Electrodeposition of Resinous Compositions Curable Through a Transesterification Curing Mechanism

Application Ser. No. 288,238, filed even date herewith discloses coating compositions which are heat-curable to give solvent-resistant coatings comprising a polymeric polyol, a polyester crosslinking agent having at least two beta-alkoxyester groups per molecule and a transesterification catalyst.

Application Ser. No. 288,240, filed even date herewith discloses coating compositions which are heat-curable to give solvent-resistant coatings comprising a polymeric polyol, a crosslinking agent having at least two beta- and/or gamma-ester-ester groups per molecule and a transesterification catalyst.

Application Ser. No. 288,241, filed even date herewith discloses coating compositions which are heat-curable to give solvent-resistant coatings comprising a polymeric polyol, a crosslinking agent having at least two beta-amido ester groups per molecule and a transesterification catalyst.

1. Field of the Invention

The present invention relates to heat-curable resinous coating compositions and to the use of these coating compositions in cationic electrodeposition. More particularly, the present invention relates to resinous coating compositions which cure through a transesterification reaction.

2. Brief Description of the Prior Art

U.S. Pat. No. 3,937,679 discloses cationic heat-curable resinous coating compositions such as hydroxyl group-containing polymers in combination with aminoplast resin curing agents. These compositions can be used in an electrodeposition process where they coat out on the cathode, and when cured, produce coatings with excellent properties. Coating compositions using aminoplast cure best in an acidic environment. However, the deposit on the cathode is basic and high curing temperatures must be used to overcome the unfavorable curing environment.

U.S. Pat. No. 4,101,486 is similar to U.S. Pat. No. 3,937,679 in that it discloses cationic electrodeposition of hydroxyl group-containing polymers, however, the curing agent is a blocked isocyanate. Coating compositions using blocked isocyanates cure very well at relatively low temperatures in a basic environment and are today widely used in industrial cationic electrodeposition. Examples of cationic electrodepositable compositions which are used industrially are those described in U.S. Pat. Nos. 4,031,050 and 4,190,564 and DE-OS No. 2,752,255. Although used extensively throughout the electrodeposition industry, blocked isocyanate-containing compositions are undesirable from the point of view of the isocyanate, some of which are undesirable to handle.

European patent application No. 0012463 discloses thermosetting resinous coating compositions which cure through a transesterification reaction. The resinous binder of the coating composition comprises a hydroxyl-containing polymer and a crosslinking agent which is a polyester containing two or more beta-hydroxyester groups per molecule. The coating composition can be made cationic and used for electrodeposition.

It is known in the art that esters containing beta-hydroxyalkyl groups transesterify very quickly. See, for example, J. Prakt. Chem. 312 (1970), 660-668. However, European patent application No. 0012463 discloses that polyesters which do not contain beta-hydroxyester groups but rather simple ester groups such as methyl esters or butyl esters do not transesterify as readily and are too sluggish to effect sufficient crosslinking at acceptable conditions.

Surprisingly, it has been found that coating compositions comprising hydroxyl group-containing polymers and a polyester crosslinking agent which do not contain beta-hydroxyester groups can be cured quite effectively.

In accordance with this invention, a method of coating a conductive substrate serving as a cathode which comprises passing electric current between an anode and a cathode in electrical contact with an aqueous resinous dispersion is provided. The resin which is dispersed in the aqueous medium comprises:

(A) a polymeric polyol which contains cationic groups,

(B) a polyester crosslinking agent having at least two gamma- and/or delta-hydroxyester groups per molecule, and

(C) a transesterification catalyst.

The polymeric polyol component of the coating compositions can be selected from a wide variety of hydroxyl group-containing polymers such as alkyd resins, polyester resins, hydroxyl group-containing acrylic polymers, hydroxyl group-containing epoxy resins and hydroxyl group-containing resins which are derived from epoxy resins such as polyepoxide-amine adducts.

The molecular weights of the polymeric polyols can vary over a wide range depending upon their type and on whether the coating composition is organic solvent based or aqueous based and also on the desired performance characteristics of the coating. Polyester, epoxy and alkyd resins can have molecular weights as low as about 500 and as high as about 10,000, preferably the molecular weights are usually in the range of about 1,000 to 5,000; the molecular weights being on a weight average basis relative to polystyrene, as determined by gel permeation chromatography. Acrylic polymers, on the other hand, can have molecular weights as high as about 100,000, and usually will be in the range of about 5,000 to 50,000 on a weight average basis relative to polystyrene, as determined by gel permeation chromatography.

The hydroxyl content of the polymeric polyol should be sufficient such that when the polyol is in combination with the curing agent, the composition will cure to a solvent-resistant coating. Generally, the hydroxyl number of the polymeric polyol will be at least about 170 and preferably will be in the range of about 180 to 300, based on resin solids.

A preferred class of polymeric polyols are hydroxyl group-containing epoxy resins or resins which are derived from epoxy resins such as polyepoxide-amine adducts which are particularly preferred. The epoxy resins which can be used in the practice of the invention are polyepoxides, that is, polymers having a 1,2-epoxy equivalency greater than 1, preferably about 2 or more. Preferred are polyepoxides which are difunctional with regard to epoxy. The preferred polyepoxides are polyglycidyl ethers of cyclic polyols. Particularly preferred are polyglycidyl ethers of polyphenols such as bisphenol A. Examples of polyepoxides are given in U.S. Pat. No. 4,260,716, column 3, line 20, to column 4, line 30, the portions of which are hereby incorporated by reference.

Besides the epoxy resins disclosed above, other epoxy-containing polymers which can be used are acrylic polymers which contain epoxy groups. These polymers are formed by polymerizing an unsaturated epoxy group-containing monomer such as glycidyl acrylate or methacrylate with one or more other polymerizable ethylenically unsaturated monomers. Examples of these polymers are described in U.S. Pat. No. 4,001,156, column 3, line 59, to column 5, line 60, the portions of which are hereby incorporated by reference.

Besides the hydroxyl group-containing epoxy resins disclosed above, hydroxyl group-containing polymers derived from epoxy resins such as polyepoxide-amine adducts can also be used. Examples of amines are ammonia, primary, secondary and tertiary amines and mixtures thereof. The reaction product of the polyepoxide and the amine can be at least partially neutralized with an acid to form a polymeric product containing amine salt and/or quaternary ammonium salt groups. Reaction conditions of polyepoxides with amines, examples of various amines and at least partial neutralization with acid are disclosed in U.S. Pat. No. 4,260,720, column 5, line 20, to column 7, line 4, the portions of which are hereby incorporated by reference.

Also, various polyepoxide-amine adducts are described in European patent application No. 0012463.

With regard to the amount of organic amine and polyepoxide which are reacted with one another, the relative amounts depend upon the extent of cationic salt group formation desired and this in turn will depend upon the molecular weight of the polymer. The extent of cationic salt group formation and the molecular weight of the reaction product should be selected such that when the cationic polymer is mixed with aqueous medium, a stable dispersion will form. A stable dispersion is one which does not settle or is one which is easily dispersible if some sedimentation occurs. In addition, the dispersion should be of sufficient cationic character that the dispersed resin particles will migrate towards the cathode when an electrical potential is impressed between an anode and a cathode immersed in aqueous dispersion.

Also, the molecular weight, structure and extent of cationic salt group formation should be controlled such that the dispersed resin will have the required flow to form a film on the substrate; in the case of electrodeposition, to form a film on the cathode. The film should be insensitive to moisture to the extent that it will not redissolve in the electrodeposition bath or be rinsed away from the coated surface after removal from the bath.

In general, most of the cationic polymers useful in the practice of the invention will have average molecular weights within the range of about 500-100,000 and contain from about 0.01 to 10, preferably about 0.1 to 5.0, preferably from about 0.3 to 3.0 milliequivalents of cationic group per gram of resin solids. Obviously one must use the skill in the art to couple the molecular weight with the cationic group content to arrive at a satisfactory polymer. The preferred polyglycidyl ethers will have molecular weights of about 500 to 10,000, more preferably 1000 to 5,000. Acrylic polymers, on the other hand, will have molecular weights as high as 100,000, preferably 5,000 to 50,000.

Besides epoxy resins and resins derived from epoxy resins, other hydroxyl group-containing polymers such as alkyd resins, polyester resins and hydroxyl group-containing acrylic polymers can also be used in the practice of the invention. Examples of these polymers and their cationic electrodepositable derivatives are shown, for example, in British Pat. No. 1,303,480 (hydroxyl group-containing acrylic polymers and polyesters) and British Pat. No. 1,159,390 (hydroxyl group-containing acrylic polymers).

Besides the cationic polymers which are designed to form aqueous-based coating compositions which may be used in coating applications such as electrodeposition, it should also be appreciated that organic solvent-based coatings employing the above polymers without cationic salt groups can also be used. Formulating coating compositions with such polymers is well known in the art and need not be described in any further detail.

The crosslinking agent of the coating composition is a polyester containing at least two gamma and/or delta-hydroxyester groups per molecule. Preferably, the polyester is substantially free of polyesters containing more than one beta-hydroxyester group per molecule. By substantially free is meant the beta-hydroxyester groups are present in amounts less than that sufficient to achieve a cured coating by themselves, i.e., a coating which can withstand 30 acetone double rubs as described infra. In general, the beta-hydroxyester groups will be present in amounts less than 5, preferably less than 2 percent by weight calculated as weight of beta-hydroxyester group per total weight of crosslinker. Usually, the crosslinkers of the invention are completely free of beta-hydroxyester groups. Examples of suitable crosslinking agents are those which are formed from reacting a polycarboxylic acid or its functional equivalent thereof with one or more 1,3- and/or 1,4-polyols.

Examples of suitable polycarboxylic acids include dicarboxylic acids such as saturated aliphatic dicarboxylic acids, for example, adipic acid and azelaic acid; aromatic acids such as isophthalic acid and terephthalic acid; ethylenically unsaturated dicarboxylic acids such as fumaric acid and itaconic acid.

Besides the acids themselves, functional equivalents of the acids such as anhydrides where they exist and lower alkyl (C.sub.1 -C.sub.4) esters of the acids can be used. Examples include succinic anhydride, phthalic anhydride and maleic anhydride.

Polycarboxylic acids or their functional equivalents having a functionality greater then two can also be used. Examples include trimellitic anhydride and polycarboxylic acids formed from reacting a dicarboxylic acid with a stoichiometric deficiency of a polyol having a functionality of 3 or more. For example, the reaction product of adipic acid with trimethylolpropane in a 3:1 molar ratio can be used. The resulting reaction product will have an acid functionality of 3.

Examples of suitable 1,3-polyols and 1,4-polyols are those of the structure: ##STR1## where R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7 and R.sub.8 can be the same or different and include hydrogen, and the radicals alkyl, cycloalkyl, aryl, alkaryl, aralkyl containing from 1 to 18 carbon atoms, including substituted radicals in which the radicals and the substituents will not adversely affect the esterification reaction with the polycarboxylic acid or its functional equivalent thereof nor adversely affect the transesterification curing reaction or the desirable properties of the coating composition. Examples of suitable substituents include chloro, hydroxy, alkoxy, carboxy and vinyl. Examples of suitable radicals include methyl, ethyl, propyl, phenyl and hydroxymethyl.

Specific examples of 1,3-polyols include 1,3-propanediol, trimethylolpropane, trimethylolethane and 1,3-butanediol.

Specific examples of 1,4-polyols include 1,4-butanediol and 2-butene-1,4-diol.

The crosslinking agent can be formed from reacting the polycarboxylic acid or its functional equivalent thereof with the 1,3- and/or 1,4-polyol at an elevated temperature, usually reflux temperature, in the presence of an esterification catalyst such as an acid or a tin compound. Usually a solvent, for example, an azeotropic solvent such as toluene or xylene, is used. Reaction is continued with water being constantly removed until a low acid value, for example, 3 or less, is obtained.

The third component in the coating compositions of the invention is a transesterification catalyst. These catalysts are known in the art and include salts or complexes of metals such as lead, zinc, iron, tin and manganese. Suitable salts and complexes include 2-ethylhexonates (octoates), naphthanates and acetyl acetonates.

The relative amounts of the polymeric polyol and the crosslinking agent which are present in the coating composition can vary between fairly wide limits depending upon the reactivity of the components and the time and temperature of curing and the properties desired in the cured coating. In general, the polymeric polyol will be present in amounts of about 20 to 95 percent, preferably about 50 to 85 percent by weight, and the cross-linking agent in amounts of about 5 to 80, preferably 15 to 50 percent by weight; the percentages by weight being based on total weight of polymeric polyol and crosslinking agent, and being determined on a solids basis.

The catalyst is present in amounts of about 0.1 to 2.0, preferably about 0.2 to 1.0 percent by weight metal based on total weight (solids) of the polymeric polyol and the crosslinking agent.

The polymeric polyol and the crosslinking agent described above can be formulated into the coating composition as two separate components such as would be the case using a crosslinking agent such as di(1,3-propanediol)adipate or di(1,4-butanediol)adipate. Alternately, the polymeric polyol and the crosslinking agent can be formulated into a coating composition as a one-component resin such as by forming a pre-condensate of the two. An example would be using an epoxy resin as the polymeric polyol and reacting a portion of the epoxy groups with a portion of the hydroxyl groups of the crosslinker. The epoxy and hydroxy functionality should be carefully controlled to avoid gelation and yet have sufficient hydroxyester groups available for curing. Therefore, in the specification and claims in which the coating composition is set forth as comprising (A) a polymeric polyol and (B) a polyester crosslinking agent having at least two gamma and/or delta-hydroxyester groups per molecule, two-component systems in which (A) and (B) are present as separate ingredients as well as one-component systems in which (A) and (B) have been prereacted to form an essentially one-component resin are intended.

The components of the coating composition can be mixed simultaneously or in any order that is convenient. If the components are a liquid and of sufficiently low viscosity, they can be mixed together neat to form the coating composition. Alternately, if the components are higher viscosity liquids or solids, the components can be mixed with a diluent to reduce the viscosity of the composition so that it may be suitable for coating applications.

By liquid diluent is meant a solvent or a non-solvent which is volatile and which is removed after the coating is applied and is needed to reduce viscosity sufficiently to enable forces available in simple coating techniques, that is, brushing and spraying, to spread the coating to controllable, desired, and uniform thickness. Also, diluents assist in substrate wetting, resinous component compatibility and coalescence or film formation. Generally, when used, the diluent will be present in the composition in amounts of about 20 to 90, preferably 50 to 80 percent by weight based on total weight of the coating composition, although more diluent may be employed depending upon the particular coating application.

Examples of suitable liquid diluents for organic solvent-based coatings will depend somewhat on the particular system employed. In general, however, aromatic hydrocarbons such as toluene and xylene, ketones such as methyl ethyl ketone and methyl isobutyl ketone, alcohols such as isopropyl alcohol, normal butyl alcohol, monoalkyl ethers of glycols such as 2-alkoxyethanol, 2-alkoxypropanol and compatible mixtures of these solvents can be used.

Besides organic solvents, water can be used as a diluent either alone or in combination with water-miscible organic solvents. When water is used, the coating composition is usually modified such as by incorporating water-solubilizing groups such as the cationic groups mentioned above to provide for the necessary solubility in water. Besides the cationic groups mentioned above, other water-solubilizing groups such as non-ionic groups, for example, ethylene oxide groups, and anionic groups such as carboxylate salt groups may be introduced into the polymeric polyol or the polyester crosslinking agent to disperse or solubilize the coating composition in water.

The coating compositions of the invention may also optionally contain a pigment. Pigments may be of any conventional type, comprising, for example, iron oxides, lead oxides, strontium chromate, carbon black, coal dust, titanium dioxide, talc, barium sulfate, as well as color pigments such as cadmium yellow, cadmium red, chromium yellow and metallic pigments such as aluminum flake.

The pigment content of the coating composition is usually expressed as the pigment-to-resin weight ratio. In the practice of the present invention, pigment-to-resin weight ratios can be as high as 2:1, and for most pigmented coatings, are usually within the range of about 0.05 to 1:1.

In addition to the above ingredients, various fillers, plasticizers, anti-oxidants, ultraviolet light absorbers, flow control agents, surfactants and other formulating additives can be employed if desired. These materials are optional and generally constitute up to 30 percent by weight of the coating composition based on total solids.

The coating compositions of the invention can be applied by conventional methods including brushing, dipping, flow coating, spraying, and, for aqueous-based compositions containing ionic salt groups, by electrodeposition. Usually, they can be applied virtually over any substrate including wood, metal, glass, cloth, leather, plastic, foam and the like, as well as over various primers. For electroconductive substrates such as metals, the coatings can be applied by electrodeposition. In general, the coating thickness will vary somewhat depending upon the application desired. In general, coatings from about 0.1 to 10 mils can be applied and coatings from about 0.1 to 5 mils are usual.

When aqueous dispersions of the coating composition are employed for use in electrodeposition, the aqueous dispersion is placed in contact with an electrically conductive anode and an electrically conductive cathode. In the case of cationic electrodeposition, the surface to be coated is the cathode. Following contact with the aqueous dispersion, an adherent film of the coating composition is deposited on the electrode being coated when a sufficient voltage is impressed between the electrodes. Conditions under which electrodeposition is carried out are known in the art. The applied voltage may be varied and can be, for example, as low as 1 volt or as high as several thousand volts, but is typically between 50 and 500 volts. Current density is usually between 1.0 ampere and 15 amperes per square foot and tends to decrease during electrodeposition indicating the formation of an insulating film.

After the coating has been applied, it is cured by heating at elevated temperatures such as at about 150.degree. to 205.degree. C. for about 10 to 45 minutes to form solvent-resistant coatings. By solvent-resistant coatings is meant that the coating will be resistant to acetone, for example, by rubbing across the coating with an acetone-saturated cloth. Coatings which are not cured or poorly cured will not withstand the rubbing action with acetone and will be removed with less than 10 acetone double rubs. Cured coatings, on the other hand, will withstand a minimum of 30 acetone double rubs, and preferably 100 acetone double rubs.

Illustrating the invention are the following examples which, however, are not to be construed as limiting the invention to their details. All parts and percentages in the examples, as well as throughout the specification, are by weight unless otherwise indicated.

The following example shows the preparation of a coating composition containing a crosslinking agent having two delta hydroxyester groups per molecule. The crosslinking agent is formed by reacting adipic acid with 1,4-butanediol in a 1:2 molar ratio. The crosslinking agent was mixed with a polymeric polyol formed from condensing an epoxy resin (polyglycidyl ether of a polyphenol) with an amine. The mixture was dissolved in organic solvent, lead octoate was added, and the solution drawn down on steel panels and the coated panels heated to give solvent-resistant coatings. The details of the Example are shown below:

The crosslinking agent was prepared from the following mixture of ingredients:

The ingredients were charged to a reaction vessel under a nitrogen blanket. The reaction mixture was heated to reflux until an acid value of 1.3 was obtained. During the reaction, 48.1 grams of aqueous layer was collected in a Dean-Stark trap.

The polymeric polyol was formed from reacting a polyglycidyl ether of bisphenol A with diethanolamine in about a 3:1 equivalent ratio. The adduct was then chain extended with a mixture of a primary and a disecondary amine namely, 3-dimethylaminopropylamine, and the adduct of 1,6-hexamethylene diamine and the glycidyl ester of Versatic acid (CARDURA E).

The ingredients were mixed together and heated in a 2-ounce glass jar to obtain a clear, yellow resin. The coating composition was drawn down on an untreated steel panel and on a zinc phosphate pretreated steel panel. The coated panels were cured for about 30 minutes at 180.degree. C. The cured coatings exhibited a medium to high gloss and had a thickness of from about 1.6 to 2.2 mils. The coating on the untreated steel was removed after 125 acetone double rubs. The coated zinc phosphate pretreated steel panel withstood 132 acetone double rubs.

The following example shows the preparation of a coating composition containing a crosslinking agent which is a gamma-hydroxy polyester formed by reacting adipic acid with trimethylolpropane in a 1:2 molar ratio. The crosslinking agent was mixed with a polymeric polyol formed from condensing a polyglycidyl ether of a polyphenol with an amine. The mixture was combined with lead octoate catalyst to form a coating composition. Steel panels were coated with the composition and the coated substrates heated to give solvent-resistant coatings. Also, the mixture was dispersed in water with the aid of acid and steel panels were cathodically electrocoated with the dispersion. The details of the Example are shown below:

The crosslinking agent was prepared from the following mixture of ingredients:

The ingredients were charged to a reaction vessel under a nitrogen blanket and heated to reflux. The reaction mixture was held at reflux temperature until an acid value of about 1.7 was obtained.

The polymeric polyol was formed from chain extending a polyglycidyl ether of bisphenol A with a polyester diol. The adduct was then reacted with a mixture of amines, namely, methyllethanolamine and the methylisobutyl diketimine of ethylene triamine.

The EPON 828, PCP 0200 and xylene were charged to a reaction vessel and heated under a nitrogen blanket to reflux and held for 30 minutes. The reaction mixture was cooled to 155.degree. C. followed by addition of the bisphenol A. Benzyldimethylamine (1.9 grams) was added and the reaction mixture exothermed. The reaction mixture was cooled to 130.degree. C. followed by the addition of the remaining benzyldimethylamine and the reaction mixture held at 130.degree. C. until it attained a reduced Gardner-Holdt viscosity (50 percent resin solids in 2-ethoxyethanol) of N.sup.+. The 2-ethoxyethanol, diketimine and methylethanolamine were then added and the reaction mixture held at 108.degree.-112.degree. C. for about one hour to complete the reaction.

To 39.55 grams (31.64 grams solids) of the polymeric polyol prepared as described immediately above was added 17.07 grams (15.60 grams solids) of the crosslinking agent prepared as described above. To a 31.78 gram sample (26.5 grams solids) of the mixture was added 0.50 grams of the lead octoate solution as described in Example I. The composition was drawn down with a drawbar over untreated and zinc phosphate pretreated steel panels and the panels heated to 360.degree. F. (182.degree. C.) for 30 minutes. The cured films were glossy with a textured surface. The coating over the untreated steel was removed after 150 acetone double rubs, whereas the coating over the zinc phosphate pretreated steel withstood 107 acetone double rubs.

When comparable coatings were made without the use of the lead catalyst, the cured coatings were removed by about 10 to 23 acetone double rubs.

An aqueous dispersion was prepared by mixing together the following ingredients:

The polymeric polyol and the crosslinking agent were blended together with warming in a steel beaker. The lead octoate was blended in, followed by the addition of the lactic acid. The reaction mixture was then thinned with the deionized water to form a 14.6 percent by weight resin solids dispersion.

Untreated steel and zinc phosphate pretreated steel panels were cathodically electrocoated in the dispersion at 200 volts for 90 seconds. The coated substrates were then heated to 400.degree. F. (204.degree. C.) for 30 minutes to form cured coatings. The coating on the untreated steel withstood 150 acetone double rubs, whereas the coating on the zinc phosphate pretreated steel withstood 140 acetone double rubs.

The following example was similar to Example II with the exception that the crosslinking agent was introduced into the resin cook.

The polymeric polyol was prepared as generally described above in Example II with the exception that the crosslinking agent was cooked in. The charge for preparing the polymeric polyol was as follows:

The EPON 828, PCP 0200 and xylene were charged to a reaction vessel under a nitrogen blanket and heated to reflux and held for 30 minutes. The reaction mixture was cooled to 155.degree. C. and the bisphenol A added. The temperature dropped to 128.degree. C. and the reaction mixture was held for about 20 minutes at this temperature. Benzyldimethylamine (1.5 grams) was added and the reaction mixture exothermed with the temperature reaching 170.degree. C. The reaction mixture was cooled to 130.degree. C., followed by the addition of 3.3 grams of benzyldimethylamine. The reaction mixture was held at a temperature of about 130.degree. C. until a Gardner-Holdt viscosity (50 percent resin solids in 2-ethoxyethanol) of P was obtained. The crosslinking agent dissolved in the 2-butoxyethanol was then added, the reaction mixture temperature dropping to 85.degree. C. The reaction mixture and methylethanolamine heated to 110.degree. C. and held for about one hour to complete the reaction. The reaction mixture had a solids content of 82.7 percent.

The resinous reaction product prepared as described immediately above was dispersed in aqueous medium as follows:

The polymeric polyol and the surfactant were mixed together in a stainless steel beaker, followed by the addition with mixing of the lactic acid. The reaction mixture was then thinned with deionized water to form a 34.2 percent solids aqueous dispersion.

A cationic electrodeposition paint was prepared from the following mixture of ingredients:

The deionized water was charged to a mixing vessel followed by the addition of the lead acetate. The polymeric polyol (34.2 percent solids) was then stirred in, followed by the addition of the pigment paste with stirring. The final paint had a total solids content of 20 percent, a pH of 6.25, pigment-to-binder ratio of 0.2:1. After stirring for two days, the pH remained at 6.25 and the specific conductivity of the dispersion was 1250 measured at 77.degree. F. (25.degree. C.). Zinc phosphate pretreated steel panels were cathodically electrocoated in the dispersion at 120 volts for 2 minutes at a bath temperature of 70.degree. F. (21.degree. C.). The coatings were cured at 350.degree. F. (177.degree. C.) for 30 minutes. The coatings were removed after 35 acetone double rubs and were evaluated for corrosion resistance which was determined by scribing the cured coated panel with an "X" and exposing the scribed panel to a salt spray fog in accordance with ASTM D-117 for 14 days. The panels were removed from the chamber, dried and the scribe mark taped with masking tape, the tape pulled off at a 45.degree. angle and the creepage from the scribe mark measured. Creepage is the rusted, darkened area of the panel where the coating has lifted from the panel surface. The scribe creepage was 1/16 inch. When untreated steel panels were cathodically electrocoated in the bath at 80 volts for 2 minutes and the coatings cured and exposed for 14 days to salt spray corrosion as described above, the scribe creepage was 3/16 of an inch.

The following example shows the preparation of a coating composition containing the crosslinking agent of Example I and the polymeric polyol of Example II. The coating composition was formulated with lead catalyst and steel panels were coated with the composition and the coated substrates heated to give solvent-resistant coatings. The specific coating formulation is shown below:

The ingredients were mixed and heated to form a homogeneous composition which was then drawn down on untreated and zinc phosphate pretreated steel panels. The coated panels were cured for 30 minutes at 400.degree. F. (204.degree. C.) to give cured coatings which withstood 150 acetone double rubs.

Coatings formulated without lead catalyst and baked at 400.degree. F. (204.degree. C.) for 30 minutes were removed by only 23 acetone double rubs on untreated steel and 16 acetone double rubs on zinc phosphate pretreated steel.