Characterization of irradiated blends of α-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 α-tocopherol into UHMWPE prior to consolidation. α-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 α-tocopherol at 0.1 and 0.3 wt% and consolidated these blends. Then we gammairradiated these blends to 100-kGy. We characterized the effect of α-tocopherol on the cross-linking efficiency, oxidative stability, wear behavior and mechanical properties of the blends. (I) The crosslink density of virgin, 0.1 and 0.3 wt% α-tocopherol blended, 100-kGy irradiated UHMWPEs were 175±19, 146±4 and 93±4 mol/m 3 , respectively. (II) Maximum oxidation indices for 100-kGy irradiated UHMWPE previously blended with 0, 0.1 and 0.3 wt% α-tocopherol that were subjected to accelerated aging at 80 °C in air for 5 weeks were 3.32, 0.09, and 0.05, respectively. (III) The pinon-disc wear rates of 100-kGy irradiated UHMWPE previously blended with 0.1 and 0.3 wt% α-tocopherol that were subjected to accelerated aging at 80 °C in air for 5 weeks were 2.10±0.17 and 5.01±0.76 mg/million cycles, respectively. (IV) Both accelerated aged, α-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 α-tocopherol-blended 100-kGy irradiated UHMWPEs were not cross-linked to the same extent as the 100-kGy irradiated, virgin UHMWPE.

Update on UHMWPE research From the bench to the bedside

“…Vitamin E is a fully biocompatible additive that is frequently used in the food industry, due to its antioxidant properties. Both techniques introduce vitamin E at trace concentrations (at a dose range under 500 ppm), and where crosslinking is not significantly affected (Oral et al 2005). Consequently, the wear resistance is reduced (Teramura et al 2008) while the microstructure changes, originated during thermal treatments, should be avoided due to the deleterous effects on UHMWPE mechanical properties.…”

Vitamin E-stabilized UHMWPE for Total Joint Implants: A Review

“…Once consolidated, the blend can be crosslinked with the use of ionizing radiation. The presence of vitamin E in UHMWPE during irradiation protects the polymer from oxidation but reduces the efficiency of crosslinking [56,57,65] while the vitamin E itself is reacted; therefore, the vitamin E concentration and the subsequent radiation dose must be optimized to obtain a simultaneously wear-and oxidation-resistant UHMWPE. The alternative method is the diffusion of vitamin E into UHMWPE after radiation crosslinking [62,63].…”

Diffusion of vitamin E in ultra-high molecular weight polyethylene

“…Once consolidated, the blend can be crosslinked with the use of ionizing radiation. However, the presence of vitamin E in UHMWPE during irradiation reduces the efficiency of cross-linking [20,21]. An alternative method is the diffusion of vitamin E into UHMWPE following radiation cross-linking [15,16].…”

“…The penetration depth was defined as the depth from the free surface of the UHMWPE cube where the vitamin E index fell below 0.02, which is close to the Vitamin E detection limit of FTIR [21].…”