UHMWPE Oxidation

Macroradicals can easily be found in the bulk of virgin UHMWPE. These radicals result from thermal decomposition of hydroperoxides (Lacoste et al. 1981) created during processing by compression molding or ram extrusion. These macroradicals, in the presence of oxygen, react readily giving hydroper-oxyl radicals and the reaction is already favored at room temperature. We must 

Ketone formation occurs via a closed process, which does not create new radicals and then does not spread oxidation. Furthermore, because the process takes place without further chain scissions, it does not induce any variation of the molecular mass and only minimum changes of the mechanical properties of UHMWPE. 

Acids are produced by scission of the polymeric chain, with a mechanism that has not yet been elucidated (Costa, Luda, and Trossarelli 1997b, Zaharadnickova, Sedlar, and Dastych 1991). Acid formation leads to a decrease in molecular mass and thus to a progressive deterioration of the mechanical properties of the UHMWPE. Acid determination, by using derivatization techniques and FTIR analyses (Lacoste and Carlsson 1992), makes it possible to quantify the molecular mass variation of the oxidized material in relation to the original molecular mass (Costa et al. 1998a). 

Thermal decomposition of hydroperoxides to form secondary alcohols. 

Hydroperoxides determination is the key factor to measure the level and the behavior of the oxidation (Carlsson et al. 1987b, Lacoste and Carlsson 1991, Shen, Yu, and McKellop 1999). Scientific studies and the American Society for Testing and Materials (ASTM) oxidation index standard (ASTM F2102) usually give the quantity of ketones and other carboxyl species present as an index of the oxidation degree. It must be pointed out that ketones, though a product of the oxidative process, do not produce polymeric chain scissions and so they do not result in substantial reduction of tiie UHMWPE mechanical properties. Quantification of ketones is reliable only if the ratio between ketones and carboxylic acids remains constant through the entire oxidative process. 

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UHMWPE Oxidation Induced by Gamma or E-Beam Irradiation... 

In 1991, John Dumbleton, PhD, in charge of Howmedica Worldwide R D, wanted to focus on decreasing the incidence of osteolysis and look more closely at UHMWPE oxidation. He beheved measurement of wear needed to be more representative of chnical conditions and decided to focus one researcher on understanding of the stracture/property relationships ofUHMWPE and developing improved materials. 

As shown in Chapter 22, UHMWPE components can undergo oxidative degradation both on the shelf and in vivo. One of the consequences of sevCTe UHMWPE oxidation is subsurface cracking that ultimately leads to delamination [24]. 

M. E Rocha, A. A. P. Mansur, and H. S. Mansur. FTIR investigation of UHMWPE oxidation submitted to accelerated aging procedure. Macroniolecular Symposia, 296 (1), 487-492 (2010). 

PC PE PES PET PF PFA PI PMMA PP PPO PS PSO PTFE PTMT PU PVA PVAC PVC PVDC PVDF PVF TFE SAN SI TP TPX UF UHMWPE UPVC Polycarbonate Polyethylene Polyether sulfone Polyethylene terephthalate Phenol-formaldehyde Polyfluoro alkoxy Polyimide Polymethyl methacrylate Polypropylene Polyphenylene oxide Polystyrene Polysulfone Polytetrafluoroethylene Polytetramethylene terephthalate (thermoplastic polyester) Polyurethane Polyvinyl alcohol Polyvinyl acetate Polyvinyl chloride Polyvinyl idene chloride Polyvinylidene fluoride Polyvinyl fluoride Polytelrafluoroethylene Styrene-acrylonitrile Silicone Thermoplastic Elastomers Polymethylpentene Urea formaldehyde Ultrahigh-molecular-weight polyethylene Unplasticized polyvinyl chloride... 

Two retrieved cross-linked UHMWPE acetabular cups were preliminary investigated by Raman microprobe spectroscopy with respect to their oxidation... 

Fig. 17.4. Typical near-surface oxidation maps as measured in the non-wear and main wear zones, and strain maps as measured in the wear zone of UHMWPE-retrieved acetabular cups (a) 25 years and 3 months exposure in vivo and (b) 17 years and 9 months exposure in vivo. The linear dimensions of the map and the collection depth of the focal plane in confocal probe configuration are also explicitly shown...

Fig. 17.5. Comparison between oxidation profiles and compressive strain profiles for a series of four retrieved and one unused UHMWPE cups (details in the inset of...

Comparing Figs. 28 and 29, accelerated aging affects the fatigue resistance of all UHMWPE samples. Again, gas-plasma and ethylene oxide sterilization as well as the nonsterilized product show little differences after aging,... 

Thermal oxidative degradation of PE and PE nanocomposites has been extensively studied over the past decades [26-30], It has been reported that the main oxidation products of PE are aldehydes, ketons, carboxylic acids, esters and lactones [26, 27], According to Lacoste and Carlsson [28], P-scission plays an important role in thermal oxidation of UHMWPE. Notably, the feasibility of intra-molecular hydrogen abstraction by the peroxy radicals for polyethylene has been questioned in frames of a thermal oxidation mechanism proposed by Gugumus [29, 30],... 

Muratoglu, O. K., Bragdon, C. R. et al., A Novel Method of Crosslinking UHMWPE to Improve Wear, Reduce Oxidation and Retain Mechanical Properties, /. Arthoplasty, 16, 149-160, 2001. 

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