ANISOTROPY IN INJECTION-MOLDED STYRENE-BUTADIENE RUBBERS .1.

Anisotropy and molecular orientation are well known phenomena in the f ield of thermoplastics, but in the case of rubber materials only a few studies have described anisotropy. In these studies, injection mouldi ng has been shown to give rise to a higher degree of anisotropy than c ompression moulding. The anisotropy in the rubber material is supposed to be due to molecular orientation and was strengthened by carbon bla ck. In order to clarify the mechanism of anisotropy in rubber material s, an extensive study has been performed. In this first paper, results from two injection-moulded styrene-butadiene rubbers, compounded both with and without carbon black, are presented. The materials had diffe rent molecular weight distributions and the compounds were injected in to centre-gated discs, 1 and 4 mm thick. The properties measured in di fferent directions are stress-strain, dynamic mechanical properties an d swelling. The results of these measurements show that anisotropy can be a very important factor to take into account. The origin of anisot ropy can be a residual orientation from the filling of the mould with the rubber melt.