Influence of thermal aging on change of crosslink density and deformation of natural rubber vulcanizates

Crosslink is the most important chemistry in a rubber vulcanizate. Degree a nd type of crosslinks of the vulcanizate determine its physical properties. Change of crosslink density and deformation of a rubber vulcanizate by the rmal aging were studied using natural rubber (NR) vulcanizates with various cure systems (conventional, semi-EV, and EV) and different curl times (und er-, optimum-, and overcure). All the NR vulcanizates were deformed by the thermal aging at 60-100 degrees C. The higher the aging temperature is, the more degree of the deformation is. The undercured NR vulcanizates after th e thermal aging were deformed more than the optimum-and overcured ones. The NR vulcanizates with the EV cure system were less deformed than those with the conventional and semi-EV cure systems. The deformation of the NR vulca nizates was found to be due to change of the crosslink density of the vulca nizates. The crosslink densities of all the vulcanizates after the extracti on of organic materials were also changed by the thermal aging. The sources to change the crosslink densities of the vulcanizates by the thermal aging were found to be dissociation of the existing sulfur crosslink and the for mation of new crosslinks by free sulfur, reaction products of curing agents , and pendent sulfide groups.