Aging of Silane Crosslinked Polyethylene

Silane crosslinked polyethylene cable insulation occasionally fails to meet the aging requirements given in technical standards. The purpose of this paper is to investigate this phenomena and establish whether the safety margin of aging tests can be increased by changes in manufacture or test procedures. Using a number of cable types with different compositions and dimensions, the evolution of the absolute values of tensile strength and elongation at break upon aging was obtained. The results show that the major changes in mechanical properties happen within the first 24-48 h. This finding is valid both for ethylene vinylsilane copolymers and for grafted silane systems. In general, the effect is more pronounced for the 100°C compatibility test. Statistical analysis shows that insulation crosslinked in a hot waterbath will exhibit this behavior to a lesser extent, thus increasing the safety margin in aging tests, compared with ambient curing. This paper demonstrates that preconditioning at 70°C has no significant impact on aging properties. In addition, only small variations in mechanical properties were seen when changing the process parameters. It is concluded that further crosslinking is the principal cause of the phenomena under investigation.