Resin composition, power cable, and method for producing power cable

Abstract

This resin composition comprises: a base polymer that includes a propylene unit; a modified polymer that includes a propylene unit and that is modified with at least one selected from unsaturated organic acids and derivatives thereof; and a thermoplastic elastomer. The resin composition is configured such that: the ratio of the volume resistivity of an inner sample of an insulating layer to the volume resistivity of an outer sample of the insulating layer is 1.0-1.5; and the degree of orientation of the outer sample of the insulating layer is not less than 10% to less than 70%.

Description

Resin composition, power cable, and method for manufacturing a power cable

[0001] This disclosure relates to a resin composition, a power cable, and a method for manufacturing a power cable.

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

[0003] Japanese Unexamined Patent Publication No. 57-69611

[0004] According to one aspect of the present disclosure, a resin composition comprising an insulating layer provided to cover a conductor of a power cable, having an inner circumferential surface facing the conductor and an outer circumferential surface opposite to the inner circumferential surface, comprising a base polymer containing propylene units, a modified polymer containing propylene units and modified with at least one selected from an unsaturated organic acid and its derivatives, and a thermoplastic elastomer, wherein the ratio of the volume resistivity of an inner sample of the insulating layer to the volume resistivity of an outer sample of the insulating layer is 1.0 or more and 1.5 or less, and The orientation of the outer sample of the insulating layer is configured to be 10% or more and less than 70%. Here, the outer sample of the insulating layer is taken from a position 0.3 mm from the outer peripheral surface toward the conductor, and the inner sample of the insulating layer is taken from a position 0.3 mm from the inner peripheral surface toward the outer peripheral surface. The volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. The orientation f of the outer sample of the insulating layer is calculated by formula (1): f = {(360 - ΣW) / 360} × 100 ... (1) Here, W is the half-width of each peak in the azimuthal width of the integrated intensity obtained by integrating the scattering intensity in the range of diffraction angle 2θ of 15° or more and 18° or less, based on the X-ray scattering image obtained by irradiating the outer sample perpendicularly with Cu Kα rays. A resin composition is provided.

[0005] Figure 1 is a schematic diagram showing the state of molecular chains in relation to the crystallinity and orientation of a polymer. Figure 2 is a schematic cross-sectional view perpendicular to the axial direction of a power cable according to one embodiment of the present disclosure. Figure 3 is a schematic diagram showing the optical system in wide-angle X-ray scattering. Figure 4 shows the temperature at the sampling position of the outer sample and the temperature at the sampling position of the inner sample during the cooling process of sample A3. Figure 5 is the azimuthal profile of the outer sample of sample A1 measured by wide-angle X-ray scattering. Figure 6 shows the temperature at the sampling position of the outer sample and the temperature at the sampling position of the inner sample during the cooling process of sample B1. Figure 7 is the azimuthal profile of the outer sample of sample B1 measured by wide-angle X-ray scattering.

[0006] [Problems this disclosure aims to solve] The present inventors focused on polypropylene as a resin component constituting the insulating layer and diligently conducted studies to improve the characteristics of power cables.

[0007] The purpose of this disclosure is to improve the insulating properties and flexibility of the insulating layer.

[0008] [Effects of this disclosure] According to this disclosure, the insulating properties and flexibility of the insulating layer can be improved.

[0009] [Description of Embodiments in this Disclosure] <Knowledge Obtained by the Inventors, etc.> First, we will briefly explain the knowledge obtained by the inventors, etc.

[0010] Figure 1 is a schematic diagram showing the state of molecular chains in relation to the degree of crystallinity and orientation of polymers. In Figure 1, solid lines represent molecular chains, and thick lines represent crystals. As shown in Figures 1(a) to (d), typical polymers have a predetermined degree of crystallinity and orientation depending on the polymer's composition and manufacturing conditions (polymer molding conditions).

[0011] In polymers, a "crystal" refers to a region where multiple molecular chains are arranged regularly at regular intervals. The higher the degree of crystallinity of a polymer, the higher its insulating properties, strength, and elasticity.

[0012] In polymers, "orientation" refers to a state where multiple molecular chains are aligned along a common direction. The lower the degree of orientation of a polymer, the more the multiple molecular chains become entangled with each other, which can potentially increase the elasticity of the polymer.

[0013] As shown in Figure 1(d), even if the crystallinity of a polymer is high, the degree of orientation of the polymer may be low. On the other hand, as shown in Figure 1(a), even if the crystallinity of a polymer is low, the degree of orientation of the polymer may be high.

[0014] In the power cable manufacturing process, an insulating layer is extruded from a molten resin composition in a heated extruder to cover the outer circumference of the conductor. At this time, the resin composition flows in the extrusion direction within the extruder, so the molecules in the insulating layer immediately after extrusion are oriented in the extrusion direction. After the insulating layer is extruded, it is cooled by a predetermined cooling method.

[0015] At this time, the outer surface of the insulating layer is cooled with a predetermined refrigerant. As a result, cooling progresses from the outer surface of the insulating layer toward the inner surface of the insulating layer in the radial direction of the conductor.

[0016] When the temperature at which the outer surface of the insulating layer is cooled is low, that is, when the rate at which the outer surface of the insulating layer is cooled is fast, the base polymer on the outside of the insulating layer is rapidly cooled. On the other hand, the base polymer on the inside of the insulating layer cools more slowly than the base polymer on the outside of the insulating layer.

[0017] Therefore, if the outer surface of the insulating layer is cooled quickly, a distribution (variation) of crystallinity occurs in the thickness direction of the insulating layer. That is, on the outside of the insulating layer, the crystallinity of the base polymer is lower because it is held in the crystallization temperature region for a shorter time. On the other hand, on the inside of the insulating layer, the crystallinity of the base polymer is higher than that of the outside of the insulating layer because it is held in the crystallization temperature region for a longer time. When such a distribution of crystallinity occurs, the insulating properties gradually decrease from the inside to the outside of the insulating layer. As a result, it becomes difficult to ensure the sufficient insulating properties required for the insulating layer of the power cable as a whole.

[0018] Conventionally, the cooling rate was made uniform in the thickness direction of the insulating layer by slowing down the cooling rate of the outer surface of the insulating layer. In other words, the cooling rate of the base polymer on the outside of the insulating layer was made similar to the cooling rate of the base polymer on the inside of the insulating layer. As a result, the degree of crystallinity of the base polymer on the outside of the insulating layer and the degree of crystallinity of the base polymer on the inside of the insulating layer were adjusted to be equally high. As a result, sufficient insulation performance required for the entire insulating layer of the power cable was ensured.

[0019] However, when the cooling rate of the outer surface of the insulating layer was slowed, as in the conventional method, the base polymer on both the outside and inside of the insulating layer was in a state as shown in Figure 1(d), for example.

[0020] In other words, because the entire thickness of the insulating layer was held in the crystallization temperature range for a long time, the crystallinity of the base polymer was high throughout the entire thickness of the insulating layer.

[0021] On the other hand, molecules are thermodynamically stable in a randomly entangled, unoriented state. Therefore, conventionally, because the molecules throughout the thickness of the insulating layer were kept at a high temperature, the orientation of the molecules throughout the thickness of the insulating layer was eliminated. In other words, the degree of orientation of the base polymer was low throughout the thickness of the insulating layer.

[0022] As described above, conventionally, the base polymer formed crystals throughout the entire thickness direction of the insulating layer, and multiple molecular chains of the base polymer were intertwined with each other. As a result, the elasticity of the base polymer of the insulating layer was uniformly increased in the thickness direction of the insulating layer.

[0023] Conventionally, as described above, the elasticity of the base polymer of the insulating layer was uniformly high in the thickness direction of the insulating layer, making it difficult to ensure sufficient flexibility of the power cable when it was bent. As a result, there were limitations on the reel diameter when winding the power cable onto a reel.

[0024] Conventional methods present the aforementioned challenges, making it desirable to achieve both the desired elasticity of the insulating layer and uniform insulation properties in the thickness direction.

[0025] Therefore, as a result of diligent research, the present inventors have succeeded in making the insulating properties of the insulating layer uniform in the thickness direction of the insulating layer and reducing the elasticity on the outside of the insulating layer by adding predetermined amounts of modified polymer (B) and thermoplastic elastomer (C) to the base polymer (A) and applying a novel manufacturing method that improves the cooling process S340.

[0026] This disclosure is based on the aforementioned findings discovered by the inventors.

[0027] <Embodiments of the Disclosure> Next, embodiments of the Disclosure will be described by listing them.

[0028] [1] A resin composition according to one aspect of the present disclosure is a resin composition comprising an insulating layer provided to cover a conductor of a power cable, having an inner circumferential surface facing the conductor and an outer circumferential surface opposite to the inner circumferential surface, comprising: a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, wherein the ratio of the volume resistivity of an inner sample of the insulating layer to the volume resistivity of an outer sample of the insulating layer is 1.0 or more and 1.5 or less, and the degree of orientation of the outer sample of the insulating layer is 10% or more and less than 70%, where, the outer sample of the insulating layer is taken from a position 0.3 mm from the outer circumferential surface toward the conductor, the inner sample of the insulating layer is taken from a position 0.3 mm from the inner circumferential surface toward the outer circumferential surface, and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. The degree of orientation f of the outer sample of the insulating layer is calculated by formula (1): f = {(360 - ΣW) / 360} × 100 ... (1) Here, W is the full width at half maximum of each peak in the azimuthal width of each peak in the azimuthal angle profile of the integrated intensity obtained by integrating the scattering intensity in the range of diffraction angle 2θ from 15° to 18°, based on the X-ray scattering image obtained by irradiating the outer sample perpendicularly with Cu Kα rays. With this configuration, the insulating properties and flexibility of the insulating layer can be improved. Hereinafter, the range in which the degree of orientation of the outer sample is "10% or more and less than 70%" is also referred to as the "specified range".

[0029] [2] A power cable according to one aspect of the present disclosure comprises: a conductor; an insulating layer provided so as to cover the outer circumference of the conductor and having an inner surface facing the conductor and an outer surface opposite to the inner surface, wherein the insulating layer comprises: a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, wherein the ratio of the volume resistivity of an inner sample of the insulating layer to the volume resistivity of an outer sample of the insulating layer is 1.0 or more and 1.5 or less, and the degree of orientation of the outer sample of the insulating layer is 10% or more and less than 70%, where the outer sample of the insulating layer is taken from a position 0.3 mm from the outer surface toward the conductor, and the inner sample of the insulating layer is taken from a position 0.3 mm from the inner surface toward the outer surface. The volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. The degree of orientation f of the outer sample of the insulating layer is calculated by formula (1): f = {(360 - ΣW) / 360} × 100 ... (1) Here, W is the full width at half maximum of each peak in the azimuthal angle profile of the integrated intensity obtained by integrating the scattering intensity in the range of diffraction angle 2θ from 15° to 18°, based on the X-ray scattering image obtained by irradiating the outer sample perpendicularly with Cu Kα rays. This configuration makes it possible to improve the insulating properties and flexibility of the insulating layer.

[0030] [3] In the power cable described in [2] above, when the total content of the base polymer, the modified polymer, and the thermoplastic elastomer in the insulating layer is 100 parts by mass, the content of the modified polymer in the insulating layer is 1 part by mass or more and 10 parts by mass or less, and the content of the thermoplastic elastomer in the insulating layer is 10 parts by mass or more and 45 parts by mass or less. This configuration makes it possible to suppress a decrease in the insulating properties and mechanical properties of the insulating layer. Hereinafter, the range of the content of the modified polymer in the insulating layer and the range of the content of the thermoplastic elastomer in the insulating layer being "1 part by mass or more and 10 parts by mass or less" and "10 parts by mass or more and 45 parts by mass or less" will also be referred to as the "specified range".

[0031] [4] In the power cable described in [2] or [3] above, the degree of orientation of the inner sample of the insulating layer is 10% or more and less than 60%. With this configuration, by making the degree of orientation of the inner sample of the insulating layer lower than the degree of orientation of the outer sample, the elasticity of the insulating layer can be made to differ in the thickness direction of the insulating layer while suppressing large variations in the thickness direction of the insulating layer. Hereinafter, the range in which the degree of orientation of the inner sample is "10% or more and less than 60%" will also be called the "specified range".

[0032] [5] In the power cable described in any one of [2] to [4] above, the thermoplastic elastomer includes a styrene-based elastomer. This configuration allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0033] [6] In the power cable described in any one of [2] to [5] above, the thermoplastic elastomer includes an olefin-based elastomer. This configuration allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0034] [7] In the power cable described in any one of [2] to [6] above, the thickness of the insulating layer is 3 mm or more. With this configuration, even when the power cable is applied to high-voltage applications, it is possible to stably achieve both the insulating properties and the flexibility of the insulating layer.

[0035] [8] A method for manufacturing a power cable according to another aspect of the present disclosure comprises the steps of: preparing a resin composition; forming an insulating layer with the resin composition so as to cover the outer circumference of a conductor, and forming an inner surface facing the conductor and an outer surface opposite to the inner surface in the insulating layer, wherein the step of preparing the resin composition comprises: a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, such that the ratio of the volume resistivity of an inner sample of the insulating layer to the volume resistivity of an outer sample of the insulating layer is 1.0 or more and 1.5 or less, and the step of forming the insulating layer comprises: extruding the insulating layer onto the outer circumference of the conductor; and cooling the insulating layer so that the degree of orientation of the outer sample of the insulating layer is 10% or more and less than 70%, wherein the outer sample of the insulating layer is taken from a position 0.3 mm from the outer surface toward the conductor, The inner sample of the insulating layer is taken from a position 0.3 mm from the inner surface toward the outer surface. The volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. The degree of orientation f of the outer sample of the insulating layer is calculated by formula (1): f = {(360 - ΣW) / 360} × 100 ... (1) Here, W is the full width at half maximum of each peak in the azimuthal width of each peak in the azimuthal profile of the integrated intensity obtained by integrating the scattering intensity in the range of diffraction angle 2θ from 15° to 18°, based on the X-ray scattering image obtained by irradiating the outer sample perpendicularly with Cu Kα rays. This configuration makes it possible to improve the insulating properties and flexibility of the insulating layer.

[0036] [Details of Embodiments of the Present Disclosure] Next, an 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, and is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0037] <An Embodiment of the Present Disclosure> (1) Resin Composition The resin composition of this embodiment is a material that constitutes the insulating layer 130 of the power cable 10 described later. The resin composition has, for example, a base polymer (A), a modified polymer (B), a thermoplastic elastomer (C), and other additives.

[0038] Hereinafter, the base polymer (A), the modified polymer (B), and the thermoplastic elastomer (C) are also referred to as "resin components".

[0039] (Base Polymer (A)) The base polymer (base resin) (A) refers to a resin component that constitutes the main component of the resin composition. The "main component" means the component with the highest content.

[0040] The base polymer (A) of this embodiment contains, for example, at least propylene units as monomer units.

[0041] That is, by analyzing the resin composition of this embodiment with a nuclear magnetic resonance (NMR) apparatus, propylene units are detected as monomer units derived from the base polymer (A).

[0042] The base polymer (A) is composed of, for example, polypropylene (also referred to as a propylene-based resin, PP) having propylene units in the main chain. Examples of polypropylene include homopolypropylene (homo-PP), random polypropylene (random-PP), block polypropylene (block-PP), and the like.

[0043] NMR analysis of the resin composition of this embodiment reveals, for example, that if the base polymer (A) is random PP or block PP, propylene units and ethylene units derived from the random PP or block PP are detected. If the base polymer (A) is homo PP, propylene units derived from the homo PP are detected.

[0044] From the viewpoint of obtaining high insulation properties in the insulating layer 130, the base polymer (A) may be random PP. Homo PP has a higher crystalline content compared to random PP and can obtain high insulation properties. However, in the insulating layer 130 containing homo PP, cracking may occur within or between crystals. For this reason, homo PP may not be able to obtain its inherent insulation properties. In contrast, random PP contains ethylene units, resulting in a lower crystalline content. However, in the insulating layer 130 containing random PP, cracking due to coarse crystallization is less likely to occur. As a result, random PP can obtain higher insulation properties compared to homo PP.

[0045] When the base polymer (A) 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, the growth of coarse spherulites can be suppressed. On the other hand, by setting the ethylene unit content to 15% by mass or less, the decrease in the melting point can be suppressed, and stable use in non-crosslinked or semi-crosslinked forms can be achieved.

[0046] In this embodiment, the stereoregularity of polypropylene is not particularly limited, but may be isotactic, for example. Here, if the stereoregularity of polypropylene is atactic, the polypropylene will not crystallize. In contrast, isotactic polypropylene is polymerized with a Ziegler-Natta catalyst and is generally available. By having isotactic stereoregularity, a decrease in the melting point can be suppressed in the composition. As a result, a predetermined crystallinity can be easily obtained, and high insulating properties can be obtained.

[0047] In this embodiment, the storage modulus of the base polymer (A) alone is, for example, 800 MPa or more and 1700 MPa or less. The measurement method and conditions for the "storage modulus of the base polymer (A) alone" are the same as those for the storage modulus of the insulating layer described later.

[0048] The melt flow rate (MFR) of the base polymer (A) is not particularly limited, but the MFR of the base polymer (A) may be, for example, 0.1 g / 10 min or more and 5.0 g / 10 min or less, or 0.1 g / 10 min or more and 2.0 g / 10 min or less. Here, "MFR" refers to the value measured in accordance with JIS K7210 at a temperature of 190°C and a load of 2.16 kg. By setting the MFR of the base polymer (A) within the above range, the phase structure described later can be easily formed in the insulating layer 130.

[0049] The melting point of the base polymer (A) is not particularly limited, but it may be, for example, 130°C to 165°C. This allows for the easy formation of the phase structure described later when the base polymer (A) is mixed with at least one of the modified polymer (B) and the thermoplastic elastomer (C).

[0050] (Modified polymer (B)) Modified polymer (B) is a resin that contains propylene units as the main chain and is modified with at least one selected from unsaturated organic acids and their derivatives.

[0051] The inclusion of propylene units in the modified polymer (B) improves the compatibility when the base polymer (A) and the modified polymer (B) are mixed.

[0052] By modifying the modified polymer (B) with an unsaturated organic acid containing polar groups, polar groups can be introduced into the resin composition. This allows space charges to be trapped by the polar groups dispersed in the insulating layer 130 formed by the resin composition. In other words, local accumulation of space charges in the insulating layer 130 can be suppressed. As a result, the insulating properties of the insulating layer 130 can be improved.

[0053] Specifically, the modified polymer (B) may be, for example, an unsaturated carboxylic acid-modified polypropylene, in which polypropylene is modified with at least one of the unsaturated carboxylic acids and their derivatives. Examples of unsaturated carboxylic acids and their derivatives include acrylic acid, methacrylic acid, crotonic acid, maleic acid, cinnamic acid, itaconic acid, citraconic acid, fumaric acid, and their anhydrides.

[0054] Among these, the modified polymer (B) may be modified with, for example, maleic anhydride. Maleic anhydride has a large number of polar groups per unit molecular weight. As a result, even if the amount of maleic anhydride in the modified polymer (B) is small, a sufficient number of polar groups can be ensured in the modified polymer (B).

[0055] The amount of modification (hereinafter simply referred to as "amount of modification") of at least one of the unsaturated carboxylic acid and its derivatives in the modified polymer (B) is not particularly limited, but the amount of modification may be, for example, 0.1% by mass or more and 10% by mass or less. Here, "amount of modification" means the copolymerization ratio (content) of at least one of the unsaturated carboxylic acid and its derivatives in the modified polymer (B). By setting the amount of modification to 0.1% by mass or more, the accumulation of space charge in the insulating layer 130 can be stably suppressed. On the other hand, by setting the amount of modification to 10% by mass or less, the compatibility between the modified polymer (B) and the base polymer (A) can be improved.

[0056] In this embodiment, the storage modulus of the modified polymer (B) alone is, for example, 1000 MPa or more and 1900 MPa or less. The measurement method and conditions for the "storage modulus of the modified polymer (B) alone" are the same as those for the storage modulus of the insulating layer described later.

[0057] The MFR of modified polymer (B) is not particularly limited, but may be, for example, 0.1 g / 10 min to 500 g / 10 min, or 1 g / 10 min to 300 g / 10 min. The measurement conditions for the MFR of modified polymer (B) here are the same as those described for the MFR of base polymer (A). By having the MFR of modified polymer (B) within the above range, the phase structure described later can be easily formed when base polymer (A) and modified polymer (B) are mixed.

[0058] The melting point of the modified polymer (B) is not particularly limited, but it may be, for example, 130°C to 165°C. This allows for the easy formation of the phase structure described later when the base polymer (A) and the modified polymer (B) are mixed.

[0059] (Thermoplastic elastomer (C)) The thermoplastic elastomer (C) has lower crystallinity compared to the base polymer (A) which has propylene units. The thermoplastic elastomer (C) can suppress excessive crystal growth of the base polymer (A) and impart flexibility to the insulating layer 130.

[0060] Examples of thermoplastic elastomers (C) include amide-based, ester-based, olefin-based, styrene-based, urethane-based, PVC-based, and fluorine-based elastomers. Among these, thermoplastic elastomer (C) may be at least one of styrene-based polymers and olefin-based elastomers.

[0061] (Styrene-based elastomer) The thermoplastic elastomer (C) may contain, for example, a styrene-based elastomer. This allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0062] In this embodiment, the styrene-based elastomer is a copolymer comprising, for example, styrene units as hard segments and at least one monomer unit from among ethylene units, propylene units, butylene units, and isoprene units as soft segments.

[0063] Examples of styrene-based elastomers include styrene-butadiene-styrene block copolymer (SBS), hydrogenated styrene-butadiene-styrene block copolymer, styrene-ethylene-styrene block copolymer (SES), styrene-ethylene-propylene copolymer (SEP), styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS), styrene-ethylene-propylene-styrene block copolymer (SEPS), styrene-isoprene-styrene copolymer (SIS), hydrogenated styrene-isoprene-styrene copolymer, hydrogenated styrene-butadiene rubber, hydrogenated styrene-isoprene rubber, and styrene-ethylene-butylene-olefin crystal block copolymer. Two or more of these may be used in combination.

[0064] In this context, "hydrogenated" means that hydrogen has been added to the double bond. For example, "hydrogenated styrene-butadiene-styrene block copolymer" refers to a polymer in which hydrogen has been added to the double bond of styrene-butadiene-styrene block copolymer. Hydrogen is not added to the double bond of the aromatic ring of styrene. "Hydrogenated styrene-butadiene-styrene block copolymer" can be rephrased as styrene-ethylene-butylene-styrene block copolymer (SEBS).

[0065] The styrene-based elastomer may be, for example, a hydrogenated material that does not contain double bonds in its chemical structure, excluding the aromatic ring. When a non-hydrogenated material is used, the resin components may degrade due to heat during molding of the resin composition. This may lead to a decrease in the properties of the resulting insulating layer 130. In contrast, using a hydrogenated material can improve resistance to heat degradation. This allows the properties of the insulating layer 130 to be maintained at a higher level.

[0066] Furthermore, the styrene-based elastomer may contain, for example, styrene units and at least one of propylene units and butene units. This allows for a stable improvement in the compatibility between polypropylene as the base polymer (A) and the styrene-based elastomer. As a result, polypropylene and the styrene-based elastomer can be uniformly mixed.

[0067] The styrene unit content (content rate; hereinafter simply referred to as "styrene unit content") in the styrene-based elastomer is not particularly limited, but the styrene unit content may be, for example, 5% by mass or more and 35% by mass or less. By setting the styrene unit content to 5% by mass or more and 35% by mass or less, it is possible to stably suppress the material from becoming excessively hard. This makes it possible to stably suppress the separation and cracking of polypropylene and thermoplastic elastomer (C).

[0068] (Olefin-based elastomer) The thermoplastic elastomer (C) may include, for example, an olefin-based elastomer. This allows for easy incorporation of short-chain branching within the molecular structure of the flexible thermoplastic elastomer (C). As a result, the compatibility between the base polymer (A) and the thermoplastic elastomer (C) can be improved.

[0069] Olefin-based elastomers as thermoplastic elastomers (C) are, for example, copolymers containing two types of olefin units. Examples of olefin-based elastomers include copolymers containing ethylene units and α-olefin units having 3 or more carbon atoms, and copolymers containing propylene units and α-olefin units having 4 or more carbon atoms.

[0070] Specifically, examples of olefin-based elastomers include ethylene propylene rubber (EPR), very low-density polyethylene (VLDPE), and propylene-1-butene copolymer (propylene-1-butene rubber (PBR)). Ethylene-1-butene copolymer (ethylene-1-butene rubber (EBR)) is a type of VLDPE.

[0071] The olefin-based elastomer may contain, for example, at least one of propylene units and butene units. This allows for a stable improvement in the compatibility between polypropylene as the base polymer (A) and the olefin-based elastomer. As a result, polypropylene and the olefin-based elastomer can be uniformly mixed.

[0072] When the olefin-based elastomer contains ethylene units, the content of ethylene units in the olefin-based elastomer is not particularly limited, but the content of ethylene units may be, for example, 5% by mass or more and 90% by mass or less. This makes it possible to stably obtain the softening effect and crystallization inhibition effect of the olefin-based elastomer.

[0073] (Characteristics of Thermoplastic Elastomer) In this embodiment, the storage modulus of the thermoplastic elastomer (C) alone is, for example, 20 MPa or more and 500 MPa or less. The measurement method and measurement conditions for the "storage modulus of the thermoplastic elastomer (C) alone" are the same as those for the storage modulus of the insulating layer described later.

[0074] The MFR of the thermoplastic elastomer (C) is not particularly limited, but may be, for example, 0.1 g / 10 min to 5.0 g / 10 min, or 0.1 g / 10 min to 2.0 g / 10 min. The measurement conditions for the MFR of the thermoplastic elastomer (C) here are the same as those described for the MFR of the base polymer (A). By having the MFR of the thermoplastic elastomer (C) within the above range, the phase structure described later can be easily formed when the base polymer (A) and the thermoplastic elastomer (C) are mixed.

[0075] The thermoplastic elastomer (C) may, for example, have no melting point or have a melting point of less than 165°C. This allows for the easy formation of the phase structure described later when the base polymer (A) and the thermoplastic elastomer (C) are mixed.

[0076] (Content of Modified Polymer (B) and Thermoplastic Elastomer (C)) The content of modified polymer (B) and thermoplastic elastomer (C) will be explained below. Here, the total content of the resin component consisting of base polymer (A), modified polymer (B), and thermoplastic elastomer (C) is set to 100 parts by mass. Of the resin component, the remainder other than modified polymer (B) and thermoplastic elastomer (C) is base polymer (A).

[0077] In this embodiment, the content of the modified polymer (B) in the resin composition (i.e., the insulating layer 130) may be, for example, 1 part by mass or more and 10 parts by mass or less. By setting the content of the modified polymer (B) to 1 part by mass or more, a decrease in the insulating properties of the insulating layer 130 can be suppressed. On the other hand, by setting the content of the modified polymer (B) to 10 parts by mass or less, a decrease in the moldability of the insulating layer 130 can be suppressed. This makes it possible to suppress a decrease in the insulating properties of the insulating layer 130 caused by a decrease in moldability.

[0078] The content of thermoplastic elastomer (C) in the resin composition (i.e., the insulating layer 130) may be, for example, 10 parts by mass or more and 45 parts by mass or less.

[0079] By including 10 parts by mass or more of thermoplastic elastomer (C), flexibility can be imparted to the insulating layer 130, and it is possible to suppress the elasticity of the insulating layer 130 from becoming excessively high throughout the thickness direction of the insulating layer 130. Furthermore, when the cooling process S340 described later is applied, it is possible to suppress the excessively high degree of orientation only on the outside of the insulating layer 130 due to the low amount of thermoplastic elastomer (C). This makes it possible to suppress large variations in the elasticity of the insulating layer in the thickness direction of the insulating layer. By suppressing the difference in elasticity within the insulating layer 130, it is possible to suppress the generation of voids caused by stress differences within the insulating layer 130. As a result, it is possible to suppress the decrease in insulating properties caused by the generation of voids in the insulating layer 130.

[0080] On the other hand, by limiting the content of thermoplastic elastomer (C) to 45 parts by mass or less, that is, by suppressing the excessive incorporation of amorphous thermoplastic elastomer (C), it is possible to suppress the decrease in the insulating and mechanical properties that are inherently required of polypropylene, which has a high melting point as the base polymer (A).

[0081] Furthermore, by keeping the content of the modified polymer (B) and thermoplastic elastomer (C) within the above-mentioned range, the phase structure described later can be easily formed.

[0082] In this way, by keeping the content of the modified polymer (B) and the thermoplastic elastomer (C) within the above-mentioned range, it is possible to stably achieve both the desired distribution of elasticity and uniformity of insulating properties in the thickness direction of the insulating layer 130.

[0083] (Inorganic Filler) In this embodiment, by including the modified polymer (B) in the resin composition, the space charge trapping effect of the modified polymer (B) can be uniformly obtained in the resin composition, as described above. As a result, high insulation properties can be stably obtained without adding an inorganic filler. Therefore, in this embodiment, the resin composition does not need to contain an inorganic filler.

[0084] On the other hand, the resin composition may contain a small amount of inorganic filler that does not cause clogging of the mesh in the extruder.

[0085] Specifically, the resin composition contains an inorganic filler, and the amount of the inorganic filler may be, for example, less than 1 part by mass when the total amount of resin components is 100 parts by mass. The lower limit of the inorganic filler content is not limited as long as an inorganic filler can be added.

[0086] Examples of inorganic fillers include magnesium oxide (MgO), silicon dioxide, zinc oxide, aluminum oxide, titanium oxide, zirconium oxide, carbon black, and mixtures of two or more of these.

[0087] The mean volume diameter (MV) of the inorganic filler is not particularly limited, but may be, for example, 1 μm or less, or 700 nm or less, or 100 nm or less. Here, "mean volume diameter (MV)" refers to the particle size of the particles d ｉ , particle volume V ｉ In this case, it can be calculated using the following formula: MV = Σ(V ｉ d ｉ ) / ΣV ｉ Furthermore, a dynamic light scattering particle size and particle size distribution analyzer is used to measure the volume-average particle size.

[0088] There are no particular limitations on the lower limit of the volume-average particle size of the inorganic filler. However, from the viewpoint of stably forming the inorganic filler, the volume-average particle size of the inorganic filler may be, for example, 1 nm or more, or 5 nm or more.

[0089] At least a portion of the inorganic filler may be surface-treated with a silane coupling agent. This can improve the adhesion of the interface between the inorganic filler and the base polymer (A), thereby improving the mechanical properties and insulating properties of the insulating layer 130.

[0090] (Crosslinking agent) In this embodiment, the resin component constituting 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.

[0091] On the other hand, the resin composition may contain a small amount of crosslinking agent so as to reduce the gel fraction (degree of crosslinking). Specifically, for example, the resin composition may contain a crosslinking agent in such a quantity that the amount of decomposition residue remaining in the insulating layer 130 due to the decomposition of the crosslinking agent in the insulating layer 130 is less than 300 ppm. When dicumyl peroxide is used as the crosslinking agent, the residue may be, for example, cumyl alcohol or α-methylstyrene.

[0092] (Other Additives) The resin composition may contain other additives as needed. Other additives include antioxidants, lubricants, and colorants.

[0093] (2) Power Cable Next, the power cable of this embodiment will be described with reference to Figure 1.

[0094] The power cable 10 in this embodiment is configured as a so-called solid-insulated power cable. The power cable 10 includes, for example, a conductor 110, an internal semiconducting layer 120, an insulating layer 130, an external semiconducting layer 140, a shielding layer 150, and a sheath 160. The power cable 10 is disclosed, for example, in WO2022 / 137750.

[0095] (Insulating layer) The insulating layer 130 is provided so as to cover the outer circumference of the internal semiconducting layer 120. The insulating layer 130 is extruded from a resin composition, for example, as described above. The insulating layer 130 has an inner circumferential surface facing the conductor 110 and an outer circumferential surface opposite to the inner circumferential surface.

[0096] In this embodiment, the insulating layer 130 has a predetermined phase structure by mixing a base polymer (A), a modified polymer (B), and a thermoplastic elastomer (C).

[0097] Specifically, the insulating layer 130 contains the modified polymer (B), which forms a structure in which the base polymer (A) and the modified polymer (B) are compatible, or a sea-island structure in which the modified polymer (B) is finely dispersed in the base polymer (A). Here, "compatible structure" means, for example, that when the phase structure is observed with a transmission electron microscope, no phase separation can be confirmed, and each component is uniformly dispersed.

[0098] The more finely the modified polymer (B) is dispersed in the insulating layer 130, the more effectively the accumulation of space charge can be suppressed. From this viewpoint, a structure in which the resin components are compatible may be formed in the insulating layer 130. Alternatively, if a sea-island structure is formed in the insulating layer 130, the diameter of the island phase formed from the modified polymer (B) may be less than 0.5 μm.

[0099] Furthermore, by including not only the base polymer (A) and the modified polymer (B) but also the thermoplastic elastomer (C) in the insulating layer 130, a sea-island structure is formed in which the thermoplastic elastomer (C) is finely dispersed within the two-component phase structure of the insulating layer 130, which includes the base polymer (A) and the modified polymer (B).

[0100] By forming the above-described phase structure in the insulating layer 130, the polar groups in the modified polymer (B) can be uniformly dispersed within the insulating layer 130. This allows for a uniform trapping effect of space charge within the insulating layer 130.

[0101] Furthermore, by forming the above-described phase structure in the insulating layer 130, the thermoplastic elastomer (C) can be uniformly dispersed in the insulating layer 130. This makes it possible to stably obtain an elastic distribution of the insulating layer 130 according to the difference in cooling rate in the thickness direction of the insulating layer 130.

[0102] In this embodiment, the insulating layer 130 is non-crosslinked or slightly crosslinked. Even if the insulating layer 130 is slightly crosslinked, the gel fraction (degree of crosslinking) is low. In this case, the insulating layer 130 is slightly crosslinked, for example, by having less than 300 ppm of decomposition residue remaining in the insulating layer 130 due to the decomposition of the crosslinking agent. In this way, the recyclability can be improved by making the insulating layer 130 non-crosslinked or slightly crosslinked.

[0103] (Specific dimensions, etc.) The specific dimensions of the power cable 10 are not particularly limited, but for example, the diameter of the conductor 110 is 5 mm or more and 60 mm or less, the thickness of the internal semiconducting 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 external semiconducting 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, and the thickness of the sheath 160 is 1 mm or more. The DC voltage applied to the power cable 10 of this embodiment is, for example, 20 kV or more.

[0104] (3) Cable characteristics In this embodiment, the following cable characteristics are ensured in the insulating layer 130.

[0105] The "outer sample" as referred to below means a sheet taken from a position 0.3 mm toward the conductor 110 in the radial direction of the conductor 110 from the outer peripheral surface of the insulating layer 130. The "inner sample" means a sheet taken from a position 0.3 mm toward the outer peripheral surface of the insulating layer 130 in the radial direction of the insulating layer 130 from the inner peripheral surface of the insulating layer 130.

[0106] (3-1) Insulating property In this embodiment, since the insulating layer 130 contains the modified polymer (B), the trap effect of space charge by the modified polymer (B) can be obtained uniformly in the insulating layer 130. As a result, the insulating property of the insulating layer 130 is uniform in the thickness direction of the insulating layer 130.

[0107] Specifically, the ratio of the volume resistivity of the inner sample of the insulating layer 130 to the volume resistivity of the outer sample of the insulating layer 130 (hereinafter, also referred to as "volume resistivity ratio") is, for example, 1.0 or more and 1.5 or less.

[0108] The "volume resistivity of the outer sample" and the "volume resistivity of the inner sample" referred to here are measured under the conditions of a temperature of 90 °C and a DC electric field of 80 kV / mm.

[0109] In this embodiment, by setting the volume resistivity ratio to 1.0 or more and 1.5 or less, sufficient insulating property required for the entire insulating layer 130 of the power cable 10 can be obtained.

[0110] The volume resistivity of the outer sample of the insulating layer 130 and the volume resistivity of the inner sample of the insulating layer 130 measured under the conditions of a temperature of 90 °C and a DC electric field of 80 kV / mm are, for example, 7.0×10 １４ Ω·cm or more, or may be 3.6×10 １５ Ω·cm or more, or may be 5.0×10 １５ Ω·cm or more, or may be 7.0×10 １５ Ω·cm or more. In the measurement of the volume resistivity, the thickness of each sample is 0.2 mm.

[0111] The DC breakdown field strength of the outer sample of the insulating layer 130 and the DC breakdown field strength of the inner sample of the insulating layer 130, measured under conditions of a temperature of 90°C, may be, for example, 160 kV / mm or more, or 200 kV / mm or more. In measuring the DC breakdown field strength, the thickness of each sample is set to 0.2 mm.

[0112] (3-2) Degree of Orientation In this embodiment, the degree of orientation of the insulating layer 130 is within a predetermined range by performing a cooling process described later after the extrusion process of the insulating layer 130.

[0113] Here, the "degree of orientation" of a polymer can be measured, for example, by wide-angle X-ray scattering, based on the peaks that appear at predetermined diffraction angles depending on the orientation of the polymer. The measurement of the "degree of orientation" by wide-angle X-ray scattering is performed, for example, by the following procedure.

[0114] As shown in Figure 3, a sheet-like sample is placed between the X-ray source and the detector. By irradiating the sample perpendicularly with X-rays, for example, Cu Kα rays, from the X-ray source, an X-ray scattering image is obtained in the detector. Here, the angle at which the X-rays diffract (scatter) from the sample with respect to the direction of X-ray incidence to the sample is called the "diffraction angle (scattering angle) 2θ". In the X-ray scattering image, the circumferential angle around the center of the X-ray irradiation is called the "azimuth angle β".

[0115] In this embodiment, when the sample obtained from the insulating layer 130 mainly consists of a base polymer (A) made of polypropylene, a peak will appear in the above-described X-ray scattering image in the range of diffraction angle 2θ of 15° to 18°, depending on the orientation of the base polymer (A). Note that the peak position may differ slightly within the above range depending on the composition of the sample.

[0116] Next, for each azimuthal angle β of the X-ray scattering image described above, the scattering intensity in the range where the diffraction angle 2θ at which a peak occurs is between 15° and 18° is integrated. By plotting the integrated scattering intensity over the range of azimuthal angle β from 0° to 360°, an azimuthal direction profile (β direction profile) is obtained.

[0117] In the β-directional profile described above, peaks are generated in a predetermined distribution depending on the orientation state of the base polymer (A) in the sample. Waveform separation is performed by fitting each peak in the β-directional profile with a Gaussian function.

[0118] After the waveform separation described above, the full width at half maximum (FWHM) W of the Gaussian function fitted to each peak is determined. Here, "FWHM" refers to the full width at half maximum (FWHM). Once the FWHM W of each peak is obtained, the degree of orientation f of the sample is calculated using the following equation (1): f = {(360 - ΣW) / 360} × 100 ... (1) Here, ΣW is the sum of the FWHM W of each peak.

[0119] Next, with reference to Figures 5 and 7, the β-direction profile and degree of orientation f corresponding to the specific orientation state of the base polymer (A) will be explained.

[0120] (Reference example: Outer sample of sample B1) The reference example corresponds to sample B1 described later, for example. In the reference example, the insulating layer 130 contains, for example, a base polymer (A), a modified polymer (B), and a thermoplastic elastomer (C) within the specified ranges described above. On the other hand, in the reference example, the rate at which the outer surface of the insulating layer 130 is cooled in the cooling step S340 after the extrusion step S320 of the insulating layer 130 is slowed down.

[0121] In the X-ray scattering image of the outer sample in the reference example, an annular peak is obtained with a radius equal to the distance from the X-ray irradiation center (2θ = 0°) to a position at a predetermined diffraction angle of 2θ. Therefore, in the outer sample in the reference example, a β-directional profile like sample B1 shown in Figure 7 is obtained.

[0122] In the β-directional profile of Figure 7, the intensity is low and gradual over a wide azimuth angle β. In the β-directional profile of Figure 7, for example, four gentle peaks are observed. The full width at half maximum of the Gaussian function fitted to the four peaks are, for example, W1' to W4'. As a result, in the outer sample shown in Figure 7, the degree of orientation f is calculated using the above-mentioned equation (1) as follows: f = {(360 - (W1' + W2' + W3' + W4')) / 360} × 100 = 7 (%)

[0123] Thus, in the reference example, as a result of slowing down the cooling rate of the outer surface of the insulating layer 130 after extrusion, the orientation degree f of the outer sample becomes less than 10%. In the reference example, since the cooling rate inside the insulating layer 130 is even slower than on the outside, the orientation degree f of the inner sample also becomes less than 10%. In other words, the crystallinity of the base polymer (A) is high and the orientation degree f of the base polymer (A) is low throughout the entire thickness direction of the insulating layer 130.

[0124] (This embodiment: outer sample of sample A1) In this embodiment, for example, the insulating layer 130 contains a base polymer (A), a modified polymer (B), and a thermoplastic elastomer (C), each within the specified range described above. Furthermore, in this embodiment, as a novel manufacturing method described later, the rate at which the outer surface of the insulating layer 130 is cooled in the cooling step S340 after the extrusion step S320 of the insulating layer 130 is increased.

[0125] In the X-ray scattering image of the outer sample measured in this embodiment, multiple peaks are obtained on a circle with a radius equal to the distance from the X-ray irradiation center (2θ = 0°) to a position at a predetermined diffraction angle of 2θ. Therefore, in the outer sample of this embodiment, a β-directional profile like sample A1 shown in Figure 5 is obtained.

[0126] In the β-direction profile of Figure 5, for example, four more distinct peaks are observed than those in Figure 7. The full width at half maximum of the Gaussian function fitted to these four peaks are, for example, W1 to W4. As a result, for the outer sample shown in Figure 5, the degree of orientation f is calculated using the above-mentioned equation (1) as follows: f = {(360 - (W1 + W2 + W3 + W4)) / 360} × 100 = 31 (%)

[0127] Thus, in this embodiment, the degree of orientation f of the outer sample of the insulating layer 130, as measured by wide-angle X-ray scattering, may be, for example, 10% or more and less than 70%, or 20% or more and 65% or less.

[0128] When the orientation degree f of the outer sample is less than 10%, for example, it is as in the reference example described above. When the orientation degree f of the outer sample is less than 10%, as in the reference example described above, the crystallinity of the base polymer (A) is high and the orientation degree f of the base polymer (A) is low throughout the thickness direction of the insulating layer 130. As a result, throughout the thickness direction of the insulating layer 130, the base polymer (A) forms crystals and multiple molecular chains of the base polymer (A) become entangled with each other. Consequently, the elasticity of the base polymer (A) of the insulating layer 130 is uniformly increased in the thickness direction of the insulating layer 130.

[0129] In contrast, in this embodiment, by setting the orientation degree f of the outer sample to 10% or more, or 20% or more, excessive entanglement of multiple molecular chains of the base polymer (A) can be suppressed at least outside the insulating layer 130. This makes it possible to lower the elasticity of the base polymer (A) at least outside the insulating layer 130.

[0130] On the other hand, if the orientation degree f of the outer sample is 70% or more, there are, for example, two cases.

[0131] In the first case, where the orientation degree f of the outer sample is 70% or more, the content of thermoplastic elastomer (C) in the insulating layer 130 is less than 10 parts by mass. Furthermore, in the first case, the cooling rate of the outer surface of the insulating layer 130 is increased in the cooling step S340 after the extrusion step S320 of the insulating layer 130. In the first case, the imparting of flexibility by thermoplastic elastomer (C) is insufficient. As a result, the degree of crystallinity is high throughout the thickness direction of the insulating layer 130. On the other hand, due to the rapid cooling of the outside of the insulating layer 130 under conditions of low thermoplastic elastomer (C) content, the orientation from the extrusion is excessively retained only on the outside of the insulating layer 130, resulting in an excessively high orientation degree f. As described above, the elasticity is excessively high on the inside of the insulating layer 130 due to the high crystallinity. In contrast, on the outside of the insulating layer 130, although the crystallinity is high, the orientation degree is excessively high, so the elasticity is within an appropriate range. In other words, the elasticity of the insulating layer 130 varies greatly in the thickness direction of the insulating layer 130. Therefore, in the first case, voids are generated due to the difference in elasticity within the insulating layer 130 as described above, i.e., the difference in stress within the insulating layer 130. As a result, in the first case, the insulating properties of the insulating layer 130 are reduced due to the generation of voids in the insulating layer 130.

[0132] In the second case, where the orientation degree f of the outer sample is 70% or more, the insulating layer 130 contains the base polymer (A), modified polymer (B), and thermoplastic elastomer (C) within the specified ranges described above. On the other hand, in the second case, in the cooling step S340 after the extrusion step S320 of the insulating layer 130, the rate at which the outer surface of the insulating layer 130 is cooled is excessively fast. In the second case, due to the excessive rapid cooling, the orientation from the extrusion is excessively retained on the outside of the insulating layer 130, and the orientation degree f becomes excessively high. As a result, the elasticity of the insulating layer on the outside of the insulating layer 130 is excessively lower than that on the inside of the insulating layer 130. That is, the elasticity of the insulating layer 130 varies greatly in the thickness direction of the insulating layer 130. In the second case, voids are generated due to the difference in elasticity within the insulating layer 130 as described above, i.e., the difference in stress within the insulating layer 130. As a result, even in the second case, the insulating properties of the insulating layer 130 decrease due to the generation of voids in the insulating layer 130.

[0133] In contrast, in this embodiment, by setting the orientation degree f of the outer sample to less than 70% or 65% or less, it is possible to suppress large variations in the elasticity of the insulating layer 130 in the thickness direction of the insulating layer 130. This makes it possible to suppress the generation of voids caused by differences in elasticity within the insulating layer 130, i.e., differences in stress within the insulating layer 130. As a result, in this embodiment, it is possible to suppress the decrease in the insulating properties of the insulating layer 130 caused by the generation of voids in the insulating layer 130.

[0134] In this embodiment, as a novel manufacturing method described later, in the cooling step S340 after the extrusion step S320 of the insulating layer 130, the rate at which the outer surface of the insulating layer 130 is cooled is increased, so that the outside of the insulating layer 130 is rapidly cooled, while the inside of the insulating layer 130 is slowly cooled. Therefore, the degree of orientation f of the inner sample of the insulating layer 130 is lower than the degree of orientation f of the outer sample.

[0135] Specifically, in this embodiment, the degree of orientation f of the inner sample of the insulating layer 130, measured by wide-angle X-ray scattering, may be, for example, 10% or more and less than 60%, or 14% or more and 40% or less.

[0136] By setting the orientation degree f of the inner sample to 10% or more, or 14% or more, it is possible to suppress large variations in the elasticity of the insulating layer 130 in the thickness direction of the insulating layer 130. This makes it possible to suppress the generation of voids caused by differences in elasticity within the insulating layer 130, i.e., differences in stress within the insulating layer 130. As a result, it is possible to suppress a decrease in the insulating properties of the insulating layer 130.

[0137] On the other hand, by setting the orientation degree f of the inner sample to less than 60% or 40% or less, as described above, the elasticity of the inner part of the insulating layer 130 can be made higher than the elasticity of the outer part of the insulating layer 130 while keeping the elasticity of the outer part of the insulating layer 130 low. In other words, the elasticity of the insulating layer 130 can be made different in the thickness direction of the insulating layer 130. This makes it possible to make the outer part of the insulating layer 130 relatively soft while making the inner part of the insulating layer 130 relatively hard. As a result, in this embodiment, it is possible to achieve both flexibility and strength of the insulating layer 130.

[0138] In this embodiment, no excessive difference in the degree of orientation f occurs in the thickness direction of the insulating layer 130.

[0139] Specifically, the ratio of the orientation degree f of the inner sample to the orientation degree f of the outer sample of the insulating layer 130, as measured by wide-angle X-ray scattering (hereinafter also referred to as the "orientation degree ratio"), is, for example, 0.3 or more and 0.7 or less, or 0.4 or more and 0.6 or less.

[0140] By setting the orientation ratio to 0.3 or higher, or 0.4 or higher, it is possible to suppress large variations in the elasticity of the insulating layer 130 in the thickness direction of the insulating layer 130. This suppresses the generation of voids caused by differences in elasticity within the insulating layer 130, i.e., differences in stress within the insulating layer 130. As a result, a decrease in the insulating properties of the insulating layer 130 can be suppressed. On the other hand, by setting the orientation ratio to 0.7 or lower, or 0.6 or lower, the elasticity of the outer part of the insulating layer 130 can be made lower than the elasticity of the inner part of the insulating layer 130. This makes the outer part of the insulating layer 130 relatively softer while making the inner part relatively harder. As a result, in this embodiment, both the flexibility and strength of the insulating layer 130 can be achieved.

[0141] (3-3) Elasticity In this embodiment, as a new manufacturing method described later, the rate at which the outer surface of the insulating layer 130 is cooled in the cooling step S340 after the extrusion step S320 of the insulating layer 130 is increased, so that the elasticity of the insulating layer 130 differs in the thickness direction of the insulating layer 130.

[0142] Specifically, the ratio of the storage modulus of the inner sample of the insulating layer 130 to the storage modulus of the outer sample of the insulating layer 130 (hereinafter also referred to as the "modulus ratio") is, for example, 1.1 or more and 2.5 or less.

[0143] The "storage modulus" referred to here is measured by dynamic mechanical analysis (DMA) in accordance with JIS K7244-4:1999.

[0144] The dynamic viscoelasticity measurement is performed under the following conditions: Measurement mode: Tensile mode Strain: 0.08% Frequency: 10 Hz Temperature range: 0°C to 200°C Heating rate: 10°C / min

[0145] The "storage modulus of the outer sample" and "storage modulus of the inner sample" in the above-mentioned specification of the elastic modulus ratio are values ​​measured at 25°C by the dynamic viscoelasticity measurement. In the dynamic viscoelasticity measurement, the thickness of each sample is 0.5 mm.

[0146] In this embodiment, by setting the elastic modulus ratio to 1.1 or higher, the outer side of the insulating layer 130 can be made relatively soft while the inner side of the insulating layer 130 is made relatively hard. As a result, in this embodiment, both the flexibility and strength of the insulating layer 130 can be achieved.

[0147] On the other hand, in this embodiment, by setting the elastic modulus ratio to 2.5 or less, the generation of voids caused by stress differences within the insulating layer 130 can be suppressed. This makes it possible to suppress a decrease in the dielectric breakdown strength (e.g., DC breakdown field strength) of the insulating layer 130.

[0148] The elastic modulus of the insulating layer 130 may gradually decrease from the inner surface of the insulating layer 130 toward the outer surface of the insulating layer 130 in the radial direction toward the conductor 110. By suppressing abrupt changes in the elasticity of the insulating layer 130 in this way, the occurrence of cracks within the insulating layer 130 caused by differences in elastic modulus can be suppressed.

[0149] The storage modulus of the inner sample of the insulating layer 130 is, for example, 650 MPa or more and 900 MPa or less. By setting the storage modulus of the inner sample to 650 MPa or more, the rigidity of the power cable 10 can be ensured. On the other hand, by setting the storage modulus of the inner sample to 900 MPa or less, the decrease in the flexibility of the power cable 10 can be suppressed.

[0150] The storage modulus of the outer sample of the insulating layer 130 is, for example, 280 MPa or more and 670 MPa or less. By setting the storage modulus of the outer sample to 280 MPa or more, it is possible to suppress the outer surface of the insulating layer 130 from becoming excessively soft. This makes it possible to suppress the occurrence of cracks in the insulating layer 130 caused by impacts during the installation of the power cable 10. On the other hand, by setting the storage modulus of the outer sample to 670 MPa or less, the flexibility of the power cable 10 can be ensured.

[0151] (4) Method for manufacturing a power cable Next, the method for manufacturing a power cable according to this embodiment will be described. Hereinafter, steps will be abbreviated as "S".

[0152] (S100: Resin composition preparation step) First, the resin composition of this embodiment is prepared.

[0153] In this embodiment, a resin component comprising a base polymer (A), a modified polymer (B), and a thermoplastic elastomer (C), along with other additives (such as antioxidants), is mixed (kneaded) in a mixer to form a mixed material. Examples of mixers include open roll mixers, barbell mixers, pressure kneaders, single-screw mixers, and multi-screw mixers.

[0154] In this embodiment, the base polymer (A), modified polymer (B), and thermoplastic elastomer (C) are blended such that the ratio of the volume resistivity of the inner sample of the insulating layer 130 to the volume resistivity of the outer sample of the insulating layer 130 is 1.0 or more and 1.5 or less.

[0155] Once the mixture is formed, it is granulated using an extruder. This forms a pellet-shaped resin composition that will constitute the insulating layer 130. A twin-screw extruder with high kneading capabilities may be used to perform the mixing and granulation processes in a single step.

[0156] (S200: Conductor preparation process) Meanwhile, a conductor 110 is prepared by twisting together multiple conductor core wires.

[0157] (S300: Cable core formation process (insulating layer formation process)) Once the resin composition preparation process S100 and the conductor preparation process S200 are completed, in the cable core formation process S300, an insulating layer 130 is formed using the above-mentioned resin composition to cover the outer circumference of the conductor 110.

[0158] The cable core formation process S300 of this embodiment includes, for example, an extrusion process S320 and a cooling process S340.

[0159] (S320: Extrusion process) In this embodiment, for example, a three-layer simultaneous extruder is used to simultaneously form the inner semiconducting layer 120, the insulating layer 130, and the outer semiconducting layer 140.

[0160] Specifically, in a three-layer simultaneous extruder, for example, the composition for the internal semiconducting layer is fed into extruder A, which forms the internal semiconducting layer 120.

[0161] The pelletized resin composition described above is fed into the extruder B for forming the insulating layer 130. At this time, the set temperature of the extruder B is set to a temperature that is 10°C to 80°C higher than the melting point of the base polymer (A). The set temperature is adjusted as appropriate based on the linear speed and extrusion pressure.

[0162] An external semiconducting layer composition containing the same material as the internal semiconducting layer resin composition that was fed into extruder A is fed into extruder C to form the external semiconducting layer 140.

[0163] Next, the extruded materials from extruders A to C are guided to a common head, and the inner semiconducting layer 120, insulating layer 130, and outer semiconducting layer 140 are simultaneously extruded onto the outer circumference of the conductor 110 from the inside out. This forms the extruded material that will become the cable core.

[0164] (S340: Cooling process) Once the extrusion process S320 is completed, the extruded material described above is cooled.

[0165] In this embodiment, the insulating layer 130 is cooled such that the degree of orientation f of the outer sample of the insulating layer 130, as measured by wide-angle X-ray scattering, is 10% or more and less than 70%. Furthermore, in this embodiment, the insulating layer 130 may be cooled such that the ratio of the storage modulus of the inner sample of the insulating layer 130 to the storage modulus of the outer sample of the insulating layer 130 is 1.1 or more and 2.5 or less. In other words, the cable core is cooled in such a way that a temperature difference is intentionally created between the inside and outside of the insulating layer 130.

[0166] Specifically, the cable cores formed in the extrusion process S320 are sequentially immersed in a water tank containing water cooled to a predetermined temperature by a chiller (hereinafter also referred to as cooling water). This rapidly cools the insulating layer 130, starting from the area closest to the outer surface of the insulating layer 130.

[0167] At this time, the cooling rate (cooling rate) at a temperature of 110°C at a position 0.3 mm from the outer surface of the insulating layer 130 toward the conductor 110, i.e., the sampling position of the outer sample, may be set to, for example, 30°C / min or more, or 75°C / min or more, or 100°C / min or more. This shortens the time that the outside of the insulating layer 130 is held in the crystallization temperature range, and suppresses excessive crystallization on the outside of the insulating layer 130. Furthermore, the outside of the insulating layer 130 can be solidified while maintaining the orientation during extrusion to some extent. That is, the degree of orientation f on the outside of the insulating layer 130 can be increased. As a result, the elasticity of the outside of the insulating layer 130 can be reduced. The "cooling rate at a temperature of 110°C" here is determined by the absolute value of the temperature gradient with respect to time at a temperature of 110°C.

[0168] At this time, the cooling rate at the sampling position of the outer sample of the insulating layer 130 at a temperature of 110°C may be set to, for example, 310°C / min or less. This suppresses excessive variation in the degree of crystallinity in the thickness direction of the insulating layer 130 and prevents the degree of orientation f on the outside of the insulating layer 130 from becoming excessively high. As a result, it is possible to suppress the occurrence of excessive differences in elasticity in the thickness direction of the insulating layer 130.

[0169] At this time, the temperature of the cooling water in the tank is set to, for example, -50°C to 50°C. This allows the outside of the insulating layer 130 to be cooled at the cooling rate described above.

[0170] On the other hand, for at least a portion of the time immediately following the extrusion process S320 in which the insulating layer 130 is extruded, the insulating layer 130 is cooled from the region close to the outer surface of the insulating layer 130 while the insulating layer 110 is heated by electromagnetic induction. Here, "electromagnetic induction heating" refers to heating the conductor 110 by the heat generated by eddy current loss and the heat generated by hysteresis loss in the conductor 110 due to electromagnetic induction. Electromagnetic induction heating allows the conductor 110 of the cable core immersed in the water tank to be heated non-contact. This allows for rapid cooling of the outside of the insulating layer 130 while suppressing cooling of the inside of the insulating layer 130. In other words, the inside of the insulating layer 130 can be cooled slowly.

[0171] At this time, for example, the conductor 110 is heated by electromagnetic induction so that its temperature is between 80°C and 110°C. This allows the inside of the insulating layer 130 to be heated to a temperature close to the crystallization temperature of polypropylene as the base polymer (A), and allows the inside of the insulating layer 130 to be slowly cooled. By slowly cooling the inside of the insulating layer 130, the orientation in at least a portion of the inside of the insulating layer 130 can be reduced compared to the orientation on the outside of the insulating layer 130. In other words, the degree of orientation of the inner sample of the insulating layer 130 can be made lower than the degree of orientation of the outer sample. As a result, the elasticity of the inside of the insulating layer 130 can be made relatively higher.

[0172] The period during which the conductor 110 is electromagnetically heated to the above temperature may be shorter than, for example, the period during which the cable core is water-cooled.

[0173] As described above, the insulating layer 130 is cooled by the cooling process S340, for example, as shown in Figure 2.

[0174] As shown in Figure 2, the temperature on the outside of the insulating layer 130 drops rapidly from the temperature immediately after the extrusion process S320 to near the temperature of the cooling water. On the other hand, the temperature on the inside of the insulating layer 130, which is close to the conductor 110, drops slowly to the temperature of the electromagnetically heated conductor 110. After the electromagnetic induction heating of the conductor 110 is stopped, the temperature on the inside of the insulating layer 130 gradually drops to the temperature of the cooling water. In this way, the outside of the insulating layer 130 is rapidly cooled, while the inside of the insulating layer 130 is slowly cooled.

[0175] In this way, by rapidly cooling the outside of the insulating layer 130, crystallization of the base polymer (A) outside the insulating layer 130 can be suppressed, and the resin component can be solidified in a near-amorphous state. In other words, the degree of crystallinity outside the insulating layer 130 can be reduced. Furthermore, by rapidly cooling the outside of the insulating layer 130, the orientation of the base polymer (A) during extrusion can be maintained in a near-amorphous state outside the insulating layer 130. In other words, the degree of orientation f outside the insulating layer 130 can be increased. As a result, the base polymer (A) outside the insulating layer 130 can be made to be in the state shown in Figure 1(a). This makes it possible to suppress excessive entanglement of multiple molecular chains of the base polymer (A) at least outside the insulating layer 130. As a result, the elasticity of the base polymer (A) at least outside the insulating layer 130 can be reduced.

[0176] On the other hand, by slowly cooling the inside of the insulating layer 130, the base polymer (A) can be crystallized inside the insulating layer 130. That is, the degree of crystallinity inside the insulating layer 130 can be made higher than that outside the insulating layer 130. Furthermore, the degree of orientation f of the inner sample of the insulating layer 130 can be made lower than that of the outer sample. As a result, as described above, the elasticity of the inside of the insulating layer 130 can be made higher than that of the outer sample while keeping the elasticity of the outside of the insulating layer 130 low. That is, the elasticity of the insulating layer 130 can be made different in the thickness direction of the insulating layer 130. As a result, the outside of the insulating layer 130 can be made relatively softer while the inside of the insulating layer 130 can be made relatively hard.

[0177] The above cable core formation process S300 forms a cable core composed of a conductor 110, an internal semiconducting layer 120, an insulating layer 130, and an external semiconducting layer 140.

[0178] (S400: Shielding layer formation process) After the cable core is formed, a shielding layer 150 is formed on the outside of the outer semiconducting layer 140 by winding, for example, copper tape around it.

[0179] (S500: Sheath formation process) Once the shielding layer 150 is formed, polyvinyl chloride is fed into the extruder and extruded to form a sheath 160 on the outer circumference of the shielding layer 150.

[0180] Based on the above, a power cable 10 as a solid-insulated power cable is manufactured.

[0181] (5) Summary of this embodiment This embodiment provides one or more of the following effects.

[0182] (a) In this embodiment, by including a predetermined amount of modified polymer (B) in the insulating layer 130, the polar groups in the modified polymer (B) can be uniformly dispersed in the insulating layer 130. This makes it possible to uniformly obtain the space charge trapping effect of the modified polymer (B) in the insulating layer 130. That is, even if there is a distribution of crystallinity and orientation in the insulating layer 130 in the thickness direction of the insulating layer 130, it is possible to suppress variations in insulating properties caused by the distribution of crystallinity and orientation.

[0183] As a result, the insulating properties of the insulating layer 130 can be made uniform in the thickness direction of the insulating layer 130. Specifically, the ratio of the volume resistivity of the inner sample of the insulating layer 130 to the volume resistivity of the outer sample of the insulating layer 130 can be set to 1.0 or more and 1.5 or less. This makes it possible to obtain sufficient insulating properties as a whole for the insulating layer 130 of the power cable 10.

[0184] (b) In this embodiment, as a novel manufacturing method, the cooling rate of the outer surface of the insulating layer 130 is increased in the cooling step S340 after the extrusion step S320 of the insulating layer 130. This makes it possible to maintain the orientation of the base polymer (A) during extrusion in a near-amorphous state on the outside of the insulating layer 130.

[0185] As a result, the degree of orientation f on the outside of the insulating layer 130 can be increased. Specifically, the degree of orientation f of the outer sample of the insulating layer 130, as measured by wide-angle X-ray scattering, can be set to 10% or more and less than 70%.

[0186] (c) In this embodiment, by setting the orientation degree f of the outer sample of the insulating layer 130 to 10% or more, excessive entanglement of multiple molecular chains of the base polymer (A) can be suppressed at least outside the insulating layer 130. This makes it possible to lower the elasticity of the base polymer (A) at least outside the insulating layer 130, and make the outside of the insulating layer 130 relatively softer. As a result, the flexibility of the power cable 10 can be improved. For example, excessive constraints on the reel diameter when winding the power cable 10 onto a reel can be avoided.

[0187] On the other hand, in this embodiment, by setting the orientation degree f of the outer sample of the insulating layer 130 to less than 70%, it is possible to suppress large variations in the elasticity of the insulating layer 130 in the thickness direction of the insulating layer 130. This makes it possible to suppress the generation of voids caused by differences in elasticity within the insulating layer 130, i.e., differences in stress within the insulating layer 130. As a result, in this embodiment, it is possible to suppress the decrease in the insulating properties of the insulating layer 130 caused by the generation of voids in the insulating layer 130.

[0188] As described in (a) to (c) above, according to this embodiment, the insulating properties of the insulating layer 130 can be made uniform in the thickness direction of the insulating layer 130, and the elasticity on the outside of the insulating layer 130 can be reduced. As a result, it is possible to obtain a power cable 10 with improved insulating properties and flexibility of the insulating layer 130.

[0189] (d) In this embodiment, in the cooling step S340 after the extrusion step S320 of the insulating layer 130, the rate at which the outer surface of the insulating layer 130 is cooled is increased so that the outside of the insulating layer 130 is rapidly cooled, while the inside of the insulating layer 130 is slowly cooled. This makes it possible to make the orientation degree f of the inner sample of the insulating layer 130 lower than the orientation degree f of the outer sample. Specifically, the orientation degree f of the inner sample of the insulating layer 130 measured by wide-angle X-ray scattering can be set to 10% or more and less than 60%.

[0190] (e) In this embodiment, by setting the orientation degree f of the inner sample to 10% or more, it is possible to suppress large variations in the elasticity of the insulating layer 130 in the thickness direction of the insulating layer 130. This makes it possible to suppress the generation of voids caused by differences in elasticity within the insulating layer 130, i.e., differences in stress within the insulating layer 130. As a result, it is possible to suppress a decrease in the insulating properties of the insulating layer 130.

[0191] On the other hand, in this embodiment, by setting the orientation degree f of the inner sample to less than 60%, as described above, the elasticity of the inner part of the insulating layer 130 can be made higher than the elasticity of the outer part of the insulating layer 130 while keeping the elasticity of the outer part of the insulating layer 130 low. In other words, the elasticity of the insulating layer 130 can be made different in the thickness direction of the insulating layer 130. This makes it possible to make the outer part of the insulating layer 130 relatively soft while making the inner part of the insulating layer 130 relatively hard. As a result, in this embodiment, it is possible to achieve both flexibility and strength of the insulating layer 130.

[0192] (f) In this embodiment, even when the thickness of the insulating layer 130 is 3 mm or more, the insulating properties of the insulating layer 130 can be made uniform in the thickness direction of the insulating layer 130, while the elasticity on the outside of the insulating layer 130 can be reduced. Therefore, even if the power cable 10 of this embodiment is applied to high-voltage applications, it is possible to stably achieve both the insulating properties and the flexibility of the insulating layer 130.

[0193] <Other Embodiments of the Disclosure> Although embodiments of the Disclosure have been described in detail above, the Disclosure is not limited to the embodiments described above and can be modified in various ways without departing from its essence.

[0194] In the embodiments described above, a case in which the power cable 10 does not need to have a waterproofing layer was explained, but this disclosure is not limited to this case. The power cable 10 may have a simple waterproofing layer. Specifically, the simple waterproofing layer is, for example, made of 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 wrapped, for example, vertically around the outer circumference of the cable core (outer than the outer semiconducting layer). The waterproofing layer may be provided outside the shielding layer, or it may also serve as the shielding layer. With such a configuration, the cost of the power cable 10 can be reduced.

[0195] The embodiments described above describe the case in which the power cable 10 is configured to be laid on land, underwater, or on the seabed, but the disclosure is not limited to this case. For example, the power cable 10 may be configured as a so-called overhead wire (overhead insulated wire).

[0196] In the above embodiment, three layers were extruded simultaneously in the cable core formation step S300, but the layers may be extruded one by one.

[0197] Next, embodiments relating to this disclosure will be described. These embodiments are examples of this disclosure and the disclosure is not limited to these embodiments.

[0198] (1) Fabrication of power cables As shown in Tables 1 and 2 below, in each of samples A1 to A8 and B1 to B9, the resin composition was mixed in a vanbar mixer and granulated into pellets using an extruder. Next, a conductor core wire made of dilute copper alloy with a diameter of 14 mm was twisted together to form a cross-sectional area of ​​2500 mm². ２A conductor was prepared. After the conductor was prepared, a resin composition for the internal semiconducting layer containing an ethylene-ethyl acrylate copolymer, the aforementioned resin composition, and a resin composition for the external semiconducting layer made of the same material as the resin composition for the internal semiconducting layer were fed into extruders A to C, respectively. The extruded materials from extruders A to C were guided to a common head, and the internal semiconducting layer, insulating layer, and external semiconducting layer were simultaneously extruded onto the outer circumference of the conductor from the inside out. At this time, the thicknesses of the internal semiconducting layer, insulating layer, and external semiconducting layer were set to 0.5 mm, 9 mm, and 0.5 mm, respectively. After the extrusion process, the extruded material was cooled using a cooling method appropriate for each sample. As a result, power cables for samples A1 to A8 and B1 to B9 were manufactured.

[0199] The conditions for each sample are as shown in Tables 1 and 2, and below.

[0200] <Samples A1-A8> (Base polymer (A)) Random polypropylene (r-PP): Stereoregularity: Isotactic Density: 0.9 g / ml, Melting point: 150°C, Heat of fusion: 60 J / g Storage modulus of elemental polypropylene at 25°C measured by dynamic viscoelasticity: 1150 MPa

[0201] (Modified Polymer (B)) Maleic anhydride-modified polypropylene (MAH-PP): Amount of maleic anhydride modification: 5% by mass, Melting point: 160°C, Storage modulus of the element at 25°C measured by dynamic viscoelasticity measurement: 1300 MPa

[0202] (Thermoplastic Elastomer (C)) Styrene-ethylene-butylene-styrene block copolymer (SEBS): Styrene unit content: 25% by mass, Melting point: None, Storage modulus of element at 25°C measured by dynamic viscoelasticity measurement: 120 MPa

[0203] Ethylene propylene rubber (EPR): Ethylene unit content: 25% by mass, Melting point: None, Heat of fusion: None, Storage modulus of elemental material at 25°C measured by dynamic viscoelasticity: 80 MPa

[0204] (Extrusion process) Extrusion temperature: 190°C

[0205] (Cooling Process) In the cooling process for samples A1 to A8, rapid cooling was performed as follows: The cable cores formed by the extrusion process were sequentially immersed in a water tank containing cooling water cooled by a chiller. This rapidly cooled the insulating layer starting from the area closest to the outer surface of the insulating layer.

[0206] In this process, for each of samples A1 to A8, the water bath temperature was adjusted within the range of -50°C to 50°C, resulting in a cooling rate of 40°C / min to 300°C / min at a temperature of 110°C outside the insulating layer.

[0207] On the other hand, during the 800-second period immediately following the extrusion process in which the insulating layer is extruded, the conductor was heated by electromagnetic induction to reach a temperature of 110°C while the insulating layer was cooled from the region closest to its outer surface.

[0208] <Samples B1-B4> Samples B1-B4 were prepared in the same manner as samples A1-A4, except that the following slow cooling was performed during the cooling process.

[0209] (Cooling Process) In the cooling process for samples B1 to B4, the cable cores formed by the extrusion process were first air-cooled for 700 seconds. Then, the cable cores formed by the extrusion process were sequentially immersed in a water tank containing cooling water cooled by a chiller. This cooled the insulating layer starting from the area closest to the outer surface of the insulating layer. The temperature of the water tank at this time was 25°C.

[0210] <Samples B5-B8> Samples B5-B8 were prepared in the same manner as Sample A3, except that the content of modified polymer (B) and thermoplastic elastomer (C) differed from those of Sample A3.

[0211] <Sample B9> Sample B9 was prepared in the same manner as Sample A3, except that the water bath temperature was adjusted to -25°C during the cooling process, resulting in a cooling rate of 312°C / min at an insulating layer temperature of 110°C.

[0212] (2) Evaluation (Temperature Change) For each of Sample A3 and Sample B1, the temperature change during the cooling process was measured as follows. Immediately after the extrusion process of each sample, thermocouples were placed (inserted) at the sampling location for the outer sample of the insulating layer and at the sampling location for the inner sample of the insulating layer, respectively. Subsequently, during the cooling process, the temperature change at each location was measured using thermocouples. As a result of the measurement, the cooling rate at a temperature of 110°C was determined at the sampling location of the outer sample.

[0213] (Sampling after the cooling process) After the cooling process, the insulating layer of each power cable sample A1-A8 and B1-B9 was thinly sliced ​​along the circumferential direction. As a result, the outer insulating layer sample was taken from a position 0.3 mm from the outer surface of the insulating layer toward the conductor. The inner insulating layer sample was taken from a position 0.3 mm from the inner surface of the insulating layer toward the outer surface. The size of each sample for dynamic viscoelasticity measurement was 5 mm in length, 42 mm in width, and 0.5 mm in thickness. The size of each sample for volume resistivity and DC breakdown field strength measurement was 50 mm in length, 50 mm in width, and 0.2 mm in thickness. The size of each sample for orientation degree measurement was 20 mm in length, 20 mm in width, and 0.5 mm in thickness.

[0214] (Storage Modulus) The storage modulus of the insulating layer in each sample was measured by dynamic viscoelasticity measurement (DMA) in accordance with JIS K7244-4:1999 under the following conditions: Measurement device: DVA-200 manufactured by IT Measurement Control Co., Ltd. Measurement mode: Tensile mode Strain: 0.08% Frequency: 10 Hz Temperature range: 0°C to 200°C Heating rate: 10°C / min

[0215] At this time, the storage modulus of the outer sample and the modulus of elasticity of the inner sample were measured at 25°C using the DMA described above.

[0216] (Volume Resistivity) The volume resistivity of the insulating layer in each sample was measured as follows. First, a 25 mm diameter flat electrode was placed on the first surface of the sheet, which served as either the outer or inner sample, and on the second surface opposite the first surface. After placing the electrodes, the sheet was immersed in silicone oil at a temperature of 90°C. In this state, a DC electric field of 80 kV / mm was applied to the sheet. The volume resistivity was then measured.

[0217] (Degree of Orientation) The degree of orientation f of the insulating layer in each sample was measured by wide-angle X-ray scattering using the arrangement shown in Figure 3 and the procedure described in "(3-2) Degree of Orientation" above. The measurement was performed under the following conditions.

[0218] Equipment: RIGAKU Nano-Viewer X-rays: Cu Kα rays X-ray source voltage: 4.0 kV X-ray source current: 30 mA Diameter of X-ray irradiation area on sample by passing through the slit: 300 μm Detector: DECTRIS PILATUS 100K Total time: 30 minutes (10 minutes x 3 times)

[0219] (DC breakdown field strength) The DC breakdown field strength of the insulating layer in each sample was measured as follows. Specifically, similar to the measurement of volume resistivity, a 25 mm diameter flat electrode was placed on the first surface of the sheet (either the outer or inner sample) and on the second surface opposite to the first surface. After placing the electrodes, the sheet was immersed in silicone oil at a temperature of 90°C. In this state, the applied voltage was increased at a rate of 4 kV / min. Subsequently, the DC breakdown field strength of the sheet was measured when dielectric breakdown occurred. A score of A (good) was given if the DC breakdown field strength was 160 kV / mm or higher in both the outer and inner samples. On the other hand, a score of B (poor) was given if the DC breakdown field strength was less than 160 kV / mm in at least one of the outer or inner samples.

[0220] (3) Results The results of the evaluation of each sample will be explained by referring to Figures 4 to 7 and Tables 1 and 2 below. In the "Cooling Method" column of each table, "Slow Cooling" means that slow cooling was performed in the same way as for sample B1. "Rapid Cooling" means that rapid cooling was performed in the same way as for sample A3.

[0221]

[0222]

[0223] <Comparison of Temperature Changes> (Sample B1) As shown in Figure 6, during the period when Sample B1 was air-cooled after the extrusion process, the temperature decreased gradually at both the outer and inner sample sampling locations. At the outer sample sampling location, the cooling rate at a temperature of 110°C was 2.5°C / min.

[0224] Subsequently, water-cooling the cable core accelerated the cooling rate at the outer sample collection site. However, the cable core was water-cooled at both the outer and inner sample collection sites after the temperature had already dropped to around 100°C. Therefore, the temperature difference between these two locations did not widen.

[0225] Thus, in sample B1, the cooling rate of the base polymer outside the insulating layer was similar to the cooling rate of the base polymer inside the insulating layer.

[0226] (Sample A3) In contrast, as shown in Figure 4, in Sample A3, the cable core was water-cooled immediately after the extrusion process. As a result, the temperature dropped rapidly at the sampling location of the outer sample. At the sampling location of the outer sample, the cooling rate at a temperature of 110°C was 150°C / min.

[0227] On the other hand, for a period of 800 seconds immediately following the extrusion process of sample A3, the conductor was electromagnetically heated to a temperature of 110°C. As a result, the temperature at the sampling location of the inner sample gradually decreased. Subsequently, by stopping the electromagnetic induction heating, the temperature at the sampling location of the inner sample began to drop below 100°C due to water cooling of the cable core. However, the temperature at the sampling location of the inner sample continued to decrease from a state where it had already dropped to around 110°C. Therefore, the temperature at the sampling location of the inner sample decreased gradually.

[0228] Thus, in sample A3, the outside of the insulating layer was rapidly cooled, while the inside of the insulating layer was slowly cooled.

[0229] <Comparison of each characteristic> (Samples B1-B4) In samples B1-B4, the content of base polymer (A), modified polymer (B), and thermoplastic elastomer (C) was the same as in samples A1-A4, but the insulating layer was cooled slowly.

[0230] Therefore, in samples B1 to B4, the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was close to 1. However, in samples B1 to B4, the orientation degree f of the outer sample and the orientation degree f of the inner sample were excessively low. Therefore, in samples B1 to B4, the storage modulus of the outer sample and the storage modulus of the inner sample were high, and the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample was close to 1.

[0231] In samples B1 to B4, the slow cooling of the insulating layer resulted in a high degree of crystallinity and a low degree of orientation f, with uniformity in the thickness direction of the insulating layer. Therefore, it is considered that the elasticity of the insulating layer was uniformly high in the thickness direction of the insulating layer in samples B1 to B4.

[0232] (Sample B5) In Sample B5, the outside of the insulating layer was rapidly cooled, similar to Sample A3, but modified polymer (B) was not added to the insulating layer.

[0233] Therefore, in sample B5, the orientation degree f of the outer sample and the orientation degree f of the inner sample were both within the specified range, but the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was greater than 1.5. In the outer sample of sample B5, the DC breakdown field strength was less than 160 kV / mm.

[0234] In sample B5, the rapid cooling of the insulating layer resulted in a distribution (variation) of crystallinity in the thickness direction of the insulating layer. Furthermore, because modified polymer (B) was not added to the insulating layer, the space charge trapping effect of modified polymer (B) was not obtained. For this reason, it is thought that in sample B5, the insulating properties of the insulating layer decreased and varied in the thickness direction of the insulating layer.

[0235] (Sample B6) In Sample B6, the outside of the insulating layer was rapidly cooled in the same manner as in Sample A3, but the content of modified polymer (B) in the insulating layer was more than 10 parts by mass.

[0236] Therefore, in sample B6, the orientation degree f of the outer sample and the orientation degree f of the inner sample were both within the specified range, but the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was greater than 1.5. In both the outer and inner samples of sample B6, the DC breakdown field strength was less than 160 kV / mm.

[0237] In sample B6, the content of modified polymer (B) in the insulating layer exceeded 10 parts by mass, resulting in reduced moldability of the insulating layer. Consequently, the insulating properties of the insulating layer decreased due to the reduced moldability. As a result, it is thought that in sample B6, the insulating properties of the insulating layer decreased and also varied in the thickness direction of the insulating layer.

[0238] (Sample B7) In Sample B7, the outside of the insulating layer was rapidly cooled in the same manner as in Sample A3, but the content of thermoplastic elastomer (C) in the insulating layer was less than 10 parts by mass.

[0239] Therefore, in sample B7, the orientation degree f of the inner sample was within the specified range, but in particular, the storage modulus of the inner sample was excessively high. On the other hand, in the outer sample of sample B7, the storage modulus was 670 MPa or less, but the orientation degree f of the outer sample of sample B7 was 70% or more. As a result, the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample was greater than 2.5. In the outer sample of sample B7, the DC breakdown field strength was less than 160 kV / mm.

[0240] In sample B7, the thermoplastic elastomer (C) content in the insulating layer was less than 10 parts by mass, resulting in insufficient flexibility provided by the thermoplastic elastomer (C). Consequently, the degree of crystallinity was high throughout the entire thickness direction of the insulating layer. On the other hand, under conditions of low thermoplastic elastomer (C), the outside of the insulating layer was rapidly cooled in the same manner as in sample A3. As a result, the orientation from the extrusion was excessively retained only on the outside of the insulating layer, and the degree of orientation f was excessively high. Thus, on the inside of the insulating layer, the elasticity was excessively high due to the high degree of crystallinity. In contrast, on the outside of the insulating layer, although the degree of crystallinity was high, the degree of orientation f was excessively high, resulting in elasticity within the appropriate range. In other words, the elasticity of the insulating layer varied greatly in the thickness direction of the insulating layer. In sample B7, voids were generated due to the difference in elasticity within the insulating layer as described above, i.e., the difference in stress within the insulating layer. As a result, it is considered that the dielectric breakdown strength of sample B7 decreased due to the generation of voids in the insulating layer.

[0241] (Sample B8) In Sample B8, the outside of the insulating layer was rapidly cooled, similar to Sample A3, but the content of thermoplastic elastomer (C) in the insulating layer was more than 45 parts by mass.

[0242] Therefore, in sample B8, the orientation degree f of the outer sample and the orientation degree f of the inner sample were both within the specified range, but the overall volume resistivity was low, and the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was greater than 1.5. In both the outer and inner samples of sample B8, the DC breakdown field strength was less than 160 kV / mm.

[0243] In sample B8, the content of amorphous thermoplastic elastomer (C) in the insulating layer exceeded 45 parts by mass, resulting in a decrease in the insulating properties inherently required of polypropylene, which has a high melting point, as the base polymer (A). As a result, it is thought that in sample B8, the insulating properties of the insulating layer were reduced and varied in the thickness direction of the insulating layer.

[0244] (Sample B9) In Sample B9, the cooling rate at the sampling location of the outer sample of the insulating layer was over 310°C / min at a temperature of 110°C.

[0245] Therefore, in sample B9, the degree of orientation f was 70% or higher. In sample B9, the storage modulus of the outer sample was less than 280 MPa. In sample B9, the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample was greater than 2.5. In the outer sample of sample B9, the DC breakdown field strength was less than 160 kV / mm.

[0246] In sample B9, the cooling rate on the outside of the insulating layer was excessively fast, resulting in an excessively high degree of orientation f on the outside of the insulating layer. Consequently, in sample B9, the elasticity of the insulating layer on the outside was excessively lower than that on the inside. In other words, the elasticity of the insulating layer varied greatly in the thickness direction of the insulating layer. In sample B9, voids were generated due to the difference in elasticity within the insulating layer as described above, i.e., the difference in stress within the insulating layer. As a result, it is thought that the dielectric breakdown strength of sample B9 decreased due to the generation of voids in the insulating layer.

[0247] (Samples A1 to A8) In contrast, in samples A1 to A8, a modified polymer (B) and a thermoplastic elastomer (C) were added to the insulating layer. In samples A1 to A8, the content of modified polymer (B) in the insulating layer was 1 part by mass or more and 10 parts by mass or less, and the content of thermoplastic elastomer (C) in the resin composition was 10 parts by mass or more and 45 parts by mass or less. Furthermore, in samples A1 to A8, the outside of the insulating layer was rapidly cooled as described above. At this time, the cooling rate at a temperature of 110°C at the sampling location of the outer sample of the insulating layer was 30°C / min or more and 310°C / min or less.

[0248] As a result, in samples A1 to A8, the ratio of the volume resistivity of the inner sample to the volume resistivity of the outer sample was between 1.0 and 1.5. In samples A1 to A8, the degree of orientation f of the outer sample was between 10% and less than 70%. In samples A1 to A8, the degree of orientation f of the inner sample was between 10% and less than 60%. In samples A1 to A8, the storage modulus of the outer sample was between 280 MPa and 670 MPa. In samples A1 to A8, the ratio of the storage modulus of the inner sample to the storage modulus of the outer sample was between 1.1 and 2.5. In samples A1 to A8, the DC breakdown field strength was 160 kV / mm or higher.

[0249] From the results of samples A1 to A8 described above, it was possible to make the insulation properties of the insulating layer uniform in the thickness direction of the insulating layer, and to reduce the elasticity on the outer side of the insulating layer. As a result, it was confirmed that it is possible to obtain a power cable with improved insulation properties and flexibility of the insulating layer.

[0250] <Note> The following are descriptions of the embodiments of this disclosure. The embodiments referred to by the numbers in brackets [] to which the following notes are dependent correspond to the embodiments described in <Embodiments of this Disclosure>.

[0251]

[10] The volume resistivity of the outer sample and the volume resistivity of the inner sample are 7.0 × 10 １４ The volume resistivity is Ω·cm or greater, where the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under the conditions of a temperature of 90°C and a DC electric field of 80kV / mm. This is the power cable described in any one of [2] to [7] above.

[0252]

[11] The ratio of the storage modulus of the inner sample of the insulating layer to the storage modulus of the outer sample of the insulating layer is 1.1 or more and 2.5 or less, where the storage modulus of the outer sample and the storage modulus of the inner sample are measured at 25°C by dynamic viscoelasticity measurement. Power cable according to any one of [2] to [7] or

[10] above.

[0253]

[12] The storage modulus of the inner sample of the insulating layer is 650 MPa or more and 900 MPa or less, and the storage modulus of the outer sample of the insulating layer is 280 MPa or more and 670 MPa or less. The power cable according to

[11] .

[0254] 10 Power cable 110 Conductor 120 Inner semiconducting layer 130 Insulating layer 140 Outer semiconducting layer 150 Shielding layer 160 Sheath

Claims

1. A resin composition comprising an insulating layer provided to cover the conductor of a power cable, having an inner circumferential surface facing the conductor and an outer circumferential surface opposite to the inner circumferential surface, comprising: a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, wherein the ratio of the volume resistivity of the inner sample of the insulating layer to the volume resistivity of the outer sample of the insulating layer is 1.0 or more and 1.5 or less, and the degree of orientation of the outer sample of the insulating layer is 10% or more and less than 70%, where, the outer sample of the insulating layer is taken from a position 0.3 mm from the outer circumferential surface toward the conductor, the inner sample of the insulating layer is taken from a position 0.3 mm from the inner circumferential surface toward the outer circumferential surface, and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. The degree of orientation f of the outer sample of the insulating layer is calculated by formula (1): f = {(360 - ΣW) / 360} × 100 ... (1) where W is the full width at half maximum of each peak in the azimuthal width of each peak in the azimuthal width of the integrated intensity obtained by integrating the scattering intensity in the range of diffraction angle 2θ from 15° to 18°, based on the X-ray scattering image obtained by irradiating the outer sample with Cu Kα rays perpendicular to the resin composition.

2. A conductor and an insulating layer provided to cover the outer circumference of the conductor, having an inner surface facing the conductor and an outer surface opposite to the inner surface, wherein the insulating layer comprises a base polymer containing propylene units, a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives, and a thermoplastic elastomer, wherein the ratio of the volume resistivity of the inner sample of the insulating layer to the volume resistivity of the outer sample of the insulating layer is 1.0 or more and 1.5 or less, and the degree of orientation of the outer sample of the insulating layer is 10% or more and less than 70%, where the outer sample of the insulating layer is taken from a position 0.3 mm from the outer surface toward the conductor, and the inner sample of the insulating layer is taken from a position 0.3 mm from the inner surface toward the outer surface, and the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm. The degree of orientation f of the outer sample of the insulating layer is calculated by formula (1): f = {(360 - ΣW) / 360} × 100 ... (1) where W is the full width at half maximum of each peak in the azimuthal width of each peak in the azimuthal angle profile of the integrated intensity obtained by integrating the scattering intensity in the range of diffraction angle 2θ from 15° to 18°, based on the X-ray scattering image obtained by irradiating the outer sample with Cu Kα rays perpendicular to the power cable.

3. The power cable according to claim 2, wherein when the total content of the base polymer, the modified polymer, and the thermoplastic elastomer in the insulating layer is 100 parts by mass, the content of the modified polymer in the insulating layer is 1 part by mass or more and 10 parts by mass or less, and the content of the thermoplastic elastomer in the insulating layer is 10 parts by mass or more and 45 parts by mass or less.

4. The power cable according to claim 2 or 3, wherein the degree of orientation of the inner sample of the insulating layer is 10% or more and less than 60%.

5. The power cable according to any one of claims 2 to 4, wherein the thermoplastic elastomer comprises a styrene-based elastomer.

6. The power cable according to any one of claims 2 to 5, wherein the thermoplastic elastomer comprises an olefin-based elastomer.

7. The power cable according to any one of claims 2 to 6, wherein the thickness of the insulating layer is 3 mm or more.

8. The process comprises the steps of: preparing a resin composition; forming an insulating layer with the resin composition so as to cover the outer circumference of a conductor, and forming an inner surface facing the conductor and an outer surface opposite to the inner surface in the insulating layer, wherein the step of preparing the resin composition is to prepare a composition comprising: a base polymer containing propylene units; a modified polymer containing propylene units and modified with at least one selected from unsaturated organic acids and their derivatives; and a thermoplastic elastomer, such that the ratio of the volume resistivity of the inner sample of the insulating layer to the volume resistivity of the outer sample of the insulating layer is 1.0 or more and 1.5 or less, and the step of forming the insulating layer comprises: extruding the insulating layer onto the outer circumference of the conductor; and cooling the insulating layer so that the orientation of the outer sample of the insulating layer is 10% or more and less than 70%, wherein the outer sample of the insulating layer is taken from a position 0.3 mm from the outer surface toward the conductor, The inner sample of the insulating layer is taken from a position 0.3 mm from the inner surface toward the outer surface, the volume resistivity of the outer sample and the volume resistivity of the inner sample are measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm, the degree of orientation f of the outer sample of the insulating layer is calculated by formula (1), f = {(360 - ΣW) / 360} × 100 ... (1) where W is the half-width of each peak in the azimuthal width in the azimuthal direction profile of the integrated intensity obtained by integrating the scattering intensity in the range of diffraction angle 2θ of 15° to 18° based on the X-ray scattering image obtained by irradiating the outer sample perpendicularly with Cu Kα rays, the half-width of each peak occurring in the azimuthal angle profile of the integrated intensity.

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