Influence of morphology and surface preparation on the weatherability of polypropylene

Abstract

Regions within injection moulded polypropylene bars having different morphologies have been exposed by milling away material from the surface then illuminating them with ultraviolet radiation (u.v.) for periods of up to 18 weeks. The crystal morphology prior to u.v. exposure was determined using X-ray diffraction. The effects of the u.v. were investigated using tensile tests and differential scanning calorimetry. Examples of bars broken in the tensile tests were examined in the scanning electron microscope. When tensile tests were conducted on samples that were exposed for a short while to u.v. after removing the surface, cracks formed that followed the machining marks and failure occurred at a fairly low extension. Some of the ductility was restored if the machine marks were polished away before conducting the u.v. exposure. The results appeared to be insensitive to surface finish at longer exposure times. The severity of the damage caused by photo-oxidation increases as the crystallinity increases.