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Polybutadiene rubber with 10% vinyl unsaturation has been crosslinked without the addition of any vulcanization chemicals. The crosslinks have been created through treatment at high temperature (240–250°C) and elevated pressure (<293 MPa) in a plunger type mold. The treatment time needed to achieve a crosslinked material is of the order of a few minutes and the reaction rate is shown to be dependent upon the temperature as well as upon the pressure applied. As a result of the severe treatment, some degradation reactions may be expected but careful FTIR analysis shows no increased oxidation. There is some consumption of the vinyl unsaturation, to an amount comparable to the number of crosslinks formed. The presence of carbon black increases the measured crosslink density, as expected, but at higher pressures the amount of extra network material, because of the carbon black, is much larger than at lower pressures, suggesting that the carbon black plays a critical role in the network formation process. The stress relaxation properties in compression of the rubber material were investigated and found to be comparable to those of a peroxide vulcanizate.

*SKECA AB, S-934 02 Ersmark, Sweden SYNOPSISThe 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 15OoC 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.

This article is concerned with the microstructural changes during peroxide and high pressure vulcanization of polybutadiene that is unfilled and filled with carbon black. The main tool is 13 C solid-state NMR; it shows that vinyl unsaturations are consumed in both filled and unfilled samples under peroxide and high pressure vulcanization. Chemical shift calculations of unvulcanized polybutadiene show good agreement with the observed peaks. Calculations of proposed structures, based on a possible reaction mechanism, suggest that a large number of peaks will appear, each at very low intensity. Nevertheless, some changes can be seen as a result of the crosslinking reaction, and the results provide support for the suggested reaction mechanism. Thus, the proposed addition crosslinking mechanism over vinyl unsaturations seems to be a reasonable explanation of the crosslink formation in high pressure vulcanization.

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