TW202532462A - Poly(vinylformal) based resin and the use thereof for coating wires - Google Patents

Abstract

The present invention relates to a composition comprising: • a poly(vinylformal) resin; • an aromatic diol compound; • an aromatic or aliphatic epoxy compound; • a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound; • a crosslinker for reacting the poly(vinylformal) resin with the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound. This composition can be used as a wire enamel.

Description

Poly(vinyl formal) based resin and its use in coating wires

The present invention relates to substantially solvent-free poly(vinyl formal) compositions that can be applied to conductors or substrates as electrical insulators.

Poly(vinyl formal) (Formvar or PVF) electromagnetic wire coating is one of the oldest product lines used in electrical insulation. Poly(vinyl formal) is known for its oil and moisture resistance on copper or aluminum conductors in oil-filled transformers. The coating has been in continuous use for over 50 years, resisting hydrolysis and chemical attack. Due to its long history and high performance, it has been difficult to replace this technology with more cost-effective alternatives.

Poly(vinyl formal) coatings suffer from a number of problems when applied to magnet wire or other solid substrates. The first is that poly(vinyl formal) polymers are primarily soluble in cresyl or phenolic solvents. Cresyl/phenolic solvents are classified as environmentally hazardous. They are also dangerous to work with, causing severe chemical burns and being toxic to the liver.

Poly(vinyl formal) solutions are normally used at very low solids content, otherwise the viscosity is too high to apply the composition to the substrate. Poly(vinyl formal) is typically applied to the substrate at 20-30% solids content. Solvents are used at 70-80% to manage the viscosity within an acceptable range. The solvent is then traditionally burned as a high-cost fuel source, which adds additional environmental emissions. Also due to the low solids content, shipping this material worldwide is expensive, as 70-80% of the material is combustible and does not add value to the end product.

CN 114634740 A relates to the technical field of functional electrochemistry, specifically to a method for preparing polyvinyl acetal enameled wire insulation varnish and solving problems reported in prior art. Acetal varnish can generate large amounts of volatile toxic, heavily polluting, and corrosive substances during production and use. The invention describes extending the service life of acetal varnish. The material comprises the following components: NMP (N-methylpyrrolidone), PMA (polymethyl alcohol), ethyl acetate, polyvinyl formal resin, and bisphenol A epoxy resin. The polyvinyl formal resin serves as the primary film-forming substance. The invention is described as imparting good oil-water resistance, excellent toughness, and conductor adhesion. This paper describes how thermal and chemical resistance are further improved by using the heat-resistant epoxy novolac resin F-44, and how this is also achieved by utilizing the electrical and chemical properties of the phenolic and epoxy resins. Finally, this paper describes how the yield and reliability of the insulating varnish are improved.

US 3,058,951 discloses a composition of matter comprising (1) an epoxy resin comprising the reaction product of a polyphenol and an epihalogenated alcohol, (2) a polyvinyl acetal resin, and (3) a polyacrylate resin selected from the group consisting of homopolymers and copolymers of esters selected from the group consisting of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, isobutyl acrylate, butyl methacrylate, and isobutyl methacrylate. Also disclosed is an article comprising a metal part having a tightly adherent, hard, tough, electrically insulating coating on its surface, the coating comprising the reaction product of: (1) 25 to 94 weight percent of an epoxy resin comprising the reaction product of a polyphenol and an epihalogen alcohol, (2) 1 to 25 percent of a polyvinyl acetal resin, and (3) 5 to 70 percent of a polyacrylate resin selected from the group consisting of homopolymers and copolymers of esters selected from the group consisting of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, isobutyl acrylate, butyl methacrylate, and isobutyl methacrylate.

US Pat. No. 3,239,598 relates to an insulated wire comprising a metal conductor and a dry, non-stick coating on the conductor, the coating comprising 100 parts by weight of polyvinyl acetal, 0.1 to 40 parts by weight of a resin material selected from the group consisting of urea formaldehyde, melamine, and phenolic formaldehyde, and 5 to 1,000 parts by weight of an epoxy resin.

What is needed is a coating that can be applied without solvent but replicates the properties of solvent-based coatings. This would eliminate the harmful solvent component and reduce shipping volume by potentially 70% or more. Poly(vinyl formal) cannot be extruded because it decomposes before melting.

The applicants have discovered a composition that can replace poly(vinyl formal) solutions while still providing mechanical and chemical resistance properties equivalent to solvent-based poly(vinyl formal) coatings. In the composition according to the present invention, the solvent from conventional poly(vinyl formal) solutions is replaced by a low-melting-point mixture prepolymer that acts as a solvent during preparation of the composition (e.g., in an extruder) but then reacts after the composition is applied to a substrate to integrate the polymer into the final thermosetting coating.

Therefore, the present invention relates to a composition comprising: a. a poly(vinyl formal) resin; b. an aromatic diol compound; c. an aromatic or aliphatic epoxide compound; d. a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxide compound; e. a crosslinking agent for reacting the poly(vinyl formal) resin with a reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxide compound.

In another embodiment, the present invention also relates to a method for preparing the above novel composition.

There are many grades of poly(vinyl formal) available commercially, with a wide distribution of molecular weights and a range of reactive functional groups (hydroxyl groups). Higher molecular weights improve chemical resistance, but this also increases the viscosity of the resulting coating. Molecular weights range from 15,000 to 80,000 Daltons, as measured by GPC. Poly(vinyl formal) with a viscosity in the range of 6 to 12 cp (centipoise) is classified as low-viscosity poly(vinyl formal), while poly(vinyl formal) with a viscosity in the range of 12 to 20 cp is classified as medium-viscosity poly(vinyl formal), where the viscosity is measured as a 5 wt% solution in ethylene dichloride at 20°C. None of these grades have a melting point due to decomposition before melting. Poly(vinyl formal) materials that can be used in the compositions according to the present invention include various grades of poly(vinyl formal) produced by Dorf Ketal Chemicals, Suketu Organics, or JNC Corporation. Medium and low viscosity poly(vinyl formal)s are used in one embodiment of the composition of the present invention. The higher the poly(vinyl formal) loading level, the higher the melt viscosity is expected. It has been found that a balance is required to control the melt viscosity, as too high a melt viscosity will have a negative impact on the way the material can be used. Loadings of up to 50 weight percent poly(vinyl formal) can be achieved in aromatic or aliphatic epoxy/aromatic diol mixtures and as low as 5 weight percent. Optimum is 10 to 25 weight percent poly(vinyl formal) for aromatic or aliphatic epoxy/aromatic diol mixtures.

Suitable aromatic diol compounds include 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), bis(4-hydroxyphenyl)sulfone (bisphenol S), bis(4-hydroxydiphenyl)methane (bisphenol F), hydroquinone, resorcinol, or o-catechol and its derivatives (e.g., toluhydroquinone, tert-butyl o-catechol). Suitable aromatic or aliphatic epoxides include diglycidyl ethers of the above aromatic diol compounds, aliphatic diglycidyl ethers derived from aliphatic diols (e.g., butanediol, hexanediol, or neopentyl glycol), or cycloaliphatic epoxides, such as the commercially available Uvicure S105. In one embodiment of the composition according to the present invention, the epoxide is an aromatic epoxide.

It has been found that the best results are obtained when the aromatic diol compound is reacted with an equivalent amount of an aromatic or aliphatic epoxide, thereby causing chain extension. To obtain certain desired properties, it may be beneficial to use a slight molar excess of the aromatic diol compound or a slight molar excess of the aromatic or aliphatic epoxide.

Typically, a catalyst is used to promote the reaction of the aromatic diol compound with the aromatic or aliphatic epoxide. Amines (such as dicyandiamide or hydrazine) are particularly useful in catalyzing the reaction. Tetramethylammonium hydroxide or tetramethylammonium chloride also work at low temperatures. Alternatively, an arylphosphonium salt catalyst can be used. Other Lewis acid catalysts also work. In one embodiment of the present invention, the catalyst is one that allows the reaction between the aromatic diol compound and the aromatic or aliphatic epoxide to begin at a temperature of at least 50°C.

Various crosslinking agents are known for reacting poly(vinyl formal) resins with reaction products obtained by reacting aromatic diol compounds with aromatic or aliphatic epoxides. Suitable crosslinking agents include phenolic resins, blocked isocyanates, melamine formaldehyde resins, and Lewis acid catalysts. The crosslinking agent enables the preparation of the composition according to the present invention at elevated temperatures. In one embodiment of the present invention, the crosslinking agent is one that initiates the reaction of the poly(vinyl formal) resin with the reaction product obtained by reacting an aromatic diol compound with an aromatic or aliphatic epoxide at a temperature of at least 150°C. Alkoxylated amine (usually melamine or urea) formaldehyde resins are also excellent crosslinkers for hydroxyl-functional polymers such as poly(vinyl formal). Melamine formaldehyde (MF) or urea formaldehyde (UF) resins can be methylated, butylated, or contain various amounts of imine groups. Methylated melamine formaldehyde resins are preferred due to their low cost and high reactivity. The crosslinker can be added at 2 to 25% of the total mixture. Similarly, mixtures of different types of crosslinkers can be used, for example, a mixture of methylated melamine formaldehyde and a blocked isocyanate.

Lewis acid catalysts based on titanium or zinc esters can be used to promote the above crosslinking reaction. Tetra-n-butyl titanium ester (TNBT) and Zn(OAc) ₂ are two examples of catalysts that can be used to promote this crosslinking. Generally speaking, any titanium ester catalyst should work. These catalysts are not necessarily required, depending on the formulation. Normally, this optional catalyst is present in an amount of 0 to 4 wt% based on the total weight of the composition.

Additionally, flow agents may be added to the formulation to improve the coating surface and reduce pinhole formation. Examples of flow agents are the Modaflow product line supplied by Allnex. If present, loading levels are typically between 0.1 and 2% based on the total weight of the composition. The amount of flow agent depends on the desired flow characteristics.

In one embodiment, the present invention relates to a composition comprising: - 5 to 50 wt% of a poly(vinyl formal) resin; - 2 to 15 wt% of an aromatic diol compound; - 40 to 85 wt% of an aromatic or aliphatic epoxy compound; - 0.1 to 4 wt% of a catalyst; - 3 to 30 wt% of a crosslinking agent, wherein the wt% are based on the total content of the composition.

The composition of the present invention can be applied to wires or solid substrates via electrostatic powder coating. After applying the composition to the substrate, heat is applied in a first step to melt the mixture, allowing uniform flow. In a second step, heat is increased to initiate a reaction between the aromatic diol and the aromatic or aliphatic epoxy compound. Subsequently, heat is further increased to initiate a crosslinking reaction of the poly(vinyl formal) resin.

The compositions of the present invention can be prepared in an extruder or similar mixing device. In the extruder or similar device, the ingredients are thoroughly mixed at a temperature at which the aromatic diol and/or aromatic or aliphatic epoxide are flowable but do not react with each other. This is normally done at a temperature below 50°C. The resulting composition can be collected at the end of the extruder and then pulverized to obtain a powder, which can be applied to a substrate via an electrostatic spray. Alternatively, the flowable material can be applied directly to a wire or any other substrate at the extruder outlet. In another alternative embodiment, the composition is collected at the extruder outlet and mixed directly with a suitable solvent to form a solution. This solution can then be applied to a wire or any other suitable substrate.

Extruders that can be used in this method can have a single-screw design, a twin-screw design, or a planetary design.

In one embodiment, the present invention also relates to a method for preparing a poly(vinyl formal) composition that can be used to coat wire. In this method, in a first step, a poly(vinyl formal) resin, an aromatic diol compound, and an aromatic or aliphatic epoxide are mixed in an extruder at a temperature at which at least one of the components is flowable but at which a reaction between the aromatic diol compound and the aromatic or aliphatic epoxide does not occur. In a next step, a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxide is added, and the temperature is raised to a temperature at which the reaction between the diol compound and the epoxide begins and propagates. When the reaction is complete, a crosslinking agent is added to the poly(vinyl formal) resin and the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound, and the resulting mixture is collected. This mixture can be applied directly to the wire, or the mixture can be combined with a suitable solvent, and the solution can then be applied to the wire.

In an alternative method, in a first step, an aromatic diol compound and an aromatic or aliphatic epoxide compound are mixed in an extruder at a temperature at which at least one of the components can flow but at which no reaction occurs between the aromatic diol compound and the aromatic or aliphatic epoxide compound. In a next step, a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxide compound is added, and the temperature is raised to a temperature at which the reaction between the diol compound and the epoxide compound begins and propagates. When this reaction is complete, a poly(vinyl formal) resin and a crosslinking agent for reacting the poly(vinyl formal) resin with the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound are added and the resulting mixture is collected. This mixture can be applied directly to the wire, or the mixture can be combined with a suitable solvent, and the solution can then be applied to the wire.

In another alternative method, in the first step, an aromatic diol compound and an aromatic or aliphatic epoxide are mixed in an extruder at a temperature at which at least one of the components can flow but at which no reaction occurs between the aromatic diol compound and the aromatic or aliphatic epoxide. In the next step, a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxide and a poly(vinyl formal) resin are added, and the temperature is raised to a temperature at which the reaction between the diol compound and the epoxide begins and propagates. When this reaction is complete, a crosslinking agent is added to the poly(vinyl formal) resin and the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound, and the resulting mixture is collected. This mixture can be applied directly to the wire, or the mixture can be combined with a suitable solvent, and the solution can then be applied to the wire.

The present invention is explained in more detail by the following examples.

Abbreviations: DICY dicyandiamide EaB elongation at break (%) RPM revolutions per minute TNBT tetrabutyl titanium ester Mod elastic modulus (kg/cm ² ) PBW parts by weight PVF poly(vinyl formal) THF tetrahydrofuran TMAH tetramethylammonium hydroxide TS tensile strength (kg/cm ² )

Measurement Methods Flexibility or mandrel testing is performed according to the procedures in IEC EN 60851-3. This describes the mandrel winding test. Coated wires are used as is and pre-stretched at 5%, 10%, 15%, 20%, 25%, and 30%. Three probes are prepared for each measurement point. Each wire is wound around a polished mandrel (a steel piece with the same diameter as the wire). Once the wire is on the mandrel, it is inspected for the presence of cracks. The absence of cracks indicates that the coated wire is flexible.

Tan δ is measured using a Dansk tangent δ instrument.

FT-IR spectra were measured using a Thermoscientific Nicolet FT-IR instrument with an ATR accessory.

Viscosity was measured using a 5 wt% solution in ethylene dichloride at 20°C.

All experiments were conducted using a Brabender Co-Rotating Clamshell twin-screw extruder model 20/40D. This extruder features four heating zones, a die adapter heating zone, and a die heating zone. Unless otherwise specified, the first heating zone is the feed zone, and the remaining zones are reaction zones. The screw design includes forward and reverse conveying elements, forward and reverse kneading sections, and toothing. The various materials were fed via either a double concave screw metering device or a single-helix screw metering device.

EPON 1001F is an epoxy resin derived from a liquid epoxy resin (condensation products of 2,2-bis(p-glycidyloxyphenyl)propane and 2,2-bis(p-hydroxyphenyl)propane and similar isomers) and bisphenol A. Its data sheet specifies a viscosity at 25°C (40 wt% solution in methyl ethyl ketone, ASTM D445) of 7 to 9.6 cP, an epoxide weight per epoxide (ASTM D1652) of 525 to 550 g/eq, and a melting point (ASTM D3461) of 75 to 80°C.

The blocked isocyanate resin is a commercially available phenol-blocked TDI resin.

Methylated melamine is a commercially available methylated melamine formaldehyde resin.

The defoamer is a commercially available acrylic flow modifier.

Powder Example 1 : The ingredients of Part 1 were dry-blended and fed through an extruder at 175°C at 25 rpm to form a homogeneous, transparent solid. This material was then powdered and mixed with Part 2. This mixture of Parts 1 and 2 was then fed through an extruder at 100°C at 200 rpm to achieve a uniform mixing without any pre-reaction between the bisphenol A and the epoxy material. This material was then ground to a particle size of 20 to 40 μm. Part 1 [ wt %] Part 2 [ wt %] Epon 1001F 59.331 Blocked isocyanate resin 14.216 Bisphenol A 12.118 Methylated melamine 4.406 PVF low viscosity 7.939 Defoaming agent 0.990 TMAH 1.0

Powder Example 2 : The ingredients of Part 1 were dry-blended and fed through an extruder at 175°C at 25 rpm to form a uniform, transparent solid. This material was then powdered and mixed with Part 2. This mixture of Parts 1 and 2 was then fed through an extruder at 100°C at 200 rpm to achieve uniform mixing without any pre-reaction between the bisphenol A and the epoxy material. This material was then ground to a particle size of 20 to 40 μm. Part 1 [g] Part 2 [g] Epon 1001F 480.241 Blocked isocyanate resin 182.949 Bisphenol A 97.711 Methylated melamine 63.408 PVF medium viscosity 192.304 Defoaming agent 12.474 TMAH 10.395

Comparative Powder Example 1 Formulation: Part 1 was dry blended and fed through an extruder at 175°C at 25 rpm to form a homogeneous, transparent solid. This material was then milled to achieve a particle size of 20 to 40 μm. Part 1 Epon 1001F 59.331 Bisphenol A 12.118 PVF low viscosity 7.939

Comparative Powder Example 2 Formulation: Part 1 was dry blended and fed through an extruder at 175°C at 25 rpm to form a homogeneous, transparent solid. This material was then milled to achieve a particle size of 20 to 40 μm. Part 1 Epon 1001F 59.331 PVF low viscosity 7.939

Wire samples : The material obtained in Powder Example 1 was electrostatically applied to 1 mm pre-annealed copper wire. The wire was oven-cured at 200°C for 20 minutes to achieve a final cure. The wire coating was smooth and uniform, with no pinholes or observable defects. The flexibility of the samples was measured by performing a 20% tensile test followed by 1x mandrel winding. Visual inspection revealed no delamination or cracking. The wire was also flattened 5:1 in a post-roll test. The flat wire was then wound around a 1 mm mandrel and visually inspected for cracks or loss of adhesion. No cracks were observed. The NEMA MW1000 3.53 alcohol toluene boiling test also passed without any problems. No swelling or foaming was observed.

The material obtained in Powder Example 2 was electrostatically applied to a 1 mm pre-annealed copper wire. The wire was cured in an oven at 200°C for 20 minutes to achieve a final cure. The wire coating was smooth and uniform, with no pinholes or observable defects. The flexibility of the samples was measured by performing a 20% stretching followed by 1x mandrel winding. Visual inspection showed no delamination or cracking. The wire was also flattened 5:1 in a post-roll test. The flat wire was then wound around a 1 mm mandrel and visually inspected for cracks or loss of adhesion. No cracks were observed. The NEMA MW1000 3.53 alcohol toluene boiling test also passed without any problems. No swelling or foaming was observed.

The material obtained in Comparative Powder Example 1 was electrostatically applied to a 1 mm pre-annealed copper wire. The wire was cured in an oven at 200°C for 20 minutes to achieve a final cure. The wire coating was smooth and uniform, with no pinholes or observable defects. However, the sample showed poor flexibility when subjected to a 20% stretching followed by 1x mandrel winding. Visual inspection revealed cracking. The wire was also flattened 5:1 in a post-roll test. The flat wire was then wound around a 1 mm mandrel and visually inspected for cracks or loss of adhesion. Cracks were observed.

The material obtained in Comparative Powder Example 2 was electrostatically applied to a 1 mm pre-annealed copper wire. The wire was cured in an oven at 200°C for 20 minutes to achieve a final cure. The wire coating was rough and had pinholes and did not form a good film.

The material obtained in Powder Example 1 was extruded at 110°C onto 1 mm pre-annealed copper wire. A coating thickness of 30 μm was achieved. The wire was cured in an oven at 200°C for 20 minutes to achieve final cure. The wire coating was smooth and uniform, with no pinholes or observable defects. The flexibility of the samples was measured by performing a 20% stretching followed by 1x mandrel winding. Visual inspection showed no delamination or cracking. The wire was also flattened 5:1 in a post-roll test. The flat wire was then wound around a 1 mm mandrel and visually inspected for cracks or loss of adhesion. No cracks were observed. The NEMA MW1000 3.53 alcohol toluene boiling test also passed without any problems. No swelling or foaming was observed.

The material obtained in Powder Example 2 was extruded at 110°C onto 1 mm pre-annealed copper wire. A coating thickness of 30 μm was achieved. The wire was cured in an oven at 200°C for 20 minutes to achieve final cure. The wire coating was smooth and uniform, with no pinholes or observable defects. The flexibility of the samples was measured by performing a 20% stretching followed by 1x mandrel winding. Visual inspection showed no delamination or cracking. The wire was also flattened 5:1 in a post-roll test. The flat wire was then wound around a 1 mm mandrel and visually inspected for cracks or loss of adhesion. No cracks were observed. The NEMA MW1000 3.53 alcohol-toluene boiling test also passed without any problems. No swelling or blistering was observed. Tan delta was measured using a Dansk testing machine and was found to be 108, indicating adequate cure.

The material obtained in Comparative Powder Example 1 was extruded at 110°C onto a 1 mm, pre-annealed copper wire. A coating thickness of 30 μm was achieved. The wire was oven-cured at 200°C for 20 minutes to achieve a final cure. The wire coating was poor. No further testing was performed.

The material obtained in Comparative Powder Example 2 was extruded at 110°C onto a 1 mm, pre-annealed copper wire. A coating thickness of 30 μm was achieved. The wire was oven-cured at 200°C for 20 minutes to achieve a final cure. The wire coating was rough, had pinholes, and did not form a good film. No further testing was performed.

An attempt was made to extrude polyvinyl formal onto 1 mm pre-annealed copper wire at 200-250°C. Decomposition of the resin was observed. No further testing was performed.

The importance of using both aromatic diol compounds and aromatic or aliphatic epoxy compounds is shown in Table 1 below. Examples A01 and A02 demonstrate properties very similar to control poly(vinyl formal) coatings used in wire enamel coatings. Examples 3 and 4 contain only aromatic or aliphatic epoxy compounds, without aromatic diol compounds, at 50% by weight, while Examples 5 and 6 are at 75% epoxy resin and 25% poly(vinyl formal). None of the samples show adequate physical properties compared to the control sample. All samples were catalyzed by tetramethylammonium hydroxide or dicyandiamide. Table 1. Composition and tensile test results: Example A01 A02 A03 A04 A05 A06 A07 Comparison Catalyst DICY DICY DICY TMAH DICY TMAH DICY Bisphenol A 1.8 2.7 Epoxy resin 1009 48.2 72.3 50 50 75 75 75 PVF 50 25 50 50 25 25 25 Tensile test on PET at 125°C, 15' + 60' 200°C cure (film produced using a BYK doctor blade coater with a 10 mil pad) TS 409 305 46 44 42 35 33 379 EaB 4.47 3.43 1.72 5.0 3.3 2.22 1.64 3.68 Mod 11510 5917 - 307 - - - 10836

Table 2 below shows the ingredients of some fully formulated compositions. Two levels of polyvinyl formal were examined. Polyvinyl formal also has low and medium molecular weights.

An aromatic diol compound, an aromatic or aliphatic epoxy compound, and a PVF compound are dry-blended and fed through an extruder at 175°C at 25 rpm to form a uniform, transparent solid. This material is then powdered and mixed with the other ingredients. This mixture is then fed through an extruder at 100°C at 200 rpm to achieve uniform mixing without any pre-reaction between the bisphenol A and the epoxy material. The material is applied to a 10-mil thick PET film, heated at 125°C for 15 minutes, then the temperature is increased to 200°C and maintained at this temperature for 60 minutes. All formulations exhibit good physical properties of modulus, tensile strength, and elongation. Testing after immersion in transformer oil also shows minimal change after 7 days at 50°C.

The dielectric properties were also examined before and after oil immersion. Again, all samples compared favorably to the control C8359 poly(vinyl formal) used on the electromagnetic wire. Table 2: Composition (in pbw) B1 B2 B3 B4 B5 B6 EPON 1001 69,3 57,8 73,1 82,1 69,3 57,8 Bisphenol A 2,6 2,2 2,7 3,1 2,6 2,2 PVF (low viscosity) 8,0 20,0 8,4 9,5 PVF (medium viscosity) 8,0 20,0 TMAH 1,0 1,0 1,0 1,0 1,0 1,0 Methylated melamine 4,4 4,4 4,4 4,4 4,4 Blocked isocyanate 14,7 14,7 14,7 14,7 14,7 Table 2A: Tensile test results B1 B2 B3 B4 B5 B6 Cntr TS 359 347 382 353 398 296 379 EaB 2,69 3.85 3.48 10.98 3.77 2.87 3.68 Mod 13703 13483 14497 10656 12831 9040 10836 Table 2B: Tensile test results after oil bath (7 days at 50°C) B1 B2 B5 B6 TS 152 533 250 249 EaB 1.27 2.54 1.46 1.61 Mod 8376 22956 13193 9040 Table 2C: Dielectric properties before and after oil bath (7 days at 50°C) B1 B2 B3 B4 B5 B6 Cntr dielectric 3090 4116 4524 3088 3136 After oil 3634 4066 4898 4800 4316

The dielectric properties were measured as dielectric breakdown (V/mil) in a standard dielectric breakdown test. For this test, the material was applied to a PET film as described above.

Claims (10)

A composition comprising: a. a poly(vinyl formal) resin; b. an aromatic diol compound; c. an aromatic or aliphatic epoxide compound; d. a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxide compound; e. a crosslinking agent for reacting the poly(vinyl formal) resin with a reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxide compound. The composition of claim 1, wherein the epoxy compound is an aromatic epoxy compound. The composition of claim 1 or 2, which has a 5% by weight solution in ethylene dichloride at 20°C and a relative humidity of 100 to 100,000 Pa at a temperature of 100°C. Viscosity in the range of s. The composition of claim 1 or 2, wherein the catalyst is such that the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound begins at a temperature of at least 50°C. The composition of claim 1 or 2, wherein the crosslinking agent is such that the reaction between the poly(vinyl formal) resin and the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxide compound begins at a temperature of at least 150°C. The composition of claim 1, comprising: a. 5 to 50 wt% of a poly(vinyl formal) resin; b. 2 to 15 wt% of an aromatic diol compound; c. 40 to 85 wt% of an aromatic or aliphatic epoxy compound; d. 0.1 to 4 wt% of a catalyst; e. 3 to 30 wt% of a crosslinking agent; wherein the wt% are based on the total content of the composition. A method for coating a metal substrate, wherein the substrate is coated with the composition of claim 1. The method of claim 7, wherein the metal substrate is a wire, foil, sheet, bar or rod. The method of claim 7 or 8, wherein the composition of claim 1 is applied to the wire in powder form using an electrostatic spray gun. A method for preparing a poly(vinyl formal) composition useful for coating wires comprises: In a first step, mixing a poly(vinyl formal) resin, an aromatic diol compound, and an aromatic or aliphatic epoxide compound in an extruder at a temperature at which at least one of the components is flowable but at which no reaction occurs between the aromatic diol compound and the aromatic or aliphatic epoxide compound; In a next step, adding a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxide compound, and raising the temperature to a value at which the reaction between the diol compound and the epoxide compound initiates and propagates; Thereafter, a crosslinking agent for reacting the poly(vinyl formal) resin with the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound is added and the combination is mixed until a uniform mixture is obtained.