Poly(vinylformal) based resin and the use thereof for coating wires

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

[0001] POLY(VINYLFORMAL) BASED RESIN AND THE USE THEREOF FOR COATING WIRES

[0002] The present invention relates to a poly(vinylformal) composition that can be applied to a conductor or substrate as electrical insulation substantially without solvent.

[0003] BACKGROUND OF THE INVENTION

[0004] Poly(vinylformal) (Formvar or PVF) magnet wire coatings are one of the oldest product lines used in electrical insulation. Poly(vinylformal) is known for its oil and moisture resistance on copper or aluminium conductors in oil filled transformers. The coating is able to withstand hydrolysis and chemical attack for over 50 years of continuous use. Due to this long history and high performance, it is hard to replace this technology with other coatings that are more cost effective.

[0005] Poly(vinylformal) coatings suffer from a number of issues on application to magnet wire or other solid substrates. The first is that poly(vinylformal) polymer is mainly soluble in cresylic acid or phenolic solvents. Cresylic / phenol solvents are classified as hazardous to the environment. They are also dangerous to work with causing severe chemical burns and are toxic to the liver.

[0006] Poly(vinylformal) solutions are normally used in very low solids content otherwise the viscosity is too high to enable the application of the composition to a substrate. Poly(vinylformal) is applied to a substrate typically at 20-30% solids content. Solvents are used at 70-80% to the manage viscosity to an acceptable range. The solvents are then traditionally burned as a source of high-cost fuel which adds to additional environmental emissions. Also due to the low solids content, it is costly to ship the material around the world where 70-80% of the material is flammable and not adding value to the final product.

[0007] CN 114634740 A relates to the technical field of functional electronic chemicals, in particular to a preparation method of polyvinyl acetal enamelled wire insulating paint and addresses the issues that in its self-reported prior art, acetal paint can generate a large amount of volatile toxic, heavy-pollution and corrosive substances in the production and use processes. The service life of the acetal paint is described to be prolonged. The material comprises the following components: NMP (N-Methyl Pyrrolidone), PMA (Polymethyl Alcohol), ethyl acetate, polyvinyl formal resin and bisphenol A epoxy resin. The polyvinyl formal resin is used as a main film forming substance. Good oil-water resistance, excellent toughness and conductor adhesiveness are described to be endowed. Heat resistance and chemical resistance are described to be further improved by adopting the heat-resistant novolac epoxy resin F-44, and meanwhile, by utilizing the effects of electronic and chemical materials of the phenolic resin and the epoxy resin, the heat resistance and the chemical resistance are described to be improved. Lastly the yield and reliability of the insulating paint are described to be improved.

[0008] US 3,058,951 discloses a composition of matter comprising (1 ) an epoxy resin comprising the reaction product of a polyhydric phenol and an epihalohydrin, (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 of manufacture comprising a metal part having on the surface thereof an intimately adherent, hard, tough electrically insulating coating comprising the reaction product of (1 ) 25-94% by weight of an epoxy resin comprising a reaction product of a polyhydric phenol and an epihalohydrin, (2) 1-25% of a polyvinyl acetal resin, and (3) 5-70% of a poly acrylate 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.

[0009] US 3,239,598 concerns an insulated wire comprising a metallic conductor and a dry, tack-free coating over said conductor comprising 100 parts by weight of polyvinyl acetal, 0.1-40 parts by weight of a resinous material selected from the group consisting of urea formaldehyde, melamine, and phenol formaldehyde, and 5-1 ,000 parts by weight of an epoxy resin.

[0010] What is desired is a coating that can be applied without solvent yet duplicate the performance of the solvent based coating. This would eliminate the hazardous solvent component and reduce shipping volume by potentially 70% or more. Poly(vinylformal) cannot be extruded due to its decomposition prior to melting.

[0011] SUMMARY OF THE INVENTION

[0012] The applicant has found a composition that can replace poly(vinylformal) solutions and still provides mechanical and chemical resistance properties that are equivalent to the solvent-based poly(vinylformal) coating. In the composition according to the present invention the solvent from the known poly(vinylformal) solution is replaced by a low melting mixture prepolymer that acts as solvent during the preparation of the composition, for example in an extruder, but then, after application of the composition to a substrate, reacts to make a polymer integrated into the final thermoset coating.

[0013] The present invention therefore relates to a composition comprising: a. a poly(vinylformal) resin; b. an aromatic diol compound; c. an aromatic or aliphatic epoxy compound; d. a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound; e. 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.

[0014] In a further embodiment, the present invention also relates to a process to prepare the above novel composition. There are a number of poly(vinylformal) grades available on the market ranging in molecular weight distributions and reactive functional hydroxyl groups. The higher the molecular weight the better the chemical resistance but this also increases the viscosity of the resulting coating. The molecular weight ranges from 15,000-80,000 Daltons as measured by GPC. Poly(vinylformal) with a viscosity in the range from 6 to 12 cps (centipoise) would be classified as low viscosity poly(vinylformal), poly(vinylformal) with a viscosity in the range from 12 to 20 cps would be classified as medium viscosity poly(vinylformal), whereby the viscosity is measured in a 5 weight-% solution in ethylene dichloride at 20°C. None of the grades have a melting point due to decomposition prior to melting. Poly(vinylformal) materials that can be used in the composition according to the present invention include various grades of poly(vinylformal) produced by DorfKetal Chemicals, Suketu Organics or JNC Corporation. Poly(vinylformal) of medium and low viscosity are used in one embodiment of the composition of the present invention. The higher the poly(vinylformal) loading level the higher the melt viscosity is expected. A balance needs to be found to control the melt viscosity, since a too high melt viscosity will have a negative impact on the way the material can be used. Loading of poly(vinylformal) as high as 50 wt.% can be achieved in the aromatic or aliphatic epoxy / aromatic diol mixture and as low as 5 wt.%. Most preferred is 10-25 wt.% poly(vinylformal) to aromatic or aliphatic epoxy / aromatic diol mixture.

[0015] Suitable aromatic diol compounds include 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), bis(4-hydroxyphenyl) sulphone (bisphenol S), bis(4- hydroxydiphenyl)methane (bisphenol F), hydroquinone, resorcinol, or catechol and their derivatives (e.g. toluhydroquinone, t-butylcatechol). Suitable aromatic or aliphatic epoxy compounds include the diglycidyl-ethers of the above aromatic diol compounds, aliphatic diglycidyl ethers derived from aliphatic diols like for example butanediol, hexanediol, or neopentylglycol, or cycloaliphatic epoxy resins, such as the commercially available Uvicure S105. In one embodiment of the composition according to the present invention, the epoxy compound is an aromatic epoxy compound. It was found that the best results are obtained when the aromatic diol compound is reacted with an equivalent amount of the aromatic or aliphatic epoxy compound leading to chain extension. To obtain certain desired properties it may be beneficial to either use a slight molar excess of the aromatic diol compound or a slight molar excess of the aromatic or aliphatic epoxy compound.

[0016] Normally, a catalyst is used to promote the reaction of the aromatic diol compound with the aromatic or aliphatic epoxy compound. Amines such as dicyandiamide or hydrazides are very useful in reaction catalysis. Tetramethyl ammonium hydroxide or chloride also work at low temperature. Alternatively, an aryl phosphonium salt catalyst can be used. Other Lewis acid catalysts work as well. In one embodiment of the present invention, the catalyst is such that the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound starts at a temperature of at least 50 °C.

[0017] Various crosslinkers are known that can be used for reacting the poly(vinylformal) resin with the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound. Suitable crosslinkers include phenolic resins, blocked isocyanates, melamine formaldehyde resins and Lewis acid catalysts. The crosslinker enables the preparation of the composition according to the present invention at elevated temperatures. In one embodiment of the present invention, the crosslinker is such that the reaction between the poly(vinylformal) resin and the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound starts at a temperature of at least 150 °C. Alkoxylated amine, typically melamine or urea, formaldehyde resins also are excellent crosslinkers of hydroxyl functional polymers such as poly(vinylformal). Melamine formaldehyde (MF) or urea formaldehyde (UF) resins can be either methylated, butylated or contain various amounts of imino groups. Methylated melamine formaldehyde resins are most preferred due to low cost and high reactivity. Crosslinkers can be added at 2 to 25% on the total mixture. Also, mixtures of different type of crosslinkers can be used, for example a mixture of methylated melamine formaldehyde in combination with blocked isocyanates. Lewis acid catalysts based on titanate or zinc can be used to promote the above crosslinking reaction. Tetra-n-butyl titanate (TNBT) and Zn(OAc)2 are two examples of catalysts that can be used to promote the crosslinking. In general, any titanate ester catalysts should work. Such catalysts are not necessarily needed depending on the formulation. Normally this optional catalyst is present in an amount of 0 - 4 wt.%, based upon the total weight of the composition.

[0018] Additionally flow agents can be added to the formulation to improve the surface of the coating and to reduce pinhole formation. An example of a flow agent is Modaflow product line supplied by Allnex. Loading levels, if present, are typically between 0.1- 2% based upon the total weight of the composition. The amount of flow agent depends on the flow characteristics needed.

[0019] In one embodiment, the invention relates to a composition comprising: from 5 to 50 wt.% of a poly(vinylformal) resin; from 2 to 15 wt.% of an aromatic diol compound; from 40 to 85 wt.% of an aromatic or aliphatic epoxy compound. from 0.1 to 4 wt. % of catalyst from 3 to 30 wt.% of a crosslinking agent wherein the wt.% are based upon the total content of the composition.

[0020] The composition of the present invention can be applied to wire or a solid substrate via electrostatic powder coating. After application of the composition to a substrate, in a first step heat is applied to melt the mixture allowing for uniform flow, in a second step the heat is increased to initiate the reaction between the aromatic diol and aromatic or aliphatic epoxy compound, and thereafter the heat is further increased to start the crosslinking reaction of the poly(vinylformal) resin.

[0021] The composition 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 where the aromatic diol and / or the aromatic or aliphatic epoxy are flowable, but do not react with each other. Normally, this is done at a temperature below 50°C. The obtained composition can be collected at the end of the extruder and then being pulverized to obtain a powder that can be applied to a substrate via electrostatic spraying. Alternatively, at the exit of the extruder the flowable material is directly applied to a wire or any other substrate. In a further alternative embodiment, the composition is collected at the exit of the extruder and directly mixed with an appropriate solvent to form a solution. This solution can then be applied to a wire or any other suitable substrate.

[0022] The extruder that can be used in such process can have a single screw design, a twin screw design or a planetary design.

[0023] In one embodiment, the present invention also relates to a process for the preparation of a poly(vinylformal) composition that can be used for coating wires.

[0024] In this process, in a first step a poly(vinylformal) resin, an aromatic diol compound, and an aromatic or aliphatic epoxy compound are mixed in an extruder at a temperature where at least one of the components is flowable, but where no reaction occurs between the aromatic diol compound and the aromatic or aliphatic epoxy compound. In a next step a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound is added, and the temperature is raised to a temperature where the reaction between the diol compound and the epoxy compound starts and propagates. When this reaction is completed, 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 is added and the obtained mixture is collected. This mixture can be applied to a wire directly, or this mixture can be combined with a suitable solvent, and then the solution can be applied to a wire.

[0025] In an alternative process, in a first step an aromatic diol compound and an aromatic or aliphatic epoxy compound are mixed in an extruder at a temperature where at least one of the components is flowable, but where no reaction occurs between the aromatic diol compound and the aromatic or aliphatic epoxy compound. In a next step a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound is added, and the temperature is raised to a temperature where the reaction between the diol compound and the epoxy compound starts and propagates. When this reaction is completed, a poly(vinylformal) resin and 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 is added and the obtained mixture is collected. This mixture can be applied to a wire directly, or this mixture can be combined with a suitable solvent, and then the solution can be applied to a wire.

[0026] In a further alternative process, in a first step an aromatic diol compound and an aromatic or aliphatic epoxy compound are mixed in an extruder at a temperature where at least one of the components is flowable, but where no reaction occurs between the aromatic diol compound and the aromatic or aliphatic epoxy compound. In a next step a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound and is added and a poly(vinylformal) resin, and the temperature is raised to a temperature where the reaction between the diol compound and the epoxy compound starts and propagates. When this reaction is completed, 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 is added and the obtained mixture is collected. This mixture can be applied to a wire directly, or this mixture can be combined with a suitable solvent, and then the solution can be applied to a wire.

[0027] The present invention is explained in more details by the following examples.

[0028] LIST of ABREVIATIONS

[0029] DICY dicyandiamide

[0030] EaB Elongation at break (%)

[0031] RPM revolutions per minute

[0032] TNBT tetra-n-butyl titanate

[0033] Mod Modulus of elasticity (kg / cm2)

[0034] PBW Parts by weight

[0035] PVF Poly(vinylformal)

[0036] THF tetrahydrofuran

[0037] TMAH tetramethylammonium hydroxide

[0038] TS Tensile strength (kg / cm2)

[0039] Measurement methods

[0040] Flexibility or Mandrel Test was performed according to the procedure IEC EN 60851-3. Therein it is described the Mandrel winding test. Coated wires were taken as such and pre-stretched at 5%, 10%, 15%, 20%, 25%, 30%. For each measurement point three probes were prepared. Each wire was wound around a polished mandrel, a piece of steel having the same diameter as the wire. Once the wire was on the mandrel, the presence of cracks was checked. The absence of cracks gives the flexibility of the coated wire.

[0041] Tan delta was measured using Dansk tangent delta instrument.

[0042] FT-IR spectra were measured using Thermoscientific Nicolet FT-IR using ATR attachment.

[0043] Viscosity was measured using a 5 wt.% solution in ethylene dichloride at 20°C. EXAMPLES

[0044] All experiments were performed using a Brabender Co-Rotating Clamshell Twin Screw Extruder Model 20 / 40D. The extruder contained 4 heating zones, a die adapter heating zone and a die heating zone. If not otherwise specified, the first heating zone is the feeding zone, and the remaining zones are reaction zones. The screw design contained forward and reverse conveying elements, forward and reverse kneading blocks, and teeth elements. The various materials were fed through either a twin concave screw volumetric feeder or a single auger screw volumetric feeder.

[0045] EPON 1001 F was an epoxy resin derived from a liquid epoxy resin (2.2-bis(p- glycidyloxyphenyl) propane condensation product with 2.2-bis(p- hydroxyphenyl)propane and similar isomers) and bisphenol A. Its datasheet specifies a viscosity at 25°C (40 weight-% solution in methyl ethyl ketone, ASTM D445) of 7 to 9.6 cP, a weight per epoxide (ASTM D1652) of 525 to 550 g / eq and a melting point (ASTM D3461 ) of 75 to 80 °C.

[0046] The blocked isocyanate resin was a commercially available phenol blocked TDI resin.

[0047] The methylated melamine was a commercially available methylated melamine formaldehyde resin.

[0048] The defoamer was a commercially available acrylic flow modifier.

[0049] Powder Example 1 :

[0050] The ingredients of Part 1 were dry blended and fed through the extruder at 175°C at 25 rpm to form a homogeneous clear solid. This material was then powdered and mixed with Part 2. This mixture of part 1 and part 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 milled to achieve a 20-40 micron particle size.

[0051] Part 1 [wt-%1 Part 2 rwt.-%1

[0052] Epon 1001 F 59.331 Blocked isocyanate resin 14.216

[0053] Bispenol A 12.118 Methylated melamine 4.406

[0054] PVF low viscosity 7.939 Defoamer 0.990

[0055] TMAH 1.0

[0056] Powder Example 2

[0057] The ingredients of Part 1 were dry blended and fed through the extruder at 175°C at 25 rpm to form a homogeneous clear solid. This material was then powdered and mixed with Part 2. This mixture of part 1 and part 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 milled to achieve a 20-40 micron particle size.

[0058] Part 1 [q] Part 2 [Qi

[0059] Epon 1001 F 480.241 Blocked isocyanate resin 182.949

[0060] Bispenol A 97.711 Methylated melamine 63.408

[0061] PVF medium viscosity 192.304 Defoamer 12.474

[0062] TMAH 10.395

[0063] Comparative Powder Example 1

[0064] Formulation: Part 1 was dry blended and fed through extruder at 175°C at 25 rpm to form a homogeneous clear solid. This material was then milled to achieve a 20- 40 micron particle size. Part 1

[0065] Epon 1001 F 59.331

[0066] Bispenol A 12.118

[0067] PVF low viscosity 7.939

[0068] Comparative Powder Example 2

[0069] Formulation: Part 1 was dry blended and fed through extruder at 175C at 25 rpm to form a homogeneous clear solid. This material was then milled to achieve a 20-40 micron particle size.

[0070] Part 1

[0071] Epon 1001 F 59.331

[0072] PVF low viscosity 7.939

[0073] Wire Samples

[0074] The material obtained in Powder Example 1 was electrostatically applied to 1 mm pre-annealed copper wire. 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 observed defects. Sample was measured for flexibility by performing a 20% stretch followed by 1x mandrel wrap. Visual observation showed no delamination or cracking. The wire was also flattened 5:1 in a post roll test. Flat wire was then wrapped around 1 mm mandrel and observed visually for cracks or loss of adhesion. No cracks were observed. NEMA MW1000 3.53 alcohol toluene boil test was also performed without any issue. No swelling or blistering was observed.

[0075] The material obtained in Powder Example 2 was electrostatically applied to 1 mm pre-annealed copper wire. 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 observed defects. Sample was measured for flexibility by performing a 20% stretch followed by 1x mandrel wrap. Visual observation showed no delamination or cracking. The wire was also flattened 5:1 in a post roll test. Flat wire was then wrapped around 1 mm mandrel and observed visually for cracks or loss of adhesion. No cracks were observed. NEMA MW1000 3.53 alcohol toluene boil test was also performed without any issue. No swelling or blistering was observed.

[0076] The material obtained in Comparative Powder Example 1 was electrostatically applied to 1 mm pre-annealed copper wire. 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 observed defects. However, the Sample showed poor flexibility by performing a 20% stretch followed by 1x mandrel wrap. Visual observation showed cracking. The wire was also flattened 5:1 in a post roll test. Flat wire was then wrapped around 1 mm mandrel and observed visually for cracks or loss of adhesion. Cracks were observed.

[0077] The material obtained in Comparative Powder Example 2 was electrostatically applied to 1 mm pre-annealed copper wire. The wire was cured in an oven at 200°C for 20 minutes to achieve final cure. The wire coating was rough and had pinholes and did not form a good film.

[0078] The material obtained in Powder Example 1 was extruded onto 1 mm pre-annealed copper wire at 110°C. Coating thickness of 30 microns 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 observed defects. Sample was measured for flexibility by performing a 20% stretch followed by 1x mandrel wrap. Visual observation showed no delamination or cracking. The wire was also flattened 5:1 in a post roll test. Flat wire was then wrapped around 1 mm mandrel and observed visually for cracks or loss of adhesion. No cracks were observed. NEMA MW1000 3.53 alcohol toluene boil test was also performed without any issue. No swelling or blistering was observed.

[0079] The material obtained in Powder Example 2 was extruded onto 1 mm pre-annealed copper wire at 110°C. Coating thickness of 30 microns 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 observed defects. Sample was measured for flexibility by performing a 20% stretch followed by 1x mandrel wrap. Visual observation showed no delamination or cracking. The wire was also flattened 5:1 in a post roll test. Flat wire was then wrapped around 1 mm mandrel and observed visually for cracks or loss of adhesion. No cracks were observed. NEMA MW1000 3.53 alcohol toluene boil test was also performed without any issue. No swelling or blistering was observed. Tangent delta was measured using Dansk tester and a tangent delta of 108 was observed signaling sufficient cure.

[0080] The material obtained in Comparative Powder Example 1 was extruded onto 1 mm pre-annealed copper wire at 110°C. Coating thickness of 30 microns was achieved. The wire was cured in an oven at 200°C for 20 minutes to achieve final cure. The wire coating was poor. No further testing was performed.

[0081] The material obtained in Comparative Powder Example 2 was extruded onto 1 mm pre-annealed copper wire at 110°C. Coating thickness of 30 microns was achieved. The wire was cured in an oven at 200°C for 20 minutes to achieve final cure. The wire coating was rough and had pinholes and did not form a good film. No further testing was performed.

[0082] Polyvinyl formal was attempted to be extruded onto 1 mm pre-annealed copper wire at 200-250°C. Decomposition of resin was observed. No further testing was performed.

[0083] The importance of using both an aromatic diol compound and an aromatic or aliphatic epoxy compound is shown in Table 1 below. Examples A01 and A02 demonstrate properties very similar to the control poly(vinylformal) coating used in wire enamel coatings. Examples 3 and 4 include only an aromatic or aliphatic epoxy compound, no aromatic diol compound, at 50% mass while Examples 5 and 6 are at 75% epoxy and 25% poly(vinylformal). None of the samples show adequate physical properties compared to the control sample. All of the samples are either catalyzed by tetramethyl ammonium hydroxide or dicyandiamide. Table 1. Compositions and tensile testing results:

[0084] In Table 2 below are the ingredients of some fully formulated compositions. Two levels of polyvinyl formal were examined. The polyvinyl formal was also of low and medium molecular weight.

[0085] The aromatic diol compound, the aromatic or aliphatic epoxy compound and the PVF-compound were dry blended and fed through the extruder at 175°C at 25 rpm to form a homogeneous clear solid. This material was then powdered and mixed with the other ingredients. This mixture 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. The material was applied on a PET film at a thickness of 10 mil, heated at 125°C for 15 minutes and thereafter the temperature was increased to 200°C, and kept at that temperature for 60 minutes All formulations gave good physical properties for modulus, tensile strength and elongation. Testing after immersion in transformer oil also showed little change after 7 days at 50°C. Dielectric properties were also examined before and after oil immersion. Again, all samples compared favorably to the control C8359 Poly(vinylformal) used on a magnet wire.

[0086] Table 2: Compositions (in pbw)

[0087] Table 2A: Results tensile testing

[0088] Table 2B: Results tensile testing after oil bath (7 days at 50°C) Table 2C: Results of dielectric properties before and after oil bath (7 days at 50°C)

[0089] The dielectric properties were measured as the dielectric breakdown (V / mil) in a standard dielectric breakdown test. For this test the material was applied on a PET film as described above.

Claims

Claims1. Composition comprising: a. a poly(vinylformal) resin; b. an aromatic diol compound; c. an aromatic or aliphatic epoxy compound; d. a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound; e. 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.

2. The composition of claim 1 , wherein the epoxy compound is an aromatic epoxy compound.

3. The composition of claim 1 or 2, having a viscosity, measured in a 5 weight-% solution in ethylene dichloride at 20 °C, in the range from 100 to 100000 Pa'S at a temperature of 100°C.

4. 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 starts at a temperature of at least 50 °C.

5. The composition of claim 1 or 2, wherein the crosslinker is such that the reaction between the poly(vinylformal) resin and the reaction product obtained by reacting the aromatic diol compound with the aromatic or aliphatic epoxy compound starts at a temperature of at least 150 °C.

6. The composition of claim 1 comprising: a. from 5 to 50 wt.% of a poly(vinylformal) resin; b. from 2 to 15 wt.% of an aromatic diol compound;c. from 40 to 85 wt.% of an aromatic or aliphatic epoxy compound. d. from 0.1 to 4 wt. % of catalyst e. from 3 to 30 wt.% of a crosslinking agent wherein the wt.% are based upon the total content of the composition.

7. A process for coating a metal substrate wherein the substrate is coated with the composition of claim 1.

8. The process of claim 7, wherein the metal substrate is a wire, a foil, a sheet, a bar or a rod.

9. The process of claim 7 or 8 wherein the composition of claim 1 is applied to the wire in a powder form using an electrostatic spray gun.

10. A process for the preparation of a poly(vinylformal) composition that can be used for coating wires, comprising:- in a first step mixing a poly(vinylformal) resin, an aromatic diol compound, and an aromatic or aliphatic epoxy compound in an extruder at a temperature where at least one of the components is flowable, but where no reaction occurs between the aromatic diol compound and the aromatic or aliphatic epoxy compound.- In a next step, adding a catalyst for the reaction between the aromatic diol compound and the aromatic or aliphatic epoxy compound, and raising the temperature to a value where the reaction between the diol compound and the epoxy compound starts and propagates.- Thereafter, adding, 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 and- mixing the composition until a homogeneous mixture is obtained.

Images