Vulcanization of Crepe Rubber by Sulfur Monochloride. I. The Gelation Method

Abstract

Abstract When a dispersion of rubber (4 per cent) in benzene is treated with sulfur monochloride (1 per cent) at room temperature, the solution becomes opaque and gelation occurs in a few minutes. With a more dilute dispersion of rubber (1 per cent), Meyer and Mark obtained a weak gel, which could be broken up by stirring to give, after one hour, a pale yellow precipitate, corresponding to the formula C10H16SCl2. By analogy with the reaction of ethylene and sulfur monochloride, these workers proposed the following reaction process: (see PDF for diagram) The so-called cold-vulcanization process, which is essentially the above reaction, thus appears to correspond to the cross-linking of adjacent isoprene units by means of thioether-bond formation. This result contrasts with the hot sulfur vulcanization process, where the reaction does not involve the sulfurization of any appreciable number of olefinic groups. The mechanism of the cold vulcanization process is unknown. However, it is known that certain commercial sulfur vulcanization accelerators speed up the above gelation process to a remarkable extent. The present work describes an attempt to investigate the kinetics of the reaction between rubber and sulfur monochloride, with the eventual view of establishing the mechanism of the process. In general, two main methods have been developed for assessing reaction velocities. They are the dilatometric method and a more arbitrary time-of-gelation method. While the former method is of importance in studying the more detailed quantitative features of the reaction, the latter method is of help as a powerful auxiliary. Although it is not yet possible to propose a clear-cut mechanism for the vulcanization process, certain kinetic features have been established, and indications of the most fruitful lines of attack are presented.