Anomalous aging phenomena in a crosslinked polyolefin cable insulation

The radiation-thermal degradation of a commercial crosslinked polyolefin (XLPO) cable insulation material was investigated as a function of dose rate and temperature in the range of 22 - 120 degrees C. Degradative changes in the material were monitored by ultimate elongation, density, gel content, O-2 consumption, infrared spectroscopy, and differential scanning calorimetry. Mechanical aging surprisingly occurred most rapidly at the lowest temperatures. This unusual phenomenon was corroborated by chemical measurements (gel content and density). When samples that had been irradiated at ambient temperature were subsequently annealed at elevated temperatures, recovery of mechanical properties and concurrent changes in gel content and density were observed. The involvement of residual radical species and hydroperoxide intermediates as well as the importance of molecular mobility in the semi-crystalline XLPO as contributors to these anomalous behaviors were evaluated and discussed. The observed inverse temperature effect, where polymer degradation occurs more rapidly at lower temperatures, represents an example in which material aging and life time prediction cannot be handled by conventional approaches, such as the commonly applied Arrhenius methodology.