Comparison of the performances and aging mechanisms of XLPE and EVA insulation during their thermal oxidative aging

Abstract

The thermal oxidation of low-density polyethylene and poly(ethylene-co-vinyl acetate) both cross-linked with triallyl isocyanurate (respectively denoted XLPE and EVA) was studied at 165 °C in air by Fourier transform infrared (FTIR) spectroscopy, dielectric spectroscopy and uniaxial tension. Monitoring the carbonyl index throughout the exposure allowed showing that these two polymers have similar oxidation reactions, taking place on their ethylene monomers, and therefore similar oxidation kinetics. In addition, in EVA, the acetate side groups thermally decompose into several degradation products during the exposure, such as: trans-vinylene double bond, acetic acid, saturated and unsaturated γ-lactone, and methane, but without interacting with the oxidation reaction. As oxidation is diffusion controlled, it leads to the development of oxidation profiles across the sheets of about 1 mm thick of these two polymers. In addition, as the oxygen transport properties (both solubility and diffusivity) are higher in EVA than in XLPE, oxidation affects the center of the EVA sheets from the early periods of exposure, whereas it is necessary to wait for high conversion ratios of the oxidation reaction to observe the same types of chain-shaped profiles in XLPE sheets. Oxidation results in dramatic changes in functional properties (both electrical and mechanical properties) for both polymers. Before thermal aging, EVA is already a poor insulating material compared to XLPE, due to the presence of polar side groups (i.e. acetates). In contrast, EVA exhibits a higher ductility, which justifies its use as nanofillers in PE-based insulating materials in order to improve their mechanical properties. The establishment of correlations between the changes in functional properties and the progress of oxidation allows the proposal of structural, electrical and mechanical end-of-life criteria for both polymers, corresponding to critical values of carbonyl index, dielectric constant, and elongation at break, respectively.