Observation of different photo-degradation behaviour in two similar polypropylenes

Abstract

The photodegradation properties of injection moulded bars 3 mm thick made from two nominally similar toughened polypropylene compounds (PP and MPP) have been found to differ remarkably in some aspects. In both of them the plot of the mass average molecular mass (M _w) versus distance from the exposed surface displayed a steep sigmoidal shape with very low values near the surface and values close to the undegraded value near the bar centre. With PP the steepest part of the M _w versus depth plot remained at almost the same position irrespective of exposure time for tests carried out for periods up to 64 weeks whereas with MPP the steepest part of the plot shifted progressively in from the exposed surface as exposure increased. The tensile strength fell rapidly with exposure time in both PP and MPP but with PP partial recovery was observed whereas with MPP the fall in strength appeared to be monotonic with exposure time.