Study On The Structure Changes Of UHMWPE Following Irradiation, Storage In Air And Heat Treatment

Ultra-high molecular weight polyethylene (UHMWPE) has been widely used as a bearing material in total joint replacement components for over 35 years. Although UHMWPE has many properties making it an excellent choice for this application, wear is the primary cause of failure in joint replacement prostheses. In this paper, UHMWPE is irradiated to doses of 80, 120, 250, 500kGy, and then post-irradiation treated at 130℃ in vacuum for 12h. The effects of radiation and post-irradiation treatment on both the molecular structure, mechanical and tribological properties of UHMWPE are investigated.The structure changes of UHMWPE following irradiation, storage in air and heat treatment are studied with various techniques. X-ray diffraction (XRD) is used to characterize the crystallinity of UHMWPE. The relative importance of crosslinking and oxidation is studied with fourier transform infrared spectroscopy (FTIR) and gel fraction obtained by extraction in refluxing boiling xylene solvent for 10h. The mechanical properties of treated and un-trteated UHMWPE are measured on an INSTRON tester, and tribological properties investigated by sliding against ion-implanted Ti6A14V disk, Si3N4 and Al2O3 ceramic balls under distilled water and saline lubrication. The morphologies of the worn surfaces are observed with optical microscope.As a result, the crystallinity, crosslinking degree and oxidation index are all increased with dose, while the toughness and the elongation at break decrease. All irradiated samples exhibite lower wear rate but higher friction coefficient compared with the un-irradiated one. The wear resistance of UHMWPE is improved with increased dose of 80, 120, 250kGy. However, with higher irradiation dose, wear resistance decrease, which is associated with the ductile-brittle transition in its mechanical properties. Heat treatment in vacuum promotes residual radicals to crosslink and improves wear of UHMWPE more. The 250kGy irradiated and heat treated samples has shown the best wear resistance. The wear of un-irradiated UHMWPE is dominated by plowing, plastic deformation and adhesion, while treated samples are characterized by mild fatigue and abrasion, to fatigue, brittle fracture and abrasion with the increased dose of 500kGy.