Mechanism of the Action of Accelerators of Vulcanization. Vulcanization of Rubber by a Sulfur Radioisotope

The problem of the mechanism of the action of accelerators and vulcanizing agents during the vulcanization of rubber has been studied in earlier works. It was shown that, during the vulcanization of natural and sodium-butadiene rubber, accelerators of the type of mercaptobenzothiazole and tetramethylthiuram disulfide enter into exchange reactions with the vulcanizing agent, sulfur. However, the role of the chemically combined sulfur, that is, the so-called bridge sulfur, in the exchange reactions with the accelerators remained unexplained. As Dogadkin and Tarasova show, in the case of sulfur vulcanizates, the following kinds of bonds between the rubber chains can occur : (1) —C—C—, formed as a result of polymerization or condensation processes (bond energy 62.7 kcal. per mole); (2) —C—S—C—, monosulfide bonds, formed during thiuram vulcanization (bond energy 54.5 kcal. per mole) ; (3) —C—S—S—C—, disulfide bonds; (4) —C—Sn—C—, polysulfides, formed during vulcanization with elemental sulfur in the absence of accelerators containing sulfur (bond energy 27.5 kcal. per mole). These authors established the presence of polysulfide bonds by extracting sulfur from vulcanizates, already extracted with cold acetone, by heating the extracted vulcanizate in a 10 per cent solution of sodium sulfite. Ten to twenty per cent of the total amount of combined sulfur was thus extracted as polysulfide, whereas rubber vulcanized with thiuram contained no sulfur at all which could be extracted by sodium sulfite. In order to study the exchange reactions between accelerators containing sulfur and chemically combined or bridge sulfur, the present investigation was made.