Degradation of Elastomers 4. Continuous Chemical Stress Relaxation of Natural Rubber Vulcanizates

Abstract

Abstract Temperature dependence of the chemical stress relaxation in oxygen of a series of natural rubber vulcanizates, extracted and unextracted, with the same pretreatment was investigated. Decrease of tension to about 50 per cent of the original was followed, using the half lives as a measure of velocity. The following observations were made: 1) Relaxation velocity of unextracted thiuram vulcanizates increases with increasing sulfur content; the activation energy decreases. 2) The relaxation velocities of extracted thiuram vulcanizates are independent of sulfur content of the mixtures and significantly greater than those of unextracted samples. This is caused by the loss of dithiocarbamate on extraction. 3) Other natural rubber vulcanizates (dithiocarbamate, MBT and peroxide vulcanizates) give the same relaxation half lives after extraction. 4) The relative importance of random scission and crosslink scission was discussed.