Electrical insulating composition and power cable

Abstract

An electrical insulating composition comprising a resin component including polyethylene and a thermoplastic elastomer, and a polyalkylene glycol, wherein the polyethylene content is 60 parts by mass or more and 85 parts by mass or less, the thermoplastic elastomer content is 15 parts by mass or more and 40 parts by mass or less, and the polyalkylene glycol content is 0.5 parts by mass or more and 1.0 part by mass or less per 100 parts by mass of the resin component.

Description

[Technical Field]

[0001] The present disclosure relates to electrical insulation compositions and power cables. [Background technology]

[0002] Due to its excellent insulating properties, polyethylene is widely used as a base resin for electrical insulating compositions that form insulating layers in power cables and the like (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 57-69611 Summary of the Invention

[0004] According to one aspect of the present disclosure, a resin component including polyethylene and a thermoplastic elastomer; and a polyalkylene glycol having a number average molecular weight of 4,000 or more and 8,000 or less, The content of the polyethylene is 60 parts by mass or more and 85 parts by mass or less, and the content of the thermoplastic elastomer is 15 parts by mass or more and 40 parts by mass or less, The content of the polyalkylene glycol is 0.5 parts by mass or more and 1.0 parts by mass or less with respect to 100 parts by mass of the resin component. An electrically insulating composition is provided. [Brief explanation of the drawings]

[0005] [Figure 1] FIG. 1 is a cross-sectional view perpendicular to the axial direction of a power cable according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a schematic diagram illustrating the occurrence of water trees in an electrical insulating composition according to one embodiment of the present invention. [Figure 3]FIG. 3 is a schematic diagram for explaining the occurrence of water trees in a conventional electrical insulating composition. [Figure 4] FIG. 4 is a schematic cross-sectional view showing a water needle test. DETAILED DESCRIPTION OF THE INVENTION

[0006] [Problem to be solved by this disclosure] When a power cable is energized in a humid or water-filled environment, water trees may occur in the insulation layer. Therefore, it is necessary to improve the water tree resistance of the insulation layer in addition to the cable properties required for power cables.

[0007] An object of the present disclosure is to provide a technology that can improve water tree resistance while maintaining various cable characteristics.

[0008] [Effects of this disclosure] According to the present disclosure, it is possible to improve water tree resistance while maintaining various cable characteristics.

[0009] [Description of the embodiments of the present disclosure] <Knowledge gained by the inventors> First, the findings of the present inventors will be explained.

[0010] From the viewpoint of practical use, power cables are required to have high insulation properties, mechanical properties, flexibility, etc., and low dielectric loss tangent. Furthermore, power cables are required to be water tree resistant, for example, because they are laid in humid or water-submerged environments. This is because, in humid environments, when a certain electric field is applied to the insulation layer of a power cable, water trees are generated in the insulation layer, which reduces the insulation properties of the power cable.

[0011] Water trees are generated, for example, by the following mechanism: In a humid or water-filled environment, water can penetrate into the insulation layer of a power cable. When water penetrates into the insulation layer while a voltage is applied to the power cable, the water condenses in areas of the insulation layer where localized electric field concentrations occur. Examples of localized electric field concentrations include voids in the insulation layer, foreign matter, and irregularities at the interface between the insulation layer and the semiconducting layer. When water condenses in such localized electric field concentrations, mechanical strain occurs around the condensed water area due to an increase in pressure of the condensed water. As a result, tree-like or bowtie-shaped water trees are generated in the insulation layer.

[0012] In the past, methods have been proposed to suppress the occurrence of water trees in the insulation layer of power cables, such as blending a thermoplastic elastomer with polyethylene. Thermoplastic elastomers can reduce the number of water trees that occur and their length by trapping foreign matter that can be the starting point for water trees. They can also trap water that penetrates the insulation layer and suppress the coagulation of water in the polyethylene.

[0013] In recent years, the specifications required for power cables laid in humid or water-submerged environments have become stricter, and therefore there is a demand for power cables with improved water tree resistance.

[0014] Conventionally, water tree resistance has been evaluated by applying an electric field to the insulation layer while the power cable is immersed in water. In this test, water pressure and an electric field are applied evenly to the insulation layer. On the other hand, a water needle test is a method for evaluating water tree resistance at a higher level. The water needle test, which will be described in detail later, is a water tree test that uses a water needle to simulate micro-foreign objects or micro-voids trapped in the electrical insulating composition.

[0015] The inventors conducted a water needle test on an electrical insulating composition containing polyethylene and a thermoplastic elastomer, and found that although the number and length of water trees can be reduced, high water tree resistance cannot be stably obtained. Further investigation into this point revealed that in the above composition, the shape of the water trees is a special shape, with some parts protruding or distorted.

[0016] Here, the phase structure and water tree formation will be explained using Figure 3. Figure 3 is a schematic diagram illustrating the formation of water trees in a conventional electrical insulating composition. As shown in Figure 3, polyethylene forms a sea phase 1', and a thermoplastic elastomer forms an island phase 2' dispersed in the sea phase 1'. When a needle 6 is inserted into such a composition and an electric field is applied, water that has penetrated the composition condenses, forming water trees 5'. The water trees 5' selectively grow through the polyethylene sea phase 1' rather than the thermoplastic elastomer island phase 2'. Because the island phases 2' are finely dispersed in the sea phase 1', the water trees 5' grow through the gaps between the island phases 2'. As a result, the water trees 5' become protruding or have a distorted shape. Such water trees 5' tend to grow during the application of voltage to a power cable, which may significantly reduce the insulation properties.

[0017] Until now, the number and length of water trees have been emphasized in terms of improving water tree resistance, but water needle testing has shown that the shape of the water trees that form is also important, and it is best to make the shape closer to a sphere, without protrusions or distortions. In other words, even if the number of water trees that form is reduced or the water trees are shortened, if the shape is distorted, it is not possible to reliably suppress the dielectric breakdown of the insulation layer caused by water trees. The fact that the shape of water trees also affects resistance could not be confirmed by immersion testing, but was first discovered by water needle testing under stricter conditions.

[0018] The inventors have investigated various compositions from the viewpoint of making water trees more spherical, and in the process have found that mixing polyethylene glycol is effective. A three-component system containing polyethylene glycol can reduce the number of water trees that occur and shorten their length, while making the shape of the water trees that do occur more spherical.

[0019] Furthermore, by adjusting the additive ratio of polyethylene, thermoplastic elastomer, and polyalkylene glycol within a predetermined range, it is possible to obtain a high level of balance between various cable characteristics and water tree resistance.

[0020] The present invention was made based on the above findings.

[0021] <Embodiments of the present disclosure> Next, embodiments of the present disclosure will be listed and described.

[0022] [1] One aspect of the present disclosure is a resin component including polyethylene and a thermoplastic elastomer; and a polyalkylene glycol having a number average molecular weight of 4,000 or more and 8,000 or less, The content of the polyethylene is 60 parts by mass or more and 85 parts by mass or less, and the content of the thermoplastic elastomer is 15 parts by mass or more and 40 parts by mass or less, The content of the polyalkylene glycol is 0.5 parts by mass or more and 1.0 parts by mass or less with respect to 100 parts by mass of the resin component. Electrical insulating compositions. This configuration makes it possible to improve water tree resistance while maintaining various cable characteristics.

[0023] [2] Another aspect of the present disclosure is A conductor; an insulating layer formed on the outer periphery of the conductor and made of an electrical insulating composition; The electrical insulating composition is a resin component including polyethylene and a thermoplastic elastomer; and a polyalkylene glycol having a number average molecular weight of 4,000 or more and 8,000 or less, The content of the polyethylene is 60 parts by mass or more and 85 parts by mass or less, and the content of the thermoplastic elastomer is 15 parts by mass or more and 40 parts by mass or less, The content of the polyalkylene glycol is 0.5 parts by mass or more and 1.0 parts by mass or less with respect to 100 parts by mass of the resin component. Power cable. With this configuration, the water tree resistance of the power cable can be improved while maintaining various cable characteristics.

[0024] [3] In the power cable according to [2] above, The thermoplastic elastomer is at least one of an olefin-based thermoplastic elastomer having an olefin unit and a styrene-based thermoplastic elastomer having a styrene unit. This configuration can further improve water tree resistance.

[0025] [4] The power cable according to the above [2] or [3], The polyethylene is a low density polyethylene. This configuration can further improve the mechanical properties of the insulating layer.

[0026] [5] In the power cable according to any one of [2] to [4], The polyalkylene glycol is polyethylene glycol. This configuration can further improve the water tree resistance of the insulating layer.

[0027] [6] The power cable according to any one of [2] to [5] above, The electrical insulating composition has a phase structure in which the polyethylene constitutes a sea phase, the thermoplastic elastomer constitutes an island phase dispersed in the sea phase, and the polyalkylene glycol is dispersed in the island phase. This configuration makes it possible to achieve a higher level of both cable characteristics and water tree resistance.

[0028] [7] The power cable according to any one of [2] to [6] above, The insulating layer has a maximum length of water trees generated by a water needle test of 200 μm or less, and the ratio of the long side to the short side of the water trees is 2.0 or less. Here, the water needle test is providing a plate electrode on a first surface of the block taken from the insulating layer; forming needle-shaped voids at positions facing each other at a distance of 1 mm from the first surface, the needle-shaped voids having tips with a curvature radius of 10 μm; A step of forming water needles in the block by injecting artificial seawater having a salinity of 3.8% by mass into the void; and applying an AC voltage of 1000 Hz and 4 kV between the plate electrode and the tip of the water needle for 30 days. With this configuration, the water tree can be adjusted to be small in the insulating layer while its shape can be made closer to a spherical shape.

[0029] [8] The power cable according to any one of [2] to [7] above, When the insulating layer was immersed in artificial seawater with a salinity of 3.8% by mass at room temperature and an AC voltage of 1000 Hz and 4 kV was applied to the insulating layer for 30 days, The density of water trees occurring in the insulating layer and having a length of 50 μm or more is 2,000 trees / cm 3 is less than. This configuration can suppress the occurrence of water trees in the insulating layer.

[0030] [Details of the embodiments of the present disclosure] An embodiment of the present disclosure will be described below. However, the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0031] [One embodiment of the present disclosure] (1) Electrical insulating composition The electrical insulating composition of this embodiment is a material that constitutes the insulating layer 130 of the power cable 10 described below. The electrical insulating composition contains at least a resin component containing polyethylene and a thermoplastic elastomer, polyalkylene glycol, and, as necessary, other additives. Hereinafter, the electrical insulating composition will also be simply referred to as the "composition."

[0032] (polyethylene) Polyethylene constitutes the main component of the resin component. Examples of polyethylene include low-density polyethylene (LDPE: density 0.88 g / cm 3 More than 0.93g / cm 3 less than 0.945 g / cm), linear low-density polyethylene (LLDPE: density 0.945 g / cm 3 below), medium density polyethylene (MDPE: density 0.93 g / cm 3 More than 0.942g / cm 3 less than 0.942 g / cm 3 (The above) etc.

[0033] As the polyethylene, LDPE is preferred from the viewpoint of realizing a predetermined phase structure in the composition, which will be described later. LDPE allows the insulating properties and mechanical properties of the insulating layer to be maintained at a high level. From the viewpoint of maintaining the insulating properties and mechanical properties of the insulating layer at a high level, the density of LDPE is 0.89 g / cm. 3 More than 0.91g / cm 3 It would be better if it was below.

[0034] The melt flow rate (MFR) of the polyethylene is preferably, for example, 0.8 g / 10 min or more and 2.3 g / 10 min or less, as measured in accordance with JIS K7210 at a temperature of 190° C. and a load of 2.16 kg.

[0035] (thermoplastic elastomer) When mixed with polyethylene, the thermoplastic elastomer is finely dispersed in the polyethylene. The thermoplastic elastomer has lower crystallinity than polyethylene, which contributes to improving the flexibility of the electrical insulating composition. Furthermore, because the thermoplastic elastomer is flexible, it easily absorbs foreign matter when mixed with polyethylene, which contributes to suppressing the occurrence of water trees. Furthermore, it suppresses the growth of water trees and contributes to reducing their maximum length.

[0036] The thermoplastic elastomer may be any component capable of improving the flexibility of the composition, and may be any known component such as an amide, ester, olefin, styrene, urethane, vinyl chloride, fluorine, etc. Among these, it is preferable to use at least one of a styrene or olefin elastomer from the viewpoint of finely dispersing the elastomer when mixed with polyethylene. Styrene-based resins can trap electrons in the aromatic rings of styrene, forming a stable resonance structure. Therefore, even if water condenses around the incorporated foreign matter, it is possible to alleviate the electric field concentration caused by the condensation. Furthermore, because they are flexible, they can suppress the occurrence of mechanical stress cracks. These factors can also suppress the occurrence of water treeing in the composition. Olefin-based resins are more easily mixed with polyethylene than styrene-based resins, and can suppress the formation of voids in the composition, thereby suppressing the occurrence of water trees in the composition.

[0037] The styrene-based thermoplastic elastomer is a copolymer containing styrene units as hard segments and at least one monomer unit selected from ethylene, propylene, butylene, isoprene, and the like as soft segments.

[0038] Examples of styrene-based thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), hydrogenated styrene-butadiene-styrene block copolymers, styrene-isoprene-styrene copolymers (SIS), hydrogenated styrene-isoprene-styrene copolymers, hydrogenated styrene-butadiene rubber, hydrogenated styrene-isoprene rubber, styrene-ethylene-butylene-olefin crystalline block copolymers, etc. Two or more of these may be used in combination.

[0039] Here, "hydrogenated" means that hydrogen has been added to the double bond. For example, "hydrogenated styrene butadiene styrene block copolymer" means a polymer in which hydrogen has been added to the double bond of a styrene butadiene styrene block copolymer. Note that no hydrogen has been added to the double bond of the aromatic ring of styrene. "Hydrogenated styrene butadiene styrene block copolymer" can be rephrased as styrene ethylene butylene styrene block copolymer (SEBS).

[0040] The content of styrene units in the styrene-based thermoplastic elastomer is not particularly limited, but from the viewpoint of the flexibility of the composition and its compatibility with polyethylene, it is preferable that it be 10% by mass or more and 45% by mass or less. Furthermore, by ensuring that the content of styrene units falls within the above range, a predetermined amount of monomer units, such as ethylene units, can be secured as soft segments, thereby improving the compatibility between the styrene-based thermoplastic elastomer and polyethylene. This allows for stable improvements in the water tree resistance and insulating properties of the composition.

[0041] Olefin-based thermoplastic elastomers (so-called TPOs) are composed of at least one olefin unit (polyethylene or polypropylene) as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment. The olefin-based thermoplastic elastomer may be a copolymer of an olefin unit and an ethylene-α-olefin copolymer unit, or a blend of an olefin and an ethylene-α-olefin copolymer. Among these, copolymers are preferred from the viewpoint of compatibility with polyethylene. The α-olefin is a linear or branched α-olefin having 2 to 8 carbon atoms, such as ethylene, propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3-methyl-1-pentene, or 1-octene. The olefin-based thermoplastic elastomers may be used alone or in combination of two or more.

[0042] From the viewpoint of compatibility with polyethylene, the MFR of the thermoplastic elastomer is preferably 1.5 g / 10 min or more and 4.5 g / 10 min or less, and may be 0.8 g / 10 min or more and 6.5 g / 10 min or less. By setting the MFR in this range, the above-mentioned phase structure can be stably formed in the resin composition.

[0043] From the viewpoint of finely dispersing the thermoplastic elastomer when mixed with polyethylene, it is preferable that the MFR of the thermoplastic elastomer has a small difference from the MFR of the polyethylene, specifically, the difference in MFR between the polyethylene and the thermoplastic elastomer is 3.0 g / 10 min or less.

[0044] (Polyalkylene glycol) Polyalkylene glycol is composed of repeating alkylene oxide units and has multiple ether groups. When mixed with a resin component containing polyethylene and a thermoplastic elastomer, polyalkylene glycol is a component that disperses more easily in the thermoplastic elastomer than polyethylene. Polyalkylene glycol easily captures water and can suppress water aggregation in polyethylene. This can suppress the generation and growth of water trees.

[0045] The behavior and dispersibility of polyalkylene glycols in resin compositions tend to vary depending on their molecular weight. According to the inventors' research, the smaller the molecular weight of a polyalkylene glycol, the more easily it dissolves at low temperatures and is more likely to generate micro-vibrations. An increase in the number of molecules that generate micro-vibrations can increase the dielectric loss tangent of the composition. On the other hand, as the molecular weight increases, the electrostatic loss tangent of the composition can be reduced, but the polyalkylene glycol becomes more compatible with polyethylene and less easily dispersed in the thermoplastic elastomer. As a result, the polyalkylene glycol's inherent water-trapping effect is less likely to be realized, and the generation of water trees cannot be stably suppressed. In this regard, the number-average molecular weight of the polyalkylene glycol is preferably 4,000 to 8,000, or even 5,500 to 6,500. Having the above number-average molecular weight allows the polyalkylene glycol to disperse in the thermoplastic elastomer and form the phase structure described below while maintaining a low dielectric loss tangent of the composition. Hereinafter, the number-average molecular weight will also be simply referred to as molecular weight. The number average molecular weight indicates a value measured by gel permeation chromatography (GPC) for polyalkylene glycol.

[0046] Examples of polyalkylene glycols that can be used include polyethylene glycol and polypropylene glycol. Among these, polyethylene glycol is preferred. Polyethylene glycol is highly hydrophilic and easily captures water, so it can more reliably suppress the occurrence of water trees.

[0047] (Other additives) The composition of the present embodiment may further contain a crosslinking agent, a crosslinking aid, an antioxidant, an inorganic filler, a lubricant, or the like, as needed.

[0048] The crosslinking agent can crosslink the composition, improving mechanical properties (such as tensile properties) and electrical properties. Examples of crosslinking agents include organic peroxides. Specific examples of organic peroxides include dicumyl peroxide, 1-(2-tert-butylperoxyisopropyl)-1-isopropylbenzene, 1-(2-tert-butylperoxyisopropyl)-3-isopropylbenzene, 1,3-bis-(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2.5-di(t-butylperoxy)hexane, and 2,5-dimethyl-2,5-(tert-butylperoxy)hexyne-3. Two or more of these may be used in combination.

[0049] The crosslinking aid can accelerate the crosslinking reaction caused by the crosslinking agent. Examples of the crosslinking aid include triallyl cyanurate, triallyl isocyanurate, triallyl isocyanurate prepolymer, trimethallyl isocyanurate, 1,3,5-triacryloylhexahydro-1,3,5-triazine, triallyl trimellitate, m-phenylenediamine bismaleimide, dipropargyl terephthalate, and N,N',N'',N'''-tetraallyl terephthalamide. These may be used alone or in combination of two or more.

[0050] The inorganic filler can improve the insulating properties by trapping space charge and suppressing local accumulation of space charge in the insulating layer 130. As the inorganic filler, for example, particles containing at least one of magnesium oxide, silicon dioxide, zinc oxide, aluminum oxide, titanium oxide, and zirconium oxide can be used.

[0051] Examples of antioxidants include 4,4'-thiobis-(6-t-butyl-3-methylphenol), 2,2-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 2,4-bis-[(octylthio)methyl]-o-cresol, 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, and bis[2-methyl-4-{3-n-alkyl(C12 or C14)thiopropionyloxy}-5-t-butylphenyl]sulfide. Two or more of these may be used in combination.

[0052] The lubricant acts to suppress aggregation of the inorganic filler and improve the fluidity of the resin composition during extrusion molding of the insulating layer 130. Known materials can be used as the lubricant in this embodiment.

[0053] (Resin composition) The composition of this embodiment contains, as resin components, 60 to 85 parts by mass of polyethylene and 15 to 40 parts by mass of thermoplastic elastomer. If the thermoplastic elastomer content is excessively high, the polyethylene content will be reduced, resulting in a low tensile strength of the insulating layer and making it impossible to meet the mechanical properties required for power cables. On the other hand, if the thermoplastic elastomer content is excessively low, the desired flexibility of the insulating layer will not be achieved. Furthermore, a low content of thermoplastic elastomer makes it difficult to absorb foreign matter, making water trees more likely to occur and grow in the polyethylene. In other words, the desired water tree resistance cannot be maintained in the insulating layer. In this regard, by mixing polyethylene and thermoplastic elastomer in the above ratio, it is possible to achieve a high level of both mechanical properties such as tensile strength and water tree resistance. To achieve a higher level of both properties, the polyethylene content should be 65 to 80 parts by mass or 65 to 75 parts by mass. The content of the thermoplastic elastomer may be 20 parts by mass or more and 35 parts by mass or less, or 25 parts by mass or more and 35 parts by mass or less.

[0054] The composition contains 0.5 to 1.0 parts by mass of polyalkylene glycol per 100 parts by mass of the resin component. If the polyalkylene glycol content is too low, the generation and growth of water trees cannot be sufficiently suppressed, resulting in significantly reduced water tree resistance. On the other hand, if the polyalkylene glycol content is too high, polar groups (e.g., ether groups) derived from the polyalkylene glycol form a dipole moment, and the energy lost due to this vibration increases. In other words, the dielectric loss tangent increases. In this regard, by setting the polyalkylene glycol content within a specified range, it is possible to maintain high water tree resistance while also maintaining a small dielectric loss tangent. From the perspective of achieving these goals at a higher level, the polyalkylene glycol content should be 0.65 to 0.85 parts by mass.

[0055] (phase structure) The composition of this embodiment has a predetermined phase structure due to the presence of a predetermined content of polyethylene, a thermoplastic elastomer, and a polyalkylene glycol. The phase structure of the composition of this embodiment will now be described with reference to Figure 2. Figure 2 is a schematic diagram illustrating the occurrence of water trees in an electrical insulating composition according to one embodiment of the present invention.

[0056] As shown in Figure 2, polyethylene forms sea phase 1. Thermoplastic elastomer is dispersed in sea phase 1 to form island phase 2. Polyalkylene glycol has higher compatibility with thermoplastic elastomer than polyethylene, so it disperses mainly in island phase 2 rather than sea phase 1. In other words, the amount of polyalkylene glycol present (dispersed amount) tends to be greater in island phase 2 than in sea phase 1. As a result, the polyalkylene glycol forms a fine phase 3. Foreign matter 4 may be mixed into the composition, and during mixing, the foreign matter 4 tends to be incorporated into the island phase 2 of the thermoplastic elastomer.

[0057] The mechanism by which the occurrence and growth of water trees is suppressed when the composition has the phase structure shown in Figure 2 is presumed to be as follows. Specifically, the foreign matter 4, which is the starting point of water tree generation, is captured in the island phase 2 and surrounded by the polyalkylene glycol microphase 3. Because polyalkylene glycol has a polar group, the electric field around the foreign matter 4 is relaxed. This makes it possible to prevent water from coagulating around the foreign matter 4 when the insulating layer 130 is immersed in water. In other words, it is possible to prevent the generation of water trees originating from the foreign matter 4. Furthermore, when the composition is immersed in water, the highly hydrophilic polyalkylene glycol can capture water that is absorbed into the composition. Furthermore, the polyalkylene glycol is dispersed in the island phases 2 that are finely dispersed in the sea phase 1 to form the fine phases 3, so the captured water can be uniformly dispersed and retained in the composition without localized aggregation. As a result, even when a voltage is applied to the power cable 10, aggregation of water at locations where the electric field is concentrated (e.g., around foreign matter 4) can be suppressed. Furthermore, since the amount of water contained in the polyethylene sea phase 1 can be reduced, aggregation of water in the sea phase 1 can also be suppressed. This can suppress the occurrence and growth of water trees. As described above, the composition of this embodiment has a predetermined phase structure, which makes it possible to suppress the occurrence of water trees and reduce the density of the water trees. It also makes it possible to suppress the growth of water trees, adjust their size to be small, and control the shape of the water trees to be spherical. These mechanisms can be realized for the first time by the three-component composition of this embodiment.

[0058] (2) Power cable Next, the power cable of this embodiment will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view perpendicular to the axial direction of the power cable according to this embodiment.

[0059] The power cable 10 of this embodiment is configured as a so-called solid insulated power cable. The power cable 10 of this embodiment is configured to be laid, for example, underwater or on the bottom of the water. The power cable 10 is used, for example, for AC power.

[0060] Specifically, the power cable 10 includes, for example, a conductor 110 , an inner semiconductive layer 120 , an insulating layer 130 , an outer semiconductive layer 140 , a shielding layer 150 , and a sheath 160 .

[0061] The power cable 10 of this embodiment has the above-mentioned remarkable water tree suppression effect, and therefore does not have, for example, a metallic water-blocking layer such as an aluminum sheath outside the shielding layer 150. In other words, the power cable 10 of this embodiment is configured with a non-completely water-blocking structure.

[0062] (Conductor (conductive part)) The conductor 110 is formed by twisting together a plurality of conductor core wires (conductive core wires) made of, for example, pure copper, copper alloy, aluminum, or aluminum alloy.

[0063] (internal semiconductive layer) The internal semiconductive layer 120 is provided so as to cover the outer periphery of the conductor 110. The internal semiconductive layer 120 is semiconductive and configured to suppress electric field concentration on the surface side of the conductor 110. The internal semiconductive layer 120 contains, for example, at least one of ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-butyl acrylate copolymer, ethylene-vinyl acetate copolymer, etc., and conductive carbon black.

[0064] (insulating layer) The insulating layer 130 is formed from the above-described electrical insulating composition so as to cover the outer periphery of the internal semiconductive layer 120. The insulating layer 130 is crosslinked, for example, by heating the electrical insulating composition after extrusion molding, as described above.

[0065] (Outer semiconductive layer) The outer semiconductive layer 140 is provided so as to cover the outer periphery of the insulating layer 130. The outer semiconductive layer 140 has semiconductivity and is configured to suppress electric field concentration between the insulating layer 130 and the shielding layer 150. The outer semiconductive layer 140 is configured, for example, from the same material as the inner semiconductive layer 120.

[0066] (shielding layer) The shielding layer 150 is provided so as to cover the outer periphery of the outer semiconducting layer 140. The shielding layer 150 is formed, for example, by winding copper tape, or is formed as a wire shield in which a plurality of annealed copper wires or the like are wound. Tape made of a material such as rubberized cloth may be wound inside or outside the shielding layer 150.

[0067] (sheath) The sheath 160 is provided so as to cover the outer periphery of the shielding layer 150. The sheath 160 is made of, for example, polyvinyl chloride or polyethylene.

[0068] (3) Cable characteristics According to the insulating layer 130 of this embodiment, the following properties can be obtained.

[0069] (Water tree resistant) In this embodiment, as described above, the insulating layer 130 contains a resin component containing polyethylene and a thermoplastic elastomer in a predetermined ratio and a predetermined amount of polyalkylene glycol, thereby achieving high water tree resistance in the insulating layer 130. As a result, the density of water tree occurrence in the insulating layer 130 can be kept low.

[0070] The density of water trees can be measured by conducting a strip test on the insulating layer 130. Specifically, the insulating layer 130 or the composition that constitutes the insulating layer 130 is immersed in artificial seawater with a salinity of 3.8% by mass at room temperature, and an AC voltage of 1000 Hz and 4 kV is applied to the insulating layer 130 or the composition for 30 days. After the voltage is applied, a test piece with a thickness of 0.2 mm is taken from the insulating layer 130, dyed, and the number of water trees with a length of 50 μm or more is counted. 3 The density of water trees is 2,000 pieces / cm. 3 It is usually less than 1,200 particles / cm 3 It may be less than 200 pieces / cm 3This makes it possible to stably suppress the dielectric breakdown of the insulating layer 130 caused by water treeing.

[0071] The density of water tree occurrence is not limited, as the lower the density, the better. If no water tree occurs, the density of occurrence is 0 trees / cm. 3 This becomes:

[0072] Furthermore, the insulating layer 130 can suppress the growth of water trees that occur, forming them into a spherical shape and shortening their length. Specifically, the insulating layer is such that the maximum length of water trees that occur in a water needle test is 200 μm or less, and the ratio of the long side to the short side of the water tree is 2.0 or less. The maximum length refers to the longest value when multiple water trees are randomly sampled. The ratio of the long side to the short side refers to the average value of multiple water trees randomly sampled.

[0073] Here, the water needle test is carried out in the following procedure as shown in FIG.

[0074] First, a hexahedral block IB is extracted from the insulating layer 130. After the block IB is extracted, a plate electrode FE is provided on the first surface S1 of the block IB. After the plate electrode FE is provided, a needle-shaped void is formed along the normal direction from the second surface S2 of the block IB, opposite the first surface S1. The needle-shaped void is formed at a position facing the first surface S1 of the block IB, spaced 1 mm apart from the first surface S1, so that the tip has a radius of curvature of 10 μm. After the void is formed in the block IB, artificial seawater with a salinity of 3.8% by mass is injected into the void. This forms a water needle WN within the block IB. In this state, an AC voltage of 1000 Hz and 4 kV is applied between the plate electrode FE and the tip of the water needle WN for 30 days. After the voltage application, the test piece of the insulating layer 130 is dyed, and the shape and maximum length of the water tree extending from the tip of the water needle WN are measured.

[0075] The shape of a water tree can be evaluated by the ratio of the long side to the short side of a single water tree; if this ratio is 2.0 or less, the water tree can be determined to be not distorted but close to a spherical shape. The maximum length of the water tree indicates the length of the longest water tree occurring in the insulating layer 130, and is preferably 200 μm or less. The shorter the maximum length of the water tree and the closer its shape is to a spherical shape, the more stably the dielectric breakdown of the insulating layer 130 caused by water treeing can be suppressed.

[0076] The shape of the water trees that occur in the composition should be spherical, and the closer the ratio is to 1, the better. In other words, the ratio of the long side to the short side should be 1.0 or more and 2.0 or less. The maximum length of the water trees that occur in the composition is not limited, as it is better the smaller it is. If no water trees occur, the maximum length of the water trees will be 0 μm.

[0077] (dielectric loss tangent) When an AC electric field is applied to the insulating layer 130, energy loss can occur. This loss can be due to, for example, leakage current, dielectric polarization, or partial discharge. Due to these losses, the current phase lags behind the lossless current flowing through an ideal electrical insulating composition. The delay angle δ in this case is called the dielectric loss angle, and the tangent is called the dielectric loss tangent (tan δ). In this embodiment, a polyalkylene glycol having a polar group is mixed, but its content is within a predetermined range, thereby maintaining a low dielectric loss tangent of the insulating layer 130. By crosslinking the insulating layer 130, the dielectric loss tangent can be maintained lower than when the insulating layer 130 is not crosslinked.

[0078] Specifically, in this embodiment, the dielectric loss tangent when an AC electric field of 4 kV / mm at a frequency of 60 Hz is applied to the composition constituting the insulating layer 130 at a temperature of 90°C may be, for example, 0.05% or less. Note that the smaller the dielectric loss tangent, the better, so there is no particular limit to its lower limit. However, the dielectric loss of the electrical insulating composition of this embodiment is, for example, 0.001% or more.

[0079] (tensile strength) The insulating layer 130 contains a predetermined amount of polyethylene and a predetermined amount of thermoplastic elastomer, and therefore has excellent mechanical properties. Specifically, the tensile strength of the insulating layer 130 is preferably, for example, 10.0 MPa or more. The higher the tensile strength of the insulating layer 130, the better, and there is no particular upper limit, but it is preferably, for example, 50 MPa or less. The method for measuring the tensile strength will be described in detail in the Examples.

[0080] (4) Power cable manufacturing method Next, a method for manufacturing the power cable of this embodiment will be described.

[0081] As shown in FIG. 2, the method for manufacturing a power cable of this embodiment includes, for example, a composition preparation step S100, a conductor preparation step S200, an extrusion step S300, a cross-linking step S400, a shielding layer formation step S500, and a sheath formation step S600.

[0082] (S100: Composition preparation step) First, an electrical insulating composition that constitutes the insulating layer 130 of this embodiment is prepared.

[0083] In this embodiment, polyethylene and a thermoplastic elastomer as resin components, a polyalkylene glycol having a predetermined number average molecular weight, and, if necessary, other additives such as a crosslinking agent and a crosslinking aid are mixed (kneaded) in a mixer to form a mixture. Examples of the mixer include an open roll, a Banbury mixer, a pressure kneader, a single-screw mixer, and a multi-screw mixer. Kneading may be performed once or multiple times.

[0084] Once the mixture is formed, it is granulated using an extruder, thereby forming a pellet-like composition that constitutes the insulating layer 130. The steps from mixing to granulation may be carried out all at once using a twin-screw extruder that has a high kneading effect.

[0085] (S200: Conductor preparation process) On the other hand, a conductor 110 is prepared by twisting together a plurality of conductor core wires.

[0086] (S300: Extrusion process) After the composition preparation step S100 and the conductor preparation step S200 are completed, the insulating layer 130 is formed from the above-mentioned resin composition so as to cover the outer periphery of the conductor 110 in the extrusion step S300.

[0087] In this embodiment, for example, a three-layer co-extruder is used to simultaneously form the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140.

[0088] Specifically, among the three-layer co-extruders, the composition for the inner semiconductive layer is charged into extruder A that forms the inner semiconductive layer 120, for example.

[0089] The above-described pellet-like composition is fed into extruder B, which forms insulating layer 130. At this time, the set temperature of extruder B is set to a temperature that is 5°C to 50°C higher than the melting point of the resin component. The set temperature is adjusted appropriately based on the linear velocity and extrusion pressure.

[0090] Furthermore, an outer semiconductive layer composition containing the same materials as the inner semiconductive layer resin composition charged into extruder A is charged into extruder C which forms the outer semiconductive layer 140 .

[0091] Next, the extrudates from extruders A to C are introduced into a common head, and inner semiconductive layer 120, insulating layer 130, and outer semiconductive layer 140 are simultaneously extruded from the inside to the outside around conductor 110. This forms an extruded material that will become the cable core.

[0092] (S400: Crosslinking process) After the extrusion step S300 is completed, the insulating layer 130 is cross-linked in the cross-linking step S400 of this embodiment. For example, the extruded material is introduced into a cross-linking pipe pressurized with nitrogen gas or the like, and in the cross-linking pipe, the insulating layer 130 is cross-linked by heating by radiation from an infrared heater or by heat transfer through a heat medium such as high-temperature nitrogen gas or silicone oil.

[0093] The cable core is then cooled, for example by water cooling.

[0094] As a result of the above, a cable core composed of the conductor 110, the inner semiconductive layer 120, the insulating layer 130 and the outer semiconductive layer 140 is formed.

[0095] (S500: Shielding layer formation process) After the cross-linking step S400 is completed, the shielding layer 150 is formed on the outside of the outer semiconductive layer 140 by winding, for example, copper tape.

[0096] (S600: Sheath forming process) After the shielding layer 150 is formed, vinyl chloride is put into an extruder and extruded to form a sheath 160 around the outer periphery of the shielding layer 150 .

[0097] In this way, the power cable 10 is manufactured.

[0098] (5) Summary of this embodiment According to this embodiment, one or more of the following effects are achieved.

[0099] (a) The electrical insulating composition of this embodiment contains, as resin components, 60 to 85 parts by mass of polyethylene, 15 to 40 parts by mass of a thermoplastic elastomer, and 0.5 to 1.0 part by mass of a polyalkylene glycol having a number-average molecular weight of 4,000 to 8,000 per 100 parts by mass of the resin component. By including the polyethylene and the thermoplastic elastomer in a predetermined ratio, the insulating layer 130 can achieve high levels of both flexibility and tensile properties. The thermoplastic elastomer can capture foreign matter, such as polyethylene, and suppress the generation of water trees originating from the foreign matter in the insulating layer 130. The polyalkylene glycol, when dispersed in the thermoplastic elastomer, can reduce electric field concentration on the foreign matter. Furthermore, the polyalkylene glycol has high hydrophilicity, so it can capture water incorporated into the composition and maintain it in a finely dispersed state without agglomerating within the composition. This suppresses water agglomeration due to electric field concentration, thereby suppressing the generation and growth of water trees. As a result, the insulation layer can improve not only the cable characteristics but also the water tree resistance.

[0100] (b) The composition has a phase structure in which polyethylene constitutes a sea phase, the thermoplastic elastomer constitutes island phases dispersed in the sea phase, and the polyalkylene glycol is dispersed in the island phases.With such a phase structure, the effect of (a) described above can be obtained more reliably.

[0101] (c) The thermoplastic elastomer may be at least one of an olefin-based thermoplastic elastomer and a styrene-based thermoplastic elastomer. The styrene-based thermoplastic elastomer can form a stable resonance structure, so even if water condenses around the trapped foreign matter, the electric field concentration caused by the condensation can be alleviated. The olefin-based thermoplastic elastomer is easily mixed with polyethylene and can suppress the generation of voids, thereby suppressing the generation of water trees originating from voids.

[0102] (d) The polyalkylene glycol may be polyethylene glycol (PEG). PEG is highly hydrophilic and easily captures water, so that the occurrence of water trees can be more reliably suppressed.

[0103] (e) According to the insulating layer 130 of this embodiment, it is preferable that the maximum length of water trees generated by a water needle test is 200 μm or less, and the ratio of the long side to the short side of the water tree is 2.0 or less. Since the insulating layer 130 is formed from the above-mentioned composition, it has excellent water tree resistance, and the growth of water trees is suppressed. Therefore, the generated water trees can be made small and their shape can be controlled to be spherical. As a result, dielectric breakdown of the insulating layer 130 caused by water trees can be stably suppressed.

[0104] (f) According to the insulating layer 130 of this embodiment, even when the insulating layer 130 is immersed in artificial seawater with a salinity of 3.8% by mass at room temperature and an AC voltage of 1000 Hz and 4 kV is applied for 30 days, the density of water trees that are generated in the insulating layer 130 and have a length of 50 μm or more is 2,000 trees / cm 3 The insulating layer 130 formed from the above-described composition has excellent water tree resistance, and therefore the occurrence of water trees is suppressed. As a result, it is possible to stably suppress the dielectric breakdown of the insulating layer 130 caused by water trees.

[0105] (g) In the insulating layer 130 of this embodiment, the polyalkylene glycol content is 0.5 to 1.0 part by mass, which allows the dielectric loss tangent to be maintained at a high level. Specifically, when an AC electric field of 4 kV / mm at a frequency of 60 Hz is applied to the insulating layer 130 at a temperature of 90°C, the dielectric loss tangent should be 0.05% or less.

[0106] <Other Embodiments of the Present Disclosure> Although the embodiments of the present disclosure have been specifically described above, the present disclosure is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the present disclosure.

[0107] In the above-described embodiment, the power cable 10 does not necessarily have a water-shielding layer. However, the present disclosure is not limited to this. The power cable 10 may have a simple water-shielding layer. Specifically, the simple water-shielding layer may be, for example, a metal laminate tape. The metal laminate tape has a metal layer made of, for example, aluminum or copper, and an adhesive layer provided on one or both sides of the metal layer. The metal laminate tape is, for example, wrapped longitudinally around the outer periphery of the cable core (outer than the outer semiconductive layer). The water-shielding layer may be provided outside the shielding layer or may also serve as the shielding layer. This configuration can reduce the cost of the power cable 10.

[0108] Although the above-described embodiment does not refer to the installation environment of the power cable 10, the power cable 10 may be configured to be installed, for example, on land, underwater, or at the bottom of the water. Alternatively, the power cable 10 may be configured, for example, as a so-called overhead electric wire (overhead insulated electric wire).

[0109] In the above embodiment, three layers are co-extruded to form the cable core, but the layers may be extruded one by one. [Example]

[0110] Next, examples according to the present disclosure will be described. These examples are examples of the present disclosure, and the present disclosure is not limited to these examples.

[0111] (1) Preparation of materials The following materials were prepared as materials for preparing a resin composition. Low-density polyethylene (LDPE) was prepared as polyethylene. The density d of LDPE was 0.90 g / cm 3 The MFR was 1.2 g / 10 min. A hydrogenated styrene butadiene styrene block copolymer (SEBS) was prepared as a thermoplastic elastomer. The styrene content of the SEBS was 17% by mass, and the MFR was 4.8 g / 10 min. The thermoplastic elastomer used was an olefin-based thermoplastic elastomer (TPO) with a hard segment of polypropylene and a soft segment of ethylene-propylene rubber. The MFR of the TPO was 0.6 g / 10 min. As polyalkylene glycols, several polyethylene glycols (PEG1 to PEG5) with different number-average molecular weights were prepared. The number-average molecular weights of each were 6,000 for PEG1, 2,000 for PEG2, 4,500 for PEG3, 8,000 for PEG4, and 10,000 for PEG5. As another additive, dicumyl peroxide (DCP) was prepared.

[0112] (2) Preparation of resin composition In this example, a resin composition was prepared by mixing the above-mentioned materials.

[0113] Specifically, for Samples 1 to 5, 70 parts by mass of LDPE and 30 parts by mass of SEBS were mixed as the resin component, and the amount of PEG added per 100 parts by mass of the resin component was appropriately changed within the range of 0 to 1.5 parts by mass to prepare electrical insulating compositions, as shown in Table 1. DCP was added in an amount of 1.6 parts by mass per 100 parts by mass of the resin component.

[0114] [Table 1]

[0115] For Samples 6 to 9, electrical insulating compositions were prepared in the same manner as for Sample 3, except that the type of polyalkylene glycol was changed from PEG1 to PEG2 to PEG5, as shown in Table 2 below.

[0116] [Table 2]

[0117] For Samples 10 to 14, the electrical insulating compositions were prepared by appropriately changing the ratio of LDPE and SEBS added as resin components and adjusting the amount of PEG to 0.75 parts by mass per 100 parts by mass of the resin components, as shown in Table 3. DCP was added in an amount of 1.6 parts by mass per 100 parts by mass of the resin components.

[0118] For Sample 15, an electrical insulating composition was prepared in the same manner as for Sample 12, except that the type of thermoplastic elastomer was changed from styrene-based to olefin-based.

[0119] [Table 3]

[0120] (3) Evaluation method The prepared electrical insulating composition samples were evaluated as follows.

[0121] (Water tree length, water tree shape) After forming the above-mentioned electrical insulating composition, an insulator was produced by pressing the electrical insulating composition by press molding for 10 minutes at 120°C. A block was taken from this insulator and subjected to the water needle test shown in Figure 4.

[0122] The conditions for the water needle test were set as follows: Radius of curvature of the tip of the water needle: 10 μm Distance from the plate electrode on the first surface of the block to the tip of the water needle: 1 mm Salinity of artificial seawater in the water needle: 3.8% by mass Frequency: 1000MHz AC voltage: 4kV

[0123] After the water needle test, the block was dried and then boiled in a methylene blue solution to stain it. After staining the block, the block was sliced ​​near the tip of the water needle to form a slice for observation. The slice for observation was then observed under an optical microscope to observe the water tree that had developed near the tip of the water needle.

[0124] At this time, the maximum length of water trees that had developed in the sliced ​​specimens for observation was measured. The "maximum length of water trees" was calculated by rounding off the longest water tree length in 10 blocks randomly selected from the insulation layer of each sample. The ratio of the long side to the short side of the shape of the water trees that had developed in the sliced ​​specimens for observation was calculated. The ratio of the long side to the short side was obtained by averaging the values ​​of the 10 randomly selected water trees.

[0125] (Water tree density) After forming the above-described electrical insulating composition, the electrical insulating composition was pressed at 120°C for 10 minutes using a press mold to produce two insulating sheets with a thickness of 1 mm. After the insulating sheets were produced, a predetermined semiconductive sheet was sandwiched between two insulating sheets to form a laminated sheet. After forming the laminated sheet, the laminated sheet was pressed at 180°C for 30 minutes using a press mold to crosslink the base resin of the insulating sheet. After crosslinking the insulating sheet, wiring was formed on the semiconductive sheet.

[0126] Next, the laminated sheet was immersed in artificial seawater at room temperature (27°C) with a salinity of 3.8% by mass, and an AC electric field with a frequency of 1000 Hz and 4 kV / mm was applied to the insulating sheet between the semiconductive sheet and the aqueous solution for 30 days.

[0127] After applying a predetermined AC electric field, the laminate sheet was dried and then boiled and dyed in a methylene blue aqueous solution. After dyeing the laminate sheet, it was sliced ​​in the stacking direction (i.e., the direction perpendicular to the main surface of the laminate sheet) to a thickness of 200 μm to form slices for observation. The slices for observation were then observed under an optical microscope to observe water trees that had occurred in the insulating sheet of the slices for observation in the direction parallel to the surface of the semiconductive sheet or in the direction perpendicular to the main surface of the semiconductive sheet. At this time, the number density of water trees that had occurred in the insulating sheet and were 50 μm or longer was measured. Note that in Tables 1 and 2 described below, the "number density of water trees" was calculated by rounding off the average value of the number concentration of water trees that had occurred in 10 randomly selected slices for observation.

[0128] (dielectric loss tangent) After forming the above-mentioned electrical insulating composition, the electrical insulating composition was pressed at 180°C for 30 minutes to produce an insulating sheet having a thickness of 0.2 mm. At this time, pressing the insulating sheet at 180°C for 30 minutes crosslinked the resin component of the insulating sheet.

[0129] Next, an AC electric field of 9 kV / mm was applied to the insulating sheet at 90° C. using a Schering bridge to measure the dielectric loss tangent. In this example, if the dielectric loss tangent was 0.05% or less, it was marked as "A," and if it exceeded 0.05%, it was marked as "B."

[0130] (tensile strength) After forming the above-described electrical insulating composition, the electrical insulating composition was pressed at 180°C for 30 minutes using a press mold to produce an insulating sheet with a thickness of 0.2 mm. Pressing the insulating sheet at 180°C for 30 minutes crosslinked the resin component of the insulating sheet. The tensile strength of the insulating sheet was then measured in accordance with JIS C3005. Specifically, the insulating sheet was pulled at a pulling rate of 200 mm / min using a JIS-3 dumbbell.

[0131] (4) Evaluation results The evaluation results are summarized in Tables 1 to 3.

[0132] As shown in Table 1, in samples 2 to 4, by setting the PEG content to 0.5 to 1.0 parts by mass per 100 parts by mass of the resin component, it was confirmed that the dielectric tangent could be kept small while the tensile strength was increased, and excellent cable characteristics could be achieved. It was also confirmed that the water tree length could be kept to 200 μm or less, and the water tree shape's long side / short side ratio could be kept to 2.0 or less. It was also confirmed that the water tree number density could be kept to 2,000 trees / cm. 3 These results confirm that a certain amount of PEG can suppress the occurrence and growth of water trees even when a voltage is applied.

[0133] In contrast, it was confirmed that mixing a thermoplastic elastomer into polyethylene in Sample 1 suppressed the generation of water trees and reduced their number density. However, because polyalkylene glycol was not added in Sample 1, it was confirmed that the water tree length exceeded 200 μm and the ratio of the long side to the short side of the water trees exceeded 2.0. It is presumed that in Sample 1, water trees were generated and grew due to the application of voltage, causing the water trees to become larger and their shapes to become distorted.

[0134] In addition, in sample 5, although PEG was added as polyalkylene glycol, the content was 1.5 parts by mass, exceeding 1.0 part by mass, so while high water tree resistance was maintained, it was confirmed that the dielectric loss tangent increased. This is presumably because the polar groups increased as the amount of PEG increased, and the dielectric loss tangent increased due to the effect of the electric field.

[0135] As shown in Table 2, in samples 3, 7, and 8, by adjusting the molecular weight of PEG to the range of 4,000 to 8,000, it was confirmed that high water tree resistance could be obtained by keeping the dielectric dissipation factor small while also keeping the water tree length small, making the water tree shape closer to a spherical shape, and further reducing the water tree generation density. In particular, in sample 3, the number average molecular weight of PEG was 6,000, within the range of 5,500 to 6,500, and it was confirmed that a reduced dielectric dissipation factor and improved water tree resistance could be achieved at a better balance than in samples 7 and 9.

[0136] On the other hand, in sample 6, the number-average molecular weight was 2,000, and an excessively low molecular weight PEG was used, which maintained high water tree resistance, but the dielectric loss tangent exceeded 0.05%. This is presumably because the PEG molecular weight was too low, making it prone to micro-vibrations at high temperatures. On the other hand, in sample 9, the number-average molecular weight was 10,000, and an excessively high molecular weight PEG was used, which was presumably because high water tree resistance could not be maintained. In particular, although the water tree length was reduced and the water tree shape was made spherical, the water tree density was confirmed to be high. This is presumably because the PEG dispersed into the polyethylene sea phase, making it difficult to exist in the island phase of the thermoplastic elastomer, thereby preventing the PEG from fully mitigating the electric field.

[0137] As shown in Table 3, in samples 11 to 13, it was confirmed that by mixing 60 to 85 parts by mass of LDPE and 15 to 40 parts by mass of thermoplastic elastomer as the resin components, it was possible to maintain high cable properties and water tree resistance.

[0138] In contrast, in Sample 10, an excessive amount of thermoplastic elastomer was added (50 parts by mass of LDPE and 50 parts by mass of thermoplastic elastomer), resulting in a tensile strength of less than 10.0 MPa, and it was confirmed that high mechanical properties could not be obtained. On the other hand, in Sample 14, no thermoplastic elastomer was added, and although the water tree length was short and the water trees were formed in a spherical shape, it was confirmed that the number density of the water trees was high. This is presumably because the absence of thermoplastic elastomer caused electric field concentration due to foreign matter and promoted water aggregation in the polyethylene matrix, making water trees more likely to occur.

[0139] Furthermore, in Sample 15, it was confirmed that the olefin-based thermoplastic elastomer, like the styrene-based thermoplastic elastomer, can also suppress the occurrence and growth of water trees. In other words, it was confirmed that the water tree resistance can be improved.

[0140] As described above, by mixing polyethylene and thermoplastic elastomer in a predetermined ratio as the resin component and adding polyalkylene glycol, it is possible to achieve a high level of both cable properties and water tree resistance.

[0141] <Additional Notes> The following additionally describes aspects of the present disclosure.

[0142] (Appendix 1) a resin component including polyethylene and a thermoplastic elastomer; and a polyalkylene glycol having a number average molecular weight of 4,000 or more and 8,000 or less, The content of the polyethylene is 60 parts by mass or more and 85 parts by mass or less, and the content of the thermoplastic elastomer is 15 parts by mass or more and 40 parts by mass or less, The content of the polyalkylene glycol is 0.5 parts by mass or more and 1.0 parts by mass or less with respect to 100 parts by mass of the resin component. Electrical insulating compositions.

[0143] (Appendix 2) A conductor; an insulating layer formed on the outer periphery of the conductor and made of an electrical insulating composition; The electrical insulating composition is a resin component including polyethylene and a thermoplastic elastomer; and a polyalkylene glycol having a number average molecular weight of 4,000 or more and 8,000 or less, The content of the polyethylene is 60 parts by mass or more and 85 parts by mass or less, and the content of the thermoplastic elastomer is 15 parts by mass or more and 40 parts by mass or less, The content of the polyalkylene glycol is 0.5 parts by mass or more and 1.0 parts by mass or less with respect to 100 parts by mass of the resin component. Power cable.

[0144] (Appendix 3) The thermoplastic elastomer is at least one of an olefin-based thermoplastic elastomer having an olefin unit and a styrene-based thermoplastic elastomer having a styrene unit. 1. A power cable as described in Appendix 2.

[0145] (Appendix 4) The polyethylene is a low-density polyethylene. 1. A power cable according to claim 2 or 3.

[0146] (Appendix 5) The density of the polyethylene is 0.89 g / cm 3 More than 0.91g / cm 3 Below is the 1. A power cable as described in Appendix 4.

[0147] (Appendix 6) The polyalkylene glycol is polyethylene glycol. 1. A power cable according to any one of Annexes 2 to 4.

[0148] (Appendix 7) The electrical insulating composition has a phase structure in which the polyethylene constitutes a sea phase, the thermoplastic elastomer constitutes an island phase dispersed in the sea phase, and the polyalkylene glycol is dispersed in the island phase. 10. A power cable according to any one of claims 2 to 6.

[0149] (Appendix 8) The insulating layer has a maximum length of water trees generated by a water needle test of 200 μm or less, and the ratio of the long side to the short side of the water trees is 2.0 or less. Here, the water needle test is providing a plate electrode on a first surface of the block taken from the insulating layer; forming needle-shaped voids at positions facing each other at a distance of 1 mm from the first surface, the needle-shaped voids having tips with a curvature radius of 10 μm; A step of forming water needles in the block by injecting artificial seawater having a salinity of 3.8% by mass into the void; Applying an AC voltage of 1000 Hz and 4 kV between the plate electrode and the tip of the water needle for 30 days; 10. The power cable according to any one of claims 2 to 7.

[0150] (Appendix 9) When the insulating layer was immersed in artificial seawater with a salinity of 3.8% by mass at room temperature and an AC voltage of 1000 Hz and 4 kV was applied to the insulating layer for 30 days, The density of water trees occurring in the insulating layer and having a length of 50 μm or more is 2,000 trees / cm 3 is less than 10. The power cable according to any one of claims 2 to 8.

[0151] (Appendix 10) The insulating layer has a dielectric loss tangent of 0.05% or less when an AC electric field of 4 kV / mm at a temperature of 90°C and a frequency of 60 Hz is applied. 10. The power cable according to any one of claims 2 to 8. [Explanation of symbols]

[0152] 1. Marine phase (polyethylene) 2 Island phase (thermoplastic elastomer) 3 Polyalkylene glycol 4 Foreign object 5 Water Tree 6 needles 10 Power Cable 110 Conductor 120 Internal semiconductive layer 130 Insulating layer 140 outer semiconductive layer 150 Shielding layer 160 Sheath

Claims

1. a resin component including polyethylene and a thermoplastic elastomer; and a polyalkylene glycol having a number average molecular weight of 4,000 or more and 8,000 or less, The thermoplastic elastomer is at least one of an olefin-based thermoplastic elastomer containing at least one olefin unit of polyethylene and polypropylene as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment, and a styrene-based thermoplastic elastomer containing a styrene unit as a hard segment and at least one monomer unit of ethylene, propylene, butylene, and isoprene as a soft segment, the content of the polyethylene is 60 parts by mass or more and 85 parts by mass or less, and the content of the thermoplastic elastomer is 15 parts by mass or more and 40 parts by mass or less, The content of the polyalkylene glycol is 0.5 parts by mass or more and 1.0 parts by mass or less with respect to 100 parts by mass of the resin component. Electrical insulating compositions.

2. A conductor; an insulating layer formed on the outer periphery of the conductor and made of an electrical insulating composition; The electrical insulating composition is a resin component including polyethylene and a thermoplastic elastomer; and a polyalkylene glycol having a number average molecular weight of 4,000 or more and 8,000 or less, The thermoplastic elastomer is at least one of an olefin-based thermoplastic elastomer containing at least one olefin unit of polyethylene and polypropylene as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment, and a styrene-based thermoplastic elastomer containing a styrene unit as a hard segment and at least one monomer unit of ethylene, propylene, butylene, and isoprene as a soft segment, the content of the polyethylene is 60 parts by mass or more and 85 parts by mass or less, and the content of the thermoplastic elastomer is 15 parts by mass or more and 40 parts by mass or less, The content of the polyalkylene glycol is 0.5 parts by mass or more and 1.0 parts by mass or less with respect to 100 parts by mass of the resin component. Power cable.

3. The polyethylene is a low-density polyethylene.

3. The power cable according to claim 2.

4. The polyalkylene glycol is polyethylene glycol.

3. The power cable according to claim 2.

5. The electrical insulating composition has a phase structure in which the polyethylene constitutes a sea phase, the thermoplastic elastomer constitutes an island phase dispersed in the sea phase, and the polyalkylene glycol is dispersed in the island phase.

3. The power cable according to claim 2.

6. The insulating layer has a maximum length of water trees generated by a water needle test of 200 μm or less, and the ratio of the long side to the short side of the water trees is 2.0 or less. Here, the water needle test is providing a plate electrode on a first surface of the block extracted from the insulating layer; forming needle-shaped voids at positions facing each other at a distance of 1 mm from the first surface, the needle-shaped voids having tips with a curvature radius of 10 μm; forming water needles in the block by injecting artificial seawater having a salinity of 3.8% by mass into the voids; Applying an AC voltage of 1000 Hz and 4 kV between the plate electrode and the tip of the water needle for 30 days; 3. The power cable according to claim 2.

7. When the insulating layer was immersed in artificial seawater with a salinity of 3.8% by mass at room temperature and an AC voltage of 1000 Hz and 4 kV was applied to the insulating layer for 30 days, The density of water trees occurring in the insulating layer and having a length of 50 μm or more is 2,000 trees / cm ３ is less than 3. The power cable according to claim 2.

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