ENVIRONMENTAL-STRESS CRACKING OF PVC - EFFECTS OF NATURAL-GAS WITH DIFFERENT AMOUNTS OF BENZENE

Craze initiation, craze growth and ultimate fracture were studied in PVC (polyvinylchloride) and PVC-CPE (PVC blend with 10% chlorinated polyethylene) under a constant load in air and in natural gas enriched with benzene. The craze initiation results are similar for PVC and PVC-CPE. The craze initiation stress is decreased dramatically at high benzene concentrations (greater-than-or-equal-to 25000 p.p.m.). Preferential and enhanced sorption of benzene molecules near surface inhomogeneities, the craze initiators, are held responsible for this phenomenon. Initial craze-growth rates in natural gas enriched with benzene increase compared to those in air. However, in all the environments studied, a limited logarithmic craze growth is observed, and growth seems limited to this constant logarithmic interval. The logarithmic craze growth and the termination of craze growth are attributed to a reduction of the stress at the craze tip. The failure mode of PVC and PVC-CPE in air and in gas with 5000-6000 p.p.m. benzene is yielding. A brittle branch appears in the failure curve (failure stress against loading time) in the more concentrated benzene vapours. This branch is attributed to a reduction in the stability of the crazes.