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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.
Anisotropy and molecular orientation are well known phenomena in the field of thermoplastics, but only a few studies have described anisotropy in rubber materials. It has been shown that injection molding gives rise to a higher degree of anisotropy than compression molding. The anisotropy in the rubber material was strengthened by carbon black and is presumably due to molecular orientation. This paper describes the anisotropy of injection‐molded ethylene‐propylene‐diene rubbers. The two polymers had different molecular weight distributions and thus different rheological properties. The compounds were injected into center‐gated 4mm thick disks. The disks were subsequently split into three layers using a water‐jet cutting technique. Measurement of mechanical and swelling properties in the different layers and directions showed that the anisotropy varied through the thickness of the disk. By X‐ray scattering it was shown that rubber molecules had a preferred direction and thus, that the anisotropy was probably predominantly due to molecular orientation created during the mold filling.
Anisotropy and molecular orientation are well‐known phenomena in the field of thermoplastics. In the case of rubber materials only a few studies have described anisotropy. Injection molding has been shown to give rise to higher anisotropy than compression molding. The anisotropy in the rubber material is assumed to be due to molecular orientation and is strengthened by carbon black. In order to understand the mechanism of anisotropy in rubber materials, an extensive study has been performed. In this paper, results from two injection‐molded ethylene‐propylene‐diene (EPDM) rubbers, compounded both with and without carbon black, are presented. The polymers had different molecular weight distributions and the compounds were injected into center‐gated 1‐ and 4‐mm‐thick disks. The properties measured in different directions were mechanical, dynamic mechanical, and swelling. These measurements show that anisotropy can be a very important factor to take into account. The origin of anisotropy is presumably the molecular orientation which arises during the filling of the mold with the rubber melt.
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