The present invention refers to an electrical insulation system with improved electrical breakdown strength.

Electrical insulations, for instance in embedded poles, instru ^¬ ment and distribution transformers or sensors, generally consist of an epoxy resin cured with an acid anhydride in the presence of an accelerator. The starting components usually are mixed together with a filler material, for example with silica flower, typically in the range of 60 to 65 % by weight of filler

material, calculated to the total weight of the electrical insulator composition; the mixture is then cured. Alternative polymers can also be used such as polyesters, polyamides, poly- butylene terephthalate , polyurethanes or polydicyclopentadiene . A large amount of filler generally decreases the overall price of the insulation, however, it also increases the stiffness, the fracture toughness, the thermal conductivity of the insulator and decreases its thermal expansion coefficient.

Cracking in epoxy based insulation is a recurrent problem in such electrical insulators. Increase of toughness is clearly benefi ^¬ cial to improve this situation. In electrical machine insulations, increasing simultaneously the glass transition temperature and the toughness of the resin generally, leads to an increase of the thermal class of the insulation, which means that the electrical machines can be run at higher current ratings.

A key property for the reliability of an electrical insulation material is that it has a high electrical breakdown strength and, therewith, good insulating properties also at high electrical field strengths. WO 2006/008422 proposes the production of an electrical insulator for high voltage use comprising a mineral filler material wherein the mineral filler material is a combi ^¬ nation of a filler material with an average grain size distribution within the micron-scale together with a selected filler material with an average grain size distribution within the nano- scale, i.e. less than 1 μιτι. However, such a combination, especi ^¬ ally for industrial potting applications, e.g. with epoxy resins, has the disadvantage that it increases the viscosity of the curable epoxy resin composition and therewith reduces its processability . It further often is difficult to incorporate the nano sized filler material homogenously within the curable electrical insulation composition.

In the production of an electrical insulator for high voltage use generally a mineral filler material is used having an average grain size distribution within the range of 1 μτ ^η -500 μπι, prefe ^¬ rably within the range of 5 μπι-100 μιτι.

It has now surprisingly been found that nano-scale sized filler materials, such as nano-scale sized silica, i.e. a filler material having an average grain size distribution within the nano-scale, when produced by a sol-gel process, can be added to the curable electrical insulation composition or to a single component thereof, using simple mixing methods, thereby providing an excellent dispersion of the nano-particles within the curable electrical insulation composition. It has been found that this is true for nano-scale sized filler materials when produced by a sol-gel process, wherein said nano-scale sized filler material is selected from the group comprising silica, zinc oxide, alumina, aluminum trihydrate (ATH) , layered silicates, titanium dioxide, metal nitrides and metal carbides.

The addition of such nano-scale sized filler materials is signi ^¬ ficantly improving the electrical properties of the insulator system, especially its electrical breakdown strength. A curable electrical epoxy resin composition containing about five parts by weight of nano-scale sized filler material produced by a sol-gel process such as nano-scale sized silica, and about 55 parts by weight of conventional micro-scale sized filler material having an average grain size distribution within the micro-scale such as conventional micro-scale sized silica, yields a cured electrical isolator composition with an improved dielectric breakdown strength by up to 50% compared to the cured electrical isolator composition containing 60 parts by weight of micro-scale sized filler only, such as micro-scale sized powdered silica.

This rather unique simultaneous increase of these different properties at low nanofiller content is highly beneficial for the development of more robust insulation systems. This effect is most likely due to the extremely good dispersion of the nanofiller as produced by a sol-gel process in the cured epoxy resin composition. Moreover, the solution presents the additional advantage when the nanofiller can be added to the hardenable epoxy resin composition or to a component thereof in the form of a masterbatch, said nanofiller masterbatch is easily mixable with the epoxy resin composition or a component thereof and thereby is preventing any air contamination so that environmental health and safety is improved.

The present invention is defined in the claims. The present invention refers to an electrical insulation system with improved electrical breakdown strength, said electrical insulation system comprising a hardened polymer component having incorporated therein a conventional filler material and a selected nano-scale sized filler material, characterized in that

(a) said hardened polymer component is selected from epoxy resin compositions, polyesters, polyamides, polybutylene

terephthalate, polyurethanes and polydicyclopentadiene, and preferably is a hardened epoxy resin system;

(b) said conventional filler material is a known filler material having an average grain size distribution within the range of 1 μτη-500 μιτι, being present in a quantity within the range of 40%-65% by weight, calculated to the total weight of the insulator system; and (c) said nano-scale sized filler material is selected from the group comprising silica, zinc oxide, alumina, aluminum tri- hydrate (ATH) , layered silicates, titanium dioxide, metal nitrides and metal carbides, wherein said nano-scale sized filler material is a pretreated nano-scale material, having been produced by a sol-gel process; wherein said nano-scale sized filler material is present within the electrical insulation system in an amount of about l%-20% by weight, calculated to the weight of the conventional filler material present in the electrical insulator system.

The present invention further refers to the hardenable electrical insulation composition for the production of the hardened electrical insulation system as defined above, said hardenable electrical insulation composition comprising a hardenable polymer component having incorporated therein a conventional filler material and a selected nano-scale sized filler material, characterized in that

(d) said hardenable polymer component is a monomeric or oligo- meric starting material of the respective component (a) being selected from hardenable epoxy resin compositions, hardenable polyesters, polyamides, polybutylene terephtha- late, polyurethanes and polydicyclopentadiene , and preferably is a hardenable epoxy resin system;

(e) said conventional filler material is a known filler material as defined herein above as component (b) , having an average grain size distribution within the range of 1 μτη-500 μιτι, and being present in a quantity within the range of 40%-65% by weight, calculated to the total weight of the hardenable insulator system; and

(f) said selected nano-scale sized filler material is a filler material as defined herein above as component (c) , having been produced by a sol-gel process; and wherein said selec ^¬ ted nano-scale sized filler material is present within the hardenable electrical insulation system in an amount of about l%-20% by weight, calculated to the weight of the conventional filler material present in the hardenable electrical insulator system.

The present invention also refers to a mixture containing the conventional filler material defined as component (b) herein above, together with the selected nano-scale sized filler material defined as component (c) herein above, wherein the selected nano-scale sized silica powder is present in an amount of l%-20% by weight, calculated to the weight of the conventional filler material present.

The present invention also refers to a masterbatch comprising a hardenable polymer component defined as component (d) herein above, and the selected nano-scale sized filler material as the only filler material, as defined as component (c) herein above.

The present invention further refers to a method of producing said hardenable electrical insulation composition, which on curing resp. hardening yields the hardened electrical insulation system with improved electrical breakdown strength.

The present invention further refers to electrical articles comprising said electrical insulation system with improved electrical breakdown strength as defined according to the present invention .

The present invention further refers to the selected pretreated nano-scale sized filler material having been produced by a sol- gel process, as defined as component (c) herein above, wherein said pretreated nano-scale sized filler material carries on it surface reactive glycidyl groups , preferably in the form of 3-glycidoxypropylsilyl groups. The polymer component of the curable electrical insulation compo sition, yielding the hardened electrical insulation system, may be selected from monomeric and/or oligomeric epoxy resin compositions, monomeric and/or oligomeric polyesters, polyamides, polybutylene terephthalate , polyurethanes and polydicyclopenta- diene. Preferably the polymer component of the hardenable electrical insulation composition is a hardenable epoxy resin composition, comprising an epoxy resin component, a hardener component and a curing agent. The expressions "hardened" or "cured", resp. "hardenable" or "curable", are equivalent and applicable for the respective polymer used. Analogously the expressions "cross-linked" or "polymerized" could be used, as is known to the expert in the art.

The conventional filler material [component (b) ] preferably has an average grain size distribution within the range of 5 μπι- 100 μιτι, preferably within the range of 5 μιη-50 μιτι, preferably within the range of 5 μπι-30 μπι. Preferably at least 70% of the particles, preferably at least 80% of the particles, have a particle size within the range indicated.

The conventional filler material is preferably selected from silica, quartz, talc, silicates such as mica, kaolin or a layere silicate, aluminum oxide, aluminum trihydrate (ATH) , titanium oxide, dolomite [CaMg (C0 ₃ ) ₂ ] _r metal nitrides, such as silicon nitride, boron nitride and aluminium nitride or metal carbides, such as silicon carbide. Mica (glimmer) and kaolin are aluminium silicates substantially composed of Si0 ₂ and A1 ₂ 0 ₃ .

The conventional filler material may be surface treated with a coupling agent known per se. The coupling agent is preferably selected from the group comprising silanes and siloxanes and preferably is a silane, for example 3-glycidoxypropyltri- methoxysilane or 3-glycidoxypropyldimethoxymethylsilane . The conventional filler material preferably is present within the insulator system in a quantity within the range of 50%-65% by weight, preferably in a quantity of about 55%-60% by weight, calculated to the total weight of the insulator system. The nano-scale sized filler material used according to the present invention is produced by a sol-gel process and is selected from the group comprising silica, zinc oxide, alumina, aluminum trihydrate (ATH) , layered silicates, titanium dioxide, metal nitrides and metal carbides. Preferred are nano-scale sized filler materials selected from the group comprising silica, zinc oxide, alumina, aluminum trihydrate (ATH) , layered silicates and titanium dioxide. Further preferred are nano-scale sized filler materials selected from the group comprising silica, zinc oxide, alumina, aluminum trihydrate (ATH) and layered silicates, and preferably selected from the group comprising silica, alumina, aluminum trihydrate (ATH) and layered silicates. Preferred are nano-scale sized filler materials selected from the group comprising silica, alumina and aluminum trihydrate (ATH) . Most preferred is nano-scale sized silica.

Nano-scale sized metal nitrides are preferably selected from silicon nitride, boron nitride and aluminum nitride. Nano-scale sized metal carbide is preferably silicon carbide. The preparation of nano-scale materials is known per se. Such sol-gel process for the production of nano-scale sized silica powder is for example described in WO 2009/015724 Al, the content of which is incorporated herein by reference. A sol-gel process for the production of nano-scale sized zinc oxide dispersions is for example described in US 6,699,316, the content of which is incorporated herein by reference. A sol-gel process for the production of nano-scale sized alpha-alumina powder is for example described in US 7,638,105, the content of which is incorporated herein by reference. A sol-gel process for the production of nano-scale sized layered silicates is for example described in Chem. Materials, 1991, 3(5), pages 772-775, the content of which is incorporated herein by reference. Such processes may be used analogously to produce the nano-scale filler materials as used according to the present invention. As nano-scale silica is the preferred nano-scale filler material, the contents of WO 2009/015724 Al is further described in more detail, whereby these details, especially with respect to average grain sizes, surface treatment, filler content of the hardenable electrical insulation composition, and production techniques for making the hardenable electrical insulation composition also apply to and include the other nano-scale filler materials mentioned herein. The expert in the art will know how to apply these techniques.

WO 2009/015724 Al describes a sol-gel process for the production of a hydrophobic, monodispersed, silicon dioxide, with an average grain size distribution within the nano-scale. The process incorporates the following steps: (a) providing an aqueous sus- pension of colloidal silicon dioxide with an average grain size within the range of 1 nm to 500 nm; (b) reacting the colloidal silicon dioxide with an organosilane and/or an organosiloxane in an aprotic cyclic ether thereby silanising the colloidal silicon dioxide; (c) separating the aqueous phase of the reaction mixture from the organic phase; (d) adding a further time to the organic phase an organosilane and/or an organosiloxane in an aprotic cyclic ether and silanising the colloidal silicon dioxide; and (e) separating the aqueous phase of the reaction mixture from the organic phase.

The selected nano-scale sized silica powder as described above, is obtained as a dry solid monodispersed powder subsequent to step (e) by eliminating the solvent from the suspension obtained in step (e) , e.g. by distilling off the solvent from the sus- pension under vacuum at elevated temperature. Such dry solid pretreated nano-scale sized silica powder [defined as component (c) ] may be used according to the present invention and can be added at any stage during the preparation of the curable electrical insulation composition, either to a single component or to an intermediate mixture of the curable electrical insulation composition .

The filler starting material used for preparing the nano-scale sized filler material in the sol-gel process, such as the colloidal silicon dioxide, used for preparing the nano-scale sized silica powder in the sol-gel process, have an average grain size preferably within the range of 2 nm - 300 nm, preferably within the range of 3 nm - 200 nm, preferably within the range of 4 nm - 150 nm, preferably within the range of 4 nm - 80 nm, and preferably within the range of 10 nm - 40 nm.

Said nano-scale sized filler material as obtained in the sol-gel process, with the preferences as mentioned herein before, is present within the curable electrical insulation composition and the cured electrical insulation system preferably in an amount of about 5%-15% by weight, preferably in an amount of about 8%-12% by weight, and preferably in an amount of about 10% by weight, calculated to the weight of the conventional filler material present in the electrical insulator system.

Preferably the nano-scale sized filler material as obtained in the sol-gel process, with the preferences as mentioned herein before, preferably the nano-scale sized silica powder, is present within the electrical insulation system in an amount of about 3% to 8% by weight, preferably at about 5% by weight, calculated to the total weight of the electrical insulator system.

The organosilane and/or an organosiloxane as used in an aprotic cyclic ether for silanising the colloidal filler material such as the silicon dioxide is a reactive organosilane and/or organosi- loxane, preferably a trialkylhalosilane such as trimethylchloro- silane, or a trialkylalkoxysilane such as trimethylmethoxysilane .

Preferably the organosilane in the aprotic cyclic ether, thereby silanising for example the colloidal silicon dioxide, is 3- glycidoxypropyltrimethoxysilane or 3-glycidoxypropyldimethoxy- methylsilane . In this way a pretreated nano-scale sized silica powder is obtained which carries on it surface reactive glycidyl groups, such as the 3-glycidoxypropylsilyl groups.

It is possible to make a homogeneous suspension of the selected nano-scale sized filler material in the form of a masterbatch by intensively mixing the dry solid nano-scale sized filler material with a part of the curable electrical insulation composition or with a component thereof, or with a part of a component thereof. Such a masterbatch, containing the nano-scale sized filler material, can be used for adding the nano-scale filler material to the curable electrical insulation composition at any stage of its preparation. Within the scope of the present invention, the nano-scale filler material is preferably used in the form of a master-batch, preferably as a masterbatch wherein the nano-scale filler maerial is dispersed in diglycidylether-bisphenol A

(DGEBA-resin) and/or diglycidylether-bisphenol F (DGEBF-resin) .

The present invention also refers to a method of producing said masterbatch containing the selected nano-scale sized filler material, characterized in that the hardenable polymer component in the form of a monomeric or an oligomeric starting material of the respective component (a) defined as component (d) herein above, and the selected nano-scale sized filler material defined as component (c) herein above, are thoroughly mixed together in a weight ratio so that the selected nano-scale sized filler material is present in the masterbatch in an amount of l%-30% by weight, preferably in an amount of l%-20% by weight, calculated to the total weight of the masterbatch.

The present invention also refers to a method of producing the hardenable electrical insulation composition as defined herein above, said composition comprising a hardenable polymer component in the form of a monomeric or oligomeric starting material of the respective component (a) ; a conventional filler material as de ^¬ fined herein above as component (b) , and the selected nano-scale sized filler material as defined herein above as component (c) , characterized in that the components of the electrical insulation composition are mixed together in any desired sequence.

The present invention further refers to a method of producing the hardenable electrical insulation composition as defined herein above, said composition comprising a hardenable polymer component in the form of a monomeric or oligomeric starting material of the respective component (a) ; a conventional filler material as defined herein above as component (b) , and the selected nano- scale sized filler material as defined herein above as component (c) , characterized in that the components of the electrical insu ^¬ lation composition are mixed together in any desired sequence, whereby said selected nano-scale sized filler material defined as component (c) is added in the form of a masterbatch at any stage during the production sequence of the electrical insulation composition .

In a further embodiment, the present invention refers to an electrical insulation system with improved electrical breakdown strength, said electrical insulation system comprising a hardened polymer component having incorporated therein a selected nano- scale sized filler material (but not containing a conventional filler material) , characterized in that, - said hardened polymer component is identical with component (a) as defined above; and

said selected nano-scale sized filler material is identical with component (c) as defined above, wherein said selected nano-scale sized filler material is present within the

electrical insulation system in an amount of about l%-20% by weight, calculated to the total weight of the electrical insulator system.

Correspondingly, the present invention further refers to a hardenable electrical insulation composition for producing an electrical insulation system, said hardenable electrical

insulation composition comprising a hardenable polymer component having incorporated therein a selected nano-scale sized filler material (but not containing a conventional filler material), characterized in that,

said hardenable polymer component is a monomeric or an oligo ^¬ meric starting material of the respective component (a) as defined above; and

said selected nano-scale sized filler material is identical with component (c) as defined above, wherein said selected nano-scale sized filler material is present within the

electrical insulation system in an amount of about l%-20% by weight, calculated to the weight of the conventional filler material present in the electrical insulator system.

The present invention also refers to a method of producing a hardenable electrical insulation composition, said hardenable composition comprising a hardenable polymer component in the form of a monomeric or oligomeric starting material of the respective component (a) having incorporated therein a selected nano-scale sized filler material as defined herein above as component (c) (but not containing a conventional filler material), charac ^¬ terized in that the components are mixed together in any desired sequence . In such an electrical insulation system, which does not contain a conventional filler material, the selected nano-scale sized filler material as defined herein above as component (c) is preferably present in an amount of about 3%-10% by weight, preferably in an amount of about 3%-8% by weight, preferably at about 5% by weight, calculated to the total weight of the electrical insulator system.

The present invention further refers to electrical articles comprising said electrical insulation system comprising a hardened polymer component defined as component (a) herein above, and the selected nano-scale sized filler material defined as component (c) herein above [excluding the presence of a conventional filler material of component (b) ] .

The filler component (b) as well as the filler component (c) can be incorporated into the respective monomeric or oligomeric starting material of component (a) analogously to any known manner to be uniformly dispersed therein, as described in the literature for other filler materials. This is within the knowledge of the expert. The non-hardened composition thus obtained, e.g. the non-hardened epoxy resin composition, can for example be processed using conventional vacuum casting and/or automated pressure gelation (APG) manufacturing processes. The dispersion is formed into the desired shape using known methods, optionally with the help of a molding tool, and then hardened out, optionally using post-curing, whereby the electrical insulation system according to the present invention is obtained.

As optional additives the composition may comprise further components selected from wetting/dispersing agents, plasticizers , antioxidants, light absorbers, and from further additives generally used in electrical applications. Preferred epoxy resins used within the context of the present invention are aromatic and/or cycloaliphatic compounds. These compounds are known per se. Said epoxy resins are reactive glycidyl compounds containing at least two 1,2-epoxy groups per molecule. Preferably a mixture of polyglycidyl compounds is used such as a mixture of diglycidyl- and triglycidyl compounds.

Epoxy compounds useful for the present invention comprise unsub- stituted glycidyl groups and/or glycidyl groups substituted with methyl groups. These glycidyl compounds preferably have a molecular weight between 200 and 1200, especially between 200 und 1000 and may be solid or liquid. The epoxy value (equiv./100 g) is preferably at least three, preferably at least four and especially at about five, preferably about 4.9 to 5.1. Preferred are glycidyl compounds which have glycidyl ether- and/or glycidyl ester groups. Such a compound may also contain both kinds of glycidyl groups, e.g. 4-glycidyloxy-benzoic acidglycidyl ester. Such compounds are known. Examples of preferred glycidyl compounds which have glycidyl ether groups are for example optionally substituted epoxy resins of formula (IV) :

D = -0-, -S02-, -CO-, -CH2-, -C(CH3)2-, -C(CF3)2- n = zero or 1 or formula (V) : Examples are glycidyl ethers derived from Bisphenol A or Bis- phenol F as well as glycidyl ethers derived from Phenol-Novolak- resins or cresol-Novolak-resins .

Cycloaliphatic epoxy resins are for example hexahydro-o-phthalic acid-bis-glycidyl ester, hexahydro-m-phthalic acid-bis-glycidyl - ester or hexahydro-p-phthalic acid-bis-glycidyl ester. Also ali ^¬ phatic epoxy resins, for example 1 , 4-butane-diol diglycidylether , may be used as a component for the composition of the present invention .

Preferred within the present invention are also aromatic and/or cycloaliphatic epoxy resins which contain at least one, preferably at least two, aminoglycidyl group in the molecule. Such epoxy resins are known and for example described in WO 99/67315.

Preferred compounds are those of formula (VI) :

D = -0 -, -S02-, -CO -, -C H 2-, -C (C H 3 )2-, -C (C F3)2- n = Zero or 1

Especially suitable aminoglycidyl compounds are N, N-diglycidyl- aniline, N, -diglycidyltoluidine , N, , ' , ' -tetraglycidyl-1 , 3- diaminobenzene, Ν,Ν,Ν' ,N'-tetraglycidyl-l, 4-diaminobenzene, Ν,Ν,Ν' , ' -tetraglycidylxylylendiamine, Ν,Ν,Ν' , N ' -tetraglycidyl- 4,4' -diaminodiphenylmethane, N, N, N ' , N ' -tetraglycidyl-3 , 3 ' -di ^¬ ethyl- 4 , 4 ' -diaminodiphenylmethane , N, , ' , ' -tetraglycidyl-3 , 3 ' - diaminodiphenylsulfone, N, N ' -Dimethyl-N, ' -diglycidyl -4 ,4 ' -diaminodiphenylmethane, N, N, N ' , N ' -tetraglycidyl-alfa, alfa ' -bis ( - aminophenyl ) -p-diisopropylbenzene and N, , ', ' -tetraglycidyl- alfa, alfa ' -bis- (3 , 5-dimethyl -4 -aminophenyl ) -p-diisopropylbenzene . Preferred aminoglycidyl compounds are also those of formul

(VII) :

or of formula (VIII) :

Further aminoglycidyl compounds which can be used according to the present invention are described in e.g. Houben-Weyl, Methoden der Organischen Chemie, Band E20, Makromolekulare Stoffe, Georg Thieme Verlag Stuttgart, 1987, pages 1926-1928.

Hardeners are known to be used in epoxy resins. Hardeners are for example hydroxyl and/or carboxyl containing polymers such as carboxyl terminated polyester and/or carboxyl containing

acrylate- and/or methacrylate polymers and/or carboxylic acid anhydrides. Useful hardeners are further cyclic anhydrides of aromatic, aliphatic, cycloaliphatic and heterocyclic polycarbonic acids. Preferred anhydrides of aromatic polycarbonic acids are phthalic acid anhydride and substituted derivates thereof, benzene-1 , 2 , 4 , 5-tetracarbonic acid dianhydride and substituted derivates thereof. Numerous further hardeners are from the literature .

The optional hardener can be used in concentrations within the range of 0.2 to 1.2, equivalents of hardening groups present, e.g. one anhydride group per 1 epoxide equivalent. However, often a concentration within the range of 0.2 to 0.4, equivalents of hardening groups is preferred. As optional additives the composition may comprise further at least a curing agent (accelerant) for enhancing the polymerization of the epoxy resin with the hardener, at least one wetting/dispersing agent, plasticizers , antioxidants, light absorbers, as well as further additives used in electrical applications.

Curing agents for enhancing the polymerization of the epoxy resin with the hardener are for example tertiary amines, such as benzyldimethylamine or amine-complexes such as complexes of ter- tiary amines with boron trichloride or boron trifluoride; urea derivatives, such as N-4-chlorophenyl-N ' , N' -dimethylurea (Monu- ron) ; optionally substituted imidazoles such as imidazole or 2- phenyl-imidazole . Preferred are tertiary amines. Other curing catalyst such as transition metal complexes of cobalt (III), copper, manganese, (II), zinc in acetylacetonate may also be used, e.g. cobalt acetylacetonate (III) . The amount of catalyst used is a concentration of about 50-1000 ppm by weight, calculated to the composition to be cured. Wetting/dispersing agents are known per se for example in the form of surface activators; or reactive diluents, preferably epoxy-containing or hydroxyl-containing reactive diluents; thixo- tropic agents or resinous modifiers. Known reactive diluents for example are cresylglycidylether, diepoxyethyl-1 , 2-benzene, bis- phenol A, bisphenol F and the diglycidylethers thereof, diep- oxydes of glycols and of polyglycols, such as neopentylglycol- diglycidylether or trimethylolpropane-diglycidylether . Preferred commercially available wetting/dispersing agents are for example organic copolymers containing acidic groups, e.g. Byk® W-9010 having an acid value of 129 mg KOH/g) . Such wetting/dispersing agents are preferably used in amounts of 0.5 % to 1.0 % based on the filler weight. Plasticizers , antioxidants, light absorbers, as well as further additives used in electrical applications are known in the art and are not critical.

The insulating composition made from epoxy resin is made by mixing all the components, optionally under vacuum, in any desired sequence and curing the mixture by heating. Preferably the hardener and the curing agent are separately added before curing. The curing temperature is preferably within the range of 50°C to 280°C, preferably within the range of 100°C to 200°C. Curing generally is possible also at lower temperatures, whereby at lower temperatures complete curing may last up to several days, depending also on catalyst present and its concentration.

The non-hardened insulating resin composition is preferably applied by using vacuum casting or automated pressure gelation (APG) manufacturing processes, optionally under the application of vacuum, to remove all moisture and air bubbles from the coil and the insulating composition. The composition may then be cured by any method known in the art.

Preferred uses of the insulation produced according to the present invention are electrical insulations, especially in the field of impregnating electrical coils and in the production of electrical components such as transformers, bushings, insulators, switches, sensors, converters and cable end seals.

Preferred uses of the insulation system produced according to the present invention are also high-voltage insulations for indoor and outdoor use, especially for outdoor insulators associated with high-voltage lines, as long-rod, composite and cap-type insulators, and also for base insulators in the medium-voltage sector, in the production of insulators associated with outdoor power switches, measuring transducers, lead-throughs , and overvoltage protectors, in switchgear construction, in power switches, dry-type transformers, and electrical machines, as coating materials for transistors and other semiconductor elements and/or to impregnate electrical components. The

following examples illustrate the invention. Definition of raw materials:

EPR 845: Bisphenol A/F epoxy, Hexion Specialty Chemicals

EPH 845: Modified carboxylic anhydride, Hexion Specialty Chemicals EPC 845: Modified tertiary amine, Hexion Specialty Chemicals Microsilica W12:Silica flower d _50% = 16 μιτι, Quarzwerke GmbH

Nanopox E470: Masterbatch of silica 5-50 nm nanoparticles

(40wt% Si0 ₂ ) dispersed in DGEBA, Nanoresins AG.

Compositions

The compositions used are given in Table 1:

Reference 1: unfilled epoxy resin composition,

Reference 2: epoxy resin composition filled with Microsilica W12, Example 1: epoxy resin composition filled with nanosilica, and Example 2 : epoxy resin composition filled with Microsilica W12 and nanosilica (Nanopox E470) .

Table 1

Reference 1 (Sample preparation)

All the components are separately preheated at a temperature of 90°C for 2 hours in an oven (Step 1) . The components EPH 845 and EPR 845 are mixed together in a mixing apparatus under a vacuum of 0.1 bar and at a temperature of 90°C for 5 minutes (Step 2) . Mixing is then continued during 10 minutes at a temperature of 90°C under normal pressure, without the application of vacuum (Step 3) . Then, EPC 845 is added and mixing is continued under vacuum for further 5 minutes under normal pressure (Step 4) . The oven, still being kept at a temperature of 90°C, is then

evacuated to a vacuum of 0.1 bar (Step 5) and kept at this vacuum and at this temperature for about 10 minutes. The curable epoxy resin composition is poured into a mould which has been preheated for about 2 hours at 130°C. The mould is then evacuated to a vacuum of 0.1 bar and kept at a temperature of 140 °C for 10 hours to cure the epoxy resin composition.

Reference 2 (Sample preparation)

The preparation is made analogous to the sample preparation of Reference 1, with the difference that Microsilica W12 is slowly added in Step 3 and the mixture is mixed for further 10 minutes after the addition has been completed.

Example 1 (Sample preparation)

The preparation is made analogous to the sample preparation of Reference 1, with the difference that Nanopox E470 is added in Step 2 together with the other components EPH 845 and EPR 845 and all the components are mixed together in a mixing apparatus under a vacuum of 0.1 bar and at a temperature of 90°C for five minutes . Example 2 (Sample preparation)

The preparation is made analogous to the sample preparation of Reference 2, with the difference that Nanopox E470 is added in Step 2 together with the other components EPH 845 and EPR 845 and the mixture is mixed for further 10 minutes after the addition has been completed. Table 2. (Properties of unfilled and microsilica filled epoxy with and without the addition of Nanopox)

Tg (°C) : glass-transission temperature in °C

E (MPa) : Young' s modulus in MPa

std dev.: Standard deviation

o _b (MPa) : Tensile strength in MPa

S _b %) : Elongation at break in % (percent )

do (kj/m ² ) : Critical energy release rate in kJ/m ²

Example 3

Analogous results are obtained when the nano-scale sized silica as used in Examples 1 and 2 is replaced by a nano-scale sized zinc oxide dispersion as described in US 6,699,316; a nano-scale sized alpha-alumina powder as described in US 7,638,105; or a nano-scale sized layered silicate as described in Chem.

Materials, 1991, 3(5), pages 772-775.

Discussion

In the case of pure epoxy (Reference 1/Example 1), one can see that the addition of Nanopox has several beneficial and

simultaneous effects on the resulting properties: increase of Tg, increase of tensile strength and critical energy release rate. Increase of Tg has usually a negative effect on critical energy release rate. Therefore, these results are surprising. The amount of EPH 845 in Example 1 (compared to Reference 1) was adjusted because in Example 1 Nanopox is added as a masterbatch constituted of silica nanoparticle dissolved in DGEBA resin. In order to maintain the same stoichiometry (in Reference 1 and Example 1) , it was necessary to adjust the amount of anhydride hardener .

Comparing the microsilica filled systems (Reference 2/Example 2 ) , one can see that the addition of Nanopox increases significantly the Tg, but also the tensile strength while maintaining good fracture properties. In this case the amount of anhydride has been adjusted to maintain the same stoichiometry and the amount of microsilica was adjusted to maintain the same overall

micro/nano silica content.