JPS6332205B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a crosslinked polyethylene power cable having an easily peelable outer semiconductive layer, and particularly to a crosslinked polyethylene power cable with excellent water resistance. Recently, in order to simplify the terminal processing and connection work of cross-linked polyethylene cables and other power cables, and to shorten the time, so-called easy-stripping type cables, in which the outer semiconducting layer can be easily peeled off from the insulator, have been developed. It is beginning to become widespread. This external semiconductive layer is often formed of a composition containing a chlorine-based polymer as a main component, as shown in Japanese Patent Publication No. 48-27111 and Japanese Patent Publication No. 55-35806. Of these, ethylene/vinyl acetate copolymer-polyvinyl chloride graftomer (EVA-
It is abbreviated as PVC graftomer. ) is the most widely used because it provides an outer semiconducting layer with excellent peelability and workability. However, this
EVA-PVC graftomer has extremely poor thermal stability compared to polymers such as polyethylene, ethylene/vinyl acetate copolymer, and ethylene/ethyl acrylate copolymer, and decomposes at high temperatures of 200°C or higher. It was not possible to raise the crosslinking temperature. In other words, in the production of cross-linked polyethylene power cables, the polyethylene is usually passed through a pipe of high-pressure steam at a temperature of about 200 degrees Celsius in order to cross-link it, and at this time the outer semiconducting layer of the cable undergoes the most severe thermal history. Therefore, the outer semiconducting layer formed from a composition containing an EVA-PVC graftomer with poor thermal stability easily deteriorates, so there is a natural limit to the crosslinking temperature, so it is necessary to increase the line speed to increase productivity. I was unable to do so. As seen in Japanese Patent Publication No. 51-32394, improvements in the thermal stability of EVA-PVC graftomers have been achieved to some extent by appropriately incorporating inorganic substances such as magnesium oxide, zinc oxide, and calcium oxide. This makes it possible to use a higher crosslinking temperature than before, approximately 230°C or higher. However, it is made by blending these inorganic substances.
When the outer semiconducting layer is formed with EVA-PVC graftomer, the inorganic substances trap acetic acid and hydrogen chloride generated during the decomposition process of the polymer and become water-soluble acetate and chloride, so the outer semiconducting layer after crosslinking is It becomes extremely hygroscopic and contains a large amount of ionic substances. The present inventors have discovered that when a large amount of ionic substances are contained in the semiconducting layer, water trees occur in the cable insulator, which causes a decrease in insulation performance.
It was confirmed that this had an effect on the long-term reliability of the cable. The present inventors have developed an EVA that does not have such problems.
-As a result of intensive research into improving the thermal stability of PVC graftomers, we found that the chemical formula: Mg ₄ Al ₂
(OH) When mixed with an inorganic compound similar to hydrotalcite represented by ₁₂ CO ₃ 3H ₂ O,
The heat resistance of PVC has been greatly improved, and even when acetic acid or hydrogen chloride is trapped, it does not form water-soluble salts, so it does not generate ionic substances, and as a result, water trees occur in the insulator. It was found that the problem of deterioration of insulation performance can be avoided. The present invention was made based on this knowledge, and the present invention is based on the above knowledge, and it is based on the fact that the outer semiconducting layer extruded on the insulator is
In a cross-linked polyethylene power cable formed of a composition based on a resin containing EVA-PVC graftomer and conductive carbon black, the composition has the chemical formula Mg ₄ Al ₂ (OH) ₁₂ CO ₃ 3H ₂ O. The amount of inorganic compounds expressed is 0.5 per 100 parts by weight of resin.
The object of the present invention is to provide a crosslinked polyethylene power cable having good water resistance and having an easily peelable outer semiconductive layer characterized by containing up to 20 parts by weight. EVA-PVC graftomers used in the present invention include Sumitomo Chemical's Sumigraft GE, GF,
There are GFL, etc., and it is desirable to use them together with EVA, polyethylene, ethylene/ethyl acrylate copolymer, etc. The appropriate proportion of the EVA-PVC graftomer in the polymer is 10 to 50%; if it is more than this, the heat resistance will be reduced, and if it is less than this, the releasability will be reduced. Examples of the conductive carbon black used in the present invention include acetylene black, which is added in an amount of 10 to 100 parts by weight per 100 parts by weight of the resin to obtain the necessary conductivity. Chemical formula Mg ₄ Al ₂ used in the present invention
The inorganic compound represented by (OH) ₁₂ CO ₃・3H ₂ O is
It is commercially available as a hydrotalcite-like compound under the trade name DHT-4A from Kyowa Kagakusha, and is used as an antacid and flame retardant, as well as a stabilizer for polyvinyl chloride. As in the present invention, there has never been an example in which the influence on water resistance has been investigated by using it in an external semiconducting layer. The amount of this compound should be adjusted depending on the amount of EVA-PVC graftomer, but it should be 0.5 to 20 parts by weight, preferably 2 to 10 parts by weight, per 100 parts by weight of the resin. When the amount is less than this, there is almost no thermal stabilizing effect of the EVA-PVC graftomer, and when it is more than this, the physical properties, especially the tear strength, are significantly reduced. In the present invention, in addition to the above-mentioned components, it is desirable to add an antioxidant such as 4,4'-thiobis6-t-butylphenol and an epoxy resin stabilizer, and it is also preferable to add an organic peroxide for crosslinking. You can also do it. These components are kneaded in a conventional manner and extrusion coated onto an insulator to obtain the cable according to the present invention. Next, an example will be described. First, the components shown in Table 1 were kneaded using a Banbury mixer, and the resulting semiconductive composition was press-molded into a 1 mm thick sheet. This was laminated with a 2 mm thick polyethylene sheet containing 2 PHR of dicumyl peroxide as a crosslinking agent, and press crosslinking was performed at 200°C for 20 minutes, and the peel strength was measured using a Schottspur tensile tester. Also, by thermogravimetric analysis, the weight loss rate when left at 200°C or 250°C for 30 minutes was determined and used as a measure of thermal stability. The test results are shown in Table 2 together with the tear strength and water absorption test results. Note that sample numbers 7 to 9 are shown for comparison.

【table】

[Table] (The numbers in the table represent parts by weight.)
Note 1. Sumigraft GF manufactured by Sumitomo Chemical Co., Ltd.

〔Example〕

A three-layer coextruded crosslinked polyethylene power cable with a cross-sectional area of 250 mm ² 6.6 KV was produced using the composition of Sample No. 1 as the outer semiconducting layer. The crosslinking conditions were set so that the temperature of the outer semiconducting layer was approximately 240°C. The outer semiconductive layer of the obtained cable had good appearance and peelability. In addition, a portion of the cable was disassembled and the copper tape shielding layer was examined for discoloration, but no discoloration was found. A 1m cable was placed in a vinyl pipe filled with water, and a voltage of 6.6KV1000Hz was applied to the conductor to conduct a water immersion test. Charge 1000
After a period of time, the insulator was taken out and disassembled, and the insulator was sliced into 1 mm thick slices, and the generation of water trees from the external semiconductive layer in the 10 slices was examined. The test results were as shown in Table 3. Comparative Example 1 A cable was manufactured in the same manner as in Example except that the composition of Sample No. 7 was used as the outer semiconductive layer.
The outer semiconductive layer of the obtained cable had good appearance and peelability, and there was no discoloration of the copper tape. A similar submergence charging test was conducted on this cable.
The occurrence of water trees from the outer semiconducting layer was investigated.
The results were as shown in Table 3. Comparative Example 2 A cable was manufactured in the same manner as in the example except that the composition of sample number 9 was used as the outer semiconductive layer.
The appearance and peelability of the outer semiconductive layer of the obtained cable were inferior to those of Example or Comparative Example 1. When we disassembled part of the cable, we found that the copper tape had turned blue. Table 3 shows the occurrence of water trees from the external semiconductive layer after the water immersion electrification test of this cable.

[Table] As is clear from the above examples, the outer semiconducting layer of the crosslinked polyethylene power cable according to the present invention can be easily peeled off, and the crosslinking temperature can be raised to increase the wire speed. This method is extremely effective because productivity is improved, and since the outer semiconducting layer has less moisture absorption and ionic substances, it is possible to provide a cable that does not generate water trees.

Claims (1)

[Claims] 1 The outer semiconducting layer extruded onto the insulator is
In a crosslinked polyethylene power cable formed from a composition whose main components are a resin containing an ethylene/vinyl acetate copolymer-polyvinyl chloride graftomer and conductive carbon black, the composition has the chemical formula Mg ₄ Al ₂ (OH ) A cross-linked polyethylene power cable having an easily peelable outer semiconductive layer, characterized in that an inorganic compound represented by ₁₂ CO ₃ .3H ₂ O is blended in 0.5 to 20 parts by weight per 100 parts by weight of resin.