Studies of space charge in electrically aged low density polyethylene

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40°C for 1500 h under an AC field of 6 MV/m (50 Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally, the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for a long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.