Vulcanization of Rubber in the Presence of Organic Accelerators

Abstract

Abstract Rubber Reviews for 1957 included an extensive survey by Craig devoted to the vulcanization of rubber with sulfur. In that survey, however, neither the thinking of Soviets nor of Germans were mentioned; likewise Japanese studies, of which a considerable number had been published in the preceding decade, were omitted. The purpose of the present survey is to supply, in some degree, the deficiency of the Craig review, particularly that part which deals with the problem of the chemistry of vulcanization of rubber by means of sulfur in the presence of accelerators. In recent years the attention of investigators, engaged in the study of vulcanization, has been focused chiefly on the clarification of elementary chemical reactions of the process, on the mechanism of the action of accelerators and on the establishment of the nature of vulcanization structures and their influence on physicomechanical properties of the vulcanizates. For the resolution of these questions, along with chemical analysis methods, attention is drawn to the physical methods of optical and electron microscopy, of isotope interchange, and of analyses of kinetics by means of radioactive sulfur. Important results were obtained through the study of sulfur in model low molecular weight compounds. Furthermore, it has been demonstrated that there are two tendencies toward interpretation of the general nature of chemical reactions of sulfur with rubber. In some studies, radical processes are assigned an important role in the explanation of structural changes in rubber during vulcanization; in others, elementary reactions are considered as proceeding in accordance with a polar (ionic) mechanism. It occurs to us that the differences in points of view, regarding the mechanism of vulcanization, are not so much a matter of approach to the interpretation of experimental factors, as this fact—that for such a complicated phenomenon as vulcanization, it is impossible to support a single mechanism. The mechanism of a reaction may vary, depending on thermodynamic considerations with regard to the reaction, the type of rubber used, and especially on the types of accelerators and activators employed. Such a conclusion was reached also by Shelton and McDonel. For this reason, in this survey we shall present material representing a number of individual viewpoints regarding vulcanization along with a discussion of some general features.