The effect of weathering on residual stresses and mechanical properties in injection-moulded semi-crystalline polymers

Abstract

Injection-moulded bars made from polypropylene (PP), polyacetal (POM) and nylon 6,6 (N6, 6), have been weathered outdoors in Jeddah, Saudi Arabia, for varying periods. In POM and N6, 6 the residual stresses became tensile near the surface, though this happened even with unexposed bars. Prolonged ageing or weathering of PP caused the residual stresses to fall to very low values. With POM and N6, 6 significant surface damage developed on weathering, and multiple surface cracks opened up during uniaxial tensile testing. With N6,6 these developed into the form of diamond-shaped ductile fracture cavities, and the failure mechanism appeared quite different to that obtained with unexposed mouldings. With weathered PP no significant surface damage was visible even in the scanning electron microscope, but prolonged weathering caused a change in the failure mechanism, with fracture usually occurring without necking and drawing with specimens weathered for two years or more.