Field of Invention

The present invention refers to a curable epoxy resin composi- tion comprising an epoxy resin component, a filler component and optionally a hardener component and further additives, which has been produced by intensively mixing together at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the additives, under defined conditions, previously to mixing theses components with the hardener component and with the remaining optional additives which are present in the curable epoxy resin composition. The resulting cured epoxy resin composition has improved mechanical properties such as flexural properties, tensile properties or fracture toughness, combined with excellent electrical properties and is specially suited for casting electrical insulations.

State of the Art

Epoxy resin compositions are commonly used as insulating materials for electrical applications because the compositions generally have a comparatively low price, are easy to process and, after curing, yield electrical insulator systems with good electrical and mechanical properties. In order to enhance specific properties, such as mechanical and electrical properties or the thermal conductivity or to reduce cost, epoxy resins are often filled with inorganic filler materials such as silica, quartz, known silicates, aluminium oxide, aluminium trihydrate, as well as other filler materials known to be used in electrical applications. The usual way to prepare filled epoxy resin compo- sitions is to mix all the components, i.e. the epoxy resin component, the hardener component, the filler material, and the optional further additives, at a temperature which is not higher than the casting temperature of the final epoxy resin composition. The obtained mixture is then poured into a mould and the mould is put into an oven for curing, yielding the cured electrical insulator. Commercial markets require that electrical devices withstand various types of tests including dielectric tests (voltage and partial discharge tests, power tests, etc.), mechanical tests (bending, torsion, vibration, pressure test, etc.) , thermal and thermo-mechanical tests, etc. For some specific applications, these tests show that cured filler containing epoxy resin compositions are often too brittle.

In order to improve the mechanical properties and especially to reduce the brittleness of filler containing epoxy resin compositions, special additives, so called tougheners, are often added to the curable epoxy resin composition. However, the incorporation of such tougheners renders the production of the epoxy resin composition more complicated and often impairs the thermal or thermo-mechanical properties of the cured product. Therefore, it would be advantageous to prepare a curable epoxy resin composition with improved mechanical properties, especially with respect to brittleness, without the use of additional additives, whereby said curable epoxy resin compo- sitions are suitable for producing therefrom electrical insulators and are compatible with conventional vacuum casting or Automated Pressure Gelation (APG) manufacturing processes.

Summary of the Invention It has now been found that cured epoxy resin compositions with significantly improved mechanical properties, such as flexural properties, tensile properties and fracture toughness, and without impairing the electrical or thermo-mechanical properties, are surprisingly obtained, when the curable epoxy resin compo- sition comprising an epoxy resin component and a filler component and optionally a hardener component as well as further additives, is produced by intensively and separately mixing together at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the further additives at a temperature higher than the casting temperature of the curable epoxy resin composition, previously to mixing theses components with the hardener component and with any remaining optional additives present in the curable epoxy resin composition. For this mixing process a standard mixing procedure can be used.

Description of the Invention

The present invention is defined in the claims. The present invention specifically refers to a curable epoxy resin composition comprising an epoxy resin component and a filler component and optionally a hardener component and further additives, characterized in that (a) said curable epoxy resin composition has been produced by separately mixing together at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the optional additives, previously to mixing theses components with the optional hardener component and with any remaining optional additives present in the curable epoxy resin composition, and that (b) said mixing together of at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the optional additives has been carried out at a temperature higher than the casting temperature of the curable epoxy resin composition.

The present invention further refers to a method of producing a curable epoxy resin composition comprising an epoxy resin component, a filler component and optionally a hardener component and further additives, characterized in that said method comprises the steps of separately mixing together at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the further additives, at a temperature higher than the casting temperature of the curable epoxy resin composition to be produced, and mixing the resulting mixture of said components with the hardener component and with any remaining optional additives present in the curable epoxy resin composition. The present invention further refers to the use of said curable epoxy resin composition for the production of insulation systems in electrical articles.

The present invention further refers to said cured epoxy resin composition, which is present in the form of an electrical insulation system, resp. in the form of an electrical insulator. The present invention further refers to the electrical articles comprising an electrical insulation system made according to the present invention.

The present invention further refers to an epoxy resin component that can be used in an curable epoxy resin composition, said curable epoxy resin composition comprising an epoxy resin com- ponent, a filler component and optionally further additives, characterized in that said epoxy resin component has been produced by separately mixing together at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the further additives at a temperature higher than the casting temperature of the curable epoxy resin composition to be produced with said epoxy resin component .

The present invention further refers to a method of producing an epoxy resin component that can be used in an curable epoxy resin composition comprising an epoxy resin component, a filler component and optionally a hardener component and further additives, characterized in that at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the further additives are separately mixed together at a temperature higher than the casting temperature of the curable epoxy resin composition to be produced with said epoxy resin component.

"At least a part of the epoxy resin component" means that at least 50% by weight, preferably 65% by weight, preferably 80% by weight, preferably 90% by weight and preferably 100% by weight of the epoxy resin component are separately mixed together with at least a part of the filler component and optionally some or all of the further additives.

"At least a part of the filler component" means that at least 30% by weight, preferably 50% by weight, preferably 70% by weight, preferably 90% by weight and preferably 100% by weight of the filler component are separately mixed with the epoxy resin component and optionally with some or with all of the further additives.

Preferably 100% by weight of the epoxy resin component present within the epoxy resin composition and 100% by weight of the filler component present within the epoxy resin composition and optionally some or all of the further additives are separately mixed together at a temperature higher than the casting temperature of the curable epoxy resin composition to be produced with said epoxy resin component.

At a temperature higher than the casting temperature of the curable epoxy resin composition to be produced with said epoxy resin component means that said temperature is within the range of 60 ⁰ C to 180 ⁰ C, preferably within the range of 70°C to 170 ⁰ C, preferably within the range of 80 ⁰ C to 160 ⁰ C, preferably within the range of 90 ⁰ C to 140 ⁰ C, and preferably within the range of 100°C to 140°C.

Said mixing process is preferably, but not necessarily, carried out under vacuum, preferably at a pressure of less than 100 mbar

(<100 mbar) , preferably less than 50 mbar (<50 mbar) , preferably less than 20 mbar (<20 mbar) and preferably less than 10 mbar

(<10 mbar) .

Said mixing process is preferably carried out for a time period within the range of 10 minutes to twenty-four hours, preferably for a time period within the range of 30 minutes to six hours, and more preferably within the range of one hour to three hours.

The epoxy resin component as used according to the present invention within the curable epoxy resin composition contains at least two 1,2-epoxy groups per molecule. Cycloaliphatic and cycloaromatic epoxy resin compounds useful for the present invention comprise unsubstituted glycidyl groups and/or glycidyl groups substituted with methyl groups. These glycidyl compounds have an epoxy value (equiv./kg) preferably of at least three, preferably at least four and especially at about five or higher, preferably about 5.0 to 6.1. Preferred are for example optionally substituted epoxy resins of formula (I) :

D = -O-, -SO2-, -CO-, -CH2-, -C(CH3)2-, -C(CF3)2- n = zero or 1

or optionally substituted epoxy resins of formula (II) )

D = -O - , -S O 2- , -C O -, -C H 2 -, -C (C H 3 )2 - , -C (C F 3 )2 - n = ze ro o r 1

Compounds of formula (I) or formula (II) wherein D is [-(CH ₂ )-] or [-C(CH ₃ J ₂ -] are preferred. Preferred further are compounds of formula (II) wherein D is [-(CH ₂ )-] or [-C(CH ₃ J ₂ -], and preferably [-C(CH ₃ J ₂ -], i.e. diglycidylether of 2, 2-bis- (4-hydroxy- phenyl) -propane [diglycidylether of bisphenol A (DGEBA)] . DGEBA is commercially available as an epoxy resin component, e.g. as Epilox A19-00 (Leuna Harze GmbH.) or similar products. DGEBA as preferably used in the present invention has an epoxy value (equiv./kg) of at least three, preferably at least four and especially at about five or higher, preferably about 5.0 to 6.1.

Preferred cycloaliphatic epoxy resin compounds are for example Araldite® CY 184 (Huntsman Advanced Materials Ltd.) , a cycloaliphatic diglycidylester epoxy resin compound having an epoxy content of 5.80-6.10 (equiv/kg) .

Further epoxy resins to be used within the scope of the present invention are for example hexahydro-o-phthalic acid-bis-glycidyl ester, hexahydro-m-phthalic acid-bis-glycidyl ester or hexa- hydro-p-phthalic acid-bis-glycidyl ester. Preferred epoxy resin compounds are liquid at room temperature or when heated to a temperature of up to about 100 ⁰ C.

The present epoxy resin composition comprises an epoxy resin component and a filler which have been treated according to the present invention. Said epoxy resin component may optionally contain additives. Said epoxy resin component can be used in any curable epoxy resin composition, such as in hardener cured epoxy resin compositions as well as in compositions that do not contain a hardener component for curing the composition. The effect of the invention is also obtained for compositions containing a homopolymerizable epoxy resin component, yielding a homopolymerized epoxy resin composition, or in a amine cured epoxy resin composition or in any other known polymerisable epoxy resin composition. A hardener is not necessary to obtain the effect of the present invention.

Numerous hardeners are known to be used as hardener component in epoxy resin compositions. The preferred hardener component is an acid anhydride. Such anhydrides are preferably aliphatic and cycloaliphatic or aromatic polycarbonic acid anhydrides. Preferred are phthalic anhydride, a tetra-hydrophtalic anhydride (THPA) , a hexahydrophtalic anhydride (HHPA) , a methylhydro- phthalic anhydride, a methyltetrahydrophthalic anhydride (MTHPA) , a methyl-hexahydrophtalic anhydride (MHHPA) , or a methyl-nadic anhydride (MNA) or a mixture thereof.

MTHPA, for example, is commercially available and exists in different forms, e.g. as 4-methyl-l, 2, 3, 6-tetrahydrophthalic anhydride or as 4-methyl-3, 4, 5, 6-tetrahydrophthalic anhydride. Although the different forms are not critical for the application in the present invention, 4-methyl-l, 2, 3, 6-tetrahydrophthalic anhydride and 4-methyl-3, 4, 5, 6-tetrahydrophthalic an- hydride are the preferred compounds to be used.

Methyltetrahydrophthalic anhydride (MTHPA) is often supplied commercially as a mixture containing MTHPA isomers as the main component, together with other anhydrides, such as tetrahydro- phthalic anhydride (THPA) , methylhexahydrophthalic anhydride

(MHHPA) and/or phthalic anhydride (PA) . Such mixtures may also be used within the scope of the present invention. The content of MTHPA within such a mixture is preferably at least 50% by weight, preferably at least 60% by weight, preferably at least 70% by weight, preferably at least 80% by weight, and preferably at least 90% by weight, calculated to the total weight of the mixture .

The hardener component within the epoxy resin composition is present preferably in concentrations within the range of 0.8 to 1.2 reactive group equivalents of the hardener component, calculated per one epoxy equivalent present in the epoxy resin component; preferably one reactive group equivalent of the hardener component, per one epoxy equivalent present in the epoxy resin component.

The filler component is preferably selected from conventional filler materials as are generally used as fillers in electrical insulations. Preferably said filler is selected from the group of filler materials comprising inorganic oxides, inorganic hydroxides and inorganic oxyhydroxides, preferably silica, quartz, known silicates, aluminium oxide, aluminium trihydrate [ATH], titanium oxide or dolomite [CaMg (CO ₃ ) ₂ ] , wollastonite, glass beads, metal nitrides, such as silicon nitride, boron nitride and aluminium nitride or metal carbides, such as silicon carbide as well as cut or continuous reinforcing fibers of known composition, length and diameters. Also a mixture of different fillers may be used. Preferred are silica and quartz, specifically silica flour, with a Si0 ₂ -content of about 95-98% by weight .

The filler material has an average grain size as known for the use in electrical insulation systems and is generally within the range of 100 nm (nanometer) up to 3 mm. Preferred, however, is an average grain size (at least 50% of the grains) within the range of about 1 μm to 300 μm, preferably from 5 μm to 100 μm, or a selected mixture of such average grain sizes. Preferred also is a filler material with a high surface area.

The filler component is present in the epoxy resin composition, depending on the final application of the epoxy resin composition, preferably within the range of about 50% by weight to about 80% by weight, preferably within the range of about 55% by weight to about 75% by weight, preferably at about 60% by weight to about 70% by weight, preferably at about 65% by weight to about 70% by weight, calculated to the total weight of the epoxy resin composition.

The filler material optionally may be present in a ,,porous" form. As a porous filler material, which optionally may be coated, is understood, that the density of said filler material is within the range of 60% to 80%, compared to the real density of the non-porous filler material. Such porous filler materials have a higher total surface area than the non-porous material. Said surface area is higher than 0.3 m ² /g (BET m ² /g) and prefe- rably higher than 0.4 m ² /g (BET) and preferably is within the range of 0.4 m ² /g (BET) to 100 m ² /g (BET), preferably within the range of 0.5 m ² /g (BET) to 80 m ² /g (BET) .

The curable epoxy resin composition according to the present invention may comprise an electrically conductive or semi- conductive filler material, such that the electrical volume resistivity of the cured epoxy resin composition is reduced, preferably within the range of about 10E9 Ω.cm to 10E14 Ω.cm, preferably within the range of about 10E10 Ω.cm to 10E14 Ω.cm, preferably within the range of about 10E12 Ω.cm to 10E14 Ω.cm.

Further conductive or semi-conductive material may be selected from doped metal oxides, antimony trioxide, zinc oxide, carbon black, or metal coated insulating filler. The semi-conductive or conductive filler is present in the epoxy resin composition, depending on the final application of the epoxy resin composition, preferably within the range of about 0.0001% by weight to about 1% by weight, preferably within the range of about 0.001% by weight to about 0.5% by weight, preferably at about 0.001% by weight to about 0.01% by weight, calculated to the total weight of the epoxy resin composition.

The curable epoxy resin composition according to the present invention may comprise further a curing agent for enhancing the polymerization of the epoxy resin with the hardener. Further additives may be selected from hydrophobic compounds including silicones, wetting/dispersing agents, plasticizers, antioxidants, light absorbers, pigments, flame retardants, fibers, tougheners and other additives generally used in electrical applications. These are known to the expert.

Preferred curing agents are for example tertiary amines, such as benzyldimethylamine or amine-complexes such as complexes of tertiary amines with boron trichloride or boron trifluoride; urea derivatives, such as N-4-chlorophenyl-N' ,N ' -dimethylurea (Monuron) ; optionally substituted imidazoles such as imidazole or 2-phenyl-imidazole . Preferred are tertiary amines, especially 1-substituted imidazole and/or N, N-dimethylbenzylamine, such as 1-alkyl imidazoles which may or may not be substituted also in the 2-position, such as 1-methyl imidazole or l-isopropyl-2- methyl imidazole. Preferred is 1-methyl imidazole. The amount of catalyst used is a concentration of about 0.1% to 2.0% by weight, calculated to the weight of the epoxy resin component present in the composition and is conventional.

Suitable hydrophobic compounds or mixtures of such compounds, especially for improving the self-healing properties of the electrical insulator may be selected from the group comprising flowable fluorinated or chlorinated hydrocarbons which contain -CH ₂ -units, -CHF-units, -CF ₂ -units, -CF ₃ -units, -CHCl-units, -C (Cl) ₂ -units, -C (Cl) ₃ -units, or mixtures thereof; or a cyclic, linear or branched flowable organopolysiloxane . Such compounds, also in encapsulated form, are known per se .

Suitable polysiloxanes are known and may be linear, branched, cross-linked or cyclic. Preferably the polysiloxanes are composed of -[Si(R) (R) 0] -groups, wherein R independently of each other is an unsubstituted or substituted, preferably fluorinated, alkyl radical having from 1 to 4 carbon atoms, or phenyl, preferably methyl, and wherein said substituent R may carry reactive groups, such as hydroxyl or epoxy groups. Non-cyclic siloxane compounds preferably on average have about from 20 to 5000, preferably 50-2000, - [Si (R) (R) O] -groups . Preferred cyclic siloxane compounds are those comprising 4-12, and preferably 4-8, - [Si (R) (R) O] -units.

The hydrophobic compound is added to the epoxy resin composition preferably in an amount of from 0.1% to 10%, preferably in an amount of from 0.25% to 5% by weight, preferably in an amount of from 0.25% to 3% by weight, calculated to the weight of the weight of the epoxy resin component present. Suitable processes for casting curable epoxy resin compositions, including the curable epoxy resin composition of the present invention, are for example the Vacuum Casting Process and the Automated Pressure Gelation (APG) Process. These processes are carried out at a casting temperature within the range of room temperature to 150 ⁰ C, preferably within the range of 50 ⁰ C to 150 ⁰ C, typically at a temperature of about 65°C. This further means that the epoxy resin component of the present invention, which has been obtained by separately mixing together at least a part of the epoxy resin component and at least a part of the filler component and optionally some or all of the further additives, at a temperature higher than the casting temperature of the curable epoxy resin composition to be produced, preferably is cooled down after mixing to at least the casting temperature of the final curable epoxy resin composition, before mixing said epoxy resin component with the hardener and with any remaining optional additives present in the curable epoxy resin composition.

The uncured epoxy resin composition is cured at a temperature of room temperature to about 280 ⁰ C, preferably within the range of 50°C to 280°C, preferably within the range of 100°C to 200°C, preferably within the range of 100 ⁰ C to 170 ⁰ C, and preferably at about 130 ⁰ C and during a curing time within the range of about 30 minutes to about 10 hours. Curing generally is possible also at lower temperatures, whereby at lower temperatures complete curing may last up to several days depending on the catalyst present and its concentration. The Vacuum Casting Process and the Automated Pressure Gelation (APG) Process when being applied to a composition of the present invention also typically include such a curing step in the mold for a time sufficient to shape the epoxy resin composition into its final infusible three dimensional structure.

Preferred uses of the electrical insulation systems produced with epoxy resin compositions according to the present invention are dry-type transformers, cable terminations, insulators for gas insulated switchgears, particularly cast coils for dry type distribution transformers, especially vacuum cast dry distribution transformers, which within the resin structure contain electrical conductors, and in the casting of electrical components such as bushings, switches, insulators, sensors, converters, cable end seals and insulators. These may be produced also for medium and high-voltage insulations for indoor and outdoor use, like breakers or switchgear applications; medium and high voltage bushings; 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, leadthroughs, and overvoltage protectors, in switchgear constructions, in power switches, and electrical machines, as coating materials for transistors and other semiconductor elements and/or to impregnate electrical components. The following examples illustrate the invention without limiting the scope of the claimed invention.

Examples 1, 3 and 5 (Reference Examples)

In Examples 1, a Reference Example as a standard silica filled epoxy resin composition was prepared according to the standard mixing procedure. All the components apart from the catalyst were mixed during 1 hour at 65°C under vacuum (<lmbar) . Then the catalyst was added and the mixture was stirred 10 minutes at

65°C under vacuum (<lmbar) , before being poured into the moulds under vacuum (30 mbar) . The moulds were then put in an oven for curing and post-curing.

In Example 3, a Reference Example as a standard aluminium oxide filled solid epoxy resin composition was prepared according to a state of the art mixing procedure. The three components (epoxy, hardener and filler) were mixed at 125 ⁰ C under vacuum (<lmbar) until the hardener was completely dissolved. The material was then poured into moulds under vacuum (about 30 mbar) and the moulds were put in an oven at 130 ⁰ C for curing. In Example 5, a Reference Example as a standard aluminium oxide filled epoxy resin based on liquid components was prepared according to a mixing method known as state of the art. All the components apart from the catalyst were first mixed together at 70 ⁰ C under vacuum (<lmbar) till an homogeneous mixture was obtained, then the catalyst was slowly added and the mixture stirred under vacuum for additional 10 minutes. The resin was poured into moulds under vacuum (30mbars), and put in an oven for curing and post-curing the resin.

The properties of the Reference Examples 1, 3 and 5 are listed in Table 1.

Examples 2, 4 and 6 In Example 2, the same components as in Reference Example 1 were used but the mixing procedure according to the present invention was applied. The epoxy resin component was first mixed with the silica filler and the flexibilizer at 140 ⁰ C during 1 hour under vacuum (<lmbar) . The mixture was then cooled to 65°C and mixed with the hardener under vacuum (<lmbar) . The catalyst was finally added, the mixture was stirred for 10 minutes and poured into moulds under vacuum (30 mbar) . The moulds were put in an oven for curing and post-curing.

In Example 4, the same components were used as in Reference Example 3 but the mixing procedure according to the present invention was used. The epoxy resin component was first pre- mixed with the filler component at 130 ⁰ C during 1 hour under vacuum (<lmbar) . Then, the hardener was added to this premix, and the mixture stirred till the hardener was completely dissolved. The final mixture was then poured into mould under vacuum (about 30 mbar), and put in an oven at 130 ⁰ C for curing and post-curing the resin.

In Example 6, the same components were used as in Example 5, but the mixing procedure according to the present invention was used. The epoxy resin was first pre-mixed with the filler and the flexibilizer at a temperature of 140°C during 1 hour; then the mixture was cooled down to 70 ⁰ C, and the hardener was added. This mixture was stirred during 1 hour at 70 ⁰ C under vacuum (<lmbar) , the catalyst was finally added and the blend stirred under vacuum for additional 10 minutes. The obtained epoxy resin composition was then poured into the moulds under vacuum (about 30 mbar) and the moulds were put in an oven at 130 ⁰ C for curing and post-curing the resin.

The properties of the Examples 2, 4 and 6 are also listed in Table 1. The comparative results as shown in Table 1 prove that the cured epoxy resin compositions made according to Examples 2, 4, and 6 exhibit significantly improved mechanical properties compared to the Reference Examples 1, 3, and 5, without impairing the thermo-mechanical properties. Table 1

(1) , (2), (3) , (5) supplied by Hexion

(4) Supplied by Polynt (8) Supplied by Huntsman

(6) Supplied by Almatis EEW = epoxy equivalent weight

(7) Supplied by Quarzwerke MW = molecular weight