Plasma surface modification of LDPE to improve insulation property

Summary form only given, as follows. Low density polyethylene (LDPE) is an excellent insulating material which has been widely used in underground power cables. However, its insulation property deteriorates in salt-containing wet circumstance. It is known that soaking a LDPE film in water under electrical stress did not change the resistivity of the polymer. However, when it was soaked in 0.9% NaCl solution without electrical stress, the decline of resistivity with soaking time was found. The application of electrical stress greatly enhanced the decrease of resistivity leading to insulation breakdown. The salt intrusion occurs through surface defects or imperfection and influences the electrical resistivity of LDPE. The deterioration of insulation characteristics follows a similar mode for fatigue. The transport path for salts and that for water seem to be different. Surface modification by plasma deposition of an ultra-thin layer of hydrophobic polymer film evidently mends surface defects that could act as salt intrusion sites. Electrochemical impedance spectroscopy (EIS) was used to investigate the salt intrusion phenomenon. The EIS measurement indicated that the deposition of the hydrophobic layer (ca. 50 nm) performed in the glow discharge of hexafluoroethane (C/sub 2/F/sub 6/) rendered a higher impedance than the untreated one. The EIS data also showed that the salt intrusion had a much stronger effect on the decrease of the impedance of the LDPE film than water absorption.