IL46449A - Thermosetting coating compositions suitable for coating metallic substrates and method for their application - Google Patents

Description

om «ΐϊ,'χ mo'wi ni'nano ni'nwn » Thermosetting coating compositions suitable for coating metallic substrates and method for their application BOHM AND HAAS COMPANY C. 44314 This invention concerns thermosetting coating compositions, useful which may be applied to be coated and methods for their application. Suitable substrates include metallic sheet which may be coated prior to various mechanical forming processes to provide items such as cans, screw cap jars and aerosol containers. The coatings for such items must be able to withstand a variety of fabrication treatments which may range from very mild treatments (e.g., the formation of can bodies) to extremely severe (e.g. , the fabrication of aerosol domes) without cracking or chipping of the coating.

The coating compositions can also be applied to previously shaped articles such as mandrel-formed two piece can bodies or toothpaste tubes.

In a typical process for the formation of a finished coated metallic item, a pigmented base white coating composition is roller-coated onto a metallic substrate and the coated substrate is subjected to a baking operation. Decorative print is then applied ove the base white coat and, while the ink is still wet, a clear over-print varnish is roller-coated over the ink to protect it. The coated metallic substrate is then subjected to a baking operation to develop hardness of the coating. The coated metallic substrates may then be stacked and stored for various time periods prior, to fabricating into the product. It is most desirable that the coating have good hot stack resistance to prevent facing coatings from marring one another.

The coated sheets are finally fabricated into various forms. The fabrication requirements will vary depending upon the end use of the sheet material, e.g., whether it is to be fabricated into cans, lids or screw caps. In addition to withstanding the various fabricating operations performed on the sheet metal, many end uses require processing or sterilization operations which involve high temperature conditions. In such instances, the coating must possess good overbake properties.

In order to obtain optimum hot stacking properties for coatings, it is desirable to use a thermosetting coating composition which results in a high degree of crosslinking when cured. However, the cured coating must also possess sufficient flexibility to permit the fabrication treatment to which it will be subjected. If the coated sheet is to be subjected to severe fabricating operations, a "soft" thermosetting resin is preferably used, i.e., a resin which is thermo-set with an aminoplast having a low crosslinking density Whereas, if the coated sheet is to be subjected to relatively mild fabricating conditions, a "hard" thermosetting resin is preferably employed.

Hitherto, the coating of metallic substrates has involved the application of primers comprising certain corrosion-protective pigments in non-aqueous vehicles based on a drying oil, such as linseed oil , a fast-drying varnish base comprising natural resins, r or a mixture of natural and synthetic resins, or an alkyd base modified with a urea-, melamine-, or phenol-formaldehyde resin. Such coating compositions are usual supplied in high-boiling solvents to insure good flow and levelling in the thin films applied during a roller-coating operation. The solvents are frequently of an inflammable character and often are the type which give off noxious fumes during the coating operation.

To cope with the fire and health hazards, protection is usually provided by way of solvent recovery systems. Moreover, recent interest in air pollution control has been aimed at reducing or eliminating organic solvent emission to the atmosphere.

There has long been a demand for thermosetting coating compositions which can be applied from an aqueous medium to avoid air pollution, fire hazards and other problems which attend the use of organic solvent coating compositions. However, earlier aqueous coating systems, such as those disclosed in U.S. Patent Nos. 2,760,886, 2,918,391 and 3,033,811, have not proved fully satisfactory for coating metals by direct-roller coating machinery because they tend to dry on the roller causing difficulties in clean up or blister when the wet products are put directly in an oven, or are not amenable to post-forming operations because of brittleness or to handling because of softness. The greatest difficulty with water based systems is to obtain rheological properties suitable for direct-roll coating of the system. Thus, a composition is required which possesses flow and levelling properties such that when applied by a direct-roll coater, it will form a uniform and smooth surface which does not contain striatioris.

The deficiency of aqueous emulsions from the standpoint of flo and levelling is well recognised in the metal coating industry, see Paint and Varnish Production, February, 1964, pages 28-33. The flow and levelling of the pigmented base coat is most critical since pigmentation noticeably reduces flow in aqueous systems.

British patent specification No. 1,392,333 describes a thermosetting aqueous coating composition which can be applied by direct-roller processes to metals to provide films with good appearance which can be post-formed and which provide, after baking, clear or pigmented coatings possessing excellent water-and solvent-resistance, excellent adhesion to a variety of substrates, and high gloss, and which we have found can be advantageously used in connection with direct-roll coaters wherein the rollers are of low Durometer hardness, e.g., about 12. However, since soft rubber rolls do not have good wear characteristics, it is desirable to provide a thermosetting resin coating composition which can be applied by a mandrel-coater or a direct-roll coater wherein the rolls are of high Durometer hardness, e.g., about 20-50 without sacrificing flow and levelling properties. This invention provides such a coating composition.

According to the invention there are provided coating compositions comprising an alkaline aqueous blend of a binder which comprises : A. a water-soluble salt of tertiary amine and water-insoluble addition copolymer, having a molecular weight of 20,000 to 200,000, the copolymer being derived from a mixture of copolymerizable monomers comprising: (1) at least one ester of a alcohol and an a, β-ethylenically unsaturated carboxylic acid (as herein defined): (2) 1 to 4 percent by weight of olefini- cally unsaturated carboxylic acid and (3) 3 to 20 percent by weight of at least one hydroxy(C2-C4)-alkyl ester of an acid of the formula: in which n is 1 or 2, and/or at least one (C^-C2)-alkyl ester of alpha methylol acrylic acid, a water-soluble condensation product of urea and/or a triazine with formaldehyde, or a water-soluble methanol or ethanol ether thereof; ) C. 1 to 5 percent by weight of poly(ethylene glycol) or poly(ethylene oxide) as water- soluble rheology modifier having a number average molecular weight of from 400 to 100,000, the percentage being based on the total weight of (A) and (B) on a solids basis, the weight ratio of A to B on a solids basis being from 85:15 to 65:35, the minimum film-forming temperature of the composition being at most 25°C, the solids content of the composition being between 30 and 80 percent by weight.

In this Specification "water soluble" includes colloidally dispersible.

Preferably B is the reaction product of a urea-formaldehyde adduct and methanol, the mole ratio of urea: formaldehyde methanol being from 1/1.75/2 to 1/3/3.5; or a melamine-forraaldehyde adduct and methanol, the mole ratio of melamine : formaldehyde :methanol being from 1/7/12 to 1/10/17, or mixture thereof. In one embodiment of the invention, in copolymer A, (1) is preferably an ester of acrylic acid and an alkanol having from 2 to 12 carbon atoms; in another embodiment, in copolymer A, (2) is methacrylic acid; and in a further embodiment, in copolymer A, (3) is at least one of hydroxyethyl acrylate or methacrylate or hydroxypropyl acrylate or methacrylate. ^ If the coated metal is to be subjected to severe fabrication treatments, such as the fabrication of aerosol domes or bottle caps, then the copolymer is preferably a "soft" copolymer. The Tg of the "soft" copolymer should preferably be below 0°C. and, preferably, below -15°C. In a preferred embodiment of the invention the coating composition may contain a volatile base, a tertiary amine being preferred, and the composition most preferably has a minimum film-forming temperature at most 15°C. When the coated sheet is to be subjected to mild fabrication treatments such as formed into can bodies, either a "soft" or "hard" copolymer may be used.

The most preferred copolymer is a copolymer of ethyl acrylate, hydroxypropyl methacrylate, metha-crylic acid and methyl methacrylate.

One of the monomers which may be utilized in a substantial proportion to prepare the addition copolymer is a flexibilizing or "soft" monomer which may be represented by the following formula: wherein R is H or alkyl having 1 to 4 carbon atoms and is the straight chain, branched chain or cyclic radical of a primary or secondary alkanol , alkoxyalkanol or alkylthiaalkanol , the alkanol having from 2 to 14 carbon atoms, examples include: ethyl, methylpropyl, n-butyl, 2-ethylhexyl , heptyl , hexyl, octyl , propyl , 2-methylbutyl , -methylbutyl , butoxybutyl , 2-methyl-pentyl , methoxymethyl , ethoxyethyl, cyclohexyl, n-hexyl , isobutyl, ethylthiaethyl , methylthiaethyl , ethyl-thiapropyl, n-octyl , 6-methylnonyl , decyl and dodecyl.

When R is alkyl and is alkyl, R^ preferably contains from 6 to 14 carbon atoms and when R is H and R^ is alkyl, R^ preferably contains from 2 to 12 carbon atoms, in order to qualify as a "soft" monomer.

Important properties of the copolymer are its toughness and flexibility, as well as its influence upon the minimum film-forming temperature (MFT) of the coating composition. The glass transition temperature (Tg) of the copolymer and consequently the selection of monomers and proportions thereof depends upon their influence on the Tg. "Tg" is a conventional criterion of polymer hardness and is described by Flory, "Principles of Polymer Chemistry", pp. 5G and 57, (1953), Cornell University Press. While actual measurement of the Tg is preferred, it may be calculated as described by Fox, Bull. Am. Physics Soc. 1, 3, p. 123 (1956).

Examples of the Tg of homopolymers and the inherent Tg thereof which permits such calculations are as follows: Homopolymer of Tg n-octyl acrylate -80°C. n-decyl methacrylate -60°C. 2-ethylhexyl acrylate -70°C. octyl methacrylate -20°C. . n-tetradecyl methacrylate -9°C. methyl acrylate 9°C. n-tetradecyl acrylate 20°C. methyl methacrylate 105°C. acrylic acid 106°C.

These or other monomers may be blended to give the desired Tg of the copolymer. As is known, for a given number of carbon atoms in the alcohol moiety, the extent and type of branching markedly influences the Tg, the straight chain products giving the lower Tg.

The coating composition of this invention similarly has a maximum MFT. MFT is determined by the method described in Resin Review, Vol. 16, No. 2 (1966). This is influenced not only by the Tg of the addition copolymer, but by the plasticizers or coalescing agents used and their amounts. At MFT values appreciably above this maximum, difficulties in obtaining a uniform coating film and lack of film integrity during deformation of the metal may be encountered.

In addition to the flexibilizing monomer, other preferable monomers may be "toughening" or "hard" monomers, including the unsaturated acid monomer, and the hydroxyalkyl methacrylates. The hardness or softness of the acid and other functional monomers is not critical because of the small amounts used.

The copolymers used in the present invention preferably have a viscosity average molecular weight from 20,000 to 200,000. In order to render the acid copolymers soluble in aqueous media for the purposes of the present invention, they are neutralized with a tertiary amine such as 2-(dimethylamino)-ethanol.

The copolymers may be prepared in a wide variety of ways. For example, they may be prepared by polymerization of the appropriate monomers in solution, or in aqueous media to provide in the latter case, a stable latex or dispersion. In some cases, it may be necessary to employ a chain-regulator in order to provide a molecular weight in the range desired. This is particularly the case whe polymerizing in aqueous media. Examples of chain-regulators that may be employed include long-chain alkyl mercaptans , e.g., tertiary dodecyl mercaptan of the formula: H3CC (CH3 ) 2CH2C (CH3 ) 2CH2C (CH3 ) 2SH isopropanol, isobutanol, long-chain alcohols, e.g., lauryl alcohol, tertiary-octyl alcohol, cumene, CCl^, C2C^4' and CBrCl-j. The amount of chain regulator that may be used depends upon the particular system and the conditions and may vary from 0 to 2 percent based on the weight of monomers. Generally, the use of 0.1 to 1 percent of bromotrichloromethane serves to provide as wide a range of molecular weights in aqueous media as is required.

While in the formation of the copolymer by emulsion copolymerization any suitable dispersant or emulsifier may be employed, it is preferable to use a salt obtained from a previously prepared batch as the dispersant in the system so that the coating composition is essentially uniform in character. The amount of such a dispersant that is used in such a latex polymerization may vary from about -J percent to 6 percent on the weight of monomers. Commonly employed water-soluble free-radical initiators, such as the persul-fates of ammonium, sodium, or potassium, or redox systems using tertiary-butyl hydroperoxide with a reducing agent such as isoascorbic acid, may be employed.

The presence of certain water-miscible solvents in the amine copolymer salt solution has been found to markedly improve the stability of aqueous solutions of the amine salts, to improve the gloss of coatings obtained therefrom and to make it possible to use a wider variety of comonomers for purposes as mentioned above. The useful solvents which are herein called "cosolvents" are soluble in water but not to such an extent that they remain essentially in the water phase to the substantial exclusion of entry into the polymer micelle. The useful cosolvents enter extensively into polymer micelle and, as a result, they show a noticeable thickening action which does not occur when the solvent remains predominantly in the water phase. The cosolvent should be volatile so that it leaves the film or coating on air-drying and is practically all removed before baking to avoid blistering on baking. The volatility should not be so high that the flash point of the aqueous compositions is undesirably low. Suitable cosolvents are 2-butoxy-ethanol, 2-ethylhexyl alcohol, tertiary butanol, n-butanol , isopropanol, isophorone, butoxyethyl acetate, butoxyethoxyethyl acetate, ethoxyethoxyethyl acetate and triethanolamine. If the polymer is prepared in other types of organic solvent, such as acetone, ethanol, or 2-ethoxyethanol , which are incapable of serving as cosolvents, the solvent used in preparation of the polymer may be removed before the acid polymer is neutralized with the amine to form the salt, since this type of solvent usually contributes no particular advantage or benefit to the final aqueous copolymer salt composition.

In order to form the salt of the acid copolymer, the acid copolymer is introduced into water, the amine is added, and then the mixture may be heated with agitation at a temperature of about 40° to 80°C. for a period of about 15 minutes to 15 hours. Preferably, a temperature of 50° to 75°C. is used. However, in some cases and particularly with the low molecular weight polymers in finely-divided condition, the reaction with the amine requires no heat and is virtually instantaneous.

If the acid copolymer has been prepared in an aqueous medium, the amine may be added directly to the aqueous dispersion of the copolymer. However, in this case, it is often desirable to eliminate ionic constituents, such as by treatment with an ion-exchange resin or by dialysis, before adding the amine. Preferably, the copolymer is of reduced particle size to facilitate the reaction with the amine in forming the salt. When the copolymer has been produced by a solution method, it may be isolated from the solution before it is introduced into the water, although it has generally been found that the solvent need not be eliminated before introduction of the copolymer into the water. When the solvent used in the polymerization is one of the co-solvents mentioned, it improves the clarity and stability (against mechanical action and on storage under normal conditions) of the aqueous solution obtained with the amine when used in the appropriate amount and improves the gloss of the films or coatings obtained.

The unsaturated carboxylic acid may be a simple monocarboxylic acid, or may be a half ester or half amide of an α,β-unsaturated dicarboxylic acid, and salts thereof with a volatile base such as ammonia, or with a volatile water-soluble amine such as dimethyl-amine, dimethylethanolamine , triethylamine , triethanol-amine, morpholine, N-methyl morpholine or picoline.

Examples of copoly erizable ethylenically unsaturated monocarboxylic or polycarboxylic acids include sorbic , cinnamic, vinyl furoic, cc-chlorosorbic , £-vinylbenzoic , acrylic, methacrylic, rnaleic, fumaric, aconitic, atropic, crotonic, and itaconic acid, or mixtures thereof, with itaconic acid and the a, β-unsaturated monocarboxylic acids, particularly methacrylic acid and acrylic acid, being preferred. Other copolymerizable acid monomers include the alkyl half esters or partial esters of unsaturated polycarboxylic acids such as of itaconic acid, rnaleic acid, and fumaric acid, or the partial amides thereof. Preferred half esters are the lower alkyl (C^ to Cg) esters such as methyl acid itaconate, butyl acid itaconate, methyl acid fumara.te, butyl acid fumarate, methyl acid maleate, and butyl acid maleate. The term "a, β-unsaturated monocarboxylic acids", includes partial esters and partial amides thereof. The hydroxyl containing monomers may be hydroxyethyl , hydroxypropyl or hydroxybutyl acrylate or methacrylate , or ethyl or methyl-a-methylol acrylate.

Other ethylenically unsaturated copolymerizable monomers may be useful in combination with the above mentioned flexibilizing monomers and toughening monomers provided they do not adversely affect the desired properties of the copolymer (e.g., unduly raise the overall Tg). These may be represented by the formula: wherein R is as above. R2 is preferably alkyl and more preferably is methyl or alkyl having from 13 to 20 carbon atoms when R is H, and is alkyl of from 1 to 5 carbon atoms or alkyl of from 15 to 20 carbon atoms when R is methyl. It can be seen from above that for alkyl acrylates and alkyl methacrylates the Tg at first decreases with an increased chain length of the alkyl group and then the Tg again increases; i.e., both hard and soft monomers are known to occur in each group of monomers. Examples of these hard monomers and other hard monomers include: vinyl aromatids such as styrene and vinyl toluene, vinyl chloride, vinyl acetate, nitrile such as acrylonitrile , methyl acrylate, tetradecyl acrylate, pentadecyl acrylate, methyl methacrylate, ethyl methacrylate, t-butyl acrylate, butyl methacrylate, and pentadecyl methacrylate. For this purpose, the cosolvent, tertiary-butanol may be used in an amount of about 5 to 10 percent based on the total weight of solution.

The proportion of amine used may be about 1 to 6 equivalents of amine per equivalent of acid in the copolymer. Generally, a minimum of one equivalent of amine is needed and ordinarily, a maximum of 2 or 3 equivalents is most suitable. A typical composition may be composed of 150 to 500 parts of water per 100 parts of polymer, about 10 to 50 parts of co-solvent per 100 parts of polymer, and 1 to 2 equivalents of the amine, based on the content of acid component in the copolymer.

The acid copolymers contain certain units in certain proportions as previously defined. The content of 1 to 4 percent acid-containing units and from 3 to 20 percent of hydroxyl-containing units both apparently cooperate in the solubilization by treatment with the dimethylaminoethanol . In general, after solubilization the pH of the aqueous system containing the solubilized copolymer has a pH in the range of 8.5 to 9.5. If insufficient amine is added to raise the pH to about 8.5 the shelf life of the composition becomes relatively short. Whereas, if an excessive amount of amine is added whereby the pH of the system is raised above 9.5, the viscosity becomes excessive for systems having solids concentrations that are most suitable for coating and impregnating applications.

By providing a relatively low proportion of acid groups in the copolymer along with a substantial proportion of hydroxyl-containing groups therein, the conversion of the copolymers into water-soluble salt molecules or micelles of colloidal dimensions is accomplished without encountering excessive viscosity.

The preparation of water-soluble salts of a tertiary amine with a water-insoluble, addition copolymer is exemplified in U.S. Patent No. 3,245,932.

The water-soluble, heat-convertible condensation products of urea or triazine (e.g., melamine) with formaldehyde and/or their derivatives obtained by reaction with ethanol or methanol can be prepared according to one of the following schemes: (1) control of reaction conditions so that the degree of polymerization is kept very low, even to the monomeric stage, and (2) introduction of hydrophilic groups into the molecules of the polymeric condensates. Tnus , they can be made by careful control of reaction conditions as set forth in Schildknecht, "Polymer Processes," Vol. X, page 295 et seq. ( Interscience Press, 1956).

The preparation of another class of compounds suitable in the present invention, such as ,N ' -bis (methoxymethyl ) urea is set forth in Bull. Chem. Soc. Japan, Vol. XI, No. 3,239 (1936). In a preferred embodiment of this invention, a mixture of a urea-formaldehyde adduct with methanol and a melamine-formaldehyde adduct with methanol is employed in the coating compositions.

The coating compositions of this invention preferably contain a material designated as a "coalescent or alternatively designated a "cosolvent". These materials aid fusion of the film during air-drying prior to baking and promote the flow of the coating composition during the baking, cycle. However, because they are volatile they do not form a part of the finished coating. They are characterized by being low in water-solubility, good solvents for the uncured polymer mixture, less volatile than water so that they remain in the film after the water has evaporated, sufficiently volatile so that they are removed from the film before the end of the baking cycle and not susceptible to hydrolysis in alkaline media either at ambient or elevated temperatures. They also serve to lower the surface tension of the aqueous system,, making it easier to wet the metal substrate. Typical examples include those coalescents previously mentioned as well as diacetone alcohol, dimethyl formamide, alkyl ethers of ethylene glycol and propylene glycol, and tributyl phosphate. The coalescent is preferably present in the coating composition in an amount of from 10 to 20 percent by weight based on the weight of the solids. The water-soluble condensation product of urea or triazine with formaldehyde is usually supplied as an 80 percent solids solution in isopropanol or an equal volume mixture of isopropanol and butanol which are coalescent. Moreover, the water-soluble copolymer salt is preferably supplied in a mixture of water and a coalescent. Therefore, the coating composition will normally contain a coalescent even without additional cosolvent being added.

The rheology modifiers which may be used in the practice of this invention are materials which are well known in the art. Polyethylene glycols are sold commercially under the name Carbowax. Their preparation is decribed by Fordyce in J. Am. Chem. Soc. , Vol. 61, pages 1905, 1910 (1939). Preferably, a polyethylene glycol is used having a molecular weight of from about 400 to 20,000. Poly (ethylene oxide) is a polyether obtained by polymerizing ethylene oxide. Such materials < are sold under the commercial designation Polyox. They may be prepared, for example, by processes as described in U.S. Patent Nos. 3,365,409; 3,167,519; 3,251,784; and 3,444,102. It is preferred that the poly(ethylene oxide) have a molecular weight which does not exceed about 100,000. It may be necessary to use conventional methods to depolymerize products produced according to these patents to obtain this molecular weight or lower ones.

Such depolymerized products are commercially available.

The rheology modifier may be added to the coating system at any stage of preparation—i.e., it may be added to the thermosetting resin emulsion during or after its preparation, it may be added to the composition at the time of mixing with the condensation product of urea or triazine with formaldehyde, etc.

Both the "Polyox" and Carbowax" products are essentially polyoxyethylene polymers having linear chains and having terminal hydroxyl groups. The terminal groups can be varied to include ester groups, ether groups, epoxy groups, or other groups and herein wherever "polyethylene glycols" are referred to, the functionally equivalent linear polyethers having terminal groups other than hydroxyls are intended to be included. Such terminal groups other than hydroxyl should have no more than about two carbon atoms. With products having the molecular weights of the invention, the nature of the end groups on the linear chain have essentially no effect upon the rheology-imparting properties of the polymer. The "Carbowax" type of polyethylene glycols are understood to be prepared by starting with water or ethylene glycol and ethylene oxide and polymerizing in the presence of an alkaline catalyst. The "Polyox" polyethylene glycols are understood to be prepared by polymerization of ethylene oxide using a different catalyst and in the absence of water, or a starter such as a glycol. The latter products may have a molecular weight of several million, having a relatively broad molecular weight distribution as compared with the lower molecular weight polyethylene glycols prepared with a starter. The water-soluble polyethylene oxide materials useful in the invention may be obtained by depolymerizxng the products having molecular weight of several million. As suggested above, the polyethylene glycols made with a starter generally have a very narrow molecular weight distribution. These facts show that the molecular weight distribution of the polyether is not particularly critical , and where a molecular weight is given, it is to be understood that this is a. number average molecular weight.

The exact mechanism by which the polyethylene glycols and poly(ethylene oxides) act to modify the rheological properties of the coating composition is not known. In the amounts used, they do not act as thickeners for the composition, but actually lower the viscosit of the compositions. It is believed that these compounds may complex with the aminoplast—i.e. , the urea- or triazine-formaldehyde condensation product, thereby decreasing the normal rapid increase in viscosity which occurs with a relatively small increase in solids encountered with the coating compositions in the absence of the rheology modifier. Thus, while the coating composition without the rheology modifier may set up very fast resulting in striations in the final coating, the coating compositions containing the rheology modifiers flow into a smooth coating before setting up. By the practice of this invention, there is obtained a synergistic effect between the urea- or triazine-formaldehyde condensation product and the rheology modifier to give improved flow. That is, the improvement in flow is considerably greater than can be attributed to the additive effect of the condensation product and the rheology modifier each by itself.

If it is desired to have the mixture of the water-soluble salt of the water-insoluble copolymer and the aminoplast stable on prolonged storage, because the aminoplast is reactive under acid conditions, the mixture may be made alkaline beyond that strictly required to neutralize and to solubilize the copolymer in water.

A volatile base, such as ammonia or a tertiary amine, may be used to make the system alkaline because tertiary amines will not react with the formaldehyde associated with the aminoplast. The tertiary amines also function as corrosion inhibitors when the coating compositions of the present invention are used for coating metal.

The tertiary amine must be sufficiently volatile that it will be driven from the film during the baking operation. However, it must not be so volatile that it "flashes" from the film or gasifies if the coating composition is applied by spraying. Particularly preferred because of the balance of properties , availability and economy is triethylamine. The pH of the mixture should be maintained in the range of 9 to 9.5 in order to ensure good storage stability. However, it is apparent that initial pH control alone is not sufficient to ensure adequate stability and retention of properties since samples neutralized to the desired pH range with ammonia have inadequate stability on prolonged storage, although satisfactory films are obtained if the ammonia-neutralized compositions are used shortly after preparation. However, if the tertiary amine is the predominant nitrogen base present, small amounts of ammonia can be used without deleterious effects., The amounts of amine used will vary depending on the specific composition employed but will generally be in the range of 1 to 5 parts by weight per 100 parts by weight of coating composition (solids basis). A preferred embodiment employs two parts by weight per 100 parts by weight of the coating composition (solids basis).

While a dispersant may be used when employing pigments, it is not essential due to the presence of the acid moieties in the colloidal dispersion. A wide variety of dispersants will satisfactorily disperse pigments, the effect of the dispersant on the properties of the final film must be considered. Many dispersing agents remain in the final film unchanged, thus seriously impairing the water-resistance of the film. Other dispersing agents will adversely affect the stabilities of the systems into which they are incorporated.

A preferred embodiment employs as dispersants the ammonium and lower amine salts of polymeric carboxylic acids. Thus, the ammonium and lower amine salts of polyacrylic and polyme hacrylic acids and similar salts of the polymeric acid obtained by copolymerizing methyl vinyl ether with maleic anhydride are suitable. A particularly preferred embodiment employs the ammonium half amide salt or the diammonium salt of a diisobutylene-maleic anhydride copolymer having a number average molecular weight of from 2,000 to 4,000. The amount of dispersant employed will vary depending on the amount and nature of the pigments used and the amount and nature of the composition employed as binder. Generally, however, from 0,3 to 3.5 parts by weight (solids basis) per 100 parts by weight of pigment, will prove to be effective for dispersing the pigment.

- - It appears that the dispersants of the type hereinbefore described decompose at the temperatures employed in the baking cycle to liberate ammonia or lower amine which is then volatilized. It is further postulated that the carboxylic residuals react either with the amide group of the copolymer or with the amino-plast or both and serves to catalyse insolubility of the composition. Regardless of the mechanism involved, the fact that it is observed that this particular class of dispersants, when employed as set forth hereinbefore, do not detract from the excellent water-resistance and-other highly desirable properties of the films proves that such catalysis does occur. It has been proven that even clears which contain a small amount of such dispersants exhibit better water- and solvent-resistances than do the same compositions without dispersant, both samples being cured under exactly the same conditions.

The amount of dispersant employed in clears varies depending on the amount of a inoplast employed. Generally, from 0.1 to about 1.0 part by weight dispersant per 100 parts by weight (solids basis) of the coating composition will effect the desired catalysis.

The coating compositions of the present invention may be employed as clears, i.e. , non-pigmented clear top coatings, or as pigmented coatings.

In the latter applications, the coatings can also be left in a "varnishless" condition, i.e., they can be left on the substrate without an additional clear varnish overprint coating. If pigmented, the ratio of pigments to coating solids may be varied widely, depending on the pigment employed and the specific application involved. Thus,, the ratio of pigment to coating solids may vary from 1 to 20 to 20 to 1. Preferably the relative weight of pigment to A plus B is from 5:95 to 60:40, the total being 100. The clears are particularly useful as "overcoats", i.e., the so-called overprint coatings which are used to protect decorative undercoats without detracting from the decorative effect. Because the clear coatings of the present invention exhibit good clarity, high gloss, excellent solvent- and water-resistance , and high adhesion to a variety of surfaces, they are admirably suited for use as overprint finishes.

The coating compositions of this · invention can be applied to a variety of substrates, the only restriction being the ability of the substrate to withstand the baking cycle which is essential in the processing of said coating compositions. Metals are particularly suitable, whether prime-coated or unprimed. Thus, iron, steel, chrome-plated steel, tin-plated steel, aluminum, copper, bronze, or brass surfaces, particularly in sheet or coil form with thicknesses of 0.05 to 0.20 inches, prove to be excellent as substrates for the coating compositions of the present invention. Ceramic surfaces and, in some instances, wood surfaces, are also suitable as substrates. For roller coating such as reverse roll coating, the thickness is from 0.05 to 5 mils in thickness, preferably 0.2 to 1.5 mils in thickness when cured.

A wide variety of pigments can be employed with the coating compositions of the present invention. The pigments employed, however, must be stable and non-reactive under alkaline conditions, i.e., a pH from about 9 to about 11. Typical pigments which are suitable include titanium dioxide, iron oxide, calcium carbonate, barytes and numerous types of clays.

The coating compositions of this invention are particularly suitable for application by a direct-roll coater or a mandrel coater although they may be applied by other means such as a reverse-roll coater or a spray gun. As is well known in the art, a single roll coater applies the coating to the substrate while the applicator roll rotates in a pool of the coating composition. The coatings may then be baked at a temperature of from 250°F. to 350°F.. for from about ¾ to 10 minutes. The baking or curing operation volatilizes all the volatile material in the film including any remaining water, traces of monomer, coalescents, and the tertiary amine. It is particularly important that the tertiary amine be volatilized since it inhibits the cure of the aminoplast. The baking operation effects the decomposition of the ammonium or amine salts of the polymeric carboxylic acids, apparently releasing the acid form of the copolymer which may then react with the other components to become insoluble. The baking operation causes the cure of the aminoplast which crosslinks and insolubilizes the entire film.

Preferred embodiments of the invention will now be described, for illustration only, in the following Examples, in which all parts and percentages are by weight and temperatures in degrees Farenheit, unless otherwise stated.

Example 1 A water-soluble salt of 2-(dimethylamino)- . ethanol with a water-insoluble copolymer of ethyl acrylate/hydroxy propyl methacrylate/methacrylic acid/ methyl methacrylate in the weight ratios of 46/5/3/46 (30-32 percent by weight total solids in a mixture of 90 percent water and 10 percent t-butyl alcohol) is blended with a water-soluble melamine-formaldehyde adduct modified by reaction with methanol, the mole ratio of melamine/formaldehyde/methanol being 1:7:12 (80 percent total solids in an equal volume mixture of isopropanol and butanol) and 1.3 percent based on the weight of total solids of a polyethylene glycol having a molecular weight of about 20,000 (Carbowax 20M). The weight ratio of the copolymer to the adduct in the mixture is 75:25. The resultant composition has a pH of 8.6, a viscosity of about 75 cp and a total solids content of 36% by weight. The weight ratio of water to alcohol in the system is 87:13. The composition is direct-roll coated on 90 pound tin-plated steel as a clear over-print varnish for can bodies at 150 feet per minute with a urethane roll of durometer 20. The flow and levelling of the composition is very good. The coated panels are baked at 325°F. for 10 minutes to obtain a uniformly smooth, hard coating having good mar resistance. The flow characteristics of the coating composition of this Example are improved slightly by the addition of 5 percent by weight of Butyl Cellosolve, based on the weight of the total solids.

Example 2 A ball mill grind is prepared by ball-milling 250 parts by weight of the composition of Example 1, 250 parts by weight titanium dioxide pigment (DuPpnt TiPure R900) and 1 part by weight of a defearning agent (Nopco NDW). After this composition has been ball-milled for 16 hours, 150 parts by weight are mixed with 133.5 parts by weight of the composition of Example 1 and an equal volume mixture of water and dimethylaminoethanol in an amount sufficient to give a viscosity of 60 to 70 seconds with a no. 2 Zahn cup. The final composition has a total solids content of about 53% by weight. The weight ratio of water to cosolvent in the mixture is 87:13. The pigmented coating is applied by a direct-roll coater having urethane rolls of high Durometer rubber (20 Durometer) onto 90 pound tin plated steel as a base white coat for can bodies at 150 feet per minute. The flow and levelling characteristics of the coating composition are very good. The coated panels are baked for 10 minutes at 325°F. Uniformly white, smooth coatings are thus obtained. The composition of this example may also be applied by a mandrel coater.

When each of these examples is repeated omitting the Carbowax, the composition does not exhibit acceptable flow and levelling characteristics.

Claims (22)

46449/2 - 31 - What We Claim is :

1. A thermosettable coating composition comprising an alkaline aqueous blend of a binder comprising: A. A water-soluble salt of tertiary amine and water-insoluble addition copolymer, having a molecular weight of 20,000 to 200,000, the copolymer being derived from a mixture of copolymerizable monomers: (1) at least one ester of a (C^-C^g )-alcohol and an a, β-ethylenically unsaturated carboxylic acid (as herein defined); (2) 1 to 4 percent by weight of olefini- cally unsaturated carboxylic acid (as herein defined) ,. and (3) 3 to 20 percent by weight of at least one hydroxy(C2-C^ )-alkyl ester of an acid of the formula: in which n is 1 or 2, and/or at least one C^-C., )-alkyl ester of alpha methylol acrylic acid, B. a water-soluble condensation product of urea and/or a triazine with formaldehyde or a water-soluble methanol or ethanol ether thereof, C. 1 to 5 percent by weight of poly (ethylene glycol) or poly( ethylene oxide) as water- soluble rheology modifier having a number average molecular weight of from 400 to 100,000, the percentage being based on the total weight of (A) and (B) on a solids bas the weight ratio of A to B on a solids basis being from 85:15 to 65:35, the minimum film-forming temperature of the composition being at most 25°C, the solids content of the composition being between 30 and 80 percent by weight.

2. A composition as claimed in Claim 1 additionally containing pigment..

3. A composition as claimed in Claim 1 or 2 wherein said tertiary amine is 2-(dimethylamino)-ethanol .

4. A composition as claimed in Claim 1 or 2 in which B comprises urea-formaldehyde reaction product or a melamine-formaldehyde reaction product, or a mixture thereof.

5. A composition as claimed in Claim 1 or 2 in which B comprises a reaction product of a urea-formaldehyde adduct and methanol, the mole ratio of urea: formaldehyde :methanol being from 1/1.75/2 to 1/3/3.5; or a melamine-formaldehyde adduct with methanol , the mole ratio of melamine: formaldehyde methanol being from 1/7/12 to 1/10/17 or a mixture thereof.

6. A composition as claimed in any preceding claim wherein (1) comprises an ester of acrylic acid and an alkanol having from 2 to 12 carbon atoms.

7. A composition as claimed in any preceding claim wherein (2) comprises methacrylic acid.

8. A composition as claimed in any preceding claim wherein (3) comprises hydroxyethyl acrylate and/or methacrylate and/or hydroxypropyl acrylate and/or methacrylate .

9. A composition as claimed in any of Claims 6-8 wherein the addition copolymer contains methyl methacrylate.

10. A composition as claimed in any of Claims 6-9 wherein the addition copolymer comprises a polymer of ethyl acrylate, hydroxypropylmethacrylate , methacrylic acid and methyl methacrylate.

11. A composition as claimed in any preceding claim in which the rheology modifier comprises polyethylene glycol having a molecular weight of from 400 to 20,000.

12. A composition as claimed in one of Claims 1-10 in which the rheology modifier comprises poly(ethylene oxide) having a molecular weight of at most 100,000.

13. A composition as claimed in any preceding claim containing from 10 to 20 percent by weight of the solids in the composition of an organic solvent for the uncured polymer mixture.

14. A composition as claimed in any preceding claim containing a pigment, the relative weight of pigment to A plus B being from 5:95 to 60:40, the total being 100.

15. A composition as claimed in any preceding claim which additionally contains a volatile base.

16. A composition as claimed in Claim 15 wherein the volatile base is a tertiary amine.

17. A composition as claimed in Claim 16 wherein the tertiary amine is triethylamine.

18. A composition as claimed in any preceding claim substantially as described in any one of the Examples.

19. A method of coating a substrate which comprises direct-roller coating or mandrel coating the substrate with a composition as claimed in Claim 1, in an amount to provide a final cured layer of 0.05 to 5 mils in thickness, and heating the coating until it is t ermoset.

20. A method as claimed in Claim 19 in which the substrate is metal sheet, which is mechanically shaped after the coating is cured.

21. A method of coating metal sheet which comprises coating the sheet with a. composition as claimed in any of claims , 6-8 in an amount to provide a final cured layer of 0.05 to 5 mils in thickness, heating the coating until it is thermoset, and mechanically shaping the sheet.

22. A substrate whenever coated by a method as claimed in any one of Claims 19-21.