Aqueous Coating Composition From Aminoplast and Reaction Product of Tertiary Amine and Polymer Containing Terminal Epoxy Groups

This invention is related to a water-borne coating composition of general utility in coating metallic substrates, for example in automotive and coil coatings applications. More particularly, the coating composition is useful as an interior and exterior can coating.

The can coating industry utilizes a variety of thermosetting coating compositions designed to fulfill various requirements dictated by the intended use. Cans are usually coated on their interior by a thin coating designed to protect the metal walls from attack by food or beverage, and vice versa, to be stored therein. Such coatings should have among other properties good adhesion to the metal walls, low extractables and sorption to prevent taste adulteration, and a rapid cure rate for an economical manufacturing process.

The coatings of the prior art are thermosetting compositions often dissolved or dispersed in organic solvents. Among commonly utilized thermosetting compositions are those based on epoxy resins crosslinked with nitrogen resins, usually in an acid catalyzed process.

Increased awareness of the environmental hazards of allowing organic solvent vapors to enter the atmosphere, the desirability of a single system that can be applied not only by the more conventional techniques of spray, roller or flow coating but also by electrodeposition, and the economy resulting from the substitution of water for some or all of the solvents in a coating composition, are all factors mitigating in favor of water-borne coating compositions.

U.S. Pat. No. 2,676,166, issued Apr. 20, 1954 to V. J. Webers, shows the conversion, through the use of tertiary amines, of random low molecular weight hydrophilic copolymers of oxirane containing monomers with other ethylenically unsaturated monomers into water soluble materials. However, these polymers contain a large number of hydroxyl and quaternary ammonium groups per molecule.

In U.S. Pat. No. 3,839,252, issued Oct. 1, 1974 to J. E. Bosso et al., there are described water dispersible products of epoxy resins with amine salts. These products are designed to contain at least one unreacted 1,2-epoxy group per molecule and also contain in the resin quaternary ammonium salt groups.

The composition of this invention is an aqueous solution or dispersion of the reaction product of a terminally functional epoxy resin with a tertiary amine and a nitrogen resin crosslinking agent. These aqueous solutions or dispersions of the polymeric quaternary ammonium hydroxide and a nitrogen resin are stable both uncatalyzed or catalyzed and can be applied to metallic substrates by spray, roller, dip or flow coating or by electrodeposition at the cathode and to paper.

According to the present invention there is provided a water-borne thermosetting coating composition consisting essentially of:

(A) 50-97 parts by weight, based on the weight of components (A) plus (C), of an epoxy resin containing, on the average, two terminal 1,2-epoxy groups per molecule and having a molecular weight of 350-5,000, reacted with

(B) an aqueous solution of 0.75-1.25 equivalents of a tertiary amine per equivalent of 1,2-epoxy groups in the epoxy resin; amd

(C) 3-50 parts by weight, based on the weight of components (A) plus (C), of a nitrogen resin crosslinking agent;

wherein components (A), (B), and (C) comprise about 0.1-50% by weight of the coating composition and the remainder is comprised of water and organic liquid(s) in a ratio of from 70:30 to 90:10 and said coating composition having a pH of 7.5 or above.

The water-borne thermosetting coating composition of this invention is a solution or dispersion of the reaction product of an epoxy resin and a tertiary amine and a nitrogen resin crosslinking agent. By utilizing aqueous solutions of highly specific tertiary amines such as dimethyl ethanol amine and epoxy resins such as the epichlorohydrin-bisphenol "A" - type epoxides, a stable, water soluble or dispersible polymeric quaternary ammonium hydroxide results. Addition of a nitrogen-resin crosslinking agent, acting also as a solubilizing agent, affords a solution or dispersion which is stable at room temperature and is infinitely dilutable with water.

Whether the coating composition is a solution or a dispersion is largely dependent on the nature of the particular nitrogen resin and amine used. Even when the composition is opaque some of the resinous components may be dissolved and when the composition appears to be a clear solution it is possible that small amounts of the components are in a dispersed state. For sake of simplicity, hereafter the term "dispersion" will be used to denote the water-borne coating composition.

The dispersion as prepared has a pH of above 10 and a non-volatile content of up to 50% by weight. It has excellent wetting properties when applied to metal substrates and, upon curing, a glossy, highly solvent resistant coating is obtained. It is also useful as a coating for paper.

The low molecular weight epoxy resins to be utilized in the present invention are commonly known in the art. One class of such resins has the generalized formula ##STR1##

wherein R is an alkylene group of 1-4 carbon atoms and n is an integer from 1-12. The epoxy resins utilized in this invention contain an average of two terminal 1,2-epoxy groups per molecule and are in the molecular weight range of 350-5,000. They can also contain substituted aromatic rings.

One such preferred epoxy resin is "Epon 1004" where R is isopropylidene, the average value of n is 5, and has an epoxy equivalent of 875-1,025. The epoxy equivalent is defined as the grams of resin containing 1 gram-equivalent of epoxide as measured by ASTM-D-1652. The coating composition containing "Epon 1004" affords a glossy, flexible, chemically resistant film. Another preferred epoxy resin is "Epon 1007" where R is isopropylidene, the average value of n is 11, has an epoxy equivalent of 2,000-2,500. The coating composition containing "Epon 1007" affords glossy, tough, flexible films upon cure.

The epoxy resin comprises 50-97 parts by weight of the film-forming components of the coating composition of this invention and preferably 65-85 parts. One preferred composition contains 75 parts of an epoxy resin.

During the preparation of the coating composition of this invention a solution of an epoxy resin in organic liquid(s) is brought in contact with an aqueous solution of a tertiary amine. A wide variety of organic liquids can be used to dissolve the epoxy resins. Among the most commonly used solvents are alcohols such as isopropanol, the butyl alcohols, 2-hydroxy-4-methylpentane, 2-ethylhexyl alcohol, cyclohexanol, glycols such as ethylene glycol, diethylene glycol, 1,3-butylene glycol, ether alcohols such as ethylene glycol mono-ethyl ether, ethylene glycol mono-butyl ether, diethylene glycol mono-methyl ether, mixtures thereof, and many aliphatic and aromatic hydrocarbons if used admixed with at least one of the above.

The reaction of tertiary amines with materials containing epoxy groups, to yield adducts containing quaternary ammonium groups, is known. Such reaction, when carried out in presence of water, can afford a product that contains both a hydroxyl group and a quaternary ammonium hydroxide. The reaction can be represented schematically as follows: ##STR2## While most tertiary amines react with epoxy resins to form quaternary ammonium hydroxides, the preparation of the quaternary ammonium hydroxides to be utilized in the water-borne coating composition of this invention is carried out utilizing at least one tertiary amine selected from the group: R.sub.1 R.sub.2 R.sub.3 N, N-methyl pyrrolidine, N-methyl morpholine, pyridine, N-methyl pyrrole, and mixtures thereof and wherein R.sub.1 and R.sub.2 are substituted or unsubstituted monovalent alkyl groups containing one or two carbon atoms in the alkyl portion and R.sub.3 is a substituted or unsubstituted monovalent alkyl group containing 1-4 carbon atoms. Some examples of R.sub.1 R.sub.2 R.sub.3 N are: trimethyl amine, dimethyl ethanol amine, methyl diethanol amine, ethyl methyl ethanol amine, dimethyl ethyl amine, dimethyl propyl amine, dimethyl 3-hydroxy-1-propyl amine, dimethylbenzyl amine, dimethyl 2-hydroxy-1-propyl amine, diethyl methyl amine, dimethyl 1-hydroxy-2-propyl amine, and mixtures thereof. Most preferably trimethyl amine or dimethyl ethanol amine is used.

While the exact mode of the reaction is not fully understood, it is believed that the efficiency of the amines utilized is related to steric factors. The amount of tertiary amine used is determined by the epoxy equivalent of the particular epoxy resin employed and is at least 0.75 equivalent, and preferably between 0.9 and 1.1 equivalents, per epoxy group. At lower levels of amine use the residual epoxy groups of the epoxy resin are subject to further reaction in the highly basic reaction medium. Such further reaction can lead to a crosslinked system prior to the addition of the nitrogen resin crosslinking agent or it can lead to storage instability and premature gelation of the coating composition. In a preferred embodiment equivalent amounts of epoxy resin and tertiary amine are used resulting in substantially complete consumption of the epoxy groups. Such a reaction, when the starting epoxy resin contains an average of two epoxy groups, affords a polymeric quaternary ammonium hydroxide having two such groups and two hydroxyl groups.

The tertiary amine and water are admixed with a solution of the epoxy resin. The exact ratio of the amine and water is not critical but one preferred ratio of amine to water is approximately 1:2 by weight. At the end of the reaction period between the epoxy resin and the tertiary amine the product is a solution at the reaction temperature. The reaction can be carried out between room temperature and below the boiling point of the reaction medium, preferably between 50.degree.-95.degree. C, and most preferably between 70.degree.-80.degree. C. In this temperature range there is a rapid rate of reaction without complicating side reactions.

Further dilution with water of the solution of the epoxy resin-tertiary amine adduct, at the reaction temperature, can be carried out. In one embodiment a quaternary ammonium hydroxide remains in solution at about 75.degree. C in a liquid medium comprising an approximately 75:25 weight ratio of organic liquid and water. Such a solution, however, has a tendency to separate into two phases upon cooling to room temperature. In another embodiment, using trimethyl amine, the solution remains in a single phase even at room temperature.

The water-borne thermosetting coating composition of this invention is a stable solution or dispersion, even at room temperature, and comprises a nitrogen resin crosslinking agent together with a polymeric quaternary ammonium hydroxide.

The nitrogenresins, often referred to as alkylated amine-aldehyde resins and alkylated amide-aldehyde resins, are well known. They are the alkylated products of amino-resins prepared by the condensations of at least one aldehyde with at least one of urea, N,N'-ethyleneurea, dicyandiamide, and aminotriazines such as melamines and guanamines. Among the aldehydes that are suitable are formaldehyde, revertible polymers thereof such as paraformaldehyde, acetaldehyde, crotonaldehyde, and acrolein. Preferred are formaldehyde and revertible polymers thereof. The amino-resins are alkylated with at least one and up to and including six alkanol molecules containing 1-6 carbon atoms. The alkanols can be straight chain, branched or cyclic.

Among the preferred nitrogen resins are partially methylated melamines, partially butylated melamines, hexaethoxymethylmelamine, hexamethoxymethylmelamine, dimethoxytetraethoxymethylmelamine, dibutoxytetramethoxymethylmelamine, butylated benzoguanamine, partially methylated urea, fully methylated urea, fully butylated urea, hexabutoxymethylmelamine, and mixtures thereof. In one preferred embodiment, utilizing a partially methylated melamine, the composition of this invention has excellent wetting properties on metals such as aluminum or tin-free-steel. The cured coatings based on this composition are glossy, show little or no cratering, and are resistant to solvent.

The nitrogen resin crosslinking agents help to solubilize the polymeric quaternary ammonium hydroxide adducts and help to stabilize the resulting water-borne coating composition. These nitrogen resins comprise 3-50 parts by weight of the film-forming components of the coating composition of this invention and preferably 15-35 parts. One preferred embodiment contains the product of 75 parts of epoxy resin with an equivalent amount of a tertiary amine and 25 parts of a nitrogen resin.

The water-borne composition of this invention is stable and is a dispersion or, depending on the particular nitrogen resin utilized, a substantially clear solution. When hexabutoxymethylmelamine is used the coating composition is a stable dispersion capable of being applied by any of the conventional techniques such as spray or roller coating or by elctrodeposition. When a partially methylated melamine or hexamethoxymethylmelamine is used the coating composition is a clear solution even at room temperature and is also capable of being applied by a variety of techniques.

The coating compositions of this invention can be cured thermally. After the composition has been applied to the substrate, baking at elevated temperatures brings about the desired crosslinking. Temperatures of 150.degree. to 260.degree. C, for 0.1 to 30 minutes, are typical baking schedules utilized.

Optionally, non-volatile acid catalysts are included in the coating composition to increase the rate of cure. By non-volatile acid catalyst is meant an acid catalyst that remains in the cured film. The acid catalyst can be particularly important when a fully alkylated nitrogen resin crosslinking agent such as hexamethoxymethylmelamine is utilized. Among useful acid catalysts are phosphoric acid, its mono- and di- esters formed with aliphatic, cycloaliphatic, and aromatic alcohols, citric acid, and para-toluenesulfonic acid. Mixtures of these can also be used. The final pH of the coating composition is determined by the concentration of the acid used and is determined on the basis of the rate of cure desired and the type of nitrogen resin crosslinking agent used.

The pH of the thermosetting coating composition is above 10 as prepared. The composition can be cured at this pH if desired. A particular advantage resulting from this ability to cure films, resulting from a liquid coating composition of high pH values, is the improved wetting of the water-borne system on metal substrates under many practical application conditions such as when unclean, oily surfaces are coated.

When acid catalysts are added to the coating composition the final pH of the system is lowered in accordance with the needs of the components of the system, the lower pH being necessary for the more fully alkylated nitrogen resins. When a completely alkylated crosslinking agent is used the final pH of the coating composition is adjusted to between 7.5 and 12, preferably to between 9 and 10. Utilizing a coating composition of this invention there is no need to lower the pH of the system to the acid side, unless very rapid cure is required. Corrosion, often common to acidic coating compositions, can be thereby avoided.

The water-borne composition can be applied by a variety of techniques and to a variety of substrates known in industry. For example, the coating composition of this invention can be utilized in the can manufacturing industry which utilizes mainly metallic cans, many of them cylindrical, made from aluminum, tin-free steel, electrolytic tin-plate, and quality-as-rolled steel, among others. Cans utilized for packaging and shipping food and beer or other beverages are mostly of the three-piece or the two-piece drawn-and-ironed (D and I) variety. Cans constructed from three pieces (body, top, and bottom) can be roller coated before the metallic sheet is formed into the body of the can or can be spray coated after partial fabrication. The D and I cans, where the metal sheet is stamped to form a cylindrical body closed at one end, are generally spray coated.

The thermosetting coating composition of this invention can also be electrocoated. In the electrodeposition process the water-borne composition is placed in contact with an electrically conductive cathode and an electrically conductive anode, with the surface to be coated being the cathode. During the process an adherent film of the coating composition is deposited on the cathode. The nitrogen resin crosslinking agent also migrates, in a possible physical entanglement with the polymeric quaternary ammonium hydroxide, to the cathode.

The conditions under which the electrocoating is carried out are similar to those used in the electrodeposition of other types of coatings. The applied voltage can be varied, can range from 1 to 1000 volts, and is typically between 25 and 500 volts. The current density is usually between about 1 milliampere and 100 milliamperes per square centimeter. The current density tends to decrease during the coating process as the coating thickness increases. The coating time can vary from 1 to 120 seconds or longer and is typically between 1 and 5 seconds for coating cans.

The concentration of the coating composition depends upon the process parameters to be used and is not generally critical. Ordinarily the film-forming components comprise 0.1-50 and preferably 5-30% for conventional coating methods and 1-20% for electrodeposition of the total, the remainder being water and organic liquid(s). The latter are present in a ratio of from 90:10 to 70:30.

The freshly deposited films are capable of being immediately crosslinked without regard to the method of coating used to obtain them.

The water-borne thermosetting coating composition of this invention is useful in a variety of applications. This coating composition finds particular utility in the can industry where the composition can be applied to the interior of two-piece drawn-and-ironed and three-piece beer and beverage cans, to the exterior of three-piece beer and beverage cans, to the interior and/or exterior ends of two- or three-piece cans or two- or three-piece sanitary cans. When the coating composition of this invention is applied to the interior of food and beer or beverage cans by spray-coating, a thin uniform film is deposited which, after curing, corresponds to a coating weight of 0.3 to 1.3 milligrams per square centimeter (2-8 milligrams per square inch).

The composition can also be used as a paper coating. A glossy coating results when the pH of the coating composition is adjusted to approximately six and the coated substrate is baked for 5-10 seconds at 150.degree. C.

The water-borne composition also has utility in automotive, appliance and coil coating applications, the final coated articles having especially desirable corrosion, saltspray, acid, base, stain, and detergent resistance properties.

The coating composition can also be used as a primer for metallic substrates. A wide variety of topcoats can be used after the primer has been partially or completely cured. Among the useful topcoats are polyvinyl chloride, vinyl chloride copolymerized with one or more of the following: fumaric acid and/or its methyl, ethyl, butyl or octyl ester, a hydroxy-modified vinyl chloride/vinyl acetate copolymer post-reacted with trimellitic anhydride, and acrylic polymers.

After application and curing the coating composition of this invention can be tested, among other properties, for flexibility, adhesion, for pasteurization and processing resistance, for resistance to boiling water and solvents, and for its effect on taste.

The following are convenient procedures for carrying out the various tests. The tests can be carried out on paper, test panels or finished cans, coated and cured in a 205.degree.-216.degree. C/1-10 minute or 150.degree. C/5-10 second cure cycle.

Pasteurization Resistance: A sample of a coated panel is immersed in water at 66.degree. C for 30 minutes. For a coating to be acceptable there must be no blistering, softening, "blush" or loss of adhesion.

Boiling Water Resistance: A coated panel is immersed in boiling water for 30 minutes. Again, no blistering, softening or loss of adhesion is permitted and no "blush" is allowed.

Flexibility: A coated panel is subjected to 180 degree bending. No cracking of the film or loss of adhesion should take place for the coating to be acceptable. No cracking or loss of adhesion is permissible during the fabrication of can ends.

Solvent Resistance: A sample coated panel is mechanically rubbed 100 times with cloth soaked in methyl ethyl ketone. A passing sample will show no dissolution of, delamination of, or penetration through, the film by the solvent after 100 strokes.

Taste Test: A simulated coated panel (aluminum foil), representing the interior surface area of a can, is inserted into a twelve-ounce soda bottle filled with spring water and the bottle is capped. The contents of the bottle are pasteurized at 66.degree. C for thirty minutes and stored in a 40.degree. C oven for five days. The bottle is cooled to room temperature and the contents are tested by a taste panel comparing to water stored without a coated panel. An acceptable coating will give no taste to the water.

Extractables: This test is carried out according to guideline extraction methods as specified in the 1966 Supplement to the Code of Federal Regulations, issued by the Food and Drug Administration (see Title 21, Section 121.2514).

Processing Resistance: A coated panel or a coated fabricated end is placed in a partially water-filld pressure cooker for ninety minutes at approximately 120.degree. C. No blistering, softening or loss of adhesion is permitted after this period. No "blush" is allowed.

The invention can be further understood by referring to the following examples in which parts and percentages are by weight, unless otherwise indicated.

Into a 500-milliliter, three-necked, round-bottomed flask, equipped with a thermometer, stirrer, and condensor, are placed "Epon 1004" (available from Shell Chemical Company), 82.5 grams; 2-butoxy-1-ethanol, 28.9 grams; and diethylene glycol mono-butyl ether, 28.9 grams. The contents are heated to 70.degree. C to obtain a clear solution. To this solution is added a mixture of dimethyl ethanol amine, 8.5 grams and water, 17.0 grams. The reaction temperature is maintained between 70 and 75.degree. C for fifteen minutes, after which time a partially methylated melamine, 80% solids in iso-propanol, (available from the Monsanto Polymers and Petrochemicals Company under the trade name of Resimene 731) 25.8 grams, is added. To the contents now between 60.degree. and 65.degree. C is added 200.5 grams of water over a thirty-minute period. A clear, viscous solution results having a Gardner-Holdt viscosity of Z6 and a pH of 12.2. The product has a solids content, determined at 204.degree. C for 10 minutes, of 27.2%. This solution is roller coated onto an aluminum panel and is cured at approximately 204.degree. C for ten minutes. The resultant coating is glossy, tough, flexible, and passes the above-described tests. The coating weight is 0.7 milligram per square centimeter.

Into a 100-gallon reactor are charged 2-buxtoxy-1ethanol, 63.1 pounds, and diethylene glycol mono-butyl ether, 63.1 pounds. The reactor is heated to 70.degree. C and "Epon 1004", 187.1 pounds, is added slowly with agitation. The contents are held between 70.degree. and 75.degree. C until all of the epoxy resin is dissolved. A mixture of dimethyl ethanol amine, 19.4 pounds, and water, 38.8 pounds, is then added over a fifteen-minute period. After an additional fifteen-minute period of agitation, hexamethoxymethylmelamine, 33.1 pounds, is added and the reaction temperature is now between 60.degree. and 70.degree. C. Maintaining the batch temperature between 55.degree. and 60.degree. C, water, 398 pounds, is added over a one-hour period, followed by a solution of phosphoric acid, 8.3 pounds, in a 3:1 by weight mixture of water and diethylene glycol mono-butyl ether, 47 pounds. This step is followed by the addition of a partially methylated melamine ("Resimene 731"), 17.2 pounds. The contents of the vessel are allowed to cool to room temperature. The final clear solution has a Gardner-Holdt viscosity of Z4, pH of 9.5, and a solids content, determined at 204.degree. C for ten mintues, of 30.4%. The solution is roller coated onto a tin-free steel panel and is cured at approximately 204.degree. C for a tin-free steel panel and is cured at approximately 204.degree. C for 10 minutes. The resultant coating is glossy, tough, flexible, and passes the above-described tests. The coating weight is 0.7 milligram per square centimeter.

The procedure of Example 2 is repeated using the following quantities: 2-butoxy-1-ethanol, 58.9 pounds; diethylene glycol mono-butyl ether, 58.9 pounds; "Epon 1004", 182.5 pounds; dimethyl ethanol amine, 18.8 pounds, dissolved in water, 37.7 pounds; a partially methylated melamine, 89% solids in iso-propanol, (available from Monsanto Polymers and Petrochemical Company, known as Resimene X-740) 51.3 pounds; 413 pounds of water; 8.1 pounds of phosphoric acid dissolved in a 3:1 by weight mixture of water and diethylene glycol mono-butyl ether, 45.8 pounds. The resulting clear solution has a Gardner-Holdt viscosity at room temperature of Z2, pH of 9.5, and solids content, determined at 204.degree. C for ten minutes, of 27.8%. This solution can be applied to various substrates as is or, in order to increase its rate of cure, the amount of nitrogen-resin crosslinking agent can be increased as follows: to 875 pounds of the total batch as prepared above are added the following: 2-butoxy-1 -ethanol, 27.6 pounds; diethylene glycol mono-butyl ether, 29.3 pounds; and 16.8 pounds of "Resimene X-740". The resulting solution has a Gardner-Holdt viscosity of T, pH of 9.5, and a solids content as determined above of 27.4%. This solution is roller coated onto quality-as-rolled steel or on tin-free steel, baked at 205.degree. C for 6 minutes. The resulting film is clear, glossy, tough, and passes the above described tests.

The procedure of Example 2 is repeated using the following quantities: 2-butoxy-1-ethanol, 52.4 pounds; diethylene glycol mono-butyl ether, 155.1 pounds; "Epon 1004", 181.9 pounds; dimethyl ethanol amine, 18.8 pounds, dissolved in water, 37.5 pounds; a mixture of nitrogen resins: dibutoxytetramethoxymethylmelamine, 39.1 pounds and a partially methylated melamine ("Cymel 370", available from the American Cyanamid Company) 44.4 pounds; water, 647.6 pounds; phosphoric acid, 10.2 pounds, dissolved in 58.1 pounds of a 75:18.5:6.5 by weight mixture of water, diethylene glycol mono-butyl ether, and 2-butoxy-1-ethanol. The resulting clear solution has a Gardner-Holdt viscosity of G+, pH of 8.0, and a solids content, determined at 204.degree. C for 10 minutes, of 20.6%. The solution is applied with an airless spray applicator and baked at 204.degree. C for 110 seconds. Satisfactory cure can also be obtained if the baking time is reduced to 55 seconds. The coatings so obtained pass all the tests above described.

A solution is prepared for electrodeposition as follows. The following ingredients are mixed and heated to 75.degree. C until dissolved: "Epon 1004," 275.9 grams; 2-butoxy-1ethanol, 90.4 grams; and methyl ethyl ketone, 10.1 grams. To this solution is added over a 5-minute period dimethyl ethanol amine, 28.6 grams, dissolved in 48.4 grams of water and the reaction temperature is maintained at 70.degree. to 75.degree. C for 30 minutes. Over the next 30-minute period at 60.degree. to 65.degree. C is added 405.2 grams of water. The reaction mixture at this point is a two-phase system. To this mixture are added a partially methylated melamine, 80% solids in iso-propanol, ("Resimene 731"), 95.8 grams, and water, 334.8 grams, over a 1-hour period while the temperature is maintained at approximately 70.degree. C. The resulting clear amber solution has a Gardner-Holdt viscosity of 0, pH of 12.3, and a solids content, determined at 204.degree. C for 10 minutes, of 23.0%. The solution has a conductivity of 1,350 micromho per centimeter. A 287.2 gram portion of this solution is diluted to 8.0 weight percent solids content by adding 538.6 grams of water. The pH now is 11.5. This dispersion is placed in a 1-liter coating tank and a 10 centimeter by 10 centimeter aluminum can endstock is placed into the solution. When the panel polarity is such that it is the cathode then, at an applied voltage of 100 volts and in a coating time of 2 seconds, the panel is found to have a smooth coating on its surface upon baking at 204.degree. C for 5 minutes. When the panel is set to be the anode, no film is deposited at all.

Coating compositions are prepared as in Example 2 by substituting a series of nitrogen resin crosslinking agents for the partially methylated melamine ("Resimene 731"). In each of the examples, see list below, a 60- gram portion of the composition is diluted with 15 grams of a 3:1:1 mixture of water:2-butoxy-1-ethanol:diethylene glycol mono-butyl ether. Each sample then is cast onto aluminum, the films baked at 204.degree. C for 10 minutes, and the samples tested for boiling water resistance, "blush," adhesion, and flexibility at the double seam of a can after processing. All samples pass these tests.

Example 6 -- hexamethoxymethylmelamine.

Example 7 -- butylated melamine formaldehyde ("Resimene X-764").

Example 8 -- dibutoxytetramethoxymethylmelamine.

Example 9 -- partially methylated melamine ("Cymel 350").

Example 10 -- partially methylated melamine ("Cymel 370").

Example 11 -- partially methylated melamine ("Cymel 373").

Example 12 -- butylated melamine-formaldehyde ("Cymel 1156").

Example 13 -- partially methylated melamine ("Cymel 735").

Example 14 -- butylated benzoguanamine ("Resimene X-760").

Into a 1-liter, round-bottomed, three-necked flask, equipped as in Example 1, are placed "Epon 1007", 182.5 grams; 2-butoxy-1-ethanol, 74.5 grams; and diethylene glycol monobutyl ether, 74.5 grams. The contents of the reactor are heated in a nitrogen atmosphere to 50.degree. C until the solution becomes clear. To this solution is added trimethyl amine, 5.4 grams, dissolved in 16.2 grams of water. To the still clear solution there is added a partially methylated melamine, 89% solids in iso-propanol, ("Resimene" X-740"), 68.4 grams, and the reaction mixture is maintained at 70.degree. C for 10 minutes. Water, 469.4 grams, is then added over a 30-minute period while the temperature of the reaction mixture is raised to 90.degree. C. At the end of this period the final product is a clear, dark amber solution. The product has a Gardner-Holdt viscosity of A and a pH of 10. A film cast from this solution onto an electrolytic tin plate panel and baked for 4.5 minutes at 204.degree. C has high gloss, is hard, and passes 55 rubs with methyl ethyl ketone in the solvent resistance test. For comparison, to a 415-gram portion of the above prepared solution is added 0.5 gram of phosphoric acid dissolved in 10.4 grams of a 75:18.5:6.5 weight ratio mixture of water:diethylene glycol mono-butyl ether:2-butoxy-1-ethanol. The pH of this solution is 4 and has a Gardner-Holdt viscosity of B. A film cast from this solution and baked as described above is hard, has high gloss, and surpasses 100 methyl ethyl ketone rubs.

To a 500-gram portion of the thermosetting composition prepared in Example 3 is added 22 grams of phosphoric acid. A film of of this solution is cast on paper with a number 13 wire-wound rod and cured at 150.degree. C for 10 seconds. The resulting coating has fair to good gloss, excellent holdout over printed and unprinted areas of the paper, and shows no blocking when tested for block resistance in a heat-sealer for 0.5 second at 150.degree. C under 40 pounds per square inches pressure. The coating passed 12 rubs of methyl ethyl ketone solvent resistance; 7 rubs or higher are considered satisfactory for such application.

To 500 grams of the thermosetting composition described in Example 3 are added 20 grams of phosphoric acid and 50 grams of partially methylated melamine, 80% solids content, (available from American Cyanamid Company as "Cyrez 933"). The solution is applied and cured as described in Example 16, resulting in a coating having good gloss and excellent holdout and is non-blocking. It passes 16 rubs with methyl ethyl ketone.