Effect of direct fluorination on surface potential behavior of polyimide film under low temperature

Polyimide film has been used as an insulation material in the excitation winding of the high temperature superconducting motors. The existence of surface charge has a great influence on the breakdown characteristic. Direct fluorination is an effective method of polymers surface modification which can improve the electrical properties of original material. Therefore, it is necessary that the surface potential behaviors of fluorinated polyimide films under low temperature were studied to clarify the effects of fluorination time and low temperature. Polyimide specimens were fluorinated in a reaction kettle at 55 °C using a F2/N2 blend with volume fraction of 80% nitrogen and 0.05 MPa for 15, 30, 45 and 60 min, respectively. The specimens without fluorination were also prepared for reference. Corona charging was performed with pulse voltage at a relative humidity of ~ 11% in atmospheric air. Then the specimens were cooled by liquid nitrogen in 10 s before and after corona charging. The surface potential was measured by the device of an electrostatic voltmeter. The experimental results indicate that the behaviors of surface potential in the surface of polyimide films are affected by the charge polarity, specimen temperature and fluorination time.