Radiochemical ageing of EPDM elastomers. 3. Mechanism of radiooxidation

The preceding paper of this series was devoted to the identification and quantification of the main chemical changes resulting from the radiochemical ageing of EPDM (77.9 % ethylene, 21.4 % propylene, 0.7 % diene) and EPR (76.6 % ethylene, 23.4 % propylene) films irradiated under oxygen atmosphere using Co-60 gamma rays. The double bond of the diene was observed to be consumed with a high radiochemical yield. The oxidation and reticulation rates were observed to be higher in the case of EPDM than in EPR. Accumulation of the major oxidation products in both polymers was shown to occur in the order of decreasing concentrations: hydroperoxides, ketones, carboxylic acids and alcohols, peroxides. On the basis of the analysis of the oxidation products formed in EPDM and EPR, and taking into account their relative concentrations, the mechanisms accounting for the EPDM gamma-degradation under oxygen atmosphere are proposed in the present paper. Two main processes are involved in the EPDM radiooxidation. The random gamma-radiolysis of the polymer provides a constant source of macroalkyl radicals mainly formed on ethylene units. The secondary radicals so formed are likely to initiate a selective oxidation of the polymer through free-radicals reactions involving the abstraction of labile hydrogen atoms. In particular, the hydroperoxides decomposition and the consumption of the ENB moieties, this latter being the most oxidisable site and the source of crosslinking, may result from hydrogen abstraction by radical species. (C) 2004 Elsevier B.V. All rights reserved.