Effects of surface fluorination on dielectric properties and surface charge behavior of water absorbed polyimide film

With the advantages of excellent physical, chemical and thermal properties, polyimide (PI) film has been widely used as insulation material in electrical and mechanical equipment. However, PI presents water absorption capacity because of the existence of carbonyl and amine groups in the molecular chain. When exposed in moisture conditions, PI film will be degraded. Therefore, the dielectric breakdown occurs and irremediable damage will appear in insulation system. The dielectric properties of PI film are largely influenced by the absorbed water and humidity. Direct fluorination is an effective method of surface modification which can improve the properties of original material from many respects, such as the wettability, barrier properties and chemical stability. In this paper, to study effects of fluorination on dielectric properties of moistened PI films, the PI specimens were surface fluorinated for 15, 30, 45 and 60 min respectively. The specimens without fluorination were also prepared for the contrast. Then the specimens were immersed in high purity water for 6, 12 and 24 hours. The water absorption, permittivity and breakdown strength under different fluorination and immersion time were measured. Meanwhile, given that the existence of surface charge had a critical influence on the breakdown characteristics, the corona charging tests were conducted at room temperature and the charge dissipation was investigated. Obtained results show that fluorination can reduce the water intake capacity and enhance the breakdown strength of PI in moisture conditions. The decay process of surface charge and the change of detrapping charges in PI films are affected by both the fluorination and the immersion time.