WATER-BASED COATING PRODUCTS

Abstract

The invention relates to a water-based coating composition comprising a vinyl polymer dispersion stabilized by a first-crosslinked oligomer DA, a vinyl polymer dispersion stabilized by a second-crosslinked oligomer DB, and at least one crosslinker X, where DA comprises a vinyl oligomer OLA and at least one vinyl polymer PA, the vinyl oligomer OLA and optionally at least one vinyl polymer PA comprising chain-suspended functional groups capable of reacting with the functional groups of at least one crosslinker X, and chain-suspended unsaturated ethylene functional groups capable of crosslinking by radical process; and DB comprises a vinyl oligomer OLB and at least one vinyl polymer PB, the vinyl oligomer OLB and / or optionally at least one vinyl polymer PB comprising chain-suspended functional groups capable of reacting with the functional groups of at least one crosslinker X. The invention also relates to a method for preparing a vinyl organic polymer dispersion stabilized by a crosslinked oligomer and the resulting coating product.

Description

[0001] WATERBORNE COATING COMPOSITION

[0002] Field of the invention

[0003] The present invention relates to a waterborne coating composition comprising a first crosslinkable oligomer stabilized vinyl polymer dispersion, a second crosslinkable oligomer stabilized vinyl polymer dispersion and at least one (carbonyl reactive) crosslinker, to a process for preparing said waterborne coating composition, and to coated substrates coated with said coating composition.

[0004] State of the Art

[0005] Due to increasingly stringent laws concerning the allowable contents of volatile organic compounds in coatings (VOC), major efforts have been made to minimize the use of organic co-solvents in waterborne coatings based on waterborne binders based on vinyl monomers. However, in waterborne coatings that use vinyl polymers as the main binder, the final hardness after curing of an applied coating film is limited by the amount of organic co-solvents in the coating formulation. For some coating applications a minimum hardness is required, for example if a high blocking-resistance or scratch-resistance is important. The hardness of the final coating can be increased by introducing a crosslinking mechanism in the coating. Coating compositions that crosslink under the influence of actinic radiation have been known for quite some time. In industrial practice the actinic radiation used is mainly ultra-violet (UV) or electronbeam (EB) radiation. In order to reduce the viscosity sufficiently to allow the coatings to be applied on a surface, the known compositions always contain low molecular weight diluents having ethylenically functional groups. These diluents are known to be irritating or toxic, so a clear need exists to develop waterborne binders that can be cured by UV, visible light or EB radiation and do not require ethylenically unsaturated diluents but nevertheless have sufficiently low viscosity to be applied by conventional means.

[0006] U.S. Patent 4,107,013 discloses a UV-curable aqueous coating containing a vinyl polymer dispersion with a so-called “core-shell” morphology and 5-35% of an emulsified low molecular weight radiation crosslinking agent wherein the vinyl polymer dispersion has a shell of copolymerized difunctional monomers containing unreacted allyl groups. The presence of the low molecular weight radiation crosslinking agent poses severe problems for the save use of such coatings.

[0007] U.S. Patent 4,244,850 discloses air-curing a waterborne coating composition containing ethylenically unsaturated resin particles, an emulsion of metal drier salts and water-immiscible organic solvent. The unsaturated resin is formed from 1 to 20% by weight of a monoethylenically unsaturated monomer having a carboxyl or 1 ,2-epoxy functionality, a portion of which has been reacted with 1 ,2-epoxy or carboxyl functionality, respectively, to provide unsaturation sites on the resin particles. The resin is not neutralized before functionalization. Again the presence of high amounts of unsaturated monomers in the coating composition poses threats to the health of those applying the coating.

[0008] European Patent Application EP330246 discloses a curable aqueous dispersion formed by first polymerizing an organic phosphate or phosphonate compound or mixtures thereof with a (meth)acrylate derivative or another unsaturated compound or a styrene derivative and then adding an ethylenically unsaturated, epoxy-containing monomer. Also in this case the presence of the ethylenically unsaturated monomers poses a health and safety risk.

[0009] U.S. Patent 4,925,893 discloses auto-oxidative and radiation curable vinylidene chloride / vinyl chloride I 2-ethylhexyl acrylate copolymer dispersions having residual unsaturation via the addition of a gel fraction of at least 5 weight % formed from a multifunctional monomer early in the polymerization and the addition of a low reactivity multifunctional compound, such as diallyl phthalate, late in the polymerization.

[0010] European Patent Application EP442653 discloses the preparation of polymers containing vinylfunctionality. An amine functional vinyl copolymer dispersion is formed by reacting a carboxyl functional copolymer dispersion with aziridines such as ethylene or propyleneimine. The amine functional latex is then reacted with a material having both an enolic carbonyl group and another functional group, for example 2-(acetoacetoxy)ethyl methacrylate, to give a methacrylate functional polymer.

[0011] European patent application EP0602763 describes a multi-staged polymer dispersion formed from a first stage polymer and a second stage polymer having p-unsaturated carbonyl functionalization which permits curing by UV radiation. The weight ratio of said first stage polymer to said second stage polymer is from about 20:80 to about 70:30. Hydrophobic monomers are preferred for the first stage polymer. The second stage polymer comprises between about 30 and 60 % by weight of at least one co-monomer containing acid functionality. The acid functionality is partially neutralized with a base and is reacted with a mono- ethylenically unsaturated epoxide. Preferably, the first stage polymer contains about 10% by weight, preferably from about 1% to about 5% by weight of a crosslinking co-monomer (allyl methacrylate). The problem associated with this synthesis route is the fact that the dispersion is not very stable, leading to coarse particles and a broad particle size distribution. The resulting polymer dispersion is difficult to filter and films cast from it have a dull and gritty appearance. Because of the occurrence of instability the maximum achievable solids content of the final dispersion is lowerthan desired, typically below about 30% by weight. Without being limited to the explanation it is believed that in the prior art process during the synthesis of the carboxyl functional second stage polymer, a significant amount of water-phase polymerization will occur. The water-phase polymerization will lead to the formation of water soluble, nonadsorbing, high molecular weight polymers that can lead to the occurrence of dispersion instability leading to flocculation. Further, it was found that the carboxyl functional second stage polymer will swell upon the addition of a neutralizing base. This will cause a strong increase in the viscosity of the dispersion. Upon reaction of the carboxyl groups with the monoethylenically unsaturated epoxide, the acid value of the second stage polymer will decrease again. This has been found to lead to further destabilization of the polymer dispersion.

[0012] European patent application EP 3892682A1 describes a coating composition based on a blend of a self-crosslinking anionic oligomer with methoxy terminated polyethylene moieties, an oligomer stabilised carbonyl functional polymer dispersion, a N-vinylpyrrolidone-co-ethyl acrylate aqueous solution polymer and an oligomer stabilised self-crosslinking polymer dispersion. More particularly, EP 3892682A1 describes an aqueous vinyl polymer dispersion comprising: an aqueous dispersion of a vinyl polymer P1 comprising 5 to 25% by weight ethylenically unsaturated monomers containing a polyethylene glycol or monoalkoxy polyethylene glycol moiety and characterized by a number average molecular weight (Mn) of from 2,000 to 120,000 g / mole; an aqueous dispersion of a vinyl polymer P2, comprising from 25 to 95% by weight of a ethylenically unsaturated monomer selected from the group of N-vinyl amides and characterized by a number average molecular weight (Mn) of from 1 ,000 to 50,000 g / mole; a film forming vinyl polymer P3 under the form of an aqueous dispersion comprising from 20 to 60 % by weight of a water dispersible crosslinkable vinyl oligomer OL and from 40 to 80% by weight of a high molecular weight vinyl polymer P4.

[0013] The crosslinker used in EP 3892682A1 comprises di- or poly-amines and di- or poly-carboxylic acid hydrazides (like adipic acid dihydrazide). The coatings based on the aqueous dispersion described in EP 3892682A1 have improved open time, however, EP 3892682A1 is silent about fast drying properties (i.e., EP 3892682A1 does not disclose nor hints to coatings having fast drying properties).

[0014] Patent application WO 2020 / 178378 A1 describes coatings with a blend of oligomer stabilised dispersions with surfactant stabilized polymer dispersions. More particularly, WO 2020 / 178378 A1 describes an aqueous vinyl polymer dispersion comprising:

[0015] - from 70 to 97.5 % by weight of a crosslinkable oligomer stabilized dispersion PD1 , said PD1 comprising from 20 to 60% by weight of water soluble or water dispersible crosslinkable oligomer OL and from 40 to 80% by weight of at least one high molecular weight vinyl polymer P1 ;

[0016] - from 2 to 29.5 % by weight of at least one non-oligomer stabilized dispersion PD2 of vinyl polymer P2; and

[0017] - from 0.5 to 28 % by weight of at least one crosslinker X reactive with the functional groups of OL and optionally with the functional groups of P1 and / or P2.

[0018] Coatings based on the the aqueous dispersion described in WO 2020 / 178378 A1 exhibit better viscosity stability compared to coatings based only on the oligomer stabilised dispersions.

[0019] Patent application WO 2009 / 007232 A1 describes a process form making an aqueous coating composition comprising a cationic vinyl oligomer, said process comprising the following steps:

[0020] I. preparation of a cationic vinyl oligomer by solution or bulk polymerisation,

[0021] II. dispersion of the cationic vinyl oligomer in an aqueous medium and neutralisation of non-ionic amine functional groups.

[0022] The cationic vinyl oligomer dispersion of WO 2009 / 007232 A1 may be used as sole stabiliser for a second phase emulsion polymerisation, to prepare a vinyl polymer in the presence of the cationic vinyl oligomer. The coatings described in WO 2009 / 007232 A1 comprising the cationic oligomer have good tannin stain blocking properties.

[0023] European Patent EP1758961 B1 describes a multistage vinyl polymer dispersion comprising an acrylic oligomer with carboxylic acid and carbonyl functionality in the presence of which a core particle consisting of ethylenically unsaturated monomers is polymerized. In a next step, part of the carboxylic acid groups of the oligomer, is reacted with an ethylenically unsaturated mono-epoxide to introduce (meth)acryloyl functionality. When resin compositions according to EP1758961 B1 are used on substrates that are not dimensionally stable, such as exterior finishes on wood, deck-stains and the like, the high crosslink density can lead to failure of the coating by cracking or flaking. Without contesting the associated advantages of the state of the art systems it is nevertheless obvious that there is still a need for a waterborne coating compositions with reduced health and safety risks in use, said waterborne coating compositions comprising stable dispersions and providing clear varnishes for exterior wood applications and wood-stain coatings with improved properties versus the state of the art coating systems.

[0024] Aim of the Invention

[0025] The present invention aims to provide a waterborne coating composition that does not present the limitations of the current state of the art coating systems.

[0026] It is the aim of the present invention to provide a stable dispersion comprising waterborne coating composition having a low minimum film forming temperature (MFFT) and providing an exterior coating on a wooden (or wood-based) substrate with high hardness as well as good chemical resistance, in particular early water resistance and gloss retention.

[0027] Summary of the Invention.

[0028] The present invention discloses a waterborne coating composition comprising a first crosslinkable oligomer stabilized vinyl polymer dispersion DA, a second crosslinkable oligomer stabilized vinyl polymer dispersion DB, and at least one (carbonyl reactive) crosslinker X, wherein:

[0029] DA comprises a vinyl oligomer OLA and at least one vinyl polymer PA, the vinyl oligomer OLA and optionally the at least one vinyl polymer PA comprising chain pendant (carbonyl) functional groups reactable with the (carbonyl reactive) functional groups of an at least one (carbonyl reactive) crosslinker X, and chain pendant ethylenically unsaturated functional groups crosslinkable (by an actinic radiation induced) free radical process; and DB comprises a vinyl oligomer OLB and at least one vinyl polymer PB, the vinyl oligomer OLB and optionally the at least one vinyl polymer PB comprising chain pendant (carbonyl) functional groups reactable with the (carbonyl reactive) functional groups of an at least one (carbonyl reactive) crosslinker X.

[0030] Preferred embodiments of the present invention disclose one or more of the following features:

[0031] • the vinyl oligomers OLA and OLB are characterized by:

[0032] - a number average molecular weight Mn within the range of from 500 to 50,000 g / mole, preferably of from 2,500 to 25,000 g / mole, more preferably of from 5,000 to 15,000 g / mole;

[0033] - a (calculated) glass transition temperature Tg (calculated using the Fox equation) of the oligomer backbone, within the range of from 10 to 150 °C, preferably of from 20 to 125 °C, more preferably of from 25 to 115 °C;

[0034] • the at least one vinyl polymers PA and PB are characterized by:

[0035] - a number average molecular weight Mn of more than 60,000 g / mole, preferably of more than 80,000 g / mole, more preferably of more than 100,000 g / mole;

[0036] - a (calculated) glass transition temperature Tg (calculated using the Fox equation) of the polymer backbone within the range of from -70 to 50 °C, preferably of from -45 to 45 °C, more preferably of from -25 to 40 °C, the calculated glass transition temperature Tg of PA and PB being at least 25 °C, preferably at least 35 °C, more preferably at least 40 °C, below the glass transition temperature of the vinyl oligomers OLA and OLB backbone, respectively; • the first crosslinkable oligomer stabilized vinyl polymer dispersion DA comprises from 20 to 60% by weight of combined PA and OLA, based on the total weight of DA, the combined PA and OLA comprising from at least 35% by weight and up to at most 75% by weight of OLA, based on the combined weight of PA and OLA, being 100% by weight;

[0037] • the second crosslinkable oligomer stabilized vinyl polymer dispersion DB comprises from 20 to 60% by weight of combined PB and OLB, based on the total weight of DB, the combined PB and OLB comprising from at least 35% by weight and up to at most 75% by weight of OLB, based on the combined weight of PB and OLB, being 100% by weight;

[0038] • the waterborne coating composition comprises from 25 to 50% by weight of the first crosslinkable oligomer stabilized vinyl polymer dispersion DA and from 50 to 75 % weight of the second crosslinkable oligomer stabilized vinyl polymer dispersion DB, based on the total weight of DA and DB, being 100% by weight;

[0039] • the (carbonyl) functional groups of any of the vinyl oligomers, OLA and OLB, and optionally of any of the vinyl polymers PA and PB, reactable with the (carbonyl reactive) functional groups of the at least one (carbonyl reactive) crosslinker X, are chain pendant (carbonyl) functional groups of the ketone, aldehyde or acetoacetyl type;

[0040] • the at least one (carbonyl reactive) crosslinker X comprises (carbonyl reactive) functional groups selected from the group consisting of primary amine, secondary amine, hydrazine, hydrazide, hydrazone, and mixtures thereof;

[0041] • the ratio of (carbonyl) functional groups of the vinyl oligomers OLA and OLB and optionally of the at least one vinyl polymer of PA and PB, to the carbonyl reactive functional groups of the at least one (carbonyl reactive) crosslinker X, is from 0.5 to 10, preferably from 0.75 to 5, more preferably from 0.95 to 2.5;

[0042] • the vinyl oligomer chain of OLA and OLB is the polymerized product of:

[0043] - from 1 to 45% by weight of at least one acid functional ethylenically unsaturated monomer M1 , preferably M2 being different from M1 ,

[0044] - from 0.5 to 20% by weight of at least one ethylenically unsaturated monomer M2 with functionality for crosslinking upon film formation,

[0045] - from 50 to 98.5% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0046] - from 0 to 8% by weight of at least one surfactant comprising an ethylenically unsaturated group M5; wherein

[0047] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into their salts, wherein the degree of neutralization is between 0.2 and 0.8; and wherein

[0048] - for the case of vinyl oligomer OLA, the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into ethylenically unsaturated pendant groups through reaction with bi-functional compound M6, M6 comprising an ethylenically unsaturated ((meth)acryloyl) group and a group reactive towards the carboxylic acid, wherein the degree of conversion is between 0.2 and 0.8; • the vinyl polymer chain of PA and PB is the polymerized product of:

[0049] - from 0 to 5% by weight of at least one acid functional ethylenically unsaturated monomer M1 , preferably M2 being different from M1 ,

[0050] - from 0 to 10% by weight of at least one ethylenically unsaturated monomer M2 with functionality for crosslinking upon film formation,

[0051] - from 80 to 100% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0052] - from 0.1 to 5% by weight of at least one multifunctional ethylenically unsaturated monomer M4 to impart a gel content; wherein:

[0053] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into salt; and one or more vinyl polymer(s) PA is (are) different from one or more vinyl polymer PB.

[0054] A yet more preferred embodiment of the present invention discloses the waterborne coating composition of the present invention comprising from 25 to 50% by weight of a first crosslinkable oligomer stabilized vinyl polymer dispersion DA, from 50 to 75% by weight of a second crosslinkable oligomer stabilized vinyl polymer dispersion DB, based on the total weight of DA and DB being 100% by weight, at least one (carbonyl reactive) crosslinker X, and at least one photo-initiator, wherein:

[0055] - DA comprises from 20 to 60% by weight of combined vinyl oligomer OLA and at least one vinyl polymer PA, based on the total weight of DA, the combined PA and OLA comprising at least 35% by weight and up to at most 75% by weight of OLA, based on the combined weight of PA and OLA, being 100% by weight, and

[0056] - DB comprises from 20 to 60% by weight of combined vinyl oligomer OLB and at least one vinyl polymer PB, based on the total weight of DB, the combined PB and OLB comprising at least 35% by weight and up to at most 75% by weight of OLB, based on the combined weight of PB and OLB, being 100% by weight, and wherein:

[0057] - the vinyl oligomer chain of OLA and OLB is the polymerized product of:

[0058] - from 1 to 45% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,

[0059] - from 0.5 to 20% by weight of at least one ethylenically unsaturated monomer M2 with carbonyl functionality for crosslinking upon film formation, preferably M2 being different from M1 ,

[0060] - from 50 to 98.5% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0061] - from 0 to 8% by weight of at least one surfactant comprising an ethylenically unsaturated group M5; and

[0062] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into their salts, wherein the degree of neutralization is between 0.2 and 0.8; and

[0063] - for the vinyl oligomer OLA, the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into ethylenically unsaturated pendant groups through reaction with bi-functional compound M6, M6 comprising an ethylenically unsaturated group and a group reactive towards the carboxylic acid, wherein the degree of conversion is between 0.2 and 0.8; and wherein:

[0064] - the vinyl polymer chain of PA and PB is the polymerized product of:

[0065] - from 0 to 5% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,

[0066] - from 0 to 10% by weight of at least one ethylenically unsaturated monomer M2 with (carbonyl) functionality for crosslinking upon film formation, preferably M2 being different from M1 ,

[0067] - from 80 to 100% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0068] - from 0.1 to 5% by weight of at least one multifunctional ethylenically unsaturated monomer M4 to impart a gel content; and

[0069] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into salt; and

[0070] - one or more vinyl polymer(s) PA is (are) different from one or more vinyl polymer PB; and wherein: the vinyl oligomer OLA and optionally the at least one vinyl polymer PA comprise:

[0071] - chain pendant functional groups (from M2) reactable with the functional groups of an at least one (carbonyl reactive) crosslinker X, and

[0072] - chain pendant ethylenically unsaturated ((meth)acryloyl) functional groups (from M6) crosslinkable by a (an actinic radiation induced) free radical process; and the vinyl oligomer OLB and optionally the at least one vinyl polymer PB comprise chain pendant (carbonyl) functional groups, from M2, reactable with the functional groups of an at least one (carbonyl reactive) crosslinker X, said (carbonyl reactive) crosslinker X having functional groups reactable with carbonyl functional groups of the vinyl oligomer OLA and OLB and optionally of the at least one vinyl polymer PA and PB.

[0073] The present invention further discloses a method for the preparation of the second crosslinkable oligomer stabilized vinyl polymer dispersion DB, the process comprising the steps of: a. making an aqueous emulsion or solution of vinyl oligomer OLB comprising carboxylic acid functionalized monomers, the amount of carboxylic acid functionalized monomers being sufficient to make the vinyl oligomer OLB water soluble or water dispersible, b. at least partially neutralizing the vinyl oligomer OLB with a base, c. adding to the at least partially neutralized vinyl oligomer OLB (formed in the previous steps) a monomer mixture, and emulsion polymerizing said monomer mixture, thereby forming a vinyl polymer PB dispersion which is less hydrophilic than vinyl oligomer OLB, thus forming a mixture of vinyl polymer PB and vinyl oligomer OLB.

[0074] The present invention also discloses a method for the preparation of the first crosslinkable oligomer stabilized vinyl polymer dispersion DA, the process comprising the steps of: a. making an aqueous emulsion or solution of vinyl oligomer OLA precursor comprising carboxylic acid functionalized monomers, the amount of carboxylic acid functionalized monomers being sufficient to make the vinyl oligomer OLA precursor water soluble or water dispersible, b. at least partially neutralizing the vinyl oligomer OLA precursor with a base, c. adding to the at least partially neutralized vinyl oligomer OLA precursor (formed in the previous step) a monomer mixture, and emulsion polymerizing said monomer mixture, thereby forming a vinyl polymer PA dispersion which is less hydrophilic than vinyl oligomer OLA precursor, d. reacting the carboxylic acid groups (formed) in vinyl oligomer OLA precursor with a bifunctional compound, said bi-functional compound comprising an ethylenically unsaturated ((meth)acryloyl) group and a group reactive towards the carboxylic acid groups (formed) in the vinyl oligomer OLA precursor, and at least partially converting the carboxylic acid groups of vinyl oligomer OLA precursor into ethylenically unsaturated ((meth)acryloyl) groups, thus forming a mixture of vinyl polymer PA and vinyl oligomer OLA.

[0075] The present invention further relates to the use of the waterborne coating composition for coating wooden (or wood-based) substrates selected from the group consisting of medium density fiberboard (MDF), high density fiberboard (HDF), particleboard, oriented strand board (OSB), solid wood, and composite materials such as Wood Plastic Composites (WPC). The present invention also relates to a coated article comprising a wooden (or wood-based) substrate being at least partly coated with the waterborne coating composition.

[0076] Detailed Description of the Invention.

[0077] In the present invention, it has been found that a waterborne coating composition comprising a first crosslinkable oligomer stabilized vinyl polymer dispersion, a second crosslinkable oligomer stabilized vinyl polymer dispersion and at least one crosslinker, allows for stable coating systems providing crosslinked coatings presenting an increased hardness combined with an outstanding water resistance. More particularly, it has been found that a waterborne coating composition comprising a first crosslinkable oligomer stabilized vinyl polymer dispersion, DA, a second crosslinkable oligomer stabilized vinyl polymer dispersion, DB, and at least one carbonyl reactive crosslinker, X, allows for stable coating systems providing crosslinked coatings presenting an increased hardness combined with an outstanding water resistance.

[0078] In the context of the present invention, “a carbonyl reactive crosslinker” refers to a crosslinker comprising (a) functional group(s) which is (are) capable or prone to react with (a) carbonyl functional group(s), i.e. a crosslinker creating (a) covalent bond(s) between molecules by targeting (a) carbonyl functional group(s).

[0079] In the context of the present invention, “oligomer stabilized vinyl polymer dispersion” refers to a dispersion of at least one vinyl polymer in an aqueous medium comprising the vinyl oligomer. The oligomer stabilized vinyl polymer dispersion is obtained by means of free-radical emulsion copolymerization of ethylenically unsaturated monomers in an aqueous medium in the presence of vinyl oligomer.

[0080] By "aqueous medium" herein is meant from 70 to 100 % by weight of water and from 0 to 30% by weight of water-miscible compound(s) (such as, for example, alcohols, glycols, glycol ethers, glycol esters, and the like), based on the weight of the aqueous medium being 100%.

[0081] "Aqueous" composition, solution, emulsion, or dispersion herein means that particles are dispersed in an aqueous medium.

[0082] In the context of the present invention, ’’crosslinkable oligomer stabilized vinyl polymer dispersion” refers to the vinyl oligomer and optionally the vinyl polymer comprises reactable functional groups enabling the formation of a network, more particularly, it refers to the vinyl oligomer and optionally the vinyl polymer comprising carbonyl reactive functional groups enabling the formation of a polymer network.

[0083] In the context of the present invention, “a carbonyl reactive functional group” refers to a functional group which is capable or prone to react with (a) carbonyl functional group(s), i.e. creating (a) covalent bond(s) between molecules by targeting (a) carbonyl functional group(s).

[0084] To make the distinction between an oligomer and a polymer in this application, the definition given by IUPAC can be used (IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"), compiled by A.D. McNaught and A. Wilkinson, Blackwell Scientific Publications, Oxford (1997)). An oligomer is a polymer molecule of intermediate relative molecular mass, the structure of which essentially comprises a small plurality of units derived from molecules of lower relative molecular mass. In the light of this invention oligomers have a number average molecular weight from 500 up to 50,000 gram / mole. As the average molecular weight for the repeating unit is about 100 gram / mole, the number of repeating units in an oligomer is from 5 to 500. Considering the fact that conventional emulsion copolymers can have number average molecular weights of at least 106or 10,000 repeating units, the criterium of “intermediate relative molecular mass” is applicable to our “oligomer” definition.

[0085] In the context of the present invention “ethylenically unsaturated monomer” or “vinyl monomer” refers to a monomer having at least one carbon-carbon double bond which can undergo free- radical polymerization. The prefix "(meth)acryl" when used to name compounds of the present invention encompasses both “acryl” and “methacryl” and refers to compounds comprising at least one CH2=CHCOO- group or CH2=CCH3 COO- group, as well as mixtures thereof and mixtures of such compounds.

[0086] The waterborne coating compositions of the present invention comprise a first crosslinkable oligomer stabilized vinyl polymer dispersion DA, a second crosslinkable oligomer stabilized vinyl polymer dispersion DB, and at least one crosslinker X, wherein:

[0087] DA comprises a vinyl oligomer OLA and at least one vinyl polymer PA, the vinyl oligomer OLA and optionally the at least one vinyl polymer PA comprising: chain pendant functional groups reactable with the functional groups of an at least one crosslinker X, and chain pendant ethylenically unsaturated functional groups crosslinkable by a free radical process; and

[0088] DB comprises a vinyl oligomer OLB and at least one vinyl polymer PB, the vinyl oligomer OLB and optionally the at least one vinyl polymer PB comprising chain pendant functional groups reactable with the functional groups of an at least one crosslinker X.

[0089] More particularly, the waterborne coating compositions of the present invention comprise a first crosslinkable oligomer stabilized vinyl polymer dispersion DA, a second crosslinkable oligomer stabilized vinyl polymer dispersion DB, and at least one carbonyl reactive crosslinker X, wherein:

[0090] DA comprises a vinyl oligomer OLA and at least one vinyl polymer PA, the vinyl oligomer OLA and optionally the at least one vinyl polymer PA comprising:

[0091] - chain pendant carbonyl functional groups reactable with the functional groups of an at least one carbonyl reactive crosslinker X, and chain pendant ethylenically unsaturated (meth)acryloyl functional groups crosslinkable by a free radical process; and

[0092] DB comprises a vinyl oligomer OLB and at least one vinyl polymer PB, the vinyl oligomer OLB and optionally the at least one vinyl polymer PB comprising chain pendant carbonyl functional groups reactable with the functional groups of an at least one carbonyl reactive crosslinker X.

[0093] In the context of the present invention, “first crosslinkable oligomer stabilized vinyl polymer dispersion DA” refers to a vinyl polymer dispersion stabilized by an oligomer comprising both carbonyl functional groups for crosslinking with a carbonyl reactive crosslinker X and pendant (meth)acryloyl functional groups for crosslinking by an actinic radiation induced free radical polymerization process. “Second crosslinkable oligomer stabilized vinyl polymer dispersion DB” refers to a vinyl polymer dispersion stabilized by an oligomer comprising carbonyl functional groups for crosslinking with a carbonyl reactive crosslinker X.

[0094] In the context of the present invention “chain pendant functional group” refers to a functional group attached to a backbone chain of the oligomer and / or polymer, said functional group, also called a side group, being capable of reacting with the functional groups of a crosslinker X. More particularly, “chain pendant carbonyl functional group” refers to a carbonyl functional group attached to a backbone chain of the oligomer and / or polymer, said carbonyl functional group being capable of reacting with the functional groups of a (carbonyl reactive) crosslinker X.

[0095] It is well known by the skilled person in the art that with “carbonyl functional group”, it is referred to a functional group with the formula C=O, i.e. a functional group consisting of an oxygen atom double-bonded to a carbon atom, the carbon atom being divalent, generally written as o

[0096] / Cv

[0097] (RI)(R2)C=O, or depicted as formula (I): . When both Ri and R2in formula (I) are a carbon-containing substituent, formula (I) is called a ketone group. When Ri is a carbon- containing substituent and R2is a hydrogen atom H, formula (I) is called an aldehyde. When Ri is a carbon-containing substituent and R2is CH2-(C=O)-R3, with R3being a carbon- containing substituent, formula (I) is referred to as an acetoacetyl group (or two carbonyl functional groups linked by an activated methylene group).

[0098] In the context of the present invention “chain pendant ethylenically unsaturated functional group” refers to an ethylenically unsaturated ((meth)acryloyl) functional group attached to a backbone chain of the oligomer and optionally the polymer, said ethylenically unsaturated functional group, also called a side group, being capable of reacting with other pendant ethylenically unsaturated functional groups, of the same or a different oligomer and optional polymer. This reaction is a free radical polymerization, induced by actinic radiation in the presence of a photo-initiator.

[0099] Preferably, the vinyl oligomer chain of OLA and OLB is the polymerized product of:

[0100] - from 1 to 45% by weight of at least one acid functional ethylenically unsaturated monomer M1 , - from 0.5 to 20% by weight of at least one ethylenically unsaturated monomer M2 with functionality for crosslinking upon film formation, preferably M2 being different from M1 ,

[0101] - from 50 to 98.5% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0102] - from 0 to 8% by weight of at least one surfactant comprising an ethylenically unsaturated group M5; wherein

[0103] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into their salts, wherein the degree of neutralization is between 0.2 and 0.8; and wherein

[0104] - for the vinyl oligomer OLA, the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into ethylenically unsaturated pendant groups through reaction with bi-functional compound M6, M6 comprising an ethylenically unsaturated ((meth)acryloyl) group and a group reactive towards the carboxylic acid, wherein the degree of conversion is between 0.2 and 0.8.

[0105] More preferably, the vinyl oligomers OLA and OLB comprise:

[0106] 1) from 1 to 45% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,

[0107] 2) from 0.5 to 20% by weight of at least one ethylenically unsaturated monomer M2 with (carbonyl) functionality for crosslinking upon film-formation, other than M1 ,

[0108] 3) from 50 to 98.5% by weight at least one ethylenically unsaturated monomer M3 other than M1 and M2,

[0109] 4) from 0 to 1% by weight of at least one multifunctional ethylenically unsaturated monomers M4 for pre-crosslinking to impart a gel content,

[0110] 5) from 0 to 8% by weight of at least one surfactant comprising an ethylenically unsaturated group M5, and

[0111] 6) a chain-transfer agent to control the molecular weight of the oligomer, wherein:

[0112] - the sum of 1) to 6) is 100 % by weight,

[0113] - the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into salt, wherein the degree of neutralization is between 0.2 and 0.8, and

[0114] - for the vinyl oligomer OLA, the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into ethylenically unsaturated ((meth)acryloyl) pendant groups through reaction with a bifunctional compound M6, M6 comprising an ethylenically unsaturated ((meth)acryloyl) group and a group reactive towards the carboxylic acid (such as an oxirane group), wherein the degree of conversion is between 0.2 and 0.8, and

[0115] - the vinyl oligomers have an acid number of at least 45 mg KOH / g, more preferably at least 50 mg KOH / g and most preferably at least 55 mg KOH / g after being partially converted into salt and for the particular case of OLA, after being partially converted into salt and in ethylenically unsaturated ((meth)acryloyl) pendant groups. In the context of the present invention “oligomer backbone” refers to the (oligomer) main chain, being the sequence of monomer units connected to each other by a -C-C- bond resulting from the copolymerization of M1 , M2, M3, optionally M4 and optionally M5.

[0116] The acid groups of the vinyl oligomers are at least partially converted into salt by adding a base. Preferably the vinyl oligomer carboxylic acid groups are converted into salt only to the extent that is required for solubilization or emulsification thereof. Preferably, the degree of neutralization is lower than 1 . The degree of neutralization is defined as the ratio of the concentration of carboxylic salt groups to the initial concentration of carboxylic acid groups. The degree of neutralization is preferably below 0.8, more preferably below 0.6, and even more preferably below 0.4. In view of the desired solubilization or emulsification, the degree of neutralization is preferably at least 0.2.

[0117] The base used, is preferably a volatile base, most preferably ammonia. Other possible bases are, for example, volatile amines, such as amino methyl propanol, dimethyl ethanolamine, 2- dimethylamino-2-methyl-1 -propanol, triethyl amine, diisopropyl ethyl amine, or monoethanol amine. Optionally, a blend of different volatile bases can be used. The advantage of using a volatile base is that after drying and curing of the film the salt-groups are converted back into the free carboxylic acid groups, imparting better water resistance to the coating.

[0118] The acid groups of the vinyl oligomer OLA precursor are at least partially converted into ethylenically unsaturated pendant groups through reaction with a bi-functional compound M6. The degree of conversion of carboxylic acid groups is defined as the ratio of the molar concentration of the functional groups of M6 to react with the carboxylic acid groups of the vinyl oligomer to the initial molar concentration of carboxylic acid groups in the vinyl oligomer. The degree of conversion is lower than 1 as this would destabilize DA. Most of the time the degree of conversion is at least 0.5, preferably even at least 0.80 and most preferably even at least 0.9.

[0119] The bi-functional compound M6 comprising an ethylenically unsaturated ((meth)acryloyl) group and a group reactive towards the carboxylic acid, is preferably an ethylenically unsaturated mono-epoxide. More particularly, the bi-functional compound M6 comprising an ethylenically unsaturated ((meth)acryloyl) group and a group reactive towards the carboxylic acid, is preferably an ethylenically unsaturated mono-epoxide including glycidyl (meth)acrylate, allyl glycidyl ether, glycidyl cinnamates, glycidyl crotonates, glycidyl itaconates, glycidyl norbornenyl ester, glycidyl norbornenyl ether and the like. The conversion is carried out in the presence of a suitable catalyst for the epoxy carboxylic acid reaction, preferably a phase-transfer catalyst.

[0120] Examples of phase-transfer catalysts that can be used for the modification are tetra-n-butyl ammonium hydroxide, methyltributylammonium hydroxide or benzyltriethylammonium hydroxide.

[0121] It is mandatory that the conversion of the mono-epoxide compound M6 proceeds to high conversion in view of the toxicity of unreacted M6. Therefore, the conversion of M6 shall be at least 95%, more preferably 99% and most preferably more than 99.5%.

[0122] In general the carboxylic acid groups of the vinyl oligomer OLA precursor are at least partially converted into pendant ethylenically unsaturated ((meth)acryloyl) groups through reaction with M6, at the end of the formation of the vinyl polymer PA in the aqueous medium comprising the carboxylic acid group containing vinyl oligomer OLA precursor i.e. the at least partially conversion is performed to the blend of vinyl polymer PA and carboxylic acid functional vinyl oligomer OLA precursor.

[0123] In the context of the present invention “vinyl oligomer OLA precursor” refers to the vinyl oligomer obtained from polymerization of M1 , M2, M3 and optionally M4 and optionally M5.

[0124] Preferably, the vinyl polymer chain of PA and PB is the polymerized product of:

[0125] - from 0 to 5% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,

[0126] - from 0 to 10% by weight of at least one ethylenically unsaturated monomer M2 with functionality for crosslinking upon film formation, preferably M2 being different from M1 ,

[0127] - from 80 to 100% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0128] - from 0.1 to 5% by weight of at least one multifunctional ethylenically unsaturated monomer M4 to impart a gel content; wherein:

[0129] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into salt; and one or more vinyl polymer(s) PA is (are) different from one or more vinyl polymer PB.

[0130] More preferably, the vinyl polymers PA and PB comprise:

[0131] 1) from O to 5% by weight of at least one an acid functional ethylenically unsaturated monomer M1 ,

[0132] 2) from 0 to 10% by weight of at least one ethylenically unsaturated monomer M2 with (carbonyl) functionality for crosslinking upon film-formation, other than M1 ,

[0133] 3) from 80 to 100% by weight of at least one ethylenically unsaturated monomer M3 other than M1 and M2, and

[0134] 4) from 0.1 to 5% by weight of at least one multifunctional ethylenically unsaturated monomers M4 for pre-crosslinking to impart a gel content,

[0135] 5) from 0 to 8% by weight of at least one surfactant comprising an ethylenically unsaturated group M5, and

[0136] 6) optionally a chain-transfer agent to control the molecular weight of the polymer, wherein:

[0137] - the sum of 1) to 6) is 100 % by weight,

[0138] - the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 , if present, are at least partially converted into salt, wherein the degree of neutralization is between 0.2 and 0.8, and

[0139] - vinyl polymer PA is different from vinyl polymer PB.

[0140] In the context of the present invention “polymer backbone” refers to the (polymer) main chain, being the sequence of monomer units connected to each other by a -C-C- bond resulting from the copolymerization of M3, M4, optionally M1 , optionally M2 and optionally M5.

[0141] The first crosslinkable oligomer stabilized dispersion DA may comprise more than one vinyl polymer PA. Also the second crosslinkable oligomer stabilized vinyl dispersion DB may comprise more than one vinyl polymer PB.

[0142] Preferably, the first crosslinkable oligomer stabilized vinyl polymer dispersion DA comprises from 20 to 60% by weight, preferably from 30 to 50% by weight, of combined PA and OLA, based on the total weight of DA, the combined PA and OLA comprising at least 35% by weight, preferably at least 40% by weight, more preferably at least 45% by weight, most preferably at least 50% by weight and up to at most 75% by weight, preferably at most 70% by weight, more preferably at most 65% by weight, most preferably at most 60% by weight of OLA, based on the combined (or total) weight of PA and OLA, being 100% by weight, wherein the OLA weight includes the total weight of M1 , M2, M3, chain transfer agent, M6, optionally M4 and optionally M5 and wherein the PA weight includes the total weight of M3, M4, optionally M1 , optionally M2, optionally M5 and optionally chain transfer agent.

[0143] In the present description, “combined PA and OLA” refers to PA and OLA forming, or having formed, a polymer network (i.e. PA and OLA being linked (or, being connected) thereby forming a polymer network. PA and OLA can for example be connected to each other by carbon-carbon double bonds (C=C) or by hydrazone crosslinks). “Combined weight of PA and OLA” refers to the total weight of PA and OLA when being combined, i.e. the total weight of PA and OLA when being linked and forming a polymer network.

[0144] The first crosslinkable oligomer stabilized vinyl polymer dispersion DA is preferably prepared by a process comprising the steps of: a. making an aqueous emulsion or solution of vinyl oligomer OLA precursor comprising carboxylic acid functionalized monomers, the amount of carboxylic acid functionalized monomers being sufficient to make the vinyl oligomer OLA precursor water soluble or water dispersible, b. at least partially neutralizing the vinyl oligomer OLA precursor with a base, c. adding to the at least partially neutralized vinyl oligomer OLA precursor (formed in the previous step) a monomer mixture, and emulsion polymerizing said monomer mixture, thereby forming a vinyl polymer PA dispersion which is less hydrophilic than vinyl oligomer OLA precursor, and d. reacting the carboxylic acid groups (formed) in vinyl oligomer OLA precursor with a bifunctional compound, said bi-functional compound comprising an ethylenically unsaturated ((meth)acryloyl) group and a group reactive towards the carboxylic acid groups (formed) in the vinyl oligomer OLA precursor, and at least partially converting the carboxylic acid groups of vinyl oligomer OLA precursor into ethylenically unsaturated ((meth)acryloyl) groups, thus forming a mixture of vinyl polymer PA and vinyl oligomer OLA.

[0145] Preferably, the second crosslinkable oligomer stabilized vinyl polymer dispersion DB comprises from 20 to 60% by weight, preferably from 30 to 50% by weight, of combined PB and OLB, based on the total weight of DB, the combined PB and OLB comprising at least 35% by weight, preferably at least 40% by weight, more preferably at least 45% by weight, most preferably at least 50% by weight and up to at most 75% by weight, preferably at most 70% by weight, more preferably at most 65% by weight, most preferably at most 60% by weight of OLB, based on the combined (or total) weight of PB and OLB, being 100% by weight, wherein the OLB weight includes the total weight M1 , M2, M3, chain transfer agent, optionally M4 and optionally M5, and wherein the PB weight includes the total weight of M3, M4, optionally M1 , optionally M2, optionally M5 and optionally chain transfer agent.

[0146] In the present description, “combined PB and OLB” refers to PB and OLB forming, or having formed, a polymer network (i.e. PB and OLB being linked (or, being connected) to form a polymer network. PB and OLB can for example be connected to each other by carbon-carbon double bonds (C=C) or by hydrazone crosslinks). “Combined weight of PB and OLB” refers to the total weight of PA and OLA when being combined, i.e. the total weight of PB and OLB when being linked and forming a polymer network.

[0147] The second crosslinkable oligomer stabilized vinyl polymer dispersion DB is preferably prepared by a process comprising the steps of: a. making an aqueous emulsion or solution of vinyl oligomer OLB comprising carboxylic acid functionalized monomers, the amount of carboxylic acid functionalized monomers being sufficient to make the vinyl oligomer OLB water soluble or water dispersible, b. at least partially neutralizing the vinyl oligomer OLB with a base, and c. adding to the at least partially neutralized vinyl oligomer OLB (formed in the previous steps) a monomer mixture, and emulsion polymerizing said monomer mixture, thereby forming a vinyl polymer PB dispersion which is less hydrophilic than vinyl oligomer OLB, thus forming a mixture of vinyl polymer PB and vinyl oligomer OLB.

[0148] Preferably, the waterborne coating composition of the present invention comprises from 25 to 50% by weight of the first crosslinkable oligomer stabilized vinyl polymer dispersion DA and from 50 to 75 % weight of the second crosslinkable oligomer stabilized vinyl polymer dispersion DB, based on the total weight of DA and DB, being 100% by weight.

[0149] Preferably, acid functional ethylenically unsaturated monomers M1 are selected from the group of carboxylic acid functional monomers such as acrylic acid, methacrylic acid, maleic acid or its half-esters, fumaric acid or its half esters, citraconic acid or its half esters, itaconic acid or its half esters, muconic acid and its half esters, and mixtures thereof. These monomers may be produced from petrochemical feedstock. Alternatively they may be derived from renewable feedstock. Bio-based acrylic acid can be produced from glycerol or hydroxypropionic acid, hydroxypropionic acid derivatives, or mixtures thereof. Itaconic acid is obtained by fermentation of sugars and bio-based methacrylic acid can be derived from itaconic acid.

[0150] In the context of the present description, “renewable feedstock” refers to natural renewable resources which will replenish to replace the portion depleted by usage and consumption, either through natural reproduction or other recurring processes (in a finite amount of time in a human time scale). Substances or mixtures of substances obtained from such renewable feedstock should have in total a bio-based carbon content of more than 20% by weight of total carbon content of the substance or mixture, the bio-based carbon content being determined using the American standard ASTM D6866-20 standard or the European standard EN 16785- 1.

[0151] As used herein, "bio-based compounds" or "bio-based monomers" (or “bio-derived monomers”) refer to compounds or monomers sourced from or made from renewable feedstock, such as, for example, biomass or plantbased sources. Biomass is a material of biological origin excluding material embedded in geological formations and / or being fossilized; very often it comes from plant materials and animal waste.

[0152] Other acid-functional, non-carboxylic group-containing monomers can for example be sulphate or sulphonic monomers. As non-limiting examples can be mentioned 2-acrylamido-2- methylpropane sulphonic acid or the alkali, ammonia or amine salt thereof, and the sodium salt of the adduct of allylglycidyl ether to sodium bisulphite, 2-sulfoethyl methacrylate, or 1- (allyloxy)-2-hydroxypropane-1 -sulfonic acid. Also phosphate or phosphonate functional monomers can be used. Non-limiting examples of such monomers are monoacryloxyethyl phosphate, SIPOMER® PAM-100 and SIPOMER® PAM-200 (both from Solvay Novecare), 10-Methacryloyloxydecyl dihydrogen phosphate (Kuraray), dimethyl (2- methacryloyloxyethyl)phosphonate and dimethyl(2-methacryloyloxypropyl)phosphonate or ethyl2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate.

[0153] Acid functional ethylenically unsaturated monomers, M1 , are more preferably selected from the group of carboxylic acid functional monomers, even more preferably from the group consisting of acrylic acid, methacrylic acid, itaconic acid, muconic acid, and mixtures thereof, most preferably from the group consisting of acrylic acid, methacrylic acid, and mixtures thereof.

[0154] Ethylenically unsaturated monomers M2, having (carbonyl) functionality for crosslinking upon film-formation can be ketone, aldehyde or acetoacetyl functional. Examples of monomers which bear carbonyl functional groups include acrolein, methacrolein, crotonaldehyde, 4- vinylbenzaldehyde, vinyl alkyl ketones of 4 to 7 carbon atoms such as vinyl methyl ketone. Further examples include acrylamido pivalaldehyde, methacrylamido pivalaldehyde, 3- acrylamidomethyl-anisaldehyde, diacetone acrylate and diacetone methacrylate, and ketocontaining amides such as diacetone acrylamide. Examples of (carbonyl functional) monomers possessing an acetoacetoxy functional group are acetoacetoxyethyl methacrylate, acetoacetoxyethyl acrylate, acetoacetoxyethyl methacrylate, acetoacetoxypropyl acrylate, allyl acetoacetate, acetoacetamidoethyl (meth)acrylate and acetoacetoxybutyl acrylate.

[0155] Preferably the (carbonyl functional) monomer M2 is selected from the group consisting of acetoacetoxyethyl methacrylate, diacetone acrylamide, and mixtures thereof.

[0156] Monomers M3 may include but are not limited to vinyl monomers such as styrene, alpha-methyl styrene (or other styrene derivatives such as t-butylstyrene, vinyl toluene, o-, m-, and p- methylstyrene, o-, m-, and p-ethylstyrene, acrylonitrile, methacrylonitrile, vinyl halides such as vinyl chloride, vinylidene halides such as vinylidene chloride, vinyl esters such as vinyl acetate, vinyl propionate, vinyl laurate; vinyl esters of versatic acid such as VEOVA™ 9 and VEOVA™ 10 (VEOVA™ is a trademark of Hexion Inc.), heterocyclic vinyl compounds; alkyl esters of ethylenically unsaturated dicarboxylic acids such as di-n-butyl maleate and di-n-butyl fumarate; esters of itaconic acid such as dimethyl or dibutyl itaconate and in particular, esters of acrylic acid and methacrylic acid of formula CH=CR1-COOR2 wherein R1 is H or methyl and R2 is optionally substituted C1 to C20, more preferably C1 to C8, alkyl, cycloalkyl, aryl or (alkyl)aryl, examples of which are methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate (all isomers), 2-ethylhexyl (meth)acrylate, isopropyl (meth)acrylate, propyl (meth)acrylate (all isomers), and hydroxyalkyl (meth)acrylates such as hydroxyethyl (meth)acrylate, 2- hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate and their modified analogues like TONE™ M-100 (TONE™ is a trademark of Dow Chemical). Polyethyleneglycol or monoalkoxy ethylene glycol modified (meth)acrylic esters can also be used, such as ethyl triglycol methacrylate, methoxypolyethylene glycol 350 methacrylate, methoxypolyethylene glycol 750 methacrylate, methoxypolyethylene glycol 1000 methacrylate, methoxypolyethylene glycol 2000 methacrylate, methoxypolyethylene glycol 5000 methacrylate (all supplied by Evonik under the tradename VISIOMER® and by BASF under the trademark BISOMER®). N- substituted compounds of acrylamide and methacrylamide such as N-tert. butyl acrylamide, N- tert. hexyl acrylamide, N-tert. octyl acrylamide, N-(1 ,5-dimethyl-1-ethyl)hexyl acrylamide, N- (1 ,1-dimethyl-2-phenyl)ethyl acrylamide, N-cyclohexyl acrylamide, N-(1- methylbutyl)acrylamide, N-ethyl acrylamide, N-ethylhexyl acrylamide, N-butyl methacrylamide, and N-cyclohexyl methacrylamide can also be used. Monomers M3 can also comprise adhesion promoting monomers. These monomers usually contain amino, ureido or N- heterocyclic groups. Examples of such monomers are dimethyl aminoethyl(meth)acrylate, diethyl aminoethyl(meth)acrylate, 3-dimethylamino-2,2- dimethylpropyl-1 -(meth)-'acrylate, N- dimethylaminomethyl(meth)acrylamide, N-(4- morpholinomethyl) (meth)acrylamide, 1 -vinyl imidazole, and N-vinyl pyrrolidone. Another type are N-alkylazetidinyl (meth)acrylates such as N-cyclohexyl azetidinyl methacrylate or N-isopropyl azetidinyl methacrylate. Further mention may be made of compounds having a pyrrolidine, piperidine, morpholine, piperazine, imidazole, pyrrolidin-(2)-one or imidazolidin-2-one (ethylene urea) ring. In this case preference is given to ureido-functional monomers such as N-(2-methacryloxyethyl) ethyleneurea, 1-[2- [[3-(allyloxy)-2-hydroxypropyl]amino]ethyl] imidazolidin-2-one, and 2-ethyleneureido-ethyl methacrylate.

[0157] The M3 monomers may be produced from petrochemical feedstock. Alternatively, they may be derived from renewable feedstock such as bio-based acrylic and methacrylic acid. The alkanols used in the (trans)esterification can also be bio-derived. Non-limiting examples of such monomers are VISIOMER® Terra C13-MA, VISIOMER® Terra C17.4-MA (available from Evonik), n-octyl acrylate and isobornyl (meth)acrylate.

[0158] Yet another feedstock for the M3 monomers may be recycled monomers, such as recycled methyl methacrylate or recycled styrene. These may be obtained by high temperature pyrolysis (i.e. pyrolyzed at temperatures above their ceiling temperature) from waste polymethyl methacrylate or polystyrene. The obtained recycled monomers can be used as such, without removal of any impurities present in the product obtained by the pyrolysis reaction. Alternatively, the pyrolysis products can be further purified (e.g. by distillation) or even converted, in the case of recycled methyl methacrylate, into other monomers, for example by means of transesterification with alcohols different from methanol.

[0159] Also dienes, such as 1 ,3-butadiene, or isoprene, or mixtures thereof; vinyl esters, such as vinyl acetate, vinyl alkanoate or their derivatives, or mixtures thereof, can be used in the M3 monomer composition.

[0160] Preferably monomers M3, M3 being different from M1 and M2, are selected from the group consisting of vinyl monomers; esters of (meth)acrylic acid of formula CH=CR1-C(O)OR2, wherein R1 is H or methyl and R2 is an optionally substituted C1 to C20 alkyl, cycloalkyl, aryl or (alkyl)aryl; and mixtures thereof. More particularly, monomers M3 are, each independently, selected from vinyl monomers, in particular styrene; esters of acrylic acid and methacrylic acid of formula CH=CR1-C(O)OR2 wherein R1 is H or methyl and R2 is optionally substituted C1 to C20, more preferably C1 to C8, alkyl, cycloalkyl, aryl or (alkyl)aryl; and mixtures thereof.

[0161] Monomers M4 are usually selected from multi-functional monomers having two or more ethylenically unsaturated groups per molecule, and may be present in monomer compositions of OLA, OLB, PA and / or PB. Examples of such monomers are 1 ,3- butadiene, isoprene, divinyl benzene, triallyl cyanurate, vinyl- or allyl-acrylate or methacrylate, diol diacrylates and diol dimethacrylates, and methylene bisacrylamide or methylene bismethacrylamide (or mixtures thereof). Use of multi-functional monomers during an emulsion polymerization will result in increased molecular weight of the polymer or partial crosslinking during the polymerization. The monomers M1 , M2, M3 and M4 can be of petrochemical origin or may also be fully or partially derived from bio-mass. In case monomers derived from bio-mass are being used, the bio-based carbon content of the polymer can be determined by means of radiocarbon analysis according to the European Standard EN 16785-1 (or according to the American standard ASTM D6866-20).

[0162] The crosslinkable vinyl oligomers OLA and OLB preferably have a (calculated) glass transition temperature (Tg) within the range of 10 to 150 °C, more preferably 20 to 125 °C and most preferably 25 to 115 °C (calculated using the Fox equation).

[0163] The (polymer backbone of the) vinyl polymers PA and PB preferably (has) have a (calculated) glass transition temperature (Tg) within the range of from -70 to 50 °C, more preferably of from -45 to 45 °C, most preferably of from -25 to 40 °C (calculated using the Fox equation). The (calculated) Tg of the respective vinyl polymers PA and PB are preferably at least 25 °C, more preferably at least 40 °C, below the Tg of the (backbone of the vinyl) oligomers OLA and OLB.

[0164] The (calculated) Tg of the vinyl oligomers OLA, OLB and vinyl polymers PA and PB herein stands for the calculated glass transition temperature and is well known to be the temperature at which a polymer changes from a glassy, brittle state to a rubbery state. Tg values are calculated using the well-known Fox equation (T. G. Fox, Bull. Am. Phys. Soc. 1 , 123 (1956)), which is well known in the art, and is represented by the formula:

[0165] 1 / Tg = W1 / Tg(1) + W2 / Tg(2) + W3 / Tg(3) + .... wherein W1 , W2, W3, etcetera, are the weight fractions of the comonomers (1), (2), and (3), (etcetera), and Tg(1), Tg(2), Tg(3) are the glass transition temperatures of their respective homopolymers. Glass transition values for homopolymers given in the Polymer Handbook, 4th edition (editors: J. Brandrup, E.H. Immergut, E.A. Grulke, John Wiley & Sons, Inc. 1999) are used to perform the calculation. The calculated Tg in degrees Kelvin may be readily converted to degrees Celsius.

[0166] In case the oligomer main chain, being the sequence of monomer units connected to each other by a -C-C- bond, is modified afterwards by a covalent reaction to one of its functional groups, such as a -COOH group, the calculated Fox Tg refers to the Tg value of the respective oligomer OLA and optionally OLB precursors.

[0167] The values of glass transition temperatures (Tg) reported in the present description for OLA, OLB, PA, and PB, are those calculated using the Fox equation described above (referred to as calculated glass transition temperatures, or calculated Tg).

[0168] The crosslinkable vinyl oligomers OLA and OLB preferably have a number average molecular weight Mn within the range of from 500 g / mole to 50,000 g / mole, more preferably 2,500 g / mole to 25,000 g / mole and most preferably 5,000 g / mole to 15,000 g / mole.

[0169] The crosslinkable vinyl polymers PA and PB preferably have a number average molecular weight (Mn) of more than 60,000 g / mole, more preferably of more than 80,000 g / mole, most preferably of more than 100,000 g / mole;

[0170] The number and weight average molecular weight (Mn and Mw) of the vinyl oligomers OLA and OLB and the vinyl polymers PA and PB is determined by using gel permeation chromatography (GPC) according to ISO 13885-1 :2008, ASTM D 3536 and ASTM D 3593. The formula for the number average molecular weight is: Mn = ZNi.Mi / ZNi, where Mn is the number average molecular weight, Z is the summation over all species, Ni is the number of molecules of species i, and Mi is the molar mass of species i.

[0171] Methods to influence the molecular weight in emulsion polymerization in order to achieve the desired number average molecular weight are well known to those skilled in the art. Molecular weight control may be provided by using chain-transfer agents such as mercaptans and halogenated hydrocarbons. Suitable mercaptans include as n-dodecyl mercaptan, n-octyl mercaptan, t-dodecyl mercaptan, mercapto ethanol, iso-octyl thioglycolurate, C2 to C8 mercapto carboxylic acids and esters thereof such as 3-mercaptopropionic acid and 2- mercaptopropionic acid.

[0172] The crosslinkable vinyl oligomer OLA and OLB precursors may be prepared by any known technique and may include directly synthesizing the oligomer in an aqueous process, i.e. in the presence of water (for example by emulsion polymerization, suspension polymerization, micro-suspension polymerization or mini emulsion polymerization), or by solution polymerization where the solvent may be water and / or any organic solvent that is miscible with water or that is removed by distillation after the vinyl oligomer is transferred to water.

[0173] When the oligomer is a vinyl macromonomer, it may be prepared by a number of processes including but not limited to the use of reversible addition fragmentation (RAFT) agents, by the use of catalytic chain transfer agents such as the cobalt-chain transfer. Also alpha-methyl styrene dimer or oligomers of alpha-methyl styrene dimer can be used as explained in US 2007 / 0043156 and US 6,872,789. Yet another method to synthesize oligomers with a well- defined molecular weight is the use of diarylethene such as diphenylethene, or high temperature processes.

[0174] Most preferably the vinyl oligomers OLA and OLB precursors are prepared by an aqueous, free radical emulsion polymerization process. This process is described in “Chemistry and Technology of Emulsion Polymerisation”, Editor A.M. van Herk, (2005), Blackwell Publishing Ltd. The free radical polymerization may be carried out as a batch or as a semi-continuous polymerization process.

[0175] By "aqueous, free radical emulsion polymerization process" herein is meant a process using (or performed in) an aqueous medium, said aqueous medium comprising more than 70% by weight of water, preferably more than 80% by weight, more preferably more than 90% by weight, even more preferably more than 95% by weight, and most preferably 100% by weight, of water, based on the total of aqueous medium being 100% by weight.

[0176] A free-radical emulsion polymerization will usually require the use of free-radical-yielding initiators to initiate the polymerization. Suitable free-radical- yielding initiators include inorganic peroxides such as K, Na or ammonium persulphate, hydrogen peroxide, or percarbonates; organic peroxides, such as acyl peroxides including e.g. benzoyl peroxide, alkyl hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide; dialkyl peroxides such as di-t-butyl peroxide; peroxy esters such as t-butyl perbenzoate and the like; mixtures may also be used. The peroxy compounds are in some cases advantageously used in combination with suitable reducing agents (redox systems) such as Na or K pyrosulphite or bisulphite, and iso-ascorbic acid. Metal compounds such as Fe-EDTA (EDTA is ethylene diamine tetra acetate) may also be used as part of the redox initiator system. Azo functional initiators may also be used such as azobis(isobutyronitrile), 2,2'-azo-bis(2-methyl butane nitrile) (ANBN); and 4,4'-azobis(4-cyanovaleric acid). It is possible to use an initiator system that partitions between the aqueous and organic phases, e.g. a combination of t-butyl hydroperoxide, iso-ascorbic acid and Fe-EDTA. The amount of initiator or initiator system to use is conventional, e.g. within the range 0.05 to 6 weight % based on the total vinyl monomer(s) used. Preferred initiators for preparing the crosslinkable oligomer OLA and OLB include ammonium persulphates, sodium persulphates, potassium persulphates, azobis(isobutyronitrile) and / or 4,4'-azobis(4-cyanovaleric acid). Most preferred initiators for preparing the crosslinkable oligomer OLA and OLB precursors include redox systems and persulphates as described above. A further amount of initiator may optionally be added at the end of the polymerization process to assist the removal of any residual vinyl monomers still present.

[0177] If the crosslinkable vinyl oligomers OLA and OLB precursors are made by means of emulsion polymerization, surfactants can be utilized in order to stabilize the vinyl copolymer in water (even if it is self-dispersible). Suitable surfactants are ionic or non-ionic surfactants. Examples of anionic emulsifiers are: potassium laurate, potassium stearate, potassium oleate, sodium decyl sulphate, sodium dodecyl sulphate, and sodium rosinate. Examples of non-ionic emulsifiers are: linear and branched alkyl and alkylaryl polyethylene glycol ethers and thioethers and linear and branched alkyl and alkylaryl polypropylene glycol ethers and thioethers, alkylphenoxypoly(ethylenoxy)ethanols such as the adduct of 1 mole of nonylphenol to 5-50 moles of ethylene oxide, or the alkali salt or ammonium salt of the sulphate or the phosphate of said adduct.

[0178] Also surfactants containing an olefinically unsaturated group that can participate in a free radical polymerization can be used. Suitable polymerizable surfactants include hemi-esters of maleic anhydride of the formula M+.- OOC-CH=CHCOOR wherein R is C6 to C22 alkyl and M+ is Na+ , K+ , Li+ , NH4+ , or a protonated or quaternary amine. Polyoxyethylene alkylphenyl ethers with an ethylenically unsaturated bond sold under the tradename NOIGEN® RN (supplied by Montello, Inc.) such as NOIGEN® RN-10, NOIGEN® RN-20, NOIGEN® RN-30, NOIGEN® RN- 40, and NOIGEN® RN-5065 or the sulphate thereof sold under the tradename HITENOL® BC (supplied by Montello, Inc.) such as HITENOL® BC-10, HITENOL® BC-1025, HITENOL® BC- 20, HITENOL® BC-2020, HITENOL® BC-30. MAXEMUL™ 6106 (available from Cargill Industrial Specialties), which has both phosphonate ester and ethoxy hydrophilicity, a nominal C18 alkyl chain with an acrylate reactive group. Other representative reactive surfactants with phosphate ester functionalities suitable for such reactions include, but are not limited to, MAXEMUL™ 61 12, MAXEMUL™ 5011 , MAXEMUL™ 5010 (all available from Cargill Industrial Specialties). Alternative reactive surfactants suitable for use with various embodiments of the present invention include sodium allyloxy hydroxypropyl sulphonate (available from Solvay as SIPOMER™ COPS-1), ADEKA REASOAP® SR / ER series such as ADEKA REASOAP® ER-10, ER-20, ER-30 and ER-40, AKEDA REASOAP® SR-10, SR-20, SR-30 (all available from ADEKA Corporation) and allylsulphosuccinate derivatives such as TREM™ LT-40 (sodium dodecyl allyl sulfosuccinate available from Henkel).

[0179] The amount of surfactant used in the synthesis of the oligomer OLA and OLB precursors are preferably 0 to 15 weight %, more preferably 0 to 8 weight %, still more preferably 0 to 5 weight %, especially 0.1 to 3 weight % and most especially 0.2 to 2 weight %. Preferably the vinyl polymers PA and PB are prepared in an aqueous process in the presence of the oligomer OLA and OLB precursor. Prior to the polymerization of the monomers building the vinyl polymer PA and PB, the crosslinkable oligomer OLA or OLB precursors may be partially or completely solubilized by adding a base to the oligomer (precursor) to (partially) neutralize the carboxylic acid groups comprised in its main chain. The neutralization of the acid functional groups is preferably from a stoichiometrically ratio of 0.6 more preferably 0.7 and most preferably 0.8. Most preferably the vinyl polymers PA and PB are prepared by an aqueous emulsion polymerization process.

[0180] Preferably the at least one vinyl polymer PA and PB is prepared by emulsion copolymerizing a mixture of ethylenically unsaturated monomers in the presence of the at least partially neutralized vinyl oligomer OLA and OLB precursor. During the neutralisation of the carboxylic acid groups on OLA and OLB vinyl oligomer precursor, the oligomer particles initially will become swollen with water until eventually the particles seem to have completely dissolved, indicating that the refractive index of the swollen particles has become the same as that of water. As it is still possible the measure a particle size using dynamic light scattering, we believe, without being bound by theory, that the oligomer chains in OLA and OLB have formed micelles that will serve as the polymerization loci for the subsequent emulsion polymerization. The at least one vinyl polymer PA and PB is characterised by the fact that it is significantly less hydrophilic than the vinyl oligomer OLA and OLB precursor. The hydrophilicity can be quantified in terms of the acid value or the concentration of acid functional monomer. When both the vinyl oligomer precursor and the vinyl polymer contain carboxylic acid functional monomers the vinyl polymer comprises significantly less carboxylic acid functional monomers that the vinyl oligomer precursor. The difference in hydrophilicity is required to obtain a particle morphology where the vinyl oligomer precursor remains at the periphery of the polymer particle during and after the polymerization of the vinyl polymer. Preferably, the vinyl oligomer precursor comprises 10 to 45 % by weight of carboxylic acid functionalised monomers and the vinyl polymer comprises less than 5 % by weight of carboxylic acid functional monomers.

[0181] The process for preparing the vinyl polymers PA and PB may be carried out in a number of modes including but not limited to polymerizing all of the oligomers OLA and OLB and monomers M3 and, optionally M1 and / or M2 and / or M4, in one batch, pre-charging a oligomer OLA or OLB to a reactor and subsequently feeding in the monomer or monomers or monomer mixtures (which may be the same or different), in one or more stages and I or using a gradient feeding technique (or vice versa), feeding both oligomer OL and monomers to a reactor (optionally pre-charged with some oligomer), preparing a polymer by feeding the monomers to the oligomer OLA or OLB which is simultaneously fed into a reactor (optionally pre-charged with some oligomer) or continuously feeding a mixture of one of the oligomer OLA or OLB and monomers into a reactor.

[0182] The carboxylic acid groups of the vinyl oligomer OLA precursor are at least partially converted into pendant ethylenically unsaturated ((meth)acryloyl) groups, through reaction with M6, at the end of the formation of the vinyl polymer PA in the aqueous medium comprising the carboxylic acid group containing vinyl oligomer OLA precursor.

[0183] The crosslinkable aqueous coating composition is preferably prepared by blending the first crosslinkable oligomer stabilized vinyl polymer dispersion DA with the second crosslinkable oligomer stabilized vinyl polymer DB and at least one (carbonyl reactive) crosslinker X. The blending ratio between DA and DB is 50:50, preferably 35:65, and more preferably 25:75.

[0184] The (carbonyl reactive) crosslinker that reacts with the M2 (carbonyl) functional groups can be added to DA and optionally DB prior to blending or after blending.

[0185] Preferably, the equivalence ratio of (the carbonyl) functional groups of the vinyl oligomer OLA and OLB and of the optionally at least one vinyl polymer of PA and PB, to the (carbonyl reactive) functional groups of the at least one (carbonyl reactive) crosslinker X, is from 0.5 to 10, preferably from 0.75 to 5, more preferably from 0.95 to 2.5. More particularly, the amount of (carbonyl reactive) crosslinker is preferably such that the equivalence ratio of functional groups provided by monomer M2 in oligomer OLA, and optionally, by monomer M2 in vinyl oligomer OLB, vinyl polymer PA and vinyl polymer PB, to crosslinker functional groups is from 0.5 to 10, more preferably from 0.75 to 5 and most preferably from 0.95 to 2.5.

[0186] The type of the at least one (carbonyl reactive) crosslinker X depends on the choice of the functionality of ethylenically unsaturated monomer M2.

[0187] Preferably, the functional groups of any of the vinyl oligomers, OLA and OLB, and optionally of any of the vinyl polymers, PA and PB, reactable with the functional groups of the at least one (carbonyl reactive) crosslinker X, are chain pendant (carbonyl) functional groups of the ketone, aldehyde or acetoacetyl type.

[0188] Preferably, the at least one (carbonyl reactive) crosslinker X having functional groups reactable with (carbonyl) functional groups of the vinyl oligomer OLA and OLB and optionally of the at least one vinyl polymer PA and PB, comprises (carbonyl reactive) functional groups selected from the group consisting of primary amine, secondary amine, hydrazine, hydrazide, hydrazone, and mixtures thereof.

[0189] More particularly, the (carbonyl reactive) crosslinker X reactive with the (carbonyl) functional groups of monomers M2 in OLA and optionally OLB, respectively, vinyl polymer PA and vinyl polymer PB, is preferably a di- or a polyamine or a di- or polyhydrazide, or mixtures thereof. Examples of diamines are aliphatic or cycloaliphatic amines having 2 to 10 primary and / or secondary amino groups and 2 to 100 carbon atoms. Suitable polyfunctional amines include but are not limited to hexamethylene diamine; 2-methyl pentamethylene diamine; 1 ,3-diamino pentane; dodecane diamine; 1 ,2- diamino cyclohexane; 1 ,4-diamino cyclohexane; paraphenylene diamine; 3-methyl piperidine; isophorone diamine; bis-hexamethylene triamine; diethylene triamine; and combinations thereof. Other polyfunctional amines, which are suitable, include those containing adducts of ethylene and propylene oxide, such as the "JEFFAMINE® " series D, ED and T of the Huntsman Chemical Company.

[0190] Examples of acid dihydrazides include, but are not limited to carbonohydrazide, oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, maleic acid dihydrazide, fumaric acid dihydrazide, itaconic acid dihydrazide, phthalic acid dihydrazide, or terephthalic acid dihydrazide. Alternatively, the crosslinking agent can be a water-soluble aliphatic dihydrazine, such as ethylene-1 , 2-dihydrazine, propylene-1 ,3-dihydrazine, and butylene-1 ,4-dihydrazine, or a polyamine such as isophorone diamine or 4,7-dioxadecane-1 ,10-diamine. Most preferred is adipic acid dihydrazide. More preferably, the waterborne coating composition of the present invention comprises from 25 to 50% by weight of a first crosslinkable oligomer stabilized vinyl polymer dispersion DA, from 50 to 75% by weight of a second crosslinkable oligomer stabilized vinyl polymer dispersion DB, based on the total weight of DA and DB being 100% by weight, at least one (carbonyl reactive) crosslinker X, and at least one photo-initiator, wherein:

[0191] - DA comprises from 20 to 60% by weight of combined vinyl oligomer OLA and at least one vinyl polymer PA, based on the total weight of DA, the combined PA and OLA comprising at least 35% by weight and up to at most 75% by weight of OLA, based on the combined weight of PA and OLA, being 100% by weight, and

[0192] - DB comprises from 20 to 60% by weight of combined vinyl oligomer OLB and at least one vinyl polymer PB, based on the total weight of DB, the combined PB and OLB comprising at least 35% by weight and up to at most 75% by weight of OLB, based on the combined weight of PB and OLB, being 100% by weight, and wherein:

[0193] - the vinyl oligomer chain of OLA and OLB is the polymerized product of:

[0194] - from 1 to 45% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,

[0195] - from 0.5 to 20% by weight of at least one ethylenically unsaturated monomer M2 with carbonyl functionality for crosslinking upon film formation, preferably M2 being different from M1 ,

[0196] - from 50 to 98.5% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0197] - from 0 to 8% by weight of at least one surfactant comprising an ethylenically unsaturated group M5; and

[0198] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into their salts, wherein the degree of neutralization is between 0.2 and 0.8; and

[0199] - for the vinyl oligomer OLA, the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into ethylenically unsaturated pendant groups through reaction with bi-functional compound M6, M6 comprising an ethylenically unsaturated group and a group reactive towards the carboxylic acid, wherein the degree of conversion is between 0.2 and 0.8; and wherein:

[0200] - the vinyl polymer chain of PA and PB is the polymerized product of:

[0201] - from 0 to 5% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,

[0202] - from 0 to 10% by weight of at least one ethylenically unsaturated monomer M2 with (carbonyl) functionality for crosslinking upon film formation, preferably M2 being different from M1 ,

[0203] - from 80 to 100% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and

[0204] - from 0.1 to 5% by weight of at least one multifunctional ethylenically unsaturated monomer M4 to impart a gel content; and

[0205] - the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into salt; and

[0206] - one or more vinyl polymer(s) PA is (are) different from one or more vinyl polymer PB; and wherein: the vinyl oligomer OLA and optionally the at least one vinyl polymer PA comprise:

[0207] - chain pendant functional groups (from M2) reactable with the functional groups of an at least one (carbonyl reactive) crosslinker X, and

[0208] - chain pendant ethylenically unsaturated ((meth)acryloyl) functional groups (from M6) crosslinkable by a (an actinic radiation induced) free radical process; and the vinyl oligomer OLB and optionally the at least one vinyl polymer PB comprise chain pendant (carbonyl) functional groups, from M2, reactable with the functional groups of an at least one (carbonyl reactive) crosslinker X, said (carbonyl reactive) crosslinker X having functional groups reactable with carbonyl functional groups of the vinyl oligomer OLA and OLB and optionally of the at least one vinyl polymer PA and PB.

[0209] To the waterborne coating composition of the present invention, one or more photo-initiator(s), preferably absorbing in the wavelengths of visible light (i.e. in the range of 380 - 700 nm, preferably in the range of 400 - 700 nm, called daylight initiators), are preferably added in order to provide radicals under exposure to visible light for the polymerization of the methacryloyl groups in vinyl oligomer OLA. Non-limiting examples of daylight initiators are Lithium phenyl- 2,4,6-trimethylbenzoylphosphinate (LAP), Diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide (TPO), Diaryliodonium and sulfonium salts, Aryl alkyl ketones and Benzophenones or heterocyclic aromatic ketones. A comprehensive review has been published by S. M. Muller et al. Chem Photo Chem (2022), 6, (2 of 12). Most preferably the visible light initiator used is a Bis-Acyl Phosphine Oxide, sometimes referred to as” BAPO” initiators. Examples of commercially available BAPO initiators are OMNIRAD® 819 DW from IGM Resins (an 44-46 % active solution of Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide) or GENOCURE® BAPO (Phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide) available as a 97% powder from Rahn AG.

[0210] The amount of one or more daylight initiator added to the coating composition is depending on the pigments used, for example adding transparent iron oxides, such as is usual in wood-stain applications will require a larger addition compared to clear wood varnishes.

[0211] The waterborne coating composition preferably comprises between 0.1 and 2.0 % by weight, preferably between 0.3 and 1 .7% by weight, more preferably between 0.5 and 1 .5% by weight of one or more photo-initiator(s). More preferably, the waterborne coating composition comprises between 0.1 and 2.0 % by weight, preferably between 0.3 and 1.7 % by weight, more preferably between 0.5 and 1 .5 % by weight of one or more daylight initiator(s).

[0212] If so desired (and less preferred), also photo-initiators having absorption maxima in the UV- spectrum (i.e. in the range of 100 - 400 nm, preferably in the range of 100 - 380 nm) can be added to the coating formulation.

[0213] Without being held by theory, we believe that polymerization of the ethylenically unsaturated pendant functional groups in vinyl polymer dispersion DA are responsible for the fast quick drying properties of the coating, whereas the hydrazone crosslink formation that occurs between vinyl polymer dispersions DA and DB is responsible for the longer time durability of the coating network.

[0214] The waterborne coating composition according to the invention permits to obtain coatings with very good decorative, protective and mechanical properties, in particular the waterborne coating composition permit to obtain coatings providing high hardness, as well as good chemical resistance, in particular early water resistance and gloss retention. The waterborne coating composition according to the invention has a low MFFT and has application properties than currently are better than provided for in the art.

[0215] The crosslinkable waterborne coating composition has usually a minimal film-formation temperature (MFFT) ranging from 0 °C to 50 °C, preferably from 5 °C to 50 °C, more preferably from 5 °C to 25 °C and most preferably from 5 °C to 15 °C.

[0216] If desired the aqueous composition of the invention can be used in combination with other aqueous polymer compositions which are not according to the invention. Examples of such aqueous polymer compositions include water-reducible alkyds, alkyd emulsions, water- reducible polyesters or polyester emulsions, polyurethane dispersions or epoxy dispersions.

[0217] The waterborne coating compositions can be pigmented or unpigmented. Preferably the waterborne coating composition is formulated to be used in pigmented coatings for exterior wood, where the pigments used are transparent iron oxides. More preferably is the use of this waterborne coating composition in clear or semi-transparent varnishes for outdoor use on wooden (or wood-based) substrates. Most preferably the waterborne coating composition is used as a coating for wooden decks (like for example a deck-stain formulation for deck-stain coatings, also referred to as wood-stain coating composition or wood-stain composition).

[0218] Atypical deck stain formulation consists of a binder, transparent iron oxide pigments, generally black, red and yellow, some rheological additives, a coalescent and additional water to adjust the viscosity. The deck-stain is usually applied by roller or brush. Several layers are applied most of the time to obtain an even finish. The waterborne coating composition of the present invention may further also be applied to a variety of other substrates including board, metals, stone, concrete, glass, cloth, leather, paper, plastics, foam and the like, by any conventional method including brushing, rolling, dipping, flow coating, spraying, flexographic printing, gravure printing, ink-jet printing, any other graphic arts application methods and the like. For these applications, the daylight initiator sometimes must be exchanged for another initiator that decomposes under UV-light of a shorter wave-length. Accordingly, in a further embodiment of the invention there is provided a coating, a polymeric film, a printing ink and / or an overprint lacquer obtainable from an waterborne coating composition of the present invention.

[0219] The waterborne coating compositions according to the invention as described here above further may comprise at least one or more conventional ingredients (or, one or more additives and auxiliaries) selected from the group consisting of pigments, dyes, emulsifiers, surfactants, plasticizers, thickeners, heat stabilizers, levelling agents, anti-cratering agents, fillers, sedimentation inhibitors, UV absorbers, antioxidants, drier salts, organic co-solvents, wetting agents and the like, and mixtures thereof. Those ingredients can be introduced at any stage of the production process of the waterborne coating compositions or subsequently.

[0220] Optionally an additional external crosslinking agent may be added to the waterborne coating compositions of the invention to aid crosslinking during or after drying. Additional external crosslinking agents include polyisocyanates, amino resins such as melamine- and ureum formaldehyde resins, polyaziridines, carbodiimides, epoxy resins and solutions of transition metal salts such as zinc or zirconium ammonium carbonate. The waterborne coating composition according to the present invention is therefore particularly suitable for coating exterior wood (for example for coating decks, (garden) furniture, (garden) fences, etc.). Preferably, the waterborne coating composition of the present invention is used for coating wooden (or wood-based) substrates selected from the group consisting of medium density fiberboard (MDF), high density fiberboard (HDF), particleboard, oriented strand board (OSB), solid wood, and composite materials (such as Wood Plastic Composites (WPC), composite materials based on a blend of (or made of) wood or wood-based materials (such as wood flour, or wood fibre) and thermoplastics (such as polyethylene, polypropylene, or polyvinyl chloride)). More preferably, the waterborne coating composition is used for coating wooden (or wood-based) substrates selected from the group consisting of medium density fiberboard (MDF), high density fiberboard (HDF), particleboard, oriented strand board (OSB), and solid wood. Even more preferably, the waterborne coating composition is used for coating wooden (or wood-based) substrates from solid wood.

[0221] The present invention also relates to a coated article comprising a wooden (or wood-based) substrate being at least partly coated with the waterborne coating composition of the present invention.

[0222] EXPERIMENTAL

[0223] TESTMETHODS

[0224] The molecular weight and molecular weight distribution were determined using size exclusion chromatography according to ASTM D 3536 and ASTM D 3593. The size-exclusion apparatus used was an Alliance system consisting of a pump, auto-sampler and He-degasser (Degasys DG-1210 from Uniflows), equipped with a PLgel 5pm MIXED-C 600x7.5 mm Column and a PL gel 5 pm guard column (50 x 7.5 mm - Polymer Laboratories). The Column oven (Separations Analytical Instruments) was set at a temperature of 30 °C. Tetrahydrofuran (THF - Extra Dry, Biosolve 206347) + 2% acetic acid (J. T. Baker® 6052) is used as eluent at a flow-rate of 0.8 ml / min. Carbon disulfide (J. T. Baker®) was used as a marker. As a refractive index detector, a Waters 410 was used. The injection Volume was 100 pl at a concentration of 1.5 mg / ml. Polystyrene standards (Polymer Laboratories, Easical PS-1 , 2010-0501 (molecular weight range 580 g / mol - 8.500.000 g / mol) and Easical PS-2, 2010-0601 (molecular weight range 580 g / mol - 400.000 g / mol)) were used for calibration using a third order polynomial. Software used for data-analysis was Empower (Waters).

[0225] Minimum film-formation temperature (MFFT) was determined according to ASTM D2354 - 10 (2018) by using a Rhopoint MFFT-Bar 60 which has a temperature range from 0 °C to 60 °C. Films were applied with a wet film thickness of 25 microns. The MFFT was the lowest temperature at which the film showed no cracks.

[0226] Particle size was determined by dynamic light scattering using a Malvern Zetasizer model Nano - S90. The Z-average mean value was reported as the particle size. The Z-average diameter is the mean hydrodynamic diameter and is calculated according to the International Standard on dynamic light scattering ISO13321.

[0227] Dynamic mechanical thermal analysis (DMTA) measurements were done in tensile mode on free-standing films (60-70 mu thickness) using a Q800 DMTA from TA Instruments. DMTA measurements are done in single-frequency mode at 1 Hz with 0.03% strain (oscillation amplitude). The temperature is varied between -80 °C and 200 °C at 5 °CZ min heating rate. Simulation of daylight curing: For aging films under simulated daylight conditions, a Q-Sun Test Chamber was used with a wavelength setting of UV-A 340 nm with an intensity of 0.45 W / m2. Tests were performed in accordance to ASTM D6695 Standard Practice forXenon-Arc Exposures of Paint and Related Coatings. Determination of pH was done according to ISO 976.

[0228] Brookfield viscosity is measured with a BROOKFIELD™ RVT viscometer at a temperature of 23 ± 1 °C, according to ISO 2555-1974.

[0229] Determination of solids content was done according to ISO 3251 : 2008 (E). Examples.

[0230] The following illustrative examples are merely meant to exemplify the present invention but they are not intended to limit or otherwise define the scope of the present invention.

[0231] Example 1. Synthesis of a methacryloyl and carbonyl functional crosslinkable oligomer stabilized vinyl polymer dispersion DA. Stage 1A: Preparation of a carboxyl and carbonyl functional oligomer OLA precursor.

[0232] Table 1 : Monomers for oligomer OLA precursor Demineralized (or deionized) water (Aa) and emulsifying agent (Ab) were charged to a jacketed 3 I glass reactor that was purged with nitrogen (5 l / h). Component (Ac) was used to rinse the feed-line for the emulsifier. The contents of the reactor were heated to 70 °C. A monomer pre-emulsion was made by adding components (Ag), (Ah), (Ai), (Aj), (Ak), (Al), (Am), (An), (Ao), (Ap) in a mixing tank. From this pre-emulsion, 5 % was added to the reactor and the contents were heated to 80 °C. When the temperature reached 80 °C, an ammonium persulfate solution made up of (Ad) + (Ae) is added and the reactor was heated to 85 °C. The remainder of the pre-emulsion was metered into the reactor over a period of 60 minutes. Simultaneously, initiator solution made up of (Ar) +(At) was added to the reactor over a period of 70 minutes. After the addition of the monomer pre-emulsion was done, the mixing tank was rinsed with component (Aq). The reactor was held at 85 °C for an additional 30 minutes. An aqueous solution of 80 grams of ammonia (25% aqueous) was then added over a period of 30 minutes. The solids content of the batch was adjusted to 24 % with demineralized water. After holding the batch at 85 °C for 2 hours, the polymer dispersion was cooled to 45 °C. A sample was taken and analyzed. The number average molecular weight, Mn was 7,550 g / mole and a weight average molecular weight of 16,840 g / mole.

[0233] Stage 1 B. Preparation of a second vinyl polymer PA in the presence of the alkaline solution of vinyl oligomer OLA precursor.

[0234] Table 2 shows the ingredients for the polymerization of vinyl polymer PA in the presence of solubilized vinyl oligomer OLA precursor of stage 1 A.

[0235] Table 2: Monomers for polymer PA

[0236] 50 % of the monomer mixture of table 2 was added to the reactor comprising the solubilized vinyl oligomer OLA precursor of stage 1 A and mixed in for 30 minutes. A reducer solution was made by adding 0.826 of erythorbic acid to 16.51 g of demineralized water. An initiator solution of 0.182 grams oft-butyl hydroperoxide (70% aqueous) in 16.51 grams of demineralized water and a Fe(ll)-EDTA complex solution consisting of 2 mg of iron sulphate heptahydrate, 5 mg EDTA disodium dihydrate dissolved in 16.51 g of demineralized water, was added to the reactor followed by 10% of the reducer solution prepared above. An exothermic reaction occurred that caused the temperature to rise to 55 - 60 °C. The temperature of the reactor is maintained at 55 - 60 °C. After 15 minutes 40% of the reducer solution was added over a period of 30 minutes. The contents of the reactor were cooled to 50 °C .The remaining part of the monomer mixture from table 2 was added and mixed for 20 minutes. A second initiator solution and EDTA-Fe complex solution were added to the reactor. 10% of reducer solution was added to the reactor. An exothermic reaction occurred. The temperature of the batch was maintained at 60 - 65 °C. After a hold period of 15 minutes the remainder of reducer solution was added over a period of 30 minutes. The batch was post-reacted at 65 °C for 20 minutes. The solids content was adjusted to approximately 44%. The resulting polymer dispersion had a solids content (determined by drying a sample for 1 hour at 125 °C) of 43.4 % and a pH of 5.4. Particle size measured by a Malvern Zetasizer was 211 nm. The acid value of the polymer was determined by titration to be 151 mg KOH / g.

[0237] Stage 1C. Modification of a carboxyl and carbonyl functional oligomer / polymer dispersion with an ethylenically unsaturated monoepoxide M6.

[0238] C Material Weight (g) a Tetrabutyl ammonium hydroxide (40% aqueous) 59.8 b Demineralized water 221.8 c Glycidyl methacrylate 132.4 d 2,6-ditert-butyl-4-methylphenol 0.26

[0239] Table 3: Raw materials for the epoxy modification of the dispersion obtained after stage 1 B, comprising OLA precursor of Stage 1A and PA.

[0240] 1 ,200 grams of the polymer dispersion obtained after stage 1 B were charged into a 3 liter jacketed reactor and purged with air (5 l / h). The contents of the reactor were heated to 80 °C. Solution (Ca) was added to the contents of the reactor over a period of 5 to 10 minutes. A mixture of (Cb), (Cc) and (Cd) was then added to the reactor in about 10 minutes. After a mild exotherm was observed, the reaction mixture was kept at 80 °C for 2 hours. The contents were cooled to 30 °C and filtered through a 60pm filter-bag. After the modification with the monoepoxide the vinyl polymer dispersion had a solid content (drying for 1 hour at 125 °C) of 40.8 % and a pH of 6.4. The viscosity at 23 °C was 0.0176 mPa.s. The particle size of the modified dispersion was 215 nm. After the modification the acid value had dropped to a value of 52.0 mg KOH / g. A film cast from this dispersion had a smooth and glossy appearance.

[0241] Example 2. Synthesis of a carbonyl functional crosslinkable oligomer stabilized vinyl polymer dispersion DB.

[0242] Stage 2.A: Synthesis of a carboxyl and carbonyl functional vinyl oligomer OLB.

[0243] Crosslinkable vinyl oligomer OLB was synthesized using the same procedure as in example 1 : Stage 1A (synthesis OLA precursor), but with a monomer composition of table 4:

[0244] Table 4: Raw material composition of OLB The crosslinkable vinyl oligomer OLB had a number average molecular weight Mnof 6,580 g / mole and a weight average molecular weight, Mwof 14,360 g / mole.

[0245] Stage 2.B: Preparation of vinyl polymer PB in the presence of the alkaline solution of vinyl oligomer OLB.

[0246] Vinyl polymer PB was synthesized according to the procedure of example 1 (stage 1 B) using the oligomer of Stage 2A (OLB) and with a monomer composition consisting of the weights as given in Table 5.

[0247] Table 5: Raw Materials used for vinyl polymer PB.

[0248] The weight ratio between the oligomer (OLB) and the polymer (PB) part in the dispersion was approximately 1 / 1.

[0249] The dispersed oligomer stabilized polymer dispersion had a solid content of 40.0 %, a pH of 8.7, a particle size of 96 nm and a viscosity of 128 mPa.s

[0250] Example 3. Waterborne coating compositions.

[0251] Clear varnishes were prepared based on combinations of the vinyl dispersions DA and DB from examples 1 and 2. As a commercial reference RESYDROL® AY 586w / 39 WA (available from Allnex Austria “AY 586” in what follows) was used as it is one of the benchmark binders used in wood-coating stains. AY 586 is an oxidatively drying, acrylic modified alkyd resin delivered as aqueous emulsion. Main properties are a solids content of 39% in water, containing 2.8% ethoxypropanol and 2.8% methoxypropoxypropanol.

[0252] The wood-stains were prepared according to table 6 wherein examples 3.3 and 3.4 are comparative examples.

[0253] Table 6: Wood-stain compositions.

[0254] Dipropylene glycol n-butyl ether. Tradename from Dow Chemical. ** Transparent Iron oxide pigments available from Krahn Chemie Benelux.

[0255] *** VOC-free silicone-containing defoamer for aqueous coatings from BYK Chemie.

[0256] **** 45% bis- acylphosphine oxide in water BAPO type photo-initiator from IGM Resins.

[0257] ***** rheology modifier that is a non-ionic polyurethane in butyl triglycol / water from Munzing Chemie. The wood-stain coating compositions were applied on glass at a wet layer thickness of 125 micron. After a flash-off of 10 minutes, they were cured. The different coatings were tested for Koenig hardness in a number of different drying scheme’s; results are represented in table 7.

[0258] Table 7: Drying performance for experiments 3.1 to 3.5 (Koenig hardness). Interestingly the formulation based on vinyl polymer DB seems to be the one with the most promising hardness development. However, the formulation consisting of 75% of binder vinyl polymer dispersion DB mixed with 25% of polymer dispersion DA has the highest hardness after illumination with a Xenon lamp for 100 hours. In the formulation based on AY 586, no effect of the presence of polymer DA can be noticed. The pure polymer DA binder almost doubles in hardness, but the film has become hard and brittle. These results illustrate the advantage of combining vinyl polymer dispersions DA and DB.

[0259] Wood-stains compositions were prepared according to table 8 wherein examples 4.3 and 4.4 are comparative examples.

[0260] Table 8: Wood-stain compositions Test results are given in Table 9.

[0261] Table 9: Drying performance for experiments 4.1 to 4.5

[0262] This table clearly shows the synergetic effect of combining binders DA and DB, especially after UV curing.

[0263] DMTA analysis.

[0264] Free-films obtained by applying the different coatings to polypropylene substrates and ageing them were also studied by means of dynamic mechanical thermal analysis (DMTA, vide supra in Testmethods). In the DMTA curve, there is always a first transition in a region around -10 °C, which can be attributed to the core particles of DA and DB, respectively PA and PB. The tan 5 peak on the righthand side can be attributed to the crosslinking OLA and OLB and gives the Tg of the formed network. The Tg values shown in below Table 10 are determined by DMTA.

[0265] Table 10: Tq <DMTA), network as a function of the time, curing at ambient temperature and under irradiation of a Xenon-lamp. *Photo-initiator (PI) OMNIRAD® 819DW

[0266] In table 10 it is shown that the coating comprising DA (25%) + polymer DB (75%) + PI shows a fast increase in Tg during the first 3 days compared to the oligomer stabilized vinyl polymer dispersions comprising only DA or DB. When this coating contains transparent iron oxides the increase of Tg in the network seems to be slowed down, probably because part of the light received by the daylight initiator has been decreased.

Claims

CLAIMS1. A waterborne coating composition comprising a first crosslinkable oligomer stabilized vinyl polymer dispersion DA, a second crosslinkable oligomer stabilized vinyl polymer dispersion DB, and at least one crosslinker X, wherein:- DA comprises a vinyl oligomer OLA and at least one vinyl polymer PA, the vinyl oligomer OLA and optionally the at least one vinyl polymer PA comprising:- chain pendant functional groups reactable with the functional groups of an at least one crosslinker X, and- chain pendant ethylenically unsaturated functional groups crosslinkable by a free radical process; and- DB comprises a vinyl oligomer OLB and at least one vinyl polymer PB, the vinyl oligomer OLB and optionally the at least one vinyl polymer PB comprising chain pendant functional groups reactable with the functional groups of an at least one crosslinker X.

2. The waterborne coating composition of claim 1 , wherein: the vinyl oligomers OLA and OLB are characterized by:- a number average molecular weight Mn within the range of from 500 to 50,000 g / mole, preferably of from 2,500 to 25,000 g / mole, more preferably of from 5,000 to 15,000 g / mole;- a calculated glass transition temperature Tg of the oligomer backbone, within the range of from 10 to 150 °C, preferably of from 20 to 125 °C, more preferably of from 25 to 115 °C; the at least one vinyl polymers PA and PB are characterized by:- a number average molecular weight Mn of more than 60,000 g / mole, preferably of more than 80,000 g / mole, more preferably of more than 100,000 g / mole;- a calculated glass transition temperature Tg of the polymer backbone within the range of from -70 to 50 °C, preferably of from -45 to 45 °C, more preferably of from -25 to 40 °C, the calculated glass transition temperature Tg of PA and PB being at least 25 °C, preferably at least 35 °C, more preferably at least 40 °C, below the glass transition temperature of the vinyl oligomers OLA and OLB backbone, respectively.

3. The waterborne coating composition of claim 1 or 2, wherein the first crosslinkable oligomer stabilized vinyl polymer dispersion DA comprises from 20 to 60% by weight, preferably from 30 to 50% by weight, of combined PA and OLA, based on the total weight of DA, the combined PA and OLA comprising at least 35% by weight, preferably at least 40% by weight, more preferably at least 45% by weight, most preferably at least 50% by weight and up to at most 75% by weight, preferably at most 70% by weight, more preferably at most 65% by weight, most preferably at most 60% by weight of OLA, based on the combined weight of PA and OLA, being 100% by weight.

4. The waterborne coating composition of claim 1 or 2, wherein the second crosslinkable oligomer stabilized vinyl polymer dispersion DB comprises from 20 to 60% by weight,preferably from 30 to 50% by weight, of combined PB and OLB, based on the total weight of DB, the combined PB and OLB comprising at least 35% by weight, preferably at least 40% by weight, more preferably at least 45% by weight, most preferably at least 50% by weigh and up to at most 75% by weight, preferably at most 70% by weight, more preferably at most 65%, most preferably at most 60% by weight of OLB, based on the combined weight of PB and OLB, being 100% by weight.

5. The waterborne coating composition according to any of claims 1 to 4, comprising from 25 to 50% by weight of the first crosslinkable oligomer stabilized vinyl polymer dispersion DA, and from 50 to 75 % weight of the second crosslinkable oligomer stabilized vinyl polymer dispersion DB, based on the total weight of DA and DB, being 100% by weight.

6. The waterborne coating composition according to any of claims 1 to 5, wherein the functional groups of any of the vinyl oligomers, OLA and OLB, and optionally of any of the vinyl polymers, PA and PB, reactable with the functional groups of the at least one crosslinker X, are chain pendant functional groups of the ketone, aldehyde or acetoacetyl type.

7. The waterborne coating composition according to any of claims 1 to 6, wherein the at least one crosslinker X having functional groups reactable with functional groups of the vinyl oligomer OLA and OLB and optionally of the at least one vinyl polymer PA and PB, comprises functional groups selected from the group consisting of primary amine, secondary amine, hydrazine, hydrazide, hydrazone, and mixtures thereof.

8. The waterborne coating composition according to any of claims 1 to 7, wherein the equivalence ratio of functional groups of the vinyl oligomer OLA and OLB and of the optionally at least one vinyl polymer of PA and PB, to the functional groups of the at least one crosslinker X, is from 0.5 to 10, preferably from 0.75 to 5, more preferably from 0.95 to 2.5.

9. The waterborne coating composition according to any of claims 1 to 8, wherein:- the vinyl oligomer chain of OLA and OLB is the polymerized product of:- from 1 to 45% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,- from 0.5 to 20% by weight of at least one ethylenically unsaturated monomer M2 with functionality for crosslinking upon film formation, preferably M2 being different from M1 ,- from 50 to 98.5% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and- from 0 to 8% by weight of at least one surfactant comprising an ethylenically unsaturated group M5;- the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into their salts, wherein the degree of neutralization is between 0.2 and 0.8; and- for the vinyl oligomer OLA, the carboxylic acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into ethylenically unsaturated pendant groups through reaction with bi-functional compound M6, M6 comprising an ethylenically unsaturated group and a group reactive towards the carboxylic acid, wherein the degree of conversion is between 0.2 and 0.8.

10. The waterborne coating composition according to any of claims 1 to 9, wherein:- the vinyl polymer chain of PA and PB is the polymerized product of:- from 0 to 5% by weight of at least one acid functional ethylenically unsaturated monomer M1 ,- from 0 to 10% by weight of at least one ethylenically unsaturated monomer M2 with functionality for crosslinking upon film formation, preferably M2 being different from M1 ,- from 80 to 100% by weight of at least one ethylenically unsaturated monomer M3, M3 being different from M1 and M2, and- from 0.1 to 5% by weight of at least one multifunctional ethylenically unsaturated monomer M4 to impart a gel content;- the acid groups of the polymerized at least one acid functional ethylenically unsaturated monomer M1 are at least partially converted into salt; and- one or more vinyl polymer(s) PA is (are) different from one or more vinyl polymer PB.

11. The waterborne coating composition according to claim 9 or 10, wherein the acid functional ethylenically unsaturated monomers M1 is selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid and mixtures thereof.

12. The waterborne coating composition according to any of claims 9 to 11 , wherein the ethylenically unsaturated monomer M2 with functionality for crosslinking upon film monomers is selected from the group consisting of acetoacetoxyethyl methacrylate, diacetone acrylamide, and mixtures thereof.

13. The waterborne coating composition according to any of claims 9 to 12, wherein the monomers M3, M3 being different from M1 and M2, are selected from the group consisting of vinyl monomers; esters of (meth)acrylic acid of formula CH=CR1- C(O)OR2, wherein R1 is H or methyl and R2 is an optionally substituted C1 to C20 alkyl, cycloalkyl, aryl or (alkyl)aryl; and mixtures thereof.

14. The waterborne coating composition according to any of claims 9 to 13, wherein the multifunctional ethylenically unsaturated monomer M4 is selected from the group consisting of 1 ,3- butadiene, isoprene, divinyl benzene, triallyl cyanurate, vinyl or allyl (meth)acrylate, diol di(meth)acrylates, methylene bis(meth)acrylamide, and mixtures thereof.

15. The waterborne coating composition according to any of claims 9 to 14, wherein the bifunctional compound M6, M6 comprising an ethylenically unsaturated group and a group reactive towards the carboxylic acid, is an ethylenically unsaturated monoepoxide.

16. The waterborne coating composition according to any of claims 1 to 15, comprising between 0.1 and 2.0 % by weight, preferably between 0.3 and 1.7% by weight, morepreferably between 0.5 and 1 .5% by weight of one or more photo-initiator(s), preferably daylight initiator(s).

17. The waterborne coating composition according to any of claims 1 to 16, comprising one or more additives and auxiliaries selected from the group consisting of pigments, dyes, emulsifiers, surfactants, plasticizers, thickeners, heat stabilizers, levelling agents, anti-cratering agents, fillers, sedimentation inhibitors, UV absorbers, antioxidants, drier salts, organic co-solvents, wetting agents and the like, and mixtures thereof.

18. Process for the preparation of the second crosslinkable oligomer stabilized vinyl polymer dispersion DB as defined in any of claims 1 to 17, the process comprising the steps of: a. making an aqueous emulsion or solution of vinyl oligomer OLB comprising carboxylic acid functionalized monomers, the amount of carboxylic acid functionalised monomers being sufficient to make the vinyl oligomer OLB water soluble or water dispersible, b. at least partially neutralising the vinyl oligomer OLB with a base, c. adding to the at least partially neutralised vinyl oligomer OLB (formed in the previous steps) a monomer mixture, and emulsion polymerising said monomer mixture, thereby forming a vinyl polymer PB dispersion which is less hydrophilic than vinyl oligomer OLB, thus forming a mixture of vinyl polymer PB and vinyl oligomer OLB.

19. Process for the preparation of the first crosslinkable oligomer stabilized vinyl polymer dispersion DA as defined in any of claims 1 to 17, the process comprising the steps of: a. making an aqueous emulsion or solution of vinyl oligomer OLA precursor comprising carboxylic acid functionalised monomers, the amount of carboxylic acid functionalised monomers being sufficient to make the vinyl oligomer OLA precursor water soluble or water dispersible, b. at least partially neutralising the vinyl oligomer OLA precursor with a base, c. adding to the at least partially neutralised vinyl oligomer OLA precursor (formed in the previous step) a monomer mixture, and emulsion polymerising said monomer mixture, thereby forming a vinyl polymer PA dispersion which is less hydrophilic than vinyl oligomer OLA precursor, d. reacting the carboxylic acid groups (formed) in vinyl oligomer OLA precursor with a bi-functional compound, said bi-functional compound comprising an ethylenically unsaturated group and a group reactive towards the carboxylic acid groups (formed) in the vinyl oligomer OLA precursor, and at least partially converting the carboxylic acid groups of vinyl oligomer OLA precursor into ethylenically unsaturated groups, thus forming a mixture of vinyl polymer PA and vinyl oligomer OLA.

20. The waterborne coating composition according to claim 17, prepared by blending the first crosslinkable oligomer stabilized vinyl polymer dispersion DA, the second crosslinkable oligomer stabilized vinyl polymer dispersion DB, the least one crosslinker X, one or more photo-initiator(s), preferably daylight initiator(s), and one or more additives and auxiliaries.

21. Use of the coating composition according to claim 20, for coating wooden or woodbased substrates selected from the group consisting of medium density fiberboard (MDF), high density fiberboard (HDF), particleboard, oriented strand board (OSB), solid wood, and composite materials.

22. A coated article comprising a wooden or wood-based substrate being at least partly coated with the waterborne coating composition according to claim 17 or with the waterborne coating composition prepared according to claim 20.