Control of water tree length and density in cable insulation polyethylenes

The goal of this paper is to study the influence of the chemical crosslinking of polyethylene on the water tree initiation and propagation in polymer insulation. For this, the water tree resistances of crosslinked and thermoplastic low density polyethylene were compared. Three types of crosslinked polyethylene systems were evaluated: one containing only peroxide and the other two having, beside peroxide, a free retarding additive system. The results were compared with those obtained on their thermoplastic correspondents. The data show that there are differences in both the water tree length and density that can be ascribed to the polyethylene systems. However, only differences in the water tree density could be ascribed to the material form (thermoplastic or crosslinked). The observed results are consistent with differences, on microscopic level, in permittivities and local breakdown strengths.