Epoxy resin composition for coating dry-type transformer coil and preparation method thereof

Abstract

The present application discloses an epoxy resin composition for coating a dry-type transformer coil and a preparation method thereof, wherein the composition comprises: a resin component and a curing agent component; wherein the curing agent component comprises: a butanol-etherified phenolic resin modified anhydride. The butanol-etherified phenolic resin modified anhydride can not only cure epoxy with anhydride, but also with free phenolic formaldehyde, and the double cross-linking curing gives the cured product better performance, so the amount of accelerator added is greatly reduced. The phenolic modified material makes the cured product not only have good toughness but also very high heat resistance.

Description

Technical Field

[0001] The present application relates to the technical field of epoxy resins, and in particular to an epoxy resin composition for coating a dry-type transformer coil and a preparation method thereof. Background Art

[0002] Epoxy resin has excellent physical, mechanical and electrical insulation properties, bonding properties with various materials, and flexibility in its use process, which are not available in other thermosetting plastics. Therefore, it can be made into coatings, composite materials, casting materials, adhesives, molding materials and injection molding materials, and is widely used in various fields of the national economy.

[0003] The prior art discloses a method for producing a low-temperature resistant epoxy composite material, which gives the composite material better toughness by increasing the carbon chain, thereby greatly improving the low-temperature resistance of the cured product. However, since such modifiers use polyglycols, the crosslinking density of the cured product is significantly reduced, and the glass transition temperature of the final product is greatly reduced, which greatly limits the scope of use of the composite material, and heat-resistant products cannot be used. Summary of the invention

[0004] In order to solve the above-mentioned deficiencies in the art, the present application aims to provide an epoxy resin composition for coating a dry-type transformer coil and a preparation method thereof.

[0005] According to one aspect of the present application, there is provided an epoxy resin composition for coating a dry-type transformer coil, the composition comprising: a resin component and a curing agent component;

[0006] Wherein, the curing agent component comprises: butanol etherified phenolic resin modified anhydride.

[0007] Furthermore, the resin component includes: 30-40 parts by weight of bisphenol A epoxy resin

[0008] Diluent 1-8 parts by weight

[0009] Toughener 1-8 parts by weight

[0010] Filler 30-50 parts by weight;

[0011] The curing agent component includes: 40-60 parts by weight of butanol etherified phenolic resin modified anhydride

[0012] Filler 40-60 parts by weight

[0013] Accelerator 0.002-0.005 parts by weight.

[0014] According to some embodiments of the present application, the preparation method of the butanol-etherified phenolic resin modified acid anhydride is to react the butanol-etherified phenolic resin with an acid anhydride;

[0015] Optionally, the acid anhydride is a liquid acid anhydride;

[0016] Optionally, the liquid acid anhydride is selected from methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, and tung oil anhydride.

[0017] According to some embodiments of the present application, the butanol-etherified phenolic resin is a butanol-etherified phenolic resin with a weight average molecular weight of 240-500 prepared by reacting phenol, formaldehyde and butanol.

[0018] According to some embodiments of the present application, the mass ratio of the butanol-etherified phenolic resin to the acid anhydride is 0.2-0.3:1, the reaction temperature is 120-180° C., and the reaction time is 3-5 h.

[0019] According to another aspect of the present application, a method for preparing the above epoxy resin composition is also provided, comprising:

[0020] preparing a curing agent component;

[0021] preparing a resin component;

[0022] The resin component and the curing agent component are placed in a drying room at 60-80° C. and preheated for 1-3 hours respectively, and then mixed to obtain the product.

[0023] According to some embodiments of the present application, the curing agent component is prepared by uniformly mixing butanol etherified phenolic resin modified anhydride, filler, and accelerator, wherein the mixing temperature is 50-75° C. and the stirring time is 10-30 min;

[0024] Preparation of resin component: bisphenol A epoxy resin, diluent, toughening agent and filler are uniformly mixed, wherein the mixing temperature is 55-75° C., and the viscosity (25° C.) of the resin component is 8000-13000 cp.

[0025] According to some embodiments of the present application, the weight ratio of the resin component to the curing agent component is 1:(0.8-1.2);

[0026] The mixed vacuum degree of the resin component and the curing agent component is -0.1 MPa.

[0027] According to another aspect of the present application, a method for preparing a heat-resistant dry-type transformer coil is provided, comprising:

[0028] The epoxy resin composition is poured onto the transformer coil and heated and cured;

[0029] The heating and curing is a curve heating, including: the first stage is heating to 70-85°C and keeping warm for 2-5h; the second stage is heating to 95-105°C and keeping warm for 1-3h; the third stage is heating to 110-140°C and keeping warm for 6-10h to obtain the dry transformer coil.

[0030] According to another aspect of the present application, a heat-resistant dry-type transformer coil prepared by the above method is provided.

[0031] Compared with the prior art, this application has at least the following beneficial effects:

[0032] According to an embodiment of the present application, the present application provides an epoxy resin composition for coating a dry-type transformer coil, the composition comprising: a resin component and a curing agent component; the curing agent component comprises: a butanol-etherified phenolic resin modified anhydride.

[0033] The present application provides a method for preparing a butanol-etherified phenolic resin modified anhydride curing agent, which is prepared from butanol-etherified phenolic resin and anhydride under specific reaction conditions. Structurally, the rigid ring of the phenolic resin provides high heat resistance, and the methylene group on the butanol provides toughness. At the same time, not only the anhydride can cure epoxy, but also the free phenolic can cure epoxy. The double cross-linking curing gives the cured product better performance. Therefore, the liquid phenolic modified anhydride of the present application as a curing agent not only makes the cured product have good toughness and high TG value, but also can significantly shorten the curing time.

[0034] The present application provides an epoxy resin composition, which comprises the butanol-etherified phenolic resin modified anhydride curing agent of the present application. When preparing the epoxy resin composition, the amount of accelerator added can be greatly reduced.

[0035] The present application provides a method for preparing an epoxy resin composition. The mixing temperature of the composition is 60-80° C., which can make the system have low viscosity, good operability, and improve the permeability of the cured product. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The present invention is a flow chart for preparing the heat-resistant dry-fried transformer coil of the present application. DETAILED DESCRIPTION

[0037] The technical solution of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0038] It is particularly important to point out that similar substitutions and modifications made to the present application are obvious to those skilled in the art, and they are all deemed to be included in the present application. Relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present application to implement and apply the technology of the present application. Obviously, the described embodiments are only some embodiments of the present application, not all embodiments.

[0039] If no specific conditions are specified in this application, the preparation shall be carried out under conventional conditions or the conditions recommended by the manufacturer. The raw materials or excipients used, as well as the reagents or instruments used, for which the manufacturers are not specified, are all conventional products that can be obtained commercially.

[0040] The application is described in detail below.

[0041] Glass transition temperature (Tg) refers to the temperature corresponding to the transition from glassy state to highly elastic state. Glass transition is an inherent property of amorphous polymer materials and a macroscopic manifestation of the change in the form of polymer motion. It directly affects the performance and process performance of the material. Therefore, it has long been the main content of polymer physics research. It mainly reflects the heat resistance of the material.

[0042] Phenolic resin is a very rigid, high TG heat-resistant material. Phenolic resin is also a high-temperature curing agent for epoxy resin. However, since thermoplastic phenolic resin is a solid material, it is generally used in high-temperature die-casting products such as copper-clad laminates and cannot be used as a casting material. The present application provides a liquid phenolic modified anhydride material, which makes the cured product not only have good toughness but also very high TG.

[0043] The phenolic modified anhydride material is prepared by reacting butanol etherified phenolic resin with liquid anhydride, and the reaction mainly utilizes the hydroxyl group on the phenolic resin to react with the anhydride group of the anhydride to obtain a liquid phenolic modified anhydride. The rigid ring on the phenolic resin provides high heat resistance, and the methylene group on the butanol provides toughness.

[0044] The traditional toughening agent selected polyols, toughening mainly relies on carbon chain to increase toughness, and there is only one very long carbon chain, so the crosslinking density of the system will be reduced, and the glass transition temperature will be greatly reduced. The butanol etherified phenolic resin of the present application is a phenolic resin obtained by the reaction of phenol, formaldehyde, and butanol, that is, the benzene ring provides heat resistance, and the carbon chain of butanol provides toughness, so it will not significantly reduce the glass transition temperature of the product while improving toughness.

[0045] The modified epoxy composition of the present application is composed of a resin component and a curing agent component.

[0046]

[0047] The curing agent component includes: 40-60 parts by weight of butanol etherified phenolic resin modified anhydride

[0048] Filler 40-60 parts by weight

[0049] Accelerator 0.002-0.005 parts by weight.

[0050] Preheat the above two components in a drying room at 60℃ for 12 hours respectively, then add them into the final mixture in a 1:1 ratio and stir thoroughly. The mixing temperature is 85-90℃, and the vacuum degree is guaranteed to be -0.1MPa for 0.5h. Then pour the mixed material into the transformer coil; cure according to the curing curve of 80℃ 5h, 100℃ 2h, and 130℃ 12-18h.

[0051] The butanol-etherified phenolic resin modified anhydride of the present application is prepared by reacting phenol, formaldehyde and butanol to obtain a butanol-etherified phenolic resin with a weight average molecular weight of 240-500. If the molecular weight is too small, the resin cross-linking density is low and the castable strength is poor. If the molecular weight is too high, the viscosity of the composition is too large, which affects the penetration and causes bubbles in the product, which not only affects the strength but also affects the partial discharge of the transformer.

[0052] In order to make the anhydride cure epoxy better, the amount of accelerator added in the prior art is generally 2%-4% of the curing agent component. Or in order to shorten the curing time, the modified anhydride epoxy resin composition also includes a latent accelerator, and the latent accelerator is 1 to 5.5 parts by weight. The modified anhydride curing agent of the present application not only can the anhydride cure epoxy, but the free phenolic formaldehyde can also cure epoxy. The double cross-linking curing gives the cured product better performance, so the amount of accelerator added is greatly reduced, and only 2‰-5‰ of the curing agent component needs to be added; at the same time, the present application increases the temperature of the early mixing material, which can improve the problem of slow initial reaction. The higher the temperature, the lower the viscosity of the product system, the better the operability, and the permeability of the product can also be improved.

[0053] The present application discloses a production method of a high-toughness and high-heat-resistant dry-type transformer, which comprises: a transformer base, a coil and an iron core vertically arranged on the base, wherein the coil is completely covered by a modified epoxy composite material, and the top of the iron core is fixed by a clamp, and the clamp is provided with a lifting ring and a pin.

[0054] The present application is described in detail below with reference to specific embodiments.

[0055] Example 1-1

[0056] Preparation of butanol etherified phenolic resin modified anhydride curing agent in this application:

[0057] (1) Phenol and formaldehyde are mixed in a ratio of 1:0.7, and a 46% sodium hydroxide solution is added with a weight ratio of 3% of phenol, and the temperature is kept constant at 75°C for 1 hour, and then butanol is added with a weight ratio of 120% of the weight of phenol, and the temperature is raised to 80°C, and the temperature is kept constant for 1.5 hours, and vacuum is applied to obtain a butanol etherified phenolic resin.

[0058] (2) The weight average molecular weight of butanol etherified phenolic resin is about 240;

[0059] (3) Butanol-etherified phenolic resin and methyltetrahydrophthalic anhydride in a mass ratio of 0.2:1 were reacted at 120° C. for 5 h to obtain a butanol-etherified phenolic resin modified anhydride curing agent.

[0060] Example 1-2

[0061] Preparation of butanol etherified phenolic resin modified anhydride curing agent in this application:

[0062] (1) Phenol and formaldehyde are mixed in a ratio of 1:0.74, and a 46% sodium hydroxide solution with a concentration of 3% by weight of phenol is added, and the temperature is kept constant at 75°C for 1 hour, and then butanol with a concentration of 120% by weight of phenol is added, and the temperature is raised to 80°C, and the temperature is kept constant for 1.5 hours, and vacuum is applied to obtain a butanol etherified phenolic resin.

[0063] (2) The weight average molecular weight of the butanol etherified phenolic resin is about 350;

[0064] (3) Butanol-etherified phenolic resin and methyl hexahydrophthalic anhydride in a mass ratio of 0.3:1 were reacted at 150° C. for 4 h to obtain a butanol-etherified phenolic resin modified anhydride curing agent.

[0065] Examples 1-3

[0066] Preparation of butanol etherified phenolic resin modified anhydride curing agent in this application:

[0067] (1) Phenol and formaldehyde are mixed in a ratio of 1:0.75, and a 46% sodium hydroxide solution with a concentration of 3% by weight of phenol is added, and the temperature is kept constant at 75°C for 1 hour, and then butanol with a concentration of 120% by weight of phenol is added, and the temperature is raised to 80°C, and the temperature is kept constant for 1.5 hours, and vacuum is applied to obtain a butanol etherified phenolic resin.

[0068] (2) The weight average molecular weight of the butanol etherified phenolic resin is about 500;

[0069] (3) Butanol-etherified phenolic resin and tung oil anhydride in a mass ratio of 0.25:1 were reacted at 180° C. for 3 h to obtain a butanol-etherified phenolic resin modified anhydride curing agent.

[0070] Comparative Example 1-1

[0071] Preparation of butanol etherified phenolic resin modified anhydride curing agent with a weight average molecular weight of 600:

[0072] Except that phenol and formaldehyde are mixed in a ratio of 1:0.8, other steps are the same as those of Example 1-3.

[0073] Comparative Example 1-2

[0074] Preparation of butanol etherified phenolic resin modified anhydride curing agent with a weight average molecular weight of 200:

[0075] Except that phenol and formaldehyde are mixed in a ratio of 1:0.68, other steps are the same as those of Example 1-3.

[0076] Comparative Examples 1-3

[0077] Preparation of phenolic resin modified anhydride curing agent:

[0078] (1) Phenolic resin;

[0079] (2) Phenolic resin and methyltetrahydrophthalic anhydride in a mass ratio of 0.2:1 were reacted at 120°C for 5 h.

[0080] Unmodified phenolic resin is connected by methyl groups. Although it has good heat resistance, it is very brittle. Butanol etherified phenolic resin not only has a rigid ring with excellent heat resistance, but also has ether bonds to improve the toughness of the material.

[0081] Comparative Examples 1-4

[0082] Preparation of butanol etherified phenolic resin modified anhydride curing agent:

[0083] (1) a butanol etherified phenolic resin having a weight average molecular weight of 350 prepared by reacting phenol, formaldehyde and butanol;

[0084] (2) Butanol etherified phenolic resin and tetrahydrophthalic anhydride in a mass ratio of 0.2:1 were reacted at 120°C for 5 hours. The curing agent reactant was in a paste state.

[0085] Example 2-1

[0086] Modified epoxy composition containing the butanol etherified phenolic resin modified anhydride curing agent of Example 1-1

[0087] It is composed of resin component and curing agent component:

[0088]

[0089] Curing agent component: 40 parts by weight of butanol etherified phenolic resin modified anhydride

[0090] Filler 40 parts by weight

[0091] Accelerator: 0.002 parts by weight.

[0092] Curing time setting: in the first stage, the temperature is raised to 70-85℃ and kept warm for 2h; in the second stage, the temperature is raised to 95-105℃ and kept warm for 1h; in the third stage, the temperature is raised to 110-140℃ and kept warm for 6h to obtain the dry transformer coil.

[0093] Example 2-2

[0094] Modified epoxy composition containing the butanol etherified phenolic resin modified anhydride curing agent of Example 1-2

[0095] It is composed of resin component and curing agent component:

[0096]

[0097] Curing agent component: 40 parts by weight of butanol etherified phenolic resin modified anhydride

[0098] Filler 60 parts by weight

[0099] Accelerator 0.005 parts by weight.

[0100] Curing time setting: in the first stage, the temperature is raised to 70-85℃ and kept warm for 2h; in the second stage, the temperature is raised to 95-105℃ and kept warm for 1h; in the third stage, the temperature is raised to 110-140℃ and kept warm for 6h to obtain the dry transformer coil.

[0101] Example 2-3

[0102] Modified epoxy composition containing the butanol etherified phenolic resin modified anhydride curing agent of Examples 1-3

[0103]

[0104] Curing agent component: 50 parts by weight of butanol etherified phenolic resin modified anhydride

[0105] Filler 50 parts by weight

[0106] Accelerator: 0.0034 parts by weight.

[0107] Curing time setting: in the first stage, the temperature is raised to 70-85℃ and kept warm for 2h; in the second stage, the temperature is raised to 95-105℃ and kept warm for 1h; in the third stage, the temperature is raised to 110-140℃ and kept warm for 6h to obtain the dry transformer coil.

[0108] Comparative Example 2-1

[0109] Modified epoxy composition containing the butanol etherified phenolic resin modified anhydride curing agent of Comparative Example 1-1

[0110] It is composed of resin component and curing agent component:

[0111]

[0112] Curing agent component: 50 parts by weight of butanol etherified phenolic resin modified anhydride

[0113] Filler 50 parts by weight

[0114] Accelerator: 0.002 parts by weight.

[0115] Curing time setting: Direct curing at 110℃ for 15h

[0116] The weight average molecular weight of the modified acid anhydride is too high, which makes the viscosity of the composition too high and affects the penetration, resulting in bubbles in the product.

[0117] Comparative Example 2-2

[0118] Modified epoxy composition containing the butanol etherified phenolic resin modified anhydride curing agent of comparative example 1-2

[0119] It is composed of resin component and curing agent component:

[0120]

[0121] Curing agent component: 50 parts by weight of butanol etherified phenolic resin modified anhydride

[0122] Filler 50 parts by weight

[0123] Accelerator: 0.002 parts by weight.

[0124] Curing time setting: in the first stage, the temperature is raised to 70-85℃ and kept warm for 2h; in the second stage, the temperature is raised to 95-105℃ and kept warm for 3h; in the third stage, the temperature is raised to 110-140℃ and kept warm for 10h to obtain the dry transformer coil.

[0125] The weight average molecular weight of the modified acid anhydride is too small, resulting in low cross-linking density of the resin and poor strength of the casting material.

[0126] Comparative Examples 2-3

[0127] Modified epoxy composition with chemically modified polycarbonic anhydride curing agent

[0128] The chemically modified polycarbonic anhydride is a reaction product of polycarbonic anhydride and diol or polyglycol or a reaction product of polycarbonic anhydride and a compound containing two carboxyl groups.

[0129] The modified epoxy composition consists of a resin component and a curing agent component:

[0130]

[0131]

[0132] Curing agent component: 40 parts by weight of chemically modified polycarbonic anhydride

[0133] Filler 60 parts by weight

[0134] Accelerator 0.005 parts by weight.

[0135] Curing time setting: in the first stage, the temperature is raised to 70-85℃ and kept warm for 2h; in the second stage, the temperature is raised to 95-105℃ and kept warm for 3h; in the third stage, the temperature is raised to 110-140℃ and kept warm for 10h to obtain the dry transformer coil.

[0136] Comparative Examples 2-4

[0137] The modified epoxy composition is composed of a resin component and a curing agent component;

[0138]

[0139] Curing agent component: 40 parts by weight of thermoplastic phenolic resin

[0140] Filler 40 parts by weight

[0141] Accelerator 0.001 parts by weight.

[0142] Curing time setting: in the first stage, the temperature is raised to 70-85℃ and kept warm for 2h; in the second stage, the temperature is raised to 95-105℃ and kept warm for 3h; in the third stage, the temperature is raised to 110-140℃ and kept warm for 10h to obtain the dry transformer coil.

[0143] The composition prepared by the component is a solid material and cannot be used as a casting material.

[0144] Application Examples

[0145] The epoxy resin composition of Example 2-1 was used to prepare a heat-resistant dry-fried transformer coil.

[0146] The epoxy resin composition prepared in Example 2-1 was poured onto the transformer coil and heated for curing. In the first stage, the temperature was raised to 75°C and kept warm for 4 hours; in the second stage, the temperature was raised to 98°C and kept warm for 2 hours; in the third stage, the temperature was raised to 115°C and kept warm for 6 hours to obtain a heat-resistant dry-type transformer coil.

[0147] Comparative Application Examples

[0148] The epoxy resin composition of Example 2-1 was used to prepare a heat-resistant dry-fried transformer coil.

[0149] The epoxy resin composition prepared in Example 2-1 was poured onto the transformer coil and heated for curing at a temperature of 110° C. for 10 hours.

[0150] This application example uses a one-stage heating method, which causes the reaction to cure quickly, and three terminal cracks appear at the terminal when the mold is opened.

[0151] Test example

[0152] The performance of the modified epoxy composition containing butanol-etherified phenolic resin modified anhydride of the present application was tested by the following method.

[0153] TG value test: test method IEC61006

[0154] Toughness test: Use a universal testing machine according to national standards ISO527 (tensile strength) ISO178 (flexural strength).

[0155] TG value℃ Bending strength N Tensile strength N Curing time Example 2-1 78 92 82 9 Example 2-2 79 97 86 9 Example 2-3 79 102 94 9 Comparative Example 2-1 67 73 64 15 Comparative Example 2-2 65 76 69 15 Comparative Examples 2-3 66 72 62 15

[0156] According to the above table data, the modified epoxy composition containing butanol etherified phenolic resin modified anhydride of the present application has a very high TG, but the toughness is still very good. The material has such a high tensile strength while maintaining a high TG, which reflects the superior performance of the material. At the same time, the curing agent of the present application can significantly shorten the curing time, and the epoxy resin composition of the present application has good heat resistance and the highest strength.

[0157] The description of the above embodiments is only used to help understand the method and core idea of ​​the present application. It should be noted that, for ordinary technicians in this technical field, several improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. An epoxy resin composition for coating a dry-type transformer coil, characterized in that: The composition comprises: a resin component and a curing agent component; in, The resin component includes: 30-40 parts by weight of bisphenol A epoxy resin Diluent 1-8 parts by weight Toughener 1-8 parts by weight Filler 30-50 parts by weight; The curing agent component includes: 40-60 parts by weight of butanol etherified phenolic resin modified anhydride Filler 40-60 parts by weight Accelerator 0.002-0.005 parts by weight; The weight average molecular weight of the butanol etherified phenolic resin is 240-500.

2. The epoxy resin composition according to claim 1, characterized in that The butanol-etherified phenolic resin modified acid anhydride is prepared by reacting butanol-etherified phenolic resin with acid anhydride.

3. The epoxy resin composition according to claim 2, characterized in that The acid anhydride is a liquid acid anhydride.

4. The epoxy resin composition according to claim 3, characterized in that The liquid acid anhydride is selected from methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride and tung oil anhydride.

5. The epoxy resin composition according to claim 2, characterized in that The mass ratio of the butanol etherified phenolic resin to the acid anhydride is 0.2-0.3:1, the reaction temperature is 120-180°C, and the reaction time is 3-5h.

6. A method for preparing the epoxy resin composition according to any one of claims 1 to 5, characterized in that: include: preparing a curing agent component; preparing a resin component; The resin component and the curing agent component are placed in a drying room at 60-80° C. and preheated for 1-3 hours respectively, and then mixed to obtain the product.

7. The preparation method according to claim 6, characterized in that: The preparation of the curing agent component is as follows: butanol etherified phenolic resin modified anhydride, filler, and accelerator are uniformly mixed, wherein the uniformly mixed mixing temperature is 50-75° C., and the stirring time is 10-30 min; Preparation of resin component: bisphenol A epoxy resin, diluent, toughening agent and filler are uniformly mixed, wherein the mixing temperature is 55-75° C. and the viscosity of the resin component is 8000-13000 cp.

8. The preparation method according to claim 6, characterized in that: The weight ratio of the resin component to the curing agent component is 1:(0.8-1.2); The mixed vacuum degree of the resin component and the curing agent component is -0.1 MPa.

9. A method for preparing a heat-resistant dry-type transformer coil, characterized in that: include: The epoxy resin composition according to any one of claims 1 to 3 is poured onto a transformer coil and heated and cured; The heating and curing is a curve heating, including: the first stage is heating to 70-85°C and keeping warm for 2-5h; the second stage is heating to 95-105°C and keeping warm for 1-3h; the third stage is heating to 110-140°C and keeping warm for 6-10h to obtain the dry transformer coil.

10. A heat-resistant dry-type transformer coil prepared by the method of claim 9.

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