Electrical Insulation System Based on Epoxy Resins for Generators and Motors

Abstract

Disclosed is an anhydride-free insulation system for current-carrying construction parts of an electric engine which comprises:

(A) a mica paper or mica tape for wrapping parts of said electric engine that are potentially current-carrying during operation of the engine, which mica paper or mica tape is impregnable via vacuum pressure impregnation with a thermally curable epoxy resin formulation and comprises a thermally activatable curing initiator for the epoxy resin formulation consisting of one or more quarternary ammonium salts of an aromatic-heterocyclic compound, which contains 1 or 2 nitrogen atoms, and a complex anion selected from the group consisting of BF.sub.4.sup., PF.sub.6.sup., SbF.sub.6.sup., SbF.sub.5(OH).sup., AsF.sub.6.sup. and Al[OC(CF.sub.3).sub.3].sub.4.sup. in an amount sufficient to cure the epoxy resin taken up by the mica paper or mica tape and the construction part of the engine during the vacuum pressure impregnation step;

(B) a thermally curable bath formulation for the vacuum pressure impregnation comprising one or more epoxy resins, which formulation is substantially or, preferably, entirely free of a thermally activatable curing initiator for the epoxy resin formulation, in particular of quarternary ammonium salts of aromatic-heterocyclic compounds containing 1 or 2 nitrogen atoms, and of a complex anion selected from the group consisting of BF.sub.4.sup., PF.sub.6.sup., SbF.sub.6.sup., SbF.sub.5(OH).sup., AsF.sub.6.sup. and [Al(OC(CF.sub.3).sub.3).sub.4] and

(C) one or more co-initiator for the thermally activatable curing initiator selected from a diarylethane derivative of formula:

##STR00001##

wherein Ar is phenyl, naphthyl, or C.sub.1-C.sub.4alkyl- or chloro-substituted phenyl, R1 is hydroxy, C.sub.1-C.sub.4alkoxy, OCOR3 or OSiR4R5R6, wherein R3 is C.sub.1-C.sub.8alkyl or phenyl, and R5, R5 and R6 are each independently of one another C.sub.1-C.sub.4alkyl or phenyl, and R2 is C.sub.1-C.sub.4alkyl or cyclohexyl or has the same meaning as Ar, wherein one or more of said co-initiators may be contained in the mica paper or mica tape (A) of the system and/or one or more of said co-initiators may be contained in the a thermally curable epoxy resin bath formulation (B) of the system and is present in said mica paper or mica tape (A) thermally curable epoxy resin bath formulation (B) in an overall amount sufficient to cure the epoxy resin taken up by the mica paper or mica tape and the construction part of the engine during the vacuum pressure impregnation step after said step.

Claims

1. An anhydride-free insulation system for current-carrying construction parts of an electric engine which comprises: (A) a mica paper or mica tape for wrapping parts of said electric engine that are potentially current-carrying during operation of the engine, which mica paper or mica tape is impregnable via vacuum pressure impregnation with a thermally curable epoxy resin formulation and comprises a thermally activatable curing initiator for the epoxy resin formulation containing one or more quarternary ammonium salts of an aromatic-heterocyclic compound, which contains 1 or 2 nitrogen atoms, and a complex anion selected from the group consisting of BF.sub.4.sup., PF.sub.6.sup., SbF.sub.6.sup., SbF.sub.5(OH).sup., AsF.sub.6.sup. and Al[OC(CF.sub.3).sub.3].sub.4.sup. in an amount sufficient to cure the epoxy resin taken up by the mica paper or mica tape and the construction part of the engine during the vacuum pressure impregnation step; (B) a thermally curable bath formulation for the vacuum pressure impregnation comprising one or more epoxy resins, which formulation is substantially or, preferably, entirely free of a thermally activatable curing initiator for the epoxy resin formulation, in particular of quarternary ammonium salts of aromatic-heterocyclic compounds containing 1 or 2 nitrogen atoms, and of a complex anion selected from the group consisting of BF.sub.4.sup., PF.sub.6.sup., SbF.sub.5(OH).sup., AsF.sub.6.sup. and Al[OC(CF.sub.3).sub.3].sub.4.sup., and (C) one or more co-initiator for the thermally activatable curing initiator selected from a diarylethane derivative of formula: ##STR00008## wherein Ar is phenyl, naphthyl, or C.sub.1-C.sub.4alkyl- or chloro-substituted phenyl, R1 is hydroxy, C.sub.1-C.sub.4alkoxy, OCOR3 or OSiR4R5R6, wherein R3 is C.sub.1-C.sub.8alkyl or phenyl, and R4, R5 and R6 are each independently of one another C.sub.1-C.sub.4alkyl or phenyl, and R2 is C.sub.1-C.sub.4alkyl or cyclohexyl or has the same meaning as Ar, wherein one or more of said co-initiators may be contained in the mica paper or mica tape (A) of the system and/or one or more of said co-initiators may be contained in the a thermally curable epoxy resin bath formulation (B) of the system and is present in said mica paper or mica tape (A) and/or in said thermally curable epoxy resin bath formulation (B) in an overall amount sufficient to cure the epoxy resin taken up by the mica paper or mica tape and the construction part of the engine during the vacuum pressure impregnation step after said step.

2. An insulation system according to claim 1, wherein the mica paper or mica tape (A) comprises the thermally activatable curing accelerator in an amount of about 0.1 to about 15 g/m.sup.2 of the mica paper or mica tape, preferably about 0.2 to about 7.5 g/m.sup.2.

3. An insulation system according to claim 2, wherein the mica paper or mica tape (A) comprises co-initator(s) (C) in an amount between 0 and about 15 g/m.sup.2.

4. An insulation system according to claim 3, wherein the entire amount of co-initiator(s) (C) is contained in the mica paper or mica tape (A) of the system.

5. An insulation system according to any one of claims 1 to 4, wherein the entire amount of co-initiator (C) is contained in the thermally curable epoxy resin bath formulation (B) of the system.

6. An insulation system according to any one claims 1 to 5, wherein the epoxy resin(s) of the thermally curable bath formulation (B) is/are derived from cycloaliphatic polycarboxylic acids and/or cycloaliphatic epoxy resins comprising at least one epoxy group, preferably two epoxy groups, fused to a cycloaliphatic ring in the molecule of the epoxy resin.

7. An insulation system according to any one claims 1 to 6, wherein the thermally activatable curing initiator comprises or, preferably, is N-benzylquinolinium hexafluoroantimonate.

8. An insulation system according to any one claims 1 to 7, wherein the co-initiator(s) (C) comprise 1,1,2,2-tetraphenyl-1,2-ethanediol (benzopinacol) or, preferably, the co-initiator is 1,1,2,2-tetraphenyl-1,2-ethanediol.

9. An insulation system according to claim 1, wherein the thermally curable bath formulation comprises the co-initiator (C).

10. A mica paper or the mica tape comprising a thermally activatable curing initiator consisting of one or more quarternary ammonium salts of an aromatic-heterocyclic compound, which contains 1 or 2 nitrogen atoms, and a complex anion selected from the group consisting of BF.sub.4.sup., PF.sub.6.sup., SbF.sub.6.sup., SbF.sub.5(OH).sup., AsF.sub.6.sup. and [Al(OC(CF.sub.3).sub.3).sub.4].

11. A mica paper or mica tape according to claim 10, which comprises the thermally activatable curing accelerator in an amount of about 0.1 to about 15 g/m.sup.2 of the mica paper or mica tape, preferably of about 0.2 to about 7.5 g/m.sup.2.

12. A mica paper or mica tape according to claim 10 or 11, which additionally comprises one or more co-initiator for said thermally activatable curing initiator selected from a diarylethane derivative of formula: ##STR00009## wherein Ar is phenyl, naphthyl, or C.sub.1-C.sub.4alkyl- or chloro-substituted phenyl, R1 is hydroxy, C.sub.1-C.sub.4alkoxy, OCOR3 or OSiR4R5R6, wherein R3 is C.sub.1-C.sub.8alkyl or phenyl, and R5, R5 and R6 are each independently of one another C.sub.1-C.sub.4alkyl or phenyl, and R2 is C.sub.1-C.sub.4alkyl or cyclohexyl or has the same meaning as Ar.

13. The use of an anhydride-free insulation system for current-carrying construction parts of an electric engine in form of a kit of parts as claimed in any one of claims 1 to 9 in the manufacture of rotors or stators of electrical generators or motors.

14. A process for using an anhydride-free insulation system for current-carrying construction parts of an electric engine as claimed in any one of claims 1 to 9 or a mica paper or mica tape according to any one of claims 10 to 12 in the manufacture of rotors or stators of electrical generators or motors, wherein (a) the potentially current-carrying parts of the rotor or stator or the construction part thereof are wrapped with a mica paper or mica tape which is impregnable via vacuum pressure impregnation with a thermally curable epoxy resin formulation and comprises a thermally activatable curing initiator for the epoxy resin formulation consisting of one or more quarternary ammonium salts of an aromatic-heterocyclic compound, which contains 1 or 2 nitrogen atoms, and a complex anion selected from the group consisting of BF.sub.4.sup., PF.sub.6.sup., SbF.sub.6.sup., SbF.sub.5(OH).sup., AsF.sub.6.sup. and Al[OC(CF.sub.3).sub.3].sub.4.sup., and, optionally, at least a partial quantity of one or more co-initiator required for the thermally activatable curing initiator to cure the epoxy resin formulation, selected from a diarylethane derivative of formula: ##STR00010## wherein Ar is phenyl, naphthyl, or C.sub.1-C.sub.4alkyl- or chloro-substituted phenyl, R1 is hydroxy, C.sub.1-C.sub.4alkoxy, OCOR3 or OSiR4R5R6, wherein R3 is C.sub.1-C.sub.8alkyl or phenyl, and R5, R5 and R6 are each independently of one another C.sub.1-C.sub.4alkyl or phenyl, and R2 is C.sub.1-C.sub.4alkyl or cyclohexyl or has the same meaning as Ar, wherein the thermally activatable curing initiator is contained by said mica paper or mica tape in an amount sufficient to cure the epoxy resin taken up by the mica paper or mica tape and the construction part of the engine during a vacuum pressure impregnation step, (b) the rotor or stator or the construction part thereof is inserted into a container, (c) the container is evacuated, (d) a thermally curable bath formulation comprising one or more epoxy resins, which formulation is substantially or, preferably, entirely free of a thermally activatable curing initiator for the epoxy resin formulation, in particular of quarternary ammonium salts of aromatic-heterocyclic compounds containing 1 or 2 nitrogen atoms, and of a complex anion selected from the group consisting of BF.sub.4.sup., PF.sub.6.sup., SbF.sub.6.sup., SbF.sub.5(OH).sup., AsF.sub.6.sup. and Al[OC(CF.sub.3).sub.3].sub.4.sup. and which comprises the remainder of the amount of the one or more co-initiator(s) required for the thermally activatable curing initiator to cure the epoxy resin formulation, is fed into the evacuated container followed by a period of applying an overpressure e.g. of dry air or nitrogen to the container containing the rotor or stator or the construction part thereof, optionally under cautious heating in order to reduce the viscosity of the thermally curable bath formulation in the container sufficiently to allow that said formulation penetrates said mica paper or mica tape and the gaps and voids existing in the structure of the rotor or stator or the construction part thereof within a desired time period forced by the pressure difference between the vacuum and the high pressure applied to the components, (e) the residual thermally curable bath formulation is removed from the container, and (f) the rotor or stator or the construction part thereof, impregnated with the thermally curable bath formulation, is removed from the container and heated after removal from the container in order to cure the thermally curable bath formulation comprised by said rotor or stator or the construction part thereof.

15. The process according to claim 14, wherein the thermally curable bath formulation comprising one or more epoxy resins is fed into the evacuated container in step (d) from a storage tank and is returned to said to a storage tank again after removal from the container in step (e) and stored in the storage tank, optionally under cooling, for further use.

Description

EXAMPLES

[0084] The following Examples serve to illustrate the invention. Unless otherwise indicated, the temperatures are given in degrees Celsius, parts are parts by weight and percentages relate to percent by weight (weight percent). Parts by weight relate to parts by volume in a ratio of kilograms to litres.

[0085] (A) Description of Ingredients Used in the Examples: [0086] CY 179: bis-(epoxycyclohexyl)-methylcarboxylate, supplier: Huntsman, Switzerland; [0087] FK XB 6079 A N-benzylquinoliniumhexafluoroantimonate, supplier: Huntsman, Switzerland; [0088] Benzopinacole: 1,1,2,2 tetraphenyl-1,2-Ethanediol, supplier: Natland, USA; [0089] MY 790-1 CH: distilled bisphenol A diglycidyl ether (BADGE), epoxy eq.: 5.7-5.9 eq./kg, supplier: Huntsman, Switzerland; [0090] HY 1102: methylhexahydrophthalic acid anydride (MHHPA), supplier: Huntsman, Switzerland; [0091] XD 4410: one-component epoxy-based VPI-resin based on BADGE, Bisphenol F diglycidyl ether (BFDGE) and 2,3-epoxypropyl-o-tolylether [0092] DY 9577: curing accelerator for epoxy anydride hardener systems based on borontrichloride-octyldimethylamine adduct (1:1) , supplier: Huntsman, Switzerland; [0093] DY 073-1: curing accelerator for epoxy anydride hardener systems based on a tertiary amine

[0094] Mica tapes are composed of mica paper, optionally containing one or more additives or resins for consolidation of the mica paper, and a light-weight glass fabric made from E-glass or a polymer film that is adhered to the mica paper with a non-reactive or reactive adhesive for mechanical support. Following tapes were used in the Examples:

[0095] New inventive mica tape containing XB 6079 A, supplier: Isovolta, Austria

[0096] Poroband ME 4020: mica tape containing zinc naphthenate, supplier: Isovolta, Austria

[0097] Poroband 0410: mica tape without accelerator, supplier: Isovolta, Austria

[0098] (B) Comparison of Properties of Comparative and Inventive Formulations Without Tape:

a) Comparative Example 1

MY 790-1CH/HY 1102/DY 9577/DY 073

[0099] This comparative example is performed in order to compare the properties of the cured neat resins (without mica tape). For curing of the Comparative Example 1, small amounts of the curing accelerators DY 9577 and DY 073-1 are used instead of Zn-naphthenate (contained in typical commercially available-tapes) because Zn-naphthenate is quite difficult to get homogenously dispersed in the epoxy/anhydride mixture.

[0100] To test the bath stability at 23 C., 1 kg of MY 790-1 CH and 1 kg of HY 1102 are mixed together in a steel vessel with an anchor stirrer at ambient temperature for 5 minutes. This mixture is then kept in an inert glass bottle for the storage test regarding bath stability at 23 C. for 80 days.

[0101] The viscosity of the mixture is determined before and after the storage at a measurement temperature of 60 C. While the initial viscosity at 60 C. is 32 mPas, the viscosity increased during the storage time of 80 days by 12%.

[0102] To test all the other properties of the cured material, to 1 kg of the mixture described above as replacement for the Zn-naphthenate that normally would promote the curing of impregnated tape, 0.8 g of DY 9577 and 0.2 g of DY 073-1 are added and mixed for another 10 minutes. This mixture is then cast in to moulds in the corresponding thicknesses to prepare plates for the various tests. After pouring the material to the moulds, these are put into an oven for 16 hours at 90 C. and 10 hours at 140 C.

b) Comparative Example 2

XD 4410

[0103] This example relates to a homopolymerisable aromatic epoxy system containing the catalyst in the composition (one-component system). It does normally go along with mica-tapes free of catalyst.

[0104] The commercial product Araldite XD 4410 is directly used to check the storage stability at 23 C. over 409 days. XD 4410 exhibits a viscosity of 78 mPas (initial at 60 C.) and an increase of less than 6% during 409 days.

[0105] The reactivity of this mixture is checked with a gel timer at 80 C. and 140 C.

[0106] To produce plates for the other tests, it is poured into moulds of corresponding thicknesses to prepare plates for the various tests. After pouring the material into the moulds, these are put to an oven for 4 hours at 125 C. and 12 hours at 170 C.

c) Inventive Example 1

[0107] The example of thermally curable bath formulation (B) for an insulation system according to the invention system is a mixture of 996.2 g resin CY 179 and 2.2 g of Benzopinacole (dissolved at 80 C. during 10 min).

[0108] The stability of this bath formulation is checked during 180 days storage at 23 C. The initial viscosity of 46 mPas increases only by 6.5% during 180 days storage at 23 C. .

[0109] To produce test plates without a mica tape, 0.8 g of N-benzyl-quinolinium-hexafluoroantimonate are dissolved at 40 C. in 499.2 g of the bath formulation, i.e. the above mentioned solution of 2.2 g of Benzopinacole in 996.2 g resin CY 179. (the resulting insulation system contains 0.22% Benzopinacole and 0.16% N-benzylquinolinium-hexafluoroantimonate in CY 179).

[0110] The reactivity of this mixture is checked with a gel timer at 80 C. and 140 C.

[0111] To produce plates for the other tests, the formulation is poured into moulds of corresponding thicknesses to prepare plates for the various tests. After pouring the material into the moulds, these are put into an oven for 2 hours at 90 C., 2 hours at 130 C. and 2 hours at 180 C.

d) Test Results

[0112] The results of the afore-mentioned tests with the curable epoxy bath formulations of Comparative Examples 1 and 2 as well as the Inventive Example 1 are summarized in Table 1 below (data determined without tape, just for illustrating the properties of the epoxy matrix of such insulation systems).

TABLE-US-00001 TABLE 1 Comparative Comparative Inventive Example 1 Example 2 Example 1 MY 790-1 100 HY 1102 100 XD 4410 100 CY 179 99.62 XB 6079A 0.16 Benzopinacole 0.22 DY 9577 0.16 DY 073-1 0.04 Working hygiene possible anhydride very good very good contact Viscosity at 60 C. 32 78 46.4 [mPa .Math. s] Viscosity increase of 12% (80 days) (without <6% (409 days) 6.5% (180 days) formulation when DY 9577/DY 073-1) (without XB 6079A) stored at 23 C. Storage tank yes no no cooling needed Number of 2 1 1 components to mix Gelation time at n.a. >>1000 min 8 h 50 min 80 C. Gelation time at n.a. 30 min 2 min 140 C. Glass transition 144 C. 130 C. 173 C. temperature T.sub.g Curing time 16 h(90 C.)/10 h(140 C.) 4 h(125 C.)/12 h(170 C.) 2 h(90 C.)/2 h(130 C.)/ 2 h(180 C.) Dissipation factor 8% 12% 5.8% tan at 155 C. 5% weight loss at 390 C. 400 C. 390 C. (TGA 20 K/min) Thermal insulation H F H class rating T.sub.g determined according to ISO 6721/94; Dielectric dissipation factor tan determined according to IEC 60250; 5% weight loss at (TGA 20 K/min): The indicated temperature is the temperature, for which the weight loss is just reaching 5% during heating a sample with a heating rate of 20 K/min.

[0113] (C) Preparation of Mica Paper and Mica Tapes According to the Invention and Application Tests Thereof:

[0114] A mica paper sheet based on uncalcined mica flakes with an areal weight of 160 g/m.sup.2 is cut in a rectangular shape of the size 200100 mm. For mica paper impregnation a solution of XB 6079 A in methyl ethyl ketone (MEK) is prepared which contains 0.25 wt % of XB 6079 A. The mica sheet is impregnated with 2.0 g of the solution and the solvent is removed in an oven at 120 C. for 1 min.

[0115] The treated mica paper is either used as it is or is combined with a glass fabric. In that case a glass fabric style 792 (23 g/m.sup.2, 2615 5.5 tex/5.5 tex) which has previously been coated with 3 g/m.sup.2 of an epoxy/acrylic resin mixture, is adhered to the mica tape using a solid epoxy resin having a melting point around 100 C. For this purpose the solid epoxy resin is evenly dispersed on the treated mica paper. Then the glass fabric is laid on top. The specimen is put into a heated press to melt the epoxy resin (130 C. for 30 s). The glass fabric and the mica paper stick together after removing from the press.

[0116] The obtained mica paper sheets and glass/mica specimens are cut in halfs to give 100100 mm samples. 4 layers of mica paper are piled with each 1.5 g evenly distributed impregnation resin between the layers giving a total resin weight of 4.5 g.

[0117] The impregnated specimens are used for monitoring the dissipation factor tan during cure in a Tettex instrument or are cured in a heated press. Cure in the Tettex instrument and tan() measurement is conducted at 155 C. Cure in the hot press is conducted following the following temperature cycle: 90 C. at 2 bar for 2 h130 C. at 2 bar for 2 h180 C., no pressure for 10 h. Excess resin is quenched out of the laminate during cure.

[0118] The cured composites are subjected to tan measurement at 155 C. and specimens for T.sub.g determination with DMA (dynamic mechanical analysis), 3 point bending mode according IEC 61006, are cut out in the shape 50 mm10 mm.

[0119] The results of the afore-mentioned tests with the curable epoxy bath formulations of Comparative Example 1 (not containing DY9577 and 073-1) with Poroband ME 4020 (Reference system 1) and Comparative Example 2 with Poroband 0410 (Reference System 2) as well as the Inventive Example 1 are summarized in Table 2 below.

TABLE-US-00002 TABLE 2 Inventive System Reference Reference mica paper mica paper/ System 1 System 2 only glass fabric Dissipation factor 4.4% 5.4% 8.0% tan (at 155 C.) Glass transition 151.4 C. 121.7 C. 160.8 C. 161.0 C. temperature T.sub.g Dissipation factor tan determined according to IEC 60250 in a Tettex instrument using a guard ring electrode at 400 V/50 Hz; T.sub.g determined with DMA, 3 point bending mode, heating rate 5 K/min according to IEC 61006 using the tan maximum as T.sub.g.

[0120] (D) Conclusions from the Examples Above:

[0121] a) Conclusions Based on the Comparisons Without Tape:

[0122] Regarding the first critical aspect of working hygiene, the anhydride-free inventive example is better than the classical anhydride-based reference, because it is does not contain a respiratory sensitizer and therefore is not regarded as a SVHC.

[0123] While the anhydride-based reference is quite low viscous, the existing anhydride-free solution according to Comparative Example 2 (XD 4410) is relatively high viscous and hence more difficult to impregnate into the mica-tape and the windings. The inventive bath formulation has a viscosity level quite similar to the anhydride-based reference and can impregnate better than the anhydride-free reference bath formulation based on XD 4410.

[0124] Regarding the bath stability, the anhydride-based reference builds up the viscosity at 23 C. during only 80 days already by 12%. To overcome this issue, a cooled storage is normally applied. The anhydride-free reference bath formulation (XD 4410) is quite stable and therefore does not need a cooling. Surprisingly with the new bath system based on CY 179 and Benzopinacole, the bath stability is quite good (6.5% build up during 180 days). Hence also no cooling would be typically required for the inventive bath composition.

[0125] A further advantage of the inventive system over the traditional reference is that there is no need for mixing the 2 components when refreshing the bath as it can be applied as one-component product. As there is no anhydride that may partly evaporate during the application process out of the bath and hence impacting the optimal mixing ratio with the reference, this issue does not happen with the inventive example resulting in a better quality consistency.

[0126] The reactivity of the inventive product is quite low at temperatures up to 80 C. but fast at temperatures around 140 C. This means that this system is quite latent and therefore stable at storage temperature but highly reactive and higher temperature.

[0127] The one component reference according to Comparative Example 2 is also quite slow at 80 C., however it is still slow at high curing temperature (gel time of 30 min at 140 C.).

[0128] The T.sub.g of the inventive system is quite high. That is positive, as there is more distance to the application critical temperature of 155 C.

[0129] The most positive and surprising finding is that the dielectric dissipation factor tan at 155 C. is even lower and hence better than that of the anhydride-based reference containing a tertiary amine or boron trichloride-octyldimethylamine adduct as curing accelerator .

[0130] A dielectric dissipation factor tan of >10% at 155 C. is the main issue of the anhydride free reference example (XD 4410) of Comparative Example 2 and the reason why such systems could not be used for class H application, although it would be even better temperature stable according to the weight loss short term experiment given in the table. In this respect the inventive example is at least as stable as the unmodified reference.

[0131] So as a conclusion the new inventive insulation system surprisingly eliminates all issues of traditional insulation system for vacuum pressure impregnation, the anhydride/SVHC/REACH issue as well the issues of already known anhydride-free systems such as high viscosity, low reactivity at high temperature, limitation to class F and a too high dielectric dissipation factor tan of more than 10%.

[0132] a) Conclusions Based on the Comparisons of Impregnated Mica Paper and Mica Tapes

[0133] Experiments exhibit a good impregnability of the mica paper and glass/mica tape specimens at low temperature. Also the compatibility with polyester-polyols, which can be used for mechanical enhancement of the impregnated mica paper and glass/mica combination, can be shown. The polyester-polyol leads to only slightly higher tan values, T.sub.g is equal to samples containing no polyester-polyol.

[0134] Tests with mica paper impregnated with different levels of N-Benzyl-Quinolinium-Hexafluoroantimonate (0.12-0.5 g/m.sup.2) show little influence of the concentration on the tan and T.sub.g values of the insulation material.