Subsurface oxidation of polyethylene

Abstract

Since the 1960s ultrahigh molecular weight polyethylene has been a primary bearing material in orthopedic prostheses. Most polyethylene components have been sterilized by exposure to gamma radiation, but in the presence of oxygen this promotes degradation and correlates with component failure in the body. To better understand the mechanism of degradation, we measured oxidation profiles for components shelf aged 5.8 and 10.9 years. Microtomed sections from each component were analyzed for ketones, which are the primary oxidation products, by FTIR spectroscopy. Maximum ketone concentrations in both cases were found several millimeters below the surface. We propose a model to explain the profiles. Alkyl radicals, which are uniformly generated in the polyethylene during irradiation, react with peroxyl radicals, which are produced by alkyl radicals with oxygen to form ketones. At the component surface ketone levels are low because high oxygen concentrations deplete alkyl radicals, creating an excess of peroxyl radicals. In the bulk material ketones are low because low oxygen concentrations limit the formation of peroxyl radicals. However, just below the surface the concentrations of alkyl and peroxyl radicals balance, promoting ketone production. A computer simulation that accounts for the coupling of these reactions with the diffusion of oxygen shows a good agreement with the measured profiles.

Oxidation of gamma-irradiated ultrahigh molecular weight polyethylene

“…Several investigators have reported on the results of modeling the simultaneous oxygen diffusion and oxidation reactions as a function of depth and time. 19,20,28,29 Several investigators have assumed that the oxygen profile reaches a pseudo steady state in a relatively short time and remains constant thereafter, 19,20 thus simplifying the problem. This assumption is probably reasonable, because the time scale of oxygen diffusion is somewhat shorter than that for the reaction sequence (discussed further below).…”

“…The predictions of oxygen penetration at 80°C are shown in Figure 2(b) and illustrate the much more rapid transport of oxygen at that elevated temperature. The oxygen-concen- 18 Oxygen diffusivity ϭ 1.14 ϫ 10 Ϫ7 (cm/s) 28 Activation energy for oxygen solubility ϭ Ϫ1.7 (kJ/mol) 18 Activation energy for oxygen diffusion ϭ 36.8 (kJ/mol) 18 tration profile after 1 day (24 h) at 80°C looks very much like the 1-week (7 days) profile at 23°C, keeping in mind that the oxygen solubility at 80°C is marginally lower than that at 23°C, as noted by the surface concentrations. The piece would be essentially saturated to a depth of 5000 m within 4 weeks.…”

Determination of Crystallinity and Crystal Structure of Hylamer™ Polyethylene after in vivo Wear

“…The former appears to be more crystalline than the latter hence confirming the role of the sterilization environment on the morphology of UHMWPE samples. Oxygen can react with the free radicals generated from the chain scission, thus promoting other chain scissions, while in an inert environment, intermolecular cross-linking can protect the material from degradation [21].…”

“…This result does not exclude the presence of oxidation species like ketones, alcohols, acids, etc. In fact, many researchers have found that the maximum concentration of these products is several millimeters below the surface [21].…”

Modification of Hexatriacontane by O2–N2 Microwave Post-Discharges

“…This initiation step turns the chain of the HTC into a free radical. In gamma irradiation processes, this initiation step is ensured by photons [41][42][43]. In this case, to understand how the scission of the C-C bond occurs, molecular reactions in the alkane must be described.…”