Thermal aging of electrical cable insulation made of cross‐linked low density polyethylene for under‐hood application in automotive industry

Abstract

The thermal aging of two electrical cable insulations made of cross-linked low density polyethylene (XLPE) only differing by their stabilization system was studied in air between 125 and 200°C. The chemical consumption of the two types of antioxidants (phenol and sulfide), the build-up of oxidation products and the consequences of thermal oxidation on the mechanical properties were respectively determined by measurement of the oxidation induction time (OIT), Fourier transform infrared spectroscopy (FTIR) and uniaxial tensile testing. It was found that the chemical consumption of each antioxidant occurs in turn almost independently, starting with the sulfide antioxidant and involving small but well detectable antagonistic effects. As soon as the antioxidant concentration becomes very low, that is, practically undetectable by OIT measurements, the thermal oxidation of XLPE is detected by FTIR spectroscopy causing a catastrophic decrease in the fracture properties. It was shown that the complete consumption of antioxidants coincides with the end of the oxidation induction period and the onset of the sudden decrease in the elongation at break. Based on all these experimental results, a simplified kinetic model was developed to account as faithfully as possible for the consumption kinetics of antioxidants, evaluate their antagonistic effects and, using a relevant end-of-life criterion, predict the lifetime of the two XLPE insulations. It was found a satisfying agreement between theory and experiment.