Radiation induced Free Radical Stability and Degradation Behavior in GUR resin and 1900 resin types UHMWPE over 20 years of aging

Radiation-induced free radical stability was evaluated using Electron Spin Resonance (ESR) for two medical-grade Ultra High Molecular Weight Polyethylene (UHMWPE) resins, GUR 4150 and 1900H, after 20 years of aging in air and vacuum at 23 °C, 37 °C, and 75 °C. Spectral analysis revealed oxygen-induced radicals in samples aged in air at 23 °C and 37 °C, with higher concentrations at 23 °C, whereas vacuum-aged samples retained primary radiation-induced radicals that decreased progressively with increasing temperature; no radicals were detected in air-aged samples at 75 °C. Overall, radical concentration decreased by about two orders of magnitude over 20 years. At 23 °C, 1900H exhibited lower FRC than GUR 4150 by about two-thirds, indicating improved oxidative stability. After air aging at 23 °C, 1900H also showed reduced carbon-centered polyenyl radicals (R1, 0.6×) and oxygen-induced polyenyl radicals (R2, 0.5×) compared to GUR 4150. X-ray diffraction (XRD) confirmed the crystalline structure of both materials through the characteristic orthorhombic peaks at Bragg angles 2θ = 22° (110) and 24° (200). The average crystallite sizes for GUR 4150 were 21.76, 20.24, and 21.84 nm, while for 1900H they were 17.14, 20.22, and 21.56 nm at 23 °C, 37 °C, and 75 °C, respectively. Fourier Transform Infrared Radiation measurements revealed oxidation index values of 0.34, 0.4, and 0.54 for GUR 4150, compared with significantly lower values of 0.5, 0.12, and 0.14 for 1900H, indicating enhanced oxidation resistance of 1900H after 20 years of aging.