EP4779655A1

EP4779655A1 - Insulating resin composition for electric power cable, and electric power cable

Info

Publication number: EP4779655A1
Application number: EP23952278.2A
Authority: EP
Inventors: Tetsuya Mieda; Shingo Mitsugi
Original assignee: Furukawa Electric Co Ltd
Current assignee: Furukawa Electric Co Ltd
Priority date: 2023-09-15
Filing date: 2023-09-15
Publication date: 2026-07-22
Also published as: WO2025057389A1; KR20260070120A; CN121889864A; JPWO2025057389A1

Abstract

This insulating resin composition for an electric power cable comprises a component (a), a component (b), a component (c), and a component (d). The component (a) is polyethylene. The component (b) is at least one resin selected from a resin (b1) and a resin (b2). The resin (b1) is a resin in which at least one modified monomer selected from unsaturated organic acids and derivatives thereof is grafted. The resin (b2) is at least one ethylene-based copolymer selected from ethylene-acrylate copolymers, ethylene-acrylic acid copolymers, and ethylene-vinyl acetate copolymers. The component (c) is a water treeing inhibitor. The component (d) is a crosslinking agent.

Description

TECHNICAL FIELD

The present disclosure relates to an insulating resin composition for electric power cable, and an electric power cable.

BACKGROUND ART

When a crosslinkable polyethylene (XLPE), which has been used conventionally in electric power cables, is operating as an insulator of the electric power cable under an environment with abundant moisture, water tree occurs. The water tree is an arborescent defect occurring due to foreign substances and bubbles (voids) in the resin.
Due to dielectrophoresis of water occurring by flowing electricity in the electric power cable, the water tree grows by the moisture in the resin concentrating at the interface of the foreign substances or the voids. Since the water tree causes destruction of electric power cables, it has been sought to suppress the growth of the water tree.
To suppress the growth of the water tree, in the XLPE resin, hydrophilic molecules, e.g., polyethylene glycols, are added to hydrophobic polyethylene. Since moisture uniformly disperses in the XLPE resin by way of the hydrophilic molecules such as polyethylene glycols, the concentration of moisture on the interface of the foreign substances and the voids is curbed, and the growth of the water tree is suppressed.
In addition, Patent Document 1 discloses a polyolefin composition for electrical insulation containing a polyolefin or a crosslinked polyolefin and a small amount of high molecular weight polyethylene glycol. Further, it also discloses technology in which a low hydrophilic material such as polypropylene glycol or a hydrophobic material do not prevent the generation of the water tree in an insulator.
In this way, the growth of the water tree is suppressed by adding hydrophilic molecules. However, since the addition of hydrophilic molecules causes the dielectric loss tangent (tanδ) of the resin to increase, the electric power transmission efficiency of the electric power cable will decline.

Citation List

Patent Documents

Patent Document 1: US Patent No. 4305849

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

An object of the present disclosure is to provide an insulating resin composition for electric power cable and an electric power cable which can suppress the growth of water tree and decrease the dielectric loss tangent.

Means for Solving the Problems

[1] An insulating resin composition for electric power cable includes: a component (a); a component (b); a component (c); and a component (d), in which the component (a) is polyethylene, the component (b) is at least one type of resin selected from a resin (b1) and a resin (b2), the resin (b1) being a resin in which at least one type of modified monomer selected from unsaturated organic acids and derivatives thereof is grafted, and the resin (b2) being at least one type of ethylene-based copolymer selected from an ethylene-acrylate copolymer, an ethylene-acrylic acid copolymer and an ethylene-vinyl acetate copolymer, the component (c) is a water tree inhibitor, and the component (d) is a crosslinking agent.
[2] In the insulating resin composition for electric power cable as described in the above [1], the component (c) is at least one compound selected from a polyalkylene glycol and derivative thereof, polyglycerin, glycerin fatty acid ester, and sorbitol ester.
[3] In the insulating resin composition for electric power cable as described in the above [1] or [2], a dielectric loss tangent of 30 kV/mm at 90°C of a crosslinked substance obtained by crosslinking the insulating resin composition for electric power cable is 1.0% or less.
[4] In the insulating resin composition for electric power cable as described in any one of the above [1] to [3], in the resin (b1), at least one type of modified monomer selected from an unsaturated dicarboxylic acid, unsaturated dicarboxylic acid anhydride, and an unsaturated dicarboxylic acid derivative is grafted to at least one type of resin selected from a polypropylene, polyethylene and olefin-based copolymer.
[5] In the insulating resin composition for electric power cable as described in any one of the above [1] to [4], a content ratio of the component (b) contained in the insulating resin composition for electric power cable is 3.0 wt% or more and 25.0 wt% or less.
[6] An electric power cable includes: a conductor; an internal semiconductor layer disposed on an outer side of the conductor, and encircling the conductor; an insulation layer disposed on an outer side of the internal semiconductor layer, encircling the internal semiconductor layer, and made by crosslinking the insulating resin composition for electric power cable as described in any one of the above [1] to [5]; and an external semiconductor layer disposed on an outer side of the insulation layer, and encircling the insulation layer.

Effects of the Invention

According to the present disclosure, it is possible to provide an insulating resin composition for electric power cable and an electric power cable which can suppress the growth of water tree and decrease the dielectric loss tangent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse sectional view showing an example of an electric power cable to which the insulating resin composition for electric power cable according to an embodiment is applied.
FIG. 2 is a schematic diagram showing a water tree test.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereinafter, details of the present invention will be described based on an embodiment.
The present inventors, as a result of conducting extensive research, have found that, even when containing a water tree inhibitor for suppressing the growth of water tree, by containing a specific component, it is possible to suppress an increase in dielectric loss tangent caused by the water tree inhibitor, and based on such knowledge, arrived at completion of the present disclosure.
The insulating resin composition for electric power cable according to an embodiment (hereinafter also referred to simply as insulating resin composition) includes: a component (a); a component (b); a component (c); and a component (d), in which the component (a) is polyethylene, the component (b) is at least one type of resin selected from a resin (b1) and a resin (b2), the resin (b1) being a resin in which at least one type of modified monomer selected from unsaturated organic acids and derivatives thereof is grafted, and the resin (b2) being at least one type of ethylene-based copolymer selected from an ethylene-acrylate copolymer, an ethylene-acrylic acid copolymer and an ethylene-vinyl acetate copolymer, the component (c) is a water tree inhibitor, and the component (d) is a crosslinking agent.
The insulating resin composition according to the embodiment contains, as constituent elements, the component (a), the component (b), the component (c) and the component (d). The crosslinked substance formed by crosslinking the insulating resin composition (hereinafter simply referred to as crosslinked substance) is an insulator, and can be suitably used in an insulation layer of an electric power cable.
The component (a) contained in the insulating resin composition is a polyethylene. The polyethylene can be manufactured by a low-pressure process or a high-pressure process, but is preferably a low-density polyethylene (LDPE) manufactured by a high-pressure process.
The component (b) contained in the insulating resin composition is at least one type of resin selected from a resin (b1) and a resin (b2). The component (b) suppresses a rise in the dielectric loss tangent of the crosslinked substance of the insulating resin composition caused by the water tree inhibitor, which is the component (c).
The resin (b1) is a resin in which at least one type of modified monomer selected from unsaturated organic acids and derivatives thereof is grafted (hereinafter simply referred to as a graft resin).
The functional group introduced to the resin by grafting of the modified monomer is preferably a functional group having a C=O bond. Thereamong, it is preferably a carbonyl group, a carboxyl group, an ester group, an acid anhydride, an amide group or an imide group. The water tree inhibitor, which is the component (c), is restrained by the hydrogen bond with C=O in the molecule grafted to the resin, whereby molecular motion due to an electric field is suppressed. In other words, since a permanent dipole of the water tree inhibitor does not follow the frequency vibration of an AC field, and orientation polarization does not occur, the energy loss due to molecular motion for molecular orientation is reduced. In addition, the C=O structure traps the space charge in the insulator, and suppresses the occurrence of interface polarization by the space charge transferring in response to electric field; therefore, the energy loss due to charge transfer is decreased. In this way, the C=O in the molecule grafted to the resin suppresses molecular motion of the water tree inhibitor, and the transfer of space charge; therefore, it is possible to efficiently decrease the dielectric loss tangent of the crosslinked substance. In addition, the graft resin can improve the dispersibility of the water tree inhibitor, and can suppress bleedout of the crosslinked substance.
The preferred modified monomer is an unsaturated dicarboxylic acid, an unsaturated dicarboxylic acid anhydride and an unsaturated dicarboxylic acid derivative. As the resin (b1) to which such a modified monomer is grafted, at least one modified monomer selected from an unsaturated dicarboxylic acid, an unsaturated dicarboxylic acid anhydride and an unsaturated dicarboxylic acid derivative is preferably grafted to at least one resin selected from a polypropylene, a polyethylene, and an olefin-based copolymer. The polyethylene is preferably a low-density polyethylene manufactured by a high-pressure process.
The unsaturated dicarboxylic acid is preferably maleic acid, fumaric acid and itaconic acid. The unsaturated dicarboxylic acid anhydride is preferably maleic acid anhydride and itaconic acid anhydride. The unsaturated dicarboxylic acid derivative is preferably monomethyl maleate, monoethyl maleate, diethyl maleate, monomethyl fumarate, dimethyl fumarate, diethyl fumarate, monoamide maleate, maleimide, N-phenylmaleimide, and N-cyclohexylmaleimide. These can be used individually or by combining two or more types. Thereamong, it is preferably a maleic acid anhydride which is a five-membered cyclic acid anhydride.
The graft amount of the modified monomer is preferably 0.25 wt% or more and 2.00 wt% or less of the graft resin. When the modified monomer is grafted in this range, since the modified monomer uniformly disperses in the insulating resin composition, it is possible to uniformly lower the dielectric loss tangent in the system.
In the case of the modified monomer being a maleic acid anhydride, the content ratio of the modified monomer contained in the insulating resin composition is preferably 0.01 wt% or more and 0.50 wt% or less. When the maleic acid anhydride is contained in the insulating resin composition in this range, it is possible to uniformly suppress the dielectric loss tangent in the system. In particular, the case of containing the maleic acid anhydride over 0.50 wt%, the graft resin sticks to the metal inner wall of a kneading device or an extrusion device, and the uniformity of the resin composition decreases. As a result, the growth of water tree may be promoted.
As a preparation method of the graft resin, it can be obtained by kneading and heating the polyethylene, the antioxidant, the modified monomer and the organic peroxide, which are the raw materials, and reacting in the extrusion device, in accordance with the method described in paragraph [0098] of the specification of Japanese Patent No. 6205032 , for example.
The resin (b2) is at least one ethylene-based copolymer selected from an ethylene-acrylate copolymer, an ethylene-acrylic acid copolymer, and an ethylene-vinyl acetate copolymer.
The ethylene-acrylate copolymer is preferably an ethylene-methylacrylate copolymer, an ethylene-ethylacrylate copolymer, or ethylene-butylacrylate copolymer.
For the content ratio of the component (b) contained in the insulating resin composition, the lower limit is preferably 3.0 wt% or more, and more preferably 4.0 wt% or more, and the upper limit is preferably 25.0 wt% or less, and more preferably 15.0 wt% or less. When the content ratio of the component (b) is within the above-mentioned range, it is possible to sufficiently lower the dielectric loss tangent of the crosslinked substance of the insulating resin composition.
For the component (b), the resin (b1) can efficiently decrease the dielectric loss tangent of the crosslinked substance compared to the resin (b2). For this reason, the insulating resin composition preferably contains the resin (b1).
The component (c) contained in the insulating resin composition is the water tree inhibitor. The water tree inhibitor that is the component (c) is a hydrophilic molecule, and can impart hygroscopic property to the crosslinked substance of the insulating resin composition. By imparting the hygroscopic property to the crosslinked substance, since the moisture in the crosslinked substance is uniformly dispersed, the concentration of moisture on the interface of the foreign substances and the voids is curbed, and the growth of water tree in the crosslinked substance can be suppressed.
The water tree inhibitor is preferably at least one compound selected from a polyalkylene glycol and derivative thereof, polyglycerin, glycerin fatty acid ester, and sorbitol ester.
The polyalkylene glycol is preferably polyethylene glycol, polypropylene glycol, a block copolymer of polyethylene glycol-polypropylene glycol, and alkylethers or carboxylic acid esters of these. Furthermore, preferred derivatives of polyalkylene glycol include alkoxy polyethylene glycol methacrylate, alkoxy polyethylene glycol acrylate, polyoxyethylene alkyl ether and polyethylene glycol fatty acid ester. These water tree inhibitors have an object of imparting a hygroscopic property to the crosslinked substance of the insulating resin composition by hydrogen bonding with water molecules, and the polyalkylene glycol derivative exhibits similar effects as polyalkylene glycol. By methacrylation, acrylation, alkoxyletherification, and esterification of the polyalkylene glycol in this way, crosslinking between the derivative of polyalkylene glycol and polyethylene progresses during crosslinking of the cable insulator, and bleedout can be reduced. The polyalkylene glycol and derivatives thereof are preferably liquids at 60°C, in the case of using by absorbing to resin pellets during dry blending. The polyglycerin is a compound in which a plurality of glycerins polymerized by ether bonding, and can assume a linear, annular or branched structure. In addition, by establishing alkyletherification or carboxylic acid esterification of these polyglycerins, the dispersibility to the insulating resin composition of the present disclosure improves. In particular, introducing to the insulating resin composition by fatty acid esterification is preferable in the point of manufacturing stability of the compound. Sorbitol is a polyalcohol derived from glucose, and a sorbitol ester is a compound produced by esterification of at least one hydroxy group of sorbitol with a fatty acid. Sorbitol can disperse in the insulating resin composition by esterification.
To suppress bleedout of the crosslinked substance of the insulating resin composition, the number average molecular weight of the water tree inhibitor is preferably 1000 or more, more preferably 3000 or more, and even more preferably 9000 or more. On the other hand, to improve the dispersibility of the water tree inhibitor during melt kneading, the number average molecular weight of the water tree inhibitor is preferably 30000 or less. In addition, for ease of handling, the number average molecular weight of the water tree inhibitor is preferably 20000 or less.
Regarding the content ratio of the water tree inhibitor contained in the insulating resin composition, the lower limit is preferably 0.1 wt% or more, and more preferably 0.2 wt% or more, and the upper limit is preferably 2.0 wt% or less, and more preferably 1.0 wt% or less. When the content ratio of the water tree inhibitor is within the above-mentioned range, it is possible to sufficiently suppress growth of water tree in the crosslinked substance of the insulating resin composition. In addition, when the content ratio of the water tree inhibitor is 1.0 wt% or less, it is possible to further suppress bleedout of the crosslinked substance.
The component (d) contained in the insulating resin composition is a crosslinking agent, and causes the polyethylene that is the component (a) to crosslink. The crosslinking agent that is the component (d) is preferably dit-hexyl peroxide (PERHEXYL D manufactured by NOF Corporation), dicumyl peroxide (PERCUMYL D manufactured by NOF Corporation), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (PERHEXA 25B manufactured by NOF Corporation), α,α'-di(t-butylperoxy)diisopropylbenzene (PERBUTYL P manufactured by NOF Corporation), t-butyl cumyl peroxide (PERBUTYL C manufactured by NOF Corporation), and di-t-butylperoxide (PERBUTYL D manufactured by NOF Corporation). These can be used individually or by combining two or more types. Thereamong, it is preferably dicumyl peroxide.
For the content ratio of the crosslinking agent contained in the insulating resin composition, the lower limit is preferably 0.1 wt% or more, and more preferably 0.5 wt% or more, and the upper limit is preferably 5.0 wt% or less, and more preferably 3.0 wt% or less. When the content ratio of the crosslinking agent is within the above-mentioned range, it is possible to favorably crosslink the insulating resin composition.
In addition, the insulating resin composition may contain an antioxidant in addition to the above-mentioned component (a), the component (b), the component (c) and the component (d). As the antioxidant, a hindered phenol-based antioxidant with the main objective of radical scavenging, and a phosphorus-based antioxidant or sulfur-based antioxidant with the main objective of peroxide decomposition are preferably jointly used.
For the content ratio of the antioxidant contained in the insulating resin composition, the lower limit is preferably 0.01 wt% or more, and more preferably 0.20 wt% or more, and the upper limit is preferably 1.00 wt% or less, and more preferably 0.60 wt% or less. When the content ratio of the antioxidant is within the above-mentioned range, it is possible to favorably suppress oxidative degradation of the crosslinked substance of the insulating resin composition.
In addition, so long as not inhibiting the growth suppression of water tree in the crosslinked substance of the insulating resin composition and the decline in the dielectric loss tangent, the insulating resin composition may contain various substances in addition to the above-mentioned components. As the various substances, stabilizers, lubricants, inorganic fillers, surface treatment agents, flame retardants, acid scavengers, voltage stabilizers, etc. can be exemplified.
The insulating resin composition can be obtained by melt kneading after dry blending the component (b), the component (c), the component (d), etc. into the component (a), by a Henschel mixer or the like. As the device for melt kneading, it is possible to use a mixer such as a single-screw or twin-screw extruder, a Banbury mixer, a kneader or the like. In particular, it is preferable to perform resin extrusion by attaching a metal mesh filter with 100 µm or less sieve size to a single-screw or twin-screw extruder, which is capable of continuous processing, with the object of removing foreign substances. For the insulating resin composition after adding the component (d), kneading is preferably performed at 120°C or higher and 135°C or lower.
The specific gravity of the insulating resin composition is preferably 0.91 or more and 0.93 or less.
In addition, the dielectric loss tangent of 30 kV/mm at 90°C of the crosslinked substance made by crosslinking the insulating resin composition is preferably 1.0% or less, and more preferably 0.5% or less. When the above-mentioned dielectric loss tangent of the crosslinked substance is within the above-mentioned range, for the electric power cable in which the crosslinked substance of the insulating resin composition is applied to the insulation layer, since the dielectric loss tangent is small, it is possible to improve the electric power transmission efficiency.
The above-mentioned dielectric loss tangent of the crosslinked substance is a value when measuring under the condition of 30 kV/mm at 90°C based on JIS C2138. It should be noted that the dielectric loss tangent has temperature dependency and applied electric field dependency, and when measuring the crosslinked substance of the above-mentioned insulating resin composition at the condition of 10 kV/mm at 23°C, the dielectric loss tangent of the crosslinked substance is 0.1% or less.
In addition, for growth suppression of the water tree in the crosslinked substance of the insulating resin composition, the coefficient of moisture absorption of the crosslinked substance is preferably 500 ppm or more, and more preferably 800 ppm or more. By imparting the hygroscopic property to the crosslinked substance, the moisture in the crosslinked substance can uniformly disperse to suppress the growth of water tree in the crosslinked substance. In addition, from the viewpoint of dielectric strength of the crosslinked substance, the coefficient of moisture absorption of the crosslinked substance is preferably 2000 ppm or less.
In addition, the melt flow rate of the crosslinked substance when measuring at a melt temperature of 190°C and load of 2.16 kgf based on JIS K7210 is preferably 0.5 g/10 min or more and 5.0 g/10 min or less, and more preferably 0.5 g/10 min or more and 3.0 g/10 min or less. When the melt flow rate is within the above-mentioned range, it is possible to suppress bleedout of the water tree inhibitor from the crosslinked substance, and possible to maintain the shape of the crosslinked substance over a long period.
The crosslinked substance obtained by crosslinking the insulating resin composition according to the embodiment is an insulator. With the electric power cable made by applying such a crosslinked substance of the insulating resin composition to the insulation layer, it is possible to suppress the growth of water tree in the insulation layer, and possible to decrease the dielectric loss tangent, and thus even if using the electric power cable at high voltage or supervoltage, the electric power cable can efficiently transmit electricity. Such an electric power cable is favorably used in underground cables and underwater cables. When the electric power cable is an AC electric power cable, the effects of water tree growth suppression and decline in dielectric loss tangent are even more effectively exhibited.
FIG. 1 is a transverse sectional view showing an example of an electric power cable made by applying the insulating resin composition according to the embodiment.
As shown in FIG. 1, the electric power cable 1 includes a conductor 2, an internal semiconductor layer 3 arranged on the outer side of the conductor 2, an insulation layer 4 arranged on the outer side of the internal semiconductor layer 3, and made by crosslinking the above-mentioned insulating resin composition, and an external semiconductor layer 5 arranged on the outer side of the insulation layer 4. The internal semiconductor layer 3 encircles the conductor 2. The insulation layer 4 encircles the internal semiconductor layer 3. The external semiconductor layer 5 encircles the insulation layer 4. In this way, with the electric power cable 1, on the conductor 2 configured from a metal such as copper or aluminum, the internal semiconductor layer 3, the insulation layer 4 and the external semiconductor layer 5 are laminated in this order.
The internal semiconductor layer 3 and the external semiconductor layer 5, for example, include an ethylene-based copolymer such as an ethylene-ethylacrylate copolymer, an ethylene-methylacrylate copolymer, an ethylene-butylacrylate copolymer or an ethylene-vinyl acetate copolymer; an olefinic elastomer; and an electrically conductive carbon black.
In addition, the electric power cable 1 may further include a metal shielding layer (not shown) arranged on the outer side of the external semiconductor layer 5, and encircling the external semiconductor layer 5. In addition, the electric power cable 1 may further include a sheath (not shown) arranged on the outer side of the metal shielding layer, and encircling the metal shielding layer.
The conductor 2 is continuously supplied to the resin extrusion port, and herein, is covered by the internal semiconductor layer 3, an insulating resin composition layer and the external semiconductor layer 5. These three layers may be simultaneously extruded and coat, or may coat sequentially. In the case of the conductor at the time of coating being heated by heat transfer from the previously coated resin, since the cooling rate of the resin in the vicinity of the conductor slows down, it is preferable to supply the conductor to the resin extrusion port with adjust the temperature of the conductor to 1°C or higher and 100°C or lower by cooling.
Coating of the insulating resin composition is performed by extruding from the resin extrusion port of the resin extruder to which a metal mesh filter with 100 µm or less sieve size is attached, with the object of removing foreign substances, onto the conductor 2 (onto the internal semiconductor layer 3). The temperature of the insulating resin composition during extrusion is preferably the melting point of the insulating resin composition or higher, and specifically is preferably 110°C or higher, and more preferably 120°C or higher. In addition, for scorch suppression, the temperature of the insulating resin composition during extrusion is preferably 140°C or lower.
After being coated on the conductor 2 with the insulating resin composition layer in this way, crosslinking reaction of the insulating resin composition layer is performed by pressurized heating to form the insulation layer 4 made by crosslinking the insulating resin composition. The electric power cable 1 is thereby obtained. Thereafter, the electric power cable 1 is cooled by a cooling pipe or a cooling water tank, and the metal shielding layer or the sheath (not shown) is formed by a known method as necessary.
Upon forming the outer packaging such as the metal shielding layer or the sheath on the electric power cable 1 after cooling, to prevent deformation of the insulation layer 4, the temperature of the insulation layer 4 is preferably 300°C or lower.
Regarding the thickness of the insulation layer 4, from the viewpoint of insulation characteristics, it is preferably 2 mm or more, more preferably 5 mm or more, and even more preferably 10 mm or more, and from the viewpoint of laying workability, it is preferably 50 mm or less, and more preferably 40 mm or less.
In addition, regarding the thickness of the internal semiconductor layer 3 and the thickness of the external semiconductor layer 5, from the viewpoint of the insulation characteristics, they are both preferably 0.1 mm or more, and more preferably 0.5 mm or more, and from the viewpoint of electric conductivity, they are both preferably 5.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less.
According to the above described embodiment, even if containing a water tree inhibitor that is the component (c) for water tree growth suppression, it is possible to suppress an increase in the dielectric loss tangent caused by the water tree inhibitor by containing the component (b). Then, by applying the crosslinked substance of the insulating resin composition capable of suppressing the water tree growth and decreasing the dielectric loss tangent, it is possible to improve the electric power transmission efficiency of the electric power cable.
Although an embodiment has been described above, the present invention is not to be limited to the above-mentioned embodiment, and encompasses every mode included in the spirit of the present disclosure and scope of the claims, and various modifications thereto within the scope of the present disclosure are possible.

EXAMPLES

Next, examples and comparative examples will be explained; however, the present invention is not to be limited to these examples.
The raw materials used in the examples and the comparative examples are as follows.

Component (a): polyethylene (CE1559; LDPE manufactured by Sumitomo Chemical Corp., MFR 0.8)
Resin (b1-1): maleic anhydride graft resin (resin grafting 2.00 wt% of maleic anhydride to CE1559)
Resin (b1-2): maleic anhydride graft resin (resin grafting 0.25 wt% of maleic anhydride to CE1559)
Resin (b1-3): maleic anhydride graft resin (resin grafting 1.00 wt% of maleic anhydride to CE1559)
Resin (b2-1): ethylene-based copolymer (NUC-6520; ethylene-ethyl acrylate copolymer manufactured by ENEOS NUC Corp., 24 wt% acrylate content, MFR 1.6)
Component (c-1): water tree inhibitor (PEG-20000; polyethylene glycol manufactured by Adeka Corp., number average molecular weight 20000)
Component (c-2): water tree inhibitor (polypropylene glycol 4000; polypropylene glycol manufactured by Adeka Corp., number average molecular weight 4000)
Component (c-3): water tree inhibitor (polypropylene glycol 400; polypropylene glycol manufactured by Adeka Corp., number average molecular weight 400)
Component (d): crosslinking agent (PERCUMYL D; dicumyl peroxide manufactured by NOF Corporation)
Antioxidant (Irganox 1010; hindered phenol-based antioxidant manufactured by BASF)

(Examples 1 to 8 and Comparative Examples 1 and 2)

After dry blending each raw material using a Henschel mixer, it was extruded by a single-screw extruder (L/D = 24, 120°C) attaching a plain weave mesh with the sieve size of 0.091 mm to obtain pellet-shaped insulating resin compositions having the compositions shown in Table 1 (hereinafter also referred to as resin pellet).

(Measurement and Evaluation)

The following measurement and evaluation were performed on the insulating resin compositions obtained in the above examples and the comparative examples. The results are shown in Table 1.

(1) Coefficient of Moisture Absorption

Test piece made by molding the resin pellets used at 120°C so as to make a sample of 100 mm length, 150 mm width and 1 mm height, crosslinking the sample at 160°C, then dividing the crosslinked substance of the sample into six pieces, was kept in a thermohygrostat bath at 70°C and 90% relative humidity for 4 hours, then extracted, and the water content thereof was measured by the Karl Fischer method of JIS K7251 method B (moisture vaporization method) to obtain the coefficient of moisture absorption.

(2) Melt flow rate

Using the resin pellets, the melt flow rate was measured based on JIS K7210, according to an extruded resin amount (g) per 10 minutes, under the condition of 190°C melting temperature and 2.16 kgf load.

(3) Water Tree Length

First, a small electric power cable was produced. First, coating was performed on the copper conductor with a threelayer head (120°C) by extruding the insulating resin composition made using the resin pellets by a single-screw extruder (L/D = 24, 120°C, full flight screw) with a plain weave mesh of 0.091 mm sieve size, together with a semiconductive material resin (NUCV-9590, semiconductive resin composition of ethylene-ethyl acrylate copolymer base manufactured by ENEOS NUC Corp.) extruded by another single-screw extruder (L/D = 24, 120°C, full flight screw) with a plain weave mesh of 0.091 mm sieve size. The coated cable passed through a pressurized crosslinking pipe, and was then sent to a water bath to cool. In this way, a small electric power cable including the copper conductor with an external form of approximately 2 mm, the internal semiconductor layer with a thickness of approximately 0.5 mm, the insulation layer with a thickness of approximately 2 mm, and the external semiconductor layer with a thickness of approximately 0.5 mm was obtained.
Next, as shown in FIG. 2, the small electric power cable 1 was immersed in a 3.5 wt% NaCl aqueous solution, and 1000 Hz AC voltage at 4 kV was applied for 200 hours between the conductor and the NaCl aqueous solution to perform the water tree test. Ten samples obtained by slicing the tested electric power cable along the axial direction thereof into 1 mm thickness round slices were observed by optical microscope, and the length of the water tree was measured.

(4) Dielectric Loss Tangent

The resin pellets used were molded for 10 minutes at 120°C so as to make film-shaped sample with 0.3 mm thickness, the sample was crosslinked for 30 minutes at 160°C, and the dielectric loss tangent was measured for the resulting crosslinked substance of the sample under the condition of 30 kV/mm at 90°C based on JIS C2138.

[Table 1]

		Example 1	Example 2	Example 3	Example 4	Example 5	Example 6	Example 7	Example 8	Comparative Example 1	Comparative Example 2
Component (a)	Polyethylene	96.1	95.1	66.2	71.2	95.0	72.2	93.0	82.2	98.1	96.2
Component (b)	Resin (b1-1)	2.0	3.0	-	-	3.0	-	-	-	-	-
	Resin (b1-2)	-	-	30.0	25.0	-	25.0	-	-	-	-
	Resin (b1-3)	-	-	-	-	-	-	-	15.0	-	-
	Resin (b2-1)	-	-	-	-	-	-	5.0	-	-	-
Component (c)	Component (c-1)	-	-	2.0	2.0	-	1.0	-	1.0	-	2.0
	Component (c-2)	0.1	0.1	-	-	0.2	-	-	-	0.1	-
	Component (c-3)	-	-	-	-	-	-	0.2	-	-	-
Component (d)	Crosslinking agent	1.5	1.5	1.5	1.5	1.5	1.5	1.5	1.5	1.5	1.5
Antioxidant		0.3	0.3	0.3	0.3	0.3	0.3	0.3	0.3	0.3	0.3
Coefficient of moisture absorption (ppm)		550	500	1800	1900	600	900	700	1300	550	1700
Resin fluidity (g/10 min)		1.0	1.0	2.0	2.0	1.0	1.5	5.0	2.0	1.0	2.0
Water tree length (mm)		200	130	160	120	100	90	90	80	120	60
Dielectric loss tangent (%)		0.9	0.4	0.4	0.3	0.4	0.3	0.7	0.2	Over 1.0	Over 1.0

As shown in Table 1, Examples 1 to 8 could achieve a suppression of the water tree growth and decrease in dielectric loss tangent, due to the insulating resin composition containing the component (a), the component (b), the component (c) and the component (d). On the other hand, Comparative Examples 1 and 2 could not achieve a suppression of the water tree growth and decrease in dielectric loss tangent due to the insulating resin composition not containing at least one component among the component (a) to the component (d).

EXPLANATION OF REFERENCE NUMERALS

1 electric power cable
2 conductor
3 internal semiconductor layer
4 insulation layer
5 external semiconductor layer

Claims

An insulating resin composition for electric power cable comprising: a component (a); a component (b); a component (c); and a component (d), wherein
the component (a) is polyethylene,

the component (b) is at least one type of resin selected from a resin (b1) and a resin (b2), the resin (b1) being a resin in which at least one type of modified monomer selected from unsaturated organic acids and derivatives thereof is grafted, and the resin (b2) being at least one type of ethylene-based copolymer selected from an ethylene-acrylate copolymer, an ethylene-acrylic acid copolymer and an ethylene-vinyl acetate copolymer,

the component (c) is a water tree inhibitor, and

the component (d) is a crosslinking agent.
The insulating resin composition for electric power cable according to claim 1, wherein the component (c) is at least one compound selected from a polyalkylene glycol and derivative thereof, polyglycerin, glycerin fatty acid ester, and sorbitol ester.
The insulating resin composition for electric power cable according to claim 1 or 2, wherein a dielectric loss tangent of 30 kV/mm at 90°C of a crosslinked substance obtained by crosslinking the insulating resin composition for electric power cable is 1.0% or less.
The insulating resin composition for electric power cable according to any one of claims 1 to 3, wherein, in the resin (b1), at least one type of modified monomer selected from an unsaturated dicarboxylic acid, unsaturated dicarboxylic acid anhydride, and an unsaturated dicarboxylic acid derivative is grafted to at least one type of resin selected from a polypropylene, polyethylene and olefin-based copolymer.
The insulating resin composition for electric power cable according to any one of claims 1 to 4, wherein a content ratio of the component (b) contained in the insulating resin composition for electric power cable is 3.0 wt% or more and 25.0 wt% or less.
An electric power cable comprising:
a conductor;

an internal semiconductor layer disposed on an outer side of the conductor, and encircling the conductor;

an insulation layer disposed on an outer side of the internal semiconductor layer, encircling the internal semiconductor layer, and made by crosslinking the insulating resin composition for electric power cable according to any one of claims 1 to 5; and

an external semiconductor layer disposed on an outer side of the insulation layer, and encircling the insulation layer.