Abstract
This work aims to investigate the electrical properties of both thermally aged and nonaged, commercially available impregnating varnish based on polyester-imide (PEI). The physical, thermal and dielectric properties of a virgin resin are initially investigated using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and Broadband Dielectric Spectroscopy (BDS) in order to quantify the behavior of the virgin PEI network. The second step consists in measuring and comparing the dielectric properties and the DC conductivity of virgin and thermally aged varnishes. The aged varnish was subjected to thermal aging at 240 °C for 4 weeks in an oxidative atmosphere. The results show large variations in both the real (e′) and the imaginary (e″) parts of the complex permittivity and in the DC conductivity (σ DC ) of the resin after aging. These results reveal an increase in these values at temperatures below 70 °C, which are close to the glass transition temperature (T g ) of the material. Above 70 °C, they show opposite trends, with a marked important decrease in the electrical properties of aged resin. The thermal treatment seems improve the resin insulation properties for high measurement temperatures (> 70 °C). The changes that occur in charge carrier density and mobility could partly explain these variations, which are related to network changes. Therefore, Fourier Transform Infrared spectroscopy (FTIR) measurements are performed in order to understand the chemical and structural changes caused by the thermo-oxidation.
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