ANISOTROPY IN INJECTION-MOLDED ETHYLENE-PROPYLENE-DIENE RUBBERS .1.

Anisotropy and molecular orientation are well-known phenomena in the f ield of thermoplastics. In the case of rubber materials only a few stu dies have described anisotropy. Injection molding has been shown to gi ve rise to higher anisotropy than compression molding. The anisotropy in the rubber material is assumed to be due to molecular orientation a nd is strengthened by carbon black. In order to understand the mechani sm of anisotropy in rubber materials, an extensive study has been perf ormed. In this paper, results from two injection-molded ethylene-propy lene-diene (EPDM) rubbers, compounded both with and without carbon bla ck, are presented. The polymers had different molecular weight distrib utions and the compounds were injected into center-gated 1- and 4-mm-t hick disks. The properties measured in different directions were mecha nical, dynamic mechanical, and swelling. These measurements show that anisotropy can be a very important factor to take into account. The or igin of anisotropy is presumably the molecular orientation which arise s during the filling of the mold with the rubber melt.