AROMATIC POLYIMIDES - SYNTHESIS, CHARACTERIZATION, AND EVALUATION OF ELECTRIC STRENGTH

Aromatic polyimides have been synthesized via the formation of polyami c acids in solution. This solution was cast into films that were therm ally imidized. The imidization process was monitored using FTIR. The d egree of imidization was also determined with thermogravimetric analys is. The electric breakdown field strength was tested under ac and dc c onditions. Ac electric breakdown field strength measured in transforme r oil was shown to follow a power-law dependence on film thickness. Th is dependence was not affected by the molecular mass of the polymer bu t changed with the degree of imidization and the electron-accepting pr operties of the diamine. High ac breakdown field values of polyimides were obtained in dibutyl phthalate medium due to the suppression of pa rtial discharges that occurs under these conditions. Measurements on f ilms with thicknesses up to 60 mum showed no thickness dependence. No significant influence of the chemical structure on the breakdown field strength was observed under these conditions. Breakdown field measure ments under dc conditions showed high values, which were in good agree ment with the peak values of the ac measurements in dibutyl phthalate. This indicates similarities in the intrinsic ac and dc breakdown proc esses.