JPS6356645B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a semiconductive composition provided on the outer periphery of a conductor and/or the outer periphery of an insulator of a power cable. The smoothness and adhesion of the interface between the semiconducting layer and the insulator in cross-linked polyethylene insulated power cables are important for the reliability of the cable, and the presence of interfacial irregularities can lead to the formation of localized high electric fields, which can lead to corona discharge or flooding. Water tree deterioration occurs and the electrical properties of the cable deteriorate. To address this problem, studies have been conducted from the viewpoint of manufacturing methods and materials for semiconductive layers.
An extruded semiconductive layer has been developed to replace the fluffy semiconductive cloth tape. Looking at the materials for extruded semiconductive layers, there are those that use ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer as the base polymer, and those that use a mixture of ethylene propylene rubber and high-density polyethylene as the base polymer. Traditionally known. In particular, semiconducting compositions based on a mixture of ethylene propylene rubber and high-density polyethylene can be The semiconducting layer is less susceptible to damage and deformation.
Additionally, since it uses a polymer with a structure similar to that of an insulator, it has features such as good interface smoothness and adhesion. Cross-linked polyethylene insulated power cables with such a semiconductive layer have significantly improved breakdown voltage, but water trees occur frequently when energized for a long period of time under severe deterioration conditions such as water seepage inside the cable. However, there is a problem that the breakdown voltage decreases. The present invention solves the problems of the prior art described above, and aims to provide a semiconductive composition that can improve the water resistance of power cables. That is, the semiconductive composition of the present invention is made of low-density polyethylene (density 0.915 to 0.930) produced by medium-low pressure polymerization.
g/cm ³ ) at a mixing ratio of 20 to 50% by weight, consisting of 100 parts by weight of a mixture of ethylene propylene rubber and medium-low pressure polymerized low-density polyethylene, and 40 parts by weight or more of a conductivity imparting material. That is. As a result of various studies on improving water resistance, the present inventor obtained experimental results showing that the desired objective can be achieved when medium-low pressure polymerized low-density polyethylene is used as a mixture in ethylene propylene rubber, and the present inventor has arrived at the present invention. Ivy. Low-density polyethylene produced by medium-low pressure polymerization is an ethylene-α-olefin copolymer with no long chain branching and a linear molecular structure, unlike conventional low-density polyethylene. As the α-olefin, those having 4 or more carbon atoms, such as butene-1, 4-methylpetene-1, hexene-1, octene-1, etc., are used. As estimated from the molecular shape, medium-low pressure polymerized low-density polyethylene exhibits almost the same physical properties as high-density polyethylene, but its density is slightly different from that of high-density polyethylene, so its density is comparable to that of conventional low-density polyethylene. be. Of course, the density can be increased by changing the amount of α-olefin, but when the density is increased, the water resistance becomes similar to that when using high-density polyethylene, and the optimal density is 0.915 to 0.930 g/cm ³ . The mixing ratio of medium-low pressure polymerized low-density polyethylene to 100 parts by weight of the mixture must be 20 to 50% by weight; if it is less than 20% by weight, sufficient mechanical strength cannot be obtained, so an insulator is extruded around the outer periphery. In this case, the semiconductive layer is likely to be damaged or deformed. 50
If the weight percentage is exceeded, water resistance will deteriorate. Representative examples of conductivity-imparting materials for imparting conductivity to the above mixture include channel black,
There are carbon blacks such as furnace black and acetylene black. It is necessary to add 40 parts by weight or more of the conductivity imparting agent to 100 parts by weight of the mixture; if it is less than 40 parts by weight, the desired conductivity cannot be obtained. The semiconductive composition thus obtained is prepared in the same manner as conventional compositions by mixing necessary additives such as anti-aging agents and processing aids. Examples of the present invention will be described below along with comparative examples. A semiconductive composition obtained by blending various components as shown in Table 1 is extrusion coated onto a stranded copper conductor having a cross-sectional area of 150 ^mm2 to form an internal semiconductive layer, followed by a low density coating. Polyethylene (density 0.920g/cm ³ , MI1.0)
100 parts by weight of dicumyl peroxide, 2.5 parts by weight of dicumyl peroxide, and 0.25 parts by weight of an antioxidant are extrusion coated to form an uncrosslinked insulating layer;
A semiconductive layer having the same composition as the internal semiconductive layer was extrusion coated on the outer periphery to form an external semiconductive layer, and then thermal crosslinking was performed to obtain a crosslinked polyethylene insulated power cable. After injecting water into the conductor of this cable and submerging the cable in water, a 50Hz, 15kV AC voltage was applied between the conductor and the water electrode. The water temperature was 90°C, and the number of days on which electricity was applied was 18 months. After applying the voltage, the insulator was spirally cut to a thickness of 0.5 mm, boiled and stained with a methylene blue aqueous solution, and the number of water trees generated at the interface between the insulator and the internal semiconductive layer was observed using a microscope. The results are shown in the lower column of Table 1.

[Table] As can be seen from Table 1, when low-density polyethylene produced by medium-low pressure polymerization is used, the number of water trees generated is significantly lower than when high-density polyethylene is used. However, when the blending ratio of low density polyethylene produced by medium and low pressure polymerization to ethylene propylene rubber is outside the scope of the present invention (Comparative Example 3), or when the density of low density polyethylene produced by medium and low pressure polymerization is outside the range of the present invention ( Comparative Example 2) shows no significant effect. Regarding the reason why the semiconductive layer using medium-low pressure polymerized low-density polyethylene has excellent water resistance,
Although it is not yet clear, it is assumed that this is because the resin becomes more flexible due to its lower density, increasing its affinity with ethylene propylene rubber. As explained above, according to the present invention, a semiconductive composition for power cables having sufficient mechanical strength and excellent water tree resistance can be obtained.

Claims (1)

[Claims] 1 Ethylene propylene rubber with a density of 0.915 to 0.930
40 parts by weight or more of a conductivity-imparting material in 100 parts by weight of a mixture of 20 to 50% by weight of low-density polyethylene produced by medium-low-pressure polymerization (g/cm ³ ), in which the mixture ratio of low-density polyethylene produced by medium-low-pressure polymerization is 20 to 50% by weight. 1. A semiconductive composition for power cables, which is characterized in that it is made by blending. 2. The semiconductive composition for power cables according to claim 1, wherein the conductivity imparting material is carbon black.