Vulcanization of Elastomers XV. The Vulcanization of Natural and Synthetic Rubber with Sulfur in Presence of Organic Bases (I)

Abstract

Abstract The results obtained from preliminary kinetic studies of the vulcanization of natural rubber (NR) with sulfur in the presence of organic bases may be summarized as follows. 1) The change of concentration of sulfur with time during reaction with NR for an initial sulfur concentration of 2.5 g per 100 g of mixture follows a fractional reaction order, n, of 0.6 at all temperatures. This result and the findings of other authors suggest that the reaction of sulfur with rubber is autocatalytic. From the temperature dependence of the rate constant the activation energy is found to be 35.3 kcal. 2) In the presence of zinc oxide and with a similar initial sulfur concentration, the reaction is first order and the activation energy, namely 35.6 kcal, is practically the same. 3) In the presence of diphenylguanidine (DPG) during cure sulfur decrease is a reaction of fractional order, n=0.75, and the activation energy is 29.8 kcal. 4) For a given initial concentration of sulfur and at constant temperature the rate of sulfur decrease during vulcanization increases with rising DPG content and reaches a limiting value; i.e., the rate constant depends on DPG concentration. Also, for various base (DPG is basic) concentrations sulfur decrease is of various fractional orders, n=0.5…0.7, etc. 5) The influence of base on the rate of sulfur decrease is interpreted broadly as a case of intermediate-catalysis (Zwischenstoff-Katalyse). 6) Vulcanization with sulfur is unaffected by tertiary amines and secondary amines have only a slight effect. 7) Primary amines, depending on basic strength, are strong accelerators of the rate of sulfur decrease. 8) In the presence of organic bases (aromatic amines) other than DPG, sulfur decrease is of fractional order where n varies between 0.5 and 0.7.