Insulating resin composition for electric power cable, and electric power cable

Abstract

This insulating resin composition for an electric power cable comprises a component (a), a component (b), a component (c), and a component (d). The component (a) is polyethylene. The component (b) is at least one resin selected from a resin (b1) and a resin (b2). The resin (b1) is a resin in which at least one modified monomer selected from unsaturated organic acids and derivatives thereof is grafted. The resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers. The component (c) is a water treeing inhibitor. The component (d) is a crosslinking agent.

Description

Insulating resin composition for power cables and power cables

[0001] The present disclosure relates to an insulating resin composition for power cables and a power cable.

[0002] Cross-linked polyethylene (XLPE), which has been used in conventional power cables, can develop water trees when used as insulation in a moisture-rich environment. Water trees are dendritic defects that arise from foreign matter or air bubbles (voids) in the resin.

[0003] Water trees grow when the water in the resin concentrates on the interfaces of foreign objects and voids due to dielectrophoresis of water caused by passing electricity through the power cable. Because water trees can cause damage to power cables, there is a need to suppress the growth of water trees.

[0004] To suppress the growth of water trees, hydrophilic molecules, such as polyethylene glycols, are added to the hydrophobic polyethylene in XLPE resin. Hydrophilic molecules such as polyethylene glycols allow water to be dispersed uniformly in the XLPE resin, preventing water from concentrating on the interfaces of foreign objects and voids, thereby suppressing the growth of water trees.

[0005] Patent Document 1 also discloses a polyolefin composition for electrical insulation containing polyolefin or crosslinked polyolefin and a small amount of high-molecular-weight polyethylene glycol, and also discloses a technology that a low-hydrophilic or hydrophobic material such as polypropylene glycol does not prevent the formation of water trees in the insulator.

[0006] In this way, the addition of hydrophilic molecules suppresses the growth of water trees. However, the addition of hydrophilic molecules increases the dielectric loss tangent (tan δ) of the resin, thereby reducing the power transmission efficiency of the power cable.

[0007] U.S. Pat. No. 4,305,849

[0008] An object of the present disclosure is to provide an insulating resin composition for power cables and a power cable that can suppress the growth of water trees and reduce the dielectric loss tangent.

[0009] [1] An insulating resin composition for a power cable, comprising components (a), (b), (c), and (d), wherein component (a) is polyethylene, component (b) is at least one resin selected from resins (b1) and (b2), wherein resin (b1) is a resin grafted with at least one modifying monomer selected from unsaturated organic acids and their derivatives, resin (b2) is at least one ethylene copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers, component (c) is a water tree inhibitor, and component (d) is a crosslinking agent. [2] The insulating resin composition for a power cable according to item [1], wherein component (c) is at least one compound selected from polyalkylene glycols and their derivatives, polyglycerin, glycerin fatty acid esters, and sorbitol esters. [3] The insulating resin composition for a power cable according to [1] or [2] above, wherein a crosslinked product obtained by crosslinking the insulating resin composition for a power cable has a dielectric loss tangent of 1.0% or less at 30 kV / mm at 90°C. [4] The insulating resin composition for a power cable according to any one of [1] to [3] above, wherein the resin (b1) is formed by adding at least one modifying monomer selected from unsaturated dicarboxylic acids, unsaturated dicarboxylic anhydrides, and unsaturated dicarboxylic acid derivatives to at least one resin selected from polypropylene, polyethylene, and olefin copolymers. [5] The insulating resin composition for a power cable according to any one of [1] to [4] above, wherein a content of the component (b) in the insulating resin composition for a power cable is 3.0 wt% or more and 25.0 wt% or less. [6] A power cable comprising: a conductor; an internal semiconductive layer disposed outside the conductor and surrounding the conductor; an insulating layer disposed outside the internal semiconductive layer and surrounding the internal semiconductive layer, the insulating layer being formed by crosslinking the insulating resin composition for a power cable according to any one of [1] to [5] above; and an external semiconductive layer disposed outside the insulating layer and surrounding the insulating layer.

[0010] According to the present disclosure, it is possible to provide an insulating resin composition for power cables and a power cable that can suppress the growth of water trees and reduce the dielectric loss tangent.

[0011] Fig. 1 is a cross-sectional view showing an example of a power cable to which an insulating resin composition for a power cable according to an embodiment of the present invention is applied, and Fig. 2 is a schematic diagram showing a water tree test.

[0012] Hereinafter, a detailed description will be given based on an embodiment.

[0013] As a result of extensive research, the present inventors have found that even when a water tree inhibitor is contained to inhibit the growth of water trees, the inclusion of a specific component can suppress an increase in the dielectric loss tangent caused by the water tree inhibitor, and have completed the present disclosure based on this finding.

[0014] An insulating resin composition for power cables (hereinafter also simply referred to as the insulating resin composition) according to an embodiment includes components (a), (b), (c), and (d), in which component (a) is polyethylene, component (b) is at least one resin selected from resins (b1) and (b2), resin (b1) is a resin grafted with at least one modifying monomer selected from unsaturated organic acids and derivatives thereof, resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers, component (c) is a water tree inhibitor, and component (d) is a crosslinking agent.

[0015] The insulating resin composition of the embodiment includes, as constituent elements, component (a), component (b), component (c), and component (d). A cross-linked product obtained by cross-linking the insulating resin composition (hereinafter also simply referred to as a cross-linked product) is an insulator and is suitably used for the insulating layer of a power cable.

[0016] The component (a) contained in the insulating resin composition is polyethylene. The polyethylene can be produced by either a low-pressure process or a high-pressure process, but low-density polyethylene (LDPE) produced by a high-pressure process is preferred.

[0017] The component (b) contained in the insulating resin composition is at least one resin selected from the group consisting of resins (b1) and (b2). The component (b) suppresses an increase in the dielectric loss tangent in a crosslinked product of the insulating resin composition, which is caused by the water tree inhibitor (component (c)).

[0018] The resin (b1) is a resin to which at least one modifying monomer selected from unsaturated organic acids and derivatives thereof has been grafted (hereinafter, also simply referred to as a graft resin).

[0019] The functional group introduced into the resin by grafting the modified monomer is preferably a functional group having a C=O bond. Among these, carbonyl, carboxyl, ester, acid anhydride, amide, and imide groups are preferred. The water tree inhibitor (component (c)) is bound by hydrogen bonds with the C=O in the molecules grafted to the resin, thereby suppressing molecular motion due to an electric field. In other words, the permanent dipole of the water tree inhibitor does not follow the frequency vibration of the AC field, preventing orientation polarization, thereby reducing energy loss due to molecular motion for molecular orientation. Furthermore, the C=O structure traps space charge within the insulator and suppresses the occurrence of interfacial polarization, which occurs when space charge moves in response to an electric field, thereby reducing energy loss due to charge transfer. Thus, the C=O in the molecules grafted to the resin suppresses the molecular motion and space charge transfer of the water tree inhibitor, thereby efficiently reducing the dielectric loss tangent of the crosslinked product. Furthermore, the graft resin can improve the dispersibility of the water tree inhibitor and suppress bleed-out of the crosslinked product.

[0020] Preferred modifying monomers are unsaturated dicarboxylic acids, unsaturated dicarboxylic anhydrides, and unsaturated dicarboxylic acid derivatives. As the resin (b1) grafted with such a modifying monomer, it is preferred that at least one modifying monomer selected from unsaturated dicarboxylic acids, unsaturated dicarboxylic anhydrides, and unsaturated dicarboxylic acid derivatives is added to at least one resin selected from polypropylene, polyethylene, and olefin copolymers. The polyethylene is preferably low-density polyethylene produced by a high-pressure process.

[0021] Preferred unsaturated dicarboxylic acids are maleic acid, fumaric acid, and itaconic acid. Preferred unsaturated dicarboxylic acid anhydrides are maleic anhydride and itaconic anhydride. Preferred unsaturated dicarboxylic acid derivatives are monomethyl maleate, monoethyl maleate, diethyl maleate, monomethyl fumarate, dimethyl fumarate, diethyl fumarate, maleic acid monoamide, maleimide, N-phenylmaleimide, and N-cyclohexylmaleimide. These can be used alone or in combination of two or more. Of these, maleic anhydride, which is a five-membered cyclic acid anhydride, is preferred.

[0022] The graft amount of the modified monomer is preferably 0.25 wt % or more and 2.00 wt % or less of the graft resin. When the modified monomer is grafted in this range, the modified monomer is uniformly dispersed in the insulating resin composition, and the dielectric loss tangent in the system can be uniformly reduced.

[0023] When the modified monomer is maleic anhydride, the content of the modified monomer in the insulating resin composition is preferably 0.01 wt% or more and 0.50 wt% or less. When maleic anhydride is contained in the insulating resin composition within this range, the dielectric loss tangent in the system can be uniformly suppressed. In particular, when maleic anhydride is contained in an amount exceeding 0.50 wt%, the graft resin may adhere to the metal inner walls of the kneading device or extrusion device, reducing the uniformity of the resin composition. As a result, the growth of water trees may be promoted.

[0024] The graft resin can be prepared, for example, by the method described in paragraph

[0098] of Japanese Patent No. 6205032, by mixing raw materials, polyethylene, an antioxidant, a modifying monomer, and an organic peroxide, in an extruder, heating, and reacting them.

[0025] The resin (b2) is at least one ethylene copolymer selected from an ethylene-acrylate copolymer, an ethylene-acrylic acid copolymer, and an ethylene-vinyl acetate copolymer.

[0026] The ethylene-acrylate copolymer is preferably an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, or an ethylene-butyl acrylate copolymer.

[0027] The lower limit of the content of component (b) in the insulating resin composition is preferably 3.0 wt % or more, more preferably 4.0 wt % or more, and the upper limit is preferably 25.0 wt % or less, more preferably 15.0 wt % or less. When the content of component (b) is within the above range, the dielectric loss tangent of the crosslinked product of the insulating resin composition can be sufficiently reduced.

[0028] Regarding the component (b), the resin (b1) can more efficiently reduce the dielectric loss tangent of the crosslinked product than the resin (b2), and therefore the insulating resin composition preferably contains the resin (b1).

[0029] Component (c) contained in the insulating resin composition is a water tree inhibitor. Component (c), the water tree inhibitor, is a hydrophilic molecule that can impart hygroscopicity to the crosslinked product of the insulating resin composition. By imparting hygroscopicity to the crosslinked product, moisture is uniformly dispersed in the crosslinked product, preventing moisture from concentrating on the interfaces of foreign matter and voids, thereby suppressing the growth of water trees in the crosslinked product.

[0030] The water tree inhibitor is preferably at least one compound selected from polyalkylene glycols and derivatives thereof, polyglycerin, glycerin fatty acid esters, and sorbitol esters.

[0031] Preferred polyalkylene glycols are polyethylene glycol, polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymers, and their alkyl ethers or carboxylic acid esters. More preferred polyalkylene glycol derivatives include alkoxypolyethylene glycol methacrylate, alkoxypolyethylene glycol acrylate, polyoxyethylene alkyl ether, and polyethylene glycol fatty acid ester. These water tree inhibitors are intended to impart moisture absorption to the crosslinked insulating resin composition through hydrogen bonding with water molecules, and polyalkylene glycol derivatives exhibit the same effect as polyalkylene glycol. By methacrylating, acrylating, alkoxy etherifying, or esterifying polyalkylene glycol, crosslinking between the polyalkylene glycol derivative and polyethylene during crosslinking of the cable insulation is promoted, thereby reducing bleedout. When used by being absorbed into resin pellets during dry blending, polyalkylene glycol and its derivatives are preferably liquid at 60°C. Polyglycerin is a compound formed by polymerizing multiple glycerin units through ether bonds and can have a linear, cyclic, or branched structure. Furthermore, alkyl-etherifying and carboxylic acid-esterifying these polyglycerols improves their dispersibility in the insulating resin composition of the present disclosure. In particular, introducing them into the insulating resin composition by fatty acid esterification is preferred in terms of compound production stability. Sorbitol is a polyhydric alcohol derived from glucose, and sorbitol esters are compounds in which one or more hydroxyl groups of sorbitol are esterified with a fatty acid. Sorbitol can be dispersed in the insulating resin composition by esterification.

[0032] In order to suppress bleed-out in the crosslinked product of the insulating resin composition, the number average molecular weight of the water tree inhibitor is preferably 1,000 or more, more preferably 3,000 or more, and even more preferably 9,000 or more. On the other hand, in order to improve the dispersibility of the water tree inhibitor during melt-kneading, the number average molecular weight of the water tree inhibitor is preferably 30,000 or less. Furthermore, from the viewpoint of ease of handling, the number average molecular weight of the water tree inhibitor is preferably 20,000 or less.

[0033] The lower limit of the content of the water tree inhibitor in the insulating resin composition is preferably 0.1 wt % or more, more preferably 0.2 wt % or more, and the upper limit is preferably 2.0 wt % or less, more preferably 1.0 wt % or less. When the content of the water tree inhibitor is within the above range, the growth of water trees in a crosslinked product of the insulating resin composition can be sufficiently suppressed. Furthermore, when the content of the water tree inhibitor is 1.0 wt % or less, bleeding out of the crosslinked product can be further suppressed.

[0034] Component (d) contained in the insulating resin composition is a crosslinking agent that crosslinks the polyethylene (component (a)). The crosslinking agent (component (d)) is preferably di-t-hexyl peroxide (Perhexyl D, manufactured by NOF Corporation), dicumyl peroxide (Percumyl D, manufactured by NOF Corporation), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (Perhexa 25B, manufactured by NOF Corporation), α,α'-di(t-butylperoxy)diisopropylbenzene (Perbutyl P, manufactured by NOF Corporation), t-butylcumyl peroxide (Perbutyl C, manufactured by NOF Corporation), or di-t-butyl peroxide (Perbutyl D, manufactured by NOF Corporation). These may be used alone or in combination of two or more. Of these, dicumyl peroxide is preferred.

[0035] The lower limit of the content of the crosslinking agent in the insulating resin composition is preferably 0.1 wt % or more, more preferably 0.5 wt % or more, and the upper limit is preferably 5.0 wt % or less, more preferably 3.0 wt % or less. When the content of the crosslinking agent is within the above range, the insulating resin composition can be well crosslinked.

[0036] The insulating resin composition may contain an antioxidant in addition to the above-mentioned components (a), (b), (c), and (d). As the antioxidant, a combination of a hindered phenol-based antioxidant, which is primarily intended to capture radicals, and a phosphorus-based or sulfur-based antioxidant, which is primarily intended to decompose peroxides, is preferred.

[0037] The lower limit of the content of the antioxidant in the insulating resin composition is preferably 0.01 wt % or more, more preferably 0.20 wt % or more, and the upper limit is preferably 1.00 wt % or less, more preferably 0.60 wt % or less. When the content of the antioxidant is within the above range, oxidation degradation of the cross-linked product of the insulating resin composition can be effectively suppressed.

[0038] In addition to the above components, the insulating resin composition may contain various substances, such as stabilizers, lubricants, inorganic fillers, surface treatment agents, flame retardants, acid scavengers, and voltage stabilizers, as long as they do not inhibit the growth of water trees and reduce the dielectric loss tangent in the crosslinked product of the insulating resin composition.

[0039] The insulating resin composition can be obtained by dry blending component (a) with components (b), (c), and (d) using a Henschel mixer or the like, followed by melt-kneading. Kneaders such as a single-screw or twin-screw extruder, a Banbury mixer, or a kneader can be used as the melt-kneading device. It is particularly preferable to extrude the resin using a single-screw or twin-screw extruder capable of continuous processing, equipped with a metal mesh filter with a mesh size of 100 μm or less to remove foreign matter. The insulating resin composition after adding component (d) is preferably kneaded at a temperature of 120°C or higher and 135°C or lower.

[0040] The specific gravity of the insulating resin composition is preferably 0.91 or more and 0.93 or less.

[0041] Furthermore, the crosslinked product obtained by crosslinking the insulating resin composition preferably has a dielectric loss tangent of 1.0% or less, more preferably 0.5% or less at 30 kV / mm at 90° C. When the dielectric loss tangent of the crosslinked product is within the above range, a power cable having an insulating layer made of the crosslinked product of the insulating resin composition has a small dielectric loss tangent, thereby improving the power transmission efficiency.

[0042] The dielectric loss tangent of the crosslinked product is a value measured at 90°C and 30 kV / mm in accordance with JIS C 2138. The dielectric loss tangent is dependent on temperature and applied electric field, and when the crosslinked product of the insulating resin composition is measured at 23°C and 10 kV / mm, the dielectric loss tangent of the crosslinked product is 0.1% or less.

[0043] Furthermore, in order to suppress the growth of water trees in a crosslinked product of the insulating resin composition, the moisture absorption rate of the crosslinked product is preferably 500 ppm or more, more preferably 800 ppm or more. By imparting moisture absorption to the crosslinked product, moisture in the crosslinked product is uniformly dispersed, thereby suppressing the growth of water trees in the crosslinked product. Furthermore, from the viewpoint of the dielectric strength of the crosslinked product, the moisture absorption rate of the crosslinked product is preferably 2000 ppm or less.

[0044] The resin fluidity of the crosslinked product is preferably 0.5 g / 10 min or more and 5.0 g / 10 min or less, more preferably 0.5 g / 10 min or more and 3.0 g / 10 min or less, when measured at a melting temperature of 190°C and a load of 2.16 kgf in accordance with JIS K 7210. When the resin fluidity is within the above range, bleeding out of the tree inhibitor from the crosslinked product can be suppressed, and the shape of the crosslinked product can be maintained for a long period of time.

[0045] The crosslinked product obtained by crosslinking the insulating resin composition of the embodiment is an insulator. In a power cable in which the crosslinked product of such an insulating resin composition is applied to an insulating layer, water tree growth in the insulating layer can be suppressed and the dielectric loss tangent can be reduced, so that the power cable can transmit electricity efficiently even when used at high voltage or extra-high voltage. Such a power cable is suitable for use as an underground cable or a submarine cable. When the power cable is an AC power cable, the effects of suppressing water tree growth and reducing the dielectric loss tangent are even more effectively exhibited.

[0046] FIG. 1 is a cross-sectional view showing an example of a power cable to which an insulating resin composition according to an embodiment of the present invention is applied.

[0047] As shown in Figure 1, power cable 1 includes conductor 2, internal semiconductive layer 3 disposed on the outside of conductor 2, insulating layer 4 disposed on the outside of internal semiconductive layer 3 and formed by crosslinking the above-mentioned insulating resin composition, and external semiconductive layer 5 disposed on the outside of insulating layer 4. Internal semiconductive layer 3 surrounds conductor 2. Insulating layer 4 surrounds internal semiconductive layer 3. External semiconductive layer 5 surrounds insulating layer 4. Thus, in power cable 1, internal semiconductive layer 3, insulating layer 4, and external semiconductive layer 5 are laminated in this order on conductor 2 made of a metal such as copper or aluminum.

[0048] The inner semiconductive layer 3 and the outer semiconductive layer 5 contain, for example, an ethylene-based copolymer such as an ethylene-ethyl acrylate copolymer, an ethylene-methyl acrylate copolymer, an ethylene-butyl acrylate copolymer, or an ethylene-vinyl acetate copolymer, an olefin-based elastomer, and conductive carbon black.

[0049] The power cable 1 may further include a metal shielding layer (not shown) that is arranged outside the outer semiconductive layer 5 and surrounds the outer semiconductive layer 5. The power cable 1 may further include a sheath (not shown) that is arranged outside the metal shielding layer and surrounds the metal shielding layer.

[0050] The conductor 2 is continuously fed to a resin extrusion port, where it is coated with an inner semiconductive layer 3, an insulating resin composition layer, and an outer semiconductive layer 5. These three layers may be extruded simultaneously or sequentially. If the conductor is heated by heat transfer from the previously coated resin during coating, the cooling rate of the resin near the conductor will be slow. Therefore, it is preferable to adjust the temperature of the conductor to between 1°C and 100°C by cooling before feeding the conductor to the resin extrusion port.

[0051] The insulating resin composition is coated by extruding it onto the conductor 2 (onto the internal semiconducting layer 3) from the resin extrusion port of a resin extruder equipped with a metal mesh filter having an opening of 100 μm or less for the purpose of removing foreign matter. The temperature of the insulating resin composition during extrusion is preferably equal to or higher than the melting point of the insulating resin composition, specifically, preferably 110° C. or higher, and more preferably 120° C. or higher. In order to suppress scorch, the temperature of the insulating resin composition during extrusion is preferably 140° C. or lower.

[0052] After the conductor 2 is thus coated with the insulating resin composition layer, a crosslinking reaction of the insulating resin composition layer is carried out by applying pressure and heat, thereby forming the insulating layer 4 by crosslinking the insulating resin composition, thereby obtaining the power cable 1. Thereafter, the power cable 1 is cooled in a cooling pipe or a cooling water tank, and a metal shielding layer or sheath (not shown) is formed by a conventional method, if necessary.

[0053] When forming an exterior covering such as a metal shielding layer or a sheath on the cooled power cable 1, the temperature of the insulating layer 4 is preferably 300° C. or less to prevent deformation of the insulating layer 4.

[0054] From the viewpoint of insulating properties, the thickness of the insulating layer 4 is preferably 2 mm or more, more preferably 5 mm or more, and even more preferably 10 mm or more, and from the viewpoint of ease of installation, the thickness is preferably 50 mm or less, and more preferably 40 mm or less.

[0055] Furthermore, from the viewpoint of insulating properties, the thickness of the internal semiconductive layer 3 and the thickness of the external semiconductive layer 5 are preferably 0.1 mm or more, and more preferably 0.5 mm or more, and from the viewpoint of conductive properties, they are preferably 5.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less.

[0056] According to the embodiment described above, even if the component (c) is a water tree inhibitor to suppress the growth of water trees, the inclusion of component (b) can suppress an increase in the dielectric loss tangent caused by the water tree inhibitor. Furthermore, by applying a cross-linked product of the insulating resin composition that can suppress the growth of water trees and reduce the dielectric loss tangent to an insulating layer, the power transmission efficiency of the power cable can be improved.

[0057] Although the embodiments have been described above, the present invention is not limited to the above embodiments, but includes all aspects encompassed by the concept and scope of the claims of the present disclosure, and can be modified in various ways within the scope of the present disclosure.

[0058] Next, examples and comparative examples will be described, but the present invention is not limited to these examples.

[0059] The raw materials used in the examples and comparative examples are as follows.

[0060] Component (a): Polyethylene (CE1559; LDPE manufactured by Sumitomo Chemical Co., Ltd., MFR 0.8) Resin (b1-1): Maleic anhydride graft resin (resin obtained by grafting 2.00 wt% of maleic anhydride onto CE1559) Resin (b1-2): Maleic anhydride graft resin (resin obtained by grafting 0.25 wt% of maleic anhydride onto CE1559) Resin (b1-3): Maleic anhydride graft resin (resin obtained by grafting 1.00 wt% of maleic anhydride onto CE1559) Resin (b2-1): Ethylene copolymer (NUC-6520; ethylene ethyl acrylate copolymer manufactured by Eneos NUC Corporation, acrylate content 24 wt%, MFR 1.6) Component (c-1): Water tree inhibitor (PEG-20000; polyethylene glycol manufactured by ADEKA Corporation, number average molecular weight 20000) Component (c-2): Water tree inhibitor (polypropylene glycol 4000; polypropylene glycol manufactured by ADEKA Corporation, number average molecular weight 4000) Component (c-3): Water tree inhibitor (polypropylene glycol 400; polypropylene glycol manufactured by ADEKA Corporation, number average molecular weight 400) Component (d): Crosslinking agent (Percumyl D; dicumyl peroxide manufactured by NOF Corporation) Antioxidant (Irganox 1010; hindered phenol-based antioxidant manufactured by BASF)

[0061] (Examples 1 to 8 and Comparative Examples 1 and 2) The raw materials were dry-blended using a Henschel mixer, and then extruded using a single-screw extruder (L / D = 24, 120°C) equipped with a plain-woven mesh having an opening of 0.091 mm, to obtain pellet-shaped insulating resin compositions (hereinafter also referred to as resin pellets) having the compositions shown in Table 1.

[0062] [Measurements and Evaluations] The insulating resin compositions obtained in the above Examples and Comparative Examples were subjected to the following measurements and evaluations. The results are shown in Table 1.

[0063] [1] Moisture absorption rate Resin pellets were used to mold a sample having a length of 100 mm, a width of 150 mm, and a height of 1 mm at 120°C, and the sample was crosslinked at 160°C. The crosslinked product was divided into 6 test pieces, which were then stored in a thermo-hygrostat at 70°C and a relative humidity of 90% for 4 hours and then removed from the chamber. The moisture content was measured by the Karl Fischer method, Method B (moisture evaporation method) of JIS K7251, to determine the moisture absorption rate.

[0064] [2] Resin fluidity Resin pellets were used to measure the resin fluidity based on the amount of resin extruded (g) per 10 minutes under conditions of a melting temperature of 190°C and a load of 2.16 kgf in accordance with JIS K7210.

[0065] [3] Water Tree Length First, a small-sized power cable was manufactured. First, an insulating resin composition was extruded using resin pellets through a single-screw extruder (L / D = 24, 120°C, full-flight screw) equipped with a plain-woven mesh with a mesh size of 0.091 mm. The extruded resin was then coated onto a copper conductor using a three-layer head (120°C) together with a semiconductive resin (NUCV-9590, an ethylene-ethyl acrylate copolymer-based semiconductive resin composition manufactured by Eneos NUC Corporation) extruded through another single-screw extruder (L / D = 24, 120°C, full-flight screw) equipped with a plain-woven mesh with a mesh size of 0.091 mm. The coated cable was passed through a pressure crosslinking tube and then sent to a water bath for cooling. This resulted in a small-sized power cable having a copper conductor with an outer diameter of approximately 2 mm, an inner semiconductive layer with a thickness of approximately 0.5 mm, an insulating layer with a thickness of approximately 2 mm, and an outer semiconductive layer with a thickness of approximately 0.5 mm.

[0066] Next, a water tree test was performed by immersing the small power cable 1 in a 3.5 wt% NaCl aqueous solution as shown in Figure 2, and applying a 4 kV 1000 Hz AC voltage between the conductor and the NaCl aqueous solution for 200 hours. After the test, the power cable was sliced ​​axially into 1 mm thick slices, and the resulting 10 samples were observed under an optical microscope to measure the length of the water trees.

[0067] [4] Dielectric loss tangent The resin pellets were molded into a film-like sample having a thickness of 0.3 mm at 120°C for 10 minutes, and the sample was crosslinked at 160°C for 30 minutes. The dielectric loss tangent of the crosslinked product was measured in accordance with JIS C 2138 under conditions of 90°C and 30 kV / mm.

[0068]

[0069] As shown in Table 1, in Examples 1 to 8, the insulating resin compositions contained components (a), (b), (c), and (d), and therefore suppression of water tree growth and reduction of dielectric loss tangent were achieved. On the other hand, in Comparative Examples 1 and 2, the insulating resin compositions did not contain at least one of components (a) to (d), and therefore suppression of water tree growth and reduction of dielectric loss tangent were not achieved.

[0070] REFERENCE SIGNS LIST 1 Power cable 2 Conductor 3 Inner semiconductive layer 4 Insulating layer 5 Outer semiconductive layer

Claims

1. An insulating resin composition for power cables comprising components (a), (b), (c) and (d), wherein the component (a) is polyethylene, the component (b) is at least one resin selected from resins (b1) and (b2), the resin (b1) is a resin grafted with at least one modifying monomer selected from unsaturated organic acids and derivatives thereof, and the resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers and ethylene-vinyl acetate copolymers, the component (c) is a water tree inhibitor, and the component (d) is a crosslinking agent.

2. The insulating resin composition for power cables according to claim 1, wherein the component (c) is at least one compound selected from the group consisting of polyalkylene glycols and derivatives thereof, polyglycerin, glycerin fatty acid esters, and sorbitol esters.

3. The insulating resin composition for power cables according to claim 1 or 2, wherein the crosslinked product obtained by crosslinking the insulating resin composition for power cables has a dielectric loss tangent of 1.0% or less at 30 kV / mm at 90°C.

4. An insulating resin composition for power cables according to any one of claims 1 to 3, wherein the resin (b1) is formed by adding at least one modified monomer selected from unsaturated dicarboxylic acids, unsaturated dicarboxylic anhydrides, and unsaturated dicarboxylic acid derivatives to at least one resin selected from polypropylene, polyethylene, and olefin-based copolymers.

5. An insulating resin composition for power cables according to any one of claims 1 to 4, wherein the content of component (b) in the insulating resin composition for power cables is 3.0 wt % or more and 25.0 wt % or less.

6. A power cable comprising: a conductor; an inner semiconductive layer disposed on the outside of the conductor and surrounding the conductor; an insulating layer disposed on the outside of the inner semiconductive layer and surrounding the inner semiconductive layer, the insulating layer being formed by crosslinking the insulating resin composition for power cables according to any one of claims 1 to 5; and an outer semiconductive layer disposed on the outside of the insulating layer and surrounding the insulating layer.

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