GB2091737A - Cationic resins useful for coating compositions - Google Patents

Abstract

A process for coating substrates comprising applying to the substrates an aqueous dispersion comprising an acid-solubilized reaction product of: (A) an amino group-containing polymer (especially a polyepoxide amine adduct), and (B) a thermally unstable acid of the structure; <IMAGE> wherein at least one member of the group X, Y and Z is selected from the group consisting of thiol, cyano, hydroxyl groups, and an unsaturated hydrocarbyl group in conjugation with the carbonyl group, and each of the other is a hydrogen or a hydrocarbyl group; provided that when either X, Y or Z is hydroxyl, the remaining groups are hydrogen. The coating compositions yield non-corrosion vapours upon baking at cure temperatures.

Description

SPECIFICATION Cationic resins useful for coating compositions The instant invention relates to a method of coating in which the coating compositions are applied to the substrate in their cationic form via non-electrophoretic means such as flow, dip, spray or roll coating.

The method of coating substrates with acid-solubilized resinous coating compositions such as those based on acid-solubilized amino group-containing resins is well known in the art. The adverse effects of using acids in solubilizing resins is also well known. One such effect is the corrosion of metals, particularly ferrous metals. Metal substrates coated with compositions comprising acid-solubilized resins wherein the acid is retained in the coating are prone to corrosion. Also, when the coated substrates are baked, the acid retained can be released into the oven and corrode the metal portions of the oven as well.

The present invention avoids or significantly reduces the aforedescribed adverse effects by formulating coating compositions with solubilizing acids which decompose, e.g., by decarboxylation, upon baking. Hence, the acids which would otherwise be harmful, decompose into non-corrosive materials such as carbon dioxide.

In accordance with the foregoing, the present invention encompasses a process for coating substrates comprising applying to said substrates an aqueous dispersion comprising an acid-solubilized resinous vehicle which is a reaction product of: (A) an amino group-containing polymer, and (B) a thermally unstable acid of the structure:

wherein at least one member of the group X, Y and Z is selected from the group consisting of thiol, cyano, hydroxyl groups, and an unsaturated hydrocarbyl group in conjugation with the carbonyl group, and each of the others is a hydrogen or a hydrocarbyl group; provided that when either X, Y or Z is hydroxyl, the remaining groups are hydrogen.

Further encompassed by the present invention is a method for preventing or reducing corrosion inside baking ovens which contain metallic components which are exposed to vapors generated by baking coated substrates and which are normally corroded by acidic vapours. The method comprises an improved process of baking substrates coated by dipping, spraying or roiling in a coating composition comprising acid-solubilized resinous vehicles, to prevent or reduce corrosion of the baking oven; the improvement which comprises coating said substrates with an acid-solubilized resinous vehicle which is the reaction product of: (A) an amino group-containing polymer, and (B) a thermally unstable acid of the structure:

wherein at least one member of the group X, Y and Z is selected from the group consisting of thiol, cyano, hydroxyl groups, and an unsaturated hydrocarbyl group in conjugation with the carbonyl group, and each of the others is a hydrogen or a hydrocarbyl group; provided that when either X, Y or Z is hydroxyl, the remaining groups are hydrogen.

The thermally unstable acid preferred herein is cyanoacetic acid.

The coating compositions used in the method of the present invention can be prepared by reacting an amino group-containing polymer, described more fully hereinafter, with a thermally unstable acid which characterizes this invention. The nature and the method of preparation of the amino groupcontaining polymers are well known in the art. Some examples of the starting polymers are amine adducts of polyepoxides containing a 1 ,2-epoxy functionality of 2 or more such as polyglycidyl ethers and acrylic polymers containing epoxy groups. Also, amino group-containing acrylic polymers prepared by polymerizing a mixture of vinyl monomers, a portion of which contain amino functionality, can be used.

Examples of polyglycidyl ethers are polyglycidyl ethers of cyclic polyols, which are preferred, such as polyphenols, more particularly bisphenol A. These can be produced, for example by etherification of a polyphenol with an epihalohydrin or dihalohydrin such as epichlorohydrin or dichlorohydrin in the presence of alkali. The polyphenol may be, for example, bis-2,2-(4-hydroxyphenyl) propane; 4,4'dihydroxybenzophenone; bis-1 , 1 -(4-hydroxyphenyl) ethane; bis-l,l -(4-hydroxyphenyl) isobutane; bis2,2-(4-hydroxytertiarybutyl-phenyl) propane; bis-(2-hydroxynaphthyl) methane; 1,5dihydroxynaphthalene or the like.

Besides the polyglycidyl ethers mentioned above, another class of epoxy-containing polymers which may be employed is acrylic polymers which contain epoxy groups. These polymers are formed by polymerizing an unsaturated epoxy-containing monomer such as glycidyl acrylate or methacrylate by itself or with one or more other polymerizable ethylenically unsaturated monomers. Examples of other ethylenically unsaturated polymerizable epoxy-containing monomers are allyl glycidyl ether, 4-vinyl cyclohexane monoepoxide, butadiene monoepoxide, vinyl glycidyl phthalate, allyl glycidyl maleate, allyl glycidyl phthalate and the like.

Examples of the ethylenically unsaturated polymerizable monomers are those having at least one

group. Examples of such monomers include vinyl acetate, methyl acrylate, ethyl acrylate, methyl methacrylate, acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, styrene, 1 ,3-butadiene, isoprene, 2-chloro-1 ,3-butadiene, vinyl chloride, vinyl fluoride, isopropenyl acetate, vinylidene chloride, methyl vinyl ether, acrolein, methyl vinyl ketone, hydroxyethyl acrylate or methacrylate and hydroxypropyl acrylate or methacrylate.

Examples of epoxy resins including the polyglycidyl ethers and epoxy-containing acrylic polymers are described in U.S. Patent No. 4,141,810 to Buchwalter et al, column 2, line 60, to column 4, line 13, the portions of which are herein incorporated by reference.

The amino compound which is reacted with the epoxy-containing polymer can be a primary, secondary or tertiary amine compound, including polyamines, or mixtures thereof. The amino group reacts with the oxirane ring in the epoxy-containing polymer to form the reaction product containing the higher alkylated amine moiety and a secondary hydroxyl moiety.

With regard to primary and secondary amine compounds, the preferred ones are those containing hydroxyl groups. Examples of hydroxyl-containing primary and secondary amines are alkanolamines, dialkanolamiones, N-alkylalkanolamines containing from 2 to 1 8 carbon atoms in the alkanol and alkyl chains, and N-arylalkanolamines. Specific examples include ethanolamine, N-methylethanolamine, diethanolamine and N-phenylethanolamine.

Primary and secondary amines which do not contain hydroxyl groups such as alkylamines, arylamines, dialkylamines and arylalkylamines and substituted amines, in which the substituents are other than hydroxyl, and in which the substituents do not detrimentally affect the epoxy-amine reaction can also be used. Mixtures of the various amines described above can also be used.

Polyamines such as ethylenediamine, diethylenetriamine, triethylene tetramine, N-(2aminoethyl)ethanolamine and piperazine can be used but their use is not preferred because they are multi-functional and have a greater tendency to gel the reaction mixture than monoamines.

For reasons of gelation, secondary amines, particularly secondary monoamines, are preferred to primary amines because the primary amines are difunctional and have a greater tendency to gel the reaction mixture. When using polyamines or primary amines, special precautions should be taken to avoid gelling. For example, the polyamine can be converted to ketimines which are relatively less prone to gelation. Also, excess amine can be used and the excess vacuum stripped at the completion of the reaction. By yet another precautionary approach, the epoxy-containing polymer can be added to the amine in a manner than ensures that excess amine will be present.

The reaction of the primary and/or secondary amine with the epoxy-containing polymer takes place upon mixing the amine with the polymer. The reaction can be conducted neat, or, optionaliy, in the presence of a suitable solvent. Reaction may be exothermic and cooling may be desired. However, heating to a moderate temperature, that is within the range of 50-1 50 C., may be used to hasten the reaction.

The amino group-containing polymers, thus obtained, attain cationic character through neutralization with the thermally unstable acids of this invention to form the corresponding amine salts.

The extent of neutralization will depend on the particular product involved. It is only necessary that sufficient acid be added to solubilize or disperse the product in water. Typically, the amount of acid used will be sufficient to provide about 30 to 100 percent of the total theoretical neutralization.

With regard to tertiary amine compounds, the preferred ones are hydroxyl-containing tertiary amines such as N,N-dialkylakanolamines, N-alkyl dialkanolamines and trialkanolamines which contain from 2 to 18 carbon atoms in the alkyl and alkanol chain. Examples of such amines include the following: N,N-dimethylethanolamine, N-methyldiethanolamine, triethanolamine and triisopropanolamine. Tertiary amine containing aryl groups such as N-phenyldiethanolamine can also be used.

Tertiary amines which do not contain hydroxyl groups such as alkylamines, arylalkalylamines and substituted alkyl and aryl tertiary amines which contain substituents other than hydroxyl and in which the substituents do not determinally affect the epoxy-tertiary amine reaction can be used. Examples include N,N-dimethylcyclohexylamine, trimethylamine, triethylamine and N-benzyldimethylamine.

Mixtures of the various tertiary amines described above can also be used.

The tertiary amine is reacted with the thermally unstable acids to form the salt and the tertiary amine salt can be reacted with the epoxy-containing polymer to form a quaternary ammonium saltcontaining polymer. The reaction is conducted by mixing the amine salt and the epoxy-containing polymer preferably in the presence of a controlled amount of water. Typically, the water is employed on the basis of about 1.75 to about 20 percent by weight based on total reaction mixture solids.

In forming the quaternary ammonium base group-containing polymers, the reaction temperatures can be varied between the lowest temperature at which reaction reasonably proceeds, for example, room temperature, or in the usual case, slightly above room temperature, to a maximum temperature of 1000C. (at atmospheric pressure). At greater than atmospheric pressure, higher reaction temperature can be used. Preferably, the reaction temperature ranges between about 60---1000C.

A solvent for the reaction is usually not necessary although a solvent such as an ester, ether or ketone may be used if desired.

Alternatively, the tertiary amines can be reacted with the epoxy-containing polymers in the presence of water to form the quaternary ammonium hydroxide group-containing polymer.

The quaternary ammonium hydroxide group-containing polymers are then subsequently acidified with the thermally unstable acids of this invention to form quaternary ammonium salt groups.

For the amino group-containing polymers derived from epoxy resins, the amount of amine and epoxy-containing polymer which are reacted with one another depend upon the extent of cationic group formation desired and this in turn will depend on the molecular weight of the polymer. The extent of cationic 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 sediment or is one which is easily redispersible if some sedimentation occurs.

There are yet other amino group-containing polymers useful in this invention. Examples include those described in British Patent 1,303,480, page 1, line 76, to page 2, line 6; page 2, line 30, to page 3, line 29, the portions of which are herein incorporated by reference.

The coating compositions of this invention are characterized in that the solubilizing acids useful herein are thermally unstable upon baking, that is, when the acid-solubilized amino group-containing polymers of the present invention are formulated into a coating composition and coated on a substrate by non-electrophoretic means and the coated substrate is baked, for example, at about 300--3750F.

(1 49-1 91 OC), only non-corrosive vapors, preferably those having a pH of about 6.5 or greater, more preferably 7.0 or greater, are generated. The non-corrosive vapors can be generated by the acid decomposing to CO2 or other non-corrosive constituents. Although CO2 could perhaps form a slightly acidic vapor, that is, some carbonic acid, this has not been found to be corrosive, particularly in view that the amino group-containing polymer may release some amine which would neutralize the slight acidity of the carbonic acid. Besides decomposing, the solubilizing acids could rearrange during baking, for example, by cyclizing, or could react with certain functionalities in the film. Acids that decarboxylate, for example, cyanoacetic acid, are preferred herein.

The thermally unstable acids are of the structure:

wherein at least one member of the group X, Y and Z is selected from the group consisting of thiol, cyano, hydroxyl groups, and an unsaturated hydrocarbyl group in conjugation with the carbonyl group, and each of the others is a hydrogen or a hydrocarbyl group; provided that when either X, Y or Z is hydroxyl, the remaining groups are hydrogen.

Illustrative of the unsaturated hydrocarbyl group in conjugation with the carbonyl carbon is an acetylenic or a vinylic group. The unsaturated hydrocarbyl group, herein contains at least two carbon atoms; the limitation on the carbon content is generally dependent on factors such as solubility, handling, processing and application conditions.

Wherein X, Y or Z is a hydrogen or a hydrocarbyl group, the carbon content of the hydrocarbyl group is generally from about 1 to 1 8 carbon atoms, preferably it is from about 1 to 10, more preferably it is from about 1 to 6, and most preferably about 1 to 4 carbon atoms. Illustrative thereof are lower alkyl, and aryl groups. Herein also, the limitation on the carbon content is generally dependent on factors such as solubility handling, processing and application conditions.

As used herein, the therm "hydrocarbyl group" may comprise any substituent provided that the substituent does not adversely affect the decomposition of the instant acids, or otherwise adversely affect the desirable solution of film-forming properties of the coating compositions derived therefrom.

Conceivably, the hydrocarbyl group may contain a carboxyl group, thus giving rise to a polycarboxylic acid such as a diacid.

In accordance with the foregoing, the preferred thermally unstable acid is selected from the group consisting of cyanoacetic acid, glycolic acid and thioglycolic acid.

Cyanoacetic acid is particularly preferred; it has been found to decarboxylate at comparatively lower temperatures. For this reason it is particularly suited for use in low temperature cure coating compositions.

Generally, the amount of the thermally unstable acids used is that which is sufficient to neutra!ize the amino group of the polymer so as to form good dispersions. At least about 20 percent, and usually 30 to 100 percent of the total theoretical neutralization of the amine moiety is used.

The acid-solubilized resinous vehicles of the present invention are usually present in coating compositions with curing agent. When used with curing agents, it is preferred that the polymers also contain active hydrogens which are reactive at elevated temperatures with a curing agent. Examples of active hydrogens are hydroxyl, thiol, primary amine, secondary amine (including imine) and carboxyl, with hydroxyl being preferred.

The curing agents are those which are capable of reacting with the active hydrogens to form a crosslinked product. Examples of suitable curing agents are phenolic resins, aminoplasts and polyisocyanates. The polyisocyanates should be capped or blocked so that they will not prematurely react with the active hydrogens.

Suitable aminoplasts for use in the invention are described in U.S. Patent 3,937,679 to Bosso et al in column 16, line 3, continuing to column 17, line 47, the portion of which is hereby incorporated by reference. As disclosed in the aforementioned portion of the '679 patent, the aminoplast can be used in combination with methylol phenol ethers. The aminoplast curing agent usually constitutes about 1-60 and preferably 5-40 percent by weight of the resinous composition based on total weight of the acidsolubilized resinous vehicle and the aminoplast.

With regard to the capped or blocked polyisocyanate curing agents, these are described in U.S.

Patent 4,104,147, column 7, line 36, continuing to column 8, line 37, the portion of which is hereby incorporated by reference. Sufficient capped or blocked polyisocyanate is present in the coating system such that the equivalent ratio of latent isocyanate groups to active hydrogens is at least 0.1:1 and preferably about 0.3 to 1:1.

While referred to as "solubilized", the resinous vehicles of this invention, typically present in water, are considered to be a complex solution, dispersion or suspension or combination of one or more of these classes in water. The term "dispersion" as used within the context of the present invention is believed to be a two-phase, transparent, translucent or opaque aqueous resinous system in which the resin is the dispersed phase and water is the continuous phase. Average particle size diameter of the resinous phase is generally less than 10, preferably less than 5 microns. The concentration of the resinous phase in the aqueous medium depends upon the particular end use of the dispersion and in general is not critical. For example the aqueous dispersion preferably contains at least 1 and usually from about 5 to 50 percent by weight resin solids.

Besides water, the aqueous medium may contain a coalescing solvent. The use of coalescing solvent may be, in some instances, for improved film appearance. These solvents include hydrocarbons, alcohols, esters, ethers and ketones. The preferred coalescing solvents include monoalcohols, glycols and polyols as well as ketones and other alcohols. Specific coalescing solvents include isopropanol, butanol, isophorone, 4-methoxy-2-pentanone, ethylene and propylene glycol, the monoethyl monobutyl and monohexyl ethers of ethylene glycol and 2-ethylhexanol. The amount of coalescing solvent is not unduly critical and is generally between 0.01 and 30 percent by weight, preferably about 0.05 to about 25 percent by weight based on total weight of aqueous medium.

In most instances, a pigment composition and, if desired, various additives such as surfactants or wetting agents are included in the dispersion. Pigment composition may be any of the conventional types comprising, for example, iron oxides, lead oxides, strontium chromate, carbon black, coal dust, titanium dioxide, talc, barium sulfate, as well as colour pigments such as cadmium yellow, cadmium red, chromium yellow and the like. Pigment content of the dispersion, usually expressed as pigment-toresin ratios, is usually within the range of 0.1 to 5:1. The other additives mentioned above are present in the dispersion in amounts of 0.01 to 3 percent by weight based on total weight of resin solids.

In the practice of this invention, aqueous dispersions or solutions of the coating compositions are used to provide corrosion-resistant substrates. The process for imparting corrosion resistance to substrates comprises coating said substrates with the instant acid-solubilized resinous vehicles by nonelectrophoretic means such as dipping, which is preferred, spraying, brushing, rolling or wiping the surface either at room temperature or at elevated temperature.

With regard to the dip coating process, the times and temperatures of immersion may vary depending on the composition and its concentration, and on the identity of the metal substrate which is being treated. In general, the metal article should be immersed at a bath temperature at about 250 C. to 500C., preferably 400 to 450C., usually, for about 5 seconds to 5 minutes, followed by removal of the article from the bath. The article is then dried. Preferably, the article is dried with forced air, and then baked at elevated temperature ranging from about 2750F. (1 350C) to 5000 F. (2600C) for about 30 seconds to 45 minutes.The time and temperature are related in that lower temperatures require longer times and higher temperatures require shorter times.

Upon baking, it is believed that solubilizing acid converts into non-acidic components. This can be shown by measuring the pH of the volatiles at baking temperatures.

The advantages of the present invention are as described herein. In the main, they are preventing or reducing corrosion of substrates and ovens.

The invention is further described by the following examples which are to be considered as illustrative rather than limiting. All parts and percentages in the examples and throughout this specification are by weight unless otherwise specified.

EXAMPLE I This example illustrates the preparation of a cyanoacetic acid-solubilized resinous composition.

The following were used in the preparation of the resinous composition: Parts by Weight Ingredients (Grams) EPON 828' 752 PCP02002 282.1 Xylene 65.6 Bisphenol A 211.8 Benzyldimethylamine 1.7 Benzyldimethylamine 3.2 Blocked isocyanate3 962.9 Keimine4 80.4 N-methylethanolamine 63.2 Polytetramethylene glycol5 204.2 1Polyglycidyl ether of bisphenol A having a molecular weight of about 380 and an epoxy equivalent of about 190, which is available from the Shell Chemical Company.

2Polycaprolactonediol sold commercially by Union Carbide Corporation; this product has a molecular weight of 543.

3Made by reacting one equivalent of toluene diisocyanate with one-half equivalent of the monobutyl ether of ethylene glycol, and then with one-half equivalent of trimethylolpropane. The blocked isocyanate was then dissolved in a 90/10 weight mixture of methylisobutyl ketone and butanol (70% by weight urethane solids).

4Methylisobutyi diketimine of diethylene triamine at about 73 percent solids in methylisobutyl ketone.

5Available from Quaker Oats as POLYMEG 650.

Into a suitable reaction vessel were charged the EPON 828, PCP 0200 and xylene. The reaction mixture was heated with a continuous reflux for about an hour and fifteen minutes, to a temperature of about 21 60C., and then cooled. At about 1 520C., the bisphenol A was added. About 23 minutes thereafter, and at a temperature of 1 530C., the first portion of the benzyldimethylamine catalyst was added and the temperature held above 1 500C. for one-half hour. The reaction mixture was cooled to 1 300 C., the second portion of the benzyidimethylamine was added, and the reaction mixture was held at that temperature for about 5 hours.The blocked isocyanate, the ketimine and the N methylethanolamine were then added and the reaction mixture held at about 11 00C. for one hour. The polytetramethylene glycol was added and the reaction mixture was allowed to cool to 11 50C. There was then obtained the desired resinous composition.

The resinous composition was then solubilized with cyanoacetic acid at 64 percent neutralization and 32 percent solids by reverse thinning it in water. The resultant aqueous dispersion was then vacuum stripped of solvent to give a resin solids of 37.7 percent.

EXAMPLE II This example illustrates the preparation of a glycolic acid-solubilized resinous composition.

The charge used in preparing the resinous composition was as follows: Ingredients Parts by Weight EPON 8291 727.6 Hydroxyl-terminated neopentyl glycol adipate (OH value = 221, MW --500) 254.7 Xylene 35.4 Bisphenol A 197.8 Benzyldimethylamine 1.6 Benzyldimethylamine 2.5 Blocked isocyanate2 982.9 Ketimine 73.6 N-methylethanolamine 59.1 2-hexoxyethanol 123.0 1An epoxy resin solution made from reacting epichlorohydrin and bisphenol A having an equivalent weight of 193-202, commercially available from Shell Chemical Company.

2Made by reacting one equivalent of toluene diisocyanate with one-half equivalent of 2ethylhexanol, and then with one-half equivalent of trimethyloipropane. The blocked isocyanate then is dissolved in a 90/10 weight mixture of methylisobutyl ketone and butanol (70% by weight urethane solids).

The resinous composition was prepared essentially in the manner of Example II and dispersed in water as follows: 2100 grams of the resinous composition was reverse thinned into a mixture of 37.6 grams of glycolic acid and 42.0 grams of a surfactant mixture. The surfactant mixture consisted of 50 percent SURFYNOL 104 (available from Air Products and Chemicals Inc.) and 50 percent of cocoimidazoline neutralized with acetic acid, dissolved in a 50/50 by volume mixture of water and ethylene glycol monobutyi ether. The resulting dispersion having 53 percent solids was thinned with 1 560.8 grams of water to form an aqueous dispersion having a resin solids content of 35.6 percent.

This dispersion was further thinned to 32 percent solids with water and vacuum stripped to remove solvent. The solids content of the vacuum-stripped dispersion was 36.2 percent.

COMPARATIVE EXAMPLE A resinous composition was prepared, acid-solubiiized and dispersed to form an aqueous dispersion in essentially the same manner as described in Example II, except that the acid used was acetic acid instead of glycolic acid.

The following was used in the preparation: Ingredients Parts by Weight (Ibs.) EPON 95.2 PCP 0200 36.0 Bisphenol A 26.8 Ingredients Parts by Weight (Ibs.) Benzyldimethylamine 0.5 Blocked isocyanate of Example II 121.3 Ketimine of Example 1 10.4 N-methylethanolamine 8.0 Polytetramethylene glycol 26.0 Xylene 35.0 Acetic acid 4.4 Surfactant mixture of Example II 7.0 Deionized water 534.5 A dispersion of 36 percent resin solids content was obtained.

EXAMPLE II The aqueous resinous dispersions of Examples I and II were pigmented and formulated into paints in an identical manner. The paints and a clear coating composition of the comparative example were placed in separate containers and heated to curing temperatures. The pH of the volatiles was monitored with time. Specifically, the test was conducted as follows: A 1/8-inch nichrome steel tube was connected to a lid of a sealed 4-ounce glass jar. A one-ounce sample of each resin was placed in the jar, the jar sealed, and placed in an oven set at 3850F. (1 960C).

The nichrome steel tube was run out of the oven. The vapors exiting the tube were tested with wet litmus paper every two minutes for the first ten minutes and every five minutes for the next 20 minutes.

The results of these tests are reported in Table I below.

TABLE I pH of Vapours of the Acid-Solubilized Time of pH Resin of: Measurement Comparative in Minutes Example Example I Example II +2:00 7.0 7.0 7.0 +4:00 7.0 7.0 7.0 +6:00 6.5 7.0 7.0 +8:00 5.5 7.0 7.0 +10:00 5.0 7.0 7.0 +15:00 5.0 7.0 7.0 +20:00 4.5 7.0 7.0 +25:00 6.5 7.0 7.0 +30:00 5.0 7.5 7.0

Claims (11)

1. A process for coating substrates comprising applying to the substrates an aqueous dispersion comprising an acid-solubilized reaction product of: (A) an amino group-containing polymer, and (B) a thermally unstable acid of the structure;

wherein at least one member of the group X, Y and Z is selected from the group consisting of thiol, cyano, hydroxyl groups, and an unsaturated hydrocarbyl group in conjunction with the carbonyl group, and each of the other is a hydrogen or a hydrocarbyl group; provided that when either X, Y or Z is hydroxyl, the remaining groups are hydrogen.

2. A process as claimed in claim 1, wherein the hydrocarbyl group contains from about 1 to 1 8 carbon atoms.

3. A process as claimed in claim 1 or 2, wherein the acid is a cyanoacetic acid, glycolic acid or thioglycolic acid.

4. A process as claimed in any of claims 1 to 3, wherein the acid-solubilized resinous vehicle is selected from the group consisting of amine salt group-containing resinous vehicles and quaternary ammonium salt group-containing resinous vehicles.

5. A process as claimed in any of claims 1 to 3, wherein the acid-solubilized resinous vehicle comprises an acid-solubilized amine containing acrylic polymer.

6. A process as claimed in any of claims 1 to 3, wherein the acid-solubilized resinous vehicle comprises an acid-solubilized polyepoxide-amine adduct.

7. A process as claimed in claim 6, wherein the polyepoxide is a polyglycidyl ether of a cyclic polyol.

8. An article coated by a process as claimed in any of claims 1 to 7.

9. A ferrous metal article coated by a process as claimed in any of claims 1 to 7.

10. In a process of baking substrates coated by dipping, spraying or rolling a composition comprising an acid-solubilized resinous vehicle, in an oven which contains metallic components which are exposed to the vapors generated by baking and which are normally corroded by acidic vapors, the improvement which comprises effecting this coating of said substrates with an acid-solubilized resinous vehicle by the process claimed in any of claims 1 to 7.

11. A process as claimed in claim 1 substantially as hereinbefore described in any one of the Examples.