ANOMALOUS AGING PHENOMENA IN A CROSS-LINKED POLYOLEFIN CABLE INSULATION

The radiation-thermal degradation of a commercial crosslinked polyolef in (XLPO) cable insulation material was investigated as a function of dose rate and temperature in the range of 22 - 120 degrees C. Degradat ive changes in the material were monitored by ultimate elongation, den sity, gel content, O-2 consumption, infrared spectroscopy, and differe ntial scanning calorimetry. Mechanical aging surprisingly occurred mos t rapidly at the lowest temperatures. This unusual phenomenon was corr oborated by chemical measurements (gel content and density). When samp les that had been irradiated at ambient temperature were subsequently annealed at elevated temperatures, recovery of mechanical properties a nd concurrent changes in gel content and density were observed. The in volvement of residual radical species and hydroperoxide intermediates as well as the importance of molecular mobility in the semi-crystallin e XLPO as contributors to these anomalous behaviors were evaluated and discussed. The observed inverse temperature effect, where polymer deg radation occurs more rapidly at lower temperatures, represents an exam ple in which material aging and life time prediction cannot be handled by conventional approaches, such as the commonly applied Arrhenius me thodology.