Patent Description:
Conventionally, AC transmission systems have been widely used to transmit power.

However, when AC transmission is used for long-distance power transmission, a great power transmission loss is caused, and therefore DC power transmission systems are sometimes used for long-distance power transmission.

As cables (DC power cables) for DC power transmission systems, oil-filled isolated cables have been widely used.

However, oil-filled isolated cables require regular maintenance of booster pumps and the like in order to keep insulating oil in the cables. When the insulating oil is flowed out, an influence on the environment will be problematic.

To overcome this, a DC power cable including an insulating layer made of an insulating resin composition is disclosed as a DC power cable that is easy to maintain and has no risk of oil leakage (see PTL <NUM> below).

<CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT> also relate to insulating resin compositions.

However, conventionally known insulating resin compositions for DC power cables, including the insulating resin composition described in the above patent literature, have the following problems.

As a DC power cable used for long-distance power transmission, a long cable, for example, a cable having a length of several kilometers to several hundred kilometers has been demanded.

However, such a long cable is difficult to produce because an initial resin pressure in extrusion is high and an allowable resin pressure set to prevent screen mesh breakage which may be caused, for example, by clogging with foreign matter is reached in a short time.

As used herein, "the amount of secondary decomposition water generated during reheating" refers to the amount of water generated when a crosslinked resin body obtained by crosslinking the resin composition is heated.

In this description, the term "reheating" is used because it is heating performed after heating for crosslinking the resin composition, and the phrase "the amount of secondary decomposition water" is used to distinguish from the amount of water generated by the heating for crosslinking the resin composition (the amount of primary decomposition water).

The present invention was made in view of the circumstances as described above. An object of the present invention is to provide an insulating resin composition for a DC power cable, which insulating resin composition has the following features: it generates appropriate torque when extruded and has excellent extrudability; its extrudate is unlikely to experience sagging which may lead to a cable insulator with low circularity; it has good scorch resistance; it can form an insulating layer (crosslinked resin body) that generates a smaller amount of secondary decomposition water during reheating in connecting cables together and that stably exhibits good DC electrical properties; and it has a stable resin pressure when extruded, has high extrusion stability, and can form an insulating layer with little variation in thickness (wall thickness deviation).

Another object of the present invention is to provide a crosslinked resin body that is obtained by crosslinking the above resin composition and can form an insulating layer that can stably exhibit good DC electrical properties, generates a smaller amount of secondary decomposition water during reheating, and can reduce performance-degrading factors such as the occurrence of water treeing.

Still another object of the present invention is to provide a DC power cable including an insulating layer made of the above crosslinked resin body.

Still another object of the present invention is to provide a member for forming a reinforcing insulating layer of a DC power cable joint, the member being made of the above resin composition.

Still another object of the present invention is to provide a DC power cable joint including a reinforcing insulating layer made of the above crosslinked resin body. Solution to Problem.

The present invention concerns an insulating resin composition for a DC power cable defined in claim <NUM>. Preferred embodiments are defined in claims <NUM> to <NUM>. Further, the present invention concerns a crosslinked resin body defined in claim <NUM>. Preferred embodiments are defined in claims <NUM> and <NUM>. Further, the present invention concerns a DC power cable defined in claim <NUM>, a member for forming a reinforcing insulating layer of a DC power cable joint defined in claim <NUM> and a DC power cable joint defined in claim <NUM>. In particular, the present invention concerns the following:.

Since the complex viscosity η*<NUM> of the low-density polyethylene that is the component (A) is <NUM> to <NUM>,<NUM> Pa·s, the resin composition of the present invention generates appropriate torque when extruded and has excellent extrudability.

Since the ratio (η*<NUM>/η*<NUM>) in the low-density polyethylene is <NUM> or more, an extrudate of the resin composition is unlikely to experience sagging, which enables the circularity of a cable insulator to be kept high.

Since the stabilizer containing a thioether antioxidant in an amount of <NUM>% by mass or more and a hindered phenol antioxidant in an amount of <NUM>% by mass or less is contained in an amount of <NUM> to <NUM> parts by mass, the resin composition has good scorch resistance.

Since the stabilizer containing a thioether antioxidant in an amount of <NUM>% by mass or less and a hindered phenol antioxidant in an amount of <NUM>% by mass or more is contained in an amount of <NUM> to <NUM> parts by mass, the amount of secondary decomposition water generated during reheating, for example, in connecting cables together can be smaller.

Since the amount of carbonyl groups relative to the total mass of the component (A), the component (B), and the component (C) is <NUM> × <NUM>-<NUM> mol/g or more, the resin composition can form an insulating layer (crosslinked resin body) that stably exhibits good DC electrical properties.

Since the amount of carbonyl groups relative to the total mass of the component (A), the component (B), and the component (C) is <NUM> × <NUM>-<NUM> mol/g or less, the resin composition has a stable resin pressure when extruded, has high extrusion stability, and can form an insulating layer with little variation in thickness (wall thickness deviation).

The resin composition of the present invention has the following features: it generates appropriate torque when extruded and has excellent extrudability; its extrudate is unlikely to experience sagging which may lead to a cable insulator with low circularity; it has good scorch resistance; it can form an insulating layer (crosslinked resin body) that generates a smaller amount of secondary decomposition water during reheating in connecting cables together and that stably exhibits good DC electrical properties; and it has a less variable resin pressure when extruded, has high extrusion stability, and can form an insulating layer with little variation in thickness (wall thickness deviation).

The crosslinked resin body of the present invention is obtained by crosslinking the above resin composition and can form an insulating layer that can stably exhibit good DC electrical properties, generates a smaller amount of secondary decomposition water during reheating, and can suppress the occurrence of water treeing.

The DC power cable of the present invention has high circularity, can exhibit good DC electrical properties, and has excellent insulation properties because of its high DC breakdown electric field (absolute value).

The DC power cable joint of the present invention has excellent insulation properties because of its high DC breakdown electric field (absolute value).

A resin composition of the present invention contains a component (A) including a low-density polyethylene, a component (B) including a modified polyethylene, and a component (C) including a stabilizer.

The component (A) of the resin composition of the present invention has a complex viscosity η*<NUM>, which is measured at <NUM> and a frequency of <NUM> rad/s, of <NUM> to <NUM>,<NUM> Pa·s, preferably <NUM> to <NUM>,<NUM> Pa·s.

A resin composition containing a low-density polyethylene having an η*<NUM> of less than <NUM> Pa·s fails to form a crosslinked resin body having sufficient mechanical strength.

A resin composition containing a low-density polyethylene having an η*<NUM> of more than <NUM>,<NUM> Pa·s generates excessively high torque when extruded (see Comparative Example <NUM> and Comparative Example <NUM> below).

When torque in extrusion is excessively high, an extruder is subjected to a heavy load, which makes it difficult to operate the extruder at a high rotation rate and limits the extrusion speed (the efficiency in cable production). In addition, an initial resin pressure in extrusion is high, and therefore an allowable resin pressure set to prevent screen mesh breakage which may be caused by clogging with foreign matter is reached in a short time, thus making it difficult to produce a long cable.

In the component (A), the ratio (η*<NUM>/η*<NUM>) of a complex viscosity η*<NUM>, which is measured at <NUM> and a frequency of <NUM> rad/s, to the complex viscosity η*<NUM>, which is measured at <NUM> and a frequency of <NUM> rad/s, is <NUM> or more, preferably <NUM> or more.

When the ratio (η*<NUM>/η*<NUM>) is less than <NUM>, a resin composition (extrudate) discharged from an extruder head tends to sag (see Comparative Example <NUM> below).

The sag leads to a cable insulator with low circularity, and the electrical insulation properties of a DC power cable to be obtained are degraded.

An example of commercially available products of the component (A) is "DFD-<NUM>" (NUC Corporation).

The component (B) of the resin composition of the present invention is a modified polyethylene obtained by grafting a polyethylene with at least one modifying monomer selected from unsaturated organic acids and derivatives thereof.

The polyethylene to be modified is preferably a low-density polyethylene that satisfies the requirements for the component (A), from the viewpoint of compatibility with the component (A).

The component (B) is a modified polyethylene obtained by grafting a polyethylene with maleic anhydride (MAH).

The content of the component (B) is <NUM> to <NUM> parts by mass, preferably <NUM> to <NUM> parts by mass, relative to <NUM> parts by mass of the component (A).

When the content of the component (B) is less than <NUM> parts by mass, it is difficult to uniformly disperse the component (B) in the component (A). In this case, space charges are likely to localize, and as a result, the increase in electric field enhanced by the space charges accumulated in a cable insulator becomes excessive (e.g., <NUM>% or more), leading to a DC power cable of low performance.

More than <NUM> parts by mass of the component (B) leads to low extrudability.

The component (B) may be prepared, for example, by mixing a low-density polyethylene, an antioxidant, a modifying monomer, and an organic peroxide in an extruder, heating the mixture to effect a reaction, and granulating the reaction product into pellets or granules.

The antioxidant used in preparing the component (B) is deactivated while the component (B) is synthesized and does not constitute the component (C) in the resin composition.

The component (C) of the resin composition of the present invention is a stabilizer including a mixture of <NUM>% to <NUM>% by weight of a hindered phenol antioxidant and <NUM>% to <NUM>% by weight of a thioether antioxidant.

By virtue of containing the stabilizer, which includes a thioether antioxidant in an amount of <NUM>% by mass or more, in an amount described below, the resin composition of the present invention has good scorch resistance.

By virtue of containing a thioether antioxidant in an amount of <NUM>% by mass or less (containing a hindered phenol antioxidant in an amount of <NUM>% by mass or more) in the stabilizer, the amount of secondary decomposition water of a crosslinked resin body formed from the resin composition of the present invention can be reduced.

In the component (C), the mixing ratio (by mass) of the hindered phenol antioxidant to the thioether antioxidant is typically <NUM>:<NUM> to <NUM>:<NUM>, preferably <NUM>:<NUM> to <NUM>:<NUM>.

Less than <NUM>% by mass of a thioether antioxidant (more than <NUM>% by mass of a hindered phenol antioxidant) provides a resin composition that cannot exhibit good scorch resistance (see Comparative Example <NUM> below).

Since thioether antioxidants react with cumyl alcohol, which is a decomposed residue of dicumyl peroxide suitable as the component (D), to generate secondary decomposition water, more than <NUM>% by mass of a thioether antioxidant (less than <NUM>% by mass of a hindered phenol antioxidant) provides a resin composition that cannot form a crosslinked resin body that generates a smaller amount of secondary decomposition water (see Comparative Example <NUM> below).

The hindered phenol antioxidant includes tetrakis[methylene-<NUM>-(<NUM>',<NUM>'-di-t-butyl-<NUM>'-hydroxyphenyl)propionate]methane (Irganox <NUM> available from BASF), <NUM>,<NUM>-hexanediol-bis[<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate] (Irganox <NUM> available from BASF), octadecyl-<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate (Irganox <NUM> available from BASF), and isooctyl-<NUM>-(<NUM>,<NUM>-dit-butyl-<NUM>-hydroxyphenyl)propionate (Irganox <NUM> available from BASF). A particularly preferred hindered phenol antioxidant is, for example, tetrakis[methylene-<NUM>-(<NUM>',<NUM>'-dit-butyl-<NUM>'-hydroxyphenyl)propionate]methane. These can be used alone or in a combination of two or more.

The thioether antioxidant includes <NUM>,<NUM>'-thiobis-(<NUM>-methyl-<NUM>-t-butylphenol) (Seenox BCS available from SHIPRO KASEI KAISHA, LTD. ) and <NUM>,<NUM>'-thiobis-(<NUM>-methyl-<NUM>-t-butylphenol) (Irganox <NUM> available from BASF). A particularly preferred thioether antioxidant is, for example, <NUM>,<NUM>'-thiobis-(<NUM>-methyl-<NUM>-t-butylphenol). These can be used alone or in a combination of two or more.

The content of the component (C) is <NUM> to <NUM> parts by mass, preferably <NUM> to <NUM> parts by mass, relative to <NUM> parts by mass of the component (A).

Less than <NUM> parts by mass of the component (C) provides a resin composition with poor scorch resistance, and a crosslinked resin body obtained by crosslinking the resin composition is provided with poor heat resistance.

More than <NUM> parts by mass of the component (C) provides a resin composition that cannot form a crosslinked resin body that generates a smaller amount of secondary decomposition water. Such an amount of the component (C) also provides a crosslinked resin body that experiences increased bleeding.

In the resin composition of the present invention, the amount of carbonyl groups introduced into the resin composition through the component (B) is <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM> mol/g, preferably <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM> mol/g, relative to the total mass of the component (A), the component (B), and the component (C).

A resin composition wherein the amount of carbonyl groups is less than <NUM> × <NUM>-<NUM> mol/g provides a crosslinked resin body that forms space charges to cause electric field distortion and thus cannot exhibit good DC electrical properties (see Comparative Example <NUM> and Comparative Example <NUM> below).

Extruding a resin composition wherein the amount of carbonyl groups is more than <NUM> × <NUM>-<NUM> mol/g forms a crosslinked resin body (an insulating layer of a cable) with great variation in thickness (wall thickness deviation), because of an unstable resin pressure (see Comparative Example <NUM> below).

A great wall thickness deviation leads to an ununiform cable diameter, which significantly makes it difficult to wind a cable on a drum and pay out the cable from the drum.

When the wall thickness deviation is great, the insulation thickness is larger than is necessary because a cable must be designed on the basis of a minimum insulation thickness.

The resin composition of the present invention may contain an organic peroxide as the component (D).

The component (D) acts as a crosslinking agent.

The component (D) preferably has a melting point of <NUM> or lower.

When the melting point of the component (D) is higher than <NUM>, it is difficult to infiltrate a molten organic peroxide into the composition containing the components (A) to (C).

The decomposition time (half-life) at <NUM> of the component (D) is preferably <NUM> to <NUM> hours.

When the decomposition time of the component (D) is less than <NUM> hour, degradation reaction rapidly proceeds during extrusion of a cable, causing scorching.

When the decomposition time of the component (D) is more than <NUM> hours, degradation reaction proceeds slowly, the degree of crosslinking is not sufficiently increased during cable processing by a conventional method, and the desired heat resistance is not obtained in a crosslinked body to be obtained.

Specific examples of the component (D) include di-t-hexyl peroxide (PERHEXYL D available from NOF Corporation), dicumyl peroxide (PERCUMYL D available from NOF Corporation), <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(t-butylperoxy)hexane (PERHEXA 25B available from NOF Corporation), α,α'-di(t-butylperoxy)diisopropylbenzene (PERBUTYL P available from NOF Corporation), t-butylcumyl peroxide (PERBUTYL C available from NOF Corporation), and di-t-butyl peroxide (PERBUTYL D available from NOF Corporation). These can be used alone or in a combination of two or more. Of these, dicumyl peroxide (melting point: <NUM> to <NUM>, half-life at <NUM>: <NUM> hours) is preferred.

The content of the component (D) is preferably <NUM> to <NUM> parts by mass, more preferably <NUM> to <NUM> parts by mass, relative to <NUM> parts by mass of the component (A).

When the amount of the component (D) is excessively small, crosslinking does not proceed sufficiently, and the mechanical properties and the heat resistance of a crosslinked body to be obtained are reduced. When the amount of the component (D) is excessively large, a resin composition provided suffers scorching when subjected to extrusion molding, leading to low electrical properties.

The resin composition of the present invention contains the components (A) to (C). When the resin composition is crosslinked (a crosslinked resin body is obtained), the resin composition further contains the component (D).

Various stabilizers and other additives may be added as long as the effects of the present invention are not impaired. Examples of stabilizers include antioxidants other than the component (C), light stabilizers, UV absorbers, and copper inhibitors. Examples of other additives include inorganic fillers, organic fillers, lubricants, and dispersants.

The resin composition of the present invention can be crosslinked with the organic peroxide that is the component (D).

A crosslinked resin body of the present invention is obtained by crosslinking the resin composition of the present invention with the organic peroxide that is the component (D).

When a sheet made of the crosslinked resin body of the present invention is subjected to space charge measurement, the increase in electric field enhanced by internally accumulated charges is preferably <NUM>% or less.

When space charges are accumulated in a cable insulator as a result of high DC voltage application, insulating properties are significantly reduced if an impulse of reverse polarity is applied or the polarity is reversed.

Thus, when the increase in electric field of the crosslinked resin body is <NUM>% or less, a DC power cable that stably exhibits good DC electrical properties can be obtained.

By crosslinking the resin composition of the present invention wherein the amount of carbonyl groups introduced into the resin composition through the component (B) is <NUM> × <NUM>-<NUM> mol/g or more relative to the total mass of the component (A), the component (B), and the component (C), a crosslinked body can be obtained wherein the increase in electric field is <NUM>% or less.

To connect cables together, the amount of secondary decomposition water generated when the crosslinked resin body of the present invention is heated needs to be <NUM> ppm or less.

When the amount of secondary decomposition water is <NUM> ppm or less, the occurrence of water treeing, which may otherwise occur when the cables are charged, is suppressed, and as a result, the breakdown of the cables can be prevented.

By crosslinking the resin composition of the present invention which contains, as the component (C), <NUM> to <NUM> parts by mass of a stabilizer containing a thioether antioxidant in an amount of <NUM>% by mass or less (containing a hindered phenol antioxidant in an amount of <NUM>% by mass or more), a crosslinked resin body that generates a smaller amount of secondary decomposition water (the amount of water generated by heating at <NUM> for <NUM> hours is <NUM> ppm or less) can be formed.

In a DC power cable of the present invention, an inner semiconductive layer and an insulating layer made of the crosslinked resin body of the present invention are disposed in layers around the surface of a conductive member.

<FIG> is a cross-sectional view of a DC power cable according to one embodiment of the present invention.

In a DC power cable <NUM> shown in <FIG>, an inner semiconductive layer <NUM>, an insulating layer <NUM> made of the crosslinked resin body of the present invention, and an outer semiconductive layer <NUM> are formed in layers around the outer peripheral surface of a conductor <NUM>. Furthermore, a metal shielding layer <NUM> and a sheath <NUM> are disposed in layers around the outer peripheral surface of the outer semiconductive layer <NUM>.

The DC power cable <NUM> of the present invention shown in <FIG> can be produced by extruding the resin composition of the present invention together with the inner semiconductive layer <NUM> covering the conductor <NUM> (the outer semiconductive layer <NUM> may be extruded together), crosslinking the resin composition to form the insulating layer <NUM> made of a crosslinked resin body, and then disposing the metal shielding layer <NUM> and the sheath <NUM> according to a conventional method.

The crosslinking method for forming the insulating layer <NUM> (crosslinked resin body) is not particularly limited. Typically, heating under pressure is used, for example.

For example, pressure heating at a temperature of <NUM> under a pressure of <NUM>/cm<NUM> is performed in a nitrogen atmosphere to effect a radical reaction in which the component (D) acts as an initiator, thereby allowing crosslinking of the resin composition to proceed.

The DC power cable <NUM> of the present invention has high circularity, exhibits good DC electrical properties, and is unlikely to experience a breakdown of the insulating layer <NUM>.

In a DC power cable joint of the present invention, a reinforcing insulating layer made of the crosslinked resin body of the present invention is formed around a joint inner semiconductive layer covering exposed portions of conductive members of the DC power cables, the exposed portions including a joint between the conductive members.

<FIG> is a longitudinal sectional view of a DC power cable joint according to one embodiment of the present invention.

A DC power cable joint <NUM> shown in <FIG> is where two DC power cables 10A and 10B are connected together. A reinforcing insulating layer <NUM> made of the crosslinked resin body of the present invention and a joint outer semiconductive layer <NUM> are formed in layers around a joint inner semiconductive layer <NUM> covering exposed portions of a conductor 11A of the DC power cable 10A and a conductor 11B of the DC power cable 10B, the exposed portions including a joint <NUM> between the conductors.

In <FIG>, 13A is an insulating layer of the DC power cable 10A, and 13B is an insulating layer of the DC power cable 10B. The insulating layers 13A and 13B are made of the crosslinked resin body of the present invention.

Furthermore, 12A, 14A, 15A, and 16A are an inner semiconductive layer, an outer semiconductive layer, a metal shielding layer, and a sheath, respectively, of the DC power cable 10A.

Furthermore, 12B, 14B, 15B, and 16B are an inner semiconductive layer, an outer semiconductive layer, a metal shielding layer, and a sheath, respectively, of the DC power cable 10B.

When the DC power cable 10A and the DC power cable 10B are connected together to form the DC power cable joint <NUM> as shown in <FIG>, a tape-shaped member formed of the resin composition of the present invention (a member for forming a reinforcing insulating layer according to the present invention) is wound around the joint inner semiconductive layer <NUM> covering the exposed portions of the conductor 11A and the conductor 11B, the exposed portions including the joint <NUM> between the conductors, and the wound body is heat-treated to cause crosslinking, thereby forming the reinforcing insulating layer <NUM> made of the crosslinked resin body of the present invention.

Although the insulating layer 13A of the DC power cable 10A and the insulating layer 13B of the DC power cable 10B are also heated when the member for forming a reinforcing insulating layer according to the present invention is heat-treated to form the reinforcing insulating layer <NUM>, the amount of secondary decomposition water generated from the insulating layers 13A and 13B can be smaller because the insulating layers 13A and 13B are made of the crosslinked resin body of the present invention.

Furthermore, accumulation of space charges in the reinforcing insulating layer <NUM> to be formed can be inhibited.

For producing the cable joint, various methods can be employed depending, for example, on the voltage class, the intended use, and the construction environment. The cable joint may be, for example, a taping molded joint (TMJ), an extruded molded joint (EMJ), or a block molded joint (BMJ).

The present invention will now be described with reference to examples.

A resin composition preheated at <NUM> and <NUM> MPa for <NUM> minutes was heated at <NUM> and <NUM> MPa for <NUM> minutes using a pressing machine to prepare a crosslinked sheet (sheet-shaped crosslinked resin body) having a thickness of about <NUM>. The thickness of the crosslinked sheet obtained was accurately measured with a micrometer.

Using an FT/IR-<NUM> infrared spectrophotometer (JASCO Corporation), the absorbance at <NUM>,<NUM>-<NUM> (baseline: <NUM>,<NUM> to <NUM>,<NUM>-<NUM>) was measured with <NUM> scans and a resolution of <NUM>-<NUM>.

Using a calibration curve prepared using a sample of known concentration, the carbonyl group content was calculated from the thickness and the absorbance of the sheet.

Using an ARES strain-controlled rotational rheometer (TA Instruments), the complex viscosity η*<NUM> at a frequency of <NUM> rad/s and the complex viscosity η*<NUM> at a frequency of <NUM> rad/s were measured at a temperature of <NUM> and a parallel plate interval of <NUM>.

Using an MKC-<NUM> Karl Fischer moisture titrator (KYOTO ELECTRONICS MANUFACTURING CO. ), measurements were made under the following conditions: temperature, <NUM>; carrier gas, (N<NUM>); flow rate, <NUM>/min; amount of sample, <NUM>.

Low-density polyethylenes (A1) to (A4) for the present invention obtained by high-pressure tubular processes and having the following MFRs, densities, complex viscosities η*<NUM>, and ratios (η*<NUM>/η*<NUM>) were provided.

MFR = <NUM>/min, density = <NUM>/cm3, η*<NUM> = <NUM>,<NUM> Pa·s, ratio (η*<NUM>/η*<NUM>) = <NUM>.

MFR = <NUM>/min, density = <NUM>/cm3, η*<NUM> = <NUM> Pa·s, ratio (η*<NUM>/η*<NUM>) = <NUM>.

Low-density polyethylenes (A5) to (A7) for comparison obtained by low-pressure processes and having the following MFRs, densities, complex viscosities η*<NUM>, and ratios (η*<NUM>/η*<NUM>) were provided.

According to the formulation shown in Table <NUM> below, <NUM> parts by mass of maleic anhydride and <NUM> parts by mass of a hindered phenol antioxidant were added to <NUM> parts by mass of the low-density polyethylene (A2), and the resultant was mixed with <NUM> parts by mass of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-bis(tertiarybutylperoxy)hexyne-<NUM> (organic peroxide, "PERHEXYNE 25B" available from NOF Corporation) in an extruder and reacted by heating, thereby preparing a modified polyethylene (B1) formed of a graft copolymer having a carbonyl group content of <NUM> × <NUM>-<NUM> mol/g.

A modified polyethylene (B2) formed of a graft copolymer having a carbonyl group content of <NUM> × <NUM>-<NUM> mol/g was prepared in the same manner as in Preparation Example B1 except that the low-density polyethylene (A3) was used in place of the low-density polyethylene (A2) according to the formulation shown in Table <NUM> below.

A modified polyethylene (B3) formed of a graft copolymer having a carbonyl group content of <NUM> × <NUM>-<NUM> mol/g was prepared in the same manner as in Preparation Example B1 except that the amounts of maleic anhydride and organic peroxide used were changed according to the formulation shown in Table <NUM> below.

A modified polyethylene (B4) formed of a graft copolymer having a carbonyl group content of <NUM> × <NUM>-<NUM> mol/g was prepared in the same manner as in Preparation Example B1 except that the amounts of maleic anhydride and organic peroxide used were changed according to the formulation shown in Table <NUM> below.

A modified polyethylene (B5) formed of a graft copolymer having a carbonyl group content of <NUM> × <NUM>-<NUM> mol/g was prepared in the same manner as in Preparation Example B1 except that the amounts of maleic anhydride and organic peroxide used were changed according to the formulation shown in Table <NUM> below.

A modified polyethylene (B6) formed of a graft copolymer having a carbonyl group content of <NUM> × <NUM>-<NUM> mol/g was prepared in the same manner as in Preparation Example B1 except that the amounts of maleic anhydride and organic peroxide used were changed according to the formulation shown in Table <NUM> below.

A mixture of <NUM>% by mass of a hindered phenol antioxidant (C11) described in (<NUM>) below and <NUM>% by mass of a thioether antioxidant (C12) described in (<NUM>) below was prepared for use as a stabilizer for the present invention.

An Irganox <NUM> hindered phenol antioxidant (BASF) was provided for use as a stabilizer for comparison.

A SEENOX BCS thioether antioxidant (SHIPRO KASEI KAISHA, LTD. ) was provided for use as a stabilizer for comparison.

According to the formulations shown in Table <NUM> below, resin compositions of the present invention were each obtained by mixing a component (A) selected from the low-density polyethylenes (A1) to (A4), a component (B) selected from the modified polyethylenes (B1) to (B4), a component (C) including the stabilizer (C1), and a component (D) including dicumyl peroxide.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that the low-density polyethylene (A5) was used in place of the low-density polyethylene (A1) according to the formulation shown in Table <NUM> below.

This comparative example is a comparative example in which a low-density polyethylene having an η*<NUM> of more than <NUM>,<NUM> Pa·s is used.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that the low-density polyethylene (A6) was used in place of the low-density polyethylene (A1) according to the formulation shown in Table <NUM> below.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that the low-density polyethylene (A7) was used in place of the low-density polyethylene (A1) according to the formulation shown in Table <NUM> below.

This comparative example is a comparative example in which a low-density polyethylene having a ratio (η*<NUM>/η*<NUM>) of less than <NUM> is used.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that <NUM> parts by mass of the stabilizer (C11) was used in place of the stabilizer (C1) according to the formulation shown in Table <NUM> below.

This comparative example is a comparative example in which a hindered phenol antioxidant alone is used as a stabilizer.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that <NUM> parts by mass of the stabilizer (C12) was used in place of the stabilizer (C1) according to the formulation shown in Table <NUM> below.

This comparative example is a comparative example in which a thioether antioxidant alone is used as a stabilizer.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that the modified polyethylene (B5) was used in place of the modified polyethylene (B1) according to the formulation shown in Table <NUM> below.

This comparative example is a comparative example in which the amount of carbonyl groups relative to the total mass of the component (A), the component (B), and the component (C) is less than <NUM> × <NUM>-<NUM> mol/g.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that the modified polyethylene (B6) was used in place of the modified polyethylene (B1) according to the formulation shown in Table <NUM> below.

This comparative example is a comparative example in which the amount of carbonyl groups relative to the total mass of the component (A), the component (B), and the component (C) is more than <NUM> × <NUM>-<NUM> mol/g.

A resin composition for comparison was obtained in the same manner as in Example <NUM> except that the modified polyethylene (B1) was not used according to the formulation shown in Table <NUM> below.

On each of the resin compositions obtained in Examples and Comparative Examples above were performed an evaluation of extrudability (a measurement of torque), an evaluation of sagging resistance, evaluation of scorch resistance, a measurement of the amount of secondary decomposition water of a crosslinked resin body formed from a resin composition, an evaluation of space charge properties of a crosslinked resin body formed from a resin composition (a measurement of an increase in electric field enhanced by internally accumulated charges), a measurement of a DC breakdown electric field of a cable including as an insulating layer a crosslinked resin body formed from a resin composition, an evaluation of extrusion stability (a measurement of resin pressure regulation and wall thickness deviation), and a measurement of a DC breakdown electric field of DC power cables connected together through a DC power cable joint including as a reinforcing insulating layer a crosslinked resin body formed from a resin composition.

Measurement methods, evaluation methods, and evaluation criteria are as described in the following (<NUM>) to (<NUM>).

Results are shown in Table <NUM> and Table <NUM> below.

The same resin compositions as those of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> were prepared, except that the dicumyl peroxide that was the component (D) was not added. Each of the resin compositions obtained was extruded using a LABO PLASTOMILL 20C200 (Toyo Seiki Seisaku-Sho, Ltd. ) with the extruder temperature set at <NUM>/<NUM>/<NUM>/<NUM> (C1/C2/C3/Die) under the following conditions: screen mesh (<NUM>/<NUM>/<NUM> from upstream to downstream), screw (L/D = <NUM>, compression ratio = <NUM>), screw speed (<NUM> rpm). The torque generated during the extrusion was measured.

The evaluation criteria are as follows: <NUM> to <NUM> N·m, acceptable; less than <NUM> N·m or more than <NUM> N·m, unacceptable.

When the torque is less than <NUM> N·m, the resin composition (extrudate) discharged from an extruder head sags due to its own weight to provide a cable insulator with low circularity, resulting in very low electrical insulation properties. When the torque is more than <NUM> N·m, the extruder is subjected to a heavy load.

Each of the resin compositions obtained in Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> was preheated at <NUM> and <NUM> MPa for <NUM> minutes and then pressed at <NUM> and <NUM> MPa for <NUM> minutes to form an uncrosslinked sheet of <NUM> × <NUM> × <NUM>.

A test specimen 5A in accordance with JIS K <NUM> was punched from the uncrosslinked sheet.

The test specimen was marked with gauge lines at intervals of <NUM> and exposed in a chamber at a temperature of <NUM> for <NUM> minutes. The gauge length after the exposure for <NUM> minutes was measured.

The evaluation criteria are as follows: the gauge length is <NUM> to <NUM>, acceptable; the gauge length is more than <NUM>, unacceptable.

When the gauge length is more than <NUM>, the resin composition (extrudate) discharged from an extruder head sags due to its own weight to provide a cable insulator with low circularity, resulting in very low electrical insulation properties.

On each of the resin compositions obtained in Example <NUM> and Comparative Examples <NUM> and <NUM>, a measurement at a testing temperature of <NUM> for <NUM> hours was performed using a moving die rheometer (MDR) in accordance with JIS K <NUM>-<NUM>. The time ts1 taken for torque to increase from a minimum value by <NUM> dNm was measured.

The evaluation criteria are as follows: ts1 is <NUM> minutes or more, acceptable; ts1 is <NUM> minutes or less, unacceptable.

The resin compositions obtained in Example <NUM> and Comparative Examples <NUM> and <NUM> were each pressed at <NUM> and <NUM> MPa for <NUM> minutes to form crosslinked sheets (crosslinked resin bodies) of <NUM> × <NUM> × <NUM>. Each of the crosslinked sheets obtained was heated at <NUM> for <NUM> hours. The amount of water generated as a result of the heating was measured by the Karl Fischer method.

The evaluation criteria are as follows: <NUM> ppm or less, acceptable; more than <NUM> ppm, unacceptable.

The resin compositions obtained in Example <NUM>, Examples <NUM> to <NUM>, and Comparative Examples <NUM> to <NUM> were each pressed at <NUM> and <NUM> MPa for <NUM> minutes into a sheet shape and then pressed at <NUM> and <NUM> MPa for <NUM> minutes to form crosslinked sheets (crosslinked resin bodies) each having a thickness of <NUM>.

Each of the crosslinked sheets obtained was evaluated for space charge properties by a pulsed electro-acoustic (PEA) method. A direct electric field at a temperature of <NUM> and a negative polarity of <NUM> kV/mm was continuously applied to the crosslinked sheet for about <NUM> hours.

The evaluation criteria are as follows: the increase in electric field (maximum electric field/applied electric field) is <NUM>% or less, acceptable; the increase in electric field is more than <NUM>%, unacceptable.

On the outer peripheral surface of a conductor (<NUM>) having a sectional area of <NUM><NUM>, an inner semiconductive layer (<NUM>) having a thickness of <NUM>, a resin layer (an uncrosslinked resin layer for forming an insulating layer (<NUM>)) having a thickness of <NUM> and made of each of the resin compositions obtained in Example <NUM>, Examples <NUM> to <NUM>, and Comparative Examples <NUM> to <NUM>, and an outer semiconductive layer (<NUM>) having a thickness of <NUM> were formed in layers by extrusion coating. The resin composition was then crosslinked by performing a heat treatment in a nitrogen atmosphere at a temperature of <NUM> for <NUM> minutes to form the insulating layer (<NUM>), thereby producing a <NUM> long miniature cable having a sectional configuration as shown in <FIG>.

The miniature cable obtained was cut crosswise at a point <NUM> backward from the front end and a point <NUM> forward from the back end. Both ends of the <NUM> central portion of the miniature cable were subjected to an end treatment and connected to a DC breakdown tester to perform a DC breakdown test. The voltage was stepped up from a starting voltage of -<NUM> kV at a rate of -<NUM> kV/min while adjusting the conductor temperature of the miniature cable to be <NUM> in an oil bath, and the electric field at which a breakdown occurred was measured.

The evaluation criteria are as follows: the breakdown electric field is -<NUM> kV/mm or less (its absolute value is <NUM> kV/mm or more), acceptable; the breakdown electric field is higher than -<NUM> kV/mm (its absolute value is less than <NUM> kV/mm), unacceptable.

In producing the miniature cable of (<NUM>) above, the resin pressure was measured over time, and the resin pressure regulation was determined by the following formula. The evaluation criteria are as follows: the resin pressure regulation is <NUM>% or less, acceptable; the resin pressure regulation is more than <NUM>%, unacceptable.

For each of the two sections (cross sections) of the miniature cable cut crosswise in (<NUM>) above, the insulating layer thickness was measured in six circumferential directions (<NUM>° intervals), and the wall thickness deviation was determined by the following formula. The evaluation criteria are as follows: the wall thickness deviation is <NUM>% or less, acceptable; the wall thickness deviation is more than <NUM>%, unacceptable.

The outer peripheral surface of a conductor (<NUM>) having a sectional area of <NUM><NUM> was coated with an inner semiconductive layer (<NUM>) having a thickness of <NUM>, an insulating layer (<NUM>) having a thickness of <NUM> and made of a crosslinked body of each of the resin compositions obtained in Example <NUM>, Examples <NUM> to <NUM>, and Comparative Example <NUM>, and an outer semiconductive layer (<NUM>) having a thickness of <NUM>. Around the outer peripheral surface of the outer semiconductive layer (<NUM>), a metal shielding layer (<NUM>) and a sheath (<NUM>) were disposed in layers to form a DC power cable. For each example, two DC power cables, ten in total, were formed.

One end of each of two DC power cables (10A, 10B) including insulating layers (13A, 13B) made of the same crosslinked resin body was cut into a substantially conical shape, and exposed conductors (11A, 11B) were then connected together with the conical ends facing each other.

Subsequently, a semiconductive tape (a member for forming a joint inner semiconductive layer) was wound around the exposed portions of the conductors (11A, 11B) connected together. A tape-shaped member for forming a reinforcing insulating layer was wound around the semiconductive tape, the tape-shaped member being made of each of the resin compositions obtained in Example <NUM>, Examples <NUM> to <NUM>, and Comparative Example <NUM> (each of the resin compositions has the same composition as that of the resin composition used to form the crosslinked resin body constituting the insulating layers (13A, 13B) of DC power cables to be connected together). The outer periphery of the member for forming a reinforcing insulating layer was coated with a semiconductive shrinkable tube (a member for forming a joint outer semiconductive layer).

The member for forming a joint inner semiconductive layer, the member for forming a reinforcing insulating layer, and the member for forming a joint outer semiconductive layer were then crosslinked by performing a heat treatment in a nitrogen atmosphere at <NUM> for <NUM> hours, thereby forming a DC power cable joint (<NUM>) in which a joint inner semiconductive layer (<NUM>) having a thickness of <NUM>, a reinforcing insulating layer (<NUM>) having a thickness of <NUM>, and a joint outer semiconductive layer (<NUM>) having a thickness of <NUM> were formed in layers around the exposed portions of the conductors (11A, 11B), as shown in <FIG> (the thickness of each layer is different from those in <FIG>).

The length of the joint outer semiconductive layer (<NUM>) was <NUM>, and a crosslinking tube having a length of <NUM>,<NUM> was used in the heat treatment.

A DC breakdown test was performed on the two DC power cables (10A, 10B) which included the insulating layers (13A, 13B) made of a crosslinked body of each of the resin compositions obtained in Example <NUM>, Examples <NUM> to <NUM>, and Comparative Example <NUM> and which were connected together through the DC power cable joint (<NUM>) in which the reinforcing insulating layer (<NUM>) was formed using a resin composition having the same composition as that of the resin composition used to form the crosslinked resin body constituting the insulating layers (13A, 13B). The voltage was stepped up from a starting voltage of -<NUM> kV at a rate of -<NUM> kV/<NUM> while adjusting the conductor temperature to be <NUM>. The breakdown electric field was measured, and the breakdown site was determined.

Cables each having an overall length of <NUM> including a DC power cable joint were evaluated using test lines.

In every two DC power cables connected together through any of the DC power cable joints, a breakdown was observed at the DC power cable joint.

As shown in Table <NUM> and Table <NUM> below, the breakdown electric fields of the two DC power cables connected together through the DC power cable joints according to Example <NUM> and Examples <NUM> to <NUM> were around -<NUM> kV/mm, which was about twice the breakdown electric field (-<NUM> kV/mm) of the two DC power cables connected together through the DC power cable joint according to Comparative Example <NUM>.

Claim 1:
An insulating resin composition for a DC power cable, the resin composition comprising:
(A) <NUM> parts by mass of a low-density polyethylene having a complex viscosity η*<NUM>, which is measured at <NUM> and a frequency of <NUM> rad/s, of <NUM> to <NUM>,<NUM> Pa s, wherein a ratio (η*<NUM>/η*<NUM>) of a complex viscosity η*<NUM>, which is measured at <NUM> and a frequency of <NUM> rad/s, to the complex viscosity η*<NUM> is <NUM> or more;
(B) <NUM> to <NUM> parts by mass of a modified polyethylene obtained by grafting a polyethylene with at least one modifying monomer selected from unsaturated organic acids and derivatives thereof, wherein the component (B) is a modified polyethylene obtained by grafting a polyethylene with maleic anhydride (MAH); and
(C) <NUM> to <NUM> parts by mass of a stabilizer including a mixture of <NUM>% to <NUM>% by weight of a hindered phenol antioxidant and <NUM>% to <NUM>% by weight of a thioether antioxidant, wherein the hindered phenol antioxidant includes tetrakis[methylene-<NUM>-(<NUM>',<NUM>'-di-t-butyl-<NUM>'-hydroxyphenyl)propionate]methane, <NUM>,<NUM>-hexanediol-bis[<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate], octadecyl-<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate, and isooctyl-<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate, and wherein the thioether antioxidant includes <NUM>,<NUM>'-thiobis-(<NUM>-methyl-<NUM>-t-butylphenol) and <NUM>,<NUM>'-thiobis-(<NUM>-methyl-<NUM>-t-butylphenol),
wherein an amount of carbonyl groups introduced into the resin composition through the component (B) is <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM> mol/g relative to a total mass of the component (A), the component (B), and the component (C),
wherein the complex viscosity is measured by using an ARES strain-controlled rotational rheometer (TA Instruments) with a parallel plate interval of <NUM>.