Characterization of irradiated blends of alpha-tocopherol and UHMWPE

Abstract

Adhesive/abrasive wear in ultra-high molecular weight polyethylene (UHMWPE) has been minimized by radiation cross-linking. Irradiation is followed by melting to eliminate residual free radicals and avoid long-term oxidative embrittlement. However, post-irradiation melting reduces the crystallinity of the polymer and hence its strength and fatigue resistance. We proposed an alternative to post-irradiation melting to be the incorporation of the antioxidant alpha-tocopherol into UHMWPE prior to consolidation. alpha-Tocopherol is known to react with oxygen and oxidized lipids, stabilizing them against further oxidative degradation reactions. We blended GUR 1050 UHMWPE resin powder with alpha-tocopherol at 0.1 and 0.3 wt% and consolidated these blends. Then we gamma-irradiated these blends to 100-kGy. We characterized the effect of alpha-tocopherol on the cross-linking efficiency, oxidative stability, wear behavior and mechanical properties of the blends. (I) The cross-link density of virgin, 0.1 and 0.3 wt% alpha-tocopherol blended, 100-kGy irradiated UHMWPEs were 175+/-19, 146+/-4 and 93+/-4 mol/m3, respectively. (II) Maximum oxidation indices for 100-kGy irradiated UHMWPE previously blended with 0, 0.1 and 0.3 wt% alpha-tocopherol that were subjected to accelerated aging at 80 degrees C in air for 5 weeks were 3.32, 0.09, and 0.05, respectively. (III) The pin-on-disc wear rates of 100-kGy irradiated UHMWPE previously blended with 0.1 and 0.3 wt% alpha-tocopherol that were subjected to accelerated aging at 80 degrees C in air for 5 weeks were 2.10+/-0.17 and 5.01+/-0.76 mg/million cycles, respectively. (IV) Both accelerated aged, alpha-tocopherol-blended 100-kGy irradiated UHMWPEs showed higher ultimate tensile strength, higher yield strength, and lower elastic modulus when compared to 100-kGy irradiated, virgin UHMWPE. These results showed that alpha-tocopherol-blended 100-kGy irradiated UHMWPEs were not cross-linked to the same extent as the 100-kGy irradiated, virgin UHMWPE.

Fig. 1

α-Tocopherol. The chroman head is responsible for abstracting free radicals and stabilizing them within the structure. The long lipophilic chain termed the ‘phytyl tail’ provides compatibility with UHMWPE.

Fig. 2

Cross-link density measurements of virgin, and 0.1 and 0.3 wt% α-tocopherol-blended, 100-kGy irradiated UHMWPE.

Fig. 3

The oxidation levels of control, 0.1 and 0.3 wt% α-tocopherol-blended UHMWPE post-irradiation and accelerated aging. Aging was done at 80 °C for 5 weeks in air.

Fig. 4

The wear rates of accelerated aged 0.1 and 0.3 wt% α-tocopherol-blended UHMWPE. Accelerated aging was done at 80 °C for 5 weeks in air. For comparison, 127-kGy irradiated and α-tocopherol-diffused UHMWPE [6], 38-kGy sterilized and 100-kGy irradiated and melted GUR 1050 [14] are also included. Accelerated aging for the 38 and 100-kGy irradiated samples taken from Muratoglu et al. [14] were done at 80 °C for 3 weeks in air.