Resin composition and power cable

Abstract

This resin composition contains a propylene-based resin (A) that has a propylene unit, a thermoplastic elastomer (B), and a modified polymer (C) that has a propylene unit and that is modified with an unsaturated carboxylic acid. The modification amount of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01 mass% or more. The amount of free monomers derived from the modified polymer (C) that have an unsaturated carboxylic acid group and that have a molecular weight of 500 or less is 1.0×10-6 mass% to 0.05 mass%.

Description

Resin composition and power cable

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

[0002] In recent years, solid insulated power cables (hereinafter abbreviated as "power cables") have been developed for DC transmission applications, and cross-linked polyethylene is widely used as a component of the insulating layer of these power cables (see, for example, Patent Document 1).

[0003] Japanese Patent Application Publication No. 11-086634

[0004] According to one aspect of the present disclosure, a thermoplastic elastomer (B) is provided which contains a propylene-based resin (A) having a propylene unit, a thermoplastic elastomer (B), and a modified polymer (C) having a propylene unit and modified with an unsaturated carboxylic acid, wherein the amount of modification of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01 mass% or more, and the content of free monomers having an unsaturated carboxylic acid group and having a molecular weight of 500 or less derived from the modified polymer (C) is 1.0 × 10 －６ The resin composition is provided in which the content of the hydroxybenzoate is from 0.05% by mass to 0.5% by mass.

[0005] According to another aspect of the present disclosure, there is provided a heat insulating material comprising: a conductor; and an insulating layer coated around the conductor and formed from a resin composition, wherein the resin composition contains: a propylene-based resin (A) having a propylene unit; a thermoplastic elastomer (B); and a modified polymer (C) having a propylene unit and modified with an unsaturated carboxylic acid, wherein the amount of modification of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01 mass% or more; and the content of free monomers having an unsaturated carboxylic acid group and having a molecular weight of 500 or less derived from the modified polymer (C) is 1.0×10 －６ The power cable is provided with a SiO 2 content of 0.05% by mass or more.

[0006] Fig. 1 is a schematic cross-sectional view perpendicular to the axial direction of a power cable according to an embodiment of the present disclosure, and Fig. 2 is a flowchart showing a method for manufacturing a power cable according to an embodiment of the present disclosure.

[0007] [Problem to be Solved by the Invention] Cross-linked polyethylene that has deteriorated over time cannot be recycled and must be incinerated, which has raised concerns about its impact on the environment.

[0008] Therefore, in recent years, propylene-based resins have been attracting attention as a resin component constituting the insulating layer. Polypropylene-based resins can achieve high insulating properties even without crosslinking. In other words, they can achieve both insulating properties and recyclability.

[0009] On the other hand, propylene-based resins sometimes fail to provide stable insulating properties in the insulating layer.

[0010] An object of the present disclosure is to provide a technology for obtaining stable insulation properties in a power cable.

[0011] Effect of the Invention According to the present disclosure, stable insulation properties can be obtained in a power cable.

[0012] [Explanation of Embodiments of the Present Disclosure] <Insights Obtained by the Inventors> First, an outline of insights obtained by the inventors will be described.

[0013] In power cables, when the insulation layer is made of a propylene-based resin, for example, space charges may be generated in the insulation layer when a high voltage is applied, resulting in a deterioration in the insulation properties of the insulation layer. This tendency becomes more pronounced in high-temperature environments. The insulation properties referred to here refer to the volume resistivity, DC breakdown field strength, space charge characteristics, etc. of the insulation layer.

[0014] In order to improve the insulating properties of the insulating layer, a method of incorporating a modified polymer modified with an unsaturated carboxylic acid has been investigated. For example, a modified propylene, in which propylene is modified with an unsaturated carboxylic acid, is used. The polar groups of the modified polymer exhibit a charge trapping effect, thereby improving the insulating properties of the resin composition.

[0015] The present inventors evaluated the electrical conductivity by appropriately changing the amount of modified polymer added and the amount of polar groups introduced into the resin composition, and found that the insulating properties differ even when the polar group content is the same. Further investigation into this point revealed that unreacted unsaturated carboxylic acids and the like remain in the modified polymer and are not bonded to the polymer main chain, and these components affect the insulating properties as free monomers.

[0016] Like the modified polymer, the free monomer has a polar group and therefore exhibits a charge trapping effect. In areas of the resin composition where the modified polymer is not dispersed, the charge trapping effect is difficult to obtain, but the free monomer disperses more easily in the resin composition than the modified polymer, and therefore exhibits a uniform charge trapping effect throughout the resin composition. However, the free monomer itself can also act as a charge carrier, and if its amount becomes too large, it can actually reduce the insulating properties.

[0017] From the above, it has been found that when adding a modified polymer to a propylene-based resin, it is advantageous to control the content of unsaturated carboxylic acid derived from the modified polymer as well as the amount of free monomer derived from the modified polymer within a predetermined range.

[0018] The present disclosure is based on the above-mentioned findings of the inventors.

[0019] <Embodiments of the Present Disclosure> Next, embodiments of the present disclosure will be listed and described.

[0020] [1] A resin composition according to one aspect of the present disclosure comprises: a propylene-based resin (A) having a propylene unit; a thermoplastic elastomer (B); and a modified polymer (C) having a propylene unit and modified with an unsaturated carboxylic acid, wherein the content of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01% by mass or more; and the content of a free monomer having an unsaturated carboxylic acid group and having a molecular weight of 500 or less derived from the modified polymer (C) is 1.0 x 10 －６ % by mass or more and 0.05% by mass or less. According to this configuration, high insulating properties can be stably obtained.

[0021] [2] A power cable according to another aspect of the present disclosure includes: a conductor; and an insulating layer that covers the conductor and is formed from a resin composition, wherein the resin composition contains a propylene-based resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) that has propylene units and is modified with an unsaturated carboxylic acid, wherein a content of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01 mass% or more, and a content of a free monomer that has an unsaturated carboxylic acid group and has a molecular weight of 500 or less, derived from the modified polymer (C), is 1.0 x 10 －６ With this configuration, a high withstand voltage can be achieved even in a high temperature environment, enabling stable DC power transmission.

[0022] [3] In the power cable according to the above [2], the thermoplastic elastomer (B) is an olefin-based thermoplastic elastomer having at least one olefin unit of polyethylene or polypropylene as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment. This configuration can further improve the insulation properties and flexibility of the insulating layer.

[0023] [4] In the power cable according to the above [2] or [3], the resin composition contains, when the total content of the propylene-based resin (A), the thermoplastic elastomer (B), and the modified polymer (C) is taken as 100 parts by mass, 55 parts by mass to 90 parts by mass of the propylene-based resin (A), 10 parts by mass to 45 parts by mass of the thermoplastic elastomer (B), and 1 part by mass to 10 parts by mass of the modified polymer (C). This configuration makes it possible to improve the flexibility of the insulating layer while stably obtaining high insulation properties.

[0024] [5] The power cable according to any one of the above [2] to [4], wherein the modified polymer (C) has a modification amount of the unsaturated carboxylic acid of 0.1 mass % or more and 10 mass % or less, and the content of the free monomer is 1.0 × 10 －４ % by mass or more and 1.0% by mass or less. This configuration makes it possible to stably obtain higher insulating properties in the insulating layer.

[0025] [Details of the embodiment of the present disclosure] Next, one embodiment of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0026] <One embodiment of the present disclosure> (1) Resin composition The resin composition of this embodiment can be used, for example, as a material for forming an insulating layer of a power cable, which will be described later. The resin composition contains a propylene-based resin (A), a thermoplastic elastomer (B), a modified polymer (C), and, as necessary, other additives. Each component will be described in detail below. Note that, hereinafter, the propylene-based resin (A) will also be referred to as component (A), the thermoplastic elastomer (B) as component (B), and the modified polymer (C) as component (C).

[0027] (Propylene-Based Resin (A)) The propylene-based resin (A) is a resin material that constitutes the main component of the resin composition and is a component having propylene units. As this component (A), at least one of a propylene homopolymer (hereinafter also referred to as homo PP) and a propylene random polymer (hereinafter also referred to as random PP) can be used. Homo PP contains propylene units, and random PP has propylene units and ethylene units.

[0028] From the viewpoint of obtaining higher insulating properties in the insulating layer, the propylene-based resin (A) is preferably random PP. Homo PP has a higher crystalline content than random PP and can obtain high insulating properties, but it can cause cracks in the crystals or between the crystals in the insulating layer, which can prevent the inherent insulating properties from being obtained. In contrast, random PP contains ethylene units, so although the crystalline content is low, it is less likely to cause cracks due to coarse crystallization in the insulating layer, and can obtain higher insulating properties than homo PP.

[0029] The stereoregularity of the propylene-based resin (A) may be isotactic, syndiotactic, or atactic. The stereoregularity is not particularly limited, but isotactic is preferable. The isotactic stereoregularity can suppress a decrease in the melting point of the resin composition. As a result, the resin composition can be used stably in a non-crosslinked or slightly crosslinked state.

[0030] As described below, the melt flow rate (MFR) of the propylene-based resin (A) is preferably 0.1 g / 10 min or more and 5.0 g / 10 min or less, and may be 0.1 g / 10 min or more and 2.0 g / 10 min or less, from the viewpoint of compatibility with the thermoplastic elastomer (B) and the modified polymer (C). By achieving such an MFR, the phase structure of the resin composition can be formed into a structure in which the components are compatible with each other or a structure in which the components are finely dispersed. This can improve the flexibility and insulating properties of the resin composition. The MFR here is a value measured in accordance with JIS K7210 at a temperature of 190°C and a load of 2.16 kg.

[0031] The melting point of the propylene-based resin (A) is not particularly limited, but is preferably 130° C. or higher and 170° C. or lower. When the propylene-based resin (A) is homo-PP, its melting point is preferably 120° C. or higher and 165° C. or lower, and when it is random-PP, its melting point is preferably 130° C. or higher and 170° C. Propylene-based resin (A) having such a melting point can achieve high compatibility when mixed with thermoplastic elastomer (B) or modified polymer (C).

[0032] In this specification, the melting point is measured as follows. First, differential scanning calorimetry (DSC) is performed on a sample in accordance with, for example, JIS-K-7121 (1987). Specifically, in a DSC device, the temperature is raised from room temperature (normal temperature, for example, 27°C) to 220°C at a rate of 10°C / min. A DSC curve is obtained by plotting the amount of heat absorbed (heat flow) per unit time against the temperature. At this time, the temperature at which the amount of heat absorbed per unit time in the sample reaches a maximum (peak) is defined as the "melting point (melting peak temperature)."

[0033] (Thermoplastic elastomer (B)) The thermoplastic elastomer (B) is a component having lower crystallinity than the propylene-based resin (A) having propylene units. The component (B) controls the crystal growth of the component (A), thereby imparting flexibility to the resin composition and the insulating layer.

[0034] The thermoplastic elastomer (B) may be any component capable of improving the flexibility of the resin composition, and known components such as amides, esters, olefins, styrenes, urethanes, vinyl chlorides, and fluorine-based components may be used. Among these, olefins are preferred from the viewpoint of compatibility with the propylene-based resin (A). Olefin-based thermoplastic elastomers (so-called TPOs) contain at least one olefin unit (polyethylene or polypropylene) as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment. The olefin-based thermoplastic elastomer may be a copolymer of an olefin unit and an ethylene-α-olefin copolymer unit, or a mixture of an olefin unit and an ethylene-α-olefin copolymer unit. Among these, copolymers are preferred from the viewpoint of compatibility with component (A). The α-olefin is a linear or branched α-olefin having 2 to 8 carbon atoms, and examples thereof include ethylene, propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3-methyl-1-pentene, and 1-octene. Among these, it is preferable that the TPO has polypropylene as the hard segment and ethylene-propylene rubber as the soft segment. The component (B) may be used singly or in combination of two or more.

[0035] The MFR of the thermoplastic elastomer (B) is preferably 0.1 g / 10 min or more and 5.0 g / 10 min or less, and may be 0.1 g / 10 min or more and 2.0 g / 10 min or less, from the viewpoint of compatibility with the propylene-based resin (A) and the modified polymer (C). By setting the MFR in this range, the phase structure of the resin composition can be formed into a compatible structure or a finely dispersed sea-island structure.

[0036] The thermoplastic elastomer (B) does not have a melting point (no melting point), or if it does have a melting point, the melting point is preferably 165° C. or lower, or may be 130° C. or higher and 155° C. or lower. Such component (B) allows the resin composition to maintain high pressure resistance at high temperatures as well as the flexibility required for the insulating layer of a power cable.

[0037] (Modified Polymer (C)) The modified polymer (C) is a component having propylene units and modified with an unsaturated carboxylic acid having a polar group. Because the component (C) has propylene units, it has excellent compatibility when mixed with the propylene-based resin (A). Furthermore, by mixing the component (C) with the component (A), a polar group can be introduced into the resin composition. The polar group allows space charge to be trapped in the insulating layer formed from the resin composition. In other words, the amount of space charge accumulation in the insulating layer can be reduced. As a result, the volume resistivity of the insulating layer can be improved, and high insulating properties can be stably ensured.

[0038] Specifically, the modified polymer (C) is unsaturated carboxylic acid-modified propylene (hereinafter simply referred to as modified PP) in which polypropylene is modified with an unsaturated carboxylic acid. Examples of unsaturated carboxylic acids include known components that can be introduced into polypropylene and have a molecular weight of 500 or less. Examples include acrylic acid, methacrylic acid, crotonic acid, maleic acid, cinnamic acid, itaconic acid, citraconic acid, and fumaric acid, as well as unsaturated carboxylic acid anhydrides such as maleic anhydride, itaconic anhydride, and citraconic anhydride. Among these, maleic anhydride is preferred. This is because maleic anhydride has a large number of polar groups per molecular weight, so even a small amount can modify polypropylene.

[0039] In the modified polymer (C), the proportion of unsaturated carboxylic acid introduced relative to the polypropylene (modification amount) is not particularly limited, but may be, for example, 0.1% by mass to 10% by mass, or 0.5% by mass to 4% by mass. By adjusting the modification amount of the (C) component to 0.1% by mass or 0.5% by mass or more, the content of unsaturated carboxylic acid in the resin composition can be easily adjusted to 0.01% or more, thereby suppressing space charge accumulation. On the other hand, by adjusting the modification amount of the (C) component to 10% by mass or less or 4% by mass or less, high compatibility with the propylene-based resin (A) and the thermoplastic elastomer (B) can be maintained. The modification amount refers to the copolymerization ratio of the unsaturated carboxylic acid bonded to the main chain polymer in the (C) component, and is expressed as the amount per 100 parts by mass of the modified polymer (C).

[0040] As described above, the modified polymer (C) may contain free monomers. The free monomers are unreacted unsaturated carboxylic acids that are not bonded to the polymer main chain during the preparation of the component (C), or their by-reaction products, and exist in a free state in the component (C). The free monomers have a chemical structure derived from unsaturated carboxylic acids. Specifically, the free monomers are components that have an unsaturated carboxylic acid group and a molecular weight of 500 or less. The molecular weight is set to 500 or less because this is the range that the unsaturated carboxylic acids and their by-reaction products can take.

[0041] The content of the free monomer in the modified polymer (C) is not particularly limited, but the content of the free monomer in the resin composition is preferably 1.0×10 －６ From the viewpoint of adjusting the content to 0.05 mass% or more, 1.0 × 10 －４The content of the free monomer in the (C) component is preferably 1.0% by mass or more and 1.0% by mass or less. The content of the free monomer in the (C) component refers to the amount of the free monomer per 100 parts by mass of the (C) component. The (C) component may be selected to contain a predetermined amount of the free monomer, or the amount may be adjusted by pretreatment, such as by extracting the free monomer to a predetermined amount. This pretreatment may involve heating the (C) component to volatilize the free monomer contained therein and evacuating the surrounding atmosphere. For example, when the free monomer is maleic anhydride or the like, the modified (C) component may be heated at a temperature higher than the boiling point of maleic anhydride (202°C) within a temperature range where thermal decomposition does not occur.

[0042] The MFR of the modified polymer (C) is preferably 0.1 g / 10 min or more and 500 g / 10 min or less, and may be 1 g / 10 min or more and 300 g / 10 min or less, from the viewpoint of compatibility with the propylene-based resin (A) and the thermoplastic elastomer (B). By setting the MFR in this range, each component in the resin composition can be finely dispersed and compatible with each other.

[0043] The melting point of the modified polymer (C) is not particularly limited, but is preferably 130° C. or higher and 165° C. or lower. When the component (C) has such a melting point, it is possible to finely disperse or dissolve each component in the resin composition when it is mixed with the propylene-based resin (A) and the thermoplastic elastomer (B).

[0044] (Other Additives) The resin composition may contain other additives as needed, such as an inorganic filler, an antioxidant, a crosslinking agent, a lubricant, and a colorant.

[0045] In this embodiment, the space charge trapping effect of the modified polymer (C) can be enhanced, so high insulation properties can be stably obtained without adding an inorganic filler. On the other hand, when an inorganic filler is added, the content thereof may be, for example, less than 1 part by mass when the total content of the resin components such as the propylene-based resin (A) is taken as 100 parts by mass. Note that the lower limit of the content of the inorganic filler is not limited as long as the inorganic filler can be added.

[0046] 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.

[0047] The mean volume diameter (MV) of the inorganic filler is not particularly limited, but may be, for example, 1 μm or less, 700 nm or less, or 100 nm or less. The "mean volume diameter (MV)" here refers to the particle diameter of the particles d ｉ , the volume of the particle V ｉ When this is the case, it can be calculated using the following formula: MV = Σ(V ｉ d ｉ ) / ΣV ｉ The volume average particle size is measured using a dynamic light scattering particle size / particle size distribution measuring device.

[0048] The lower limit of the volume average particle diameter of the inorganic filler is not particularly limited, but from the viewpoint of stably forming the inorganic filler, the volume average particle diameter of the inorganic filler may be, for example, 1 nm or more, or may be 5 nm or more.

[0049] At least a portion of the inorganic filler may be surface-treated with a silane coupling agent, which can improve the adhesion at the interface between the inorganic filler and the propylene-based resin (A) or the like, and can improve the mechanical properties and insulating properties of the insulating layer 130.

[0050] As the antioxidant, known antioxidants such as phenol-based, sulfur-based, and amine-based antioxidants can be used. When the resin composition contains an antioxidant, the content of the antioxidant is not limited. However, the content of the antioxidant is preferably 0.1 parts by mass or more and 1.0 parts by mass or less when the total content of the resin components in the resin composition, in this case the propylene-based resin (A), the thermoplastic elastomer (B), and the modified polymer (C), is taken as 100 parts by mass.

[0051] Furthermore, from the viewpoint of recycling, the resin composition is preferably uncrosslinked, but may contain a crosslinking agent for crosslinking. However, even if crosslinking is performed, it is preferable to perform crosslinking so that the gel fraction (degree of crosslinking) is low. Specifically, it is preferable to perform crosslinking at a degree of crosslinking such that the crosslinking agent residue in the resin composition is less than 300 ppm. When dicumyl peroxide is used as the crosslinking agent, the residue is, for example, cumyl alcohol, α-methylstyrene, etc.

[0052] The resin composition may also contain a lubricant to improve the fluidity of the resin composition during the extrusion process of the insulating layer 130. Examples of lubricants include fatty acid metal salts and fatty acid amides. Examples of fatty acid metal salts include magnesium stearate, zinc stearate, aluminum stearate, and magnesium montanate. Examples of fatty acid amides include oleic acid amide and stearic acid amide. Two or more of these may be used in combination.

[0053] (Phase Structure of Resin Composition) The resin composition is formed by mixing a propylene-based resin (A), a thermoplastic elastomer (B), a modified polymer (C), and, if necessary, other additives. Since the components are easily mixed with each other, the resin composition has a phase structure in which the components (B) and (C) are finely dispersed in the component (A), or in which the components are compatible with each other. This allows space charges to be trapped more uniformly in the resin composition, resulting in more stable insulation. In addition, the resin composition can meet the flexibility required for an insulating layer.

[0054] From the viewpoint of finer dispersion or compatibility of the components in the resin composition, it is preferable that the MFRs of the components are close to each other and that the difference between these MFRs is small. Specifically, it is preferable that the difference between the highest and lowest MFRs of the components is 300 g / 10 min or less. By combining components that result in such a difference in MFR, it is possible to finely disperse or compatibility of the components during mixing.

[0055] (Content of unsaturated carboxylic acid in resin composition) An unsaturated carboxylic acid having a polar group derived from the modified polymer (C) is introduced into the resin composition. The content of the unsaturated carboxylic acid in the resin composition can be adjusted by the amount of modification of the unsaturated carboxylic acid in the (C) component or the amount of the (C) component added. In this embodiment, the content of the unsaturated carboxylic acid in the resin composition is 0.01% by mass or more, and may be 0.03% by mass or more. By setting the content in this range, it is possible to reduce the accumulation of space charge in the resin composition and improve the insulation properties.

[0056] On the other hand, the upper limit of the unsaturated carboxylic acid content is not particularly limited, but it is preferably 0.5% by mass or less, and may be 0.1% by mass or less. Generally, as the unsaturated carboxylic acid content increases, the content of the modified polymer (C) increases, and the ratio of the propylene-based resin (A) and the thermoplastic elastomer (B) decreases, which may result in the resin composition being unable to maintain the flexibility and the desired strength required for the insulating layer. Furthermore, as the content of the (C) component increases, the amount of free monomer in the resin composition also increases, making it difficult to adjust the content to the range of 0.05% by mass or less, as described below. In this regard, by adding the (C) component so that the unsaturated carboxylic acid content in the resin composition is 0.5% by mass or less, the flexibility and strength of the resin composition can be maintained at a high level. Furthermore, the content of free monomer in the resin composition can be easily adjusted to a predetermined range, which suppresses the deterioration of insulating properties due to the free monomer, thereby maintaining high insulating properties.

[0057] The content of the unsaturated carboxylic acid will be described in detail in the Examples, but can be determined, for example, by directly measuring the resin composition by NMR.

[0058] (Free Monomer Content in Resin Composition) The resin composition contains a free monomer derived from the modified polymer (C), and the free monomer content is 1.0 × 10 －６% by mass or more and 0.05% by mass or less. As described above, the free monomer has an unsaturated carboxylic acid group and exhibits a charge trapping effect, but if the amount is large, the free monomer itself may behave as a charge carrier and may actually reduce the insulating properties. In this regard, in this embodiment, by adjusting the content of the free monomer within a predetermined range, the charge trapping effect of the free monomer can be obtained while suppressing its action as a charge carrier. In other words, the insulating properties of the resin composition can be maintained at a high level. From the viewpoint of further improving the insulating properties, the content of the free monomer is set to 1.0 x 10 －５ % by mass or more and 0.04% by mass or less, and may be 5.0 × 10 －４ The content of the free monomer in the resin composition may be 0.03% by mass or more and 0.03% by mass or less. As will be described in detail in the Examples, the content of the free monomer in the resin composition can be determined, for example, by dissolving the resin composition in an organic solvent, eluting the free monomer, and measuring it by 1H-NMR. Furthermore, for example, when the amount of the free monomer is extremely small, it can be extracted using an organic solvent and measured by liquid chromatography.

[0059] (Melting Point of Resin Composition) The resin composition has a predetermined melting point by containing the propylene-based resin (A), thermoplastic elastomer (B), and modified polymer (C) with different melting points. The melting point of the resin composition also serves as an indicator of the addition ratio of each component. From the viewpoint of containing each component and achieving a high level of insulation, flexibility, and other properties required for the insulating layer in a well-balanced manner, it is preferable that the melting point of the resin composition be 130°C or higher and 170°C or lower.

[0060] (Blending ratio) The blending ratio of the propylene-based resin (A), the thermoplastic elastomer (B) and the modified polymer (C) in the resin composition is such that the content of unsaturated carboxylic acid in the resin composition is 0.01 mass% or more and the content of free monomer is 1.0 × 10 －６ For example, the content of each component can be adjusted appropriately depending on the amount of modification of the unsaturated carboxylic acid in component (C) used or the amount of free monomer contained in component (C) so that a high level of insulation property, flexibility, and the like can be obtained in a well-balanced manner.

[0061] For example, when the total content of the propylene-based resin (A), thermoplastic elastomer (B), and modified polymer (C) in the resin composition is taken as 100 parts by mass, the content of component (A) may be 55 parts by mass or more and 90 parts by mass or less, the content of component (B) may be 10 parts by mass or more and 45 parts by mass or less, and the content of component (C) may be 1 part by mass or more and 10 parts by mass or less. By setting the contents in these ranges, it is easy to adjust the melting point of the resin composition within a predetermined range, and also to adjust the content of unsaturated carboxylic acid and the content of free monomer in the resin composition within a predetermined range.

[0062] (2) Power Cable Next, the power cable of this embodiment will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view of the power cable according to this embodiment, taken perpendicular to the axial direction.

[0063] The power cable 10 of this embodiment is configured as a so-called solid insulated power cable. The power cable 10 of this embodiment is configured to be laid, for example, on land (in a conduit), underwater, or at the bottom of a body of water. The power cable 10 is used, for example, for direct current.

[0064] Specifically, the power cable 10 includes, for example, a conductor 110 , an inner semiconductive layer 120 , an insulating layer 130 , an outer semiconductive layer 140 , a shielding layer 150 , and a sheath 160 .

[0065] (Conductor (Conductive Portion)) The conductor 110 is formed by twisting together a plurality of conductor core wires (conductive core wires) containing, for example, pure copper, copper alloy, aluminum, or aluminum alloy.

[0066] (Internal Semiconductive Layer) The internal semiconductive layer 120 is provided so as to cover the outer periphery of the conductor 110. The internal semiconductive layer 120 is semiconductive and configured to suppress electric field concentration on the surface side of the conductor 110. The internal semiconductive layer 120 contains, for example, at least one of an ethylene-based copolymer such as an ethylene-ethyl acrylate copolymer, an ethylene-methyl acrylate copolymer, an ethylene-butyl acrylate copolymer, or an ethylene-vinyl acetate copolymer, a thermoplastic elastomer, or the above-mentioned low-crystalline resin, and conductive carbon black.

[0067] (Insulating Layer) The insulating layer 130 is formed from the resin composition described above so as to cover the outer periphery of the internal semiconducting layer 120. For example, the insulating layer 130 is formed by extruding the resin composition.

[0068] (Outer Semiconductive Layer) The outer semiconductive layer 140 is provided so as to cover the outer periphery of the insulating layer 130. The outer semiconductive layer 140 has semiconductivity and is configured to suppress electric field concentration between the insulating layer 130 and the shielding layer 150. The outer semiconductive layer 140 is made of, for example, the same material as the inner semiconductive layer 120.

[0069] (Shielding Layer) The shielding layer 150 is provided so as to cover the outer periphery of the outer semiconducting layer 140. The shielding layer 150 is formed, for example, by winding copper tape, or is formed as a wire shield wound with a plurality of annealed copper wires or the like. Tape made of a material such as rubberized cloth may be wound inside or outside the shielding layer 150.

[0070] (Sheath) The sheath 160 is provided so as to cover the outer periphery of the shielding layer 150. The sheath 160 is made of, for example, polyvinyl chloride or polyethylene.

[0071] If the power cable 10 of this embodiment is an underwater cable or an underwater cable, it may have a metal waterproof layer such as an aluminum sheath or an iron wire armor outside the shielding layer 150.

[0072] On the other hand, the power cable 10 of the present embodiment may not have a water-proof layer outside the shielding layer 150. In other words, the power cable 10 of the present embodiment may have a non-completely water-proof structure.

[0073] (Specific Dimensions, etc.) Specific dimensions of the power cable 10 are not particularly limited, but may be, for example, the diameter of the conductor 110 is 5 mm or more and 60 mm or less, the thickness of the inner semiconductive layer 120 is 0.5 mm or more and 3 mm or less, the thickness of the insulating layer 130 is 3 mm or more and 35 mm or less, the thickness of the outer semiconductive layer 140 is 0.5 mm or more and 3 mm or less, the thickness of the shielding layer 150 is 0.1 mm or more and 5 mm or less, 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.

[0074] (3) Cable Characteristics In this embodiment, by forming the insulating layer 130 from the above-described resin composition, high insulating properties can be stably obtained in the insulating layer 130.

[0075] Specifically, the insulating layer 130 of this embodiment satisfies the following insulating requirements, measured under high temperature and high electric field conditions. The measurement is performed, for example, on a sheet taken from the center of the insulating layer 130 in the thickness direction. The thickness of the insulating layer 130 sheet is, for example, 0.2 mm.

[0076] The amount of space charge accumulation measured for the insulating layer 130 under conditions of a temperature of 90°C and a DC electric field of 40 kV / mm may be, for example, 100% or less, or may be 35% or less. The amount of space charge accumulation measured under conditions of a temperature of 90°C and a DC electric field of 80 kV / mm may be, for example, 100% or less, or may be 35% or less.

[0077] However, the amount of space charge accumulation is determined by the current integration charge method. In the current integration charge method, charge is accumulated in a measuring capacitor connected in series with a sheet as a sample, and the amount of charge, which is the integrated value of the current, is evaluated. Specifically, a DC electric field of 40 kV / mm or 80 kV / mm is continuously applied to the sample at a temperature of 90°C, and the amount of charge Q after 300 seconds has elapsed is measured. ３００ and the charge amount Q immediately after application (0 seconds) ０ Based on this, the amount of space charge accumulation is calculated using the following formula: Amount of space charge accumulation = (Q ３００ / Q ０ −1) × 100

[0078] The volume resistivity of the insulating layer 130 sheet measured under conditions of a temperature of 90° C. and a DC electric field of 40 kV / mm, or under conditions of a temperature of 90° C. and a DC electric field of 80 kV / mm, is, for example, 1.0×10 １４ It is preferable that the resistivity is Ω cm or more, and 8.2 × 10 １４ It may be Ω·cm or more.

[0079] The DC breakdown field strength of the sheet of insulating layer 130 measured under a temperature condition of 90° C. may be, for example, 160 kV / mm or more, or may be 200 kV / mm or more.

[0080] (4) Method for Manufacturing Power Cable Next, a method for manufacturing a power cable according to this embodiment will be described with reference to Fig. 2. Fig. 2 is a flowchart showing a method for manufacturing a power cable according to an embodiment of the present disclosure. Hereinafter, steps will be abbreviated as "S".

[0081] (S100: Resin Composition Preparation Step) First, a resin composition for forming the insulating layer 130 is prepared.

[0082] In this embodiment, a resin composition is prepared by mixing, for example, a propylene-based resin (A), a thermoplastic elastomer (B), a modified polymer (C), and, if necessary, other additives (such as an antioxidant). The ratio of each component added is determined so that the content of unsaturated carboxylic acid and free monomer derived from component (C) falls within a predetermined range when components (A), (B), and (C) are mixed. Specifically, the ratio of each component added is determined depending on the amount of modification of the unsaturated carboxylic acid in component (C) and the content of free monomer. For example, the amount of component (A) added is 55 to 90 parts by mass, the amount of component (B) added is 10 to 45 parts by mass, and the amount of component (C) added is 1 to 10 parts by mass.

[0083] (S200: Conductor Preparation Step) On the other hand, the conductor 110 formed by twisting together a plurality of conductor core wires is prepared.

[0084] (S300: Cable core forming process (extrusion process, insulating layer forming process)) After the resin composition preparation process S100 and the conductor preparation process S200 are completed, the insulating layer 130 is formed using the above-mentioned resin composition so as to cover the outer periphery of the conductor 110 to a thickness of, for example, 3 mm or more.

[0085] At this time, in this embodiment, for example, a three-layer co-extruder is used to simultaneously form the inner semiconductive layer 120, the insulating layer 130, and the outer semiconductive layer 140.

[0086] Specifically, for example, a composition for the inner semiconductive layer is fed into extruder A of the three-layer co-extruder, which forms the inner semiconductive layer 120. The resin composition described above is fed into extruder B, which forms the insulating layer 130. For example, a composition for the outer semiconductive layer containing the same materials as the resin composition for the inner semiconductive layer fed into extruder A is fed into extruder C, which forms the outer semiconductive layer 140. Next, the extrudates from extruders A to C are guided to a common head, and the inner semiconductive layer 120, insulating layer 130, and outer semiconductive layer 140 are simultaneously extruded from the inside to the outside around the conductor 110. This forms an extruded material that will become the cable core.

[0087] The extrusion is then cooled, for example with water.

[0088] By the above-described cable core forming step S300, a cable core composed of the conductor 110, the inner semiconductive layer 120, the insulating layer 130 and the outer semiconductive layer 140 is formed.

[0089] (S400: Shielding Layer Forming Step) After the cable core is formed, the shielding layer 150 is formed on the outside of the outer semiconductive layer 140 by winding, for example, copper tape.

[0090] (S500: Sheath Forming Step) After the shielding layer 150 is formed, the sheath 160 is formed around the outer periphery of the shielding layer 150 by feeding vinyl chloride into an extruder and extruding it.

[0091] In this manner, the power cable 10 is manufactured as a solid insulated power cable.

[0092] (5) Effects of the Present Embodiment The present embodiment provides one or more of the following effects.

[0093] (a) The resin composition of the present embodiment contains the above-described components (A) to (C), and has a content of unsaturated carboxylic acid derived from the modified polymer (C) of 0.01 mass% or more and a content of free monomer derived from the modified polymer (C) of 1.0 × 10 －６ The content of the component (C) is set to be 0.01% by mass or more and 0.05% by mass or less. The component (C) allows an unsaturated carboxylic acid having a polar group to be introduced into the resin composition. By setting the content of the unsaturated carboxylic acid in the resin composition to 0.01% by mass or more, space charges can be trapped and their accumulation can be suppressed. Furthermore, free monomers derived from the component (C) are introduced into the resin composition, and in this embodiment, the content is set to 1.0 × 10 －６ The content of the component (C) is 0.05% by mass or more and 0.1% by mass or less. This allows the charge trapping effect of the free monomer to be obtained while suppressing its function as a charge carrier. Moreover, since the free monomer can be uniformly dispersed in the resin composition, the charge trapping effect can be obtained uniformly and stably. As a result, the resin composition can stably exhibit the space charge trapping effect of component (C) even in high-temperature environments, thereby suppressing local accumulation of space charge. Therefore, by forming the insulating layer 130 from the above-mentioned resin composition, high insulating properties can be stably obtained. Specifically, the insulating layer 130 can be made to withstand high voltage even at high temperatures. In other words, the power cable 10 of this embodiment enables stable DC transmission.

[0094] The (b) resin composition allows the thermoplastic elastomer (B) component and the modified polymer (C) to be finely dispersed or compatible with the propylene-based resin (A). In other words, the (B) component and the (C) component can be uniformly distributed in the resin composition. As a result, the (B) component can suppress excessive crystal growth of the (A) component, thereby imparting flexibility to the resin composition. Furthermore, the (C) component can trap space charges more uniformly in the resin composition. In other words, the resin composition can exhibit higher and more stable flexibility and insulating properties.

[0095] (c) The thermoplastic elastomer (B) may be an olefin-based thermoplastic elastomer having polyethylene or polypropylene olefin units as hard segments and ethylene-α-olefin copolymer units as soft segments, or may be an olefin-based thermoplastic elastomer having polypropylene as hard segments and ethylene-propylene rubber as soft segments. By using such a component (B), the above-mentioned effect (a) can be more reliably obtained.

[0096] The (d) resin composition preferably contains 55 to 90 parts by mass of component (A), 10 to 45 parts by mass of component (B), and 1 to 10 parts by mass of component (C), where the total content of the propylene-based resin (A), thermoplastic elastomer (B), and modified polymer (C) is taken as 100 parts by mass. This allows the amount of modification of the unsaturated carboxylic acid derived from component (C) and the content of free monomer to be adjusted within the above-mentioned ranges in the resin composition. As a result, the effect of (a) described above can be more reliably achieved.

[0097] (e) The modified polymer (C) has a modification amount of unsaturated carboxylic acid of 0.1% by mass or more and 10% by mass or less, and a free monomer content of 1.0 × 10 －４ The content of component (C) is preferably from 1.0% by mass to 1.0% by mass. By using such component (C), it is easy to adjust the content of unsaturated carboxylic acid and the content of free monomer in the resin composition to fall within the above ranges. As a result, the effect (a) described above can be more reliably obtained.

[0098] (f) The difference between the maximum and minimum MFR values ​​of the propylene-based resin (A), the thermoplastic elastomer (B), and the modified polymer (C) is preferably 300 g / 10 min or less. By setting the MFRs to have such a correlation, the components can be more reliably finely dispersed or compatible when mixed.

[0099] (g) The resin composition is preferably non-crosslinked, which allows the resin composition to be recycled.

[0100] <Other Embodiments of the Present Disclosure> Although the embodiments of the present disclosure have been specifically described above, the present disclosure is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the present disclosure.

[0101] In the above-described embodiment, the power cable 10 does not necessarily have a water-shielding layer. However, the present disclosure is not limited to this case. The power cable 10 may have a simple water-shielding layer. Specifically, the simple water-shielding layer may be, for example, a metal laminate tape. The metal laminate tape has a metal layer made of, for example, aluminum or copper, and an adhesive layer provided on one or both sides of the metal layer. The metal laminate tape is, for example, wrapped longitudinally around the outer periphery of the cable core (outer than the outer semiconductive layer). The water-shielding layer may be provided outside the shielding layer or may also serve as the shielding layer. This configuration can reduce the cost of the power cable 10.

[0102] In the above-described embodiment, the power cable 10 is configured to be laid on land, underwater, or on the bottom of the water, but the present disclosure is not limited to this. For example, the power cable 10 may be configured as a so-called overhead electric wire (overhead insulated electric wire).

[0103] In the above embodiment, three layers are simultaneously extruded in the cable core forming step S300, but each layer may be extruded one by one.

[0104] Next, examples according to the present disclosure will be described. These examples are examples of the present disclosure, and the present disclosure is not limited to these examples.

[0105] (1) Materials The materials used in preparing the resin compositions are listed below.

[0106] Random polypropylene (PP1) was prepared as the propylene-based resin (A), an olefin-based thermoplastic elastomer (TPO) was prepared as the thermoplastic elastomer (B), and maleic acid-modified polypropylenes (MAH-PP1) to (MAH-PP9) were prepared as the modified polymer (C). Each of MAH-PP1 to MAH-PP9 had a predetermined amount of free monomer, or was prepared by treating it to have a predetermined amount. The physical properties of each component are as follows: PP1: melting point 160°C, MFR 0.6 g / 10 min TPO: olefin-based thermoplastic elastomer with a hard segment made of polypropylene and a soft segment made of ethylene-propylene rubber, melting point 140°C, MFR 3.2 g / 10 min MAH-PP1: MFR 12 g / 10 min, melting point 160°C, maleic acid modification amount 0.8 mass%, free monomer amount 2.0 x 10 －６ MAH-PP2: MFR 12 g / 10 min, melting point 160°C, maleic acid modification amount 0.8 mass%, free monomer amount 1.0 × 10 －３ MAH-PP3: MFR 12 g / 10 min, melting point 160°C, maleic acid modification amount 0.8 mass%, free monomer amount 2.0 × 10 －２ MAH-PP4: MFR 12 g / 10 min, melting point 160°C, maleic acid modification amount 0.8 mass%, free monomer amount 0.6 mass% MAH-PP5: MFR 12 g / 10 min, melting point 160°C, maleic acid modification amount 0.8 mass%, free monomer amount 0.8 mass% MAH-PP6: MFR 10 g / 10 min, melting point 160°C, maleic acid modification amount 0.1 mass%, free monomer amount 4.0 × 10 －３ MAH-PP7: MFR 13 g / 10 min, melting point 160°C, maleic acid modification amount 0.2 mass%, free monomer amount 4.0 × 10 －３ MAH-PP8: MFR 18 g / 10 min, melting point 160°C, maleic acid modification amount 1.0 mass%, free monomer amount 4.0 × 10 －３ MAH-PP9: MFR 20 g / 10 min, melting point 160°C, maleic acid modification amount 2.0 mass%, free monomer amount 4.0 × 10 －３ mass%

[0107] As another additive, a hindered phenol, pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] (molecular weight 1178), was prepared as an antioxidant.

[0108] (2) Preparation of Resin Compositions The above materials were fed to an extruder in the amounts shown in Tables 1 and 2 below, and then heated, mixed, and granulated in the extruder to prepare Samples 1-A to 5-A and Samples 1-B to 4-B. In each sample, the amount of antioxidant added was 0.1 parts by mass.

[0109]

[0110]

[0111] (3) Fabrication of Power Cable Next, a conductor was prepared by twisting together conductor core wires made of a dilute copper alloy with a diameter of 14 mm. After the conductor was prepared, an inner semiconductive layer resin composition containing an ethylene-ethyl acrylate copolymer, an insulating layer resin composition prepared in Tables 1 and 2, and an outer semiconductive layer resin composition made of the same materials as the inner semiconductive layer resin composition were loaded into extruders A to C, respectively. The extrudates from extruders A to C were guided to a common head, and the inner semiconductive layer, insulating layer, and outer semiconductive layer were simultaneously extruded from the inside to the outside around the conductor. This produced a power cable sample having a conductor, inner semiconductive layer, insulating layer, and outer semiconductive layer from the center to the periphery.

[0112] (4) Evaluation Samples were cut out from the insulating layer of the produced power cable, and the resin composition constituting the insulating layer was evaluated for the amount of modification with unsaturated carboxylic acid (maleic acid), the content of free monomer, space charge characteristics, volume resistivity, and DC breakdown strength. Each evaluation method was as follows.

[0113] (Amount of Modification of Unsaturated Carboxylic Acid) The content of unsaturated carboxylic acid (maleic acid) in the insulating layer was evaluated by an elution test using an organic solvent. Specifically, first, a sheet-shaped sample piece was collected from the insulating layer of the power cable. Next, this sample piece was dissolved in hot p-xylene as an organic solvent, and reprecipitated with acetone and collected. The unsaturated carboxylic acid moieties contained in this resin composition were methyl esterified, and 1H-NMR measurement was performed. The amount of modification of unsaturated carboxylic acid was determined from the obtained spectrum. Specifically, in the obtained spectrum, the area S1 of the peak derived from the polymer skeleton and the area S2 of the peak derived from the unsaturated carboxylic acid were determined, and the ratio of S2 to the sum of S1 and S2 (S2 / S1+S2) was defined as the total content of unsaturated carboxylic acid.

[0114] (Free Monomer Content) The free monomer content in the insulating layer was evaluated by an immersion test in an organic solvent. Specifically, a sheet-shaped sample piece was first taken from the insulating layer of the power cable. Next, this sample piece was dissolved in an organic solvent, and the free monomer content was determined by 1H-NMR measurement. When the amount of free monomer was small, it was extracted using an organic solvent and determined by liquid chromatography measurement.

[0115] (Space Charge Characteristics) The space charge characteristics of the insulating layer were evaluated by the amount of space charge accumulated in the insulating layer. The amount of space charge accumulated was measured by the current integration charge method. Specifically, a sheet-shaped sample piece was first taken from the insulating layer of the power cable. Next, this sample piece was connected in series to a measuring capacitor, and then charge was accumulated in the measuring capacitor, and the amount of charge, which is the integrated value of the current, was measured. In this example, a DC electric field of 40 kV / mm or 80 kV / mm was continuously applied to the sample at a temperature of 90°C, and the amount of charge Q after 300 seconds was measured. ３００ and the charge amount Q immediately after application (0 seconds) ０Based on this, the space charge accumulation amount at a temperature of 90°C and a DC electric field of 40 kV / mm and the space charge accumulation amount at a temperature of 90°C and a DC electric field of 80 kV / mm were calculated using the following formula. The case where the space charge accumulation amount was 35% or less was rated as A (best), the case where the space charge accumulation amount was more than 35% and 100% or less was rated as B (good), and the case where the space charge accumulation amount was more than 100% was rated as C (poor). Space charge accumulation amount=(Q ３００ / Q ０ −1) × 100

[0116] (Volume Resistivity) The volume resistivity of the insulating layer was measured using a sheet-shaped sample taken from the insulating layer of a power cable in the same manner as in the case of the space charge characteristics. Specifically, the sample was immersed in silicone oil at a temperature of 90°C, and a DC electric field of 40 kV / mm or 80 kV / mm was applied to the sample using a flat electrode with a diameter of 25 mm, thereby measuring the volume resistivity. When the volume resistivity was 8.2 × 10 １４ A value of A (best) is used when the volume resistivity is 1×10 １４ Ω・cm or more 8.2×10 １４ A volume resistivity of less than Ω cm is rated as B (good), and a volume resistivity of 1×10 １４ When the resistance was less than Ω·cm, it was evaluated as C (poor).

[0117] (DC Breakdown Strength) The DC breakdown strength of the insulating layer was measured using a sheet-shaped sample taken from the insulating layer of a power cable, similar to the space charge characteristics. Specifically, the sample was first immersed in silicone oil at a temperature of 90 ° C., and a flat electrode with a diameter of 25 mm was used to increase the applied voltage at a rate of 4 kV / min. Then, when the sample reached dielectric breakdown, the applied voltage was divided by the thickness of the sample to determine the DC breakdown strength of the sample. When the DC breakdown strength was 200 kV / mm or more, it was evaluated as A (best), when the DC breakdown strength was 160 kV / mm or more but less than 200 kV / mm, it was evaluated as B (good), and when the DC breakdown strength was less than 160 kV / mm, it was evaluated as C (poor).

[0118] (5) Evaluation Results The above-described evaluations were carried out for each sample, and the evaluation results are summarized in Tables 1 and 2. In Tables 1 and 2, the MAH content indicates the content of unsaturated carboxylic acid in the resin composition, and the free MAH amount indicates the content of free monomer in the resin composition.

[0119] As shown in Table 1, in Samples 1-A to 5-A, the resin compositions were prepared so that the content of unsaturated carboxylic acid (MAH content) in the resin composition was the same at 0.04 mass%, but the content of free monomer (free MAH amount) was different. In Sample 1-A, the content of unsaturated carboxylic acid was the predetermined amount, but the content of free monomer was 1 × 10 －７ % by mass, 1.0 x 10 －６ % by mass, the amount of space charge accumulation was high, the volume resistivity was low, and it was confirmed that high insulation properties could not be obtained. This is thought to be because the amount of free monomer was too small, and the charge trapping effect was not sufficiently obtained. On the other hand, in Samples 2-A to 5-A, the content of free monomer was 1×10 －６ It was confirmed that the content of the unsaturated carboxylic acid in the resin composition was 50% by mass or more, and that the free monomer together with the unsaturated carboxylic acid provided a charge trapping effect, reducing the accumulation of space charge even in high-temperature environments and improving the volume resistivity and DC breakdown field strength. In other words, it was confirmed that high insulating properties could be stably obtained in the resin composition. This revealed that the insulating layer could have high voltage resistance even in high-temperature environments.

[0120] Furthermore, it was confirmed that, according to Samples 2-A to 5-A, by setting the free monomer content to 0.05% by mass or less, it is possible to reduce the amount of space charge accumulation and increase the volume resistivity, thereby improving the insulating properties at high temperatures. Furthermore, since Samples 3-A and 4-A exhibited higher insulating properties than Sample 5-A, it was found that the free monomer content should be 0.04% by mass or less, and preferably 0.03% by mass. It was also confirmed that, when the free monomer content exceeds 0.05% by mass, the amount of space charge accumulation increases, the volume resistivity decreases, and the insulating properties are significantly reduced. This is thought to be due to the free monomer acting as a charge carrier.

[0121] As shown in Table 2, in Samples 1-B to 4-B, the free monomer content was 2×10 －４ Resin compositions were prepared with different unsaturated carboxylic acid contents, but the same mass %. In Sample 1-B, the unsaturated carboxylic acid modification amount was 0.005 mass %, less than 0.01 mass %, resulting in high space charge accumulation, low volume resistivity, and poor insulating properties. Although Sample 1-B contained a predetermined amount of free monomer, the unsaturated carboxylic acid modification amount was low, preventing sufficient polar groups from being introduced into the resin composition, presumably preventing high insulating properties. On the other hand, Samples 2-B to 4-B, in which the unsaturated carboxylic acid modification amount was 0.01 mass % or greater, provided the charge trapping effect of the unsaturated carboxylic acid, reduced space charge accumulation even in high-temperature environments, and improved volume resistivity and DC breakdown field strength. These results demonstrate that Samples 2-B to 4-B exhibit high insulating properties.

[0122] Furthermore, Samples 2-B to 4-B confirmed that the higher the content of unsaturated carboxylic acid in the resin composition, the more improved the insulating properties at high temperatures. Specifically, it was confirmed that the unsaturated carboxylic acid content is preferably 0.01% by mass or more, and preferably 0.03% by mass. It was also confirmed that if the content of unsaturated carboxylic acid is excessively high, the content of modified polymer (C) increases, reducing the proportion of propylene-based resin (A) and thermoplastic elastomer (B), making it impossible to obtain the flexibility and mechanical strength required for the insulating layer. From the perspective of achieving a good balance between insulating properties and flexibility at high temperatures, it was found that the content of unsaturated carboxylic acid should be 0.1% by mass or less.

[0123] It was confirmed that in Samples 1-A to 5-A and Samples 1-B to 4-B, the thermoplastic elastomer (B) or modified polymer (C) could be finely dispersed or mixed in the propylene-based resin (A). This enabled the strength and flexibility required for the insulation layer of power cables to be obtained.

[0124] As described above, it has been confirmed that by mixing a propylene-based resin (A), a thermoplastic elastomer (B), and a modified polymer (C) in a resin composition and adjusting the content of unsaturated carboxylic acid contained in the modified polymer (C) and the content of free monomer derived from the modified polymer (C) within a predetermined range, it is possible to suppress the accumulation of space charge and improve the volume resistivity and DC breakdown field strength even in high-temperature environments. In other words, it has been confirmed that high insulating properties can be stably obtained in the resin composition. As a result, the insulating layer can withstand high voltage even in high-temperature environments, enabling stable DC transmission using a power cable.

[0125] <Supplementary Notes> The following provides supplementary notes on aspects of the present disclosure.

[0126] (Note 1) A thermoplastic elastomer composition comprising: a propylene-based resin (A) having a propylene unit; a thermoplastic elastomer (B); and a modified polymer (C) having a propylene unit and modified with an unsaturated carboxylic acid, wherein the amount of modification of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01% by mass or more; and the content of free monomers having an unsaturated carboxylic acid group and having a molecular weight of 500 or less derived from the modified polymer (C) is 1.0 x 10 －６ % by mass or more and 0.05% by mass or less.

[0127] (Appendix 2) A conductive material comprising: a conductor; and an insulating layer covering the conductor and formed from a resin composition, wherein the resin composition contains a propylene-based resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with an unsaturated carboxylic acid, wherein the amount of modification of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01 mass% or more, and the content of free monomers having an unsaturated carboxylic acid group and having a molecular weight of 500 or less derived from the modified polymer (C) is 1.0 x 10 －６ % by mass or more and 0.05% by mass or less.

[0128] (Supplementary Note 3) In Supplementary Note 2, the thermoplastic elastomer (B) is an olefin-based thermoplastic elastomer having at least one olefin unit of polyethylene and polypropylene as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment.

[0129] (Appendix 4) In Appendix 2 or Appendix 3, the resin composition contains 55 parts by mass or more and 90 parts by mass or less of the propylene-based resin (A), 10 parts by mass or more and 45 parts by mass or less of the thermoplastic elastomer (B), and 1 part by mass or more and 10 parts by mass or less of the modified polymer (C), when the total content of the propylene-based resin (A), the thermoplastic elastomer (B), and the modified polymer (C) is 100 parts by mass.

[0130] (Appendix 5) In any one of Appendices 2 to 4, the modified polymer (C) has an unsaturated carboxylic acid modification amount of 0.1% by mass or more and 10% by mass or less, and a free monomer content of 1.0 × 10 －４ The content is 0.1 mass % or more and 0.2 mass % or less.

[0131] (Supplementary Note 6) In any one of Supplementary Notes 2 to 5, the propylene-based resin (A) has a melting point of 130°C or more and 170°C or less, and a melt flow rate of 0.1 g / 10 min or more and 5.0 g / 10 min or less.

[0132] (Supplementary Note 7) In any one of Supplementary Notes 2 to 6, the propylene-based resin (A) is a random polypropylene.

[0133] (Supplementary Note 8) In any one of Supplementary Notes 2 to 7, the thermoplastic elastomer (B) has no melting point or a melting point of 165°C or less, and a melt flow rate of 0.1 g / 10 min or more and 5.0 g / 10 min or less.

[0134] (Supplementary Note 9) In any one of Supplementary Notes 2 to 8, the modified polymer (C) has a melting point of 130°C or more and 165°C or less, and a melt flow rate of 0.1 g / 10 min or more and 500 g / 10 min or less.

[0135] (Supplementary Note 10) In any one of Supplementary Notes 2 to 9, the melting point of the resin composition is 130°C or higher and 170°C or lower.

[0136] (Appendix 11) A method for manufacturing a conductor, comprising: preparing a resin composition for forming an insulating layer; and forming an insulating layer from the resin composition so as to cover an outer periphery of a conductor, wherein the resin composition is prepared by mixing a propylene-based resin (A) having a propylene unit, a thermoplastic elastomer (B), and a modified polymer (C) having a propylene unit and modified with an unsaturated carboxylic acid, the resin composition being such that the amount of modification of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01% by mass or more, and the content of free monomers having an unsaturated carboxylic acid group and a molecular weight of 500 or less derived from the modified polymer (C) is 1.0 x 10 －６ % by mass or more and 0.05% by mass or less.

[0137] (Appendix 12) In Appendix 11, the modified polymer (C) has a modification amount of the unsaturated carboxylic acid of 0.1% by mass or more and 10% by mass or less, and a content of the free monomer of 1.0×10 －４ The content is 0.1 mass % or more and 0.2 mass % or less.

[0138] (Appendix 13) In Appendix 11 or Appendix 12, in the step of preparing the resin composition, the propylene-based resin (A), the thermoplastic elastomer (B), and the modified polymer (C) are mixed so that, when the total content of these is 100 parts by mass, the content of the propylene-based resin (A) is 55 parts by mass or more and 90 parts by mass or less, the content of the thermoplastic elastomer (B) is 10 parts by mass or more and 45 parts by mass or less, and the content of the modified polymer (C) is 1 part by mass or more and 10 parts by mass or less.

[0139] REFERENCE SIGNS LIST 10 Power cable 110 Conductor 120 Inner semiconductive layer 130 Insulating layer 140 Outer semiconductive layer 150 Shielding layer 160 Sheath

Claims

1. A thermoplastic elastomer composition comprising: (A) a propylene-based resin having propylene units; (B) a thermoplastic elastomer; and (C) a modified polymer having propylene units and modified with an unsaturated carboxylic acid, wherein the content of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01% by mass or more; and the content of a free monomer having an unsaturated carboxylic acid group and having a molecular weight of 500 or less derived from the modified polymer (C) is 1.0 x 10 －６ % by mass or more and 0.05% by mass or less.

2. A conductive material comprising: a conductor; and an insulating layer covering the conductor and formed from a resin composition, wherein the resin composition contains a propylene-based resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with an unsaturated carboxylic acid, wherein the content of the unsaturated carboxylic acid derived from the modified polymer (C) is 0.01 mass% or more, and the content of free monomers having an unsaturated carboxylic acid group and a molecular weight of 500 or less derived from the modified polymer (C) is 1.0 x 10 －６ % by mass or more and 0.05% by mass or less.

3. The power cable according to claim 2, wherein the thermoplastic elastomer (B) is an olefin-based thermoplastic elastomer having olefin units of at least one of polyethylene and polypropylene as hard segments and ethylene-α-olefin copolymer units as soft segments.

4. A power cable according to claim 2 or claim 3, wherein the resin composition contains 55 parts by mass or more and 90 parts by mass or less of the propylene-based resin (A), 10 parts by mass or more and 45 parts by mass or less of the thermoplastic elastomer (B), and 1 part by mass or more and 10 parts by mass or less of the modified polymer (C), when the total content of the propylene-based resin (A), the thermoplastic elastomer (B), and the modified polymer (C) is 100 parts by mass.

5. The modified polymer (C) has a modification amount of unsaturated carboxylic acid of 0.1% by mass or more and 10% by mass or less, and a free monomer content of 1.0 x 10 －４ The power cable according to claim 2 , wherein the content of the SiO 2 is 0.1 mass % or more and 0.1 mass % or less.