The Effect of Crosslinking on Morphology and Electrical Properties in LDPE Intended for Power Cables
Doctoral thesis, 2010
Crosslinked polyethylene (XLPE) is used as underground cable insulation due to its outstanding electrical properties and high temperature dimension stability. XLPE is often made by peroxide initiated crosslinking of low density polyethylene (LDPE). The introduction of unsaturations, i.e. vinyl groups, into LDPE has a major effect on the crosslinking performance. Peroxide radicals trigger a reaction of the vinyl groups, hence forming crosslinks with the radical reactivity preserved, in contrast to ordinary combination crosslinking where the radicals are consumed as the crosslink is formed.
The main object of this thesis was to investigate the influence of two crosslinking mechanisms and increasing crosslink density on selected electrical properties. Two LDPEs crosslinked via mainly combination crosslinking or mainly reacted vinyl groups, respectively, were used. The electrical mechanisms investigated were water treeing, electrical treeing and electrical breakdowns. Treeing are fairly slow degradation mechanisms that form under electrical stress, making high voltage insulating materials more susceptible to electrical breakdowns. Water trees are tree shaped structures consisting of water filled voids, whereas electrical trees consists of hollow tubes that grow by local degradation of the polymer via partial discharges.
The polymer morphology changes significantly at increasing crosslinking densities in both LDPEs, i.e. the degree of crystallinity decreases and the morphology transforms from consisting of fully developed spherulites to an axialitic structure, thereafter to randomly distributed lamella stacks. Water tree growth is significantly enhanced at increasing crosslink density, which is believed to be accounted for by the morphology change and increasing amount of amorphous areas. A corresponding effect could be seen for the growth of electrical trees. No distinct morphology dependence could however be established for the inception of electrical trees. Crosslinking with different peroxides and the addition of various antioxidants alters the electrical properties, which emphasizes the importance of a controlled polymeric system when used as an insulating material in power cables.
These findings can be used to produce tailor-made polymeric insulation materials to maximize the electrical performance of power cables.