The role of physical structure and morphology in the photodegradation behaviour of polypropylene

Photo-oxidatative degradation of isotactic polypropylene (PP) has been examined in samples made by injection and compression moulding and using different moulding conditions. Samples were exposed to ultraviolet radiation (UV) in the laboratory for periods of up to 48 weeks. The extent of chemical degradation was assessed by gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FTIR); the specimens were tested in tension and the fracture surfaces were inspected by scanning electron microscopy. The processing pathways defined the structural characteristics of the starting materials, as determined by light microscopy and X-ray diffraction. The investigation conducted here indicates that the fractional crystallinity is the main structural parameter controlling the rate of degradation of polypropylene. The mechanical properties of degraded PP were shown to depend not only on the extent of chemical degradation but also on the character of the polymer physical structure, especially the spherulite size. With most samples studied here a partial recovery in tensile properties was observed after 6-9 weeks exposure. Analysis of the fracture surfaces showed that improvement in the tensile properties coincided with the appearance of a ductile zone inside the surface embrittled layer, indicating that the cracks in the surface layer were arrested on reaching the ductile material in the interior. Surface cracks were formed spontaneously after 9 weeks of UV exposure and the pattern and concentration of these cracks also depends on the processing type and conditions. In injection moulded samples, surface cracks were of the form of circular arcs radiating from the injection gate and they were correlated with the flow lines generated during mould filling. (C) 1997 Published by Elsevier Science Limited.