The flow is a pressure driven, laminar shear type. Mixing in MM is expressed by the numbers of striations, N s ,

generated by n. ofMM elements, each having n. channels: N, = n:·. Over 30 MM designs have been developed [39]. Their efficiency is determined comparing: (1) the length-to-

139

diameter ratio, un (required to produce the same degree of homogeneity); (2) the associated pressure drop, ~Prel; (3) the holdup volume, ~vrel; and (4) the relative dimensions of the device, Drel and Lre,.

4.1.4. CHAOTIC MIXING The term chaotic mixing" was introduced by Ottino [40] do describe laminar, distributive mixing with continuous or periodic translation of cavity walls. This flow stretches, folds, and transports a drop more effectively than a steady-state translation. The experiments also showed presence of "mixing islands" where little mixing took place.

4.1.5. MIXING IN EXTENSIONAL FLOW FIELD The elongational flow field exists anywhere where the streamlines are not parallel. Thus, this type of deformation is common during processing. However, in commercial mixers the shear mixing dominates.

Blending in extensional flow field. Mixing in extensional flow field is particularly advantageous in systems where the viscosity ratio A ;:= 3.8. The following factors are important [41, 42]:

(i) Diameter of the convergence, dc, (ii) The ratio of the reservoir-to-convergence diameters, C '" dr/dc, (iii) The capillary length-to-diameter ratio, R '" LID, (iv) The initial drop size, d, (v) Extensional viscosity ratio, As'" TJstVTJsm, (vi) Absolute value of the elongational stress, 0"11, and (vii) Number of passages through the convergence.

Extensional Flow Mixer. Recently an extensional flow mixer, EFM, was designed. The design was based on the microrheological principles incorporating the following principles [43]. In EFM the liquid mixture is exposed to the extensional fields and semi­quiescent zones. The extensional flow field intensity progressively increases as the mixture flows through a series of radially placed slit restrictions. The slit gap is adjustable to permit generation the extensional stresses that are required for given systems - the intensity of the flow field changes inversely with the radial distance from the center, viz. 0"11 - IIR .. Several models were designed with the throughput capability from 50 to 40,000 kg/hr. Fig. 7. shows a cross-section of an EFM. EFM is a general purpose mixer that can be used to generate fine emulsions, disperse "fish eyes" in polymeric film. It can be used as an in-line dispersive mixer in injection- or blow-molding machines, etc.

The pressure drop across EFM, JJ.P, can be calculated following Binding's theory. Power-law dependence for the shear and extensional viscosity was assumed. The derived relationships are predictive, from the slit geometry, and the rheological characteristics of the resin. For a set of 38 experimental runs (using either PE, PP, or PS), the experimental vales, ~Pexp' followed the computed values, ~P calc: ~Pexp = 0.0512 + 1.0044 ~Pcalc with the correlatIon coefficient, R = 0.9772. The derived dependencies make it also possible to separate the pressure loss due to shear and elongation, Pe and Ps, respectively. As shown Fig. 8, in EFM Pe > Ps [42,43].

140

Figure 7. Cross-section of the EFM, assembly. The molten polymer blend enters through the adapter plate #1. The melt is directed by the spiral mandrel, part #2 to the gap space between it and the EFM body, part #3, then it enters the space limited by the upper (part #7) and lower (part #6) convergent-divergent, c-d, plates. The melt flows from the rim toward the center, undergoing the convergent and divergent mixing, before sorting out through the central passage in the lower plate #6 via the central bore in the plate holder #5. The gap between c-d plates is controlled by turning the adjusting plate #4.

Performance ofEFM attached to a singe-screw extruder, SSE, was evaluated examining its suitability for: l. Dispersion of viscous polymer in low viscosity matrix (where A. > 4); 2. Impact-modification of engineering resins; 3. Elimination of gel particles from either R-TPO or EVAc reactor powders; 4. Dissolution of UHMWPE in HOPE, etc. For comparison, the blends were also prepared using a co-rotating, intermeshing twin-screw extruder, TSE, equipped with high dispersion screws [42, 43 ].

141

10',--------------,--------------,

-Total pressure drop - -Extensional pressure drop ..... Shear pressure drop

PP; T = 200'C, convergence gap = 1 mm

10' ~------------~------------~ 10' 10' Flow rate, Q (kg/h) 10 3

Fig. 8. Pressure drop across EFM and its extensional and shear components vs. throughput forPP.

4.2. COMMERCIAL MIXERS

4.2.1. Introduction The melt mixers are either batch or continuous. The former requires lower investment cost, but is more labor-intensive, has low output and poor batch-to-batch reproducibility. The continuous mixers are: extruders, continuous shaft mixers and specialty machines.

To ascertain high-quality compounding, the machine design should be based on fundamental principles for balanced dispersive and distributive mixing. It should be flexible and reliable (long lifetime of parts). The mixer should have high torque capability, a wide selection of mixing configurations, as well as reliable feeding and granulation equipment. Furthermore, it should be easy to automate, providing for on-line quality control, flexibility and user friendliness. Several compounders have segmented barrel, ability to change screw-rotation direction, and can either extrude a product or recycle scrap.

4.2.2. Batch mixers A counter rotating twin shaft internal mixers were developed by the year 1916. These machines are enclosed and can be pressurized, so that fine powders and additives do not drift away. Batch mixing is useful for short-runs (it can complete a cycle in minutes) in which the materials and additives are often changed. They are used for small scale compound evaluation, as well as for the production of either color-concentrate masterbatches, heat-sensitive materials, materials with tailored identity, alloying resins of varying melt indices, etc. The heat conduction by the mixing shafts is a serious drawback. During mixing of elastomers or commodity resins at low-T, a dynamic equilibrium may be reached. However, for high temperature blending of engineering polymers, there is much higher temperature at the chamber wall than that on the shafts -- the temperature difference as large as ~ T = 100°C was observed.

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4.2.3. Extruders Extrusion is one of the most important fonning method in polymer processing. Virtually all polymers go through extruder at least once. Furthennore, most fonning operations involve extrusion, viz. extrusion (of profiles, films, sheets, fibers, wire or paper coating), injection or blow molding, thennofonning, etc. The extrusion is accomplished, by a screw rotating in a cylindrical barrel. Extruder may be used to perfonn the following operations: Primary (melt, pump & fonn); Secondary (devolatilize & mix); and Tertiary (conduct chemical reaction). According to the principal element of their construction, the compounding extruders are divided, into:

Single-, Twin-, & Multi-screw, Single-, Twin-, & Multi-shaft,

=> Gear or Disk (e.g., Maxwell's, or Tadmor's) Special (e.g., Gelimat, or Patfoort).

It is also important to distinguish extrusion compounding from extrusion fonning.

Single-screw extruders, SSE. The single screw extruder is a relatively inexpensive machine for small or medium size production lines. For throughputs, Q ~ 10 ton/hour, the cost of a SSE and a TSE is comparable. SSE is characterized by a simple design, raggedness, reliability, it is easy to operate and maintain and its theoretical description is well documented. However, SSE is difficult to scale up, show notoriously poor mixing, broad residence time distribution and relatively long residence time. To make it more versatile, special mixing screws, and/or add-on mixing devices can be used.

Three extruder zones are distinguished: I. Solid conveying - its length L = 4-8D (D is the screw diameter); 2. Melting-L = 1O-16D; and 3. Conveying- L = 6-IOD. The standard screw has a single parallel flight with pitch = 1D (pitch angle 4> = 17.66°), and length-to-diameter ratio 20 :::; LID :::; 36. The flight width is about 0.1 D, the channel depth in the feed section is 0.1-0.15D, and the channel depth ratio is 2 to 4. The compression ratio, KR, is defined as the screw flight volume ratio at the entrance to that at the exit. For standard screws, KR varies from 1.5 (rigid PVC) to 6 (PA). The rotational screw speed ranges from N = 20 (rigid PVC) to 360 rpm (PE). The die pressure: P =: 70 to 200 MPa.

Mathematical modeling of the flow through SSE considers that the screw and the barrel are unwound. The screw is stationary and the barrel moves over it at the correct gap height and the pitch angle. The initial models assumed: (i) steady state, (ii) constant melt density and thennal conductivity, (iii) conductive heat only perpendicular to the barrel surface, (iv) laminar flow of Newtonian liquid without a wall slip, (v) no pressure gradient in the melt film, and (vi) temperature effect on viscosity was neglected. Later models introduced non-Newtonian and non-isothennal flows. Present computer programs make it possible to simulate the flow in three dimensions, 3D [39].

Twin-screw extruder, TSE. The "classical" TSE developed by the mid-1950's, was mainly dispersive mixer. In these machines, the dispersive mixing has been controlled by assemblies of mixing blocks, whereas the distributive mixing by their width - a set of narrower blocks generates more distributive, and less dispersive mixing [44].

Table 3. Summary of Extrusion Characteristics: SSE and TSE.

Function SSE TSE Counter-rotatin Co-rotatin

flow mechanism continuous discrete C-sections figure-of-eight shear (continuous)

pumping variable good, positive good, positive efficiency die restriction often severe smaller effects smaller effects LID ratios >20 variable ::; 26 variable::; 50 compression decrease

channel various designs various designs

screw speeds 20-100 rpm ::; 500 rpm ::; 1200 rpm heating mode mainly by shear controllable low shear near-adiabatic residence time large spread, narrow distribution, narrow distribution,

wide distribution

often easy to control often easy to control

transport drag-induced good positive positive displacement displacement

flow pattern simple shear complex, shear & complex, shear & extension extension

disadvantages poor mixing cost, feed control, cost, feed control, theoretical description theoretical description

TSE's are claimed to be superior to SSE's because they provide (also see Table 3): => better feeding and more positive conveying characteristics,

shorter residence times and narrower residence time distribution, => improved kinetics and melt temperature control, => high and controlled deformational stresses, => positive pumping action, => reduced melt slippage,

self-wiping action, => generation of high extrusion pressures with a very short backup length.

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TSE's are classified from the point of view of the screw rotation (co- and counter­rotating) and degree of intermeshing (intermeshing, tangential, and separated). Co­rotation induces lower and more uniform shear and elongation stresses that counter­rotation. By contrast, counter-rotation, owing to high stresses in the calendering gap, has higher dispersive flow. The fully intermeshing machines are self wiping, what leads to narrow distribution of residence time. By contrast, the low intermeshing machines (tangential and separated) have broad distribution of residence times, and low-intensity uniform stress field. The low pressure at the die may require either a single screw extruder or a gear pump to generate sufficient flow.

Seven types TSE's are on the market: I Counter-rotating, non-intermeshing (mostly tangential), lengthwise & crosswise

(l&c) open - CRNI II Counter-rotating, partially intermeshing, l&c-open (rare) III Counter-rotating, partially intermeshing, I-open & c-closed (rare)

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IV Counter-rotating, fully intenneshing, l&c closed -ICRR V Co-rotating, non-intenneshing, l&c-open (rare) VI Co-rotating, partially intenneshing, l&c-open VII Co-rotating, fully intenneshing, screws: I-open & c-closed, discs: l&c open

(most popular) - CORI The acronyms of the three most popular types are indicated in bold.

CRNI was originally developed for rubbers and foodstuffs. It is used as either a stirred tank reactor or a twin-rotor continuous mixer. With one screw longer than the other, it separates the mixing and pumping functions. The low stress field in the mixing section is responsible for the absence of dispersive mixing, while the interchange of material between the screws provides for good distributive mixing. These machines are well suited for polymerization of miscible or low-.., systems. CRN!' s are also used for compounding PVC, in the coagulation technology, reactive extrusion, anionic polymerization of caprolactam, halogenation of PO, grafting MA and styrene onto EVAc, etc.

ICRR evolved from the positive displacement screw pump. This extruder is fully, axially and radially, closed. The throughput is determined by the intenneshing geometry and screw rotation. At low speed, ICRR has been used for PVC compounding and fonning. The machines are used for compounding, devolatilization, and reactive extrusion. ICRR's have narrow distribution of residence times and better precision in controlling rapid reactions between liquid reagent and molten polymer than co-rotating ones. The older machines had high "calendering" stresses between screws and low stresses outside this region. Owing to the calendering pressures the screws could rub against the barrel causing premature wear. Thus, short barrel, slower speeds, and large intenneshing gaps are used. There is significant elongational flow field at the entrance to the calendering zone.

During the last few years, significant progress was made in the TSE engineering designs. The screw profile was deepen and the screw separation was increased to reduce the callendering pressure. The new generation of ICRR operates at similar screw speed as CORI.

CORI's are the most popular TSE's. Their advantages arise from the fact that in the intenneshing zone, the surfaces move in opposite directions. As a result, the melt free surface is continuously renewed, the screws clean each other, and there is little possibility for the material to go through the gap. Thus, there is no screw bending due to the calendering pressure, what makes it possible to use longer barrel (LID ::; 50), operate at high screw speeds (::; 1200 rpm) and higher output.

CaRl's were first used to polymerize butadiene and s-caprolactam, then as reactors for addition polymerization (polyacrylates, PA-6, POM, or TPU), polycondensation (PA-66, PAr, PEST, PEl), grafting (PO + silane, MA, acetic anhydride), as well as mechano­chemical degradation, e.g., visbreaking ofPP. These machines are preferentially used for polymer alloying, blending, and compounding - the compounder's ABC. They are operated in starve-fed mode requiring high perfonnance feeders. The material transport depends mainly on the drag flow, with local contribution from the screw pumping in the so-call pressure zones. The present tendency is to: I. use slender screw profile, 2. maximize torque capability, 3. increase screw speed, and develop new kneading and mixing elements to improve either the distributive or dispersive mixing.

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There are several reports comparing the efficiency for compounding, mixing, or reactive blending in different types ofISE. Most of these studies suffer from the same aspect­the compared machines were not operated at a comparable level of performance. A summary of these observations is given in Table 4.

Table 4. Comparison of co- and counter-rotating TSE performance.

Function Co-rotating TSE Counter-rotatin TSE transfer of material considerable, axially open, little transfer, C-shape chambers, plug between screws melt conveyed by frictional flow

forces range of pressures pressure controlled by wide range, high pressure within the (stresses) and restrictions, screws calendar gap, lower rotational speeds rotational speeds suspended in melt, higher

rotational speeds feed intake region controllable melting calendering gap problems

process residence time narrower, faster cleaning broader, slower melting and cleaning distribution time, better devolatilization pumping capability not as good better dispersion of small good for erosive-type of better for breaking strong aggregates particle aggregates mixing dispersing capability better for dispersing glass better for dispersing agglomerates

fibers distributive mixing better for blending mainly dispersive mixer dispersing lubricants same lubricant content at higher lubricant concentration in PO different throughputs obtained at high power consumption radical better mechanical high stresses in the calendering gap copolymerization properties of the blend may lead to degradation

4.2.4. Other compounding machines Planetary roller extruder. Six evenly spaced planetary screws, revolve around the central screw, intermeshing with it and with the barrel. Thus, the planetary barrel section has helical grooves corresponding to the helical flights on the planetary screws. The section is usually separated, with a flange connection to the feed section, in which the material moves forward as in a single screw extruder. When the pre-plasticated material reaches the planetary section, it is exposed to intensive mixing by the rolling action between the planetary screws, central screw and the barrel. The small clearance between the planetary screws and the mating surfaces generates large material surface that helps devolatilization, heat exchanged and temperature control. The machine has been used for PVC­compounding (intensive mixing during a short residence time), but seldom it was used for polymer blending or recycling.

Disk extruders. There are several types of screwless extruders. These machines employ a disk or a drum to plasticate, mix, and extrude. Most designs are based on viscous drag flow. They include: Maxwell's elastic melt extruder, Westover's stepped disk and drum extruders, and Diskpack [39]. The Diskpack extruder has the capability of performing all the elementary steps of plastics processing by combination of differently shaped rotating

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disks in a drum-like housing. The extruder has been used for reactive processing, blending, compounding, mixing, and devolatilizing [45].

Special extruders. Patfoort extruder or FN-Plastifier is a short (LID = 5) single-screw extruder, developed for polymer blending and recycling. It has a three-start screw extended from the feed zone 2/3 over the screw length. The frontal part of the screw is smooth, and it ends with a flat disk. The material is transported and partially fluxed in the grooved part of the screw, then melted between the smooth part of the screw and the barrel. Pumping is generated by the normal stresses between the flat part of the screw-end and the die (the Maxwell's extruder principle). The residence time is short (measured in seconds) and relatively narrow. Large thrust bearings are needed for adjustment of the die gap that controls the normal stresses, i.e., throughput and the blend's morphology [46].

4.2.5. Add-on devices for improved mixing Many devices were developed to improve mixing capabilities of the SSE's. Nowadays, as demands for better performance increase, TSE's must perform a growing number of operations. Instead of extending the TSE length, specific add-on 's can be used as well. These devices can be classified as: 1. Internal modifications (screw & barrel), and 2. External devices.

Internal modifications. Here belong the screw modifications, as well as the devices that need to be attached to the screw, e.g., Barmag's torpedo, RAPRA's cavity transfer mixer (CTM), a multi-screw planetary unit. All three examples also require modification of the corresponding barrel section. By contrast, the developed in the University of Twente torpedo with perforated, freely rotating sleeve, can be used without affecting the barrel, e.g., to improve mixing of the injection molding extruders.

External devices. Here belong the motionless, distributive mixers, capable to ·provide excellent T -homogenization at minimum cost. They are recommended for the film or fiber production lines. Other self-contained, mixing devices that can be placed between regular extruder and the forming die are gear pumps, planetary mixing units, self-contained CTM units, etc. EFM is the only device where mixing is accomplished in the extensional flow field. The device does not have moving parts. The flow pattern provides for self-cleaning, thus it provides trouble-free long service. Having adjustable gap, it can be optimized for different materials and performances.

4.3. EVOLUTION OF MORPHOLOGY DURING PROCESSING

The performance most frequently demanded from blends are: toughness, strength, rigidity, processability, heat deflection temperature, and reduced permeability. Designing a blend with specific performance characteristics, mean identifying the best blend's composition and morphology. Different applications are known to demand different morphology, viz. for toughness, the elastomeric component should be dispersed as spherical drops with micron or sub-micron diameter, while reduced permeability the minor phase should be dispersed in the form of relatively large, thin lamellas [24], etc.

An internal mixer with a glass window was used by Shih et at. [47] to observe mixing of different polymeric compositions prepared from HDPE, PBT, and PAr. Four sequential characteristic states were identified: 1. Elastic solid pellets, 2. Deformable solid pellets, 3. Transition material: either 3.1 liquid with suspended solid particles, 3.2 fractured or semi-liquid material, or 3.3 dough-like material, and 4. Viscoelastic fluid. Very high

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torque was associated with state 3.1. The same behavior was postulated to take place inside extruders. These observations are of particular value as a guide for compounding polymers with widely different melting points or viscosities. This type of system must be sequentially introduced to the machines.

Lindt and Ghosh computed the early stages of blend morphology development in SSE. The authors assumed that due to friction on the barrel, the solid pellets of two polymers change into lamellas, which flowing through, progressively thin down until the onset of the capillary instability. The measured thickness of the lamellae at the bottom and on the top of screw channel was in good agreement with the model predictions [48, 49]. This work sows how little mixing takes place in a SSE.

This is not so for TSE. Several mathematical models for predicting variations of blend morphology during compounding in a TSE have been proposed [50-54]. To start with, all the models require information about the screw configuration, extrusion conditions and polymer properties. First, the pressure profile and local strains in the extruder are computed, then, using this information, the average drop size along the screw is calculated.

PE·DROP DIAMETER VS. SCREW LENGTH 10 .

l--05, N150 • I \ - - ·05, N200 • -·_ .. 05, N250 " \ -----_..-::.

10 , - ,

10 400 800

Position along the screw, L (mm)

Fig. 9. Variation of an average drop diameter of 5 vol% PE in PS inside a TSE. Throughput Q = 5 kg/min; screw speed N = 150, 200, and 250 rpm. Points are experimental, curves are computed [53].

Dilute PEIPS blends (5 vol% of either PE in PS or PS in PE) were prepared in a CORl [51-53]. The screws were designed to extend the melting zone over several screw diameters. Thus, the pellets of the minor-phase polymer deformed into large, irregularly shaped entities that disintegrated into drops. To measure the morphology evolution during extrusion, a special quenching barrel section was used. The cooling system made it possible to inject chilled water, quenching the specimen within few seconds. Owing to the clamshell design it could be easily open for removal of quenched specimens [52].

Evolution of the average drop diameter along the screw was computed from the microrheological rules supplemented by the coalescence kinetics, see Eqs 19-21. The model assumed: steady state shear flow, viscosity dependent on shear stress and temperature, and interfacial tension coefficient dependent on temperature. The

148

experimental drop diameter vs. location along the screw favorably compared with the computed predictions (see Fig. 9). The model did not have any adjustable parameter.

5. CONCLUSIONS

In this chapter the three principal elements of the polymer blending technology were outlined. The interested reader should consult the cited literature

The thermodynamics helps to defme the optimum conditions for miscibility. The miscibility is of interest only for two reasons: (l) it provides a basis for designing compatibilizers, and (2) it may lead through spinodal decomposition to co-continuity of phases with excellent performance characteristics.

The flow behavior is tightly coupled to morphology, which in turn controls the performance. The knowledge of the blend rheology is required for predicting the evolution of the material structure during mixing, blending or compounding.

While the thermodynamics and rheology form the foundation for designing commercially viable compositions with advanced performance characteristics, the third element - the mixing equipment must do the work. In the text, several possibilities were addressed based on the availability of equipment, and combination of mixing and pumping devices. As always, the last word belongs to the process economics.

6. NOMENCLATURE

ABC BR CMC CORl CRNI CTM EFM EGMA EVAc EVAc-MA GF HDPE HIPS ICRR KR LCP LCST LDPE LLDPE MA MM MW MWD NDB NPDB P(HB-b-I-S)

alloying-blending-compounding butyl rubber critical micelles concentration co-rotating, fully interrneshing TSE counter-rotating, non-intermeshing TSE cavity transfer mixer extensional flow mixer ethylene-glycidyl methacrylate copolymer copolymer from ethylene and vinyl acetate copolymer from ethylene, vinyl acetate, and methacrylic acid glass fiber, or glass fiber reinforced plastic high density polyethylene high impact polystyrene intermeshing, counter-rotating TSE compression ratio liquid crystalline polymer lower critical solution temperature low density polyethylene linear low density polyethylene maleic anhydride motionless mixer molecular weight molecular weight distribution negatively deviating blends negatively-positively deviating blends block copolymer of hydrogenated butadiene, isoprene, and styrene

PA PA-6 PA-66 PAr PBT PC PCW PDB PE PEl PEST PET Phenoxy PMMA PNDB PO POM PP PP-MA PPE PS PVAc PVAI PVC PVME R-TPO SB SBR SEBS SH SSE SSSE TPU TSE UHMW-PE

7. REFERENCES

polyamide poly-e-caprolactam polyhexamethylene-adipamide polyarylate polybutylene terephthalate polycarbonate of bis-phenol-A post-consumer waste positively deviating blends polyethylene polyetherimide thermoplastic polyesters, viz. PET, PBT, PEN, etc. polyethylene terephthalate polyhydroxyether of bisphenol-A polymethylmethacrylate positively-negatively deviating blends polyolefin polyoxymethylene isotactic polypropylene (aPP - atactic; sPP - syndiotactic) maleated polypropylene polyphenyleneether polystyrene polyvinyl acetate polyvinyl alcohol polyvinyl chloride polyvinylmethylether reactor-blended thermoplastic olefinic elastomer styrene-butadiene copolymer styrene-butadiene elastomer styrene-ethylenelbutene-styrene three block copolymer strain hardening single-screw extruder solid-state shear extrusion thermoplastic urethanes twin-screw extruder ultrahigh molecular weight polyethylene (over 3 Mg/mol)

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COMPATIBILIZATION OF HETEROGENEOUS POLYMER MIXTURES FROM THE PLASTICS WASTE STREAMS

HANS-JOACHIM RADUSCH, JIANMIN DING Martin Luther University Halle-Wittenberg Institute of Materials Technology D-06099 Halle-Saale; and GUNERl AKOV ALI Middle East Technical University, Depts. Chemistry and Polymer Science and Technology, 06531 Ankara

1. Heterogeneous Polymer Blends from Plastics Recycling

The most economic and ecologically advantageous process to recycle plastics waste is the material recycling [1]. This simple and non-polluting method, is applicable for recycling plastics waste both of a single-type and a mixture of similar polymers. The waste may originate either from a plastics-producing plant, or from household refuse. In the latter case, it is mainly composed of plastics from the packaging sources.

During recycling, at first, the plastics waste has to be separated, cleaned and shredded, then the fractional streams are re-melted and compounded/mixed to form homogenous materials. This method ensures a short material cycle and effective recovery of material properties with low energy consumption. Highly commingled and polluted plastics waste should not be considered for the regular recycling. These materials are usually recycled either for recovery of: low molecular weight components (e.g., by pyrolysis or solvolysis); energy (e.g., by incineration), or they are used as a source of carbon in blast furnaces, [2].

Basically, if the packaging plastics waste is used for material recycling, polymer blends are generated. Because of thermodynamic immiscibility of most polymer pairs, the polymer mixtures from the plastics waste usually show the typical appearance and behavior of heterogeneous polymeric mixture. Different polymeric components generate mixtures with a more or less coarse morphology. Between the phases there are usually weak interactions. Besides the thermodynamic immiscibility, the differences in the rheological properties may also provide reason for generation of heterogeneous blends (even when the components are similar in chemical structure) [3,4].

Plastics waste collected from the municipal or household refuse, contains similar

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G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 153-189. © 1998 Kluwer Academic Publishers.

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types and fractions of polymers. Depending on the extend of plastics use in packaging and of the geographic location, different polymer types may be found. The municipal plastics waste not only has to be washed and cleaned from contaminants, but also, to avoid the "downcycling," it must be separated into different fractions. Usually, after the initial segregation and shredding, the plastic waste is segregated into a light and a heavy fractions by the float-sink fractionation. These fractions contain different polymers. Typical compositions are listed in Table 1.

TABLE I. An example of typical plastics waste compositions from the packaging refuse

Component Total (%) Light fraction Heavy fraction (%) (%)

Reference [5] [6] [6]

LDPE 60-70 33.5

HDPE 59.0

PP 5-10 7.5

PS 10-15 41

PVC 5-10 49

PET 5 4

Others 6

The light and heavy fractions (obtained by a float-sink separation method from the polymers in municipal waste stream), comprise different types of polymeric materials. Thus, the light fraction is made mainly of different types of the polyethylene (PE), polypropylene (PP) and expanded polystyrene (EPS). In this fraction, the largest proportion is made of the high (HDPE) and low density polyethylene (LDPE). Since EPS can easily be separated from other components in the light fraction, the resulting mixtures of different polyolefins that are relatively simple to recycle. When EPS is not removed from the fraction, blends of polyolefins with antagonistically immiscible polystyrene (PS) must be envisaged. In the heavy fraction usually there are: polyvinylchloride (PVC), polystyrene (PS) and polyethylene terephthalate (PET), different thermoset resins and composites as well as contaminants.

Thus, recycling of post-consumer waste may lead to different blends: - of different polyolefins (light fraction) - of polyolefins with EPS (light fraction) - ofPS, PVC and PET (heavy fraction)

The most efficient recycling method would be the one requiring minimum

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segregation efforts. However, if "downcycling" is to be avoided, this would require a considerable amount of effort to compatibilize and well mechanically disperse the immiscible and heterogeneous polymers. Recycling and re-using of mixed plastics waste by simply re-melting and re-processing them (e.g. by injection molding), usually results in reduction of mechanical properties. Investigations have shown that specimens of recycled mixed plastics waste almost always show reduced yield stress and low maximum strain at break: - these mixtures are brittle. Because of differences in rheological properties, surface tension, and thermodynamic immiscibility an adequate homogenization of such a waste material is rarely feasible.

Table 2 shows typical values of mechanical properties of the recycled melt-mixed plastics waste obtained from municipal refuse. The main characteristic of these blends is low value of the tensile and impact strength, as well as of the elongation at break:. The latter being much lower than corresponding values of the virgin homopolymers.

TABLE 2. Mechanical properties of recycled mixed plastics waste

Parameter References

[5] [7] [8] [9]

Tensile strength, N/mm2 14.2 20.5 21.9 13.9-15.6

Modulus of elasticity, 933 950 250-N/mm2 1280

Flexural modulus, 811 N/mm2

Compression modulus, 794 N/mm2

Elongation at break:, % 20.7 6.8 2.8-15

Impact strength, kJ/m2 17.1 11.5 5.2-17.4

Notched impact strength, 7.3 3.9-6.7 kJ/m2

Hence, there is a considerable deterioration of mechanical properties when commingled recycled mixed plastics waste is directly compounded. Because of this deterioration, the possibilities of direct use of recycled plastics is limited. However, they may be used for less demanding applications - this is the origin of the expression

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"downcycling" . When the heterogeneous recycled polymer blends are to be used for applications

without a downcycling, the materials must show improved mechanical properties. These can be generated ascertaining fine phase dispersion and intensive interactions between the phases. The method that leads to such improvement of properties is known in polymer blends industry as compatibilization.

2. Principles of Polymers Compatibilization

2.1. Polymer Miscibility

Most polymer pairs are thermodynamically immiscible [10]. By definition, a binary polymer system is miscible if the free energy of mixing (~G mix) is negative (Eq 1) and its second derivative is positive (Eq 2).

(1)

(2)

Because of the large molecules involved, the combinatorial entropy of mixing is vanishingly small. The energetic interactions between segments of different polymers (e.g., between PS and PE monomeric units) are also weak. The free volume effects usually reduces these interactions. At constant concentration and pressure, the phase separation may occur either by decreasing and/or increasing the temperature. Thus, phase diagrams with lower and upper critical solution temperature (LCST and UCST, respectively) are known for polymer blends - the former being more common.

Numerous polymer blends from plastics waste recycling have been studied for miscibility and phase behavior [11-14]. There are large differences in miscibility reported for polymer pairs common for plastics recycling of packaging waste. An overview of miscibility of different polymer pairs [15] is summarized in Table 3. It is evident that polymer components present in packaging plastics waste (see Table 1) are immiscible. Only some pairs of polyethylenes may show limited miscibility, viz. HDPE with some types of LLDPE. However, as a rule, even different polyethylenes should be considered immiscible with each other (for example, HDPE with LDPE). From the thermodynamic point of view, even miscible polyolefins may differ in their crystallization tendencies that can lead to phase separation. Furthermore, large differences in rheological behavior of the components can make formation of homogenous mixtures more difficult.

Mixtures of polyolefins and polystyrene - relevant components in the packaging plastics waste recycling problem - are antagonistically immiscible. The phase diagram

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TABLE 3. Miscibility of some common polymer types, [15]

Poly- LOPE HOPE PP PS PVC PET ABS PMMA PA PC mer

LDPE 1 HOPE 1 1 PP 6 6 1 PS 6 6 6 1 pvc 6 6 6 6 1 PET 6 6 6 5 6 1 A.S 6 6 6 6 3 5 1 PMMA 6 6 6 4 1 6 1 1 PA 6 6 6 5 6 5 6 6 1 PC 6 6 6 6 5 1 2 1 6 1 1 = hIghly mIscIble, 6 = not mIscIble (ImmIsCIble)

60,------------------------. van Kl'8Velen

a 0,0 0,1 0,2 0,3 0,4 0,5 ~,6 0,7 0,8 0,9 1,0

Figure 1. Miscibility gap of the polymer system PP/PS.

based on the Huggins-Flory theory shows a large miscibility gap (Fig. 1). Other polymer pairs that have been reported miscible, e.g. PMMAlPVC or PMMAIABS, are not

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present in appreciable amounts in the packaging waste. In short, to obtain improved performance of recycled polymers they should be compatibilized.

The compatibilization strategies have been developed by the polymer blends industry for virtually any pair of immiscible polymers. The material recycling should profit from this waste pool of information.

2.2. Methods of Compatibilization

Compatibilization is a well-defined process of modification of the interfacial properties in immiscible polymer blends, resulting in formation of the interphase and stabilization of the desired morphology, thus leading to the creation of a polymer alloy.4\.. compatibilizing agent, or compatibilizer, can either be added to the polymer mixture as a third component or generated in situ during reactive compatibilization process. The added compatibilizer should migrate to the interface, reducing the interfacial tension coefficient and the size of the phase domains, as well as creating improved adhesion in the solid state.

Intensified phase interactions and a controlled phase morphology lead to improvements in mechanical properties of the blends. An additional, but often forgotten, function of compatibilizers is stabilization of the blend morphology against coalescence and agglomeration of the dispersed particles that can take place during the following processing and forming steps. Compatibilization means that systems with acceptable properties can be obtained from mixtures of immiscible polymeric components.

Different methods of compatibilization can be used, such as, addition of a compatibilizer, reactive compatibilization, i.e., in situ formation of the compatibilizing agents, as well as by crosslinking (total or partial of at least one phase), by addition of suitable fillers, by formation of ion-ion or ion-dipole complexes and by modifying interfaces, e.g., by application of plasma and/or by use of suitable plasma polymers [16]. In the past, the most common compatibilization method was addition of the block or graft copolymers. The addition takes place during melt mixing in the compounding equipment. Different groups of compatibilizers are known and used for such applications. Some examples are given in Table 4.

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T A BLE 4. Examples 0 f f h 'bT h d some 0 t e common compatl I lzatlOn met 0 s

Method of Blend Examples

compatibilization

Addition of polymeric PE/PS S-B, S-EP, S-[-S, S-l-HBD, S-compatibilizers EB-S, S-B-S, PS-g-PE

PPIPS S-EB-S

PPIPE EPM,EPDM

PVCIPS, CPE, PCL-b-PS, PE-g-PVC PVCIPE, PVCIPP

PPIPA 6 S-EB-S

PET/PE S-EB-S, EGMA, EAE-GMA

Addition of co-reactive PP/PS maleated PS and carboxylated polymers/monomers PP, functionalized polymers

PEIPVC polyfunctional monomers plus peroxide, MSA

Addition of reactive low PPIPS Peroxides (DCP), molecular agents Bismaleimide (BMl),

Sulfonylazide

PEIPVC TAC, TAlC plus peroxide

PEIPS/O-VBA TAlC plus peroxide

Addition of surfactants PEIPVC, non-ionic compatibilizers PSlRubber (Ethylene oxide-propylene

oxide mix polymer), Sodium oleate, n-alcansulfonate, dodecyl benzoate, Mono chlorine acetic acid, etc.

Trans-reactions PBTIPC, PAIPET, PAIPC

Addition of fillers Chemical reaction with both polymers

Salt formation Sulfonation, Carboxylation, Zinc stearate

The principle of activity of copolymeric compatibilizers is that they create interactions between their blocks or graft branches and the corresponding polymer components in a

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heterogeneous blend. These compatibilizers consist e.g. of a block A, which has a high affinity to the polymer A, and a block B, which has an affinity to the polymer B. In the case of a graft copolymer as compatibilizer, the backbone chain is miscible with one and the graft branches are miscible with the other polymer. Figure 2 shows possible arrangements of block and graft copolymers at the interface between two polymers. Most frequently, di-block or three-block copolymers have been used as compatibilizers.

2.2.1. Compatibilization by Block and Graft Copolymers

The block or graft copolymeric compatibilizers are prepared separately and introduced at the lowest possible concentrations into a mixture of immiscible polymers. Often as little as 0.5 to 2.0 wt% of the copolymer is sufficient to compatibilize the system. However, frequently 10 to 20 wt% of is required to obtain optimum physical performance of the blend, [17]. The compatibilizer should not form its own phase, since this would result in a decreased efficiency and economy. The copolymeric compatibilizer must be located at the interface between two phases and has to diffuse into both of them. Thus one part of the compatibilizer has to be better soluble in one polymeric component of the blend and the other part of the compatibilizer has to be dissolved preferentially in the other.

f ~~ ~~

Backbono

Inlerlace

Dlbloc:c and Irlblock capo ymer Graft copolymer

Figure 2. Models of polymeric compatibilizers at interface.

The structure and characteristics of the compatibilizer, i.e., chain length, molecular weight and molecular weight distribution, strongly influences the compatibilizing effects, and should be optimized. The block or the graft branches, respectively, must have optimum length. The molecular masses of the blocks and of the graft branches should not be too small, since this may prevent them from diffusing from the interphase

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to the two immiscible homopolymeric domains. The larger the total molecular mass of the compatibilizer the higher is the tendency for the inter-diffusion. A-B block copolymers with 50/50 compositions are reported to offer the best results. The optimum of molecular mass depends on the characteristics of the blend components. Fayt et al. [18] reported that for LDPEIPS blends the optimal molecular weight of the compatibilizer should be between 18 and 400 kg/mol. Figure 3 shows results ofFayt et al. with diblock copolymers of PS and hydrogenated polybutadiene (hPBD) with different molecular masses. It can be seen that a shorter diblock copolymer results in a higher ultimate tensile strength, while a higher total molecular weight leads to a more ductile material with values of the elongation at break higher than the additive values. The mechanical properties of the PS-rich blends strongly depend on the molecular weight of the diblock copolymer used as compatibilizer .

.... /""

i 50 ..-

....... >t .......... ....

,§ ... )10

10

o 20 IoQ 60 eo lOll IQ~...,20;::---f;;IoO--t.60;--~.AQ -:;':100

\"0 \"0

LDPE/PS BLENDS

Figure 3. Ultimate tensile strength and elongation at break of LDPEIPS blends with 9 % PSIhPBD diblock copolymer: MW,LDPE = 40, Mw.PS = 100, MW,PSIhPBD = 80 e) and 270 kg/mol (x), without compatibilizer (e). [18]

Diblock copolymers are better compatibilizers than graft copolymers and linear or star­shaped three-block copolymers [19]. The reason is, that a diblock copolymer is subjected to less drastic conformational restraints at the interface, and its constitutive blocks can penetrate more deeply the corresponding homopolymer phases, providing them with a stronger mutual anchorage. However, in contrast to graft copolymers, upon increasing concentration, diblock copolymers are known for their tendency to form micelles and mesophases [20]. Therefore, their interfacial activity may be limited by the tendency to form their own phases, instead of locating at the interphase [21].

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2.2.2. Compatibilization by Reactive Agents

Reactive blending is a very effective technology for compatibilization of polymer blends composed of immiscible components. Interchain copolymer formation by reactive compounding is particularly useful for compatibilization of immiscible polymer blends so that a product may be obtained with combinations of desirable properties arising from both polymers. Compatibilization in this sense refers to "operational compatibility" as defined by Gaylord [22] in which the blend exhibits useful technological properties over the lifetime of a molded part.

Chemical modification may result in carboxyl-containing polymers that are useful as compatibilizing agents in the preparation of blends with, e.g., polyamides. Carboxylation of polymers is usually conducted by first generating free radical on the polymer molecules, then reacting these with either acrylic acid, maleic anhydride, styrene-maleic anhydride, or Diels-alder adducts of maleic anhydride [23]. Thus, in this case the reactive blending is performed by using low molecular reactive agents that functionalize selected polymers.

1. Using low molecular reactive agents to produce in situ copolvmers

Often, it is sufficient to create copolymers by grafting reactive groups. In turn, these groups react during the melt mixing process. In the course of the mixing, e.g., in an extruder, a reactive group create free radicals at the different macromolecules. These free radical recombine, what leads to compatibilizing graft copolymers. Compatibilization is also realized by inter-polymeric bonding through the reactive agent that is able to react with both polymeric species. The in situ formed copolymers reduce the interfacial tension between the phases, which results in a decrease of the particle size of the dispersed phase and an improved interactions between the phases. Small amount of a reactive agent can be sufficiently effectively. Hence, this method is interesting because of the relatively low costs involved.

There are many examples in the literature where low molecular reactive agents were used for the compatibilization of immiscible polymer components [22-25].

2. Using functionalized polYmers in the mixture

Interphase adhesion can be achieved by several mechanisms. A common and efficient method is through the use of a block or graft copolymer of two homopolymers involved that can act as an emulsifying agent and lower the interfacial tension coefficient, as discussed above. Addition (or production by i.e. plasma) of suitable agents that may not necessarily be copolymeric in nature but contain similar groups as parent homopolymers

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can also be used for compatibilization, [26]. These agents can be produced directly during the melt mixing process of polymeric components in presence of functionalized polymers. Thus, covalent or ionic bonds are formed during mixing, which help to disperse one polymer in the other producing better performing blends.

An effective way of compatibilization is formation of covalent bonds. This requires the presence of reactive functionality, e.g. nucleophilic on one polymer and electrophilic on the other. For the generation of ionic bonds, ionic sites on both polymers or acidic sites on one polymer and basic sites on the other polymer may be used. The reactivity of the functional groups has to be sufficiently high to react at melting conditions in the mixing equipment. Furthermore, the reaction must take place within the residence time of the material in the mixing section. After the reactive mixing the bonds must be sufficiently stable to guarantee the desired properties. A large interface during the melt mixing is necessary for efficient interchain copolymer formation. Most commercially available polymers suitable for reactive blending have reactive end groups (such as carboxylic, amine, and hydroxyl) that can be used for the formation of compatibilizing species.

Examples of formation and the use of functionalized reactive copolymers can be found in [17]. Maleic anhydride (MA) is the most frequently used agent to generate of reactive polymer components by grafting reactions. Carboxyl-containing polymers are of particular interest since they may serve as effective compatibilizing agents acting through covalent, ionic or hydrogen bonding. Some of these are particularly useful as compatibilizers in blends from plastics waste recycling. Few examples are given below: Hohlfeld compatibilized PSILLDPE blends by using maleated PE (LLDPE-g-MA) and oxazoline-functionalized PS (styrene-isopropenyl oxazoline copolymer SIPO). Increased tensile strength and elongation at break in comparison to the unmodified blends were reported [27]. Similarly, Saleem and Baker investigated the compatibilization of PEIPS by carboxylic acid-functionalized PE and oxazoline­functionalized PS (SIPO). The generation of a PE-PS copolymer was detected by selective solvent extraction, FTIR, SEM, DSC, and mechanical testing [28]. Radical initiated coupling ofPS and PE by TAlC and di-cumyl peroxide (DCP) was described by van Ballegooie. 0-Vinyl benzaldehyde was incorporated to make PS more susceptible to radical grafting. The phase morphology of the blends was finer and mechanical properties were improved in comparison to the unmodified blends [25].

Mashita et al. [29] described interchain copolymer formation in compatibilized PP blends with: PP-g-MA, PET, and glycidyl methacrylate grafted ethylene vinyl acetate (EV Ac-g-GMA). The compatibilized blends had markedly higher impact strength and tensile properties than blends with either non-functionalized PP or without EV Ac-g­GMA.

PPIP A-6 blends were compatibilized during melt mixing process by formation of a PA-6-PP copolymer using PP-g-MA. The resulting blends had a finer phase morphology and improved mechanical properties as compared to blends without

164

compatibilizer [30]. A similar study was reported by Akovali et ai., [31]. Thus, PE was modified in plasma at various conditions (to produce free/grafted active agents to enhance compatibilization). It was shown that mechanical properties of the blends prepared with PVC and modified PE were much better than those prepared with virgin polymers. Pertinent elements of this study (still in progress) will be discussed in more detail in following sections.

2.2.3. Compatibilization by Use of Interfacially Active Additives

An important criterion for the performance of blends is the specific surface of the disperse phases, i.e., the shape and size of the dispersed particles. In heterogeneous polymer blends, interactions are possible only through the interfaces. Besides the thermodynamic interactions, the specific surface determines the adhesion between the phases and therefore the properties of the blend.

It should be noted that, optimum degree of dispersion depends on the application and the fracture mechanics of the major blend component. Thus, to toughen PS that fracture by the crazing and cracking mechanism, relatively coarse dispersion of elastomeric particles is required.(1-2 /lm). On the other hand, for such polymers like polycarbonate or PP, the shear banding mechanism requires very fine dispersion of the second phase, say 20 to 50 nm. Optimum size of the dispersion of other polymers is situated in between these two limits. In consequence, for plastics waste recycling generation of a fine dispersion is one 0 tthe principal aims of mixing processes. In blends of immiscible polymers, the interfacial tension coefficient (v12) is usually high, what results is a coarse morphology. Reduction of this coefficient can be achieved by adding either polymeric compatibilizers, or low moleculr weight additives.

As stated before, an addition of wmpatibilizer requires miscibility of one part of copolymer in ole polymeric phase, and miscibility of the second part in another. When the dissolved copolymer parts have molecular weight above the entanglement molecular weight value, the polymeric compatibilizer offers not only the interfacial activity, but also a mechanism for enhanced adhesion and stress transfer in the solid state. By contrast, the low molecular weight additives are forced by the laws of thermodynamics to diffuse into the interfacial space between the two principal polymeric phases. When properly selected, it also diffusses into the two phases, bringing their chemical potential closer, i.e., reducing the interfacial tension coefficient. It has been shown that the interfacial tension coefficient reduction upon addition of an interfacial tension active additive has identical mathematical form as the diameter of the disppersed phase- reduction of V 12 enables the shear stress to break drops into smaller ones more efficiently. This can also be concluded from the capillarity number:

K = 11 . Y . d / V12 (3) The capillarity number expresses the steady-state relation between the shear stress ( as

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the product of the viscosity TJ and the shear rate y ) , and the interfacial stress ( ratio of the interfacial tension coefficient v12 , and the equilibrium drop diameter, d ). Especially in studies of plastics recycling,different types of surface active sustances were tried in PEIPS/PVC blends [32]. These were used ionic and non-ionic additives, such as lauryl alcohol, copolymers of propylene oxide-ethylene oxide, as well as polysiloxane polyalkylene oxide. Their activity was rationalised by Funke, e.g. for non­ionic addives to PEIPVC mixtures[32]. As shown in Figure 4, the author assumed that the non-polar alkyl groups of the surfactant are incorporated into PE and that the polar, non-ionic segments were attached to PVC. The polarity of PVC is reduced and the potential threshold decreases.

Anionit: sur/adant pvc

Figure 4. Effect of a non-ionic additive on a PE/PVC interface.

The data showed that low molecular weight additives do generate finer phase morphology. They also may stabilize the phase morphologies. However, they do not increase interactions at the interface. For PE/PS/PVC blends, the tensile strength and modulus of elasticity did not change essentially, but elongation at break and impact strength values were improved [33].

3. Compatibilization ofPEIPS and PPIPS Blends

3.1. Introduction

The immiscible polymer systems PE/PS and PPIPS were selected as model systems for plastics waste recycling. Compatibilization of these blends during melt compounding was investigated. Copolymers, reactive agents and low molecular weight additives were used. Performance of resulting compatibilized blends was related to different mechanisms of activity of the compatibilizers, enabling better understanding and interpretation of data.

166

3.2. Materials And Technology

Polystyrene PS S 134 (BSL GmbH Schkopau), polyethylene PE A 21 (BSL GmbH Schkopau) and isotactic polypropylene PP KF 6100 H (Shell AG) were used. Their characteristic parameters are shown in Table 5.

Substances shown in Table 6 were used as compatibilizing agents. Copolymer compatibilizers (CoA - CoH) containing styrene and olefinic segments were either commercial or research copolymers. Bis-( 4-maleimido-phenyl)-methane (BMI) was used as a low molecular reactive coupling agent and dicumyl peroxide (DCP) as a free radical initiator. Already Tawney et al. have shown that bis-maleimides can react in the presence of peroxides, both with PP and PS [34]. Therefore, it should be possible to generate in situ graft copolymers during melt-mixing ofPP and PS in presence ofDCP and a suitable bis-maleimide. Another low molecular reactive additive with an azido

TABLE 5. Polymers used in compatibilization studies

Polymer Parameter

Density (kg/mJ) Melt flow index (g/1O min)

PEA21 920 20 (5/190)

PP KF 6100 902 34 (5/230)

PS S 134 1055 14 (5/230)

decaline were studied for the detection of graft products with PS compound, BA - it was chosen for the possibility of its reaction with PP [35] and PS [36]. Di-amino naphthalene (DN) was used as a low molecular weight interfacial tension modifier. The compound is characterized by low tendency to migration.

The compatibilizers were mixed in the laboratory Brabender Plasticorder PS 2000-6 at IS5-190/C, mixing rate of SO min-1 and the total mixing time of 12 min. First either PE or PP was added to the mixer, then PS and last the compatibilizing agent. The blends had constant PS content of 30 wt%. To examine the compatibilizing efficiency, the compatibilizers content varied. After mixing, the blends were pressed into sheets 120x120xl mm3 and 120x120x4 mm3 in a hydraulic press (COLLIN Type 6202) at a pressure of 10 bar and 35 bar, respectively. From these sheets specimens for the mechanical characterization were prepared.

For the investigation of the changes in the molecular structure and molecular interactions, FTIR spectroscopy (Perkin & Elmer FTIR 2000) was used. The in-situ formation ofPP-PS copolymers have been proved by FTIR investigations. Unsoluble residues of PP in decaline were investigted specifically for detection of graft

products produced with PS.

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The phase morphology of the blends was characterized by the optical (Zeiss Amplival pol) and scanning electron microscopy (SEM JEOL JSM 35C). The mechanical properties were measured using tensile (Zwick tensile tester 1464) and Charpy impact testers (Zwick impact tester 5102).

TABLE 6. Compatibilizers used for polyolefin blends with 30 wt% PS

Type Agent Abbr.

Copolymers Kraton G (Shell) CoA Scona TPSE (Buna) CoB Tuffprene (Asay) CoC PE-g-Styrene (Leuna) CoD EV Ac-g-Styrene (Leuna) CoE EV Ac-g-StyreneIMA (Leuna) CoF EPM-g-Styrene/MA (Leuna) CoG PP-g-PS (4 % Styrene) CoH

Reactive agents Bis- (4-maleimido-phenyl) methane BMI Di-cumyle peroxide DCP 2,6-bis- (4-azidobenzyliden)-4- BA methyl-cyclohexanone

Interfacial tension 1,5-di-amino-naphthalene DN modifier

3.3. Results and Discussion

3.3.1. Molecular Interactions

The molecular interactions were evaluated by FTIR spectroscopy. As an example, Figure 5 shows the FTIR spectrum of the insoluble PP residue from the extracted blend, after modification by BMI. Typical absorption bands of PS occur in the PP spectrum at 1601 cm-! and 699 cm-! caused by absorption of the skeleton oscillations of the benzene ring and the absorption of the aromatic substitution on it. This indicates that PS chains are grafted onto the PP molecules. Hence, no absorption band occurs in the FTIR spectrum of the unmodified PP/PS blend. For quantification of the grafting, the PS content was estimated from the area under the peaks at 1601 cm-! and 699 cm-! using a calibration curve. The amount of the grafted PS strongly depends on the BMI, but not as much on the DCP content. For example, at 1 wt% ofBMI in the blend up to 2.5 wt% of

168

PS was grafted. Addition of DCP does not change this amount significantly (Fig. 6). Also addition of BA to PPIPS blends was found to generate copolymers in-situ. These copolymers contributed to the blend compatibilization.

I r I • ~ .. I r\~ r-.. l=-- rP .~~ b~ ~ I e"" r , , r IVfV d Vl ~ d

I 1'\ I /1

I I 'v I f\

.. ' 1\ h Ii ~W

r tv , I

~AI rJ BMI

I I

1/ ) f ,..,. '''' .... ... 700

Figure 5. FTIR spectrum of insoluble PP residues from PPIPS blends modified with 0 % (a), 0.2 % (b), 0.5 % (c), 1 % (d) BMI.

3 ,---------~--~----~----~---.

2,5 f--------~-----'-----~,........,"-----I

~ ~ 2 ~--~---~--~~~~~--+--~ a. "0 ~1 ,5 f----------,L-~'-----------+-----I

(;

0,5 f---fi<------~--____:----------+-----I

0,2 0.4 0,6 9,8 BMI, "to

1,2

Figure 6. Grafted PS fraction as functions of the BMI and DCP content

Figure 7. Optical micrographs of PP/PS blends. a) PP/PS 70/30 unmodified, b) modified with 5 % PP-g-PS, c) 10 % PP-g-PS, d) 1% BMI, e) 1% BA, f) 0.5 % DN

169

170

Figure 8. SEM micrographs ofPPIPS blends, PPIPS 70/30. a) unmodified PPIPS blend, b) 10 % PP-g-PS, c) I % BMI, d) I % BMI + 0,2 % DCP, e) 1% BA, t) 0.5 % DN

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3.3.2. Phase Morphology

The changes in the phase morphology caused by the compatibilizers effects are presented in Figure 7. The optical micrographs show markedly changes in the phase morphology of the modified vs. unmodified blends. The blend modified with copolymer CoH (Fig. 7 b, c) show a finer phase morphology. Depending on concentration of copolymer, the particle size gets smaller as compared with that of unmodified blend. Thus, the particle size is reduced most in the blend with 10 wt% CoHo

The PPIPS blends compatibilized with the reactive agent BMI shows very fine PS particles (Fig. 7 d). The results indicate that in PPIPS blends with PP matrix, only a small amount of BMI is needed for refinement of the phase morphology. The in situ formation of PP-PS copolymers (as proved by FTIR studies) is the main reason of the reduced interfacial tension coefficient and paralleled with it reduction of drop size.

It is very interesting to see that the low molecular substance BA also reduces the particle size (Fig. 7 e). The type of morphology is the same as for 10 wt% of the PP-g­PS copolymer (CoH). In the case of the interfacial tension modifier DN much finer morphology was also found (in comparison to the unmodified PPIPS blend). However, the particle size was markedly coarser than in the blends modified with the other reactive agents or copolymers (Fig. 7f).

3.3.3. Phase Interactions

The scanning electron microscopic (SEM) investigations gave information about the phase interactions at the interface and about the particle size and shape. Figure 8 shows the SEM micrographs of the PP blends with 30 wt% of PS. In the unmodified blends (Fig. 8 a) the PS drops are large, varying from 1 to more that 10 :m. Furthermore, as the SEM shows, the drops are loose within the PP matrix (that shrunk on crystallization). Because of the high surface tension of the dispersed PS phase, the drops are a nearly perfect spheres. The detachment of the particles from the matrix is clearly visible. Addition of compatibilizing copolymer engenders fine phase morphology as well as enhanced interactions between the dispersed and matrix phases (Fig. 8 b). The particles have a good contact to the matrix and the fracture surface of the specimens show that break passed through the PS drop. This is an indication of high adhesion between the phases caused by interdiffusion of the corresponding parts of the copolymer molecules. The SEM micrographs of the reactive compounded blends (Fig. 8 c, d, e) show that the in-situ generated copolymers strongly influence the interactions between the dispersed and the matrix in the polymer system PPIPS, very intensively. The particle diameter is < 5 /lm and the phase interaction seems very high in the case of added BMI and DCP (Fig. 8 d). The particles are smaller than in the blends with copolymer. This effect is caused by the proportion of copolymer. The amount of in situ generated copolymer in presence of BMI is larger than the externally added amount of copolymer. The addition

172

of the reactive agent BA results in a finer phase morphology and intensified interactions (Fig. 8 e) but not not at the same level as BMI and DCP. The micrographs show the changes of the particle surface caused by the in-situ chemical reactions during melt mixing but the phase adhesion is not as high as in the blends modified by BMI or copolymers. PPIPS blends modified with the surfactants show a refinement of morphology but the adhesion of the dispersed phases remained are not improved essentially(Fig. 8 f). Clearly, the voids between the matrix and dispersed particles are visible indicating an insufficient phase adhesion.

3.3.4. Mechanical Behavior

The mechanical behavior of the PEIPS blends were evaluated using the tensile and impact strength tests. Figures 9 to 11 show selected results of tensile measurements. The tensile strength and elongation at break values strongly depend on the deformation mechanisms determined by the phase morphology and adhesion between the phases. Figure 9 shows three possible types of stress-strain diagrams of the PPIPS blends. Unmodified PPIPS blends are characterized by a brittle material behavior expressed by a low elongation at break and poor tensile strength (curve 1). Compatibilized PPIPS blends have a mechanical behavior which can be derived into two different types: blends with higher tensile stress but unchanged brittleness in comparison to the unmodified blend (curve 2), and blends with higher deformability but only moderate or not improved tensile strength (curve 3). The reason for this behavior can be found in the mechanisms of deformation of heterogeneous blends. In the first case (curve 1) the particles are large and have no adhesion to the matrix. The particle distance is high and in the volume between the particles there is a three-dimensional stress distribution resulting from the mechanical loading of the specimen. Here only the matrix is deformed since the PS drops have lose contact with the interface. The resulting voids are the source of crazes and micro cracks, which grow into macro cracks. If the phase morphology is fine disperse, the particle distances are reduced and the stress state is changed to a one-dimensional stresses. If the dispersed phase has no intensive interactions with the matrix, than the PP matrix fibrillates between the drops. This mechanism leads to higher specimen deformability, thus higher maximum extension at break.

The particles are strongly coupled to the matrix in blends either prepared via interchain reactions (using reactive agents and functionalized polymers) or compatibilized by addition of copolymers that can inter-diffuse and entangle within the two phases. In this case, the deformation of the matrix was limited by the dispersed PS drops having higher modulus. Fibrillation was not possible, thus high tensile stresses at low elongation at break resulted.

40

30 '" Q. :;;

:i ~

US 20

10

°0 5 10 15 20 25

Elongation, 0/0

Figure 9. Stress-strain behavior ofPP/PS = 70130 blends. 1) unmodified, 2) compatibilized with a reactive agent (BMI), 3) compatibilized with

interfacial tension modifier (DN).

173

174

Figs, 10 and II show the tensile strength and elongation at break for PPIPS and PEIPS blends compatibilized by addition of copolymers (Table 6).

30 N

E 25 E --z .£ 20

'6l c: 15 ~ 1i) ~ 10 'u; c: ~ 5

/ - -- - -- - - - ~- - -- - ,;.-- -- -- - - -, ,.. ,.. , - - -~- - - ,..-, f"""'f""'r- : ;. ,.. - - - -

- -""'

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GJstrength

; Delonaation

l' II"

4

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3 ::> co III

2,5 "" 0 ::>

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0,5 *' 0

0 o Co A Co B Co C Co 0 Co E Co F Co G Co H

Figure J 0, Tensile strength and elongation at break of copolymer modified PPIPS 70/30, blends, with 5 wt% of compatibilizer.

12

1: 10 E Z 8

~ <: 6 ~ "' 4

"' <: {!

2

/ _1"'"11- -, - : ,;.. - - -: - - ~- -- -.:. - - -

" -" - - - - -~ - -,.; ~ -, - - - - ... -.. - I- . - ~ -

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1 11~strength -

.,

l Delongation 1 Til III

25

20 m 0 :::l

<0

1S !!t 6' :::l

!!t 10 0-m

D> ?'

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0 0

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o CoA CoB CoC CoD CoE CoF CoG CoH

Figure J r Tensile strength and elongation at break of copolymer modified PEIPS 70/30 blends with 5 wt% compatibilizer.

175

Evidently, different copolymers have different effects on the PP/PS blend. The tensile strengths was the highest for blends with copolymer CoH (PP-g-PS). This may be explained by the highest molecular affinity between the components. Also the maleated EPM-g-PS copolymer (CoG), TPSE (CoB) and the SEBS Kraton G (CoA) showed highly improved tensile strength. However, the elongation at break for these modified blends was low, exception being CoA that simultaneously improved tensile strength and elongation at breaks. Better elongation was obtained when EV Ac copolymers (CoE or CoF) were used. However, now the tensile strength was even lower than that of un­modified PP/PS blends.

In summary, only the CoA (Kraton G) significantly improved the tensile strength, and produced satisfactory elongation of break.

When reactive agents are used as compatibilizers, the tensile strength of PP/PS was improved (Fig. 12). Especially, the blends reactively compounded with BMI or BMI and DCP had about twice as large tensile strength as the non-compatibilizedmixture -the elongation at break also was improved. The reason was probably an intensification of interactions between the phases and a better dispersion. Booth effects originated from the copolymers generated during the reactive melt mixing.

When no reactive agent, but DN (a low molecular interfacial tension modifier) was used, the elongation at break increased to a relatively high level (dependent on DN concentration), but the tensile was unaffected (Fig. 13). Evidently, the low molecular interfacial tension modifier contributes only to a finer dispersion of the blend, but does not promote adhesion between the phases.

3,5

40 3

m N 2,5 0

E ::I

30 (0

E gj, Z 2 0

::I

~ ~ c 20

1,5 0-

g iD III

(/) -'" Q) '# ~ c

10 0,5 ~

0

0 05 %BMI 06% eMl' 0' %OCP OS%BA withOut agent

Figure 12. Tensile strength and elongation at break of unmodified PP/PS 70/30 blend and reactively compounded blends.

176

30.---~--~--,---,---,---,---,---, 25

20 m 0 ~ co

15 ~ 0 ~

e 10

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5

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Figure 13. Concentration dependence of the tensile strength and elongation at break for PPIPS 70/30 blend modified by addition of

interfacial tension modifier, DN.

The impact strength is an important property, especially for recycled plastics. Figure 14 shows the impact strength ofPPIPS blends modified with different copolymers. Only a slight improvement of the impact strength was obtained for blends modified by compatibilizers CoF (EV Ac-g-StyreneIMA) and CoG (EPM-g-Styrene/MA). That means that maleated compatibilizers showed only a small improvement of PPIPS toughness.

....

en

i ~ 0.5

Figure 14. Impact strength of copolymer modified PPIPS 70/30 blends with 5 wt% of compatibilizer.

14

'" 12

~ 10 "" t 6 c Q)

iii 6 "0 '" 0- 4 .s

2

0 o CoA CoB CoC CoD CoE CoF CoG CoH

Figure 15. Impact strength of copolymer modified PP/PS 70/30 blends with 5 wt% of compatibilizer.

177

By contrast, the compatibilizer CoA (SEBS Kraton G) had an extraordinary effect on the impact strength (Fig. 15). It is obvious that the chemical composition and the molecular parameters of this copolymer is favorable for the PEIPS blend.

178

3

N 2,5 .§. "' .0.:

.cO 2 a, c

1,5 OJ

iii u '" a. E

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0 without agent 1 %BMI 1 %DN PS PP

Figure 16. Impact strength ofPP, PS, PPIPS 70/30 blends and PPIPS blends modified with the reactive agent BMI and the interfacial tension

modifier, DN.

Reactive agents usually did not improved the impact strength (Fig. 16). Because of the chemical coupling reactions, strong interactions between the phases were obtained, but the method did not provided for an elastic energy dissipation mechanism that would lead to higher impact strength. When the dispersity of the polymer system was regulated by addition of the interfacial tension modifier, a fine disperse morphology was created without strong interactions at the interface. Under these circumstances, energy dissipation at impact load was possible, which resulted in higher impact strength (Fig. 16).

The results of these mechanical tests showed that compatibilization may not necessarily be result in improvement of the mechanical properties. This probably is due to the control of deformation characteristics by different modes of compatibilization. The results indicate that compatibilization offers a promising method for the improvement of the mechanical properties of blends containing immiscible polymers.

4. Compatibilization of PEIPVC Mixtures by Plasma

4.1. Introduction

Low density polyethylene (LDPE) and polyvinyIchloride (PVC) system constitute the greatest proportion of plastic packaging waste and attract considerable interest. The recovery and reuse of these materials is important, firstly as a viable solution to environmental problems, and secondly for the economy involved. The economic argument would even stronger if the mixture could be reprocessed without separation.

179

LDPE and PVC are strongly immiscible (Table 3). There is a vast volume of literature available describing diverse physical and chemical methods for their compatibilization [37]. Plasma has proved to be an efficient method for modification of surfaces [38] and production of new polymers [39]. By means of the plasma technology appropriate chemical groups can be created, enhancing specific interactions. Plasma modification effects only a thin surface layer of the polymer, leaving the bulk of the system intact

4.2. Materials and Technology

LDPE (Petilen type F.2-12, with melt flow index of 2.0g/l0 min and density of 920 kg/m\ PVC (Petvinyl type S 23/29 with K value of 54-58 in cyclohexanone at 25EC and relative viscosity of 1.354-1.417 in the same solvent), vinyl chloride monomer (VCM) and an oligomeric copolymer of vinyl chloride and polyethylene (VCO with 30% Cl) were products of Petkim-Petrochem. Ind. Inc., Turkey. The latter was a side product ofVCM, and its characteristics were not disclosed. Carbon tetrachloride used in this study was a reagent grade Merck product.

The plasma system consisted of a standard vacuum pump and a tubular Pyrex reactor with a 13.56 MHz RF generator coupled to it using external copper electrodes [26]. During the experiments, surfaces of PVC powder and PE granules were modified in carbon tetrachloride and vinyl chloride plasma for the first polymer, and in acetylene plasma for the latter. The plasma treatment conditions are presented in Table 7. Plasma is known to yield mainly surface modifications with a minimum (if any) of plasma polymerization. The oligomer VCO was also used without application of plasma to prepare reference blends. After plasma modification, the polymers were melt mixed at different composition in the Brabender Plasticorder, at 175EC. Samples from unmodified polymers were prepared under identical conditions. The test specimens were prepared by pressing at 190EC, then annealing. For the mechanical tests, an Instron TM 1102 tester at room temperature was used with crosshead speed of 12.4 mmlmin. The dynamic mechanical tests were performed using a TA 983 dynamic mechanical analyzer (DMA), at a resonant frequency of 0.2 Hz.

4.3. Results and Discussion

The FTIR spectra of plasma treated and untreated polymer films were presented elsewhere [26].

The steady-state mechanical test results for untreated parent homopolymers (label~d as U) and their blends with 50 and 75% LDPE have shown that both of these blends have lower moduli and toughness than either of the pure polymers( Figure.17), which probably is due to poor interfaces between the phase separated domains.

180

TABLE 7. Plasma treatment conditions employed

Polymer Monomer Type of' Used Time Average Average Abbrevia Surface in Plasma Treatment Power (min) Monomer Pressure -tion Treated (W) Flow Rate (mbar)

(mLimin) A.PVCl PVC b Acetylene LW/LQ 5 30 58.3 0.5

PVC b Acetylene LW/HQ 5 30 126.4 0.7 A. PVC2

A. PVC3 PVC b Acetylene HW/LQ 10 30 70.7 0.5

A. PVC4 PVC b Acetylene HW/HQ 10 30 136.0 0.6

C. PE-I LDPE' CC14 LW/LQ 7 30 55.7 0.4

C. PE-3 LDPE' CC14 HW/LQ 7 30 58.2 0.4

C. PE-4 LDPE' CC14 HW/HQ 14 30 102.0 0.5 V. PE-2 LDPE' VCM LW/HQ 10 15 47.6 0.5

a L W / HW: low / high watts, LQ / HQ: low / high flow rates; respectively ; b Powder, C Granules

12-

i ~

10

'ii r IC.. a 8 • uoon :S OIl> 0 UOO.75 .. v 6 ti

" uo.tOO ~ 4 .. .. v uaaoe ~

2

~ 0 0.00 0.10 0.20 0.30 O.a.o 0.50

Stral" (cIJIJe~

Figure 17. Stress-strain curves of untreated LDPE-PVC blends with 0, 50, 75 and 100% PE. For the U0050 blend, 2 pph stabilizer was added; strain rate is: 1.24 ern/min .(Reproducedfrom reference 26 with kind permission of

J Wiley and Sons Ltd).

181

The results for acetylene-modified PVC blends with LDPE are shown in Figure 18. The treatment was expected to bring the surface properties of PVC closer to these of PE. As the data in Fig. 18 shows, the treatment was not helpful for modification of the mechanical performance. These results are probably due to a number of reasons that will not be discussed here. However, the macro behavior of these systems and the degree of compatibilization are obviously poor. Studies with acetylene plasma are still continuing by using different plasma conditions.

Blends of PVC with the plasma-modified LDPE surfaces (by either carbon tetrachloride or VCM) led to somewhat different mechanical test results. It is known that VCM, if plasma polymerized alone, yields a PVC-like molecules [40]. Although there is no study available either for the effects of VCM or carbon tetrachloride plasma treatment of PE, the former is hoped to produce PVC-like grafts on PE, while the latter a chlorinated hydrocarbon-like grafts and products (after a small scale decomposition reactions of stable species produced in plasma). Extensive characterization of plasma treated surfaces by ESCA is being conducted, but results are not yet complete.

5

4

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IC I. AtGi'S ::. 0 Al!.G7~

~ A8075

0 A4075'

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StraIn (CID,em)

Figure 18. Stress-strain curves of acetylene plasma treated LDPE-virgin PVC blends with 75% PE. For these blends, 2 pph heat stabilizer was added. Strain rate: 1.24 cm/min. (Reproduced from reference 26 with kind permission of J Wiley and Sons Ltd)

Figures 19 and 20 present mechanical test results for blends with 75 and 50 wt% modified and unmodified LDPE, respectively. From this group of results, it is seen that

182

improvement obtained in mechanical properties can be appreciable if blends prepared with and without modified PE are compared. The extend of improvement can be small (as in the case of carbon tetrachloride plasma) or large (for VCM plasma treatment and for addition of VCO). The modification is evidenced by increases in both strength and toughness C they are 65% and 225%; respectively (Figs. 19-22). These results show the efficiency of incorporation of a Cl-rich texture to the surface of PE by plasma, or by the use of a compatibilizer of similar nature in increasing the degrees of compatibility in these systems. The differences observed for VCO and carbon tetrachloride are explainable by the differences in efficiencies achieved in plasma as well as the differences in the products involved. Any variation in plasma operational parameters can lead to certain variations in the products, which can lead to differences in the mechanical properties (as for plasma modification at low watts and low flow rates CI075, to be compared with high watts and high flow rates, C4075 - see Fig. 19 below).

10

B II ~ ... 6 .. ... I': !'! • UOO7e .. w

4 ~ C1Q715 ~ • C3075 .. ..

C4075 • 0 ... 2 ~ V207l5

Q LOO(:>

+ uo015

0 0.00 0.10 0.20 0.30 OAO 0.50

stralrl (cm/em)

Figure 19. Stress-strain curves of Carbon Tetra Chloride plasma treated LDPE / VCO added virgin PVC blends with 75% PE ;Strain rate: 1.24

cm/min.(Reproducedjrom reference 26 with kind permission of J Wiley and Sons Ltd)

Figure 21 shows the effects of surface modification on the elongation at break values for 75 wt% PE blends. If there is a Cl-containing material (i.e., PVC) in the system, a proper chlorine-rich texture produced on PE (e.g., VCM plasma modification) increases the interactions in the blends, what leads to appreciable improvement of mechanical

183

properties. The increase is considered to be due to the dual characteristics of the PE surfaces, since it contain groups common to both polymers. In the case of direct use of VCO oligomer resulted in even more pronounced increases. The ultimate tensile strength values of same samples (Fig. 22) showed a parallel result: VCM plasma modification or use ofVCO led to high improvements.

6r-------------------------~

1i III.

5

~ 4

IS 4» .. ~ 2 w

" ': 2 ~

t. 0:3050 o C4060

... V2Q5(1

V L.OOSO

O~~--~-L~~~~~~~--~

0.00 0.05 0.10 0.15

Strain (em/em)

Figure 20. Stress-strain curves of Carbon tetrachloride and VCM plasma treated, VCO added chloride and VCM plasma treated, VCO added and untreated LDPE-PVC blends with 75 wt% PE and untreated LDPE-PVC blends with 50% PE .Strain rate: 1.24 em/min. (Reproduced from reference

26 with permission)

184

o.~Q (;~ 14.!.~ 21~ 413'X. '01'110

~ ().6(1

j 03)

'i Q..i<) .iii :;: ~

1 0.:20 l-ii 0.;0

000 .... 7.Q?~ t..OQ7~ .c t07., C307~ C4()7(i

Tr",tIttorlt lJp.

Figure 21. Elongation at break values versus type of treatment applied in plasma for LDPE-virgin pve blends with 75% PE

(Reproducedjrorn reference 26 with kind permission of J Wiley and Sons Ltd)

V?075 L0075 C1075 C3075 C407S

Tr.atmollt TJpe

Figure 22. Ultimate strength values versus type of treatment applied in plasma for LDPE-virgin pve blends with 75% PE. 75% PE blend from virgin components is 75% PE blend from virgin components is used as reference.

It is of interest to see whether there is any plasticization effect involved for these samples that may be due to the direct effect of either veo or low molecular weight plasma products. To check this, a series of DMA temperature scans were performed for all blend samples. As shown in Fig. 23, there is no plasticization effect; even for the veo case.

185

It should be noted that, although unmodified blends with 75% PE were prepared with certain ease, the blending and processing of modified counterparts of these samples were made even easier. In addition, the higher PVC containing blends with unmodified PE (i.e., 50 wt%) could not be processed into an acceptable film at all, while a modification of PE surfaces by plasma (or addition of veo to the system) always produced an easy to process blend.

The improvements obtained in mechanical properties result from improved interaction between the two polymers. As pointed out by Datta and Lohse [41], the changes in mechanical properties may not always be taken as an evidence for compatibilization. There is a complex correlation between the morphology and the physical properties of a blend, which is not completely understood. In the present case, the complications were minimized by using a standard processing procedure for all samples, and by keeping the amount of modifications at a minimum.

8~ ... ~": ~ lilt c:a:~u.[llltJl;i

~-I·«IX.rr4l~

rna : c: 1t.:tV!li W ....... I;';t"i Trar.;;uc. 'Nm'AI lllan D.lt: ... ~ .-~u-o" 0': :&II

-iitc4' ··-'-'··;j:;Q--·:I{.D-~-.-~-'r""-""--;.r.;---·\irl.. ...... b'!: "1~1I1"Jo"1J1't1·1l1

Figure 23. DMA curves of75% PE containing LDPE-PVe samples.(Reproducedfrom reference 26 with permission of J. Wiley and Sons Ltd.)

186

f :!! -25 E

G.

i = -50 Q

=-

-75

25 50 75 100

-100 1..-_.1--_-'--_-'-_-'

" LOPE

c uo

{;. C1

o C3

.. C4

• V2

• LO

Figure 24. Percent deviation from ideal case for elongation at break as a function of PE contents for the LDPE-PVC blends. (Reproduced from reference 26 with kind permission of J. Wiley and Sons Ltd.)

The effects of compatibilization may be expressed by using the concept of deviation from ideality [42]. The ideal condition is defined as the one that fit with that of the straight line connecting logarithm of performance of neat two homopolymers, viz. PE and PVC. This behavior ignores all synergistic and antagonistic effects. In Fig. 24 the logarithmic averages of ultimate tensile elongation is plotted as percentage deviation from the calculated ideality. As expected, the largest deviation is observed for the unmodified blends, and the smallest for either the VCM plasma modified or VCO added systems.

187

Summary

In polymer recycling of plastics packaging waste, polymer blends are characterized by inherent high degree of immiscibility. The properties can be greatly improved by compatibilization, following several different methods. In polymer recycling, the compatibilization can be achieved either using proper copolymers, adding functionalized copolymers, low molecular reactive agents or interfacial tension modifiers, as well as by plasma surface modification of the components. The effect of different compatibilizers or modifications on the polymer systems of PPIPS, PEIPS and PEIPVC were investigated by characterizing the phase morphologies, phase interactions and mechanical properties. Various compatibilizers used or modifications applied showed specific effects that differently influenced the properties. It was concluded that the degree of phase dispersion and the intensity of phase coupling are the main effects responsible for the mechanical performance.

188

5. References

1. Bauermeister, n, Maiburg, U., Huber, 1., Gutzer, Wirtschaftlichkeit und stofflich-okologischer Nutzwert rohstofflichen und energetischen Verfahren der PolymerwerkstofJe '94, Merseburg, Proceedings, 526

W., Vick, S. (1994), von werkstofflichen, Kunststoffverwertung,

2. Radusch, H.-J. (1997). Future Perspectives and Strategies of Polymer Recycling, NATO-AS/ on Polymer Recycling, Antalya 3. Radusch, H.-J., Marinow, S. (1986) Plaste und Kautschuk 33,269 4. Utracki, L. A., Melt Flow of Polyethylene Blends (1989), in: Multiphase Polymers and /onomers, L. A. Utracki, R A. Weiss (Eds.), American Chemical Society, Washington 5. Thomas, G. (1993) Recycle '93 Conference, Davos, 12/5 6. Brandrup, 1. (1995) Die Wiederverwertung von Kunststoffen, Hanser Verlag Miinchen, Wien 7. Vezzoli, A., Beretta, C. A., Lamperti, M. (1993) In: Recycling of Plastic Materials. Ed.: La Mantia, F. P. ChemTech Pub., Toronto 8. van Ness, K. E., Nosker, T. 1. (1992) In: Plastics Recycling: Products and Processes. Ed.: Ehring, R. 1. Hanser Pub., Miinchen 9. Starke, L., Funke, S., K'lzsch, N. (1992) KunststofJe 31,821

10. Krause, S. (1978) Compatibility in Polymer-Polymer Systems, in Paul, D. R., Newman, S., Polymer Blends Academic Press New York

11. Kolzsch, N. (1987) Untersuchungen zum Einsatz Grenzflachenaktiver Copolymere in PolyethylenIPolystyren-Mischungen, Thesis, Technical University Merseburg

12. Funke, Z. (1984) Untersuchungen zur Verbesserung der Mischbarkeit von Polymeren durch Anwendung Oberflachenaktiver Substanzen, Thesis, Technical University Merseburg

13. Fayt, R, Jerome, R Teyssie, P. (1986) A nge w. Makromol. Chern. 187,837 14. Ram, A., Narkis, M., Kost, 1. (1977) Polym. Eng. Sci. 17,274 15. Lemmens, 1. (1995) Vertraglichmacher fiihr Kunststoffe, in: Die Wiederverwertung von KunststofJen, 1. Brandrup et al. (Eds.), Hanser Verlag Miihnchen Wien

16. Akovali, G., AsIan S. (1993) J Appl. Polym. Sci. 50.1747 17. Brown, S. B. (1992) Reactive Extrusion, in: Xanthos, M. (Ed.) Reactive Extrusion, Principles and Practice, Hanser Pub., New York

18. Fayt, R., Jerome, R., Teyssie, P. (1989) Interface Modification in Polymer Blends, in: Multiphase Polymers: Blends and /onomers, L. A. Utracki, R. A. Weiss (Eds.), American Chemical Society, Washington 19. Gallot, B. (1978) Adv. Polym. Sci. 29, 85 20. Heikens, D., Hoen, N., Barentsen, W., Piet, P., Ladan, J. (1978) J Polym. Sci. Symp., 62, 309

189

21. Fayt, R., Jerome, R., Teyssie, P. (1981)J Polym. Sci. Polym. Lett. Ed. 19,79 22. Gaylord, N. G., Mehta, M., Kumar, V., Tazi, M. (1989), J Appl. Polym. Sci. 38,359 23. Gaylord, N. G. (1992) Reactive Extrusion in the Preparation of Carboxyl­Containing Polymers and their Utilization as Compatibilizing Agents, in: Xanthos, M. (Ed.) Reactive Extrusion, Principles and Practice, Hanser Publ. Munich 24. Hajian, M. (1984) Europ. Polym. J, 20,135 25. Van Ballegooie, P., Rudin, A. (1988) Polym. Eng. Sci. 28, 1434 26. Akovali, G., Torun T. T., Polymer International (1997).42.307. 27. Hohlfeld, R. W. (1086) US-Pat. 4,590,241, Dow 28. Saleem, M., Baker, W. E. (1990) J Appl. Polym. Sci. 39,655 29. Mashita, K., Fujii, T., Oomae, T. (1986) EPA, 177,151, Sumitomo Chemical 30. Ide, F., Hasegawa, A. (1974) J Appl. Polym. Sci. 18,963 31. Akovali, G., Torun T. T., Bayramli, E., Eriny N. K. (1998), Polymer. 39. 1363-1368

32. Funke, Z., Starke, L. (1992) Acta Polym., 43,21 33. Funke, Z., Starke, L. (1984) Wiss. Zt. TH Leuna-Merseburg, 26, 482 34. Tawney, P.O., Conger, R. P., Van Buskirk, P. R. (1960), Belg. Pat. 590534 35. De Benito, J. L., Ibarra, L., Gonzales, L. (1990) Kautsch. Gummi Kunstst., 43,697 36. Bisio, L. A. and Xanthos, M. (1994), How to Manage Plastics Waste- Technology and Market Opportunities, Hanser Pub., Cincinnati 37. Reiser, A., Leyshon, L .1., Johnston, (1971), Trans. Farad. Soc. 67,2389 38. Strobel, M., Lyons, C. S., Mittal, K. L.(1994) Plasma Surface Modification of Polymers: Relevance to Adhesion. VSP, Utrecht 39. Biederman, H., Osada, Y., (1992) Plasma Polymerization Processes, Elsevier, The Netherlands 40. Boenig, H. V.,(1982) ; Plasma Science and Technology, Cornell University Press, New York. 41. Datta, S., Lohse, J. D.,(1996); Polymeric Compatibilizers-Uses and Benefits in Polymer Blends, Hanser Pub., Cincinnati

MORPHOLOGY DEVELOPMENT DURING PROCESSING OF RECYCLED POLYMERS

HANS-JOACHIM RADUSCH Martin Luther University Halle- Wittenberg Institute of Materials Technology D-06099 Halle-Saale

1. Heterogeneous Polymer Blends from Plastics Waste

Plastics waste is found in refuse from industry and household. Its state varies from one source to the next. The scrap from polymer processing is often clean, whereas that from the building sites or municipal waste streams usually is contaminated. Depending on the type of waste and the technological possibilities, four methods of utilization of plastics waste are distinguished:

1. Mechanical recycling (re-melting) 2. Chemical recycling (decomposition into raw materials by

solvolysis, hydrogenation, thermolysis, etc.) 3. Energy recovery (pyrolysis) 4. Use in blast furnace as a carbon donor

In the first method the material is re-melted and recycled either as homopolymers or polymer mixtures, whereas in the methods 2 to 4 the polymeric character is lost. When mixed plastics are used, polymer blends with two or more components are formed. For this case serious problems can be encountered, due to the rheological and thermodynamic properties of the polymeric components.

The polymeric part of the household waste consists of more than 90 wt% of thermoplastics (mainly from plastics packaging) and less than 10% of thermosets. The composition of the household plastics waste and packaging plastics, respectively, from different countries is shown in Table 1. The data were gathered in a study ordered by the Association of Plastics Manufacturers in Europe (AP ME) [1] and in other investigations [2-7). The results show that polyolefins (PE, PP) have the highest proportion in the municipal waste. The other polymeric components present in larger quantities are

191

G. Akovali et al. (eds.), Frontiers in the Science and TechnologyoJPolymer Recycling, 191-211. © 1998 Kluwer Academic Publishers.

192

polystyrene (PS), polyvinylchloride (PVC) and polyethylene terephthalate (PET).

TABLE 1. Composition of household plastics waste (*) and packaging plastics (**) from different regions

Reference: f11 f11 f11 f21 f31 f41

Region EC NL* D** USA

Component wt% wt% wt% wt% wt% wt%

HDPE 21.1 19 28

LDPE I 39.7 36 31

PP 12.5 13.l 18 11

PS (+ 15 12 13 11.1 8 16

PVC 10 7.5 11 11.2 5 4.7

PET 5 9 6 0.7 12 11

Others '; 9 J.l 2

f51

J**

wt%

16.3

25.6

17.9

20.7

7.9

4.7

flQ

Shredding and re-melting is the easiest way to recycle clean, commingled plastics waste. However, the plastics waste from the municipal waste streams not only have to be cleaned, but they also should be separated into different fractions. Usually, after shredding the light and heavy fractions of polymer waste are separated by the floatation method. The light fraction contains polymeric foams, PP as well as high and low density polyethylenes. The latter polymers constitute the major portion of the fraction. The heavy fraction contains PVC, PS, PET, the thermosets and contaminants. If it is possible to separate polystyrene foam from the other components of the light fraction, the remaining mixture is made of po1yo1efins only. These resins (HDPE, LDPE, LLDPE, PP) are characterized by a certain chemical affinity but widely different rheological behavior and different crystallization tendencies.

The performance of polymer blends largely depend on the degree of dispersion. For example, mixtures of PO and PS are antagonistically immiscible. Their blends have coarse phase morphology and poor mechanical prop~rties [8]. However, the morphology and performance can be enhanced by a judicious selection of the compatibilization method.

2. Mechanisms of Morphology Development during Melt Mixing

2.1 Fundamentals of Morphology Formation

Morphology development during melt mixing of polymer blends is determined by: - Thermodynamic miscibility of the components - Relative rheology of the components - Mixing mechanisms - Technological mixing parameters

193

Thermodynamic miscibility is a function of the chemical nature or the macromolecules, their molecular parameters and concentration as well as the temperature, pressure and the stress level. In principle, molecular mixing is possible if the technological process provides a sufficiently high shear stress. During melt mixing of miscible polymers, the mutual diffusion enhance homogenization of the blend. However, polymer miscibility is rare, and the blends resulting from commingled plastics waste must be assumed immiscible. For immiscible systems the melt mixing results in limited degree of dispersion - interdiffusion does not take place. The phase morphology results from the dispersing and coalescing processes that depend on the rheological and thermodynamic properties of the system, as well as composition

The development of the phase morphology in heterogeneous polymer blends is complex. The thermodynamic miscibility of the components depends on the stress, temperature and concentration. First, a macroscopic mixing process must be carried out. The mixing process must include the size reduction and spatial redistribution regardless of whether real interfaces are involved or not. These processes are commonly referred as dispersive and distributive mixing, respectively. The dispersive mixing is a process of breaking up the dispersed phase by stresses acting at the interface. The stresses are transmitted through the matrix from a moving surface (e.g., a screw) and act against the cohesive forces when the dispersed phase is a solid (e.g., a filler) or the interfacial tension when it is a fluid (e.g., another melt). The mixing mechanisms in polymer melts can be divided into [9]:

- Mass convection - Laminar convection - Diffusion - Desagglomeration - Destruction

While mass convection, laminar convection and diffusion contribute to the distribution in the system, desagglomeration and destruction are the principal dispersive mechanisms. In practice, some of these mechanisms are acting together.

The aim of melt mixing of the plastics waste is development of an ideal coincidental

194

mixture of the dispersed polymer phase in a polymeric matrix. There are always two elementary steps: reduction of the size from the initial dimension to the level required for the optimization of performance, and uniform distribution of the resulting fragments in space. Note that the same processes are present during mixing of miscible or immiscible blends.

A distinguished significance has to be attached to the microrheological relations between the components. In immiscible polymer blends the rheological ratios are important criterion for the development of phase morphology. The theoretical and experimental studies have shown that the following relations and criterion are important for the development of morphology during melt mixing of polymer blends [10-15]:

a) The ratio of the viscosity of the dispersed (d) phase, 11d' and the continuous (c) phase,11 c:

(1)

b) The ratio of the melt elasticity of the dispersed phase, td, and the continuous phase tc:

(2)

c) The ratio between the local deforming stress acting on the dispersed drop, 11c r, and the iI}terfacial stress (r djd) (known as the capillarity number):

(3)

d) Relative duration of the deformation process usually expressed by the ratio of strain to the capillarity number, (r he).

e) Coalescence.

During the flow the interfacial tension coefficient is not constant. Its value depends on the total interfacial area as well as on the difference of the first normal stresses of the dispersed and continuous phase. The latter dependence was formulated by Van Oene for the deformation under simple shear stress [10]:

(4)

195

Mixing is more difficult when the difference between viscosities of the two phases is large. The drop deformation in shear flow is possible only for the viscosity ratios, Y'1 < 3.8. Under this condition deformation of drops into elongated prolate ellipsoids and breaking them into fine droplets is possible. Otherwise the drops can only deform - the resulting morphology is coarse. Figure I shows the basic forms of the deformation and

t -cp {ZJ,#f ~~- ~=2'10"

cp (lJ;1t' ~~ ~ = 0.7

cp (2J,#f ~ rg ... (i) ~ = 1

cp (2J -0 B ~=6

Figure 1. Deformation and break-up of drops for liquid mixtures having different viscosity ratios [17].

dispersion of drops as a function of the viscosity ratio. In the case of low viscosity dispersed phase a break-up of the deformed ellipsoids is observed. Formation of long fibrils and their disintegration (when the capillarity number value is reduced) corresponds to the capillarity instability mechanism described by Rayleigh [13], Tomotika, and more recently by Elmendorp [14] (see Figure 2).

The melt elasticity ratio is said to be important for the texture of the mixture and the shape of the immiscible domains. Furthermore, it is to be noted that the relaxation is time-dependent. A stable deformed droplet can only be obtained if the ratio of the characteristic relaxation time ratio of the dispersed and continuous phase is larger than one:

(5)

196

r·,· ',,9

r=::>Z§?1

~ ~

Figure 2. Break-up mechanism of threads in polymer blends with a low viscous dispersed phase.

L

In addition to the dispersion processes, these of coalescence must be taken into account. Both processes: dispersion and coalescence are simultaneous. The coalescence depends on the concentration of the dispersed phase, the mean drop size and the molecular mobility of the interface between the matrix and dispersed phase. The viscosity ratio, 8, is essential. Thus an increase of the matrix viscosity results in better dispersion since the coalescence is hindered. In the opposite case, the coalescence increases, and the effect is intensified by the normal stress effects. The drops moving in a capillary are also subjected to radially variable stresses, that create a concentration gradient over the capillary cross-section, what leads to enhanced coalescence in the middle of the strand. The number of collisions per unit volume and time can be expressed as [15]:

(6)

Studies of the morphology formation in thermally sensitive polymer blends have shown that the duration of the thermomechanical load is an additional factor [16]. The thermomechanically induced degradation often results in marked changes of the viscosity ratio and the interfacial tension coefficient, resulting in morphological changes.

2.2. Case Discussion

Many studies have been carried out to verify validity of Eqs (1 )-(3) [17-20]. One of the most important correlation is the dependency of the capillarity number, K, on the viscosity ratio, 1..11. This relationship is presented in Figure 3. Several authors found that deformation and burst of droplets is the easiest when viscosity ratio is: 0.1 < 1..11 < 1.5.

197

Different shape of the curves for the shear and elongational flow indicates that elongation is more effective than shear for the drop break-up. Especially for AT] > 3.8, the difference between the two types of flow is of paramount importance. Thus, at high viscosity ratios the dispersion is possible only in the extensional flow fields.

103 r-----------------------------------, -- Shear flow - - - Elongational flow

1 0-1 '--_J...-_J...-_J...-_-'--_-'--_-'--_-'-_-'-_~

10-6 10" 10-2 10° '1

Figure 3. Equilibrium drop size as a function of the viscosity ratio AT] and the capillarity number K.

Taking into consideration the dominant influence of the viscosity ratio, the following cases can be distinguished for the morphology formation:

a) The dispersed phase has the lower viscosity, A <1: The dispersed drops are deformed by the shear or elongational stresses. The deformation of the dispersed phase is the same as the matrix. When the viscosity ratio 8 is near 1, the drops preferentially deform into threads that upon the release of stresses disintegrate by the capillarity disintegration mechanism (see Fig. 2). The result is finely dispersed phase in the continuous matrix. As an example, in Figure 4 the morphology of a PBTIPC blend is shown. Here low viscosity PBT was melt mixed at 250/C with PC - the viscosity ratio was T]BPT / T]pc = 0.04. The detail in Figure 4A show threads of PBT that are broken-up into droplets and short fibers frayed out at the ends. Finely dispersed blend results from this process (Fig. 4B) [21].

b) The dispersed phase has higher viscosity, AT] > 3.8: Here the shear stresses cause flow of the matrix but are unable to break the dispersed viscous drops. The morphology remains coarse and the drops are in form of prolate ellipsoids aligned with the flow field. In this region of the viscosity ratios the

198

A B

Figure 4. Morphology formation in PBTIPC blends, where T]PBT < T]pc.

Optical micrograph in polarized light. (A): Detail of a PBTIPC (10/90) blend quenched during mixing. (B): Phase morphology of a PBTIPC (20/80) blend after mixing.

dispersion is only possible in the extens~Jnal flow field. As an example, in Fig. 5 PP/PS blend morphology is shown. The components were mixed at 190/C. Their

viscosity ratio was AT] = T]pg/T]pp > 1. Due to high surface tension the PS drops are nearly spherical. Because of the immiscibility, the PS spheres have poor contact with the PP matrix - they easily separate during specimen preparation for the electron microscopy.

c) The volume fraction of the dispersed phase and the matrix is about the same, and the viscosities are approximately equal:

In this case, the matrix and the dispersed phase are equally deformed. Because of the mass convective processes, the phase co-continuity (similar to the interpenetrating

199

networks) is obtained. An example of this morphology is shown in Fig. 6. If PP is mixed in the melt with uncrosslinked EPDM rUbber, and the volume fractions and viscosities of the components are similar, then the mixture shows a co-continuous phase morphology as illustrated in Fig. 6.

3. Morphology and Performance of HDPEILDPE Blends

The observations discussed on the preceded pages will be illustrated using the melt mixed blends of HDPE with LDPE (see Table 2). This system was selected as typical light fraction of the plastics waste stream. The necessity to mix different types of PE'S is one of the main problems in polymer recycling. The material parameters of PE and process variables determine the morphology and performance.

TABLE 2. Material parameters ofHDPE and LDPE

HDPE LDPE Parameter

Type A 61 FA. BUNA GmbH A 15 CC. LEUNA GmbH

DSC peak temperature 404 378 (K)

Density (kglm3) 954 919

Melt flow index (g/10 0.15 6,0 min)

3.1. Experimental

Blends containing HDPEILDPE = 50/50 were mixed using a laboratory roll mill, a single-screw extruder, and a twin-screw extruder. The mixing process was controlled by varying the screw speed, and temperature of the feeding, melting, and metering zones.

The quality of the extrudates was evaluated by means of the optical microscopy. The blend components were identified by selective melting using a thermo-optical microscopic method. The optical micrographs of thin cross sections were made after melting the LDPE phase at 120/C in a heating device coupled to a polarization microscope. The micrographs were evaluated photometrically. The calculated standard deviation s of the measured values of the light intensity I is a quantitative criterion of

200

Figure 5. SEM micrograph of PP blend with 30 wt"10 PS; 'lps > 'lPP

Figure 6. SEM micrograph of a PPIEPDM blend. <jlpp = <jlEPDM; 'lPP " 'lEPDM

201

the state of distribution and dispersity of the mixture:

(7)

A high s-value corresponds to a poor mixing. For a mixture with constant composition a maximum value of the standard deviation Smax at minimum mixing can be determined. To evaluate the mixing efficiency the determined s-values corresponding to the different process parameters were normalized by a linear transformation to the homogenization parameter Hm:

(8)

This parameter increases with improved dispersion and reaches value of 1 at the ideal stochastic distribution. The dispersive mixing effect was evaluated quantitatively by accounting for the shear deformation in the screw channel of the metering zone in a single screw extruder [18]. Morphology of the blends was studied using an optical microscope in polarized light. The same selective melting technology was applied as for evaluating the mixing quality. The specific inner surface, Sv was taken as the quantitative value characterizing the phase morphology. Its value was calculated using an image analysis:

Sv = 2X1L.z/xs (9)

The isothermal rheological measurements (at T = 190,205 and 220/C) were carried out in a capillary viscometer, HKV 7901, at shear rates varying from 10 to 2500 (lis). The constant-stress viscosities of the component polymers and their blends were determined from the primary flow curves 't = f( y). These were used to calculate the constant-stress viscosity ratios and capillarity numbers.

3.2. Results and Discussion

The mixing process of the partially crystalline polyethylenes can be discussed considering the melting behavior and composition of the mixture. In the extruder feed zone a mixture of pelletized polymers is fed, compressed and transported into the melting zone. Here the component with the lower melting point (LDPE) melts, forming a molten film around the HDPE granules. When the temperature becomes sufficiently high, the HDPE cores start melting. If the volume fraction of HDPE is smaller than that of LDPE, <1>HDPE < <1>WPE' initially relatively large HDPE droplets are formed. On the other hand, when <1>HDPE > <1>WPE a phase inversion must take place, so HDPE becomes a

202

matrix in which LDPE drops are dispersed. After melting of both components their rheological properties and concentration have

major influence on the dispersion and coalescence processes that lead to the final morphology. If LDPE with lower viscosity forms the dispersed phase, the drops are co­deformed with the HDPE matrix (see Figure 7 A). Under these conditions, the deformation, forming of threads, and dispersive mixing are at the easiest. By contrast, when the more viscous HDPE forms the dispersed phase, the initially large drops may only be deformed. They "swim" in the low viscosity LDPE matrix (see Fig. 7B), forming coarse dispersion .

• : PI P2

J!1 '-0

t -1 0

B: PI P2

r7T-...•... , .... 1J:2L

~ ........

'-10

V"IT.t'" v.,lT.r' a"l T.r' S a'21T.j'

'-0 :'1.,-'1'2-0

"" >"'2 '1"IT.t' < '1'21T·t' a"lT.r' < a ,2IT.t'

'-0: r,,- 'I., -0

Figure 7. Deformation behavior of the dispersed polymer melt HDPE(P2)ILDPE(Pl ):

(A) <I>WPE < <I>lIDPE and llwPE <lllIDPE; (B) <I>WPE><I>lIDPE and llwPE<lllIDPE

Thus, depending on the composition, the mixing efficiency can be quite different for the identical compounding conditions. The progress of the dispersion processes in a single­screw extruder as a function of shear strain is shown in Figure 8 and 9. The thermo­optical method of selective melting of thin cuts of the HDPEILDPE blends allowed to record these morphological changes. For their quantification the specific surface of the dispersed phase, Sv, was used. The variation of Sv as a function of the mixing intensity is shown in Fig. 9. The results correspond to the morphologies illustrated in Fig. 8. With the increasing shear strain, the dispersion and distribution processes increase, hence the specific surface of the dispersed phas increases and the morphology becomes finer. As

-- 20~m

Figure 8. Morphology of HOPEI LOPE blends as a function of mixing intensity (Numbers correspond to these in Figure 9)

203

204

shown in Figure 8/1, at low shear strain and short residence time in the extruder, the mixing is incomplete, and the morphology is coarse. Because of the selective melting of LDPE only the crystalline HDPE phase can be seen in the micrographs. Under the polarized light, molten LDPE and amorphous portions of the blends are dark. With increasing shear deformation the phase morphology becomes finer (Figures 8/4 to 8). However, it is not possible to dissolve the phase morphology completely, i.e., under the conditions used, molecular level mixing of these two polyethylenes was not possible.

" 13

mm" HOPE (A61)A.OPE (Al5) = 5G'5O 12

" 10

<n' 9

8

800 900 1000 noo 1200 1300 UIXI 1500 1600 1700 _ 1900 r

Figure. 9. Specific surface of dispersed HDPE phase as a function of shear strain y; HDPE / LDPE = 50:50. Topt = 120/C. (Numbers correspond to the morphological micrographs of Figure 8).

The mixing processes are controlled by the thermodynamic and rheological properties of components [18], as well as by the composition and the process variables (temperature, pressure, stress field, residence time, etc.). The HDPEILDPE viscosity ratios at different temperatures and different shear stress level are shown in Table 3 and Figure 10.

When HDPE is dispersed in LDPE, within the full range of temperature and shear stress the viscosity ratio ranges from

205

A= 5.5 to 50. For such large values of A the deformation of drops and break is not possible. The blend morphology is expected to be poor. The viscosity ratio is influenced by the temperature, but within the explored range the changes are insufficient to affect the dispersion process - only a coarse morphology is to be expected.

TABLE 3. Viscosity ratios of polyethylene blends

Viscosity ratio hd,LDPE Ihc,HDPE

s kPa T=463 K T=493K T=463 K T =493 K 10 50 38 0.02 0.02 50 10 8 0.1 0.125

100 83 57 0.12 0.175 200 8 1 55 0.124 0.19

100

10

1+463 K iff ~ +493K J '1 HDPfJlDPE f":::::::: .-.

-r 1 t-f-

,

===1 'llDPfJHDPE +463 ~ r +493K

0,1

I I II II I I 0 ,01 1 10 cr, kPa 100 1000

Figure 10. Influence of viscosity ratio, shear stress and temperature on the morphology ofHDPEILDPE blends.

206

Ae

••

...

. " . .,

...

...

.. . " 330 340 350 3110 370 3110 380 ~ 410 420 430

T.K

Figure 11. DSC melting curves ofHDPEILDPE blends, Vh = 2.5 Klmin.

In the second case, when HDPE is the matrix, in the whole range of shear stresses and temperatures the viscosity ratio y = 0.02 to 0.19. In this case the deformation of LDPE drops into thin threads is possible, followed by burst of these into small droplets. The mixing process is efficient, resulting in a fine phase morphology. Would the polymers be miscible, at sufficiently long residence time in the extruder, the diffusion processes could lead to the molecular level of mixing. However, since HDPE and LDPE are thermodynamically immiscible, the molecular level of mixing for the investigated system is not expected.

207

The phase micro-structure, also affected by the rheological properties and the mixing conditions, can be analyzed using the differential scanning calorimetry (DSC). The melting curves are shown in Figure 11. Blends with high HDPE content and low viscosity ratio are well homogenized and show only one melting peak, typical for HDPE. Crystallization from the homogeneous melt may allow the incorporation of LDPE segments into the HDPE lamellas, i.e., partial co-crystallization. Blends with high LDPE content show two or three peaks, corresponding to different crystalline phases. These blends have an adverse viscosity ratio and due to this they are not mix well - coarse HDPE drops are dispersed in the LDPE matrix. Co-crystallization may be take place only at the interface between the phases. This may the source of an additional peak in the DSC diagram. Another possible source of the third peak may be a bimodality of crystalline perfection at the interface. Thus, insufficient dispersion of the HDPE droplets, a stepwise concentration profile at the interface, and existence of the LDPE matrix are the reasons for the multiple DSC crystallization peaks in this concentration range ofLDPE/HDPE blends.

E :c

0,8

0,6

0,4

0,2

o 100

I

/ ~

1000

~ Io-"~

.;:sSE

.. ROLL

+TSE m y 10000 100000

Figure 12. Correlation between the quality of mixture H and the efficiency of the mixing equipment y.

From the technological point of view it is necessary to select the melt mixing method that would guarantee sufficient mixing. Beside the thermodynamic miscibility and the rheological properties of the components also the parameters of the mixing process (viz. geometry of the mixer, temperature, pressure, residence time within the active mixing

208

zone, etc.) influence the mixing efficiency. The material and technological parameters can be correlated in a diagram using the homogenization parameter Hm. and the total strain imposed during the mixing (a product of shear rate and mixing time ~):

y = y. ~ (10)

While Hm characterizes the homogeneity of the material, y describes the mixing efficiency. The dependence of the mixing degree Hm on the total strain is shown in Figure 12. The efficiency of mixing was compared for the three mixing units: roll mill, single-screw extruder and twin-screw extruder. Clearly the conventional single-screw extruder produces only low strain and limited mixing. Twin-screw extruder provided higher degree of homogeneity. This was due to higher total strain and longer residence time. In the roll mill the total strain can be extended independently of the rolls rotation speed, i.e., a good homogenization of the blends was possible. Similar relations are valid for other batch-type mixers, viz. melt kneaders. The relationships shown in Figure 12 are useful for a judicious selection of the mixing devices and optimization of the process variables.

4. Summary

The development of morphology during mixing of polymer blends is controlled by the rheological and thermodynamic behavior of the polymeric components. Characteristic ratios of material parameters are useful for the prediction of mixing behavior and resulting type of morphology in binary polymeric systems. These tools are equally applicable to blends from virgin polymers as these from plastics waste. The problem in the plastics waste recycling processes (especially of plastics waste from packaging or household refuse) is the lack of consistency in composition and types of polymers. Usually more than two components are involved in the mixed waste fractions smearing the relations.

As an example, a model polymer blend consisting of HDPE and LDPE was chosen. The mixing and morphology development were investigated on the basis of the rheological behavior of the components. The phase morphology and phase behavior were evaluated using microscopic and calorimetric methods. The results showed that mixing of low viscosity LDPE as the minor component into high viscosity HDPE leads to finer dispersion than vice versa. The results and methods of investigations used are transferable to other polymer systems and polymer blends.

209

5. Notation

HDPE high density polyethylene LDPE low density polyethylene LLDPE linear low density polyethylene PBT polybutylene terephthalate PC polycarbonate PP polypropylene PS polystyrene A Amplitude d particle diameter

H.n homogeneity I intensity K stress ratio L wave length N number R radius Nw N22 normal stresses s standard deviation Sv specific surface T temperature Vb heating rate ( shear strain

shear strain rate (G interfacial tension coefficient 0 shear viscosity 80 shear viscosity ratio 84 elasticity ratio F shear stress J relaxation time 4 melt elasticity N volume fraction

continuous phase

d disperse phase

210

6. References

1. N.N.: (1992) Information ofSEMA-Group for PWMI (European Centre for Plastics in the Environment). Brussels

2. Brandrup. J. (1991) Kunststoffe in der Verpackung. Kunststoffe. 81. 273 3. N.N. (1997) Kunststoff fur Verpackungszwecke in ltalien. Gurnrni. Fasern.

Kunststoffe. 50. 343 4. Liesener. R.N. (1995) Kunststoffrecycling in den USA. in Brandrup. J. (ed.) Die

Wiederverwertung von Kunststoffen. Hanser Verlag. MUnchen. 5. Iijima. R. (1995) Der gegenw@rtige Stand und die Zukunftsaussichten des

Kunststoffrecycling in Japan. in Brandrup. J. (ed.) Die Wiederverwertung von Kunststoffen. Hanser Verlag. MUnchen.

6. Vezzoli. A. Beretta. C.A. Lamperti. M. (1993). in La Mantia. F.P. (ed.) Recycling of Plastic Materials. ChemTech Pub. Toronto

7. van Ness. K.E. Nosker. T.J. (1992) in Ehring. R. J. (ed.) Plastics Recycling: Products and Processes. Hanser Pub. MUnchen

8. Starke. L. Funke. S. Kolzsch. N. (1992) Gemischte Kunststoffabfalle stofflich verwerten. Kunststoffe 82. 31

9. Mei8ner. K. Nolte. K. (1990) Zur Modellierung verarbeitungstechnischer Prozesse. Plaste und Kautschuk 29. 198

10. Van Oene. H. (1972) Modes of dispersion of viscoelastic fluids in flow. J. Colloid Interface Sci. 40. 448

11. Cox. R.G. (1969) The deformation of a drop in a general time-dependent fluid flow. J. Fluid Mech. 37

12. Lyngaae-Jrrgensen. J. (1981) Domain stability during capillary flow of well dispersed two phases polymer blends. ACS. Org. Coat. Plast. Chern. 15. 174

13. Rayleigh. J.W.S. (1892) Phil. Mag. 34. 145 14. Elmendorp. J.J. (1986) A study on polymer blending microrheology. PhD Thesis.

Technical University Delft

15. Smoluchowski. M.Z. (1917) Phys. Chern. 92.129 16. Radusch. H.-J. Hendrich. R. Michler. G. Naumann. I. (1992) Morpho­

logiebildungs- und Dispergiermechanismen bei der Herstellung und Verarbeitung von Polymerblends. Angew. Makrornol. Chern. 194. 159

17. Utracki. L.A. Shi. Z.H. (1992) Development of polymer blend morphology. Polyrn. Eng. Sci. 24. 1824

18. Rumscheidt. F.D. Mason. S.G. (1961) Particle motions in sheared suspensions.

211

J. Colloid Sci. 16.238 19. Torza. S. Cox. R.C. Mason. S.G. (1972)J. Colloidlnterf Sci. 38.395 20. Grace. H.P. (1982) Dispersion phenomena in high viscosity immiscible fluid

systems. Chern. Eng. Cornrnun. 14.225 21. Androsch. R. (1993). PhD Thesis. Technical University Merseburg 22. Radusch. H.-J. Marinow. S. (1986) Untersuchungen zum Schme1ze- mischen von

Po1yethylen hoher Dichte mit Polyethylen niedriger Dichte. Plaste und Kautschuk 33.269

23. Radusch. H.-J. (1984) Rheologische Eigenschaften von PE-HDIPE-ND­Mischungen. Plaste und Kautschuk. 31. 262

Chapter.3 REPROCESSING OF SINGLE TYPE POLYMERS

DERIVATION AND VALIDATION OF MODELS TO PREDICT THE PROPERTIES OF MIXTURES OF VIRGIN AND RECYCLED POLYMERS

C. A. BERNARDO

Department of Polymer Engineering Universidade do Minho. 4800 Guimariies, PORTUGAL

Abstract

The theoretical basis for the prediction of the properties of mixtures of virgin and reprocessed polymers is presented in this text. The theory is developed using degradation equations, that describe the loss of properties of the polymer as a function of the number of processing cycles. The single pass-property loss concept is also utilized to derive algorithms that can be applied to properties with quite complex decay behaviors. Various degradation equations, corresponding to important properties in materials characterization and quality control, such as melt flow index and tensile and impact strengths, are determined. The effect of reprocessing on the degradation of the fibers length, the molecular weight and the mechanical properties of reinforced thermoplastics is explained with the help of the algorithms derived.

1. Introduction

In the last few years the interest in the recycling of plastics has increased significantly, namely as a result of a growing conscience of their environmental impact. Accordingly, public authorities have put a great effort into the recovery of plastics waste, essentially from municipal (post-consumer), industrial and agricultural sources. Although it has not received a large attention, primary recycling, also known as reprocessing, has been one of the major ways ofre-utilizing plastics. Primary recycling is done in industry, using standard processing operations and homogeneous, non-contaminated, production-line scrap, to fabricate parts with properties similar to those of the products from which it was generated [1]. The economic importance of this type of recycling is evident, because raw-materials often comprise 80% of the cost of a fmished part, and scrap can be a significant fraction of its weight. For instance, in an injection molding operation, the sprue and runner system of a multi-cavity shot can represent more than 50% of the material used, although values in the order of 10 to 15% are more common.

In primary recycling the properties of the products must be maintained, and, for this, the entire process, including storing and handling, must be adequately controlled. However, the major reason for the loss of properties is the different degradation effects that can occur during the processing of polymers. Repeated processing can degrade both non-reinforced and reinforced polymers. This degradation, which is mostly thermo-oxidation, increases with temperature and shear, and leads to substantial alterations in the molecular structure. The more significant are: reduction in average molecular weight due to chain-scission, increase in

215

G. Akovali et al. (eds.), Frontiers in the Science and Technology a/Polymer Recycling, 215-247. © 1998 Kluwer Academic Publishers.

216

molecular weight by cross-linking, fonnation of unsaturations by thennal or chemical attack, and cyclizations by side-chain reactions.

In some polymers, volatile degradation products can themselves cause new degradation reactions, as is the case of Hel released in the processing of poly (vinyl chloride). These alterations are manifested by changes in the viscosity of the polymer melt, and by variations of the color and the mechanical properties. Other properties of the material, such as dimensional stability and fIre retardancy, can also be profoundly affected by reprocessing. The dependencies of two of these properties, notched impact strength and melt flow index, on the number of cycles (that is, the number of times the material is granulated and processed) are presented in Fig. 1 for four different polymers. These dependencies are nonnally known as degradation curves, as no virgin polymer is added in each new cycle. It can be observed that the curves vary with the nature of the polymer, indicating that different degradation mechanisms may occur in each case.

o a.. -c: a..

.

-~ PVC - impact strength I

I

I

•

O.4~

O.2~ -­

I

I PC - imp~ct strength

O.O~---!,r----+--,--,-+--t----! o 2 4 6 8 10 12

Number of cycles

Figure 1. Schematic dependence of the impact strength and melt flow index on the number of cycles (adapted from reference [1], and also based on results from this work).

Most of the published studies of primary recycling of thennoplastics have dealt with the effect of reprocessing on the mechanical properties. Various polymers have been

217

investigated, namely, glass reinforced nylon 6.6 [2,3], reinforced and unreinforced polycarbonate [4-10] acrylonitrile-butadiene-styrene and polybutylene terephtalate [9], high­impact polystyrene [11], polyphenylene sulphide [12], polypropylene [10, 13], and polycarbonate/ABS [14] and polycarbonate/polysulphone [15] blends. Although the number of studies is significant, the available information is still insufficient. In fact, for these studies to be of practical importance to the processor, they should include algorithms that allow the prediction of the loss of properties after a given number of reprocessing steps. Furthermore, in primary recycling virgin polymer is normally added to the scrap. Hence, the algorithms must be able to compute the minimum amount of virgin polymer that must be added, to obtain products without significant loss of properties. They should also be able to predict the value of the property after a very large number of reprocessing cycles (steady­state property). Some researchers have derived these algorithms [4, 16-18] and a few used experimental data to validate them [4, 7, 16]. Recently, in a series of publications, the author and co-workers presented a theoretical analysis of reprocessing, and used the derived algorithms to interpret the experimental degradation curves [19, 20, 21]. However, a complete theoretical analysis of the most important situations that can occur in primary recycling has not been presented yet.

The present text analyses, in a systematic way, most of the situations that can occur in primary recycling and are pertinent to the prediction of the properties of mixtures of virgin and reprocessed polymers. The algorithms are applicable to properties that can and cannot be defmed without one processing step. Experimental information on the effect of reprocessing on a number of properties of general use and engineering thermoplastics is utilized to validate these algorithms. The combination of theoretical and experimental information allows some insight on the nature of the degradation mechanisms.

2. Derivation of prediction models

The dependence of the properties of the reprocessed polymer (regrind) on the number of cycles can be worked out from diagrams such as the one shown in Fig. 2. The figure represents a continuous operation of plastics processing, such as injection molding, incorporating a recycling/granulation step.

In Fig. 2, R, V, F and 0 represent, respectively, the reprocessed, the virgin, the feed

and the output material streams, PI' Po' Pn and P: being the corresponding properties. The

fraction of reprocessed and virgin material are defined by the ratios: r = RIF and k = V/F, related through the material balance: F = R + V. The values of the property after the first, second, ith and nth processing, when no virgin polymer is added (k = 0), are represented by PI, P2, ... , Pi, ... , Pn. It should be noticed that the degradation curves, like those shown in

Fig.l, are obtained for k = O. As stated before, these curves, are of great importance in reprocessing studies.

218

,--____ ---, RepIDC88Sed

.------t~l Granulation : PDlymer (P,)

R (I = R/F)

Output (P.*) I L Feed (F' J • Virgin pDlymel (P.) .... -----1 Processing.... ....

O=F I I F=R+V V(lc=VlFl

Figure 2. Diagrammatic representation of the primary recycling of a polymer.

Reprinted from "The recycling of thennoplastics: prediction of the properties of mixtures of virgin and reprocessed polymers" Bernardo, C. A., Cunha, A. M. and Oliveira, M. 1., Polym. Eng. Sci. 36, 511-519 © 1996 Society of Plastics Engineers

In the following derivation two simplifying hypotheses are assumed. The fIrst, is

that granulation does not affect the properties of the material (and therefore Pr = P: ). As it will be shown ahead, this hypothesis is substantially true, even in the case of glass reinforced polypropylene and polycarbonate, where it could be expected that the length of the fIbers would diminish with the granulation [18]. The second is that the fraction of regrind (and, hence, also the fraction of virgin material, as r + k = 1) is always constant for the same reprocessing sequence. The main objective is to obtain equations that allow the property loss to be calculated as a function of the number of cycles and the fraction of regrind. Additionally, it is also important to predict the value of the property when the number of cycles is very large (approaches infmity). The corresponding equation is called a steady-state algorithm.

As referred to before, in the present text the word cycle means a sequence of operations that includes mixture of polymers in the feed, processing, separation of sprues and runner system, and granulation. The derivation that will be presented next corresponds to a process, like injection molding, in which the regrind from a given cycle is identifIed and stored separately and is only processed when all the regrind from the previous cycle has been used. A linear or a logarithmic law will be assumed to explain the properties of mixtures of the reprocessed and virgin polymer.

2.1 LINEAR LAW OF MIXTURES

According to the linear law of mixtures, the property in the feed stream containing the regrind of the fIrst processing will be:

PI = kPO+ rPI (1)

asr=l-k,

219

PI = kPO +(I-k)PI (2)

For the feed stream of the next cycle:

Repeating the process over n cycles, P n, the value of the property of the feed stream that contains the regrind of the nth processing (e.g., at the inlet of the (n+l)th cycle), will be:

n-I Pn = kL{l-k)i Pi +(I-k)n pn (4)

i = 0

This equation can be directly applied to properties, such as the melt flow index or the fiber's length, that can be determined in the virgin polymer, hence in the feed stream of the first cycle. However, in the case of other properties, such as the tensile or the impact

strengths, that can only be measured after processing to produce test pieces, Po cannot be determined a priori. Thus, admitting that the value of the properties of the components of the

* mixture are additive at the outlet of the processing step, and as PI coincides with PI:

P; = k PI + (1- k)PI

p. 3 k PI + k (1- k )P2 + (1- k)2 P3

and, finally: n-I

P; = k L(1- k)i-IPi + (1- ky-I Pn i=1

P• where n is the value of the property at the outlet of the nth processing step.

(Ia)

(2a)

(3a)

(4a)

The value of Pn or P; can be calculated substituting Pi by their experimental

values taken from the degradation curves. However, if the analytical representation of these

curves is known, it is possible to obtain Pn or P; directly.

2.1.1 Linear property loss

When the property of the polymer varies linearly with the number of processing cycles (e.g., the case of poly carbonate in Fig. I):

(5) thus:

Pn n-I i n k .~ (1- k) (I - ti) + (I - k) (1 - tn)

1=0 (6)

220

Equation (6) allows the calculation of P n for any number of cycles, if t and k are known. Since it is possible to obtain a closed-solution form of the sum, this equation is equivalent to:

[ n+\] k - t (I - k) - (\ - k)

k (7)

Equation (7) is easier to utilize and allows the determination of the steady state algorithm, when n ~ 00

P 00 k - t (\ - k) (8)

Po k

Similarly, when the property can only be measured at the outlet stream of the fIrst processing and decays linearly:

then, from equation (4)

n-\ i-I n \ k .L (1 - k) [\ - t (i - \)] + (i - k) - [I - t(n - \)]

1=\

which leads to:

P n k - t [(1 - k) - (1 _ k) n ]

PI k

In this case, the steady state algorithm is:

k-t (l-k)

k

(5a)

(6a)

(7a)

(8a)

As is to be expected, for large values ofn, equations (8) and (8a) are formally identical.

2.1.2 Exponential decay property loss

If the property loss follows decays exponentially:

with b >0, (9)

then, from equation (4), doing the appropriate mathematical manipulations, it can be concluded that:

and, when n ~ 00

k + (1- k)n+l (e-bn _ e-b(n+l))

1- (1- k) e-b

k

1- (1- k) e -b

221

(10)

(11)

The exponential decay is a particular case of a proportional property loss, such as:

with 0 < d < 1 (12)

From equation (12), equations identical to (10) and (11) are obtained if e-b is replaced by d.

The property that decays exponentially to an asymptotic value (as, for example, the impact strength of reinforced nylon in Fig. 1), has a great practical importance. In this case

(13)

where Pais the asymptotic value, and ao is the difference between the original and the

asymptotic value of the property:

or

then, doing a change in variables, such as:

Pi - Pa =~Pi

and, as ~P 0 (which is equivalent to Po - P a ) coincides with ao, equation (13) becomes:

-bi bP. = bPo e

1

(14)

(15)

(16)

(17)

Then, after substituting (17) in equation (4), and doing the appropriate mathematical manipulations, an expression similar to equation (10) can be obtained:

8P n P n - Pa

8 Po Po - Pa

n+1 -bn -b(n+l) k + (1 - k) (e - e )

-b 1 - (I - k) e

(18)

From equation (18) it can be easily concluded that the value of the property in the feed stream at the inlet of the (n+ l)th cycle will be given by:

222

Pn n+1 -bn -b(n+l)

a 0 k + (1- k) (e - e )

p -b o 1 - (1- k) e

(19)

Identically to (8), the steady-state algorithm will be:

P", = Pa + ao k P P P · -b

o 0 0 1 - (I - k) e (20)

Repeating the derivation for the properties that can only be determined after one processing cycle, and for an exponential decay, the relative value of the property at the outlet of the nth processing will be:

and when n -+ 00

n -b(n-I) -bn k + (I - k) (e - e )

-b 1 - (I - k) e

k

-b 1- (1- k) e

When the property decays exponentially to an asymptotic value:

(lOa)

(lla)

(13a)

where a l is now the difference between the of the value of the property after one processing and the asymptotic value:

and

Then, the et::;ations corresponding to (19) and (20) become:

n -b(n-l) -bn a l k + (1 - k) (e - e ) -- -b PI 1 - (1- k) e

-b PI 1- (1- k) e

(14a)

(19a)

(20a)

As in the case of linear decay, for large values of n, equations (II) and (20) are formally identical to equations (1Ia) and (20a), respectively.

223

2. 2 LOGARITHMIC LAW OF MIXTURES

If the mixture obeys a logarithmic law, the property value at the feed stream of the ftrst cycle, that contains the regrind of the ftrst processing, will be:

(21)

then, repeating steps (2) through (4):

{O-I } P n = P~ n pik(l-k)i p~l-k)n 1=1

(22)

or, in the case of the property only being determinable after one processing,

(22a)

The value of Po or P: can again be calculated substituting Pi by their experimental

values taken from degradation curves. For brevity, only the analytical representation of the

degradation curves with greater practical interest will be used in this section to obtain Po and

P: directly.

2.2.1 Exponential decay property loss

If the property decays according to an exponential law, then again

P P -bi i = 0 e with b >0,

then, substituting in equation (22):

Pn k -b k(l-k) -2b k(l_k)2 -(n-l)b k(l-k)n-l -nb (l_k)n - = -.Po (Poe) (Poe) ... (Poe ) (Poe) (23) Po Po

or

: n = [ Po -1+k+k(l-k)+k(1-k)2+ .... +k(1-k)n-l +(I-k)n ] .

o 2 3 k n-l ( k)n .[e -b t(1-k)+2k(1-k) +3k(1-k) +··+(n-l)k(I-) +n 1- (24)

and, as

2 n-l n -1+k+k(k-l)+k(l-k) +···+k(l-k) +(1-k) =

224

then:

but:

[ 2 n-I] n = -I + k I + (1- k) + (1- k) + ... +(1- k) + (1- k) =

n n-I n n I - (1- k) n = - I + k L (1- k) + (1- k) = - I + k + (1- k) = 0,

i=O I-(l-k)

- n-I inn-I i n Pn [-b].~ ik (I-k) +n (I-k) [-b].~ i(l-r) r +n r - = e I-I = e I-I Po

n-l inn. .L i(1 - r) r + n r = L rl 1=1 i=1

n+1 r-r

1- r

(25)

The relative value of the property of the feed stream that contains the regrind of the nth processing (e.g., at the inlet of the (n+l)th cycle), will then be:

- n i ~~~+~ :: = {e -b )i~{ or :: = (e -b) I -r (26)

and when n ~ ex) , the steady-state algorithm will be:

r

:: = {e-b )l-r (27)

When the property cannot be defmed without a processing step, then using again equation (9) and substituting in equation (22a):

* k -b k (I-k) -2b k(l_k)2 P = P (PI e) (PI e) + .....

n

n-2 n-I -(n-2)b k(l-k) -b(n-I) (I-k) ..... + (PI e (PI e )

or

* :: = [PI-I+k+k(l-k)+k(l-k)2+k(l-k)3+ ... +k(l-k)n-2+(I-k)n-1 ] .

. [e -b ] k(l-k)+2k(l-k)2 + ..... +(n_2)k(l_k)n-2 +(n-I xl-kl-1

and, as the fIrst term is again equal to 1:

(23a)

(24a)

fmally:

* Pn -b k(l-k)+2k(l-b)2+ ... + (n-2)k(l-k)n-2 +(n-l)(l-k)n-l -=(e ) PI

n-2· 1 n-2· 1

[ -b].L ik(l-k)I+(n-I)(I-k)n- [-b].L i(l-r)r l +(n-l)rn-= e 1=1 = e 1=1

And, when n ~ OC!

n-l . L rl

-b i=1 (e ) or

L -b l-r

(e )

~ -b l-r

(e )

225

(2Sa)

(26a)

(27a)

When the property decays exponentially to an asymptotic value, the degradation -bi

equation will again be Pi = ao e + Pa . Hence, doing the same change in variables as in

the case of the linear law of mixtures, an equation identical to (17) will be obtained: -bi

Pi = Po e . After substituting this expression in equation (22), and doing the appropriate

mathematical manipulations:

n . '" rl (-b) ..... = e 1=1 = (28)

Finally, the value of the property in the feed stream at the inlet of the (n+l)th cycle will be:

or (29)

Identically to (20), the steady-state algorithm will be:

r

Poo = ~+ ~(e-b)l-r Po Po Po

(30)

Repeating the derivation for the properties that can only be determined after one processing cycle, and for an exponential decay:

226

n-I i

( -b).L r e 1=1 (28a) API PI - Pa

and the value of the property in the outlet of the nth cycle will be given by:

(29a)

When n ..... 00, the steady-state algorithm will be:

P"" p. -= -+ (30a) PI PI

which is formally identical to equation (30).

2.3 UNIFIED EQUATIONS FOR THE INLET AND OUTLET PROPERTIES

The theory presented herein allows the prediction of the value of all the properties relevant to primary recycling of thermoplastics, provided their degradation curves are known. This methodology can be easily extended to technologies other than injection molding, like extrusion, using the same equations and adapting the production protocol.

The complexity arising from the duplication of some of the equations, when the property cannot be determined directly from the virgin polymer, can be obviated by a change of variables. In this case, n' will be defmed as the number of reprocessing cycles and

Po' as the value of the property at the outlet of the n'th reprocessing cycle. Hence, n' is equal •

to the number of cycles minus one. Then, recalling that P D is the value of the property at the outlet of the nth processing:

• n = n'+ I, Po = Po'+ I = PD' and PI = Po

In this way, equations (xx a) will be formally identical to (xx) if they are expressed in terms ofn'. For example, (7a) becomes:

, k _ t [(I _ k) _ (I _ t) n'+ I] Po' _ _ _

, -Po k

(7a')

Identical expressions can be obtained for all the other equations.

2.4 SINGLE-PASS PROPERTY LOSS

An alternative methodology for the determination of the fmal product properties was proposed by Throne [18] using a single-pass property loss and the two mentioned laws of mixtures. The major advantage of this methodology is to allow the determination of

227 ...

analytical expressions for P and P"" in the case of complex equations for the property loss n and the law of mixtures. However, as it is not based on the determination of degradation curves, it is difficult to use it to interpret the nature of the degradation mechanisms. We will use this methodology to derive equations with great practical interest. Additionally, the usefulness of the equations will be further increased by a modification that enables their application to properties that can only be determined after one processing operation.

According to Throne [18], the protocol to follow is:

1. Determine the value of P; from Po using single-pass property value loss;

2. Obtain the value of the property at the inlet of the second processing, PI, using the chosen law of mixtures;

3. Determine the value of P; from p), using the single-pass property value loss;

4. Repeat the process for several cycles until a proper series form for the nth cycle can be found;

5. Obtain a closed-solution form of the series, leading to an equation for P n ;

6. Determine the steady-state algorithm, as the number of cycles approaches infmity.

The algorithm derived in this work considers a power law property loss and a logarithmic law of mixtures. Thus, in this case:

• -z Pn = c Pn-l (31)

where c and z are positive real numbers. The value of the property after the frrst processing (in which no regrind is added at the inlet) is given by:

or, making c. n z-l c ro

pt = cPO = cP6 (32)

(33)

If a logarithmic law of mixtures is applicable to the property under study, its mean value at the inlet of the second processing will be:

(34)

and,asr+k=l:

(35)

and, applying again the power law at the outlet of the second processing:

228

[ ] z [ (I)] rz+1 rz2_rz+z c c~ Po = c c rz P:: z- P; = c Po

[ rz+1 (rz+l) (Z-I)] rz+1 = c Po Po = c. Po

(36)

and, at the inlet of the third processing:

-p (p*)r pk (Crz+1p)r pl-r = Cr(rz+l) p 2= 2 0= * 0 0 * 0 (37)

Identically, at the outlet of the third processing, applying the power law property loss again:

(38)

and, at the inlet of the fourth processing:

- • r k [(rz)2+rz+1 ]r I-r r3i+r2z+r r. [(rz)2+rz+1] P3 = (P3 ) Po = c. Po Po = c. Po = c. Po (39)

Repeating the derivation for n cycles, the value of the property at the inlet of the (n+ l)th processing will be:

n n-I r L (rz)

p = c* i = 1 n

.P o

as it is possible to obtain a closed-fonn solution for the sum, then, with r = 1 - k:

1- rz .P o

At steady-state, when n ~ 00 , for rz > 0:

(1- k) - [(1- k) z]n 1- (1- k) z

~ 1- (l-k)z

or P",=c. P o

P o

(40)

(41)

(42)

When the properties can only be measured after one processing operation to obtain test pieces, Po cannot be detennined a priori. In this case, the above methodology can still be

applied, admitting that the logarithms of the values of the properties of the components of the mixture are additive at the outlet of each processing operation. This implies that the single­pass property value loss should be applied to each individual component of the mixture Pn:

p = c P~-I n

(32a)

229

It should be noticed that the counters of the individual components increase one unit in each passage through the processing equipment, that is, PI becomes P2, P2 becomes

* P3 ' etc. Then, as PI = PI, after mixing the polymer processed one time with virgin

polymer, and processing the second time, the value of the property at the outlet will be:

P; = Pz pl-r = (cPifptr

or, making c®

* P2 = c@ PI

Similarly, at the outlet of the third processing:

P* _ [pr pl_r]r pl-r _ pr2pr-r2p l_r _ 3- 32 I - 3 2 1-

2 2 2 = (C®+IPlt (c®Plt-r Ptr = c® +r PI = c~zr+l) PI

and, at the outlet of the fourth processing:

P; = [(PJ pj-r)r pi-r r pl-r = c~z2r2+zr+1) PI

Repeating the derivation, at the outlet of the nth processing is:

r f(rz)n-2 * i=O

Pn =C® PI

as it is again possible to obtain a closed solution for the sum, then, with r = 1 - k:

n-I I-(rz) * r """------""

Pn = c® I-rz PI

At steady-state, for rz >0:

or

or

l-[(l-k)zr-J (I-k) ~---~"-

P _ l-(l-k)z n - c®

(36a)

(38a)

(40a)

(4Ia)

(42a)

As is to be expected, the steady-state equations (42) and (42a) are formally identical, as found in all the previous cases.

Depending on the combination of the parameters k, c* or c® and z, the variation of

the property value with the number of cycles, as given by equations (41) or (41a), follows distinct trends. Five main behaviors can be considered, as shown in Fig. 3 a) to e). These behaviors cover most of the observed dependencies of the properties of mixtures of virgin and recycled polymers on the number of processing cycles. The figure corresponds to equation (41), but a similar behavior is observed when equation (41a) is used with the same combination of the above parameters.

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For c. > 1, and independently of the value ofk, PnlPO is always >1, increasing with n

to infmity, if (1- k)z > 1 (Fig. 3a), or to an asymptotic value if (1- k)z < 1 (Fig. 3b). In the

general case, Pn/PO decreases with n. This decrement is described by the equations when c* <1.

However, depending on the value of (1- k)z, different behaviors are observed. If that value is smaller than 1, the decrease is towards a positive asymptotic value (Fig. 3c). When (1 - k) z is greater than 1, the decrease is towards zero, although two distinct patterns are

possible: exponential (Fig. 3d), or showing an inflection point (Fig. 3e), depending on ll1-kLz (J-k)z

being smaller or greater than In c* ' respectively.

As will be seen next, these distinct behaviors are quite useful to describe complex degradation patterns, namely when the values of the property increase with reprocessing instead of diminishing, as is usually the case.

3. Experimental

3.1 MATERIALS

Table 1 shows the relevant properties of the polymers used in the experiments to validate the theoretical models. A low-density polyethylene, Lusotene JLB 1004, an high-density polyethylene, Sinelene JJQ 2103, and a polypropylene, Garblene JEG 3101, supplied by Borealis and a polypropylene reinforced with 20% of glass fibers, Propathene HW60 GR20, obtained from leI were studied in this work. Two grades of polycarbonate, non reinforced -Xantar 22R (PC) - and 20% glass fibers reinforced - Xantar G24R (RPC) - supplied by DSM were also studied. These two grades had the same matrix in terms of molecular weight and additives.

TABLE I. Reference properties of the polymers studied

Polymer

LDPE

HDPE

PP

GRPP

Xantar 22R

MFI Mw (g/600s) (g/mol)

21.3

17.2

4.2

3.0

2.37 26000

Xantar G24R 1.06 26000

Fiber length, Yield stress Lw (rrun) (MPa)

0.11

0.36

10

12

34

70

56.8

104.2

Strain at break (%)

77.9

2.86

Modulus (GPa)

2.44

6.20

Impact strength

(kJ/m2)

5

6.3

18.9

7.21

232

In Table 1 the reference properties were measured, either in the granule form or, when the property could not be measured in that form, after the fIrst molding operation.

3.2 INJECTION MOULDING AND REPROCESSING

ISO tensile and impact test pieces were molded with all the polymers studied. To avoid the deleterious effect of humidity on degradation [4,5], the materials were dehumidifIed (drying air with a dew point of - 40°C) before each molding stage at 120°C during 4 hours. Sequences of a minimum of 5 to a maximum of 12 processing cycles were carried out in a Krauss Maffei KM60-120A injection molding machine. The melt and the mold temperatures were those indicated by the manufacturers as the more appropriate for the different polymers. The injection pressure was always lower than 120 MPa, and the holding and back pressures were set at 70 MPa and the at 7 MPa, respectively. These molding conditions were determined in a previous study as those that caused the minimum damage to the glass fIbers.

In each cycle 15 tensile and 15 impact specimens were sampled for testing and the remaining grinded in a Colortronic M 200L granulator and fed back into the injection molding machine. Recycling sequences were performed using different fractions of virgin and recycled polymer, one without addition of virgin material (k = 0) and others with the addition of 10, 30 and 50 % (w/w) of virgin polymer in each cycle (k = 0.1, 0.3 and 0.5, respectively). In each sequence, the value of k was kept always constant.

3.3 TESTING

The following techniques were used to assess the effect of reprocessing on the degradation of the materials:

Melt flow measurements were performed according to ASTM D-1238-86 with an extrusion plastometer Davenport model 3.

The molecular weight (Mw) of the polycarbonate was measured with a Waters 150 CV Gel Permeation Chromatography apparatus. The samples were dissolved in tetrahydrofuran and, for the reinforced grade, the fIbers were removed from the solution by fIltration prior to the injection. During the tests the temperature was maintained at 35°C and polystyrene standards were used for the calibration.

The fIber length measurements were carried out with an Olympus optical microscope connected to a Leica Quantimet 500 image analyzer. The ground polymers were pyrolised at 500 °C directly onto glass slides, spread over a slide with an immersion liquid and covered with another slide. The average fIber length (L,.)was determined from a minimum of 2000 measurements. The effect of the grinding on the fIber breakage was assessed by comparing the fIber length before and after granulation in the frrst cycle.

Tensile tests were performed, according to ISO 179: 1993, in a Instron 1122 testing machine at standard conditions of temperature and humidity. From these tests the stress at maximum load, the strain at break and the secant modulus at 2 % were determined.

233

Charpy impact tests were done, according to ISOIR 527:1996, at room temperature, in a

Rosand IFW 5 instrumented impact machine at a velocity of 3 ID.S -1.

4. Validation of the prediction models

The degradation curves determined for the melt flow indexes of all the polyolefins studied are presented in Fig. 4.

It can be observed that the MFI of the high density and the low density polyethylene decreases with the number of cycles whilst that of the two grades of polypropylene increases.

LDPE

o o 2 4 6 8 10 12

Number of cycles

Figure 4. Dependence of the MFI of the polyolefms on the number of cycles.

234

It is interesting to compare the results for the reinforced and non-reinforced polypropylene (k=O). It can be observed that the MFI of the former increases much more rapidly with the number of reprocessing. A similar trend was observed for reinforced and non-reinforced polycarbonate in the present and other studies [6]. Although the MFI of polyethylene decreases with reprocessing, it can be observed that in the frrst cycle the reduction is very small. A similar initial effect has already been reported for polycarbonate and was considered to be an induction period [16]. In our case, a more likely explanation is the simultaneous occurrence of two competitive degradation mechanisms, one leading to an increase in MFI (breakage of molecular chains) and the other causing its decrease (reticulation). Apparently, the rate of the frrst of these mechanisms decreases markedly after an initial attack to the active molecular centers, and becomes slower than the reticulation.

The MFI of mixtures of virgin and recycled polypropylene (k=O.I, k=O.3) were also determined and are presented in Fig. 5 together with the degradation curve (k=O).

3.0.--------------------,

"i 2.0 :E

pp

k=O

1.0 4-....I.--+~-I__-'--+_-_+----'-+_-'-__l

o 2 4 6 8 10 12

Number of cycles

Figure 5. Dependence of the MFI of polypropylene on n for different k values

Reprinted from "The recycling of thermoplastics: prediction of the properties of mixtures of virgin and reprocessed polymers" Bernardo, C. A., Cunha, A. M. and Oliveira, M. J., Polym. Eng. Sci. 36, 511-519 © 1996 Society of Plastics Engineers

235

The results presented in Fig. 5 were used to evaluate the theoretical models derived in section 2. The equations were fitted to the experimental data with a linear optimization program, that uses the least square method and the Hook-Jeeves algorithm [22]. The sum of the least square deviations were taken as the criteria for the applicability of the models. Only the algorithms that led to the best fittings are presented in the text.

The results of the fitting of equations (10), (19) and (41), designated, respectively, by models 1,2 and 3, are presented in Table 2. The least square deviations show that models 1 and 2 are adequate and practically equivalent, and that model 3 is not as good. Modell, that involves only one parameter, was selected to draw the theoretical curves presented in Fig. 5. As the fitting was made simultaneously to all the experimental data for the 3 values of k, the value of the parameter b (0.116) can be considered as representative of the model.

TABLE 2. Fitting of theoretical models to experimental data (MFI of polypropylene)

Model

I - Exponential decay, linear law of mixtures

2 - Exponential decay to an asympto­tic value, linear law of mixtures

3 - Power law loss, logarithmic law of mixtures

Least square deviations

1.61 x 10-2

1.48 x 10-2

7.40 x 10-2

Parameters

b= 0.116

ao b=0.132; Po =0.922

c. = 0.884 Igll ° minl-O.02; z = 1.02

The theoretical curves presented in Fig. 4 were also obtained in the same way. In this case, models 2 and 3 gave the best fit to the data of the two polypropylenes and the two polyethylenes, respectively. The sum of the least square deviations and parameters obtained for the best models are presented in Table 3.

TABLE 3. Fitting of the theoretical models to the MFI data of the polyolefins (k = 0)

Material Model Least square deviations Parameters

LDPE 3 5.02 x 10-3 c. = 1.27 I gil ° minIO.II ; z = 0.89

HDPE 3 1.24 x 10-3 c. = 0.9841g/10 minIO.OI ; z = 0.99

PP 2 1.l4x 10-3 b = 0.118; a o - = 0.987 Po

2 1.03 x 10-3 b = 0.204; a o

GRPP - = 0.975 Po

236

The present results show that the MFI is quite well suited to evaluate the degradation of polyolefins with recycling, given its sensitivity to repeated processing.

Melt flow index, tensile and impact polycarbonate data were used to test the applicability of equations (41) and (41a), presented theoretically in Fig. 3. It should be noticed that, through all this work, when the property cannot be determined from the virgin polymer, equations (xxa), were used to fit the data. Consequently, the counting of the processing cycles started with n= 1.

Fig. 6 shows the evolution of the MFI of the two grades of polycarbonate studied with the number of processing cycles, together with the best fitting of equation (41). The values of the parameters c. and z are also presented. As the average molecular weight decreases as the number of cycles increase (see Fig. 9), the MFI values increase with n. The increase of the reinforced grade is faster. This may indicate that the presence of the fibers accelerates the change of the rheological properties with reprocessing.

The values ofPnlPo are highly dependent on k. The fmal result could be the complete

degradation of the polymer (small values ofk) or a steady state (high values ofk).

~ "C .5

20

~ 15 q:: :: Q)

::i!

~ 10,

c ' a.. I

I

-I !

I

.. RPC, k=O I • RPC, k=o.si

:::~ I ! I

5~ - - ~

1£k.='.:::;.~1 =7. -;. i t I I I I i

o +------+-~+~__+__'-t~--j o 2 4 6 8 10

Number of cycles

Figure 6. Dependence of the MFI of polycarbonate on the number of cycles. Eq. (41) was fitted to the experimental data. PC:c. = 1.841, z = 0.826; RPC: c. = 2.481, z =0.810

Reprinted fom "The effect of fiber reinforcement on the properties of reprocessed polycarbonate" Bernardo, C.A., Cunha, A.M. and Oliveira, M. 1., Polym. Eng. Sci. 36, 511-519 © 1997 Rapra Technology Ltd

237

A different behavior of the same algorithm is exhibited in Fig. 7 and Fig. 8, where the experimental values and predictions of the impact strength (peak energy) and strain at break are shown for glass fiber reinforced PC. The respective values of the parameters C® and z are also presented. As referred above, in this case the property cannot be detennined from the virgin polymer, the counting starts at n = 1 and equation (41a) was used to fit the data.

Both examples depend noticeably on k. The impact strength data (Fig. 7) are in agreement with the example of Fig. 3c), where a steady degradation level is achieved after a certain number of reprocessing cycles. In fact, C® < 1 and, as z is small, (l-k)z, is always smaller than 1. The loss of strain at break (Fig. 8) follows the trend of Fig. 3e). PnlPo tends to

zero after a typical induction time in the first processing cycles where the degradation is less important. As predicted by equation (41a), there is an inflection point at n == 4.

1.0,-~-----------------'

0.8 .c -C) c: ~ 1;) 0.6 o CO a. E

I 0.4 ,..... a.. C­a..

0.2

o

.. RPe, k=O

2 4 6 8 10

Number of cycles

Figure 7. Dependence of the impact strength of reinforced polycarbonate on the number of cycles. Eq. (41a) was fitted to the experimental data: c e = 0.743, z = 0.785

Reprinted from "An algorithm for predicting the properties of products incorporating recycled polymers" Bernardo, C. A , Cunha, A. M. and Oliveira, M. 1., Advances in Polymer Technology 15, 215-221 © 1996 John Wiley & Sons, Inc.

238

1.0 -.----;----------,

0.8

k=O

k=O.5 '0.4

..... a.. --c a..

0.2

0.0 +--'--+--'"---,1--"--+----"--+----'--1

o 2 4 6 8 10

Number of cycles

Figure 8. Dependence of the strain at break of reinforced polycarbonate on the number of cycles. Eq. (41a) was fitted to the experimental data:Ce = 0.902, z = 1.950.

Each of the curves presented in figures 4 to 8 reflects a specific combination of polymer and processing conditions, as it is this combination that determines the way in which a given property varies with reprocessing. The behaviors depicted in the figures are quite diverse. They show properties whose values increase and decrease with the number of cycles, properties that can and cannot be determined without one processing step, and properties that are not very and are very sensitive to the value of k. These behaviors are representative of most of the experimental reprocessing curves described in the literature.

Both the figures and the results of Tables 2 and 3 indicate that the equations derived in section 2. can be adequately fit to all types of reprocessing data. Equations (41) or (41a) could be fitted successfully to properties with quite distinct decay behaviors. The same versatility was observed when equations (29) or (29a) were used instead. As will be shown ahead, once the best parameters are determined, the algorithms can be utilized to predict the smallest values of k (fraction of virgin polymer) that are compatible with the desired properties.

239

5. Application of the models to reprocessing studies

The two grades of polycarbonate investigated in this work were used to further elucidate the application of the algorithms in reprocessing studies. Fig. 9 and Fig. 10 present the degradation curves corresponding to the variation of the molecular weight (Mw ) and fiber length (Lw) with

the number of processing cycles.

+' .c Ol .~

1.0

0.8

i;; 0.6 :; o Q)

'0 ~

* PC, k=O

• RPC, k=O

.0.4 ~~~~

~ -c: 0...

0.2 -~ ~ ~ ~

0.0 +--'-----l-----'--+--'--+---'-_+_--"----1

o 2 4 6 8 10

Number of cycles

Figure 9. Dependence of Mw of polycarbonate on the number of cycles. Eq. (41) was fitted to the experimental data. PC:c. =0.961, Z =1.318; RPC: c. =0.920, Z =1

Reprinted fom "The effect of fiber reinforcement on the properties of reprocessed polycarbonate" Bernardo, C.A., Cunha, A.M. and Oliveira, M. 1., Polym. Eng. Sci. 36, 511-519 © 1997 Rapra Technology Ltd.

The molecular weight decrement for both materials is not very important during the first two cycles. This can be due to the presence of heat stabilizers that remain effective during those cycles. After the second cycle, the molecular weight shows a clear decrease, which is more severe for the reinforced material (RPC). The viscous heating generated during processing promotes chain scission and this effect was certainly increased in the RPC. As both grades studied have the same matrix and were processed under identical conditions, this enhanced degradation can only be attributed to the presence of the fibers.

240

Fig. 10 evidences that the reprocessing of the RPC causes a noticeable decrease on the average fiber length, although there is indication that they tend to an asymptotic length. As it was found that the brittleness of the material is extremely high in the 5th cycle (see Fig. 8), the reprocessing was stopped after 6 cycles. This prevented the decrease of the length of the fibers to a reach a clear asymptotic value.

No evidence was found in this work that the grinding operation causes fiber breakage. Thus, the observed decrease in Lw should result from the stress fields developed in the material

during the processing. As, under identical stress fields, the flexural stresses in the fibers increase linearly with the length, the longer fibers are more susceptible to breakage than the smaller ones. This helps to understand the sharper decrease in fiber length of the first cycles that becomes progressively smoother in the later ones.

1.0------------,

.c -Ol c:

0.8

~0.6 ~ .c u::

00.4 Il. e-ll.

0.2

o 2 4

.. RPC, k=O

• RPC, k=O.5

6 8 10

Number of cycles

Figure 10. Dependence ofLw of poly carbonate on the number of cycles. Eq. (19) was fitted to the experimental data: b = 0.363, aolPo = 0.700

241

The data discussed so far reinforce the idea that the increase of MFI of the RPC observed in Fig. 6 is due to a combined effect of fiber breakage and polymer chain scission. While the former manifests itself more strongly in the first cycles, the latter certainly dominates in later cycles, when the fibers break at a slower rate.

The variation of the tensile strength of both grades of polycarbonate with recycling is shown in Figure 11. As the figure illustrates, the decrease is significant for both grades. However, for PC (k = 0) there is little variation in the frrst two cycles. Afterwards, the tensile strength decreases abruptly, being only 40% of the original value at the 6th cycle. As is suggested by the MFI and molecular weight measurements, the sharp property loss coincides with a loss of molecular weight.

1.2~----------------~

1.0

,.... Q:.O.4 c: a.. • RPC, k=O

• RPC, k=O.5 0.2

)I( PC, k=O

x PC, k=O.5

0.0 0 2 4 6 8 10

Number of cycles Figure 11. Dependence of the tensile strength of polycarbonate on the

number of cycles. Eq. (41a) was fitted to the experimental data PC: CEIl = 0.995, z = 3.85; RPC: C EIl= 0.927, Z = 1.64

Reprinted fom "The effect of fiber reinforcement on the properties of reprocessed polycarbonate" Bernardo, C.A., Cunha, A. M. and Oliveira, M.J., Polym. Eng. Sci. 36, 511-519 © 1997 Rapra Technology Ltd.

The molecular degradation of the polymer has an even more catastrophic effect on the impact strength and the ductility. This is illustrated in Fig. 7 and 8 and also in Fig. 12, that shows the impact peak deflection curve of PC and RPC. As can be observed in Fig. 8, the ductility

242

decrease is clearly more abrupt after the 3rd cycle. For example, between the 3rd and 4th cycles, the strain at break falls from 80 to "" 20 % of the original value and is almost nil in the 5th cycle. Fig. 12 shows that he capacity to absorb energy decreases strongly with reprocessing, tending to o for the unreinforced grade after the fifth cycle.

The fibers modify the material degradation behavior during recycling, as is evidenced by comparing the degradation curves of both materials (Figures 11 and 12, k = 0). They enhance the decrease of the tensile strength. as can be seen in Figure 11. This could be explained by the severe fiber breakage experienced during the first cycles when the chain scission is yet not very important. However, as the recycling progresses, the fiber breakage slows down and the molecular weight reduces more severely, and, at n == 6, the tensile strength of RPC seems to become higher than that of the PC. On the other hand, the fibers have a positive effect on the degradation of the impact properties with reprocessing. As Fig. 12 shows, the impact peak deflection of the RPC deteriorates at a slower rate than that of the PC. The model indicates that it might tend to an asymptotic value at higher values ofn. The experimental data, however, are not enough to prove that effect clearly.

1.0,..-..-,,------------,

5 0.8 1S Q)

'i ~

~0.6 Q) Q.

~ Q. EO.4

.,.... Q.. -. If 0.2

0.0

.. RPC, k=O . • RPC, k=O.S

• PC, k=O

x PC, k=0.5

0 2 4 6 8 10

Number of cycles

Figure 12. Dependence of the impact peak deflection on the number of cycles. Eq. (19a) was fitted to the experimental data. PC: c e = 0.871, Z = 1.52; RPC: Ce = 0.941, Z = 1.46

Reprinted fom "The effect of fiber reinforcement on the properties of reprocessed polycarbonate" Bernardo, C.A., Cunha, A. M. and Oliveira, M. 1., Polym. Eng. Sci. 36, 511·519 © 1997 Rapra Technology Ltd.

243

6. The practical interest of the reprocessing curves

The behavior of a given material under reprocessing can be expressed by means of curves that relate the fraction of the original property with the number of cycles, for different contents of virgin polymer (parameter k). These curves are shown in Fig. 13a) and Fig. 13b). Fig. 13a) shows an example of MFI curves for the polypropylene used in this study. In this case, an exponential decay of the property, coupled with a linear law of mixtures and the data of Table 3 were used to derive the theoretical curves.

x Q) u c:

0.8

·i 0.6 o

;0:::: -Q) ::2:

I

c: 0.4 a..

" I

"- O.2~ I I

o.of o

·---·--·------------k-;;-O:8·-~

0.6

0.4

0.2

o

-"--+---'-_-f---~-+---'-' +-'- -+----1----1

2 4 6 8 10 12

Number of cycles

Figure 13a). Theoretical curves for the MFI of mixtures of virgin and reprocessed polypropylene. Loss of property versus number of cycles for different contents of virgin polymer.

A different representation of the same data is shown in Fig. 13b), that depicts the variation of the content of virgin polymer with n for different values of the property loss.

244

1.0 r--------------------------,

~ = 0.99 n

0.8 - - - .- - - .--~

ctS '':::: Q)

CiS 0.6 E c 0> L-.s;

'0 0.4 c o -

0.95

0.90

~ 0.75 L-

LL 0.2

0.50

0.0 t----'---I'-----'--+-----'----t"""---L--.-t---'--

o 2 4 6 8 10

Number of cycles

Figure J 3b). Fraction of virgin polymer versus number of cycles for different values of loss of the original property.

Reprinted "The recycling of thermoplastics: prediction of the properties of mixtures of virgin and reprocessed polymers" Bernardo, C.A , Cunha, A. M. and Oliveira, M.J., Polym. Eng. Sci. 36, 11-519 © 1996 Society of Plastics Engineers

The above representation may be more helpful to the processors. In fact, it allows the direct determination of the fraction of virgin material that can be incorporated, so that, after a given number of reprocessing cycles, the selected property would not deteriorate below a specified value. This type of curves (in principle, one for each property) could be provided by the raw material manufacturers.

245

7. Conclusions

The present work shows that the variation of the properties of mixtures of virgin and recycled polymers with repeated processing can be predicted with mathematical models based on experimental degradation curves. These curves also provide useful information on the mechanisms of degradation induced by reprocessing.

The models, that were tested for a number of relevant properties, allow the determination of the maximum content of recycled polymer, processed a given number of times, which is compatible with the maintenance of the desired properties.

8. Acknowledgments

Most of the results upon which the present work is based were taken from references [19] to [21]. Thanks are due to Drs. A. M. Cunha and M. J. Oliveira who were the co-authors of those publications. Without them, their knowledge and friendship, this work of many years would not have been possible. Thanks are also due to Prof. Estelita Rodrigues Vaz, from the Mathematics Department of Minho University, for helping in the derivation of some of the algorithms shown in section 2. The collaboration of other colleagues of the Polymer Engineering Department in the development of the WINRECIC computer program must also be acknowledged. Last but not least, the author expresses his gratitude to the many students who performed the experimental work that provided the data presented in this text.

9. Nomenclature

1Io Parameter of eq. (13), 1Io = Po - Pa; difference between the original and the asymptotic value of the property

a l Parameter ofeq. (13a), a] =Po - PI; difference between the value of the property after une processing and the asymptotic value

b Parameter of eq. (9) - exponent of the degradation curve

c Parameter of the power law property loss; real number < 1

C. Parameter ofeq. (41

cEI) Parameter ofeq. (4la)

d Parameter ofeq. (12)

F Feed stream

Counter of the number of cycles

k Virgin material fraction in the mixture at the inlet of each processing

Isd Least square deviation

Lw Second moment of the fiber length distribution

Mw Weight average molecular weight

MFI Melt Flow Index

246

n Number of cycles (processing)

n' Number of cycles (reprocessing)

o Outlet stream

P a Parameter ofeq. (13) - asymptotic limit

Po Value of the property of the virgin polymer

P n Value of the property in the nth cycle (degradation curve, no virgin polymer added to

the feed)

p ~. Value of the property of the mixture at the outlet of the n' th reprocessing cycle

* P n Value of the property of the mixture at the outlet of the nth processing

P n Value of the property of the mixture that contains polymer processed n times, at the inlet of the (n+l)th processing

P r Value of the property of the reprocessed polymer

P.., Value of the property of the mixture at steady-state

M 0 Difference between the value of the property of the original polymer and the asymptotic value - coincides with Clo

M; Difference between the value of the property at the ith cycle and the asymptotic value - change in variables, Eq. (16)

R Recycled material stream

r Recycled material fraction in the mixture at the inlet of each processing

t Parameter ofEq. (Sa) and Eq. (Sa) - slope of the degradation curve

V Virgin material stream

z Exponent of the power law property loss; real number < 1

10. References

1. Leidner, J. (1981) Plastics Waste, Marcel Dekker Inc., New York.

2. Filbert, Jr., W. C. (1968) Glass reinforced 6 6 nylon - the effect of moulding variables on fibre length and the relation of fibre length to physical properties, SPE ANTEC Tech. Papers 14, 836-841.

3. Yang, H. W., Farris, R., and Chien, 1. C (1979) Study of the effect of regrinding on the cumulative damage to the mechanical properties of fiber-reinforced nylon 6.6, J. Appl. Polym. Sci. 23, 3375-3382.

4. Shea, 1. W., Nelson E. D., and Cammons, R. R. (1975) The effect of recycling on the properties of the injection molded polycarbonate, SPE ANTEC Tech. Papers 21, 614-617.

5. Shea, J. W., Aloisio, C. J., and Cammons, R. R. (1977) Effects of composition, processing, and environmental exposure on mechanical integrity of polycarbonate connectors, SPE ANTEC Tech. Papers 35, 326-329.

247

6. Abbas, K. B. (1980) Reprocessing of thermoplastics. II. Polycarbonate, Polym. Eng. Sci. 20,376-382.

7. Abbas, K. B. (1981) Degradational effects on bisphenol-A polycarbonate extruded at high shear stresses, Polymer 22,836-841.

8. Eguiazabal, J.I., and Nazabal, J. (1989) Effect of processing on the properties of bisphenol-A polycarbonate, Eur. Polym. J. 25, 891-893.

9. Driscoll, S. B. (1977) Thermoplastic resin regrind study, SPE ANTEC Tech. Papers 23, 536-538.

10. Bernardo, C. A., Cunha, A. M., and Oliveira, M. J. (1993) The effect of recycling on the properties of thermoplastics composites, in G. Akovali (ed), The Interfacial Interactions in Polymeric Composites, Kluwer Academic Publishers, Dordrecht, pp. 443-448.

11. Kalfoglou, N. K., and Chaffey, C. E. (1979) Effects of extrusion on the structure and properties of high-impact polystrene, Polym. Eng. Sci. 19, 552-557.

12. Spinney, L. J., Orroth, S. A., and Malloy, R. (1979) Regrind study on a polyphenylene sulfide molding compound, SPE RETEC, 126-129.

13. La Mantia, F. P. (1991) Techniques and problems in plastics recycling, MACPLAS Int., 53-57.

14. Eguiazabal, J. I., and Nazabal, J. (1990) Reprocessing polycarbonate/acrylonitrile­butadiene-styrene blends: influence on physical properties, Polym. Eng. Sci. 30, 527-531.

15. Sanchez, P., Remiro, P. M., and Nazabal, J. (1992) Influence of reprocessing on the mechanical properties of a commercial polysulfone/polycarbonate blend, Polym. Eng. Sci. 32, 861-867.

16 Abbas, K. B., Knutsson, A. B., and Berglund, S. H. (1978) New thermoplastics from old, Chemtech. 8, 502-508.

17. Schott, N. R., Lak, L., and Smoluk, G. (1974) Recycle calculations in plastics extrusion, SPE ANTEC Tech. Papers 20, 43-46.

18. Throne, J. (1987) Effect of recycle on properties and profits: algorithms, Adv. Polym. Techn. 7, 347-360.

19. Bernardo, C. A., Cunha, A. M., and Oliveira, M. J. (1996), The recycling of thermoplastics: prediction of the properties of mixtures of virgin and reprocessed polyolefms, Polym. Eng. Sci. 36, 511-519.

20. Bernardo, C. A., Cunha, A. M., and Oliveira, M. J. (1996) An algorithm for predicting the properties of products incorporating recycled polymers, Adv. Polym. Tech. 15,215-221.

21. Bernardo, C. A., Cunha, A. M., and Oliveira, M. J. (1997) The effect of the fibre reinforcement on the properties of reprocessed polycarbonate, Polymer Recycling 2, 1-14.

22. WINRECIC - Primary Recycling Modelling Package, version 3.0 (1996), Universidade do Minho, Guimaraes.

REPROCESSING OF POLY(VINYL CHLORIDE), POLYCARBONATE AND

POLYETBYLENETEREPBTBALATE

F.P. LA MANTIA

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy

1. Introduction

Poly ethyleneterephthalate - PET- and Poly(vinyl chloride) - PVC - are two of the most frequently encountered polymers in post-consumer plastics. In particular, these polymers can be used for manufacturing the same products (bottles, for example) and therefore are often found in the same stream, even when the collection is done with separation. The recycling of PET and PVC is usually carried out by separating them and reprocessing the two homogeneous polymers. Indeed, the heterogeneous recycling of two polymers is at present almost impossible because of many problems: - as their melting points are very different, at the processing temperature of PET severe

degradation phenomena of PVC take place; - the degradation of PET, in tum, is drastically accelerated by hydrochloric acid, that is

the main product of the degradation of PVC; - PVC is not sensitive to the presence of water, while PET can be easily degraded by

hydrolytic chain scission and must be carefully dried before processing; - the two polymers are strongly incompatible and, even if all the previous problems were

overcome, the recycled material would have very poor properties [I].

For this reason ,the separation of the two polymers is very drastic and the amount of PVC must be very small (less than 50 ppm) in the recycled PET. On the contrary, PET can be contained in relatively large amounts (about 0.3%) in the recycled PVC. Larger amounts, however, can lead to a rapid obstruction of the filters of the extruder. The recycling of these two polymers, when present in the same stream, starts therefore with a careful separation before the reprocessing operations. Moreover, because the plastic separate collection always yields a polluted product, cleaning of the foreign bodies and separation from other plastics is necessary.

Polycarbonate - PC - is much easier to collect separately from other plastic products because it is used in well determined applications, like electric and electronic items, windows, etc. The recycling of PC can become, however, very difficult because of the presence of humidity. In this case, as for PET, dramatic degradation phenomena take place with consequent reduction of the properties of the recycled material.

In this chapter some separation equipment will be described before discussing the relationships between the properties of the recycled material and the reprocessing operations of these three polymers. A brief outlook of recycling plants will also be done.

249

G. AkovaIi et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 249-269. © 1998 Kluwer Academic Publishers.

250

2. Equipment for Separation of PET and PVC

The separation of the plastic materials into homogeneous components depends on the type of collection and then on the local laws regulating the collection of plastic waste. As an example, in Italy a separate collection of plastic containers is obligatory and the final stream is composed ofHDPE, PET and PVc. The separation ofHDPE from the other two polymers is very easy by flotation in water, because its density is lower than that of water while the density of PET and PVC is larger. However, other methods can be used, employing detectors able to separate PET from HDPE after separation of PVC in a first set of detectors. On the contrary, this technique can not be applied to the separation of PET and PVC that show similar density. Several apparatuses have been proposed for achieving this separation (2). Apart the manual separation, the automatic separation processes include various systems employing different detectors. The more important commercial detectors are based on X-ray sources, on fluorescence and on near-infrared spectrometry. All these methods work on different characteristics of these two polymers. The X-ray works on the crystallinity of PET versus the amorphous state of PVc. Some systems are based on the recognition of the chlorine atom in the PVC chains. Other detectors can also detect the colored and transparent bottles for a separate recycling of these two different products.

In Figs. 1 and 2 typical examples of two separation plants of plastic bottles (HDPE, PET and PVC) are presented.

In the fITst scheme, see Fig. 1 taken from [2], after washing and after eliminating the ferrous contaminants, an aluminium detector separates the bottles from aluminium while the next detector separates the PVC bottles. Further detectors separate the colored PET and the remaining PVC. In geneml, more detectors for PVC are needed in order to obtain a PVC-free PET (less than 50 ppm). In the second scheme, Fig. 2, the fITst two detectors separate PVC and the others the bottles of colored and transparent PET. Because the aluminium is being progressively substituted by polymers for the cap valves, the aluminirrm detector is missing.

3. Recycling of PET

The scheme of a typical plant for recycling of bottles of PET is reported in Fig. 3 [3]. The bottles can come from the sepamtion previously described or from a separate collection of PET bottles. The scheme reported takes into account all the operations necessary for the whole bottle.

PET bottles recycling plants, are very different from those that reclaim other polymers like, for example, PE film, since the body of the bottles must be separated from various materials: label paper, PE base cups, cap valves (polypropylene - PP - or aluminirrm), etc. After one or more shredding and grinding sections, the flakes are washed twice to eliminate glues, paper, etc. The residue is PET, PE, PP and, occasionally, aluminirrm of the cups. A subsequent flotation process separates out the PE and PP (the only materials lighter than water) which are then dried and reused. When it is necessary, PET and aluminirrm are sepamted in an electrostatic separator and PET, ready for reprocessing, is stored. Unlike the plants for recycling of polyolefmes, the extrusion step is missing and the material does not undergo severe degradation phenomena. Of course, this does not mean that the molecular structure, the morphology and the properties remain unchanged with respect to those of the virgin material. In fact, as will be shown in the following, the material undergoes more processing operations and then some degradation occurs, even if the PET is reprocessed in dry conditions. Moreover, the same sample can

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3.1 PROPERTIES-REPROCESSING RELATIONSHIPS

The main problem in the recycling of PET, like for other polycondensation polymers, is the hydrolytic chain scission. It is therefore necessary to dry the polymer before each reprocessing operation. As already said, the recycling of PET does not imply, in general, a melt processing and then the degradation phenomena occur, in part, during grinding and, in part, during the next reprocessing steps.

In Figs. 4 - 7 some mechanical properties of a bottle grade PET are reported as a function of the processing steps. In these figures, V means virgin. P means first processing, R recycled and the figures accompanying R stand for the number of recycling steps [4,5]. The processing steps - extrusions - have been carried out on samples without any predrying, but in the same figures some values relative to samples processed in dry conditions are also reported. This is indicated by H and D, respectively.

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All the above results suggest that PET undergoes hydrolytic chain scission during processing but also that some degradation occurs even when the material is processed after careful predrying. Thermomecbanical degradation is therefore responsible for the degradation of dry PET.

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To understand the mechanism of degradation of PET better, some samples (dry and humid) were processed in a closed mixer at two different rotational speeds. The Newtonian viscosity is plotted in Fig. 10 against the processing time. The degradation increases with time and with the rotational speed. Thermal degradation is then enhanced by the presence of the mechanical stress which acts as a "catalyst" of the thermal degradation.

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Reprinted from Polym. Deg. Stab. 45, La Mantia, F.P. and Vinci, M. "Recycling of polyethyleneterephthalate" pp 121-125 (1994), with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington OX5 1 GB, UK

258

The main feature of the recycling of polymers is the decrease of the molecular weight which leads to materials with lower viscosity. This means that the recycled polymers can not, in general, be processed in the same processing conditions used for the same virgin material.

4. Reprocessing of PVC

PVC is one the most used polymers and is used in a wide series of products. The plants for recycling PVC are strongly dependent on the type of product to be recycled. Bottles, pipes, windows and cables are the main PVC products recycled at present.

4.1 PVC PIPES

A scheme for regeneration of PVC pipes is reported in Fig. 11 [8]. PVC and PE pipes are manually pre-sorted and then are crushed in a press. PVC pipes are broken while the ductile PE pipes do not break and are eliminated from the stream. The material is again ground and ferrous contaminants are separated. The PVC is then micronised and homogenized.

4.2 PVC WINDOWS

A similar scheme is applied to e complete windows, Fig. 12 a-c [9]. In this case the main problem is the separation of PVC from iron, glass, rubber and wood. After crushing and separation of ferrous parts, Fig. 12a, the materials having a particle size larger than 15 rom are returned to the shredder for further crushing while those less than 4 rom in diameter, mainly glass are discarded. The fraction with intermediate particle size, is sent to the separation of glass and wood, Fig. 12b. This separation is carried out by means of vibrating tables by exploiting the different densities of the three components. After a final stage of separation of ferrous parts, Fig. 12c, the material is crushed, washed, dried and separated in different fractions depending on the particle size. After separation of the rubber parts, white grades are separated from the colored pieces and stored according to their size.

4.3 PVC CABLES

Separate collection of cables is a very easy and economic source of metals and plastics. Insulators for cables are mainly made of PVC and LDPE, but rubber can be also present. A typical layout of a plant for recycling PVC insulators from cables is reported in Figs. 13 a and b [10].

After cutting, grinding and after a first separation of the metals, the plastics are sent, Fig. 13a, to a floating section for separating PVC and PE. After a passage in an electrostatic separator to eliminate the last particles of metals, Fig. 13b, PVC is blended with additives (stabilizers, lubricants, colorants), compounded and extruded in a twin screw extruder. The recycling of LDPE insulators from cables is subjected to a similar processing.

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Figure 13b. Recycling plant of PVC cables

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4.4 PROPERTIES-REPROCESSING RELATIONSHIPS

The main disadvantage of PVC is the limited thermal stability, that requires the addition of heat stabilizers to prevent a large extent of degradation. Because the stabilizer in PVC products is consumed both during processing and sometimes during the lifetime, the thermal stability is remarkably reduced in recycled PVC, which undergoes more processing steps.

Fig. 14 a and b [11] reports the typical variation of the torque, Fig. l4a, and of the concomitant formation of gel, Fig. l4b, as a function of the processing time in a mixing test.

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After an initial decrease due to the heating of the material, the torque tends to be constant and then starts to rise. This behavior is more impressive when the temperature increases. Indeed, unexpectedly the torque for long times becomes larger at higher temperatures.

The curves of formation of the gel are similar to the curves of the torque. Indeed, the gel content increases remarkably when the torque starts to rise. For long mixing times the gel content increases with temperature. The increase of the torque is a sign of the change of the structure and therefore of all the properties. In particular, under the action of an external stress - heat, light, mechanical forces - a loss of hydrochloric acid takes place with formation of conjugated double bonds. The new molecular structures, which includes also the presence of gel, give rise to an increase of the viscosity of the melt.

The Dynamic Thermal Stability Time (DTST) is a good index for evaluating the thermal stability. DTST is the time at which the torque in a mixing test at constant temperature starts to rise. The material, therefore, must be processed for times lower than DTST.

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Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Ma and La Mantia, F.P. J Appl. Polym. Sci. 59,759-767 © 1996 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc

DTST values of two different grades of PVC, RBPVC - recycled PVC from bottles - and RPPVC - recycled PVC from pipes - have been determined from the torque­time curves, Figs. 15 and 16 respectively [12]. The tests were carried out at 190°C and at a rotational speed of 60 rpm. It can be observed that the DTST is less than 4 min for the first sample and about 6 min for the sample recycled from pipes.

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Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Ma and La Mantia, F. P. J. App/. Po/ym. Sci. 59, 759-767 © 1996 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc

The DTST values are, of course, dependent on the processing conditions and in particular on temperature and mechanical stress. The effect of these two parameters is put in evidence in Fig. 17 [5] where the DSDT values of a RBPYC sample are reported as a function of the temperature for three different rotational speeds.

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265

DTST decreases with increasing temperature and rotational speed, that is, with t.'1e mechanical stress. However, the effect of temperature is more dramatic for the lower rotational speeds. In fact, for 20 and 50 rpm the DTST is halved when the temperature increases from 180 to 220°C. The effect of the mechanical stress is to increase the rate of formation of macro-radicals, acting as a "catalyst" of the thermal degradation.

To enhance the processability is therefore necessary to increase the DTST value by adding suitable stabilizing systems which have been consumed both during processing and during the lifetime of the PVC products.

The effect of a stabilizer system has been evaluated on the same two different grades of PVC: RBPVC and RPPVC. The curves of torque vs time for the samples with 1-3 phr of stabilizer are reported in the same Figs. 15 and 16 [12]. The stabilizer is a dibasic lead hydrophosphite hemihydrate. DTST ofRBPVC increases to about 14 min by adding 3 phr of stabilizer, while DTST of RPPVC increases to about 20 min by using 2 phr stabilizer. The presence of small amounts of stabilizer remarkably improves the thermal stability of PVC and enhances the processability of the recycled PVC samples. Provided that the processing time is below the DTST, a small decrease of the mechanical properties of the recycled PVC can be observed, in particular if the reprocessing is carried out at high temperature and rotational speed [5]. In these conditions some improvement of the mechanical properties can be achieved by using a stabilizer agent.

Modulus, stress and strain at yield are reported in Figs. 18-20 as a function of the stabilizer content for two different processing temperatures. Modulus, stress and strain at yield increase with the content of stabilizer but, for all these properties, the improvement is evident but not very large.

The properties of the recycled PVC therefore can be maintained similar to those of the virgin material provided that the processing time is lower than the DTST in the processing conditions and small amount of stabilizer is added.

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5. Reprocessing of PC

PC is widely employed in many applications like doors and windows components, electric and electronic products, appliances, etc., because of its interesting thermomechanical and impact strength properties. The processing is quite difficult due to the high processing and to the almost Newtonian behavior. Moreover, the material must be carefully dried before processing in order to avoid hydrolytic scission. Most of these applications have a long life cycle, but the increasing amount of products made of PC makes its recycling an important consideration.

Multiple reprocessing operations lead mostly to a reduction of the viscosity and, thus, of the molecular weight [7,13-15] while modest formation of oxygenated groups can

267

give rise to yellowing of the material [7,14]. The degradation of PC is mainly due to the temperature but the mechanical stress applied to the melt can also play an important role. The Newtonian viscosity of some samples processed in a mixer in different conditions is reported in Fig. 21 [7].

300°C, 30 rpm 10 min

270°C, 30 rpm 30

300°C, 30 rpm 30

300°C, 30 rpm 120

300°C, 150 rpm 30

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I I

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200 400 600 800

Newtonian viscosity, Pa*s

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The viscosity decreases when the mixing time and the temperature increase. The Newtonian viscosity is drastically reduced when the time increases from 10 to 120 min and the sample processed at 300°C shows a lower viscosity than mixed at 270 DC. This confirms that the degradation is mainly due to the temperature, but, at the same temperature, the viscosity of the sample processed at 150 rpm is lower than that of the sample mixed at 30 rpm. This result indicates that the mechanical stress enhances the amount of degradation and then acts like a "catalyst" of the thermal degradation.

Like PET, the presence of water during reprocessing causes a drastic decrease of the molecular weight of PC because of the hydrolytic chain scission. This decrease is reflected in a reduction of the mechanical properties. The impact strength of two different grades of PC, extrusion and injection molding, is reported in Fig. 22 as a function of the number of reprocessing operations.

Both extrusion and injection molding have been carried out on humid samples and on samples pre-dried before each processing step [7]. The decrease of the impact strength becomes significant only after a large number of processing steps when the sample is reprocessed after predrying. On the contrary, the impact strength decreases if PC is processed in humid conditions, even after only one recycling step.

In conclusion, PC shows a good resistance to the degradation processes provided that a careful predrying is made before each reprocessing step. In this case, the mechanical properties of recycled PC are similar to those of the virgin polymer even after several recycling steps.

268

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Number of processings

Figure 22. Impact strength of PC vs number of reprocessing operations. E means extrusion grade, I, injection molding grade, D dry, H humid

6. Acknowledgment

This work has been fmancially supported by MURST 60%

7. References

l. La Mantia, F.P. (1996) Basic concepts on the recycling of homogeneous and heterogeneous plastics, in F.P. La Mantia (ed), Recycling of pvc & Mixed Plastic Waste, ChemTec Publisher, Toronto. pp. 63-76.

2. Sereni, E (1993) Techniques for selection and recycle of post-consumer bottles, in F.P. La Mantia (ed), Recycling of Plastic Materials, ChemTec Publisher, Toronto, pp.99-109.

3. La Mantia F.P. (1990) Techniques and problems in plastic recycling, Macplas International, May, 53-57.

4. La Mantia, F.P. and M. Vinci, M. (1994) Recycling of polyethyleneterephthalate, Polym. Deg. Stab. 45, 121-125.

5. La Mantia, F.P. (1997) unpublished results.

6. Marrone, M. and La Mantia, F.P. (1996) Re-stabilization of recycled polypropylenes, Polymer Recycling 2, 17-26.

7. La Mantia, F.P. and Conte, F. (1997) Recycling of extrusion and moulding grade polycarbonate, submitted for publication to J. Appl. Polym. Sci.

8. Voituron, G. (1996) Recycling PVC bottles and pipes by coextrusion, in F.P. La Mantia (ed), Recycling of pvc & Mixed Plastic Waste, Chemtec Publisher, Toronto, pp.51-62.

269

9. Uhlen H. (1996) Recycling of complete PVC windows, in F.P. La Mantia (ed), Recycling of pvc & Mixed Plastic Waste, Chemtec Publisher, Toronto, pp. 43-51.

10. Farina, G. (1996) MEIE '96 Versailles, 17-18 June.

11. Scott G., Tahan M., Vyvoda J., (1976) Chem. & Ind. pp. 903-.

12. Wenguang, Ma and La Mantia, F.P. (1996) Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC, J. Appl. Polym. Sci. 59,759-767.

13. Abbas, K.B. (1980) Reprocessing of thermoplastics: II polycarbonate, Polym. Eng. Sci. 20,376-382.

14. Eguizabal, J.I., Nazabal J., (1989) Eur. Polym. J. 25, 891-893.

15. Bernardo C.A., Cunha A.M., Oliveira, M.J. (1996) An algorithm for predicting the properties of products incorporating recycled polymers, Adv. Polym. Techno!. 15, 215-221.

REPROCESSING OF POLYOLEFINS

Changes in Rheology and Reprocessing Case Studies

Abstract

A. T. P. ZAHAVICH Uniplast Industries, Inc. Forest Ave. Orillia, Ontario, Canada L3V 6R9

1. VLACHOPOULOS Department of Chemical Engineering McMaster University Hamilton, Ontario, Canada L8S 4L7

Plastics recycling has received significant attention within the past decade. Positive or negative, this attention has provided an impetus for the plastics industry to advance recycling technology. Polyethylene resins have been identified as primary materials for solid waste minimization and recycling. This paper is concerned with the effects of recycling on the rheological properties of high density polyethylene blow molding resins. Properties such as shear and elongational viscosity and elastic modulus are examined. The changes that have been observed are analyzed in terms of known degradation mechanisms such as chain scission and cross-linking, and their relationship to the Phillips and Ziegler-Natta catalyst systems. Case studies are used to illustrate the practical challenges faced by manufacturers who use recycled polyethylene, including discussions on rheological considerations in woodjilled polymers and rotomolded structures.

1. The Reclamation and Reprocessing System

When a product containing polymeric materials is considered unfit for use, the industrial, commercial, institutional or household consumer of the product will decide on the next event the polymer will experience. The consumer will either direct the product into the general trash stream for permanent disposal or into a recycling stream.

Recycling represents the last opportunity for delaying or diverting polymeric materials from permanent disposal and consists of 2 parts, reclamation and reprocessing.

271

G. Akovali et aI. (eds.), Frontiers in the Science and Technology a/Polymer Recycling, 271·297. © 1998 Kluwer Academic Publishers.

272

Reclamation is concerned with the collection and separation of products and/or their constituent materials. Reprocessing involves the forming of reclaimed polymeric materials into useful products either independently or mixed with virgin polymers. Post consumer recycling has been significantly developed in the past 10 years to aid in the reduction of municipal solid waste (MSW).

1.1 THE RECLAMA nON SYSTEM

If a product is directed into a recycling stream it is collected and sent to a dismantling and/or separation center. Dismantling centers separate multi-component products such as automobiles, computers or some household appliances, into sub-component bins or if possible into specific material bins [I). At an automobile dismantling center HDPE washer and coolant bottles may be sorted directly into an HDPE bin, whereas dashboards and instrument panels, which are made of plastics with imbedded metallic sub-components, are separated into an instrument panel bin. Even a seemingly pure stream of single material components may require higher levels of separation depending on the materials they may have contained. An example of this is HDPE and its use not just in coolant and washer bottles but also in gasoline tanks. The HDPE gasoline tanks are usually separated from the other HDPE components because of the residual gasoline trapped in the polymer structure.

Household packaging reclamation is typically done in conjunction with general trash collection. Various collection schemes have been piloted and are in place including curbside collection and drop off depots. One of the first and most successful is the Blue Box program initiated in Ontario. Regardless of the system, it has been demonstrated that collection costs and resale revenues are the two most critical factors in the planning of a reclamation program [2,3].

The extent of household collection for plastic materials is restricted to bottles and in some cases film. The materials for bottles are typically PET and HDPE, though PVC is also inadvertently collected because it is often used in similar applications as PET and HDPE. Polyethylene is the only material involved in the collection of films and is typically found in grocery carry-out sacks and shrink wrap.

1.1.1 Legislation. public issues and reclamation

Scientists. engineers and designers are often guided by government regulations, who are in tum driven by public pressure. Recycling plastics is one of the highest profile yet least well defined activities in terms of government legislation. The impact of social pressure and policy cannot be ignored, especially in relation to the technical analysis in this paper.

The type of reclamation system used and its success is largely dependent on the jurisdiction and social policies of the day in that jurisdiction. The effect of public pressure cannot be underestimated and its power is best illustrated by the following example. Prior to 1988 immense pressure was being applied to governments to ban plastics from packaging. In 1988 a number of studies were published which showed the effect of a plastics ban. One of the more prolific studies was done in Germany [4). A chart summarizing the results from that study is shown in Figure 1.

use of energy 1''',S>IoIJ:>:l''lH

cost of packaging

volume of waste

weight of waste

1CXl

273

What effect would it have on our lives if we banned all plastics packaging?

19) :m 23J 3XJ

percentage increase

Figure 1. Effect of banning plastics packaging . Summary by the German Society for Research into Packaging Markets [4]

Other examples of the influence of public pressure on plastics recycling involve the TETRA PAK© and MacDonald's restaurant "clamshells". The TETRA PAK, a multi­layer, multi-material container was targeted for banning because of its complex materials structure of paper, plastic and aluminium foil. This product, originally developed in the 1960's, was designed for transporting large quantities of fresh milk to underdeveloped countries without the need for refrigeration or preservatives. In the late 1970's juice companies saw an opportunity to reduce transportation costs by using a downscaled version commonly referred to as a juice box. Since the product could not be reused or effectively recycled, with the plastic component being the prime for its non-recyclability, it was considered a burden on the solid waste system. Public pressure almost eliminated it from the store shelves in early 1991 but the manufacturers and juice companies fought back with aggressive advertising [5,6]. This case clearly demonstrates the conflicts within the waste management hierarchy. While the TETRA PAK was designed for source reduction, arguments were made that it did not fit the criteria for reuse and recycling.

A move by MacDonald's Canada in 1990 virtually shut down the Canadian Polystyrene Recycling Association (CPRA) facility in Mississauga, Ontario. A switch by MacDonald's Canada from polystyrene foam "clamshell" containers back to wax paper wrappers for hamburgers was done in response to pressure to reduce the volume of packaging generated by the clamshells [7] . This was done in spite of the industries' efforts to develop a reclamation and reprocessing system for the plastic. This case demonstrates the dilemma faced by researchers and investors to advance the recycling of plastics. A plan

274

to reclaim and reprocess the packaging was developed and actually being implemented, then the primary source of stock for recycling was eliminated.

The previous examples illustrate how public pressure influences the recycling of plastics. However, the greatest effect is from government regulation. World-wide, governments in the late 1980's and early 1990's took aggressive action to control waste through legislation. However, there is no consistent standard and this makes plastics recycling and research into recycling a challenge. The differences in regulations between regions, such as states and provinces, are just as great as the differences between countries. Differences in the definition of plastics reprocessing can also dramatically alter the direction of research as illustrated by the judgement in the spring of 1994 in the courts in Washington State, which ruled that a pyrolysis project for plastics did not fall within the recycling legislation and was considered incineration.

1.2 SEPARATION TECHNOLOGY

When a product is deposited into the recycling stream it becomes intermingled with any number of and any combination of other materials and products. Gross separation, where identification is generally driven by the main constituent of the product (i.e. plastics, metals and paper) is relatively straightforward. For example, a pop bottle may be made of plastic or glass but the distinct difference in properties between the materials makes separation easy. In contrast, automotive fluff may have glass, polymeric and other materials, yet when it is reduced to baseball sized chunks how can these materials be distinguished?

The degree of separation for waste is dictated by the end use or application of the separated materials. The end use for a material is dictated by the acceptance of the product on the market. Regardless of whether the product uses recycled, virgin or both types of material, if it performs, and is cost effective, then the material used is considered acceptable. The desire to have a highly separated, clean stream for reprocessing in packaging is often driven by the performance requirements that consumers and marketing representatives place on products.

In 1994, the most automated, commercially available process was part of a fully integrated bottle sorting system. Developed by Magnetic Separation Systems, in Nashville, Tennessee. It uses electromagnetic radiation and mechanical sorters after an optic system. Figure 2 shows a schematic of the system. Plastic bottles which have been grossly separated from the reclamation stream are debaled and singulated. Each bottle then passes through an optic and electromagnetic sensor where a polymeric "fingerprint" is recorded and compared to a data-bank. Optional color identification is also done using video technology. The bottles are sorted according to the information stored in a microprocessor. With this system, bottles of different polymers and colors can be sorted at a rate of 2300 kglhr [8].

1.2.1 Cleaning reclaimed plastics

Once products made from plastics have been separated, they are reduced in size by shredding or grinding and cleaned prior to entering the reprocessing stage. A flow diagram ofthe QUANTUM Chemical HDPE bottle reclaiming system is shown in Figure 3.

5,000 LB/HR PLASTIC BOTTLE SEPARATION SYSTEM

Figure 2. Bottle Sort'" system

Figure 3. QUANTUM Chemical reclaiming process

(Reprinted with permission from the author [9])

275

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lXlHT!n. CABt£T

276

Most reclaimed plastics are cleaned to remove loose dirt, labels and ink. Pneumocyclones, hydrocyclones and dryers usually make up the cleaning system. Sometimes detergents are used but often hot water is the only cleaning agent. Contamination is the presence of a compound or particulate which was not originally intended to be part of the material system. The effects of contaminants are typically measured subjectively in terms of visual, odor or other sensory means, though they may also modify the physical or melt flow characteristics of the system. Typical contaminants found during reclamation include particulates, chemical compounds and incompatible polymers. Particulates can range from dirt, cellulose, metals, severely oxidized polymers and gels.

The possible chemical compounds found in reclaimed materials are limited only by the compounds used in the product or by the environment to which the product is exposed [10]. Buric acid, a by product of spoiled milk, is considered a contaminant because of the odor associated with it. Motor oil in bottles, perfumes used in detergents and tackifiers used in stretch film are also considered as contaminants in reclaimed material. Most of the compounds are residues that manage to migrate into amorphous regions of the polyolefin material. While they may not be strongly bonded to molecular chains, the highly random packing of the molecules tends to entrap the compounds. Some chemical compounds may be vented and most particulates can be screened out of a polymer during extrusion., even at elevated temperatures, with the polymer in a molten state, traces of these compounds still remain within the polymer structure.

Closed loop systems are currently being developed in order to accommodate the complexities and toxic nature of contaminants. In one project, trials involving FDA approved products were initiated in 1993 with a major US airline. A closed loop process is being tested where PS food trays are collected from flights, ground and re-extruded back into food trays. The FDA has approved the trial, based on the elimination of unidentified source material entering the reclamation stage. Similar, non-food application trials are ongoing in the agricultural industry. Banana bags in Costa Rica are part of a closed loop to prevent the insecticide laced bags from entering the post consumer recycling stream [II]. The bags are shredded and re-extruded back into new bags.

1.3 REPROCESSING SYSTEM

After a plastic has been separated and cleaned it is reprocessed. There are three categories of reprocessing, primary, secondary and tertiary. Primary reprocessing can be defined as the reforming of polymer scrap (sprues, edge trim etc.) into a product with the same level of specifications as the original one. An HDPE blow molded detergent bottle, blow molded back into a fabric softener bottle or a PET pop bottle reprocessed into a tennis ball bottle are classified as primary processes. Secondary reprocessing involves reforming a polymer into a lower valued product, such as HDPE blow molded bottles compression molded into stall liners for livestock, or the shredding of PET pop bottles into fibers for use as clothing insulation. The distinction between primary and secondary lies in the type of process used and source of material. While primary reprocessing is restricted to the original forming process, using scrap, secondary reprocessing usually involves a different product, an alternative forming process and waste plastics which have to be cleaned. In plant primary

277

or secondary recycling has existed virtually since the time plastics were first polymerized, and this is referred to as post industrial recycling. It is usually done in conjunction with virgin polymers at relatively low percentages «20%) of reclaimed material. While coextrusion is not a new technology, it is increasingly being used as a method for using recycled material in food approved (FDA) applications. A three layer, single resin system is used where the outer skin and inner food contact layer are virgin resins and the middle layer is recycled resin [12].

The third type, tertiary reprocessing, involves the breaking down of plastics, either thermally or chemically, back to feedstock material such as crude oil. The feedstock can be repolymerized into plastics, used in other chemical products or used as an energy source. Tertiary recycling became prominent in the 1970's during the energy crisis, when plastics were considered as an energy source. In the last 5 years this form of recycling received more attention and, in reaction to legislation such as the Dual System, it is expanding rapidly. It is expected that a significant capacity to convert plastics back to crude oil will come on stream in Europe by the end of 1997.

A schematic diagram of how the three types of reprocessing categories fit into the life of a polymer is shown in Figure 4

Resin producer Industry I ~c~h~e~m~i~c~al~f~ee~d~s~t~o~ck~~~m~o~n~o~m~e~r~~~~m~e~r ~ L-__________ .-____ ~

chemicals i TERTIARY

PRIMARY SECONDARY

Landfill/incinerator

Figure 4. Schematic representation of the reprocessing categories

278

1.3.1 Commingled reprocessing systems

Except for some processes, the current market for recycled polymers demands virtually 100% separation of the plastic waste into specific polymers, The exception is the commingled process [13]. Commingled systems were at first considered as a panacea for recycling plastics. The ability to combine a variety of polymers with other non-polymer materials seemed a viable approach to avoid costs associated with separation. However, the process used for commingled materials only produces large simple parts which, in most cases, are intended to compete directly against wood. It has been demonstrated that they can be nailed and sawed in a manner similar to wood.

Other commingled processes under development include PP/wood flour and HDPEI rubber products, where the thermoplastic is used as a matrix for a fine ground non­thermoplastic material. Both systems are intended for use in sheet extrusion and compression molded products [14,15].

2. The Life Cycle of a Thermoplastic

The life cycle of a polyethylene resin begins with polymerization in a reactor. When the reaction is terminated, molten or powdered product is passed through a melt blending extruder where antioxidants, processing stabilizers, processing aids and other additives are added. Thermal exposure and extrusion shear rates « 500 S·I) are generally kept low at

this stage' to minimize the onset of degradation. The polymer melt is then passed through a pelletizer and batches of the pellets are shipped to the manufacturer of the primary product application. In many applications the material is either dry or melt blended with other materials, such as pigments, anti-static, flame retardant or UV light stabilizing additives. The material is then extruded and/or molded into the primary product. Extrusion shear rates tend to be around 500 S·I, though in some final molding stages shear rates> 1000S·1

can be experienced by the polymer. Thermal exposure can be quite severe depending on the type of process and the desire for optimizing throughput. Since the viscosity varies inversely with temperature, a higher temperature will allow for a greater throughput.

Once the product is used, the polyolefin will take one of 2 paths. It will either be discarded in the form of the primary product (likely into a landfill, possibly in a waste-to­energy system) or it will be diverted from the waste stream. If it is discarded and left exposed to the atmospheric environment, the product will degrade due to UV exposure, wear, and thermal-oxidative aging. Arguably, complete degradation will not occur until the carbon or hydrogen atoms are liberated from each other. If the material is collected and landfilled, the speed of the degradation process will be severely reduced as the material is virtually entombed and not exposed to the degrading elements. It is worth noting that degradation for virtually all landfilled material, and not polyolefins alone, is also stagnant under these conditions [16]. If the material is collected and used in a waste-to-energy system, the energy stored in the molecule, in terms of its calorific value, is transformed into heat. and can be used in a district heating system.

279

If the polyolefin is collected and diverted from the waste stream it will be reclaimed through a separation, cleaning and pelletizing system. The material is separated using near infrared analysis (NIR) and/or a flotation system, shredded into flakes, passed through a blending extruder and pelletized [9,17]. Antioxidants and processing stabilizers are added at the blending extruder, similar to those added when the resin is first manufactured. Again shear rates and thermal exposure are minimized, such that the shear rates are < 500 S-I and melt temperatures are kept as low as possible. Prior to the pelletizer, the molten polymer passes through a screen pack. All extruders use screen packs to filter out particulates and breakdown gels. In reclamation processes this stage takes on a greater importance because the frequency of the particulates is substantially higher and the composition is less predictable. As discussed previously, particulates are generally considered contaminants which could adversely affect the performance of a primary product.

In general, the path the resin follows will determine its heat, shear and contamination history. The life cycle process as a flow chart is shown in figure 5.

virgin resin from reactor

20 kg

100%

No

150 kg *-------'-Consumer HOPE

extrsuion 96/4

190 kg

Yes

30 kg

extrusion

75/25

Delayed diversion +_-1

Figure 5. Life cycle ofHDPE

20 kg

40 kg

280

For illustrative purposes, to demonstrate the state of polyolefin recycling in 1992, the life of 200 kg of virgin HDPE bottle resin is studied. The assumptions are that 10% of the resin is used in milk bottles, which is a homopolymer product, 8% is used in motor oil bottles, a difficult to clean product, and the remainder is used in other consumer bottles. A macroscopic perspective suggests that consumer products use about 5% post consumer recycled material and motor oil bottles about 25%. Some products would use more and some none at all. This does not take post industrial or internal scrap levels into consideration in terms of reclaimed content. It can be concluded that, at the current levels of recycling, a large fraction of the polyolefin molecules will be exposed to at most 4 thermal and shearing cycles.

Depending on the quality of the reclaimed material, the polyolefin will then be re­extruded into another primary product application or into a durable good. In a primary product, the reclaimed polyolefin could be blended with virgin material at ratios as high as 50/50 and, in some cases, tests are being done using 100% reclaimed material. The heating and shearing exposures experienced at this stage are similar to those in the initial product extrusion. If the material is incorporated into a durable good, it is temporarily diverted from both the recycling and the waste streams. Durable goods are characterized as large part products which have longer term use/reuse in an application. Garbage cans, injection molded beer crates and compression molded sheet for truck liners are considered durable goods, in contrast to single use blow molded bottles or stretch wrap for pallets. The manufacture of a durable good will expose the polyolefin to another thermal and shear deformation process. The life of the resin in a value added application, such as a blow molded detergent bottle or milk bottle, tends to be limited to the first use before it enters the solid waste/recycling stream.

The random nature of the recycling system can lead to extreme lot to lot variation in batches of reclaimed material. If the life cycle described earlier is repeated a number of times, it can be shown that the variability in the source material for recycling will increase dramatically. This puts tremendous pressure on the recycler to maintain consistency. It has been shown that, while lots of reclaimed material may have average physical and rheological properties close to the virgin polymer, their consistency is unpredictable [18]. The presence of contaminants produced numerous extrusion, odor and smoke problems.

3. The Effects of Reprocessing on the Rheology of Polyethylene

The effects of degradation and contamination can be highly interactive on the melt flow and physical characteristics. In terms of molecular structure and depending on the catalyst system, the average molecular weight (MW) may increase or decrease and the distribution (MWD) may narrow or broaden. It has also been shown that changes in MW and MWD affect elastic properties, based on the effect of branching, and may appear as a change in swell characteristics. The efficiency of the original polymerization process can also contribute to molecular changes during extrusion. The level of unsaturation in the virgin polymer will have a direct relationship with the change in branching along the polymer chain. Groups which are present after polymerization is terminated will react with unsaturated groups along the backbone chain and form branches. This type of modification tends to occur in a very few number of extrusion passes.

281

3.1 CHANGES IN VISCOSITY

In a series of experiments by Zahavich and Vlachopoulos [19], it was shown that there was little effect of shear and temperature, over 8 extrusion passes, on the viscosity of a virgin HDPE, homopolymer, blow molding resin, as shown in Figure 6. Similar results were observed with other properties such as pumping efficiency and tensile strength. However, a decrease in the capillary extrudate swell ratio was observed, with most of the drop occurring within the first four extrusion passes. This result was consistent regardless of the severity of the extrusion conditions.

In the same work, a more comprehensive study was done to examine the effects of multiple extrusion passes during reprocessing. The virgin homopolymer resin, a virgin butene-ethylene copolymer, a non-colored post-consumer recycled (PCR) HDPE and a mixed color PCR were used in this study. In North America, natural PCR is primarily homopolymer HDPE used in milk bottles. Mixed color PCR is a composite of homopolymer HDPE and copolymer HDPE used in detergent bottles. The four resins were subjected an extrusion shear rate of 560 s,\ at a melt temperature of 215°C for 4 extrusion passes. The changes in viscosity were expanded to include a study of changes in elongational viscosity and storage and loss modulus, G'(ro) and G"(ro). The latter were included due to their relationship with molecular weight characteristics.

The relationship between viscoelastic properties and MW characterization has been reported widely [20]. However, it has usually been in the direction of MW predicting viscoelasticity. In the last 15 years Tuminello [21,22], Yu [23] and others have successfully used dynamic measurements to describe MW and MWD properties. More specifically, Zeichner [24] and Shang [25] have used the terminal zone crossover point, Gc(w), ofG'(w)

and G"(w) to determine the polydispersity index, PI (PI = Mw ), of polypropylene, where Mn

6 PI = 10 (Pa) (1)

Gc(ro ) at

Gc(w) = G'(w) = G"(w) (2

Although this technique is not widely applied to PE, it was used in this study for comparison purposes, as in the work by Hinsken [26] and Moss [27]. For post consumer resins this technique may allow for MWD comparisons independent of the purity of the material, where pigments and residual contaminants may be present.

The second study also included two application specific performance properties. During the multiple pass extrusions a study of melt strength was done by measuring the sag of the molten extrudate strand as it passed from the extruder to the take off apparatus.

Environmental stress crack resistance (ESCR) is critical to many blow molded bottle applications. Changes in MW which may affect rheology may also affect ESCR, therefore it was a logical step to observe changes in ESCR with multiple extrusion passes.

282

1~0~_-_-_-_--_-_-_-_-_-_--_-_-_-_-_-_-_--_-_-_-_-_-_-_--_-_-_-_-_-_-_~_~_~_~_~_~_~_~_~_~_~_~_~_

------------------------------------OP~ ;3-

1st Pass .... 41hP~ ..... 8thP~

100~----------~--~~~--~~------~------~----~~~ 10 100 Shear rate (1/s)

a) extrusion shear rate = 460 sot, melt temperature = 190°C

1000

10000~_-_--_-_-_-_--_-_-_-_--_-_-_--_-_-_-_--_-_-_-_--_-_-_-_--_-_-_-_--_-_-_-_--_-_-_-_--_-_-_-_--_-_-_-,

-e­OP~ ;3-

1st Pass .... 4th Pass ..... 8th Pass

100~----------~--~~~~~~ ______ ~ ____ ~~ __ ~ __ ~~ 10 100

Shear rate (1/s)

b) extrusion shear rate = 460 5-1, melt temperature = 240°C

Figure 6. Viscosity curves for HDPE homopolymer multiple passes

1000

283

The steady state shear viscosity was determined with a capillary rheometer using the Rabinowitsch correction factor to correct for the shear rate at the wall. The elongational viscosity was determined from the pressure loss at the entrance to the capillary die using the Bagley correction factor. Dynamic viscosity and shear modulus measurements were recorded using a plate on plate geometry through a frequency sweep from 0.6 to 126 rad/s. An example of a typical set of viscosity curves is presented in Figure 7.

In general, the four resins showed a small difference in the shear viscosity, as determined by the capillary rheometer, from the "as is" or 0 pass state to the 4 pass state. In terms of the complex viscosity, the virgin copolymer did not show a great difference from 0 to 4 passes, whereas the other three polymers showed a more significant difference, especially at the lower frequencies studied.

The viscosity curves for the virgin copolymer appeared to shift down, while the viscosity curves for the other three resins appeared to shift upward.

The behavior of the elongational viscosity changed for all four materials but the differences for the PCR materials were less defined. The nature of the change was different for the virgin copolymer, with a change in the slope of the curve, to a greater dependency on shear rate. The other resins had an upward shift in the elongational viscosity curve, with virtually the same slope. The storage and loss modulus curves followed the opposite pattern as described by the elongational viscosity.

The virgin copolymer G'-G" curves shifted down, with essentially the same slopes, from 0 to 4 passes, while the other three materials shifted up slightly but had a significant decrease in slope from 0 to 4 passes.

There was clearly a distinction in the viscosity and viscoelastic properties between the virgin copolymer and the other three materials. The natural PCR material behaved in a similar fashion to the virgin homopolymer, but with less definition in the changes from 0 to 4 passes.

The mixed color PCR behaved more like the virgin homopolymer than the virgin copolymer. However, the changes were not as defined as the changes observed in the natural PCR.

3.2 CHANGES IN POL YDISPERSITY INDEX AND MOLECULAR WEIGHT

The primary intent of studying the modulus data was to use the information to examine changes in the molecular weight distribution. As described earlier, it has been reported that the G' - G" crossover, Go, point in the terminal zone can be used to measure the polydispersity index, PI (eq. 1). The breadth of the MWD has a direct relationship with PI and an inverse relationship with Go. Consequently, as Go shifts down it indicates a broadening of the MWD.

In all cases the PI increased when the number of extrusion passes increased. The magnitude of the increase was significantly larger for both the virgin homopolymer and

284

lrnro~~-------------------------------------------------------.

100000

1000

• o pass

• 4 pass

10 100

shear rate. strain rate (5-1)

a) shear and elongational viscosity

1000 ..----------------------------------------------r1000

~ 100

~ j?;­·iii 8 UI .> x ~ 10 E 8

0.1

.. pass crossover

1.0 frequency (raclls)

• o pass

• 4 pass

b) complex viscosity, storage and loss modulus Figure 7. Rheology curves for virgin copolymer

100

10

10.0

'iii 0.. 0 0 0 ~

UI ~

"3 -g E

285

natural PCR. The virgin copolymer had the least amount of change. A summary of the Gc

and PI determined in this study is given in Table 1.

Table 1. Summary of modulus crossover and polydispersity index

Experiment design Modulus crossover and

polydis )ersity index

resin G'-G" Polydispersity source pass

Index crossover

(pa) (PI)

homopolymer virgin 0 30083 3.3 homopolymer virgin 4 12503 8.0

natural pcr 0 29307 3.4 natural pcr 4 12920 7.7

copolymer virgin 0 31638 3.2 copolymer virgin 4 22437 4.5

mixed colr pcr 0 28463 3.5 mixed color per 4 13665 7.3

3.3 CHANGES IN MELT STRENGTH

A graphic summary of the extrudate sag is given in Figure 8. It can be observed that at 0 passes the virgin copolymer exhibited superior sag resistance compared to the other three resins, in particular the natural PCR. However, at 4 passes the reverse is true. The virgin copolymer sag resistance dropped dramatically and the natural PCR sag resistance increased significantly. The virgin homopolymer also demonstrated an increase in sag resistance, while the mixed color PCR maintained the same characteristics from 0 to 4 passes.

3.4 CHANGES IN ESCR

This response provided the most dramatic effect from multiple extrusion passes. The virgin homopolymer, the mixed color PCR and the virgin copolymer decreased in ESCR, and the natural PCR showed little change The magnitude of the decrease for the homopolymer and mixed color PCR was about 20%, with the virgin copolymer dropping by over 60%.

A summary of the results obtained in this study is given in Table 2, where F50 represents the 50% failure point.

286

o~================================~

-10

E ~ C> .. '"

-20

-30 +---~--~--~--r---~~~~---T--~--~-------r--~ 0.0 0.5 1.0 1.5

span(m) • virgin homopolymer, 0 pass 0 natural peR, 0 pass

• virgin homopolymer, 4 pass • natural peR, 4 pass

a) virgin homopolymer, natural PCR

o~================================~

-10

E ~ C> .. .,

-20

-30 +---------------~r---~----------_r--~--~----~--~--~ 0.0 0.5 1.0

span (m)

• virgin copolymer, 0 pass 0 mixed colour PCR, 0 pass

• virgin copolymer, 4 pass • mixed colour peR, 4 pass

b) virgin copolymer and mixed colour PCR Figure 8_ Sag plots for experiment set 2

1.5

287

Table 2. ESCR response to multiple extrusion passes

Experiment design ESCR

resin I source I pass F50

homopolymer virgin 0 7.25 homopolymer virgin 4 5.75

natural pcr 0 6.00 natural pcr 4 6.75

copolymer virgin 0 90.0 copolymer virgin 4 19.0

mixed color pcr 0 10.0 mixed color pcr 4 8.0

3.5 ANALYSIS

In general, the conclusion from this work supports the fmdings of Moss [27] and Hinsken [26] and adds PCR materials and more application specific responses, such as swell and ESCR, to the analysis. The natural PCR material behaves much like the virgin homopolymer, confirming that the source of the natural resin is primarily homopolymer based bottles. The 0 pass characteristics of the natural PCR are very similar to the virgin homopolymer, in particular the viscoelastic modulus, which may suggest the presence of some virgin homopolymer resin material blended into the natural resin. The mixed colour PCR has similar tendencies as the virgin homopolymer and natural PCR. However, the distinction between the 0 and 4 pass curves for the mixed color PCR are not as clear as for the other 2 materials. This indicates that this PCR stream is more of a composite of copolymer and homopolymer bottles, though mixed color PCR is often marketed as a copolymer based material.

Aside from the change in polydispersity, which was also observed by Moss, the greatest effect of multiple passes was observed in the swell and ESCR properties. The change in viscosity, as measured with a capillary rheometer, showed little effect regardless of the material. However, the complex viscosity at the lower frequencies and the elongational viscosity, as determined from the Bagley correction factor, did show some effect from multiple passes.

Swell represents a recovery of stored elastic energy. The molecular structure, in particular molecular weight and long chain branching, has a profound effect on the swell characteristics, but to date there is no consistent theory to explain the relationships. Early investigations into the phenomenon of swell showed that HDPE appeared to behave in a similar fashion to polystyrene (PS), where an increase in the breadth of the MWD of PS

288

produced an increase in swell. However, work by Mendelson [28] showed that there was some doubt as to the strength of the relationship in the case of HDPE. This study led to an investigation by Shroff and Shida [29] which attempted to explain the anomalous work of Mendelson. The conclusions of this work are best described by a direct quote from the final sentence:

" ... Thus the effect of increasing MW is opposite of broadening MWD such that when polymers vary in both MW and MWD, die swell and compliance may decrease or increas, depending on whether increase in MW is predominating or whether effect due to broadening of MWD is predominating. "

Shida's earlier work with Nakajima [30] did not study the relationship between MWD and swell rather; they examined the mechanism of swell in terms of elastic recovery. In their study, they used two resins. The first was a Phillips based HDPE and the other was the same resin passed through an extruder 3 times at 240°C, not unlike the first set of experiments done by Zahavich and Vlachopoulos [19]. The technology at the time suggested that for this Phillips catalyst resin the re-extrusion process caused preferential chain scission. This was thought to reduce the higher molecular weight fractions without increasing low molecular weight components, and it was also suspected that a small amount of long branches were created. This statement is in direct contrast to the conclusions of Moss and Zweifel [27]. The conflicting evidence in these papers show that a generalized link between swell, MWD and MW and the degradation of HDPE would probably not be a useful exercise. Taking into consideration these past studies, an analysis on swell mechanisms relative to changes in molecular structure resulting from multiple passes, should be done for a specific resin and take into account the primary polymerization process, the predominant degradation mechanism, and the effect of residual components in the resin i.e. pigments, dirt etc.

The results of the sag study provided a unique opportunity to study the changes in melt strength. Sag in blow molding is best described as the extension of the molten parison due to gravitational force. Any variance in the sag properties of a resin will have a direct effect on the shape and physical characteristics of the bottle.

Observed changes in sag with multiple passes is evidence that some higher MW material in the copolymer is lost while some chain lengthening occurs in the homopolymer and natural PCR. Previously reported work on polyethylene used in blown film has shown an exponential relationship between melt strength and the zero shear viscosity, 170 [31]. As 170 increased, the melt strength increased and it was found that

1/3.4 melt strength = kTJ 0

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-3.4 TJO - MW

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From this link it was concluded that the melt strength was directly proportional to MW. In a later study with polypropylene the same relationship was reported [32].

289

The findings in these 2 studies are consistent with observed changes in sag and changes in the low frequency complex viscosity. It has been established that chain scission is the dominant degradation mechanism in the copolymer resin and crosslinking dominates the modification of the homopolymer. A drop in low frequency complex viscosity and sag for this particular copolymer and increase for the homopolymer, after four extrusion passes,

could be attributed to a reduction in the MW of the copolymer and an increase in MW for the homopolymer. Natural PCR appears to behave much like a virgin homopolymer providing more evidence that this material is primarily composed of homopolymer resin. On the other hand, as discussed earlier, the mixed color bottle appears to behave more as a composite of copolymer and homopolymer material. The relatively small change in the sag for this material would lead to the conclusion that neither degradation mechanism is dominant in this resin.

The observed change in the PI provides a relative measure of the changes in the MWD. The change in the position and slope of the loss modulus curve may provide some evidence of the nature of the broadening of the distribution. A general increase in PI, driven by a drop in Gc, showed a broadening of the MWD for all materials. The inconsistent behavior of the copolymer, relative to the other three resins would indicate that, while the MWD is broadening, the average MW is decreasing for the copolymer and increasing for the homopolymer and natural PCR. This observation provides some explanation on the change in the sag behavior of the four resins. As the average MW decreases the resistance to sagging decreases, as occurred with the copolymer, while the opposite occurred with the homopolymer and natural PCR. The mixed color average MW changed very little and the sag also changed very little.

The magnitude of the changes in extrusion performance were quite low and arguably may be insignificant but there may be some molecular based explanation for the behavior of the different materials. The observed increase in pressure drop for the homopolymer/natural materials is consistent with a crosslinking mechanism. On the other hand, the drop in pressure for the copolymer suggests a loss of higher molecular weight material through chain scission. A lack of change in pressure drop for the mixed color PCR is similar to the observations from the sag study. In effect, the amount of crosslinking may be offset by a similar degree of chain scission, resulting in little change in the extrusion performance.

In many applications the ESCR property is a crucial performance requirement. The large decrease associated with the virgin copolymer can have a significant impact on the suitability of using this material, in the original primary application, after 4 passes. The high ESCR performance of this material is based on the high MW fraction and short chain branching, which is directly attributed to the presence of the copolymer. The interlamellae interaction of the high MW chains provides a strong resisting force to the cleaving action associated with stress cracking in an aggressive media. As the high MW chains are consumed during chain scission, the resistance to cleaving also declines and the ESCR drops. The other materials do not have as large amount of high MW material initially and as result do not have as high initial ESCR. At the same, in earlier sections it has been suggested that crosslinking is occurring in the other materials. Crosslinked material tends to produce a rigid structure which is brittle. An increase in brittleness tends to lead to a

290

decrease in ESCR because of a greater number of microvoids and larger surface area exposed to the aggressive media. Both the ESCR and the tensile properties, studied in the first set of experiments, are sensitive to the forming process in terms of part shape and cooling rate and can have a large variance in the data. The drop in ESCR for the copolymer is significantly larger than the variance.

4. Thermoplastic Recycling System Case Studies

4.1 ACCOUNTING FOR PCR V ARlABILITY IN SCHEDULING

The design, scheduling and production of products containing reprocessed resin is greatly dependent upon the available supply of the material and the lot to lot variability of the supply. The following case study is an illustration of how the supply and variability concerns can be addressed.

In Figure 9 an extrusion flowchart is presented to show where key decisions are made involving scheduling equipment and selecting materials. If PCR is a component of a blend system, there should be in place checks to account for the quality of the PCR. Typically for companies registered to the ISO-9000 standard or who have an effective quality system in place, a certificate of analysis is required prior to receiving each shipment of virgin resin. The certificate of analysis may include the melt index (MI), density and additive levels for the lot of resin. Since the variability in a property for a particular lot of resin from a reactor is known, there is an implicit expectation that the behavior of a virgin resin is reasonably predictable.

In the case of PCR, the MI and density are not usually indicative of the true properties for a lot of material and a certificate of analysis may not be relevant. Therefore, in order to account for the variability, a procedure should be in place to assess the rheological and physical performance of the material prior to its incorporation into production. It could be argued that a thorough analysis of the viscosity and viscoelastic characteristics should be done. In practice, time constraints and the lack of analytical equipment may force a manufacturer to adopt a more applied approach to assess the suitability of a lot of PCR for a specific application. Nonetheless, prior to a full scale run with large parts or multilayer coextrusion equipment, it is prudent to assess an incoming lot of PCR to allow for the possibility of making minor blend or manufacturing changes.

4.1.1 Establishing a Procedure for Assessing Material

A quality supplier of PCR will homogenize a lot of material to the best of his ability without exposing the plastic to excessive shear and degradation. However, the true performance of the material often may only be assessed by testing the resin in a specific application. In the flowchart in Figure 9 two areas (labeled 1 and 2) where testing can occur are identified.

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4.1.2 Small Scale Assessment

At point 1 a small scale trial can be done on a small extrusion line where run time and volume of resin is minimized. At this stage, a gross assessment of the lot of resin can be carried out in terms of sensory quality, level of contaminants and extrusion performance.

Sensory quality refers to color consistency and odors. If the planned application for the material is not sensitive to them then this may not carry significant weight. However, if opacity, color or odor are concerns, this trial can be used to accept or reject the material. Streaky parts or film may indicate a non homogeneous mixture which could cause problems for color matching. Brownish or yellowish tints could indicate the onset of severe oxidation, which could have an effect on other properties.

Odors can present problems with the acceptance of finished parts. The strength attributes of a product may be acceptable but an unusual odor may cause a customer to reject it. Rancid odors typically result from materials contained by a plastic package. Some additives, such as slip agents in films, can also turn rancid if the additive is past its shelf life. Sensory qualities can be assessed both in the small scale production of a finished part or from a cursory sample of the pelletized peR.

During a small scale trial the level of contaminants remaining in peR can be readily observed. In particular, the size and frequency of gels and dirt particles may not only affect the performance of the finished product but also lead to excessive scrap levels during manufacturing. Similar to the sensory assessment, the level of contaminants can be evaluated visually. These observations should also include an assessment of build up on dies or screen packs, and in film production, tear-off or bubble loss.

During the extrusion or molding of a part, extrusion pressures and temperatures should be monitored and compared against a benchmark. The gross shear thinning behavior of peR can quickly be determined by observing extrusion pressure. For example, a material rich in a high density or linear low density component will generally extrude at higher pressures than low density materials. If typical shear rates are achieved in the small scale assessment melt fracture may also be observed. At this point a decision can be made in terms of the economic benefit of adding processing aid to eliminate the presence of melt fracture.

Melt strength can also be determined at this stage by observing sag or bubble stability. Referring to the discussion in section 3.5, the melt strength of the peR could adversely affect the molding or film manufacturing process. Thus, a given lot of peR may have to be enhanced with virgin material to maintain a desired level of melt strength.

If a peR is to be blended, its mixing capability must also be determined. A standard blend ratio can be established using a virgin resin and this blend can be a part of the small scale trial procedure. Surging extrusion pressures or a "mottled" appearance (often referred to as "lensing") to the surface of a finished part can be indicative of a blend that won't mix well. Screw design and temperature profiles may be used to overcome lensing or "unmelts" but, if the viscosity of the peR and virgin resin are not well matched, mixing could become a significant problem.

293

4.1.3 Full Scale Assessment

Point 2 on the flowchart represents the second area for assessment, a full scale trial. The full scale assessment is essentially a confirmation trial, that may immediately precede the production run. Minor changes in blends or coextruded layer ratios may be made at this stage. Sensory quality and extrusion performance should not be an issue unless there is a significant variance within a lot of material.

Operators should be trained to observe changes in extrusion performance and product quality that may occur with PCR. They should also be given the tools to make decisions about what adjustments may have to be made to move a PCR based product back into the specification range. Adjustments of temperature profiles and additive levels are typical with PCR

With an assessment procedure in place, such as the one described in this case study, surprises should be minimized, yet the time to assess shouldn't be onerous. PCR can be treated as another raw material but one with a lot to lot variance that is wider than virgin material.

4.2 RHEOLOGICAL CONSIDERATIONS IN PCR THERMOPLASTIC APPLICATIONS

As discussed in the previous section, any number of factors have to be considered when approving a lot of PCR for a specific application. Aesthetic and physical properties such as optics and contaminant levels are usually the primary criteria for acceptance. Extrudablity and formability are often considered as secondary criteria and any deficiencies may be overcome by modifying manufacturing parameters. The following are examples of specific applications where rheology is a consideration when incorporating PCR into a thermoplastic structure.

4.2.1 Mononlayer Thermoplastic Structures

Blending PCR into a monolayer structure may require a matching of rheological properties of the peR to a virgin resin. If key property performance targets have been established and it has been determined that a lot of PCR meets those targets, then the rheology of the PCR can be enhanced with virgin resins to maximize output.

In blow molding, film blowing and casting the shear thinning behavior and melt strength of a polymer are critical to maximizing output. The previous case study dealt with lot to lot variability. Once a lot of material is approved, if it is assumed to be homogeneous, then the viscosity and melt strength should also be consistent throughout the lot. Any extruder, screw and die system selected for running the PCR will most likely be designed around the rheological characteristics of a virgin polymer. Therefore, to maximize the output of the PCR, its viscosity and melt strength would have to be modified to match it to the virgin polymer. It is worth noting that, as the rheology of PCR is similar to that of the virgin resin, the same blending principles are applicable.

Viscosity modification could be achieved by either blending the PCR with a virgin resin or by altering the temperature profile of the extruder. However, modifying a temperature profile is often limited in its effectiveness and may cause other problems such

294

as oxidation and die build up. Blending PCR with virgin material is a viable solution but only if it is economically feasible and the material is readily available.

Consider a scenario where a PCR tends towards melt instability at high speeds or at normal melt temperatures. Excessive parison sag and bubble or web instability are indicative of a lack of melt strength. Therefore, the addition of a low MI resin will improve the elongational viscosity of a PCR that has a tendency to excessive shear thinning and increase its melt strength. This should improve the cycle time for blow molding, improve bubble stability in blown film or increase the line speed before the onset of draw resonance in cast film.

4.2.2 Coextruded, Rotomolded and Commingled Structures

In many instances the state of a batch of PCR does not meet the needs of a specific application or the availability of a consistent supply is not sufficient to satisfy the required volumes. In these situations coextruded, commingled or alternative applications may be used to consume the batch of PCR.

Coextruded structures are an attractive alternative for two reasons. First, deficiencies in certain properties associated with PCR can be supplemented by layers of virgin material. There are increasing demands for PCR in retail bags and courier pouches but, at the same time, the physical and aesthetic properties must be maintained. A three layer coextruded structure allows PCR to be sandwiched in the core layer between two virgin skin layers. The virgin resins can improve tear and impact strength, maintain gloss and at the same time hide the high level of gels inherent to PCR.

A second opportunity with coextruded structures is also related to the use of virgin resin skin layers. Recent approval for food contact of coextruded films with PCR core layers has been granted based on the precedent of food contact approval for virgin resins. There is some concern about migration of compounds from the core to the skins, and this has led to tight controls over the source of the PCR for the core layers.

Since it has been shown that the shear viscosity characteristics of PCR polyethylene materials closely match the characteristics of the virgin resins used in the skins, coextrusion with PCR is no more challenging than coextrusion with virgin resins. However, the presence of gels or changes in melt strength can cause problems, because they can induce the generation of microvoids between layers or instability in the blow molding parisons or blown film bubbles.

Rotomolded structures represent another opportunity for batch PCR applications. The challenges associated with this process involve developing a homogeneous resin which flows consistently during melting and fusion. This is essential to minimize the level of inclusions and microvoids which could lead to areas of high stress concentrations. By compounding a film grade polyethylene PCR material with a certain type of virgin polyethylene, a group of researchers has succeeded in producing a resin having excellent rotomolding characteristics [33].

295

As mentioned in section 1. 3.1, there is a growing market for commingled applications which use PCR as a matrix for other materials. Two applications of note involve the use cellulose fibers and wood flour in thermoplastic PCR materials.

In one case, post industrial cellulose fibers are blend in a proprietary binding process to produce a homogeneous feedstock. A sophisticated die design, which accounts for the different flow characteristics of the two dissimilar materials in the feedstock allows for the continuous formation of intricate profiles. The profiles are cut and assembled into a reusable, recyclable pallet. The challenge in this process centers around maintaining a consistent flow regime through the die system.

The use of wood flour in thermoplastics is not a new technology but is often used in proprietary processes. Recent advances in extruder design are leading to an increase in the use of these types of compounds. Rheologically, traditional screw designs for PCR thermoplastics create too much shear heat. Co-rotating twin screws allow for the distribution of the fragile, heat sensitive wood fibers without charring the material. These applications typically require low viscosity PCR materials and are often used in injection molding products [34].

5. Concluding Remarks

In this paper it has been shown how multiple extrusion cycles affect the rheological properties of HDPE blow molding resins. While competing degradation mechanisms appear to be related to the type of catalyst systems involved, the resultant change in properties varies. In all materials analyzed in this study the shear viscosity showed little effect from multiple extrusion passes. Other properties such as elongational viscosity, sag and ESCR changed differently depending upon the catalyst system. It was also shown that the PCR materials behaved similar to the virgin resins. A variety of case studies were used to demonstrate how varying rheological and physical properties can be overcome. Identifying opportunities during scheduling or incorporating PCR in specific applications, such as coextruded, rotomolded and wood filled structures, can minimize the effects of PCR variability and can ensure the consistent performance of PCR based products.

6. References

1 Brooke, L., Kobe, G. and Sawyer, C. (1990) Recyclability, Automotive Industries, September.

2. Pilot Project - Barhaven Demonstration Project (1990) Canadian PlastiCS, March.

3. Bond, B.E. (1990) Recycling Plastics in Akron, Ohio, Proceedings of the SPE -RETEC, Toronto, Canada.

4. Packaging Without Plastics (1987) German Society for Research into Packaging Markets.

5. Lynch, M. (1990) Drinking boxes now recyclable, Canadian Plastics, July/August.

296

6. Deloitte and Touche (1991) Summary Report - Energy and environmental impact profiles in Canada of TETRA BRIK ASEPTIC carton and glass bottle packaging system, Toronto.

7. Did the earth get a break today or did the public misdirect MacDonald's? (1991) Today's Generation, March.

8. Kenny, G.R and Bruner, RS. (1993) Experience and Advances in Automated Separation of Plastics for Recycling, Proceedings of the SPE-RETEC, Chicago.

9. Ehrig, R (1992) Plastics Recycling, Hanser, New York.

10. Ezrin, M., Wyatt, D., Lavigne, G. and Garton, A. (1994) Quantification and control of contaminants in recycled HDPE, proceedings of the SPE-ANTEC'94, San Francisco, 2922-2926.

11. Portugues, M.M. (1994) Situaci6n del reciclaje de los materiales plasticos utilizados para el cuItivo de la banana en Costa Rica, 1 Coloquio de Reciclado de Plasticos, Guadalajara, Mexico.

12. Waters, K. (1989) Bottles, plastic coextrustion, multilayer and high barier, Packaging's Encyclopedia, 60.

13. Renfree R W. et al (1989) Physical characteristics and properties of profile extrusions produced from post consumer commingled plastic wastes, Proceedings of SPE-ANTEC'89, 1809-1812.

14. Engleman, P.v., et al (1992) Extrusion - compression of commingled resin blends, Plastics Engineering, February, 27-31.

15. Meyers, G.E. and Chahyadi, 1.S. (1991) Wood flour/polyppropylene composites, Inter. J. Polymer Materials 15, 21-44.

16. Rathje, W.L. (1989) Rubbish!, The Atlantic Monthly, December.

17. Paudich, c.w. and Ritzman, H.B. (1993) Rapid identification of plastics utilizing fast NIR-spectroscopy, Proceedings of the ReC'93, Geneva, vol. II, 268-271.

18. Gibbs, M.L. (1990) Post consumer recycled HDPE: is suitable for blowmolding?, Plastics Engineering, July, 55-59.

19. Zahavich, A.T.P, Latto, B., Takacs, E. and Vlachopoulos, 1. (1997) The effect of multiple extrusion passes during the recycling of high density polyethylene, Advances in Polymer Techology 16, 11-24.

20. Han, C.D. (1976) Rheology in Polymer Processing, Academic Press, New York.

21. Tuminello, W.H. (1986) Molecular weight and molecular weight distribution from dynamic measurements of polymer melts, Polymer Engineering and Science 26, 1339-1347.

22. Tuminello, W.H. and Cudre-Mauroux, N. (1991) Determining molecular weight distributions from viscosity versus shear rate flow curves, Polymer Engineering and Science, 31, 1496-1507.

297

23. Yu, T.L. and Ma, S.C. (1992) Polymer molecular weight from loss modulus, Polymer Joumal 24, 1321-1328.

24. Zeichner, G.R and Macosko, C.W. (1982) On line viscoelastic measurements for polymer melt processing, Proceedings of SPE-ANTEC'82, San Francisco, 79-81.

25. Shang, S.W. (1993) The precise determination of polydispersity index in rheological testing of polypropylene, Advances in Polymer Techology 12, 389-401.

26. Hinsken, H., Moss, S., Paquet, J-R and Zweifel, H. (1991) Degradation of polyolefins during melt processing, Polymer Degradation and Stability 34, 279-293.

27. Moss, S. and Zweifel, H. (1989) Degradation and Stabilization of high density polyethylene during multiple extrusions, Polymer Degradation and Stability 25, 229-245.

28. Mendelson, RA and Finger, F. L. (1975) High density polyethylene melt elasticity - some anomalous observations on the effects of molecular structure, J. of Applied Polymer Science 19, 1061-1077.

29. Shroff, R, Shida, M. (1977) Effect of molecular weight and molecular weight distribution on the elasticity of polymer melts, Proceedings of SPE-ANTEC, 285-289.

30. Nakajima, N. and Shida, M. (1966) Viscoelastic behavior of polyethylene in capillary flow expressed with three material functions, Trans. Soc. Rheol. 10, 299-316.

31. Ghijsels, A, Ente, J. and Raadsen, J. (1990) Melt strength behavior of polyethylene and its relation to bubble stability in film blowing, Int. Polymer Processing 5, 284-286.

32. Ghijsels, A, De Clippeleir, J. (1994) Melt strength behavior of polypropylene Int. Polymer Processing 9, 252-257.

33. Nichols, K., Voldner, E., Vlachopoulos, J. Takacs, E. and Kontopoulos, M. Recycled Rotational Molding Resin, U.S. patent pending.

34. Schut, J. (1997) Wood-filled thermoplastics go commercial, Plastics Formulating and Compounding, January/February, 35-36.

Chapter.4 REPROCESSING OF MIXTURE OF POLYMERS

SEPARATION TECHNOLOGIES

JACOB LEIDNER ORTECH Corp. 2395 Speakman Dr. Mississauga, Ontario Canada.

DR. GRAHAM BODEN School of Applied Sciences University ofWolverhampton Wulfruna St. Wolverhampton WVllSB U.K.

Separation and cleaning is usually a central part of a plastics recycling process. Separation processes utilize differences in some of the properties of the materials to be separated. Some of the properties used to separate plastics are density, surface energy, appearance, colour, solubility, low temperature behaviour and melt flow properties. Separation can be carried out manually or using automated process.

1. Introduction

In order to produce a recycled resin with the properties approximating those of the virgin material and, therefore, recover most of the inherent value, plastic waste to be recycled has to be uniform, well defined in its composition and free of non - plastic impurities. Separation and cleaning is usually a central part of a plastics recycling process. Some typical separation needs which are encountered are :

301

G. AJrovali et al. (eds.). Frontiers in the Science and Technology of Polymer Recycling, 301-332. © 1998 K1uwer Academic Publishers.

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PET bottles - separation of PET from other plastic bottles, especially PVC, removal of non-PET components of the bottles such as label, label adhesive and cap. Automotive body trim waste - the waste produced in manufacturing of body trim consists of aluminum or stainless steel with a plastic (usually PVC) bonded to it. The metal is the more valuable component of the waste. Insulated copper wire - copper is the valuable component of the waste. Plastic can be recycled if different types of plastic present in the waste stream are separated. Supported PVC - materials such as vinyl upholstery and vinyl wallpaper consist of about 50% of plasticized PVC and 50% of supporting fabric or paper. PVC can be re-used if separated from fabric. Plastic collected through curbside collection - different types of plastics have to be separated. Waste polyethylene coated cardboard - paper can be recycled through pulping operation. The remaining polyethylene film can be recycled if the remaining fibre adhering to it is removed. Waste automotive plastic bumpers - plastic can be recycled but the paint has to be removed first.

Separation processes utilize differences in some of the properties of the materials to be separated. Some of the properties used to separate plastics are :

density surface energy appearance colour solubility low temperature brittleness flow properties

303

Separation processes utilize these differences in properties directly, in the sorting processes the relevant property is measured and decision is made in which stream to place a given component of the waste. Sorting can,be done manually or automatically.

2. Sorting.

Sorting has been described in a number of publications [1],[2],[3]. EPA [1] conducted a study to compare automated and manual sorting of plastic bottles.

2. 1. MANUAL SORTING

Fig 1 shows a layout of a manual sorting line. The facility processes approximately 3000 pounds of material per hour ( based on 3 months average, including downtime ).

The facility receives baled waste consisting of plastics containers and produces bales of : Clear PET and green PET

Natural HDPE Mixed colour HDPE PVC rich material PP

The facility also sells approximately 3000 pounds of bailing wire per month and disposes of 50 tons of non - marketable waste ( 10.5 % of total input) per month. The facility operates on two shifts and each shift employs

nine sorters three inspectors a supervisor a bale breaker a part time mechanic

304

The bale is deposited by a fork lift and the bailing wire is manually cut. The bale is lifted by a forklift and then dropped on the floor to open the bale. The bale is further broken manually and bottles separated. Conveyor carries the waste to the vibratory screen where the small components such as caps are separated. Vibrating screen further separates clumped bottles. Material is then transferred to the main conveyor. Sorting personnel on both sides of the conveyor remove their assigned materials and deposit them in the assigned bins. A return line is located under the main conveyor belt and used by sorters for deposit of PET which has not been removed by PET sorters. When the bins are filled their content is deposited onto adjacent conveyor belts. Inspectors check for any bottles which are in the wrong stream. These bottles are removed for re-sorting. The sorted material is either sent for bailing or for grinding [1].

2.2 AUTOMATED SORTING LINE

Automated sorting utilizes sensors to identify different types of plastics present in the waste stream. The system described in [1] and [3] utilizes three types of sensors. Primary sensor based on near infrarared spectroscopy identifies three groups of materials

PET and PVC Natural HDPE and PP Mixed colour HOPE

An X-ray based sensor provides a very specific identification of PVC. Machine vision is then used to separate materials based on colour. Combination of these sensors allows for separation of plastic bottles into the following groups

PVC rich stream Clear PET Green PET Natural HOPE Mixed colour HDPE

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Figure 2 shows a layout of an automated bottle sorting facility. The facility processes approximately 3600 pounds of waste per hour ( based on 3 months average, including downtime ).

The facility sells approximately 4000 pounds of bailing wire and disposes of 60 tons of waste per months. The facility operates on two shifts and each shift employs

eight inspectors a supervisor a balebreaker part time mechanic

A bale of plastics containers is deposited on a bale table where the bailing wires are manually cut. The bale is broken and the clumps of the bottles separated by machines. Containers are conveyed over a screen where the small components such as caps are separated out. The bottles are then passed by detection sensors. It is important for the automated system that the bottles are presented to the sensors one at a time. When an appropriate material is detected a computer activates an air jet which then transfers the bottle onto a sorting conveyor belt. Seven inspectors are located on these sorting lines to remove trash and to correct machine sorting errors. There is also an additional PVC sorting station positioned on a clear PET line [1].

2.3. COMPARISON OF MANUAL AND AUTOMATED SORTING FACILITIES

Table I compares operating costs of automated and sorting facilities.

Table 1. Fixed and variable costs for sorting of comingled plastics bottles. Values are in cents per pound of sorted material excluded disposed waste [1] .

Automated sorting Manual sorting facility facility

Fixed operating costs - amortization 1.0 0.4 - building lease 0.1 0.2 Total fixed operating costs 1.1 0.6 Variable operating costs - labour and benefits 2.2 4.0 - parts and supplies 0.2 0.2 - utilities 0.1 0.1 Total variable operatinJ2; costs 2.5 4.2

The automated facility operates with lower total cost per unit weight of sorted material. It is interesting to note though that even in the automated facility the labour cost constitutes a considerable portion of an overall operating cost. The accuracy of the automated system has been also found to be better.

When deciding on automated versus manual sorting system the following main criterial have to be considered

quality and market value of the resulting product cost of labour cost of capital

The high labour cost environment will tend to favour automated sorting while the low labour cost environment will tend to favour manual sorting.

2.4. IDENTIFICATION OF POLYMER TYPES

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Identification of a polymer is an essential component of sorting - both automated as well as manual. It is also usually an important tool in making a decision to purchase or receive plastics waste for recycling as well as in evaluating its commercial value. Some of the different requirements one faces are:

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- to establish generic type of a polymer where a large number of different types are possible (for example in sorting of plastics from the municipal solid waste)

- to define composition ( for example amount of a filler) when generic type of the polymer is known

- to distinguish between two possible polymers ( for example between PET and PVC)

309

- to distinguish between a limited number of possible polymers (for example in recycling of carpet where only a limited number of polymers could be used)

-to identify an undesirable element (for example lead stabilizer in PVC)

The methods utilized to accomplish the above can be grouped into

- lab scale - requiring properly equipped analytical lab and skilled personnel

- use of specialized, portable or hand held equipment delivering rapid results and requiring only minimal skills from the operator

- methods which can be incorporated into automated sorting lines - sorting by shape or appearance as practiced in manual sorting

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The ideal identification method would be -able to identify the sample - utilize equipment which is robust and portable - rapid « 5 seconds) -require minimum or no sample preparation - be applicable to a large variety of sample sizes / colours / shapes. - be capable of automation (interpretation and sorting).

2.4.1. Laboratory Identification Methods A variety of methods exist for the identification and molecular

characterisation of polymers The following list indicates some of these and their area of specificity.

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-Infrared spectroscopy ( specific chemical groups and their assemblies) - Ultra-violet spectroscopy (chromophores) - Nuclear magnetic resonance (molecular structure, especially H and C atoms) - Raman spectroscopy (weak. scattering of radiation from small particles) - Microscopy (optical features, can be coupled with infrared) - X-Ray diffraction (crystallinity) - X-Ray fluorescence (presence of heteroatoms e.g. CI) - Thermal gravimetric analysis (decomposition under the action of heat) - Differential scanning calorimetry (energy changes e.g. melting, transitions) - Pyrolysis I Infrared spectroscopy (thermal decomposition coupled with IR spectroscopy) - Pyrolysis I Gas chromatography I Mass spectrometry (molecular fragments from decomposition) - High performance liquid chromatography (additives) - Gel permeation chromatography (molecular size and distribution) - Melt flow indexer (process ability of polymers as melts) Although a combination of these methods may be needed for complete characterisation of a sample, they have severe limitations in the context of recycling. These may be summarised as :

- Most do not give identification evidence - The techniques are usually expensive - Skilled technician I expert interpretation is often necessary - Sample preparation is usually needed - The techniques are laboratory-based, immobile and non-robust - They are mostly slow (typically 2- 30 minutes)

2.4.2. Labelling Perhaps the most cost-effective solution to the problem of identification is to effectively "label" the material at the point of manufacture. In some

cases processors are making mouldings with an internationally-agreed code corresponding to the polymer type as follows: 1 - PET 2 - HDPE 3 - PVC 4 - LDPE 5 - PP

6 - PS 7 - Others

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Some manufacturers are beginning to put the name of the polymer on labels e.g. on bottles and parts of cars. More sophisticated tagging is possible through the incorporation of fluorescent additives into the polymer [4] but the difficulty of obtaining universal agreement and consideration of all the health and food implications make this somewhat impractical, and perhaps a system of bar coding on the polymer product might be more appropriate and adaptable.

2.4.3. Rapid Identification Methods Density Determination. Tests that may be useful for the identification of an occasional sample by a skilled operator would probably not be suitable for continuous screening of mixed polymers in a technological scale process. What is needed are rapid robust techniques that can be used in industrial situations with the minimum of skilled intervention and sample preparation. Ideally too they would be portable and inexpensive. The type of information sought and obtained in screening polymers varies greatly. It may be sufficient to use a relatively non-selective property such as density. Density cannot give unequivocal information about the identity of a polymeric sample. Neither the elemental composition nor the polymer identity can be stated with certainty from a determination of density. Nevertheless it has been used to screen incoming mixed polymer into density bands that could reasonably be graded as "polyolefins" and "chlorine-containing polymers" etc. Density becomes less suitable where the material may be foamed, blended or filled. The method also requires comminution of the sample before testing, and separation from the flotation liquids afterwards. Industrial scale sorting on the basis of density is a feasible method, especially useful for the routine removal of chloro polymers prior to pyrolysis.

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Infra-red Techniques. The use of Fourier Transform (FTIR) instruments with their inherent speed advantage is normal for this type of application. In FT instruments all frequencies of radiation reach the detector simultaneously, allowing complete spectra to be obtained in a fraction of a second. Co-addition of serial spectra improves the signal-to-noise ratio producing results with high sensitivity and precision in very short collection times.

Most IR spectra run in the laboratory are transmission spectra, but the usual methods for preparing polymer samples are :

- a solution in a suitable solvent - a thin film or slice - a thin film evaporated onto a sodium chloride plate - a powdered sample dispersed and compressed into a potassium

bromide disc Such methods are not directly suitable for use in recycling technology.

To avoid significant sample preparation time there have been two main approaches to rapidly obtain infrared spectra of polymers samples that might be of uneven size and shape. Both pyrolysis and reflectance methods have been employed. Pyrolysis has been used for many years as a method of producing volatile degradation products of the polymer. In the laboratory, the process is usually coupled with gas chromatographic separation of these products. Mass spectroscopy or FTIR may then be used if specific identification is required. In the context of polymer recycling neither the chromatographic separation nor the specific identification offered by mass spectrometry is necessary. However FTIR of the total pyrolysate can produce an infrared spectrum that is characteristic of the polymer or polymer mixture from which it was produced. Nicolet Instruments Corporation in association with Toyota have developed a sampling accessory called HotShot which can pyrolyse polymer samples and identify them with high (96%) accuracy in less than 10 seconds. This was exhibited at the Pittsburgh Analytical Conference in March 1997. The device uses a powerful light source to flash pyrolyse a 1 cm spot on the polymer sample. The sample does not directly contact the probe. The pyrolysate is carried on a nitrogen gas stream through a heated transfer line to a gas cell which can be measured on a conventional FTIR

313

spectrometer. The identification of the polymer is performed by comparing the spectrum against a library of 100 known pyrolysed polymers. The probe is hand-operated through two buttons to start the analysis and complete the identification. The portability of the probe and its simple controls would make it very convenient in recycling situations. Because the sample is pyrolysed to a depth of 1-2mm, surface preparation e.g. removal of paint is not necessary, and it will operate on filled dark materials. It can be used for crosslinked polymers and blends, such as ABS, PUR and HIPS. There are several types of reflectance methods:

- Diffuse Reflectance is where the incident beam is absorbed then reflected at all angles to the front surface of the material.

-Specular Reflectance is where the incident beam is reflected from the front surface of the sample.

- Attenuated Total Reflectance (ATR) is another form of reflectance requiring contact of the polymer surface with an IR-transparent medium of high refractive index. Each of these methods have found application in recycling. Diffuse reflectance infrared FT spectroscopy (DRIFTS) can be used to obtain infrared spectra of polymers in which the polymer sample is scraped off the sample surface and analysed directly. A pre-cut circle of silicon carbide paper which has an adhesive reverse side, is pressed on to a special steel plug which fits directly into the sample mounting of the DRIFTS attachment on the spectrometer. The polymer sample is abraded with the silicon carbide topped plug, and the spectrum run of this sample. A large number of scans may be necessary to obtain a reasonable signal-to-noise ratio, but satisfactory spectra can nevertheless be produced with minimal sample preparation. This method might be satisfactory for a laboratory wishing to carry out occasional inexpensive identifications but would not be fast or simple enough for automation in a recycling plant. Specular reflectance is inherently inefficient because only a small proportion of the incident radiation is reflected from the sample and measured by the detector. The reflectance R of a surface is related to its refractive index n by the formula

R = (n - 1)2/ (n + 1)2 (1)

314

This gives a figure of 4% of the incident beam that is the maximum that is measurable, for a sample with a typical refractive index of 1.5. In addition it is necessary to align the sample at a definite position, and the surface should be smooth and free of surface coatings or contaminants. In A TR the sample is pressed against an optically flat surface of a prism of materials such as germanium and thallium bromo-iodide. Such optical materials are fragile and expensive, and some sample preparations are essential. These factors make its application in industrial situations highly unlikely. The wavelength range used is either near or mid range infrared. Near IR (NIR) uses wavelengths in the range 5000 to 10000 cm- I

(wavelength 1-2J.1m). In this region the absorptions are based on overtones of the normal spectrum.

Mid-range IR (MIR) operates in the spectral range 500 to 5000 cm- I (2-20J.1m) corresponding to the region typically used to obtain IR spectra that are very characteristic of organic molecules. NIR techniques have some advantages, for example :

- diffuse reflectance is a quantifiable technique, because of the favourable scattering/absorption ratio

- remote measurements via fibre optics is possible - signal-to-noise ratio is relatively high - minimal sample preparation is necessary - radiation is not absorbed in this wavelength range by glass or

moisture. NIR is widely employed in quality control applications. It can be employed as the method of identifying polymeric packaging material where the material is usually transparent or light coloured. In contrast to the almost total lack of signal from dark polymers, the NIR spectra obtained with diffuse reflectance on polymers used in commercial packaging are of good quality and have been successfully used to identify unfilled packaging polymers such as PE, PP, PS, PET and PVc. Bruker Ltd manufacture a NIR spectrometer (Vector 22/N) that can be used with various fibre optic probes and identification software. Although reasonable spectra are obtained for unfilled polymers, it has the severe disadvantage that black polymeric materials cannot be identified by NIR, because of the lack of spectral contrast [5]. MIR is the most convenient

315

and reliable technique that can identify both filled and unfilled polymer samples. MIR is usually used in association with specular reflectance, and although appearing to have a number of apparent disadvantages has been commercially developed into systems for the identification of polymers. The disadvantages are :

-It is used as a pure surface method (specular), with negligible penetration.

-Specular reflectance is innately inefficient ( only - 4% of the incident light is measurable).

-Surface dirt, paint etc. has to be removed prior to measuring. -Fibre optic remote sensing is not possible.

In spite of these considerations MIR proved to be by far the most effective of a range of techniques studied in a wide ranging assessment of recognition methods that also considered NIR, FT Raman spectroscopy, Pyrolysis Mass Spectrometry, Pyrolysis Infrared Spectroscopy and Laser Induced Emission Spectral analysis [6]. The mid-range region of the spectrum is normally selected as the region of choice, corresponding to the primary absorption range and including the fingerprint region. Specular reflectance MIR spectra can be obtained with an acquisition time of less than 1 second using data resolution of 8 cm-1

[7]. Milling the surface may be necessary for a painted or dirty surface, but this can be done in a few seconds with hand held abraders. Although the reflection spectra obtained do not resemble absorption spectra, they can be transformed mathematically to give comparable results. Even without the transformation the data obtained is sufficient to set up a specific identification procedure that can recognise subtle differences in polymer structure and composition. MIR can identify and separate polymers of a similar type e.g. different form of polyamides or blends that only vary in proportion of polymers present [8]. A number of rapid identification methods are now marketed, based on MIR with sealed specular reflectance attachments. Both large scale and portable systems are available. The method can be automated and used in conjunction with a reference collection of spectra can visually display the most probable polymer type. The Bruker system PIID 22 has been developed and tested in conjunction with the European Automobile Manufacturers Association

316

and BMW. It has been used to identify polymers, copolymers and polymer blends and in development tests with 29 different polymers and blends was verified to have an error rate of <1 % [6]. A system designed to be sufficiently mobile to be transported in a small estate car to a factory or reprocessing site as well as a larger version, designed for fixed installation have been developed by Wolfson Electrostatics at the University of Southampton in a research project with Ford Motor Company.

2.4.5. Other Techniques Other techniques that can be used in some aspects of identification for recycling include:

- X-ray fluorescence and absorption for detection of heteroatoms of moderately high atomic mass especially chlorine atoms in PVC

-Thermal analysis in the recycling of polyolefins to assess crystallinity. -Fourier transform Raman spectroscopy does not need specialist

sampling, but is limited by the need for powerful lasers, and it also fails with darkly coloured samples.

-Laser induced emission is a rapid non-contact method [9],that is extremely useful in identifying heteroatoms present in additives in polymers.

-A hand-held device called a "triboelectric pen" is commercially available from Wolfson Electrostatics. This operates on the phenomenon of the frictional electrical charge that develops when the plastic material is rubbed. Each pen is designed to distinguish selected pairs of polymers e.g. PP and ABS. The particular advantage here is the simplicity and portability of the method. It is claimed to have success rates of 90-98% depending on the relative humidity [10].

Future development of methods based on molecular recognition using a combination of methods is possible. Information from Pyrolysis / Mass spectrometry, MIR and NMR methods might be combined to give a comprehensive data base that could give unequivocal reliable information about the sample. The cost of a battery of such techniques could only be justified with very large scale operations and in the immediate future the

317

development of intelligent devices based on MIR seems to be the most likely prospect.

3. Density Separation

Different generic types of plastics will exhibit different densities as shown in Table 2

Table 2. Densities of virgin plastics

Generic type of Density, G/cm.J plastic

PP 0.916 -0.925 LDPE 0.920 -0955 HDPE 0.940-0.980 PS 1.05-1.22 PVC 1.30-1.36 PET 1.33 -1.40

These density differences can be used to separate plastics into different generic types as well as to separate out non - plastic contaminants. Figure 3 shows a schematic of such a separation process [11].

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Wast. plastics mixture

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r--- wat.r-alcohol d-O.91

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Fig 3 Schematic of a conceptual float I sink separator [11].

Water is used to separate PS and PVC from HDPE, LDPE and PP. PS is separated from PVC using brine with specific gravity of 1.2. HDPE is separated from LDPE and PP in water / alcohol mixture with specific gravity of 0.93 and the LDPE is separated from PP using another water alcohol mixture with specific gravity of 0.91. This separation system although conceptually very simple suffers from a number of drawbacks

plastics are often compounded with fillers which changes their densities cross-contamination of liquids will change densities of separation media there is a need for cleaning of salt solution from the separated plastics.

Although the system is not used in its entirety, separation in aqueous medium is often utilized as a first step of a separation process especially when combined with a washing step.

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An interesting variation of the sink / float separation utilizing super- or near-critical liquid has been proposed [12], [13]. At the critical conditions of temperature and pressure the difference between liquid and gas phase disappears. Near - and super - critical liquids are compressible and their density can be varied by adjusting pressure. The advantages of some of the near - and super - critical fluids ( for example CO2) are

variable density ( at 2980 K and pressure of 935 to 10000 psi, density of C02 can be varied between 0.78 and 1 g/cm3 ) low viscosity - carbon dioxide exhibits viscosity 10 times lower than that of water allowing for rapid settling of components being separated. poor solvency - some non - polar liquids such as carbon dioxide are poor solvents for plastics being separated. high vapour pressure results in rapid evaporation once the pressure is removed. No additional drying equipment is required. minimal environmental impact [13].

Altland et al [12] proposed a process in which near-and super - critical liquids are used. The process consists of five elements

air classifier removes foamed materials float / sink separation using water separates PP, LDPE, and HDPE from PS, PVC and PET. float / sink separation using brine separates PS from PVC and PET. liquid C02 is used to separate light fraction into PP, LDPE andHDPE liquid SF6 is used to separate PVC from PET

The process worked reasonably well on a lab scale. Table 3 shows some of the experimental results.

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Table 3 Quality offloatl sink separation [12].(Reprinted from Brian L. Altland,

Separation Optimum Light Purity Heavy Purity medium density fraction of light fraction of

fraction heavy % fraction

Water 0.998 poleolefins 100 Non- 100 olefins

CO2 0.936 PP 100 LOPFJHO 100 PE

CO, 0.956 PPILOPE 100 HOPE 100 MeOHlwater 0.956 PPILOPE 69 HOPE 71 MeOHlwater 0.928 PPILOPE 99 HOPE 99 Salt water 1.19 PS 100 PVCIPET 100 SF6 1.330 1.330 83 PET 94 SF6 1.347 1.347 96 PET 99

It has to be noted that 1 % of PVC in PET would not have been commercially acceptable. The estimated cost of float / sink separation using near - or super - critical fluids is $ 0.08 per pound for C02 and $0.27 per pound for SF6.

4. Air Separation

Air classification utilizes rising stream of air to separate different materials. Terminal velocity is the property which determines if the separation is possible. Terminal velocity is the speed with which the particle falls once the steady speed has been reached. The properties which govern terminal velocity are density, size and shape of the. particle. This method of separation is useful if the materials being separated are

the same or similar shape and size but different densities

the same or similar densities but different shapes and sizes.

Air classification might, for example, be successful in separating residual copper from plastic insulation after the waste wire has been shredded finally enough to liberate copper particles. The shapes and sizes of particles are similar but the densities are considerably different.

Stessel et al. [14] modeled air classification of shredded plastics and showed that the use of pulsatile air flow improves separation. The optimum frequency of pulsing is between 1 and 2 Hz.

5. Flotation

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Flotation separation utilizes differences in surface wettability of materials. This process is being widely used by mining industry and has been proposed for the separation of plastics waste [15], [16]. A schematic of flotation column is shown in Fig 4' . In the process the materials to be separated are treated with various chemicals which make some fractions preferentially wettable. The separation occurs in a column where the mixture to be separated is contacted with water in which air bubbles are dispersed. Air bubbles attach themselves to the hydrophobic particles floating them to the surface while hydrophilic particles sink to the bottom. In another version of this process, when both of the materials to be separated are either hydrophilic or hydrophobic but exhibit different critical surface tensions of wetting (defined as a surface tension of a liquid in which the solid exhibits transition from hydrophilic to hydrophobic behaviour ), a liquid medium can be selected such that one material is wettable by it and the other is not. Air bubbles will attach themselves to the material which is not wettable while the wettable material will sink. The process is referred to as gamma separation [17].

Saitoh et al. showed that treatment of plastics surfaces with various surface active agents such as salts of lignin sulfonate or organic colloids such as gelatin selectively changes surface tension of these plastics.

322

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Fig 4. Schematic of flotation column [17]. B. Yarar, Flotation, in Kirk-Othmer Encyclopedia of Chemical Technology, Copyright 1994, John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc. All Rights Reserved.

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Untreated PP, PE, PS, and PVC show contact angles somewhere between 80 and 95 degrees. After treatment contact angle of water on PP was around 75 degrees, PE 55 degrees, PS 40 degrees and PVC 25 degrees, making it possible to separate these materials by flotation process [16] .

Buchanan et al. [18] investigated feasibility of gamma separation for separation of PET from PVc. After treatment in alkaline solution (pH= 11) the critical surface tension of wetting of PET was 43 dynes /cm and that of PVC was 39 dynes/cm. The solution of water, alcohol and methylisobutyl carbinol (MIBC ) had surface tension of 40.9 dynes/cm which allowed for the separation of two solids.

6. Solvent Separation

Solvent based processes can be used to separate plastics into different generic type groups, to separate plastics from non - plastic materials or even to remove fillers which have been incorporated in plastic. The solvent base separation processes are based on

one material being soluble in a solvent while the other is not difference in solubilities at different temperatures removal of different components with different solvents.

After the polymers have been solubilized and separated they can be removed from solution by evaporation of the solvent, flash devolatalization cooling or precipitation of polymer by addition of a non - solvent. Lynch et al. [18] describes preliminary experiments with a solvent based separation process. Tetrahydrofuran (THF) was used as a solvent to separate mixture consisting of PVC, PP, LDPE, HDPE and PET. Materials were removed as follows

At room temperature PVC and PS At700 C LDPE At 1600 C PP and HDPE At 1900 C PET

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A mixture of plastics was placed in a dissolution column through which a solvent was circulated. After extraction at a given temperature polymer was recovered by rapid transfer of hot and pressurized solution into a low pressure chamber to volatalize the solvent and precipitate the polymer. The solvent was then recovered by condensation.

Another example of the solvent based separation process is the process described by Hafner [19].The process is meant to separate PVC from non-vinyl components such as fabric ( for example in vinyl upholstery) or paper ( such as in vinyl wallpaper ). PVC is dissolved in a solvent, solution is separated from the non - soluble components by filtration and then the polymer is precipitated by addition of a non -solvent. Solvent, non - solvent and plasticizers are recovered by fractional distillation. The advantage of solvent separation is the usually good separation which can be achieved. The disadvantage, however, is the need to use solvents and to prevent their emissions to the atmosphere. Leidner [20] proposed a process for recycling of PVC which overcomes that disadvantage. In that process plasticized PVC is dissolved in a hot plasticizer and the solution is filtered or centrifuged to remove non-PVC components. The resulting product can be used for compounding with additional rigid PVC to bring PVC I plasticizer ratio to the desired level. The process is smilar to the solvent separation except that the solvent (plasticizer in this case) is not removed but becomes part of the recycled material.

7. Melt Filtration

Melt filtration is utilized to remove small amounts of impurities from material being extruded. The removal of these impurities usually does not affect mechanical properties of the recycled material but improves other properties such as - for example - appearance of the part made using recycled material. The simplest melt filtration device is formed by a several layers of wire screen discs having the same diameter as an extruder barrel. The outer screens are coarse and act as support for the inner fine filtration screens. The screen pack is backed by a thick,

325

perforated metal disc known as a breaker plate. To replace clogged up screen pack the breaker plate has to be un - bolted and the screen pack removed and replaced with a new pack ..

There are two main types of commercial screen changers -reciprocating and continuous. The reciprocating screen changer consists of two screen packs of which only one is positioned in a melt flow. When the screen gets plugged up the hydraulic mechanism pushes the plugged up screen pack out and replaces it with the new pack. Some designs use valves which redirect flow from one pack to the other. Continuous screen changer utilize long ribbons of screens which are pulled across the breaker plate. The speed of such a movement can be adjusted so as to maintain constant pressure drop across the screen. Another design (so called solid state changer) utilizes melt flow and pressure drop across the screen to move the screen past the breaker plate. Miranda et al. [21] studied effect of melt filtration on the properties of recycled modified polypropylene automotive bumper coated with polyurethane paint. They showed that the use of 100 mesh screen was effective in producing material with good surface finish and high gloss.

ORTECH has proposed a concept of separation of a plastic from a non - plastic material by a melt separation process [22]. The process is meant especially for waste materials such as automotive body trim and insulated copper wire. The schematic of the experimental device is shown in Fig 5. The device is a heated cylindrical die with upper movable and lower stationary dies. The insulated wire is placed between the two plungers, heated and the pressure is applied to the upper plunger. As the plastic melts it gets extruded through the clearance between the plunger and the die, flows into the grooves cut in the plungers and is removed form the die through the openings in the die. A typical sample is shown in Fig 6.

326

. "

UPPER PLUNGER

PLASTIC FLOW CHANNEL PLAST K: EXIT CHANNEL

LOWER PLUN<£R PLASTIC FLOW CHANNEL

Fig 5. Experimental device for separation of plastic from a non - plastic material by melt filtration [22].

327

Fig 6 Sample of insulated copper wire before and after separation [22].

Compressed copper billets can be further milled and some of the residual plastic or fabric removed by air lift. Typical purity of separation is given in Table 6.

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Table 6. Copper content after melt filtration process [22].

Insulation Original Copper content Copper copper after melt content after

content, % separation, % milling and air lift, %

Thermoplastic 75.9 98.8 99.5 Thermoplastic 75.9 98.8 99.7 Crosslinked 85.1 98.7 99.9

plastic Crosslinked 64.5 99.5 99.5

rubber Plastic with 67.5 97.8 99.9

fabric

8. Cryogenic Separation

Cryogenic separation utilizes the differences in low temperature brittleness of different materials. Materials are chilled using liquid nitrogen and then ground. A typical example of commercial application of cryogenic separation is separation of PVC from aluminum in a waste automotive trim. When cooled and hammer milled the PVC breaks into small particles while aluminum remains in large pieces. The two components can be then separated by sieving or air separation.

9. Separation of Resin From Fibers in Waste FRP.

In the manufacturing of composites a certain amount of a waste is generated. Such a waste consists of a crosslinked resin and glass fibers. ORTECH has developed composite recycling process for one of its clients. Fig 7. shows a schematic of this process. Waste composite material is fed into a low speed, high torque shredder where it is shredded into elongated pieces approximately 8cm by 8cm. The shredded material is deposited on a vibrating non-perforated screen where free fibres are separated, picked up an airlift and further

329

IWILLOW~FREE FIBRE !-30mm)

_---11---_ FIBRE ("'24mm)

POW[ER !OI\Opm}

Fig 7 Schematic of a composite recycling process [23].

330

separated from fines in an air classifier known as a willow. The shredded pieces are fed into a vertical pulveriser. The pulverizer does not contain a classifying screen so that material goes through it only once. Shredded material is deposited on a 10 mesh vibrating screen where it is separated into free fibres which are removed by airlift, small particles which pass through the screen and are deposited on 35 mesh vibrating screen and large particles which are returned into the pulveriser. Additional fibres are removed by airlift from the 35 mesh screen and the unders are deposited on a 60 mesh vibrating screen. Short fibres are again removed by an airlift and both unders and overs are transferred to the mill where they are ground into approximately 10 ~m powder. The final product of the process are glass fibres resembling commercial chopped and milled glass fibres and a powder consisting of milled glass and powdered resin. Both of these products can be used as a replacement for glass fibres as well as filler in sheet molding compounds (SMC). Up to 25 % of recycled fibres have been used in commercial applications [24].

References.

1. Burgiel , J., Butcher, W., Halpern, R., Oliver, D., Tangora, P., Beck, R. W. (1994) Cost evaluation 0/ automated and manual post - consumer plastic bottle sorting system, US Environmental Protection Agency, Cincinati, Ohio 45268, EPN6001-94/165,. 2. Dinger, P. (1992)Automated sorting/or mixed plastics, Bio Cycle, pp. 80-82 3. Kenny, G. R., Bruner,R. S. (1994) Experience and advances in automated separation o/plastics/or recycling, Journal of Vinyl Technology, vol 16, no 3 pp. 181-186 4. Corbett, E.C., Frey, J., Grose, I., Hendra, P.I. (1994) An investigation into the applicability 0/ luminescent tagging to polymer recovery, Plastics, Rubber and Composites Processing and Applications, 21, p.5-11.

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5. Zachmann, G. (1995) A rapid and dependable identification system for black polymeric materials. Journal of Molecular Structure, 348, p.453-456. 6. Vornberger,K. & Willenberg, B.(1994) Rapid identification of plastics, Kunststoffe EuroPlastics, 84(5), pp 586-589. 7. Graham, 1., Hendra, PJ., & Mucci, P. (1995) Rapid identification of plastics components recovered from scrap automobiles. Plastics, Rubber and Composites Processing and Applications, 24, p.55-67. 8. Zachmann, G. & Turner, P. (1997) Fast and reliable identification of black plastics. Spectroscopy Europe, 9(1). 9. Lorentzen, c.J., Carlhoff, c., Hahn, U., & Jogwich, M. (1992) Applications of laser induced emission spectral analysis for industrial process and quality control, Journal of Analytical Atomic Spectroscopy, 9, pp.1029-1035. 10. Hearn, G.L., Mucci, P.E.R, Eyres, A., & Amner, J.A. (1996) The triboelectric pen: an electrostatic method for the identification of plastics in recycling. Transactions / Journal of the Industry Applications Society (31st Annual Meeting, San Diego CA October 1996) pp.1955-1958. 11. Holman, J. L., Stephenson, J. B., Adam, M. J.(1974) Recycling of plastics from urban and industrial refuse, Report ofInvestigations 7955, US Bureau of Mines, Washington, DC, 12. Altland, B. L., Cox, D., Beckerman, E. J. (1995) Optimization of the high pressure, near-critical liquid - based microsortation of recyclable post - consumer plastics, Resources, Conservation and Recycling, 15, pp.203-217 13. Super, M., Enick, R, M. (1991) Separation for thermoplastics by density using near - and supercritical fluids as a precursor to recycling., Annual Technical Conference of SPE, pp. 2130-2133 14. Stessel, R 1., Pe1z, S. (1994) Air classification of mixed plastics, National Waste Processing Conference Proceedings, ASME pp 333-339 15. Buchan, R Yarar, B. (1995) Recovering plastics for recycling by mineral processing techniques, JOM, Feb pp. 52-55

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16. K. Saitoh, K., Nagamo, 1., Izumi, S. (1976) New separation technique Jor waste plastics, Proceedings of the Fifth Mineral Waste Utilizaton Symposium, Chicago, Ill. Apr 1976. pp. 322-328 17. B. Yarar (1994) Flotation, in Kirk -Othmer Encyclopedia of Chemical Technology, Fourth Edition, John Wiley & Sons, New York, vol 11 pp. 81-107 18. Lynch, J. c., Nauman, E. B. (1989) Separation oj comingled plastics by selective dissolution, RETEC SPE, New Developments in Plastics Recycling, October 30 1989 19. E. A. Hafner (1974) Vinyl chloride polymer recovery process, US Pat. 3,836,486, 20. J. Leidner (1993) ProcessJor recycling oJsupported or contaminated PVC, US Pat 5,232,606, 21. Miranda, V., Lai, F. S. (1994) Recycling oJpainted modified polypropylene auto bumpers by melt filtration, Annual Technical Conference of SPE, 1994, pp. 2888-2891 22. J. Leidner (1975) Reclamation oj copper from insulated copper wire scrap, ORTECH's internal report. 23. B. Sims, B. Booth, C., Lakshmanan, V. J. (1993) ProcessJor separatingfibresJrom composite materials, US Pat 5,251,827 24. T. W. Harth (1995) Reclaming SMCJor use in Neon and other current vehicles, Proceedings of Auto Recycle 95, Dearborn, Michigan, Nov 1995

REPROCESSING OF COMMINGLED POLYMERS AND RECYCLING OF POLYMER BLENDS

L. A. Utracki National Research Council Canada, Industrial Materials Institute, 75 de Mortagne, Boucherville, QC, Canada J4B 6Y 4

ABSTRACT

Out of the three methods of plastic waste recycling, that is, polymer recycling, feedstock recycling, and energy recovery, the former is the most desirable. The polymer recycling usually involves: segregation, washing, shredding and extruding. During melt-extrusion, the material undergoes devolatilization, stabilization, compatibilization, alloying, filtering and pelletization. The ABC of the polymer recycling technology, the alloying, blending, and compounding, is summarized in three parts: (i) The physics and chemistry of polymer blending, (ii) Compounding and processing of polymer blends, and (iii) Polymer blends' recycling.

1. INTRODUCTION

1.1. World Plastics Production

In 1996, the world production of thermoplastics (Europe - 33.1 %; N. America - 33.0%; S. America - 3.6%; Asia - 28.5%, and Africa - 1.8%) reached 131 million ton/year. By the year 2000, this amount is supposed to increase by another 20 million ton/year [1]. At the end of the life-time most of this mass becomes a solid waste. Recycling is considered to be either a reduction of waste, alleviation of disposal problems, or means for the recovery of material and/or energy. This chapter focuses on the material recovery as the most pertinent.

1.2. The methods of Plastics Recycling

Over the years, three methods of plastics' recycling have been developed [2]: 1. Polymer recycling (alternatively known as mechanical recycling). The method is the

most desirable, and should be used whenever it is ecologically, commercially, and technically feasible.

2. Feeedstock recycling (or thermolysis). The method is used to recycle soiled, mixed plastics and rubbers into monomers, feedstock gasses, oils, and condensates, from which new polymers can be synthesized. It should be used only when and where the first method cannot be carried out.

3. Energy recovery (or incineration). This is the last recourse, to be used when and where neither of the two previous methods is viable.

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G. Akovali et af. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 333-354. © 1998 Kluwer Academic Publishers.

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When designing a new plastics part, the recyclability should be incorporated into the list of perfonnance criteria. The material selection should consider the total cost, viz. that of the materials, forming, assembling, decorating, customer satisfaction, esthetics, service life­spans, ease of disposal, and recycling. Thus recyclability, as well as the recommended strategy for recycling, should come in the beginning of the process, not at the end.

For example, General Electric Company proposed that engineering resins should be reused sequentially in progressively lesser critical applications - this strategy of recycling is also known as downcycling. The company also demonstrated that such engineering resins recovered from automobiles as, e.g., modified polyphenyleneether (PPE), bis-phenol-A polycarbonate (PC), or polybutyleneterephthalate (PBT), can successfully be recycled after 10 years of weathering.

Another method of polymer recycling is known as upgrading. The process includes several elements of the polymer blends technology, viz. alloying (i.e., compatibilization and/or impact modification), blending to the desired morphology, and compounding with other additives (e.g., stabilizers and fillers) - the three ABC elements of plastics recycling.

The main technical difficulties of recycling are the variability of composition and the level of contamination present in the waste material. For this reason, it is convenient to group the commingled plastics waste into three categories [3]:

• The easiest to recycle is the in-house generated, processing scrap. At present, virtually all these materials are reused by blending (up to 40 wt%) with virgin resins.

• To the second category belong clean, industrial scraps. These materials are collected in a full range of chemical homogeneity, from a single resin (e.g., off-spec resins) to commingled (e.g., PE and PVC from wire-&-cable plants). The former type is usually upgraded by ABC, or downcycled. The second category resins are alloyed either in the molten or solid state.

• The most difficult is the post-consumer waste, PCW. The plastics that belong to this category, first must be separated then cleaned. Alternatively, they may be processed, e.g., into a variety of plastics woods.

TABLE 1. List price of selected resins in US $/ton (Plast. News, Nov. 1996).

Resin Virgin Recycled ABS 1958 - 2002 792 - 902 PC 3014 - 3190 1144 - 1298 HDPE 902 - 946 704 - 814 LLDPE 968 - 1012 638 - 748 LDPE 1012 - 1056 594 - 704 PET-bottle 1188 - 1254 528 - 638 PP 792 - 880 550 - 638 PS-crysta1 990 - 1034 770 - 880 PS-high heat 1012 - 1034 946 - 1012 PVC-flakes 704 - 770 286 - 462

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1.3. Commercial recycled resins.

A comparative list of prices of the virgin and recycled resins is shown in Table 1. The prices of recycled plastics are quite competitive. Evidently, while the quoted prices of commodity resins are reasonably close to these of virgin materials, the situation vis-a-vis condensation polymer is atrocious. The only way to make profit on the latter materials is to upgrade them by the recyclers ABC and supply compounds with well defmed and reproducible set of properties to a well a specific market niche.

TABLE 2. Commercially available Recycled Resins and Blends.

Resin Comment Manufacturer

Aurum™ Fully recyclable polyimide Mitsui Toatsu

BayblencfIM Blends of ABS & recycled PC Bayer A.-G.

Dylite™ EPS EPS with 25% recycles Arco Chem. Co.

Naxe/[TM Recycled PC MRC Polymers

Orgablen,jfM Recycled P AlPP alloys Elf Atochem

Retain™ Recycled PSIPO blends Dow Plastics

UltramidTMRC Recycled PA's + GF BASF A.-G.

Note: For explanation of the abbreviations used in the text, see NOMENCLATURE at the end of the Chapter

• PA-RC Comparison of recycled and virgin 0 PA

grades available from BASF, viz.: • AS5-RC f] ASS Ultramid Terlu & I • PS·RC

f:2I PS

(f) PA .2 l- I- iii ::J 0::: 0 U "0 Z I >ps

100 0 ~

CD ps "0 PA :;, -'2 CI C\J

::::E 10

Property

Fig. 1. Properties of selected virgin and recycled resins from BASF (after [4 J).

336

Several recycled resins that are available on the market are listed in Table 2. As the data in Fig. 1 demonstrate, their perfonnance is slightly (by about 10%) reduced [4]. In spite of the price advantage, the recycled resins still constitute only a small fraction of the thennoplastics' consumption - it varies from 16.5% for PET, 5.0% for HDPE, 2.2% for LDPE, and to a fraction of one percent of PS, PVC, and other resins.

Numerous products are commercially available from recycled resins and blends, viz. injection molded from recycled PC multi-pallet systems (GE); agricultural films comprising 30 wt% recycled PE (Dow); fleece jackets, sweaters and other knitwear from recycled PVCIPET (Reprise Plast.); Styropor - EPS for concrete and other industrial applications (BASF) from recycled PS; Remo-Tec are moldings of recycled PU foams to desired shape (Hennecke), etc.

2. POST-CONSUMER WASTE, PCW, RECYCLING.

2.1. Recycling plants.

Complete PCW recycling plants are available from, e.g., AKW, Erema, Krauss-Maffei, National Recovery Technology, Officina Meccanica Prealpina, Sikoplast, Sorema, etc. Approximate cost of a plant having capacity 1 tonlhr is $3-5 million. The number of manufacturers for separate machines, viz. sorters, washers, shredders, aggregators, classifiers, extruders, and others, runs into thousands. The cost of recycling in the USA has been estimated as (in US $Iton): scrap - 220, QC - 50, grinding - 100 to 250, washing - 50 to 100, and pelletizing - 250. In 1995, there were 1550 plastics recycling companies in the USA that processed 630 ktons of plastics: 40% PET, 54% PE and 6% of PP, PS and other materials.

Specialty reprocessing plants have been also developed. For example, fully automatic plant was designed for separating copper wire, PE and PVC into clean, ready to re-use materials. Similar specialized plants have been designed to clean and separate PCW from the agricultural scrap (mostly PE and PVC films), from household (a variety of packaging materials), or from the industrial waste streams comprising thennosets and thermoplastics, etc.[5].

An automatic line (built by Sorema) that recycle up to 50 tons of PET bottles a day, comprises: pre-washing, X-ray sorting (to remove PVC down to below 15 parts per million), grinding, washing in hot caustic solution, rinsing (PO's are separated), drying and sizing. The high quality recycled PET is then used either in textile industry, or as an inner layer in co-fonned food containers. Similar processing line has been used by Linpac for recycling variety of plastics. The line starts with wet grinding and washing, followed by five successive hydrocyclones capable to separate EPS, PO, PS, PVC, and PET, followed by rinsing, hot air drying and air classification. Recyclates from different sources are blended and upgraded by ABC into reproducible, high perfonnance grades. There is no downcycling in this operation.

The methods of recycling depend very much on the type of the waste stream and the degree of cleanliness. The relatively clean PCW can be processed by similar methods as those used for industrial scrap. Numerous techniques have been developed for reclaiming the most readily identifiable constituents of such mixed waste streams as soft drink bottles (PET) and milk containers (HOPE). After separation of these containers, the remaining part ofPCW (known as tailings) is made ofPE's (50-75 wt%), styrenics (about 25 wt% ofPS, ABS and their blends) and other plastics. These tailings can be cleaned, comminuted,

337

melted, upgraded by blending, and melt-fonned. They can also be reinforced, foamed, or stretch-fonned into a variety of products for specific applications. Alternatively, PCW tailings (mainly PE) can be used to produce "plastics lumber," known to have excellent long-tenn outdoor perfonnance.

The experience indicates that it is easier to recycle unfilled than filled polymers. In all cases re-stabilization of the resin is required. Some polymers (e.g., PI, PPS, POM, and LCP) are less susceptible to degradation than others. By contrast, the condensation polymers, viz. PA, and PEST, are difficult to reprocess without loss of perfonnance. In most cases, the engineering resins and polymer blends require upgrading.

2.2. Stabilization.

Recycled plastics need to be re-stabilized. Since the resin may already suffer from degradation, a higher concentration of stabilizer should be added than that introduced during the initial pelletization [6]. Each type of polymer requires a specific type of additives. In consequence, stabilization of PCW requires addition of different cocktails of stabilizers to different types of commingled resins.

Light stability for LLDPE/LDPE 120 blends for irrigation pipes

o 2 4 6 8 10

Time of exposure to light & weather (kh)

Fig. 2. Comparison of the stabilization effect on a virgin and recycled LLDPEILDPE blends with and without carbon black (after [7]).

The stabilizers are used to perfonn specific functions. Thus, there are antioxidants, viz. sterically hindered phenols [e.g., pentaerythritol ester] and/or sterically hindered amines; (hydro )-peroxide decomposers, viz. phosphite [e.g., tris(2,4-di-tert-butyl phenyl)­phosphite]; radical scavengers such as thio-derivatives; heat stabilizers [e.g., calcium-zinc type for PVC]; light stabilizers and UV blockers [e.g. aluminum flakes or carbon black] for outdoor storage and applications, etc.

It is noteworthy that additives suitable for one type of resin may have detrimental effects on another resin and/or on its additives. For example, it has been observed that the interfacial properties in P AlPO blends can be seriously affected by incorporation of unsuitable

338

additive into a blend. When PA scrap was mixed with PO containing phosphine stabilizers, a rigid membrane was be formed at the interface by a chemical reaction between -NH2 groups of P A and phosphine acidic functionalities. The membrane has two serious effects: it increases the melt viscosity (thus, it reduces the throughput), and it makes dispersing the PO in PA much more difficult (what reduces the performance). The detrimental effects are most likely when heat and light stabilizers are present in the recyclates.

Commingled PO blends are relatively easy to re-stabilize using about 0.06 wt% of a hindered phenol antioxidant, and about 0.09 wt% phosphite. It was found advantageous to neutralize the mixture by addition of a sufficient amount of calcium hydroxide. For other compositions of PCW, others stabilizers, viz., thio-propionic acid, benzophenones, oxalides, benzotriazoles, or sterically hindered amines may have to be used [8].

Ciba-Geigy developed proprietary antioxidant and co-stabilizer formulations, e.g., available under the trade name Recyclostab 1M. These formulations combine several stabilizers, e.g., 10 parts hindered amine light stabilizer, 3 parts hindered phenol antioxidant, 7 parts phosphite, and calcium hydroxide. As shown in Fig. 2, the performance of LLDPEILDPE blends is strongly affected not only by a standard stabilization (using 0.08 wt% of hindered phenol stabilizer and 0.03 wt% oftri(di-tert-butyl phosphite), but also by addition of carbon black and hindered amine light stabilizer, HALS. The reduction of performance was caused by exposure of the stored pipes to sunlight before placing them underground [7].

For recyclable PVC window profiles, Rh6ne-Poulenc developed Rhodistab™ , a formulated stabilizer based on calcium-zinc.

Stabilization of condensation-type polymers may not be sufficient. Polyesters and polyamides may suffer from the hydrolytic depolymerization during the usage and storage of the formed parts. Thus, before stabilizing the mixture, it is advantageous to increase the molecular weight by polycondensation under high vacuum (inefficient and expensive), or by solid-state condensation. The latter reaction involves two steps: first, in the molten state addition of di-functional agents [viz. di-glycidyls] and a catalyst [e.g., SnO], second, condensation carried out at temperature below the melting point of the resin, T m' but above the glass transition temperature, Tg•

2.3. Elimination of odor from PCW recycles

Five years ago, du Pont de Nemours patented a method that eliminates the unwanted odors during recycling of PO (in particular the HDPE milk bottles). The noxious odors of recycled plastics, viz. HDPE, LDPE, LLDPE, ULDPE, PP, PET, vinyl or acrylic copolymers were reported to be reduced by incorporating 0.001-30 phr of polyethyleneimine, H-[-(CH2)n-NH-]m-H (PEtI). The polyethyleneimine was found to have good aldehyde scavenging capabilities - it does not hide the odor, but efficiently it eliminates it in a chemical reaction. To ascertain good dispersion ofPEtI within the recycled scrap, an acidic compatibilizer, e.g., maleated PO, may have to be used. The maleic group reacts with the PEt! amine end-group, forming PO-branch-PEd chains. Blending of the commingled scrap, a compatibilizer, and PEtI has been carried out in a corotating twin­screw extruder [9].

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2.4. Compatibilization

2.4.1. General principles of compatibilization Compatibilization of PCW follows similar pattern as that of any polymer blend. Thus, to improve performance, the process should improve the degree of dispersion, prevent destruction of the generated morphology during the subsequent processing steps, and improve adhesion between the phases in the solid state that controls the performance [10].

Compatibilization of PCW is primarily accomplished by addition of reactive compatibilizers. Since immiscibility and degradation invariably lead to brittle, commingled resins, introduction of a toughening agent is recommended. Most often, the compatibilization and impact modification are simultaneously accomplished by addition of a multi-polymer compatibilizer-cum-impact modifier. To this category belong the "universal compatibilizers" described below.

The physical compatibilization by high stress shearing is used for PO recycling with growing frequency. Here, an elastomeric block copolymer may also be added to improve toughens. Such mechanical compatibilization generates a fine, non-equilibrium type, highly oriented morphology. The created transient structure is usually locked by enhanced crystallinity. The process may take place either in the molten or solid state - see below.

Finally, a direct, reactive compatibilization by addition of an active, low molecular weight coupler can be used to upgrade and compatibilize mixtures of engineering resins. The process may be expensive (the required tapered block copolymers usually are dear), thus used only in specific cases.

The listed above different strategies usually lead to different blend morphology, resulting in different sets of properties. Thus, these are not alternatives, but rather opportunities to be explored for given set of materials and market potentials.

2.4.2. Universal Compatibilizers. There are numerous, so call "universal" compatibilizers for PCW available on the market. Typically, these are multi-component copolymers, with parts that either are soluble in some components of the blend, chemically bond to chain ends, or have a tendency for hydrogen bonding. Because of the "universality," these materials are rather expensive to use. A better chance offers the proprietary compatibilizers-cum-impact modifiers, formulated for specific types of polymer mixtures, viz. Blendex™ (polybutadiene-type compatibilizer for styrenics, PVC, TPU, PET), EXL (an acrylic-based additive for PEST), Fusabond™ (maleated-PO compatibilizer for POIPET blends), Vector™ (is SBS-type block copolymer with stabilizers, designed for POIPS commingled mixtures), and many others.

2.4.3. Mechanical compatibilization of PCW in the melt. During the 1970's Patfoort developed an extruder with the residence time of T > 7 seconds [11]. The machine generates such a high shear stress that extruded, immiscible polymers (e.g., PCW comprising PS and PE) showed good mechanical properties. Apparently, during the extrusion there was enough chain scission and recombination to generate in situ a sufficient amount of copolymer, capable of stabilizing the systems. Several other machines of this type have been developed since. They all involve intensive mechanical shearing, that produces extensive chain scission. Recombination of the free radicals in situ generates sufficient concentration of copolymer, to compatibilize the system. The generated under high stress, non-equilibrium morphology is then locked by quenching. Best

340

perfonnance has been observed for systems with co-continuous morphology. For example, the Newplast process is based on a "homomicronizer" - the machine resembles an old kinetic energy mixer, where short, stubby blades, rotating at high speeds, melt and homogenize the PCW within 35-120 sec [12].

The following description ofPCW recycling into plastic blends with good and reproducible set of perfonnance characteristics, was recently published. First the waste plastics were shredded, metals and heavy elements were removed, then the blend was washed, paper was removed, dried and ground. Next, the composition was verified by FTIR and adjusted, the additives and stabilizers were added. Then, the material was mechanically compatibilized by compounding in a high speed mechanical shearing machine, with the screw speed 30x higher than that of a TSE. The mass was kinetically heated to 230°C, in-situ generating the compatibilizing copolymers. The compound was fed to a melt extruder, degassed, filtered, and pelletized. A blend comprising 63.7 wt% HOPE, 8.75 wt% PVC, 5 wt% PET, 8,75 wt% PP, 8.75 wt% PS and 5 wf'1o ABS had higher tensile strength at yield than HOPE (but lower at break), higher Young modulus, the same flexural modulus, and lower elongation at break. The ABSIPVC scrap showed similar perfonnance to PP [13].

2.5. Solid-state recycling.

The methods of PCW recycling in solid state continue to gain acceptance. Intensive mechanical dispersion of blend components, co-reaction between them, then interlocking them into a stable, desired morphology can produce well perfonning blends without a need of melting. Stabilization of morphology can be achieved either by chemical or physical means. Owing to high stresses and low temperature, the fonned during the process free radicals recombine to generate compatibilizing copolymers. At least four methods of solid­state recycling have been described. • Ball-milling each resin for 8-24 hrs in a shaker ball-mill at acceleration 12.3 g, a

frequency 29 Hz and T = -150°C. Next, these powders were milled together for another 24 hr period. The fmal powder had the particle size of about 2 mm. Placed under vacuum, it was consolidated for 28 hrs under P = 69 MPa and at T - T m = 3 to 100°C. The solid state alloying resulted in homogenous materials, with high hardness and tensile strength [14].

• High-stress extrusion of commingled, uncompatibilized PCW through Patfoort extruder (residence time ca. 10 sec) was followed by rolling.

• Commingled resins' perfonnance was upgraded without going through the melting and compatibilization stage by solid-state sheet rolling process.

The solid-state shear extrusion, SSSE, utilizes Berstorff co-rotating twin-screw extruder. The first zone is slightly heated, while the kneading zone was modified for intensive cooling to IS-60°C. Within this zone, the particles are repeatedly broken and refonned by impact, compression, attrition, and shear. The process changes PCW into powder with the particle size, 20 :-::; d :-::; 2000 J.!ffi. The powder was used directly for injection molding of articles having light, unifonn color. The pulverized PO's (viz. HOPEILLOPE, HOPEIPP, etc.) showed about 20% improved impact strength, and about 10% decreased elongation at break in comparison to melt-mixed blends. The moldings had good surface appearance and did not delaminate upon breaking [15-17]. The process can easily be adopted for incorporation of upgrading additives: compatibilizers, impact modifiers, stabilizers, pigment, dyes, etc.

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3. Plastics Waste from the Automotive Industry.

The industry started to use plastics in 1946. Since then, its content steadily increases (see Fig. 3). As shown in Table 3, about 25 types of plastics are used. However, it is noteworthy that the plastics used in automotive industry are mainly in form of alloys, blends, and composites. For example, in Saturn: front fenders and rear quarter panels are from PA/PPE, door outer skins are from PC/ABS, the bumper fascias are TPO, etc. Thus, in Table 3 the statistics refer only to the matrix resins [18].

TABLE 3. Relative magnitude of polymer consumption in automotive industry.

Polymer PP PA PE PVC TPO ABS PC PEST PPE PC/ABS Acrylics, TPV, PVDF, POM, TPU, SMA, PPS, PP A, etc.

Use (in wt%) 27 14 12 12 10 8

4.5 4.2 4

1.8 <1

The automotive industry has a strong commitment to recycling. It has been decided that new cars must be designed for easy dismantling of plastic body parts. The industry favors: (1) recyclable resins, (2) reduction of the variety of resins, and (3) resins that can easily be compatibilized and recycled.

Subaru was the fIrst car with fully recyclable PP/TPE bumpers of all car models. The bumpers are chopped into sections and placed between rollers rotating at different speeds. This removes 99% of paints and coatings. The sections are then granulated, ready for re­use. Also Nissan aims to achieve total bumper-to-bumper recycling ofPPEIPA. Fiat has an alternative policy of 100% recycling its TPO bumpers into air ducts, liners, etc. The PP battery cases are being recycled as fender liners. For at least fIve years, bumper recycling has been commercial in Europe.

A different strategy was adopted by Ford. The company recycles the PCIPBT XenoyTM alloy bumpers (after removing 99.7% of paint) into taillight assemblies. Alternatively, by adding 25 wt% of the recycled material to virgin resin, it is used to mold bumpers, guide brackets, etc. In 1996, the company used 17.5 kton, or 17%, of post-consumer recycled resin. The recyclates content is to grow steadily to 50% in the year 2000 [19].

342

--ell 120 8.5 ..::c - '"d I. ~ ~ '" CJ e.

110 7.5 n ..,::. '" ·s = '" ~ ~ -.S 100 6.5 :::R e

'" Q .~ / .... ..... '" / n ~ = Q. 90 5.5 '"t

/ --.- Wt/car ..... ~ Q

~ <'C>

..... - ...... - %/car riQ.

-= =-ell -.;:; 80 4.5 ~ 1975 1985 1995

Year

Fig. 3. Increase o/polymeric content in North American/amity cars: 1978-2000.

Recycling automotive plastics is less troublesome than that of municipal PCW. The industry is self-regulatory, with good set of rules that make recycling easier, viz. removable large plastic parts, reduction of resin diversity in a given part (e.g., PPITPO frame and PP fabrics and fluff for seats and interior door panels). Thus, recycling of specific parts, e.g., bumpers or fenders, can be carried out using well-tailored process. Since these parts are molded of polymer alloys, the key to the reprocessing is the recovery of morphology, hence re-compatibilization and re-stabilization.

In automobiles, besides the large plastics parts, there is a substantial quantity of polymers that cannot be easily separated, viz. the interior trim made of: PP, ABS, PC/ABS, PPEIPA, POM, etc. Recycling these commingled resins is more challenging, owing to lower value and high diversity of compositions. However, in some cases recycling can be simple - it was found that contamination of PP by as much as 15% of POM do not have detrimental effects on tensile properties, and it increases the modulus. The blends of PCIABS as well as those of PPEIP A have also been profitably recycled.

A general method for recycling commingled automotive plastics waste includes: steam devolatilization in a TSE, admixing additives, and filtering out unwanted solids and paint flakes. For consistent performance characteristics, the extrusion is close-loop controlled. The extruded materials are reused in combination with virgin resins having the same properties and characteristics. The process was found suitable for recycling automotive parts comprising PC, PEST, ABS, PA, etc. As a property enhancing compatibilizer a copolymer of butadiene, (meth)acrylates and styrene, 5-15 wt% MBS, can be used [20].

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4. RECYCLING OF POLYMER BLENDS

4.1. Principles of blend recycling.

To recycle polymer blends it is important to re-generate the morphology and to re-stabilize the ingredients. Thus, it is necessary to provide adequate mixing (preferably with significant component of the extensional flow field), compatibilize, and stabilize the system. Frequently, the recycled blends must be re-compatibilized and impact-modified. There are two reasons for this: (i) contamination of the composition by other polymeric ingredients, and (ii) degradation of the usually less stable compatibilizers. As a rule, recycled blends must be re-stabilized. The originally incorporated stabilizers can be lost through migration, extraction, or degradation during the processing and usage of the plastics, as well as during the storage of the waste. The exposed to weathering polymeric blends may contain high concentration of peroxy radicals that should be removed before the alloys are formed into new parts.

The recycled blends are frequently upgraded by compatibilization, toughening, and re­stabilization. As described in Chapter 2.4 of this book, the aim of compatibilization is to generate polymeric alloys with optimum, stable and reproducible properties. The compatibilization must ascertain reproduction of the original blend's morphology, stabilize the structure against possible damage during the forming stage, and to secure adhesion between the phases in the solid state. The compatibilization-cum-impact modification is achieved either by means of addition of several ingredients, addition of a multi-polymer, [e.g., ethylene-glycidyl methacrylate, triglycidylisocyanurate, etc.], by reactive processing that generates a multitude of species, or by high stress, mechanical compatibilization. • The first and the easiest task of compatibilization, to decrease of the interfacial tension

coefficient, is accomplished when within the interphasial region between the polymeric ingredients the miscibility is improved.

• The second task, the protection of morphology against destruction during the subsequent processing, is accomplished by either: (i) increasing the thickness of the interphase, (ii) partially crosslinking any of the three principal phases, and/or (iii) introducing to a dispersion of polymer B in A an additional polymer C that has the ability to form a protective layer around the dispersed phase. The thermodynamic condition for complete wetting of B by C is: v AB > V AC + V BO where vij is the interfacial tension coefficient between substances i and j. Alternatively, this condition can be expressed by the three binary thermodynamic interaction parameters, X - they should follow a similar dependence. One of the commercial alloys stabilized by this mechanism is XenoyTM, here A = PET, B = PPE/SEES, and C = PC.

• The third task, the improvement of adhesion between phases in the solid state, can be achieved by providing good bonding across the interphase. This is the case involving either addition or formation of a copolymer that has one part dissolved in one phase and second in another - a covalent bond is formed between the phases. Another method involves the principles of "gluing" different polymeric domains together. This method is particularly useful in multicomponent systems comprising scrap. For example, polyetherimine, PErm, can be used as a universal adhesive [21]. The thermodynamics indicates that for good adhesion in solid state there should be balance of the surface tension coefficients: vdispersed = v rnatrix [22].

344

For the sake of simplicity, in the following discussion the three categories of recyclable materials will be distinguished:

• Polymers and/or their mixtures that belong to the same chemical family, arid have similar chemical constitution, e.g., styrenics, polyolefins, polyesters, etc. The recyclables belonging to the same category require limited compatibilization.

• Commingled plastics belonging to different chemical families, e.g., mixtures of PO's with either PA's or PEST's, or multicomponent mixtures comprising PO, PS, PVC, and engineering thermoplastics, etc. Usually these systems need extensive compatibilization and impact modification.

• Recyclable polymer blends. Here, the recyclability is conditional on re-generation of morphology, often by means of re-compatibilization, re-stabilization, and re-compounding.

Few examples of recyclates that belong to each of these categories will be discussed in the following sections.

4.2. Recycling with limited compatibilization.

In polymer blends technology it has been know that useful properties of mixed resins can be obtained without a need for compatibilization when the components are nearly miscible with each other [viz. several members of PO family, blends of PS with styrenic copolymers, etc.] and concentration of the minor-phase polymer does not exceed 15 wt%. Similarly, immiscible polymer blends also do not require compatibilization if the dispersed phase resin does not exceed 10 vol%. Blends having co-continuous morphology may also offer good performance with limited if any compatibilization. Materials developed for the esthetic, not structural reasons, viz. compositions showing nacreous or wood-grain effects, also may require little compatibilization.

Styrenics. A large percentage of expanded polystyrene, EPS, is being recycled. Also laminated trays of EPS, with such a barrier polymer as ethylene vinyl alcohol, EV AI, are re-granulated and re-extruded with virgin PS at least 20 times. The re-extrusion generated PSIEV AclEV AI blends that, after foaming, produced articles with excellent mechanical properties [23].

In one of the more interesting examples, PS, having high melt viscosity, at T = 200-220°C was compounded with PO into blends having stable co-continuous morphology. Within the optimum composition range (see Fig. 4), the blends had high compression modulus, yield stress and compression strength. The performance was unaffected by PO-contaminants, viz. pigments, additives, and other plastics. Further enhancement of properties was achieved by uniaxial drawing. The materials were reported recyclable [24].

Polyolejins. Biaxially oriented PPIPEIEPR, films laminated with PVDC and an adhesive layer in between, were recycled by soaking the film at T = 25-l40°C in a caustic solution comprising 0.1-50 wt% NaOH and 0.05-1 wt% of a wetting agent. Next, the PO layer was separated from the PVDC layer, and washed. The recovered polymers were reprocessed by blending them with virgin polymers, and subsequently re-used [25].

345

1.8

ooE • 35 • ~ Q / ~---.- 1.4 '<

e'::I ~ .-~

"'C ~ -- --25

~ 1.0 /

~ ./

15

PS (wtO/O) 40

Fig. 4. Composition dependence of modulus, E, and yield stress, 0)', for blends of recycled PS/PO blends. The phase inversion concentration depends on the relative shear viscosity at the processing stress [24].

Blends of two or more polyolefins, PO, were recycled in solid state at the processing temperature, Tp, such that TmJ <Tp < Tm2 (T m:s are the melting points of the blends' components, with Tm, - Tm, > lO°e). The components were: PE, PP, PS, polydienes­either virgin, recycled or mixed. The preferred PO's were: PE and PP, and preferred blends were those containing at least one PE, particularly a substantially linear ethylene polymer [26].

Properties of commingled PO's can also be enhanced without chemical compatibilization by mixing and enhanced nucleation that leads to physical interlocking of structure, hence physical compatibilization. For example, LDPE and/or LLDPE with 2-60 wt% recycled HDPE, was compounded with 0.1-1.5 wt% Zno and 0.1-2 wt% glycerol mono-stearate. The resulting blends could be formed into foamed cushioning material or films. For these applications, respectively ~ 30 and ~ 40 wt% of recycled HDPE was used. The blends could also be extruded, molded, or cast to form films having up to 100% increase in elongation and 50%-90% increase in transverse film strength over LDPE alone. The products were used fore packaging [27].

4.3. Recyclable blends

There is a growing tendency to develop polymer blends that will preserve the desired performance characteristics upon reprocessing. Few examples of the commodity and engineering resin blends are given. More detailed discussion can be found in [3].

Recyclable blends of syndiotactic polystyrene, sPS, were prepared by blending sPS with a copolymer of styrene with either maleic anhydride (MA), or with glycidyl methacrylate (GMA), and with an elastomer, e.g., SEBS, SBS, SBR, EPDM. The compositions showed good impact resistance, elongation and retention of physical properties upon

346

recycling [28]. Similarly, blends ofPA or PARA with polyesteretherimide, PEEl, showed good resistance to thermal aging [29).

Blends that resist loss of impact strength upon being subjected to recycling at temperatures T = 250-350°C (or to thermal aging at T = 50-200°C), were prepared from PPE, SBS and antioxidant/metal deactivator (di-octylamine) [30, 31). Similarly, recyclable blends were made in two steps: (l) PPE was capped with salicylic acid ester, then blended with SEBS. (2) The modified PPE was blended with SBS, then dispersed in a matrix resin selected from between: PEI, PA, PEST, or PS [32].

4.4. Recycling where re-compatibilization is necessary

Over the years, several strategies for recycling polymer blends have been developed. The pragmatism of a given situation dictates selection of one or another. Most polymers are immiscible, some as PS with PO antagonistically so. For this reason compatibilizers need to be used to facilitate dispersing the minor-phase resins, and to improve mechanical performance. Addition of hydrogenated S-B copolymers (SEBS) may homogenize PSIPO mixtures, making them suitable for high-quality applications. However, this is costly and often a compromise must be reached using less expensive additive, viz. SBR.

Some companies, e.g., Mann Organization in the UK, aims to match not only at the type, but even the grade performance of polymer blends. Others (like General Electric Company) believe in downcycling, where the lower blends' performance is still acceptable in less demanding applications [33]. Still other recycling companies do not attempt to reproduce the original blends at all, but rather to generate new materials with consistent performance characteristics, capable to satisfy identified applications. In many cases, the adopted method is dictated by the source of PCW, type of the resin and local market demands. Thus, engineering blends, from housings of the electronic equipment, are the most suitable for the total properties recovery. The automobile scrap (due to contamination) is easier to recycle by downcycling, whereas the commingled plastics usually follow the third path.

In the following text examples of recycled polymer blends will be given, first for the commodity resin, then for the engineering and specialty resin blends. It is noteworthy that the methods of compatibilization and re-compatibilization are the same. In particular, when recycling is to reproduce the original blends' performance, the same compatibilization method should be used, hence support of the original blends' manufacturer is essential.

Commodity resins. Outside the range of phase co-continuity the POIPS blends need to be compatibilized. There are several methods of compatibilization out of which the addition of styrene-elastomer block copolymers and reactive radical co-grafting are most common. For example, PCW comprising 55-75 wt% PO, 5-25 wt% PS, 5-15 wt% PVC, and 0-10 wt% of other thermoplastics, was compatibilized with 3-20 wt% SB, and stabilized by adding 0.1-0.5 wt% of pentaerythritol ester and tris(2,4-di-tert-butyl phenyl) phosphite at a ratio of 5: 1 to 1:5. The recycled mixtures showed good long-term performance [8].

SEBS was used to compatibilize PP with either PS or HIPS. The blends showed good impact and flexural strength [34]. Addition of SEBS was found to be expensive but useful in many recycling processes. Other compatibilizers for commingled polymeric scrap containing PS and PO and other resins, are: CSR, SBR, (SB)n block copolymers, polybutylene-1, a copolymer ofbutylene-1 and ethylene, isotactic polybutene [3], etc.

Halide (co)po1ymers, such as PVC or PVDC, recovered from commingled polymer scrap were contaminated by low melting point PAIs [e.g., PA-6, PA-1212 or PARA]. The

347

recyclate was compatibilized and impact modified by addition of an acrylic copolymer [e.g., of ethylene, alkyl (meth)acrylate, vinyl acetate, (meth)acrylic acid, CO and MA]. To prevent degradation, the blends were processed at T < 220°C, then formed by extrusion or injection molding into a variety of articles [35].

PP and PET, recovered from PCW, were successfully recycled by incorporation of maleated SEBS (SEBS-MA) as a reactive compatibilizer, responsible for improved degree of dispersion, stabilization of morphology and amelioration of adhesion in the solid state. The ratio of the two resins was 2:3 to 3:2, i.e., covering the most probable range of co­continuous morphology. The resulting alloys had high modulus and impact strength while maintaining tensile strength, elongation, and flexural modulus. They were found suitable for molding battery containers, fuel cells, and other automotive components [36].

Engineering resins. PET recovered from PCW was used to produce polyester foam. The process comprised three steps: (1) blending branched PET, ::;; 25 wt% bPET, with recycled, cleaned PET, and either a chain extender or a crosslinking agent, (2) injecting a blowing agent into the melt stream, and (3) extruding the mixture to an area of low pressure, thereby facilitating the development of a stable closed cell foam structure. The method could accommodate other PEST resins or their blends, e.g., PET with either PBT or PEN [37].

It is noteworthy that reprocessing of either PEST, PC, or their mixtures can be facilitated by adding 0.2-0.5 wt% of titanium and zirconium esters that re-polymerize, copolymerize and bond these polyesters to fibers, flakes or rubber crumbs. The method has been successfully used to blend 80% recycled PET with recycled PC. The agent is re-activated every time the mixture is re-processed, thus the alloy's properties improve during recycling [38]

4.5. Recycling commingled polymers and their blends

Mixtures of polar polymers, such as: PVC, PC, PMMA, TPU, PA, PEST, PGI, SAN, or ABS could be compatibilized by incorporation of two copolymers, the first containing vinyl alcohol, the second an anhydride. For example, blends of TPU with Phenoxy, EV AI, capo, modified cellulose, and/or polyalkylene oxide, had attractive physical, optical and barrier properties, and were melt-processable without degradation. They could be transformed into films, sheets, or bottles with good barrier properties. Blends containing PA were used for films, tubes, toys, gears, bearings, shafts, curtain sliders, door rollers, etc. The blends with elastomers were reported suitable for improved wiper blades [39].

Blends of recycled thermoplastics recovered from domestic and commercial waste, and comprising: PE and/or PP, PS, PVC, and ::;; 10 wt% of other thermoplastics, were compatibilized and impact modified by incorporation of 3-20 wt% SBS. The blends were re-stabilized against light and thermo-oxidative degradation by addition of pentaerythritol ester and phosphite [8].

Either PA-6 or PA-6,66, was blended with 1-50 wt% of a P AlLDPE laminated film scrap. As a compatibilizer, either an ethylene-acrylic acid-t-alkyl acrylate, ethylene-glycidyl acrylate, allyl glycidyl ether-t-alkyl acrylate, ethylene-acrylic acid (or ester)-MA, or EMA, was used. The blends could also contain inorganic fillers, other resins, elastomers, and additives. The materials had good impact and notched impact strength, combined with stiffness and toughness. They have been used for the same applications as new materials, instead of having to be downcycled [40].

348

Commingled simulated PCW's (comprising PE's, PP, PVC, PET, and PS) were compatibilized by adding either HDPE-MA or SEBS-MA. The blends were reinforced with glass fibers or vitrified fly ash. The performance depended on compatibilizer - it was superior for blends with HDPE-MA [41].

4.6. Biodegradable blends

Owing to the increasing concern for environment, a growing number of patents deal with the issue of environment contamination by PCW plastics. While re-use of polymers is the preferred method, another proposed solution is biodegradability. The approach may be advantageous in specific areas of applications. For many packaging applications, an incorporation of commingled PCW polymers into a biodegradable resin may be an appropriate solution. The use of plastics films in agriculture is growing. In arid areas, the seedlings are being planted in fields covered with dark PO films. An ideal film should prevent excessive moisture loss and weeds growth, then, when it is no longer needed, it may disintegrate under the influence of either UV irradiation, bacteria, or fungi.

Table 4. Biodegradable blends with polysaccharides.

Thermoplastics Polysaccharide Ref. PVAl Saponified vinylacetate grafted starch [42] Either PS, PE, PP, EPR, NR, SBR, Either cellulose or starch, with a biological [43] PI, PB, or CA Agent, viz. bacteria, fungi or enzymes LDPE and acrylate copolymer as Starch [44] compatibilizer Either PE or PP and ethylene- Maleated starch (improved film properties [45] methylacrylate-maleic anhydride by drawing uni- or bi-directionally, with copolymer as compatibilizer DR ::;;7) PA-6, PA-69, PA-66, PA-12, Hydroxypropyl-starch or urea-starch [46] PEST, etc. PA, PEST, POM Amylose and gelatins [47] Latex ofNR or a synthetic polymer, Starch [48] and fillers Polyesters or polyamides from Thermomechanically converted starch [49] unsaturated fatty acids & diamines or dimer diol-based glycols SMA or maleated EPR, PS, PE, Either starches or proteins [50] PP, TPU, PEST, PA, etc. TPU, PS, PO, EV Ac, EV AI, EAA, Polysaccharide, fatty acid peroxide, [51 ] EMAc, PV AI, EBAc, PMMA and benzophenone, ferric hydroxy stearate, PEG copper stearate, and hindered phenol LLDPE and ionomer Starch [52] PE, PP, PB, PMP, EPR, EVAc, Starch and ::;; 10 wt% peracid containing at [53] EEA, EAA, PS, SBR, SBS, PVC, least one peroxy carboxylic group, -PVDC, PVF, PVDF, POM, PEG, COOOH, and another functional group, PPG, PV AI, PV Ac, PMA, PEA, viz., carboxyl, aldehyde or cyanate PA, PEST, CA, and their blends

Excepting polymers having ester groups in the main chain, the synthetic polymers with MW > I kg/mol are resistant to microorganisms' attack. By contrast, most natural

349

polymers, e.g., polysaccharides, proteins, or lipids, are biodegradable. Preparation of biodegradable blends usually involves combining a thermoplastic resin with a biodegradable one. The blending must produce sufficient dispersion that after disintegration of the biodegradable part the remaining thermoplastic will not contaminate the environment.

Polymers with pure carbon backbones, viz. PE or PP, is resistant to bio-degradation, but may be rendered susceptible to UV. The simplest method of sensitization involves mixing it with metallic oxides or salts, viz. Fe20 J• Incorporation of a ketone side group, -C(R)(COR')-, into polymer chain provides a fine control of the degradation process. Depending on the concentration of these groups, as well as on the nature of Rand R' moieties, the UV susceptibility may be adjusted at will [54, 55]. In 1996, DuPont commercialized Biomax™ biodegradable PET that contains three proprietary aliphatic monomer units to introduce sites susceptible to hydrolysis. The broken down PET chains are biodegraded by naturally occurring microorganisms.

Table 5. Biodegradable blends with synthetic biodegradable polymers.

Thermoplastics 10-40 wt% CPE EVAl PEG, EVAc, EV AI, EPDM, PMMA, imidized-PMMA, SBR, PC, PSF, PI, PPE-blends, siloxanes, silicones, etc. Polyolefins, PO

PA-II, PA-12, PA-6, EAA, EMA, EV Ac, EV AI

Biodegradable polymer Poly(b-hydroxybutyric acid), PHBA 3-hydroxy butyrate-valerate copolymer Polylactic acid, PLA, polyglycolides, polybutyric acid or copolymers of butyric and valeric acid

PHB and block copolymer having poly(methacrylic acid) backbone with hydroxybutyric and hydrovaleric acid groups High molecular weight polyethyleneglycol, PEG

Ref. [56] [57] [58]

[59]

[60]

The biodegradable polymers can be either natural or synthetic. In the former category the polysaccharides (starch, cellulose, dextran, glucose, galactan, mannan, etc.) are the most frequently used. To the latter type belong such well known polymers as PV AI, CA, poly­lactams (e.g., polycaprolactam), polyglycols (e.g., polyethylene glycol), poly(aspartic acid), poly(butylene succinate-co-adipate), as well as poly(3-hydroxy butyrate) or PHB, poly«(3-hydroxybutyric acid) or PHBA, poly(hydroxy-valerate) or PHV, polylactic acid or PLA, polyglycolides, polybutyric acid, their copolymers and mixtures. Several examples of biodegradable blends comprising either natural or synthetic degradable polymer are listed, respectively, in Tables 4 and 5 [3].

5. CONCLUSIONS

The polymer blends technology plays a major role in all aspects of polymer recycling. Blending is essential for homogenization of in-house generated plastics scrap. Importance of the technology increases with the complexity of the system. The commingled plastics scrap, either from the processing plants, or from PCW, can be profitably recycled following the good practices of polymer alloying.

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• The blending technology requires mixing equipment that may provide adequate dispersive and distributive mixing. Since dispersive mixing in extensional flow field is more efficient, the compounders for recyclable commingled scrap should be selected with this aspect in mind. The fundamental aspect of blending is generation of the optimized morphology - the type and the degree of dispersion.

• The mechanical mixing must be supported by compatibilization. The latter process must ascertain adequate decrease of the interfacial tension coefficient (to generate desired degree of dispersion). It also must stabilize the morphology against possible shear coalescence during forming (e.g., high speed injection molding). Furthermore, it must provide sufficient adhesion between the phases in solid state that the stresses will be transferred across the interface.

• Blends that comprise high glass transition amorphous polymers and semi-crystalline ones are notoriously brittle. The brittleness does not have anything to do either with the source of polymer (recycled or virgin), or with compatibilization. Compatibilized mixtures of brittle polymers show nearly as bad performance as non-compatibilized ones. To improve the impact strength, these systems must be impact modified. Impact­modification (or toughening) is an integral part of the polymer blending technology. Many multi-component commercial compatibilizers serve also role of impact modifiers.

• The fourth essential element of the technology is stabilization against degradation -during processing as well during the life-time of the product. It should be assumed that the recycled plastics are in advanced stage of degradation. Thus, the stabilizers must first eliminate or compensate for the damage already done, then they must provide protection for the processing and life time use. In short, recycled polymers require more stabilizer than virgin resins.

The blending technology is flexible, capable to provide tailored performance to any mixture. The difficulties are related to stability of the polymeric materials' supply, stability of their composition, and adequate stability of the market.

6. NOMENCLATURE

ABC ABS bPET CA COPO

CPE CR CSR EAA EBAc EEA EGMA EMA EMAc EPDM EPR EPR-MA

alloying-blending-compounding acrylonitrile-butadiene-styrene branched polyethyleneterephthalate cellulose acetate poly(carbon monoxide-co-polyolefin), a linear, alternating terpolymer chlorinated polyethylene chloroprene, or neoprene, rubber chi oro sulfonated rubber ethylene acrylic acid copolymer ethylene butyl acrylate copolymer elastomeric copolymer from ethylene and ethyl acrylate ethylene-glycidyl methacrylate copolymer ethylene-maleic anhydride copolymer copolymer from ethylene and methacrylic acid ethylene-propylene-diene terpolymer ethylene-propylene rubber maleated ethylene-propylene rubber, EPR

EPS EVAc EVAI FTIR GF GMA HALS HDPE HDPE-MA HIPS LCP LDPE LLDPE MA MBS MW MWD NR PA PA-6 PA-46 PA-66 PARA PB PBT PC PCW PE PEA PEEl PEG PEl PEN PEST PET PEtI PGI PHB PHBA PHV PI PLA Phenoxy PMA PMMA PMP PO POM PP PPA PP-MA PPE PPG PPS PS PSF PVAc PVAI

polystyrene foam; expanded PS copolymer from ethylene and vinyl acetate copolymer of ethylene and vinyl alcohol Fourier-transform infrared spectroscopy glass fiber, or glass fiber reinforced plastic glycidyl methacrylate hindered amine light stabilizer high density polyethylene maleated high density polyethylene high impact polystyrene liquid crystalline polymer low density polyethylene linear low density polyethylene maleic anhydride copolymer from methylmethacrylate, butadiene, and styrene molecular weight molecular weight distribution natural rubber polyamide poly-s-caprolactam poly(tetramethylene adipamide) polyhexamethylene-adipamide aromatic (mainly amorphous) polyamide polybutadiene polybutylene terephthalate polycarbonate of bis-phenol-A post-consumer waste polyethylene polyetheramide polyesteretherimide polyethyleneglycol polyetherimide poly( ethylene 2,6-naphthalene dicarboxylate) thermoplastic polyesters, viz. PET, PBT, PEN, etc. polyethylene terephthalate polyethyleneimine polyglutarimide polybutyric acid poly(l3-hydroxybutyric acid) poly(hydroxy-valerate) polyimide polylactic acid polyhydroxyether of bisphenol-A polymethylacrylate polymethylmethacrylate poly-4-methyl-l-pentene, also TPX polyolefin polyoxymethylene isotactic polypropylene (aPP - atactic; sPP - syndiotactic)

maleated polypropylene polyphenyleneether polypropylene glycol polyphenylsulfide polystyrene polysulfone polyvinyl acetate polyvinyl alcohol

351

352

PVC PVDC PVF PVDF QC R-TPO SAN SB (SB)n SBR SBS SEBS SEBS-MA SMA sPS SSE SSSE TEM TPO TPU TSE UHMW-PE ULDPE UV

7. REFERENCES

polyvinyl chloride polyvinylidene chloride polyvinyl fluoride polyvinylidene fluoride quality control reactor-blended thermoplastic olefinic elastomer styrene-acrylonitrile styrene-butadiene copolymer multi-block styrene-butadiene copolymer styrene-butadiene elastomer styrene-butadiene-styrene three block copolymer styrene-ethylenelbutene-styrene three block copolymer maleated SEBS styrene-maleic anhydride syndiotactic polystyrene single-screw extruder solid-state shear extrusion transmission electron microscopy thermoplastic olefinic elastomer thermoplastic urethanes twin-screw extruder ultrahigh molecular weight polyethylene (over 3 Mg/mol) ultra low density polyethylene ultraviolet

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40. Timmermann, R, Dujardin, R., Orth, P., Ostlinning, E., Schulte, H., Dhein, R., and Grigat, E., Europ. Pat. Appl., 583,595,23 Feb 1994; Jap. Pat., 61 92,570, 12 July 1994, Appl. 20 July 1992, to Bayer.

41. Xanthos, M., Dey, S. K., and Sebastian, D. H. (1996) Proceedings Polymer Processing Society and American Institute of Chemical Engineers Joint Meeting, Chicago, Nov. 11-14.

354

42. Yoshitake, T., Tasaka, T., and Sato, R., Jap. Pat., 050,254; 050,264, 08 May 1978, Appl. 18 Oct 1976, to Kuraray Company, Limited.

43. Guttag, A, u.s. Pat., 5,120,089, 1992, Appl. 28 Feb 1990; u.s. Pat., 5,346,929, 13 Sep 1994, Appl. 18 Mar 1992.

44. Willett, J. L., u.s. Pat., 5,087,650, 11 Feb 1992, Appl. 1990, to Fully Compo Plastics, Incorporated.

45. Tomka, I., Ger. Offen., 4,116,404, 19 Nov 1992, Appl. 18 May 1991; Tomka, 1., Meissner, J., and Menard, R., Ger. Offen., 4,134,190,22 Apr 1993, Appl. 16 Oct 1991.

46. Buehler, F., Schmid, E., and Schultze, H. 1., Europ. Pat. Appl., 536,679, 14 Apr 1993, Appl. 08 Oct 1991; u.s. Pat., 5,346,936, 13 Sep 1994, Appl. 15 June 1992, Ger. Appl. 17 June 1991; Meier, P., Ger. Pat., 4,139,468,03 June 1993, Appl. 29 Nov 1991, to Ems-Inventa Aktiengesellschaft.

47. Meier, P., Ger. Pat., 4,139,468,03 June 1993, Appl. 29 Nov 1991, to Ems-Inventa Aktiengesellschaft.

48. Munk, W. G., Ger. Offen., 4,204,083 AI, 04 Mar 1993, Appl. 09 Aug 1991, to Nordmann Rassmann GmbH & Company.

49. Ritter, W., Bergner, R., and Schafer, M., Ger. Offen.,4,121,111 AI, 07 Jan 1993, Appl., 26 June 1991, to Henkel K.-G.a.A

50. Vaidya, U. R., and Bhattacharya, M, u.s. Pat., 5,321,064, 14 June 1994, Appl. 12 May 1992, to Regents of the University of Minnesota, Minneapolis.

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53. Hsu, H.-W., Liuo, S.-C., Jiang, S.-F., Chen, J.-H., Lin, H.-M., Hwu, H.-D., Chen, M.-L., Lee, M.-S., and Hu, T., u.s. Pat., 5,308,897, 14 Jan 1994, Appl. 03 May 1993, to Industrial Technology Research Institute, Taiwan.

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26 May 1982, Appl. 18 Nov 1980, to Imperial Chemical Industries, Limited. 57. Webb, A, Carlson, A W., and Galvin, T. J., PCT Int. Appl., 001,733, 06 Feb

1992, Appl. 1990, to ICI Americas, Incorporated. 58. Kharas, G. B., and Nemphos, S. P., Europ. Pat. Appl, 515,203,25 Nov 1992,

Appl. 24 May 1991, to Novacor Chemicals. 59. Ballard, D. G. H., and Buckmann, A J. P., PCT Int. Appl., 93 17,064,02 Sep

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1991.

NON-CONVENTIONAL PROCESSING TECHNIQUES FOR POLYMER RECYCLING

M. J. BEVIS

Consultant Director Wolfson Centre for Materials Processing BruneI University Uxbridge, Middlesex, UB8 3PH, U.K.

The main categories of plastics waste are identified together with some of the established and possible routes for secondary recycling. The discussion concentrates on practical methods that have largely, but not exclusively, been adopted by industrial firms.

1. Proposed categories of waste plastics

The recycling of waste plastics may be conveniently divided into the following categories: I) Scrap in the form of offcuts, rejects, sprues, etc., arising in the manufacture of plastics products. Most of this waste material is recycled by blending it with virgin plastic, subject to the careful control of the levels of contamination in the plastic regrind, the proportion of regrind to virgin materials, and the deterioration in the physical properties which may be caused by repeated thermal and mechanical processing. II) Single grades of contaminated plastic which can be collected from consumers or processors; for example, used fertiliser sacks, crates, packaging materials and in-house contaminated scrap. III) Single grades of in-house or consumer scrap as in (II), but contaminated with metal attachments. For example, laminated aluminium-polystyrene sheet, milk bottles, reject mouldings or off cuts containing metal transfers, electroplate, batteries, etc. IV) Mixtures of two or more contaminated plastics arising as industrial scrap. For example, sweepings from factory floors, scrap cables, carpet trim, laminated film and containers, brown and white products, automotive products. V) Light industrial scrap and pre-segregated municipal scrap- PET, PVC or PE from bottle banks, etc. VI) Mixed contaminated plastics as found in municipal refuse.

Data relating to the nature and quantities of plastics wastes arising from industrial processes and municipal refuse have been reviewed [1,2], together with a consideration of the economics and appropriateness of primary, secondary, tertiary and quaternary recycling respectively.

Intense exploration and development of recovery processes occurred in the 1960's and 1970's, encouraged by a shortage of polymers. An important series of articles [3] was published in Kunsttoffe- German Plastics in 1978, and provided detailed descriptions of developments then in plastics recycling. A publication of 1981 [4] describes 335 processes for reclaiming organic and polymer wastes, many of which are relevant to plastics recycling. The influences that encourage the recovery and re-use of plastics are now longer term, and related to concern for the environment and associated legislation. The plastics recycling industry has matured. Recycling processes and the infrastructure for the

355 G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 355-370. © 1998 Kluwer Academic Publishers.

356

collection of scrap, its comminution, purification and conversion into saleable products have been established, and are the basis of large scale business activity such as the production of PE film and PET fibres.

There are significant high added value recycling options in other product areas, and these include niche businesses that rely on non-conventional recycling technology. The following sections consider aspects of primary recycling, and the six potential routes for secondary recycling of plastics featured in Figure 1.

In many cases, the plastics produced by these routes can be modified by the addition of compatibilisers, cross-linking agents, or expanded for injection moulding and extrusion applications using foam technology, including multi-component extrusion and injection moulding, when good surface properties are required. In the latter case, sequenced screw loading provides for the incorporation of recyclate as a core layer, thereby producing multi-component mouldings using a conventional injection moulding machine (see Figure 2), as a consequence of the relatively low cost incorporation of a special two component dosing unit [5].

2. Primary Recycling

The utilisation of waste arisings in Category 1 is mainly through primary recycling, which is routinely practised by more than one half of the industrial firms involved in the conversion of thermoplastics, and recognised as an essential requirement for viable business. Figure 3 is a schematic diagram of a typical thermoplastics recycling line, that typically prepares plastics for a wide range of conversion processes, for example, profile extrusion, injection moulding, etc.

There have been significant developments in the efficiency of extruders, melt filtration devices and pelletisers, which give rise to a more reproducible product and realise better quality from traditional as well as increasingly diverse feedstocks.

The development of practice where the producer of "refined" feedstock from scrap plastics then goes on to convert the recyclate into a saleable product, provides the basis for some large and successful industrial operations. In this situation, the convertor is aware of the deficiencies of the feedstock plastic, and influences the appropriate addition of compounding additives and appropriate sourcing and securing of scrap. The technology used for the production of feedstock and conversion into demanding end products is non­trivial, and the attainment of consistency in [mal product quality is often remarkable, and emanates from attention to detail in compound formulation and recycling process technology.

Most of the wastes falling into Categories II to VI are currently disposed of by landfill even though many processes have been developed for the recovery and re-use of a very wide range of polymeric wastes. A small proportion is used for secondary or mechanical recycling, but nevertheless, provides the polymeric feedstock for a substantial number of successful industrial firms. The practice overall makes economic sense and satisfies the requirements of niche businesses. The non-conventional methods for recycling plastics naturally concentrate in this field of use, where the performance criteria for use are often less critical than for virgin plastics, but where some of the inherent properties of plastics at reduced cost is an advantage.

An increasing proportion of scrap plastics is being used for tertiary or quaternary recycling, where the income derived from the products produced, for example chemical feedstock and energy respectively, contribute towards offsetting the costs of providing for alternatives to landfilling for a substantial proportion of the very large tonnage of mixed contaminated plastics in municipal refuse and industrial waste.

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The efficient conversion of thennoplastics into the majority of saleable artefacts calls for reproducible rheological and mechanical properties and colour. It is difficult to upgrade most scrap arising in Categories II to VI at a cost where the recycled product can compete with the virgin materials or recycled materials arising from Category 1. The alternative to efficient modem methods of conversion is to convert poorly characterised scrap plastics into products which would nonnally be made from wood or concrete. With very few exceptions, this has proved unsuccessful commercially in the longer tenn, and previously well publicised plastics recycling methods have not proved viable in widescale use. The successful ventures are primarily based on efficient materials handling technology; they are thoroughly researched in tenns of plastics compound fonnulation and conversion technology, and the saleable artefacts represent high added value and exploit the advantages of low density thennoplastics. The usage of the thennoplastic scrap may not be considered simply as a substitute for wood or concrete.

3. Secondary Recycling

The optimum primary recycling of thennoplastics compounds is often excluded because of the relatively poor reproducibility of the colour or the rheological and mechanical properties of recycled scrap. These characteristics may arise because of the mixing of different grades of plastics or because of contamination, or thennal or photo­degradation. The secondary recycling into a down graded application is, therefore, the more likely, and these may be conveniently placed into six potential processing routes (Figure 1). Integrated extrusion processing, Route 3, is relatively new and may result in the use of the recyclate as a component polymer in blends or as a filler in the case of composites. This route may provide for the enhancement of mechanical or other functional properties.

Route 1 Thick-section artefacts result from the application of the processes referred to; the

production of sintered plastic particle board [6] and, in addition, there are some other good examples of compression mOUlding. Several flow moulding machines have been developed specially for plastics recycling. These include the Reverzer, which came into prominence in Japan in the 1970s, and Klobbie recycling machines for the manufacture of very thick section artefacts from relatively low grade scrap, and Remaker machines for the manufacture of relatively thick-sectioned non-linear product [7]. Schematic diagrams of Remaker machines are shown in Figure 4, and with adaptions and improvements, are produced by Kunstoffmaschinen Ettlinger GmbH, Konigsbrunn, Gennany. Examples of products produced by Ettlinger recycling machines are illustrated in Figure 5 and represent a large plant pot and drainage gully respectively, as produced from household waste plastic and a mixture of household waste plastics and thirty per cent detergent bottle regrind, noting that a recent development includes machines newly designed for the manufacture of heavy duty plastic pallets.

Route 2 Comminution to fine particle size «500 microns) in order to reduce the effects of

impurity particles on the mechanical properties and/or appearance of thin-section artefacts. The FN Industry homogenising extruder [7], for example, produced pellets from mixed waste suitable for conversion into relatively thin-walled artefacts.

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Route 3 Integrated extrusion processes [8] provide for the production of composite materials

which use the intrinsic properties of the plastics recyclate, or other waste as a compounding additive, to add value to the composite based on a matrix of virgin or recycled plastic. The method is based on modified twin-screw extrusion technology requiring assembly of well­defined process steps, which conditions the additive phase, prior to its incorporation into the polymeric matrix. Hence in the early stages of the process only the additive is present, which is then combined with the polymer further downstream. This concept is the inverse of conventional extrusion compounding procedures.

The schematic diagram shown in Figure 6 identifies the principal roles of a co­rotating twin screw extruder as an integrated extrusion process. The use of natural fibres (wheat straw) and comminuted thermoset matrix composites scrap, are two documented examples of the process [9]. The twin-screw configuration and operating conditions can be optimized to provide for final recyclate size reduction, chemical treatment and incorporation in a polymer matrix, whilst preserving the structural integrity of the fibres in recycled composites.

Specialised polymer compounding technology has been developed which enables waste materials, such as scrap or residue derived from industrial or agricultural sources to be physically and chemically modified, then combined with polymer within a single process unit. The technology integrates the additive preparation and combination stages in a more cost effective manner than is possible by separate unit operations and in addition, permits greater control of microstructure in the ultimate composite. By this means, the effectiveness of potentially reinforcing additives can be enhanced

This process technology has been successfully applied to the re-use of polyester and phenolic based thermoset scrap and natural fibres, originating from annual crops such as linseed flax and wheat straw. In particular, the early stages of the process alter the physical form of the additive phase, either by controlled fracture and size reduction of brittle thermoset composites or by pulping of natural fibre species. Surface treatment can be applied during this stage and/or later, whilst blending additive and polymer matrix components. A key benefit of this approach is that treatment is constantly available to coat new surfaces which are inevitably created during the compounding process.

Route 4. Several wash-sink/float processes are in use and readily available for secondary

recycling operations and, in addition to the action of water and detergents, function on the basis that polyolefins float inwater and most contamination and other polymers sink. Wash granulators which combine comminution and washing are also available.

A rising current separator that provides for the separation of PE, PVC and copper in comminuted cable scrap has also been described [10], see Figure 7.

Future developments in plastics separation technology may be based on improvements in selective froth floatation techniques which have been pioneered in Japan [11]. Some of the original patents are referred to in Reference 11, though these patents are not known to be the basis of commercially successful plastics separation businesses.

Route 5 Melt mtration systems can very effectively remove particulate contaminants from

polymer melts, for example, continuous systems provided by Process Developments [10] and by Gneuss. Kauferle (Remaker) produced a prototype device capable of causing separation of the melt from high concentrations of aluminium from polystyrene in laminates, or copper from polyethylene in telephone wire, without the need for disposable filter screens. The solid and particle contaminant-free thermoplastic exited from one port on the separation head, while a very high concentration of contaminant embedded in

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thermoplastic exited from a second port. Perfection of the device identified in Figure 3, which, apparently, was withdrawn from the market, would offer substantial potential for recycling of laminates and other metal containing sources of thermoplastic scrap.

Route 6 Processes that do not rely on the melting of plastic recyclate relate to large scale

predominantly tertiary and quaternary recycling. Thermoset matrix composites recycling also falls within this description, and the principal recycling routes for thermoset matrix composites are outlined below.

4. Thermoset Matrix Composite Recycling

Recycling is one of the biggest issues facing the composites industry, particularly for large-volume applications. Increasingly stringent environmental regulations are likely to restrict the use of composites in favour of materials that can be recycled cost effectively. Indeed, in order to counter the arguments which may lead to legislation that would limit the nature of composites products for sale, the composites industry as a whole will have to encompass viable primary, secondary, tertiary and quaternary recycling capabilities.

The recycling of thermoset-matrix composites presents much more significant difficulties, principally centred on the irreversibility of cross-linking, the fibre attrition associated with comminution, and a polymer content that may be less than 30% of the total weight. The bulk of the material is often glass-fibre reinforcement or a filler, including fire retardants and resin diluents. In tertiary recycling the problem is not just one of recycling the polymer.

Greater stability in the supply of scrap is associated with large sources of standardised composite scrap arising from co-operative industry ventures. For example, ERCOM, a consortium of large European composite manufacturers in partnership with a number of leading raw materials suppliers, shred components manufactured from polyester and vinylester-based sheet and thermoset-moulding compounds to a range of well-defined particle sizes. The resultant fibre and powder fractions can be used in the production of new moulding compound components, and can also be used as reinforcing material for thermoplastics and other materials.

Addition of reground process scrap back into uncured resin formulations has been reviewed for a wide range of thermoset-matrix composites, including polyurethanes, phenolics and epoxies, in addition to unsaturated polyester moulding compounds. A comprehensive review ofthermoset recycling [12] also encompasses the use of hydrolysis, glycolysis and pyrolysis as effective routes for recycling of selected thermosets. Combustion with heat recovery is proposed as a route for utilising the energy content of the matrix polymer. The behaviour of a range of composites during combustion, the form of the ash product and the emissions during combustion have been investigated systematically [13]. Two industrial processes that utilise the energy content and the inert materials arising in the ash have been proposed. In cement manufacture thermoset-matrix composites may be burned in a cement kiln to utilise their energy content and the mineral materials utilised in the cement klinker. Alternatively, polymeric materials filled with calcium carbonate may be of use as a fuel substitute and sulfur oxide removing agent in coal-fired fluidised bed combustion.

Thermosetting polyester-matrix composites represent a large percentage of composite manufacturing, particularly in the automotive sector, and SMCs have been the subject of considerable research and success in recycling. Comprehensive assessments of the mechanical properties of SMCs containing large proportions of comminuted recyclate have been reported, and confirm that incorporation for new product manufacture is viable,

368

Table 1: Mechanical properties of polypropylene/thermoset recyclate composites (at 23°C).

Composition Wt .. Glass in Tensile Strength Tensile Modulus Notched Composite (MPa) (GPa) Charpy Impact

Strength (J/mm')

PP - 26.5 (0.3) 1.8 (O.I) 7.7 (0.7) PP/CaCO, (*) - 20.4 (O.I) 2.1 (0.2) 3.2 (0.5) PP/Glass Fibre 30 72.3 (0.6) 7.7 (0.4) 5.4 (0.2)

PPIDMC: Untreated 5 18.4 (0.9) 2.7 (0.3) 2.4 (0.2) PPIDMC: Treated 5 29.1 (O.I) 3.1 (O.I) 2.4 (0.2)

PP/GWP: Untreated 24 28.9 (0.2) 5.2 (0.6) 4.7 (0.2) PP/GWP: Treated 24 62.1 (O.I) 5.1 (0.2) 7.4 (0.2)

*30% by weight PP-Polypropylene, DMC-Dough Moulding Compound Recyclate, GWP-Glass (Woven Fabric) Phenolic Recyclate, ( )- Standard Deviation

369

and justify major recovery and reuse initiatives of the form summarised in Figure 8. The diagram summarises the options that are available for recovery and reuse of SMCs and, within limitations, can be applied to other thermosetting matrices, including epoxies, phenolics, polyurethane and urea-formaldehyde.

5. Utilisation of Thermoset Recyclate by Integrated Extrusion Technology

The mechanical properties of polypropylene composites [9] containing 30% by weight of thermoset recyclate are shown in Table 1. It should be noted that the DMC recyclate used in these compositions contained only 15% by weight of chopped glass fibres, in addition to 55% by weight of mineral fillers, with the remainder being cured polyester resin. The phenolic material originated from industrial scrap made from 80% woven glass impregnated with phenolic resin. These results reflect both the differences in glass level within these recyclate variants, the form of glass fibre present and the influence of interfacial bonding between additive and matrix phases. The performance of PP composites containing treated phenolic recyclate with high glass fibre content is most significant. Stiffness, strength and toughness greatly exceed polymer filled with calcium carbonate, at an equivalent loading, and approach the properties of commercial glass fibre­reinforced PP material. The exceptional impact strength of the phenolic recyclate composite is attributed to the presence of woven glass mat fragments which effectively inhibit crack propagation, whereas particulates of DMC recyclate are inherently weak and offer little resistance to crack development.

Pelletised compound has been successfully injection moulded into complex automotive parts.

6. Concluding Remarks

The processes identified above relate principally to extensions of polymer melt processing technology. There have been many developments in plastics recycling which are generally considered to be more radical and, in conclusion, two radical innovations in the utilisation of plastics scrap are identified.

Large tonnage applications for selected recycled composite materials are being developed as shot-blasting media for selective paint removal, for use as soil conditioners, and also for use as functional additives such as flame or smoke retardants. Thermoplastics are used in analogous ways, as exemplified by "Biobead", a patented sewage treatment pellet. Designed for use in modem aerated ftltration plants, the pellets create a biological mass that aids the breakdown of sewage effluent. Scrap polymer used in the manufacture of "Biobead" include low grade co-extruded film, and large quantities of heavily contaminated materials that could not otherwise be used. In essence, the production of shot-blasting material and "Biobead" depend on the availability of low cost scrap, and are fairly typical of the niche businesses that foster the use of non-conventional polymer recycling processes.

Acknowledgements

The author is indebted to colleagues C. E. Bream, P. R. Hornsby, K. Tarverdi and K. S. Williams for their technical contributions to this report.

370

References

1. Dennison, M.T. and Lovell, J.S., "The future of plastics recycling in Europe", Paper presented in the DeWitt Petrochemical Review, Houston, USA, 1994. 2. Dennison, M.T. and Mennicken, T., "Plastics recycling in Europe", PACIA 96 Convention, Brisbane, Australia, 1996. 3. Kunsttoffe-German Plastics, 67, 266 (1978). 4. Sittig, M.," Organic and Polymer Waste Reclaiming Encyclopedia", Noyes Data Corporation. S. Addmix, Unit 20, Cryogenics Business Centre, Dalymeyer Road, London, NW 1 0, 2XA, UK. 6. Wood, R, Plastics Machinery and Equipment, 4, 11, 19, (1982). 7. Bevis, MJ. , "Plastics recycling", Proc. of Plastics-strategies for the eighties, Lausanne, Switzerland. Published by Modem Plastics Intern. (1981). 8. Hornsby, P.R and Tarverdi, K.T., U.K. Patent application (1996). 9. Bream, C.E., Hinrichsen, E., Hornsby, P.R, Tarverdi, K.T. and Williams, K.S. "Integrated compounding technology for the preparation of polymer composites containing waste materials"; Paper presented at the July PPE Conference, Bradford University, U. K, 1997. 10. Allan, P.S., Bevis, MJ. and Irving, C.N., Conservation and Recycling, 6, 3, 10, (1983) 11. Saitoh, K., Nagano, I. and Izumi, U.M., Recovery and Conservation, 2, 127, (1976). 12. Farrisey, W.J. in Ehrig, RH. (Ed.) "Thermosets" in Plastics Recycling Products and Processes, Hanser, Munich, Germany, (1992) 13. Pickering, S.J. and Benson, M., "Disposal of engineering plastics", Paper 42, Proc. 2nd. Int.Conf. Plastics Recycling, Plastics and Rubber Institute, London, 1991.

REPROCESSING AND PROPERTIES OF HOMOPOLYMER BLENDS OF VIRGIN AND RECYCLED POLYMERS

Abstract

F.P. LA MANTIA

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita' di Palermo, Viale delle Scienze, 90128 Palermo, Italy

The blends of the same vugm and recycled polymer, (homopolymer or "monopolymer" blends), show rheological and mechanical properties in most cases between those of the two components athough, in general, lower than those expected on the basis of additivity rules. In some cases, however, a minimum observed in some property versus composition curves is correlated with the change of the crystalline morphology induced by structural differences of the two components and due to different molecular weights and, in particular, the presence of oxygenated groups. In this chapter, the main properties of some homopolymer blends will be reviewed by considering, in particular, the influence of composition, structure and morphology of the recycled components on the properties of these blends.

1. Introduction

An easy and frequently used method to process recycled polymers is to blend them with the same virgin material in order to produce an inexpensive material with adequate properties. The rheological and mechanical properties of these homopolymer or "monopolymer" blends do not obey, in general, an additivity rule, (as expected given the same chemical nature of the components), but rather these properties are well below those expected on the basis of a linear rule. In few cases, a minimum in the property-composition curve is also observed. The first feature is mainly due to the different molecular weights of the two components and to their different structures [1, 3-5], while the change of the crystalline morphology achieved by the macromolecules of the recycled components has been considered responsible for the minima in the property-composition curves [2]. Degradation phenomena occuring during the product lifetime or during the recycling operations, indeed, can strongly decrease the molecular weight and induce the formation of new functional groups. The knowledge of the main property-composition relationships of these homopolymer blends is therefore a necessary step for increasing the amount of recycled components without depressing mechanical properties.

In this chapter, the main properties of some homopolymer blends will be reviewed by considering, in particular, the influence of composition, structure and morphology of the recycled components on the properties of these blends. Blends incorporating polymers recycled more than once as well as theories predicting the properties of these blends [6-9] will not be taken into consideration.

371

G. Akovali et al. (eds.), Frontiers in the Science and Technologyofl'olymer Recycling, 371-385. © 1998 Kluwer Academic Publishers.

372

2. Poiyoiefins

2.1 POLYETHYLENE

"Monopolymer" blends made of the same components but with different molecular weights may not be compatible and this behaviour is enhanced when one of the components contains also different chemical groups. This can occur mainly when one of the components originates from the recycling of products exposed to UV radiation. Polyethylene recycled from greenhouses (RPE) contains oxygenated groups as a result of photo-oxidation, cross-links and small amounts of mineral fillers, additives, stabilizers and other polymers. The properties of incompatible blends are determined mostly from the dimensions and dispersion of the minor phase in the matrix. Good homogenisation and small domain size can improve mechanical properties and can be achieved by severe mixing conditions, for example by increasing the number of extrusion passes.

480-.--------------------------------r 16 EB,% EY,%

420 14

A EB

360 e EY 12

Number of extrusions

300-~--------------~---------------+ 10 1 2 3

Figure I. Elongation at break and at yield of a RPENPE blend as a function of the number of extrusion passes

Adapted, by permission, from La Mantia F.P., Ma Wenguang (1996) "Recycling of post-consumer polyethylene greenhouse films: monopolymer blends of recycled and virgin polyethylene", Polym. Networks & Blends,S, 173-179. Copyright ChemTec Publishing.

The values of elongation at break, EB, and at yield, EY, of a blend with 80% of RPE and 20% of virgin PE (VPE) are reported in Fig. 1 as a function of the number of passes in a single screw extruder [3]. Both curves appear to show a maximum after two extrusion steps. This behaviour is probably due to a balance between the better homogenisation of the blend, which improves with increasing extrusion steps, and the degradation, which increases with the severity of the thermomechanical treatment. All the results that will be shown below have been determined on blends extruded twice.

The average mechanical properties of compression molded sheets of RPENPE blends versus VPE concentration are shown in Figs. 2 and 3 [3]. The thickness of the specimens is about 0.50 mm. All the properties of the blends are intermediate

16~----------------------------~

14 Stress, MPa

12 ~----~----~---~-~--~~--10 81--~~~-r--~~----~~

6

4

o Yield Break

2~-----r----~~----~-----r-----+ o 20 40 60 80 100

VPE,% Figure 2 . Stress at yield and at break as a function of VPE content

600 -: EY,%

540 - EB,%

480 ~

420 A EB 0 EY

360 X EB calculated by mOM

300 -0 20 40 60 80 100

VPE,%

Figure 3 . Elongation at yield and at break as a function of VPE content

373

20

18

16

14

12

10

Adapted, by permission, from La Mantia F.P., Ma Wenguang (1996) "Recycling of post-consumer polyethylene greenhouse films: monopolymer blends of recycled and virgin polyethylene", Polym. Networks & Blends, 5, 173-179. Copyright ChemTec Publishing.

374

between those of the two pure polymers and increase with increasing VPE content. The stress and strain at yield are only slightly influenced by the composition. The properties of the blends are, however, slightly lower than those expected on the basis of a linear additivity rule. In particular, the elongation at break at high VPE concentrations shows lower values. This means that small contents of RPE can significantly reduce the ultimate properties of virgin PE.

The values of elongation at break are very close to the values calculated by the inverse rule of mixture (IROM):

IIEBb = VrlEBr + V vlEBv

as seen in Fig. 3, where EBb is the value of the elongation at break of the blends, EBr and EBv are the values of the elongation of the recycled and virgin component respectively, and V r and V v the corresponding volume fractions. These results are similar to those reported in [10] on blends of recycled and virgin HDPE. The reasons for this behavior may be due to the different structure of the photo-oxidized polymer and to the different molecular weights that can give rise to some incompatibility between the two phases. It is, however, worth noting that small amount of cross­linked polymer in the photo-oxidized recycled material worsens the reproducibility of the vaues of the mechanical properties and gives rise to the deviation from the additivity rule.

The values of the mechanical properties of these blends change with the type of processing but the comments made on the property-composition curves of the compression molded sheets hold also for injection moulded bars and for blown films. Almost all tensile properties are somewhat slightly lower than those predicted by the linear additivity rule.

Finally, it is worth noting that in the case of the "monopolymer" blends discussed above, the properties of the two components are similar and also molecular structure and morphology do not present significant difefrences. As will be discussed below, this similarity is the main reason of the almost linear dependence of most mechanical properties on composition.

2.2 POL VPROPYLENE

The "monopolymer" blends of recycled and virgin polypropylene (PP) can be very useful in the strategy of PP recycling since this material undergoes dramatic degradation phenomena, and then a drastic drop of mechanical properties, during reprocessing. The poor resistance to thermomechanical stress gives rise to secondary materials with lower molecular weight and modified morphology [11). These features imply that polypropylene needs extensive stabilization before each reprocessing operation. The preparation of blends of virgin and recycled PP can, at least in part, overcome these problems and give rise to materials with adequate mechanical properties. Similarly to all "monopolymer" blends all properties are strongly dependent on composition and on the properties of the recycled component; however, in the case of "monopolymer" PP blends this feature is more dramatic because of the drastic changes in the structure and morphology of the recycled PP.

2.2.1 Viscosity and processability

The decrease of the molecular weight leads to drastic reduction of the viscosity of the recycled material and then of the viscosity of the "monopolymer" blends. In Fig. 4, [5], the viscosity at two fixed shear rates of blends of virgin PP (VPPE) and of the same sample after one extrusion (El) and after five extrusions (E5) is reported

375

as a function of the content of the recyced component (RE). The viscosity of the extruded PP is lower than that of the virgin sample and decreases with increasing number of extrusion passes indicating significant degradation phenomena. The viscosity of the blends is between those of the two unblended components and lower than that predicted from linear and logarithmic additivity rules. Due to the different non-Newtonian behavior [11], the values of the viscosity of the two components approach those of the blends with increasing the shear rate and the viscosity of the blends is only slightly different than that of the component. At low shear rates, when the viscosity is mostly dependent on the molecular structure and the melt morphology of the blend, the difference between the viscosities is larger and also the negative deviation from linear rules is larger. With decreasing the molecular weight of the recycled sample the negative deviation becomes larger and larger. These features suggest some incompatibility of the two components in the molten state due to the different molecular weight since no significant formation of oxygenated groups has been shown in the extruded materials.

lE+04 -r-------------------------------------------------------------~ Viscosity, Pa*s

lE+03

RE,% lE+02 -,~------------~------~------~------~

o 25 50 75 100

Figure 4. Viscosity ofVPPEIRE blends as a function of the RE content at two fixed shear rates (15 and 150 s-I). Open symbols refer to E I and full symbols to E5

Adapted, by permission, from Marrone M., La Mantia F.P. (1996) "Monopolymer blends of virgin and recycled polypropylene", Polymer Recycling, 2, 9-17. Copyright RAPRA

2.2.2 Mechanical properties

The change of molecular structure and morphology strongly influences mechanical properties. In Figs. 5 and 6 the modulus of PP "monopolymer" blends is reported as a function of the content of the secondary material [5]. The blends are made of virgin extrusion grade PP (VPPE) with the same material extruded 1, 3, 5 times or with an injection molding grade PP (RI) molded 1 or 3 times. The modulus of the recycled RE sample is higher because of the higher crystallinity induced by the lower molecular weight [5,11], but does not increase monotonically with the number of extrusion steps because of the concomitant reduction of the molecular weight. The modulus values of the blends are in both systems higher than those of the virgin components. While for the VPPEIRI system a linear increase is observed, the values

376

of the modulus of the VPPEIRE blends increase at a low concentration of the extruded material and then flatten-out. A small maximum is also observed.

1200 -r--------------------,

E,MPa ---iJ- El

e E3

1000 e E5

800

RE(%)

600+-------~--------~------~------~ o 25 50 75 100

Figure 5. Modulus ofVPPEIRE blends as a function ofRE content Adapted, by permission, from Marrone M., La Mantia F.P. (1996) "Monopolymer blends of virgin and recycled polypropylene", Polymer Recycling, 2, 9-17. Copyright RAPRA

A more complicated behavior is shown in the blends made of RI and of the same virgin sample (VPPI), Fig. 7 [5]. In this case, the modulus of the recycled component is very similar to that of the virgin polymer. This behavior can be explained by considering first the decrease of molecular weight and, second that the increase of crystallinty is smaller for samples with low molecular weight [7]. The more important point is, however, the small maximum observed at intermediate compositions.

The elongation at break of the VPPEIRE and VPPIIRI blends is plotted in Figs. 8 and 9, respectively; as a function of the recycled component content. In the first case the elongation at break of the blends is intermediate and a drastic drop of the ductility of the blends occurs only at high content of the component extruded 5 times. On the contrary, the elongation at break versus composition curves of the VPPIIRI system show a minimum at low content of the recycled material.

The higher values of the modulus and the lower values of the elongation at break can be correlated with the change of crystalline morphology. The VPPIIRI blend with 25% ofRII shows spherulites smaller than those of the unblended polymers [5]. The first crystalline morphology stiffen the blend giving rise to materials having higher values of modulus and lower values of the elongation at break.

377

1000 --r--------------------~ E,MPa

900

800 0 11

~ 13

700

RI, % 600

0 25 50 75 100

Figure 6. Modulus ofVPPEIRI blends as a function ofR! content Adapted, by permission, from Marrone M., La Mantia F.P. (1996) Monopolymer blends of virgin and recycled polypropylene, Polymer Recycling, 2, 9-17. Copyright RAPRA

E,MPa 1400 -

1200

1000 -

800

o 25 50

o 11

~ I3

RI,%

75 100

Figure 7. Modulus ofVPPIIRI blends as a function ofR! content Adapted, by permission, from Marrone M., La Mantia F.P. (1996) "Monopolymer blends of virgin and recycled polypropylene", Polymer Recycling, 2, 9-17. Copyright RAPRA

378

1000~----------------------------~ EB,%

100 [J El

¢ E3

[> E5

RE,%

10+-------r-----~------~------~ o 25 50 75 100

Figure 8. Elongation at break ofVPPEIRE blends as a function ofRE content Adapted, by pennission, from Marrone M., La Mantia F.P. (1996) Monopolymer blends of virgin and recycled polypropylene, Polymer Recycling, 2, 9-17. Copyright RAPRA

The presence of deep minima, see Fig.1 0, taken from [2], in the elongation at break versus composition curves was already observed for PP "monopolymer" blends where the recycled component was degraded in a mixer (M) at different mixing speeds or in extruder (E). The elongation at break versus composition curves show a minimum which becomes more and more pronounced with increasing extent of degradation. For these blends a small amount of recycled component gives rise to fragile blends. It is worth noting that the recycled samples, although strongly degraded, show high values of elongation at break contrarly to that shown in Fig. 4.4.8. This point can be attributed to the presence of significant amount of oxygenated groups which hinder further crystallization promoted by the reduced molecular weight. It is well known, indeed, that for semicrystalline polymers brittleness increases with crystallinity.

Figure 9. Elongation at break ofVPPVRl blends as a function of the RI content Adapted, by permission, from Marrone M., La Mantia F.P. (1996) Monopolymer blends of virgin and recycled polypropylene, Polymer Recycling, 2, 9-17. Copyright RAPRA

l 10

10 o

CD w

• PP2/PP3M250 • PP2/PP2M150 1:. PP2/PP2Ml00 .... PP2/PP2M75 a PP2/PP2 E

50 100

Figure. 1 o. Elongation at break of virgin and degraded PP blends as a function of the virgin PP content Reprinted from Valenza A., La Mantia F.P. (1988) Recycling of polymer waste: part II - stress degraded polypropylene, Polym. Deg. Stab., 20, 63-73 with kind permission from Elsevier Science Ltd., The Boulvard, Langford Lane, Kidlington OXS I GB, UK.

380

The presence of the minimum has been correlated with the crystalline morphology of these blends [2]. The blend with the less degraded recycled component shows spherulites of about the same size throughout the entire sample. The blend with a more degraded component, PPIPPM250, shows islands of spherulites of different dimensions. Probably the high melting temperture crystals of the higher molecular weight sample act as nucleating agents for the low molecular weight chains. This morphology with weak interspherulitic boundaries induces a premature brittle fracture of these samples.

In conclusion, the mechanical properties and, in particular, the values of the elongation at break, for the PP "monopolymer" blends depend on the morphology of these samples; in tum, the morphology is dramatically modified by changing the structural parameters and by the presence of oxygenated groups, besides the effects of processing conditions. This behavior makes very difficult the prediction of the properties of the virgin/recycled PP blends and the preparation of brittle blends from ductile components is always possible. This fact is well known in the industry where usually only small amounts of recycled material to the virgin component are added. Very small minima have also been observed in the modulus versus composition curves of blends of recycled and virgin PET [12]. The same blends show, on the contrary, intermediate values for the other tensile properties.

The incorporation of degraded and recycled material into blends with virgin component can also worsen the photo-oxidative resistance of the virgin polymer. This occurs because degradation frequently leads to the formation of oxygenated groups, particularly sensible to UV radiation, hence inducing a rapid degradation of the polymer chains.

The carbonyl and hydroxyl indices, good parameters for evaluating the extent of photo-oxidation of polymers, have been measured, as a function of the irradiation time, for blends of virgin and recycled photo-oxidized polypropylene [13]. The blends show increasing values of these indices with increasing content of recycled polymer; the rate of formation of these oxygenated groups is also increasing with the concentration of degraded component. In particular, even after a long irradiation period, the content of carbonyl groups of the pure polymer does not reach the initial value of the blend with 50% of recycled polymer. This behavior suggests that "monopolymer" blends with large amount of photo-oxidized component can not be used outdoors for long times.

3. Polyvinylchloride

The blending of virgin and recycled resins can be remarkably influenced by the different size of the two materials. This problem is more important for the homopolymer blends of virgin and recycled PVC. Indeed, virgin PVC is in general in the form of micronized powder whereas the particle size of the recycled polymer depends on the post-consumer products to be recycled and can be in either powdered form or flakes or thick particles. These latter cases are usually encountered in the recycling of industrial scrap.

The properties of homopolymer blends of recycled and virgin PVC samples depend, as for other polymers, on the molecular weight of the two components. Moreover, in the case of PVC the presence of additives, plasticizers, stabilizers, impact modifiers, etc, that can also influence the properties of a "monopolymer" blend, in particular, when the formulation of the two components is different.

3.1 INFLUENCE OF THE PARTICLE SIZE OF THE RECYCLED RESIN

The properties of the PVC homopolymer blends have been investigated by considering size, content and formulation of the recycled component (RPVC) [4]. A virgin pipe

381

grade PVC powder, VPPVC, has been blended in a twin screw extruder with PVC recycled from pipes, RPPVC, and from bottles, RBPVC. The recycled PVC samples contain ony small amounts of CaC03 while RBPVC contains lubricants and impact modifiers. The recycled particles are in the form of thin flakes from bottles and thick particles from pipes. Because of their different shape, size and apparent density, powder and particles segregate during gravity hopper feeding in the extruder so that the composition of the resulting material changes with time.

Before investigating the effects of composition and formulation, the influence of the dimensions of the PVC recycled particles and the type of the blending apparatus has been considered. Flakes obtained from the granulation of bottles through 3, 5 and 9 mm screen have been blended with VPPVC powder in the ratio 80/100 and processed in the twin extruder. For comparison the same blend made with flakes of 9 mm maximum dimension has been prepared in a mixer. The elongation at break, EB, property strongly affected by the inhomogeneity of the material, has been considered to provide information about the best blending conditions and is reported in Table 4.1, [4].

Table 1 Influence of the maximum size ofRBPVC and type of the mixing apparatus on the elongation at break ofVPPVCIRBPVC (100/80) blend

Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Ma and La Mantia, F.P. J. Appl. Polym. Sci., 59, 759-767 © 1996 John

Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc

Maximum size and EB,% Average EB, %

mixing apparatus

9 mm, extruder 20-120 60

5 mm, extruder 40-110 70

3 mm, extruder 70-115 80

9 mm, mixer 75-115 85

The range of elongation at break values is quite large but undoubtedly is decreasing with decreasing size of the recycled particles. The scatter of the same data can be attributed to the different composition of the sample caused by the insufficient dry blending and inconsistent feeding ofthe extruder. The data reported in the same Table indicate that the elongation at break of samples prepared in the internal mixer are much more reproducible and the average value is higher than that obtained in the extruder. This better performance can be explained by considering that in the internal mixer the composition is more constant over the whole sample. However, some remarkable scatter still present can be explained by considering that these blends, although of the same chemical species, are incompatible two-phase materials containing inhomogeneities as a result of the different molecular weight and the presence of small amounts of additives.

3.2 PROCESSABILITY

The processability of the homopolymer blends depends, of course, on the rheological behaviour of the two components and their compatibility. The torque recorded at the end of mixing, when steady-state conditions are attained, is a good parameter to evaluate processability, see Fig. 11, [4]. In the investigated case, the processability of the pipe grade PVC decreases by adding recycled PVC. Both the recycled PVC

382

samples reduce significantly the values of the torque of VPPVC, RBPVC being particularly effective. It is also worth to mention that the processing temperature of blends with recycled PVC from bottles is lower than that of virgin pipe grade PVC allowing a significant energy saving. This effect can not be generalized and is due to the different viscosity of the components and the presence of lubricants and impact modifiers in the RBPVC.

3.3 MECHANICAL PROPERTIES

The mechanical properties of all blends presented below have been measured on samples with RPVC particles granulated through a 3 mm screen. Modulus, tensile stress, elongation at break and impact strength of VPPVCIRBPVC and VPPVCIRPPVC blends are shown in Figs.l2 - 15 as a function of the content of recycled PVC [4]. All the reported properties of the blends are between those of the two components and show an almost additive behavior.

It is interesting to note that impact strength and elongation at break of the VPPVCIRBPVC blends are remarkably better than those of the virgin materials for pipe, while the other properties are not significantly modified. This is due, of course, to the modifier agents present in the RBPVC. These modifier agents improve, then, some critical properties of the virgin pipe grade PVC without appreciably worsening other characteristics. Also, the blends with the two PVC pipe grade components show slightly better elongation at break and impact strength. The different composition of the two samples can explain this behavior.

26

25 o RPPVC

e Il. RBPVC

24 Z ~

~ = 23 0'" r..

0 ~

22

21 RPVC,%

20 0 10 20 30 40 50 60

Figure .11. Torque of PVC monopolymer blends as a function of the recycled component Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Maand La Mantia, F.P. J Appl. Polym. Sci., 59, 759-767 © 1996 John

Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

383

2T-------------------------------------, E,GPa [J RBPVC <> RPPVC

1.8

1.6

RPVC,%

1.4 -t----........ "'T"""--...,.....--""T"""--""T"""--~--....., o 10 20 30 40 50 60

Figure 12. Modulus of PVC monopolymer blends as a function of the recycled component Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Ma and LaMantia, F.P. J. Appl. Polym. Sci., 59, 759-767 © 1996 John

Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

45~----------------------------------~ TS,MPa

[J RBPVC RPPVC

40'-==~==~--__ 4-__ ~

35

RPVC,% 30~---...,.....---~----~----~----~~--~

o 10 20 30 40 50 60

Figure 13. Stress at break of PVC monopolymer blends as a function of the recycled component Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Ma and La Mantia, F.P. J. Appl. Polym. Sci., 59, 759-767 © 1996 John

Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc

384

80 EB,%

60 CJ

40

20

RPVC,% O+-----~----~----~~----T_----~----~ o 10 20 30 40 50 60

Figure 14. Elongation at break of PVC monopolymer blends as a function of the recycled component Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Ma and La Mantia, F.P. J. App/. Po/ym. Sci., 59, 759-767 © 1996 John

Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc

Figure 15. Impact strength of PVC monopolymer blends as a function of the recycled component Reprinted from "Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC" Wenguang, Ma and La Mantia, F.P. J. Appl. Po/ym. Sci., 59, 759-767 © 1996 John

Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

385

4. Conclusions

The blends of the same vrrgm and recycled polymer, (homopolymer or "monopolymer" blends), show rheological and mechanical properties in most cases between those of the two components athough are, in general, lower than those expected on the basis of additivity rules. In some cases, however, a minimum observed in some property versus composition curves is correlated with the change of the crystalline morphology induced by structural differences of the two components and due to different molecular weights and, in particular, the presence of oxygenated groups. Thus, in order to avoid a dramatic decrease of the properties small amounts of the recycled component should be used.

Acknowledgment

This work has been financially supported by CNR 96.02714.CT03.

References 1. Valenza A, La Mantia F.P., Polym. Deg. Stab., 19,135-145, (1987) 2. Valenza A, La Mantia F.P., Polym. Deg. Stab., 20, 63-73, (1988) 3. La Mantia F.P., Ma Wenguang, Polym. Networks & Blends, 5, 173-179, (1996) 4. Ma Wenguang, La Mantia F.P., J. Appl. Polym. Sci., 59,759-767, (1996) 5. Marrone M., La Mantia F.P., Polymer Recycling, 2, 9-17, (1996) 6. Abbas K.B., Knutsson AB., Berglund S.H., Chemtech, 8,502-508, (1978) 7. Throne J.L., Adv. Polym. Technol., 7, 347-360, (1987) 8. Bernardo C.A, Cunha AM., Oliveira, M.J., Polym. Eng. Sci., 36, 511-519,

(1996) 9. Bernardo C.A, Cunha AM., Oliveira M.J., Adv. Polym. Technol., 15, 215-221,

(1996) 10. Pattankul C., Selke S., Lai C., Miltz J., J. Appl. Polym. Sci., 43, 2147-2150,

(1991) 11. Marrone M., La Mantia F.P., Polymer Recycling, 2, 17-26, (1996) 12. Valenza A, La Mantia F.P, AJSE, 13,497-502, (1988)

Chapter.5 RECOVERY OF CHEMICALS AND ENERGY

PVC-RECYCLING WITH CHLORINE RECOVERY

Abstract.

GEORG MENGES Institut fur Kunstoffverabeitung RWTH-Aachen

Becouse of its inherent thermal instability the presence of PVC in plastics waste compli­cates reycling of that waste resulting in generation of coal - like residue leading often to breakdown of processing equipment. Various approaches to dealing with PVC containing waste have been considered. These include dehalogenation through degradative extru­sion, combustion in refuse incinerators with recovery of HCl and various gasification processes. Recovered hydrogen chloride can be utilized as a feedstock for manufacturing of PVC thus partly closing recycling loop.

1. Introduction

PVC has a special position in the recycling of plastics [1]. As a result of the relatively weak bonding of chlorine to the polymer chain, the chlorine atom is split off at low tem­peratures of about 150°C and hence complicates the material recycling of plastics wastes containing PVC. Corrosion, which could be initiated by the released chlorine or hydro­chloric acid produced from it is less of a problem than the coal like polymer residue, which remains suspended in the machine or is distributed in the melt and further in the product. Many times it produces breakdown of the equipment and necessitates cleaning of the screw, barrel and dies. Therefore as a rule, PVC must be carefully removed from mixed plastic waste before most types of recycling - material or chemical. Because the significant difference between the density of PVC and that of other thermoplastics sepa­ration with one of the well known methods is possible. It is, however relatively expen­sive. A more important reason is political. Most environmental campaigners consider that, as a large volume chlorine containing product, PVC should be banned. In many debates the following arguments are used:

• PVC is non recyclable • VCM, the monomer for PVC production is carcinogenic. • PVC has also been called the 'largest source of dioxins',

389

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 389-406. © 1998 Kluwer Academic Publishers.

390

.... ~ e..:. .s:: en ~ s:

~

~ e;.

:S CI .~

~

......... __ . __ ._ .. _ .. .... ... ... ... .. ..... ...... ...... ....... _._ .. _--" ..... _ .. .• _-- .... 19.0

100

50-

" " ............. __ .............. __ .... \ 50 % PE j. \ , -' 1

~ .... - WeI«ht _ .... Weigbi ....

, ;": :: "" .1 _ •

!~ i 1 i '; I

I

. ............ _ .l " I

'----.~yj

" .: ~.

~ "'\ ; ~ /}.\ ::

:' i ':.\ :\ . ,' ': \::

: , ~ : I :,' .l\::

• J \ :.\, ~/

I- 9.5

. ..... -" . .., .\~. :.::- . .. -.-O~~~-r--~~~~~--~~--~--~~--~ o

100

50-

o o

100

Temperature rC)

-_ ........ _ .. ..... . ..... ..... .... .. .... __ ._ .. .... _ ... . -.-. ...... . - .. ... :~ ... ....... ..... - --.- r- 15,0

_ ._ ..... _--_ ..... _-.,

~ ...... Wet&ht - -- Wei«hll,.

Theorfl.i<aI ... . Weight

. .. w~1oeI

\.

" "-" '-:-'

; ; ,I ,:

50 % PP H , ., :: .: .,

.1 ., "~)o.... ........ :':~

'" j! ~ i \~f ::

: -l. , . , \ . 1 ' , ', \ -I

./ -'.\ :i . , \\ :i

/ ,\;\ \ ..... / '\ ;

:>~~ ~/:I ", \...

f,:

c~~~.~::~~_ .. __

\00

Temperature rC)

~7,5

o

Figure 1. Non-isothermal TG and DTG curves of mixtures with 50% PVC [3] .

391

because it is considered to cause dioxin emissions in municipal waste incineration. It has been proven however, that PVC does not increase the PCDD levels in emissions from such plants, the same way the addition of PVC to the waste does not have signifi­cant influence on the PCDD concentration. So far, it has generally been fairly easy to rebut these attacks, but they did have positive effect. The PVC industry has developed and now operates is own material-recycling plants for waste from large volume, pure grade PVC products. So far these plants do not pay for themselves because the returned quantities of pure grade PVC waste are not large enough for an economically viable op­eration.

Some other plans for dealing with PVC waste which is highly soiled and mixed whith other materials are intensively studied. One such a study for an incineration plant with the special target to recover chlorine as a source for new PVC [1] monomer production has been completed. PVC is produced by radical polymerisation from vinyl chloride. Its structural formula is as follows:

xCH2 =CH I CJ

vinyl chloride

=> [ CH2 - CH ], (1)

I CI

polyvinyl chloride [PVC]

The polymer consists of 56% by weight of chlorine. The chlorine atom has a relatively weak bond to the polymer chain. At temperatures above 280°C plastic decomposes vir­tually spontaneously and the chlorine atom is liberated from the main chain. At tempera­tures of 350°C this process may take place in seconds - for example in suitable screw mixers. Fig. 1 shows a thermogram of a melt of PVC mixed with polyolefins. The spontaneous decomposition of PVC at 280°C is clearly evident from the sharp drop downwards in the upper curves. It produces a diene - type of a polymer which degrades at temperatures above 400°C to produce certain amount of carbon.

2. Recovery of Chlorine as Feedstock Recycling

2.1. DEHALOGENATION [4].

The large amount of mixed plastic which is produced by sorting packaging waste from household waste in Germany, for example, contains a significant amount of PVc. Con­sidering the problems of reprocessing, chemical recycling to produce a new raw material seems to be the correct approach. However, in all the larger pilot plants for chemical re­cycling used up to now, the permissible content of PVC or halogen-containing plastics is restricted to a chlorine content of < 0.5 %, partly for technical reasons (corrosion) and partly because of the licensing conditions. The packaging waste collected by Duale Sys­tem Deutschland (DSD) has to be sorted to ensure that the PVC content does not exceed this low amount, which has a significant effect on cost. At present this is only a problem

392

for the company which has to pay for the recycling, the DEUTSCHE GESELLSCHAFf FUR KUNSTSTOFFRECYCLING (DKR), a subsidiary of the DSD. A second, even more problematic type of waste which will have to be recycled in the future is the so called "light fraction of shredder waste" a product produced from the re­cycling of old cars, of which> 3000,000 tonnes are generated in Germany each year. Depending on the age of the cars, the content of PVC may be expected to be > 10 % by weight of the total plastics content. In this case, the costs of removing the PVC are more critical as there are no grants available. We need to find an economic method either for sorting out the halogen containing materials or a method for removing the halogens from the plastic waste in the least expensive way possible. A plan for this will be discussed later under degradative extrusion. Unlike the well researched subject of the stabilisation of PVC, only a few researchers have investigated the dehalogenation of plastic waste. The studies which have been per­formed [2] have come up with the following results:

1 . A satisfactory reaction rate (minutes) can only be achieved at melt tempera­tures of more than 380°C unless screw-mixers with special shear sec­tions are used.

2. Regardless of the initial chlorine content, residual contents of only 200 to 400 ppm are obtained and these cannot be further reduced even after long-term thermal treatment. In the presence of aromatic polymers the concentrations of only 800 to 1000 ppm are obtained.

3. About half of the residual chlorine is inorganically bonded. The reason for this is probably the presence of PVC stabilizers. The preferred stabilizers in PVC used in packaging applications are based on calcium, zinc and tin. All these metal compounds are optimized for reaction with HCI. The metal chlorides formed, SnCI2, CaCI2, or MgCl2, are thermally stable up to high temperatures and cannot be washed out [6].

2.2. THE REUSE OF THE RECOVERED CHLORINE BY POL YMERIZA nON OF PVC WITH OXYCHLORINATION.

Oxychlorination is a common procedure used for the production of the monomer of PVC - the vinyl chloride - from hydrogen chloride or hydrogen chloride solutions in water (hydrochloric acid). As the reaction diagram in Fig. 2 shows, hydrochloric acid and oxygen are used to convert ethylene into ethylene dichloride and then the PVC monomer - vinyl chloride - is produced by means of pyrolysis. At present, this procedure is used to use up waste hy­drogen chloride from various chemical processes. An important precondition is that the HCI must be as clean as possible.

393

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394

3. The Common Plants for the Chemical Recycling and the Disposal of Plastic Waste with the Recovery of Chlorine or its Compounds.

3.1. COMBUSTION IN REFUSE INCINERATION PLANTS AND RECOVERY FROM THE FLUE GASES.

All usual refuse incineration plants are equipped to cope with a certain amount of chlo­rine in the feedstock in a suitable dilution. In principle, therefore it should be possible to process PVC containing waste in refuse incineration plants, paying attention to the suit­ability of the dilution. However, this would mean the possible throughput of PVC would be so small that this method will have to be restricted to special waste. In refuse incin­eration plants, chlorine, which is always present as a proportion of household waste, even if it does not contain any PVC, and which is carried into the flue gases in the form of HCI, is washed out of the flue gases by water and neutralised. In the newer plants chlo­rine or hydrochloride in the flue gases is neutralised by adding lime to the oven or spraying in the flue gases and then the combination is filtered out. The chloride or hydro­chloric acid solutions obtained in this way are usually disposed of. The first hydrochloric acid recovery plant in the world in a refuse incineration plant started operation in the Hamburg BorsigstraBe incineration plant in 1993 [7]. This plant produces high-quality hydrochloric acid from the washed out raw hydrochloric acid. The process is described by M. Schaub from SULZER CHEMTECH AG [8] . Hydrochloric acid which meets the appropriate standards commands considerable prices. Fig. 3 is a block diagram of the method for incorporating the HCI recovery plant in this refuse incineration plant. Fig. 4 shows the plant for processing the raw acid to produce a high-quality hydro-chloric acid based on the conversion of calcium chloride. It consists of the following main operations:

• rectification: HCI distilled out using a CaCl2 solution • fluorine elimination: (not yet installed in Hamburg) • absorption: production of the acid from the HCI gas issued from the distil­

lation area • evaporation: re-concentration of the dilute CaCl2 solution produced by

the recti ficati on • filtration: precipitation of the by products to be discarded

The plant in Hamburg has been in operation for two years and has completely fulfilled all expectations. It generates 2.5 t/h of hydrochloric acid with a purity of >99.9% and a con­centration of > 30 %, which satisfies all standard requirements.

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3.2 SPECIAL WASTE INCINERA nON PLANT FOR PVC WASTE WITH RECOV­ERy OF HCL FROM THE FLUE GASES.

Status of the procedure. It was not due to an acute necessity, as at present the amount of PVC waste returned is insufficient, but solely due to the public pressure with regard to PVC over recent years, that a theoretical study for the recovery of chlorine from waste PVC by incinerating the waste in a rotary tube furnace was undertaken. The study was funded by an association of PVC raw material producers in Germany (AGPU) and performed by reputable plant suppliers [2]. The study involves an incineration plant with downstream chlorine recov­ery from the flue gas as shown in Figure 5. Originally, the plant was designed to process 240 000 tonnes a year - this was subsequently reduced due to the inadequate amount of returned PVC waste. It is planned now to install a smaller version equipped as a special waste incineration plant to be constructed in BUNA at Merseburg (East Germany) which would be able to process highly concentrated PVC waste and recover the chlorine. The calculations revealed that a minimum volume of 80 000 tonnes a year would be required if the operation was to be at all economically viable. The plant would, however, require an investment of about 200 million DM which, together with the operating costs, would have to be reflected in the price of the products. It is estimated that these costs alone would come to 500 DM/t as a gate fee. The other disadvantage is that only this low ca­pacity can be met - because of the current very low amounts of waste PVC, which will be probably even smaller in the future. Extremely long transportation distances to this central plant from all over Europe would increase the costs even further. For these rea­sons, the raw materials manufacturers represented by association (AGPU) which are mainly from Germany and its western neighbours could not yet reach a decision on the construction of the plant. The complicating factor is the fact that, without a common European legislation, it is not possible to force all European suppliers to make a propor­tional contribution. Nevertheless it seems to be worthwhile to describe this process. As Fig. 5. ) shows, the plant consists of the following:

• rotary tube incineration plant • heat boiler and flue gas precipitation and • HCI processing • turbine house • water purification

The shredded PVC waste (20 - 30 mm particle size) is burnt continuously together with brown coal dust, sand and calcium-chloride solution. Brown coal dust and sand improve the burn-out and the recovery of chlorine; sand also bonds with heavy metals etc. in the slag. The third additive is a 30% calcium chloride solution, recirculated from the waste water unit. Under incineration conditions calcium chloride is converted to calcium sili­cate and HC!.

398

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By this measure the yield of HCI is improved significantly. The incineration takes place at a process temperature of 1200°C. The residence time is less than 50 minutes. The molten slag, mainly consisting of CaSiOJ, Si02, and Ti02 is discharged through a water sealed gap between kiln and after-burning chamber. Slag is solidified in a water bath. After extraction of water soluble impurities it is a neutral, leaching-resisting mate­rial, which can be used in construction. Flue gas of rotary kiln enters the after-burning chamber, where the combustion is com­pleted. Next station of the flue gas is a boiler, where feed water is converted into super­heated 40 bar steam. The 360°C hot steam is used for generation of electricity in the neighbouring power station. The flue gas leaving the boiler has a temperature of 250° and is led through an electro­static precipitator. Separation of HCI from flue gas is carried out in a 3-stage quench/absorption operation. The flue gases are then sent to a flue gas precipitator of a standard design. 20 % hydrochloric acid produced is mixed with a concentrated calcium chloride solution and then subjected to azeotropic distillation in a distillation column. 95% of the HCI leaves the column at the top while a dilute calcium chloride solution collects in the bot­tom - this is reconcentrated by evaporation and remains in the circuit. Fig. 5.1.5 shows how this type of plant for the recovery of chlorine from flue gases would look like.

3.3. DEGRADATIVE EXTRUSION AS A PROCEDURE FOR THE SIMULTANE­OUS PROCESSING, DEHALOGENA TION AND LIQUFICA TION OF MIXTURES OF DIFFERENT THERMOPLASTICS [10] .

This procedure is in the direct competition to the incineration plant for the PVC recovery. The process can recycle mixed plastics with a high PVC content, like that produced from building waste. LINDE-KCA DRESDEN GmbH (a subsidiary of LINDE AG) offers a plant of this type with throughput rates of up to 8 tIh. The plant may also be operated efficiently as a stand-alone plant for small quantities of waste. It recovers the chlorine in the form of aqueous HCI directly during depolymerisation.

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it is sheared and heated to approximately 400°C within a few seconds to degrade the polymers to waxes and separate chlorine and other halogen atoms from hydrocarbons. In a subsequent degassing zone, the halogens are siphoned off, together with any other volatile materials. The hydrogen halides are easy to separate, for example by feeding the stream of the gases into a water bath. The hydroen chloride is diluted in the water bath and hence may be recovered as a monomer for the production of new vinyl chloride. The rest of the melt is of low viscosity polluted only with small amounts of halogenated plas­tics « 0.2% CI ); it solidifies at about 100° into a waxy product. If the halogen content in the feed product is very high (e.g. > 80 % PVC in the feed­stock), only a type of coke is left in the screw. It sticks to the screw and barrel walls but it is immediately scraped oft the walls of the screw machine by the closely intermeshing screws and discharged as a free-flowing fine powder. Experiments have found that other types of screw machine are not suitable as the residue is baked solid and sticks to walls so that, single-screw machines, for example, become clogged up in a relatively short time. Co-rotating twin screw compounders have been found to be suitable for all mixtures of plastics, including pure PVC, because the closely intermeshing screws prevent the resi­due from sticking to the wall by scraping them oft all screw and cylinder surfaces the minute they are deposited, pulverising them and incorporating them in the melt, if there is any left, or discharging them as a dry powder in the cases of very high content of pure PVC. The discharge, which is free of dioxins, may be disposed for use as a fuel in brown coal de gasifiers or refuse incineration plants. As this plant is not yet used on an industrial scale, the costs have to be estimated on the basis of the extensive studies performed [10] but they are probably less than 300 DM/t. As these types of plants are able simultaneously and fully automatically without further sorting to convert mixed, contaminated plastic waste into a manageable, sterile form (granules, powder, liquid) to separate off and distill the halogens to a residual content of < 0.2% in a single operation, the prospects of their being used for waste plastic with a high chlorine content are very good. Only inorganic, bonded, non-volatile chlorine com­pounds remain in the products. This procedure exploits the fact that the halogen atoms have a relatively weak bond with the polymer's main chains so that at temperatures of over 270° they separate spontaneously from the hydrocarbon chain in the form of hydro­gen halide. At melt temperatures of 350 to 400°C and high shear forces, the bonds are so weak that spontaneous dehalogenation to a dechlorination level of <0.2 % takes place within seconds, as soon as the ambient pressure is achieved.

3.4. GASIFICATION PROCEDURE.

Among the large number of gasification procedures only a few have been found to be suitable for the conversion of plastic waste and only one is being considered for use in the recovery of chlorine from PVC waste.

402

3.5. MOLTEN METAL INC.'S catalytic extraction processing (CEP) [11]

The CATALYTIC EXTRACTION PROCESSING (CEP) by MOLTEN METAL Inc. has been the subject of intensive discussion with regard to its particular suitability for proc­essing PVC with chlorine recovery by AGPU in direct competition to the offer of STE­INMULLER a/o. at BUNA. These plants which use the heat content and the catalytic capabilities of molten metal for the decomposition of organic waste are specially adapted for a wide variety of special waste. Fig. 7 shows a plant which is able to gasify any kind of chlorinecontaining waste and is specially designed to wash the hydrochloric acid out of the otherwise clean gases to recover it as a feedstock for the polymerisation of new Pvc.

3.5.1 Status of the procedure The company has an experimental reactor with a throughput rate of 2 tIh which has al­ready been subjected to experiments using 100% PVC waste. The chlorine-containing waste is added to the molten metal bath either in shredded form or as a gas, which is pro­duced from the waste in an upstream carbonisation oven, together with an exact stoi­chiometric quantity of oxygen. The oxygen reacts with the carbon to form carbon mon­oxide which also heats the bath. This also prevents carbon dioxide from entering the out­going gas. Lead, zinc, tin, cadmium and mercury are volatile in a steel bath. The outgo­ing gas mixture is then separated from the outgoing HCl gas in a cleaning plant and the metal is precipitated by condensation and recovered. The residue is used as a fuel gas for energy production. According to MOL TEN METAL Inc., the costs for the treatment of this kind of waste should be about half those of special wasteincineration, which is partially due to the re­covery of relatively clean elements [4]. Although the two commercial plants already in operation in the USA are planned for use with other types of industrial waste, on-going discussions are being held in BUNA concerning the construction of a plant of this type for the recovery of chlorine or HCl from waste PVC as a feedstock for new PVC.

4. Comparison of Suitable Procedures

Only a few of the plants currently in use for the disposal or recycling of chemical mate­rials from old plastics are designed for the recovery of chlorine from the recycling or combustion of PVC.

4.1 MODERN INCINERATION PLANTS FOR COMMUNITY REFUSE.

Modem incineration plants for community refuse such as those in Hamburg-BorsigstraBe would be particularly suitable.

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for political reasons combustion is not yet a feasible solution in Germany at present. This means the only appropriate methods are speCIally constructed systems, such as:

4.2. SPECIAL WAS'f'E INCINERATION PLANT BASED ON A ROTARY TUBE (FOR EXAMPLE PLANT TO BE BUILD FOR BUNA BY STEINMULLER)

Advantages:

Disadvantages:

The plant is a multi - purpose plant; it may be used for all types of waste

For economically viable operations, the plant must be operated con­tinuously. The plant should be able to process> 80,000 tonnes of any kind of waste a year to achieve an acceptable disposal price which would give handling costs of approximately 500 DM/t. From the point of view of the operating costs, a plant size of 240,000 tonnes a year, like the one originally planned, would be favourable; this would reduce the handling costs to approximately 350 DM/t.

4.3. CATALYTIC EXTRACTION PROCESSING (CEP) PLANT (MOL TEN METAL, INC.) Advantages:

Disadvantages:

The plant is specially designed for PVC waste and the like and is there­fore smaller, so continuous operation is not necessary. The cost of the gas cleaning is lower, as only CO, water and HCI are produced in addi­tion to the metal vapours. The ashes are mixed with the slag, the carbon is burnt. The manufacturers state that the handling costs are about half those of an incineration plant.

As this is a special plant which requires a considerable investment and the HCI recovery is an integral component, this type of plant would have to be constructed centrally for use by the whole of Germany and this would give use to high transport costs and logistical expenditure.

405

4.4. PLANT FOR DEGRADATIVE EXTRUSION (LINDE AG)

Advantages:

Disadvantages:

The plant utilises commercial equipment which is sold with throughput rates of a several 100 kilogram/h up to 35 tIh. The processing of the HCl solution does not have to be integrated in the plant. In addition, it may be operated intermittently. These plants may be decentralised in vinyl chloride plants. This would be of benefit with regard to availabil­ity, utilisation of the plant and the handling costs. In addition the pro­ducers of vinyl chloride, who are generally the same as PVC producers, usually already have plants for the conversion of HCl into vinyl chlo­ride.

The carbon powder formed from the residue which is discharged from the extruder has to be disposed of separately; for example in a refuse incineration plant or fixed bed gasification plant.

In view of the strong public pressure, it should be assumed that at least in Germany a plant for the recovery of the chlorine will come into operation, because it is thought that the use of a chlorine circuit will improve the image of the PVC production industry.

References 1. G. Menges, PVC Recycling Management, Chapter 8 in White Book on Chlorine, G. J. Martens editor, International Union of Pure and Applied Chemistry (IUAPAC) 1997 2. W. Frey, Hel recycling from PVC, AGPU e.V. conference, AGPU e.V. Pleimestr. 3, 531289 Bonn, 1993 3. R. Knumann, H. Bockhorn, Investigation of the kinetics of pyrolysis of pvc by TGMS analysis, Combustion Science and Technology, 1992 4. J. Brandrup, Requirementsfor plastic waste preparation offeedstockfor petrochemical recycling, in The recycling of Plastics, Brandrup, Bittner, Michaeli, Menges editors, Chapter 5.1, C. Hanser Verlag Munich, Vienna, 1995 5. G. Menges, V. Lackner, Degradative extrusion of plastics, in the Recycling of Plas­tics, Brandrup, Bittner, Michaeli, Menges editors, Chapter 6.2.1. C. Hanser Verlag Mu­nich, Vienna, 1995 6. Ph. Brunner, 1. Zobrist, Mull Abfall15 (1983) p. 221 7. H. Hantelmann, D. Slowieja, Industrial hydrochloric acid from refuse incineration, Sulzer Technical Review 1/95 pp 22,23, 1995 8. M. Schaub, Recovery of hydrochloric acid from thermal processing of waste PVC, R'95 conference, Geneva 1995, Sulzer Chemtech AG. PO Box 65, CH-8404 Winther­thur. 9. P. Hornig, Feedstock recycling of pvc in recovery of HCI by incineration, paper 7-

406

1, Recycle '94 Global Forum and Exposition, March 14-18 1994, Davos, Switzerland 10. Menges, G. Lackner, V., Degradative extrusion of plastics, in the Recycling of Plas­tics, Brandrup, Bittner, Michaeli, Menges editors, Chapter 6.2.1. C. Hanser Verlag Mu­nich, Vienna, 1995 11. G. Carr, Molten metal technology, APClAPME Technology Exchange on Advanced Recycling, May 23-26, 1994 12. I. C. Yates Molten metal gears LIP for first commercial applications, Chemical

Technology Europe, pp. 10,11, September I October 1995

THERMOLYTIC PROCESSES

Abstract

M.XANTHOS Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA and Polymer Processing Institute, Castle Point, Hoboken, NJ 07030, USA

J. LEIDNER ORTECH Corp. 2395 Speakman Dr., Mississauga, Ont., Canada L5K IB3

Thermolysis as applied to plastics wastes includes refinery and pyrolytic processes that produce monomers, fuels or gases. Thermolysis is not combustion but involves rather a complex set of reactions that depends both on the plastics involved and the precise nature of the thermolytic process used. Important classes of thermolytic processes include high temperature pyrolysis to monomers, lower temperature refinery processes to produce naphtha-like materials and thermal oxidation /gasification. Such processes are, in principle, applicable to certain single contaminated plastics or mixtures with high selectivity to monomers, or certain significantly contaminated commingled waste streams. Following a presentation of fundamentals on polymer thermal degradation and pyrolysis, this section examines existing and developmental technologies in high and low temperature pyrolysis, refinery processes (steam cracking, coking, hydrocracking, catalytic cracking) and gasification/thermal oxidation. Details on selected processes are provided.

1. Introduction

Tertiary or quaternary recycling, (recovery of chemicals or energy), should only be considered when other types of recycling are not economically or technologically feasible. In tertiary recycling, waste plastics are converted to either monomers or fuels or petrochemical feedstocks. Conversion to monomers by solvolytic methods is feasible for condensation polymers but often requires pure polymer streams. Sorting and cleaning of the waste stream increases the cost of the process.

Thermal decomposition of polymers to produce petrochemical feedstock is often considered for addition polymers such as PE, PP, PS and PVc. These processes may

407

G. Akovali et aI. (eds.), Frontiers in the Science and Technology of Polymer Rec.vc/ing, 407-423. © 1998 Kluwer Academic Publishers.

408

accept mixed and contaminated waste streams without or with minimal pre-cleaning and the products are useful as monomers and liquid or gaseous fuels, (Fig I ).

Thermal recycling of plastics waste can be carried out by pyrolysis or various refinery processes. Specific examples of thermal recycling processes include: 1. High Temperature Pyrolysis (Back to Monomer, BTM): Products are mixtures of monomers (ethylene, propylene, but also aromatics such as benzene, toluene); products are equivalent to those obtained from a naphtha cracker and after separation can be used for polymerization. 2. Low temperature «60(/C) Pyrolysis (Back-to-Feedstock, BTF) Products are waxes, (C 20-30 hydrocarbons), to be fed in a naphtha cracker; product composition depends on the use of steam or hydrogen instead of steam. 3. Gasification (thermal oxidation) Products are CO, CO2, H , H 0, HCI (the latter to be removed); product mixture may be

2 2

used to make methanol or converted to higher liquid hydrocarbons by the Fischer-Tropsch process.

Suitable waste plastics feedstocks are single contaminated polymers with high selectivity to monomers or commingled waste plastics. Some mixed polymer waste streams and their estimated volume, (US 1993 statistics), amenable to recovery/recycling by thermolytic processes are shown in Table 1, [1].

TABLE I • Polymer Streams Available for Recovery/Recycling in million tonnes [l]

Year l22Q 1995 2000 2010

Automotive Shredder Residue 0.81 0.95 1.04 1.36

Carpets 0.95 1.04 1.14 1.36 Wire and Cable Covers 0.18 0.22 0.27 0.36 Tailings from municipal solid 0.09 2.30 2.70 3.60 waste Total 2.03 4.51 5.15 6.68

A summary of the current status of eXlstmg and developmental thermal recycling technologies along with information on targeted feedstocks can be found in Reference [2].

2. Pyrolysis - General

Pyrolytic processes involve the thermal degradation of polymeric substances during which oxygen is completely or largely excluded. The products may be gases, liquids, and solid residues, chars, and inorganic fillers. Materials, (monomers and other organic chemicals), fuels (liquids and gases), or both materials and fuels may be

409

Plastic

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410

recovered. Although thermolytic processes can be carried out specifically for waste plastics they tend to be more practical when a feedstock is a general refuse of which plastic is only one, (usually minor), component.

Pyrolysis is not combustion but involves rather a complex set of reactions that depend both on the plastics involved and the precise nature of the thermolytic process used. Possible reaction pathways are:

Decomposition into monomers, e.g. PMMA and PTFE; Fragmentation of the principle chains into organic moieties of variable size, e.g. PE and PP; Simultaneous decomposition and fragmentation, e.g. PS and PIB; Elimination of simple inorganic moieties leaving charred residues, e.g. PVC; Elimination of side chains, followed by crosslinking.

In general, higher temperatures favor the production of gaseous products and lower temperatures that of liquid products. The course of the pathways can be modified by addition of controlled quantities of hydrogen or oxygen, the presence of catalysts and additives (stabilizers, plasticizers, and pigments), or the particle size of feedstock. The process is usually endothermic; some of the required heat may be supplied by combustion of the thermolysis products.

Pyrolysis can be carried out in a variety of reactor systems. Types of reactors, both developmental and in use, are summarized in Table 2 . [3].

3. Fundamentals of Polymer Thermal Degradation

Thermal degradation of polymers can take place through: a) chain scission where the breakage of the backbone yields free radical segments, and b) non-chain scission reactions involving elimination of a small molecule and double bond formation.

Examples of chain scission reactions, [4], include: - random homolytic cleavage to a complex mixture of low MW degradation products. PE and PP degrade in this manner. In addition to random cleavage, other intra and intermolecular reactions and secondary reactions in the gas phase may occur. The type of PE (LD, LLD, HD), its MW, temperature, interaction with other polymers or metals dictate the reaction products that may include alkanes, olefins, diene fragments, etc. - depolymerization where monomers units are released at an active chain end. Whether a polymer will thermally depolymerize to yield monomer depends to a large extent on its ceiling temperature, Tc, (the temperature at which the rates of propagation and depolymerization are equal). Polymers with relatively low Tc that can easily depolymerize are poly(a-methyl styrene) - 61°C, PMMA - 220°C, PS - 310°C. In commercial feedstock recycling operations, PMMA is almost quantitatively depolymerized to monomer at temperatures above 300°C. For PS, in addition to depolymerization some random scission to benzene, toluene, etc. can take place.

411

Dehydrohalogenation which results from the breakage of a carbon-halogen bond and the subsequent liberation of HCl, as in PVC, is an example of a non-chain scission reaction. PVC dehydrochlorination leads to the formation of polyenes, cyclic structures

TABLE 2. Pvrolvtic Reactor Systems [3]

Reactor System Investigators

Conveyors Wayne Technology

Extruders APV Union Carbide Japan Steel Works Voest-Alpine

Fluidized Beds Japan Gasoline Co. Nippon Zeon Occidental Sumitomo Machinery Toyo Engineering Tsukishima Ebara University of Hamburg University of Waterloo West Virginia Univ.

Molten Salt Reactors Ruhrchemie Univ. of Tennessee

Rotating Drum! Monsanto Landgard Furnaces Kobe Steel

Fischer Menges Sanyo Electrical Veba Oel Technologie Dr. Otto Noell

Stirred Tank Mitsubishi Heavy Industries Reactors Mitsui Engineering

KWU

Verticle Retorts Andco-Torrax Firestone Union Carbide

412

POSSIBLE MECHANISM FOR PE DEGRADATION

R-CHz- CHz- CHz- CHz-R ~ R- CHz- CHz- CHz- CHz- + R­Initiation

R- CHz- CHz- CHz- CHz- ~ R- CHz- CHz- + CHz = CHz ~- scission - Propagation

R- CHz- CHz- CHz- CHz- ~ C~- CHz-CH= CHz + R­Random Propagation

R- +R- ~ R-R Termination

MECHANISM OF THERMAL DECOMPOSITION OF PS

R-M-M-M-M-R ~ 2R-M-M­Initiation

R-M-M- ~ R-M- + M R-M- ~ R- + M

Propagation 2R- ~ R-R Termination

HC) ELIMINATION REACTION OF PVC

R-CHC) - CHz- CHCI- CHz-R ~ R- CH- CHz- CHCI- CHz-R + CI­Initiation

R-CHCI - CHz- CHCI- CHz-R + CI- ~ R-CHCI - CH- CHCI- CHz-R + HCI

R-CHCI - CH- CHCI- CHz-R ~ R-CH=CH-CHCI- CHz-R + CI-

R-CH=CH-CHCI- CHz-R + CI- ~ R-CH=CH-CHCI- CH-R + HCI Propagation

Figure J. Thermal Degradation Mechanisms [5]

413

and aromatic compounds. Possible mechanisms of thermal degradation reactions in the absence of oxygen for PE, PS and PVC are shown in Fig.,) , [5].

In all the above thermal degradation reactions, type of atmosphere and presence of accelerators or contaminants will affect kinetics and products. For example, metals are known to accelerate the depolymerization of polyolefins as well as that of PTFE and POM to their respective monomers. The presence of oxygen has a significant effect on the polyolefin degradation mechanism resulting in the formation of hydroperoxides and accelerated cleavage of the polymer chains, [6]. Molecular weight of PVC may increase through crosslinking reactions by heating at 190°C in the presence of nitrogen or may decrease by heating in the presence of air, [7].

4. High Temperature Pyrolysis Technologies

High temperature pyrolysis or back to monomers processes produce a mixture of monomers such as ethene and propene but can also be used to produce aromatic compounds like benzene and toluene. The products can be used after separation and purification to produce new polymers. Developed since the 1970' s three different processes have been established: the Kaminsky or Hamburg University process, the Batelle process and the Union Carbide process [5,8].

The Hamburg University process is a low temperature fluidized bed pyrolysis system meant for pyrolysis of plastics waste. The original objective of the process was the production from plastics waste of a gas with high heating value and liquid hydrocarbons containing mainly benzene, toluene and xylene. Production of olefins from plastics waste with high content of polyolefins and use of these olefins as feedstock in the refinery processes was also considered. The schematic of the process is shown in

Fig 3. The sand is used as fluidized medium and steam preheated to 400 to 5000 C acts as a fluidizing gas. The fluid bed reactor is externally heated by propane burners. The gas/steam mixture passes through a multi - step cooling / separation process. First, solids such as soot and dust are precipitated in a cyclone. Next, water is condensed in a

water cooling system. The liquid products are removed by chilling to -10 to _200 C. Another cyclone and electrostatic precipitator remove hydrocarbon droplets from the pyrolysis gas, [9,10]. Examples of experimental conditions and yields are shown in Table 3 .. '.

Experiments show that the ratio of the gas and liquid can be controlled by the

pyrolysis temperature. The highest amount of gas was generated at 7000 C; both higher and lower pyrolysis temperatures resulted in lower generation of gases. The composition of the pyrolysis products also depends on the choice of a fluidizing gas - pyrolysis gas or steam. Use of steam tends to increase the amount of olefins and reduce the aromatics [9].

The process employs a simple reactor extensively tested over 20 years of development with a variety of waste polymers and polymer mixtures including PE and PP, PS, PVC, PMMA, rubbers, tires etc. [8]. Overall economics, however, appear

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unattractive, particularly as related to problems with removal of HCl. Among other pyrolytic processes, the Batelle process employs a Circulating Fluidized Bed as main reactor. Pilot plant scale experiments have been conducted with PE, PS, PVC and various polymer mixtures with nitrogen or steam as fluidization media.

TABLE 3. The Effect of the Choice of Fluidizing Gas on Pyrolysis Products [9]

Temperature (oC ) 700 700

Starting material LLDPE HDPE Throughput (kg/hr) 1.4 1. 1 Fluidizing gas Pyrolysis gas steam Gases (wt%) 57 76 Aliphatics (wt %) 1.5 3.7 BTX aromatics 24 11 Other aromatics 12.5 4.8 Total oil (wt % ) 38 20 Distillation residue 3.9 2.0 (wt %)

Solids (wt %) 1.0 1.6 (Repnnted from W. Kaminsky, B. Schlesselmann, C. SImon,

"Olefins from polyolefins and mixed plastics by pyrolysis", Journal of Analytical and Applied Pyrolysis 32 (1995) pp. 19-27, with kind permission of Elsevier Science - NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Nederlands)

Types of products (hydrogen, methane, ethylene, propylene, butene), and yield depend very much on the operating temperature (680 to 770°C). The Union Carbide process employing a Tubular Reactor has not been practiced; one disadvantage is that carrier gas needs to be preheated to more than 1000°C.

Recent work at the University of Twente, Netherlands [5] focused on the design of a Rotating Cone Reactor to meet BTM process requirements such as: short solid and gas phase residence time, high heat transfer, no carrier gas, economic scale-up, uniform reactor temperature. Problems still not solved include: removal of HCI, requirements for very small particle size of feedstock, removal of heavy metals and additives.

5. Recycling of Plastics Waste Through Refinery Processes

In low temperature pyrolytic processes, high boiling point liquids or waxes are produced that may serve as potential feedstocks for refinery operations (steam cracking, hydrocracking, coking, fluid catalytic cracking). Use of waste plastics as a feedstock to refineries offers an interesting recycling option - existing facilities are utilized, although some waste preparation and removal of non-plastic impurities might be required. These

416

facilities are located in various locations and the waste can be easily delivered to them. In addition, either fuels or petrochemicals can be produced, [11]. Details on low­temperature pyrolysis technologies for feedstock preparation can be found in references [12,13].

The yields of specific products from any pyrolytic process that uses plastic wastes as a feed depend on a variety of factors: residence time, temperature, particle size of waste feed and atmosphere (oxygen, air, oxygen-free, steam). Depending on the combination of conditions chosen, varying amounts of gas, liquid, and solids can be produced from a feed. The gaseous and liquid fuels, and their quality will be significantly influenced by the impurities in the feed.

The two major process concepts in refining are carbon rejection (also known as coking) and addition of hydrogen ( hydroprocessing ).

Coking is a refining process in which a heavy crude is heated to around 450°C under low positive pressure resulting in viscosity reduction of the crude and deposition of a coke. Light fraction is removed for distillation and heavy fraction is mixed with feed and returned to the coker for further processing. There is a number of versions of a coking process: - delayed coking; cracking takes place in heated drums. Two drums are used, while one of them is operative the other is being cleaned from the residual coke. - fluid coking; uses fluidized bed, liquid material is sprayed into fluidized bed of hot coke particles. Two vessels are used - reactor and burner and the coke particles are circulated between the two. Heat generated by partial incineration of coke is used for cracking operation. The fluid coking process produces less coke and more of the liquid products than delayed coking. - flexicoking is similar to fluid coking but the coke is gasified producing surplus heating gas. Fiexicoking is used with heavier feedstocks which produce more coke. - catalytic cracking; the process is similar to delayed or fluid coking but utilizes catalysts such as zeolites [14].

In order to process plastics in the coking operation plastics waste has to be shredded and dispersed in a feed material. Up to 10% of plastics waste in the feedstock can be usually tolerated, 20 % might be possible as well [15].

Hydroprocessing includes processes in which petroleum feedstock is reacted with hydrogen resulting in lower boiling products. The reaction takes place at high pressures and temperature over 350°C. Hydrogenation processes tend to produce higher levels of liquid products than coking processes but with significantly higher costs and potential operating difficulties. Whether these can be justified depends on the value assigned to liquid fuels.

Cracking of plastic wastes to gasoline and fuel oil in fluid catalytic cracker (FCC) should be more attractive than other pyrolysis processes except when the pyrolysis process is highly selective to high valued monomers, [3]. Most likely the presence of fillers in the waste can be compensated for by catalyst additions. Studies have shown that, in an FCC unit, the following significant product yields are obtained: Polystyrene Aromatic Naphtha; Polypropylene Aliphatic Naphtha & Distillate; Polyethylene LPG and Aliphatic Naphtha.

417

6. Thermal Oxidation/Gasification

Gasification is the partial oxidation of hydrocarbons in a restricted supply of oxygen at temperatures up to 1600°C and pressures up to 150 bar. Products are CO, CO2,

H , H 0, HCI (the latter to be removed). Partial oxidation as a gasification process per se 2 2

or as part of refinery processes has been considered in a variety of R&D programs [3]. For example:

Shell International proposed a chemical recycling scheme for MSW consisting of a feed preparation unit and the Shell downstream gasification process. According to publications from Argonne National Laboratory, gasification could be used to convert auto shredder residue (ASR) into low calorific value gas (-3.7x106 J/m3) containing CO, H2, and light hydrocarbons.

Research in Germany suggests that degradative extrusion of mixed waste poly olefin plastics in the presence of oxygen can be used as the first step for the production of suitable feedstock for hydrogenation or gasification and production of synthesis gas.

Details on the principles and chemistry of gasification as well as recent industrial applications for treatment of plastics wastes in Europe can be found in [16]. The following section gives details on some of the earlier work in this area.

6.1. PUROX SYSTEM

Union Carbide developed a pyrolysis reactor schematically shown in Fig. 4 The reactor uses pure oxygen rather than air and hence the Purox name. Before being fed into the reactor the waste is shredded and magnetic separator removes metals. The waste is fed at the top of the reactor. As it descends it is first dried and then pyrolysed by raising hot gases. Gas produced by the process is cleaned in two steps :

removal of suspended oils by electrostatic precipitators removal of water vapor by condensation.

Oil is moved back to the pyrolysis unit. The incineration temperature of 17000 C is adequate to melt and sterilize molten residue.

1 tonne of refuse typically requires 0.2 tonnes of oxygen and produces 0.7 tonnes of fuel gas. The volume of solid residue is under 3% of the volume of the initial waste. The main features of the process are :

-The gas can be used either as a fuel or for production of chemicals such as methanol. Because of the use of pure oxygen the fuel gas is not diluted with

6 3 nitrogen and, therefore, has a fairly high calorific value of 12xlO Jist m . The

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419

-The solid residue has a very low volume and is sterile reducing disposal cost. -There is no effluent to the atmosphere and the produced gas is low in sulfur and ash. -The condensed water contains organics and has to be further treated, [17-19].

6.2. ANDCO - TORRAX PYROLYSIS SYSTEM

Figure 5. shows a schematic of the Andco-Torrax waste pyrolysis system. The waste is not shredded or in any way pre-treated. The waste is fed into the top of a vertical reactor. The seal is formed by the compaction of the waste preventing escape of the gases. Preheated air is fed to the combustion zone. Just like in the Purox system descending waste is dried and pyrolysed by rising gases. The slag is melted, removed and quenched. Some of the oil droplets formed by the pyrolysis process are scrubbed by the descending waste and returned to the combustion zone. The main difference between Andco - Torrax and Purox system is in the condition of the produced gas. The gases

leaving Andco - Torrax reactor are very hot (400 to 5500 C ) but have much lower

heating value of around 6x106 Jist m3. Because of its low calorific value and high temperature that gas can be economically used to produce hot water or steam on - site only - some supplemental fuel might be required [19-21].

6.3. CIRCULATING FLUIDIZED BED PYROLYSIS SYSTEM.

A fluidized bed system producing gases with characteristics of those produced by the Purox system but not requiring oxygen feed has been developed in Japan (Fig. 6.) Two fluidized beds are used - pyrolysis and regenerator bed. Solid waste is fed into pyrolysis unit where it is fluidized using super-heated steam. Sand is used as fluidizing medium and as a heat transfer medium. Gaseous product of pyrolysis is removed from the reactor, some of it is used for generation of steam. Carbonaceous products of pyrolysis are moved with sand to the regeneration bed where these residues are incinerated generating heat needed for the pyrolysis process. The combustion unit is

operating at around 9500 C while the pyrolysis bed operates at a temperature 100 to

I500 C lower [22].

7. Concluding Remarks

Most of the open issues related to the use of pyrolytic processes as an alternative recycling process are controlled by the overall economics of the process under consideration. These are related to:

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Capital investment; Thermal efficiency (i.e. heat requirements);

• Purity and compositioJ,l of the resulting products; • Selectivity to desired products;

Transportation costs for both the feedstock and the degradation products; • Variability in feedstock composition; • Scope of preliminary physical separation. In the absence of cost-effective sortationlreclamation technologies, pyrolytic

processes are potentially applicable to commingled streams containing mostly hydrocarbon polymers. Therefore, in principle, pyrolytic processes may be applicable to Municipal Solid Waste "tailings", automotive shredder residue fluff, and perhaps mixed textiles. However, the limitations imposed by PVC contamination and other impurities will significantly limit the applicability of a specific process or require an additional pretreatment step.

References 1. AL Bissio et aI., "Polymer Streams Available for RecoverylRecycling", Chapter 2, pp. 7-12 in. L. Bisio and M. Xanthos, Eds.., "How to Manage Plastics Waste: Technology and Market Opportunities", Carl Hanser Verlag, Munich, New York (1994). 2. T. R. Curlee and S. Das, "Back to Basics? The Viability of Recycling Plastics by Tertiary Processes", Working Paper #5, Yale Program on Solid Waste Policy, Yale University, New Haven, CT, Sept. 1996. 3. AL Bisio and M. Xanthos, "Pyrolytic Processes", Chapter 12, pp. 125-143 in. L. Bisio and M. Xanthos, Eds., "How to Manage Plastics Waste: Technology and Market Opportunities", Carl Hanser Verlag, Munich, New York (1994). 4. J.R. Fried, "PoLymer Science and TechnoLogy", pp. 232-243, Prentice Hall PTR, Englewoods Cliffs, NJ (1995). 5. M. Smits, Ed., "PoLymer Products and Waste Management", Chapter 2, pp. 41-67, International Books, Utrecht, The Netherlands (1996). 6. R. Gachter and H. Muller, "PLastics Additives", pp. 4-6, Carl Hanser Verlag, Munich, (1987). 7. 1. Brandrup, "Preparation of Feedstock for Petrochemical Recycling", Chapter 5.1., pp. 393-412 in BrandruplBittnerlMengeslMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 8. W. Kaminsky and H. Sinn, "Pyrolytic Techniques", Chapter 5.3.1, pp. 434-443, in BrandruplBittnerlMengeslMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 9. W. Kaminsky et aI., J. of AnaL. Appl. PyroL. ,32 (1995) 27, pp 19-27 10. N. Grittner et al. ,J. of Anal. AppL. PyroL. ,25 ( 1993) , pp 293-299. 11. R. D. Leaversuch , Modern Plastics, July 1991 pp 40-43. 12. J.H. Brophy and S. Hardman, "Low Temperature Pyrolysis for Feedstock Preparation, Chapter 5.2.2, pp. 422-433 in BrandruplBittnerlMengeslMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 13. K. Niemann, "Hydrogenation", Chapter 5.3.2, pp. 444-454 in

423

BrandruplBittnerlMengeslMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 14. J. Speight, "Refinery Processes Survey" , in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons ,Fourth Edition, Vol 18, pp 433-469. 15. P. Mapleston ,Modern Plastics, Nov 1993 , pp 58-61. 16. M. Gebauer and D. Stannard, "Gasification of Plastics Wastes", Chapter 5.3.3, pp. 455-479, in BrandruplBittnerlMengeslMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 17. J. E. Anderson, "The oxygen refuse converter - a system for producing fuel gas, oil, molten metal and slag from refuse" , Material Incineration Conference, Miami, Florida, May 12 - 15 , 1974, pp. 337-346. 18. T. F. Fisher et al., "The Purox system" ,National Waste Processing Conference, Boston, Mass. ,May 22-26,1976, pp 125-132. 19. C. R. Brunner, Handbook of Incineration Systems, McGraw-Hill, Inc., New York, (1991). 20. E. Legille et al. , "A Slagging pyrolysis solid waste conversion system", Conversion of Refuse to Energy, 1st International Conference and Technical Exhibition, Montreux, Switzerland, Nov 3-5 , 1975, pp 1-6. 21. P. E. Davidson et al., "The Andco-Torrax High Temperature Slagging Pyrolysis System", ACS Symposium Series, v 76, 1978, pp 47-62. 22. B. B. Crocker et aI., "Incinerators" , in Kirk Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, New York, Third Edition, Vol 13, pp. 182-206.

SOLVOLYSIS

Abstract

M.XANTHOS Polymer Processing Institute, Castle Point, Hoboken, NJ 07030, USA, and Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA and S.H. PATEL Polymer Processing Institute, Castle Point, Hoboken, NJ 07030, USA

Solvolysis as applied to plastics wastes includes depolymerization processes such as alcoholysis, hydrolysis, acidolysis, aminolysis and various interchange reactions that produce oligomers or monomers. Solvolytic techniques fall under the categories of chemical or tertiary recycling options. Suitable candidates are mostly step-growth thermoplastics and thermosets such as polyesters, polyamides and polyurethanes. Following a presentation of fundamentals on solvolytic techniques, this section examines existing and developmental technologies applicable to post-consumer and post-industrial scrap. Details on selected processes are provided.

1. Introduction

Solvolytic reactions generally deal with the breaking of C-X bonds, where X­represents hetero (non-carbon) atoms such as 0, N, P, S, Si, halogen, etc. In this section, we will consider solvolysis reactions applicable to high volume polymers containing only C - 0 and/or C - N bonds in their backbone (not as a side group or branching), viz. polyesters (PET), polyamides (nylon 6 and nylon 6,6) and polyurethanes (PUR), [1-4]. Other suitable polymers include polycarbonates, unsaturated polyesters, polyacetals, [5]. These step-growth polymers are often synthesized by reversible reactions and it is feasible to convert them back to their monomers or oligomers/chemicals by various solvolytic processes such as:

Glycolysis; Hydrolysis;

• Methanolysis; Aminolysis; Transesterification (ester exchange); Alcoholysis; Hydroglycolysis; Acidolysis; Transamidation (amide exchange).

425

G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 425-436. © 1998 Kluwer Academic Publishers.

426

Solvolytic processes are mostly applicable to thermoplastic and thermoset polymers produced by step growth reactions as shown in Table 1. [1].

Table . U.S. Production of Chain-!!rowth and Step-growth Plastics [1] Types of plastics % U.S. Plastics Sales

(1991)

Chain-I!rowth Thermoplastics LDPE 20 HDPE 15.1 PVC 15.0 PP 13.4 PS 8.0 Stvrenics 1.9 ABS 1.8 Acrvlics 1.1 TPE, SAN, Vinyl, Other <l.O(each)

SteD- I!rowth Thermoplastics Polvester) 4.2 paM, PC, Nylon <l.O(each)

SteD-l!rowth Thermosets PUR 4.9 Phenolics 4.2 UF,MF 2.4 Uns. Polyester 1.8 Epoxies, Alkyds <1.0

Thus, solvolysis, as the predominant conversion route for step-growth plastics, is, in principle, applicable to only 20% of all plastics sold in the U.S. Step growth polymers are predominant in at least three critical waste feedstocks containining mixed plastics for which generic separation is either difficult or non-economic; these streams identified in our earlier studies, [1], include automotive shredder residue (e.g. PUR, SMC, PET, Nylons), textile/carpets (e.g. polyesters, nylons) and post-consumer plastics obtained from Municipal Solid Waste, MSW (e.g. PET, PE, PVC).

2. Polyethylene Terephthalate (PET)

2.1. SYNTHESIS OF PET

PET is a step-growth (condensation) polymer derived from terephthalic acid (TPA) or dimethyl terephthalate (DMT) and ethylene glycol (EG) according to the

427

following chemical reactions:

(1)

(2)

Polyesters are synthesized by either direct esterification (Eq. 1) or transesterification (Eq. 2) reactions. In direct esterification, terephthalic acid is reacted with ethylene glycol to produce polymer and water as a by-product. Reaction conditions involve an esterification step in the presence of catalyst (usually antimony trioxide) and a polycondensation step. In transesterification, dimethyl terephthalate is reacted with ethylene glycol to produce polymer and methyl alcohol as a by-product; the transesterification stq) in the presence of catalyst, (usually metal carboxylates), is followed by a polycondensation step in the presence of catalyst (usually antimony trioxide).

2.2. PET SOL VOL YSIS REACTIONS

The principal solvolytic reactions for PET are hydrolysis with reaction products terephthalic acid and ethylene glycol, methanolysis with reaction products dimethyl terephthalate and ethylene glycol, and glycolysis with reaction products a mixture of polyols.

Hydrolysis

TPA + EG (Neutral) (excess) Na-acetate

TPA + EG (Acid catalyzed)

TPA + EG (Base catalyzed)

Methanolysis Cat.

PET + CH30H ~ ~ DMT +EG

428

Glycolysis

PET + PG Cat,. 6

propylene glycol excess

polyols + PG + EG

The preferred chemical recycle method is methanolysis. Post consumer bottles are first ground and cleaned, and then reacted with methanol, (3-times the stoichiometric ratio), in the presence of catalyst, (e.g. zinc acetate), at 185°C and elevated pressure for about 80-90 minutes. DMT is centrifuged and purified by distillation whereas methanol and EG are distilled off; yields are about 90%.

In glycolysis, PET is depolymerized with ethylene glycol or propylene glycol (PG) at ambient pressure and high temperature in the presence of a catalyst. The end product is intended to be the glycol terephthalate, but quantities of oligomers are also produced. Purification of the product is difficult and renders this process less desirable than methanolysis. Hydrolysis involving depolymerization in water at 150-250°C, under pressure and in the presence of catalyst should be, in principle, the easiest of the chemical methods. However, the TPA produced is difficult to purify compared with DMT, rendering the process less economical than methanolysis.

Some examples from the variety of reagents and catalysts employed in the depolymerization of PET are listed in Table 2.

T bl 2 a e . S I I . fP I h I o VOlYSIS 0 OIyet lylene h h I T erepr t a ate: R eagents an dC atalysts A. Glycolysis REAGENTS

EG,PG,DPG Aromatic Polyols, PEG, PPG, Glycerol DEG, Neopentyl Glycol (NPG), Butylene Glycol, Thiodiethylene Glycol Bis (2-hydroxy-ethoxy-ethyl) glutarate Oxyalkylene Glycol (polyethoxylated nonyl phenol) Aromatic Polycarboxylic Acids, Mixed ether-ester triol CATALYSTS

Zn, Pb, Zn, Mn, Co-acetate Amines , Alkoxides Na-acetate trihydrate, Benzenesulfonate, Ti(OBu)4

B. Hydrolysis REAGENTS

H20IEGlNaOH, H20lNaOH, CH3OHIDMSOINaOH

CATALYSTS

Na-Acetate, H2S04, NH40H

429

More details and corresponding references can be found in [1]. The following are examples of solvolytic processes described in the patent literature: Glycolysis: A patent issued to DuPont describes the use of aq. NaOH in EG at 90-150°C and at atmospheric pressure to decompose in-plant scrap to dis odium salt (95.7% yield) and then to TPA [6]. In an earlier patent also to DuPont, [7], waste fiber PET is reacted with EG and a benzenesulfonate catalyst to give bis-hydroxyethyl terephthalate (BHET). A final polymer made with 45% of recovered monomer gave fiber properties equivalent to that of virgin. In another patent issued to Fiber Industries [8]) a continuous glycolysis process is described for scrap PET of fiber formable grade using EG at atmospheric pressure conditions to obtain low molecular weight PET oligomer mixture, which may be employed directly as feed material to make high viscosity PET suitable for films and fibers. Methanolysis: Eastman Kodak Co. patented a batch methanolysis process for recovering EG and DMT from ground scrap PET bottles including contaminants such as poly olefin bottom cups, aluminum bottle caps, labels, and adhesive by dissolving the scrap in oligomers of the same monomer and passing superheated methanol through the solution [9] Hydrolysis: Celanese (Mexico) patented a continuous neutral hydrolysis process in a twin screw extruder to depolymerize PET waste using high pressure steam at 200-300°C and -15 atm. pressure [10]. Transesterification: A general equation describing transesterification is shown below:

o 0 II II

R1-C-OR2 + HOR3 ~ RI-C-OR3 + R2 0H

Degradative transesterification reaction of PET pellets (from PET bottles) with 2-ethylhexanol (20% excess) at 220°C using a tin catalyst for 4-6 hours resulted in cost effective production of dioctyl terephthalate (DOTP), a plasticizer for flexible PVC [11]. In another patent, Texaco, Inc. [12] has described the preparation of mixtures of aromatic polyols containing ester functionalities suitable for use in rigid foams by reacting a dimethyl terephthalate waste stream, containing methyl p-formylbenzoate, over a metal oxide catalyst and subsequently transesterifying the product with polyalkylene glycol in the presence of heat.

General characteristics of the main PET solvolysis reactions are summarized below: Glycolysis - Rate depends heavily on temperature and products; varies with ratio of PET/EG. - Below 220°C reaction requires catalysts (Mn, Mg, Zn salts). - Difficult to purify oligomer melt. - Mainly suitable for clean scrap. Methanolysis - Less than one hour reaction time in the presence of methanol and catalyst. - Relatively insensitive to contaminants.

430

Hydrolysis Uncatalyzed reaction with water at >200°C is too slow; high pressure with excess water, (waterlPET >5:1), at temp. 250-260"C promote high yields of TA, (TA solubility increases with temperature promoting homogeneous reaction); TA is crystallized and EG distilled or extracted.

3. Polyurethanes (PUR)

3.1. SYNTHESIS OF POLYURETHANES

Polyurethanes (PUR) are step-growth polymers synthesized by the reactions of polyols and polyisocyanates, generally in the presence of basic or organometallic catalysts.

o

OCN-RI-NCO> Ho-Rz-<lH ... OCNfr-NHJl-O-R10H (3)

jiisacyanate) (dial) PUR n

3.2. PUR SOL VOL YSIS REACTIONS

Important solvolytic reactions for PUR are summarized below:

Hydrolysis

I I

--R'NHCO-:-0- R-O +CONHR'NHCO+ OR ---HO ....L.H H.LOH HO .L..H

I I t~ I

431

Alcoholysis

I I I

---R NHCcr.-O-R'-O -TCONHRNHCO -TOR' """

R"O 7H H -+- OR" R"O 7-H

~ ---RNH-CO + HOR'OH + CO-HNRNH-CO + HO-R'--.-

I I I OR" OR" OR"

Glycolysis

W i -R3NH -C-OR4--- + HORSOH -. --R3NH-C-ORs OH +

HOR4--

Aminolysis

(Equilibrium strongly favors formation of substituted ureas. Rate is proportional to base strength of the amine. )

Interchange reactions

o II

RINH-C-OR2

(Equilibrium favoring formation of aliphatic rathe: than aromatic urethanes)

General characteristics of the reactions as applicable to crosslinked scrap are as follows: Hydrolysis: needs defined scrap; reaction yields mixtures of products and CO2 is evolved Alcoholysis: more defined, stable products than hydrolysis (no CO2 evolved); product is mixture of polyols and low MW PUR; PUR:glycol ratio is 1: 1 for depolymerization of rigid foams and 3:1 for flexible foams.

Examples from a plethora of reagents and catalysts that have been employed in the depolymerization of PUR are listed in Tables 3. and 4. Mere details and corresponding references can be found in [1]. The following are examples of solvolytic processes described in the patent literature: Hydrolysis: A continuous hydrolysis process for foam scrap using a specially designed twin screw extruder was patented by Bayer AG in 1977 [13]. PUR waste is shredded, compressed and evacuated to remove air, after which it is mixed with hot water and fed

432

into the extruder at 300°C to give -100% yield of polyethers and -90% yield of diamines [14,15]. Also, a continuous process using a vertical reactor has been designed which yields 60-80% recovery of polyol from scrap at 288°C with residence time of 10-28 minutes [16, p. 249].

T bl 3 a e lyCOlySl co OlYSIS 0 olyuret anes: GI I . siAl hi· fP I h R eagents an dC atalYsts REAGENTS

EG, PG, DEG, DPG Thiols, 1,2-butane diol, 1,4-butane diol, 1,5-pentanediol Polyether glycol, Polyether triol, 3-methylpentane, 1,5-pentanediol, 1,6-hexane diol

Glycerol:KOH:DMSO, Glycerol:KOH, H20:DEG:LiOH

CATALYSTS

Ti(OBu)4, Ti(i-Pro)4

Group I-IV Metal Alkoxides Acid or Base Triethy lenediamine Diethanolamine Potassium Acetate Chlorides of Ti, Cr, Z:r Ti-butylate

Table 4. HydrolysislHydroglycolysis of Polyurethanes REAGENTS

Superheated Steam, Steam/NaOH, DEG/Steam, DEG/Steam/NaOH, GlyceroVSteam

H20IDMSO

H20lNaOH, H20/KOH, H20INH3 DEGIH20lNaOH, Glycerol1H20lNaOH, DIPGIH20INaOH

Hydro~lycolysis: The term "hydro glycolysis" refers to the combined use of polyols (diols, triols, etc.}lalcohols and water as the reagents to bring about chemical degradation of step-growth polymers. In one example PUR foam waste was dissolved in EG at 185-220°C under N2; water and alkali metal hydroxides (e.g. NaOH) were added to the

solution, refluxed at 175-220°C to give amines and alcohol [17]. In another example, through the solution of PUR foam waste in glycol ether at

185-220°C in the presence of NaOH, superheated steam was bubbled to produce alcohol and amines. The solution volume is maintained by adding alcohol (EG) to replace that removed by steam. Substantial amounts of polyol are recovered [18]. Aminolysis: Polyether alcohols were recovered with high space-time yields and low by­product formation by slowly heating the coarsely chopped PUR waste with aniline to 120°C, then reacting it with propylene oxide in the presence of KOH [19].

433

Rigid PUR foam wastes were simultaneously aminolyzed with NH3, ethylene

diamine, diethylene triamine, hexamethylenediamine, or ethanolamine and alkoxylated by ethylene oxide, propylene oxide, butylene oxide, phenyl glycidyl ether, or styrene oxide optionally in the presence of a hydroxyl containing tert-amine (dimethyl ethanol

amine) catalyst at 160-190°C and 4 kglcm2 pressure for 2.5 hours to give brown polyols [20].

Wastes of cellular, flexible and elastomeric PUR were treated with diethanolamine at 140-190°C to give polyols having diethanolamino groups for use in the manufacture of rigid PUR foams [21].

4. Nylons

4.1. SYNTHESIS OF NYLONS

Nylon 6 - Nylon 66 Nylon 6 is manufactured using e-caprolactam as starting material either in a

batch reactor or a continuous process:

+ H20 ~ H tHN --(CHzJs~ tOH . n

(4)

c-caprolactarn Nylon 6

Nylon 66 is manufactured from adipic acid and hexamethylene diamine:

HOOC...,CHr.-COOH + H2N-(CH2t NH2= Hop...,mt.~ -NH ""CH2t NH f: + H20 (S)

Adipic acid Hexamethylene diamine Nylon 66

4.2. NYLON SOLVOLYSIS REACTIONS

Important solvolytic reactions for nylons are summarized below: Hvdrolysis

434

Methanolysis

o II

............... NH-C """ + HO-CH3

Glvcolysis

I

----R)-NHr-CO-R2----I

H:O-R3-0H

----R)-NH2 + HO-Rr-O-CO-R2""""""-

Aminolysis o 0 II II

R}-C-NHR2 + NH3 ~ R}-C-NH2 + H2NR2

o II

OR R}-C-NHR2 + H 2N-R3--' R}-CO-NH-R3+H2NR2

Transamidation (Amide Exchange)

o 0 II II

o 0 II II

R-C-NHR + RI-C-NHR) -----I~~ R-C-NHR) + R)-C-NHR

Acidolysis

~ -""'-"'C-NH"""-- + RCOOH ----. """--C-OH + R -C-NH......--

The following are examples of solvolytic processes described in the patent literature:

In a patent assigned to BASF [22], a process for the continuous recovery of caprolactam from nylon 6 fibers containing carpet waste has been described. Nylon 6 fiber scrap obtained from the carpet separation is fed to a depolymerizing reactor at temperature of at least the melting point of nylon 6 in the presence of catalyst and superheated steam to produce caprolactam containing distillate which is separated from other volatiles and purified.

Zimmer AG, has developed a process that converts nylon 6 from worn-out carpets containing 40-80% nylon to caprolactam. The Zimmer process uses an acid­catalyzed reaction at 280-400°C to produce a caprolactam product that is filtered and

435

purified by chemical treatment and distillation. By adding a proprietary shredding and pretreatment step, Zimmer has demonstrated the process feasibility in the lab. A semicommercial plant with 10-20,000 metric tons/year capacity had been planned [23].

General characteristics of the main nylon solvolysis reactions are summarized below: Nylon 6 Hydrolysis Good yields only at 300°C and pressures of 100 bar; costly, need to separate additives. Acidolysis 300 DC, 1-2 hours with phosphoric acid as catalyst, conversion to 90-95%; distil lactam, treatment needed for acid containing residues. Nylon 6,6 Neutral hydrolysis Difficult; need excess water, 8 hours at 280°C. Acidic hydrolysis 50% sulphuric acid at 115°C and atmospheric pressure; large quantities of inorganic salts. Alkaline hydrolysis 200-220°C in autoclave 20 bar, 10-15% aqueous NaOH, three hours for high yields.

5. Concluding Remarks

There exists a variety of technically feasible routes on a developmental or commercial scale to recover chemicals/monomers that can be repolymerized [24] Economic viability of chemical recycling methods depends largely on availability of scrap, its nature, quantity and cost, and end-use applications. With respect to PET, solvolytic methods, although practiced for fiber or film scrap, are not currently applied to bottle scrap due to unfavorable process economics and the current low price of virgin resin vs recycled grades. In addition, hydrolysis and glycolysis routes are difficult, due to sensitivities to waste impurities. Solvolytic techniques appear to be both technically and economically feasible for recovering monomer from nylon 6 based carpets and for production scrap and automotive flexible PUR foams. For rigid PUR foams chemical recycling seems to be developing more slowly.

References 1. S.H. Patel and M. Xanthos, "Solvolysis", Chapter 11, pp. 91-124, in A.L. Bisio and M. Xanthos, Eds., "How to Manage Plastics Waste: Technology and Market Opportunities", Carl Hanser Verlag, Munich, New York (1994). 2. P. Klein, "Solvolysis of Polyethylene Terephthalate" In

BrandruplBittnerlMengeslMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 3. M. Kopietz and U. Seeliger, BrandruplBittnerlMengeslMichaeli, Eds.,

"Depolymerization of Polyamides" in "Recycling and Recovery of Plastics", Carl

436

Hanser Verlag, Munich, New York (1996). 4. G. Bauer, " Solvolysis of Polyuretrhanes" in BrandrupIBittnerlMengesIMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 5. G. Reuschel, "Acidolysis of Polyoxymethylene" in BrandrupIBittnerlMengeslMichaeli, Eds., "Recycling and Recovery of Plastics", Carl Hanser Verlag, Munich, New York (1996). 6. R.J. England, US 3544622, E.I. DuPont (1970) 7. J.T. MacDowell, US 3222299, E.I. Dupont (1965) 8. H.S. Ostrowski, US 3884850, Fiber industries (1975) 9. AA Neujokas and K. Ryan, US 5051528, Eastman Kodak (1991) 10. J.W. Mandoki, US 4605762, Celanese, Mexicana (1986) 11. L.A Dupont and V. Gupta, Davos Recycle '92, Maack Business Services, 15/3-(1992) 12. M.E. Brennan, US 4444920, Texaco Inc. (1984) 13. E. Grigat and H. Hetzel, US 4051212, Bayer (1977) 14. K.W. Steiner, Kunstst. Ger. Plast., 74 (4), 2 (1984) 15. E. Grigat, Mater. Tech., 4, 141 (1978) 16. R.J. Ehrig, Ed., "Plastics Recycling; Products and Processes", Hanser, New York (1989). 17. J.L. Gerlock et al., US 4317939, Ford Motor Co., (1982) 18. J.L. Gerlock et al., US 4316992, Ford Motor Co., (1982) 19. B.R. Guettes et aI., DD 226575, VEB Synth. Schwarz. (1985) 20. M. Ionescu et al., RO 89944, Comb. Petro. Mid. (1986) 21. V.L. Petru et aI., RO 82464, Comb. Petro. Mid. (1983 22. T.F. Corbin et aI., US 5169870, BASF Corp. (1992) 23. S.K Shelly et aI., Chern. Eng. July, 30 (1992) 24. T. R. Curlee and S. Das, "Back to Basics? The Viability of Recycling Plastics by Tertiary Processes", Working Paper #5, Yale Program on Solid Waste Policy, Yale University, New Haven, CT, Sept. 1996.

FLUIDIZED BED INCINERATOR WITH ENERGY RECOVERY SYSTEM AS A MEANS OF PLASTICS RECYCLING

S. SUZUKI Kobe Steel Ltd., London Office, Alton House 177 High Holborn, WCIV 7AA London, UK

T.MINOURA Kobe Steel Ltd, Mechanical Engineering Research Laboratory 5-5 Takatsukadai l-chome, Nishi-ku Kobe 651-22, Japan

Abstract Although in a number of countries incineration of plastics is not acceptable because of the public opinion, we believe that incineration with energy recovery is a better solution, for the present at least, than other processes such as reducing to monomers the post consumer plastic waste. In Japan where the public have accepted incineration, a great number of incineration plants have been in operation. Our company alone has constructed more than 20 fluidized bed incineration plants since 1981. The incineration must meet the following conditions:

Highly efficient energy recovery Low emission of toxic gas and dust User friendly system at low cost.

With these in mind a new fluidized bed demonstration plant for the incineration of municipal solid waste with a capaciry of 20 tonne/day has recently been developed in

Japan. With this plant NOx concentration in the exhaust gas is very low. Ash is treated by a plasma melting technique to generate no significant amount of dioxins. Furthermore the slag shows leaching values of heavy metals well below the legal limits for landfill. We would like to demonstrate in this chapter that a safe, efficient, and economical incineration plant can be built for municipal solid waste which contains plastics.

1. Introduction

For two decades there has been growing concern about the management of the resources of our planet to ensure a sustainable quality of life in the future. There are two aspects. One is that there is a finite limit to the non-renewable resources available. The other is the fate of products and their components. Detrimental effect of chemicals on the ozone

437

G. Akovali et al. (eds.), Frontiers in the Science and Technology oJPolymer Recycling, 437-447. © 1998 Kluwer Academic Publishers.

438

layer is one instance and the accumulation of waste a more obvious one. The disposal of solid municipal waste presents an increasingly serious problem. Not only is there pressure to find adequate land-fill sites, it is also clear that degradation of the buried wastes leads to the emission of harmful or dangerous gases and contamination of ground water. Clearly it would be advantageous to the overall resource depletion on the planet if components of the waste could be re-used in some way. Indeed this NATO Advanced Study Institute is devoted to consideration of the Frontiers in the Science and Technology for Polymer Recycling. The implication is that the polymers will be re-used in materials applications.

This approach of recycling for re-use seems eminently sensible and simple. However, when the re-sale value of the recycled products and the costs of conversion and extra energy required are compared, there are, in reality, very few recycling operations which are economically feasible and around which sustainable businesses can be developed. Society must be willing to provide a continuing subsidy or to find some more cost effective form of re-use. The approach outlined above turns out in practice to be very complicated.

In the first place why do we feel that we must recycle plastics? Is this because we want to conserve our natural resources, in this case oil or coal, for future generations? Or are we concerned about the environmental problems which are caused by plastics waste? Certainly plastics represent a dominant component among packaging materials, some 40% according to a recent survey, and being durable thus represent a major part of municipal solid waste. This easily draws them to the attention of the general public.

I[ the purpose is the conservation of the non-renewable resources, first our efforts should be concentrated on the reduction of the amount of plastics materials in each application, i.e. source reduction. The benefits of source reduction are obvious, since it results directly in less environmental damage. Indeed this option has been undertaken in recent years by the users of packaging materials, because it makes economic sense. For example, the thickness of film packaging materials today is much smaller than it was a decade ago. Excessive wrapping should be avoided. Secondly we have to re-use the same products if at all possible. The culture of completely discarding the products after use must be changed. The minimisation of the amount of plastics in circulation is more directly beneficial than is recycling.

There are in fact two regimes in which plastics can be recycled. The first deals with waste generated inside factories, and the second deals with the recycling of plastic components in discarded or disused products. The recycling of plastics generated in the course of processing plastics into the final product should be a relatively simple and effective operation in a well managed factory. The waste raw materials are of known composition and little contaminated. Thus they can be reintroduced into the manufacturing system along with new feedstock, and furthermore collection of the waste presents little difficulty.

Recovering plastic waste from products at the end of their useful life cycle presents two major problems. The first is recovering the discarded products and return to the recycling plant. The second is the separation of plastic components from the product and then chemical identification. Design of the products can have a major impact on the ease

439

of separation of the components. The ease of dismantling can help with the cost of recycling. Chemical coding can also help enormously to sort the components into similar polymer types. But above these two problems is the difficulty of ensuring the discarded products are returned for collection. There is still much to do to educate our society in this respect. Some countries, notably Germany, have introduced legislation to assist the process. It is to be hoped that this will be a world wide trend.

Even when the plastics are sorted there is a major challenge to develop recycling process which can produce a recyclate which has a value and a demand in the market place. The availability of markets which can bear the economic cost of the recyclate is the final hurdle in presenting the recyclate for re-use.

After plastics have been collected and sorted, the alternative to re-use as materials is to burn them in a proper station to generate energy. The costs added in the recycle for use as a material are eliminated and a product is obtained which has a simple market value. In Japan where land is scarce and thus the cost of landfill is high, incineration of plastics in municipal solid waste has been accepted by the general public, though still surrounded by some controversy about emissions.

Currently Japan produces 50 million tonnes of municipal solid waste per year. The disposal of the waste is a major problem for local authorities, since in law they are responsible for collection and disposal. Currently about 70% of municipal solid waste is incinerated. The first generation of incinerators was built without energy recovery simply as a means of disposal. More recently incinerators with energy recovery systems have become the norm. Various types are being developed by a number of companies. Nineteen Japanese companies are now participating in the development of the next generation incinerator which will remove dioxins by gasification and melting processes.

We believe there is a growing consensus in Japan that as far as plastics in municipal waste is concerned, the most sensible option is to burn it safely, assuming of course that the usage of plastics operates under a minimisation strategy. Even though technology may be available the extra cost of collection, sorting, washing and processing in the light of the volatile markets for the recyclates often does not make economic sense. Furthermore these operations can cause damage to the environment, using more solvents or producing unnecessary by-products, or consuming more energy.

2. Incineration of Municipal Solid Waste

There are two main types of incineration system for municipal solid waste, the stoker­type and the fluidized bed systems.

2.1. STOKER-TYPE

In the stoker-type furnace, municipal solid waste is fed on to grates by a hopper and burnt by the air which is usually blown from the bottom. The resulting ash which drops from the grates is collected at the bottom and carried away by a riddling conveyor. There are mainly three zones; drying, combustion and burn-out. First the waste is dried

440

by the heat coming from the main combustion zone on the drying grate and passed through a movable grate to the combustion zone. The movable grate is generally used to stir and mix the waste for efficient combustion and also to riddle the ash, improving the ventilation of air in the furnace. The components which were un burnt in this zone are transferred to the burn-out grate where, with an auxiliary fuel burner, complete combustion takes place.

In general this type of furnace is suitable for the combustion of large amounts of municipal solid waste. Plants have been built in Japan typically with a capacity of more than 200 t/day. It is known that the introduction of secondary air can reduce efficiently the amount of CO in the emissions, and various devices are being developed by companies.

Low oxygen combustion is effective to reduce NOx in the emissions, but this results in high temperature in the furnace. To avoid the temperature of the furnace wall going too high, damaging the wall materials, in some models water pipes are imbedded in the wall.

2.2. FLUIDIZED BED

In the fluidized bed system, hot air is blown vertically upwards together with inactive particles, normally sand, and municipal solid waste as fuel is supplied to the combustor by a hopper. When the waste falls on top of the fluidized bed, the temperature of which

is around 800°C, combustion takes place instantaneously. As mentioned earlier, a heat recovery system is attached to a modern fluidized bed incineration plant. The ash and incombustibles are discharged from the bottom together with sand and the latter is recycled to the system. After heat recovery, flue gas is treated to remove impurities. The fly ash is also collected and treated.

In comparison with the stoker type combustor, the fluidized bed system is more suitable for the incineration of plastic materials which have high calorific values; waste plastics are good fuel that should not be discarded. Roughly speaking I tonne of municipal solid waste has the energy equivalent value of 0.4 tonnes of coal. In Japan, about 5 weight per cent of municipal waste comes from plastics. Their calorific values are relatively high (30,000-40,000 kJ/kg) compared with about 8,000 kJ/kg for municipal waste and plastics contribute to raising the calorific value of waste. Furthermore the high combustion temperature of the fluidized bed prevents efficiently the formation of toxic by-products in the emissions.

In real incineration systems, high energy recovery rates, low emissions of toxic gases & solid particles, simplicity of operation, and low running costs are required. To achieve these, technologies are being developed by a number of companies. Here, we show, as an example, our demonstration fluidized bed incineration plant with a capacity of 20 t/day which is now operating at Kakogawa works in Japan [I]. In this system sand is axially circulated, and uniform and strong fluidization is maintained. Usually a heat recovery system is installed to extract heat from the flue gas which comes from the freeboard. The corrosion of heat exchanger tubes by HCI in the flue gas limits the temperature of the steam produced and consequently the efficiency of energy recovery.

441

It has to be operated at a steam temperature of around 300°C. We have therefore designed a plant with the tubes installed in the fluidized bed. Though the temperature of the fluidized bed zone is lower than that in the freeboard zone, the concentration of HCI is also lower (about lI50 of that in the freeboard zone) and the temperature of sand is still high enough to recover energy. Thus we have been able to raise the temperature of the

steam up to 450°C. It compares with the temperature of the steam recovered from the

flue gas, which is about 300°C. Consequently the efficiency of electric generation is now

° more than 20%, higher then the figure of about 15% which is available at 300 C. The fluidized bed is divided into two zones by baffles which allows control of the superficial velocity in the energy recovery zone independently from that in the combustion zone. This in turn allows control of the temperature of the superheated steam. The flow diagram of this plant is shown in Fig. 1 . The freeboard includes a boiler which produces saturated steam. The fluidized bed is equipped with a super heater to raise the

° temperature of the steam to 450 C. In Table 1. a set of typical operating values are given.

TABLE 1 . Emissions Energy Recovery Flue gas Zone

01 15.3% 8.9%

CO) 4.6% 11.1%

CO 3 4.5xlO ppm 22ppm

HCl 12mg/Nm 3

507mg/Nm 3

Temperature 798°C 659°C

3. Treatment of Emissions

In the emissions formed by the incineration of municipal solid waste, HCI, SOx, NOx, and dioxins are major problems. These can be reduced, however, by conventional methods and therefore no mention is specifically made here of each technique. The recent general trend is to combine all the techniques and make the whole process more compact and of course more efficient. An example is the bag reactor. In the bag reactor,

which is a modified bag filter, HCI can be treated with Ca(OH)2 and at the same time the dioxins in the ash are reduced by activated carbon [2].

As far as toxicity is concerned, the treatment of fly ash which contains dioxins is becoming a more urgent problem.

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4. Treatment of Fly Ash

Fly ash is usually collected in a bag filter. It contains various organic toxic compounds including dioxins, as well as inorganic metal oxides and heavy metals. The size of these particles is very small, typically about 40 Ilm. To avoid secondary pollution, it is imperative to treat the ash before re-use or before burying at land-fill site. One treatment method is plasma melting (Fig. 2.. ) [3]. The ash is heated by a plasma arc generated from a plasma torch. The high temperature of the plasma causes the decomposition of dioxins. Vaporised materials and Hcl in the exhaust gas are treated in the gas treatment system. The molten slag is discharged from the bottom of the plasma furnace.

The concentration of dioxins in the slag is greatly reduced compared with the dioxins concentration in the fly ash. In a typical case more than 98% of dioxins are decomposed by plasma melting. In terms of TEQ (Toxic Equivalent Quantity), the dioxins concentration is only 0.01 ng (Table 2. ).

TABLE 2 . Dioxins balance (TEQ %) Input 100.000 Exhaust gas 1.493 Dust 0.089 Slag 0.000 Decomposition 98.418

The concentrations of heavy metals too are reduced. The results of a leaching test are shown in Table 3 •

TABLE 3. Leachate levels (mgll) from )lasma melting slag Total Hg <0.0005 Pb <0.01 Cd <0.005 Cr+6 <0.02

As <0.01 Organic P <0.1 PCB <0.0005 CN <0.01

These values are well below the Japanese standards.

444

Cooling Watert==~ IGas Supply I System

Power Supply

L.=:::;>~

Rapidly Coolled Slag Discharge System

Slag Discharge System L..-___ .....

~ Wale<Coolied ~ .IJ. Slag Discharge System

.~.uoISlag

Figure 2. Flow diagram of plasma melting process

IDF

445

When plasma melting is applied, the plasma arc can cause the dissociation and

recombination of oxygen and nitrogen thus forming NOx• To avoid this, town gas is injected into at the slag hole. The effects of this operation are a) the volume of the exhaust gas is reduced and b) the temperature of the tapping hole gas increases. By this

non-catalytic process, the concentration of NOx decreases from typically 1500 ppm to 140 ppm after 40 minutes.

Another method is swirling flow melting [4]. A typical structure is shown in Figure 3. Ash is introduced at the top of the furnace. Strong swirling flow is generated by the

° combustion air, and the temperature of the furnace is maintained at about 100 C higher

than the melting point of the ash. A typical value is 1250°C. Melted ash collides with

the furnace wall by centrifugal force. The wall temperature is around 1300°C. Then it flows down along the wall as molten liquid and is discharged. In this method too, dioxins concentration is very low. The results of a typical analysis are given in Table 4.

TABLE 4 .. Dioxins Balance (TEQ %)

Input 100.000 Exhaust gas 0.86 Dust 0.22 Slag 0.03 Decomposition 98.89

5. Conclusion

The general public are increasingly concerned about environmental issues and they are likely to purchase products which can be recycled. This may lead and has led, in some countries, to new legislation setting up unrealistic recycling rates as targets for products. However, as we pointed out in the introduction, recycling is not always the best solution to the environmental problems. We should also consider incineration with energy recovery as an acceptable option. This agrees with one of the conclusions in the recent report from the European Commission, DGXI [5]. The technology described in this article shows that post consumer, plastics packaging materials in municipal solid waste can be burnt without toxic emissions and with a high efficiency energy recovery rate.

One problem remains, however. It is the emission of CO2. Its effect of climate warming has been highly publicised, though there is a small group of people who are sceptical

about it. There are a few research projects on the disposal of CO2, notably in Japan,

including the discharge of solid CO2 into the oceans or feeding CO2 gas into algae to

produce carbohydrates. The problems of CO2 disposal will be solved eventually, if not

446

Swirl Vane

Swirling Flow Melting Section (Combustion Chamber)

Cooling Water Inlet q

Burner

Do SJag

Figure 3 Swirling flow melting furnace

~..-u----J ¢::J Primary Air

Slag Separating Section

Slag Extracting Section

447

immediately. We, scientists or engineers, should communicate accurate information, which is comprehensive and rational, regarding environmental issues to the general public to protect our planet for our future generations.

6. Acknowledgment

The authors wish to thank our colleague, Sir Geoffrey Allen, FEng, FRS, who is a member of the Royal Commission on Environmental Pollution in the UK, for valuable advice and discussions.

7. References

1. M.Sakano, Y. Shiraishi, T. Ito, T., Minoura, T.Suzuki, (1997) Combustion of Fluidized Bed Refuse Incinerator with a High Efficiency Energy Recovery System, Fourth International Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal, 7 - 10 July 1997. 2. M. Hiraoka,. (1993) Waste Treatment and Elimination of Dioxins, Kankyo Kogai Shinbunsha, Tokyo. pp 466-467. 3. Y. Higashi, T. Suzuki, Y.Shimizu, M.Yamada,. (1996) Plasma Melting Process for Incineration Ash of Municipal Solid Waste, Haikibutsugakkai Ronbunshu 7 193-201. 4. T. Suzuki, M. Matsuda, T. Yoshigae. (1995) Application and Scale-up of a Swirling Flow Melting Furnace. Kobelco Technology Review 18,57-61. 5. Cost-benefit Analysis of the Different Municipal Solid Waste Management Systems: Objectives and Instruments for the Year 2000 EUROPEAN COMMISSION, DGXI, Brussels.

Chapter.6 THE WAY FORWARD

FUTURE PERSPECTIVES AND STRATEGIES OF POLYMER RECYCLING

HANS-JOACHIM RADUSCH Martin Luther University Halle-Wittenberg Institute of Materials Technology D-06099 Halle-Saale

1. Sustaiuable Development

A "Sustainable Society" in particular a "Sustainable Economy" are respectively societal and economic systems "which meet the needs of the present without compromising the ability of future generations to meet their own needs."[1] This concept is the backbone of the Tokyo Declaration issued by the United Nations' World Commission on Environment and Development (UN-WCED) 1987. The report of this commission "Our common future" ("Brundtland Report") has developed the guiding principle of a "Sustainable Development", what means a lasting future effect in economy and society paying attention to comprehensive environmental points of view.

Basing on this, the United Nations Conference on Environment and Development in Rio de Janeiro 1992 has instituted "Sustainable Development" as the only possible way to ensure the long-term survival of mankind.

The main idea of this principle is to regulate the managment of the resources of the earth in such a way, that coming generations will find yet these ressources necessaryly for satisfaction of their needs.

In accordance with the Rio Declaration the people are standing in the center of efforts for sustainable development. They have a right on "healthy and productive life in harmony with the nature". Environmental protection is accepted as "an element of the development process" as to see in Table l. The aim of the Rio Declaration is not the termination of economical activity of man, not even a stop of economical growth; but the economical development should be directed to a new ideal, integrating economical and environmental policy.

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G. Akovali et aI. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 451-467. © 1998 Kluwer Academic Publishers.

452

I. 2.

3. 4. 5. 6. 7. 8.

9. 10.

II.

12.

13. 14. 15. 16. 17. 18.-27.

TABLE I: Rio Declaration 1992 [2]

Man in center: Right on healthy and productive life States have the right to use their own ressources in the frame of their own environmental and developing policy Realization of developing and environmental needs of present future generations Environmental protection is a component ofthe development process Elimination of poverty Priority for developing countries and countries with most vulnerable environment Partnership, technology and money transfer Reduction of non-effective production and consumption structures; population policy Extension of existing first beginnings of sustainable development Involvment of all concerned citizens; access to informations, legal and administrative proceedings Effective environmental laws co-ordinated to environmental and development policy, economical and social tenable Open economical system, which leads to economical growth and sustainable development in all countries Liability for environmental damages Warding off "exports" of environmental damages Using the precaution principle at weighty or permanent damages Internalization of environmental costs, cause principle Check on environmental impact Additional dutys of states: - mutual information - integration of women, young persons and natives - protection of environment in the case of annexation, war - inseparableness of peace, development and environmental protection - peaceful solution of conflicts

Concerning the general acceptance of this ideal, the task of the business concerns and other institutions is to check, which steps are to be taken, to cope with this job as good as possible. Because of the limited resources of organic raw materials (oil, gas, biomass) and energy (excluding solar energy) all recycled materials will be an important source of starting materials for industrial production.

In this sense also plastics recycling is a very complex matter and it is to discuss in its entirety of economical, political, technological, energetical, material and environmental aspects. World-wide, all countries have to introduce market instruments and regulations which direct plastics production and processing industry and users of plastics materials towards a sustainable society.

453

2. The Position and Importance of Plastics Recycling in Sustainable Development

1993 in Europe were used all over 24.36 Mio t polymeric raw materials. This amount is divided in the following user fields:

TABLE 2. Polymer applications in Europe 1993, data acc. [3]

Polymer application Fraction, %

Packaging sector 41 %

Building trade 20%

Electrical engineering / electronics 12%

Car industry 7%

Agriculture 4%

Others 16%

In spite of many different applications of polymers in all fields of our life only 0.6 % of the whole waste amount are falling to plastics. All over, in Western Europe in 1993 2.8 billion refuse of all type was proved (household, agricultural waste, industrial waste, electricallelectronical scrap, building refuse etc.). From this amount the plastics waste is only 16.2 million t. In the waste stream the largest fraction is the household plastics waste, like shown in table 3:

TABLE 3. Plastics fraction in waste stream, 1993 [3]

Origin of waste Whole plastics waste Plastics waste fraction amount, 1000 t from all waste in this

field, %

Agriculture 636 0.11

Building industry 753 0.33

Household refuse 10 928 7,88

Trade, industry 2636 0.87

Car industry 842 7.0

Electrical engineering/electronics 518 12.7

454

But the plastics are responsable only for 8 % of the whole household refuse. The second-largest plastics waste source is the field trade and industry, but also in this section plastics cause only 0.87 % of the whole waste amount. F or the different fields different options of utilization were used like shown in table 4.

TABLE 4. Origin of plastics waste and waste amounts recycled in the different processes in Western Europe 1993, (Amounts in 1000 t)[3]

Recycling process

Origin of waste Material Energetical Incineration Deposit recycling utilization without

energy use

Agriculture 70 13 9 635

Building industry 15 11 1 753

Household refuse 179 2108 510 8131

Trade, industry 609 246 60 1620

Car industry 39 26 4 773

Electrical engineering! 3 21 8 486 electronics

In all 915 2425 592 12279

It results that Europe-wide the largest part of the plastics waste is still going to the landfill (1993 more than 12 million t). Only 915 000 t were re-used materially. Above all the plastics waste from trade and industry and agriculture, respectively, were gone into material recycling processes. More than 24 % of all plastics waste from this fields was utilized in material recycling. The reason for this comparatively high recycling rate is, that in this fields mainly type-clean waste is to recycle. The fraction of plastics waste utilized by material recycling achieved in 1993 in Western Europe was only 6 %. If one wants to relief the deposits from plastics waste, in future beside material recycling also all other possibilities of recycling have to be used in a higher degree.

In plastics recycling the commercial business is determined by economy and ecology in the frame of the social market. General obliging aims of environmental protection determine the ecological decisions in connection with plastics

455

recycling. As an instrument for the evaluation of the rightness of the decisions are serving eco-balance, describing the whole life cycle of products, processes and service. Working with plastics waste is subjected to these ecological criterions. Utilization of plastics waste is not only an end it itself but also an environmental goal. On the other hand it is not possible to make recycling "at any price". Beside economical limits also ecological frontiers are existing, if the environmental impact from the recycling processes including transport are higher than alternative utilization processes (e.g. energetical utilization) or even higher than environmental impact at new-production of plastics. In accordance with the German Waste Laws from 1986 [4] and 1993 [5] the waste recycling, i.e. also the plastics recycling, is granted priority before other disposal, if 1. recycling is possible technically 2. the resulting costs in comparison to other processes of disposal are not

unreasonable 3. a market exists for the resulting materials or energies, or a market can be

generated 4. the resulting environmental impact is lower in comparison to other disposal

processes.

3. Economic Efficiency and Material-Ecological Benefit of Different

Methods of Plastics Recycling

3.1. RECYCLING TECHNOLOGIES (OVERVIEW)

Mixed plastics wastes from packaging refuse are recycled by different technologies depending on technological, economical and also subjective factors. In principle the main procedures of plastics recycling can be classified into the following groups:

a) Material recycling

al) Mechanical recycling - materials recycling by re-melting of the collected mixed plastics waste

fraction from houshold refuse - re-melting of type-clean plastics waste fractions separated from mixed

plastics waste - re-melting of type-clean plastics (industrial waste)

456

a2) Chemical recycling (Raw material recycling) - thermal decomposition of mixed plastics waste

-- hydrogenation -- pyrolysis -- gasification -- smouldering-bum process -- thermoselect process

- solvolytical processes -- hydrolysis (special polymers) -- alcoholysis (special polymers)

b) Energetical utilization - incineration of mixed household waste - incineration of collected plastics waste fraction

c) Special utilization - using plastics waste in blast furnace as carbon donator - using plastics waste in rotary sintering kiln

d) Deposit

3.2. THE DETERMINANTS OF ECONOMICAL AND ECOLOGICAL FEASIBILITY

The assessment of different procedures for plastics recycling is possible using different concrete criterions expressing some entierty aspects. As such criterions can be used [6]:

1. Material economical and ecological benefit 1.1. material cycle

- retention of material - output of material

1.2. usability of recycled plastics - applicationallimits - downcycling, diffusion of pollutants

1.3. energy balance - energy consumption - degree of effectiveness of the process

457

- energy conservation regarding to production of new plastics material

1.4. Environmental compatibility

2. Economy 2.1. Costs

- remains and new formed pollutant - necessary cleaning of product gas, waste gas and waste water

- need for landfill; residues - usability of waste products

- pre-process costs - costs for treatment and utilization

2.2. Refunds - refund for collection - refund for utilization . - selling proceeds

3. Practicability

Considering the different criterions of evaluation it was realized a study on economical and ecological assessment of recycling procedures for recycling of packaging plastics waste by [6]. Some ofthis criterions are discussed below.

3.3. COMPARATIVE DISCUSSION OF CRITERIONS FOR PLASTICS WASTE RECYCLING PROCESSES

3.3.1. Materials circulation

The circulation of the waste material is to evaluate from the point of view that the circle formed by the streams of the material should be as narrow as possible and closed too. The most effective type of circulation is reached, if the material can be preserved without changing of the chemical nature of the material after recycling. Here the primary material, i.e. the resources of material, are most saved. But also the transformation of waste material into starting components for polymer· synthesis is an acceptable way in recycling. Considering the preservation or saving of the material, the material recycling is the most effective procedure. The usable fractions are about 70 to 80 % [6]. Also raw-material recycling processes achieve an output of 70 up to 80 %, exceptional the pyrolysis of mixed plastics waste. Hydrogenation of a clean

458

polyolefine fraction even gives an output of more than 90 % [6], but the usable material fraction is reduced to 70 up to 80 % because of the necessity of separation the heavy fraction from the mixed plastics waste. In the result this is not better than using mixed plastics waste in hydrogenation process. Using thermal processes of refuse treatment, no circulation of material is possible. Principally only the thermoselect process enables a preservation of the material. Another variant of preservation of the material is the surrender into another circle of material. This is the case, if plastics waste is used in the blast furnish process. Here not the easy energetical use is realized, but the carbon chain of the polymer molecule is used as a carbon donator. By this method coke or mineral oil is saved [11 ].

3.3.2. Usability of recycled plastics

The usability of recycled plastics is to evaluate by the universality of using and the quality of the resulting products from the different recycling processes. With regard to this two criterions the hydrogenation process is the best variant. The resulting Syncrude oil is a high-grade petro-chemical secondary raw material and it is characterized by a wide spectrum of re-using possibilities. The usability of recycled plastics from material recycling processes is strongly limited because of the downcycling problem. The incompatibility of the polymer components is the reason for a pronounced phase morphology of recyclates. This is connected with a decrease of the most mechanical properties, and the possibilities of application are limited. Even if a type-clean separation of mixed plastics waste is realized, only about 18 until 35 % of plastics waste is usable in material recycling processes [6]. Using plastics waste in the gasification process the resulting product is a gas fraction (syngas). The usability of the syngas is given in different chemical processes, e.g. for methanole production or as heating gas. In this case the usability is not high because of the limited heating value [8]. Pyrolysis of mixed plastics waste to produce oil fractions is unfavourable because in the resulting pyrolysis oil pollutants are abducted. Especially the organic bonded chlorine is a big problem. Practically the products are not to commercialize. Using plastics waste for incineration, really it is not possible to speak about usability of a recycled material, but by energetical re-using of plastics waste energy as long-distance thermal energy or electric current is the result of the process. If the recycling processes are considered as a re-using cascade, all of the products from the other recycling processes are usable for energy generation at last.

459

3.3.3. Energy balance

From the point of view of the energy balance a very complex field is to discuss. It is to investigate the energy consumption for collection, sorting, separation and the real recycling process. If the plastics waste is only low polluted and not very mixed, the energy balance is at best for material recycling. On the other hand the energy balance can be very unfavourable if the material recycling is connected with a very high expenditure of sorting and separation. Also the raw-material recycling process shows a very different picture. Caused by the specific process conditions the pyrolysis requiers the lowest expenditure of energy in these group. But if mixtures of plastics waste are used from which the heavy fraction was removed, thus at hydrogenation, pyrolysis and gasification unfavourable expenditures of energy are expected like at material recycling of strong polluted and mixed plastics waste. A very unfavourable energy balance is expected for incineration of household refuse and for the thermoselect process too. To realize the thermoselect process addition of pure oxygen is necessary and the produced syngas is consumed for 50 -60 % in own need, thus the thermoselect process has to be ranked as last. Discussing the energetical degree of effectiveness of the energetical re-using processes of plastics waste, Berghoff [9] gives the following data:

- incineration ofhoushold refuse 16 % - smouldering-burn process - thermoselect process

13% 7%

TABLE 5. Overview of energy balance data for re-using of a polyolefine fraction, acc.[lO]

Re-using process Energy preservation

Material recycling (Plastics low mixed and 74% polluted)

Material recycling (Plastics strong mixed 40 % until 0 % and polluted)

Hydrogenation (incl. separation of the <35% heavy fraction)

Hydrogenation (without separation of the 34 until 45 % heavy fraction)

Gasification about 38 %

Pyrolysis « 35 %) lower process energy than hydrogenation or gasification (no data)

Incineration 27 - 28 %

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Table 5 shows data of the energy balance for different recycling processes. Here the energy conservation results from the polyolefine fraction under consideration of the life cycle including the first-production of the plastics material. [1 0]

3.3.4. Environmental compatibility

Concerning the environmental impact, toxicity, air pollution, water pollution and soil pollution are to evaluate, but also effects on the environment like Greenhouse effect, destruction of the ozon layer, disturbances by odour or noise and residues from processes are to take into account. These effects are very complex and evaluation or weighting is rather difficult. Some aspects of the environmental impact are discussed below. Taking into consideration toxicity, air and water pollution, emmissions and residues, material recycling shows the highest environmental compatibility. Merely at very heavy mixed and polluted plastics waste a worsening is to recognize above all because of the energy caused emmissions at granulation, washing, drying, separation. At the raw-material recycling processes waste products from gas-washing and waste water cleaning and solid residues are accumulating. The following data of solid residues from raw-material recycling of packaging plastics waste are given in [6]:

- Hydrogenation - Gasification - Pyrolysis (fluid bed)

3 - 10 % 3 -10%

50 - 60 %

Using clean polyolefine fractions in pyrolysis, the toxic substances and the solid residues are essentially lower. Also the fluid bed sand is possible to recycle partly. At gasification of plastics waste solid residues are accumulated as slag. This integrates the advantage that a displacement of toxic substances into other mediums does not occur. Concerning gas-cleaning the raw-material re-using of plastics waste is more favourable than energetical utilization processes, because ofthe higher amount of smoke gases in energetical utilization processes. Therefore the main criterion is here the volume of waste gas, to clean at the end of the process. The highest volume of waste gas is resulting from incineration of household refuse.

Under ecological aspects utilization of plastics waste in blast furnace represents a meaningful process variant. 60 % of the plastics waste are re-used materially as

461

carbon donator at reduction of iron ore and additional 20 % are re-used as energy by generation electric current using the blast furnace gas [11]. Only low expenditures for treatment and inblowing in blast furnace are faced to this high degree of utilization. Dioxines does not result from plastics waste utilization in blast furnace. Measurements of dioxane and furane emmissions showed that all values are in the region of the detection limit ofO,00Ing/m3 [12].

The generation and release of dioxines is a very sensible question. The material recycling is harmless at the temperatures used there. Also at hydrogenation a dioxin generation does not occure because of the reducing hydrogen atmosphere. At pyrolysis only a fully deairation prevents the dioxine generation. A high impermeability of the chemical plant is necessary. At gasification the high temperatures used destroy the generated dioxines for the present. Because of the presence of oxygene the generation of dioxine is given especially in a critical temperature range at cooling of the generated gases. By quenching of the gas the dioxine generation can be reduced, because of the very fast transmission of the hot gas through the critical temperature range. The same situation is valid for the thermoselect process, but here the dioxine generation is more probably because of the presence of other materials like metals and chlorine. A total quench to about 90 0 C tries to hinder the dioxine generation.

4. Benefit-Cost Comparison

In global scale the scene of re-using plastics waste is very complex and a lot of problems are unsolved especially in connection to the environment. Also it is not possible to give today a broadly accepted comprehensive methodology which allows to evaluate precisely the advantages and disadvantages of the different methods of plastics recycling and re-using, respectively.

Bauermeister et al. [6] have tried to summarize the aspects discussed above in an effectivity ranking for the re-using processes of plastics waste valid for german relations without considering re-using plastics waste in blast furnace. The results of Bauermeister et al. [6] were completed and are presented in table 6. The effectivity ranking corresponds to a cost hierarchy of the processes. Re-using of plastics waste in blast furnace process and rotary sintering kiln in the cement production process were not considered by [6]. As reported in [11,12] the use of plastics waste in the blast furnace process is very effective and enables to utilize large amounts of waste. In addition to the process specifical costs in the effectivity ranking also the largely process independent steps of collection and separation going on ahead the main

462

TABLE 6. Results of an effectivity ranking of plastics waste re-using,

Effectivity Re-using process

Very high I. Material recycling of type-clean plastics waste

2. Blast furnace process

High l. Hydrogenation 2. Material recycling of mixed plastics waste 3. Material recycling of mixed refuse 4. Gasification 5. Pyrolysis of type clean plastics waste

Middle to low l. Pyrolysis of mixed plastics waste 2. Thermoselect process 3. Smouldering-bum process 4. Incineration of household refuse

Poor Deposit of residues

recycling process are influencing on the costs. The costs are compared to the proceeds from selling the re-products produced and remuneration for collection services, correct re-using and disposal of residues. Table 7 represents an overview over the cost-benefit analysis of plastics waste utilization on the basis of the study of Bauermeister et al. [6].

In Germany the service of refuse disposal by deposition and incineration is payed in the form of "refuse removal charges", but the service of utilization plastics waste is payed by the DSD (Duales System Deutschland) from the charges of the so called "green point". On the basis of this system the material recycling can be economically in spite of the high total costs because of the high selling proceeds. Looking at hydrogenation and gasification, in these processes the costs are the same as in material recycling but here only low selling proceeds are possible, that means for raw-material recycling exists a high gap of rentability. Energetical utilization, i.e. direct incineration of plastics waste has the disadvantage of the higher environmental impact in comparison to the incineration of oil or gas from raw-material recycling processes. Every step to higher environmental compatibility of incineration decreases its energetical usability and is coupled with an appropriate increase of costs.

Hence, incineration of waste, realized as high-tech incineration, does not represent no competition to material utilization, but it is a completion with the goal of

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destroying of pollutants, to transform the non-reusable waste into a material state compatible to the environment. In that light incineration is just as expensive as raw-material utilization. On the other hand, a to high expenditure for separation and cleaning of mixed and polluted plastics waste is always connected with an additional need of a lot of energy. This let us conclude that perfect recycling or perfect material circulation, respectively, is not possible and should not been realized at any price.

The results in industry and scientificalliterature show, that there is no easy way to solve the question for the most satisfying utilization of plastics waste. None of the discussed processes fulfills all requests to a most economical and most ecological process, but the different processes can coexist in an effective way if the main premise is the sustainable development.

5. Environmental and Economical-Political Conclusions and Future Developments

In chapter 3 it was shown that there is a large gap between generated waste amount and quantity of recycled waste. Today the most waste amount is going to deposit, and only 915 000 t plastics waste has been recycled by material recycling. In Western Europe just 2.5 million t plastics waste was re-used by energetical processes. It is assumed that this fraction will increase markedly in future. In Germany the trend is going to the blast furnace process. Today there are costs of about 200 DMlt reducing to zero if the blast furnaces are reconstructed but in chemical (raw material) recycling we have costs of about 500 DMlt [13]. In this connection a planned plant for chemical recycling of 300 000 tla plastics waste by BASF will not been built [14]. The material recycling cannot solve the quantity problem of plastics waste. 1993, from the in Western Europe all over 9.5 million t plastics waste available for utilization, only for 3.6 million t is existing a market and appropriate products under pure technical points of views . Considering the economy this fraction is to set markedly lower yet. Estimations assume that under economical aspects only 2.3 million t plastics waste are at disposal for pure material recycling. It is evident, that other processes are necessary for an efficient utilization of plastics waste. Chemical recycling or utilization in blast furnace are the future alternatives for utilization of large amounts of plastics waste with ecological benefit and ecological compatibility.

Altogether a cascade of plastics waste recycling may be proposed as shown in figure 1:

Mixed and polluted plastics

I Increasing limitation of applicatillfij

Utilization in blast furnace

Landfill

Degree of pollution

Figure 1. Plastics waste recycling cascade

465

In the field of packaging plastics, trends are visible which contribute to a more ecological application of the types of plastics used there. Thus, in Germany in connection with a packaging order the PVC fraction of the packaging plastics is reduced drastically as shown in figure 2. The most used plastics for packaging are PE and PP.

Considering the principle of sustainable development as well as the technological and economical possibilities it is possible to make the following conclusions for realizing a satisfying plastics waste utilization:

1. Avoidance of plastics waste ranks before plastics recycling. This is valid all over for the packaging industry. Optimization of packaging or complete renunciation of needless packaging is necessary.

2. Economical instruments are necessary to promote plastics recycling. Circulation economics is an important contribution to sustainable development. Circulation concepts contra linear materials flow.

466

16 ,---

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Figure 2. Fractions of plastics in packaging before and after substitution (Data acc. [15])

3. Collection of plastics waste as type-clean as possible for material utilization. No material recycling of strong mixed and polluted as well as small-part plastics waste.

4. Construction and design of plastics products following the principle of the best possible approach for dismantling; type clean collection of dismanteled parts and material recycling of these.

5. Decreasing of type diversity of plastics in packaging field. Renunciation of polyvinyl chloride in packaging.

6. Fractions of plastics waste which are not possible or not meaningful to recycle in material (mechanical) recycling processes are used in blast furnace or recycled in feedstock (chemical) recycling. Here hydrogenation has the priority followed by gasification and pyrolysis. Pyrolysis of type clean plastics waste will be suited only in exceptional cases.

7. All plastics waste residues and strong mixed and polluted fractions causing a high expenditure of treatment are going to the energetical utilization.

8. Final treatment and well-ordered deposit of utilization residues.

6. References

1. Schlegel, W. (1994) PVC in Sustainable Development, Paper, Arbeitsgemeinschaft PVC und Umwelt, Bonn

2. N.N. (1992) Reihe "Umweltpolitik", Environmental Ministry ofFRG

467

3. Chapelle, A. (1995) Kunststoff-Recycling in Western Europe, KunststofJe, 10, 1636

4. Koller, H. (1991) Leitfaden Abfallrecht, Berlin, Gesetz uber die Vermeidung und Entsorgung von Abfallen (Abfallgesetz) vom 27.8.1986

5. Gesetz zur Vermeidung von Ruckstanden, Verwertung von Sekundar­rohstoffen und Entsorgung von Abflillen 1993

6. Bauermeister, D., Maiburg, D., Huber, J., Gutzer,W., Vick,S. (1994) Wirtschaftlichkeit und stofflich-okologischer Nutzwert von werkstofflichen, rohstofflichen und energetischen Verfahren der Kunststoffverwertung, PolymerwerkstofJe '94, Merseburg, Proceedings, 526

7. Niemann,K. (1995) Hydrierung, in: Die Wiederverwertung von KunststofJen, J. Brandrup et al. (eds.), Hanser Verlag Munchen Wien

8. Gebauer,M., Stannard, D. (1995) Vergasung von Altkunststoffen, in: Die Wiederverwertung von KunststofJen, J. Brandrup et al. (eds.), Hanser Verlag Munchen Wien

9. Berghoff, R. (1993) Mullverbrennung-Schwelbrennverfahren-Thermoselect­verfahren, Landesamt fur Wasser und Abfall NRW, Dusseldorf

10. Tosch,W., Pollack,H. (1992) PVC und Okobilanzen, in: Z. Umweltchem. akotox. 2,4

11. Niemoller, B. (1995) Reduktion im Hochofen, in: Die Wiederverwertung von KunststofJen, J. Brandrup et al. (eds.), Hanser Verlag Munchen Wien

12. N.N. (1995) Kunststoffe im Hochofen, KunststofJe, 3, 362 13. Wacker, M. (1995) Rohstoffliches Recycling von Kunststoffabfallen in

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bei Verpackungen, DSD, Bonn

GENERAL DISCUSSION - PARTICIPANTS' VIEW

During the last session of the NATO Advanced Study Institute (ASI) on "Frontiers in the Science & Technology of Polymer Recycling", or for short, "Polymer Recycling", a general discussion took place in which all participants were involved. The aim of the session was to provide an opportunity for the participants to express ideas regarding the topic, as well as to generate new insights on the future trends and key issues in Polymer Recycling. The participant's reactions were quite diverse. A large portion of the discusssion was focused on the book that is to result from the AS!. Below, the more interesting ideas advanced during the meeting on the future R&D in Polymer Recycling are summarised.

The discussion was structured according to the principal ASI themes, namely: 1. present situation of plastics recycling, 2. relevant fundamental issues, 3. reprocessing of homo polymers, 4. reutilization of post-consumer plastics, and 5. recovery of energy and chemicals.

Stoiko Fakirov: An important discussion for the near future is the nature of the driving force for plastics recycling. Will it be environmental concern, as seems to be the case in Europe; or just economics, that probably control these activities in the United States?

Tun9 Sava~91: The key issues are collection, sorting and supply of plastics waste to the reprocessors; certainly, to me, these will determine the future of plastics recycling.

Jacob Liedner: An important challenge is to find innovative uses for collected plastics waste. For example, in the near future, compact disks could be a very good source of polycarbonate. However, the success will depend on finding good markets for the reprocessed polymer or its blends.

Wieslaw Sulkowski: I would like to stress the problem of the environmental impact of plastics, which has to be faced at a global scale. For instance, some countries are trying to solve their problems by exporting plastics wastes to other countries which certainly is not an acceptable solution.

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G. Akovali et al. (eds.), Frontiers in the Science and Technology of Polymer Recycling, 469-471. © 1998 Kluwer Academic Publishers.

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Antonio Cunha: Different couuntries have different rules for plastics recycling. This is due to the relative strength of environmental, economic and industrial pressure groups. Even in the European Union I do not believe that this problem can be solved at a global scale.

Stoiko Fakirov: Legislation is certainly an important subject. However, we have no legislators in this room. Why not discuss things more relevant to us: who is going to finance recycling research?

Marino Xanthos: Although we are academicians and technologists from industry, we stilll have a responsibility towards society to discuss the social and legal implications of recycling. I believe that a chapter on plastics recycling legislation in the different NATO countries should be included in the book.

Lezsek Utracki: I suggest that now we have to move to fundamental issues pertinent to recycling. One of the problems in recycling commingled polymers is the variability of composition. One would expect that future trends will be directed toward development of more sturdy methods of blending immiscible polymers. Since addition of compatibilizers is expensive, one may expect that these effords will be directed toward either reactive blending, solid state compounding of a technology that combines these two.

Graham Boden: Should we encourage the use of polymer-bound stabilizers to extend the life time of plastics and produce environmentally friendly materials, or is the expense not justified?

Lezsek Utracki: That could be a good idea, but there are two difficulties: the cost of polymer grafting, and the possibility of neutralization of the polymer-bound stabilizers by additives used in other plastics that are commingled with.

Carlos Bernardo: I will initiate discussions on the next topic, reprocessing of homopolymers, by presenting some ideas for future R&D work in the engineering area. For example, research on the on-line closed-loop control of properties for mixtures of virgin and recycled polymers, and modelling and control of aesthetic properties, such as colour, gloss and finishing; will be important. Significant research will also be devoted to the reprocesing of blends and thermoplastic matrix composites. These will require studies of the effect of proces variables on interfaces, on fibre break-up, on morphology" etc.

Jose Covas: From the engineering point of view, design for recycling already is, and will be, an important issue.

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Jacob Liedner: I think we should now discuss the reprocessing of commingled plastics.

Graham Boden: The tagging of plastics might assist in the identification and sorting of scrap. Has research been done in this field?

Mike Bevis: Four years ago I anticipated in a think-tank in the UK, where many proposals were made on the tagging of polymers. However, although a lot of work was done, no system was implemented!

Erwin Mlecknik: Recycling of thermosets is important for some industries and I think that this should be also included in the book.

Jacob Liedner: I referred to it in my " Separation" lecture. However, the most important is to find the right applications for the recycled plastic waste. Here, imagination is necessary. For example, some years ago in Canada, two companies produced "plastic lumber". One sold it as a substitute for wood and went broke, the other marketed it as a material for transport containers that can be steam-cleanable and made profit.

Guneri Akovali: What messages will there be for the future?

Shigeko Suzuki: We should communicate to the public the message that incineration of plastics waste can be clean.

Voices: Not in my back-yard!

Tun9 Sava§91: Life cycle (cradle-to grave) assessment shows that the use of plastics, including recycling, is less energy consumung than other materials. Let us rather pass this message.

INDEX

Air Separation 320 Andco- Torrax System 419 Antioxidants 93 APME 191 Availability 5

Benefit- Cost Comparision 461 Biodegradables 103, 107,348 Biopol 116 Blend Elasticity 135 Brundland Report 451

Catalytic Extraction Processing (CEP) 404 Chaotic Mixing 139 Chimmassorb 95 Classification 7 Compatibilization 123, 124, 153,339 Compounding 136 Compounders 145 Commingled Polymers 333,347 Combustion 394 Consumption of Plastics 19,30,36 Control of Solid Waste Act 28 Collection 23 Crysttalline Starch 108 Cyrogenic Separation 330

Decree Ronchi 24 Degradation Curves 216 Degradative Extrusion 405 Delphi Survey 44 Disposal Bans 38 Distrubitive Mixing 138 Dynamic Flow 134

Eco- Emballages 22 Economic Model 51, 53

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474

Ecoprofiles 10 Embodied Energy 58,61 Environmental Protection Act 22 Exponential Decay Property Loss 220, 223

Forced Deposit Laws 38 Flotation 321 Flue Gases 394 Fly Ash 443 Full Scale Assessment 293 Future Perspectives 451

Gasification 401 Green Point 21,22,27

Identification 305 Incineration 394,397,402,404,437 Interface 127, 160 Interphase 124, 127 Irganox 93

Labelling 3 10 Lalonde Decree 22 Legislation 45, 272 Life Cycle Assesment (LCA) 10,57, 27[ Linear Law of Mixtures 218 Linear Property Loss 219 Logarithmic Law of Mixtures 222 Lower Critical Solutin Temperature (LCST) 124

Mandatory Coding 39 Microrheology 133 Miscibility 124, 156 Miscibility Window 123 Miscibility Gap 157 Mixers 141 Models 215 Morphology 146, 191 Municipal Solid Waste (MSW) 5

National Packaging Protocol 31

Phase Morphology 171 Phase Interactions 171 Plasma 178 Post Consumer Plastics Recycling 31, 37, 74, 336 Primary Recycling 7,41,215,356 Properties - Reprocessing Relationship 262 Purox System 417, 413 Pyrolysis 408,411

Quaternary Recycling 7, 41, 409

Reactive Agents 163 Regulations 17, 34 Recyclable Blends j45 Recyclate Stabilizer Systems 96 Recovery-Recycling Options 65 Recovery-Recycling- Dispoal Options 67 Reprocessing 33, 76, 79, 243, 249, 266, 271, 276, 371 Residual Value 7 Restabilization 73, 79 Rheology 131 Rio Declaration 451 Rome Treaty 23

Secondary Recycling 7,41,360 Separation 301 Single Pass Property Loss 226 Starch Based Materials 106 Small Scale Assessment 292 Sorting 303 Solvent Separation 323 Sorema 336 Solvolysis 425 Stoker Type 439

Targets 48 Tertiary Recycling 7, 41, 60, 409 Thermoplastic Starch 109 Thermoplastic Starch Composites 110 Thermoset Recycling 367 Thermolysis 407 Tinuvin 94, 95

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476

Tip Spinning 133 Tokyo Declaration 451 Topfer Regulation 23

Upgrading 73 U. V Absorbers 95

Van Oene 194 Viscosity Ratio 205