Solutions of Polyvinyl Acetals

The present invention is directed to certain novel solutions of polyvinyl formal.

Polyvinyl formal (Formvar) is normally employed with phenolic solvents as wire enamels for coating magnet wires.

These wire enamels have a number of disadvantages. Thus polyvinyl formals must be wet by a hydrocarbon diluent prior to adding the primary solvent. It is possible however, but not practical, to obtain a high concentration of solids, e.g., 16 to 22% solids is the rule in conventional cresylic acid since the viscosity becomes too high above this range, for conventional application. The solvent systems are expensive and the phenolic solvents are toxic and are pollutants.

It has now been found that novel solvents comprising certain monoalkyl ethers of diethylene glycol and triethylene glycol or the monophenyl ether of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol or propylene glycol can be used as solvents for polyvinyl formals. In some cases it is necessary to have a diluent, e.g. an aromatic hydrocarbon present as will be discussed hereinafter.

Technical Bulletin No. 6070B of Monsanto Polymers and Petrochemicals Co. entitled "Butvar Polyvinyl Butyral, Formvar Polyvinyl Formal, Properties and Uses" shows the solubility properties of Formvar resins. The entire disclosure of the Technical Bulletin No. 6070B is hereby incorporated by reference and relied upon.

Bulletin No. 6070B shows that polyvinyl formal with either 68% or 82% formal content is insoluble in Cellosolve (monoethylether of ethylene glycol) while polyvinyl formal with 68% formal content is soluble in Methyl Cellosolve (monomethyl ether of ethylene glycol) but that polyvinyl formal with 82% formal content is not soluble in Methyl Cellosolve. It is not stated what is the concentration at which the solubility was tested but apparently it was quite low since the solubility of various polyvinyl butyrals was tested at either 5% solids or 10% solids depending on the particular polyvinyl butyral.

The Technical Bulletin No. 6070B also mentions that polyvinyl formals are soluble in mixtures of alcohols and aromatic hydrocarbons and the viscosity of a 15% solution of various polyvinyl formals in 60:40 toluene-ethanol at 25.degree. C is recorded. It is further disclosed that solutions of polyvinyl formals show marked increases in viscosity as resin solids increase. In the case of polyvinyl butyral it is disclosed that the solvent can be a mixture of diacetone alcohol, butyl alcohol, 95% ethyl alcohol and either xylene or toluene and that an unspecified part of the diacetone alcohol can be replaced by Cellosolve for lower viscosity. There is not mentioned any combination of a glycol monoether with another solvent for dissolving polyvinyl formals, which are more difficult to dissolve than polyvinyl butyrals.

The specific ether solvents with which the present invention is concerned are the monomethyl ether of diethylene glycol (Methyl Carbitol), the monoethyl ether of diethylene glycol (Carbitol), the monomethyl ether of triethylene glycol, the monoethyl ether of triethylene glycol, the monophenyl ether of ethylene glycol, the monophenyl ether of diethylene glycol, the monophenyl ether of triethylene glycol, the monophenyl ether of tetraethylene glycol, and the monophenyl ether of propylene glycol. The polyvinyl formals are only soluble in the monophenyl ether of ethylene glycol alone, the monophenyl ether of diethylene glycol alone, the monophenyl ether of triethylene glycol, the monophenyl ether of tetraethylene glycol, or the monophenyl ether of propylene glycol (or in a mixture of these monophenyl ethers).

The suitable monoalkyl ethers can be expressed by the formula

where R.sub.1 is alkyl of 1 to 2 carbon atoms and x is 2 or 3.

The suitable phenyl ethers can be expressed by the formula

where R.sub.2 is phenyl, and y is 1, 2, 3, or 4 and n is 2 or 3.

The compounds of formula I must be used with a diluent while the compounds of formula II can be used alone or with a diluent.

The amount of diluent will vary depending on the diluent and the solids concentration desired. In general with the compounds of formula I there is used 10 to 95% of the compound and 90 to 5% of the diluent. (The same amount of diluent can be used, if desired, with the compounds of formula II and in addition to the diluents mentioned below there can also be used the compounds of formula I as diluents for the compounds of formula II, the compounds of formula I being less expensive than those of fromula II.)

The proportions of the compound of formula I (or formula II) and diluent should be such as to provide a solution of polyvinyl formal having at least 20% solids and more preferably 25 to 35% solids, more preferably at least 30% solids.

Some of the formulations have the desired concentrations, both in the cold, e.g., at 77.degree. F (25.degree. C) or hot, e.g., at 140.degree. F (60.degree. C) or above while other formulations can only be used hot. The preferred formulations are those which impart the desired solubility of at least 20% solids, more preferably at least 25% solids in the cold since they have the broadest range of application.

For hot melt and extrusion work the temperature can be raised, e.g., to 120.degree.-140.degree. C and the solids can be as high as 70% or even higher. The limiting factor in solids content is the viscosity which can be tolerated by the equipment.

As diluents there are especially preferred aromatic hydrocarbons such as benzene, toluene, xylene, ethyl benzene and other alkylated benzenes, alkylated naphthalenes. There can be present a minor amount of aliphatic hydrocarbons in the aromatic hydrocarbon. In the event that the hydrocarbon mixture does not sufficiently dissolve in the compound of formula I or II in the cold there can be added enough xylene or toluene (or even cyclohexanone) to render it more soluble. Examples of mixtures of hydrocarbons are enamel wire naphtha, Solvesso 100 (an aromatic naphtha, primarily monoalkyl benzenes, boiling in the range of 315.degree.-350.degree. F), Solvesso 150 (a heavy aromatic naphtha which is primarily a mixture of tetramethyl benzene with dialkyl and trialkyl benzenes boiling in the range 360.degree.-400.degree. F), a blend of 70% Solvesso 150 with 30% heavy aromatic naphtha (the blend being known as Solvesso N-150 and having a boiling range of 372.degree.-500.degree. F), W.E.S. oil (a blend of aromatic hydrocarbons which distills 10% to not below 165.degree. C, 70% to not above 190.degree. C and 95% to not above 235.degree. C). W.E.S. oil is an example of an enamel wire naphtha. Solvesso 100 can be used with the ethers of formula I in the hot but in the cold it is necessary to add xylene (or toluene) to get the desired solubility. Solvesso 150 can be blended with the ethers of formula I hot but not cold.

Other diluents include Cellosolve, Cellosolve acetate (ethoxyethyl acetate), lower alkyl acetates, e.g., methyl acetate, ethyl acetate, lower alkanols, such as those having 1 to 3 carbon atoms, e.g., methyl alcohol, ethyl alcohol, isopropyl alcohol, ketones including lower alkanones, e.g., acetone, methyl ethyl ketone and methyl isobutyl ketone, cyclohexanone, Butyl Carbitol (butyl ether of diethylene glycol), Hexyl Carbitol (hexyl ether of diethylene glycol). The alkanols were only useful with the compounds of formula II as diluents in polyvinyl formula enamels.

Propasol DM solvent (monomethyl ether of dipropylene glycol) can also be used as a diluent together with the aromatic solvent to give a composition in which the polyvinyl formal is soluble in the hot.

The wire enamels of the present invention can have the conventional additives employed in coating magnet wire and other electrical conductors. Thus there can be employed phenolic resins, e.g., phenol-formaldehyde resins, such as p-phenylphenol-formaldehyde, meta-para cresol-formaldehyde, phenol-formaldehyde, alkylphenol-formaldehyde resins. There can also be added conventional blocked isocyanates, e.g., the reaction product of glycerine and toluene diisocyanate blocked with phenol, the reaction product of diethylene glycol and trimethylol propane with toluene diisocyanate blocked with phenol Mondur S (butylene glycol and trimethylol propane with toluene diisocyanate blocked with a phenol), trimethylol propane and 1,3-butylene glycol reacted with toluene diisocyanate blocked with phenol. Other conventional additives such as polyesters or resimenes may be added.

Examples of suitable polyvinyl formals (all are free flowing powders) are those set forth below. The properties are taken from Technical Bulletin No. 6070B.

The solvent systems of the invention are particularly useful with the polyvinyl formals of relatively high formal content, e.g., 82%, which are normally most difficult to dissolve in solvents and which are the most important polyvinyl formals for use as wire coating.

Unless otherwise indicated all parts and percentages are by weight.

The compositions of the invention can comprise, consist essentially of or consist of the materials set forth.

The wire enamels can be applied to copper, silver, aluminum or other wires using conventional coating procedures and wire speeds and temperatures, e.g., 400.degree. to 800.degree. F. In the working examples the temperature of application to the wire was 140.degree. F and the wire speed was 45 ft/min.

In place of the Carbitol materials there of course can be used the same solvents sold under other trademarks such as Dowanol.

There is no problem in determining whether any particular combination of solvent and diluent is suitable. All that is necessary is to mix the solvent and diluent and add polyvinyl formal at the desired concentration and heat to 60.degree. C to see if solution occurs. To determine whether the polyvinyl formal is soluble at 25.degree. C the solution is then cooled to 25.degree. C. (If the polyvinyl formal is not soluble in the hot mixture there is no need to test its solubility in the cold as it will not be soluble.

The novel solvent systems of the invention have the following advantages over the present day commercial phenolic solvent containing wire enamels:

1. The polyvinyl formal powder does not need to be wet by a hydrocabon diluent prior to the addition of the primary solvent.

2. The final coatings are at much higher percentage solids, e.g., 25-35%, and when heated to application temperature have lower viscosities than those of equal solids in conventional solvent systems.

3. The current conventional solvent systems are more expensive.

4. The new glycol ether (formula I or formula II) based solvent systems are less toxic than the conventional phenol, cresol and xylenol based solvent systems. Thus, the end result is that the polyvinyl formal solutions in glycol ether based solvent systems of the invention are more readily processed, handled and ecologically advantageous.

As previously pointed out, except for the phenyl glycol ethers of formula II, the polyvinyl formal powders used are not soluble in the component parts of the systems described. However, when the component parts are blended, the polyvinyl formal is readily dissolved and forms a stable, clear solution at room temperature (or in some cases only at elevated temperature).

Parts one through five were charged into a three liter reaction kettle equipped with an agitator, thermometer and condenser. The batch was heated to 70.degree. C and held until three was all dissolved. Parts 6 through 11 were then added. The reaction temperature dropped during the addition to 60.degree. C. The reaction mixture was held for 1 hour at this temperature then filtered through a paper cone. The viscosity of this mixture at 140.degree. C was 3375 cps. The percent solids as determined at two grams for 2 hours at 400.degree. F was 31.2 percent.

A 3.2 mil coating of this material was applied to 18AWG copper wire. The coated wire had a good appearance. The properties are as shown in Table I.

A similar wire enamel solution was held for two weeks at 130.degree. F. The only changes observed were a darkening of the reaction mixture's color and very slight advance in viscosity from 3375 centipoises to 3500 centipoises.

A viscosity study conducted in an open pot at 140.degree. F on the product of Example I showed an increase in viscosity of 4000 centipoises over a 7 hour time span. The initial increase from 3750 centipoises to 7375 centipoises took only one and one half hours. The solution viscosity remained relatively stable thereafter, advancing only 275 centipoises in the next five and one half hours. In contrast similar conventional systems with phenolic solvents have also shown an initial doubling of viscosity, however upon prolonged holding at elevated temperatures the viscosity continued to advance exponentially.

The blending procedure was carried out as stated in Example I with parts 1 through 4 being charged then heated slightly to aid in dissolving 3. When 3 was dissolved in the solvents, the rest of the materials were added and blended into the solution thoroughly prior to filtration.

The percent solids determined by baking three grams for one half hour at 400.degree. F were 21.6 percent.

The viscosity determined on the Gardner-Holdt scale at 77.degree. F was Z2 3/4.

The solution was applied to 18 AWG copper wire and tested as a magnet wire coating. A 2.9 mil build was obtained at 45 feet per minute with good appearance. The electrical test properties are as shown in Table I.

This coating was blended as described in Example I.

The viscosity at 140.degree. F was determined to be 3700 centipoises.

The solids were 28.4 percent.

When used to coat 18 AWG copper wire this enamel gave a 2.8 mil build at 45 fee per minute. The appearance was rated as good. Other test results are shown in Table I.

The batch was blended as in Example One.

The viscosity was 2500 cps at 130.degree. F.

The solids were calculated to be 21.2 percent.

This batch when cooled had a very slight haze which did not worsen after standing at room temperature for 24 hours. This haze disappeared upon very slight heating.

The batch was blended as in Example I.

The percent solids was 21.2 percent.

The batch was soluble hot, and remained in solution upon cooling for a short period. However, upon leaving the batch sit in a covered container over night, it become cloudy.

Butyl Carbitol was tried in a blend as in Examples IV and V (i.e., it replaced the Methyl Carbitol or Ethyl Carbitol) and was insoluble hot and at room temperature.

Examples IV and V were blended together in equal portions to produce a clear stable solution, with a viscosity of 2475 centipoises at 130.degree. F and 21.2 percent solids.

To Example VII enough of Example VI was added to produce a solvent system containing 40 percent Methyl Carbitol, 40 percent Ethyl Carbitol and 20 percent Butyl Carbitol based on the total Carbitols. The solution remained clear at room temperature.

When enough of Example VI was added to Example VII to give equal portions of methyl, ethyl and butyl Carbitols in the solvent system, the mixture became insoluble.

A series of diluents were tried in determining the scope of the invention. In Table II are representative examples of several classes of compounds and their effects on the polyvinyl formal solvent system.

All solutions were compounded at 20 percent calculated solids using 6/95E Formvar Powder at 15 centipoises. The solvent system contained 30 percent Methyl Carbitol, 30 percent Ethyl Carbitol and 40 percent of the diluent of choice. The standard formula used therefore read:

Table III lists other various solvent systems tried and the results obtained. All the reactions were compounded at 20 percent calculated solids.

From Table III it is clear that Methoxy Triethylene Glycol compares to Methyl Carbitol as does Ethoxy Triethylene Glycol to Ethyl Carbitol.

Also from Table III it is obvious that the Phenyl Glycol Ethers are good solvents for Formvar. Not only was the 6/95E Formvar Powder soluble in straight phenyl glycol ethers, but an equal amount of Ethyl Alcohol was added without precipitating out the Formvar solution. With Methyl Carbitol a diluent is needed and the Formvar powder is precipitated by even a slight addition of Ethyl Alcohol. Similar results were obtained when the phenyl ether of tetraethylene glycol and the phenyl ether of propylene glycol were used, as shown in Table III.