JPS6141086B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electrically insulating composite material used as an insulating material for electric motors used in refrigerators, freezers, room coolers, etc. The purpose of this invention is to reduce the amount of oligomers extracted by refrigerants, and to form a coated resin film. The present invention relates to a composite material for electrical insulation with excellent adhesion of polyethylene terephthalate film and polyethylene terephthalate film. In general, polyethylene terephthalate film is the most commonly used insulating material for slot insulation, interphase insulation, interstage insulation wedges, etc. of refrigerator motors, considering its heat resistance, moisture resistance, and mechanical strength. With the recent trend of increasing the heat resistance of electric motors, the use of polyethylene terephthalate films continues to increase.
However, this polyethylene terephthalate film has the property of easily deteriorating in the presence of moisture and alkaline substances, and furthermore, this deterioration is accelerated as the temperature rises, reducing the mechanical strength, especially the tensile strength, of the polyethylene terephthalate film, and It was found that the amount of oligomers extracted from terephthalate film was increased. Therefore, sufficient consideration is required to use the polyethylene terephthalate film as an insulating material for a cooler motor having a heat resistance of Class E or higher. In other words, polyethylene terephthalate film consists of low molecular weight incomplete polymers called oligomers.
Since it contains 1.5 to 2.0% by weight, when used at high temperatures, the oligomers may be extracted into the refrigerant (e.g., a mixture of refrigeration oil and Freon 22), and other refrigerants may peel off and contaminate the refrigerant. . As a result, since the refrigeration cycle of current coolers uses a method of expansion through capillary tubes, the above-mentioned degraded products and oligomers can clog the capillary tubes.
This will significantly reduce cooling capacity. In order to solve these problems, a resin solution having an imide group, such as polyimide varnish or polyamide imide varnish, was applied onto a polyethylene terephthalate film to create a dried composite material. Since the adhesion (adhesiveness) between the imide group-containing resin films is low, the imide group-containing resin film easily peels off during electrical work, making it difficult to put it into practical use. In order to improve the adhesion between the polyethylene terephthalate film and the imide group-containing resin film, attempts have been made to roughen the surface of the polyethylene terephthalate film by etching it to improve the adhesion. Etching processing on both sides is expensive, and its use is quite limited in practice, so it has not been widely used. Resins with imide groups that are effective in improving the refrigerant resistance of polyethylene terephthalate films include polyimide and polyamideimide resins, but these resins are highly hygroscopic such as N-methyl-2-pyrrolidone, dimethylacetamide, and dimethylformamide. The main ingredient is a solvent with rich properties. Therefore, during the heating and drying process after coating, it tends to absorb moisture from the air, and when it absorbs moisture, some of these resins precipitate from the solvent and turn yellowish white.
The disadvantage was that it was impossible to form a strong film. The present invention has been made in view of the above points, and is an electrically insulating composite formed by applying a polyamideimide resin varnish soluble in a cresol solvent to both sides of a polyethylene terephthalate film, and then heating and drying it to adhere a resin film. Regarding materials. According to the present invention, an electrically insulating composite material with excellent adhesion between a resin film and a polyethylene terephthalate film and a low amount of oligomer extraction can be obtained. The varnish applied to both sides of the polyethylene terephthalate film is a polyamide-imide resin that is soluble in cresol solvents, and it adheres extremely well to the polyethylene terephthalate film, which generally has very poor adhesion to resin films.
Moreover, it not only has extremely excellent heat resistance, but also has the effect of suppressing moisture and alkaline substances from entering and diffusing into the refrigerant. Examples of polyamide-imide resins soluble in cresol solvents used in the present invention include isocyanurate ring-containing polyisocyanates, diisocyanates, lactams, tricarboxylic anhydrides, and general formulas other than tricarboxylic anhydrides.

[Formula] [X and X' may be the same or different, carboxyl group or acid anhydride group, Y is carboxyl group, acid anhydride group, hydroxyl group, or amino group, R is (-R ₁ )― _n Z― (R ₂ )― _l , aromatic,
Aliphatic, alicyclic or heterocyclic residues, provided that R ₁ and R ₂
may be the same or different and are aromatic, aliphatic, alicyclic or heterocyclic residues, Z is -CH ₂ -,
—CO—, —SO ₂ — or —O—, m and l are integers of 1 or 2, and n is an integer of 1 or more], and the isocyanurate ring-containing polyisocyanate is added to the total isocyanate equivalent. 0 to 30 equivalent percent of general formula other than tricarboxylic anhydride

A polyamide-imide resin obtained by reacting a compound represented by the formula in an amount of 0 to 30 equivalent percent of the total carboxyl equivalents in a cresol solvent is used. Examples of the isocyanurate ring-containing polyisocyanate include tolylene diisocyanate, xylylene diisocyanate, 4,4'-diphenyl ether diisocyanate, and naphthylene-1,5-
Diisocyanate, aromatic diisocyanate such as 4,4'-diphenylmethane diisocyanate, aliphatic diisocyanate such as ethylene diisocyanate, 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 1,12-dodecane diisocyanate, cyclobutene 1 , 3-diisocyanate, cyclohexane 1,
Isocyanurate ring-containing compounds obtained by trimerization of 3- and 1,4-diisocyanates, alicyclic diisocyanates such as isophorone diisocyanate, and polyisocyanates such as triphenylmethane-4,4′,4″-triisocyanate. Polyisocyanate is used. Considering heat resistance etc., tolylene diisocyanate, 4,
It is preferable to use an isocyanurate ring-containing polyisocyanate obtained by the trimerization reaction of an aromatic diisocyanate such as 4'-diphenylmethane diisocyanate or the trimerization reaction of isophorone diisocyanate. A suitable method for producing isocyanurate ring-containing polyisocyanate is disclosed in JP-A-55
- Shown in Publication No. 75417. As the diisocyanate, aromatic diisocyanate, aliphatic diisocyanate, or alicyclic diisocyanate used as a raw material for the above-described isocyanurate ring-containing polyisocyanate is used. In consideration of heat resistance and the like, aromatic diisocyanates such as tolylene diisocyanate and 4,4'-diphenylmethane diisocyanate are preferred. Isocyanurate ring-containing polyisocyanates are used as branching components, and the isocyanurate ring core provides excellent heat resistance. The isocyanurate ring-containing polyisocyanate is used in an amount ranging from 0 to 30 equivalent percent of the total isocyanate equivalent weight. When the amount exceeds 30 equivalent percent, the degree of branching increases, and gelation may occur during synthesis before the desired molecular weight is reached. Lactam, which is an important raw material for cresol solvent solubilization, can generally be anything as long as it reacts with isocyanate groups or acid anhydride groups and is mainly soluble in cresol solvents, but depending on the solubility,
Considering reactivity and cost, ε-caprolactam is preferred. The amount of lactam to be used is not particularly limited, but in consideration of heat resistance and the like, it is preferably less than 100 equivalent percent of the total isocyanate equivalents. As the tricarboxylic anhydride, trimellitic anhydride, butane-1,2,4-tricarboxylic anhydride, etc. are used. In consideration of heat resistance and the like, trimellitic anhydride is preferred. General formula other than tricarboxylic anhydride

The compound represented by the formula has at least two carboxyl groups or acid anhydride groups that can be made into a resin by forming an amide bond and/or imide bond with a polyisocyanate, and further has a hydroxyl group or an amino group as necessary. It is something that we have. Considering flexibility, heat resistance, abrasion resistance, freon resistance, etc., trimesic acid, tris(2-carboxyethyl)isocyanurate, 3,3',4,4'-benzophenonetetracarboxylic acid di Anhydrides are preferred, and reaction products of the above-mentioned isocyanurate ring-containing polyisocyanates such as tolylene diisocyanate trimer and isophorone diisocyanate trimer with trimellitic anhydride, such as polyimide polycarboxylic acid, are used. General formula other than tricarboxylic anhydride

The compound represented by the formula is used in an amount of 0 to 30 equivalent percent of the total carboxyl equivalents. When the amount exceeds 30 equivalent percent, the degree of branching increases, and gelation may occur during synthesis before the desired molecular weight is reached. Here, 1 equivalent of acid anhydride group, hydroxyl group, and amino group of the acid component is treated as 1 equivalent of carboxyl group. From the viewpoint of heat resistance and flexibility, the amounts of the isocyanate component and acid component to be used are such that the equivalent ratio of the isocyanate group to the carboxyl group is preferably 1.5 to 0.7, more preferably 1.5 to 0.7. Preferably it is in the range of 0.85 to 1.15. The reaction may be carried out by charging all the raw materials at the same time, or by charging them in stages depending on the purpose. The reaction temperature is 200 to 220°C for the main reaction after all ingredients have been charged. The progress of the reaction can be determined by observing the generated carbon dioxide gas bubbles and the viscosity of the solution. As the cresol solvent, in addition to cresol, phenol, xylenol, etc. can be used, and a mixed solvent may also be used. Some of the synthesis solvents include aromatic organic solvents with high boiling points, such as xylene and NISSEKI HISOL.
100, 150 (trademark of aromatic hydrocarbon manufactured by Nippon Petrochemical Co., Ltd.), cellosolve acetate (trademark of ethylene glycol monoethyl ether monoacetate manufactured by Dow Chemical Company), etc. can also be used. The polyamide-imide resin composition thus obtained is further diluted with the above-mentioned cresol solvent, for example, to a resin content of 5 to 40% by weight and used as a varnish. In this case, xylene, NISSEKI
Chlorinated solvents such as HISOL-100, cellosolve acetate, and trichlene may be used in combination. The polyamide-imide resin soluble in a cresol solvent in the present invention provides a substantially linear polymer when at least lactam, diisocyanate and tricarboxylic acid anhydride are used as starting materials. The manufacturing method of such polyamide-imide resin is disclosed in Japanese Patent Publication No. 29730/1973 and Japanese Patent Application Laid-open No. 1973-29730.
This is shown in Publication No. 116591, etc. Branched polymers are obtained when an isocyanurate ring-containing polyisocyanate is used as a starting material in addition to at least a lactam, a diisocyanate and a tricarboxylic anhydride. The manufacturing method for such branched polyamide-imide resin was disclosed in Japanese Patent Application Laid-open No. 1986-
Publication No. 75417, Japanese Patent Application Laid-open No. 1983-41218, Japanese Patent Application Publication No. 1987-41218
-95919, US Pat. No. 3,238,181, etc. In addition to at least lactam, diisocyanate and tricarboxylic anhydride as starting materials, a ketopolycarboxylic anhydride such as 3,3',4,4'-benzophenonetetracarboxylic dianhydride is used as a branching component. , a branched polymer is obtained. A method for producing such a branched polyamide-imide resin is shown in Japanese Patent Application No. 30482/1982. In addition to at least lactam, diisocyanate, and tricarboxylic acid anhydride as starting materials, trifunctional or higher functional polycarboxylic acids as branching components, such as trimesic acid, tris(2-carboxylethyl)isocyanurate, or isocyanurate ring-containing polycarbonate, When using a reaction product of isocyanate and trimellitic anhydride, etc.
It is known to give branched polymers. In consideration of flexibility, heat resistance, abrasion resistance, cost, etc., a branch soluble in a cresol solvent obtained by using at least a lactam, diisocyanate, tricarboxylic anhydride, and isocyanurate ring-containing polyisocyanate as a starting material. Polyamideimide resins are preferred. Epoxy resin, alkoxy-modified melamine resin, phenol formaldehyde resin, xylene formaldehyde resin, isocyanurate ring-containing polyisocyanate, organic acid metal salt, polyether resin, polyamide for the polyamide-imide resin varnish that is soluble in the cresol solvent used in the present invention. resin,
Additives such as polyimide resin, polyhydantoin resin, polysulfonic acid resin, guanidine carbonate, benzotriazole, furan resin, phenoxy resin, urethane elastomer, polybutadiene resin, nitrile butadiene rubber, acrylic rubber, etc. are added at 0.1 to 25% by weight based on the resin content. It can be used and modified at a ratio of In particular, in order to accelerate the curing speed and improve coating film formation properties, thermosetting resins, especially epoxy resins, alkoxy-modified melamine resins,
Thermosetting resins such as phenol formaldehyde resins are preferred. If polyamide-imide resin that is soluble in cresol solvents is applied to both sides of a polyethylene terephthalate film and then heated and dried to form a resin film on both sides, it is possible to not only improve its heat resistance but also to prevent moisture in the refrigerant. Since the intrusion of alkaline substances and alkaline substances is reduced, the deterioration of the polyethylene terephthalate film is small, and a composite material with excellent adhesion that can prevent the occurrence of foaming (blistering) after thermal deterioration can be obtained. The thickness of the dried resin film obtained by coating both sides of a polyethylene terephthalate film and heating and drying is preferably in the range of 0.004 mm to 0.025 mm. Applying and drying a polyamide-imide resin varnish soluble in a cresol solvent onto the surface of a polyethylene terephthalate film takes, for example, 10
The polyethylene terephthalate film is immersed in a polyamide-imide resin varnish adjusted to a resin solution of a weight percent, and then dried preferably at a temperature of 80 to 180°C for 0.5 to 2 hours. The present invention will be explained using comparative examples and examples. Comparative Example 1 Polyamide-imide varnish (Hitachi Chemical Co., Ltd.) was applied to both sides of a polyethylene terephthalate film (Toray Industries, Inc. S-type Pull Mirror #250, thickness 0.25 mm).
Comparative Sample 1 was prepared by applying a dimethylacetamide solution (resin content: 8% by weight) manufactured by HI-400 (trade name: HI-400) and heating and drying to form a resin film with a thickness of 0.006 mm. Comparative Example 2 The polyamide-imide varnish described above was applied to a polyethylene terephthalate film whose surface had been etched by sand matting, and the resin film thickness was reduced by heating and drying.
Comparative sample 2 was formed to a thickness of 0.007 mm. Example 1 (1) Synthesis of polyamideimide resin soluble in cresol solvent (i) Synthesis components of isocyanurate ring-containing polyisocyanate Gram tolylene diisocyanate 600 Xylene 600 2-dimethylaminoethanol (catalyst) 1.8 The above components Place in a four-necked flask equipped with a thermometer and stirrer, raise the temperature to 140°C in a nitrogen stream, and react at the same temperature until the content of isocyanate groups (initial concentration: 48% by weight) reaches 25% by weight. advanced. In the infrared spectrum of this product, absorption of isocyanurate rings is observed at 1710 cm ^-1 and 1410 cm ^-1 , and absorption of isocyanurate rings at 2260 cm -1 and 1410 cm -1 is observed.
Absorption of isocyanate groups was observed at cm ^-1 . (ii) Synthesis of polyamideimide resin soluble in cresol solvents

[Table] Ring-containing polyisocyanates
(50% solution by weight)

[Table] Place the above ingredients except trimellitic anhydride into four flasks equipped with a thermometer, stirrer, and fractionator tube, raise the temperature to 180°C in a nitrogen stream, and react for 90 minutes. Next, trimellitic anhydride is added and the temperature is raised to 210°C. 210℃
The mixture was kept warm and the reaction proceeded for 15 hours. A varnish was obtained by adjusting the resin concentration to 30% by weight with cresol. The viscosity of this product was 250 poise. In the infrared absorption spectrum, imide group absorption was observed at 1780 cm ^-1 and amide group absorption was observed at 1650 cm ^-1 . (iii) Manufacture of composite material Polyethylene terephthalate film (Toray Industries, Inc. S-type Pull Mirror #250, thickness
0.25mm) was coated with a diluted cresol solution (resin content: 9% by weight) of the cresol-soluble polyamideimide resin varnish synthesized in () and dried by heating at 160°C for 2 hours until the resin film thickness was 0.006mm. A composite material was obtained. Example 2 (1) Preparation of epoxy resin-containing polyamide-imide resin varnish (i) Synthesis of polyamide-imide resin soluble in cresol solvents

[Table] Place the above ingredients except trimesic acid and trimellitic anhydride into a four-necked flask equipped with a thermometer, stirrer, and fractionator tube, raise the temperature to 180°C in a nitrogen stream, and react for 90 minutes. .
Then lower the temperature to 160℃ and add trimesic acid,
Trimellitic anhydride is added and the temperature is raised to reflux of the cresol. The reaction proceeded at this temperature for 10 hours. A polyamide-imide varnish was obtained by adjusting the resin concentration to 23% by weight with cresol. The solution viscosity of this product was 83 poise (30°C), and the reduced specific viscosity was 0.27 (0.5 g/100 ml solution of dimethylformamide). (ii) Preparation of epoxy resin-containing polyamide-imide resin varnish Resin content of polyamide-imide resin obtained in (i)
A uniform varnish was obtained by adding 15 parts by weight of Epikote 1004 (an epoxy resin manufactured by Ciel, trade name) to 100 parts by weight with stirring. (iii) Manufacture of composite material Polyethylene terephthalate film (Toray Industries, Inc. S-type Pull Mirror #250, thickness
0.25 mm) was coated with the epoxy resin-containing polyamide-imide varnish obtained in step (i) (adjusted to a resin content of 9% by weight with a mixed solvent of cresol and xylene) and dried by heating at 160°C for 40 minutes. A composite material with a resin film thickness of 0.008 mm was obtained. Example 3 (i) Preparation of polyamide-imide resin varnish containing phenol formaldehyde resin Hytanol 2084 (phenol formaldehyde resin manufactured by Hitachi Chemical Co., Ltd., trade name) was added to 100 parts by weight of the resin content of the polyamide-imide resin obtained in Example 1 (i). ) was added with stirring to obtain a uniform varnish. (ii) Manufacture of composite material Polyethylene terephthalate film (Toray Industries, Inc. S-type Pull Mirror #250, thickness 0.25
mm) was coated with the phenolic resin-containing polyamide-imide varnish obtained in step (i) (resin content was applied to both sides of
(adjusted to a concentration of 8.5% by weight) and dried by heating at a temperature of 155℃ for 50 minutes to reduce the thickness.
A composite material that forms a resin film of 0.007 mm was manufactured. Example 4 (i) Preparation of polyamide-imide resin varnish containing alkoxy-modified melamine resin Melan 20 (Hitachi Chemical Co., Ltd.) was added to 100 parts by weight of the resin content of the polyamide-imide resin obtained in Example 2 (1) (i). A homogeneous varnish was obtained by adding 5 parts by weight of alkoxy-modified melamine resin (trade name) manufactured by Co., Ltd. with stirring. (ii) Manufacture of composite material Example 3 The alkoxy-modified melamine resin-containing polyamide-imide resin varnish obtained in (i) (resin in a mixed solvent of cresol, cellosolve acetate and xylene) was applied to both sides of the polyethylene terephthalate film used in the manufacture of the composite material. (adjusted to a concentration of 8% by weight) and 160%
A composite material was produced which formed a resin film with a thickness of 0.006 mm by heating and drying at a temperature of ℃ for 50 minutes. Using the composite materials of Comparative Examples and Examples as test samples, the adhesion between the polyethylene terephthalate film and the applied resin film was measured at initial and Freon-22
Comparison was made after aging by heating in gas (R-22) at 140°C for 4 days. The adhesion value was determined by making cross-cut cuts in accordance with the JIS K 5400-1979 cross-cut test method, pasting adhesive tape on top of the cuts, and comparing the adhesion of the resin film by a forced peel test using the adhesive tape. . In addition, a blister test was conducted at a temperature of 130°C to examine the state of foaming between the polyethylene terephthalate film and the applied resin film, which occurs due to the rapid volatilization of the refrigerant (R-22) that entered the composite material during rapid heating. This was done by heating for 10 minutes.
Table 1 shows the adhesion test results using the above test method.
Shown below.

【table】

[Table] The polyethylene terephthalate film alone used in Examples 1 to 4, the composite material obtained in Comparative Examples 1 and 2, the composite material obtained in Examples 1 to 4 in an R-22 gas atmosphere (R- Collect 30g of 22)
It deteriorated by heating at a temperature of 120℃ for 1500 hours. Figure 1 shows the results of measuring the amount of oligomer extracted by R-22 at predetermined intervals. 1 indicates a polyethylene terephthalate film, 2 and 3 indicate composite materials of Comparative Examples 1 and 2, and 4
-7 (4, 5, 6, 7) are respectively Example 1,
2, 3, and 4 composite materials are shown. As will be seen, it was treated with a polyamideimide resin varnish that is soluble in cresol solvents. The amount of oligomer extracted from polyethylene terephthalate film is 50% or less compared to polyethylene terephthalate film not treated with polyamideimide resin varnish soluble in cresol solvents. 130 in the same R-22 gas atmosphere as the above test method
Degraded by heating at a temperature of ℃ for 1500 hours and R
Figure 2 shows the results of measuring the amount of oligomer extracted by -22. The numbers indicate the same as in Figure 1.
Similar to the case of heat deterioration at 120℃, the amount of oligomer extracted from polyethylene terephthalate film treated with polyamide-imide resin varnish soluble in cresol solvents is the same as that of polyethylene terephthalate films not treated with polyamide-imide resin varnish soluble in cresol solvents. It is shown that it is less than about 50% compared to . Furthermore, the amount of oligomer extracted at the heating deterioration temperatures of 120°C and 130°C is almost the same, indicating that it is stable with respect to R-22. In the electrically insulating composite material of the present invention,
By providing a resin film obtained by coating and drying a polyamide-imide resin soluble in a cresol solvent on both sides of a polyethylene terephthalate film, the problem of peeling of the resin film seen with conventional polyamide-imide resins can be significantly improved. Due to the strong adhesion between the polyethylene terephthalate film and this resin coating, it can withstand electrical work. In addition, if this is applied to the electric motors of refrigerators, room coolers, refrigerators, etc., it will have a particularly excellent effect in a refrigerant atmosphere, and can suppress the ingress and diffusion of moisture, alkaline substances, and refrigerants, thereby reducing deterioration. In addition, due to the strong adhesion, there is no foaming (blistering) caused by the refrigerant, which improves mechanical properties such as elongation and tensile strength, as well as the extraction of oligomers from the polyethylene terephthalate film by the refrigerant.A polyamide-imide resin film is formed. Therefore, this resin film prevents the extraction of oligomers and significantly reduces the amount of oligomers extracted. In addition, in the process of applying a polyamide-imide resin solution and then heating and drying it, the conventional polyamide-imide resin solution easily absorbs moisture from the air immediately after being coated on the base polyethylene terephthalate film. The disadvantage is that a part of the imide resin precipitates from the solvent and becomes yellowish white, making it impossible to form a strong film.For this reason, the coating to heat drying process of polyamide-imide resin varnish must be carried out under strict moisture-proof atmosphere conditions. However, when using the polyamideimide resin varnish that is soluble in a cresol-based solvent in the present invention, since cresol or an organic solvent with low hygroscopicity is used as the solvent, Coating ~
A strong resin film can be easily formed without being affected by moisture such as water during heat drying, the manufacturing process can be simplified, and a homogeneous product can be obtained.

[Brief explanation of the drawing]

FIGS. 1 and 2 are graphs showing the relationship between the amount of oligomer extracted by R-22 of the composite material and time measured in Examples.

Claims (1)

[Claims] 1. An electrically insulating composite material made by applying a polyamide-imide resin varnish soluble in a cresol solvent to both sides of a polyethylene terephthalate film and then heating and drying the varnish. 2 A polyamide-imide resin varnish soluble in a cresol solvent has a general formula other than isocyanurate ring-containing polyisocyanate, diisocyanate, lactam, tricarboxylic anhydride, and tricarboxylic anhydride [Formula] [X and X' are the same and may be different, carboxyl group or acid anhydride group, Y is carboxyl group, acid anhydride group,
hydroxyl group or amino group, R is (-R ₁ )- _n Z(-R ₂ )- _l ,
Aromatic, aliphatic, alicyclic or heterocyclic residues, provided that
R ₁ and R ₂ may be the same or different, and are aromatic, aliphatic, alicyclic or heterocyclic residues, and Z is -
CH ₂ —, —CO—, —SO ₂ — or —O—, m and l
is an integer of 1 or 2, and n is an integer of 1 or more. 2. The electrically insulating composite material according to claim 1, which is a polyamide-imide resin obtained by reacting a compound represented by the formula in an amount of 0 to 30 equivalent percent of the total carboxyl equivalents in a cresol solvent. 3 The isocyanurate ring-containing polyisocyanate is 4,4'-diphenylmethane diisocyanate,
The composite material for electrical insulation according to claim 2, which is an isocyanurate ring-containing polyisocyanate obtained from tolylene diisocyanate and isophorone diisocyanate. 4. Composite material for electrical insulation according to claim 2 or 3, wherein the diisocyanate is 4,4'-diphenylmethane diisocyanate, 4,4'-diphenyl ether diisocyanate, tolylene diisocyanate, or xylylene diisocyanate. . 5. The electrically insulating composite material according to claim 2, 3, or 4, wherein the lactam is ε-caprolactam. 6 The compound represented by the general formula Claims 2, 3, 4 or 5 which are reaction products of isocyanate and polycarboxylic acid containing an acid anhydride group
Composite material for electrical insulation as described in . 7 Claims 2, 3, and 4 in which the tricarboxylic anhydride is trimellitic anhydride
Composite material for electrical insulation according to item 5, item 6, or item 6. 8. Claims 1, 2, 3, 4, and 5, wherein the polyamide-imide resin varnish is a polyamide-imide resin containing a thermosetting resin.
Composite material for electrical insulation according to item 6, item 7, or item 7. 9. The composite material for electrical insulation according to claim 8, wherein the thermosetting resin is an epoxy resin, an alkoxy-modified melamine resin, or a phenol formaldehyde resin.