Insulating resin composition for power cable and power cable

Abstract

The invention provides an insulating resin composition for a power cable and the power cable. The insulating resin composition for the power cable can inhibit growth of a water tree and can inhibit increase of dielectric loss tangent even in an environment with a large amount of moisture. The insulating resin composition for power cables contains a component (a), a component (b), and a component (c), the component (a) being an unmodified polyolefin resin, the component (b) being a resin containing, as a constituent element, at least one compound selected from unsaturated organic acids and derivatives thereof, and the component (c) being a carboxylic acid ester-containing ethylene copolymer.

Description

Technical Field

[0001] The present invention relates to an insulating resin composition for power cables and the power cables. Background Art

[0002] Insulating resins of power cables laid in environments with a lot of moisture, such as underground or on the seabed, branch-like defects called water trees are likely to occur. These defects are caused by foreign matter or bubbles (voids) in the resin.

[0003] Water trees are formed and grow due to the dielectric migration of water generated by electricity in power cables, which causes moisture in the cable insulation resin to concentrate at foreign matter or gap interfaces. Water trees can cause damage to power cables, so it is required to suppress the growth of water trees.

[0004] In order to suppress the growth of water trees, hydrophilic molecules such as polyethylene glycol are sometimes added to hydrophobic polyethylene in cross-linked polyethylene (XLPE) resins, or polyethylene glycol derivatives are modified with olefins or acrylic acid to improve their compatibility with XLPE. By introducing a hydrophilic molecular structure such as polyethylene glycol, water can be evenly dispersed in the XLPE resin, thereby suppressing the concentration of water on foreign matter or void interfaces, thereby suppressing the generation and growth of water trees.

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

[0006] However, the existing water tree suppression type XLPE resin obtained by adding polyethylene glycol or polyethylene glycol derivatives can contain much more water molecules than the XLPE resin without adding polyethylene glycol or polyethylene glycol derivatives. Therefore, if the power cable using the existing water tree suppression type XLPE resin as an insulator is used for a long time in an environment with a lot of water, a large amount of water molecules migrated from the outside of the cable into the XLPE resin will cause the dielectric loss tangent (tanδ) of the XLPE resin to increase, resulting in a decrease in the power transmission efficiency of the power cable.

[0007] This increase in dielectric loss tangent caused by the addition of hydrophilic molecules is observed not only in XLPE resins, but also in insulators based on uncrosslinked polypropylene. Therefore, water-tree-suppressed XLPE resins that do not contain highly hydrophilic polyethylene glycol or polyethylene glycol derivatives are preferred.

[0008] [Background Technology Literature]

[0009] [Patent Document]

[0010] Patent Document 1: U.S. Patent No. 4305849 Summary of the invention

[0011] [Problems to be solved by the invention]

[0012] An object of the present invention is to provide an insulating resin composition for a power cable and a power cable, which can suppress the growth of water trees and suppress the increase in dielectric loss tangent even in an environment with a large amount of water.

[0013] [Technical means to solve the problem]

[0014] [1] An insulating resin composition for a power cable, comprising a component (a), a component (b) and a component (c), wherein the component (a) is an unmodified polyolefin resin, the component (b) is a resin containing at least one compound selected from unsaturated organic acids and derivatives thereof as a constituent element, and the component (c) is an ethylene-based copolymer containing a carboxylic acid ester.

[0015] [2] The insulating resin composition for power cables according to [1], wherein the component (a) is unmodified polypropylene.

[0016] [3] The insulating resin composition for a power cable according to [2], further comprising a component (d), wherein the component (d) is an olefin-based copolymer or a styrene-based copolymer.

[0017] [4] The insulating resin composition for power cables according to [1], wherein the component (a) is unmodified polyethylene.

[0018] [5] The insulating resin composition for a power cable according to [4], further comprising a component (e), wherein the component (e) is a crosslinking agent.

[0019] [6] The insulating resin composition for power cables according to [5], wherein the crosslinking agent is an organic peroxide.

[0020] [7] The insulating resin composition for power cable according to any one of [1] to [6], wherein the component (b) is at least one resin selected from polyolefin grafted with at least one compound selected from maleic anhydride and its derivatives, and maleic anhydride copolymers.

[0021] [8] The insulating resin composition for power cable according to any one of [1] to [7], wherein the component (c) is at least one ethylene-based copolymer selected from ethylene-methacrylate copolymers, ethylene-acrylate copolymers and ethylene-vinyl acetate copolymers.

[0022] [9] A power cable comprising: a conductor; an inner semiconductive layer disposed on the outer side of the conductor and surrounding the conductor; an insulating layer disposed on the outer side of the inner semiconductive layer and surrounding the inner semiconductive layer, and formed from the insulating resin composition for a power cable according to any one of [1] to [8]; and an outer semiconductive layer disposed on the outer side of the insulating layer and surrounding the insulating layer.

[0023] [Effects of the Invention]

[0024] According to the present invention, there can be provided an insulating resin composition for a power cable and a power cable capable of suppressing the growth of water trees and suppressing an increase in dielectric loss tangent even in an environment with a large amount of water. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a cross-sectional view showing an example of a power cable to which the insulating resin composition for a power cable according to the embodiment is applied.

[0026] Figure 2 This is a schematic diagram showing a water tree test. DETAILED DESCRIPTION

[0027] Hereinafter, detailed description will be given based on the embodiments.

[0028] The inventors and others have repeatedly studied the inhibition of water tree growth and the inhibition of the increase of dielectric loss tangent, and obtained the following insights. In an insulating resin composition, ethylene copolymers containing carboxylic acid esters can inhibit the growth of water trees while inhibiting the absorption of water, compared with polyethylene glycol or polyethylene glycol derivatives with higher hydrophilicity. However, similar to polyethylene glycols traditionally used as water tree inhibitors, ethylene copolymers containing carboxylic acid esters can also cause the dielectric loss tangent of the insulating resin composition to increase. In response to this problem, the inventors and others found that the increase in the dielectric loss tangent of the insulating resin composition caused by the formulation of ethylene copolymers containing carboxylic acid esters without polyethylene glycol or polyethylene glycol derivatives can be suppressed by a resin containing at least one compound selected from unsaturated organic acids and their derivatives as a constituent element, and further found that: the growth of water trees in the insulating resin composition can be suppressed, and the increase in dielectric loss tangent can be suppressed even in an environment with more water. The present invention is completed based on the above insights.

[0029] The insulating resin composition for power cable of the embodiment comprises component (a), component (b) and component (c), wherein component (a) is an unmodified polyolefin resin, component (b) is a resin containing at least one compound selected from unsaturated organic acids and derivatives thereof as a constituent element, and component (c) is a carboxylic acid ester-containing ethylene copolymer.

[0030] The insulating resin composition for power cables of the embodiment includes component (a), component (b), and component (c) as constituent elements. The insulating resin composition is an insulator.

[0031] The component (a) contained in the insulating resin composition is an unmodified polyolefin resin, and among them, unmodified polypropylene or unmodified polyethylene is preferred.

[0032] As unmodified polypropylene, homopolypropylene, random polypropylene or block polypropylene is preferred. As random polypropylene, it refers to a random copolymer of ethylene or α-olefin and propylene, and the propylene component is preferably 90wt% or more. As block polypropylene, homopolypropylene, a copolymer of ethylene and α-olefin is preferred. Here, α-olefin is preferably an olefin with a carbon number of 3 to 8. When component (a) is unmodified polypropylene, the insulating resin composition is recyclable.

[0033] The unmodified polyethylene may be produced by a low-pressure process or a high-pressure process, but low-density polyethylene (LDPE) produced by a high-pressure process is generally preferred.

[0034] The component (b) contained in the insulating resin composition is a resin containing at least one compound selected from unsaturated organic acids and their derivatives as a constituent element, and is at least any one of a resin in which the compound is grafted onto a polymer and a copolymer resin of the compound and other monomers. The component (b) suppresses the increase in the dielectric loss tangent of the insulating resin composition caused by the component (c) which is a water tree inhibitor. Since the insulating resin composition contains the component (b), the increase in the dielectric loss tangent of the insulating resin composition can be suppressed even if the insulating resin composition is exposed to an environment with a large amount of moisture. The blending amount of the component (b) contained in the insulating resin composition is preferably 1.0wt% or more and 20.0wt% or less. This is because, when the blending amount of component (b) is less than 1.0wt%, the increase in dielectric loss tangent is insufficiently suppressed, and when the blending amount of component (b) exceeds 20.0wt%, when the conductor is coated by extrusion, the resin solidifies on the wall surface inside the extruder to form a gel, and the gel mixed into the insulator deteriorates the dielectric loss tangent of the power cable or becomes the starting point for the formation of water trees.

[0035] As unsaturated organic acid, unsaturated dicarboxylic acid, unsaturated dicarboxylic anhydride and unsaturated dicarboxylic acid derivatives are preferred. As unsaturated dicarboxylic acid, maleic acid, fumaric acid and itaconic acid are preferred. As unsaturated dicarboxylic anhydride, maleic anhydride (MAH) and itaconic anhydride are preferred. As unsaturated dicarboxylic acid derivatives, monomethyl maleate, monoethyl maleate, diethyl maleate, monomethyl fumarate, dimethyl fumarate, diethyl fumarate, maleic acid monoamide, maleimide, N-methylmaleimide, N-phenylmaleimide, N-cyclohexylmaleimide and triallyl isocyanurate are preferred. They can be used alone or in combination of two or more. Among them, as unsaturated organic acid, cyclic acid anhydride of five-membered ring, i.e. maleic anhydride, is preferred.

[0036] Examples of polymers grafted with unsaturated organic acids and their derivatives include polyolefins represented by polyethylene or polypropylene, olefin copolymers represented by ethylene-methacrylate copolymers or ethylene-acrylate copolymers, ethylene-vinyl acetate copolymers, and styrene copolymers represented by styrene-butadiene block copolymers (SBR), styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-ethylene-butylene-styrene block copolymers (SEBS), and styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

[0037] As the polymer grafted with an unsaturated organic acid and its derivatives, a particularly preferred polymer is, from the viewpoint of resin compatibility, polypropylene if the unmodified polyolefin resin of component (a) is unmodified polypropylene, and polyethylene if the unmodified polyolefin resin of component (a) is unmodified polyethylene.

[0038] The grafting amount of the unsaturated organic acid and its derivative in the component (b) is preferably 0.01wt% or more and 2.00wt% or less in the grafted resin of the component (b). By using the resin grafted with the unsaturated organic acid within this range, the unsaturated organic acid can be uniformly dispersed in the insulating resin composition, thereby uniformly reducing the dielectric loss tangent in the system.

[0039] Moreover, when the unsaturated organic acid and its derivatives are maleic anhydride, the content of maleic anhydride contained in the insulating resin composition is more preferably 0.01wt% or more and less than 0.50wt%. If the insulating resin composition contains maleic anhydride within this range, the dielectric loss tangent in the system can be uniformly suppressed. In particular, when the content of maleic anhydride exceeds 0.50wt%, the maleic anhydride grafted resin is solidified on the metal inner wall of the mixing device or the extrusion device, resulting in a scorching phenomenon. Moreover, the solidification of the maleic anhydride grafted resin sometimes reduces the uniformity of the resin composition, resulting in the inability to effectively suppress the growth of water trees. Furthermore, if the content is less than 0.47wt%, the unevenness of the resin composition caused by the resin solidification will be less likely to occur.

[0040] As a method for preparing the grafted resin, the method described in paragraph

[0098] of the specification of Patent No. 6205032 can be used, for example, to mix the high molecular weight resin, antioxidant, modified monomer and organic peroxide as raw materials in an extruder and heat them to react and obtain the grafted resin. In addition, commercially available maleic anhydride grafted resins can also be purchased and used. For example, ADMER (registered trademark) manufactured by Mitsui Chemicals, MODIC (registered trademark) manufactured by Mitsubishi Chemical, UMEX (registered trademark) manufactured by Sanyo Chemical Industries, TOUGHTEC (registered trademark) M manufactured by Asahi Chemicals, and OREVAC (registered trademark) manufactured by SK Functional Polymer can be exemplified.

[0041] Preferred monomers constituting the copolymer resin of unsaturated organic acid and its derivatives include ethylene monomers, propylene monomers, butene monomers, olefin monomers having 5 or more carbon atoms, styrene monomers, methacrylate monomers, acrylate monomers, and vinyl acetate monomers.

[0042] Component (b) is preferably at least one resin selected from polyolefins grafted with at least one compound selected from maleic anhydride and its derivatives, and maleic anhydride copolymers. Furthermore, the grafted resin of unsaturated organic acids and their derivatives is preferably maleic anhydride grafted polyethylene resin (MAH-g-PE) and maleic anhydride grafted polypropylene resin (MAH-g-PP). The copolymer resin of unsaturated organic acids and their derivatives is preferably ethylene-maleic anhydride copolymer (Et-MAH copolymer) resin.

[0043] As styrene-maleic anhydride copolymers, XIRAN (registered trademark) manufactured by Polyscope and ARASTAR manufactured by Arakawa Chemical Industries, Ltd. can be exemplified, and as copolymers of α-olefin and maleic anhydride, DIAKALNA (registered trademark) manufactured by Mitsubishi Chemical Corporation and FUSABOND (registered trademark) manufactured by Dow Chemical Company can be exemplified, etc. Moreover, as terpolymers of ethylene-acrylate-maleic anhydride, LOTADER (registered trademark) MAH manufactured by SK Functional Polymer can be exemplified, and as terpolymers of ethylene-vinyl acetate-maleic anhydride, OREVAC (registered trademark) T manufactured by SK Functional Polymer can be exemplified, etc.

[0044] The component (c) contained in the insulating resin composition is an ethylene copolymer containing a carboxylate, and is a component that inhibits the growth of water trees. Among them, the component (c) is preferably at least one ethylene copolymer selected from ethylene-methacrylate copolymers, ethylene-acrylate copolymers, and ethylene-vinyl acetate copolymers (EVA). As ethylene-methacrylate copolymers, ethylene-methyl methacrylate copolymers (EMMA), ethylene-ethyl methacrylate copolymers, and ethylene-butyl methacrylate copolymers are preferred. As ethylene-acrylate copolymers, ethylene-methyl acrylate copolymers, ethylene-ethyl acrylate copolymers (EEA), and ethylene-butyl acrylate copolymers are preferred.

[0045] Suitable ethylene-methacrylate copolymers, ethylene-acrylate copolymers and ethylene-vinyl acetate copolymers include ACRYFT (registered trademark) manufactured by Sumitomo Chemical Co., Ltd., EEA resin and EVA resin manufactured by ENEOS NUC, REXPEARL (registered trademark) EEA and NOVATEC (registered trademark) EVA manufactured by Japan Polyethylene Co., Ltd., and EVAFLEX (registered trademark) manufactured by Mitsui Dow Polychemicals Co., Ltd.

[0046] Regarding the content ratio of component (c) contained in the insulating resin composition, in order to effectively suppress water trees, the lower limit is preferably 1.0wt% or more, and in order to uniformly suppress water trees, the lower limit is preferably 2.0wt% or more. In addition, in order to suppress the water absorption of the insulating resin composition, the upper limit of the content ratio of the component (c) is preferably 40.0wt% or less, and in order to effectively suppress the increase in dielectric loss tangent caused by hydrophilic molecules, the upper limit is preferably 20.0wt% or less.

[0047] In addition to the components (a) to (c), the insulating resin composition further comprises an olefin copolymer or a styrene copolymer as a component (d) as an elastomer. If the insulating resin composition contains component (d), the flexibility of the insulating resin composition is improved, so it is preferred. In particular, when the unmodified polyolefin resin of component (a) is unmodified polypropylene, the formulation of these copolymers is effective for improving the flexibility of the insulating resin composition.

[0048] The olefin copolymer is a copolymer of ethylene or propylene and an α-olefin. The α-olefin is preferably an α-olefin having a carbon number of 3 or more and 8 or less. The olefin copolymer is preferably an ethylene-propylene copolymer.

[0049] The olefin copolymer can be formulated as the olefin thermoplastic elastomer described below. The polymerization catalyst of the olefin copolymer is preferably a Ziegler-Natta catalyst, a metallocene catalyst, or a constrained geometry catalyst. The olefin copolymer can be prepared by gas phase, solution, or slurry polymerization.

[0050] As the olefin-based thermoplastic elastomer, a resin obtained by multi-stage gas phase polymerization is preferred. The resin obtained by multi-stage gas phase polymerization contains polypropylene and an olefin-based copolymer, specifically, a resin composition containing polypropylene and ethylene-α-olefin copolymer rubber or propylene-α-olefin copolymer rubber, which improves the impact strength (especially the impact strength at low temperature, etc.) while maintaining the rigidity and heat resistance of polypropylene. The resin obtained by multi-stage gas phase polymerization is generally called heterophasic copolymers, impact copolymers, reactor alloys, reactor thermoplastic elastomers, or reactor TPO (Thermoplastic polyolefin).

[0051] In multi-stage gas phase polymerization, the powdered resin components generated in each stage are mixed in the reactor during polymerization. By melt-kneading them, compared with the existing method of melt-kneading by mixing polypropylene and elastomer particles, a resin with a microscopic phase separation structure can be obtained, and the microscopic phase separation structure has a finer sea of ​​propylene polymer components such as polypropylene and islands of elastomer components (copolymer rubber components). The average particle size of the elastomer component in the insulating resin composition is preferably less than 5 μm, and more preferably less than 1 μm. Here, the average particle size is obtained by, for example, taking a freeze-fracture surface of the resin using a transmission electron microscope (TEM) to obtain the average value of the long diameter of the island of the elastomer component.

[0052] As commercially available products of such olefin-based thermoplastic elastomers obtained by multi-stage gas phase polymerization, a series of resins called Catalloy process resins are preferred, such as Adflex, Hifax, Softell, and Adsyl (all registered trademarks) (all manufactured by LyondellBasell Industries, Ltd.). In addition, TAFMER (registered trademark) PN (manufactured by Mitsui Chemicals, Inc.), TAFSELEN (registered trademark) (manufactured by Sumitomo Chemical Co., Ltd.), NUCON (registered trademark) (manufactured by Japan Polypropylene Co., Ltd.), PRIME TPO (registered trademark) (manufactured by Prime Polymer Co., Ltd.), and the like are also preferably used.

[0053] When the unmodified polyolefin resin of component (a) is unmodified polypropylene, if the insulating resin composition further contains a crosslinking agent as component (e), the main reaction of the polypropylene is a decomposition reaction rather than a crosslinking reaction, which is not preferred. However, when the unmodified polyolefin resin of component (a) is unmodified polyethylene, if the insulating resin composition further contains a crosslinking agent as component (e), polyethylene is crosslinked, thereby being able to obtain a crosslinked body of the insulating resin composition, which will increase the insulation breakdown voltage of the insulating resin composition, which is preferred. The crosslinking agent is preferably an organic peroxide.

[0054] Among them, the crosslinking agent is preferably di-tert-hexyl peroxide (PERHEXYL D manufactured by NOF Corporation), dicumyl peroxide (PERCUMYL D manufactured by NOF Corporation), 2,5-dimethyl-2,5-di(tertiary butyl peroxide)hexane (PERHEXA 25B manufactured by NOF Corporation), α,α'-bis(tertiary butyl peroxy)diisopropylbenzene (PERBUTYL P manufactured by NOF Corporation), tertiary butyl isopropyl peroxide (PERBUTYL C manufactured by NOF Corporation), di-tertiary butyl peroxide (PERBUTYLD manufactured by NOF Corporation). These crosslinking agents may be used alone or in combination of two or more. Dicumyl peroxide is particularly preferred.

[0055] The content ratio of the crosslinking agent contained 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 ratio of the crosslinking agent is within the above range, when the insulating resin composition is extruded onto a conductor using an extruder at a temperature of 120° C. to 135° C., an insulating layer can be formed without unnecessary gelation of the resin, and a crosslinking reaction can be well performed even in a crosslinking step after the insulating layer is formed.

[0056] In addition to the above components, the insulating resin composition may also contain an antioxidant. As the antioxidant, hindered phenolic and semi-hindered phenolic antioxidants whose main purpose is to capture free radicals, and phosphorus-based or sulfur-based antioxidants whose main purpose is to decompose peroxides are preferred. These antioxidants may be used alone or in combination of two or more.

[0057] Regarding the content ratio of the antioxidant contained in the insulating resin composition, the lower limit is preferably 0.01wt% or more, more preferably 0.20wt% or more, and the upper limit is preferably 1.00wt% or less, more preferably 0.60wt% or less. When the content ratio of the antioxidant is within the above range, oxidative degradation in the crosslinked product of the insulating resin composition can be well suppressed.

[0058] Moreover, as long as the inhibition of water tree growth and the reduction of dielectric loss tangent in the insulating resin composition are not hindered, the insulating resin composition may contain various substances in addition to the above-mentioned components. As various substances, stabilizers, lubricants, inorganic fillers, surface treatment agents, flame retardants, acid scavengers, voltage stabilizers, etc. can be listed. However, the median diameter (D50) of the inorganic filler is preferably less than 10 μm and less than 10.0 wt%. If these values ​​are exceeded, the inorganic filler may become the core of the occurrence of water trees. By maintaining the insulating resin composition at 600 ° C in a nitrogen atmosphere, the organic components will decompose and volatilize, and the inorganic filler can be obtained in the form of residue, so the inorganic filler can be measured by laser diffraction scattering method.

[0059] The insulating resin composition can be obtained by melt-kneading raw materials such as component (a), component (b), and component (c). As a melt-kneading device, an existing mixer such as a single-axis or double-axis extruder, a Banbury machine, a kneader, etc. can be used. In particular, in order to remove foreign matter in the resin, it is preferred to install a metal mesh filter with a mesh of 100 μm or less on a single-axis or double-axis extruder capable of continuous kneading processing to perform resin extrusion. Moreover, in the insulating resin composition to which a crosslinking agent as component (e) is added, in order to suppress the crosslinking of the resin, it is preferably kneaded at 120° C. or more and 135° C. or less.

[0060] The specific gravity of the insulating resin composition is preferably 0.90 or more and 0.93 or less.

[0061] Furthermore, the dielectric loss tangent of the insulating resin composition is preferably 1.0% or less, and more preferably 0.5% or less, when measured under the conditions of 90°C and 30 kV / mm according to the rubber and plastic insulated wire test method of JIS C 3005. If the dielectric loss tangent of the insulating resin composition is within the above range, the power cable to which the insulating layer composed of the insulating resin composition is applied has a small dielectric loss tangent, so that energy loss is reduced, thereby improving power transmission efficiency.

[0062] Furthermore, in order to suppress the growth of water trees in the insulating resin composition, after the insulating resin composition with a thickness of 1 mm is stored in a constant temperature and humidity chamber at a temperature of 70° C. and a relative humidity of 90% for 24 hours, the moisture absorption rate is preferably 100 ppm or more, and more preferably 150 ppm or more. By imparting hygroscopicity to the insulating resin composition, the water in the insulating resin composition can be evenly dispersed, thereby suppressing the growth of water trees in the insulating resin composition. Furthermore, from the viewpoint of suppressing the increase in the dielectric loss tangent, the moisture absorption rate of the insulating resin composition is preferably 800 ppm or less, and more preferably 350 ppm or less.

[0063] This insulating resin composition is suitable as an insulating material for power cables laid in an environment with a lot of water, such as underground or on the seabed. The insulating resin composition is applied to the power cable with an insulating layer, which can inhibit the growth of water trees in the insulating layer and inhibit the increase of the dielectric loss tangent even in an environment with a lot of water. Therefore, even if the power cable is used under high voltage or ultra-high voltage, the power cable can efficiently transmit power.

[0064] Figure 1 This is a cross-sectional view of an example of a power cable to which the insulating resin composition according to the embodiment is applied.

[0065] like Figure 1 As shown, the power cable 1 includes a conductor 2, an inner semiconductive layer 3 disposed outside the conductor 2, an insulating layer 4 disposed outside the inner semiconductive layer 3 and formed of the insulating resin composition, and an outer semiconductive layer 5 disposed outside the insulating layer 4. The inner semiconductive layer 3 surrounds the conductor 2. The insulating layer 4 surrounds the inner semiconductive layer 3. In the insulating resin composition whose component (a) is unmodified polypropylene, the insulating layer 4 is a non-crosslinked product (non-crosslinked resin) composed of the insulating resin composition, and in the insulating resin composition whose component (a) is unmodified polyethylene, the insulating layer 4 is a crosslinked product (crosslinked resin) obtained by crosslinking the insulating resin composition. The outer semiconductive layer 5 surrounds the insulating layer 4. In this way, in the power cable 1, the inner semiconductive layer 3, the insulating layer 4, and the outer semiconductive layer 5 are sequentially laminated on the conductor 2 composed of metal such as copper or aluminum.

[0066] The inner semiconductive layer 3 and the outer semiconductive layer 5, for example, contain an olefin-based elastomer and conductive carbon black in a non-crosslinked resin, and contain an ethylene-based copolymer such as ethylene-ethyl acrylate copolymer, ethylene-methacrylate copolymer, ethylene-butyl acrylate copolymer or ethylene-vinyl acetate copolymer and conductive carbon black in a crosslinked resin.

[0067] Furthermore, the power cable 1 may further include a metal shield layer (not shown) disposed outside the outer semiconductive layer 5 and surrounding the outer semiconductive layer 5. Furthermore, the power cable 1 may further include a sheath (not shown) disposed outside the metal shield layer and surrounding the metal shield layer.

[0068] The conductor 2 is continuously supplied to the resin extrusion port, where it is coated with the inner semiconductive layer 3, the insulating layer 4 (insulating resin composition layer), and the outer semiconductive layer 5. These three layers may be extruded and coated simultaneously, or may be coated sequentially. If the conductor is heated by heat transfer from the first coated resin during coating, the cooling rate of the resin near the conductor will be slow, so it is preferred to adjust the temperature of the conductor to 1°C or more and 100°C or less by cooling, and then supply the conductor to the resin extrusion port.

[0069] The coating of the insulating resin composition is performed by extruding the insulating resin composition from the resin extruder equipped with a metal mesh filter with a mesh size of 100 μm or less to remove foreign matter toward the conductor 2 (on the inner semiconductive layer 3). The temperature of the insulating resin composition during extrusion is preferably equal to or higher than the melting point of the insulating resin composition. In particular, in the case of a cross-linked resin, the temperature is preferably equal to or higher than 110°C, more preferably equal to or higher than 120°C, and preferably equal to or lower than 140°C in order to suppress scorching. In the case of a non-cross-linked resin, the temperature is preferably equal to or higher than 140°C, more preferably equal to or higher than 180°C, and preferably equal to or lower than 300°C in order to prevent thermal decomposition of polypropylene.

[0070] After the conductor 2 is coated with the insulating resin composition layer in this manner, if it is a cross-linking resin, the insulating resin composition layer is subjected to a cross-linking reaction by pressurizing and heating to form an insulating layer 4 in which the insulating resin composition is cross-linked, and if it is a non-cross-linking resin, the insulating resin composition layer is slowly cooled by air cooling at normal pressure or pressurizing and room temperature to 150° C. to form the insulating layer 4. In this way, the power cable 1 is obtained. Then, the power cable 1 is cooled by a cooling pipe or a cooling water tank, and a metal shield layer or a sheath not shown is formed by a usual method as needed.

[0071] The thickness of the insulating layer 4 is preferably 2 mm or more, more preferably 5 mm or more, and further preferably 10 mm or more from the viewpoint of insulation properties, and is preferably 50 mm or less, more preferably 40 mm or less from the viewpoint of laying workability.

[0072] Moreover, regarding the thickness of either the inner semiconductive layer 3 or the outer semiconductive layer 5, from the viewpoint of insulation properties, it is preferably 0.1 mm or more, more preferably 0.5 mm or more, and from the viewpoint of conductive properties, it is preferably 5.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less.

[0073] According to the above-described embodiment, even if an ethylene copolymer containing a carboxylic acid ester as component (c) is contained in order to suppress the growth of water trees, the increase in dielectric loss tangent caused by component (c) can be suppressed by containing a resin containing at least one compound selected from unsaturated organic acids and derivatives thereof as component (b) as a constituent element. In addition, by applying an insulating resin composition that can suppress the growth of water trees and reduce dielectric loss tangent to the insulating layer of a power cable, the power transmission efficiency of the power cable can be improved.

[0074] Although the embodiments have been described above, the present invention is not limited to the embodiments, but includes the concept of the present invention and various aspects included in the claims, and various changes can be made within the scope of the present invention.

[0075] Example

[0076] Next, examples and comparative examples will be described, but the present invention is not limited to these examples.

[0077] The raw materials used in Examples and Comparative Examples are as follows.

[0078] Component (a) is as follows.

[0079] • Component (a-1): Unmodified polypropylene (B241; random polypropylene manufactured by Prime Polymers, MFR 0.5)

[0080] • Component (a-2): Unmodified polyethylene (CE1559; polyethylene manufactured by Sumitomo Chemical Co., Ltd., MFR0.6)

[0081] Component (b) is as follows.

[0082] • Component (b-1): Maleic anhydride grafted PE resin (MODIC L553; manufactured by Mitsubishi Chemical Corporation, maleic anhydride 1.00 wt% grafted PE resin)

[0083] • Component (b-2): Maleic anhydride grafted PP resin (ADMER QE800; manufactured by Mitsui Chemicals, maleic anhydride 0.4wt% grafted PP resin, MFR9.1)

[0084] • Component (b-3): Ethylene-maleic anhydride copolymer (FUSABOND M603; manufactured by Dow Chemical Company, maleic anhydride 18 wt%, MFR 25.0)

[0085] Component (c) is as follows.

[0086] • Component (c-1): Ethylene-acrylate copolymer (NUC-6520; EEA manufactured by ENEOS NUC, containing 24 wt% acrylate, MFR 1.6)

[0087] • Component (c-2): Ethylene-methyl methacrylate copolymer (ACRYFT WH102; EMMA manufactured by Sumitomo Chemical Co., Ltd., containing 17 wt% methyl methacrylate, MFR 0.25)

[0088] • Component (c-3): Ethylene-vinyl acetate copolymer (NUC-3888; EVA manufactured by ENEOS NUC, containing 21wt% vinyl acetate, MFR 1.8)

[0089] The component (d) is as follows.

[0090] • Component (d-1): Olefin elastomer (CATALLOY Q100F; TPO manufactured by SunAllomer, a thermoplastic elastomer composed of propylene-α-olefin copolymer and polypropylene, MFR 0.6)

[0091] • Component (d-2): Ethylene-propylene copolymer (ESPRENE SPO P-4801, EPR manufactured by Sumitomo Chemical Co., Ltd.)

[0092] • Component (d-3): Styrene elastomer (SEPTON 8066; SEBS manufactured by Kuraray, styrene content 33wt%, MFR no fluidity)

[0093] Component (e) is as follows.

[0094] • Ingredient (e-1): Crosslinking agent (PERCUMYL D; diisopropylbenzene peroxide manufactured by NOF Corporation)

[0095] Components other than the above components are as follows.

[0096] • Antioxidant (IRGANOX 1010; manufactured by BASF, hindered phenol antioxidant)

[0097] • Polyethylene glycol (PEG-20000: polyethylene glycol manufactured by ADEKA, number average molecular weight 20000)

[0098] (Examples 1 to 14 and Comparative Examples 1 to 2)

[0099] According to the formulation described in Tables 1 and 3, the raw materials were dry-mixed using a Henschel mixer and then extruded using a single-screw extruder (L / D=24, resin ejection temperature 200°C) equipped with a plain woven net with a mesh size of 0.091 mm to obtain a granular insulating resin composition (hereinafter also referred to as resin pellets) having the composition (parts by mass) shown in Tables 1 and 3.

[0100] (Examples 21-31 and Comparative Examples 11-12)

[0101] Resin pellets having the compositions (parts by mass) shown in Tables 2 and 4 were obtained in the same manner as in Example 1 except that the resin ejection temperature was changed to 120°C.

[0102] [Measurement and evaluation]

[0103] The insulating resin compositions obtained in the examples and comparative examples were subjected to the following measurements and evaluations. The results are shown in Tables 1 to 4.

[0104] [1] Moisture absorption rate

[0105] Use resin pellets to form a sheet sample with a length of 100 mm, a width of 150 mm, and a thickness of 1 mm. In Examples 1 to 14 and Comparative Examples 1 to 2 based on unmodified polypropylene, the sample was obtained by press molding at 230°C. In Examples 21 to 31 and Comparative Examples 11 to 12 based on unmodified polyethylene, the sample was obtained by press molding at 120°C for 10 minutes and then cross-linking at 160°C for 30 minutes. The sheet sample thus obtained was divided into 6 parts to obtain a test piece. Next, the test piece was kept in a constant temperature and humidity chamber at 70°C and a relative humidity of 90% for 24 hours and then taken out, and the water content was measured using the Karl Fischer method of Method B (moisture vaporization method) of JIS K7251 to determine the moisture absorption rate.

[0106] [2] Water tree length

[0107] First, a small power cable was manufactured. An insulating resin composition was extruded using a single-shaft extruder (L / D=24, full-screw) equipped with a plain woven net with a mesh size of 0.091 mm and loaded with resin pellets, and an inner semiconductive layer and an outer semiconductive layer were extruded using another single-shaft extruder (L / D=24, full-screw) equipped with a plain woven net with a mesh size of 0.091 mm and loaded with a semiconductive material resin, thereby using a three-layer head to sequentially coat the copper conductor with an inner semiconductive layer, an insulating layer (insulating resin composition), and an outer semiconductive layer. At this time, the inner semiconductive layer, the insulating layer, and the outer semiconductive layer were extruded at 200°C in Examples 1 to 14 and Comparative Examples 1 to 2 based on unmodified polypropylene, and at 120°C in Examples 21 to 31 and Comparative Examples 11 to 12 based on unmodified polyethylene.

[0108] In addition, the semiconductive material resin of Examples 1 to 14 and Comparative Examples 1 to 2 was obtained by dry-mixing 30 parts of EVA450 (ethylene-vinyl acetate copolymer resin manufactured by Mitsui Dow Polychemicals), 70 parts of elastomer Q200F (TPO manufactured by SunAllomer, MFR0.8), 30 parts of Denka Black (carbon black manufactured by Denka), and 0.5 parts of antioxidant (NOCRAC 300, hindered phenol antioxidant manufactured by Ouchi Shinko Chemical Co., Ltd.) in a Henschel mixer, extruding and pelletizing with a single-screw extruder (L / D=24, 200°C). In addition, the semiconductive material resin of Examples 21 to 31 and Comparative Examples 11 to 12 was obtained by NUCV-9590 (a semiconductive resin composition based on ethylene-ethyl acrylate copolymer manufactured by ENEOS NUC).

[0109] Next, the coated cables were air-cooled in Examples 1 to 14 and Comparative Examples 1 to 2, and then cooled in a water tank. In Examples 21 to 31 and Comparative Examples 11 to 12, cross-linking was performed by heating at 260°C through a 0.5 MPa pressurized cross-linking tube, and then cooled in a water tank. Finally, a small power cable having a copper conductor with an outer diameter of about 2 mm, an inner semiconductive layer with a thickness of about 0.5 mm, an insulating layer with a thickness of about 2 mm, and an outer semiconductive layer with a thickness of about 0.5 mm was obtained.

[0110] Next, we will Figure 2The small power cable 1 obtained as shown is immersed in a 3.5wt% NaCl solution, and a 1000Hz AC voltage of 4kV is applied between the conductor and the NaCl solution for 200 hours to perform a water tree test. Ten samples obtained by cutting the tested power cable into 1mm thick discs are observed with an optical microscope, and the maximum length of the generated water tree is measured. If the water tree length is less than 150μm, it is considered that the growth of the water tree can be suppressed and it is judged to be qualified. If the water tree length is more than 150μm, it is considered that the growth of the water tree cannot be suppressed and it is judged to be unqualified.

[0111] [3] Dielectric loss tangent (tanδ)

[0112] Using the power cable manufactured as described above, the dielectric loss tangent before moisture absorption is measured according to the rubber and plastic insulated wire test method of JIS C 3005 at 90°C and 30 kV / mm. Next, after the power cable is stored in a constant temperature and humidity chamber at 70°C and 90% relative humidity for 24 hours, the dielectric loss tangent after moisture absorption is measured according to the rubber and plastic insulated wire test method of JIS C 3005 at 90°C and 30 kV / mm, in the same manner as described above, for the power cable formed by winding an aluminum tape on the surface of the outer semi-conductive layer to prevent volatilization of moisture caused by heating during measurement. Whether before or after moisture absorption, when measured at 90°C and 30 kV / mm, the dielectric loss tangent is preferably less than 1.00%, and more preferably less than 0.50%.

[0113] [Table 1]

[0114]

[0115] [Table 2]

[0116]

[0117] [Table 3]

[0118]

[0119] [Table 4]

[0120]

[0121] As shown in Table 1, in the embodiment, since the insulating resin composition includes components (a), (b) and (c), the growth of water trees can be suppressed, and the increase of dielectric loss tangent can be suppressed even in an environment with a lot of moisture. On the other hand, in the comparative example, since the insulating resin composition does not include at least one of components (a) to (c), the growth of water trees cannot be suppressed, and the increase of dielectric loss tangent in an environment with a lot of moisture cannot be suppressed. In particular, in Comparative Examples 1 and 11, since the insulating resin composition does not include component (b), the dielectric loss tangent before and after moisture absorption increases. Moreover, in Comparative Examples 2 and 12, since the insulating resin composition contains polyethylene glycol, the moisture absorption is large, the dielectric loss tangent before moisture absorption increases, and the dielectric loss tangent after moisture absorption increases significantly.

[0122] Reference numerals

[0123] 1: Power cable

[0124] 2: Conductor

[0125] 3: Internal semi-conductive layer

[0126] 4: Insulation layer

[0127] 5: External semi-conductive layer

Claims

1. An insulating resin composition for power cable, characterized in that: comprising component (a), component (b) and component (c), The component (a) is an unmodified polyolefin resin, The component (b) is a resin containing at least one compound selected from unsaturated organic acids and their derivatives as a constituent element, The component (c) is a carboxylic acid ester-containing vinyl copolymer.

2. The insulating resin composition for power cable according to claim 1, characterized in that: The component (a) is unmodified polypropylene.

3. The insulating resin composition for power cable according to claim 2, characterized in that: The invention further comprises a component (d), wherein the component (d) is an olefin-based copolymer or a styrene-based copolymer.

4. The insulating resin composition for power cable according to claim 1, characterized in that: The component (a) is unmodified polyethylene.

5. The insulating resin composition for power cable according to claim 4, characterized in that: The composition further comprises a component (e), wherein the component (e) is a cross-linking agent.

6. The insulating resin composition for power cable according to claim 5, characterized in that: The crosslinking agent is an organic peroxide.

7. The insulating resin composition for power cable according to claim 1, characterized in that: The component (b) is at least one resin selected from polyolefins grafted with at least one compound selected from maleic anhydride and its derivatives, and maleic anhydride copolymers.

8. The insulating resin composition for power cable according to claim 1, characterized in that: The component (c) is at least one ethylene copolymer selected from the group consisting of ethylene-methacrylate copolymer, ethylene-acrylate copolymer and ethylene-vinyl acetate copolymer.

9. A power cable, characterized in that: include: conductor; an inner semiconductive layer disposed outside the conductor and surrounding the conductor; an insulating layer, which is arranged outside the inner semiconductive layer, surrounds the inner semiconductive layer, and is formed of the insulating resin composition for a power cable according to any one of claims 1 to 8; and The outer semiconductive layer is disposed on the outer side of the insulating layer and surrounds the insulating layer.

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