The Solubilization of Sulfur in the Latex Vulcanization Process

Abstract

Abstract Recent work has shown that optimum properties of rubber vulcanizates are obtained when the vulcanizing ingredients are very well dispersed in the rubber, preferably dispersed to the molecular state, in other words, dissolved in the rubber. The process by which the vulcanization of latex is carried out seems to contradict this. Here the vulcanization ingredients are present in the water phase, and vulcanization of the rubber particles can be accomplished by heating the latex. Apparently molecular dispersion in the rubber phase is not necessary. In the case of sulfur, if we assume that the sulfur particles have the same size as the latex particles and that the density of sulfur is 2 and that of rubber is 1, then only one sulfur particle per 100 rubber particles is needed to obtain a sulfur concentration of 2 phr. The question is, how the sulfur is transferred from that one particle to the 100 rubber particles. It can only be through the water phase. However, sulfur is not soluble in water nor in aqueous ammonia. The experiments described below show that sulfur can indeed “dissolve” in the water phase (serum), be it in very small amounts, and subsequently is transferred into the rubber phase at concentrations sufficient for curing.