Page 248 - Handbook of Plastics Technologies

ELASTOMERS

4.40 CHAPTER 4

FIGURE 4.25 Vulcanizate properties, ______ 300 percent modulus (MPa); ______,
–1
De Mattia flex fatigue life (kHz × 10 ); -O-O-O-O-, percent retention of ultimate elon-
gation after two days at 100°C.
rubber as a model substrate. Natural rubber was the first elastomer and, therefore, the
search for understanding of vulcanization originated with work on natural rubber. Most of
what we have discussed so far vis à vis vulcanization has been related to natural rubber.
The chemistry of the accelerated vulcanization of BR, SBR, and EPDM appears to
have much in common with the vulcanization of natural rubber. Before the formation of
cross-links, the rubber is first sulfurated by accelerator-derived polysulfides (Ac-S -Ac) to
x
give macromolecular, polysulfidic intermediates (rubber-S -Ac), which then form cross-
x
links (rubber-S -rubber). As in the case of natural rubber, the average length of a cross-
x
link (its sulfidic rank, the value of x in the cross-link, rubber-S -rubber) increases with the
x
ratio of sulfur concentration to accelerator concentration (S/Ac) used in the compounded
rubber mix. However, in the case of BR or SBR, the cross-link sulfidic rank is not nearly
as sensitive to S/Ac as it is in the case of natural rubber. Model compound studies of the
vulcanization of EPDM (e.g., wherein ethylidenenorbornane was used as a model for
EPDM) indicate that the polysulfidic rank of the EPDM cross-links probably responds to
changes in S/Ac in a natural rubber-like fashion.
Reversion (when defined as the loss of cross-links during nonoxidative thermal vulca-
nizate aging) is a problem associated mainly with natural rubber or synthetic isoprene
polymers. It can occur only under severe conditions in butadiene rubber; in SBR, instead
of the softening associated with the nonoxidative aging of natural rubber, one can observe

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