The radio-thermal oxidation of silane cross-linked polyethylene (Si-XLPE) was studied in air under different γ dose rates (6.0, 8.5, 77.8, and 400 Gy.h −1 ) at different temperatures (21, 47, and 86 °C). The changes in the physico-chemical and mechanical properties of Si-XLPE throughout its exposure were determined by FTIR spectroscopy, differential scanning calorimetry (DSC), swelling measurements, rheometry in rubbery (DMTA) and in molten states, and uniaxial tensile testing. It was found that oxidation leads to the build-up of a wide variety of carbonyl and hydroxyl products (mostly carboxylic acids and hydroperoxides) and an efficient chain scission process that catastrophically reduces the concentration in elastically active chains and the elongation at break from the early periods of exposure. A new analytical model was derived from the current radio-thermal mechanistic scheme without making the usual assumption of thermal stability of hydroperoxides. After an initial period where the oxidation kinetics occurs with a constant rate, this model allows also predicting the auto-acceleration of the oxidation kinetics when the hydroperoxide concentration reaches a critical value of about 1.6 × 10 −1 mol.L −1 . Choosing this critical value as a structural end-of-life criterion allows a more direct assessment of the lifetime of Si-XLPE in the various radio-thermal environments under study, except at the highest temperature (i.e. 86 °C) where the kinetic model can still be noticeably improved.

This paper investigates the evolution of electrical properties with aging for two XLPE based materials having different chemical compositions. It is shown that aging and material composition can significantly affect the electrical response, suggesting the suitability of dielectric spectroscopy for polymer diagnostics. Furthermore, the dielectric response is correlated with the evolution of mechanical properties, in particular elongation at break (EaB), in order to evaluate the possible scaling-up of the current cable qualification standard.

This study focuses on the degradation of a silane cross-linked polyethylene (Si-XLPE) matrix filled with three different contents of aluminum tri-hydrate (ATH): 0, 25, and 50 phr. These three materials were subjected to radiochemical ageing at three different dose rates (8.5, 77.8, and 400 Gy·h−1) in air at low temperatures close to ambient (47, 47, and 21 °C, respectively). Changes due to radio-thermal ageing were investigated according to both a multi-scale and a multi-technique approach. In particular, the changes in the chemical composition, the macromolecular network structure, and the crystallinity of the Si-XLPE matrix were monitored by FTIR spectroscopy, swelling measurements in xylene, differential scanning calorimetry, and density measurements. A more pronounced degradation of the Si-XLPE matrix located in the immediate vicinity of the ATH fillers was clearly highlighted by the swelling measurements. A very fast radiolytic decomposition of the covalent bonds initially formed at the ATH/Si-XLPE interface was proposed to explain the higher concentration of chain scissions. If, as expected, the changes in the elastic properties of the three materials under study are mainly driven by the crystallinity of the Si-XLPE matrix, in contrast, the changes in their fracture properties are also significantly impacted by the degradation of the interfacial region. As an example, the lifetime was found to be approximately halved for the two composite materials compared to the unfilled Si-XLPE matrix under the harshest ageing conditions (i.e., under 400 Gy·h−1 at 21 °C). The radio-thermal oxidation kinetic model previously developed for the unfilled Si-XLPE matrix was extended to the two composite materials by taking into account both the diluting effect of the ATH fillers (i.e., the ATH content) and the interfacial degradation.

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