Corona Aging of Polyimide Films at Different Atmospheric Relative Humidity

In order to investigate the effect of atmospheric relative humidity (RH) on corona discharge aging of polyimide (PI) films, a tracing monitoring was performed on the different aging phases of PI film at different RH values. The experiments on discharge activities, dielectric properties and surface morphologies were carried out in this work. It revealed that as RH below 69% both discharge quantity and discharge frequency experienced rapid growth stage, stationary stage and secondary development stage, but as RH exceeds 84%, both of them increased linearly with time. Moisture content rate ($\Delta$wt%) increased significantly at the beginning of the corona aging and leveled off at stationary and secondary development stages; As RH reached 84%, $\Delta$wt% almost linearly increased along with the aging time; Moreover, it indicated that the electrothermal surface damage caused by corona discharge was deemed as the main cause of the rapid decrease of surface resistivity $(p_{\mathrm{s}})$. With the corona erosion, water gradually entered the film, eventually leading to the sharp decrease of $\rho_{\mathrm{v}}$ till breakdown.