Water-based pigment dispersion

Abstract

Provided are pigment particles and a water-based pigment dispersion which are capable of obtaining a water-based ink for ink-jet printing that is excellent in marker resistance, is capable of realizing a printed material that is suppressed in curling, and is excellent in redispersibility, and a water-based ink for ink-jet printing containing the water-based pigment dispersion or the pigment particles.SOLUTION: Wherein the crosslinked polymer contains a structure derived from a water-dispersible polymer having an acid group and a structure derived from a polyfunctional epoxy crosslinking agent having a hydroxy group, a degree of neutralization of the water-dispersible polymer having an acid group is not less than 30 mol% and not more than 75 mol%, and a value of a mass ratio M of the crosslinked polymer to the water-dispersible polymer as calculated by the following formula (1M) is not less than 0.45: Mass of pigments / (mass of pigments + mass of crosslinked polymer) Formula (1M) SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to pigment particles, a water-based pigment dispersion, and a water-based ink for ink-jet printing containing the pigment particles or the water-based pigment dispersion. [Background technology]

[0002] Inkjet recording is a method of directly ejecting ink droplets from minute nozzles and depositing them on a recording medium to produce a recorded product with characters or images. This method has become extremely popular due to its many advantages, including ease of full-color printing, low cost, the ability to use plain paper as the recording medium, and non-contact with the recording medium. In particular, from the perspective of weather resistance and water resistance of the recorded product, the use of pigments as colorants has become mainstream.

[0003] For example, Patent Document 1 describes an aqueous pigment dispersion containing a pigment (A), a pigment dispersing resin (B) having a specific structure (X) and an anionic group, and an aqueous medium (C), wherein the pigment dispersing resin (B) has the anionic group in an amount of 0.2 to 1.5 mmol / g relative to the total amount of the pigment dispersing resin (B), with the aim of providing an aqueous pigment dispersion that can be used to produce an inkjet printing ink that maintains excellent storage stability and is capable of forming clear printed images with little color unevenness, etc., and also describes an inkjet printing ink containing the aqueous pigment dispersion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-155270 Summary of the Invention [Problem to be solved by the invention]

[0005] Water-based inks for inkjet printing lose fluidity when the aqueous medium evaporates, causing them to adhere to nozzles and lead to clogging. To redisperse and remove the adhered ink, water-based inks for inkjet printing are required to have the ability to disperse again in an aqueous medium after the aqueous medium evaporates (redispersibility). On the other hand, when printing on a highly water-absorbent print medium such as plain paper, the pigment remains on the surface of the print medium. Therefore, water-based inks for inkjet printing are required to be able to produce printed matter that is excellent in marker resistance. Furthermore, when printing on plain paper, the high water absorbency of plain paper results in high print density, but there is a problem in that the water contained in the water-based ink for inkjet printing causes the plain paper fibers to expand, and the expanded fibers then dry, causing curling and deformation in the printed matter. Therefore, water-based inks for inkjet printing are required to be able to produce recorded matter with reduced curling.

[0006] The present invention relates to pigment particles and a water-based pigment dispersion that can provide a water-based ink for inkjet printing that is excellent in highlighter resistance and curl suppression and that has excellent redispersibility, and to a water-based ink for inkjet printing that contains the pigment particles or the water-based pigment dispersion. [Means for solving the problem]

[0007] The present inventors have found that the above-mentioned problems can be solved by providing an aqueous pigment dispersion containing a pigment, a crosslinked polymer as a dispersant, and an aqueous medium, wherein the crosslinked polymer contains a structure derived from a water-dispersible polymer having an acid group and a structure derived from a polyfunctional epoxy crosslinker having a hydroxyl group, the degree of neutralization of the water-dispersible polymer having an acid group being within a specific range, and the ratio (mass ratio) of the content of the pigment to the total content of the pigment, the water-dispersible polymer having an acid group, and the crosslinker being a specific value or more.

[0008] That is, the present invention relates to the following [1] to [4]. [1] An aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium using a crosslinked polymer, the crosslinked polymer contains a structure derived from a water-dispersible polymer having an acid group and a structure derived from a multifunctional epoxy crosslinking agent having a hydroxyl group; the degree of neutralization of the water-dispersible polymer having an acid group is 30 mol % or more and 75 mol % or less, An aqueous pigment dispersion having a mass ratio M calculated by the following formula (1M) of 0.45 or more. Mass of pigment / (mass of pigment + mass of cross-linked polymer) Equation (1M) [2] A water-based ink for ink-jet recording, comprising the water-based pigment dispersion according to [1] above. [3] Pigment particles having a crosslinked polymer on the surface thereof, the crosslinked polymer contains a structure derived from a monomer having an acid group and a structure derived from a polyfunctional epoxy crosslinking agent having a hydroxyl group, and the degree of neutralization of the acid group derived from the monomer having an acid group is 30 mol % or more and 75 mol % or less; Pigment particles having a mass ratio M calculated by the following formula (1M) of 0.45 or more: Mass of pigment / (mass of pigment + mass of cross-linked polymer) Equation (1M) [4] A water-based ink for ink-jet printing, comprising the pigment particles described in [3] above. [Effects of the Invention]

[0009] The present invention can provide pigment particles, a water-based pigment dispersion, and a water-based ink for inkjet printing containing the pigment particles or the water-based pigment dispersion, which can produce recorded materials that are excellent in highlighter resistance and suppressed curling, and which can provide a water-based ink for inkjet printing that is excellent in re-dispersibility. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Water-based pigment dispersion] The aqueous pigment dispersion of the present invention comprises a pigment dispersed in an aqueous medium by a crosslinked polymer, the crosslinked polymer containing a structure derived from a water-dispersible polymer having an acid group and a structure derived from a polyfunctional epoxy crosslinker having a hydroxyl group, the degree of neutralization of the water-dispersible polymer having an acid group is 30 mol % or more and 75 mol % or less, and the value of the mass ratio M calculated by the following formula (1M) is 0.45 or more. Mass of pigment / (mass of pigment + mass of cross-linked polymer) Equation (1M)

[0011] In this specification, "aqueous" means that water accounts for the largest proportion of the medium in which the pigment is dispersed. Furthermore, "recording" is a concept that includes printing or printing characters or images. The aqueous pigment dispersion of the present invention, when blended in an aqueous ink, can realize recorded matter that is excellent in marker resistance and suppressed curling, and therefore can be suitably used as an aqueous pigment dispersion for flexographic printing, gravure printing, or inkjet recording. Furthermore, since the water-based ink containing the water-based pigment dispersion of the present invention has excellent redispersibility, it is preferably used as a water-based ink for ink-jet recording.

[0012] The aqueous ink containing the aqueous pigment dispersion of the present invention and the aqueous ink containing pigment particles are excellent in marker resistance, can produce printed matter with reduced curling, and have excellent redispersibility. The reasons for this are unclear, but are thought to be as follows. In the aqueous pigment dispersion and pigment particles of the present invention, the ratio obtained by the above formula (1M) is 0.45 or more, so that the proportion of solids other than the pigment, such as polymers, is relatively low. Therefore, even when the water in the aqueous ink containing the aqueous pigment dispersion or pigment particles of the present invention dries and high-boiling-point solvents such as glycerin remain, thickening of the ink is suppressed and fluidity is maintained. As a result, it is believed that the aqueous ink has excellent redispersibility after drying. On the other hand, when a water-based ink produced from the water-based pigment dispersion or pigment particles according to the present invention is printed on plain paper, the resulting print has excellent marker resistance and prevents the paper from curling after printing. By setting the degree of neutralization of the water-dispersible polymer having acid groups to 30 mol % or more and 75 mol % or less, even if the ink film dried on plain paper as a printing medium comes into contact with the aqueous medium again, for example, if it is rubbed with an aqueous marker pen, the pigment dispersion is prevented from redispersing in the aqueous medium. Therefore, it is believed that the ink film formed by the aqueous pigment dispersion or aqueous ink produced from the pigment particles according to the present invention has excellent marker resistance. Furthermore, the cellulose that makes up plain paper, a printing medium, has a strong crystalline structure, with cellulose molecules held together by hydrogen bonds. When printed with water-based ink, the cellulose fibers swell as the intermolecular hydrogen bonds are loosened by the water. When dried, the water is removed from the hydrated cellulose, causing intermolecular hydrogen bonds to form between the cellulose molecules, resulting in curling of the paper. In pigment dispersions that are fixed to the cellulose, hydroxyl groups derived from the crosslinked polymer are interposed between the cellulose molecules during drying, which is thought to inhibit the formation of intermolecular hydrogen bonds and prevent paper curling.

[0013] [Pigments] The pigment may be either an inorganic pigment or an organic pigment. Examples of inorganic pigments include carbon black and metal oxides. Carbon black is preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of white inks include metal oxides such as titanium dioxide, zinc oxide, silica, alumina, and magnesium oxide. Examples of organic pigments include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments. In the achromatic ink, achromatic pigments such as white, black, and gray can be used, while in the chromatic ink, chromatic pigments such as yellow, magenta, cyan, red, blue, orange, and green can be used. The pigments can be used alone or in combination of two or more. In the present invention, the pigment is dispersed in a crosslinked polymer and contained in the form of particles in an aqueous medium, or the crosslinked polymer is present on the surface of the pigment in the pigment particles of the present invention.

[0014] [Crosslinked polymer] The crosslinked polymer contains a structure derived from a water-dispersible polymer having an acid group (hereinafter also referred to as "water-dispersible polymer") and a structure derived from a polyfunctional epoxy crosslinker having a hydroxyl group, and the degree of neutralization of the water-dispersible polymer is 30 mol % or more and 75 mol % or less. The crosslinked polymers can be used alone or in combination of two or more.

[0015] The aqueous pigment dispersion of the present invention is prepared by dispersing a pigment in an aqueous medium with a crosslinked polymer. The pigment particles of the present invention are pigment particles having a crosslinked polymer present on the surface of the pigment. The forms of the pigment and crosslinked polymer present in the aqueous pigment dispersion of the present invention, the aqueous ink for inkjet recording containing the aqueous pigment dispersion, and the pigment particles and the aqueous ink for inkjet recording containing the pigment particles are common to both, and include, for example, a state in which the crosslinked polymer is adsorbed to the pigment, and a state in which the crosslinked polymer contains the pigment and encapsulates it (capsules). Of these, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the form of crosslinked polymer particles containing the pigment (hereinafter also referred to as "pigment-containing crosslinked polymer particles") is preferred, and the form of pigment-encapsulated crosslinked polymer particles in which the crosslinked polymer encapsulates the pigment is more preferred. The pigment particles are preferably used in an aqueous medium. There are no particular restrictions on the aqueous medium, but it is preferable that the aqueous medium contains water as the main component.

[0016] <Water-dispersible polymer having acid groups> A water-dispersible polymer having an acid group functions as a pigment dispersant that exhibits a pigment dispersing effect. Examples of the acid group that the water-dispersible polymer has include a carboxy group, a sulfonic acid group, and a phosphate group, with a carboxy group being preferred. In the present invention, the term "water-dispersible polymer" refers to a polymer that can be dispersed in water or an aqueous medium at room temperature (25°C).

[0017] The water-dispersible polymer may be at least one selected from polyester, polyurethane, and vinyl polymers, but from the viewpoints of redispersibility, highlighter resistance, and curl suppression, vinyl polymers obtained by addition polymerization of vinyl monomers are preferred. Examples of vinyl monomers include vinyl compounds, vinylidene compounds, and vinylene compounds.

[0018] From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the water-dispersible polymer is preferably a vinyl polymer containing a structural unit derived from an acid group-containing monomer, more preferably a carboxy group-containing vinyl polymer, and even more preferably a vinyl polymer containing a structural unit derived from a carboxy group-containing monomer and a structural unit derived from a hydrophobic monomer. When the water-dispersible polymer contains a structural unit derived from a carboxy group-containing monomer and a structural unit derived from a hydrophobic monomer, it is preferably one obtained by copolymerizing raw material monomers containing a carboxy group-containing monomer and a hydrophobic monomer.

[0019] (Carboxy group-containing monomer) Preferred examples of the carboxyl group-containing monomer include carboxylic acid monomers. Examples of the carboxylic acid monomer include one or more selected from the group consisting of unsaturated monocarboxylic acids such as (meth)acrylic acid and crotonic acid; and unsaturated dicarboxylic acids such as itaconic acid, maleic acid, fumaric acid, citraconic acid, and α,β-unsaturated dicarboxylic acids. Among these, the carboxyl group-containing monomer is preferably (meth)acrylic acid, more preferably acrylic acid, from the viewpoints of redispersibility, highlighter resistance, and curl suppression. The unsaturated dicarboxylic acid may be an anhydride. In this specification, the term "(meth)acrylic" such as (meth)acrylic acid is a concept that includes both acrylic and methacrylic.

[0020] (hydrophobic monomer) In the present invention, the term "hydrophobic monomer" refers to a monomer that dissolves in 100 g of ion-exchanged water at 25° C. to saturation in an amount of less than 10 g. Specific examples of hydrophobic monomers include alkyl (meth)acrylates and aromatic group-containing monomers described in paragraphs

[0020] to

[0022] of JP 2018-83938 A. Among these, from the viewpoints of redispersibility, highlighter resistance, and curl suppression, the hydrophobic monomer is preferably at least one selected from alkyl (meth)acrylates having an alkyl group preferably having 1 to 18 carbon atoms, more preferably having 1 to 10 carbon atoms, and aromatic group-containing monomers having an aromatic group preferably having 6 to 22 carbon atoms, more preferably having 6 to 18 carbon atoms, more preferably at least one selected from ethyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, styrene, α-methylstyrene, and benzyl (meth)acrylate, more preferably at least one selected from styrene, α-methylstyrene, and benzyl (meth)acrylate, even more preferably at least one selected from styrene and α-methylstyrene.

[0021] The raw material monomers for the water-dispersible polymer can be used singly or in combination of two or more.

[0022] <Content of each structural unit in water-dispersible polymer> The content of structural units derived from carboxyl group-containing monomers in the water-dispersible polymer is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and preferably 55% by mass or less, more preferably 45% by mass or less, even more preferably 35% by mass or less. The content of structural units derived from hydrophobic monomers in the water-dispersible polymer is preferably 45% by mass or more, more preferably 55% by mass or more, even more preferably 65% ​​by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less. From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the total content of the structural units derived from carboxyl group-containing monomers and the structural units derived from hydrophobic monomers in the water-dispersible polymer is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, and is 100% by mass or less, preferably 100% by mass. The mass ratio of the content of structural units derived from carboxyl group-containing monomers to the content of structural units derived from hydrophobic monomers in the water-dispersible polymer [carboxyl group-containing monomers / hydrophobic monomers] is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and is preferably 1.2 or less, more preferably 1.0 or less, even more preferably 0.7 or less.

[0023] <Production of water-dispersible polymer> The water-dispersible polymer can be produced, for example, by copolymerizing raw material monomers including an acid group-containing monomer and a hydrophobic monomer by a known polymerization method, preferably a solution polymerization method. There are no limitations on the solvent used in the solution polymerization method, but polar solvents such as water, aliphatic alcohols having 1 to 3 carbon atoms, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred. During polymerization, known polymerization initiators and polymerization chain transfer agents can be used. As the polymerization initiator, an azo compound such as 2,2'-azobis(2,4-dimethylvaleronitrile) is preferred, and as the polymerization chain transfer agent, a mercaptan compound such as 2-mercaptoethanol is preferred. Although preferred polymerization conditions vary depending on the type of polymerization initiator, the polymerization temperature is preferably 50° C. to 90° C., and the polymerization time is preferably 1 hour to 10 hours. The polymerization atmosphere is preferably a nitrogen gas atmosphere or an inert gas atmosphere such as argon.

[0024] <Degree of neutralization of water-dispersible polymer> From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the degree of neutralization of the acid groups in the water-dispersible polymer is 30 mol% or more, preferably 32 mol% or more, more preferably 33 mol% or more, and 75 mol% or less, preferably 65 mol% or less, more preferably 60 mol% or less, and even more preferably 55 mol% or less. Here, the degree of neutralization is the value obtained by dividing the number of molar equivalents of the neutralizing agent by the number of molar equivalents of acid groups derived from the water-dispersible polymer, i.e., the percentage of "number of molar equivalents of neutralizing agent / number of molar equivalents of acid groups derived from the water-dispersible polymer." Normally, the degree of neutralization does not exceed 100 mol %, but in the present invention, since it is calculated from the number of molar equivalents of the neutralizing agent, it exceeds 100 mol % when an excess amount of the neutralizing agent is used. When a divalent or higher neutralizing agent is used, the "number of molar equivalents of the neutralizing agent" is synonymous with the number of functional group equivalents.

[0025] <Acid value of water-dispersible polymer> The acid value of the water-dispersible polymer is preferably 100 mgKOH / g or more, more preferably 150 mgKOH / g or more, even more preferably 200 mgKOH / g or more, and preferably 400 mgKOH / g or less, more preferably 350 mgKOH / g or less, even more preferably 300 mgKOH / g or less. When the acid value of the water-dispersible polymer is within the above range, the amount of acid groups crosslinked by the polyfunctional epoxy crosslinking agent and the amount of acid groups neutralized can be ensured, and the dispersion stability of the pigment dispersed by the crosslinked polymer can be ensured. The acid value of the water-dispersible polymer can be determined by neutralization titration as described in the examples, or can be calculated from the mass ratio of the monomers constituting the water-dispersible polymer. The acid value of the water-dispersible polymer can also be calculated from the acid value of the crosslinked polymer (described later) and the degree of crosslinking of the crosslinked polymer (described later) using the following formula: Acid value of water-dispersible polymer (mgKOH / g) = 100 × (acid value of cross-linked polymer) / (100 - (degree of cross-linking of cross-linked polymer)) Here, the unit of the acid value of the crosslinked polymer is "mgKOH / g", and the unit of the degree of crosslinking of the crosslinked polymer is "mol %".

[0026] <Weight average molecular weight of water-dispersible polymer> From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the weight-average molecular weight of the water-dispersible polymer is preferably 1,000 or more, more preferably 5,000 or more, and even more preferably 10,000 or more, and is preferably 30,000 or less, more preferably 25,000 or less, and even more preferably 20,000 or less. The weight average molecular weight of the water-dispersible polymer is measured by the method described in the examples.

[0027] <Multifunctional epoxy crosslinking agent with hydroxyl groups> The polyfunctional epoxy crosslinking agent having a hydroxyl group is preferably water-soluble, and from the viewpoint of efficiently reacting with the acid group in the water-dispersible polymer in an aqueous medium to form a crosslinked structure, the water-solubility of the polyfunctional epoxy crosslinking agent is preferably 90% by mass or more, more preferably 92% by mass or more, and 100% by mass or less, preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 96% by mass or less. The polyfunctional epoxy crosslinking agent may be used alone or in combination of two or more. When two or more polyfunctional epoxy crosslinking agents are used in combination, the water solubility of the polyfunctional epoxy crosslinking agent means the water solubility of the mixture of the polyfunctional epoxy crosslinking agents. The water solubility of the polyfunctional epoxy crosslinking agent is measured by the method described in the Examples.

[0028] From the viewpoint of suppressing curling, the number of hydroxyl groups possessed by the polyfunctional epoxy crosslinking agent is 1 or more, preferably 1.5 or more, more preferably 2 or more, and preferably 6 or less, more preferably 4 or less, and even more preferably 3 or less. When polyfunctional epoxy crosslinking agents having different numbers of hydroxyl groups are used in combination, the "number of hydroxyl groups possessed by the polyfunctional epoxy crosslinking agent" refers to a weighted average value. From the viewpoints of redispersibility and curl suppression, the number of epoxy groups possessed by the polyfunctional epoxy crosslinking agent is 2 or more, preferably 3 or more, more preferably 3.5 or more, and even more preferably 4 or more, and is preferably 10 or less, more preferably 8 or less, and even more preferably 6 or less. When polyfunctional epoxy crosslinking agents having different numbers of epoxy groups are used in combination, the "number of epoxy groups possessed by the polyfunctional epoxy crosslinking agent" refers to a weighted average value.

[0029] The polyfunctional epoxy crosslinking agent having a hydroxyl group is preferably a polyglycidyl ether having a hydroxyl group, more preferably a polyglycidyl ether of a polyhydric alcohol having a hydroxyl group. The preferred range of the number of hydroxyl groups in the polyglycidyl ether is the same as that described above. The preferred range of the number of glycidyl ether groups in the polyglycidyl ether is the same as that described above for the number of epoxy groups. The epoxy equivalent of the multifunctional epoxy crosslinker is preferably 90 g / eq or more, more preferably 100 g / eq or more, even more preferably 110 g / eq or more, and preferably 300 g / eq or less, more preferably 220 g / eq or less, even more preferably 180 g / eq or less.

[0030] Examples of polyfunctional epoxy crosslinking agents include sorbitol polyglycidyl ether (epoxy equivalent: 173 g / eq, water solubility: 94% by mass), glycerol polyglycidyl ether (epoxy equivalent: 141 g / eq, water solubility: 99% by mass), polyglycerol polyglycidyl ether (epoxy equivalent: 168 g / eq, water solubility: 100%), and polyglycerol polyglycidyl ether (epoxy equivalent: 183 g / eq, water solubility: 100%). Among these, sorbitol polyglycidyl ether and polyglycerol polyglycidyl ether are preferred, with sorbitol polyglycidyl ether being more preferred.

[0031] <Degree of cross-linking of cross-linked polymer> The degree of crosslinking of the crosslinked polymer is preferably 40 mol% or more, more preferably 50 mol% or more, even more preferably 60 mol% or more, and preferably 85 mol% or less, more preferably 80 mol% or less, even more preferably 75 mol% or less. Here, the degree of crosslinking of the crosslinked polymer means the percentage of the value obtained by dividing the molar equivalent number of epoxy groups derived from the polyfunctional epoxy crosslinking agent by the molar equivalent number of acid groups derived from the water-dispersible polymer.

[0032] <Acid value of cross-linked polymer> From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the acid value of the crosslinked polymer is preferably 40 mgKOH / g or more, more preferably 50 mgKOH / g or more, even more preferably 60 mgKOH / g or more, and is preferably 170 mgKOH / g or less, more preferably 150 mgKOH / g or less, even more preferably 130 mgKOH / g or less. Here, the acid value of the crosslinked polymer can be calculated from the degree of crosslinking of the crosslinked polymer using the following formula, and indicates the amount of acid groups remaining in the water-dispersible polymer that are not involved in the formation of a crosslinked structure by reaction with the polyfunctional epoxy crosslinking agent. Acid value of cross-linked polymer (mgKOH / g) = (acid value of water-dispersible polymer) × (100 - (degree of cross-linking of cross-linked polymer)) / 100 Here, the acid value of the water-dispersible polymer is expressed in units of "mgKOH / g," and the degree of crosslinking of the crosslinked polymer is expressed in units of "mol %."

[0033] The content of the pigment in the aqueous pigment dispersion of the present invention is preferably 5% by mass or more, more preferably 6% by mass or more, even more preferably 7% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less.

[0034] The content (concentration) of the crosslinked polymer in the aqueous pigment dispersion of the present invention is preferably 3% by mass or more, more preferably 4% by mass or more, even more preferably 5% by mass or more, and preferably 15% by mass or less, more preferably 13% by mass or less, even more preferably 10% by mass or less.

[0035] In the aqueous pigment dispersion of the present invention, from the viewpoints of redispersibility, highlighter resistance, and curl suppression, the value of the mass ratio M calculated by the following formula (1M) is 0.45 or more, preferably 0.48 or more, more preferably 0.51 or more, even more preferably 0.54 or more, still more preferably 0.58 or more, and is preferably 0.89 or less, more preferably 0.85 or less, even more preferably 0.80 or less, and still more preferably 0.75 or less. Mass of pigment / (mass of pigment + mass of cross-linked polymer) Equation (1M)

[0036] [Method for producing aqueous pigment dispersion] The method for producing the water-based pigment dispersion preferably includes the following steps 1 to 3. Step 1: A step of neutralizing a water-dispersible polymer having an acid group to adjust the degree of neutralization of the water-dispersible polymer to 30 mol % or more and 75 mol % or less. Step 2: A step of dispersing a pigment in an aqueous medium using the water-dispersible polymer with a neutralization degree of 30 mol % to 75 mol % obtained in Step 1 as a polymer dispersant to obtain an uncrosslinked polymer pigment dispersion (a dispersion in which an uncrosslinked polymer and a pigment are dispersed in an aqueous medium). Step 3: A step of crosslinking the uncrosslinked polymer pigment dispersion obtained in Step 2 with a polyfunctional epoxy crosslinking agent to obtain an aqueous pigment dispersion in which the water-dispersible polymer is crosslinked.

[0037] [Process 1] Step 1 is a step of neutralizing the water-dispersible polymer with an alkaline compound. In step 1, first, the water-dispersible polymer is neutralized by adding a neutralizing agent.

[0038] <Neutralization> A portion of the acid groups of the water-dispersible polymer is neutralized to a degree of neutralization of 30 mol % or more and 75 mol % or less. The neutralizing agent is preferably one or more alkaline compounds selected from the group consisting of alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, and amine compounds, with alkali metal hydroxides being preferred. Neutralization is preferably carried out in an aqueous medium using an alkaline compound. The alkaline compound includes an alkali metal hydroxide, and the alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide. From the viewpoint of promoting sufficient and uniform neutralization, the neutralizing agent is preferably used as an aqueous solution of the neutralizing agent. From the same viewpoint as above, the concentration of the aqueous solution of the neutralizing agent is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 50% by mass or less, more preferably 25% by mass or less. The aqueous medium may contain an organic solvent, and the organic solvent is preferably an aliphatic alcohol having from 1 to 3 carbon atoms, a ketone having from 3 to 8 carbon atoms, an ether, an ester, or the like, more preferably a ketone having from 4 to 8 carbon atoms, and even more preferably methyl ethyl ketone. When the water-dispersible polymer is synthesized by solution polymerization, the solvent used in the polymerization may be used as is.

[0039] [Process 2] Step 2 is a step of dispersing a pigment in an aqueous medium using a water-dispersible polymer as a polymer dispersant to obtain an uncrosslinked polymer pigment dispersion. The aqueous medium used here is not particularly limited, but one containing water as the main component is preferred. To improve the pigment wettability and the pigment adsorption of the water-dispersible polymer, an organic solvent may be further added to the aqueous medium. Examples of the organic solvent include the same organic solvents as those used in step 1. In the dispersion treatment in step 2, the pigment particles can be atomized to the desired particle size by main dispersion using shear stress alone, but from the viewpoint of obtaining a uniform uncrosslinked polymer pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion. As a dispersing machine used for preliminary dispersion, a commonly used mixing and stirring device such as an anchor blade or a disperser blade can be used. Examples of means for applying shear stress used in this dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersers such as paint shakers and bead mills. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When the dispersion treatment is carried out using a high-pressure homogenizer, the average particle size of the pigment particles in the non-crosslinked polymer pigment dispersion can be adjusted by controlling the treatment pressure and the number of passes. From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more and 300 MPa or less, and the number of passes is preferably 3 or more and 30 or less.

[0040] It is preferable that the organic solvent in the uncrosslinked polymer pigment dispersion obtained in step 2 has been substantially removed, but it may remain as long as it does not impair the object of the present invention. The obtained non-crosslinked polymer pigment dispersion is a dispersion of pigment-containing polymer particles in a medium mainly composed of water. Here, the form of the pigment-containing polymer particles is not particularly limited, as long as the particles are formed from at least the pigment and the water-dispersible polymer. For example, the form includes a particle form in which the pigment is encapsulated in the water-dispersible polymer, a particle form in which the pigment is uniformly dispersed in the water-dispersible polymer, a particle form in which the pigment is exposed on the surface of the water-dispersible polymer particle, and the like, and mixtures thereof are also included.

[0041] The non-volatile component concentration (solid content concentration) of the obtained uncrosslinked polymer pigment dispersion is preferably 10% by mass or more, more preferably 15% by mass or more, from the viewpoint of improving the dispersion stability of the uncrosslinked polymer pigment dispersion and facilitating the preparation of an aqueous ink, and is preferably 30% by mass or less, more preferably 25% by mass or less. The solid content concentration of the uncrosslinked polymer pigment dispersion is measured by the method described in the examples.

[0042] The content of the pigment in the uncrosslinked polymer pigment dispersion is preferably 5% by mass or more, more preferably 8% by mass or more, even more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less. The content of the water-dispersible polymer in the uncrosslinked polymer pigment dispersion is preferably 2% by mass or more, more preferably 3% by mass or more, even more preferably 4% by mass or more, and is preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less. The mass ratio k of the pigment content to the total content of the pigment and water-dispersible polymer in the uncrosslinked polymer pigment dispersion [pigment / (pigment+water-dispersible polymer)] is preferably 0.47 or more, more preferably 0.57 or more, even more preferably 0.67 or more, from the viewpoints of redispersibility, highlighter resistance, and curl suppression, and is preferably 0.94 or less, more preferably 0.90 or less, even more preferably 0.80 or less.

[0043] The cumulant average particle size (hereinafter also referred to as "average particle size") of the pigment-containing polymer particles in the uncrosslinked polymer pigment dispersion is preferably 50 nm or more, more preferably 60 nm or more, even more preferably 70 nm or more, from the viewpoint of reducing coarse particles and improving the jetting properties of the resulting water-based ink, and is preferably 200 nm or less, more preferably 160 nm or less, even more preferably 150 nm or less. The average particle size of the pigment-containing polymer particles is measured by the method described in the examples. Furthermore, the average particle size of the pigment-containing polymer particles in the aqueous pigment dispersion and the aqueous ink is approximately the same as the average particle size in the uncrosslinked polymer pigment dispersion, and the preferred embodiment of the average particle size is the same as the preferred embodiment of the average particle size in the uncrosslinked polymer pigment dispersion.

[0044] [Step 3] Step 3 is a step of crosslinking the uncrosslinked polymer pigment dispersion obtained in step 2 with a polyfunctional epoxy crosslinking agent to obtain an aqueous pigment dispersion in which the water-dispersible polymer is crosslinked. In step 3, some of the acid groups of the water-dispersible polymer that constitutes the pigment-containing polymer particles in the uncrosslinked polymer pigment dispersion are crosslinked to form a crosslinked structure on the surface layer of the pigment-containing polymer particles, thereby converting the uncrosslinked polymer pigment dispersion into a predetermined water-based pigment dispersion. This water-based pigment dispersion is composed of a component derived from the water-dispersible polymer and a component derived from the polyfunctional epoxy crosslinker, and is in a form in which the pigment is dispersed in an aqueous medium by the crosslinked polymer dispersant.

[0045] [Water-based ink for inkjet printing] The water-based ink for ink-jet printing of the present invention (hereinafter also referred to as "the ink of the present invention") contains the above-mentioned pigment particles or water-based pigment dispersion. The ink of the present invention preferably contains an organic solvent from the viewpoint of improving the ink jetting properties. Examples of the organic solvent include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, the organic solvent is preferably at least one selected from polyhydric alcohols and polyhydric alcohol alkyl ethers, more preferably at least one selected from ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, glycerin, trimethylolpropane, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, diethylene glycol diethyl ether, and triethylene glycol monobutyl ether, and even more preferably at least one selected from glycerin and triethylene glycol monobutyl ether.

[0046] The ink of the present invention preferably contains a surfactant from the viewpoint of improving the ejection properties of the ink. Examples of the surfactant include nonionic surfactants, anionic surfactants, and amphoteric surfactants. Of these, nonionic surfactants are preferred, and at least one surfactant selected from acetylene glycol surfactants and silicone surfactants is more preferred, with acetylene glycol surfactants being even more preferred. Examples of acetylene glycol surfactants include acetylene diols such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, and 2,4-dimethyl-5-hexyn-3-ol, and ethylene oxide adducts of these acetylene diols. Examples of commercially available acetylene glycol surfactants include the "Surfynol" series and the "Olfine" series manufactured by Nissin Chemical Industry Co., Ltd. As the silicone surfactant, polyether-modified silicone is preferred. Examples of commercially available silicone surfactants include the KF series (KF-353, KF-355A, KF-642, KF-6011, etc.) manufactured by Shin-Etsu Chemical Co., Ltd., the Silface SAG series manufactured by Nissin Chemical Industry Co., Ltd., and the BYK series manufactured by BYK Japan K.K.

[0047] The ink of the present invention may further contain, if necessary, various additives commonly used in water-based inks for ink-jet printing, such as a humectant, wetting agent, penetrant, dispersant, viscosity modifier, antifoaming agent, preservative, antifungal agent and antirust agent.

[0048] From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the content of the pigment in the ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, and is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 7% by mass or less. From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the total content of the pigment and crosslinked polymer in the ink of the present invention is preferably 2% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 14% by mass or less. In the ink of the present invention, from the viewpoints of redispersibility, highlighter resistance, and curl suppression, the value of the mass ratio M calculated by the following formula (1I) is 0.45 or more, preferably 0.48 or more, more preferably 0.51 or more, even more preferably 0.54 or more, still more preferably 0.58 or more, and is preferably 0.89 or less, more preferably 0.85 or less, even more preferably 0.80 or less, and still more preferably 0.75 or less. Mass of pigment / (mass of pigment+mass of crosslinked polymer) Equation (1I) From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the content of organic solvent in the ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 7% by mass or more, and is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less. From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the content of surfactant in the ink of the present invention is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.3% by mass or more, and is preferably 3% by mass or less, and more preferably 2% by mass or less. From the viewpoints of redispersibility, highlighter resistance, and curl suppression, the water content in the ink of the present invention is preferably 40% by mass or more, more preferably 45% by mass or more, and even more preferably 50% by mass or more, and is preferably 75% by mass or less, more preferably 70% by mass or less, and even more preferably 65% ​​by mass or less.

[0049] <Method of manufacturing water-based ink for inkjet printing> The ink of the present invention can be prepared by adding an organic solvent and a surfactant to the aqueous pigment dispersion of the present invention as needed, dispersing the mixture by a known method, and filtering the mixture as needed to remove coarse particles. In producing the ink of the present invention, when an alkali metal hydroxide is further contained in addition to the above-described aqueous pigment dispersion, the alkali metal hydroxide added other than the aqueous pigment dispersion is added to the alkali metal hydroxide that neutralizes the carboxy group of the crosslinked polymer contained in the aqueous pigment dispersion.

[0050] <Inkjet recording> The ink of the present invention can be loaded into a known ink jet recording device and ejected as ink droplets onto a recording medium to record characters and images. Ink jet recording apparatuses include thermal and piezo types, but the thermal type is preferred, and the ink of the present invention is preferably used for thermal ink jet recording. As the recording medium, plain paper can be used. Alternatively, for example, low-water-absorbency recording paper such as coated paper, or non-water-absorbency resin film such as polyethylene terephthalate (PET) film or polypropylene film can also be used. The terms "low water absorption" and "non-water absorption" refer to a state in which the amount of water absorption of the recording medium is 10 g / m2 or less when the recording medium is in contact with pure water for 100 ms.2 This means that the water absorption is 10 g / m or less. 2 Ultra-high absorbency recording media are highly absorbent, and plain paper is classified as highly absorbent. [Example]

[0051] In the following Production Examples, Examples, and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The methods for measuring each physical property are as follows.

[0052] [Measurement method] [Weight average molecular weight and acid value of polymer] (1) Measurement of polymer weight average molecular weight The molecular weight of the water-dispersible polymer (hereinafter also referred to as "polymer a") was measured by gel permeation chromatography (GPC system manufactured by Tosoh Corporation (HLC-8120GPC), columns manufactured by Tosoh Corporation (TSK-GEL, α-M × 2), flow rate: 1 mL / min) using a solution of phosphoric acid and lithium bromide dissolved in N,N-dimethylformamide to concentrations of 60 mmol / L and 50 mmol / L, respectively, as the eluent. Monodisperse polystyrene of known molecular weight was used as a standard substance.

[0053] (2) Measurement of the acid value of the polymer Using an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), the water-dispersible polymer was dissolved in a titration solvent consisting of a mixture of toluene and acetone (2:1), and titrated with a 0.1 N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was taken as the endpoint. The acid value (mg KOH / g) was calculated from the titration volume of the potassium hydroxide solution up to the endpoint.

[0054] [Solids Concentration of Polymer Water Dispersion A, Uncrosslinked Polymer Pigment Dispersion k, and Water-Based Pigment Dispersion M] Approximately 10 g of sodium sulfate, brought to a constant weight in a desiccator, was weighed into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm), and the combined mass of the container and the sodium sulfate was precisely weighed. Approximately 1 g of the sample was added thereto and mixed, and then the combined mass of the container, the sodium sulfate, and the sample was precisely weighed and maintained at 105°C for 2 hours to remove volatiles. The mixture was then left in the desiccator for a further 15 minutes, and the combined mass of the container, the sodium sulfate, and the sample after devolatilization was precisely weighed to calculate the mass of the sample after devolatilization. The mass of the sample after devolatilization was then divided by the mass of the sample added to the container to obtain the solids concentration (%).

[0055] [Cumulant Average Particle Size of Pigment-Containing Polymer Particles] Cumulant analysis was performed using a laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.) to measure the cumulant average particle size. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 integration times. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The measured concentration was 5 x 10 -3 The results were calculated in mass % (solid concentration).

[0056] [Water solubility of multifunctional epoxy crosslinking agent] At room temperature (25°C), 90 parts by mass of deionized water and 10 parts by mass of a multifunctional epoxy crosslinker were added to a glass tube (25 mmφ×250 mmh), and the glass tube was left to stand for 1 hour in a thermostatic bath with the water temperature adjusted to 25°C. The glass tube was then shaken at 1500 rpm for 1 minute using a small shaker (manufactured by AS ONE Corporation, product name: Test Tube Mixer TRIO (High Type) HM-1N) and then left to stand again in the thermostatic bath for 10 minutes. The undissolved material was then recovered, dried, and weighed, and the water solubility (mass%) was calculated using the following formula: Water solubility (mass%) = 100 × (10 - mass parts of undissolved matter) / 10

[0057] [Manufacturing example] [Production of Polymer a] Production Example 1-1 (Production of Polymer a1) A raw material monomer mixture was prepared by mixing 30.8 parts of acrylic acid, 59.2 parts of styrene, and 10.0 parts of α-methylstyrene. 10 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), 0.3 parts of 2-mercaptoethanol as a polymerization chain transfer agent, and 10% of the raw material monomer mixture were mixed in a reaction vessel, and the inside of the vessel was thoroughly purged with nitrogen gas. Separately, a mixture of the remaining raw monomer mixture (90%), 2.7 parts of 2-mercaptoethanol, 40 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65, 2,2'-azobis(2,4-dimethylvaleronitrile)) (hereinafter referred to as "V-65") was placed in a dropping funnel. Under a nitrogen atmosphere, the temperature of the raw monomers in the reaction vessel was raised to 65°C while stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After stirring for 2 hours after the completion of the dropping, a solution of 0.15 parts of V-65 dissolved in 2.5 parts of MEK was added, and the mixture was aged at 65°C for 2 hours and then at 70°C for 2 hours to obtain a solution of polymer a1 having carboxy groups. The resulting polymer a1 solution was dried under reduced pressure and then pulverized using a coffee mill to obtain polymer a1. The weight-average molecular weight and acid value of polymer a1 are shown in Table 1.

[0058] [Table 1]

[0059] [Production of Polymer Aqueous Dispersion A] Production Example 2-1 (Production of Polymer Aqueous Dispersion A1) A heat-resistant bottle was charged with 80.0 parts of polymer a1, 292.4 parts of deionized water, and 27.6 parts of a 5N aqueous solution of sodium hydroxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) as a neutralizing agent (set so that the ratio of the number of moles of sodium hydroxide to the number of moles of carboxyl groups in polymer a1 was 34 mol% (neutralization degree: 34 mol%)). The mixture was heated at 90°C for 5 hours while stirring at 150 rpm to neutralize and disperse the polymer, obtaining an aqueous polymer dispersion A1 (solid concentration: 21.2%).

[0060] Production Examples 2-2, 2-3, and 2-4' (Production of Polymer Aqueous Dispersions A2, A3, and A4') In Production Example 2-1, the same procedure as in Production Example 2-1 was carried out except that the amount of 5N aqueous sodium hydroxide solution and the amount of deionized water added were changed as shown in Table 2, and polymer aqueous dispersions A2, A3, and A4' having the degrees of neutralization and solid concentrations shown in Table 2 were obtained.

[0061] [Table 2]

[0062] <Production of Uncrosslinked Polymer Pigment Dispersion k> Production Example 3-1 (Production of Uncrosslinked Polymer Pigment Dispersion k1) To 157.9 parts of polymer aqueous dispersion A1, 370.2 parts of deionized water and 100.0 parts of carbon black (manufactured by Cabot Corporation, product name: Monarch 880) were added, and the mixture was stirred for 60 minutes using a Disper (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) at 20°C with the Disper blade rotating at 7,000 rpm. The resulting mixture was dispersed 10 times using a Microfluidizer (manufactured by Mizuho Industries Co., Ltd., product name: M-110-EH) at a pressure of 200 MPa to obtain a dispersion. The resulting dispersion was filtered through a 25 mL needleless syringe (Terumo Corporation) fitted with a 5 μm pore size syringe filter (Fujifilm Wako Pure Chemical Industries, Ltd., product name: Minisart®, S7594-FMOSK) to remove coarse particles, yielding uncrosslinked polymer pigment dispersion k1 (mass ratio k: 0.76, solids concentration: 21.2%). The mass ratio k was calculated using the following formula (1k): Mass of pigment / (mass of pigment + mass of polymer a1) Equation (1k)

[0063] Production Examples 3-2, 3-3, 3-4', 3-5, 3-6, and 3-7' (Production of Uncrosslinked Polymer Pigment Dispersions k2, k3, k4', k5, k6, and k7') In Production Example 3-1, except that the type and / or amount of polymer aqueous dispersion A was changed as shown in Table 3, the same procedure as in Production Example 3-1 was carried out to obtain uncrosslinked polymer pigment dispersions k2, k3, k4', k5, k6, and k7' having the mass ratio k values ​​and solid content concentrations shown in Table 3.

[0064] [Table 3]

[0065] [Production of Water-Based Pigment Dispersion M] Production Example 4-1 (Production of Water-Based Pigment Dispersion M1) 100 parts of uncrosslinked polymer pigment dispersion k1 (solids concentration: 21.2%) was placed in a screw-cap glass bottle, and 2.60 parts of sorbitol polyglycidyl ether (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-614B, epoxy equivalent: 173 g / eq, water solubility: 94%) (an amount sufficient to crosslink 70 mol % of the total carboxy groups in polymer a1 (an amount sufficient to achieve a crosslinking degree of 70 mol %)) and 15.8 parts of deionized water were added. The bottle was then sealed and heated at 70°C for 5 hours while stirring. The temperature was then lowered to room temperature, and the mixture was filtered using a 25 mL needleless syringe equipped with a 5 μm pore size syringe filter to obtain aqueous pigment dispersion M1 (mass ratio M: 0.68, solids concentration: 20.1%, pigment concentration: 13.4%). The mass ratio M was calculated using the following formula (1M): Mass of pigment / (mass of pigment + mass of cross-linked polymer) Equation (1M)

[0066] Production Examples 4-2, 4-3, 4-4', 4-5, 4-6, 4-7', 4-8, 4-9, and 4-10' (Production of Water-Based Pigment Dispersions M2, M3, M4', M5, M6, M7', M8, M9, and M10') Water-based pigment dispersions M2, M3, M4', M5, M6, M7', M8, M9, and M10' having the mass ratio M values, solid content concentrations, and pigment concentrations shown in Table 4 were obtained in the same manner as in Production Example 4-1, except that in Production Example 4-1, the type of uncrosslinked polymer pigment dispersion k, the type and amount of polyfunctional epoxy crosslinker added, and / or the amount of deionized water added were changed as shown in Table 4.

[0067] [Table 4]

[0068] (Details of multifunctional epoxy crosslinker) Denacol EX-614B: Nagase ChemteX Corporation, sorbitol polyglycidyl ether, epoxy equivalent: 173 g / eq, water solubility: 94% by mass Denacol EX-313: Nagase ChemteX Corporation, glycerol polyglycidyl ether, epoxy equivalent: 141 g / eq, water solubility: 99% by mass Denacol EX-512: Nagase ChemteX Corporation, Polyglycerol polyglycidyl ether, epoxy equivalent: 168 g / eq, water solubility: 100% by mass Denacol EX-810: Nagase ChemteX Corporation, ethylene glycol diglycidyl ether, epoxy equivalent: 113 g / eq, water solubility: 100% by mass In Table 6 below, "Deconal" will be omitted.

[0069] Example 1 37.3 parts of aqueous pigment dispersion M1, 5.0 parts of glycerin (manufactured by Kao Corporation, product name: concentrated glycerin for cosmetics), 0.5 parts of an acetylene glycol surfactant (manufactured by Nissin Chemical Industry Co., Ltd., product name: Olfine E1010), 7.0 parts of triethylene glycol monobutyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.), and 50.2 parts of deionized water were mixed and filtered through the syringe filter with a pore size of 5 μm to obtain ink 1 (pigment concentration: 5.0%).

[0070] Examples 2 to 7, Comparative Examples 1 to 3 Inks 2 to 7 and 8' to 10' with a pigment concentration of 5.0% were obtained in the same manner as in Example 1, except that the type and amount of aqueous pigment dispersion M and / or the amount of deionized water were changed as shown in Table 5.

[0071] [Evaluation method] [Redispersibility evaluation] 5 g of ink was placed in a glass petri dish whose mass had been measured in advance and left in a thermostatic chamber at 60°C for 2 hours. The dish was removed from the thermostatic chamber and returned to room temperature (25°C), after which the mass of the dish after drying was measured and the mass of the dried ink (a) was calculated. The water in the dried ink had evaporated, resulting in a paste-like state. Next, 5 g of fresh ink with the same composition was added to the petri dish containing the dried ink, and the mixture was left to stand at room temperature (25°C) for 30 minutes. The dish was tilted and left standing for 20 seconds to allow the ink to flow out, after which the mass of the dish was measured, and the mass b of the ink remaining in the dish without flowing out was measured. The re-dispersion rate of the ink was calculated using the following formula: The closer the re-dispersion rate is to 100%, the better the ink re-dispersibility is. Redispersion rate (%) = [1 - (mass of ink remaining in the dish b) / (mass of dried ink a)] x 100

[0072] [Evaluation of marker resistance] The resulting ink was filled into the black cartridge of an inkjet printer (product name: Business Printer PX-105, manufactured by Seiko Epson Corporation), and a 20mm x 20mm solid image was printed on plain paper (product name: Xerox Business 4200 Copy Paper, manufactured by Xerox Corporation) at room temperature of 23°C and relative humidity of 50%RH, using the paper type: plain paper mode, gray screen, and standard print quality. Immediately after printing, the solid image surface was traced with a commercially available yellow highlighter pen (manufactured by Pentel Co., Ltd., product name: Knock-type Handy Line S), and the area transferred to the white paper was measured for print density using an optical densitometer, SpectroEye (manufactured by Gretag Macbeth). Lower print density in the transferred area indicates better marker resistance.

[0073] [Evaluation of curl] A solid image measuring 230 mm x 150 mm was printed using the same method as in the marker resistance evaluation. The resulting print was placed on a horizontal table with the printed side facing up and left for one day under the same environmental conditions. After leaving it for one day, the heights of the four corners of the print from the horizontal table were measured and the average value (mm) was calculated to evaluate the curl of the print. The smaller the average value (mm), the less curl the print had.

[0074] [Table 5]

[0075] [Table 6]

[0076] As is clear from Table 6, the water-based ink for ink-jet printing of the present invention is excellent in highlighter resistance, can produce recorded matter with reduced curling, and has excellent redispersibility. In contrast, a printed matter produced using an inkjet printing water-based ink in which the crosslinked polymer contained a structure derived from a water-dispersible polymer with a degree of neutralization of more than 75 mol% (80 mol%) exhibited poor highlighter resistance (Comparative Example 1). Furthermore, a water-based inkjet printing ink obtained using an aqueous pigment dispersion in which the mass ratio M was less than 0.45 (0.41) exhibited poor redispersibility (Comparative Example 2). Furthermore, a printed matter produced using an inkjet printing water-based ink in which the crosslinked polymer contained a structure derived from a polyfunctional epoxy crosslinker without a hydroxyl group exhibited significant curl (Comparative Example 3).

Claims

1. An aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium using a crosslinked polymer, the crosslinked polymer contains a structure derived from a water-dispersible polymer having an acid group and a structure derived from a multifunctional epoxy crosslinking agent having a hydroxyl group; the degree of neutralization of the water-dispersible polymer having an acid group is 30 mol % or more and 75 mol % or less; An aqueous pigment dispersion having a mass ratio M calculated by the following formula (1M) of 0.45 or more: Mass of pigment / (mass of pigment+mass of crosslinked polymer) Formula (1M)

2. The water-based pigment dispersion according to claim 1 , wherein the water-dispersible polymer having an acid group comprises a structural unit derived from a carboxy group-containing monomer and a structural unit derived from a hydrophobic monomer.

3. 3. The aqueous pigment dispersion according to claim 1, wherein the polyfunctional epoxy crosslinking agent having a hydroxyl group is a polyglycidyl ether having a hydroxyl group.

4. The aqueous pigment dispersion according to claim 1 or 2, wherein the water-solubility of the hydroxyl group-containing polyfunctional epoxy crosslinking agent is 90 mass % or more.

5. 3. The aqueous pigment dispersion according to claim 1, wherein the hydroxyl group-containing polyfunctional epoxy crosslinking agent has 1.5 or more hydroxyl groups and 3.5 or more epoxy groups.

6. A water-based ink for ink-jet recording, comprising the water-based pigment dispersion according to claim 1 or 2.

7. A pigment particle having a crosslinked polymer on the surface of the pigment, the crosslinked polymer contains a structure derived from a monomer having an acid group and a structure derived from a polyfunctional epoxy crosslinking agent having a hydroxyl group, and the degree of neutralization of the acid group derived from the monomer having an acid group is 30 mol % or more and 75 mol % or less; Pigment particles having a mass ratio M calculated by the following formula (1M) of 0.45 or more: Mass of pigment / (mass of pigment+mass of crosslinked polymer) Formula (1M)

8. The pigment particles according to claim 7 , wherein the crosslinked polymer comprises constitutional units derived from a carboxy group-containing monomer and constitutional units derived from a hydrophobic monomer.

9. 9. The pigment particles according to claim 7, wherein the polyfunctional epoxy crosslinking agent having a hydroxyl group is a polyglycidyl ether having a hydroxyl group.

10. 9. The pigment particles according to claim 7, wherein the water solubility of the hydroxyl group-containing polyfunctional epoxy crosslinking agent is 90% by mass or more.

11. 9. The pigment particles according to claim 7, wherein the number of hydroxyl groups contained in the hydroxyl-containing polyfunctional epoxy crosslinking agent is 1.5 or more and the number of epoxy groups contained in the polyfunctional epoxy crosslinking agent is 3.5 or more.

12. The pigment particles according to claim 7 or 8, which are used in an aqueous medium.

13. A water-based ink for ink-jet printing, comprising the pigment particles according to claim 7 or 8.

14. A method for producing an aqueous pigment dispersion, comprising the following steps 1 to 3: Step 1: A step of neutralizing a water-dispersible polymer having an acid group to adjust the degree of neutralization of the water-dispersible polymer to 30 mol % or more and 75 mol % or less. Step 2: A step of dispersing a pigment in an aqueous medium using the water-dispersible polymer having a neutralization degree of 30 mol % to 75 mol % obtained in Step 1 as a polymer dispersant to obtain a dispersion in which the uncrosslinked polymer and the pigment are dispersed in the aqueous medium (hereinafter referred to as an uncrosslinked polymer pigment dispersion). Step 3: A step of crosslinking the uncrosslinked polymer pigment dispersion obtained in Step 2 with a polyfunctional epoxy crosslinking agent to obtain an aqueous pigment dispersion in which the water-dispersible polymer is crosslinked.

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