Waterborne radiation-curable coating composition

Abstract

The present invention concerns an aqueous radiation-curable polyurethane acrylate dispersion comprising: a) a polyurethane acrylate polymer which is the reaction product of: i) from 10 wt.% to 50 wt.% of an isocyanate monomer; ii) from 5 wt.% to 20 wt.% of a polyunsaturated monomer or polyunsaturated oligomer, which has an iodine value of at least 50 g of I2 / 100g of respectively polyunsaturated monomer or polyunsaturated oligomer, and at least one hydroxyl functional group; iii) from 10 wt.% to 55 wt.% of an acrylate functional oligomer which contains at least one (meth)acrylate group and at least one isocyanate-reactive group; and iv) from 1 wt.% to 10 wt. % of a dispersing agent which contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action; dispersed in b) an aqueous continuous phase; and a coating composition and kit-of-parts with a drying agent; and a coated substrate.

Description

[0001] P81927047W001

[0002] 1 WATERBORNE RADIATION-CURABLE COATING COMPOSITION

[0003] Field of the Invention

[0004] The present invention relates to an aqueous radiation-curable coating composition. More specifically, it relates to an aqueous, radiation-curable polyurethane acrylate dispersion, and a coating composition comprising adispersion which has improved ability for through-cure, leading to coatings with higher hardness and chemical resistance than those known in the art.

[0005] Background

[0006] Aqueous radiation-curable coating compositions comprising polyurethane dispersions are well-known in the art. Radiation curing, especially UV-curing, offers many advantages over other curing techniques, for example fast curing times, relatively low energy expenditure, avoiding the use of volatile organic chemicals (VOC’s) and improved coating properties such as hardness, scratch resistance and chemical resistance. Such coating compositions are well-known for application to many industrial substrates including ceramics, plastics, wood and metal. They are commonly used for coating wood trim, window frames, furniture components and similar objects, especially wooden furniture components. UV-curing is most commonly used for clear or white coatings but also for colored coating compositions. However, pigments present in colored coating compositions, in particular yellow, black and red, and to a lesser extent green and blue pigments, absorb at the same wavelengths as commonly used UV- photoinitiators. The presence of such pigments impedes the applied radiation in reaching the photoinitiators, resulting in incomplete cure of a colored coating composition. The effect is more pronounced away from the open side of the coating, i.e. away from the side which is exposed to the radiation source. Thus through-cure of such colored coating compositions by UV radiation is difficult, leading to softer, less chemical resistant coatings.

[0007] Another drawback of known radiation-curable coatings is that curing only occurs on areas directly exposed to the radiation during a conventional industrial radiation-cure step. A complex molded substrate may have areas which are shaded from the P81927047W001

[0008] 2 radiation. Even if multiple radiation sources are used to address this, complex substrate designs may still have areas which are not directly exposed to radiation or are exposed to insufficient radiation to effect adequate curing. Coating at these shaded areas therefore suffers inferior mechanical and chemical properties.

[0009] A method to mitigate this is the subject of US2013 / 041072, which describes the use of an aqueous dispersion of polyurethane acrylates based on both aromatic polyepoxy (meth)acrylates and oligo- and polyesters containing unsaturated fatty acids. In such a system, two curing mechanisms occur: i) radiation initiated radical polymerization and ii) oxidative curing. Significantly higher mechanical strength and better chemical resistances are reported compared with known polyurethane acrylate dispersions. The use of a siccative, e.g. a salt of lead, cobalt, iron, manganese or copper, to accelerate oxidative cross-linking is mentioned; less than 0.1 wt.% of Borchi Oxy Coat is exemplified. However, no advantage of using this was demonstrated.

[0010] JPH03126781 A describes oxidatively curable water-based coatings based on either a dispersed fatty acid modified polyester (alkyd) or a fatty acid modified acrylic latex. These are mixed with a multifunctional acrylic monomer and a dispersible cobalt drier. A mechanism is described wherein the fatty acids upon drying generate radicals that react with the multifunctional acrylic monomers. However, the alkyd or acrylic latex used is not acrylate functional and is not suitable for curing by use of radiation.

[0011] US2003 / 0191273 describes polyurethane dispersions based on fatty acid dialkanolamides prepared from saturated fatty acids, with improved hardness in clear and white pigmented coatings. An acrylate functional polyester was used to make the polyurethane dispersion radiation curable. A transesterification between castor oil and soybean oil was used to introduce oxidatively drying compounds.

[0012] US2008 / 0242757 describes a composition curable by UV-A radiation, comprising an aqueous polyurethane dispersion; an aqueous polyester acrylate / urethane dispersion; one or more photoinitiators, and water or a mixture of water and solvent. P81927047W001

[0013] 3

[0014] The objects of the invention are met by the first embodiment, which provides an aqueous radiation-curable polyurethane acrylate dispersion comprising: a) a polyurethane acrylate polymer which is the reaction product of: i) from 10 wt.% to 50 wt.% of an isocyanate monomer; ii) from 5 wt.% to 20 wt.% of a polyunsaturated monomer or polyunsaturated oligomer, which has an iodine value of at least 50 g of I2 / 100g of respectively polyunsaturated monomer or polyunsaturated oligomer, and at least one hydroxyl functional group; ill) from 10 wt.% to 55 wt.% of an acrylate functional oligomer which contains at least one (meth)acrylate group and at least one isocyanate-reactive group; and iv) from 1 wt.% to 10 wt.% of a dispersing agent which contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action; dispersed in b) an aqueous continuous phase.

[0015] In a second embodiment is provided an aqueous radiation-curable coating composition comprising: a) an aqueous radiation-curable polyurethane acrylate dispersion comprising: a polyurethane acrylate polymer which is the reaction product of: i) an isocyanate monomer; ii) a polyunsaturated monomer or polyunsaturated oligomer, which has an iodine value of at least 50 g of I2 / 100g of respectively polyunsaturated monomer or polyunsaturated oligomer, and at least one hydroxyl functional group; iii) an acrylate functional oligomer which contains at least one (meth)acrylate group and at least one isocyanate-reactive group; and iv) a dispersing agent which contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action; dispersed in an aqueous continuous phase; b) a drying agent; c) optionally a pigment; P81927047W001

[0016] 4 d) optionally a photoinitiator; e) optionally one or more additives; wherein the drying agent is present in an amount of from 0.2 wt.% to 10 wt.% of the total weight of respectively the coating composition.

[0017] In a third embodiment is provided a kit of parts comprising:

[0018] A. a polymer component, which comprises: a) an aqueous radiation-curable polyurethane acrylate dispersion comprising: a polyurethane acrylate polymer which is the reaction product of: i) an isocyanate monomer; ii) a polyunsaturated monomer or polyunsaturated oligomer, which has an iodine value of at least 50 g of I2 / 100g of respectively polyunsaturated monomer or polyunsaturated oligomer, and at least one hydroxyl functional group; iii) an acrylate functional oligomer which contains at least one (meth)acrylate group and at least one isocyanate-reactive group; and iv) a dispersing agent which contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action; dispersed in an aqueous continuous phase; b) optionally a pigment; a) optionally a photoinitiator; b) optionally one or more additives;

[0019] B. a drying component, which comprises a drying agent and a solvent; wherein the drying agent is present in an amount of from 0.2 wt.% to 10 wt.% of the total weight of the kit of parts.

[0020] In a fourth embodiment is provided coated substrate comprising a substrate and a coating, which coated substrate is obtainable by: a) applying an aqueous radiation-curable coating composition as defined herein or a mixture comprising the polymer component A and drying component B of the kit of parts as defined herein to the substrate; b) allowing water to evaporate from respectively the aqueous radiation-curable coating composition or mixture; and P81927047W001

[0021] 5 c) applying UV radiation and / or electron radiation respectively to the aqueous radiation-curable coating composition or mixture.

[0022] Detailed description

[0023] An isocyanate monomer may be an aromatic, araliphatic, aliphatic or cycloaliphatic isocyanate. In one embodiment, the isocyanate monomer is a polyisocyanate monomer. Preferably, the polyisocyanate is a diisocyanate. Examples of polyisocyanate (diisocyanate) monomers include 1 ,3-cyclohexane-diisocyanate, 1 - methyl-2,4-diisocyanato-cyclohexane, 1 -methyl-2, 6-diisocyanato-cyclohexane, tetramethylene-diisocyanate, 4,4'-diisocyanatodiphenylmethane, 2,4'- diisocyanatodiphenylmethane, 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, a,a,a',a'-tetramethyl-m- or p-xylylenediisocyanate, 1 ,5-pentamethylene-diisocyanate, 1 ,6-hexamethylene-diisocyanate, 1 -isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane (isophorone-diisocyanate or IPDI), 4,4'-diisocyanato- dicyclohexylmethane, 1 ,3-bis(iso-cyanato-methyl)benzene (XDI), l,3-bis(1-isocyanato- 1 -methylethyl)-benzene (TMXDI), 4-isocyanatomethyl-1 ,8-octane-diisocyanate (triisocyanatononane, TIN), or a homologue or oligomer of one of these diisocyanate monomers listed with 2-imidodicarbonic diamide (biuret), carbodiimide, isocyanurate, carbamoylcarbamate (allophanate), iminooxadiazinedione and / or uretdione groups. Typically, the diisocyanate monomer is 1 ,5-pentamethylene-diisocyanate, 1 ,6- hexamethylene-diisocyanate, 1 -isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane also known as (isophorone-diisocyanate or IPDI) or 4,4'- diisocyanato-dicyclohexylmethane.

[0024] The isocyanate monomer is typically present in an amount of from 10 wt.% to 50 wt.% of the solid components of the polyurethane acrylate dispersion. Preferably, it is present in an amount of from 20 wt.% to 35 wt.% for example from 25 wt.% to 30 wt.% of the solid components of the polyurethane acrylate dispersion.

[0025] The polyurethane acrylate polymer may comprise a mixture of isocyanate monomers, for example a mixture of two or more of the above-described diisocyanate monomers. P81927047W001

[0026] 6

[0027] The polyunsaturated monomer is typically a polyunsaturated fatty acid ester or cardanol. The polyunsaturated oligomer is an oligomer comprising at least one polyunsaturated monomer as defined herein. The polyunsaturated oligomer typically comprises a di-, tri-, tetra- or hexa-ol; a polyunsaturated monomer or polyunsaturated fatty acid; and optionally a further saturated aliphatic or aromatic di- or tri-acid or a mixture thereof.

[0028] The polyunsaturated fatty acid ester may be the esterification or transesterification product of an unsaturated fatty acid with an at least bifunctional polyol compound, preferably a tri- or tetra-functional hydroxyl component. Suitable at least bifunctional polyol compounds include, for example, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, 1 ,2-propanediol, 1 ,3-propanediol, 1 ,4-butanediol, neopentyl glycol, 2-ethyl-2-butylpropanediol, trimethylpentanediol, 1 ,3-butylene glycol, 1 ,4-cyclohexanedimethanol, 1 ,6-hexanediol, 1 ,2- and 1 ,4-cyclohexanediol, hydrogenated bisphenol A (2,2-bis(4- hydroxycyclohexyl)propane), diols derived from dimer fatty acids, 2,2-dimethyl-3- hydroxypropionic acid (2,2-dimethyl-3-hydroxypropyl ester), glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, ditrimethylolpropane, castor oil, partly dehydrated castor oil, pentaerythritol and dipentaerythritol. Preferred are glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, ditrimethylolpropane, pentaerythritol and dipentaerythritol.

[0029] Unsaturated fatty acids may be derived from oils, typically naturally occurring vegetable or animal oils. In such oils the fatty acid is present as part of an ester. Suitable fatty acids derived from oils include, for example, linseed oil fatty acid, soy bean oil fatty acid, sunflower oil fatty acid, rapeseed oil fatty acid, tall oil fatty acid and herring oil fatty acid. These fatty acids are typically a mixture of the acids occurring in the named oil. Soy bean oil, safflower oil, linseed oil or sunflower oil, are preferred. The oil-derived fatty acid mixture is typically a distilled product which predominantly (>60 wt. %) contains oleic, linoleic acid, licanic acid, arachidonic acid, palmitoleic acid, ricinoleic acid and linolenic acid; unsaturated fatty acids which correspond in their composition with respect to the fatty acid present in oils. An unsaturated fatty acid ester is an ester of one of the above-defined unsaturated fatty acids. P81927047W001

[0030] 7

[0031] The polyunsaturated oligomer may further comprise other constituent monomers, for example a monounsaturated monomer, for example a monounsaturated fatty acid ester, or a saturated monomer, for example a saturated fatty acid ester.

[0032] Saturated aliphatic or aromatic di- and tri-acids include, for example, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, adipic acid, hexahydrophthalic acid, malonic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, dodecanedioic acid, hydrogenated dimer fatty acids, trimellitic acid and analogous anhydrides thereof.

[0033] Preferred polyunsaturated monomers or polyunsaturated oligomers are reaction products of polyunsaturated fatty acids for example linoleic acid or linolenic acid, with castor oil or glycerol and optionally a further saturated aliphatic or aromatic diacid and / or reaction products of polyunsaturated oils with castor oil. Preferred polyunsaturated fatty acids are those polyunsaturated fatty acid mixtures which can be obtained from plant or animal oils, such as e.g. soy bean oil, safflower oil, linseed oil, sunflower oil or olive oil. Particularly preferred is an oligomer containing soy bean fatty acid, sebacic acid and glycerol.

[0034] In one embodiment the polyunsaturated monomer or polyunsaturated oligomer ii) typically comprises a polyunsaturated fatty acid, a polyunsaturated fatty acid ester or a polyunsaturated phenol. Typically the unsaturated phenol is cardanol. Typically the polyunsaturated oligomer ii) comprises one or more of cardanol, soybean fatty acid, soybean oil, safflower fatty acid, safflower oil, linseed fatty acid and linseed oil.

[0035] The polyurethane acrylate polymer may comprise more than one polyunsaturated monomer or polyunsaturated oligomer.

[0036] Iodine value is a measure of the degree of unsaturation. The iodine value of the polyunsaturated monomer or polyunsaturated oligomer is typically at least 75 g of I2 / 100g; preferably at least 100 g of I2 / 100g. The iodine value polyunsaturated monomer or polyunsaturated oligomer is typically at most 250 g of I2 / 100g; preferably at most 220 g of I2 / 100g; more preferably at most 200 g of I2 / 100g. P81927047W001

[0037] 8

[0038] The polyunsaturated monomer or polyunsaturated oligomer typically has an OH number of from 15 to 300 mg of KOH / g polyunsaturated monomer or polyunsaturated oligomer respectively. Preferably it has an OH number of from 50 to 180 mg of KOH / g, particularly preferably 70 to 140 mg of KOH / g.

[0039] In one embodiment, the polyunsaturated monomer or polyunsaturated oligomer ii) is present in an amount of from 5 wt.% to 20 wt.% of the total weight of the solid components of the composition. Typically, it is present in an amount of from 5 wt.% to 15 wt.% of the total weight of the non-aqueous components of the composition, preferably from 6 wt.% to 14 wt.%, more preferably from 7 wt.% to 13 wt.%. In case both polyunsaturated monomer and polyunsaturated oligomer are present, typically the combined weight of polyunsaturated monomer and polyunsaturated oligomer is from 5 wt.% to 20 wt.% of the total weight of the solid components of the composition; preferably from 5 wt.% to 15 wt.%, more preferably from 6 wt.% to 14 wt.%, for example from 7 wt.% to 13 wt.%.

[0040] As used herein the solid components of the dispersion or coating composition means the total of all solid components present in the dispersion or coating composition, which explicitly excludes water and any solvents present. Total solid components of the dispersion or coating composition excluding water and any solvents present includes all components of the coating composition which remain when the dispersion or coating composition is applied to a substrate and is dried. It therefore includes reactive diluents, cross-linkers and any additives, for example pigments and fillers, which do not evaporate from the dispersion or coating composition during drying. Accordingly, wt.% of the total solid components of the dispersion or coating composition means the weight expressed as a percentage of the total weight of the dispersion or coating composition excluding water and any solvents present. It therefore corresponds to the wt.% of the coating resulting from the dried dispersion or coating composition.

[0041] The acrylate functional oligomer contains at least one (meth)acrylate group and at least one isocyanate reactive group. As used herein (meth)acrylate means methacrylate or acrylate or a mixture of methacrylate and acrylate. The (meth)acrylate group provides the polymer with a C=C double bond which can react to cross-link the polymer via radical polymerization initiated by radiation; typically in the presence of a photoinitiator. P81927047W001

[0042] 9

[0043] An isocyanate-reactive group is any nucleophilic group which reacts with an isocyanate moiety. An isocyanate-reactive group is, for example, a hydroxyl or amino group. Typically it is hydroxyl.

[0044] A typical acrylate functional oligomer containing at least one (meth)acrylate group and at least one isocyanate reactive group is a (meth)acryloyl dihydroxy compound, poly(meth)acryloyl dihydroxy compound or a (meth)acryloyl mono-hydroxy compound, for example a poly(meth)acryloyl mono-hydroxy compound. A polyacrylol monohydroxyl compound is preferred.

[0045] An acrylate functional oligomer containing at least one (meth)acrylate group and at least one isocyanate reactive group may be an oligomer obtained from the reaction of a polyglycidyl compound with (meth)acrylic acid. An acrylate functional oligomer containing at least one (meth)acrylate group and at least one isocyanate reactive group may be obtained from partial esterification of an aliphatic or aromatic polyol with (meth)acrylic acid. Preferably, it may be obtained from partial esterification of an aliphatic polyol with (meth)acrylic acid. The reaction product of such a polyol with ethylene oxide or propylene oxide or a mixture thereof, or reaction product of such polyol with a lactone, which adds to these polyols in a ring-opening reaction may also be used. The acrylate functional oligomer may be a partial esterification product of an aliphatic or aromatic polyol or a mixture thereof with (meth)acrylic acid. Preferably, it is a partial esterification product of an aliphatic polyol with (meth)acrylic acid. The partial esterification product of (meth)acrylic acid with a tri-, tetra-, penta- or hexahydric polyol or a mixture thereof is preferred. Examples include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate and hydroxybutyl (meth)acrylate. Preferred examples are those comprising at least two (meth)acryl functions such as glycerol diacrylate, trimethylolethane diacrylate, trimethylolpropane diacrylate, trimethylolbutane diacrylate, glycerol diacrylate, pentaerythritol triacrylate, ditrimethylolpropane triacrylate, dipentaerythritol pentaacrylate and their (poly)ethoxylated and / or (poly)propoxylated equivalents.

[0046] The acrylate functional oligomer is typically present in an amount of from 10 wt.% to 55 wt.% of the solid components of the polyurethane acrylate dispersion. Preferably it is present in an amount of from 10 wt.% to 45 wt.%, preferably from 15 wt.% to 40 wt.%, P81927047W001

[0047] 10 for example from 18 wt.% to 35 wt.% of the solid components of the solid components of the polyurethane acrylate dispersion.

[0048] A dispersing agent may be selected from those commonly used in polyurethane dispersions. The dispersing agent contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action. A dispersing agent may be anionic, for example, sulfonium, phosphonium, carboxylate, sulfonate, phosphonate; cationic, for example ammonium; or non-ionic, for example polyether. Typically it is anionic or nonionic or a mixture thereof.

[0049] The dispersing agent is typically a polyol comprising a functional group that can exhibit an ionic or non-ionic hydrophilic nature. Preferably it is a polyol containing one or more anionic salt groups, for example a carboxylate or sulfonate salt group or an acid group which may be converted to an anionic salt group, for example carboxylic acid or sulfonic acid group. Preferred are hydroxycarboxylic acids with from 1 to 3 hydroxyl groups and from 1 to 3 carboxylic acid groups. Examples of these hydroxycarboxylic acids include citric acid, malic acid, lactic acid and tartaric acid. The most preferred hydroxycarboxylic acids are the a,a-dimethylolalkanoic acids, which comprise 2 hydroxyl groups and one carboxylic acid group. Examples are 2,2-dimethylolpropionic acid and 2,2-dimethylolbutanoic acid.

[0050] The polyurethane acrylate polymer may comprise more than one dispersing agent comprising at least one isocyanate reactive group and at least one group having a hydrophilizing action.

[0051] In one embodiment, the dispersing agent is ionic and the aqueous, radiation-curable polyurethane acrylate dispersion further comprises a counterion to the dispersing agent.

[0052] The dispersing agent is typically present in an amount of from 1 wt.% to 10 wt.% of the solid components of the polyurethane acrylate dispersion. Preferably it is present in an amount of from 1 .5 wt.% to 8 wt.%, more preferably from 2 wt.% to 6 wt.%, for example from 2.5 wt.% to 5 wt.% of the solid components of the solid components of the polyurethane acrylate dispersion. P81927047W001

[0053] 11

[0054] The polyurethane acrylate polymer is dispersed in an aqueous medium. Accordingly, the dispersion comprises a continuous phase b) which is aqueous. The polyurethane acrylate polymer particles form a discrete phase. In its simplest form the aqueous medium is water. However, it may comprise additional solvents or solutes.

[0055] In one embodiment, the polyurethane acrylate polymer is the reaction product of, in addition to i) to iv), v) a polyol. The polyol v) typically is a polyester polyol, polyether polyol, polycarbonate polyol, fatty dimer diol, polybutadiene polyol, silicone polyol or polyacrylate polyol, or a mixture thereof. Preferred are polyester polyol, polyether polyol, polycarbonate polyol or simply a polyol. Typically, the number average molecular weight, Mnis less than 3000 g / mol; in particular less than 2000 g / mol. Examples are ethyleneglycol, diethyleneglycol, propyleneglycol, dipropyleneglycol, neopentyl glycol, 1 ,3-propane diol, 2-ethyl-2-butyl-1 ,3-propanediol, 1 ,3-butanediol, 1 ,4- butanediol, 1 ,5-pentane-diol, 1 ,6-hexanediol, 1 ,7-heptanediol, 1 ,8-octanediol, 2-ethyl- 1 ,6-hexanediol, cyclohexane dimethanol, trimethylolpropane, di-trimethylol propane, glycerol, pentaerythritol, dipentaerythritol, polyols derived from castor oil, and mixtures thereof.

[0056] The polyol is typically present in an amount up to 40 wt.%_of the solid components of the polyurethane acrylate dispersion. Preferably it is present in an amount of from 5 wt.% to 30 wt.%, more preferably from 10 wt.% to 25 wt.%, for example from 15 wt.% to 20 wt.% of the solid components of the solid components of the polyurethane acrylate dispersion.

[0057] In one embodiment, the polyurethane acrylate polymer has a number of acrylate groups / g of from 1 to 5 mmol / g. Typically, the polyurethane acrylate polymer has a number of acrylate groups / g of from 1.5 to 3.5 mmol / g; preferably, from 2.0 to 3.0 mmol / g. The number of acrylate groups / g is measured by NMR.

[0058] In one embodiment, the polyurethane acrylate polymer has an iodine value of at least 5 g of I2 / 100g. Preferably, the polyurethane acrylate polymer has an iodine value of at P81927047W001

[0059] 12 least 10 g of I2 / 100g. Typically, the polyurethane acrylate polymer has an iodine value of at most 25 g of I2 / 100g, preferably at most 20 g of I2 / 100g, for example at most 15 g of l2 / 100g.

[0060] In one embodiment the polyurethane acrylate polymer is the reaction product of, in addition to i) to iv), vi) a chain extender. Typical chain extenders are active hydrogencontaining. Water can act as chain extender. Alternative chain extenders typically comprise active amino groups capable of making a chain extension of the remaining isocyanate end-groups of the pre-polymer. The chain extender is suitably a water- soluble aliphatic, alicyclic, aromatic or heterocyclic primary or secondary polyamine or hydrazine. Typically, it has up to 12 carbon atoms. The total amount of chain extender added is generally calculated according to the amount of residual isocyanate groups present in the polyurethane acrylate polymer. The ratio of isocyanate groups in the polymer to the amine groups in the chain extender is typically in the range of from 1 :0.7 to 1 :1 , for example from 1 :0.9 to 1 :1 , preferably about 1 :1 on an equivalent basis. The object is to obtain a fully reacted polyurethane polymer with no residual free isocyanate groups.

[0061] The polyamine used preferably has an average functionality of from 2 to 4, more preferably 2 to 3. Examples of chain extenders include hydrazine, ethylene diamine, piperazine, 1 ,4-butanediamine, 1 ,6-hexane-diamine, 1 ,8-octanediamine, 1 ,10- decanediamine, 1 ,12-dodecanediamine, 2-methylpentamethylenediamine, diethylene triamine, triethylene triamine, isophorone diamine (or 1 -amino, 3-aminomethyl-3, 5,5- trimethyl-cyclohexane), bis(4-aminocy-clohexyl)methane, bis(4-amino-3- methylcyclohexyljmethane, polyethylene amines, polyoxyethylene amines and polyoxypropylene amines (e.g. Jeffamines from TEXACO), and mixtures thereof. A mixture of ethylene diamine and diethylene triamine is preferred.

[0062] A reactive diluent may be present in the aqueous radiation-curable polyurethane acrylate dispersion. A suitable reactive diluent is ethylenically unsaturated and comprises no functionality which is capable to react with an isocyanate group. In one embodiment, the radiation-curable polyurethane acrylate dispersion further comprises c) a reactive diluent which contains at least one at least one (meth)acrylate group and P81927047W001

[0063] 13 no isocyanate reactive groups. A reactive diluent comprises no functionality which is capable to react with an isocyanate group.

[0064] Typically a reactive diluent is an aliphatic or aromatic polyol which has been esterified with (meth)acrylic acid and contains substantially no residual hydroxyl functionality in the molecule. Suitable are the esterification products of (meth)acrylic acid with tri-, tetra-, penta- and / or hexahydric polyols and mixtures thereof. Suitable is the reaction product of such a polyol with ethylene oxide or propylene oxide or a mixture thereof, or a reaction product of such a polyol with a lactone which adds to the polyol in a ringopening reaction. Examples of suitable lactones are y-butyrolactone, 5-valerolactone and £-caprolactone. Preferred polyols are alkoxylated polyols having no more than two alkoxy groups per hydroxyl functionality, and e-caprolactone-modified polyols. These modified or unmodified polyols are preferably totally esterified with acrylic acid, methacrylic acid or mixtures thereof until substantially no residual hydroxyl functionality remains. Examples of poly-unsaturated compounds from this category are trimethylolpropane tri-acrylate, glycerol tri-acrylate, pentaerythritol tetra-acrylate, di-tri- methylolpropane tetra-acrylate, di-pentaerythritol hexa-acrylate and their (poly)ethoxylated and / or (poly)propoxylated equivalents, as well as mixtures thereof. The reactive diluent may be a (meth)acrylic(meth)acrylates or a mixture thereof.

[0065] The reactive diluent is typically present in an amount up to 40 wt.% of the of the solid components of the polyurethane acrylate dispersion. Preferably it is present in an amount of from 5 wt.% to 30 wt.%, more preferably from 10 wt.% to 25 wt.%, for example from 15 wt.% to 20 wt.% of the solid components of the solid components of the polyurethane acrylate dispersion.

[0066] The aqueous radiation-curable coating composition comprises a drying agent. The drying agent present acts to generate radicals which initiate polymerization of unreacted C=C double bonds in the polyurethane polymer and, if present, reactive diluent. Known drying agents capable of promoting the oxidative polymerization reaction of the polyunsaturated oligomer or polyunsaturated monomer and the radical polymerization reaction of the acrylate functional groups in the polyurethane and reactive diluent may be used. Typicallly, a drying agent is a catalyst compound suitable for the catalytic autoxidation reaction of an autoxidisable resin, in particular an alkyd P81927047W001

[0067] 14 resin. Such drying agents are well known in the art. Examples include salts containing cobalt (Co), cerium (Ce), iron (Fe), manganese (Mn) and vanadium (V) as the cation; and halides, nitrates, sulphates, and carboxylates such as acetates, ethylhexanoates, octanoates, neodecanoates, and naphthenates, or aceto aceto nates, as the anion. The catalytic activity of the polyvalent metal during decomposition of the (hydro)peroxide relies on the repeated transition of the metal ion from the lower to the higher oxidation state and back again, leading to reduction and oxidation of the hydroperoxides to catalyze and accelerate oxidation of the unsaturated oil component of the composition. For this reason, transition metals are commonly employed in such drying agents, since transition metals are capable of switching from a lower valence state to a higher valence state in a redox reaction with fatty acid peroxides present in the alkyd. In principle any such drying agent may be used. Preferred are iron complexes or manganese complexes, for example Borchi®Oxy Coat, also defined as CAS no. 478945-46-9 and a manganese complex with Borchi® Dragon ligand (1 ,4,7-trimethyl- 1 ,4,7-triazacyclononane; CAS no. 96556-05-7).

[0068] In the coating composition as defined above, or a kit of parts as defined above, the drying agent is present in an amount of from 0.2 to 10 wt.% of the total weight of the coating composition or kit; typically in an amount of from 0.3 to 8.0 wt.%; preferably from 0.4 wt.% to 6.0 wt.%; more preferably from 0.5 wt.% to 8.0 wt.% of the total weight of the coating composition or kit of parts.

[0069] Optionally, the aqueous radiation-curable coating composition comprises a pigment. In principle any pigment suitable for an aqueous coating composition may be used. The pigments which can be used in the compositions include inorganic and organic pigments.

[0070] Inorganic pigments include compounds of metals such as iron, zinc, titanium, lead, bismuth, chromium, copper, cadmium, calcium, zirconium, cobalt, magnesium, aluminum, nickel and other transition metals. Carbon black and graphite are also considered herein as inorganic pigments. Examples of suitable inorganic pigments are: iron oxides, including red iron oxides, yellow iron oxides, black iron oxides and brown iron oxides, carbon black, iron hydroxide, graphite, black micaceous iron oxide, aluminum flake pigments, pearlescent pigments, calcium carbonate, calcium P81927047W001

[0071] 15 phosphate, calcium oxide, calcium hydroxide, bismuth oxide, bismuth hydroxide, bismuth carbonate, copper carbonate, copper hydroxide, basic copper carbonate, cupric oxide, cuprous oxide, silicon oxide, zinc carbonate, barium carbonate, barium hydroxide, strontium carbonate, zinc oxide, zinc phosphate, zinc chromate, barium chromate, chrome oxide, titanium dioxide, zinc sulfide, antimony oxide and lead chrome.

[0072] Examples of organic pigments are: monoazo (arylide) pigments, such as PY3, PY65, PY73, PY74, PY97 and PY98, disazo (diarylide) pigments, disazo condensation products, benzimidazolone, B -naphthol, naphthol, metal-organic complexes, isoindoline, isoindolinone, quinacridone, perylene, perinone, anthraquinone, diketo- pyrrolo pyrrole, dioxazine, triacrylcarbonium, phthalocyanine pigments, such as cobalt phthalocyanine, copper phthalocyanine, copper semichloro- or monochlorophthalocyanine, copper phthalocyanine, metal-free phthalocyanine, copper polychlorophthalocyanine, phthalocyanine blue, organic azo compounds, organic nitro compounds, polycyclic compounds, such as phthalocyanine pigments, quinacridone pigments, perylene and perinone pigments, diketopyrrolo-pyrrole(DPP) pigments, thioindigo pigments, dioxazine pigments, quinophthalone pigments, triacrylcarbonium pigments and diaryl pyrrolopyroles such as PR254.

[0073] Pigments may be grouped according to the color they produce and may thereby described, based on their color index as Pigment Yellows (PY), Pigment Oranges (PO), Pigment Reds (PR), Pigment Violets (PV), Pigment Browns (PBr), Pigment Blacks (PBk), Pigment Blues (PB) and Pigment Greens (PG).

[0074] The coating composition typically comprises a mixture of two or more pigments. As mentioned above the invention allows previously unavailable through-curing of coatings having the pigment colours yellow, black and red, and to a lesser extent green and blue. Accordingly, a coating composition comprising a yellow, black, red, green or blue pigment as described above is preferred; especially a coating composition comprising a yellow, black or red pigment. Further, common inorganic pigments typically have strong absorption in the UV part of the electromagnetic spectrum, i.e. the same wavelengths at which UV photoinitiators absorb. Accordingly, a coating composition comprising an inorganic pigment as described above is preferred. Preferred pigments P81927047W001

[0075] 16 include titanium dioxide, yellow and red iron oxide, phthalocyanine and carbon based pigments.

[0076] Optionally, the coating composition and polymer component of the kit of parts may each comprise one or more photoinitiators. A photoinitiator is activated by high-energy electromagnetic radiation, such as for example visible light or in particular UV radiation, e.g. light of wavelength 200 to 700 nm. The photoinitiator initiates polymerisation by means of the acrylate groups. Any photoinitiator suitable for an aqueous coating composition may be used. For example, the photoinitiator may be selected from the group consisting of unimolecular (type I) and bimolecular (type II) ketone compounds. Suitable type I photoinitiators include benzoins, benzoin derivatives, in particular benzoin ether, benzil ketals, acylphosphine oxides, for example 2,4,6-trimethylbenzoyl diphenylphosphine oxides, bisacylphosphine oxides, phenylglycoxylic acid esters, camphorquinone, a-aminoalkyl phenones, a,a-dialkoxyacetophenones and a- hydroxyalkylphenones. Suitable type II photoinitiators include benzophenones in combination with tertiary amines, alkyl benzophenones, 4,4'- bis(dimethylamino)benzophenone (Michler’s ketone), anthrone and halogenated benzophenones or mixtures of the cited types.

[0077] A coated substrate is obtained by applying the aqueous radiation-curable coating composition to the substrate and allowing water to evaporate from the aqueous radiation-curable coating composition, i.e. allowing the coating composition to dry. Other volatile components present, for example solvents and neutralizing agents, also typically evaporate. The aqueous radiation-curable coating composition may be applied to a substrate by conventional techniques known in the art, for example by spraying, rolling, flooding, printing, knife-coating, pouring, brushing or dipping. Preferably, it is applied by brush, roller or spaying. Drying may be supplemented by applying radiation, for example UV-radiation or electron radiation (also known as electron beam or e- beam), or applying heat to effect curing of the coating. Typically, curing is carried out at ambient temperature. Typically, UV radiation and / or electron radiation, i.e. either UV radiation or electron radiation or both UV radiation and electron radiation are applied to the aqueous radiation-curable coating composition. If UV radiation is used, preferably a photoinitiator is present in the coating composition. If no UV radiation is used, typically no photoinitiator is present in the coating composition. Preferably, UV radiation is P81927047W001

[0078] 17 applied; optionally with additional electron radiation.

[0079] The substrate may be any suitable substrate, such as for example wood, polymer, composite, metal or mineral substrate. The substrate may be a primed or bare substrate. A single layer or multiple successive layers of coating composition may be applied to the substrate.

[0080] A kit of parts comprises a polymer component (A) and a drying component (B). The polymer component and drying component are prepared and stored separately and are not typically combined until shortly before application of the coating composition. The kit of parts is formulated in order that the drying agent and the aqueous radiation- curable polyurethane acrylate dispersion are separated until shortly before application. The drying agent in the drying component acts to accelerate cross-linking (or curing) of the aqueous radiation-curable polyurethane acrylate dispersion in component. Such cross-linking or curing is detrimental to storage stability (shelf life) of the polymer component and is only desired after application of the mixture to a substrate.

[0081] Therefore, separate storage of components mitigates cross-linking in the polymer component. In other words, storage stability or shelf-life, of polymer component is increased by formulation as a kit of parts. Moreover, each of the polymer component and drying component independently have been found to be storage stable. On mixing the polymer component and drying component, a coating composition is formed, which coating composition has an acceptable pot life. The mixing ratio of polymer component to drying component is dependent principally on the relative amount of active agents in each component; namely the amount of polymer and amount of drying agent. For practical reasons formulation is typically carried out such that the ratio of weight of polymer component to weight of drying component is from 1 :10 to 20:1 , typically from 1 :1 to 10:1 , for example 2:1 to 5:1 . The skilled person will understand that formulation of active agent concentrations to achieve such ratio is preferred.

[0082] The kit of parts is applied by first mixing the polymer component and the drying component. The resulting mixture has a pot life, for example of the order of hours, days or even weeks; typically several days. The mixture is then applied as described above for the aqueous radiation-curable coating composition. P81927047W001

[0083] 18

[0084] The aqueous radiation-curable coating composition may be cured by photoinitiation, i.e. by radiation with visible light or LIV light. In one embodiment, the composition further comprises d) a photo-initiator. Further, for the same reason, the polymer component of the kit of parts of the present invention optionally comprises a photoinitiator.

[0085] The photo-initiator may be one photo initiator or a mixture of two or more thereof. Photo-initiators generate free radicals when exposed to radiation energy in the visible light or UV light wavelength range. Any suitable photo-initiator known in the art may be used, depending on the wavelength. Suitable photo-initiators include benzoin derivatives, benzile ketales, a-hydroxyalkylphenones, monoacylphosphine oxide (MAPO) and bisacylphosphine oxides (BAPO), such as diphenyl(2,4,6- trimethylbenzoyl)phosphine oxide, 1 -hydroxy-cyclohexyl-phenyl-ketone, bis(2,4,6- trimethylbenzoyl)-phenylphosphineoxide, bis(2,6-dimethoxybenzoyl)-2,4,4- trimethylpentyl-phosphine oxide, 2-hydroxy-2-methyl-1 -phenyl-propan-1 -one, 2-methyl- 1 [4-(methylthio)phenyl]-2-morpholono-propan-1 -one, a phenyl glyoxylic acid methyl ester. Mixtures of these compounds may also be employed.

[0086] The photo-initiator may be present in an amount of from 0.3 to 15 wt%, for example from 1 .5 to 10 wt%, preferably from 2.0 to 5 wt% based on the total weight of the solid components of the coating composition.

[0087] The aqueous radiation-curable coating composition, or polymer composition of the kit of parts, optionally further comprises an additive. Suitable additives include an extender pigment, light stabilizer, other stabilizer, defoaming agent, matting agent, wetting agent, coalescing solvent, co-solvent, surfactant, plasticizer, pH modifier, thickener, leveling agent, anti-setting agent, biocide or flow agent. Typically, more than one such additive is present. More than one additive of any specified type may be present.

[0088] The present disclosure is illustrated by the following Examples. EXAMPLES

[0089] Raw materials The raw materials used are listed in Table 1 .

[0090] Table 1 : P81927047W001

[0091] 20

[0092] Measurement Techniques

[0093] Ethanol resistance

[0094] The coating composition was applied as a 120 pm wet film using a bar applicator on MDF panels sealed with a 100% UV sealer to provide a non-porous, non-reactive surface. The film was dried for 1 hour at 40 °C before being UV-cured (except Comparative Examples 2, 3, 4, 5 and 6, which were not UV cured; see Table 5) using 100 W / cm2Hg and 100 W / cm2Ga UV-lamps at a line speed of 10 m / min. The film was aged at 23 °C and the ethanol resistance was measured over time. Ethanol resistance was measured according to EN12720, where a filter paper pad soaked in 48 v / v% ethanol was placed on the coating surface for 1 hour. Afterwards the filter paper was removed, and the surface wiped. Evaluation of the stain was performed the following day. The remaining stains were assessed visually using a 1 -5 scale; where 1 means severe damage to the surface, in part completely to the substrate; and 5 means no visible change to the surface.

[0095] Hardness

[0096] The coating composition was applied as a 120 pm wet film using a bar applicator on glass panels. The film was dried for 1 hour at 40 °C before being UV-cured (except Comparative Examples 2, 3, 4, 5 and 6, which were not UV cured; see Table 5) using 100 W / cm2Hg and 100 W / cm2Ga UV-lamps at a line speed of 10 m / min. The film was aged at 23 °C and the pendulum hardness (Kbnig hardness), determined as the number of oscillations on 3 places of the surface, was measured over time. Results presented are the mean value of each measured sample.

[0097] Acrylate Conversion

[0098] Acrylate Conversion is a measure of the degree of cure. Acrylate conversion was measured on a (UV-cured) film prepared in an analogous way as for the hardness test. Part of the film was removed from the glass panel to measure each side with ATR- FTIR. As reference peak, the 1635 cm-1peak corresponding to the carbonyl group in P81927047W001

[0099] 21 an ester was used. To measure the degree of cure, the 1410 cm-1peak was used corresponding to unsaturated C=C bond in an acrylic group. These measurements were performed over time to follow post-cure of acrylic groups. As starting values, a film prepared in the same way as described but without UV-curing was used. For Comp. Ex 1 and 6, the starting value was instead taken right after UV-cure (at Day 0) rather than after 4 days.

[0100] Preparation of the unsaturated oligomer

[0101] The synthesis of Olig 1 was performed in a 1 L round-bottomed glass reactor fitted with an overhead stirrer, a thermocouple, distillation attachment and nitrogen inlet. The reactor was heated using a temperature-controlled heating mantle. All reactants (see Table 2) were added to the reactor and the temperature was slowly heated to 235 °C over the course of 4-6 hours. When an acid value below 40 mgKOH / g is reached and at a temperature of above 120 °C, 450 ppm TYTAN™ TNBT catalyst was added. The reaction was stopped when an acid value < 1 mgKOH / g was measured.

[0102] For the synthesis of Olig2 the reactants were charged into the reactor and the temperature was quickly heated to 210 °C at which point 1500 ppm LiOH was added, and the temperature was raised to 240 °C. A sample was taken and diluted 1 :3 in 96% ethanol. When the sample in ethanol mixture was clear the reaction was cooled to room temperature.

[0103] Olig3 was produced by a method analogous to Olig 1 . Compositions of Olig 1 , Olig2, and Olig3 are given in table 2.

[0104] Table 2: Preparation of the polymer dispersion The synthesis was performed in a 1 L round bottomed glass reactor fitted with an overhead stirrer, a thermocouple and a reflux condenser. The reactor was heated using a temperature-controlled heating mantle. To the reactor, the unsaturated oligomer (Olig 1 , Olig2, Olig3 or Cardolite), SR444F, DMPA, SR 454 as reactive diluent if used, solvent if used and a separate polyol (castor oil or TMP) was added to the reactor (see Table 3). To all samples 450 ppm BHT was added at this stage to inhibit radical polymerization during synthesis. The temperature of the reactor was increased to 75°C. When that temperature was reached, the isocyanate H12MDI was added to the reactor followed by 10% of the neutralizing agent, TEA, as catalyst. The reaction was held at 75°C for 2-3 hours until stable NCO-value was measured. At that point the remaining TEA was added to the reactor and was let to homogenize for 10 minutes before the content was dispersed into water. After dispersing, the chain extenders (EDA and / or DETA) were added to the dispersion. The compositions are summarized in Table 3.

[0105]

[0106] Coating formulation

[0107] Each polymer dispersion (PD1 a to PD5) was formulated into a coating by adding the components described in Table 4. To achieve full properties and conversion of acrylic groups after UV-cure, 1 wt% of either Borchi® Dragon (Examples 2, 4, 5, 6 and 7) or 1 wt% of Borchi® OxyCoat 1 101 (Examples 1 and 3) was added before application of the coating (examples), or no drying agent was added (Comparative Examples 1 , 2 and 3). Yellow iron oxide-based pigment paste was added to the formulations corresponding to 9.75 wt% iron oxide. This correlates to a highly pigmented coating usually not possible to use with ordinary UV-curable PLIDs.

[0108] Table 4: Ethanol resistance, pendulum (Kbnig) hardness, acrylate conversion at the surface and acrylate conversion at the substrate were measured as described above. The results are provided in Table 5. 25

[0109] Table 5:

[0110] Negative acrylate conversion at the surface indicates a migration of unreacted acrylate P81927047W001

[0111] 26 groups from the substrate side of the coating to the surface of the coating. For Comp Ex. 1 and Comp. Ex. 3, the initial acrylate conversion measurement was taken at Day 0, rather than at Day 4. PD = polymer dispersion no.; BOC = Borchi® OxyCoat 1101 ; BD = Borchi® Dragon; UC = UceCoat 7700

[0112] From results of acrylate conversion at substrate interface on day 4 for Example 2, it is safe to assume all samples have almost zero through cure from UV initiated cure, and that the pigment loading in the formulations to the largest extent block all UV-radiation from passing through the film.

[0113] Examples 2 and 3 and Comparative Example 2 all use Polymer Dispersion PD1 b. Results show that Example 2 (drying agent, UV cure) greatly exceeds performance of Comparative Example 2 (no drying agent, no UV cure) and Example 3 (drying agent, no UV cure) from day 4. It also shows an increase in acrylate conversion both at the substrate and surface interface. Example 3 also show improved performance over Comparative Example 2. These results indicate an improvement in cure when a drying agent is present, and a further improvement in cure when the coating composition is additionally subjected to UV irradiation.

[0114] All examples also show a development of ethanol resistance (except Example 6), pendulum hardness and acrylate conversion (except Example 4, which was not measured, and Example 6 which showed high acrylate conversion already at day 4) both at substrate and surface interface, which is not seen in any of Comparative Examples 1 , 2 and 3.

Claims

P81927047W00127CLAIMS1 . An aqueous radiation-curable polyurethane acrylate dispersion comprising: a) a polyurethane acrylate polymer which is the reaction product of: i) from 10 wt.% to 50 wt.% of an isocyanate monomer; ii) from 5 wt.% to 20 wt.% of a polyunsaturated monomer or polyunsaturated oligomer, which has an iodine value of at least 50 g of I2 / 100g of respectively polyunsaturated monomer or polyunsaturated oligomer, and at least one hydroxyl functional group; ill) from 10 wt.% to 55 wt.% of an acrylate functional oligomer which contains at least one (meth)acrylate group and at least one isocyanate-reactive group; and iv) from 1 wt.% to 10 wt. % of a dispersing agent which contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action; dispersed in b) an aqueous continuous phase.

2. An aqueous radiation-curable polyurethane acrylate dispersion according to claim1 , wherein the isocyanate monomer is a polyisocyanate monomer.

3. An aqueous radiation-curable polyurethane acrylate dispersion according to claim2, wherein the polyisocyanate monomer is 1 ,5-pentamethylene-diisocyanate, 1 ,6- hexamethylene-diisocyanate, 1 -isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane or 4,4'-diisocyanato-dicyclohexylmethane.

4. An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 3, wherein the polyunsaturated monomer or polyunsaturated oligomer ii) is present in an amount of from 6 wt.% to 14 wt.% of the total weight of the solid components of the polyurethane acrylate dispersion.

5. An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 4, wherein the polyurethane acrylate polymer is the reaction product of, in addition to i) to iv), v) a polyol.

6. An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 5, wherein the polyurethane acrylate polymer has a number of acrylate groups / g of from 1 .5 to 3.5 mmol / g.

7. An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 6, wherein the polyurethane acrylate polymer has an iodine value of at least 5 g of I2 / 100g.

8. An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 7, wherein the polyunsaturated monomer or polyunsaturated oligomer ii) comprises a polyunsaturated fatty acid, a polyunsaturated fatty acid ester or a polyunsaturated phenol.

9. An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 8, wherein the polyurethane acrylate polymer is the reaction product of, in addition to i) to iv), vi) a chain extender.

10. An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 9, wherein the dispersing agent iv) is ionic and the aqueous radiation-curable polyurethane acrylate dispersion further comprises a counterion to the dispersing agent.11 . An aqueous radiation-curable polyurethane acrylate dispersion according to any one of claims 1 to 10, which further comprises c) a reactive diluent which contains at least one (meth)acrylate group and no isocyanate-reactive groups.

12. An aqueous radiation-curable coating composition comprising: a) an aqueous radiation curable polyurethane-acrylate dispersion, comprising a polyurethane acrylate polymer which is the reaction product of: i) an isocyanate monomer; ii) a polyunsaturated monomer or polyunsaturated oligomer, which has an iodine value of at least 50 g of I2 / 100g of respectively polyunsaturated monomer or polyunsaturated oligomer, and at least one hydroxyl functional group;iii) an acrylate functional oligomer which contains at least one (meth)acrylate group and at least one isocyanate-reactive group; and iv) a dispersing agent which contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action; dispersed in an aqueous continuous phase; b) a drying agent; c) optionally a pigment; d) optionally a photoinitiator; and e) optionally one or more additives; wherein the drying agent is present in an amount of from 0.2 wt.% to 10 wt.% of the total weight of the coating composition.

13. A kit of parts comprising:A. a polymer component, which comprises: a) an aqueous radiation curable polyurethane-acrylate dispersion, comprising a polyurethane acrylate polymer which is the reaction product of: i) an isocyanate monomer; ii) a polyunsaturated monomer or polyunsaturated oligomer, which has an iodine value of at least 50 g of I2 / 100g of respectively polyunsaturated monomer or polyunsaturated oligomer, and at least one hydroxyl functional group; iii) an acrylate functional oligomer which contains at least one (meth)acrylate group and at least one isocyanate-reactive group; and iv) a dispersing agent which contains at least one isocyanate-reactive group and at least one group having a hydrophilizing action; dispersed in an aqueous continuous phase; b) optionally a pigment; c) optionally a photoinitiator; d) optionally one or more additives; andB. a drying component, which comprises a drying agent and a solvent, wherein the drying agent is present in an amount of from 0.2 wt.% to 10 wt.% of the total weight of the kit of parts.

14. A coated substrate comprising a substrate and a coating, which coated substrate is obtainable by: a) applying an aqueous radiation-curable coating composition as defined in claim 12, or a mixture comprising the polymer component A and drying component B of the kit of parts as defined in claim 13, to the substrate; b) allowing water to evaporate from respectively the aqueous radiation-curable coating composition or mixture; and c) applying UV radiation and / or electron radiation respectively to the aqueous radiation-curable coating composition or mixture.

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