Effect of plasma polymer coating on the dielectric strength of polyethylene

The authors have shown previously (1987) that a thin layer of plasma-polymerized ethylene deposited on the surface of high-density polyethylene suppresses the injection of carriers from the electrodes. Here they report the effect of this coating on the dielectric strength of high-density polyethylene. The polyethylene used is additive-free blown film with a density of 0.950 g/cm³ and a melt flow rate of 0.9 g/10 min. The plasma-polymer coated sample studied is shown to have a higher breakdown strength than one without plasma-coating. The plasma-polymerized ethylene is amorphous, with many localized states in the forbidden band. These states act as trapping sites for electrons injected from the cathode. The trapped electrons, in turn, suppress the succeeding carrier injection from the cathode. The trapping centers seem to have an injection-suppressing effect even for the application of short-duration voltages such as impulse voltages. It is concluded that the suppression of electron injection by the coating brings about an increase in breakdown voltage.