Pendent Accelerator Groups in the Aging of Rubber

Abstract

Abstract As pendent accelerator groups are believed to be precursors to the formation of crosslinks in accelerated sulfur vulcanization, their original composition and concentration are largely responsible for the characteristics and properties of the final vulcanizate, including aging. This means that aging of rubber vulcanizates is inherently tied into the curing system. Dual labeled experiments using tritium-labeled accelerator and sulfur-35 were carried out to determine the number of sulfur atoms in the original pendent groups. As attached pendent accelerator groups are built up in the initial part of the vulcanization reaction before a network is formed, a dual vulcanization system was used, first curing with dicumyl peroxide followed by the addition of labeled sulfur and accelerator by infusion from benzene solution. Analyses were made by liquid scintillation counting after degrading the vulcanizates in p-xylene solution with t-butyl hydroperoxide using osmium tetroxide as a catalyst. The initial number of sulfur atoms in the pendent groups was found to be 3 for an efficient low sulfur system and 16–17 for a conventional high sulfur system. Oxygen absorption measurements have shown that vulcanizates having a low amount of sulfur in the original pendent groups resulted in superior aging. The presently proposed scheme of vulcanization requires the formation of a zinc perthiomercaptide as the actual sulfurating agent. Reaction with rubber gives rise to attached pendent groups which are the precursors to the formation of crosslinks. Results have shown, however, that pendent accelerator groups were formed not only in the presence of zinc but also in its absence where no zinc complex is possible. This would mean that the mechanism for the formation of pendent groups, at least in the absence of zinc, may be different from that presently accepted.