Waterborne coating composition for corrosion protection

Abstract

The present invention relates to an aqueous resinous dispersion comprising a mixture of a hydrophilically modified epoxy-based resin, a crosslinker and a co-crosslinker. The present invention also relates to an aqueous coating composition comprising said resinous dispersion, providing improved corrosion resistance and color change resistance; a process for the preparation of the aqueous resinous dispersion and coating composition; and the use of said coating composition for coating metal substrates.

Description

TECHNICAL FIELD

[0001] The present invention relates to aqueous resin dispersions providing improved corrosion resistance and color change resistance and to aqueous coating compositions comprising said resin dispersions; to methods for the preparation of the aqueous resin dispersions and coating compositions; and to the use of said coating compositions for coating metal substrates. BACKGROUND

[0002] Most metals, except noble metals or those that form a coherent oxide layer (e.g. aluminum), have a coating film applied to them when exposed to environmental conditions to prevent or at least delay corrosion. If the coating film has sufficient adhesion to the metal substrate and if the permeability of the coating film to oxygen and water is low, the application of a coating film to the metal surface can provide a barrier between the metal substrate and the damaging environment. In coating films, coatings in liquid or powder form play an important role. One aspect that has to be considered when selecting a coating material is how to avoid ingredients in the coating composition that can react with the metal surface and thus can damage the metal substrate, e.g. chloride ions that can provide seeds for future corrosion attack.

[0003] Many organic polymers are suitable for forming a coating film on a metal surface. It is desirable that these films have sufficient hardness, sufficient adhesion as described above, and sufficient elasticity to enable the coating film to adapt to possible deformations of the coated metal article. Epoxy resins are one of the most commonly used materials in anticorrosive coating formulations (High-Performance Coatings, 2008, A.S. Khanna, "the word epoxy has become synonymous with anti-corrosion in today's industrial environment"). Reaction products prepared from epoxy resins and amines have become standard materials for cathodic electrocoating of vehicle bodies.

[0004] An aqueous resin composition (D) is disclosed in WO 2015 / 093299 A1, which is obtained by dispersing a vinyl ester resin (A) and a polyurethane resin having an aromatic ring (B) in an aqueous medium (C) and adding a carbodiimide crosslinking agent (E) thereto. The vinyl ester resin (A) is prepared by reacting a polymerizable ethylenically unsaturated acid compound (a2) with at least one epoxy resin (a1) selected from the group consisting of a phenol-aldehyde resin type epoxy resin and a bisphenol type epoxy resin. The polyurethane resin (B) is obtained by reacting a polyol having an aromatic ring (b1-1) and a polyol having a hydrophilic group (preferably an anionic group) (b1-2) with a polyisocyanate (b2). The carbodiimide crosslinking agent (E) preferably has 2 or more carbodiimide groups per molecule.

[0005] US2010 / 0129659A1 discloses a coated product prepared by a 3-coat 1-bake process; the process comprises the steps of forming a cured coating film (A1) of an anodic electrodeposition coating composition (A) on a metal object to be coated; forming a first colored coating film (B1) by applying a first colored water-based coating composition (B) thereon; forming a second colored coating film (C1) by applying a second colored water-based coating composition (C) on the uncured first colored coating film (B1); forming a transparent coating film (D1) by applying a transparent coating composition (D) on the uncured second colored coating film (C1); simultaneously curing the uncured first colored coating film (B1), the uncured second colored coating film (C1) and the uncured transparent coating film (D1). The cationic electrodeposition coating composition (A) contains a modified epoxy resin (a1) containing cationic amino groups, wherein the modification is carried out by the addition of a xylene formaldehyde resin, which is prepared by condensation of xylene and phenol with formaldehyde in the presence of an acidic catalyst, which modification imparts plasticity and hydrophobicity to the epoxy resin.

[0006] WO2013 / 191826A1 discloses an aqueous mixture comprising: an aqueous polyolefin dispersion comprising a melt blend product of one or more base polymers and one or more stabilizers in the presence of water and optionally one or more neutralizing agents, wherein the polyolefin dispersion has a volume average particle size of 400 nm to 1500 nm and a pH of 8 to 11; and one or more crosslinking agents selected from the group consisting of phenol-formaldehyde resins, hydroxyalkylamide resins, amino-formaldehyde resins, epoxy group-containing resins, and combinations thereof.

[0007] EP1805260A1 relates to a self-adhesive coating composition for producing an electrical steel sheet core, comprising A) 100 parts by weight of at least one epoxy resin based on bisphenol A, bisphenol F or a mixture thereof, 100% solids, B) 0.1 to 200 parts by weight of nanoparticles having an average radius of 2 to 600 nm, C) 0 to 25 parts by weight of at least one curing agent selected from dicyandiamide, blocked isocyanate and Lewis acid or selected from phenol-formaldehyde resins, carboxylic acids, acid anhydrides and Lewis acids, 100% solids, D) 0.1 to 10 parts by weight of at least one additive, and E) 50 to 200 parts by weight of water or at least one organic solvent.

[0008] WO2018130700A1 discloses an aqueous resin dispersion D comprising a mixture of a hydrophilically modified epoxy-based resin P and a resol resin crosslinking agent R and a co-crosslinking agent E, wherein the hydrophilically modified epoxy-based resin P is prepared in a reaction or series of reactions comprising at least one step in which a promoting reaction is carried out.

[0009] US 5 177 161 A discloses can coating applications comprising water dilutable coatings based on a condensation product of a bisphenol A type epoxy resin of type 7 and a phenol based phenol formaldehyde resin, which is methylated and then reacted with sodium monochloroacetate and then dispersed in water. The epoxy resin and the phenol formaldehyde resin moieties are connected by ether linkages.

[0010] Further water dilutable can coating resins based on addition products of a carboxylated phenol formaldehyde resin A and an epoxy resin B are known from DE 197 56 749 B4. These addition products have at least one ester group per molecule, which is formed by the reaction of a carboxyl group of the phenol formaldehyde resin A with an epoxy group of the epoxy resin B. These products have a lighter color than those according to US 5 177 161 A.

[0011] In the state of the art so far, the corrosion resistance of steel sheets with the coating films of the state of the art is still not satisfactory or the corrosion resistant coating film discolors when baked at temperatures above 160°C. In the worst case, neither the corrosion protection nor the discoloration resistance is sufficient when baked.

[0012] Methylol phenol formaldehyde resins are widely used as the main crosslinker for metal coatings to protect the interior surface of food cans from corrosion by the fillings. For such applications, the tendency of methylol phenol formaldehyde resins to discolor when cured at temperatures between 150 and 250°C is known and not regarded as a problem. Furthermore, the yellowing behavior of methylol phenol formaldehyde resins crosslinkers is not a technical obstacle if the metal coating is pigmented with dark pigments. However, the use of methylol phenol formaldehyde resins is problematic for light colored metal coatings or clear coatings which should not change their color when oven cured. Therefore, there is still a need for coating compositions with a high level of metal protection and a low tendency to discolor when cured at high temperatures.

[0013] Object of the invention

[0014] It is the object of the present invention to provide a water dispersion for coating compositions which is free of the disadvantages of the prior art.

[0015] It is the object of the present invention to provide a coating composition with improved corrosion resistance and improved discoloration resistance at high baking temperatures compared to prior art coating systems. SUMMARY

[0017] The present invention discloses a water borne resin dispersion D comprising a mixture of a hydrophilically modified epoxy based resin P, a blocked isocyanate crosslinker IB and a co-crosslinker E. Wherein the hydrophilically modified epoxy based resin P comprises 1-hydroxy-2-phenoxy-ethyl; and wherein the co-crosslinker E is selected from:

[0018] - a compound E1 having at least two hydroxyl groups which react at elevated temperatures above 80°C with acid compounds present in the coating composition to form esters and release water, and

[0019] - ester E2 having at least two ester groups, which are formed from at least a dihydric alcohol E22 and an acid E21, which is not stable at elevated temperatures above 80°C.

[0020] The preferred embodiments of the present application disclose one or more of the following features:

[0021] - the blocked isocyanate crosslinker IB is preferably a reaction product of a polyisocyanate and a hydroxy-functional urethane C, which is a reaction product of a cyclic organic carbonate with an aliphatic monoamine, diamine, triamine, tetraamine and / or alkanolamine, which amine has at least one primary or secondary amino group;

[0022] - the hydroxy-functional urethane C is preferably 2-hydroxyethyl bis(2-hydroxyethyl)carbamate or 2-hydroxypropyl bis(2-hydroxyethyl)carbamate;

[0023] - the co-crosslinker E1 is preferably a β-hydroxyalkylamide having the following formula:

[0024] (OH-CHR1-CH2-NR2-CO) m -A-(CO-NR2-CH2-CHR1-OH) n

[0025] wherein:

[0026] - R1 is hydrogen or C1-C5 alkyl;

[0027] - R2 is hydrogen, C1-C5 alkyl or CH2-CHR1-OH;

[0028] - A is a chemical bond or a polyvalent organic radical derived from a saturated, unsaturated or aromatic hydrocarbon radical, including substituted hydrocarbon radicals containing 2 to 20 carbon atoms; and

[0029] - m is an integer from 1 to 2; n is an integer from 0 to 2, and m+n is at least 2;

[0030] - the co-crosslinker E1 is preferably N,N,N',N'-tetra-(2-hydroxyethyl)-adipamide or N,N,N',N'-tetra-(2-hydroxypropyl)-adipamide;

[0031] - the hydrophilically modified epoxy-based resin P is a non-ionic modified epoxy-based resin Pn, which is a reaction product of a diepoxide, a dihydric aromatic compound and an epoxy-functional non-ionic emulsifier F; the epoxy-functional non-ionic emulsifier F is;

[0032] - an epoxy-functional polyalkylene oxide homopolymer or copolymer; and / or

[0033] - an epoxy-functional sugar alcohol; and / or

[0034] - a reaction product of a hydroxyl-functional polyalkylene oxide homopolymer or copolymer or a sugar alcohol segment with at least a difunctional epoxide compound; wherein:

[0035] - the polyalkylene oxide homopolymer is a polyethylene oxide or a polypropylene oxide;

[0036] - the polyalkylene oxide copolymer is a polyethylene oxide-propylene oxide copolymer;

[0037] - the polyalkylene oxide homopolymer and copolymer comprise 20-150 C2-C3 alkylene oxide units;

[0038] - the non-ionically modified epoxy-based resin Pn is preferably characterized by an epoxy equivalent weight (EEW) of 200-2000 g / eq;

[0039] - the aqueous resin dispersion D preferably comprises:

[0040] - 40-90 wt% of a hydrophilically modified epoxy-based resin P, preferably a non-ionically modified epoxy-based resin Pn;

[0041] - 5-55 wt% of a blocked isocyanate crosslinker IB; and

[0042] - 0.1-5 wt% of a co-crosslinker E, preferably E1, more preferably a beta-hydroxyalkylamide; based on the total weight of P, IB and E;

[0043] - the aqueous resin dispersion D preferably comprises one or more catalysts selected from the group consisting of salts, chelate compounds and organometallic compounds of elements from groups 4, 7, 8, 9, 12, 13, 14 and 15 and periods 4, 5 and 6 according to the new IUPAC nomenclature system, and strong amines;

[0044] - the hydrophilically modified epoxy-based resin P and / or the blocked isocyanate crosslinker IB in the aqueous resin dispersion D of the present invention are obtained from renewable feedstocks and the biobased carbon content is greater than 20 wt% of the total carbon content of the epoxy-based resin P and the blocked isocyanate crosslinker IB combined, the biobased carbon content determined using the ASTM D6866-20 standard, or the epoxy-based resin P and / or the blocked isocyanate crosslinker IB are derived from recycled monomers, preferably the hydrophilically modified epoxy-based resin P and / or the blocked isocyanate crosslinker IB in the aqueous resin dispersion D of the present invention are obtained from renewable feedstocks and the biobased carbon content is greater than 20 wt% of the total carbon content of the epoxy-based resin P and the blocked isocyanate crosslinker IB combined, the biobased carbon content determined using the ASTM D6866-20 standard.

[0045] The present application also discloses a coating composition comprising 35-55 wt% of a non-volatile compound and 45-65 wt% of water and a co-solvent selected from the group consisting of alcohols, ketones, esters, glycols, glycol ethers, glycol esters, and mixtures thereof; the coating composition preferably further comprises one or more additives selected from the group consisting of defoamers, leveling agents, coalescing agents, flow modifiers, biocides, pigments, rheological additives, and wetting agents.

[0046] The present application also discloses a process for preparing a coated metal substrate comprising the following steps:

[0047] - applying the coating composition to at least one side of the optionally pretreated and / or primer comprising metal substrate with a coating thickness adjusted to obtain a dry coating thickness of at least 10 pm;

[0048] - flashing off the water and the co-solvent at a temperature of at least 20 °C for at least 1 minute;

[0049] - baking the applied coating composition at a temperature of at least 100 °C for a time of at least 20 seconds to form a metal substrate coated with a crosslinked coating.

[0050] The present application also discloses the use of the coating composition for coating a metal substrate. DETAILED DESCRIPTION

[0052] The aqueous resin dispersion D of the present application comprises a mixture of a hydrophilically modified epoxy-based resin P, a crosslinker IB and a co-crosslinker E, wherein the hydrophilically modified epoxy-based resin P comprises a 1-hydroxy-2-phenoxy-ethyl group (1-hydroxy-2-phenoxy-ethyl is an ether linkage).

[0053] More specifically, the aqueous resin dispersion D of the present application comprises a mixture of a hydrophilically modified epoxy-based resin P, a crosslinker IB and a co-crosslinker E, wherein the hydrophilically modified epoxy-based resin P is prepared in a reaction or series of reactions comprising at least one step of an advancement reaction, which is a reaction of a phenolic compound having a phenolic hydroxyl group with a compound having at least two reactive epoxy groups, resulting in the formation of a 1-hydroxy-2-phenoxy-ethyl group (reaction of an epoxy group and a phenolic hydroxyl group resulting in the formation of a 1-hydroxy-2-phenoxy-ethyl group or ether linkage).

[0054] Alternatively and preferably, where possible, the hydrophilically modified epoxy-based resin P and / or the crosslinker IB are obtained from renewable feedstocks or recycled monomers, preferably the hydrophilically modified epoxy-based resin P and / or the crosslinker IB are obtained from renewable feedstocks and their bio-based carbon content amounts to more than 20 wt% of the total carbon content of the epoxy-based resin P and isocyanate crosslinker IB, the bio-based carbon content being determined using the ASTM D6866-20 standard.

[0055] The hydrophilically modified epoxy-based resin P is selected from the group consisting of an at least partially neutralized anionically modified epoxy-based resin Pa, a non-ionically modified epoxy-based resin Pn, and an at least partially neutralized anionically and non-ionically modified epoxy-based resin Pan.

[0056] One of the features of the hydrophilically modified epoxy-based resin P according to the present application is that all variants Pa, Pn and Pan are always prepared by a reaction or a series of reactions comprising at least one promoting reaction, also referred to as a melt reaction, i.e. a reaction of a phenolic compound having a phenolic hydroxyl group with a compound having at least two epoxy groups.

[0057] The at least partially neutralized anionically modified epoxy-based resin Pa comprising anionic groups is prepared by at least partially neutralizing the acid groups of a product obtained by a first multi-step process or by a second multi-step process.

[0058] The first multi-step process for preparing the at least partially neutralized anionically modified epoxy-based resin Pa comprises the steps a1, b1, c1, d1, e1 and f1, wherein:

[0059] - in step a1 a phenol-formaldehyde resin is prepared from phenol and formaldehyde under acid catalysis and unreacted phenol is separated from the reaction product of step a1 ;

[0060] - in step b1 the phenol-formaldehyde resin of step a1 is subjected to a promoting reaction under catalysis by adding an epoxy resin having on average at least two functional epoxy groups per molecule, preferably an epoxy resin based on bisphenol A;

[0061] - in step c1 the reaction product of step b1 is dissolved in an organic solvent to form a solution, wherein the solvent is selected from the group consisting of linear or branched aliphatic alcohols, linear or branched aliphatic ethers, linear or branched aliphatic ketones, and mixtures thereof with aromatic hydrocarbons;

[0062] - in step d1 the solution of the reaction product of step c1 is then reacted with formaldehyde in the presence of a base to form a hydroxymethyl compound;

[0063] - in step e1 after addition of further base a halogenated alkanoic acid, preferably 2-chloroacetic acid, is added to the product of d1 and after complete reaction of the halogenated alkanoic acid the reaction product of step e1 is purified by acidification with aqueous acid, the organic layer containing the reaction product is separated, and the separated liquid is washed with distilled water; and

[0064] - in step f1 the solvent of the purified reaction product of step e1 is removed by distillation under reduced pressure, and then water and a tertiary amine as neutralizing agent are added to obtain an aqueous solution with only a small amount of residual solvent.

[0065] The second multi-step process for preparing the at least partially neutralized anionically modified epoxy-based resin Pa comprises steps a2, b2 and c2, wherein:

[0066] - in step a2, an ester of a phosphorus-based acid and an epoxy-functional compound having at least one epoxy group per molecule is prepared, wherein the phosphorus-based acid has at least two acidic hydrogen atoms per molecule and is selected from inorganic acidic phosphorus compounds and organic acidic phosphorus compounds, and the epoxy-functional compound is an epoxy compound having at least two epoxy groups per molecule, and wherein the reaction is carried out in such a way that the ester produced by the reaction of step a2 has a specific number of epoxy groups which is not higher than 0.1 mol / kg and on average at least one acidic hydrogen atom per molecule;

[0067] - in step b2, a promoted reaction is carried out using at least difunctional epoxide and aromatic dihydroxy compound in the presence of a catalyst to obtain a polyether compound having epoxy groups;

[0068] - in step c2, the polyether compound of step b2 is dissolved in a solvent and the ester prepared in step a2 is added under stirring until a homogeneous mixture is obtained, and then the solvent is removed from the mixture by distillation under reduced pressure.

[0069] The phosphorus-based acid used in step a2 has at least two acidic hydrogen atoms per molecule and is selected from inorganic acidic phosphorus compounds and organic acidic phosphorus compounds. The former group includes orthophosphoric acid H3PO4, diphosphoric acid H4P2O7, triphosphoric acid H5P3O 10 and higher homologues thereof, phosphorous acid H3PO3, diphosphorous acid H4P2O5 and higher homologues thereof, and hypophosphorous acid H3PO2 and higher homologues thereof. Particularly preferred are orthophosphoric acid, a mixture of di- and higher oligomers of orthophosphoric acid, phosphorous acid and higher oligomers thereof. The latter group includes alkanephosphonic acids R 1 -PO3H2, aromatic phosphonic acids R 2 -PO3H2and the corresponding phosphinic acids H3PO2, wherein R 1 is a linear, branched or cyclic alkyl group having 1 to 20 carbon atoms, R 2 is an optionally substituted aromatic group having 6 to 20 carbon atoms.

[0070] Particularly preferred are methylphosphonic acid and phenylphosphonic acid.

[0071] The epoxy-functional compound used in step a2 is preferably a glycidyl ether of a phenolic compound, preferably bisphenol A diglycidyl ether or bisphenol F diglycidyl ether, or an oligomeric or polymeric epoxy resin based on these bisphenols.

[0072] The non-ionically modified epoxy-based resin Pn is prepared by a process comprising steps a3 and b3, wherein:

[0073] - In step a3, the emulsifier F is prepared from a non-ionic part comprising a polyoxyethylene homo- or copolymer segment or a sugar alcohol segment and a non-hydrophilic and compatibilizing part comprising structural units derived from at least difunctional epoxide compound, preferably bisphenol A diglycidyl ether or oligomers thereof, by coupling of the at least difunctional epoxide compound with a hydroxy-functional polyoxyethylene homo- or copolymer or with a sugar alcohol catalyzed with a strong Brønsted acid or Lewis acid, preferably tetrafluoroboric acid HBF4, boron trifluoride BF3, or complexes thereof with dialkyl ethers or amines. Alternatively, the emulsifier F comprises an epoxide-functional polyoxyethylene homo- or copolymer or an epoxide-functional sugar alcohol.

[0074] - In step b3, the emulsifier F is incorporated into the epoxy resin by a facilitated reaction, wherein a diepoxide, a dihydroxy aromatic compound and the emulsifier F of step a3 are reacted in the presence of a phosphine or amine catalyst and wherein the stoichiometry is chosen such that the facilitated reaction product has an epoxy end group.

[0075] A sugar alcohol is a compound of the formula HO-CH2-[-CH(OH)] n -CH2-OH (wherein n is an integer from 1 to 24) or ethers derived therefrom; well-known compounds include glycerol, erythritol, threitol, arabitol, xylitol, ribitol, mannitol, sorbitol, galactitol, fucitol, iditol, inositol, volemitol, isomaltitol, maltitol, lactitol, maltotriitol and maltotetraitol.

[0076] The modified epoxy-based resin Pan comprising anionic groups and non-ionic groups is preferably prepared by mixing an at least partially neutralized anionically modified epoxy-based resin Pa and a non-ionically modified epoxy-based resin Pn. Preferably, the mixing ratio is chosen such that the mass ratio Pa / Pn is 0.2 / 0.8 to 0.8 / 0.2.

[0077] Preferably, the hydrophilically modified epoxy-based resin P is a non-ionically modified epoxy-based resin Pn; more preferably, the hydrophilically modified epoxy-based resin P is a non-ionically modified epoxy-based resin Pn which is (completely) free of anionic groups, i.e. comprises 0.0% of anionic groups.

[0078] The hydrophilically modified epoxy-based resin P is obtained from petrochemical feedstocks.

[0079] Alternatively and preferably, where possible, the hydrophilically modified epoxy-based resin P is obtained from renewable feedstocks. Particularly preferred renewable feedstocks are those extracted from wood biomass such as lignin and tannins and cashew nut shell liquid (CNSL), which is a source of phenolic derivatives such as anacardic acids, cardanol, coniferyl alcohol and 2-methyl coniferyl alcohol. Also preferred are feedstocks from lignocellulosic biomass such as cellulose and hemicellulose, which are further depolymerized and dehydrated to 5-hydroxymethyl-2-furfural, which is then further derivatized to, for example, 2,5-furandicarboxylic acid or 2,5-furandimethanol. Other preferred feedstocks are terpenes and terpenoids, such as limonene and carvacrol, and other phenolic compounds, such as eugenol, ferulic acid and sinapic acid. Other renewable feedstocks for the epoxy-based resin are abietic acid (and its mono- and di-glycidyl ethers), isosorbide (and its di-glycidyl ether), and oligo- or polyglycerol (and its glycidyl ethers). Epoxy groups can also be introduced into the hydrophilically modified epoxy-based resin P by epichlorohydrin, which is derived partially or completely from (bio)renewable glycerol. The exact content of biobased carbon in these epoxy resins can be determined by the method described in ASTM D6866-20, in which carbon resulting from contemporary biomass-based inputs is distinguished from carbon originating from fossil-based inputs, and the biobased carbon content is reported as a fraction of the total organic carbon content (TOC). Other standardized methods to determine the fraction of renewable carbon include ISO 16620-2 and CEN 16640.

[0080] Another alternative to reduce the carbon footprint of the hydrophilically modified epoxy-based resin P of the present invention is to use recycled monomers for its preparation. A polymer such as poly(bisphenol A carbonate) can be depolymerized to yield monomers (i.e. bisphenol A), which can then be further used for the preparation of the epoxy-based resin of the present invention.

[0081] In yet another alternative, the hydrophilically modified epoxy-based resin P is derived from petrochemical feedstocks and / or renewable feedstocks, and / or from recycled monomers.

[0082] In the context of the present specification, “biobased carbon content” means biogenic carbon content.

[0083] The crosslinker IB is a blocked isocyanate comprising the addition reaction product of a polyisocyanate compound I and an isocyanate blocking agent B.

[0084] Particularly suitable polyisocyanate compounds I for blocked polyisocyanates IB include 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4-trimethyl-1,6-hexamethylene diisocyanate, 2,4,4-trimethyl-1,6-hexamethylene diisocyanate, 1,12-dodecane diisocyanate, cyclohexane-1,3- and -1,4-diisocyanate, 1 -isocyanato-2-isocyanatomethyl cyclopentane, 1 -isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate or IPDI), bis(4-isocyanatocyclohexyl)methane, 1,3- and 1,4-bis(isocyanatomethyl)-cyclohexane, bis-(4-isocyanato-3-methyl-cyclohexyl)-methane, alpha, alpha, alpha', alpha'-tetramethyl-1,3- and / or -1,4-xylene diisocyanate, 1 -isocyanato-1 -methyl-4(3)-isocyanatomethyl cyclohexane, 2,4- and / or 2,6-hexahydrotoluene diisocyanate, 1,3- and / or 1,4-phenylene diisocyanate, 2,4- and / or 2,6-toluene diisocyanate, 2,4- and / or 4,4'-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, 4,4',4"-triphenylmethane diisocyanate. Mixtures of diisocyanates can be used.

[0085] The blocked polyisocyanate IB can be obtained from petrochemical and / or renewable feedstocks, and / or derived from recycled monomers. Preferably, the blocked polyisocyanate IB is obtained from petrochemical and / or renewable feedstocks.

[0086] Examples of suitable higher polyisocyanates are triphenylmethane-4,4',4'-triisocyanate, 1,2,4-benzene triisocyanate and polymethylene polyphenylisocyanate. Suitable higher polyisocyanates can also be formed by dimerization of diisocyanates to form uretdiones or by trimerization to form isocyanurates. The reaction of carbamates with isocyanates to form uretonimines and the reaction of ureas with isocyanates to form biurets can be used to generate higher polyisocyanates.

[0087] Isocyanate prepolymers, such as the reaction products of polyisocyanates with polyols, such as neopentyl glycol and trimethylolpropane, or with polymeric polyols, such as polycaprolactone diols and triols, in which the equivalent ratio of isocyanate groups to hydroxyl groups (NCO / OH) is greater than 1, can also be used.

[0088] Preferred diisocyanates are 1,6-hexamethylene diisocyanate, 1,5-pentamethylene diisocyanate, isophorone diisocyanate, bis-(4-isocyanatocyclohexyl)-methane, 2,4- and / or 2,6-toluene diisocyanate, 2,4'- and / or 4,4'-diphenylmethane diisocyanate and a,a,a',a'-tetramethyl-1,3- and / or -1,4-xylylene diisocyanate, and oligomers of the mentioned diisocyanates, preferably 1,6-hexamethylene diisocyanate, isophorone diisocyanate, bis-(4-isocyanatocyclohexyl)-methane, 2,4- and / or 2,6-toluene diisocyanate, 2,4'- and / or 4,4'-diphenylmethane diisocyanate and a,a,a',a'-tetramethyl-1,3- and / or -1,4-xylylene diisocyanate, and oligomers of the mentioned diisocyanates.

[0089] Alternatively and preferably, where possible, the polyisocyanate is obtained from renewable raw materials. A particularly preferred renewable raw material is bio-based acetone (i.e. for the production of isophorone diisocyanate). Other preferred polyisocyanates derived in part from renewable raw materials are, for example, 1,5-pentaethylene diisocyanate (and its trimer, under the trade name also known as Eco N 7300), diisocyanates of L-lysine methyl or ethyl ester, isosorbide diisocyanate, furan diisocyanate, bis(4-isocyanato-2-methoxyphenoxy)alkanes, bis(4-isocyanato-2,6-dimethoxyphenoxy)alkanes, 2,4-diisocyanato-1 -pentadecylbenzene, diisocyanates and polyisocyanates based on fatty acids, dimer fatty acids and vegetable oils, 1 -isocyanato-10-[(isocyanatomethyl)thio]decane and products under the trade name TOLONATE TM XFLO 100.

[0090] The blocking agent B is added to the isocyanate groups of the polyisocyanate compound I to block the isocyanate groups, thus creating a urethane structure for the specific case of a hydroxyl-functional blocking agent B. If an isocyanate is used in stoichiometric excess relative to the blocking agent B, the unreacted isocyanate can also react with the -NH group in the urethane group to form a uretonimine structure.

[0091] The blocked polyisocyanate IB in the form of a urethane structure or a uretonimine structure is stable at room temperature; however, when heated at the baking temperature of the coating film, typically 100-200°C, the blocking agent B dissociates to regenerate free isocyanate groups.

[0092] Representative endcapping agents B are selected from the group consisting of oximes, lactams, phenols, reactive methylene compounds, pyrazoles (or pyrazole derivatives), thiols, imidazoles, amines, imines, triazoles, hydroxylamines, (aliphatic, cycloaliphatic or aromatic) monoalcohols and hydroxyl-functional carbamates.

[0093] Preferred endcapping agents B are aliphatic monoalcohols, oximes, pyrazole derivatives, reactive methylene compounds or hydroxyl-functional carbamates. More preferably, endcapping agent B is a hydroxyl-functional carbamate.

[0094] Suitable aliphatic monoalcohols for use as endcapping agent B include methanol, ethanol, 2-propanol, n-butanol, sec-butanol, 2-ethylhexanol, 1- or 2-octanol, nonanol, 3,3,5-trimethylhexanol, decanol and lauryl alcohol, cyclopentanol, cyclohexanol, benzyl alcohol, 2,2,2-trifluoroethanol, 2,2,2-trichloroethanol, 2-(hydroxymethyl)furan, 2-methoxyethanol, methoxypropanol, 2-ethoxyethanol, n-propoxyethanol, 2-butoxyethanol, 2-ethoxyethoxyethanol, 2-ethoxybutoxyethanol, butoxyethoxyethanol, 2-butoxyethyl ethanol, 2-butoxyethoxyethanol, N,N-dibutyl-2-hydroxyacetamide, N-hydroxysuccinimide, N-morpholinoethanol, 2,2-dimethyl-1,3-dioxolane-4-methanol, 3-oxazolidinethanol, 2-hydroxymethylpyridine, furfuryl alcohol, 12-hydroxystearic acid, triphenylsilanol, 2-hydroxyethyl methacrylate, and ether alcohols such as ethylene glycol, 1,2-propanediol, monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether or diethylene glycol monobutyl ether.

[0095] Suitable cycloaliphatic monoalcohols include, for example, cyclopentanol and cyclohexanol.

[0096] Suitable aromatic monoalcohols include phenol, cresol, ethylphenol, n-propylphenol, isopropylphenol, n-butylphenol, sec-butylphenol, t-butylphenol, n-hexylphenol, 2- ethylhexylphenol, n-octylphenol, n-nonylphenol, di-n-propylphenol, diisopropylphenol, isopropylcresol, di-n-butylphenol, di-sec-butylphenol, di-t-butylphenol, di-n-octylphenol, di-2-ethylhexylphenol, di-n-nonylphenol, nitrophenol, bromophenol, chlorophenol, fluorophenol, dimethylphenol, styrenated phenol, methyl salicylate, methyl 4-hydroxybenzoate, benzyl 4-hydroxybenzoate, 2-ethylhexyl hydroxybenzoate, 4-[(dimethylamino)methyl]phenol, 4-[(dimethylamino)methyl]nonylphenol, bis(4-hydroxyphenyl)acetic acid, 2-hydroxypyridine, 2- or 8-hydroxyquinoline and 2-chloro-3-pyridinol.

[0097] Preferably, the monoalcohols used as endcapping agent B for endcapping isocyanate IB are aliphatic monoalcohols, including hexanol, 2-ethylhexanol and ethylene glycol monoethyl ether.

[0098] Aliphatic diols such as 1,2-ethanediol or 1,2-propanediol are also preferred endcapping agents B, more preferably 1,2-propanediol, wherein the primary hydroxyl group provides higher reactivity towards isocyanates than the secondary hydroxyl group.

[0099] Other preferred endcapping agents B are diol esters of acrylic or methacrylic acid. It is particularly preferred to use 2-hydroxyethyl methacrylate as endcapping agent B.

[0100] Suitable amines include dibutylamine, diphenylamine, aniline, N-methylaniline, carbazole, bis(2,2,6,6-tetramethylpiperidyl)amine, di-n-propylamine, diisopropylamine, isopropylethylamine, 2,2,4- or 2,2,5-trimethylhexamethylenamine, N-isopropylcyclohexylamine, dicyclohexylamine, bis(3,5,5-trimethylcyclohexyl)amine, piperidine, 2,6-dimethylpiperidine, t-butylmethylamine, t-butylethylamine, t-butylpropylamine, t-butylbutylamine, t-butylbenzylamine, t-butylaniline, 2,2,6-trimethylpiperidine, 2,2,6,6-tetramethylpiperidine (dimethylamino)-2,2,6,6-tetramethylpiperidine, 2,2,6,6-tetramethyl-4-piperidine, 6-methyl-2-piperidine, and 6-aminohexanoic acid.

[0101] Suitable hydroxylamines include N,N-diethylhydroxylamine.

[0102] Suitable imidazoles include imidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, and 2-ethylimidazole.

[0103] Suitable imines include ethyleneimine, polyethyleneimine, 1,4,5,6-tetrahydropyrimidine, and guanidine.

[0104] Suitable triazoles include 1,2,4-triazole and benzotriazole.

[0105] Suitable thiols include butyl mercaptan, dodecyl mercaptan, and hexyl mercaptan.

[0106] Suitable oximes include, for example, formaldoxime, acetaldoxime, acetoxime, methyl ethyl ketoxime (or methylethylketoxime), cyclohexanone oxime, diacetyl monoxime, benzophenone oxime, 2,2,6,6-tetramethylcyclohexanone oxime, diisopropyl ketoxime, methyl tert-butyl ketoxime, diisobutyl ketoxime, methyl isobutyl ketoxime, methyl isopropyl ketoxime, 2,4-dimethyl-3-pentanone oxime, methyl 2,4-dimethylpentanone oxime, methyl 3-ethylheptanone oxime, 2,6-dimethyl-4-heptanone oxime, methyl isoamyl ketoxime, n-amyl ketoxime, 2,2,4,4-tetramethyl-1,3-cyclobutanedione oxime, 4,4'-dimethoxybenzophenone oxime, and 2-heptanone oxime. Preferably, the oxime is methylethylketoxime.

[0107] Suitable pyrazoles include pyrazole, 3,5-dimethylpyrazole, 3,5-diisopropylpyrazole, 3,5-diphenylpyrazole, 3,5-di-tert-butylpyrazole, 3-methylpyrazole, 4-benzyl-3,5-dimethylpyrazole, 4-nitro-3,5-dimethylpyrazole, 4-bromo-3,5-dimethylpyrazole, 4-nitro-3,5-dimethylpyrazole, and 3-methyl-5-phenylpyrazole. Preferably, the pyrazole derivative is 3,5-dimethylpyrazole.

[0108] Suitable reactive methylene compounds include Ci-C4dialkyl malonates, such as dimethyl malonate, diethyl malonate, diisopropyl malonate, di-n-butyl malonate, di-tert-butyl malonate, di-2-ethylhexyl malonate, n-butyl methyl malonate, n-butyl ethyl malonate, sec-butyl methyl malonate, sec-butyl ethyl malonate, tert-butyl methyl malonate, tert-butyl ethyl malonate, diethyl methylmalonate, dibenzyl malonate, diphenyl malonate, benzyl methyl malonate, ethylphenyl malonate, tert-butylphenyl malonate, isopropylidene malonate; acetoacetic acid alkyl esters, such as methyl acetoacetate, ethyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, n-butyl acetoacetate, tert-butyl acetoacetate, benzyl acetoacetate, phenyl acetoacetate, 2-acetoacetyloxyethyl methacrylate, acetylacetone; and cyanoethyl acetate. Preferably, the reactive methylene compound is diethyl malonate.

[0109] Suitable lactams include acetanilide, N-methylacetamide, acetoamide, ε-caprolactam, δ-valerolactam, γ-butyrolactam, pyrrolidone, 2,5-piperazinedione, and lauryllactam.

[0110] Preferably, the hydroxy-functional urethanes C used as capping agents B for the isocyanate IB are obtained by reaction of:

[0111] cyclic organic carbonates, including propylene carbonate, ethylene carbonate, butylene carbonate, glycerol carbonate, allyloxymethyl carbonate, and hydrogen carbonate, prepared from the diglycidyl ethers of bisphenol A or polypropylene glycol, preferably alkylene carbonates selected from ethylene carbonate, propylene carbonate, and butylene carbonate, more preferably alkylene carbonates selected from ethylene carbonate and propylene carbonate, with aliphatic mono- and / or di- and / or tri- and / or tetra- and / or alkanol amines, all of which amines have primary and / or secondary and / or, if appropriate, tertiary unreactive amino groups, including cyclohexylamine, N-methylbutylamine, N-methylbenzylamine, piperidine, piperazine, morpholine, benzylamine, diethylenetriamine, ethanolamine, diethanolamine, and polyoxyalkylene amines and diamines, or with

[0112] - secondary amino compounds (alkanolamines) which are obtained by reacting 1 mole of an aliphatic diamine having primary amino groups with 2 moles of a monoepoxide compound or with 2 moles of an aliphatic monoamine having primary amino groups, and / or an aliphatic diamine having primary and tertiary unreactive amino groups with 1 mole of a diepoxide compound. Mixtures of different amines can also be used.

[0113] More preferably, the hydroxy-functional carbamates C used as capping agents B (for the capped isocyanates IB) are reaction products of ethylene carbonate or propylene carbonate with alkanolamines.

[0114] Even more preferably, the hydroxy-functional carbamates C are 2-hydroxyethyl bis(2-hydroxyethyl)carbamate or 2-hydroxypropyl bis(2-hydroxyethyl)carbamate.

[0115] In a preferred embodiment, the capped isocyanates IB are prepared in a two-step reaction, wherein in the first step a di-functional isocyanate I is reacted with an organic hydroxyl compound having at least three hydroxyl groups (monohydric alcohol) to form a partially capped isocyanate, wherein the NCO / OH equivalent ratio is not less than 3. In the following step, the partially capped isocyanate is then reacted with a capping agent B (preferably aliphatic monohydric alcohols, oximes, pyrazole derivatives, reactive methylene compounds and / or hydroxy-functional carbamates) to cap all remaining isocyanate groups.

[0116] It is further preferred to use chain-extending alcohols, such as reaction products of triols or tetraols with lactones, whereby triols or tetraols are prepared in which the hydroxyl groups of the molecule are separated by oligomeric or polymeric polylactone chains.

[0117] A similar effect can be achieved by reacting triols or tetraols with methyl-oxirane (propylene oxide), but the reaction with ethylene oxide is less preferred, because oligomeric or polymeric oxyethylene chains make the coating more hydrophilic and weaken the corrosion protection.

[0118] In another preferred embodiment, the capped isocyanates IB are prepared by reacting a polyisocyanate I with a hydroxy-functional carbamate C and other capping agents (preferably aliphatic monohydric alcohols, oximes, pyrazole derivatives and / or reactive methylene compounds).

[0119] The isocyanate groups of the polyisocyanate compound I used in the aqueous dispersion D can be completely or only partially blocked with a blocking agent B. In the context of the present specification, completely (or fully) blocked polyisocyanate IB refers to a polyisocyanate compound I in which all isocyanate groups are blocked with a blocking agent B (such that no free isocyanate groups remain in IB). Partially blocked polyisocyanate IB is a polyisocyanate compound I comprising isocyanate groups, preferably comprising on average at most one isocyanate group per molecule of polyisocyanate compound I, which is connected to a binder resin (the binder resin together with the (co)crosslinker comprises the so-called binder) by reaction of said isocyanate group with an isocyanate-reactive group of said binder resin; the remaining isocyanate groups of the polyisocyanate compound I are blocked with a blocking agent B. Thus, the isocyanate groups of the partially blocked polyisocyanate IB consist of isocyanate groups which react with the binder resin (preferably on average at most one isocyanate group per molecule of I) and isocyanate groups (or multiple isocyanate groups) blocked with a blocking agent B, such that no free isocyanate groups remain in IB.

[0120] The partially blocked polyisocyanate IB can be readily prepared by reacting a polyisocyanate compound I having at least two isocyanate groups in the molecule with a blocking agent B, wherein the amount of blocking agent B is sufficient to allow the resulting product to contain unblocked isocyanate groups, preferably on average not more than one unblocked isocyanate group per molecule of I, which is connected (or bonded) to the hydrophilically modified epoxy-based resin P by reaction of said isocyanate group with an isocyanate-reactive group of P (the reactive group of the binder resin, such as a hydroxyl group, is capable of reacting with and covalently connecting to a non-blocked isocyanate group of a crosslinker).

[0121] The blocking agent B can be obtained from petrochemical and / or renewable feedstocks.

[0122] - the co-crosslinker E is preferably at least di-functional and is a compound E1 having at least two hydroxyl groups which react with acidic compounds present in the coating composition (prepared from the aqueous resin dispersion D, see below) at elevated temperature (above 80°C) to form esters and release water, or

[0123] - esters E2 having at least two ester groups, formed from at least a diol E22 and an acid E21, the acid E21 being unstable at elevated temperature (above 80°C). In this case, the crosslinking reaction is a metathesis reaction between the ester E2 and an acidic compound present in the coating composition, forming the at least diol E22 and an ester of said acidic compound, and releasing the acid E21, which in the case of a beta-keto acid E21 decomposes into a ketone and carbon dioxide. Useful compounds E2 have the general formula (R 1 -C(=0)-CR 2 R 3-C(=0)-0- x R 4 wherein R 1 is an alkyl group having 1 to 8 carbon atoms, R 2 and R 3 are independently of each other hydrogen or an alkyl group having 1 to 8 carbon atoms, and R 4 is a residue of an aliphatic alcohol having 2 to 4 hydroxyl groups and 2 to 40 carbon atoms; preferably E2 is selected from the group consisting of ethylene glycol bisacetoacetate, diethylene glycol bisacetoacetate, propylene glycol bisacetoacetate, 1,4-butanediol bisacetoacetate, 2,2,4-trimethylpentanediol bisacetoacetate; and bisacetoacetates of mixtures of dimeric aliphatic alcohols, glycerol triacetoacetate, trimethylolpropane triacetoacetate; and the corresponding diesters of 3-oxopentanoic acid, 3-oxohexanoic acid, 3-oxoheptanoic acid, 2-methylacetoacetic acid, 2,2-dimethylacetoacetic acid, 2- ethylacetoacetic acid and 2-methyl-2-ethylacetoacetic acid. In the coating composition, the acid groups can be present in the binder component or in additives, in particular in the rheological additive, or in auxiliaries such as catalysts and biocides.

[0124] A preferred example of a compound E1 is a β-hydroxyalkylamide of the formula

[0125] (OH-CHR1-CH2-NR2-CO) m -A-(CO-NR2-CH2-CHR1-OH) n

[0126] wherein:

[0127] R1is hydrogen or a C1-C5alkyl group;

[0128] R2is hydrogen, a C1-C5alkyl group or CH2-CHR1-OH;

[0129] A is a chemical bond or a polyvalent organic group derived from a saturated, unsaturated or aromatic hydrocarbon group, including substituted hydrocarbon groups containing 2 to 20 carbon atoms; and

[0130] m is an integer from 1 to 2, n is an integer from 0 to 2, and m+n is at least 2.

[0131] Suitable β-hydroxyalkylamides are described in the literature, for example those mentioned in patents US 4,727,111, US 4,788,255, US 4,076,917, US 5,266,628, EP 322834 and EP 473380.

[0132] More preferably, the compound E1 is a β-hydroxyalkylamide of the formula

[0133] (OH-CHR1-CH2-NR2-CO) m -A-(CO-NR2-CH2-CHR1-OH) n

[0134] wherein:

[0135] - R1 is hydrogen or C1 alkyl;

[0136] - R2 is CH2-CHR1-OH;

[0137] - A is a saturated hydrocarbon group containing 4 carbon atoms; and

[0138] - m is equal to 1 and n is equal to 1.

[0139] Most preferably, the compound E1 is N,N,N',N'-tetra-(2-hydroxyethyl)- hexanediamide, commercially available under the trade name XL552, or N,N,N',N'-tetra-(2-hydroxypropyl)-hexanediamide, commercially available under the trade name QM1260.

[0140] The co-crosslinker E can be obtained from petrochemical feedstocks and / or renewable feedstocks.

[0141] The coating composition of the present application is prepared from an aqueous resin dispersion D comprising a mixture of a hydrophilically modified epoxy-based resin P, a blocked isocyanate IB and a co-crosslinker E.

[0142] Preferably, the aqueous resin dispersion D of the present application comprises:

[0143] - 40-90 wt%, more preferably 45-85 wt%, of a hydrophilically modified epoxy-based resin P;

[0144] - 5-55 wt%, more preferably 10-50 wt%, of a blocked isocyanate crosslinker IB; and

[0145] - 0.1-5 wt%, preferably 1-3 wt%, of a co-crosslinker E;

[0146] based on the total weight of P, IB and E, wherein the sum of the weight percentages (wt%) of P, IB and E does not exceed 100%.

[0147] More preferably, the aqueous resin dispersion D of the present application comprises:

[0148] - 40-90 wt%, more preferably 45-85 wt%, of a hydrophilically modified epoxy-based resin P, which is a non-ionic modified epoxy-based resin Pn;

[0149] - 5-55 wt%, more preferably 10-50 wt%, of a blocked isocyanate crosslinker IB; and

[0150] - 0.1 to 5 wt%, preferably 1 to 3 wt%, of a co-crosslinker E, which is a beta-hydroxyalkylamide El;

[0151] based on the total weight of P, IB and El, wherein the sum of the weight percentages (wt%) of Pn, IB and El does not exceed 100%.

[0152] Preferably one or more co-solvents are added selected from the group consisting of aliphatic alcohols, ketones, esters, glycols, glycol ethers, glycol esters, and mixtures thereof, preferably one or more additives are added selected from the group consisting of antifoams, flow and levelling agents, coalescing agents, flow modifiers, biocides, pigments and rheological additives.

[0153] If a pigmented coating composition is prepared, wetting agents and anti-settling agents can also be added. Representative alcohols include ethanol, n-propanol, isopropanol, n-butanol, and isobutanol; representative ketones include acetone, 2-butanone, cyclohexanone, methyl aryl ketones, ethyl aryl ketones, and methyl isoamyl ketone; representative esters include ethyl acetate and butyl acetate; representative glycols include ethylene glycol and propylene glycol; representative glycol ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and methoxypropanol; representative glycol esters include butanediol acetate and methoxypropanol acetate.

[0154] Preferably, the coating composition comprises 35 to 55 wt%, more preferably 40 to 50 wt%, of non-volatile compounds, 45 to 65 wt%, more preferably 50 to 60 wt%, of water, and a co-solvent selected from the group consisting of alcohols, ketones, esters, glycols, glycol ethers, glycol esters, and mixtures thereof.

[0155] Preferably, one or more crosslinking catalysts are added to the coating composition, which are selected from the group consisting of salts, chelate compounds and organometallic compounds of the elements of groups 4, 7, 8, 9, 12, 13, 14 and 15 and periods 4, 5 and 6 according to the new IUPAC nomenclature system, and strong amines.

[0156] Preferred are water-soluble compounds, such as salts of these elements which dissociate into ions in aqueous systems, and chelates of these elements, wherein the chelate-forming agent can be an organic hydroxy acid, such as lactic acid, 2,2-dimethylolpropionic acid, an amino acid such as N,N,N',N'-ethylenediaminetetraacetic acid, nitrilotriacetic acid, beta-alanine, or a polyfunctional amine or hydroxylamine. Other compounds which can be used are organometallic compounds, such as alkoxymetal oxides, and metal salts of organic or hydroxy acids. Particularly preferred are the methanesulfonates, lactates and dimethylolpropionates of bismuth, tin, lead and titanium.

[0157] Stronger amines are more preferably tertiary amines and most preferably (poly)cyclic tertiary amines such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,4-diazabicyclo[2.2.2]octane (DABCO).

[0158] Based on the hydrophilically modified epoxy-based resin P comprising the blocked isocyanate crosslinker IB and the co-crosslinker E, the coating composition preferably comprises up to 5 wt.-% of one or more crosslinking catalysts.

[0159] The coating composition can comprise one or more organic and inorganic pigments and optionally one or more fillers.

[0160] Examples of inorganic pigments are iron oxide pigments, titanium oxide pigments, zinc oxide pigments, chromium oxide pigments co-precipitated with nickel and nickel titanate, yellow pigments from lead sulfochromate or bismuth vanadate lead, orange pigments from lead sulfochromate molybdate and carbon black.

[0161] Examples of suitable organic pigments are azo pigments, metal complex pigments, anthraquinone pigments, phthalocyanine pigments, polycyclic pigments, especially those of the sulfur indigo, quinacridone, dioxazine, pyrrolo, naphthalene tetracarboxylic acid, perylene, isoindoline (ketone), flavanthrone, pyranthrone or iso- anthanthrone series.

[0162] Examples of fillers which can be used are kaolin, talc, mica, other silicates, quartz, cristobalite, wollastonite, perlite, diatomite, fibrous fillers, aluminum hydroxide, barium sulfate or calcium carbonate.

[0163] In the process of the present application, the inventive aqueous resin dispersion D comprising the mixture of the hydrophilically modified epoxy-based resin P, the blocked isocyanate crosslinker IB and the co-crosslinker E is transferred into a blender, such as a high speed stirrer, a planetary mixer or a ball mill, and water, a co-solvent, additives, pigments and fillers are added. The blending is carried out for at least 5 minutes until a homogeneous blend (or coating composition) is obtained.

[0164] The coating composition of the present application is preferably applied to a metal substrate, more preferably to a corrosion-resistant pretreated metal substrate, even more preferably to a phosphatized steel sheet or to a pretreated steel sheet comprising a zirconium-based, vanadium-based, titanium-based or silane-based conversion coating.

[0165] The coating composition can be applied to the substrate using any suitable procedure known in the art and is preferably applied by spraying or dipping.

[0166] The water and optional co-solvent are then flashed off at a temperature of at least 20°C for at least 1 minute, preferably at a temperature of at least 20°C for at least 5 minutes, after which the coating is baked in a ventilated convection oven at a temperature of at least 140°C, preferably 140-230°C, more preferably 150-220°C, even more preferably 160-210°C, most preferably 170-200°C for at least 20 seconds, preferably 1-25 minutes, more preferably 2-20 minutes, even more preferably 4-18 minutes, yet even more preferably 6-15 minutes, most preferably 8-12 minutes.

[0167] Alternatively, the coating can be cured by infrared radiation (e.g. near infrared, short infrared or mid infrared) or by induction or by a combination thereof. In embodiments using infrared or induction systems, the baking cycle is in the range of 2-160 seconds, depending on the heating system or combination of heating systems. Example

[0168] Example 1 : Synthesis of blocked isocyanate crosslinker

[0169] 190 g of hydroxyl urethane are prepared in a known manner (according to EP 0 476 514: Table 1, HC 2) from diethanolamine and propylene carbonate. 470 g of butyl diglycol and 0.4 g of bismuth neodecanoate (28% Bi metal) are added at 30°C and homogenized. 690 g of oligomeric diphenylmethane diisocyanate (according to DIN EN ISO 11909, isocyanate content 32.0%) are added stepwise to keep the temperature below 100°C. When the amount of free isocyanate is determined (by titration) to be less than 0.1 %, the product is diluted in 80 g of methoxypropanol and 60 g of deionized water. The resulting product has a solids content of 90.2% (1 g of sample dried at 125°C for 10 minutes), a dynamic viscosity of 12500 mPa.s (measured at 23°C, shear rate of 25 s -1 ).

[0170] Example 2: Preparation of blocked isocyanate nonionic dispersion

[0171] 2.1 : Preparation of hydrophilic epoxy resin.

[0172] 1 kg of polyethylene glycol PEG 4000 (average molar mass of about 4000 g / mol) is heated to 120°C and the dissolved water is removed by distillation under reduced pressure and nitrogen flow. 110 g of bisphenol A diglycidyl ether are added, followed by 1.7 g of a 50% mass fraction w(HBF4) aqueous solution of tetrafluoroboric acid in solution. When a constant value of the epoxy group specific content is reached (about 0.1 -0.2 mol / kg), 1100 g of water are added to dilute to a solids mass fraction of about 50%.

[0173] 2.2. Accelerating the reaction

[0174] A reaction vessel equipped with a thermometer, stirrer, reflux condenser, falling film condenser and a vacuum pump was charged with 2400 g of bisphenol A diglycidyl ether, 720 g of bisphenol A and 1200 g of the hydrophilic epoxy resin of example 2.1 and heated to 125°C under stirring at a reduced pressure of 10 kPa (100 mbar) to remove all volatile components. Then 1.3 g of triphenylphosphine were added and the temperature was further increased to 160°C and the mixture was continuously stirred until the specific amount of epoxy groups was 1.87 equivalents / kg ("epoxy equivalent" 534 g / equivalent).

[0175] 2.3. Addition of blocked isocyanate and dispersion in water

[0176] The reaction mixture obtained in example 2.2 was then cooled to 120°C and 729 g of the blocked isocyanate from example 1 were added. The viscosity was then reduced by adding 410 g of methoxy propanol and the solution was cooled to 80°C. Then 1200 g of deionized water were added to the reaction vessel and the mixture was dispersed at 70°C for 3 hours to obtain a water dilutable resin dispersion. In addition, 3500 g of deionized water were added to the vessel within 2 hours and finally the mass fraction of solids was adjusted to 42.1% (1 g sample, dried at 125°C for 10 minutes) by further addition of deionized water and the viscosity was reduced to 1070 mPa.s (measured at 23°C, shear rate of 100 s -1 ). The resulting Z-average particle size (Z-average according to ISO 22412) was 246 nm.

[0177] Example 3: Preparation of a blocked isocyanate nonionic dispersion

[0178] 3.2. Acceleration of the reaction.

[0179] A reaction vessel equipped with a thermometer, stirrer, reflux condenser, falling film condenser and a vacuum pump was charged with 2400 g of bisphenol A diglycidyl ether, 670 g of bisphenol A and 1750 g of the hydrophilic epoxy resin of example 2.1 and heated to 125°C under stirring at a reduced pressure of 10 kPa (100 mbar) to remove all volatile components. Then 1.3 g of triphenylphosphine were added and the temperature was further increased to 160°C and the mixture was continuously stirred until the specific amount of epoxy groups was 1.96 equivalents / kg ("epoxy equivalent" 510 g / equivalent).

[0180] 3.3. Addition of blocked isocyanate and dispersion in water

[0181] The reaction mixture obtained in Example 3.2 was then cooled to 120°C and 2945 g of the capped isocyanate from Example 1 were added. The viscosity was then reduced by adding 410 g of methoxy propanol and the solution was cooled to 80°C. Then 1850 g of deionized water were added to the reaction vessel and the mixture was dispersed for 3 hours at 70°C to obtain a water dilutable resin dispersion. In addition, 5000 g of deionized water were added to the vessel over 2 hours and finally the solids mass fraction was adjusted to 42.3% (1 g sample, dried for 10 minutes at 125°C) and the viscosity was reduced to 930 mPa.s (measured at 23°C, shear rate of 100 s -1 ) by further addition of deionized water. The resulting Z-average particle size (Z-average according to ISO 22412) was 244 nm.

[0182] Comparative Example 1 : Synthesis of a resol crosslinker

[0183] In a reaction vessel equipped with a thermometer, stirrer, reflux condenser, falling film condenser and a vacuum apparatus, 453.6 g of n-butanol were heated to 50°C. Then, 397.3 g of bisphenol A, 48.5 g of a sodium hydroxide solution (mass fraction of the solid of 45%), 357.6 g of a formaldehyde aqueous solution (mass fraction of dissolved formaldehyde of 37%) and 95.4 g of paraformaldehyde were added and reacted for 10 hours at 50°C. Then, 447 g of n-butanol were added again and the pH was adjusted to 3.5 by adding 72.2 g of a phosphoric acid aqueous solution (mass fraction of dissolved phosphoric acid of 75%). Azeotropic distillation was carried out at atmospheric pressure and a temperature of 94-105°C for 8 hours to strip the water phase. Then, the residual n-butanol was stripped at 120°C. In addition, 212 g of n-butanol were added, followed by a second azeotropic distillation at atmospheric pressure for 5 hours. The remaining n-butanol was also stripped to reach a distillation temperature of 115°C and the water was further stripped by azeotropic distillation until the reaction mixture was compatible with n-heptane at a ratio of resin to n-heptane of 1 :2. The salt was removed by washing the obtained resin with 187 g of deionized water. The residual water was removed by azeotropic distillation at atmospheric pressure and a temperature of 105°C, followed by removal of all volatile components at a reduced pressure of 10 kPa (100 mbar) at 85°C. The obtained polymer resin was dissolved in methoxy propanol to form a solution with a mass fraction of solids of 80% (sample mass: 1 g, drying conditions: 60 minutes, 125°C), the dynamic viscosity at 23°C was 1200 mPa.s.

[0184] Comparative Example 2: Preparation of a resol modified non-ionic dispersion

[0185] A hydrophilic epoxy resin was prepared according to examples 2.1 and 2.2. The resulting reaction mass was then cooled to 120°C and 820 g of the phenol formaldehyde resin from comparative example 1 was added. The viscosity was then reduced by adding 318 g of methoxy propanol and the solution was cooled to 80°C. Then 1130 g of deionized water was added to the reaction vessel and the mixture was dispersed at 70°C for 3 hours to obtain a water dilutable resin dispersion. In addition, 3500 g of deionized water was added to the vessel over 2 hours and finally the mass fraction of solids was adjusted to 41.8% (1 g sample, dried at 125°C for 10 minutes) and the viscosity was reduced to 1020 mPa.s (measured at 23°C, shear rate of 100 s -1 ) by adding additional deionized water. The resulting Z-average particle size (Z-average according to ISO 22412) was 220 nm.

[0186] Example 4: Preparation of a white single-coat and its application on a steel sheet

[0187] 4.1 : Pigment paste, dispersed in water on a bead mill.

[0188] A pigment paste was obtained by dispersing the components shown in table 1 in water on a bead mill.

[0189] Table 1

[0190]

[0191] 4.2. Coating composition

[0192] In table 2, the individual components of the coating compositions according to the present application (examples 5-10) and according to the prior art (examples 11-13 are comparative examples) are shown, wherein:

[0193] DOWANOL TM DPnB is n-butoxypropoxypropanol;

[0194] VXW 6393 is a mineral oil based antifoam agent (Allnex Austria GmbH);

[0195] AS1130 is an acrylic HASE thickener (acid value 90 mg / g, based on as delivered form; BASF SE);

[0196] XL 552 is N,N,N',N'-Tetrakis(2-hydroxyethyl)-adipamide (Ems Chemie AG), 20% by weight dissolved in water.

[0197] Table 2

[0198]

[0199]

[0200] The components disclosed in Table 2 were mixed in the given order with a laboratory mixer. 24 hours after mixing, the coating compositions of Examples 5-13 were applied to zinc phosphate primed steel panels (SFS 26S 6800 OC, Chemetal GmbH) by means of an airless spray gun. After flashing for 10 minutes at 23 °C, the coated panels were cured for 10 minutes at 180 °C. The dry film thickness of the coating was 35 pm in each case. 26S 6800 OC, Chemetal GmbH) by means of an airless spray gun. After flashing for 10 minutes at 23 °C, the coated panels were cured for 10 minutes at 180 °C. The dry film thickness of the coating was 35 pm in each case.

[0201] Example 14: Corrosion and color performance

[0202] The color of the coated panels of each of the coating compositions of Examples 5-13 was measured using a colorimeter "Spectro-guide" (Byk Gardner GmbH) after conditioning for 7 days at 23 °C and 50% relative humidity. According to the CIE L*a*b* system, a higher "b" value indicates a more yellowish (discolored) coating film. The panels were then scribed in the middle of the test panel and exposed to a salt spray chamber test ("SST"; DIN EN ISO 9227). Blistering of the artificially aged panels was recorded according to DIN EN ISO 4628-2.

[0203] Table 3 shows the color and blistering after exposure to the salt spray test.

[0204] Comparative Example 2 and the inventive Example 2 have comparable amounts of crosslinker (resorcinol formaldehyde resin or blocked isocyanate, respectively) based on the amount of epoxy resin. A direct comparison between the coating compositions of Example 5 and Comparative Example 11, Example 6 and Comparative Example 12, and Example 7 and Comparative Example 13 shows that the corrosion performance of Comparative Examples 11-13 is at the same level as Examples 5-7 (coating compositions of the invention). It is also clearly shown that the addition of a beta-hydroxyalkylamide in the anticorrosive monocoat significantly improves the anticorrosive effect in all cases. However, in addition to the retained anticorrosive performance, all coating compositions of the invention based on blocked isocyanate (Examples 5-10) show a lighter color (smaller b value) at the same pigment / binder ratio of 130% (wt / wt ratio) than the coating compositions based on resorcinol formaldehyde resin (Comparative Examples 11-13). In the context of this specification, the pigment / binder ratio means the sum of pigments and fillers (by weight) divided by the sum of solid binder resin and blocked isocyanate crosslinker (by weight) in percent; the solid content of additives and co-crosslinkers is not considered.

[0205] Table 3

[0206]

Claims

1. An aqueous resin dispersion D comprising a mixture of 40 to 90 wt% of a hydrophilically modified epoxy-based resin P, 5 to 55 wt% of a blocked isocyanate crosslinker IB and 0.1 to 5 wt% of a co-crosslinker E, based on the total weight of the P, IB and E; wherein the hydrophilically modified epoxy-based resin P is prepared in a reaction or series of reactions comprising at least one step in which a promoting reaction is carried out, which promoting reaction is a reaction in which a phenolic compound having a phenolic hydroxyl group is reacted with a compound having at least two reactive epoxy groups, and wherein the hydrophilically modified epoxy-based resin P comprises 1-hydroxy-2-phenoxy-ethyl; wherein the blocked isocyanate crosslinker IB is a blocked isocyanate comprising the addition reaction product of a polyisocyanate compound I and an isocyanate blocking agent B; and wherein the co-crosslinker E is selected from the group consisting of: - a compound having at least two hydroxyl groups E1 which react at elevated temperature above 80°C with acid compounds present in the coating composition to form esters and release water, and - an ester E2 having at least two ester groups formed from at least one dihydric alcohol E22 and an acid E21 which is unstable at elevated temperature above 80°C.

2. The aqueous resin dispersion D according to claim 1, wherein the blocked isocyanate crosslinker IB is the reaction product of: - a polyisocyanate with a monohydric alcohol, or - a polyisocyanate with an oxime, or - a polyisocyanate with a pyrazole derivative, or - a polyisocyanate with a reactive methylene compound, or - a polyisocyanate with a lactam, or - a polyisocyanate with a phenol, or - a polyisocyanate with a mercaptan, or - a polyisocyanate with an imidazole, or - a polyisocyanate with an amine, or - a polyisocyanate with an imine, or - a polyisocyanate with a triazole, or - a polyisocyanate with a hydroxylamine, or - a polyisocyanate with a hydroxyl-functional carbamate C, or - a polyisocyanate with a hydroxyl-functional carbamate C and a monohydric alcohol, and / or an oxime, and / or a pyrazole derivative, and / or a reactive methylene compound, and / or a lactam, and / or a phenol, and / or a mercaptan, and / or an imidazole, and / or an amine, and / or an imine, and / or a triazole, and / or a hydroxylamine.

3. The aqueous resin dispersion D according to claim 1 or 2, wherein the blocked isocyanate crosslinker IB is the reaction product of a polyisocyanate with a hydroxyl-functional carbamate C.

4. The aqueous resin dispersion D according to claim 2 or 3, wherein the hydroxyl-functional carbamate C is the reaction product of a cyclic organic carbonate with an aliphatic monoamine, a diamine, a triamine, a tetraamine and / or an alkanolamine, the amine having at least one primary or secondary amino group.

5. The aqueous resin dispersion D according to claim 4, wherein the alkanolamine is the reaction product of: - 1 mole of an aliphatic diamine having a primary amino group with 2 moles of a monoepoxide or with 2 moles of an aliphatic monoamine having a primary amino group, and / or - an aliphatic diamine having primary and tertiary unreactive amino groups with 1 mole of a diepoxide.

6. The aqueous resin dispersion D according to any one of claims 2-5, wherein the hydroxyl-functional carbamate C is a reaction product of ethylene carbonate or propylene carbonate with an alkanolamine.

7. The aqueous resin dispersion D according to any one of claims 2-6, wherein the hydroxyl-functional carbamate C is 2-hydroxyethyl bis(2-hydroxyethyl)carbamate or 2-hydroxypropyl bis(2-hydroxyethyl)carbamate.

8. The aqueous resin dispersion D according to any one of claims 1-7, wherein the blocked isocyanate crosslinker IB is a partially blocked polyisocyanate comprising on average a maximum of 1 isocyanate group per molecule of polyisocyanate compound I, which is attached to the hydrophilically modified epoxy-based resin P by reaction of the isocyanate group with an isocyanate-reactive group of P; the remaining isocyanate groups of the polyisocyanate compound I are blocked with a blocking agent B.

9. The aqueous resin dispersion D according to any one of claims 1-8, wherein the co- crosslinker E is E1, E1 being a beta-hydroxyalkylamide having the following formula: wherein: - R1 is hydrogen or a C1-C5 alkyl group; - R2 is hydrogen, a C1-C5 alkyl group or CH2-CHR1-OH; - A is a chemical bond or a polyvalent organic group derived from a saturated, unsaturated or aromatic hydrocarbon group, including substituted hydrocarbon groups containing 2 to 20 carbon atoms; and - m is an integer from 1 to 2 and n is an integer from 0 to 2, with m+n being at least 2. (OH-CHR1-CH2-NR2-CO) m -A-(CO-NR2-CH2-CHR1-OH) n 10. The aqueous resin dispersion D according to any one of claims 1-9, wherein the co- crosslinker E is E1, E1 being a beta-hydroxyalkylamide having the following formula: wherein: - R1 is hydrogen or a C1 alkyl group; - R2 is CH2-CHR1-OH; - A is a saturated hydrocarbon group containing 4 carbon atoms; and - m is equal to 1 and n is equal to 1.

11. The aqueous resin dispersion D according to any one of claims 1-10, wherein the co- crosslinker E is E1, E1 being N,N,N',N'-tetra-(2-hydroxyethyl)-adipamide or N,N,N',N'-tetra-(2-hydroxypropyl)-adipamide.

12. The aqueous resin dispersion D according to any one of claims 1-11, wherein the hydrophilically modified epoxy-based resin P is a non-ionic modified epoxy-based resin Pn. The epoxy equivalent weight (EEW) is from 200 to 2000 g / eq.

14. The aqueous resin dispersion D according to claim 13, wherein the epoxy-functional non- ionic emulsifier F is: - an epoxy-functional polyalkylene oxide homopolymer or copolymer; and / or - an epoxy-functional sugar alcohol; and / or - a reaction product of a hydroxyl-functional polyalkylene oxide homopolymer or copolymer or sugar alcohol segment with at least a difunctional epoxide; wherein: - the polyalkylene oxide homopolymer is a polyoxyethylene or polyoxypropylene, - the polyalkylene oxide copolymer is a polyoxyethylene-propylene copolymer, - the polyalkylene oxide homopolymer and copolymer comprise from 20 to 150 C2-C3 alkylene oxide units.

15. The aqueous resin dispersion D according to any one of claims 1-14, comprising: (OH-CHR1-CH2-NR2-CO) m -A-(CO-NR2-CH2-CHR1-OH) n ​ ​ ​ ​ ​ ​ ​ 13. The aqueous resinous dispersion D according to claim 12, wherein the non-ionically modified epoxy-based resin Pn is a reaction product of a diepoxide, a diaromatic compound and an epoxy-functional non-ionic emulsifier F, and characterized in that ​ ​ ​ ​ ​ ​ ​ ​ ​ - 40-90 wt% of a hydrophilically modified epoxy-based resin P which is a non- ionically modified epoxy-based resin Pn; - 5-55 wt% of a blocked polyisocyanate crosslinker IB; and - 0.1-5 wt% of a co-crosslinker E which is a beta-hydroxyalkylamide E1; based on the total weight of Pn, IB and E1.

16. The aqueous resin dispersion D according to any one of claims 1-15, comprising one or more catalysts selected from the group consisting of salts, chelate compounds and organometallic compounds of elements of groups 4, 7, 8, 9, 12, 13, 14 and 15 and periods 4, 5 and 6 of the Periodic Table of Elements according to the new IUPAC nomenclature, and strong amines.

17. The aqueous resin dispersion D according to any one of claims 1-16, wherein the hydrophilically modified epoxy-based resin P and / or blocked isocyanate crosslinker IB are obtained from renewable feedstocks and the biobased carbon content, determined using ASTM D6866-20 standard, totals more than 20 wt% of the total carbon content of the epoxy-based resin P and blocked isocyanate crosslinker IB, or wherein the epoxy-based resin P and / or blocked isocyanate crosslinker IB are derived from recycled monomers.

18. A coating composition comprising the aqueous resin dispersion D according to any one of claims 1-17 and one or more additives selected from the group consisting of defoamers, leveling agents, coalescing agents, flow modifiers, biocides, pigments, rheological additives and wetting agents.

19. The coating composition according to claim 18, comprising 35-55 wt% of non- volatile compounds, 45-65 wt% of water and a co-solvent selected from the group consisting of alcohols, ketones, esters, glycols, glycol ethers, glycol esters, and mixtures thereof.

20. A method of producing a coated metal substrate comprising the steps of: - applying the coating composition according to claim 18 or 19 to at least one side of a metal substrate, optionally pretreated and / or comprising a primer, at a coating thickness adjusted to obtain a dry coating thickness of at least 10 pm; - flashing off water and co-solvent at a temperature of at least 20 °C for at least 1 minute; - baking the applied coating composition at a temperature of at least 100 °C for a time of at least 20 seconds to form a metal substrate coated with a crosslinked coating.

21. Use of the coating composition according to any one of claims 18-19 for coating a metal substrate.

Images