AC breakdown strength and dielectric response of water-treed low density polyethylene

Results on AC breakdown strength and on dielectric behavior of water-treed plane samples of low density polyethylene (LDPE) are presented in this paper. Water trees were grown at room temperature, under an electric field of 4 kV/mm, 5 kHz, and having a 0.1 M NaCl solution as electrolyte in contact with a face of the sample having needle-like defects created by pressing a sheet of abrasive paper. The breakdown tests have been carried out under an AC voltage ramp, at 50 Hz. The dielectric spectroscopy was the tool used to analyze the dielectric behavior of LDPE samples with and without water trees, over a frequency range of 10 mHz-1 MHz, at room temperature. The results show a good correlation of the breakdown strength decrease with the increase of water tree average length and relative volume. The dielectric spectra of water-treed samples emphasize an increase of the sample permittivity with the water tree growth, as well as three relaxation peaks in the loss tangent frequency variation obeying the Havriliak-Negami law. Possible electrical equivalent circuits for treed and non-treed samples are also discussed.