JPS64769B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a resin composition for direct current electrical insulation used for insulating high voltage direct current wires, cables, their connections, terminal parts, and other direct current circuits. Electric wires and cables with insulators mainly composed of polyolefin polymers such as polyethylene and cross-linked polyethylene are used for AC high pressure transmission and
Widely used for power distribution. However, since AC power transmission involves dielectric loss, it is necessary to compensate for the line charging current, and it is widely known that DC power transmission is more advantageous for transmitting large currents over long distances.
However, in an experiment in which a cable with cross-linked polyethylene as an insulator was used for 250KV DC power transmission, it was announced that the insulation performance lacked stability (IEEJ/Electric Power Technology Joint Study Group Material PE-
76-14, Test method for insulation performance of DC cables, 1976
December 14th). Therefore, it has been desired to improve these drawbacks and develop an insulating resin composition suitable for DC power transmission. The reason for the lack of stability in insulation performance is that the accumulation of space charges in the insulator caused by direct current causes tree-like discharge deterioration, so one solution is to It is known that the problem can be solved to some extent by creating a synthetic resin layer that is less likely to be injected with charge (Japanese Patent Application Laid-Open No. 51-111).
No. 110678). However, conventionally, many ultra-high voltage DC power cables have a three-layer structure, and in order to add one more new resin layer to this, it is necessary to invest in equipment such as installing a new extruder. Since an increase in the number of layers leads to an increase in the defective rate, there was a desire to develop a technology that would not require changes to existing equipment. In the previous invention (Japanese Patent Laid-Open No. 54-159680), the inventors of the present application focused on the technology development with a focus on the characteristics as an insulating material for DC high voltage, and developed a tree during DC energization, a tree during polarity reversal, The main purpose of the study was to improve trees during impulse energization, and it was found that polyethylene, which is an extremely excellent insulating material for AC high-voltage power cables, was considered undesirable for DC high-voltage power cables. did it. Trees refer to insulation deterioration parts observed in the form of branches or bushes that occur in an insulator when high voltage is applied. However, polyamide generally has a high melting point, and its cocondensate has a lower melting point, but is expensive and not easy to obtain.Polyethylene polymers and polyamides have poor compatibility, so it takes a long time to knead them. is necessary. Then,
We are searching for a polymer that has good compatibility with polyethylene polymers due to polyamides, has a processing temperature close to that of polyethylene polymers, and is suitable as a DC high voltage insulation material in terms of electrical properties. We have reached the unexpected result that an ionomer consisting of an ethylene-organic acid copolymer and a metal ion, which has not been considered as an electrical insulating material, is an effective compound. That is, when we conducted an experiment in which we added an ionomer to a conventional polyolefin composition and dispersed it evenly in an insulator, we surprisingly found that the accumulation of space charges could be alleviated without impairing the insulation properties.
They discovered that insulation deterioration caused by tree discharge can be suppressed, and succeeded in developing an insulating resin composition suitable for high-voltage DC power transmission. Polyester resins are known as other resins that are difficult to inject charges into (Japanese Patent Application Laid-open No. 110678/1989), but polyethylene terephthalate, a commonly available polyester resin, has a high melting point of 260°C, and polyethylene electric wires It is not preferable because it does not melt at the temperature at which it is molded. The ionomer used as a modifier in the present invention has been used since 1965 when DuPont put on the market a product prepared by adding a metal salt to an ethylene-methacrylic acid copolymer. It is used as the name for a polymer that is chemically bonded by adding a salt. This resin is characterized by its excellent adhesion, particularly with metals, inorganic materials, and polyamide resins, as well as good oil resistance, toughness, deep drawability, and transparency. However, since ionomers contain polar groups and metal ions, there have been no examples of them being applied as electrical insulating materials. data are shown in detail, but there is no description of ionomers (see pages 580-607). In the field of electric wires and cables, there are examples where it is used as an adhesive or adhesion layer between a metal layer such as aluminum and an insulating layer such as polyolefin in a laminated sheath cable, but it has never been used as an electrical insulating layer. According to the present invention, 10 to 300 parts by weight, preferably 30 to 300 parts by weight of one or several kinds of mixtures selected from various ionomers are added to 100 parts by weight of the polyolefin resin.
Contains 100 parts by weight, and optionally contains antioxidants,
By using a resin composition containing the required amounts of heat deterioration inhibitors, ultraviolet stabilizers, crosslinking agents, crosslinking aids, foaming agents, etc., we can improve the stability of polyolefin insulated cables for DC high voltages and insulation materials for DC high voltages. succeeded in improving it. The polyolefins mentioned here include not only low-density polyethylene and high-density polyethylene, but also ethylene copolymers with an ethylene content of 50% or more, such as ethylene-propylene copolymers, ethylene-butene copolymers, ethylene-4-methylpentene copolymers, etc. 1 copolymer, ethylene-α-olefin copolymers such as ethylene-hexene copolymer, ethylene-vinyl ester copolymer, ethylene-alkyl acrylate copolymer, and modified or modified products of these. Refers to anything that can be used for electrical insulation. An ionomer is a copolymer of an α-olefin and a polymerizable monomer having a carboxylic acid, in which carboxyl groups are used to create bonds between molecular chains with metal ions. If the amount is less than 10 polymerization parts, an improvement in tree resistance can be seen, but the effect is not sufficient, and if it is more than 300 polymerization parts, the elongation of positive impulse tree becomes very large, and the heat aging resistance Significant decline in sex. Stabilizers applicable to this invention include, but are not limited to, 4,4'-thiobis(6-t-butyl-3-methylphenol) and 2,6-di-t-butyl-p-cresol. All stabilizers that are normally effective for polyolefins can be used. Crosslinking agents applicable to this invention include dicumyl peroxide, 2,5-dimethyl-2,5-
It is desirable to use bis(t-butylperoxy)hexane, t-butylcumyl peroxide, etc., which have a decomposition rate of 100 to 200°C with a half-life of 1 minute. Examples and comparative examples will be described below based on Tables 1 and 2. The various compositions shown in Table 1 were press-molded into sheets with a thickness of 5 mm, and
Punch out a sample at the corner, insert the needle-shaped electrode 2 whose tip has a half diameter of curvature of 5 x 10 ^-3 mm or less as shown in Figure 1, and apply silver paint to the surface of the sample facing the needle end. The insulation thickness was set at 5 mm, and a DC voltage was applied using the apparatus shown in FIG. 2 to observe the occurrence of trees. Sample 1 was placed in a container 8 filled with silicone oil, a DC voltage generated by DC voltage generator 3 was applied to the sample, and the length of the tree was measured using microscope 5. 4 is a light source, 5
is a filter, and 7 is a vacuum switch. The effect of polarity reversal, which is a phenomenon specific to DC insulation materials such as DC cables, is that -40KV negative polarity is
After applying for a minute, the application was stopped, and after 1 second, a positive impulse voltage of +40 KV was applied, and the length of the tree generated at the needle end was measured and determined. The 50% DC tree generation voltage and the 50% impulse tree generation voltage are voltages at which trees are generated in 50% of the sample after 1 hour of application. Next, the various compositions shown in Table 2 were press-molded into sheets with a thickness of 1 mm, punched using a JIS No. 3 dumbbell cutter, heat-aged in a gear oven at 150°C for 7 days, and then stretched in accordance with JIS-K6760. Judgment was made by measuring elongation.

【table】

【table】

[Table] As shown in Comparative Examples 1 and 4, polyethylene has a polar effect, so the difference between positive polarity and negative polarity is 50%.
The difference between the DC tree generation voltage and the 50% impulse tree generation voltage is large, and the tree elongation is also large when the polarity is reversed, which confirms that the insulation performance of insulation materials for DC power transmission lacks stability. However, as shown in Examples 1 to 9, by blending either or both of ionomer A and ionomer B with polyolefin, this polar effect can be reduced and the space charge, which is considered to be a factor in tree formation and elongation, can be reduced. By alleviating the accumulation, we were able to obtain an excellent DC electrical insulation material. Comparative Example 2, in which 5 parts by weight of ionomer was blended, did not sufficiently alleviate the accumulation of space charges, and Comparative Example 3, in which 350 parts by weight of ionomer was blended, was insufficient.
In this case, the positive impulse tree is significantly elongated, making it unsuitable as an insulating material for high voltage DC applications. In Example 9 and Comparative Example 4, 2 parts by weight of dicumyl peroxide was added to cause crosslinking, but the presence or absence of crosslinking did not affect the effects of the present invention. When using only HDPE, EVA, and EEA without the addition of an ionomer, the same results as when using only LDPE (Comparative Examples 1 and 4) were obtained. On the other hand, as shown in Comparative Example 6 and Examples 10, 11, and 12 in Table 2, as the amount of ionomer added increases, the mechanical properties after heat aging decrease significantly. This deterioration in mechanical properties can be prevented by incorporating a large amount of an appropriate antioxidant, but this is not preferable since it results in a deterioration in the degree of crosslinking. The method for producing a DC high voltage cable and DC high voltage insulation material using the resin composition for DC electrical insulation of the present invention can be carried out by conventional methods, and if necessary, it can be carried out by partially modifying the extrusion processing conditions. Regarding the structure of the cable, only the conventional insulating material layer is replaced with the composition of the present invention.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the shape of a sample and the arrangement of electrodes used for observing the state of tree formation, and FIG. 2 is a schematic diagram of an experimental apparatus for tree observation.

Claims (1)

[Scope of Claims] 1. A resin composition for direct current electrical insulation, characterized in that 100 parts by weight of a polyolefin resin composition is blended with 21 to 300 parts by weight of one selected from various ionomer resins or a mixture thereof. 2. The polyolefin resin composition according to claim 1, wherein the polyolefin resin composition is mainly composed of a polyolefin resin, and a small amount of an oxidation stabilizer, a thermal deterioration inhibitor, an ultraviolet stabilizer, and/or other stabilizers are blended therein. Composition. 3. The composition according to claim 1 or 2, wherein the polyolefin resin composition contains a crosslinking agent, a crosslinking aid, and/or a foaming agent and a foaming aid in the required amount. 4. The composition according to claim 1, 2 or 3, wherein the polyolefin resin composition contains a required amount of carbon black and/or other inorganic additives.