JP2025006506A - Pigment dispersion and aqueous coating composition, and methods for producing same - Google Patents

Abstract

To provide a pigment dispersion having low viscosity and excellent pigment dispersibility regardless of the type of pigment.SOLUTION: There is provided a pigment dispersion which contains a copolymer (A), a hydrocarbon group-containing acidic compound (B), a pigment (C) and an aqueous solvent (D), wherein the copolymer (A) has a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain and the hydrocarbon group-containing acidic compound (B) has a saturated or unsaturated hydrocarbon group having 5 to 23 carbon atoms (b1) and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphoric acid group, a sulfonic acid group and a phenol group.SELECTED DRAWING: None

Description

The present invention relates to a pigment dispersion and an aqueous paint composition, as well as methods for producing the same.

In recent years, there has been a demand to reduce emissions of volatile organic compounds (VOCs) from factories and other facilities in order to protect the environment. Water-based paint compositions have been proposed as one method of reducing VOC emissions. However, pigments generally do not wet well in aqueous solvents and have poor dispersibility.

In this regard, Patent Document 1 discloses an aqueous coating composition that contains a copolymer of multiple monomers including a polymerizable unsaturated monomer having a cationic functional group represented by a specific formula, a pigment, and an acrylic resin containing an acid group and a hydroxyl group.

JP 2014-5399 A

However, it is difficult to disperse the pigment sufficiently and stably with the copolymer described in Patent Document 1.

In order to solve the above problems, the present invention provides the following aspects.
[1]
A composition comprising: a copolymer (A), a hydrocarbon group-containing acidic compound (B), a pigment (C), and an aqueous solvent (D);
The copolymer (A) is
a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain;
The hydrocarbon group-containing acidic compound (B) is
A pigment dispersion having (b1) a saturated or unsaturated hydrocarbon group having 5 to 23 carbon atoms, and (b2) at least one acid group selected from the group consisting of a carboxy group, a phosphoric acid group, a sulfonic acid group, and a phenol group.
[2]
The pigment dispersion liquid according to the above-mentioned [1], wherein a ratio of an equivalent of an acid group contained in the hydrocarbon group-containing acidic compound (B) to a total equivalent of a tertiary amino group and a nitrogen-containing heterocyclic group contained in the copolymer (A) (acid group/tertiary amino group and nitrogen-containing heterocyclic group) is 0.5 or more and 2 or less.
[3]
The copolymer (A) further has a third segment (a3) derived from a polymerizable unsaturated monomer other than the nitrogen-containing polymerizable unsaturated monomer and the polymerizable unsaturated monomer having a polyoxyalkylene chain,
In the copolymer (A),
The mass proportion of the first segment (a1) is 5 mass% or more and 30 mass% or less,
The mass ratio of the second segment (a2) is 20 mass% or more and 80 mass% or less,
The pigment dispersion liquid according to the above [1] or [2], wherein the mass ratio of the third segment (a3) is 10 mass % or more and 60 mass % or less.
[4]
The third segment (a3) includes a segment (a31) derived from a hydroxyl group-containing polymerizable saturated monomer,
The pigment dispersion liquid according to the above-mentioned [3], wherein the mass ratio of the segment (a31) in the copolymer (A) is 10 mass % or more and 30 mass % or less.
[5]
The pigment dispersion according to the above item [1] or [2],
and a film-forming resin.
[6]
A step of adding and mixing a hydrocarbon group-containing acidic compound (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) to obtain a second mixture;
and adding and mixing a pigment (C) to the second mixture,
The copolymer (A) is
a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain;
The hydrocarbon group-containing acidic compound (B) is
A method for producing a pigment dispersion liquid, comprising: a saturated or unsaturated hydrocarbon group (b1) having 5 to 23 carbon atoms; and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphoric acid group, a sulfonic acid group, and a phenol group.
[7]
A method for producing an aqueous coating composition, comprising the step of mixing the pigment dispersion produced by the method described in [6] above with a coating film-forming resin.

According to the present disclosure, a pigment dispersion liquid that has low viscosity and excellent pigment dispersibility is provided, regardless of the type of pigment.

Pigment dispersants are additives used to disperse pigments uniformly in solvents, and like surfactants, they have hydrophilic and hydrophobic sites. There are various types of hydrophilic sites. Hydrophilic sites can ionize to produce anions or cations, and may not have ionic dissociation properties.

Pigments usually have acidic, basic or inactive sites on their surface, and exhibit acidic, basic, hydrophobic or hydrophilic properties. These properties cannot be clearly distinguished for each pigment, and may have complex properties. It is difficult to select a pigment dispersant suitable for pigments with such various properties. Therefore, a pigment dispersant that can be used for any pigment is desired. However, if one pigment dispersant (compound) has multiple functional groups that can correspond to each of the above sites, problems such as an increase in the viscosity of the pigment dispersion due to association, etc. may occur. Similarly, when multiple types of pigment dispersants are used in combination, the viscosity of the pigment dispersion may increase, and problems may occur in terms of the affinity between the pigment dispersant and the solvent. An increase in the viscosity of the pigment dispersion makes it difficult to prepare a paint composition and may also be a factor in reducing the dispersibility of the pigment in the pigment dispersion.

The smaller the particle size of the pigment in the coating film (i.e., the higher the dispersibility of the pigment in the paint composition and, ultimately, in the pigment dispersion), the better the color development of the coating film. In general, the smaller the particle size of the pigment (the higher the dispersibility of the pigment), the higher the viscosity of the pigment dispersion. In other words, if the dispersibility of the pigment in the pigment dispersion is increased in order to improve the color development of the coating film, the viscosity of the pigment dispersion increases. This makes it difficult to mix the pigment dispersion with the coating film-forming resin, which can result in a decrease in pigment dispersibility in the coating composition. Usually, there is a trade-off between improved color development of the coating film (improved pigment dispersibility) and lowering the viscosity of the pigment dispersion.

The present disclosure has discovered a combination of compounds (pigment dispersants) that can disperse pigments well in polar solvents (aqueous solvents) without causing an increase in viscosity, regardless of the nature of the pigment. The paint composition obtained using the pigment dispersion according to the present disclosure has high color development.

The pigment dispersion according to the present disclosure includes a copolymer (A), a hydrocarbon group-containing acidic compound (B) (hereinafter sometimes referred to as "organic acid (B)"), a pigment (C), and an aqueous solvent (D). The copolymer (A) has a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain. The organic acid (B) has a saturated or unsaturated hydrocarbon group (b1) having 4 to 18 carbon atoms, and at least one acid group (b2) selected from the group consisting of a carboxylic acid group, a phosphoric acid group, a sulfonic acid group, and a phenol group.

In the copolymer (A), the tertiary amino group and/or nitrogen-containing heterocyclic group (hereinafter sometimes referred to as "nitrogen-containing group") of the first segment (a1) and the polyoxyalkylene chain of the second segment (a2) are both arranged as side chains. The main chain of the copolymer (A) is composed of carbon-carbon bonds. In the organic acid (B), the acid group (b2) is usually located at the end of the hydrocarbon group (b1).

The nitrogen-containing group in the first segment (a1) of the copolymer (A) generates a cation in an aqueous solvent. The acid group of the organic acid (B) generates an anion in an aqueous solvent. Therefore, the copolymer (A) and the organic acid (B) can exist as counter ions of each other in an aqueous solvent. The saturated or unsaturated hydrocarbon group (hereinafter sometimes simply referred to as "hydrocarbon group") of the organic acid (B) basically does not have ionic dissociation properties.

The combination of the copolymer (A) and the organic acid (B) improves the dispersibility of various pigments without increasing the viscosity, and the reason for this is thought to be as follows, without being bound by any particular theory.
When the pigment (C) has acidic properties (hereinafter, for convenience, it may be referred to as an "acidic pigment"; typically, carbon black), the copolymer (A) can be adsorbed to the pigment (C) by the nitrogen-containing group. Specifically, the nitrogen-containing group is adsorbed to the pigment (C), and the polyoxyalkylene chain is arranged on the outside of the pigment (C). Furthermore, the polyoxyalkylene chain is arranged so as to extend toward the outside of the pigment (C) due to chemical repulsion and steric repulsion with the hydrophobic main chain of the copolymer (A). The polyoxyalkylene chain extending toward the outside can suppress aggregation of the pigments (C) that have adsorbed the copolymer (A).

Furthermore, the acid group of the organic acid (B) may be present as a counter ion near the nitrogen-containing group. This maintains the charge balance, further suppressing aggregation of the pigments (C).

In addition, the main chain of the copolymer (A) and the hydrocarbon group of the organic acid (B) are both hydrophobic and have a high affinity. Therefore, the pigment (C) can strongly adsorb the organic acid (B) in addition to the copolymer (A). On the other hand, the polyoxyalkylene chain has a high affinity with the aqueous solvent (D). In other words, the pigment (C) that has adsorbed the copolymer (A) and the organic acid (B) is dispersed in the aqueous solvent (D) with the hydrophobic parts of the copolymer (A) and the organic acid (B) surrounded by the hydrophilic polyoxyalkylene chain, so it is difficult to aggregate and is thought to be able to be dispersed while maintaining this state.

From another perspective, the copolymer (A) having a polyoxyalkylene chain disperses the organic acid (B) having a relatively long-chain hydrocarbon group uniformly in the aqueous solvent (D). The organic acid (B) having a hydrocarbon group (b1) can reduce the surface tension of the aqueous solvent (D). The reduction in the surface tension of the aqueous solvent (D) improves the wettability of the pigment (C) to the aqueous solvent. As described above, the organic acid (B) is uniformly dispersed in the aqueous solvent (D) together with the pigment (C), so the surface tension reduction effect is further enhanced, and the wettability of the pigment (C) to the aqueous solvent (D) can be further improved. As a result, the cohesive force between the pigments (C) is further weakened, and the pigment dispersibility in the aqueous solvent (D) is further improved. The dispersion stability of the pigment (C) can also be improved.

By using the organic acid (B) in combination, the copolymer (A) can be selected relatively freely, so long as it has the first segment (a1) and the second segment (a2). In other words, when preparing the aqueous paint composition, the design freedom is expanded, and a copolymer (A) that is more suitable for the pigment (C) can be used.

When the pigment (C) has basic properties (hereinafter, for convenience, it may be referred to as "basic pigment"), the organic acid (B) is adsorbed to the pigment (C) by the acid group (b2). A nitrogen-containing group, which is a counter ion, may be present in the vicinity of the acid group. In other words, in this case, the copolymer (A) and the organic acid (B) can be present in the vicinity of each other. Therefore, the polyoxyalkylene chain of the copolymer (A) is arranged so as to extend toward the outside of the pigment (C) due to chemical and steric repulsion with the hydrocarbon group (b1) of the organic acid (B) and the hydrophobic main chain of the copolymer (A), suppressing the aggregation of the pigments. In addition, as in the case of acidic pigments, the effect of the organic acid (B) in reducing the surface tension of the aqueous solvent improves the pigment dispersibility and further the dispersion stability of the pigment (C).

When the pigment (C) has hydrophobic properties (hereinafter, for convenience, it may be referred to as a "hydrophobic pigment"; typically, phthalocyanine blue), the organic acid (B) is adsorbed to the pigment (C) by hydrophobic interaction with the hydrocarbon group (b1). Therefore, the dispersibility and dispersion stability of the pigment (C) are improved by the same action as in the case of a basic pigment.

As described above, various pigments (C) can be dispersed by using a copolymer (A) having a group that easily adsorbs to an acidic pigment in combination with an organic acid (B) having a group that easily adsorbs to a basic or hydrophobic pigment. Furthermore, since the copolymer (A) and the organic acid (B) exist as counter ions, the charge balance is maintained, the effect of reducing the surface tension is enhanced, and the pigment (C) can exist stably in a finely divided state in the pigment dispersion. In addition, since the nitrogen-containing group and the acid group are provided in separate compounds, the occurrence of an interaction between the two is suppressed, and an increase in the viscosity of the pigment dispersion is suppressed. Since the polyoxyalkylene chain is non-ionic dissociable, it is difficult for an interaction to occur between the nitrogen-containing group in the same compound.

The pigment dispersion according to the present disclosure has a low viscosity, even though it contains finely divided pigments. Therefore, when preparing an aqueous paint composition, the pigment dispersion can be easily mixed with a coating film-forming resin, and the pigment dispersibility in the aqueous paint composition is improved.

[Pigment dispersion]
The pigment dispersion according to the present disclosure includes a copolymer (A), an organic acid (B), a pigment (C), and an aqueous solvent (D). The copolymer (A) has a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having a nitrogen-containing group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain. The organic acid (B) has a hydrocarbon group (b1) having 4 to 18 carbon atoms and an acid group (b2). The pigment dispersion includes the aqueous solvent (D) and is aqueous.

The above-mentioned copolymer (A) and organic acid (B) allow various pigments (C) to be well dispersed in the aqueous solvent (D) without causing an increase in viscosity. In other words, the pigment dispersion according to the present disclosure has a low viscosity, and the pigment is well dispersed in the pigment dispersion (in other words, the average particle size of the pigment is small).

The copolymer (A) and the organic acid (B) can be blended so that the ratio of the equivalent of the acid group (b2) of the organic acid (B) to the total equivalent of the nitrogen-containing group of the copolymer (A) (acid group/nitrogen-containing group) is 0.5 or more and 2 or less. When the equivalent ratio (acid group/nitrogen-containing group) is 0.5 or more, the effect of improving the pigment dispersibility is more easily obtained. When the equivalent ratio (acid group/nitrogen-containing group) is 2 or less, the dispersion effect of the organic acid (B) by the copolymer (A) is more enhanced, and the effect of improving the dispersion stability is more easily obtained. The equivalent ratio (acid group/nitrogen-containing group) may be 0.6 or more, or may be 0.8 or more. The equivalent ratio (acid group/nitrogen-containing group) may be 1.8 or less, or may be 1.5 or less.

For example, the pigment (C) is blended in an amount of 30 parts by mass or more and 3000 parts by mass or less per 100 parts by mass of the solid content of the copolymer (A). The blended amount of the pigment (C) may be 45 parts by mass or more, or may be 60 parts by mass or more. The blended amount of the pigment (C) may be 2800 parts by mass or less, or may be 2700 parts by mass or less.

The solid content concentration of the pigment dispersion is, for example, 5% by mass or more and 90% by mass or less. The solid content concentration of the pigment dispersion may be 8% by mass or more, or 10% by mass or more. The solid content concentration of the pigment dispersion may be 88% by mass or less, or 85% by mass or less.

The solids concentration of the pigment dispersion is measured according to JIS K 5601-1-2 Heat Residue Measurement Method.

Pigment (C) is finely dispersed in the pigment dispersion. The average particle size of pigment (C) in the pigment dispersion can be, for example, 90% or more and 1000% or less of the primary particle size of the pigment (C). When the average particle size of pigment (C) in the pigment dispersion is within this range, it can be said that the aggregation of pigment (C) is suppressed and the pigment is microparticulated. The average particle size of pigment (C) in the pigment dispersion can be 900% or less, or 800% or less of the primary particle size of the pigment (C).

The average particle size refers to the volume average particle size D50. The volume average particle size D50 is the 50% average particle size (D50) in the volume-based particle size distribution measured using a laser diffraction/scattering particle size distribution measuring device (for example, product name: UPA-150, manufactured by Microtrac).

Pigment dispersions have various viscosities depending on the type of pigment. It is desirable for the viscosity of the pigment dispersion to be low so that the coating composition can be easily prepared. The viscosity of the pigment dispersion at 25°C and a rotation speed of 5 rpm may be, for example, 5000 cps or less, or 2000 cps or less. The lower limit of the viscosity of the pigment dispersion is appropriately set depending on the type and size of the pigment (C), and is not particularly limited.

The pigment dispersion according to the present disclosure has a sufficiently lower viscosity than other pigment dispersions in which the same pigment is dispersed to a similar particle size using a pigment dispersant other than the copolymer (A) and/or the organic acid (B). For example, the viscosity of the pigment dispersion according to the present disclosure may be about 1/2 or less, or about 1/3 or less, of the viscosity of the other pigment dispersions described above. Alternatively, the pigment dispersion according to the present disclosure may have a smaller pigment particle size than yet another pigment dispersion in which the same pigment is dispersed to a degree that does not cause an increase in viscosity using a pigment dispersant other than the copolymer (A) and/or the organic acid (B). For example, the average particle size of the pigment contained in the pigment dispersion according to the present disclosure may be about 80% or less, about 75% or less, or about 70% or less of the average particle size of the pigment contained in the yet another pigment dispersion described above.

(A) Copolymer The copolymer (A) has a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having a nitrogen-containing group (hereinafter may be referred to as a "nitrogen-containing monomer"), and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain (hereinafter may be referred to as a "hydrophilic monomer").

The first segment (a1) has at least one nitrogen-containing group. The nitrogen-containing group generates a cation in an aqueous solvent and can be adsorbed to an acidic pigment. In addition, the nitrogen-containing group does not have an active hydrogen that chemically reacts with a component (typically a curing agent) blended in the aqueous coating composition, and therefore is unlikely to inhibit the curing reaction of the aqueous coating composition.

The second segment (a2) has at least one polyoxyalkylene chain. The polyoxyalkylene chain inhibits the pigments from approaching each other, contributing to the suppression of aggregation and increasing the hydrophilicity of the copolymer (A).

The copolymer (A) may further have a third segment (a3) derived from a polymerizable unsaturated monomer other than the nitrogen-containing monomer and the hydrophilic monomer.

The mass percentage of the first segment (a1) may be 5% by mass or more and 30% by mass or less, the mass percentage of the second segment (a2) may be 20% by mass or more and 80% by mass or less, and the mass percentage of the third segment (a3) may be 10% by mass or more and 60% by mass or less.

When the mass proportion of the first segment (a1) is 5% by mass or more, the adsorption performance to the pigment is improved. When the mass proportion of the first segment (a1) is 30% by mass or less, the hydrophilicity of the copolymer (A) is prevented from becoming excessively high, and the deterioration of the water resistance of the coating film is prevented. The mass proportion of the first segment (a1) may be 7% by mass or more, 10% by mass or more, or 12% by mass or more. The mass proportion of the first segment (a1) may be 27% by mass or less, 25% by mass or less, or 22% by mass or less.

When the mass proportion of the second segment (a2) is 20% by mass or more, the dispersibility of the pigment adsorbed with the copolymer (A) is improved. When the mass proportion of the second segment (a2) is 80% by mass or less, the hydrophilicity of the copolymer (A) is prevented from becoming excessively high, and the deterioration of the water resistance of the coating film is prevented. The mass proportion of the second segment (a2) may be 30% by mass or more, 35% by mass or more, or 40% by mass or more. The mass proportion of the second segment (a2) may be 75% by mass or less, 70% by mass or less, or 65% by mass or less.

The mass proportion of the third segment (a3) may be, for example, 10% by mass or more and 60% by mass or less. The mass proportion of the third segment (a3) may be 15% by mass or more, or 20% by mass or more. The mass proportion of the third segment (a3) may be 55% by mass or less, or 50% by mass or less.

The third segment (a3) may contain a segment derived from a hydroxyl-containing polymerizable unsaturated monomer (hereinafter referred to as hydroxyl-containing segment (a31)). The hydroxyl group can chemically react with the components blended in the aqueous coating composition. The copolymer (A) adsorbed to the pigment reacts chemically with the curing agent, thereby suppressing the pigment from falling off (bleeding out) from the coating film. The mass ratio of the hydroxyl-containing segment (a31) may be 10% by mass or more and 30% by mass or less. When the mass ratio of the hydroxyl-containing segment (a31) is in this range, the effect of suppressing bleed-out is easily exerted, and the viscosity of the pigment dispersion is less likely to increase. The mass ratio of the hydroxyl-containing segment (a31) may be 12% by mass or more, or 15% by mass or more. The mass ratio of the hydroxyl-containing segment (a31) may be 25% by mass or less, or 20% by mass or less.

The content of the first segment (a1) relative to all structural units can be calculated by dividing the charged mass of the nitrogen-containing polymerizable unsaturated monomer used in synthesizing the copolymer (A) by the total charged mass of all raw material monomers. Similarly, the content of all structural units of the other segments can be calculated by dividing the charged mass of the monomer forming the segment used in synthesizing the copolymer (A) by the total charged mass of all raw material monomers.

The weight average molecular weight of copolymer (A) may be 10,000 or more and 40,000 or less. The weight average molecular weight of copolymer (A) may be 15,000 or more and may be 20,000 or more. The weight average molecular weight of copolymer (A) may be 38,000 or less and may be 35,000 or less.

The weight average molecular weight can be calculated from a chromatogram measured by gel permeation chromatography, based on the molecular weight of standard polystyrene. The acid value and hydroxyl value are calculated from the monomer composition used in the preparation, in accordance with JIS regulations.

(Nitrogen-containing polymerizable unsaturated monomer)
The nitrogen-containing polymerizable unsaturated monomer (nitrogen-containing monomer) has at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a polymerizable unsaturated group in one molecule. The tertiary amino group and the nitrogen-containing heterocyclic group are basic. Therefore, the (a1) first segment derived from the nitrogen-containing monomer can be well adsorbed to a pigment having an acidic site (e.g., carbon black).

Examples of tertiary amino group-containing monomers include N,N-dialkylaminoalkyl (meth)acrylates such as N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, N,N-di-t-butylaminoethyl (meth)acrylate, and N,N-dimethylaminobutyl (meth)acrylate; and N,N-dialkylaminoalkyl (meth)acrylamides such as N,N-dimethylaminoethyl (meth)acrylamide, N,N-diethylaminoethyl (meth)acrylamide, and N,N-dimethylaminopropyl (meth)acrylamide. These may be used alone or in combination of two or more.

Among these, N,N-dialkylaminoalkyl(meth)acrylate may be used, and N,N-dimethylaminoethyl(meth)acrylate and N,N-diethylaminoethyl(meth)acrylate may be used.

In this specification, "(meth)acrylic" is a concept that includes both acrylic and methacrylic.

Examples of nitrogen-containing heterocyclic groups include pyridine ring groups, quinoline ring groups, thiazole ring groups, oxazole ring groups, benzothiazole ring groups, imidazole ring groups, pyrazole ring groups, imidazoline ring groups, pyrimidine ring groups, pyrazine ring groups, triazole ring groups, and tetrazole ring groups.

Examples of polymerizable unsaturated monomers (vinyl monomers) having a pyridine ring include 2-vinylpyridine and 4-vinylpyridine. Examples of polymerizable unsaturated monomers having a thiazole ring include 2-vinylthiazole and 4-methyl-5-vinylthiazole. Examples of polymerizable unsaturated monomers having an oxazole ring include 2-phenyl-5-vinyloxazole. Examples of polymerizable unsaturated monomers having a benzothiazole ring include 2-vinylbenzothiazole, 2-[2-(1-naphthyl)vinyl]benzothiazole, and 2-[2-(dimethylamino)vinyl]benzothiazole. Examples of polymerizable unsaturated monomers having an imidazole ring include 1-vinylimidazole, 2-methyl-1-vinylimidazole, 2-vinylimidazole, 4-vinylimidazole, 2-phenyl-1-vinylimidazole, 1-vinylcarbazole, and 2-(1H-imidazol-1-yl)ethyl (meth)acrylate. Examples of polymerizable unsaturated monomers having a pyrazole ring include 1-vinylpyrazole and 3-vinylpyrazole. Examples of polymerizable unsaturated monomers having an imidazoline ring include 1-vinyl-2-imidazoline, 1-vinyl-2-methylimidazoline, 2-vinyl-2-imidazoline, and 2-(1H-imidazolin-1-yl)ethyl (meth)acrylate. Examples of polymerizable unsaturated monomers having a pyrimidine ring include 5-vinylpyrimidine and 2,4-dichloro-6-vinylpyrimidine. Examples of polymerizable unsaturated monomers having a pyrazine ring include 2-vinylpyrazine, 2,5-dimethyl-3-vinylpyrazine, and 2-methyl-5-vinylpyrazine. Examples of vinyl monomers having a triazole ring include 2,4-diamino-6-vinyltriazine. Examples of polymerizable unsaturated monomers having a tetrazole ring include 1-vinyl-1H-tetrazole, 2-vinyl-2H-tetrazole, 5-vinyl-1H-tetrazole, and 1-methyl-5-vinyl-1H-tetrazole. These may be used alone or in combination of two or more.

Among these, it may be at least one of a vinyl monomer having a pyridine ring group and a vinyl monomer having an imidazole ring group.

Among them, the nitrogen-containing monomer may be an N,N-dialkylaminoalkyl(meth)acrylate, which further improves the dispersibility of the pigment. A preferred N,N-dialkylaminoalkyl(meth)acrylate is represented by the following general formula:

In the formula, A represents hydrogen or a methyl group, ^B1 and ^B2 each independently represent a linear or branched hydrocarbon group having 1 to 4 carbon atoms, and n is an integer of 1 to 3. In the formula, A may be a methyl group, ^B1 and ^B2 may be a hydrocarbon group having 1 or 2 carbon atoms, and n may be 1 or 2.

(Polymerizable unsaturated monomer having a polyoxyalkylene chain)
The polymerizable unsaturated monomer having a polyoxyalkylene chain (hydrophilic monomer) has a polyoxyalkylene chain and a polymerizable unsaturated group in one molecule. The second segment (a2) derived from the hydrophilic monomer imparts hydrophilicity to the copolymer (A).

Examples of polyoxyalkylene chains include polyoxyethylene chains, polyoxypropylene chains, polyoxyethylene blocks, and chains containing polyoxypropylene blocks. The proportion of polyoxyethylene chains in all polyoxyalkylene chains may be 65% by mass or more, 70% by mass or more, 75% by mass or more, or 80% by mass or more. In one embodiment, the polyoxyalkylene chain is composed of polyoxyethylene.

The molecular weight of the polyoxyalkylene chain is, for example, 200 or more and 5,000 or less. From the viewpoint of dispersion stability, the molecular weight of the polyoxyalkylene chain may be 300 or more, 400 or more, 800 or more, or 1,000 or more. The molecular weight of the polyoxyalkylene chain may be 3,500 or less, or 2,500 or less. In particular, if the molecular weight of the polyoxyalkylene chain is 800 or more, long-term dispersion stability is likely to be obtained.

Examples of hydrophilic monomers include acrylates having a polyoxyethylene chain, such as tetraethylene glycol (meth)acrylate, methoxytetraethylene glycol (meth)acrylate, ethoxytetraethylene glycol (meth)acrylate, n-butoxytetraethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, and ethoxypolyethylene glycol (meth)acrylate; and acrylates having a polyoxypropylene chain, such as tetrapropylene glycol (meth)acrylate, methoxytetrapropylene glycol (meth)acrylate, ethoxytetrapropylene glycol (meth)acrylate, n-butoxytetrapropylene glycol (meth)acrylate, and polypropylene glycol (meth)acrylate. These may be used alone or in combination of two or more.

(Other polymerizable unsaturated monomers)
The other polymerizable unsaturated monomer (hereinafter sometimes referred to as "other raw material monomer") has a polymerizable unsaturated group in one molecule, but does not have a polyoxyalkylene chain or a nitrogen-containing group. The third segment (a3) derived from the other raw material monomer can impart various functions to the copolymer (A).

It is desirable that the other raw material monomers do not have acid groups such as carboxy groups, phosphate groups, sulfonic acid groups, and phenol groups. This is because if an interaction occurs between the acid groups and the nitrogen-containing groups, intermolecular forces will be generated in the copolymer (A), which may cause the viscosity of the pigment dispersion to become excessively high.

The other raw material monomers may have a hydroxyl group and a polymerizable unsaturated group in one molecule. The hydroxyl group can chemically react with the curing agent blended in the aqueous paint composition, which can improve the water resistance of the resulting coating film. Furthermore, the copolymer (A) adsorbed to the pigment (C) can chemically react with the curing agent, which can help prevent the pigment from falling off (bleeding out) from the coating film.

Examples of hydroxyl group-containing polymerizable unsaturated monomers (hereinafter sometimes referred to as "hydroxyl group-containing monomers") include monoesters of (meth)acrylic acid and 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 monoesters of (meth)acrylic acid and dihydric alcohols having 2 to 8 carbon atoms; and allyl alcohol. These may be used alone or in combination of two or more.

Other raw material monomers besides the hydroxyl group-containing monomers include, for example, alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, and tert-butyl (meth)acrylate; vinyl aromatic compounds such as styrene, α-methylstyrene, and vinyl toluene; (meth)acrylonitrile, (meth)acrylamide, and vinyl acetate. These may be used alone or in combination of two or more.

(B) Organic Acid The organic acid (B) has a hydrocarbon group (b1) and an acid group (b2). The acid group (b2) is typically located at the terminal of the hydrocarbon group (b1).

The hydrocarbon group (b1) may be saturated or unsaturated. The hydrocarbon group (b1) may be linear, alicyclic, or may have an aromatic ring. In particular, from the viewpoint of color development, the hydrocarbon group (b1) may be linear. The chain-like hydrocarbon group (b1) may be linear or branched.

The number of carbon atoms in the hydrocarbon group (b1) is 5 to 23. When the organic acid (B) has a relatively long hydrocarbon group (b1), the hydrophobic pigment is more easily adsorbed to the organic acid (B), the polyoxyalkylene chain is more easily extended outward, and the surface tension of the aqueous solvent is more easily reduced. In addition, when preparing the aqueous paint composition, the organic acid (B) is prevented from dissociating from the pigment to which the organic acid (B) is adsorbed, and the dispersibility of the pigment in the aqueous paint composition is also maintained. On the other hand, if the number of carbon atoms in the hydrocarbon group (b1) is excessively large, it is difficult to uniformly disperse the pigment in the aqueous solvent. The number of carbon atoms in the hydrocarbon group (b1) may be 6 or more, 7 or more, or 12 or more. The number of carbon atoms in the hydrocarbon group (b1) may be 21 or less, or 19 or less.

The acid group (b2) is at least one selected from the group consisting of a carboxy group, a phosphate group, a sulfonic acid group, and a phenol group. The organic acid (B) may have a plurality of the same or different acid groups (b2), or may have one acid group (b2).

Examples of organic acids (B) having a carboxy group include monovalent, divalent and trivalent organic carboxylic acids. Examples of monovalent organic carboxylic acids include caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, isostearic acid, undecylenic acid, petroselinic acid, oleic acid, ricinoleic acid, linoleic acid, linolenic acid and cyclohexane carboxylic acid. Examples of divalent organic carboxylic acids include fumaric acid, maleic acid, adipic acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid and 1,4-cyclohexane dicarboxylic acid. Examples of trivalent organic carboxylic acids include aconitic acid. Other examples of organic acids (B) having a carboxy group include amino acids and their derivatives (e.g. trimethylated amino acids such as trimethylglycine).

Examples of organic acids (B) having a phosphate group include alkylphosphonic acids (e.g., octylphosphonic acid), alkenylphosphonic acids (e.g., oleyl phosphate), and phenylphosphinic acid.

Examples of organic acids (B) having a sulfonic acid group include paraphenylbenzenesulfonic acid, β-naphthalenesulfonic acid, naphthol-1-sulfonic acid, p-toluenesulfonic acid, p-benzenechlorosulfonic acid, and 5-chloro-α,α-bis(3,5-dichloro-2-hydroxyphenyl)toluenesulfonic acid.

Examples of organic acids (B) having a phenol group include nonylphenol, t-butylphenol, and t-butylcatechol.

(C) Pigment Examples of the pigment include color pigments, extender pigments, anti-rust pigments, and luster pigments.
The color pigment may be inorganic or organic. The color pigment may be chromatic or achromatic. Examples of organic color pigments include azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Examples of inorganic color pigments include yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These may be used alone or in combination of two or more.

Examples of extender pigments include calcium carbonate, barium sulfate, clay, and talc. These may be used alone or in combination of two or more.

Examples of luster pigments include mica pigments such as interference mica, white mica, and colored mica; graphite pigments; glass flake pigments; and metal pigments such as aluminum, copper, zinc, iron, nickel, tin, aluminum oxide, chromium oxide, and alloys containing these. These may be used alone or in combination of two or more. Luster pigments may be colored.

(D) Aqueous Solvent The aqueous solvent (D) accounts for 50% by mass or more of the total solvent in the pigment dispersion. Examples of the aqueous solvent (D) include water and a mixture of water and a hydrophilic solvent.

Examples of hydrophilic solvents include glycol-based solvents such as ethylene glycol, propylene glycol, butanediol, pentanediol, diethylene glycol, dipropylene glycol, and triethylene glycol; glycol ether-based solvents such as ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monobutyl ether, diethylene glycol dibutyl ether, triethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, and propylene glycol monomethyl ether acetate; alcohol-based solvents such as methanol, ethanol, and isopropyl alcohol; ketone-based solvents such as acetone; and N-methyl-2-pyrrolidone. These may be used alone or in combination of two or more.

(others)
The pigment dispersion may contain other components such as a defoamer, an ultraviolet absorber, a light stabilizer, an antioxidant, a surface conditioner, a film-forming assistant, and a rust inhibitor.

[Method of producing pigment dispersion]
The pigment dispersion according to the present disclosure is produced by a method including a step of adding an organic acid (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) and mixing the resulting mixture to obtain a second mixture, and a step of adding a pigment (C) to the second mixture and mixing the resulting mixture.

By first mixing the copolymer (A) with the aqueous solvent (D) and then mixing the organic acid (B), the organic acid (B) becomes more easily dissolved and/or dispersed in the aqueous solvent (D), and the dispersibility of the pigment (C) is further improved.

Each mixing is carried out, for example, using a disperser (agitator). Examples of dispersers include low-speed agitators such as a propeller mixer, paddle mixer, and anchor mixer; and high-speed agitators such as a homo mixer, disperser mixer, and ultra mixer.

The obtained pigment dispersion may be further mixed by a dispersing machine using dispersion media (beads). This promotes the fine particle formation of the pigment (C). Specific examples of dispersing machines include Ultra Apex Mill, product name: Dual Apex Mill (Kotobuki Industries Co., Ltd.), Paint Conditioner, Picograin Mill, product name: Eco Mill (Asada Iron Works Co., Ltd.), Star Mill ZRS, Star Mill, Nano Getter, product name: Max Nano Getter (Ashizawa Finetech Co., Ltd.), product name: Micro Media (Buehler Co., Ltd.), product name: MSC Mill (Nippon Coke & Engineering Co., Ltd.), product name: NPM (Shinmaru Enterprises Co., Ltd.), and Sand Grind (SG) Mill.

Examples of materials for the dispersion media include alumina, zirconia, silicon carbide, silicon nitride, glass, steel, stainless steel, and ceramics. The particle size of the dispersion media may be, for example, 0.3 mm or less, 0.1 mm or less, or 0.05 mm or less. This can further promote the fine particle size of the pigment (C).

The agitator rotation speed of the dispersing machine may be, for example, 500 rpm or more and 5000 rpm or less. The mixing time may be, for example, 30 minutes or more and 200 minutes or less. The mixing temperature may be, for example, 5°C or more and 45°C or less.

[Water-based paint composition]
The aqueous coating composition may contain the above-mentioned pigment dispersion and a coating film-forming resin. The aqueous coating composition may further contain a curing agent capable of reacting with the coating film-forming resin and an aqueous solvent. The aqueous solvent may be the same as the aqueous solvent exemplified as that contained in the pigment dispersion. The aqueous coating composition may be further diluted with a solvent suitable for coating.

The pigment (C) is sufficiently and stably dispersed even in the aqueous coating composition. Therefore, the resulting coating film has excellent color development. Color development is determined by comprehensively evaluating the gloss, transparency, depth of color, etc. according to the type of pigment. "Having excellent color development" can also be said to mean that the color expected from the pigment is expressed in the coating film.

The pigment dispersion is blended so that, for example, the pigment (C) is present in an amount of 1 part by mass or more and 30 parts by mass or less per 100 parts by mass of the solid content of the film-forming resin contained in the aqueous paint composition. The blended amount of pigment (C) may be 2 parts by mass or more, or may be 3 parts by mass or more. The blended amount of pigment (C) may be 25 parts by mass or less, or may be 20 parts by mass or less.

The solids concentration of the aqueous paint composition is, for example, 3% by mass or more and 50% by mass or less. The solids concentration of the aqueous paint composition may be 5% by mass or more, or 7% by mass or more. The solids concentration of the aqueous paint composition may be 45% by mass or less, or 40% by mass or less.

The solids concentration of the aqueous coating composition is measured according to JIS K 5601-1-2 Heat Residue Measurement Method.

(Film-forming resin)
Examples of the film-forming resin include acrylic resin, acrylic silicone resin, polyester resin, polyurethane resin, epoxy resin, fluororesin, and silicone resin. These may be used alone or in combination of two or more. Among them, acrylic resin is preferable.

The coating film-forming resin may contain a hydroxyl-containing resin. Examples of hydroxyl-containing resins include hydroxyl-containing acrylic resins, hydroxyl-containing polyester resins, polycarbonate polyol resins, polyether polyol resins, and polycaprolactone polyol resins. These may be used alone or in combination of two or more. Of these, hydroxyl-containing acrylic resins may be used.

The hydroxyl value (OHV) of the hydroxyl-containing resin may be, for example, 20 mgKOH/g or more and 180 mgKOH/g or less. When the hydroxyl value of the hydroxyl-containing resin is 20 mgKOH/g or more, the breaking strength of the coating film is likely to be high. When the hydroxyl value of the hydroxyl-containing resin is 180 mgKOH/g or less, the hydrophilization of the coating film is suppressed, and the water resistance is likely to be improved. The hydroxyl value of the hydroxyl-containing resin may be 30 mgKOH/g or more, or 40 mgKOH/g or more. The hydroxyl value of the hydroxyl-containing resin may be 150 mgKOH/g or less, 140 mgKOH/g or less, 100 mgKOH/g or less, or 80 mgKOH/g or less.

The weight average molecular weight (Mw) of the hydroxyl-containing resin (A) may be, for example, 2,000 or more and 50,000 or less.

The weight average molecular weight is determined by the GPC method using polystyrene as the standard. The acid value and hydroxyl value are calculated from the monomer composition used in the preparation, based on JIS regulations.

The film-forming resin may be included as an emulsion, a dispersion, or dissolved in a solvent. For example, an acrylic resin dispersion can be prepared by solution polymerizing the above-mentioned α,β-ethylenically unsaturated monomer and dispersing it using a basic compound. A water-soluble acrylic resin can be prepared, for example, by solution polymerizing the above-mentioned α,β-ethylenically unsaturated monomer and dispersing it using a basic compound.

The emulsion of the hydroxyl-containing acrylic resin can be prepared, for example, by emulsion polymerization of α,β-ethylenically unsaturated monomers. Examples of the α,β-ethylenically unsaturated monomers include (meth)acrylic acid esters, α,β-ethylenically unsaturated monomers having an acid group, and α,β-ethylenically unsaturated monomers having a hydroxyl group. The monomers can be used alone or in combination of two or more.

Examples of (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate. (Meth)acrylic acid esters refer to acrylic acid esters and methacrylic acid esters.

Examples of α,β-ethylenically unsaturated monomers having an acid group include acrylic acid, methacrylic acid, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, isocrotonic acid, α-hydro-ω-((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-1,6-hexanediyl)), maleic acid, fumaric acid, itaconic acid, 3-vinylsalicylic acid, 3-vinylacetylsalicylic acid, 2-acrylamido-2-methylpropanesulfonic acid, p-hydroxystyrene, and 2,4-dihydroxy-4'-vinylbenzophenone.

Examples of α,β-ethylenically unsaturated monomers having a hydroxyl group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ε-caprolactone.

Other α,β-ethylenically unsaturated monomers may be used in combination. Examples of other α,β-ethylenically unsaturated monomers include polymerizable amide compounds, polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds, polymerizable alkoxysilyl compounds, and other polymerizable compounds.

The emulsion polymerization method is not particularly limited. For example, an emulsifier is dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as an alcohol or ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and an α,β-ethylenically unsaturated monomer and a polymerization initiator are added dropwise while heating and stirring. The α,β-ethylenically unsaturated monomer may be emulsified in advance with an emulsifier.

Polymerization initiators and emulsifiers that are commonly used by those skilled in the art can be used. If necessary, the molecular weight can be adjusted using chain transfer agents such as mercaptans (e.g., lauryl mercaptan) and α-methylstyrene dimer. The reaction temperature, reaction time, and the like can be appropriately selected within the ranges commonly used by those skilled in the art. The obtained acrylic resin emulsion is neutralized with a base as necessary.

The hydroxyl-containing acrylic resin obtained by emulsion polymerization may have a number average molecular weight of 3,000 or more. The hydroxyl-containing acrylic resin may have a hydroxyl value (solid content hydroxyl value) of 20 mgKOH/g or more and 180 mgKOH/g or less. The hydroxyl-containing acrylic resin may have an acid value (solid content acid value) of 1 mgKOH/g or more and 80 mgKOH/g or less.

(Hardening agent)
The aqueous coating composition may include a curing agent that reacts with the film-forming resin to form a cured coating therewith.

Examples of the curing agent include melamine resin, blocked isocyanate compound, epoxy compound, aziridine compound, carbodiimide compound, oxazoline compound, and metal ion. These may be used alone or in combination of two or more. Among them, at least one of melamine resin and blocked isocyanate compound may be used.

The melamine resin may be water-soluble or water-insoluble. The melamine resin contains a structure in which hydrogen atoms or substituents (alkyl ether groups, methylol groups, etc.) are bonded to the periphery of a melamine nucleus (triazine nucleus) via three nitrogen atoms. The melamine resin is generally composed of a polynuclear body in which multiple melamine nuclei are bonded to each other. The melamine resin may be a mononuclear body consisting of one melamine nucleus.

Commercially available melamine resins may be used. Examples of commercially available melamine resins include the Cymel series (trade name) manufactured by Allnex, specifically Cymel 202, Cymel 204, Cymel 211, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 250, Cymel 251, Cymel 254, Cymel 266, Cymel 267, Cymel 272, Cymel 285, Cymel 301, Cymel 303, Cymel 325, Cymel 327, Cymel 350, Cymel 370, Cymel 701, Cymel 703, and Cymel 1141; and the U-Ban (trade name) series manufactured by Mitsui Chemicals. These may be used alone or in combination of two or more.

Blocked isocyanate compounds can be prepared by adding a blocking agent having an active hydrogen to a polyisocyanate such as trimethylene diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, or isophorone diisocyanate.

The content of the curing agent may be 10% by mass or more and 80% by mass or less of the resin solids contained in the aqueous coating composition. The content of the curing agent may be 15% by mass or more. The content of the curing agent may be 60% by mass or less.

(others)
The aqueous coating composition may contain other components such as a defoamer, an ultraviolet absorber, a light stabilizer (e.g., a hindered amine), an antioxidant, a surface conditioner, a film-forming assistant, and a rust inhibitor.

[Method of producing aqueous coating composition]
The aqueous coating composition according to the present disclosure is produced by a method comprising a step of mixing the pigment dispersion produced by the above-mentioned method with a coating film-forming resin.

The mixing conditions and mixing equipment may be the same as those mentioned in the method for producing the pigment dispersion.

[Painted items]
The aqueous coating composition according to the present disclosure provides a coated article having a coating film with excellent color development. The coated article includes a substrate and a base coat coating film formed from the aqueous coating composition. The coated article may include a substrate, a base coat coating film formed from the aqueous coating composition, and a clear coating film disposed on the base coat coating film.

The base coat film may be one layer or two or more layers. At least one base coat film is formed from the aqueous paint composition according to the present disclosure. The components contained in each base coat film may be the same or different.

(Substrate)
Examples of the material of the object to be coated include metals, plastics, and foams. In particular, the object may be a metal (particularly a casting), and may be a metal that can be electrocoated. Examples of such metals include iron, copper, aluminum, tin, zinc, and alloys containing these metals.

The shape of the object to be coated is not particularly limited, and may be flat or three-dimensionally shaped. Specific examples of objects to be coated include automobile bodies and parts thereof, such as passenger cars, trucks, motorcycles, and buses.

The metal substrate may be subjected to a chemical conversion treatment using a phosphoric acid-based chemical conversion treatment agent, a zirconium-based chemical conversion treatment agent, or the like, and may be subjected to electrodeposition coating. The electrodeposition coating composition may be of the cationic type or the anionic type. The cationic type electrodeposition coating composition can form a coating film with excellent corrosion resistance.

The metal substrate may have an electrocoating film and an intermediate coating film formed thereon. The intermediate coating film is usually formed for the purpose of improving the adhesion and durability of the multi-layer coating film. The intermediate coating composition may contain, for example, a film-forming resin, a hardener, a color pigment, and an extender pigment. Examples of the film-forming resin and hardener include those contained in the aqueous coating composition according to the present disclosure.

(Base coat film)
At least one layer of base coat film is formed from the above-mentioned aqueous coating composition. The thickness of the base coat film may be 0.2 μm or more and 50 μm or less. The thickness of the base film may be 3 μm or more. The thickness of the base film may be 40 μm or less, 30 μm or less, or 20 μm or less.

(Clear coating)
The clear coating film protects the base coating film. The thickness of the clear coating film may be, for example, 10 μm or more and 80 μm or less. The thickness of the clear coating film may be 20 μm or more. The thickness of the clear coating film may be 60 μm or less.

The clear coating film is formed by a clear coating composition. The clear coating composition may be solvent-based, water-based, or powder-type. From the viewpoint of transparency or acid etching resistance, the solvent-based clear coating composition may contain an acrylic resin and/or a polyester resin as a coating film-forming resin, and an amino resin and/or an isocyanate as a curing agent. The solvent-based clear coating composition may also contain an acrylic resin and/or a polyester resin having a carboxylic acid and/or an epoxy group.

The clear coating composition may contain the above-mentioned various pigments to the extent that the transparency and the effect of the coating composition according to the present disclosure are not impaired. The clear coating composition may contain various additives as necessary. Examples of additives include ultraviolet absorbers, antioxidants, defoamers, surface conditioners, and pinhole inhibitors.

[Method of manufacturing coated articles]
The method for producing a coated article will be described using as an example a coated article having a substrate, a base coat film, and a clear coat film.

A coated article having a base coat film and a clear coat film can be obtained, for example, by sequentially coating an aqueous coating composition according to the present disclosure and a clear coating composition on a substrate, and then curing both compositions simultaneously. After coating the aqueous coating composition, preheating may be performed before coating the clear coating composition.

Examples of painting methods include air spray painting, airless spray painting, electrostatic spray painting, multi-stage painting using air electrostatic spray painting (preferably two-stage painting), and a combination of air electrostatic spray painting and a rotary atomizer-type electrostatic paint sprayer.

The curing of each coating composition is carried out, for example, at a heating temperature of 80°C to 180°C (preferably 100°C to 160°C) for a heating time of 5 to 60 minutes (preferably 10 to 30 minutes).

The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are by weight unless otherwise specified.

Production Example 1-1 Production of Copolymer (A-1) 78 parts of dipropylene glycol monomethyl ether was charged into a reaction vessel equipped with a stirrer, a temperature controller, a cooling tube, and a dropping device, and the temperature was raised to 120° C. with stirring to reflux.
Next, a solution of 15 parts of 2-(dimethylamino)ethyl methacrylate, 60 parts of a monomer having a polyoxyalkylene structure (product name: PME-1000, manufactured by NOF Corp., methoxypolyethylene glycol-methacrylate), 19 parts of 2-hydroxyethyl methacrylate and 6 parts of butyl acrylate, 0.5 parts of Kayaester O (manufactured by NOF Corp.), and 76 parts of dipropylene glycol monomethyl ether was added dropwise over 3 hours and reacted. As a post-shot, a polymerization initiator solution in which 0.3 parts of Kayaester O (manufactured by NOF Corp.) was dissolved in 11 parts of dipropylene glycol monomethyl ether was further added dropwise over 0.5 hours as a polymerization initiator, and the mixture was polymerized by stirring for 0.5 hours to obtain a copolymer (A-1). Next, the solvent was distilled off by vacuum distillation at 90° C., and then 100 parts of ion-exchanged water was added relative to 1,000 parts of the copolymer (A-1), and the mixture was stirred to obtain an aqueous solution of the copolymer (A-1) (solid content 50% by mass).
The resulting copolymer (A-1) had a hydroxyl value of 82 mg KOH/g and a weight average molecular weight of 30,000.

[Production Examples 1-2 to 1-14] Production of Copolymers (A-2) to (A-12), (a-1), and (a-2) Copolymers (A-2) to (A-12), (a-1), and (a-2) were prepared in the same manner as in Production Example 1-1, except that the monomers shown in the following table were used in the amounts shown in Table 1.

[Production Example 2] Production of acrylic resin emulsion 633 parts of deionized water was added to a reaction vessel, and the temperature was raised to 80° C. while mixing and stirring in a nitrogen stream. Separately, 75.65 parts by mass of styrene (ST), 178.96 parts by mass of methyl methacrylate (MMA), 75.94 parts by mass of n-butyl acrylate (BA), 64.45 parts by mass of 2-ethylhexyl acrylate (2-EHA), and 105 parts by mass of hydroxyethyl methacrylate (HEMA) were mixed to prepare a first-stage monomer mixture. Next, this monomer mixture was mixed with 25 parts of Aqualon HS-10 (polyoxyethylene alkylpropenyl phenyl ether sulfate, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), 25.0 parts of Adeka Reasoap NE-20 (α-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]-ω-hydroxyoxyethylene, manufactured by Asahi Denka Co., Ltd.), and 400 parts of deionized water to prepare a monomer emulsion. Separately, an initiator solution consisting of 1.2 parts of ammonium persulfate and 500 parts of deionized water was prepared. The monomer emulsion and the initiator solution were dropped in parallel into the above reaction vessel over a period of 1.5 hours. After completion of the dropwise addition, the mixture was aged at the same temperature for 1 hour.

Separately, 53.65 parts by mass of styrene (ST), 178.96 parts by mass of methyl methacrylate (MMA), 75.94 parts by mass of n-butyl acrylate (BA), 64.45 parts by mass of 2-ethylhexyl acrylate (2-EHA), 105 parts by mass of hydroxyethyl methacrylate (HEMA), and 22 parts by mass of acrylic acid were mixed to prepare a second-stage monomer mixture. Next, this monomer mixture was mixed with 10 parts of Aqualon HS-10 and 250 parts of deionized water to prepare a monomer emulsion. Separately, an initiator solution consisting of 3.0 parts of ammonium persulfate and 500 parts of deionized water was prepared. The monomer emulsion and the initiator solution were dropped in parallel into the above reaction vessel over a period of 1.5 hours. After the end of the dropwise addition, the mixture was aged at the same temperature for 2 hours.

The reaction mixture was then cooled to 40°C and filtered through a 400 mesh filter. Finally, 100 parts of deionized water and 1.6 parts of dimethylaminoethanol were added to the reaction mixture to adjust the pH to 6.5. In this way, an acrylic resin emulsion was obtained with an average particle size of 150 nm, a solid content of 35 mass%, a solid content acid value of 20 mgKOH/g, and a hydroxyl value of 100 mgKOH/g.

[Production Example 3] Production of water-soluble acrylic resin 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether were added to a reaction vessel, and the temperature was raised to 105 ° C. while mixing and stirring in a nitrogen stream. Next, a monomer mixture containing 13.1 parts of methyl methacrylate, 68.4 parts of ethyl acrylate, 11.6 parts of 2-hydroxyethyl methacrylate, and 6.9 parts of methacrylic acid was prepared, and an initiator solution consisting of 100 parts of the monomer mixture, 10.0 parts of tripropylene glycol methyl ether, and 1 part of tertiary butyl peroxy 2-ethylhexanoate was dropped into the reaction vessel in parallel over 3 hours. After the dropwise addition, the mixture was aged at the same temperature for 0.5 hours.

Furthermore, an initiator solution consisting of 5.0 parts of tripropylene glycol methyl ether and 0.3 parts of tertiary butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel over a period of 0.5 hours. After the addition was completed, the mixture was aged at the same temperature for 2 hours.

After 16.1 parts of the solvent was distilled off at 110°C under reduced pressure (70 torr) using a solvent remover, 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added to obtain a water-soluble acrylic resin solution. The solid content of the resulting water-soluble acrylic resin solution was 30% by mass, with a solid acid value of 40 mg KOH/g, a hydroxyl value of 50 mg KOH/g, and a viscosity of 140 poise (E-type viscometer 1 rpm/25°C).

The weight average molecular weight was measured using a GPC device "HLC8220GPC" (product name, manufactured by Tosoh Corporation) and four columns "Shodex KF-606M" and "Shodex KF-603" (both product names, manufactured by Showa Denko K.K.), under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 0.6 cc/min, detector: RI.

The acid value and hydroxyl value were calculated from the monomer composition used in the preparation, based on JIS regulations.

Details of the organic acids (B) used in the examples and comparative examples are shown in Table 2.

[Example 1] Production of pigment dispersion (blue) 8.3 parts (solid content mass) of copolymer (A-1) and 60 parts of ion-exchanged water were added to a stirrer and stirred to obtain a first mixture. 2.2 parts of organic acid (B-1) were added to the first mixture and stirred, and 0.5 parts of defoamer (BYK-011, manufactured by ALTNA) were further added and stirred to obtain a second mixture. 15 parts of phthalocyanine blue (blue pigment, manufactured by Toyo Color Co., Ltd., product name: LIONOL BLUE 7186-PM) were added to the second mixture and stirred, and ion-exchanged water was further added so that the total amount became 100 parts. The obtained mixture was dispersed in a batch-type SG mill filled with 0.5 mm zirconia beads as a dispersion medium at a volume filling rate of 70%, to obtain a pigment dispersion (blue).

[Examples 2 to 19, Comparative Examples 1 to 8] Preparation of Pigment Dispersion Liquid (Blue) Pigment dispersion liquids (blue) were prepared in the same manner as in Example 1, except that the type or amount of copolymer (A) and the type or amount of organic acid (B) were changed as shown in Table 3.

[Production of Example 20] 2.8 parts (solid content mass) of copolymer (A-1) and 19 parts of ion-exchanged water were added to a stirrer and stirred to obtain a first mixture. 0.8 parts of organic acid (B-1) were added to the first mixture and stirred, and then 0.5 parts of defoamer (BYK-011, manufactured by ALTNA) were added and stirred to obtain a second mixture. 73.0 parts of titanium oxide (white pigment, manufactured by Ishihara Sangyo Kaisha, Ltd. CR-97) were added to the second mixture and stirred, and ion-exchanged water was added so that the total amount became 100 parts. The obtained mixture was dispersed in a paint conditioner filled with 0.6 mm zircon beads as a dispersion medium at a volume filling rate of 70%, to obtain a pigment dispersion (white).

[Example 21 and Comparative Examples 9 and 10] Production of Pigment Dispersion Liquid (White) Pigment dispersion liquids (white) were obtained by the same procedure as in Example 20, except that the type or amount of copolymer (A) and the type or amount of organic acid (B) were changed as shown in Table 4.

[Example 22] Production of pigment dispersion (black) 12.5 parts (solid content mass) of copolymer (A-1) and 50.0 parts of ion-exchanged water were added to a stirrer and stirred to obtain a first mixture. 3.2 parts of organic acid (B-1) were added to the first mixture and stirred, and then 0.5 parts of defoamer (BYK-011, manufactured by ALTNA) were added and stirred to obtain a second mixture. 17.5 parts of carbon black (black pigment, Raven 5000 manufactured by BILRA CARBON) were added to the second mixture and stirred, and ion-exchanged water was added so that the total amount became 100 parts. The obtained mixture was dispersed in a batch-type SG mill filled with 0.5 mm zircon beads as a dispersion medium at a volume filling rate of 70%, to obtain a pigment dispersion (black).

[Example 23 and Comparative Examples 11 and 12] Production of Pigment Dispersion Liquid (Black) Pigment dispersion liquids (black) were obtained in the same manner as in Example 22, except that the type or amount of the copolymer (A) and the type or amount of the organic acid (B) were changed as shown in Table 5.

[Example 24] Production of pigment dispersion (red) 7.8 parts (solid content mass) of copolymer (A-1) and 45.0 parts of ion-exchanged water were added to a stirrer and stirred to obtain a first mixture. 2.0 parts of organic acid (B-1) were added to the first mixture and stirred, and then 0.5 parts of defoamer (BYK-011, manufactured by ALTNA) were added and stirred to obtain a second mixture. 34.5 parts of diketopyrrolopyrrole (red pigment, Irgazin Rubine L 4025 manufactured by BASF) were added to the second mixture and stirred, and ion-exchanged water was added so that the total amount became 100 parts. The resulting mixture and the resulting pigment dispersion were dispersed in a batch-type SG mill filled with 0.6 mm zircon beads as a dispersion medium at a volume filling rate of 70%, to obtain a pigment dispersion (red).

Comparative Example 13 Production of Pigment Dispersion (Red) A pigment dispersion (red) was obtained by the same procedure as in Example 24, except that the type or amount of copolymer (A) and the type or amount of organic acid (B) were changed as shown in Table 6.

[evaluation]
The pigment dispersions of the Examples and Comparative Examples were evaluated as follows.
(1) Average particle size The pigment dispersion was diluted infinitely with ion-exchanged water, and the 50% volume particle size (D50) of the pigment in the pigment dispersion was measured by dynamic light scattering using a particle size distribution measuring device (product name: UPA-150, manufactured by Microtrac). It can be said that the smaller the particle size, the higher the pigment dispersibility.

(2) Viscosity The viscosity was measured using a viscosity measuring device (product name: VISCOMETER TV-25, manufactured by Toki Sangyo Co., Ltd.) at 25° C. and a rotation speed of 5 rpm.

(3) Color development of coating film (3-1) Pigment dispersion containing blue pigment, red pigment, and black pigment A clear base was prepared by mixing 210 parts of the water-soluble acrylic resin (solid content 30%) of Production Example 3 and 30 parts of Cymel 327 (mixed alkylated melamine resin, manufactured by Allnex, solid content 90%). The pigment dispersion was mixed into the clear base so that the mass of the pigment was 10 parts per 90 parts by mass of the solid content of the clear base, to obtain an aqueous coating composition for evaluation.

The resulting aqueous coating composition for evaluation was applied to a 15 cm x 10 cm glass plate and cured by heating at 140°C for 20 minutes to obtain a coating film for evaluation. The appearance of the resulting coating film for evaluation was visually inspected and the color development was evaluated according to the following criteria. The evaluation results are shown in Tables 3 to 5, 7, and 8. A rating of more than B can be said to be excellent in color development.

Evaluation criteria for coating film formed from pigment dispersion (blue) A: Transparent and strong blue A-: Transparent, but slightly weak blue B: Slightly cloudy and slightly weak blue C: Cloudy and weak blue

Evaluation criteria for coating film formed from pigment dispersion (black) A: Glossy and highly jet black B: Between criteria A and C C: White and dull

Evaluation criteria for coating film formed with pigment dispersion (red) A: Transparent, deep red B: Slightly cloudy, slightly less red C: Cloudy

(3-2) Pigment dispersion containing white pigment The pigment dispersion was applied to a 15 cm x 10 cm glass plate and heated at 140°C for 20 minutes to obtain a coating film for evaluation. The obtained coating film for evaluation was allowed to stand at 23°C, and then the 60° gloss value was measured using a macrogloss glossmeter (manufactured by BYK Gardner). The 60° gloss value was evaluated according to the following criteria. The evaluation results are shown in Table 6. The higher the 60° gloss value, the higher the white color development. A rating of more than B can be said to be excellent in color development.

Evaluation criteria for coating film formed with pigment dispersion (white) A: 60° gloss value of 80 or more B: 60° gloss value of 60 or more and less than 80 C: 60° gloss value of less than 60

In the pigment dispersions of Examples 1 to 24, the average pigment particle size was small and the viscosity was kept low.

In Comparative Examples 1 and 2, the average particle size of the pigment (C) was large. The pigment dispersions of Comparative Examples 1 and 2 did not contain organic acid (B), so it is believed that the hydrophobic pigment phthalocyanine blue was not sufficiently dispersed. In Comparative Example 3, when oleylamine was used instead of organic acid (B), the average particle size of the pigment (C) was large. It is believed that the oleylamine used in Comparative Example 3 could not function as a counter ion of the copolymer (A), the charge balance was lost, and the aggregation of the pigments (C) was not suppressed. In Comparative Example 4, when polyoxyethylene oleyl ether was used instead of organic acid (B), the average particle size of the pigment (C) could not be sufficiently reduced, and the viscosity of the pigment dispersion increased. It is believed that the polyoxyethylene oleyl ether used in Comparative Example 4 could not function sufficiently as a counter ion of the copolymer (A), so the aggregation of the pigments (C) was not sufficiently suppressed, and further, a specific interaction occurred between polyoxyethylene oleyl ether and phthalocyanine blue, resulting in an extremely high viscosity. In Comparative Examples 5 and 6, the average particle size of the pigment (C) was large, and the viscosity of the pigment dispersion was also high. It is believed that the organic acid used in Comparative Examples 5 and 6 had a short hydrocarbon group, and therefore aggregation between the pigments was not sufficiently suppressed. In Comparative Example 7, the average particle size of the pigment (C) was large. It is believed that this is because the copolymer (A) used in Comparative Example 7 did not have a nitrogen-containing group, and therefore could not function as a counter ion for the organic acid (B), and therefore aggregation between the pigments (C) was not suppressed.

The coating films formed from the aqueous coating compositions produced using the pigment dispersions of Comparative Examples 1 to 7 all had particularly low transparency.

In Comparative Example 8, the pigment could not be dispersed. This is thought to be because the copolymer used in Comparative Example 8 did not have a polyoxyalkylene chain, and therefore had low affinity with the aqueous solvent, so the copolymer itself could not be dispersed in the aqueous solvent, and as a result, the pigment (C) could not be dispersed.

In Comparative Examples 9 to 13, no organic acid (B) was used.
In Comparative Example 9, the average particle size of the pigment (C) was large, and in particular, an increase in viscosity was confirmed. Since no organic acid (B) was used in Comparative Example 9, it is believed that titanium oxide, which is generally basic, had difficulty in adsorbing the resin component, and therefore was difficult to disperse. Furthermore, titanium oxide has a large specific gravity and is prone to settling, and in addition, it was insufficiently dispersed, which is believed to have led to an excessive increase in viscosity. In Comparative Example 10, it was also difficult to disperse titanium oxide, and an evaluation test could not be performed.

In Comparative Examples 11 and 12, carbon black, an acidic pigment, was used, and although it could be dispersed by copolymer (A), the average particle size of pigment (C) was relatively large and dispersibility was insufficient. In Comparative Example 12, an increase in viscosity was also confirmed.

In Comparative Example 13, the average particle size of the pigment was relatively small, but the color development was poor. This is thought to be because the pigment's dispersion state in the pigment dispersion liquid was unstable, causing partial aggregation of the pigment during the production of the aqueous paint composition.

Example 25 Production of Aqueous Coating Composition and Coated Article (1) Production of Aqueous Coating Composition 160 parts of the acrylic resin emulsion of Production Example 2, 1 part of dimethylaminoethanol, 33 parts of the water-soluble acrylic resin of Production Example 3 (solids content 30% by mass), 38 parts of a melamine resin (Cymel 204, manufactured by Mitsui Cytec Co., Ltd., mixed alkylated melamine resin, solids content 80%), 10 parts of a bifunctional polyether polyol (Primepol PX-1000, manufactured by Sanyo Chemical Industries, Ltd.), and 30 parts of the pigment dispersion of Example 1 (solids content 26%) were mixed and diluted with deionized water to produce an aqueous coating composition with a solids concentration of 20%.

(2) Production of Coated Articles A zinc phosphate-treated dull steel plate having a thickness of 0.8 mm, length of 30 cm, and width of 40 cm was electrocoated with a cationic electrodeposition coating composition "Power Top U-50" (manufactured by Nippon Paint Automotive Coatings) to a dry film thickness of 20 μm, and baked at 160° C. for 30 minutes. An intermediate coating composition "OP-30P Middle Gray" (manufactured by Nippon Paint Automotive Coatings, polyester-melamine paint, pre-diluted for 25 seconds (measured at 20° C. using a No. 4 Ford cup)) was air spray coated on the resulting coated plate using an air spray gun W-101-132G manufactured by Anest Iwata to a dry film thickness of 35 μm, and baked and cured at 140° C. for 30 minutes. In this way, a coated article having an electrodeposition coating film and an intermediate coating film was obtained.

An aqueous base paint composition (Nippon Paint Automotive Coatings, aqueous base AR-3020-1 (gray metallic)) was air-sprayed onto the substrate at a room temperature of 23°C and a humidity of 68% to a dry film thickness of 12 μm. After setting for 4 minutes, the coating was preheated at 80°C for 5 minutes. Next, the aqueous paint composition prepared above was air-sprayed onto the substrate at a room temperature of 23°C and a humidity of 68% to a dry film thickness of 12 μm. After setting for 4 minutes, the coating was preheated at 80°C for 5 minutes.

Next, a clear paint (PolyureExcel O-1200 (product name), a two-component acrylic urethane organic solvent-based clear paint containing a polyisocyanate compound, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied using a rotary atomizing electrostatic coater so that the dry film thickness was 35 μm. Finally, the product was heated at 80°C for 20 minutes to obtain a coated article with a multi-layer coating. The resulting coating was transparent and had a strong bluish tint.

The present disclosure includes the following aspects.
[1]
A composition comprising a copolymer (A), a hydrocarbon group-containing acidic compound (B), a pigment (C), and an aqueous solvent (D),
The copolymer (A) is
a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain;
The hydrocarbon group-containing acidic compound (B) is
A pigment dispersion having (b1) a saturated or unsaturated hydrocarbon group having 5 to 23 carbon atoms, and (b2) at least one acid group selected from the group consisting of a carboxy group, a phosphoric acid group, a sulfonic acid group, and a phenol group.
[2]
The pigment dispersion liquid according to the above-mentioned [1], wherein a ratio of an equivalent of an acid group contained in the hydrocarbon group-containing acidic compound (B) to a total equivalent of a tertiary amino group and a nitrogen-containing heterocyclic group contained in the copolymer (A) (acid group/tertiary amino group and nitrogen-containing heterocyclic group) is 0.5 or more and 2 or less.
[3]
The copolymer (A) further has a third segment (a3) derived from a polymerizable unsaturated monomer other than the nitrogen-containing polymerizable unsaturated monomer and the polymerizable unsaturated monomer having a polyoxyalkylene chain,
In the copolymer (A),
The mass proportion of the first segment (a1) is 5 mass% or more and 30 mass% or less,
The mass ratio of the second segment (a2) is 20 mass% or more and 80 mass% or less,
The pigment dispersion according to the above [1] or [2], wherein the mass ratio of the third segment (a3) is 10 mass % or more and 60 mass % or less.
[4]
The third segment (a3) includes a segment (a31) derived from a hydroxyl group-containing polymerizable saturated monomer,
The pigment dispersion liquid according to [3] above, wherein the mass ratio of the segment (a31) in the copolymer (A) is 10 mass % or more and 30 mass % or less.
[5]
Any one of the pigment dispersions [1] to [4] above,
and a film-forming resin.
[6]
A step of adding and mixing a hydrocarbon group-containing acidic compound (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) to obtain a second mixture;
and adding and mixing a pigment (C) to the second mixture,
The copolymer (A) is
The copolymer has a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain,
The hydrocarbon group-containing acidic compound (B) is
A method for producing a pigment dispersion liquid, comprising: a saturated or unsaturated hydrocarbon group (b1) having 5 to 23 carbon atoms; and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphoric acid group, a sulfonic acid group, and a phenol group.
[7]
A method for producing an aqueous coating composition, comprising the step of mixing the pigment dispersion produced by the method described in [6] above with a coating film-forming resin.

The pigment dispersion of the present invention can be used in a variety of aqueous paint compositions because it can disperse pigments sufficiently and stably regardless of the type of pigment and without causing an increase in viscosity.

Claims (7)

A composition comprising a copolymer (A), a hydrocarbon group-containing acidic compound (B), a pigment (C), and an aqueous solvent (D),
The copolymer (A) is
a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain;
The hydrocarbon group-containing acidic compound (B) is
A pigment dispersion having (b1) a saturated or unsaturated hydrocarbon group having 5 to 23 carbon atoms, and (b2) at least one acid group selected from the group consisting of a carboxy group, a phosphoric acid group, a sulfonic acid group, and a phenol group. The pigment dispersion according to claim 1, wherein the ratio of the equivalent of the acid group contained in the hydrocarbon group-containing acidic compound (B) to the total equivalent of the tertiary amino group and the nitrogen-containing heterocyclic group contained in the copolymer (A) (acid group/tertiary amino group and nitrogen-containing heterocyclic group) is 0.5 or more and 2 or less. The copolymer (A) further has a third segment (a3) derived from a polymerizable unsaturated monomer other than the nitrogen-containing polymerizable unsaturated monomer and the polymerizable unsaturated monomer having a polyoxyalkylene chain,
In the copolymer (A),
The mass proportion of the first segment (a1) is 5 mass% or more and 30 mass% or less,
The mass ratio of the second segment (a2) is 20 mass% or more and 80 mass% or less,
The pigment dispersion according to claim 1 or 2, wherein a mass ratio of the third segment (a3) is 10 mass % or more and 60 mass % or less. The third segment (a3) includes a segment (a31) derived from a hydroxyl group-containing polymerizable saturated monomer,
The pigment dispersion liquid according to claim 3 , wherein a mass ratio of the segment (a31) in the copolymer (A) is 10 mass % or more and 30 mass % or less. The pigment dispersion according to claim 1 or 2,
and a film-forming resin. A step of adding and mixing a hydrocarbon group-containing acidic compound (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) to obtain a second mixture;
and adding and mixing a pigment (C) to the second mixture,
The copolymer (A) is
a first segment (a1) derived from a nitrogen-containing polymerizable unsaturated monomer having at least one of a tertiary amino group and a nitrogen-containing heterocyclic group, and a second segment (a2) derived from a polymerizable unsaturated monomer having a polyoxyalkylene chain;
The hydrocarbon group-containing acidic compound (B) is
A method for producing a pigment dispersion liquid, comprising: a saturated or unsaturated hydrocarbon group (b1) having 5 to 23 carbon atoms; and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphoric acid group, a sulfonic acid group, and a phenol group. A method for producing an aqueous coating composition, comprising a step of mixing the pigment dispersion produced by the method according to claim 6 with a coating film-forming resin.