Variables Controlling the Cross-Linking Reactions in Rubber

Abstract

Abstract During vulcanization, sulfur reacts with rubber and compounding ingredients in various ways to give diversified reaction products. Only the sulfur that forms cross-links between rubber molecules contributes to the transformation of rubber from the plastic to the elastic state. Reactions of sulfur which lead to products other than intermolecular cross-links are considered side reactions of secondary importance to vulcanization. This study was initiated to determine in a relatively quantitative manner the effect of time, temperature, and concentration of reactants on the yield of intermolecular cross-links in the 2-mercaptobenzothiazole-accelerated sulfur-vulcanization of Hevea rubber. This investigation was considered a necessary step in a study of the over-all mechanism of the vulcanization reactions. When conditions necessary for the greatest yield of cross-links per unit of combined sulfur are followed, a linear relationship exists between stress at 200 per cent elongation (a measure of the relative number of cross-links) and combined sulfur. Under conditions that lead to a maximum yield of crosslinks, a linear relationship exists between retractive force and zinc sulfide.