The Solubility of Sulfur in Butadiene-Styrene Copolymer
Abstract The solubility of sulfur in rubber has been studied sufficiently for the various phenomena connected with solubility to be well known. Among the important factors are the available concentration of sulfur at any temperature, the diffusion of sulfur in the rubber, blooming of both vulcanized and unvulcanized rubber, and the effect of supersaturation on the formation of bloom. Similar studies have not been reported on synthetic rubbers which are vulcanized with sulfur. The present paper deals with the solubility of sulfur in a butadiene-styrene copolymer. This work was conducted with a copolymer prepared from 75 parts of butadiene and 25 parts of styrene. The polymer contained about 6 per cent of acetone-extractable material, consisting largely of phenylnaphthylamines and soap-forming acids, particularly stearic acid. It was the general type proposed for large-scale production, and was characterized by lack of tack, nerviness, and short break, especially when hot. The general method employed was one already described, and later employed in a modified form. In general, it consists in the observation, with a low-power microscope, of known amounts of finely divided sulfur dispersed in rubber when very slowly heated and cooled in a hot stage. The solubility temperature is taken as the temperature at which all the sulfur disappears, and the limit of supersaturation is determined by the temperature at which the sulfur reappears when the rubber is lowly cooled.