Practical Guide to Latex Technology

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Practical Guide to Latex Technology

Rani Joseph



Practical Guide to LatexTechnology

Rani Joseph

A Smithers Group Company

Shawbury, Shrewsbury, Shropshire, SY4 4NR, United KingdomTelephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118

http://www.polymer-books.com



3.1.7 Other Compounding Ingredients ........................................... 33

3.2 Preparation of Aqueous Dispersions and Emulsions .......................... 33

3.2.1 Dispersion of Water Insoluble Solids .................................... 343.2.2 Evaluation of the Quality of Dispersion ................................ 37

3.3 Preparation of Emulsions ................................................................... 37

References ................................................................................................... 39

4 Dipping and Casting .................................................................................... 41

4.1 Dipping ............................................................................................. 41

4.1.1 Types of Dipping Processes ................................................... 41

4.1.2 Glove Production .................................................................. 424.1.2.1 Batch Dipping Process .............................................. 42

4.1.2.2 Continuous Dipping Process..................................... 43

4.1.3 The Manufacturing Process ................................................... 43

4.1.3.1 Material Inputs......................................................... 44

4.1.3.2 Ceramic Formers/Moulds ......................................... 44

4.1.3.3 Latex Concentrate .................................................... 45

4.1.4 Rubber Chemicals ................................................................. 47

4.1.4.1 Packing Materials .................................................. 474.1.4.2 Compounding ........................................................ 47

4.1.4.3 Coagulant Dipping................................................. 48

4.1.4.4 Latex Dipping ........................................................ 48

4.1.4.5 Beading .................................................................. 48

4.1.4.6 Leaching ................................................................ 48

4.1.4.7 Vulcanisation ......................................................... 49

4.1.4.8 Post Leaching ......................................................... 49

4.1.4.9 Slurry Dip .............................................................. 49

4.1.4.10 Stripping ................................................................ 49

4.1.4.11 Tumbling ............................................................... 49

4.1.4.12 Quality Control ..................................................... 49

4.1.4.13 Glove Packing ........................................................ 50

4.1.4.14 Glove Sterilisation .................................................. 50

4.1.4.15 Finished Gloves ...................................................... 50

4.1.5 Glove Properties .................................................................... 50

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Contents

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4.1.6 Defects and Remedies ........................................................... 51

4.2 Latex Casting .................................................................................... 55

4.2.1 Latex Casting using Plaster Mould ........................................ 554.2.2 Latex Casting using a Metal Mould ...................................... 57

References ................................................................................................... 58

Additional Reading ..................................................................................... 58

5 Latex Foam, Thread and Adhesives ............................................................. 59

5.1 Latex Foam Rubber ........................................................................... 59

5.1.1 The Dunlop Process .............................................................. 60

5.1.2 The Talalay Process ............................................................... 675.1.3 Testing of Latex Foam ........................................................... 71

5.1.3.1 Indentation Hardness Index ..................................... 71

5.1.3.2 Measurement of Dimensions .................................... 71

5.1.3.3 Flexing Test .............................................................. 71

5.1.3.4 Ageing ...................................................................... 72

5.1.3.5 Compression Set ....................................................... 72

5.2 Latex Rubber Thread ....................................................................... 72

5.2.1 Production Process ................................................................ 745.2.1.1 Activation and Maturation of Compound ................ 74

5.2.1.2 Extrusion.................................................................. 75

5.2.1.3 Application of Talcum Powder ................................ 75

5.2.1.4 Ribbon Forming ...................................................... 76

5.2.1.5 Silicone Coated Thread Production .......................... 76

5.2.1.6 Curing of the Thread ................................................ 76

5.2.1.7 Cooling Drums ......................................................... 76

5.2.1.8 Festooning Machine ................................................. 76

5.2.1.9 Final Inspection ........................................................ 77

5.2.2 Technical Specifications of Latex Extruded Rubber Thread ... 77

5.2.2.1 Special Properties ..................................................... 77

5.2.3 Testing of Latex Thread ........................................................ 78

5.3 Latex Adhesives ................................................................................. 78

5.3.1 Formulatory Ingredients for Latex-based Adhesives .............. 80

5.3.1.1 Polymers................................................................... 80

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5.3.1.2 Adhesion Modifiers .................................................. 80

5.3.1.3 Plasticisers ................................................................ 80

5.3.1.4 Crosslinking Agents.................................................. 805.3.1.5 Fillers ....................................................................... 81

5.3.1.6 Tackifiers .................................................................. 81

5.3.1.7 Other Additives ........................................................ 81

5.3.2 Latex-based Adhesives for Paper ........................................... 81

5.3.3 Testing the Quality of the Adhesive ....................................... 82

5.3.3.1 Testing Devices ......................................................... 82

References ................................................................................................... 83

6 Synthetic Lattices ......................................................................................... 85

6.1 Styrene Butadiene Latices .................................................................. 86

6.2 Nitrile Latices .................................................................................... 87

6.3 Polychloroprene Latices ..................................................................... 87

6.4 Polyvinyl Chloride Lattices ................................................................ 88

References ................................................................................................... 89

Abbreviations ...................................................................................................... 91

Index ................................................................................................................... 95

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viii




ix

P

reface

At the invitation of Smithers Rapra Technology Limited, this book was conceived as

a brief introduction to the technology of natural rubber and synthetic rubber lattices.

The intention is not to provide a completely comprehensive text but to give an abridged

version of the technologies for the production of important latex products since there

are many excellent books dealing with the details of such technologies. Latex-basedtechnology forms a sizable fraction of natural and synthetic rubber technology and,

thus, it is presumed that such an introduction to the important technologies would

be beneficial to the technical personnel who use them.

This book begins by giving a short history of natural rubber latex in Chapter 1. Other

topics discussed in this chapter are the tapping of the natural rubber trees and storage

and conversion of natural rubber latex to marketable forms. If natural rubber latex

is to be kept without destabilisation it has to be properly preserved. Furthermore to

produce useful articles from natural rubber latex, the concentration of rubber in thelatex should be almost double that in the field latex. Preservation and concentration

of natural rubber latex is discussed in Chapter 2. Latex compounding has similarities

and dissimilarities with dry rubber compounding. The most widely used latex

compounding ingredients are discussed in Chapter 3. One of the main attractive

features of products manufactured from latex is the low level of energy consumption

compared to dry rubber production. Dipping is one of the most widely used techniques

for manufacture of latex products such as latex gloves, balloons, rubber bands, and so

on. The dipping and casting techniques are discussed in Chapter 4. The technologies

related to three other products, namely, foam, thread and adhesives are discussed in

Chapter 5. A short introduction to important lattices such as styrene-co-butadienerubber, acrylonitrile-co-butadiene, polychloroprene, polyvinyl chloride, and so on,

are discussed in Chapter 6.

The subject matter is illustrated with many photographs from actual production

sites. Mathematical/chemical equations are avoided to make the subject readable

even to the layman

Rani Joseph



x




2


and Malaya (now Malaysia). Malaya was later to become the biggest producer of

rubber. Now Thailand is the biggest producer of natural rubber. One hundred years

ago, the Congo Free State in Africa was also a significant source of natural rubber

latex, mostly gathered by forced labour. Liberia and Nigeria also started productionof rubber during the same period. In India, commercial cultivation of natural rubber

was introduced by the British planters, although the experimental efforts to grow

rubber on a commercial scale in India were initiated as early as 1873 at the Botanical

Gardens, Calcutta. The first commercial Hevea plantations in India were established

at Thattekadu in Kerala in 1902. In the 19th and early 20th centuries, it was often

called ‘India rubber’.

Propagation of Hevea brasiliensis is carried out using seeds or vegetative parts.

During the early days of the rubber plantation industry, propagation was done onlythrough seeds. At present seeds are used only for propagation of root stocks. In some

cases polyclonal seeds are used for propagation. Polyclonal seeds are produced in

polyclonal seed gardens where clones of desirable characteristics such as yield, disease

resistance, and vigour are planted in such a way as to maximise cross pollination

Vegetative propagation is done through vegetative parts like buds, leaves and stem

cuttings. Vegetative propagation of a rubber tree is also done through budding,

which involves the replacement of the shoot system of a plant with the bud of a more

desirable plant [2].

The name rubber was given by Joseph Priestly in 1770 based on the use of rubber toerase lead pencil marks from paper. Later it was used for preparing waterproof gum

fabrics on a commercial scale by Charles Macintosh in 1823.

The rubber tree grows in tropical climates and is now cultivated in many countries.

Asia has continued to dominate the world supply of natural rubber, averaging more

than 90% of the total world production. The top three natural rubber producing

countries in the world are Thailand, Indonasia and Malaysia. Other countries

producing rubber include India, Sri Lanka, Vietnam, Liberia, China and Japan.

The world natural rubber production was 6.8 million tons during 1998-2000. It hasreached about 7.9 million tons in 2010. The annual growth rate would be about

1.3% in the current decade, which is significantly below the 2.9% during the previous

decade. Due to the reduction in the production of natural rubber and the increase in

consumption, the rubber price has shoot up globally.

1.2 Tapping

Tapping is the process of controlled wounding of the tree to extract latex. Latex

vessels run down from right to left and tapping is down from left to right to maximiseproduction which is usually about ½ cup of latex, which is 30-35% rubber and the rest

http://en.wikipedia.org/wiki/Malaysia

http://en.wikipedia.org/wiki/Congo_Free_State

http://en.wikipedia.org/wiki/Africa

http://en.wikipedia.org/wiki/Forced_labour

http://en.wikipedia.org/wiki/Liberia

http://en.wikipedia.org/wiki/Nigeria

http://en.wikipedia.org/wiki/India

http://en.wikipedia.org/wiki/Calcutta

http://en.wikipedia.org/wiki/Kerala

http://en.wikipedia.org/wiki/Kerala

http://en.wikipedia.org/wiki/Calcutta

http://en.wikipedia.org/wiki/India

http://en.wikipedia.org/wiki/Nigeria

http://en.wikipedia.org/wiki/Liberia

http://en.wikipedia.org/wiki/Forced_labour

http://en.wikipedia.org/wiki/Africa

http://en.wikipedia.org/wiki/Congo_Free_State

http://en.wikipedia.org/wiki/Malaysia



3

An Introduction to Natural Rubber Latex

is water. The rubber content varies widely with the age of the tree and the species [3].

Tapping is started when the tree attains an age of seven to eight years. Trees produce

their best yields in the first five years of tapping. The cut must be neither too deep,

nor too thick. Either will reduce the productive life of the tree. Tapping is done earlymorning when it is not raining or the trees are not wet. [Tapping is done for about

150 days for d2 tapping (every other day) and for about 100 days for d3 tapping

(once in every three days) out of 365 days a year]. A tapper taps 400-500 trees in

about three to three and a half hours, takes a break and then collects the latex four

to five hours after tapping. After four to five hours, the latex vessels become blocked

and the latex coagulates leaving a white strip of latex on the tapping cut of the tree.

This is known as ‘tree lace’. Each time before tapping, the tapper must remove the

tree lace. If done carefully and with skill, this tapping panel will yield latex for up to

five years. Then the opposite side will be tapped allowing this side to heal over. Thespiral allows the latex to run down to a collecting cup. The tapping is done at night

or in the early morning before it becomes hot, so that the latex will drip longer before

coagulating and sealing the cut as shown in Figures 1.2, 1.3 and 1.4.

Depending on the final product, additional chemicals can be added to the latex in order

to preserve the latex for a longer period of time. Ammonia solution helps to prevent

natural coagulation and allows the latex to remain in its liquid state for a long time.

This form of latex is used as the raw material for latex concentrates, which are used

for the manufacture of dipped rubber products, latex foam, thread, and so on. Field

latex is used for the manufacture of ribbed smoked sheets (RSS) of different gradessuch as RSS 1, RSS 2, RSS 3, and so on, and high quality technically specified rubber

(TSR) such as Indian standard natural rubber ISNR1, ISNR 3, ISNR 5, and so on (in

India). Naturally coagulated latex in the collecting cup, sometimes referred to as ‘cup

lump’, is collected for processing into block rubber, which is also referred to as TSR.

Figure 1.2 Latex collections after tapping



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Figure 1.3 Tapping of rain guarded rubber tree

Figure 1.4 Tapped latex in the collection cup

1.2.1 Tapping Notations

Tapping notations are sets of symbols and numbers describing the mode of tapping

and its frequency. The notation consists of three parts to indicate:

• Tapping method,

• Panel notation – panel position and type, and

• Yield stimulated tapping notations – stimulation followed for cut tapping and

puncture tapping.



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1.2.1.1 Tapping Method

The notations for tapping methods include notations for symbol of cut, length of the

tapping cut, direction, frequency, and so on. Symbol of cut involves S (spiral cut), V(V-cut), C (circumference, symbol C is used for two or more unspecified cuts on a

tree tapped on the same tapping day) and Mc (mini cut, 5 cm or less in length). The

length of the cut is the relative proportion of the trunk circumference that is embraced

by the tapping cut. It is represented by a fraction such as 1/2S (one-half spiral cut),

1/4S (one-quarter spiral cut), 1/3V (one-third V cut), Mc2 (mini cut in 2 cm), and so

on. No symbol for direction is used when tapping is downward only (for example,

1/2S). For upward tapping, the symbol is an upward arrow (↑) written immediately

after the cut notation (e.g., 1/2S↑). Bidirectional tapping on the same tree is denoted

by both upward and downward arrows (↑↓

). The notation for frequency of tappingdescribes the interval between two tappings and is expressed as one fraction or a

series of fractions such as d/1 (tapping daily), d/2 (every other day), d/0.5 (twice a

day tapping), d/2 6 d/7 (tapping every other day, for six days, followed by one day of

rest), and so on. Change of tapping cut is represented as 1/4S→1/2S (one-quarter spiral

cut tapped downward changed to one-half spiral cut tapped downward). Tapping

intensity values provide parameters for comparison and evaluation of tapping systems.

1.2.1.2 Panel Notation

The panel is the area of bark in which tapping cut is located. Panel notation indicates

the panel position and renewal scission of the panel such as BO-1 (first basal panel

of virgin bark), BO-2 (second basal panel of virgin bark), BI-1 (first renewed bark

of BO-1), and so on.

1.2.1.3 Yield Stimulated Tapping Notations

The complete stimulation notation consists of three parts. The first part denotesstimulant and its concentration; the second, the place of application, quantity of

stimulant and method of application; and the third, the number of applications and

periodicity. Full stops must be inserted between these units to differentiate them

clearly. For example ET2.5%.Pa2(1).16/y(2w)½S↑d/2: stimulated with ethephon (ET)

at 2.5% concentration applied on panel (Pa) with 2 g of stimulant per application

on a 1 cm band in 16 applications per year at fortnightly intervals, half spiral cut

upward once in two days.

Tapping and stimulation notations are presented together as complete notation, with

no full stop inserted between them.



6


1.3 Latex Collection and Storage

Natural rubber is collected from the field as latex and field coagulum. Latex accounts

for 70-80% and the rest is field coagulum. The ratio varies with factors such as climate,stability of latex, yield stimulants used, and so on. The field coagulum includes cup

lump (remains of the latex in the cup), tree lace (remains on the tapping cut), earth

lump (a small amount of latex spilt on the ground which coagulates and is then

usually collected once or twice a month), and so on. The field coagulum is processed

into crepe rubber, estate brown crepe, TSR, and so on.

1.3.1 Latex Collection

Latex flow usually ceases in two to three hours after tapping, except in some very

high yielding crops. The latex from the collection cup is collected in buckets. The

buckets containing latex are always kept exposed to sunlight. Latex has a tendency

to undergo pre-coagulation in four to six hours. This tendency is also related to many

factors such as high magnesium content contamination with water, and so on [4].

Pre-coagulation during storage can be prevented by the use of anti-coagulants such

as formalin, ammonia, sodium sulfite, and so on. Formalin is 40% formaldehyde

in water, normally for preservation, a 2% solution of formalin in water is added

to latex (100 ml/10 litres of latex). Ammonia is normally available as liquefied gas

or as a 25% solution. Ammonia (25% solution; 40 ml) is diluted to with water to

1 litre and 100 ml of this stock solution is added to 10 litres of latex. Sodium sulfite

is available as a white powder of 98% purity. As it is unstable, a freshly prepared

solution has to be used – 100 ml of a 5% solution of sodium sulfite in water can be

added to 10 litres of latex for preservation.

Hardening of natural rubber in storage can be prevented by the addition of

hydroxylamine hydrochloride/hydroxylamine sulfate along with ammonia. It is used

for making constant viscosity or low viscosity rubber. The solution of hydroxylamine

hydrochloride/hydroxylamine sulfate can be prepared by dissolving 50 g ofhydroxylamine hydrochloride/hydroxylamine sulfate in 3 litres of a 1% solution

of ammonia. This mix will be sufficient as an anticoagulant for 100 litres of latex.

A combination of a 0.4-0.5% solution of boric acid mixed with 0.07% ammonia

can act as a long-term anti-coagulant and results in a lighter coloured rubber. The

selection of the anti-coagulant is based on the form to which the rubber is going to be

converted. For example, for sheet rubber, ammonia is preferred because it evaporates

when the rubber is stored in open condition. Excess sodium sulfate may lead to bubble

formation in the sheets and may retard the drying of the sheets. For concentrated latex,

formalin if used as preservative, may later react with ammonia (used for long-term



7


preservation) resulting in the formation of hexamethylene tetramine. For conversion

to TSR, the latex has to be preserved for a long time, which needs a higher dosage of

ammonia. But excess ammonia will require a higher amount of acid for coagulation

for conversion to TSR.

The rubber latex once it is tapped should not be contaminated with bacteria, so it

is of utmost importance that good hygiene should be maintained in the field. This

applies to collection cups, tapping knife, buckets, storage tanks and barrels. Coconut

shell cups are universally used in rubber plantations – they should be clean and free

from fibre. Alternatively plastic cups (700 ml) made of polyethylene are being used in

place of the coconut shell cup. The latex is channeled to the cups by using galvanised

iron spouts. The scrap adhering to the spout has to be removed daily. Buckets made

of galvanised iron are commonly used for the collection of latex. Normally two typesof buckets are used: a small one for collection of the latex from the trees and then a

big one to transport the latex back to the processing centre.

1.3.2 Collection of Field Coagulum

Field coagulum removed from the tapping cut is the tree lace and that from the

collection cup is known as cup lump. Earth scrap is normally collected once in a

month while tree lace and cup lump are collected everyday. These materials are highly

prone to oxidative degradation so they have to be stored and processed correctly.

1.4 Conversion to Marketable Forms

The different marketable forms of natural rubber are:

• Ribbed sheets

• Crepe rubber

• TSR

• Latex concentrate

1.4.1 Ribbed Sheets

Ribbed sheets can be of different types such as RSS, air dried sheets, sun dried sheets,

and so on, depending upon the method of drying. The procedure involves sieving,

bulking, addition of necessary chemicals, coagulation dripping, sheeting and drying.



9


1.4.4 Technically Specified Rubber

Technically specified rubber can be made from latex and field coagulum. The steps

involved are latex sieving, bulking, coagulation, passing the rubber through the crepemaking machine then drying it and then piling it up to convert it to bales. When

the raw material used is field coagulum, the procedure involves soaking in water to

remove impurities and them breaking the coagulum into small pieces using a hammer

mill and then the washed coagulum is dried in an oven and then it is made into bales.

References

1. J.M. Bonner and A.W. Galston, The Botanical Review, 1947, 13, 10, 543.

2. T.K. San in the Proceedings of the RRIM Planters Conference, Eds., J.C. Rajarao

and F.K. Yoon, Kuala Lumpur, Malaysia, 1972, p.59.

3. A.S. Raghavendra in Physiology of Trees, Ed., A.S. Raghavendra, John Wiley

and Sons, New York, NY, USA, 1991, p.403.

4. J.A. Barney in Rubber Research Institute of Malaysia Planting Manual No.13,

Rubber Research Institute of Malaysia, Kuala Lumpur, Malaysia, 1968, p.10.



10




11

Natural rubber (NR) latex is the workhorse of the latex products manufacturing

industry. This was the first rubber latex available for product manufacture and even

now it is the most widely used one, in spite of the availability of a variety of synthetic

rubber lattices, mainly on account of the favorable combination of mechanical

properties, processability, and so on. The fresh latex obtained from the plantation isnot suitable for storage and marketing and, thus, it has to be processed appropriately

to make it suitable for storage, marketing and further processing to finished goods.

The main processing operations are preservation and concentration, details of which

are discussed here.

2.1 Preservation

2.1.1 Chemical Composition of Fresh Latex

NR latex is a colloid, like milk. The dispersed phase is mainly rubber and the dispersion

medium is water containing certain dissolved materials such as carbohydrates, proteins

and mineral matter. A typical composition of fresh latex in weight percentage is given

in Table 2.1.

Table 2.1 Composition of natural rubber latex (NRL)

Constituents Percentage (%)

Rubber 30–35

Proteins 1–1.5

Resins 1–2

Carbohydrates 1

Mineral matter 1

Water 59.5–66.0

The rubber particles are predominantly pear-shaped rather than spherical. The size

of the particles varies between wide limits, the range being 20 to 2000 nm and the

2

Natural Rubber Latex: Preservationand Concentration



12


majority will be nearer to 100 nm. The particle surface is covered by a protein-lipid

envelope, which makes the rubber particles hydrophilic. Fresh latex also contains

non-rubber particles called lutoids, which comprise a fluid substance bound by a

membrane made of proteins. The fluid contains proteins, amino acids and mineralmatter. The lutoid particles are more prone to coagulation by acids. The pH of fresh

latex is in the range of 6.5-7.0.

2.1.2 Spontaneous Coagulation and Putrefaction

Fresh latex coagulates within a few hours of leaving the tree. Overnight storage of

latex results in the formation of a solid mass of coagulum, often larger in volume

than the original. Moreover, putrefaction occurs with the development of a badodour. The main reason for spontaneous coagulation is the development of acidity

through the interaction of microorganisms with the various non-rubber constituents

present in the latex. Some of the bacteria feed on the carbohydrates in latex and

convert them into volatile fatty acids such as formic acid and acetic acid while some

of the other strains of bacteria attack the proteins decomposing them into simple

products. Both of these lead to progressive destabilisation of the latex, which slowly

thickens and finally coagulates. This process is known by many different names such

as premature coagulation, pre-coagulation and auto or spontaneous coagulation.

The decomposition of proteins by bacteria (putrefaction) leads to the formation of

gases such as hydrogen sulfide and sulfur dioxide, which give a bad odour to thelatex coagulum.

The yeast cells present in the latex cause fermentation of the serum constituents such

as carbohydrates and a lot of carbon dioxide gas is produced causing the coagulum

to expand.

2.1.3 Preservatives

If latex is to be stored for a long period of time for further processing and marketing,

it is necessary to prevent spontaneous coagulation and putrefaction. This is done by

the addition of certain chemicals known as preservatives. These materials, apart from

destroying or deactivating microorganisms, very often enhance the colloidal stability

of latex. Ammonia was the first and even now is the most popular preservative for

NR latex. Certain other chemicals are also used along with a lower concentration

of ammonia. These are called secondary preservatives. The basic requirement of

any preservative is that it shall preserve latex against spontaneous coagulation and

putrefaction and, thus, will destroy or deactivate bacteria. It should also contribute

to the colloidal stability of latex, particularly by increasing the magnitude of the



13

Natural Rubber Latex: Preservation and Concentration

electric charge on the particles and the electro-kinetic potential (zeta potential) at

the rubber-serum interface. This is usually achieved by increasing the pH of the latex

and, thus, the preservative should preferably be an alkali. It should also deactivate

or remove traces of metal ions present in the latex. An ideal preservative should becheap, readily available, and easy to handle with no human toxicity and have no

reaction with the rubber or the container material.

2.1.3.1 Ammonia as a Preservative

Ammonia fulfills the entire primary and most of the secondary requirements of

an ideal preservative. It is a good bactericide and is effective at concentrations

above 0.35%. For maximum benefit ammonia should be added to the latex soonafter tapping. Being alkali ammonia enhances the negative charge on the particle

and the zeta potential and, thus, improves the stability of latex. Some of the metal

ions, which affect the stability of latex and the quality of latex are either removed

or deactivated by ammonia. Thus, magnesium is precipitated by ammonia as

magnesium ammonium phosphate, provided enough phosphate ions are also present.

These precipitate sediments as sludge together with any sand or other particles,

which may be present. Copper ions in the latex are deactivated by ammonia by

complex formation.

For effective preservation the concentration of ammonia shall be 0.6% to 1.0% byweight of latex. The colloidal condition of latex is maintained almost indefinitely.

During storage, the higher fatty acid esters present in the latex get hydrolysed into

ammonium soaps which improve the mechanical stability of the ammoniated latex.

2.1.3.2 Low-ammonia Preservative Systems

Though ammonia is the most effective preservative for NR latex, it has certain

disadvantages. The most serious disadvantage is the pungent odour of ammonia,

especially when used at concentrations above the 0.3% level. This causes human

discomfort although ammonia is generally considered non-toxic. Again the formation

of simple salts and soaps through the hydrolysis of serum constituents results in a

progressive loss of zinc oxide stability of latex which is of great importance in

industrial applications. Moreover, the presence of ammonia necessitates the use of

an equivalent quantity of coagulant for subsequent processing. Because of these

problems, preservation systems comprising lower concentrations of ammonia, in

combination with other chemicals, have been introduced. The most important

chemicals used for this purpose include tetramethylthiuram disulfide (TMTD),

zinc oxide, zinc diethyldithiocarbamate (ZDEC), and lauric acid. Of these a system



15


2.1.4.2 Dosage of Preservatives

The efficiency of preservation depends on the quantity of preservative added and

the time lag between tapping and addition of preservative. As field latex containsmore water and non-rubber constituents than concentrated latex, it requires more

preservative. When field latex is to be preserved using ammonia alone for a single

day, 0.4% to 0.5% is sufficient, while for longer periods the minimum concentration

should be 1%. This again depends upon the total solids (TS) content of the latex.

For latex having less than 35% TS, a higher dosage of ammonia may be required.

If a secondary preservative is also used, the concentration of ammonia can be

reduced suitably.

2.1.4.3 Sludge Removal

Sludge (magnesium ammonium phosphate) is formed in latex as a result of the

reaction between the naturally occurring magnesium ions and phosphate ions in the

presence of ammonia. The phosphate ions are formed in latex through the hydrolysis

of naturally occurring phospholipids. But the phosphate ions, thus formed may not

be enough to precipitate all the magnesium ions, especially when the magnesium

availability in latex is higher than normal. In such cases the excess magnesium ions can

be removed from the latex by the addition of diammonium hydrogen orthophosphate.

The sludge, thus formed settles down to the bottom of the bulking tank and when

the latex is removed from the tank through a tap fitted a few centimetres above the

bottom, latex devoid of sludge is obtained.

2.2 Concentration

Concentration of latex is necessary because of reasons such as preference for high

rubber content by the manufacturing industries, economy in transportation and a

higher degree of purity. The process of latex concentration involves the removal of a

substantial quantity of serum from field latex, thus making it richer in rubber.

The methods used for concentration of lattices are evaporation, electro-decantation,

creaming and centrifuging. Evaporation involves the removal of water only and,

thus, the ratio of non-rubber constituents to rubber and the particle size distribution

remain unaffected. On the other hand, the other three processes involve partial

removal of non-rubber constituents and smaller particles of rubber. Because of

this, the particle size range is reduced and a higher degree of purity is obtained.

Although, electro-decantation was developed after the other methods, even now



16


the process is not used commercially. In India, only centrifuging and creaming are

used commercially.

2.2.1 Creaming

2.2.1.1 Principle

In any dispersion, the disperse particles cream or sediment under the influence of

gravity. In the case of latex, the rubber particles being lighter than the serum tend

to cream up. The velocity of creaming depends on a number of factors and can be

deduced approximately from Stoke’s law, stated mathematically as Equation 2.1:

V = 2g (Ds – Dr) r2

/9n (2.1)Where: V is the velocity with which the particles rise (cm/s)

g is the acceleration due to gravity (cm/s2)

Ds is the density of the serum (g/ml)

Dr is the density of the rubber particle (g/ml)

r is the radius of the particle (cm), and,

n is the coefficient of viscosity of the serum (cP)

From Equation 2.1 it is clear that the velocity of creaming can be increased by

increasing the particle size or the difference between the densities of the rubber and

serum or by decreasing the viscosity of the serum. No control over gravitational force

can be exercised when creaming is practiced.

2.2.1.2 Creaming Agents

The most widely used creaming agents for NR latex are sodium alginate, ammonium

alginate or tamarind seed powder. Tamarind seed powder, being cheap and readilyavailable, is the most preferred creaming agent in India. The amount used is 0.1%

to 0.3% of the dry material by weight of latex. The amount actually varies with the

type of creaming agent used, age and the dry rubber content (DRC) of the latex. The

optimum amount is that which produces a cream of the highest DRC, although it

may not produce a serum that is clear. If more than the optimum amount is used, the

induction time is prolonged and the rate of creaming is decreased. If a lower quantity

is used, the rate of creaming is decreased and a low DRC is obtained. In most cases

the optimum quantity is found out by trial and error. Creaming agents generally swell

in water and often produce viscous solutions at low concentrations.



17


2.2.1.3 Creaming Process

Creaming agents are generally added to latex as 3% solutions in water. Sodium

and ammonium alginates are soluble in warm water, while tamarind seed powderis cooked by boiling slurry in water for about one hour. The solutions are sieved

to remove insoluble/uncooked material. For 200 kg of latex, 600 g of tamarind

seed powder cooked in 20 litres of water will be enough in normal cases. Freshly

prepared solutions are preferred. Soaps such as ammonium or potassium oleate or

even commercial washing soaps can be used as secondary creaming agents. A 10%

solution of the soap is prepared and for 200 kg of latex, 100 g of the soap solution

is required. For well-aged latex, the quantity of soap may be reduced.

Ammoniated field latex, which has been stored for a minimum period of three weeks,

is used for creaming. The latex is placed in the creaming tank and the required

quantity of creaming agent and soap are added as solutions and the latex stirred well

for about one hour to ensure homogeneous distribution of the creaming agent in the

latex. The creaming tank is closed and the latex is allowed to remain undisturbed

till the desired level of creaming has been achieved. Although a minimum period of

48 hours is usually required for satisfactory creaming, no fixed time can be assigned

for all conditions. There is an induction period of several hours before any creaming

is visible. After that creaming is rapid and then slows down. When the desired level

of separation has been achieved, the skim layer is drained off through the outlet valve

at the bottom. The cream is homogenised by stirring, the DRC and ammonia contentadjusted and the latex is packed.

The essential equipment for creaming consists of one or more vertical tanks of

appropriate size and a stirrer. The bottom of the tank should preferably have a slope

with an outlet valve fitted at the lowest point. The tank is made of mild steel or

masonry or concrete. If made of mild steel, it should be coated with a bituminous

paint to prevent the latex from coming in direct contact with the metal. If made of

concrete or masonry, the inner side is preferably lined with glazed tiles.

2.2.1.4 Factors Affecting Creaming Efficiency

Other than the optimum quantity of creaming agent, the following factors are also

found to influence efficiency of creaming:

• Age of latex: Creaming takes place rapidly and more efficiently in preserved and

aged latex than in freshly preserved latex. About three weeks’ ageing is preferred.

• Temperature: An increase in temperature improves the efficiency.



19


certain mechanical adjustments. The temperature of the latex up to around 50 °C

increases the efficiency of separation.

2.2.2.2 Concentrating Process

Different models of centrifuges are used for concentration of latex. Of these, the most

widely used type is the Alfa Laval LRB 510 (Sweden). The other popular models are

from Westfalia (Germany) and Westlake (China). The basic design of the different

makes is similar. The machine consists of a rotating bowl in which a set of concentric,

metallic separator discs are enclosed. Latex enters the bowl through a central feed

tube and passes to the bottom of the bowl through a distributor. A series of small

holes on the separator discs, positioned at definite distances from the centre, allowthe latex to get distributed and broken up into a number of thin conical shells within

the bowl, which rotates at a speed of around 600 rpm. By maintaining a very small

clearance between successive conical shells, the maximum distance, which a particle

has to traverse in order to pass from the skim to the cream is made very small.

The degree of ammoniation of the latex prior to centrifuging depends upon the time

since collection and ranges from 0.25%, if centrifuging is immediate, to 1.0% if

the period is two days or more. Only the minimum required quantity of ammonia

should be added as most of the ammonia added to the field latex goes to the skim,

which makes coagulation of skim more difficult. Moreover, if the concentration ofammonia in latex is high, a larger quantity escapes into the atmosphere and causes

human discomfort. Usually latex is ammoniated and kept overnight before being

centrifuged, thus giving time for the sludge to settle down. When the machine runs,

the cream centrifuges inwards to the axis of rotation and then empties from the bowl

through the holes into a stationary gully. The skim flows outwards away from the

axis and leaves the bowl through orifices, which are formed by regulating screws

and passes out of the centrifuge through a second stationary gully. The parts of the

machine, which come into contact with the latex should preferably be of stainless

steel for avoiding contamination and preventing corrosion. The cream is separately

collected in a bulking tank, its ammonia content is adjusted to a minimum of 0.6%

of the latex (0.6 g of ammonia in 100 g of latex) and packed in drums.

Use of a latex clarifier for removing sludge from the latex prior to feeding it into

the centrifuge is being practiced in many countries. The clarifier is a centrifuge of

lower speed, usually around 2800 rpm. The advantages of using a clarifier are higher

output and longer running time for the centrifuge, more efficient sludge removal

and labour savings because it is easier to clean the bowl. Also, storage of field latex

before centrifuging for the purpose of removing sludge can be avoided and a better

assessment of the sludge content of field latex can be made.



20


2.2.2.3 Efficiency of the Process

Efficiency of the centrifuging process is defined as the fraction of the total rubber

recovered as concentrate and may be calculated according to Equation 2.2:

E = Wc x Dc / Wf x Df (2.2)

Where: Wc is the weight of the cream

Dc is the DRC of the cream

Wf is the weight of the field latex, and,

Df is the DRC of the field latex.

The usual efficiency achieved in commercial units is 0.85 to 0.90, meaning that forevery 100 kg of rubber fed into the machine, only 85 to 90 kg is obtained as cream.

The operating variables that affect the efficiency are feed rate, angular velocity of

the machine, length of regulating screws and DRC of the field latex. A reduction in

feed rate of field latex to the centrifuge or an increase in the speed of the centrifuge

bowl causes an increase in efficiency. A shorter screw increases the DRC of the

cream, but reduces efficiency, since the proportion by volume of the input, which

emerges as skim increases. Non-rubber content in the cream will be less. When the

DRC of the field latex is high, efficiency increases. Figure 2.1 shows a commonly

used centrifuging machine.

Figure 2.1 Centrifuge



21


The centrifuging process involves different steps and the first step is latex storage,

which involves the storage of fresh field latex after proper stabilisation. Then the

latex is treated with the required amount of diammonium hydrogen phosphate to

remove the sludge as magnesium ammonium phosphate. The latex from feed tanks isdelivered by spouts to centrifuging nets through a filter net box. After that the latex is

fed in to the machine and concentrated and after all the impurities and skim milk have

been rejected by the centrifuge, the DRC of the latex will be between 60% to 62%.

Stabilised latex is delivered to stored tanks and mixed carefully with some ammonia

and other preservatives. It is stored in tanks for 15 to 20 days to get a product with

good stability. Before and after being stabilised, the latex is checked based on required

standards (see Table 2.2). After the last quality check, latex is pumped into storage

tanks or drums for consumption/marketing.

2.2.3 Skim Latex and Recovery of Skim Rubber

When field latex is centrifuged, a large volume of serum containing a very small

proportion of rubber is obtained as a by-product. This is known as skim latex. As

the efficiency of the centrifuging process is only 85% to 90%, about 10% to 15%

of the incoming rubber goes into the skim. Thus, for economic reasons, it is essential

to recover the rubber present in the skim latex. Normally, the rubber content of

the skim varies between 2.5% and 10%. The average size of the rubber particles is

smaller than that of field latex since larger particles separate more readily into the

concentrate fraction. In addition to the water soluble substances in the serum, latex

contains proteins which are mainly present as an adsorbed layer over the surface of

the particles. As the particles in the skim are relatively small, the protein content per

unit weight of rubber is higher in the skim latex. This not only renders coagulation

more difficult, but also has a marked effect on the properties of the skim rubber. Skim

also contains ammonia, which adds to the cost and difficulty of acid coagulation.

With increasing efficiency of concentration, the DRC of the skim fraction decreases,

making coagulation more difficult.

The usual method of recovery of skim rubber is by spontaneous or acid coagulation.

Coagulation by surface active quaternary ammonium compounds has also been

reported. In spontaneous coagulation, the skim latex is de-ammoniated by bubbling

air through it and it is then kept in tanks for several days for coagulation to

occur by fermentation. In the acid method, coagulation is effected in batches by

the addition of dilute sulfuric acid (20% to 50%). About 1 kg of sulfuric acid

is required for recovering 4 to 5 kg of skim rubber. Ammonium stearate when

added to skim latex can accelerate the formation of skim coagulum after acid

addition [2]. In either case the coagulum obtained is processed into crepe by

conventional methods, taking care to give it a thorough washing. Due to the



22


presence of more serum solids, the rubber obtained from skim latex has inferior

properties. Compounds made from skim rubber are scorchy. Ageing resistance of

vulcanisates made from skim rubber is found to be poor, as the rubber contains

high levels of metallic impurities.

Methods for the production of good quality rubber from skim latex are now available.

One of the methods involves enzymic deproteinisation of skim latex followed

by coagulation and further processing. Another method consists of spontaneous

coagulation followed by alkaline hydrolysis of proteins. Creaming of skim latex

using tamarind seed powder followed by coagulation also improves the quality of

skim rubber.

The latex and skim storage tanks are made of either mild steel or concrete. If it is of

mild steel, the inside of the tank should be provided with a protective coating of an

alkali resistant material such as wax or bituminous paint and if made of concrete/

masonry, the inside should be lined with glazed tiles.

2.3 Quality Standards

Latex is the principal raw material for the manufacture of a number of rubber

products and thus, its quality has to be controlled by widely recognised standards.

NR latex concentrate was being marketed to strict technical specifications even beforethe introduction of any standards for dry NR, because if the quality of latex is poor

it will lead to all sorts of defects in the products such as pinholes in condoms for

example. Whereas a buyer may attempt to assess the quality of a sample of dry NR

by visual inspection, it would be unwise to do so with latex. Visual inspection can

neither reveal the rubber content, which determines the price nor detect any tendency

for instability. The Government of India have amended the Rubber Rules 1955 to

make it obligatory to grade and market solid block rubber and latex concentrate

produced in the country to conform with such standards as are specified by the

Bureau of Indian Standards (BIS) from time to time. The use of the BIS certification

mark is governed by the provisions of the Bureau of Indian Standards (Certification

Marks) Act and the rules and regulations made as part of it. The presence of this

mark on products covered by an Indian Standard conveys the assurance that they

have been produced to comply with the requirements of that standard, through a

well-defined system of inspection, testing and quality control. The BIS specifications

for preserved latex concentrates are given in IS 5430-1981 [3] for centrifuged latex

[types high ammonia (HA), medium ammonia (MA) and low ammonia (LA)], IS

11001-1984 [4] for double centrifuged latex and IS 13101-1991 for creamed latex

(types 1 and 2) [5]. The specifications for centrifuged and creamed latex are given

in Table 2.2 and 2.3.



24


Table 2.3 Requirements for creamed, ammonia-preserved NR latex

Characteristics Type 1 Type 2 Method of test

DRC (% by mass, min) 64 55 IS 3708: Part 1 [7]NRS (% by mass, max) 2.0 2.0 –

Total alkalinity as ammonia

(% by mass, min)

0.67 0.74 IS 3708: Part 4 [10]

Mechanical stability time, (s, min) 650 450 IS 3708: Part 6 [12]

VFA number (max) 0.2 0.2 IS 3708: Part 7 [13]

NRS: Non-rubber substances

2.4 Packing and Despatch

Shipment of latex is done either in tankers or in drums. In India shipment in drums

is more common. BIS has specified the code of packaging of NR latex in drums (IS

5190 [15]). This code prescribes the method of packing and marking of latex in clean,

disinfected and painted drums. Light duty, mild steel 200 litre drums conforming to the

sizes and dimensions given in IS 1783: Part 1 [16] and IS 1783: Part 2 [17], free from

rust and other contamination may be used. Those used previously for other purposes

may also be used provided they are carefully cleaned to make them free from traces

of rust and other contamination. Shipment in tankers has the advantage of savingon the cost of containers and freight. Also, it ensures large, uniform consignments

of latex. But to operate the system, installations are needed at both the shipping and

receiving ends and a distribution system must be organised at the receiving terminal.

Special arrangements must be made with the shipping line to provide tanks, which

are suitably cleaned and surface coated for the transport of latex. These involve

considerable capital expenditure, which cannot be justified unless large volumes are

regularly handled.

References

1. S.N. Angove and N.M. Pillai, Transactions of the Institution of the Rubber

Industry, 1965, 41, 41.

2. T.C. Khoo, C.O. Ong and A.R. Rais in the Proceedings of Rubber Research

Institute of Malaysia, Rubber Growers Conference, Kuala Lumpur, Malaysia,

1991, p.495.

3. IS 5430, Ammonia Preserved Concentrated Natural Rubber Latex, 1981.



25


4. IS 11001, Double Centrifuged Natural Rubber Latex, 1984.

5. IS 13101, Natural Rubber Latex, Creamed Ammonia Preserved , 1991.

6. IS 3708: Part 11, Methods of Test for Natural Rubber Latex – Part 11:

Determination of Magnesium (Direct Titration Method) [NRL: 18], 2001.


Determination of Dry Rubber Content , 1985.

8. IS 9316: Part 3, Methods of Test for Rubber Latex – Part 3: Determination of

Coagulum Content (Sieve Residue) (RL:3), 1987.

9. IS 3708: Part 2, Methods of Test for Natural Rubber Latex – Part 2:Determination of Sludge Content , 1985.

10. IS 3708: Part 4, Methods of Test for Natural Rubber Latex – Part: 4 Natural

Rubber Latex Concentrate – Determination of Alkalinity, 1985.


Determination of KOH Number, 1985.


Determination of Mechanical Stability, 1985


Determination of Volatile Fatty Acid Number, 2005.

14. IS 3708: Part 9, Methods of Test for Natural Rubber Latex: Part 9

Determination of Total Ash, 2005.

15. IS 5190, Code of Packaging of Natural Rubber Latex in Drums, 1993.

16. IS 1783: Part 1, Drums, Large, Fixed Ends – Part 1: Grade A Drums, 1993.

17. IS 1783: Part 2, Drums, Large, Fixed Ends, – Part 2: Grade B Drums, 1988.



27

The different compounding ingredients used in rubber latex can be grouped into curing

agents, sulfur, accelerators, antioxidants, fillers, pigments, stabilisers, thickening and

wetting agents, and other ingredients such as: heat sensitisers, plasticisers, viscosity

modifiers, and so on.

3.1 Compounding Ingredients

3.1.1 Curing Agent: Sulfur

Sulfur is the universal vulcanising agent for natural rubber and also for synthetic rubbers,

which contain olefinic unsaturation in the polymer chain, whether these polymers are in

latex form or in the form of dry rubber. Sulfur is the main vulcanising agent for natural

rubber, synthetic polyisoprene, styrene-butadiene rubber, acrylonitrile-butadiene rubber,polybutadiene rubber, and so on. The crosslinks formed during sulfur vulcanisation

of olefinically unsaturated rubber are of three types: monosulfidic, disulfidic and

polysulfidic. The relative properties of above crosslinks have an implication in the

mechanical and ageing behaviour of vulcanisates. Monosulfidic and disulfidic crosslinks

give better ageing resistance compared to polysulfidic linkage, whereas the initial tensile

properties are better for a rubber vulcanisates with polysulfidic linkage. When the

amount of sulfur used is high, a higher percentage of polysulfidic linkage is formed.

Sulfur to be used for latex compound should be of good quality and easily dispersed

in water. Colloidal sulfur is preferred for latex compounds, which is obtained by a

reaction between hydrogen sulfide and sulfur dioxide in an aqueous medium [1].

Thiurams, for example, tetramethylthiuram disulfide (TMTD) with disulfidic linkage

can be used as a vulcanising agent in olefinically unsaturated rubber in the absence

of elemental sulfur (sulfurless curing). This type of curing is superior to conventional

curing for heat resistance, oxidative aging resistance, and so on.

Butyl xanthogen disulfide (at 4 phr) in presence of zinc oxide can be used for

vulcanising rubber latex without elemental sulfur. Vulcanisate properties of thissystem are inferior to those obtained using the thiuram system.

3

Latex Compounding Ingredients



28


3.1.2 Accelerators

The rate of sulfur vulcanisation can be increased by the addition of accelerators. The

most important class of accelerators used in latex industry are metallic and aminedialkyl dithiocarbamate, thiazoles and thiurams function as secondary accelerators.

Dithiocarbamates are a class of accelerators used as primary accelerators in latex

compounds. It can be in the form of alkali metal salts such as sodium diethyl

dithiocarbamate (SDC) or zinc salts such as zinc dimethyl dithiocarbamate (ZMDC).

An important difference between the ammonia and alkali metal salts compared to the

polyvalent metallic ions is that the former are soluble in water, whereas the latter are

not. Water insoluble solids are incorporated in latex as dispersions in water. Table 3.1

shows the preparation of a sulfur dispersion.

Table 3.1 Preparation of sulfur* dispersion (50%)

Ingredient Parts by weight

Sulfur 100

Dispersol F Conc 4.0

Distilled water 96.0*Ball milled for 72 hours

Commonly used dialkyl dithiocarbamates in latex compounds are zinc diethyldithiocarbamate (ZEDC), SDC, and piperidinium pentamethylene dithiocarbamate [2].

The accelerating activity of various dithiocarbamates differs considerably. ZDEC is of

intermediate activity and it tends to cause gradual thickening in ammonia preserved

natural rubber latex (NRL) under normal storage conditions due to the slow liberation

of zinc ions. An exception to this rule is zinc pentamethylene dithiocarbamate. Latex

films turn brown in the presence of dithiocarbamate and copper due to the formation

of copper dithiocarbamate.

Xanthates are very reactive accelerators. They are active even at room temperature.They are somewhat unstable and are invariably accompanied by a bad odour. This

may be due to a small amount of carbon disulfide, which is evolved during their

decomposition on storage. Alkali metal xanthates are water soluble whereas heavy

metal salts are insoluble. Typical examples are zinc isopropyl xanthate, sodium

isopropyl xanthate, zinc-n-butyl xanthate.

Thiazoles are used as secondary accelerators in combination with dithiocarbamates.

They impart lower compression set and higher modulus and load bearing capacity.

Two thiazoles which are most commonly used in latex compounding are sodium

mercaptobenzthiazole (SMBT), and zinc mercaptobenzothiazole (ZMBT). SMBT



29


are usually prepared by dissolving mercaptobenzothiazoles in a slight excess of

sodium hydroxide solution. The most suitable thiazole accelerator for latex work

is the water insoluble ZMBT. This may be prepared by a reaction between sodium

mercaptobenzthiozle solution and zinc sulfate solution. ZMBT can be used in placeof ZDEC to get desirable technological properties such as high modulus, and so on.

Thiurams are used as a secondary accelerator along with dithiocarbamates. Some

typical examples are tetramethylthiuram monosulfide (TMTM), tetraethylthiuram

disulfide, dipentamethylenethiuram disulfide (DPTD), dipentamethylenethiuram tetra

sulfide. All these accelerators are insoluble in water and so they are incorporated

in latex as dispersions. The cure with these accelerators is comparatively slow but

their activity can be improved by incorporating sulfur bearing compounds such as

thiourea in the compound.

3.1.3 Antioxidants

The ageing characteristic of rubber latex vulcanisates is better compared to dry rubber,

since it is not subjected to any degradation during processing (in latex processing there

is no mastication or exposure to high temperatures). For NRL products the aging

resistance is further improved by the presence of naturally occurring rubber constituents

which function as antioxidants. Similarly some of the vulcanisation chemicals such as

ZDEC/zinc mercaptoimidazole, and so on, also improves aging resistance.

Two types of antioxidants are used in rubber compounding: the amine type and the

phenolic type.

Amine type antioxidants cause discoloration/staining on ageing and because they are

resinous in nature it is difficult to disperse them in the rubber latex.

Phenolic antioxidants are the most commonly used in latex, compounding examples

are styrenated phenol (SP), substituted cresols, and so on. Water insoluble liquid

antioxidants are incorporated in to the latex as an emulsion in water. The emulsifiedantioxidant droplets are adsorbed on to the rubber particles as the compound matures.

Even if this does not occur they will be expected to disperse rapidly in the rubber

phase when the latex is dried down to form a solid deposit.

3.1.4 Fillers and Pigments

Inorganic fillers and pigments are added to the latex in order to make it less expensive

and to stiffen the product or to colour it. These fillers don’t have any reinforcing effectwhen they are added to latex as they do in dry rubber. If the compounded latex with



31


3.1.4.5 Barytes

Barytes is precipitated barium sulfate, which has been used with NRL to give filled

compounds with good extensibility and elongation at break. The main disadvantageof this pigment is its tendency to sediment rapidly. This is because of its high specific

gravity.

3.1.4.6 Carbon Black

Carbon black is used as black pigment in latex compounding. The carbon blacks

are added to latex in the form of dispersion or slurries after adjustment of the pH to

alkaline. Wet ground mica is also used as a filler in latex compounding.

3.1.5 Stabilisers

Surfactants: These are substances which lower the surface free energy against air and

aqueous media, along with interfacial free energy against immiscible organic liquids.

One method to classify these agents is based on function:

• Wetting agent

• Dispersing agent

• Dispersion stabilisers

• Emulsifiers

• Foam promoters and foam stabilisers

The disadvantage of this classification is that there exists a considerable degree of

overlap among different categories. For example, potassium oleate is classified asa foam promoter for latex foam and as a stabiliser for synthetic and NRL. Most

surfactants are tolerably efficient in the majority of functions but may be outstandingly

efficient in just one respect.

Chemically surfactants are classified as amphoteric, anionic, cationic, and non-ionic,

and so on, depending upon the active entity present.

Anionic surfactants: In this case the surface activity is attributed to the anion –

examples are carboxylates, sulfates, sulfonates, and so on.



32


Unsaturated straight chain aliphatic carboxylates derived from oleic acid find

application as colloidal electrolytes in emulsion polymerisation. The oleates are

also used as an emulsifier in water immiscible oils, and as a foam promoter in the

manufacture of latex foams.

Fugitive soaps: These are ammonium soaps. They lose free amines by vulcanisation.

Ammonia is the most volatile base but is rather too volatile for some applications,

alternatives include morpholine and triethanolamine. Rosin acid soaps also find

applications as emulsifiers, colloidal electrolytes and foam promoters.

Sulfonates: are much more sensitive to acids and heavy metal ions than are the

carboxylates. They mainly function as wetting agents and examples of this group are

sodium diisopropyl naphthalene sulfonate, sodium dibutyl naphthalene sulfonate,

and so on. A well-known compound sodium naphthalene formaldehyde sulfoxylate

is prepared by reacting two molecules of sodium naphthanate sulfonate with

formaldehyde. This substance is a deflocculating and dispersing agent and it finds

application in preparing dispersions of insoluble powder. Sodium salts or esters of

sulfonic acid are another group and they find application as wetting and dispersion

stabilisers.

Sulfate: In general, substances of this class are all strongly surface active and find

application as wetting agents and dispersion stabilisers. The typical examples of

straight chain alkyl sulfonates include sodium dodecyl sulfate, sodium hexadecylsulfate and a mixture of them.

3.1.6 Thickening and Wetting Agents

Thickening agent: It is frequently necessary to increase the viscosity of latex

compounds. Thus, dipping mixes may require to be thickened so that thicker deposits

of rubber are obtained or spreading mixes are thickened to prevent the latex from

striking through the fabric.

Latex compounds may be thickened in two ways: (i) by filling the mix or (ii) by

adding thickening agents. The tolerance of latex for these fillers is limited and their

addition may produce undesirable effects in the rubber. It may prove necessary,

therefore to add thickening agents, among which a wide range of natural products

are available, e.g., gums, casein, glue and gelatine. These are all somewhat

unpredictable in effect, are subject to bacterial attack and although they may

cause high initial increase in viscosity, this effect decreases on prolonged storage.

Furthermore, they have marked effects on the ‘handle’ of the rubber article and on

its resistance to water.



34


be realised, however, that if this procedure is adopted it will be found that the time

required to produce a reasonable dispersion of the most difficult ingredient, is the

same as that required to reduce the particle size of the most intractable component.

For example it is difficult to prepare a sulfur dispersion and so it is given 72 hoursof ball milling so if sulfur is also mixed together with other chemicals then at least

72 hours has to be given so that a good dispersion of sulfur together with other

ingredients is obtained. For the production of high quality ‘pure gum’ rubber articles,

it is recommended that all dispersions be prepared individually.

Dispersions prepared individually should be mixed together prior to addition of latex

since, even though no obvious flocculation is apparent, the particles may aggregate

and the quality of the mix be impaired.

3.2.1 Dispersion of Water Insoluble Solids

The treatment required to produce high-quality dispersions of water-insoluble solids

depends on the physical nature of the materials. Those which have been prepared by

drying from a colloidal state (e.g., clays) do not usually require prolonged milling.

Their primary particles are small but have aggregated and can sometimes be re-

dispersed merely by stirring with water containing a dispersing agent, followed by

passing the paste through a colloid mill or by brief ball milling.

Other materials, however, require actual grinding of the particles and for this purpose

ball or gravel milling is necessary, the later being reserved for difficult materials such

as sulfur.

Ball milling: The container is rotated about its cylindrical axis in a horizontal

plane at such a speed that the charge is tumbled. In this example of the ball mill

(Figure 3.1), the grinding charge consists of unglazed porcelain or glass balls, their

size being governed by the diameter of the container. A small laboratory mill may

use balls of 1.2–1.5 cm diameter. Larger mills require balls of an average diameter

of about 2.5 cm.

The rate of grinding by the mill is related to the diameter of the container. If the mill

rotates too rapidly, centrifugal force will cause the charge to adhere to the container

walls and no grinding results.

Large mills must rotate more slowly than small ones and the following table of

optimum speeds assists the operator in arranging the milling operation. Slower speeds

may be used but the time of milling will be extended since the grinding is achieved

by a definite number of rotations of the mill. Table 3.2 shows the ball mill size and

suggested speed of operation.



35


Figure 3.1 Typical ball mill

Table 3.2 Ball mill size and suggested speed

Internal diameter of mill

container (cm)

Suggested speed of

container (rpm)

10 93

15 76

20 66

25 59

30 53

38 48

46 44

53 41

61 38

Colloid mill: A colloid mill essentially consists of two circular plates, one of which is

stationary and the other is rotating at a very high speed (1,000 to 20,000 rpm). The

clearance between the two plates is generally adjusted to within 0.025 to 0.200 mm.

Most colloidal mills are provided with water cooling devices to prevent overheating

of the material being dispersed.

The solid powder is first made into a slurry with the required amount of water and

dispersing agent and fed into the space near the axis of the mill and is carried outwards

between the discs by the centrifugal pressure set up. Besides its use in the preparation

of aqueous dispersions of soft materials (e.g., china clay), the colloid mill is also used

to wet the powders before ball milling. The powder is made into slurry and then passed

through the colloid mill. By this treatment, the material will be wetted properly and

after ball milling will produce a satisfactory dispersion.

Dispersing agents: The selection and amount of dispersing agent are determined by

the physical properties of the material to be dispersed. The functions of these agents



36


are to wet the powder, to prevent or reduce frothing and to obviate re-aggregation of

the particles. The concentration of the dispersing agent should be maintained at the

minimum required to produce the desired effect and need rarely exceed 2% except

in special circumstances.

Dispersol F Conc: Dispersol F Conc is a very effective dispersing agent recommended

for use in the preparation of aqueous dispersions of the water insoluble solid

ingredients used in the various types of latex compounds. By using a sufficient

quantity of this dispersing agent, the water insoluble compounding ingredients

in powder form can be dispersed in water by an appropriate mechanical milling

process, e.g., ball milling. The mechanical action necessary to secure good dispersions

depends on the physical nature of the material to be dispersed. Sulfur for example,

which is a hard material and occurs in relatively large ultimate particles requiresactual grinding in a ball mill for a long period. For materials such as accelerators

and zinc oxide, ball milling for a shorter period or colloid mill may be used. When

re-aggregation of the ultimate particles is very weak, for example, in the case of

good quality china clay, simple mechanical stirring in the presence of Dispersol F

Conc will suffice. For resinous materials such as Accinox B, there is a tendancy for

it to adhere to things and so the addition of an inert material, such as China clay

is necessary to prevent this.

Tables 3.3–3.6 show the formulae and methods for the preparation of the aqueous

dispersions of the common compounding ingredients used in latex.

Table 3.3 Preparation of sulfur* dispersion (50%)


Sulfur 100



Table 3.4 Preparation of Accicure ZDC* dispersion (50%)


Accicure ZDC 100





37


Table 3.5 Preparation of zinc oxide* dispersion (40%)


Zinc oxide 100Dispersol F 3.0

Distilled water 147.0

*Ball milled for 24 hours

Table 3.6 Preparation of Accinox B* Dispersion (20%)


Accinox B 50.0

China Clay 50.0


Distilled water 147.0

*Ball milled for 24 hours

3.2.2 Evaluation of the Quality of Dispersion

Consistency in quality and stability of the dispersions are highly desirable. Suitable

tests should be done to assess the quality of the dispersion before addition to the

latex. In doubtful cases it is advisable to carry out a small-scale test also with the

compounded latex prepared by using the dispersions under test.

A drop of the dispersion is allowed to fall on the surface of the water taken in a tall

glass cylinder. The drop must not fall to the bottom, but should disperse well in the

water phase, leaving a cloudy trail.

3.3 Preparation of Emulsions

As for dispersions, distilled or softened water should also be used for the

preparation of emulsions of the water immiscible liquids, which are used in latex

compounds. An emulsion is defined as a system in which a liquid is colloidally

dispersed in another liquid. The emulsions for latex use should be of the oil-in-

water type, in which the water is the continuous phase and the suspended droplets

carry a negative charge.



38


Equipment used for the preparation of an emulsion consists of a tank and a high

speed stirrer. Very fine and stable emulsions can be prepared by a device, which

imparts a shearing action, e.g., a colloidal mill or a homogeniser. In an homogeniser

the liquid is mixed with the required amounts of water and the emulsifying agentis forced through a fine orifice under high pressure (6.9 MPa to 34 MPa). Thus,

liquid mix is subjected to a high shearing force, which breaks down the particles to

the required size.

Various synthetic emulsifying agents are available on the market, but for latex

use, soaps have been found to be quite satisfactory. Often the addition of the

ingredients to the soap solution with stirring will produce a satisfactory emulsion.

However, a better and more effective method of emulsification, is to produce the

soap in situ during the mixing of the components. In this method, the cationicpart of the soap (ammonia, KOH or amine) is dissolved in water and the anionic

part (oleic, stearic or rosin acid) is dissolved in the liquid to be emulsified. Soap

is formed when these solutions are mixed. Often this technique is modified in

that the water solution is added in small amounts to the non-aqueous phase,

producing at first a ‘water-in-oil’ emulsion, which undergoes an inversion to an

‘oil-in-water’ emulsion on further addition of the water-solution. Table 3.7 and

Table 3.8 give the formulations used for the preparation of liquid paraffin and

SP emulsions.

In the formulation in Table 3.7, oleic acid is mixed with liquid paraffin and the mixtureadded to the water containing the concentrated ammonia solution. The two phases

are mixed by agitation and the stability and dispersion of the emulsion is improved

by being passed through a homogeniser. A further improvement in quality is obtained

by replacing one part of water in the formulation by one part of Vulcastab VL.

In the formulation in Table 3.8, Part A is heated to about 60 °C and then added with

high speed stirring to part B also at the same temperature. The stirring is continued

until the emulsion attains the required temperature.

Table 3.7 Preparation of aqueous emulsion of liquidparaffin (50% emulsion)


Liquid paraffin 50.0

Oleic acid 2.5

Concentrated ammonia solution 2.5

Water 45.0



39


Table 3.8 Preparation of SP (50% emulsion)


Part ASP 50.0

Oleic acid 2.0

Part B

Triethanolamine 1.5

Water 46.5

References

1. B.L. Babitskii and L.E. Vinitskii, Soviet Rubber Technology, 1961, 20, 28.

2. G.G. Winspear in The Vanderbilt Latex Handbook, R.T. Vanderbilt Co., Inc.,

New York, NY, USA, 1954, p.136.

3. A. Lamm and G. Lamm, Rubber Chemistry and Technology, 1962, 35, 4, 848.



40




41

4.1 Dipping

Latex dipping is a process by which thin walled polymer products are produced

by first immersing a former in a latex which has been suitably compounded and

then subsequently withdrawing the former from the latex slowly in such a way asto leave a uniform deposit upon the former [1]. The thickness of the deposit can be

increased by the repetition of the process. The formation of the product is completed

by leaching, drying and if necessary, subjecting it to appropriate treatments, of which

the most obvious is vulcanisation. In many cases the product may also be subjected

to appropriate post treatments. In many cases it is the practice to form a rolled bead

at the open end of the article. The purpose of the bead is principally to reinforce the

thin film against tear-initiation from the edge of the open end. It also prevents very

thin walled articles from adopting various distorted configurations.

The dry rubber is processed after mechanical shearing (mastication) or dissolved

in suitable solvents whereas the latex polymer need not be broken down, thus

retaining its original high molecular weight which results in products with a higher

modulus. Applications involving latex, incur lower machinery costs and lower power

consumption, since the compounding materials may be simply stirred into the latex

using conventional liquid mixing equipment. However, addition of chemicals into

the latex is cumbersome as these have to be made into dispersions using ball mills,

attritor grinding mills or pearl mills (uses balls like pearls for the dispersion).

4.1.1 Types of Dipping Processes

Latex dipping processes are conveniently classified according to whether or not any

colloid-destabilisation agents are used to assist in the formation of a polymer deposit

upon the former, and if such an agent is used, the nature of that agent.

If no destabilisation agent is used, then the process is known as simple or straight dipping.

If a direct coagulant or coacervant is used to promote the formation of the deposit then

the process is known as coagulant dipping [2]. Heat sensitised dipping is a process in

which a latex compound is formulated in such a way as to be heat sensitive and the

4

Dipping and Casting



42


formation of the deposit is facilitated by heating the former prior to immersion in the

latex. If the formation of the deposit is facilitated by the establishment of electric field,

which causes the latex particles to accumulate in the vicinity of the former, it is known

as electrodeposition.

Gloves make up for about 50% of the total consumption of natural rubber latex (NRL)

and, thus, a description of the glove making process will explain most of the processes

involved in the manufacture of dipped goods. The following section describes the glove

production process and will give an insight into manufacturing of dipped rubber goods.

4.1.2 Glove Production

Latex gloves are manufactured either by:

• A batch dipping process, or

• A continuous dipping process

4.1.2.1 Batch Dipping Process

The batch dipping process is presently only being used for the manufacture of irregular

shaped articles or where the output required is small. Industrial gloves are generallymade using a batch process as shown in Figure 4.1 and household gloves are made

using a combination of batch and continuous processes.

Figure 4.1 Batch dipping



43

Dipping and Casting

4.1.2.2 Continuous Dipping Process

Surgical and examination gloves are presently made on high-speed continuous dipping

lines with very high output as shown in Figure 4.2.

Figure 4.2 Continuous dipping

4.1.3 The Manufacturing Process

The production process and the steps involved are similar in both batch and continuous

processes and the flow diagram is shown in Figure 4.3.

End

FinishedGloves

Quality

Control

Former

Cleansing

Coagulant

Dipping Drying

LayexDipping

LeachingVulcanisingPost

LeachingSlurry

Dipping

Stripping

Tumbling

MANUFACTURING PROCESS FLOW

LatexConcentrate

Compounding

Start

Beading

Figure 4.3 Manufacturing process flow chart



44


4.1.3.1 Material Inputs

The materials needed for the manufacture of a surgeon’s or examination glove are

specific and include: profiled ceramic formers/moulds, centrifuged latex, rubberchemicals, dusting powder and various other processing aids. Packing materials such

as primary wrappers (wallets), inner cartons or dispensers and master shippers (used

for export) are required to pack the surgeons/examination gloves.

4.1.3.2 Ceramic Formers/Moulds

Ceramic formers are the mould on which the gloves are formed. The dimensions of

the glove and its texture are largely defined by the size and texture of the former.Suitable formers are used to make different sized gloves and with varying textures.

A typical specification for a glove former for surgeons and examination is shown in

Table 4.1 and Table 4.2.

Table 4.1 Typical specification of a former for making surgeon’s gloves

Glove size (inch) 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

Total length (mm) 420.0 420.0 420.0 420.0 420.0 420.0 420.0 420.0

Palm circumference(mm)

149.0 153.0 176.0 187.5 189.5 214.0 226.0 239.5

Wrist circumference(mm)

128.5 138.0 147.0 158.0 163.0 174.0 186.0 192.0

Shankcircumference9 (mm)

164.0 174.0 182.0 197.0 204.0 220.0 230.0 240.6

Finger lengths:

Little finger (mm) 46.5 50.5 54.0 57.0 80.5 63.5 66.5 70.5Ring finger (mm) 55.0 60.5 64.0 69.0 72.0 77.0 81.5 85.0

Middle finger (mm) 64.0 69.0 73.0 77.5 83.0 87.8 93.0 96.0

Index finger (mm) 53.5 59.0 63.0 67.0 70.0 75.6 79.0 82.0

Thumb finger (mm) 50.0 52.5 57.5 61.5 62.5 69.3 72.5 76.0

Distance fromthumb crotch/indexfinger tip (mm)

82.0 87.5 84.0 101.5 104.0 114.7 119.6 12.0



45

Dipping and Casting

Fingercircumference (10

mm above crotch):little finger (mm)

43.0 47.0 51.0 52.0 55.0 59.0 60.0 62.0

Ring finger (mm) 46.5 51.5 59.0 60.0 63.0 65.0 68.5 71.0

Middle finger (mm) 51.0 54.0 61.0 61.5 67.0 69.0 71.0 75.5

Index finger (mm) 48.0 53.0 56.0 58.0 63.6 67.0 69.0 74.5

Thumb finger (mm) 56.0 65.0 70.6 72.0 76.5 80.6 84.0 90.0

Breadth of socket(mm)

75.0 75.0 75.0 75.0 75.0 75.0 75.0 75.0

Width of socket(mm)75.0 75.0 75.0 75.0 75.0 75.0 75.0 75.0

Breadth of socketslot (mm)

56.0 56.0 56.0 56.0 56.0 56.0 56.0 56.0

Width of socketslot (mm)

23.0 23.0 23.0 23.0 23.0 23.0 23.0 23.0

Rotation radius(mm)

46.0 50.0 55.0 55.0 58.0 63.0 689.0 69.0

Size marking (inch) 5½ 6 6½ 7 7½ 8 8½ 9

Texture Finger tip to wrist front side

Table 4.2 Typical specification of a former for an examination glove

Sizes XS S M L XL

Height (mm) 400 400 400 400 400

Palm circumference (mm) 165 177 200 224 245

Wrist circumference (mm) 151 166 180 193 204

Beading circumference (mm) 181 193 204 209 220

Weight of the former (g) 650 ± 30 700 ± 30 850 ± 35 920 ± 35 955 ± 40

4.1.3.3 Latex Concentrate

The latex from the tree is collected and then concentrated to 60% dry rubber content

(DRC) in a latex centrifuging factory. The concentrated latex is the basic raw material

for the dipping process and, thus the quality of latex is of prime importance. The

concentrated latex specification taken from ASTM D1076 [3] is given in Table 4.3.



46


Table 4.3 Requirements for specified latex categories

Property Category 1 Category 2 Category 3 Category 4

Total solid (% min) 61.3 66.0 61.3 44.0DRCA (% min) 59.8 64.0 59.8 42.0

Total solid content – DRC

(% max)

2.0 2.0 2.0 2.0

Protein content (µg/g latex) 0.60 min 0.55 min 0.29 max 0.60 min

Total protein µg/g of latex deter-

mined by ASTM D5712 [4]

– – – 200 max

Hevea antigenic protein deter-

mined by ASTM D6499 [5]

– – – None

detectableTotal alkalinity calculated

as ammonia as % latex

0.60 min 0.55 min 0.29 max 0.60 min

Sludge content (% max) 0.10 0.10 0.10 0.10 min

Coagulam content (% max) 0.050 0.050 0.050 0.050

KOH number (maxB) 0.0.80 0.0.80 0.0.80 0.0.80

Mechanical stability (s) (min) 650 650 650 650

Copper content (% of the

total solids) (max)

0.0008 0.0008 0.0008 0.0008

Manganese content (% of the

total solids) (max)

0.0008 0.0008 0.0008 0.0008

A = DRC by definition is the acid coagulable portion of latex after washing and

dryingB = It is accepted that the KOH number for boric acid preserved lattices will be

higher than normal, equivalent to the amount of boric acid in the latex.

Category 1 = Centrifuged Hevea natural latex preserved with ammonia only or

by formaldehyde followed by ammonia.

Category 2 = Creamed Hevea natural latex preserved with ammonia only or byformaldehyde followed by ammonia.

Category 3 = Centrifuged Hevea natural latex preserved with low ammonia with

other necessary preservatives.

Category 4 = Centrifuged, or centrifuged and creamed, guayule latex, or other

NRL, containing less than 200 µg total protein per gram dry

weight of latex, with ammonia or other hydroxide, with other

necessary preservatives and stabilisers.

max: maximum

min: minimumKOH: Potassium hydroxide



47

Dipping and Casting

4.1.4 Rubber Chemicals

Rubber chemicals used in the manufacture of gloves can be classified as:

• Direct, or

• Indirect

Direct chemicals are those that are added directly to the latex during compounding

and, thus, undergo a greater level of control. The curatives, stabilisers, antioxidants,

antiozonants and pigments are in this category of direct chemicals. Indirect chemicals

are those that are used during the processing of gloves, for example the coagulant,

the stripping agent, the wetting agent and the dusting powder.

4.1.4.1 Packing Materials

Gloves are packed in a primary and secondary packaging, which includes the dispenser

boxes and the master/shipper carton. Sterile gloves are packed individually or in pairs

in special medical grade paper called the ‘wallets’.

4.1.4.2 Compounding

Compounding involves the addition of rubber chemicals such as curing agents,

rubber accelerators, antioxidants, stabilisers, pigments, and other chemicals to the

concentrated latex. The chemicals added are mainly solids and thus, have to be

ground before adding to the latex. The chemicals are ground in ball mills, pearl

mills or attritors. The compounded latex is aged – usually 48 hours is given to get

the required maturation, before feeding it on to the dipping line for manufacture.

Formulation of the compound latex is a very critical factor and plays an important

role in manufacture.

A typical formulation, specifying the range of additives, for surgeon’s glove is given

in Table 4.4.

Table 4.4 A typical formulation for surgeon’s gloves

Ingredients Parts per hundred rubber (phr)

Natural rubber latex (60%) 100

50% Sulfur 0.50–1.25

50% ZnO 0.25–0.50

50% ZDEC/ZDBC 0.50–0.85



48


50% ZMBT 0.25–0.45

50% Antioxidant 0.50–1.00

40% Wax emulsion 1.00–3.0050% Titanium dioxide 0.25–0.50

10% KOH 1.00–2.00

ZDBC = zinc dibutyl dithiocarbamate

ZDEC = zinc diethyl dithiocarbamate

ZMBT = zinc mercaptobenzothiazole

ZnO = zinc oxide

4.1.4.3 Coagulant Dipping

The formers are cleaned, dried and then dipped in a coagulant bath containing suitable

quantities of calcium nitrate, calcium carbonate (CaCO3) and a suitable wetting agent.

4.1.4.4 Latex Dipping

The compounded latex is fed into the latex tank, which is maintained at a

temperature lower than the ambient temperature. The coagulant coated dry formers

are then dipped into the latex bath. The angle of dip of the formers into the bath

is critical and should maintain the meniscus surrounding the former in a convex-

to-air condition.

4.1.4.5 Beading

The latex film is gelled in a gelling oven and then passed through edge rollers, which

curl the latex film at the cuff forming a rolled bead. The beading is necessary to

facilitate the gripping of the gloves when putting them on (donning).

4.1.4.6 Leaching

Leaching is the process by which the latex film is dipped in a bath of hot water

maintained at a temperature of around 80 °C. This process removes the excess

chemicals in the latex film. This process is done before curing and is also called pre-

cure leaching.



49

Dipping and Casting

4.1.4.7 Vulcanisation

Vulcanisation or curing is the process by which the latex film gets dried and chemically

crosslinked to form the glove. The curing takes place in a long continuous tunneloven maintained at around 130 °C.

4.1.4.8 Post Leaching

The glove film after curing is leached again in hot water to remove the water-soluble

protein and chemicals. This is an important step in the manufacture, which keeps

the residual protein level of gloves to a minimum.

4.1.4.9 Slurry Dip

The formers with the gloves on them are then dipped in wet slurry containing modified

cornstarch. The cornstarch is dried in an oven and it forms the donning powder on

the gloves. The starch powder is bio-absorbable and, thus soft on the skin.

4.1.4.10 Stripping

The gloves are then stripped off the formers and put in crates or bins. The gloves are

segregated by size and then put in the appropriate crates or bins.

4.1.4.11 Tumbling

The gloves are dried in a tumble drier where the excess moisture and powder isremoved. The gloves are then divided into lots. The lots are then transferred for

further processing.

4.1.4.12 Quality Control

The gloves, in lots, ordered by size, are either 100% inspected or audited randomly

and then released for further processing.



50


4.1.4.13 Glove Packing

Gloves are either packed in bulk or in sterile pouches. Bulk packing involves stuffing

the gloves into packs of 100, in dispenser boxes. Sterile packing involves wrappingthe gloves, left and right, into primary packing called wallets and then into a pouch

made of either paper or plastic. The pouches are then packed into inner cartons or

shelf boxes of generally 50 pairs. The shelf boxes are then packed in shipper cartons.

4.1.4.14 Glove Sterilisation

Gloves are sterilised either by ethylene oxide (ETO) or gamma irradiation. ETO

sterilisation involves subjecting the gloves to ETO under controlled conditions. ETOkills the microorganisms and makes the gloves sterile and doesn’t affect the quality of

the gloves. Gamma irradiation involves subjecting the gloves to gamma rays. Gamma

rays kill the microorganisms but does not affect the rubber glove.

4.1.4.15 Finished Gloves

The gloves are despatched as per the planning schedule. Every consignment is audited

and released for shipment by the quality assurance department.

4.1.5 Glove Properties

The performance requirements of gloves are:

• Freedom from holes

• Physical dimensions

• Physical properties

• Amount of powder

• Protein content

• Powder free residue

• Antigenic protein content

• Sterility



51

Dipping and Casting

4.1.6 Defects and Remedies

Technically, the art and science of handling problems with latex processing is more

intricate than regular rubber compounding and requires a good background incolloidal systems. Physically rubber latex is quite different from the dry rubber form

but the properties differ a little from the dry rubber counterpart. Gloves being a thin

material, the thickness ranging from 0.10 mm at the cuff to 0.20 mm at the fingers,

are liable to have many film imperfections, resulting in defective gloves.

Defects can be classified broadly into:

• Functional, or

• Cosmetic

Functional defects are those that affect the barrier properties and, thus, the

performance characteristics of the glove. They are:

• Pinholes – very small holes, as the name suggests, which result from bubbles in

the latex tank, excess of CaCO3, mould imperfections, and so on.

• Weak spots – areas of weakness that could lead to a hole if left unattended.

• Visual holes or tears – holes that are big enough to be detected visually and cutsand tears.

• Bead imperfections – no bead or improper beading, resulting in difficulty during

donning.

• Lower tensile properties – the reason could be the compound recipe or the

processing parameters.

• Dimensional variation – process parameters or former dimensions could be the

reason.

• Variation in powder content.

• Increased protein levels – insufficient leaching, high protein latex.

Cosmetic defects are those that do not compromise the barrier and performance

characteristics of the glove, but they do affect the appearance and visual characteristics

of the glove. Some examples are:

• Coagulum – lumps of latex coagulated on the surface of the glove.



52


• Dirt – specks of dirt on the surface of the glove.

• Stains – stains due to oil or grease on the glove.

The possible defects, their causes and the remedy for dipped goods are given in

Table 4.5.

Table 4.5 The possible defects, its cause and the remedy for the dipped goods

Defect Probable cause Remedy

Pin hole Bubbles in the

coagulant tank

Scoop out the bubbles.

Check the temperature of coagulant

tank as per the quality plan. If notwithin specification adjust the RFF line

valve accordingly

Check the coagulant circulation pump.

No air is being sucked.

– Excess powder in the

coagulant tank

Check the powder percentage in the

coagulant tank. If it is in excess take

the required quantity of coagulant

from the tank and replace it with a

coagulant solution without CaCO3 powder.

– Insufficient wetting in

the coagulant tank

Check the wetting agent level in the

coagulant tank. If it is not within the

specification, add or leave out the

addition of Teric accordingly.

– Dirty moulds Replace the moulds and clean. Replace

the mould with a clean one.

– Excess webbing in the

latex tank(Observe the latex film

between fingers)

Add the required quantity of Bevaloid

642 (5%), an anti-web agent.

– Bubbles in the latex

tank

Scoop out the bubbles on the latex.

Check the latex agitation system.

Check the feeding of latex into the

dip tank.

Weak spot/

thin spot

Low percentage

of acid/insufficient

wetting of coagulant

Add the required quantity of acid.

Check the wetting agent level (froth

level) and add more if required.



53

Dipping and Casting

– Insufficient drying of

the coagulant before

the latex dip

Check the coagulant tank temperature.

Check the coagulant drying oven

temperature.Adjust the temperature as per the

specification.

– Excess webbing in the

latex

Add the required quantity of 5%

Bevaloid 642 emulsion (anti-web agent)

– Oil in the cleaning

tanks

Clean out the tank and fill with fresh

solution

– Dirty moulds Remove the moulds and clean them.

– Foreign particles in the

coagulant solution

Scoop out the foreign particles by using

a cup sieve.Tear/knock Moulds getting to

close on the belt and

touching each other

(a) Adjust the mould or remove it.

(b) Check the condition of the holder. If

it is damaged replace it.

– Mould with gloves

touching any other

part of machinery if

they get too close

Adjust the belt so that the mould

doesn’t touch any other part of the

machinery

– Insufficient

vulcanisation of theproducts

Check the temperature of cure oven as

per the specification.Check the thickness of the product. If

it is less than it should be – increase it

by adding calcium nitrate (coagulant)

solution or increase the TSC of latex.

Reduce the speed of machine.

Check the CTR of the latex and adjust.

– Strip tear Check the nail of the strippers.

Strip out glove in the proper way.

Black spot/ stains

Stability ofcompounded latex is

too low

Clean the chain

– Dried grease particles

falling from the chain

Filter the solution by using a cup sieve.

– Foreign particles

struck to the mould

Clean the mould.

– Dirt on the leach tank Drain the tank and refill it with fresh

water.



54


Lumps/Scum Stability of the

compounded latex is

too low

Increase the stability by adding 10%

KOH into the latex dip tank.

– The pH of latex dip

too low

(a) Add 3% KOH – 2 litres into the

latex tank every 15 min/30 min

(b) Add 500 ml of 10% KOH solution

CTR = Chloroform test rate

TCS = Total solids content

A typical formulation for manufacturing rubber bands and toy balloons produced

by a dipping process is given in Table 4.6.

Table 4.6 Formulation for making rubber bands and toy balloons

Rubber band Toy balloon

Item Parts by

weight

Item Parts by

weight

60% Creamed latex 167 60% Centrifuged latex 167

Vulcanised D paste

(10% sol)

1 20% Potassium oleate 1.5

50% Sulfur dispersion 3 10% KOH 1.0

50% ZDEC dispersion 2 50% Sulfur dispersion 2.0

50% ZnO dispersion 0.5 50% ZDEC dispersion 2.0

Anti-oxidant – SP emulsion

(40%)

2 50% ZnO dispersion 0.75

33% TiO2 dispersion 6 50% Mineral oil emulsion 10

50% China clay dispersion 8 30% Colour emulsion As required

30 % SP emulsion 2.5

SP = styrenated phenol

TiO2 = titanium dioxideZDEC = zinc diethyl dithiocarbamate

Toy balloons are sometimes manufactured using pre-vulcanised latex. Pre-vulcanised

latex remains unchanged in its fluidity but it is in the vulcanised state. The latex

mixed with stabiliser, accelerator, sulfur and anti-oxidants is heated for about two to

three hours at 65–70 ºC with proper stirring. Other ingredients such as colour oils,

and so on, can be added to it and it can then be used for making products such as

toy balloons. The use of a hot aging oven can be avoided when using pre-vulcanised

latex. The product needs to be dried and could be easily removed from the former

after proper dusting with corn starch to avoid sticking. Formulations for industrial

gloves, surgeon’s gloves and examination gloves are given in Table 4.7.



55

Dipping and Casting

Table 4.7 Formulations for industrial gloves, surgeon’s gloves andexamination gloves

Ingredients Industrialgloves (g)

Examinationgloves (g)

Surgeon’sgloves (g)

60% Centrifuged latex 167 167 167

10% KOH 0.5 0.5 0.5

10% Potassium oleate 1.5 – 0.5

20% Vulcastab VL – 0.5 0.5

50% Sulfur dispersion 4.0 2.0 1.5

40% TiO2 dispersion – 5.0 –

50% ZDEC dispersion 2.0 1.5 1.550% ZnO dispersion 2.0 1.0 1.0

30% Colour emulsion As required – –

30% SP emulsion 4.0 – –

30% Wingstay L – 2.5 2.5

30% Liquid paraffin

emulsion

– – 3.0

4.2 Latex Casting

Latex casting, also called latex moulding, is a process in which a solid object is

formed from the latex by gelation inside a mould cavity [6]. The product will have

as its external appearance, that of the interior surface of the cavity of the mould.

The casting or moulding process can be used for making thin walled articles as well

as solid articles Normally it is used for making thin walled articles because drying

of latex takes a long time. Nowadays this process is not used much for commercial

applications, even though many products can be made by this process, because

plasticised PVC is used for such products. Products made by this process includerubber toys, bathing caps, advertising displays, and so on.

The casting process can be divided in to two types depending upon the type of mould

used. One uses plaster moulds and other one uses metal moulds.

4.2.1 Latex Casting using Plaster Mould

The first step is the preparation of the mould. It is made from plaster of Paris, which is

chemically calcium sulfate dihydrate when set [6]. It is mixed with water to make a paste



57

Dipping and Casting

4.2.2 Latex Casting using a Metal Mould

The second type of material used for making moulds is light alloy. In this case the

gelation does not occur by the mechanism mentioned for plaster moulds, so heatsensitised latex has to be used with light alloy moulds, which if used with care can be

used for a long time.

Latex casting can be done in two ways:

• Slush moulding, or

• Rotational moulding

In slush moulding the latex compound is poured through the hole, into the mouldand the mould will be rotated so that a thin film of rubber is formed on the inner

surface of the mould. After that excess compound is poured through the hole and the

hole will be plugged using a stopper made from the same material used for making

the mould. Again the mould is rotated to get a uniform thickness. The mould with

the deposit is heated in an oven so that the product gets cured.

In rotational moulding the moulds are mounted on rotating equipment. The required

amount of latex compound is poured in to the mould, it is then closed and rotated

at several axes simultaneously. This method is restricted to hollow articles without

any hole. Rotational moulding gives products with a uniform thickness. The deposit

thickness does not depend on the colloidal stability of the latex compound. The

disadvantage of this process is the rotation of the mould during gelling of the latex.

This method is used when thickness uniformity is very important.

Typical compound formulations can be based on pre-vulcanised or unvulcanised

latex compound. Pre-vulcanised latex is preferred for making solid articles. If pre-

vulcanised latex is used then the vulcanisation of the product can be avoided. The

product can be dried and taken out from the mould but the strength is always low

for film prepared from pre-vulcanised latex compound. Filler loading increases thehardness and modulus of the product. The colloidal stability of the latex compound

is important, a non-ionic stabiliser such as polyethylene oxide condensate is always

used in compounds for casting. Typical formulations for casting in plaster and metal

moulds are given in Table 4.8.

Table 4.8 Typical formulations for casting in plaster and metal moulds

Ingredients Casting in plaster

mould (g)

Casting in metal

mould (g)

60% Creamed latex 167 16710% KOH 0.75 0.75



58


10% Potassium oleate 0.75 0.75

20% Vulcastab VL 0.5 0.5

50% Sulfur dispersion 3.0 3.550% ZDEC dispersion 2.0 2.0

50% ZnO dispersion 1.5 4.0

30% SP emulsion 2.5 2.5

50% Filler dispersion As required As required

20% Ammonium acetate solution – 7.5

References

1. T.D. Pendle and A.D.T. Gorton, NR Technical Bulletin, The Malaysian Rubber

Producers Association, 1980.

2. C.W. Stewart, Journal of Colloid and Interface Science, 1973, 43, 122.

3. ASTM D1076, Standard Specification for Rubber-Concentrated, Ammonia

Preserved, Creamed, and Centrifuged Natural Latex, 2010.

4. ASTM D5712, Standard Test Method for Analysis of Aqueous ExtractableProtein in Natural Rubber and Its Products Using the Modified Lowry

Method , 2010.

5. ASTM D6499, Standard Test Method for the Immunological Measurement of

Antigenic Protein in Natural Rubber and its Products, 2012.

6. C.M. Blow and S.C Stokes, Natural Rubber Latex and its Applications: No.2

Latex Casting , The British Rubber Development Board, London, UK, 1952.

Additional Reading

MRPRA, Technical Information Sheet L36, Malaysian Rubber Producers’ Research

Association, Brickendonbury, Hertford, UK, 1979.



59

5.1 Latex Foam Rubber

Latex foam is a cellular rubber product made from rubber latex and the cells

are partially intercommunicating and partially non-intercommunicating. The

product has a smooth surface, which was formed by the contact with the mouldsurface. The first patent for the process of mechanical agitation was taken out

in 1930 [1]. It was claimed that latex containing 30% m/m could be made to

foam by mechanical agitation and the foam structure formation could be assisted

by the addition of soap. Then, the foam structure has to be stabilised, dried

and vulcanised.

One of the important developments in latex foam was the introduction of the

Dunlop process. In this process after the formation of the latex foam, it is

allowed to set in the mould using a delayed action gelling agent such as sodium

silicofluoride. Murphy records the first production of latex foam and its curing

in steam [2]. The first foam mattress was made in 1931. In 1932, foam products

were made on a commercial scale. Seat cushions, bus seats, and so on, were first

made during 1932.

In 1936 several alternative methods were suggested, none of these methods except the

Talalay process were successful [3]. The method involved the addition of hydrogen

peroxide or a low boiling liquid and then subjecting the compound to reduced pressure.

This process is the forerunner of the Talalay process, which is the only alternative to

the Dunlop process. During the period immediately after World War II there was arapid growth in the use of latex foam rubber. Synthetic based foam also was developed

during this period. In the early 1950s a general purpose, flexible polyurethane foam

was introduced, which was produced by foaming and crosslinking of polyurethane

oligomers and this was a serious competitor to latex foam rubber. Another threat

was from plasticised polyvinyl chloride foam. The introduction of carpet underlay

using latex foam boosted the use of latex foam.

Latex foam is produced by the Dunlop and the Talalay processes and these are

described in more depth in the next sections.

5

Latex Foam, Thread and Adhesives



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InitialCompounding

Maturation FinalCompounding

Foaming/Whipping

R ef i ni n g

P o ur i n gi n t o

h o t m o ul d

MouldLevelling

GellingCuringWashing

D r y i n g

P o s t c u r i n g

Figure 5.1 Flow chart diagram for typical foam production

The different steps involved in the batch production process are [5]:

• Preparation of dispersions, emulsions and aqueous solutions

• De-ammoniation of latex

• Compounding

• Maturation

• Foaming (whipping)

• Refining (slow speed whipping)

• Addition of gelling agent

• Pouring of the sensitised compounding into the mould

• Gelling

• Curing

• Removel of the product from the mould

• Washing, drying and finishing

The different steps involved in the production process are shown in Figures 5.2–5.14.



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Figure 5.2 A typical ball mill used for making dispersions

of water insoluble solid ingredients

Figure 5.3 De-ammoniation process

The first step is the expansion of the latex compound by the introduction of air

by whipping. In the compound formulation substances such as soaps are added to

promote foaming.



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Figure 5.4 Foaming in Hobart mixer

Figure 5.5 Refining

Figure 5.6 Mould filling



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Figure 5.7 Foam levelling

The latex phase of the foam is then converted to gel. This part of the process is

conventionally referred to as the foam setting and finally the rubber phase of the

foam is vulcanised.

Figure 5.8 Mould closing



65


Figure 5.9 Curing

Figure 5.10 Foam stripping

Figure 5.11 Washing



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Figure 5.12 Squeezing

Figure 5.13 Post curing

Figure 5.14 Final product (foam mattress)



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Commonly observed defects in latex foam are given in Table 5.2.

Table 5.2 Defects in latex foam

Commonly observed defects in

latex products

Remedy

Shrinkage 10–20% Build in a suitable allowance in the mould.

Foam collapse A gelling period that is too long can be

overcome by increasing the amount of the

secondary gelling agent.

Settling The amount of gelling agent used is not

sufficient.

Complete distortion of foam Foam collapse or incomplete vulcanisation.Adjustment of the level of curative is required.

Improper skin Insufficient mould temperature – it should be

more than 45 °C.

Lower strength Improper curing.

Bad odour Inadequate washing.

5.1.2 The Talalay Process

The process for making latex foam by the Talalay process is similar in some aspects

to that for the Dunlop process. It is different from the Dunlop process in that the

chemical gelling agent in the latter process for setting the foam is replaced by carbon

dioxide in the former and, thus, is more environmentally friendly. However, as in the

Dunlop process this also requires accurate control in which rubber/water and water/

air interfaces collapse and are manipulated by the matrix temperature.

Talalay of BF Goodrich Sponge Products was the pioneer of the Talalay process.

There have been minor changes in the process over the years and the modern Talalay

process can be divided in to the following operations:

• Preparation of dispersion, emulsion and solutions

• De-ammoniation in the case of natural rubber latex

• Compounding

• Maturation

• Foaming



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• Pouring the foamed compound into the mould and vacuum expansion

• Freezing

• Gelling

• Curing

• Removal of the product from the mould

• Washing, drying and finishing

The compound preparation is similar to that for the Dunlop process. Usually the Talalay

process uses SBR latex or a blend of SBR latex and natural rubber latex, probablybecause the process is more popular outside the natural rubber (NR) producing

countries in the world. It may be observed that the basic process used is more or less

same as that of the Dunlop process expect for the gelling stage. Silicofluoride gelling

agent is avoided in this process. Since the latex base is likely to be ammonia free, a

small amount of ammonia is added to improve the gelling. KOH and Vulcastab VL

(polyethylene oxide condensate) are added as the stabilisers. A small amount of process

oil is added to improve the flexibility of the product. Sulfur along with the accelerator

and ZnO function as the curing system while Nonox SP is used as the antioxidant. The

Talalay process is generally not used for the manufacture of specialty foams based on

nitrile or Neoprene latex. A typical formulation using SBR latex is given in Table 5.3.

Table 5.3 A typical formulation using SBR latex

Ingredients Parts by weight (g)

Dry Wet

SBR latex (50%) 100 200

Polystyrene co-agglomerated with SBR latex (50%) 17.5 35

Potassium oleate solution (20%) 0.5 2.5

Process oil emulsion (40%) 2.0 5.0

Nonox SP emulsion (50%) 1.0 2.0

Sulfur dispersion (50%) 1.5 3.0

Vulcastab VL solution (20%) 0.25 1.25

ZnO dispersion (50%) 5.0 10.0

ZDEC dispersion (50%) 1.25 2.5

ZMBT dispersion (50%) 0.6 1.2

Ammonia solution (35%) 2.0 6.0

KOH solution (10%) 0.1 1.0



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In the Talalay process, in one method the expansion is brought about by the chemical

decomposition of hydrogen peroxide by an enzyme. The latex compound after

maturation is mixed with the required amount of hydrogen peroxide and a slurry of

bakers yeast preferably at a low temperature (about 10 °C) to delay the decompositionof the peroxide. The mixture is then quickly placed in a specially designed mould

(Figure 5.15). The enzyme catalase present in the yeast decomposes hydrogen peroxide

to liberate oxygen which expands the compound in to froth. Due to the difficulty in

controlling chemical frothing, in the modern process this is replaced by a combination

of mechanical frothing and vacuum expansion. Gelling agents are not used. The

partially expanded froth is placed in the mould and as the mould is closed vacuum

is applied so that the froth expands and fills the mould. Products of varying density

can be made by this method by adjusting the froth density in the mixture. Due to the

complicated design of the mould it is difficult to apply a mould releasing agent in theTalalay mould. This is overcome by the use of an internal lubricant added to the froth

prior to entering the mould. The usual lubricant is a small amount of dilute hydrogen

peroxide solution, which is blended with froth just prior to feeding it in to the mould.

1

3

2

4

5

6

7

Figure 5.15 A typical Talalay mould. 1 – Heating and cooling passages, 2 – Rubbergasket, 3 – Mould cover live plate, 4 – Semi-permeable paper gasket, 5 – Vacuum

moat, 6 – Heating and cooling channels, 7 – Mould bottom live plate

The gelling and curing are controlled by heatind and cooling process and so the

mould should have provisions for effective heating and cooling rapidly and reliably.

The mould is fitted with channels through which a glycol/water mixture at a precisely

controlled temperature is circulated, and the heat is conducted into and out of the

foam by a series of closely spaced pins penetrating the foam from both surfaces of

the mould. Four glycol/water streams are used to get the following temperatures:



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• Cold at 30 °C,

• Low intermediate at −40 °C,

• High intermediate −38 °C, and

• Hot is at 110 °C.

The mould periphery is fitted with a double groove with a vacuum moat between the two

grooves. The outer groove is provided with a permanent temperature resistant rubber

gasket, which seals the mould cavity with an air-tight fit when the mould is closed. The

inner groove is fitted with a replaceable semi-permeable paper gasket through which air

or gas can pass but froth cannot pass. When the required amount of a partially foamed

compound is metered in to the mould, the mould closes and a vacuum is applied to themoat which withdraws the air from the mould, through the paper gasket. This causes

the foam to expand and fills the mould cavity. An automatic valve then operates to

circulate the glycol/water mixture through the passage in the mould, which causes the

expanded foam to freeze rapidly. The rapid rise in surface tension destabilises the air/

water system and this together with the growth of ice crystals, causes the air bubbles

to connect together resulting in the formation of an open cell foam. There is a chance

of collapse of the foam during destabilisation of the air/water interphase but this is

prevented as the froth is in the frozen state. With the cold glycol/water mixture still

circulating, the vacuum is removed and the carbon dioxide is pumped into the moat.Here it passes through the paper and the frozen foam. The pH falls from 12 to 9.5

and the rubber–water phase breaks down due to precipitation of zinc soap from the

destabilisation of zinc amines, and the formation of zinc carbonate. When the rubber

is coagulated in the stable foam structure, the mould and its contents are rewarmed

with the intermediate glycol/water mixture, which is passed through the mould. The

final stream raises the temperature to 110 °C and the foam is kept at this temperature

for curing. The time schedule of the various processes is given in Table 5.4.

Table 5.4 Time schedule for various operations in the Talalay process

Operations Time (min)

Vacuum expansion 2

Freezing (cold) 8

Carbon dioxide gassing 5

Low intermediate 2

High intermediate 2

Hot (cure) 10

Drying (post cure) 7.5



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At the cure temperature, as the ammonium carbonate breaks down into ammonia and

carbon dioxide, the pH rises and it causes the reformation of the potassium oleate

soap, which aids the removal of the foam from the mould. Furthermore as the lid

of the mould is hotter than the base, the product gets withdrawn from the pins inthe base and is held on to the pins in the lid, where it is easier to remove it. In order

to get a high contact area and to have an efficient heat transfer a number of closely

spaced pins are provided in the mould. The compound should be designed to have

high hot wet tear strength to avoid damage during stripping of the product from the

mould. Finally the product is washed and dried.

5.1.3 Testing of Latex Foam

Testin g of latex foam is done according to the IS specification IS 1741–1960 [6]. Theimportant tests are described in the next sections.

5.1.3.1 Indentation Hardness Index

The indentation hardness index is the load in kilograms required to give an indentation

in a sample which is equivalent to 40% of the original thickness of the sample under

specified conditions. The test should be carried out not less than 48 hours after

vulcanisation and drying. The samples are conditioned at a temperature of 27 ± 2 °C

for six hours at a relative humidity of 65 ± 5%. The size of the sample should besuch that a margin of not less than 5 cm should remain outside the area immediately

below the indentor. Sheets less than 20 mm thick should be superimposed using two

or more plies to bring the sample to as near to 25 mm as possible. The diameter of

the indentor should be 305 ± 0.25 mm with a 25 ± 1 mm radius at the outer edge.

The indentor is applied to the sample at a uniform speed of 14 mm per second.

5.1.3.2 Measurement of Dimensions

The thickness of a sample up to and including 25 mm is measured by means of adial gauge having a circular foot of 6.5 cm2 in area and exerting a total pressure of

3 g on the sample. If the thickness is greater than 25 mm it should be measured with

a steel rule or vernier calipers. The dimensions other than thickness are measured

using vernier calipers or a steel rule.

5.1.3.3 Flexing Test

The test involves submitting a sample to a continued flexing with an indentor for

250,000 cycles at four cycles per second and measuring the loss in hardness and



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thickness. The samples are conditioned in the same way as those for the indentation

hardness test. The indentor should be circular, 305 mm in diameter with a 13 mm

radius on the bottom edge.

5.1.3.4 Ageing

The ageing test consists of subjecting samples to controlled deterioration by air at an

elevated temperature and at atmospheric pressure, after which the physical properties

are measured and compared with those of unaged samples. The deterioration is

measured by observing the change in physical properties concerned in the service

application of the article or it may be determined by the visual examination. The

ageing is done at 70 ± 1 °C for 168 hours.

5.1.3.5 Compression Set

The test consists of maintaining the test piece under specified conditions of time,

temperature and constant deflection. After measuring the initial thickness of the test

piece using a dial gauge having a circular foot of 6.5 cm2 in area and exerting a total

pressure of 3 g on the sample, the samples are placed in the compression device and

compressed to 50% of the initial thickness. The sample is then placed in an oven kept

at 70 ± 1 °C for 22 hours. The sample is then allowed to recover for 30 minutes and thefinal thickness is measured. Compression set (%) is calculated by using Equation 5.1:

Compression set (%) = [(To−Tr)/ To] × 100 (5.1)

Where: To − Initial thickness of the test piece in mm, and

Tr − Thickness of the test piece after recovery in mm.

5.2 Latex Rubber Thread

The different steps involved in the manufacture of latex thread are extrusion of the latexcompound, coagulation, washing/drying, talcum coating, dusting, band formation/

ribbon formation, vulcanisation, festooning, packing, testing and quality control.

The main ingredients in a latex thread formulation are [7]:

• NRL

• Vulcanising agent

• Fillers



73


• Stabilisers

• Antidegradent

• Accelerators

• Activator

Sulfur is used as vulcanising agent for thread rubber. The fillers used are aluminium

silicate, kaolinite clay, titanium dioxide (TiO2), and so on, and their use is found to

reduce set properties, shrinkage and cost.

Stabilisers are used to improve the mechanical stability during processing and finally

they get leached out when the product is washed with water. Commonly used stabilisersare soaps such as potassium oleate, potassium laurate and non-ionic stabilisers such as

Vulcastab VL, which is a polyethylene oxide condensate. Non-staining antioxidants

such as substituted phenols are used in latex thread. Accelerators are used in latex

compound for increasing the rate of vulcanisation. The accelerator used should be

FDA approved and non-toxic and free from nitrosamines. Commonly used accelerators

are tetramethylthiuram disulfide (TMTD), zinc dibutyldithiocarbamate (ZDBC), zinc

dibenzyldithiocarbamate zinc isopropyl xanthate (ZIX), and so on.

ZnO is used as the activator in latex compounds. In dry rubber compounding normally

4–5 phr of ZnO is used in combination with stearic acid. In latex due to the presence

of free fatty acids and due to the addition of soaps such as stearate, ZnO alone is

used as the accelerator and is used at a lower dosage.

A typical formulation for elastic thread is given in Table 5.5.

Table 5.5 Formulation for producing elastic thread

Item Parts by weight (g)

60% Natural rubber latex 16710% KOH 1.0

10% Potassium laurate 1.0

50% Sulfur dispersion 3.5

50% ZMBT dispersion 2.0

40% Antioxidant SP emulsion 4.0

33% TiO2 dispersion 2.0

50% ZnO dispersion 2.5

Colour 0.6



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5.2.1 Production Process

A flow diagram for a latex thread production is shown in Figure 5.16.

STEPS IN THE PREPARATION OF

A LATEX THREAD MIX

DISPERSIONS LATEX SOLUTIONS

EMULSIONSMIXING

MATURATION

COOL TO BELOW 20

°c

FILTER

HOMOGENIZE

DEAERATE (76mmhg)

FILTER

HEADER TANK

Quality Control

Post Curing andPacking

Header Tank

Pressure Regulator

Manifold

Coagulant Bath

Capillaries

Rollers

Washing Bath

Curing Oven

Drying Oven

THE EXTRUDED LATEX THREAD PROCESS

TalcDusting

Figure 5.16 Latex thread production

Centrifuged latex in drums is initially filtered before loading it into to the latex

storage tanks. During the period of storage, the latex is continuously stirred to

prevent coagulation. Samples are periodically drawn from the tank to analyse

its properties before compounding. The required quantity of latex, as per theformulation, is removed to the latex weighing tank for compound preparation.

All water insoluble solids have to be made into aqueous dispersions and all water

insoluble liquids have to be made into emulsions before they are incorporated into

the latex.

Compounding ingredients after the quality check are ground/filtered and stored

individually as good dispersions. Certain dispersions are stored in special jacketed

tanks with provision for cooling. Emulsions are prepared in specially designed

heating tanks and stored in the jacketed tanks under controlled conditions. Thesedispersions are periodically checked for the specified chemical properties. An

appropriate amount of the dispersions are mixed together with the latex in a latex

weighing tank to produce an inactive compound, which is then stored in the inactive

compound tank.

5.2.1.1 Activation and Maturation of Compound

The non-activated compounding process commences when the latex is mixed with

the chemical dispersions in the inactive tank. These dispersions are transferred



75


with the help of special process pumps and are mixed constantly with a stirrer.

Samples of compounds are drawn at periodic intervals to confirm and ensure the

product quality. The inactivated compound is homogenised in a homogeniser and

transferred to the activation tank. Activation chemicals are added to activate thecompound under controlled conditions and after homogenisation is transferred to

the maturation tank and kept under controlled conditions until it is time for the

extrusion process.

5.2.1.2 Extrusion

In order to attain uniformity and consistency these activated and matured compounds

are transferred to the top container from which the compound will be extruded. Thecompound feeding system consisting of diaphragm pumps, feeds the compound to the

extrusion header. The header is fitted with calibrated glass capillary tubes through

which the compound is extruded for producing latex rubber thread in talc coated and

silicone coated forms. The gravity extrusion is achieved through a specially designed

compound feeding system to attain consistency of the thread size. The extruded

threads are drawn through a bath, which contains the coagulant (acetic acid) and are

then passed over rollers to attain the desired tension. The threads are then leached

in hot water at different temperatures through five sets of water baths. The required

tension is maintained with the help of different speed variable equipment like speed

helical bevel gear motor (Stober) settings achieved through rollers. For the productionof silicone-coated rubber thread (SCR), threads are then dipped through a specially

formulated silicone emulsion and then taken by conveyor belt for drying. But for

the production of talc-coated rubber thread (TCR) the threads are taken directly to

the drying oven by an anti-static conveyor belt. The threads are then passed through

a drying oven by an anti-static conveyor belt. Hot air is circulated inside the drying

oven at a temperature of 90 °C to 130 °C. The air flow has been designed in such

a manner that the fine threads are not disturbed while passing through the drying

oven. Special anti-static belts are used to reduce the static electricity on the rubber

threads. The oven is totally insulated with rock wool and monolux panels to reducethe heat loss.

5.2.1.3 Application of Talcum Powder

For the production of TCR threads, the threads are taken to the talcum application

set up, where the threads are coated with talc. The excess talc is removed in the

thread shaker machine and the fine talcum dust particles are collected in the dust

extraction system. But for the production of SCR the threads are directly taken to

the ribbon forming machine.



77


before packing. The packed cartons are weighed and labelled. Then they are ready

for despatch after the final inspection.

5.2.1.9 Final Inspection

The final inspection is to ensure that the quality of the product in line with International

standards. The following tests are conducted in the laboratory. Using a tensometer

(universal tester/material testing machine), physical properties (tensile strength,

elongation at break and permanent set), heat aging resistance, ability to separate,

shelf life of the product, property retention of thread, and so on, are measured.

5.2.2 Technical Specifications of Latex Extruded Rubber Thread

Specifications are correct now and are subject to change with time. The material

should meet these properties at the time of shipment. These properties may change

with time of storage and the change in properties depends on storage conditions. The

specifications are given in Table 5.6.

Table 5.6 Technical specifications of latex extruded rubber thread

Properties Specification limit

Modulus @ 300% 200–260 g/mm2

Tensile strength > 2000 g/mm2

Elongation at break > 600%

Permanent set 8% maximum

Heat resistance 80% minimum modulus retention

Diameter tolerance ± 3%

5.2.2.1 Special Properties

An ozone resistant version of the high modulus thread is presently available. However,

it has a tendency to stain due to the presence of antiozonant. This staining is not usually

a problem for furniture webbing applications. Typical properties are given in Table 5.7.

Table 5.7 Typical properties

Properties Specification limit

Modulus at 300% 250 g/mm2

Tensile strength 1900 g/mm2



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Elongation at break 500%

Schwartz value 155 g/mm2

Diameter tolerance ± 3%Permanent set < 8%

Thermal resistivity index 50%

5.2.3 Testing of Latex Thread

The testing of latex thread is done according to ASTM D2433–07 [8]. The important

tests are:

• Density: The mass of a unit volume of thread measured at a temperature of23 ± 1 °C expressed as kg/m3.

• Count: The number of threads that when placed side-by-side measures 25.4 mm

by its diameter. The count of a square thread is calculated by dividing 25.4 by

the length of one of its sides expressed in mm.

• Metric yield: Metric yield is the unstretched length in metres of 1 kg of the thread.

• Tensile strength: The stress at which the thread breaks when it is stretched under

the specified conditions is expressed as its tensile strength. The value is expressedin pascals.

• Elongation at break: The increase in length of the thread at break when it is

stretched under the specified conditions expressed as the percentage increase of

the original length.

• Stress retention: The residual force (or stress) expressed as a percentage of the

original force (or stress) on the thread after the test specimen has been maintained

at a constant elongation (usually 150%) for a specified time.

5.3 Latex Adhesives

Adhesives are substances capable of holding materials together by surface attachment.

The principal attribute of adhesives is their ability to form strong bonds with the

surfaces of a wide range of materials and to retain bond strength under expected use

conditions. Although most adhesives do not have excellent bulk properties and it is,

therefore, important to keep adhesive films thin, some materials such as epoxies have

bulk properties, which qualify them as engineering materials and, thus they can beused in multifunctional applications.



79


Adhesives may come from either natural or synthetic sources. The types of materials

that can be bonded are vast but they are especially useful for bonding thin materials.

Adhesives cure (harden) by either evaporating a solvent or by chemical reactions that

occur between two or more constituents. Adhesives are advantageous for joining thinor dissimilar materials, minimising weight, and when a vibration dampening joint is

needed. A disadvantage of adhesives is that they do not form an instantaneous joint,

unlike most other joining processes, because the adhesive needs time to cure.

The first synthetic adhesive was produced in 1869. This material was incorrectly

termed nitrocellulose and was created by a reaction between nitric acid, sulfuric

acid, and cellulose. Today, this product is known as cellulose nitrate. In 1912, Leo

Baekeland produced phenol-formaldehyde resins, a basic material for many of today’s

adhesives. High strength, electrometric adhesives were available in 1928 when areaction that produced polychloroprene was developed. Later in the 1930s, pressure

sensitive tapes were developed.

The first metal bonding adhesive was developed by Nicholas de Bruyne in 1941. This

material was used in the construction of aircraft. Later in the decade, epoxy resin

adhesives were introduced. During the 1960s the extremely strong cyanoacrylate

adhesives were developed. These products, called super glues, became adhesive when

exposed to moisture in the air. Other adhesives that were developed during this time

include silicones and anaerobic adhesives. Since that time, most of the advances in

adhesive technology have been the result of formulation modifications using variedpolymers.

Latex-based adhesives are nowadays widely used in various application areas such as

milk cartons, envelopes, books, gummed tape, bonding of ceramic tiles, in the footwear

industry, as wood adhesives, and so on. The main advantage of these latex-based

adhesives over solution-based adhesives is low cost, absence of flammable and toxic

solvents. Other advantages include: it is possible to develop formulations for a wide

range of total solid contents and viscosities, polymers with a high molecular weight

and superior resistance to deterioration during ageing can be used and it is easy to

vary the tendencies of adhesives to wet solids and to penetrate porous substrates.Natural rubber latex (NRL) with a solid content as high as 60% has a very low

viscosity (0.15–0.20 Pa.s), where as a rubber solution of a lower solid content (17%)

has a viscosity of (60–100 Pa.s). This means that the coating can be easily applied

over a wider area due to the lower viscosity at the higher solid content. Pumping is

easier for water-based adhesives due to their lower viscosity.

Along with all these advantages, there are also some disadvantages such as susceptibility

to freezing, the tendency of aqueous lattices to shrink textiles and to wrinkle paper,

inferior water resistance and the electrical properties of dried latex film, and so on.

http://en.wikipedia.org/wiki/Chemical_synthesis

http://en.wikipedia.org/wiki/Curing_(chemistry)

http://en.wikipedia.org/wiki/Curing_(chemistry)

http://en.wikipedia.org/wiki/Chemical_synthesis



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5.3.1 Formulatory Ingredients for Latex-based Adhesives

5.3.1.1 Polymers

The polymer selected should be able to form a uniform feeling at the working

temperature. Polymer selection also depends on the nature of the substrate and

the service conditions to which the bonded substrates undergo. Substrates may be

porous or non-porous. For the porous substrates, the nature of the polymer is not

very important, because the bond is mainly mechanical in nature. If the substrate is

non-porous then the polymer should be of matched polarity with the substrate. The

non-polar polymer lattices may be of polyisoprene or of an SBR type. Polymers of high

polarity may be based on acrylonitrile-butadiene copolymers, various acrylic polymers,

styrene vinyl pyridine butadiene copolymers or various carboxylated polymers.

5.3.1.2 Adhesion Modifiers

Various types of additives, which are added to latex-based adhesives for improving

the adhesion characteristics of the polymer are called adhesion modifiers. These

may be of aqueous solutions and dispersions of resins, tackifiers, cooked starches

or uncooked starches. The type of resin used in adhesives can vary, but the most

common include wood rosin and its derivatives, mainly esters, hydrocarbon resins,

cumarone resins and phenolic resins. The resin has to be converted into very fineparticles, which then have to be prevented from associating together again. Water

soluble resins are added by dissolving them in water, and water insoluble resins are

added as emulsions or dispersions.

5.3.1.3 Plasticisers

For adhesive bonds, which need some degree of flexibility polymeric plasticisers

are used. Along with giving flexibility they also impart some amount of tack to the

adhesive film. Polymeric plasticisers are also added as diluents, as they reduce thecost. Polymer solvents such as benzene, carbon tetrachloride, toluene, and so on,

are sometimes used as fugitive plasticisers so that the solvent should plasticise the

polymer during the application of the adhesive and is then subsequently lost from

the adhesive film by evaporation.

5.3.1.4 Crosslinking Agents

Conventional crosslinking agents are added to the adhesive formulation to form a

crosslinked structure. The advantages of crosslinking is that reduced sensitivity of



81


bond strength and flexibility to changes of temperature, improves resistance to ageing

and increased resistance to deterioration by water and organic solvents.

5.3.1.5 Fillers

Different types of fillers such as barytes, clay, ground limestone, gypsum, kaolinite or

whiting, are added to latex-based adhesives to impart different types of properties.

5.3.1.6 Tackifiers

Different types of thickeners such as water soluble organohydrocolloids, for example,

cellulose ether, polyacrylates, polyvinyl alcohol and proteinacious substances are usedin latex-based adhesives.

5.3.1.7 Other Additives

Various types of ingredients such antioxidants, anti-foaming agents, surface active

substances, anti-freeze and anti-freeze-thaw stabilisers, corrosion inhibitors, flame

retardants and colorants are also used if necessary.

5.3.2 Latex-based Adhesives for Paper

NRLcan be used as a paper adhesive [9]. A typical formulation is given in Table 5.8.

Table 5.8 Formulation for paper adhesive

Ingredients Parts by mass (g)

Dry Actual

NR (as 60% m/m ammonia preserved) 100 167Ammonium caseinate (15% m/m aqueous emulsion) 1.5 10

Sodium methelene-bis(naphthalene sulfonate)

(10% m/m aqueous solution)

0.5 5

Sulfur (50% m/m aqueous dispersion) 2 4

ZDBC (50% m/m aqueous dispersion) 1 2

ZnO (50% m/m aqueous dispersion) 1 2

N ,N -di-2-napthyl- p-phenylenediamine

(50% m/m aqueous dispersion)

1 2



82


5.3.3 Testing the Quality of the Adhesive

5.3.3.1 Testing Devices

A wide range of testing devices are shown in Figure 5.17 and have been devised to

evaluate the fracture resistance of bonded structures in pure mode I, pure mode II or

in mixed mode [10]. Most of these devices are beam type specimens.

Double cantilever Beam (DCB) and Tapered Double Cantilever Beam (TDCB) tests

Peel tests

Wedge tests

MMDB and End Notch Flexure (ENF) tests

Symmetrical and Dissym etrical Crack Lap Shear tests (CLS and DLS)

Figure 5.17 Testing devices. MMDB: Mixed mode delaminating beam tests, DLS:

Double lap shear



83


• Double cantilever beam tests (DCB) measure the mode I fracture resistance of

adhesives in a fracture mechanics framework. These tests consist of opening

an assembly of two beams by applying a force at the ends of the two beams.

The test is unstable (i.e., the crack propagates along the entire specimen oncea critical load is attained) and a modified version of this test characterised by a

non-constant inertia was proposed and is called the tapered double cantilever

beam (TDCB) specimen.

• Peel tests measure the fracture resistance of a thin layer bonded on to a thick

substrate or of two layers bonded together. It consists of measuring the force

needed for tearing an adherent layer from a substrate or for tearing two adherent

layers one from the other. Whereas the structure is not symmetrical, various mixed

mode can be introduced in these tests. This is one of the common methods ofevaluating paper strength in library and archival preservation.

• Wedge tests measure the mode I dominated fracture resistance of adhesives used to

bond thin plates. These tests consist of inserting a wedge in between two bonded

plates. A critical energy release rate can be derived from the crack length during

testing. This test is a mode I test but some mode II components can be introduced

by bonding plates of different thicknesses.

• Mixed-mode delaminating beam tests (MMDB) consist of a bonded bilayer with

two starting cracks loaded on four points. The test presents roughly the sameamount of mode I and mode II, with a slight dependence on the ratio of the

thicknesses of the two layers.

• End notch flexure tests (ENF) consist of two bonded beams built-in on one side

and loaded by a force on the other. As no normal opening is allowed, this device

allows testing in essentially a mode II condition.

Crack lap shear tests (CLS) are application-oriented fracture resistance tests. It consists

of two plates bonded on a limited length and loaded in tension on both ends. The

test can either be symmetrical or asymmetrical. In the first case two cracks can beinitiated and in the second only one crack can be propagated.

References

1. E.A. Murphy and E.W.B. Owen, inventors; Dunlop Rubber Company Ltd.,

assignee; GB 332526, 1930.

2. E.A. Murphy, E.W. Madge, S.D. Taylor and D.W. Pounder, inventors; Dunlop

Rubber Company Ltd., assignee; GB 471899, 1937.

http://en.wikipedia.org/wiki/Preservation_(library_and_archival_science)

http://en.wikipedia.org/wiki/Preservation_(library_and_archival_science)



84


3. J. Talalay, inventor; no assignee; GB 619619, 1949.

4. W.H. Chapman, D.W. Pounder and E.A. Murphy, inventors; Dunlop Rubber

Company Ltd., assignee; GB 332, 1930.

5. R.L. Kelly in the Proceedings of the 123rd ACS Rubber Division Meeting ,

Toronto, Canada, Spring 1983, Paper No.12.

6. IS 1741, Latex Foam Rubber Products, 1960.

7. G.H.R. Weiss, NR Technology, 1979, 10, 80.

8. ASTM D2433–07, Standard Test Methods for Rubber Thread , 2007.

9. D.C. Blackley, Polymer Latices, Science and Technology Volume 3, Chapman

and Hall, London, UK, 1997, p.489.

10. L. De Lorenzis and G. Zavarise in the Proceedings of the Fourth International

Conference on FRP Composites in Civil Engineering (CICE2008), 2008,

Zurich, Switzerland.



85

There are different types of lattices such as natural rubber lattices, synthetic lattices,

artificial lattices pre-vulcanised lattices, and so on. Natural rubber latex (NRL)

is obtained from the rubber tree. Synthetic lattices are produced by the emulsion

polymerisation reaction of olefinic monomers having one or more carbon-carbon

double bonds. Particle sizes are smaller than the natural lattices, although by followingvarious polymerisation techniques this property may be varied over a wide range.

Particle sizes are spherical. Artificial lattices are prepared by redispersing rubber

solution in an organic solvent in to an aqueous medium by high speed stirring in

the presence of good emulsifying agent. Pre-vulcanised lattices are produced by the

vulcanisation of latex before converting it into the final product.

Synthetic lattices are produced by the emulsion polymerisation in an aqueous medium

using soap as an emulsifier. The major ingredients in an emulsion polymerisation

recipe are monomers, emulsifier (soap), water soluble initiators such as potassiumpersulfate, water (medium of reaction), modifier (to control the molecular weight)

and short stop (any chemical which can terminate the polymerisation reaction) will

be added when the polymerisation attains the required percentage conversion. To

conduct the polymerisation at lower temperatures such as 5 °C, redox type initiators

are used. An example of a redox initiator system is a combination of a reducing agent

and an oxidising agent, for example, ferrous sulfate and an alkyl hydroperoxide

(ROOH). Synthetic lattices differ from NRL in properties such as particle shape, size

and size distribution, and so on. Synthetic lattices have a lower particle size and lower

distribution of particle size and they are more or less spherical compared to NRL.

The important properties of synthetic lattices are total solid content, pH, viscosity,surface tension, particle size, particle size distribution, coagulum content, mechanical

stability and residual monomer content.

Synthetic lattices can be produced by a batch, a semi-continuous or a continuous

process. In batch processes all the ingredients in the recipe are fed in to the reactor

and the reaction is allowed to continue until the required percentage conversion

is attained. A semi-continuous reaction is a modification of the batch process and

is the most widely used emulsion polymerisation technique. In this case the feed

materials are fed in to a single reactor and the monomer and other ingredients are

6

Synthetic Lattices



87

Synthetic Lattices

Water 180 180 Medium

Fatty acid soap

(Emulsifier)

4.5 4.5 Tackifier

KCl - 0.3 Stabiliser

Sodium naphthalene

sulfonate

- 0.3 Surfactant

Potassium per sulfate 0.3 - Initiator

p-Menthane

hydroperoxide

- 0.06 Redox oxidant

Ferrous sulfate - 0.01 Reducing agent

Sodium formaldehydesulfoxylate - 0.05

Sodium salt of EDTA - 0.05 EDTA = ethylene

diamine tetra-acetic acid

(Chelating agent)

t -Dodecyl mercaptan 0.28 0.2 Regulator

Hydroquinone 0.05 - Polymerisation short stop

Sodium dimethyl

dithiocarbamate

- 0.05 Polymerisation short stop

Polymerizationtemperature (°C)

50 5 Reaction temperature

Conversion (%) 72 60 Percentage conversion of

monomer to polymer

6.2 Nitrile Latices

Nitrile latex is prepared by the emulsion polymerisation of acrylonitrile and butadiene.

Acrylonitrile content may vary from 10–45% with an average value of 33%. Nitriles

are of low, medium or high acrylonitrile content indicating levels of 25, 33 and45% respectively. Nitrile latex is used in applications where high oil and abrasion

resistances are required. The main applications are in the textile (non-woven), and

paper industries, gloves, surface coatings and adhesives.

6.3 Polychloroprene Latices

CR is the polymer of 2-chlorobutadiene. Emulsion polymerisation is used for the

production of sulfur modified CR rubber latex by a batch process using an anionicsurfactant system of salts of rosin or disproportionated rosin acids. The initiator system



88


used is potassium persulfate. The reaction is carried out at 40–50 °C. The reaction is

allowed to go to full conversion or short stops may be used, depending on the type of

latex to be produced [1] redox initiators are used for low temperature polymerisation.

A typical recipe for the polymerisation of chloroprene is given in Table 6.2.

Table 6.2 Recipe for the polymerisation of chloroprene

Ingredient Parts by weight (g)

Chloroprene 100

Wood rosin 4.0

Sulfur 0.6

Water 160

Sodium naphthalene sulfonate 0.7

Sodium hydroxide (KOH) 0.8

Potassium persulfate 0.4

The rosin acid from wood rosin will react with KOH and form soap. Normally on

storage, the pH of the latex falls slowly due to the liberation of hydrochloric acid

because of the slow hydrolysis of the carbon-chlorine bond.

CR products have good resistance to oils, solvents, ozone, sunlight and oxidation, and

flex cracking. These are used in a wide varieties of applications including: concreteadditives, dipped goods, sealants, modified bitumins, and so on.

6.4 Polyvinyl Chloride Lattices

Polyvinyl chloride (PVC) is the most common thermoplastic material. It possesses

various properties such as good strength, abrasion resistance, good resistance to

flammability, chemical and water resistance, the ability to be softened by plasticisers

to give a wide range of hardness, and so on. Two types of PVC homopolymer are

produced of which one is used in the general latex applications, and the other for

plastisol production. The initiators used in the production are water soluble, redox

types and persulfate/bisulfate. PVC lattices are available with solid content up to

58% and particle sizes in the range 80–200 nm.

PVC lattices are used in various applications in textiles, paper and board coating,

and beater addition and impregnation.



89

Synthetic Lattices

References

1. A.M. Neal and L.R. Mayo, Synthetic Rubber, Eds., G.S. Whitby, C.C. Davies

and R.F. Dunbrook, John Wiley and Sons, New York, NY, USA, 1954.



90




91

bbreviations

A

↑↓ Bidirectional tapping on the same tree

1/2S One-half spiral cut

1/2S↑ One-half spiral cut, upwardly tapped

1/3V One-third V-cut1/4S One-quarter spiral cut

1/4S→1/2S One-quarter spiral cut tapped downward changed to half

spiral cut tapped downward

ABS Acrylonitrile–butadiene -styrene

ASTM American Society for Testing Materials

BI-1 First renewed bark of BO-1

BIS Bureau of Indian Standards

BO-1 First base panel of virgin bark

BO-2 Second base panel of virgin bark

C Circumference

CaCO3 Calcium carbonate

CLS Crack lap shear tests

CR Polychloroprene rubber

CTR Chloroform test rate

d/0.5 Tapping twice a day

d/1 Tapping daily

d/2 Tapping every other day

d/2 6 d/7 Tapping every other day for six days followed by one day

of rest

d/3 Tapping once in every three days

Dc Dry rubber content of cream

DCB Double cantilever beam tests

Df Dry rubber content of the field latex



92


DLS Double lap shear

DPTD Dipentamethylenethiuram disulfide

Dr Density of the rubber particleDRC Dry rubber content

Ds Density of the serum

EDTA Ethylenediaminetetraacetic acid

ENF End notch flexure test(s)

ET Ethephon

ETO Ethylene oxide

FDA Food and Drug Administration of the USA

HA High ammonia

IS Indian standard

ISNR Indian standard natural rubber

KOH Potassium hydroxide

LA Low ammonia

LATZ Low ammonia tetramethylthiuram disulfide zinc oxide

LPG Liquified petroleum gas

m/m Mass/mass

MA Medium ammonia

max Maximum

Mc Mini cut

Mc2 Mini cut in 2 cm

min Minimum

MMDB Mixed mode delaminating beam tests

n Coefficient of viscosity of the serum

NR Natural rubberNRL Natural rubber latex

NRS Non-rubber substances

Pa Panel

phr Parts per hundred rubber

PLC Pale latex crepe

ppm Parts per million

PVC Polyvinyl chloride



93

Abbreviations

r Radius of the particle

rpm Revolutions per minute

rpm Rotations per minuteRSS Ribbed smoked sheet(s)

S Spiral cut

SBR Styrene-butadiene rubber

SCR Silicon coated rubber thread

SDC Sodium diethyl dithiocarbamate

SFS Sodium formaldehyde sulfoxylate

SMBT Sodium mercaptobenzothiazole

SP Styrenated phenol

TCR Talc coated rubber thread

TDCB Tapered double cantilever beam

TiO2 Titanium dioxide

TMTD Tetramethylthiuram disulfide

TMTM Tetramethylthiuram monosulfide

To Initial thickness of the test piece

Tr

Thickness of the test piece in mm after recovery

TS Total solid(s)

TSC Total solids content

TSR Technically specified rubber

V V-cut

VFA Volatile fatty acid

Wc Weight of the cream

Wf Weight of the field latex

ZBEC Zinc dibenzyl dithiocarbamateZDBC Zinc dibutyl dithiocarbamate

ZDC Zinc diethyl dithiocarbamate

ZDEC Zinc diethyl dithiocarbamate

ZIX Zinc isopropyl xanthate

ZMBT Zinc mercaptobenzothiazole

ZMDC Zinc dimethyl dithiocarbamate

ZnO Zinc oxide



94




95

I

ndex

A

Abrasion, 88–89Abrasion resistance, 89

Absorption, 14, 56Acceleration, 16Accelerator, 29, 54, 69, 74Acid, 6–8, 12–14, 21, 23, 25, 32, 38–39, 46, 52, 74, 76, 80, 87–89Acid number, 25Acidity, 12, 30Acrylic, 81Acrylic polymers, 81Acrylonitrile, 27, 33, 81, 87–88Acrylonitrile-butadiene-styrene, 87

Activation, 75–76Activator, 74Additives, 47, 81–82, 89Adhesion, 81Adhesive, 79–83Ageing, 17, 22, 27, 29, 73, 80, 82Agents, 16–18, 27, 30–36, 38, 41, 47–48, 52–53, 60–62, 68–70, 73–74, 81–82, 86–88Aggregate, 34Aggregation, 36Aging, 27, 29, 54, 78

Agitation, 18, 38, 52, 60Aliphatic, 32Alkali, 13–14, 22, 28, 33Alkaline, 22, 31Alkalinity, 18, 23–25, 46Alloy, 57Aluminium, 74Amphoteric, 31Anaerobic, 80Analysis, 58

Anion, 31



96


Anionic, 31, 38, 88surfactant, 88

Antioxidants, 27, 29, 47, 54, 74, 82

Application, 5, 30, 32, 73, 76, 80–81, 84, 87Aqueous, 14, 27, 30–31, 33, 35–36, 38, 58, 61–62, 75, 80–82, 86

phase, 38Aqueous solution, 14, 61, 82Assembly, 84Assessment, 19Atmosphere, 19Atmospheric, 73Atmospheric pressure, 73

B

Bacteria, 7, 12Balls, 28, 34–37, 41, 47, 63Barium sulfate, 30–31Barrier, 51Barrier properties, 51Biological, 8Bleaching, 8Blend, 69

Block, 3, 22Blocked, 3Board, 58, 89Boiling, 17, 60Bond, 79, 81–82, 84, 89

strength, 79, 82Bonded, 80–81, 83–84Bonding, 80, 84, 87Breaking, 9Bubble, 6

Bulk, 50, 79Bureau of Indian Standards, 22Butadiene, 27, 33, 61, 81, 87–88By-product, 21

C

Calcium carbonate, 30, 48Calcium sulfate, 55Cantilever, 83–84Capacity, 14, 28

Capillary, 76



97

Index

Carbon black, 30–31Carbon dioxide, 12, 68, 71–72Cardboard, 56

Cast, 8, 56Casting, 41, 43, 45, 47, 49, 51, 53, 55–58Cationic, 31, 38Cavity, 55, 71Cell, 71Cellular, 60Cellulose, 33, 80, 82Centrifuge, 19–21Cermaic, 44, 80Chain, 27, 32, 53, 87

transfer agent, 87Chemicals, 3, 7–8, 11–13, 23, 29, 34, 41, 44, 47–49, 68, 70, 75–76, 80, 86–87, 89

composition, 11properties, 23, 75structure, 87

Chemistry, 39Classification, 31Clay, 30, 35–37, 54, 74, 82Coagulation, 3, 6–9, 12, 19, 21–22, 33, 73, 75Coated, 17, 24, 48, 76–77

Coatinga, 22, 73, 80, 87–89Coefficient, 16Collapse, 68, 71Colloid, 11, 34–36, 41, 58Complex, 13Component, 34, 87Composite, 14Composition, 11, 23Compound, 27, 29, 32, 41, 47, 51, 56–57, 60, 63, 69–76Compounding, 27–31, 33, 35–37, 39, 41, 43, 47, 51, 61–62, 68, 74–75

Compressed, 73Compression, 28, 73Compression set, 28, 73Concentrated, 6, 14–15, 18, 21, 24, 38, 45, 47, 58, 61Concentration, 5, 8, 11–15, 17–19, 21, 23, 25, 36Condensation, 33Conjugation, 61Consistency, 37, 76Constant viscosity, 6Construction, 80

Consumption, 2, 18, 21, 41–42



98


Container, 13, 34–35, 76Contaminated, 7Contamination, 6, 19, 24

Continuous, 37, 42–43, 49, 61, 86–87Conversion, 7, 86, 88–89Converting, 86Conveyor belt, 76Cooling, 35, 70, 75, 77Copper, 13, 23, 28, 46Cord, 87Costs, 21, 24, 41, 74, 80–81Cover, 70Crack, 83–84

Cracking, 89Crosslinked, 49, 81Crosslinking, 60, 81Crosslinking agents, 81Crystallisation, 87Cure, 29, 53, 71–72, 80

temperature, 72Cured, 57Curing, 27, 47–49, 56, 60, 62, 66–71, 75, 77

agent, 27

system, 69Current, 2Cylinder, 14, 37Cylindrical, 34

D

Damage, 72Decompositon, 12, 28, 70Defect, 52Degradation, 7–8, 29

Density, 16, 70, 79Deposit, 29, 41–42, 57Depth, 60Derivatives, 81Design, 19, 70Deterioration, 73, 80, 82Determination, 25Development, 1, 12, 58Dilute, 21, 61, 70Diphenyl guanidine, 61

Dipped, 3, 42, 48–49, 52, 76, 89



99

Index

Dipping, 32, 41–43, 45, 47–49, 51, 53–55, 57, 87Disease, 2Dispersing, 30–32, 34–36

Dispersion, 11, 16, 28, 31–38, 41, 54–55, 58, 68–69, 74, 82Dissolving, 6, 29, 81Distortion, 68Distribution, 15, 17, 24, 86Dosage, 7, 15, 18, 74Drawn, 75–76Dried, 7–9, 29, 48–49, 53–54, 56–57, 60, 72, 80Drier, 8, 49Dripping, 7Drive, 77

Drying, 6–9, 34, 41, 43, 46, 53, 55–56, 62, 69, 71–73, 75–76Dunlop, 60–61, 68–69, 84–85Edge, 41, 48, 72–73Efficiency, 15, 17–21Elastic, 74Electric, 13, 42Electrical, 80Electricity, 76–77Electrodeposition, 42Elevated temperature, 73

Elongation, 31, 33, 78–79at break, 31, 33, 78–79

Emulsifier, 32, 86–88Emulsion, 1, 29, 32, 37–39, 48, 53–55, 58, 68–69, 74, 76, 82, 86–88

polymerisation, 1, 32, 86–88Energy, 30–31, 84Engineering, 79, 85Enzyme, 70Epoxy, 80

resin, 80

Equipment, 17–18, 38, 41, 57, 76Ethylene, 50, 88Evaluation, 5, 37Evaporation, 15, 81Expanded, 70–71Expansion, 63, 69–71Export, 44Exposure, 29Extraction, 76Extruded, 75–76, 78

Extrusion, 73, 76



100


F

Fabric, 32

Factory, 45Feed, 12, 19–21, 86Feeding, 19, 47, 52, 70, 76Fibre, 7Filled, 31, 56Filler, 30–31, 57–58, 87Filling, 32, 64Films, 28, 41, 48–49, 51–52, 57, 79–81Filter, 21, 53, 75Finishing, 62, 69

Flame retardants, 82Flammability, 89Flexibility, 69, 81–82Flexible, 60Flow, 6, 14, 43, 61–62, 75–76Fluid, 12Fluidity, 54Foam, 3, 31–32, 60–62, 64–68, 70–72, 74, 76, 78, 80, 82, 84–85, 87Foamed, 69, 71Force, 16, 18, 34, 38, 79, 84Foreign, 53Formaldehyde resins, 33, 80Formation, 6–7, 12–14, 21, 28, 33, 41–42, 60, 71, 73Formic acid, 8, 12Forming, 48, 76–77Formulation, 38, 47, 54, 61, 63, 69, 73–75, 80–82Fraction, 5, 20–21Fracture, 83–84Framework, 84Free energy, 31Frequency, 4–5Frozen, 71

G

Gamma irradiation, 50Gas, 6, 12, 14, 71, 77Gases, 12Gauge, 72–73Gear, 76Gel, 65

Gelation, 55–57, 61



101

Index

Gelled, 48Gelling, 48, 57, 60–62, 68–70Glass, 34, 37, 56, 76Glue, 32Government, 22Grade, 22, 25, 47Grinding, 34, 36, 41Growth, 2, 8, 60, 71Gum, 1–2, 34

H

Hammer, 9

Handle, 13–14, 32Handling, 14, 51Hardening, 6Hardness, 33, 57, 72–73, 87, 89Heat, 27, 41, 57, 70, 72, 76, 78Heat resistance, 27, 78Heated, 8, 38, 54, 57Heating, 42, 70, 75Heavy metal, 28, 32High impact polystyrene, 87

High molecular weight, 41, 80High pressure, 14, 38High temperature, 77High-speed, 43Hole, 51–52, 56–57Homogeneous, 17Homopolymer, 89House, 8Humidity, 72Hydrocarbon, 1, 81

Hydrochloric acid, 89Hydrogen peroxide, 60, 70Hydrolysis, 13, 15, 22, 89Hydrophilic, 12

I

Ice, 71Immersion, 42Immiscible, 31–32, 37Impact, 87

Impregnation, 33, 89



102


Impurities, 8–9, 14, 21–22Indian Standard, 3, 22Induction, 16–18

Industry, 2, 11, 14, 24, 28, 80Initiation, 41Initiator, 86–88Inorganic, 29–30Insoluble, 17, 28–29, 32, 34, 36, 63, 75, 81Inspection, 22, 78Instability, 22Institute, 9, 24Institution, 24Insulated, 76–77

Intensity, 5Interaction, 12Interface, 13, 33, 58Interphase, 71Ionic, 31, 57, 74Irradiation, 30, 50

J

Joining, 56, 80 Joint, 80

K

Kinetic, 13Knife, 7

L

Labour, 2, 19Laminating, 8Latex, 1–58, 60–89, 91

Latices, 85, 87–88Layer, 17–18, 21, 84Leaching, 41, 43, 48–49, 51Linkage, 27Lipid, 12Liquid, 3, 29, 33, 37–38, 41, 55, 60, 77Load, 28, 72, 84Loading, 57, 75Long-term, 6



103

Index

Loss, 13–14, 18, 72, 76Losses, 77Low temperature, 70, 89

Lubricant, 70Lubricate, 56Lubricated, 56

M

Machine, 8–9, 18–21, 53, 76–78Machinery, 8, 41, 53Manufacture, 3, 11, 22, 32, 42, 44, 47, 49, 69, 73Manufacturing, 11, 14–15, 42–43, 54Margin, 72Marking, 24, 45Materials, 3, 7, 9, 12–14, 16–17, 22, 30, 34–36, 41, 44–45, 47, 51, 57, 78–80, 86, 89Matrix, 33, 68Measurement, 58, 72Mechanical properties, 11Mechanism, 56–57, 77Membrane, 12Mercaptobenzthiazole, 28Mesh, 14Metallic, 19, 22, 28Mill, 9, 33–36, 38, 63Milled 28, 36–37Milling, 34–36Mix, 6, 32–34, 38, 75Mixed, 6, 21, 34, 38, 54–55, 70, 75–76, 83–84Mixer, 64Mixing, 38, 41, 75Mixture, 30, 32, 38, 70–71, 87Modification, 86Modified, 38, 49, 58, 84, 88–89

Modifier, 86Modulus, 28–29, 33, 41, 57, 78Moisture, 8, 49, 80Molecular weight, 41, 80, 86Monomer, 86–88Motor, 76Mould, 8, 44, 51–53, 55–57, 60, 62, 64–65, 68–72Moulding, 55–57Multifunctional, 79



104


N

Nail, 53

Natural rubber, 1–3, 5–7, 9, 11, 13, 15, 17, 19, 21, 23–25, 27–28, 33, 42, 47, 58,68–69, 74, 80, 86Neoprene, 69Neutralisation, 8, 23Nickel, 77Nitrile rubber, 61, 87Non-ionic, 31, 57, 74Non-polar, 81Non-toxic, 13, 74Notch, 83–84

O

Organic, 31, 82, 86Organic solvent, 86Orifice, 38Outlet, 17Output, 19, 42–43Oven, 9, 48–49, 53–54, 56–57, 73, 75–77Overlap, 31Oxidant, 54, 88

Oxidation, 89Oxidative, 7, 27Oxidative degradation, 7Oxygen, 70Ozone, 78, 89

P

Packaging, 24–25, 47Paints, 17, 22, 30Panel, 3–5

Paper, 2, 47, 50, 70–71, 80, 82, 84–85, 87–89Particles, 8, 11–13, 15–16, 18–19, 21, 29–30, 33–34, 36, 38, 42, 53, 76, 81, 86, 89

size, 15–16, 30, 34, 86distribution, 15, 86

Parts per hundred rubber, 47Paste, 34, 54–55, 61Pearl, 41, 47Penetration, 33Performance, 50–51Periodicity, 5

Permanent, 71, 78–79



106


Pressure, 14, 35, 38, 60, 72–73, 75, 80Procedure, 7–9, 34Process, 2, 8, 12, 15–21, 36, 41–43, 45, 48–49, 51, 54–57, 60–63, 65, 68–71, 75–76,

86–88Processability, 11Processing, 3, 7, 11–14, 22, 29, 44, 47, 49, 51, 74Producers, 2, 23, 58Product, 3, 11, 21, 29, 33, 41, 53–57, 60, 62, 67, 69, 72, 74, 76–78, 80, 86–87Production, 2, 8, 18, 22, 34, 42–43, 60–62, 75–77, 87–89Propagation, 2Properties, 11, 21–23, 27, 29–30, 35, 46, 50–51, 73–75, 77–80, 82, 86–87, 89Protective coating, 22Protein, 12, 21, 46, 49–51, 58

Pump, 52Purity 6, 15

Q

Quality, 3, 8, 13, 18, 21–22, 27, 30, 33–34, 36–38, 43, 45, 49–50, 52, 56,73, 75–78, 83assurance, 50control, 22, 49, 73, 75

Quaternary, 21

R

Radiation, 30, 77Radius, 16, 45, 72–73Ratio, 6, 15, 56, 84Raw material, 3, 9, 14, 22, 45Reaction, 13, 15, 27, 29, 80, 86, 88–89Reaction temperature, 88Reactor, 86–87Recipe, 51, 86–87, 89

Recovery, 21, 73Reduction, 2, 18, 20, 33Reinforcement, 30Relative, 5, 27, 72Relative humidity, 72Replacement, 2, 61Residue, 25, 50Resin, 33, 80–81Resorcinol formaldehyde, 33Retaining, 41

Retention, 78–79



107

Index

Ring, 44–45Risk, 14Rod, 56

Rollers, 8, 48, 75–77Room temperature, 28Rotating, 18–19, 35, 57Rotation, 18–19, 45, 57Rotational, 57Rubber, 1–9, 11–13, 15–25, 27–29, 32–34, 39, 41–42, 44–45, 47, 50–51, 54–55,

57–58, 60–61, 65, 68–71, 73–74, 76, 78, 80, 84–88, 90compounding, 29, 51, 74industry, 24phase, 29, 65

Rubbery, 30, 87

S

Sample, 22, 72–73Scale, 2, 37, 60, 87Sealed, 14Sealing, 3Sensitivity, 81Separation, 17–19, 77

Serum, 8, 12–13, 15–16, 18, 21–22Setting, 65, 68Shape, 86Shaped, 11, 42Shear, 83–84Sheet, 6, 58Shell, 7Shrink, 80Shrinkage, 68, 74Silicon, 77

Silicones, 56, 76–77, 80oil, 56Soaking, 8–9Sodium hydroxide, 29, 89Sole, 8Solid, 12, 22, 29, 33, 35–36, 46, 55, 57, 63, 80, 86, 89Solids content, 54Soluble, 17, 21, 28, 30, 33, 49, 81–82, 86, 89Solution, 3, 6, 14, 17, 29, 38, 52–54, 58, 61, 69–70, 80, 82, 86Solvent, 80–81, 86Spacing, 18



108


Specific gravity, 31Specification, 44–45, 52–53, 58, 72, 78Speed, 18–20, 33–35, 38, 43, 53, 62, 72, 76–77, 86

Spherical, 11, 86Spontaneous, 12, 21–22Spreading, 32Stabilisation, 21Stabilised, 21, 60Stabilisers, 27, 31–32, 46–47, 54, 57, 69, 74, 82, 88Stability, 6, 12–13, 21, 24–25, 30, 37–38, 46, 53–54, 57, 61, 74, 86Staining, 29, 74, 78Standards, 3, 8, 21–22, 58, 78, 85Static, 76–77

Stearic acid, 74Steel, 14, 17, 19, 22, 24, 72Sterilisation, 50Sticking, 54Stiffness, 33Stimulation, 4–5Stirrer, 17, 38, 76Stirring, 17–18, 34, 36, 38, 54, 86Storage, 6–7, 11–13, 18–19, 21–22, 28, 32, 75, 78, 89Strain, 87

Strength, 33, 57, 68, 72, 78–80, 82, 84, 89Stress, 79Stretched, 79Strip, 3, 53Structure, 1, 60, 71, 81, 84, 87Styrene-acrylonitrile, 87Styrene-butadiene, 27, 33, 61, 87

rubber, 27, 61, 87Substituted, 29, 74Substrate, 81, 84

Sulfonate, 32, 82, 87–89Sulfonated, 33Sulfur, 12, 27–29, 34, 36, 47, 54–55, 58, 61, 69, 74, 82, 88–89Sunlight, 6, 8, 89Supply, 2Surface, 12, 21, 24, 31–32, 37, 51–52, 55–57, 60, 71, 79, 82, 86, 88

activity, 31free energy, 31tension, 71, 86

Surfactant, 88

Swell, 16



109

Index

Synthetic rubber, 11, 90Systems, 2, 5, 13–14, 22, 24, 27, 37, 51–52, 69, 71, 76–77, 86–88

TTalc, 76Tank, 14–15, 17, 19, 22, 38, 48, 51–54, 75–76Tap, 15Tapping, 2–7, 13, 15Technical, 22, 58, 78Temperature, 8, 17, 19, 28, 38, 48, 52–53, 68, 70–73, 76–77, 79, 81–82, 88–89Tensile properties, 27, 51, 87Tensile strength, 33, 78–79Tension, 71, 76, 84, 86Test, 23–25, 37, 54, 58, 72–73, 79, 84–85

Method, 58specimen, 79

Testing, 22, 72–73, 78–79, 83–84Thermoplastic, 89Thickness, 8, 41, 51, 53, 56–57, 72–73Thin film, 41, 57Threads, 3, 60, 62, 64, 66, 68, 70, 72–80, 82, 84–85Tiles, 17, 22, 80Time, 3, 7, 12, 15–19, 22–24, 34, 55–57, 71, 73, 76, 78–80, 87Tip, 44–45Titanium dioxide, 30, 48, 54, 74Titration, 25Toxic, 13, 74, 80Transfer, 72, 87

agent, 87Transparency, 33Transportation, 15Tube, 19

U

Ultraviolet, 8Uniformity, 56–57, 76Unsaturated, 27, 32Unsaturation, 27Unstable, 6, 28, 84

V

Vacuum, 69–71

Velocity, 16, 20



110


Vibration, 80Vinyl pyridine, 81, 87Viscosity, 6, 16, 18, 27, 32–33, 80, 86

Viscous, 16Volatile, 12, 14, 25, 32Volume, 12, 20–21, 79, 85Vulcanisation, 27–29, 32, 41, 49, 53, 56–57, 68, 72–74, 77, 86

W

Washing, 17, 21, 46, 62, 66, 68–69, 73, 75Water, 3, 6, 8–9, 11, 15–17, 21, 27–30, 32–39, 48–49, 53, 55–56, 63, 68, 70–71,

74–77, 80–82, 86, 88–89Water-based adhesives, 80Web, 52–53Weighing, 75Weight, 11, 13–14, 16, 18, 20–21, 28, 36–39, 41, 45–46, 54, 61, 69, 74, 80, 86, 89Weight loss, 14Well, 14, 17, 22, 32, 37, 55, 87Wetting, 27, 31–33, 47–48, 52Wetting agent, 31, 33, 47–48, 52Wood, 80–81, 89

Y

Yield, 2–6, 79Yielding, 6

Z

Zinc oxide, 13–14, 27, 36–37, 48, 61



Published by Smithers Rapra Technology Ltd, 2013

Practical Guide to Latex Technology is an introduction to the technology of natural

rubber and synthetic rubber lattices. The intention is not to provide a completely

comprehensive text but to offer an abridged version of the technologies used

for the production of important latex products. Latex-based technology forms a

sizable fraction of natural and synthetic rubber technology and an introduction

to the important technologies is beneficial to all practicing technical personnel.

The book begins with a short history of natural rubber latex, formation in

the tree and the tapping, storage and conversion of latex to marketable

forms. It discusses preservation and concentration of natural rubber latex and

the most widely used latex compounding ingredients Dipping and casting

Practical Guide to Latex Technology

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Transcript of Practical Guide to Latex Technology