Water-based pigment dispersions and water-based inks

Abstract

The present invention provides an aqueous pigment dispersion that does not result in a decrease in print quality due to white streaks or color unevenness when printed on coated paper, and that has good abrasion resistance, as well as an aqueous ink containing the aqueous pigment dispersion. [Solution] An aqueous pigment dispersion comprising a pigment and a polymer dispersant, wherein the polymer dispersant contains a polymer comprising a structural unit (A) derived from an alkyl (meth)acrylate, a structural unit (B) derived from a sulfonic acid having a polymerizable unsaturated group, and a structural unit (C) derived from a carboxylic acid having a polymerizable unsaturated group, and the alkyl group of the alkyl (meth)acrylate is a linear, branched, or alicyclic alkyl group having 5 to 24 carbon atoms.

Description

Technical Field

[0001] The present invention relates to an aqueous pigment dispersion and an aqueous ink.

Background Art

[0002] The inkjet recording method is a recording method in which ink droplets are directly ejected from a fine nozzle and adhered to a recording medium to obtain characters and images. This method has many advantages such as easy full-colorization, low cost, the ability to use plain paper as a recording medium, and non-contact with the printed material, so it has become extremely popular. In recent years, due to the spread of digital printing, it has been used not only for consumer printing but also for commercial printing and industrial printing using low-absorbency coated paper and the like. The demand for aqueous inkjet inks is increasing more and more for the purpose of increasing the printing speed, improving the image quality, and further reducing the environmental impact.

[0003] For example, in Patent Document 1, an object is to provide a dispersion liquid or the like that enables the easily redispersion of a solidified colorant and can improve clogging prevention and ejection stability. It contains water, a colorant, and a dispersion resin for dispersing the colorant. The dispersion resin has a structural unit A containing a hydrophobic monomer, a structural unit B containing a hydrophilic acrylic acid monomer, and a structural unit C containing an acrylamide monomer having a sulfonic acid group. The weight average molecular weight of the dispersion resin is 5000 to 100,000, and the content of the structural unit A is 60 mol% or more with respect to the total amount of the dispersion resin. A dispersion liquid is disclosed.

[0004] Patent Document 2 discloses a dispersion that enables easy redispersion of solidified colorants and improves stability during drying and re-dispensing, comprising water, a colorant, and a dispersion resin for dispersing the colorant, wherein the dispersion resin comprises constituent unit A containing a hydrophobic monomer, constituent unit B containing a hydrophilic acrylic acid monomer, and constituent unit C containing a hydrophilic vinyl monomer having a sulfonic acid group, the weight-average molecular weight of the dispersion resin is 10,000 to 100,000, and the content of constituent unit A is 60 mol% or more of the total amount of the dispersion resin.

[0005] Furthermore, Patent Document 3 discloses an aqueous inkjet ink that aims to solve problems such as scratch resistance and glossiness of recorded images formed using pigment ink, wherein resin A contributes to the dispersion of the pigment, resin B does not contribute to the dispersion of the pigment compared to resin A, resin A is a copolymer that does not precipitate in the pH range of 4.0 to 6.0, and is at least one copolymer selected from the group consisting of copolymers having vinyl alcohol monomer units, copolymers having monomer units having sulfonic acid groups, and copolymers in which the monomer units contributing as hydrophilic groups are only (meth)acrylic acid units, and resin B is a copolymer having (meth)acrylic acid monomer units that precipitate in the pH range of 4.0 to 6.0. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-020902 [Patent Document 2] Japanese Patent Publication No. 2022-052812 [Patent Document 3] Japanese Patent Publication No. 2010-090191 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Water-based inks containing aqueous pigment dispersions described in Patent Documents 1 and 2 had problems when printed on coated paper, a low-liquid-absorbent media with a smooth surface that does not spread easily. This resulted in insufficient ink wetting and a decrease in print quality due to white streaks and uneven coloring. Furthermore, the ink described in Patent Document 3 did not provide satisfactory abrasion resistance.

[0008] The present invention aims to provide an aqueous pigment dispersion and an aqueous ink containing the aqueous pigment dispersion that, when printed on coated paper, does not result in a decrease in print quality due to white streaks or color unevenness, and has good abrasion resistance. [Means for solving the problem]

[0009] The present invention relates to the following [1] and [2]. [1] An aqueous pigment dispersion comprising a pigment and a polymer dispersant, The polymer dispersant contains a polymer comprising a structural unit (A) derived from an alkyl (meth)acrylate, a structural unit (B) derived from a sulfonic acid having a polymerizable unsaturated group, and a structural unit (C) derived from a carboxylic acid having a polymerizable unsaturated group. An aqueous pigment dispersion wherein the alkyl group of the (meth)acrylate alkyl ester is a linear, branched, or alicyclic alkyl group having 5 to 24 carbon atoms. [2] A water-based ink containing the water-based pigment dispersion described in [1] above. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an aqueous pigment dispersion and an aqueous ink containing the aqueous pigment dispersion that do not cause a decrease in print quality due to white streaks or color unevenness even when printed on coated paper, and that have good abrasion resistance. [Modes for carrying out the invention]

[0011] <Water-based pigment dispersion> The aqueous pigment dispersion of the present invention is an aqueous pigment dispersion comprising a pigment and a polymer dispersant, wherein the polymer dispersant contains a polymer comprising a constituent unit (A) derived from an alkyl (meth)acrylate, a constituent unit (B) derived from a sulfonic acid having a polymerizable unsaturated group, and a constituent unit (C) derived from a carboxylic acid having a polymerizable unsaturated group, and the alkyl group of the alkyl (meth)acrylate is a linear, branched, or alicyclic alkyl group having 5 to 24 carbon atoms. In this invention, "aqueous system" means that water accounts for the largest proportion by mass in the medium. Water-based ink is sometimes simply referred to as "ink." A polymer containing the above-mentioned constituent units (A) to (C) may be referred to as "the polymer according to this embodiment."

[0012] By using the above-described configuration for the aqueous pigment dispersion, even when printing on coated paper with a smooth surface that does not easily wet, the deterioration of print quality due to white streaks and color unevenness is suppressed, and the dispersion also exhibits excellent abrasion resistance. The reason for this is not entirely clear, but it is presumed to be due to the following reasons. The polymer according to this embodiment comprises a structural unit (C) having a highly hydrophilic carboxyl group and a structural unit (B) having a more hydrophilic sulfonic acid group, and further comprises a structural unit (A) having a linear, branched, or alicyclic alkyl chain with 5 to 24 carbon atoms. The inclusion of structural units (B) and (C) mitigates the aggregation rate associated with the drying of ink droplets. The linear, branched, or alicyclic alkyl groups with 5 to 24 carbon atoms in constituent unit (A) exhibit excellent adsorption to the pigment surface. Therefore, when an aqueous pigment dispersion is used in an aqueous ink, even when the ink becomes solvent-rich during the drying process, it can suppress pigment aggregation due to unstable dispersion of the pigment. This results in superior ink wetting and spreading on coated paper, which is expected to reduce white streaks and color unevenness. Furthermore, compared to polymers containing aromatic groups, polymers having linear, branched, or alicyclic alkyl groups with 5 to 24 carbon atoms exhibit lower interaction between the polymer hydrophobic parts, resulting in improved adhesion between the polymer hydrophobic parts and the coated paper, and superior abrasion resistance.

[0013] (Polymer dispersant) The polymer dispersant contains a polymer comprising a constituent unit (A) derived from an alkyl (meth)acrylate, a constituent unit (B) derived from a sulfonic acid having a polymerizable unsaturated group, and a constituent unit (C) derived from a carboxylic acid having a polymerizable unsaturated group, wherein the alkyl group of the alkyl (meth)acrylate is a linear, branched, or alicyclic alkyl group having 5 to 24 carbon atoms.

[0014] [Component Unit (A)] The polymer according to this embodiment contains a constituent unit (A) derived from an alkyl (meth)acrylate, wherein the alkyl group of the alkyl (meth)acrylate is a linear, branched, or alicyclic alkyl group having 5 to 24 carbon atoms. The number of carbon atoms in the alkyl group of the (meth)acrylate alkyl ester is 5 or more, preferably 6 or more, and more preferably 22 or less, more preferably 18 or less, even more preferably 14 or less, and even more preferably 11 or less, from the viewpoint of the ink's wettability and abrasion resistance. The alkyl group structure of the (meth)acrylate alkyl ester is not particularly limited, and linear, branched, or alicyclic alkyl groups can be used. Examples of linear alkyl groups include n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group, n-dodecyl group (lauryl group), stearyl group, and behenyl group. Examples of branched alkyl groups include 2-ethylhexyl group, isohexyl group, isodecyl group, 3,5,5-trimethylhexyl group, and isostearyl group. From the viewpoint of abrasion resistance, the number of carbon atoms in the linear or branched alkyl group is preferably 8 to 18, more preferably 10 to 14. Examples of the alicyclic alkyl group (cycloalkyl group) include a cyclohexyl group, a cyclooctyl group, a 3,3,5-trimethylcyclohexyl group, an isobornyl group, an adamantyl group, and the like. From the viewpoint of abrasion resistance, the number of carbon atoms of the alicyclic alkyl group is preferably 6 or more and 14 or less, more preferably 6 or more and 11 or less. Among the above, from the viewpoint of the wetting and spreading property of the ink, the alkyl group of the (meth)acrylic acid alkyl ester is preferably an alicyclic alkyl group and a branched-chain alkyl group. From the viewpoint of abrasion resistance, an alicyclic alkyl group and a linear alkyl group are preferable. Furthermore, from the viewpoint of achieving both the wetting and spreading property and the abrasion resistance of the ink, it is preferable to have an alicyclic alkyl group (cycloalkyl group), more preferably a cyclohexyl group and an isobornyl group, and still more preferably a cyclohexyl group. The alkyl group does not include an arylalkyl group.

[0015] If the alkyl group of the (meth)acrylic acid alkyl ester has 5 or more and 24 or less carbon atoms, it may be only one kind or two or more kinds. In the structural unit (A), the content of the structural unit derived from the (meth)acrylic acid alkyl ester having an alicyclic alkyl group is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably 99% by mass or more, and more preferably 100% by mass, from the viewpoints of the wetting and spreading property and the abrasion resistance of the ink.

[0016] [Structural unit (B)] The polymer according to the present embodiment includes a structural unit (B) derived from a sulfonic acid having a polymerizable unsaturated group (hereinafter also simply referred to as "unsaturated sulfonic acid"). Examples of the polymerizable unsaturated group include a vinyl group, a vinylidene group, a vinylene group, etc., and from the viewpoints of the wetting and spreading property and the abrasion resistance of the ink, a vinyl group is preferable. "Derived from a sulfonic acid having a polymerizable unsaturated group" includes not only the origin of the sulfonic acid having a polymerizable unsaturated group itself but also the origin of a sulfonate of a sulfonic acid having a polymerizable unsaturated group. Examples of unsaturated sulfonic acids include styrene sulfonic acid, vinyl sulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid, from the viewpoint of availability and cost-effectiveness. Examples of salts include ammonium salts and alkali metal salts. Among the above, vinyl sulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid are preferred as unsaturated sulfonic acids from the viewpoint of ink wetting spreadability and abrasion resistance. The unsaturated sulfonic acid that constitutes the constituent unit (B) may be only one type or two or more types. That is, constituent unit (B) may consist only of constituent units derived from one type of unsaturated sulfonic acid, or it may consist of constituent units derived from two or more types of unsaturated sulfonic acid. For example, constituent unit (B) may consist only of constituent units derived from vinyl sulfonic acid, or it may consist of constituent units derived from vinyl sulfonic acid and constituent units derived from acrylamide-2-methylpropanesulfonic acid.

[0017] [Component unit (C)] The polymer according to this embodiment contains a constituent unit (C) derived from a carboxylic acid having a polymerizable unsaturated group (hereinafter also simply referred to as "unsaturated carboxylic acid"). Examples of polymerizable unsaturated groups include vinyl groups, vinylidene groups, vinylene groups, etc., and from the viewpoint of ink wettability and abrasion resistance, vinyl groups are preferred. "Derived from carboxylic acids having polymerizable unsaturated groups" includes not only carboxylic acids themselves that have polymerizable unsaturated groups, but also carboxylic acid salts that have polymerizable unsaturated groups. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, and 2-methacryloyloxymethylsuccinate. Examples of salts include ammonium salts and alkali metal salts. Among the above, acrylic acid and methacrylic acid are preferred as unsaturated carboxylic acids from the viewpoint of ink wetting and abrasion resistance, availability, and cost-effectiveness. The unsaturated carboxylic acid that constitutes the constituent unit (C) may be one type or two or more types. That is, the constituent unit (C) may consist only of constituent units derived from one type of unsaturated carboxylic acid, or it may consist of constituent units derived from two or more types of unsaturated carboxylic acids. For example, the constituent unit (C) may consist only of constituent units derived from acrylic acid, or it may consist of constituent units derived from acrylic acid and constituent units derived from methacrylic acid.

[0018] [Other constituent units] The polymer according to this embodiment may further have a polymerizable monomer (D) derived from polymerizable monomers other than the polymerizable monomers (A) to (C). Examples of polymerizable monomers constituting polymerizable monomer (D) include alkyl (meth)acrylate esters in which the alkyl group has fewer than 5 or more than 24 carbon atoms, or which have an arylalkyl group, and monomers containing an aromatic hydrocarbon group. The total content of constituent units (D) in the polymer according to this embodiment is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably less than 1% by mass, and more preferably 0% by mass, from the viewpoint of ink wettability and abrasion resistance. In particular, the content of monomer-derived constituent units (d1) containing aromatic hydrocarbon groups in the polymer is preferably 3% by mass or less, more preferably 1% by mass or less, even more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, from the viewpoint of improving adhesion between the polymer hydrophobic parts and the coated paper and improving abrasion resistance, by suppressing the affinity between polymer hydrophobic parts due to π-π interactions between some aromatic groups that do not adsorb to the pigment surface. Examples of monomers containing aromatic hydrocarbon groups include arylalkyl (meth)acrylate esters such as benzyl (meth)acrylate, as well as aromatic hydrocarbons having a vinyl group. Examples of aromatic hydrocarbons having a vinyl group include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, p-tert-butylstyrene, 2,4-dimethylstyrene, vinylnaphthalene, and vinylanthracene.

[0019] The content of constituent unit (A) in the polymer according to this embodiment is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more, from the viewpoint of abrasion resistance, and preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less, from the viewpoint of ink wetting spreadability. The content of unsaturated sulfonic acid-derived constituent units (B) in the polymer according to this embodiment is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 7% by mass or more, from the viewpoint of ink wetting spreadability, and preferably 60% by mass or less, more preferably 45% by mass or less, even more preferably 30% by mass or less, even more preferably 15% by mass or less, and even more preferably 12% by mass or less, from the viewpoint of abrasion resistance. The content of unsaturated carboxylic acid-derived constituent units (C) in the polymer according to this embodiment is preferably 3% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of ink wetting spreadability, and preferably 60% by mass or less, more preferably 45% by mass or less, and even more preferably 30% by mass or less, from the viewpoint of abrasion resistance. The total content of unsaturated sulfonic acid-derived constituent units (B) and unsaturated carboxylic acid-derived constituent units (C) in the polymer according to this embodiment is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, from the viewpoint of abrasion resistance, and preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less, from the viewpoint of ink wetting spreadability.

[0020] In the polymer according to this embodiment, the mass ratio of the content of constituent unit (A) to the total content of constituent unit (B) and constituent unit (C) [content of constituent unit (A) / [content of constituent unit (B) + content of constituent unit (C)]] is preferably 0.2 or more, more preferably 0.8 or more, and even more preferably 1.2 or more from the viewpoint of abrasion resistance, and preferably 15 or less, more preferably 10 or less, even more preferably 4 or less, and even more preferably 2.5 or less from the viewpoint of ink wetting spreadability.

[0021] The acid value of the polymer according to this embodiment is preferably 100 mg KOH / g or more, more preferably 115 mg KOH / g or more, even more preferably 150 mg KOH / g or more, and even more preferably 200 mg KOH / g or more, from the viewpoint of ink wetting spreadability, and preferably 420 mg KOH / g or less, more preferably 400 mg KOH / g or less, even more preferably 300 mg KOH / g or less, and even more preferably 250 mg KOH / g or less, from the viewpoint of abrasion resistance. The acid value of the polymer according to this embodiment can also be calculated from the mass ratio of the constituent monomers.

[0022] The weight-average molecular weight of the polymer according to this embodiment is preferably 5,000 or more, more preferably 10,000 or more, even more preferably 17,000 or more, and even more preferably 23,000 or more, from the viewpoint of ink wettability and abrasion resistance, and similarly preferably 120,000 or less, more preferably 96,000 or less, even more preferably 50,000 or less, and even more preferably 30,000 or less. The weight-average molecular weight of the polymer according to this embodiment can be measured by the method described in the examples.

[0023] At least a portion of the sulfonic acid group of constituent unit (B) and the carboxyl group of constituent unit (C) of the polymer according to this embodiment may be neutralized with a neutralizing agent from the viewpoint of ink wettability. The neutralizing agent is preferably one or more selected from the group consisting of alkali metal hydroxides, organic amines, and ammonia. Examples of alkali metal hydroxides include sodium hydroxide and potassium hydroxide. Examples of organic amines include alkanolamines, alkylamines, aminoalkanediols, alkoxyamines, and heterocyclic amines. Among these, one or more selected from the group consisting of alkanolamines and alkylamines are preferred, with alkanolamines being more preferred. Preferred alkanolamines include those miscible with water and having 2 to 8 carbon atoms, such as monoethanolamine, monoisopropanolamine, monoisobutanolamine, N-methylethanolamine, 2-dimethylaminoethanol, N-methyldiethanolamine, and triethanolamine. More preferably, one or more selected from the group consisting of alkali metal hydroxides and organic amines, and even more preferably sodium hydroxide, potassium hydroxide, and 2-dimethylaminoethanol. The degree of neutralization of the sulfonic acid group and carboxyl group in the polymer according to this embodiment is preferably 15 mol% or more, more preferably 20 mol% or more, and even more preferably 30 mol% or more, from the viewpoint of ink wettability and abrasion resistance, and similarly, preferably 100 mol% or less, more preferably 95 mol% or less, and even more preferably 90 mol% or less.

[0024] Here, the degree of neutralization can be determined by the following formula as the equivalent amount of neutralizing agent used relative to the sulfonic acid groups and carboxyl groups of the polymer according to this embodiment. When the equivalent amount of neutralizing agent used is 100 mol% or less, it is synonymous with the degree of neutralization. When the equivalent amount of neutralizing agent used exceeds 100 mol%, it means that the neutralizing agent is in excess relative to the sulfonic acid groups and carboxyl groups of the polymer according to this embodiment, and in this case, the degree of neutralization of the polymer according to this embodiment is considered to be 100 mol%. Amount of neutralizing agent used (mol%) = [(Mass of neutralizing agent added (g) / Equivalent amount of neutralizing agent) / [(Acid value of polymer according to this embodiment (mgKOH / g) × Mass of polymer according to this embodiment (g)) / (56.1 × 1000)] × 100

[0025] (Composition of aqueous pigment dispersion) The solid content concentration of the aqueous pigment dispersion of the present invention is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, from the viewpoint of the degree of freedom in designing the ink formulation, and preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less, from the viewpoint of the dispersion stability of the aqueous pigment dispersion. The ratio of pigment to polymer dispersant (pigment / polymer dispersant) in the aqueous dispersion is preferably 40 / 60 or more, more preferably 50 / 50 or more, even more preferably 60 / 40 or more, and even more preferably 70 / 30 or more, from the viewpoint of ink wetting spreadability, and preferably 95 / 5 or less, more preferably 90 / 10 or less, even more preferably 85 / 15 or less, and even more preferably 80 / 20 or less, from the viewpoint of abrasion resistance.

[0026] The water content in the aqueous pigment dispersion of the present invention is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, from the viewpoint of dispersion stability of the aqueous pigment dispersion and reduction of environmental impact, and preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less, from the viewpoint of freedom in designing the ink formulation.

[0027] The method for producing the aqueous pigment dispersion of the present invention preferably involves dispersing a pigment, a polymer according to this embodiment, and, if necessary, a neutralizing agent, a surfactant, etc., using a known method. The following methods can be used to produce aqueous pigment dispersions. A pigment mixture containing a pigment, a polymer according to this embodiment, an organic solvent, and water is dispersed to obtain a dispersion. Subsequently, the organic solvent is removed from the dispersion, and coarse particles are removed by filtration to obtain an aqueous pigment dispersion in which pigment-containing polymer particles are dispersed in an aqueous medium.

[0028] The cumulant average particle size of the pigment-containing polymer particles in the aqueous pigment dispersion of the present invention is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 90 nm or more, from the viewpoint of ink wetting spreadability and abrasion resistance, and similarly, preferably 600 nm or less, more preferably 300 nm or less, even more preferably 200 nm or less, and even more preferably 160 nm or less. The cumulant average particle size of pigment-containing polymer particles in an aqueous pigment dispersion can be measured by the method described in the examples.

[0029] The polymer dispersant may further contain known pigment dispersants other than the polymer according to this embodiment, but from the viewpoint of ink wettability and abrasion resistance, the content of the polymer according to this embodiment in the polymer dispersant is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 100% by mass.

[0030] (Pigment) The pigments used in this invention may be either inorganic or organic pigments, and lake pigments and fluorescent pigments may also be used. Furthermore, these may be used in combination with extender pigments as needed. Specific examples of inorganic pigments include metal oxides such as carbon black, titanium dioxide, iron oxide, red iron oxide, and chromium oxide, as well as pearlescent pigments. Carbon black is particularly preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and slene pigments. Specific examples of preferred organic pigments include one or more products from each product code selected from CI Pigment Yellow, CI Pigment Red, CI Pigment Orange, CI Pigment Violet, CI Pigment Blue, and CI Pigment Green. Examples of extender pigments include silica, calcium carbonate, and talc. For achromatic inks, achromatic pigments such as white, black, and gray can be used, while for chromatic inks, chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used. The above pigments can be used individually or in combination of two or more.

[0031] <Water-based ink> The aqueous ink of the present invention comprises the aqueous pigment dispersion of the present invention. The water-based ink of the present invention contains a water-based pigment dispersion that does not cause a decrease in print quality due to white streaks or color unevenness even when printed on coated paper, and has good abrasion resistance, making it suitable for use as a water-based ink for inkjet printing. The water-based ink of the present invention may further contain, as necessary, various additives such as water-soluble organic solvents, fixing resins, surfactants, humectants, wetting agents, wetting / penetrating agents, viscosity modifiers, defoaming agents, preservatives, fungicides, and rust inhibitors. The water-based ink of the present invention is preferably obtained by mixing the water-based pigment dispersion of the present invention with a water-soluble organic solvent and a surfactant, from the viewpoint of ink wettability and abrasion resistance. There are no particular restrictions on the method of mixing each component.

[0032] (Water-soluble organic solvent) The water-based ink of the present invention preferably contains a water-soluble organic solvent from the viewpoint of ink wetting spreadability, abrasion resistance, and discharge stability. In the present invention, "water-soluble organic solvent" refers to an organic solvent in which the amount of solvent dissolved when dissolved in 100 mL of water at 25°C is 5 mL or more. Water-soluble organic solvents can be used individually or in combination of two or more. Examples of water-soluble organic solvents include polyhydric alcohols; ethers such as polyhydric alcohol alkyl ethers and polyhydric alcohol aryl ethers; nitrogen-containing heterocyclic compounds; amides; amines; and sulfur-containing compounds. Among these, one or more selected from the group consisting of polyhydric alcohols and polyhydric alcohol alkyl ethers are preferred.

[0033] Examples of polyhydric alcohols include diols such as ethylene glycol, diethylene glycol, propylene glycol (1,2-propanediol), 1,3-propylene glycol, dipropylene glycol (isomer mixture), 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-hexanediol, triethylene glycol, polyethylene glycol, and polypropylene glycol.

[0034] Examples of polyhydric alcohol alkyl ethers include (poly)alkylene glycol monoalkyl ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monoisobutyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethylhexyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol monobutyl ether, triethylene glycol dimethyl ether, tetraethylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, and dipropylene glycol dimethyl ether. Examples of polyhydric alcohol aryl ethers include ethylene glycol monophenyl ether and ethylene glycol monobenzyl ether.

[0035] (Surfactants) Examples of surfactants include nonionic surfactants, anionic surfactants, and amphoteric surfactants. Among these, nonionic surfactants are preferred from the viewpoint of ink wetting spreadability and abrasion resistance. Examples of nonionic surfactants include polyoxyalkylene alkyl ether type surfactants, acetylene glycol type surfactants, polyhydric alcohol type surfactants, fatty acid alkanolamides, silicone type surfactants, and fluorine type surfactants. Among these, from the viewpoint of ink wetting spreadability and abrasion resistance, one or more nonionic surfactants selected from the group consisting of acetylene glycol type surfactants and silicone type surfactants are preferred. Preferred acetylene glycol-based surfactants include acetylene glycol having 8 to 22 carbon atoms and ethylene oxide adducts of the acetylene glycol, with 2,4,7,9-tetramethyl-5-decine-4,7-diol or its ethylene oxide adduct being more preferred. Examples of silicone-based surfactants include polyether-modified silicones, amino-modified silicones, carboxy-modified silicones, fatty acid-modified silicones, alcohol-modified silicones, aliphatic alcohol-modified silicones, epoxy-modified silicones, fluorine-modified silicones, and alkyl-modified silicones. Examples of commercially available nonionic surfactants include the KF series from Shin-Etsu Chemical Co., Ltd., the BYK series from BIC Chemie Japan Co., Ltd., the Surfinol series from Nisshin Chemical Industry Co., Ltd. and Air Products & Chemicals, and the Acetyleneol series from Kawaken Fine Chemical Co., Ltd.

[0036] (Composition and physical properties of water-based inks) The total content of pigment and polymer dispersant in the aqueous ink of the present invention is preferably 4% by mass or more, more preferably 5.5% by mass or more, even more preferably 6.7% by mass or more, and even more preferably 7.5% by mass or less, from the viewpoint of scratch resistance, and preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less, from the viewpoint of ink wettability.

[0037] The pigment content in the aqueous ink of the present invention is preferably 4.0% by mass or more, more preferably 5.0% by mass or more, and even more preferably 5.5% by mass or more, from the viewpoint of print density, and preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 7% by mass or less, and even more preferably 6.5% by mass or less, from the viewpoint of ink dispersion stability.

[0038] The content of the polymer dispersant in the aqueous ink of the present invention is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, even more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more, from the viewpoint of scratch resistance, and preferably 7% by mass or less, more preferably 6% by mass or less, even more preferably 4% by mass or less, and even more preferably 3% by mass or less, from the viewpoint of ink wettability. The ratio of pigment to polymer dispersant in the ink (pigment / polymer dispersant) is similar to that of the pigment to polymer dispersant in the aqueous dispersion.

[0039] The content of the water-soluble organic solvent in the water-based ink of the present invention is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of ink wettability and abrasion resistance, and similarly, preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 20% by mass or less.

[0040] The water content in the water-based ink of the present invention is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, from the viewpoint of ink dispersion stability and reduction of environmental impact, and preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less, from the viewpoint of freedom in designing the ink formulation.

[0041] (Inkjet printing) The aqueous ink of the present invention can be suitably used as an aqueous ink for inkjet printing. In particular, it is suitable for printing on coated paper as a printing substrate. In inkjet printing methods, a piezoelectric system is preferred as the method for ejecting water-based ink from the viewpoint of ejection performance. Examples of coated paper include general-purpose glossy paper and multi-color foam glossy paper. [Examples]

[0042] In the following manufacturing examples, embodiments, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" respectively, unless otherwise specified. The measurement and calculation methods for each physical property are as follows.

[0043] (1) Measurement of the weight-average molecular weight of polymer dispersant a The results were obtained by gel permeation chromatography. The measurement conditions are shown below. GPC device: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolum Super AW-H" manufactured by Tosoh Corporation. Eluent: A solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively. Flow rate: 0.5mL / min Standard material: Monodisperse polystyrene kits with known molecular weight [PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick C (F-288, F-40, F-4, A-5000, A-500)] (all manufactured by Tosoh Corporation) Measurement sample: 0.1 g of the polymer according to this embodiment was mixed with 10 mL of the eluent in a glass vial, stirred with a magnetic stirrer at 25°C for 10 hours, and filtered through a syringe filter "DISMIC-13HP" (made of PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0044] (2) Measurement of solid content concentration 10.0 g of sodium sulfate, which had been stabilized in a desiccator, was weighed into a 30 mL polypropylene container (φ=40 mm, height=30 mm). Approximately 1.0 g of the sample was added and mixed, then accurately weighed and maintained at 105°C for 2 hours to remove volatile components. After standing in the desiccator for another 15 minutes, the mass was measured. The mass of the sample after removal of volatile components was taken as the solid content, and the solid content concentration (%) was obtained by dividing it by the mass of the added sample.

[0045] (3) Measurement of the average particle size of aqueous pigment dispersion Cumulant analysis was performed using the "ELS-8000" laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd.), and the average particle size of the resulting cumulant was used to measure the average particle size of the aqueous pigment dispersion or aqueous ink. The sample to be measured had a particle concentration of 5 × 10⁻⁶ -3 A dispersion solution diluted with water to a concentration of % (converted to solid content) was used. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 cumulative measurements. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent.

[0046] (Preparation of polymer dispersant a) <Preparation Example 1> In a reaction vessel equipped with a stirrer, reflux condenser, and dropping tank, 6.3 parts of hexyl acrylate, 1.0 part of acrylamide-2-methylpropanesulfonic acid, 2.7 parts of acrylic acid, and 6.6 parts of N,N-dimethylformamide (hereinafter referred to as "DMF") as the polymerization solvent were added as the initial charge, and the reaction vessel was stirred for 10 minutes while maintaining the temperature at 70°C. Next, a mixture containing 56.7 parts hexyl acrylate, 9.0 parts acrylamide-2-methylpropanesulfonic acid, 24.3 parts acrylic acid, 60 parts DMF, 0.2 parts 2-mercaptoethanol as a chain transfer agent, and 1.0 part 2,2'-azobis(2,4-dimethylvalenonitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65) as a polymerization initiator was continuously added dropwise to a reaction vessel over 5 hours. After the addition was complete, the polymerization reaction was carried out for 1 hour, and then the polymerization reaction was terminated by cooling to room temperature. After the reaction was complete, the polymerization solvent was removed by vacuum distillation, diluted with tetrahydrofuran, and precipitated as a solid by dropwise addition to hexane. The precipitated solid was collected and dried under reduced pressure at 60°C for 24 hours to obtain polymer dispersant a1 (weight-average molecular weight: 22000).

[0047] <Preparation Examples 2-14> Polymer dispersants a2 to a14 were obtained in the same manner as in Preparation Example 1, except that the composition of the raw material monomers constituting polymer dispersant a was changed to the conditions shown in Table 1. 10% by mass of each monomer was initially charged, and 90% by mass was added dropwise.

[0048] Polymer dispersants a2 to a12 are polymers according to this embodiment, while polymer dispersants a13 and a14 are polymers for comparison. In Table 1, the unit of measurement for all values ​​except those in the "Polymer Dispersant a" column, the "A / (B+C)" column, and the "Properties" column is "parts". Furthermore, the value in the "A / (B+C)" column represents the mass ratio of the amount of alkyl (meth)acrylate (A) to the sum of the amounts of unsaturated sulfonic acid (B) and unsaturated carboxylic acid (C), and corresponds to the mass ratio of the content of constituent unit (A) to the sum of the content of constituent unit (B) and constituent unit (C) in polymer dispersant a [Content of constituent unit (A) / [Content of constituent unit (B) + Content of constituent unit (C)]].

[0049] [Table 1]

[0050] (Manufacturing of aqueous pigment dispersions) <Manufacturing Example 1> (Process 1) 25 parts of polymer dispersant a1 and 180 parts of deionized water were added to a reaction vessel equipped with a stirrer. Then, 10.0 parts of 5N sodium hydroxide aqueous solution (16.9% sodium hydroxide solids, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration) was added so that the degree of neutralization of the sulfonic acid groups and carboxylic acid groups of polymer dispersant a1 was 40 mol%. The reaction vessel was heated to 80°C and stirred for 5 hours, and then cooled to 25°C to obtain a varnish solution of polymer dispersant a1.

[0051] (Process 2) To the obtained varnish solution, 145 parts of deionized water and 50 parts of methyl ethyl ketone were added, and then 75 parts of cyanide pigment (manufactured by DIC Corporation, product name: Fastogen Blue LA5380, Pigment Blue 15:3) were added to obtain a pigment mixture. In Tables 2 and 3, Pigment Blue 15:3 is indicated as "PB15:3". The resulting pigment mixture was mixed for 1 hour at 6000 rpm and 20°C using a disperser blade (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disperser). Then, it was subjected to a 10-pass dispersion treatment at a pressure of 200 MPa using a microfluidizer (manufactured by Microfluidics, high-pressure homogenizer, product name: M-140K) to obtain a pigment dispersion.

[0052] (Step 3) To the obtained pigment dispersion, 135 parts of deionized water were added, and methyl ethyl ketone was removed under reduced pressure at 60°C. After removing some of the water, the mixture was filtered through a membrane filter (Sartorius, product name: Minisart syringe filter, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, thereby obtaining an aqueous pigment dispersion A1 with a solid content of 20% (average particle size: 112 nm). The results are shown in Table 2.

[0053] <Manufacturing Example 2> In Production Example 1, aqueous pigment dispersion A2 was obtained using the same procedure as in Production Example 1, except that 25 parts of polymer dispersant a2 obtained in Production Example 2 were used instead of polymer dispersant a1 obtained in Production Example 1. The results are shown in Table 2.

[0054] <Manufacturing Example 3> In Production Example 1, aqueous pigment dispersion A3 was obtained using the same procedure as in Production Example 1, except that 25 parts of polymer dispersant a3 obtained in Production Example 3 and 10.1 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%) were used instead of polymer dispersant a1 obtained in Production Example 1. The results are shown in Table 2.

[0055] <Manufacturing Example 4> In Production Example 1, aqueous pigment dispersion A4 was obtained using the same procedure as in Production Example 1, except that 25 parts of polymer dispersant a4 obtained in Production Example 4 and 16.5 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%) were used instead of polymer dispersant a1 obtained in Production Example 1. The results are shown in Table 2.

[0056] <Manufacturing Example 5> In Production Example 2, aqueous pigment dispersion A5 was obtained using the same procedure as in Production Example 2, except that 10 parts of polymer dispersant a2, 4.0 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%), and 90 parts of cyanide pigment were used. The results are shown in Table 2.

[0057] <Manufacturing Example 6> In Production Example 2, aqueous pigment dispersion A6 was obtained using the same procedure as in Production Example 2, except that 50 parts of polymer dispersant a2, 20.1 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%), and 50 parts of cyanide pigment were used. The results are shown in Table 2.

[0058] <Manufacturing Examples 7-8> In Production Example 2, aqueous pigment dispersions A7 to A8 were obtained using the same procedure as in Production Example 2, except that 7.0 parts of 8N potassium hydroxide aqueous solution (KOH) (potassium hydroxide solid content 33.8%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration: degree of neutralization 40 mol%) or 8.5 g of 2-dimethylaminoethanol (DMAE) (Fujifilm Wako Pure Chemical Industries, Ltd., degree of neutralization 90 mol%) were used instead of 5N sodium hydroxide aqueous solution. The results are shown in Table 2.

[0059] <Manufacturing Examples 9-13> In Production Example 2, aqueous pigment dispersions A9 to A13 were obtained using the same procedure as in Production Example 2, except that the cyan pigment was replaced with 75 parts magenta pigment (Heubach Color Japan Co., Ltd., product name: Inkjet Magenta E02, Pigment Red 122 / Pigment Violet 19 solid solution), 75 parts red pigment (Fuji Pigment Co., Ltd., product name: FUJI FAST CARMINE 522-1D, Pigment Red 150), 75 parts yellow pigment (Sanyo Pigment Co., Ltd., product name: Fast Yellow 7423, Pigment Yellow 74), 75 parts yellow pigment (Heubach Color Japan Co., Ltd., product name: Inkjet Yellow 4GC, Pigment Yellow 155), or 75 parts black pigment (Cabot, product name: Monarch 717, Pigment Black 7). The results for aqueous pigment dispersions A9 to A12 are shown in Table 2, and the results for aqueous pigment dispersion A13 are shown in Table 3. Note that in Table 2, Pigment Red 122 is shown as "PR122", Pigment Violet 19 as "PV19", Pigment Red 150 as "PR150", Pigment Yellow 74 as "PY74", and Pigment Yellow 155 as "PY155", while in Table 3, Pigment Black 7 is shown as "PBk7".

[0060] <Manufacturing Example 14> In Production Example 2, aqueous pigment dispersion A14 was obtained using the same procedure as in Production Example 1, except that 25 parts of polymer dispersant a5 obtained in Production Example 5 and 10.0 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%) were used instead of polymer dispersant a2 obtained in Production Example 2. The results are shown in Table 3.

[0061] <Manufacturing Example 15> In Production Example 2, aqueous pigment dispersion A15 was obtained using the same procedure as in Production Example 1, except that 25 parts of polymer dispersant a6 obtained in Production Example 6 and 10.0 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%) were used instead of polymer dispersant a2 obtained in Production Example 2. The results are shown in Table 3.

[0062] <Manufacturing Example 16> In Production Example 2, aqueous pigment dispersion A16 was obtained using the same procedure as in Production Example 1, except that 25 parts of polymer dispersant a7 obtained in Production Example 7 and 4.9 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%) were used instead of polymer dispersant a2 obtained in Production Example 2. The results are shown in Table 3.

[0063] <Manufacturing Example 17> In Production Example 2, aqueous pigment dispersion A17 was obtained using the same procedure as in Production Example 2, except that 25 parts of polymer dispersant a8 obtained in Production Example 8 and 8.6 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%) were used instead of polymer dispersant a2 obtained in Production Example 2. The results are shown in Table 3.

[0064] <Manufacturing Example 18> In Production Example 2, aqueous pigment dispersion A18 was obtained using the same procedure as in Production Example 2, except that 25 parts of polymer dispersant a9 obtained in Production Example 9 and 11.1 parts of 5N sodium hydroxide aqueous solution (degree of neutralization: 40 mol%) were used instead of polymer dispersant a2 obtained in Production Example 2. The results are shown in Table 3.

[0065] <Manufacturing Examples 19-23> In Production Example 2, aqueous pigment dispersions A19 to A23 were obtained using the same procedure as in Production Example 2, except that polymer dispersant a10 obtained in Production Example 10, polymer dispersant a11 obtained in Production Example 11, polymer dispersant a12 obtained in Production Example 12, polymer dispersant a13 obtained in Production Example 13, or polymer dispersant a14 obtained in Production Example 14 were used instead of polymer dispersant a2 obtained in Production Example 2. The results are shown in Table 3.

[0066] [Table 2]

[0067] [Table 3]

[0068] (Preparation of water-based ink) <Example 1> To obtain the ink composition shown in Table 4 (total of 100 parts), 8.0 parts of aqueous pigment dispersion A1 (40 parts as aqueous pigment dispersion) as the total of pigment and polymer dispersant a1, 20.0 parts of propylene glycol, 5.0 parts of 1,2-hexanediol, 0.5 parts of acetylene glycol-based surfactant (manufactured by Nisshin Chemical Industry Co., Ltd., product name: Surfinol 465), and deionized water were added and stirred, and filtered through a membrane filter (manufactured by Sartorius, product name: Minisart syringe filter, pore size: 5 μm, material: cellulose acetate) to obtain aqueous ink 1.

[0069] <Examples 2-22 and Comparative Examples 1-3> Water-based inks 2 to 25 were obtained in the same manner as in Example 1, except that the type or amount of water-based pigment dispersion A, water-soluble organic solvent, and surfactant were changed to those shown in Tables 4 to 5. The resin emulsion used in the preparation of water-based ink 23 in Comparative Example 3, shown in Table 5, is a styrene-acrylic emulsion (DSM NeoResins, "NeoCryl A-1091"). In Tables 4 and 5, the units for "Aqueous Pigment Dispersion A" other than the "Type" and "Evaluation Results" columns, and for the remainder, are in "parts".

[0070] <Creating printed materials> An inkjet printer (ImageXpert, product name: Jetxpert) equipped with a FUJIFILM Dimatix piezo-driven inkjet head, product name: Samba G3L, was filled with water-based ink. After adjusting the head voltage to 30V, frequency to 20kHz, push-pull drive waveform, and ink ejection volume to 3.0pL, a solid image (Duty 100%) was printed on gloss-coated paper (Oji Paper Co., Ltd., product name: OK Topcoat+) at a resolution of 1200 x 1200 dpi.

[0071] <Abrasion resistance> The resulting print was dried in a constant temperature bath set to 70°C for 1 minute, and a friction test was performed on the solid image area (Duty 100%) of the print using an AB-201 Sutherland type ink lab tester (manufactured by Tester Sangyo Co., Ltd.). The friction material used was the bottom surface of a weight (bottom surface area 50 cm²) on "OK Topcoat+" coated paper. 2 A load of 500g was applied using a tool, and the material was rubbed 20 times (back and forth). The condition of the printed surface of the rubbed material was visually inspected and evaluated according to the following evaluation criteria.

[0072] (Evaluation Criteria) A: No scratches can be seen on the surface of the printed material by visual inspection. B: A slight scratch is visible on the surface of the printed material, but there is no exposed surface of the printing medium. C: While scratches on the surface of the printed material are visible to the naked eye, there is no exposed surface on the printing medium, and it is usable for practical purposes. D: Visual inspection revealed scratches on the surface of the printed material, and some parts of the printing medium's surface were exposed, posing a practical problem.

[0073] <Wetting properties> White streaks and color unevenness were visually evaluated in the solid image areas of evaluation prints obtained on gloss-coated paper. The evaluation criteria are as follows: A: There are no white streaks or color inconsistencies visible in the printed material. B: Slight white streaks or color inconsistencies are visible in the printed material. C: White streaks or color unevenness may be visible in the printed material, but it is still usable for practical purposes. D: The printed material has many white streaks or color inconsistencies, which poses a practical problem.

[0074] [Table 4]

[0075] [Table 5]

[0076] As is clear from Tables 4 and 5, the water-based inks of Examples 1 to 22 exhibit superior wetting and abrasion resistance compared to the water-based inks of Comparative Examples 1 to 3. [Industrial applicability]

[0077] According to the present invention, it is possible to provide an aqueous pigment dispersion and an aqueous ink containing the aqueous pigment dispersion that do not cause a decrease in print quality due to white streaks or color unevenness even when printed on coated paper, and that have good abrasion resistance.

Claims

1. A water-based pigment dispersion comprising a pigment and a polymer dispersant, The polymer dispersant contains a polymer comprising a structural unit (A) derived from an alkyl (meth)acrylate, a structural unit (B) derived from a sulfonic acid having a polymerizable unsaturated group, and a structural unit (C) derived from a carboxylic acid having a polymerizable unsaturated group. An aqueous pigment dispersion wherein the alkyl group of the (meth)acrylate alkyl ester is a linear, branched, or alicyclic alkyl group having 5 to 24 carbon atoms.

2. The aqueous pigment dispersion according to claim 1, wherein the mass ratio of the content of constituent unit (A) to the sum of the content of constituent unit (B) and the content of constituent unit (C) in the polymer [content of constituent unit (A) / [content of constituent unit (B) + content of constituent unit (C)]] is 0.2 or more and 15 or less.

3. The aqueous pigment dispersion according to claim 1 or 2, wherein the content of the constituent unit (A) in the polymer is 30% by mass or more and 90% by mass or less.

4. The aqueous pigment dispersion according to claim 1 or 2, wherein the content of the constituent unit (B) in the polymer is 3% by mass or more and 60% by mass or less.

5. The aqueous pigment dispersion according to claim 1 or 2, wherein the content of the constituent unit (C) in the polymer is 3% by mass or more and 60% by mass or less.

6. The aqueous pigment dispersion according to claim 1 or 2, wherein the content of constituent units derived from monomers containing aromatic hydrocarbon groups in the polymer is 3% by mass or less.

7. The aqueous pigment dispersion according to claim 1 or 2, wherein the alkyl group is an alicyclic alkyl group.

8. A water-based ink comprising the aqueous pigment dispersion described in claim 1 or 2.

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