Title of Invention	AQUEOUS ACRYLIC COATING COMPOSITION
Abstract	This invention relates to an aqueous coating composition comprising a dispersion in (i) an aqueous medium of an acrylic addition polymer having a acid value of at least 20, (ii) an acrylic addition polymer having an acid value lower than that of polymer (i). Preferably, the composition also comprises a cross-linked e.g. a polyisocyanate or a melamine resin.

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DESCRIPTION AQUEOUS ACRYLIC COATING COMPOSITION This invention relates to an aqueous coating composition containing acrylic addition polymers. It also relates to a process for producing the coating composition, to a process of coating using the composition and to a coated substrate obtainable by the coating process. One major known class of coating compositions comprises acrylic addition polymers as film formers. These can be carried in an organic solvent or an aqueous medium. The polymer can have various functional groups such as hydroxyl groups. These compositions can also contain a crosslinkcr which reacts with functional groups on the polymer so as to crosslink the final coating film. The crosslinker can be for example a polyisocyanate or a melamine formaldehyde resin. The conmpositions are applied as a layer to the surface of a substrate and either left to dry and cure at room temperature or else heated to initiate or speed the drying and curing process. During drying and curing the solvent or water evaporates and the polymer and cross-linker react together so as to produce a crosslinked coating film. Due to environmental considerations there is a general trend in the coatings industry towards coating compositions with reduced organic solvent content. Coatings with a lower organic solvent content emit lower levels of solvent when they are used and so are less polluting of the atmosphere. One way to achieve a lower solvent content is to use waterbome compositions. One method of incorporating acrylic addition polymers into water is to make them carboxyl (-COOH) functional by the incorporation of some carboxyl functional ethylenically unsaturated monomer such as acrylic acid during their manufacture, and neutralising at least some of the carboxyl groups in the aqueous composition by adding a base such as alkali metal hydroxide, ammonia or an amine. The resulting neutralised carboxyl groups stabilise the polymer in dispersion in water. A problem, which we have discovered, is that in practice dilution with water to achieve suitable viscosities for application results in a solids content that is too low to achieve adequate film build per coat. For spray applications a solids content of 35 - 60% is preferred. We have found that using a combination of acrylic polymers of different acid values and different hydrophobic monomer contents result in a water borne composition which can have a viscosity suitable for application whilst at the same time having a sufficiently high solids content to produce good film build per coat. In this specification AV is often used as an abbreviation for acid value. Acid value •is the mass of potassium hydroxide in milligrams required to neutralize the acid groups in 1 gram of said resin. According to the present invention in one aspect there is provided an aqueous thermosetting coating composition comprising a dispersion in an aqueous mediam of (i) an acrylic addition polymer having an acid value of at least 20, and (ii) an acrylic addition polymer having an acid value lower than that of polymer (i) and (iii) a base, wherein polymer (i) and/or polymer (ii) contain additional functional groups, and wherein polymer (i) has a lower hydrophobic monomer content than polymer (ii) and the overall styrene and substituted styrene content of polymers (i) and (ii) is 15.0% to 23.8%. Preferably the overall styrene and substituted styrene content of polymers (I) and (ii) is 23.75% or less, more preferably 22.5% or less, yet more preferably 21.25% or less. According to a second aspect of the invention there is provided an aqueous thermosetting coating composition comprising a dispersion in an aqueous medium of (i) an acid the addition polymer having a acid value of at least 20, (ii) an acrylic addition polymer having an acid value lower than that of polymer (i) and (iii) a base, wherein polymer (i) and/or polymer (ii) contain additional functional groups, and wherein (i) has a lower hydrophobic monomer content than polymer (ii), and polymer (i) has a hydrophobic monomer content of 10% by weight or less. In either aspect of the invention, preferably the AV of polymer (i) is from 35 to 150, more preferably from 50 to 120, even more preferably from 75 to 100, and ideally 95. Preferably the AV of polymer (ii) is less than 20, more preferably less than 10, and even more preferably 5 or less. Preferably polymer (i) and polymer (ii) are present at a ratio of 1:99 to 99:1 by weight, more preferably 1:99 to 1:1, even more preferably l:19to3:7. The acrylic addition polymers are derived from polymerisable ethylenically unsaturated monomers such as vinyl or acrylic monomers. The polymers comprise structural imits. At least one of the polymers also comprises carboxyl functional units. At least one of the polymers also comprises other functional imits. Preferably both of the polymers also comprise other functional units. Preferred acrylic addition polymers have a number average molecular weight as measured by gel pemieation chromatography of 700 to 10,000, more preferably 1,000 to 6,000, most preferably 1, 500 to S, 000. Preferred acrylic addition polymers, have a theoretical glass transition temperature (Fox Tg) of -30 to l00C, more preferably 0 to 80"C. When used herein, the term acrylic monomer refers to acrylic or methacrylic acid or their esters. The term (meth) acrylate refers to both the acrylate and methacrylate equally and the term (meth)acrylic acid refers to acrylic or methacrylic acid equally. Structural units are derived from monomers which are non-functional, that is they do not have reactive functional groups. Examples of non-functional monomers are alkyl esters of (raeth)acrylic acid and non-functional vinyl monomers. Examples of suitable alkyl esters of (meth)acrylic acid are CI-12 alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (metb)acrylate, t-butyl (meth)acrylate, and n-propyl (meth)aciylate. Examples of non-functional vinyi monomers are styrene and alpha-methyl styrene. Structural units can also be derived from non-functional chain transfer agents.. chain transfer agents are compounds which are used in the manufuture of acrylic addition polymers to control their molecular weight Examples of known chain transfer agents include merapto compounds. Examples of mercaptan compounds that can be used to produce structural units include octyl mercaptan, dodecyl mercaptan and pentaeiythritol tetraO-mercqitcqnDirionate). Preferabbly polyinar (10 has a hydrophobic monomer content of at least 10% by weight more prefetably at least 20% by weight Polymer (i) has lower hydrophobic monoiner content than polymer (ii). Preferably polymer (i) has a hydrophobic monomer content of 10% or less. More preferably polymer (i) has a hydrophobic monomer content of 5% or less. Hydrophobic monomers are stynne, substituted styrenes (e.g. alpha-methyl styrene, vinyl toluene, t-butylstyrcne) and alkyl esters of (meth) acrylic acid where the alkyl group contains six or more carbon atoms. Preferred hydrophobic monomers are styrene, alpha-methyl styrene, isobomyl. methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate and lauryl methacrylate. In the first aspect of the invention, the combined weight of styrene and substituted styrene in polymers (I) and (ii) is 30% or less of the total weight of polymers (I) and (ii). This is particularly important if the coating is a topcoatdesigned for exterior use. Topcoats having high levels of styrene tend to have poor resistance to ultaviolet radiation leading to gloss loss, cracking and colour change. The carboxyl fimctional units can be derived from unsaturated carboxylic acid functional monomers, carboxylic acid fimctional chain transfer agents or carboxylic acid functional initiators. Examples of carboxyiic acid functional monomers are acrylic acid and methactylic acid. Examples of carboxylic acid functional chain transfer agents are mercaptoacetic acid, 3-mercaptopropionic acid and 2-mercaptopropionic acid. An example of an acid functional initiator is 4,4"-azobis (4-cyBnopentanoic acid). It is also possible to produce carboxyl functional units by modifying other functional units on the polymer. For example hydroxyl functional groups can be reacted with cyclic anhydrides such as phthalic anhydride or hexahydrophthalic anhydride. Preferably the carboxyl functional units are derived from (meth)acrylic acid and/or 3-mercaptopropionic acid. Other functinal units are derived from monomers which carry reactive groups other than carboxyl groups, such as hydroxyl groups, acetoacetate groups, alkoxysilane groups and epoxy groups. An example of a monomer carrying an acetoacetate group is acetoacetyl methacrylate. An example of a monomer carrying an epoxy group is glycidyl (meth)acrylate. An example of a monomer carrying an alkoxysilane group is 3-(trimethoxysilyl)propyl methacrylate. The composition preferably comprise a crorss- linker for the addition polymer. The ciosslinker is a compound havmg on average at least two functional groups per molecule that will react with functional groups on the polymer. Examples of suitable crosslinkers are melamine formaldehyde resins, polyamines, polyamides,ketimines, aldimines and polyisocyanates. When at least one of the polymers has acetoacetate functional groups suitable crosslinkers include polyamines. Polyamines are compounds containiag at least two primary and/or secondary amine groups per molecule. Examples of polyamines are the EPI-CURE (TM) range of polyamines available from Shell Chemicals. The amine groups can be blocked, for example by reacting primary amine groups with aldehydes or ketones. Examples of blocked polyamines are Desmophen LS 2965 and Desmophen LS 2142 available from Bayer. When at least one of the polymers has alkoxysilane functional groups crosslinking can occur without the need for a separate crosslinker. However alkoxy silane functional crosslinkers can be added, for example to increase the crosslink density of the coating. Examples of silane functional crosslinkers include the reaction products of polyisocyanates and aminosilanes as disclosed in EP 571 073 and the reaction products of polyacrylates and aminosilanes as disclosed in US 4 429 082. When at least one of the addition polymers has epoxy groups suitable crosslinkers include polyamines and polyamides. Polyamides are obtained by reacting dimeiised fatty acids with polyamines. Examples of polyamides include tiie EPI-CURE (TM) range of polyamides available from Shell Chemicals. Preferably at least one of the polymers contains hydroxyl functional imits. More preferably both of the polymers contains hydroxyl functional units. If only one of the polymers contains hydroxyl functional groups, preferably it is the polymer with the lower AV ie. polymer (ii). The optional hydroxyl functional units can be derived from hydroxyl functional vinyl or acrylic monomers or from hydroxyl functional chain transfer agents. An example of a hydroxyl functional vinyl monomer is vinyl alcohol. Examples of hydroxyl functional acrylic monomos are hydroxyetbyl (meth)acrylate, and hydroxypiopyl (meth)acrylate and hydroxybutyl (meth)acrylate. An example of a hydroxyl functional chain transfer agent is mercaptoethanol. Other examples of suitable hydroxyl functional acrylic monomers are the reaction products of glycidyl (meth)acrylate with mono-carboxylic acids, such as versatic acid and the reaction product of (meth)acrylic acid with monoepoxy compounds such as Cardura E (the glycidyl ester of versatic acid; trade mark of Shell). Preferably, at least one of the polymers comprises 10 to 50% by weight of hydroxyl functional units, more preferably 10 to 40% by weight. More preferably, both of the polymers comprises 10 to S0% by weight of hydroxyl functional units, more preferably 10 to 40% by weight Preferably at least one of the polymers has a hydroxyl value of S to 500 mgKOH/g of polymer, more preferably 50 to 250. More prefiaably both of the polymers have a hydroxyl value of 5 to 500 mgKOH/g of polymer, more preferably 50 to 250. Preferably at least one of the addition polymers contains hydroxyl functional units and the compositions comprise a crosslinker. More preferably both of the addition polymers contain hydroxyl functional units and the compositions comprise a crosslinker. Suitable crosslinkers include melamine formaldehyde resins and polyisocyanates. Polyisocyanates are preferred. Melamine formalddiyde resins are the reaction products of raelamine and fonnaldehyde. Examples include partially methylated melamines and hexamethoxy methyl melamine. Polyisocyanates are well known in the coatings art. Polyisocyanates are compounds having two or more isocyanate groups per molecule. Suitable polyisocyanates are aliphatic or aromatic polyisocyanates. Examples of suitable aliphatic diisocyanates are hexamethylene diisocyanate, isophorone diisocyanate and l,3-bis(isocyanato-l-methylethyl)benzeoe (TMXDI). Examples of suitable aromatic diisocyanates are toluene diisocyanate and 4,4"-diphgiylmethane diisocyanate. Other suitable polyisocyanates include the isocyanurate trimers, allophanates and uretdiones of diisocyanates such as those described above as well as the reaction products ofthese diisocyanates with polyols.Polyols are compounds having three or more hydroxyl groups. Suitable polyols include trimethylol propane, glycerol and pentaerythritol. Many such polyisocjranates are commercially available, for example under the Desmodur trademark from Bayer, the Tolonate trademark from Rhodia and the Cythane trademark from Cytec Industries. The polyisocyanates can optionally be blocked by reacting them with a blocking compound. A blocking compound is one which reacts reversibly with isocyanate groups to form an adduct which is stable at ordinary temperatures but which breaks down so as to regenerate the isocyanate groups at elevated temperatures, such as those used to cure the coating composition. In order to make them dispersible in water, the polyisocyanates may also comprise a dispersion stabiliser, such as the product of the reaction between a polyisocyanate and a polyethylene glycol. Suitable stabilisers and mixtures of these stabilises and polyisocyanates are known from European Patent EP-B-0 206 059 and European Patent EP-B-0 516 277. Polyisocyanate crosslinkers are preferably used in an amount such that the ratio of isocyanate groups on the polyisocyanate to the number of hydroxy! groups on the polymer is in the range 0.8 to 2 . The conxpositions also comprise a base that at least partially neutralises the carboxyl groups on the addition polymers. Ammonia or an amine or mixtures thereof are the preferred bases, while alkali metal hydroxide bases are useful but less preferred. Examples of suitable amines are dimethylethanol amine, 2-amino-2-methyl-l-isopanol and trietfaylamine. Preferably, the amount of base present is such as to be enable of neutralising between 30% and 100% of the carboxyl groups on the addition polymers. The aqueous medium comprises predominantly water, optionally also containing organic solvent. Preferably the aqueous medium comprises at least 50% by weight of water and most preferably at least 70%. Suitable water miscible organic solvents include alcohols such as butanol, ether-alcohols such as propylene glycol monomethyl ether and ester alcohols such as propylene glycol methyl ether acetate. The compositions can also contain catalysts for the crosslinking reaction. Suitable catalysts include tin catalysts such as dibutyl tin dilaurate and amine catalysts such as triethylamine. TTie compositions can also contain other conventional paint additives such as reactive diluents, pigments, fillets. UV absorbers and flow aids. The compositions can also comprise additiowl polymeric components such as film forming polymers. Examples are polyester polymers or polyurethane polymers. Preferably the polymeric components of the composition, excluding any crosslinkers, are made up of at least 40% by weight of acrylic addition polymers, more preferably at least 50% and most preferably at least 75%. The acrylic addition polymers can be produced by conventional means. In general they can be produced by contacting a mixture of the appropriate monomers including any chain transfer agent with a polymerisation initiator at a temperature at which polymerisation occurs. The process for preparing the addition polymers can be carried out in organic solution.or. akematively it can be carried out in the aqueous For example, the initiator can be fed into the solvent at the polymerisation temperature simultaneously with the monomer misdure. When the polymer is made by emulsion polymerisation, the monomer mixture can be fed together with the initiator and a surfectant into the stirred aqueous medium held at the polymerisation temperature. Sufficient base to at least partially neutralise the carboxyl groups on the polymer can be added before, during or after polymerisation but is typically added afterwards. Typical polymerisation temperatures are 50 to 140°C when the SUBSTITUTE SHEET (RULE 26) process is carried out in solvent and 50 to 95""C when it is carried out by emulsion polymerisation in water. Initiators can include for example typical free radical and redox types such as hydrogen peroxide, t-butyl hydroperoxide, di>t-butyl peroxide, butylperoxy-2-ethyl hexanoate. benzoyl peroxide, 2,4-dichlorbenzoyl peroxide, t-butylpcracetate, 2,2" azobis (2-methylbutyranitrile), ammoniiun persulphate, sodium persulphate, potassium persulphate, sodium and potassium peiphosphates, and redox initiators such as posulphate/sodium formaldehyde sulphoxylate, cumene hydroperoxide/sodium metabisulphite, potassium persulphate/sodium bisulphite, cumene hydroperoxide/iion (II) sulphate and the like. Polymerisation initiators are usually added in amounts between about O.l and 6% by weight of the monomers polymerised, preferably between O.S and S%. A preferred process for preparing the coating composition comprises (a) contacting a mixture of polymer (i) and polymer (ii) in organic solvent with a base so as to at least partially neutralise the carboxyl groups on the polymer and (b) forming a dispersion of the at least partially neutralised polymer in an aqueous medium. This creates what is commonly known as a secondary dispersion. This type of dispersion is preferred over a primary dispersion. A primary dispersion results from a polymerisation process that leads directly to a polymer dispersed in water, for example emulsion polymerisation. Any crosslinker and any other components can be dispersed into the aqueous dispersion of addition polymer. Generally, when the crosslinker is an unblocked polyisocyanate then the composition is preferably made by adding the polyisocyanate, optionally as a solution in an organic solvent, to the aqueous dispersion shortly before use. This minimises the problems with the potlife of the SUBSTITUTE SHEET (RULE 26) compositions. In a particularly preferred embodiment the crosslinkcr and other components can also be mixed with the polymer in organic solvent before the polymer is dispersed in water. The coating composition of the invention can be applied as a layer to the surface of a substrate and then allovtred or caused to dry and cure. According to the present invention there is provided a process for coating a substrate that comprises the steps of applying a layer of a coating composition according to the present invention to a surface of the substrate and thereafter causing or allowing the layer to cure. The compositions are particularly useful as vehicle refinish primers or topcoats. Primers are somewhat heavily pigmented compositions that are applied over the bare substrate or over the pre-existing coating before the new topcoat is aj^lied. Topcoats are the final coating applied to give the vehicle its colour and gloss as vrell as providing protection from the elements and physical damage. The coating composition can be applied by conventional means such as brushing, rollovoating or spraying, preferably by spraying. The applied layer can be allowed to cure at ambient temperature in the case where the polymer and crosslinker react together at ambient temperatures. This is generally the case for example when the polymer has hydroxyl groups and the crosslinker is a polyisocyanate. Alternatively the layer can be baked at elevated temperatures, for example SO-130 C, either to accelerate curing or to cause curing when the crosslinker will only react with the polymer at elevated temperatures. This is generally the case when the polyner has hydroxyl groups and the crosslinker is a blocked polyisocyanate or a melamine formaldehyde. According to the present invention there is also provided a coated article obtainable by the process. The invention will now be illustrated by means of the following examples. In the examples the following abbreviations are used: AA acrylic acid AMS alpha-methylstyrene AV acid value (mgKOH/gNV) BA butyl aciylate Cythane 3174 adduct of 1,3-bis isocyanate 1-methylethyl) benzene (TMXDI) and 2-ethyl-2- (hydroxymethyl)-l,3-propanediol (trimethyl propane) available from Cytec. DMAE N,N-dimethylethanolamine HBA 4-hydroxybutyl acrylate HTD LV isocyanurate trimer of 1,6-diisocyanatohexane available fiom Rhodia as TolonateHDTLV MiAK methyl isoamyl ketone MMa methyl methaciylate MPA 3-mercaptopropionic acid POM 1-octanethiol EXAMPLES 1- Synthesis of acrylic copolymers 1 to 17 Actylic copolymers 1 to 17 as set out in table 1 were synthesised using the follovving procedure. The charge was heated to reflux (approximately 140°C) in a reaction vessel fitted with stiner, heating mantle, water condenser and nitrogen blanket. The charge was held at reflux and stirred wliilst the feed mixture was fed into the reaction vessel at a uniform rate over 180 minutes. The mixture was stirred for a further 15 minutes. Initiator shot 1 was added over 10 minutes, the mixture stirred for one hour. Initiator shot 2 was added over 10 minutes, the mixture was stired for a further one hour and then allowed to cool. EXAMPLE I TO XIV Formulations of polymer only systems (i.e. without a cross-linker) that have been studied are given in table 3. The polymer numbers are the polymers shown in table 1. The polymer systems were evaluated using the following procedure. The polymers and DMAE were mixed thoroughly. Demineralised water was then added slowly to the mixture whilst it was stired with a spatula. The mass of water required to reduce the viscosity of the system to between 25 and 30 seconds in a DIN 4 flow cup was recorded. The results are shown in tabular form in Table 4, and in graph forai in Figure 1 of the accompanying drawings. These compositions could all be converted to thermosetting coating compositions by the addition of a suitable crosslinker, for example a polyisocyanate. and AV. This benefit is illustrated graphically in Figure 1 of the accompanying drawings. EXAMPLES XV TO XXV The formulations are given in Table 5. Fonnulations of polymers with cross-linker systems were prepared and tested as follows:- The polymos and DMAE were mixed thoroughly in the proportions shown in Table 5. The polymer numbers correspond to the poljrmers of Table I. The isocyanate was then added with stirring until homogeneous. Demineralised water was then added slowly to the mixture whilst it was stirred with a spatula. The mass of water required to reduce the viscosity of the system to between 25 and 30 seconds in a DIN 4 flow cup was recorded. Table 5 - polymer and crosslinker systems - formulations ISO is a mixture of 80% Cythane 3174 and 20% HTD LV The results are shown in tabular form in Table 6, and in graph form in Figure 2 of the accompanying drawings. Table 6 demonstrates that the blends consistently give higher solids than a single resin of the same average monomer composition and AV. Examples XXVI to XXXI The fonnulations are given in table 7. Formulations of polymers with crosslinker systems were prepared and tested as follows:- The polymers and DMAE were mixed thoroughly in the proportions shown in table 7. The polymer numbers correspond to the polymers of table 1. The isocyanate was then added with stirring until homogeneous. Demineralised water was then added slowly to the mixture whilst it was stirred with a spatula. The mass of water to reduce the viscosity of the system to between 25 and 30 seconds in a DIN 4 cup was recorded. lAELEI Table 7 - polymer and crosslinker systems - formulations ISO is a mixture of 80% by weight Cythane 3174 and 20% by weight HDT LV The results are shown in table 8 TABLE 8 Table 8 - polymer and crosslinker systems - results Example XXXIl 80g of polymer 16,20g of polymer 17 and 2.92g of DMAE were mixed until homogeneous. 130g of demineralised water were then added slowly whilst the mixture was stined with a spatula. 3S.4g of Bayhydur 3100 (hydrophilicaily modified trimer of 1,6-diisocyanatohexane available from Bayer) were then added with stirring following by enough demineralised water to reduce the vicosity of the mixture to between 25 and 30 seconds in a DIN 4 flow cup. 2S.7g were required giving a solids content of 35.6%. The overall styrene and substituted styrene content lofpolymers (i) and (ii) was 16.2% . PrepaTation nf a polyester polvol 23.3 parts by weight of trimethylol propane, 9.5 parts by weight of neopentyl glycol and 37.0 parts of hexahydrophtfaalic anhydride were heated with siining under a nitrogen atmosphere until the mixture began to exotheim (at about 90"C) v^ien heated was stopped. The temperature of the mixture rose to about 140"C due to the exodienn. When the exothenn was over and temperature began to fall, heating was resumed and the temperature was raised to 205 "C. As the temperature was raised, water was removed via a fractionating column. When the mixture was at 205 "C and the evolution of watn- had slowed, the mixture was allowed to cool. The fractionating column was replaced with a Dean & Stark column filled with methyl isobutyl ketone. Methyl isobutyl ketone (3.2 parts) was added to the mixture and the temperature was raised to reflux to remove further water. When the acid value of the mixture reached 30-35 mg KOH/g non-vol. the heat was removed. The mixture was allowed to cool to 120""C. Methyl amyl ketone (27.0 parts) was then added and the mixture was stirred until a homogenous solution had formed. Examples XXXIII to XXXIII Six cold rolled steel test panels were sanded with P180 sandpaper and solvent wiped with white spirit. The panels were then coated with 20-25 microns of chromate free etch primer PS6S-713 (available from ICI Autocolor, used as instructed on the product data sheet), followed by 100 microns of Hidur Rapide undercoat PS65-693 (available from ICI Autocolour, used as instructed on the product data sheet). 100 parts by weight of Aquabase medium coarse aluminium basic P968-9987 was mixed with 10.3 parts by weight of Aquabase activator P93S-2018 and 3 parts by weight of Aquabase thimier P275-366 (all available from ICI Autocolor). This was spray applied to the panels to give a film build up of 12-IS microns. The panels were then coated with the cleaicoat compositions given in table 9 below. Compositions XXXIII to XXXVII were prepared by first mixing all of the ingredients except the polyisocyanate until homogenous. The polyisocyanate was added with stirring to give a homogeneous mixture. Demineralised water was then added slowly to the mixture whilst it was stirred with a spatula. The mass of water required to reduce the viscosity of system to between 25 and 30 seconds in a DIN 4 flow cup was recorded. All compositions were prepared immediately prior to spraying. Composition XXXHI required lS8.4g of water giving a solids content of 35.1%, composition XXXTV required I24.4g of water giving a solids content of 40.6%, composition XXXV required 179.9g of water giving a solids content of 33.3%, composition XXXVI required 172.3g of water giving a solids content of 35.0% and composition XXXVII required 256.2g of water giving a solids content of 26.8%. Composition XXVIII had a solids content of 50.3%. PI 90-643 was prepared as instructed on the product data sheet. The clearcoats were spray applied to give a film build of about 60 microns. All of the compositions required two coats to achieve the required film build except for example XXXVII that required three coats. After application the coated panels were left at room temperature for thirty minutes before being low-baked at 60 °C for thirty minutes. The 20° gloss of each coating was measured using a Erichsen (trademark) glossmeter and the hardness was measured lasing a Leitz (trademark) Knoop Hardness Tester. The results are shown in table 10. The solvent content of each composition, expressed as the mass of organic solvent present per l00g of non-volatile material, is also shown in this table. WE CLAIM: 1. An aqueous thermosetting coating composition comprising a dispersion in an aqueous medium of (i) an acrylic addition polymer having a acid value of at least 20, (ii) an acrylic addition polymer having an acid value lower than that of polymer (i) and (iii) a base, wherein polymer (i) and/or polymer (ii) contain additional functional groups, and wherein (i) has a lower hydrophobic monomer content than polymer (ii) and the overall styrene and substituted styrene content of polymers (i) and (ii) is 15.0% to 23.8%. 2. The composition as claimed in claims 1, in which the overall styrene and substituted styrene content of polymers (i) and (ii) is from 15.0% to 22.5%. 3. The composition as claimed in any one of the preceding claims, in which the overall styrene and substituted styrene content of polymers (i) and (ii) is from 16.1% to 21.3%. 4. The coating composition as claimed in any one of the preceding claims, in which polymer (i) has a hydrophobic monomer content of 10% by weight or less. 5. An aqueous thermosetting coating composition comprising a dispersion in an aqueous medium of (i) an acrylic addition polymer having a acid value of at least 20, (ii) an acrylic addition polymer having an acid value lower than that of polymer (i) and (iii) a base, wherein polymer (i) and/or polymer (ii) contain additional functional groups, and wherein (i) has a lower hydrophobic monomer content than polymer (ii), and polymer (i) has a hydrophobic monomer content of 10% by weight or less. 6. The coating composition as claimed in any one of the preceding claims, in which polymer (i) has a hydrophobic monomer content of 5% by weight or less. 7. The coating composition as claimed in any one of the preceding claims, in which polymer (ii) has a hydrophobic monomer content of at least 10% by weight. 8. The coating composition as claimed in any one of the preceding claims, in which polymer (ii) has a hydrophobic monomer content of at least 20% by weight. 9. The composition as claimed in any one of the preceding claims, in which polymer (i) has an acid value of 35 to 150. 10. The composition as claimed in any one of the preceding claims, in which polymer (i) has an acid value of 50 to 120. 11. The composition as claimed in any one of the preceding claims, in which polymer (i) has an acid value of 75 to 100. 12. The coating composition as claimed in any one of the preceding claims, in which polymer (i) has an acid value of 95. 13. The coating composition as claimed in any one of the preceding claims, in which polymer (ii) has an acid value of less than 20. 14. The coating composition as claimed in any one of the preceding claims, in which polymer (ii) has an acid value of less than 10. 15. The coating composition as claimed in any one of the preceding claims, in which polymer (ii) has an acid value of 5 or less. 16. The coating composition as claimed in any one of the preceding claims, in which polymer (i) and polymer (ii) are present at a ratio of 1:99 to 99:1. 17. The coating composition as claimed in claim 16, in which polymer (i) and polymer (ii) are present at a ratio of 1:99 to 1:1. 18. The coating composition as claimed in claim 17, in which polymer (i) and polymer (ii) are present at a ratio of 1:19 to 3:7. 19. The coating composition as claimed in any one of the preceding claims in which polymer (i) and/or polymer (ii) contain additional functional groups selected from hydroxyl groups, aceto acetate groups, alkoxy silane groups and epoxy groups. 20. The coating composition as claimed in claim 19 also comprising a crosslinker. 21. The coating composition as claimed in claim 19 or 20, in which the additional functional groups are hydroxyl groups. 22. The coating composition as claimed in claim 19, in which the additional functional groups are hydroxyl groups and the cross linker is a polyisocyanate or a melamine resin. 23. The coating composition as claimed in claim 19, in which the crosslinker is a polyisocyanate. 24. A process for preparing a coating composition as claimed in any of the preceding claims which comprises forming a dispersion components (i) to (iii) as claimed in any one of claims 1 to 20. 25. An article such as a substrate coated with a coating composition as claimed in any one of claims 1 to 24. 26. An article such as a substrate as claimed in claim 25, in which said composition has been cured. 27. A process for forming a coated article such as a substrate, which comprises coating, said article with the composition as claimed in any one of claims 1 to 26 and curing said composition. 28. A coated article such as a substrate obtained by a process as claimed in claim

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