INVERSE TEMPERATURE AND ANNEALING PHENOMENA DURING DEGRADATION CROSS-LINKED POLYOLEFINS

The radiation-thermal degradation of several types of commercial cable insulation materials (semi-crystalline crosslinked polyolefins) was i nvestigated as a function of temperature in the range of 22-120 degree s C. Mechanical property deterioration surprisingly occurred most rapi dly at the lowest temperatures. This unusual phenomenon was corroborat ed by fundamental differences in the degradation mechanism at elevated and ambient temperatures. Annealing at elevated temperatures of sampl es that had been aged at or near room temperature led in some cases to significant recovery of mechanical properties (elongation at break) a nd concurrent changes in gel content (additional crosslinking) and den sity. The importance of molecular mobility in the semi-crystalline mat erials and the nature of the additional crosslinking reaction as contr ibutors to these anomalous behaviors are evaluated and discussed. The observed inverse temperature/aging dependence is due to a combination of the semi-crystalline morphology of the materials, and a specific cr osslinking reaction, which can act as a repair mechanism at elevated t emperatures, but is dormant at ambient conditions.