JPH0367288B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is designed to reduce deterioration of dendritic insulation (hereinafter referred to as water tree) that occurs when a polyolefin or a composition mainly composed of a crosslinkable polyolefin is used as an electrical insulation material. In order to prevent as much as possible the occurrence of the electrically insulating composition layer, we do not add any special additives to the electrically insulating composition, nor do we add any special processes. The composition for a semiconducting layer and a power cable using the same, more specifically, the composition for a semiconducting layer contains a polyolefin or a crosslinkable polyolefin and a required amount of a highly electrokinetic polyolefin according to claim 1. Carbon black and a small amount of polyethylene glycol are added to the polyolefin in an amount of 0.1 to 20 parts by weight, more preferably 0.2 to 5 parts by weight.
It is characterized by containing part by weight. Regarding the cable, the material forming each layer is new, and the arrangement and manufacturing method of the layers shown in the claims are well known. The explanation will be omitted. Conventionally, compositions for semiconductive layers have mainly consisted of 100 parts by weight of polyolefin or crosslinkable polyolefin and 50 to 100 parts by weight of acetylene black or furnace black as conductive carbon black. Because it contains a large amount of carbon black, it has a high viscosity when melted, which makes extrusion molding difficult, and the adhesive strength between it and the electrical insulating layer is low, and a gap is created between the two layers due to heat cycles, which causes electric field irregularities. This causes partial discharge, which causes deterioration of the electrical insulation layer. In order to improve these shortcomings, JISK6221−
According to the 1970s, 8 to 30 parts by weight of highly conductive carbon black with a DBP oil absorption of 250 ml/100 g or more and a surface area of BET formula (N ₂ ) of 500 m ² /g or more is blended with 100 parts by weight of polyolefin or crosslinkable polyolefin. The conductive composition was published in 1985-15056.
It was proposed as a number. This composition uses a highly conductive carbon black called Ketsuzen Black EC, and requires less carbon black than conventional semiconductive compositions that use acetylene black or furnace black. .
Therefore, the viscosity during melting is low and extrusion molding is easy. However, during extrusion of highly conductive carbon black, as the extrusion time elapses, scum forms and accumulates at the die opening, scratching the extruded surface of the inner semiconductive layer, and furthermore, it essentially scratches the extruded surface of about 5 to 10 microns. Due to the presence of a large number of fine particles, protrusions are generated at the interface of the semiconducting layer where it contacts the electrically insulating layer, which becomes an electric field irregularity and causes water tree deterioration (dendritic deterioration). Therefore, the performance of semiconductive materials using highly conductive carbon black is unreliable, which is a serious drawback. It has already been disclosed in Japanese Patent Application No. 104699/1987 that adding a small amount of relatively high molecular weight polyethylene glycol to an electrical insulating composition is highly effective in preventing water trees from occurring in electrical insulators. However, it was disclosed in Japanese Patent Application No. 12605/1983 that if polyethylene glycol is added to a conventional semiconductive composition, considerable effects can be obtained even without adding polyethylene glycol to the insulating layer. As a result of further investigation, we surprisingly found that by adding and mixing a small amount of relatively high molecular weight polyethylene glycol into a semiconductive composition to which highly conductive carbon black was added, the following results were obtained. An unexpected improvement in performance was obtained. (a) It has been revealed that the generation and accumulation of scum at the die opening, which is considered to be a serious defect in semiconductive materials containing highly conductive carbon black, can be drastically reduced. (b) It is possible to set the extrusion temperature lower than before. (c) As a result, troubles caused by scorching (early crosslinking) are completely eliminated. (d) Good extrusion processability improves cable production speed. (E) The adhesion between the insulating layer and the semiconductive layer is significantly improved compared to when the semiconductive layer contains a large amount of carbon, and no voids are created between the two layers during use as a cable. Unlike the case of No. 55-15056, there is no difficulty in peeling due to excessive adhesiveness, and it is possible to peel off at the interface if necessary. (f) Installing a fine screen with 300 mesh or more in the extruder makes it possible to easily remove coarse carbon particles. (g) Since the interface between the semiconducting layer and the insulating layer becomes extremely smooth, not only water trees but also electrical trees are significantly suppressed. (h) Another feature of this technology is that since it is a semiconductive layer, the presence of small amounts of additives does not cause any electrical or mechanical problems, and the polyethylene glycol added is inexpensive and Always readily available. (li) Addition of polyethylene glycol increases the extrusion capacity of the extruder. Therefore, it has already been disclosed in JP-A-56-28231 and JP-A-57-126004 that the rotation speed of the extruder can be lowered in order to obtain the same amount of extrusion; The effect was further increased by decreasing the amount. This significantly reduces the heat generated by the resin composition, reduces the number of operation stoppages caused by increases in extruder temperature, and also produces secondary effects such as improved productivity and energy savings. In the present invention, polyolefins include polyethylene, polypropylene, ethylene-α-olefin copolymers, polybutene, polyisobutylene, poly-4-methylpentene, and copolymers containing these basic units as main components, such as ethylene,
A vinyl acetate copolymer, an ethylene/alkyl acrylate copolymer, and a polymer obtained by crosslinking these polymers are encapsulated. Furthermore, high molecular weight polyethylene glycol refers to polyethylene glycol having 30 or more carbon atoms, preferably 80 or more carbon atoms.
The amount added is from 0.1 parts by weight to 20 parts by weight, preferably from 0.2 to 5 parts by weight, based on the weight of the polyolefin or crosslinked polyolefin. Below 0.1 part by weight, the effect decreases as the molecular weight decreases. Since polyethylene glycol is inexpensive, adding it in large amounts has little effect on cost, but adding more than 20 parts by weight is not preferred in terms of difficulty in uniform mixing and deterioration of adhesion between the insulating layer and the semiconductive layer. Since the present invention does not distinguish between crosslinking and non-crosslinking, it does not limit the use or non-use of crosslinking agents, and does not preclude the use of other stabilizers, inorganic fillers, and other additives. The experimental method for generating and quantifying the dross is described below using a Brabender extruder (Type PL) equipped with a screw of 25 mm in diameter and 20 L/D.
-2000), the amount generated per unit time is determined from the ratio to the amount of semiconductive material extruded at a temperature of 125°C from a die with an opening of 0.7 mm in length and 25 mm in width. Examples and Comparative Examples The semiconductive composition uses EEA (ethylene/ethyl acrylate copolymer) or EVA (ethylene/vinyl acetate copolymer) as a base resin, and contains carbon black, an antioxidant, and a crosslinking agent. A predetermined amount of polyethylene glycol was added, and various samples were prepared by changing the amount of polyethylene glycol added. A tape extrusion test was conducted on these using a Brabender extruder.
Table 1 shows typical results as Examples, and Comparative Examples in which polyethylene glycol was added in a small amount and the effect of preventing generation of scum was insufficient. When the amount of polyethylene glycol added was 0.05 parts by weight (Comparative Example 2), a reduction in scum was observed, but the prevention effect was insufficient, and when the amount added was 0.20 parts by weight (Example 1), there was no effect. The effectiveness is fully recognized as the amount of dregs generated is halved compared to the case of addition. Although no change is observed in the amount of dregs generated when the amount is 2.0 parts by weight or more, many other effects caused by polyethylene glycol can be expected. However, if the amount of polyethylene glycol added exceeds 20 parts by weight, the physical properties and extrusion processability necessary for a semiconductive resin composition will deteriorate, so it is not preferable to add more than that amount. The polyethylene glycol used in this example was PEG20000 manufactured by NOF Corporation. 【table】

Claims (1)

[Claims] 1. A power cable consisting of an inner semiconductive layer, an electrically insulating layer, an outer semiconductive layer, and a sheath layer around a conductor, in which the inner and/or outer semiconductive layers are made of polyolefin or crosslinkable polyolefin. 100 parts by weight and 0.1 to 20 parts by weight of polyethylene glycol according to JIS K6221-1970
A power cable mainly comprising 8 to 30 parts by weight of highly conductive carbon black having a DBP oil absorption of 250 ml/100 g or more and a surface area of BET formula (N ₂ ) of 500 m ² /g or more.