M.P. Luda

Oxidation in orthopaedic UHMWPE sterilized by gamma-radiation and ethylene oxide

Ten new polyethylene prosthetic components (PEs), ethylene oxide (EtO)- and gamma-sterilized, were studied by means of infrared (IR) analysis in order to evaluate oxidation. Using attenuate total reflection infrared spectroscopy the authors demonstrate that all samples show surface oxidation, formed during processing and shaping, related to mechano-degradation. By means of Fourier transform infrared (FTIR) microscopy and derivatization techniques, the EtO-sterilized PEs present different behaviour in comparison to the gamma-radiated. The former show low levels of depth oxidation that could be related to the degradation which occurred during preparation of slices for IR analysis. The gamma-sterilized PEs show higher oxidation levels, variable from sample to sample, on the surface and in the bulk. The authors demonstrate that oxidation is described both by carbonyl species distribution and by hydroperoxide concentration. The hydroperoxides could be responsible for oxidative PEs ageing in in vivo service. The different oxidation levels and distribution could depend on the conditions of gamma-sterilization, in particular on the rate of the dose absorbed, not previously reported for ready-to-implant PEs. Chain scission of gamma-radiated UHMWPE, estimated by IR analysis, results in reduction of molecular weight and consequently in less abrasive resistance. The scheme of UHMWPE oxidation due to gamma-sterilization demonstrates the inhomogeneous distribution of reactive species in PEs responsible for the unpredictable performance in vivo.

In vivo UHMWPE biodegradation of retrieved prosthesis.

Sixty-two ultra high molecular weight polyethylene prosthetic components (PEs) (31 tibial plateaux and 31 cups), sterilised by gamma rays or ethylene oxide (EtO), were retrieved after 1-12 years depending on different medical reasons and were studied by FTIR spectroscopy with derivatisation of oxidised species. Esters, acids and hydroperoxides were found under the surface of the EtO sterilised PEs up to 2 mm depth. The behaviour of gamma ray sterilised PEs is more complex due to the oxidation following the sterilisation process. Ester and acid formation might arise from the diffusion of components of synovial liquid or from the oxidation process, whereas hydroperoxide formation is thought to be due to the oxidation. Abrasion and delamination process is discussed considering the topological distribution of degradation products.

Ultra high molecular weight polyethylene—II. Thermal- and photo-oxidation

Abstract Thermal- and photo-oxidation of ultra high molecular weight polyethylene (UHMWPE) has been studied using FTIR and derivatisation techniques. The species responsible for the initiation of the oxidative process are peroxides, produced by mechano-oxidative degradation, which evolve through a non-radical concerted mechanism and form ester and acid species. A second oxidative process takes place. It has an induction period and is similar to those described in the literature for linear low density polyethylene (LLDPE). The main difference between thermo- and photo-oxidation processes is the larger amount of acids formed in photo-oxidation. As far as the mechanism is concerned β-scission prevails in thermo-oxidative conditions whereas Norrish I and Norrish II reactions are the most relevant in photo-oxidation.

Analysis of products diffused into UHMWPE prosthetic components in vivo.

Ten UHMWPE hip inserts, five ethylene oxide and five gamma-ray sterilised in air, were retrieved during surgical revision after aseptic failure. Time in situ varied from 6 to 23 years. First implant was carried out for degenerative arthritis in all cases. The retrieved inserts were cut into two parts perpendicular to the articulate surface and a series of 150 microm thick slices were obtained from the cross-section. These were studied by FTIR microscopy and the absorbed products were extracted with cyclohexane for identification by GC/MS and Py/GC/MS. All retrieved UHMWPE components, independent of the sterilisation method, showed species adsorbed on the surface, which were mainly synovial liquid protein components. In addition, species such as cholesterol, fatty esters of cholesterol and squalene, also originating from synovial liquid, were found in the bulk. The concentration of the different species varies depending on the individual patient.

Thermal decomposition of fire retardant brominated epoxy resins

Abstract Brominated epoxy resin (BER) containing 20% Br has been prepared by curing a mixture of diglycidylethers of bisphenol A (DGBBA)/tetrabromobisphenol A (DGEBTBA) and 4,4′-diaminodiphenylsulphone (DDS). Its thermal stability has been studied by high resolution thermogravimetry and IR spectroscopy. Programmed heating condition using inert atmosphere has been applied to collect gaseous and high boiling product, which have been identified by means of gas chromatography–mass spectrometry (GC–MS). The mechanism of the thermal decomposition of the resin is discussed.

Epoxy resins cured with aminophenylmethylphosphine oxide—II. Mechanism of thermal decomposition

The thermal decomposition behaviour of the mixed epoxy resin, tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) and diglycidylether of bisphenol A (DGEBA), in a 1:1 ratio, cured with 3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane (M-DEA) or bis(m-aminophenyl)methylphosphine oxide (BAMPO) or with mixtures of the hardeners was studied. On heating, the resins rapidly decompose above 290 °C in an exothermic process involving dehydration, bond scission and chain fragment volatilisation. However, as the temperature increases, unsaturations created in the bond scission process polymerise to give a residue which, above 400 °C, undergoes aromatisation to form a thermostable ‘char’. In the presence of the phosphorous-containing hardener (BAMPO), the volatilisation is reduced and aromatisation is accelerated which leads to a larger amount of stable ‘char’. This is suggested to be a reason for the fire-retardant action of BAMPO which, however, at high BAMPO content is overwhelmed by flame quenching by volatilising phosphorus-containing moieties from BAMPO decomposition.

Ultra-high molecular weight polyethylene: I. Mechano-oxidative degradation

Mechano-oxidative degradation occurs during microtoming of unstabilised ultra-high molecular weight polyethylene (UHMWPE). The oxidised species formed are shown to be esters and, to minor extent, acids. A mechanism is proposed in which the displacement of the blade of the microtome creates primary radicals in pairs by stretching of the interpenetrating chains. Oxidation of one of these radicals, followed by coupling of the pair, yields a dialkyl peroxide which in turn has been shown to decompose to ester via a concerted mechanism.

Kinetic aspects of water sorption in polyester-resin/glass-fibre composites

Abstract Solubility and diffusivity of water in sheet-moulding compound (SMC) materials based on polyester-resin/glass-fibre composites with variable content of fibres, filler (CaCO 3 ) and other low-molecular-weight additives have been studied. The main scope is the finding of a realistic approach to describing water sorption behaviour as a function of the hydrophilic character of the matrix, the mechanical properties of the material and the water solubility of fibres, fillers and low-molecular-weight compounds (solutes). The model previously proposed for simultaneous water sorption in and solute release from moderately hydrophobic materials has been generalised here in order to take into account experimental findings on chemical and physical processes induced by water penetration into the composites. A method is developed that is based on the use of thermodynamic and kinetic parameters of water sorption/desorption for the physico-chemical characterisation of SMC materials.

Characterization and reprocessing of greenhouse films

Films for greenhouses are an attractive source of post-consumer plastic materials because they are mainly made of polyethylene and can be easily collected in large amounts in small zones. The types of polymers for this application are, however, increasing and the films contain not only additives and stabilisers, but also fertiliser and pesticide residues. Finally, the extent of photooxidative degradation undergone during the use can strongly influence the recycling operations and the final properties of the secondary material. In this work, a complete characterisation of post-consumer films for greenhouses has been carried out and the properties of the recycled material have been correlated with the number of reprocessing steps and compared with those obtained by reprocessing virgin scraps of the same composition. The presence of small amounts of low molecular weight compounds (photooxidized species and pesticide residues) does not compromise the use of the recycled plastic in many applications. The mechanical properties decrease with the number of reprocessing steps and with increasing level of photooxidative degradation but are good enough for many applications.

Mechanistic study of thermal behaviour and combustion performance of epoxy resins. II. TGDDM/DDS system

Abstract The kinetics of curing of tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) with 50% or 85% of stoichiometric 4,4′-diaminodiphenylsulfone (DDS) was studied by differential scanning calorimetry. It was found that the activation energy of the process increases with advancement of curing, which is caused by concomitant homopolymerisation of TGDDM accompanied by cyclisation. The structure of the cured formulations was characterised by infrared and 13 C NMR spectroscopy. The mechanistic study of thermal decomposition of TGDDM/DDS systems was carried out by thermogravimetry and differential scanning calorimetry in inert atmosphere and by thermal volatilisation analysis in vacuo . It was found that above 200 °C elimination of water from secondary alcohol groups takes place with formation of aliphatic unsaturation. Breaking of the resulting weakened allylic CN bonds leads to chain scission with formation of primary amine and quinolin-type chain ends. Scission of non-allylic CN bonds followed by evolution of acetone was observed at about 300 °C. The volatilisation of sulfur-containing species and formation of a thermostable char occurred on further heating to 400 °C. The poorer fire resistance of TGDDM/DDS formulations in comparison with homopolymerised TGDDM is discussed on the basis of a lower yield of char and a higher yield of gases.