Abstract
Samples of ultra-high molecular weight polyethylene (UHMWPE) and nylon 66, of grades suitable for use in medical prostheses, were subjected to irradiation using a 60Co source. The effects of a 10 Mrad dose on percentage crystallinity, tensile behaviour and dynamic mechanical response were established 3 days after irradiation, and irradiated samples were then retested at intervals over a one year period. Crystallinity in UHMWPE was found to increase progressively during this period with consequent changes in mechanical behaviour. It is proposed that the observed ageing process arises from the breakdown of metastable groups in the polymer, which are formed on irradiation, with consequent release of long inter-lamella tie chains. This removes an inhibition to crystal perfection processes, which can occur at room temperature in polyethylene. Infra-red spectroscopic evidence indicates that the carbonyl concentration in irradiated UHMWPE initially rises, but falls again during the latter part of the ageing period. Unsaturation remains relatively constant. It is proposed that the metastable species introduced on irradiation is peroxy or hydroperoxy and that this decomposes on ageing to a carbonyl group, with concurrent chain scission. With nylon 66 the evidence for ageing is much less conclusive. This is thought to be due to the reduced chain mobility and relatively low molecular weight of this polymer.
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