On the assessment of oxidative stability of post-irradiation stabilized highly crosslinked UHMWPEs by thermogravimetry

Abstract

The characterization of the oxidative resistance of contemporary UHMWPE formulations currently relies upon time-consuming accelerated aging protocols and subsequent FTIR assessment. The potential of thermogravimetric analysis (TGA) for this purpose, however, remains mostly unknown. To study radiation and stabilization-induced changes in the decomposition process, TGA curves corresponding to virgin, e-beam irradiated, and post-irradiation stabilized UHMWPE materials were registered and analyzed. TGA traces of most materials exhibited similar features, with two weight increases before an initially non-linear decomposition and a portion of linear volatilization starting near 400 °C. After irradiation, the onset of the second weight increase shifted to lower temperatures and the linear volatilization stage began at higher temperatures. While post-irradiation annealing did not introduce significant changes in TGA curves, remelting delayed the onset of the second weight gain. Furthermore, this weight increase disappeared after vitamin E diffusion. Energies of activation of the initially predominant Thermoxidation and the subsequent thermodegradation processes were also calculated from fraction of conversion and reciprocal of temperature plots. As opposed to TGA, FTIR assessment of oxidation following accelerated aging at 120 °C for 36 h appeared to be less sensitive to discriminate between the different materials. This study confirms TGA as a promising technique to characterize the oxidative resistance of medical grade UHMWPEs.