Application of a sealed tube test to the study of degraded insulation resulting from thermal aging of cables with PVC jacket

The cause of dark amber spots formed in XLPE insulation of thermally aged power distribution cables with PVC (polyvinyl chloride) jacket was studied. Heating tests in sealed glass tubes showed that the spots are due to hydrogen chloride (HCl) evolved by PVC acting at localized sites of transition metals in the insulation. Small slices of insulation were used with controlled environments of HCl, air and water. At 175°C, in the presence of the right gaseous components, dark spots formed in about 1 h, compared to several days or weeks for aging in complete cable sections at temperatures of 130°C and above. Both air and water were required, in addition to HCl and transition metals, for dark, highly degraded (oxidized) spots to form. The mechanism is the ionization of HCl by water, which in turn converts the neutral metal into the ionic form in which they are active catalysts for oxidation of polyolefins. Several polyethylenes were tested by the tube method for the presence of metal contaminants that cause spot formation. The method is also being used to study the origin of the contaminants. EP (ethylene-propylene) insulated cable thermally aged with PVC jacket degraded to a black color uniformly throughout the insulation. This result is also discussed