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Multifactor surface aging of anhydride cured epoxides was studied. Three systems based on diglycidyl ether of bisphenol A (DGEBA) and cycloaliphatic epoxy were investigated. They were submitted either to a solar irradiation alone, or to a tangential ac electric field, or to both stresses in a laboratory setup or outdoors. The ac voltage was applied between two electrodes inserted into the sample surface. Before aging, the applied ac voltage was lower than the inception voltage of discharges. The main structural changes of the surface of the three epoxy networks were determined by Fourier transform infrared (FTIR), attenuated total reflection (ATR) and X-ray microanalysis. The evolution of the surface conductivity was investigated by current-voltage measurements and by the method of the surface potential decay. Solar irradiation of the different resins led to the buildup of IR bands characteristic of hydroxyl and carboxylic groups. These groups are very hydrophilic and have a strong impact on the electrical properties of the surface of the resins, especially in wet medium. Under both electric and climatic stresses, decarboxylation or decarbonylation of the photo-oxidized surface occurred at the polymer-electrode interface and were attributed to electrode reactions. Tracking was observed only after electrical aging alone. This unexpected result shows that a homogeneous, slightly conducting surface may prevent the formation of tracks on the surface of aged insulators.

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 [degrees] C, 135 MV/m in model capacitors filled with benzyltoluene saturated with [O.sub.2] 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.

The effect of partial discharges in benzyltoluene upon impregnated polypropylen films has been studied in a sphere/plane electrode geometry. It is shown that discharges even with a small apparent charge ([approximately] 1 pC) cause a significant decrease of the breakdown strength of the film. The addition of an epoxy compound to the liquid notably reduces the film degradation characterized by the breakdown voltage, in spite of the increase of the repetition rate and apparent charge of the discharges.

Discharges in model capacitors have been studied with a digital analyzer. The dielectric was polypropylene impregnated with an organic liquid. We have investigated the effect of the nature of the liquid (aliphatic or aromatic hydrocarbon) and the influence of parameters such as voltage, pressure and temperature on the main characteristics of the discharges. Discharges were also monitored during aging tests of the models. We conclude that aging of the models we tested at high electric fields (approximately 135 V/Micrometer) is not due to discharges but to the chemical degradation of polypropylene submitted to electrical and thermal stresses.

The aging of impregnated PP film has been carried out on capacitor models at 80 [degrees] C under 50 Hz ac electrical stress (137 [V.sub.rms]/[mu]m). The damage was estimated from the local measurement of the breakdown voltage of the PP film. The effects of impurities (electrolyte, water, oxygen) added to the impregnating liquid were observed. The relatively low breakdown voltages of the aged film measured in the electrically stressed region were no longer found when the liquid contained epoxides. A chemical degradation analyzed by FT-IR microspectroscopy and gel permeation chromatography was observed and found only in the region of lower dielectric strength.