Page 237 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.29

FIGURE 4.20 The history of vulcanization by sulfur.

Accelerated-sulfur vulcanization is the most widely used method. For many applica-
tions, it is the only rapid cross-linking technique that can, in a practical manner, give the
delayed action required for processing, shaping, and forming before the formation of the
intractable vulcanized network. It can be used to vulcanize natural rubber (NR), synthetic
isoprene rubber (IR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), butyl rubber
(IIR), chlorobutyl rubber (CIIR), bromobutyl rubber (BIIR), and ethylene-propylene-di-
ene-monomer rubber (EPDM). The reactive moiety for all of these elastomers can be rep-
resented by

H
>C-C=CH-

Typically, a recipe for the vulcanization system for one of the above elastomers con-
tains 2 to 10 phr of zinc oxide, 1 to 4 phr of fatty acid (e.g., stearic), 0.5 to 4 phr of sulfur,
and 0.5 to 2 phr of accelerator. Zinc oxide and the fatty acid are vulcanization-system acti-
vators. The fatty acid, with zinc oxide, forms a salt that can form complexes with acceler-

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