Effect of stabilizers on failure mechanisms in weathered polypropylene

Abstract

The degradation of the mechanical properties of injection moulded bars made from polypropylene stabilized by carbon black (PP(CB)) or by a hindered amine (PP(HALS)) has been compared to that of the unstabilized polymer. The unstabilized grade became brittle within a few weeks exposure outdoors in Jeddah, Saudi Arabia, whereas the stabilized materials retained significant ductility for extended periods of weathering. PP(HALS) deteriorated very little in the first year but then degraded rapidly. PP(CB) deteriorated much more rapidly than PP(HALS) in the first year but the rate of deterioration slowed down, so that it still retained substantial ductility after 2 years and showed no further significant change thereafter. Scanning electron microscopy indicated 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. In unstabilized PP the surface facing the sun became so fragile that fracture in subsequent tensile tests appeared to nucleate near to the unexposed face, presumably because stress transfer from the heavily degraded exposed face into the interior was not efficient.