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 a polypropylene; the component (b) is an elastomer; the component (c) is at least one resin selected from among a resin (c1) and a resin (c2); the resin (c1) is a resin resulting from grafting with at least one modified monomer selected from among unsaturated organic acids and derivatives thereof; the resin (c2) is at least one ethylene-based copolymer selected from among ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers; and the component (d) is a water tree inhibitor.

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, is used as the insulator for power cables. However, since XLPE has no thermoplastic properties, it cannot be recycled when the cables are discarded. Therefore, development of insulating resins using polypropylene, a thermoplastic resin with no cross-linking properties, is underway.

[0003] However, like XLPE, thermoplastic insulating resins also suffer from water treeing. Water trees are dendritic defects that arise from foreign matter or air bubbles (voids) in the resin. Water trees grow when the water in the resin concentrates at the interfaces of foreign matter or voids due to dielectrophoresis of water caused by the flow of electricity through the power cable. Water trees can cause destruction of power cables, so there is a need to suppress their growth.

[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, hydrophilic molecules are added to XLPE to suppress the growth of water trees. However, even in the case of polypropylene, which is a thermoplastic resin, adding hydrophilic molecules increases the dielectric loss tangent (tan δ) of the resin, which reduces 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, which is recyclable, can suppress the growth of water trees, and can reduce the dielectric loss tangent, and a power cable.

[0009] [1] An insulating resin composition for a power cable, comprising components (a), (b), (c), and (d), wherein the component (a) is polypropylene, the component (b) is an elastomer, the component (c) is at least one resin selected from resins (c1) and (c2), the resin (c1) is a resin grafted with at least one modifying monomer selected from unsaturated organic acids and derivatives thereof, the resin (c2) is at least one ethylene copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers, and the component (d) is a water tree inhibitor. [2] The insulating resin composition for a power cable according to the above item [1], wherein the resin (c1) is a maleic anhydride graft resin. [3] The insulating resin composition for a power cable according to the above [1] or [2], wherein the component (d) is at least one compound selected from polyalkylene glycols and derivatives thereof, polyglycerin, glycerin fatty acid esters, and sorbitol esters. [4] The insulating resin composition for a power cable according to any one of the above [1] to [3], wherein the resin fluidity of the insulating resin composition for a power cable at 230°C is 10.0 g / 10 min or less. [5] The insulating resin composition for a power cable according to any one of the above [1] to [4], wherein the component (b) is an olefin-based copolymer or a styrene-based copolymer. [6] The insulating resin composition for a power cable according to any one of the above [1] to [5], wherein the resin (c1) is formed by adducting maleic anhydride to at least one resin selected from polypropylene, polyethylene, and a styrene-based copolymer. [7] 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 made of the insulating resin composition for a power cable according to any one of [1] to [6] 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 are recyclable, capable of suppressing the growth of water trees and reducing 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 polypropylene is used from the viewpoint of recycling an insulating resin composition for a power cable, and that even when a water tree inhibitor is contained in the polypropylene to suppress the growth of water trees, the increase in dielectric loss tangent caused by the water tree inhibitor can be suppressed by containing a specific component, 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 polypropylene, component (b) is an elastomer, component (c) is at least one resin selected from resins (c1) and (c2), resin (c1) is a resin grafted with at least one modifying monomer selected from unsaturated organic acids and derivatives thereof, resin (c2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers, and component (d) is a water tree inhibitor.

[0015] The insulating resin composition of the embodiment includes, as constituent elements, component (a), component (b), component (c), and component (d). The insulating resin composition is a non-crosslinked insulator and is suitable for use in an insulating layer of a power cable.

[0016] Component (a) contained in the insulating resin composition is polypropylene. The polypropylene is preferably homopolypropylene, random polypropylene, or block polypropylene. The random polypropylene is preferably a random copolymer of ethylene or an α-olefin and propylene, and the propylene component is preferably 90 wt % or more. The block polypropylene is preferably a copolymer of homopolypropylene, ethylene, and an α-olefin. Here, the α-olefin is preferably an olefin having 3 to 8 carbon atoms.

[0017] The lower limit of the content of component (a) in the insulating resin composition is preferably 10.0 wt % or more, more preferably 30.0 wt % or more, and the upper limit is preferably 90 wt % or less. When the content of component (a) is within the above range, the recyclability of the insulating resin composition is good, and when the content of component (a) is 30.0 wt % or more, the heat resistance of the insulating resin composition can be improved.

[0018] Component (b) contained in the insulating resin composition is an elastomer. The elastomer increases the flexibility of the insulating resin composition containing component (a), polypropylene. The elastomer, component (b), is preferably an olefin copolymer or a styrene copolymer. When the elastomer is an olefin copolymer or a styrene copolymer, the compatibility of component (a) with polypropylene is good, so the interface between component (a) and component (b) is tightly adhered, reducing fracture originating from the interface. Furthermore, the elastomer component of component (b) absorbs impact and strain, increasing resin strength.

[0019] The olefin copolymer is a copolymer of ethylene or propylene and an α-olefin. The α-olefin is preferably an α-olefin having 3 to 8 carbon atoms, and an ethylene-propylene copolymer is more preferred. When the α-olefin has 3 to 8 carbon atoms, the compatibility of component (a) with polypropylene is particularly good, resulting in close contact at the interface between components (a) and (b), reducing fracture originating at the interface. Furthermore, the elastomer component of component (b) absorbs impact and strain, increasing resin strength. These may be blended as an olefin thermoplastic elastomer, as described below. The polymerization catalyst for the olefin copolymer is preferably a Ziegler-Natta catalyst, a metallocene catalyst, or a constrained geometry catalyst. The olefin copolymer may be produced using a gas phase, solution, or slurry polymer process.

[0020] The olefin-based thermoplastic elastomer is preferably a resin obtained by multistage gas phase polymerization. Resins obtained by multistage gas phase polymerization improve impact strength (especially impact strength at low temperatures) while maintaining the rigidity and heat resistance of polypropylene. Resins obtained by multistage gas phase polymerization are generally called heterophasic copolymers, impact copolymers, reactor-made alloys, reactor-made thermoplastic elastomers, or reactor TPOs (thermoplastic alloys), with reactor TPOs being the most common. Such resins are resin compositions containing polypropylene and an olefin-based copolymer, specifically, polypropylene and an ethylene-α-olefin copolymer rubber or a propylene-α-olefin copolymer rubber.

[0021] The reaction catalyst used in carrying out the multistage gas phase polymerization is preferably a Ziegler-Natta catalyst or a metallocene catalyst, and the monomer used in carrying out the multistage gas phase polymerization is preferably an α-olefin monomer such as ethylene, propylene, butene, or octene.

[0022] Resins obtained by multistage gas-phase polymerization are prepared by blending finely powdered resin components produced in each stage in a reactor during polymerization. By melt-kneading the resulting mixture, a resin with a microphase-separated structure is obtained, with a sea of ​​finer propylene polymer components such as polypropylene and islands of elastomer components (copolymer rubber components), compared to conventional methods of blending and melt-kneading pellets of polypropylene and elastomer. The average particle size of the elastomer components in the insulating resin composition is preferably 5 μm or less, more preferably 1 μm or less. Here, the average particle size is determined by the average major axis of the elastomer component islands, obtained by photographing a freeze-fracture surface of the resin using, for example, a transmission electron microscope (TEM).

[0023] Suitable commercially available propylene-based resins obtained by such multistage gas phase polymerization include a series of resins known as Cattalloy process resins, such as Adflex, Hifax, Softell, and Adsyl (registered trademarks) (all manufactured by LyondellBasell). Other suitable products include Tafmer (registered trademark) PN (manufactured by Mitsui Chemicals, Inc.), Tafthren (registered trademark) (manufactured by Sumitomo Chemical Co., Ltd.), Newcon (registered trademark) (manufactured by Japan Polypropylene Corporation), and Prime TPO (registered trademark) (manufactured by Prime Polymer Co., Ltd.).

[0024] Preferred styrene copolymers include styrene-butadiene block copolymers (SBR), styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), hydrogenated styrene-butadiene rubber (HSBR), styrene-ethylene-butylene-styrene block copolymers (SEBS), styrene-ethylene-propylene-styrene block copolymers (SEPS), styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS), styrene-ethylenebutylene-olefin crystalline block copolymers (SEBC), and styrene-butadiene block copolymers (SBR), each of which is obtained by hydrogenating a butadiene polymer block and an isoprene polymer block. These styrene copolymers may be used alone or in combination of two or more. In these styrene copolymers, the component (a) has particularly good compatibility with polypropylene, so the interface between the components (a) and (b) is closely attached, which reduces fracture originating from the interface, and the styrene elastomer component (b) absorbs impact and strain, thereby increasing the resin strength.

[0025] The lower limit of the content of component (b) in the insulating resin composition is preferably 10.0 wt % or more, more preferably 20.0 wt % or more, and even more preferably 30.0 wt % or more, and the upper limit is preferably 90.0 wt % or less. When the content of component (b) is within the above range, the insulating resin composition has good flexibility, and when the content of component (b) is 20.0 wt % or more, the flexibility of the insulating resin composition can be sufficiently improved, and when the content of component (b) is 30.0 wt % or more, the cold resistance of the insulating resin composition can be improved.

[0026] The component (c) contained in the insulating resin composition is at least one resin selected from the group consisting of resins (c1) and (c2). The component (c) suppresses an increase in the dielectric loss tangent in the insulating resin composition, which is caused by the water tree inhibitor, component (d).

[0027] The resin (c1) 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).

[0028] The functional group introduced into the resin by grafting the modified monomer is preferably a functional group having a C═O bond. Among these, a carbonyl group, a carboxyl group, an ester group, an acid anhydride group, an amide group, or an imide group is preferred. The water tree inhibitor (component (d)) is restrained by hydrogen bonding with the C═O in the molecule grafted to the resin, thereby suppressing molecular motion due to an electric field. Thus, the C═O in the molecule grafted to the resin suppresses the motion of the water tree inhibitor, thereby efficiently reducing the dielectric loss tangent of the insulating resin composition. Furthermore, the graft resin can improve the dispersibility of the water tree inhibitor and suppress bleed-out of the insulating resin composition.

[0029] A preferred modifying monomer is an unsaturated dicarboxylic acid anhydride. As the unsaturated dicarboxylic acid anhydride, maleic anhydride and itaconic anhydride are preferred, with maleic anhydride being more preferred. These may be used alone or in combination of two or more. The resin (c1) grafted with such a modifying monomer is preferably one in which maleic anhydride is added to at least one resin selected from polypropylene, polyethylene, and styrene copolymers. The polyethylene is preferably low-density polyethylene produced by a high-pressure process.

[0030] When the graft amount of the modified monomer is 0.1 wt % or more and 10.0 wt % or less in the graft resin, the dielectric loss tangent of the insulating resin composition is sufficiently reduced, and when it is 0.5 wt % or more and 2.0 wt % or less, component (c) is uniformly dispersed in the insulating resin composition, so that the dielectric loss tangent in the system can be uniformly reduced.

[0031] 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.

[0032] Resin (c2) is at least one ethylene copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers.

[0033] The ethylene-acrylate copolymer is preferably an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, or an ethylene-butyl acrylate copolymer.

[0034] The content of component (c) in the insulating resin composition preferably has a lower limit of 3.0 wt % or more, more preferably 4.0 wt % or more, and an upper limit of 25.0 wt % or less, more preferably 15.0 wt % or less. When the content of component (c) is within the above range, the dielectric loss tangent of the insulating resin composition can be sufficiently reduced.

[0035] Regarding component (c), resin (c1) can more efficiently reduce the dielectric loss tangent of the insulating resin composition than resin (c2). Therefore, the insulating resin composition preferably contains resin (c1). Among them, resin (c1) is preferably a maleic anhydride grafted resin.

[0036] Component (d) contained in the insulating resin composition is a water tree inhibitor. Component (d), the water tree inhibitor, is a hydrophilic molecule and can impart hygroscopicity to the insulating resin composition. By imparting hygroscopicity to the insulating resin composition, moisture in the insulating resin composition is uniformly dispersed, thereby suppressing moisture concentration at foreign matter and void interfaces and inhibiting the growth of water trees in the insulating resin composition.

[0037] 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.

[0038] Preferred polyalkylene glycols are polyethylene glycol, polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymers, and their alkyl ethers or carboxylic acid esters. These water tree inhibitors are intended to impart hygroscopicity to the insulating resin composition through hydrogen bonding with water molecules, and polyalkylene glycol derivatives exhibit the same effect as polyalkylene glycols. When used by absorbing them into resin pellets during dry blending, polyalkylene glycols and their derivatives are preferably liquid at 60°C. Polyglycerin is a compound in which multiple glycerin units are polymerized through ether bonds and can have a linear, cyclic, or branched structure. Furthermore, alkyl-etherifying or carboxylic acid-esterifying these polyglycerins improves their dispersibility in the insulating resin composition of the present disclosure. In particular, introducing them into the insulating resin composition via fatty acid esterification is preferred from the standpoint of compound manufacturing 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.

[0039] In order to suppress bleed-out in 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.

[0040] 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 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 insulating resin composition can be further suppressed.

[0041] Furthermore, since polypropylene (component (a)) has tertiary carbons, its molecular chains are susceptible to radical scission. Therefore, in order to prevent molecular chain scission due to overheating during extrusion, which would result in a decrease in the viscosity of the insulating resin composition, the insulating resin composition preferably contains an antioxidant in addition to the above-mentioned components (a), (b), (c), and (d). As the antioxidant, it is preferable to use a hindered phenol-based antioxidant, which is primarily intended to capture radicals, in combination with a phosphorus-based antioxidant or sulfur-based antioxidant, which is primarily intended to decompose peroxides.

[0042] The lower limit of the antioxidant content in the insulating resin composition is preferably 0.01 wt % or more, more preferably 0.2 wt % or more, and the upper limit is preferably 1.0 wt % or less, more preferably 0.6 wt % or less. When the antioxidant content is within the above range, the viscosity of the insulating resin composition can be prevented from decreasing.

[0043] 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 impair the recyclability, suppress the growth of water trees, or reduce the dielectric loss tangent of the insulating resin composition.

[0044] The insulating resin composition can be obtained by dry blending components (a), (b), (c), and (d) using a Henschel mixer or the like, followed by melt-kneading. Examples of melt-kneading equipment that can be used include single-screw or twin-screw extruders, Banbury mixers, and kneaders. 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 having a mesh size of 100 μm or less for the purpose of removing foreign matter.

[0045] The resin temperature during melt-kneading is preferably equal to or higher than the melting point of polypropylene, specifically, preferably 140°C or higher. When the insulating resin composition is applied to the insulating layer of a power cable, the resin temperature during melt-kneading is preferably 160°C or higher, more preferably 180°C or higher, and even more preferably 200°C or higher, in order to improve adhesion between the resin layers of the power cable. When the resin temperature during melt-kneading is 270°C or lower, resin sagging during extrusion can be suppressed, and when the resin temperature during melt-kneading is 240°C or lower, thermal degradation of the resin can be suppressed.

[0046] Furthermore, the dielectric loss tangent of the insulating resin composition at 30 kV / mm at 90°C is preferably 0.50% or less, more preferably 0.10% or less. When the dielectric loss tangent of the insulating resin composition is within the above range, a power cable having the insulating resin composition applied to its insulating layer has a small dielectric loss tangent, thereby improving power transmission efficiency. The dielectric loss tangent of the insulating resin composition is a value measured in accordance with JIS C 2138 under conditions of 90°C and 30 kV / mm. The dielectric loss tangent is dependent on temperature and applied electric field.

[0047] In addition, in order to suppress the growth of water trees in the insulating resin composition, the moisture absorption rate of the insulating resin composition is preferably 500 ppm or more, and more preferably 800 ppm or more. By imparting moisture absorption to the insulating resin composition, moisture in the insulating resin composition is uniformly dispersed, thereby suppressing the growth of water trees in the insulating resin composition. In addition, from the viewpoint of the dielectric strength of the insulating resin composition, the moisture absorption rate of the insulating resin composition is preferably 2000 ppm or less.

[0048] Furthermore, the resin fluidity of the insulating resin composition when measured at a melting temperature of 230°C and a load of 2.16 kgf in accordance with JIS K 7210 is preferably 10.0 g / 10 min or less, more preferably 1.0 g / 10 min or less, from the viewpoint of maintaining shape, and the lower the value, the better, from the viewpoint of suppressing bleed-out of the water tree inhibitor (d). Furthermore, from the viewpoint of processability, the resin fluidity of the insulating resin composition when measured at a melting temperature of 250°C and a load of 2.16 kgf in accordance with JIS K 7210 is preferably 0.1 g / 10 min or more.

[0049] The insulating resin composition of the embodiment is an insulator. In a power cable in which such an insulating resin composition is applied to an insulating layer, the growth of water trees 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 the growth of water trees and reducing the dielectric loss tangent are even more effectively exhibited.

[0050] Furthermore, the insulating resin composition is not crosslinked, i.e., non-crosslinked. Therefore, the insulating resin composition is recyclable. For example, when a power cable having an insulating layer made of the insulating resin composition is disposed of, the insulating resin composition used in the insulating layer can be recycled. Furthermore, since the insulating resin composition contains polypropylene, which has a higher melting point than polyethylene, as component (a), a power cable having an insulating resin composition used in the insulating layer can carry a larger current than an insulating resin composition using polyethylene, and can suppress the growth of water trees and an increase in dielectric loss tangent.

[0051] 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.

[0052] As shown in Figure 1, power cable 1 includes conductor 2, internal semiconductive layer 3 arranged on the outside of conductor 2, insulating layer 4 arranged on the outside of internal semiconductive layer 3 and made of the above-mentioned insulating resin composition, and external semiconductive layer 5 arranged 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.

[0053] The inner and outer semiconductive layers 3 and 5 are made of a conductive resin composition containing, for example, a thermoplastic elastomer resin containing polypropylene and an ethylene-α-olefin copolymer or a propylene-α-olefin copolymer, or a styrene-based elastomer resin, obtained by multistage gas phase polymerization, and conductive carbon black. The conductive resin composition may contain an ethylene-ethyl acrylate resin or an ethylene-vinyl acetate resin to adjust the dispersibility of the carbon black.

[0054] 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.

[0055] The conductor 2 is continuously fed to a resin extrusion nozzle, where it is coated with an inner semiconductive layer 3, an insulating layer 4 (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 by cooling to between 1°C and 100°C before feeding the conductor to the resin extrusion nozzle. In this way, a power cable 1 is obtained. The power cable 1 is then cooled in a cooling pipe or a cooling water tank, and, if necessary, a metal shielding layer or sheath (not shown) is formed by a conventional method.

[0056] 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 a mesh size 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 140°C or higher. Furthermore, in order to improve adhesion between the resin layers, the temperature of the insulating resin composition during extrusion is preferably 160°C or higher, more preferably 180°C or higher, and even more preferably 200°C or higher. The temperature of the insulating resin composition during extrusion is preferably 270°C or lower to prevent resin sagging during extrusion, and preferably 240°C or lower to prevent thermal degradation of the resin.

[0057] When forming an exterior covering such as a metal shielding layer or sheath on the cooled power cable 1, the temperature of the insulating layer 4 is preferably 300°C or lower in order to prevent deformation of the insulating layer 4, and more preferably lower than the extrusion temperature of the inner semiconductive layer 3, the insulating layer 4, and the outer semiconductive layer 5 onto the conductor 2.

[0058] 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.

[0059] 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.

[0060] According to the embodiment described above, even if a water tree inhibitor (component (d)) is contained to suppress the growth of water trees, the inclusion of component (c) can suppress an increase in the dielectric loss tangent due to the water tree inhibitor. Furthermore, since the insulating resin composition containing polypropylene (component (a)) is non-crosslinked, the insulating resin composition is recyclable. Furthermore, by applying an 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, and the insulating resin composition used in the insulating layer can be recycled when the power cable is disassembled.

[0061] 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.

[0062] Next, examples and comparative examples will be described, but the present invention is not limited to these examples.

[0063] The raw materials used in the examples and comparative examples are as follows.

[0064] Component (a): Polypropylene (B241; random polypropylene manufactured by Prime Polymer Co., Ltd., MFR 0.5) Component (b-1): Elastomer (Septon 8066; SEBS manufactured by Kuraray Co., Ltd., styrene content 33 wt%, no MFR flowability) Component (b-2): Elastomer (Q100F; TPO (thermoplastic elastomer composed of propylene-α-olefin copolymer and polypropylene) manufactured by Sunallomer Co., Ltd., MFR 0.6) Component (b-3): Elastomer (Tafmer DF740; butene-ethylene copolymer manufactured by Mitsui Chemicals, Inc.) Component (b-4): Elastomer (Tafmer P-0280; ethylene-propylene copolymer manufactured by Mitsui Chemicals, Inc.) Component (c1-1): Maleic anhydride graft resin (resin obtained by grafting 1.00 wt% of maleic anhydride onto CE1559, MFR 1.4 (190°C)) Component (c1-2): Maleic anhydride graft resin (Admer QE800; 0.4 wt% maleic anhydride graft PP resin, MFR 9.1, manufactured by Mitsui Chemicals, Inc.) Component (c1-3): Maleic anhydride graft resin (Tuftec M1913: 0.9 wt% maleic anhydride graft SEBS resin, MFR 5, manufactured by Asahi Kasei Corporation) Component (c2): Ethylene-ethyl acrylate copolymer (DPDJ-6182: ethylene ethyl acrylate copolymer, MFR 1.5, manufactured by Eneos NUC Corporation) Component (d-1): Water tree inhibitor (PEG-20000; polyethylene glycol, number average molecular weight 20,000, manufactured by Adeka Corporation) Component (d-2): Water tree inhibitor (polypropylene glycol 4000; polypropylene glycol, number average molecular weight 3,000, manufactured by Adeka Corporation) Component (d-3): Water tree inhibitor (PGL10PSW; polyglycerin manufactured by Daicel Chemical Industries, Ltd., average degree of polymerization 10) Antioxidant 1: (Irganox 1010; hindered phenol-based antioxidant manufactured by BASF) Antioxidant 2: (DSTDP (distearyl thiodipropionate); sulfur-based manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

[0065] (Examples 1 to 11 and Comparative Examples 1 to 3) The raw materials were dry-blended using a Henschel mixer, and then extruded using a single-screw extruder (L / D=24, 200°C) equipped with a plain-woven mesh having an opening of 0.091 mm, to obtain insulating resin compositions in pellet form (hereinafter also referred to as resin pellets) having the compositions shown in Table 1.

[0066] [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.

[0067] [1] Moisture absorption rate Resin pellets were used to mold a sample measuring 100 mm in length, 150 mm in width, and 1 mm in height. The molded body was divided into 6 test pieces, which were then stored in a constant temperature and humidity chamber at 70°C and 90% relative humidity for 4 hours. The test pieces were then removed and their moisture contents were measured by the Karl Fischer method, Method B (moisture evaporation method) of JIS K7251, to determine the moisture absorption rate.

[0068] [2] Resin fluidity (MFR) Using resin pellets, resin fluidity was measured in terms of the amount of resin (g) extruded per 10 minutes under conditions of a melting temperature of 230°C and a load of 2.16 kgf in accordance with JIS K7210 unless otherwise specified.

[0069] [3] Water Treeing First, a small-sized power cable was manufactured. First, an insulating resin composition was extruded using resin pellets in a single-screw extruder (L / D = 24, 200°C, full-flight screw) equipped with a plain-woven mesh with a mesh size of 0.091 mm. The resin composition was then coated onto a copper conductor using a three-layer extruder (200°C) together with a semiconductive resin extruded in another single-screw extruder (L / D = 24, 200°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 pressurized cooling 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.

[0070] The semiconductive resin used was prepared by dry-blending 30 parts of EVA450 (ethylene-vinyl acetate copolymer resin manufactured by Mitsui Dow Polychemicals), 70 parts of Elastomer Q200F (TPO manufactured by SunAllomer Co., Ltd., MFR 0.8), 30 parts of Denka Black (carbon black manufactured by Denka Co., Ltd.), and 0.5 parts of an antioxidant (Nocrac 300, a hindered phenol-based antioxidant manufactured by Ouchi Shinko Chemical Co., Ltd.) in a Henschel mixer, extruding the mixture in a single-screw extruder (L / D=24, 200°C), and pelletizing it.

[0071] Next, a small-sized power cable 1 was immersed in a 3.5 wt% NaCl aqueous solution as shown in Figure 2, and a 1000 Hz AC voltage of 4 kV was applied between the conductor and the NaCl aqueous solution for 200 hours to perform a water tree test. After the test, the power cable was sliced ​​axially into 1 mm thick slices, and the resulting 10 samples were observed with an optical microscope to measure the water tree lengths. Water tree lengths of less than 150 μm were marked with a circle (◯), and water tree lengths of 150 μm or more were marked with an ×.

[0072] [4] Dielectric Loss Tangent The resin pellets were used to form a film having a thickness of 0.3 mm, and the dielectric loss tangent of the film was measured in accordance with JIS C 2138 at 90° C. and 30 kV / mm.

[0073]

[0074] As shown in Table 1, in Examples 1 to 11, the insulating resin compositions contained polypropylene components (a), (b), (c), and (d), and therefore were recyclable, and were able to suppress water tree growth and reduce dielectric loss tangent. On the other hand, in Comparative Examples 1 to 3, the insulating resin compositions did not contain at least one of components (a) to (d), and therefore were unable to suppress water tree growth and reduce dielectric loss tangent.

[0075] REFERENCE SIGNS LIST 1 Power cable 2 Conductor 3 Inner semiconductive layer 4 Insulating layer 5 Outer semiconductive layer

Claims

1. Contains components (a), (b), (c) and (d), The component (a) is polypropylene, The component (b) is an elastomer, The component (c) is at least one resin selected from resins (c1) and (c2), the resin (c1) is a resin to which at least one modifying monomer selected from unsaturated organic acids and derivatives thereof is grafted, and the resin (c2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers, The component (d) is a water tree inhibitor. An insulating resin composition for power cables.

2. The insulating resin composition for a power cable according to claim 1 , wherein the resin (c1) is a maleic anhydride grafted resin.

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

4. 2. The insulating resin composition for a power cable according to claim 1, wherein the resin fluidity of the insulating resin composition for a power cable at 230° C. is 10.0 g / 10 min or less.

5. 2. The insulating resin composition for a power cable according to claim 1, wherein the component (b) is an olefin-based copolymer or a styrene-based copolymer.

6. 2. The insulating resin composition for a power cable according to claim 1, wherein the resin (c1) is formed by adding maleic anhydride to at least one resin selected from the group consisting of polypropylene, polyethylene, and a styrene-based copolymer.

7. A conductor; an inner semiconducting layer disposed outside the conductor and surrounding the conductor; An insulating layer comprising the insulating resin composition for power cable according to any one of claims 1 to 6, which is disposed outside the internal semiconductive layer and surrounds the internal semiconductive layer; an outer semiconductive layer disposed outside the insulating layer and surrounding the insulating layer; A power cable comprising: