Water-based pigment dispersions

Abstract

To provide a water-based pigment dispersion that has high storage stability, is used in water-based ink to show high re-dispersibility, and can improve image density even in inkjet recording onto plain paper, a water-based ink containing the same, and a method for producing a water-based pigment dispersion.SOLUTION: A water-based pigment dispersion has a polymer (A) having a crosslinked structure of a water-insoluble polymer having a carboxy group (a) and a polyfunctional epoxy compound, and a pigment. At least some of carboxy groups of the polymer (A) are neutralized with an alkali metal compound, and [1] the water-insoluble polymer (a) has an acid value of 180-320 mgKOH / g and [2] the amount of the alkali metal compound to the pigment is 0.07-0.19 in the case of organic pigment or 0.15-0.45 in the case of carbon black.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an aqueous pigment dispersion, an aqueous ink containing the aqueous pigment dispersion, and a method for producing 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 contact the recording medium. In recent years, in particular, methods that use pigments as colorants have become widely used from the viewpoint of the weather resistance and water resistance of the recorded matter.

[0003] In the case of aqueous pigment dispersions and aqueous inks using the same, functional groups with high affinity to water are added to the pigment surface, and the pigment is dispersed in an aqueous medium or ink vehicle without or with a dispersant. However, unlike inks using dyes that are uniformly dissolved in aqueous media or ink vehicles, it is difficult for inks using pigment dispersions to maintain the dispersion state of pigment particles for a long period of time, and polymers and pigments may become stuck at the ink ejection nozzle.

[0004] Various proposals have been made for aqueous pigment dispersions that can solve this problem. For example, Patent Document 1 discloses an aqueous pigment dispersion that has redispersibility capable of suppressing solidification of the pigment or polymer in the ink ejection nozzles and that can ensure storage stability at high temperatures, the aqueous pigment dispersion comprising a pigment and a polymer dispersant formed by crosslinking a water-insoluble polymer (A) having a carboxy group with a water-insoluble polyfunctional epoxy compound, wherein 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) have a specific relationship. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2018-65997 A Summary of the Invention [Problem to be solved by the invention]

[0006] Although the water-based pigment dispersion of Patent Document 1 has excellent storage stability, there is room for improvement in image density when inkjet recording is performed on plain paper. An object of the present invention is to provide an aqueous pigment dispersion which has excellent storage stability, and which, when used in an aqueous ink, has high redispersibility and can improve image density even in inkjet recording on plain paper; an aqueous ink containing the aqueous pigment dispersion; and a method for producing the aqueous pigment dispersion. [Means for solving the problem]

[0007] The present inventors have found that the above problems can be solved by an aqueous pigment dispersion in which a pigment is dispersed using a polymer (A) having a specific structure as a dispersant. That is, the present invention provides the following [1] to [3]. [1] An aqueous pigment dispersion containing a water-insoluble polymer (a) having a carboxy group and a polymer (A) having a crosslinked structure formed by a polyfunctional epoxy compound, and a pigment, wherein at least a portion of the carboxy groups of the polymer (A) are neutralized with an alkali metal compound, and the aqueous pigment dispersion satisfies the following [Condition 1] and [Condition 2]: [Condition 1]: The acid value of the water-insoluble polymer (a) is 180 mgKOH / g or more and 320 mgKOH / g or less. [Condition 2]: The amount p of the alkali metal compound relative to the pigment in the aqueous pigment dispersion, as represented by the following formula (1), is 0.07 or more and 0.19 or less when the pigment is an organic pigment, and is 0.15 or more and 0.45 or less when the pigment is carbon black. Amount of alkali metal compound p (mmol / g) = [molar amount of alkali metal compound in aqueous pigment dispersion (mmol) / mass of pigment in aqueous pigment dispersion (g)] (1)

[0008] [2] A water-based ink comprising the water-based pigment dispersion according to [1] above and a water-soluble organic solvent. [3] A method for producing the aqueous pigment dispersion according to [1] above, comprising the following steps 1 to 3: Step 1: A step of neutralizing at least a part of the carboxy groups of the water-insoluble polymer (a) with an alkali metal compound to obtain an aqueous dispersion of the water-insoluble polymer (a). Step 2: A step of dispersing the aqueous dispersion of the water-insoluble polymer (a) obtained in step 1 and a pigment to obtain an aqueous pigment dispersion (i) of polymer particles containing the pigment, the pigment being contained in the water-insoluble polymer (a). Step 3: A step of adding a polyfunctional epoxy compound to the pigment water dispersion (i) obtained in step 2, and carrying out a crosslinking treatment to obtain an aqueous pigment dispersion of polymer (A) particles having a crosslinked structure containing a pigment. Effect of the Invention

[0009] According to the present invention, it is possible to provide an aqueous pigment dispersion which has excellent storage stability, and when used in an aqueous ink, has high redispersibility and can improve image density even in inkjet recording on plain paper; an aqueous ink containing the aqueous pigment dispersion; and a method for producing the aqueous pigment dispersion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [Water-based pigment dispersion] The aqueous pigment dispersion of the present invention contains a water-insoluble polymer (a) having a carboxy group and a polymer (A) having a crosslinked structure formed of a polyfunctional epoxy compound, and a pigment, in which at least a portion of the carboxy groups of the polymer (A) are neutralized with an alkali metal compound, and the aqueous pigment dispersion satisfies the following [Condition 1] and [Condition 2]. [Condition 1]: The acid value of the water-insoluble polymer (a) is 180 mgKOH / g or more and 320 mgKOH / g or less. [Condition 2]: The amount p of the alkali metal compound relative to the pigment in the aqueous pigment dispersion, as represented by the following formula (1), is 0.07 or more and 0.19 or less when the pigment is an organic pigment, and is 0.15 or more and 0.45 or less when the pigment is carbon black. Amount of alkali metal compound p (mmol / g) = [molar amount of alkali metal compound in aqueous pigment dispersion (mmol) / mass of pigment in aqueous pigment dispersion (g)] (1) In this specification, the term "water-based" means that water accounts for the largest proportion by mass of the medium in which the pigment is dispersed. Moreover, "recording" refers to printing or imprinting characters or images. The aqueous pigment dispersion of the present invention is suitable as an aqueous pigment dispersion for use in inkjet recording inks, gravure printing inks, flexographic printing inks, and the like, and is particularly preferred for use in inkjet recording inks because of its excellent redispersibility.

[0011] The aqueous pigment dispersion of the present invention has excellent storage stability, and when used in an aqueous ink, it has high redispersibility and can improve image density even in ink-jet recording on plain paper. The reason for this is unclear, but is thought to be as follows. In the aqueous pigment dispersion of the present invention, the water-insoluble polymer (a) acting as a dispersant for the pigment has a large acid value of 180 to 320 mgKOH / g [Condition 1]. This large acid value of the polymer (a) is derived from the carboxyl group, and the carboxyl group portion forms many crosslinked structures by crosslinking treatment, and the polymer is firmly fixed to the pigment surface. Therefore, even when an ink containing this aqueous pigment dispersion dries and the ratio of the organic solvent in the ink increases, the polymer is unlikely to be detached from the pigment and the pigments are unlikely to be strongly aggregated together. Therefore, when the ink comes into contact with a liquid component again, the dried material can be loosened by the liquid component and become liquid, which is considered to enable high redispersibility to be exhibited.

[0012] On the other hand, it is believed that the image density on plain paper can be improved by maximizing the effect of the calcium salt contained in a small amount in the plain paper. In the present invention, by making the amount p of the alkali metal compound relative to the pigment satisfy [Condition 2], the neutralization amount of the carboxyl group of the polymer (a) used to disperse the pigment is reduced, the charge repulsion force required for dispersing the pigment is minimized, and when the ink contacts (lands on) the plain paper, a strong cohesive force can be exerted on the pigment particles. In other words, when the ink contacts the plain paper, the liquid component of the ink is absorbed into the paper fibers, and the ratio of pigment particles in the ink increases, and the viscosity increases quickly, so that the ink does not penetrate into the inside of the plain paper, which is believed to be able to achieve a high image density.

[0013] <Pigments> The pigment used in the present invention may be either carbon black or an organic pigment, including lake pigments and fluorescent pigments, and may also be used in combination with an extender pigment, if necessary. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and threne pigments.

[0014] There are no particular limitations on the hue, and any of achromatic pigments such as black and gray, and chromatic organic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used. Specific examples of preferred organic pigments include one or more product numbers selected from CI Pigment Yellow, CI Pigment Red, CI Pigment Orange, CI Pigment Violet, CI Pigment Blue, and CI Pigment Green. Examples of the extender pigment include silica, calcium carbonate, and talc. The above pigments can be used alone or in combination of two or more kinds.

[0015] The pigment is dispersed in the aqueous medium by a polymer (A) (hereinafter simply referred to as "polymer (A)") having a crosslinked structure formed by a water-insoluble polymer (a) having a carboxy group and a polyfunctional epoxy compound. From the viewpoint of improving storage stability and image density, the pigment is preferably dispersed in any of the following forms: particles of a water-insoluble polymer (A) containing the pigment, i.e., a form in which the polymer (A) encompasses the pigment; a form in which a part of the pigment is exposed on the surface of a particle consisting of the polymer (A) and the pigment; a form in which the polymer (A) is adsorbed to a part of the pigment; a form in which the pigment adheres to a particle of the polymer (A); or a mixture of these forms.

[0016] <Polymer (A)> The polymer (A) has a crosslinked structure obtained by reacting a water-insoluble polymer (a) having a carboxy group (hereinafter simply referred to as "polymer (a)") with a polyfunctional epoxy compound. That is, the polymer (A) is composed of a structural unit derived from the water-insoluble polymer (a) having a carboxy group and a structural unit derived from the polyfunctional epoxy compound. Furthermore, the polymer (A) has carboxy groups derived from the polymer (a), and at least a portion of the carboxy groups is neutralized with an alkali metal compound (neutralizing agent). In the present invention, by using an alkali metal compound as a neutralizing agent for the carboxy group, the charge repulsion force of the polymer (A) particles that is generated after neutralization can be increased, and it is considered that even when the aqueous pigment dispersion or aqueous ink is stored at high temperatures for a long period of time, thickening and aggregation can be suppressed and storage stability can be improved. In the water-based pigment dispersion of the present invention, since the pigment ratio x is adjusted to fall within a specific range, it is believed that most of the polymer (A) acting as a dispersant is present by being adsorbed onto the pigment surface.

[0017] Polymer (a) is water-insoluble, and "water-insoluble" means that the polymer does not dissolve in water when neutralized under conditions that satisfy the "amount p of alkali metal compound relative to pigment" set in [Condition 2] described below. Specifically, when a neutralized polymer is mixed with water, it is determined to be "water-insoluble" if (i) the insoluble portion can be confirmed with the naked eye, (ii) even if the insoluble portion is too small to be confirmed with the naked eye, the Tyndall phenomenon can be observed when observed with laser light or normal light, or (iii) the average particle size can be observed when measured using the laser particle analysis system described in the examples.

[0018] [Condition 1]: Acid value of polymer (a) The acid value of the polymer (a) is derived from the carboxy group, and the acid value is 180 mgKOH / g or more and 320 mgKOH / g or less. From the viewpoint of enhancing the storage stability of the aqueous pigment dispersion of the present invention, the acid value of the polymer (a) is preferably 200 mgKOH / g or more, more preferably 220 mgKOH / g or more, even more preferably 230 mgKOH / g or more, and preferably 310 mgKOH / g or less, more preferably 300 mgKOH / g or less, even more preferably 290 mgKOH / g or less. If the acid value is within the above range, the amount of carboxyl groups is sufficient. In addition, it is also preferable from the viewpoint of the balance between the affinity with the aqueous medium and the interaction with the pigment when the polymer (a) becomes the polymer (A). The acid value of the polymer (a) can be measured by the method described in the Examples, or can be calculated from the mass ratio of the constituent monomers.

[0019] [Water-insoluble polymer (a) having a carboxy group] The polymer (a) is preferably a vinyl polymer obtained by copolymerizing a vinyl monomer mixture (hereinafter also referred to as a "monomer mixture") containing (a-1) a carboxyl group-containing vinyl monomer (hereinafter also referred to as "component (a-1)") and (a-2) a hydrophobic vinyl monomer (hereinafter also referred to as "component (a-2)"). This vinyl polymer has a structural unit derived from component (a-1) and a structural unit derived from component (a-2). This vinyl polymer may further contain a structural unit derived from (a-3) a nonionic monomer (hereinafter also referred to as "component (a-3)") and further a structural unit derived from a macromonomer.

[0020] [(a-1) Carboxy Group-Containing Vinyl Monomer] The (a-1) carboxy group-containing vinyl monomer is used as a monomer component of polymer (a) constituting polymer (A) in the aqueous pigment dispersion of the present invention and the aqueous ink containing the same, as a source of pigment dispersion stability. The component (a-1) may be a vinyl carboxylate monomer. The vinyl carboxylate monomer may be one or more selected from acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, 2-methacryloyloxymethylsuccinic acid, etc., and is preferably one or more selected from acrylic acid and methacrylic acid.

[0021] [(a-2) Hydrophobic vinyl monomer] The (a-2) hydrophobic vinyl monomer is preferably used as a monomer component of the polymer (a) from the viewpoints of improving the dispersion stability of the pigment and improving the storage stability of the aqueous pigment dispersion and the image density of the aqueous ink. Here, "hydrophobic" means that the amount of the monomer dissolved is less than 10 g when the monomer is dissolved to saturation in 100 g of ion-exchanged water at 25° C. The amount of the hydrophobic monomer (a-2) dissolved is preferably 5 g or less, more preferably 1 g or less, from the viewpoint of the adsorption of the polymer to the pigment. Preferred examples of the component (a-2) include alkyl (meth)acrylates having an alkyl group with 1 to 22 carbon atoms or an aryl group with 6 to 22 carbon atoms, and aromatic group-containing monomers.

[0022] The alkyl (meth)acrylate preferably has an alkyl group having 1 to 18 carbon atoms, more preferably 1 to 10 carbon atoms. Examples of the alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, amyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, isopropyl (meth)acrylate, isobutyl (meth)acrylate, tertiary butyl (meth)acrylate, isoamyl (meth)acrylate, isooctyl (meth)acrylate, isodecyl (meth)acrylate, isododecyl (meth)acrylate, and isostearyl (meth)acrylate. The term "(meth)acrylate" refers to one or more selected from acrylate and methacrylate. The term "(meth)" below has the same meaning.

[0023] The aromatic group-containing monomer is preferably a vinyl monomer having an aromatic group with 6 to 22 carbon atoms, which may have a substituent containing a hetero atom, more preferably a styrene-based monomer or an aromatic group-containing (meth)acrylate. The molecular weight of the aromatic group-containing monomer is preferably less than 500. The styrene-based monomer is preferably at least one selected from styrene, α-methylstyrene, vinyltoluene, vinylpyridine, divinylbenzene, and the like, and more preferably at least one selected from styrene and α-methylstyrene. Moreover, the aromatic group-containing (meth)acrylate is preferably at least one selected from phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, and the like, and more preferably benzyl (meth)acrylate. The component (a-2) may contain two or more of the above monomers. In this case, it is preferable to use two or more styrene-based monomers, and it is more preferable to use styrene and α-methylstyrene in combination.

[0024] [(a-3) Nonionic Monomer] The nonionic monomer (a-3) can be used as a monomer component of the polymer (a) from the viewpoint of adjusting the dispersion stability of the pigment. Examples of the (a-3) nonionic monomer include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 3-hydroxypropyl (meth)acrylate; polyalkylene glycol (meth)acrylates such as polypropylene glycol (n=2-30, n indicates the average number of moles of oxypropylene groups added. The following n indicates the average number of moles of oxyalkylene groups added.) (meth)acrylate and polyethylene glycol (n=2-30) (meth)acrylate; methoxypolyethylene glycol (n=1-30) (meth)acrylate; phenoxy (ethylene glycol-propylene glycol copolymer) (n=1-30, ethylene glycol therein: n=1-29) (meth)acrylate, and the like. Among these, polypropylene glycol (n=2-30) (meth)acrylate and phenoxy (ethylene glycol-propylene glycol copolymer) (meth)acrylate are preferred, and polypropylene glycol (n=2-30) (meth)acrylate is more preferred. Commercially available examples of the component (a-3) include the NK Ester M series from Shin-Nakamura Chemical Co., Ltd., the Blenmar PE, PME, PP, and AP series from NOF Corporation, as well as 50PEP-300, 50POEP-800B, and 43PAPE-600B.

[0025] From the above viewpoints, the polymer (a) is preferably such that the (a-1) component is one or more selected from acrylic acid and methacrylic acid, and the (a-2) component is one or more selected from styrene and α-methylstyrene, and is more preferably a styrene-α-methylstyrene-acrylic acid copolymer.

[0026] The content of each component in the monomer mixture during production of polymer (a) (content as unneutralized amount; the same applies hereinafter) or the content of the constitutional units derived from each component in polymer (a) is as follows, from the viewpoints of improving the dispersion stability of the pigment, the storage stability of the aqueous pigment dispersion, and the image density of the obtained aqueous ink. The content of the (a-1) component is preferably 15 mass% or more, more preferably 20 mass% or more, even more preferably 25 mass% or more, and preferably 70 mass% or less, more preferably 60 mass% or less, even more preferably 50 mass% or less. The content of the (a-2) component is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

[0027] When the (a-3) component is contained, the content of the (a-3) component is preferably 30 mass % or less, more preferably 20 mass % or less, even more preferably 10 mass % or less, and still more preferably 5 mass % or less. The mass ratio of [component (a-1) / component (a-2)] is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.35 or more, still more preferably 0.38 or more, and is preferably 2 or less, more preferably 1.5 or less, even more preferably 1 or less, and still more preferably 0.8 or less.

[0028] [Production of polymer (a)] The polymer (a) is produced by copolymerizing the raw material monomer mixture by a known polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. Among these polymerization methods, the solution polymerization method is preferred. Alternatively, a commercially available product may be used as the polymer (a), and a specific example of the commercially available product is Joncryl manufactured by BASF.

[0029] The weight average molecular weight of the polymer (a) is preferably at least 4,000, more preferably at least 6,000, and even more preferably at least 8,000, and is preferably at most 80,000, more preferably at most 50,000, even more preferably at most 30,000, and even more preferably at most 20,000. From the viewpoint of enhancing adsorption to the pigment and exhibiting storage stability, the weight average molecular weight of the polymer (a) is preferably within the above range. The weight average molecular weight is measured by the method described in the Examples.

[0030] [Neutralization] From the viewpoint of improving the storage stability of the water-based pigment dispersion and the image density of the resulting water-based ink, it is preferable that at least a portion of the carboxyl groups of the polymer (a) is neutralized with an alkali metal compound. The alkali metal compound is used in the form of water or a compound that generates alkali metal ions in a medium containing water when preparing the water-based pigment dispersion of the present invention. Examples of the compound that generates an alkali metal ion include one or more selected from alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide, alkali metal salts of carbonate such as disodium carbonate, sodium hydrogen carbonate, and dipotassium carbonate, and alkali metal salts of boric acid such as sodium borate, etc. Two or more types of alkali metal compounds may be used in combination. Among these, alkali metal hydroxides are preferred, and as the metal, sodium and potassium are preferred, and sodium is more preferred. That is, as the alkali metal compound, sodium hydroxide and potassium hydroxide are preferred, and sodium hydroxide is more preferred.

[0031] In the present invention, it is preferable not to use a water-soluble amine compound or ammonia, but a water-soluble amine compound or ammonia can be used within a range that does not impair the effects of the present invention. Even when a water-soluble amine compound or ammonia is used, when calculating the neutralization degree of the polymer, only the amount of the alkali metal compound is used, and the amount of the water-soluble amine compound or ammonia used is not included. The water-based ink containing the water-based pigment dispersion of the present invention may further contain a water-soluble amine compound.

[0032] From the viewpoint of ensuring dispersion stability before reacting with a polyfunctional epoxy compound, the degree of neutralization of the polymer (a) is preferably 20 mol % or more, more preferably 30 mol % or more, even more preferably 40 mol % or more, still more preferably 50 mol % or more, and is preferably 150 mol % or less, more preferably 140 mol % or less, and even more preferably 130 mol % or less. Here, the degree of neutralization (mol %) of the polymer (a) is calculated by the following formula. Degree of neutralization of polymer (a) (mol %)=[number of moles of alkali metal compound in aqueous pigment dispersion / number of moles of carboxyl group of polymer (a) in aqueous pigment dispersion]×100 In the present invention, when the alkali metal compound is used in excess of the number of moles of carboxy groups in the polymer (a), the degree of neutralization may exceed 100 mol %.

[0033] [Multifunctional epoxy compound] The polymer (A) according to the present invention has a crosslinked structure formed by reacting a water-insoluble polymer (a) having a carboxy group with a polyfunctional epoxy compound (hereinafter also referred to as an "epoxy compound"). This crosslinked structure allows the polymer to be firmly adsorbed or fixed on the pigment surface, suppressing aggregation of the pigment in an aqueous medium and further suppressing swelling of the polymer, which is believed to improve storage stability. The epoxy compound (crosslinking agent) may be water-soluble or water-insoluble, but is preferably water-insoluble from the viewpoint of more efficient crosslinking reaction with the carboxyl group of the polymer (a) in a medium mainly composed of water. When a water-insoluble epoxy compound is used, its water solubility (mass%) is preferably 50 mass% or less, more preferably 40 mass% or less, and even more preferably 35 mass% or less. Here, the water solubility (mass%) refers to the solubility (mass%) when 10 mass parts of the epoxy compound are dissolved in 90 mass parts of water at room temperature of 25°C.

[0034] The epoxy compound is preferably a compound having two or more epoxy groups in the molecule, more preferably a compound having two or more glycidyl ether groups, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 8 carbon atoms. From the viewpoints of improving the efficiency of the crosslinking reaction, the storage stability of the resulting water-based pigment dispersion, and the redispersibility of the water-based ink, the molecular weight of the epoxy compound is preferably 120 or more, more preferably 150 or more, and even more preferably 200 or more, and is preferably 2,000 or less, more preferably 1,000 or less, and even more preferably 800 or less. From the same viewpoints as above, the epoxy equivalent of the epoxy compound is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and preferably 300 or less, more preferably 200 or less, even more preferably 170 or less. The number of epoxy groups in the epoxy compound is 2 or more per molecule from the same viewpoint as above, and preferably 6 or less per molecule, and from the viewpoint of market availability, is more preferably 4 or less, and even more preferably 3 or less.

[0035] The epoxy compound has a [number of carbon atoms / number of oxygen atoms] ratio (hereinafter also referred to as the "C / O ratio") of preferably 2.0 or more, more preferably 2.2 or more, even more preferably 2.4 or more, still more preferably 2.6 or more, and preferably 4.0 or less, more preferably 3.9 or less, even more preferably 3.8 or less, and still more preferably 3.7 or less, from the same viewpoints as above. The [C / O] ratio in an epoxy compound can be calculated according to JIS M 8813:2006 "Coals and cokes - Elemental analysis method". In addition, if the structure and composition of the epoxy compound are known, the number of carbon atoms and oxygen atoms can be calculated from that information.

[0036] Specific examples of the epoxy compound include one or more selected from polyglycidyl ethers such as cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether. Among these, cyclohexanedimethanol diglycidyl ether (trade name: Denacol EX-216L, epoxy equivalent: 150, [C / O] ratio: 3.5, water solubility: 0% by mass), polyethylene glycol diglycidyl ether (trade name: Denacol EX-850L, epoxy equivalent: 145, [C / O] ratio: 2.0, water solubility: 100% by mass), 1,6-hexanediol diglycidyl ether (trade name: Denacol EX-212L, epoxy equivalent: 150, [C / O] ratio: 3.0, water solubility: 0% by mass), neopentyl glycol diglycidyl ether (trade name: Denacol EX-216L, epoxy equivalent: 150, [C / O] ratio: 3.5, water solubility: 0% by mass),

[0043] Preferably, one or more selected from the group consisting of diglycidyl ether (trade name: Denacol EX-211, epoxy equivalent: 138, [C / O] ratio: 2.8, water solubility: 0 mass%), trimethylolpropane polyglycidyl ether (trade name: Denacol EX-321, epoxy equivalent: 140, [C / O] ratio: 2.4 to 2.5, water solubility: 27 mass%), and pentaerythritol polyglycidyl ether (trade name: Denacol EX-411, epoxy equivalent: 229, [C / O] ratio: 2.1, water solubility: 0 mass%) are used, and cyclohexanedimethanol diglycidyl ether is more preferred.

[0037] [Crosslinked structure of polymer (A)] The polymer (A) used in the present invention has a crosslinked structure formed by a water-insoluble polymer (a) having a carboxy group and a multifunctional epoxy compound. This crosslinked structure is considered to be formed by the polymer (a) constituting the polymer (A) and the epoxy compound forming an ester group by a ring-opening reaction of the epoxy group of the epoxy compound, and the two are bonded together. At this time, the polymer (a) has multiple carboxy groups in the molecule, and the epoxy compound also has two or more epoxy groups in the molecule, so that the two-dimensional structure of the polymer (a) becomes a three-dimensional structure by crosslinking with the epoxy compound.

[0038] From the viewpoint of improving the storage stability of the aqueous pigment dispersion and the redispersibility of the aqueous ink, the degree of crosslinking of the polymer (A) is preferably 20 mol % or more, more preferably 25 mol % or more, even more preferably 30 mol % or more, still more preferably 35 mol % or more, and is preferably 90 mol % or less, more preferably 85 mol % or less, even more preferably 80 mol % or less, and still more preferably 75 mol % or less. Here, the degree of crosslinking (mol %) is calculated by "(mol number of epoxy groups of the epoxy compound in the water-based pigment dispersion / mol number of carboxy groups of the polymer (a) in the water-based pigment dispersion)×100".

[0039] [Neutralization] At least a part of the carboxyl groups of the polymer (A) is neutralized with an alkali metal compound from the viewpoint of improving the storage stability of the aqueous pigment dispersion and the image density of the aqueous ink, i.e., at least a part of the carboxyl groups is in a state in which a carboxylate anion and an alkali metal cation are ionic-bonded to each other. The alkali metal compound is as explained in the section on the neutralization of the carboxy group of the polymer (a) above, and is preferably an alkali metal hydroxide, more preferably sodium hydroxide or potassium hydroxide, and even more preferably sodium hydroxide.

[0040] From the viewpoint of improving the storage stability of the resulting aqueous pigment dispersion and aqueous ink, the degree of neutralization of the polymer (A) is preferably 70 mol % or more, more preferably 80 mol % or more, even more preferably 90 mol % or more, and is preferably 500 mol % or less, more preferably 300 mol % or less, even more preferably 200 mol % or less. Here, the degree of neutralization (mol %) of the polymer (A) is calculated by the following formula. Degree of neutralization of polymer (A) (mol %)=[number of moles of alkali metal compound in aqueous pigment dispersion / number of moles of carboxyl group of polymer (A) in aqueous pigment dispersion]×100 Here, the number of moles of carboxy groups in the polymer (A) is the difference obtained by subtracting the number of moles of the polyfunctional epoxy compound used in preparing the polymer (A) from the number of moles of carboxy groups in the water-insoluble polymer (a) having a carboxy group used in preparing the polymer (A). In the present invention, the degree of neutralization of polymer (A) is calculated from the number of moles of carboxy groups in polymer (A) and the number of moles of the alkali metal compound, and therefore, when the amount of the alkali metal compound used is in excess of the number of moles of carboxy groups in polymer (A), the value may exceed 100 mol %.

[0041] The crosslinked structure of polymer (A) is formed by obtaining an intermediate by finely dispersing polymer (a) and a pigment in an aqueous medium until the particle size reaches a predetermined size, and then reacting some or all of the carboxy groups of polymer (a) with a polyfunctional epoxy compound to form chemical bonds, thereby forming the form of water-insoluble polymer (A) particles containing the pigment.

[0042] [Condition 2]: Amount of alkali metal compound relative to pigment p The water-based pigment dispersion of the present invention satisfies the following [Condition 2] from the viewpoint of improving the storage stability of the water-based pigment dispersion and the image density of the water-based ink. [Condition 2]: The amount p of the alkali metal compound relative to the pigment in the aqueous pigment dispersion, as represented by the following formula (1), is 0.07 or more and 0.19 or less when the pigment is an organic pigment, and is 0.15 or more and 0.45 or less when the pigment is carbon black. Amount of alkali metal compound p (mmol / g) = [molar amount of alkali metal compound in aqueous pigment dispersion (mmol) / mass of pigment in aqueous pigment dispersion (g)] (1) In the present invention, by satisfying [Condition 2], the repulsive force due to the electric charge obtained by neutralizing the carboxyl group of the polymer (A) can be increased to such an extent that even if the volatile components in the water-based ink evaporate once and the pigment particles aggregate, the water-based ink can be redispersed and regain a dispersed state when it comes into contact with a liquid component again. At this time, the interaction with the calcium salt present in the plain paper can be maximized, and the image density can be increased. When the pigment is an organic pigment, the amount p of the alkali metal compound relative to the pigment is preferably 0.1 or more, more preferably 0.11 or more, even more preferably 0.12 or more, still more preferably 0.13 or more, and is preferably 0.18 or less, more preferably 0.17 or less, even more preferably 0.16 or less, and still more preferably 0.15 or less. Furthermore, when the pigment is carbon black, the p is preferably 0.18 or more, more preferably 0.2 or more, even more preferably 0.22 or more, still more preferably 0.25 or more, and is preferably 0.42 or less, more preferably 0.4 or less, even more preferably 0.35 or less, and still more preferably 0.32 or less.

[0043] From the viewpoint of improving the storage stability of the water-based pigment dispersion and the redispersibility of the water-based ink, it is preferable that the water-based pigment dispersion of the present invention satisfies the following [Condition 3]. [Condition 3]: The pigment ratio x in the aqueous pigment dispersion, represented by the following formula (2), is 0.85 or more and 0.98 or less when the pigment is an organic pigment, and is 0.82 or more and 0.95 or less when the pigment is carbon black. Pigment ratio x = [mass of pigment in aqueous pigment dispersion / [mass of pigment in aqueous pigment dispersion + mass of polymer (A) in aqueous pigment dispersion]] (2) When the pigment is an organic pigment, the pigment ratio x is preferably 0.88 or more, more preferably 0.90 or more, even more preferably 0.91 or more, and preferably 0.97 or less, more preferably 0.96 or less, even more preferably 0.95 or less. When the pigment is carbon black, the pigment ratio x is preferably 0.83 or more, more preferably 0.84 or more, even more preferably 0.85 or more, and is preferably 0.94 or less, more preferably 0.92 or less, even more preferably 0.91 or less.

[0044] [Method of manufacturing water-based pigment dispersion] The water-based pigment dispersion of the present invention can be efficiently produced by a method including the following steps 1 to 3. Step 1: A step of neutralizing at least a part of the carboxy groups of the water-insoluble polymer (a) with an alkali metal compound to obtain an aqueous dispersion of the water-insoluble polymer (a). Step 2: A step of dispersing the aqueous dispersion of the water-insoluble polymer (a) obtained in Step 1 and a pigment to obtain an aqueous pigment dispersion (i) of pigment-containing polymer particles (hereinafter also referred to as "pigment-containing polymer particles") in which the pigment is contained in the water-insoluble polymer (a). Step 3: A step of adding a polyfunctional epoxy compound to the pigment water dispersion (i) obtained in step 2, and carrying out a crosslinking treatment to obtain an aqueous pigment dispersion of polymer (A) particles having a crosslinked structure containing a pigment (hereinafter also referred to as "pigment-containing crosslinked polymer (A) particles").

[0045] (Process 1) In step 1, at least a portion of the carboxy groups of the polymer (a) is neutralized with an alkali metal compound to obtain an aqueous dispersion of the polymer (a) (hereinafter also referred to as "aqueous polymer dispersion (D)"). The neutralization in step 1 is preferably carried out so that the pH is from 7 to 11. The degree of neutralization of the alkali metal compound and polymer (a) used for neutralization is as described above. From the viewpoint of promoting sufficient and uniform neutralization, the alkali metal compound is preferably used as an aqueous solution, the concentration of which is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and is preferably 50% by mass or less, more preferably 25% by mass or less.

[0046] (Process 2) In step 2, the aqueous dispersion of the water-insoluble polymer (a) obtained in step 1 and a pigment are subjected to a dispersion treatment to obtain an aqueous pigment dispersion (i) of pigment-containing polymer particles. In the dispersion treatment in step 2, the pigment particles can be atomized to a desired particle size only by main dispersion using shear stress. However, from the viewpoint of obtaining a uniform aqueous 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 dispersing blade can be used.

[0047] Examples of dispersing machines used in the dispersion include kneading machines such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersing machines 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 aqueous dispersion (i) 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.

[0048] When the pigment mixture contains an organic solvent, the organic solvent can be removed by a known method to obtain the pigment water dispersion (i). It is preferable that the organic solvent in the obtained pigment water dispersion (i) is substantially removed, but it may remain, for example, at about 0.1 mass % or less, as long as it does not impair the object of the present invention. The non-volatile component concentration (solid content concentration) of the obtained pigment water dispersion (i) is preferably 10% by mass or more, more preferably 15% by mass or more, from the viewpoint of improving dispersion stability and facilitating the preparation of an aqueous ink, and is preferably 30% by mass or less, more preferably 25% by mass or less. The solids concentration of the pigment water dispersion (i) is measured by the method described in the examples.

[0049] The cumulant average particle size of the pigment water dispersion (i) is preferably 50 nm or more, more preferably 60 nm or more, and even more preferably 70 nm or more, from the viewpoints of reducing coarse particles and improving storage stability and image density, and is preferably 200 nm or less, more preferably 160 nm or less, and even more preferably 150 nm or less. The cumulant average particle size of the pigment water dispersion (i) is measured by the method described in the Examples. The pigment ratio x' in the pigment water dispersion (i) obtained in step 2 is the same as the preferred range of the pigment ratio x in the above-mentioned water-based pigment dispersion.

[0050] (Step 3) In step 3, a polyfunctional epoxy compound is added to the pigment aqueous dispersion (i) obtained in step 2, and the polymer (a) in which the pigment is dispersed is crosslinked to obtain an aqueous pigment dispersion of pigment-containing crosslinked polymer (A) particles. The preferred epoxy compound and the degree of crosslinking are as described above. From the viewpoints of completion of the crosslinking reaction and economic efficiency, the temperature of the crosslinking treatment is preferably 50° C. or higher, more preferably 70° C. or higher, and preferably 95° C. or lower, more preferably 92° C. or lower. From the same viewpoints as above, the time of the crosslinking treatment is preferably 0.5 hours or higher, more preferably 1 hour or higher, and preferably 10 hours or lower, more preferably 6 hours or lower. From the viewpoint of improving dispersion stability and facilitating the production of ink, the non-volatile component concentration (solid content concentration) of the aqueous pigment dispersion is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less. The solid content concentration of the aqueous pigment dispersion is measured by the method described in the Examples.

[0051] (Water-based pigment dispersion) From the viewpoint of improving the storage stability and the image density of the water-based ink, the content of the pigment in the water-based pigment dispersion is preferably 4% by mass or more, more preferably 8% by mass or more, even more preferably 10% by mass or more, and is preferably 25% by mass or less, more preferably 22% by mass or less, even more preferably 20% by mass or less. From the viewpoint of improving storage stability, the content of the polymer (A) in the aqueous pigment dispersion is preferably 0.1 mass % or more, more preferably 0.2 mass % or more, even more preferably 0.4 mass % or more, and is preferably 6 mass % or less, more preferably 3 mass % or less, even more preferably 2 mass % or less.

[0052] [Water-based ink] The water-based ink of the present invention contains the water-based pigment dispersion of the present invention and a water-soluble organic solvent. The term "water-soluble" in the context of a water-soluble organic solvent means the property of being miscible with water in any ratio. Examples of the water-soluble organic solvent include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, sulfur-containing compounds, etc. Among these, from the viewpoint of improving the image density of the water-based ink, one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers are preferred. As the polyhydric alcohol, polyhydric alcohols having a boiling point of 250° C. or higher, such as glycerin, triethylene glycol, and tripropylene glycol, are more preferred, and a combination of glycerin and triethylene glycol is even more preferred.

[0053] The water-based ink of the present invention may further contain, as necessary, various additives that are commonly used in inks, such as a fixing aid, a humectant, a wetting agent, a penetrating agent, a surfactant, a viscosity modifier, an antifoaming agent, an antiseptic, an antifungal agent, and an antirust agent. The fixing aid may be an emulsion containing water-insoluble polymer particles that do not contain a pigment. Examples of the water-insoluble polymer particles include particles of condensation resins such as polyurethane and polyester; and vinyl resins such as (meth)acrylic resins, styrene resins, styrene-(meth)acrylic resins, butadiene resins, styrene-butadiene resins, vinyl chloride resins, vinyl acetate resins, and acrylic silicone resins.

[0054] (Pigment content) The content of the pigment in the water-based ink is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 2.5% by mass or more from the viewpoint of improving image density, and is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less from the viewpoint of reducing the ink viscosity when the solvent evaporates and improving storage stability at high temperatures. (Polymer (A) content) From the viewpoint of improving storage stability and redispersibility, the content of polymer (A) in the water-based ink is preferably 0.1 mass % or more, more preferably 0.2 mass % or more, even more preferably 0.3 mass % or more, and is preferably 5 mass % or less, more preferably 3 mass % or less, even more preferably 2 mass % or less, and still more preferably 1 mass % or less. (Water-soluble organic solvent content) From the viewpoints of improving storage stability and image density, improving fixability and glossiness, and moisturizing properties, the content of the water-soluble organic solvent in the water-based ink is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. From the viewpoints of fixability, storage stability, and redispersibility, the water content in the water-based ink is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less.

[0055] (Water-based ink properties) From the viewpoint of improving storage stability and image density, the viscosity of the water-based ink at 32°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 5 mPa·s or more, and preferably 12 mPa·s or less, more preferably 9 mPa·s or less, even more preferably 7 mPa·s or less. The viscosity of the water-based ink can be measured using an E-type viscometer. From the viewpoint of improving storage stability and image density, the pH of the water-based ink is preferably 7.0 or more, more preferably 7.2 or more, and even more preferably 7.5 or more. From the viewpoint of member resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less, and even more preferably 9.5 or less. The pH of the water-based ink can be measured by a conventional method. EXAMPLES

[0056] In the following Preparation Examples, Production Examples, Examples and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The various physical properties were measured by the following methods.

[0057] (1) Measurement of weight-average molecular weight of water-insoluble polymer (a) The measurement was performed by gel permeation chromatography (Tosoh Corporation, GPC apparatus (HLC-8320GPC), Tosoh Corporation columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolum Super AW-H), flow rate: 0.5mL / min) using a solution of phosphoric acid and lithium bromide dissolved in N,N-dimethylformamide to a concentration of 60mmol / L and 50mmol / L, respectively, as an eluent, and using a 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), Tosoh Corporation) as a standard substance. The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25° C. for 10 hours, and filtering with a syringe filter (DISMIC-13HP PTFE 0.2 μm, manufactured by Advantec Co., Ltd.).

[0058] (2) Measurement of the acid value of water-insoluble polymer (a) The measurements were performed according to JIS K0070, except that the measurement solvent was changed from a mixed solvent of ethanol and ether to a mixed solvent of ethanol and toluene [ethanol:toluene=1:1 (volume ratio)].

[0059] (3) Measurement of cumulant average particle size of pigment dispersion 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 between the incident light and the detector of 90°, and 100 integrations. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The concentration of the measurement sample was 5×10 -3 % (converted into solids concentration).

[0060] (4) Measurement of solids concentration of pigment dispersion Using an infrared moisture meter (FD-230, manufactured by Kett Electric Laboratory Co., Ltd.), 5 g of the measurement sample was dried at a drying temperature of 150°C and measurement mode 96 (monitoring time 2.5 minutes, moisture content fluctuation range 0.05%), and the moisture content (mass%) of the pigment dispersion was measured. The solid content concentration (mass%) was calculated using the following formula. Solid content concentration (mass%) = 100-moisture content (mass%)

[0061] Preparation Example 1 (Preparation of Water-Insoluble Polymer (a1)) A raw material monomer mixture was prepared by mixing 30.8 parts of acrylic acid (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., reagent), 59.2 parts of styrene (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., reagent), and 10.0 parts of α-methylstyrene (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., reagent). Into a reaction vessel, 10 parts of methyl ethyl ketone (MEK), 0.3 parts of 2-mercaptopropionic acid (polymerization chain transfer agent), and 10% of the raw material monomer mixture were placed and mixed, and the atmosphere was thoroughly replaced with nitrogen gas. Meanwhile, the remaining raw material monomer mixture, 0.27 parts of the polymerization chain transfer agent, 40 parts of MEK, and 1.1 parts of an azo radical polymerization initiator (V-501; 4,4'-azobis(4-cyanovaleric acid) manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were 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 0.15 parts of the polymerization initiator was dissolved in 2.5 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 solution of a water-insoluble polymer (a1) having a carboxy group (weight average molecular weight: 11,800, acid value 240mgKOH / g).

[0062] Preparation Examples 2 to 5 Solutions of water-insoluble polymers (a2) to (a5) having a carboxy group were obtained in the same manner as in Preparation Example 1, except that the composition of the raw material monomers was changed as shown in Table 1. The results are shown in Table 1.

[0063] [Table 1]

[0064] Production Example 1-1 (Step 1: Production of polymer aqueous dispersion (D1)) 271 parts of ion-exchanged water and 48.6 parts of 5N aqueous sodium hydroxide solution (sodium hydroxide solid content 16.9%) as an alkali metal compound (neutralizing agent) were added to 80 parts of water-insoluble polymer (a1) having a carboxy group obtained by drying the polymer solution obtained in Preparation Example 1 under reduced pressure, and the ratio of the number of moles of sodium hydroxide to the number of moles of carboxy groups in the water-insoluble polymer (a1) was 60 mol% (neutralization degree 60 mol%). The resulting aqueous solution was heated at 90°C for 5 hours while stirring at 150 rpm to obtain a polymer aqueous dispersion (D1) (polymer (a1) concentration 20%).

[0065] Manufacturing Examples 1-2 to 1-7 Polymer aqueous dispersions (D2) to (D7) were obtained in the same manner as in Production Example 1-1, except that the type of water-insoluble polymer (a), the amount of sodium hydroxide aqueous solution, or the amount of ion-exchanged water was changed as shown in Table 2. The results are shown in Table 2.

[0066] [Table 2]

[0067] Production Example 2-1 (Step 2: Production of Pigment Water Dispersion (i-1)) 400 parts of ion-exchanged water and 100 parts of magenta pigment (PV19: Pigment Violet 19, manufactured by Dainichi Seika Chemicals Co., Ltd.) were added to 26.3 parts of the polymer aqueous dispersion (D1) obtained in Production Example 1-1, and the mixture was stirred for 60 minutes using a disperser (Ultra Disperser, manufactured by Asada Iron Works Co., Ltd.) at 20° C. with a disperser blade rotating at 7,000 rpm. The resulting mixture was subjected to 10-pass dispersion treatment at a pressure of 200 MPa using a Microfluidizer (trade name, manufactured by Microfluidics Co., Ltd.) to obtain a dispersion treatment product. The obtained dispersion was filtered using a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a filter having a pore size of 5 μm (manufactured by Sartorius Stedim Biotech, Mini Sart Syringe Filter, filter diameter: 26 mm, material: cellulose acetate) to remove coarse particles, and then ion-exchanged water was added to give a solids concentration of 20%, thereby obtaining a pigment water dispersion (i-1). In the resulting pigment water dispersion (i-1), the pigment ratio x', represented by the following formula, was 0.95. Pigment ratio x' = [mass of pigment in pigment water dispersion / [mass of pigment in pigment water dispersion + mass of polymer in pigment water dispersion]] In addition, the cumulant average particle size of the pigment water dispersion (i-1) was measured 1 hour after preparation and was found to be 121 nm.

[0068] Production Examples 2-2 to 2-8 and Comparative Production Examples 2-1 to 2-10 Pigment aqueous dispersions (i-2) to (i-8), (i-C2) to (i-C8), and (i-C10) were obtained in the same manner as in Production Example 2-1, except that the type of polymer aqueous dispersion, the type of pigment, or the amount of ion-exchanged water was changed as shown in Table 3. The results are shown in Table 3. In addition, in Comparative Production Examples 2-1 and 2-9 in Table 3, the obtained products could not be filtered through a filter having a pore size of 5 μm, and the corresponding pigment water dispersions (i-C1) and (i-C9) could not be obtained. Details of the pigments listed in Table 3 are as follows: PV19: Magenta pigment (Dainichiseika Color & Chemicals Co., Ltd., Pigment Violet 19) PB15:3: Copper phthalocyanine pigment (DIC Corporation, Pigment Blue 15:3) PY74: Yellow pigment (Sanyo Pigment Co., Ltd., Pigment Yellow 74) M717: Black pigment (Cabot Corporation, MONARCH 717 Carbon Black)

[0069] [Table 3]

[0070] Example 1-1 (Step 3: Production of Water-Based Pigment Dispersion 1) 100 parts (20% solids content) of the pigment water dispersion (i-1) obtained in Production Example 2-1 was placed in a screw-capped glass bottle, and 0.26 parts of cyclohexanedimethanol diglycidyl ether (Denacol EX-216L, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 150, [C / O] ratio: 3.5, water solubility: 0%) was added as a polyfunctional epoxy compound so that 40 mol % of the total carboxy groups in the water-insoluble polymer (a1) were crosslinked (degree of crosslinking: 40 mol %). The bottle was then sealed and immersed in a hot water bath at 90° C. while stirring with a stirrer. After 2 hours, the mixture was removed from the hot water bath and allowed to cool to room temperature, then filtered with a 25 mL needleless syringe (Terumo Corporation) equipped with a 5 μm pore size filter (Mini Sart Syringe Filter, Sartorius Stedim Biotech, filter diameter: 26 mm, material: cellulose acetate), and ion-exchanged water was added to adjust the solids concentration to 20%, preparing aqueous pigment dispersion 1. The cumulant average particle size of aqueous pigment dispersion 1 was measured 1 hour after preparation and was found to be 132 nm. In addition, 5 g of the obtained aqueous pigment dispersion 1 was taken out and placed in a 10 mL screw tube (manufactured by Maruemu Co., Ltd.), which was then sealed and allowed to stand for 24 hours in a thermostatic chamber (manufactured by Espec Corporation, PU-1KT) set at 70°C. After that, the cumulant average particle size was measured again and was found to be 133 nm. The cumulant average particle size after standing at 70°C for 7 days indicates the stability of the aqueous pigment dispersion, and is preferably within 1.05 times the cumulant average particle size measured 1 hour after preparation. An aqueous pigment dispersion with a cumulant average particle size exceeding 1.05 times means that the particle size increases over time, and the quality deteriorates during the manufacturing and distribution processes.

[0071] Examples 1-2 to 1-11 and Comparative Examples 1-1 to 1-9 Water-based pigment water dispersions 2 to 11 and C1 to C9 were obtained in the same manner as in Example 1-1, except that in step 3 shown in Example 1-1, the type or amount of the pigment water dispersion (i) or the epoxy compound was changed as shown in Tables 4 and 5. The results are shown in Tables 4 and 5. In Tables 4 and 5, EX-850L is a multifunctional epoxy compound manufactured by Nagase ChemteX Corporation, Denacol EX-850L (diethylene glycol diglycidyl ether, epoxy equivalent: 145, [C / O] ratio: 2.0, water solubility: 100% by mass), and EX-321 is a multifunctional epoxy compound manufactured by Nagase ChemteX Corporation, Denacol EX-321 (trimethylolpropane polyglycidyl ether, epoxy equivalent: 140, [C / O] ratio: 2.4 to 2.5, water solubility: 27% by mass). Moreover, the pigment ratio x in Tables 4 and 5 is a value represented by the above formula (2).

[0072] [Table 4]

[0073] [Table 5]

[0074] From Tables 4 and 5, it can be seen that the aqueous pigment dispersions obtained in Examples 1-1 to 1-11 have superior storage stability compared to the aqueous pigment dispersions obtained in Comparative Examples 1-1 to 1-2 which do not satisfy Condition 1 (acid value of the water-insoluble polymer (a)) and the aqueous pigment dispersion obtained in Comparative Example 1-6 in which the polymer used to disperse the pigment does not have a crosslinked structure.

[0075] Example 2-1 (Preparation of Water-Based Ink 1) Using the obtained aqueous pigment dispersion 1, the content of the pigment in the aqueous ink was 5.6 parts, the content of glycerin (Kao Corporation, concentrated glycerin for cosmetics) as an organic solvent was 18 parts, the content of triethylene glycol (Fujifilm Wako Pure Chemical Industries, Ltd., reagent) was 10 parts, the content of nonionic surfactant (Kawaken Fine Chemicals Co., Ltd., Acetylenol E100, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethylene oxide 10 mole adduct) was 0.5 parts, and ion-exchanged water was added to make the total amount 100 parts. Next, the mixture was stirred with a magnetic stirrer for 2 hours to mix well, and filtered with a 25 mL needleless syringe equipped with a filter with a pore size of 1.2 μm (Sartorius Stedim Biotech, Mini Sart Syringe Filter, filter diameter: 26 mm, material: cellulose acetate) to obtain aqueous ink 1.

[0076] Examples 2-2 to 2-11 and Comparative Examples 2-1 to 2-9 Water-based inks 2 to 11 and C1 to C9 were obtained in the same manner as in Example 2-1, except that in the production of water-based ink 1 in Example 2-1, each water-based pigment dispersion shown in Tables 6 and 7 was used instead of water-based pigment dispersion 1.

[0077] The water-based inks obtained in the Examples and Comparative Examples were evaluated for redispersibility of the ink and image density when printed on plain paper by the following methods. The results are shown in Tables 6 and 7.

[0078] (Ink redispersibility) A vehicle for evaluating redispersibility was prepared by adding ion-exchanged water to 5 parts of glycerin, 10 parts of triethylene glycol, and 0.5 parts of a nonionic surfactant (Acetylenol E100) to make a total volume of 100 parts, and stirring thoroughly with a magnetic stirrer. Using a micropipette, 10 μL of water-based ink was dropped onto a glass slide, and the water-based ink was evaporated to dryness by leaving it to stand for 24 hours in an environment of 60°C and 40% RH. After the specified time had passed, the glass slide was removed and left to stand for 1 hour in a glass desiccator equipped with blue silica gel at the bottom, and after returning to room temperature, the glass slide was removed from the desiccator. 200 μL of the vehicle for evaluating redispersibility was dropped onto the solid matter on the slide glass, and one minute after the vehicle for evaluating redispersibility was dropped, the solid matter was visually observed and evaluated according to the following evaluation criteria. In addition, an evaluation was also performed using the same procedure, with the sample left to stand for 48 hours in an environment of 60°C and 40% RH. (Evaluation Criteria) ◯: The solid matter was uniformly redispersed. Δ: The solid matter was redispersed, but a residue remained on the slide glass. ×: The solid matter did not redisperse and the liquid portion remained transparent. If the test result is ◯ after 48 hours of standing, it is optimal, but if the test result is ◯ after 24 hours of standing, it will not cause any problems in practical use. The results are shown in Tables 6 and 7.

[0079] (Image density on plain paper prints) An inkjet printer (Seiko Epson Corporation, model number: EW-M-660FT, piezoelectric type) was filled with the water-based inks 1 to 12 and the comparative water-based inks C1 to C8, respectively, and a solid image was printed on plain paper (Xerox Corporation, product name: XEROX 4200) under the following printing conditions: paper type: plain paper, print quality: standard, at room temperature of 23° C. and relative humidity of 50%, to obtain a plain paper print. The image density of the plain paper prints was measured using a spectrophotometer (X-Rite, model number: Spectro Eye) under the following measurement conditions: observation light source D65, observation field of view 2 degrees, density standard DIN16536, and the numerical values ​​of the color density components of each color were read. Measurements were taken at different locations, and the average of a total of 10 points was calculated. The higher the numerical value, the higher the image density. The results are shown in Table 5. Although a high image density is preferable, in practical use, there is no problem if the image density is 0.90 or more for color inks (organic pigments) and 1.05 or more for black inks (carbon black).

[0080] [Table 6]

[0081] [Table 7]

[0082] From Tables 6 and 7, it can be seen that the water-based inks obtained in Examples 2-1 to 2-11 can achieve both excellent redispersibility and high image density, as compared to the water-based inks obtained in Comparative Examples 2-1 to 2-9. The water-based inks obtained in Comparative Examples 2-1 to 2-2 do not satisfy condition 1 (acid value of water-insoluble polymer (a)), and the water-based ink obtained in Comparative Example 2-6 has poor redispersibility because the polymer used to disperse the pigment does not have a crosslinked structure. The water-based inks obtained in Comparative Examples 2-3 to 2-5 and 2-7 to 2-9 did not satisfy condition 2 (amount of alkali metal compound relative to pigment), and therefore had low image density. [Industrial Applicability]

[0083] The aqueous pigment dispersion of the present invention has excellent storage stability, and an aqueous ink containing the aqueous pigment dispersion has high redispersibility and can provide a recorded matter having a high image density even in ink-jet recording on plain paper.

Claims

1. An aqueous pigment dispersion containing a water-insoluble polymer (a) having a crosslinked structure formed by a carboxy group and a polyfunctional epoxy compound, and a pigment, wherein at least a portion of the carboxy groups of the polymer (A) are neutralized with an alkali metal compound, and the aqueous pigment dispersion satisfies the following [Condition 1], [Condition 2], and [Condition 3]: [Condition 1]: The acid value of the water-insoluble polymer (a) is 180 mgKOH / g or more and 320 mgKOH / g or less. [Condition 2]: The amount p of the alkali metal compound relative to the pigment in the aqueous pigment dispersion, as represented by the following formula (1), is 0.07 or more and 0.19 or less when the pigment is an organic pigment, and is 0.15 or more and 0.45 or less when the pigment is carbon black. Amount of alkali metal compound p (mmol / g)=[molar amount of alkali metal compound in aqueous pigment dispersion (mmol) / mass of pigment in aqueous pigment dispersion (g)] (1) [Condition 3]: The pigment ratio x in the aqueous pigment dispersion, represented by the following formula (2), is 0.85 or more and 0.98 or less when the pigment is an organic pigment, and is 0.85 or more and 0.95 or less when the pigment is carbon black. Pigment ratio x=[mass of pigment in water-based pigment dispersion / [mass of pigment in water-based pigment dispersion+mass of polymer (A) in water-based pigment dispersion]] (2) the pigment is at least one selected from carbon black and organic pigments, The aqueous pigment dispersion has a crosslinking degree of the polymer (A) calculated by the following formula of 40 mol % or more and 90 mol % or less. Degree of crosslinking (mol %)=(number of moles of epoxy groups of epoxy compound in aqueous pigment dispersion / number of moles of carboxy groups of polymer (a) in aqueous pigment dispersion)×100

2. The aqueous pigment dispersion according to claim 1 , wherein the degree of neutralization of the water-insoluble polymer (a) calculated by the following formula is from 20 mol % to 150 mol %. Degree of neutralization (mol %)=[number of moles of alkali metal compound in aqueous pigment dispersion / number of moles of carboxyl group of polymer (A) in aqueous pigment dispersion]×100

3. 3. The aqueous pigment dispersion according to claim 1, wherein the polyfunctional epoxy compound has a [number of carbon atoms / number of oxygen atoms] ratio of 2.0 or more and 4.0 or less.

4. 4. The water-based pigment dispersion according to claim 1, wherein the pigment is in the form of water-insoluble polymer (A) particles containing the pigment.

5. 5. The water-based pigment dispersion according to claim 1, wherein the water-insoluble polymer (a) is a styrene-α-methylstyrene-acrylic acid copolymer.

6. The aqueous pigment dispersion according to any one of claims 1 to 5, wherein the degree of neutralization of the polymer (A) calculated by the following formula is 70 mol % or more and 500 mol % or less. Degree of neutralization (mol %) of polymer (A)=[number of moles of alkali metal compound in aqueous pigment dispersion / number of moles of carboxyl group of polymer (A) in aqueous pigment dispersion]×100

7. 7. A water-based ink comprising the water-based pigment dispersion according to claim 1 and a water-soluble organic solvent.

8. The water-based ink according to claim 7, which is for ink-jet recording.

9. A method for producing the aqueous pigment dispersion according to any one of claims 1 to 6, comprising the following steps 1 to 3: Step 1: A step of neutralizing at least a part of the carboxy groups of the water-insoluble polymer (a) with an alkali metal compound to obtain an aqueous dispersion of the water-insoluble polymer (a). Step 2: A step of dispersing the aqueous dispersion of the water-insoluble polymer (a) obtained in step 1 and a pigment to obtain an aqueous pigment dispersion (i) of polymer particles containing the pigment in which the pigment is contained in the water-insoluble polymer (a). Step 3: A step of adding a polyfunctional epoxy compound to the pigment water dispersion (i) obtained in step 2, and carrying out a crosslinking treatment to obtain an aqueous pigment dispersion of polymer (A) particles having a crosslinked structure containing a pigment.

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