Corona discharges aging of recycled PET filled with inorganic reinforcements

Polyethylene Terephthalate (PET) is one of the most promising candidate thermoplastics that can replace epoxy resin as insulating material for electric and electronic applications, thanks to its good processability and excellent properties which remain relatively stable in a large range of temperatures. In this paper, two different micrometric fillers, short glass fibers as well as small and large mica platelets have been used to form composites. One of the possible applications is the mechanical support for the HV terminal system of internal combustion engines. In order to evaluate the dielectric endurance of these materials, corona resistance tests were performed in an oven using a point-to-plane configuration in air at several constant temperatures between the room temperature and 170°C. Tests were carried out at 7 kV rms and at a frequency of 300 Hz to accelerate the degradation process. In order to check the effect of time, experiments were also conducted at different times between 24 and 96 hours. The dielectric strength has been evaluated at room temperature before and after erosion according to the ASTM-D149 standard. The dielectric response was evaluated using a broadband dielectric spectroscopy (BDS) at room temperature before and after erosion. Dielectric breakdown strength data obtained before exposure to corona discharges shows that there is a slight difference between the composites filled with large mica platelets and those reinforced with small mica platelets. However, the data obtained after erosion revealed that recycled PET filled with large mica platelets perform better than those filled with small mica platelets. In addition, it was found that an increase of temperature induces a decrease of the residual breakdown voltage. Results demonstrated that the recycled PET based composites reinforced with large mica platelets has good resistance to corona discharges at elevated temperature as compared to small mica platelets filled recycled PET. Data obtained by BDS show a slight increase in relative permittivity for recycled PET filled with large mica platelets as compared to the recycled PET reinforced with large mica platelets. It was also found that an increase of temperature or time results in an increase of dielectric losses mainly at low frequencies. It can be concluded that BDS and dielectric breakdown strength after erosion process were useful to distinct the performance of composites filled with small and large mica platelets, which was not possible with only short term dielectric breakdown measurements.