# Patent

## Title
Copper body power hybrid package and method of manufacture

## Abstract

A novel electrical insulating oil and oil-filled electrical appliances that are impregnated with the novel insulating oil. The electrical insulating oil is quite suitable for use in oil-filled electrical appliances in which insulating materials or dielectric materials made of plastics are employed. The electrical insulating oil comprises (a) at least one member of alkylbiphenyls and alkylnaphthalenes and (b) at least one member of monoolefins and diolefins each having two condensed or noncondensed aromatic nuclei.

## Background
(1) Field of the Invention

This invention relates to a novel electrical insulating oil and oil-filled electrical appliances which is impregnated with the insulating oil.

More particularly, the invention relates to an electrical insulating oil and oil-filled electrical appliances in which the insulating oil comprises a mixture of alkylbiphenyl and/or alkylnaphthalene and monoolefin and/or diolefin having two aromatic nuclei. The electrical insulating oil of the invention is quite suitable for use in oil-filled electrical appliances in which insulating materials or dielectric materials made of plastics such as polyolefins are employed.

(2) Description of the Prior Art

Electrical appliances such as oil-filled capacitors, oil-filled power cables and transformers have recently been made to withstand high electric voltages while being small in size. With this tendency, various kinds of plastic films are used together with conventional insulating paper.

In the conventional art, refined mineral oils, polybutenes, alkylbenzenes, polychlorinated biphenyls and the like are used as electrical insulating oils; however, they have several drawbacks. For example, the use of polychlorinated biphenyls was discontinued because it constitutes a public health hazard that is characteristic of halogenated aromatic hydrocarbons. Furthermore, the conventional electrical insulating oils are not satisfactorily compatible with the foregoing plastic materials such as polyolefin films which are recently used in oil-filled electrical appliances.

With the requirements of high-voltage withstanding and size reduction, it is necessary that the electrical insulating oil has a high dielectric breakdown voltage, a low dielectric loss tangent, and good hydrogen gas absorbing capacity.

The hydrogen gas absorbing capacity indicates the stability of the insulating oil against corona discharge (partial discharge) under high electric voltage conditions. The higher the gas-absorbing capacity, the smaller the likelihood of corona discharge, which leads to the advantage of the insulating oil having excellent stability or durability.

Meanwhile, in order to meet the requirement of high-voltage use, plastic films such as polyolefin films, polystyrene films and polyester films are used to replace either partially or completely the conventional insulating paper as insulating materials or dielectric materials for electrical appliances such as oil-filled electric cables and capacitors. In view of their dielectric strength, dielectric loss tangent and dielectric constant, polyolefin films, especially polypropylene and cross-linked polyethylene films, are preferred as the plastic films.

When these polyolefin films are impregnated with insulating oils, some oils cause the films to swell to some extent. If a film becomes swollen, the thickness of the insulating layer increases. As a result, the resistance to the flow of insulating oil increases in electrical cables, and insufficient impregnation with insulating oil occurs in electric capacitors, causing the formation of voids (unimpregnated portions) and the undesirable lowering of the corona discharge voltage.

In connection with the above-mentioned conventional electrical insulating oils, the values of the dielectric breakdown voltages (BDV) and the dielectric loss tangents (tan .delta.) are satisfactory to a certain extent, but the hydrogen gas absorbing capacity or corona discharge characteristics and the stability of the dimensions of polypropylene films are not satisfactory.

In view of the above-described conventional state of the art, it is the primary object of the present invention to provide an improved non-halogenated electrical insulating oil and oil-filled electrical appliances which are impregnated with the improved insulating oil and are free from the above-described disadvantages in the conventional art.

Another object of the present invention is to provide an electrical insulating oil which has an excellent dielectric constant and other electrical properties, which has a good hydrogen gas absorbing capacity, and which is highly compatible with plastic film insulating materials.

It is a further object of the present invention to provide oil-filled electrical appliances which have excellent corona discharge characteristics, dielectric breakdown voltage and other advantageous electrical characteristics, and have a long service life.

The present invention is, therefore, concerned with a novel and improved electrical insulating oil and electrical appliances which are impregnated with this oil.

The electrical insulating oil of the invention comprises:

(a) at least one member of alkyl (including cycloalkyl) biphenyls and alkyl (including cycloalkyl) naphthalenes and

(b) at least one member of monoolefins and diolefins each having two condensed or noncondensed aromatic nuclei, excluding bicyclic monoolefins which are unsaturated dimers and unsaturated codimers of styrenes such as styrene, .alpha.-methylstyrene and their monomethyl nuclear substituted compounds.

The present invention will be described in more detail.

In the above item (a), the alkyl group in the alkylbiphenyl is exemplified by such alkyl groups as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, isobutyl and amyl groups, and a cycloalkyl group such as cyclohexyl group. A plurality of alkyl groups can exist, however, the total number of carbon atoms in the alkyl groups is preferably 1 to 10. These alkylbiphenyls can be used singly or in a mixture of two kinds or more. As preferable components for use in preparing the electrical insulating oil of the present invention, the alkylbiphenyls have viscosities of not higher than 30 cSt (3.times.10.sup.-5 m.sup.2 /s), preferably not higher than 10 cSt (10.sup.-5 m.sup.2 /s) at 40.degree. C. One of the most preferable compounds is monoisopropylbiphenyl.

The above alkylbiphenyl can be prepared by high temperature radical reaction of benzene, or by alkylation of benzene with chlorobenzene to obtain biphenyl and further alkylating the biphenyl with an olefin such as ethylene or propylene or with a halogenated hydrocarbon such as chloroethane or chloropropane.

The alkyl group of the alkylnaphthalene in the above item (a) is exemplified by such alkyl groups as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, isobutyl and amyl groups, and a cycloalkyl group such as cyclohexyl group. A plurality of the alkyl groups can exist, however, the total number of carbon atoms in the alkyl and cycloalkyl groups is preferably in the range of 1 to 10.

These alkylnaphthalenes can be used singly or in a mixture of two or more kinds. As a preferable component of the insulating oil of the invention, the alkylnaphthalene has a viscosity of not higher than 30 cSt (3.times.10.sup.-5 m.sup.2 /s), preferably not higher than 10 cSt (10.sup.-5 m.sup.2 /s) at 40.degree. C. One of the most preferable compounds is diisopropylnaphthalene.

The above alkylnaphthalene can be prepared by alkylation of naphthalene with olefins such as propylene and butene or a halogenated hydrocarbon such as propylchloride.

Incidentally, a mixture of the alkylbiphenyl and the alkylnaphthalene can be of course used as the components of item (a).

The compounds which are used together with the above-described alkylbiphenyl and/or alkylnaphthalene of item (a) are the compounds of the foregoing item (b), that is, monoolefins and/or diolefins each having two condensed or noncondensed aromatic nuclei, excluding bicyclic monoolefins of unsaturated dimers and unsaturated codimers of styrenes such as styrene, .alpha.-methylstyrene and their monomethyl nuclear substituted compounds.

The compounds excluded from the above item (b) are represented by any one of the following general formula (I) to (III): ##STR1## wherein each of R.sub.1 to R.sub.4 is a hydrogen atom or a methyl group and the total number of carbon atoms in R.sub.1 to R.sub.4 is an integer from zero to 4.

More particularly, the olefins to be excluded from item (b) are exemplified by 1,3-diphenylbutene-1, 1,3-diphenylbutene-2, 4-methyl-2,4-diphenylpentene-1, 4-methyl-2,4-diphenylpentene-2, 1,3-di(methylphenyl)butene-1, and 1,3-di(methylphenyl)butene-2.

Among the olefins of item (b) except the above monoolefins, there are monoolefins each having two condensed or noncondensed aromatic nuclei that are represented by the following general formulae (IV), (V) and (VI): ##STR2## wherein any one of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is an aryl group or an aralkyl group and the others are a hydrogen atom or an alkyl group, respectively; n is an integer from 0 to 3; and when R.sub.4 is an aryl group or an aralkyl group, n is 1. Further, the symbol " . . . " represents the existence or nonexistence of a bond, and when it represents the existence of a bond, R.sub.1 and R.sub.3 are alkylene groups forming a 5- to 7-membered ring. As stated above, unsaturated dimers and unsaturated codimers of styrenes such as styrene, .alpha.-methylstyrene and their monomethyl nuclear substituted compounds are excluded. ##STR3## wherein R.sub.5 is an alkenylene group or a cycloalkenylene group which is exemplified by a divalent substituent group obtained by removing two hydrogen atoms from olefinic hydrocarbons such as ethylene, propylene, butenes, cyclopentene and cyclohexene, and the aliphatic unsaturated double bond thereof is not conjugated with the aromatic nuclei. Further, m and n are representing integers from 0 to 3, and R.sub.6 of m in number and R.sub.7 of n in number are respectively the same or different from each other and each of them is a hydrogen atom or an alkyl group. ##STR4## wherein R.sub.8 is an alkenyl group or a cycloalkenyl group, m and n are representing integers from 0 to 3, and R.sub.9 of m in number and R.sub.10 of n in number are respectively the same or different from each other and each of them is a hydrogen atom or an alkyl group.

Among the aromatic olefins represented by the above formulae (IV) to (VI) that are used together with the alkyl biphenyls and/or alkylnaphthalenes of item (a), when R.sub.1 or R.sub.2 in formula (IV) is an aryl group or an aralkyl group, the compounds of formula (IV) are represented by the following general formula (IV-1), in which Ar denotes an aryl group or an aralkyl group. ##STR5##

In the case where R.sub.3 is an aryl group or an aralkyl group in general formula (IV), the compounds are represented by the following general formula (IV-2). ##STR6##

Further, when R.sub.4 is an aryl group or an aralkyl group in general formula (IV), the compounds are represented by the following general formula (IV-3). ##STR7##

In the above formulae (IV-1) to (IV-3), when Ar is an aryl group, it is exemplified by a phenyl, tolyl, xylyl, ethylphenyl, cumenyl group or the like. When Ar is an aralkyl group, Ar is, for example, a benzyl, 1- or 2-phenylethyl, 1- or 2-tolylethyl, 1- or 2-xylylethyl, 1- or 2-ethylphenylethyl, 1- or 2-cumenylethyl or 1-, 2- or 3-phenylpropyl group. In such cases, each of R.sub.1 to R.sub.4 in formulae (IV-1) to (IV-3) is a hydrogen atom or an alkyl group which is exemplified by a methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl group. The symbol " . . . " in formulae (IV-1) and (IV-3) represents either the existence or nonexistence of a bond, and when it represents the existence of a bond, R.sub.1 and R.sub.3 are alkylene groups forming a 5- to 7-membered ring.

In the case where Ar is an aryl group in the above formula (IV-1), the compounds are exemplified by stilbene, 4-methylstilbene, 1,2-diphenylpropene-1, 1,2-diphenyl-1-methylpropene-1, 1,2-diphenylcyclohexene and 2,3-diphenylbutene-2.

In the case where Ar is an aralkyl group in the above formula (IV-1), the compounds are exemplified by 1,3-diphenylpropene, 1,4-diphenylbutene-1 and phenylbenzylcyclohexene.

In the case where Ar is an aryl group in the above formula (IV-2), the compounds are exemplified by 1,1-diphenylethylene, 1-phenyl-1-(4'-ethylphenyl)ethylene and 1,1-diphenylpropene-1.

In the case where Ar is an aralkyl group in the above formula (IV-2), the compounds are exemplified by 2,3-diphenylpropene and 1,2-diphenylbutene-2.

In the case where Ar is an aryl group in the above formula (IV-3), the compounds are exemplified by 2-isopropenylbiphenyl, 4-isopropenyl-biphenyl, 2-isopropenyl-4'-isopropylbiphenyl, cyclohexenyl-biphenyl and cyclopentenyl-biphenyl.

In the case where Ar is an aralkyl group in the above formula (IV-3), the compounds are exemplified by 1-phenyl-1-(4'-vinylphenyl)ethane, 1-(4-methylphenyl)-1-(4'-vinylphenyl)ethane, 1-phenyl-1-(4'-isopropenylphenyl)ethane, phenyl-(4'-vinylphenyl)methane and phenyl-(cyclohexenylphenyl)methane.

In the foregoing general formula (V), the symbol R.sub.5 is an alkenylene group or a cycloalkenylene group and the aliphatic unsaturated double bond of the group is not conjugated with any of the aromatic nuclei of the aromatic olefin. The R.sub.5 is exemplified by butenylene, methylbutenylene, pentenylene, cyclopentenylene and cyclohexenylene. The symbols R.sub.6 and R.sub.7 denote a hydrogen atom or an alkyl group such as a methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl group.

The aromatic olefins represented by the formula (V) are exemplified by 1,4-diphenylbutene-2, 1,4-diphenylpentene-2 and 1,4-diphenyl-2-methylpentene-2.

In the aromatic olefins represented by the general formula (VI), the symbol R.sub.8 denotes an alkenyl group such as a vinyl, allyl, propenyl, isopropenyl and butenyl group, or a cycloalkenyl group such as a cyclopentenyl and cyclohexeneyl group. The symbols R.sub.9 and R.sub.10 denote a hydrogen atom or an alkyl group such as a methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl group.

The aromatic olefins represented by the general formula (VI) are exemplified by .alpha.-vinylnaphthalene, isopropenylnaphthalene, allylnaphthalene and 1-cyclopent-2-enylnaphthalene.

In the aromatic olefins of the foregoing item (b) which are components of the electrical insulating oil of the present invention, the diolefins having two aromatic nuclei are represented by the following general formulae (VII), (VIII) and (IX). ##STR8## wherein R.sub.1, R.sub.2 and R.sub.3 are hydrocarbon residual groups, respectively; each of m and n is 0 (zero) or a positive integer; R.sub.1 of m in number and R.sub.3 of n in number are either the same or different substituent groups; and the total number of aliphatic double bonds in the substituent groups is 2 in each formula.

In the case where R.sub.1 or R.sub.3 is an unsaturated group, it is an alkenyl or cycloalkenyl group, and is exemplified by a vinyl, propenyl, isopropenyl, allyl, butenyl, and cyclohexenyl group.

In the case where R.sub.1 or R.sub.3 is a saturated group, it is an alkyl or cycloalkyl group, and is exemplified by a methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl and cyclohexyl group.

In the case where R.sub.2 is an unsaturated group, it is an alkenylene or cycloalkenylene group, and is exemplified by a divalent substituent group which is obtained by removing two hydrogen atoms from an olefinic hydrocarbon such as ethylene, propylene, butenes, cyclopentene, and cyclohexene.

Furthermore, in the case where R.sub.2 is a saturated group, it is an alkylene or cycloalkylene group, and is exemplified by divalent substituent groups which are obtained by removing two hydrogen atoms from a saturated hydrocarbon such as methane, ethane, propane, butanes and cyclohexane.

The following compounds are exemplified as those represented by the foregoing general formulae (VII), (VIII) and (IX). Compounds represented by formula (VII):

Compounds represented by formula (VIII):

Compounds represented by formula (IX):

The above compounds are shown as examples of the components which can be used in the preparation of the insulating oil composition of the present invention, and the materials which may be used for the present invention are by no means restricted to the above exemplary compounds.

These aromatic olefins can be prepared by various chemical synthesis methods.

For instance, vinylnaphthalene is prepared by reacting formylnaphthalene with a Grignard reagent such as methylmagnesium iodide, and then dehydrating. Phenyl(vinylphenyl)ethane is prepared by reacting diphenylethane with acetyl chloride in the presence of a Friedel-Crafts catalyst to obtain phenyl(acetylphenyl)ethane, reducing by sodium borohydride, and then dehydrating. Phenyl(isopropenylphenyl)ethane is prepared by reacting phenyl(acetylphenyl)ethane with a Grignard reagent such as methylmagnesium iodide, and then dehydrating. 1,2-Diphenylethylene is prepared by reacting benzaldehyde with benzylmagnesium bromide, and then dehydrating. 1,2-Diphenylpropene is also prepared by a similar method. 1,1-Diphenylethylene is prepared by reacting diphenyl ketone with a Grignard reagent such as methylmagnesium iodide, and then dehydrating.

Furthermore, the aromatic diolefins are prepared by obtaining a Grignard reagent having a vinyl group and an aromatic ring from, for example, bromostyrene, reacting the reagent with an aromatic ketone such as acetophenone, and dehydrating the obtained alcohol.

Still further, the aromatic olefins used in the present invention are prepared by employing a reaction of dehydrogenation, oxidative dehydrodimerization or decomposition.

More particularly, in a method employing dehydrogenation, a saturated aromatic hydrocarbon or an aromatic monoolefin corresponding to or a little higher than the aromatic olefins of the invention is dehydrogenated in the presence of a suitable dehydrogenation catalyst with suppressing side reactions of excess decomposition and polymerization.

In the reaction, the dehydrogenation catalyst is not restricted to any specific one. For example, the dehydrogenation catalysts are exemplified by one or a mixture of oxides of metals such as Cr, Fe, Cu, K, Mg and Ca or precious metals such as Pt and Pd, or these metal oxides or precious metals which are supported on a carrier such as alumina.

The reaction temperature of the dehydrogenation is in the range of 350.degree. to 650.degree. C., preferably 400.degree. to 600.degree. C. The LHSV (liquid hourly space velocity) of the dehydrogenation is in the range of 0.2 to 10, preferably 0.5 to 3.0. In the dehydrogenation; steam, nitrogen gas or hydrogen gas can be introduced into the reaction system in order to reduce partial pressures and to avoid the formation of carbon. Further, if necessary, a suitable diluent can be used. When the rate of dehydrogenation is not so high, raw materials themselves conveniently serve as a diluent.

Through the above procedures, for example, diphenylethylene is obtained from diphenylethane; vinylphenylphenylethane, from ethylphenyl-phenylethane; and vinylphenylphenylethylene, from ethylphenyl-phenylethane or ethylphenylphenylethylene. Further, isopropenyl biphenyl is obtained from isopropyl biphenyl; and isopropenyl-isopropylnaphthalene or diisopropenylnaphthalene, from diisopropylnaphthalene.

The aromatic monoolefins used in the present invention can also be prepared by oxidative dehydrodimerization method. In this method, methyl-substituted monocyclic aromatic hydrocarbon such as toluene, xylene, ethyltoluene and vinyltoluene are subjected to dimerization (coupling) together with dehydrogenation.

For example, 1,2-diphenylethylene is obtained from toluene, and 1,2-di(methylphenyl)ethylene, from xylene. In this reaction, a saturated aromatic hydrocarbon corresponding the obtained olefin, for example, 1,2-diphenylethane from toluene, is simultaneously obtained, which is convenient for preparing the electrical insulating oil of the present invention.

Any suitable catalyst can be used for this oxidative dehydrodimerization. For example, usable catalysts are copper chromite catalysts containing Ni, Ta or Ti as disclosed in Japanese Patent Publication No. 49-6312 (1974), the catalysts of oxides of metals such as Bi, Pb, Te, Ba, Tl and Cd or their mixture as disclosed in Japanese Patent Publication No. 49-20561 (1974), and composite oxide catalyst of Tl as disclosed in U.S. Pat. No. 4,243,825. Further, alkali metal oxides as promoters can be added to these catalysts.

This reaction can be carried out in the presence of molecular oxygen with the above-described catalyst. The molar ratio of oxygen/methyl-substituted aromatic hydrocarbon is in the range of 0.01 to 5.0, preferably 0.05 to 1.0. Meanwhile, the reaction can be performed stoichiometrically without the presence of molecular oxygen, in which oxidation treatment in addition to usual treatment to remove deposited carbon, is necessary because the oxide catalyst is reduced with the progress of reaction.

The reaction temperature is in the range of 300.degree. to 800.degree. C., and preferably 500.degree. to 700.degree. C. The contact time is in the range of 0.01 second to several minutes, and preferably 0.1 to 30 seconds. The pressure in this reaction is not restricted and can range from a reduced pressure to 100 atmospheric pressure (98 bar), but preferably in the range of 0.1 to 5.0 atmospheric pressure (0.098 to 4.9 bar).

Further, the aromatic olefins used in the present invention can also be prepared by decomposition such as thermal cracking and catalytic cracking, in which, for example, triarylalkanes, diaralkyl aromatic hydrocarbons and polymers of styrenes are employed as raw materials.

In the thermal cracking of the above raw materials, the reaction temperature is set in the range of 300.degree. to 700.degree. C., and preferably in the range of 330.degree. to 600.degree. C. When the reaction temperature is too low, the rate of decomposition becomes very low. On the other hand, when the reaction temperature is too high, the raw material is decomposed to monocyclic hydrocarbons. Accordingly, in order to obtain the aromatic hydrocarbons used in the present invention at a higher yield, it is advisable that the thermal cracking is performed at a relatively higher temperature with a shorter retention time.

In the catalytic cracking, silica, silica gel, silica-alumina, kaolin, zeolite (with or without de-aluminum treatment), and organic or inorganic sulfonic acid can be used. The reaction is preformed in a liquid phase or gas phase, and the reaction temperature is in the range of 300.degree. to 700.degree. C., and preferably in the range of 330.degree. to 600.degree. C.

The above-mentioned monoolefin and/or diolefin having two condensed or noncondensed aromatic nuclei is/are employed as a mixture with the alkylbiphenyl, alkylnaphthalene or their mixture. Accordingly, provided the monoolefin and/or diolefin can be mixed and dissolved into the alkylbiphenyl, alkylnaphthalene or their mixture and produces a liquid mixture at ordinary temperatures, the olefin itself can be either liquid or solid. The above olefin having two aromatic nuclei can be used singly or in a mixture of two or more kinds together with the alkylbiphenyl, alkylnaphthalene or their mixture.

In the present application, as described above, the electrical insulating oil is prepared by mixing the alkylbiphenyl, alkylnaphthalene, or their mixture of item (a) and the aromatic olefin of item (b). The viscosity of the thus prepared insulating oil of the invention is preferably not higher than 30 cSt (3.times.10.sup.-5 m.sup.2 /s) at 40.degree. C. and more preferably not higher than 10 cSt (10.sup.-5 m.sup.2 /s) at 40.degree. C. Accordingly, in order to obtain a mixture having a viscosity of the above value, components are suitably selected from the alkylbiphenyls and/or alkylnaphthalenes of item (a) and the aromatic olefins of item (b).

Although the alkylbiphenyl and alkylnaphthalene themselves have excellent electrical properties and good biodegradability, thermal stability and oxidation stability, when they are used in a mixture with the aromatic olefins of the present invention, the hydrogen gas absorbing capacity can be further improved. In addition, in spite of the mixing with the unsaturated compounds of the aromatic olefins, no deterioration in biodegradability, thermal stability and oxidation stability is observed in practical uses, while various electrical properties can be improved.

The mixing ratio of the alkylbiphenyl and/or alkylnaphthalene of item (a) and the aromatic olefin of item (b) is arbitrary. However, a ratio of 0.01 to 50% by weight of the aromatic olefin with respect to the mixture of both component materials is preferable in view of their synergistic effects. The more preferable quantity of the aromatic olefin is 1.0 to 30% and most preferable quantity is 5.0 to 30% by weight.

The electrical insulating oil of the present invention is made of a mixture having the above-described composition; however, the present invention is not restricted to the foregoing composition. That is, in order to improve desired electrical characteristics without impairing the general electrical properties, other conventional electrical insulating oils such as polybutene, mineral oils, alkylbenzenes, diarylalkanes or aromatic ethers such as ditolyl ether can be added to the insulating oil of the present invention in an adequate quantity. When polybutene is added, the volume resistivity and dielectric loss tangent can be improved. The addition of mineral oils can improve the dielectric breakdown voltage, and the addition of alkylbenzenes or other aromatic insulating oils can improve the dielectric breakdown voltage, dielectric loss tangent and pour point.

In order to improve further the oxidation stability, several known antioxidants can be added to the electrical insulating oil of the present invention. As such antioxidants, there are phenol compounds such as 2,6-di-tert-butyl-p-cresol, 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 4,4'-butylidenebis(3-methy l-6-tert-butylphenol), 4,4'-thiobis(3-methyl-6-tert-butylphenol), stearyl-.beta.-(3,5-di-tert-butyl-4-hydroxyphenol)propionate, tetrakis[methylene-3(3',5'-di-tert-butyl-4'-hydroxyphenyl)-propionate]meth ane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, and 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenol)butane; sulfur compounds such as dilauryl thiodipropionate, distearyl thiodipropionate, laurylstearyl thiodipropionate, and dimyristyl thiodipropionate; and phosphorous compounds such as triisodecylphosphite, diphenylisodecylphosphite, triphenylphosphite, and trinonylphenylphosphite.

These antioxidants can be added to the electrical insulating oil singly or in combination of two kinds or more. The addition quantity of the antioxidant is 0.001 to 5% by weight and preferably 0.01 to 2.0% by weight of the electrical insulating oil.

Furthermore, in order to impart a nonflammable property and other desirable effects to the electrical insulating oil of the present invention, several known additives such as phosphoric esters and epoxy compounds can be added to the electrical insulating oil.

The electrical insulating oil of the present invention is good for general uses and, in particular, it is advantageous for the impregnation of oil-filled electrical appliances such as electric capacitors, power cables and transformers.

As described at the beginning of this specification, the requirements of high-voltage withstanding and size reduction of such oil-filled electrical appliances have become severe in recent years. In order to meet these requirements, plastics are used to replace either partially or totally the conventional insulating paper as insulating materials or dielectric materials for the oil-filled electrical appliances. More particularly, as electrical insulating materials (dielectric materials) of electric capacitors, there is proposed the use of a combination of insulating paper and plastic films such as stretched or nonstretched polypropylene, polymethylpentene, or polyester film; the use of these plastic films singly; the use of embossed or roughened films of these plastic films to facilitate impregnation with the insulating oil; or the use of metallized plastic films, wherein the metallic layer serves as an electrode. Capacitors are made by winding these films together with an electrode material.

In the case of oil-filled cables, the electrical insulating materials are made of polyolefin film such as cross-linked or non-cross-linked polyethylene film, stretched or nonstretched polypropylene film, and polymethylpentene film; paper-polyolefin laminated film made by the extrusion of polyolefin onto paper; composite film which is made by cross-linking insulating paper with silane-grafted polyethylene in the presence of a silanol condensation catalyst; or an artificial paper sheet which is made by mixing wood pulp and polyolefin fiber. Cables are made by winding tapes of these films around electric conductors.

The above capacitors and cables are impregnated or filled with the insulating oil of the present invention according to conventional methods.

The electrical insulating oil of the present invention is excellent in compatibility with plastic materials. Accordingly, the electrical insulating oil is quite suitable for use in oil-filled electrical appliances such as electric capacitors and electric cables in which plastic materials are used for either part or all of the insulating material or dielectric material.

More particularly, when an electric capacitor is provided with an insulating (dielectric) material that is partially or totally made of plastics, especially polyolefin, and when it is impregnated with the electrical insulating oil of the present invention, the insulating material can be fully and completely impregnated with the electrical insulating oil because swelling of the insulating material is slight, and voids (unimpregnated portions) are not formed. Accordingly, corona discharge due to the convergence of electric fields to the voids hardly occurs, and dielectric breakdown can be well avoided. Furthermore, the electrical insulating oil of the present invention has excellent hydrogen gas absorbing capacity and corona discharge resistance under high-voltage stress, so that it is possible to obtain both a long service life and high-voltage use of the electrical appliances.

In the case of electric power cables, a change in dimensions of the insulating material due to swelling is small, and resistance to the insulating oil flow can be made low so that oil impregnation can be performed in a short time. Of course, it will be understood that, because of the ease of impregnation, voids are hardly formed and the dielectric breakdown voltage becomes higher. When a cable is made by using an insulating material of a laminated film or composite film made of plastic material and paper, peeling, creasing and buckling of the insulating material upon bending of the cable do not occur even when the insulating material has been in contact with the electrical insulating oil for a long time. Further, as in the case of the electric capacitor, a power cable having a good corona discharge resistance can be obtained due to the excellent hydrogen gas absorbing capacity of the electrical insulating oil. Accordingly, it is also possible to obtain a long service life and high-voltage use, as for the capacitors.

According to the present invention, the above-described advantageous features can be improved by impregnation with the electrical insulating oil consisting of a plurality of specific component materials, owing to the synergistic effect between the component materials. Further, the good electrical characteristics, biodegradability, thermal resistance, and oxidation stability of each component material can be well maintained, and at the same time, the viscosity and pour point of the electrical insulating oil composition can be adjusted within desired ranges. Therefore, the manufacture of oil-filled electrical appliances is facilitated, and oil-filled electrical appliances exhibiting high performance under any use conditions can be obtained. In addition, the components of the electrical insulating oil of the present invention are non-halogenated hydrocarbons, so that the oil does not constitute any public health hazard.

In the following, the electrical insulating oil and electrical appliances impregnated therewith according to the present invention will be described in more detail with reference to several examples.

The monoolefins and diolefins having two condensed or noncondensed aromatic nuclei of the present invention can be prepared by several known methods as described above. For reference purposes, however, the preparation of some of compounds of item (b) employed in the following Examples will be described.

To a 5 liter reaction vessel equipped with a stirrer, reflux condenser and dropping funnel were added 2 liters of carbon tetrachloride and 467 g of anhydrous aluminum chloride, and the contents were cooled by ice while being stirred. This was followed by the addition of 275 g of acetyl chloride through the dropping funnel and additional stirring for 1 hour. To this was added 546 g of 1,1-diphenylethane, and the contents were stirred for 4 hours. After the reaction, the aluminum chloride was deactivated by diluted hydrochloric acid and the reaction mixture was rinsed with an aqueous solution of sodium carbonate. The reaction medium was then removed by distillation to obtain 502 g of ketone in a yield of 74.7%.

To a 2 liter reaction vessel equipped with a stirrer, reflux condenser and dropping funnel were added 600 ml of isopropyl alcohol and 84 g of sodium borohydride, and the isopropyl alcohol was refluxed by heating the vessel. The ketone (500 g) was added dropwise for 1 hour to this mixture and the reaction mixture was stirred further with refluxing of the isopropyl alcohol.

After the reaction, the catalyst was deactivated by adding water. The reaction product was separated by ether extraction and was dried by anhydrous sodium sulfate. The ether was distilled off to obtain 480 g of alcohol in a yield of 95.2%.

A 500 ml three neck flask was equipped with a dropping funnel, 40 g of potassium hydrogensulfate was fed into the flask, and it was heated to 230.degree. to 240.degree. C. under a reduced pressure. The above-obtained alcohol (480 g) was then added through the dropping funnel. The alcohol was dehydrated to produce an olefin, which olefin was immediately collected by distillation into an outer receptacle. By removing water from the obtained olefin, 332 g of 1-phenyl-1-(4'-vinylphenyl)ethane was obtained in a yield of 75.2% (b.p. 149.degree. C./10 mmHg, 113.degree. C./2 mmHg).

The chemical structure of the final product was identified by elemental analysis, IR spectrum analysis and NMR spectrum analysis.

To a 5 liter reaction vessel equipped with a stirrer, reflux condenser and dropping funnel were added 71 g of metallic magnesium and 2 liters of diethyl ether, which was dried by metallic sodium. While cooling the contents by ice with stirring, 410 g of methyl iodide was slowly added dropwise, which was followed by the dropping of 500 g of a ketone [1-phenyl-1-(4'-acetylphenyl)ethane] obtained in like manner as in the foregoing Preparation Example 1. After the above dropwise addition, the mixture was allowed to react for 30 min. with stirring. Following the reaction, the reaction mixture was poured into a mixture of iced water and sulfuric acid to recover the layer of ether. After that, the ether was evaporated off to obtain 495 g of alcohol in a yield of 92.4%.

In like manner as in the foregoing Preparation Example 1, the above 495 g of alcohol was dehydrated to produce 310 g of 1-phenyl-1-(4'-isopropenylphenyl)ethane in a yield of 67.7% (b.p. 153.degree. C./10 mmHg, 116.degree. C./2 mmHg).

The chemical structure of the final product was identified by elemental analysis, IR spectrum analysis and NMR spectrum analysis.

1-Phenyl-1-(4'-ethylphenyl)ethane was dehydrogenated in the presence of a catalyst and steam under the following conditions and obtained an oil of the following composition. Conditions of Dehydrogenation: chromium oxide Composition of the Obtained Oil: Compounds            % by weight ______________________________________ 1-phenyl-1-(4'-ethylphenyl)ethane 23.8 1-phenyl-1-(4'-ethylphenyl)ethylene 39.9 1-phenyl-1-(4'-vinylphenyl)ethane 5.0 1-phenyl-1-(4'-vinylphenyl)ethylene 28.1 Others               3.2 Total                100.0 ______________________________________ Characteristics

Samples of electrical insulating oils were prepared according to the compositions indicated in the following Table 1 and Table 2. In these Tables, Examples 1, 18 to 22, 25, and 42 to 44 are comparative examples and others are examples according to the present invention.

In all examples, 0.2% by weight of BHT (2,6-di-tert-butyl-p-cresol) was added to the electrical insulating oils as antioxidant. The viscosities of all insulating oils became within the range of 4.5 to 6.5 cSt at 40.degree. C.

The electrical insulating oils were subjected to electrical characteristics tests, the results of which are shown in the following Table 3 and Table 4. The tests were performed in accordance with JIS C 2101 (Methods for Testing Electrical Insulating Oil). __________________________________________________________________________ Example Alkylbiphenyl    Aromatic Olefin No.  Name        wt. % Name               wt. % __________________________________________________________________________ 1    Monoisopropylbiphenyl 100  --                 -- 2    "           90   1-Phenyl-1-(4'-vinylphenyl)ethane 10 3    "           90   1-(4-Methylphenyl)-1-(4'-vinylphenyl)- 10 ethane 4    "           90   1-Phenyl-1-(4'-isopropenylphenyl)- 10 ethane 5    "           90   Phenyl-(4'-vinylphenyl)methane 10 6    "           90   2-Isopropenylbiphenyl 10 7    "           90   .alpha.-Vinylnaphthalene 10 8    "           95   trans-Stilbene      5 9    "           90   4-Methylstilbene (cis, trans mixture) 10 10   "           90   1,2-Diphenylpropene (cis, trans mixture) 10 11   "           95   1,1-Diphenylethylene 5 12   "           90   "                  10 13   "           80   "                  20 14   "           90   1-Phenyl-1-(4'-ethylphenyl)ethylene 10 15   "           90   1,4-Diphenylbutene-2 10 16   "           90   1-Cyclopent-2-enylnaphthalene 10 17   "           90   1-Phenyl-1-(4'-vinylphenyl)ethylene 10 18   "           90   1,3-Diphenylbutene-1 10 19   "           90   2,4-Diphenyl-4-methylpentene-1 10 20   "           90   1-Hexadecene       10 21   "           90   1-Decene           10 22   Monoisopropylbiphenyl 80   --                 -- Diisopropylbiphenyl 20 23   Monoisopropylbiphenyl 90   1,1-Diphenylethylene 10 24   "           90   1-Phenyl-1-(4'-vinylphenyl)ethane 10 __________________________________________________________________________ __________________________________________________________________________ Example Alkylnaphthalene Aromatic Olefin No.  Name        wt. % Name               wt. % __________________________________________________________________________ 25   Diisopropylnaphthalene 100 --                 -- 26   "           90  1-Phenyl-1-(4'-vinylphenyl)ethane 10 27   "           90  1-(4-Methylphenyl)-1-(4'-vinylphenyl)- 10 ethane 28   "           90  1-Phenyl-1-(4'-isopropenylphenyl)- 10 ethane 29   "           90  Phenyl-(4'-vinylphenyl)methane 10 30   "           90  2-Isopropenylbiphenyl 10 31   "           90  .alpha.-Vinylnaphthalene 10 32   "           95  trans-Stilbene      5 33   "           90  4-Methylstilbene (cis, trans mixture) 10 34   "           90  1,2-Diphenylpropene (cis, trans mixture) 10 35   "           95  1,1-Diphenylethylene 5 36   "           90  "                  10 37   "           80  "                  20 38   "           90  1-Phenyl-1-(4'-ethylphenyl)ethylene 10 39   "           90  1,4-Diphenylbutene-2 10 40   "           90  1-Cyclopent-2-enylnaphthalene 10 41   "           90  1-Phenyl-1-(4'-vinylphenyl)ethylene 10 42   "           90  1,3-Diphenylbutene-1 10 43   "           90  2,4-Diphenyl-4-methylpentene-1 10 44   "           90  1-Hexadecene       10 __________________________________________________________________________ __________________________________________________________________________ Electrical Characteristics __________________________________________________________________________ Example No. Test Item   2     3     4     5     6 __________________________________________________________________________ Pour Point (.degree.C.)* <-50  <-50  <-50  <-50  <-50 Kinematic Viscosity 4.6   4.7   4.7   4.6   4.8 (cSt at 37.8.degree. C.) Dielectric Breakdown >70   >70   >70   >70   >70 Voltage (kV/2.5 mm)** Dielectric Loss 0.018 0.016 0.017 0.020 0.015 Tangent (% at 80.degree. C.) Volume Resistivity 8.3 .times. 10.sup.14 8.1 .times. 10.sup.14 8.5 .times. 10.sup.14 9.3 .times. 10.sup.14 9.8 .times. 10.sup.14 (.OMEGA. .multidot. cm at 80.degree. C.) Dielectric Constant 2.52  2.51  2.50  2.52  2.52 __________________________________________________________________________ Example No. Test Item   7     11    13    17    23 __________________________________________________________________________ Pour Point (.degree.C.) <-50  <-50  <-50  <-50  <-50 Kinematic Viscosity 4.9   4.6   4.5   4.7   6.6 (cSt at 37.8.degree. C.) Dielectric Breakdown >70  >70   >70   >70   >70 Voltage (kV/2.5 mm) Dielectric Loss 0.024 0.025 0.026 0.020 0.026 Tangent (% at 80.degree. C.) Volume Resistivity 9.5 .times. 10.sup.14 8.9 .times. 10.sup.14 9.0 .times. 10.sup.14 8.7 .times. 10.sup.14 8.5 .times. 10.sup.14 (.OMEGA. .multidot. cm at 80.degree. C.) Dielectric Constant 2.53  2.53  2.52  2.50  2.50 __________________________________________________________________________ Notes: *<-50 means "not higher than -50" **> 70 means "higher than 70" __________________________________________________________________________ Electrical Characteristics __________________________________________________________________________ Example No. Test Item  26    27    28    29    30 __________________________________________________________________________ Pour Point (.degree.C.) <-50  <-50  <-50  <-50  <-50 Kinematic Viscosity 6.3   6.4   6.5   6.4   6.5 (cSt at 37.8.degree. C.) Dielectric Breakdown >70   >70   >70   >70   >70 Voltage (kV/2.5 mm) Dielectric Loss 0.026 0.029 0.030 0.028 0.025 Tangent (% at 80.degree. C.) Volume Resistivity 8.1 .times. 10.sup.14 8.3 .times. 10.sup.14 7.9 .times. 10.sup.14 8.8 .times. 10.sup.14 8.9 .times. 10.sup.14 (.OMEGA. .multidot. cm at 80.degree. C.) Dielectric Constant 2.41  2.45  2.43  2.46  2.50 __________________________________________________________________________ Example No. Test Item  31    35    37    38    41 __________________________________________________________________________ Pour Point (.degree.C.) <-50  <-50  <-50  <-50  <-50 Kinematic Viscosity 6.6   6.1   6.0   6.4   6.3 (cSt at 37.8.degree. C.) Dielectric Breakdown >70  >70   >70   >70   >70 Voltage (kV/2.5 mm) Dielectric Loss 0.029 0.030 0.026 0.029 0.030 Tangent (% at 80.degree. C.) Volume Resistivity 9.0 .times. 10.sup.14 8.6 .times. 10.sup.14 8.6 .times. 10.sup.14 9.0 .times. 10.sup.14 8.3 .times. 10.sup.14 (.OMEGA. .multidot. cm at 80.degree. C.) Dielectric Constant 2.52  2.46  2.44  2.45  2.45 __________________________________________________________________________

The following tests were carried out with regard to the electrical insulating oils containing alkylbiphenyls that are shown in the foregoing Table 1.

A polypropylene film of 16.mu. in thickness was cut into a certain configuration and each cut film was immersed into each insulating oil at 80.degree. C. for 72 hours. After that the cut film was taken out and the ratio of change in volume (%) of before and after the immersion was measured.

The results of this test are shown in the following Table 5, in which if the resultant value is small, i.e., the ratio of volume change is small, the tendency to swell the polypropylene film is small giving good size stability of the polypropylene film, and it is understood that the adaptability of the insulating oil to the polypropylene is good.

As will be understood from the results shown in Table 5, the electrical insulating oils according to the present invention have good adaptability to polypropylene. Meanwhile, the insulating oils of Examples 20 and 21 containing an aliphatic olefin such as 1-hexadecene or 1-decene showed a large ratio of volume change, from which it will be understood that these oils have no adaptability to polypropylene.

Two sheets of polypropylene films (thickness: 16.mu.) were put together in layers to obtain a dielectric material. The dielectric material and aluminum foil as an electrode were wound together according to the conventional method to obtain model capacitors for oil impregnation.

These model capacitors were impregnated with the foregoing electrical insulating oils in vacuum to prepare oil-filled capacitors of about 0.5 .mu.F electrostatic capacitance.

Corona starting voltages (CSV) and corona ending voltages (CEV) were then determined by applying electric voltage to the capacitors thus prepared. The temperature of the test was 30.degree. C. and the results of the test are shown also in the following Table 5.

Meanwhile, similar oil-filled capacitors were applied with a constant alternating voltage of 3.6 kV until the capacitors were broken to determine breakdown times. The results of them are also shown in Table 5, in which each value was calculated such that seven capacitors impregnated with the same oil were tested and the maximum value and minimum value were neglected and the average of the other five breakdown times was adopted as the resultant value. The breakdown times are relative values to that of the non-olefinic insulating oil of 100% alkylbiphenyl as 1.0. ______________________________________ Volume Change             Breakdown Time Example Ratio (%)   C S V   C E V (Relative No.    of Film     (kV)    (kV)  Value) ______________________________________ 1      7.3         2.6     2.1   1.0 2      7.7         3.5     2.9   40.3 3      7.2         3.5     2.9   35.2 4      7.3         3.5     2.9   18.6 5      7.3         3.5     2.9   41.8 6      7.5         3.5     2.9   17.5 7      7.3         3.5     2.9   25.9 8      7.1         3.3     2.6   5.2 9      7.3         3.4     2.7   9.9 10     7.2         3.4     2.7   9.5 11     7.4         3.3     2.7   10.1 12     7.6         3.4     2.9   20.7 13     7.4         3.6     2.9   26.4 14     7.3         3.4     2.9   17.3 15     7.5         3.4     2.7   8.6 16     7.5         3.4     2.7   6.7 17     7.4         3.5     2.9   41.5 18     7.4         2.9     2.3   3.6 19     7.3         3.0     2.4   4.6 20     11.6        2.7     2.2   1.1 21     13.9        --      --    -- 22     7.4         2.4     2.0   1.0 23     7.3         3.3     2.7   20.6 24     7.6         3.4     2.7   37.3 ______________________________________

As will be understood from the results shown in Table 5, the capacitors which are impregnated with the insulating oils of the invention have quite excellent electrical properties as compared with those impregnated with only monoisopropylbiphenyl. Furthermore, the adaptability of the insulating oil to the plastic film is also satisfactory.

The insulating oils of Example Nos. 20 and 21 containing aliphatic olefins have no adaptability to plastic films, so that these oils will not be employed in preparing oil-impregnated electrical appliances using plastic films.

The following tests were carried out in connection with the electrical insulating oils containing alkylnaphthalenes that are shown in Table 2.

A dielectric material was made of a 28.mu. thick, 62 mm wide polypropylene film and 14.mu. thick, 62 mm wide insulating paper, which were put together in layers. Model capacitors were made by the ordinary method with winding the above dielectric material together with 7.mu. thick, 50 mm wide aluminum foil.

These model capacitors were impregnated with the foregoing electrical insulating oils in vacuum to obtain oil-filled capacitors of about 0.6 .mu.F in electrostatic capacitance.

Corona starting voltages (CSV) and corona ending voltages (CEV) were then measured by applying electric voltages to the capacitors thus prepared. The temperature of measuring was 30.degree. C. and the test results are shown in the following Table 6.

Furthermore, similar oil-filled capacitors were applied with a constant alternating voltage of 3.1 kV until the capacitors were broken to obtain breakdown times. The results thereof are also shown in Table 6, in which each value was calculated such that seven capacitors impregnated with the same oil were tested and the maximum value and minimum value were neglected and the average of the remaining five breakdown times was adopted as the resultant value. The breakdown times are relative values to that of the insulating oil of 100% alkylnaphthalene as 1.0. ______________________________________ Example  C S V       C E V   Breakdown Time No.      (kV)        (kV)    (Relative Value) ______________________________________ 25       1.9         1.1     1.0 26       3.2         2.3     41.5 27       3.1         2.3     37.0 28       3.1         2.2     27.1 29       3.1         2.2     40.8 30       3.0         2.1     27.3 31       3.0         2.2     29.8 32       2.7         1.9     3.1 33       3.0         2.0     4.8 34       2.8         1.9     8.0 35       2.8         1.9     11.7 36       3.1         2.2     29.3 37       3.1         2.3     34.1 38       2.9         1.9     13.4 39       2.8         1.9     4.0 40       2.8         1.9     4.4 41       3.2         2.3     41.6 42       2.6         1.6     2.5 43       2.7         1.7     2.9 44       2.0         1.2     1.0 ______________________________________

From the results shown in Table 6, it will be understood that the values of both CSV and CEV of the capacitors that are impregnated with the insulating oil of the present invention, are all high and that the life of the capacitors can be much prolonged. Furthermore, it is quite apparent that the expected life of the capacitors prepared according to the present invention containing the aromatic olefins is quite excellent as compared with the capacitors containing aliphatic olefins.

As described above, the electrical insulating oil of the present invention is excellent in adaptability to plastic films, is improved in dielectric strength, and is quite stable against the energy of electric discharge. Especially, the electrical insulating oil of the present invention can be advantageously used for electrical appliances containing the insulating (dielectric) material at least partially made of polyolefin film such as polypropylene film or the like.

## Claims

1. An electrical insulating oil comprising: biphenyls and alkyl naphthalenes, and monoolefins and diolefins having two condensed or noncondensed aromatic nuclei, excluding bicyclic monoolefins of unsaturated dimers and unsaturated codimers of styrenes such as styrene, .alpha.-methylstyrene and their monomethyl nuclear substituted compounds.

2. The electrical insulating oil in claim 1, wherein the total number of carbon atoms in alkyl groups of said alkylbiphenyl is in the range of 1 to 10.

3. The electrical insulating oil in claim 1, wherein the total number of carbon atoms in alkyl groups of said alkylnaphthalene is in the range of 1 to 10.

4. The electrical insulating oil in claim 1, wherein the mixing ratio of said aromatic monoolefins and/or diolefins in said insulating oil is in the range from 0.01 to 50% by weight.

5. The electrical insulating oil in claim 1, wherein the viscosity of said electrical insulating oil is not higher than 30 cSt (3.times.10.sup.-5 m.sup.2 /s) at 40.degree. C.

6. The electrical insulating oil in claim 1, wherein said aromatic monoolefins are the compounds represented by the following general formulae (IV) to (VI): ##STR9## wherein any one of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is an aryl group or an aralkyl group and the others are a hydrogen atom or an alkyl group, n is an integer from 0 to 3, when R.sub.4 is an aryl group or an aralkyl group, n is 1, the symbol " . . . " represents either the existence or nonexistence of a bond, and when it represents the existence of a bond, R.sub.1 and R.sub.3 are alkylene groups forming a 5- to 7-membered ring, ##STR10## wherein R.sub.5 is an alkenylene group or a cycloalkenylene group, and the aliphatic unsaturated double bond thereof is not conjugated with the aromatic nuclei, m and n are integers of 0 to 3, and R.sub.6 of m in number and R.sub.7 of n in number are the same or different from each other and each of them is a hydrogen atom or an alkyl group, ##STR11## wherein R.sub.8 is an alkenyl group or a cycloalkenyl group, m and n are integers of 0 to 3, R.sub.9 of m in number and R.sub.10 of n in number are the same or different from each other, and each of them is a hydrogen atom or an alkyl group.

7. The electrical insulating oil in claim 1, wherein said aromatic diolefins are the compounds represented by the following general formulae (VII) to (IX): ##STR12## wherein R.sub.1, R.sub.2 and R.sub.3 are hydrocarbon residual groups, each of m and n is 0 (zero) or a positive integer, R.sub.1 of m in number and R.sub.3 of n in number are either the same or different substituent groups, and the total number of double bonds in the substituent groups is 2 in each formula.

8. An oil-filled electrical appliance which is impregnated with an electrical insulating oil comprising: biphenyls and alkyl naphthalenes, and monoolefins and diolefins having two condensed or noncondensed aromatic nuclei, excluding bicyclic monoolefins of unsaturated dimers and unsaturated codimers of styrenes such as styrene, .alpha.-methylstyrene and their monomethyl nuclear substituted compounds.

9. The oil-filled electrical appliance in claim 8, wherein the total number of carbon atoms in alkyl groups of said alkylbiphenyl is in the range of 1 to 10.

10. The oil-filled electrical appliance in claim 8, wherein the total number of carbon atoms in alkyl groups of said alkylnaphthalene is in the range of 1 to 10.

11. The oil-filled electrical appliance in claim 8, wherein the mixing ration of said aromatic monoolefins and/or diolefins in said insulating oil is in the range from 0.01 to 50% by weight.

12. The oil-filled electrical appliance in claim 8, wherein the viscosity of said electrical insulating oil is not higher than 30 cSt (3.times.10.sup.-5 m.sup.2 /s) at 40.degree. C.

13. The oil-filled electrical appliance in claim 8, wherein said aromatic monoolefins are the compounds represented by the following general formulae (IV) to (VI): ##STR13## wherein any one of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is an aryl group or an aralkyl group and the others are a hydrogen atom or an alkyl group, n is an integer from 0 to 3, when R.sub.4 is an aryl group or an aralkyl group, n is 1, the symbol " . . . " represents either the existence or nonexistence of a bond, and when it represents the existence of a bond, R.sub.1 and R.sub.3 are alkylene groups forming a 5- to 7-membered ring, ##STR14## wherein R.sub.5 is an alkenylene group or a cycloalkenylene group, and the aliphatic unsaturated double bond thereof is not conjugated with the aromatic nuclei, m and n are integers of 0 to 3, and R.sub.6 of m in number and R.sub.7 of n in number are the same or different from each other and each of them is a hydrogen atom or an alkyl group, ##STR15## wherein R.sub.8 is an alkenyl group or a cycloalkenyl group, m and n are integers of 0 to 3, R.sub.9 of m in number and R.sub.10 of n in number are the same or different from each other, and each of them is a hydrogen atom or an alkyl group.

14. The oil-filled electrical appliance in claim 8, wherein said aromatic diolefins are the compounds represented by the following general formulae (VII) to (IX): ##STR16## wherein R.sub.1, R.sub.2 and R.sub.3 are hydrocarbon residual groups, each of m and n is 0 (zero) or a positive integer, R.sub.1 of m in number, R.sub.2, and R.sub.3 of n in number are either the same or different substituent groups, and the total number of double bonds in the substituent groups is 2 in each formula.

15. The oil-filled electrical appliance in claim 8, wherein said electrical appliance is one member selected from the group consisting of oil-filled capacitors, oil-filled cables and transformers.

16. The oil-filled electrical appliance in claim 8, wherein the insulating material or dielectric material used in said oil-filled electrical appliance is insulating paper, synthetic resin film or their combination.

17. The oil-filled electrical appliance in claim 16, wherein said synthetic resin film is polyethylene film or polypropylene film.