NO314475B1 - Electrically insulating polymeric material and its use - Google Patents

Description

This invention relates to an electrically insulating polymer material and its use. More specifically, the present invention relates to a material which is an excellent electrical insulator with regard to volume resistivity, breakdown strength and other electrical properties.

It is required per definition that insulation materials for wires or cables must have high volume resistivity, high breakdown strength, low dielectric constant and low loss factor, and are typically made of polyethylene. Although oil-filled insulators (hereinafter referred to as OF) are used for high-capacity power supply cables and provide satisfactory electrical insulation, the oil-filled insulators are accompanied by certain disadvantages, including that they must be constantly supplied with oil to replace the amount of oil that leaks out of them. In recent years, cross-linked olefinic polymers such as cross-linked polyethylene have become popular as they show an elevated level

of heat resistance and physical strength.

Techniques for cross-linking include electron beam-induced cross-linking and chemical cross-linking using peroxides and the like, the former of which requires extensive equipment and is consequently expensive. Although chemical cross-linking is less expensive, cross-linking agents can remain in the products and gradually reduce the volume resistivity of the products, causing water trees and other problems.

In an effort to improve the electrical insulation of insulating materials and eliminate the problem of water tar, Japanese Patent Application No. 5-15007 has proposed a method for introducing polyolefin modified with maleic anhydride into polyethylene for the purpose of introducing hydrophilic groups. Japanese published patent application No. 4-11646 describes a method for improving the electrical insulation of an insulating material by introducing double bonds in advance in polyolefin to be cross-linked, thereby reducing the degree of use of cross-linking agent. However, none of these methods appeal satisfactorily for improving volume resistivity and other aspects of electrical insulation and heat resistance.

The inventors of the present invention have found some time ago that highly electrically insulating materials can be produced by using maleic anhydride in a very limited concentration. Although these materials are far superior to normally cross-linked polyolefinic materials in terms of electrical insulation, they are still not satisfactory in terms of volume resistivity and other properties of electrical insulation.

In the meantime, methods have been proposed regarding the use of carboxyl compounds and/or aromatic compounds with polyolefin to improve the final product's electrical insulation. Examples of such proposed methods include the use of polystyrene grafted to polyolefin to improve the impulse impact strength of the final product (Japanese Patent Application No. 2-165506), the use of a mixture of polyethylene and polystyrene also to improve the impulse impact strength of the final product product (Japanese Laid-open Patent Application No. 63-301427), use of a mixture of polyolefin and polystyrene modified with maleic anhydride to improve the electrical insulation of the final product (Japanese Laid-open Patent Application No. 62-100909) and use of a mixture of polyolefin and aromatic carboxylic acid to improve the impact strength of the final insulation product (Japanese Laid-Open Patent Application No. 60-23904).

However, none of these methods can satisfactorily improve an insulation material of the type in question either with regard to volume resistivity or impact strength. Nor are they satisfactory with regard to improving the electrical insulation of the material after cross-linking.

It is therefore an object according to the present invention to provide electrically insulating polymer material comprising an olefinic polymer as the main component and functional group-containing monomeric units of the monomers Dl through D4 in an amount of 5x10" 7 to 5x10" 3 mol per g of the mixture, and also comprising ethylenic bonds in an amount not less than 0.8 per 1,000 carbon atoms and/or aromatic ring-containing monomer units of the monomer D5 in an amount of 5x10"<7> to 5x10"<3> mol per g of the mixture, characterized in that said monomers Dl through D5 have an ethylenic bond, said monomer Dl is a carbonyl group or carbonyl group derivative containing monomer, said monomer D2 is a hydroxyl group containing monomer, said monomer D3 is a nitro group containing monomer, said monomer D4 is a nitrile group-containing monomer, and said monomer D5 is an aromatic ring-containing monomer. The polymer blend operates as an excellent electrical insulator with respect to volume resistivity, breakdown strength and other electrical properties.

As a result of intensive research efforts, the inventors of the present invention have achieved the present invention. Figures 1(a) and (b) are schematic illustrations of an electrode system used to test the volume resistivity of test pieces of a mixture according to the present invention, where figure 1(a) is a plan sketch of the electrode system and figure 1(b) is a cross section hence. Figure 2 is a schematic illustration of an electrode system used for testing the impact strength of test pieces of a mixture according to the present invention. Figure 3 is a schematic illustration of a water tree observation system used to test samples of a mixture according to the present invention. Figure 4 is a diagram showing the performance of Examples 1 to 6 of the present invention and the performance of Comparative Examples 1, 4 and 5, expressed as the ratio between the concentration of the functional group (horizontal axis) and the volume resistivity (vertical axis). Figure 5 is a diagram showing the performance of Examples 7 through 10 according to the present invention and 13 through 15 and the performance of Comparative Examples 1, 4, 5 and 10. Figure 6 shows a schematic cross-section of a cable according to the present invention .

The invention will now be described in greater detail.

For the purpose of the above-mentioned first aspect of this invention, electrically insulating polymer material is provided, characterized in that said material comprises a polymer having said functional group-containing monomers Dl through D4, a polymer having monomeric units containing two or more ethylenic bonds and/or a polymer having said aromatic ring-containing monomer D5.

More specifically, for the purposes of the above-mentioned first feature of this invention, said material comprises at least one of component A), component (B) and component (C) as defined below: Component (A): at least one of (A1) and (A2 ) below;

(Al): an olefinic polymer, and

(A2): an arbitrary copolymer of olefin and at least one of said functional group-containing monomers Dl to D5 inclusive or a graft copolymer obtained by modification of an olefinic polymer with at least one of said functional group-containing monomers Dl to D5 inclusive,

Component (B): at least one of (B1) through (B3) below;

(Bl): a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said

monomer and ethylene,

(B2): an arbitrary copolymer of a monomer containing two or more ethylenic bonds and at least one of said functional group-containing monomers Dl through D5, an arbitrary copolymer of a monomer containing two or more ethylenic bonds, ethylene and at least one of said functional group-containing monomers Dl through D5 or a graft copolymer obtained by modification of a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with at least one of said functional group-containing monomers

Dl through D5, and

(B3): a compound containing two or more ethylenic bonds,

Component (C): at least one of (Cl) and (C2) below;

(Cl): an olefinic polymer containing at least one aromatic ring, and

(C2): an arbitrary copolymer of olefin, at least one of said functional group-containing monomers Dl through D4, and said aromatic ring-containing monomer D5 or a graft copolymer obtained by modifying an olefinic polymer containing aromatic rings with at least one of said functional group-containing monomers D1 through D4, or a graft copolymer obtained by modifying an arbitrary copolymer of olefin and at least one of said functional group-containing monomers D1 through D4 with said aromatic ring-containing monomer D5.

For the purpose of the present invention, said olefinic polymer of the component (Al) shall be a homopolymer or copolymer of hydrocarbons expressed by the general formula CnH2n which may be selected from ethylene, propylene, 1-butene, 2-butene, isobutylene, 1 -pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 2-methyl-2-butene, 1-hexene, 2,3-dimethyl-2-butene, 4-methyl-1 -pentene, 1-heptene, 1-octene, 1-nonene, 1-decene, etc.

Examples of the olefinic polymers that can be used for the purpose of the present invention include high, medium and low density polyethylene, linear low density polyethylene, very low density polyethylene, polypropylene, polybutene, polypentene, poly-4-methyl-l-pentene, ethylene- Of-olefin copolymer, ethylene-propylene copolymer rubber (EPR), low-density polyethylene obtained by a high-pressure radical method and ethylene copolymer obtained by a high-pressure radical method. Of these olefinic polymers, low density polyethylene obtained by a high pressure radical method, high, medium or low density polyethylene, linear low density polyethylene and polypropylene are preferred.

For the purpose of the present invention, functional group containing monomers having an ethylenic bond can be the monomer D1: a carbonyl group or carbonyl group derivative containing monomer, the monomer D2: a hydroxyl group containing monomer, the monomer D3: a nitro group containing monomer, the monomer D4: a nitrile group containing monomer and the monomer D5: an aromatic cyclic monomer. Specific examples of such monomers will be listed below.

Examples of the carbonyl group or carbonyl group derivative containing monomer D1 include unsaturated carboxylic acids derived from α,β-unsaturated carboxylic acids, unsaturated carboxylates derived from α,β-unsaturated carboxylates and vinyl ester monomers. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid. Examples of unsaturated carboxylates include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, cyclohexyl acrylate, cyclohexyl methacrylate, lauryl acrylate, lauryl methacrylate, stearyl acrylate, stearyl methacrylate, methyl maleate, ethyl maleate, dimethyl maleate , diethyl maleate, methyl fumarate, ethyl fumarate, glycidyl acrylate and glycidyl methacrylate.

Examples of vinyl esters which can be used for the purpose of the present invention include vinyl propionate, vinyl acetate, vinyl caproate, vinyl caprylate, vinyl laurate, vinyl stearate and vinyl trifluorate, of which vinyl acetate is most preferred.

Examples of acid anhydrides that can be used for carbonyl group derivative-containing monomers for the purpose of the present invention include maleic anhydride, itaconic anhydride, methylmaleic anhydride, dimethylmaleic anhydride, phenylmaleic anhydride, diphenylmaleic anhydride, chloromaleic anhydride, dichloromaleic anhydride, fluoromaleic anhydride, difluoromaleic anhydride, bromomaleic anhydride and dibromomaleic anhydride, of which is most preferred.

Other monomers which can be used for the purpose of the present invention include carbon monoxide, methyl vinyl ketone, isopropenyl vinyl ketone, ethyl vinyl ketone, phenyl vinyl ketone, t-butyl vinyl ketone, isopropyl vinyl ketone, methyl propenyl ketone, methyl isopropenyl ketone and cyclohexyl vinyl ketone.

Examples of the hydroxyl group-containing monomer D2 include vinyl alcohol, 1-hydroxypropyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate and hydroxyethyl (meth)acrylate.

Examples of the nitro group-containing monomer D3 include 2,4-dinitro-phenyl acrylate, 2-nitrostyrene, m-nitrostyrene, o-nitrostyrene, p-nitrostyrene, p-nitrophenyl methacrylate, m-nitrophenyl methacrylate, 2,4-dinitrophenyl methacrylate and 2 ,4,6-tri-nitrophenyl methacrylate.

Examples of the nitrile group-containing monomer D4 include acrylonitrile, methacrylonitrile, α-methoxyacrylonitrile, vinylidene cyanide, cinnamic nitrile, crotononitrile, O-phenylcrotononitrile, fumaronitrile, arylacetonitrile, 2-butenenitrile and 3-butenenitrile.

For the purposes of the present invention, the aromatic ring-containing monomer D5 must be a compound containing a monocyclic or polycyclic aromatic ring and with ethylenic bonds.

Examples of said aromatic ring-containing monomer D5 which can be used for the purpose of the present invention are preferably aromatic compounds containing a monocyclic, dicyclic or tricyclic aromatic ring and include styrene and its derivatives, arylbenzene, arylbiphenyl, methylstyrene, arylbenzoate, vinylnaphthalene, 4-phenyl -l-butene, benzyl methacrylate, 1,1-diphenylethylene, 4-phenyl-1-tolylethylene, 1-phenyl-l-styrylethane, 1-tolyl-l-styrylethane, 2,4-diphenyl-l-butene, 2,4 -diphenyl-1-pentene and 2,4-di-phenyl-4-methyl-1-pentene.

Of the above examples, styrene is most preferable with regard to the electrical quality of the final product.

The concentration of the functional group containing monomers must be between 5xl0"<7> and 5xl0"<3> mol, preferably between lxlO"<6> and lxlO<*4> mol and most preferably between lxlO"<6> and lxlO<*5> moles for each gram of polymer mixture containing no cross-linking agent.

If the concentration of the functional groups in the monomers D1 to D4 including maleic anhydride is less than 5x10" mol per gram of the polymer mixture, the volume resistivity of the final product will not be improved. If the same is true with the functional group in the monomer D5, the impact strength will of the final product also not be improved. If, on the other hand, the concentration is higher than 5xl0"<3> mol per grams of the polymer mixture, the volume resistivity of the final product is degraded.

Copolymers which can be used for an arbitrary copolymer of olefin and at least one of said functional group-containing monomers Dl to D5 inclusive or a graft copolymer obtained by modifying an olefinic polymer with at least one of said functional group-containing monomers Dl to D5 inclusive as defined for component (A2) includes linear low density polyethylene modified with acrylic acid or maleic anhydride, high, medium or low density polyethylene modified with acrylic acid or maleic anhydride, copolymer of ethylene and acrylic acid or maleic anhydride, copolymer of ethylene and carbon monoxide, ethylene methyl vinyl ketone copolymer, ethylene methyl isopropenyl ketone copolymer, ethylene hydroxyethyl (met )-acrylate copolymer, ethylene-2-nitrostyrene copolymer, ethylene-m-nitrostyrene copolymer, ethylene-p-nitrophenyl methacrylate copolymer, ethylene (meth)acrylonitrile copolymer, ethylene-arylacetonitrile copolymer and low density polyethylene modified with styrene.

A homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene as defined for component (Bl) must contain double bonds in sufficient concentration after polymerisation. According to the existence of the ethylenic bonds, the final product shows a satisfactory cross-linked effect. For component (B), a liquid oligomer or copolymer with an average molecular weight of 1,000 to 200,000 will be used.

A homopolymer derived from a monomer containing two or more ethylenic bonds as defined in the former half of (B1) is a diene polymer of 4-10 carbon atoms. The diene polymer can be cyclic or straight chain, as long as it has two or more double bonds. However, butadiene oligomers or polybutadiene having an average molecular weight of 1,000 to 20,000 is most preferable because it is excellent in electrical insulation and crosslinking efficiency after crosslinking.

Examples of such compounds include 1,3-butadiene, 1,3-pentadiene, 1,4-pentadiene, 2-methyl-1,3-butadiene, 1,3-hexadiene, 1,4-hexadiene, 1,5-hexadiene , 2,4-hexadiene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-heptadiene, 1,4-heptadiene, 3-(2-propenyl)-cyclopentene and 2-(cyclopentyl)-1,3-butadiene. Trienes and tetraenes which can be produced from dienes by polymerization can also be used.

Of the possible compounds above, the monomer of 1,3-butadiene would be most preferable, due to its cross-linking effect.

Compounds which can be used as an arbitrary copolymer of monomer containing two or more ethylenic bonds and ethylene as defined in the last half of (Bl) include ethylene aryl (meth)acrylate copolymer and ethylene vinyl (meth)acrylate copolymer. Alternatively, a mixture of two or more of such copolymers can be used.

Examples of an arbitrary copolymer of a monomer containing two or more ethylenic bonds and at least one of said functional group-containing monomers Dl through D5, an arbitrary copolymer of a monomer containing two or more ethylenic bonds, ethylene and at least one of said functional group-containing monomers Dl through D5 or a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with at least one of said functional group-containing monomers Dl through D5 as defined in ( B2) comprises polybutadiene modified with acrylic acid or maleic anhydride, maleic anhydride-butadiene copolymer, maleic acid modified ethylene aryl (meth)acrylate copolymer and maleic acid modified ethylene vinyl (meth)acrylate copolymer.

Of these, polybutadiene modified with acrylic acid or maleic anhydride is most preferred.

Examples of compounds which can be used for a compound containing two or more ethylenic bonds as defined in (B3) include methacrylate monomers having several functional groups such as trimethylolpropane trimethacrylate, ethylene glycol dimethacrylate and diethylene glycol dimethacrylate; vinyl monomers having multiple functional groups such as triaryl isocyanurate, diaryl phthalate and vinyl butyrate; bismaleimides such as N,N'-m-phenylenebismaleimide and N,N'-ethylenebismaleimide; dioximes such as e.g. β-quinone dioxime; divinyl compounds such as divinylbenzene, 1,5-hexadien-3-yne, hexatriene, divinyl ether, divinyl sulfone; and diaryl compounds such as e.g. arylstyrene, 2,6-diacrylatephenol, diarylcarbinol.

For component (B) defined as (B1), (B2) or (B3), before cross-linking, the mixture contains a concentration of less than 0.8 double bonds per 1,000 carbon atoms. Preferably, the number of double bonds will be between 0.8 and 4.0 per 1,000 carbon atoms.

Furthermore, when it contains the double bonds, the number of terminal vinyl groups will preferably be between 0.5 and 3.0 per 1,000 carbon atoms.

An olefinic polymer containing at least one aromatic ring as defined in (Cl) above is a copolymer of a monomer containing a monocyclic or polycyclic aromatic ring and olefin.

For the purposes of the present invention, a monomer containing at least one aromatic ring will preferably be an aromatic compound with a mono-, di- or tricyclic aromatic ring and selected from compounds comprising styrene, styrene derivatives, arylbenzene, arylbiphenyl, methylstyrene, arylbenzoate, vinylnaphthalene, 4 -phenyl-l-butene, bezyl methacrylate, 1,1-diphenylethylene, 1-phenyltolylethylene, 1-phenyl-l-styrylethane, 2,4-diphenyl-l-butene, 2,4-diphenyl-l-pentene and 2,4 -diphenyl-4-methyl-1-pentene.

Olefins which can be used in component (Cl) for the purposes of this invention include α-olefins which have 3-12 carbon atoms such as e.g. ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene and 1-decene.

Of olefinic polymers containing aromatic rings, an ethylene styrene random copolymer is most preferred.

An arbitrary copolymer of olefin, at least one of said functional group-containing monomers Dl, and said aromatic ring-containing monomer D5 or a graft copolymer obtained by modifying an olefinic polymer containing aromatic rings with at least one of said functional group-containing monomers Dl through D4, or a graft copolymer obtained by modifying an arbitrary copolymer of olefin and at least one of said functional group-containing monomers Dl through D4 with said aromatic ring-containing monomer D5 as defined in component (C2) above is selected from compounds comprising ethylene-styrene-maleic anhydride random copolymer, maleic anhydride-modified ethylene styrene copolymer, ethylene-arylbenzene copolymer, maleic anhydride-modified ethylene-benzyl methacrylate copolymer and maleic anhydride-modified ethylene-arylstyrene.

Examples of copolymers of at least one of the monomers containing a styrene derivative and ethylene which can be used for the purpose of the present invention include ethylene-vinyl acetate-styrene copolymer, ethylene-vinyl acetate-α-methylstyrene copolymer, ethylene-ethyl acrylate-styrene copolymer and ethylene -ethyl acrylate-o-methyl-styrene copolymer.

For component (C) defined as the components (Cl) or (C2), the concentration of said aromatic-containing monomer in the mixture will be between 5x10" and 5xl0"<3> mol, preferably between lxlO"<6> and lxlO"<4> mol and most preferably between lxlO"<6> and 5xl0"<5> mol per grams of the polymer mixture.

Some preferred combinations of possible components of a polymer mixture according to the present invention will now be described. (1) According to claim 4, there is provided a mixture of an olefinic polymer (Al) as component (A) and a graft copolymer (B21) obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with a functional group-containing monomer Dl as component (B). An example of such a combination is obtained from unmodified low-density polyethylene (Al) obtained by high-pressure radical polymerization and maleic anhydride-modified liquid polybutadiene (B21). (2) According to claim 5, there is provided a combination of an olefinic polymer (A1) and a graft copolymer (A21) obtained by modification of an olefinic polymer with a functional group containing monomer D1 as component (A) and a graft copolymer ( B21) obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with a functional group containing monomer D1 as component (B). An example of such a combination is obtained from unmodified low-density polyethylene (Al) obtained by high-pressure radical polymerization, maleic anhydride-modified low-density polyethylene (A21) and maleic anhydride-modified liquid polybutadiene (B21). (3) According to claim 6, a combination of an olefinic polymer (A1) and a graft copolymer (A21) obtained by modifying an olefinic polymer with a functional group containing monomer D1 as component (A) and a compound (B3) with two or more ethylenic bonds as component (B). An example of such a combination is obtained from unmodified low density polyethylene (Al) obtained by high pressure radical polymerization, maleic anhydride modified low density polyethylene (A21) and divinylbenzene

(B3).

(4) According to claim 7, a combination of a graft copolymer (B21) obtained by a modification of a homopolymer derived from a monomer with two or more ethylenic bonds or a copolymer of said monomer and ethylene with a functional group containing monomer D1 is obtained component (B) and an olefinic polymer (Cl) containing at least one aromatic ring as component (C). An example of such a combination is obtained from maleic anhydride modified liquid polybutadiene (B21) and ethylene styrene arbitrarily

copolymer (Cl).

(5) According to claim 8, a combination of a graft copolymer (A21) obtained by modifying an olefinic polymer with a functional group containing monomer D1 as component (A) and an olefinic polymer (Cl) containing at least one aromatic ring as component is obtained (C). An example of such a combination is obtained from maleic anhydride modified low density

polyethylene (A21) and ethylene styrene arbitrary copolymer (Cl).

(6) According to claim 9, a combination of an olefinic polymer (Cl) containing at least one aromatic ring and a graft copolymer (C21) obtained by modifying an olefinic polymer with a functional group containing monomer D1 as component (C) is provided. An example of such a combination is obtained from ethylene styrene random copolymer (Cl) and ethylene-styrene maleic anhydride graft copolymer (C21).

For the purposes of the present invention, although a composition according to the present invention may well be used by itself as an electrically insulating material, it is preferred to crosslink it to improve the volume resistivity and breakdown strength. Although any suitable crosslinking technique including the use of a radical generating agent such as an organic peroxide, electron beam induced crosslinking and silane crosslinking may be used, the use of a radical generating agent is preferred in view of cost effectiveness.

Examples of radical generating agents that can be used for the purpose of the present invention include peroxides such as benzoyl peroxide, lauryl peroxide, dicumyl peroxide, t-butyl hydroperoxide, a,a-bis-(t-butyl peroxide-isopropyl)benzene, di-t-butyl peroxide, 2, 5-di-(t-butylperoxy)hexane, 2,5-di-(t-butylperoxy)hexene; azobisisobutylonitrile, 2,3-dimethyl-2,3-diphenylbutane, 2,3-diethyl-2,3-diphenylbutane and 2,3-diethyl-2,3-di-(bromophenyl)butane.

In the cross-linking method, the volume resistivity and other electrical properties of the mixture can be improved by cross-linking the mixture after preparing a given amount of component (B).

The radical-generating agents for modifying component (B) with a functional group containing monomer as described above can mostly be used as cross-linking agents for the purpose of the present invention. Of the cross-linking agents listed above, dicumyl peroxide, 2,5-di-(t-butylperoxy)hexane and 2,5-di-(t-butylperoxy)hexene can be recommended.

The crosslinking agent is used in an amount between 0.01 and 5 parts by weight, preferably between 0.1 and 3 parts by weight, for a total of 100 parts by weight of the mixture.

The mixture according to the present invention may comprise one or more chemicals selected from inorganic fillers, organic fillers, antioxidants, lubricants, organic or inorganic pigments, ultraviolet prevention agents, photostabilizers, dispersants, anti-copper corrosion agents, neutralizing agents, plasticizing agents and nucleating agents.

An insulating polymer mixture according to the present invention can be used for insulating materials for wires, cables and capacitors, for insulating high voltage areas in X-ray generators and cables and other applications.

A wire or a cable that is produced will comprise an insulation layer made of a polymer mixture according to the present invention or a material obtained by cross-linking said mixture. More specifically, a wire or cable that is manufactured will include an insulating layer covering its conductor. If necessary, a bundle of wires can be used for the conductor, and a semi-conductive layer can be arranged between the conductor and the insulating layer. A flame-retardant polymer layer can be placed on the outside of the insulation layer.

A wire or a cable is produced by coating a bundle of copper wires with a polymer material containing carbon powder or metal powder to form a semi-conductive layer, on which an insulating layer of a polymer material according to the present invention is formed, on which a coating of metal foil or further is arranged a semi-conductive layer and then at the very end a coating of a flame-retardant or rat-repellent polymer layer is formed. Alternatively, a wire or cable can be produced by combining dozens of coated copper wires, each comprising a single copper wire coated with a semi-conductive layer of a polymer material containing carbon or metal powder and an insulating layer of a polymer material according to the present invention, forming a very outer coating of a flame-retardant or rat-repellent polymer layer. A polymer material according to the present invention is particularly effective for high voltage and can be suitably used for direct current power cables.

FUNCTIONS

The present invention provides an electrically insulating polymer material comprising an olefinic polymer as the main component and a functional group-containing monomer, and also comprising ethylenic bonds and/or an aromatic ring of monomer.

The functional group operates as a joint that blocks the movement of electrical charges and consequently improves the volume resistivity of the material.

The aromatic ring has the effect of absorbing high energy electrons, causing them to emit their energy in the form of heat and release them at a low energy level (electron energy absorption effect). The energy level for high-energy electrons that can trigger electrical breakdown can thus be significantly reduced to increase the material's breakdown strength.

The electrical properties of an insulation material made from a polymer material according to the present invention can be improved by cross-linking, particularly with regard to volume resistivity, the phenomenon of water wood and impact resistance. The double bonds in a diene polymer will improve the efficiency of the cross-linking, since they operate as cross-linking sites. The degradation residues of the cross-linking agent are taken up in the main chain of the polymer. The decomposition residues floating in the main volume can reduce the volume resistivity, as they undergo ionic decomposition during the generation of electric charges of heat and the electric field. Such a harmful effect of the degradation residues can be prevented by incorporating them into the main chain of the polymer, and consequently the volume resistivity of the material is improved.

The breakdown of the electrical properties of an insulating material can be effectively avoided by the three effects mentioned above. In addition, the volume resistivity and impact strength of the material is significantly improved after cross-linking.

The present invention will now be described further by means of examples.

TEST PROCEDURES

(1) Test method for vnlnm resistivity

An electrode system was used as shown in figures l(a) and l(b). The test pieces were tested by applying a direct voltage of 3300 volts (V) at room temperature and at 90°C in a nitrogen atmosphere. A Reed-type vibrating ammeter (TR8411 available from Advantest, Co., Ltd.) was used to measure the volume resistivity. Each specimen had a thickness of 0.3 mm and an effective electrode surface area of 19.6 cm 2 . The volume resistivity was determined by the electrical current observed 10 minutes after the voltage was applied. Each specimen was tested four times and the average of the readings was used.

In figures l(a) and l(b) which illustrate the electrode system, 11 shall denote a main electrode (500), 12 a protective electrode (inner diameter 750, outer diameter 800), 13 a test piece and 14 a high voltage electrode (800).

(2) Test method for impact resistance

An electrode system with fixed electrodes or so-called McKewon electrodes was used as shown in Figure 2. The electrode system comprises a substrate 24 made of polymethyl methacrylate and drilled in the center to a diameter of 12.7 mm and the electrodes 21 of stainless steel balls with a diameter of 12.7 mm. Each sample piece 22 was cut into a disc of 8 to 10 mm and placed between the electrodes. The gap between the test piece and the electrodes was filled with deaerated epoxy resin which was then cured. The McKewon electrodes were dipped in silicone oil contained in a container, which was then placed in a thermostat for observation. The applied voltage for this test had an impulse waveform of 1.2/50 piS and negative polarity. The waveform was observed using an oscilloscope, and the reading of the wave peak where the breakdown occurred was recorded for each test. An average of more than 20 readings was used.

In figure 2 which illustrates the measurement system for the impact strength test, 21 shall denote a pair of stainless steel balls, 22 a test piece, 23 epoxy resin and 24 a substrate made of polymethyl methacrylate.

(3) Observation of water trees

A water tree observation system was used with a configuration as described in Figure 3. A voltage of 10 kV and 10 kHz was applied to each sample at room temperature.

After the voltage was applied, the water tree was pigmented and observed microscopically. The following grades were used.

In Figure 3 which illustrates the water tree observation system, 31 shall denote test pieces that are observed for water trees, 32 a conducting plate, 33 water, 34 an earthing electrode, 35 containers and 36 an electrode for applied voltage.

MATERIALS

Knmpnnent (A); pnlynlefin

A1 -1: Polyethylene (LD) from the high-pressure radical method (density = 0.919 g/cm<3>,

MI = 1.0 g/10 min., trade name: Nisseki Rexlon W200 - available from Nippon Petrochemicals Co., Ltd.)

Al-2: Linear low density polyethylene (LL) (density = 0.921 g/cm3, MI = 1.0 g/10

min., trade name: Nisseki Linirex AF3280 - available from Nippon Petrochemicals Co., Ltd.)

Al-3: Polypropylene (PP) (density = 0.905 g/cm<3>, MI = 1.5 g/10 min., trade name: Nisseki Polypro F120K - available from Nippon Petrochemicals Co., Ltd.)

A2-1: Maleic anhydride-modified polyethylene from the high-pressure radical method (MAn-LD) (0.75 parts by weight of maleic anhydride and 0.05 parts by weight of 2,5-dimethyl-2,5-di(tert-butyl peroxide) were added to 100 parts by weight of LD, and they were preliminarily mixed using a Henschel mixer and obtained

react at 200°C in a biaxial extruder. When examined by infrared spectroscopic analysis, 1 g of the polymer contained maleic anhydride in an amount of 2.66x10"<5> mol).

A2-2: Acrylic acid-modified polyethylene according to the high-pressure radical method (Ac-LD)

(after compounding acrylic acid and dicumyl peroxide with LD, they were allowed to react at 200°C in an extruder. When examined by infrared spectroscopic analysis, 1 g of the polymer contained acrylic acid in an amount of 3.38x10"<6> mol).

A2-3: Ethylene maleic anhydride random copolymer (Et-MAn) [A 380 g of n-hexane and 11 g of maleic anhydride in acetone solution (0.11 g of maleic anhydride) were placed in a 3.8 L autoclave equipped with stirrers and with a nitrogen cover and 2 ,5-dimethyl-2,5-di(tert-butyl peroxide) was added thereto as a polymerization initiator. Then, 1700 g of ethylene was introduced therein and the mixture polymerized at 1600 kg f/cm 3 and 190°C for 1 hour to produce an ethylene maleic anhydride copolymer. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 1.58 x 10"<3> moles of maleic anhydride].

A2-4: Ethylene-vinyl alcohol random copolymer (Et-VOL)

[made in the same manner as above. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 2.4x1c"<4>mol of the alcohol group.]

A2-5: Ethylene-nitrostyrene copolymer (Et-Nst)

[made in the same manner as above. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 2.38x10"<4 >mol of the nitro group.]

A2-6: Ethylene-acrylonitrile copolymer (Et-AN)

[made in the same manner as above. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 2.3x10^ moles of the nitro group.]

Component (R):

B1 -1: Polybutadiene (PB 1) [average molecular weight 3,000, specific weight 0.89 g/cm<3>' trade name: Nisseki Butadiene B-3000, available from Nippon Petrochemicals Co., Ltd.]

B1 -2: Butadiene polymer (PB2) [specific gravity 0.89 g/cm<3>, trade name: JSR

RB820 - available from Japan Synthetic Rubber Co., Ltd.]

Bl-3: Copolymer of 1,3-butadiene and cyclododeca-1,5,9-hexatriene (PB3)

[1,3-Butadiene and cycloidodeca-1,5,9-hexatriene were used in a molar ratio of 9:1 in the presence of toluene and a 3-dimensional catalyst of cobalt triacetylacetate, triethylaluminum and water and allowed to react for polymerization at 7°C. Then the low molecular weight components were removed to produce a copolymer with an average molecular weight of 2500. When examined by infrared spectroscopic analysis, the vinyl bond was in an amount of 43%.]

Bl-4: Liquid isoprene (PI) [average molecular weight 29,000, 740 poise/

38°C, trade name: Kuraprene LIR-30 - available from Kuraray Co.,Ltd.]

B2-1: Maleic anhydride-modified polybutadiene (MAn-PBl) [average molecular weight 3000, specific gravity 0.89 g/cm<3.> When examined by infrared spectroscopic analysis, 1 g of the polymer contained 4.32x10"<4> moles of maleic anhydride, trade name: Nisseki Polybutadiene M-2000 - available from Nippon Petrochemicals Co., Ltd.]

B2-2: Acrylic acid-modified polybutadiene (MAn-PBl) [average molecular weight 2,000, specific gravity 0.91 g/cm . When examined by infrared spectroscopic analysis, 1 g of the polymer contained 2.20x10"<4> moles of acrylic acid, trade name: Nisseki Polybutadiene M-2000 - available from Nippon Petrochemicals Co., Ltd.]

B2-3: A mixture of ethylene-vinyl alcohol copolymer (EVOL) [produced by copolymerization and hydrolysis of ethylene and vinyl ester. 1 g of the copolymer contained 1.13x10"<3> mol of the residual alcohol group) and polybutadiene (PB1).

B2-4: Maleic anhydride-butadiene copolymer (MAn-PB3) [quintylized by heating and modification in an autoclave. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 8.95x10~<5 >mol of maleic anhydride.]

B2-5: Maleic anhydride-modified liquid polyisoprene (MAn-PI)

[produced by heating and modification in an autoclave. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 2.87x10"4 moles of maleic anhydride.]

B2-6: Acrylic acid-modified polybutadiene (MAn-PB 1) [produced by heating and modification in a monoaxial extruder. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 3.25x10"<4> moles of acrylic acid.]

B2-7: Maleic anhydride-modified butadiene polymer (MAn-PB2)

[produced by heating and modification in a single-axis extruder. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 3.44x10^ moles of maleic anhydride.]

B3: Divinylbenzene (DVB)

Component (C):

C1-1 & Ethylene styrene random copolymer (Et-St) [two samples with different Cl-2: styrene concentrations were prepared by high-pressure radical- the polymerization method. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 0.016 g of styrene (1.54x10^ mol) for Cl-1 and 0.164 g of styrene (1.58x10"<3> mol) for Cl-2, respectively.]

C2-1: Ethylene styrene-maleic anhydride random copolymer (Et-St-MAn)

[prepared by introducing styrene monomer and maleic anhydride into an autoclave equipped with a stirrer and then reacting for polymerization after adding ethylene to the mixture. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 0.121 g of styrene (1.16x10 3 mol) and 0.107 g of maleic anhydride (1.09x10"<3> mol).]

C2-2: Maleic anhydride-modified ethylene styrene copolymer (MAn-Et-St)

[prepared by adding dicumyl peroxide to an ethylene-styrene copolymer prepared by the high-pressure radical polymerization method and reacted with maleic anhydride. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 0.3616 g of styrene (3.4x10"4 mol) and 2.0x10"3 g of maleic anhydride (2.04x10^ mol).]

C2-3: Polypropylene copolymer (PP-St-MAn) [prepared by causing polypropylene to react with styrene and maleic anhydride, using a radical generating agent. When examined by infrared spectroscopic analysis, 1 g of the polymer contained 4.03x10"3 g of styrene (3.86xl0~<5> mol) and 1.96xl0"<3> g of maleic anhydride (1.94xl0 5 mol).]

[EXAMPLES 1 THROUGH 38]

[COMPARISON EXAMPLES 1 THROUGH 8]

For each example, a set of chemicals selected from components (A), (B) and (C) was used in a ratio as indicated in one of Tables 1-1 through 1-3. In addition, 2 parts by weight of dicumyl peroxide were used as cross-linking agent in each example and each comparative example, except examples 31-43. The ingredients were melted and mixed in a two-axis extruder. The liquid and modified test pieces were introduced through a bent inlet.

The polymer was then pressed into a foil and tested for volume resistivity, impact strength and water wood.

Figure 4 is a graph showing the performance of Examples 1 through 6 and the performance of Comparative Examples 1, 4 and 5, expressed as the ratio of functional group concentration (horizontal axis) to volume resistivity (vertical axis). The markings 0 and • are the results of the examples according to the present invention, and the other markings □ and ■ are the results of the comparative examples. Furthermore, the markings 0 and □ shall be the results at room temperature and the other markings • and ■ are the results at 90°C.

It can be seen that the volume resistivity of examples 1 to 6 was significantly improved at both room temperature (RT) and 90°C by introducing the functional groups B2-1 into the mixtures. Comparative example 1 where no functional group was introduced, and comparative examples 4 and 5 where the polymer was simply modified with the functional group A2-1, on the other hand, have poor volume resistivity which was noticeably changed between RT and 90°C.

Figure 5 is a graph showing the performance (marks 0, •, _ and A) of Examples 7 through 10, and 13 through 15 and the performance (the other marks □ and ■) of Comparative Examples 1, 4, 5 and 10. Note that the volume resistivity was improved by using A1-1 and A2-1 for (A) and also by adding B2-1 to Cl-1.

[MANUFACTURE OF TEST CABLES]

Sample cables with a configuration as shown in Figure 6 were made using the material of Example 3. When tested, they gave remarkably good results.

In Figure 6 illustrating a sample cable, 41 denotes a conductor which is a bundle of conductive metal wires, 42 denotes an inner semi-conductive layer, 43 denotes an insulating polymer material layer, 44 denotes an outer semi-conductive layer, 45 denotes an aluminum foil and 46 denotes a protective layer (made of polyolefin containing an inorganic flame retardant).

[BENEFITS ACCORDING TO THE INVENTION]

The present invention provides an electrically insulating polymer material comprising an olefinic polymer as a main component and also

comprising specific functional group-containing monomer units and/or aromatic ring-containing monomer units. Such a polymer material is excellent with respect to volume resistivity, breakdown strength and other electrically insulating properties. It is also sensitive to cross-linking, and if cross-linked, it will not lose its remarkable volume resistivity, impact strength and other electrical

insulating properties. A polymer mixture according to the present invention and a cross-linked material obtained from such a mixture can thus be used to form an insulating layer for wires or cables in general, and in particular for direct current power cables.

A polymer material according to the present invention finds a number of different applications including insulating materials for wires or cables in general, in particular for direct current power cables to be used for electrical appliances, transport equipment, facilities and factories.

Claims (16)

1. Electrically insulating polymer material comprising an olefinic polymer as the main component and functional group-containing monomeric units of the monomers Dl through D4 in an amount of • 5x10" -7 ti• l 5x10* *3 mol per g of the mixture, and also comprising ethylenic bonds in an amount of not less than 0.8 per 1,000 carbon atoms and/or aromatic ring-containing monomer units of the monomer D5 in an amount of 5xl0"<7> to 5xl0"3 mol per g of the mixture, characterized in that said monomers Dl to and with D5 has an ethylenic bond, said monomer D1 is a carbonyl group or carbonyl group derivative containing monomer, said monomer D2 is a hydroxyl group containing monomer, said monomer D3 is a nitro group containing monomer, said monomer D4 is a nitrile group containing monomer, and said monomer D5 is a aromatic ring-containing monomer. 2. Electrically insulating polymer material according to claim 1, characterized in that said material comprises a polymer that has said functional group-containing monomers Dl through D4, a polymer that has monomeric units containing two or more ethylenic bonds and/or a polymer that has said aromatic ring-containing monomer D5. 3. Electrically insulating polymer material according to claim 1, characterized in that said material comprises at least one of component A), component (B) and component (C) as defined below: Component (A): at least one of (A1) and (A2) below; (A1): an olefinic polymer, and (A2): an arbitrary copolymer of olefin and at least one of said functional group-containing monomers Dl through D5 inclusive or a graft copolymer obtained by modification of an olefinic polymer with at least one of said functional group-containing monomers Dl through D5 inclusive, Component (B): at least one of (Bl) through (B3) below ; (Bl): a homopolymer derived from a monomer containing two or several ethylenic bonds or a copolymer of said monomer and ethylene, (B2): an arbitrary copolymer of a monomer containing two or several ethylenic bonds and at least one of said functional group-containing monomers Dl to D5 inclusive, an arbitrary copolymer of a monomer containing two or more ethylenic bonds, ethylene and at least one of said functional group-containing monomers Dl to D5 inclusive or a graft copolymer obtained by modification of a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with at least one of said functional group-containing monomers Dl through D5, and (B3): a compound containing two or more ethylenic bonds, Component ( C): at least one of (Cl) and (C2) below; (Cl): an olefinic polymer containing at least one aromatic ring, and (C2): an arbitrary copolymer of olefin, at least one of said functions group-containing monomers Dl to D4 inclusive, and said aromatic ring-containing monomer D5 or a graft copolymer obtained by modification of an olefinic polymer containing aromatic rings with at least one of said functional group-containing monomers Dl to D4 inclusive, or a graft copolymer obtained by modification of an arbitrary copolymer of olefin and at least one of said functional group-containing monomers Dl through D4 with said aromatic ring-containing monomer D5. 4. Electrically insulating polymer material according to claim 1, characterized in that said material comprises: (Al) an olefinic polymer, (B21) a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with said functional group-containing monomer D1. 5. Electrically insulating polymer material according to claim 1, characterized in that said material comprises: (Al) an olefinic polymer, (A21) a graft copolymer obtained by modifying an olefinic polymer with said functional group-containing monomer D1, and (B21) a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with said functional group-containing monomer D1. 6. Electrically insulating polymer material according to claim 1, characterized in that said material comprises: (Al) an olefinic polymer, (A21) a graft copolymer obtained by modifying an olefinic polymer with said functional group-containing monomer D1, and (B3) a compound with two or more ethylenic bonds. 7. Electrically insulating polymer material according to claim 1, characterized in that said material comprises: (B21) a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic bonds or a copolymer of said monomer and ethylene with said functional group-containing monomer D1, and (Cl) an olefinic polymer containing at least one aromatic ring. 8. Electrically insulating polymer material according to claim 1, characterized in that said material comprises: (A21) a graft copolymer obtained by modifying an olefinic polymer with said functional group-containing monomer Dl, and (Cl) an olefinic polymer containing at least one aromatic ring. 9. Electrically insulating polymer material according to claim 1, characterized in that said material comprises: (Cl) an olefinic polymer containing at least one aromatic ring, and (C21) a graft copolymer obtained by modifying an olefinic polymer with said functional group-containing monomer Dl. 10. Electrically insulating polymer material according to claim 3, characterized in that the olefinic polymer as component (A) is selected from the group consisting of polyethylene produced by the high-pressure polymerization method, high-density polyethylene, medium-density polyethylene, linear low-density polyethylene or polypropylene. 11. Electrically insulating polymer material according to claim 3, characterized in that the homopolymer derived from a monomer with two or more ethylenic bonds or the copolymer of said monomer and ethylene as component (B) is selected from the group consisting of liquid polybutadiene, ethylene acrylic acid copolymer or ethylene vinyl methacrylate copolymer. 12. Electrically insulating polymer material according to claim 1, characterized in that the functional group-containing monomer D1 is (meth)acrylic acid or maleic anhydride. 13. Electrically insulating polymer material according to claim 3, characterized in that the olefin polymer containing at least one aromatic ring as component (C) is an arbitrary copolymer of ethylene styrene or a graft copolymer obtained by modifying an olefin copolymer with said aromatic ring-containing monomer D5. 14. Use of an electrically insulating polymer mixture according to claim 1, or a material produced by cross-linking said polymer mixture, for the production of wire or cable with an insulating layer. 15. Use of an electrically insulating polymer mixture according to claim 1 or a material produced by cross-linking said polymer mixture, for the production of direct current power cable with an insulating layer 16. Use of a material produced by cross-linking an electrically insulating polymer mixture according to any one of claims 1 to 7, for the production of wire or cable with an insulating layer.