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

gamma irradiation of ultra-high molecular weight polyethylene (UHMWPE) resu lts in both immediate and time dependent property changes. The time depende nt changes are believed to arise from long lived free radicals reacting wit h oxygen and causing chain scission. Samples of UHMWPE have been irradiated in air and in vacuum at ambient temperatures to various dose levels, and s ubjected 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 spl itting values, and comparison with literature this is identified as arising primarily from alkyl radicals with smaller contribution from allyl and per oxy radicals. There is some evidence that with time the relative proportion of allyl radicals increases, but comparison of observed and theoretical sp ectra shows that the allyl radical is less significant than the alkyl radic al in the first few months of ageing. Material irradiated to a received dos e of 11.25 Mrad and stored under vacuum for 24 h had a radical concentratio n of 2.03 x 10(19)/g. The number of radicals decreases with time when the i rradiated samples are exposed to air. After 70 days post-irradiation air ex posure 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 con stant regeneration of peroxy radicals throughout the cycle of oxidative age ing. Heat treatment or 'annealing' at 85 degrees C is effective in reducing the number of radicals within an irradiated specimen. Air annealing of 50 mu m thick irradiated specimens for 24 h is sufficient for removal of all a lkyl radicals, leaving only peroxy radicals. A further 100 h annealing is n ecessary for complete removal of all radicals and the material is then full y stabilised. When annealing in vacuum, there is still evidence of alkyl ra dicals after 24 h of treatment. Vacuum annealing causes radicals to react w ithout oxidation, presumably to form crosslinks. Annealing under this condi tion requires a longer period for complete elimination of radicals present, indicating that the rate of crosslinking is slower than the rate of oxidat ion. (C) 1998 Elsevier Science Ltd. All rights reserved.