Resin composition for power cable insulation, and power cable

Abstract

Provided are: a resin composition for power cable insulation, in which even if the resin composition is coated on a conductor of a power cable by means of melt extrusion, it is possible to suppress sagging under the cable's own weight; and a power cable obtained using the resin composition for insulating a power cable. The resin composition for power cable insulation contains: a polypropylene (A) that is random polypropylene having an MFR value at 230ºC of 2.0 g / 10 min or more; an elastomer (B) that is a styrene-based elastomer having an MFR value at 230ºC of 2.0 g / 10 min or less; a polyethylene (C) that is low density polyethylene; and an antioxidant (E). The resin composition has an MFR value at 230ºC of 0.1-2.0 g / 10 min, and has a self-weight extension of 150% or less in a resin sagging test.

Description

Resin composition for power cable insulation and power cable

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

[0002] Polypropylene has a higher melting point than cross-linked polyethylene (XLPE), and therefore has high heat resistance without the need for inorganic fillers or cross-linking of polymer chains, allowing a larger current to flow through the power cable conductor, thereby increasing the power transmission efficiency of the power cable. Therefore, since there is no need to add inorganic fillers, the weight of the insulation does not increase, and the burden of cable installation work does not increase.

[0003] However, polypropylene is highly crystalline and, as it is, is hard and unsuitable for use as a wire coating, so it is necessary to add an elastomer such as ethylene propylene rubber (EPR) or olefin-based thermoplastic elastomer (TPO) to impart flexibility.

[0004] For example, Patent Document 1 describes a resin composition containing propylene, ethylene, and styrene, having a melting point of 140°C or higher and 150°C or lower, and a heat of fusion of 55 J / g or higher and 90 J / g or lower.

[0005] The resin composition is used to cover the conductor of a power cable by melt-extruding the resin composition at a temperature equal to or higher than its melting point. However, when melt-extruding the resin composition of Patent Document 1, the resin composition may sag under its own weight when the conductor is covered with the molten resin composition, causing deformation of the power cable insulator. Furthermore, if the deformation becomes too great, the resin composition may fall off the conductor.

[0006] International Publication No. 2021 / 090579

[0007] An object of the present disclosure is to provide a resin composition for power cable insulation that can prevent sagging under its own weight even when used to coat a power cable conductor by melt extrusion, and a power cable using the resin composition for power cable insulation.

[0008] [1] A resin composition for power cable insulation comprising polypropylene (A), which is a random polypropylene having an MFR of 2.0 g / 10 min or more at 230°C; elastomer (B), which is a styrene-based elastomer having an MFR of 2.0 g / 10 min or less at 230°C; polyethylene (C), which is a low-density polyethylene; and antioxidant (E), wherein the resin composition has an MFR of 0.1 g / 10 min or more and 2.0 g / 10 min or less at 230°C and an elongation under its own weight of 150% or less in a resin sagging test. [2] The resin composition for power cable insulation according to [1] above, further comprising acid-modified polyethylene (D). [3] The resin composition for power cable insulation according to [2] above, wherein the acid-modified polyethylene (D) is maleic anhydride-grafted polyethylene. [4] The resin composition for power cable insulation according to any one of [1] to [3] above, wherein the blending amount of the elastomer (B) relative to the resin composition for power cable insulation is 20 wt % or more and less than 40 wt %. [5] The resin composition for power cable insulation according to any one of [2] to [4] above, wherein a total blending amount of the polyethylene (C) and the acid-modified polyethylene (D) relative to the resin composition for power cable insulation is 5 wt % or more and 20 wt % or less. [6] The resin composition for power cable insulation according to any one of [2] to [5] above, wherein the polyethylene (C) has an MFR of 10.0 g / 10 min or less at 190°C, and the acid-modified polyethylene (D) has an MFR of 10.0 g / 10 min or less at 190°C. [7] A power cable comprising: a conductor; an inner semiconducting layer disposed on the outside so as to surround the conductor; an insulating layer disposed on the outside so as to surround the inner semiconducting layer, the insulating layer being made of the resin composition for power cable insulation according to any one of [1] to [6] above; and an outer semiconducting layer disposed on the outside so as to surround the insulating layer.

[0009] According to the present disclosure, it is possible to provide a resin composition for power cable insulation that can prevent sagging under its own weight even when used to coat a power cable conductor by melt extrusion, and a power cable using the resin composition for power cable insulation.

[0010] Fig. 1 is a front view showing the shape of a No. 3 dumbbell die described in JIS K 6251, which is used to punch out a sheet-shaped resin composition in a resin sagging test. Fig. 2 is a cross-sectional view showing an example of a power cable to which the resin composition for power cable insulation of the embodiment is applied.

[0011] Hereinafter, a detailed description will be given based on an embodiment.

[0012] The present inventors have focused on and investigated the following points.

[0013] Polypropylene melts at 130°C to 175°C, while elastomers such as EPR, TPO, and styrene-based elastomers soften at even lower temperatures. Therefore, when a power cable insulating resin composition made of these resins is used as an insulator to coat a power cable conductor, the coating resin may sag under its own weight immediately after the conductor coating is completed by melt extrusion. This can result in deformation of the insulator, and if the deformation progresses further, the insulator itself may fall off the conductor.

[0014] The problem is that the thicker the insulator, the greater its own weight and therefore the greater the deformation. Specifically, when the insulator thickness exceeds 10 mm, deformation of the insulator is likely to occur. In order to suppress such problems caused by resin deformation, if a power cable insulating resin composition having low resin fluidity at the extrusion temperature is used as the insulator for the conductor coating, a large torque is applied to the extruder, which significantly reduces the flexibility of the processing step.

[0015] Thus, it is desirable for a resin composition for power cable insulation made of a thermoplastic resin to have high fluidity during extrusion in order to reduce the load on the extruder, whereas it is desirable for the resin composition for power cable insulation to have low fluidity in order to suppress deformation of the insulator that has coated the conductor during melt extrusion, which are contradictory properties that are required.

[0016] As a result of further investigation based on this finding, it was found that, when a resin composition for use in power cable insulation contains polypropylene (A) and elastomer (B), a specified polyethylene (C), and an antioxidant (E), each having an MFR within a specified range, and the resin composition for use in power cable insulation has an MFR within a specified range and a self-weight elongation in a resin sagging test within a specified range, even when the resin composition for use in power cable insulation is used to coat a power cable conductor by melt extrusion, the coated resin composition for use in power cable insulation can be prevented from sagging under its own weight. The present disclosure has been completed based on this finding.

[0017] The resin composition for power cable insulation of an embodiment includes polypropylene (A), which is a random polypropylene having an MFR at 230°C of 2.0 g / 10 min or more, elastomer (B), which is a styrene-based elastomer having an MFR at 230°C of 2.0 g / 10 min or less, polyethylene (C), which is a low-density polyethylene, and antioxidant (E), and has an MFR at 230°C of 0.1 g / 10 min or more and 2.0 g / 10 min or less, and a self-weight elongation in a resin sagging test of 150% or less.

[0018] A resin composition for power cable insulation (hereinafter also simply referred to as a resin composition) according to an embodiment includes, as basic components, polypropylene (A), elastomer (B), polyethylene (C), and antioxidant (E). The resin composition is an insulator.

[0019] The polypropylene (A) contained in the resin composition is a random polypropylene having an MFR (molten resin fluidity) at 230°C of 2.0 g / 10 min or more. The MFR of the polypropylene (A) is a value measured in accordance with JIS K 7210 at a melting temperature of 230°C and a load of 2.16 kgf. Among these, the random polypropylene preferably has a melting point (Tm) of 130°C or more and 150°C or less as measured by differential scanning calorimetry (DSC) in accordance with JIS K 7121. If the MFR is less than 2.0 g / 10 min, the fluidity of the resin composition during melt extrusion is low, making it difficult to coat the conductor of a power cable.

[0020] When multiple random polypropylenes are blended as polypropylene (A), it is sufficient that the MFR of one or more random polypropylenes is within the above range, and it is preferable that the MFR of all the random polypropylenes is within the above range.

[0021] The random polypropylene of polypropylene (A) contains ethylene units as a copolymer component, and the ethylene units in the random polypropylene are 5 wt% or less. The ethylene units are entangled at the molecular level with the polyethylene (C) contained in the resin composition, resulting in the resin composition exhibiting so-called thixotropy, which means that the resin composition does not flow under low load stress but begins to flow when stress exceeding the yield value destroys the entanglement of the polymer chains. Therefore, the resin composition exhibits excellent flow characteristics as a power cable insulator, with high fluidity during extrusion and suppressed deformation due to resin sagging, or sagging under its own weight after coating.

[0022] The random polypropylene of polypropylene (A) may be a commercially available product, and examples thereof include "Prime Polypro (registered trademark) F227D" manufactured by Prime Polymer Co., Ltd., "PF621S" manufactured by SunAllomer Co., Ltd., and "Novatec (registered trademark) FX4EA," "Novatec (registered trademark) EG7FTB," "Novatec (registered trademark) MG3F," "Novatec (registered trademark) FW4BA," and "Wintec (registered trademark) WFX6" manufactured by Japan Polypropylene Corporation.

[0023] The elastomer (B) contained in the resin composition is a styrene-based elastomer having an MFR at 230°C of 2.0 g / 10 min or less. The MFR of the elastomer (B) is a value measured in accordance with JIS K 7210 at a melting temperature of 230°C and a load of 2.16 kgf. When the MFR is 2.0 g / 10 min or less, the elastomer (B) behaves like an organic filler during extrusion of the resin composition, allowing the resin composition to exhibit excellent flow properties as a power cable insulator. From this perspective, the MFR is preferably 1.0 g / 10 min or less, more preferably 0.1 g / 10 min or less, and most preferably 0 g / 10 min, i.e., no flowability.

[0024] When a plurality of styrene-based elastomers are blended as the elastomer (B), it is sufficient that the MFR of one or more of the styrene-based elastomers falls within the above range, and it is preferable that the MFR of all of the styrene-based elastomers falls within the above range.

[0025] The styrene-based elastomer of the elastomer (B) preferably contains a structural unit derived from styrene and a structural unit derived from an α-olefin having from 3 to 30 carbon atoms. The ratio of the styrene component constituting the styrene-based elastomer is preferably 10.0% by mass or more, more preferably 15.0% by mass or more, and even more preferably 20.0% by mass or more in order to obtain good compatibility with other components in the resin composition, and is preferably 60.0% by mass or less, more preferably 50.0% by mass or less, and even more preferably less than 40.0% by mass in order to obtain more preferable flow properties for the resin composition as a power cable insulator.

[0026] The styrene-based elastomer of the elastomer (B) is preferably styrene-butadiene block copolymer (SBR), styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS), hydrogenated styrene-butadiene rubber (HSBR), styrene-ethylene-butylene-styrene block copolymer (SEBS), styrene-ethylene-propylene-styrene block copolymer (SEPS), styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS), styrene-ethylene-butylene-olefin crystalline block copolymer (SEBC), or styrene-butadiene block copolymer (SBR), with SEBS and SEEPS being more preferred. These styrene-based elastomers may be used alone or in combination of two or more.

[0027] The styrene-based elastomer of the elastomer (B) may be a commercially available product, and examples thereof include "Septon (registered trademark) 2005," "Septon (registered trademark) 4033," "Septon (registered trademark) 4055," "Septon (registered trademark) 4077," "Septon (registered trademark) 4086," and "Septon (registered trademark) 8006" manufactured by Kuraray Co., Ltd., and "Tuftec (registered trademark) H1051," "Tuftec (registered trademark) H1053," and "Tuftec (registered trademark) H1272" manufactured by Asahi Kasei Corporation.

[0028] In order to suppress deformation of the resin composition when heated, the blending amount of the elastomer (B) with respect to the entire resin composition is preferably 20 wt % or more, and in order to ensure the flowability of the resin composition when kneaded, the blending amount of the elastomer (B) with respect to the entire resin composition is preferably less than 40 wt %.

[0029] The polyethylene (C) contained in the resin composition is low-density polyethylene (LDPE). In low-density polyethylene, the repeating unit ethylene is randomly branched. Alternatively, the polyethylene (C) may be linear low-density polyethylene (LLDPE) obtained by copolymerizing ethylene with a few percent of an α-olefin. Due to the branching in the molecular chain, the polyethylene (C) is intricately entangled with the ethylene component of the random polypropylene of the polypropylene (A), allowing the resin composition to have excellent flow properties as a power cable insulator.

[0030] From the viewpoint of achieving even better flow properties, the polyethylene (C) preferably has an MFR of 10.0 g / 10 min or less, more preferably 5.0 g / 10 min or less at 190° C. The MFR of the polyethylene (C) is a value measured in accordance with JIS K 7210 at a melting temperature of 190° C. under a load of 2.16 kgf.

[0031] When a plurality of low-density polyethylenes are blended as the polyethylene (C), it is sufficient that the MFR of one or more low-density polyethylenes falls within the above range, and it is preferable that the MFR of all the low-density polyethylenes falls within the above range.

[0032] The low-density polyethylene of the polyethylene (C) may be a commercially available product, and examples thereof include "NUC-8008", "NUC-8009", "NUC-8351", "NUC-8505", "NUC-8160", "DND-2450", and "DXM-446" manufactured by Eneos NUC Corporation.

[0033] The antioxidant (E) contained in the resin composition prevents oxidation of the resin composition. The antioxidant (E) is preferably a hindered phenol-based antioxidant whose main purpose is to capture radicals. Furthermore, it is preferable to use a hindered phenol-based antioxidant in combination with a sulfur-based antioxidant whose main purpose is to decompose peroxides, or a phosphorus-based antioxidant, since this effectively prevents oxidation of the resin composition.

[0034] The hindered phenol-based antioxidant may be a commercially available product, such as "Irganox (registered trademark) 1010," "Irganox (registered trademark) 259," "Irganox (registered trademark) 1076," or "Irganox (registered trademark) 1135," manufactured by BASF. The sulfur-based antioxidant may be a commercially available product, such as "SEENOX (registered trademark)-BCS," manufactured by Shipro Chemical Co., Ltd., or "Irganox (registered trademark) 1081," manufactured by BASF. The phosphorus-based antioxidant may be a commercially available product, such as "Irgafos (registered trademark) 168," manufactured by BASF, or "ADK STAB (registered trademark) 2112," or "ADK STAB (registered trademark) PEP-36," manufactured by ADEKA Corporation.

[0035] Furthermore, the resin composition preferably further contains an acid-modified polyethylene (D) in addition to the above basic components. The acid-modified polyethylene (D) is polyethylene grafted with an organic acid or a derivative thereof. The acid-modified polyethylene (D) is preferably maleic anhydride-grafted polyethylene. The maleic anhydride-grafted polyethylene of the acid-modified polyethylene (D) is a modified polyethylene in which maleic anhydride is uniformly grafted onto polyethylene, and is effective in improving the compatibility between the polypropylene (A), the elastomer (B), and the polyethylene (C).

[0036] The graft amount of maleic anhydride is preferably 0.01 wt % or more and 2.00 wt % or less in the graft resin. By using a resin grafted with maleic anhydride in this range, maleic anhydride is uniformly dispersed in the resin composition, and the compatibility between the components of the resin composition can be further improved.

[0037] In order to provide the resin composition with even better flow properties, the acid-modified polyethylene (D) preferably has an MFR of 10.0 g / 10 min or less, more preferably 5.0 g / 10 min or less at 190° C. The MFR of the acid-modified polyethylene (D) is a value measured in accordance with JIS K 7210 at a melting temperature of 190° C. and a load of 2.16 kgf.

[0038] When a plurality of polyethylenes are blended as the acid-modified polyethylene (D), it is sufficient that the MFR of one or more polyethylenes falls within the above range, and it is preferable that the MFR of all polyethylenes falls within the above range.

[0039] Examples of methods for preparing maleic anhydride-grafted modified polyethylene include a method in which low-density polyethylene, an antioxidant, maleic anhydride monomer, and an organic peroxide are mixed and heated in an extruder to cause a reaction, followed by granulation into pellets or granules.

[0040] The organic peroxide used in preparing the maleic anhydride-grafted modified polyethylene is deactivated during the synthesis of the maleic anhydride-grafted modified polyethylene, and does not become a component constituting the maleic anhydride-grafted modified polyethylene in the resin composition.

[0041] The maleic anhydride graft-modified polyethylene of the acid-modified polyethylene (D) may be a commercially available product, and examples thereof include "MODIC (registered trademark) L504," "MODIC (registered trademark) L553," and "MODIC (registered trademark) M142" manufactured by Mitsubishi Chemical Corporation, and "Fusabond (registered trademark) E226" manufactured by The Dow Chemical Company.

[0042] From the viewpoint of further improving the flow properties of the resin composition, the total amount of polyethylene (C) and acid-modified polyethylene (D) relative to the entire resin composition is preferably 5 wt % or more, and more preferably 10 wt % or more. From the viewpoint of further suppressing deformation due to resin sagging after coating, the total amount of polyethylene (C) and acid-modified polyethylene (D) relative to the entire resin composition is preferably 20 wt % or less, and more preferably 15 wt % or less.

[0043] In addition to the above components, the resin composition may contain various other components, such as stabilizers, lubricants, surface treatment agents, acid scavengers, and voltage stabilizers, as long as the above effects are not impaired.

[0044] Other components include inorganic fillers (inorganic flame retardants) and flame retardants. However, adding an excessive amount of inorganic filler to a resin composition increases the specific gravity of the resin composition, making it more susceptible to deformation due to resin sagging after coating. Furthermore, it increases the weight of the power cable, reducing workability during installation. Therefore, when the resin composition contains an inorganic filler, the amount of inorganic filler relative to the total resin composition is preferably 5 wt% or less, regardless of the particle size of the inorganic filler, and the resin composition may be free of inorganic fillers. Furthermore, metal hydroxide particles aggregate in the resin composition, thereby reducing the breakdown voltage of the resin composition. Therefore, the amount of metal hydroxide particles is preferably 5 wt% or less, and the resin composition may be free of metal hydroxide particles. Furthermore, if the resin composition contains a halogen-based flame retardant, the DC insulation properties of the resin composition will deteriorate when used as a DC insulating material. Therefore, the amount of halogen-based flame retardant is preferably 5 wt% or less, and the resin composition may be free of halogen-based flame retardants.

[0045] The resin composition can be obtained by melt-kneading components such as polypropylene (A), elastomer (B), polyethylene (C), and antioxidant (E). Examples of melt-kneading devices that can be used include single-screw or twin-screw extruders, Banbury mixers, and kneaders. It is particularly preferable to extrude the resin using a single-screw or twin-screw extruder capable of continuous processing, equipped with a metal mesh filter having an opening of 100 μm or less for the purpose of removing foreign matter.

[0046] The temperature of the resin composition during melt-kneading is preferably equal to or higher than the melting point of the polypropylene (A), specifically, preferably equal to or higher than 130° C. Furthermore, in order to enhance the compatibility between the components constituting the resin composition, the temperature of the resin composition during melt-kneading is preferably equal to or higher than 160° C., more preferably equal to or higher than 180° C., and even more preferably equal to or higher than 200° C. Furthermore, if the temperature of the resin composition during melt-kneading is equal to or lower than 270° C., thermal degradation of the resin composition can be suppressed, and if it is equal to or lower than 250° C., it is more preferable.

[0047] The resin composition has an MFR of 0.1 g / 10 min or more and 2.0 g / 10 min or less, preferably 1.0 g / 10 min, at 230° C. The MFR of the resin composition is a value measured in accordance with JIS K 7210 at a melting temperature of 230° C. and a load of 2.16 kgf.

[0048] The resin composition has a self-weight elongation in a resin sagging test of 150% or less, more preferably 125% or less. FIG. 1 is a front view showing the shape of a No. 3 dumbbell die, as defined in JIS K 6251, used to punch out a sheet-shaped resin composition in the resin sagging test. In the resin sagging test, a 1 mm-thick sheet of resin composition is first punched out using the dumbbell die shown in FIG. 1 , and a measurement sample is obtained by marking the sheet with a 20 mm gauge distance. The measurement sample is then vertically hung in a windowed thermostatic chamber at 220°C, with one side fixed. The gauge distance after 5 minutes is measured, and the self-weight elongation (%) can be calculated using the formula ((gauge distance - 20 mm) / 20 mm) x 100. The resin sagging test of a resin composition using a dumbbell-shaped measurement sample allows for estimation of the deformation due to the self-weight of an insulating layer formed by melt extrusion.

[0049] From the viewpoint of recycling, the resin composition preferably does not contain a crosslinking agent, and more preferably is a non-crosslinked resin composition. The term "non-crosslinked resin composition" refers to a resin composition that has not been actively subjected to a crosslinking treatment, and includes a resin composition having a crosslinked structure resulting from an unavoidable crosslinking reaction.

[0050] By incorporating polypropylene (A), which has a higher melting point than cross-linked polyethylene, such a resin composition can be used at higher temperatures than conventional cross-linked polyethylene, allowing power cables to be used at higher temperatures and improving the power transmission efficiency of the power cable. Furthermore, the molecular chains of the styrene-based elastomer elastomer (B) and the low-density polyethylene polyethylene (C) are entangled, which allows the resin composition to exhibit good fluidity during extrusion and maintain the coating shape even at temperatures above the melting point after coating. In other words, when the resin composition is used to coat a power cable conductor at a melt viscosity that allows extrusion, there is little deformation of the insulator due to resin sagging. Therefore, the resin composition is suitable as an insulator for power cables.

[0051] FIG. 2 is a cross-sectional view showing an example of a power cable to which the resin composition of the embodiment is applied.

[0052] 2, the power cable 1 includes a conductor 2, an internal semiconductive layer 3 disposed on the outside so as to surround the conductor 2, an insulating layer 4 made of the above-mentioned resin composition disposed on the outside so as to surround the internal semiconductive layer 3, and an external semiconductive layer 5 disposed on the outside so as to surround the insulating layer 4. In this way, in the power cable 1, the internal semiconductive layer 3, the insulating layer 4, and the external semiconductive layer 5 are laminated in this order on the conductor 2 made of a metal such as copper or aluminum.

[0053] The inner and outer semiconductive layers 3 and 5 are made of a conductive resin composition containing, for example, polypropylene, conductive carbon black, and a thermoplastic elastomer resin containing an ethylene-α-olefin copolymer or a propylene-α-olefin copolymer, or a styrene-based elastomer resin. The conductive resin composition may contain an ethylene-ethyl acrylate resin or an ethylene-vinyl acetate resin to adjust the dispersibility of the carbon black.

[0054] The power cable 1 may further include a metal shielding layer (not shown) arranged on the outside to surround the outer semiconductive layer 5. The power cable 1 may further include a sheath (not shown) arranged on the outside to surround the metal shielding layer.

[0055] The conductor 2 is continuously fed to a resin extrusion port, where it is coated with an inner semiconductive layer 3, an insulating layer 4 (a resin composition layer), and an outer semiconductive layer 5, to obtain a power cable 1. These three layers may be extruded simultaneously or sequentially. The power cable 1 is then cooled in a cooling pipe or a cooling water tank, and a metal shielding layer or sheath (not shown) is formed as needed by a conventional method.

[0056] The resin composition is coated by extruding the resin composition onto the conductor 2 (onto the internal semiconductive layer 3) from the resin extrusion port of a resin extruder equipped with a metal mesh filter with a mesh size of 100 μm or less to remove foreign matter. The temperature of the resin composition during extrusion is preferably equal to or higher than the melting point of the resin composition, specifically, 130°C or higher. Furthermore, in order to improve adhesion between the conductor 2 and the internal semiconductive layer 3 and between the layers, the temperature of the resin composition during extrusion is preferably 160°C or higher, more preferably 180°C or higher, and even more preferably 200°C or higher. Furthermore, in order to suppress deformation due to resin sagging after coating, the temperature of the resin composition during extrusion is preferably 270°C or lower, and furthermore, in order to suppress thermal degradation of the resin composition, it is preferably 250°C or lower.

[0057] When forming an exterior covering such as a metal shielding layer or sheath on the cooled power cable 1, the temperature of the insulating layer 4 is preferably 300°C or lower in order to suppress deformation of the insulating layer 4, and more preferably lower than the extrusion temperature of the inner semiconductive layer 3, the insulating layer 4, and the outer semiconductive layer 5 onto the conductor 2.

[0058] From the viewpoint of insulating properties, the thickness of the insulating layer 4 is preferably 2 mm or more, more preferably 5 mm or more, and even more preferably 10 mm or more. From the viewpoint of installation workability, the thickness of the insulating layer 4 is preferably 50 mm or less, and more preferably 40 mm or less.

[0059] From the viewpoint of insulating properties, the thickness of each of the inner and outer semiconductive layers 3 and 5 is preferably 0.1 mm or more, and more preferably 0.5 mm or more. From the viewpoint of conductive properties, the thickness of each of the inner and outer semiconductive layers 3 and 5 is preferably 5.0 mm or less, and more preferably 3.0 mm or less.

[0060] According to the embodiment described above, the resin composition contains polypropylene (A) and elastomer (B), each having an MFR within a predetermined range, polyethylene (C), which is a predetermined polyethylene, and antioxidant (E), and the MFR of the resin composition is within the predetermined range, and the self-weight elongation of the resin composition in a resin sagging test is within the predetermined range. Therefore, even if the resin composition is used to coat a power cable conductor by melt extrusion, the coated resin composition can be prevented from sagging under its own weight.

[0061] Although the embodiments have been described above, the present invention is not limited to the above embodiments, but includes all aspects encompassed by the concept and scope of the claims of the present disclosure, and can be modified in various ways within the scope of the present disclosure.

[0062] Next, examples and comparative examples will be described, but the present invention is not limited to these examples.

[0063] The raw materials used in the examples and comparative examples are as follows.

[0064] The polypropylene (A) was as follows: Polypropylene (A1): Wintec (registered trademark) WFX6 (manufactured by Japan Polypropylene Corporation, MFR 2.0 g / 10 min) Polypropylene (A2): Prime Polypropylene (registered trademark) B241 (manufactured by Prime Polymer Co., Ltd., MFR 0.5 g / 10 min)

[0065] The elastomers (B) are as follows: Elastomer (B1): Septon (registered trademark) 4086 (manufactured by Kuraray Co., Ltd., SEBS, MFR 0 g / 10 min) Elastomer (B2): Septon (registered trademark) 4077 (manufactured by Kuraray Co., Ltd., SEEPS, MFR 0 g / 10 min) Elastomer (B3): Septon (registered trademark) 4033 (manufactured by Kuraray Co., Ltd., SEEPS, MFR 0.1 g / 10 min) Elastomer (B4): Tuftec (registered trademark) H1051 (manufactured by Asahi Kasei Corporation, SEBS, MFR 0.8 g / 10 min) Elastomer (B5): Tuftec (registered trademark) H1053 (manufactured by Asahi Kasei Corporation, SEBS, MFR 1.8 g / 10 min) Elastomer (B6): Tuftec (registered trademark) H1517 (manufactured by Asahi Kasei Corporation, SEBS, MFR 3.0 g / 10 min)

[0066] The polyethylenes (C) were as follows: Polyethylene (C1): NUC-8009 (manufactured by Eneos NUC Corporation, MFR 9.0 g / 10 min) Polyethylene (C2): NUC-8008 (manufactured by Eneos NUC Corporation, MFR 4.8 g / 10 min) Polyethylene (C3): NUC-8351 (manufactured by Eneos NUC Corporation, MFR 14.0 g / 10 min) Polyethylene (C4): DXM-446 (MFR 2.1 g / 10 min)

[0067] The acid-modified polyethylene (D) was as follows: Acid-modified polyethylene (D1): prepared from NUC-8009 (maleic anhydride grafted LDPE, MFR 9.6 g / 10 min) Acid-modified polyethylene (D2): MODIC (registered trademark) L553 (manufactured by Mitsubishi Chemical Corporation, maleic anhydride grafted LDPE, MFR 1.5 g / 10 min)

[0068] Acid-modified polyethylene (D1) was prepared as follows: Specifically, 100 parts by mass of NUC-8009, a low-density polyethylene, 0.5 parts by mass of maleic anhydride, 0.05 parts by mass of tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane (Irganox (registered trademark) 1010, manufactured by BASF) as a hindered phenol-based antioxidant, and 0.02 parts by mass of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexyne-3 (Perhexyne (registered trademark) 25B, manufactured by NOF Corporation) as an organic peroxide were dry-blended in a Henschel mixer, and then the mixture was melt-kneaded and grafted in a single-screw extruder (L / D = 24, 160°C) equipped with a plain-woven mesh having an opening of 0.091 mm to produce acid-modified polyethylene (D1).

[0069] The antioxidants (E) were as follows: Antioxidant (E1): Irganox (registered trademark) 1010 (manufactured by BASF, tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane) Antioxidant (E2): Irgafos (registered trademark) 168 (manufactured by BASF, tris(2,4-di-t-butylphenyl)phosphite)

[0070] (Examples 1 to 12 and Comparative Examples 1 to 5) The above-mentioned raw materials were dry-blended in a Henschel mixer, and then extruded using a single-screw extruder (L / D = 24, 230°C) equipped with a plain-woven mesh having an opening of 0.091 mm to obtain pellet-shaped resin compositions (hereinafter also referred to as resin pellets) having the compositions shown in Tables 1 and 2.

[0071] [Measurements and Evaluations] The resin compositions obtained in the above Examples and Comparative Examples were subjected to the following measurements and evaluations. The results are shown in Tables 1 and 2.

[0072] [1] Molten resin fluidity (MFR) In accordance with JIS K 7210, the polypropylene (A), elastomer (B), and resin composition were measured at a melting temperature of 230°C, and the polyethylene (C) and acid-modified polyethylene (D) were measured at a melting temperature of 190°C under a load of 2.16 kgf for 10 minutes, and the MFR was defined as the amount of resin that flowed out for 10 minutes.

[0073] [2] Resin sagging First, resin pellets were used and heated to 240 ° C. in a press to obtain a resin sheet having a thickness of 1 mm. Next, the resin sheet was punched out using a No. 3 dumbbell mold shown in FIG. 1 and described in JIS K 6251, and a measurement sample was obtained by marking the sheet with a gauge distance of 20 mm. Next, the measurement sample was hung vertically with one side fixed in a windowed thermostatic chamber at 220 ° C., and the gauge distance after 5 minutes was measured. The weight elongation (%) was calculated using the formula ((gauge distance - 20 mm) / 20 mm) × 100.

[0074] [3] Wall thickness deviation: First, a power cable was manufactured. A single-screw extruder (L / D = 24, 220°C, full-flight screw) equipped with a plain-weave mesh with a mesh size of 0.091 mm was used to manufacture the power cable. Resin pellets were extruded through the single-screw extruder, and then coated onto a horizontally fed copper conductor using a three-layer head heated to 220°C, along with a semiconductive resin extruded through another single-screw extruder. The coated cable was air-cooled for 5 minutes and then transferred to a water bath for cooling. This resulted in a power cable with a conductor with an outer diameter of approximately 30 mm, an inner semiconductive layer with a thickness of approximately 1 mm, an insulating layer with a thickness of approximately 10 mm, and an outer semiconductive layer with a thickness of approximately 1 mm.

[0075] The semiconductive resin used was prepared by dry-blending 30 parts of EVA450 (ethylene-vinyl acetate copolymer resin manufactured by Mitsui Dow Polychemicals), 70 parts of Elastomer Q200F (TPO manufactured by SunAllomer Co., Ltd., MFR 0.8), 30 parts of Denka Black (carbon black manufactured by Denka Co., Ltd.), and 0.5 parts of an antioxidant (Nocrac 300, hindered phenol-based antioxidant manufactured by Ouchi Shinko Chemical Co., Ltd.) in a Henschel mixer, extruding the mixture in a single-screw extruder (L / D=24, 200°C), and pelletizing it.

[0076] The minimum value of the maximum diameter of the power cable divided by the minimum diameter of the power cable in the longitudinal cross section of the obtained power cable was taken as the wall thickness deviation. When the wall thickness deviation ratio is 1, the cross section of the power cable is a perfect circle. When the coating resin, centered on the insulating layer, sags under its own weight during the period from the time of coating by melt extrusion to cooling, the wall thickness deviation becomes large. Power cables with a wall thickness deviation ratio exceeding 1.10 were rejected. Power cables with a large wall thickness deviation have areas where the insulating coating is thin and the insulating performance is lower than designed. This means that there is a possibility of insulation breakdown during use of the power cable.

[0077]

[0078]

[0079] As shown in Tables 1 and 2, the above examples contained polypropylene (A) and elastomer (B), a specified polyethylene (C), and antioxidant (E), each having an MFR within a specified range, and the resin composition's own-weight elongation in a resin sagging test was within a specified range, resulting in a thickness deviation of 1.10 or less and passing the test. On the other hand, the above comparative examples did not satisfy at least one of the following: the MFR of polypropylene (A) was within the specified range, the MFR of elastomer (B) was within the specified range, the polyethylene (C) was a specified polyethylene, the resin composition contained antioxidant (E), the MFR of the resin composition was within the specified range, and the resin composition's own-weight elongation in a resin sagging test was within the specified range, resulting in a thickness deviation of more than 1.10 and failing the test.

[0080] Furthermore, in Comparative Examples 2 and 4, the resin compositions had no fluidity in the MFR measurement. Furthermore, the surface of the resin composition extruded during the production of the power cable was not smooth, and the interface between the insulating layer and the semiconducting layer was irregular. The irregular interface between the insulating layer and the semiconducting layer is likely to cause dielectric breakdown starting from this interface.

[0081] REFERENCE SIGNS LIST 1 Power cable 2 Conductor 3 Inner semiconductive layer 4 Insulating layer 5 Outer semiconductive layer

Claims

1. A resin composition for power cable insulation comprising polypropylene (A), which is a random polypropylene having an MFR at 230°C of 2.0 g / 10 min or more; elastomer (B), which is a styrene-based elastomer having an MFR at 230°C of 2.0 g / 10 min or less; polyethylene (C), which is a low-density polyethylene; and antioxidant (E), wherein the MFR at 230°C is 0.1 g / 10 min or more and 2.0 g / 10 min or less; and the elongation under its own weight in a resin sagging test is 150% or less.

2. The resin composition for power cable insulation according to claim 1, further comprising an acid-modified polyethylene (D).

3. The resin composition for power cable insulation according to claim 2, wherein the acid-modified polyethylene (D) is maleic anhydride-grafted modified polyethylene.

4. The resin composition for power cable insulation according to claim 1, wherein the blending amount of the elastomer (B) relative to the resin composition for power cable insulation is 20 wt % or more and less than 40 wt %.

5. A resin composition for power cable insulation according to claim 2, wherein the total amount of the polyethylene (C) and the acid-modified polyethylene (D) in the resin composition for power cable insulation is 5 wt % or more and 20 wt % or less.

6. The resin composition for power cable insulation according to claim 2, wherein the polyethylene (C) has an MFR at 190°C of 10.0 g / 10 min or less, and the acid-modified polyethylene (D) has an MFR at 190°C of 10.0 g / 10 min or less.

7. A power cable comprising: a conductor; an inner semiconductive layer arranged on the outside so as to surround said conductor; an insulating layer arranged on the outside so as to surround said inner semiconductive layer, said insulating layer being made of the resin composition for power cable insulation according to any one of claims 1 to 6; and an outer semiconductive layer arranged on the outside so as to surround said insulating layer.

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