JP7777906B2 - Water-based paint composition for wood - Google Patents

Description

The present invention relates to an aqueous wood coating composition that has excellent pigment sedimentation stability and filling properties, as well as excellent water resistance and other performance characteristics.

When using wood materials such as plywood, various coatings are applied as needed depending on the application. Until now, hardwood has been the mainstream wood material, but due to environmental protection and a decrease in the supply of hardwood, softwood materials are increasingly being used.

However, coniferous wood has many fine irregularities on its surface compared to hardwood, making it difficult to achieve a sufficient smoothness with conventional sealer compositions. Furthermore, coniferous wood often has knot holes, which are defects inherent to the coniferous tree, and these defects are often filled with putty, sealant, etc. in addition to painting.

For example, Patent Document 1 discloses a curable composition suitable for applications such as sealers, which contains (A) a synthetic resin emulsion, (B) a highly water-absorbent resin, (C) an extender pigment, and (D) a polyisocyanate compound.

Japanese Patent Application Laid-Open No. 2020-63403

However, when the curable composition of Patent Document 1 is used as a putty to fill the defects, volume shrinkage during drying and hardening can result in insufficient filling of the repaired area, and the resulting wood material may have insufficient performance, such as water resistance.

The problem that this invention aims to solve is to provide an aqueous wood coating composition that exhibits pigment settling stability, almost no volume shrinkage upon heat curing, excellent filling properties, and excellent water resistance and other performance characteristics.

As a result of extensive research into resolving the above-mentioned problems, the inventors discovered that the above-mentioned problems could be resolved by a composition containing acrylic resin particles having a specific glass transition temperature, a surfactant having a specific triblock structure, a polyisocyanate compound, and a pigment, leading to the completion of the present invention.

That is, the present invention provides:
1. An aqueous wood coating composition containing acrylic resin particles (A) having a glass transition temperature of 20°C or higher, a surfactant (B) having a weight-average molecular weight of 2,400 or higher and a triblock structure consisting of a polyoxypropylene group and two polyoxyethylene groups sandwiching it, a polyisocyanate compound (C), and a pigment (D), wherein the pigment (D) accounts for 600% by mass or more of the total solid content of components (A), (B), and (C);
2. The aqueous wood coating composition according to item 1, wherein the acrylic resin particles (A) are crosslinked acrylic resin particles.
3. The aqueous wood coating composition according to item 1 or 2, wherein the acrylic resin particles (A) are core-shell type acrylic resin particles.
4. The aqueous wood coating composition according to any one of items 1 to 3, wherein the weight-average molecular weight of the polyoxypropylene group of the surfactant (B) is 1,450 or more.
5. The aqueous wood coating composition according to any one of items 1 to 4, wherein the surfactant (B) has a polyoxyethylene group content in the molecule of 20% by mass or more.
6. The aqueous wood coating composition according to any one of items 1 to 5, wherein the polyisocyanate compound (C) is an alicyclic polyisocyanate and/or a derivative of an alicyclic polyisocyanate.
7. The aqueous wood coating composition according to any one of items 1 to 6, wherein the polyisocyanate compound (C) is a nonionic group-containing polyisocyanate compound.

The coating composition of the present invention can provide an aqueous coating composition that exhibits almost no volumetric shrinkage upon curing, excellent application properties such as filling and workability, excellent pigment settling stability, and excellent coating film performance such as water resistance.

The present invention relates to an aqueous wood coating composition (hereinafter sometimes simply referred to as the present coating composition) that contains acrylic resin particles (A) with a glass transition temperature of 20°C or higher, a surfactant (B) with a weight-average molecular weight of 2,400 or higher and a triblock structure consisting of a polyoxypropylene group sandwiched between two polyoxyethylene groups, a polyisocyanate compound (C), and a pigment (D), wherein the pigment (D) accounts for 600% by mass or more of the total solids content of components (A), (B), and (C).

The present invention will be described in detail below.
<Coating composition>
Acrylic resin particles (A)
The acrylic resin particles (A) are characterized by having a glass transition temperature of 20° C. or higher.

Acrylic resin particles (A) are typically obtained by copolymerizing a polymerizable unsaturated monomer. The polymerizable unsaturated monomer is a monomer having a polymerizable unsaturated group.

In this specification, a polymerizable unsaturated group refers to an unsaturated group that can undergo radical polymerization. Examples of polymerizable unsaturated groups include vinyl groups, vinylidene groups, acryloyl groups, and methacryloyl groups.

In this specification, "(meth)acrylate" means "acrylate or methacrylate." "(meth)acrylic acid" means "acrylic acid or methacrylic acid." "(meth)acryloyl" means "acryloyl or methacryloyl." "(meth)acrylamide" means "acrylamide or methacrylamide."

Examples of polymerizable unsaturated monomers include polymerizable unsaturated monomers having one polymerizable unsaturated group per molecule and polymerizable unsaturated monomers having two or more polymerizable unsaturated groups per molecule.

Examples of polymerizable unsaturated monomers having one polymerizable unsaturated group per molecule include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, tridecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, "isostearyl acrylate" (trade name, manufactured by Osaka Organic Chemical Industry Ltd.), cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclododecyl (meth)acrylate, and tricyclodecanyl (meth)acrylate. alkyl or cycloalkyl (meth)acrylates such as acrylate; polymerizable unsaturated monomers having an isobornyl group such as isobornyl (meth)acrylate; polymerizable unsaturated monomers having an adamantyl group such as adamantyl (meth)acrylate; polymerizable unsaturated monomers having a tricyclodecenyl group such as tricyclodecenyl (meth)acrylate; aromatic ring-containing polymerizable unsaturated monomers such as benzyl (meth)acrylate, styrene, α-methylstyrene, and vinyltoluene; polymerizable unsaturated monomers having an alkoxysilyl group such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, γ-(meth)acryloyloxypropyltrimethoxysilane, and γ-(meth)acryloyloxypropyltriethoxysilane; perfluorobutylethyl (meth)acrylate, perfluorooctylethyl (meth)acrylate perfluoroalkyl(meth)acrylates such as those mentioned above; polymerizable unsaturated monomers having a fluorinated alkyl group such as fluoroolefins; polymerizable unsaturated monomers having a photopolymerizable functional group such as a maleimide group; vinyl compounds such as N-vinylpyrrolidone, ethylene, butadiene, chloroprene, vinyl propionate, and vinyl acetate; hydroxyl group-containing polymerizable unsaturated monomers such as monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, ε-caprolactone-modified products of the monoesters, N-hydroxymethyl (meth)acrylamide, allyl alcohol, and (meth)acrylates having a polyoxyethylene chain with a hydroxyl group at the molecular terminal; (meth)acrylic acid, maleic acid, crotone Examples of suitable polymerizable unsaturated monomers include carboxyl group-containing polymerizable unsaturated monomers such as β-carboxyethyl acrylate and β-carboxyethyl acrylate; nitrogen-containing polymerizable unsaturated monomers such as (meth)acrylonitrile, (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and adducts of glycidyl (meth)acrylate and amines; epoxy group-containing polymerizable unsaturated monomers such as glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl (meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate, and allyl glycidyl ether; and (meth)acrylates having a polyoxyethylene chain terminated in an alkoxy group.

These monomers can be used alone or in combination of two or more depending on the performance required of the acrylic resin particles (A).

Of the above polymerizable unsaturated monomers, the acrylic resin particles (A) preferably contain a hydroxyl group-containing polymerizable unsaturated monomer as a copolymerization component, from the viewpoint of reactivity with the polyisocyanate compound (C).

Examples of polymerizable unsaturated monomers having two or more polymerizable unsaturated groups in one molecule include allyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol, methylpropane tri(meth)acrylate ... Examples of suitable monomers include taerythritol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, glycerol di(meth)acrylate, 1,1,1-trishydroxymethylethane di(meth)acrylate, 1,1,1-trishydroxymethylethane tri(meth)acrylate, 1,1,1-trishydroxymethylpropane tri(meth)acrylate, methylene bis(meth)acrylamide, ethylene bis(meth)acrylamide, triallyl isocyanurate, diallyl terephthalate, and divinylbenzene. These monomers can be used alone or in combination of two or more.

The polymerizable unsaturated monomer having two or more polymerizable unsaturated groups per molecule functions to impart a crosslinked structure to the copolymer. When using a polymerizable unsaturated monomer having two or more polymerizable unsaturated groups per molecule, the proportion used can be determined appropriately depending on the degree of crosslinking in the copolymer. However, it is generally preferable that the proportion be approximately 0.1 to 30% by mass, particularly approximately 0.5 to 10% by mass, and even more particularly approximately 1 to 7% by mass, based on the total amount of the polymerizable unsaturated monomer having two or more polymerizable unsaturated groups per molecule and the polymerizable unsaturated monomer having one polymerizable unsaturated group per molecule.

From the standpoint of water resistance and blocking resistance, the acrylic resin particles (A) are preferably crosslinked acrylic resin particles synthesized using a polymerizable unsaturated monomer having two or more of the above-mentioned polymerizable unsaturated groups per molecule as a copolymer component.

Acrylic resin particles (A) can be obtained by emulsion polymerization of a polymerizable unsaturated monomer mixture.

Anionic emulsifiers and nonionic emulsifiers are suitable for use as emulsifiers.

Examples of the anionic emulsifier include sodium salts and ammonium salts of alkylsulfonic acid, alkylbenzenesulfonic acid, alkylphosphate, etc. Examples of the nonionic emulsifier include polyoxyethylene oleyl ether, polyoxyethylene stearyl ether, polyoxyethylene lauryl ether, polyoxyethylene tridecyl ether, polyoxyethylene phenyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene monolaurate, polyoxyethylene monostearate, polyoxyethylene monooleate, sorbitan monolaurate, sorbitan monostearate, sorbitan trioleate, polyoxyethylene sorbitan monolaurate, etc.
Also usable are polyoxyalkylene group-containing anionic emulsifiers having an anionic group and a polyoxyalkylene group such as a polyoxyethylene group or a polyoxypropylene group in one molecule; and reactive anionic emulsifiers having an anionic group and a radically polymerizable unsaturated group in one molecule.

Examples of the reactive anionic emulsifier include sodium salts of sulfonic acid compounds having radically polymerizable unsaturated groups such as allyl groups, methallyl groups, (meth)acryloyl groups, propenyl groups, and butenyl groups, and ammonium salts of such sulfonic acid compounds.

The amount of the emulsifier used is preferably within the range of approximately 0.1 to 15% by mass, particularly approximately 0.5 to 10% by mass, and even more particularly approximately 1 to 5% by mass, based on the total amount of all monomers used.

Polymerization initiators include, for example, persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; benzoyl peroxide, octanoyl peroxide, lauroyl peroxide, stearoyl peroxide, cumene hydroperoxide, tert-butyl peroxide, tert-butyl peroxylaurate, tert-butylperoxyisopropyl carbonate, tert-butyl peroxyacetate, and diisopropylbenzene hydroperoxide. Organic peroxides include azo compounds such as azobisisobutyronitrile, azobis(2,4-dimethylvaleronitrile), azobis(2-methylpropionitrile), azobis(2-methylbutyronitrile), 4,4'-azobis(4-cyanobutanoic acid), dimethylazobis(2-methylpropionate), azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and azobis{2-methyl-N-[2-(1-hydroxybutyl)]-propionamide}. These polymerization initiators can be used alone or in combination. Furthermore, if necessary, a reducing agent such as sugar, sodium formaldehyde sulfoxylate, or an iron complex can be used in combination with the above polymerization initiators to form a redox initiator.

The amount of the polymerization initiator used is generally preferably within the range of approximately 0.1 to 5% by mass, and particularly approximately 0.2 to 3% by mass, based on the total amount of all monomers used. The method of adding the polymerization initiator is not particularly limited and can be selected appropriately depending on the type and amount. For example, the initiator can be added in advance to the monomer mixture or aqueous medium, or it can be added all at once or dropwise during polymerization.

The acrylic resin particles (A) are preferably core-shell acrylic resin particles having a core-shell structure from the viewpoints of pigment sedimentation stability and water resistance.

In the present invention, the "shell portion" of a core-shell acrylic resin particle refers to the polymer layer present in the outermost layer of the resin particle, the "core portion" refers to the polymer layer present in the inner layer of the resin particle excluding the shell portion, and the "core-shell structure" refers to a structure having the core portion and shell portion.

The core-shell structure is generally a layered structure in which the core is completely coated with the shell, but depending on the mass ratio of the core to the shell, the amount of monomer in the shell may be insufficient to form a layered structure. In such cases, the above-described complete layered structure is not necessary, and a structure in which only a portion of the core is coated with the shell may also be used. The concept of a multilayered structure in the core-shell structure also applies when a multilayered structure is formed in the core of core-shell acrylic resin particles.

The core-shell acrylic resin particles can be obtained by emulsion polymerizing a polymerizable unsaturated monomer mixture to obtain an emulsion of core copolymer (I), then adding a polymerizable unsaturated monomer mixture to this emulsion and further emulsion polymerizing it to prepare shell copolymer (II).

The above-mentioned monomer mixture may contain components such as the polymerization initiator, chain transfer agent, reducing agent, and emulsifier as needed.

The above-mentioned monomer mixture can be added dropwise as is, but it is preferable to add it dropwise as a monomer emulsion obtained by dispersing the monomer mixture in an aqueous medium.

The emulsion polymerization for preparing the emulsion of the core copolymer (I) can be carried out by a conventionally known method. For example, it can be carried out by emulsion polymerizing a mixture of polymerizable unsaturated monomers using a polymerization initiator in the presence of an emulsifier.

One example of a method for polymerizing the monomer mixture that forms the shell copolymer (II) is to add the monomer mixture or an emulsion thereof, either all at once or gradually dropwise, to the emulsion of the core copolymer (I), and then heat the mixture to an appropriate temperature while stirring.

The core-shell acrylic resin particles thus obtained have a multilayer structure with copolymer (I) as the core and copolymer (II) as the shell.

In addition, core-shell acrylic resin particles can be made into acrylic resin particles consisting of three or more layers by adding a step of supplying a polymerizable unsaturated monomer (one type or a mixture of two or more types) that forms another resin layer and carrying out emulsion polymerization between the steps of obtaining the core copolymer (I) and obtaining the shell copolymer (II).

When the acrylic resin particles (A) are core-shell type, from the viewpoint of resin particle stability, the mass ratio of the core portion to the shell portion (core portion/shell portion) is preferably 90/10 to 50/50, particularly 85/15 to 60/40, and even more particularly 80/20 to 65/35.

From the viewpoint of resin stability and water resistance, the acid value of the acrylic resin particles (A) is preferably within the range of 1 to 50 mgKOH/g, particularly 5 to 30 mgKOH/g.

If the acid value is less than 1 mgKOH/g, the dispersion stability of the particles may be poor, and if it exceeds 50 mgKOH/g, the coating film may become too hydrophilic and have poor water resistance.
As a monomer for imparting an acid value to the acrylic resin particles (A), it is preferable to contain a carboxyl group-containing polymerizable unsaturated monomer, from the viewpoint that the neutralizing agent volatilizes, the hydrophilicity is significantly reduced after film formation, and water resistance is improved.

Acrylic acid and/or methacrylic acid are particularly suitable as carboxyl group-containing polymerizable unsaturated monomers.

In this specification, the acid value (mg KOH/g) is the amount of acid groups contained in 1 g of sample (solid content in the case of resin) expressed as mg of potassium hydroxide when converted to potassium hydroxide. The molecular weight of potassium hydroxide is assumed to be 56.1.

Acid value is measured in accordance with JIS K-5601-2-1 (1999). The sample is dissolved in a mixed solvent of toluene and ethanol in a 2:1 volume ratio, titrated with potassium hydroxide solution using phenolphthalein as an indicator, and calculated using the following formula.

Acid value (mgKOH/g) = 56.1 x V x C/m
V: titration volume (ml), C: concentration of titrant (mol/l), m: weight of solid content of sample (g)
The hydroxyl value of the acrylic resin particles (A) is preferably within the range of 5 to 150 mgKOH/g, particularly 10 to 100 mgKOH/g, from the viewpoints of reactivity with the polyisocyanate compound (C) and water resistance.

If the hydroxyl value is less than 5 mg KOH/g, water resistance may be poor, and if it exceeds 150 mg KOH/g, film-forming properties may be poor.

The monomer used to impart a hydroxyl value to the acrylic resin particles (A) preferably contains the above-mentioned hydroxyl group-containing polymerizable unsaturated monomer.

Of the above, particularly suitable hydroxyl group-containing polymerizable unsaturated monomers are monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 4 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate.

From the viewpoint of blocking resistance and film-forming ability, the glass transition temperature (Tg) of the acrylic resin particles (A) is preferably 20°C to 80°C, particularly 25°C to 70°C, and even more particularly 30°C to 60°C.

If the glass transition temperature (Tg) is below 20°C, blocking may occur when plywood is loaded, and if the glass transition temperature (Tg) is above 80°C, film-forming properties may be poor.

In this specification, the glass transition temperature (°C) of the copolymer can be calculated using the following formula:

1/Tg(°K)=(W1/T1)+(W2/T2)+...
Tg (°C) = Tg (°K) - 273
In each formula, W1, W2, ... represent the weight percentage of each monomer used in the copolymerization, and T1, T2, ... represent the Tg (°K) of the homopolymer of each monomer. Note that T1, T2, ... are values taken from pages III-139 to 179 of Polymer Handbook (Second Edition, edited by J. Brandup and EH Immergut).

Furthermore, when the Tg of the monomer homopolymer is unclear, the glass transition temperature (°C) is taken as the static glass transition temperature. For example, using a differential scanning calorimeter "DSC-220U" (Seiko Instruments Inc.), the sample is placed in a measuring cup and vacuum suction is used to completely remove the solvent. After that, the heat change is measured in the range of -20°C to +200°C at a heating rate of 3°C/min, and the first change point in the baseline on the low temperature side is taken as the static glass transition temperature.

When the acrylic resin particles (A) are core-shell type, from the viewpoint of water resistance, it is preferable that the acid value of the core portion is in the range of 0 to 50 mgKOH/g, particularly 0 to 30 mgKOH/g, the hydroxyl value of the core portion is in the range of 5 to 150 mgKOH/g, particularly 10 to 100 mgKOH/g, and the Tg of the core portion is in the range of 20 to 80°C, particularly 30 to 60°C.

Furthermore, from the viewpoint of the stability of the resin particles, it is preferable that the acid value of the shell portion is in the range of 10 to 100 mgKOH/g, particularly 30 to 70 mgKOH/g, the hydroxyl value of the shell portion is in the range of 5 to 150 mgKOH/g, particularly 10 to 100 mgKOH/g, and the Tg of the shell portion is in the range of 20 to 80°C, particularly 25 to 60°C.

In addition, from the viewpoint of the film-forming properties and water resistance of the resulting coating film, the acrylic resin particles (A) preferably have a weight-average molecular weight of 50,000 or more, particularly in the range of 100,000 to 500,000 (excluding cases where the acrylic resin particles (A) are crosslinked resin particles).
In this specification, the average molecular weight is a value obtained by converting the retention time measured using a gel permeation chromatograph into the molecular weight of polystyrene using the retention time of standard polystyrenes with known molecular weights measured under the same conditions.

Specifically, for example, a gel permeation chromatograph using the "HLC-8120GPC" (product name, manufactured by Tosoh Corporation) can be used, with four columns: one "TSKgel G4000HXL," two "TSKgel G3000HXL," and one "TSKgel G2000HXL" (all product names, manufactured by Tosoh Corporation). A differential refractometer can be used as the detector, with the mobile phase being tetrahydrofuran, the measurement temperature being 40°C, and the flow rate being 1 mL/min.

From the viewpoint of resin particle stability, the average particle diameter of the acrylic resin particles (A) is preferably within the range of 50 to 500 nm, particularly 60 to 400 nm, and even more particularly 70 to 300 nm.

If the average particle size is less than 50 nm, the resin particles may become unstable, resulting in poor paint stability; if it exceeds 500 nm, dispersion stability may be poor.

The average particle size can be measured by a common measuring means such as laser light scattering.
In this specification, the average particle size of resin particles is a value measured at 20° C. using a submicron particle size distribution analyzer after dilution with deionized water in a conventional manner. As the submicron particle size distribution analyzer, for example, a "COULTER N4 type" (trade name, manufactured by Beckman Coulter, Inc.) can be used.

To improve the mechanical stability of the acrylic resin particles (A), the acid groups, such as carboxyl groups, of the acrylic resin particles can be neutralized with a neutralizing agent. There are no particular restrictions on the neutralizing agent, as long as it can neutralize the acid groups. Examples of such neutralizing agents include sodium hydroxide, potassium hydroxide, trimethylamine, 2-(dimethylamino)ethanol, 2-amino-2-methyl-1-propanol, triethylamine, and aqueous ammonia. These neutralizing agents are preferably used in an amount such that the pH of the aqueous dispersion of acrylic resin particles (A) after neutralization is approximately 6.5 to 9.0.

Resin particles other than the acrylic resin particles (A) can also be used in the aqueous coating composition of the present invention, if necessary. Specific examples include resin particles (excluding the acrylic resin particles (A)) made of at least one type selected from acrylic resin, acrylic/styrene resin, urethane resin, phenolic resin, vinyl chloride resin, vinyl acetate resin, vinyl acetate/acrylic resin, ethylene/vinyl acetate resin, epoxy resin, epoxy ester resin, polyester resin, alkyd resin, acrylonitrile/butadiene resin, styrene/butadiene resin, polybutadiene, polyisoprene, silicone resin, and fluororesin, as well as modified versions of these resins, such as carbonate-modified urethane resin, acrylic resin-modified epoxy resin, alkyd-modified epoxy resin, polybutadiene-modified epoxy resin, (poly)amine-modified epoxy resin, and urethane-modified epoxy resin. These resin particles may also be rubber.

The above resin particles may be prepared by blending multiple resins, or may be a blend of multiple types of resin particles. These resin particles can usually be incorporated into the composition of the present invention in the form of an emulsion.

Surfactant (B)
The surfactant (B) is a nonionic surfactant, a block copolymer having a block structure of an addition polymer of ethylene oxide, propylene oxide, and ethylene oxide, and a copolymer having a triblock structure consisting of a polyoxypropylene group sandwiched between two polyoxyethylene groups. Although the surfactant (B) is highly hydrophilic overall, it has amphiphilicity because the polyoxypropylene chain is more lipophilic than the polyoxyethylene chain.

From the viewpoint of pigment dispersibility, it is preferable that the surfactant (B) in this coating composition has a weight-average molecular weight in the range of 2,400 to 13,000, particularly 2,600 to 10,000, and even more particularly 2,800 to 9,000.

Furthermore, in each block structure, from the viewpoint of pigment dispersibility, it is preferable that the weight-average molecular weight of the polyoxypropylene group is 1,200 to 4,000, particularly 1,450 to 3,250, and even more particularly 1,450 to 2,250, and the content of polyoxyethylene groups in the molecule is preferably within the range of 10 to 90% by mass, particularly 20 to 80% by mass, and even more particularly 30 to 70% by mass.

By including surfactant (B), the viscosity of the coating composition is less temperature-dependent, and even in temperatures between 30 and 60°C, the viscosity remains high without decreasing. This means that the pigment is less likely to settle during storage (especially in summer) and the curing process, resulting in excellent storage stability and filling properties. Furthermore, even when coating plywood, the plywood may be warm shortly after the completion of the bonding process prior to plywood production, but the coating viscosity remains high, resulting in excellent filling properties.

Polyisocyanate compound (C)
The polyisocyanate compound (C) is a compound having at least two isocyanate groups in one molecule, and examples thereof include aliphatic polyisocyanates, alicyclic polyisocyanates, araliphatic polyisocyanates, aromatic polyisocyanates, and derivatives of the polyisocyanates.

Examples of the aliphatic polyisocyanates include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, dimer acid diisocyanate, methyl 2,6-diisocyanatohexanoate (common name: lysine aliphatic diisocyanates such as 2-isocyanatoethyl 2,6-diisocyanatohexanoate, 1,6-diisocyanato-3-isocyanatomethylhexane, 1,4,8-triisocyanatooctane, 1,6,11-triisocyanatoundecane, 1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-triisocyanatohexane, and 2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane.

Examples of the alicyclic polyisocyanate include 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), 4-methyl-1,3-cyclohexylene diisocyanate (common name: hydrogenated TDI), 2-methyl-1,3-cyclohexylene diisocyanate, alicyclic diisocyanates such as 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name: hydrogenated xylylene diisocyanate) or a mixture thereof, methylenebis(4,1-cyclohexanediyl)diisocyanate (common name: hydrogenated MDI), and norbornane diisocyanate; 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)- 2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, 6-(2-isocyanatoethyl)- Examples include alicyclic triisocyanates such as (2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane, and 6-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane.

Examples of the araliphatic polyisocyanate include araliphatic diisocyanates such as methylenebis(4,1-phenylene)diisocyanate (common name: MDI), 1,3- or 1,4-xylylene diisocyanate or mixtures thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name: tetramethylxylylene diisocyanate) or mixtures thereof; and araliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene.

Examples of the aromatic polyisocyanate include aromatic diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 2,4-tolylene diisocyanate (common name: 2,4-TDI), 2,6-tolylene diisocyanate (common name: 2,6-TDI), or mixtures thereof, 4,4'-toluidine diisocyanate, 4,4'-diphenyl ether diisocyanate, etc.; aromatic triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene, etc.; and aromatic tetraisocyanates such as 4,4'-diphenylmethane-2,2',5,5'-tetraisocyanate, etc.

Furthermore, examples of derivatives of the polyisocyanates include dimers, trimers, biurets, allophanates, uretdiones, uretimines, isocyanurates, oxadiazinetriones, polymethylene polyphenyl polyisocyanates (crude MDI, polymeric MDI), crude TDI, and the like.

The above polyisocyanates and their derivatives may be used alone or in combination of two or more.

Of these polyisocyanates, alicyclic polyisocyanates and derivatives of alicyclic polyisocyanates are preferred.

Furthermore, from the viewpoint of pot life, isophorone diisocyanate and its derivatives are particularly preferred among alicyclic polyisocyanates and alicyclic polyisocyanate derivatives.

The polyisocyanate compound may also be a prepolymer obtained by reacting the above-mentioned polyisocyanate or a derivative thereof with a compound reactive with the polyisocyanate under conditions of excess isocyanate groups. Examples of compounds reactive with the polyisocyanate include compounds having active hydrogen groups such as hydroxyl groups and amino groups. Specific examples include polyhydric alcohols, low-molecular-weight polyester resins, amines, and water.

The polyisocyanate compound may also be a polymer of an isocyanate group-containing polymerizable unsaturated monomer, or a copolymer of the isocyanate group-containing polymerizable unsaturated monomer and a polymerizable unsaturated monomer other than the isocyanate group-containing polymerizable unsaturated monomer.

Preferable examples of the polyisocyanate compound include hydrophilic polyisocyanate compounds (C1) for use in aqueous paints, such as hydrophilized polyisocyanate compounds in which hydrophilic groups have been introduced into polyisocyanate compounds, and water-dispersible polyisocyanate compounds in which polyisocyanate compounds can be dispersed in water using a surfactant.

Hydrophilic groups include anionic groups such as acid groups and nonionic groups containing polyoxyalkylene (polyether chain) units. Examples of acid groups include carboxyl groups, phosphate groups, and sulfonic acid groups.

Among hydrophilized polyisocyanate compounds, nonionic group-containing polyisocyanate compounds are preferred from the standpoint of water resistance, and polyisocyanate compounds having polyoxyalkylene (polyether chain) units as the nonionic group are particularly preferred.

In terms of water resistance, the solids content of the acrylic resin particles (A), surfactant (B), and polyisocyanate compound (C) in this coating composition is preferably such that, relative to the total solids content of the acrylic resin particles (A), surfactant (B), and polyisocyanate compound (C), the solids content of the acrylic resin particles (A) is 50 to 99% by mass, particularly 60 to 95% by mass, and even more particularly 60 to 90% by mass; the solids content of the surfactant (B) is 1 to 50% by mass, particularly 5 to 40% by mass, and even more particularly 10 to 30% by mass; and the solids content of the polyisocyanate compound (C) is 0.1 to 20% by mass, particularly 1 to 20% by mass, and even more particularly 3 to 15% by mass.

Pigment (D)
The pigment (D) is contained mainly to improve the ability of the coating composition to fill defects and recesses in wood materials.

Examples of pigment (D) include extender pigments, coloring pigments, etc.

As extender pigments, inorganic and organic pigments known as extender pigments can be used without any particular restrictions, and examples include clay, silica, kaolin, barium sulfate, talc, calcium carbonate, white carbon, diatomaceous earth, magnesium aluminum carbonate flakes, mica flakes, etc.

Of the above extender pigments, calcium carbonate is preferably used.

Examples of commercially available extender pigments include:

Examples of calcium carbonate include "NS-300, 400" (manufactured by Nippon Funka Kogyo Co., Ltd.), "CaCO3 Super SS" (manufactured by Kyushu Calcium Co., Ltd.), "CaCO3 Sunlight No. 300" (manufactured by Takehara Kogyo Co., Ltd.), and "Calcium Carbonate NN-200" (manufactured by Nitto Funka Kogyo Co., Ltd.).

Examples of talc include "Microace L-1" (manufactured by Nippon Talc Co., Ltd.), "Talc MG115," "Talc MS410," and "Talc RL119" (manufactured by Fuji Talc Kogyo Co., Ltd.), "Talc TST-1," "Talc TST-2," and "FC-1 Talc" (manufactured by Fukuoka Talc Kogyo Co., Ltd.).

In addition to improving filling properties, coloring pigments can be used to achieve the desired color.

Specific examples of coloring pigments include titanium white, zinc molybdate, calcium molybdate, carbon black, graphite, iron black, Prussian blue, ultramarine, cobalt blue, copper phthalocyanine blue, indanthrone blue, yellow lead, synthetic yellow iron oxide, red iron oxide, transparent red iron oxide, bismuth vanadate, titanium yellow, zinc yellow, monoazo yellow, ochre, disazo, isoindolinone yellow, metal complex azo yellow, and quinoxazone. Examples of suitable quinacridones include quinacridone yellow, benzimidazolone yellow, monoazo red, unsubstituted quinacridone red, azo lake (Mn salt), quinacridone magenta, anthanthrone orange, dianthraquinonyl red, perylene maroon, perylene red, diketopyrrolopyrrole chrome vermilion, chlorinated phthalocyanine green, brominated phthalocyanine green, pyrazolone orange, benzimidazolone orange, dioxazine violet, and perylene violet.

Pigments (D) can be used alone or in combination of two or more.

In this coating composition, from the viewpoints of the ability to fill recesses in the substrate and the solids concentration of the composition, the solids content of the pigment (D) is preferably 600% by mass or more, particularly 700% by mass or more, and even more particularly 800% by mass or more, based on the total solids content of the acrylic resin particles (A), surfactant (B), and polyisocyanate compound (C).

Coating Composition In addition to the acrylic resin particles (A), surfactant (B), polyisocyanate compound (C), and pigment (D), the coating composition may further contain, as necessary, coating additives such as a rheology control agent, a suspending agent, an antifoaming agent, a pigment dispersant, an anti-flocculating agent, and a leveling agent.

This coating composition contains water as a volatile component, and may also contain solvents other than water as needed. Organic solvents such as alcohols can be used as solvents other than water.

This coating composition can be produced by mixing the above components.

This coating composition can be prepared by mixing all of the components simultaneously or sequentially. However, because the polyisocyanate compound (C) is reactive with the acrylic resin particles (A) and surfactant (B) even at room temperature, it is preferable from the standpoint of storage stability to premix the components other than component (C) to form the base component, to which the polyisocyanate compound (C) is added as a curing agent to prepare a two-component coating composition.

The viscosity of this coating composition can be adjusted to the desired viscosity appropriate for application. Specifically, for example, when used as a sealer or putty for filling defects in coniferous wood, the viscosity, measured at 60 revolutions using a Brookfield type rotational viscometer, is preferably in the range of 1 to 30 Pa·s, and particularly 5 to 20 Pa·s.

Viscosity can be adjusted by adding and mixing water, organic solvents, rheology control agents, etc. as needed.

From the viewpoint of filling properties, the solids concentration of this coating composition is preferably 75% by mass or more, and particularly 80% by mass or more.

From the viewpoint of storage stability of the paint, it is preferable that the pH of the aqueous paint composition of the present invention be within the range of 5.0 to 10.0, preferably 6.0 to 9.0.

This paint composition has excellent workability, including filling and application properties, and is good at filling substrates with recesses. Furthermore, it exhibits excellent properties, such as minimal volume shrinkage upon hardening. Therefore, it can be particularly well suited for use as a sealer or putty composition for smoothing wood materials with defects such as knotholes or recesses.

The uses of this coating composition are not particularly limited, but it is preferably applied to wood that has defects or recesses on its surface. Specifically, it can be suitably applied to the surfaces of wooden materials such as plywood, flooring, wall materials, ceilings, handrails, and furniture.

This coating composition can be applied to the wood to which it is to be applied, filled in, and then cured to form a coating film or cured body on the wood surface and in the filled area.

The applied and filled coating composition can usually be cured at a temperature of 0 to 180°C, preferably 60 to 140°C, for a curing time of 30 seconds to 1 hour.

When applying this coating composition to wood as a wood sealer composition or wood putty composition, it can be applied using a coating machine such as a natural reverse coater, sponge reverse coater, or knife coater.

Knife coaters are most preferred for filling knot holes in softwood plywood.

When using a knife coater, the blade is placed against a dummy plate set in the knife coater, the composition of the present invention is dripped on the front side of the blade, and the substrate is conveyed, easily filling recesses such as knotholes. From the viewpoint of the composition's filling ability, the substrate conveyance speed is preferably 10 to 30 m/min, and the blade angle is preferably 30° to 60°.

After application with a knife coater, forced drying (for example, at 100°C for 30 seconds) allows the paint composition to efficiently fill knotholes and depressions as a dried and hardened coating or hardened product, eliminating the surface irregularities that are a drawback of softwood plywood, and allowing the wood to be moved on to the next process, such as painting.

While there are no restrictions on the wood that can be used, including softwood, hardwood, and various types of plywood, the present invention is particularly suited to softwood that has knotholes or other depressions or defects on its surface.

After application, the present coating composition can be used to form a coating film on the cured portion, and can also be laminated.
The separate paint refers to a paint to be applied on top of the coating film (sealer layer) of the present paint composition, and includes a top coat paint to be applied to the top surface of the wood, as well as a primer paint and an intermediate coat paint. Known paints can be used in appropriate combination as these paints.

The present invention will be explained in more detail below using examples. Here, "parts" and "%" mean "parts by mass" and "% by mass," respectively. Note that the "parts by mass" of raw materials in the following production examples, examples, and comparative examples refer to the parts by mass of the solid content (or sometimes referred to as the active ingredient) of the raw materials.

Production of acrylic resin particles (A) Production Example 1
Deionized water and Newcol 707SF (trade name, Nippon Nyukazai Co., Ltd., anionic surfactant, solids content 30% by mass) in the amounts shown in Table 1 were charged into a polymerization apparatus equipped with a stirrer, thermometer, and reflux condenser. After thorough nitrogen replacement, the temperature was raised. While stirring at approximately 100 rpm, the internal temperature was maintained at 82°C. Component (A) shown in Table 1 was emulsified using a homomixer (hereinafter referred to as component (A) emulsion; the same applies to component (B)). An aqueous solution of initiator 1 (Table 1) was added dropwise over 3 hours to polymerize the mixture. After the dropwise addition, the mixture was allowed to react at 82°C for 0.5 hours, and then an aqueous solution of initiator 2 was added dropwise over 0.5 hours. After the dropwise addition, the mixture was allowed to react at 82°C for 1 hour, and then cooled to 25°C. Finally, a neutralizing agent shown in Table 1 was added to obtain an emulsion of acrylic resin particles (A-1) (Tg: 20.9°C, acid value: 13.0 mgKOH/g, hydroxyl value: 64.7 mgKOH/g) with a solid content of 40.0 mass%.

The above emulsion had a viscosity (measured using a Brookfield viscometer at 60 rpm and 20°C) of 400 mPa·s, a pH of 7.3, an average particle size of 100 nm, and a weight-average molecular weight of 140,000.

Production Example 2
Deionized water and Newcol 707SF in the amounts shown in Table 1 were charged into a polymerization apparatus equipped with a stirrer, thermometer, and reflux condenser. After thorough nitrogen replacement, the temperature was raised. While stirring at approximately 100 rpm, the internal temperature was maintained at 82°C. 5% by mass of the component (A) shown in Table 1 emulsified using a homomixer and an aqueous initiator 1 solution (see Table 1) were added dropwise to polymerize. After completion of the dropwise addition, the reaction was continued for 0.5 hours at 82°C. The remaining 95% by mass of the (A) component emulsion and the initiator 2 aqueous solution (see Table 1) were then added dropwise over 3 hours to polymerize. After completion of the dropwise addition, the reaction was continued for 0.5 hours at 82°C. The emulsion of component (B) and the initiator 3 aqueous solution shown in Table 1 were then added dropwise over 1 hour to allow the reaction. After completion of the dropwise addition, the reaction was continued for 1 hour at 82°C, and then cooled to 25°C. Finally, a neutralizing agent shown in Table 1 was added to obtain an emulsion of acrylic resin particles (A-2) (Tg: 21.5°C, acid value: 13.0 mgKOH/g, hydroxyl value: 64.7 mgKOH/g) with a solid content of 40.0 mass%.

The above emulsion had a viscosity (measured with a Brookfield viscometer at 60 rpm and 20°C) of 400 mPa·s, a pH of 7.4, and an average particle size of 110 nm.

Production Example 3
An emulsion of acrylic resin particles (A-3) (Tg: 35.0°C, acid value: 13.0 mgKOH/g, hydroxyl value: 64.7 mgKOH/g) having a solids concentration of 40.0 mass% was obtained in the same manner as in Production Example 2, except that the formulation was as shown in Production Example No. 3 in Table 1.

The above emulsion had a viscosity (measured using a Brookfield viscometer at 60 rpm and 20°C) of 410 mPa·s, a pH of 7.4, and an average particle size of 110 nm.

Production Example 4
An emulsion of acrylic resin particles (A-4) (Tg: 35.0°C, acid value: 13.0 mgKOH/g, hydroxyl value: 64.7 mgKOH/g) having a solids concentration of 40.0 mass% was obtained in the same manner as in Production Example 2, except that the formulation was as shown in Production Example No. 4 in Table 1.

The above emulsion had a viscosity (measured using a Brookfield viscometer at 60 rpm and 20°C) of 400 mPa·s, a pH of 7.3, an average particle size of 130 nm, and a weight-average molecular weight of 180,000.

Production Example 5
An emulsion of acrylic resin particles (A-5) (Tg: 11.3°C, acid value: 13.0 mgKOH/g, hydroxyl value: 64.7 mgKOH/g) having a solids concentration of 40.0 mass% was obtained in the same manner as in Production Example 1, except that the formulation was as shown in Production Example No. 5 in Table 1.

The above emulsion had a viscosity (B-type viscometer, 60 rpm, 20°C) of 400 mPa·s, a pH of 7.2, an average particle size of 120 nm, and a weight-average molecular weight of 150,000.

Acrylic resin particles (A-5) are for comparative purposes.

Table 1 also shows the Tg (°C), acid value, and hydroxyl value of each of the obtained acrylic resin particles (A). The core Tg/shell Tg/total particle Tg (°C) of the core-shell type acrylic resin particles (A-2), (A-3), and (A-4) is 21.7/21.1//21.5 for acrylic resin particles (A-2), 39.4/25.1//35.0 for acrylic resin particles (A-3), and 39.5/25.1//35.0 for acrylic resin particles (A-4).

Preparation of Coating Composition Example 1
The raw materials were mixed using a disperser to obtain the solid content shown in Table 2 below, and the solid content concentration of the paint was adjusted to 75 mass% with deionized water to obtain Paint Composition No. 1. The viscosity of the resulting paint composition (measured with a Brookfield viscometer at 60 rpm and 23°C) was 5.0 Pa s.

Examples 2 to 16 and Comparative Examples 1 to 5
Coating composition Nos. 2 to 21 were obtained in the same manner as in Example 1 except for the formulations shown in Table 2 below. Coating composition Nos. 17 to 21 are comparative coating compositions.

The raw materials in the table other than the acrylic resin particles (A) are as follows:

Surfactant (B)
L62: Weight average molecular weight 2500, weight average molecular weight of polyoxypropylene group 1750, content of polyoxyethylene group in molecule 20% by mass
L64: Weight average molecular weight 2900, weight average molecular weight of polyoxypropylene group 1750, content of polyoxyethylene group in molecule 40% by mass
F68: Weight average molecular weight 8,400, weight average molecular weight of polyoxypropylene group 1,750, content of polyoxyethylene group in molecule 80% by mass
P84: Weight average molecular weight 4200, weight average molecular weight of polyoxypropylene group 2250, content of polyoxyethylene group in molecule 80% by mass
L44: Weight average molecular weight 2200, weight average molecular weight of polyoxypropylene group 1200, content of polyoxyethylene group in molecule 40% by mass
All five pigments listed above are manufactured by ADEKA Corporation.
Calcium carbonate: Sunlight No. 300 (manufactured by Takehara Kogyo Co., Ltd.)
Talc: Microace L-1 (manufactured by Nippon Talc Co., Ltd.)
Polyisocyanate compound (C)
Desmodur Z4470BA; isocyanurate of isophorone diisocyanate
Sumidur N3300: isocyanurate of hexamethylene diisocyanate Bayhydur 304: isocyanurate of hexamethylene diisocyanate having a polyoxyethylene unit, hydrophilized polyisocyanate compound All three of the above, manufactured by Sumika Covestro Urethane Co., Ltd. Defoamer BYK-028: silicone defoamer manufactured by BYK-Chemie Preparation of test coated boards Softwood plywood (manufactured by Hayashi Veneer Sangyo Co., Ltd., 120 mm thick) was cut into 30 cm x 30 cm pieces, and four artificial holes 2.5 cm in diameter and 2.0 mm deep were drilled using a Bosch DIY power tool PMR 500 power trimmer, and the surface was polished with sandpaper #180 to obtain wood material for test coated boards.

Each of the coating compositions obtained in the above examples and comparative examples was applied to wood test panels using a knife coater at a speed of 20 m/min, an angle of 45°, and a press pressure of 0.2 MPa. The test panels were then dried at 140°C for 1 minute to obtain test panels corresponding to each coating composition.

The test and evaluation methods are as follows:

Pigment settling stability For each paint composition, 4 kg of the base agent (a composition obtained by excluding the polyisocyanate compound from the paint formulation) was placed in a 4 L can and stored in a constant temperature room at 40°C for one week, after which the state of pigment settling at the bottom was checked with a spatula.

⊚: No pigment sediment was observed. ◯: A small amount of pigment sediment adhering to the spatula was observed. Δ: Pigment sediment was present, but it became uniform upon stirring. ×: Pigment sediment was present and it did not become uniform even upon stirring. Pot life Each coating composition was left in a thermostatic chamber at 23°C, and the condition of the coating was observed after 1 hour.

◎: No change from the beginning, fluidity maintained 〇: Viscosity is slightly higher than the beginning, but fluidity is still there and painting is possible Filling The state of the cured coating filled into the artificial holes of each test painted panel was observed. It is excellent because the convex swelling makes the filled part of the coating smooth after sanding. If there are dents due to wall thinning, it will have a negative effect on the smoothness after sanding.

◎: Fully bulged in a convex shape ○: Slightly bulged in a convex shape △: Slightly dented ×: Dentated Blocking resistance Immediately after removing from the dryer, each coated test board was cooled for about 1 minute, and an uncoated wooden test board was placed on top of the coated surface. A 50 kg load was applied, and the board was left in a thermostatic chamber at 50°C for 1 hour, after which the overlapping wooden test boards were peeled off and the condition of the coating film in the filled area was observed.

◎: No abnormalities were observed. ○: A small amount of paint film was attached to the overlapping wooden test boards, but most of it remained in the filled area. ×: Most of the paint film had peeled off from the filled area of the artificial hole in the test board.

Water Resistance Each test coated plate was immersed in water at 70° C. for 2 hours, and then allowed to stand at room temperature for 1 hour and 24 hours, after which the state of the coating film in the filled area was observed.

◎: No abnormalities after 1 hour at room temperature. ○: After 1 hour at room temperature, the sample absorbed water and swelled, but after 24 hours at room temperature, the sample returned to its pre-test state. ×: The coating dissolved in water and disappeared from the filled portion of the artificial hole.

It is possible to provide an aqueous coating composition that exhibits almost no volumetric shrinkage upon curing, excellent application properties such as filling and workability, excellent pigment settling stability, and excellent coating film performance such as water resistance.

Claims (7)

A composition comprising: acrylic resin particles (A) having a glass transition temperature of 20°C or higher; a surfactant (B) having a weight-average molecular weight of 2,400 or higher and a triblock structure composed of a polyoxypropylene group and two polyoxyethylene groups sandwiching the polyoxypropylene group; a polyisocyanate compound (C); and a pigment (D),
An aqueous wood coating composition comprising a pigment (D) in an amount of 600 mass% or more relative to the total solid content of components (A), (B), and (C). The aqueous wood coating composition according to claim 1, wherein the acrylic resin particles (A) are crosslinked acrylic resin particles. The aqueous wood coating composition according to claim 1 or 2, wherein the acrylic resin particles (A) are core-shell acrylic resin particles. The aqueous wood coating composition according to any one of claims 1 to 3, wherein the weight-average molecular weight of the polyoxypropylene group in surfactant (B) is 1,450 or more. The aqueous wood coating composition according to any one of claims 1 to 4, wherein the content of polyoxyethylene groups in the surfactant (B) molecule is 20% by mass or more. The aqueous wood coating composition according to any one of claims 1 to 5, wherein the polyisocyanate compound (C) is an alicyclic polyisocyanate and/or a derivative of an alicyclic polyisocyanate. The aqueous wood coating composition according to any one of claims 1 to 6, wherein the polyisocyanate compound (C) is a nonionic group-containing polyisocyanate compound.