Influence of packaging conditions on the properties of gamma-irradiated UHMWPE following accelerated ageing and shelf ageing

The ageing behaviour of ultra-high molecular weight polyethylene (UHMWPE) h as been studied following gamma irradiation (25 or 40 kGy) in air or in ine rt atmosphere (vacuum packed). Accelerated ageing procedures used elevated temperature (70 degrees C) and/or pressurised oxygen (5 bar). Shelf ageing for up to six months was also performed. The variation in polymer propertie s with depth into the polymer was determined using density measurements, in fra-red spectroscopy and differential scanning calorimetry. Tensile propert ies of the irradiated polymer after accelerated ageing were also determined . Accelerated ageing in air for 20 days or pressurised oxygen for 4 days re sulted in peaks in polymer density, crystallinity and degree of oxidation a t the polymer surface. Accelerated ageing in pressurised oxygen for 8 days resulted in peaks in these properties 500 mu m below the polymer surface. W here gamma irradiation was performed in inert atmosphere the amount of poly mer degradation following accelerated ageing (in oxidising conditions) was generally lower but still significant. Differences were also observed in te nsile properties between material gamma-irradiated in air and in vacuum. Th is study indicated that performing gamma sterilisation procedures under ine rt conditions can reduce the level of UHMWPE degradation when exposed to an oxidising environment. (C) 1999 Elsevier Science Ltd. All rights reserved.