Thermal aging of enameled wire: dielectric markers and structural properties drift correlation

This work aims to investigate and correlate the evolution of dielectric properties to structural changes induced by thermal aging of enameled wires. The samples of standard and corona PEI/PAI enameled wires were subjected to $180 ^{\circ}\mathrm{C}$, $200 ^{\circ}\mathrm{C}$ and $220 ^{\circ}\mathrm{C}$ thermal aging up to 1600 h. To assess the influence of the thermal degradation on the insulation properties, dielectric spectroscopy and voltage breakdown measurement were performed at room temperature on unaged and aged samples. DSC were performed to determine the evolution of glass transition temperature (Tg) to assess the structural evolution of the enamels. The results of capacitances show an increase after aging, since the first hours of aging. The breakdown voltages show a decrease in the values. A decrease in the PEI layer thickness with increasing aging times and temperatures confirms the thermal degradation suggested by the insulation properties weakening. The enamel degradation could be due to the thermo-oxidation of the PEI layer. It consists of competition mechanisms of crosslinking and chain scission activated by thermal aging under oxidizing atmosphere.