Kinetics of sulfur-free thiuram vulcanization

Sulfur-free thiuram vulcanization has been investigated at temperatures from 160° to 190°C over 0.5 to 600 min. Continuous measurements in a VUREMO curemeter were used to estimate the extent of crosslinking, which was plotted against cure time. Simultaneously the values of the network chain density were calculated from swelling measurements on the vulcanizates. The cure curves show clearly an induction period (t_i), then fast crosslinking, a partial degradation, a “long-time” crosslinking, and finally a slow, limited degradation. Apart from the induction period, the kinetic graphs are satisfactory represented by a rate equation assuming three independent first-order reactions: fast crosslinking, degradation, and slow crosslinking. The rate equation contains seven kinetic parameters. Over the temperature range studied, there is no difference between the values of activation energy for fast crosslinking, for degradation, for slow crosslinking, and for t. Due to the presence of thiourea, the values of the induction period, the rate constant, and the extent of slow crosslinking are decreased. Simultaneously the activation energies calculated from degradation and slow crosslinking are significantly increased. On the basis of the above results, the mechanism of the sulfur-free thiuram vulcanization, in which ionic and radical reactions take place, is discussed.