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

Abstract

A resin composition which contains a base polymer (A) having a propylene unit and a modified polymer (B) having a propylene unit and modified by an unsaturated carboxylic acid, wherein the total content of the unsaturated carboxylic acid derived from the modified polymer (B) is at least 0.0003%, and when the molecular weight distribution is measured, the proportion of the components which have a molecular weight less than or equal to 1×104 is less than 1.3% and the proportion of the components which have a molecular weight greater than or equal to 6×105 is at least 16%.

Description

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

[0001] The present disclosure relates to a resin composition, a power cable, and a method for producing 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 polymer composition comprising a base polymer (A) having a propylene unit and a modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid is provided, wherein the total content of the unsaturated carboxylic acid derived from the modified polymer (B) is 0.0003% or more, and when the molecular weight distribution is measured, the molecular weight is 1×10 ４ The proportion of the following components is less than 1.3% and the molecular weight is 6×10 ５ The resin composition is provided in which the proportion of the above components is 16% or more.

[0005] According to another aspect of the present disclosure, there is provided a method for producing a laminated laminated laminate comprising the steps of: a conductor; and an insulating layer covering the conductor and formed from a resin composition; wherein the resin composition contains a base polymer (A) having a propylene unit; and a modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid; a total content of the unsaturated carboxylic acid derived from the modified polymer (B) contained in the resin composition is 0.0003% or more; and, when a molecular weight distribution of the resin composition is measured, the molecular weight is 1×10 ４ The proportion of the following components is less than 1.3% and the molecular weight is 6×10 ５ The power cable has a ratio of the above components of 16% 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. Fig. 2 is a schematic configuration diagram of an extruder used in a power cable manufacturing method according to an embodiment of the present disclosure. Fig. 3 is a schematic diagram for explaining the shape of a screw flight. Fig. 4 is a schematic diagram for explaining a retention suppression member. Fig. 5 is a flowchart showing a power cable manufacturing method 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, polymer components containing propylene units, such as polypropylene, have been attracting attention as resin components constituting the insulating layer. Polypropylene can achieve high insulating properties even without cross-linking. In other words, it can achieve both insulating properties and recyclability.

[0009] On the other hand, polymer components containing propylene units 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 polymer component containing propylene units, 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, methods of blending modified polymers having polar groups have been investigated, such as modified propylene obtained by modifying propylene with unsaturated carboxylic acid.

[0015] However, it was confirmed that the insulating properties of the resin composition and the insulating layer were not stable simply by adding a modified polymer. Further investigation into this point revealed that when the insulating properties were unstable, the proportion of high molecular weight components in the resin composition tended to be low and the proportion of low molecular weight components tended to be high.

[0016] While high-molecular-weight components contribute to maintaining high insulating properties, low-molecular-weight components may reduce insulating properties. Low-molecular-weight components are thought to cause localized space charge accumulation in the resin composition, and to migrate within the resin composition, particularly in high-temperature environments, further increasing the accumulation. Therefore, fluctuations in the proportions of high-molecular-weight and low-molecular-weight components may make it difficult to obtain the space charge trapping effect of the modified polymer even when it is added to the resin composition. As a result, the insulating layer may not consistently achieve high insulating properties.

[0017] The proportions of high-molecular-weight and low-molecular-weight components are determined depending on the type and addition ratio of polymer components and modified polymers, each with a predetermined molecular weight distribution. However, it has been confirmed that the proportions of each molecular weight component, which should be determined at the material selection stage, can sometimes vary after extrusion.

[0018] Further investigation into this point revealed that when the resin composition is heated during extrusion, the polymer components and other components are thermally decomposed, resulting in a lower molecular weight. As a result of this thermal decomposition, the proportion of high molecular weight components decreases and the proportion of low molecular weight components increases, causing the respective proportions to deviate from their original values.

[0019] Moreover, it was found that this fluctuation in the ratio is not large in the early stages of extrusion, but becomes more pronounced as the extrusion continues for a long time. In other words, the longer the extrusion is carried out to produce a long power cable, the more likely the ratio of molecular weight components is to fluctuate, making it difficult to stably obtain high insulation properties.

[0020] Therefore, the present inventors have investigated methods for suppressing thermal decomposition of a resin composition. As a result, they have found that thermal decomposition of a resin composition can be suppressed by extruding the resin composition while adjusting the inside of the extruder to an inert gas atmosphere. Furthermore, if the resin composition remains in the extruder and the heating time becomes longer, thermal decomposition is likely to be accelerated. From this, they have found that thermal decomposition of a resin composition can be further suppressed by configuring the screw and extrusion port of the extruder so that the resin composition is less likely to remain. Then, in the extruded resin composition, a molecular weight of 6 × 10 relative to the base resin in the resin composition is 6 × 10 ５ The proportion of high molecular weight components having a molecular weight of 1×10 or more is 16% or more, and the proportion of high molecular weight components having a molecular weight of 1×10 or more is 16% or more. ４ It has been found that high insulating properties can be stably achieved by controlling the proportion of low molecular weight components to less than 1.3%, which is the same or less than the above range.

[0021] On the other hand, from the viewpoint of obtaining the properties of the modified polymer modified with unsaturated carboxylic acid, it has been found that it is advantageous to adjust the type and amount of the modified polymer used so that the total content of unsaturated carboxylic acid units contained in the base resin is 0.0003% or more.

[0022] The present disclosure is based on the above-mentioned findings of the inventors.

[0023] <Embodiments of the Present Disclosure> Next, embodiments of the present disclosure will be listed and described.

[0024] [1] A resin composition according to one embodiment of the present disclosure comprises a base polymer (A) having a propylene unit and a modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid, wherein the total content of the unsaturated carboxylic acid derived from the modified polymer (B) is 0.0003% or more, and when a molecular weight distribution is measured, the molecular weight is 1×10 ４ The proportion of the following components is less than 1.3% and the molecular weight is 6×10 ５ The resin composition has a ratio of the above components of 16% or more. With this configuration, high insulating properties can be stably obtained.

[0025] [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 base polymer (A) having a propylene unit and a modified polymer (B) that has a propylene unit and is modified with an unsaturated carboxylic acid, wherein a total content of the unsaturated carboxylic acid derived from the modified polymer (B) contained in the resin composition is 0.0003% or more, and when a molecular weight distribution of the resin composition is measured, the molecular weight is 1×10 ４ The proportion of the following components is less than 1.3% and the molecular weight is 6×10 ５ The power cable has a ratio of the above components of 16% or more. With this configuration, high voltage resistance can be achieved even in high temperature environments, enabling stable DC power transmission.

[0026] [3] In the power cable according to the above [2], the resin composition contains 90 parts by mass or more and 99 parts by mass or less of the base polymer (A) and 1 part by mass or more and 10 parts by mass or less of the modified polymer (B), where the total content of the base polymer (A) and the modified polymer (B) is taken as 100 parts by mass. This configuration makes it possible to stably obtain high insulation properties.

[0027] [4] The power cable according to [2], wherein the resin composition further contains a thermoplastic elastomer (C), and the resin composition contains 55 to 90 parts by mass of the base polymer (A), 1 to 10 parts by mass of the modified polymer (B), and 10 to 45 parts by mass of the thermoplastic elastomer (C), where the total content of the base polymer (A), the modified polymer (B), and the thermoplastic elastomer (C) is 100 parts by mass. This configuration makes it possible to improve the flexibility of the insulating layer while stably obtaining high insulating properties.

[0028] [5] The power cable according to the above [4], wherein the thermoplastic elastomer (C) is a styrene-based elastomer containing a styrene unit. With this configuration, higher insulation properties can be stably obtained in the insulating layer.

[0029] [6] The power cable according to any one of [2] to [5] above, wherein the modified polymer (B) has a number average molecular weight of 20,000 or more. This configuration allows the amount of low-molecular-weight components contained in the resin composition to be reduced.

[0030] [7] The power cable according to any one of the above [2] to [6], wherein the number average molecular weight of the base polymer (A) is 80,000 or more. This configuration makes it possible to maintain a high proportion of high molecular weight components in the resin composition while further reducing the proportion of low molecular weight components.

[0031] [8] A method for producing a power cable according to another aspect of the present disclosure is a method for producing a power cable including a conductor and an insulating layer formed from a resin composition and covering the conductor, the method comprising: a preparation step of preparing the resin composition; and an insulating layer formation step of extruding the resin composition so as to cover the conductor and form the insulating layer, wherein in the preparation step, a base polymer (A) having propylene units and a modified polymer (B) having propylene units and modified with an unsaturated carboxylic acid are added to the resin composition so that the total content of the unsaturated carboxylic acid derived from the modified polymer (B) is 0.0003% or more in the resin composition, and the resulting mixture is heated and mixed in an inert gas atmosphere. This configuration makes it possible to produce a power cable that stably exhibits high insulation properties.

[0032] [9] In the power cable manufacturing method described in [8] above, in the preparation step, the resin composition is prepared using an extruder including a cylindrical cylinder, a rotatable screw inserted into the cylinder from a first direction, a discharge section attached to the cylinder from a second direction and provided with a hole for discharging the resin composition, and an atmosphere adjustment section adjusting the interior of the cylinder to an inert gas atmosphere. This configuration makes it possible to more stably obtain high insulation properties.

[0033] [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.

[0034] <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 includes a base polymer (A) having propylene units, a modified polymer (B) having propylene units and modified with an unsaturated carboxylic acid, and, as necessary, a thermoplastic elastomer (C) and other additives. Each component will be described in detail below.

[0035] (Base Polymer (A)) The base polymer (A) is a resin material that constitutes the main component of the resin composition and is a propylene-based resin containing propylene units. Examples of this base polymer (A) include a propylene homopolymer (hereinafter also referred to as homo-PP) and a propylene random polymer (hereinafter also referred to as random PP). Homo-PP contains propylene units, and random PP has propylene units and ethylene units. The content of ethylene units in random PP may be, for example, 0.5% by mass or more and 15% by mass or less. By setting the content of ethylene units to 0.5% by mass or more, spherulite growth (the generation of coarse crystals) can be suppressed and high insulation properties can be maintained. On the other hand, by setting the content of ethylene units to 15% by mass or less, a decrease in melting point can be suppressed and stable use in non-crosslinked or slightly crosslinked states can be achieved.

[0036] From the viewpoint of obtaining higher insulating properties in the insulating layer, it is preferable that the base polymer (A) is random PP. Homo PP has a higher crystalline content than random PP and can obtain high insulating properties, but it may cause cracks in the crystals or between the crystals in the insulating layer, which may 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.

[0037] The stereoregularity of the propylene-based resin may be isotactic, syndiotactic, or atactic. The stereoregularity is not particularly limited, but isotactic is preferable. The isotactic stereoregularity can suppress a decrease in melting point in a composition obtained by mixing the base polymer (A) and the modified polymer (B). As a result, the resin can be stably used in a non-crosslinked or slightly crosslinked state.

[0038] The number average molecular weight of the base polymer (A) is not particularly limited, but is preferably 80,000 or more, and may be 80,000 or more and 300,000 or less. When the base polymer (A) has such a number average molecular weight, the molecular weight is 1×10 ４ The ratio of the low molecular weight component, which is equal to or less than 1000 ppm, is small, and the ratio of the high molecular weight component contained in the resin composition can be adjusted to be large and the ratio of the low molecular weight component to be small.

[0039] The number average molecular weight is calculated from the molecular weight distribution of the base polymer (A) measured by gel permeation chromatography (GPC) based on a calibration curve prepared using polystyrene (PS) as a standard sample. Details will be described later in the Examples.

[0040] The melt flow rate (MFR) of the base polymer (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 modified polymer (B) and the thermoplastic elastomer (C), as described below. By achieving such an MFR, the phase structure of the resin composition can be formed into a compatible structure or a sea-island structure, as described below. 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.

[0041] The melting point of the base polymer (A) is not particularly limited, but is preferably 130° C. or higher and 165° C. or lower. A base polymer (A) having such a melting point is likely to form a phase structure described below when mixed with the modified polymer (B).

[0042] (Modified polymer (B)) The modified polymer (B) is a resin having propylene units and modified with an unsaturated carboxylic acid having a polar group. Because the modified polymer (B) has propylene units, it has excellent compatibility when mixed with the base polymer (A). Furthermore, by mixing the modified polymer (B) with the base polymer (A), polar groups can be introduced into the resin composition. The polar groups allow space charges 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.

[0043] Specifically, the modified polymer (B) 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 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.

[0044] In the modified polymer (B), the ratio of unsaturated carboxylic acid introduced to polypropylene (modification amount) is not particularly limited, but is, for example, 0.1% or more and 10% or less. By setting the modification amount of unsaturated carboxylic acid in polypropylene to 0.1% or more, it is easy to adjust the total content of unsaturated carboxylic acid in the resin composition to 0.0003% or more, and space charge accumulation can be suppressed. On the other hand, by setting the modification amount of unsaturated carboxylic acid to 10% or less in polypropylene, compatibility with the base polymer (A) can be improved. The modification amount refers to the copolymerization ratio of unsaturated carboxylic acid in the modified polymer (B).

[0045] The number average molecular weight of the modified polymer (B) is not particularly limited, but is preferably 20,000 or more, and may be 20,000 or more and 300,000 or less. According to the modified polymer (B) having such a number average molecular weight, the molecular weight is 1 × 10 ４ The proportion of low molecular weight components, which is equal to or less than 1000 ppm, is small, and the proportion of low molecular weight components contained in the resin composition can be kept low.

[0046] From the viewpoint of compatibility with the base polymer (A), the MFR of the modified polymer (B) 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. By setting the MFR in this range, it becomes easier to form the phase structure described later in the resin composition.

[0047] The melting point of the modified polymer (B) is not particularly limited, but is preferably 130° C. or higher and 165° C. or lower. A modified polymer (B) having such a melting point is likely to form a phase structure described below when mixed with the base polymer (A).

[0048] (Thermoplastic Elastomer (C)) The resin composition of this embodiment may contain a thermoplastic elastomer (C) in addition to the base polymer (A) and modified polymer (B) described above. The thermoplastic elastomer (C) has lower crystallinity than the base polymer (A) containing propylene units, and can control the crystal growth of the base polymer (A) and impart flexibility to the resin composition and insulating layer. Known components such as amide-based, ester-based, olefin-based, styrene-based, urethane-based, PVC-based, and fluorine-based thermoplastic elastomers can be used as the thermoplastic elastomer (C). Among these, styrene-based elastomers are preferred. Styrene-based elastomers contain styrene units and can trap electrons with aromatic rings to form a stable resonance structure, thereby further improving the insulating properties of the insulating layer. Furthermore, when mixed with the base polymer (A) or modified polymer (B), the thermoplastic elastomer (C) is likely to form the phase structure described below, thereby further improving the flexibility of the resin composition and the like.

[0049] Specifically, the styrene elastomer is a copolymer containing, for example, a styrene unit as a hard segment and at least one monomer unit selected from ethylene, propylene, butylene, isoprene, and the like as a soft segment.

[0050] Examples of styrene-based elastomers include styrene-butadiene-styrene block copolymers (SBS), hydrogenated styrene-butadiene-styrene block copolymers, styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS), styrene-ethylene-propylene-styrene block copolymers (SEPS), styrene-isoprene-styrene copolymers (SIS), hydrogenated styrene-isoprene-styrene copolymers, hydrogenated styrene-butadiene rubber, hydrogenated styrene-isoprene rubber, styrene-ethylene-butylene olefin crystalline block copolymers, etc. Two or more of these may be used in combination.

[0051] Here, "hydrogenated" means that hydrogen has been added to the double bond. For example, "hydrogenated styrene butadiene styrene block copolymer" means a polymer in which hydrogen has been added to the double bond of a styrene butadiene styrene block copolymer. Note that hydrogen has not been 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).

[0052] As the styrene-based elastomer, one that does not contain a double bond in the chemical structure excluding the benzene ring is preferred. If a styrene-based elastomer containing a double bond is used, the resin component may be thermally deteriorated during molding of the resin composition, which may result in a decrease in the properties of the resulting insulating layer. In this regard, a styrene-based elastomer that does not contain a double bond has high resistance to thermal deterioration, and therefore can maintain the properties of the insulating layer at a higher level.

[0053] The content of styrene units in the styrene-based elastomer is not particularly limited, but from the viewpoint of controlling the crystal growth of the base polymer (A) and softening the insulating layer, it is preferable that the content be 5% by mass or more and 50% by mass or less. Furthermore, by ensuring that the content of styrene units falls within the above range, a predetermined amount of monomer units, such as ethylene units, can be secured as soft segments, thereby improving the compatibility between the styrene-based elastomer and the base polymer (A). This allows for stable improvement in the insulating properties of the insulating layer.

[0054] The number average molecular weight of the thermoplastic elastomer (C) is not particularly limited, but is preferably 20,000 or more, and may be 20,000 or more and 300,000 or less. According to the thermoplastic elastomer (C) having such a number average molecular weight, the molecular weight of 1 × 10 ４ The proportion of low molecular weight components, which is equal to or less than 1000 ppm, is small, and the proportion of low molecular weight components contained in the resin composition can be kept low.

[0055] From the viewpoint of compatibility with the base polymer (A), the MFR of the thermoplastic elastomer (C) 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. By setting the MFR in this range, the above-mentioned phase structure can be easily formed in the resin composition.

[0056] The thermoplastic elastomer (C) preferably does not have a melting point, or if it does, the melting point is preferably lower than 165° C. A thermoplastic elastomer (C) having such a melting point is easily compatible with the base polymer (A) and easily forms a phase structure described below.

[0057] (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.

[0058] In this embodiment, the space charge trapping effect of the modified polymer (B) can be uniformly obtained in the resin composition, so that high insulating properties can be stably obtained without adding an inorganic filler. As will be described later, from the viewpoint of suppressing clogging of the mesh during the extrusion process of the resin composition, it is preferable that the resin composition does not contain an inorganic filler, but an inorganic filler may be added in a small amount that does not cause clogging of the mesh.

[0059] When inorganic fillers are added, depending on the manufacturing conditions, including the particle size, surface treatment conditions, and content of the inorganic filler, the mesh may gradually become clogged due to aggregation of the inorganic filler during the extrusion process of the insulation layer. If the mesh becomes clogged, the resin pressure during the extrusion process may increase. This may result in variations in the insulation properties of the insulation layer along the length of the cable or a decrease in the moldability of the insulation layer itself. Therefore, it is recommended that the inorganic filler content be set to a level that does not cause clogging of the mesh.

[0060] Specifically, the content of the inorganic filler is preferably less than 1 part by mass when the total content of the resin components such as the base polymer (A), the modified polymer (B), and the thermoplastic elastomer (C) is taken as 100 parts by mass. The lower limit of the content of the inorganic filler is not limited as long as the inorganic filler can be added.

[0061] 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.

[0062] Examples of methods for forming magnesium oxide include a gas phase method in which magnesium vapor is brought into contact with oxygen, and a seawater method in which magnesium oxide is formed from seawater as a raw material. The method for forming the inorganic filler in this embodiment may be either the gas phase method or the seawater method.

[0063] Examples of silicon dioxide include at least one of fumed silica, colloidal silica, precipitated silica, and deflagration silica. Among these, fumed silica is preferred as silicon dioxide.

[0064] 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.

[0065] The lower limit of the volume average particle size of the inorganic filler is not particularly limited, but 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 may be 5 nm or more.

[0066] 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 base polymer (A), etc., and can improve the mechanical properties and insulating properties of the insulating layer 130.

[0067] As the antioxidant, known antioxidants such as phenol-based, sulfur-based, and amine-based antioxidants can be used. Because antioxidants have a relatively low molecular weight, they tend to migrate when the resin composition is exposed to a high-temperature environment, which can lead to localized space charge accumulation. In this regard, from the perspective of suppressing the migration of the antioxidant, it is preferable that the antioxidant has a chemical structure with large steric hindrance. Among phenol-based antioxidants, hindered phenol-based antioxidants are preferable. Furthermore, from the perspective of suppressing the migration of the antioxidant, it is preferable that the molecular weight of the antioxidant is large, specifically, 500 or more. The upper limit of the molecular weight is not particularly limited.

[0068] Examples of hindered phenol antioxidants include 2,2,6,6-tetra-t-butyl-4,4-dihydroxybiphenyl, N,N-(hexane-1,6-diyl)bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propanamide], 2,2-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], ethylenebis(oxyethylene)bis[3-(5-t-butyl-4-hydroxy-m-tolyl)propionate], pentaerythrityl-tetrakis[3-(3,5-di 4,4'-thiobis(3-methyl-6-t-butylphenol), 2,2-methylenebis(4-ethyl-6-t-butylphenol), octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, diethyl [{3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl}methyl]phosphonate, etc. Two or more of these may be used in combination.

[0069] When the resin composition contains an antioxidant, the content of the antioxidant is not limited, but may be 0.1 to 1.0 parts by mass when the total content of the resin components in the resin composition, in this case the base polymer (A) and the modified polymer (B), is taken as 100 parts by mass.

[0070] 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.

[0071] 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.

[0072] (Amount of Unsaturated Carboxylic Acid in Resin Composition) By including the modified polymer (B) in the resin composition, unsaturated carboxylic acids are introduced into the resin composition as polar groups derived from the modified polymer (B). The total content of unsaturated carboxylic acids contained in the resin composition can be adjusted by the amount of modification of the unsaturated carboxylic acid in the modified polymer (B) and the content of the modified polymer (B). In this embodiment, the total content of unsaturated carboxylic acids contained in the resin composition is 0.0003% or more, and preferably 0.003% or more. The upper limit of the total content is not particularly limited, and may be appropriately changed so that the proportion of low-molecular-weight components in the resin composition satisfies a predetermined range. Generally, since the modified polymer (B) has a lower molecular weight than the base polymer (A), the content of the modified polymer (B) increases, and the total content of unsaturated carboxylic acids increases, resulting in a higher proportion of low-molecular-weight components. Therefore, the total content of unsaturated carboxylic acids may be appropriately adjusted within a range in which the proportion of low-molecular-weight components does not exceed a predetermined value. For example, the upper limit of the total content of unsaturated carboxylic acids may be 0.5% or less, and may even be 0.25% or less. By setting the content to 0.0003% or more, it is possible to suppress the accumulation of space charge in the resin composition. On the other hand, by setting the content to 0.5% or less, it is possible to ensure a predetermined content of the base polymer (A) while suppressing the accumulation of space charge, and it is possible to maintain high dielectric breakdown strength in the resin composition. The total content of unsaturated carboxylic acids, as will be described in detail in the Examples, is a value calculated from the peak area derived from the polymer skeleton and the peak area derived from the unsaturated carboxylic acid based on the spectrum obtained by 1H-NMR measurement.

[0073] (Molecular Weight Distribution of Resin Composition) When the molecular weight distribution of the resin composition is measured, the resin composition has a molecular weight distribution with a predetermined width derived from the base polymer (A) and modified polymer (B) contained in the resin composition. When the resin composition contains a thermoplastic elastomer (C), the resin composition has a molecular weight distribution with a predetermined width derived from the base polymer (A), modified polymer (B), and thermoplastic elastomer (C). In this embodiment, in the molecular weight distribution of the resin composition, the molecular weight is 6×10 ５ The proportion of high molecular weight components having a molecular weight of 1×10 or more is 16% or more, and the molecular weight is 1×10 ４The proportion of low molecular weight components that are 1.3% or less is less than 1.3%. The proportion of high molecular weight components may be 19% or more, or 20% or more. The proportion of low molecular weight components may be 0.9% or less, or 0.8% or less.

[0074] The proportion of high molecular weight components indicates the proportion of high molecular weight components in the entire resin composition, and in the molecular weight distribution, the proportion of high molecular weight components in the entire molecular weight distribution is 6×10 ５ The ratio of the area of ​​the low molecular weight component indicates the ratio of the low molecular weight component to the entire resin composition, and in the molecular weight distribution, the ratio of the area of ​​the low molecular weight component to the area of ​​the entire molecular weight distribution is 1 × 10 ４ The ratio is the area ratio of the following regions. The proportion of each molecular weight component can be adjusted by the content of the modified polymer (B) or thermoplastic elastomer (C) which has a lower molecular weight than the base polymer (A) or by their number average molecular weights. The lower limit of the proportion of the low molecular weight component is not particularly limited, but is, for example, 0.0001%.

[0075] (Phase Structure of Resin Composition) The resin composition has a predetermined phase structure by mixing the base polymer (A) and the modified polymer (B), or these polymers with the thermoplastic elastomer (C). Specifically, in a two-component system containing the base polymer (A) and the modified polymer (B), a structure in which they are compatible with each other is formed, or a sea-island structure in which the modified polymer (B) is finely dispersed in the base polymer (A). From the viewpoint of suppressing the accumulation of space charge and stably achieving high insulation properties in the resin composition, it is preferable that the modified polymer (B) is finely dispersed, and in the case of a sea-island structure, it is preferable that the diameter of the island phase formed by the modified polymer (B) is less than 0.5 μm. Alternatively, it is preferable that the phase structure is a structure in which the respective components are compatible with each other. On the other hand, in a three-component system further containing the thermoplastic elastomer (C), the phase structure is a sea-island structure in which the thermoplastic elastomer (C) is finely dispersed in the phase structure of the above-mentioned two-component system. The term "compatible" means that when the phase structure is observed with a transmission electron microscope, no phase separation can be confirmed and each component is uniformly dispersed.

[0076] From the viewpoint of forming the above-mentioned phase structure, it is preferable that the MFRs of the components contained in the resin composition are close and that the difference between these MFRs is small. Specifically, if the resin composition contains a base polymer (A) and a modified polymer (B), the difference between their MFRs should be 300 g / 10 min or less. Furthermore, if the resin composition further contains a thermoplastic elastomer (C), the difference between the highest and lowest viscosities of the components should be 300 g / 10 min or less. By combining components that result in such a difference in MFR, the components can be finely dispersed during mixing.

[0077] (Blending ratio) The blending ratio of the base polymer (A), the modified polymer (B) and the thermoplastic elastomer (C) in the resin composition is such that the total content of unsaturated carboxylic acids in the resin composition is 0.0003% or more, and the molecular weight distribution is 6×10 ５ The proportion of high molecular weight components of 16% or more, and the molecular weight of 1 × 10 ４ The proportion of the following low-molecular-weight components is not particularly limited as long as it is less than 1.3%. The content of each component may be adjusted depending on the modification amount of the unsaturated carboxylic acid in the modified polymer (B) used or the number-average molecular weight of the modified polymer (B) and the thermoplastic elastomer (C).

[0078] Specifically, when the resin composition contains a base polymer (A) and a modified polymer (B), the content of the base polymer (A) is preferably 90 parts by mass or more and 99 parts by mass or less, and the content of the modified polymer (B) is preferably 1 part by mass or more and 10 parts by mass or less, when the total content of these is taken as 100 parts by mass. On the other hand, when the resin composition further contains a thermoplastic elastomer (C), the content of the base polymer (A) is preferably 55 parts by mass or more and 90 parts by mass or less, the content of the modified polymer (B) is preferably 1 part by mass or more and 10 parts by mass or less, and the content of the thermoplastic elastomer (C) is preferably 10 parts by mass or more and 45 parts by mass or less, when the total content of these is taken as 100 parts by mass. By setting the contents in these ranges, it is easy to adjust the total content of unsaturated carboxylic acids and the proportion of low-molecular-weight components in the resin composition to the above-mentioned ranges.

[0079] (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.

[0080] 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.

[0081] 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 .

[0082] (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.

[0083] (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.

[0084] (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.

[0085] (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.

[0086] (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.

[0087] (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.

[0088] 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.

[0089] 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.

[0090] (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.

[0091] (3) Cable characteristics In this embodiment, as described above, the resin composition contains at least the base polymer (A) and the modified polymer (B), and the total content of unsaturated carboxylic acids in the resin composition and the proportion of high molecular weight components and low molecular weight components in the molecular weight distribution satisfy predetermined requirements, thereby making it possible to stably obtain high insulation properties in the insulating layer 130.

[0092] 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.

[0093] 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 25% 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 30% or less.

[0094] 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

[0095] 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 １５ It is preferable that the resistance is Ω cm or more, and 7.0 × 10 １５ It may be Ω·cm or more.

[0096] 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.

[0097] (3) Extruder Next, prior to the method for manufacturing a power cable according to this embodiment, an extruder used in manufacturing the power cable will be described with reference to Fig. 2. Fig. 2 is a schematic configuration diagram of the extruder used in the method for manufacturing a power cable according to an embodiment of the present disclosure.

[0098] 2, the extruder 200 is configured to include a cylindrical cylinder 210 to which the resin composition material is supplied, a hopper 220 for supplying the material into the cylinder 210, a screw 230 inserted from a first direction (left side in FIG. 2) of the cylinder 210 and rotatably arranged, a rotation drive mechanism 240 for rotating the screw 230, a discharge section 250 attached to a second direction (right side in FIG. 2) of the cylinder 210 and provided with a hole for discharging the resin composition, and an atmosphere adjustment section 260 for adjusting the inside of the cylinder 210 to an inert gas atmosphere. Note that, hereinafter, the first direction is also referred to as the upstream side in the heating and mixing of the resin composition, and the second direction is also referred to as the downstream side.

[0099] The cylindrical cylinder 210 has a space therein for accommodating and mixing the materials. In the cylindrical cylinder 210, the materials supplied to the internal space are mixed by the screw 230. The screw 230 is inserted from the end of the cylinder 210 in the first direction and is disposed at the axial center of the cylinder 210. The screw 230 is connected to a rotation drive mechanism 240 and rotatably supported. The screw 230 is configured to be rotated by the rotation drive mechanism 240 and to extrude the materials toward the discharge section 250 while mixing them. The screw 230 may be a single-shaft or dual-shaft screw. In FIG. 2, two screws 230 are disposed parallel to each other in the depth direction of the page, and only one of the screws 230 is shown. Note that, for example, a known rotary motor or the like can be used as the rotation drive mechanism 240.

[0100] The screw 230 has screw flights 232 arranged in a spiral shape on the surface of the screw body 231. The cross-sectional shape of the screw flights 232 is not particularly limited, but may be tapered as shown in FIG. 3. FIG. 3 is a schematic diagram illustrating the shape of the screw flights, showing a cross-section along the axial direction of the screw 230. In FIG. 3, the right side is the tip direction of the screw 230 (the second direction of the cylinder 210), and the left side is the end direction of the screw 230 (the first direction of the cylinder 210). When the screw flights 232 have a rectangular shape as shown by the dashed line in FIG. 3, the resin composition tends to remain at the end side of the screw flight 232, and the remaining resin composition may be thermally decomposed. Therefore, from the viewpoint of suppressing the retention of the resin composition on the surface of the screw 230, the cross-sectional shape of the screw flight 232 may have a tapered side on the end side as shown in FIG. 3. In other words, it is preferable that the angle formed between the side surface of the screw flight 232 on the first direction side and the surface of the screw body 231 is an obtuse angle. The angle may be, for example, 120° to 145°.

[0101] The discharge section 250 is disposed at the end of the cylinder 210 in the second direction. The discharge section 250 has, for example, a plurality of holes penetrating in the thickness direction, and is configured to extrude the resin composition prepared by mixing inside the cylinder 210 to the outside. For example, a breaker plate can be used as the discharge section 250. Note that a mesh or the like may be disposed between the cylinder 210 and the discharge section 250 in order to remove foreign matter contained in the resin composition.

[0102] As shown in FIG. 4 , a retention suppression member 270 may be disposed at the end of the cylinder 210 in the second direction, in contact with the discharge section. FIG. 4 is a schematic diagram illustrating the retention suppression member, showing a cross-sectional view of the end of the cylinder 210 in the second direction. As shown in FIG. 4 , the retention suppression member 270 suppresses the resin composition from accumulating within the cylinder 210 and promotes extrusion from the discharge section 250. The retention suppression member 270 has a plurality of tapered holes 271 that penetrate the cylinder 210 in the thickness direction and whose diameters decrease toward the thickness direction. The retention suppression member 270 is disposed so that the smaller diameter sides of the tapered holes 271 communicate with the holes 251 in the discharge section 250. The retention suppression member 270 can suppress the resin composition from accumulating at the edges of the holes 251 in the discharge section 250 or at the corners between the inner wall of the cylinder 210 and the discharge section 250. The retention suppression member 270 may be made of the same material as the breaker plate, for example.

[0103] An atmosphere adjustment unit 260 is connected to the cylinder 210, which adjusts the interior of the cylinder 210 to an inert gas atmosphere. The atmosphere adjustment unit 260 is configured to supply an inert gas to the interior of the cylinder 210. Air may be mixed into the interior of the cylinder 210 as materials are supplied from the hopper 220, but the atmosphere adjustment unit 260 can adjust the interior of the cylinder 210 to an inert gas atmosphere. This makes it possible to prevent the materials to be mixed and the resulting resin composition from being oxidized and thermally decomposed by air. In other words, it is possible to prevent an increase in the proportion of low-molecular-weight components in the resin composition while maintaining a high proportion of high-molecular-weight components.

[0104] The inert gas is not particularly limited, but it is preferable to use, for example, nitrogen gas or argon gas.

[0105] The extruder 200 may include a heating section (not shown) for heating the inside of the cylinder 210. Any conventionally known heating section may be used.

[0106] (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. 5. Fig. 5 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".

[0107] (S100: Resin Composition Preparation Step) First, a resin composition for forming the insulating layer 130 is prepared.

[0108] In this embodiment, a base polymer (A), for example, a propylene-based resin, a modified polymer (B), such as modified polypropylene, and, as necessary, a thermoplastic elastomer (C) and other additives (such as antioxidants) are supplied to an extruder 200 shown in FIG. 2 . The materials supplied from a hopper 220 are then mixed while being heated inside a cylinder 210. At this time, an inert gas atmosphere is maintained inside the cylinder 210 by an atmosphere adjustment unit 260. The resin composition obtained by heating and mixing is then extruded from a discharge unit 250 and granulated. This produces a pellet-shaped resin composition that will form the insulating layer 130.

[0109] The content of each component may be adjusted appropriately depending on the degree of modification of the unsaturated carboxylic acid in the modified polymer (B), the number average molecular weight, and the number average molecular weight of the thermoplastic elastomer (C). For example, when a base polymer (A) and a modified polymer (B) are used, 90 to 99 parts by mass of the base polymer (A) and 1 to 10 parts by mass of the modified polymer (B) may be added. Furthermore, when a thermoplastic elastomer (C) is further used, 55 to 90 parts by mass of the base polymer (A), 1 to 10 parts by mass of the modified polymer (B), and 10 to 45 parts by mass of the thermoplastic elastomer (C) may be added.

[0110] In the resin composition preparation step S100, the components are heated, mixed, and extruded in an inert gas atmosphere, thereby suppressing fluctuations in the proportions of high-molecular-weight components and low-molecular-weight components that occur when the resin composition is heated.

[0111] (S200: Conductor Preparation Step) On the other hand, the conductor 110 formed by twisting together a plurality of conductor core wires is prepared.

[0112] (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.

[0113] 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.

[0114] Specifically, for example, a composition for the internal semiconductive layer is fed into extruder A, which forms the internal semiconductive layer 120, of the three-layer co-extruder. The above-described pellet-shaped resin composition is fed into extruder 200 shown in FIG. 2, which serves as extruder B for forming the insulating layer 130. At this time, the interior of the cylinder 210 of extruder 200 is adjusted to an inert gas atmosphere. The set temperature of extruder B is set, for example, to a temperature that is 10°C to 80°C higher than the desired melting point. The set temperature may be adjusted appropriately based on the linear speed and extrusion pressure. A composition for the external semiconductive layer, containing the same materials as the resin composition for the internal semiconductive layer fed into extruder A, is fed into extruder C, which forms the external semiconductive layer 140.

[0115] Next, the extrudates from the extruders A to C are introduced into a common head, and the inner semiconductive layer 120, the insulating layer 130, and the 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.

[0116] The extrusion is then cooled, for example with water.

[0117] In the cable core forming process S300, the pellet-shaped resin composition is mixed and extruded while being heated in an inert gas atmosphere, thereby suppressing fluctuations in the proportions of high-molecular-weight components and low-molecular-weight components due to heating. As a result, the proportion of high-molecular-weight components in the insulating layer 130 can be maintained high while the proportion of low-molecular-weight components can be suppressed low. Moreover, even when extrusion is performed continuously over a long period of time, fluctuations in the proportions can be suppressed.

[0118] 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.

[0119] (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.

[0120] (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.

[0121] In this manner, the power cable 10 is manufactured as a solid insulated power cable.

[0122] (4) Effects of the Present Embodiment The present embodiment provides one or more of the following effects.

[0123] (a) The resin composition of this embodiment contains a base polymer (A) having propylene units and a modified polymer (B) having propylene units and modified with an unsaturated carboxylic acid, and the total content of the unsaturated carboxylic acid is 0.0003% or more. The modified polymer (B) allows a polar unsaturated carboxylic acid to be introduced into the resin composition. By making the total content of the unsaturated carboxylic acid 0.0003% or more, space charges can be trapped in the resin composition and their accumulation can be suppressed. Furthermore, when the molecular weight distribution of the resin composition is measured, the molecular weight of the resin composition is 1×10 ４ The proportion of low molecular weight components below 1.3% and the molecular weight is 6×10 ５The proportion of the high molecular weight component is 16% or more. In other words, the proportion of the high molecular weight component that contributes to insulation is increased, while the proportion of the low molecular weight component that migrates through the resin composition and causes localized space charge accumulation is reduced. This resin composition not only achieves high insulation, but also stably exhibits the space charge trapping effect of the modified polymer (B) even in high-temperature environments, thereby suppressing localized space charge accumulation. Therefore, by forming the insulating layer 130 from the resin composition, high insulation can be stably achieved. Specifically, the insulating layer 130 can be made to withstand high voltages even at high temperatures. In other words, the power cable 10 of this embodiment enables stable DC transmission.

[0124] (b) When preparing the resin composition, the base polymer (A), the modified polymer (B), and, if necessary, the thermoplastic elastomer (C) are supplied to the cylinder 210 of the extruder 200 and heated and mixed in an inert gas atmosphere. When the heating and mixing is performed in an oxygen-containing atmosphere, the components may decompose due to heating, which may lead to a decrease in molecular weight. Furthermore, if the prepared resin composition remains inside the cylinder 210 and is heated for a long period of time, the decrease in molecular weight may further progress. In this regard, by performing the mixing in an inert gas atmosphere, the decomposition of the components can be suppressed, and a decrease in the proportion of high molecular weight components and an increase in the proportion of low molecular weight components can be suppressed. This makes it easier to adjust the proportions of the molecular weight components within the above ranges, and the above-mentioned effect (a) can be more reliably achieved.

[0125] (c) Furthermore, when preparing the resin composition, it is advisable to configure the screw flight 232 of the screw 230 in the extruder 200 so that the first direction side is tapered. If the screw flight 232 is rectangular, the resin composition is blocked by the screw flight 232 and tends to remain inside the cylinder 210. In this regard, configuring the screw flight 232 so that the side surface on the first direction side is tapered can suppress the retention of the resin composition and can suppress a decrease in the proportion of high molecular weight components and an increase in the proportion of low molecular weight components due to prolonged heating. This makes it possible to more reliably obtain the above-mentioned effect (a).

[0126] (d) Furthermore, when preparing the resin composition, it is preferable to dispose a retention suppression member 270 having a plurality of tapered holes 271 that penetrate the extruder 200 in the thickness direction and whose diameter decreases from the first direction to the second direction between the cylinder 210 and the discharge section 250 of the extruder 200. When the resin composition is extruded from the discharge section 250, it may be blocked by the edges of the holes 251 of the discharge section 250 and remain inside the cylinder 210. In this regard, disposing the retention suppression member 270 having the tapered holes 271 makes it easier to extrude the resin composition to the outside without it remaining inside the cylinder 210. This makes it possible to suppress fluctuations in the proportions of the various molecular weight components, and more reliably obtain the above-mentioned effect (a).

[0127] (e) The resin composition preferably contains 90 to 99 parts by mass of the base polymer (A) and 1 to 10 parts by mass of the modified polymer (B), where the total content of the base polymer (A) and the modified polymer (B) is taken as 100 parts by mass. By using such amounts, the proportion of low-molecular-weight components can be reduced while keeping the total content of unsaturated carboxylic acids within the above range, and the above-mentioned effect (a) can be more reliably obtained.

[0128] (f) The resin composition further contains a thermoplastic elastomer (C), preferably containing 55 to 90 parts by mass of the base polymer (A), 1 to 10 parts by mass of the modified polymer (B), and 10 to 45 parts by mass of the thermoplastic elastomer (C) when the total content of the base polymer (A), the modified polymer (B), and the thermoplastic elastomer (C) is taken as 100 parts by mass. The thermoplastic elastomer (C) can suppress excessive crystal growth of the base polymer (A) having propylene units, thereby imparting flexibility to the resin composition. Furthermore, by setting the content of each component within the above range, the flexibility-improving effect of the thermoplastic elastomer (C) can be obtained, while the total content of unsaturated carboxylic acids and the ratio of each molecular weight component can be adjusted within a predetermined range, thereby more reliably achieving the above-mentioned effect (a).

[0129] (g) The thermoplastic elastomer (C) is preferably a styrene-based elastomer, which can trap electrons in the aromatic ring to form a stable resonance structure, thereby further improving the insulating properties of the resin composition.

[0130] (h) The modified polymer (B) preferably has a number-average molecular weight of 20,000 or more. The base polymer (A) preferably has a number-average molecular weight of 80,000 or more. Such base polymer (A) and modified polymer (B) can more reliably reduce the proportion of low-molecular-weight components contained in the resin composition, thereby further suppressing the accumulation of space charge.

[0131] (i) Furthermore, the phase structure of the resin composition, in a two-component system containing a base polymer (A) and a modified polymer (B), is preferably a structure in which the base polymer (A) and the modified polymer (B) are compatible with each other, or a sea-island structure in which the modified polymer (B) is finely dispersed in the base polymer (A). In the case of a sea-island structure, the diameter of the island phase formed by the modified polymer (B) is preferably less than 0.5 μm. Furthermore, in a three-component system further containing a thermoplastic elastomer (C), the phase structure is preferably a sea-island structure in which the thermoplastic elastomer (C) is finely dispersed within the two-component phase structure described above. Such a phase structure allows each component to be uniformly dispersed in the resin composition, allowing space charges to be trapped more uniformly within the resin composition. This allows for more stable insulation.

[0132] (j) The resin composition may be free of inorganic fillers. "Free of inorganic fillers" here means that inorganic fillers are not intentionally added. For example, the content of inorganic fillers may be less than 1 part by mass per 100 parts by mass of the resin component. This can prevent clogging of the mesh when the resin composition is extruded. As a result, it is possible to prevent the insulating properties of the insulating layer 130 formed by extruding the resin composition from varying along its length. It is also possible to prevent a decrease in the moldability of the insulating layer 130 itself. In other words, when a long power cable 10 is continuously manufactured by an extrusion process, stable insulating properties can be obtained for the entire long power cable 10.

[0133] (k) The resin composition is preferably non-crosslinked, which allows the resin composition to be recycled.

[0134] (l) The resin composition may further contain an antioxidant from the viewpoint of preventing deterioration, and the antioxidant may be a hindered phenol-based antioxidant. The molecular weight of the antioxidant may be 500 or more. Hindered phenol-based antioxidants have greater steric hindrance than other antioxidants, and thus can suppress migration within the resin composition. Furthermore, by setting the molecular weight to 500 or more, migration within the resin composition can be suppressed. Therefore, when the resin composition is exposed to a high-temperature environment, localized accumulation of space charge due to migration of the antioxidant can be suppressed.

[0135] <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.

[0136] 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.

[0137] 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).

[0138] 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.

[0139] 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.

[0140] (1) Materials The materials used in preparing the resin compositions are listed below.

[0141] Random polypropylene (PP1) was prepared as the base polymer (A), maleic acid-modified polypropylenes (MAH-PP1) to (MAH-PP3) were prepared as the modified polymer (B), and a hydrogenated styrene-based thermoplastic elastomer (SEBS1) was prepared as the thermoplastic elastomer (C). The physical properties of each component are as follows:・ PP1: number average molecular weight 140,000, MFR 0.5 g / 10 min, melting point 145 ° C. ・ MAH-PP1: number average molecular weight 58,000, maleic acid modification amount 0.3 mass%, MFR 6 g / 10 min, melting point 160 ° C. ・ MAH-PP2: number average molecular weight 41,000, maleic acid modification amount 0.8 mass%, MFR 110 g / 10 min, melting point 158 ​​° C. ・ MAH-PP3: number average molecular weight 15,000, maleic acid modification amount 1.8 mass%, MFR 900 g / 10 min, melting point 145 ° C. ・ SEBS1: number average molecular weight 150,000, styrene amount 15 mass%, MFR 0.2 g / 10 min, no melting point

[0142] 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.

[0143] (2) Preparation of Resin Compositions Samples 1 to 6 were prepared by feeding the above materials to an extruder in the amounts shown in Table 1 below, heating, mixing, and granulating them using the extruder. The extrusion conditions were changed as follows when preparing the resin compositions. Sample 1 was extruded using the extruder described above, with the interior of the cylinder purged with nitrogen gas and a screw having a tapered side surface on the first direction side of the screw flight. Sample 2 was extruded in the same manner as Sample 1, except that a screw having a rectangular cross-sectional shape was used. Sample 3 was extruded in the same manner as Sample 1, except that the interior of the cylinder was not purged with nitrogen gas. Sample 4 was extruded in the same manner as Sample 1, except that the nitrogen gas was not purged and the screw flights were rectangular. Sample 5 was extruded in the same manner as Sample 4, except that the materials used and the amounts of the materials used were changed. Sample 6 was extruded in the same manner as Sample 5, except that the interior of the cylinder was purged with nitrogen gas and a screw having a tapered side surface on the first direction side of the screw flight was used. In each sample, the blend amount of the antioxidant was set to 0.1 parts by mass.

[0144]

[0145] (3) Power Cable Fabrication Next, a conductor was prepared by twisting together 14 mm diameter conductor core wires made of a dilute copper alloy. After the conductor was prepared, a resin composition for the inner semiconductive layer containing an ethylene-ethyl acrylate copolymer, a resin composition for the insulating layer prepared in (2) above, and a resin composition for the outer semiconductive layer composed of the same materials as the resin composition for the inner semiconductive layer were loaded into extruders A to C, respectively. The extrudates from extruders A to C were fed 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 resulted in the fabrication of a power cable sample having a conductor, inner semiconductive layer, insulating layer, and outer semiconductive layer from the center to the periphery. In this example, extrusion was performed continuously, and evaluation sample A was cut out from the power cable after 240 hours. For comparison, evaluation sample B was cut out from the power cable immediately after extrusion (0 hours after extrusion). The resin compositions for the insulating layer were extruded under the extrusion conditions shown in Table 1.

[0146] (4) Evaluation Evaluation Sample A and Evaluation Sample B were cut out from the manufactured power cables and evaluated for the molecular weight distribution of the resin composition constituting the insulating layer, the total content of unsaturated carboxylic acid (maleic acid), space charge characteristics, volume resistivity, and DC breakdown strength. Each evaluation method was as follows.

[0147] (Molecular Weight Distribution) The molecular weight distribution of the resin composition forming the insulating layer was measured by measuring the number average molecular weight of the base resin based on a calibration curve prepared by GPC using PS as a standard sample under the following conditions. In this example, based on the obtained molecular weight distribution, the molecular weight was measured at a ratio of 1 × 10 to the total area of ​​the entire molecular weight distribution. ４ Calculate the area ratio of the following regions, and calculate the molecular weight of 1 × 10 ４ Similarly, the ratio of the molecular weight of the low molecular weight component to the total area of ​​the entire molecular weight distribution was calculated. ５ The ratio of the area of ​​the above regions was calculated, and the molecular weight was 6 × 10 ５The proportions of the above high molecular weight components were determined. Apparatus: HLC-8321GPC / HT manufactured by Tosoh Corporation Eluent: 1,2,4-trichlorobenzene Temperature: 140°C Concentration: 1.0 mg / mL Flow rate: 1.0 mL / min The calibration curve for PS was created based on the results within the molecular weight range of 1,000 to 5.5 million.

[0148] (Total Content of Unsaturated Carboxylic Acid) The total content of unsaturated carboxylic acid (maleic acid) in the resin composition forming the insulating layer was calculated from the spectrum obtained by methyl esterifying the unsaturated carboxylic acid moieties contained in the resin composition and then performing 1H-NMR measurement. Specifically, in the spectrum obtained, 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 taken as the total content of unsaturated carboxylic acid.

[0149] (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) ０ Based 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. When the space charge accumulation amount was 30% or less, it was rated as A (best), when the space charge accumulation amount was more than 30% and 100% or less, it was rated as B (good), and when the space charge accumulation amount was more than 100%, it was rated as C (poor). Space charge accumulation amount=(Q ３００ / Q ０ −1) × 100

[0150] (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 7 × 10 １５ A value of A (best) is used when the volume resistivity is 1×10 １５ Ω・cm or more 7×10 １５ A volume resistivity of less than Ω cm is rated as B (good), and a volume resistivity of 1×10 １５ When the resistance was less than Ω·cm, it was evaluated as C (poor).

[0151] (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).

[0152] (5) Evaluation Results The results for Evaluation Sample A, which was collected 240 hours after extrusion, are summarized in Table 1 above, and the results for Evaluation Sample B, which was collected immediately after extrusion, are summarized in Table 2 below.

[0153]

[0154] As shown in Tables 1 and 2, for Samples 1 to 6, the proportion of high molecular weight components was lower and the proportion of low molecular weight components was higher 240 hours after extrusion than immediately after extrusion, indicating a change in molecular weight distribution. However, for Samples 1 to 3, the change in molecular weight distribution was small, and it was found that in Evaluation Sample A after 240 hours, the proportion of low molecular weight components was maintained at less than 1.3% and the proportion of high molecular weight components was maintained at 16% or more. Furthermore, the total content of unsaturated carboxylic acids in the resin composition was 0.0003% or more. As a result, it was found that even after 240 hours of extrusion, the amount of space charge accumulation was kept low and high volume resistivity was achieved, similar to immediately after extrusion. Furthermore, it was found that all samples had high DC breakdown field strength. In other words, when long power cables were fabricated with Samples 1 to 3, the variation in insulation properties along the length could be suppressed, and high insulation properties could be stably achieved.

[0155] Furthermore, when comparing Samples 1-A to 3-A, it was confirmed that, despite the same materials and compositions, the proportions of high-molecular-weight and low-molecular-weight components varied depending on the extrusion conditions. Specifically, it was confirmed that Sample 2-A, in which the interior of the cylinder was purged with nitrogen gas, was able to achieve a higher proportion of high-molecular-weight components and a lower proportion of low-molecular-weight components compared to Sample 3-A, in which the screw flights were tapered. This indicates that creating an inert gas atmosphere inside the cylinder is more effective in suppressing the reduction in molecular weight of the polymer component than suppressing the retention of the resin composition inside the cylinder. Furthermore, it was found that Sample 1-A, which employed nitrogen gas purging and tapered screw flights, was able to more reliably suppress fluctuations in molecular weight distribution compared to Samples 2-A and 3-A, which employed either of these.

[0156] In Sample 4, the proportion of high molecular weight components was high and the proportion of low molecular weight components was low in Evaluation Sample B immediately after extrusion. However, the proportion of high molecular weight components significantly decreased and the proportion of low molecular weight components increased in Evaluation Sample A 240 hours after extrusion. In other words, it was found that the molecular weight distribution of the resin composition fluctuated significantly over the course of extrusion. This is thought to be due to the accelerated decomposition of the resin components by heating and mixing the materials without nitrogen purging. Furthermore, the use of a screw with rectangular screw flights likely caused the resin composition to remain inside the cylinder, and excessive heating of the retained resin components accelerated their decomposition. As a result, Sample 4 was confirmed to have a large amount of space charge accumulation and low volume resistivity and DC breakdown field strength.

[0157] In Sample 5, MAH-PP3, which has a relatively small number-average molecular weight, was used as the modified polymer (B), and the resin composition was formulated so that the proportion of low-molecular-weight components was initially high. Therefore, it was confirmed that the proportion of high-molecular-weight components was relatively low and the proportion of low-molecular-weight components was relatively high in Evaluation Sample B immediately after extrusion. Furthermore, nitrogen purging and a screw with a tapered screw flight were not used. Therefore, it was confirmed that the proportion of high-molecular-weight components was even lower and the proportion of low-molecular-weight components was even higher in Evaluation Sample A 240 hours after extrusion, compared to Evaluation Sample B immediately after extrusion. As a result, it was confirmed that the space charge accumulation amount was large and the volume resistivity and DC breakdown field strength were low.

[0158] In Sample 6, by purging with nitrogen gas and using a tapered screw flight, it was possible to suppress the decomposition of the resin components during extrusion and the fluctuation in molecular weight distribution due to the retention of the resin composition, but the proportion of high molecular weight components in the resin composition was originally low and the proportion of low molecular weight components was also originally high, despite the same composition as Sample 5. Therefore, it was confirmed that the amount of space charge accumulation was large and the volume resistivity and DC breakdown field strength were low.

[0159] Regarding Samples 1 to 6, the phase structure of the resin composition was observed with a transmission electron microscope, and it was confirmed that a sea-island structure was formed in which the thermoplastic elastomer (C) was finely dispersed as an island phase in a sea phase in which the base polymer (A) and the modified polymer (B) were compatible with each other.

[0160] As described above, by using at least a base polymer (A) having propylene units and a modified polymer (B) having propylene units in a resin composition, and by ensuring that the total content of unsaturated carboxylic acids and the ratio of high molecular weight components to low molecular weight components in the resin composition meet certain requirements, it has been confirmed that space charge accumulation can be suppressed and volume resistivity and DC breakdown field strength can be improved 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 voltages even in high-temperature environments, enabling stable DC transmission using a power cable.

[0161] <Supplementary Notes> The following provides supplementary notes on aspects of the present disclosure.

[0162] (Note 1) A polymer composition comprising a base polymer (A) having a propylene unit and a modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid, wherein the total content of the unsaturated carboxylic acid derived from the modified polymer (B) is 0.0003% or more, and when the molecular weight distribution is measured, the molecular weight is 1×10 ４ The proportion of the following components is less than 1.3% and the molecular weight is 6×10 ５ A resin composition, wherein the proportion of the above components is 16% or more.

[0163] (Supplementary Note 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 base polymer (A) having a propylene unit; and a modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid, wherein a total content of the unsaturated carboxylic acid derived from the modified polymer (B) contained in the resin composition is 0.0003% or more, and when the molecular weight distribution of the resin composition is measured, the molecular weight is 1×10 ４The proportion of the following components is less than 1.3% and the molecular weight is 6×10 ５ A power cable in which the proportion of the above components is 16% or more.

[0164] (Appendix 3) In Appendix 2, preferably, when the total content of the base polymer (A) and the modified polymer (B) is 100 parts by mass, the resin composition contains 90 parts by mass or more and 99 parts by mass or less of the base polymer (A) and 1 part by mass or more and 10 parts by mass or less of the modified polymer (B).

[0165] (Appendix 4) In Appendix 2, preferably, the resin composition further contains a thermoplastic elastomer (C) containing a styrene unit, and when the total content of the base polymer (A), the modified polymer (B), and the thermoplastic elastomer (C) is taken as 100 parts by mass, the resin composition contains 55 parts by mass or more and 90 parts by mass or less of the base polymer (A), 1 part by mass or more and 10 parts by mass or less of the modified polymer (B), and 10 parts by mass or more and 45 parts by mass or less of the thermoplastic elastomer (C).

[0166] (Appendix 5) In any one of Appendices 2 to 4, preferably, the modified polymer (B) has a number average molecular weight of 20,000 or more.

[0167] (Supplementary Note 6) In any of Supplementary Notes 2 to 5, preferably, the number average molecular weight of the base polymer (A) is 80,000 or more.

[0168] (Supplementary Note 7) In any one of Supplementary Notes 2 to 6, the melt flow rate of the base polymer (A) is 0.1 g / 10 min or more and 5.0 g / 10 min or less.

[0169] (Supplementary Note 8) In any one of Supplementary Notes 2 to 7, the melt flow rate of the modified polymer (B) is 0.1 g / 10 min or more and 500 g / 10 min or less.

[0170] (Supplementary Note 9) In Supplementary Note 4, the melt flow rate of the thermoplastic elastomer (C) is 0.1 g / 10 min or more and 5.0 g / 10 min or less.

[0171] (Appendix 10) A method for manufacturing a power cable including a conductor and an insulating layer that covers the conductor and is formed from a resin composition, the method comprising: a preparation step of preparing the resin composition; and an insulating layer forming step of extruding the resin composition so as to cover the conductor and form the insulating layer, wherein in the preparation step, a base polymer (A) having propylene units and a modified polymer (B) having propylene units and modified with an unsaturated carboxylic acid are added to the resin composition so that a total content of the unsaturated carboxylic acid derived from the modified polymer (B) is 0.0003% or more, and the resulting resin composition is heated and mixed in an inert gas atmosphere.

[0172] (Appendix 11) In Appendix 10, preferably, in the preparation step, the resin composition is prepared using an extruder including: a cylindrical cylinder; a screw inserted from a first direction of the cylinder and arranged to be freely rotatable; a discharge section attached to a second direction of the cylinder and provided with a hole for discharging the resin composition; and an atmosphere adjustment section adjusting the inside of the cylinder to an inert gas atmosphere.

[0173] (Appendix 12) In Appendix 11, preferably, the screw comprises a screw body and a screw flight arranged spirally on the surface of the screw body, and the screw flight is configured so that the side surface on the first direction side is tapered.

[0174] REFERENCE SIGNS LIST 10 Power cable 110 Conductor 120 Inner semiconductive layer 130 Insulating layer 140 Outer semiconductive layer 150 Shielding layer 160 Sheath 200 Extruder 210 Cylinder 220 Hopper 230 Screw 231 Screw main body 232 Screw flight 240 Rotation drive mechanism 250 Discharge section 251 Hole section 260 Atmosphere adjustment section 270 Retention suppression member 271 Tapered hole section

Claims

1. A base polymer (A) having propylene units; A modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid, The total content of the unsaturated carboxylic acid derived from the modified polymer (B) is 0.0003% or more, and When the molecular weight distribution was measured, the molecular weight was 1×10 ４ The ratio of the following components is less than 1.3%, and the molecular weight is 6×10 ５ The proportion of the above components is 16% or more. Resin composition.

2. A conductor; an insulating layer formed from a resin composition and covering the conductor; Equipped with The resin composition comprises A base polymer (A) having propylene units; A modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid, The total content of the unsaturated carboxylic acid derived from the modified polymer (B) contained in the resin composition is 0.0003% or more, and When the molecular weight distribution of the resin composition was measured, the molecular weight was 1×10 ４ The ratio of the following components is less than 1.3%, and the molecular weight is 6×10 ５ The proportion of the above components is 16% or more. Power cable.

3. The resin composition contains 90 parts by mass or more and 99 parts by mass or less of the base polymer (A) and 1 part by mass or more and 10 parts by mass or less of the modified polymer (B), when the total content of the base polymer (A) and the modified polymer (B) is 100 parts by mass.

3. The power cable according to claim 2.

4. The resin composition further contains a thermoplastic elastomer (C), and when the total content of the base polymer (A), the modified polymer (B), and the thermoplastic elastomer (C) is 100 parts by mass, the resin composition contains 55 parts by mass or more and 90 parts by mass or less of the base polymer (A), 1 part by mass or more and 10 parts by mass or less of the modified polymer (B), and 10 parts by mass or more and 45 parts by mass or less of the thermoplastic elastomer (C).

3. The power cable according to claim 2.

5. The thermoplastic elastomer (C) is a styrene-based elastomer containing a styrene unit.

5. The power cable according to claim 4.

6. The modified polymer (B) has a number average molecular weight of 20,000 or more.

6. The power cable according to claim 2 .

7. The number average molecular weight of the base polymer (A) is 80,000 or more.

6. The power cable according to claim 2 .

8. A method for producing a power cable comprising: a conductor; and an insulating layer formed from a resin composition and covering the conductor, A preparation step of preparing the resin composition; and forming an insulating layer by extruding the resin composition so as to cover the conductor, thereby forming the insulating layer. In the preparation step, a base polymer (A) having a propylene unit and a modified polymer (B) having a propylene unit and modified with an unsaturated carboxylic acid are added to the resin composition so that the total content of the unsaturated carboxylic acid derived from the modified polymer (B) is 0.0003% or more, and the mixture is heated and mixed in an inert gas atmosphere. A method for manufacturing power cables.

9. In the preparation step, the resin composition is prepared using an extruder including a cylindrical cylinder, a screw inserted from a first direction of the cylinder and rotatably arranged, a discharge section attached to the second direction of the cylinder and provided with a hole for discharging the resin composition, and an atmosphere adjustment section for adjusting the inside of the cylinder to an inert gas atmosphere. A method for producing a power cable according to claim 8.

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