The reaction of sulfur and sulfur compounds with olefinic substances. Part XII. Effects of vulcanizing additives on the reaction of sulfur with 2,6‐dimethylocta‐2,6‐diene, and their bearing on the mechanism of sulfur vulcanization of natural rubber

Abstract

AbstractThe effects of zinc oxide, of zinc oxide and stearic acid, and of zinc oxide, stearic acid, and 2‐mercaptobenzothiazole on the products of interaction of sulfur with the diisoprene, 2,6‐dimethylocta‐2,6‐diene at 140°C. have been investigated in detail. Zinc oxide alone produces little change in the composition of the complex product mixture, but in the presence of stearic acid, and more so when the thiazole is also present, significant changes are produced in the distribution of combined sulfur which reflect more efficient use of the sulfur in crosslinking reactions, and which can be related rationally to important effects exerted by these additives on the sulfur‐vulcanization of natural rubber. It has recently been advanced that sulfur‐olefin interaction proceeds by a polar mechanism in which the dominant feature is the additive reactivity of persulfenium ionic species (T) towards double bonds. Evidence is now presented that the chemical and technological effects of soluble zinc salts and of thiazole accelerators stem from their action in enhancing the role of reactions promoted by the complementary persulfenyl anionic species (T) which involve the displacement of hydrogen from, and the addition of sulfur to, carbon atoms adjacent to double bonds. Essentially similar mechanistic considerations appear to apply to systems accelerated by amines and by metal dithiocarbamates and xanthates.