Method for producing water-based pigment dispersion

Abstract

To provide a method for producing a water-based pigment dispersion that, when used as a color material of inkjet ink, achieves both of high image density on plain paper and glossiness and fixability on photograph paper and coat paper, and is further excellent in re-dispersibility and filterability, and provide a water-based ink.SOLUTION: A method for producing a water-based pigment dispersion includes a step 1 for mixing and dispersing a pigment and a water-insoluble polymer (a) having a carboxy group in aqueous medium, to obtain a pigment water dispersion (I) with a pigment ratio x of 0.84-0.98, a step 2 for adding a water-insoluble polymer (a') having a carboxy group to the resultant pigment water dispersion (I), to obtain a pigment water dispersion (II), and a step 3 for adding a polyfunctional epoxy compound to the resultant pigment water dispersion (II) for crosslinking at 50°C or higher. The polymer (a') has the same structure as the polymer (a). The difference between the neutralization level of the polymer (a) and the neutralization level of the polymer (a') is 10-145 mol%.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing an aqueous pigment dispersion, and to an aqueous ink. [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] However, when the inkjet recording ink using a pigment is printed on plain paper, the pigment penetrates into the paper fibers after the ink droplets land, resulting in a problem of lower image density compared to ink using a dye. In order to solve this problem, the amount of dispersant used relative to the pigment is reduced in an attempt to reduce the dispersibility of the pigment dispersion so that the pigment quickly aggregates after the ink droplets land, but this leads to problems such as impaired ink storage stability and ejection properties, and further impaired gloss and fixability when printed on photo paper or coated paper. In addition, reducing the amount of dispersant used relative to the pigment can also deteriorate the filterability of the pigment dispersion obtained when preparing the pigment dispersion, which can cause production problems.

[0004] Various proposals have been made as methods for producing water dispersions for ink-jet printing that can solve these problems. For example, Patent Document 1 discloses a method for producing a water dispersion for inkjet printing which is excellent in dispersion stability and ejection stability and can impart a high print density, the method comprising a step 1 of obtaining a pigment treated with water-insoluble vinyl polymer (A) particles in an aqueous solvent, and a step 2 of dispersing a mixture containing the obtained pigment, an organic solvent, the water-insoluble vinyl polymer (B), and water, and then removing the organic solvent to obtain an aqueous dispersion of water-insoluble vinyl polymer particles containing the pigment. Patent Document 2 discloses a method for producing a colorant aqueous dispersion used in a water-based ink for ink-jet recording, which has excellent jetting properties, excellent printing density, and low viscosity, comprising a step (I) of dispersing a colorant in a water-soluble polymer (x) and water to obtain an aqueous dispersion, and a step (II) of adding a water-insoluble polymer (y) to the obtained aqueous dispersion and further dispersing the mixture to obtain a dispersion containing polymer particles (A) containing a colorant. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2005-42095 A [Patent Document 2] JP 2011-144348 A Summary of the Invention [Problem to be solved by the invention]

[0006] With regard to the ink-jet inks using the pigment dispersions obtained by the methods of Patent Documents 1 and 2, further improvements in image density, fixability, and gloss have been desired. An object of the present invention is to provide a method for producing an aqueous pigment dispersion which, when used as a colorant in an inkjet ink, can achieve both high image density on plain paper and gloss and fixability on photo paper or coated paper, and which further has excellent redispersibility for obtaining good ejection stability, and excellent filterability for increasing productivity, and to provide an aqueous ink. [Means for solving the problem]

[0007] The present inventors have discovered that the above-mentioned problems can be solved by performing the mixing of a polymer dispersant and the pigment, dispersion, and crosslinking treatment in three separate steps under specific conditions when obtaining an aqueous pigment dispersion. That is, the present invention provides the following [1] and [2]. [1] A method for producing an aqueous pigment dispersion, comprising the following steps 1 to 3: Step 1: A step of mixing and dispersing a pigment and a water-insoluble polymer (a) having a carboxy group in an aqueous medium to obtain a pigment water dispersion (I) having a pigment ratio x of 0.84 or more and 0.98 or less, as represented by the following formula (1): Pigment ratio x = [mass of pigment in pigment water dispersion (I)] / [mass of pigment in pigment water dispersion (I) + mass of water-insoluble polymer (a) in pigment water dispersion (I)] (1) Step 2: A step of adding a water-insoluble polymer (a') having a carboxy group to the pigment water dispersion (I) obtained in step 1 to obtain a pigment water dispersion (II). Here, the water-insoluble polymer (a') has the same composition as the water-insoluble polymer (a), and differs only in the degree of neutralization, and the difference between the degree of neutralization a of the water-insoluble polymer (a) and the degree of neutralization a' of the water-insoluble polymer (a') [neutralization degree a - neutralization degree a'] is 10 mol % or more and 145 mol % or less. Step 3: A step of adding a polyfunctional epoxy compound to the pigment water dispersion (II) obtained in step 2, and crosslinking the mixture at a temperature of 50° C. or higher to obtain an aqueous pigment dispersion. [2] A water-based ink comprising the water-based pigment dispersion obtained in [1] above and a water-soluble organic solvent. Effect of the Invention

[0008] According to the present invention, it is possible to provide a method for producing an aqueous pigment dispersion which, when used as a coloring material for an ink-jet ink, can achieve both high image density on plain paper and gloss and fixability on photo paper or coated paper, and further has excellent redispersibility for obtaining good ejection stability, and excellent filterability for increasing productivity, and a aqueous ink. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Method of manufacturing water-based pigment dispersion] The method for producing the water-based pigment dispersion of the present invention includes the following steps 1 to 3. Step 1: A step of mixing and dispersing a pigment and a water-insoluble polymer (a) having a carboxy group in an aqueous medium to obtain a pigment water dispersion (I) having a pigment ratio x of 0.84 or more and 0.98 or less, as represented by the following formula (1): Pigment ratio x = [mass of pigment in pigment water dispersion (I)] / [mass of pigment in pigment water dispersion (I) + mass of water-insoluble polymer (a) in pigment water dispersion (I)] (1) Step 2: A step of adding a water-insoluble polymer (a') having a carboxy group to the pigment water dispersion (I) obtained in step 1 to obtain a pigment water dispersion (II). Here, the water-insoluble polymer (a') has the same composition as the water-insoluble polymer (a), and differs only in the degree of neutralization, and the difference between the degree of neutralization a of the water-insoluble polymer (a) and the degree of neutralization a' of the water-insoluble polymer (a') [neutralization degree a - neutralization degree a'] is 10 mol % or more and 145 mol % or less. Step 3: A step of adding a polyfunctional epoxy compound to the pigment water dispersion (II) obtained in step 2, and crosslinking the mixture at a temperature of 50° C. or higher to obtain an aqueous pigment dispersion. 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.

[0010] According to the manufacturing method of the present invention, it is possible to obtain an aqueous pigment dispersion that is excellent in redispersibility and filterability, and that can achieve both high image density on plain paper and gloss and fixability on photo paper or coated paper. The reason for this is unclear, but is thought to be as follows. The manufacturing method of the present invention is characterized in that the manufacturing process includes an operation of adding water-insoluble polymers (a) and (a') having the same structure and a carboxyl group separately in steps 1 and 2, and the degree of neutralization of the water-insoluble polymer (a) used in step 1 is greater than the degree of neutralization of the water-insoluble polymer (a') used in step 2. This is believed to allow the pigment to be finely ground when the pigment is mixed and dispersed, and the water-insoluble polymers (a) and (a') to be efficiently adsorbed thereto. The reason why the water-insoluble polymer can be efficiently adsorbed to the pigment is believed to be that the higher the hydrophilicity of the water-insoluble polymer, i.e., the higher the neutralization degree of the water-insoluble polymer, the higher the wettability of the pigment. Furthermore, in the manufacturing method of the present invention, a multifunctional epoxy compound is added in step 3 and crosslinked at a temperature of 50°C or higher, so that the polymer serving as a dispersant can be firmly bound to the pigment. This is believed to improve the dispersion stability, redispersibility and filterability of the resulting aqueous pigment dispersion, as well as to achieve high image density on plain paper and improved gloss and fixability on photo paper and coated paper.

[0011] <Process 1> Step 1 is a step of mixing and dispersing a pigment and a water-insoluble polymer (a) having a carboxy group in an aqueous medium to obtain a pigment water dispersion (I) in which the pigment ratio x in the pigment water dispersion (I), represented by the following formula (1), is 0.84 or more and 0.98 or less. Pigment ratio x = [mass of pigment in pigment water dispersion (I)] / [mass of pigment in pigment water dispersion (I) + mass of water-insoluble polymer (a) in pigment water dispersion (I)] (1)

[0012] [Pigments] The pigments used in the present invention may be either inorganic or organic pigments, including lake pigments and fluorescent pigments, and may also be used in combination with extender pigments, if necessary. Specific examples of inorganic pigments include carbon black, metal oxides such as titanium oxide, iron oxide, red iron oxide, and chromium oxide, and pearlescent pigments. In particular, carbon black is preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and threne pigments.

[0013] There are no particular limitations on the hue, and any of achromatic pigments such as white, 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.

[0014] In the present invention, the pigment is dispersed in the water-based pigment dispersion by the water-insoluble polymer (a) or (a') having a carboxy group and a polymer having a crosslinked structure formed by a polyfunctional epoxy compound. From the viewpoint of improving redispersibility and filterability and improving image density, gloss and fixability of the resulting recorded matter, the pigment is preferably in the form of water-insoluble polymer particles containing the pigment, i.e., a form in which the polymer encompasses the pigment, a form in which a part of the pigment is exposed on the surface of a particle consisting of the polymer and the pigment, a form in which the polymer is adsorbed to a part of the pigment, a form in which the pigment is attached to a particle of polymer (A), or a mixture of these. In the explanation of the pigment dispersion form, the polymer means all of the water-insoluble polymers (a) and (a') having a carboxy group and the polymer obtained through steps (1) to (3).

[0015] [Water-insoluble polymer (a) having a carboxy group] The acid value of the water-insoluble polymer (a) having a carboxy group (hereinafter simply referred to as "polymer (a)") is derived from the carboxy group, and the acid value is preferably 140 mgKOH / g or more and 300 mgKOH / g or less. From the viewpoint of enhancing the storage stability of the aqueous pigment dispersion, the acid value of the polymer (a) is preferably 150 mgKOH / g or more, more preferably 170 mgKOH / g or more, even more preferably 190 mgKOH / g or more, and is preferably 290 mgKOH / g or less, more preferably 280 mgKOH / g or less, even more preferably 260 mgKOH / g or less. If the acid value is within the above range, the amount of carboxy groups is sufficient. 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.

[0016] 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.

[0017] [(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 according to 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.

[0018] [(a-2) Hydrophobic vinyl monomer] The hydrophobic vinyl monomer (a-2) 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 redispersibility and filterability of the aqueous pigment dispersion and the image density of the resulting recorded matter. 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.

[0019] 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.

[0020] 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.

[0021] [(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.

[0022] 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.

[0023] 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 and improving the redispersibility and filterability of the aqueous pigment dispersion, and the image density of the obtained recorded matter, etc. The content of the (a-1) component is preferably 15 mass% or more, more preferably 18 mass% or more, even more preferably 20 mass% or more, and is 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 82% by mass or less, even more preferably 80% by mass or less.

[0024] 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 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, still more preferably 0.35 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.

[0025] [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.

[0026] 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.

[0027] [Neutralization] At least a portion of the carboxyl groups of the polymer (a) are neutralized with an alkali metal compound from the viewpoints of improving the redispersibility and filterability of the aqueous pigment dispersion, and the image density, gloss, and fixability of the resulting recorded matter. 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.

[0028] 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.

[0029] The degree of neutralization a of polymer (a) is, from the viewpoint of ensuring dispersion stability before reaction with the polyfunctional epoxy compound, and from the viewpoint of improving the image density, gloss, and fixability of the resulting recorded matter, preferably 40 mol % or more, more preferably 45 mol % or more, even more preferably 50 mol % or more, still more preferably 55 mol % or more, and is preferably 150 mol % or less, more preferably 140 mol % or less, even more preferably 130 mol % or less, and still more preferably 125 mol % or less. Here, the degree of neutralization a (mol %) of the polymer (a) is calculated by the following formula. Degree of neutralization a (mol %) of polymer (a)=[mol number of alkali metal compounds in pigment water dispersion (I) / mol number of carboxy groups of polymer (a) in pigment water dispersion (I)]×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 %. The amount (mmol / g) of the neutralizer per 1 g of pigment in the pigment water dispersion (I) obtained in step 1 is, from the viewpoint of efficiently producing the pigment water dispersion (I), preferably 0.05 to 1 mmol / g, more preferably 0.1 to 0.5 mmol / g, even more preferably 0.13 to 0.3 mmol, and still more preferably 0.14 to 0.2 mmol.

[0030] (aqueous medium) The aqueous medium used in step 1 is a medium containing water as a main component, and constitutes a pigment water dispersion (I) together with the pigment and the polymer (a). The aqueous medium may contain a medium other than water. The medium other than water may be any medium that forms a homogeneous phase at 20° C., and specific examples thereof include alcohols having 1 to 4 carbon atoms, ketones having 3 to 6 carbon atoms, ethers, esters, etc. The content of the medium other than water in the aqueous medium is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 8% by mass or less, and still more preferably 0% by mass.

[0031] In step 1, the pigment, polymer (a), and aqueous medium can be mixed and dispersed in any order. For example, there can be mentioned (i) a method in which an aqueous medium, an alkali metal compound, and polymer (a) are mixed and dispersed, and then a pigment is further mixed and dispersed, and (ii) a method in which an aqueous medium and an alkali metal compound are mixed, then a polymer (a) is further mixed, and then a pigment is further mixed and dispersed, and the above method (i) is preferred.

[0032] There is no particular limitation on the method for mixing and dispersing the components in step 1. In the dispersion treatment, the pigment particles can be atomized to a desired particle size only by main dispersion using shear stress, but from the viewpoint of obtaining a uniform pigment aqueous dispersion, it is preferable to perform pre-dispersion and then 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 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 performed 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 dispersion 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.

[0033] In step 1, when the pigment and the polymer (a) are mixed in an aqueous medium, the pigment ratio x in the pigment aqueous dispersion (I), represented by the following formula (1), is 0.84 or more and 0.98 or less from the viewpoint of improving image density. Pigment ratio x = [mass of pigment in pigment water dispersion (I)] / [mass of pigment in pigment water dispersion (I) + mass of polymer (a) in pigment water dispersion (I)] (1) From the viewpoint of improving storage stability and image density, the pigment ratio x is preferably 0.80 or more, more preferably 0.82 or more, even more preferably 0.85 or more, still more preferably 0.88 or more, and is preferably 0.99 or less, more preferably 0.98 or less, and even more preferably 0.97 or less.

[0034] In step 1, the pigment water dispersion (I) obtained is preferably in the form of an emulsion. The average particle size of the pigment water dispersion (I) is, from the viewpoint of increasing the adsorption rate of the polymer (a) to the pigment, preferably 50 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and still more preferably 90 nm or more, and from the viewpoint of increasing the dispersion stability, preferably 200 nm or less, more preferably 180 nm or less, even more preferably 150 nm or less, and still more preferably 140 nm or less. The average particle size of the pigment water dispersion can be determined by the method described in the Examples.

[0035] In step 1, the ratio of the total mass of the pigment and polymer (a) to the total mass of each component during mixing is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more from the viewpoint of economic efficiency. Also, from the viewpoint of efficiently atomizing the average particle size of the resulting pigment water dispersion (I) to 70 nm to 200 nm, the ratio is preferably 45% by mass or less, more preferably 35% by mass or less, and even more preferably 40% by mass or less. Among these, in the case of the mixing and dispersing method (i) above, it is preferable to use a high-pressure homogenizer in a range in which the ratio of the total mass of the pigment and polymer (a) to the total mass of each component during the mixing process is in the range of 15 mass% or more and 45 mass% or less.

[0036] <Process 2> Step 2 is a step of adding a water-insoluble polymer (a') having a carboxy group (hereinafter also referred to as "polymer (a')") to the pigment water dispersion (I) obtained in step 1 and mixing them to obtain a pigment water dispersion (II). The acid value of the polymer (a') is derived from the carboxy group, and the acid value is preferably 140 mgKOH / g or more and 300 mgKOH / g or less. From the viewpoint of enhancing the storage stability of the aqueous pigment dispersion, the acid value of the polymer (a') is preferably 150 mgKOH / g or more, more preferably 170 mgKOH / g or more, even more preferably 190 mgKOH / g or more, and is preferably 290 mgKOH / g or less, more preferably 280 mgKOH / g or less, even more preferably 260 mgKOH / g or less. If the acid value is within the above range, the amount of carboxyl groups is sufficient. 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.

[0037] Here, the polymer (a') has a different degree of neutralization from the polymer (a) contained in the pigment water dispersion (I), but has the same structure as the polymer (a) contained in the pigment water dispersion (I) except for the degree of neutralization. That is, the only difference between step 1 and step 2 is the degree of neutralization of the water-insoluble polymer having a carboxy group used. Polymers having the "same composition" mean that the difference in weight-average molecular weight of the polymers is within 10,000 in terms of polystyrene, the monomer species constituting the polymers are the same, and the difference in the composition ratio of each monomer species is 10% by mass or less. The weight-average molecular weight can be determined by the method described in the Examples. In the present invention, it is preferable to use polymers (a) and (a') that are all identical in weight average molecular weight, monomer species constituting the polymer, and compositional ratio of each monomer species, and to divide them into smaller portions according to the amounts used in step 1 and step 2 and adjust the neutralization degree of each portion.

[0038] From the viewpoints of ensuring the stability of the pigment water dispersion (II) and the dispersion stability before the reaction with the polyfunctional epoxy compound, and of improving the image density, gloss, and fixability of the obtained recorded matter, the degree of neutralization a' of the polymer (a') is preferably 5 mol % or more, more preferably 10 mol % or more, even more preferably 20 mol % or more, still more preferably 25 mol % or more, and is preferably 55 mol % or less, more preferably 50 mol % or less, even more preferably 45 mol % or less, and still more preferably 40 mol % or less. Here, the degree of neutralization a' (mol %) of the polymer (a') is calculated by the following formula. Degree of neutralization of polymer (a') a' (mol %) = [number of moles of alkali metal compound neutralizing polymer (a') added in step 2 / number of moles of carboxyl groups of polymer (a') added in step 2] x 100

[0039] In the method for producing the aqueous pigment dispersion of the present invention, the difference between the neutralization degree a of the polymer (a) and the neutralization degree a' of the polymer (a') [neutralization degree a-neutralization degree a'] is 10 mol % or more and 145 mol % or less, thereby achieving fine particle size of the pigment, and improving the redispersibility and filterability of the aqueous pigment dispersion obtained, as well as the image density, glossiness, and fixability of the resulting recorded matter. [Degree of neutralization a-degree of neutralization a'] is preferably 15 mol% or more, more preferably 20 mol% or more, even more preferably 25 mol% or more, and preferably 120 mol% or less, more preferably 115 mol% or less, even more preferably 100 mol% or less, still more preferably 90 mol% or less, still more preferably 70 mol% or less, and still more preferably 50 mol% or less. From the viewpoint of achieving a balance between various performances of the water-based pigment dispersion and the water-based ink obtained, it is preferably 15 mol% or more and 120 mol% or less, more preferably 20 mol% or more and 100 mol% or less, still more preferably 20 mol% or more and 90 mol% or less, and still more preferably 20 mol% or more and 70 mol% or less.

[0040] The polymer (a') added in step 2 is preferably in the form of an emulsion. The medium of this emulsion is preferably the aqueous medium described in step (1). Furthermore, the average particle size of this emulsion is preferably 5 nm or more, more preferably 10 nm or more, even more preferably 15 nm or more, and even more preferably 20 nm or more from the viewpoint of increasing the adsorption rate to particles present in the pigment water dispersion (I), and is preferably 200 nm or less, more preferably 180 nm or less, even more preferably 150 nm or less, and even more preferably 140 nm or less from the viewpoint of ensuring the dispersion stability of the emulsion. There is no particular limitation on the method for obtaining the emulsion of polymer (a'), and examples thereof include the above method (i) used in step 1. In step 2, the rate at which the polymer (a') is added to the pigment water dispersion (I) can be arbitrarily selected within the range of 1 g or more and 10 kg or less per second. The resulting pigment water dispersion (II) contains a pigment, an aqueous medium, and polymers (a) and (a'). The aqueous medium in the pigment water dispersion (II) has the same meaning as the aqueous medium described in step (1). It is preferred that no new pigment is added in step 2.

[0041] <Process 3> Step 3 is a step of adding a polyfunctional epoxy compound to the pigment water dispersion (II) obtained in step 2, and crosslinking the mixture at a temperature of 50° C. or higher to obtain an aqueous pigment dispersion. In step 3, the added polyfunctional epoxy compound reacts with the carboxy groups of the water-insoluble polymers (a) and (a') having a carboxy group contained in the pigment water dispersion (II), forming a three-dimensional network structure in the water-insoluble polymers (a) and (a') having a carboxy group. That is, in step 3, an aqueous pigment dispersion can be obtained that is composed of the pigment, the aqueous medium, and the water-insoluble polymers (a) and (a') having a carboxy group and the polymer dispersant having the polyfunctional epoxy compound as a structural unit.

[0042] In step 3, the ratio of the mass of the pigment present in the system to the sum of the mass of the polymer ((a)+(a')) also present in the system and the mass of the pigment, i.e., the pigment ratio x' in the aqueous pigment dispersion represented by the following formula (2), is preferably 0.6 or more, more preferably 0.65 or more, even more preferably 0.7 or more, from the viewpoint of improving the redispersibility and filterability of the obtained aqueous pigment dispersion, and the image density, gloss and fixability of the obtained recorded matter, and is preferably 0.85 or less, more preferably 0.8 or less, even more preferably 0.75 or less. Pigment ratio x' = [mass of pigment present in the system in step 3] / [mass of pigment present in the system in step 3 + mass of polymer ((a) + (a')) present in the system in step 3] (2)

[0043] [Multifunctional epoxy compound] The polyfunctional epoxy compound (hereinafter also referred to as "epoxy compound") reacts with the water-insoluble polymers (a) and (a') having a carboxy group, which are dispersed in the pigment aqueous dispersion (II), to form a crosslinked structure in the polymer. This crosslinked structure allows the polymer to be firmly adsorbed or fixed on the pigment surface, suppressing aggregation of the pigment in the aqueous medium and further suppressing swelling of the polymer, which is believed to improve the redispersibility and filterability of the resulting aqueous pigment dispersion, as well as the image density, glossiness, and fixability of the resulting recorded matter.

[0044] 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.

[0045] From the same viewpoint as above, the ratio of [number of carbon atoms / number of oxygen atoms] of the epoxy compound (hereinafter also referred to as "[C / O] ratio") is preferably 1.8 or more and 4.0 or less. The [C / O] ratio is more preferably 2.0 or more, even more preferably 2.4 or more, still more preferably 2.5 or more, and more preferably 3.9 or less, even more preferably 3.8 or less, still more preferably 3.7 or less. 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.

[0046] 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, diethylene glycol 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), 1,6-hexanediol diglycidyl ether (trade name: Denacol EX-212L, epoxy equivalent: 150, [C / O] ratio: 3.0), diethylene glycol diglycidyl ether (trade name: Denacol EX-850L, epoxy equivalent: 145, [C / O] ratio: 2.0, triethylene glycol diglycidyl ether (trade name: Denacol EX-850L, epoxy equivalent: 145, [C / O] ratio: 2.0), One or more selected from methylolpropane polyglycidyl ether (trade name: Denacol EX-321, epoxy equivalent: 140, [C / O] ratio: 2.4 to 2.5) and pentaerythritol polyglycidyl ether (trade name: Denacol EX-411, epoxy equivalent: 229, [C / O] ratio: 2.1) (all manufactured by Nagase ChemteX Corporation) are preferred, and cyclohexanedimethanol diglycidyl ether is more preferred.

[0047] Regarding the amount of the epoxy compound added in step 3, from the viewpoints of improving the redispersibility and filterability of the resulting aqueous pigment dispersion and improving the image density, gloss, and fixability of the resulting recorded matter, it is preferable to add the polyfunctional epoxy compound in an amount such that the crosslinking degree (mol %) represented by the following formula is 35 mol % or more and 90 mol % or less. Degree of crosslinking (mol %)=(mol number of epoxy groups of the multifunctional epoxy compound added to the system in step 3 / mol number of carboxy groups of the water-insoluble polymer (a) present in the system in step 3)×100 The degree of crosslinking is preferably from 40 mol % to 85 mol %, and more preferably from 50 mol % to 75 mol %.

[0048] From the viewpoints of completion of the crosslinking reaction and economic efficiency, the temperature of the crosslinking treatment is 50° C. or higher, preferably 60° C. or higher, more preferably 70° C. or higher, even more preferably 80° C. or higher, and preferably 96° C. or lower, more preferably 94° C. or lower, and even 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. This crosslinking treatment promotes the reaction between the carboxyl groups and the epoxy groups, forming a three-dimensional network structure in the water-insoluble polymer having the carboxyl groups.

[0049] In the present invention, any step used in the production of a known water-based pigment dispersion can be incorporated before, after, or between each of steps 1, 2, and 3. Examples of the optional step include steps of solvent distillation, filtration, aging, etc. Among these, it is preferable to incorporate a filtration step, and it is more preferable to incorporate a filtration step between step 2 and step 3. The type of filtration step is not particularly limited, and the mesh size of the filter used in the filtration step is preferably 1.2 μm or less from the viewpoint of improving the properties of the resulting recorded matter, particularly the glossiness of photo paper prints.

[0050] (Water-based pigment dispersion) The pigment content in the aqueous pigment dispersion obtained by the method of the present invention is, from the viewpoints of improving redispersibility and filterability and improving the image density of the obtained recorded matter, 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. Furthermore, from the viewpoint of improving redispersibility and filterability, the content of the water-insoluble polymer ((a)+(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. From the viewpoints of improving redispersibility and filterability and improving the image density, gloss and fixability of the obtained recorded matter, the average particle size of the aqueous pigment dispersion is preferably 60 nm or more, more preferably 80 nm or more, even more preferably 90 nm or more, and is preferably 200 nm or less, more preferably 180 nm or less, even more preferably 160 nm or less. The average particle size of the water-based pigment dispersion can be determined by the method described in the Examples.

[0051] [Water-based ink] The water-based ink of the present invention contains the water-based pigment dispersion obtained by the method 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, gloss, and fixability of the resulting recorded matter, 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. 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.

[0052] (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. (Polymer content) From the viewpoint of improving the image density, gloss and fixability of the obtained recorded matter, the content of the polymer ((a)+(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 moisture retention and the image density, gloss and fixability of the resulting recorded matter, 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, even more preferably 20% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less. The water content in the water-based ink is, from the viewpoints of image density, gloss, and fixability of the obtained recorded matter, 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.

[0053] (Water-based ink properties) From the viewpoint of improving storage stability, image density, and fixability, 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

[0054] 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.

[0055] (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.).

[0056] (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)].

[0057] (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). The average particle size was measured one hour after the pigment dispersion was prepared.

[0058] (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%)

[0059] Preparation Example 1 (Preparation of Water-Insoluble Polymer (a)) 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).

[0060] Preparation Example 2 A solution of a water-insoluble polymer (a2) having a carboxy group was 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.

[0061] [Table 1]

[0062] Production Example 1-1 (Production of Polymer Aqueous Dispersion (D1)) 292 parts of ion-exchanged water and 27.5 parts of 5N aqueous sodium hydroxide solution (sodium hydroxide solid content 16.9%) as a 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 neutralized to 34 mol% (neutralization degree 34 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) (solid content concentration 20%).

[0063] Manufacturing Examples 1-2 to 1-5 Polymer aqueous dispersions (D2) to (D5) were obtained in the same manner as in Production Example 1-1, except that the type of water-insoluble polymer (a), the amount of the aqueous sodium hydroxide solution, and the amount of ion-exchanged water were changed as shown in Table 2. The results are shown in Table 2.

[0064] [Table 2]

[0065] Example 1-1 (Preparation of Pigment Water Dispersion (I-1)) (Process 1) 352 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 with 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 a solids concentration of 20%, thereby obtaining a pigment aqueous dispersion (I-1). In the resulting pigment water dispersion (I-1), the pigment ratio x was 0.95. The cumulant average particle size of the pigment water dispersion (I-1) was measured and found to be 121 nm. The results are shown in Table 3.

[0066] Examples 1-2 to 1-4 and Comparative Examples 1-1 to 1-3 (Process 1) Pigment aqueous dispersions (I-2) to (I-4) and (I-C1) to (I-C2) were obtained in the same manner as in Example 1-1, except that the type and amount of the polymer aqueous dispersion and the amount of ion-exchanged water were changed as shown in Table 3. The results are shown in Table 3. In addition, in Comparative Example 1-3 in Table 3, the obtained product could not be filtered through a filter having a pore size of 5 μm, and the corresponding pigment water dispersion (I-C3) could not be obtained.

[0067] [Table 3]

[0068] Example 2-1 (Production of Water-Based Pigment Dispersion 1) (Step 2: Production of Pigment Water Dispersion (II-1)) 84 parts of the pigment water dispersion (I-1) obtained in Example 1-1 and 16 parts of the polymer water dispersion (D1) obtained in Production Example 1-1 were sealed in a pressure-resistant glass bottle and stirred with a magnetic stirrer at 20° C. for 60 minutes to obtain a pigment water dispersion (II-1). (Step 3: Production of Water-Based Pigment Dispersion 1) To the pigment aqueous dispersion (II-1) obtained in step 2, 1.41 parts of cyclohexanedimethanol diglycidyl ether (Denacol EX-216L, epoxy equivalent: 150, manufactured by Nagase ChemteX Corporation) was added as a polyfunctional epoxy compound, and the temperature was raised to 90°C and stirred for 2 hours, and then the temperature was lowered to 20°C to obtain aqueous pigment dispersion 1.

[0069] Examples 2-2 to 2-8 and Comparative Examples 2-1 to 2-6 (Step 2: Production of Pigment Water Dispersions (II-2) to (II-8), (II-C1) to (II-C6)) In (step 2) of Example 1-1, the type and amount of the pigment water dispersion (I), the type and amount of the polymer water dispersion (D), and the type and amount of the polyfunctional epoxy compound were changed as shown in Tables 4 and 5, and the same procedures as in Example 2-1 were carried out to obtain pigment water dispersions (II-2) to (II-8) and (II-C1) to (II-C6). (Step 3: Production of Water-Based Pigment Dispersions 2 to 8 and C1 to C6) In (step 3) of Example 1-1, water-based pigment water dispersions 2 to 8 and C1 to C6 were obtained in the same manner as in Example 1-1, except that the type of pigment water dispersion (II) and the amount of epoxy compound were changed as shown in Tables 4 and 5. The EX-850L used as the polyfunctional epoxy compound in Example 2-3 is diethylene glycol diglycidyl ether (Denacol EX-850L, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 145).

[0070] The filterability and redispersibility of the water-based pigment dispersions obtained in the Examples and Comparative Examples were evaluated by the following methods. The results are shown in Tables 4 and 5.

[0071] (Filterability of Water-Based Pigment Dispersions) The obtained aqueous pigment dispersion was filtered through a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a filter with a pore size of 1.2 μm (manufactured by Sartorius Stedim Biotech, Mini Sart Syringe Filter, filter diameter: 26 mm, material: cellulose acetate), and the amount of liquid passing through one filter until it became clogged was measured. The same procedure was repeated three times using different filters, and the average value was taken as the amount of liquid passing through. The larger the amount of liquid passing through, the better the filterability. 〇: The amount of liquid passing is 20g or more △: The amount of liquid passing is 5g or more and less than 20g ×: Less than 5g of liquid passed If the evaluation is △ or higher, there is no problem with the filtration performance in practical use.

[0072] (Redispersibility of Water-Based Pigment Dispersions) 10 μL of the obtained aqueous pigment dispersion was dropped into the center of the bottom of a screw tube No. 5 (manufactured by Maruemu Co., Ltd.) using a micropipette, and the tube was left uncovered at 40°C and 25% RH for 24 hours to evaporate the aqueous ink to dryness. 10 mL of ion-exchanged water was added to the solid matter remaining at the bottom of the screw tube, and the mixture was stirred at 150 rpm for 1 minute. The redispersibility of the solid matter was visually observed and evaluated according to the following evaluation criteria. (Evaluation Criteria) ◯: The solids were uniformly redispersed. Δ: The solid matter was redispersed, but some residue was left behind. ×: The solid matter was not redispersed. If the evaluation is △ or higher, there is no problem with redispersibility in practical use.

[0073] [Table 4]

[0074] [Table 5]

[0075] It can be seen from Tables 4 and 5 that the water-based pigment dispersions obtained in Examples 2-1 to 2-8 are superior in filterability and redispersibility to the water-based pigment dispersions obtained in Comparative Examples 2-1 to 2-6. It is clear that Comparative Examples 2-1 and 2-2 have poor redispersibility because the polymer used in step 1 and the polymer used in step 2 are different.

[0076] Examples 3-1 to 3-8 and Comparative Examples 3-1 to 3-6 (Production of Water-Based Inks) Using the obtained aqueous pigment dispersion, the pigment content in the aqueous ink was adjusted to 5.6 parts, the content of glycerin (Kao Corporation, concentrated glycerin for cosmetics) as an organic solvent was adjusted to 18 parts, the content of triethylene glycol (Fujifilm Wako Pure Chemical Industries, Ltd., reagent) was adjusted to 10 parts, and the content of nonionic surfactant (Kawaken Fine Chemicals Co., Ltd., Acetylenol E100, 10 mole ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol) was adjusted to 0.5 parts, and ion exchanged water was added to make the total amount 100 parts. The mixture was then stirred for 2 hours using a magnetic stirrer to mix thoroughly, and filtered using a 25 mL needleless syringe fitted with a 1.2 μm pore size filter (Mini Sart Syringe Filter, manufactured by Sartorius Stedim Biotech, filter diameter: 26 mm, material: cellulose acetate) to obtain water-based inks 1 to 8, and C1, C3, C4, and C6. In addition, since the results of the filtration test for the water-based pigment dispersions C2 and C4 were poor, the corresponding inks I-C2 were not prepared. Using the obtained water-based ink, the image density in printing on plain paper, and the glossiness and fixability in printing on photo paper were evaluated by the following methods.

[0077] (Image density on plain paper prints) An inkjet printer (Seiko Epson Corporation, model number: EW-M-660FT, piezoelectric type) was filled with each water-based ink, and a solid image was printed on plain paper (Xerox Corporation, product name: XEROX 4200) under the following printing conditions: room temperature of 23° C., relative humidity of 50%; paper type: plain paper, print quality: standard, 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 6. (Evaluation Criteria) ◎: 1.00 or higher ○: 0.96 or more and less than 1.00 △: 0.93 or more and less than 0.96 ×: Less than 0.93 Although a high image density is preferable, in practical use, there is no problem if the color ink (organic pigment) is rated △ or higher.

[0078] (Glossiness of photo paper prints) An inkjet printer (Seiko Epson Corporation, model number: EW-M-660FT, piezoelectric type) was filled with water-based ink, and a solid image was printed on photo paper (Seiko Epson Corporation, product name: photo paper, model number: KA4100PSKR) under the following printing conditions: room temperature of 23° C., relative humidity of 50%; paper type: photo paper, print quality: clean, to obtain a photo paper print. The 60° gloss of the print obtained above was measured five times using a glossmeter (manufactured by Nippon Denshoku Industries Co., Ltd., product name: HANDY GLOSS METER, product number: PG-1) and the average value was calculated. The larger the value, the better the gloss. (Evaluation Criteria) ◎: 55 or more ○: 45 to 55 △: 35 to less than 45 ×: Less than 35 If the evaluation is △ or higher, there is no problem with gloss in practical use.

[0079] (Fixation of photo paper prints) Ten seconds after printing, the print was rubbed with a finger and the boundary between the print surface and the paper was then evaluated using the following four-level rating scale. (Evaluation Criteria) Good: The printed material does not come off and the area around the printed area does not get dirty. △: The printed matter is rubbed off, the surrounding area becomes dirty, and dirt also adheres to fingers. ×: The printed matter was badly rubbed off, the surrounding area was badly stained, and the fingers were also considerably stained. If the evaluation is △ or higher, there is no problem with the fixing property in practical use.

[0080] [Table 6]

[0081] It can be seen from Table 6 that the water-based inks obtained in Examples 3-1 to 3-8 are superior in image density, glossiness, and fixability to the water-based inks obtained in the comparative examples. [Industrial Applicability]

[0082] The aqueous pigment dispersion obtained by the present invention has high redispersibility and excellent filterability, resulting in high productivity. Furthermore, when the aqueous pigment dispersion according to the present invention is used as a colorant for an ink-jet ink, it is possible to achieve both high image density on plain paper and gloss and fixability on photo paper or coated paper.

Claims

1. A method for producing an aqueous pigment dispersion, comprising the following steps 1 to 3: Step 1: A step of mixing and dispersing a pigment and a water-insoluble polymer (a) having a carboxy group in an aqueous medium to obtain a pigment water dispersion (I) having a pigment ratio x of 0.84 or more and 0.98 or less, as represented by the following formula (1): Pigment ratio x=[mass of pigment in pigment water dispersion (I)] / [mass of pigment in pigment water dispersion (I)+mass of water-insoluble polymer (a) in pigment water dispersion (I)] (1) Step 2: A step of adding a water-insoluble polymer (a') having a carboxy group to the pigment water dispersion (I) obtained in step 1 to obtain a pigment water dispersion (II). Here, the water-insoluble polymer (a') has the same constitution as the water-insoluble polymer (a), and differs only in the degree of neutralization, and the difference between the degree of neutralization a of the water-insoluble polymer (a) and the degree of neutralization a' of the water-insoluble polymer (a') [neutralization degree a - neutralization degree a'] is 10 mol % or more and 145 mol % or less. Step 3: A step of adding a polyfunctional epoxy compound to the pigment water dispersion (II) obtained in step 2 and crosslinking the mixture at a temperature of 50° C. or higher to obtain an aqueous pigment dispersion. The degree of neutralization a is 40 mol % or more and 150 mol % or less, The degree of neutralization a' is 5 mol% or more and 55 mol% or less, In step 1, the ratio of the total mass of the pigment and the water-insoluble polymer (a) to the total mass of each component during the mixing treatment is 15% by mass or more and 45% by mass or less; A method for producing an aqueous pigment dispersion, wherein the water-insoluble polymers (a) and (a') have an acid value of 140 mgKOH / g or more and 300 mgKOH / g or less.

2. The method for producing an aqueous pigment dispersion according to claim 1, wherein in step 3, the polyfunctional epoxy compound is added so that the degree of crosslinking (mol %) represented by the following formula is 35 mol % or more and 90 mol % or less. Degree of crosslinking (mol %)=(mol number of epoxy groups of multifunctional epoxy compound / mol number of carboxy groups of water-insoluble polymer (a) present in the system in step 3)×100

3. The method for producing the water-based pigment dispersion according to claim 1 or 2, which is for ink-jet recording.

4. The method for producing an aqueous pigment dispersion according to any one of claims 1 to 3, wherein step 2 is a step of adding and mixing a water-insoluble polymer (a') having a carboxy group to the aqueous pigment dispersion (I) obtained in step 1 to obtain an aqueous pigment dispersion (II).

5. 5. A water-based ink comprising the water-based pigment dispersion obtained by the method according to any one of claims 1 to 4 and a water-soluble organic solvent.

Images