Accelerated AC degradation of impregnated PP films

The influence of the liquid, its impurities and additives on the ac degradation of impregnated polypropylene (PP) films has been investigated. Aging of polypropylene was performed at 80°C, 135 MV/m in model capacitors filled with benzyltoluene saturated with O₂ in order to increase the aging rate. The thermal oxidation of the liquid has been studied by gas chromatography, the oxidation of PP by FTIR microspectroscopy, the degradation rate of PP films has been evaluated by the variation of their mean breakdown voltage with aging time. All of them are strongly enhanced by the electric field. The oxidation of the liquid which takes place during these experiments influences only slightly the PP degradation rate. It is concluded that these severe conditions of aging do not change the chemical process. They only enhance drastically its propagation rate. Since aging of PP films is due to the scission of macromolecular chains initiated by radicals formed at the electrodes, the influence of the liquid, its components or additives is explained by their ability to react with radicals which initiate the scission of the macromolecules.