Photooxidation of anhydride-cured epoxies: FTIR study of the modifications of the chemical structure

Anhydride-cured epoxies are used in electrical engineering to manufacture insulators. The purpose of this study is to determine the modifications undergone by these resins when exposed to solar-type radiation (radiation with a wavelength of more than 300 nm). Two epoxy–anhydride systems have been studied. Using transmission Fourier transform infrared (FTIR) and attenuated total reflection (ATR), we have determined the structural modifications and the groups formed during photooxidation. The degradation is very heterogeneous, more significant on the surface than in the bulk of the material, and a considerable decrease in almost all the initial functional groups is observed. The flexibilized system, richer in ester groups but containing fewer phenyl groups, degrades more rapidly during the first hours of exposure than the nonflexibilized system. Two explanations accounting for this faster degradation in the first hours of exposure are drawn from ultraviolet absorbance analysis and degradation mechanisms considerations. After 40 h of irradiation, the disappearance rate of the aromatic groups depends only on their initial concentration, whereas the formation kinetics for the hydroxyl groups is limited by the diffusion of oxygen in the material. Chemical treatments using SF4, NH3, and NaCl have revealed the formation of hydrophilic products, acids, and alcohols. Their presence and particularly high concentration at the surface could very well be responsible for the degradation of the insulating properties of the electrical insulators. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1221–1237, 1998