The properties and crystallization behaviour of photo-degraded polypropylene

Abstract

Photo-oxidatative degradation in isotactic polypropylene (PP) has been examined in samples made by both injection and compression moulding and using different moulding conditions. Samples were exposed to ultraviolet (UV) radiation in the laboratory for periods up to 48 weeks. The specimens were tested in tension and the fracture surfaces were inspected by scanning electron microscopy. The extent of chemical degradation was assessed by gel permeation chromatography and Fourier transform infrared spectroscopy. The structural characterization of the as-moulded and UV-exposed materials was conducted by differential scanning calorimetry, X-ray diffraction, pole figure analysis, and polarized light microscopy. Filled grades of PP containing talc were also investigated. The mechanical properties of degraded PP were shown to be highly dependent on aspects like the extent and depth-profile of degradation, the character of the polymer physical structure, and the presence of filler and nucleating agent. Injection moulded bars containing weld lines were also investigated, and their mechanical properties were generally poorer than those for equivalent samples without weld regions. With most samples studied here a partial recovery in tensile properties was observed after 6-9 weeks exposure. Analysis of the fracture surfaces indicated that in the specimens with better properties the surface cracks did not propagate into the undegraded interior. An increase in the polymer crystallinity was observed during exposure, caused by rearrangement of molecule segments released by oxidative scission in the non-crystalline region. This process, called chemi-crystallization, continues until a large number of chemical defects is present within the molecules, and it results in surface cracks formed spontaneously during exposure. In injection moulded samples the pattern of surface cracks featured circular arcs radiating from the injection gate and they were correlated with the flow lines generated during mould filling. The concentration of surface cracks in these samples varied with the location in the test bar, according to local variations in crystal orientation. The crystallization of photodegraded PP from the melt was investigated under isothermal and non-isothermal conditions. The kinetics of crystallization, degree of crystallinity, and morphology were shown to depend on the molecular weight and on the presence of chemical impurities within the molecules. The crystal y-phase was formed in specimens exposed for more than 18 weeks. Double peaks were normally observed in DSC heating experiments of unfilled polypropylene and were assigned to crystal re-organization during heating. The existence of different molecule species was detected by DSC and light microscopy, and might also have contributed to the melting behaviour.