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.

The radio-thermal ageing of silane-crosslinked 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 electrical properties of Si-XLPE throughout its exposure were determined using Fourier transform infrared spectroscopy coupled with chemical gas derivatization, hydrostatic weighing, differential scanning calorimetry, dielectric spectroscopy and current measurements under an applied electric field. From a careful analysis of the oxidation products, it was confirmed that ketones are the main oxidation products in Si-XLPE. The analytical kinetic model for radio-thermal oxidation was thus completed with relatively simple structure–property relationships in order to additionally predict the increase in density induced by oxidation, and the adverse changes in two electrical properties of Si-XLPE: the dielectric constant ε′ and volume resistivity R. After having shown the reliability of these new kinetic developments, the lifetime of Si-XLPE was determined using a dielectric end-of-life criterion deduced from a literature compilation on the changes in R with ε′ for common polymers. The corresponding lifetime was found to be at least two times longer than the lifetime previously determined with the conventional end-of-life criterion, i.e., the mechanical type, thus confirming the previous literature studies that had shown that fracture properties degrade faster than electrical properties.

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.

The radiothermal ageing of silane-crosslinked low-density PE (Si-XLPE) films was studied in the air under three different γ dose rates (8.5, 77.8, and 400 Gy·h−1) at a low temperature close to ambient (47, 47, and 21 °C, respectively). Changes in crystalline morphology were investigated using a multi-technique approach based on differential scanning calorimetry (DSC), wide- (WAXS) and small-angle X-ray scattering (SAXS), and density measurements. In particular, the changes in four structural variables were accurately monitored during radiothermal ageing: crystallinity ratio (XC), crystalline lamellae thickness (LC), long period (Lp), and interlamellar spacing (La). Concerning the changes in XC, a perfect agreement was found between DSC and WAXS experiments. Successive sequences of self-nucleation and annealing (SSA) were also performed on aged Si-XLPE samples in the DSC chamber in order to assess the thickness distribution of crystalline lamellae. This method allowed the thermally splitting of the melting domain of Si-XLPE into a series of elementary melting peaks, with each one characterised by a distinct thickness of crystalline lamellae. DSC (used with the SSA method) showed a slight increase in LC during the oxidation of Si-XLPE, while SAXS confirmed a catastrophic decrease in La. The critical value of the interlamellar spacing characterising the ductile/brittle transition of Si-XLPE was found to be of the same order of magnitude as that for linear polyethylene (LaF≈6 nm). This structural end-of-life criterion can now be used for predicting the lifetime of Si-XLPE in a nuclear environment.

This study focuses on the degradation of a silane cross-linked polyethylene (Si-XLPE) matrix filled with three different aluminum tri-hydrate (ATH) contents: 0, 25, and 50 phr. These materials are subjected to radiochemical ageing in air under three different dose rates (8.5, 77.8, and 400 Gy h −1 ) at two temperatures (47 and 21°C). Changes due to ageing have been investigated by FTIR spectroscopy, DSC analysis, and density measurements in order to more precisely define their chemical composition and microstructure and to evidence the possible influence of ATH fillers on the oxidation kinetics of the Si-XLPE matrix.