Electrically insulating polymer composition and wire or cable using the same

Abstract: An improved electrically insulating polymer composition is provided, wherein said polymer composition comprising an olefinic polymer as a principal ingredient and functional group containing monomeric units of monomers D1 through D4 at a rate of 5x10⁻⁷ to 5x10⁻³ mols per gram of the composition, and also comprising ethylenic linkages at a rate of not smaller than 0.8 per 1,000 carbon atoms and/or aromatic ring containing monomeric units of monomer D5 at a rate of 5x10⁻⁷ to 5x10⁻³ mols per gram of the composition, wherein said monomers D1 through D5 have an ethylenic linkage, the monomer D1 is a carbonyl group or carbonyl group derivative containing monomer, the monomer D2 is a hydroxyl group containing monomer, the monomer D3 is a nitro group containing monomer, the monomer D4 is a nitrile group containing monomer, and the monomer D5 is an aromatic ring containing monomer. And there is also provided a wire or a cable having an electrically insulating polymer composition as defined above or an insulating layer made of such a composition or a material obtained by crosslinking such a composition.

Description: This invention relates to an electrically insulating polymer composition and a wire or a cable using such a composition. More particularly, the present invention relates to a composition that provides an excellent electrical insulator in terms of volume resistivity, breakdown strength and other electrical properties. It also relates to a polymer composition for providing an excellent electrical insulator that does not degrade in terms of volume resistivity, breakdown strength and other electrical properties as well as to a wire, a cable or a DC power cable comprising an electrically insulating polymer composition or an insulation layer formed by such a composition or a material obtained by crosslinking such a composition. 2. Background Art Insulating materials for wires or cables are, by definition, required to show a high volume resistivity, a high breakdown strength, a low dielectrical constant and a low loss tangent and typically made of polyethylene. While oil-fill insulators (hereinafter referred to as OFs) are used for large capacity power supply cables and provide satisfactory electrical insulation, the OF is accompanied by certain draw-backs including that it should be constantly supplied with oil to offset the quantity of oil that leaks out of it. In recent years, crosslinked olefinic polymers such as crosslinked polyethylene are getting popularity as they show an enhanced level of thermal resistance and physical strength. Techniques for crosslinking include electron beam induced crosslinking and chemical crosslinking using peroxides and the like, of which the former requires large equipment and hence costly. While chemical crosslinking is less costly, crosslinking agents can remain in the products and gradually reduce the volume resistivity of the products , and give rise to water trees and other problems. In an attempt to improve the electrical insulation of insulating materials and eliminate the problem of water trees, Japanese Patent Publication No. 5-15007 proposes a method of introducing polyolefin that is modified with maleic anhydride into polyethylene for the purpose of introducing hydrophilic groups. Japanese Patent Laid-open Publication No. 4-11646 discloses a method of improving the electrical insulation of an insulating material by introducing double bonds in advance into polyolefin to be crosslinked and thereby reducing the rate of the use of crosslinking agent. However, neither of these methods operates satisfactorily for the improvement of volume resistivity and other aspects of electrical insulation and thermal resistance. The inventors of the present invention had found sometime ago that electrically highly insulating materials can be produced by using maleic anhydride at a very limited rate. While these materials surpass normally crosslinked polyolefinic materials by far for electrical insulation, they are still not satisfactory in terms of volume resistivity and other properties of electrical insulation. Meanwhile, methods have been proposed concerning the use of carboxylic compounds and/or aromatic compounds with polyolefin in order to improve the electrical insulation of the final product. Examples of such proposed methods include the use of polystyrene grafted to polyolefin in order to improve the impulse breakdown strength of the final product (Japanese Patent Publication No. 2-165506), the use of a blend of polyethylene and polystyrene also in order to improve the impulse breakdown strength of the final product (Japanese Patent Laid-open Publication No. 63-301427), the use of a blend of polyolefin and polystyrene modified with maleic anhydride in order-to improve the electrical insulation of the final product (Japanese Patent Laid-open Publication No. 62-100909) and the use of a blend of polyolefin and aromatic carboxylic acid in order to improve the breakdown strength of the final insulating product (Japanese Patent Laid-open Publication No. 60-23904). A polymer composition having improved insulating properties is further described in EP-A-0 463 402. This composition comprises an ethylene (co)polymer containing polar groups having a dipole moment of more than 0.8 debye which may be selected from ketone groups, nitrile groups and nitro groups. A specific example of such copolymer is an ethylene-acrylonitrile copolymer. However, any of these methods cannot satisfactorily improve an insulating material of the type under consideration in terms of both the volume resistivity and the breakdown strength. They are not satisfactory either in terms of improvement in the electrical insulation of the material after crosslinking. It is therefore an object of the present invention to provide a polymer composition that operates as an excellent electrical insulator in terms of volume resistivity, breakdown strength and other electrical properties. It is also an object of the invention to provide a composition capable of being crosslinked to a large extent that operates as an excellent electrical insulator that does not degrade after crosslinking in terms of volume resistivity, breakdown strength and other electrical properties as well as to a wire, a cable or a DC power cable having an insulation layer formed by such a composition or a material obtained by crosslinking such a composition. SUMMARY OF THE INVENTION As a result of intensive research efforts, the inventors of the present invention achieved the present invention. According to an aspect of the invention, the above objects are achieved by providing an electrically insulating polymer composition comprising an olefinic polymer and at least one type of functional group containing monomer units selected from monomer units derived from monomers D1 through D3 at a rate of 5x10 -7 to 5x10 -3 mols per gram of the composition, and also comprising ethylenic linkages at a rate of not smaller than 0.8 per 1,000 carbon atoms and/or aromatic ring containing monomer units derived from monomer D4 at a rate of 5x10 -7 to 5x10 -3 mols per gram of the composition, wherein said monomers D1 through D4 have an ethylenic linkage, 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 nitrile group containing monomer, and said monomer D4 is an aromatic ring containing monomer. According to a second aspect of the invention, there is provided a wire or a cable having an electrically insulating polymer composition as defined above or an insulating layer made of such a composition or a material obtained by crosslinking such a composition. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a) and (b) are schematic illustrations of an electrode system used for testing the volume resistivity of specimens of a composition according to the present invention, wherein Fig. 1(a) is a plan view of the electrode system and Fig. 1(b) is a sectional view thereof. Figure 2 is a schematic illustration of an electrode system used for testing the breakdown strength of specimens of a composition according to the present invention. Figure 3 is a schematic illustration of an water tree observing system used for testing specimens of a composition according to the present invention. Figure. 4 is a graph showing the performances of Examples 1 through 6 according to the present invention and those of Comparative Examples 1, 4 and 5, expressed in terms of the relationship between the functional group concentration (horizontal axis) and the volume resistivity (vertical axis). Figure 5 is a graph showing the performances of Examples 7 through 10 according to the present invention and 13 through 15 and those of Comparative Examples 1, 4, 5 and 10. Figure 6 is a schematic cross sectional view of a cable according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, the invention will be described in greater detail. For the purpose of the above first aspect of the invention and for introducing said functional group containing monomers and said ethylenic linkages into said composition, said electrically insulating polymer composition preferably comprises a polymer having said functional group containing monomers D1 through D3, a polymer having monomeric units containing two or more ethylenic linkages and/or a copolymer having said aromatic ring containing monomer D4. Particularly, for the purpose of the above first aspect of the invention, said composition may preferably 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): a random copolymer of olefin and at least one of said functional group containing monomers D1 through D4 or a graft copolymer obtained by modifying an olefinic polymer with at least one of said functional group containing monomers D1 through D4, component (B): at least one of (B1) through (B3) below; (B1): a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene, (B2): a random copolymer of a monomer containing two or more ethylenic linkages and at least one of said functional group containing monomers D1 through D4, a random copolymer of a monomer containing two or more ethylenic linkages, ethylene and at least one of said functional group containing monomers D1 through D4 or a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene with at least one of said functional group containing monomers D1 through D4, and (B3): a compound containing two or more ethylenic linkages, component (C): at least one of (C1) and (C2) below; (C1): an olefinic polymer containing at least one aromatic ring, and (C2): a random copolymer of olefin, at least one of said functional group containing monomers D1 through D3, and said aromatic ring containing monomer D4 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 D3, or a graft copolymer obtained by modifying a random copolymer of olefin and at least one of said functional group containing monomers D1 through D3 with said aromatic ring containing monomer D4. For the purpose of the present invention, said olefinic polymer of the component (A1) is a homopolymer or copolymer of hydrocarbons expressed by general formula C n H 2n which can 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 and so on. Examples of the olefinic polymers that can be used for the purpose of the present invention include high, medium or low density polyethylene, linear low density polyethylene, very low density polyethylene, polypropylene, polybutene, polypentene, poly-4-methyl-1-pentene, ethylene-α-olefin copolymer, ethylene-propylene copolymeric 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 preferable. For the purpose of the present invention, a functional group containing monomers having an ethylenic linkage may be the monomer D1: carbonyl group or carbonyl group derivative containing monomer, the monomer D2: hydroxyl group containing monomer, the monomer D3: nitrile group containing monomer and the monomer D4: aromatic ring containing 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 vinylester monomers. Examples of unsaturated carboxylic acid 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 vinylester that 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 trifulorate, of which vinyl acetate is most preferable. Examples of acid anhydrides to 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 maleic anhydride is most preferable. Other monomers that can be used for the purpose of the present invention include carbon monoxide, methylvinylketone, isopropenylvinylketone, ethylvinylketone, phenylvinylketone, t-butylvinylketone, isopropylvinylketone, methylpropenylketone, methylisopropenylketone and cyclohexylvinylketone. Examples of the hydroxyl group containing monomer D2 include vinylalcohol, 1-hydroxypropyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate and hydroxyethyl(meth)acrylate. Examples of the nitrile group containing monomer D3 include acrylonitrile, methacrylonitrile, α-methoxyacrylonitrile, vinylidenecyanide, cinnamonitrile, crotononitrile, α-phenylcrotononitrile, fumaronitrile, arylacetonitrile, 2-butenenitrile and 3-butenenitrile. For the purpose of the present invention, the aromatic ring containing monomer D4 is a compound containing a monocyclic or polycyclic aromatic ring and having ethylenic linkages. Examples of said aromatic ring containing monomer D4 that 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, aryl benzoate, vinylnaphthalene, 4-phenyl-1-butene, benzil methacrylate, 1,1-diphenylethylene, 4-phenyl-1-tolylethylene, 1-phenyl-1-styrylethane, 1-tolyl-1-styrylethane, 2,4-diphenyl-1-butene, 2,4-diphenyl-1-pentene and 2,4-diphenyl-4-methyl-1-pentene. Of the above examples, styrene is most preferably in terms of the electrical performance of the final product. The rate at which the functional group containing monomers need to be between 5x10 -7 and 5x10 -3 mols, preferably between 1x10 -6 and 1x10 -4 mols and most preferably between 1x10 -6 and 1x10 -5 mols for each one gram of polymer composition not containing any crosslinking agent. If the concentration of the functional groups of the monomers of D1 through D3 including maleic anhydride is less than 5x10 -7 mols per one gram of the polymer composition, the volume resistivity of the final product would not be improved. If the same is true with the functional group of the monomer of D4, the breakdown strength of the final product would not be improved either. If, on the other hand, the concentration is greater than 5x10 -3 mols per one gram of polymer composition, the volume resistivity of the final product is degraded. Copolymers that can be used for a random copolymer of olefin and at least one of said functional group containing monomers D1 through D4 or a graft copolymer obtained by modifying an olefinic polymer with at least one of said functional group containing monomers D1 through D4 as defined for the component (A2) include 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-methylvinylketone copolymer, ethylene-methylisopropenylketone copolymer, ethylene-hydroxyethyl(meth)acrylate 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 linkages or a copolymer of said monomer and ethylene as defined for the component (B1) needs to contain double bonds to a sufficient rate after polymerization. According to the existence of the ethylenic linkages, the final product shows a satisfactory crosslinked effect. For 5 the component (B), a liquid oligomer or copolymer having an average molecular weight of 1,000 to 200,000 will be used. A homopolymer derived from a monomer containing two or more ethylenic linkages as defined in the former half of (B1) is a diene polymer having 4 to 10 carbon atoms. The diene polymer may be cyclic or straight chain so long as it has two or more double bonds. However, butadiene oligomer or polybutadiene having an average molecular weight of 1,000 to 20,000 is most preferable because it is excellent in terms of 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 that can be prepared from dienes by polymerization can also be used. Of the above candidate compounds, monomer of 1,3-butadiene is most preferable because of its crosslinking effect. Compounds that can be used a random copolymer of monomer containing two or more ethylenic linkages and ethylene as defined in the latter half of (B1) include ethylene-aryl(meth)acrylate copolymer and ethylene-vinyl(meth)acrylate copolymer. Alternatively, a mixture of two or more of such copolymers may be used. Examples of a random copolymer of a monomer containing two or more ethylenic.linkages and at least one of said functional group containing monomers D1 through D4, a random copolymer of a monomer containing two or more ethylenic linkages, ethylene and at least one of said functional group containing monomers D1 through D4 or a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene with at least one of said functional group containing monomers D1 through D4 as defined in (B2) include polybutadiene modified with acrylic acid or maleic anhydride; maleic anhydride-butadiene copolymer, maleic acid-modified ethylene-aryl(math)acrylate copolymer and maleic acid-modified ethylene-vinyl(meth)acrylate copolymer. Of these, polybutadiene modified with acrylic acid or maleic anhydride is most preferable. Examples of compounds that can be used for a compound containing two or more ethylenic linkages as defined in (B3) include methacrylate monomers having a number of functional groups such as trimethylolpropane trimethacrylate, ethyleneglycol dimethacrylate and diethyleneglycol dimethacrylate; vinyl monomers having a number of functional groups such as triarylisocyanurate, diarylphthalate and vinylbutyrate; bismaleimides such as N, N'-m-phenylenebismaleimide and N,N'-ethylenbismaleimide; dioximes such as P-quinonedioxime; divinyl compounds such as divinylbenzene, 1,5-hexadiene-3-in, hexatriene, divinylether, divinylsulfone; and diaryl compounds such as arylstyrene, 2,6-diacrylphenol, diarylcarbinol. For component (B) defined as (B1), (B2) or (B3), before being crosslinked, the composition contains at a rate not less than 0.8 double bonds per 1,000 carbon atoms. Preferably, the number of double bonds is between 0.8 and 4.0 per 1,000 carbon atoms. In addition, when containing the double bonds, preferably the number of terminal vinyl group is between 0.5 and 3.0 per 1,000 carbon atoms. An olefinic polymer containing at least one aromatic ring as defined in (C1) above is a copolymer of a monomer containing a monocyclic or polycyclic aromatic ring and olefin. For the purpose of the present invention, a monomer containing at least one aromatic ring is preferably an aromatic compound having a mono-, di- or tricyclic aromatic ring and selected from compounds including styrene, styrene derivatives, arylbenzene, arylbiphenyl, methylstyrene, aryl benzoate, vinylnaphthalene, 4-phenyl-1-butene, benzyl methacrylate, 1,1-diphenylethylene, 1-phenyl-tolylethylene, 1-phenyl-l-styrylethane, 2,4-diphenyl-1-butene, 2,4-diphenyl-1-pentene and 2,4-diphenyl-4-methyl-1-pentane. Olefines that can be used in component (C1) for the purpose of the invention include α-olefines having 3 to 12 carbon atoms such as 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 preferable. A random copolymer of olefin, at least one of said functional group containing monomers D1, and said aromatic ring containing monomer D4 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 D3, or a graft copolymer obtained by modifying a random copolymer of olefin and at least one of said functional group containing monomers D1 through D3 with said aromatic ring containing monomer D4 as defined in component (C2) above is selected from compounds including ethylene-styrene-maleic anhydride random copolymer, maleic anhydride modified ethylene-styrene copolymer, ethylene-arylbenzen copolymer, maleic anhydride modified ethylene-benzyl methacrylate copolymer, and maleic anhydride modified ethylene-arylstyrene. Examples of copolymers of at least one of monomers containing a styrene derivative and ethylene that can be used for the purpose of the present invention include ethylene/vinylacetate/styrene copolymer, ethylene/vinylacetate/α-methylstyrene copolymer, ethylene/ethyl acrylate/styrene copolymer and ethylene/ethyl acrylate/α-methylstyrene copolymer. For component (C) defined as components (C1) or (C2), the concentration of said aromatic containing monomer in the composition is between 5x10 -7 and 5x10 -3 mols, preferably between 1x10 -6 and 1x10 -4 mols and most preferably between 1x10 -6 and 5x10 -5 mols per 1 gram of the polymer composition. Now, some preferable combinations of candidate components of a polymer composition according to the present invention will be described. (1) As claimed in claim 4, a combination of an olefinic polymer (A1) as component (A) and a graft copolymer (B21) obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene with a functional group containing monomer D1 as component (B) is provided. An example of such a combination is obtained from unmodified low density polyethylene (A1) obtained by high pressure radical polymerization and maleic anhydride modified liquid polybutadiene (B21). (2) As claimed in claim 5, 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 graft copolymer (B21) obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene with a functional group containing monomer D1 as component (B) is provided. An example of such a combination is obtained from unmodified low density polyethylene (A1) obtained by high pressure radical polymerization, maleic anhydride modified low density polyethylene (A21) and maleic anhydride modified liquid polybutadiene (B21). (3) As claimed in 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) having two or more ethylenic linkages as component (B) is provided. An example of such a combination is obtained from unmodified low density polyethylene (A1) obtained by high pressure radical polymerization, maleic anhydride modified low density polyethylene (A21) and divinylbenzene (B3). (4) As claimed in claim 7, a combination of a graft copolymer (B21) obtained by modifying a homopolymer derived from a monomer having two or more ethylenic linkages or a copolymer of said monomer and ethylene with a functional group containing monomer D1 as component (B) and an olefinic polymer (C1) containing at least one aromatic ring as component (C) is provided. An example of such a combination is obtained from maleic anhydride modified liquid polybutadiene (B21) and ethylene-styrene random copolymer (C1). (5) As claimed in 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 (C1) containing at least one aromatic ring as component (C) is provided. An example of such a combination is obtained from maleic anhydride modified low density polyethylene (A21) and ethylene-styrene random copolymer (C1). (6) As claimed in claim 9, a combination of an olefinic polymer (C1) 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 (C1) and ethylene-styrene-maleic anhydride graft copolymer (C21). For the purpose of the present invention, while a composition according to the present invention may well be used by itself as an electrically insulating material, it is preferable to crosslink it in order to improve the volume resistivity and the breakdown strength. While any appropriate crosslinking techniques 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 preferable 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 benzoylperoxide, laurylperoxide, dicumylperoxide, t-butylhydroperoxide, α,α-bis(t-butylperoxideisopropyl)benzene, di-t-butylperoxide, 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, 2,3-diethyl-2,3-di(p-methylphenyl)butane and 2,3-diethyl-2,3-di(bromophenyl)butane. In the crosslinking process, the volume resistivity and other electrical properties of the composition can be improved by crosslinking the composition after preparing a given amount of component (B). The radical generating agents for modifying component (B) with an functional group containing monomer as described above may mostly used as crosslinking agents for the purpose of the present invention. Of the crosslinking agents listed above, dicumylperoxide, 2,5-di-(t-butylperoxy)hexane and 2,5-di-(t-butylperoxy)hexene are recommendable. The crosslinking agent is used by an amount between 0.01 to 5 parts by weight, preferably between 0.1 and 3 parts by weight, for a total of 100 parts by weight of the composition. The composition according to the present invention may include one or more chemicals selected from inorganic fillers, organic fillers, antioxidants, lubricants, organic or inorganic pigments, ultraviolet preventing agents, photostabilizers, dispersing agents, anti-copper-corrosion agents, neutralizing agents, plasticizers, and nucleating agents. An insulating polymer composition according to the present invention can be used for insulating materials for wires, cables and capacitors, for the insulation of high voltages areas of X-ray generators and cables and other applications. A wire or a cable according to the present invention is one comprising an insulation layer made of a polymer composition according to the present invention or a composition obtained by crosslinking said composition. More specifically, a wire or a cable according to the present invention is one comprising an insulation layer covering its conductor. If necessary, a bundle of wires may be used for the conductor and a semiconducting layer may be arranged between the conductor and the insulation layer. A flame retarding polymer layer may be formed outside the insulation layer. For the purpose of the present invention, a wire or a cable may be prepared by coating a bundle of copper wires with a polymer composition containing carbon powder or metal powder to form semiconducting layer, forming thereon an insulation layer of a polymer composition according to the present invention, arranging thereon a coat of metal sheet or still another semiconducting layer and then forming an outermost coat of a flame retarding or rat repellent polymer layer. Alternatively, a wire or a cable may be prepared by combining several to tens of several covered copper wires, each comprising a single copper wire coated with a semiconducting layer of a polymer composition containing carbon or metal powder and an insulation layer of a polymer composition according to the present invention, and forming an outermost coat of a flame retarding or rat repellent polymer layer. A polymer composition according to the present invention is particularly effective for high voltage and can be suitably used for DC power cables. [Functions] The present invention provides an electrically insulating polymer composition comprising an olefinic polymer as a principal ingredient and a functional group containing monomer, and also comprising ethylenic linkage and/or an aromatic ring containing monomer. The functional group operates as a trap site that blocks the movement of electrical charges and consequently improve the volume resistivity of the composition. The aromatic ring has an effect of taking up highly energized electrons, causing them to emit their energy in the form of heat and releasing them with a low energy level (electron energy absorption effect). Thus, the energy level of highly energized electrons that can trigger electrical breakdown can be significantly reduced to enhance the breakdown strength of the composition. The electrical properties of an insulating material prepared from a polymer composition according to the present invention can be improved by crosslinking particularly in terms of volume resistivity, the phenomenon of water tree and breakdown strength. The double bonds in a diene polymer improves the efficiency of crosslinking since they operates as crosslinking sites. The decomposition residues of the crosslinking agent are taken up in the main chain of the polymer. The decomposition residues floating in the bulk can reduce the volume resistivity as they are subjected to ionic decomposition to produce electrical charges by heat and the electrical field. Such a detrimental effect of decomposition residues can be prevented by taking them up into the main chain of the polymer and consequently the volume resistivity of the composition is improved. The degradation in the electrical properties of an insulating material can be effectively avoided by the above three effects of the present invention. Additionally, the volume resistivity and the breakdown strength of the composition are remarkably improved after crosslinking. Now, the present invention will be described further by way of examples. (Test Methods) (1) Test Method for Volume Resistivity An electrode system as shown in Figs. 1(a) and 1(b) was used. The specimens were tested by applying a DC voltage of 3,300 V at room temperature and at 90°C in a nitrogen atmosphere. A vibrating reed type ampere meter (TR8411 available from Advantest, Co.,Ltd.) was used for gauging 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 start of applying the voltage. Each specimen was tested four times and the average of the readings was used. In Figs. 1(a) and 1(b) illustrating the electrode system, 11 denotes a main electrode (50), 12 denotes a guard electrode (inner diameter 75, outer diameter 80), 13 denotes a specimen and 14 denotes a high voltage electrode (80). (2) Test Method for Breakdown Strength An electrode system having fixed electrodes or so-called McKewon electrodes as shown in Fig. 2 was used. The electrode system comprises a substrate 24 made of polymethylmethacrylate and bored at the center to a diameter of 1/2 inch and electrodes 21 of stainless steel balls with a diameter of 1/2 inch. Each specimen 22 was cut to a dice of 8 to 10 mm and arranged between the electrodes. The gap between the specimen and the electrodes was filled with deaerated epoxy resin, which was then hardened. The McKewon electrodes were dipped into silicon oil contained in a vessel, which was then put into a thermostat for observation. The voltage used for this breakdown test had an impulse waveform of 1.2/50 µS and negative polarity. The waveform was observed by means of an oscilloscope and the reading of the wave crest where breakdown occurred was recorded for each test. The average of more than twenty readings was used. In Fig. 2 illustrating the gauging system for the breakdown strength test, 21 denotes a pair of stainless steel balls, 22 denotes a specimen, 23 denotes epoxy resin and 24 denotes a substrate made of polymethylmethacrylate. (3) Observation of Water Trees A water tree observing system having a configuration as described in Fig. 3 was used. A voltage of 10 kV and 10 kHz was applied to each specimen at room temperature. After the application of the voltage, the water tree was pigmented and microscopically observed. The following ratings were used. strongly observable x (poor) mildly observable Δ (good) slightly observable o (excellent) In Fig. 3 illustrating the water tree observing system, 31 denotes specimens being observed water trees, 32 denotes a conducting plate, 33 denotes water, 34 denotes a grounding electrode, 35 denotes vessels and 36 denotes a voltage applying electrode. [Materials] Component (A): polyolefin A1-1: high pressure radical method polyethylene (LD) [density=0.919g/cm 3 , MI=1.0g/10min. trade name: Nisseki Rexlon W2000 - available from Nippon Petrochemicals Co., Ltd.] A1-2: linear low density polyethylene (LL) [density=0.921g/cm 3 , MI=1.0g/10min. trade name: Nisseki Linirex AF3280 - available from Nippon Petrochemicals Co., Ltd.] A1-3: polypropyrene (PP) [density=0.905g/cm 3 , MI=1.5g/10min. trade name: Nisseki Polypro F120K - available from Nippon Petrochemicals Co., Ltd.] A2-1: maleic anhydride modified high pressure radical method polyethylene (MAn-LD) [A 0.75 parts by weight of maleic anhydride and a 0.05 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxide) were added to a 100 parts by weight of LD and they were preliminary mixed by means of a Henschel mixer and made to react at 200°C in a biaxial extruder. When examined by infrared spectroscopic analysis, a 1 g of the polymer contained maleic anhydride by 2.66x10 -5 mols.] A2-2: acrylic acid modified high pressure radical method polyethylene (Ac-LD) [After compounding acrylic acid and dicumylperoxide with LD, they were made to react at 200°C in an extruder. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained acrylic acid by 3.38x10 -6 mols.] A2-3: ethylene-maleic anhydride random copolymer (Et-MAn) [A 380g of n-hexane and a 11g of maleic anhydride in acetone solution (0.11g of maleic anhydride) were put in an 3.8 liter autoclave provided with a stirrer and having a plenum of nitrogen and 2,5-dimethyl-2,5-di(tert-butylperoxide) was added thereto as a polymerization starter. Thereafter, a 1.700 g of ethylene was introduced therein and the mixture was polymerized at 1,600kgf/cm 2 and 190°C for an hour to produce an ethylene-maleic anhydride copolymer. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained maleic anhydride by 1.58x10 -3 mols.] A2-4: ethylene-vinyl alcohol random copolymer (Et-VOL) [Prepared in a manner same as above. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained the alcohol group by 2.4x10 -4 mols.] A2-5: ethylene-nitrostyrene copolymer (Et-Nst) (not according to the invention) [Prepared in a manner same as above. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained the nitro group by 2.38x10 -4 mols.] A2-6: ethylene-acrylonitrile copolymer (Et-AN) [Prepared in a manner same as above. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained the nitro group by 2.38x10 -4 mols.] Component (B): B1-1: polybutadiene (PB1) [average molecular weight 3,000, specific gravity 0.89g/cm 3 trade name: Nisseki Butadiene B-3000 - available from Nippon Petrochemicals Co., Ltd.] B1-2: butadiene polymer (PB2) [specific gravity 0.89g/cm 3 trade name: JSR RB820 - available from Japan Synthetic Rubber Co., Ltd.] B1-3: copolymer of 1,3-butadiene and cyclododeca-1,5,9-hexatriene (PB3) [1,3-butadiene and cyclododeca-1,5,9-hexatriene were used at a molar ratio of 9:1 in the presence of toluene and a 3-dimensional catalyst of cobalt triacetylacetate, triethylaluminum and water and made to react for polymerization at 7°C. Thereafter, the low molecular components were removed to produce a copolymer having an average molecular weight of 2,500. When examined by infrared spectroscopic analysis, the vinyl bonding amounted to 43%.] B1-4: liquid polyisoprene (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-PB1) [average molecular weight 3,000, specific gravity 0.89g/cm 3 When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained maleic anhydride by 4.32x10 -4 mols. trade name: Nisseki Polybutadiene M-2000 - available from Nippon Petrochemicals Co., Ltd.] B2-2: acrylic acid modified polybutadiene (MAn-PB1) [average molecular weight 2,000, specific gravity 0.91g/cm 3 When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained acrylic acid by 2.20x10 -4 mols. trade name: Nisseki Polybutadiene M-2000 - available from Nippon Petrochemicals Co., Ltd.] B2-3: a mixture of ethylene-vinylalcohol copolymer (EVOL) (Prepared through copolymerization and hydrolysis of ethylene and vinylester. A 1 gram of the copolymer contained the residual alcohol group by 1.13x10 -3 mols.) and polybutadiene (PB1) B2-4: maleic anhydrides-butadiene copolymer (MAn-PB3) [Prepared through heating and modification in an autoclave. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained maleic anhydride by 8.95x10 -5 mols.] B2-5: maleic anhydrides modified liquid polyisoprene (MAn-PI) [Prepared through heating and modification in an autoclave. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained maleic anhydride by 2.87x10 -4 mols.] B2-6: acrylic acid modified polybutadiene (MAn-PB1) [Prepared through heating and modification in an monoaxial extruder. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained acrylic acid by 3.25x10 -4 mols.] B2-7: maleic anhydrides modified butadiene polymer (MAn-PB2) [Prepared through heating and modification in an monoaxial extruder. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained maleic anhydride by 3.44x10 -4 mols.] B3: divinylbenzene (DVB) Component (C): C1-1 & ethylene-styrene random copolymer (Et-St) [Two C1-2: samples with different styrene concentrations were prepared by the high pressure radical polymerization method. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained styrene by C1-1 0.016 g (1.54x10 -4 mols) and C1-2 0.164 g (1.58x10 -3 mols)] C2-1: ethylene-styrene-maleic anhydride random copolymer (Et-St-MAn) [Prepared by introducing styrene monomer and maleic anhydride into an autoclave provided with a stirrer and then made to react for polymerization after throwing ethylene into the mixture. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained styrene by 0.121 g (1.16x10 -3 mols) and maleic anhydride by 0.107 g (1.09x10 -3 mols).] C2-2: maleic anhydride modified ethylene-styrene copolymer (MAn-Et-St) [Prepared by adding dicumylperoxide to an ethylene-styrene copolymer produced by the high pressure radical polymerization method and caused them to react with maleic anhydride. When examined by infrared spectroscopic analysis, a 1 gram of the polymer contained styrene by 0.3616 g (3.4x10 -4 mols) and maleic anhydride by 2.0x10 -3 g (2.04x10 -4 mols).] 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, a 1 gram of the polymer contained styrene by 4.03g (3.86x10 -5 mols) and maleic anhydride by 1.96x10 -3 g (1.94x10 -5 mols).] [Examples 1 through 38][Comparative Examples 1 through 8] For each example, a set of chemicals selected from components (A), (B) and (C) were used to a ratio as indicated in one of Table 1-1 through 1-3. In addition, 2 parts by weight of dicumylperoxide was used as a crosslinking agent in each examples and each comparative examples except for examples 31-43. The ingredients were molten and mixed in a biaxial extruder. Liquid and modified specimens were introduced through a bent inlet. Subsequently, the polymer was pressed into a sheet and tested for volume resistivity, breakdown strength and water tree. Fig. 4 is a graph showing the performances of Examples 1 through 6 and those of Comparative Examples 1, 4 and 5, expressed in terms of the relationship between the functional group concentration (horizontal axis) and the volume resistivity (vertical axis). The marks ○ and  are the results of the examples according to the present invention, and the other marks □ and ▪ are the results of the comparative ones. Further, the marks ○ and □ are the results at room temperature and the other marks  and ▪ are the results at 90°C. It will be seen that the volume resistivity of Examples 1 through 6 were remarkably improved at both room temperature (RT) and 90°C by introducing functional groups of B2-1 into the compositions. On the contrary, 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 of A2-1 have poor volume resistivities which were notably changed between RT and 90°C. Fig. 5 is a graph showing the performances (marks ○, , Δ and ▴) of Examples 7 through 10 and 13 through 15 and those (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 C1-1. [Preparation of Sample Cables] Sample cables having a configuration as shown in Fig. 6 were prepared by using the composition of Example 3. When tested, they performed remarkably well. In Fig. 6 illustrating a sample cable, 41 denotes a conducting member which is a bundle of conducting metal wires, 42 denotes an internal semiconducting layer, 43 denotes an insulating polymer composition layer, 44 denotes an external semiconducting layer, 45 denotes an aluminum foil and 46 denotes a protective member (made of polyolefin containing an inorganic flame retarding agent). [Advantages of the Invention] The present invention provides an electrically insulating polymer composition comprising an olefinic polymer as a principal ingredient and also comprising specific functional group containing monomeric units and/or aromatic ring containing monomeric units. Such a polymer composition is excellent in the volume resistivity, the breakdown strength and other electrically insulating properties. It is also sensitive to crosslinking and, if crosslinked, it does not lose its remarkable volume resistivity, the breakdown strength and other electrically insulating properties. Thus, a polymer composition according to the present invention and a crosslinked material obtained from such a composition can be used to form an insulating layer for wires or cables in general and DC power cables in particular. A polymer composition according to the present invention finds a wide variety of applications including insulating materials for wires or cables in general and DC power cables in particular to be used for electrical appliances, transportation equipment, plants and factories.

Claims: An electrically insulating polymer composition comprising an olefinic polymer and at least one type of functional group containing monomer units selected from monomer units derived from monomers D1 through D3 at a rate of 5x10 -7 to 5x10 -3 mols per gram of the composition, and also comprising ethylenic linkages at a rate of not smaller than 0.8 per 1,000 carbon atoms and/or aromatic ring containing monomer units derived from monomer D4 at a rate of 5x10 -7 to 5x10 -3 mols per gram of the composition, wherein said monomers D1 through D4 have an ethylenic linkage, 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 nitrile group containing monomer, and said monomer D4 is an aromatic ring containing monomer. An electrically insulating polymer composition according to claim 1, wherein said composition comprises a polymer having said functional group containing monomer units derived from monomers D1 through D3, a polymer having monomer units containing two or more ethylenic linkages and/or a polymer having said aromatic ring containing monomer D4. An electrically insulating polymer composition according to claim 1, wherein said composition 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): a random copolymer of olefin and at least one of said functional group containing monomers D1 through D4 or a graft copolymer obtained by modifying an olefinic polymer with at least one of said functional group containing monomers D1 through D4, component (B): at least one of (B1) through (B3) below; (B1): a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene, (B2): a random copolymer of a monomer containing two or more ethylenic linkages and at least one of said functional group containing monomers D1 through D4, a random copolymer of a monomer containing two or more ethylenic linkages, ethylene and at least one of said functional group containing monomers D1 through D4 or a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene with at least one of said functional group containing monomers D1 through D4, and (B3): a compound containing two or more ethylenic linkages, component (C): at least one of (C1) and (C2) below; (C1): an olefinic polymer containing at least one aromatic ring, and (C2): a random copolymer of an olefin, at least one of said functional group containing monomers D1 through D3, and said aromatic ring containing monomer D4 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 D3, or a graft copolymer obtained by modifying a random copolymer of olefin and at least one of said functional group containing monomers D1 through D3 with said aromatic ring containing monomer D4. An electrically insulating polymer composition according to claim 3, wherein said composition comprises; (A1) an olefinic polymer, (B21) a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene with said functional group containing monomer D1. An electrically insulating polymer composition according to claim 3, wherein said composition comprises; (A1) 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 linkages or a copolymer of said monomer and ethylene with said functional group containing monomer D1. An electrically insulating polymer composition according to claim 3, wherein said composition comprises; (A1) an olefinic polymer, (A21) a graft copolymer obtained by modifying an olefinic polymer with said functional group containing monomer D1, and (B3) a compound having two or more ethylenic linkages. An electrically insulating polymer composition according to claim 3, wherein said composition comprises; (B21) a graft copolymer obtained by modifying a homopolymer derived from a monomer containing two or more ethylenic linkages or a copolymer of said monomer and ethylene with said functional group containing monomer D1, and (C1) an olefinic polymer containing at least one aromatic ring. An electrically insulating polymer composition according to claim 3, wherein said composition comprises; (A21) a graft copolymer obtained by modifying an olefinic polymer with said functional group containing monomer D1, and (C1) an olefinic polymer containing at least one aromatic ring. An electrically insulating polymer composition according to claim 3, wherein said composition comprises; (C1) 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 D1. An electrically insulating polymer composition according to claim 3, wherein the olefinic polymer as component (A) is one selected from the group consisting of high pressure polymerization method polyethylene, high density polyethylene, medium density polyethylene, linear low density polyethylene or polypropylene. An electrically insulating polymer composition according to claim 3, wherein the homopolymer derived from a monomer having two or more ethylenic linkages or the copolymer of said monomer and ethylene as component (B) is one selected from the group consisting of liquid polybutadiene, ethylene-acrylic acid copolymer or ethylene-vinyl(meth)acrylate copolymer. An electrically insulating polymer composition according to claim 1, wherein the functional group containing monomer D1 is (meth)acrylic acid or maleic anhydride. An electrically insulating polymer composition according to claim 3, wherein the olefin polymer containing at least one aromatic ring as component (C) is a random copolymer of ethylene-styrene or a graft copolymer obtained by modifying an olefin copolymer with said aromatic ring containing monomer D4. A wire or a cable having an insulating layer formed by an electrically insulating polymer composition according to any of the claims 1 to 13 or a material prepared by crosslinking said polymer composition. A wire or a cable according to claim 14, wherein the insulating layer is formed by a material prepared by crosslinking a polymer composition according to any of the claims 1 to 13. A DC power cable having an insulating layer formed by an electrically insulating polymer composition according to any of the claims 1 to 13 or a material prepared by crosslinking said polymer composition.

Inventors: IKEDA MASAAKI, UMESHIMA YUKA, YOKOYAMA JUNICHI, IKEDA, MASAAKI, UMESHIMA, YUKA, YOKOYAMA, JUNICHI
Applicants: NIPPON PETROCHEMICALS CO LTD, NIPPON PETROCHEMICALS CO., LTD.
Application year: 1995
Main classification: H01B3
IPC codes: H01B3