CA2213146A1 - Aqueous coating composition - Google Patents

Abstract

An aqueous coating composition comprises a) an organic polymer in aqueous solution which carries free or neutralized acid groups, or free and neutralized acid groups, and b) a crosslinker of the structural formula I

(see fig. I) where R1, R2, R3 and R4 independently are hydrogen or alkyls, aryls and/or aralkyls of 1 to 24 C atoms, or mixtures of two or more thereof, X is oxygen, sulfur and/or -N(R5)-, where R5 is alkyl, aryl or aralkyl of 1 to 24 C atoms or is hydrogen and Z is an n-functional alkyl, aryl or aralkyl which is unsubstituted or substituted by further groups, and n is a number which is at least 2, wherein the molar mass of the crosslinker is below 10,000.

Description

Aqueous co~t;n~ c~ )o~,lion The invention relates to an aqueous coating co"~posilion cGmplisillg a polymer which has free and/or neutralized acid groups and a low molecular mass crosslinker which has at least two centers capable of reaction with anionic groups.

~queous dispersion colllplising polyurethanes as binders are generally known (cf. D. G. Oertel, KunststoJ~handbuch 7, 2nd ed., 1983, Carl Hanser Verlag Munich Vienna, pp. 24 to 25 and pp. 571 to 574). Also known is the use of polyulelllal1e dispersions as coating materials, for example as paints or plilllillg inks.

In view of processing and economic efficiency, and in the light of the desirable properties of the coatings produced with the coating materials, these aqueous polyurethane dispersions must be made subject to a number of requirements, which to date are not met to the re~luiled extent by ~y~ ls known from the prior art.

Such requirements relating to the service prope~lies are, in particular:

- The coating materials should be able to be stored over a prolonged ., period without alteration in their properties (for example the rheological propellies) or the prope-lies of the coatings produced therewith.

5 - The coating materials should as far as possible contain small amounts of solvents, leveling agents or other volatile organic constituents, so as to minimi7e the emissions of organic compounds during the application and drying of the coating materials.

10 - Following application to the workpiece, the coating material should cure or dry rapidly so that the workpiece is ready for use or can be processed further after just a short time.

- The coating materials should have a very low tend-ency to foam in the course of processing.

On high-quality coatings and surfaces made from polyu,etllane coating materials, the following requirements are placed:

- Smooth surface and high gloss - Resistance to moisture, steam and chemicals such as dilute alkalis and acids, and to organic solvents or surfactants - Insellsili~ity to m~çh~nical stresses such as impact, shock or friction - No inherent color or defects such as bubbles or cracks In general, to produce coatings best able to withstand mechanical and chemical stresses, the polymers present in the coating material are subjected to chemical crosslinking.

The solubility or dispersibility of polymeric binder components in water is frequently ensured by neutralized acid groups which are incorporated s chemically into the polymers, examples being carboxylate, sulfonate or phosphonate groups having as counterions, in general, ammonium cations based on ammonia or on organic amines which are volatile at least on heating. In the course of the drying of sheetlike structures and coatings produced from polymer solutions of this kind and, in particular, polymer o dispersions, which drying usually proceeds under the action of heat, it is true that a large proportion of the carboxylate groups originally present is converted into carboxyl groups as a result of the elimin~tion of the amine or ammonia cation. The carboxyl-con~inin~ polymers then present often exhibit a level of water resi~lance which is still not entirely satisfactory.
Moreover, these carboxyl groups catalytically accelerate the unwanted hydrolytic degradation of polymers containing ester groups, as a result of which the mech~nic~l level of said sheetlike structures, especially in a moist environment, can be impaired rapidly.

It is therefore advantageous to carry out crosslinking of the water-soluble or water-dispersible polymers by way of the free or neutralized acid groups on the polymer. In this context, firstly, crosslinking brings about higher mechanical stability of the coating and enhanced resistance to chemicals; secondly, crosslinking is in general associated with a hydrophobicization of the ionic groups, so that the sensitivity to water or aqueous solvents is reduced in the crosslinked coating.

Attempts to carry out such crosslinking are already known from the prior art.

-For instance, the crosslinking of carboxyl groups with polyaziridine compounds is described, for example, in US-A-4,301,053 and in US-A-5,430,049.

Crosslinking with polycarbo-liimides is known, for example, from EP-Al 0 121 083, EP-B1 0 198 343 and EP-A1 0 277 361.

Finally, cro.cslinking with polyepoxides is mentioned in EP-~2 0 512 710 and in EP-A1 0 578 068.

The polyaziridine, polycarbodiimide and polyepoxide crosslinkers described all have the serious disadvantage that they are toxicologically objectionable. Furthermore, compositions comprising them are not stable on storage.

EP-Al 0 663 412 describes an efficient crosslinking method in which a carboxyl-cont~ining polyurethane dispersion is combined with an acetoacetate group-cont~ining dispersion of a vinylic polymer. The coating compositions described give rise to crosslinked films which have a high .l,cas~lle of resi.st~nce, especially to solvents. However, these compositions have the disadvantage that they neces.s~.ily co~ "ise two different dispersions in a selected ratio, so that the paint formulator is restric-ted in his or her freedom when it comes to coatings formulation. Owing to the limited combinability of both dispersions, the profile of propellies of 2S the resulting coating co~ )osilion will always be a collli)ro~llise betweenthe pro~ellies of the two individual dispersions. Moreover, the skilled worker is forced toward a specific combination of a polyurethane dispersion and the dispersion of a vinylic polymer.

It is an object of the present invention, theleÇole, to provide an aqueous coating composition which on the one hand is toxicologically unobjectionable and on the other hand provides the skilled worker with freedom in formulation to such an extent that the crosslinker system can be employed with as large as possible a number of different polymers and as large as possible a number of different dispersions.

We have found that this object is achieved, surprisingly, by using polymers with free and/or neutralized acid groups in coating compositions together with a crosslinker which carries aceto~cet~te groups or derivatives o thereof. The coating combination is highly unobjectionable from a toxicological standpoint, is stable on storage and can be employed with a large number of binder combinations.

The invention therefore provides an aqueous coating composition COIllyl iSillg a) an organic polymer in aqueous solution or dispersion which carries free and/or neutralized acid groups, and 20 b) a crosslinker of the structural formula I

O O
Rl R~ H~C /C~ C--XtZ (I) R3 R4 n where Rl, R2, R3 and R4 independently are hydrogen or alkyls, aryls and/or aralkyls of 1 to 24 C atoms, or mixtures of two or more - 30 thereof, -X is oxygen, sulfur and/or -N(~5)-, where Rs is alkyl, aryl or aralkyl of 1 to 24 C atoms or is hydrogen and Z iS an n-functional alkyl, aryl or aralkyl which is unsubstituted or sub~liluled by further groups, and n is a number which is at least 2, the molar mass of the crosslinker being below 10,000, preferably below 5000 and, with particular pie~elellce, below 1000.

The upper lirnit for n is about 100, in general about 80. However, it is plerelled if n has values which are markedly below these levels, for in~l~nce about 20, 30, 40, 50 or less, for example 10. The lower limit for n is two, since this constitutes the minimum number of functional groups required for cros.elinking. In a prel~l.ed embodiment, n can adopt values from 2 to about 8, with values from 2 to about 6 or from 2 to about 4 being particularly preferred. The values of n need not be even numbers; in the case of relatively high molecular mass crosslinkers in particular n can be a statistical mean value for the groups participating in the crosslinking, which need not nPcess~rily be an even number.

For the purposes of the present invention the combined phrase "neutralized acid groups" refers to acid groups which carry negative charges, such as carboxylates, sulfonates or phosphonates (together with the lespe.;live c~ulllelions) which are bonded covalently with the polymer backbone.

The combined term "free acid groups" relates to acid groups bonded covalently to the polymer backbone, for example carboxyl, sulfo and/or phosphono groups. These groups can in general be converted by neutralization with a base into anionic groups, ie. into acid anions and thus into neutralized acid groups of the first-mentioned type.

As organic polymers which carry free and/or neutralized acid groups and s can be used for the purposes of the invention, which polymers are in aqueous solution or in aqueous dispersion, it is possible to employ a large number of different polymers. In general, the solubility of the otherwise normally hydrophobic polymer chain in water is brought about by the introduction of carboxyl, sulfo or phosphono groups. For o hydrophilicization plerelellce is generally given to carboxyl and/or sulfo groups, hydrophilicization by carboxyl groups in particular being very important.

A further option for increasing the hydrophilicity of the polymers and 15 thus of increasing the dispersibility in water is the incorporation of nonionic polyether units together with free and/or neutralized acid groups into the polymer. For the plepalalion of the dispersion it is possible if desired for the acid groups to remain entirely or at least substantially in the free state, if the hydrophilicity introduced by the polyether chain is 20 sufficient to render the polymer chain dispersible in water.
Hydrophilicizing polyether chains of this kind are generally composed to ~ the extent of at least 50% by weight of oxyethylene units.

Through the introduction of such groups which impart water-solubility or 25 at least dispersibility it is possible to render a wide variety of polymer chains soluble in water or at least dispersible in water.

Polymers for which such hydrophilicization is possible include, for example, the poly~l~Lhalle resins. Such polyurethane resins are generally 30 plepared in the form of polyester- or polyetherpolyurethane resins.

The preparation of aqueous dispersions comprising water-dispersible polyurethane resins is customarily carried out by reacting A) polyfunctional isocyanates having 4 to 30 C atoms with B) polyols, for example polyester- and polyetherpolyols, and C) if desired, further compounds carrying at least two isocyanate-reactive groups, the isocyanate-reactive groups preferably being hydroxyl groups or primary or secondary arnino groups, and D) conlpoullds other than B) and C) which contain at least one, preferably at least two isocyanate groups or isocyanate-reactive groups and, furthermore, at least one free and/or neutralized acid group and then dispersing the resulting polyurethane in water.

lS Suitable isocyanates A) are the polyisocyanates customarily employed in polyurethane chemistry.

Particular mention may be made of the diisocyanates X(NCO)2, where X
is an aliphatic hydrocall,oll radical of 4 to 12 C atoms, a cycloaliphatic or aromatic hydrocarbon radical of 6 to 15 carbon atoms or an araliphatic hydrocarbon radical of 7 to 15 carbon atoms. Examples of such isocyanates are tetramethylene diisocyanate, hexamethylene diisocyanate, dodecamethylene diisocyanate, 1,4-diisocyanatocyclohexane, 1-isocyanato-3,5,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), 2,2-bis(4-isocyanatocyclohexyl)propalle, llil.letllylhexane diisocyanate, 1,4-di-isocyalla~obenzene, 2,4-diiso~;yandlotoluene, 2,6-diisocyanatotoluene, 4,4'-diisocyanatodiphenylrnethan~, tetramethylxylylene diisocyanate (TMXDI), 2,4'-diisocyanato-diphenylmethane, p-xylylene diisocyanate, the isomers of bis(4-isocyanatocyclohexyl)methane, such as the trans/trans, the cis/cis and the cis/trans isomer, and mixtures con~inin~ these compounds.

Particularly important mixtures of these isocyanates are the mixtures of the respective structural isomers of diisocyanatotoluene and diisocyanatodiphenylmethane, with the mixture of 80 mol-% 2,4-diisocyanatotoluene and 20 mol-% 2,6-diisocyanatotoluene being particularly ayplopliate. Mixtures of aromatic isocyanates such as 2,4-diisocyanatotoluene and/or 2,6-diisocyanatotoluene with aliphatic and/or cycloaliph~tic isocyanates such as hexamethylene diisocyanate or IPDI are also particularly advantageous, the prefel.ed mixing ratio of aliphatic to aromatic isocyanates being from 4:1 to 1:4 As component A) it is also possible to employ isocyanates which in addition to free isocyanate groups include further capped isocyanate groups, for example uretdione or carbodiimide groups.

It is also possible if desired to use isocyanates having only one isocyanate group, in which case the yl~yollion thereof should not generally exceed 10 mol-%. These isocyanates generally carry further functional groups, for example olefinically unsaturated groups or carbonyl groups, and serve to introduce these functional groups into the polyurethane. By this means it iS possible to bring about further modification of the polyurethane, for example grafting of the polyurethane by free-radical polymerization of olefinically ~nsalul~led compounds in the presence of a polyurethane which carries olefinically unsalu-~ted groups, or, if desired, it is possible for crosslinking or other polymer-analogous reactions to take place.

Branched or crosslinked polyurethanes can be prepared, for example, using trifunctional or polyfunctional isocyanates. Examples of commercial compounds are the isocyallul~te or the biuret of hexamethylene diisocyanate.

~ - 10-Suitable polyols B) are ptincipally polyols of relatively high molecular weight, preferably diols, which have a molecular weight of from about 500 to 5000, preferably from about 1000 to 3000.

The polyols B) are, in particular, polyesterpolyols, which are known, for example, from Ullm~nn.c Encyklopadie der technischen Chemie, 4th Edition, Volume 19, pp. 62 to 65. It is plerelled to employ polyesterpolyols obtainable by reacting dihydric alcohols with dibasic carboxylic acids. Instead of the free carboxylic acids it is also possible to o employ the cc,lre~pollding polycarboxylic anhydrides or the collesponding polycarboxylic esters of lower alcohols or mixtures thereof to prepare the polyesterpolyols. The polycarboxylic acids can be aliphatic, cycloaliphatic, araliphatic, aromatic or heterocyclic and, if desired, can be substituted by halogen atoms and/or unsaturated. Examples are: suberic acid, azelaic s acid, phthalic acid, isophthalic acid, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, tetrachlorophthalic anhydride, endomethylenetetrahydrophthalic anhydride, glutaric anhydride, maleic acid, maleic anhydride, fumaric acid or dimeric fatty acids. Particularly plefelled examples are succinic acid, adipic acid, dodecanedicarboxylic acid and/or sebacic acid.

Examples of suitable polyhydric alcohols are ethylene glycol, 1,2-propallediol, 1,3-propanediol, 1,4-butanediol, 1,4-butenediol, 1,4-butynediol, 1,5-pe ,~ iol, neopentylglycol, bis(hydroxymethyl)cyclohexane, 2-methyl-25 propane-1,3-diol, methylpentanediols, diethy1ene g1ycol, triethylene glycol and other polyethers, for example tetl..ctllylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycols, dibutylene glycols or polybutylene glycols, and also polyethers having an even higher number of carbon atoms between the ether oxygens, for example polytetrahydrofuran.

Preference is given, for example, to neopentylglycol, ethylene glycol, butane-1,4-diol, hexane-1,4-diol, octane-1,4-diol and dodecane-1,12-diol.

Likewise suitable are lactone-based polyesterdiols, which are homo- or s copolymers of lactones, preferably adducts, with terminal hydroxyl groups, of lactones with appro~liate difunctional starter molecules. Examples of such lactones are ~-caprolactone"~-propiolactone, y-butyrolactone and/or methyl-~-caprolactone .

o In addition, polyetherdiols are also suitable as component B). They are obtainable in particular by polymerizing ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran, styrene oxide or epichlorohydrin with itself, for example in the presence of BF3, or by addition reaction of these compounds, if desired in a mixture or in succession, with starter components having reactive hydrogen atoms, such as alcohols or amines, for exarnple water, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-bis(4-hydroxyphenyl)propane or aniline.

Also suitable are polyhydroxyolefins, preferably those having 2 terminal hydroxyl groups, for example, o~ -dihydroxypolybutadiene, ~
dihydroxypolymethacrylic esters or cY,~-dihydroxypolyacrylic esters as component B). Examples of further suitable components B) are polyacetals, polysiloxanes and alkyd resins.

The polyols can also be employed as ~ ules in any desired proportions.

The hardness and the modulus of elasticity of the polyuleLllalles can be increased by using as coml)ollellt C), for example, low molecular mass diols having a molecular weight of from about 62 to about 500, preferably from about 62 to about 200 g/mol. Examples of such monomers are the low molecular mass polyols described in the context of the synthesis of the polyester component B). Particular l)lerelence is given to the short-chain alkanediols, such as neopentylglycol, and the unbranched diols having 2 to 12 C atoms and an even number of C
atoms.

The hydroxyl number of the OH-cont~ining components B) and/or C) should in general be from about 20 to about 300, preferably from about 50 to about 200, mg of KOH/g of solids.

Examples of other compounds which can be employed as component C) are di- or polyamines such as, for example, hydrazine, ethylene~ mine, propylenedi~mines, butylene~ mines or higher alkylenedi~mines. Also suitable are compounds having both a p~ laly, secondary or tertiary s amino group and, for example, a hydroxyl group. Examples of these include ethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, ethyldiethanolamine and the like. In general these compounds should have at least two isocyanate-reactive groups, it also being possible to employ monofunctional compounds in minor amounts of, for example, not more than about 10% by weight, but prefer-ably less than 5% by weight.

In order to achieve dispelsibility in water a further component D), in addition to the components A) to C) already described, is included in the reaction for prcp~illg the polyurethanes.

This component comprises compounds which carry at least one isocyanate group or isocyanate-reactive group and, furthermore, at least one free or neutralized acid group. Suitable monomers having such free and/or neutralized acid groups are usually aliphatic, cycloaliphatic, araliphatic or aromatic carboxylic acids, sulfonic acids or phosphonic acids which carry at least one isocyanate group or isocyanate-reactive group. Plcr~ ce is given to the dihydroxyalkylcarboxylic acids, especially those having from 3 to 10 carbon atoms, as are described, for example, in US-A 3,412,054.
To introduce carboxyl groups into polyurethane resins it has been found suitable to use as component D), in particular, dimethylolpropionic acid, lactic acid, malic acid, tartaric acid, ~-hydroxycaproic acid, castor oil, fatty acid or ~-caprolactone.

o Also suitable are corresponding dihydroxysulfonic acids and dihydroxylalkanephosphonic acids, for example 2,3-dihydroxypropanephosphonic acid or bisglycol phosphate.

Another possibility of introducing acid groups into polyutethane resins is to employ as component D), for plepa~ g the polyurethanes, a polyester resin which already carries acid groups. Such polyester resins can generally be obtained by polycondensation of polyhydroxy components with polycarboxylates, the free or neutralized acid group being introduced into the polyester chain either in the form of the polyhydroxy component or as polycarboxylate. In this case it should be ensured that the free or neutralized acid group, especially insofar as it is a carboxylate group, plays little or no part in the polycondensation process. If necessary, the group must be prevented from such a reaction by means of appropliate protective mea~. res. It is likewise possible to use mixtures of different 25 compounds carrying free and/or neutralized acid groups in order to synthesize the polyester resin.

Examples of such polyesters are dihydroxy compounds having a molecular weight of from about 500 to 10,000 g/mol and at least 2 carboxylate groups, as are known, for example, from DE-A1 39 11 827. They are obtainable by reacting dihydroxy compounds with tetracarboxylic dianhydrides, such as pyromellitic dianhydride or cyclo-pentanetetracarboxylic dianhydride, in a molar ratio of from 2:1 to 1.05:1 in a polyaddition reaction. Particularly suitable dihydroxy compounds are the diols in accordance with components B) and/or C).

In this way it is likewise possible instead of the carboxyl groups to introduce sulfo groups, by carrying out reaction not with the tetracarboxylic dianhydrides but with dicarboxysulfonic acids, for example o sulfoisophthalic acid, sulfoterephthalic acid or sulfosuccinic acid, or esters thereof, with diols.

This form of the reaction is also particularly suitable for introducing sulfonate groups into the polyester chain. To this end, polyfunctional alcohols, generally with a molecular weight of from 62 to 1000, are polycondensed by known methods with polycarboxylic acids having a molecular weight of from about 98 to 500 or with anhydrides of such polycarboxylic acids together with structural components which ha~e sulfonate groups. Examples of such components having sulfonate groups are 5-sulfoisophthalic acid, sulfobenzoic acid, sulfophthalic acid, dimethyl-sulfoisophthalic acid, 3-hydroxy-5-sulfobenzoic acid and 2-hydroxy-5-sulfobenzoic acid or the lithillm, potassium, sodium, magnesium, calcium or tertiary ammonium salts thereof. Sulfonatediols may also be suitable.
The sulfonic acid groups are p1ereldbly neutralized by lithium, potassium or sodium hydroxide or c~l,onate or bicarbonate or, with particular preference, by tertiary amines. Examples of suitable tertiary amines for neutralizing the sulfonic acid groups are triethylamine, N,N-dimethylethanolamine, N-methyldiethanolamine or other tertiary amines.

Also possible in principle is the use of corresponding acids having free carboxyl and/or sulfo groups in carrying out the polycondensation reaction, with or without the subsequent, at least partial neutralization of the carboxylic and/or sulfonic acid groups introduced into the polyester resins in this way. During the polycondensation it is possible for suitable s organic solvents to be present, for example N-methylpyrrolidone, diethylene glycol dimethyl ether, methyl ethyl ketone, methyl isobutyl ketone, acetone, xylene, toluene, butyl acetate or mixtures of these or other solvents. The organic solvents used can be removed from the reaction mixture prior to, during or after the dispersion step, in whole or o in part, possibly by azeotropic means and/or by applying reduced ples;,~le or by means of an intensified stream of inert gas. The polyester resins thus obtainable, carrying carboxyl and/or sulfo groups, should in general be prepa~ed such that they comprise at least two groups which are reactive toward urethane groups, usually at the end of the polymer chain.

The components employable as component D) can be incorporated into the polyurethane either with neutralized or with free acid groups or as a mixture of both forms. Which form is chosen for incorporation depends on the particular component D) employed. In general, however, the procedure is such that the incolyol~lion of compounds which have already been neutralized, and therefore carry acid anions, into the polyurethane is only possible when the acid anion dissolves at least substantially in the reaction mixture for preparing the polyurethane.

For neutralization, thererole, the acid groups are neutralized with a basic neutralizing agent prior to, or preferably, after their incorporation into the polyurethane chain. Examples of basic neutralizing agents are, in general, the alkali metals such as Li,' Na or K and the al~aline earth metals such as Ca, Mg, Ba or Sr, although they are not preferred for the purposes of the present invention. More suitable, and prerelled for the purposes of the present invention, are all salts of the abovementioned metals which are capable of undergoing reactions in which the acid groups are neutralized, in particular the carbonates, the hydrogen carbonates Ot the hydroxides, for example LiOH, NaOH, KOH or Ca(OH)2.
s Likewise suitable for neutralization, and particularly preferred for the purposes of the present invention, are organic bases cont~ining nitrogen, for exatrtple ammonia and amines, such as trimethylamine, triethylamine, tributylamine, dimethylaniline, triphenylamine, dimethylethanolamine, 10 methyldiethanolamine or triethanolamine, and mixtures thereof.
Neutralization with the nitrogen-cont~ining organic bases can be carried out in organic or in aqueous phase. Compounds of component D) neutralized with nitrogen-cont~inin~ bases are therefore generally suitable, even in neutralized form, for incorporation into the polyurethane in organic solution.

If neutralization of the acid groups is desired, the neutralizing agent can be added in an amount such that a sufficient ~lopollion of the acid groups, generally from about 0.1 to 100%, is neutralized. If desired, moreover, an even smaller proportion of the acid groups can be neutral-ized, for exarrtple 0.05 or 0.01%, provided the polyurethane is nevertheless soluble in water or at least dispersible in water.

The polyurethane resin can be ple~ared in stages by, for example, ftrst of all reacting only certain starting materials, carrying isocyanate-reactive groups, with an excess of polyisocyanates to give prepolymers having isocyanate end groups, which can then be reacted with further starting materials. In this way it is possible, for exatnple, to synthesize block copolymers. More sirnple in its execution, however, is the reaction between a hydroxyl-cont~ining starting-material mixture and the polyisocyanates, in which case it is possible to vary the stoichiometric ratio between NCO groups and OH groups within wide ranges. In order to obtain OH end groups the NCO/OH ratio should be < 1, while to produce polyurethane resins with NCO end groups a ratio > 1 should be s chosen. The molecular weight of the polyurethane resins prepared can likewise be regulated by way of the NCO/OH ratio, with ratios of 1 or near to 1 leading to higher molecular weights than ratios far removed from 1. In general, the NCO/OH ratio should be from about 0.5:1 to 5:1.

Instead of or together with the polyurelllane resins it is possible to employ polyester resins whose polymer backbone carries free and/or neutralized acid groups for the purposes of the present invention.

These include polyester resins as obtainable by polycondensation of diols with polyfunctional carboxylic acids and with other reaction components carrying free and/or neutralized acid groups. The acid groups in this case can be introduced either by using an app~opliate polyol component which carries free and/or neutralized acid groups, for example dihydroxyalkyl- or dihydroxyarylcarboxylic or -sulfonic acids, or by using polycarboxylic acids or mixed polycarboxysulfonic acids, in which case, however, at least one free or neutralized acid group must remain after the polycondensation. The latter group of compounds that can be employed for the purposes of the invention for synthesizing the polyester resins 25 includes, for example, trimellitic anhydride.

The polyester resins callyhlg free and/or neutralized acid groups can carry the acid groups in the chain and/or at the ends; if appropliate, the polyester may carry functional groups other than the acid groups, for example hydroxyl groups.

~ - 18 -For the purposes of the present invention it is likewise possible to employ those polyester resins as were disclosed earlier as component D) for incorporation into the pol~/u~ellla1~e resin. The molecular weight of the polyester resins is in general from about 500 to about lS,000, plererence being given to molecular weights of from about 1000 to about lO,000 and, in particular, from about lS00 to about 7000.

The polyester resins can likewise be prel)~ed with incorporation of hydrophilic nonionic units, for example with incorporation of o polyetherdiols, in order to render the resulting polyesters water-soluble or at least water-dispersible. The polyetherdiols are generally pleparGd predol..in~ ly, ie. to an extent of at least about 50% by weight, from ethylene oxide, and have molecular weights of from about 180 to about 5000.

The purpose of incorporating such hydrophilic nonionic units is to render the polyester chain dispersible in water even when only a small number of acid groups is present on the polymer backbone and/or when only a small propo-lion of the acid groups, if any, have been neutralized, ie. are in the ionic form.

The acid groups of the polyester resin can be neutralized in the manner described using either inorganic or organic neutralizing agents.

In addition to its application to polyurethane resins or polyester resins carrying free and/or neutralized acid groups, the process according to the invention is also suitable for the crosslinking of binders which constitute the polymerization product of olefinically unsslu1~ted monomers.

The polymers prep~uGd from the olefinically u11salulated monomers can I g carry sulfonate, carboxylate and/or phosphonate groups, with the use of polymers carrying sulfonate and/or carboxylate groups, and in particular the use of polymers carrying carboxylate groups, being p-efelled in the novel coating materials.

The polymers having free and/or neutralized acid groups are prepared by the conventional copolymerization of olefinically unsaturated monomers, with monomers carrying free or neutralized acid groups generally being copolymerized together with other monomers. The concomitant use of 10 monomers cont~ining acid groups is done in order to incorporate into the copolymers carboxyl and/or sulfo groups whose hydrophilicity ensures the solubility or dispersibility of the polymers in water, possibly following partial or complete neutralization of the acid groups The acid groups are preferably carboxyl groups The amount of the concomitantly used monomers having free acid groups, and the degree of neutralization of the polymers carrying free acid groups that are obtained to start with, generally gives the finished binder an acid number of from about 5 to 150 mg of KOH/g of solids Dependmg on the molecular weight of the polymers and on their content of acid anions and/or of free acid groups, especially carboxyl groups, even the aqueous systems Co.~ -g the polymers are true dispersions, colloidally disperse or molecularly disperse dispersions, but in general are so-called partial dispersions, ie. aqueous sy~Lellls which are part molecularly disperse and 2S part colloidally di~el~e.

Suitable comonomers having free acid groups are, in principle, all olefinically unsallllaled, polymerizable compounds having at least one carboxyl, sulfo and/or phosphono group, for example olefinically uns~ ated mono- or dicarboxylic acids of the molecular weight range from 72 to 207, such as acrylic acid, methacrylic acid, maleic acid, itaconic acid or ole~lnically unsaturated compounds con~inin~ sulfo groups, for example 2-acrylamido-2-methylpropanesulfonic acid, or any desired mixtures of such olefinically unsaturated acids. In addition to these monomers it is also possible to employ further olefinically unsaturated comonomers in synthesizing the polymer, which make little or no contribution to its hydrophilicity. In general such compounds are the esters of c~,~-unsatutated mono- or polycarboxylic acids, particular ~rerelellce being given to the esters of acrylic or of methacrylic acid. It o is also possible to use aromatic compounds having olefinically unsalulated double bonds, for exarnple styrene or alkylstyrenes, as comonomers.

Yet another possibility is the copolymerization of esters of carboxylic acids with llnsa~ul~ted alcohols, for example vinyl acetate, vinyl propionate, vinyl butyrate ranging right up to the vinyl esters of higher fatty acids, for example vinyl myristate or vinyl stearate. In general, however, use is made of the esters of acrylic acid or of methacrylic acid, with the alcohol residue usually having from about 1 to 18, preferably about 1 to 8, carbon atoms. Such esters include, for example, methyl acrylate, ethyl acrylate, isoplo~yl acrylate, n-propyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate or n-stearyl acrylate. If desired it is also possible to employ comonomers which are functionalized by other func-tional groups. Examples of such comonomers include those having epoxide groups, for h~lance glycidyl acrylate or glycidyl methacrylate. Hydroxyl-cont~inin~ monomers can also be copolymerized, for example hydroxyethylacrylate, hydro~yplo~yl acrylates or hydroxybutyl acrylates or the colle~l)ollding derivatives of methacrylic acid.

In general the proportion of monomers Cont~ining acid groups in the 30 entire monomer mixture is from about 2 to 98% by weight, plerelred proportions being from S to 50% by weight, and, in particular, from lO
to 30 % by weight.

The polymers are customarily plepared by bulk, solution, emulsion or s suspension polymerization techniques. The olefinic polymers are preferably plepared in organic solution or in aqueous emulsion. Continuous or discontinuous polymerization techniques are possible. Of the discontinuous techniques, mention may be made of the batchwise and of the feed technique, the latter being plGÇell~d.

In order to obtain particular effects it is possible to meter in some of the monomers more slowly, more rapidly, starting earlier, starting later, ending earlier and/or ending later. By means of the effects which occur in this case it is possible to impart different properties to the polymer.
Examples of suitable solvents for the polymerization are aromatics, such as benzene, xylene, chlorobenzene, esters, such as ethyl acetate, butyl acetate, methylglycol acetate, ethylglycol acetate, methoxypropyl acetate, ethers, such as butylglycol, tetrahydrofuran, dioxane, ethylene chloride or trichloromonofluoroethane .

The free-radical initiated polymerization can be triggered by initiators whose half-lives (time in which the initial level of radicals is halved by decoll~posilion) at from 80 to 180~C are from O.Ol to 400 minutes.
Copolymerization generally takes place within the stated temperature 2S range, preferably from lO0 to 160~C, under a ple~ule of from 103 to 2 x 104 mbar, the precise polymerization tel"pe,alu~e depending on the nature of the initiator. The initiators are employed in amounts of from 0.05 to about 6~ by weight, based on the overall amount of monomers.
Examples of suitable initiators are aliphatic azo compounds, such as azoisobutyronitrile, and also peroxides, for example dibenzoyl peroxide, t-butyl perpivalate, t-butyl per-2-ethylhexanoate, t-butyl pell,eluoate, t-butyl hydroperoxide, di-t-butyl peroxide, cumene hydroperoxide, and also dicyclohexyl and dibenzyl peroxodicarbonate. If desired, customary regulators can be employed to regulate the molecular weight. In addition to the pure polyolefin polymers it is also possible to employ acrylate-grafted polyester resins or polyurethane resins in the novel process, with either the acrylate polymer or the polyester resin or the polyurethane resin, or both, carrying free and/or neutralized acid groups.

10 Acrylate-grafted resins of this kind can be produced by polymerizing a monomer ~ u~e cont~ining acid groups in a polyester resin or polyurethane resin. In general it is advantageous in this context if the polyester resin or polyurethane resin has been prepared using unsatlllated building blocks, for example unsaturated dicarboxylic acids or unsaturated diols. In such cases, following the polymerization of the acrylate monomers, true graft copolymers are obtained in which the polyacrylate chain is bonded covalently to the polyester or polyurethane chain.

The polymers described here, carrying free and/or neutralized acid groups, are only part of the possible range of polymers that can be employed. In principle, suitable polymers are all those which carry free or neutralized acid groups on the polymer backbone, the most suitable, for example, being carboxylate, sulfonate or phosphonate groups. Other polymers which comply with these characteristics can of course also be employed in a mixture with the abovementioned polymers, with binary, ternary or even higher ~ ures being possible. The polymers described in detail can also be employed, individually or in a ~ni~lu~e with one another, in the context of the invention.

The water-dispersed polymers cont~ining acid groups have an acid group ~ CA 02213146 1997-08-26 content of from about 20 to about 2000, preferably from 50 to 1000 and, with particular prere~ ce, from about 100 to about 800 mmol/kg of solid resm.

To crosslink the binders that are in aqueous solution or aqueous dispersion, the novel coating composition includes a crosslinker of the formula I

( Rl R~ HC! C/C, C XtZ
R3 R~ n In this formula, Rl, R2, R3 and R4 independently are hydrogen or allyls, aryls and/or aralkyls of 1 to 24 C atoms, or mixtures of two or more thereof. X is oxygen, sulfur and/or -N(Rs)-, where Rs is alkyl, aryl or aralkyl of 1 to 24 C atoms or is hydrogen. Z is an n-functional alkyl, aryl or aralkyl which is unsubstituted or sub~LiLuLed by further groups, and n is a number which is at least 2, the molar mass of the crosslinker of the formula I being below 10,000, preferably below 5000 and, with particular prcreleQce~ below 1000.

The crosslinker employed in the novel coating compositions can be 2S plcpalcd by known methods of organic chemistry, for example by reacting suitably sub~LiLuLed diketenes with approl,liate nucleophiles in accordance with the following equation:

The diketene starting material can be obtained from a wide variety of )n ~ RI~X--Z

Rl R2 ~ ~ n ketenes by ketene dimerization, which is known to the skilled worker (see for example Rompp Chemie-Lex~k~n, 9th ed. (1990), p. 976 and 2224 and literature cited therein). Plerelellce here is given to employing firstly diketene itself, where Rl, R2, R3 and R4 are each hydrogen, and also s the diketene plepaled from stearylketene, where Rl and R3 are alkyl with 16 C atoms and R2 and R4 are hydrogen.

The nucleophile Z(XH)n then effects ring opening of the lactone ring of the diketene, the compound corresponding to the target structure I being o formed after tautomerization. The compound Z(XH)n is preferably an alcohol, a thiol or an amine. It is prefelled at this point to employ alcohols or amines, since otherwise it is possible that there may be in.~t~nCeS of odor pollution (thiols).

15 Z iS preferably a linear or, with particular prererellce, a branched alkyl radical having n-fold XH functionality with respect to the diketene. It is particularly advantageous if the compound Z(XH)n is a known, readily available, polyhydric alcohol. Ple~lled examples in this context are ethylene glycol, propylene glycols, butylene glycols, diethylene glycol, dipropylene glycols, dibutylene glycols, higher alkylglycols, higher dimeric or oligomeric alkylglycols (polyether), glycerol, trimethylolplopane, pentaerythritol, glucose, other carbohydrates, oligomeric carbohydrates, partially esterified carbohydrates, for example partially esterified glucose, - 2s -oligomeric or polymeric polyfunctional alcohols, for example oligo- or polyglycerol, oligo- or polytrimethylolpropane, preferably with the proviso that the functionality of the compound Z(XH)n as crosslinker does not exceed about 50, preferably about 20 and, with particular pleferellce, about 8, while n is not greater than 8. It is likewise possible, but not preferred, to employ compounds Z(XH)n having a functionality with respect to diketenes of more than 8 and to react such a compound with an amount of diketene which is such that Z(XH)n reacts on average with preferably not more than 8 diketene molecules. The number of ketene o molecules employed per molecule of compound Z(XH)n, however, should preferably be equivalent to the functionality of the nucleophile employed, so as to avoid the formation of byproducts.

The crosslinker present in the coating compositions can be of such s composition that Z is an n-functional alkyl, aryl or aralkyl which is unsubstituted or subsliluted by other groups. For the purposes of the invention, however, it is advantageous if Z is an n-functional alkyl. It is likewise advantageous if n can adopt values from 2 to about 50 and, in particular, from 2 to about 20. Particular plererellce, however, is,given to the use as colllpound Z(XH)n of low molecular mass polyols, for example ethylene glycol, propylene glycols, butylene glycols, pentaerythritol, glycerol or trimethylolpropane, the use of trimethylolplopalle being prefelled. When these low molecular mass polyols are employed n generally adopts values which correspond to the number of the OH
groups in the lespe~ te low molecular mass polyol, ie. from 2 to about 8.

A further possibility for preparing the crosslinkers used in the novel coating compositions is to react a substance of the formula 1, in which X
iS generally oxygen or sulfur and Z is a monovalent alkyl or aryl and where n = 1 with the above-described polyhydric alcohols in a transesterification reaction. In such a case alcohol exchange leads to the crosslinker of the formula I that is required in the novel coating compositions, while the monohydric alcohol ZXH origin~ting from the starting material is removed from the reaction mixture by customary method or remains in the mixture.

The novel coating compositions comprise the crosslinker of the structural formula I preferably in the form in which Rl to R4 are hydrogen.

It is also preferable for the crosslinker present in the coating compositions to be of such a composition that X is oxygen or -N(Rs)- where Rs is alkyl or aryl of 1 to 24 C atoms or is hydrogen. It is particularly prefelled if X is oxygen.

The molar mass of the crosslinker of the structural formula I that is present in the coating compositions is advantageously from about 200 to about 600, it being preferred for the molar mass to be from about 300 to about 450.

The invention likewise provides a process for pleparil,g a coating composition, in which a) an organic polymer in aqueous solution which carries free and/or neutralized acid groups, and b) a crosslinker of the ~ ;lui~l formula I

where R1, R2, R3 and R4 independently are hydrogen and/or alkyls, aryls or aralkyls of 1 to 24 C atoms or mixtures of two or more thereof, o X is oxygen, sulfur and/or -N(R5)-, where Rs is alkyl, aryl and/or aralkyl of 1 to 24 C atoms or is hydrogen, and Z is an n-functional alkyl, aryl or aralkyl which is unsubstituted or subsliluled by further groups, and n is a number which is at least 2, the molar mass of the crosslinker being below 10,000, preferably below S000 and, with particular pl~r~lellce, below 1000, are mlxed.

In a yrefelled embodiment of the invention a crosslinker of the formula I
is employed in which n can adopt values from 2 to about 20, preferably from 2 to about 8 and, with particular plererence, from 2 to about 6.

2S If desired, it is possible within the scope of the process according to theinvention for further, ~;u~lc....~.y paint additives to be mixed in for the ~,lep~dlion of the novel coating composition.

The novel coating compositions thus obtained are notable for particularly good stability on storage and extremely low toxicity. The individual components can be mixed in any desired points in time prior to the use of the dispersion as coating material. Accordingly, the coating composition can be applied as a one- or two-component coating material, but preferably as a one-component coating material. The term one-component coating material for the purposes of the present invention refers to a coating material in which crosslinker and polymer are present together in the coating composition and can be stored over prolonged petiods without crosslinking and without detriment to the quality of the subsequent coating.

The coating compositions prepared in this way are generally applied to the workpiece that is to be coated by techniques which are customary in the coatings industry, for example by rolling, knife coating, spraying, spreading, flowcoating or dipping. The subsequent drying and/or curing of the coating material can be effected both by cold curing (ie. by drying at from 0~C to 80~C, preferably at room te,l,~e,~lu,e), or by stoving procedures (ie. by drying at usually from 80~C to 280~C). The ambient telllpelalule may also be below 0~C, provided the coating composition forms a homogeneous film under these conditions. Drying at > about 20 100~C pre~upl)oses a certain degree of pre-drying at ambient temperature or at < about 100~C.

Opti~ llll stability of the film coating composition is produced when the film is thermally activated. The plefe.led drying le-npe.alure, therefore, is 25 above 100~C. If a lel~lL)el~lule of about 180~C is exceeded in the course of drying, then in certain circumstances yellowing phenomena may occur in the film, so that the y~ere~le~ drying range is from about 100~C to about 180~C and, in particular, from about 120~C to 160~C.

30 In prep~ing polymer resins which carry free and/or neutralized acid -groups, the crosslinker can be added after polymerization is over before, during or after the dispersing stage. Provided the crosslinker is not deactivated in the course of the polymerization reaction, however, it can also be added even before or during the polymerization reaction.

The crosslinker can also be incorporated into the aqueous dispersion, which may already include further, customary paint additives.

The crosslinker can also be blended with one or more cuslo"~a,y paint o additives, the corresponding formulation then being mixed with the aqueous dispersion.

The molar ratio of the acid groups in the aqueous dispersion to the reactive centers of the crosslinker is from 10:1 to 0.2:1, preferably from 5:1 to 0.4:1 and, with particular p,efefence, from 2:1 to 0.5:1.

The novel coating composition can comprise further customary paint additives. These include, in particular, thickeners, pigments, organic solvents in propo~Lions of not more than 20%, dyes, emulsifiers, SUlra~;~nl~, heat stabilizers, leveling assistants, wetting agents, fillers, sedimentation inhibitors, flame l~lalda"ls, UV absolbels and/or antioxidants, which can be added simultaneously or in succession at any point in time during the p~epalaLion of the coating composilion provided they do not react with the crosslinker and/or with the binder to deactivate the groups that are required for crosslinking.

The novel coating compositions can be applied to a large number of substrates, for exarnple wood, metal, glass, fabric, leather, concrete, paper, plastic, polymer foam and the like.

-The invention therefore also provides a method of coating substrates by applying an aqueous coating composition of the present invention to the substrate.

s The invention likewise provides, therefore, coated articles obtainable by the coating method described.

EXAMPLES

o I. Dispersions a) Comparison dispersion 1 The co~npa~isoll dispersion employed is an aqueous poly-ester-polyurethane dislJe,~ion pr~.afed from 400 parts of a hydroxyl-termin~ted polyester based on isophthalic acid/adipic acid and 1,6-hexanediol (1:1) having a molar mass of 2000 g/mol, 40.2 parts of dimethylolpropionic acid, 72.1 parts of 1,4-butanediol, 316.8 parts of isophorone diisocyanate, 21.4 parts of dimethylethanolarnine and 8.6 parts of diethylenetriamine, whicb has a solids content of about 30% by weight.

b) Dispersion 1 The novel dispersion 1 was p~e~afed by adding 4.2 parts of ~ netl,~lolplopalle tris(acetoaGel~te) to 300 parts of comparison dispersion 1. The solids content was 31% by weight.

c) Comparison dispersion 2 The co~"p~ison dispersion 2 was prel)ared by adding 1.5 parts trimethylolpropane to 300 parts of comparison dispersion 1.

d) Comparison dispersion 3 Comparison dispersion 3 was plepaled by adding 3.3 parts pentaerythritol tris(glycidylether) (Basoset~ 162) to 300 parts of conlpalison dispersion 1.

II. Test ~ tl~ds lO a) Forming a film on sheet metal The dispersions were applied using a film-drawing frame to sheet metal parts of grade St 1405 in a wet film thickness of 150-200 ~m, predried at room telnpel~lu~-, for 10 minutes, and then stoved.

b) Erichsen indentation The test was carried out in accordance with ISO 1520.

C) Crosshatch The test was carried out in accordance with DIN 53 151.

d) Surface hardness The film hardness (pendulum hardness) was determined in accordance withDIN 53 157 using a Konig insLlulllent. The number of double strokes was measured.

e) Acetone test A cotton pad soaked with solvent was rubbed back and forth under slight ples~ule over a selected point on the coating of the metal sheet coated in s a) (1 x back, 1 x forward = 1 double stroke DS). This test was carried out over from 50 to 100 DS. Insofar as the ~llm has not been abraded up to that point, it is regarded as being crosslinked or fully cured.

o f) 28% strength sulfuric acid test The test was carried out according to the specification from the company Dr. Kurt Herberts (DKH), Wuppertal, as follows: a small cotton pad which had been soaked in 28% strength sulfuric acid was placed on the 15 test piece. After 4 hours in a convection oven at 60~C, the sample was then assessed in accordance with DIN 53 230 Table 1 (from 0 to 5; 0 = very good, 5 = poor).

g) 38% strength sulfuric acid test and 1% and 5% strength sodium hydroxide test The test was carried out in analogy to the method for f), with storage at room L~ el~lult; for 24 hours.

III. T~ results The dispersions 1 and l(C) were applied to metal sheets and cured under the stated conditions. Following stoving, both films are visually coln~arable, clear, glossy and colorless. The test results are set out in Table 1 below.

Table 1 Drying 30 min/150~C 30 min/150~C
Erichsen value (mm) > 10 > 10 Crosshatch (0-5**) 0 0 Pendulum hardness 98 101 (No. of DS) Acetone test (No. of > 100 30 DS) H2SO4 test, 28% 0 strength;
4h/60~C (0-5**) H2SO4 test, 38% 0 0 strength;
24h/room temp.
(0-5**) NaOH test, 1% 0 0 strength;
24h/room temp.
(0-5**) NaOH test, 5 % 2 2 strength;
24h/room temp.
(0-5**) * To 100 parts of the ~ ;Un prepared under 1. there were added in each case 8 parts of ethylene glycol Ib ~1 ether.
~ 0: best score The results of Table 1 illustrate the improvement in solvent resistance by the novel llealll-ellt of the dispersion with the low molecular mass crosslinker, with otherwise substantially unch~nged properties.

In a further test different crosslinkers were compared to each other. The test results are given in the following table 2 ~ CA 02213146 1997-08-26 Table 2 Dispersion* ~ 2~ 3~
Drying 30 min/140~C 30 min/140~C 30 min/140~C
Crosshatch (0-5**) immediate 0-1 0-1 0-1 after 2 weeks 0-1 0-1 0-1 after 4 weeks 0 0 0 after 8 weeks 0 0 0 10 Pen-1nlllm ha~ ess (No. of DS) immediate 89 84 82 after 2 weeks 85 94 90 after 4 weeks 94 87 93 after 8 weeks 90 90 96 Acetone test (No.
of DS) immediate 40 3 6 after 2 weeks >50 5 6 after 4 weeks >50 7 7 after 8 weeks 44 6 14 H2SO4 test 38 % strength, 24h/room temp.
(0-5**) immediate 0 0 0 after 2 weeks 0 0 0 after 4 weeks 0 0 0 after 8 weeks 0 0 0 NaOH test 1 % strength, 24h/room temp.
(0-5**) immediate 5 5 5 after 2 weeks 2 5 5 after 4 weeks 0 5 5 after 8 weeks 0 5 5 , ~

To 100 parts of the ~ ;on prepared under 1 2,2 parts of ethylene glycol l,.onob"ether were added.
J'~ 0: best result 5 The results of table 2 clarify, that just in case of the dispersion I, according to the invention, an increased resistance to solvents and alkali is obtained, while further p~opellies are comparable.

Claims (10)

1. An aqueous coating composition comprising:

a) an organic polymer in aqueous solution or dispersion which carries free or neutralized acid groups, or free and neutralized acid groups and b) a crosslinker of the structural formula I

(I) where R1, R2, R3 and R4 independently are hydrogen or alkyls, aryls or aralkyls of 1 to 24 C atoms, or mixtures of two or more thereof, X is oxygen, sulfur or -N(R5)-, or any combination of two or more of oxygen, sulfur and -N(R5)-, where R5 is alkyl, aryl or aralkyl of 1 to 24 C atoms or is hydrogen and Z is an n-functional alkyl, aryl or aralkyl which is unsubstituted or substituted by further groups, and .

n is a number which is at least 2, wherein the molar mass of the crosslinker is below 10,000.

2. A coating composition as claimed in claim 1, wherein R1 to R4 are hydrogen.

3. A coating composition as claimed in claim 1, wherein X is oxygen.

4. A coating composition as claimed in claim 1, wherein Z is an n-functional alkyl.

5. A coating composition as claimed in claim 1, wherein n can adopt values from 2 to 6, in particular from 2 to 4.

6. A coating composition as claimed in claim 1, wherein the molar mass of the crosslinker is from 200 to 600.

7. A process for preparing a coating composition, which comprises mixing a) an organic polymer in aqueous solution which carries free or neutralized acid groups, or free and neutralized acid groups, and b) a crosslinker of the structural formula I

(I) where R1, R2, R3 and R4 independently are hydrogen or alkyls, aryls or aralkyls of 1 to 24 C atoms, or mixtures of two or more thereof, X is oxygen, sulfur or -N(R5)-, or any combination of two or more of oxygen, sulfur and -N(R5)-, where R5 is alkyl, aryl or aralkyl of 1 to 24 C atoms or is hydrogen and Z is an n-functional alkyl, aryl or aralkyl which is unsubstituted or substituted by further groups, and n is a number which is at least 2, wherein the molar mass of the crosslinker is below 10,000.

8. A process as claimed in claim 7, wherein further, customary paint additives are mixed in.

9. A method of coating substrates, which comprises applying an aqueous coating composition as claimed in claim 1 to the substrate.

10. A coated article obtainable by a method as claimed in claim 9.