Study of Vulcanization and Degradation Chemistry in Natural Rubber by Solid-State 13C NMR and Physical Property Measurements

Sulfur-vulcanization and heat aging of natural rubber was investigated by solid-state 13C NMR along with the measurements of physical properties. 13C NMR detects all of the sulfurization occurring in natural rubber network including inter- and intra-molecular crosslink and pendant linkages, while physical measurements such as modulus at small strain and equilibrium swelling postulate detect only an amount of physically effective crosslinks. By comparing those two approaches, the efficiency of sulfur bonding in contributing to physical strength can be determined. It was found that the efficiency became lower for the samples with higher curing temperature, longer cure time, and also at higher sulfur level. It was assumed by 13C chemical shift analysis that all of the sulfurized isoprene units were cis-to-trans isomerized during the vulcanization and heat aging, even for ineffective sulfurization. Furthermore, it was found that the configuration of those units remained in the trans-1,4 form after the desulfurization at either reversion or heat aging. The nature of the trans-1,4 form in the isoprene series was assumed to be more rigid compared to that of a cis-1,4 form. This resulted in an introduction of chain scission at large deformation for the rubber with higher trans-1,4 content. Comparing several samples with different cure temperatures, cure degrees, and sulfur levels, it was postulated that such cis-to-trans isomerization is a main factor governing a degradation chemistry at elevated cure temperature, overcuring and heat aging.