The role of long lived free radicals in the ageing of irradiated ultra high molecular weight polyethylene

Abstract

γ irradiation of ultra-high molecular weight polyethylene (UHMWPE) results in both immediate and time dependent property changes. The time dependent changes are believed to arise from long lived free radicals reacting with oxygen and causing chain scission. Samples of UHMWPE have been irradiated in air and in vacuum at ambient temperatures to various dose levels, and subjected to a series of ageing and heat treatment regimes, following which the electron spin resonance spectra have been recorded. Material irradiated in air initially shows a seven peak spectrum, and by fitting hyperfine splitting values, and comparison with literature this is identified as arising primarily from alkyl radicals with smaller contribution from allyl and peroxy radicals. There is some evidence that with time the relative proportion of allyl radicals increases, but comparison of observed and theoretical spectra shows that the allyl radical is less significant than the alkyl radical in the first few months of ageing. Material irradiated to a received dose of 11.25 Mrad and stored under vacuum for 24 h had a radical concentration of 2.03×10 19/g. The number of radicals decreases with time when the irradiated samples are exposed to air. After 70 days post-irradiation air exposure all alkyl radicals had reacted, leaving only the more stable peroxy radicals. Following reaction of the alkyl radicals there is little further reduction in the number of spins during ageing suggesting that there is constant regeneration of peroxy radicals throughout the cycle of oxidative ageing. Heat treatment or ‘annealing’ at 85°C is effective in reducing the number of radicals within an irradiated specimen. Air annealing of 50 μm thick irradiated specimens for 24 h is sufficient for removal of all alkyl radicals, leaving only peroxy radicals. A further 100 h annealing is necessary for complete removal of all radicals and the material is then fully stabilised. When annealing in vacuum, there is still evidence of alkyl radicals after 24 h of treatment. Vacuum annealing causes radicals to react without oxidation, presumably to form crosslinks. Annealing under this condition requires a longer period for complete elimination of radicals present, indicating that the rate of crosslinking is slower than the rate of oxidation.