Vulcanization of Elastomers. 18. Vulcanization of Natural Rubber with Sulfur in the Presence of Mercaptobenzothiazole. III

Abstract

Abstract The present work is concerned with the influence of the concentration of sulfur and the zinc oxide content on the kinetics of the vulcanization of natural rubber with sulfur in the presence of mercaptobenzothiazole, zinc stearate and zinc oxide. The following results were obtained. 1. The velocity constant of the sulfur decrease, kSI, is inversely proportional to the initial concentration of sulfur. 2. The velocity constant of the change with time of the reciprocal equilibrium swelling, kQI, is inversely proportional to the square root of the initial concentration of sulfur. 3. The limiting value of the reciprocal equilibrium swelling, 1/Q∞ (degree of crosslinking), at constant stearate content, is proportional to the square root of the initial concentration of sulfur. 4. The amount of accelerator consumed during vulcanization increases with increase in the initial amount of sulfur. The decrease in the accelerator is a first order reaction having approximately the same rate as the decrease of sulfur. 5. The velocity constants of the sulfur decrease and the change with time of the reciprocal equilibrium swelling are practically independent of the amount of zinc oxide. Other situations exist, of course, for compounds without zinc oxide. 6. Empirical equations were derived to express the dependence of the constants kSI, kQI and 1/Q∞ on the composition of the mixture.