Dielectric Properties of XLPE Obtained by High-Energy Irradiation in Different Media

Polyethylene (PE) has good dielectric properties (excellent dielectric strength, high insulation resistance, and a low dissipation factor) and is, therefore, widely used as an insulating material for power and communication cables, although with limitations in the temperature range. Crosslinking the PE to become XLPE increases the temperature range of the insulation and chemical corrosion resistance. There are two main crosslinking methods for cable insulation: chemical crosslinking involves adding chemicals or initiators such as silane or peroxide to generate free radicals, which form the crosslinking. Physical crosslinking mainly involves subjecting the polymer to high-energy radiation. In this study, XLPE was created by high-energy irradiation of low-density polyethylene (LDPE) in different media: air, deionized distilled (DD) water, and nitrogen. The role of irradiation media on dielectric properties and charge-trapping behavior has been studied through dielectric loss analysis and thermally stimulated discharge current (TSDC) measurements. The polar groups introduced in initially apolar PE by irradiation were regarded as tracer ones. Changes in magnitude, position, and activation energy of dielectric relaxations were found to be strongly dependent on the two main radiation-induced processes, e.g., crosslinking and oxidative degradation. Gel and infrared results confirmed that the domination of one or the other of these competitive processes is strongly determined by radiation dose and media. Oxidation, e.g., an increase in the amount of polar carbonyl groups, is accentuated for the samples irradiated in air, leading to a significant increase in the magnitude of the observed dielectric relaxations. On the other hand, crosslinking and net formation are much more extensive for the samples irradiated in DD water and/or nitrogen, and play a significant role only in the case of the dielectric β relaxation due to its nature and cooperative character; the increase in the position and apparent activation energy for this relaxation process can be clearly related to the restricted chain mobility in the amorphous phase induced by crosslinking and net formation.