Kinetics and mechanism of accelerated sulfur vulcanization

As a step in the general development of a kinetic theory of sulfur vulcanization, rate data published by Zapp and co‐workers on the accelerated vulcanization of Butyl rubber are analyzed. Rate constants are calculated, some with remarkable accuracy, and these compare admirably with those previously calculated by the writer from data on unaccelerated Buna S vulcanization by Blake and co‐workers. In accordance with theory, acceleration is found to be due to a decrease in activation energy, which more than compensates for a considerable decrease in frequency factor predicted by theory. As in the case of unaccelerated cure, the reaction order is unity, at least initially, and reflects the disappearance of the sulfur during the reaction. The rate determining step is the reaction magnified image where Z· is a radical derived from the accelerator. Although the sulfur combines with the polymer and with zinc oxide, these two entities thus do not enter into the rate controlling step. The importance of classifying polymer reactions according to the participation or nonparticipation of the polymer in the rate control is stressed. Tentative arguments are advanced for kinetic and chemical details of the chain reaction between ZSSSSSSSS·. and rubbers. Some parallels are drawn between vulcanization, polymerization of sulfur, and the cyclization of rubber.