The Mobility of Sulfur Bonds in Soft Rubber and Ebonite

Much attention has been devoted to the problem of the sulfur and oxygen bonds in vulcanized rubber in numerous studies of both Soviet and foreign scholars. In the Soviet Union, the works of a large group of investigators, including B. A. Dogadkin, A. D. Zai˘onchovskii˘, P. A. Rebinder, A. P. Pisarenko, A. S. Kuzminskii˘, and others, have dealt with this problem. The nature of the sulfur structures formed in soft vulcanized rubbers depends on a number of factors, including the nature of the accelerators. It was shown in one work that sulfur vulcanization in the absence of sulfur-bearing accelerators leads to the formation of relatively weak polysulfide bonds. We assume that here a group of different polysulfide bonds is formed at the expense of the passage of octatomic ring sulfur structures into open linear sulfur radicals. For vulcanized rubbers which contain sulfur and diphenylguanidine in their recipe, the group of polysulfide bonds evidently constitutes an excellent criterion of their characteristic vulcanization structures. A number of rubber products, including cable and insulation rubbers, which do not contain sulfur as a vulcanizing agent are also known. In this case vulcanization takes place at the expense of the decomposition of tetramethylthiuram disulfide (thiuram vulcanization). Here stronger intermolecular monosulfide sulfur bonds are formed at the expense of the atomic sulfur, which is liberated during the decomposition of thiuram: