Effect of photo-oxidation cracks on behaviour of thick polypropylene samples

Photo-degradation of isotactic polypropylene (PP) has been examined in thick samples made by extrusion. Samples were exposed to ultraviolet radiation (UV) for periods of up to 7 weeks. The study of degradation of PP was assessed by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction. Surface cracks appeared spontaneously after 5 days of UV exposure. These cracks in the surface layer of degraded PP were inspected by scanning electron microscopy (SEM). The tests under tension were performed to characterize the behaviour of degraded PP, in particularly the evolution of Young's modulus with respect to UV exposure. Linear elastic fracture mechanics was used for modelling the surface cracks caused by photo-degradation in order to evaluate the decrease of Young's modulus due to these cracks. The photo-degradation of PP in terms of Young's modulus lowering can be split into four stages: incubation period (3-5 days), period of decrease of Young's modulus caused by surface cracks (5 days-3 weeks), period of decrease of Young's modulus caused by surface cracks and chemi-crystallization (3-5 weeks) and period of stabilization (5-7 weeks). (C) 2004 Elsevier Ltd. All rights reserved.