Effects of direct fluorination on surface charge of polyimide/Al2O3 nanocomposite films

With advantage of excellent thermostability, mechanical property, polyimide films have been widely used in the aerial, nuclear, microelectronic industry. However, The dielectric breakdown of the polyimide films caused by surface charge often occurs and becomes one of main problems to the safety and reliability of insulation system. Addition of nanoparticles into polyimide films can improve the insulating properties compared with undoped material. Fluorination as the change of the chemical component in surface layer of polymers should give rise to the corresponding change in electrical properties of the surface layer thus influence the charge injection from electrodes when they are used as an insulator. In this paper, the study aimed at clarifying the effect of nanoparticles and fluorination on the surface charge accumulation and decay behaviors of polyimide films have been carried out. Polyimide/Al₂O₃ nanocomposite films were prepared with the weight percent of 0, 1, 3, 5, 7 wt% respectively. Samples were surface fluorinated in a laboratory vessel at about 328K (55 °C) using a F₂/N₂ mixture with 20% F₂ by volume and 0.05 MPa (500 mbar) for 30 minutes. Corona charging tests were performed at room temperature with a relative humidity of ~ 40%. The charge distribution was measured by means of an electrostatic voltmeter. Obtained results show the dependence of the charge density as well as the charge decay rate upon the nanoparticle content and the fluorination, varying as a function of the charging time. It is suggested that the nanoparticles and the fluorination can significantly affect the decay rate of the surface charge in polyimide nanocomposite films.