Process for Electrocoating Water Reducible Quaternary Ammonium Salt Containing Polymers

1. Field of the Invention

This invention relates to a water dispersible coating composition which comprises:

(i) an acrylic polymer having active hydrogen functionality and quaternary ammonium salt functionality; and

(ii) a blocked polyisocyanate.

The water dispersible coating composition of this invention can be applied by any means known within the coatings art, but is especially suited for use in cathodic electrodeposition.

2. Description of the Prior Art

The use of quaternary ammonium salt containing polymers as water reducible coatings has been known in the art. For example, U.S. Pat. No. 3,935,087 teaches resins prepared by reacting a portion of the hydroxyl groups of a hydroxyl-containing epoxy group-containing organic material with a partially-capped or blocked organic polyisocyanate, followed by reacting at least a portion of the epoxy groups in a manner so as to add pendant quaternary onium salt groups to render the resin water dispersible or solubilized.

U.S. Pat. No. 4,071,428 teaches electrodepositable compositions which comprise the reaction product of a polymeric tertiary amine and a 1,2 epoxy containing material in the presence of acid and/or water to form quaternary ammonium base group-containing polymers. This approach has the drawback that the epoxy could also react with other functional groups on the polymer backbone, such as hydroxyl groups or primary and secondary amine groups.

This invention relates to a water dispersible coating composition which comprises:

(i) an acrylic polymer which comprises the free radical addition product of (a) at least one quaternary ammonium salt containing monomer having the structure: ##STR1## wherein R.sub.1 is H or alkyl of 1 to 3 carbons, Z is N-H or O; R.sub.2 is a divalent methylene radical --(CH.sub.2).sub.n -- wherein n is an integer from 1 to about 10; R.sub.3 and R.sub.4 can be the same or different and as alkyl or substituted alkyl, preferably of 1 to about 20 carbons or aryl or substituted aryl, preferably of 6 to about 18 carbons and R.sub.5 and R.sub.6 can be the same or different and are hydrogen, alkyl or substituted alkyl, preferably of 1 to about 20 carbons or aryl or substituted aryl, preferably of 6 to about 18 carbons; and X.sup.- is an anion; and (b) at least one active hydrogen functional unsaturated monomer; and (c) at least one other ethylenically unsaturated monomer; and

(ii) a blocked polyisocyanate which is stable in the presence of the acrylic polymer at room temperature but reactive with the acrylic polymer at elevated temperatures.

It is especially preferred to have R.sub.6 be hydrogen and R.sub.3, R.sub.4 and R.sub.5 be lower alkyl of 1 to 4 carbons. Substituted alkyl or aryl groups are those having one or more hydrogen atoms replaced with alkyl, aryl, aroxy, alkoxy or halogen groups and can contain acyl groups.

The quaternary ammonium salt groups on the acrylic polymer backbone have a catalytic effect providing lower temperature cures with the blocked polyisocyanate.

This invention also relates to a method of electrocoating an electrically conductive surface serving as a cathode in an electrical circuit comprising said cathode, an anode, and an aqueous electrodepositable composition wherein the aqueous electrodepositable composition comprises the coating described above.

Accordingly, it is an object of this invention to provide improved water reducible coatings. It is a further object of this invention to provide water reducible coatings having excellent exterior durability. Another object of this invention is to provide water reducible coatings having excellent low temperature cure response with blocked isocyanates. Another object of this invention is to provide a method of electrocoating an electrically conductive surface. These and other objects of this invention will be apparent from the following discussions.

The acrylic polymers useful in the practice of this invention contain one or more active hydrogen containing groups as determined by the Zerewitinoff Method as described by Kohler in J. Am. Chem. Soc, 49, 3181 (1927). Some such suitable compounds are, for example, acrylic polymers made from unsaturated monomers which contain one or more of any of the following types of active hydrogen containing groups, among other, --OH, --NH.sub.2, --NH, --CONH--, --COOH, --SH and the like. It is especially preferred in the practice of this invention to use a hydroxy functional monomer as the active hydrogen functional monomer.

The acrylic polymers are prepared by conventional free radical addition polymerization techniques and can use any suitable unsaturated monomers. Methods of producing acrylic polymers are well known and are not a part of the present invention. If desired the free radical polymerization can be catalyzed by conventional catalysts known in the art such as azo, peroxy or redox catalysts. Typically the acrylic polymers are produced by heating the unsaturated monomers at temperatures ranging from about 180.degree. F. to 450.degree. F. and especially 200.degree. F. to about 300.degree. F. to effect the polymerization. It is normally preferred to prepare the acrylic polymers by solution polymerization in organic solvents. Solvents having a solubility of at least about 30 grams per liter of water are preferred. Especially preferred are the glycol ethers and glycol ether esters such as ethylene glycol monoethyl ether, diethylene glycol monobutyl ether, 2-ethoxyethyl acetate etc. If it is desired to achieve a relatively low molecular weight acrylic polymer, a chain transfer agent such as a mercaptan may be employed to achieve this result.

Especially preferred in the practice of this invention is an acrylic polymer which comprises the free radical addition product of (a) from about 1 to about 10% of at least one quaternary ammonium salt containing monomer having the structure: ##STR2## wherein R.sub.1 is H or alkyl of 1 to 3 carbons, Z is N-H or O; R.sub.2 is a divalent methylene radical --(CH.sub.2).sub.n -- wherein n is from 1 to about 10; R.sub.3 and R.sub.4 can be the same or different and as alkyl or substituted alkyl, preferably of 1 to about 20 carbons or aryl or substituted aryl, preferably of 6 to about 18 carbons and R.sub.5 and R.sub.6 can be the same or different and are hydrogen, alkyl or substituted alkyl, preferably of 1 to about 20 carbons or aryl or substituted aryl, preferably of 6 to about 18 carbons; and X.sup.- is an anion; and (b) from about 1 to about 20% of at least one tertiary amine containing unsaturated monomer; and (c) from about 5 to about 30% of at least one active hydrogen functional unsaturated monomer; and (d) from about 40 to about 93% of at least one other ethylenically unsaturated monomer.

The quaternary ammonium salt containing unsaturated monomers having the structure (I) are readily prepared by reacting tertiary amines having the structure: ##STR3## with 1,2 epoxies having the structure: ##STR4## in the presence of water and/or acid. When this reaction takes place in the presence of water, the anion X.sup.- will typically be the hydroxide ion, and when this reaction takes place in the presence of acid the anion X.sup.- will typically be the anion obtained by removing a hydrogen atom from the acid. Other methods known in the art for preparing quaternary ammonium salts such as alkylation of tertiary amines can also be used to prepare the monomers of this invention.

An especially preferred unsaturated monomer having this structure is: ##STR5## which can be prepared by the reaction of ethylene oxide and dimethylaminopropyl methacrylate in the presence of acetic acid. Other useful monomers include, for example, the quaternary ammonium salt containing unsaturated monomers which are the reaction product obtained by admixing propylene oxide and dimethylaminoethyl acrylate in the presence of water or aqueous acid.

The quaternary ammonium containing unsaturated monomers of this invention are readily prepared by mixing approximately stoichiometrically equivalent amounts of the tertiary amine acid salt and the epoxide in the presence of water and maintaining the reaction at temperatures between about 30.degree. to about 110.degree. C. Alternatively, the amine and the epoxy can be reacted first followed by acidification. Any suitable water soluble acid can be used including inorganic acids such as phosphoric or sulfuric, as well as organic acids such as acetic acid, lactic acid, citric acid etc. The organic acids are especially preferred. When low molecular weight epoxies, such as ethylene oxide or propylene oxide are used it is generally preferred to maintain the reaction mixture at elevated pressure and at lower reaction temperatures to minimze the volatility of the epoxies.

Useful active hydrogen functional unsaturated monomers include the hydroxy-functional monoesters typically prepared by the reaction of diols with unsaturated acids such as hydroxyesters of acrylic or methacrylic acid wherein the hydroxyalkyl group contains up to about 6 carbon atoms such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, diethylene glycol methacrylate and so forth. Corresponding esters of other unsaturated acids, for example, crotonic acid, maleic acid, and similar acids of up to about 6 carbon atoms can also be employed. Also useful as active hydrogen functional unsaturated monomers are the ethylenically unsaturated amides such as acrylamide, methacrylamide, and other similar materials.

If desired, primary or secondary amine functionality can also be incorporated as active hydrogen functionality into the acrylic polymer. Primary or secondary amines will impart a cationic charge to the polymer when neutralized by an acid and, by virtue of their active hydrogens, also become a site for accepting cross-links when the coating containing the blocked isocyanate is cured.

Typical primary and secondary amine compounds which can be incorporated into the acrylic polymers include amine-containing esters of monofunctional unsaturated acids of the formula: ##STR6## wherein R.sub.7 is hydrogen, alkyl of 1 to 10 carbon atoms, or aryl of 6 to about 12 carbon atoms; R.sub.8 is --(CH.sub.2).sub.n -- with n having a value of about 1 to 6; and R.sub.9 is hydrogen or an alkyl group of from 1 to about 3 carbon atoms. Examples of such materials include acrylic or methacrylic esters of alcohols having pendant primary or secondary amines such as the esters of ethanol amine, t-butylamino ethanol, and the like.

Useful tertiary amine containing unsaturated monomers include the acrylates, such as 2-(N,N-dimethylamino)ethylacrylate, 2-(N,N-diethylamino)ethylacrylate, 2-(N,N-dimethylamino)propylacrylate and the like; the methacrylates such as 2-(N,N-dimethylamino)ethylmethacrylate, 2-(N,N-diethylamino)ethylmethacryl ate and the like; the acrylamides, such as 2-(N,N-dimethylamino)ethylacrylamide and 2-(N,N-dimethylamino)ethylmethacrylamide and the like; vinyl compounds such as N-vinyl piperidine and the like.

When the acrylic polymer contains primary, secondary or tertiary amine groups, these groups can be, if desired, neutralized with acid to provide additional water solubility and to facilitate deposition upon the cathode during cathodic electrodeposition. The total amount of quaternary ammonium salt and/or neutralized amine need only be sufficient to provide water dispersibility to the polymer. If it is desired to neutralize some of the primary, secondary or tertiary amine groups in the polymer this can be readily accomplished by reacting all or part of the amino groups with an acid such as acetic acid, lactic acid, phosphoric acid, formic acid, citric acid or the like.

Other useful ethylenically unsaturated monomers include those which are substantially free of tertiary amine or quaternary ammonium salt functionality. These include the unsaturated hydrocarbons such as isoprene, butadiene, ethylene, styrene, substituted styrenes, etc.; vinyl compounds such as vinyl toluene, vinyl acetate, vinyl chloride, etc; esters of unsaturated acids such as butyl acrylate, butyl methacrylate, methyl methacrylate, isobutyl methacrylate, and so forth.

The blocked isocyanates which may be employed in this invention may be any isocyanate where the isocyanate groups have been reacted with a compound so that the resultant blocked isocyanate is stable in the presence of the water reducible acrylic polymer at room temperature but is reactive with the acrylic polymer at elevated temperatures, usually between about 150.degree. F. and about 600.degree. F. The blocked polyisocyanate will typically be at a level of about 0.3 to about 3.0 equivalents of isocyanate for each equivalent of active hydrogen.

In the preparation of the blocked organic polyisocyanate, any suitable organic polyisocyanate may be used. Representative examples are the aliphatic compounds such as trimethylene, tetramethylene, pentamethylene, hexamethylene, 1,2-propylene, 1,2-butylene, 2,3-butylene, 1,3-butylene, ethylidine and butylidene diisocyanates; the cycloalkylene compounds such as 3-isocyanotomethyl-3,5,5-trimethylcyclohyexylioscyanate, and the 1,3-cyclopentane, 1,4-cyclohexane, and 1,2-cyclohexane diisocyanates; the aromatic compounds such as m-phenylene, p-phenylene, 4,4'-diphenyl, 1,5-naphthalene and 1,4-naphthalene diisocyanates; the aliphatic-aromatic compounds such as 4,4'-diphenylene methane, 2,4- or 2,6-toluene, or mixtures thereof, 4,4'-toluidine, and 1,4-xylylene diisocyanates; the nuclear substituted aromatic compounds such as dianisidine diisocyanate, 4,4'-diphenylether diisocyanate and chloro-diphenylene diisocyanate; the triisocyanates such as triphenyl methane-4,4',4'-triisocyanate, 1,3,5-triisocyanate benzene and 2,4,6-triisocyanate toluene; and the tetraisocyanates such as 4,4'-diphenyl-dimethyl methane-2,2'-5,5'-tetraisocyanate; the polymerized polyisocyanates such as tolylene diisocyanate dimers and trimers, and other various polyisocyanates containing biuret linkages and/or allophanate linkages.

The blocked isocyanate may also be a prepolymer prepared by reacting a partially blocked isocyanate with an active hydrogen containing molecule. For instance, the prepolymer may be derived from a polyol including polyether polyols, or polyester polyols, which are reacted with excess partially blocked polyisocyanates. In this procedure, a less than stoichiometric amount of blocking agent is added to a polyfunctional isocyanate thereby blocking some but not all of the isocyanate groups available. The remaining isocyanate groups can then be reacted with a compound containing at least two active hydrogens to prepare the prepolymer.

As the blocking agent, any of the blocking agents known in the art may be used in the practice of this invention. Typical examples of them are phenol types (e.g. phenol, cresol, xylenol, nitrophenol, chlorophenol, ethyl phenol, t-butylphenol, hydroxy benzoic acid, hydroxy benzoic acid esters, 2,5-di-t-butyl-4-hydroxy toluene, etc.), lactam types (e.g. .epsilon.-caprolactam, .delta.-valerolactam, .gamma.-butyrolactam, .beta.-propiolactam, etc.), active methylene types (e.g. diethyl malonate, dimethyl malonate, ethyl acetoacetate, methyl acetoacetate, acetyl acetone, etc.), alcohol types (e.g. methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, n-amyl alcohol, t-amyl alcohol, lauryl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, methoxymethanol, glycolic acid, glycolic acid esters, lactic acid, lactic acid esters, methylol urea, methylol melamine, diacetone alcohol, ethylene chlorohydrine, ethylene bromhydrine, 1,3-dichloro-2-propanol, acetocyanhydrine, etc.), mercaptan types (e.g. butyl mercaptan, hexyl mercaptan, t-butyl mercaptan, t-dodecyl mercaptan, 2-mercapto-benzothiazole, thiophenol, methyl thiophenol, ethyl thiophenol, etc.), acid amide types (e.g. acetoanilide, acetoanisidine amide, acrylamide, methacrylamide, acetic amide, stearic amide, benzamide, etc.), imide types (e.g. succinimide, phthalimide, maleimide, etc.), amine types (e.g. diphenylamine, phenylnaphthylamine, xylidine, N-phenyl xylidine, carbazole, aniline, naphthylamine, butylamine, dibutylamine, butyl phenylamine, etc.), imidazole types (e.g. imidazole, 2-ethylimidazole, etc.), urea types (e.g. urea, thiourea, ethylene urea, ethylene thiourea, 1,3-diphenyl urea, etc.), carbamate types (e.g. N-phenyl carbamic acid phenyl ester, 2-oxazolidone, etc.), imine types (e.g. ethylene imine, etc.), oxime types (e.g. formaldoxime, acetaldoxime, acetoxime, methylethyl ketoxime, diacetylmonoxime, benzophenonoxime, chlorohexanonoxime, etc.), sulfurous acid salt types (e.g. sodium bisulfite, potassium bisulfite, etc.), and hydroxamic acid ester or acyl hydroxamate types (e.g. benzyl methacrylo hydroxamate (BMH), allyl methacrylo hydroxamate, etc.). The preparation and use of hydroxamic acid ester or acyl hydroxamates as blocking agents is thoroughly disclosed in U.S. Pat. No. 4,008,192 issued to Mijs et al, the teaching of which is hereby incorporated by reference.

Among the possible blocking agents, it is especially preferred to use the lactam types, the active methylene types, the oxime types, and the hydroxamic acid ester or acyl hydroxamate type blocking agents.

The blocked isocyanates are typically prepared by mixing the isocyanate and the blocking agent while maintaining the temperature between about 50.degree. to about 230.degree. F.

The coating of this invention comprising the acrylic polymer and the blocked isocyanate can be dispersed in water and, usually, in combination with a water-miscible solvent. The concentration of the coating in water depends upon the process parameters to be used and is in general not critical, but in the case of electrodeposition, the major proportion of the aqueous composition will normally be water, e.g., the composition may contain about 1 to about 75% and typically 1 to about 50% by weight solid coating with the remainder being water.

The coating of this invention may be applied by any means, including electrodeposition, spray, flow coating, curtain coating, roller coating, dipping, brushing, or other method known in the art.

In most instances, a pigment composition and, if desired, various additives such as catalysts, flow agents, dispersants, solvents, and other materials may be included in the coating composition. The pigment composition may be of any conventional type, comprising, for example, one or more pigments such as iron oxides, lead oxides, strontium chromate, carbon black, titanium dioxide, talc, barium sulfate, zinc oxide, cadmium yellow, cadmium red, chromic yellow and the like.

In electrodeposition processes employing the aqueous coating composition described herein, the aqueous composition is placed in contact with an electrically conductive anode and an electrically conductive cathode, with the surface to be coated being the cathode. Upon passage of electric current between the anode and the cathode, while in contact with the bath containing the coating composition, an adherent film of the coating composition is deposited on the cathode. The applied voltage may be varied greatly within the practice of this invention for electrodeposition and can be, for example, as low as 1 volt or as high as several thousand volts, although typically between 50 volts and 500 volts. The current density is usually between 1 ampere and 15 amperes per square foot, and tends to decrease during electrodeposition.

The electrodeposition method is applicable to the coating of any electrically conductive substrate, and especially metal such as steel, aluminum, copper, or surfaces which have been coated with conductive primers or filaments to impart a conductive surface.

After application by whatever means desired, the coating is cured at elevated temperatures by any convenient method such as in baking ovens or with banks of infared heat lamps. Curing temperatures are preferrably from about 150.degree. F. to about 450.degree. F. although somewhat higher or lower temperatures can be employed if desired.

The following examples have been selected to illustrate specific embodiments and practices of advantage to a more complete understanding of the invention. Unless otherwise stated, "parts" means parts by weight.

A four necked flask equipped with Friedricks condenser, calcium chloride drying tube, mechanical stirrer, nitrogen gas inlet, and heating mantle was charged with 729.5 parts benzylmethacrylo hydroxamate (BMH) (prepared as described in U.S. Pat. No. 4,008,192, column 6, lines 28-56) and heated to 180.degree. F. until the BMH melted. To the molten BMH was added 626 parts Hylene.RTM.W (saturated version of methylene di-para-phenylene isocyanate sold by E. I. duPont) and 600 parts n-methyl pyrrolidinone over a 2 hour period. The reaction mixture was maintained at 180.degree. F. for an additional 2 hours. The temperature was then reduced to 130.degree. F. and 9.5 parts dibutyl tin dilaurate was added to the reaction mixture. The reaction mixture was held at 130.degree. F. until infrared analysis indicated a constant isocyanate level. 43 parts of trimethyol propane was gradually added to the reaction mixture over a 2 hour period. The reaction mixture was held at this temperature until infrared analysis indicated that essentially all of the isocyanate was reacted. The BMH-blocked isocyanate prepolymer product was a 70.12 percent weight solids solution in the n-methyl pyrrolidinone solvent.

A reaction vessel equipped as described in Example 1 was charged with 372.45 parts BMH and heated to approximately 180.degree. F. until the BMH was melted. A solution of 370.5 parts Desmodur.RTM.N 100 isocyanate (biuret of hexamethylene diisocyanate obtained by reacting one mole of water and three moles of hexamethylene diisocyanate--sold by the Mobay Company) in 158.8 parts isopropyl acetate was slowly added to the molten BMH over a two-hour period. The reaction mixture was held at about 180.degree. F. until infrared analysis indicated essentially all of the isocyanate was reacted.

A reaction vessel equipped with stirrer, condenser, and 2 addition dropping funnels was charged with 470 parts 2-butoxy ethanol and heated to 250.degree. F. A mixture of 375 parts styrene; 645 parts butylacrylate; 270 parts hydroxyethylacrylate; 180 parts dimethylaminoethyl methacrylate; and 30 parts Vazo 64 (2,2'-azobis-(isobutyronitrile) free radical initiator manufactured by E. I. duPont,) was placed in one of the dropping funnels. The other dropping funnel was charged with 60 parts of a 50% aqueous solution of methacrylamidopropyl(2-hydroxyethyldimethylammonium acetate).

The contents of the 2 addition dropping funnels were added simultaneously to the heated solvent over about a 3 hour period. The mixture was maintained at 250.degree. for about 15 minutes and then a solution of 7.6 parts butylcellosolve and 3.8 parts Vazo 64 was added to the reaction mixture in 4 separate additions each 15 minutes apart. The reaction mixture was held at 250.degree. for approximately 3 hours and then the reaction vessel was attached to a Dean-Stark trap to remove the water. The acrylic vehicle produced by this reaction was approximately 72% weight solids.

Additional acrylic resins were prepared using the process of Example 3 having the following charge weight percent compositions (based on weight solids):

In order to test pigmented formulations a grind paste was prepared by mixing on a laboratory mill:

Representative examples of coating formulations useful in cathodic electrodeposition are described in Examples 7 and 8.

A cathodically electrodepositable coating was prepared by mixing:

This coating was cathodically electrodeposited on a zinc phosphate treated steel panel cathode for 60 seconds at 130 volts rinsed with water and then baked for 30 minutes at 260.degree. F. to give a hard glossy finish with excellent solvent resistance.

A cathodic electrocoating was prepared by mixing:

This coating was electrodeposited onto an iron phosphate treated steel panel which was used as cathode, rinsed with water, and baked for 30 minutes at 280.degree. F. to yield a hard, glossy coating.

200 parts of the acrylic from Example 6, 63 parts of the blocked isocyanate from Example 2; and 1 part of acetic acid were stirred together and then reduced with deionized water to give an aqueous solution at 26.5 percent by weight solids. The coating was then sprayed or drawn down over various plastic substrates and baked 30 minutes at 180.degree. F. The cured films showed excellent adhesion and resistance to moisture when applied over polystyrene, ABS, SMC, Norel, and Lexan plastic substrates.

While this invention has been described by a number of specific embodiments, it is obvious that other variations and modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.