EFFECT OF STABILIZERS ON RESIDUAL-STRESSES IN WEATHERED POLYETHYLENE

The degradation of mechanical properties of injection moulded bars of polyethylene stabilized by carbon black or by a hindered amine has been compared to that of the unstabilized polymer. The residual stress distribution in the material stabilized by the hindered amine changed very little during prolonged exposure in Jeddah, Saudi Arabia, whereas in the carbon black-stabilized grade the sense of the residual stress distribution reversed during weathering, stresses near the surface becoming tensile and those in the interior compressive. An explanation for this observation invokes the influence of the temperature gradient that forms when the moulding is heated from one side. Scanning electron microscopy of the fracture surfaces produced during tensile testing and of the moulded surfaces, which become damaged during testing, indicates that a brittle surface layer forms even in the stabilized materials but that this does not necessarily lead to brittle failure of the moulding as long as the material in the interior remains ductile.