JP7812605B2 - Insulating resin composition for DC power cables - Google Patents

Description

The present invention relates to an insulating resin composition for DC power cables, a crosslinked resin obtained by crosslinking this resin composition, a DC power cable having an insulating layer made of this crosslinked resin, a member for forming a reinforcing insulating layer at a DC power cable joint used when connecting DC power cables, and a DC power cable joint having a reinforcing insulating layer made of the crosslinked resin.

Conventionally, DC power cables with an insulating layer formed from an insulating resin composition have been introduced as DC power cables that are easy to maintain and do not pose a risk of oil leakage (see Patent Document 1 below).

Furthermore, as an insulating resin composition for a DC power cable, the torque during extrusion is appropriate and the extrusion processability is excellent, and in the extrusion molded product, sagging that reduces the roundness of the cable insulation is unlikely to occur, the scorch resistance is good, and the amount of secondary decomposition water generated during reheating when connecting cables is small, making it possible to form an insulating layer that stably exhibits good DC electrical characteristics, and the resin pressure during extrusion is stable and the extrusion stability is excellent, making it possible to form an insulating layer with small thickness variation. The insulating resin composition further includes: (A) a complex viscosity η ^* ₁₀₀ at 130°C and a frequency of 100 rad/s of 600 to 1300 Pa·s, and a ratio of the complex viscosity η ^* _0.1 at 130°C and a frequency of 0.1 rad/s to the complex viscosity η ^* ₁₀₀ (η ^* _0.1 /η ^* ₁₀₀ The applicant has introduced a resin composition comprising: 100 parts by mass of a low-density polyethylene having a molecular weight (Mw) of 4 or more; (B) 5 to 12 parts by mass of a modified polyethylene in which at least one modifying monomer selected from unsaturated organic acids and derivatives thereof has been grafted onto polyethylene; and (C) 0.01 to 0.8 parts by mass of a stabilizer consisting of a mixture of 40 to 60% by weight of a hindered phenol-based antioxidant and 60 to 40% by weight of a thioether-based antioxidant, wherein the amount of carbonyl groups introduced into the resin composition by component (B) is 7 to 13 μmol/g relative to the total mass of components (A), (B), and (C) (see Patent Document 2 listed below).

In the resin composition described in Patent Document 2, maleic anhydride (MAH) is described as a suitable modifying monomer for obtaining component (B).
By producing a resin composition containing modified polyethylene modified with maleic anhydride, the crosslinked resin obtained by crosslinking the resin composition is less likely to accumulate space charge under DC voltage and can exhibit excellent insulating properties against DC voltage.
As a crosslinking agent for crosslinking the resin composition, an organic peroxide is used.

Japanese Patent Application Publication No. 8-7653 Patent No. 6205032

However, maleic anhydride is a deleterious substance, and there are concerns that it may be harmful to the human body during the preparation of modified polyethylene.
Furthermore, while prior art techniques require measurement of space charge characteristics using a cable, there is a demand for a simple method for evaluating insulating materials that does not require the fabrication of a cable.

The present invention has been made based on the above circumstances.
An object of the present invention is to provide an insulating resin composition for DC power cables, which contains modified polyethylene modified with a less harmful modifying monomer and which can give a crosslinked resin product having excellent insulating properties against DC voltage (having insulating properties equal to or greater than those of a crosslinked resin product of a resin composition containing modified polyethylene modified with maleic anhydride).
Another object of the present invention is to establish a method for simply evaluating whether an insulating material has excellent insulating properties against DC voltage, and to provide an insulating resin composition for DC power cables that has been evaluated by this method as having excellent insulating properties against DC voltage.

In order to achieve the above objective, the inventors conducted extensive research and discovered a simple method for evaluating whether a resin composition containing modified polyethylene modified with maleic anhydride, as described in Patent Document 2, has insulating properties equivalent to or greater than those of a crosslinked resin composition. They also discovered that by incorporating modified polyethylene into acetophenone, a decomposition residue of an organic peroxide crosslinking agent, into which polar groups with high binding energy to positive ions formed by bonding hydrogen ions have been introduced (grafted), the movement of space charges in the resulting crosslinked resin composition is suppressed, thereby inhibiting their accumulation. Based on these findings, the present invention was completed.

(1) That is, the insulating resin composition for a DC power cable of the present invention is characterized in that the space charge change rate measured as an evaluation of space charge characteristics by a pulse electrostatic discharge (PEA) method is 3% or less .

(2) The resin composition of the present invention contains a modified polyethylene in which a modifying monomer having a terminal double bond is grafted onto polyethylene,
The modified polyethylene preferably has a polar group introduced by the modifying monomer, the polar group having a bond energy of 0.44 eV or more with a positive ion formed by bonding a hydrogen ion to acetophenone.
Hereinafter, the modified monomer capable of introducing the above polar group will be referred to as a "specific modified monomer."

According to the inventors' investigations, there is a correlation between the binding energy with the positive ions and the charge change rate (described later), which is an index of charge transfer, and the higher the binding energy with the positive ions, the smaller the charge change rate (the more suppressed the charge transfer).
A resin composition is produced containing modified polyethylene having a polar group whose bond energy with the positive ion is 0.44 eV or more, and the resulting crosslinked resin product can capture charge carriers such as acetophenone, which is a decomposition residue of organic peroxide, and suppress the movement of charges. This makes it possible to suppress the amount of charge accumulation, and ultimately to exhibit excellent insulation properties against DC voltage.

(3) In the resin composition of (2) above, the specific modifying monomer is preferably a nitrogen-containing (meth)acrylic acid derivative containing two or more nitrogen atoms in the molecule.

(4) In the resin composition (3) above, it is preferable that the nitrogen atom-containing (meth)acrylic acid derivative is a ^{(meth)acrylamide represented by the general formula: H2C=C(R1)CONR2R3 (wherein R1} _is ^{a hydrogen atom} or a methyl group, ^R2 is a hydrogen atom or an alkyl group, and ^R3 is a monovalent organic group containing at least one nitrogen atom).

(5) In the resin composition described in (2) above, it is particularly preferred that the specific modifying monomer is dimethylaminopropylacrylamide (DMAPAA).

(6) The resin compositions described above in (2) to (5) preferably contain (A) polyethylene, (B) the modified polyethylene, and (C) an antioxidant.

(7) It is preferable that the resin composition of (6) above further contains (D) an organic peroxide.

(8) The resin composition of (6) or (7) above preferably contains 8 parts by mass or more of (B) modified polyethylene per 100 parts by mass of (A) polyethylene.

(9) In the resin compositions of (6) to (8) above, it is preferable that the concentration of the polar group introduced by the specific modified monomer is 4 μmol/g or more .

(10) The resin composition of (5) above, comprising: (A) 100 parts by mass of polyethylene;
(B) 8 to 18 parts by mass of the modified polyethylene;
(C) 0.01 to 0.8 parts by mass of an antioxidant;
(D) 0.1 to 5 parts by mass of an organic peroxide,
The concentration of the polar group introduced by the specific modified monomer is preferably 4 to 8 μmol/g.

(11) The crosslinked resin product of the present invention is characterized in that it is obtained by crosslinking the resin composition of the present invention.

(12) The DC power cable of the present invention is characterized in that an internal semiconductive layer and an insulating layer made of the crosslinked resin of the present invention are laminated on the surface of a conductive member.

(13) The reinforcing insulating layer-forming member of the present invention is a repair tape-shaped member that is wound around a connection portion internal semiconducting layer that covers exposed portions of conductive members, including the connection portions of the conductive members of DC power cables, when connecting DC power cables together, and forms a reinforcing insulating layer on the connection portion internal semiconducting layer by crosslinking, and is characterized by being made of the resin composition of the present invention.

(14) The DC power cable connection of the present invention is formed by connecting two DC power cables together, and is characterized in that a reinforcing insulating layer made of the crosslinked resin of the present invention is formed on a connection internal semiconductive layer that covers exposed portions of the conductive members of the DC power cables, including the connection portions of the conductive members.

The resin composition of the present invention makes it possible to obtain a crosslinked resin product that has excellent insulation properties against DC voltage, equivalent to or better than those of a crosslinked resin product made from a resin composition containing modified polyethylene modified with maleic anhydride.

1 is a cross-sectional view showing an example of a DC power cable of the present invention. 1 is a longitudinal cross-sectional view showing an example of a DC power cable connection part of the present invention. 10 is a flowchart showing an example of a procedure for calculating an average value of binding energy <E>.

The resin composition of the present invention contains component (A) consisting of polyethylene, component (B) consisting of modified polyethylene, component (C) consisting of an antioxidant, and component (D) consisting of an organic peroxide.

<Component (A)>
The polyethylene that is component (A) of the resin composition of the present invention is not particularly limited, and conventionally known high-pressure low-density polyethylenes can be suitably used.
An example of a commercially available product of component (A) is "NUC-8160" (manufactured by ENEOS NUC Corporation).

<(B) component>
The component (B) constituting the resin composition of the present invention is a modified polyethylene having a polar group having a bond energy of 0.44 eV or more with a positive ion formed by bonding a hydrogen ion to acetophenone represented by the following formula:

The "positive ion formed by bonding a hydrogen ion to acetophenone" is a positive ion used to determine the bond energy with the polar group of component (B), and is not an essential component of the resin composition of the present invention. However, this positive ion is inevitably present in the crosslinked product of the resin composition of the present invention as a decomposition residue (charge carrier) of component (D).

The bond energy between the polar groups (polar groups derived from the modifying monomer) in the modified polyethylene and the positive ions can be calculated.

Assuming that the space charge change rate increases due to the diffusion of charge carriers within the modified polyethylene, if the bond strength of the charge carriers to the polar groups of the modified polyethylene is strong, the charge carriers will be less likely to diffuse, and the space charge change rate can be kept low.
In the present invention, the strength of the bond between the polar group of the modified polyethylene and the positive ion, which is a charge carrier, is quantified as bond energy, and calculated by molecular simulation.

To give a specific example, in the case of a modified polyethylene in which maleic anhydride is grafted onto a polyethylene main chain, the bond energy between the polar group derived from maleic anhydride and the positive ion can be obtained by randomly arranging the positive ions around the modified polyethylene molecules and determining the intermolecular force.
In this case, a molecular simulation is used to perform a thorough conformational search (100 or more, and if possible, 1000 or more configurations) to calculate the total energy for each structure (configuration), thereby obtaining a distribution of total energies for all structures.
The distribution of bond energies is obtained by subtracting the energies of the isolated molecules of the modified polyethylene and the positive ions from the distribution of the total energy obtained.
This distribution of binding energy ranges from low energy to high energy states. In an actual experimental environment, for example at room temperature, it is possible to assume a high energy state corresponding to that temperature, and this state follows the Boltzmann distribution. Therefore, the average value of the binding energy <E> is given by the following formula:

In the above formula, E(x) is the binding energy in an arbitrary structure x, k _B is the Boltzmann constant, T is the absolute temperature, and N is a normalization constant.

The "bond energy" defined in this invention (claim 2) refers to the "average bond energy <E>" calculated from the distribution of bond energies using the above formula.

Figure 3 shows a flowchart illustrating an example of the procedure for calculating the average bond energy <E>. However, the calculation procedure is not limited to this.

Below is the bond energy (average bond energy <E> calculated using the above method) between the polar group derived from the compound and the positive ion for each modified polyethylene in which the following compounds have been grafted onto the polyethylene main chain:

Dimethylaminopropylacrylamide (0.46 eV)
N-methyl-N-[3-(dimethylamino)propyl]acrylamide (0.46 eV)
Maleic anhydride (0.46 eV)
2-(2-ethoxy)ethoxyethyl acrylate (0.42 eV)
2-(dimethylamino)ethyl acrylate (0.42 eV)
2-allylmalonic acid (0.40 eV)
Dimethylaminoethylacrylamide (0.40 eV)
Diethylaminoethylacrylamide (0.38 eV)
2-ethylhexyl acrylate (0.32 eV)
triallyl isocyanurate (0.32 eV)
Diallyl adipate (0.30 eV)
Aminopropylacrylamide (0.30 eV)
N-allyloxyphthalimide (0.30 eV)
Hydroxymethylacrylamide (0.28 eV)
For reference, the bond energy between ungrafted polyethylene and the positive ions was 0.22 eV.

Polar groups with a bond energy of 0.44 eV or more with positive ions are introduced by specific modified monomers (modified monomers that can introduce such polar groups and have double bonds at their terminals are specific modified monomers).

Component (B), a modified polyethylene in which the polar groups have been introduced by modifying polyethylene with a specific modifying monomer, is prepared, and by incorporating the resulting component (B) in a specified proportion, charge carriers such as acetophenone, which is a residue of component (D) described below, are captured, thereby suppressing charge migration.

The specific modified monomer has a double bond at the end of its molecule.
This is advantageous in that the reactivity during modification (graft reaction) is higher than that of a monomer that does not have a double bond at the molecular end, such as maleic anhydride.
The use of specific highly reactive modified monomers is expected to suppress the formation of high molecular weight gels resulting from the cross-linking reaction of polyethylene as a side reaction.
Specific examples of the specific modifying monomer include nitrogen atom-containing (meth)acrylic acid derivatives containing two or more nitrogen atoms in the molecule, such as dimethylaminopropyl(meth)acrylamide and N-methyl-N-[3-(dimethylamino)propyl]acrylamide.
Of these, dimethylaminopropylacrylamide is particularly preferred.

The content of component (B) is usually 5 parts by mass or more, and preferably 5 to 18 parts by mass, per 100 parts by mass of component (A).
If the content of component (B) is less than 5 parts by mass, it becomes difficult to uniformly disperse component (B) in component (A), and as a result, the movement of space charges cannot be sufficiently suppressed in the crosslinked product of the resulting resin composition, and the space charges that accumulate upon application of DC charges prevent the product from exhibiting sufficient performance as an insulating material for DC power cables (see Comparative Example 2 described below).
On the other hand, even if the content of component (B) exceeds 18 parts by mass, the improvement in effect commensurate with the increase in the amount added is not obtained.

The component (B) can be prepared by mixing polyethylene, a specific modified monomer, an antioxidant, and an organic peroxide in an extruder, heating them to cause a reaction, and then granulating them into pellets or granules.
The antioxidant used in preparing component (B) is deactivated during the synthesis of component (B) and does not become a component of component (C) in the resin composition.

<(C) component>
The component (C) constituting the resin composition of the present invention is not particularly limited, and examples thereof include conventionally known antioxidants (such as phenol-based antioxidants, amine-based antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants) that are used in insulating resin compositions for power cables.
The content of component (C) is set to 0.01 to 0.8 parts by mass, and preferably 0.2 to 0.6 parts by mass, per 100 parts by mass of component (A).
If the content of component (C) is less than 0.01 parts by mass, the scorch resistance of the resulting resin composition will be poor, and the heat resistance of the crosslinked resin product obtained by crosslinking the resin composition will be poor.
On the other hand, if the content of component (C) exceeds 0.8 parts by mass, the resulting resin composition cannot form a crosslinked resin product with a small amount of secondary decomposition water, and bleeding from the resulting crosslinked resin product also increases.

<Component (D)> The organic peroxide, which is the component (D) constituting the resin composition of the present invention, acts as a crosslinking agent.
Specific examples of component (D) include di-t-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-butylcumyl peroxide (Perbutyl C, manufactured by NOF Corporation), and di-t-butyl peroxide (Perbutyl D, manufactured by NOF Corporation), which can be used alone or in combination of two or more. Of these, dicumyl peroxide is preferred.
The content of component (D) is preferably 0.1 to 5 parts by mass, and more preferably 0.5 to 3 parts by mass, per 100 parts by mass of component (A).
If the blending amount is too small, crosslinking will not be sufficient, resulting in poor mechanical properties and heat resistance of the resulting crosslinked product, whereas if the blending amount is too large, scorching will occur during extrusion molding of the resulting resin composition, resulting in poor electrical properties.

<Concentration of polar groups introduced by specific modified monomers>
In the resin composition of the present invention, the concentration (graft amount) of the polar group introduced by the specific modified monomer is usually 2 μmol/g or more, preferably 2 to 8 μmol/g, and a suitable example is 4 μmol/g.
A crosslinked resin product obtained from a resin composition having a polar group concentration of less than 2 μmol/g is unable to sufficiently suppress the movement of space charges, and is therefore unable to exhibit sufficient performance as an insulating material for a DC power cable (see Comparative Example 2 described below).

Here, the "polar group concentration" refers to the number of moles of polar groups in component (B) introduced by a specific modifying monomer relative to the total mass of components (A), (B), and (C).
The concentration of the polar group introduced by the modifying monomer (graft amount) is a measured value as described below.

<Resin composition>
The resin composition of the present invention contains components (A) to (C), and when crosslinking the resin composition (to obtain a crosslinked resin product), it contains component (D).
Various stabilizers and other additives may also be contained within the range that does not impair the effects of the present invention. Examples of stabilizers include light stabilizers, ultraviolet absorbers, and copper inhibitors. Examples of other additives include inorganic fillers, organic fillers, lubricants, and dispersants.

<Crosslinked resin>
The resin composition of the present invention can be crosslinked with the organic peroxide, which is component (D).
The crosslinked resin of the present invention can be obtained by crosslinking the resin composition of the present invention with an organic peroxide, which is component (D).

The charge change rate of a sheet made of the crosslinked resin of the present invention is preferably 10% or less, and more preferably 3% or less.
When space charges accumulate in the cable insulation due to the application of a high DC voltage, the insulation properties deteriorate significantly when an impulse of the opposite polarity is applied or when the polarity is reversed.
Therefore, by keeping the charge change rate in the crosslinked resin product at 10% or less, it is possible to obtain a DC power cable that stably exhibits good DC electrical characteristics.
By crosslinking the resin composition of the present invention, in which the concentration (graft amount) of the polar group introduced by a specific modified monomer is 2 μmol/g or more, the charge change rate of the resulting crosslinked product can be made 10% or less.

<DC power cable>
The DC power cable of the present invention comprises an inner semiconductive layer and an insulating layer made of the crosslinked resin of the present invention laminated on the surface of a conductive member.
FIG. 1 is a cross-sectional view showing an example of a DC power cable according to the present invention.
In a DC power cable 10 shown in FIG. 1 , an inner semiconductive layer 12, an insulating layer 13 made of the crosslinked resin of the present invention, and an outer semiconductive layer 14 are laminated on the outer peripheral surface of a conductor 11, and further, a metal shielding layer 15 and a sheath 16 are laminated on the outer peripheral surface of the outer semiconductive layer 14.

The DC power cable 10 of the present invention shown in Figure 1 can be produced by extruding the resin composition of the present invention together with the inner semiconductive layer 12 that covers the conductor 11 (at this time, the outer semiconductive layer 14 may also be extruded simultaneously), crosslinking the resin composition to form the insulating layer 13 made of a crosslinked resin, and then providing the metal shielding layer 15 and sheath 16 according to conventional methods.

The cross-linking method for forming the insulating layer 13 (cross-linked resin body) is not particularly limited, but typically, a pressurized and heated treatment or the like is used.
For example, the resin composition is heated and pressurized at a pressure of 10 kg/cm@2 and a temperature of 280 DEG C. in a nitrogen atmosphere to promote crosslinking of the resin composition by a radical reaction using component (D) as an initiator.

The DC power cable 10 of the present invention exhibits excellent DC electrical characteristics and is less susceptible to breakdown of the insulating layer 13.

<DC power cable connection>
The DC power cable joint of the present invention comprises a reinforcing insulating layer made of the crosslinked resin product of the present invention formed on a joint internal semiconductive layer that covers exposed portions of conductive members of a DC power cable, including the joint portion of the conductive members.

FIG. 2 is a vertical cross-sectional view showing an example of a DC power cable joint according to the present invention.
The DC power cable connection part 20 shown in FIG. 2 is formed by connecting two DC power cables 10A, 10B, and is formed by laminating a reinforcing insulating layer 23 made of the crosslinked resin of the present invention and a connection part external semiconductive layer 24 on a connection part internal semiconductive layer 22 that covers exposed portions of the conductors, including a connection part 17 between the conductor 11A of the DC power cable 10A and the conductor 11B of the DC power cable 10B.

In the figure, 13A is an insulating layer of a DC power cable 10A, and 13B is an insulating layer of a DC power cable 10B, and the insulating layers 13A and 13B are made of the crosslinked resin of the present invention.
Also, 12A, 14A, 15A and 16A respectively denote the inner semiconductive layer, the outer semiconductive layer, the metal shielding layer and the sheath of the DC power cable 10A.
Furthermore, 12B, 14B, 15B and 16B respectively denote the inner semiconductive layer, the outer semiconductive layer, the metal shielding layer and the sheath of the DC power cable 10B.

When connecting DC power cables 10A and 10B to form a DC power cable connection 20 as shown in FIG. 2, a tape-shaped member (reinforcing insulating layer-forming member of the present invention) made from the resin composition of the present invention is wrapped around the connection internal semiconducting layer 22, which covers the exposed portions of the conductors, including the connection 17 between conductors 11A and 11B, and crosslinked by heat treatment to form a reinforcing insulating layer 23 made from the crosslinked resin of the present invention.

When the reinforcing insulating layer-forming member of the present invention is heat-treated to form the reinforcing insulating layer 23, the insulating layer 13A of the DC power cable 10A and the insulating layer 13B of the DC power cable 10B are also heated. However, since the insulating layers 13A, 13B are made of the crosslinked resin body of the present invention, the amount of secondary decomposition water generated from the insulating layers 13A, 13B can be reduced.
Furthermore, the accumulation of space charges in the reinforcing insulating layer 23 can also be suppressed.

A variety of methods can be used to manufacture cable connections depending on the voltage class, application, installation environment, etc., including tape mold joints (TMJ), extrusion mold joints (EMJ), and block mold joints (BMJ).

The present invention will be explained below using examples.

<Analysis method>
(1) MFR: Measured in accordance with JIS K 7210 at a measurement temperature of 190° C. and a load of 21.18 N.

(2) Density: Measured in accordance with JIS K 7112.

(3) Measurement of the concentration of polar groups introduced by modified monomers:
(3-1) Preparation of Samples A resin composition preheated at 120°C and 1 MPa for 5 minutes was heated in a press at 180°C and 15 MPa for 15 minutes to prepare a crosslinked sheet (sheet-shaped crosslinked resin product) with a thickness of approximately 0.2 mm. This crosslinked sheet was immersed in chloroform to remove unreacted modified monomer.

(3-2) Measurement of infrared absorption spectrum:
Measurements were taken using an infrared spectrophotometer "FT/IR-4200" (manufactured by JASCO Corporation) in the wavenumber range of 500 cm ^-1 to 4000 cm ^-1 under conditions of 16 accumulations and a resolution of 4 cm ^-1 , and the ratio of the absorbance of the absorption peak characteristic of the modified monomer near 1650 to 1750 cm ^-1 (baseline 1600 to 1840 cm ^-1 ) to the absorbance of the absorption peak characteristic of polyethylene near 2020 cm ^-1 (baseline 1980 to 2110 cm ^-1 ) was recorded.

(3-3) Calculation of polar group concentration:
Using a calibration curve prepared from samples of known concentrations, the concentration of the polar group introduced by the modified monomer was calculated from the modified monomer/polyethylene ratio.

<Calculation conditions for the bond energy between a polar group and a positive ion>
(1) Software used:
- Cloud service Matlantis (PFP version 1.1.0, Grimme D3 dispersion force correction)

(2) References:
So Takamoto, Chikashi Shinagawa, Daisuke Motoki, Kosuke Nakago, Wenwen Li, Iori Kurata, Taku Watanabe, Yoshihiro Yayama, Hiroki Iriguchi, Yusuke Asano, Tasuku Onodera, Takafumi Ishii, Takao Kudo, Hideki Ono, Ryohto Sawada, Ryuichiro Ishitani, Marc Ong, Taiki Yamaguchi, Toshiki Kataoka, Akihide Hayashi, and Takeshi Ibuka, PFP: Universal Neural Network Potential for Material Discovery. arXiv:2106.14583v1 Condensed Matter

(3) Structural optimization calculation:
・Optimization algorithm: FIRE (atomic coordinates), Exp Cell Filter (periodic structure)
Calculation convergence condition: 0.05 eV/Å or less PE periodic structure: C ₁₂ H ₂₄ , a=15.2533 Å, b=20.0 Å, c=20.0 Å, rectangular parallelepiped

(4) Binding energy sampling:
Number of states: 2000
Boltzmann distribution: 300 Kelvin

<Preparing polyethylene>
A low-density polyethylene (A1) obtained by a high-pressure tubular method and having an MFR of 2.0 to 3.0 g/10 min and a density of 0.92 g/cm ³ was prepared.

<Preparation of modified polyethylene>
(1) Preparation Example B1 (for the present invention):
To 100 parts by mass of low-density polyethylene (A1), 1.1 parts by mass of dimethylaminopropylacrylamide and 0.05 parts by mass of a hindered phenol-based antioxidant "Irganox 1010" (manufactured by BASF) were added, and the mixture was reacted by mixing and heating in an extruder using 0.05 parts by mass of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexyne-3 "Perhexyne 25B" (manufactured by NOF Corporation), which is an organic peroxide, to obtain a modified polyethylene (B1) consisting of a graft copolymer.
Here, the bond energy (average value of the calculated bond energy <E>) between the polar group of the modified polyethylene (B1) introduced by dimethylaminopropylacrylamide and the positive ion formed by bonding a hydrogen ion to acetophenone is 0.46 eV.

(2) Preparation example B2 (for comparison):
A modified polyethylene (B2) consisting of a graft copolymer was obtained in the same manner as in Preparation Example B1, except that 0.7 parts by mass of hydroxymethyl acrylamide was used instead of dimethylaminopropyl acrylamide.
Here, the bond energy (average value of the calculated bond energy <E>) between the polar group of the modified polyethylene (B2) introduced by hydroxymethylacrylamide and the positive ion formed by bonding a hydrogen ion to acetophenone is 0.28 eV.

(3) Preparation example B3 (for comparison):
A modified polyethylene (B3) consisting of a graft copolymer was obtained in the same manner as in Preparation Example B1, except that 1.2 parts by mass of 2-ethylhexyl acrylate was used instead of dimethylaminopropylacrylamide.
Here, the bond energy (average value of the calculated bond energy <E>) between the polar group of the modified polyethylene (B3) introduced by 2-ethylhexyl acrylate and the positive ion formed by bonding a hydrogen ion to acetophenone is 0.32 eV.

(4) Preparation example B4 (for comparison):
A modified polyethylene (B4) consisting of a graft copolymer was obtained in the same manner as in Preparation Example B1, except that 1.1 parts by mass of 2-(2-ethoxy)ethoxyethyl acrylate was used instead of dimethylaminopropylacrylamide.
Here, the bond energy (average value of the calculated bond energy <E>) between the polar group of the modified polyethylene (B4) introduced by 2-(2-ethoxy)ethoxyethyl acrylate and the positive ion in which a hydrogen ion is bonded to acetophenone is 0.42 eV.

(5) Preparation example B5 (for reference):
A modified polyethylene (B5) consisting of a graft copolymer was obtained in the same manner as in Preparation Example B1, except that 0.7 parts by mass of maleic anhydride, which is a modified monomer not having a double bond at its terminal, was used instead of dimethylaminopropylacrylamide.
Here, the bond energy (average value of the calculated bond energy <E>) between the polar group of the modified polyethylene (B5) introduced with maleic anhydride and the positive ion formed by bonding a hydrogen ion to acetophenone is 0.46 eV.

<Preparing the antioxidant>
As component (C), a thioether-based antioxidant "Seenox BCS" (manufactured by Shipro Kasei Co., Ltd.) was prepared.

<Preparation of organic peroxide>
As component (D), a crosslinking agent made of dicumyl peroxide was prepared.

< Comparative Example 6, Examples 2 and 3 >
Resin compositions were obtained by mixing 100 parts by mass of low-density polyethylene (A1), modified polyethylene (B1) in the amounts shown in Table 1 below, 0.2 parts by mass of component (C), and 1.5 parts by mass of component (D). The concentrations of polar groups introduced by the modifying monomer measured for each of the obtained resin compositions are also shown in Table 1.

<Comparative Example 1>
According to the formulation shown in Table 1 below, a comparative resin composition was obtained in the same manner as in Comparative Example 6 , except that component (B) was not used.

<Comparative Example 2>
A comparative resin composition was obtained in the same manner as in Comparative Example 6 , except that the amount of modified polyethylene (B1) used was changed to 3 parts by mass according to the formulation shown in Table 1. The concentration of polar groups introduced by the modifying monomer in this resin composition was 1 μmol/g.

<Comparative Example 3>
A comparative resin composition was obtained in the same manner as in Comparative Example 6 , except that 18 parts by mass of modified polyethylene (B2) was used instead of modified polyethylene (B1) according to the formulation shown in Table 1 below. The concentration of polar groups introduced by the modifying monomer in this resin composition was 8 μmol/g.

<Comparative Example 4>
A comparative resin composition was obtained in the same manner as in Comparative Example 6 , except that 18 parts by mass of modified polyethylene (B3) was used instead of modified polyethylene (B1) according to the formulation shown in Table 1 below. The concentration of polar groups introduced by the modifying monomer in this resin composition was 8 μmol/g.

<Comparative Example 5>
A comparative resin composition was obtained in the same manner as in Comparative Example 6 , except that 18 parts by mass of modified polyethylene (B4) was used instead of modified polyethylene (B1) according to the formulation shown in Table 1 below. The concentration of polar groups introduced by the modifying monomer in this resin composition was 8 μmol/g.

<Reference example 1>
A reference resin composition was obtained in the same manner as in Comparative Example 6 , except that 15 parts by mass of modified polyethylene (B5) was used instead of modified polyethylene (B1) according to the formulation shown in Table 1 below. The concentration of polar groups introduced by the modifying monomer in this resin composition was 7 μmol/g.

<Evaluation>
For each of the resin compositions obtained in the above Examples, Comparative Examples, and Reference Examples, the space charge characteristics of the crosslinked resin products obtained from the resin compositions were evaluated (measurement of the charge change rate). The measurement method, evaluation method, and evaluation criteria are as follows. The results are also shown in Table 1 below.

(1) Measurement of space charge change rate (evaluation of space charge characteristics):
Each of the resin compositions obtained in Examples 2 to 3 , Comparative Examples 1 to 6 , and Reference Example 1 was pressed at a temperature of 120°C and a pressure of 1 MPa for 5 minutes to form a sheet, and then pressed at a temperature of 180°C and a pressure of 15 MPa for 15 minutes to obtain a crosslinked sheet (crosslinked resin product) having a thickness of 0.2 mm.
For each of the obtained crosslinked sheets, the space charge change rate was measured as an evaluation of the space charge characteristics by a pulse electrostatic discharge (PEA) method.
Specifically, a negative DC electric field of 40 kV/mm was applied to the crosslinked sheet continuously for 30 minutes at a temperature of 23°C, and the space charge change rate was measured using the formula [(C ₀ - C ₃₀ )/C ₀ ] x 100, calculated from the maximum charge density (unit: C/m ³ ) value at the start of application (C ₀ ) and the maximum charge density value at the end of application (C ₃₀ ).
The smaller this space charge change rate, the less space charge moves and accumulates when a high DC voltage is applied, and the better the space charge characteristics.
The evaluation criteria were as follows: a charge change rate of 3% or less was rated as "pass", and a charge change rate of more than 3% was rated as "fail".

The results of Examples 2 and 3 and Comparative Examples 2 and 6 in Table 1 show that the space charge change rate of the crosslinked sheet depends on the concentration of polar groups introduced by the dimethylaminopropylacrylamide used, and that the space charge change rate is less than 10% at a concentration of 2 μmol/g or higher. Furthermore, when the monomer concentration is 4 μmol/g or higher, there is almost no change in the space charge change rate of the crosslinked sheet, and it is also clear that the effect of improving space charge characteristics is saturated.
In contrast, the results of Comparative Examples 3 to 5 show that even when the concentration of polar groups introduced by the modified monomer used was 8 μmol/g, the effect of improving the space charge characteristics was insufficient.

10, 10A, 10B DC power cable 11, 11A, 11B Conductor 12, 12A, 12B Inner semiconductive layer 13, 13A, 13B Insulating layer 14, 14A, 14B Outer semiconductive layer 15, 15A, 15B Metal shielding layer 16, 16A, 16B Sheath 17 Conductor connection portion 20 DC power cable connection portion 22 Internal semiconductive layer of connection portion 23 Reinforcing insulating layer 24 External semiconductive layer of connection portion

Claims (14)

An insulating resin composition for DC power cables, having a space charge change rate of 3% or less as measured and calculated by the pulse electrostatic discharge (PEA) method as follows:
[Method for measuring and calculating space charge change rate]
The resin composition is pressed at a temperature of 120°C and a pressure of 1 MPa for 5 minutes to form a sheet, and then pressed at a temperature of 180°C and a pressure of 15 MPa for 15 minutes to prepare a crosslinked sheet having a thickness of 0.2 mm. A negative DC electric field of 40 kV/mm is continuously applied to the obtained crosslinked sheet for 30 minutes under a temperature condition of 23°C, and the maximum charge density value (C ₀ ) and the maximum charge density at the end of application (C ₃₀ ) was measured, and the ₀ -C ₃₀ )/C ₀ ] × 100 to calculate the space charge change rate. The resin composition contains a modified polyethylene in which a modifying monomer having a double bond at a terminal is grafted onto polyethylene,
2. The insulating resin composition for a DC power cable according to claim 1, wherein the modified polyethylene has a polar group introduced by the modifying monomer, the polar group having a bond energy of 0.44 eV or more with a positive ion formed by bonding a hydrogen ion to acetophenone. The insulating resin composition for DC power cables according to claim 2, wherein the modifying monomer is a nitrogen-containing (meth)acrylic acid derivative containing two or more nitrogen atoms in the molecule. An insulating resin composition for DC power cables as described in claim ³ , wherein the nitrogen atom-containing (meth)acrylic acid derivative is a (meth)acrylamide represented by the general formula: _H2C =C( ^R1 ) ^CONR2R3 (wherein ^R1 is a hydrogen atom or a methyl group, ^R2 is a hydrogen atom or an alkyl group, and ^R3 is a monovalent organic group containing at least one nitrogen atom). The insulating resin composition for DC power cables according to claim 2, characterized in that the modified monomer is dimethylaminopropylacrylamide (DMAPAA). (A) polyethylene;
(B) the modified polyethylene;
The insulating resin composition for a DC power cable according to any one of claims 2 to 5, further comprising (C) an antioxidant. The insulating resin composition for DC power cables according to claim 6, further comprising (D) an organic peroxide. 8. The insulating resin composition for a DC power cable according to claim 6, comprising 8 parts by mass or more of the modified polyethylene (B) per 100 parts by mass of the polyethylene (A). 9. The insulating resin composition for a DC power cable according to claim 6, wherein the concentration of the polar group introduced by the modified monomer is 4 μmol/g or more . (A) 100 parts by mass of polyethylene;
(B) 8 to 18 parts by mass of the modified polyethylene;
(C) 0.01 to 0.8 parts by mass of an antioxidant;
(D) 0.1 to 5 parts by mass of an organic peroxide,
6. The insulating resin composition for a DC power cable according to claim 5, wherein the concentration of the polar group introduced by the modified monomer is 4 to 8 μmol/g. A crosslinked resin obtained by crosslinking the resin composition described in any one of claims 1 to 10. A DC power cable characterized by having an internal semiconductive layer and an insulating layer made of the crosslinked resin described in claim 11 laminated on the surface of a conductive member. A repair tape-shaped component for forming a reinforcing insulating layer on a DC power cable joint, which is wound around a semiconducting layer inside the joint that covers exposed portions of the conductive members, including the joints of the conductive members of the DC power cables, and forms a reinforcing insulating layer on the semiconducting layer inside the joint by crosslinking, when connecting DC power cables together, the component comprising the resin composition of any one of claims 1 to 10. A DC power cable connection formed by connecting two DC power cables, characterized in that a reinforcing insulating layer made of the crosslinked resin product according to claim 11 is formed on a connection internal semiconductive layer that covers exposed portions of the conductive members of the DC power cables, including the connection portions of the conductive members.