Water-based pigment dispersion

Abstract

To provide an aqueous pigment dispersion having superior filterability even following prolonged storage.SOLUTION: An aqueous pigment dispersion comprises crosslinked polymer particles containing a pigment, and a polyoxyethylene glycol monoether (I) represented by a formula (1). The content of the polyoxyethylene glycol monoether (I) is 0.1 mass% or more and 4.0 mass% or less.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] The inkjet recording method is a recording method in which ink droplets are directly ejected from a very fine inkjet nozzle onto a recording medium and adhered to obtain a recording in which characters and images are recorded. This method is easy and inexpensive to achieve full color, and in addition to paper recording media such as plain paper and coated paper, synthetic resin films and the like can be used as the recording medium, and there are many advantages such as non-contact with the recording medium, so it has become very popular. In recent years, in order to impart weather resistance and water resistance to the obtained recordings, pigments are widely used as colorants, and aqueous inks that impose less burden on the environment are used. In such aqueous inks, an aqueous pigment dispersion in which pigments are finely dispersed in an aqueous medium is used in the preparation of the ink. However, since aqueous inks and aqueous pigment dispersions contain a large amount of water and have a high water activity, deterioration due to the generation of microorganisms such as filamentous fungi becomes a problem. Therefore, in order to overcome such problems, studies have been made to enhance the antiseptic properties of aqueous inks and aqueous pigment dispersions.

[0003] For example, Patent Document 1 describes a method for producing an aqueous dispersion for inkjet recording, which has a step of heat sterilizing a water-dispersion containing 10 to 30% by weight of water-insoluble polymer particles containing a colorant, 3 to 10% by weight of a compound represented by a specific formula (R 10 -O(CH2CH2O) a -H, where R 10 represents a hydrocarbon group having 3 to 6 carbon atoms and a is a number from 2 to 5.) and a preservative at 60°C or higher.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] However, it has been found that the technology described in Patent Document 1 may result in the formation of aggregates after long-term storage of the inkjet recording aqueous dispersion. When such an inkjet recording aqueous dispersion is used in ink preparation, the sedimentation of the aggregates can reduce the filter's filtration efficiency. Therefore, there is a need for an aqueous pigment dispersion that exhibits excellent filtration efficiency and produces fewer aggregates even after long-term storage. The object of this invention is to provide an aqueous pigment dispersion that exhibits excellent filterability even after long-term storage. [Means for solving the problem]

[0006] The present inventors have found that the above problem can be solved by providing an aqueous pigment dispersion containing crosslinked polymer particles containing a pigment and a polyoxyethylene glycol monoether represented by a specific formula, wherein the content of the polyoxyethylene glycol monoether in the aqueous pigment dispersion is within a predetermined range. In other words, the present invention contains crosslinked polymer particles containing a pigment, and a polyoxyethylene glycol monoether (I) represented by the following formula (1), RO(CH2CH2O) n H (1) (In the formula, R represents a hydrocarbon group with 3 to 6 carbon atoms, and n represents the average number of moles of ethylene oxide added, which is between 2 and 5.) The present invention provides an aqueous pigment dispersion having a polyoxyethylene glycol monoether (I) content of 0.1% by mass or more and 4.0% by mass or less. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an aqueous pigment dispersion that exhibits excellent filterability even after long-term storage. [Modes for carrying out the invention]

[0008] [Water-based pigment dispersion] The aqueous pigment dispersion of the present invention contains crosslinked polymer particles containing pigment and a polyoxyethylene glycol monoether (I) represented by the following formula (1), RO(CH2CH2O) n H (1) (In the formula, R represents a hydrocarbon group with 3 to 6 carbon atoms, and n represents the average number of moles of ethylene oxide added, which is between 2 and 5.) The content of the polyoxyethylene glycol monoether (I) is 0.1% by mass or more and 4.0% by mass or less. In this invention, "aqueous system" means that water accounts for the largest proportion by mass in the medium.

[0009] According to the present invention, excellent filtration performance is maintained even after long-term storage. The reason for this is not entirely clear, but we believe it is as follows. The aqueous pigment dispersion of the present invention contains crosslinked polymer particles containing pigment and a polyoxyethylene glycol monoether represented by a specific formula. This polyoxyethylene glycol monoether can impart a high antiseptic effect against filamentous fungi and the like to the aqueous pigment dispersion of the present invention. On the other hand, in the present invention, since the polymer present on the surface of the crosslinked polymer particles containing pigment has a crosslinked structure, swelling of the polymer can be suppressed even in the presence of the polyoxyethylene glycol monoether. Therefore, when the aqueous pigment dispersion of the present invention is stored for a long period of time at high temperatures such as in summer, even if the crosslinked polymer particles containing pigment come into contact with each other, entanglement of polymer chains can be suppressed and the generation of aggregates can be suppressed, and as a result, it is possible to provide an aqueous pigment dispersion with excellent filterability. Hereinafter, the filterability of a filter after storing an aqueous pigment dispersion at high temperatures for an extended period (for example, 6 months at 40°C) will also be simply referred to as "filterability."

[0010] <Pigment-containing cross-linked polymer particles> The pigment-containing crosslinked polymer particles according to the present invention (hereinafter also referred to as "pigment-containing crosslinked polymer particles") are formed from at least a pigment and a crosslinked polymer (A), and are dispersed in an aqueous medium. The form of the pigment-containing crosslinked polymer particles is not particularly limited, as long as the particles are formed from at least a pigment and a crosslinked polymer (A), and are particles formed in which the crosslinked polymer (A) is adsorbed onto the pigment in an aqueous pigment dispersion. Examples of the form of pigment-containing crosslinked polymer particles include particle forms in which the pigment is encapsulated in the crosslinked polymer (A), particle forms in which the pigment is uniformly dispersed in the crosslinked polymer (A), particle forms in which the pigment is exposed on the surface of the crosslinked polymer (A) particles, and mixtures thereof are also included.

[0011] (Pigment) The pigments used in this invention may be either inorganic or organic pigments, and lake pigments and fluorescent pigments may also be used. Furthermore, these may be used in combination with extender pigments as needed. Specific examples of inorganic pigments include metal oxides such as carbon black, titanium dioxide, iron oxide, red iron oxide, and chromium oxide, as well as pearlescent pigments. Carbon black is particularly preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of organic pigments include azo pigments such as azo lake pigments and insoluble azo pigments (e.g., acetoacetate allylide monoazo pigments, acetoacetate allylide disazo pigments, pyrazolone pigments, condensed disazo pigments, and metal chelate azo pigments); and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and slene pigments. For achromatic inks, achromatic pigments such as white, black, and gray can be used, while for chromatic inks, chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used. Specific examples of preferred organic pigments include one or more products from each product code selected from the group consisting of CI Pigment Yellow, CI Pigment Red, CI Pigment Orange, CI Pigment Violet, CI Pigment Blue, and CI Pigment Green. Examples of extender pigments include silica, calcium carbonate, and talc. The above pigments can be used individually or in combination of two or more.

[0012] (Cross-linked polymer (A)) The crosslinked polymer (A) constituting the pigment-containing crosslinked polymer particles according to the present invention (hereinafter also referred to as "crosslinked polymer (A)") is preferably a reaction product of a polymer (a) having acid groups and a crosslinking agent. In this case, the crosslinked polymer (A) preferably has a structure that includes components derived from the polymer (a) having acid groups and components derived from the crosslinking agent. Such a crosslinked structure is thought to be a three-dimensional structure formed by the polymer (a) having acid groups and components derived from the crosslinking agent. The crosslinked polymer (A) may be used alone or in combination of two or more types.

[0013] [Polymers containing acidic groups (a)] Examples of the acidic group in polymer (a) include a carboxyl group (-COOM), a sulfonic acid group (-SO3M), and a phosphate group (-OPO3M2). In the above chemical formula, M represents a hydrogen atom, an alkali metal, an ammonium, or an organic ammonium. Among these, the acidic group is preferably a carboxyl group. The polymer (a) having an acid group may be either water-soluble or water-insoluble. Regarding the "water-solubility" and "water-insolubility" of the polymer, when the polymer dried at 105°C for 2 hours until a constant weight is dissolved in 100 g of water at 25°C until saturation, if the dissolved amount exceeds 10 g, it is judged as "water-soluble", and if it is 10 g or less, it is judged as "water-insoluble". Further, when the acid group of the polymer (a) having an acid group is neutralized with a neutralizing agent, it is judged by the dissolved amount measured under the condition that the neutralizing agent is present under the condition that the mass ratio of the polymer (a) having an acid group to the neutralizing agent is the same as that in the aqueous pigment dispersion of the present invention.

[0014] Examples of the polymer skeleton of the polymer (a) having an acid group include vinyl polymers obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds), polyesters, and polyurethanes. The polymer (a) having an acid group may be appropriately synthesized or a commercially available product may be used. Among these, from the viewpoint of improving the filterability, the polymer (a) having an acid group is preferably a vinyl polymer having an acid group, and more preferably a vinyl polymer containing a structural unit derived from the monomer (a-1) having an acid group. Examples of such vinyl polymers include homopolymers of the monomer (a-1) having an acid group, copolymers of the monomer (a-1) having an acid group and the hydrophobic monomer (a-2), and copolymers of the monomer (a-1) having an acid group, the hydrophobic monomer (a-2), and the nonionic monomer (a-3). Here, the "hydrophobicity" of the hydrophobic monomer (a-2) means that when the monomer is dissolved in 100 g of ion-exchanged water at 25°C until saturation, the dissolved amount is less than 10 g. The nonionic monomer (a-3) is a monomer having a high affinity for water and water-soluble organic solvents, and is, for example, a monomer containing a hydroxy group or a polyalkylene glycol chain. When the polymer (a) having an acid group is a copolymer, it may be any of a random copolymer, a block copolymer, an alternating copolymer, and a graft copolymer.

[0015] As the monomer (a-1) having an acidic group, one or more selected from the group consisting of acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, and 2-methacryloyloxymethylsuccinic acid are preferred. Among these, from the viewpoint of improving filterability, one or more selected from the group consisting of acrylic acid and methacrylic acid are preferred.

[0016] The hydrophobic monomer (a-2) is preferably one or more selected from aromatic group-containing monomers, (meth)acrylates having hydrocarbon groups derived from aliphatic alcohols, and macromers, and more preferably one or more selected from aromatic group-containing monomers and macromers. In this specification, "(meth)acrylate" means one or more selected from the group consisting of acrylates and methacrylates. The same applies to "(meth)acrylate" below.

[0017] The aromatic group-containing monomer is preferably a vinyl monomer having an aromatic group with 6 to 22 carbon atoms, and more preferably one or more selected from the group consisting of styrene monomers and aromatic group-containing (meth)acrylates. The molecular weight of the aromatic group-containing monomer is preferably less than 500. The styrene monomer is preferably one or more selected from the group consisting of styrene and 2-methylstyrene. The aromatic group-containing (meth)acrylate is preferably one or more selected from the group consisting of benzyl (meth)acrylate and phenoxyethyl (meth)acrylate, and more preferably benzyl (meth)acrylate.

[0018] The (meth)acrylate having a hydrocarbon group derived from an aliphatic alcohol preferably has a hydrocarbon group derived from an aliphatic alcohol having 1 to 22 carbon atoms, more preferably has an alkyl group having 1 to 22 carbon atoms, and even more preferably has an alkyl group having 6 to 18 carbon atoms. Examples 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.

[0019] Macromers may be used as hydrophobic monomers. The macromer is a compound having a number average molecular weight of 500 to 100,000 and having a polymerizable functional group at one end. From the viewpoint of improving filterability, it is preferably a compound having a number average molecular weight of 1,000 to 10,000. The number-average molecular weight is measured using gel permeation chromatography with chloroform containing 1 mmol / L dodecyldimethylamine as the solvent, with polystyrene used as the standard substance. The polymerizable functional group present at one end of the macromer is preferably a methacryloyloxy group and an acryloyloxy group, and more preferably a methacryloyloxy group. From the viewpoint of improving filterability, the macromer is preferably one or more selected from the group consisting of aromatic group-containing monomer-based macromers and silicone-based macromers, and more preferably an aromatic group-containing monomer-based macromer. Examples of aromatic group-containing monomers that constitute the aromatic group-containing monomer macromer include the aforementioned aromatic group-containing monomers, with one or more selected from the group consisting of styrene and benzyl (meth)acrylate being preferred, and styrene being more preferred. Specific examples of commercially available styrene macromers include AS-6(S), AN-6(S), and HS-6(S) (all manufactured by Toagosei Co., Ltd.). Examples of silicone-based macromers include organopolysiloxanes having a polymerizable functional group at one end.

[0020] Examples of nonionic monomers (a-3) include polyalkylene glycol mono(meth)acrylates such as polyethylene glycol mono(meth)acrylate and polypropylene glycol mono(meth)acrylate; and alkoxy polyalkylene glycol mono(meth)acrylates such as methoxy polyethylene glycol mono(meth)acrylate and octoxy polyethylene glycol mono(meth)acrylate. Among these, alkoxy polyalkylene glycol mono(meth)acrylate is preferred, and polypropylene glycol mono(meth)acrylate is more preferred. Specific examples of commercially available nonionic monomers include the "NK Ester" series from Shin-Nakamura Chemical Industry Co., Ltd. and the "Bremmer" series from NOF Corporation. Each monomer of the acidic polymer (a) can be used individually or in combination of two or more.

[0021] The content of constituent units derived from the monomer (a-1) having an acid group in the polymer (a) having an acid group is preferably 5% by mass or more, more preferably 8% by mass or more, and even more preferably 10% by mass or more, from the viewpoint of improving filterability, and from the same viewpoint as above, preferably 45% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. The content of constituent units derived from the hydrophobic monomer (a-2) in the polymer (a) having an acid group is preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 65% ​​by mass or more, from the viewpoint of improving filterability, and from the same viewpoint as above, preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less. When the polymer (a) having an acidic group further contains structural units derived from a nonionic monomer (a-3), the content of structural units derived from a nonionic monomer (a-3) in the polymer (a) having an acidic group is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more, from the viewpoint of improving filterability, and preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 12% by mass or less, from the same viewpoint as above.

[0022] The acid value of polymer (a) having acid groups is preferably 50 mg KOH / g or more, more preferably 80 mg KOH / g or more, even more preferably 180 mg KOH / g or more, and even more preferably 230 mg KOH / g or more, and from the same viewpoint as above, preferably 800 mg KOH / g or less, more preferably 500 mg KOH / g or less, and even more preferably 300 mg KOH / g or less. The acid value of polymer (a) having acid groups can be determined by the method described in the examples, but it can also be calculated from the mass ratio of the constituent monomers.

[0023] The weight-average molecular weight of polymer (a) having an acid group is preferably 5,000 or more, more preferably 7,000 or more, and even more preferably 10,000 or more, from the viewpoint of improving filterability, and also preferably 500,000 or less, more preferably 300,000 or less, even more preferably 100,000 or less, and even more preferably 50,000 or less, from the same viewpoint as above. The weight-average molecular weight can be determined by the method described in the examples.

[0024] Examples of commercially available polymers (a) having acid groups include polyacrylic acid such as "Aron AC-10SL" (manufactured by Toagosei Co., Ltd.) and styrene / acrylic resins such as "Joncryl 67", "Joncryl 611", "Joncryl 678", "Joncryl 680", "Joncryl 690", and "Joncryl 819" (all manufactured by BASF Japan Ltd.).

[0025] The polymer (a) having an acid group can be produced by copolymerizing the above-mentioned monomer (a-1) and, optionally, the raw material monomers (a-2) and (a-3) using a known polymerization method. Solution polymerization is preferred as the polymerization method. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as water, aliphatic alcohols with 1 to 3 carbon atoms, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, etc. are more preferred. During polymerization, known polymerization initiators and polymerization chain transfer agents can be used. As polymerization initiators, azo radical polymerization initiators such as 2,2'-azobis(2,4-dimethylvaleronitrile) are preferred, and as polymerization chain transfer agents, mercaptan compounds such as 2-mercaptoethanol are preferred. The preferred polymerization conditions vary depending on the type of polymerization initiator, but the polymerization temperature is preferably between 50°C and 90°C, and the polymerization time is preferably between 1 hour and 10 hours. Furthermore, the polymerization atmosphere is preferably an inert gas atmosphere such as a nitrogen gas atmosphere or argon.

[0026] [Crosslinking agent] The crosslinking agent is a polyfunctional epoxy compound having two or more crosslinkable functional groups in its molecule, preferably two or more epoxy groups, from the viewpoint of improving filterability. More preferably, such a polyfunctional epoxy compound is a compound having two or more glycidyl ether groups in its molecule, even more preferably a polyglycidyl ether compound of a polyhydric alcohol, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having hydrocarbon groups with 3 to 20 carbon atoms. The epoxy group equivalent of the polyfunctional epoxy compound is preferably 90 g / eq. or more, more preferably 100 g / eq. or more, even more preferably 110 g / eq. or more, and preferably 300 g / eq. or less, more preferably 200 g / eq. or less.

[0027] Examples of polyfunctional epoxy compounds include one or more selected from the group consisting of polypropylene glycol diglycidyl ether (water solubility: 31% by mass), 1,4-butanediol diglycidyl ether (water solubility: 100% by mass), 1,6-hexanediol diglycidyl ether (water solubility: 0% by mass), trimethylolpropane polyglycidyl ether (water solubility: 27% by mass), pentaerythritol polyglycidyl ether (water solubility: 0% by mass), neopentyl glycol diglycidyl ether, hydrogenated bisphenol A diglycidyl ether (water solubility: 0% by mass), ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, sorbitol polyglycidyl ether, glycerol polyglycidyl ether, and polyglycerol polyglycidyl ether. Among these, from the viewpoint of improving filterability, one or more selected from the group consisting of 1,4-butanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, and glycerol polyglycidyl ether is more preferred, and trimethylolpropane polyglycidyl ether is even more preferred.

[0028] The polyfunctional epoxy compound may be water-insoluble or water-soluble, but from the viewpoint of efficiently reacting with the acid groups in the polymer (a) having carboxyl groups in an aqueous medium to form a crosslinked structure and improve filterability, the water solubility of the polyfunctional epoxy compound is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, even more preferably 60% by mass or less, even more preferably 50% by mass or less, even more preferably 40% by mass or less, and even more preferably 30% by mass or less. Here, "water solubility" refers to the solubility rate (mass %) when 10 parts by mass of the crosslinking agent are dissolved in 90 parts by mass of water at room temperature (25°C).

[0029] The degree of crosslinking of the crosslinked polymer (A), represented by the following formula (2), is preferably 10 mol% or more, more preferably 20 mol% or more, even more preferably 30 mol% or more, even more preferably 40 mol% or more, even more preferably 50 mol% or more, and even more preferably 60 mol% or more, and preferably 80 mol% or less, more preferably 75 mol% or less, from the viewpoint of improving filterability. Degree of crosslinking = [(Molar equivalents of crosslinkable functional groups of the crosslinking agent) / (Molar equivalents of acid groups in polymer (a) having acid groups)] × 100 (2)

[0030] The acid value of the crosslinked polymer (A) is preferably 40 mg KOH / g or more, more preferably 50 mg KOH / g or more, even more preferably 60 mg KOH / g or more, and even more preferably 65 mg KOH / g or more, and from the same viewpoint as above, it is preferably 200 mg KOH / g or less, more preferably 150 mg KOH / g or less, even more preferably 130 mg KOH / g or less, even more preferably 100 mg KOH / g or less, and even more preferably 80 mg KOH / g or less. The acid value of the crosslinked polymer (A) can be determined by the method described in the examples, but it can also be calculated using the following formula (3). Acid value of crosslinked polymer (A) (mgKOH / g) = [Acid value of polymer (a) having acid groups (mgKOH / g) × [(100 - degree of crosslinking (mol%)) / 100]] (3) In the present invention, the degree of crosslinking (mol%) of the crosslinked polymer (A) is represented by the above formula (2).

[0031] It is preferable that the acidic groups of the crosslinked polymer (A) are neutralized with a neutralizing agent. The neutralizing agent is preferably one or more selected from the group consisting of alkali metal hydroxides, ammonia, and organic amines, more preferably one or more selected from the group consisting of alkali metal hydroxides and organic amines, even more preferably an alkali metal hydroxide, and even more preferably sodium hydroxide. From the viewpoint of improving filterability, the degree of neutralization of the crosslinked polymer (A) is preferably 15 mol% or more, more preferably 20 mol% or more, and even more preferably 25 mol% or more, and from the same viewpoint as above, preferably 65 mol% or less, more preferably 50 mol% or less, and even more preferably 35 mol% or less. The degree of neutralization is determined by calculating the equivalent amount of neutralizing agent used using the following formula. If the sum of the equivalent amount of neutralizing agent used and the degree of crosslinking of the crosslinked polymer (A) exceeds 100 mol%, the degree of neutralization of the crosslinked polymer (A) is the value obtained by subtracting the degree of crosslinking (mol%) from 100 mol%. If the sum of the equivalent amount of neutralizing agent used and the degree of crosslinking of the crosslinked polymer (A) does not exceed 100 mol%, the degree of neutralization of the crosslinked polymer dispersant A is equivalent to the equivalent amount of neutralizing agent used. Amount of neutralizing agent used (mol%) = {[Mass of neutralizing agent added (g) / Equivalent amount of neutralizing agent (g / mol)] / [(Acid value of polymer (a) with acidic groups before neutralization (mgKOH / g) × Mass of polymer (a) with acidic groups before neutralization (g)) / (56.1 × 1,000)]} × 100

[0032] <Polyoxyethylene glycol monoether (I)> The aqueous pigment dispersion of the present invention contains a polyoxyethylene glycol monoether (I) represented by the following formula (1) from the viewpoint of imparting preservative properties to the aqueous pigment dispersion. RO(CH2CH2O) n H (1) (In the formula, R represents a hydrocarbon group with 3 to 6 carbon atoms, and n represents the average number of moles of ethylene oxide added, which is between 2 and 5.)

[0033] In formula (1) above, R is preferably an alkyl group having 3 to 6 carbon atoms, and more preferably an alkyl group having 3 to 4 carbon atoms, from the viewpoint of preservative and filterability. Specific examples of R include one or more selected from the group consisting of ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, t-butyl group, hexyl group, and isohexyl group. Among these, R is more preferably one or more selected from the group consisting of propyl group, isopropyl group, butyl group, isobutyl group, and t-butyl group, even more preferably one or more selected from the group consisting of propyl group, isopropyl group, butyl group, isobutyl group, and t-butyl group, and even more preferably a butyl group. In formula (1) above, n is preferably 2 to 4, more preferably 2 to 3, and even more preferably 3, from the viewpoint of antiseptic and filterability. Preferred examples of the polyoxyethylene glycol monoether (I) represented by formula (1) above include polyethylene glycol monoalkyl ethers such as diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and tetraethylene glycol monobutyl ether, from the viewpoint of preservative and filterability, but one or more selected from the group consisting of diethylene glycol monobutyl ether and triethylene glycol monobutyl ether are more preferred, and triethylene glycol monobutyl ether is even more preferred.

[0034] <Water> The aqueous pigment dispersion of the present invention contains water. As the water used in the water-based ink of the present invention, pure water or ion-exchanged water is preferred from the viewpoint of preventing the contamination of unintended substances.

[0035] The aqueous pigment dispersion of the present invention may further contain additives such as water-soluble organic solvents commonly used in aqueous inks as wetting agents, penetrating agents, etc., surfactants, viscosity modifiers, defoamers, rust inhibitors, preservatives, and fungicides.

[0036] (Manufacturing of aqueous pigment dispersions) The aqueous pigment dispersion of the present invention is preferably obtained by a method comprising the following steps 1 to 3. Step 1: Disperse a mixture containing an acidic polymer (a), an organic solvent, a pigment, water, and optionally a neutralizing agent; remove the organic solvent from the resulting dispersion to obtain a pigment aqueous dispersion (d) containing polymer particles containing the pigment. Step 2: A step in which a polymer (a) having acid groups contained in the pigment aqueous dispersion (d) obtained in Step 1 is reacted with a crosslinking agent to crosslink the polymer (a) with the crosslinking agent, thereby obtaining a pigment aqueous dispersion (D) containing crosslinked polymer particles containing pigment. Step 3: A step to obtain an aqueous pigment dispersion by mixing the pigment aqueous dispersion (D) obtained in Step 2 with polyoxyethylene glycol monoether (I), and then heating and sterilizing the mixture.

[0037] [Process 1] Step 1 involves dispersing a mixture containing an acidic polymer (a), an organic solvent, a pigment, water, and optionally a neutralizing agent, and removing the organic solvent from the resulting dispersion to obtain a pigment aqueous dispersion (d) containing polymer particles containing the pigment. In step 1, from the viewpoint of improving filterability, it is preferable to disperse a mixture of an aqueous dispersion of the acidic polymer (a) and a pigment to obtain an aqueous pigment dispersion (d). It is preferable to use the aforementioned neutralizing agent to prepare the aqueous dispersion of the acidic polymer (a). The equivalent amount of neutralizing agent used is preferably 10 mol% or more, more preferably 30 mol% or more, and even more preferably 50 mol% or more, from the viewpoint of improving filterability, and preferably 90 mol% or less, more preferably 80 mol% or less, and even more preferably 70 mol% or less, from the same viewpoint as above. The equivalent amount of neutralizing agent used can be determined by the following formula. When the equivalent amount of neutralizing agent used is 100 mol% or less, it is equivalent to the degree of neutralization, and when the equivalent amount of neutralizing agent used in the following formula exceeds 100 mol%, it means that the neutralizing agent is in excess of the acidic groups of polymer (a) having acidic groups, and in this case the degree of neutralization of polymer (a) having acidic groups is considered to be 100 mol%. Amount of neutralizing agent used (mol%) = {[Mass of neutralizing agent added (g) / Equivalent amount of neutralizing agent (g / mol)] / [(Acid value of polymer (a) with acidic groups before neutralization (mgKOH / g) × Mass of polymer (a) with acidic groups before neutralization (g)) / (56.1 × 1,000)]} × 100

[0038] In step 1, the dispersion treatment can be performed solely by shear stress to atomize the pigment to the desired particle size. However, from the viewpoint of obtaining a uniform aqueous dispersion, it is preferable to perform pre-dispersion followed by the main dispersion. For pre-dispersion, commonly used mixing and stirring devices such as anchor blades and disperser blades can be used. Means of applying shear stress for this dispersion include, for example, kneaders such as roll mills and kneaders; high-pressure homogenizers such as microfluidizers; and media-type dispersers such as paint shakers and bead mills. Among these, it is preferable to use a high-pressure homogenizer or a bead mill from the viewpoint of reducing the particle size of the pigment. When performing dispersion processing using a high-pressure homogenizer, the average particle size of the pigment-containing polymer particles can be adjusted by controlling the processing pressure and the number of passes. From the viewpoint of productivity and economic efficiency, the processing pressure is preferably 60 MPa to 300 MPa, and the number of passes is preferably 3 to 30. Furthermore, it is preferable to obtain a pigment aqueous dispersion (d) after passing through a filter or the like, for the purpose of removing coarse particles and the like.

[0039] [Process 2] Step 2 is a step in which a polymer (a) having acid groups contained in the pigment aqueous dispersion (d) obtained in Step 1 is reacted with a crosslinking agent to crosslink the polymer (a) with the crosslinking agent, thereby obtaining a pigment aqueous dispersion (D) containing crosslinked polymer particles containing pigment (hereinafter also simply referred to as "pigment aqueous dispersion (D)" 9). In step 2, the temperature of the crosslinking reaction is preferably 40°C to 95°C, and the duration of the crosslinking reaction is preferably 0.5 hours or more, more preferably 1 hour or more, and preferably 10 hours or less, more preferably 7 hours or less.

[0040] [Step 3] Step 3 is a step in which the aqueous pigment dispersion (D) obtained in Step 2 is mixed with the polyoxyethylene glycol monoether (I) represented by formula (1), and then heat-sterilized to obtain an aqueous pigment dispersion. The preservative properties of the aqueous pigment dispersion can be improved by including step 3 and performing heat sterilization in the presence of polyoxyethylene glycol monoether (I). In step 3, there are no particular restrictions on the method of mixing the pigment aqueous dispersion (D) obtained in step 2 with the polyoxyethylene glycol monoether (I). For example, conventionally known mixing devices such as homomixers, homodispersers, wave rotors, homogenizers, dispersers, paint conditioners, ball mills, magnetic stirrers, and mechanical stirrers can be used for mixing. From the viewpoint of improving the sterilization effect, it is preferable to add polyoxyethylene glycol monoether (I) before heating. Furthermore, if the aqueous pigment dispersion of the present invention further contains a preservative, from the viewpoint of improving the sterilization effect, it is preferable to add the preservative before the heat sterilization treatment and then perform the heat sterilization treatment. The temperature for the heat sterilization treatment is preferably 60°C or higher, more preferably 70°C or higher, and preferably 95°C or lower, more preferably 80°C or lower. The heat sterilization treatment time is preferably 1 hour or more, more preferably 3 hours or more, and preferably 24 hours or less, more preferably 12 hours or less. Heat sterilization is preferably carried out, for example, by placing the pigment aqueous dispersion (D) and polyoxyethylene glycol monoether (I) in a sealed container and holding it at the aforementioned temperature and for the aforementioned time, either standing or stirring.

[0041] The average particle size of the pigment-containing crosslinked polymer particles in the aqueous pigment dispersion of the present invention is preferably 50 nm or more, more preferably 70 nm or more, and even more preferably 90 nm or more, from the viewpoint of improving filterability, and preferably 600 nm or less, more preferably 500 nm or less, even more preferably 300 nm or less, even more preferably 200 nm or less, and even more preferably 150 nm or less, from the same viewpoint as above. The average particle size of the pigment-containing crosslinked polymer particles can be measured by the method described in the examples.

[0042] (Composition of aqueous pigment dispersion) The solid content concentration of the aqueous pigment dispersion of the present invention is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 18% by mass or more, from the viewpoint of print density, and preferably 30% by mass or less, more preferably 27% by mass or less, and even more preferably 25% by mass or less, from the viewpoint of improving filterability. The solid content concentration of the aqueous pigment dispersion is measured by the method described in the examples. The pigment content in the aqueous pigment dispersion of the present invention is preferably 5% by mass or more, more preferably 7% by mass or more, and even more preferably 10% by mass or more, from the viewpoint of printing density, and preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less, from the viewpoint of improving filterability. The mass ratio of the pigment content to the total content of the pigment and crosslinked polymer (A) in the aqueous pigment dispersion of the present invention [pigment / (pigment + crosslinked polymer (A))] is preferably 0.50 or higher, more preferably 0.60 or higher, and even more preferably 0.70 or higher, from the viewpoint of improving filterability, and preferably 0.90 or lower, more preferably 0.85 or lower, and even more preferably 0.80 or lower, from the same viewpoint as above. The content of crosslinked polymer (A) in the aqueous pigment dispersion of the present invention is the total content of the polymer (a) having acid groups before crosslinking and the crosslinking agent.

[0043] The content of polyoxyethylene glycol monoether (I) in the aqueous pigment dispersion of the present invention is 0.1% by mass or more, preferably 0.3% by mass or more, more preferably 0.5% by mass or more, and even more preferably 0.7% by mass or more, from the viewpoint of preservative properties, and from the viewpoint of improving filterability, it is 4.0% by mass or less, preferably 3.5% by mass or less, more preferably 3.0% by mass or less, even more preferably 2.5% by mass or less, even more preferably 2.0% by mass or less, and even more preferably 1.5% by mass or less. The mass ratio of the content of polyoxyethylene glycol monoether (I) to the content of pigment-containing crosslinked polymer particles in the aqueous pigment dispersion of the present invention [polyoxyethylene glycol monoether (I) / pigment-containing crosslinked polymer particles] is preferably 0.01 or more, more preferably 0.02 or more, and even more preferably 0.04 or more from the viewpoint of preservation, and preferably 0.20 or less, more preferably 0.17 or less, even more preferably 0.12 or less, and even more preferably 0.07 or less from the viewpoint of improving filterability.

[0044] From the viewpoint of reducing environmental impact, the water content in the aqueous pigment dispersion of the present invention is preferably 60% by mass or more, more preferably 65% ​​by mass or more, even more preferably 70% by mass or more, and preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less.

[0045] (Preparation of water-based ink) The aqueous pigment dispersion of the present invention can be used as an aqueous ink by mixing it with water-soluble organic solvents, dispersants, defoamers, rust inhibitors, surfactants, etc., which are commonly used in aqueous inks. The content of pigment-containing crosslinked polymer particles in the aqueous ink is preferably 1% by mass or more, more preferably 3% by mass or more, and preferably 10% by mass or less, more preferably 8% by mass or less. The content of polyoxyethylene glycol monoether (I) in the aqueous ink is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, even more preferably 0.5% by mass or more, and even more preferably 0.7% by mass or more, from the viewpoint of preservative properties, and from the viewpoint of improving filterability, it is preferably 4.0% by mass or less, more preferably 3.0% by mass or less, even more preferably 2.0% by mass or less, and even more preferably 1.5% by mass or less. From the viewpoint of reducing environmental impact, 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 preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less. [Examples]

[0046] In the following preparation examples, manufacturing examples, examples, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" unless otherwise specified. The measurement methods for each physical property are as follows.

[0047] (1) Measurement of the weight-average molecular weight of polymer (a) having an acid group The results were obtained by gel permeation chromatography. The measurement conditions are shown below. GPC device: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolumn SuperAW-H" manufactured by Tosoh Corporation. Eluent: A solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively. Flow rate: 1mL / min Standard material: Monodisperse polystyrene kits with known molecular weight, manufactured by Tosoh Corporation: "PStQuick B (F-550, F-80, F-10, F-1, A-1000)" and "PStQuick C (F-288, F-40, F-4, A-5000, A-500)".

[0048] (2) Measurement of the acid value of polymer (a) and crosslinked polymer (A) having acid groups Using a potentiometric automatic titrator (manufactured by Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), in the case of polymer (a) having acidic groups without a crosslinking structure, the polymer (a) having acidic groups was dissolved in a titration solvent of toluene and acetone (toluene:acetone = 2:1 (volume ratio)). In the case of crosslinked polymer (A), an aqueous pigment dispersion was dispersed in the titration solvent, and the titration was performed by potentiometric titration with a 0.1N potassium hydroxide / ethanol solution, with the inflection point on the titration curve as the endpoint. The acid value (mgKOH / g) was calculated from the amount of potassium hydroxide solution titrated to the endpoint.

[0049] (3) Measurement of the solid content concentration of the polymer solution Using an infrared moisture meter (FD-230, manufactured by Kett Scientific Research Institute Co., Ltd.), 1.0 g of the sample was dried at a drying temperature of 150°C and measurement mode 96 (monitoring time 2.5 minutes / variation range 0.05%). The moisture content (%) of the sample was then measured, and the solid content concentration (%) was calculated using the following formula. Solid content concentration (%) = 100 - Moisture content of the sample (%)

[0050] (4) Measurement of the solid content concentration of aqueous pigment dispersions 10.0 g of sodium sulfate, which had been stabilized in a desiccator, was weighed into a 30 mL ointment container. Approximately 1.0 g of the sample was added and mixed, then accurately weighed. The mixture was kept at 105°C for 2 hours to remove volatile components, and then left in a desiccator for another 15 minutes before the mass was measured. The solid content was calculated by dividing the mass of the sample after removing volatile components by the mass of the added sample.

[0051] (5) Measurement of the average particle size of pigment-containing crosslinked polymer particles in aqueous pigment dispersions Cumulant analysis was performed using a laser particle analysis system (ELS-8000, manufactured by Otsuka Electronics Co., Ltd.), and the resulting average cumulant particle size was taken as the average particle size of the cross-linked polymer particles containing pigment in the aqueous pigment dispersion. The measurement sample contained particles with a concentration of 5 × 10⁻⁶-3 A dispersion solution diluted with water to a concentration of % (converted to solid content) was used. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 cumulative measurements. The refractive index of water (1.333) was input as the refractive index of the dispersion medium.

[0052] <Preparation of polymer (a) having acidic groups> Preparation Example 1-1 A mixture of raw material monomers was prepared by mixing 19.2 parts of acrylic acid, 70.8 parts of styrene, and 10.0 parts of α-methylstyrene. In a reaction vessel, 10.0 parts of methyl ethyl ketone (hereinafter also referred to as "MEK"), 0.3 parts of 2-mercaptoethanol as a polymerization chain transfer agent, and 10% of the raw material monomer mixture were added and mixed, and the mixture was thoroughly purged with nitrogen gas. Meanwhile, a mixture of the remaining 90% of the raw material monomer mixture, 0.27 parts of 2-mercaptoethanol, 40.0 parts of MEK, and 1.1 parts of an azo radical polymerization initiator (V-65, 2,2'-azobis(2,4-dimethylvaleronitrile)) (hereinafter also referred to as "V-65"), was placed in a dropping funnel. Under a nitrogen atmosphere, the reaction vessel was heated to 65°C while stirring the raw material monomers, and the mixture in the dropping funnel was added dropwise over 3 hours. After stirring for 2 hours from the end of the dropwise addition, a solution of 0.15 parts of the azo radical polymerization initiator (V-65) dissolved in 2.5 parts of MEK was added, and the mixture was aged at 65°C for 2 hours, and then at 70°C for another 2 hours to obtain a solution of polymer (a1) having a carboxyl group. The weight-average molecular weight and acid value of polymer (a1) are shown in Table 1.

[0053] Preparation Examples 1-2, 1-3, 1-5 Solutions of polymers (a2), (a3), and (a5) containing a carboxyl group were obtained in the same manner as in Preparation Example 1-1, except that the amount of raw material monomer charged was changed as shown in Table 1. The weight-average molecular weight and acid value of each polymer are shown in Table 1.

[0054] Preparation Examples 1-4 A mixture of raw material monomers was prepared by mixing 13.8 parts of methacrylic acid, 56.2 parts of benzyl acrylate, 40.0 parts of styrene macromer (AS-6S, manufactured by Toagosei Co., Ltd., number average molecular weight: 6,000, solid content concentration 50%) (hereinafter referred to as "styrene macromer") (20.0 parts as solid content), and 10.0 parts of polypropylene glycol monomethacrylate (Bremmer PP-800, manufactured by NOF Corporation, average number of moles of propylene oxide added: 13, terminal: hydroxyl group). 10 parts of MEK, 0.3 parts of 2-mercaptoethanol, and 10% of the raw material monomer mixture were added to the reaction vessel and mixed, followed by thorough nitrogen gas purging. Meanwhile, a mixture of the remaining (90%) of the raw material monomer mixture, 0.27 parts of 2-mercaptoethanol, 40 parts of MEK, and 1.1 parts of azo radical polymerization initiator (V-65) was placed in a dropping funnel, and the process was carried out in the same manner as in Preparation Example 1-1 to obtain a solution of polymer (a4) having a carboxyl group. The weight-average molecular weight and acid value of polymer (a4) are shown in Table 1.

[0055] [Table 1]

[0056] <Preparation of polymer aqueous dispersion (P)> Preparation Example 2-1 The solution of the carboxyl group-containing polymer (a1) obtained in Preparation Example 1-1 was dried under reduced pressure and then ground using a coffee mill to obtain carboxyl group-containing polymer (a1). 80.0 parts of the carboxyl group-containing polymer (a1), 289.3 parts of deionized water, and 30.3 parts of 5N sodium hydroxide aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) as a neutralizing agent were added to a heat-resistant bottle (the amount of neutralizing agent used (the ratio of moles of sodium hydroxide to moles of carboxyl groups in the carboxyl group-containing polymer (a1)) was set to 60 mol% (degree of neutralization: 60 mol%)). The mixture was neutralized and dispersed by heating at 90°C for 5 hours while stirring at 150 rpm to obtain a polymer aqueous dispersion (P1) (solid content concentration: 20.0%).

[0057] Preparation Examples 2-2 to 2-5 In Preparation Example 2-1, polymer aqueous dispersions (P2) to (P5) (degree of neutralization: 40 mol% or 60 mol%, solid content concentration: 20.0%) were obtained in the same manner as in Preparation Example 2-1, except that the type of polymer (a) having an acid group, the amount of deionized water, and the amount of 5N sodium hydroxide aqueous solution added were changed as shown in Table 2.

[0058] [Table 2]

[0059] <Preparation of Pigment Aqueous Dispersion (d)> Manufacturing Example 1-1 To 124.9 parts of the polymer aqueous dispersion (P1) obtained in Preparation Example 2-1, 370.2 parts of ion-exchanged water and 100.0 parts of CI Pigment Red 122 (dimethylquinacridone pigment, "IRGAPHOR MAGENTA DMQ" manufactured by Ciba Specialty Chemicals Co., Ltd.) (hereinafter referred to as "PR122") were added, and the mixture was stirred for 60 minutes at 20°C with the disperser blades rotating at 7,000 rpm using a disperser ("Ultra Disperser" manufactured by Asada Iron Works Co., Ltd.). The obtained mixture was subjected to a 10-pass dispersion treatment at a pressure of 200 MPa using a microfluidizer (product name: Microfluidics, model name: M-110-EH) to obtain the dispersed product. The resulting dispersion was filtered using a 25 mL needleless syringe (manufactured by Terumo Corporation) fitted with a syringe filter with a pore size of 5 μm (Sartorius "Minisart", model number: S7594-FMOSK) to remove coarse particles, thereby obtaining a pigment aqueous dispersion (d1) (solid content concentration: 21.0%).

[0060] Manufacturing Examples 1-2 to 1-10 In Production Example 1-1, pigment aqueous dispersions (d2) to (d10) (solid content concentration: 21.0%) were obtained in the same manner as in Production Example 1-1, except that the type and amount of polymer aqueous dispersion (P) were changed as shown in Table 3. PO34: CI Pigment Orange 34 (Pyrazolone-based pigment, Clariant Chemicals "PERMANENT ORANGE RL-01") PO43: CI Pigment Orange 43 (Perinon pigment, Clariant "PV Fast Orange GRL") PG7: CI Pigment Green 7 (Phthalocyanine pigment, "PV Fast Green GNX" manufactured by Clariant Chemicals Japan) PV23: CI Pigment Violet 23 (Dioxazine pigment, Clariant "Hostaperm Violet RL SPEC.") PR166: CI Pigment Red 166 (Condensed disazo pigment, Clariant's "Chromophtal Scarlet RT")

[0061] [Table 3]

[0062] <Manufacturing of Pigment Aqueous Dispersion (D)> Manufacturing Example 2-1 100.0 parts (solid content concentration: 21.0%) of the pigment aqueous dispersion (d1) obtained in Production Example 1-1 were placed in a screw-top glass bottle, and 1.09 parts of trimethylolpropane polyglycidyl ether (Denacol EX-321, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 139 g / eq., water solubility: 27%) (hereinafter referred to as "EX-321") were added as a crosslinking agent (crosslinking 70 mol% of the total carboxyl groups of the polymer (a1) having carboxyl groups (degree of crosslinking: 70 mol%)), and 4.0 parts of deionized water were added. The bottle was then tightly sealed and heated at 70°C for 5 hours while stirring with a stirrer. After that, it was cooled to room temperature and filtered using a 25 mL needleless syringe fitted with the aforementioned 5 μm pore size syringe filter to obtain the pigment aqueous dispersion (D1) (solid content concentration: 21.0%).

[0063] Manufacturing Examples 2-2 to 2-19 In Production Example 2-1, pigment aqueous dispersions (D2) to (D19) (solid content concentration: 21.0%) were obtained in the same manner as in Production Example 2-1, except that the type of pigment aqueous dispersion (d), the type and amount of crosslinking agent, and the amount of ion-exchanged water added were changed as shown in Table 4. The crosslinking agents used in manufacturing examples 2-7 to 2-13 are shown below. EX-214L: "Denacol EX-214L" manufactured by Nagase ChemteX Corporation, 1,4-butanediol diglycidyl ether, epoxy equivalent: 115 g / eq., water solubility: 100% EX-850: Denacol EX-850 manufactured by Nagase ChemteX Corporation, polyethylene glycol diglycidyl ether, epoxy equivalent: 122 g / eq., water solubility: 100% EX-614B: Denacol EX-614B manufactured by Nagase ChemteX Corporation, sorbitol polyglycidyl ether, epoxy equivalent: 173 g / eq., water solubility: 94% EX-622: "Denacol EX-622" manufactured by Nagase ChemteX Corporation, sorbitol polyglycidyl ether, epoxy equivalent: 191 g / eq., water solubility: insoluble EX-313: Denacol EX-313, manufactured by Nagase ChemteX Corporation. It contains glycerol polyglycidyl ether, epoxy equivalent weight: 141 g / eq., and water solubility: 99%. EX-512: Denacol EX-512 manufactured by Nagase ChemteX Corporation, polyglycerol polyglycidyl ether, epoxy equivalent: 168 g / eq., water solubility: 100% EX-521: Denacol EX-521 manufactured by Nagase ChemteX Corporation, polyglycerol polyglycidyl ether, epoxy equivalent: 183 g / eq., water solubility: 100%

[0064] [Table 4]

[0065] Example 1 As shown in Table 5, 95.2 parts of a pigment aqueous dispersion (D1), 1.0 part of triethylene glycol monobutyl ether (hereinafter also referred to as "BTG") as polyoxyethylene glycol monoether (I), and 3.8 parts of deionized water were mixed and sterilized at 70°C for 5 hours. After that, the mixture was filtered through the 5 μm syringe filter to obtain an aqueous pigment dispersion (1) (solid content concentration: 20.0%). The average particle size of the pigment-containing crosslinked polymer particles in the aqueous pigment dispersion (1) was 120 nm.

[0066] Examples 2-13, 15-23 In Example 1, aqueous pigment dispersions (2) to (13) and (15) to (23) (all with a solid content concentration of 20.0%) were obtained in the same manner as in Example 1, except that the type of pigment aqueous dispersion (D), the amount of polyoxyethylene glycol monoether (I) added, or the amount of ion-exchanged water added were changed as shown in Table 5.

[0067] Example 14 In Example 1, an aqueous pigment dispersion (14) (solid content concentration: 20.0%) was obtained in the same manner as in Example 1, except that the aqueous pigment dispersion (D1) was replaced with an aqueous pigment dispersion (D4) as shown in Table 5, and compound (I) was replaced with diethylene glycol monobutyl ether (hereinafter referred to as "BDG").

[0068] Comparative Example 1 In Example 1, an aqueous pigment dispersion (C1) (solid content concentration: 20.0%) was obtained in the same manner as in Example 1, except that the aqueous pigment dispersion (D1) was replaced with an aqueous pigment dispersion (d5) as shown in Table 5.

[0069] The filterability of each aqueous pigment dispersion obtained in the examples and comparative examples was evaluated by the following method. The results are shown in Table 5. 100 g of each aqueous pigment dispersion obtained in the examples or comparative examples was added to a 100 mL poly container and stored in a sealed state at 40°C for 6 months. After that, each aqueous pigment dispersion, cooled to room temperature (25°C), was filtered using a 25 mL needleless syringe (manufactured by Terumo Corporation) fitted with a syringe filter with a pore size of 5 μm (Sartorius "Minisart", model number: S7594-FMOSK), and the amount of liquid that passed through (g) until one syringe filter became blocked was measured. A higher amount of liquid that passed through (g) indicates better filterability.

[0070] [Table 5]

[0071] Table 5 shows that the aqueous pigment dispersions in the examples exhibit superior filterability even after long-term storage compared to the comparative examples. [Industrial applicability]

[0072] According to the present invention, it is possible to provide an aqueous pigment dispersion with excellent filterability in which the generation of aggregates is suppressed even after long-term storage.

Claims

1. It contains crosslinked polymer particles containing pigment, and polyoxyethylene glycol monoether (I) represented by the following formula (1), RO(CH 2 CH 2 O) n H (1) (In formula (1), R represents a hydrocarbon group having 3 to 6 carbon atoms, and n represents the average number of moles of ethylene oxide added, which is between 2 and 5.) The content of the polyoxyethylene glycol monoether (I) is 0.1% by mass or more and 4.0% by mass or less. The solid content concentration is 10% by mass or more and 30% by mass or less. An aqueous pigment dispersion in which the mass ratio of the content of polyoxyethylene glycol monoether (I) to the content of pigment-containing crosslinked polymer particles [polyoxyethylene glycol monoether (I) / pigment-containing crosslinked polymer particles] is 0.01 or more and 0.07 or less.

2. The aqueous pigment dispersion according to claim 1, wherein the crosslinked polymer (A) constituting the crosslinked polymer particles containing the pigment is a reaction product of a polymer (a) having an acid group and a crosslinking agent, and the degree of crosslinking represented by the following formula (2) is 10 mol% or more and 80 mol% or less. Degree of crosslinking = [(Molar equivalents of crosslinkable functional groups of the crosslinking agent) / (Molar equivalents of acid groups in polymer (a) having acid groups)] × 100 (2)

3. The aqueous pigment dispersion according to claim 2, wherein the acid value of the crosslinked polymer (A) is 40 mg KOH / g or more and 200 mg KOH / g or less.

4. The aqueous pigment dispersion according to claim 2 or 3, wherein the acid group of the crosslinked polymer (A) is neutralized with one or more selected from the group consisting of alkali metal hydroxides, ammonia, and organic amines.

5. The aqueous pigment dispersion according to claim 2 or 3, wherein the polymer (a) having an acid group is a vinyl polymer containing a constituent unit derived from a monomer (a-1) having an acid group, and the monomer (a-1) having an acid group is one or more selected from the group consisting of acrylic acid and methacrylic acid.

6. The aqueous pigment dispersion according to claim 2 or 3, wherein the crosslinking agent is a polyfunctional epoxy compound having two or more epoxy groups in its molecule.

7. The aqueous pigment dispersion according to claim 6, wherein the polyfunctional epoxy compound is a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 20 carbon atoms.

8. The aqueous pigment dispersion according to claim 1 or 2, wherein the polyoxyethylene glycol monoether (I) is triethylene glycol monobutyl ether.

9. The aqueous pigment dispersion according to claim 2, wherein the crosslinked polymer (A) constituting the crosslinked polymer particles containing the pigment is a reaction product of a polymer (a) having an acid group and a crosslinking agent, and the degree of crosslinking represented by formula (2) is 30 mol% or more and 80 mol% or less.

10. The aqueous pigment dispersion according to claim 2, wherein the acid value of the crosslinked polymer (A) is 50 mg KOH / g or more and 200 mg KOH / g or less.

11. The aqueous pigment dispersion according to claim 2, wherein the crosslinking agent is one or more selected from the group consisting of 1,4-butanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, and glycerol polyglycidyl ether.

12. A method for producing an aqueous pigment dispersion containing crosslinked polymer particles containing a pigment and a polyoxyethylene glycol monoether (I) represented by the following formula (1), The following steps 1 to 3 are included: Step 1: Disperse a mixture containing an acidic polymer (a), an organic solvent, a pigment, water, and optionally a neutralizing agent; remove the organic solvent from the resulting dispersion to obtain a pigment aqueous dispersion (d) containing polymer particles containing the pigment; Step 2: A step in which a polymer (a) having acid groups contained in the pigment aqueous dispersion (d) obtained in Step 1 is reacted with a crosslinking agent to crosslink the polymer (a) with the crosslinking agent, thereby obtaining a pigment aqueous dispersion (D) containing crosslinked polymer particles containing pigment; Step 3: A step to obtain a sterilized aqueous pigment dispersion by heating and sterilizing an aqueous pigment dispersion obtained by mixing the pigment aqueous dispersion (D) obtained in Step 2 with polyoxyethylene glycol monoether (I) at a temperature of 60°C to 95°C for 1 hour to 24 hours. In the aqueous pigment dispersion, the content of the polyoxyethylene glycol monoether (I) is 0.1% by mass or more and 4.0% by mass or less, and the solid content concentration is 10% by mass or more and 30% by mass or less. A method for producing an aqueous pigment dispersion, wherein the mass ratio of the content of polyoxyethylene glycol monoether (I) to the content of pigment-containing crosslinked polymer particles [polyoxyethylene glycol monoether (I) / pigment-containing crosslinked polymer particles] is 0.01 or more and 0.07 or less. RO(CH 2 CH 2 O) n H (1) (In formula (1), R represents a hydrocarbon group having 3 to 6 carbon atoms, and n represents the average number of moles of ethylene oxide added, which is between 2 and 5.)

13. The method for producing an aqueous pigment dispersion according to claim 12, wherein the crosslinked polymer (A) constituting the crosslinked polymer particles containing the pigment is a reaction product of a polymer (a) having an acid group and a crosslinking agent, and the degree of crosslinking represented by formula (2) is 30 mol% or more and 80 mol% or less. Degree of crosslinking = [(Molar equivalents of crosslinkable functional groups of the crosslinking agent) / (Molar equivalents of acid groups in polymer (a) having acid groups)] × 100 (2)

14. The method for producing an aqueous pigment dispersion according to claim 13, wherein the acid value of the crosslinked polymer (A) is 50 mg KOH / g or more and 200 mg KOH / g or less.

15. The method for producing an aqueous pigment dispersion according to claim 13 or 14, wherein the crosslinking agent is one or more selected from the group consisting of 1,4-butanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, and glycerol polyglycidyl ether.

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