Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L53/00—Compositions of block copolymers containing at least one sequence of a polymer obtained by reactions only involving carbon-to-carbon unsaturated bonds; Compositions of derivatives of such polymers
  • C08L53/02—Compositions of block copolymers containing at least one sequence of a polymer obtained by reactions only involving carbon-to-carbon unsaturated bonds; Compositions of derivatives of such polymers of vinyl-aromatic monomers and conjugated dienes
  • C08L53/025—Compositions of block copolymers containing at least one sequence of a polymer obtained by reactions only involving carbon-to-carbon unsaturated bonds; Compositions of derivatives of such polymers of vinyl-aromatic monomers and conjugated dienes modified
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
  • C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
  • C08L23/10—Homopolymers or copolymers of propene
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
  • C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
  • C08L23/10—Homopolymers or copolymers of propene
  • C08L23/12—Polypropene
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B13/00—Apparatus or processes specially adapted for manufacturing conductors or cables
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
  • H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
  • H01B3/28—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances natural or synthetic rubbers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
  • H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
  • H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
  • H01B3/44—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins
  • H01B3/441—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins from alkenes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
  • H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
  • H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
  • H01B3/44—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins
  • H01B3/442—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins from aromatic vinyl compounds
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B7/00—Insulated conductors or cables characterised by their form
  • H01B7/02—Disposition of insulation

Definitions

• the present disclosure relates to a resin composition and a power cable.
• cross-linked polyethylene has excellent insulating properties, it has been widely used as a resin component that makes up the insulating layer in power cables and the like (for example, Patent Document 1).
• a resin composition constituting an insulating layer of a power cable Contains propylene units and styrene units
• the resin composition comprises In the elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25° C., a sea phase having an elastic modulus of more than 100 MPa; A first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; A second island phase having an elastic modulus of 100 MPa or less and a size of more than 100 nm and less than 2000 nm;
• a resin composition comprising:
• FIG. 1 is a schematic cross-sectional view perpendicular to the axial direction of a power cable according to an embodiment of the present disclosure.
• FIG. 2 shows the elastic modulus distribution of sample A1 obtained by measuring the elasticity of a small area at 25° C. using a scanning probe microscope.
• FIG. 3 shows the elastic modulus distribution of sample B5 obtained by measuring the elasticity of a small area at 25° C. using a scanning probe microscope.
• FIG. 4 shows the elastic modulus distribution of sample B2 obtained by measuring the elasticity of a small area at 25° C. using a scanning probe microscope.
• FIG. 5 shows the elastic modulus distribution of sample B3 obtained by measuring the elasticity of a small area at 25° C. using a scanning probe microscope.
• FIG. 6 is a schematic cross-sectional view showing a water needle test.
• the present inventors have focused on propylene-based resin (polypropylene) as a resin component constituting an insulating layer, and have conducted extensive research to improve the characteristics of power cables.
• the objective of this disclosure is to suppress the occurrence of water trees originating from minute foreign objects or minute voids in the insulating layer while ensuring the bending resistance of the insulating layer.
• Propylene-based resins tend to have a high crystal content and are hard.
• the inventors therefore considered mixing a propylene-based resin with a low-crystalline resin as the resin component that constitutes the insulation layer in order to control the crystallinity of the propylene-based resin and improve the flexibility of the insulation layer of the power cable.
• the inventors discovered the following new problem in the insulating layer containing a propylene-based resin and a low-crystalline resin.
• the water tree resistance of the insulation layer may decrease depending on the phase structure of the insulation layer as follows:
• the phase structure of the insulating layer is a co-continuous structure
• the micro-foreign matter or micro-voids when micro-foreign matter or micro-voids are mixed into the insulating layer, the micro-foreign matter or micro-voids may be incorporated into the relatively flexible phase (hereinafter also referred to as the flexible phase) of the co-continuous structure that contains the low-crystalline resin.
• the flexible phase when water trees are generated starting from the micro-foreign matter or micro-voids, the water trees are likely to propagate along the flexible phase of the co-continuous structure.
• phase structure of the insulating layer is a sea-island structure having only a sea phase and small island phases with a size of 100 nm or less as a soft phase, coarse crystals grow in the sea phase containing the propylene-based resin. Therefore, when minute foreign objects or minute voids are mixed into the insulating layer, water trees are likely to propagate at the interface of the coarse crystals.
• the bending resistance of the insulating layer can decrease depending on the phase structure of the insulating layer as follows:
• phase structure of the insulating layer is a sea-island structure having only a sea phase and a large island phase having a size of more than 100 nm as a soft phase
• the elasticity of the sea phase containing the propylene-based resin remains high. Therefore, when the power cable is bent, cracks occur in the sea phase or the crystals separate from each other. This may reduce the insulating properties of the insulating layer.
• the inventors have succeeded in optimizing the phase structure of the insulating layer, while ensuring the bending resistance of the insulating layer, and suppressing the occurrence of water trees originating from minute foreign objects or minute voids in the insulating layer.
• a resin composition according to one embodiment of the present disclosure A resin composition constituting an insulating layer of a power cable, Contains propylene units and styrene units,
• the resin composition comprises In the elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25° C., a sea phase having an elastic modulus of more than 100 MPa; A first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; A second island phase having an elastic modulus of 100 MPa or less and a size of more than 100 nm and less than 2000 nm; Equipped with.
• a sea phase having an elastic modulus of more than 100 MPa
• a first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less
• a power cable according to one embodiment of the present disclosure, A conductor; an insulating layer provided so as to cover an outer periphery of the conductor; Equipped with The insulating layer includes a propylene unit and a styrene unit,
• the insulating layer is In the elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25° C., a sea phase having an elastic modulus of more than 100 MPa; A first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; A second island phase having an elastic modulus of 100 MPa or less and a size of more than 100 nm and less than 2000 nm; Equipped with.
• this configuration by optimizing the phase structure of the insulating layer, it is possible to ensure the bending resistance of the insulating layer while suppressing the occurrence of water trees originating from tiny foreign objects or tiny voids in the insulating layer.
• the water needle test is providing a plate electrode on a first surface of the block taken from the insulating layer; forming a needle-shaped gap having a tip with a radius of curvature of 10 ⁇ m at a position facing the first surface with a gap of 1 mm therebetween; forming a water needle in the block by injecting artificial seawater having a salinity of 3.8% by mass into the gap; applying an AC voltage of 1000 Hz and 4 kV between the plate electrode and the tip of the water needle for 200 hours; Includes. According to this configuration, even if minute foreign matter or minute voids are mixed into the insulating layer, excessive propagation of water trees can be suppressed.
• the AC breakdown field strength of the insulating layer at 25° C. after a predetermined bending test is 50 kV/mm or more.
• the bending test is a first step of bending the power cable so that a bending ratio of a bending radius of the power cable to an outer diameter of the insulating layer is 7 or less; a second step of bending the power cable in a direction opposite to the bending direction of the first step at a bending ratio equal to the bending ratio of the first step; Includes. According to this configuration, even if the power cable is bent, it is possible to maintain high insulation properties of the insulating layer.
• the insulating layer further comprises a butene unit,
• the content of the butene units in the insulating layer is equal to or greater than 9 mass % and less than 20 mass %. According to this configuration, the above-mentioned sea-island structure can be stably formed.
• the power cable according to any one of [2] to [5] above,
• the insulating layer is A propylene-based resin, a first styrenic elastomer comprising propylene units or butene units and styrene units; a second styrenic elastomer containing propylene units or butene units and styrene units and different from the first styrenic elastomer; has. According to this configuration, two island phases having different sizes can be stably formed in the insulating layer.
• Resin Composition constitutes, for example, the insulating layer 130 of the power cable 10 described below.
• the resin composition of this embodiment contains, for example, a propylene-based resin and a low-crystalline resin as resin components.
• resin component here refers to the resin material (polymer) that constitutes the main component of the resin composition.
• main component refers to the component that is present in the greatest proportion.
• the propylene-based resin of the present embodiment constitutes the main component of the insulating layer and contains at least propylene units as the main chain.
• Examples of the propylene-based resin include homopolypropylene (homoPP), random polypropylene (randomPP), and block polypropylene (blockPP).
• the propylene-based resin is random PP or block PP, propylene units and ethylene units are detected. If the low-crystalline resin is homo PP, propylene units are detected.
• the propylene-based resin may be random PP.
• Homo PP has a larger amount of crystals than random PP, and can obtain high insulation.
• random PP contains ethylene units, and therefore has a smaller amount of crystals.
• cracks due to coarse crystallization are less likely to occur. As a result, random PP can obtain high insulation properties compared to homo PP.
• the ethylene unit content in the random PP may be, for example, 0.5% by mass or more and 15% by mass or less.
• the ethylene unit content 0.5% by mass or more it is possible to suppress the growth of coarse spherulites.
• the ethylene unit content 15% by mass or less it is possible to suppress the decrease in melting point and to stably realize use in a non-crosslinked or slightly crosslinked state.
• the stereoregularity of the polypropylene is not particularly limited, but may be, for example, isotactic.
• Isotactic polypropylene is polymerized with a Ziegler-Natta catalyst and is a versatile material.
• the stereoregularity of the polypropylene can suppress a decrease in melting point in a composition in which a propylene-based resin and a low-crystalline resin are mixed. This makes it possible to obtain the high insulating properties inherent to the propylene-based resin.
• the storage modulus of the propylene-based resin alone is, for example, 300 MPa or more and 2500 MPa or less.
• the "storage modulus” referred to here is measured by dynamic mechanical analysis (DMA) in accordance with JIS K7244-4:1999.
• the dynamic viscoelasticity measurement is carried out under the following conditions. Measurement mode: Tensile mode Distortion: 0.08% Frequency: 10Hz Temperature range: 0°C to 200°C Heating rate: 10°C/min The above-mentioned "storage modulus” is a value measured at 25°C by the dynamic viscoelasticity measurement.
• the rigidity of the insulating layer 130 can be ensured.
• the flexibility of the insulating layer 130 can be ensured.
• the melt flow rate (MFR) of the propylene-based resin is not particularly limited, but may be, for example, 0.1 g/10 min or more and 10.0 g/10 min or less, or 0.1 g/10 min or more and 5.0 g/10 min or less.
• the "MFR" here is a value measured in accordance with JIS K7210 at a temperature of 230°C and a load of 2.16 kg.
• the melting point of the propylene-based resin is not particularly limited, but may be, for example, 130°C or higher and 165°C or lower. This makes it easy to form the sea-island structure described below when the propylene-based resin is mixed with the low-crystalline resin.
• the low crystalline resin of the present embodiment is configured to control the crystallinity of the propylene-based resin and impart flexibility to the insulating layer.
• the low crystalline resin does not have a melting point, or the melting point of the low crystalline resin is less than 100° C.
• the heat of fusion of the low crystalline resin may be, for example, 50 J/g or less, or 30 J/g or less.
• the low crystalline resin in this embodiment is, for example, a styrene-based elastomer.
• the styrene-based elastomer can form the island phase described below.
• the styrene-based elastomer which is a low-crystalline resin, contains styrene units as hard segments and at least propylene units or butene units as soft segments.
• the styrene-based elastomer may contain at least one monomer unit, such as an ethylene unit or an isoprene unit, as other soft segments.
• styrene-based elastomers include: Styrene butadiene styrene block copolymer (SBS), Hydrogenated styrene butadiene styrene block copolymer, Styrene ethylene propylene copolymer (SEP), Styrene-isoprene-styrene copolymer (SIS), Hydrogenated styrene-isoprene styrene copolymer, Hydrogenated styrene butadiene rubber, Hydrogenated styrene isoprene rubber, Styrene ethylene butene olefin crystalline block copolymer, Among these, two or more kinds may be used in combination.
• SBS Styrene butadiene styrene block copolymer
• SEP Styrene ethylene propylene copolymer
• SIS Styrene-isoprene-sty
• hydrolyzed here means that hydrogen has been added to the double bonds.
• hydrogenated styrene butadiene styrene block copolymer means a polymer in which hydrogen has been added to the double bonds of a styrene butadiene styrene block copolymer. Note that no hydrogen has been added to the double bonds of the aromatic rings of styrene.
• Hydrodrogenated styrene butadiene styrene block copolymer can be rephrased as styrene ethylene butene styrene block copolymer (SEBS).
• hydrogenated materials that do not contain double bonds in the chemical structure excluding aromatic rings may be used.
• non-hydrogenated materials there is a possibility that the resin components may be thermally degraded during molding of the resin composition, and the properties of the resulting molded product may be reduced.
• hydrogenated materials it is possible to improve resistance to thermal degradation. This allows the properties of the molded product to be maintained at a higher level.
• the resin composition contains two styrene-based elastomers (a first styrene-based elastomer and a second styrene-based elastomer) as low-crystalline resins.
• the first styrene-based elastomer contains propylene units or butene units and styrene units.
• the second styrene-based elastomer contains propylene units or butene units and styrene units, and is different from the first styrene-based elastomer.
• the content of at least one monomer unit in the second styrene-based elastomer is different from the content of at least one monomer unit in the first styrene-based elastomer. This allows two island phases of different sizes to be stably formed in the insulating layer 130.
• the content of propylene units or butene units in the second styrene-based elastomer is, for example, less than the content of propylene units or butene units in the first styrene-based elastomer. Since the second styrene-based elastomer contains relatively few propylene units or butene units, which have good compatibility with propylene-based resins, the second styrene-based elastomer can be aggregated to form a large island phase containing the second styrene-based elastomer (second island phase described below).
• the first styrene-based elastomer contains relatively many propylene units or butene units, the first styrene-based elastomer can be finely dispersed to form a small island phase containing the first styrene-based elastomer (first island phase described below).
• the content of propylene units or butene units in the first styrene-based elastomer is, for example, 40% by mass or more and 80% by mass or less.
• the content of propylene units or butene units in the first styrene-based elastomer 40% by mass or more, the first styrene-based elastomer can be finely dispersed.
• the rigidity of the insulating layer 130 can be ensured.
• the content of propylene units or butene units in the second styrene-based elastomer is, for example, 5% by mass or more and less than 40% by mass.
• the second styrene-based elastomer can be aggregated to stably form large island phases containing the second styrene-based elastomer (second island phases described below).
• the content of styrene units in the second styrene elastomer is, for example, greater than the content of styrene units in the first styrene elastomer. This allows the compatibility of the second styrene elastomer with the propylene resin to be lower than the compatibility of the first styrene elastomer. As a result, the island phase containing the second styrene elastomer (the second island phase described below) can be made larger than the island phase containing the first styrene elastomer (the first island phase described below) with ease.
• the content of styrene units in the first styrene-based elastomer may be, for example, 20% by mass or less, or 10% by mass or less.
• the first styrene-based elastomer can be finely dispersed.
• the first styrene-based elastomer can be finely dispersed while efficiently softening the insulating layer 130.
• the content of styrene units in the second styrene-based elastomer is, for example, more than 20% by mass and not more than 70% by mass.
• the second styrene-based elastomer can be aggregated.
• the flexibility of the insulating layer 130 can be ensured.
• the storage modulus of the styrene-based elastomer alone is, for example, 0.1 MPa or more and 100 MPa or less.
• the measurement method and conditions for the "storage modulus of the styrene-based elastomer alone" are the same as those for the storage modulus of the propylene-based resin described above.
• the content of the monomer units in the resin composition (insulating layer 130 described below) is measured by an NMR device, where the total content of the monomer units in the resin composition is taken as 100 mass %.
• the content of butene units in the resin composition is, for example, 9% by mass or more and less than 20% by mass. This allows the sea-island structure described below to be stably formed in the insulating layer 130. The phase structure will be described in detail later.
• the content of styrene units in the resin composition is, for example, 0.1% by mass or more and less than 15% by mass.
• the insulating properties of the insulating layer 130 can be stabilized.
• the flexibility of the insulating layer 130 can be ensured.
• the resin composition may contain, as a monomer unit other than propylene units, butene units, and styrene units, for example, ethylene units derived from a propylene-based resin or a low-crystalline resin.
• the content of each of the first styrene-based elastomer and the second styrene-based elastomer as low crystalline resins in the resin composition is adjusted based on the content of the above-mentioned monomer units in the resin composition when the resin components are mixed.
• the resin component constituting the insulating layer 130 may be non-crosslinked from the viewpoint of recycling.
• the resin composition may not contain a crosslinking agent.
• the resin composition may contain a small amount of a crosslinking agent so that the gel fraction (degree of crosslinking) is low.
• the resin composition may contain a crosslinking agent so that the crosslinking agent residue in the insulating layer 130 is less than 300 ppm.
• the "crosslinking agent residue (decomposition residue)" here refers to the decomposition product generated when the crosslinking agent is decomposed by the crosslinking reaction.
• the residue is, for example, cumyl alcohol, ⁇ -methylstyrene, etc.
• the resin composition may contain, for example, an antioxidant, a heat stabilizer, a copper inhibitor, a lubricant, and a colorant.
• the resin composition does not need to contain a metal hydroxide, which acts as a flame retardant.
• the content of the metal oxide in the resin composition may be 1 part by mass or less when the content of the resin component in the resin composition is 100 parts by mass. This makes it possible to obtain stable insulation properties for the insulating layer 130.
• the power cable 10 of this embodiment is configured as a so-called solid insulated power cable.
• the power cable 10 of this embodiment is configured to be laid, for example, on land (in a conduit), underwater, or at the bottom of the water.
• the power cable 10 may be used, for example, for alternating current or direct current.
• the power cable 10 has, for example, a conductor 110, an inner semiconductive layer 120, an insulating layer 130, an outer semiconductive layer 140, a shielding layer 150, and a sheath 160.
• the conductor 110 is formed by twisting together a plurality of conductor core wires (conductive core wires) containing, for example, pure copper, a copper alloy, aluminum, or an aluminum alloy.
• the internal semiconductive layer 120 is provided so as to cover the outer periphery of the conductor 110.
• the internal semiconductive layer 120 has semiconductivity and is configured to suppress electric field concentration on the surface of the conductor 110.
• the internal semiconductive layer 120 contains, for example, at least one of an ethylene-based copolymer such as an ethylene-ethyl acrylate copolymer, an ethylene-methyl acrylate copolymer, an ethylene-butyl acrylate copolymer, and an ethylene-vinyl acetate copolymer, an olefin-based elastomer, and the above-mentioned low crystalline resin, and conductive carbon black.
• an ethylene-based copolymer such as an ethylene-ethyl acrylate copolymer, an ethylene-methyl acrylate copolymer, an ethylene-butyl acrylate copolymer, and an ethylene-vinyl acetate copolymer, an olefin
• the insulating layer 130 is provided so as to cover the outer periphery of the internal semiconducting layer 120.
• the insulating layer 130 is, for example, extrusion molded from a resin composition as described above. The phase structure of the insulating layer 130 and the cable characteristics will be described in detail later.
• the outer semiconductive layer 140 is provided so as to cover the outer periphery of the insulating layer 130.
• the outer semiconductive layer 140 has semiconductivity and is configured to suppress electric field concentration between the insulating layer 130 and the shielding layer 150.
• the outer semiconductive layer 140 is configured of, for example, the same material as the inner semiconductive layer 120.
• the shielding layer 150 is provided so as to cover the outer periphery of the outer semiconductive layer 140.
• the shielding layer 150 is formed, for example, by winding a copper tape, or is formed as a wire shield in which a plurality of soft copper wires or the like are wound. Note that a tape made of a material such as a rubber-coated cloth may be wound on the inside or outside of the shielding layer 150.
• the sheath 160 is provided so as to cover the outer periphery of the shielding layer 150.
• the sheath 160 is made of, for example, polyvinyl chloride or polyethylene.
• the power cable 10 of this embodiment is an underwater cable or a bottom cable, it may have a metal waterproof layer such as aluminum sheath or iron wire armor outside the shielding layer 150.
• the power cable 10 of this embodiment may not have a water-proof layer outside the shielding layer 150, for example.
• the power cable 10 of this embodiment may be configured with a non-completely water-proof structure.
• the diameter of the conductor 110 is 5 mm or more and 60 mm or less
• the thickness of the inner semiconductive layer 120 is 0.5 mm or more and 3 mm or less
• the thickness of the insulating layer 130 is 3 mm or more and 35 mm or less
• the thickness of the outer semiconductive layer 140 is 0.5 mm or more and 3 mm or less
• the thickness of the shielding layer 150 is 0.1 mm or more and 5 mm or less
• the thickness of the sheath 160 is 1 mm or more.
• the AC voltage applied to the power cable 10 of this embodiment is, for example, 20 kV or more.
• Phase structure of insulating layer The inventors discovered a phase structure of insulating layer 130 that can solve the above-mentioned new problem by performing micro-area elasticity measurement of insulating layer 130 containing a resin composition.
• micro-area elasticity measurement is performed using a scanning probe microscope (SPM). For example, a sheet of a given thickness cut from the center of the thickness direction of the insulating layer 130 is used in this measurement.
• SPM scanning probe microscope
• the elastic modulus is measured under the conditions of tapping 16,000 times within a 1 ⁇ m square area of the sample at 25° C. using a cantilever with a tip made of silicon (single crystal) and with a radius of curvature of 1 nm or more and less than 20 nm.
• the phase structure of the insulating layer 130 is a co-continuous structure.
• the propylene units or butene units in the styrene-based elastomer easily mix with the propylene-based resin.
• the portion where the styrene-based elastomer and the propylene-based resin mix has an elastic modulus of, for example, 200 MPa or less, which is an intermediate elastic modulus between the elastic modulus of the propylene-based resin and the elastic modulus of the styrene-based resin.
• the elastic modulus distribution with a threshold elastic modulus of 100 MPa, it is possible to measure the size of the island phases when the phase structure of the insulating layer 130 is a sea-island structure. That is, since styrene-based elastomers exhibit an elastic modulus of less than 100 MPa in micro-area elasticity measurements, it is possible to measure the size of the island phases derived from the styrene-based elastomers by observing with a threshold elastic modulus of 100 MPa.
• the insulating layer 130 has, in the elastic modulus distribution obtained by micro-area elasticity measurement using an SPM at 25°C, a sea phase having an elastic modulus of more than 100 MPa, a first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less, and a second island phase having an elastic modulus of 100 MPa or less and a size of more than 100 nm and 2000 nm or less.
• the resin composition constituting the insulating layer 130 satisfies, for example, the following requirements, so that the above-mentioned phase structure can be stably formed in the insulating layer 130.
• the resin composition contains a propylene-based resin and two styrene-based elastomers as low-crystalline resins.
• C The new mixing method described below is applied.
• phase structure of the insulating layer 130 of this embodiment will be described with reference to Figures 2 to 5.
• the phase structure of the insulating layer 130 of this embodiment will be compared with that of a reference example when the resin composition constituting the insulating layer 130 contains propylene units, butene units, and styrene units.
• Figures 2 to 5 show the elastic modulus distribution obtained by micro-area elasticity measurement using an SPM at 25°C.
• the gray areas indicate areas having an elastic modulus below the threshold value indicated at the top of each figure.
• Reference Example 1 Co-continuous structure
• the phase structure of the insulating layer 130 is the cocontinuous structure shown in Figure 3.
• the co-continuous structure of Reference Example 1 has, for example, a first continuous phase CP1 having an elastic modulus of more than 200 MPa and a second continuous phase CP2 having an elastic modulus of less than 200 MPa.
• the first continuous phase CP1 is derived from a propylene-based resin.
• the second continuous phase CP2 is mainly derived from a styrene-based elastomer as a low-crystalline resin.
• the first continuous phase CP1 and the second continuous phase CP2 are each long and continuous.
• the length between both ends of the second continuous phase CP2 is, for example, 5 ⁇ m or more.
• the "length between both ends of the second continuous phase CP2" refers to the length of a straight line connecting the first end of the second continuous phase CP2 to the second end, which is the furthest from the first end.
• the phase structure of the insulating layer 130 is a co-continuous structure as described above, when minute foreign matter or microvoids are mixed into the insulating layer 130, the minute foreign matter or microvoids may be incorporated into the flexible second continuous phase CP2 containing the styrene-based elastomer. In this case, when water trees are generated starting from the minute foreign matter or microvoids, the water trees are likely to propagate along the second continuous phase CP2 of the co-continuous structure.
• the phase structure of the insulating layer 130 is, for example, a sea-island structure having only the micro-island phase SIP shown in Figure 4.
• the sea-island structure of Reference Example 2 has, for example, a sea phase SP having an elastic modulus of more than 100 MPa and a microisland phase SIP having an elastic modulus of less than 100 MPa.
• the sea phase SP is derived from a propylene-based resin.
• the microisland phase SIP is derived from a styrene-based elastomer as a low-crystalline resin.
• the size of the microisland phase SIP is, for example, 100 nm or less.
• phase structure of the insulating layer 130 is a sea-island structure having only the micro-island phase SIP as described above, coarse crystals grow in the sea phase SP containing the propylene-based resin. Therefore, when minute foreign matter or minute voids are mixed into the insulating layer 130, water trees tend to propagate at the interface of the coarse crystals.
• Reference Example 3 Sea-island structure having only coarse island phases
• the phase structure of the insulating layer 130 is, for example, a sea-island structure having only the coarse island phase LIP shown in Figure 5.
• the sea-island structure of Reference Example 3 has, for example, a sea phase SP having an elastic modulus of more than 100 MPa and a fine island phase SIP having an elastic modulus of less than 100 MPa.
• the sea phase SP is derived from a propylene-based resin.
• the coarse island phase LIP is derived from a styrene-based elastomer as a low-crystalline resin.
• the size of the coarse island phase LIP is, for example, more than 100 nm.
• phase structure of the insulating layer 130 is a sea-island structure having only coarse island phases LIP as described above, the elasticity of the sea phase SP containing the propylene-based resin remains high. Therefore, when the power cable 100 is bent, cracks occur in the sea phase SP, and the crystals separate from each other. This may result in a decrease in the insulating properties of the insulating layer 130.
• the phase structure of the insulating layer 130 will be a sea-island structure similar to that of Reference Example 3.
• the resin composition constituting the insulating layer 130 satisfies, for example, the above-mentioned requirements (A), (B), and (C), and the phase structure of the insulating layer 130 becomes, for example, a sea-island structure as shown in FIG. 2 .
• the resin composition (A) contains a propylene-based resin and two styrene-based elastomers as low-crystalline resins, making it possible to form two island phases (first island phase IP1 and second island phase IP2) that are different in size.
• the compatibility of each styrene-based elastomer with the propylene-based resin is ensured, and excessive aggregation of each styrene-based elastomer can be suppressed.
• the sea-island structure of this embodiment includes, for example, a sea phase SP, a first island phase IP1, and a second island phase IP2.
• Marine phase SP has an elastic modulus of over 100 MPa. Marine phase SP is derived from propylene-based resin. Marine phase SP is spread throughout the entire elastic modulus distribution.
• the first island phase IP1 has an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less.
• the "size of the first island phase IP1" refers to the maximum length of a straight line connecting the first end of the first island phase IP1 to the second end, which is the furthest end.
• the first island phase IP1 is, for example, mainly derived from the first styrene-based elastomer. That is, the first island phase IP1 contains at least styrene units. The styrene units in the first island phase IP1 can be observed with a transmission electron microscope (TEM) by staining a sample taken from the insulating layer 130 with a heavy metal.
• TEM transmission electron microscope
• the second island phase IP2 has an elastic modulus of 100 MPa or less and a size of more than 100 nm and less than 2000 nm.
• the method for measuring the "size of the second island phase IP2" is the same as the method for measuring the "size of the first island phase IP1" described above.
• the second island phase IP2 is derived mainly from the second styrene-based elastomer. That is, the second island phase IP2 contains at least styrene units. The styrene units in the second island phase IP2 can be observed by TEM using a method similar to that described for the first island phase IP1.
• the ratio of the total area of the first island phase IP1 and the second island phase IP2 to the unit area of the elastic modulus distribution (hereinafter also referred to as the "island phase total area ratio”) is, for example, 10% or more and 70% or less.
• the island phase total area ratio 10% or more, it is possible to suppress the formation of coarse crystals of the sea phase SP. This makes it possible to make the insulating layer 130 flexible.
• the island phase total area ratio 70% or less it is possible to ensure the insulating properties of the sea phase SP.
• the ratio of the area of the first island phase IP1 to the unit area of the elastic modulus distribution (hereinafter also referred to as the "first island phase area ratio”) is, for example, 1% or more and 40% or less.
• the first island phase area ratio is, for example, 1% or more and 40% or less.
• the ratio of the area of the second island phase IP2 to the unit area of the elastic modulus distribution (hereinafter also referred to as the "second island phase area ratio") is, for example, 5% or more and 30% or less.
• the second island phase area ratio is, for example, 5% or more and 30% or less.
• the diameter of the circular island phase free region (region within the dashed line in the figure) that does not contain the first island phase IP1 and the second island phase IP2 is, for example, 5 nm or more and 400 nm or less.
• the diameter of the island phase free region is 5 nm or more, the insulating properties of the sea phase SP can be ensured.
• the diameter of the island phase free region is 400 nm or less, the formation of coarse crystals of the sea phase SP can be suppressed. This makes it possible to make the insulating layer 130 flexible.
• the insulating layer 130 since the insulating layer 130 has the above-mentioned sea-island structure, it is possible to suppress the occurrence of water trees originating from minute foreign objects or minute voids in the insulating layer 130 while ensuring the bending resistance of the insulating layer 130.
• the inventors evaluated the water tree resistance of the insulating layer 130 by performing a water tree test (hereinafter also referred to as a "water needle test") using a water needle that simulates a micro-foreign object or micro-void that has been trapped in the insulating layer 130.
• a hexahedral block IB is extracted from the insulating layer 130. After the block IB is extracted, a flat electrode FE is provided on the first surface S1 of the block IB.
• a needle-shaped gap is formed along the normal direction from the second surface S2 opposite the first surface S1 of the block IB. At this time, the needle-shaped gap is formed at a position facing the first surface S1 of the block IB with a distance d of 1 mm, so that the tip has a radius of curvature of 10 ⁇ m.
• the tip of the water needle WN is sharpened to simulate a minute foreign object or a minute void that has been introduced into the insulating layer 130.
• a high electric field is applied locally near the tip of the water needle WN. Therefore, the water needle test of this embodiment makes it much easier for water trees to occur compared to conventional water tree tests in which an electric field is applied evenly to the insulating layer.
• the maximum length of the water tree in the insulating layer 130 that occurs in the water needle test is, for example, 200 ⁇ m or less. In other words, even in tests in which water treeing is likely to occur as described above, the maximum length of the water tree in the insulating layer 130 can be shortened.
• the insulating layer 130 has the above-mentioned sea-island structure, and thus has resistance to a predetermined bending test.
• the "bending test” referred to here includes, for example, a first step of bending the power cable 10 so that the bending ratio (hereinafter also simply referred to as the "bending ratio") of the bending radius of the power cable 10 to the outer diameter of the insulating layer 130 is 7 or less, and a second step of bending the power cable 10 in the direction opposite to the bending direction of the first step at the same bending ratio as the bending ratio of the first step.
• the bending ratio hereinafter also simply referred to as the "bending ratio”
• the bending ratio of the bending radius of the power cable to the outer diameter of the insulation layer is set to, for example, approximately 20.
• the bending ratio in the bending test of this embodiment is smaller than the bending ratio in the bending test of the normal cable standard. Therefore, in the bending test of this embodiment, the bending stress applied to the insulating layer 130 is strong. As a result, defects are more likely to occur in the insulating layer 130 due to the bending test.
• the AC breakdown field strength remains high even after the bending test described above.
• the AC breakdown field strength of the insulating layer 130 at room temperature (e.g., 25°C) after the above-mentioned bending test may be, for example, 50 kV/mm or more, or 60 kV/mm or more.
• the "AC breakdown field strength" referred to here is measured at room temperature (25°C) by applying an AC voltage of commercial frequency (e.g., 60 Hz) to a 0.4 mm thick sample at 10 kV for 10 minutes, and then increasing the voltage by 1 kV in increments and applying the voltage for 10 minutes in a repeated cycle.
• an AC voltage of commercial frequency e.g. 60 Hz
• the AC breakdown field strength of the insulating layer 130 at room temperature (e.g., 25°C) before the bending test described above is also naturally, for example, 50 kV/mm or more, or 60 kV/mm or more.
• the power cable manufacturing method of this embodiment includes, for example, a conductor preparation process S120, a resin composition preparation process S140, a cable core formation process 300, a shielding layer formation process S400, and a sheath formation process S500.
• a conductor 110 is prepared by twisting together a plurality of conductor core wires.
• the resin composition is mixed so that the elastic modulus distribution obtained by measuring the elasticity of a micro-area using an SPM at 25°C contains a sea phase SP having an elastic modulus of more than 100 MPa, a first island phase IP1 having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less, and a second island phase IP2 having an elastic modulus of 100 MPa or less and a size of more than 100 nm and 2000 nm or less.
• a propylene-based resin is mixed with two styrene-based elastomers as low-crystalline resins. That is, a first styrene-based elastomer containing propylene units or butene units and styrene units, and a second styrene-based elastomer containing propylene units or butene units and styrene units and different from the first styrene-based elastomer are mixed into the propylene-based resin.
• the propylene-based resin when the resin composition contains propylene units, butene units, and styrene units, the propylene-based resin is mixed with the first styrene-based elastomer and the second styrene-based elastomer as low crystalline resins so that the content of butene units in the resin composition is 9% by mass or more and less than 20% by mass.
• the resin composition is mixed using, for example, a twin-screw mixer.
• a twin-screw mixer By using a twin-screw mixer, the resin composition can be mixed with a higher shear force than with a kneader.
• the resin composition is mixed at a temperature of, for example, less than 220°C. If the resin composition is mixed at a temperature of 220°C or higher, the viscosity of the resin composition decreases due to the high temperature. This makes it difficult to apply a predetermined shear force to the resin composition.
• the mixing temperature is set to less than 220°C, so that the viscosity of the resin composition can be maintained high. This makes it possible to apply a high shear force to the resin composition. As a result, the above-mentioned sea-island structure can be stably formed.
• the installation conditions of the kneading disks in the twin-screw mixer are adjusted, and the rotation speed of the twin-screw mixer is adjusted.
• the shear force in the twin-screw mixer can be changed, for example, by changing the kneading disks or changing the rotation speed.
• the number of kneading disks installed is set to 2 to 5, and each rotation speed is set to 20 rpm or more and less than 200 rpm.
• the filling rate of the resin composition in the cylinder of the twin-screw mixer (ratio of the volume of the resin composition to the cylinder volume) is increased to ensure the residence time of the resin composition in the cylinder.
• the filling rate of the resin composition in the cylinder of the twin-screw mixer is set to 20% or less to reduce the load on the equipment (torque applied to the screw) and ensure the discharge volume.
• the filling rate of the resin composition in the cylinder of the twin-screw mixer is set to more than 20%, so that the residence time of the resin composition in the cylinder can be sufficiently ensured. This allows the above-mentioned sea-island structure to be stably formed.
• the above-mentioned sea-island structure can be formed in the resin composition.
• the resin composition After mixing the resin composition, the resin composition is granulated using an extruder. This forms the resin composition in pellet form that will form the insulating layer 130. Note that the steps from mixing to granulation may be performed in one go.
• the insulating layer 130 is formed so as to cover the outer periphery of the conductor 110 using the above-mentioned resin composition.
• the insulating layer 130 is extruded using the resin composition described above, so that the above-mentioned sea-island structure is also formed in the insulating layer 130.
• a three-layer co-extruder is used to simultaneously form the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140.
• the composition for the inner semiconductive layer is fed into extruder A, which is one of the three-layer co-extruders that forms the inner semiconductive layer 120.
• extruder B which forms insulating layer 130.
• the set temperature of extruder B is set, for example, to a temperature 10°C to 50°C higher than the desired melting point.
• the set temperature is adjusted appropriately based on the linear speed and extrusion pressure.
• extruder C which forms the outer semiconductive layer 140, is fed a composition for the outer semiconductive layer, which contains the same materials as the resin composition for the inner semiconductive layer fed into extruder A.
• extrusions from extruders A to C are guided to a common head, where the inner semiconductive layer 120, insulating layer 130, and outer semiconductive layer 140 are simultaneously extruded from the inside to the outside around the conductor 110. This forms the extrusion material that will become the cable core.
• the extruded material is then cooled, for example with water.
• a shielding layer 150 is formed on the outside of the outer semiconducting layer 140, for example by wrapping copper tape around it.
• the insulating layer 130 has a sea phase SP with an elastic modulus of more than 100 MPa, a first island phase IP1 with an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less, and a second island phase IP2 with an elastic modulus of 100 MPa or less and a size of more than 100 nm and 2000 nm or less in the elastic modulus distribution obtained by micro-area elasticity measurement using an SPM at 25°C.
• the propylene-based resin can be efficiently made flexible, and the insulating layer 130 as a whole can be made flexible. This makes it possible to suppress cracks in the sea phase SP even if bending stress is applied to the insulating layer 130 when the power cable 10 is bent. As a result, the insulating properties of the insulating layer 130 can be ensured even if the power cable 10 is bent.
• the maximum length of the water tree in the insulating layer 130 is 200 ⁇ m or less. In other words, even in a test in which water treeing is likely to occur, the maximum length of the water tree in the insulating layer 130 can be shortened. As a result, even if a micro-foreign object or a micro-void is mixed into the actual insulating layer 130, excessive propagation of the water tree can be suppressed.
• the AC breakdown field strength of the insulating layer 130 at room temperature is 50 kV/mm or more.
• the AC breakdown field strength can be maintained high.
• the power cable 10 may have a simple water-shielding layer.
• the simple water-shielding layer is made of, for example, a metal laminate tape.
• the metal laminate tape has, for example, a metal layer made of aluminum or copper, and an adhesive layer provided on one or both sides of the metal layer.
• the metal laminate tape is, for example, wrapped vertically around the outer periphery of the cable core (outer periphery than the outer semiconductive layer).
• the water-shielding layer may be provided outside the shielding layer, or may also serve as the shielding layer. With such a configuration, the cost of the power cable 10 can be reduced.
• the power cable 10 is configured to be laid on land, underwater, or on the bottom of the water, but the present disclosure is not limited to this case.
• the power cable 10 may be configured as a so-called overhead electric wire (overhead insulated electric wire).
• each layer was extruded one at a time.
• SEBS2 Hydrogenated styrene butadiene styrene block copolymer
• SEBS3 Hydrogenated styrene butadiene styrene block copolymer
• VLDPE Very low density polyethylene
• Butene unit content 21% by mass
• MFR 6.7g/10min (230°C, 2.16kg)
• Hardness A86
• Melting point 66° C.
• Heat of fusion 10 J/g
• samples A1 to A3 In samples A1 to A3, r-PP, SEBS1, and SEBS2 were mixed so that the content of each monomer unit was the value shown in Table 1 below. At this time, a twin-screw mixer was used to mix the resins at a temperature of 200°C. Kneading disks were installed at three locations, each rotating at 100 rpm, and the filling rate of the resin composition in the cylinder was 50%. This method is also referred to as the "new mixing method" below.
• samples B1 to B7 In samples B1 to B7, the resin compositions were mixed in the same manner as in sample A1, except that the low crystalline resin used and the content of each monomer unit were different.
• sample B8 For sample B8, the resin composition was mixed in the same manner as for sample A1, except that the mixing method was different.
• the resin composition was mixed using a kneader at a temperature of 220°C.
• the extrudates from the extruders A to C were guided to a common head, and the inner semiconductive layer, the insulating layer, and the outer semiconductive layer were simultaneously extruded from the inside to the outside around the conductor. At this time, the thicknesses of the inner semiconductive layer, the insulating layer, and the outer semiconductive layer were set to 0.5 mm, 3.5 mm, and 0.5 mm, respectively. After the extrusion process, the extruded material was cooled. As a result, the power cables of samples A1 to A3 and B1 to B8 were manufactured.
• S.P.M. Micro-area elasticity measurements were performed using an SPM. In the measurements, a sheet having a thickness of 1 mm cut from the center of the thickness direction of the insulating layer of each sample was used. In the micro-area elasticity measurements, the elastic modulus was measured at 25° C. under the condition that a cantilever having a tip made of silicon (single crystal) and a curvature radius of 7 nm was tapped 16,000 times within a 1 ⁇ m square range of the sample.
• an elastic modulus distribution with an elastic modulus threshold of 200 MPa and an elastic modulus distribution with an elastic modulus threshold of 100 MPa were obtained.
• the field of view was 1.0 ⁇ m square.
• phase structure in which each phase is continuous without being separated was determined to be a "co-continuous structure (B1)," and the phase structure in which each phase is separated was determined to be an "island-sea structure.”
• the size of the island phase was measured in the elastic modulus distribution with a threshold elastic modulus of 100 MPa.
• a sea-island structure comprising a first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm to 100 nm, and a second island phase having an elastic modulus of 100 MPa or less and a size of more than 100 nm to 2000 nm, was evaluated as "A” as the phase structure of the present disclosure.
• a sea-island structure comprising only the first island phase was evaluated as "B2.”
• a sea-island structure comprising only the second island phase was evaluated as "B3.”
• the block was dried and then boiled in an aqueous solution of methylene blue for staining. Once the block was stained, the block was sliced near the tip of the water needle to form a slice for observation. The slice for observation was then observed under an optical microscope to confirm that water trees had developed near the tip of the water needle in the slice for observation.
• an AC voltage of commercial frequency 60 Hz was applied to the insulating layer sheet at 10 kV for 10 minutes, and then the voltage was increased by 1 kV and applied for 10 minutes repeatedly.
• the electric field strength was measured when the insulating layer sheet experienced dielectric breakdown. As a result, AC breakdown strength of 50 kV/mm or more was rated as good (A), and AC breakdown strength of less than 50 kV/mm was rated as poor.
• the power cable was bent so that the bending ratio of the bending radius of the power cable to the outer diameter of the insulation layer (outer diameter of the power cable) was 7 or less.
• the power cable was bent in the opposite direction to the bending direction in the first step, at the same bending ratio as in the first step.
• sample collection and AC breakdown test For each sample of power cable after the bending test, an insulating layer sheet was taken in the same manner as in the above-mentioned evaluation immediately after manufacture. For the insulating layer sheet taken after the bending test, an AC breakdown test was performed in the same manner as in the above-mentioned evaluation immediately after manufacture.
• samples B1, B5 and B6 In the samples B1 and B5, the insulating layer contained 20 mass % or more of butene units. In the sample B6, the insulating layer did not contain styrene units and contained 20 mass % or more of butene units.
• the phase structure of the insulating layer in samples B1, B5, and B6 was a bicontinuous structure B1.
• phase structure of the insulating layer was the co-continuous structure B1, so when water trees occurred in the water needle test, they were thought to have propagated along the flexible phase of the co-continuous structure.
• sample B2 In sample B2, the insulating layer contained only SEBS1 having a high content of butene units as the low-crystalline resin, and the content of butene units in the insulating layer 130 was less than 20 mass %.
• the phase structure of the insulating layer in sample B2 was a sea-island structure B2 that had minute first island phases but no coarse second island phases.
• phase structure of the insulating layer was a sea-island structure B2 that had only minute first island phases, and so coarse crystals grew in the sea phase. For this reason, it is believed that when water treeing occurred in the water needle test, the water treeing propagated at the interface of the coarse crystals.
• samples B3 and B4 In samples B3 and B4, the insulating layer contained only SEBS2 having a small content of butene units as the low-crystalline resin, and the content of butene units in the insulating layer was less than 9 mass %.
• the phase structure of the insulating layer was a sea-island structure B3 that had coarse second island phases but no minute first island phases.
• samples B3 and B4 the maximum length of the water tree in the water needle test was 200 ⁇ m or less.
• the AC breakdown field strength after the bending test was less than 50 kV/mm.
• sample B7 In sample B7, the insulating layer contained only VLDPE containing no styrene units as the low crystallinity resin.
• sample B7 had a sea-island structure B3, which consisted only of coarse island phases, similar to samples B3 and B4.
• sample B8 In sample B8, the composition of the monomer units of the insulating layer was the same as in sample A1, but the novel mixing method described above was not applied.
• phase structure of the insulating layer in sample B8 was a sea-island structure B3, which only contained coarse island phases, similar to samples B3 and B4.
• the maximum length of the water tree in the water needle test was 200 ⁇ m or less.
• the AC breakdown field strength after the bending test was less than 50 kV/mm.
• sample B8 the new mixing method was not applied, so the shear force during mixing was insufficient and the resin composition was not mixed sufficiently. For this reason, in sample B8, the sea-island structure A as in sample A1 was not formed. As a result, cracks occurred in the sea phase during the bending test, which is thought to have reduced the insulation properties after the bending test.
• samples A1 to A3 In the samples A1 to A3, the insulating layer contained SEBS1 and SEBS2 as the low crystalline resin. In the samples A1 to A3, the content of butene units in the insulating layer was 9 mass% or more and less than 20 mass%. Furthermore, a new mixing method was applied to the samples A1 to A3.
• the phase structure of the insulating layer in samples A1 to A3 was a sea-island structure A that had minute first island phases and coarse second island phases.
• the phase structure of the insulating layer was made to be an island-sea structure A, which had minute first island phases and coarse second island phases, and it was confirmed that the insulating properties of the insulating layer could be ensured even when the power cable was bent.
• phase structure of the insulating layer into a sea-island structure A, which has a minute first island phase and a coarse second island phase, it was possible to suppress excessive propagation of water trees in the insulating layer from the tip of a water needle simulating a minute foreign object or minute void.
• a resin composition constituting an insulating layer of a power cable Contains propylene units and styrene units, The resin composition comprises In the elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25° C., a sea phase having an elastic modulus of more than 100 MPa; A first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; A second island phase having an elastic modulus of 100 MPa or less and a size of more than 100 nm and less than 2000 nm; A resin composition comprising:
• the insulating layer is In the elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25° C., a sea phase having an elastic modulus of more than 100 MPa; A first island phase having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; A second island phase having an elastic modulus of 100 MPa or less and a size of more than 100 nm and not more than 2000 nm; A power cable.
• the maximum length of the water tree in the insulating layer generated by the water needle test is 200 ⁇ m or less.
• the water needle test is providing a plate electrode on a first surface of the block taken from the insulating layer; forming a needle-shaped gap having a tip with a radius of curvature of 10 ⁇ m at a position facing the first surface with a gap of 1 mm therebetween; forming a water needle in the block by injecting artificial seawater having a salinity of 3.8% by mass into the gap; applying an AC voltage of 1000 Hz and 4 kV between the plate electrode and the tip of the water needle for 200 hours; 3.
• the power cable according to claim 2 is provided a plate electrode on a first surface of the block taken from the insulating layer; forming a needle-shaped gap having a tip with a radius of curvature of 10 ⁇ m at a position facing the first surface with a gap of 1 mm therebetween; forming a water needle in the block by injecting artificial seawater having a salinity
• the AC breakdown field strength of the insulating layer at 25° C. after a predetermined bending test is 50 kV/mm or more.
• the bending test is a first step of bending the power cable so that a bending ratio of a bending radius of the power cable to an outer diameter of the insulating layer is 7 or less; a second step of bending the power cable in a direction opposite to the bending direction of the first step at a bending ratio equal to the bending ratio of the first step;
• the insulating layer further comprises a butene unit, The power cable according to any one of Supplementary Note 2 to Supplementary Note 4, wherein the content of the butene unit in the insulating layer is 9 mass% or more and less than 20 mass%.
• the insulating layer is A propylene-based resin, a first styrenic elastomer comprising propylene units or butene units and styrene units; a second styrenic elastomer containing propylene units or butene units and styrene units and different from the first styrenic elastomer;
• the power cable according to any one of Supplementary Note 2 to Supplementary Note 6,

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