APPLICATION OF COMPUTED X-RAY TOMOGRAPHY SCANNING IN THE STUDY OF THERMOOXIDATIVE DEGRADATION OF THICK-WALLED FILLED NATURAL-RUBBER VULCANIZATES

The aging of five thick-walled natural rubber compounds has been studied by computed X-ray tomography scanning and crosslink density measurements. The compounds were compounded as ordinary carbon-black-filled rubbers with sulfur and peroxide as curing agents. The rubber samples were aged in air at 70, 100, and 150-degrees-C for 1000 h. The relatively new technique of computed X-ray tomography scanning proved to be a good method for studying the aging procedure, and especially for following the crack propagation in the surface. Antioxidants (TMQ and 6PPD) had a low effect on the resistance toward oxidative degradation and crosslinking under these conditions. Surprisingly, the efficient sulfur-vulcanized material had a poor resistance toward thermal degradation. When the crosslink density and the computed X-ray tomography scanning results were compared, it was assumed that the computed X-ray tomograph detected oxygen in the surface, both as elementary oxygen and as oxygen in degradation products, i.e., in carbonyls. The results agree well with the theory that oxidative aging is limited by the ability of the oxygen to diffuse into the material.