Preparation and properties of ODPA-ODA-SDA polyimide film and its application in corona resistance

High-frequency trend of electrical equipment puts forward higher requirements on the dielectric properties of insulating materials, especially under corona ageing environment. Therefore, a facile and effective method for improved corona resistance is essential for the development of high-frequency power systems. In this article, 4,4′-Oxydiphthalic anhydride (ODPA) was chosen as the dianhydride while 4,4′-Diaminodiphenyl ether (ODA) and 4,4′-diaminodiphenyl sulphide (SDA) as diamines. Consequently, thermally imidized ODPA-ODA-SDA copolyimides were synthesized, and the influence of ODPA and SDA on the dielectric properties was discussed. High-frequency corona ageing test was also conducted to investigate the relationship between dielectric properties and the corona resistance. From dielectric frequency spectra analysis, the ODPA-ODA-SDA copolyimide was considered to exhibit the best dielectric properties when SDA accounted for 30% mole fraction of diamine, the dielectric loss of which was 24.03% lower than that of ODPA-ODA at 20 kHz. Meanwhile, the better corona resistance property and mechanical properties were obtained, especially the corona ageing lifetime was 2.26 times higher than that of ODPA-ODA. To further improve the corona resistance property, a series of modified-PI/Al2O3 nanocomposite films based on ODPA-ODA-SDA matrix were prepared by in-situ polymerization. When the content of Al2O3 nanoparticles reached 12 wt%, the thermal stability was improved and the corona ageing lifetime was 6.14 times longer than that of ODPA-ODA. This experiment provides new ideas and directions for reducing mechanical and thermal performance losses as well as achieving high-efficiency corona resistance, which is expected to promote the development of advanced insulating films.