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The Effect of Aging on Mechanical Properties of Melt-Annealed Highly Cross-Linked UHMWPE
Bhambri, S. K., Gsell, R., Kirkpatrick, L., Swarts, D. F., Blanchard, C. R., Crowninshield, R. D.
Abstract
The effects of aging on the physical and mechanical properties of UHMWPE gamma sterilized in air have been studied extensively and significant changes in these properties have been reported. Age-related changes in material properties due to oxidative degradation are now understood to be a significant factor affecting wear of articulating surfaces. Efforts have been made to reduce the oxidative degradation of UHMWPE by packaging in an inert environment or vacuum or using alternate sterilization methods such as ethylene oxide or gas plasma. In the case of high energy irradiation-induced crosslinking of UHMWPE, the free radicals produced during irradiation can be reduced by subsequent melt annealing. In this study, the effect of accelerated laboratory aging on mechanical and fracture properties of melt-annealed and gas plasma sterilized in air electron-beam crosslinked GUR 1050 UHMWPE, was studied and compared to conventional gamma sterilized in nitrogen controls.
Highly crosslinked UHMWPE was produced by high energy e-beam irradiation of compression molded GUR 1050 at 110 kGy. The e-beam irradiation was followed by a melt-annealing process to reduce free radicals and all crosslinked specimens were double cycle gas plasma sterilized in air. Control specimens, obtained from the same manufacturing lot of compression molded slabs of GUR 1050, were gamma irradiated in nitrogen at 37±3 kGy and used as non-aged controls. Part of these specimens were accelerated aged in the laboratory in oxygen at 70 deg C and 5 atm for 14 days (Sanford and Saum technique) and referred to as aged-controls. Crystallinity was measured by wide angle x-ray diffraction (WAXS) and mechanical tests were conducted using servohydraulic universal test machines. Tensile tests were conducted per ASTM E8 Test Method using ASTM F648 Type IV specimens and J-integral fracture toughness tests were conducted on 12.7mm thick compact-tension specimens per ASTM Test Method E1820.
Aging resulted in an increase in crystallinity both for control (by 10%) and crosslinked UHMWPE (by 5%). The ultimate tensile strength (UTS) of the aged gamma control specimens decreased significantly (by 43%) relative to the non-aged gamma controls while both yield strength and elongation of the non-aged gamma controls and aged gamma controls increased slightly (by 14% and 16% respectively). The aged and non-aged crosslinked UHMWPE showed no significant difference in these properties. The aged crosslinked UHMWPE demonstrated yield and tensile strength values comparable to the aged gamma controls. J-R curves for gamma control specimens indicated that the crack growth resistance for the aged condition was lower compared to the non-aged condition while no significant difference in crack growth resistance was observed for melt-annealed crosslinked UHMWPE for aged and non-aged conditions. J-integral fracture toughness (JIC) for non-aged gamma control specimens decreased from 66.4 kJ/m2 to 22.9 kJ/m2 (by 66%) upon aging. Highly crosslinked UHMWPE showed a slight increase in fracture toughness from 30.5 to 34.2 kJ/m2 indicating no detrimental effect of aging on the fracture toughness of crosslinked polyethylene. Fracture toughness for aged crosslinked polyethylene was significantly higher compared to the aged gamma controls. These results indicate that melt annealed highly crosslinked UHMWPE is very resistant to oxidation-induced degradation in material properties associated with aging.
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