Kinetics of Thiazole-Accelerated Sulfur Vulcanization of Natural Rubber

Abstract

Abstract Thiazole-accelerated sulfur vulcanization of natural rubber gum stocks has been investigated at temperatures from 110° to 140° C for times over the range 1 to 104 min. Swelling measurements on the vulcanizates were used to estimate the contribution of chemical reactions to the network chain density, (ρMc−1)chem which was plotted against cure time. The kinetic graphs show clearly an induction period (t0), then fast crosslinking, a slow limited degradation, and finally a “long-time” crosslinking reaction. Apart from the induction period, each of the kinetic graphs is satisfactorily represented by a rate equation assuming three independent additive reactions: first order crosslinking, first order partial degradation, and a long-time reaction assumed to be zero order. The method has been used to compare MBT and MBTS at equal concentration. One consistent difference is the increase of induction period with MBTS in place of MBT, accompanied by a somewhat greater increase of time of cure to maximum network density. Also the completed contributions (Xand Δ) from the initial crosslinking reaction and the degradation reaction, respectively, are greater with MBTS than with MBT. Over the temperature range studied there is no difference between MBT and MBTS in the activation energies for crosslinking, or for t0−1 or for the long-time reaction, but MBTS has a higher activation energy for degradation than MBT. With rising temperature of cure, Δ increases towards X with each accelerator.