Water-based pigment dispersion

Abstract

To provide a water-based pigment dispersion having excellent dischargeability even in thermal type inkjet recording, a method for producing the water-based pigment dispersion, and a water-based ink for inkjet recording comprising the water-based pigment dispersion.SOLUTION: There are provided: a water-based pigment dispersion that comprises a pigment, a cross-linked polymer dispersant A, and a water-based medium, wherein the cross-linked polymer dispersant A includes a structure derived from a polymer (a) having carboxyl groups and a structure derived from a polyfunctional epoxy compound (b), wherein at least a part of the carboxyl groups of the cross-linked polymer dispersant A is neutralized with an alkali metal hydroxide, and a degree of cross-linking of the cross-linked polymer dispersant A and an acid value of the polymer (a) satisfy the following condition 1; a method for producing the water-based pigment dispersion; and a water-based ink for inkjet recording that comprises the water-based pigment dispersion. Condition 1: the value of [(degree of cross-linking of cross-linked polymer dispersant A) / 100]×(acid value of polymer (a)) is 230 mgKOH / g to 360 mgKOH / g.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an aqueous pigment dispersion, a method for producing the aqueous pigment dispersion, and an aqueous ink for ink-jet printing containing the aqueous pigment dispersion. [Background technology]

[0002] Inkjet recording is a method of directly ejecting ink droplets from fine nozzles and depositing them on a recording medium to obtain a recorded matter on which characters or images are recorded. This method has become extremely popular due to its many advantages, including the ease and low cost of full coloring, the ability to use plain paper as a recording medium, and the fact that it does not come into contact with the recording medium. In particular, from the viewpoint of the weather resistance and water resistance of the recorded matter, the use of pigments as coloring materials has become mainstream. On the other hand, water-based inks for ink-jet recording, which use a pigment as a coloring material, have a problem in that they are inferior in ejection properties because the pigment particles are dispersed in the water-based medium. Therefore, various proposals have been made to improve the properties of water-based inks for ink-jet printing, which use pigments as colorants.

[0003] For example, Patent Document 1 aims to provide an aqueous pigment dispersion or the like that has excellent redispersibility capable of suppressing clogging of ink ejection nozzles due to solidification of the pigment or polymer in the nozzles, and that can ensure storage stability at high temperatures. The aqueous pigment dispersion disclosed herein is composed of a polymer dispersant formed by crosslinking a water-insoluble polymer (A) having a carboxy group with a water-insoluble polyfunctional epoxy compound (b), and a pigment, in which at least a portion of the carboxy groups of the water-insoluble polymer (A) are neutralized with an alkali metal compound, and the acid value, degree of neutralization, and degree of crosslinking of the neutralized water-insoluble polymer (A) satisfy specific conditions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-65997 A Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, there has been an increasing demand for water-based pigment dispersions with improved ejection properties when used in inkjet recording inks. Patent Document 1 describes that the water-based pigment dispersion can suppress clogging of ejection nozzles when used in water-based inks for piezo inkjet recording, but it has been found that the water-based pigment dispersion is still insufficient for application to thermal inkjet recording, in which thermal energy is applied to the ink to make it fly. An object of the present invention is to provide an aqueous pigment dispersion which is excellent in ejection property even in thermal inkjet recording, a method for producing the aqueous pigment dispersion, and an aqueous ink for inkjet recording which contains the aqueous pigment dispersion. [Means for solving the problem]

[0006] The present inventors have found that the above-mentioned problems can be solved by an aqueous pigment dispersion containing a pigment, a crosslinked polymer dispersant A, and an aqueous medium, wherein the crosslinked polymer dispersant contains a structure derived from a polymer having a carboxy group and a structure derived from a polyfunctional epoxy compound, at least a portion of the carboxy groups of the polymer constituting the crosslinked polymer dispersant are neutralized with an alkali metal hydroxide, and further, the degree of crosslinking of the crosslinked polymer dispersant A and the acid value of the polymer having a carboxy group satisfy specific conditions.

[0007] That is, the present invention relates to the following [1] to [3]. [1] An aqueous pigment dispersion comprising a pigment, a crosslinked polymer dispersant A, and an aqueous medium, The crosslinked polymer dispersant A contains a structure derived from a polymer (a) having a carboxy group and a structure derived from a polyfunctional epoxy compound (b), At least a portion of the carboxyl groups of the crosslinked polymer dispersant A is neutralized with an alkali metal hydroxide, The crosslinking degree of the crosslinked polymer dispersant A and the acid value of the polymer (a) satisfy the following condition 1. Condition 1: The value of [(degree of crosslinking of crosslinked polymer dispersant A) / 100]×(acid value of polymer (a)) is 230 mgKOH / g or more and 360 mgKOH / g or less. Here, the degree of crosslinking of the crosslinked polymer dispersant A is the percentage of the molar equivalent number of epoxy groups derived from the polyfunctional epoxy compound (b) constituting the crosslinked polymer dispersant A divided by the molar equivalent number of carboxy groups derived from the polymer (a) constituting the crosslinked polymer dispersant A [(the molar equivalent number of epoxy groups derived from the polyfunctional epoxy compound (b)) / (the molar equivalent number of carboxy groups derived from the polymer (a)]. [2] A water-based ink for ink-jet recording, comprising the water-based pigment dispersion according to [1] above. [3] A method for producing the aqueous pigment dispersion according to [1] above, comprising the following steps 1 and 2: Step 1: A step of dispersing a pigment mixture containing a pigment, a polymer (a) having a carboxy group, and an alkali metal hydroxide by applying shear stress to obtain a pigment water dispersion (d). Step 2: A step of reacting the carboxy group of the polymer (a) having a carboxy group in the pigment aqueous dispersion (d) obtained in step 1 with the polyfunctional epoxy compound (b) to form a crosslinked polymer dispersant A, thereby obtaining an aqueous pigment dispersion containing a pigment, the crosslinked polymer dispersant A, and an aqueous medium. Effect of the Invention

[0008] The present invention can provide an aqueous pigment dispersion that is excellent in ejection property even in thermal inkjet recording, a method for producing the aqueous pigment dispersion, and an aqueous ink for inkjet recording that contains the aqueous pigment dispersion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Water-based pigment dispersion] The aqueous pigment dispersion of the present invention is an aqueous pigment dispersion containing a pigment, a crosslinked polymer dispersant A, and an aqueous medium, the crosslinked polymer dispersant A contains a structure derived from a polymer (a) having a carboxy group and a structure derived from a polyfunctional epoxy compound (b), at least a portion of the carboxy groups of the crosslinked polymer dispersant A are neutralized with an alkali metal hydroxide, and the degree of crosslinking of the crosslinked polymer dispersant A and the acid value of the polymer (a) satisfy the following condition 1: Condition 1: The value of [(degree of crosslinking of crosslinked polymer dispersant A) / 100]×(acid value of polymer (a)) is 230 mgKOH / g or more and 360 mgKOH / g or less. Here, the degree of crosslinking of the crosslinked polymer dispersant A is the percentage of the molar equivalent number of epoxy groups derived from the polyfunctional epoxy compound (b) constituting the crosslinked polymer dispersant A divided by the molar equivalent number of carboxy groups derived from the polymer (a) constituting the crosslinked polymer dispersant A [(the molar equivalent number of epoxy groups derived from the polyfunctional epoxy compound (b)) / (the molar equivalent number of carboxy groups derived from the polymer (a)].

[0010] In the present invention, the term "aqueous" means that water accounts for the largest proportion by mass of the medium in which the pigment is dispersed. Furthermore, the term "recording" is a concept including printing and printing characters or images. The aqueous pigment dispersion of the present invention has excellent ejection properties even when used as an aqueous ink in thermal ink-jet recording, and therefore the aqueous pigment dispersion of the present invention can be suitably used as an aqueous ink for thermal ink-jet recording. In addition, in this specification, the ink ejection properties in thermal ink-jet recording are also simply referred to as "ink ejection properties."

[0011] The water-based pigment dispersion of the present invention also exhibits excellent ejection properties in thermal ink-jet recording. The reason for this is not clear, but is thought to be as follows. The aqueous pigment dispersion of the present invention contains a pigment, a crosslinked polymer dispersant A, and an aqueous medium. In the aqueous pigment dispersion of the present invention, the pigment is dispersed in an aqueous medium by a crosslinked polymer dispersant A containing a structure derived from a polymer (a) having a carboxy group and a structure derived from a polyfunctional epoxy compound (b). At least a part of the carboxy groups present in the crosslinked polymer dispersant A is neutralized with an alkali metal hydroxide, and further, the crosslinking degree of the crosslinked polymer dispersant A and the acid value of the polymer (a) having a carboxy group satisfy specific conditions. As a result, a strong charge repulsion force is generated between the crosslinked polymer dispersant A due to the strong basicity of the alkali metal hydroxide, and the crosslinked polymer dispersant A can be firmly adsorbed on the pigment surface, so that the detachment of the crosslinked polymer dispersant A from the pigment can be suppressed, and therefore the dispersion stability of the pigment in the aqueous pigment dispersion and the aqueous ink for inkjet recording containing the aqueous pigment dispersion is improved, and as a result, the generation of aggregates on the heater of a thermal inkjet recording device can be suppressed, and it is considered that excellent dischargeability can be exhibited even in thermal inkjet recording.

[0012] <Pigments> The pigment according to the present invention may be either an inorganic pigment or an organic pigment. Examples of inorganic pigments include carbon black and metal oxides. In the case of black ink, carbon black is preferred. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. In the case of white ink, examples of metal oxides include 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 may be used alone or in combination of two or more. In the present invention, the pigment is dispersed in the crosslinked polymer dispersant A and contained in the aqueous medium in the form of particles.

[0013] <Crosslinked polymer dispersant A> The crosslinked polymer dispersant A according to the present invention contains a structure derived from a polymer (a) having a carboxy group and a structure derived from a polyfunctional epoxy compound (b), and at least a portion of the carboxy groups of the crosslinked polymer dispersant A is neutralized with an alkali metal hydroxide. The crosslinked structure of the crosslinked polymer dispersant A is formed by the reaction of a portion of the carboxy groups present in the polymer (a) with the polyfunctional epoxy compound (b).

[0014] The aqueous pigment dispersion of the present invention is prepared by dispersing a pigment in an aqueous medium with a crosslinked polymer dispersant A. The form of the pigment and the crosslinked polymer dispersant A in the aqueous pigment dispersion of the present invention and the aqueous ink for inkjet recording containing the aqueous pigment dispersion are common to both, and examples of such forms include a state in which the crosslinked polymer dispersant A is adsorbed on the pigment, and a pigment-encapsulated (capsulated) state in which the crosslinked polymer dispersant A contains the pigment. Among these, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the form of a crosslinked polymer particle containing a pigment (hereinafter also referred to as a "pigment-containing crosslinked polymer particle") is preferred, and the form of a pigment-encapsulated crosslinked polymer particle in which the crosslinked polymer dispersant A encapsulates the pigment is more preferred.

[0015] (Polymer (a) having a carboxy group) The polymer (a) having a carboxy group (hereinafter simply referred to as "polymer (a)") constituting the crosslinked polymer dispersant A functions as a pigment dispersant that exhibits a pigment dispersing effect.

[0016] The polymer (a) is preferably one or more selected from the group consisting of polyester, polyurethane, and vinyl polymer, and from the viewpoint of improving the dispersion stability of the pigment and improving the ejection property of the ink, a vinyl polymer obtained by addition polymerization of a vinyl monomer is more preferable. The vinyl monomer is preferably one or more selected from the group consisting of a vinyl compound, a vinylidene compound, and a vinylene compound.

[0017] From the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the polymer (a) is more preferably a vinyl polymer containing a structural unit derived from a carboxyl group-containing monomer (a-1), and even more preferably a vinyl polymer containing a structural unit derived from a carboxyl group-containing monomer (a-1) and a structural unit derived from a hydrophobic monomer (a-2). When the polymer (a) contains a structural unit derived from a carboxyl group-containing monomer (a-1) and a structural unit derived from a hydrophobic monomer (a-2), it is preferable that the polymer (a) is obtained by copolymerizing raw material monomers containing the carboxyl group-containing monomer (a-1) and the hydrophobic monomer (a-2). In this case, the polymer (a) may further contain a structural unit derived from a macromonomer (a-3) and / or a structural unit derived from a nonionic monomer (a-4).

[0018] [Carboxy group-containing monomer (a-1)] The carboxyl group-containing monomer (a-1) is preferably a carboxylic acid monomer. The carboxylic acid monomer is preferably one or more selected from the group consisting of unsaturated monocarboxylic acids such as acrylic acid, methacrylic 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 (a-1) is more preferably one or more selected from the group consisting of acrylic acid, methacrylic acid, and maleic acid, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink. The unsaturated dicarboxylic acid may be an anhydride.

[0019] [Hydrophobic monomer (a-2)] In the present invention, the term "hydrophobic monomer" refers to a monomer that, when dissolved in 100 g of ion-exchanged water at 25° C. until saturated, dissolves in an amount of less than 10 g. Specific examples of the hydrophobic monomer (a-2) include the alkyl (meth)acrylate and aromatic group-containing monomers described in paragraphs

[0020] to

[0022] of JP-A-2018-83938. Among these, the hydrophobic monomer (a-2) is, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, preferably one or more selected from the group consisting of alkyl (meth)acrylate having an alkyl group having 1 to 18 carbon atoms, more preferably one or more selected from the group consisting of alkyl (meth)acrylate having an alkyl group having 1 to 10 carbon atoms, and preferably one or more selected from the group consisting of aromatic group-containing monomers having 6 to 22 carbon atoms, more preferably one or more selected from the group consisting of ethyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, styrene, α-methylstyrene, and benzyl (meth)acrylate, more preferably one or more selected from the group consisting of styrene, α-methylstyrene, and benzyl (meth)acrylate. In this specification, the term "(meth)acrylic" such as alkyl (meth)acrylate is a concept that includes both acrylic and methacrylic.

[0020] [Macromonomer (a-3)] Examples of the macromonomer (a-3) include compounds having a polymerizable functional group such as a methacryloyloxy group at one end and having a number average molecular weight of 500 or more and 100,000 or less, preferably 1,000 or more and 10,000 or less. As the macromonomer (a-3), an aromatic group-containing monomer-based macromonomer is preferable. Examples of the aromatic group-containing monomer constituting the aromatic group-containing monomer-based macromonomer include the aromatic group-containing monomers described in paragraph

[0021] of JP 2018-83938 A. Among them, the aromatic group-containing monomer constituting the aromatic group-containing monomer-based macromonomer is preferably one or more selected from the group consisting of styrene and benzyl (meth)acrylate, more preferably styrene, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink. Examples of commercially available styrene-based macromonomers include "AS-6(S)", "AN-6(S)", and "HS-6(S)" manufactured by Toagosei Co., Ltd.

[0021] [Nonionic monomer (a-4)] The nonionic monomer (a-4) is preferably one or more selected from the group consisting of (meth)acrylic acid monoesters of polyalkylene glycols such as polypropylene glycol and polyethylene glycol, (meth)acrylic acid monoesters of alkoxypolyalkylene glycols, and (meth)acrylic acid monoesters of phenoxy (ethylene glycol / propylene glycol copolymers). Among these, the nonionic monomer (a-4) is more preferably one or more selected from the group consisting of monopolyethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, and monopolypropylene glycol (meth)acrylate, and further preferably monopolypropylene glycol methacrylate (average number of moles of oxypropylene groups added: 2 to 30). Commercially available examples of the nonionic monomer (a-4) include the NK Ester M series manufactured by Shin-Nakamura Chemical Co., Ltd., and the Blenmer PE series, Blenmer PME series, Blenmer PP series, Blenmer AP series, Blenmer 50PEP-300, Blenmer 50POEP-800B, and Blenmer 43PAPE-600B manufactured by NOF Corporation.

[0022] The above monomers (a-1) to (a-4) can each be used alone or in combination of two or more. In the present invention, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the polymer (a) is a vinyl-based polymer containing a structural unit derived from a carboxyl group-containing monomer (a-1), and the carboxyl group-containing monomer (a-1) is preferably at least one selected from the group consisting of acrylic acid, methacrylic acid, and maleic acid. Furthermore, the polymer (a) is more preferably a vinyl-based polymer containing a structural unit derived from at least one carboxyl group-containing monomer (a-1) selected from acrylic acid, methacrylic acid, and maleic acid and a structural unit derived from a hydrophobic monomer (a-2). The polymer (a) is preferably a vinyl polymer containing one or more structural units derived from acrylic acid, methacrylic acid, and maleic acid as the carboxyl group-containing monomer (a-1) and one or more structural units derived from styrene, α-methylstyrene, and benzyl (meth)acrylate as the hydrophobic monomer (a-2), and the polymer (a) is even more preferably a styrene / maleic acid copolymer containing one or more structural units derived from maleic acid as the carboxyl group-containing monomer (a-1) and one or more structural units derived from styrene as the hydrophobic monomer (a-2). The styrene / maleic acid copolymer will be described later.

[0023] (Content of each structural unit in polymer (a) having a carboxy group) The content of structural units derived from the carboxy group-containing monomer (a-1) in the polymer (a) is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, still more preferably 30% by mass or more, and preferably 75% by mass or less, more preferably 70% by mass or less, even more preferably 65% ​​by mass or less, still more preferably 60% by mass or less, still more preferably 55% by mass or less, and still more preferably 50% by mass or less. The content of structural units derived from hydrophobic monomer (a-2) in polymer (a) is preferably 25% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, still more preferably 45% by mass or more, even more preferably 50% by mass or more, 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 improving the dispersion stability of the pigment and improving the ejection properties of the ink, the total content of the constituent units derived from the carboxy group-containing monomer (a-1) and the constituent units derived from the hydrophobic monomer (a-2) in the polymer (a) is preferably 65% ​​by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, still more preferably 95% by mass or more, and preferably 100% by mass or less. The mass ratio of the content of the constituent units derived from the carboxy group-containing monomer (a-1) to the content of the constituent units derived from the hydrophobic monomer (a-2) in the polymer (a) [carboxy group-containing monomer (a-1) / hydrophobic monomer (a-2)] is preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, and is preferably 5 or less, more preferably 4 or less, even more preferably 3 or less, still more preferably 2 or less, and even more preferably 1 or less.

[0024] Furthermore, in the case where the polymer (a) contains a structural unit derived from the macromonomer (a-3) and / or a structural unit derived from the nonionic monomer (a-4), the content of the structural units derived from the monomers (a-1) to (a-4) in the polymer (a) is as follows, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink. The content of structural units derived from the carboxy group-containing monomer (a-1) in the polymer (a) is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, and even more preferably 50% by mass or less. The content of structural units derived from hydrophobic monomer (a-2) in polymer (a) is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less, even more preferably 50% by mass or less, still more preferably 45% by mass or less, and even more preferably 40% by mass or less. The content of the structural units derived from the macromonomer (a-3) in the polymer (a) is preferably 3 mass% or more, more preferably 5 mass% or more, even more preferably 7 mass% or more, and preferably 30 mass% or less, more preferably 20 mass% or less, even more preferably 15 mass% or less. The content of the structural units derived from the nonionic monomer (a-4) in the polymer (a) is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 40% by mass or less, more preferably 30% by mass or less. Furthermore, when the polymer (a) contains a structural unit derived from macromonomer (a-3), the mass ratio of the content of the structural unit derived from carboxy group-containing monomer (a-1) to the total content of the structural units derived from hydrophobic monomer (a-2) and the structural units derived from macromonomer (a-3) [carboxy group-containing monomer (a-1) / [hydrophobic monomer (a-2)+macromonomer (a-3)]] is preferably 0.1 or more, more preferably 0.5 or more, even more preferably 0.7 or more, and is preferably 3.5 or less, more preferably 3 or less, even more preferably 2.5 or less, still more preferably 2 or less, and even more preferably 1.5 or less.

[0025] (styrene / maleic acid copolymer) The styrene / maleic acid copolymer contains structural units derived from maleic acid and structural units derived from styrene, and has a portion in which the structural units derived from maleic acid and the structural units derived from styrene are alternately copolymerized. The structural units of maleic acid and the structural units of styrene are regularly arranged, which is thought to result in more uniform adsorption of polymer (a) constituting crosslinked polymer dispersant A to the pigment, thereby improving the dispersion stability of the pigment and improving the ejection properties of the ink. When polymer (a) is a styrene / maleic acid copolymer, the molar ratio of the content of structural units derived from styrene to the content of structural units derived from maleic acid in the styrene / maleic acid copolymer [styrene / maleic acid] is preferably 0.2 or more, more preferably 0.6 or more, even more preferably 0.8 or more, still more preferably 1 or more, and is preferably 3 or less, more preferably 2.8 or less, even more preferably 2.5 or less, and still more preferably 2 or less. When polymer (a) is a styrene / maleic acid copolymer, the content of structural units derived from maleic acid in the styrene / maleic acid copolymer is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, still more preferably 30% by mass or more, and preferably 75% by mass or less, more preferably 70% by mass or less, even more preferably 65% ​​by mass or less, still more preferably 60% by mass or less, still more preferably 55% by mass or less, and still more preferably 50% by mass or less. When the polymer (a) is a styrene / maleic acid copolymer, the styrene / maleic acid copolymer may contain constituent units derived from monomers other than the constituent units derived from styrene and the constituent units derived from maleic acid, but the total content of the constituent units derived from styrene and the constituent units derived from maleic acid in the styrene / maleic acid copolymer is preferably 80 mass% or more, more preferably 90 mass% or more, and preferably 100 mass% or less, and even more preferably 100 mass%.

[0026] (Preparation of polymer (a) having carboxy group) The polymer (a) can be produced by copolymerizing the above-mentioned monomer (a-1) and, if necessary, raw material monomers including (a-2) to (a-4) by a known polymerization method. As the polymerization method, a solution polymerization method is preferable. The solvent used in the solution polymerization method is not limited, 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 the polymerization, a known polymerization initiator or polymerization chain transfer agent 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, an inert gas atmosphere such as argon, etc.

[0027] When the polymer (a) is a styrene / maleic acid copolymer, the styrene / maleic acid copolymer may be one synthesized by a known method. The styrene / maleic acid copolymer may be one derived from a styrene / maleic anhydride copolymer. The styrene / maleic anhydride copolymer may be one synthesized by a known method or may be a commercially available product. There is no particular restriction on the bonding form between the styrene-derived structural unit and the maleic anhydride-derived structural unit in the styrene / maleic anhydride copolymer, but it is preferable that there is a portion in which the styrene-derived structural unit and the maleic anhydride-derived structural unit are alternately copolymerized. Commercially available styrene / maleic anhydride copolymers include, for example, the XIRAN series manufactured by Polyscope, the DYLARK series manufactured by NOVA Chemicals, and the SMA series manufactured by Cray Valley. In addition, the degree of polymerization of the styrene / maleic acid copolymer, the bonding form between the styrene-derived structural units and the maleic acid-derived structural units in the styrene / maleic acid copolymer, and the molar ratio [styrene / maleic acid] can be considered to be the same as those of the styrene / maleic anhydride copolymer.

[0028] The acid value of the polymer (a) is preferably 300 mgKOH / g or more, more preferably 330 mgKOH / g or more, even more preferably 350 mgKOH / g or more, and preferably 800 mgKOH / g or less, more preferably 700 mgKOH / g or less, even more preferably 600 mgKOH / g or less, even more preferably 500 mgKOH / g or less, even more preferably 480 mgKOH / g or less. If the acid value of the polymer (a) is within the above range, the amount of carboxyl groups crosslinked by the polyfunctional epoxy compound (b) and the amount of carboxyl groups neutralized can be secured, the dispersion stability of the pigment dispersed by the crosslinked polymer dispersant A can be guaranteed, and the affinity of the crosslinked polymer dispersant A to the aqueous medium and the pigment can be balanced, and the dischargeability can be improved even in thermal inkjet recording. The acid value of the polymer (a) can be determined by the neutralization titration method described in JIS K0070-1992, as described in the Examples, or can be calculated from the mass ratio of the monomers constituting the polymer (a). The acid value of the polymer (a) can also be calculated from the acid value of the crosslinked polymer dispersant A described below and the degree of crosslinking of the crosslinked polymer dispersant A described below, according to the following formula. Acid value of polymer (a) (mg KOH / g) = 100 × (acid value of cross-linked polymer dispersant A) / (100 - (degree of cross-linking of cross-linked polymer dispersant A)) Here, the acid value of the crosslinked polymer dispersant A is expressed in units of "mgKOH / g", and the degree of crosslinking of the crosslinked polymer dispersant A is expressed in units of "mol %". From the viewpoint of improving the dispersion stability of the pigment and improving the jetting properties of the ink, the number average molecular weight of the polymer (a) is preferably 1,500 or more, more preferably 1,800 or more, and even more preferably 2,000 or more, and is preferably 20,000 or less, more preferably 18,000 or less, even more preferably 15,000 or less, still more preferably 10,000 or less, still more preferably 5,000 or less, and even more preferably 4,000 or less. The number average molecular weight of the polymer (a) is measured by the method described in the Examples.

[0029] When the polymer (a) is a styrene / maleic acid copolymer, the acid value of the styrene / maleic acid copolymer is preferably 300 mgKOH / g or more, more preferably 330 mgKOH / g or more, even more preferably 350 mgKOH / g or more, and preferably 600 mgKOH / g or less, more preferably 500 mgKOH / g or less, even more preferably 480 mgKOH / g or less. When the polymer (a) is a styrene / maleic acid copolymer, the number average molecular weight (Mn) of the styrene / maleic acid copolymer is preferably 1,500 or more, more preferably 1,800 or more, even more preferably 2,000 or more, and preferably 10,000 or less, more preferably 5,000 or less, even more preferably 4,000 or less.

[0030] (Multifunctional epoxy compound (b)) The polyfunctional epoxy compound (b) constituting the crosslinked polymer dispersant A according to the present invention may be water-insoluble or water-soluble. From the viewpoint of efficiently reacting with the carboxyl group in the polymer (a) in an aqueous medium to form a crosslinked structure, the water-solubility of the polyfunctional epoxy compound (b) is preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less, and still more preferably 30% by mass or less.

[0031] The polyfunctional epoxy compound (b) is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a compound having two or more glycidyl ether groups in the molecule, even more preferably a polyglycidyl ether compound of a polyhydric alcohol, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 8 carbon atoms. The epoxy equivalent of the polyfunctional epoxy compound (b) is preferably 90 g / eq or more, more preferably 100 g / eq or more, even more preferably 110 g / eq or more, and is preferably 300 g / eq or less, more preferably 220 g / eq or less, more preferably 180 g / eq or less.

[0032] As the polyfunctional epoxy compound (b), for example, one or more selected from the group consisting of polyglycidyl ether compounds such as polypropylene glycol diglycidyl ether (water soluble rate: 31 mass%), 1,6-hexanediol diglycidyl ether (water soluble rate: 0 mass%), trimethylolpropane polyglycidyl ether (water soluble rate: 27 mass%), pentaerythritol polyglycidyl ether (water soluble rate: 0 mass%), and diglycidyl ether of hydrogenated bisphenol A (water soluble rate: 0 mass%) can be preferably mentioned, but ethylene glycol diglycidyl ether (water soluble rate: 100 mass%) and sorbitol polyglycidyl ether (water soluble rate: 94 mass%) can also be used. Among these, trimethylolpropane polyglycidyl ether is more preferable.

[0033] (Degree of cross-linking of cross-linked polymer dispersant A) The degree of crosslinking of the crosslinked polymer dispersant A is preferably 40 mol% or more, more preferably 50 mol% or more, even more preferably 53 mol% or more, and preferably 90 mol% or less, more preferably 86 mol% or less, even more preferably 80 mol% or less. Here, the degree of crosslinking of the crosslinked polymer dispersant A means the percentage of the molar equivalent number of epoxy groups derived from the polyfunctional epoxy compound (b) constituting the crosslinked polymer dispersant A divided by the molar equivalent number of carboxy groups derived from the polymer (a) constituting the crosslinked polymer dispersant A.

[0034] <Alkali metal hydroxide> At least a part of the carboxyl groups of the crosslinked polymer dispersant A according to the present invention is neutralized with an alkali metal hydroxide. That is, the crosslinked polymer dispersant A has a structure derived from the polymer (a) having a carboxyl group, in which the carboxyl groups in the crosslinked polymer dispersant A form an alkali metal salt. The alkali metal hydroxide is preferably at least one selected from the group consisting of sodium hydroxide and potassium hydroxide. From the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the degree of neutralization of the crosslinked polymer dispersant A is preferably 5 mol % or more, more preferably 10 mol % or more, even more preferably 15 mol % or more, still more preferably 20 mol % or more, and is preferably 50 mol % or less, more preferably 40 mol % or less, and even more preferably 30 mol % or less. Here, the neutralization degree of the crosslinked polymer dispersant A is the percentage of the value obtained by dividing the molar equivalent number of the alkali metal hydroxide in the aqueous pigment dispersion by the molar equivalent number of the carboxy group derived from the polymer (a) constituting the crosslinked polymer dispersant A [(molar equivalent number of alkali metal hydroxide) / (molar equivalent number of carboxy group derived from polymer (a)]. However, when the sum of the percentage of the value [(molar equivalent number of alkali metal hydroxide) / (molar equivalent number of carboxy group derived from polymer (a)] and the crosslinking degree of the crosslinked polymer dispersant A exceeds 100 mol%, the neutralization degree of the crosslinked polymer dispersant A is a value obtained by subtracting the crosslinking degree (mol%) from 100 mol%. Note that, when the sum of the percentage of the value [(molar equivalent number of alkali metal hydroxide) / (molar equivalent number of carboxy group derived from polymer (a)] and the crosslinking degree of the crosslinked polymer dispersant A does not exceed 100 mol%, the neutralization degree of the crosslinked polymer dispersant A is synonymous with the neutralization degree of the polymer (a). At least a portion of the carboxyl groups of the crosslinked polymer dispersant A may be neutralized with a compound other than an alkali metal hydroxide. Suitable examples of the compound other than an alkali metal hydroxide include ammonia and amines. Incidentally, even if the water-based pigment dispersion of the present invention and the water-based ink for ink-jet recording containing the water-based pigment dispersion of the present invention are neutralized with a compound other than an alkali metal hydroxide, the neutralization with the compound other than the alkali metal hydroxide is not included in the degree of neutralization of the present invention.

[0035] <Condition 1> In the water-based pigment dispersion of the present invention, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the degree of crosslinking of the crosslinked polymer dispersant A and the acid value of the polymer (a) satisfy the following condition 1. Condition 1: The value of [(degree of crosslinking of crosslinked polymer dispersant A) / 100]×(acid value of polymer (a)) is 230 mgKOH / g or more and 360 mgKOH / g or less. Here, the degree of crosslinking of the crosslinked polymer dispersant A is, as described above, the percentage of the molar equivalent number of epoxy groups derived from the polyfunctional epoxy compound (b) constituting the crosslinked polymer dispersant A divided by the molar equivalent number of carboxy groups derived from the polymer (a) constituting the crosslinked polymer dispersant A [(the molar equivalent number of epoxy groups derived from the polyfunctional epoxy compound (b)) / (the molar equivalent number of carboxy groups derived from the polymer (a)]. As described above, the acid value of polymer (a) can also be calculated from the acid value and degree of crosslinking of crosslinked polymer dispersant A. Therefore, condition 1 can also be expressed by the following formula using the acid value and degree of crosslinking of crosslinked polymer dispersant A. Condition 1: The value of [(degree of crosslinking of crosslinked polymer dispersant A) × (acid value of crosslinked polymer dispersant A) / (100 - (degree of crosslinking of crosslinked polymer dispersant A))] is 230 mg KOH / g or more and 360 mg KOH / g or less.

[0036] Condition 1 expresses the ratio of the carboxy groups of the polymer (a) that contribute to the formation of a crosslinked structure through a reaction with the polyfunctional epoxy compound (b) in relation to the acid value attributable to the carboxy groups of the polymer (a). The value of condition 1, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, is 230 mgKOH / g or more, preferably more than 230 mgKOH / g, more preferably 245 mgKOH / g or more, even more preferably 250 mgKOH / g or more, still more preferably 255 mgKOH / g or more, still more preferably 260 mgKOH / g or more, and is 360 mgKOH / g or less, preferably 330 mgKOH / g or less, more preferably 310 mgKOH / g or less, still more preferably 306 mgKOH / g or less, still more preferably 300 mgKOH / g or less, and still more preferably 290 mgKOH / g or less.

[0037] (Ratio [(Degree of crosslinking of crosslinked polymer dispersant A) / (Degree of neutralization of crosslinked polymer dispersant A)]) In the aqueous pigment dispersion of the present invention, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, it is preferable that the ratio of the degree of crosslinking of the crosslinked polymer dispersant A to the degree of neutralization of the crosslinked polymer dispersant A [(degree of crosslinking of crosslinked polymer dispersant A) / (degree of neutralization of crosslinked polymer dispersant A)] is 1 or more and 7 or less.

[0038] In the aqueous pigment dispersion of the present invention, when the ratio [(degree of crosslinking of crosslinked polymer dispersant A) / (degree of neutralization of crosslinked polymer dispersant A)] is 1 or more, the proportion of the carboxy groups of polymer (a) that are in the form of alkali metal salts relative to the crosslinked structure of crosslinked polymer dispersant A is not too high, and the affinity of crosslinked polymer dispersant A to the vehicle component of the aqueous ink for inkjet recording is prevented from becoming too high, and detachment of crosslinked polymer dispersant A from the pigment can be prevented. On the other hand, when the ratio [(degree of crosslinking of crosslinked polymer dispersant A) / (degree of neutralization of crosslinked polymer dispersant A)] is 7 or less, the proportion of the carboxy groups of polymer (a) that are in the form of alkali metal salts relative to the crosslinked structure of crosslinked polymer dispersant A is not too low, and charge repulsion is exerted, pigment aggregation can be suppressed, and ink ejection properties can be improved. From this viewpoint, the ratio [(degree of crosslinking of crosslinked polymer dispersant A) / (degree of neutralization of crosslinked polymer dispersant A)] is more preferably 1.5 or more, even more preferably 2 or more, still more preferably 2.5 or more, and more preferably 6.5 or less, even more preferably 6 or less, still more preferably 5.5 or less, still more preferably 5 or less, still more preferably 4.5 or less, still more preferably 4 or less.

[0039] (Acid value of cross-linked polymer dispersant A) From the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the acid value of the crosslinked polymer dispersant A is preferably 40 mgKOH / g or more, more preferably 50 mgKOH / g or more, even more preferably 60 mgKOH / g or more, still more preferably 70 mgKOH / g or more, and is preferably 250 mgKOH / g or less, more preferably 230 mgKOH / g or less, and even more preferably 220 mgKOH / g or less. Here, the acid value of the crosslinked polymer dispersant A can be calculated from the degree of crosslinking of the crosslinked polymer dispersant A using the following formula, and indicates the amount of carboxy groups remaining in the polymer (a) that are not involved in the formation of a crosslinked structure through the reaction with the polyfunctional epoxy compound (b). Acid value of cross-linked polymer dispersant A (mgKOH / g) = (acid value of polymer (a)) x (100 - (degree of cross-linking of cross-linked polymer dispersant A)) / 100 Here, the unit of the acid value of the polymer (a) is "mgKOH / g", and the unit of the crosslinking degree of the crosslinked polymer dispersant A is "mol %". When the polymer (a) constituting the crosslinked polymer dispersant A is a styrene / maleic acid copolymer, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the acid value of the crosslinked polymer dispersant A is preferably 40 mgKOH / g or more, more preferably 50 mgKOH / g or more, even more preferably 60 mgKOH / g or more, still more preferably 70 mgKOH / g or more, and is preferably 250 mgKOH / g or less, more preferably 230 mgKOH / g or less, and even more preferably 220 mgKOH / g or less.

[0040] [Method of manufacturing water-based pigment dispersion] The water-based pigment dispersion of the present invention is preferably produced by a method including the following steps 1 and 2. Step 1: A step of dispersing a pigment mixture containing a pigment, a polymer (a) having a carboxy group, and an alkali metal hydroxide by applying shear stress to obtain a pigment water dispersion (d). Step 2: A step of reacting the carboxy group of the polymer (a) having a carboxy group in the pigment aqueous dispersion (d) obtained in step 1 with the polyfunctional epoxy compound (b) to form a crosslinked polymer dispersant A, thereby obtaining an aqueous pigment dispersion containing a pigment, the crosslinked polymer dispersant A, and an aqueous medium. In the method for producing the water-based pigment dispersion of the present invention, it is preferable to satisfy the above-mentioned condition 1 by adjusting the amount of carboxy groups in the polymer (a) and the amount of the polyfunctional epoxy compound (b) in the above step 2. When a styrene / maleic anhydride copolymer is used as the polymer (a) having a carboxy group, the styrene / maleic anhydride copolymer is converted into a styrene / maleic acid copolymer in step 1.

[0041] [Process 1] Step 1 is a step of dispersing a pigment in an aqueous medium using the polymer (a) as a dispersant to obtain a pigment aqueous dispersion (d). The aqueous medium in step 1 means that, among the dispersion medium components other than the pigment, the polymer (a), and the alkali metal hydroxide, water accounts for the largest proportion by mass. In order to improve the wettability of the pigment and the adsorption of the polymer (a) to the pigment in step 1, the aqueous medium may contain an organic solvent. Suitable examples of the organic solvent include alcohols having 1 to 3 carbon atoms and ketones having 3 to 6 carbon atoms. In the dispersion treatment in step 1, the pigment particles may be atomized to a desired particle size only by main dispersion using shear stress. However, from the viewpoint of obtaining a uniform pigment aqueous dispersion (d), 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 dispersing blade can be used. Examples of means for applying shear stress used in the present 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 pigment water dispersion (d) 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.

[0042] In step 1, at least a part of the carboxy groups of the polymer (a) is neutralized with at least an alkali metal hydroxide as a neutralizing agent. That is, at least a part of the carboxy groups of the polymer (a) constituting the crosslinked polymer dispersant A becomes an alkali metal salt, and as a result, at least a part of the carboxy groups of the crosslinked polymer dispersant A becomes an alkali metal salt. The neutralization of the polymer (a) is preferably carried out by an operation of mixing the polymer (a) with an alkali metal hydroxide. The neutralizing agent is preferably used in the form of an aqueous neutralizing solution in order to promote sufficient and uniform neutralization. The neutralizing agent may contain a compound other than an alkali metal hydroxide. Specific examples of the compound other than an alkali metal hydroxide include ammonia and amines, as described above. When the compound other than an alkali metal hydroxide is ammonia or an amine, it is preferable that the ammonia or amine is removed in any step of the operation for producing the aqueous pigment dispersion of the present invention. From the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the degree of neutralization of the carboxyl groups of the polymer (a) in step 1 is preferably 5 mol % or more, more preferably 10 mol % or more, even more preferably 15 mol % or more, still more preferably 20 mol % or more, and is preferably 50 mol % or less, more preferably 40 mol % or less, and even more preferably 30 mol % or less. The degree of neutralization of the carboxyl groups of the polymer (a) in step 1 is calculated based on neutralization with an alkali metal compound, and neutralization with a compound other than the alkali metal hydroxide, such as ammonia or an amine, is not used in calculating the degree of neutralization of the carboxyl groups of the polymer (a) in step 1.

[0043] In step 1, it is preferable that the organic solvent in the resulting pigment water dispersion (d) has been substantially removed, but it may remain as long as it does not impair the object of the present invention. The pigment water dispersion (d) obtained by step 1 is obtained by dispersing the pigment in an aqueous medium with the polymer (a). The form of the pigment and the polymer (a) in the pigment water dispersion is not particularly limited as long as particles are formed by at least the pigment and the polymer (a). For example, the form includes a particle form in which the pigment is encapsulated in the polymer (a), a particle form in which the pigment is uniformly dispersed in the polymer (a), a particle form in which the pigment is exposed on the particle surface of the polymer (a), and mixtures thereof.

[0044] The non-volatile component concentration (solid content concentration) of the pigment water dispersion (d) obtained in step 1 is, from the viewpoint of improving the dispersion stability of the pigment in the pigment water dispersion (d) and of facilitating the preparation of a water-based ink for ink-jet printing, preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 30% by mass or less, more preferably 25% by mass or less. The solids concentration of the pigment water dispersion (d) is measured by the method described in the examples.

[0045] The content of the pigment in the pigment water dispersion (d) is preferably 5% by mass or more, more preferably 7% by mass or more, even more preferably 9% 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, and still more preferably 15% by mass or less. The content of the polymer (a) in the pigment water dispersion (d) is preferably 2 mass% or more, more preferably 4 mass% or more, even more preferably 5 mass% or more, and preferably 15 mass% or less, more preferably 12 mass% or less, even more preferably 10 mass% or less. The mass ratio of the pigment to the polymer (a) in the pigment water dispersion (d) [pigment / polymer (a)], from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, is preferably 0.5 or more, more preferably 0.8 or more, even more preferably 1 or more, still more preferably 1.3 or more, and is preferably 5 or less, more preferably 3 or less, even more preferably 2 or less, and still more preferably 1.7 or less.

[0046] [Process 2] Step 2 is a step of reacting the carboxy group of the polymer (a) having a carboxy group in the pigment aqueous dispersion (d) obtained in step 1 with the polyfunctional epoxy compound (b) to obtain an aqueous pigment dispersion in which the pigment is dispersed in an aqueous medium with the crosslinked polymer dispersant A. The reaction between the carboxy group of the polymer (a) and the polyfunctional epoxy compound (b) forms the crosslinked polymer dispersant A containing a crosslinked structure.

[0047] The reaction temperature between the carboxy group of the polymer (a) and the polyfunctional epoxy compound (b) is, from the viewpoint of completing the reaction and from the viewpoint of economy, preferably 50° C. or higher, more preferably 60° C. or higher, and preferably 90° C. or lower, more preferably 80° C. or lower. The reaction time is preferably 0.5 hours or higher, more preferably 1 hour or higher, and preferably 15 hours or lower, more preferably 8 hours or lower.

[0048] In producing the aqueous pigment dispersion of the present invention, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, it is preferable to satisfy the above condition 1 by adjusting the amount of carboxy groups in the polymer (a) and the amount of epoxy groups in the polyfunctional epoxy compound (b) in step 2. In producing the aqueous pigment dispersion of the present invention, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, it is preferable to appropriately set the degree of neutralization in step 1 or the degree of crosslinking in step 2, it is more preferable to appropriately set the degree of neutralization in step 1 and the degree of crosslinking in step 2, and it is even more preferable to set the degree of neutralization in step 1 and the degree of crosslinking in step 2 so that they satisfy the numerical range of the above-mentioned ratio [(degree of crosslinking of crosslinked polymer dispersant A) / (degree of neutralization of crosslinked polymer dispersant A)].

[0049] 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 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less. The content of the crosslinked polymer dispersant A in the aqueous pigment dispersion of the present invention is preferably 6% by mass or more, more preferably 7% by mass or more, even more preferably 8% by mass or more, and is preferably 23% by mass or less, more preferably 17% by mass or less, even more preferably 12% by mass or less. The mass ratio of the pigment to the crosslinked polymer dispersant A in the aqueous pigment dispersion of the present invention [pigment / crosslinked polymer dispersant A] is, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, still more preferably 0.8 or more, and is preferably 3 or less, more preferably 2 or less, and even more preferably 1 or less.

[0050] From the viewpoints of reducing coarse particles and improving the dispersion stability of the pigment to improve the ejection properties of the ink, the average particle size of the pigment particles in the aqueous pigment dispersion of the present invention is preferably 50 nm or more, more preferably 60 nm or more, even more preferably 70 nm or more, and is preferably 200 nm or less, more preferably 160 nm or less, even more preferably 150 nm or less, and still more preferably 130 nm or less. The average particle size of the pigment particles in the aqueous pigment dispersion is measured by the method described in the Examples.

[0051] [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 water-based pigment dispersion. The ink of the present invention preferably contains an organic solvent from the viewpoint of improving the ejection properties of the ink. 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 the group consisting of polyhydric alcohols and polyhydric alcohol alkyl ethers, more preferably at least one selected from the group consisting of ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, glycerin, trimethylolpropane, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, and diethylene glycol diethyl ether, and even more preferably at least one selected from the group consisting of triethylene glycol, polyethylene glycol, and diethylene glycol monobutyl ether.

[0052] From the viewpoint of improving the ejection properties of the ink of the present invention, it is preferable that the ink of the present invention contains a surfactant. Examples of the surfactant include nonionic surfactants, anionic surfactants, amphoteric surfactants, etc., and the like, but nonionic surfactants are preferable, and one or more surfactants selected from the group consisting of acetylene glycol surfactants and polyoxyalkylene alkyl ether surfactants are more preferable. Preferred examples of the acetylene glycol surfactant include acetylene glycol having from 8 to 22 carbon atoms and ethylene oxide adducts of the acetylene glycol, and more preferred is 2,4,7,9-tetramethyl-5-decyne-4,7-diol or an ethylene oxide adduct thereof. A specific example of the polyoxyalkylene alkyl ether surfactant is polyoxyethylene lauryl ether. The ink of the present invention preferably contains a combination of an acetylene glycol surfactant and a polyoxyethylene alkyl ether.

[0053] The ink of the present invention may further contain, if necessary, various additives which are usually used in water-based inks for ink-jet printing, such as a humectant, a wetting agent, a penetrating agent, a dispersant, a viscosity modifier, an antifoaming agent, an antiseptic, an antifungal agent and an antirust agent.

[0054] From the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, 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, even more preferably 3% by mass or more, and is preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 7% by mass or less, and even more preferably 5% by mass or less. From the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, the total content of the pigment and crosslinked polymer dispersant A in the ink of the present invention is preferably 2% by mass or more, more preferably 3% by mass or more, even more preferably 5% by mass or more, and is preferably 17% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less. The mass ratio of the pigment to the crosslinked polymer dispersant A in the ink of the present invention [pigment / crosslinked polymer dispersant A] is, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection properties of the ink, preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, still more preferably 0.8 or more, and is preferably 3 or less, more preferably 2 or less, and even more preferably 1 or less. From the viewpoint of improving the ejection properties of the ink, the content of the organic solvent in the ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 7% by mass or more, and is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less. From the viewpoint of improving the ejection properties of the ink, the content of the surfactant in the ink of the present invention is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, even more preferably 0.5% by mass or more, and is preferably 3% by mass or less, more preferably 2.5% by mass or less, even more preferably 2% by mass or less. In producing the ink of the present invention, when an alkali metal hydroxide is further contained in addition to the above-mentioned aqueous pigment dispersion, the alkali metal hydroxide added other than to the aqueous pigment dispersion is added to the alkali metal hydroxide that neutralizes the carboxy group of the crosslinked polymer dispersant A contained in the aqueous pigment dispersion.

[0055] The average particle size of the pigment particles in the water-based ink for ink-jet printing of the present invention is preferably the same as the average particle size of the pigment particles in the water-based pigment dispersion of the present invention. Therefore, the preferred embodiment of the average particle size of the pigment particles in the water-based ink for ink-jet printing of the present invention is also preferably the same as the preferred embodiment of the average particle size of the pigment particles in the water-based pigment dispersion of the present invention.

[0056] (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. Inkjet recording apparatuses include thermal and piezoelectric types, with the thermal type being preferred, and the ink of the present invention is preferably used for thermal type inkjet recording. Examples of the recording medium include low water-absorbent recording paper such as coated paper, and non-water-absorbent resin films such as PET film and polypropylene film. Plain paper can also be used as the recording medium. The terms "low water absorption" and "non-water absorption" refer to a water absorption capacity of the recording medium of 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 Most recording media are highly absorbent, and plain paper is classified as highly absorbent. EXAMPLES

[0057] In the following Preparation 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.

[0058] (1) Acid value of polymer (a) The measurements were performed according to the neutralization titration method described in JIS K0070-1992, except that the measurement solvent was changed from a mixed solvent of ethanol and ether to a mixed solvent of acetone and toluene [acetone:toluene=1:1 (volume ratio)].

[0059] (2) Measurement of number average molecular weight (Mn) of polymer (a) The measurement was performed by gel permeation chromatography under the following conditions. GPC equipment: Tosoh Corporation "HLC-8320GPC" Columns: Tosoh Corporation's "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolum Super AW-H" Eluent: N,N-dimethylformamide with phosphoric acid and lithium bromide dissolved at concentrations of 60mmol / L and 50mmol / L, respectively. Flow rate: 0.5mL / min Standard material: Monodisperse polystyrene kit 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)] (manufactured by Tosoh Corporation) Measurement sample: 0.1 g of polymer (a) 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 with a syringe filter (DISMIC-13HP, PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.) and used.

[0060] (3) Measurement of solids concentration of pigment water dispersion (d) and water-based pigment dispersion Approximately 10 g of sodium sulfate that had been kept at a constant weight in a desiccator was weighed out 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 a 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 volatile matter. 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 volatile matter removal was precisely weighed to calculate the mass of the sample after volatile matter removal. The mass of the sample after volatile matter removal was divided by the mass of the sample added to the container to obtain the solid concentration (%).

[0061] (4) Measurement of pH of water-based pigment dispersion The pH of the water-based pigment dispersion at 20° C. was measured using a tabletop pH meter “F-71” (manufactured by Horiba, Ltd.) using a pH electrode “6337-10D” (manufactured by Horiba, Ltd.).

[0062] (5) Measurement of average particle size of water-based pigment dispersion Using a laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.), particle size was measured by dynamic light scattering, and cumulant analysis was performed. The obtained cumulant average particle size was taken as the average particle size of the aqueous pigment dispersion. For the measurement sample, the aqueous pigment dispersion was weighed into a screw tube (Maruemu Co., Ltd., No. 5) and the solids concentration was 2 × 10 -4 % (converted into solids concentration) was added to the solution, which was then stirred with a magnetic stirrer for 1 hour at 25° C. The measurement conditions were a temperature of 25° C., an angle between the incident light and the detector of 90°, and 100 cumulative measurements. The refractive index of water (1.333) was entered as the refractive index of the dispersion solvent.

[0063] Preparation Example 1 (Preparation of polymer (a1) having a carboxy group) A monomer mixture was prepared by mixing 79.7 parts of acrylic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 60.3 parts of benzyl methacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 40.0 parts of styrene macromer (product name: AS-6S, manufactured by Toagosei Co., Ltd., toluene solution with 50% solid content) (solid content 20.0 parts), and 40.0 parts of monopolypropylene glycol methacrylate (manufactured by NOF Corporation, product name: Blenmer PP-800, average number of moles of propylene oxide added: 13, terminal: hydroxyl group). In a reaction vessel, 20 parts of methyl ethyl ketone (hereinafter referred to as "MEK") (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 0.3 parts of 2-mercaptoethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) as a polymerization chain transfer agent, and 10% of the monomer mixture were mixed and thoroughly substituted with nitrogen gas. Separately, a mixture of the remainder of the monomer mixture (90% of the monomer mixture), 0.27 parts of the polymerization chain transfer agent (2-mercaptoethanol), 60 parts of MEK, and 2.2 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (product name: V-65, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as an azo radical polymerization initiator was placed in a dropping funnel, and the monomer mixture in the reaction vessel was heated to 65°C while stirring under a nitrogen atmosphere, and the mixture in the dropping funnel was dropped over 3 hours. After 2 hours at 65°C from the end of the dropping, a solution in which 0.3 parts of the polymerization initiator was dissolved in 5 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and 70°C for 2 hours to obtain a MEK solution of a polymer (a1) having a carboxy group (number average molecular weight: 13,000, acid value: 310 mgKOH / g).

[0064] Example 1 (Production of Water-Based Pigment Dispersion D1) (Process 1) 66.6 parts of the MEK solution of the polymer (a1) having a carboxy group obtained in Preparation Example 1 was dried under reduced pressure, and mixed with 78.6 parts of MEK, and 21.6 parts of a 5N aqueous sodium hydroxide solution (content of sodium hydroxide: 16.9%) was added as a neutralizing agent to neutralize the ratio of the molar equivalent value of sodium hydroxide to the molar equivalent value of the carboxy group derived from the polymer (a1) having a carboxy group to 24.8 mol% (degree of neutralization: 24.8 mol%), and 600 parts of ion-exchanged water was added. Next, 100 parts of carbon black pigment (trade name: Monarch 800, manufactured by Cabot Specialty Chemicals) was added, and the resulting pigment mixture was stirred for 60 minutes at 20°C using a Disper (manufactured by Asada Iron Works, Ltd., trade name: Ultra Disper) under conditions of rotating the Disper blade at 7,000 rpm, and then dispersed with a Microfluidizer (manufactured by Microfluidics, trade name) at a pressure of 200 MPa for 10 passes. To the obtained dispersion, 330 parts of ion-exchanged water was added and stirred. After that, MEK was completely removed at 60° C. under reduced pressure. Further, some of the water was removed, and the mixture was filtered with a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a 5 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm, manufactured by FUJIFILM Corporation) to remove coarse particles, thereby obtaining a pigment water dispersion (d1) with a solid content concentration of 20%. (Process 2) 200 parts of the pigment water dispersion (d1) (solid content concentration: 20%) obtained in step 1 was taken into a screw-top glass bottle, and 9.4 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-321, water solubility: 27% (catalog value), epoxy equivalent: 140 g / eq (catalog value)) (hereinafter referred to as "EX-321") as a polyfunctional epoxy compound (b) (crosslinking agent) and 37.4 parts of ion-exchanged water were added, sealed, and heated at 70 ° C. for 5 hours while stirring with a stirrer. Thereafter, the temperature was lowered to room temperature, and the above-mentioned 5 μm acetyl cellulose filter was used to filter the water-based pigment dispersion D1 (crosslinking degree of crosslinked polymer dispersant A1: 77.1 mol%) with a solid content concentration of 20% was obtained. (Preparation of Water-Based Ink 1 for Ink-Jet Printing) To 38 parts of the obtained aqueous pigment dispersion D1, 50.5 parts of ion exchanged water, 10 parts of polyethylene glycol 400 (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., reagent) as a solvent, 1 part of a propylene glycol solution of an acetylene glycol surfactant (trade name: Surfynol 104PG-50, manufactured by Air Products, active content 50%), and 0.5 parts of (trade name: Emulgen 120, manufactured by Kao Corporation, polyoxyethylene lauryl ether) were added, and the resulting mixture was filtered using a syringe equipped with the above-mentioned 5 μm acetyl cellulose filter, to obtain an aqueous ink 1 for ink-jet recording.

[0065] Examples 2 to 7 and Comparative Examples 1 and 2 In step 1 of Example 1, the same operation as in Example 1 was carried out except that the type of polymer (a) or the amount of neutralizing agent shown in Table 1 was changed, thereby obtaining pigment water dispersions (d2) to (d9) each having a solid content concentration of 20%. Next, in step 2 of Example 1, the same operation as in Example 1 was carried out except that the type of pigment water dispersion (d), the amount of polyfunctional epoxy compound (b), and the amount of ion-exchanged water shown in Table 1 were changed, thereby obtaining water-based pigment dispersions D2 to D7, DC1, and DC2 each having a solid content concentration of 20%. Next, the same operations as in Example 1 were carried out except that the aqueous pigment dispersions shown in Table 1 were changed in the preparation of the aqueous ink for inkjet printing in Example 1, thereby obtaining aqueous inks for inkjet printing 2 to 7, C1, and C2, respectively.

[0066] Among the polymers (a) shown in Table 1, the polymers (a2) and (a3) ​​having a carboxy group are as follows. Polymer (a2): styrene / maleic anhydride copolymer (manufactured by Polyscope, product name: XIRAN 2000, styrene / maleic anhydride = 2 (molar ratio) (catalog value), acid value: 355 mg KOH / g (catalog value), number average molecular weight: 2,700 (catalog value)) Polymer (a3): styrene / maleic anhydride copolymer (manufactured by Polyscope, product name: XIRAN 1000, styrene / maleic anhydride = 1 (molar ratio) (catalog value), acid value: 480 mg KOH / g (catalog value), number average molecular weight: 2,300 (catalog value))

[0067] <Evaluation of ejection properties> The jetting properties of each of the water-based inks for ink-jet printing obtained in the Examples and Comparative Examples were evaluated by the following method. The results are shown in Table 1. An inkjet recording device "LPP-6010N" (manufactured by LG Electronics) with a thermal inkjet head was modified to allow printing by pulling out the liquid supply tube and inserting the tube into an ink tank. The ink tank was filled with each water-based ink for inkjet recording, and the printing conditions were set to best mode (resolution: 1,600 dpi vertical x 1,600 dpi horizontal) and the duty was set to 100%. The device was placed in an environment of 25±1°C temperature and 30±5% relative humidity, one sheet of A4 size plain paper was printed, and the print density was measured with an optical densitometer. Printing was continued while measuring the print density with the optical densitometer every 100 sheets, and the number of sheets that could be printed when the print density of the first sheet was 0.04 lower for two consecutive prints was used as an index of ejection properties, and the number of sheets was evaluated according to the following evaluation criteria. In the following evaluation criteria, the more sheets that could be printed, the better the ejection properties. The plain paper used was HP All-in-One printing paper (manufactured by Hewlett-Packard), and the optical densitometer used was SpectroEye (manufactured by GretagMacbeth). (Evaluation Criteria) A: 15,000 or more sheets of A4 size plain paper B: A4 size plain paper, 10,000 to 15,000 sheets C: A4 size plain paper, 5,000 to 10,000 sheets D: A4 size plain paper, 1,000 to 5,000 sheets E: Less than 1,000 sheets of A4 size plain paper

[0068] [Table 1]

[0069] From Table 1, it can be seen that the water-based ink for ink-jet printing containing the water-based pigment dispersion of the Example is superior in ejection properties to the water-based ink for ink-jet printing containing the water-based pigment dispersion of the Comparative Example.

Claims

1. An aqueous pigment dispersion containing a pigment, a crosslinked polymer dispersant A, and an aqueous medium, The crosslinked polymer dispersant A comprises a structure derived from a polymer (a) having a carboxyl group and a structure derived from a polyfunctional epoxy compound (b), At least a portion of the carboxyl groups of the crosslinked polymer dispersant A are neutralized with an alkali metal hydroxide, An aqueous pigment dispersion wherein the degree of crosslinking of the crosslinked polymer dispersant A and the acid value of the polymer (a) satisfy the following condition 1. Condition 1: The value of [(degree of crosslinking of crosslinked polymer dispersant A) / 100] × (acid value of polymer (a)) is 230 mg KOH / g or more and 360 mg KOH / g or less. Here, the degree of crosslinking of the crosslinked polymer dispersant A is the percentage of the value obtained by dividing the molar equivalents of epoxy groups derived from the polyfunctional epoxy compound (b) constituting the crosslinked polymer dispersant A by the molar equivalents of carboxyl groups derived from the polymer (a) constituting the crosslinked polymer dispersant A [(molar equivalents of epoxy groups derived from the polyfunctional epoxy compound (b)) / (molar equivalents of carboxyl groups derived from the polymer (a))].

2. The aqueous pigment dispersion according to claim 1, wherein the acid value of the crosslinked polymer dispersant A is 40 mg KOH / g or more and 250 mg KOH / g or less.

3. The degree of crosslinking of crosslinked polymer dispersant A is 40 mol% or more and 90 mol% or less. The aqueous pigment dispersion according to claim 1.

4. The aqueous pigment dispersion according to claim 1, wherein the acid value of polymer (a) is 300 mg KOH / g or more and 800 mg KOH / g or less.

5. The aqueous pigment dispersion according to claim 1, wherein polymer (a) is a vinyl polymer containing structural units derived from carboxyl group-containing monomer (a-1), and the carboxyl group-containing monomer (a-1) is one or more selected from the group consisting of acrylic acid, methacrylic acid, and maleic acid.

6. The aqueous pigment dispersion according to claim 1, wherein polymer (a) is a styrene / maleic acid copolymer.

7. The aqueous pigment dispersion according to claim 1, wherein the water solubility of the polyfunctional epoxy compound (b) is 45% by mass or less.

8. The aqueous pigment dispersion according to claim 1, wherein the ratio of the degree of crosslinking of the crosslinked polymer dispersant A to the degree of neutralization of the crosslinked polymer dispersant A [(degree of crosslinking of crosslinked polymer dispersant A) / (degree of neutralization of crosslinked polymer dispersant A)] is 1 or more and 7 or less. Here, the degree of neutralization of the crosslinked polymer dispersant A is the percentage of the value obtained by dividing the molar equivalents of alkali metal hydroxide in the aqueous pigment dispersion by the molar equivalents of carboxyl groups derived from polymer (a) constituting the crosslinked polymer dispersant A [(molar equivalents of alkali metal hydroxide) / (molar equivalents of carboxyl groups derived from polymer (a))] (however, if the sum of the percentage of this value [(molar equivalents of alkali metal hydroxide) / (molar equivalents of carboxyl groups derived from polymer (a))] and the degree of crosslinking of the crosslinked polymer dispersant A exceeds 100 mol%, the degree of neutralization of the crosslinked polymer dispersant A is the value obtained by subtracting the degree of crosslinking (mol%) from 100 mol%).

9. An aqueous ink for inkjet recording, comprising an aqueous pigment dispersion according to any one of claims 1 to 8.

10. Pigment particles comprising a pigment and a crosslinked polymer dispersant A, The pigment and crosslinked polymer dispersant A are in the form of crosslinked polymer particles containing the pigment. The crosslinked polymer dispersant A comprises a structure derived from a polymer (a) having a carboxyl group and a structure derived from a polyfunctional epoxy compound (b), At least a portion of the carboxyl groups of the crosslinked polymer dispersant A are neutralized with an alkali metal hydroxide, Pigment particles wherein the degree of crosslinking of the crosslinked polymer dispersant A and the acid value of the polymer (a) satisfy the following condition 1. Condition 1: The value of [(degree of crosslinking of crosslinked polymer dispersant A) / 100] × (acid value of polymer (a)) is 230 mg KOH / g or more and 360 mg KOH / g or less. Here, the degree of crosslinking of the crosslinked polymer dispersant A is the percentage of the value obtained by dividing the molar equivalents of epoxy groups derived from the polyfunctional epoxy compound (b) constituting the crosslinked polymer dispersant A by the molar equivalents of carboxyl groups derived from the polymer (a) constituting the crosslinked polymer dispersant A [(molar equivalents of epoxy groups derived from the polyfunctional epoxy compound (b)) / (molar equivalents of carboxyl groups derived from the polymer (a))].

11. An inkjet recording water-based ink comprising the pigment particles described in Claim 10.

12. The water-based inkjet recording ink according to claim 9, which is for thermal inkjet recording.

13. A method for producing an aqueous pigment dispersion according to any one of claims 1 to 8, comprising the steps 1 and 2 described below. Step 1: A process to obtain a pigment aqueous dispersion (d) by applying shear stress to a pigment mixture containing a pigment, a polymer having a carboxyl group (a), and an alkali metal hydroxide to disperse it. Step 2: A step to obtain an aqueous pigment dispersion containing a pigment, a crosslinked polymer dispersant A, and an aqueous medium by reacting the carboxyl group of a polymer (a) having a carboxyl group in the aqueous pigment dispersion (d) obtained in Step 1 with a polyfunctional epoxy compound (b) to form a crosslinked polymer dispersant A.