Photo-degradation of polyethylene and polypropylene at slow strain-rate

An investigation has been conducted of the fracture behaviour of polyethylene (PE) and polypropylene (PP) in the form of notched tensile bars when deformed at a slow strain-rate (similar to 1.3 x 10(-7) s(-1)) while under ultraviolet (UV) irradiation. The radiation accelerates failure with both materials, although then are differences in the mechanisms by which this occurs. In PE cracking occurs predominantly at the crack tip and is probably directly accelerated by stress-aided photo-oxidation, whereas in PP orientation of the molecules at the crack tip appears to provide some resistance to photo-oxidative chain scission. In PP failure is more influenced by cracking of the photo-degraded surface layer. As a consequence the crack opening displacement (GOD) develops more rapidly in PE when exposed to UV, whereas there is far less influence of UV in the COD with PP. Pre-exposure to UV prior to the slow strain-rate test has been shown to cause failure after a much shorter test period. The effect of the direction of the UV irradiation (side-on or edge-on) was not very great, though this may be because an extended source was used and the illumination was not collimated. (C) 1997 Elsevier Science Limited.