Charge trapping in corona-charge polyethylene films

A model for the process of surface potential decay of insulating films following charging by corona ions is developed. Account is taken of charge trapping and release at sites both in the surface and in the bulk where trapping can reduce the mobility significantly. The model is particularly applicable to polyethylene films and is used to interpret measurements of surface potential on this polymer after 'instantaneous' charging with ions. There are both deep and shallow surface traps, the deep ones of concentration approximately 1015 m-2 being filled preferentially. Moreover negative charge penetrates more readily into the bulk. Within the bulk, positive carriers are found to have a trap-free mobility at 22 degrees C of about 5*10-15 m2 V-1 s-1, and negative carriers a mobility of between one and two order greater. Bulk traps exist for both sign of carrier and the concentration is estimated to have an upper limit CF 1024 m-3. Effective mobility becomes markedly time-dependent during times corresponding to the transit time of carriers across the sample.