Radiation sterilization and the wear rate of polyethylene

The resistance to sliding wear was measured in bovine serum for two polyethylenes with molecular weights of about 10(5) and 10(6) that had been irradiated in air and nitrogen with gamma-ray dosages up to 20 Mrad. Molecular weight measurements were performed after irradiation as well. Wear generally increased with dosage and contact stress, becoming measurable in many cases only after a critical dose (or stress) was exceeded. The most significant effect noted was that the irradiation changed the pressure dependence of the wear rate. Thus, whether or not a sterilizing (or resterilizing) dose will measurably increase the wear depends on the contact stress and, therefore, on the specific application. The increase in wear rate appeared to be due to a combination of scission and oxidation, suggesting the practical advisability of radiation sterilization under an inert atmosphere, as confirmed by comparative measurements at the higher dosages.