Radiochemical oxidation of ethylene–propylene elastomers in the presence of some phenolic antioxidants (original) (raw)
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DSC and oxygen uptake studies of γ-irradiated ethylene–propylene elastomers
1999
The speci®c heat capacity of irradiated ethylene±propylene copolymer was examined by dierential scanning calorimetry. Additional measurements on oxidative stability of g-exposed EPR were performed by isothermal and isobaric oxygen uptake. The c p dierences between vacuum ± and air ± radiolysis of polymers are pointed out. The oxidative irradiation environment generates peroxy radicals that are involved in the air-degraded samples. The c p dependencies on temperature determined for the two manners of irradiation are not alike. The assessment of the thermal behavior by DSC and oxygen uptake con®rms the key role of RO 2 . in the radiolytic and thermal degradation of these polymers. #
Radiation Physics and Chemistry (1977), 1985
In part one of this series the effects of a phenolic, an amine and a thioester antioxidant on the thermo-oxidative stability of irradiated and unirradiated low-density polyethylene was reported. In this paper the effects of combined phenolic and thioester stabilizers are described. Isothermal thermogravimetric analysis was used to study the systems. Pronounced synergism was observed with the induction periods, the time when the initial weight loss begins and the 5% weight loss. At about 50% of each stabilizer increases greater than twofold were observed both with the unirradiated and irradiated polymers. The rate constants for oxygen uptake were decreased. However, the rates of degradation at 5% weight loss fell between the values of the two pure stabilizers with no pronounced synergism in either case. In the absence of oxygen little effect of either antioxidant or their mixtures was observed. The corresponding activation energies were somewhat higher, however, with the irradiated samples containing antioxidants. Dynamic thermogravimetry was used for this study. A kinetic analysis indicated that there were somewhat different modes of degradation at lower-and highertemperature ranges.
EPR study of the formation of radicals in PP with antioxidants irradiated with gamma rays
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Thermo-oxidative degradation and radio-processing of ethylene vinyl acetate elastomers
Polymer Degradation and Stability, 2003
Thermal and radiation degradation of three ethylene vinyl acetate elastomers containing 28 and 33% VA were studied by IR spectroscopy and thermal analysis. Kinetic parameters: induction time, oxidation rate, reaction order and activation energy of oxidation were calculated on the basis of the modification in the carbonyl band absorbance during thermal degradation. Temperatures that characterize various processes occurring during degradation are pointed out. The progress in radiochemical degradation was assessed by changes in the 1735 cm À1 band relative to a standard (1475 cm À1 ). A mechanism of degradation is proposed. #
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1998
Following gamma irradiation in air which causes bond scission and yields large concentrations of peroxy radicals, maximum oxidation and an increase in crystallinity occurs on the surface of ultrahigh molecular weight polyethylene. Here, bimolecular reactions of peroxy radicals generate carbonyls, mostly ketones. On the polymer surface, peroxy radicals continue to react over time periods of years to generate carbonyls and chain scission. Peroxy radicals in the interior of the polymer abstract hydrogens and form hydroperoxides, inducing chain reactions and a slow but continue increase of ketone. Within the polymer sample, to a decreasing depth with increasing dose, a reduced concentration of oxygen is available to react with radiolytic radicals, so that more efficient crosslinking and a low level of hydroperoxide chain reaction occur. After long periods of time a surface maximum in carbonyl concentration is produced. Heating polyethylene in high pressures of oxygen accelerates the oxidative process.