Resin composition and power cable

Abstract

A resin composition containing a propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with an unsaturated carboxylic acid, the modified amount of the unsaturated carboxylic acid from the modified polymer (C) in the resin composition being 0.01 mass% or more, and the content of a free monomer having an unsaturated carboxylic acid group and having a molecular weight of 500 or less from the modified polymer (C) in the resin composition being 1.0 x 10 ‑6 mass% or more and 0.05 mass% or less.

Description

Technical Field

[0001] This disclosure relates to resin compositions and power cables. Background Technology

[0002] In recent years, solid-insulated power cables (hereinafter referred to as "power cables") have been developed for DC power transmission applications. In these power cables, cross-linked polyethylene (XLPE) is widely used as a component constituting the insulation layer (e.g., Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 11-086634. Summary of the Invention

[0006] According to one aspect of this disclosure, a resin composition is provided comprising an propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids, wherein the amount of unsaturated carboxylic acid modification from the modified polymer (C) in the resin composition is 0.01% by mass or more, and the content of free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and having a molecular weight of 500 or less in the resin composition is 1.0 × 10⁻⁶. -6 More than 0.05% by mass and less than 0.05% by mass.

[0007] According to another aspect of this disclosure, a power cable is provided, having a conductor and an insulating layer formed of a resin composition covering the conductor. The resin composition contains an propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids. The amount of unsaturated carboxylic acid modification from the modified polymer (C) in the resin composition is 0.01% by mass or more, and the content of free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and a molecular weight of 500 or less in the resin composition is 1.0 × 10⁻⁶. -6 More than 0.05% by mass and less than 0.05% by mass. Attached Figure Description

[0008] Figure 1 This is a schematic cross-sectional view orthogonal to the axial direction of a power cable according to one embodiment of the present disclosure.

[0009] Figure 2 This is a flowchart illustrating a method for manufacturing a power cable according to one embodiment of the present disclosure. Detailed Implementation

[0010] [The problem the invention aims to solve]

[0011] Cross-linked polyethylene that has deteriorated over the years cannot be recycled and can only be incinerated. Therefore, there are concerns about its environmental impact.

[0012] Therefore, in recent years, acrylic resins, as a component of insulating layers, have attracted much attention. According to polypropylene resins, even non-crosslinked resins can achieve high insulation properties. That is, they can balance insulation and recyclability.

[0013] On the other hand, in acrylic resins, insulation properties are sometimes not stably obtained in the insulating layer.

[0014] The purpose of this disclosure is to provide a technique for consistently achieving insulation in power cables.

[0015] [The effects of the invention]

[0016] According to this disclosure, insulation can be stably obtained in power cables.

[0017] [Description of embodiments of this disclosure]

[0018] <Insights gained by the inventors, etc.>

[0019] First, an overview of the insights gained by the inventors and others is provided.

[0020] In power cables, when the insulation layer is made of acrylic resin, space charge can be generated within the insulation layer under conditions such as high voltage, sometimes reducing its insulating properties. This tendency is more pronounced at high temperatures. Furthermore, the insulation properties referred to here include the volume resistivity, DC breakdown electric field strength, and space charge characteristics of the insulation layer.

[0021] To improve the insulation properties of insulating layers, methods are being investigated for using modified polymers derived from unsaturated carboxylic acids. Examples of such modified polymers include modified propylene derived from unsaturated carboxylic acids. Based on these modified polymers, the polar groups exhibit charge-trapping properties, thereby improving the insulation properties of the resin composition.

[0022] The inventors appropriately varied the amount of modified polymer added, altering the amount of polar groups introduced into the resin composition and evaluating conductivity. The results showed that even with the same amount of polar groups, the insulation properties differed. Further investigation revealed that unreacted unsaturated carboxylic acids and other components, not bonded to the polymer backbone, remained in the modified polymer, and these components, acting as free monomers, affected the insulation properties.

[0023] Like modified polymers, free monomers possess polar groups and thus exhibit charge-trapping properties. In regions of the resin composition where modified polymers are not dispersed, charge-trapping effects are difficult to achieve, but free monomers are more easily dispersed in the resin composition, thus exhibiting a uniform charge-trapping effect. However, free monomers themselves can sometimes act as charge carriers; when their amount increases, it can actually reduce insulation properties.

[0024] As can be seen from the above, when adding modified polymers to propylene resins, it is preferable to control the content of unsaturated carboxylic acids from the modified polymer within a specified range, while also controlling the amount of free monomers from the modified polymer within a specified range.

[0025] This disclosure is based on the above-mentioned insights discovered by the inventors, etc.

[0026] <Implementation Methods of this Disclosure>

[0027] Next, embodiments of this disclosure will be described.

[0028] [1] One aspect of the resin composition disclosed herein comprises an propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids, wherein the content of the unsaturated carboxylic acid from the modified polymer (C) in the resin composition is 0.01% by mass or more, and the content of free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and having a molecular weight of 500 or less in the resin composition is 1.0 × 10⁻⁶. -6 More than 0.05% by mass and less than 0.05% by mass.

[0029] Based on this configuration, high insulation can be stably obtained.

[0030] [2] Another aspect of this disclosure relates to a power cable having a conductor and an insulating layer formed of a resin composition covering the conductor, the resin composition containing an propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids, wherein the content of the unsaturated carboxylic acid from the modified polymer (C) in the resin composition is 0.01% by mass or more, and the content of free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and having a molecular weight of 500 or less in the resin composition is 1.0 × 10⁻⁶. -6 More than 0.05% by mass and less than 0.05% by mass.

[0031] Based on this configuration, high voltage resistance can be achieved even in high-temperature environments, enabling stable DC power transmission.

[0032] [3] In the power cable described in [2] above, the thermoplastic elastomer (B) is an olefin-based thermoplastic elastomer having at least one olefin unit of polyethylene and polypropylene as a hard segment and an ethylene-α olefin copolymer unit as a soft segment.

[0033] Based on this structure, the insulation and flexibility of the insulation layer can be further improved.

[0034] [4] In the power cable described in [2] or [3] above, when the total content of the acrylic resin (A), the thermoplastic elastomer (B) and the modified polymer (C) is set to 100 parts by mass, the resin composition contains 55 parts by mass and 90 parts by mass of the acrylic resin (A), 10 parts by mass and 45 parts by mass of the thermoplastic elastomer (B), and 1 part by mass and 10 parts by mass of the modified polymer (C).

[0035] Based on this structure, it is possible to achieve high insulation performance while improving the flexibility of the insulation layer.

[0036] [5] In the power cable described in any one of [2] to [4] above, the amount of unsaturated carboxylic acid modification in the modified polymer (C) is 0.1% by mass or more and 10% by mass or less, and the content of the free monomer is 1.0 × 10⁻⁶. -4 Quality percentage above 1% and below 1.0% of the total mass.

[0037] Based on this configuration, higher insulation can be stably obtained in the insulating layer.

[0038] [Details of the embodiments disclosed herein]

[0039] Next, one embodiment of the present disclosure will be described with reference to the accompanying drawings. Furthermore, the invention is not limited to these illustrations, as shown in the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0040] <One embodiment of this disclosure>

[0041] (1) Resin composition

[0042] The resin composition of this embodiment can be used, for example, as a material for forming the insulation layer of the power cable described later. The resin composition contains an acrylic resin (A), a thermoplastic elastomer (B), a modified polymer (C), and other additives added as needed. Each component will be described in detail below. Hereinafter, the acrylic resin (A) will be referred to as component (A), the thermoplastic elastomer (B) as component (B), and the modified polymer (C) as component (C).

[0043] (Acrylic resin (A))

[0044] Acrylic resin (A) is a resin material constituting the main component in a resin composition, and is a component having propylene units. As component (A), at least one of propylene homopolymer (hereinafter also referred to as homopolymer PP) and propylene random polymer (hereinafter also referred to as random PP) can be used. Homopolymer PP contains propylene units, and random PP has propylene units and ethylene units.

[0045] From the perspective of achieving higher insulation performance in the insulating layer, acrylic resin (A) is preferably atactic polypropylene (PP). While homopolymer PP, with its higher crystallinity compared to atactic PP, can achieve high insulation, intracrystalline and intercrystalline cracking can occur within the insulating layer, sometimes preventing the attainment of its intended insulation properties. In contrast, atactic PP, containing ethylene units, has a lower crystallinity, making it less prone to cracking due to coarse crystallization within the insulating layer, thus achieving higher insulation performance compared to homopolymer PP.

[0046] Furthermore, examples of stereoregularity for the acrylic resin (A) include isotactic, syndiotactic, and atactic structures. Stereoregularity is not particularly limited, but isotacticity is preferred. By achieving isotacticity, the decrease in the melting point of the resin composition can be suppressed. As a result, stable use in non-crosslinked or micro-crosslinked states is possible.

[0047] The melt flow rate (MFR) of the acrylic resin (A), as described below, is preferably 0.1 g / 10 min or more and 5.0 g / 10 min or less from the viewpoint of compatibility with the thermoplastic elastomer (B) and the modified polymer (C), and may also be 0.1 g / 10 min or more and 2.0 g / 10 min or less. By employing such an MFR, the phase structure of the resin composition can be formed into a structure in which each component is compatible and finely dispersed. This improves the flexibility and insulation properties of the resin composition. Furthermore, the MFR here refers to the value measured according to JIS K7210 at a temperature of 190°C and a load of 2.16 kg.

[0048] Furthermore, the melting point of the acrylic resin (A) is not particularly limited, but is preferably 130°C or higher and 170°C or lower. When the acrylic resin (A) is a homopolymer PP, its melting point is preferably 120°C or higher and 165°C or lower; when the acrylic resin (A) is a random PP, its melting point is preferably 130°C or higher and 170°C or lower. With acrylic resin (A) having such a melting point, high compatibility can be achieved when mixed with thermoplastic elastomer (B) and modified polymer (C).

[0049] In addition, the melting point is determined as follows in this specification.

[0050] First, for the sample, differential scanning calorimetry (DSC) is performed, for example, according to JIS-K-7121 (1987). Specifically, in the DSC apparatus, the temperature is increased from room temperature (room temperature, e.g., 27°C) to 220°C at a rate of 10°C / min. The DSC curve is then obtained by plotting the heat endothermic per unit time relative to the temperature. The temperature at which the heat endothermic per unit time in the sample reaches its maximum value (peak) is then set as the "melting point (melting peak temperature)".

[0051] (Thermoplastic elastomer (B))

[0052] Thermoplastic elastomer (B) is a component with lower crystallinity compared to propylene-based resin (A) containing propylene units. Based on component (B), the crystal growth of component (A) can be controlled, imparting flexibility to resin compositions and insulating layers.

[0053] As the thermoplastic elastomer (B), any component that can improve the softness of the resin composition can be used, such as known components like amide-based, ester-based, olefin-based, styrene-based, polyurethane-based, polyvinyl chloride-based, and fluorine-based components. Among these, from the viewpoint of compatibility with acrylic resin (A), olefin-based components are preferred. The olefin-based thermoplastic elastomer (so-called TPO) is configured to include at least one olefin unit from polyethylene and polypropylene as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment. The olefin-based thermoplastic elastomer can be a copolymer type of olefin units and ethylene-α-olefin copolymer units, or a mixture type of olefin units and ethylene-α-olefin copolymer units. Among these, from the viewpoint of miscibility with component (A), copolymer types are preferred. The α-olefin is a straight-chain or branched α-olefin with 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, 1-octene, etc. TPO preferably contains polypropylene as the hard segment and ethylene-propylene rubber as the soft segment. Additionally, component (B) can be used alone or in combination with two or more components.

[0054] From the viewpoint of compatibility with acrylic resin (A) and modified polymer (C), the molecular weight ratio (MFR) of the thermoplastic elastomer (B) is preferably 0.1 g / 10 min or more and 5.0 g / 10 min or less, or it can be 0.1 g / 10 min or more and 2.0 g / 10 min or less. By employing such an MFR, the phase structure of the resin composition can be formed into a compatible structure and a finely dispersed island structure.

[0055] The thermoplastic elastomer (B) has no melting point (no melting point), or even if it has a melting point, the melting point is preferably below 165°C, or it may be above 130°C and below 155°C. According to such a composition (B), it is possible to maintain high pressure resistance of the resin composition at high temperatures while maintaining high flexibility required for the insulation layer of power cables.

[0056] (Modified polymer (C))

[0057] The modified polymer (C) is a component containing propylene units and modified with unsaturated carboxylic acids having polar groups. Because of the propylene units, component (C) exhibits excellent compatibility when mixed with propylene-based resin (A). Furthermore, component (C) can introduce polar groups into the resin composition by mixing with component (A). Based on these polar groups, space charge can be trapped in the insulating layer formed from the resin composition. That is, the amount of space charge accumulation in the insulating layer can be reduced. As a result, the volume resistivity in the insulating layer can be increased, stably ensuring high insulation performance.

[0058] Specifically, the modified polymer (C) is polypropylene modified with unsaturated carboxylic acids, which is also referred to as modified PP. As unsaturated carboxylic acids, known components with a molecular weight of 500 or less that can be introduced into polypropylene can be cited. Examples include acrylic acid, methacrylic acid, crotonic acid, maleic acid, cinnamic acid, itaconic acid, citraconic acid, and fumaric acid, as well as unsaturated carboxylic 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 unit molecular weight, so even a small amount can modify polypropylene.

[0059] In the modified polymer (C), the proportion (modification amount) of the unsaturated carboxylic acid introduced relative to the polypropylene is not particularly limited, but is preferably 0.1% by mass or more and 10% by mass or less, or 0.5% by mass or more and 4% by mass or less. By setting the modification amount of component (C) to 0.1% by mass or more or 0.5% by mass or more, the content of unsaturated carboxylic acid contained in the resin composition can be easily adjusted to 0.01% or more, which can suppress the accumulation of space charge. On the other hand, by setting the modification amount of component (C) to 10% by mass or less or 4% by mass or less, high compatibility with acrylic resin (A) and thermoplastic elastomer (B) can be maintained. In addition, the modification amount indicates the copolymerization ratio of unsaturated carboxylic acid in component (C) that is bonded to the polymer that forms the main chain, and indicates the amount per 100 parts by mass of the modified polymer (C).

[0060] As mentioned above, the modified polymer (C) sometimes contains free monomers. Free monomers are unreacted unsaturated carboxylic acids or their by-reaction products that were not bonded to the polymer backbone during the preparation of component (C) and exist in a free state within component (C). Free monomers have a chemical structure derived from unsaturated carboxylic acids. Specifically, free monomers are components having an unsaturated carboxylic acid group and a molecular weight of 500 or less. Furthermore, the reason for setting the molecular weight to 500 or less is that this is the range that unsaturated carboxylic acids and their by-reaction products may have.

[0061] The content of free monomers in the modified polymer (C) is not particularly limited, but the content of free monomers in the resin composition is adjusted to 1.0 × 10⁻⁶. -6 From the perspective of mass percentage of 1% or more and mass percentage of 0.05% or less, 1.0 × 10⁻⁶ is preferred. -4 The content of free monomers in component (C) is 1.0% to 1.0% by mass. The free monomer content indicates the amount of free monomers per 100 parts by mass of component (C). Alternatively, component (C) can be selected as containing a predetermined amount of free monomers, or the predetermined amount can be adjusted through pretreatment such as free monomer extraction. As a pretreatment, it is preferable to heat component (C) to volatilize the free monomers contained therein and to evacuate the ambient atmosphere. For example, in the case where the free monomer is maleic anhydride, it is preferable to heat the modified component (C) at a temperature higher than the boiling point of maleic anhydride (202°C) without thermal decomposition.

[0062] From the viewpoint of compatibility with acrylic resin (A) and thermoplastic elastomer (B), the MFR of the modified polymer (C) is preferably 0.1 g / 10 min or more and 500 g / 10 min or less, or 1 g / 10 min or more and 300 g / 10 min or less. By employing such an MFR, it is possible to achieve fine dispersion or compatibility of the components in the resin composition.

[0063] Furthermore, the melting point of the modified polymer (C) is not particularly limited, but is preferably 130°C or higher and 165°C or lower. Based on the (C) component having such a melting point, when mixed with acrylic resin (A) and thermoplastic elastomer (B), the components can be finely dispersed or compatible in the resin composition.

[0064] (Other additives)

[0065] The resin composition may contain other additives as needed. These other additives may include inorganic fillers, antioxidants, crosslinking agents, lubricants, and colorants.

[0066] In this embodiment, since the space charge trapping effect of the modified polymer (C) can be obtained more effectively, high insulation can be stably achieved even without the addition of inorganic fillers. On the other hand, when inorganic fillers are added, if the total content of resin components such as acrylic resin (A) is set to 100 parts by mass, the content of inorganic fillers is preferably less than 1 part by mass, for example. Furthermore, there is no particular limitation on the lower limit of the content of inorganic fillers, as long as inorganic fillers can be added.

[0067] Examples of inorganic fillers include, for example, magnesium oxide (MgO), silicon dioxide, zinc oxide, aluminum oxide, titanium oxide, zirconium oxide, carbon black, and at least one of mixtures of two or more of these.

[0068] Furthermore, the mean volume diameter (MV) of the inorganic filler is not particularly limited; for example, it is preferably 1 μm or less, but can also be 700 nm or less, or even 100 nm or less. Additionally, regarding the "mean volume diameter (MV)" mentioned here, let the particle size be d... i The volume of the particle is V i When, it can be obtained from the following formula.

[0069]

[0070] In addition, the volume average particle size was determined using a dynamic light scattering particle size distribution measuring device.

[0071] Furthermore, there is no particular limitation on the lower limit of the volume average particle size of the inorganic filler. However, from the viewpoint of stably forming the inorganic filler, the volume average particle size of the inorganic filler is preferably 1 nm or more, and may also be 5 nm or more.

[0072] Furthermore, at least a portion of the inorganic filler can be surface-treated with a silane coupling agent. This improves the interfacial adhesion between the inorganic filler and the acrylic resin (A), thereby enhancing the mechanical properties and insulation of the insulating layer 130.

[0073] As antioxidants, known antioxidants such as phenolic, sulfur-based, and amine-based antioxidants can be used. When the resin composition contains an antioxidant, the content of the antioxidant is not particularly limited. However, when the total content of the resin components in the resin composition, namely, propylene resin (A), thermoplastic elastomer (B), and modified polymer (C), is set to 100 parts by mass, the content of the antioxidant is preferably 0.1 parts by mass or more and 1.0 parts by mass or less.

[0074] Furthermore, from a recycling perspective, the resin composition is preferably non-crosslinked, but it may contain a crosslinking agent for crosslinking purposes. However, even if crosslinking is performed, it is preferable to crosslink in a manner that reduces the gel fraction (degree of crosslinking). Specifically, it is preferable to crosslink with a degree of crosslinking where the crosslinking agent residue in the resin composition is less than 300 ppm. In addition, when dicumyl peroxide is used as a crosslinking agent, the residue may be, for example, cumyl alcohol, α-methylstyrene, etc.

[0075] Furthermore, to improve the flowability of the resin composition during the extrusion process of the insulating layer 130, the resin composition may contain a lubricant. Examples of lubricants include fatty acid metal salts or fatty acid amides. Examples of fatty acid metal salts include magnesium stearate, zinc stearate, aluminum stearate, and magnesium lignite. Examples of fatty acid amides include oleamide or stearamide. Additionally, two or more of these can be used in combination.

[0076] (Phase structure of the resin composition)

[0077] The resin composition is formed by mixing an acrylic resin (A), a thermoplastic elastomer (B), and a modified polymer (C), along with other additives added as needed. Because the components readily mix, the resin composition has a phase structure in which components (B) and (C) are finely dispersed within component (A), or a phase structure in which the components are compatible. This allows for more uniform trapping of space charge within the resin composition, resulting in more stable insulation. Furthermore, the resin composition can meet the flexibility required for an insulating layer.

[0078] From the viewpoint of achieving finer dispersion or compatibility of components in a resin composition, it is preferable that the mean free float (MFR) of each component is close and that the deviation of these MFRs is small. Specifically, it is preferable that the difference between the highest and lowest MFR of each component is less than 300 g / 10 min. By combining components with such a difference in MFR, it is possible to achieve fine dispersion or compatibility of the components during mixing.

[0079] (The content of unsaturated carboxylic acids in the resin composition)

[0080] An unsaturated carboxylic acid with polar groups from the modified polymer (C) is introduced into the resin composition. The content of the unsaturated carboxylic acid in the resin composition can be adjusted by the amount of unsaturated carboxylic acid modification in component (C) and the amount of component (C) added. In this embodiment, the content of the unsaturated carboxylic acid in the resin composition is 0.01% by mass or more, or 0.03% by mass or more. By setting it to such a content, the accumulation of space charge in the resin composition can be reduced, thereby improving insulation.

[0081] On the other hand, there is no particular upper limit to the content of unsaturated carboxylic acids, but it is preferably 0.5% by mass or less, and can also be 0.1% by mass or less. Generally speaking, the higher the content of unsaturated carboxylic acids, the higher the content of modified polymer (C), and the lower the proportion of acrylic resin (A) and thermoplastic elastomer (B). As a result, it is sometimes impossible to maintain the softness of the resin composition and the desired strength required for the insulation layer. In addition, due to the increase in the content of component (C), the free monomers in the resin composition also increase, making it difficult to adjust to the range of 0.05% by mass or less as described later. In this regard, by adding component (C) such that the content of unsaturated carboxylic acids in the resin composition is 0.5% by mass or less, it is possible to maintain high softness and strength of the resin composition. Moreover, it is possible to easily adjust the content of free monomers in the resin composition to a specified range, suppress the reduction in insulation caused by free monomers, and maintain high insulation.

[0082] In addition, the content of unsaturated carboxylic acids is detailed in the examples and can be determined, for example, by directly measuring the resin composition using NMR.

[0083] (The content of free monomers in the resin composition)

[0084] The resin composition contains free monomers from the modified polymer (C), with the free monomer content being 1.0 × 10⁻⁶. -6 The content is ≥ 0.05% by mass and ≤ 0.05% by mass. As mentioned above, although the free monomer has an unsaturated carboxylic acid group and exhibits a charge-trapping effect, when its amount increases, the free monomer itself will act as a charge carrier, sometimes resulting in a decrease in insulation. In this embodiment, by adjusting the content of the free monomer to a specified range, it is possible to obtain the charge-trapping effect brought by the free monomer while suppressing its role as a charge carrier. That is, it is possible to maintain a high level of insulation in the resin composition. From the viewpoint of further improving insulation, the content of the free monomer can also be 1.0 × 10⁻⁻⁻⁶. 5 For substances exceeding 5.0% by mass and below 0.04% by mass, a value of 5.0 × 10⁻⁶ is also acceptable. -4 The content of free monomers in the resin composition is greater than 0.03% by mass and less than 0.03% by mass. Furthermore, the content of free monomers in the resin composition is detailed in the examples. For example, the free monomers can be dissolved by dissolving the resin composition in an organic solvent, and then... 1 The determination can be made by H-NMR. Alternatively, in cases where the free monomer is present in trace amounts, it can be extracted using organic solvents and then determined by liquid chromatography.

[0085] (Melting point of the resin composition)

[0086] The resin composition contains acrylic resin (A), thermoplastic elastomer (B), and modified polymer (C) with different melting points, and has a specified melting point. The melting point of the resin composition also serves as an indicator of the addition ratio of each component. From the viewpoint of obtaining the insulation, flexibility, and other properties required for the insulating layer by containing each component in a high-level balance, the melting point of the resin composition is preferably 130°C or higher and 170°C or lower.

[0087] (Mix ratio)

[0088] For the proportions of acrylic resin (A), thermoplastic elastomer (B), and modified polymer (C) in the resin composition, as long as the content of unsaturated carboxylic acid in the resin composition is 0.01% by mass or more, and the content of free monomer is 1.0 × 10⁻⁶, it is acceptable. -6 There are no particular limitations on the content of components, which is more than 0.05% by mass and less than 0.05% by mass. For example, it is preferable to adjust the content of each component appropriately according to the amount of unsaturated carboxylic acid modification in component (C) or the amount of free monomer contained in component (C) so as to obtain insulation, flexibility, etc. in a good balance at a high level.

[0089] Regarding the resin composition, for example, when the total content of acrylic resin (A), thermoplastic elastomer (B), and modified polymer (C) is set to 100 parts by mass, the content of component (A) is preferably 55 parts by mass or more and 90 parts by mass or less, the content of component (B) is preferably 10 parts by mass or more and 45 parts by mass or less, and the content of component (C) is preferably 1 part by mass or more and 10 parts by mass or less. By using such contents, the melting point of the resin composition can be adjusted to a specified range, while the content of unsaturated carboxylic acids and free monomers in the resin composition can be easily adjusted to a specified range.

[0090] (2) Power cables

[0091] Next, use Figure 1 The power cable of this embodiment will be described. Figure 1 This is a cross-sectional view of the power cable involved in this embodiment, orthogonal to the axial direction.

[0092] The power cable 10 in this embodiment is configured as a so-called solid-insulated power cable. Furthermore, the power cable 10 in this embodiment is configured to be laid on land (in a conduit), in water, or underwater. Additionally, the power cable 10 is used for direct current (DC), for example.

[0093] Specifically, the power cable 10 has, for example, a conductor 110, an inner semiconducting layer 120, an insulation layer 130, an outer semiconducting layer 140, a shielding layer 150, and a sheath 160.

[0094] (Conductor (Conductive Part))

[0095] Conductor 110 is formed, for example, by twisting together multiple conductor cores (conductive cores) containing pure copper, copper alloy, aluminum or aluminum alloy.

[0096] (Internal semiconductive layer)

[0097] An internal semiconductive layer 120 is provided to cover the outer periphery of the conductor 110. Furthermore, the internal semiconductive layer 120 is semiconductive, configured to suppress electric field concentration on the surface side of the conductor 110. The internal semiconductive layer 120 may contain at least one of, for example, ethylene-based copolymers such as ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-butyl acrylate copolymer, and ethylene-vinyl acetate copolymer, thermoplastic elastomers, and the aforementioned low-crystallinity resins, and may also contain conductive carbon black.

[0098] (Insulating layer)

[0099] The insulating layer 130 is configured to cover the outer periphery of the inner semiconductive layer 120 and is formed from the aforementioned resin composition. For example, the insulating layer 130 is formed by extruding the resin composition.

[0100] (Outer semiconductive layer)

[0101] The outer semiconductive layer 140 is configured to cover the outer periphery of the insulating layer 130. Furthermore, the outer semiconductive layer 140 is semiconductive and configured to suppress electric field concentration between the insulating layer 130 and the shielding layer 150. The outer semiconductive layer 140 is, for example, made of the same material as the inner semiconductive layer 120.

[0102] (Shielding layer)

[0103] The shielding layer 150 is configured to cover the outer periphery of the outer semiconductive layer 140. The shielding layer 150 is formed, for example, by winding copper tape, or by forming a wire shield with multiple soft copper wires wound around it. Alternatively, a tape made of adhesive tape or the like can be wound around the inner and outer sides of the shielding layer 150.

[0104] (jacket)

[0105] The sheath 160 is configured to cover the outer periphery of the shielding layer 150. The sheath 160 is made of, for example, polyvinyl chloride or polyethylene.

[0106] In addition, if the power cable 10 of this embodiment is an underwater cable or a submarine cable, it may have a metal water-blocking layer such as an aluminum cladding layer or wire armor at a position further outward than the shielding layer 150.

[0107] On the other hand, the power cable 10 of this embodiment may, for example, not have a water-blocking layer at a position further outward than the shielding layer 150. That is, the power cable 10 of this embodiment may also be constructed with a non-completely water-blocking structure.

[0108] (Specific dimensions, etc.)

[0109] The specific dimensions of the power cable 10 are not particularly limited. 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 insulation 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.

[0110] (3) Various characteristics of cables

[0111] In this embodiment, by forming the insulating layer 130 with the above-described resin composition, high insulation can be stably obtained in the insulating layer 130.

[0112] Specifically, the insulating layer 130 of this embodiment, for example, meets the following insulation performance requirements measured under high temperature and high electric field conditions. Furthermore, this measurement is performed, for example, using a sheet sampled from the center portion of the insulating layer 130 in the thickness direction. The thickness of the insulating layer 130 sheet at this time is, for example, 0.2 mm.

[0113] Regarding the insulating layer 130, the space charge accumulation measured under conditions of 90°C and a DC electric field of 40 kV / mm is preferably 100% or less, and may also be 35% or less. Furthermore, the space charge accumulation measured under conditions of 90°C and a DC electric field of 80 kV / mm is preferably 100% or less, and may also be 35% or less.

[0114] The space charge accumulation is determined using the current-integrated charge method. In this method, charge is accumulated in a measuring capacitor connected in series with the sheet material used as the sample, and the amount of charge is evaluated as the integral value of the current. 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 charge Q is calculated based on the amount of charge after 300 seconds. 300 The amount of space charge accumulated is calculated using the following formula, based on the amount of charge Q0 immediately after application (0 seconds).

[0115]

[0116] Furthermore, the volume resistivity of the insulating layer 130 sheet, measured at a temperature of 90°C and a DC electric field of 40 kV / mm or at a temperature of 90°C and a DC electric field of 80 kV / mm, is preferably, for example, 1.0 × 10⁻⁶. 14 Ω·cm or higher, or 8.2×1014 Ω·cm or higher.

[0117] Furthermore, the DC breakdown electric field strength of the insulating layer 130 sheet, measured at a temperature of 90°C, is preferably 160 kV / mm or higher, or may be 200 kV / mm or higher.

[0118] (4) Manufacturing method of power cables

[0119] Next, use Figure 2 The manufacturing method of the power cable according to this embodiment will be described. Figure 2 This is a flowchart illustrating a method for manufacturing a power cable according to one embodiment of the present disclosure. Hereinafter, the steps will be abbreviated as "S".

[0120] (S100: Resin composition preparation process)

[0121] First, a resin composition for forming the insulating layer 130 is prepared.

[0122] In this embodiment, a resin composition is prepared by mixing, for example, an propylene resin (A), a thermoplastic elastomer (B), and a modified polymer (C), along with other additives (antioxidants, etc.) added as needed. Here, it is preferable to determine the addition ratio of each component when mixing components (A) to (C) in a manner that ensures the content of unsaturated carboxylic acids and free monomers from component (C) falls within a specified range. Specifically, it is preferable to determine the addition ratio of each component based on the amount of unsaturated carboxylic acid modification and the content of free monomers in the component (C) used. For example, it is preferable to add 55 parts by mass and 90 parts by mass of component (A), 10 parts by mass and 45 parts by mass of component (B), and 1 part by mass and 10 parts by mass of component (C).

[0123] (S200: Conductor preparation process)

[0124] On the other hand, a conductor 110 is prepared to be formed by twisting together multiple conductor cores.

[0125] (S300: Cable core forming process (extrusion process, insulation layer forming process))

[0126] Once the resin composition preparation step S100 and the conductor preparation step S200 are completed, the above-described resin composition is used to form an insulating layer 130 by covering the outer periphery of the conductor 110 with a thickness of, for example, 3 mm or more.

[0127] In this embodiment, for example, a three-layer co-extruder is used to simultaneously form an inner semiconductive layer 120, an insulating layer 130, and an outer semiconductive layer 140.

[0128] Specifically, an internal semiconductive layer composition, for example, is fed into extruder A, which forms the internal semiconductive layer 120, in a three-layer co-extruder. The same resin composition is fed into extruder B, which forms the insulating layer 130. An external semiconductive layer composition, for example, containing the same material 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. Then, the extrudates from extruders A through C are guided to a common die head, and the internal semiconductive layer 120, the insulating layer 130, and the external semiconductive layer 140 are simultaneously extruded from the inside to the outside around the conductor 110. This forms the extruded material that becomes the cable core.

[0129] Then, for example, the material is extruded by cooling with water.

[0130] Through the above cable core forming process S300, a cable core consisting of a conductor 110, an inner semi-conductive layer 120, an insulating layer 130, and an outer semi-conductive layer 140 is formed.

[0131] (S400: Shielding layer formation process)

[0132] Once the cable core is formed, a shielding layer 150 is formed outside the outer semiconductive layer 140, for example, by winding copper tape.

[0133] (S500: Sheath Forming Process)

[0134] Once the shielding layer 150 is formed, a sheath 160 is formed on the outer periphery of the shielding layer 150 by feeding vinyl chloride into an extruder and extruding it.

[0135] Through the above processes, a power cable 10, which is a solid insulated power cable, is manufactured.

[0136] (5) Effects of this implementation method

[0137] According to this embodiment, one or more of the effects shown below are achieved.

[0138] (a) The resin composition of this embodiment is configured to contain components (A) to (C) above, wherein the content of unsaturated carboxylic acids from the modified polymer (C) is 0.01% by mass or more, and the content of free monomers from the modified polymer (C) is 1.0 × 10⁻⁶. -6 The content is 0.05% by mass or more and 0.05% by mass or less. According to component (C), an unsaturated carboxylic acid having a polar group can be introduced into the resin composition. By making the content of the unsaturated carboxylic acid in the resin composition 0.01% by mass or more, space charge can be captured and its accumulation suppressed. Furthermore, although a free monomer from component (C) is introduced into the resin composition, in this embodiment, its content is 1.0 × 10⁻⁶. -6The content is 1% or more and 0.05% or less by mass. Therefore, while suppressing the effect of free monomers as charge carriers, charge trapping is achieved. Furthermore, since the free monomers are uniformly dispersed in the resin composition, the charge trapping effect can be obtained uniformly and stably. As a result, even at high temperatures, the space charge trapping effect of component (C) can be stably maintained in the resin composition, suppressing the local accumulation of space charge. Therefore, by forming the insulating layer 130 with the above-described resin composition, high insulation performance can be stably obtained. Specifically, even at high temperatures, the insulating layer 130 can have high withstand voltage. That is, stable DC power transmission can be achieved with the power cable 10 of this embodiment.

[0139] (b) In the resin composition, the thermoplastic elastomer (B) and the modified polymer (C) can be finely dispersed in the acrylic resin (A), or they can be made compatible. That is, components (B) and (C) can be uniformly distributed in the resin composition. As a result, the excessive crystallization growth of component (A) can be suppressed by component (B), and the resin composition can be endowed with softness. Furthermore, space charge can be more uniformly captured in the resin composition by component (C). That is, the resin composition can exhibit softness and insulation to a higher degree and with greater stability.

[0140] (c) The thermoplastic elastomer (B) is preferably an olefin-based thermoplastic elastomer having polyethylene or polypropylene olefin units as hard segments and ethylene-α-olefin copolymer units as soft segments, or it may be an olefin-based thermoplastic elastomer having polypropylene as a hard segment and ethylene-propylene rubber as a soft segment. With such a composition (B), the effects of (a) described above can be obtained more reliably.

[0141] (d) When the total content of the acrylic resin (A), thermoplastic elastomer (B), and modified polymer (C) is set to 100 parts by mass, the resin composition preferably contains 55 parts by mass and 90 parts by mass of component (A), 10 parts by mass and 45 parts by mass of component (B), and 1 part by mass and 10 parts by mass of component (C). Therefore, in the resin composition, the amount of unsaturated carboxylic acid modification from component (C) and the content of free monomers can be adjusted to the above-mentioned ranges. As a result, the effects described in (a) can be obtained more reliably.

[0142] (e) The modified polymer (C) preferably has an unsaturated carboxylic acid modification amount of 0.1% by mass or more and 10% by mass or less, and a free monomer content of 1.0 × 10⁻⁶. -4The content of unsaturated carboxylic acid and free monomer in the resin composition is 1.0% to 1.0% by mass. Based on this composition (C), the content of unsaturated carboxylic acid and free monomer in the resin composition can be easily adjusted to the above-mentioned range. As a result, the effects of (a) described above can be obtained more reliably.

[0143] (f) The MFR of the acrylic resin (A), thermoplastic elastomer (B), and modified polymer (C) preferably has a maximum and minimum difference of 300 g / 10 min or less. By setting the MFR in such a way that such a correlation is formed, the components can be more reliably dispersed or compatible during mixing.

[0144] (g) The resin composition is preferably non-crosslinked. Therefore, the resin composition can be recycled.

[0145] <Other embodiments of this disclosure>

[0146] The embodiments of this disclosure have been described in detail above, but this disclosure is not limited to the above embodiments and various changes can be made without departing from its spirit.

[0147] In the above embodiments, the case where the power cable 10 may not have a water-blocking layer has been described, but this disclosure is not limited to this case. The power cable 10 may also have a simple water-blocking layer. Specifically, the simple water-blocking layer is, for example, made of a metal laminate. The metal laminate has, for example, a metal layer made of aluminum or copper and an adhesive layer disposed on one or both sides of the metal layer. The metal laminate is, for example, wound longitudinally around the outer periphery of the cable core (outer periphery than the outer semiconductive layer). In addition, this water-blocking layer may be disposed at a position further outward than the shielding layer, or it may also serve as a shielding layer. With this configuration, the cost of the power cable 10 can be reduced.

[0148] In the above embodiments, the power cable 10 has been described as being laid on land, in water, or underwater, but this disclosure is not limited to this. For example, the power cable 10 may also be configured as a so-called overhead wire (overhead insulated wire).

[0149] In the above embodiment, three-layer co-extrusion is performed in the cable core forming process S300, but it can also be extruded layer by layer.

[0150] Example

[0151] Next, embodiments related to this disclosure will be described. These embodiments are examples of this disclosure, and this disclosure is not limited to these embodiments.

[0152] (1) Regarding materials

[0153] The materials used to prepare the resin composition are listed below.

[0154] As an acrylic resin (A), atactic polypropylene (PP1) was prepared; as a thermoplastic elastomer (B), an olefin-based thermoplastic elastomer (TPO) was prepared; and as a modified polymer (C), maleic acid-modified polypropylene (MAH-PP1) to (MAH-PP9) were prepared. For MAH-PP1 to MAH-PP9, substances with a specified amount of free monomer or substances treated to achieve a specified amount were prepared respectively. The physical properties of each component are described below.

[0155] PP1: Melting point 160℃, MFR 0.6g / 10min

[0156] • TPO: An olefin-based thermoplastic elastomer with polypropylene as the hard segment and ethylene-propylene rubber as the soft segment; melting point 140℃; MFR 3.2g / 10min

[0157] MAH-PP1: MFR 12g / 10min, melting point 160℃, maleic acid modification 0.8% by mass, free monomer content 2.0×10 -6 quality%

[0158] MAH-PP2: MFR 12g / 10min, melting point 160℃, maleic acid modification 0.8% by mass, free monomer content 1.0×10 -3 quality%

[0159] MAH-PP3: MFR 12g / 10min, melting point 160℃, maleic acid modification 0.8% by mass, free monomer content 2.0×10 -2 quality%

[0160] MAH-PP4: MFR 12g / 10min, melting point 160℃, maleic acid modification 0.8% by mass, free monomer 0.6% by mass

[0161] MAH-PP5: MFR 12g / 10min, melting point 160℃, maleic acid modification 0.8% by mass, free monomer 0.8% by mass

[0162] MAH-PP6: MFR 10g / 10min, melting point 160℃, maleic acid modification 0.1% by mass, free monomer content 4.0×10 -3 quality%

[0163] MAH-PP7: MFR 13g / 10min, melting point 160℃, maleic acid modification 0.2% by mass, free monomer content 4.0×10 -3 quality%

[0164] MAH-PP8: MFR 18g / 10min, melting point 160℃, maleic acid modification 1.0% by mass, free monomer content 4.0×10⁻⁶ -3 quality%

[0165] MAH-PP9: MFR 20g / 10min, melting point 160℃, maleic acid modification 2.0% by mass, free monomer content 4.0×10⁻⁶ -3 quality%

[0166] In addition, hindered phenolic pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (molecular weight 1178) was prepared as an antioxidant as another additive.

[0167] (2) Preparation of resin composition

[0168] The materials described above were fed into an extruder in the amounts shown in Tables 1 and 2 below, and were heated, mixed, and granulated in the extruder to produce samples 1-A to 5-A and samples 1-B to 4-B. In each sample, the amount of antioxidant was set to 0.1 parts by mass.

[0169] Table 1

[0170]

[0171] Table 2

[0172]

[0173] (3) Fabrication of power cables

[0174] Next, a conductor was prepared by twisting together conductor cores made of a thin copper alloy with a diameter of 14 mm. Once the conductor was prepared, an inner semiconductive layer resin composition containing an ethylene-ethyl acrylate copolymer, an insulation layer resin composition prepared in Tables 1 and 2, and an outer semiconductive layer resin composition made of the same material as the inner semiconductive layer resin composition were fed into extruders A to C, respectively. The extrudates from extruders A to C were guided to a common die head, and the inner semiconductive layer, insulation layer, and outer semiconductive layer were simultaneously extruded from the inside to the outside of the conductor. Thus, a sample of a power cable having a conductor, an inner semiconductive layer, an insulation layer, and an outer semiconductive layer from the center outwards was produced.

[0175] (4) Evaluation

[0176] Samples were cut from the insulation layer of the manufactured power cable. For the resin composition constituting the insulation layer, the modification amount of unsaturated carboxylic acid (maleic acid), the content of free monomers, space charge characteristics, volume resistivity, and DC breakdown strength were evaluated. The evaluation methods are described below.

[0177] (Modification amount of unsaturated carboxylic acids)

[0178] The content of unsaturated carboxylic acid (maleic acid) in the insulation layer was evaluated using a leaching test with an organic solvent. Specifically, firstly, sheet-like samples were collected from the insulation layer of the power cable. Next, the sample sheet was dissolved in hot p-xylene as the organic solvent, and the substance that had been redeprecipitated using acetone was recovered. The unsaturated carboxylic acid sites in the resin composition were then methylated. 1 H-NMR determination was performed, and the amount of unsaturated carboxylic acid modification was determined based on the obtained spectrum. Specifically, in the obtained spectrum, the peak area S1 from the polymer backbone and the peak area S2 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.

[0179] (Content of free monomers)

[0180] The content of free monomers in the insulation layer was evaluated using an immersion test in an organic solvent. Specifically, firstly, sheet-like samples were collected from the insulation layer of the power cable. Then, the sample sheet was dissolved in an organic solvent and subjected to... 1 The content of free monomers was determined by ¹H-NMR. Alternatively, when the free monomers were present in trace amounts, extraction with an organic solvent was performed, and the content was determined by liquid chromatography.

[0181] (Space charge properties)

[0182] The space charge characteristics of the insulation layer were evaluated by the amount of space charge accumulated in the insulation layer. The amount of space charge accumulated was determined by the current-integrated charge method. Specifically, firstly, a sheet-like sample was collected from the insulation layer of a power cable. Then, the sample was connected in series with a measuring capacitor, and the charge was accumulated in the measuring capacitor. The amount of charge was measured as the integral value of the current. In this embodiment, 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 The space charge accumulation at 90°C and 40 kV / mm DC electric field, and the space charge accumulation at 90°C and 80 kV / mm DC electric field, were calculated using the following formulas, along with the initial charge Q0 immediately after application (0 seconds). The following conditions were evaluated: A (best), space charge accumulation exceeding 35% but below 100%, B (good), and space charge accumulation exceeding 100%, C (poor).

[0183]

[0184] (Volume resistivity)

[0185] The volume resistivity of the insulation layer was measured using sheet-like samples taken from the insulation layer of power cables, similar to the space charge characteristics. Specifically, the sample sheet was immersed in silicone oil at 90°C, and a DC electric field of 40 kV / mm or 80 kV / mm was applied to the sample sheet using a 25 mm diameter flat electrode, thereby measuring the volume resistivity. The volume resistivity was 8.2 × 10⁻⁶. 14 For values ​​above Ω·cm, let's define A (optimal), and set the volume resistivity to 1×10⁻⁶. 14 Ω·cm or higher and less than 8.2×10 14 The case with Ω·cm is designated as B (good), and the volume resistivity is less than 1×10. 14 The Ω·cm value was set as C (poor) and evaluated.

[0186] (DC breakdown strength)

[0187] The DC breakdown strength of the insulation layer was measured using sheet-like specimens collected from the insulation layer of power cables, similar to the space charge characteristics. Specifically, the specimen was first immersed in silicone oil at 90°C, and a 25mm diameter flat electrode was used to increase the applied voltage at a rate of 4 kV / min. Then, when the specimen reached insulation breakdown, the DC breakdown strength was determined by dividing the applied voltage by the thickness of the specimen. Cases with a DC breakdown strength of 200 kV / mm or higher were designated A (best), those with a DC breakdown strength of 160 kV / mm or higher but less than 200 kV / mm were designated B (good), and those with a DC breakdown strength less than 160 kV / mm were designated C (poor).

[0188] (5) Evaluation Results

[0189] The above evaluation was performed on each sample, and the evaluation results are summarized in Tables 1 and 2, respectively. In Tables 1 and 2, MAH content represents the content of unsaturated carboxylic acids in the resin composition, and free MAH content represents the content of free monomers in the resin composition.

[0190] As shown in Table 1, in Samples 1-A to 5-A, resin compositions were prepared by uniformly setting the content of unsaturated carboxylic acids (MAH content) in the resin composition to 0.04% by mass, while varying the content of free monomers (free MAH content). In Sample 1-A, it was confirmed that although the content of unsaturated carboxylic acids was the specified amount, the content of free monomers was 1 × 10⁻⁶. -7 mass%, less than 1.0 × 10 -6The free monomer content is 1% by mass, resulting in high space charge accumulation and low volume resistivity, thus failing to achieve high insulation properties. This can be attributed to insufficient free monomers, which cannot adequately capture charge. Furthermore, samples 2-A to 5-A were found to contain 1 × 10⁻⁶ free monomers. -6 With a mass percentage of over 90%, it can achieve charge trapping through the combination of free monomers and unsaturated carboxylic acids, reducing space charge accumulation and increasing volume resistivity and DC breakdown electric field strength even at high temperatures. In other words, it has been confirmed that high insulation can be stably obtained in the resin composition. Therefore, it is evident that the insulation layer can achieve high withstand voltage even at high temperatures.

[0191] Furthermore, based on samples 2-A to 5-A, it was confirmed that by keeping the free monomer content below 0.05% by mass, the space charge accumulation can be reduced, the volume resistivity increased, and the insulation performance at high temperatures improved. Moreover, since higher insulation performance was achieved in samples 3-A and 4-A compared to sample 5-A, it is understood that the free monomer content is preferably below 0.04% by mass, and more preferably below 0.03% by mass. It was also confirmed that when the free monomer content exceeds 0.05% by mass, the space charge accumulation is high, the volume resistivity decreases, and the insulation performance significantly decreases. This can be attributed to the free monomer acting as a charge carrier.

[0192] Furthermore, as shown in Table 2, in samples 1-B to 4-B, the content of free monomers was uniformly set at 2 × 10⁻⁶. -4 Resin compositions were prepared by varying the content of unsaturated carboxylic acids, with each composition being % by mass. In Sample 1-B, it was confirmed that due to the modification amount of unsaturated carboxylic acids being 0.005% by mass (less than 0.01% by mass), there was high space charge accumulation, low volume resistivity, and low insulation. In Sample 1-B, although a specified amount of free monomer was contained, the low modification amount of unsaturated carboxylic acids prevented the introduction of sufficient polar groups into the resin composition, thus failing to maintain high insulation. On the other hand, in Samples 2-B to 4-B, since the modification amount of unsaturated carboxylic acids was 0.01% by mass or more, the charge trapping effect of the unsaturated carboxylic acids was obtained, reducing space charge accumulation even at high temperatures and increasing volume resistivity and DC breakdown electric field strength. Therefore, high insulation can be obtained in Samples 2-B to 4-B.

[0193] Furthermore, based on samples 2-B to 4-B, it was confirmed that the higher the content of unsaturated carboxylic acid in the resin composition, the better the insulation performance at high temperatures. Specifically, it was confirmed that the content of unsaturated carboxylic acid is preferably 0.01% by mass or more, and more preferably 0.03% by mass. It was also confirmed that when the content of unsaturated carboxylic acid is excessively increased, the content of the modified polymer (C) increases, thereby reducing the ratio of acrylic resin (A) and thermoplastic elastomer (B), making it impossible to obtain the required flexibility and mechanical strength of the insulation layer. Therefore, from the viewpoint of obtaining a good balance of insulation performance and flexibility at high temperatures, it is preferable that the content of unsaturated carboxylic acid is 0.1% by mass or less.

[0194] Furthermore, in samples 1-A to 5-A and samples 1-B to 4-B, it was confirmed that the thermoplastic elastomer (B) and modified polymer (C) could be finely dispersed in or compatible with the propylene resin (A). Therefore, it can be concluded that the required strength and flexibility for the insulation layer of power cables can be obtained.

[0195] As described above, it has been confirmed that by mixing acrylic resin (A), thermoplastic elastomer (B), and modified polymer (C) into the resin composition, and adjusting the content of unsaturated carboxylic acid contained in the modified polymer (C) and the content of free monomers from the modified polymer (C) to a specified range, even under high-temperature environments, the accumulation of space charge can be suppressed, and the volume resistivity and DC breakdown electric field strength can be improved. That is, it has been confirmed that high insulation can be stably obtained in the resin composition. Therefore, even under high-temperature environments, the insulation layer can achieve high withstand voltage, enabling stable DC power transmission through power cables.

[0196] <Postscript>

[0197] The following is the manner in which this note is published.

[0198] (Postscript 1)

[0199] A resin composition comprising an propylene-based resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids.

[0200] The amount of unsaturated carboxylic acid from the modified polymer (C) in the resin composition is 0.01% by mass or more.

[0201] The content of free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and a molecular weight of less than 500 in the resin composition is 1.0 × 10⁻⁶. -6 More than 0.05% by mass and less than 0.05% by mass.

[0202] (Postscript 2)

[0203] A power cable having:

[0204] conductors, and

[0205] An insulating layer formed of a resin composition, covering the conductor.

[0206] The resin composition contains an propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids.

[0207] The amount of unsaturated carboxylic acid from the modified polymer (C) in the above resin composition is 0.01% by mass or more.

[0208] The content of free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and a molecular weight of 500 or less in the above resin composition is 1.0 × 10⁻⁶. -6 More than 0.05% by mass and less than 0.05% by mass.

[0209] (Note 3)

[0210] In Appendix 2,

[0211] The above-mentioned thermoplastic elastomer (B) is an olefin-based thermoplastic elastomer having at least one olefin unit selected from polyethylene and polypropylene as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment.

[0212] (Postscript 4)

[0213] In Appendix 2 or Appendix 3,

[0214] When the total content of acrylic resin (A), thermoplastic elastomer (B) and modified polymer (C) is set to 100 parts by mass, the resin composition contains 55 parts by mass and 90 parts by mass of acrylic resin (A), 10 parts by mass and 45 parts by mass of thermoplastic elastomer (B), and 1 part by mass and 10 parts by mass of modified polymer (C).

[0215] (Note 5)

[0216] In any of the notes 2 to 4

[0217] The modified polymer (C) contains an amount of unsaturated carboxylic acid of 0.1% to 10% by mass and a free monomer content of 1.0 × 10⁻⁶. -4 The mass percentage is above 0.5% and below 0.1%.

[0218] (Note 6)

[0219] In any of the notes 2 to 5

[0220] The above-mentioned acrylic resin (A) has a melting point of 130°C or higher and 170°C or lower, and a melt flow rate of 0.1 g / 10 min or higher and 5.0 g / 10 min or lower.

[0221] (Note 7)

[0222] In any of the notes 2 to 6

[0223] The aforementioned propylene resin (A) is atactic polypropylene.

[0224] (Note 8)

[0225] In any of the notes 2 to 7

[0226] The above-mentioned thermoplastic elastomer (B) does not have a melting point or a melting point below 165°C, and a melt flow rate of more than 0.1 g / 10 min and less than 5.0 g / 10 min.

[0227] (Note 9)

[0228] In any of the notes 2 to 8

[0229] The above-mentioned modified polymer (C) has a melting point of 130°C or higher and 165°C or lower, and a melt flow rate of 0.1 g / 10 min or higher and 500 g / 10 min or lower.

[0230] (Postscript 10)

[0231] In any of the notes 2 to 9

[0232] The melting point of the above resin composition is above 130°C and below 170°C.

[0233] (Postscript 11)

[0234] A method for manufacturing a power cable, comprising:

[0235] The process of preparing a resin composition for forming an insulating layer; and

[0236] The process of forming an insulating layer by means of the above-described resin composition in a manner that covers the outer periphery of a conductor.

[0237] In the process of preparing the above-mentioned resin composition, the propylene resin (A) having propylene units, the thermoplastic elastomer (B), and the modified polymer (C) having propylene units and modified with unsaturated carboxylic acids are mixed such that the amount of unsaturated carboxylic acid modification from the modified polymer (C) in the above-mentioned resin composition is 0.01% by mass or more, and the content of free monomers having unsaturated carboxylic acid groups and having a molecular weight of 500 or less from the modified polymer (C) is 1.0 × 10⁻⁶. -6 Mixing shall be carried out in a manner that is between 0.05% and 1% by mass.

[0238] (Postscript 12)

[0239] In Appendix 11,

[0240] The modified amount of the unsaturated carboxylic acid in the modified polymer (C) is 0.1% by mass or more and 10% by mass or less, and the content of the free monomer is 1.0 × 10⁻⁶. -4 The mass percentage is above 0.5% and below 0.1%.

[0241] (Postscript 13)

[0242] In Appendix 11 or Appendix 12,

[0243] In the process of preparing the above-mentioned resin composition, when the total content of the above-mentioned acrylic resin (A), the above-mentioned thermoplastic elastomer (B) and the above-mentioned modified polymer (C) is set to 100 parts by mass, the acrylic resin (A) is mixed in such a way that the content of the above-mentioned acrylic resin (A) is 55 parts by mass or more and 90 parts by mass or less, the content of the above-mentioned thermoplastic elastomer (B) is 10 parts by mass or more and 45 parts by mass or less, and the content of the above-mentioned modified polymer (C) is 1 part by mass or more and 10 parts by mass or less.

[0244] Explanation of reference numerals in the attached figures

[0245] 10: Power cables;

[0246] 110: Conductor;

[0247] 120: Internal semiconductive layer;

[0248] 130: Insulation layer;

[0249] 140: External semiconductive layer;

[0250] 150: Shielding layer;

[0251] 160: Sheath.

Claims

1. A resin composition comprising an propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids. The content of the unsaturated carboxylic acid from the modified polymer (C) in the resin composition is 0.01% by mass or more. The resin composition contains 1.0 × 10⁻⁶ free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and a molecular weight of less than 500. -6 More than 0.05% by mass and less than 0.05% by mass.

2. A power cable, comprising: Conductors, and An insulating layer formed of a resin composition, covering the conductor. The resin composition comprises an propylene resin (A) having propylene units, a thermoplastic elastomer (B), and a modified polymer (C) having propylene units and modified with unsaturated carboxylic acids. The content of the unsaturated carboxylic acid from the modified polymer (C) in the resin composition is 0.01% by mass or more. The resin composition contains 1.0 × 10⁻⁶ free monomers from the modified polymer (C) having unsaturated carboxylic acid groups and a molecular weight of less than 500. -6 More than 0.05% by mass and less than 0.05% by mass.

3. The power cable according to claim 2, wherein, The thermoplastic elastomer (B) is an olefin-based thermoplastic elastomer having at least one olefin unit selected from polyethylene and polypropylene as a hard segment and an ethylene-α-olefin copolymer unit as a soft segment.

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

5. The power cable according to any one of claims 2 to 4, wherein, The modified polymer (C) contains an amount of unsaturated carboxylic acid of 0.1% to 10% by mass and a free monomer content of 1.0 × 10⁻⁶. -4 The mass percentage is above 0.5% and below 0.1%.

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