GB1558616A

GB1558616A - Aqueous wire enamel dispersions and their manufacture

Info

Publication number: GB1558616A
Application number: GB34737/76A
Authority: GB
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1975-08-21
Filing date: 1976-08-20
Publication date: 1980-01-09
Also published as: DE2537207A1; NL7609174A; CH603773A5; IT1062531B; SE7609130L; FR2321529A1; ES450858A1; FR2321529B1; SE431553B

Description

(54) AQUEOUS WIRE ENAMEL DISPERSIONS AND THEIR MANUFACTURE (71) We. BASF AKTIEN GESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following Statement:- The present invention relates to aqueous wire enamel dispersions based on polyurethane-forming starting components, and to their manufacture.

It is known that coatings based on polyurethanes have advantages for insulating electrical conductors. They result in good moisture resistance and aging resistance, but above all the coatings can be soldered.

Such polyurethane coatings are usually produced by applying a solution, which contains a mixture of a hydroxyl-containing polyester and a blocked polyisocyanate, to the wire to be coated, and then baking the coating, so that the polyisocyanate is released and the polyester becomes crosslinked. This process has the disadvantage that the manufacture of enamel and its application entails handling large amounts of organic solvents, and that on baking the solvents are released into the atmosphere and pollute the environment.

German Published Application 1,729,201 discloses aqueous dispersions of polyurethane powders containing salt-like groups. Such dispersions contain finished polyurethanes, the reaction between the starting components, to form a polyurethane, having been carried out before dispersing the material in water.

Such dispersions are unsuitable for use as coating agents for insulating electrical conductors. If they contain crosslinked polyurethanes. the latter cannot be applied to wires as a level coating; if the polyurethanes are of the non-crosslinked linear type, it is true that they can be used to coat wires, but the coatings are unsuitable for electrical insulation purposes since they are too soft and have a low softening range and inaequate heat resistance.

The present invention seeks to enable wire enamels based on polyurethanes to be applied from aqueous dispersion to give coatings with good mechanical and electrical properties.

According to the invention there is provided an aqueous wire enamel dispersion which comprises: A) from 40 to 80% by weight of water, B) from 60 to 20% by weight of mixture of (a) from 70 to 10% by weight of one or more polyesters or polyethers having a hydroxyl number greater than 20 and (b) from 30 to 90% by weight of one or more blocked polyisocyanates, the mixture being dispersed in water, in the form of particles of mean diameter from 0.05 to 2 ,u, and C) from 0.1 to 15% by weight of a dispersant which is a water-soluble high molecular weight organic compound with polar groups.

A preferred process for the manufacture of such a dispersion comprises mixing a solution, of from 10 to 80 X" strength by weight, of the component B in an inert organic solvent with a solution of the dispersant C in water to form an emulsion, subjecting this emulsion to a shearing motion to provide a mean particle diameter for the component B particles of from 0.05 to 2 it, and at the same time distilling off the organic solvent or an azeotrope of the solvent and water.

In a further preferred process (a) a solution of the component B, with or without a dispersant C and further additive(s), in an inert organic solvent having a freezing point of from +30 to -500C is prepared, (b) this solution is solidified by cooling, (c) the solvent is removed by sublimation under reduced pressure, (d) the resulting solid, solvent-free product is comminuted mechanically to a mean particle diameter of the component B particles of from 0.05 to 2 ,u, and (e) the powder obtained is disperesed in water together with a dispersant C if it does not already contain such a dispersant and optionally together with further additive(s).

Mixtures of hydroxyl-containing polyesters or polyethers and blocked polyisocyanates are used as starting components for the manufacture of the dispersions of the invention. These are conventional starting materials for the manufacture of polyurethanes. Details of the materials, the process of manufacture and the properties are to be found in the chapter on "Polyurethanes" in Houben Well, Methoden der organischen Chemie, Volume XIV/2 (1963), pages 77-98.

Preferred polyesters are reaction products of aliphatic or aromatic dicarboxylic acids, e.g. adipic acid, terephthalic acid, isophthalic acid, phthalic acid or azelaic acid, or their derivatives, with diols, preferably aliphatic diols, e.g.

ethylene glycol, diethylene glycol, triethylene glycol or neopentyl glycol, preferably in the presence of polyhydric (such as trihydric) alcohols, e.g. trimethylolpropane, glycerol, trimethylolbenzene or tris-hydroxyethyl-isocyanurate, as crosslinking agents. Examples of suitable polyethers are polyethylene glycol, polypropylene glycol and polytetrahydrofuran.

The hydroxyl number of the polyesters and polyethers used is greater than 20 and is preferably from 25 to 400 and especially from 60 to 300. The molecular weight of the polyesters and polyethers used is preferably from 500 to 5,000 and especially from 1,000 to 2,000.

The other component is a blocked polyisocyanate or mixture thereof.

Preferably, it is based on an aromatic polyisocyanate having an isocyanate content of from 5 to 30% by weight.

Examples of polyisocyanates are hexamethylenediisocyanate, 4,4' diphenylmethane diisocyanate, toluylene diisocyanate, isophoronediisocyanate, naphthalenediisocyanate, and more highly condensed products, e.g. the product obtained by trimerizing toluylenediisocyanate or by reacting 1 mole of trimethylolpropane with 3 moles of toluylenediisocyanate. Examples of blocking agents are lactams. e.g. E caprolactam, phenols e.g. phenol and the cresols, dicarboxylic acid esters, e.g. maleic acid esters and malonic acid esters, glycols, imidazoles and phthalimide.

The weight ratio of polyester and/or polyether to blocked polyisocyanate is from 10:90 to 70:30, preferably from 20:80 to 60:40.

The aqueous dispersions of the invention contain from 0.1 to 15% by weight of a dispersant, which is a water-soluble high molecular weight organic compounds with polar groups. Conventional dispersants of this type may be used, e.g.

polyvinylpyrrolidone, compolymers of vinyl propionate and vinylpyrrolidone, polyacrylic acid, partially hydrolyzed copolymers of acrylic esters and acrylonitrile, polyvinyl, alcohol, cellulose ethers, gelatine or mixtures of these materials.

In addition, the compositions may contain one or more further additives.

These may be, for example, fillers, flow control agents, thickeners, neutralizing agents and anti-thixotropic agents. Having regard to the amounts of the other components, they may be present in the compositions in amounts of up to about 40 ,^ by weight in total. Possible further additives also include materials which increase the elasticity and hardness of the finished coatings, e.g. polyamides, epoxy resins, phenolic resins, melamine resins or polyvinylformals, preferably in amounts of from 2 to 30 Sn by weight. Aliphatic polyamides, as well as acrylic copolymers, e.g. a copolymer of an acrylic ester, acrylonitrile and acrylic acid, are particularly suitable for this purpose.

These additives may be added to the starting solution used to manufacture the aqueous dispersion or only to the finished aqueous dispersion.

In one preferred process for manufacturing the aqueous dispersion, the starting component B is dissolved in an inert organic solvent to give a solution of from 10 to 80, preferably from 20 to 60% strength by weight. The solvent used should preferably boil below 1500C and especially below 105"C.

Examples of suitable solvents are tetrahydrofuran, dioxane, acetone, methylene chloride, xylenes, benzene, toluene and ethylglycol acetate.

This solution is then mixed with a solution of the dispersant C in water. Particularly suitable dispersants are polyvinyl alcohols having a residual acetate content of less than 35 mole%, preferably of from 5 to 30 mole '. The amount of the dispersant is so chosen that the finished wire enamel contains from 0.1 to 15% by weight, preferably from 0.3. to 3% by weight.

On mixing the two solutions, an emulsion is produced, and the choice of the ratios determines which solution is the continuous phase and which is the disperse phase. The emulsion is subjected to a shearing motion.

This is advantageously achieved by vigorous stirring, for which the speed of rotation of the outer extremity of the blades of the stirrer is suitably from 50 to 500 minute preferably from 100 to 400 m/minute. At the same time, the organic solvent, or an azeotrope thereof with water, is distilled off.

For this purpose, the emulsion is advantageously heated to above the boiling point of the solvent or azeotrope. However, it is also possible to strip off the solvent by applying reduced pressure. It is important that during the distillation the emulsion should be subjected to strong shearing forces, since only in this way is a sufficiently fine dispersion obtained. The maximum particle size should preferably be less than 5 ; the mean diameter is from 0.05 to 2 p and preferably from 0.1 to 1 p.

In the other preferred process of manufacture, a solution of a mixture of the polyester or polyether and the blocked polyisocyanate in an inert organic solvent is again prepared first. The solvent should have a freezing point of from +30 to50 C.

Examples of suitable solvents are xylenes, glacial acetic acid, cyclohexane, ethylene bromide, neopentyl alcohol ortert-butanol, dioxane being preferred. In addition to the reactants for the formation of the polyurethane, part or all of the dispersant C and of any further additives may be added at this stage. Once again, the solids content of the solution is preferably from 10 to 80, and especially from 20 to 60, per cent by weight.

The solution is then cooled to below its freezing point, and is thereby solidified. This can be carried out batchwise, but in industrial-scale operation it is preferred to use a continuous method in which the solution is applied, e.g. to an endless belt where it is cooled and solidified, the solidified product finally being detached from the belt. The solidified product is then freed from the solvent under reduced pressure. For this purpose, the pressure is set to a value below the vapor pressure of the solvent at the freezing point, and preferably to a value from 3 to 10 mm Hg lower than the vapor pressure of the solvent at the triple point. In this way, the autogeneous temperature in the material being dried is below the freezing point. The solvent can also be sublimed from the solidified mixture by passing a stream of gas, e.g. air or nitrogen, over the surface of the solid phase, the stream of gas advantageously being set to temperatures equal to or moderately above the freezing point of the solution. This process step can also be carried out continuously, if a reduced pressure tower with a plurality of trays rotating about a longitudinal axis is used, the material to be dried being slowly conveyed onto the next-lowest tray. The solvent can be condensed and recycled.

The dried product is obtained in the form of loose foam-like aggregates which disintegrate even when touched lightly, but above all on crushing or grinding, to give a loose powder having a mean particle size of from about 5 to 200 ,*4. This powder can be further comminuted by milling. Finally, the powder is dispersed in water, using from 40 to 80 parts by weight, of water for from 20 to 60 parts by weight of powder together with a dispersant C if it does not contain enough of such a dispersant and optimally together with one or more further additives. The powder can be further comminuted in the aqueous phase, e.g. by dispersion milling so that a dispersion in which the particles have a mean diameter of from 0.05 to 2 p and preferably from 0.1 to 1 p are produced, in which case more than 99 < of the particles have a diameter of less than 1 p.

In addition to these preferred methods of manufacture, the dispersion according to the invention can also be manufactured by dry milling of the solid starting materials, dispersing the product in water and then wet milling the mixture. It is also possible to dissolve the starting materials in suitable solvents, precipitate them by addition of non-solvents, and wash the material obtained and then disperse it in water. The preferred processes are particularly advantageous because they produce, in a simple manner, the desired fine particle size necessary to achieve uniform smooth coatings.

The aqueous wire enamel dispersions of the invention are used for coating electrical conductors.

If appropriate, the dispersions can be further diluted with water to an optimum concentration. To produce the coatings, the dispersions may be applied by conventional enamelling machinery and are then baked at oven temperatures of from 250 to 500"C, preferably from 350 to 4500 C. This eliminates the blocking agent, and the polyisocyanate is liberated and can crosslink the polyesters or polyethers by reaction with the hydroxyl groups.

In the following Examples, which illustrate the invention, parts and percentages are by weight.

EXAMPLE 1 375 parts of a polyester of adipic acid, 1,4butanediol and trimethylolpropane, having a hydroxyl number of 165, and 480 parts of a phenol-blocked polyisocyanate obtained from I mole of trimethylolpropane and 3 moles of toluylenediisocyanate (having an isocyanate content of 13 4To) are dissolved in 750 parts of tetrahydrofuran in a stirred flask surmounted by a distillation attachment. 45 parts of ethylglycol acetate as a flow control agent, 684 parts of a 5% strength aqueous solution of a polyvinyl alcohol having a viscosity of 5 cp (in 2% strength aqueous solution), a hydroxyl number of 653 and a residual acetate content of 27 ", and a further 240 parts of water, were added to the preceding solution, whilst stirring vigorously (stirrer speed 450 rpm). An emulsion formed. The internal temperature of the flask was raised to 99"C in the course of 3 hours, during which 800 parts of liquid were distilled off.

The dispersion in which the mean diameter of the polyester and blocked polyisocyanate particles was from 0.05 to 2 1, was then cooled and 170 parts of a 50 /a strength dispersion of a polyamide obtained from 39 of hexamethylenediamine adipate, 33% of caprolactam and 24% of 4,4'diaminodicyclohexylmethane were added.

0.8 mm thick copper wires were coated with the dispersion at 3800C, using a coating speed of from 8 to 14 m/min. The coated wires had the following properties: increase in diameter: 5060 p heat shock resistance (DIN 46,453): 185"C soldering time at 375"C (DIN 46,416): 2-3 sec.

softening point (DIN 53,180): 15Q--210"C pencil hardness (DIN 46,453): lH-2H EXAMPLE 2 100 parts of a polyester obtained from phthalic anhydride and trimethylolpropane and having a hydroxyl number of 265, and 207 parts of the blocked polyisocyanate from Example 1, were dissolved in 307 parts of methylene chloride. 21 parts of ethylglycol acetate and 307 parts of a 5% strength aqueous solution of the dispersing agent of Example 1 were then added to the solution, with vigorous stirring. The temperature was then raised to 850C in the course of 2 hours, during which 310 parts of solvent distilled off. The dispersion in which the polyester and blocked polyisocyanate particles had a mean diameter of from 0.05 to 2 p was then cooled and defoamed overnight. Wires were coated, as described in Example 1, with this dispersion. Their properties were as follows: increase in diameter: 50,60 ,u softening point: 255"C heat shock resistance: 130"C soldering time at 3750 C: 3 - 4 sec.

pencil hardness: 7H EXAMPLE 3 233.6 g of a polyester (prepared from phthalic anhydride and trimethylolpropane in the weight ratio of about 1:1) having a hydroxyl number of 140, and 166.4 g of a blocked polyisocyanate (a reaction product of 1 mole of trimethylolpropane and 3 moles of toluylenediisocyanate, blocked with phenol) were dissolved in 600 g of dioxane.

The solution was poured into a metal vessel and cooled with a solid carbon dioxide/acetone mixture. The mixture was then freeze-dried for 12 hours at a pressure of less than 1 mm Hg, in such a way that the dioxane did not melt. To remove the residual dioxane, the temperature was raised to 50"C for 4 hours. A loose foam was produced, which was pulverized by means of a conventional laboratory mill. To produce a dispersion, 350 g of powder, 10.5 g of a copolymer of vinylpyrrolidone and vinyl propionate, and 8.75 g of polyvinyl alcohol were dispersed in 650 g of water and the mixture was then milled in a laboratory mill at a throughput of 30 1 per hour. The polyester and blocked polyisocyanate particles had a mean diameter of from 0.05 to 2 p. After adding 3.5 g of triethanolamine to the resulting thixotropic dispersion, the mixture was used to coat 1 mm thick copper wires on a horizontal wire-coating machine at 4500 C, using a coating speed of from 6 to 12 m per minute. The resulting coatings had the following properties: soldering time at 3750C (DIN 46,416): 1-2 sec.

heat shock resistance (DIN 46,453): 1300C softening point (DIN 46,453): 2lO-2300C pencil hardness (DIN 46,453): 1--2H breakdown voltage (DIN 53,481): 100--150 V/,a WHAT WE CLAIM IS: 1. An aqueous wire enamel dispersion which comprises A) from 40 to 80% by weight of water, B) from 60 to 20% by weight of a mixture of a) from 70 to 10% by weight of one or more polyesters or polyethers having a hydroxyl number greater than 20 and b) from 30 to 90 ," by weight of one or more blocked polyisocyanates, the mixture being dispersed in water, in the form of particles of mean diameter from 0.05 to 2 z and c) from 0.1 to 150,,, by weight of a dispersant which is a water-soluble high molecular weight organic compound with polar groups.

2. An aqueous wire enamel dispersion as claimed in Claim 1, wherein component B a) is a polyester obtained from an aliphatic or aromatic dicarboxylic acid, an aliphatic diol and, if desired, a trihydric alcohol, which polyester has a hydroxyl number of

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Claims

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alcohol having a viscosity of 5 cp (in 2% strength aqueous solution), a hydroxyl number of 653 and a residual acetate content of 27 ", and a further 240 parts of water, were added to the preceding solution, whilst stirring vigorously (stirrer speed 450 rpm). An emulsion formed. The internal temperature of the flask was raised to 99"C in the course of 3 hours, during which 800 parts of liquid were distilled off.

The dispersion in which the mean diameter of the polyester and blocked polyisocyanate particles was from 0.05 to 2 1, was then cooled and 170 parts of a 50 /a strength dispersion of a polyamide obtained from 39 á of hexamethylenediamine adipate, 33% of caprolactam and 24% of 4,4'diaminodicyclohexylmethane were added.

0.8 mm thick copper wires were coated with the dispersion at 3800C, using a coating speed of from 8 to 14 m/min. The coated wires had the following properties: increase in diameter: 5060 p heat shock resistance (DIN 46,453): 185"C soldering time at 375"C (DIN 46,416): 2-3 sec.

softening point (DIN 53,180): 15Q--210"C pencil hardness (DIN 46,453): lH-2H EXAMPLE 2 100 parts of a polyester obtained from phthalic anhydride and trimethylolpropane and having a hydroxyl number of 265, and 207 parts of the blocked polyisocyanate from Example 1, were dissolved in 307 parts of methylene chloride. 21 parts of ethylglycol acetate and 307 parts of a 5% strength aqueous solution of the dispersing agent of Example 1 were then added to the solution, with vigorous stirring. The temperature was then raised to 850C in the course of 2 hours, during which 310 parts of solvent distilled off. The dispersion in which the polyester and blocked polyisocyanate particles had a mean diameter of from 0.05 to 2 p was then cooled and defoamed overnight. Wires were coated, as described in Example 1, with this dispersion. Their properties were as follows: increase in diameter: 50,60 ,u softening point: 255"C heat shock resistance: 130"C soldering time at 3750 C: 3 - 4 sec.

pencil hardness: 7H EXAMPLE 3

233.6 g of a polyester (prepared from phthalic anhydride and trimethylolpropane in the weight ratio of about 1:1) having a hydroxyl number of 140, and 166.4 g of a blocked polyisocyanate (a reaction product of 1 mole of trimethylolpropane and 3 moles of toluylenediisocyanate, blocked with phenol) were dissolved in 600 g of dioxane.

The solution was poured into a metal vessel and cooled with a solid carbon dioxide/acetone mixture. The mixture was then freeze-dried for 12 hours at a pressure of less than 1 mm Hg, in such a way that the dioxane did not melt. To remove the residual dioxane, the temperature was raised to 50"C for 4 hours. A loose foam was produced, which was pulverized by means of a conventional laboratory mill. To produce a dispersion, 350 g of powder, 10.5 g of a copolymer of vinylpyrrolidone and vinyl propionate, and 8.75 g of polyvinyl alcohol were dispersed in 650 g of water and the mixture was then milled in a laboratory mill at a throughput of 30 1 per hour. The polyester and blocked polyisocyanate particles had a mean diameter of from 0.05 to 2 p. After adding 3.5 g of triethanolamine to the resulting thixotropic dispersion, the mixture was used to coat 1 mm thick copper wires on a horizontal wire-coating machine at 4500 C, using a coating speed of from 6 to 12 m per minute. The resulting coatings had the following properties: soldering time at 3750C (DIN 46,416): 1-2 sec.

heat shock resistance (DIN 46,453): 1300C softening point (DIN 46,453): 2lO-2300C pencil hardness (DIN 46,453): 1--2H breakdown voltage (DIN 53,481): 100--150 V/,a WHAT WE CLAIM IS: 1. An aqueous wire enamel dispersion which comprises A) from 40 to 80% by weight of water, B) from 60 to 20% by weight of a mixture of a) from 70 to 10% by weight of one or more polyesters or polyethers having a hydroxyl number greater than 20 and b) from 30 to 90 ," by weight of one or more blocked polyisocyanates, the mixture being dispersed in water, in the form of particles of mean diameter from 0.05 to 2 z and c) from 0.1 to 150,,, by weight of a dispersant which is a water-soluble high molecular weight organic compound with polar groups.
2. An aqueous wire enamel dispersion as claimed in Claim 1, wherein component B a) is a polyester obtained from an aliphatic or aromatic dicarboxylic acid, an aliphatic diol and, if desired, a trihydric alcohol, which polyester has a hydroxyl number of

from 25 to 400 and a molecular weight of from 500 to 5,000.
3. An aqueous wire enamel dispersion as claimed in Claim 1, wherein component B a) is an aliphatic polyether having a hydroxyl number of from 25 to 400.
4. An aqueous wire enamel dispersion as claimed in any of Claims 1 to 3, wherein the component B b) is an aromatic polyisocyanate which has an isocyanate content of from 5 to 30% by weight and is blocked with a lactam, a phenol, a glycol, a dicarboxylic acid ester, an imidazole or phthalimide.
5. An aqueous wire enamel dispersion as claimed in any of Claims 1 to 4, wherein the component C is a polyvinyl alcohol with a residual acetate content of less than 35 mole 0/ n
6. An aqueous wire enamel dispersion as claimed, in any of Claims I to 5 which also contains up to 40% by weight in total of a filler, flow control agent, thickener, neutralizing agent and/or anti-thixotropic agent.
7. An aqueous wire enamel dispersion as claimed in any of Claims 1 to 5, which, in addition to components A, B and C, contains from 2 to 30 /n by weight of an aliphatic polyamide or of an acrylic copolymer.
8. An aqueous wire enamel dispersion as described in any of the Examples.
9. A process for the manufacture of an aqueous wire enamel dispersion as claimed in Claim 1, wherein a solution of from 10 to 80% strength by weight of the component B in an inert organic solvent is mixed with a solution of the dispersant C in water so as to form an emulsion, this emulsion is subjected to a shearing motion to provide a mean particle diameter for the component B particles of from 0.05 to 2 p and at the same time the organic solvent or an azeotrope of the solvent and water is distilled off.
10. A process as claimed in Claim 9, wherein an organic solvent boiling below 150"C is used.
11. A process as claimed in Claim 9 or 10, wherein the shearing motion is effected by stirring the emulsion at a rate of rotation of the stirrer of from 50 to 500 m/min.
12. A process for the manufacture of an aqueous wire enamel dispersion as claimed in Claim 1, wherein (a) a solution of the component B, with or without a dispersant C and further additive(s), in an inert organic solvent having a freezing point of from +30 to -500C is prepared, (b) this solution is solidified by cooling, (c) the solvent is removed by sublimation under reduced pressure.

(d) the resulting solid, solvent-free product is comminuted mechanically to a mean particle diameter of the component B particles of from 0.05 to 2 p, and.

(e) the powder obtained is dispersed in water, together with a dispersant C if it does not already contain enough of such a dispersant and optionally together with further additive(s).
13. A process for the manufacture of an aqueous wire enamel dispersion carried out substantially as described in Example 1 or 2.
14. A process for the manufacture of an aqueous wire enamel dispersion carried out substantially as described in Example 3.
15. An aqueous wire enamel dispersion when manufactured by a process as claimed in any of Claims 9 to 14.
16. Use of an aqueous wire enamel dispersion as claimed in any of Claims 1 to 8 or 15 for coating electrical conductors.
17. Electrical conductors which have been coated with an aqueous wire enamel dispersion as claimed in any of Claims 1 to 8 or 15 followed by baking at 250 to 500"C to cure the coating.