Page 295 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.87

Chlorinated hydrocarbons are used as plasticizers in rubber articles (e.g., at a level of
20 phr) to lower flammability (e.g., chlorinated paraffins in combination with antimony
trioxide).
Flame Retardants. Hydrocarbon elastomers are flammable and thus require flame
retardants if their service conditions include the possibility of fire. Alumina trihydrate,
magnesium hydroxide, and zinc borate are used, because they give off blanketing vapors
at high temperatures. Also, typical flame-retardant systems include chlorinated paraffins or
brominated aromatic resins in combination with antimony trioxide.
Blowing Agents. Blowing agents are used to produce cellular rubber (e.g., sponge
rubber). These additives give off gas during vulcanization to form bubbles in the vulcani-
zate. Usually, highly plasticized compounds are used.
At one time, sodium bicarbonate (e.g., in combination with oleic acid) was used to give
off carbon dioxide during curing. However, it was difficult to disperse very finely and uni-
formly to give a uniform fine cellular structure.
Organic blowing agents that liberate nitrogen are more commonly used. They are dis-
persed more easily and give greater processing safety and regularity of the foam. Common
examples are sulfonyl hydrazides, certain N-nitroso compounds (e.g., dinitrosopentameth-
ylenetetramine), and azo dicarbonamides.
Peptizers. Certain elastomers such as NR must be broken down (reduced in molecu-
lar weight) by mastication, for example in an internal mixer or (less commonly) on an
open two-roll mill. With NR, this can be done purely by mechanical means but, as the tem-
perature rises due to mixing, the viscosity drops, and the mechanochemical action is
greatly reduced (because there is not enough shear stress). Certain additives can facilitate
the breakdown. They are called peptizers and are used in small concentration (0.05 to
0.15 phr) for breaking down the elastomer (generally NR) before adding the general com-
pounding ingredients. An appropriate peptizer is zinc pentachlorothiophenate, with or
without a zinc soap activator. The activator increases the temperature range for the pepti-
zation process. The soap also reduced the effective viscosity and lowers the mastication
temperature, possibly because of its lubricant activity.

4.5.5 Processing of Vulcanizable Elastomers
Many of the production methods used for rubbers are similar to those used for plastics.
However, rubber processing technology is also different in certain respects. Processing
rubber into finished goods consists of compounding, mixing, shaping, generally molding,
and vulcanizing. Rubber is always compounded with additives: vulcanization chemicals,
and usually fillers, antidegradants, oils or plasticizers, and so on. It is through compound-
ing that the specific rubber vulcanizate obtains its characteristics (properties, cost, and
processability) to satisfy a given application.
4.5.5.1 Mixing
Mastication. The first step in rubber compounding and mixing is mastication (break-
down of the polymer). This is especially essential for natural rubber. During the mixing of
the rubber polymer or polymers with other ingredients, the rubber must be more plastic
than elastic so as to accept the additives during mixing. Some rubbers have molecular
weights that are large enough to permit entanglements that act as cross-links during the
deformation motion of the material in the internal mixer on a two-roll mill. Working the
rubber, especially in the presence of peptizers, reduces the molecular weight sufficiently to
permit good mixing.
In early times, rubber breakdown and subsequent compounding was done on open roll
mills. A schematic representation of such a mill is represented by Fig. 4.29. The rolls ro-

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