Recycling of Plastic Materials

Recycling_of_Plastic_Materials/Recycling of Plastic Materials/98038_01.pdfPoly(ethylene terephthalate) Film Recycling

W. De Winter

Agfa-Gevaert N.V., Research & Development, Septestraat,B-2640 Mortsel, Belgium

INTRODUCTIONThe impact of man-made polymers on the environment is a problem of high pri-

ority in most industrialised countries. Mainly due to a build-up of disposedwaste in landfills, and due to campaigns in the press about mistakes made in themanagement of waste treatment, public opinion is focusing on this problem. Thefact that the corresponding percentage by volume is higher, due to the low pack-ing density of wastes, makes the problem more visible. Although plastics con-stitute not even 10 wt% of the total amount of wastes, both residential andindustrial, found in landfills (see Figure 1), public attention to them is increas-ing. A possible explanation1 of such a reaction suggests that there is a lack ofcompatibility of plastics with the environment, despite the fact that the majorityof products used in present daily life are made of materials which have also beenmanufactured by a chemical process.

The plastic waste in landfills consists of about two-thirds polyolefines, andonly ca. 15 % of styrene polymers, ca.10 % of polyvinyl chloride, and less than10% of all other polymers, including poly(ethylene terephthalate) (PET).

The largest use of PET is in the fiber sector. PET film and PET bottles repre-sents only about 10 % each of the total PET volume produced annually.2 It is alsogenerally known that the total ECO-balance, considering energy consumption,atmospheric and water pollution, as well as solid waste content, is by a factor 2to 5 more favorable for PET film than for its greatest competitors in the packag-ing sector, namely glass and aluminium.3

In addition, PET is one of the largest recycled polymers by volume,4 because itis suitable for practically all recycling methods.1 PET recycling by the followingtechnological processes is discussed below:

W. De Winter 1

direct re-use re-use after modification monomer recovery incineration and re-use in a modified way.

In addition, attention will be given to some other attempts for recycling whichhave not been thoroughly evaluated so far, like biodegradability andphotodegradation.

This paper is limited to the discussion of PET-film recycling. A global review ofPET-recycling in the sectors of fibres, films, and bottles was published earlier.2

2 PET Film Recycling

Figure 1. Composition of landfill-waste (domestic and industrial).

DIRECT RE-USEOver 50 % of the PET film produced in the world is used as a photographic

filmbase. The manufacturers of these materials, mainly Agfa-Gevaert, East-man Kodak, du Pont de Nemours, Fuji, Minnesota Mining & Manufacturing,and Konishiroku have long been interested in PET film recovery. An importantmotivation for the efforts made by these companies is the fact that photographicfilms are usually coated with one or more layers containing some amount ofrather expensive silver derivatives, which have been recovered since the early20th century, when cellulosics were used as a film base. Silver recovery makesPET-base recovery more economical.5,6 In a typical way of operation, PET filmrecycling is coupled with the simultaneous recovery of silver, as represented inFigure 2.

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Figure 2. Combined recovery of silver and PET.

In the first step of the process, photographic emulsion layers containing silverare washed with, for example, NaOH, and after separation, silver is recoveredon one side, and cleaned PET-waste on the other side.2 Important in this processis that the washed PET-film scrap is clean enough to be recovered by directre-extrusion, although careful analysis remains necessary.

Direct recycling of PET-waste in the molten state, before re-extrusion toPET-film, is of course the most economical process thinkable, as recoveredPET-scrap can be substituted for virgin PET-granulate without requiring anyadditional steps. It is well-known that PET in the molten state gives rise simul-taneously to polymer build-up and to polymer degradation, so that reaction con-ditions for this process have to be controlled very carefully in order to obtain anend-product with desired physical, chemical and mechanical properties, likecolor, molecular weight, and molecular weight distribution.

A large number of reaction parameters have to be kept under permanent con-trol (temperature, environmental atmosphere, holding time in a melt state,amount of impurities, type of used catalysts and stabilizers, etc.). The order ofaddition of the PET flakes is very important. A typical flowsheet of abatch-PET-process7 is represented in Figure 3. In such a process, the PET-flakescan be added after polymerization, before the melt enters the film extruderscrew (Figure 3, indication 1). Such a procedure, however, has two main draw-backs:

a highly viscous melt is difficult to filter (to eliminate possible gels ormicrogels)

resulting low-boiling or volatile side-products cannot be discarded any-more.

In order to eliminate these disadvantages, several alternative operationmodes have been worked out in the past. A method to add recycled PET during


the esterification step (Figure 3, indication 2) has been described by du Pont.8 Insuch a way filtration can take place in the low-viscosity phase, and volatiles canstill be eliminated during the prepolymerisation phase.

Although PET-recycling by direct re-use is by far the most economical process,it is only useful in practice for well characterized PET-wastes, having exactlyknown chemical composition (catalysts, stabilizers, impurities). Therefore, thisprocess is the most suited for the recovery of in-production wastes, but it maynot be ideal for customer-recollected PET-film. An industrial process for X-rayfilm-recycling was worked out by the IPR-company9 and introduced to the mar-ket under the name REPET on the basis of a triple motivation:

availability of the waste chips on a repetitive basis suitable purity very competitive price.

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Figure 3. Batch process flow sheet.

RE-USE AFTER MODIFICATIONSimilar to the method described under direct re-use, in which PET-flakes are

added during the esterification process, PET-polymer is broken down intolow-molecular, low-viscous fractions. Such method could already be viewed as amethod of re-use after modification. Because the intermediate products are notseparated at any moment of the process, the degree of purity of PET-scrap mustbe high.

For PET-wastes having a higher degree of contamination, other technologicalprocesses are applied, including further degradation by either glycolysis,methanolysis, or hydrolysis,10 yielding products which can be isolated. The prin-ciples of chemical processes on which these methods are based are schematicallyrepresented in Figure 4.


Figure 4. PET degradation by glycolysis, methanolysis, and hydrolysis.

Glycolysis can be considered as a method for direct re-use, whereasmethanolysis and hydrolysis are mainly taken into consideration for monomerrecovery, as discussed below.

The du Pont Company published11 many details concerning the glycolytic recy-cling of PET. Less costly ingredients than those required for hydrolysis ormethanolysis, and more versatility than direct remelt recycling are quoted asthe reasons for glycolysis choice. Goodyear has also developed the PET recyclingprocess based on glycolysis which is called REPETE.12

Glycolytic recycling of PET, which can be done in a continuous or in a batchprocess, is preferentially performed by addition of a PET waste to a boiling eth-ylene glycol, which leads to the formation of low-molecular weight intermedi-ates and eventually to crystallizable diglycol terephthalate (DGT). The rate ofthe degradation reactions is primarily controlled by varying the holding timeand temperature, which depends on a choice of suitable catalysts (e.g., titaniumderivatives),12,13 and by adjusting the PET/glycol ratio. It is also necessary toavoid side reactions which might occur, e.g., by adding buffers or by keepingdown reaction time and temperature.

The low-molecular weight depolymerizates can be introduced directly into apolymerization system,14 preferentially after filtration. In this method, particu-lar care has to be taken in order to avoid glycol ether formation, which may leadto PET of inferior properties. The glycolytic degradation can also be pushed tofurther completion, leading to DGT-recovery, rather than to direct re-use.

In addition to the glycolytic recovery of PET for production of a new PET-film,granulate, or monomer (EG and DGT), alternative methods have been describedfor the preparation of so-called PETGs (i.e., glycol-modified PET), which can beused for different purposes.15,10 Depending on the type of glycol (or polyol) usedfor depolymerization, and on the nature of dicarboxylic acid used for subsequentpolycondensation, the obtained polyester may be used as a saturated polyesterresin (e.g. for films, fibres or engineering plastics), unsaturated polyester resin,mixed with vinyl-type monomers, or alkyd resin, where polycondensation is per-formed in the presence of tri- or poly-functional organic acids.

Although this method for producing unsaturated resin, e.g., for use in regularcastings or in fiber-reinforced laminates, has been thoroughly studied byPET-film manufacturers, it is believed that the method is not currently used inproduction.16

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MONOMER RECOVERYAlthough monomer recovery is the oldest recycling method and can be used to

recover PET-waste having a high degree of impurity, it is regrettable that it isnot the most economical method. The earliest methods of PET synthesizingwere based preferentially on the use of dimethyl terephthalate (DMT), whichcould be better purified than terephthalic acid (TPA), therefore methanolysis isdiscussed before hydrolysis. The chemical principles of both processes are al-ready given in Figure 4.

Methanolysis of PET-wasteThe waste is treated with methanol (in a ratio 1/2 to 1/10), usually under pres-

sure at high temperatures (160-310oC) in the presence of transesterification and(or) depolymerization catalysts.17 Once the reaction is completed, DMT isrecrystallised from the EG-methanol mother liquor, and distilled to obtain poly-merization-grade DMT. Also EG and methanol are purified by distillation. East-man Kodak has been using such a process for recycling of X-ray films for 25years, and it is still improving the process,18 e.g., by using superheated methanolvapor, to allow the use of ever more impure PET-waste. Important factors whichhave to be dealt with in this process are avoiding coloration and keeping downthe formation of ether-glycols.

Hydrolysis of PET-waste19

Although aromatic polyesters are rather resistant to water under atmosphericconditions, compared with other polymers, they can be completely hydrolyzedby water at higher temperatures (and) under pressure. For practical purposes,however, particularly to speed up the process, use has to be made of catalysts.Acidic as well as alkaline catalysts have been studied and worked out in prac-tice.

Figure 5 gives a flow chart of both processes. While both systems are com-pletely realistic, their usefulness under practical production conditions remainscontroversial. As far as acid hydrolysis is concerned, the large acid consumptionand the rigorous requirements of corrosion resistance of the equipment makeprofitability questionable. In addition, the simultaneous (with TPA) regenera-tion of ethylene glycol is difficult, ecologically undesirable (requiring the use oforganic halogenated solvents), and not economical. Concerning alkaline hydro-lysis, the profitability is strongly determined by the necessity of expensive filtra-


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Figure 5. Flow chart of acid- and base-catalyzed PET degradation.

tion and precipitation steps. To our knowledge, recycling of PET-waste byhydrolysis is not practiced on a production scale at present. This situation evenpersists in spite of the fact that the majority of newer industrial PET-synthesisplants are based on the TPA-process rather than on the DMT-process.20

INCINERATIONAnother approach which can be used to recycle plastics, particularly when

they contain a large amount of impurities and other combustible solids (if such isa case, it is important to keep them away from landfills), is more recently calledquaternary recycling, and consists of the energy recovery from the wastes byburning.21 Research along this line has been performed, particularly in Europeand Japan, since the early 1960s. Strong emphasis has been laid on an optimiza-tion of incinerators with regard to higher temperature of their operation and re-duction of the level of air pollution.

PET has a calorific value of ca. 30.2 MJ/kg, which is about equivalent to that ofcoal. It is thus ideally suited for the incineration process. The combustion ofplastics, however, requires 3 to 5 times more oxygen than for conventional incin-eration, produces more soot, develops more excessive heat, and incinerationequipment had to be adapted in order to cope with these problems.

Several processes have been worked out to overcome these technological draw-backs.22-27 Examples include Leidners continuous rotary-kiln process, Balikosprocess for glass-reinforced PET, Crown Zellerbach Corporations combined sys-tem for wood fibre and PET to provide steam to power equipment, and ETH-Zu-richs fluidized bed system for pyrolysis, especially of photographic film, i.e., incombination with silver recovery. The latter system raises the additional prob-lem of the formation of toxic halogenated compounds, stemming from the pres-ence of silver halides.

Typical operation conditions take place at temperatures around 700oC. Atlower temperatures, waxy side-products are formed, leading to clogging. Athigher temperatures, in turn, the amount of the desirable fraction ofmononuclear aromatics decreases. A representative sample, pyrolysed underoptimized conditions, yields, in addition to water and carbon, aromatics likebenzene and toluene, and a variety of carbon-hydrogen and carbon-oxygengases. Studies have been performed1 to avoid formation of dioxines and disposalof residual ashes containing heavy metals and other stabilizers.


be resolved; however, quite a few residual hurdles will have to be taken25 beforean economically feasible and ecologically accepted industrial technical processwill be available.

BIO- AND PHOTO-DEGRADATIONAlthough there certainly has never been a great incentive for making unstable

polymers, the idea of making photo- or bio-degradable polymers has long ex-isted,28,29 and quite a bit of effort has gone into research along these lines. Forsuch a process, of course, limitations with regard to the percentage of allowableimpurities do not exist.

PhotodegradationSpecial photodegradable polymers30 were synthesized for the purpose of hav-

ing them destroyed after use (e.g., in a landfill). Another approach was the incor-poration of suitable groups (e.g., carbonyl) in the polymer backbone in order tomake polymer photodegradable by sunlight or UV (see Figure 6). A problemarises due to the fact that light exposure conditions on a landfill cannot be regu-lated. The main difficulty, however, seems to be practically insurmountable: it is

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Figure 6. Photodegradable monomers and polymers.31

At present it seems that most problems arising during incineration of PET can

hardly possible to combine rapid degradation upon exposure to light in a landfillafter use with a good light-stability of the film during service. This contradictioin terminis is probably the reason why this method never really caught on.29 An-other problem is a combination of desired properties with favorable economics.

BiodegradationThe main difference between biodegradation and photodegradation lies in the

possibility to create in a landfill an environment completely different from thatencountered under normal storage conditions; e.g., microorganisms which candestroy plastic films may be added to a landfill.

In spite of the fact that substantial research time was spent on studies in thisfield, it is claimed32 that surprisingly little is understood about the molecu-lar-level interaction between polymers and microorganisms. This can be ex-plained by a poorly defined environment (in a landfill), and by a large number ofcomplex parameters involved in the process: methods of evaluation based solelyon changes in physical properties are thus unsuitable for forming conclusions,similar to the evaluations based only on biogas production. Specifically for poly-esters, however, a number of interesting data are available. Esterases (ester-hy-drolyzing enzymes) and also some microorganisms are known to biodegradepolyesters at a reaction rate depending upon the polyester structure.29,33 Whilemany aliphatic polyesters, specifically poly(hydroxy fatty acids) - e.g., theBIOPOL34-36 packaging material commercialized by ICI - are suited forbiodegradation, the aromatic polyesters (e.g., PET) do not possess this prop-erty.32,37-39

Another approach consists of mixing small amounts of biodegradable poly-mers, e.g., polysaccharides, with a regular polymer (e.g., a polyolefin), in order tomake the end-product destroyable as well. Examples of polysaccharides/poly-ethylene have been commercialized.38 Mixtures of starch with other polymers,40


including PET, have been studied,34 but no commercialization of the latter mix-ture is known so far. The fact, however, that the starch additive is only needed insmall amounts, which hardly alters the properties of an original polymer, mightshow some promise for future applications. One has to realize, however, that thethermal stability of starch derivatives above 230oC is limited, whereas thePET-film extrusion temperature is in the range of 280oC. There also remainsome controversies about the completeness of the degradation of polymer/starchmixtures.

Although the development of biodegradable plastics is still in progress, it is be-coming evident that the enormous market potential, forecast some years ago, re-quires a real breakthrough in order to be attained.41,42 The main reason for thissetback is probably the fact that organic polymers do not biodegrade fastenough.43,44

CONCLUSIVE REMARKS

From the data presented in this overview, it seems obvious that there ex-ists a clear hierarchy in PET-film recycling technologies. The most impor-tant criteria of classification are, first of all, the degree of purity ofPET-scrap to be handled, and secondly, the economics of the process.

For the cleanest PET grade, the most economical process, i.e., direct re-usein extrusion, is self-explanatory.

For less clean PET samples, it is still possible to re-use them after the modi-fication step (partial degradation, e.g., by glycolysis) at a reasonably lowprice.

More contaminated PET-film waste must be degraded into the startingmonomers, which can be separated and re-polymerized afterwards, ofcourse, at a higher cost. At present, only the methanolysis process is ex-ploited industrially, as opposed to hydrolysis processes, which are kept inreserve.

Finally, the most heavily contaminated PET-shreds have to be incinerated.Here, however, economics may not be favorable enough for industrial de-velopment. As an alternative, those PET-shreds are brought to a landfill.Perhaps in future more attention will be given to modification of PET-filmsin such a way that they may become biodegradable, if the process can be ac-celerated or if a real breakthrough becomes available.

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REFERENCES1 F. P. Boettcher, ACS Polymer Preprints, 32 (2), 114 (1991).2. W. De Winter, Die Makromol. Chem., Macromolecular Symposia No. 57, 253 (1992).3. Anon., Plastics Bulletin, 174, 6 (Jan-1992).4. N. Basta et al., Chem. Eng., 97, 37 (Nov-1990).5. Brit. Pat. 1.476.539 (1977) to Barber-Colman Co.6. Anon., Manufacturing Chemist, 66, (Mar-1987).7. L. Hellemans, R. De Saedeleer, and J. Verheijen, US Pat. 4,008,048 (1977)

to Agfa-Gevaert.L. Jeurissen and F. De Smedt, Brit. Pat. 1,486,409 (1977) to Agfa-Gevaert.J. Tempels, Brit. Pat. 1,432,776 (1976) to Agfa-Gevaert.

8. W. Fisher, US Pat. 2,933,476 (1960) to du Pont.9. J. Burke, in Plastics Recycling as a Future Business Opportunity, Technomic

Publishing Co, Pennsylvania, USA, (1986).10. K. Datye, H. Raje, and N. Sharma, Resources and Conservation, 11, 117 (1984).11. D. Gintis, Die Makromol.Chem., Macromolecular Symposia, 57, 185 (1992).12. R. Richard et al, ACS Polymer Preprints, 32 (2), 144 (1991).13. A. Petrov and E. Aizenshtein, Khim. Volokna, 21 (4), 16 (1979).14. US Pat. 3,884,850 to Fiber Ind.15. Anon., Mod. Plast. Int., 20, 6 (1990).16. A. M. Thayer, Chem. Eng. News, (Jan. 13, 1989).17. Brit. Pat. 784,248 (1957) to du Pont.18. Anon., Eur. Chem. News, 30 (Oct. 28, 1991).19. H. Ludewig, Polyester Fibers, Chemistry and Technology, Wiley Int. Publ., 1971.20. H. Schumann, Chemiefasern Textil, 11, 1058 (1990).

U. Thiele, Kunststoffe, 79 (11), 1192 (1989).21. T. Randall Curlee, The Economic Feasibility of Recycling, Praeger Publishers,

New York, 1986.22. Leidner, Polymer Plastics Techn. & Eng., 10 (2), 199 (1978).23. S. Baliko, Energiagazdalkodos, 28 (11), 496 (1987).24. D. Vaughan, M. Anastos, and H. Krause, Rpt. Battelle Columbus Lab.,

EPA-670/2-74-083, (Dec-1974).25. R. Hagenbucher et al, Kunststoffe, 80 (4), 535 (1990).26. K. Niemann and U. Braun, Plastverarbeiter, 43 (1), 92 (1992).27. W. Kaminsky et al., Chem. Ing. Techn., 57 (9), 778 (1985).28. Guillet, Chem. Eng. News, 48, 61 (May 11, 1970).29. F. Rodriguez, Chem. Techn., 409, (Jul-1971).30. G. Smets, Chem. Magazine, 481, (Sep-1989).31. Brit. Pat. 1,128,793 (1968) to E. Kodak.32. G. Loomis et al., ACS-Polymer Preprints, 32 (2), 127 (1991).33. R. Klausmeier, Soc. Chem. Ind., London, Monogr., 23, 232 (1966).34. Anon., Neue Verpackung, 1, 50 (1991).35. J. Emsley, New Scientist, 50, 1 (Oct. 19, 1991).


36. A. Steinbuchel, Nachr. Chem. Techn. Lab., 39 (10), 1112 (1991).37. P. Klemchuk, Mod. Plastics Int., 82, (Sep-1989).38. J. Evans and S. Sikdar, Chemtech, 38, (Jan-1990).39. K. Joris and E. Vandamme, Technivisie, 179, 5, (1992).40. R. Narayan, Kunststoffe, 79, 1022 (1989).41. N. Holy, Chemtech, 26, (Jan-1991).42. A. Calders, Technivisie, 156, 8 (Nov-1990).43. Anon., Mod. Plast., 20 (1), 72 (1990).44. H. Pearce, Scient. European, 14, (Dec-1990).

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Recycling_of_Plastic_Materials/Recycling of Plastic Materials/98038_02.pdfThe Importance and Practicability of Co-Injected,Recycled Poly(ethylene terephthalate)/Virgin

Poly(ethylene terephthalate) Containers

Eberhard H. Neumann

Nissei ASB GmbH, Mndelheimer Weg 58, D-4000 Dsseldorf 30, Germany

INTRODUCTIONIn several European countries, packaging items, whether they are made of

plastic, paper, metal, etc., are under governmental and public pressure.Well-known are:

actions to ban all plastic bags in one southern European country the boycott of plastic packages in certain alpine villages Denmarks ban of metal cans for beverages Switzerlands removal of all PVC-packages Germanys set-up of a mandatory deposit on beverage bottles made of plas-

tic and limiting sales on non-refillables.The list of restrictions on packages and their markets in Europe could be end-less. Increasing environmental concerns, overloaded landfills and inadequatelyequipped or even not existing garbage incineration units are calling for solu-tions.

Out of many proposals two solutions are always highlighted in public discus-sions:

refillable and returnable packaging articles to reduce the amount of house-hold refuse

recycling of post-consumer packages.Regarding recycling systems for post-consumer plastic bottles, companies

have already installed plants in North America which are profitable operations.However, these systems are leading to a converting technology which trans-forms discarded plastics into a range of second-use commodities, as well as low

E. H. Neumann 17

cost base specialties.This paper is intended to show a technology, whereby discarded plastic bottles

for beverages, food and household items - being post-consumer ware - can beused to manufacture the same range of packaging articles (bottles) for whichthey were originally made. Additional goal is to assure the highest level of safetyoffered by the original products made from virgin plastic.

BASIC TECHNOLOGYInjection molding technology involves the injection of molten plastic into one or

several cavities via a hot-runner system (melt-channel distribution system) andrapid cooling of a preform to a low temperature. At this point the freshly manu-factured article can be ejected from the cavity. In multilayer technology, morethan one plastic resin is injected into the cavity.

The different resins are molten in separate injection units, conveyed in sepa-rate hot-runner channels, under pressure and high temperature, to an injectionnozzle, which is the gate area for the molten plastic, into the cavity.

This injection nozzle consists of an outside and an inside tube. Both plasticstreams are brought together after being released from the nozzle and they bondtogether because of a high pressure (up to 300 bar) and a high melting tempera-ture (see Figure 1).

MANUFACTURING PROCESSOF MULTILAYER BOTTLES CONTAINING REGRIND

For the process, a machine used had one injection unit for virgin PET and an-other injection unit for reground PET flakes. The individual steps of the processare described below.

Drying of PET resin and PET flakes (Figure 2)The drying of a virgin PET resin and reground PET flakes at temperature lev-

els of 160-180oC to below 0.005% moisture content is essential for the productionof amorphous multilayer polyester bottles. Polyester is an effective desiccant.The water absorption depends on a relative humidity, residence time, tempera-ture, and dimension of the flakes.

When flakes containing moisture are heated up to the melting temperature,hydrolytic degradation occurs lowering a viscosity of the melt that results in en-hanced ability of preforms/bottles to crystallize (milky appearance).

18 PET Co-injection Molding

The dryer and the hopper for the virgin PET chips are of standard design. Theunit for the reground PET flakes is of almost the same design but it needs an in-ternal agitator (propeller) which prevents the amorphous PET flakes from stick-ing together. To be exact this is not a problem inherent in the highly stretchedbottle sidewalls (these flakes do not show this property). The problem is ratherthat ground-up flakes come from non-stretched portions of PET bottles, i.e. theneck or bottom.

Above the glass transition temperature (74oC) the flakes will stick together be-cause of adhesional forces. It is therefore necessary to reduce a contact time be-tween the individual flakes and keep them constantly moving in a hopper toavoid the above mentioned sticking process and bridge building in the hopper.This is carried out by the agitator installed inside the hopper. The flakes coming

E. H. Neumann 19

Figure 1. A diagram of an injection molding equipment.

from less stretched bottle regions, i.e. the neck and bottom recrystallize duringthe drying process. Crystallized flakes will not stick together.

The dried reground PET flakes are fed via a feeding-extruder into the throat ofthe satellite injection (2nd and 3rd) unit of the multilayer single stage machine.

An extruder feeder is necessary because PET flakes are bulky and cause prob-lems when they are fed by gravity into the screw of the injection unit.

Co-injection molding of virgin and reground PET flakesThe virgin PET as well as the reground PET flakes are melted inside the sepa-

rate injection units by means of external electrical heating of the injection barreland the applied shear forces of the screw, driven by a hydraulic motor (see Fig-ure 1).


Figure 2. Close-loop drying system for PET regrind

The PET melt, either coming from virgin PET or from PET flakes, is accumu-lated in the head-space of the barrel head and released into the hot runner(melt-channel system) under high injection pressure and at a certainmelt-stream velocity. The melt is kept in a molten stage, inside the hot runnersystem because of the electric heating systems. The two melt-streams (virginand reground PET) are kept, up to the injection nozzle, in separate hot runnerchannels.

At the end of the hot-runner channels, at the gate area of the injection cavity,two nozzles are installed like a double tube or as one tube with a second smallertube inside it.

A portion of a virgin PET melt, forming the outside layers, is first injected intothe preform cavity. Under controlled pressure another melt-portion comingfrom the reground PET is injected into the core of the virgin PET melt cake. Sub-sequently both injection units inject further melt (from virgin and regroundPET) simultaneously during the cavity fill process. This three phase filling pro-cess results in a preform made from the following layers: inside/middle/outside(virgin PET/regrind PET/virgin PET).

During the cavity filling process, the layers do not mix together because the in-dividual melt-layers have a high melt viscosity and are not subjected to a turbu-lent flow.

The adhesion between the individual preform layers is as good as if the layerswere welded together and formed monolayer preform (made out of one meltstream).

Conditioning and stretch-blow-moldingThermal conditioning is the next step of the multilayer preform production.

The purpose of thermal conditioning of a given preform is to provide the neces-sary temperature distribution in a preform. After leaving the injection mold andundergoing cooling process, the preform has a cross-sectional temperature dis-tribution of an upside-down U-shape, which means that the middle of the pre-form wall shows higher temperatures than the two outside skin-layers.

Since PET stretch properties are influenced by the temperature level abovethe glass transition point of PET, the preform requires more even temperaturedistribution, otherwise the middle layer will stretch at a different rate from theskin-layers. Thermal conditioning can be carried out by allowing the preform toequilibrate before stretch-blow-molding or by applying thermal energy from the

E. H. Neumann 21

outside by moving the heater-pots around the preform and/or by allowing aheated core rod to plunge into the center of the preform, thus influencing thethermal profile of a preform wall from the inside.

After thermal conditioning is accomplished, the preform is transferred into theblow mold of the stretch-blow-molding station. Here the preform is axiallystretched by using a stretch rod and circumferentially inflated by air pressure,to match the shape of the blow mold. The final bottle is cooled down due to thecontact heat losses on the metal surface of the blow mold.

The stretch-blow-molding process leads to a biaxial orientation of themacromolecules resulting in better mechanical properties and lowering the gaspermeation of bottles.

Double-layer preformsThe injection molding technique using double layer technology is an alterna-

tive method. By this method the first preform is made of a thin layer of virginPET. In the next step, melt from reground PET flakes is injected into the exte-rior of the preform layer made of virgin PET. Such technology requires two injec-tion preform molds and subsequently a five station machine, compared with astandard machine which has four.

TRIALS OF CO-INJECTING VIRGIN PET AND REGROUND PETFLAKES

The following trials were carried out at NISSEI ASB headquarters in Komoro,Japan.

Quality of the raw materialsVirgin PET had an intrinsic viscosity of 0.78 dl/g and was crystallized and solid

state-polycondensed (upgraded) by the chemical manufacturer. PET regroundflakes came from an unknown PET source. The flakes consisted of 100% PET be-fore the reground bottle flakes went through a separation and clean-up process.The flakes were not regranulated since this step involves separate machineryand would add to the cost of the reground polymer material. The intrinsic viscos-ity was 0.03 to 0.05 dl/g lower compared to that of virgin PET. Such difference isnegligible. Only extreme variation in viscosities of melt-layers can lead to pro-cessing problems. The size of flakes was in general between 9 and 49 mm2.


E. H. Neumann 23

Figure 3. Bottle shape and the thickness of layers.

The trial processingFor the trial approach, a NISSEI ASB-250 T-Series machine (multilayer type)

was used. The processing parameters, as chosen, were within standard set-ups.The bottle shape for these trials and its layer distributionare shown in Figure 3.

Trial resultsThe manufactured bottles were of a high quality. When supplied with a clean

regrind, the transparency was similar to that of comparable monolayer bottles.Physical and mechanical properties of the multilayer bottles were the same asthe monolayer types (creep under CO2 pressure, top-load, impact strength).Standard treatment like drop testing and bottle squeezing did not result indelamination of the layer structure. In conclusion, all trial results indicated thatmultilayer bottles had comparable properties to those having monolayer struc-tures.

COST SAVINGSThe cost saving in PET bottle production by co-injection technology, using an

inexpensive polymer layer, is about 20% as calculated in Table 1. The calcula-tion includes a higher investment cost of machinery for the co-injection technol-ogy as compared to the monolayer bottle production which employs lessexpensive monolayer machinery.

CONTAMINATION ASPECTSSeveral aspects regarding potential contamination of the middle layer made

from the post-consumer PET bottle flakes, are discussed below.

Bacteriological contaminationThe majority of micro-organisms found on PET bottle surfaces is washed away

during the cleaning process of the post-consumer reground PET flakes. The re-maining minor amounts do not survive the drying and processing temperaturein the injection molding unit, which reaches 180 and 270oC, respectively.

Contamination by foreign substancesThere is a possibility that PET bottles are used for storage of substances which

can cause danger to human health. Analysis was conducted, using health-en-dangering substances, to evaluate the effect of PET surfaces exposure. The mi-gration and leaching of these substances were examined. It was found that, even


with strong toxic substances, like pesticides, the migration into PET surfaces isextremely low and furthermore, the re-migration rate (possible leaching of sub-stances) is so low (a small fraction of the migration rate) that the values werewell below the average daily intake specified by FDA.

E. H. Neumann 25

Table 1Manufacturing cost comparison of PET/PET-regrind bottles

Container: round bottle, free-standing Material: PET/regrind Content: 1000 ml

Equipment/operation Output Resin price (DM/kg) Bottle weight(g)Price per bot-

tle (DM)

NISSEI ASB-650 N PET 8cavities (monolayer) 1,371

Virgin-injec-tion-stretch-blow PET3.20

35 0.1767

NISSEI ASB-650 NT 8cavities (multilayer) 1,371

Regrind PET (bottleflakes) 1.30 35 0.1435-0.1501

Machinery type NISSEI ASB

Unit DM/h 650 N (monolayer M/C) 650 NT (multilayerM/C)

650 NT (multilayer

M/C)

1. Equipment cost amortization 48.57 100% 62.17 60% 70% PET-regrind

2. Interest 8% p.a. 3.89 virgin PET 4.97PET-regrind

30% virgin PET

3. Energy consumption 0.15 DM/kWh (main equipment) 11.55 12.30 40%

4. Energy consumption 0.15 DM/kWh (auxiliary equip-ment)

3.75 3.75 virgin PET

5. Rent for space 8 DM/m2 per month: 500 h 0.28 0.35

6. Labour (1/3 operator) 33 DM/h 11.00 11.00

7. Maintenance 20% of machine price 9.71 12.43

8. Entire cost per hour 88.75 106.97 106.97

9. Cost and output per hour (production cost) = DM/PC 0.0647 0.0780 0.0780

10. Total resin cost including scrap = DM/PC 0.112 0.0721 0.0655

Furthermore, the PET flakes are exposed to a melting temperature in the in-jection barrel which would degrade most organic substances in the middle layer.In a new multilayer PET bottle, this middle layer is shielded by layers made ofvirgin PET. The contents of the bottle (food) are therefore separated by a layer ofvirgin PET from the middle layer.

In the case of inorganic toxic substances which might have migrated into thePET surfaces in the ppb-range and furthermore survived the processing tem-peratures, these extremely small fractions will be diluted with the melt cake inthe injection barrel and hot runner system.

CONCLUSIONS

Pressure from environmentalists and legislations require creative solu-tions for re-using of packages. Discarded plastic packages can be repro-cessed into articles similar to the originals.

Co-injection technology, allowing for the injection of a layer of melt ofreground PET flakes into the center of the preform wall, offers a technologyof using low cost post-consumer regrind PET bottle resin.

The layer of the melt of reground PET flakes is completely insulated in themiddle of the bottle and therefore has no contact with the filled-in productsuch as food, nor with an outside environment. Contamination of filled-inproducts is therefore excluded.

Cost savings by this method are about 20%.


Recycling_of_Plastic_Materials/Recycling of Plastic Materials/98038_03.pdfRecycling of Post-consumer GreenhousePolyethylene Films: Blends with Polyamide 6

F. P. La Mantia and D. Curto

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali,Universit di Palermo, Viale delle Scienze, 90128 Palermo, Italy

INTRODUCTIONProblems connected with recycling of polymeric materials arise mainly from

the heterogeneous nature of plastics waste and from degradation of polymersoccuring either during their processing or their lifetime.

As for the first point, important only when mixtures of plastics have to be recy-cled, it is well established that a strong incompatibility is typical for polymersusually found in plastics wastes (PE, PET, PVC, PS, PP). This incompatibilitygives rise to materials which are very difficult to process and/or have inferiormechanical properties. Regarding the degradation experienced by polymericmaterials, the formation of lower molecular weight fractions, possible presenceof crosslinking, and oxygenated groups can further worsen properties of the re-cycled materials.

Polyethylenes are the most popular polymers and as such are the most fre-quently recycled. In particular, films from greenhouses, although highly de-graded by UV radiation, are recycled by means of well-established processes,leading to materials frequently used to manufacture films and molded productswith low mechanical properties.

Even more important problems in the recycling of greenhouse films arise fromthe presence of products of photooxidation, which modify both structure andmorphology of polyethylene, strongly affecting its mechanical properties.1-5 Es-pecially, the presence of low molecular weight chains and of oxygenated groupsaffects the properties of a recycled material.

In previous works,6,7 a possibility of use of photooxidized polyethylene inblends with nylon 6, to improve blend compatibility, was demonstrated.

F.P. La Mantia and D. Curto 27

In this paper, after a short review of the method, the properties and morphol-ogy of blends and coextruded films made out nylon 6 and recycled photooxidizedpolyethylene will be presented.

State of ArtBlends of polyamides and polyolefines are potentially very interesting, but, be-

cause of the strong incompatibility of both polymers, the product of blendingusually has poor properties. Many efforts have been devoted to compatibilizethese blends.8-12 Most of the methods consider functionalized polyolefines whichcan react with the amino groups of polyamides, giving rise to copolymers, stabi-lizing the blend. Such functionalization, in general a long and expensive step, ismostly performed by chemical modification of the polyolefine structure.

In a previous study,6 we have demonstrated that photooxidized polyethyleneoffer similar results.

A low density polyethylene sample with different degree of photooxidation (seeTable 1) was blended with nylon 6 (LDPE 25 wt%). The results of the Molau testare shown in Figure 1. The solution is clear in the case of blend PEPh0/Ny. Thesolution consists of nylon 6, which is soluble in formic acid. The upper part of thesame test tube is a suspension of polyethylene particles. The tests regardingblends containing photooxidized polyethylenes show, on the contrary, an in-creasing and persistent turbidity which represents a suspension of colloidal par-ticles.

28 Greenhouse PE Film Recycling by Blending with PA

Table 1Physico-chemical properties of polyethylene samples

Sample code Photooxidation time* MFI Gel C=O

(h) (g/10 min) (%) (mol L-1)

PEPh0 0 0.08 0 0

PEPh24 24 - 11 0.07

PEPh48 48 - 19 0.11

PEPh72 72 - 25 0.28* photooxidation was carried out using eight UVB fluorescent lamps for 24, 48, and 72 h. The cycle adopted was:

8 h UV radiation at T = 60oC and 4 h condnesation at T = 50oC

Following Molau and other authors,11-13 thepresence of colloidal suspension is attributable tothe existence of LDPE/Ny 6 graft copolymers, act-ing as interfacial agents. In our case, we believethat a chemical reaction between carbonyl groups(formed during photooxidation of polyethylene)and amino groups of nylon 6 occurred during meltblending.

The suspension turbidity increases along withan increase in the photooxidation time of polyeth-ylene, and it is also parallel to the concentration ofcarbonyl groups.

The SEM micrographs of samples of blends con-taining virgin and the most extensivelyphotooxidized polyethylene (Figures 2 and 3) con-firm such an interpretation. In the blend contain-ing virgin polyethylene, the polyamide forms thecontinuous phase and the polyethylene the dis-crete phase. The polyethylene particles have aver-age dimensions ranging from 5 to 10 m m. The


Figure 1. Molau test. Fromleft to right:PEPh0/Ny; PEPh24/Ny;PEPh48/Ny and PEPh72/Ny.

Figure 2. SEM micrograph: PEPh0/Ny. Figure 3. SEM micrograph: PEPh72/Ny.

micrographs of blends containing photooxidized polyethylenes show an almosthomogeneous phase and are hardly distinguishable as a result of gooddispersibility.

Elastic modulus, tensile strength and elongation at break are reported inFigure 4. Tensile strength and elastic modulus increase with an increase in thephotooxidation time, whereas the elongation at break increases as thephotooxidation time increases until it begins to decrease again, which occurs af-ter 50 h of photooxidation. The elongation decrease, as will be explained in thefollowing, can be attributed to a very low value of the elongation at break of amore extensively photooxidized polyethylene.

The SEM micrographs and the results of the mechanical properties are consis-tent with the proposed chemical reactions between carbonyl groups and aminoend-groups forming a graft copolymer. This copolymer acts as interfacial agentbetween continuous and dispersed phases. The presence of such an interfacialagent causes smaller dimensions of the dispersed particles and a good adhesionbetween discrete and continuous phases.

These results suggest the possibility of using degraded (photooxidized) poly-ethylene, from recycling of films for greenhouses, as a functionalized polymer toobtain compatibilized nylon/polyethylene blends.


Figure 4. Mechanical properties of blends. PEPh0/Ny, PEPh24/Ny, PEPh48/Ny, and PEPh72/Ny.

EXPERIMENTAL

MaterialsThe materials used in this work were: virgin polyethylene (V), nylon 6 (Ny),

and two samples of recycled polyethylene (R1 and R2). These latter materialswere obtained from films for greenhouses which were photooxidized to a greatextent.

The main physico-chemical properties of the raw materials are reported inTable 2.

The blends were prepared by melt extrusion in a Brabender laboratory sin-gle-screw extruder (D = 19 mm, L/D = 25) at 100 rpm with a die temperature of260oC. The R2/Ny blend was also prepared by melt mixing of homopolymers inthe same Brabender Plasticorder equipped with a mixer head model W 50 EH at260oC and 100 rpm for 15 min. The mixing time was sufficient to attain a practi-cally constant value of torque. Homopolymers were subjected to the same treat-ment.

All blends were prepared with 80 wt% of nylon 6. For comparison, a blend with20% of nylon 6 was also prepared.

Coextruded films were manufactured by using two layers coextruder Toyplast(Crespi, Italy) with a blowing unit. The coextruder was operated at a flow rate ofabout 20 kg/h. The thickness of the nylon layer was adjusted by the extruderrate, maintaining a layer ratio of about 4.

The die temperature was 215oC for all the runs. Runs were carried out with anaxial draw ratio of 50 and a blow up ratio of 2 for all the blends.


Table 2Raw materials

Sample Supplier Mw Mw/Mn Gel %

V Enichem Polimeri 250,000 7.2 0

R1 - - - 40

R2 - - - 56

Ny Snia 37,000 2.1 -

Structural studiesThe gel content of two samples of recycled polyethylene was determined by

means of a Soxhlet extractor. Approximately 0.3 g of any photooxidized polyeth-ylene sample was extracted by refluxing p-xylene close to its boiling point for 48h.

Samples of all blends, fractured under liquid nitrogen, were observed, using ascanning electron microscope, Philips Model 505. The surface of the specimenswas coated with gold.

Molau tests11 were carried out by dissolving 200 mg of sample in 10 mL of 80 %formic acid.

Mechanical propertiesTensile properties and peel adhesion were determined, using an Instron ma-

chine Model 1122 at room temperature. A crosshead speed of 5 cm/min and agauge length of 3 cm were used to obtain the stress-strain curves.

The specimens used for tensile tests were cut out of sheets obtained by com-pression molding at T = 240oC (180oC for the pure polyethylene). All the reportedresults are an average of at least 10 measurements.

Impact strength was determined at room temperature on notched samples us-ing a Fractoscope (CEAST) in an Izod mode. Ball drop measurements were car-ried out using a Ceast apparatus. Before testing, the specimens wereequilibrated in ambient conditions (T = 20oC and 60% R.H.) for at least 3 days.

RESULTS AND DISCUSSION

BlendsFigure 5 shows the results of the Molau tests of the following blends: V/Ny,

R1/Ny, and R2/Ny prepared by extrusion. The solution is clear for the blendV/Ny, consisting of nylon 6 dissolved in formic acid. The upper part is a suspen-sion of polyethylene particles. The tests of blends containing recycledphotooxidized polyethylene show a turbidity which is due to a suspension of col-loidal particles. As already pointed out, this colloidal suspension is caused bythe presence of polyethylene/nylon graft copolymers formed during melt extru-sion in chemical reaction between carbonyl groups of the recycled photooxidizedpolyethylene and amino groups of nylon 6.


also increases. The R2 sample has a larger amount of C=O groups and thus itsblend shows a more intense turbidity. The results of Molau test of R2/Ny blend,prepared by extrusion and melt mixing, are reported in Figure 6. The blend pre-pared by melt mixing shows a more intense turbidity.

The influence of a composition was tested for blend, having 80 wt% polyethyl-ene. The Molau test (Figure 6) shows that at the low content of nylon, no turbid-ity is observed.


Figure 5. Molau test. From left to right:V/Ny, R1/Ny, R2/Ny prepared by melt ex-trusion.

Figure 6. Molau test. From left to right:R2/Ny by melt extrusion, R2/Ny (Ny =20%), and R2/Ny by melt mixing.

As the degradation degree of the polyethylene rises, the suspension turbidity

sion ranging from 5 to 30 m m. Furthermore, a negligible adhesion between thetwo phases is observed.

The micrographs of the samples with recycled PE show that the dimensions ofthe discrete phase decreases with an increase in the degradation degree of thepolyolefines (Figures 7b and c) and with severity of the mixing process (mixingvs. extrusion) (Figures 7c and d). Moreover, the adhesion improves with the


Figure 7. SEM micrographs: (a) V/Ny, (b) R1/Ny, (c) R2/Ny, prepared by extrusion, (d) R2/Ny pre-pared by melt mixing.

The SEM micrographs of the same samples are given in Figure 7a-d. The blendcontaining virgin PE (Figure 7a) has polyethylene particles of average dimen-

same trend. In particular, the blend with R2 prepared by melt mixing (Figure7d) shows an almost homogeneous phase.

All the above results indicate that the formation of graft copolymers increaseswith:

the photooxidation degree of the polyethylene the content of nylon the intensity of the mixing.The presence of graft copolymers, acting as reinforcing interphase between the

two polymers, improves some mechanical properties of these blends, comparedwith blends which were not compatibilized.

Modulus, E, tensile strength, TS, elongation at break, EB, and impactstrength, IS, for all blends are reported in Table 3.

Modulus and impact strength can be increased by the use of recycled degradedpolyethylene and, for blends with the same components, by increasing the mix-ing intensity (mixer vs. extruder).


Table 3Mechanical properties of polyethylene/nylon blends

Sample E (MPa) TS (MPa) EB (%) IS (J/m)

V/Ny 700 30 140 110

R1/Ny (extr) 800 25 30 120

R2/Ny (extr) 1000 30 10 125

R2/Ny (mix) 1200 46 25 135

Table 4Elongation at break of polyethylene samples

Sample EB (%)

V 490

R1 180

R2 110

Tensile strength is only significantly improved when the blending process isperformed in the mixer. The elongation at break is drastically worsened whenrecycled polyethylene is used. In this case, however, it should be noted that theelongation at break of the polyethylene dramatically decreases with an increasein the photooxidation degree (see Table 4).

The elongation at break of blends R1/Ny and R2/Ny drastically decreases, dueto a low elongation at break of the recycled polyethylene. Nevertheless, theblend with R2 prepared by melt mixing shows a value higher than that of thesame blend obtained by extrusion. This is probably due to the larger amount ofcompatibilizer formed during this severe processing.

The mechanical properties of R2/Ny blend are similar to those reported forblends of nylon with a functionalized polyethylene (FPE) (these latter data aretaken from Ref. 13) (see Table 5). Even if the results relative to this blend havebeen obtained for dry samples, both modulus and impact strength of the blendwith more photooxidized PE are better than those of the blend with thefunctionalized PE, whereas TS and EB are only slightly lower.


Table 5Mechanical properties of compatibilized PE/Ny blends

Sample E (MPa) TS (MPa) EB (%) IS (J/m)

FPE/Ny* 79 52 40 70

R2/Ny (mix) 120 46 25 135*FPE - functionalized polyethylene (taken from Ref.13)

Table 6Mechanical properties of coextruded films

TS (MPa) EB (%)BD (cN) Peeling(cN/m)draw transv draw transv

V/Ny 31 28 240 450 105 80

R2/Ny 21 23 500 440 165 900

Coextruded filmsIn Table 6, the mechanical properties of two coextruded films are reported for

samples cut out in the draw direction and in the transverse direction. The ten-sile properties in the draw direction of the V/Ny coextruded film are signifi-cantly larger than those of the R2/Ny film, whereas in the transverse directionthey are similar. This unusual behavior is probably due to the different rheologi-cal properties of two PE samples. Indeed, the film blowing operation has beencarried out using the same processing parameters, which give rise, in this case,to more oriented film in the case of virgin polyethylene. This hypothesis is con-firmed by the results of the ball drop, which are better for the more balancedR2/Ny film.

The more interesting result is, however, related to the improvement of a peel-ing value for the recycled material. This means that two phases at the exit of thedie adhere much better in presence of photooxidized polyethylene because of in-teraction between the carbonyl groups of the R2 sample and the NH2 groups ofthe nylon.

CONCLUSIONSThe above results show that use of the recycled polyethylene in blends with ny-

lon, giving rise to PE/Ny graft copolymers during processing, improves the me-chanical properties of the resultant material.

The graft copolymers act as compatibilizing agents and the properties of ny-lon-rich blends are very similar to those of blends compatibilized byfunctionalized polyethylene.

A good adhesion between the two layers of coextruded films helps to avoid aneed for the addition of a third layer binding two incompatible phases.

ACKNOWLEDGMENTThis work has been financially supported by MURST 60% and by Plastionica

SpA.

REFERENCES1. J. F. Heacock, F. B. Mallory, and F. B. Gay, J. Polym. Sci., 6, 2921 (1968).2. J. M. Adams, J. Polym.Sci., 8, 1279 (1970).3. M. U. Amin, G. Scott, and L. M. K. Tlillekeratne, Eur. Polym. J., 11, 85 (1975).4. F. P. La Mantia, Radiat. Phys. Chem., 23, 699 (1984).5. F. P. La Mantia, Eur. Polym. J., 20, 10 (1984).


6. D. Curto, A. Valenza, and F. P. La Mantia, J. Appl. Polym. Sci., 39, 865 (1990).7. F. P. La Mantia and D. Curto, Polym. Deg. Stab., 36, 131 (1992).8. F. Ide and A. Hasegawa, J. Appl. Polym. Sci., 18, 963 (1974).9. S. Cimmino, L. DOrazio, R. Greco, G. Maglio, M. Malinconico, C. Mancarella,

E. Martuscelli, R. Palumbo, and G. Ragosta, Polym. Eng. Sci., 24, 48 (1984).10. S. Cimmino, F. Coppola, L. DOrazio, R. Greco, G. Maglio, M. Malinconico,

C. Mancarella, E. Martuscelli, and G. Ragosta, Polymer, 27, 1874 (1986).11. G. E. Molau, J. Polym. Sci., A3, 1267 (1965); Kolloid Z.Z. Polym, 238, 493 (1970).12. G. Illing in Polymer Blends: Processing Morphology and Properties, Eds.,

E. Martuscelli, R. Palumbo and Kryszewski, Eds., New York, 167 (1980).13. H. K. Chuang and C. D. Han, J. Appl. Polym. Sci., 30, 165 (1985).


Recycling_of_Plastic_Materials/Recycling of Plastic Materials/98038_04.pdfRecycling of Plastics from Urban Solid Wastes:Comparison Between Blends from Virgin and

Recovered from Wastes Polymers

E. Gattiglia, A. Turturro, A. Serra,S. Delfino*, and A. Tinnirello*

Centro di Studi Chimico-Fisici di Macromolecole Sintetiche e Naturalie Istituto di Chimica Industriale, Corso Europa 30, 16132 Genova, Italy

* Enichem Polimeri, Centro Tecnologico, Via Iannozzi 1,20192 S. Donato Milanese (Mi), Italy

INTRODUCTIONThe recycling of plastic wastes is not a recent problem for plastic users and pro-

ducers. Since long, industrial scraps are recycled within the production cycle it-self or recovered as lower grade materials and refabricated into new products.On the contrary, plastics recovered from urban solid wastes are being consid-ered only since recent times. A general problem of waste management is becom-ing more and more relevant. Traditional disposal policies such as landfill andincineration are facing increasing opposition due to the ecological drawbacksand rising costs. Recycling, i.e. reprocessing and reshaping of plastic wastes intoa new object seems the logical solution to close the plastic cycle which will befully exploited in the future.

The major problem still facing the viable recycle of plastic wastes is their chem-ical multiplicity: several different polymers are present in plastic wastes andmany of them are mutually incompatible, making the reprocessing of them, as awhole, practically impossible. On the other hand, the separation of the individ-ual plastics is extremely expensive and must consequently be simplified asmuch as possible. Therefore, many separation methods currently used, such asfor instance flotation, cannot provide the individual components but only frac-tions which may contain several different polymers. Such heterogeneous mix-tures are often scarcely compatible, giving rise to processability and quality

E. Gattiglia et al. 39

problems.

By using the flotation method, generally two fractions are obtained from theplastic wastes: a light fraction, floating on water, and a heavy fraction (density>1 g/cm3). The first fraction is essentially made of low and high density poly(eth-ylene) (LPDE and HDPE), polypropylene (PP) and high impact polystyrene(PS); the heavy fraction is formed by polyvinylchloride, cross-linked resins, highmelting thermoplastics.

As well known, polymers of such fractions are incompatible, so the mechanicalproperties of their blends are expected to be poor. Nevertheless, the light frac-tion is at present used to produce different kinds of products. In order to obtainproducts with expected mechanical properties, people often overdesign them orlimit their use to safe applications, therefore reducing market potentiality.

From the fundamental point of view, a multitude of studies on polyolefinblends have been reported in literature: binary blends of LDPE/HDPE andLDPE or HDPE mixed with PP, polyvinylchloride, PS, etc.1-11 as well as ternarymixtures of LDPE/HDPE/PP1,12 have been examined. Several aspects have beeninvestigated, such as crystallization and melting, rheology, mechanical proper-ties and morphology. Since polyolefins represent the large majority of polymersin the plastic wastes, some of the above works were specifically focussed on thewastes.1,3-5

In this study we will report results concerning a wide analysis of:

quaternary blends of virgin LDPE/HDPE/PP/PS, in which LDPE is the ma-jor component, as in the plastic wastes

a light fraction from the urban solid wastes a blend of HDPE with the heavy fraction recovered from wastes as well.13,14

The aims of our research are the following:

to understand why the incompatible polymers of the light fraction give rise,without compatibilizing agents, to a material with useful final properties

to establish the lowest content of the major component (LDPE) in the blendrequired to obtain always a product with good properties

to estimate the influence of possible impurities on the mechanical perfor-mance of the light fraction from the municipal solid wastes

to test a possible use of the heavy fraction as filler for virgin HDPE.

40 Recycling of Plastics from Urban Solid Wastes

EXPERIMENTAL

MaterialsThe virgin polymers used in this study are described in Table 1 as well as the

composition of their mixtures. Note that LDPE is a blend of two LDPEs havingdifferent molecular weights and PS is a high impact PS, containing some elasto-mers selected to improve similarities with real wastes components.


Table 1Some physical characteristics of homopolymers and compositions

of the studied mixtures

Polymer Trade name MFI Density Mixture

(g/10 min) (g/cm3) 1 2 3 4

LDPE Riblene CF 2200 2 0.917 70 63 49 42

LDPE Riblene EF 2200 0.7 0.917 20 18 14 12

HDPE Eraclene H ZB 5515 0.3 0.963 10 9 7 6

PP Moplen X 305 7.5 0.895 - 8 24 32

PS BASF Polystirol 454C 1.53 1.003 - 2 6 8

Table 2Blends of the plastic fractions from urban solid wastes

PolymerComposition wt%

Mix 5* Mix 6

Light fraction

LDPE + HDPE 91 -

PP 3 -

PS 6 -

HDPE Eraclene HZB 5515 - 75

Heavy fraction - 25*composition estimated by thermal analysis

By the flotation method, a light and a heavy fractions were separated from ur-ban plastic wastes in an industrial plant. In Table 2, Mix 5 corresponds to thelight fraction,15 whose composition was estimated by thermal analysis; Mix 6 isa blend of virgin HDPE with 25 wt% of the heavy fraction.

Blend PreparationBefore blending, the light fraction films were ground to pieces of a few mm2 and

the heavy fraction to particles of about 100 m m size. The blends were prepared ina Krauss-Maffei double screw extruder at T = 180oC and a rotation speed of20 rpm. The extrudates were injection molded with a Negri and Bossi nb 55 ma-chine, at T = 220oC in a cold mold and specimens for mechanical tests were pro-duced.

Rheological measurementsThe shear viscosity was measured according to ASTM D3835 in a gas pressure

viscometer at T = 190oC. Die of diameter 0.5 mm and L/D ratio of 10 was used.The melting flow index, MFI, was determined according to ASTM D 1238/c in aKarl Frank model 73694.

DensityThe homopolymer and blend densities were measured using a gradient column

filled with water and isopropyl alcohol in such a proportion as to obtain a densityrange from 1.000 to 0.870 g/cm3, at a room temperature.

Mechanical propertiesThe tensile properties were determined with an Instron Dynamometer model

1122 at a crosshead speed of 5 mm/min according to ASTM D638. The flexuralmodulus was measured in an Arquati Dynamometer AG7E at a speed of 100mm/min and a distance of 50.8 mm according to ASTM D790. All results are av-erage of at least 10 measurements. The IZOD impact tests were performed ac-cording to ASTM D256 on a CEAST pendulum model 6545/000 at T = 30oC, 0oCand 23oC. The last results are average of 15-20 measurements.

MorphologyThe samples were cryogenically fractured and the surface fractures were gold

coated and observed in a Scanning Electron Microscope, model CambridgeStereoscan MK 250.


Thermal analysisThe melting and crystallization behaviors were studied in a Mettler TA 3000

DSC. The non-isothermal crystallization was performed as follows: heating at20oC/min up to 200oC, 3 min. of dwelling time, cooling down at cooling rates vari-able from 30 to 1oC/min.

RESULTS AND DISCUSSION

RheologyThe rheological behavior of polyolefin blends has been widely studied by many

authors. It was found that the shear viscosity exhibits maxima and minimawhen plotted as a function of composition. In general, the viscosity vs. shearstress trends for the binary blends, which present negative (PS/LDPE, PS/PP,PS/HDPE)16-21 and positive (PP/HDPE)22 deviations, cannot be super-imposedinto a master curve by a simple horizontal shift. In other words, the shape of de-pendence changes with composition.


Figure 1. Viscosity of homopolymers vs. shear rate at 190oC.

In our case, also considering the rather complicate composition of the mix-tures, we do not intend to fully study the rheological characteristics of these ma-terials, but simply to check whether such blends may present melt propertieswhich can prevent processability with usual machinery. Figures 1 and 2 showthe viscosity, h , vs. shear rate, &g , of homopolymers and their blends, respectively.The addition of PP and PS reduces the viscosity of the blends in the range of


Figure 2. Viscosity of blends vs. shear rate at 190oC.

Table 3Density and melt flow index measured at 190oC

and calculated by the additivity law

MixtureMFI (g/10 min) Density

experimental calculated (g/cm3)

Mix 1 1.19 1.57 0.9234

Mix 2 1.31 2.04 0.9261

Mix 3 1.81 2.01 0.9261

LDPE CF/LDPE EF = 77.8/22.2 1.71 0.92 -

shear rates under investigation and then increases the value of the melt flow in-dex. In Table 3 experimental MFI values are compared with calculated valuesassuming the additivity law. The experimental values are lower than calculatedsuggesting that weak interactions among components exist, due to a similarchemical structure of blend ingredients. Therefore, it seems that theprocessability of blends does not present particular problem.

DensityIn Table 3 the density values of the Mix 1, 2, and 3 are presented. These values

are about 0.5% higher than those calculated according to the (additivity rule)weighted average of contributions of several components, assuming that the de-gree of crystallinity of crystallizable polymers is not affected by the blending. Itcan be thus deduced that the blends have a compact structure, without holes orvoids.

MorphologyThe morphologies of the fracture surfaces of the Mix 2, Mix 3, and Mix 4 are

shown in Figures 3, 4, and 5. The morphology of Mix 1, not reported here, looks


Figure 3. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 81/9/8/2 blend.




guishable in such composition. As it will be discussed later, HDPE certainlygives rise to homodomains inside the LDPE matrix because it crystallizes beforeLDPE. However, from the point of view of the morphology LDPE/HDPE 90/10mixture will be considered homogeneous.

Mix 2 presents a ductile fractured PE matrix with inclusions of PP and PS noteasily recognizable. The same morphological characteristics were observed inthe case of Mix 5 from plastic wastes. The morphology of Mix 3 (Figure 4) pres-ents clearly visible PS and PP domains. PS domains break with a typical rough,globular surface while PP domains, whose size distribution is broad, on fracture,expose a circular cross-section with smooth surface. On increasing the amountof PP (Figure 5) to 32% (Mix 4) this component almost tends to form a co-contin-uous matrix with PEs; PS is still visible as separated spherical particles.

It is important to notice that in all blends PS and PP domains are well lockedinto the matrix. The interfacial adhesion is good and no voids are observed at thephase boundary. Of course the structures of the blends are not in thermody-namic equilibrium, as can be seen by melting and molding the samples in differ-ent processing conditions. However, no morphological modifications orshrinkage phenomena were observed by annealing of the mixtures at about100oC for more than two months.


Figure 6. Optical micrograph of LDPE/HDPE/PP/PS = 63/7/24/6 blend at 180oC. Parallel Nicols.Magnification 200 .

like a single phase material, meaning that the two components are indistin-

Phase segregation is present also in the melt, as can be observed by the opticalmicroscope. Figure 6 clearly shows segregated PS droplets at 180oC, even at afew percent of PS as in Mix 3. On the contrary, PP domains in the molten stateare visible only when its content is higher than 10 wt%. However, cooling downslowly one can follow the crystallization of PP as shown in Figure 7 for Mix 3 at140oC.

Crystallization behaviorSince the crystallization process is very important in controlling the morphol-

ogy and thus the mechanical properties, we will discuss in more detail the meltbehavior during cooling. In Figures 8 and 9 the effect of cooling rate on the crys-tallization temperature, Tc, of the LDPE, HDPE, and PP pure and in mixture, isshown.

The point of interest here is the fact that for cooling rate higher than 1oC/min.the Tc of HDPE is higher than PP and both are, as known, well above that of theLDPE. Only at very low cooling rates, PP crystallizes before HDPE and probablyat very fast cooling rate (> 40oC/min), they crystallize at the same time. InFigure 8, the crystallization peaks of PP and HDPE components of Mix 2 merge


Figure 7. Optical micrograph of LDPE/HDPE/PP/PS = 63/7/24/6 blend at 140oC. Crossed Nicols.Magnification 200 .


Figure 8. Crystallization temperatures of pure homopolymers and LDPE/HDPE/PP/PS = 81/9/8/2blend as a function of cooling rate.

Figure 9. Crystallization temperatures of pure homopolymers and LDPE/HDPE/PP/PS = 54/6/32/8blend as a function of cooling rate.

multaneous crystallization of HDPE and PP is only due to particular conditionsof cooling and blend compositions. In fact, increasing the PP content (Mix 4) twopeaks occur, when the cooling rate is less than 10oC/min (Figure 9).

A comparison between the crystallization behaviors of the Mix 5, preparedfrom the light fraction of plastic wastes, and Mix 2, from virgin polymers onlyputs into evidence that LDPE of Mix 5 crystallizes earlier than LDPE of Mix 2.This agrees well with the smaller crystalline grains observed in optical micro-scope and may be attributed to some nucleating power of the present impurities.The Tcs of PP and HDPE in the blends are lower than those of the purehomopolymers. On the contrary the Tc of LDPE in the blend is a few degreeshigher than that of the single component and the difference increases on in-creasing the cooling rate. This behavior is understandable considering the fa-vorable effect of the already crystallized HDPE and PP on the nucleation processof the LDPE. Therefore, the LDPE matrix crystallizes always when HDPE andPP are solid and PS is below its glass transition. During this crystallization avolume reduction occurs and the matrix shrinks over the domains of the dis-persed phases clinging them together very solidly. This is the main reason of agood contact between the matrix and the different polymer domains, observed inthe electron microscope analysis.

Mechanical properties

Tensile behavior

Values of tensile modulus, E, yielding stress, s y, tensile strength, s b, and elon-gation at break, e b, of the homopolymers and mixtures are presented in Table 4.Modulus values of the homopolymers scatter by about 5% whereas for blendsby about 8%. The scatter of s b and s y data ranges from 12% for homopolymersto about 18% in the case of blends values; e b results have wider scatter rangingfrom 20 to 27%.

Reducing the amount of LDPE in a blend, the modulus and the yield stress in-crease, whereas s b does not practically change and e b seems to reach a maximumfor the Mix 3 having 70% PE. The Mix 5, prepared with the light fraction of plas-tic wastes, shows practically the same tensile modulus and strength as purehomopolymers (Mix 2); however, samples break before reaching the high defor-mation of Mix 2, probably due to defects created by impurities not completely re-moved during the flotation process. The increase of the E modulus, reducing thepercentage of LDPE, is well below the additivity rule prediction, as shown in


into one and only one Tc is detected in the range of cooling rate examined. This si-

Figure 10, even if the morphology does not reveal any kind of holes or voids be-tween the dispersed phase domains and the matrix.

If we consider the mixtures as a matrix-filler composite in which the matrix isthe LDPE/HDPE blend and the fillers are PP and PS taken together, it should bepossible to compare our mechanical data with the model developed for poly-mer-filler systems by Nielsen. We recognize that this approximation is quitesimplistic, because the difference between the moduli of LDPE and PP-PS com-ponents is not as high as in the case of a polymer matrix and an inorganic filler.Nevertheless, we think that this approach can be attempted making the follow-ing assumptions:

the matrix is a mixture of LDPE and HDPE in the constant weight ratio 9/1for all blends, and its modulus is that experimentally measured for the Mix1 ( 151 MPa)

the filler consists PP and PS in a constant ratio 4/1 and its modulus is takenas the weighted average ( 739 MPa) of the moduli of two components.


Table 4Tensile characteristics of injection molded specimens

Sample E (MPa) s y (MPa) s b (MPa) e b (%)

LDPE CF 119 - 12 613

LDPE EF 143 - 13.8 548

HDPE 640 - 30.4 960

PP 704 - 25.2 720

PS 850 - 15.1 60

Mix 1 151 10.6 13.4 353

Mix 2 196 11.5 13.5 377

Mix 3 264 16.1 14.4 440

Mix 4 330 17.9 13.9 147

Mix 5 190 - 12.0 80

Mix 6 1026 - 22.2 -

For a polymer-filler system, the Nielsen model23,24 is described by the followingequation:

E = E (1+ AB ) / (1 B )b m f ff - f y [1]

where:

A = K f 1E - [1a]

B= (E E 1) / (E E + A)f m f m/ /- [1b]

y = 1+ (1 ) / ( )max max2

f- fF F [1c]


Figure 10. Comparison between experimental and calculated tensile moduli of the mixtures.

and: Eb, Em, Ef are the blend, matrix, and filler modulus, respectively; KE is theEinsteins constant, depending on the geometry and size of the filler particles, asobserved from the SEM morphology; f is the correction factor related to the Pois-sons ratio, n , of the matrix; f f is the volume fraction of the filler; F max is the maxi-mum packing fraction of the filler.

The SEM pictures offer evidence that the geometry of the dispersed phase iscomplex, due to the presence of ellipsoid and sphere shaped domains. We haveapplied the Eq [1] taking into account spheres and ellipsoids with aspect ratior = 4. According to Nielsen,24 KE = 2.5 and f max = 0.60 for random loose packedspheres and KE = 3.08, f = 0.6 for random packed rods or ellipsoids were used.Moreover, f = 0.9 was assumed on the base of the matrix n = 0.4. Figure 10 showsthe experimental data of Eb and the calculated values as a function of the volumefraction, f f, of the filler (PP + PS). The trends are similar and suggest that PPand PS have a reinforcing effect on the PE matrix and the stress transfer at lowstrain is good. However, the experimental values are higher than the calculatedvalues, and a difference between them increases with increasing f f. This must beattributed to the geometrical shape of the dispersed PP domains, which tend tobecome more and more elongated.

Flexural modulus

The flexural modulus and the impact strength are very important propertiesin the field of applications of recovered plastics. Table 5 presents data on the me-chanical properties for single components and blends. The increase of flexuralmodulus by addition of more rigid polymers is evident, confirming that thestress transfer between the phases is good also in the flexural mode.The flexuralmodulus of Mix 5 is practically equal to that of Mix 2 if one considers the experi-mental uncertainty.

Impact resistance

The impact strength, reported in Table 5, shows a very strong dependence onphase heterogeneity and on the presence of rigid inclusions in LDPE. Althoughthe impact properties of the blends are generally decreased, when LDPE contentdecreases, the effect is dramatic at low temperature which requires special at-tention if such application is required. Nevertheless the absolute values are stillacceptable for most applications. Data of mixtures containing less than 50%LDPE (Mix 3 and Mix 4) indicate very poor impact properties suggesting that


50% LDPE is the lower limit for acceptable properties balance. Once again, thedetrimental influence of the impurities is the cause of a low impact resistance at30oC (220 J/m) of Mix 5 compared to Mix 2, which does not break. This suggeststhat more precise separation and washing stages are needed before processingthe blend.

HDPE/heavy fraction blendThe HDPE/heavy fraction blend, 75/25 was examined to evaluate a possibility

of reusing of a heavy fraction as a filler of HDPE to obtain extrudates with ap-propriate rigidity. As indicated before, the heavy fraction was first ground to


Table 5Flexural modulus and impact strength of injection molded specimens

Sample Flexural modulus IZOD (J/m)

(MPa) -23oC 0oC 30oC

LDPE CF 182 n.b. n.b. n.b.

LDPE EF 216 n.b. n.b. n.b.

HDPE 1035 196* 212* 268*

PP 1143 4 24 33

PS 1709 44 53 72

Mix 1 223 58 n.b. n.b.

Mix 2 273 31 78* n.b.

Mix 3 511 19 29* 136*

Mix 4 607 20 24 200

Mix 5 256 - - 220

Mix 6 3381 - - 56n.b. = no break*partially broken

particles of about 100 m m and then blended with HDPE pellets in a double-screwextruder at 180oC. At this temperature, all the components (PVC, PET, nylons,crosslinked polymers, etc.) are still solid in the molten HDPE. Morphological ob-servation by SEM shows that the particles distribution is not homogenous. Thisis probably the main reason for poor performance of a blend at high deforma-tions. In fact, both the tensile and the flexural moduli of HDPE improve signifi-cantly, as shown in Tables 4 and 5, while the strength at break and the impactresistance of the blend are much lower compared to pure HDPE. Perhaps, achange of some parameters of the blending process and optimization of a fillercontent or reduction of a percentage of the heavy fraction offer a possibility toprepare HDPE-based materials, having technological properties suitable forextrudates, like pipes used for general purposes.

CONCLUSIONSAlthough LDPE, HDPE, PP, and PS are incompatible polymers and their

blends show phase separation, the mixtures containing more than 60% LDPEmaintain good mechanical properties.

In general, the introduction of HDPE, PP, PS in LDPE causes an increase instiffness, flexural modulus and strength at break and an obvious reduction inthe ultimate elongation and impact resistance. The latter seems to be the mostcritical characteristic to be evaluated for applications. The reason for this be-havior is explained by a very good adhesion between the LDPE matrix and thedispersed phases, obtained during a melt solidification process of the matrix.The presence of a LDPE matrix as a binding agent is indispensable for a transferof the mechanical stress. As a consequence, with LDPE content decreasing be-low 50 %, the mechanical properties of the material become affected.

The materials can be grossly schematized as composites in which PE is the ma-trix while PP and PS are the fillers, with a good adhesion at the phases inter-face.Within this phase frame, the tensile modulus fits reasonably well to thetheoretical prediction.

Compared to blends made from virgin materials, the real waste recovered plas-tics present obvious problems due to the presence of impurities but the overallproperties are still acceptable.


The properties can be modulated by varying the LDPE content in the mixturewhich allows one to reprocess and reuse the light fraction from plastic wastesfrom various sources whose original composition is outside the proposed opti-mized range.

The possible alternative approach for a reuse of the heavy fraction, as filler ofthe HDPE, can be considered. Approach is promising but needs further work tooptimize the processing steps, with special attention to the optimization of afiller size and blending conditions.

ACKNOWLEDGMENTSSpecial thanks to Dr. S. Astengo for a part of the experimental work and to

Mr. G. Dondero for precious assistance with the Scanning Electron Microscope.

REFERENCES1. R. E. Robertson and D. R. Paul, J. Appl. Polym. Sci., 17, 2579 (1973).2. D. R. Paul, C. E. Vonson, and E. C. Locke, Polym. Eng. Sci., 12, 157 (1972).3. O. Laguna, O. Castellanos, and E. P. Collar, Resources, Conservation and Recycling, 2,

37 (1988).4. O. Laguna, E. P. Collar, and J. Taranco, J. Polymer Eng., 7, 169 (1987).5. O. Laguna, E. P. Collar, and J. Taranco, J. Appl. Polym. Sci., 38, 667 (1989).6. A. P. Plochocki, Polymer Blends, vol. 2, Eds., D. R. Paul and S. Newmann, Academic

Press, New York, (1978).7. A. K. Gupta and S. N. Purwar, J. Polym. Sci., 30, 1799 (1985).8. T. Kyn and P. Vadhar, J. Appl. Polym. Sci., 32, 5575 (1986).9. K. Min, J. L. White, and J. F. Fellers, J. Appl. Polym. Sci., 29, 2117 (1984).10. D. W. Clegg, A. A. Collyer, and K. Morton, Polymer Comm., 24, 10 (1983).11. R. Wycisk, W. M. Trochimczuk, and J. Matlys, Eur. Polym. J., 26, 5 (1990).12. L. Bohn, Rubber Chem. Technol., 41, 495 (1968).13. S. Astengo, Thesis, University of Genoa, (1989).14. A. Serra, Thesis, University of Genoa, (1991).15. C. Perrone, Poliplasti (Milan), 5, 72 (1987).16. Y. Shimomura, J. E. Spruiell, and J. L. White, Polym. Eng. Rev., 2, 417 (1983).17. C. D. Han and J. E. Kim, Trans. Soc. Rheology, 19, 254 (1975).18. L. A. Utracki and M. R. Kamal, Polym. Eng. Sci., 22 (2), 96 (1982).19. C. D. Han and Y. W. Kim, J. Appl. Polym. Sci., 19, 2831 (1975).20. B. L. Lee and J. L. White, Trans. Soc. Rheology, 19, 481 (1975).21. N. Alle and J. Lyngaae - Jrgensen, Rheolog. Acta, 19, 94 (1980).


Cambridge, Mass. (1977).23. T. B. Lewis and L. E. Nielsen, J. Appl. Polym. Sci., 14, 1449 (1970).24. L. E. Nielsen, Mechanical Properties of Polymers and Composites, Vol. 2,

Marcel Dekker, New York, (1974).


22. M. Kasajima, A. Suganuma, D. Kunii, and K. Ito, Proceed. Intl. Conf. Polym. Process.,

Recycling_of_Plastic_Materials/Recycling of Plastic Materials/98038_05.pdfManagement of Plastic Wastes: Technicaland Economic Approach

O. Laguna Castellanos, E. Prez Collar,and J. Taranco Gonzlez

Instituto de Ciencia y Tecnologa de Polmeros, U.E.I.,Tecnologa de Plsticos. Grupo de Ingeniera de Polmeros,

c/Juan de la Cierva, 3, 28006 Madrid, Spain

INTRODUCTIONIt seems that a general agreement has been reached on the real recycling possi-

bilities of plastic wastes, considering the place and the manner in which wastesare generated and restriction to thermoplastic polymers. The basic principles ofrecycling are included in studies conducted during the 1980s which consideredthe technical validity of recycling of various plastic wastes. Most of these studieshave been carried out by corporate research and they are covered by patents orsold in the form of utilities.1-12

Plastic wastes or scraps are generated from two main sources: industrialwastes and post-consumer wastes. The problem of the industrial wastes was ad-dressed from the beginning by the companies which generated them in order toimprove the economics of the process. Present public interest in the environ-mental impact has affected further strategies of big companies. Their knowl-edge on the recyclability of their industrial wastes was applied to solve the earlysteps of the recyclability of the products manufactured by these companies.13-15

It is important to mention that economic aspects play a secondary role under theexpectations of regulations and laws which control recycling of plastic wastes.

The second and the biggest source of plastic wastes generation, which createsthe real problem, is termed post-consumer wastes. Three kinds of wastes aregenerated: municipal, agricultural, and uncontrolled plastic wastes. The lastgroup of plastic wastes must not be considered in the sense of a technical prob-

O. Laguna Castellanos et al. 59

lem because they can only be avoided due to common consent of users of public

facilities. The agricultural plastic wastes can be collected and well classified inthe place of their generation. After their proper characterization, theirrecyclability can be determined. Municipal plastic wastes are the most visible. Asocial cost is inherent in the disposal of all kinds of wastes and so it is generallyrecognized to the extent that wastes can be reduced and treated for reuse or re-cycling in specific ways considered attractive by consumers.

On the other hand, many advantages are derived from the application of poly-mers in the packaging industry, even considering waste disposal:

In economic terms, the costs of the packaging industries would be in-creased by approximately four hundred percent by the weight if non-plasticmaterials were used.

An increase by two hundred and fifty percent by volume of wastes wouldbecome real if plastic packaging materials were not used.

Approximately two hundred percent increase in energy consumption andcosts of materials is feasible for packaging without plastics. As a conse-quence, plastics offer many advantages in packaging materials.

The total amount of plastics present in municipal solid wastes is estimatedto be currently about 7% of total waste.

The present paper shows the scheme followed in the development of a strategyof study and determination of recycling feasibility of the plastic waste fraction.

Recycling of urban plastic wastesThe initial step, i.e., source identification, was previously discussed.1 Once the

source is identified, the first question regards the composition of the source: if plastics are mixed with other materials (glass, paper, organic), a separa-

tion is needed if plastics are dirty (clays or similar contaminants), wastes must be cleaned

if the plastic waste consists of a polymer blend, the situation is much morecomplex.

The problem was studied by comparison between the polymer blend of theplastic waste fraction and polymer blend, having similar composition, but ob-tained from virgin polymers. The technology was applied to the film plasticwaste fraction present in urban solid wastes from Madrid (Spain) and was de-veloped during the 1983-1986 period.

60 Management of Plastic Wastes

The first step of separation of plastic wastes from the overall waste stream wassuccessfully achieved in an industrial plant having a capacity of 7 104 tons/yearwhich was designed by ENADIMSA (Spain) and operated in Madrid accordingto the layout given in Figure 1. The economic evaluation, once the recyclability

O. Laguna Castellanos et al. 61

Figure 1. Flow diagram of treatment and separation plant for municipal solid wastes.

was determined, was made taking into account the actual sale price of thewastes based on the amortization costs and profitability of the plant during the1983-86 period. A preliminary economic study was conducted using a classicmethod based on Engines and Machinery. The results obtained were then takenas starting data for more realistic evaluation, based on the state of the Spanishpolymers market in the 1985-1988 period. Only the main results of these workswill be presented here, with the goal of showing that the plastics recycling busi-ness is also very attractive from an economical point of view.

EXPERIMENTAL

MaterialsThe film plastic wastes were supplied by ENADIMSA from the Municipal

Treatment Plant of Urban Solid Wastes in Valdemingmez (Madrid). The iden-tification of polymers present in plastic wastes was carried out by IR and DSCmethods. The results are compiled in Table 1.

A flotation method was applied in order to separate ninety percent of thepolyolefins present in the film plastic wastes fraction from rejectable materials.The physical properties of virgin polymers supplied by Repsol Qumica S.A.(wastes, and of polymers chosen as interfacial modifiers) are collected inTable 2.

Procedures and UtilitiesThe study of the rheological behavior of wastes and HDPE/LDPE system was

carried out using a torque-rheometer.16

62 Management of Plastic Wastes

Table 1Average composition of the plastic film wastes of urban origin (1985)

Polymer % Remarks

PVC 4 half is removed by flotation in water

HDPE 20

LDPE 68

Solid waste 8 insoluble solids

Blends, having 15/85, 50/50, and 85/15 of HDPE/LDPE ratio, were preparedaccording to an experimental desi

Recycling of Plastic Materials

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