Journal of Optoelectronics and Advanced Materials Vol. 6, No. 3, September 2004, p. 1077 - 1080 WATER TREEING IN CHEMICALLY CROSSLINKED POLYETHYLENE F. Ciuprina * , G. Teissèdre a , J. C. Filippini a , P. V. Notingher, A. Campus b , T. Zaharescu c “Politehnica” University of Bucharest, ELMAT, Splaiul Independentei 313, sector 6, 77206 Bucharest, Romania a LEMD – CNRS/UJF, Grenoble, France b Borealis Polymers NV, Belgium c Advanced Research Institute for Electrical Engineering, Bucharest, Romania The water tree resistance of chemically crosslinked polyethylene (XLPE) and of low density polyethylene (LDPE) were compared in order to elucidate whether the crosslinking itself influences or not the water tree propagation in polymer insulation. For this purpose, water trees were grown in compression molded disks, obtained from pellets of either thermoplastic (LDPE) or chemically crosslinkable polyethylene provided by Borealis. Two types of pellets of crosslinkable polyethylene were evaluated: one containing only peroxide (XLPE1) and another having, besides peroxide, a tree retarding additive system (TRXLPE). The results obtained indicate that there is no significant difference between the average of water tree lengths in XLPE1 (286 μm) and those in LDPE (269 μm). This is in agreement with the result of a previous study in which water treeing in irradiation crosslinked samples was analysed. On the other hand, the water tree lengths in TRXLPE (122 μm) are much smaller than in LDPE and XLPE1. (Received April 29, 2004; accepted June 22, 2004) Keywords: Crosslinked polyethylene, Water treeing 1. Introduction Crosslinked polyethylene (XLPE) has been extensively used in the last years in underground transmission and distribution cables, replacing LDPE which was previously used for extruded cable insulation. The main reason was that by crosslinking, the thermal and the dimensional stability is improved, without affecting the electrical properties of the polymer. Besides its mechanical resistance and electrical performance, another property that is considered when choosing the insulating material for power cables is its water tree resistance, because in MV cables the main cause of the insulation breakdown is the aging due to water treeing [1]. Does the crosslinking itself play a role in water treeing in polyethylene insulation? This is the question that our study, started four years ago, is trying to answer. The previous research we performed on irradiation crosslinked samples [8] has led us to the conclusion that polyethylene crosslinking does not create any network able to hinder the migration of water through the polymer under the action of the electric field and, therefore, no consistent influence of the irradiation crosslinking on water tree growth could be observed. Since the most common technique used today for the cable insulation manufacturing, is not irradiation, but crosslinking by dicumyl peroxide, the latter technique has been widely studied.. It has been shown that this method creates crosslinking by-products which can affect the insulation properties. It has, for example, been shown that acetophenone, which is one of the decomposition by-products of the peroxide used for this reaction, is a water tree inhibitor. In fact, if acetophenone and the other by-products are extracted from XLPE by vacuum heating, the original water tree resistance of XLPE decreases. Experiments performed on two samples of the same base polymer, * Corresponding author: florin@elmat.pub.ro