Pigment dispersion, aqueous coating composition, and method for producing same

Abstract

This pigment dispersion contains a copolymer (A), a hydrocarbon group-containing acidic compound (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 or 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) has a saturated or unsaturated C5-23 hydrocarbon group (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.

Description

Pigment dispersion and aqueous coating composition, and methods for producing the same

[0001] The present invention relates to a pigment dispersion and an aqueous coating composition, as well as methods for producing the same.

[0002] In recent years, there has been a demand for reducing emissions of volatile organic compounds (VOCs) from factories and other facilities in order to protect the environment. Water-based coating compositions have been proposed as a method for reducing VOC emissions. However, pigments generally have poor wettability in aqueous solvents and poor dispersibility.

[0003] In this regard, Patent Document 1 discloses an aqueous coating composition characterized by containing a copolymer of multiple monomers including a polymerizable unsaturated monomer having a cationic functional group represented by a specific formula, a pigment, and an acid group- and hydroxyl group-containing acrylic resin.

[0004] JP 2014-5399 A

[0005] However, with the copolymer of Patent Document 1, it is difficult to disperse the pigment sufficiently and stably.

[0006] In order to solve the above problems, the present invention provides the following aspects: [1] A pigment dispersion comprising: 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 (b1) having 5 to 23 carbon atoms, and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphate group, a sulfonic acid group, and a phenol group. [2] The pigment dispersion according to the above [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. [3] The pigment dispersion according to the above [1] or [2], wherein 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, and in the copolymer (A), the mass proportion of the first segment (a1) is 5% by mass or more and 30% by mass or less, the mass proportion of the second segment (a2) is 20% by mass or more and 80% by mass or less, and the mass proportion of the third segment (a3) ​​is 10% by mass or more and 60% by mass or less. [4] The pigment dispersion according to the above [3], wherein the third segment (a3) ​​comprises a segment (a31) derived from a hydroxyl group-containing polymerizable saturated monomer, and the mass proportion of the segment (a31) in the copolymer (A) is 10 mass% or more and 30 mass% or less. [5] An aqueous coating composition comprising the pigment dispersion according to the above [1] or [2], and a coating film-forming resin.[6] A method for producing a pigment dispersion, comprising the steps of adding a hydrocarbon group-containing acidic compound (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) and mixing them to obtain a second mixture, and adding a pigment (C) to the second mixture and mixing them, 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 (b1) having 5 to 23 carbon atoms, and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphate 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.

[0007] According to the present disclosure, a pigment dispersion liquid having low viscosity and excellent pigment dispersibility is provided regardless of the type of pigment.

[0008] Pigment dispersants are additives used to uniformly disperse pigments in solvents, and like surfactants, they have hydrophilic and hydrophobic moieties. There are various types of hydrophilic moieties. The hydrophilic moieties can be ionized to generate anions or cations, and some do not have ionic dissociation properties.

[0009] Pigments typically have acidic, basic, or inactive sites on their surface, exhibiting acidic, basic, hydrophobic, or hydrophilic properties. These properties are not clearly distinguishable for each pigment, and some pigments may have complex properties. Selecting a pigment dispersant suitable for pigments with such diverse properties is difficult. Therefore, a pigment dispersant that can be used with any pigment is desired. However, if a single pigment dispersant (compound) contains multiple functional groups corresponding to the above sites, problems such as increased viscosity of the pigment dispersion due to association can arise. Similarly, when multiple types of pigment dispersants are used in combination, the viscosity of the pigment dispersion can increase or problems can arise in terms of the affinity between the pigment dispersant and the solvent. An increased viscosity of the pigment dispersion can make it difficult to prepare a coating composition and can also contribute to reduced pigment dispersibility in the pigment dispersion.

[0010] The color development of a coating film improves as the particle size of the pigment in the coating film decreases (i.e., as the pigment dispersibility in the coating composition and, ultimately, in the pigment dispersion increases). Generally, the smaller the particle size of the pigment (the higher the pigment dispersibility), the higher the viscosity of the pigment dispersion. In other words, increasing the pigment dispersibility in the pigment dispersion to improve the color development of the coating film increases the viscosity of the pigment dispersion. This makes it difficult to mix the pigment dispersion with the film-forming resin, which can result in a decrease in the pigment dispersibility in the coating composition. Generally, there is a trade-off between improving the color development of the coating film (improving pigment dispersibility) and reducing the viscosity of the pigment dispersion.

[0011] The present disclosure has discovered a combination of compounds (pigment dispersants) that can disperse pigments well in polar solvents (aqueous solvents) without increasing viscosity, regardless of the nature of the pigment. Paint compositions obtained using the pigment dispersions according to the present disclosure have high color development properties.

[0012] The pigment dispersion according to the present disclosure comprises 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) comprises 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) comprises 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 carboxylic acid group, a phosphoric acid group, a sulfonic acid group, and a phenol group.

[0013] 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) consists of carbon-carbon bonds. In the organic acid (B), the acid group (b2) is usually located at the terminal of the hydrocarbon group (b1).

[0014] 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.

[0015] The combination of copolymer (A) and organic acid (B) improves the dispersibility of various pigments without increasing viscosity. Without being bound by any particular theory, the reason for this is thought to be as follows: When pigment (C) has acidic properties (hereinafter, for convenience, sometimes referred to as an "acidic pigment"; typically, carbon black), copolymer (A) can be adsorbed to pigment (C) via the nitrogen-containing groups. Specifically, the nitrogen-containing groups adsorb to pigment (C), and the polyoxyalkylene chains are positioned on the outside of pigment (C). Furthermore, due to chemical and steric repulsion with the hydrophobic main chain of copolymer (A), the polyoxyalkylene chains are arranged so as to extend toward the outside of pigment (C). This outward-extending polyoxyalkylene chain can suppress aggregation of pigments (C) that have adsorbed copolymer (A).

[0016] Furthermore, the acid group of the organic acid (B) may exist as a counter ion near the nitrogen-containing group, thereby maintaining charge balance and further suppressing aggregation of the pigments (C).

[0017] 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 firmly adsorb not only the copolymer (A) but also the organic acid (B). Meanwhile, 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 portions of the copolymer (A) and the organic acid (B) surrounded by the hydrophilic polyoxyalkylene chain, making it less likely to aggregate and allowing it to be dispersed while maintaining this state.

[0018] From another perspective, the copolymer (A) having a polyoxyalkylene chain uniformly disperses the organic acid (B) having a relatively long-chain hydrocarbon group 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, since the organic acid (B) is uniformly dispersed together with the pigment (C) in the aqueous solvent (D), 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, further improving the pigment dispersibility in the aqueous solvent (D). The dispersion stability of the pigment (C) can also be improved.

[0019] By using the organic acid (B) in combination, the copolymer (A) can be selected relatively freely as long as it has the first segment (a1) and the second segment (a2). In other words, when preparing an aqueous coating composition, the degree of freedom in design is expanded, and a copolymer (A) more suitable for the pigment (C) can be used.

[0020] When the pigment (C) has basic properties (hereinafter, for convenience, sometimes referred to as a "basic pigment"), the organic acid (B) is adsorbed to the pigment (C) by the acid group (b2). A nitrogen-containing group serving as a counter ion may be present near the acid group. In other words, even in this case, the copolymer (A) and the organic acid (B) can be present in close proximity to each other. Therefore, due to chemical and steric repulsion between the polyoxyalkylene chains of the copolymer (A) and the hydrocarbon group (b1) of the organic acid (B) and the hydrophobic main chain of the copolymer (A), the polyoxyalkylene chains of the copolymer (A) are arranged to extend toward the outside of the pigment (C), thereby suppressing aggregation of the pigments. Furthermore, as with acidic pigments, the organic acid (B) reduces the surface tension of the aqueous solvent, improving the pigment dispersibility and even the dispersion stability of the pigment (C).

[0021] 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) due to 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.

[0022] As described above, by using a copolymer (A) having a group readily adsorbable to an acidic pigment and an organic acid (B) having a group readily adsorbable to a basic or hydrophobic pigment in combination, various pigments (C) can be dispersed. Furthermore, the copolymer (A) and the organic acid (B) exist as counter ions to each other, maintaining charge balance and enhancing the effect of reducing surface tension, allowing the pigment (C) to exist stably in a finely divided state in the pigment dispersion. In addition, by providing the nitrogen-containing group and the acid group in separate compounds, interactions between the two are suppressed, thereby suppressing an increase in the viscosity of the pigment dispersion. Because the polyoxyalkylene chain is non-ionic dissociative, interactions with nitrogen-containing groups within the same compound are unlikely to occur.

[0023] The pigment dispersion according to the present disclosure has a low viscosity despite containing a finely divided pigment, which allows for easy mixing of the pigment dispersion with a film-forming resin during preparation of an aqueous coating composition, thereby improving the pigment dispersibility in the aqueous coating composition.

[0024] [Pigment Dispersion] The pigment dispersion according to the present disclosure comprises a copolymer (A), an organic acid (B), a pigment (C), and an aqueous solvent (D). The copolymer (A) comprises 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) comprises a hydrocarbon group (b1) having 5 to 23 carbon atoms and an acid group (b2). The pigment dispersion comprises the aqueous solvent (D) and is aqueous.

[0025] The copolymer (A) and organic acid (B) allow various pigments (C) to be well dispersed in the aqueous solvent (D) without increasing the viscosity. That is, 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).

[0026] The copolymer (A) and the organic acid (B) can be blended, for example, so that the ratio of the equivalent of the acid group (b2) in the organic acid (B) to the total equivalent of the nitrogen-containing groups in 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 pigment dispersibility is more easily achieved. When the equivalent ratio (acid group / nitrogen-containing group) is 2 or less, the effect of dispersing the organic acid (B) by the copolymer (A) is more enhanced, and the effect of improving dispersion stability is more easily achieved. 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.

[0027] The pigment (C) is blended in an amount of, for example, 30 parts by mass or more and 3,000 parts by mass or less per 100 parts by mass of the solid content of the copolymer (A). The blending amount of the pigment (C) may be 45 parts by mass or more, or may be 60 parts by mass or more. The blending amount of the pigment (C) may be 2,800 parts by mass or less, or may be 2,700 parts by mass or less.

[0028] 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.

[0029] The solid content concentration of the pigment dispersion is measured in accordance with JIS K 5601-1-2 Heating residue measurement method.

[0030] The pigment (C) is finely dispersed in the pigment dispersion. The average particle size of the pigment (C) in the pigment dispersion can be, for example, 90% to 1000% of the primary particle size of the pigment (C). When the average particle size of the pigment (C) in the pigment dispersion is within this range, aggregation of the pigment (C) is suppressed, and it can be said that the pigment (C) is microparticulated. The average particle size of the pigment (C) in the pigment dispersion can be 900% or less, or 800% or less of the primary particle size of the pigment (C).

[0031] The average particle size refers to the volume average particle size D50, which is the 50% average particle size (D50) in the volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer (for example, trade name: UPA-150, manufactured by Microtrac).

[0032] The pigment dispersion has various viscosities depending on the type of pigment. A low viscosity of the pigment dispersion is desirable in terms of facilitating the preparation of a coating composition. The viscosity of the pigment dispersion at 25°C and a rotation speed of 5 rpm may be, for example, 5,000 cps or less, or 2,000 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.

[0033] The pigment dispersion according to the present disclosure has a viscosity significantly lower than that of other pigment dispersions prepared by dispersing the same pigment 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 approximately half or less, or even one-third or less, of the viscosity of the other pigment dispersions. Alternatively, the pigment particle size of the pigment dispersion according to the present disclosure may be smaller than that of yet another pigment dispersion prepared by dispersing the same pigment to a degree that does not increase the 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 approximately 80% or less, or approximately 75% or less, or approximately 70% or less of the average particle size of the pigment contained in the yet another pigment dispersion.

[0034] (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").

[0035] The first segment (a1) has at least one nitrogen-containing group. The nitrogen-containing group generates a cation in an aqueous solvent and can adsorb to the acidic pigment. In addition, the nitrogen-containing group does not have an active hydrogen that chemically reacts with components (typically, a curing agent) blended in the aqueous coating composition, and therefore is unlikely to inhibit the curing reaction of the aqueous coating composition.

[0036] The second segment (a2) has at least one polyoxyalkylene chain. The polyoxyalkylene chain inhibits pigment particles from approaching each other, contributing to the suppression of aggregation and enhancing the hydrophilicity of the copolymer (A).

[0037] 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.

[0038] The mass proportion of the first segment (a1) may be 5% by mass or more and 30% by mass or less, the mass proportion of the second segment (a2) may be 20% by mass or more and 80% by mass or less, and the mass proportion of the third segment (a3) ​​may be 10% by mass or more and 60% by mass or less.

[0039] When the mass proportion of the first segment (a1) is 5% by mass or more, the pigment adsorption performance 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 a decrease in 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.

[0040] When the mass proportion of the second segment (a2) is 20% by mass or more, the dispersibility of the pigment having adsorbed copolymer (A) is improved. When the mass proportion of the second segment (a2) is 80% by mass or less, the hydrophilicity of copolymer (A) is prevented from becoming excessively high, and a decrease in 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.

[0041] 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.

[0042] The third segment (a3) ​​may contain a segment derived from a hydroxyl group-containing polymerizable unsaturated monomer (hereinafter referred to as the hydroxyl group-containing segment (a31)). The hydroxyl group can chemically react with components blended into the aqueous coating composition. The copolymer (A) adsorbed to the pigment chemically reacts with the curing agent, thereby preventing the pigment from falling off (bleeding out) from the coating film. The mass proportion of the hydroxyl group-containing segment (a31) may be 10% by mass or more and 30% by mass or less. When the mass proportion of the hydroxyl group-containing segment (a31) is within this range, the bleed-out suppression effect is easily exerted, and an increase in the viscosity of the pigment dispersion is unlikely to occur. The mass proportion of the hydroxyl group-containing segment (a31) may be 12% by mass or more, or 15% by mass or more. The mass proportion of the hydroxyl group-containing segment (a31) may be 25% by mass or less, or 20% by mass or less.

[0043] 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 monomers forming the segments used in synthesizing the copolymer (A) by the total charged mass of all raw material monomers.

[0044] The weight average molecular weight of the copolymer (A) may be 10,000 or more and 40,000 or less. The weight average molecular weight of the copolymer (A) may be 15,000 or more, or 20,000 or more. The weight average molecular weight of the copolymer (A) may be 38,000 or less, or 35,000 or less.

[0045] The weight average molecular weight can be calculated from a chromatogram measured by gel permeation chromatography using the molecular weight of standard polystyrene as a standard. The acid value and hydroxyl value are calculated from the monomer composition used in the preparation, based on the JIS regulations.

[0046] (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 first segment (a1) derived from the nitrogen-containing monomer can be well adsorbed to a pigment having an acidic site (e.g., carbon black).

[0047] 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.

[0048] 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.

[0049] In this specification, the term "(meth)acrylic" is a concept that includes both acrylic and methacrylic.

[0050] Examples of the nitrogen-containing heterocyclic group include a pyridine ring group, a quinoline ring group, a thiazole ring group, an oxazole ring group, a benzothiazole ring group, an imidazole ring group, a pyrazole ring group, an imidazoline ring group, a pyrimidine ring group, a pyrazine ring group, a triazole ring group, and a tetrazole ring group.

[0051] 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.

[0052] Among these, at least one of a vinyl monomer having a pyridine ring group and a vinyl monomer having an imidazole ring group may be used.

[0053] In particular, 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:

[0054] In the formula, A represents hydrogen or a methyl group, and B １ and B ２ each independently represents 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, B １ and B ２ may be a hydrocarbon group having 1 or 2 carbon atoms, and n may be 1 or 2.

[0055] (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).

[0056] Examples of the polyoxyalkylene chain include a polyoxyethylene chain, a polyoxypropylene chain, a polyoxyethylene block, and a chain containing a polyoxypropylene block. 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.

[0057] 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, when the molecular weight of the polyoxyalkylene chain is 800 or more, long-term dispersion stability is likely to be obtained.

[0058] 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.

[0059] (Other polymerizable unsaturated monomers) The other polymerizable unsaturated monomers (hereinafter sometimes referred to as "other raw material monomers") have a polymerizable unsaturated group in one molecule but do not have a polyoxyalkylene chain or a nitrogen-containing group. The third segment (a3) ​​derived from the other raw material monomers can impart various functions to the copolymer (A).

[0060] The other raw material monomers desirably do not have an acid group such as a carboxy group, a phosphate group, a sulfonic acid group, or a phenol group, because if an interaction occurs between the acid group and the nitrogen-containing group, an intermolecular force is generated in the copolymer (A), which can result in an excessively high viscosity of the pigment dispersion.

[0061] The other raw material monomer 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 coating composition, thereby improving the water resistance of the resulting coating film. Furthermore, the copolymer (A) adsorbed to the pigment (C) chemically reacts with the curing agent, which makes it easier to prevent the pigment from falling off (bleeding out) from the coating film.

[0062] Examples of hydroxyl group-containing polymerizable unsaturated monomers (hereinafter sometimes referred to as "hydroxyl group-containing monomers") include 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 monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms; and allyl alcohol. These may be used alone or in combination of two or more.

[0063] Examples of raw material monomers other than the hydroxyl group-containing monomer include 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.

[0064] (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).

[0065] 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 linear hydrocarbon group (b1) may be linear or branched.

[0066] The hydrocarbon group (b1) has 5 to 23 carbon atoms. When the organic acid (B) has a hydrocarbon group (b1) of a certain length, the hydrophobic pigment is more likely to adsorb to the organic acid (B), the polyoxyalkylene chain is more likely to extend outward, and the surface tension of the aqueous solvent is more likely to decrease. In addition, when preparing an aqueous paint composition, dissociation of the organic acid (B) from the pigment to which the organic acid (B) has been adsorbed is suppressed, thereby maintaining the dispersibility of the pigment in the aqueous paint composition. On the other hand, if the hydrocarbon group (b1) has an excessively large number of carbon atoms, it becomes difficult to uniformly disperse the pigment in the aqueous solvent. The hydrocarbon group (b1) may have 6 or more carbon atoms, 7 or more carbon atoms, or 12 or more carbon atoms. The hydrocarbon group (b1) may have 21 or less carbon atoms, or 19 or less carbon atoms.

[0067] The acid group (b2) is at least one selected from the group consisting of a carboxyl group, a phosphoric acid 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).

[0068] Examples of the organic acid (B) having a carboxy group include monovalent, divalent, and trivalent organic carboxylic acids. Examples of the 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 cyclohexanecarboxylic acid. Examples of the 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-cyclohexanedicarboxylic acid. Examples of the trivalent organic carboxylic acid include aconitic acid. Other examples of the organic acid (B) having a carboxy group include amino acids and their derivatives (e.g., trimethylated amino acids such as trimethylglycine).

[0069] Examples of the organic acid (B) having a phosphoric acid group include alkylphosphonic acids (such as octylphosphonic acid), alkenylphosphonic acids (such as oleyl phosphate), and phenylphosphinic acids.

[0070] Examples of the organic acid (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.

[0071] Examples of the organic acid (B) having a phenol group include nonylphenol, t-butylphenol, and t-butylcatechol.

[0072] (C) Pigment Examples of pigments include color pigments, extender pigments, anti-rust pigments, and luster pigments. Color pigments may be inorganic or organic. Color pigments 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 lead yellow, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These may be used alone or in combination of two or more.

[0073] Examples of extender pigments include calcium carbonate, barium sulfate, clay, and talc, which may be used alone or in combination of two or more.

[0074] Examples of the luster pigment 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. The luster pigment may be colored.

[0075] (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.

[0076] Examples of hydrophilic solvents include glycol solvents such as ethylene glycol, propylene glycol, butanediol, pentanediol, diethylene glycol, dipropylene glycol, and triethylene glycol; glycol ether 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 solvents such as methanol, ethanol, and isopropyl alcohol; ketone solvents such as acetone; and N-methyl-2-pyrrolidone. These may be used alone or in combination of two or more.

[0077] (Others) The pigment dispersion may contain other components such as a defoaming agent, an ultraviolet absorber, a light stabilizer, an antioxidant, a surface conditioner, a film-forming aid, and a rust inhibitor.

[0078] [Method for producing pigment dispersion] The pigment dispersion according to the present disclosure is produced by a method including the steps of adding an organic acid (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) and mixing them to obtain a second mixture, and adding a pigment (C) to the second mixture and mixing them.

[0079] 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.

[0080] Each mixing step is carried out using, for example, a disperser (a stirrer). Examples of dispersers include low-speed stirrers such as a propeller mixer, a paddle mixer, and an anchor mixer, and high-speed stirrers such as a homomixer, a disperser mixer, and an ultra mixer.

[0081] The obtained pigment dispersion may be further mixed using a disperser using dispersion media (beads). This promotes the microparticulation of the pigment (C). Specific examples of dispersers include Ultra Apex Mill (trade name: Dual Apex Mill (Kotobuki Industries Co., Ltd.)), Paint Conditioner, Picograin Mill (trade name: Eco Mill (Asada Iron Works Co., Ltd.)), Star Mill ZRS, Star Mill, Nano Getter (trade name: Max Nano Getter (Ashizawa Finetech Co., Ltd.)), Micromedia (Buehler Co., Ltd.), MSC Mill (Nippon Coke & Engineering Co., Ltd.), NPM (Shinmaru Enterprises Co., Ltd.), and Sand Grind (SG) Mill.

[0082] Examples of materials for the dispersion media include alumina, zirconia, silicon carbide, silicon nitride, glass, steel, stainless steel, and ceramics. The particle diameter 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 microparticulation of the pigment (C).

[0083] The agitator rotation speed of the disperser 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, and the mixing temperature may be, for example, 5°C or more and 45°C or less.

[0084] [Aqueous Coating 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. Examples of the aqueous solvent include the same aqueous solvents exemplified as those contained in the pigment dispersion. The aqueous coating composition may be further diluted with a solvent suitable for application.

[0085] The pigment (C) is sufficiently and stably dispersed in the aqueous coating composition. Therefore, the resulting coating film has excellent color development. Color development is determined by comprehensively evaluating gloss, transparency, color depth, etc. depending on 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.

[0086] The pigment dispersion is blended so that the pigment (C) is present in an amount of, for example, 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 coating 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.

[0087] The solid content concentration of the aqueous coating composition is, for example, 3% by mass or more and 50% by mass or less. The solid content concentration of the aqueous coating composition may be 5% by mass or more, or 7% by mass or more. The solid content concentration of the aqueous coating composition may be 45% by mass or less, or 40% by mass or less.

[0088] The solids concentration of the aqueous coating composition is measured in accordance with JIS K 5601-1-2 Heating Residue Measurement Method.

[0089] (Film-forming resin) Examples of film-forming resins include acrylic resins, acrylic silicone resins, polyester resins, polyurethane resins, epoxy resins, fluororesins, and silicone resins. These may be used alone or in combination of two or more. Among these, acrylic resins are preferred.

[0090] The coating film-forming resin may contain a hydroxyl group-containing resin. Examples of hydroxyl group-containing resins include hydroxyl group-containing acrylic resins, hydroxyl group-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. Among these, hydroxyl group-containing acrylic resins are preferred.

[0091] 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, hydrophilization of the coating film is suppressed, and 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.

[0092] The weight average molecular weight (Mw) of the hydroxyl group-containing resin (A) may be, for example, 2,000 or more and 50,000 or less.

[0093] The weight average molecular weight is determined by GPC using polystyrene as a standard. The acid value and hydroxyl value are calculated from the monomer composition used in the preparation, based on the JIS regulations.

[0094] The film-forming resin may be contained 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 the polymer using a basic compound. A water-soluble acrylic resin can be prepared by solution polymerizing the above-mentioned α,β-ethylenically unsaturated monomer and solubilizing the polymer using a basic compound.

[0095] The emulsion of the hydroxyl group-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.

[0096] 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.

[0097] 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.

[0098] Examples of the α,β-ethylenically unsaturated monomer 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.

[0099] Other α,β-ethylenically unsaturated monomers may be used in combination. Examples of the 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.

[0100] 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 an 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 with heating and stirring. The α,β-ethylenically unsaturated monomer may be previously emulsified with an emulsifier.

[0101] Polymerization initiators and emulsifiers commonly used by those skilled in the art can be used. If necessary, a chain transfer agent such as a mercaptan (e.g., lauryl mercaptan) or α-methylstyrene dimer may be used to adjust the molecular weight. The reaction temperature, reaction time, and the like can be appropriately selected within ranges commonly used by those skilled in the art. The resulting acrylic resin emulsion is neutralized with a base, if necessary.

[0102] 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 mg KOH / g or more and 180 mg KOH / g or less. The hydroxyl-containing acrylic resin may have an acid value (solid content acid value) of 1 mg KOH / g or more and 80 mg KOH / g or less.

[0103] (Curing Agent) The aqueous coating composition may contain a curing agent, which reacts with the film-forming resin to form a cured coating therewith.

[0104] Examples of curing agents include melamine resins, blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more. Among these, at least one of melamine resins and blocked isocyanate compounds may be used.

[0105] The melamine resin may be water-soluble or water-insoluble. The melamine resin has a structure in which hydrogen atoms or substituents (such as alkyl ether groups and methylol groups) are bonded to the periphery of a melamine nucleus (triazine nucleus) via three nitrogen atoms. The melamine resin is generally composed of a polynuclear compound in which multiple melamine nuclei are bonded to each other. The melamine resin may also be a mononuclear compound consisting of a single melamine nucleus.

[0106] 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, Inc. These may be used alone or in combination of two or more.

[0107] The blocked isocyanate compound 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.

[0108] The content of the curing agent may be 10% by mass or more and 80% by mass or less of the resin solid content 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.

[0109] (Others) The aqueous coating composition may contain other components such as antifoaming agents, ultraviolet absorbers, light stabilizers (for example, hindered amines), antioxidants, surface conditioners, film-forming aids, and rust inhibitors.

[0110] [Method for producing aqueous coating composition] The aqueous coating composition according to the present disclosure is produced by a method comprising the step of mixing the pigment dispersion produced by the method described above with a coating film-forming resin.

[0111] The mixing conditions and mixing equipment may be the same as those mentioned in the method for producing the pigment dispersion.

[0112] [Coated Article] The aqueous coating composition according to the present disclosure can be used to obtain a coated article having a coating film with excellent color development. Such a coated article comprises a substrate and a base coat film formed from the aqueous coating composition. The coated article may comprise a substrate, a base coat film formed from the aqueous coating composition, and a clear coating film disposed on the base coat film.

[0113] The base coat film may consist of one layer or two or more layers. At least one base coat film is formed from the aqueous coating composition according to the present disclosure. The components contained in each base coat film may be the same or different.

[0114] (Substrate) Examples of the material of the substrate include metal, plastic, and foam. Among them, metal (particularly, casting) may be used, and metal that can be electrocoated may be used. Examples of such metals include iron, copper, aluminum, tin, zinc, etc., and alloys containing these metals.

[0115] The shape of the substrate is not particularly limited, and may be flat or three-dimensional. Specific examples of the substrate include automobile bodies and parts thereof, such as passenger cars, trucks, motorcycles, and buses.

[0116] The metal substrate may have been subjected to a chemical conversion treatment using a phosphate-based chemical conversion treatment agent, a zirconium-based chemical conversion treatment agent, or the like, and may have been subjected to electrodeposition coating. The electrodeposition coating composition may be either a cationic type or an anionic type. A cationic electrodeposition coating composition can form a coating film with excellent corrosion resistance.

[0117] The metal substrate may have an electrodeposition coating 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 coating composition for the intermediate coating may contain, for example, a film-forming resin, a curing agent, a color pigment, and an extender pigment. Examples of the film-forming resin and curing agent include those contained in the aqueous coating composition according to the present disclosure.

[0118] (Base Coat Film) At least one 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 coating film may be 3 μm or more. The thickness of the base coating film may be 40 μm or less, 30 μm or less, or 20 μm or less.

[0119] (Clear Coating Film) 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.

[0120] 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 viewpoints of transparency, acid etching resistance, etc., the solvent-based clear coating composition may contain an acrylic resin and / or polyester resin as a film-forming resin and an amino resin and / or isocyanate as a curing agent. The solvent-based clear coating composition may also contain an acrylic resin and / or polyester resin having a carboxylic acid and / or an epoxy group.

[0121] The clear coating composition may contain the above-mentioned various pigments to the extent that the transparency and the effects of the coating composition according to the present disclosure are not impaired. The clear coating composition may contain various additives as needed. Examples of additives include ultraviolet absorbers, antioxidants, antifoaming agents, surface conditioners, and pinhole inhibitors.

[0122] [Method for manufacturing a coated article] A method for manufacturing a coated article will be described using as an example a coated article comprising an object to be coated, a base coat film, and a clear coat film.

[0123] A coated article comprising a base coat film and a clear coat film can be obtained, for example, by sequentially applying the aqueous coating composition and the clear coating composition according to the present disclosure to a substrate, and then simultaneously curing both. After applying the aqueous coating composition, preheating may be performed before applying the clear coating composition.

[0124] Examples of the coating method include air spray coating, airless spray coating, electrostatic spray coating, multi-stage coating (preferably two-stage coating) using air electrostatic spray coating, and coating that combines air electrostatic spray coating with a rotary atomizing electrostatic coater.

[0125] 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).

[0126] 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.

[0127] [Production Example 1-1] Production of Copolymer (A-1) A reaction vessel equipped with a stirrer, temperature controller, condenser, and dropping device was charged with 78 parts of dipropylene glycol monomethyl ether, and the mixture was heated to 120°C with stirring and refluxed. 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 Corporation, methoxypolyethylene glycol methacrylate), 19 parts of 2-hydroxyethyl methacrylate, and 6 parts of butyl acrylate, 0.5 parts of Kayaester O (manufactured by NOF Corporation), and 76 parts of dipropylene glycol monomethyl ether was added dropwise over 3 hours and allowed to react. As a post-shot, a polymerization initiator solution prepared by dissolving 0.3 parts of Kayaester O (manufactured by NOF Corporation) in 11 parts of dipropylene glycol monomethyl ether was added dropwise over 0.5 hours, and the mixture was allowed to polymerize with continued stirring for 0.5 hours to obtain Copolymer (A-1). Subsequently, the solvent was distilled off at 90°C under reduced pressure, and then 100 parts of ion-exchanged water was added to 1000 parts of copolymer (A-1) and stirred to obtain an aqueous solution of copolymer (A-1) (solid content: 50% by mass). The obtained copolymer (A-1) had a hydroxyl value of 82 mgKOH / g and a weight-average molecular weight of 30,000.

[0128] [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 table below were used in the amounts shown in Table 1.

[0129]

[0130] [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 alkylpropenylphenyl ether sulfate, manufactured by Dai-ichi 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 added dropwise in parallel to the reaction vessel over 1.5 hours. After completion of the dropwise addition, the mixture was aged at the same temperature for 1 hour.

[0131] Additionally, a second-stage monomer mixture was prepared by mixing 53.65 parts by weight of styrene (ST), 178.96 parts by weight of methyl methacrylate (MMA), 75.94 parts by weight of n-butyl acrylate (BA), 64.45 parts by weight of 2-ethylhexyl acrylate (2-EHA), 105 parts by weight of hydroxyethyl methacrylate (HEMA), and 22 parts by weight of acrylic acid. This monomer mixture was then 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 initiator solution were added dropwise to the reaction vessel over 1.5 hours. After the completion of the dropwise addition, the mixture was aged at the same temperature for 2 hours.

[0132] 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 having an average particle size of 150 nm, a solid content of 35% by mass, an acid value of the solid content of 20 mgKOH / g, and a hydroxyl value of 100 mgKOH / g.

[0133] [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 this monomer mixture, 10.0 parts of tripropylene glycol methyl ether, and 1 part of tert-butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel in parallel over 3 hours. After completion of the dropwise addition, the mixture was aged at the same temperature for 0.5 hours.

[0134] Further, 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 0.5 hours. After the completion of the addition, the mixture was aged at the same temperature for 2 hours.

[0135] Using a solvent remover, 16.1 parts of the solvent was distilled off under reduced pressure (70 torr) at 110°C, and then 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added to obtain a water-soluble acrylic resin solution. The resulting water-soluble acrylic resin solution had a solids content of 30% by mass, a solids acid value of 40 mgKOH / g, a hydroxyl value of 50 mgKOH / g, and a viscosity of 140 poise (E-type viscometer, 1 rpm, 25°C).

[0136] The weight average molecular weight was measured using a GPC apparatus "HLC8220GPC" (trade name, manufactured by Tosoh Corporation) and four columns "Shodex KF-606M" and "Shodex KF-603" (both trade names, manufactured by Showa Denko K.K.), under the conditions of a mobile phase of tetrahydrofuran, a measurement temperature of 40°C, a flow rate of 0.6 cc / min, and a detector of RI.

[0137] The acid value and hydroxyl value were calculated from the monomer composition used in the preparation, based on the JIS standard.

[0138] Details of the organic acids (B) used in the examples and comparative examples are shown in Table 2.

[0139]

[0140] Example 1: Preparation of Pigment Dispersion (Blue) 8.3 parts (solids mass) of copolymer (A-1) and 60 parts of ion-exchanged water were placed in a mixer 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 an antifoaming agent (BYK-011, manufactured by ALTNA) were then 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 added to bring the total to 100 parts. The resulting mixture was dispersed in a batch SG mill filled with 0.5 mm zirconia beads as dispersion media at a volumetric filling rate of 70%, to obtain a pigment dispersion (blue).

[0141] [Examples 2 to 19, Comparative Examples 1 to 8] Preparation of Pigment Dispersions (Blue) Pigment dispersions (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.

[0142] Preparation of Example 20: 2.8 parts (solids mass) of copolymer (A-1) and 19 parts of ion-exchanged water were placed in a mixer and stirred to obtain a first mixture. 0.8 parts of organic acid (B-1) was added to the first mixture and stirred, followed by 0.5 parts of an antifoaming agent (BYK-011, manufactured by ALTNA Corporation) and stirring to obtain a second mixture. 73.0 parts of titanium oxide (white pigment, CR-97 manufactured by Ishihara Sangyo Kaisha) was added to the second mixture and stirred, and ion-exchanged water was added to make a total of 100 parts. The resulting mixture was dispersed in a paint conditioner filled with 0.6 mm zircon beads as a dispersion medium at a volumetric filling rate of 70%, to obtain a pigment dispersion (white).

[0143] [Example 21 and Comparative Examples 9 and 10] Preparation of Pigment Dispersion Liquid (White) Pigment dispersion liquids (white) were obtained in the same manner 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.

[0144] Example 22: Preparation of a pigment dispersion (black) 12.5 parts (solids mass) of copolymer (A-1) and 50.0 parts of ion-exchanged water were placed in a mixer and stirred to obtain a first mixture. 3.2 parts of organic acid (B-1) were added to the first mixture and stirred, followed by 0.5 parts of an antifoaming agent (BYK-011, manufactured by ALTNA) and stirring to obtain a second mixture. 17.5 parts of carbon black (black pigment, Laven 5000 manufactured by BILRA CARBON) were added to the second mixture and stirred, and ion-exchanged water was added to bring the total to 100 parts. The resulting mixture was dispersed in a batch SG mill filled with 0.5 mm zircon beads as dispersion media at a volumetric filling rate of 70%, to obtain a pigment dispersion (black).

[0145] [Example 23 and Comparative Examples 11 and 12] Preparation 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 copolymer (A) and the type or amount of organic acid (B) were changed as shown in Table 5.

[0146] Example 24: Preparation of Pigment Dispersion (Red) 7.8 parts (solids mass) of copolymer (A-1) and 45.0 parts of ion-exchanged water were placed in 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, followed by 0.5 parts of an antifoaming agent (BYK-011, manufactured by ALTNA Corporation) and stirring 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 to bring the total to 100 parts. The resulting mixture was dispersed in a batch SG mill filled with 0.6 mm zircon beads as dispersion media at a volume filling rate of 70%, to obtain a pigment dispersion (red).

[0147] Comparative Example 13 Production of Pigment Dispersion Liquid (Red) A pigment dispersion liquid (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.

[0148] [Evaluation] The pigment dispersions of the Examples and Comparative Examples were evaluated as follows. (1) Average particle size The pigment dispersions were 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 analyzer (product name: UPA-150, manufactured by Microtrac). It can be said that the smaller the particle size, the higher the pigment dispersibility.

[0149] (2) Viscosity The viscosity was measured at 25° C. and a rotation speed of 5 rpm using a viscosity measuring device (trade name: VISCOMETER TV-25, manufactured by Toki Sangyo Co., Ltd.).

[0150] (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 Corporation, solid content 90%). The pigment dispersion was mixed with 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, thereby obtaining an aqueous coating composition for evaluation.

[0151] The resulting aqueous coating composition for evaluation was applied to a 15 cm x 10 cm glass plate and heat-cured 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 above B can be said to indicate excellent color development.

[0152] Evaluation criteria for coating films formed with pigment dispersion (blue) A: Transparent and strong blue tint A-: Transparent, but slightly weak blue tint B: Slightly cloudy and slightly weak blue tint C: Cloudy and weak blue tint

[0153] Evaluation criteria for coating films formed with pigment dispersion (black) A: Glossy and highly jet black B: Between criteria A and C C: White and dull

[0154] Evaluation criteria for coating films formed with pigment dispersion (red) A: Transparent, deep red B: Slightly cloudy, slightly less red C: Cloudy

[0155] (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.

[0156] Evaluation criteria for coating films formed with pigment dispersion (white) A: 60° gloss value of 80 or more B: 60° gloss value of 60 or more but less than 80 C: 60° gloss value of less than 60

[0157]

[0158]

[0159]

[0160]

[0161]

[0162]

[0163] In the pigment dispersions of Examples 1 to 24, the average particle size of the pigment was small and the viscosity was kept low.

[0164] In Comparative Examples 1 and 2, the average particle size of the pigment (C) was large. Since the pigment dispersions of Comparative Examples 1 and 2 did not contain organic acid (B), 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. This is believed to be because the oleylamine used in Comparative Example 3 was unable to function as a counter ion for copolymer (A), disrupting the charge balance and preventing aggregation of the pigments (C). In Comparative Example 4, when polyoxyethylene oleyl ether was used instead of organic acid (B), it was not possible to sufficiently reduce the average particle size of the pigment (C), and the viscosity of the pigment dispersion increased. Since the polyoxyethylene oleyl ether used in Comparative Example 4 was unable to sufficiently function as a counter ion for copolymer (A), aggregation of the pigments (C) was not sufficiently prevented, and it is believed that 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 acids used in Comparative Examples 5 and 6 had short hydrocarbon groups and therefore did not sufficiently suppress aggregation of the pigments. 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 did not suppress aggregation of the pigments (C).

[0165] The coating films formed using the aqueous coating compositions produced using the pigment dispersions of Comparative Examples 1 to 7 all had particularly low transparency.

[0166] The pigment could not be dispersed in Comparative Example 8. 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, making it impossible for the copolymer itself to be dispersed in the aqueous solvent, and as a result, it was also impossible to disperse the pigment (C).

[0167] 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 an increase in viscosity in particular was confirmed. Since no organic acid (B) was used in Comparative Example 9, it was difficult for titanium oxide, which is generally basic, to adsorb the resin component, and it is thought that this made dispersion difficult. Furthermore, titanium oxide has a high specific gravity and therefore tends to settle, and in addition, dispersion was insufficient, which is thought to have led to an excessive increase in viscosity. In Comparative Example 10, it was also difficult to disperse titanium oxide, and evaluation tests could not be performed.

[0168] 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.

[0169] In Comparative Example 13, although the average particle size of the pigment was relatively small, the color development was poor. This is thought to be because the dispersed state of the pigment in the pigment dispersion was unstable, causing partial aggregation of the pigment during the production of the aqueous coating composition.

[0170] 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 (solids content 30% by mass) of the water-soluble acrylic resin of Production Example 3, 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%.

[0171] (2) Production of Coated Articles A zinc phosphate-treated dull steel plate, 0.8 mm thick, 30 cm long, and 40 cm wide, was electrodeposited with the 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. The resulting coated plate was air-sprayed with the intermediate coating composition "OP-30P Middle Gray" (manufactured by Nippon Paint Automotive Coatings, polyester-melamine paint, pre-diluted to 25 seconds (measured at 20°C using a No. 4 Ford cup)) using an Anest Iwata air spray gun W-101-132G to a dry film thickness of 35 μm, and then baked and cured at 140°C for 30 minutes. In this way, a coated article having an electrodeposition coating and an intermediate coating was obtained.

[0172] An aqueous base paint composition (Nippon Paint Automotive Coatings, aqueous base AR-3020-1 (gray metallic)) was applied to the substrate by air spray at a room temperature of 23°C and a humidity of 68% so as to give 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 applied by air spray at a room temperature of 23°C and a humidity of 68% so as to give a dry film thickness of 12 μm. After setting for 4 minutes, the coating was preheated at 80°C for 5 minutes.

[0173] Next, a clear paint (Polyurexcel O-1200 (trade 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 atomizer 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 having a multi-layer coating film. The resulting coating film had a transparent appearance and a strong bluish tint.

[0174] The present disclosure includes the following aspects: [1] A pigment dispersion comprising: 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 (b1) having 5 to 23 carbon atoms, and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphate group, a sulfonic acid group, and a phenol group. [2] The pigment dispersion of the above [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. [3] The pigment dispersion of the above [1] or [2], wherein 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, and in the copolymer (A), the mass proportion of the first segment (a1) is 5% by mass or more and 30% by mass or less, the mass proportion of the second segment (a2) is 20% by mass or more and 80% by mass or less, and the mass proportion of the third segment (a3) ​​is 10% by mass or more and 60% by mass or less. [4] The pigment dispersion according to [3] above, wherein the third segment (a3) ​​comprises a segment (a31) derived from a hydroxyl group-containing polymerizable saturated monomer, and the mass proportion of the segment (a31) in the copolymer (A) is 10 mass% or more and 30 mass% or less. [5] An aqueous coating composition comprising the pigment dispersion according to any one of [1] to [4] above, and a coating film-forming resin.[6] A method for producing a pigment dispersion, comprising the steps of adding a hydrocarbon group-containing acidic compound (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) and mixing them to obtain a second mixture, and adding a pigment (C) to the second mixture and mixing them, 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 (b1) having 5 to 23 carbon atoms, and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphate 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.

[0175] The pigment dispersion of the present invention can disperse pigments sufficiently and stably regardless of the type of pigment and without causing an increase in viscosity, and therefore can be used in a variety of aqueous coating compositions.

[0176] This application claims priority based on Japanese Patent Application No. 2023-107340, filed on June 29, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A pigment dispersion comprising: 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 (b1) having 5 to 23 carbon atoms, and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphate group, a sulfonic acid group, and a phenol group.

2. The pigment dispersion liquid 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.

3. The pigment dispersion liquid described in claim 1 or 2, wherein 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, and in the copolymer (A), the mass proportion of the first segment (a1) is 5 mass% or more and 30 mass% or less, the mass proportion of the second segment (a2) is 20 mass% or more and 80 mass% or less, and the mass proportion of the third segment (a3) ​​is 10 mass% or more and 60 mass% or less.

4. The pigment dispersion liquid described in claim 3, wherein the third segment (a3) ​​contains a segment (a31) derived from a hydroxyl group-containing polymerizable saturated monomer, and the mass proportion of the segment (a31) in the copolymer (A) is 10 mass% or more and 30 mass% or less.

5. An aqueous coating composition comprising the pigment dispersion according to any one of claims 1 to 4 and a coating film-forming resin.

6. A method for producing a pigment dispersion, comprising the steps of: adding a hydrocarbon group-containing acidic compound (B) to a first mixture containing an aqueous solvent (D) and a copolymer (A) and mixing the mixture to obtain a second mixture; and adding a pigment (C) to the second mixture and mixing the mixture, 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 (b1) having 5 to 23 carbon atoms, and at least one acid group (b2) selected from the group consisting of a carboxy group, a phosphate 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 according to claim 6 with a film-forming resin.

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