No abstract

Electrical breakdown of new ethylene copolymers, ethylene-acrylic acid copolymer and its blend polymer, was studied. The DC breakdown strength (FBDC) of ethylene-acrylic acid copolymer (EAA: 4.6wt%) was 10.4 MVicm, which was higher than that of polyethylene homopolymer (PE), 7.4 MV/cm at -60°C. The impulse breakdown strength (F~imp) of EAA was also higher than that of PE at -60°C. The increase of FBDC may be caused by the effect of space charge and/or carrier scattering due to polar groups. The increase in acrylic acid(AA)concentration reduced Fe of EAA.PE and EAA blend polymer (PE+EAA) was prepared for a specimen with a low €AA concentration. All FB values of (PE+EAA)4.6wt% were much lower than those of EAA with the same amount of AA content in the whole temperature region. It was found that FBlmp of (PE+EAA) 0.1-lwt% was much higher than FBlmp of EAA4.6wt% at -6O"C, while FBDC of (PE+EAA)O.l-lwt% was higher than FBDC of PE and much lower than FBDC of EAA at -60°C. Since ethylene and AA functional groups are, respectively, hydrophobic and hydrophilic, EAA copolymer has a bad compatibility with PE. Interfacial defects may be formed between PE phase and EAA phase in PE+EAA blend polymer with a high EAA concentration. The FB of PE+EAA with a low EAA concentration is high at -60°C since EAA groups act as scattering centers. Therefore, it is effective for the improvement of PE to introduce a low concentration of EAA.Cross-linked polyethylene (XLPE) has been used widely for wire and cable insulation of rated voltages to 275 kV. Furthermore, its practical use in a power transmission line operated at a rated voltage of 500 kV with joints is now being studied in Japan. This development is due to the improvement of manufacturing processes such as dry curing, three-layer common crosshead for extrusion. and the elimination of contaminants in the insulating materials. In order to develop further the XLPE insulation system and to design the new insulation materials which endure under an even much higher electric stress and has a better reliability, it is necessary to improve the intrinsic electrical properties inherent to materials. Recently, many papers have been were reported on the relationship between the molecular structure of polyethylene and the electric breakdown phenomena [l]. The authors have studied so far the electrical breakdown of ethylene copolymers which contain halogen groups in their chemical structure [2]. The copolymerization of monomers with halogen groups gave high breakdown strength and a possibility to the new materials. But, the combustion of the materials with halogen groups leads to the generation of a harmful gas. From the above background, in this paper, we have investigated the breakdown characteristics of the new copolymers, i.e. ethylene acrylic acid copolymers and EVA which are replacing halogen groups with polar carbonyl groups. Experimental results revealed that breakdown strengthes of all tested samples except ethylene-acrylic acid copolymer(EAA) were not improved in comparison with that of...