Structural effects on treeing phenomena in XLPE

Two low density polyethylene (LDPE) grades with low (material A) and high vinyl content (material B), respectively, were crosslinked to varying network densities. The crosslinks present in material B mainly consist of reacted vinyl groups, in contrast to the ordinary combination crosslinks present in material A. The crosslinked network was analyzed by gel content, swelling and differential scanning calorimetry (DSC) measurements. The morphology was observed with a scanning electron microscope (SEM) and the water tree growth was analyzed by means of a water needle test. It was found that the water tree length was enhanced at increasing crosslinking degree until a certain network density was reached, where the water tree length decreased again. This change was found to correspond to a change in morphology. No significant differences could be observed between the two materials, consisting of the two types of crosslinks.