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

Multifactor surface aging of anhydride cured epoxides was studied. Three sy stems based on diglycidyl ether of bisphenol A (DGEBA) and cycloaliphatic e poxy were investigated. They were submitted either to a solar irradiation a lone, or to a tangential ac electric field, or to both stresses in a labora tory setup or outdoors. The ac voltage was applied between two electrodes i nserted into the sample surface. Before aging, the applied ac voltage was l ower than the inception voltage of discharges. The main structural changes of the surface of the three epoxy networks were determined by Fourier trans form infrared (FTIR), attenuated total reflection (ATR) and X-ray microanal ysis. The evolution of the surface conductivity was investigated by current -voltage measurements and by the method of the surface potential decay. Sol ar irradiation of the different resins led to the buildup of IR bands chara cteristic of hydroxyl and carboxylic groups. These groups are very hydrophi lic and have a strong impact on the electrical properties of the surface of the resins, especially in wet medium. Under both electric and climatic str esses, decarboxylation or decarbonylation of the photo-oxidized surface occ urred at the polymer-electrode interface and were attributed to electrode r eactions. Tracking was observed only after electrical aging alone. This une xpected result shows that a homogeneous, slightly conducting surface may pr event the formation of tracks on the surface of aged insulators.