The Effect of Crosslinking on Morphology and Water Treeing in LDPE

Licentiatavhandling, 2008

Crosslinked polyethylene (XLPE) is used as underground cable insulation due to its outstanding electrical properties and high temperature dimension stability. XLPE is made by peroxide initiated crosslinking of low density polyethylene. In recent years, polyethylene with unsaturations, i.e. vinyl groups introduced in the polymer chain, has been found to have a major impact on the crosslinking performance. Peroxide radicals trigger a “polymerization” reaction of the vinyl groups, hence forming crosslinks with the radical reactivity preserved, in contrast to ordinary combination crosslinking where the radical is consumed as the crosslink is formed. Consequently, vinyl containing LDPE will reach high crosslinking degree at lower peroxide consumption, since both combination crosslinking and polymerization of vinyl groups occur. The main object of this thesis has been to investigate the influence of differences in the crosslinked network structure for two polymers crosslinked via mainly combination crosslinking or mainly polymerization of vinyl groups, and the effect of increasing crosslinking degrees on the electrical properties, here with focus on water treeing in XLPE. This is a mechanism where a tree shaped structure consisting of water filled voids grows under electrical stress, making the cables more susceptible to electrical breakdown. The morphology changes significantly after crosslinking in both materials. Thermal fractionation studies show that the lamella thickness is diminished at increasing crosslinking degree, and scanning electron microscopy (SEM) show the change in the supermolecular structure from consisting of fully developed banded spherulites to axialites, thereafter to randomly distributed lamella stacks. DSC studies show that melt temperature, crystallization temperature and degree of crystallinity are all reduced with increasing crosslinking degree as well. Water trees are known to grow in the amorphous areas, especially in the areas between the supermolecular structures. When the degree of crystallinity decreases and the structure of the spherulites distorts during crosslinking, there will be more free space for the water trees to grow in; hence the water tree length increases. When barely any of these structures can be seen and the crosslinked network is dense, a drop in water tree length is noticed. The water tree growth might then be prevented by the restriction in mobility of the chain sections between the crosslinks. The difference in crosslinked network structure for the two materials in the study does not influence the water treeing properties. These findings can be used to produce tailor-made polymeric insulation materials to maximize the electrical performance of power cables.