Photo-oxidation and electrical aging of anhydride-cured epoxy resins

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.