Patent Description:
Patent Document <NUM> discloses a direct-current cable including a conductive portion and an insulating layer that covers an outer periphery of the conductive portion. The insulating layer contains resin composition, as a main constituent, formed by adding inorganic filler of magnesium oxide or carbon black, and greater than or equal to <NUM> parts by weight and less than or equal to <NUM> parts by weight of a metal salt of a fatty acid, based on <NUM> parts by weight of polyethylene, to polyethylene.

Patent Document <NUM> discloses a direct-current cable including an insulating layer formed by cross-linking polyethylene composition and provided on an outer periphery of a conductor. The polyethylene composition contains polyethylene (A), ethylene-methacrylate copolymer (B), and inorganic filler (C) selected from either of magnesium oxide and carbon black. <CIT> relates to a crosslinked polyolefin composition, direct-current power cable, and process for construction of direct-current power line.

The direct-current cable of the disclosure is according to claim <NUM>.

The composition of the disclosure is according to claim <NUM>.

<FIG> is a cross-sectional view illustrating an example of a direct-current cable.

However, it is desired to improve long-term insulating performance of an insulating layer against applied direct-current voltage.

Thus, it is a purpose to provide a direct-current cable in which long-term insulating performance of an insulating layer against applied direct-current voltage and space-charge characteristics of an insulating layer are good.

According to the disclosure, a direct-current cable in which long-term insulating performance of an insulating layer against applied direct-current voltage and space-charge characteristics of an insulating layer are good can be provided.

First, embodiments of the invention are described in order.

<FIG> illustrates an example of a direct-current cable. <FIG> is a cross-sectional view that is perpendicular to an axial direction of a direct-current cable <NUM>.

An outer periphery of a conductive portion <NUM> is covered by an insulating layer <NUM> in the direct-current cable <NUM>. Further, an inner semi-conducting layer <NUM> is formed between the conductive portion <NUM> and the insulating layer <NUM> in the direct-current cable <NUM>. Further, an outer periphery of the insulating layer <NUM> is covered by a shielding layer <NUM>, and an outer periphery of the shielding layer <NUM> is covered by a covering layer <NUM> in the direct-current cable <NUM>. Further, an outer semi-conducting layer <NUM> is formed between the insulating layer <NUM> and the shielding layer <NUM> in the direct-current cable <NUM>.

The conductive portion <NUM> is formed by twisting a plurality of conductive core wires.

As the material constituting the conductive core wire, although not specifically limited, copper, aluminum, copper alloy, aluminum alloy or the like may be used.

As the material constituting the inner semi-conducting layer <NUM>, although not specifically limited, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate copolymer or the like may be used.

The insulating layer <NUM> contains cross-linked base resin and inorganic filler.

The polyethylene may be either of low density, intermediate density and high density. Further, the polyethylene may be either of straight-chain and branched.

The cross-linked base resin is cross-linked by a cross-linking agent containing organic peroxide.

As the organic peroxide, dicumyl peroxide, t-butyl cumyl peroxide, di-t-butyl peroxide, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-bis(t-butylperoxy)hexane, <NUM>,<NUM>-bis(t-butylperoxyisopropyl)benzene or the like may be used.

It is preferable that the organic peroxide does not contain an aromatic ring. With this, the long-term performance against direct-current can be maintained.

The base resin further contains polyethylene-graft-maleic anhydride. With this, the long-term insulating performance of the insulating layer <NUM> against applied direct-current voltage and the space-charge characteristics of the insulating layer <NUM> can be improved.

The mass ratio of the polyethylene-graft-maleic anhydride with respect to the polyethylene is less than or equal to <NUM>/<NUM>. With this, the long-term insulating performance of the insulating layer <NUM> against applied direct-current voltage can be improved. The mass ratio of the polyethylene-graft-maleic anhydride with respect to the polyethylene is, generally, greater than or equal to <NUM>.

The BET specific surface area of the inorganic filler is greater than or equal to <NUM><NUM>/g and less than or equal to <NUM><NUM>/g, and preferably, greater than or equal to <NUM><NUM>/g and less than or equal to <NUM><NUM>/g. If the BET specific surface area of the inorganic filler is less than <NUM><NUM>/g, the long-term insulating performance of the insulating layer <NUM> against applied direct-current voltage and the space-charge characteristics of the insulating layer <NUM> are lowered. On the other hand, if the BET specific surface area of the inorganic filler exceeds <NUM><NUM>/g, the long-term insulating performance of the insulating layer <NUM> against applied direct-current voltage is lowered.

The mean volume diameter of the inorganic filler is less than or equal to <NUM>, and preferably, less than or equal to <NUM>. If the mean volume diameter of the inorganic filler exceeds <NUM>, the long-term insulating performance of the insulating layer <NUM> against applied direct-current and the space-charge characteristics of the insulating layer <NUM> are lowered. The mean volume diameter of the inorganic filler is, generally, greater than or equal to <NUM>.

Here, when the BET specific surface area of the inorganic filler is greater than or equal to <NUM><NUM>/g, it is preferable that the mean volume diameter of the inorganic filler is greater than or equal to <NUM>. With this, the long-term performance against direct-current can be maintained.

The mass ratio of the inorganic filler with respect to the base resin is <NUM> to <NUM>, and preferably, <NUM> to <NUM>. If the mass ratio of the inorganic filler with respect to the base resin is less than <NUM> or exceeds <NUM>, the long-term insulating performance of the insulating layer <NUM> against applied direct-current and the space-charge characteristics of the insulating layer <NUM> are lowered.

As the inorganic filler, although not specifically limited, magnesium oxide powder, aluminum oxide powder, silica powder, magnesium silicate powder, aluminum silicate powder, carbon black or the like may be used, and two or more of them may be used in combination. Among them, in a view of the space-charge characteristics of the insulating layer <NUM>, the magnesium oxide powder is preferable.

A surface process by a silane coupling agent may be performed on each of the magnesium oxide powder, the aluminum oxide powder, the silica powder, the magnesium silicate powder and the aluminum silicate powder. With this, the long-term insulating performance of the insulating layer <NUM> against applied direct-current and the space-charge characteristics of the insulating layer <NUM> can be improved.

As the silane coupling agent, although not specifically limited, Vinyltrimethoxysilane, Vinyltriethoxysilane, <NUM>-(<NUM>,<NUM>-Epoxycyclohexyl)ethyltrimethoxysilane, <NUM>-Glycidoxypropylmethyldimethoxysilane, <NUM>-Glycidoxypropyltrimethoxysilane, <NUM>-Glycidoxypropylmethyldiethoxysilane, <NUM>-Glycidoxypropyltriethoxysilane, <NUM>-Methacryloxypropylmethyldimethoxysilane, <NUM>-Methacryloxypropyltrimethoxysilane, <NUM>-Methacryloxypropylmethyldiethoxysilane, <NUM>-Methacryloxypropyltriethoxysilane, <NUM>-Acryloxypropyltrimethoxysilane, N-(<NUM>-Aminoethyl)-<NUM>-aminopropylmethyldimethoxysilane, N-(<NUM>-Aminoethyl)-<NUM>-aminopropyltrimethoxysilane, N-(<NUM>-Aminoethyl)-<NUM>-aminopropyltriethoxysilane, <NUM>-Aminopropyltrimethoxysilane, <NUM>-Aminopropyltriethoxysilane, <NUM>-Triethoxysilyl-N-(<NUM>,<NUM>-dimethylbutylidene)propylamine or the like may be used, and two or more of them may be used in combination.

Here, the inorganic filler whose surface is treated by a silane coupling agent and the inorganic filler whose surface is not treated by a silane coupling agent may be used together in combination.

A grinding process may be performed on the inorganic filler. For example, a grinding process by jet grinding may be performed on the inorganic filler, whose particle size becomes larger as being adhered with each other when performing the surface treatment using the silane coupling agent.

The insulating layer <NUM> may further contain an anti-oxidizing agent. With this, thermal aging resistance of the insulating layer <NUM> can be improved.

As the anti-oxidizing agent, although not specifically limited, <NUM>,<NUM>-Thiodiethylene-bis[<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate], Pentaerythrityl-tetrakis(<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate), Octadecyl <NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate, <NUM>,<NUM>-Bis(n-octylthiomethyl)-o-cresol, <NUM>,<NUM>-Bis(n-octylthio)-<NUM>-(<NUM>-hydroxy-<NUM>,<NUM>-di-t-butylanilino)-<NUM>,<NUM>,<NUM>-triazine, Bis[<NUM>-methyl-<NUM>-{<NUM>-n-alkyl (C12 or C14) thiopropionyloxy}-<NUM>-t- butylphenyl]sulfide, <NUM>,<NUM>'-Thiobis(<NUM>-methyl-<NUM>-t-butylphenol) or the like may be used, and two or more of them may be used in combination.

The insulating layer <NUM> may further contain lubricant, a coloring agent or the like.

As the material constituting the outer semi-conducting layer <NUM>, although not specifically limited, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate copolymer or the like may be used.

As the material constituting the shielding layer <NUM>, although not specifically limited, copper or the like may be used.

As the material constituting the covering layer <NUM>, although not specifically limited, polyvinyl chloride or the like may be used.

The direct-current cable <NUM> may be applied for power transmission of direct-current power or the like.

Next, an example of a method of manufacturing the direct-current cable <NUM> is described.

The inner semi-conducting layer <NUM>, the insulating layer <NUM> and the outer semi-conducting layer <NUM> are formed by extrusion molding a raw material of the inner semi-conducting layer <NUM>, the composition containing the base resin, the inorganic filler and the cross-linking agent as a raw material of the insulating layer <NUM> and a raw material of the outer semi-conducting layer <NUM> at the same time at the outer periphery of the conductive portion <NUM>, and heating it to a predetermined temperature to cross-link the base resin. Next, the shielding layer <NUM> is formed by winding a conductive wire such as a copper tape, or an annealed copper wire around the outer periphery of the outer semi-conducting layer <NUM>. Further, the covering layer <NUM> is formed at an outer periphery of the shielding layer <NUM> by extrusion molding a raw material of the covering layer <NUM>.

As the method of manufacturing the composition, although not specifically limited, a method or the like may be used in which the base resin, the inorganic filler, if necessary, the anti-oxidizing agent, the lubricant, the coloring agent and the like are kneaded to manufacture pellets, and thereafter, the cross-linking agent is heated and impregnated to the pellets.

Here, the composition may be extrusion molded by removing aggregates by using a screen mesh.

Further, the raw material of the inner semi-conducting layer <NUM>, the above described composition and the raw material of the outer semi-conducting layer <NUM> may be extrusion molded at the same time.

Next, examples of the invention are described. Here, a term "parts" means "parts by weight".

<NUM> parts of low density polyethylene (LDPE) with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM> as the base resin, <NUM> parts of magnesium oxide powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> as the inorganic filler, and <NUM> parts of <NUM>,<NUM>'-thiobis(<NUM>-methyl-<NUM>-t-butylphenol) as the anti-oxidizing agent were heated and kneaded at about <NUM> to manufacture pellets. Next, <NUM> parts of dicumyl peroxide as the cross-linking agent was heated and impregnated to the obtained pellets at about <NUM> to obtain composition.

Composition was obtained similarly as Example <NUM> except that the amount of the inorganic filler was changed to <NUM> part.

Composition was obtained similarly as Example <NUM> except that the amount of the inorganic filler was changed to <NUM> parts.

Composition was obtained similarly as Example <NUM> except that magnesium oxide powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> whose surface was treated by vinyltrimethoxysilane as the silane coupling agent was used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of LDPE with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM>, and <NUM> parts of polyethylene-graft-maleic anhydride (MA-g-PE) with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM> were used, as the base resin.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-bis(t-butylperoxy)hexane was used, as the cross-linking agent.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of LDPE with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM>, and <NUM> parts of ethylene-ethyl acrylate copolymer (poly(E-co-EA)), in which the content of units originated from ethyl acrylate was <NUM> mass%, with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM> were used, as the base resin, and silica powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> was used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that silica powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> was used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of LDPE with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM>, and <NUM> parts of poly(E-co-EA), in which the content of units originated from ethyl acrylate was <NUM> mass%, with a density of <NUM>/mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM> were used, as the base resin, alumina powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> was used, as the inorganic filler, and <NUM>,<NUM>-bis(t-butylperoxyisopropyl)benzene was used, as the cross-linking agent.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of LDPE with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM>, and <NUM> parts of poly(E-co-EA) whose EA concentration was <NUM>% with a density of <NUM> / mm<NUM>, and MFR (Melt Flow Rate) of <NUM> / <NUM> were used, as the base resin, and carbon black (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> was used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that <NUM> part of magnesium oxide powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> whose surface was treated by vinyltrimethoxysilane as the silane coupling agent, and <NUM> parts of silica powder with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> were used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of magnesium oxide powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> whose surface was treated by vinyltrimethoxysilane as the silane coupling agent, and <NUM> parts of alumina powder with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> were used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that the inorganic filler was not used.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of magnesium oxide powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> was used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that <NUM> parts of alumina powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> was used, as the inorganic filler.

Composition was obtained similarly as Example <NUM> except that magnesium oxide powder (inorganic filler <NUM>) with a BET specific surface area of <NUM><NUM> / g, and a mean volume diameter of <NUM> was used, as the inorganic filler.

Here, the BET specific surface area conformed JIS Z8830 and ISO <NUM>, and was measured by a fluid process. The mean volume diameter was measured using a laser diffraction particle size analyzer.

Characteristics of inorganic fillers <NUM> to <NUM> contained in the compositions of Examples <NUM> to <NUM> and Comparative examples <NUM> to <NUM> are illustrated in Table <NUM>.

Characteristics of the compositions of Examples <NUM> to <NUM> and Comparative examples <NUM> to <NUM> are illustrated in Table <NUM>.

Each of the compositions of Examples <NUM> to <NUM> and Comparative examples <NUM> to <NUM> was press molded to obtain a sheet with thickness T of <NUM>.

Next, specific volume resistance, long-term insulating performance against applied direct-current voltage and space-charge characteristics of each of the sheets were evaluated.

Specific volume resistance was measured by soaking the sheet in silicone oil of <NUM>, and applying a direct electric field of <NUM> kV / mm to the sheet using a flat plate electrode with a diameter of <NUM>.

Using the sheet, a V-t curve was obtained by soaking the sheet in silicone oil of <NUM>, applying a direct electric field V<NUM> [kV / mm] of <NUM> to <NUM> kV / mm to the sheet using a flat plate electrode with a diameter of <NUM> and measuring a period "t" [h] until dielectric breakdown occurs in the sheet. Next, life exponent "n" was obtained from the formula <MAT> and long-term insulating performance against applied direct-current voltage was evaluated. Here, when "n" was greater than or equal to <NUM>, it was determined to be "A", when "n" was greater than or equal to <NUM> and less than <NUM>, it was determined to be "B", and when "n" was less than <NUM>, it was determined to be "C".

Space-charge characteristics of the sheet were evaluated using a Pulsed Electro Acoustic Nondestructive Test System (manufactured by Five Lab). Specifically, space-charge characteristics of the sheet was evaluated by continuously applying a direct electric field V<NUM> of <NUM> kV / mm to the sheet under atmospheric pressure at <NUM> for an hour, measuring maximum electric field V<NUM> in the sheet, and obtaining Field Enhancement Factor FEF defined by the formula <MAT>.

Here, when the FEF was less than <NUM>, it was determined to be "A" and when the FEF was greater than or equal to <NUM>, it was determined to be "B".

Evaluated results of the specific volume resistance, the long-term insulating performance against applied direct-current current and the space-charge characteristics of each of the sheets manufactured from the compositions of Examples <NUM> to <NUM>, and Comparative examples <NUM> to <NUM> are illustrated in Table <NUM>.

From Table <NUM>, for each of the sheets manufactured from the compositions of Examples <NUM> to <NUM>, respectively, it can be understood that the specific volume resistance is high, and the long-term insulating performance against applied direct-current voltage and the space-charge characteristics are good.

On the other hand, as the sheet manufactured from the composition of Comparative example <NUM> does not contain inorganic filler, the specific volume resistance, the long-term insulating performance against applied direct-current voltage and the space-charge characteristics are lowered.

For the sheet manufactured from the composition of Comparative example <NUM>, as the mass ratio of the inorganic filler <NUM> with respect to the base resin is <NUM>, the long-term insulating performance against applied direct-current voltage and the space-charge characteristics are lowered.

For the sheet manufactured from the composition of Comparative example <NUM>, as the BET specific surface area and the mean volume diameter of the inorganic filler <NUM> are <NUM><NUM> / g and <NUM>, respectively, the specific volume resistance, the long-term insulating performance against applied direct-current voltage and the space-charge characteristics are lowered.

For the sheet manufactured from the composition of Comparative example <NUM>, as the BET specific surface area of the inorganic filler <NUM> is <NUM><NUM> / g, the long-term insulating performance against applied direct-current voltage is lowered.

Claim 1:
A direct-current cable comprising:
a conductive portion; and
an insulating layer covering an outer periphery of the conductive portion,
the insulating layer containing cross-linked base resin and inorganic filler,
the base resin containing polyethylene,
a BET specific surface area of the inorganic filler being greater than or equal to <NUM><NUM> / g and less than or equal to <NUM><NUM> / g, and a mean volume diameter of the inorganic filler being less than or equal to <NUM>, the mean volume diameter being measured using a laser diffraction particle size analyzer,
the mass ratio of the inorganic filler with respect to the base resin being greater than or equal to <NUM> and less than or equal to <NUM>, and
the cross-linked base resin being cross-linked by a cross-linking agent containing organic peroxide,
wherein the base resin further contains polyethylene-graft-maleic anhydride and the mass ratio of the polyethylene-graft-maleic anhydride with respect to the polyethylene is less than or equal to <NUM>/<NUM>.