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

Aromatic polyimides have been synthesized via the formation of polyamic acids in solution. This solution was cast into films that were thermally imidized. The imidization process was monitored using FTIR. The degree of imidization was also determined with thermogravimetric analysis. The electric breakdown field strength was tested under ac and dc conditions. Ac electric breakdown field strength measured in transformer oil was shown to follow a power-law dependence on film thickness. This dependence was not affected by the molecular mass of the polymer but changed with the degree of imidization and the electron-accepting properties of the diamine. High ac breakdown field values of polyimides were obtained in dibutyl phthalate medium due to the suppression of partial discharges that occurs under these conditions. Measurements on films 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 measurements under dc conditions showed high values, which were in good agreement with the peak values of the ac measurements in dibutyl phthalate. This indicates similarities in the intrinsic ac and dc breakdown processes.