Page 619 - Carrahers_Polymer_Chemistry,_Eighth_Edition

582 Carraher’s Polymer Chemistry

Plant fibers such as cotton, abaca, agave, flax, hemp, kapok, jute, kenaf, and ramie are still in use

but even cotton is no longer “king.”
Regenerated proteins from casein (lanital), peanuts (ardil), soybeans (aralac), and zine (vicara)

are used as specialty fibers. Regenerated and modified cellulose products, including acetate, are still
widely used today and the production of fibers is similar to that described above for synthetic fi ber

production. Most regenerated cellulose (rayon) is produced by the viscose process where an aqueous
solution of the sodium salt of cellulose xanthate is precipitated in an acid bath. The relatively weak

fibers produced by this wet spinning process are stretched to produce strong rayon.
18.5 ELASTOMERS
Before World War II, hevea rubber accounted for more than 99% of all elastomers used, but syn-
thetic elastomers account for more than 70% of all rubber used today. Natural rubber and many
synthetic elastomers are available in latex form. The latex may be used, as such, for adhering carpet
fibers or for dipped articles, such as gloves, but most of the latex is coagulated and the dried coagu-

lant used for the production of tires and mechanical goods.
More than 5.5 billion pounds of synthetic rubber is produced annually in the United States. The
principle elastomer is the copolymer of butadiene (75%) and styrene styrene–butadiene–rubber (SBR)
produced at an annual rate of more than 1 million tons by the emulsion polymerization of butadiene
and styrene. The copolymer of butadiene and acrylonitrile (Buna-H, NBR) is also produced by the
emulsion process at an annual rate of about 200 million pounds. Likewise, neoprene is produced by
the emulsion polymerization of chloroprene at an annual rate of more than 125,000 tons. Butyl rub-
ber is produced by the low-temperature cationic copolymerization of isobutylene (90%) and isoprene
(10%) at an annual rate of about 150,000 tons. Polybutadiene (BR), polyisoprene, and ethylene–
propylene copolymer rubber (EPDM) are produced by the anionic polymerization of about 600,000,
100,000, and 350,000 tons, respectively. Many other elastomers are also produced.

18.5.1 ELASTOMER PROCESSING

The processing of elastomeric (rubbery) material is quite varied dependent on the end use, form
of the material (that is dry or in solution) and material processed. Latex forms of rubber can be
properly mixed with additives using simple (or more complex) stirring and agitation. The mixing/
agitation should be such as to not cause a separation or breakdown of the latex or foam formation.
Straight, coagulant, and heat-assisted dipping processes are commonly used to produce a variety of
tubes, gloves, and so on. Latexes are also used to make thread for the garment industry, and adhe-
sives for shoes, carpets, and tape formation.
Following we will look at the processing involving bulk rubber. The manufacture of rubber prod-
ucts from this material can be divided into four steps:

1. Mastication
2. Incorporation or compounding
3. Shaping, and
4. Vulcanization

The shaping and vulcanization steps are combined in a number of processes such as transfer and
injection molding or may be separated as in the extrusion and subsequent vulcanization sequence.
An outline of these steps is given in Figure 18.4.

We will look at the processing of natural dry rubber first since its processing is similar to other
elastomers and because of its historical importance. Natural rubber is a dispersion of rubber par-
ticles in water. Unvulcanized raw rubber obtained by coagulation and drying has large chains with
5
chain lengths in the order of 10 carbons.

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