Diffusion of Sulfur in Rubber. Relation to Vulcanization

Abstract

Abstract A survey of the literature failed to disclose any previous attempt to determine the. rate of diffusion of sulfur through rubber. The present paper is intended to fill this gap and to show the importance of diffusion and solubility of sulfur in rubber on control of the vulcanization reaction. The vulcanization of rubber cannot be considered as a homogeneous chemical reaction since the reactants, i. e., sulfur, accelerators and activators, are seldom completely in solution or uniformly dispersed. The limited solubility of sulfur, many accelerators, and zinc soaps in rubber leads to the presence of these substances partly in solution and partly in the dispersed state during vulcanization. Local high concentrations of these substances occur in solution around the dispersed phase, owing to their increased solubility which results from the temperature rise during vulcanization, coupled with their slow rate of diffusion through rubber. This paper presents the results of a detailed study of the effects of solubility, diffusion, and crystallization of sulfur on heterogeneity of vulcanization. Results showing the effect of various methods of mixing and handling on the physical properties of certain rubber compounds are presented. The diffusion of sulfur into rubber to form a solution during vulcanization, followed by the separation of globular sulfur on cooling, and its transition into dendritic and finally into rhombic sulfur on standing, was observed under the microscope by Loewen and Endres. Several factors which influence the migration and crystallization of sulfur internally and as a surface bloom on rubber were studied by Rimpel, Endres, Twiss, Loewen, and Graffe. The absorption of sulfur by rubber at elevated temperatures was studied by Skellon and by Venable and Green, and the solubility of sulfur at various temperatures was investigated by Venable and Green, Endres, Kelly and Ayers, Loewen, Morris, and Williams.