Power cable

Abstract

The power cable of the present application has: a conductor; and an insulation layer provided so as to cover the outer periphery of the conductor and composed of a resin composition containing polyethylene, the insulation layer containing crosslinked polyethylene in the polyethylene that is insoluble in xylene at 120°C and un-crosslinked polyethylene in the polyethylene that is soluble out to xylene at 120°C, the number average molecular weight of the un-crosslinked polyethylene being 5000 or more and 25000 or less.

Description

Technical Field

[0001] This disclosure relates to a power cable. Background Technology

[0002] Cross-linked polyethylene is widely used as a resin component for the insulation layer of power cables due to its excellent insulation properties (e.g., Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 57-69611. Summary of the Invention

[0006] According to one aspect of this disclosure, a power cable is provided, which has:

[0007] conductors; and

[0008] An insulating layer, which is provided to cover the outer periphery of the conductor, is composed of a resin composition comprising polyethylene.

[0009] The aforementioned insulating layer comprises cross-linked polyethylene that is insoluble in xylene at 120°C and uncross-linked polyethylene that can dissolve in xylene at 120°C.

[0010] The number average molecular weight of the aforementioned uncrosslinked polyethylene is above 5000 and below 25000. Attached Figure Description

[0011] Figure 1 This is a schematic cross-sectional view of a power cable orthogonal to the axial direction according to one embodiment of the present disclosure.

[0012] Figure 2 This is a schematic diagram showing the molecular weight distribution.

[0013] Figure 3 This is a flowchart illustrating a method for manufacturing a power cable according to one embodiment of the present disclosure.

[0014] Figure 4 This is a cross-sectional view illustrating a cross-linking device according to one embodiment of the present disclosure. Detailed Implementation

[0015] [The problem this disclosure aims to solve]

[0016] The purpose of this disclosure is to improve the heat aging resistance of the insulation layer.

[0017] [The Effects of This Disclosure]

[0018] According to this disclosure, the heat aging resistance of the insulation layer can be improved.

[0019] [Description of embodiments of this disclosure]

[0020] <Insights gained by the inventors, etc.>

[0021] First, an overview of the insights gained by the inventors and others is provided.

[0022] The insulation layer of a typical power cable is not composed solely of fully cross-linked polyethylene, but also 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 xylene-insoluble components to the total mass of polyethylene in the insulation layer.

[0023] Even when the gel fraction of the insulation layer is low, meaning that the content of uncrosslinked polyethylene in the insulation layer is high, it has almost no effect on the initial mechanical and electrical properties. Therefore, uncrosslinked polyethylene in the insulation layer has not received much attention in the past.

[0024] However, the inventors' research revealed that the uncrosslinked polyethylene in the insulation layer affects the heat aging resistance of the insulation layer.

[0025] The molecular chains of uncrosslinked polyethylene are more mobile than those of crosslinked polyethylene. Therefore, uncrosslinked polyethylene exhibits microscopic or macroscopic Brownian motion when heated, making it more susceptible to thermal aging than crosslinked polyethylene. During this thermal aging process, hydrogen atoms are stripped from the molecular chains of uncrosslinked polyethylene, generating free radicals. These free radicals combine with oxygen to form aging sites. Consequently, uncrosslinked polyethylene becomes a target for aging.

[0026] On the other hand, heat-aged uncrosslinked polyethylene can also become a catalyst for further aging of crosslinked polyethylene. Specifically, when uncrosslinked polyethylene is exposed to heat for extended periods and generates free radicals, these free radicals may remove hydrogen from the crosslinked polyethylene, causing the main chain of the crosslinked polyethylene to break. Thus, the heat-aged portion of the uncrosslinked polyethylene can extend the aging process to the crosslinked polyethylene, thereby accelerating aging.

[0027] As a result, the presence of uncrosslinked polyethylene in the insulation layer may affect the thermal aging of the insulation layer.

[0028] Therefore, in order to investigate the effect of uncrosslinked polyethylene on the thermal aging of the insulation layer, the inventors conducted a thermal aging test at a higher temperature and for a longer period of time than previous thermal aging tests (heating at 180°C for 21 days). The results showed that the elongation and volume resistivity of the insulation layer after the thermal aging test at 180°C for 21 days depended on the molecular weight of the uncrosslinked polyethylene in the insulation layer.

[0029] In order to adjust the molecular weight of the uncrosslinked polyethylene remaining in the insulation layer, the inventors further studied the crosslinking process as a new manufacturing method. As a result of further in-depth research, the inventors successfully obtained a composition that improved the heat aging resistance of the insulation layer by applying the new manufacturing method and optimizing the molecular weight of the uncrosslinked polyethylene in the insulation layer.

[0030] This disclosure is based on the above-mentioned insights discovered by the inventors, etc.

[0031] <Implementation Methods of this Disclosure>

[0032] Next, embodiments of this disclosure will be described.

[0033] [1] One embodiment of the power cable disclosed herein has:

[0034] conductors; and

[0035] An insulating layer, which is provided to cover the outer periphery of the conductor, is composed of a resin composition comprising polyethylene.

[0036] The aforementioned insulating layer comprises cross-linked polyethylene that is insoluble in xylene at 120°C and uncross-linked polyethylene that can dissolve in xylene at 120°C.

[0037] The number average molecular weight of the aforementioned uncrosslinked polyethylene is above 5000 and below 25000.

[0038] This structure can improve the heat aging resistance of the insulation layer.

[0039] [2] In the power cable described in [1] above,

[0040] The gel content of the above-mentioned insulating layer is more than 60% and less than 90%.

[0041] According to this configuration, by setting the gel fraction of the insulating layer to 60% or more, excessive formation of heat-aged uncrosslinked polyethylene can be suppressed. On the other hand, by setting the gel fraction of the insulating layer to 90% or less, uncrosslinked polyethylene that functions as an aging target can be adequately ensured.

[0042] [3] In the power cables described in [1] or [2] above,

[0043] The weight-average molecular weight of the aforementioned uncrosslinked polyethylene is above 20,000 and below 100,000.

[0044] This structure can improve the heat aging resistance of the insulation layer.

[0045] [4] In any of the power cables described in any of [1] to [3] above,

[0046] The aforementioned insulating layer contains antioxidants with a molecular weight of 300 or more and 1100 or less.

[0047] Based on this composition, the oxidative aging of uncrosslinked polyethylene can be stably suppressed.

[0048] [5] In any of the above-mentioned [1] to [4] power cables,

[0049] After heating the sheet obtained from the above insulating layer at 180°C for 21 days, the tensile elongation at break of the sheet, measured at 25°C according to JISC3005:2014, is 50% or more.

[0050] This configuration allows the power cable to maintain the required flexibility over a long period of time.

[0051] [6] In any of the above-mentioned [1] to [5] power cables,

[0052] The sheet obtained from the above insulating layer was heated at 180°C for 21 days. The volume resistivity of the sheet, measured under conditions of 90°C and a DC electric field of 10 kV / mm, was 1 × 10⁻⁶. 12 Ω·cm or higher.

[0053] This structure enables the insulation properties of the insulation layer required for power cables to be maintained for a long period of time.

[0054] [Detailed Description of Embodiments of this Disclosure]

[0055] Next, an embodiment of the present disclosure will be described with reference to the following accompanying drawings. Furthermore, the present disclosure is not limited to these illustrations, but is shown in the claims and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0056] <One embodiment of this disclosure>

[0057] (1) Resin composition

[0058] The resin composition of this embodiment is the material constituting the insulation layer 130 of the power cable 10 described later, and is the material before crosslinking. The resin composition includes, for example, a base resin, a crosslinking agent, an antioxidant, and other additives.

[0059] (Base resin)

[0060] The base resin (base polymer) refers to the resin component that constitutes the main component of the resin composition. In this embodiment, the base resin includes, for example, polyethylene.

[0061] Polyethylene, as a constituent of the base resin, can be exemplified by low-density polyethylene (LDPE: density 0.91 g / cm³). 3 Above and below 0.93 g / cm³3 Linear low-density polyethylene (LLDPE: density 0.92 g / cm³) 3 Above and 0.945 g / cm 3 The following), medium-density polyethylene (MDPE: density 0.93 g / cm³). 3 Above and below 0.942 g / cm³ 3 High-density polyethylene (HDPE: density 0.942 g / cm³) 3 Above and 0.97 g / cm 3 (The following are examples). Two or more of these can also be used in combination.

[0062] In this embodiment, the base resin can be at least one of LDPE and LLDPE. This allows for improved mechanical properties while simultaneously enhancing the insulation of the insulation layer 130 of the power cable 10.

[0063] There is no particular limitation on the melting point of polyethylene. However, the melting point of polyethylene can be, for example, above 90°C and below 135°C.

[0064] The number-average molecular weight of the polyethylene before crosslinking (i.e., the polyethylene used as raw material before being fed into the extruder) can, for example, be 30,000 or more and 60,000 or less. The weight-average molecular weight of the polyethylene before crosslinking can be 150,000 or more and 250,000 or less.

[0065] Depending on how the polyethylene is cross-linked in the cross-linking process S400 described later, the gel fraction of the cross-linked insulating layer 130 and the molecular weight of the uncross-linked polyethylene remaining in the cross-linked insulating layer 130 will change.

[0066] In addition to polyethylene as the main component, the aforementioned base resin may also include at least one of copolymers of olefins and polar monomers, and α-olefin copolymers. Examples of copolymers of olefins and polar monomers include ethylene-ethyl acrylate copolymers and ethylene-methyl acrylate copolymers. Examples of α-olefin copolymers include ultra-low density polyethylene (VLDPE) and ethylene-propylene rubber. Two or more of these may also be included in the base resin.

[0067] The content of at least one of the copolymers of olefins and polar monomers and α-olefin copolymers in the base resin is, for example, more than 0 parts by mass and less than 20 parts by mass relative to 100 parts by mass of the base resin.

[0068] (Cross-linking agent)

[0069] Crosslinking agents are, for example, organic peroxides. Examples of organic peroxides include dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and 1,3-bis(tert-butylperoxyisopropyl)benzene. Two or more of these can also be used in combination.

[0070] The content of the crosslinking agent in the resin composition is not particularly limited, for example, it is 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, the insulation layer 130 can be fully crosslinked. On the other hand, by setting the content of the crosslinking agent to 4.0 parts by mass or less, during the manufacturing process of the power cable 10, it is possible to suppress the residual thermal aging components of the base resin inside the extruder and suppress contamination of the extruder.

[0071] (Antioxidants)

[0072] The antioxidant is configured, for example, to inhibit the oxidation of polyethylene in the extrusion process S300 and the crosslinking process S400, and to inhibit the thermal aging of polyethylene after the manufacture of the power cable 100. Details regarding antioxidants suitable for this embodiment will be described later.

[0073] (Other additives)

[0074] The resin composition may further include other additives, such as copper inhibitors, lubricants, and colorants.

[0075] (2) Power cables

[0076] Next, refer to Figure 1 The power cable of this embodiment will be described.

[0077] The power cable 10 in this embodiment is configured as a so-called solid-insulated power cable. The power cable 10 can be used for alternating current (AC) or direct current (DC).

[0078] 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.

[0079] (Conductor (Conductive Part))

[0080] The conductor 110 is formed, for example, by stranding together multiple conductor cores (conductive cores) containing pure copper, copper alloys, aluminum or aluminum alloys.

[0081] (Internal semiconductive layer)

[0082] An internal semiconducting layer 120 is disposed such that it covers the outer periphery of the conductor 110. The internal semiconducting layer 120 is semiconducting and is configured to suppress electric field concentration near the surface of the conductor 110. The internal semiconducting layer 120 comprises, for example, at least one of: ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-butyl acrylate copolymer, and ethylene-vinyl acetate copolymer; and conductive carbon black.

[0083] (Insulating layer)

[0084] The insulating layer 130 is provided in such a way that it covers the outer periphery of the inner semiconductive layer 120. The insulating layer 130 is, for example, formed by extrusion molding of the above-described resin composition.

[0085] In this embodiment, through the crosslinking process S400, described later as a novel manufacturing method, a portion of the polyethylene in the insulating layer 130 is crosslinked, while another portion remains in an uncrosslinked state. Furthermore, in this embodiment, the molecular weight of the uncrosslinked polyethylene in the insulating layer 130 is optimized through the crosslinking process S400, described later as a novel manufacturing method. This will be explained in detail later.

[0086] (Outer semiconductive layer)

[0087] The outer semiconductive layer 140 is disposed such that it covers the outer periphery of the insulating layer 130. The outer semiconductive layer 140 is semiconductive and is 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.

[0088] (Shielding layer)

[0089] The shielding layer 150 is provided to cover the outer periphery of the outer semiconductive layer 140. The shielding layer 150 is formed, for example, by winding copper strip, or is configured as a wire shielding layer with multiple soft copper wires wound around it. A strip made of adhesive tape or the like may also be wound on the inner or outer side of the shielding layer 150.

[0090] (jacket)

[0091] The sheath 160 is provided in such a way as to cover the outer periphery of the shielding layer 150. The sheath 160 is made of, for example, polyvinyl chloride or polyethylene.

[0092] (3) Features of the insulating layer in this embodiment

[0093] Next, the features of the insulating layer 130 in this embodiment will be described.

[0094] (Molecular weight of uncrosslinked polyethylene)

[0095] In this embodiment, through the crosslinking process S400, which is described later as a new manufacturing method, the insulating layer 130 comprises, for example, crosslinked polyethylene that is insoluble in xylene at 120°C and uncrosslinked polyethylene that can dissolve in xylene at 120°C.

[0096] The inventors conducted in-depth research and discovered the optimal molecular weight range of uncrosslinked polyethylene that can suppress the thermal aging of the insulation layer 130.

[0097] Here, refer to Figure 2 The molecular weight distribution of the uncrosslinked polyethylene in the insulating layer 130 of this embodiment will be explained. Figure 2 This is a schematic diagram illustrating the molecular weight distribution. The term "molecular weight distribution" as used herein refers to, for example,... Figure 2 As shown, the distribution curve is obtained by plotting the differential distribution values ​​relative to the molecular weight, which correspond to the number of molecules. Figure 2 The horizontal axis is represented logarithmically. Figure 2 In this context, "aEb" represents a × 10 b .

[0098] The molecular weight distribution of the uncrosslinked polyethylene remaining in the insulating layer 130 is determined, for example, by the following steps. First, a sheet obtained from the insulating layer 130 is immersed in xylene, and the xylene in this state is heated at 120°C for 5 hours to 24 hours. As a result, the uncrosslinked polyethylene dissolves into the xylene. Next, the xylene solution from which the uncrosslinked polyethylene has dissolved is filtered through filter paper heated to 120°C. The filtered xylene solution is then evaporated at 120°C. This yields a powder of the uncrosslinked polyethylene dissolved in the xylene. After separating the uncrosslinked polyethylene, the powder is dissolved in a specified eluent for molecular weight determination, and the molecular weight distribution of the uncrosslinked polyethylene is determined by gel permeation chromatography (GPC). In this case, the molecular weight distribution of the uncrosslinked polyethylene is determined based on a calibration curve prepared using polystyrene (PS) as a standard sample.

[0099] like Figure 2 As shown, the molecular weight distribution of the raw material polyethylene is widely distributed from low to high molecular weight. As mentioned above, the number average molecular weight Mn0 of the raw material polyethylene is, for example, above 30,000 and below 60,000.

[0100] Comparative Example 1 illustrates the molecular weight distribution of uncrosslinked polyethylene when a conventional crosslinking process is performed. In Comparative Example 1, the polyethylene constituting the insulating layer is sufficiently crosslinked because it is crosslinked at an appropriate temperature. On the other hand, in Comparative Example 1, the amount of uncrosslinked polyethylene remaining is too small. Therefore, in Comparative Example 1, the molecular weight distribution of the uncrosslinked polyethylene is 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 in Comparative Example 1 is, for example, less than 5000.

[0101] As in Comparative Example 1, when the number average molecular weight of uncrosslinked polyethylene is less than 5000, there is too little low-molecular-weight uncrosslinked polyethylene that is prone to thermal aging. Therefore, it is possible that it is not uncrosslinked polyethylene, but rather crosslinked polyethylene, that directly becomes the aging target. As a result, the insulation layer is prone to thermal aging.

[0102] Comparative Example 2 illustrates the molecular weight distribution of uncrosslinked polyethylene at excessively low or high crosslinking temperatures. At excessively low crosslinking temperatures, free radicals are difficult to generate, resulting in insufficient crosslinking of the polyethylene constituting the insulating layer. Conversely, at excessively high crosslinking temperatures, the crosslinking agent volatilizes during the crosslinking process, or only the outermost surface of the insulating layer is over-crosslinked, thus the polyethylene is not sufficiently crosslinked throughout the entire insulating layer. Therefore, in Comparative Example 2, most of the raw polyethylene was not consumed during crosslinking, resulting in excessive residual uncrosslinked polyethylene. In Comparative Example 2, the molecular weight of the uncrosslinked polyethylene was distributed over a wide range, with the peak of the molecular weight distribution located in the high molecular weight range. As a result, the number-average molecular weight MnR2 of the uncrosslinked polyethylene in Comparative Example 2 was, for example, greater than 25,000.

[0103] As in Comparative Example 2, when the number average molecular weight of uncrosslinked polyethylene is greater than 25,000, as mentioned above, excessive amounts of uncrosslinked polyethylene remain across a wide molecular weight range. Therefore, uncrosslinked polyethylene with excessive residues across this range is prone to thermal aging. Over-thermally aged uncrosslinked polyethylene may further attack crosslinked polyethylene. Alternatively, when the number average molecular weight of uncrosslinked polyethylene is greater than 25,000, the difference in aging behavior between crosslinked and uncrosslinked polyethylene becomes smaller. From this perspective, crosslinked polyethylene is also prone to direct thermal aging. As a result, the thermal aging of the entire insulation layer may be accelerated.

[0104] In contrast, in this embodiment, through the crosslinking process S400, described later as a novel manufacturing method, crosslinked polyethylene is sufficiently formed in the insulating layer 130, and an appropriate amount of uncrosslinked polyethylene remains. Thus, as... Figure 2 As shown, the molecular weight distribution of the uncrosslinked polyethylene in this embodiment is, for example, between the molecular weight distribution of Comparative Example 1 and the molecular weight distribution of Comparative Example 2.

[0105] Specifically, in this embodiment, the number average molecular weight Mn of the uncrosslinked polyethylene in the insulating layer 130 is, for example, 5000 or more and 25000 or less.

[0106] In this embodiment, by setting the number-average molecular weight Mn of the uncrosslinked polyethylene to 5000 or higher, it is possible to suppress the excessive formation of crosslinked polyethylene while allowing a certain amount of low-molecular-weight uncrosslinked polyethylene, which is prone to thermal aging, to remain. Therefore, low-molecular-weight uncrosslinked polyethylene can be used as an aging target, and the aging attack on crosslinked polyethylene can be suppressed. As a result, thermal aging of the insulation layer can be suppressed.

[0107] On the other hand, in this embodiment, by setting the number-average molecular weight Mn of the uncrosslinked polyethylene to 25,000 or less, crosslinked polyethylene can be sufficiently formed, and excessive residue of uncrosslinked polyethylene, which is prone to heat aging across a wide molecular weight range, can be suppressed. This suppresses further attack on the crosslinked polyethylene caused by heat-aged uncrosslinked polyethylene. Furthermore, by setting the number-average molecular weight Mn of the uncrosslinked polyethylene to 25,000 or less, the difference in aging behavior between crosslinked and uncrosslinked polyethylene can be maintained. The state of uncrosslinked polyethylene's easy aging can be maintained, and direct heat aging of crosslinked polyethylene can be suppressed. As a result, heat aging throughout the insulation layer 130 can be suppressed.

[0108] In this embodiment, the weight-average molecular weight (Mw) of the uncrosslinked polyethylene in the insulating layer 130 can 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 used as an aging target, suppressing aging attack on crosslinked polyethylene. On the other hand, by setting the weight-average molecular weight (Mw) of the uncrosslinked polyethylene to 100,000 or less, excessive thermal aging of uncrosslinked polyethylene across a wide molecular weight range can be suppressed. Thus, further attack on crosslinked polyethylene caused by thermally aged uncrosslinked polyethylene can be suppressed.

[0109] In this embodiment, the ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn of the uncrosslinked polyethylene in the insulating layer 130, Mw / Mn, can be, for example, 2 or more and 8 or less, or 3 or more and 7 or less.

[0110] The "Mw / Mn" mentioned here is also known as the Polydispersity Index, and is defined as an indicator (numerical value) representing the degree of broadening of the molecular weight distribution. The larger the Mw / Mn, the wider the molecular weight distribution.

[0111] In this embodiment, by setting the Mw / Mn ratio of the uncrosslinked polyethylene to 2 or more, or 3 or more, low molecular weight uncrosslinked polyethylene that functions as an aging target can be sufficiently ensured. This allows for stable suppression of aging attack on crosslinked polyethylene. On the other hand, in this embodiment, by setting the Mw / Mn ratio of the uncrosslinked polyethylene to 8 or less, or 7 or less, excessive formation of thermally aged uncrosslinked polyethylene across a wide molecular weight range can be suppressed. This allows for stable suppression of further attack on crosslinked polyethylene caused by thermally aged uncrosslinked polyethylene.

[0112] (Gel fraction)

[0113] In this embodiment, the gel fraction (degree of crosslinking) of the insulating layer 130 is appropriately adjusted through the crosslinking process S400, which is described later as a new manufacturing method.

[0114] Specifically, in this embodiment, the gel fraction of the insulating layer 130 can be, for example, 60% or more and 90% or less. By setting the gel fraction of the insulating layer 130 to 60% or more, excessive formation of heat-aged uncrosslinked polyethylene can be suppressed. This allows for the stable suppression of further attack on crosslinked polyethylene caused by heat-aged uncrosslinked polyethylene. On the other hand, by setting the gel fraction of the insulating layer 130 to 90% or less, the uncrosslinked polyethylene functioning as an aging target can be sufficiently ensured. This allows for the stable suppression of aging attack on crosslinked polyethylene.

[0115] (Cross-linked state)

[0116] In this embodiment, the polyethylene constituting the insulating layer 130 is cross-linked using an organic peroxide. Cross-linking using organic peroxides is sometimes referred to as chemical cross-linking or thermal cross-linking.

[0117] That is, the insulating layer 130 in this embodiment is not crosslinked using the so-called silane crosslinking method. The "silane crosslinking method" described herein is performed according to the following steps: 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 brought into contact with water in the presence of a silanol condensation catalyst, thereby crosslinking the polyethylene. As a result, in the silane crosslinking method, silicon atoms (and oxygen atoms) are contained at the crosslinking points of the polyethylene. In this silane crosslinking method, it is difficult to increase the gel fraction. Furthermore, when the insulating layer 130 is thick, water has difficulty penetrating into the interior of the insulating layer 130, thus making it difficult to achieve uniform crosslinking of the entire insulating layer 130.

[0118] In contrast, in this embodiment, the polyethylene constituting the insulating layer 130 is thermally cross-linked using an organic peroxide as described above. Therefore, the cross-linked polyethylene in the insulating layer 130 does not contain silicon atoms (and oxygen atoms) at the cross-linking points. This allows for the maintenance of a strong bond within the cross-linked polyethylene in the insulating layer 130. The gel fraction of the insulating layer 130 can be stably maintained within the aforementioned range. Furthermore, even when the insulating layer 130 is relatively thick, a uniform cross-linking state can be obtained throughout the insulating layer 130 while retaining a specified amount of uncross-linked polyethylene.

[0119] (4) About antioxidants

[0120] In this embodiment, the antioxidant is not limited. However, in this embodiment, based on the relationship between the uncrosslinked polyethylene remaining in the insulating layer 130 and the antioxidant, the antioxidant may have, for example, the following composition.

[0121] In this embodiment, the insulating layer 130 may, for example, contain an antioxidant with a molecular weight of 300 or more and 1100 or less.

[0122] Here, during thermal aging, hydrogen is stripped from the polymer molecular chains, generating free radicals. These free radicals from thermal aging combine with oxygen, forming aging sites. Before oxygen combines with these free radicals, antioxidants deactivate them. This inhibits polymer oxidation.

[0123] However, if the molecular weight of the antioxidant is too low, it may deactivate free radicals generated in the crosslinking agent, or free radicals in the polymer backbone generated by the free radicals of the crosslinking agent abstracting hydrogen from the polymer backbone. Therefore, it may be difficult to obtain the desired degree of crosslinking (gel fraction). On the other hand, if the molecular weight of the antioxidant is too high, the antioxidant is difficult to move within the polymer. Therefore, the antioxidant is difficult to disperse uniformly in the polymer. Consequently, the frequency with which the antioxidant functions against free radicals generated in the polymer molecular chains during thermal aging decreases. Based on these reasons, there is an optimal range of antioxidant molecular weights depending on the polymer to which the antioxidant functions.

[0124] Therefore, in this embodiment, by setting the molecular weight of the antioxidant to 300 or higher, the deactivation of free radicals generated in the crosslinking agent or in the polyethylene backbone can be suppressed. This allows for the stable acquisition of the desired degree of crosslinking (gel fraction). On the other hand, by setting the molecular weight of the antioxidant to 1100 or lower, although the detailed mechanism is not yet clear, the antioxidant can be stably dispersed in uncrosslinked polyethylene that meets the above-mentioned number average molecular weight. Stable dispersion of the antioxidant allows the uncrosslinked polyethylene to retain the antioxidant. As a result, oxidative aging of the uncrosslinked polyethylene can be stably suppressed.

[0125] In this embodiment, the antioxidant may be, for example, a phenol.

[0126] Antioxidants can be, for example, hindered phenols with at least some bulky substituents. Examples of bulky substituents include tert-butyl. If the antioxidant is not a hindered phenol, i.e., does not have bulky substituents, it may deactivate free radicals generated in the crosslinking agent, or free radicals in the polyethylene backbone resulting therefrom. In contrast, by having bulky substituents, excessive movement of the antioxidant itself can be suppressed. Thus, the deactivation of free radicals generated in the crosslinking agent, or free radicals in the polyethylene backbone resulting therefrom, can be suppressed.

[0127] In phenolic antioxidants, bulky substituents can be configured, for example, as so-called "double hindered" substituents, at adjacent positions on both sides of the hydroxyl group relative to the phenolic skeleton.

[0128] Alternatively, in phenolic antioxidants, bulky substituents can be configured, for example, as so-called "single hindered" substituents, only at the ortho position relative to the hydroxyl group of the phenolic skeleton. Specifically, the antioxidant can, for example, have a hydrogen atom or an alkyl group with 1 or more but less than 3 carbon atoms at one ortho position relative to the hydroxyl group of the phenolic skeleton. That is, at least one ortho position relative to the hydroxyl group of the antioxidant can be free of bulky substituents. This reduces steric hindrance around the hydroxyl group of the antioxidant. By reducing steric hindrance, the antioxidant molecule can move more easily. As a result, the antioxidant can function not only in uncrosslinked polyethylene but also in complexly crosslinked polyethylene.

[0129] Alternatively, in this embodiment, the antioxidant may be, for example, an amine.

[0130] In this embodiment, the melting point of the antioxidant can be, for example, below 200°C. Therefore, in the resin composition preparation step S100, which mixes the polyethylene as the base resin and the antioxidant, and the crosslinking step S400, the antioxidant can be melted into the base resin. As a result, the antioxidant can be uniformly dispersed in the base resin.

[0131] There is no specific lower limit to the melting point of antioxidants, but the melting point of antioxidants is, for example, above 0°C.

[0132] The following substances can be listed as examples of antioxidants used in this embodiment.

[0133] ·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℃)

[0134] ·4,4'-Thiobis(6-tert-butyl-m-cresol) (Molecular weight 359, melting point 160℃)

[0135] • Bis[4-(1-phenyl-1-methylethyl)phenyl]amine (molecular weight 406, melting point 100℃)

[0136] ·1,2,3,4-Butanetetracarboxylic acid tetra(1,2,2,6,6-pentamethylpiperidin-4-yl) ester (molecular weight 847, melting point 65℃)

[0137] · 1,2,3,4-Butanetetracarboxylic acid tetra(2,2,6,6-tetramethyl-4-piperidinyl) ester (molecular weight 791, melting point 125~135℃)

[0138] 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, the heat aging resistance of the insulating layer 130 can be stably obtained. On the other hand, by setting the content of the antioxidant to 5 parts by mass or less, the desired degree of crosslinking (gel fraction) can be stably obtained without hindering crosslinking.

[0139] (5) Cable characteristics

[0140] In this embodiment, by satisfying the above requirements through the insulating layer 130, the following characteristics of the insulating layer 130 can be obtained.

[0141] In this embodiment, even when subjected to a heat aging test at a higher temperature and for a longer period of time than in previous heat aging tests, the insulation layer 130 exhibits good heat aging resistance.

[0142] Specifically, in this embodiment, after a heat aging test at 180°C for 21 days, the sheet obtained from the insulating layer 130 has a tensile elongation at break of 50% or more, as measured at 25°C according to JISC3005:2014.

[0143] The sheet obtained from the insulation layer 130 was subjected to a thermal aging test at 180°C for 21 days, which is equivalent to several decades at normal operating temperatures, according to the rule of halving every 8°C.

[0144] In the determination of tensile elongation at break according to JISC3005:2014, the tensile elongation at break ε is determined, for example, under test conditions of tensile speed of 200 mm / min and distance between marks of 25 mm, and is calculated by the following formula (1).

[0145]

[0146] Here, L1 is the length between the markings at the time of breakage (mm), and L0 is the distance between the markings (mm).

[0147] In this embodiment, the insulation layer 130 meets the aforementioned tensile elongation at break after undergoing a thermal aging test under harsh conditions. Even for power cables 10 that generate heat due to prolonged energization, thermal aging in terms of elongation of the insulation layer 130 can be suppressed. As a result, the required flexibility of the power cable 10 can be maintained for a long period of time.

[0148] In this embodiment, after a thermal aging test at 180°C for 21 days, the sheet obtained from the insulating layer 130 has a volume resistivity of 1×10⁻⁶, measured under conditions of 90°C and a DC electric field of 10 kV / mm. 12 Ω·cm or higher.

[0149] In this embodiment, the insulation layer 130 meets the aforementioned volume resistivity after undergoing a thermal aging test under harsh conditions. Even in power cables 10 that generate heat due to prolonged energization, thermal aging of the insulation layer 130 can be suppressed. As a result, the insulation performance of the insulation layer 130 required by the power cable 10 can be maintained for a long period of time.

[0150] In this embodiment, even under conventional heat aging tests, the insulation layer 130 naturally exhibits good heat aging resistance.

[0151] Specifically, in this embodiment, after the sheet obtained from the insulating layer 130 is heated at 120°C for 7 days for thermal aging test, the tensile elongation at break of the sheet, measured at 25°C according to JISC3005:2014, is, for example, more than 300%, or may exceed 500%.

[0152] (6) Manufacturing method of power cables

[0153] Next, refer to Figure 3 The manufacturing method of the power cable according to this embodiment will be described.

[0154] like Figure 3 As shown, the manufacturing method of the power cable in this embodiment includes, for example, a resin composition preparation step S100, a conductor preparation step S200, an extrusion step S300, a crosslinking step S400, a shielding layer formation step S500, and a sheath formation step S600.

[0155] (S100: Resin composition preparation process)

[0156] First, a resin composition constituting the insulating layer 130 of this embodiment is prepared.

[0157] In this embodiment, a mixer is used to mix (knead) a base resin containing polyethylene, a crosslinking agent, an antioxidant, and other additives to form a mixed material. Examples of mixers include open roller mixers, Banbury mixers, pressure kneaders, single-screw mixers, and multi-screw mixers. The mixing can be performed once or in multiple stages.

[0158] In this embodiment, the polyethylene used as the pre-crosslinking polyethylene can, for example, be a polyethylene with 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. Therefore, in the crosslinking step S400 described later, the desired molecular weight distribution of the uncrosslinked polyethylene can be stably formed.

[0159] In this embodiment, an antioxidant, for example, can be a material with a molecular weight of 300 or more and 1100 or less. By setting the molecular weight of the antioxidant to 300 or more, the deactivation of free radicals generated in the crosslinking agent or in the polyethylene backbone generated therefrom can be suppressed in the crosslinking step S400 described later. On the other hand, by setting the molecular weight of the antioxidant to 1100 or less, the antioxidant can be stably dispersed in uncrosslinked polyethylene that meets the desired number average molecular weight in the crosslinking step S400 described later.

[0160] After the mixture is formed, it is granulated using an extruder. This forms a granular resin composition constituting the insulating layer 130. Alternatively, a twin-screw extruder with high mixing action can be used to perform the mixing and granulation processes simultaneously.

[0161] (S200: Conductor preparation process)

[0162] On the other hand, a conductor 110 is prepared to be formed by twisting together multiple conductor cores.

[0163] (S300: Extrusion process)

[0164] After the resin composition preparation step S100 and the conductor preparation step S200 are completed, in the extrusion step S300, the above-mentioned resin composition is used to form an insulating layer 130 in such a way as to cover the outer periphery of the conductor 110.

[0165] 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.

[0166] Specifically, in a three-layer co-extruder, in extruder A that forms the internal semi-conductive layer 120, for example, a composition for the internal semi-conductive layer is fed in.

[0167] The aforementioned granular resin composition is fed into extruder B, which forms the insulating layer 130. At this time, the set temperature of extruder B is set to a temperature at least 5°C higher than the melting point of the base resin and less than 50°C. The set temperature is adjusted appropriately based on the linear speed and extrusion pressure.

[0168] Furthermore, in the extruder C that forms the outer semiconductive layer 140, an outer semiconductive layer composition containing the same material as the resin composition for the inner semiconductive layer fed into the extruder A is fed.

[0169] Next, the extrudates from extruders A through C are fed into a common die 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 the extruded material that becomes the cable core.

[0170] (S400: Crosslinking process)

[0171] After the extrusion process S300 is completed, in the crosslinking process S400 of this embodiment, at least a portion of the insulating layer 130 is crosslinked using a crosslinking agent. The crosslinking process S400 of this embodiment is performed, for example, by a so-called dry crosslinking method.

[0172] In this embodiment, cross-linked polyethylene that is insoluble in xylene at 120°C is formed in the polyethylene of the insulating layer 130, while uncross-linked polyethylene that can dissolve in xylene at 120°C remains. Furthermore, the number average molecular weight of the uncross-linked polyethylene is set to be 5000 or more and 25000 or less.

[0173] In this embodiment, for example, the crosslinking temperature is changed in two stages to crosslink the insulating layer 130.

[0174] Here, we consider the case where the crosslinking temperature is not changed during the crosslinking process and is kept constant in a single stage.

[0175] In this scenario, if an appropriate crosslinking temperature is set in a single stage, crosslinking proceeds normally; that is, the crosslinking agent causes the hydrogen-de-hydrogenated portions of the polyethylene to combine with other polyethylene molecules. Therefore, it is difficult for uncrosslinked polyethylene to remain in the insulation layer 130. As a result, the peak of the molecular weight distribution of the uncrosslinked polyethylene is located in the low molecular weight range.

[0176] Alternatively, if the crosslinking temperature is set too low in a single stage, free radicals cannot be sufficiently generated. Therefore, although a large amount of uncrosslinked polyethylene remains, crosslinked polyethylene cannot be sufficiently formed in the insulation layer 130. As a result, the molecular weight distribution peaks of the uncrosslinked polyethylene are located in the high molecular weight range.

[0177] Alternatively, if the crosslinking temperature is set too high in a single stage, the crosslinking agent evaporates, or only the outermost surface of the insulating layer 130 is crosslinked. Therefore, most of the raw polyethylene is not consumed in the crosslinking process, resulting in a higher amount of uncrosslinked polyethylene remaining. In this case, the peak of the molecular weight distribution of the uncrosslinked polyethylene is also located in the high molecular weight range.

[0178] In contrast, in this embodiment, by changing the crosslinking temperature in two stages, it is possible to retain uncrosslinked polyethylene that meets the above-mentioned number-average molecular weight in the insulating layer 130.

[0179] Specifically, the crosslinking process S400 in this embodiment includes, for example, a first crosslinking process S420 and a second crosslinking process S440.

[0180] In the first crosslinking step S420 and the second crosslinking step S440, for example, using Figure 4 The crosslinking device 30 shown is an example. Figure 4 As shown, the crosslinking device 30 of this embodiment includes, for example, a crosslinking tube 320 and a plurality of zone heaters 340.

[0181] Within the cross-linking tube 320, for example, an extruded material serving as the cable core of the power cable 10 is threaded through. The cross-linking tube 320 is pressurized by nitrogen or the like.

[0182] The zone heater 340 is arranged, for example, to surround the outer periphery of the crosslinking tube 320. The zone heater 340 is configured, for example, to heat the infrared heater inside the crosslinking tube 320 by infrared radiation.

[0183] For example, multiple zone heaters 340 are provided. These multiple zone heaters 340 are arranged, for example, along the axial direction of the crosslinking pipe 320. The multiple zone heaters 340 are configured, for example, to allow individual adjustment of their heating temperature.

[0184] The aforementioned crosslinking device 30 is divided into two regions, for example, based on the crosslinking temperature reached by the zone heaters 340. Specifically, the crosslinking device 30, for example, has a first crosslinking region 32 heated to a first crosslinking temperature and a second crosslinking region 34 heated to a second crosslinking temperature sequentially along the insertion direction of the power cable 10. In each of the first crosslinking region 32 and the second crosslinking region 34, the crosslinking time is adjusted by the number of zone heaters 340 (i.e., the axial length of each region).

[0185] (S420: First crosslinking process)

[0186] In the first crosslinking process S420, a portion of crosslinked polyethylene is formed in the insulating layer 130 at a first crosslinking temperature in the first crosslinking region 32 of the crosslinking apparatus 30. This ensures that the required amount of crosslinked polyethylene is achieved in the insulating layer 130.

[0187] In the first crosslinking step S420, polyethylene is crosslinked, and the crosslinking agent is retained in the polyethylene so that, compared with the case where the first crosslinking step S420 is not performed, the crosslinking agent is less likely to volatilize at the second crosslinking temperature described later. That is, by making the molecules of the crosslinked portion of the polyethylene less mobile, the volatilization of the crosslinking agent caused by heat can be suppressed.

[0188] As mentioned above, if the crosslinking temperature is directly increased, the crosslinking agent tends to volatilize. Therefore, it is difficult to form fully crosslinked polyethylene.

[0189] In contrast, in this embodiment, the first crosslinking temperature is set lower than the second crosslinking temperature (described later) in the first crosslinking step S420, so that the crosslinking agent is retained in the polyethylene. Therefore, even if the crosslinking temperature is increased to the second crosslinking temperature in the second crosslinking step S440, the 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 even in the second crosslinking step S440.

[0190] Specifically, the first crosslinking temperature is set to, for example, 180°C or higher and 275°C or lower. By setting the first crosslinking temperature to 180°C or higher, it is possible to ensure a certain amount of crosslinked polyethylene and to retain the crosslinking agent within the polyethylene. On the other hand, by setting the first crosslinking temperature to 275°C or lower, excessive volatilization of the crosslinking agent in the first crosslinking step S420 can be suppressed.

[0191] The crosslinking time of the first crosslinking step S420 is set, for example, to be more than 1 minute and less than 30 minutes. By setting the crosslinking time of the first crosslinking step S420 to more than 1 minute, it is possible to ensure a certain amount of crosslinked polyethylene and to retain the crosslinking agent in the polyethylene. On the other hand, by setting the crosslinking time of 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.

[0192] (S440: Second crosslinking process)

[0193] In the second crosslinking process 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 apparatus 30. On the other hand, hydrogen is recombined into the polyethylene, at least a portion of which has been dehydrogenated, thereby leaving uncrosslinked polyethylene in the insulating layer 130.

[0194] In other words, it makes it difficult for the molecular chains of further cross-linked or uncross-linked polyethylene to bind to the hydrogen-free portion of the polyethylene. That is, it can inhibit the excessive formation of cross-linking points and suppress the excessive cross-linking of polyethylene.

[0195] Therefore, in the second crosslinking process S440, it is possible to stably balance the formation of crosslinked polyethylene and the residue of uncrosslinked polyethylene.

[0196] Specifically, the second crosslinking temperature is set, for example, to 280°C or higher and 400°C or lower. By setting the second crosslinking temperature to 280°C or higher, it is possible to stably balance the formation of crosslinked polyethylene and the residue of uncrosslinked polyethylene. On the other hand, by setting the second crosslinking temperature to 400°C or lower, excessive volatilization of the crosslinking agent in the second crosslinking step S440 can be suppressed.

[0197] The crosslinking time of the first crosslinking step S420 is set, for example, to be 1 minute or more and less than 50 minutes. By setting the crosslinking time of the second crosslinking step S440 to be 1 minute or more, it is possible to stably balance the formation of crosslinked polyethylene and the residue of uncrosslinked polyethylene. On the other hand, by setting the crosslinking time of the second crosslinking step S440 to be less than 50 minutes, it is possible to suppress excessive crosslinking of polyethylene in the second crosslinking step S440.

[0198] Through the first crosslinking process S420 and the second crosslinking process S440 described above, uncrosslinked polyethylene with a number average molecular weight of 5,000 or more and 25,000 or less can remain in the insulating layer 130.

[0199] Then, the cable core that has passed through the cross-linking device 30 is cooled by water cooling.

[0200] Through the above processes, a cable core consisting of conductor 110, inner semiconducting layer 120, insulation layer 130 and outer semiconducting layer 140 can be formed.

[0201] (S500: Shielding layer formation process)

[0202] After the crosslinking process S400 is completed, a shielding layer 150 is formed on the outside of the outer semiconductive layer 140, for example by winding copper tape.

[0203] (S600: Sheath forming process)

[0204] After the shielding layer 150 is formed, a sheath 160 is formed on the outer periphery of the shielding layer 150 by extruding vinyl chloride into an extruder.

[0205] Through the above processes, a power cable 10, which is a solid insulated power cable, can be manufactured.

[0206] (7) Summary of this implementation method

[0207] According to this embodiment, one or more of the following effects are achieved.

[0208] In this embodiment, the number average molecular weight Mn of the uncrosslinked polyethylene in the insulating layer 130 is 5000 or more and 25000 or less.

[0209] In this embodiment, by setting the number-average molecular weight Mn of the uncrosslinked polyethylene to 5000 or higher, it is possible to suppress the excessive formation of crosslinked polyethylene while allowing a certain amount of low-molecular-weight uncrosslinked polyethylene, which is prone to thermal aging, to remain. Therefore, low-molecular-weight uncrosslinked polyethylene can be used as an aging target, that is, it can function as a sacrificial component for thermal aging. By using uncrosslinked polyethylene as an aging target, the aging attack on crosslinked polyethylene can be suppressed. As a result, the thermal aging of the insulation layer can be suppressed.

[0210] On the other hand, in this embodiment, by setting the number-average molecular weight Mn of the uncrosslinked polyethylene to 25,000 or less, crosslinked polyethylene can be sufficiently formed, and excessive residue of uncrosslinked polyethylene that is prone to heat aging across a wide molecular weight range can be suppressed. This suppresses excessive heat aging of uncrosslinked polyethylene across a wide molecular weight range. Further attack on crosslinked polyethylene caused by heat-aged 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 aging behavior between crosslinked and uncrosslinked polyethylene can be maintained. The state of uncrosslinked polyethylene being prone to aging can be maintained, and direct heat aging of crosslinked polyethylene can be suppressed. As a result, heat aging throughout the entire insulation layer 130 can be suppressed.

[0211] As described above, according to this embodiment, the heat aging resistance of the insulation layer 130 can be improved. That is, even if the power cable 10 heats up due to long-term energization, it can maintain the various characteristics required for the power cable 10 (flexibility and insulation, etc.).

[0212] (8) Variations of this embodiment

[0213] The above embodiments can be modified as needed, as shown in the following variations. Hereinafter, only elements different from those described in the above embodiments will be described; elements substantially the same as those described in the above embodiments will be marked with the same reference numerals and their descriptions will be omitted.

[0214] In a modified example, the power cable 10 is configured for direct current (DC) applications. In another modified example, the insulation layer 130 may contain an inorganic filler.

[0215] (Inorganic filler)

[0216] Inorganic fillers are used to capture space charge in the insulating layer 130 and suppress the local accumulation of space charge in the insulating layer 130. As a result, the insulation properties of the insulating layer 130 can be improved.

[0217] Inorganic fillers include, for example, at least one of zinc oxide, titanium oxide, magnesium oxide, silicon dioxide, carbon black, carbon nanotubes, graphene, etc.

[0218] In this embodiment, at least a portion of the inorganic filler may be surface-treated with a silane coupling agent.

[0219] (Modified polymer)

[0220] In this embodiment, the insulating layer 130 serves as the base resin, which may include, for example, a modified polymer. The modified polymer in this embodiment is, for example, a resin containing olefin units and modified with at least one of unsaturated organic acids and their derivatives. The modified polymer may, for example, be unsaturated carboxylic acid-modified polyethylene. Among these, the modified polymer may, for example, be modified with maleic anhydride. By including such a modified polymer, the compatibility (adhesion) between the polar inorganic filler and the base resin can be improved, as can the dispersibility of the inorganic filler.

[0221] <Other embodiments of this disclosure>

[0222] 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.

[0223] In the above embodiment, three-layer co-extrusion is performed in the extrusion process S300, but it can also be performed layer by layer.

[0224] Example

[0225] Next, embodiments of the present disclosure will be described. These embodiments are examples of the present disclosure, and the present disclosure is not limited to these embodiments.

[0226] (1) Fabrication of power cables

[0227] As described below, power cables of samples A1 to A9 and B1 to B5 were manufactured.

[0228] <Samples A1~A9>

[0229] In samples A1 to A9, a resin composition containing the following materials was mixed using a Banbury mixer and granulated into granules by an extruder.

[0230] (Base resin)

[0231] Low-density polyethylene (LDPE): 100 parts by weight

[0232] Density: 0.92 g / cm³ 3

[0233] Melting point: 105℃

[0234] Number average molecular weight: 39,000

[0235] Weight-average molecular weight: 182,000

[0236] (Cross-linking agent)

[0237] 1.5 parts by weight of dicumyl peroxide

[0238] (Antioxidants)

[0239] As an antioxidant, it is formulated with 0.15 parts by weight of any of the following materials.

[0240] (Antioxidant A)

[0241] 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

[0242] Molecular weight: 699

[0243] Melting point: 160℃

[0244] (Antioxidant B)

[0245] 4,4'-Thiobis(6-tert-butyl-m-cresol)

[0246] Molecular weight: 359

[0247] Melting point: 160℃

[0248] (Antioxidant C)

[0249] 1,2,3,4-Butanetetracarboxylic acid tetra(1,2,2,6,6-pentamethylpiperidin-4-yl) ester

[0250] Molecular weight: 847

[0251] Melting point: 65℃

[0252] (Antioxidant D)

[0253] Pentaerythritol tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]

[0254] Molecular weight: 1178

[0255] Melting point: 125℃

[0256] (Antioxidant E)

[0257] Butylated hydroxytoluene

[0258] Molecular weight: 220

[0259] Melting point: 70℃

[0260] Next, a cross-sectional area of ​​600 mm² will be formed by stranding a conductor core made of diluted copper alloy with a diameter of 5 mm. 2 The conductor is prepared. After preparing the conductor, the resin composition for the inner semiconductive layer containing ethylene-ethyl acrylate copolymer, the resin composition for the insulating layer, and the resin composition for the outer semiconductive layer made of the same material as the resin composition for the inner semiconductive layer are respectively fed into extruders A to C. In addition, the temperature of extruder B is set to a temperature 20°C higher than the melting point of LDPE, which is the base resin.

[0261] The extrudates from extruders A through C are fed into a common die head, where an inner semiconductive layer, an insulating layer, and an outer semiconductive layer are simultaneously extruded from the inside out around the conductor. The thicknesses of the inner, insulating, and outer semiconductive layers are set to 0.5 mm, 9 mm, and 0.5 mm, respectively.

[0262] Next, using the aforementioned crosslinking apparatus, the crosslinking temperature was varied in two stages to crosslink the insulation layer. The conditions for the first and second crosslinking processes for each sample are described in Tables 1 and 2 below. After the crosslinking process, the power cable was cooled by water cooling at 25°C.

[0263] Through the above processes, power cables of samples A1 to A9 were manufactured.

[0264] <Samples B1~B5>

[0265] Samples B1 to B5 were manufactured in the same manner as samples A1 to A9, except that the crosslinking process, which is a new manufacturing method, was not used. That is, in samples B1 to B5, the crosslinking temperature was not changed, and the crosslinking temperature was kept constant in a single stage. The conditions of the crosslinking process for each sample are described in Table 3 below.

[0266] (2) Evaluation

[0267] (Image Acquisition)

[0268] Multiple insulation sheets were obtained by thinly slicing the insulation layer of the power cable. The thickness of each sheet was 1 mm. The insulation sheets were obtained from the center of the insulation layer along its thickness direction.

[0269] (Molecular weight distribution of uncrosslinked polyethylene)

[0270] The sheet obtained from the insulating layer was impregnated in xylene, and the xylene in this state was heated at 120°C for 24 hours. Then, the xylene solution from which the uncrosslinked polyethylene was dissolved was filtered through filter paper heated to 120°C. The filtered xylene solution was then evaporated at 120°C to obtain uncrosslinked polyethylene powder.

[0271] After separating the uncrosslinked polyethylene, the uncrosslinked polyethylene powder was dissolved in the following eluent, and the molecular weight distribution of the uncrosslinked polyethylene was determined by GPC under the following conditions. The molecular weight distribution of the uncrosslinked polyethylene was determined based on a calibration curve prepared by GPC using PS as the standard sample. The number-average molecular weight Mn, weight-average molecular weight Mw, and their ratio Mw / Mn of the uncrosslinked polyethylene were determined.

[0272] Device: Tosoh HLC-8321GPC / HT

[0273] Eluent: o-dichlorobenzene

[0274] Temperature: 145℃

[0275] Concentration: 0.1wt% / vol%

[0276] Flow rate: 1.0 ml / min

[0277] In addition, the PS calibration curve is based on results for a molecular weight range of 1,000 to 5.5 million.

[0278] (Thermal Aging Test 1)

[0279] The sheet obtained from the insulating layer is placed in a specified constant temperature bath and heated at 120°C for 7 days.

[0280] (Elongation after heat aging test 1)

[0281] Following heat aging test 1, the tensile elongation at break of the sheet was measured according to JISC3005:2014 under test conditions of 25℃, tensile speed 200mm / min, and mark spacing 25mm. The results showed that a tensile elongation at break of 300% or higher after heat aging test 1 was rated as "good," while a tensile elongation at break of less than 300% was rated as "poor."

[0282] (Thermal Aging Test 2)

[0283] The sheet obtained from the insulating layer is placed in a specified constant temperature bath and heated at 180°C for 21 days.

[0284] (Elongation after heat aging test 2)

[0285] Following heat aging test 2, the tensile elongation at break of the sheet was measured according to JISC3005:2014 under test conditions of 25℃ temperature, 200mm / min tensile speed, and 25mm mark spacing. The results showed that a tensile elongation at break of 50% or more after heat aging test 2 was rated as "good," while a tensile elongation at break of less than 50% was rated as "poor."

[0286] (Volume resistivity after thermal aging test 2)

[0287] After thermal aging test 2, the volume resistivity of the sheet was measured at a temperature of 90℃ and a DC electric field of 10kV / mm. The result showed that the volume resistivity after thermal aging test 2 was 1.0×10⁻⁶. 12 Conditions above Ω·cm are rated as "good," and the volume resistivity after heat aging test 2 is less than 1.0 × 10⁻⁶. 12 The Ω·cm rating is "unacceptable".

[0288] (Gel fraction)

[0289] Using the obtained tablets, the gel fraction was determined through the following steps. First, the mass of the tablets before xylene impregnation was measured. Next, the entire tablet was impregnated in xylene, and the xylene in this state was heated at 120°C for 1 day. Then, the remaining tablets were removed from the xylene, and only the insoluble portion of the tablets was dried in the atmosphere at 120°C for 1 day. Afterward, the mass of the insoluble portion of the remaining tablets after xylene impregnation was measured. The "gel fraction" was calculated as the ratio of the mass of the insoluble portion of the tablets after xylene impregnation to the mass of the tablets before xylene impregnation.

[0290] (3) Results

[0291] Please refer to Tables 1 to 3 below to describe the evaluation results for each sample. In each table, "aE+b" represents a × 10⁻⁶. b .

[0292] [Table 1]

[0293]

[0294] [Table 2]

[0295]

[0296] [Table 3]

[0297]

[0298] <Sample B1>

[0299] In sample B1, the gel fraction was low, and the number average molecular weight of the uncrosslinked polyethylene was greater than 25,000. Therefore, in sample B1, melting occurred after heat aging test 2.

[0300] In sample B1, due to the excessively low crosslinking temperature, the gel fraction was low, meaning that crosslinked polyethylene was not sufficiently formed. Consequently, an excessive amount of uncrosslinked polyethylene remaining across a wide molecular weight range underwent thermal aging. Therefore, the over-aged uncrosslinked polyethylene attacked the crosslinked polyethylene. As a result, the sheet melted in thermal aging test 2, which can be considered to be under more stringent conditions than the previous thermal aging test 1.

[0301] <Samples B2 and B3>

[0302] In samples B2 and B3, the gel fraction was within the specified range (above 60% and below 90%), but the number average molecular weight of the uncrosslinked polyethylene was less than 5000. Therefore, in samples B2 and B3, the elongation at break after heat aging test 2 was less than 50%, and the volume resistivity after heat aging test 2 was less than 1×10⁻⁶. 12 Ω·cm.

[0303] In samples B2 and B3, the polyethylene constituting the insulating layer was fully cross-linked due to cross-linking of the polyethylene at appropriate temperatures. However, too little low-molecular-weight uncross-linked polyethylene remained. Therefore, it was not the uncross-linked polyethylene, but rather the cross-linked polyethylene, that directly became the aging target. As a result, the tensile elongation at break and volume resistivity decreased in heat aging test 2, which is considered to be more stringent than the previous heat aging test 1.

[0304] <Sample B4>

[0305] In sample B4, the gel fraction was low, and the number average molecular weight of the uncrosslinked polyethylene was greater than 25,000. Therefore, in sample B1, the sheet melted after heat aging test 2.

[0306] In sample B4, the crosslinking agent evaporated during the crosslinking process due to the excessively high crosslinking temperature. Therefore, the polyethylene in the entire insulating layer was not sufficiently crosslinked. Consequently, an excessive amount of uncrosslinked polyethylene remaining across a wide molecular weight range underwent thermal aging. Thus, the over-aged uncrosslinked polyethylene attacked the crosslinked polyethylene. Alternatively, because the number average molecular weight of the uncrosslinked polyethylene was greater than 25,000, the difference in aging behavior between crosslinked and uncrosslinked polyethylene became smaller. Therefore, the crosslinked polyethylene directly underwent thermal aging. As a result, the sheet melted in thermal aging test 2, which is considered to be under more stringent conditions than the previous thermal aging test 1.

[0307] <Sample B5>

[0308] In sample B5, the gel fraction was within the specified range (above 60% and below 90%), but the number average molecular weight of the uncrosslinked polyethylene was less than 5000. Therefore, in sample B5, the elongation at break after heat aging test 2 was less than 50%, and the volume resistivity after heat aging test 2 was less than 1×10⁻⁶. 12 Ω·cm.

[0309] In sample B5, the crosslinking reaction rate and frequency were high due to the high temperature at which the crosslinking agent was not volatile. However, although the polyethylene was crosslinked, only the uncrosslinked polyethylene molecules bonded to each other before the low-molecular-weight uncrosslinked polyethylene molecules could move. Therefore, the uncrosslinked polyethylene also contained uncrosslinked polyethylene molecules with a molecular weight that had only increased to the point of dissolving to xylene. The amount of uncrosslinked polyethylene decreased due to their bonding, resulting in too little residual low-molecular-weight uncrosslinked polyethylene. Therefore, it was not the uncrosslinked polyethylene, but the crosslinked polyethylene, that directly became the aging target. As a result, the tensile elongation at break and volume resistivity were considered to be reduced in the more stringent heat aging test 2 than in the previous heat aging test 1.

[0310] <Samples A1~A9>

[0311] 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 5000 or more and 25000 or less. Therefore, in samples A1 to A9, the elongation at break after heat aging test 2 was 50% or more, and the volume resistivity after heat aging test 2 was 1×10⁻⁶. 12 Ω·cm or higher.

[0312] In samples A1 to A9, the crosslinking temperature was changed in two stages. As a result, uncrosslinked polyethylene with the above-mentioned number-average molecular weight could remain in the insulating layer.

[0313] In samples A1 to A9, by setting the number-average molecular weight of uncrosslinked polyethylene to 5000 or higher, low molecular weight uncrosslinked polyethylene can be used as an aging target. Conversely, in samples A1 to A9, by setting the number-average molecular weight of uncrosslinked polyethylene to 25000 or lower, further attack on crosslinked polyethylene caused by heat-aged uncrosslinked polyethylene can be suppressed. The results confirm that the heat aging resistance of the insulation layer can be improved in samples A1 to A9.

[0314] Antioxidant Dependence

[0315] Samples A1, A6-A9, manufactured under the same conditions except for the antioxidant, were compared.

[0316] In samples A1, A6, and A7, where the molecular weight of the antioxidant is set to be above 300 and below 1100, the gel fraction is in the range of above 60% and below 90%, which is greater than that of sample A9, where the molecular weight of the antioxidant is set to be 300.

[0317] In samples A1, A6, and A7, where the molecular weight of the antioxidant is between 300 and 1100, the elongation at break after heat aging test 2 is greater than that of sample A8, where the molecular weight of the antioxidant is greater than 1100. Furthermore, in samples A1, A6, and A7, the volume resistivity after heat aging test 2 is less than 1×10⁻⁶. 12 In the range above Ω·cm, its volume resistivity is higher than that of sample A8.

[0318] There were no differences in evaluation results between samples A1 and A6, which used phenolic antioxidants, and sample A7, which used amine antioxidants.

[0319] In samples A1, A6, and A7, by setting the molecular weight of the antioxidant to 300 or higher, the deactivation of free radicals generated in the crosslinking agent or in the polyethylene backbone was suppressed. This confirmed that the desired degree of crosslinking (gel fraction) could be stably obtained in samples A1, A6, and A7.

[0320] In samples A1, A6, and A7, by setting the molecular weight of the antioxidant to below 1100, the antioxidant could be stably dispersed in uncrosslinked polyethylene that met the aforementioned number-average molecular weight. This effectively suppressed the oxidative aging of the crosslinked polyethylene. As a result, samples A1, A6, and A7 were confirmed to exhibit better heat aging resistance of the insulation layer.

[0321] <Postscript>

[0322] The following describes the manner in which this disclosure is made. The manner in which the numbers in brackets [] refer to the manner described in <Embodiments of this Disclosure>. [7]

[0324] According to any one of [1] to [6] above, the power cable wherein,

[0325] The aforementioned cross-linked polyethylene does not contain silicon atoms at the cross-linking point. [8]

[0327] A method for manufacturing a power cable, comprising:

[0328] A process for preparing a resin composition comprising polyethylene and a crosslinking agent;

[0329] The process of forming an insulating layer by covering the outer periphery of a conductor using the above-described resin composition; and...

[0330] The process of crosslinking at least a portion of the above-mentioned insulating layer using the above-mentioned crosslinking agent;

[0331] In the process of crosslinking at least a portion of the above-mentioned insulating layer,

[0332] In the aforementioned polyethylene within the insulating layer, cross-linked polyethylene that is insoluble in xylene at 120°C is formed, while uncross-linked polyethylene that can dissolve in xylene at 120°C remains.

[0333] The number average molecular weight of the aforementioned uncrosslinked polyethylene is above 5000 and below 25000. [9]

[0335] According to the power cable manufacturing method described above [8], wherein,

[0336] In the process of preparing the above resin composition,

[0337] The above-mentioned polyethylene, in its pre-crosslinking state, has a number average molecular weight of 30,000 or more and 60,000 or less.

[10]

[0339] According to the manufacturing method of the power cable described in [8] or [9] above, wherein,

[0340] The process of crosslinking the above-mentioned polyethylene includes:

[0341] The first crosslinking step, which forms a portion of the aforementioned crosslinked polyethylene at a first crosslinking temperature; and,

[0342] At a second crosslinking temperature higher than the first crosslinking temperature, while further forming the crosslinked polyethylene, hydrogen is recombined to at least a portion of the polyethylene that has lost hydrogen, thereby leaving the uncrosslinked polyethylene as a second crosslinking process.

[11]

[0344] According to the power cable manufacturing method described above

[10] , wherein,

[0345] In the aforementioned first crosslinking process,

[0346] The polyethylene is crosslinked, and the crosslinking agent is retained in the polyethylene so that the crosslinking agent is less likely to volatilize at the second crosslinking temperature compared to the case where the first crosslinking process is not performed.

[0347] Explanation of reference numerals in the attached figures

[0348] 10: Power cables;

[0349] 30: Crosslinking device;

[0350] 32: First cross-linking region;

[0351] 34: Second cross-linking region;

[0352] 100: Power cable;

[0353] 110: Conductor;

[0354] 120: Internal semiconductive layer;

[0355] 130: Insulation layer;

[0356] 140: External semiconductive layer;

[0357] 150: Shielding layer;

[0358] 160: Sheath;

[0359] 320: Cross-linked tube;

[0360] 340: Zone heater.

Claims

1. A power cable, comprising: conductors; and An insulating layer, which is disposed in a manner covering the outer periphery of the conductor, is composed of a resin composition comprising polyethylene. The insulating layer comprises cross-linked polyethylene that is insoluble in xylene at 120°C and uncross-linked polyethylene that can dissolve in xylene at 120°C. The number average molecular weight of the uncrosslinked polyethylene is above 5000 and below 25000.

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

3. The power cable according to claim 1 or claim 2, wherein, The uncrosslinked polyethylene has a weight-average molecular weight of 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 with a molecular weight of 300 or more and 1100 or less.

5. The power cable according to any one of claims 1 to 4, wherein, After the sheet obtained from the insulating layer is heated at 180°C for 21 days, the tensile elongation at break of the sheet, measured at 25°C according to JISC3005:2014, is 50% or more.

6. The power cable according to any one of claims 1 to 5, wherein, The volume resistivity of the sheet obtained from the insulating layer, after being heated at 180°C for 21 days and measured at 90°C with a DC electric field of 10 kV / mm, was 1 × 10⁻⁶. 12 Ω·cm or higher.

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