L. Trossarelli

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.

Study of the mechanism of intumescence in fire retardant polymers: Part I—Thermal degradation of ammonium polyphosphate-pentaerythritol mixtures

Abstract Interactions are shown to take place between ammonium polyphosphate and pentaerythritol in thermally degrading mixtures. Up to 500°C, gaseous products are evolved on programmed heating above 200°C in two steps with maximum rate temperatures differing by about 100–150°C. Swelling of the degrading mass seems, however, to be mainly due to gases evolved in the step occurring at the higher temperature. A strong dependence of the degradation process on the experimental conditions is observed.

Study of the mechanism of intumescence in fire retardant polymers: Part V—Mechanism of formation of gaseous products in the thermal degradation of ammonium polyphosphate

On heating, ammonium polyphosphate eliminates ammonia and water in two successive steps. The first (T = 165–280°C) involves a limited weight loss (ca. 3%) and the formation of some branched structures. The transition from one crystalline form of ammonium polyphosphate to another, more stable, form, may prevent further elimination of NH3 and H2O which occurs in the second step (T > 280°C). The final product of degradation is characterised by a crosslinked POP structure formed by elimination of H2O between POH groups freed by the evolution of NH3.

Photooxidative degradation of acrylic and methacrylic polymers

Abstract The photooxidative stability of poly(methyl acrylate), poly(ethyl acrylate), poly(ethyl methacrylate) and poly(butyl methacrylate) has been investigated under conditions of artificial solar light irradiation. Molecular and chemical changes induced by the light treatment were followed by size exclusion chromatography and Fourier transform infrared spectroscopy. The acrylate units were found to be more reactive towards oxidation, in comparison with the methacrylate ones. With short alkyl side groups chain scissions prevailed over cross-linking reactions both in acrylate and methacrylate samples. The degradation of poly(butyl methacrylate) proceeds in a completely different way, with extensive cross-linking and simultaneous fragmentation reactions. In all the samples the structure formed as result of oxidation reactions were similar. It has also been found that the first effect of degradation to be detected is that connected with the changes of samples molecular weight distributions.

Study of the mechanism of intumescence in fire retardant polymers: Part II—Mechanism of action in polypropylene-ammonium polyphosphate-pentaerythritol mixtures

Abstract It is shown that, by the addition of a typical intumescent mixture of ammonium polyphosphate and pentaerythritol to polypropylene, the mechanism of intumescence which develops on heating is not significantly affected by dispersion of the intumescent mixture in the polymer. On the other hand, in these mixtures, polypropylene seems to evolve, by thermal degradation, a smaller amount of flammable products than when it is heated alone. The ammonium polyphosphate-pentaerythritol additive is shown to induce fire retardant characteristics in polypropylene by means of a ‘condensed phase’ mechanism.

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.

Study of the mechanism of intumescence in fire retardant polymers. Part IV: Evidence of ester formation in ammonium polyphosphate - pentaerythritol mixtures

It is shown that, on heating ammonium polyphosphate-pentaerythritol mixtures, phosphoric ester bonds are formed first by alcoholysis of the polyphosphate chain. Further intramolecular alcoholysis and/or esterification leads to cyclic pentaerythritol phosphate structures inserted in the polyphosphate chains. Evidence is also given of intermolecular dehydration of pentaerythritol in mixtures with a large excess of alcoholic hydroxy groups over phosphorus atoms. The intumescent behaviour of the mixtures is discussed in relation to the final structure of the product of reaction which depends on the composition of the original mixture.

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.

Study of the mechanism of intumescence in fire retardant polymers: Part III—Effect of urea on the ammonium polyphosphate-pentaerythritol system

Abstract Urea, which is commonly used as a ‘blowing’ co-additive in intumescent coatings, is shown to depress intumescence when it is added to ammonium polyphosphate-pentaerythritol mixtures incorporated into the bulk of polypropylene. Concurrently, the fire retardant properties of the intumescent additive are depressed in the presence of urea although, in this case, a smaller amount of flammable hydrocarbons is evolved in the thermal degradation of the polymer.