WO2022043180A1

WO2022043180A1 - Method for uv curing of water-based polyurethane paint dispersions without uv-c-activatable surface initiators

Info

Publication number: WO2022043180A1
Application number: PCT/EP2021/073036
Authority: WO
Inventors: Axel Becker; Oliver Starzmann; Michael Stengle; Frederik FEIL
Original assignee: Basf Se; Ist Metz Gmbh
Priority date: 2020-08-28
Filing date: 2021-08-19
Publication date: 2022-03-03
Also published as: EP4204462A1

Abstract

The invention relates to a method for producing a paint coating on a substrate by UV curing of a water-based paint formulation comprising an aqueous dispersion of urethane (meth)acrylates, said method comprising the steps in the sequence (i) to (iv): (i) applying the paint formulation comprising the aqueous dispersion of urethane (meth)acrylates to the substrate; (ii) drying the paint formulation; (iii) curing the dried paint formulation by irradiating with UV-C radiation in the 180 to 280 nm wavelength range, affording an at least partially cured paint film; (iv) optionally irradiating the at least partially cured paint film with UV radiation having a wavelength of ≥ 350 nm for further curing of the paint film; wherein the total content in the paint formulation of photoinitiators activatable by UV radiation having a wavelength of < 350 nm is < 0.01% by weight based on the solids content of the paint formulation.

Description

Method for UV curing of water-based polyurethane paint dispersions without UV-C-activatable surface initiators

Description

The present invention relates to a method for producing a paint coating on a flat substrate by UV curing of a water-based paint formulation comprising an aqueous dispersion of urethane (meth)acrylates, in particular on substrates such as paper, wood, woodbase materials, metals, and plastics.

Coating systems cured by exposure to actinic radiation are formulations comprising oligomers or polymers having a plurality of ethylenically unsaturated double bonds, commonly present in the form of acrylate groups or methacrylate groups, and optionally low-molecular-weight ethylenically unsaturated monomers. For production of the coatings, the formulations are applied to the substrate surface to be coated and cured by irradiating with actinic radiation or else with electron beams. What takes place here is a free-radical polymerization of the ethylenically unsaturated double bonds to form a crosslinked polymer film.

Whereas the presence of photoinitiators is not necessary for the curing of coating compositions with electron beams, the curing of photosensitive mixtures with UV radiation involves the addition to the coating compositions of photoinitiators that absorb the UV radiation, generate radicals or protons, and initiate photopolymerization or photocrosslinking.

The amount of photoinitiator added for this purpose is > 0.5% by weight of the curable unsaturated compounds, often > 1 % by weight. UV curing under an inert gas atmosphere allows the amount of photoinitiator required to be reduced. The presence of atmospheric oxygen inhibits the photopolymerization, since reactive oxygen, which is itself a diradical, scavenges the radicals formed from the photoinitiator or during the free-radical polymerization, thereby taking them out of the polymerization reaction.

WO 01/27207 A1 discloses the curing of a paint formulation comprising 25 parts by weight of polyether acrylate and 75 parts by weight of an aqueous polyurethane dispersion using a high- pressure mercury lamp at a power of 120 W/cm.

It is known that a photocurable paint formulation comprising a photoinitiator is often more unstable than a corresponding formulation without photoinitiator. Since photoinitiators rank among the costly constituents of UV-curable paint formulations, it is advantageous for the amounts of photoinitiators in UV-curable paint formulations to be only small. Moreover, photoinitiators and fragments thereof that form on irradiation with UV light often result in yellowing of the paint films or obtrusive odors due to volatile substances. There may also be toxicological concerns about the use of photoinitiators. Furniture manufacturers accordingly impose strict requirements on the type and amount of photoinitiators used in UV-curable paint coatings. The object of the invention is to provide water-based UV-curable paint formulations in which photoinitiators or particular types of photoinitiators are present only in small amounts or are absent altogether, and also to provide a method for producing paint coatings on a flat substrate by UV curing of the water-based paint formulations, wherein curing may in particular be carried out in air too.

The object is achieved by a method for producing a paint coating on a substrate by UV curing of a water-based paint formulation comprising an aqueous dispersion of urethane (meth)acrylates, said method comprising the steps in the sequence (i) to (iv):

(i) applying the paint formulation comprising the aqueous dispersion of urethane (meth)acry- lates to the substrate;

(ii) drying the paint formulation;

(iii) curing the dried paint formulation by irradiating with UV-C radiation in the 180 to 280 nm wavelength range, affording an at least partially cured paint film;

(iv) optionally irradiating the at least partially cured paint film with UV radiation having a wavelength of > 350 nm for further curing of the paint film; wherein the total content in the paint formulation of photoinitiators activatable by UV radiation having a wavelength of < 350 nm is < 0.01% by weight based on the solids content of the paint formulation.

The curing of the paint formulation with UV-C light in step (iii) uses according to the invention a UV radiation source that in the UV-C range from 180 to 280 nm, in particular in the range from 180 to 240 nm, has a distinctly higher power output compared with conventional UV-C radiation sources. The radiation source used is according to the invention a mercury lamp, in particular, a medium-pressure mercury lamp.

The mercury lamp has according to the invention an electrical power consumption (power input) of > 150 W/cm, preferably > 180 W/cm.

In a preferred embodiment, an undoped mercury lamp is used.

The mercury lamp used according to the invention is constructed in the form of a hollow cylinder, the jacket tube of which is made of a material that is transparent to UV radiation. The hollow cylinder is sealed at its end faces, with the result that the interior of the hollow cylinder forms a closed volume in which a filling gas is enclosed. The hollow cylinder has electrical connections at least at its end faces for transferring energy into the filling gas in the interior of the hollow cylinder. The filling gas consists of at least one noble gas and a proportion of mercury, it being possible for the filling gas to additionally be doped with proportions of other metals. Doping may be for example with lead, iron, gallium, indium or other metals, according to individual requirement.

The jacket tube of the mercury lamp has in accordance with the invention high transparency for shortwave UV radiation in the 180 to 240 nm range. It is preferable that the jacket tube is made of a quartz material having high transparency to IIV-C. The jacket tube has high thermal stability. The quartz material used can for example be one of the Suprasil series type from Heraeus, for example Suprasil 3001 13002 I 300.

The filling gas has in operation a working pressure within a range from 1 bar to 10 bar, preferably within a range from 1.5 bar to 8 bar, more preferably within a range from 1.7 bar to 5 bar.

The working voltage per cm arc length is within a range from 5 V/cm to 30 V/cm, preferably from 8 V/cm to 20 V/cm, more preferably within a range from 10 V/cm to 15 V/cm.

The electrical power input of the mercury lamp per cm arc length is in general within a range from 150 to 500 W/cm, preferably within a range from 180 to 500 W/cm, more preferably within a range from 200 to 500 W/cm, even more preferably within a range from 200 to 400 W/cm, in particular within a range from 200 to 350 W/cm.

The current and voltage per cm arc length are in accordance with the invention optimized for a maximum output in the 180 to 240 nm UV range; the conversion efficiency for this wavelength range is > 13%, preferably > 15%, more preferably > 17%, based on the power input. The UV-C output per cm arc length in the 180 to 240 nm range is thus in accordance with the invention at least 19.5 W/cm, preferably at least 22.5 W/cm, and more preferably at least 25.5 W/cm or at least 23.4 W/cm, preferably at least 27 W/cm and more preferably at least 30.6 W/cm. At a conversion efficiency of 15%, it is in general within a range from 22.5 to 75 W/cm, preferably within a range from 27 to 75 W/cm, more preferably within a range from 30 to 75 W/cm, even more preferably within a range from 30 to 60 W/cm, and in particular within a range from 30 to 52.5 W/cm.

Because of the rotational symmetry of the radiation emissions from the cylindrical lamp, it is expedient to deflect the radiation that is not directly incident on the process plane so as to achieve a higher irradiation effect and thus greater efficiency in the polymerization. The radiation unit accordingly has reflectors for deflecting light of the shortwave radiation emitted by the radiation source into the process plane. The reflectors reflect UV radiation in the 180 to 240 nm range, preferably in the 180 nm to 300 nm range, more preferably in the 180 nm to 400 nm range. The reflectance of the reflectors in the 180 to 240 nm range is in particular greater than or equal to 80%, preferably greater than 90%, more preferably greater than 95%. The reflectors may be metallic reflectors, for example aluminum reflectors, dielectric reflectors or combinations of metallic and dielectric reflectors.

The reflectors may have focusing, scattering or collimating properties. Further optical elements for the deflection of light may be provided, for example lenses or diffractive elements.

It has been found that the IIV-C curing (iii) of the dried paint film with the mercury lamp used according to the invention can be carried out in the absence of so-called surface photoinitiators activatable with IIV-C light and in the presence of oxygen, i.e. without an inert gas atmosphere.

In a preferred embodiment of the method of the invention, curing step (iii) is carried out in air.

Photoinitiators activatable by UV radiation having a wavelength of > 350 nm, which generally serve as initiators for the surface curing of pigmented paints, generally have an absorption band with a maximum within a range from 260 to 340 nm. These include in particular alpha-hydroxy- alkylphenones and alpha-dialkoxyacetophenones such as 1 -hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenyl-1 -propanone, 2- hydroxy-1 -{4-[4-(2-hydroxy-2-methylpropio- nyl)benzyl]phenyl}-2-methylpropan-1-one, 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1- propanone or 2, 2-dimethoxy-1 -phenylethanone; phenylglyoxalic esters such as methyl phenylglyoxalate; benzophenones such as benzophenone, 2-hydroxybenzophenone, 3-hydroxybenzo- phenone, 4-hydroxybenzophenone, 2-methylbenzophenone, 3-methylbenzophenone, 4- methylbenzophenone, 2,4-dimethylbenzophenone, 3,4-dimethylbenzophenone, 2,5-dime- thylbenzophenone, 4-benzoylbiphenyl, or 4- methoxybenzophenone; benzoins such as benzoin, benzoin ethyl ether, benzoin isopropyl ether, and benzoin methyl ether; thioxanthones such as 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, and 2,4-dichlo- rothioxanthone.

The abovementioned photoinitiators are present in amounts of < 0.01% by weight based on the solids content of the paint formulation and are preferably not present at all.

The average particle size of the aqueous dispersions of urethane (meth)acrylates is in general in the range of from 20 to 140 nm, preferably of from 40 to 90 nm.

The aqueous dispersions of urethane (meth) acrylates in general have a particle size distribution that is characterized by a dw value within a range of from 10 to 70 nm, preferably of from 20 to 70 nm, or that is characterized by a dso value within a range of from 15 to 120 nm, preferably of from 35 to 90 nm, or that is characterized by a dgo value of from 30 to 220 nm, preferably of from 70 to 100 nm. In particular, the particle size distribution of the aqueous dispersions of urethane (meth)acrylates is characterized by a d value of from 10 to 70 nm, a dso value of from 15 to 120 nm and a dgo value of from 30 to 220 nm. In very preferred ambodiments, dw is from 20 to 70 nm, dso is from 35 to 90 nm and dgo is from 70 to 100 nm. The particle size distribution is determined by means of dynamic light scattering at 25°C using a Beckmann Coulter Delsa Nano S particle size analyser.

Suitable water-dispersible urethane (meth)acrylates are generally obtainable from components

(a) to (d) and optionally (e):

(a) one or more di- or polyisocyanates;

(b) one or more (meth)acrylates having at least one isocyanate-reactive group;

(c) diols or polyols;

(d) one or more compounds having at least one ionic or hydrophilizing group and at least one isocyanate-reactive group;

(e) optionally further compounds having isocyanate-reactive groups.

They optionally also comprise

(f) low-molecular-weight poly(meth)acrylates having no isocyanate-reactive groups.

The isocyanate component (a) is preferably selected from

(a1) saturated aliphatic and saturated alicyclic diisocyanates,

(a2) polyisocyanates that are obtainable by oligomerization of a diisocyanate and have an average isocyanate functionality of at least 2.5, preferably a functionality within a range from 2.5 to 4, wherein the diisocyanate includes a saturated alicyclic structural unit,

(a3) polyisocyanates that are obtainable by oligomerization of a C2 to Cs alkylene diisocyanate and have an average isocyanate functionality of at least 2.5, preferably within a range from 2.5 to 6, and especially within a range from 2.8 to 4.0.

The diisocyanates (a1) are preferably saturated aliphatic and saturated alicyclic diisocyanates having 4 to 20 carbon atoms. Examples of such diisocyanates having 4 to 20 carbon atoms are aliphatic diisocyanates such as tetramethylene diisocyanate, pentamethylene 1 ,5-diisocyanate, hexamethylene diisocyanate (1,6-diisocyanatohexane), octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, tetradecamethylene diisocyanate, trimethylhexane diisocyanate or tetramethylhexane diisocyanate, cycloaliphatic diisocyanates such as 1,4-, 1,3- or 1,2-diisocyanatocyclohexane, 4,4'- or 2,4'-di(isocyanatocyclohexyl)methane, 1- isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane (isophorone diisocyanate), 1,3- or 1 ,4-bis(isocyanatomethyl)cyclohexane or 2,4- or 2, 6-diisocyanato-1 -methylcyclohexane, and also 3(or 4),8(or 9)-bis(isocyanatomethyl)tricyclo[5.2.1.0²’⁶]decane isomer mixtures.

Particular preference is given to hexamethylene diisocyanate, 1,3-bis(isocyanatomethyl)cyclo- hexane, isophorone diisocyanate, 4,4'-di(isocyanatocyclohexyl)methane, and 2,4'-di(isocyanato- cyclohexyl)methane and mixtures thereof, and very particular preference to isophorone diisocyanate, 4,4'-di(isocyanatocyclohexyl)methane, and hexamethylene diisocyanate. In a specific embodiment, the diisocyanate compounds (a1) are selected from saturated alicyclic diisocyanates and in particular from isophorone diisocyanate and bis(4-isocyanatocyclo- hexyl)methane and mixtures thereof.

The polyisocyanates (a2) are oligomers obtainable by oligomerization of diisocyanates having at least one, in particular just one, cycloaliphatic structural unit, e.g. 1 or 2 cyclohexane rings, and which have preferably 4 to 20 carbon atoms. Such diisocyanates are also referred to hereinafter as cycloaliphatic diisocyanates. Examples of cycloaliphatic diisocyanates are 1 ,4-, 1 ,3- or 1 ,2-diisocyanatocyclohexane, 4,4'- or 2,4'-di(isocyanatocyclohexyl)methane, 1-isocyanato-3,3,5- trimethyl-5-(isocyanatomethyl)cyclohexane (isophorone diisocyanate), 1 ,3- or 1 ,4-bis(isocy- anatomethyl)cyclohexane or 2,4-, or 2, 6-diisocyanato-1 -methylcyclohexane and also 3(or 4),8(or 9)-bis(isocyanatomethyl)tricyclo[5.2.1.0²’⁶]decane isomer mixtures.

Suitable oligomers (a2) are i. isocyanurates of the abovementioned cycloaliphatic diisocyanates, ii. polyisocyanates that have uretdione groups and are based on the abovementioned cycloaliphatic diisocyanates, iii. polyisocyanates that have biuret groups and are based on the abovementioned cycloaliphatic diisocyanates, iv. polyisocyanates that have urethane and/or allophanate groups and are based on the abovementioned cycloaliphatic diisocyanates, v. polyisocyanates that have oxadiazinetrione groups and are based on the abovementioned cycloaliphatic diisocyanates, vi. polyisocyanates that have iminooxadiazinetrione groups and are based on the abovementioned cycloaliphatic diisocyanates.

Preferred polyisocyanate compounds (a2) are selected from isocyanurates of cycloaliphatic diisocyanates, for example isocyanurates of isophorone diisocyanate, of

1.4-, 1 ,3- or 1 ,2-diisocyanatocyclohexane, of 1 ,3-, 1 ,4-bis(isocyanatomethyl)cyclohexane, of

1 .4-bis(isocyanatomethyl)cyclohexane, of 2, 4-diisocyanato-1 -methylcyclohexane or of 2,6-diiso- cyanato-1 -methylcyclohexane.

Such oligomers (a2) typically have a number-average molecular weight within a range from 400 to 1800 daltons, in particular within a range from 500 to 1600 daltons. The degree of oligomerization is typically within a range from 2.5 to 8, in particular within a range from 3 to 6.

The average isocyanate functionality of the oligomers (a2) is preferably within a range from 2.5 to 6 and in particular within a range from 2.8 to 4.5 and especially within a range from 2.8 to 4.0. Average isocyanate functionality is understood as meaning the average number of isocyanate groups in the oligomer (number average).

Preference is given to oligomers (a2) having an isocyanate equivalent weight within a range from 180 to 500 g/mol NCO, in particular within a range from 200 to 400 g/mol NCO. The diisocyanate content is preferably below 10% by weight based on oligomer (a2).

The polyisocyanate compounds (a2) are particularly preferably selected from isocyanurates of isophorone diisocyanate, in particular those having an average isocyanate functionality within a range from 2.5 to 6, in particular within a range from 2.8 to 4.5, and especially within a range from 2.8 to 4.0, and an isocyanate equivalent weight preferably within a range from 200 to 400 g/mol NCO.

The polyisocyanates (a3) are allophanates, isocyanurates, and uretdiones of a C2 to Cs alkylene diisocyanate, such as tetramethylene diisocyanate, 1 ,5-pentamethylene diisocyanate, hexamethylene diisocyanate (1 ,6-diisocyanatohexane), octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, tetradecamethylene diisocyanate, trimethylhexane diisocyanate or tetramethylhexane diisocyanate.

Such oligomers (a3) typically have a number-average molecular weight within a range from 400 to 1600 daltons, in particular within a range from 450 to 1400 daltons. The degree of oligomerization is typically within a range from 2.5 to 8, in particular within a range from 3 to 6.

Preference is given to oligomers (a3) having an isocyanate equivalent weight within a range from 120 to 400 g/mol NCO, in particular within a range from 150 to 300 g/mol NCO.

The diisocyanate content in oligomers (a3) is preferably below 10% by weight based on the total amount of oligomers (a3).

Preferred polyisocyanate compounds (a3) are selected from isocyanurates of a C4 to Cs alkylene diisocyanate, in particular isocyanurates of hexamethylene diisocyanate. The polyisocyanate (a3) is particularly preferably selected from an isocyanurate of hexamethylene diisocyanate having an average isocyanate functionality within a range from 2.5 to 6, in particular within a range from 2.8 to 4.5, and especially within a range from 2.8 to 4.0, and preferably having an isocyanate equivalent weight within a range from 150 to 300 g/mol NCO.

As a general rule, the isocyanate component (a) essentially contains no isocyanate components different from the abovementioned isocyanates (a1), (a2), and (a3), i.e. the proportion of isocyanate compounds different from the abovementioned isocyanates typically does not exceed 5% by weight, in particular 1 % by weight, based on the total weight of isocyanate component (a). Component (b) comprises at least one compound having at least one, in particular just one, eth- ylenically unsaturated double bond in the form of an acrylate or methacrylate group and also at least one, in particular just one, functional group F that reacts with an isocyanate group to form a covalent bond.

The functional group F is preferably selected from -OH, -NH2, and -NRH, where R is a saturated or unsaturated aliphatic hydrocarbon radical having 1 to 10 carbon atoms. The functional group F is particularly preferably an -OH group.

Preferred components (b) are selected from hydroxy C2 to Cs alkyl esters of acrylic acid, hydroxy C2 to Cs alkyl esters of methacrylic acid, diesters of C3 to Cs alkanetriols with acrylic acid and diesters of C3 to Cs alkanetriols with methacrylic acid, component (b) being selected in particular from hydroxy C2 to C4 alkyl esters of acrylic acid. Compound (b) is especially 2-hydroxy- ethyl acrylate.

Component (c) can be either (c1) nonionic low-molecular-weight diols or polyols having a molecular weight of < 350 g/mol or (c2) nonionic polymeric diols or polyols having a number-average molecular weight of > 350 g/mol.

Suitable as component (c1) are aliphatic and cycloaliphatic diols. Examples of aliphatic diols are ethylene glycol, propane-1 , 2-diol, propane-1 , 3-diol, butane-1 ,2-diol, butane-1 ,3-diol, butane-1 ,4- diol, butane-2, 3-diol, pentane-1 , 2-diol, pentane-1 , 3-diol, pentane-1 ,4-diol, pentane-1 ,5-diol, pen- tane-2, 3-diol, pentane-2,4-diol, hexane-1 , 2-diol, hexane-1 , 3-diol, hexane-1 ,4-diol, hexane-1 , 5- diol, hexane-1 , 6-diol, hexane-2,5-diol, heptane-1 , 2-diol, heptane-1 ,7-diol, octane-1 , 8-diol, oc- tane-1 , 2-diol, nonane-1 ,9-diol, decane-1 , 2-diol, decane-1 , 10-diol, dodecane-1 , 2-diol, dodecane- 1 ,12-diol, 1 ,5-hexadiene-3,4-diol, neopentyl glycol (2, 2-dimethylpropane-1 , 3-diol), 2,2-dieth- ylpropane-1 , 3-diol, 2-methyl-2-ethylpropane-1 , 3-diol, 2-methylpentane-2,4-diol, 2,4-dime- thylpentane-2,4-diol, 2-ethylhexane-1 , 3-diol, 2,5-dimethylhexane-2,5-diol, 2,2,4-trimethylpen- tane-1 , 3-diol, pinacol, diethylene glycol, triethylene glycol, dipropylene glycol, and tripropylene glycol.

Examples of cycloaliphatic diols are cyclopentane-1 , 2-diol and -1 , 3-diol, cyclohexane-1 , 2-diol, - 1 , 3-diol, and -1 ,4-diol, 1 ,1-, 1 ,2-, 1 ,3-, and 1 ,4-bis(hydroxymethyl)cyclohexanes, 1 ,1-, 1 ,2-, 1 ,3-, and 1 ,4-bis(hydroxyethyl)cyclohexanes, and bis(4-hydroxycyclohexane)isopropylidene. Preference among these is given to cyclohexane-1 , 2-diol, -1 , 3-diol, and -1 ,4-diol, 1 ,3-bis(hydroxyme- thyl)cyclohexane, 1 ,4-bis(hydroxymethyl)cyclohexane, bis(4-hydroxycyclohexyl)methane, and bis(4-hydroxycyclohexane)isopropylidene.

Suitable polymeric polyol components (c2) are polyesterols, polyetherols and polycarbonate polyols. So as to achieve a minimal tendency to yellowing, the polymeric polyol component (c2) preferably contains no aromatic groups. The polymeric polyol component (c2) preferably has a number-average molecular weight within a range from 350 to 10 000 g/mol, in particular within a range from 400 to 8000 g/mol, especially within a range from 450 to 5000 g/mol.

In particular, the polymeric polyol component (c2) comprises at least one aliphatic polyester polyol. Preferred polyesterols are those based on aliphatic or cycloaliphatic dicarboxylic acids with aliphatic diols. Preferred polyester polyols have OH values, determined in accordance with DIN 53240-2:2007-11 , within a range from 5 to 220, in particular within a range from 10 to 200, mg KOH/g. The acid value is preferably below 20 mg KOH/g, in particular below 10 mg KOH/g.

Suitable aliphatic diols for preparation of the polyester polyols have preferably 2 to 40 carbon atoms, in particular 4 to 10 carbon atoms. Examples of aliphatic diols are ethylene glycol, pro- pane-1 , 2-diol, propane-1 , 3-diol, butane-1 , 2-diol, butane-1 ,3-diol, butane-1 ,4-diol, butane-2,3- diol, pentane-1 , 2-diol, pentane-1 , 3-diol, pentane-1 ,4-diol, pentane-1 ,5-diol, pentane-2, 3-diol, pentane-2,4-diol, hexane-1 , 2-diol, hexane-1 , 3-diol, hexane-1 ,4-diol, hexane-1 ,5-diol, hexane-

1.6-diol, hexane-2,5-diol, heptane-1 , 2-diol, heptane-1 ,7-diol, octane-1 , 8-diol, octane-1 , 2-diol, nonane-1 ,9-diol, decane-1 , 2-diol, decane-1 , 10-diol, dodecane-1 , 2-diol, dodecane-1 ,12-diol, 1 ,5- hexadiene-3,4-diol, neopentyl glycol (2, 2-dimethylpropane-1 , 3-diol), 2, 2-diethylpropane-1 , 3-diol, 2-methyl-2- ethylpropane-1 , 3-diol, 2-methylpentane-2,4-diol, 2,4-dimethylpentane-2,4-diol, 2-ethylhexane- 1 , 3-diol, 2,5-dimethylhexane-2,5-diol, 2,2,4-trimethylpentane-1 ,3-diol, pinacol, diethylene glycol, triethylene glycol, dipropylene glycol, and tripropylene glycol. Also suitable are higher polyethylene glycols HO(CH2CH2O)n-H, higher polypropylene glycols HO(CH[CH3]CH2O)n-H, where n is an integer and n > 4, e.g. 4 to 20, polyethylene polypropylene glycols, in particular those having 4 to 20 repeat units, wherein the sequence of the ethylene oxide and propylene oxide units may be blockwise or random, polytetramethylene glycols, in particular those having 4 to 20 repeat units, and polypropane-1 ,3-diols, in particular those having 4 to 20 repeat units.

Suitable cycloaliphatic diols for preparation of the polyester polyols are cyclopentane- 1 , 2-diol and -1 , 3-diol, cyclohexane-1 , 2-diol, -1 , 3-diol, and -1 ,4-diol, 1 ,1-, 1 ,2-, 1 ,3-, and 1 ,4-bis(hy- droxymethyl)cyclohexanes, 1 ,1-, 1 ,2-, 1 ,3-, and 1 ,4-bis(hydroxyethyl)cyclohexanes, bis(4-hy- droxycyclohexane)methane, and bis(4-hydroxycyclohexane)isopropylidene. Preference among these is given to cyclohexane-1 , 2-diol, -1 , 3-diol, and -1 ,4-diol, 1 ,3-bis(hydroxymethyl)cyclohex- ane and 1 ,4-bis(hydroxymethyl)cyclohexane, and bis(4-hydroxycyclohexane)methane.

Preferred diols for preparation of the polyester polyols are acyclic aliphatic diols having 2 to 8 carbon atoms, for example ethylene glycol, propane-1 , 2-diol, propane-1 , 3-diol, 2,2-di- methylethane-1 , 2-diol, 2, 2-dimethylpropane-1 , 3-diol (neopentyl glycol), 2,2-diethylpropane-1 ,3- diol, 2-methyl-2-ethylpropane-1 , 3-diol, butane-1 , 2-diol, butane-1 , 3-diol, butane-1 , 4-diol, hexane-

1 .6-diol or diethylene glycol. Particularly preferred diols for preparation of the polyester polyols are ethylene glycol, propane-1 , 2-diol, propane-1 , 3-diol, neopentyl glycol, butane-1 , 4-diol, and diethylene glycol. The dicarboxylic acid structural units used for preparation of the polyester polyols may be the free acids or ester-forming derivatives thereof. Derivatives are preferably understood as meaning the relevant anhydrides, monoalkyl esters, and dialkyl esters, preferably monoalkyl esters and di-Ci to C4 alkyl esters, particularly preferably monomethyl and dimethyl esters and the corresponding monoethyl and diethyl esters, also monovinyl and divinyl esters and also mixed esters for example having different Ci to C4 alkyl components, especially mixed methyl ethyl esters.

In the context of this document, Ci to C4 alkyl means methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl, sec-butyl and tert-butyl, preferably methyl, ethyl and n-butyl, more preferably methyl and ethyl, and most preferably methyl.

Suitable dicarboxylic acids that may be used for preparation of the polyester polyols are in particular aliphatic dicarboxylic acids/alkanedicarboxylic acids having 2 to 12 carbon atoms and cycloaliphatic dicarboxylic acids/cycloalkanedicarboxylic acids having 7 to 14 carbon atoms.

Examples of aliphatic dicarboxylic acids that may be used for preparation of the polyester polyols are oxalic acid, malonic acid, maleic acid, fumaric acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecane-a, co-dicarboxylic acid, do- decane-a, co-dicarboxylic acid, and derivatives thereof. Examples of cycloaliphatic dicarboxylic acids are cis- and trans-cyclohexane-1 ,2-dicarboxylic acid (hexahydrophthalic acids), cis- and trans-cyclohexane-1 ,3-dicarboxylic acid, cis- and trans-cyclohexane-1 ,4-dicarboxylic acid, 1 ,2-, 1 ,3- or 1 ,4-cyclohex-4-enedicarboxylic acid (tetrahydrophthalic acids), cis- and trans-cyclopen- tane-1 ,2-dicarboxylic acid, cis- and trans-cyclopentane-1 ,3-dicarboxylic acid, and derivatives thereof.

Preferred dicarboxylic acids are saturated aliphatic dicarboxylic acids, in particular those having 3 to 10 carbon atoms such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecane-a, co-dicarboxylic acid, and dodecane-a, co-dicarboxylic acid.

Also suitable as polyesterols are lactone-based polyesterols, in particular those based on an aliphatic lactone having 4 to 8 carbon atoms, for example butyrolactone, valerolactone or caprolactone.

Component (c2) is preferably selected from aliphatic polyesters, in particular from aliphatic polyester polyols formed from at least one C3 to C12 alkanedicarboxylic acid and at least one C3 to C10 alkanediol. In a particularly preferred embodiment of the invention, component (c2) is a polyester polyol that is formed from adipic acid and neopentyl glycol and in particular has an OH value within a range from 20 to 200 mg KOH/g. In particular, the aliphatic polyester preferably has in this embodiment a number-average molecular weight within a range from 450 to 5000 g/mol. Particularly preferably, the aliphatic polyester has in this embodiment a number-average molecular weight within a range from 480 to 2500 g/mol, more preferably within a range from 500 to 2000 g/mol.

Compounds (d) have at least one, e.g. one or two, ionic or ionizable groups I' and at least one, e.g. one or two, functional groups F’ that react with an isocyanate group to form a covalent bond.

The functional group F' is preferably selected from -OH, -NH2, and -NR'H, where R' is a saturated or unsaturated hydrocarbon radical having 1 to 10 carbon atoms. In particular, the functional group F' is an OH group.

The ionic or ionizable group I' is preferably selected from acid groups in the acid or salt form, in particular from carboxyl groups. Examples of acid groups are -COOH, -SO3H or -PO3H. The acid groups may be in their anionic forms and accordingly have a counterion. Examples of counterions are alkali metal and alkaline earth metal ions, e.g. Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺ or Ba²⁺. It is additionally possible for the counterion present to be ammonium ions or quaternary ammonium ions derived from ammonia or amines, especially tertiary amines, for example ammonium, methylammonium, dimethylammonium, trimethylammonium, ethylammonium, diethylammonium, triethylammonium, tributylammonium, diisopropylethylammonium, benzyldimethylammonium, monoethanolammonium, diethanolammonium, triethanolammonium, hydroxyethyldimethylammonium, hydroxyethyldiethylammonium, monopropanolammonium, dipropanolammonium, tripropanolammonium, piperidinium, piperazinium, N,N'-dimethylpiperazinium, mor- pholinium, pyridinium, tetramethylammonium, triethylmethylammonium, 2-hydroxyethyltrime- thylammonium, bis(2-hydroxyethyl)dimethylammonium or tris(2-hydroxyethyl)methylammonium.

Examples of compounds (d) are those of the formula

(F')n-R-I' and salts thereof, where n is 1 , 2 or 3 and in particular 2;

F' is as defined above and is in particular OH;

I' is an ionic or ionizable group, in particular an acid group, and especially a carboxyl group;

R is a saturated aliphatic or alicyclic hydrocarbon radical having 2 to 20 carbon atoms.

Particularly preferred compounds (d) are selected from aliphatic dihydroxycarboxylic acids, in particular those having 3 to 10 carbon atoms and salts thereof. Examples of such aliphatic dihydroxycarboxylic acids are 2,3-dihydroxypropanoic acid, 2,2-dimethylolpropionic acid, 2,2-dime- thylolbutyric acid, and 2,2-dimethylolpentanoic acid, preference being given to 2,2-dime- thylolpropionic acid and dimethylolbutyric acid. The further compounds (e) can be poly(C2 to C3 alkylene ether) compounds having an average OH functionality within a range from 0.9 to 1.2. This is understood as meaning polyethylene glycol ethers, polypropylene glycol ethers, and polyethylene glycol-co-propylene glycol ethers having an average of 0.9 to 1.2, in particular 1 , hydroxyl group in the molecule. Through compound (e), not only is better dispersibility and stability in the aqueous polymer composition achieved. Compound (e) also results in the dispersion drying more rapidly and exhibiting better film-formation.

The poly(C2 to C3 alkylene ether) compounds generally have a number-average molecular weight within a range from 250 to 2500 g/mol.

Preferred compounds (e) are Ci- to C4-alkyl polyethylene glycols, in particular methyl polyethylene glycols, having a number-average molecular weight within a range from 250 to 2500 g/mol.

In addition to urethane (meth)acrylates, the paint formulation may comprise, as UV-curable paint constituents, low-molecular-weight poly(meth)acrylates of low-molecular-weight diols or polyols (low-molecular-weight diols or polyols being ones having a molecular weight of

< 350 g/mol). These low-molecular-weight poly(meth)acrylates (f) are dispersible in water (maximum water solubility 10 g/L) and have no isocyanate-reactive groups. Preferred poly(meth)acry- lates (f) are selected from diacrylates of C2 to C10 alkanediols, triacrylates of C3 to C10 alkanetriols, tetraacrylates of C4 to C10 alkanetetraols, tetraacrylates of bis(dihydroxy-C3 to C10 alkyl) ethers, hexaacrylates of bis(trihydroxy-C4 to C10 alkyl) ethers, and the corresponding methacrylates.

Particularly preferred low-molecular-weight poly(meth)acrylates (f) are selected from dipropylene glycol diacrylate, tripropylene glycol diacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol tetraacrylate, bis(dimethylolpropane) tetraacrylate, and bis(pentaerythritol) hexaacrylate.

In particular embodiments of the invention, the paint formulation comprises low-molecular- weight poly(meth)acrylates (f) of this type. In this case, their content is generally 5% to 50% by weight, preferably 5% to 30% by weight, based on the sum of urethane (meth)acrylates and poly(meth)acrylates (f). Aside from this, the paint formulation essentially comprises no further UV-curable paint constituents, i.e. at least 90% by weight, preferably at least 95% by weight, and especially 100% by weight, of the UV-curable paint constituents is urethane (meth)acrylates and poly(meth)acrylates (f).

In further embodiments of the invention, the paint formulation does not comprise any low-molec- ular-weight poly(meth)acrylates (f). In this case, at least 90% by weight, preferably at least 95% by weight, and in particular 100% by weight, of the UV-curable paint constituents is urethane (meth) acrylates.

The paint formulations may comprise further auxiliaries and additives customary in coatings, such as leveling agents, defoamers, UV absorbers, dyes, pigments, matting agents and/or fillers.

In one embodiment of the invention, the water-based paint formulation is a clearcoat formulation and only curing step (iii) is carried out. The clearcoat formulations preferably comprise no photoinitiators at all.

In a further embodiment of the invention, the water-based paint formulation is a pigmented paint formulation and curing steps (iii) and (iv) are carried out. In this case, the paint formulation comprises at least one photoinitiator activatable through UV radiation having a wavelength of > 350 nm.

Curing step (iv) can be carried out by irradiating with UV LEDs in the 350 nm to 420 nm range, for example at 365 nm, 385 nm, 395 nm or 405 nm, or with a gas-discharge lamp, for example an undoped mercury lamp or with a doped mercury lamp having gallium or indium doping, iron doping or lead doping or having another suitable doping. In a preferred embodiment, it is carried out with UV LEDs, in a particularly preferred embodiment with UV LEDs at 365 nm.

Photoinitiators activatable by UV radiation having a wavelength of > 350 nm generally have an absorption band with a maximum in the 360 to 420 nm range. These include in particular acylphosphine oxides such as (2,4,6-trimethylbenzoyl)diphenylphosphine oxide, [ethoxy(phe- nyl)phosphoryl]-(2,4,6-trimethylphenyl)methanone, and bis(2,4,6-trimethylbenzoyl)phe- nylphosphine oxide, also bisacylphosphine oxides and acylphosphine sulfides. Clearcoats of the invention do not comprise these photoinitiators.

Depending on the desired color impression, the pigments may be inorganic pigments, for example aluminum oxide, iron pigments such as iron(lll) oxide, chromium(lll) oxide, titanium(IV) oxide, zirconium(IV) oxide, zinc oxide, zinc sulfide, zinc phosphate, mixed metal oxide phosphates, molybdenum sulfide, cadmium sulfide, graphite, vanadates such as bismuth vanadate, chromates such as lead(IV) chromates, molybdates such as lead(IV) molybdate, and mixtures thereof, or organic pigments. Examples of organic pigments are color pigments and pearlescent pigments such as azo pigments, disazo pigments, naphthol pigments, benzimidazolone pigments, azo condensation pigments, metal complex pigments, isoindolinone pigments, quinoph- thalone pigments and dioxazine pigments, polycyclic pigments such as indigo, thioindigo, quina- cridone, phthalocyanines, perylenes, perinones, anthraquinones, for example aminoanthraquinones or hydroxyanthraquinones, anthrapyrimidines, indanthrones, flavanthrones, py- ranthrones, anthanthrones, isoviolanthrones, diketopyrrolopyrroles and also carbazoles, for example carbazole violet, and the like. Further examples of organic pigments can be found in the following monograph: W. Herbst, K. Hunger “Industrielle Organische Pigmente” [Industrial organic pigments], 2nd edition, 1995, VCH Verlagsgesellschaft, ISBN: 3-527-28744-2.

Suitable fillers and matting agents include silicates, for example silicates obtainable by hydrolysis of silicon tetrachloride such as Aerosil® from Degussa, siliceous earth, talc, aluminum silicates, magnesium silicates, calcium carbonates, etc. Matting agents are in particular untreated or treated silicas, in particular precipitated and fumed silicas, for example Acematt® types 3300, 3600, OK 412, OK500, OK607, and TS100 from Evonik Industries.

Suitable stabilizers include typical UV absorbers such as oxanilides, triazines, and benzotriazole (the latter obtainable as Tinuvin® products from BASF SE) and benzophenones. These may be used alone or together with suitable free-radical scavengers, for example sterically hindered amines such as 2,2,6,6-tetramethylpiperidine, 2,6-di-tert-butylpiperidine or derivatives thereof, for example bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate. Stabilizers are typically used in amounts of 0.1% to 5.0% by weight based on the “solid” components present in the preparation.

In particular embodiments of the invention, the paint formulation is a pigment-containing paint formulation that, in addition to the polyurethane (meth)acrylate, comprises a pigment, preferably at least one inorganic pigment. The pigment content is here preferably within a range from 1% to 50% by weight based on the composition. The mass ratio of pigment to polyurethane (meth)acrylate is preferably within a range from 1 :20 to 1:1 and in particular within a range from 1 :10 to 1 :5.

The aqueous paint formulations are preferably paint formulations for the coating of wood, paper, textile, leather, nonwoven, plastics surfaces, glass, ceramic, mineral building materials, such as cement moldings and fiber-cement panels, and of coated or uncoated metals.

Application of the aqueous paint formulation to the substrate to be coated may be carried out in a known manner, by spraying, troweling, knifecoating, brushing, rolling, roller coating or pouring. It is likewise possible to apply the paint formulations to a substrate by means of a printing process such as offset printing, flexographic printing, intaglio printing, screen printing or inkjet printing, or by means of a similar method. The coverage is generally within a range from 1 to 250 g/m², preferably within a range from 10 to 150 g/m², and more preferably within a range from 25 to 125 g/m², based on the solids content of the aqueous paint formulation. In the case of offset printing, it is also possible for the amount applied to be < 1 g/m².

The aqueous paint formulations are in particular paint formulations for the coating of metals, plastics surfaces such as PVC or melamine, cement or concrete surfaces, for example of concrete roof tiles and fiber cement panels, and also for the coating of wood and woodbase materials, and also of paper, for example furniture decals. The aqueous paint formulations and the method of the invention are suitable in particular for the coating of wood and woodbase materials and wood-containing substrates, such as fiberboard, and also for the coating of substrates containing cellulose fibers, for example paper, paperboard or cardboard. Wood substrates can for example be pure-wood substrates, for example oak, spruce, pine, beech, maple, walnut, makore, chestnut, plane, robinia, ash, birch, pine, and elm, pure-wood woodbase materials, such as cross-laminated timber, block plywood, glued laminated timber or similar, laminated plywood, veneer wood materials such as veneer plywood, laminated veneer lumber, veneer strip lumber, flexible plywood, or else chipboard materials such as MDF panels, HDF panels or OSB panels, and also cork.

The invention is illustrated by the examples hereinbelow.

Examples

Test methods:

Scratch resistance:

Scotch-Brite SLIFN nonwoven; 500 g weight

The Scotch-Brite SLIFN nonwoven is used to scratch across the paint surface with the appropriate weight. 20 double strokes (movement back and forth) are executed and the gloss of the coating before and after scratching is determined at a 60° measurement angle.

Gloss 60°: BYK Gardner Micro-Tri-Gloss

Substrate: White primed MDF board

Film thickness: 200 pm from box-type doctor blade

Drying: 20 min at 50°C

Double-bond conversion in clearcoat:

Raman microscope (A = 523 nm, 100x lens, 30 s) Aliphatic (2925-2945 cm’¹) vs. C=C (1631-1641) Reference: Liquid paint

Film thickness: 200 pm from box-type doctor blade Drying: 20 min at 50°C

Chemical resistance:

The chemical resistance was determined on cured films on white MDF panels. The chemical resistance toward various substances was determined by the IKEA R2 method. 24 h water

24 h fat on scratch (3 N)

1 h coffee

Substrate: White primed MDF board

Film thickness: 200 pm from box-type doctor blade Drying: 20 min at 50°C

Curing conditions according to standard method:

1ST type E-60-2*1-MBS-5-L-ELC

Distance from substrate: 7.5 cm

Clearcoat:

Belt speed 10 m/min

Medium-pressure Hg lamp

Pigmented paint:

Belt speed 10 m/min

Medium-pressure Ga/Hg lamp

Curing according to the invention: Belt speed 10 m/min

Distance from substrate: 3 cm

Clearcoat:

Lamp: IIV-C lamp from 1ST Metz GmbH with special glass for high transmissions up to 250 nm, UVC-optimized through appropriate current/voltage parameters

Reflectors: Optimized reflection in UVC

IIV-C dose: 1150 mJ/cm²

Pigmented paint:

IIV-C lamp from 1ST Metz GmbH

IIV-C dose: 1150 mJ/cm² combined with 22 W 365 nm LED

In the experiments, water-based UV paints based on two UV-curable polyurethane dispersions (PU dispersions) were formulated. Clearcoats (examples 1 and 2) as well as pigmented paints (examples 3 and 4) were produced.

For the standard curing method, the paint formulations comprised a photoinitiator (Omnirad 184 in the clearcoats and additionally Omnirad 819 DW in the pigmented paints).

For curing according to the invention, the clearcoat formulations did not comprise a photoinitiator and the pigmented paint formulations comprised solely Omnirad 819 DW (deep-curing initiator) for deep curing with the additional LED light source.

Table 1 : Formulation of UV paint for experiments with standard curing for 100 g paint with photoinitiator

100 100

² Laromer UA 9135 Aqua is a UV-curable, water-based PU dispersion comprising the reaction product from HDI, IPDI, polyesterol from adipic acid and neopentyl glycol, polyetherol, dimethylolpropionic acid, and 2-hydroxyacrylate and also low-molecular-weight poly(meth)acrylates based on TMP, glycerol, pentaerythritol, and ethoxylates or propoxylates thereof.

Table 2: Properties of the clearcoat cured according to the standard method

The pigmented formulations according to example 2 were cured under the same conditions. Unlike with the clearcoat according to example 1 , a medium-pressure Ga lamp was additionally used for deep curing of the pigmented paint film (Table 3).

Table 3: Properties of the pigmented paint cured according to the standard method

Curing: Hg lamp 200 W/cm combined with

Ga lamp 200 W/cm

For comparison, a formulation corresponding to example 1 , but without a photoinitiator, under- went curing.

Table 4: Clearcoat formulations for experiments with standard curing for 100 g paint without photoinitiator

100 Table 5: Properties of the clearcoat cured according to the standard method

The results with and without photoinitiator show clear differences both in the scratch resistance and in the double-bond conversion of the cured films. This demonstrates the need to use a photoinitiator with standard curing. Experiments in accordance with the invention:

The experiments in accordance with the invention used a newly developed UV-C-optimized medium-pressure mercury lamp with improved output in the IIV-C range.

The new vacuum IIV-C lamp from 1ST Metz GmbH was used without an inert gas atmosphere. The spectral output of the new 1ST lamp at a power consumption of 240 W/cm is shown below:

Distance of lamp housing from transport belt: 3 cm.

The clearcoat formulation cured with the new IIV-C lamp does not comprise a photoinitiator (Ta- ble 4). The cured clearcoat film exhibited properties (Table 6) comparable to those of the standard-cured clearcoat film comprising a photoinitiator (example 1).

Table 6: Properties of the clearcoat cured according to the method of the invention (UV- C-optimized medium-pressure Hg lamp from 1ST Metz GmbH)

Pigmented paint formulations corresponding to examples 5 and 6 were produced without a surface initiator (Omnirad 184), using only the deep-curing initiator Omnirad 819 DW (Table 7). Curing was effected using the UV-C-optimized medium-pressure Hg lamp from 1ST for surface curing in combination with a 22 W 365 nm LED lamp for deep curing (belt speed 10 m/min).

Table 7: Pigmented paint formulation for curing experiments in accordance with the invention for 100 g paint with only a deep-curing photoinitiator (Omnirad 819 DW)

100

Table 8: Properties of the pigmented paint cured according to the method of the invention (UV-C-optimized medium-pressure Hg lamp from 1ST Metz GmbH in combination with 22 W 365 nm LED lamp)

The experiments demonstrate that, with the UV-C-optimized medium-pressure mercury lamp, it is possible to cure water-based UV paints comprising aqueous PU dispersions without using a surface initiator and in the presence of atmospheric oxygen without loss of scratch resistance or chemical resistance. This achieves a double-bond conversion that is at least comparable to that achieved with standard curing using a surface initiator.

With the new method it is possible to comply with the restrictions on photoinitiators specified in IKEA IOS-MAT 0066.

Claims

23 Claims

1 . A method for producing a paint coating on a substrate by UV curing of a water-based paint formulation comprising an aqueous dispersion of urethane (meth)acrylates, said method comprising the steps in the sequence (i) to (iv):

(i) applying the paint formulation comprising the aqueous dispersion of urethane (meth)acrylates to the substrate;

(ii) drying the paint formulation;

(iii) curing the dried paint formulation by irradiating with IIV-C radiation in the 180 to 280 nm wavelength range, affording an at least partially cured paint film;

2. The method according to claim 1 , wherein the power consumption of the mercury lamp per cm arc length is > 150 W/cm, preferably > 180 W/cm, and the conversion efficiency for UV-C radiation in the 180 to 240 nm wavelength range is at least 13% based on the power consumption.

3. The method according to claim 2, wherein the conversion efficiency for UV-C radiation in the 180 to 240 nm wavelength range is at least 15 % and the power consumption is from 200 to 500 W/cm.

4. The method according to any one of claims claim 1 to 3, wherein curing step (iii) is carried out in air.

5. The method according to any of claims 1 to 4, wherein the paint formulation does not comprise any photoinitiators activatable by UV radiation having a wavelength of < 350 nm.

6. The method according to any of claims 1 to 5, wherein the urethane (meth)acrylates are obtainable from components (a) to (d) and optionally (e), and optionally additionally include component (f):

(a) one or more di- or polyisocyanates;

(b) one or more (meth)acrylates having at least one isocyanate-reactive group;

(c) nonionic diols or polyols;

(e) optionally further compounds having isocyanate-reactive groups; (f) optionally low-molecular-weight poly(meth)acrylates having no isocyanate-reactive groups.

7. The method according to claims 1 to 6, wherein the aqueous dispersion of urethane (meth)acrylates has a particle size distribution characterized by an average particle size within a range from 20 to 140 nm.

8. The method according to claim 7, wherein component (c) is selected from (c1) nonionic low-molecular-weight diols or polyols having a molecular weight of < 350 g/mol and (c2) nonionic polymeric diols or polyols having a number-average molecular weight of

> 350 g/mol.

9. The method according to any of claims 6 to 8, wherein the water-based paint formulation, besides the polyurethane (meth)acrylates obtainable from (a) to (d) and optionally (e), additionally comprises low-molecular-weight poly(meth)acrylates (f) from diols or polyols.

10. The method according to any of claims 1 to 9, wherein at least 90% by weight of the UV- curable paint constituents of the water-based paint formulation is urethane (methacrylates and optionally low-molecular-weight poly(meth)acrylates.

11. The method according to any of claims 1 to 10, wherein the paint formulation is a clearcoat formulation and only curing step (iii) is carried out.

12. The method according to any of claims 1 to 10, wherein the paint formulation is a pigmented paint formulation and curing steps (iii) and (iv) are carried out.

13. The method according to claim 12, wherein the paint formulation comprises at least one photoinitiator activatable through UV radiation having a wavelength of > 350 nm.

14. The method according to either of claims 12 or 13, wherein curing step (iv) is carried out by irradiating with UV LEDs in the 350 nm to 420 nm range or with a doped mercury lamp doped with gallium, indium, iron or lead or with a combination thereof, or with an undoped mercury lamp.

15. The method according to any of claims 12 to 14, wherein curing step (iv) is carried out by irradiating with UV LEDs at 365 nm, 385 nm, 395 nm or 405 nm.

16. The method according to any of claims 1 to 15, wherein the mercury lamp used in curing step (iii) is an undoped medium-pressure mercury lamp.

17. The method according to any of claims 1 to 16, wherein the substrate is a wood substrate. The method according to claim 17, wherein the wood substrate is selected from pure- wood substrates, such as oak, spruce, pine, beech, maple, walnut, makore, chestnut, plane, robinia, ash, birch, pine, and elm, pure-wood woodbase materials, such as crosslaminated timber, block plywood, glued laminated timber, and laminated plywood, veneer wood materials such as veneer plywood, laminated veneer lumber, veneer strip lumber, and flexible plywood, chipboard materials such as MDF panels, HDF panels and OSB panels, and also cork. A UV-curable, water-based paint formulation comprising an aqueous dispersion of urethane (meth)acrylates, optionally low-molecular-weight poly(meth)acrylates having no iso- cyanate-reactive group, and optionally pigments, and also optionally further customary auxiliaries and additives, wherein the total content in the paint formulation of photoinitiators activatable by UV radiation having a wavelength of < 350 nm is < 0.01% by weight based on the solids content of the paint formulation. The UV-curable, water-based paint formulation according to claim 19, wherein at least 90% by weight of the UV-curable paint constituents of the water-based paint formulation is urethane (meth)acrylates and optionally low-molecular-weight poly(meth)acrylates having no isocyanate-reactive group.