Resin composition and power cable

Abstract

This resin composition contains a propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B), and has a melting point of 110°C or higher. When the molecular weight distribution is measured, the proportion of components having a molecular weight of 1×104 or less is 3% or less.

Description

Resin composition and power cable

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

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

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

[0004] According to one aspect of the present disclosure, a thermoplastic elastomer (B) is included, the thermoplastic elastomer (B) having a melting point of 110°C or higher, and a molecular weight of 1 x 10 when the molecular weight distribution is measured. ４ A resin composition is provided in which the proportion of the following component is 3% or less:

[0005] According to another aspect of the present disclosure, there is provided a conductive material comprising: a conductor; and an insulating layer that covers the conductor and is formed from a resin composition, wherein the resin composition contains a propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B), and has a melting point of 110°C or higher, and when a molecular weight distribution is measured, the molecular weight is 1 x 10 ４ A power cable is provided in which the proportion of the following component is 3% or less:

[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 has been confirmed that simply adding a modified polymer does not stabilize the insulating properties of the resin composition or insulating layer. Research into this issue has revealed that modified polymers contain low-molecular-weight components due to the generation of radicals and molecular chain scission during their manufacturing process, and that these low-molecular-weight components reduce insulating properties. Low-molecular-weight components tend to behave as charge carriers, especially in high-temperature environments. Therefore, it is believed that low-molecular-weight components cause localized space charge accumulation in the resin composition, and that they migrate throughout the resin composition, especially in high-temperature environments, further increasing the accumulation. Therefore, even if a modified polymer is added to a propylene-based resin, the influence of the low-molecular-weight components can make it difficult to achieve the space charge trapping effect of the modified polymer. As a result, the insulating layer may not consistently achieve high insulating properties.

[0016] For this reason, the inventors investigated methods for improving insulation without adding modified polymers and focused on softening components added to the resin composition that forms the insulation layer. Propylene-based resins tend to be highly crystalline and hard by themselves, so they may not satisfy the flexibility required for the insulation layer of power cables. Various elastomers and wax components are used as softening components.

[0017] However, when using softening ingredients, the following points are of concern.

[0018] First, the melting point of the soft component is lower than that of the propylene-based resin. The lower the melting point of the soft component, the higher the proportion of regions that become amorphous or melt in the resin composition under high-temperature conditions. Therefore, when a soft component with a low melting point is mixed, charge migration is likely to occur under high-temperature conditions, which may lead to a decrease in insulation properties.

[0019] Second, the molecular weight of the flexible component is lower than that of the propylene-based resin. Generally, resins and elastomers have a specific molecular weight distribution depending on the type. The lower the molecular weight of the flexible component, the higher the proportion of low-molecular-weight components in the molecular weight distribution. Low-molecular-weight components have a lower melting point than high-molecular-weight components, and they migrate within the resin composition, causing localized space charge accumulation, which can lead to a decrease in insulation properties in high-temperature environments.

[0020] For this reason, we focused on thermoplastic elastomers with high melting points and low molecular weight components as softening components to be added to propylene-based resins. We then investigated the properties of resin compositions containing propylene-based resins and thermoplastic elastomers, which have a melting point of 110°C or higher and a molecular weight of 1x10 ４ It has been found that by controlling the proportion of low molecular weight components to 3% or less, it is possible to suppress the accumulation of space charges in high temperature environments and achieve high insulating properties.

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

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

[0023] [1] A resin composition according to one embodiment of the present disclosure comprises a propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B), the resin composition having a melting point of 110°C or higher, and a molecular weight of 1 x 10 when the molecular weight distribution is measured. ４ The ratio of the following components is 3% or less: According to this configuration, high insulating properties can be stably obtained.

[0024] [2] A power cable according to another aspect of the present disclosure includes a conductor; and an insulating layer that covers the conductor and is formed from a resin composition, wherein the resin composition contains a propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B), and has a melting point of 110°C or higher, and a molecular weight of 1 x 10 when a molecular weight distribution is measured. ４ The ratio of the following components is 3% or less: This configuration makes it possible to achieve high withstand voltage even in a high-temperature environment, enabling stable DC power transmission.

[0025] [3] In the power cable according to the above [2], the thermoplastic elastomer (B) is at least one of an olefin-based thermoplastic elastomer having an olefin unit and a styrene-based thermoplastic elastomer having a styrene unit. This configuration improves the flexibility of the insulating layer while stably achieving high insulating properties.

[0026] [4] In the power cable according to the above [2] or [3], the melting point of the thermoplastic elastomer (B) is 110° C. or higher and 160° C. or lower. This configuration makes it easy to adjust the melting point of the resin composition to 110° C. or higher, and high insulation properties can be stably obtained.

[0027] [5] The power cable according to any one of the above [2] to [4], wherein the thermoplastic elastomer (B) has a molecular weight of 1×10 ４ The ratio of the following components is 4.0% or less: According to this configuration, the ratio of low molecular weight components can be adjusted to be low, and high insulating properties can be stably obtained.

[0028] [6] The power cable according to any one of [2] to [5] above, wherein the melting point of the propylene-based resin (A) is 130° C. or higher and 170° C. or lower. This configuration makes it easy to adjust the melting point of the resin composition to 110° C. or higher, and high insulation properties can be stably obtained.

[0029] [7] The power cable according to any one of [2] to [6], wherein the resin composition contains 50 to 90 parts by mass of the propylene-based resin (A) and 10 to 50 parts by mass of the thermoplastic elastomer (B). This configuration reduces the proportion of low-molecular-weight components, and allows for stable high insulation.

[0030] [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.

[0031] <One embodiment of the present disclosure> (1) Resin composition The resin composition of this embodiment can be used, for example, as a material for forming an insulating layer of a power cable, which will be described later. The resin composition contains a propylene-based resin (A) having propylene units, a thermoplastic elastomer (B), and, as necessary, other additives. Hereinafter, the propylene-based resin (A) will also be referred to as component (A), and the thermoplastic elastomer (B) will also be referred to as component (B).

[0032] The resin composition is formed by mixing a propylene-based resin (A) and a thermoplastic elastomer (B) having different melting points and molecular weight distributions, and the melting point is 110° C. or higher and the molecular weight is 1×10 ４ The proportion of the following components (hereinafter also referred to as low-molecular-weight components) is 3% or less. The resin composition has a predetermined phase structure due to the mixture of components (A) and (B). The phase structure may be, for example, a structure in which component (B) is finely dispersed in component (A) (a so-called sea-island structure), or a structure in which components (A) and (B) are compatible with each other.

[0033] (Melting Point of Resin Composition) By including a thermoplastic elastomer (B) having a melting point lower than that of the propylene-based resin (A), the resin composition has a melting point lower than that of the inherent melting point of the (A) component. The melting point of the resin composition serves as an indicator of the addition ratio of the (A) component and the (B) component. By achieving a melting point of the resin composition of 110°C or higher, the ratio of the (B) component having a low melting point can be controlled to a predetermined amount. This reduces the effects of amorphization or melting of the (B) component in high-temperature environments. The upper limit of the melting point of the resin composition is not particularly limited, but is preferably 170°C or lower. Furthermore, as described below, when an olefin-based thermoplastic elastomer is used as the (B) component, the melting point of the resin composition is preferably 130°C or higher and 170°C or lower. Furthermore, when a styrene-based thermoplastic elastomer is used as the (B) component, the melting point of the resin composition is preferably 110°C or higher and 170°C or lower.

[0034] In this specification, the melting point is measured as follows. First, differential scanning calorimetry (DSC) is performed on a sample in accordance with, for example, JIS-K-7121 (1987). Specifically, in a DSC device, the temperature is raised from room temperature (normal temperature, for example, 27°C) to 220°C at a rate of 10°C / min. A DSC curve is obtained by plotting the amount of heat absorbed (heat flow) per unit time against the temperature. At this time, the temperature at which the amount of heat absorbed per unit time in the sample reaches a maximum (peak) is defined as the "melting point (melting peak temperature)."

[0035] (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 of a predetermined width derived from the propylene-based resin (A) and the thermoplastic elastomer (B) contained in the resin composition. In this embodiment, in the molecular weight distribution of the resin composition, the molecular weight of the resin composition is 1×10 ４ The proportion of low molecular weight components that is 0.0001% or less is 3% or less. The proportion of low molecular weight components may be 2.5% or less, or may be 1.5% or less. The lower limit of the proportion of low molecular weight components is not particularly limited, but is, for example, 0.0001%.

[0036] The low molecular weight component has a molecular weight of 1×10 in the molecular weight distribution of the resin composition. ４ The following components are shown. The low molecular weight component is a component derived from the thermoplastic elastomer (B) having a relatively low molecular weight, or a component derived from the thermal decomposition product of the component (B) or the propylene-based resin (A). The proportion of the low molecular weight component indicates the proportion of the low molecular weight component in the entire resin composition, and in the molecular weight distribution, the proportion of the low molecular weight component with a molecular weight of 1 × 10 relative to the area of ​​the entire molecular weight distribution is ４ The molecular weight distribution is the ratio of the area of ​​the following region. As described above, low molecular weight components have a low melting point and may cause a decrease in insulating properties in a high-temperature environment. In this regard, in this embodiment, the decrease in insulating properties can be suppressed by setting the ratio of low molecular weight components in the resin composition to 3% or less. The measurement of molecular weight distribution will be described later in the examples.

[0037] The molecular weight distribution of the resin composition is determined by the molecular weight distribution of the propylene-based resin (A) and the thermoplastic elastomer (B). The molecular weight range is not particularly limited, but is, for example, 1×10 ３ 1x10 or more ８ The ratio of the low molecular weight component in the resin composition can be adjusted by the content of the low molecular weight thermoplastic elastomer (B) and the molecular weight of the (B) component used.

[0038] (Constituent Components) Next, the propylene-based resin (A), thermoplastic elastomer (B) and other additives that constitute the resin composition will be described.

[0039] (Propylene-Based Resin (A)) The propylene-based resin (A) is a resin material that constitutes the main component of the resin composition and is a component containing propylene units. This (A) component can be at least one of a propylene homopolymer (hereinafter also referred to as homo-PP) and a propylene random polymer (hereinafter also referred to as random PP). Homo-PP contains propylene units, and random PP contains propylene units and ethylene units. The content of ethylene units in random PP is preferably, for example, 0.5% by mass or more and 15% by mass or less, and may be 0.5% by mass or more and 10% 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 a non-crosslinked or slightly crosslinked state can be achieved.

[0040] From the viewpoint of obtaining higher insulating properties in the resin composition, the propylene-based resin (A) is preferably random PP. Homo PP has a higher crystalline content than random PP and can obtain high insulating properties, but it may cause cracks in the crystals or between 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.

[0041] The stereoregularity of the propylene-based resin (A) may be isotactic, syndiotactic, or atactic. The stereoregularity is not particularly limited, but isotactic is preferable. The isotactic stereoregularity can suppress a decrease in the melting point of the resin composition. As a result, the resin composition can be used stably in a non-crosslinked or slightly crosslinked state.

[0042] The melting point of the propylene-based resin (A) is not particularly limited, but is preferably 130° C. or higher and 170° C. or lower. When the propylene-based resin (A) is homo-PP, its melting point is preferably 120° C. or higher and 165° C. or lower, and when it is random-PP, its melting point is preferably 130° C. or higher and 170° C. By using a propylene-based resin (A) having such a melting point, it is possible to increase the melting point of the resin composition while reducing the proportion of low-molecular-weight components.

[0043] The number average molecular weight of the propylene-based resin (A) is not particularly limited, but is preferably 5.0 × 10 ４ It is preferable that the value is 8.0 × 10 or more. ４ Above 5.0 x 10 ５ When the component (A) has such a number average molecular weight, the amount of low molecular weight components is small, and the proportion of low molecular weight components in the resin composition can be adjusted to be smaller.

[0044] The number average molecular weight of the propylene-based resin (A) is measured by gel permeation chromatography (GPC) based on a calibration curve prepared using polystyrene (PS) as a standard sample, and the number average molecular weight is calculated from the molecular weight distribution. The number average molecular weight of the thermoplastic elastomer (B) can also be calculated in the same manner. A detailed calculation method will be described later in the Examples.

[0045] From the viewpoint of compatibility with the thermoplastic elastomer (B), the melt flow rate (MFR) of the propylene-based resin (A) is preferably 0.1 g / 10 min or more and 5.0 g / 10 min or less, and may be 0.1 g / 10 min or more and 2.0 g / 10 min or less. By achieving such an MFR, the phase structure of the resin composition can be formed into a structure in which the components are compatible with each other or a structure in which the components are finely dispersed. This can improve the flexibility and insulating properties of the resin composition. The MFR here is a value measured in accordance with JIS K7210 at a temperature of 190°C and a load of 2.16 kg.

[0046] (Thermoplastic elastomer (B)) The thermoplastic elastomer (B) has lower crystallinity than the propylene-based resin (A) having propylene units, and can control the crystal growth of the (A) component and impart flexibility to the resin composition and insulating layer.

[0047] The thermoplastic elastomer (B) tends to have a lower melting point and a higher proportion of low-molecular-weight components than the propylene-based resin (A), which tends to lower the melting point of the resin composition and increase the proportion of low-molecular-weight components in the resin composition. In this regard, in this embodiment, it is preferable to select component (B) that has a higher melting point and a lower proportion of low-molecular-weight components.

[0048] Specifically, the melting point of the thermoplastic elastomer (B) is preferably 110° C. or higher and 160° C. or lower. Component (B) having such a melting point makes it easy to adjust the melting point of the resin composition to 110° C. or higher without excessively lowering the melting point.

[0049] Furthermore, when the molecular weight distribution of the thermoplastic elastomer (B) was measured, the molecular weight was 1×10 ４ It is preferable that the proportion of low-molecular-weight components having a molecular weight distribution of 4.0% or less is 4.0% or less. By using component (B) having such a molecular weight distribution, the proportion of low-molecular-weight components in the resin composition can be stably adjusted to 3% or less. Note that, from the viewpoint of reducing the proportion of low-molecular-weight components in the resin composition, the lower the proportion of low-molecular-weight components contained in component (B), the better. The lower limit is not particularly limited, but it is preferable that it be, for example, 0.001%.

[0050] The number average molecular weight of the thermoplastic elastomer (B) is not particularly limited, but is preferably 5.0×10 ４ It is preferable that the value is 8.0 × 10 or more. ４ Above 7.0 x 10 ５ Component (B) having such a number average molecular weight has a small proportion of low molecular weight components, and the proportion of low molecular weight components contained in the resin composition can be kept low.

[0051] The thermoplastic elastomer (B) may be any component capable of improving the flexibility of the resin composition, and known components such as amide-based, ester-based, olefin-based, styrene-based, urethane-based, vinyl chloride-based, and fluorine-based components may be used. Among these, at least one of olefin-based and styrene-based components is preferably used from the viewpoint of achieving fine dispersion or compatibility when mixed with the propylene-based resin (A). Olefin-based components are preferred from the viewpoint of further improving insulating properties. Compared to styrene-based components, olefin-based components have higher compatibility with the propylene-based resin (A), and it is easier to select components with higher melting points and molecular weights, thereby further improving insulating properties.

[0052] Olefin-based thermoplastic elastomers (so-called TPOs) are composed of at least one olefin unit (polyethylene or polypropylene) as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment. The olefin-based thermoplastic elastomer may be a copolymer of an olefin unit and an ethylene-α-olefin copolymer unit, or a mixture of an olefin and an ethylene-α-olefin copolymer. Among these, copolymers are preferred from the viewpoint of compatibility with component (A). The α-olefin is a linear or branched α-olefin having 2 to 8 carbon atoms, such as ethylene, propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3-methyl-1-pentene, or 1-octene. Among these, TPOs preferably have polypropylene as a hard segment and ethylene-propylene rubber as a soft segment. The olefin-based thermoplastic elastomers may be used alone or in combination of two or more.

[0053] The styrene-based thermoplastic elastomer is a copolymer containing styrene units as hard segments and at least one monomer unit selected from ethylene, propylene, butylene, isoprene, and the like as soft segments.

[0054] Examples of styrene-based thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), hydrogenated styrene-butadiene-styrene block copolymers, 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.

[0055] 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 no hydrogen has been added to the double bond of the aromatic ring of styrene. "Hydrogenated styrene butadiene styrene block copolymer" can be rephrased as styrene ethylene butylene styrene block copolymer (SEBS).

[0056] As the styrene-based thermoplastic elastomer, one that does not contain a double bond in the chemical structure excluding the benzene ring is preferred. If a double bond is used, the resin component may be thermally deteriorated during molding of the resin composition, which may reduce the properties of the resulting insulating layer. In this regard, a double bond-free elastomer has high resistance to thermal deterioration, allowing the insulating layer to maintain its properties at a higher level.

[0057] The content of styrene units in the styrene-based thermoplastic elastomer is not particularly limited, but from the viewpoint of controlling the crystal growth of the propylene-based resin (A) and softening the insulating layer, it is preferable that the content be 5% by mass or more and 35% 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 thermoplastic elastomer and component (A). This allows for stable improvement in the insulating properties of the insulating layer.

[0058] The melting points of the olefin-based thermoplastic elastomer and the styrene-based thermoplastic elastomer are not particularly limited. For example, the melting point of the olefin-based thermoplastic elastomer is preferably 130°C or higher and 160°C or lower. For example, the melting point of the styrene-based thermoplastic elastomer is preferably 110°C or higher and 130°C or lower. Components having such melting points make it easy to adjust the melting point of the resin composition to 110°C or higher and 170°C or lower without excessively lowering the melting point.

[0059] The MFR of the thermoplastic elastomer (B) is preferably 0.1 g / 10 min or more and 5.0 g / 10 min or less, and may be 0.1 g / 10 min or more and 2.0 g / 10 min or less, from the viewpoint of compatibility with the propylene-based resin (A). By setting the MFR in this range, the above-mentioned phase structure can be stably formed in the resin composition.

[0060] From the viewpoint of finely dispersing or miscible when mixed with the propylene-based resin (A), it is preferable that the MFR of the thermoplastic elastomer (B) has a small difference from the MFR of the propylene-based resin (A). Specifically, it is preferable that the difference in MFR between the components (A) and (B) in the resin composition is 300 g / 10 min or less.

[0061] (Other Additives) The resin composition may contain other additives as needed, such as an antioxidant, a crosslinking agent, a lubricant, and a colorant.

[0062] As the antioxidant, for example, known antioxidants such as phenol-based, sulfur-based, amine-based, etc. The content of the antioxidant is not particularly limited, but it is preferably 0.1 parts by mass or more and 1.0 parts by mass or less when the total content of the propylene-based resin (A) and the thermoplastic elastomer (B) is 100 parts by mass.

[0063] 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.

[0064] The resin composition may also contain a lubricant to improve the fluidity of the resin composition during the extrusion process of the insulating layer. As the lubricant, for example, a conventionally known component such as a fatty acid metal salt or a fatty acid amide can be used. These may be used alone or in combination of two or more.

[0065] In addition, the resin composition of this embodiment can maintain high insulating properties by controlling the melting point and the proportion of low-molecular-weight components within a predetermined range, so there is no need to add inorganic fillers. As will be described later, inorganic fillers can cause clogging of the mesh during the extrusion process of the resin composition, and the remaining resin composition may be excessively heated, promoting the formation of low-molecular-weight components. In this regard, by not adding inorganic fillers, the proportion of low-molecular-weight components can be adjusted to a low level.

[0066] (Blending ratio) The blending ratio of the propylene-based resin (A) and the thermoplastic elastomer (B) in the resin composition is not particularly limited as long as it can achieve a melting point of 110°C or higher. The content of each component may be adjusted depending on the components (A) and (B) used. Specifically, when the total content of the propylene-based resin (A) and the thermoplastic elastomer (B) is taken as 100 parts by mass, the content of the component (A) may be 50 parts by mass or more and 90 parts by mass or less, and the content of the component (B) may be 10 parts by mass or more and 50 parts by mass or less. By adjusting the contents in this range, it is easy to adjust the melting point of the resin composition and the ratio of the low-molecular-weight component within the above-mentioned ranges.

[0067] (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.

[0068] 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.

[0069] 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 .

[0070] (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.

[0071] (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.

[0072] (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.

[0073] (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.

[0074] (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.

[0075] (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.

[0076] 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.

[0077] 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.

[0078] (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.

[0079] (3) Cable Characteristics In this embodiment, as described above, the resin composition contains a propylene-based resin (A) and a thermoplastic elastomer (B), and the melting point of the resin composition and the proportion of low molecular weight components in the molecular weight distribution satisfy predetermined requirements, thereby enabling high insulation properties to be stably obtained in the insulating layer 130.

[0080] 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.

[0081] 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 40% or less.

[0082] 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 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

[0083] 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, 5.0×10 １４ It is preferable that the resistance is Ω cm or more, and 1.0 × 10 １５ It may be Ω·cm or more.

[0084] 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.

[0085] (4) 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.

[0086] The extruder 200 is a device for preparing the above-mentioned resin composition. In the extruder 200, the resin composition is prepared by mixing the components while heating them. At this time, the resin components may be thermally decomposed by heating, resulting in a lower molecular weight. For example, when the heating and mixing is performed in an oxygen-containing atmosphere, the resin components may be thermally decomposed. Furthermore, for example, if the resin components remain in the extruder 200 and the heating time becomes long, the resin components may be thermally decomposed. For this reason, the proportion of low-molecular-weight components in the resin composition may be higher than the original value of the contained components.

[0087] In this embodiment, from the viewpoint of adjusting the proportion of low molecular weight components in the resin composition to a predetermined range, it is preferable that the extruder 200 is configured so that the resin composition is extruded while adjusting the inside of the extruder to an inert gas atmosphere, or that the screw and extrusion port of the extruder are configured so that the resin composition is less likely to stagnate. The specific configuration of the extruder 200 will be described below.

[0088] 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.

[0089] 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.

[0090] 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°.

[0091] 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.

[0092] 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 250. 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.

[0093] 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.

[0094] The inert gas is not particularly limited, but may be, for example, nitrogen gas or argon gas.

[0095] 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.

[0096] (5) 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".

[0097] (S100: Resin Composition Preparation Step) First, a resin composition for forming the insulating layer 130 is prepared.

[0098] In this embodiment, for example, a propylene-based resin (A), a thermoplastic elastomer (B), and, as necessary, other additives (such as an antioxidant) are supplied to an extruder 200 shown in FIG. 2 . Then, the materials supplied from a hopper 220 are mixed while being heated inside a cylinder 210. At this time, an inert gas atmosphere is adjusted 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.

[0099] The content of each component may be appropriately adjusted so that the melting point of the resin composition is 110° C. or higher and the proportion of low-molecular-weight components is 3% or less. For example, the content of the propylene-based resin (A) may be 50 parts by mass or more and 90 parts by mass or less, and the content of the thermoplastic elastomer (B) may be 10 parts by mass or more and 50 parts by mass or less.

[0100] In the resin composition preparation step S100, the components are heated, mixed, and extruded in an inert gas atmosphere, thereby suppressing and reducing the proportion of low molecular weight components that are caused by heating the resin composition.

[0101] (S200: Conductor Preparation Step) On the other hand, the conductor 110 formed by twisting together a plurality of conductor core wires is prepared.

[0102] (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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] The extrusion is then cooled, for example with water.

[0107] In the cable core forming step S300, the pellet-shaped resin composition is mixed and extruded while being heated in an inert gas atmosphere, thereby suppressing an increase in the proportion of low molecular weight components due to heating, and as a result, the proportion of low molecular weight components in the insulating layer 130 can be kept low.

[0108] 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.

[0109] (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.

[0110] (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.

[0111] In this manner, the power cable 10 is manufactured as a solid insulated power cable.

[0112] (6) Effects of the Present Embodiment The present embodiment provides one or more of the following effects.

[0113] (a) The resin composition of the present embodiment contains a propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B), and the resin composition has a melting point of 110°C or higher, and when the molecular weight distribution is measured, the molecular weight is 1 x 10 ４ The proportion of the following low-molecular-weight components is 3% or less. The resin composition may contain low-molecular-weight components derived from component (B), which may become amorphous or melt in high-temperature environments. The melted low-molecular-weight components may migrate within the resin composition, causing space charge accumulation, behaving as charge carriers, and potentially reducing insulation. In this regard, by increasing the melting point of the resin composition to 110°C or higher, melting of the resin components and the associated flow can be suppressed, allowing them to be fixed within the resin composition. In other words, the low-molecular-weight components can be maintained in a finely dispersed or compatible state within the resin composition. Furthermore, by reducing the proportion of low-molecular-weight components in the resin composition, the resulting reduction in insulation can be further suppressed. As a result, the resin composition can maintain high insulation even in high-temperature environments. Therefore, by forming the insulating layer 130 from the above-described resin composition, the insulating layer 130 can be made to withstand high voltages even at high temperatures. As a result, the power cable 10 of this embodiment enables stable DC transmission.

[0114] In the (b) resin composition, the thermoplastic elastomer (B) can be finely dispersed or mixed with the propylene-based resin (A). That is, the (B) component can be uniformly distributed in the resin composition. As a result, the (B) component can suppress excessive crystal growth of the (A) component, imparting flexibility to the resin composition. That is, the resin composition can exhibit high and stable flexibility.

[0115] (c) The thermoplastic elastomer (B) may be at least one of an olefin-based thermoplastic elastomer and a styrene-based thermoplastic elastomer. The olefin-based thermoplastic elastomer can be finely dispersed or miscible in the propylene-based resin (A) and has a higher melting point than the styrene-based thermoplastic elastomer, thereby more reliably achieving the above-mentioned effect (a). On the other hand, the styrene-based thermoplastic elastomer can trap electrons in the aromatic ring to form a stable resonance structure, thereby further improving the insulating properties of the resin composition.

[0116] (d) The melting point of the thermoplastic elastomer (B) is preferably 110° C. or higher and 160° C. or lower. When such component (B) is mixed with the propylene-based resin (A) to prepare a resin composition, the melting point of the resin composition can be adjusted to 110° C. or higher without excessively lowering the melting point. This makes it possible to more reliably obtain the above-mentioned effect (a).

[0117] (e) Thermoplastic elastomer (B) has a molecular weight of 1×10 ４ It is preferable that the proportion of the following component be 4.0% or less. By using such component (B), when preparing the resin composition, the proportion of low-molecular-weight components can be more reliably adjusted to 3% or less. This makes it possible to more reliably obtain the above-mentioned effect (a).

[0118] The melting point of the (f) propylene-based resin is preferably 130° C. or higher and 170° C. or lower. When the (A) component is mixed with the thermoplastic elastomer (B), the melting point of the resin composition can be maintained at 110° C. or higher, while the flexibility required for a power cable can be achieved.

[0119] The (g) resin composition preferably contains 50 to 90 parts by mass of component (A) and 10 to 50 parts by mass of component (B), where the total content of the propylene-based resin (A) and the thermoplastic elastomer (B) is taken as 100 parts by mass. By using these blending amounts, the melting point of the resin composition can be more reliably adjusted to 110°C or higher, and the proportion of low-molecular-weight components in the resin composition can be more reliably adjusted to 3% or less, thereby more reliably achieving the above-mentioned effect (a).

[0120] (h) The number average molecular weight of the thermoplastic elastomer (B) is 8.0 × 10 ４ Such component (B) has a low proportion of low molecular weight components, so that when the component (B) is mixed with the propylene-based resin (A) to prepare a resin composition, the proportion of low molecular weight components in the resin composition can be reduced. This makes it possible to more reliably obtain the above-mentioned effect (a).

[0121] (i) The resin composition may be substantially free of modified polymers or inorganic fillers that improve insulation properties. The resin composition is formulated so that the melting point and the ratio of low-molecular-weight components fall within a predetermined range, thereby achieving high insulation properties without the inclusion of modified polymers or inorganic fillers. Furthermore, if the resin composition contains an inorganic filler, the resin composition is likely to clog the mesh when extruded from the extruder 200, making the resin composition more likely to remain in the extruder 200. In this regard, the absence of inorganic fillers can more reliably suppress the retention of the resin composition and the resulting reduction in molecular weight. Furthermore, modified polymers tend to be susceptible to thermal degradation, but the absence of modified polymers can suppress degradation of the resin composition during the long-term production of power cables. This suppresses thermal degradation of the resin composition, reducing the variation in properties along the length of the insulation layer and maintaining high properties, for example, when producing long-length power cables.

[0122] (j) When preparing the resin composition, the propylene-based resin (A), the thermoplastic elastomer (B), and, if necessary, other additives may be supplied to the cylinder 210 of the extruder 200 and heated and mixed in an inert gas atmosphere. Alternatively, the screw flight 232 of the screw 230 in the extruder 200 may be configured to have a tapered shape on the first direction side. Alternatively, a retention suppression member 270 having a plurality of tapered holes 271 penetrating the thickness direction and having a diameter that decreases from the first direction to the second direction may be disposed between the cylinder 210 and the discharge section 250 of the extruder 200. By using at least one of these configurations, thermal decomposition of the resin components and reduction in molecular weight during preparation of the resin composition can be suppressed. This makes it possible to more reliably adjust the proportion of low-molecular-weight components in the resin composition to 3% or less, thereby achieving the above-mentioned effect (a).

[0123] <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.

[0124] 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.

[0125] 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).

[0126] 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.

[0127] 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.

[0128] (1) Materials The materials used in preparing the resin compositions are listed below.

[0129] Propylene random polymer (PP1) was prepared as the propylene-based resin (A), and olefin-based elastomers (TPO1) to (TPO2) and styrene-based thermoplastic elastomers (SEBS1) to (SEBS2) were prepared as the thermoplastic elastomer (B). The physical properties of each component are as follows. The melting point, number average molecular weight, MFR, and proportion of low molecular weight components were measured in advance using the methods described below. PP1: Melting point 160°C, number average molecular weight 1.5 x 10 ５ TPO1: an olefin-based thermoplastic elastomer having a hard segment of polypropylene and a soft segment of ethylene-propylene rubber, melting point 135°C, a proportion of low molecular weight components of 5.1%, and a number average molecular weight of 9.0 x 10 ４ TPO2: olefin-based thermoplastic elastomer with a hard segment of polypropylene and a soft segment of ethylene-propylene rubber, melting point 140°C, a proportion of low molecular weight components of 1.5%, and a number average molecular weight of 1.6 x 10 ５ , MFR 3.2 g / 10 min SEBS1: Melting point 115°C, proportion of low molecular weight components 5.4%, number average molecular weight 1.2 × 10 ５ , MFR 4.5 g / 10 min, styrene content 10 mass% SEBS2: melting point 120°C, proportion of low molecular weight components 2.0%, number average molecular weight 1.5 × 10 ５ , MFR 3.7 g / 10 min, styrene content 12% by mass

[0130] As another additive, pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], a hindered phenol, was prepared as an antioxidant.

[0131] (2) Preparation of Resin Compositions Samples 1 to 5 were prepared by feeding the above materials to an extruder in the amounts shown in Table 1 below, and then heating, mixing, and granulating them using the extruder. The extrusion conditions were changed as follows when preparing the resin compositions. For Samples 1 to 4, the extruder described above was used, the inside of the cylinder was purged with nitrogen gas, and a screw with a tapered side surface on the first direction side of the screw flight was employed for extrusion. For Sample 5, on the other hand, the inside of the cylinder was not purged with nitrogen gas, and a rectangular screw was employed for extrusion. The amount of antioxidant blended in each sample was 0.1 parts by mass.

[0132]

[0133] (3) Fabrication of Power Cable Next, a conductor was prepared by twisting together conductor core wires made of a dilute copper alloy with a diameter of 14 mm. After the conductor was prepared, an inner semiconductive layer resin composition containing an ethylene-ethyl acrylate copolymer, an insulating layer resin composition prepared in Table 1, and an outer semiconductive layer resin composition made of the same materials as the inner semiconductive layer resin composition were respectively fed into extruders A to C. The extrudates from extruders A to C were guided to a common head, and the inner semiconductive layer, insulating layer, and outer semiconductive layer were simultaneously extruded from the inside to the outside around the conductor. This 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.

[0134] (4) Evaluation The prepared resin compositions and samples cut out from the insulating layers of the manufactured power cables were evaluated for their melting points, proportions of low molecular weight components, space charge characteristics, volume resistivity, and DC breakdown strength. Each evaluation method was as follows.

[0135] (Melting Point) The melting point of the resin composition prepared for each sample was determined by DSC measurement. DSC measurement was performed in accordance with JIS-K-7121 (1987). Specifically, a PerkinElmer DSC8500 (input compensation type) was used as the DSC device. The reference sample was, for example, α-alumina. The mass of the measurement sample was 8 to 10 g. In the DSC device, the temperature was raised from room temperature (27°C) to 220°C at a rate of 10°C / min. A DSC curve was obtained by plotting the amount of heat absorbed per unit time (heat flow) against the temperature. The temperature at which the amount of heat absorbed per unit time for each measurement sample reached its maximum (highest peak) was defined as the "melting point."

[0136] (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 The proportions of the following low 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.

[0137] (Space Charge Characteristics) The space charge characteristics of the insulating layer were evaluated by the amount of space charge accumulated in the insulating layer. The amount of space charge accumulated was measured by the current integration charge method. Specifically, a sheet-shaped sample piece was first taken from the insulating layer of the power cable. Next, this sample piece was connected in series to a measuring capacitor, and then charge was accumulated in the measuring capacitor, and the amount of charge, which is the integrated value of the current, was measured. In this example, a DC electric field of 40 kV / mm or 80 kV / mm was continuously applied to the sample at a temperature of 90°C, and the amount of charge Q after 300 seconds was measured. ３００ and the charge amount Q immediately after application (0 seconds) ０Based on this, the space charge accumulation amounts at a temperature of 90°C and a DC electric field of 40 kV / mm and at a temperature of 90°C and a DC electric field of 80 kV / mm were calculated using the following formula. In this example, when the space charge accumulation amount at a temperature of 90°C and a DC electric field of 40 kV / mm was 25% or less, it was evaluated as A (best), when the space charge accumulation amount was more than 25% and 100% or less, it was evaluated as B (good), and when the space charge accumulation amount was more than 100%, it was evaluated as C (poor). Furthermore, when the space charge accumulation amount at a temperature of 90°C and a DC electric field of 80 kV / mm was 40% or less, it was evaluated as A (best), when the space charge accumulation amount was more than 40% and 100% or less, it was evaluated as B (good), and when the space charge accumulation amount was more than 100%, it was evaluated as C (poor). Space charge accumulation amount=(Q ３００ / Q ０ −1) × 100

[0138] (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 1×10 １５ A value of A (best) is used when the volume resistivity is 5×10 １４ Ω・cm or more 1×10 １５ A volume resistivity of less than 5×10 Ω cm is rated as B (good). １４ When the resistance was less than Ω·cm, it was evaluated as C (poor).

[0139] (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).

[0140] (5) Evaluation Results The above-described evaluations were carried out for each sample, and the evaluation results are summarized in Table 1.

[0141] As shown in Table 1, Samples 1 and 2 used TPO1 and SEBS1, which have high melting points but high proportions of low-molecular-weight components. Therefore, although the melting point of the resin composition could be raised to 110°C or higher, the proportion of low-molecular-weight components could not be maintained at 3% or less. As a result, it was confirmed that the insulating properties were poor in high-temperature environments. It is presumed that these samples contained a high proportion of low-molecular-weight components, which melted in high-temperature environments and flowed through the resin composition, causing localized charge accumulation. In particular, Sample 2 had a lower melting point than Sample 1, which is presumed to have caused the low-molecular-weight components to melt more easily, resulting in a further deterioration in insulating properties.

[0142] On the other hand, in Samples 3 and 4, TPO2 and SEBS2, which have high melting points and low proportions of low-molecular-weight components, were used, and it was confirmed that the melting point of the resin composition could be 110°C or higher and the proportion of low-molecular-weight components could be 3% or lower. In particular, in Sample 3, it was confirmed that the use of TPO2, which has a higher melting point than SEBS2, made it possible to further increase the melting point of the resin composition, and as a result, to further improve the insulating properties in high-temperature environments. This is presumably because the melting of the resin components contained in the resin composition in high-temperature environments could be further suppressed, and the dispersion and compatibility of the low-molecular-weight components could be maintained.

[0143] Furthermore, Sample 5 used the same materials as Sample 3, but it was confirmed that the proportion of low-molecular-weight components was higher than that of Sample 3. As a result, it was confirmed that the insulating properties in a high-temperature environment were lower than those of Sample 3. The reason for the higher proportion of low-molecular-weight components than Sample 3 is presumed to be that Sample 5 did not use nitrogen gas purging or tapered screw flights in the extruder that heat-mixed the materials, which made the resin composition more likely to remain in the extruder and reduced its molecular weight due to thermal decomposition. It is presumed that these low-molecular-weight components melted and flowed in a high-temperature environment, causing localized charge accumulation and reducing the insulating properties.

[0144] As described above, it has been confirmed that by using a propylene-based resin (A) having propylene units and a thermoplastic elastomer (B) in a resin composition and by ensuring that the melting point and the proportion of low-molecular-weight components of the resin composition meet certain requirements, it is possible to suppress the accumulation of space charge and improve the volume resistivity and DC breakdown field strength even in high-temperature environments. In other words, it has been confirmed that high insulating properties can be stably obtained in the resin composition. This allows the insulating layer to withstand high voltages even in high-temperature environments, enabling stable DC transmission using a power cable.

[0145] <Preferred Aspects of the Present Disclosure> Preferred aspects of the present disclosure will be additionally described below.

[0146] (Note 1) A thermoplastic elastomer (B) is included, and the melting point is 110°C or higher. When the molecular weight distribution is measured, the molecular weight is 1 x 10 ４ A resin composition comprising the following components in an amount of 3% or less:

[0147] (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 propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B), and has a melting point of 110°C or higher, and a molecular weight of 1 x 10 when the molecular weight distribution is measured. ４A power cable containing the following components in a proportion of 3% or less:

[0148] (Supplementary Note 3) In Supplementary Note 2, preferably, the thermoplastic elastomer (B) is at least one of an olefin-based thermoplastic elastomer having an olefin unit and a styrene-based thermoplastic elastomer having a styrene unit.

[0149] (Supplementary Note 4) In Supplementary Note 3, preferably, the thermoplastic elastomer (B) is an olefin-based thermoplastic elastomer having an olefin unit.

[0150] (Supplementary Note 5) In any one of Supplementary Notes 2 to 4, preferably, the melting point of the thermoplastic elastomer (B) is 110°C or higher and 160°C or lower.

[0151] (Supplementary Note 6) In any one of Supplementary Notes 2 to 5, preferably, the thermoplastic elastomer (B) has a molecular weight of 1×10 ４ The proportion of the following components is 4.0% or less.

[0152] (Supplementary Note 7) In any one of Supplementary Notes 2 to 6, preferably, the propylene-based resin (A) is a propylene random polymer.

[0153] (Supplementary Note 8) In any one of Supplementary Notes 2 to 7, preferably, the melting point of the propylene-based resin (A) is 130°C or higher and 170°C or lower.

[0154] (Appendix 9) In any one of Appendices 2 to 8, preferably, the resin composition contains 50 parts by mass or more and 90 parts by mass or less of the propylene-based resin (A) and 10 parts by mass or more and 50 parts by mass or less of the thermoplastic elastomer (B).

[0155] (Appendix 10) In any one of Appendices 2 to 9, preferably, the number average molecular weight of the thermoplastic elastomer (B) is 5.0 × 10 ４ That's all.

[0156] (Appendix 11) In any one of Appendices 2 to 10, preferably, the propylene-based resin (A) has a number average molecular weight of 5.0 × 10 ４That's all.

[0157] (Appendix 12) A method for producing 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 propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B) are added so that the melting point of the resin composition becomes 110°C or higher, and when the molecular weight distribution of the resin composition is measured, a molecular weight of 1 x 10 ４ A method for manufacturing a power cable, comprising heating and mixing the following components so that the ratio of each component is 3% or less:

[0158] (Supplementary Note 13) In Supplementary Note 12, preferably, in the preparation step, the heating and mixing is carried out in an inert gas atmosphere.

[0159] (Appendix 14) In Appendix 12 or 13, 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 discharge port for discharging the resin composition; and an atmosphere adjustment section adjusting the inside of the cylinder to an inert gas atmosphere.

[0160] (Appendix 15) In Appendix 14, 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.

[0161] 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 thermoplastic elastomer (B) containing a propylene-based resin (A) having a propylene unit, having a melting point of 110°C or higher, and having a molecular weight of 1 x 10 when the molecular weight distribution is measured. ４ A resin composition comprising the following components in an amount of 3% or less:

2. A conductive material comprising: a conductor; and an insulating layer covering the conductor and formed from a resin composition, wherein the resin composition contains a propylene-based resin (A) having a propylene unit and a thermoplastic elastomer (B), and has a melting point of 110°C or higher, and a molecular weight of 1 x 10 when the molecular weight distribution is measured. ４ A power cable containing the following components in a proportion of 3% or less:

3. The power cable according to claim 2, wherein the thermoplastic elastomer (B) is at least one of an olefin-based thermoplastic elastomer having an olefin unit and a styrene-based thermoplastic elastomer having a styrene unit.

4. The power cable according to claim 2 or 3, wherein the melting point of the thermoplastic elastomer (B) is 110°C or higher and 160°C or lower.

5. The thermoplastic elastomer (B) has a molecular weight of 1 x 10 ４ The power cable according to any one of claims 2 to 4, wherein the proportion of the following component is 4.0% or less:

6. The power cable according to any one of claims 2 to 5, wherein the melting point of the propylene-based resin (A) is 130°C or higher and 170°C or lower.

7. The power cable according to any one of claims 2 to 6, wherein the resin composition contains 50 parts by mass or more and 90 parts by mass or less of the propylene-based resin (A) and 10 parts by mass or more and 50 parts by mass or less of the thermoplastic elastomer (B).

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