Evolution of morphology in UHMWPE following accelerated aging: The effect of heating rates

Abstract

Accelerated aging methods are used to evaluate the oxidative stability of UHMWPE components for total joint replacements. In this study, we traced the evolution of the crystalline morphology during accelerated thermal aging of UHMWPE in air with the intent of explaining previous, counterintuitive heating rate effects. GUR4150HP extruded rod stock material was machined into miniature (0.5 mm thick) specimens that were either gamma irradiated in air or in nitrogen (27 +/- 3 kGy) or left unirradiated (control). Accelerated aging in an air furnace (at 80 degrees C, atmospheric pressure) was performed on half of the test samples at a heating rate of 0.1 degrees C/min and at 5 degrees C/min for the remaining half. Although the initial heating rate, as measured by changes in density, did influence the absolute degradation rate by up to 214%, the heating rate effect did not appear to influence the relative ranking of UHMWPE in terms of its oxidative stability. The heating rate effect is more consistent with a kinetic mechanism of the oxidation process than it is with a previously hypothesized diffusion mechanism. UHMWPE morphology, as characterized using a transmission electron microscope (TEM), demonstrated considerable rearrangement of the crystalline regions as a result of the accelerated aging. The stacking of the lamellae observed after accelerated aging was not consistent with the morphology of naturally aged UHMWPE components. The observed differences in crystalline morphology likely result from the enhanced mobility of the polymer chains due to thermal aging and may be analogous to an annealing process.

Comparative micro‐Raman study on standard, cross‐linked and vitamin E‐blended polyethylene acetabular cups after long‐term in vitro testing and ageing

“…Several studies have reported an increase in PE crystallinity upon ageing, and this behaviour has been related to a rearrangement of the polymeric chains as a result of their increased mobility due to thermal ageing . However, for EtO‐sterilized samples, this effect appeared less pronounced; on the other hand, thermally unstabilized XLPE samples submitted to accelerated ageing did not display any increase in crystallinity, as well as XLPE and UHMWPE components after 2 years of real‐time shelf ageing .…”

Effect of long‐term natural aging on the thermal, mechanical, and viscoelastic behavior of biomedical grade of ultra high molecular weight polyethylene

“…Also, the aging of polymer that performed at 80°C for 23 days has found to be equivalent to approximately 8 years shelf aging 1. Meanwhile, many researchers have highlighted certain limitations of accelerated thermal aging techniques 18, 19. For example, thermal aging of UHMWPE is considered as a severe oxidative process compared to natural aging.…”

Ultra High Molecular Weight Polyethylene and its Reinforcement with Carbon Nanotubes in Medical Devices

“…The chain scission that occurs during aging of UHMWPE leads to shorter molecules with higher mobility. Thus they can pack more efficiently into the lamellae leading to an increase in overall crystallinity and density (Kurtz [1999b]). The chain scission also affects the tie molecules by decreasing the effectiveness in supporting load leading to the breakage of the polymer long chains, reducing the molecular weight, increasing the brittleness, and weakening the properties of aged UHMWPE (Fouad [2010]).…”