Resin composition and power cable

Abstract

The resin composition is a resin composition for forming an insulating layer of a power cable, and contains propylene units and styrene units. In the elastic modulus distribution obtained by micro-area elasticity measurement using a scanning probe microscope at 25°C, the resin composition comprises a sea phase having an elastic modulus of more than 100 MPa, first island phases having an elastic modulus of 100 MPa or less and a size of 0.1 to 100 nm, and second island phases having an elastic modulus of 100 MPa or less and a size of more than 100 nm and 2000 nm or less.

Description

[Technical Field]

[0001] The present disclosure relates to a resin composition and a power cable. [Background technology]

[0002] Cross-linked polyethylene has excellent insulating properties and has therefore been widely used as a resin component constituting the insulating layer in power cables and the like (for example, Patent Document 1). [Prior art documents] [Patent documents]

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

[0004] According to one aspect of the present disclosure, A resin composition constituting an insulating layer of a power cable, Contains propylene units and styrene units, The resin composition comprises The elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C was a sea phase having an elastic modulus of more than 100 MPa; First island phases having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; second island phases having an elastic modulus of 100 MPa or less and a size of more than 100 nm and not more than 2000 nm; Equipped with A resin composition is provided. [Brief explanation of the drawings]

[0005] [Figure 1] 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. [Figure 2]FIG. 2 shows the elastic modulus distribution of sample A1 obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C. [Figure 3] 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. [Figure 4] FIG. 4 shows the elastic modulus distribution of sample B2 obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C. [Figure 5] FIG. 5 shows the elastic modulus distribution of sample B3 obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C. [Figure 6] FIG. 6 is a schematic cross-sectional view showing a water needle test. DETAILED DESCRIPTION OF THE INVENTION

[0006] [Problem to be solved by this disclosure] The present inventors have focused on propylene-based resin (polypropylene) as a resin component constituting the insulating layer, and have conducted extensive research to improve the properties of power cables.

[0007] An object of the present disclosure is to suppress the occurrence of water trees originating from minute foreign particles or minute voids in an insulating layer while ensuring bending resistance of the insulating layer.

[0008] [Effects of this disclosure] According to the present disclosure, it is possible to suppress the occurrence of water trees originating from minute foreign particles or minute voids in the insulating layer while ensuring the bending resistance of the insulating layer.

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

[0010] The propylene-based resin itself tends to have a large amount of crystals and to be hard.

[0011] Therefore, the inventors considered mixing a propylene-based resin with a low-crystalline resin as the resin component constituting the insulating layer in order to control the crystallinity of the propylene-based resin and improve the flexibility of the insulating layer of the power cable.

[0012] However, the inventors have discovered the following new problem in an insulating layer containing a propylene-based resin and a low-crystalline resin.

[0013] (Water trees originating from minute foreign particles or minute voids) In conventional water tree tests, an electric field is applied to the insulation layer of a power cable while the cable is immersed in water. In this water tree test, water pressure is applied uniformly to the insulation layer, and an electric field is applied uniformly.

[0014] However, it has been found that even in power cables in which water trees did not occur in the above-mentioned water tree test, when tiny foreign objects or tiny voids are mixed into the insulation layer, the water tree resistance of the insulation layer may decrease depending on the phase structure of the insulation layer as follows.

[0015] (i) When the insulating layer has a bicontinuous phase structure, if minute foreign particles or microvoids are mixed into the insulating layer, the minute foreign particles or microvoids may be incorporated into the relatively flexible phase (hereinafter also referred to as the flexible phase) containing the low-crystalline resin in the bicontinuous structure. In this case, when water trees are generated starting from the minute foreign particles or microvoids, the water trees are likely to propagate along the flexible phase of the bicontinuous structure.

[0016] (ii) When the insulating layer has a sea-island structure consisting of a sea phase and a soft phase consisting of only small islands with a size of 100 nm or less, coarse crystals grow in the sea phase containing the propylene-based resin. Therefore, when minute foreign matter or minute voids are mixed into the insulating layer, water trees tend to propagate at the interfaces of the coarse crystals.

[0017] (Deterioration of insulation when bent) Conventionally, the insulating properties of an insulating layer have been evaluated by, for example, measuring the AC breakdown field strength, which has been performed on a sheet taken from the insulating layer of a power cable immediately after manufacture, without applying any particular stress.

[0018] However, it was found that even for power cables with a high AC breakdown field strength, the AC breakdown field strength decreases when the power cable is bent. Specifically, it was found that the bending resistance of the insulating layer can decrease depending on the following phase structure of the insulating layer.

[0019] (iii) When the insulating layer has a sea-island structure consisting of a sea phase and a flexible phase consisting of large island phases with a size of more than 100 nm, the elasticity of the sea phase containing the propylene-based resin remains high. Therefore, when the power cable is bent, cracks may occur in the sea phase or the crystals may separate. This may result in a decrease in the insulating properties of the insulating layer.

[0020] Therefore, the inventors conducted extensive research into the above-mentioned new problem, and as a result, by optimizing the phase structure of the insulating layer, they succeeded in suppressing the occurrence of water trees originating from minute foreign particles or minute voids in the insulating layer while ensuring the bending resistance of the insulating layer.

[0021] The present disclosure is based on the above-mentioned findings discovered by the inventors.

[0022] <Embodiments of the present disclosure> Next, embodiments of the present disclosure will be listed and described.

[0023] [1] A resin composition according to one embodiment of the present disclosure comprises: A resin composition constituting an insulating layer of a power cable, Contains propylene units and styrene units, The resin composition comprises The elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C was a sea phase having an elastic modulus of more than 100 MPa; First island phases having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; second island phases having an elastic modulus of 100 MPa or less and a size of more than 100 nm and not more than 2000 nm; Equipped with. According to 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 minute foreign objects or minute voids in the insulating layer.

[0024] [2] A power cable according to one embodiment of the present disclosure includes: A conductor; an insulating layer provided to cover the outer periphery of the conductor; Equipped with the insulating layer includes a propylene unit and a styrene unit; The insulating layer is The elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C was a sea phase having an elastic modulus of more than 100 MPa; First island phases having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; second island phases having an elastic modulus of 100 MPa or less and a size of more than 100 nm and not more than 2000 nm; Equipped with. According to 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 minute foreign objects or minute voids in the insulating layer.

[0025] [3] In the power cable according to [2] above, The maximum length of the water tree in the insulation layer generated by the water needle test is 200 μm or less. Here, the water needle test is providing a plate electrode on a first surface of the block taken from the insulating layer; forming needle-shaped voids at positions facing each other at a distance of 1 mm from the first surface, the needle-shaped voids having tips with a curvature radius of 10 μm; A step of forming water needles in the block by injecting artificial seawater having a salinity of 3.8% by mass into the void; applying an AC voltage of 1000 Hz and 4 kV between the plate electrode and the tip of the water needle for 200 hours; Includes. With this configuration, even if minute foreign matter or minute voids are mixed into the insulating layer, excessive propagation of water trees can be suppressed.

[0026] [4] In the power cable according to the above [2] or [3], The insulating layer has an AC breakdown field strength of 50 kV / mm or more at 25°C after a predetermined bending test. Here, 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 the same bending ratio as the bending ratio of the first step; Includes. With this configuration, it is possible to maintain high insulation properties of the insulating layer even when the power cable is bent.

[0027] [5] The power cable according to any one of [2] to [4] above, the insulating layer further comprises butene units; The content of the butene units in the insulating layer is 9% by mass or more and less than 20% by mass. According to this configuration, the above-mentioned sea-island structure can be stably formed.

[0028] [6] The power cable according to any one of [2] to [5] above, The insulating layer is A propylene-based resin, a first styrenic elastomer containing 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; It has. According to this configuration, two island phases having different sizes can be stably formed in the insulating layer.

[0029] [Details of the embodiments of the present disclosure] Next, one embodiment of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0030] <One embodiment of the present disclosure> (1) Resin composition The resin composition of this embodiment constitutes, for example, the insulating layer 130 of the power cable 10 described below.

[0031] The resin composition of this embodiment contains, for example, a propylene-based resin and a low-crystalline resin as resin components. The term "resin component" as used herein refers to a resin material (polymer) that constitutes the main component of the resin composition. The term "main component" refers to the component with the highest content.

[0032] By mixing the propylene-based resin with the low-crystalline resin, excessive crystal growth of the propylene-based resin can be inhibited, and the flexibility of the insulating layer 130 can be improved.

[0033] (Propylene-based resin) The propylene-based resin of this 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).

[0034] By analyzing the resin composition of this embodiment using a nuclear magnetic resonance (NMR) device, for example, if the propylene-based resin is random PP or block PP, propylene units and ethylene units are detected, and if the low-crystalline resin is homo PP, propylene units are detected.

[0035] From the viewpoint of obtaining high insulating properties in the insulating layer 130, the propylene-based resin may be random PP. Homo PP has a higher crystalline content than random PP and can obtain high insulating properties. However, an insulating layer 130 containing homo PP may cause cracks within or between crystals. For this reason, homo PP may not obtain the insulating properties it inherently possesses. In contrast, random PP contains ethylene units, resulting in a low crystalline content. However, an insulating layer 130 containing random PP is less likely to crack due to coarse crystallization. As a result, random PP can obtain higher insulating properties than homo PP.

[0036] When the propylene-based resin is random 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. By setting the ethylene unit content to 0.5% by mass or more, it is possible to suppress the growth of coarse spherulites. On the other hand, by setting the ethylene unit content to 15% by mass or less, it is possible to suppress a decrease in melting point and to stably realize use in a non-crosslinked or slightly crosslinked state.

[0037] In this embodiment, the stereoregularity of the polypropylene is not particularly limited, but may be, for example, isotactic. Isotactic polypropylene is polymerized using a Ziegler-Natta catalyst and is a versatile material. The isotactic stereoregularity can suppress a decrease in melting point in a composition in which a propylene-based resin and a low-crystalline resin are mixed. This allows the high insulating properties inherent to the propylene-based resin to be obtained.

[0038] In this embodiment, the storage modulus of the propylene-based resin alone is, for example, 300 MPa or more and 2500 MPa or less.

[0039] The "storage modulus" referred to here is measured by dynamic mechanical analysis (DMA) in accordance with JIS K7244-4:1999.

[0040] 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℃ / min The above-mentioned "storage modulus" is a value measured at 25°C by the dynamic viscoelasticity measurement.

[0041] By setting the storage modulus of the propylene-based resin alone to 300 MPa or more, it is possible to ensure the rigidity of the insulating layer 130. On the other hand, by setting the storage modulus of the propylene-based resin alone to 2500 MPa or less, it is possible to ensure the flexibility of the insulating layer 130.

[0042] 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 refers to a value measured in accordance with JIS K7210 at a temperature of 230°C and a load of 2.16 kg. By setting the MFR of the propylene-based resin within the above range, a sea-island structure, which will be described later, can be easily formed in the insulating layer 130.

[0043] 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, which makes it possible to easily form a sea-island structure, which will be described later, when the propylene-based resin is mixed with the low-crystalline resin.

[0044] (Low crystalline resin) The low-crystalline resin of this embodiment is configured to control the crystallinity of the propylene-based resin and impart flexibility to the insulating layer. For example, 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.

[0045] The low crystalline resin of this embodiment is, for example, a styrene-based elastomer, which can form island phases, which will be described later.

[0046] The styrene elastomer as the low-crystalline resin contains styrene units as hard segments and at least propylene units or butene units as soft segments. The styrene elastomer may contain at least one monomer unit selected from ethylene units and isoprene units as other soft segments.

[0047] Examples of styrene 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, Two or more of these may be used in combination.

[0048] "Hydrogenated" here means that hydrogen has been added to the double bond. For example, "hydrogenated styrene butadiene styrene block copolymer" means a polymer in which hydrogen has been added to the double bond of a styrene butadiene styrene block copolymer. Note that no hydrogen has been added to the double bond of the aromatic ring of styrene. "Hydrogenated styrene butadiene styrene block copolymer" can be rephrased as styrene ethylene butene styrene block copolymer (SEBS).

[0049] Among styrene-based elastomers, hydrogenated materials that do not contain double bonds in the chemical structure excluding aromatic rings may be used. When non-hydrogenated materials are used, the resin components may be thermally degraded during molding of the resin composition, which may result in a deterioration in the properties of the resulting molded article. In contrast, the use of hydrogenated materials can improve resistance to thermal degradation, thereby allowing the molded article to maintain its properties at a higher level.

[0050] In this embodiment, 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.

[0051] The content of propylene units or butene units in the second styrene elastomer is, for example, lower than the content of propylene units or butene units in the first styrene elastomer. Since the second styrene elastomer contains relatively few propylene units or butene units, which are compatible with propylene resins, the second styrene elastomer can be aggregated to form large island phases containing the second styrene elastomer (second island phases, described below). On the other hand, since the first styrene elastomer contains relatively many propylene units or butene units, the first styrene elastomer can be finely dispersed to form small island phases containing the first styrene elastomer (first island phases, described below).

[0052] Specifically, 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. By setting the content of propylene units or butene units in the first styrene-based elastomer to 40% by mass or more, the first styrene-based elastomer can be finely dispersed. On the other hand, by setting the content of propylene units or butene units in the first styrene-based elastomer to 80% by mass or less, the rigidity of the insulating layer 130 can be ensured.

[0053] On the other hand, the content of propylene units or butene units in the second styrene elastomer is, for example, 5% by mass or more and less than 40% by mass. By making the content of propylene units or butene units in the second styrene elastomer 5% by mass or more, it is possible to ensure the flexibility of the insulating layer 130. On the other hand, by making the content of propylene units or butene units in the second styrene elastomer less than 40% by mass, it is possible to aggregate the second styrene elastomer and stably form large island phases containing the second styrene elastomer (second island phases described below).

[0054] The content of styrene units in the second styrene elastomer is, for example, higher than the content of styrene units in the first styrene elastomer. This allows the compatibility of the second styrene elastomer with the propylene-based resin to be lower than that of the first styrene elastomer. As a result, the island phases containing the second styrene elastomer (the second island phases described below) can be made larger than the island phases containing the first styrene elastomer (the first island phases described below).

[0055] Specifically, the content of styrene units in the first styrene elastomer may be, for example, 20% by mass or less, or 10% by mass or less. By setting the content of styrene units in the first styrene elastomer to 20% by mass or less, the first styrene elastomer can be finely dispersed. Furthermore, by setting the content of styrene units in the first styrene elastomer to 10% by mass or less, the first styrene elastomer can be finely dispersed and the insulating layer 130 can be efficiently made flexible.

[0056] On the other hand, the content of styrene units in the second styrene elastomer is, for example, more than 20% by mass and not more than 70% by mass. By making the content of styrene units in the second styrene elastomer more than 20% by mass, the second styrene elastomer can be aggregated. On the other hand, by making the content of styrene units in the second styrene elastomer not more than 70% by mass, the flexibility of the insulating layer 130 can be ensured.

[0057] In this embodiment, 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. By setting the storage modulus of the styrene-based elastomer alone to 0.1 MPa or more, the rigidity of the insulating layer 130 can be ensured. On the other hand, by setting the storage modulus of the styrene-based elastomer alone to 400 MPa or less, the flexibility of the insulating layer 130 can be ensured.

[0058] The MFR of the styrene-based elastomer is not particularly limited, but may be, for example, 0.1 g / 10 min or more and 30.0 g / 10 min or less, or 0.1 g / 10 min or more and 10.0 g / 10 min or less. The measurement conditions for the MFR of the styrene-based elastomer here are the same as those described for the MFR of the propylene-based resin. When the MFR of the styrene-based elastomer is within the above range, the sea-island structure described below can be easily formed in the insulating layer 130.

[0059] (Monomer unit content in resin composition) 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 %.

[0060] The content of propylene units in the resin composition is, for example, more than 50% by mass and not more than 80% by mass. The propylene-based resin phase can be formed as a sea phase throughout the entire insulating layer. This makes it possible to ensure the insulating properties of the propylene-based resin as the main component while making the insulating layer 130 non-crosslinked. On the other hand, by setting the content of propylene units to not more than 80% by mass, it is possible to ensure island phases containing a low-crystalline resin in the insulating layer 130. This makes it possible to suppress excessive crystal growth of the propylene-based resin.

[0061] In this embodiment, when the resin composition contains propylene units, butene units, and styrene units, 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 a sea-island structure, which will be described later, to be stably formed in the insulating layer 130. The phase structure will be described in detail later.

[0062] In this embodiment, the content of styrene units in the resin composition is, for example, 0.1% by mass or more and less than 15% by mass. By setting the content of styrene units to 0.1% by mass or more, it is possible to stabilize the insulating properties of the insulating layer 130. On the other hand, by setting the content of styrene units to less than 15% by mass, it is possible to ensure the flexibility of the insulating layer 130.

[0063] The resin composition may contain, as a monomer unit other than the propylene unit, butene unit, and styrene unit, for example, an ethylene unit derived from a propylene-based resin or a low-crystalline resin.

[0064] In this embodiment, 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.

[0065] (Crosslinking agent) In this embodiment, the resin component that constitutes the insulating layer 130 may be non-crosslinked from the viewpoint of recycling. In this case, the resin composition may not contain a crosslinking agent.

[0066] On the other hand, the resin composition may contain a small amount of crosslinking agent to reduce the gel fraction (degree of crosslinking). Specifically, the resin composition may contain a crosslinking agent so that the crosslinking agent residue in the insulating layer 130 is less than 300 ppm. Here, "crosslinking agent residue (decomposition residue)" refers to decomposition products generated by the decomposition of the crosslinking agent due to the crosslinking reaction. When dicumyl peroxide is used as the crosslinking agent, the residue is, for example, cumyl alcohol, α-methylstyrene, etc.

[0067] (Other additives) In addition to the above-mentioned resin components, the resin composition may contain, for example, an antioxidant, a heat stabilizer, a copper inhibitor, a lubricant, and a colorant.

[0068] The resin composition may not contain a metal hydroxide, which acts as a flame retardant. Alternatively, even if the resin composition contains a metal oxide, 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 allows the insulating layer 130 to have stable insulating properties.

[0069] (2) Power cable Next, the power cable of this embodiment will be described with reference to FIG.

[0070] 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 AC or DC.

[0071] Specifically, the power cable 10 includes, for example, a conductor 110 , an inner semiconductive layer 120 , an insulating layer 130 , an outer semiconductive layer 140 , a shielding layer 150 , and a sheath 160 .

[0072] (Conductor (conductive part)) The conductor 110 is formed by twisting together a plurality of conductor core wires (conductive core wires) made of, for example, pure copper, copper alloy, aluminum, or aluminum alloy.

[0073] (internal semiconductive layer) The internal semiconductive layer 120 is provided so as to cover the outer periphery of the conductor 110. The internal semiconductive layer 120 is semiconductive and configured to suppress electric field concentration on the surface 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, or the above-mentioned low-crystalline resin, and conductive carbon black.

[0074] (insulating layer) The insulating layer 130 is provided so as to cover the outer periphery of the internal semiconductive layer 120. The insulating layer 130 is formed, for example, by extrusion molding using a resin composition as described above. The phase structure of the insulating layer 130 and the cable characteristics will be described in detail later.

[0075] (Outer semiconductive layer) The outer semiconductive layer 140 is provided so as to cover the outer periphery of the insulating layer 130. The outer semiconductive layer 140 has semiconductivity and is configured to suppress electric field concentration between the insulating layer 130 and the shielding layer 150. The outer semiconductive layer 140 is configured, for example, from the same material as the inner semiconductive layer 120.

[0076] (shielding layer) The shielding layer 150 is provided so as to cover the outer periphery of the outer semiconducting layer 140. The shielding layer 150 is formed, for example, by winding copper tape, or is formed as a wire shield in which a plurality of annealed copper wires or the like are wound. Tape made of a material such as rubberized cloth may be wound inside or outside the shielding layer 150.

[0077] (sheath) The sheath 160 is provided so as to cover the outer periphery of the shielding layer 150. The sheath 160 is made of, for example, polyvinyl chloride or polyethylene.

[0078] If the power cable 10 of this embodiment is an underwater cable or an underwater cable, it may have a metal waterproof layer such as aluminum sheath or iron wire armor outside the shielding layer 150.

[0079] On the other hand, the power cable 10 of this embodiment may not have a water-shielding layer outside the shielding layer 150. In other words, the power cable 10 of this embodiment may have a non-completely water-shielding structure.

[0080] (Specific dimensions, etc.) Although there are no particular limitations on the specific dimensions of the power cable 10, for example, the diameter of the conductor 110 is 5 mm to 60 mm, the thickness of the inner semiconductive layer 120 is 0.5 mm to 3 mm, the thickness of the insulating layer 130 is 3 mm to 35 mm, the thickness of the outer semiconductive layer 140 is 0.5 mm to 3 mm, the thickness of the shielding layer 150 is 0.1 mm to 5 mm, and 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.

[0081] (3) Phase structure of the insulating layer The inventors have discovered a phase structure of the insulating layer 130 that can solve the above-mentioned new problems by measuring the elasticity of a micro-area of ​​the insulating layer 130 that contains a resin composition.

[0082] The "micro-area elasticity measurement" referred to here is performed using a scanning probe microscope (SPM). For example, a sheet having a predetermined thickness cut from the center of the insulating layer 130 in the thickness direction is used in this measurement. In the micro-area elasticity measurement, the elastic modulus is measured under the following conditions: at 25°C, a cantilever made of silicon (single crystal) and having a tip with a radius of curvature of 1 nm or more and less than 20 nm is tapped 16,000 times within a 1 μm square area of ​​the sample.

[0083] Such micro-area elasticity measurement makes it possible to obtain the elastic modulus distribution in the insulating layer 130. This makes it possible to grasp the phase structure of the insulating layer 130 based on the elastic modulus distribution.

[0084] For example, by observing the elastic modulus distribution using a threshold value of 200 MPa, it is possible to determine whether the phase structure of the insulating layer 130 is a co-continuous structure. Here, 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 intermediate between the elastic modulus of the propylene-based resin and the elastic modulus of the styrene-based resin. When such a phase of 200 MPa or less is connected as a continuous phase, water trees tend to propagate through the phase. Therefore, by observing the elastic modulus distribution using a threshold value of 200 MPa, it is possible to determine whether the insulating layer 130 has a co-continuous structure in which water trees tend to propagate.

[0085] Furthermore, for example, by observing the elastic modulus distribution using a threshold elastic modulus of 100 MPa, it is possible to measure the size of island phases when the phase structure of insulating layer 130 is a sea-island structure. That is, since styrene-based elastomers exhibit an elastic modulus of less than 100 MPa in microarea elasticity measurements, it is possible to measure the size of island phases derived from styrene-based elastomers by observing using a threshold elastic modulus of 100 MPa.

[0086] In this embodiment, the insulating layer 130 has, in the elastic modulus distribution obtained by micro-area elasticity measurement using an SPM at 25°C, for example, 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.

[0087] In this embodiment, the resin composition constituting the insulating layer 130 can stably form the above-described phase structure in the insulating layer 130 by, for example, satisfying the following requirements. The (A) resin composition contains a propylene-based resin and two styrene-based elastomers as low-crystalline resins. (B) When the resin composition contains propylene units, butene units, and styrene units, the content (mass percent concentration) of butene units in the resin composition is 9 mass % or more and less than 20 mass %. (C) The new mixing method described below is applied.

[0088] The phase structure of the insulating layer 130 of this embodiment will be described below with reference to FIGS. 2 to 5. As an example, 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. FIGS. 2 to 5 show the elastic modulus distribution obtained by measuring the elasticity of a small area using an SPM at 25°C. In FIGS. 2 to 5, the gray areas indicate areas having an elastic modulus below the threshold value indicated at the top of each figure.

[0089] (Reference Example 1: Co-continuous structure) For example, in Reference Example 1, in which the content of butene units in the resin composition is 20 mass% or more, regardless of whether the resin composition contains two styrene-based elastomers as low-crystalline resins, the phase structure of the insulating layer 130 is a bicontinuous structure as shown in Figure 3.

[0090] As shown in the elastic modulus distribution in Figure 3, the bicontinuous 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.

[0091] In the bicontinuous structure, the first continuous phase CP1 and the second continuous phase CP2 are each long and continuous. In an elastic modulus distribution with a threshold elastic modulus of 200 MPa, the length between both ends of the second continuous phase CP2 is, for example, 5 μm or more. Here, 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 farthest end.

[0092] When the insulating layer 130 has a bicontinuous phase structure as described above, the micro-foreign matter or micro-void may be incorporated into the flexible second continuous phase CP2 containing a styrene-based elastomer when a micro-foreign matter or micro-void is mixed into the insulating layer 130. In this case, when a water tree originates from the micro-foreign matter or micro-void, the water tree is likely to propagate along the second continuous phase CP2 having a bicontinuous structure.

[0093] (Reference Example 2: Sea-island structure with only microisland phases) For example, in Reference Example 2, in which the resin composition contains only a styrene-based elastomer with a high butene unit content as the low-crystalline resin and the content of butene units in the resin composition is less than 20 mass%, 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.

[0094] As shown in the elastic modulus distribution 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 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 fine island phase SIP is derived from a styrene-based elastomer as a low-crystalline resin. The size of the fine island phase SIP is, for example, 100 nm or less.

[0095] When the insulating layer 130 has 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 micro-voids are mixed into the insulating layer 130, water trees tend to propagate at the interfaces of the coarse crystals.

[0096] (Reference Example 3: Sea-island structure with only coarse island phases) For example, in Reference Example 3, in which the resin composition contains only a styrene-based elastomer with a low butylene unit content as the low-crystalline resin and the butene unit content in the resin composition is less than 9 mass%, 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.

[0097] In the elastic modulus distribution 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.

[0098] When the insulating layer 130 has 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 may occur in the sea phase SP, or the crystals may separate. This may result in a decrease in the insulating properties of the insulating layer 130.

[0099] In addition, for example, even when the resin composition constituting the insulating layer 130 does not contain a styrene-based elastomer, or when the resin composition constituting the insulating layer 130 satisfies the above-mentioned (A) and (B) but does not satisfy (C), the phase structure of the insulating layer 130 will be a sea-island structure similar to that of Reference Example 3.

[0100] (Embodiment: Sea-island structure with two island phases of different sizes) In contrast, in this embodiment, 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.

[0101] That is, in this embodiment, the resin composition (A) contains a propylene-based resin and two styrene-based elastomers as low-crystalline resins, and thus can form two island phases (first island phase IP1 and second island phase IP2) of different sizes.

[0102] In this embodiment, by setting the butene unit content in the (B) resin composition to 9% by mass or more, compatibility of each styrene-based elastomer with the propylene-based resin is ensured, and excessive aggregation of each styrene-based elastomer can be suppressed. This allows the first island phase IP1 to be finely dispersed in the sea phase SP. Furthermore, excessive aggregation of the second island phase IP2 can be suppressed.

[0103] In this embodiment, by setting the content of butene units in the (B) resin composition to less than 20% by mass, it is possible to prevent each styrene-based elastomer from becoming excessively compatible with the propylene-based resin, thereby preventing the formation of a co-continuous structure.

[0104] In this embodiment, by applying a novel mixing method described below, it is possible to apply a high shear force to the resin composition and ensure a sufficient residence time of the resin composition in the cylinder, thereby stably forming the sea-island structure of this embodiment.

[0105] Specifically, as shown in the elastic modulus distribution in FIG. 2, 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.

[0106] The marine phase SP has an elastic modulus of over 100 MPa. The marine phase SP is derived from a propylene-based resin. The marine phase SP spans the entire elastic modulus distribution.

[0107] The first island phase IP1 has an elastic modulus of 100 MPa or less and a size of 0.1 nm to 100 nm. Here, 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 farthest end.

[0108] The first island phase IP1 is 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 using a transmission electron microscope (TEM) by staining a sample taken from the insulating layer 130 with a heavy metal.

[0109] The second island phase IP2 has an elastic modulus of 100 MPa or less and a size of more than 100 nm and not more 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.

[0110] The second island phase IP2 is primarily derived from the second styrene 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 the same method as described for the first island phase IP1.

[0111] 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 "total island phase area ratio") is, for example, 10% or more and 70% or less. By setting the total island phase area ratio to 10% or more, the formation of coarse crystals in the sea phase SP can be suppressed. This makes it possible to make the insulating layer 130 flexible. On the other hand, by setting the total island phase area ratio to 70% or less, the insulating properties of the sea phase SP can be ensured.

[0112] The ratio of the area of ​​the first island phases IP1 to the unit area of ​​the elastic modulus distribution (hereinafter also referred to as "first island phase area ratio") is, for example, 1% or more and 40% or less. By setting the first island phase area ratio to 1% or more, the first island phases IP1 can be sufficiently dispersed in the sea phase SP. This makes it possible to make the insulating layer 130 flexible. On the other hand, by setting the total first island phase area ratio to 40% or less, the insulating properties of the sea phase SP can be ensured.

[0113] 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 "second island phase area ratio") is, for example, 5% or more and 30% or less. By setting the second island phase area ratio to 5% or more, the formation of coarse crystals of the sea phase SP can be suppressed. On the other hand, by setting the total second island phase area ratio to 30% or less, the insulating properties of the sea phase SP can be ensured.

[0114] Furthermore, in the sea phase SP, the diameter of the circular island-phase-free region (the region within the dashed line in the figure) that does not contain the first island phase IP1 or the second island phase IP2 is, for example, 5 nm or more and 400 nm or less. When the diameter of the island-phase-free region is 5 nm or more, the insulating properties of the sea phase SP can be ensured. On the other hand, when the diameter of the island-phase-free region is 400 nm or less, the formation of coarse crystals in the sea phase SP can be suppressed. This makes it possible to make the insulating layer 130 flexible.

[0115] (4) Cable characteristics In this embodiment, since the insulating layer 130 has the above-mentioned sea-island structure, the bending resistance of the insulating layer 130 can be ensured while suppressing the occurrence of water trees originating from minute foreign matter or minute voids in the insulating layer 130.

[0116] (Water tree resistant) The inventors evaluated the water tree resistance of the insulating layer 130 by performing a water tree test (hereinafter also referred to as the "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.

[0117] The "water needle test" here is performed in the following procedure, as shown in Figure 6.

[0118] First, a hexahedral block IB is cut from the insulating layer 130. After the block IB is cut, a plate electrode FE is provided on a first surface S1 of the block IB.

[0119] After the plate electrode FE is provided, a needle-shaped gap is formed along the normal direction from the second surface S2 opposite to the first surface S1 of the block IB, with a distance d of 1 mm from the first surface S1 of the block IB, so that the needle-shaped gap has a tip with a radius of curvature of 10 μm.

[0120] After forming a void in block IB, artificial seawater with a salinity of 3.8% by mass is injected into the void, forming a water needle WN within block IB.

[0121] In the above state, an AC voltage of 1000 Hz and 4 kV is applied between the plate electrode FE and the tip of the water needle WN for 200 hours.

[0122] After the water needle test, the length of the water tree that has occurred at the tip of the water needle WN is measured.

[0123] As described above, in the water needle test of this embodiment, the tip of the water needle WN is sharpened to simulate a minute foreign particle or minute void trapped in the insulating layer 130. As a result, 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 the conventional water tree test in which an electric field is applied uniformly to the insulating layer.

[0124] In this embodiment, the maximum length of water trees in the insulating layer 130 that occur in the water needle test is, for example, 200 μm or less. That is, even in the test described above in which water trees are likely to occur, the maximum length of water trees in the insulating layer 130 can be shortened.

[0125] (Insulation after bending test) In this embodiment, the insulating layer 130 has the above-described sea-island structure, and therefore has resistance to a predetermined bending test.

[0126] The "bending test" here includes, for example, a first step of bending the power cable 10 so that the bending ratio (hereinafter also simply referred to as "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.

[0127] In a bending test according to a normal cable standard, the bending ratio of the bending radius of the power cable to the outer diameter of the insulating layer is set to, for example, about 20.

[0128] In contrast, the bending ratio in the bending test of this embodiment is smaller than the bending ratio in the bending test of a 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.

[0129] In this embodiment, the AC breakdown field strength is maintained high even after the bending test described above.

[0130] That is, in this embodiment, the AC breakdown field strength of the insulating layer 130 at room temperature (for example, 25° C.) after the above-mentioned bending test may be, for example, 50 kV / mm or more, or 60 kV / mm or more.

[0131] The "AC breakdown field strength" referred to here is measured at room temperature (25°C) by applying an AC voltage of 10 kV at a commercial frequency (e.g., 60 Hz) to a 0.4 mm thick sample for 10 minutes, followed by increasing the voltage by 1 kV in increments of 1 kV and applying the voltage for 10 minutes, and repeating this cycle.

[0132] In this embodiment, the AC breakdown field strength of the insulating layer 130 at room temperature (for example, 25° C.) before the bending test is also naturally, for example, 50 kV / mm or more, or 60 kV / mm or more.

[0133] (5) Power cable manufacturing method Next, a method for manufacturing the power cable of this embodiment will be described.

[0134] The method for manufacturing a power cable of this embodiment includes, for example, a conductor preparing step S120, a resin composition preparing step S140, a cable core forming step 300, a shielding layer forming step S400, and a sheath forming step S500.

[0135] (S120: Conductor preparation process) A conductor 110 is prepared by twisting together a plurality of conductor core wires.

[0136] (S140: Resin composition preparation step) On the other hand, a resin composition containing propylene units and styrene units is prepared.

[0137] In this case, the resin composition is mixed so that, in the elastic modulus distribution obtained by measuring the elasticity of a micro-area using an SPM at 25°C, the resin composition comprises 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.

[0138] Specifically, in this embodiment, a propylene-based resin is mixed with two styrene-based elastomers as low-crystalline resins: 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.

[0139] In this embodiment, 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.

[0140] At this time, in this embodiment, for example, a twin-screw mixer is used to mix the resin composition. By using a twin-screw mixer, the resin composition can be mixed with a higher shear force than with a kneader.

[0141] In this embodiment, the resin composition is mixed at a temperature below 220°C, for example. 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. In contrast, in this embodiment, the mixing temperature is set to below 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.

[0142] In this embodiment, for example, the installation conditions of the kneading discs in the twin-screw mixer are adjusted, and the rotation speed of the twin-screw mixer is adjusted. Here, the shear force in the twin-screw mixer can be changed, for example, by changing the kneading discs or the rotation speed. For example, if the kneading discs are installed in more than five locations and each rotation speed is 200 rpm or higher, the temperature is likely to rise during mixing. In contrast, in this embodiment, for example, the number of kneading discs installed is set to 2 to 5 locations, and each rotation speed is set to 20 rpm or higher and less than 200 rpm. This makes it possible to suppress the temperature rise to 220°C or higher during mixing. By suppressing the temperature rise, it is possible to suppress a decrease in the viscosity of the resin composition. As a result, a high shear force in the twin-screw mixer can be maintained.

[0143] In this embodiment, for example, the filling rate of the resin composition in the cylinder of the twin-screw mixer (the ratio of the volume of the resin composition to the volume of the cylinder) is increased to ensure the residence time of the resin composition in the cylinder. Typically, 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 rate. In contrast, in this embodiment, the filling rate of the resin composition in the cylinder of the twin-screw mixer is set to more than 20%, which ensures a sufficient residence time of the resin composition in the cylinder. This allows the above-mentioned sea-island structure to be stably formed.

[0144] In this manner, in this embodiment, the above-described sea-island structure can be formed in the resin composition.

[0145] After mixing the resin composition, the resin composition is granulated using an extruder, thereby forming pellets of the resin composition that will form the insulating layer 130. Note that the steps from mixing to granulation may be performed all at once.

[0146] (S300: Cable core forming process (extrusion process, insulating layer forming process)) After the conductor preparation step S120 and the resin composition preparation step S140 are completed, the insulating layer 130 is formed so as to cover the outer periphery of the conductor 110 using the resin composition described above.

[0147] In this embodiment, the insulating layer 130 is extrusion-molded using the resin composition described above, so that the insulating layer 130 also has the above-described sea-island structure.

[0148] At this time, in this embodiment, for example, a three-layer co-extruder is used to simultaneously form the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140.

[0149] Specifically, for example, the composition for the inner semiconductive layer is charged into extruder A, which forms the inner semiconductive layer 120, of the three-layer co-extruder.

[0150] The above-described pellet-shaped resin composition is fed into extruder B, which forms insulating layer 130. The set temperature of extruder B is set, for example, to a temperature that is 10°C to 50°C higher than the desired melting point. The set temperature is adjusted appropriately based on the linear velocity and extrusion pressure.

[0151] An outer semiconductive layer composition containing the same materials as the inner semiconductive layer resin composition charged into extruder A is charged into extruder C which forms the outer semiconductive layer 140 .

[0152] Next, the extrudates from extruders A to C are introduced into a common head, and inner semiconductive layer 120, insulating layer 130, and outer semiconductive layer 140 are simultaneously extruded from the inside to the outside around conductor 110. This forms an extruded material that will become the cable core.

[0153] The extrusion is then cooled, for example with water.

[0154] By the above-described cable core forming step S300, a cable core composed of the conductor 110, the inner semiconductive layer 120, the insulating layer 130 and the outer semiconductive layer 140 is formed.

[0155] (S400: Shielding layer formation process) Once the cable core is formed, a shielding layer 150 is formed on the outside of the outer semiconducting layer 140 by wrapping, for example, copper tape.

[0156] (S500: Sheath forming process) After the shielding layer 150 is formed, vinyl chloride is put into an extruder and extruded to form a sheath 160 around the outer periphery of the shielding layer 150 .

[0157] In this manner, the power cable 10 is manufactured as a solid insulated power cable.

[0158] (6) Summary of this embodiment According to this embodiment, one or more of the following effects can be achieved.

[0159] (a) In this embodiment, the insulating layer 130 comprises, in the elastic modulus distribution obtained by micro-area elasticity measurement using an SPM at 25°C, 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.

[0160] By dispersing the flexible and minute first island phases IP1 in the sea phase SP containing the propylene-based resin, 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 cracking in the sea phase SP even when 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 when the power cable 10 is bent.

[0161] In this state, by further dispersing the flexible and large second island phases IP2 in the sea phase SP containing the propylene-based resin, even if water trees originate from minute foreign particles or minute voids, the propagation of the water trees can be stopped within the second island phases IP2, thereby suppressing excessive propagation of water trees caused by minute foreign particles or minute voids in the insulating layer 130.

[0162] As described above, according to this embodiment, it is possible to suppress the occurrence of water trees originating from minute foreign particles or minute voids in the insulating layer 130 while ensuring the bending resistance of the insulating layer 130.

[0163] (b) In this embodiment, in a water probe test that simulates minute foreign particles or minute voids trapped in the insulating layer 130, the maximum length of the water tree in the insulating layer 130 is 200 μm or less. That is, even in tests in which water trees are likely to occur, the maximum length of the water tree in the insulating layer 130 can be shortened. As a result, even if minute foreign particles or minute voids are trapped in the actual insulating layer 130, excessive propagation of the water tree can be suppressed.

[0164] (c) In this embodiment, after a bending test with a bending ratio of 7 or less, the AC breakdown field strength of the insulating layer 130 at room temperature (e.g., 25°C) is 50 kV / mm or more. That is, even in a test in which a bending stress stronger than that in a bending test according to a normal cable standard is applied to the insulating layer 130, the AC breakdown field strength can be maintained high. As a result, even when an actual power cable 10 is bent, the insulating properties of the insulating layer 130 can be maintained at a high level.

[0165] <Other Embodiments of the Present Disclosure> Although the embodiments of the present disclosure have been specifically described above, the present disclosure is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the present disclosure.

[0166] In the above-described embodiment, the power cable 10 does not necessarily have a water-shielding layer. However, the present disclosure is not limited to this. The power cable 10 may have a simple water-shielding layer. Specifically, the simple water-shielding layer may be, for example, a metal laminate tape. The metal laminate tape has a metal layer made of, for example, aluminum or copper, and an adhesive layer provided on one or both sides of the metal layer. The metal laminate tape is, for example, wrapped longitudinally around the outer periphery of the cable core (outer than the outer semiconductive layer). The water-shielding layer may be provided outside the shielding layer or may also serve as the shielding layer. This configuration can reduce the cost of the power cable 10.

[0167] In the above-described embodiment, 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. For example, the power cable 10 may be configured as a so-called overhead electric wire (overhead insulated electric wire).

[0168] In the above embodiment, three layers are simultaneously extruded in the cable core forming step S300, but each layer may be extruded one by one. [Example]

[0169] Next, examples according to the present disclosure will be described. These examples are examples of the present disclosure, and the present disclosure is not limited to these examples.

[0170] (1) Power cable production Power cables of samples A1 to A3 and B1 to B8 were manufactured according to the following procedure.

[0171] (1-1) Material preparation process The following materials were prepared as the resin components of each sample.

[0172] (Propylene-based resin) Random Polypropylene (r-PP): Stereoregularity: Isotactic Ethylene unit content: 3% by mass, MFR: 1.3g / 10min (230℃, 2.16kg), Density: 0.9g / ml, Melting point: 145℃, Heat of fusion: 100J / g

[0173] (Low crystalline resin) Hydrogenated Styrene Butadiene Styrene Block Copolymer (SEBS1): Butene unit content: 65% by mass, Styrene unit content: 5% by mass, MFR:6.0g / 10min(230℃, 2.16kg), Hardness: A45, Melting point: None, Heat of fusion: None

[0174] Hydrogenated Styrene Butadiene Styrene Block Copolymer (SEBS2): Butene unit content: 25% by mass, Styrene unit content: 30% by mass, MFR:5.0g / 10min(230℃, 2.16kg), Hardness: A84, Melting point: None, Heat of fusion: None

[0175] Hydrogenated Styrene Butadiene Styrene Block Copolymer (SEBS3): Butene unit content: 65% by mass, Styrene unit content: 12% by mass, MFR: 4.5g / 10min (230℃, 2.16kg), Hardness: A42, Melting point: None, Heat of fusion: None

[0176] Very low density polyethylene (VLDPE): ethylene-1-butene copolymer, Butene unit content: 21% by mass, MFR:6.7g / 10min(230℃, 2.16kg), Hardness: A86, Melting point: 66°C, Heat of fusion: 10J / g

[0177] (1-2) Resin composition mixing process The resin compositions for each sample were mixed using the materials described above.

[0178] (Samples A1 to A3) For 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. A twin-screw mixer was used to mix the resins at a temperature of 200°C. Kneading disks were installed in 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 below as the "new mixing method."

[0179] (Samples B1 to B7) In samples B1 to B7, 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.

[0180] (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: For Sample B8, the resin composition was mixed using a kneader at a temperature of 220°C.

[0181] (1-3) Post-process While the resin composition was mixed as described above, a 200 mm cross-sectional area cable was formed by twisting together conductor core wires made of dilute copper alloy with a diameter of 17 mm. 2 The conductor was prepared. After the conductor was prepared, an inner semiconductive layer resin composition containing an ethylene-ethyl acrylate copolymer, the above-mentioned resin composition, and an outer semiconductive layer resin composition composed of the same materials as the inner semiconductive layer resin composition were fed into extruders A to C, respectively. The extrudates from extruders A to C were guided to a common head, and the inner semiconductive layer, insulating layer, and outer semiconductive layer were simultaneously extruded from the inside to the outside around the conductor. The thicknesses of the inner semiconductive layer, insulating layer, and outer semiconductive layer were 0.5 mm, 3.5 mm, and 0.5 mm, respectively. After the extrusion process, the extruded material was cooled. As a result, power cables Samples A1 to A3 and B1 to B8 were produced.

[0182] (2) Evaluation Each sample was evaluated as follows.

[0183] (NMR) A portion of the insulating layer of each of the above samples was dissolved in orthochlorobenzene, and then analyzed in this state using a JEOL NMR device in a temperature environment of 130°C. 1 H-NMR and 13 The composition ratio of the monomer units was calculated from the peak area ratio obtained by C-NMR.

[0184] (SPM) Micro-area elasticity measurements were performed using an SPM. For these measurements, a 1 mm thick sheet cut from the center of the insulating layer of each sample was used. For the micro-area elasticity measurements, the elastic modulus was measured at 25°C by tapping a 1 μm square area of ​​the sample 16,000 times using a cantilever made of silicon (single crystal) with a tip with a curvature radius of 7 nm.

[0185] In the micro-area elasticity measurement, an elastic modulus distribution with a threshold of 200 MPa and an elastic modulus distribution with a threshold of 100 MPa were obtained. For each elastic modulus distribution, the field of view was 1.0 μm square.

[0186] In the elastic modulus distribution with a threshold of 200 MPa, the phase structure in which each phase is continuous without separation was judged to be a "co-continuous structure (B1)," and the phase structure in which each phase is separated was judged to be an "island-sea structure."

[0187] When a phase structure 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. As a result, 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 an "A" phase structure according to 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."

[0188] (Water needle test) As shown in FIG. 6, a block was taken from the insulating layer of each sample and subjected to the water needle test described above.

[0189] The conditions for the water needle test were set as follows: Radius of curvature of the tip of the water needle: 10 μm Distance from the plate electrode on the first surface of the block to the tip of the water needle: 1 mm Salinity of artificial seawater in the water needle: 3.8% by mass Frequency: 1000MHz AC voltage: 4kV

[0190] After the water needle test, the block was dried and then boiled in a methylene blue solution to stain it. After staining the block, the block was sliced ​​near the tip of the water needle to form a slice for observation. The slice for observation was then observed under an optical microscope to observe the water tree that had developed near the tip of the water needle.

[0191] At this time, the maximum length of water trees that had developed in the slices for observation was measured. The "maximum length of water trees" was calculated by rounding off the length of the longest water tree in 10 blocks randomly extracted from the insulation layer of each sample.

[0192] As a result, when the maximum length of the water tree was 200 μm or less, it was evaluated as good (A), and when the maximum length of the water tree was more than 200 μm, it was evaluated as poor (B).

[0193] (Insulation evaluation) (i) Insulation evaluation immediately after manufacturing Immediately after the production of the power cables, sheets with a thickness of 0.4 mm were taken from the insulating layer of each sample.

[0194] At room temperature (25°C), an AC voltage of 10 kV at commercial frequency (60 Hz) was applied to the insulating layer sheet for 10 minutes, and then the voltage was increased by 1 kV in increments of 10 minutes, and this was repeated. The electric field strength was measured when the insulating layer sheet underwent dielectric breakdown. As a result, an AC breakdown strength of 50 kV / mm or more was evaluated as good (A), and an AC breakdown strength of less than 50 kV / mm was evaluated as poor.

[0195] (ii) Insulation evaluation after bending test (7D bending test) First, a bending test was carried out on each sample power cable.

[0196] In the first step of the bending test, a power cable with an outer diameter of 26 mm was pressed along half the circumference of a SUS ring with a radius of 180 mm. In other words, 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. In the subsequent second step, the power cable was bent in the opposite direction to the first step, but at the same bending ratio as the first step.

[0197] (Sample collection and AC breakdown test) After the bending test, the insulating layer sheets were sampled from each sample power cable in the same manner as in the evaluation immediately after manufacture described above. The insulating layer sheets sampled after the bending test were subjected to an AC breakdown test in the same manner as in the evaluation immediately after manufacture described above.

[0198] (3) Results The results of evaluation of each sample will be described with reference to Table 1 below and FIGS.

[0199] [Table 1]

[0200] As a result of the above-mentioned NMR analysis, it was confirmed that each sample satisfied the content of each monomer unit shown in Table 1. Other evaluation results of each sample will be explained below.

[0201] (Samples B1, B5 and B6) In samples B1 and B5, the insulating layer contained 20 mass % or more of butene units. In sample B6, the insulating layer did not contain styrene units and the insulating layer contained 20 mass % or more of butene units.

[0202] Therefore, as shown in the elastic modulus distribution in FIG. 3, the phase structure of the insulating layer in samples B1, B5 and B6 was a bicontinuous structure B1.

[0203] As a result, the AC breakdown field strength after the bending test was 50 kV / mm or more for samples B1, B5, and B6. However, the maximum length of the water tree in the water needle test for samples B1, B5, and B6 exceeded 200 μm.

[0204] In samples B1, B5, and B6, the phase structure of the insulating layer was the co-continuous structure B1, so when water trees occurred in the water needle test, it is thought that the water trees propagated along the flexible phase of the co-continuous structure.

[0205] (Sample B2) In sample B2, the insulating layer contained only SEBS1, which has a high butene unit content, as the low-crystalline resin, and the butene unit content in the insulating layer 130 was less than 20 mass %.

[0206] Therefore, as shown in the elastic modulus distribution in FIG. 4, in sample B2, the phase structure of the insulating layer was a sea-island structure B2 that had minute first island phases but no coarse second island phases.

[0207] As a result, the AC breakdown field strength of sample B2 after the bending test was 50 kV / mm or more. However, the maximum length of the water tree in sample B2 in the water needle test exceeded 200 μm.

[0208] In sample B2, the phase structure of the insulating layer was a sea-island structure B2 consisting of only minute first island phases, and coarse crystals grew in the sea phase. Therefore, when water trees occurred in the water needle test, they were thought to have propagated at the interfaces of the coarse crystals.

[0209] (Samples B3 and B4) In samples B3 and B4, the insulating layer contained only SEBS2, which has a low content of butene units, as the low-crystalline resin, and the content of butene units in the insulating layer was less than 9 mass %.

[0210] Therefore, as shown in the elastic modulus distribution in FIG. 5, in samples B3 and B4, 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.

[0211] As a result, in samples B3 and B4, the maximum length of the water tree in the water needle test was 200 μm or less. However, in samples B3 and B4, the AC breakdown field strength after the bending test was less than 50 kV / mm.

[0212] In samples B3 and B4, cracks and the like occurred in the sea phase during the bending test, which is thought to be why the insulating properties decreased after the bending test.

[0213] (Sample B7) In sample B7, the insulating layer contained only VLDPE containing no styrene units as the low crystalline resin.

[0214] Therefore, sample B7 had a sea-island structure B3 including only coarse island phases, similar to samples B3 and B4.

[0215] As a result, in sample B7, cracks and the like occurred in the sea phase, as in samples B3 and B4, and therefore the AC breakdown field strength after the bending test was less than 50 kV / mm.

[0216] (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.

[0217] Therefore, in sample B8, the phase structure of the insulating layer was the sea-island structure B3 having only coarse island phases, similar to samples B3 and B4.

[0218] As a result, in sample B8, the maximum length of the water tree in the water needle test was 200 μm or less, but in sample B8, the AC breakdown field strength after the bending test was less than 50 kV / mm.

[0219] In sample B8, the new mixing method was not applied, resulting in insufficient shear force during mixing and insufficient mixing of the resin composition. As a result, the sea-island structure A seen in sample A1 was not formed in sample B8. As a result, cracks occurred in the sea phase during the bending test, which is thought to have reduced the insulating properties after the bending test.

[0220] (Samples A1 to A3) In Samples A1 to A3, the insulating layer contained SEBS1 and SEBS2 as low-crystalline resins. In 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 Samples A1 to A3.

[0221] Therefore, as shown in FIG. 2, in samples A1 to A3, the phase structure of the insulating layer was a sea-island structure A having minute first island phases and coarse second island phases.

[0222] As a result, the AC breakdown field strength after the bending test was 50 kV / mm or more for Samples A1 to A3. Furthermore, the maximum length of the water tree in the water needle test for Samples A1 to A3 was 200 μm or less.

[0223] In samples A1 to A3, the phase structure of the insulating layer was a sea-island structure A having 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.

[0224] Furthermore, it was confirmed that by changing the phase structure of the insulating layer to a sea-island structure A, which has a minute first island phase and a coarse second island phase, excessive propagation of water trees from the tip of a water needle, which simulates a minute foreign object or a minute void, in the insulating layer could be suppressed.

[0225] <Additional Notes> The following additionally describes aspects of the present disclosure.

[0226] (Appendix 1) A resin composition constituting an insulating layer of a power cable, Contains propylene units and styrene units, The resin composition comprises The elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C was a sea phase having an elastic modulus of more than 100 MPa; First island phases having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; second island phases having an elastic modulus of 100 MPa or less and a size of more than 100 nm and not more than 2000 nm; Equipped with Resin composition.

[0227] (Appendix 2) A conductor; an insulating layer provided to cover the outer periphery of the conductor; Equipped with the insulating layer includes a propylene unit and a styrene unit; The insulating layer is The elastic modulus distribution obtained by measuring the elasticity of a small area using a scanning probe microscope at 25°C was a sea phase having an elastic modulus of more than 100 MPa; First island phases having an elastic modulus of 100 MPa or less and a size of 0.1 nm or more and 100 nm or less; second island phases having an elastic modulus of 100 MPa or less and a size of more than 100 nm and not more than 2000 nm; Equipped with Power cable.

[0228] (Appendix 3) The maximum length of the water tree in the insulation layer generated by the water needle test is 200 μm or less. Here, the water needle test is providing a plate electrode on a first surface of the block taken from the insulating layer; forming needle-shaped voids at positions facing each other at a distance of 1 mm from the first surface, the needle-shaped voids having tips with a curvature radius of 10 μm; A step of forming water needles in the block by injecting artificial seawater having a salinity of 3.8% by mass into the void; applying an AC voltage of 1000 Hz and 4 kV between the plate electrode and the tip of the water needle for 200 hours; Contains 1. A power cable as described in Appendix 2.

[0229] (Appendix 4) The insulating layer has an AC breakdown field strength of 50 kV / mm or more at 25°C after a predetermined bending test. Here, 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 the same bending ratio as the bending ratio of the first step; Contains 1. A power cable as defined in Appendix 2 or Appendix 3.

[0230] (Appendix 5) the insulating layer further comprises butene units; The content of the butene units in the insulating layer is 9% by mass or more and less than 20% by mass. 10. The power cable according to claim 2, wherein the power cable is a

[0231] (Appendix 6) The first island phase and the second island phase contain at least the styrene unit. 1. A power cable according to any one of appendices 2 to 5.

[0232] (Appendix 7) The insulating layer is A propylene-based resin, a first styrenic elastomer containing 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; have 10. A power cable according to any one of claims 2 to 6.

[0233] (Appendix 8) The content of the propylene unit or the butene unit in the second styrene-based elastomer is less than the content of the propylene unit or the butene unit in the first styrene-based elastomer. 10. A power cable as described in Appendix 7.

[0234] (Appendix 9) The content of the styrene units in the second styrene-based elastomer is greater than the content of the styrene units in the first styrene-based elastomer. 1. A power cable as defined in Annex 7 or Annex 8.

[0235] (Appendix 10) preparing a resin composition containing propylene units and styrene units; forming an insulating layer from the resin composition so as to cover the outer periphery of the conductor; Equipped with In the step of preparing the resin composition, The resin composition is mixed so that, in the elastic modulus distribution obtained by measuring the elasticity of a micro-area using a scanning probe microscope at 25°C, the resin composition has 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. Power cable manufacturing method. [Explanation of symbols]

[0236] 10 Power Cable 110 Conductor 120 Internal semiconductive layer 130 Insulating layer 140 outer semiconductive layer 150 Shielding layer 160 Sheath

Claims

1. A resin 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 not more than 2000 nm; Equipped Resin composition.

2. 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 not more than 2000 nm; Equipped Power cable.

3. The maximum length of the water tree in the insulating layer generated by the water needle test is 200 μm or less. Here, the water needle test is providing a plate electrode on a first surface of the block taken from the insulating layer; forming a needle-shaped gap having a tip with a radius of curvature of 10 μm 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 3. The power cable according to claim 2.

4. The AC breakdown field strength of the insulating layer at 25° C. after a predetermined bending test is 50 kV / mm or more. Here, 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 4. The power cable according to claim 2 or 3.

5. The insulating layer further comprises a butene unit, The content of the butene unit in the insulating layer is 9 mass % or more and less than 20 mass %. The power cable according to claim 2 or 3.

6. 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; have The power cable according to claim 2 or 3.

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