Weathering of injection-moulded glassy polymers: changes in residual stress and fracture behaviour

Abstract

Injection-moulded bars made from poly(methyl methacrylate) (PMMA) polystyrene (PS), polycarbonate (PC) and poly(vinyl chloride) (PVC), have been weathered outdoors in Jeddah, Saudi Arabia, for varying periods. The residual stresses in the bars have been found to vary considerably with exposure and in some cases the surface stress has reversed, becoming tensile. With PS and PVC, significant surface damage developed on exposure and the presence of surface flaws together with tensile residual stresses at the surface can be expected to reduce the resistance to fracture, especially in the case of PS which suffers considerable molecular weight reduction near the surface. The changes in residual stress in PMMA and PC were smaller, and the surface deterioration, examined by scanning electron microscopy, was much less than with PS and PVC. The performance of PMMA and PC in fracture tests generally declined less with weathering than did that of PS and PVC but the materials used were all commercial grades designed for use in Europe and the results obtained here should not be taken to indicate the relative weatherability of the different polymers. The significance of the results relate to the mechanism of failure involving the interaction of reversed residual stresses and surface flaws.