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A study of free radicals in irradiated/aged UHMWPE materials

UHMWPE bar stock, resin powder and fibers that were irradiated with an electron beam and then stored in air for three years were investigated for long-lived free radicals. Significant concentrations of free radicals were found in all UHMWPE materials. Peroxy radicals are predominant in the UHMWPE materials that are highly accessible to oxygen, while more primary radicals remain in the UHMWPE materials that are isolated from oxygen. Based on this study, it is expected that irradiation of UHMWPE in an inert environment would result in long-lived primary radicals, which could be a potential cause of long-term oxidation. Furthermore, a highly crystalline UHMWPE may be more susceptible to long-term oxidation as more free radicals can be trapped in crystalline regions.