SEQUENTIALLY CROSSLINKING ULTRA HIGH MOLECULAR POLYETHYLEN(UHMWPE) PRESERVES THE MICROSTRUCTURE WHILE PROVIDING OXIDATION RESISTANCE

Abstract

Remelted highly cross linked UHMWPEs have no detectable free radicals but the mechanical and fatigue properties are reduced because remelting changes the microstructure. Annealed highly cross linked UHMWPEs maintain the microstructure and mechanical properties but contain free radicals. A novel sequential irradiation and annealing process preserves the microstructure while providing enhanced oxidation resistance.

6_B_Material_e_Methods: GUR 1020 polyethylene was sequentially cross linked using three separate gamma radiation doses of 3 Mrad with an annealing step at 130 degrees C after each irradiation (SXL). Density was measured according to ASTM D1505. Crystallinity and thermal properties were determined according to ASTM D3417. Crystallite size/lamellar structure was determined by small angle x-ray scattering. Accelerated aging was carried out in an oxygen bomb under 5 atmospheres of oxygen at 70 degrees C for 14 days.

SXL density was 939.2 kg/cubic meter, identical to that for unirradiated UHMWPE and UHMWPE irradiated in nitrogen to 3 Mrad (gamma-N2). SXL crystallinity was 61.7%, compared to 61.3% and 59.2% for gamma-N2 and virgin UHMWPE, respectively. The long period spacing, crystal thickness and amorphous thickness were 38.2, 23.6 and 14.6 nm respectively for SXL and 38.9, 23.0 and 15.9 for gamma-N2. There was no statistical difference. Accelerated aging resulted in a white band for gamma-N2 with an oxidation index of 1.27. The response of SXL was the same as virgin UHMWPE e.g. crystallinity and density were unchanged with no white band formation and an oxidation index of 0.09.

By avoiding remelting, sequential irradiation and annealing preserves polyethylene microstructure. The sequential process allows more efficient cross linking of free radicals resulting in an oxidation resistance equivalent to that of virgin UHMWPE.