Hydrophobic property of XLPE filled with calcium carbonate

The aging of crosslinked polyethylene (XLPE) is often initiated by a process of thermal oxidation that gradually degrades the insulation until failure occurs. Oxidation on the surface causes the loss of hydrophobicity which is accelerated by increasing temperature and moisture. After the hydrophobicity on the surface has diminished, water can permeate into the bulk more readily. To investigate the process of wettability of the surface and its dependence on the level of calcium carbonate filler, XLPE specimens were immersed in saline water solutions at levels of salinity from 5.5 to 10000 /spl mu/S/cm, at temperatures from 0 to 98/spl deg/C and durations from 5 to 150 h. It was found that at a fixed salinity level the contact angle decreased with increasing temperature and was more pronounced for 15 and 25% filler levels than when no filler was used. The contact angle decreased slightly with increasing salinity of the water from 5.5 to 1000 /spl mu/S/cm and rapidly from 1000 to 10000 /spl mu/S/cm for all three filler levels. The contact angle decreased rapidly with increasing time of immersion up to 50 h and slowly thereafter up to 150 h for all filler levels. The contact angle was found to be constant at 86/spl deg/ after exposure to air for 50 h at five temperatures from 0 to 98/spl deg/C. On the other hand after immersing in a saline solution in the range 5.5 to 10000 /spl mu/S/cm for 50 h, the contact angle decreased with increasing time of immersion and increasing the temperature from 0 to 98/spl deg/C for all filler levels. The recovery of hydrophobicity depends on filler concentration, temperature and salinity level of the aqueous solution. Mechanisms and reactions of oxidation responsible for the observed phenomena are suggested.