Power cable

Abstract

This power cable is provided with: a conductor; and an insulating layer that is provided so as to cover the outer periphery of the conductor and that is configured from a resin composition containing polyethylene. The insulating layer contains, among polyethylene, a crosslinked polyethylene that is insoluble in xylene at 120° C, and uncrosslinked polyethylene that can be eluted in xylene at 120° C. The uncrosslinked polyethylene has a number average molecular weight of 5000 to 25000.

Description

power cables

[0001] The present disclosure relates to power cables.

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

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

[0004] According to one aspect of the present disclosure, there is provided a power cable comprising: a conductor; and an insulating layer that is provided to cover the outer periphery of the conductor and is made of a resin composition containing polyethylene, wherein the insulating layer contains, among the polyethylene, cross-linked polyethylene that is insoluble in xylene at 120°C and uncross-linked polyethylene that is soluble in xylene at 120°C, and the number average molecular weight of the uncross-linked polyethylene is 5,000 or more and 25,000 or less.

[0005] 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 diagram showing molecular weight distribution. Fig. 3 is a flowchart showing a method for manufacturing a power cable according to an embodiment of the present disclosure. Fig. 4 is a cross-sectional view showing a cross-linking device according to an embodiment of the present disclosure.

[0006] [Problem to be Solved by the Present Disclosure] An object of the present disclosure is to improve the heat aging resistance of an insulating layer.

[0007] [Advantages of the Present Disclosure] According to the present disclosure, it is possible to improve the heat aging resistance of the insulating layer.

[0008] [Explanation of Embodiments of the Present Disclosure] <Insights Obtained by the Inventors> First, an outline of insights obtained by the inventors will be described.

[0009] The insulation layer of a typical power cable does not consist of completely cross-linked polyethylene but contains a certain amount of uncross-linked polyethylene. The proportion of cross-linked polyethylene in the insulation layer is measured as the "gel fraction," which is the ratio of the mass of components insoluble in xylene to the total mass of polyethylene in the insulation layer.

[0010] Even when the gel fraction of the insulating layer is low, i.e., when the content of uncrosslinked polyethylene in the insulating layer is high, the initial mechanical and electrical properties are hardly affected, and for this reason, the uncrosslinked polyethylene in the insulating layer has not received much attention in the past.

[0011] However, the inventors have found through their investigations that the uncrosslinked polyethylene in the insulating layer affects the heat aging resistance of the insulating layer.

[0012] The molecular chains of uncrosslinked polyethylene are more mobile than those of crosslinked polyethylene. Therefore, uncrosslinked polyethylene undergoes micro- or macro-Brownian motion due to heat, making it more susceptible to thermal degradation than crosslinked polyethylene. During this thermal degradation, hydrogen is extracted from the molecular chains of uncrosslinked polyethylene, generating radicals. Oxygen binds to the radicals of uncrosslinked polyethylene generated by the above-mentioned thermal degradation, forming degradation points. In this way, uncrosslinked polyethylene can become a degradation target.

[0013] On the other hand, thermally degraded uncrosslinked polyethylene can also be an attacking factor that further degrades crosslinked polyethylene. That is, when radicals are generated in uncrosslinked polyethylene exposed to heat for a long period of time, the radicals may abstract hydrogen from the crosslinked polyethylene and sever the main chain of the crosslinked polyethylene. In this way, the thermally degraded portion of the uncrosslinked polyethylene can spread the degradation to the crosslinked polyethylene, accelerating the degradation.

[0014] As a result, the presence of uncrosslinked polyethylene in the insulating layer may affect the thermal aging of the insulating layer.

[0015] Therefore, in order to investigate the influence of uncrosslinked polyethylene on thermal aging of the insulating layer, the inventors conducted a thermal aging test (heating at 180°C for 21 days) in which the insulating layer was heated at a higher temperature and for a longer time than in conventional thermal aging tests. As a result, it was found that the elongation and volume resistivity of the insulating layer after the thermal aging test in which the insulating layer was heated at 180°C for 21 days depended on the molecular weight of the uncrosslinked polyethylene in the insulating layer.

[0016] The inventors further investigated the crosslinking process as a new manufacturing method in order to adjust the molecular weight of the uncrosslinked polyethylene remaining in the insulating layer. As a result of further intensive investigation, the inventors applied the new manufacturing method and succeeded in obtaining a configuration in which the heat aging resistance of the insulating layer is improved by optimizing the molecular weight of the uncrosslinked polyethylene in the insulating layer.

[0017] The present disclosure is based on the above-mentioned findings of the inventors.

[0018] <Embodiments of the Present Disclosure> Next, embodiments of the present disclosure will be listed and described.

[0019] [1] A power cable according to one aspect of the present disclosure comprises: a conductor; and an insulating layer covering the outer periphery of the conductor and made of a resin composition containing polyethylene, wherein the insulating layer contains, among the polyethylene, cross-linked polyethylene that is insoluble in xylene at 120°C and uncross-linked polyethylene that is soluble in xylene at 120°C, and the number average molecular weight of the uncross-linked polyethylene is 5,000 or more and 25,000 or less. This configuration can improve the heat aging resistance of the insulating layer.

[0020] [2] The power cable according to [1] above, wherein the insulating layer has a gel fraction of 60% or more and 90% or less. By setting the gel fraction of the insulating layer to 60% or more, it is possible to suppress the excessive production of thermally degraded uncrosslinked polyethylene. On the other hand, by setting the gel fraction of the insulating layer to 90% or less, it is possible to ensure a sufficient amount of uncrosslinked polyethylene that functions as a degradation target.

[0021] [3] In the power cable according to the above [1] or [2], the weight-average molecular weight of the uncrosslinked polyethylene is 20,000 or more and 100,000 or less. This configuration can improve the heat aging resistance of the insulating layer.

[0022] [4] In the power cable according to any one of the above [1] to [3], the insulating layer contains an antioxidant having a molecular weight of 300 or more and 1100 or less. This configuration makes it possible to stably suppress oxidative degradation of the uncrosslinked polyethylene.

[0023] [5] The power cable according to any one of [1] to [4] above, wherein a sheet taken from the insulating layer is heated at 180°C for 21 days, and then the sheet has a tensile breaking elongation of 50% or more as measured at 25°C in accordance with JIS C3005:2014. This configuration makes it possible to maintain the flexibility required for a power cable for a long period of time.

[0024] [6] In the power cable according to any one of the above [1] to [5], a sheet taken from the insulating layer is heated at 180°C for 21 days, and then the volume resistivity of the sheet measured under conditions of a temperature of 90°C and a DC electric field of 10 kV / mm is 1 x 10 １２ This configuration makes it possible to maintain the insulating properties of the insulating layer required for power cables for a long period of time.

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

[0026] <One embodiment of the present disclosure> (1) Resin composition The resin composition of this embodiment is a material that constitutes the insulating layer 130 of the power cable 10 described below, and is a material before cross-linking. The resin composition contains, for example, a base resin, a cross-linking agent, an antioxidant, and other additives.

[0027] (Base Resin) The base resin (base polymer) refers to a resin component that constitutes the main component of a resin composition. The base resin of this embodiment contains, for example, polyethylene.

[0028] The polyethylene constituting the base resin is low-density polyethylene (LDPE: density 0.91 g / cm ３ 0.93g / cm or more ３ less than 0.92 g / cm), linear low-density polyethylene (LLDPE: density 0.92 g / cm ３ 0.945g / cm or more ３hereinafter), medium density polyethylene (MDPE: density 0.93 g / cm ３ 0.942g / cm or more ３ less than 0.942 g / cm), high density polyethylene (HDPE: density 0.942 g / cm ３ 0.97g / cm or more ３ Two or more of these may be used in combination.

[0029] In the present embodiment, the base resin may be at least one of LDPE and LLDPE, which can improve the insulation properties and mechanical properties of the insulating layer 130 of the power cable 10.

[0030] The melting point of the polyethylene is not particularly limited, but may be, for example, 90°C or higher and 135°C or lower.

[0031] The number average molecular weight of the polyethylene before crosslinking (i.e., the polyethylene as a raw material before being fed into the extruder) may be, for example, 30,000 or more and 60,000 or less. The weight average molecular weight of the polyethylene before crosslinking may be 150,000 or more and 250,000 or less.

[0032] In the cross-linking step S400 described below, the gel fraction of the insulating layer 130 after cross-linking, the molecular weight of the uncross-linked polyethylene remaining in the insulating layer 130 after cross-linking, etc. will change depending on how the above-mentioned polyethylene is cross-linked.

[0033] The base resin may contain, in addition to polyethylene as the main component, at least one of a copolymer of an olefin and a polar monomer, and an α-olefin copolymer. Examples of copolymers of an olefin and a polar monomer include ethylene-ethyl acrylate copolymer and ethylene-methyl acrylate copolymer. Examples of α-olefin copolymers include very low density polyethylene (VLDPE) and ethylene propylene rubber. Two or more of these may be combined and contained in the base resin.

[0034] The content of at least one of the copolymer of olefin and polar monomer and the α-olefin copolymer in the base resin is, for example, 0 to 20 parts by mass per 100 parts by mass of the base resin.

[0035] (Crosslinking Agent) The crosslinking agent is, for example, an organic peroxide. Examples of the organic peroxide include dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and 1,3-bis(t-butylperoxyisopropyl)benzene. Two or more of these may be used in combination.

[0036] The content of the crosslinking agent in the resin composition is not particularly limited, but is, for example, 0.1 parts by mass or more and 4.0 parts by mass or less relative to 100 parts by mass of the base resin. By setting the content of the crosslinking agent to 0.1 parts by mass or more, it is possible to sufficiently crosslink the insulating layer 130. On the other hand, by setting the content of the crosslinking agent to 4.0 parts by mass or less, it is possible to prevent thermal degradation components of the base resin from remaining inside the extruder during the manufacturing process of the power cable 10, and thereby prevent contamination of the extruder.

[0037] (Antioxidant) The antioxidant is configured to suppress oxidation of polyethylene in the extrusion step S300 and the cross-linking step S400, and to suppress thermal aging of polyethylene after production of the power cable 100. The antioxidant suitable for this embodiment will be described in detail later.

[0038] (Other Additives) The resin composition may further contain other additives such as a copper inhibitor, a lubricant, and a colorant.

[0039] (2) Power Cable Next, the power cable of this embodiment will be described with reference to FIG.

[0040] The power cable 10 of this embodiment is configured as a so-called solid insulated power cable. The power cable 10 may be used for either AC or DC.

[0041] 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 .

[0042] (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.

[0043] (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 near the surface of the conductor 110. The internal semiconductive layer 120 contains, for example, at least one of ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-butyl acrylate copolymer, ethylene-vinyl acetate copolymer, etc., and conductive carbon black.

[0044] (Insulating Layer) The insulating layer 130 is provided so as to cover the outer periphery of the internal semiconducting layer 120. The insulating layer 130 is formed by extrusion molding using the above-mentioned resin composition, for example.

[0045] In this embodiment, a cross-linking step S400 as a novel manufacturing method described later is performed to cross-link some polyethylene in the insulating layer 130, while other polyethylene remains uncross-linked. Furthermore, in this embodiment, the cross-linking step S400 as a novel manufacturing method described later optimizes the molecular weight of the uncross-linked polyethylene in the insulating layer 130. This will be described in detail later.

[0046] (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 configured of, for example, the same material as the inner semiconductive layer 120.

[0047] (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.

[0048] (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.

[0049] (3) Characteristics of the Insulating Layer of the Present Embodiment Next, the characteristics of the insulating layer 130 of the present embodiment will be described.

[0050] (Molecular weight of uncrosslinked polyethylene) In this embodiment, by the crosslinking process S400 as a new manufacturing method described later, the insulating layer 130 contains, among polyethylenes, for example, crosslinked polyethylene that is insoluble in xylene at 120°C and uncrosslinked polyethylene that can be dissolved in xylene at 120°C.

[0051] As a result of extensive research, the inventors have found an optimum range for the molecular weight of uncrosslinked polyethylene that can suppress thermal aging of insulating layer 130 .

[0052] Here, the molecular weight distribution of the uncrosslinked polyethylene in the insulating layer 130 in this embodiment will be described with reference to FIG. 2. FIG. 2 is a schematic diagram showing the molecular weight distribution. The "molecular weight distribution" referred to here refers to a distribution curve obtained by plotting the differential distribution value corresponding to the number of molecules against the molecular weight, as shown in FIG. 2, for example. The horizontal axis in FIG. 2 is expressed in logarithm. "aEb" in FIG. 2 is a×10 ｂ means.

[0053] The molecular weight distribution of the uncrosslinked polyethylene remaining in the insulating layer 130 is measured, for example, by the following procedure. First, a sheet taken from the insulating layer 130 is immersed in xylene, and the xylene in this state is heated at 120°C for 5 to 24 hours. This causes the uncrosslinked polyethylene to elute into the xylene. Next, the xylene solution into which the uncrosslinked polyethylene has been eluted is filtered using filter paper heated to 120°C. The filtered xylene solution is volatilized at 120°C. This yields a powder of uncrosslinked polyethylene that has been eluted into the xylene. After separating the uncrosslinked polyethylene, the powder is dissolved in a predetermined eluent for molecular weight measurement, and the molecular weight distribution of the uncrosslinked polyethylene is measured by gel permeation chromatography (GPC). Here, the molecular weight distribution of the uncrosslinked polyethylene is measured based on a calibration curve prepared using polystyrene (PS) as a standard sample.

[0054] As shown in Figure 2, the molecular weight distribution of the raw material polyethylene is broad, ranging from low molecular weight to high molecular weight. As described above, the number average molecular weight Mn0 of the raw material polyethylene is, for example, 30,000 or more and 60,000 or less.

[0055] Comparative Example 1 shows the molecular weight distribution of uncrosslinked polyethylene when a conventional crosslinking process is carried out. In Comparative Example 1, the polyethylene of the insulating layer is crosslinked at an appropriate temperature, so the polyethylene constituting the insulating layer is sufficiently crosslinked. On the other hand, in Comparative Example 1, the amount of remaining uncrosslinked polyethylene is excessively small. Therefore, in Comparative Example 1, the molecular weight of the uncrosslinked polyethylene is distributed in the low molecular weight range, and the peak of the molecular weight distribution of the uncrosslinked polyethylene is located in the low molecular weight range. As a result, the number average molecular weight MnR1 of the uncrosslinked polyethylene of Comparative Example 1 is, for example, less than 5,000.

[0056] When the number average molecular weight of the uncrosslinked polyethylene is less than 5,000 as in Comparative Example 1, the amount of low-molecular-weight uncrosslinked polyethylene, which is susceptible to thermal degradation, is excessively small. Therefore, the crosslinked polyethylene, rather than the uncrosslinked polyethylene, may directly become the target of degradation. As a result, the insulating layer is susceptible to thermal aging.

[0057] Comparative Example 2 shows the molecular weight distribution of uncrosslinked polyethylene when the crosslinking temperature is excessively low or excessively high. When the crosslinking temperature is excessively low, radicals are not generated, and the polyethylene constituting the insulating layer is not sufficiently crosslinked. On the other hand, when the crosslinking temperature is excessively high, the crosslinking agent volatilizes during the crosslinking process, or only the outermost surface of the insulating layer is excessively crosslinked, and the polyethylene is not sufficiently crosslinked throughout the insulating layer. Therefore, in Comparative Example 2, much of the raw polyethylene is not consumed in the crosslinking, and an excessive amount of uncrosslinked polyethylene remains. In Comparative Example 2, the molecular weight of the uncrosslinked polyethylene is distributed over a wide range, and the peak of the molecular weight distribution of the uncrosslinked polyethylene is located in the high molecular weight range. As a result, the number average molecular weight MnR2 of the uncrosslinked polyethylene of Comparative Example 2 is, for example, greater than 25,000.

[0058] When the number-average molecular weight of the uncrosslinked polyethylene exceeds 25,000, as in Comparative Example 2, excess uncrosslinked polyethylene remains across a wide molecular weight range, as described above. Therefore, excess uncrosslinked polyethylene remaining across a wide molecular weight range is susceptible to thermal degradation. Excessively thermally degraded uncrosslinked polyethylene may further attack the crosslinked polyethylene. Alternatively, when the number-average molecular weight of the uncrosslinked polyethylene exceeds 25,000, the difference in degradation behavior between crosslinked polyethylene and uncrosslinked polyethylene becomes smaller. From this perspective, the crosslinked polyethylene is also susceptible to direct thermal degradation. As a result, thermal aging may progress throughout the entire insulating layer.

[0059] In contrast, in the present embodiment, cross-linked polyethylene is sufficiently formed and an appropriate amount of uncross-linked polyethylene remains in the insulating layer 130 by the cross-linking step S400 as a novel manufacturing method described later. As a result, as shown in Figure 2, the molecular weight distribution of the uncross-linked polyethylene of this embodiment is located, for example, between the molecular weight distributions of Comparative Example 1 and Comparative Example 2.

[0060] Specifically, in this embodiment, the number average molecular weight Mn of the uncrosslinked polyethylene in the insulating layer 130 is, for example, 5,000 or more and 25,000 or less.

[0061] In this embodiment, by setting the number average molecular weight Mn of the uncrosslinked polyethylene to 5,000 or more, it is possible to suppress the excessive formation of crosslinked polyethylene while leaving a certain amount of low-molecular-weight uncrosslinked polyethylene, which is susceptible to thermal degradation. This allows the low-molecular-weight uncrosslinked polyethylene to be targeted for degradation, thereby suppressing the attack of degradation on the crosslinked polyethylene. As a result, it is possible to suppress thermal aging of the insulating layer.

[0062] On the other hand, in this embodiment, by setting the number average molecular weight Mn of the uncrosslinked polyethylene to 25,000 or less, sufficient crosslinked polyethylene is formed, and excess residual uncrosslinked polyethylene, which is susceptible to thermal degradation across a wide molecular weight range, can be suppressed. This makes it possible to suppress further attack on the crosslinked polyethylene caused by thermally degraded uncrosslinked polyethylene. Furthermore, by setting the number average molecular weight Mn of the uncrosslinked polyethylene to 25,000 or less, it is possible to maintain the difference in degradation behavior between crosslinked polyethylene and uncrosslinked polyethylene. This makes it possible to maintain the uncrosslinked polyethylene in a state susceptible to degradation and suppress direct thermal degradation of the crosslinked polyethylene. As a result, it is possible to suppress the progression of thermal aging throughout the entire insulating layer 130.

[0063] In this embodiment, the weight-average molecular weight Mw of the uncrosslinked polyethylene in the insulating layer 130 may be, for example, 20,000 or more and 100,000 or less. By setting the weight-average molecular weight Mw of the uncrosslinked polyethylene to 20,000 or more, low-molecular-weight uncrosslinked polyethylene can be targeted for degradation, and degradation attacks on the crosslinked polyethylene can be suppressed. On the other hand, by setting the weight-average molecular weight Mw of the uncrosslinked polyethylene to 100,000 or less, excessive thermal degradation of uncrosslinked polyethylene over a wide molecular weight range can be suppressed. This makes it possible to suppress further attack on the crosslinked polyethylene caused by thermally degraded uncrosslinked polyethylene.

[0064] In this embodiment, the ratio Mw / Mn of the weight average molecular weight Mw to the number average molecular weight Mn of the uncrosslinked polyethylene in the insulating layer 130 may be, for example, 2 or more and 8 or less, or 3 or more and 7 or less.

[0065] The "Mw / Mn" referred to here is a value also called polydispersity index, and is defined as an index value (numerical value) indicating the extent of the broadness of the molecular weight distribution. The larger the Mw / Mn, the broader the molecular weight distribution.

[0066] In this embodiment, by setting the Mw / Mn of the uncrosslinked polyethylene to 2 or more, or 3 or more, it is possible to ensure a sufficient amount of low-molecular-weight uncrosslinked polyethylene that functions as a degradation target. This makes it possible to stably suppress degradation attacks on the crosslinked polyethylene. On the other hand, in this embodiment, by setting the Mw / Mn of the uncrosslinked polyethylene to 8 or less, or 7 or less, it is possible to suppress excessive production of thermally degraded uncrosslinked polyethylene over a wide molecular weight range. This makes it possible to stably suppress further attacks on the crosslinked polyethylene caused by thermally degraded uncrosslinked polyethylene.

[0067] (Gel Fraction) In this embodiment, the gel fraction (degree of cross-linking) of the insulating layer 130 is appropriately adjusted by a cross-linking step S400 as a new manufacturing method described later.

[0068] Specifically, in this embodiment, the gel fraction of the insulating layer 130 may be, for example, 60% or more and 90% or less. By making the gel fraction of the insulating layer 130 60% or more, it is possible to suppress the excessive production of thermally degraded uncrosslinked polyethylene. This makes it possible to stably suppress further attack on the crosslinked polyethylene caused by the thermally degraded uncrosslinked polyethylene. On the other hand, by making the gel fraction of the insulating layer 130 90% or less, it is possible to ensure a sufficient amount of uncrosslinked polyethylene that functions as a degradation target. This makes it possible to stably suppress the attack of degradation on the crosslinked polyethylene.

[0069] (Crosslinked State) In this embodiment, the polyethylene constituting the insulating layer 130 is crosslinked by an organic peroxide. Crosslinking by an organic peroxide is also called chemical crosslinking or thermal crosslinking.

[0070] That is, the insulating layer 130 of this embodiment is not crosslinked by the so-called silane crosslinking method. The "silane crosslinking method" referred to here is performed by the following procedure. First, a hydrolyzable silane coupling agent is grafted onto polyethylene in the presence of an organic peroxide to obtain silane-grafted polyethylene. Then, the silane-grafted polyethylene is crosslinked by contacting it with moisture in the presence of a silanol condensation catalyst. As a result, in the silane crosslinking method, silicon atoms (and oxygen atoms) are contained at the crosslinking points of the polyethylene. This type of silane crosslinking method makes it difficult to increase the gel fraction. Furthermore, when the insulating layer 130 is thick, moisture does not easily penetrate into the insulating layer 130, making it difficult to uniformly crosslink the entire insulating layer 130.

[0071] In contrast, in this embodiment, the polyethylene constituting the insulating layer 130 is thermally cross-linked with an organic peroxide as described above, so that the cross-linked polyethylene in the insulating layer 130 does not contain silicon atoms (and oxygen atoms) at the cross-linking points. This allows the cross-linked polyethylene in the insulating layer 130 to maintain strong bonds. The gel fraction of the insulating layer 130 can be stably kept within the above-mentioned range. Furthermore, even if the insulating layer 130 is thick, a uniform cross-linked state can be obtained throughout the entire insulating layer 130, while leaving a certain amount of uncross-linked polyethylene.

[0072] (4) Antioxidant In the present embodiment, the antioxidant is not limited. However, in the present embodiment, the antioxidant may have, for example, the following composition, based on the relationship between the uncrosslinked polyethylene remaining in the insulating layer 130 and the antioxidant.

[0073] In this embodiment, the insulating layer 130 may contain, for example, an antioxidant having a molecular weight of 300 or more and 1,100 or less.

[0074] During thermal degradation, hydrogen is extracted from the molecular chain of the polymer, generating radicals. Oxygen bonds with the radicals in the molecular chain generated by thermal degradation, forming degradation points. Before oxygen can bond with the radicals, the radicals are deactivated by the antioxidant, thereby suppressing oxidation of the polymer.

[0075] However, if the molecular weight of the antioxidant is too low, it may deactivate radicals generated in the crosslinker or radicals in the polymer main chain generated by the crosslinker radicals abstracting hydrogen atoms from the polymer main chain. This may make it difficult to achieve the desired degree of crosslinking (gel fraction). On the other hand, if the molecular weight of the antioxidant is too high, the antioxidant becomes less mobile in the polymer. This makes it difficult for the antioxidant to disperse uniformly throughout the polymer. This reduces the frequency with which the antioxidant functions against radicals generated in the molecular chains of thermally degraded polymers. For these reasons, there is an optimal range for the molecular weight of the antioxidant depending on the polymer for which the antioxidant is intended to function.

[0076] Therefore, in this embodiment, by setting the molecular weight of the antioxidant to 300 or more, it is possible to suppress deactivation of radicals generated in the crosslinking agent or radicals generated thereby in the main chain of polyethylene. This makes it possible to stably obtain a desired degree of crosslinking (gel fraction). On the other hand, by setting the molecular weight of the antioxidant to 1100 or less, it is possible to stably disperse the antioxidant in uncrosslinked polyethylene that satisfies the above-mentioned number average molecular weight, although the detailed mechanism is unknown. The stable dispersion of the antioxidant allows the antioxidant to be retained in the uncrosslinked polyethylene. As a result, it is possible to stably suppress oxidative degradation of the uncrosslinked polyethylene.

[0077] In this embodiment, the antioxidant may be, for example, phenolic.

[0078] The antioxidant may be, for example, a hindered phenol having a bulky substituent at least in part. Examples of the bulky substituent include a tert-butyl group. If the antioxidant is not a hindered phenol, i.e., does not have a bulky substituent, there is a possibility that the radicals generated in the crosslinking agent or the radicals generated thereby in the main chain of polyethylene may be deactivated. In contrast, if the antioxidant has a bulky substituent, excessive movement of the antioxidant itself can be suppressed. This makes it possible to suppress the deactivation of the radicals generated in the crosslinking agent or the radicals generated thereby in the main chain of polyethylene.

[0079] In the phenolic antioxidant, the bulky substituents may be arranged, for example, as so-called "double hindered" substituents at the ortho positions on both sides of the hydroxyl group of the phenol skeleton.

[0080] Alternatively, in a phenolic antioxidant, the bulky substituent may be, for example, a so-called "single-hindered" substituent, located only at one ortho position relative to the hydroxyl group of the phenol skeleton. Specifically, the antioxidant may have, for example, hydrogen or an alkyl group having 1 to 3 carbon atoms at one of the ortho positions relative to the hydroxyl group of the phenol skeleton. That is, a bulky substituent does not need to be located at least on one side of the ortho position relative to the hydroxyl group of the antioxidant. This reduces steric hindrance around the hydroxyl group of the antioxidant. Reducing steric hindrance allows the antioxidant molecules to move more easily. As a result, the antioxidant can function not only in uncrosslinked polyethylene but also in complexly crosslinked crosslinked polyethylene.

[0081] Alternatively, in this embodiment, the antioxidant may be, for example, an amine-based antioxidant.

[0082] In this embodiment, the melting point of the antioxidant may be, for example, 200° C. or lower. This allows the antioxidant to melt in the base resin in the resin composition preparation step S100, in which the polyethylene base resin and the antioxidant are mixed, and the crosslinking step S400. As a result, the antioxidant can be uniformly dispersed in the base resin.

[0083] The lower limit of the melting point of the antioxidant is not particularly limited, but the melting point of the antioxidant is, for example, 0° C. or higher.

[0084] Examples of antioxidants used in this embodiment include the following: 1,3,5-tris[[4-(1,1-dimethylethyl)-3-hydroxy-2,6-dimethylphenyl]methyl]-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (molecular weight: 699, melting point: 160°C) 4,4'-thiobis(6-tert-butyl-m-cresol) (molecular weight: 359, melting point: 160°C) Bis[4-(1-phenyl-1-methylethyl)phenyl]amine (molecular weight: 406, melting point: 100°C) 1,2,3,4-butanetetracarboxylic acid tetrakis(1,2,2,6,6-pentamethylpiperidin-4-yl) (molecular weight: 847, melting point: 65°C) 1,2,3,4-butanetetracarboxylic acid tetrakis(2,2,6,6-tetramethyl-4-piperidinyl) (molecular weight 791, melting point 125 to 135°C)

[0085] The content of the antioxidant in the resin composition is, for example, 0.02 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin. By setting the content of the antioxidant to 0.02 parts by mass or more, it is possible to stably obtain the heat aging resistance of the insulating layer 130. On the other hand, by setting the content of the antioxidant to 5 parts by mass or less, it is possible to stably obtain the desired degree of crosslinking (gel fraction) without inhibiting crosslinking.

[0086] (5) Cable Characteristics In this embodiment, the insulating layer 130 satisfies the above requirements, and therefore the following characteristics of the insulating layer 130 can be obtained.

[0087] In this embodiment, even when a heat aging test is performed in which the insulating layer 130 is heated at a higher temperature and for a longer time than in a conventional heat aging test, the insulating layer 130 exhibits good heat aging resistance.

[0088] Specifically, in this embodiment, after a heat aging test in which a sheet taken from the insulating layer 130 is heated at 180°C for 21 days, the tensile elongation at break of the sheet measured at 25°C in accordance with JIS C3005:2014 is, for example, 50% or more.

[0089] A heat aging test in which a sheet taken from the insulating layer 130 was heated at 180°C for 21 days corresponds to several decades at normal operating temperatures according to the 8°C half-life rule.

[0090] In the measurement of tensile elongation at break in accordance with JIS C3005:2014, the tensile elongation at break ε is measured under test conditions of, for example, a pulling speed of 200 mm / min and a gauge length of 25 mm, and is calculated by the following formula (1): ε={(L1-L0) / L0}×100 (1) Here, L1 is the length (mm) between the gauge lines at break, and L0 is the gauge length (mm).

[0091] In this embodiment, the insulating layer 130 satisfies the above-mentioned tensile breaking elongation after a heat aging test under strict conditions, so that even if the power cable 10 generates heat due to the passage of current over a long period of time, it is possible to suppress heat aging in the elongation of the insulating layer 130. As a result, it is possible to maintain the flexibility required for the power cable 10 for a long period of time.

[0092] In this embodiment, a sheet taken from the insulating layer 130 was subjected to a heat aging test in which the sheet was heated at 180°C for 21 days. After that, the volume resistivity of the sheet measured under conditions of a temperature of 90°C and a DC electric field of 10 kV / mm was 1×10 １２ It is Ω·cm or more.

[0093] In this embodiment, the insulating layer 130 satisfies the above-mentioned volume resistivity after a heat aging test under strict conditions, so that even if the power cable 10 generates heat due to the passage of current over a long period of time, heat aging of the insulating properties of the insulating layer 130 can be suppressed. As a result, the insulating properties of the insulating layer 130 required for the power cable 10 can be maintained for a long period of time.

[0094] In this embodiment, even when a conventional heat aging test is performed, the insulating layer 130 naturally exhibits good heat aging resistance.

[0095] Specifically, in this embodiment, after a heat aging test in which a sheet taken from the insulating layer 130 is heated at 120°C for 7 days, the tensile elongation at break of the sheet measured at 25°C in accordance with JIS C3005:2014 may be, for example, 300% or more, or may be greater than 500%.

[0096] (6) Method for Manufacturing the Power Cable Next, a method for manufacturing the power cable of this embodiment will be described with reference to FIG.

[0097] As shown in FIG. 3, the method for manufacturing a power cable according to this embodiment includes, for example, a resin composition preparation step S100, a conductor preparation step S200, an extrusion step S300, a cross-linking step S400, a shielding layer formation step S500, and a sheath formation step S600.

[0098] (S100: Resin Composition Preparation Step) First, a resin composition that constitutes the insulating layer 130 of this embodiment is prepared.

[0099] In this embodiment, a base resin containing polyethylene, a crosslinking agent, an antioxidant, and other additives are mixed (kneaded) in a mixer to form a mixture. Examples of the mixer include an open roll, a Banbury mixer, a pressure kneader, a single-screw mixer, and a multi-screw mixer. Kneading may be performed once or multiple times.

[0100] In this case, in this embodiment, the polyethylene before cross-linking may be, for example, polyethylene having a number average molecular weight of 30,000 or more and 60,000 or less and a weight average molecular weight of 150,000 or more and 250,000 or less. This makes it possible to stably form the desired molecular weight distribution of the uncross-linked polyethylene in the cross-linking step S400 described below.

[0101] In this case, in this embodiment, a material having a molecular weight of 300 or more and 1100 or less may be used as the antioxidant. By setting the molecular weight of the antioxidant to 300 or more, it is possible to suppress deactivation of radicals generated in the crosslinking agent or radicals generated thereby in the main chain of polyethylene in the crosslinking step S400 described below. On the other hand, by setting the molecular weight of the antioxidant to 1100 or less, it is possible to stably disperse the antioxidant in uncrosslinked polyethylene that satisfies the desired number average molecular weight in the crosslinking step S400 described below.

[0102] Once the mixture is formed, it is granulated using an extruder, thereby forming a pellet-like resin composition that constitutes the insulating layer 130. The steps from mixing to granulation may be carried out all at once using a twin-screw extruder that has a high kneading effect.

[0103] (S200: Conductor Preparation Step) On the other hand, the conductor 110 formed by twisting together a plurality of conductor core wires is prepared.

[0104] (S300: Extrusion Step) After the resin composition preparation step S100 and the conductor preparation step S200 are completed, the insulating layer 130 is formed from the above-described resin composition so as to cover the outer periphery of the conductor 110 in the extrusion step S300.

[0105] 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.

[0106] Specifically, among the three-layer co-extruders, an extruder A for forming the inner semiconductive layer 120 is charged with, for example, a composition for the inner semiconductive layer.

[0107] The above-described pellet-shaped resin composition is fed into extruder B, which forms insulating layer 130. At this time, the set temperature of extruder B is set to a temperature that is 5° C. to 50° C. higher than the melting point of the base resin. The set temperature is adjusted appropriately based on the linear speed and extrusion pressure.

[0108] Furthermore, an outer semiconductive layer composition containing the same materials as the inner semiconductive layer resin composition charged into extruder A is charged into extruder C, which forms the outer semiconductive layer 140 .

[0109] 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.

[0110] (S400: Cross-linking Step) After the extrusion step S300 is completed, in the cross-linking step S400 of this embodiment, at least a part of the insulating layer 130 is cross-linked with a cross-linking agent. The cross-linking step S400 of this embodiment is performed by, for example, a so-called dry cross-linking method.

[0111] In this embodiment, the polyethylene in the insulating layer 130 is cross-linked polyethylene that is insoluble in xylene at 120° C., and uncross-linked polyethylene that is soluble in xylene at 120° C. remains. The number average molecular weight of the uncross-linked polyethylene is set to 5,000 or more and 25,000 or less.

[0112] In this embodiment, for example, the cross-linking temperature is changed in two stages to cross-link the insulating layer 130 .

[0113] Here, let us consider the case where the crosslinking temperature is not changed during the crosslinking process, but is maintained constant in one step.

[0114] In this case, if an appropriate crosslinking temperature is set in one step, crosslinking proceeds normally, i.e., other polyethylene bonds to the portion of polyethylene where hydrogen has been removed by the crosslinking agent. Therefore, uncrosslinked polyethylene is unlikely to remain in the insulating layer 130. As a result, the peak of the molecular weight distribution of the uncrosslinked polyethylene is located in the low molecular weight range.

[0115] Alternatively, if the crosslinking temperature is set too low in one step, sufficient radicals are not generated, and therefore, a large amount of uncrosslinked polyethylene remains, but not enough crosslinked polyethylene is formed in the insulating layer 130. As a result, the peak of the molecular weight distribution of the uncrosslinked polyethylene is located in the high molecular weight range.

[0116] Alternatively, if the crosslinking temperature is set too high in one step, the crosslinking agent may volatilize or only the outermost surface of the insulating layer 130 may be crosslinked. As a result, much of the raw polyethylene is not consumed in the crosslinking process, and a large amount of uncrosslinked polyethylene remains. In this case, too, the peak of the molecular weight distribution of the uncrosslinked polyethylene will be located in the high molecular weight range.

[0117] In contrast to this, in this embodiment, by changing the crosslinking temperature in two stages, it is possible to leave uncrosslinked polyethylene satisfying the above-mentioned number average molecular weight in the insulating layer 130 .

[0118] Specifically, the cross-linking step S400 of this embodiment includes, for example, a first cross-linking step S420 and a second cross-linking step S440.

[0119] In the first cross-linking step S420 and the second cross-linking step S440, for example, a cross-linking device 30 shown in Fig. 4 is used. As shown in Fig. 4, the cross-linking device 30 of this embodiment includes, for example, a cross-linking pipe 320 and a plurality of zone heaters 340.

[0120] For example, an extruded material that will become the cable core of the power cable 10 is inserted into the bridging pipe 320. The inside of the bridging pipe 320 is pressurized with nitrogen gas or the like.

[0121] The zone heater 340 is provided, for example, to surround the outer periphery of the cross-linking pipe 320. The zone heater 340 is configured, for example, as an infrared heater that heats the inside of the cross-linking pipe 320 by radiating infrared rays.

[0122] For example, a plurality of zone heaters 340 are provided. The plurality of zone heaters 340 are arranged, for example, along the axial direction of the cross-linking pipe 320. The plurality of zone heaters 340 are configured, for example, to be able to individually adjust the heating temperature.

[0123] The above-described cross-linking device 30 is divided into two regions, for example, according to the cross-linking temperature to which the cross-linking is performed by the zone heaters 340. Specifically, the cross-linking device 30 has, for example, a first cross-linking region 32 that is heated to a first cross-linking temperature and a second cross-linking region 34 that is heated to a second cross-linking temperature, in this order in the insertion direction of the power cable 10. In each of the first cross-linking region 32 and the second cross-linking region 340, the cross-linking time is adjusted by the number of zone heaters 340 (i.e., the axial length of each region).

[0124] (S420: First Cross-Linking Step) In the first cross-linking step S420, a portion of cross-linked polyethylene is formed in the insulating layer 130 at a first cross-linking temperature in the first cross-linking region 32 of the cross-linking device 30. This ensures a certain amount of cross-linked polyethylene required for the insulating layer 130.

[0125] In the first cross-linking step S420, the polyethylene is cross-linked to retain the cross-linking agent in the polyethylene so that the cross-linking agent is less likely to volatilize at the second cross-linking temperature described below than when the first cross-linking step S420 is not performed. In other words, by making it difficult for the molecules in the cross-linked portion of the polyethylene to move, it is possible to suppress the volatilization of the cross-linking agent due to heat.

[0126] However, as mentioned above, if the crosslinking temperature is directly increased, the crosslinking agent becomes more likely to volatilize, making it difficult to form a sufficiently crosslinked polyethylene.

[0127] In contrast to this, in the present embodiment, the first crosslinking temperature in the first crosslinking step S420 is set lower than the second crosslinking temperature described below, and the crosslinking agent is retained in the polyethylene, so that even if the crosslinking temperature is raised to the second crosslinking temperature in the second crosslinking step S440 described below, volatilization of the crosslinking agent in the second crosslinking step S440 can be suppressed. As a result, a certain amount of crosslinked polyethylene can be formed in the second crosslinking step S440 described below.

[0128] Specifically, the first crosslinking temperature is set to, for example, 180° C. or higher and 275° C. or lower. Setting the first crosslinking temperature to 180° C. or higher makes it possible to secure a certain amount of crosslinked polyethylene and to retain the crosslinking agent in the polyethylene. On the other hand, setting the first crosslinking temperature to 275° C. or lower makes it possible to suppress excessive volatilization of the crosslinking agent in the first crosslinking step S420.

[0129] The crosslinking time in the first crosslinking step S420 is, for example, 1 minute or more and less than 30 minutes. By setting the crosslinking time in the first crosslinking step S420 to 1 minute or more, it is possible to ensure both a certain amount of crosslinked polyethylene and retention of the crosslinking agent in the polyethylene. On the other hand, by setting the crosslinking time in the first crosslinking step S420 to less than 30 minutes, it is possible to suppress excessive crosslinking of the polyethylene in the first crosslinking step S420.

[0130] (S440: Second Crosslinking Step) In the second crosslinking step S440, crosslinked polyethylene is further formed in the insulating layer 130 at a second crosslinking temperature higher than the first crosslinking temperature in the second crosslinking region 34 of the crosslinking device 30. Meanwhile, uncrosslinked polyethylene remains in the insulating layer 130 by recombining hydrogen with at least a portion of the polyethylene that has been desorbed.

[0131] In other words, it is possible to make it difficult for molecular chains of further crosslinked polyethylene or uncrosslinked polyethylene to bond to the portion of the polyethylene where hydrogen has been eliminated, i.e., it is possible to suppress the formation of excessive crosslinking points and the excessive progress of crosslinking of the polyethylene.

[0132] In this way, in the second cross-linking step S440, it is possible to stably achieve both the formation of cross-linked polyethylene and the remaining uncross-linked polyethylene.

[0133] Specifically, the second crosslinking temperature is set to, for example, 280° C. or higher and 400° C. or lower. Setting the second crosslinking temperature to 280° C. or higher makes it possible to stably achieve both the formation of crosslinked polyethylene and the remaining uncrosslinked polyethylene. On the other hand, setting the second crosslinking temperature to 400° C. or lower makes it possible to suppress excessive volatilization of the crosslinking agent in the second crosslinking step S440.

[0134] The crosslinking time in the first crosslinking step S420 is, for example, 1 minute or more and less than 50 minutes. By setting the crosslinking time in the second crosslinking step S440 to 1 minute or more, it is possible to stably achieve both the formation of crosslinked polyethylene and the remaining uncrosslinked polyethylene. On the other hand, by setting the crosslinking time in the second crosslinking step S440 to less than 50 minutes, it is possible to suppress excessive crosslinking of polyethylene in the second crosslinking step S440.

[0135] By the first cross-linking step S420 and the second cross-linking step S440, it is possible to leave uncross-linked polyethylene in the insulating layer 130 that satisfies the number average molecular weight of 5,000 or more and 25,000 or less.

[0136] Thereafter, the cable core that has passed through the bridging device 30 is cooled by water cooling.

[0137] As a result of the above, a cable core is formed, which is composed of the conductor 110, the inner semiconductive layer 120, the insulating layer 130 and the outer semiconductive layer 140.

[0138] (S500: Shielding Layer Forming Step) After the cross-linking step S400 is completed, the shielding layer 150 is formed on the outside of the outer semiconducting layer 140 by winding, for example, a copper tape.

[0139] (S600: 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.

[0140] In this manner, the power cable 10 is manufactured as a solid insulated power cable.

[0141] (7) Summary of the Present Embodiment According to the present embodiment, one or more of the following effects are achieved.

[0142] In this embodiment, the number average molecular weight Mn of the uncrosslinked polyethylene in the insulating layer 130 is 5,000 or more and 25,000 or less.

[0143] In this embodiment, by setting the number average molecular weight Mn of the uncrosslinked polyethylene to 5000 or more, it is possible to suppress the excessive formation of crosslinked polyethylene while leaving a certain amount of low-molecular-weight uncrosslinked polyethylene that is susceptible to thermal degradation. This allows the low-molecular-weight uncrosslinked polyethylene to serve as a degradation target, that is, to function as a sacrificial component for thermal aging. By using the uncrosslinked polyethylene as a degradation target, it is possible to suppress the attack of degradation on the crosslinked polyethylene. As a result, it is possible to suppress thermal aging of the insulating layer.

[0144] On the other hand, in this embodiment, by setting the number average molecular weight Mn of the uncrosslinked polyethylene to 25,000 or less, sufficient crosslinked polyethylene is formed, and excess residual uncrosslinked polyethylene, which is susceptible to thermal degradation across a wide molecular weight range, can be suppressed. This makes it possible to suppress excessive thermal degradation of uncrosslinked polyethylene across a wide molecular weight range. Further attack on the crosslinked polyethylene caused by thermally degraded uncrosslinked polyethylene can be suppressed. Furthermore, by setting the number average molecular weight Mn of the uncrosslinked polyethylene to 25,000 or less, the difference in degradation behavior between crosslinked polyethylene and uncrosslinked polyethylene can be maintained. This makes it possible to maintain the uncrosslinked polyethylene in a state susceptible to degradation and suppress direct thermal degradation of the crosslinked polyethylene. As a result, it is possible to suppress the progression of thermal aging throughout the entire insulating layer 130.

[0145] As described above, according to the present embodiment, it is possible to improve the heat aging resistance of the insulating layer 130. That is, even if the power cable 10 generates heat due to the passage of current over a long period of time, it is possible to maintain the various properties (such as flexibility and insulation properties) required of the power cable 10.

[0146] (8) Modifications of the Present Embodiment The above-described embodiment can be modified as necessary as shown in the following modifications. Only elements different from the above-described embodiment will be described below, and elements that are substantially the same as the elements described in the above-described embodiment will be assigned the same reference numerals and their description will be omitted.

[0147] In a variant, the power cable 10 is configured for direct current use, for example. In a variant, the insulating layer 130 may include, for example, an inorganic filler.

[0148] (Inorganic Filler) The inorganic filler acts to trap space charges in the insulating layer 130 and suppress local accumulation of space charges in the insulating layer 130. This can improve the insulating properties of the insulating layer 130.

[0149] The inorganic filler includes, for example, at least one of zinc oxide, titanium oxide, magnesium oxide, silicon dioxide, carbon black, carbon nanotubes, graphene, and the like.

[0150] In this embodiment, at least a portion of the inorganic filler may be surface-treated with a silane coupling agent.

[0151] (Modified Polymer) In this embodiment, the insulating layer 130 may contain, for example, a modified polymer as the base resin. The modified polymer in this embodiment is, for example, a resin containing an olefin unit and modified with at least one selected from unsaturated organic acids and derivatives thereof. The modified polymer may be, for example, unsaturated carboxylic acid-modified polyethylene. Among these, the modified polymer may be modified with, for example, maleic anhydride. The inclusion of such a modified polymer can improve the compatibility (adhesion) between the polar inorganic filler and the base resin, thereby improving the dispersibility of the inorganic filler.

[0152] <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.

[0153] In the above embodiment, three layers are simultaneously extruded in the extrusion step S300, but each layer may be extruded one by one.

[0154] 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.

[0155] (1) Production of Power Cables Power cables of samples A1 to A9 and B1 to B5 were produced as follows.

[0156] <Samples A1 to A9> For samples A1 to A9, resin compositions containing the following materials were mixed in a Banbury mixer and granulated into pellets in an extruder.

[0157] (Base resin) Low-density polyethylene (LDPE): 100 parts by mass Density: 0.92 g / cm ３ Melting point: 105°C Number average molecular weight: 39,000 Weight average molecular weight: 182,000

[0158] (Crosslinking agent) Dicumyl peroxide 1.5 parts by mass

[0159] (Antioxidant) As an antioxidant, 0.15 parts by mass of any of the following materials was blended.

[0160] (Antioxidant A) 1,3,5-tris[[4-(1,1-dimethylethyl)-3-hydroxy-2,6-dimethylphenyl]methyl]-1,3,5-triazine-2,4,6(1H,3H,5H)-trione Molecular weight: 699 Melting point: 160°C

[0161] (Antioxidant B) 4,4'-thiobis(6-tert-butyl-m-cresol) Molecular weight: 359 Melting point: 160°C

[0162] (Antioxidant C) 1,2,3,4-butanetetracarboxylic acid tetrakis(1,2,2,6,6-pentamethylpiperidin-4-yl) Molecular weight: 847 Melting point: 65°C

[0163] (Antioxidant D) Pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] Molecular weight: 1178 Melting point: 125°C

[0164] (Antioxidant E) Dibutylhydroxytoluene Molecular weight: 220 Melting point: 70°C

[0165] Next, a wire with a cross-sectional area of ​​600 mm2 was formed by twisting together conductor core wires made of dilute copper alloy with a diameter of 5 mm. ２After the conductors were prepared, the resin composition for the inner semiconductive layer containing an ethylene-ethyl acrylate copolymer, the above-mentioned resin composition for the insulating layer, and the resin composition for the outer semiconductive layer made of the same materials as the resin composition for the inner semiconductive layer were charged into extruders A to C, respectively. The temperature of extruder B was set to a temperature 20°C higher than the melting point of the LDPE base resin.

[0166] The extrudates from extruders A to C were introduced into a common head, and an inner semiconductive layer, an insulating layer, and an outer semiconductive layer were simultaneously extruded from the inside to the outside around the conductor, with thicknesses of the inner semiconductive layer, the insulating layer, and the outer semiconductive layer set to 0.5 mm, 9 mm, and 0.5 mm, respectively.

[0167] Next, the insulating layer was crosslinked using the above-mentioned crosslinking device, changing the crosslinking temperature in two stages. The conditions for the first crosslinking step and the second crosslinking step for each sample are as shown in Tables 1 and 2 below. After the crosslinking step, the power cable was cooled by water at 25°C.

[0168] In this manner, power cables samples A1 to A9 were manufactured.

[0169] <Samples B1 to B5> Samples B1 to B5 were produced in the same manner as Samples A1 to A9, respectively, except that the crosslinking step as the novel production method was not applied. That is, for Samples B1 to B5, the crosslinking temperature was not changed and was maintained constant in one step. The conditions for the crosslinking step for each sample are as shown in Table 3 below.

[0170] (2) Evaluation (Sheet Collection) The insulation layer of the power cable was thinly sliced ​​to obtain multiple sheets of the insulation layer. The thickness of the sheets was 1 mm. The position of the sheet of the insulation layer was the center of the insulation layer in the thickness direction.

[0171] (Molecular weight distribution of uncrosslinked polyethylene) A sheet taken from the insulating layer was immersed in xylene, and the xylene in this state was heated at 120°C for 24 hours. Next, the xylene solution in which the uncrosslinked polyethylene had been eluted was filtered using filter paper heated to 120°C. The filtered xylene solution was volatilized at 120°C to obtain powder of uncrosslinked polyethylene.

[0172] After separation of the uncrosslinked polyethylene, the uncrosslinked polyethylene powder was dissolved in the eluent described below, and the molecular weight distribution of the uncrosslinked polyethylene was measured by GPC under the conditions described below. The molecular weight distribution of the uncrosslinked polyethylene was measured based on a calibration curve prepared by GPC using PS as a standard sample. As a result, the number average molecular weight Mn of the uncrosslinked polyethylene, the weight average molecular weight Mw of the uncrosslinked polyethylene, and their ratio Mw / Mn were determined. Apparatus: Tosoh HLC-8321GPC / HT Eluent: o-dichlorobenzene Temperature: 145°C Concentration: 0.1 wt% / vol% Flow rate: 1.0 ml / min The calibration curve for PS was prepared based on results within the molecular weight range of 1,000 to 5.5 million.

[0173] (Heat Aging Test 1) A sheet taken from the insulating layer was placed in a predetermined thermostatic oven and heated at 120°C for 7 days.

[0174] (Elongation after Heat Aging Test 1) After heat aging test 1, the tensile breaking elongation of the sheet was measured under test conditions of a temperature of 25°C, a pulling speed of 200 mm / min, and a gauge length of 25 mm in accordance with JIS C3005: 2014. As a result, a tensile breaking elongation of 300% or more after heat aging test 1 was evaluated as "good", and a tensile breaking elongation of less than 300% after heat aging test 1 was evaluated as "poor".

[0175] (Heat Aging Test 2) A sheet taken from the insulating layer was placed in a predetermined thermostatic oven and heated at 180°C for 21 days.

[0176] (Elongation after Heat Aging Test 2) After heat aging test 2, the tensile breaking elongation of the sheet was measured under test conditions of a temperature of 25°C, a pulling speed of 200 mm / min, and a gauge length of 25 mm in accordance with JIS C3005: 2014. As a result, a case where the tensile breaking elongation after heat aging test 2 was 50% or more was evaluated as "good", and a case where the tensile breaking elongation after heat aging test 2 was less than 50% was evaluated as "poor".

[0177] (Volume Resistivity After Heat Aging Test 2) After heat aging test 2, the volume resistivity of the sheet was measured under conditions of a temperature of 90°C and a DC electric field of 10 kV / mm. As a result, the volume resistivity after heat aging test 2 was 1.0 × 10 １２ A volume resistivity of 1.0×10 Ω cm or more after heat aging test 2 is rated as "good." １２ A resistance less than Ω·cm was evaluated as "poor".

[0178] (Gel Fraction) The gel fraction was measured using the collected sheet according to the following procedure. First, the mass of the sheet before immersion in xylene was measured. Next, the entire sheet was immersed in xylene, and the xylene in this state was heated at 120°C for one day. Next, the remaining sheet was removed from the xylene, and only the insoluble portion of the sheet was dried in the air at 120°C for one day. Thereafter, the mass of the insoluble portion of the sheet remaining after immersion in xylene was measured. As a result, the ratio of the mass of the insoluble portion of the sheet remaining after immersion in xylene to the mass of the sheet before immersion in xylene was calculated as the "gel fraction."

[0179] (3) Results The evaluation results of each sample are explained with reference to Tables 1 to 3 below. In each table, "aE+b" is a×10 ｂ means.

[0180]

[0181]

[0182]

[0183] <Sample B1> Sample B1 had a low gel fraction and the number average molecular weight of the uncrosslinked polyethylene exceeded 25000. Therefore, in Sample B1, the sheet melted after Heat Aging Test 2.

[0184] In Sample B1, the crosslinking temperature was excessively low, resulting in a low gel fraction, i.e., insufficient crosslinked polyethylene was formed. Furthermore, excessive uncrosslinked polyethylene remained over a wide range of molecular weights and was thermally degraded. As a result, the excessively thermally degraded uncrosslinked polyethylene attacked the crosslinked polyethylene. As a result, it is believed that the sheet melted in Heat Aging Test 2, which is more severe than the conventional Heat Aging Test 1.

[0185] <Samples B2 and B3> In samples B2 and B3, the gel fraction was within the specified range (60% or more and 90% or less), but the number average molecular weight of the uncrosslinked polyethylene was less than 5000. Therefore, in samples B2 and B3, the tensile elongation at break after heat aging test 2 was less than 50%, and the volume resistivity after heat aging test 2 was less than 1×10 １２ The resistance was less than Ω·cm.

[0186] In Samples B2 and B3, the polyethylene in the insulating layer was crosslinked at an appropriate temperature, resulting in sufficient crosslinking of the polyethylene constituting the insulating layer. However, the amount of remaining low-molecular-weight uncrosslinked polyethylene was excessively small. Therefore, the crosslinked polyethylene, rather than the uncrosslinked polyethylene, was the direct target of degradation. As a result, it is believed that the tensile elongation at break and volume resistivity were low in Heat Aging Test 2, which is more severe than the conventional Heat Aging Test 1.

[0187] <Sample B4> Sample B4 had a low gel fraction and the number average molecular weight of the uncrosslinked polyethylene exceeded 25000. Therefore, in Sample B1, the sheet melted after heat aging test 2.

[0188] In Sample B4, the crosslinking temperature was excessively high, causing the crosslinking agent to volatilize during the crosslinking process. As a result, the polyethylene was not sufficiently crosslinked throughout the insulating layer. Furthermore, excessive uncrosslinked polyethylene remained over a wide molecular weight range and underwent thermal degradation. As a result, the excessively thermally degraded uncrosslinked polyethylene attacked the crosslinked polyethylene. Alternatively, because the number-average molecular weight of the uncrosslinked polyethylene exceeded 25,000, the difference in degradation behavior between the crosslinked and uncrosslinked polyethylenes was small. Therefore, the crosslinked polyethylene was directly thermally degraded. As a result, it is believed that the sheet melted in Heat Aging Test 2, which had more severe conditions than the conventional Heat Aging Test 1.

[0189] <Sample B5> In sample B5, the gel fraction was within the specified range (60% or more and 90% or less), but the number average molecular weight of the uncrosslinked polyethylene was less than 5000. Therefore, in sample B5, the tensile breaking elongation after heat aging test 2 was less than 50%, and the volume resistivity after heat aging test 2 was less than 1×10 １２ The resistance was less than Ω·cm.

[0190] In sample B5, crosslinking was performed at a high temperature within the range where the crosslinking agent would not volatilize, resulting in a high crosslinking reaction rate and high crosslinking frequency. However, although the polyethylene was crosslinked, only the uncrosslinked polyethylenes bonded to each other before the low-molecular-weight uncrosslinked polyethylenes moved. As a result, the uncrosslinked polyethylene included uncrosslinked polyethylenes whose molecular weights only increased to a level that allowed them to dissolve in xylene. Because the number of uncrosslinked polyethylenes decreased due to their bonding, the remaining low-molecular-weight uncrosslinked polyethylenes were excessively small. Therefore, crosslinked polyethylenes, rather than uncrosslinked polyethylenes, were directly targeted for degradation. As a result, it is believed that the tensile elongation at break and volume resistivity were lower in Heat Aging Test 2, which had more severe conditions than the conventional Heat Aging Test 1.

[0191] <Samples A1 to A9> In contrast, in Samples A1 to A9, the gel fraction was within the specified range (60% or more and 90% or less), and the number average molecular weight of the uncrosslinked polyethylene was 5,000 or more and 25,000 or less. As a result, in Samples A1 to A9, the tensile elongation at break after Heat Aging Test 2 was 50% or more, and the volume resistivity after Heat Aging Test 2 was 1×10 １２ It was Ω·cm or more.

[0192] In Samples A1 to A9, the crosslinking temperature was changed in two stages, which allowed uncrosslinked polyethylene satisfying the above-mentioned number average molecular weight to remain in the insulating layer.

[0193] In Samples A1 to A9, by setting the number average molecular weight of the uncrosslinked polyethylene to 5,000 or more, it was possible to target low-molecular-weight uncrosslinked polyethylene for degradation. On the other hand, in Samples A1 to A9, by setting the number average molecular weight of the uncrosslinked polyethylene to 25,000 or less, it was possible to suppress further attack on the crosslinked polyethylene caused by thermally deteriorated uncrosslinked polyethylene. As a result, it was confirmed that the heat aging resistance of the insulating layer was improved in Samples A1 to A9.

[0194] <Antioxidant Dependence> Samples A1 and A6 to A9 produced under the same conditions except for the antioxidant were compared.

[0195] In samples A1, A6, and A7, in which the molecular weight of the antioxidant was 300 or more and 1100 or less, the gel fraction was in the range of 60% or more and 90% or less, and was larger than that of sample A9, in which the molecular weight of the antioxidant was 300.

[0196] In samples A1, A6, and A7, in which the molecular weight of the antioxidant was 300 or more and 1100 or less, the tensile elongation at break after heat aging test 2 was in the range of 50% or more and was larger than that of sample A8, in which the molecular weight of the antioxidant was more than 1100. Furthermore, in samples A1, A6, and A7, the volume resistivity after heat aging test 2 was 1×10 １２ Ω·cm or more, and was higher than that of sample A8.

[0197] There was no difference in the evaluation results between Samples A1 and A6, which used a phenol-based antioxidant, and Sample A7, which used an amine-based antioxidant.

[0198] In Samples A1, A6, and A7, it was possible to suppress deactivation of radicals generated in the crosslinking agent or radicals generated thereby in the main chain of polyethylene by using an antioxidant with a molecular weight of 300 or more. As a result, it was confirmed that the desired degree of crosslinking (gel fraction) could be stably obtained in Samples A1, A6, and A7.

[0199] In samples A1, A6, and A7, the antioxidant molecular weight was set to 1,100 or less, allowing the antioxidant to be stably dispersed in uncrosslinked polyethylene satisfying the above-mentioned number-average molecular weight. This enabled stable suppression of oxidative degradation of the crosslinked polyethylene. As a result, it was confirmed that samples A1, A6, and A7 exhibited better heat aging resistance of the insulating layer.

[0200] <Supplementary Notes> The following supplementary notes are provided regarding aspects of the present disclosure. The aspects referenced by the numbers in brackets [ ] to which the supplementary notes below depend correspond to the aspects described in <Embodiments of the present disclosure>.

[0201] [7] The power cable according to any one of [1] to [6] above, wherein the cross-linked polyethylene does not contain silicon atoms at cross-linking points.

[0202] [8] A method for manufacturing a power cable, comprising: a step of preparing a resin composition containing polyethylene and a crosslinking agent; a step of forming an insulating layer from the resin composition so as to cover an outer periphery of a conductor; and a step of crosslinking at least a part of the insulating layer with the crosslinking agent, wherein in the step of crosslinking at least a part of the insulating layer, crosslinked polyethylene that is insoluble in xylene at 120°C is formed from the polyethylene in the insulating layer and uncrosslinked polyethylene that is soluble in xylene at 120°C remains, and the number average molecular weight of the uncrosslinked polyethylene is 5,000 or more and 25,000 or less.

[0203] [9] The method for producing a power cable according to the above [8], wherein in the step of preparing the resin composition, the polyethylene having a number average molecular weight of 30,000 or more and 60,000 or less in a state before crosslinking is used.

[0204]

[10] The method for manufacturing a power cable according to the above [8] or [9], wherein the step of crosslinking the polyethylene comprises: a first crosslinking step of forming a part of the crosslinked polyethylene at a first crosslinking temperature; and a second crosslinking step of further forming the crosslinked polyethylene at a second crosslinking temperature higher than the first crosslinking temperature, while recombining hydrogen with at least a part of the hydrogen desorbed from the polyethylene, thereby leaving the uncrosslinked polyethylene.

[0205]

[11] The method for manufacturing a power cable according to the above

[10] , wherein in the first crosslinking step, the polyethylene is crosslinked so that the crosslinking agent is less likely to volatilize at the second crosslinking temperature than when the first crosslinking step is not performed, thereby retaining the crosslinking agent in the polyethylene.

[0206] REFERENCE SIGNS LIST 10 Power cable 30 Cross-linking device 32 First cross-linking region 34 Second cross-linking region 100 Power cable 110 Conductor 120 Inner semi-conductive layer 130 Insulating layer 140 Outer semi-conductive layer 150 Shielding layer 160 Sheath 320 Cross-linking pipe 340 Zone heater

Claims

1. A power cable comprising: a conductor; and an insulating layer that is provided so as to cover the outer periphery of the conductor and is made of a resin composition containing polyethylene, wherein the insulating layer contains, among the polyethylene, cross-linked polyethylene that is insoluble in xylene at 120°C and uncross-linked polyethylene that is soluble in xylene at 120°C, and the number average molecular weight of the uncross-linked polyethylene is 5,000 or more and 25,000 or less.

2. The power cable according to claim 1, wherein the gel fraction of the insulating layer is between 60% and 90%.

3. The power cable according to claim 1 or 2, wherein the weight average molecular weight of the uncrosslinked polyethylene is 20,000 or more and 100,000 or less.

4. The power cable according to any one of claims 1 to 3, wherein the insulating layer contains an antioxidant having a molecular weight of 300 or more and 1,100 or less.

5. A power cable according to any one of claims 1 to 4, wherein a sheet taken from the insulating layer is heated at 180°C for 21 days, and then the tensile elongation at break of the sheet measured at 25°C in accordance with JIS C3005:2014 is 50% or more.

6. After heating a sheet taken from the insulating layer at 180°C for 21 days, the volume resistivity of the sheet measured under conditions of a temperature of 90°C and a DC electric field of 10 kV / mm was 1 x 10 １２ The power cable according to any one of claims 1 to 5, having a resistance of Ω·cm or more.