JP7511442B2 - Water-based ink - Google Patents

Description

The present invention relates to water-based ink and a method for producing water-based ink.

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.
In water-based inks using pigments, functional groups with high affinity for water are added to the pigment surface, and the pigment is dispersed in an aqueous medium or ink vehicle without or with a dispersant. However, unlike inks using dyes that are uniformly dissolved in an aqueous medium or ink vehicle, it is difficult to maintain the dispersed state of pigment particles in the inks using pigments for a long period of time, and this results in problems such as redispersibility.

Therefore, various proposals have been made for water-based inks for ink-jet recording and the like.
For example, Patent Literature 1 discloses a water-based ink for ink-jet printing that is excellent in print density and gloss and can suppress bronzing, and that contains an aqueous dispersion containing water-insoluble crosslinked polymer particles (A) containing a colorant and core-shell polymer particles (B).
Patent Document 2 discloses an aqueous ink that maintains print density while exhibiting excellent ejection stability (ejection recovery property) and image fastness, the aqueous ink comprising pigment-containing water-insoluble crosslinked polymer particles (A) and a polymer emulsion (B), in which the water-insoluble crosslinked polymer constituting the crosslinked polymer particles (A) is obtained by crosslinking a polymer having a carboxylic acid monomer-derived constituent unit and a hydrophobic monomer-derived constituent unit having an acid value of 200 mg KOH/g or more with an epoxy compound, and the polymer constituting the polymer emulsion (B) contains a carboxylic acid monomer-derived constituent unit and a hydrophobic monomer-derived constituent unit.

JP 2008-63500 A JP 2019-14883 A

The water-based ink of Patent Document 1 can achieve both image density when printed on plain paper and gloss when printed on photo paper, but there is room for improvement in redispersibility.
Furthermore, although the water-based ink of Patent Document 2 has redispersibility and image fastness when printed on coated paper, there is room for improvement in image density when printed on plain paper.
An object of the present invention is to provide a water-based ink that has excellent redispersibility and can improve image density even when printed on plain paper, and a method for producing the water-based ink.
Here, the redispersibility refers to the ease of dispersion of a pigment when the pigment that has settled in an aqueous ink is dispersed again, or the ease of flowability when the ink comes into contact with a liquid component again after drying. In particular, in inkjet recording, it is required that the ink that has adhered to the periphery of the nozzle hole that ejects the ink droplets and dried is redissolved or redispersed in the ink.

The present inventors have found that the above-mentioned problems can be solved by using, as a crosslinking agent, in an aqueous ink containing pigment-containing crosslinked polymer particles (A) and a crosslinked polymer emulsion (B) each crosslinked with an epoxy compound having a different [number of carbon atoms/number of oxygen atoms] ratio in the molecule.
That is, the present invention provides the following [1] and [2].
[1] An aqueous ink containing a pigment-containing crosslinked polymer particle (A), a crosslinked polymer emulsion (B), and water, wherein the polymer constituting the pigment-containing crosslinked polymer particle (A) is obtained by crosslinking a polymer (a) having a constituent unit derived from a carboxyl group-containing vinyl monomer and a constituent unit derived from a hydrophobic vinyl monomer with an epoxy compound (a), and the polymer constituting the crosslinked polymer emulsion (B) is obtained by crosslinking a polymer (b) having a constituent unit derived from a carboxyl group-containing vinyl monomer and a constituent unit derived from a hydrophobic vinyl monomer with an epoxy compound (b),
1. A water-based ink, wherein the ratio q(a) of [number of carbon atoms/number of oxygen atoms] of the epoxy compound (a) is 2.7 or more and 3.8 or less, and the ratio q(b) of [number of carbon atoms/number of oxygen atoms] of the epoxy compound (b) is 1.6 or more and less than 2.7.
[2] The method for producing the water-based ink according to [1] above, comprising mixing an aqueous dispersion of crosslinked polymer particles (A) containing a pigment, a crosslinked polymer emulsion (B), an organic solvent, and water.

The present invention provides a water-based ink that has excellent redispersibility and can improve image density even when printing on plain paper, and a method for producing the water-based ink.

[Water-based ink]
The water-based ink of the present invention is a water-based ink containing a crosslinked polymer particle (A) containing a pigment, a crosslinked polymer emulsion (B), and water (hereinafter, also referred to as the "ink of the present invention").
a polymer constituting a pigment-containing crosslinked polymer particle (A) (hereinafter also simply referred to as "crosslinked polymer particle (A)") is obtained by crosslinking a polymer (a) (hereinafter also simply referred to as "polymer (a)") having a structural unit derived from a carboxy group-containing vinyl monomer and a structural unit derived from a hydrophobic vinyl monomer with an epoxy compound (a);
The polymer constituting the crosslinked polymer emulsion (B) is obtained by crosslinking a polymer (b) (hereinafter, also simply referred to as "polymer (b)") having a constituent unit derived from a carboxy group-containing vinyl monomer and a constituent unit derived from a hydrophobic vinyl monomer with an epoxy compound (b),
The ratio of [number of carbon atoms/number of oxygen atoms] (hereinafter also referred to as “[C/O] ratio”) q(a) of the epoxy compound (a) is 2.7 or more and 3.8 or less, and the [C/O] ratio q(b) of the epoxy compound (b) is 1.6 or more and less than 2.7.

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 ink of the present invention is suitably used as an ink for ink-jet recording, an ink for gravure printing, an ink for flexographic printing, etc., but is preferably used as an ink for ink-jet recording because of its particularly excellent redispersibility.

The ink of the present invention has excellent redispersibility and can improve image density even when printed on plain paper. The reason for this is not clear, but is thought to be as follows.
The ink of the present invention contains crosslinked polymer particles (A) containing a pigment and a crosslinked polymer emulsion (B), and the [C/O] ratio of the epoxy compound (a) used to obtain the crosslinked polymer particles (A) is greater than the [C/O] ratio of the epoxy compound (b) used to obtain the crosslinked polymer emulsion (B). That is, the polymer constituting the crosslinked polymer particles (A) is crosslinked with the epoxy compound (a) containing more carbon atoms, and the polymer constituting the emulsion (B) is crosslinked with the epoxy compound (b) containing more oxygen atoms (fewer carbon atoms).
Therefore, the outermost surface of the pigment-containing crosslinked polymer particles (A) becomes more hydrophobic, which is believed to facilitate aggregation on the plain paper surface and improve image density, whereas the outermost surface of the polymer particles of emulsion (B) becomes more hydrophilic, which is believed to improve redispersibility.

<Pigment-containing crosslinked polymer particles (A)>
In the present invention, the crosslinked polymer particle (A) containing a pigment means any one of the following forms: a form in which the polymer (A) constituting the crosslinked polymer particle (A) containing a pigment (hereinafter also simply referred to as "crosslinked polymer (A)") encompasses the pigment; a form in which a part of the pigment is exposed on the surface of a particle made of the crosslinked polymer (A); a form in which the crosslinked polymer (A) is adsorbed to a part of the pigment; a form in which the pigment adheres to a particle of the crosslinked polymer (A); and a mixture of these forms.

[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.

The hue is not particularly limited, 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 C.I. Pigment Yellow, C.I. Pigment Red, C.I. Pigment Orange, C.I. Pigment Violet, C.I. Pigment Blue, and C.I. Pigment Green.
Examples of the extender pigment include silica, calcium carbonate, and talc.
The above pigments may be used alone or in combination of two or more.

In order to improve the redispersibility and image density of the water-based ink, the pigment is dispersed in the water-based ink by a crosslinked polymer (A) having a crosslinked structure in which a polymer (a) having a structural unit derived from a carboxyl group-containing vinyl monomer and a structural unit derived from a hydrophobic vinyl monomer is crosslinked with an epoxy compound (a).

[Polymer (a)]
The polymer (a) has a structural unit derived from (a-1) a carboxy group-containing vinyl monomer (hereinafter also referred to as "component (a-1)") and a structural unit derived from (a-2) a hydrophobic vinyl monomer (hereinafter also referred to as "component (a-2)"). The polymer (a) 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.

[(a-1) Carboxy Group-Containing Vinyl Monomer]
The carboxyl group-containing vinyl monomer (a-1) is used as a monomer component of the polymer (a) in the ink of the present invention, as a source for realizing dispersion stability of the pigment.
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., with one or more selected from acrylic acid and methacrylic acid being preferred.

[(a-2) Hydrophobic vinyl monomer]
The hydrophobic vinyl monomer (a-2) is 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 image density of the water-based ink.
Here, "hydrophobic" means that when the monomer is dissolved to saturation in 100 g of ion-exchanged water at 25° C., the amount of the dissolved hydrophobic monomer (a-2) 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.

The alkyl (meth)acrylate preferably has an alkyl group having from 1 to 18 carbon atoms, more preferably from 1 to 10 carbon atoms. Examples thereof 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)" used below has the same meaning.

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.
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.

[(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. Hereinafter, n indicates the average number of moles of the 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, etc. 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 manufactured by Shin-Nakamura Chemical Co., Ltd., the Blenmar PE, PME, PP, and AP series manufactured by NOF Corporation, as well as 50PEP-300, 50POEP-800B, and 43PAPE-600B.

From the above viewpoints, it is preferable that the (a-1) component of polymer (a) 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 it is more preferable that the polymer (a) is a styrene-α-methylstyrene-acrylic acid copolymer.

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 structural unit derived from each component in polymer (a) is as follows, from the viewpoint of improving the dispersion stability of the pigment and improving the redispersibility and image density of the water-based ink.
The content of the (a-1) component is preferably 15 mass% or more, more preferably 20 mass% or more, even more preferably 25 mass% or more, and 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 is preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

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

[Production of Polymer (a)]
The polymer (a) is produced by copolymerizing a monomer mixture containing the above-mentioned components (a-1) and (a-2), and further the component (a-3) as required, 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). A specific example of a commercially available product is Joncryl manufactured by BASF.

The acid value of the polymer (a) is derived from the carboxy group, and from the viewpoint of improving the storage stability, redispersibility and image density of the water-based ink, the acid value is preferably 180 mgKOH/g or more, more preferably 200 mgKOH/g or more, even more preferably 220 mgKOH/g or more, and preferably 300 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 the carboxy group is sufficient, and the dispersion stability of the pigment is ensured.
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.
The weight average molecular weight of the polymer (a) is preferably 4,000 or more, more preferably 6,000 or more, even more preferably 8,000 or more, and is preferably 80,000 or less, more preferably 50,000 or less, even more preferably 30,000 or less, and still more preferably 20,000 or less. From the viewpoint of enhancing adsorption to the pigment and exhibiting storage stability, the weight average molecular weight of the polymer (a) is preferably in the above-mentioned range.
The weight average molecular weight is measured by the method described in the Examples.

[Neutralization]
From the viewpoint of improving the redispersibility of the water-based ink and the image density, it is preferable that at least a part of the carboxyl groups of the polymer (a) is neutralized with an alkali metal compound.
The alkali metal compound is used in the form of a compound that generates alkali metal ions in water or a medium containing water when preparing the pigment-containing crosslinked polymer particles (A) that constitute the ink 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.

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 ink of the present invention may contain a water-soluble amine compound.

From the viewpoint of ensuring dispersion stability before the reaction with the epoxy compound, the degree of neutralization of the polymer (a) is preferably 15 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and is preferably 180 mol % or less, more preferably 160 mol % or less, even more preferably 140 mol % or less, and still more preferably 120 mol % or less.
The degree of neutralization (mol %) of the polymer (a) is calculated by the following formula.
Degree of neutralization (mol %) of polymer (a)=[(mol number of alkali metal compounds in the ink of the present invention−mol number of alkali metal compounds brought from emulsion B in the ink of the present invention)/mol number of carboxy groups of polymer (a) in the ink of the present invention]×100

<Crosslinked Polymer Emulsion (B)>
The crosslinked polymer emulsion (B) is used in order to achieve both high levels of redispersibility and high image density in the ink of the present invention.
The polymer (b) constituting the crosslinked polymer emulsion (B) has a constituent unit derived from (b-1) a carboxy group-containing vinyl monomer (hereinafter also referred to as "component (b-1)") and a constituent unit derived from (b-2) a hydrophobic vinyl monomer (hereinafter also referred to as "component (b-2)").
The components (b-1) and (b-2) are the same as the carboxy group-containing vinyl monomer (a-1) and the hydrophobic vinyl monomer (a-2) in the polymer (a), respectively, and specific examples thereof are also the same, so that the description thereof will be omitted.
The component (b-1) is preferably at least one selected from acrylic acid and methacrylic acid.
The component (b-2) is preferably an alkyl (meth)acrylate, and more preferably an alkyl (meth)acrylate having an alkyl group having 1 to 10 carbon atoms.

The polymer (b) may further contain a structural unit derived from a (b-3) nonionic monomer (hereinafter also referred to as "component (b-3)"), and/or a structural unit derived from a polyfunctional monomer having two or more polymerizable double bonds.
The component (b-3) is the same as the nonionic monomer (a-3) in the polymer (a), and specific examples and preferred examples thereof are also the same, so a description thereof will be omitted.
Specific examples of polyfunctional monomers having two or more polymerizable double bonds include diacrylate compounds such as polyethylene glycol diacrylate, triethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,6-butylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol diacrylate, 1,9-nonanediol diacrylate, and polypropylene glycol diacrylate, as well as dimethacrylate compounds, tri(meth)acrylate compounds, and tetra(meth)acrylate compounds.

From the viewpoint of improving the storage stability, redispersibility, and image density of the aqueous ink, the acid value of the polymer (b) before crosslinking is preferably 10 mgKOH/g or more, more preferably 20 mgKOH/g or more, even more preferably 25 mgKOH/g or more, and is preferably 150 mgKOH/g or less, more preferably 100 mgKOH/g or less, even more preferably 80 mgKOH/g or less.
From the same viewpoints as above, the weight average molecular weight of the polymer (b) is preferably 8,000 or more, more preferably 16,000 or more, even more preferably 30,000 or more, and preferably 500,000 or less, more preferably 300,000 or less, even more preferably 200,000 or less.
A part of the carboxy groups of the polymer (b) can be neutralized, if necessary, with a neutralizing agent such as an alkali metal compound.

The polymer (b) is produced by a known polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. Among these polymerization methods, emulsion polymerization is preferred.
The polymerization initiator used in emulsion polymerization is preferably one that is soluble in the medium in which emulsion polymerization is carried out. In the present invention, it is preferable that the polymer (b) is emulsion-polymerized using an aqueous medium and a polymerization initiator soluble in the aqueous medium. That is, the polymer (b) of the present invention preferably contains a component derived from the polymerization initiator soluble in the aqueous medium. Examples of the polymerization initiator include potassium persulfate, ammonium persulfate, and hydrogen peroxide, and examples of the emulsifier include sodium lauryl ether sulfate and other known anionic, nonionic, or amphoteric surfactants.
When a polymer emulsion containing polymer (b) is produced by emulsion polymerization, the pH tends to be acidic due to the presence of carboxyl groups on the surface of the polymer particles, and viscosity increase and aggregation are likely to occur. Therefore, neutralization is performed using a neutralizing agent such as an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. The amount of the neutralizing agent added is appropriately determined so that the pH of the polymer emulsion is in the range of 7.5 to 9.5, preferably 7.5 to 8.5.
From the viewpoint of achieving both high levels of redispersibility of the water-based ink and high image density, the crosslinked polymer emulsion (B) is in a state in which a crosslinked polymer obtained by crosslinking a polymer (b) with an epoxy compound (b) is suspended in an aqueous medium in a finely divided state.
The crosslinking reaction between the polymer (b) and the epoxy compound (b) is preferably carried out in a system that does not contain a pigment.

[Epoxy Compound]
In the present invention, the polymer (a) and the polymer (b) undergo a crosslinking reaction with an epoxy compound to form a crosslinked structure, which is believed to allow the polymer to be firmly adsorbed or fixed to the pigment surface, suppressing aggregation of the pigment in the water-based ink and further suppressing swelling of the polymer, thereby improving storage stability and redispersibility.
The epoxy compound (crosslinking agent) may be water-soluble or water-insoluble, but from the viewpoint of more efficient crosslinking reaction with the carboxyl group of the polymer (a) or (b) in a medium mainly composed of water, the crosslinking agent of the polymer (a) is preferably a water-insoluble epoxy compound, and the crosslinking agent of the polymer (b) is preferably a water-soluble epoxy compound. When a water-insoluble epoxy compound is used, its water solubility (mass%) is preferably 50 mass% or less, more preferably 40 mass% or less, and even more preferably 35 mass% or less. Here, the water solubility (mass%) refers to the solubility (mass%) when 10 mass parts of the epoxy compound are dissolved in 90 mass parts of water at room temperature of 25°C.

The epoxy compound is preferably a polyfunctional epoxy compound, more preferably a compound having two or more epoxy groups in the molecule, even more preferably a compound having two or more glycidyl ether groups, and still 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 and the storage stability and redispersibility of the resulting water-based ink, the molecular weight of the epoxy compound is preferably 120 or more, more preferably 150 or more, even more preferably 200 or more, and is preferably 2,000 or less, more preferably 1,000 or less, 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.

The [number of carbon atoms/number of oxygen atoms] ratio ([C/O] ratio) q(a) of the epoxy compound (a) used for crosslinking the polymer (a) (hereinafter also simply referred to as "epoxy compound (a)") is 2.7 or more and 3.8 or less from the same viewpoint as above. The [C/O] ratio q(a) of the epoxy compound (a) is preferably 2.8 or more, more preferably 2.9 or more, even more preferably 3.0 or more, and preferably 3.9 or less, more preferably 3.8 or less, even more preferably 3.7 or less.

Suitable examples of the epoxy compound (a) include one or more selected from cyclohexanedimethanol diglycidyl ether ([C/O] ratio: 3.5), 1,6-hexanediol diglycidyl ether ([C/O] ratio: 3.0), neopentyl glycol diglycidyl ether ([C/O] ratio: 2.8), resorcinol diglycidyl ether ([C/O] ratio: 3.0), and hydrogenated bisphenol A diglycidyl ether ([C/O] ratio: 5.3).
Among these, one or more selected from cyclohexanedimethanol diglycidyl ether, 1,6-hexanediol diglycidyl ether, and neopentyl glycol diglycidyl ether are preferred, and cyclohexanedimethanol diglycidyl ether (trade name: Denacol EX-216L, epoxy equivalent: 150, [C/O] ratio: 3.5, water solubility: 0 mass%) is more preferred.

The [C/O] ratio q(b) of the epoxy compound (b) (hereinafter also simply referred to as "epoxy compound (b)") used for crosslinking the polymer (b) is 1.6 or more and less than 2.7 from the same viewpoint as above. The [C/O] ratio q(b) of the epoxy compound (b) is preferably 1.7 or more, more preferably 1.8 or more, even more preferably 1.9 or more, and is preferably 2.5 or less, more preferably 2.3 or less, even more preferably 2.1 or less.

Suitable examples of the epoxy compound (b) include one or more selected from diethylene glycol diglycidyl ether ([C/O] ratio: 2.0), 1,4-butanediol diglycidyl ether ([C/O] ratio: 2.5), triethylene glycol diglycidyl ether ([C/O] ratio: 2.0), pentaerythritol polyglycidyl ether ([C/O] ratio: 2.1), glycerin diglycidyl ether ([C/O] ratio: 1.8), glycerol polyglycidyl ether ([C/O] ratio: 1.8), and the like. Among these, diethylene glycol diglycidyl ether (trade name: Denacol EX-850L, epoxy equivalent: 145, [C/O] ratio: 2.0, water solubility: 100%) and 1,4-butanediol diglycidyl ether (trade name: Denacol EX-214L, epoxy equivalent: 120, [C/O] ratio: 2.5, water solubility: 100%) are more preferable.
The [C/O] ratio in the epoxy compound can be calculated according to JIS M 8813:2006 "Coals and cokes - Elemental analysis method". In addition, when the structure and composition of the epoxy compound are known, the number of carbon atoms and the number of oxygen atoms can be calculated from the information.

From the viewpoint of maintaining the storage stability of the water-based ink and achieving both high levels of redispersibility and image density, the difference in the [C/O] ratio between epoxy compound (a) and epoxy compound (b), [q(a)-q(b)], is preferably 0.6 or more, more preferably 0.8 or more, even more preferably 1.0 or more, and is preferably 2.5 or less, more preferably 2.3 or less, even more preferably 2.1 or less, and even more preferably 1.9 or less.

[Cross-linked structure of polymer]
The polymer constituting the crosslinked polymer particle (A) containing the pigment used in the present invention or the polymer constituting the crosslinked polymer emulsion (B) has a crosslinked structure by the epoxy compound (a) or the epoxy compound (b), respectively. The crosslinked structure is formed by the polymer (a) or (b) and the epoxy compound (a) or the epoxy compound (b) forming an ester group by the ring-opening reaction of the epoxy group of the epoxy compound, and the two are bonded together, and at this time, the polymer (a) or the polymer (b) has a plurality of carboxy groups in the molecule, and the epoxy compound also has two or more epoxy groups in the molecule, so that the two-dimensional structure of the polymer (a) or the polymer (b) becomes a three-dimensional structure by crosslinking with the epoxy compound.
The polymer may be either water-soluble or water-insoluble, but becomes a water-insoluble polymer by crosslinking. Here, "water-insoluble" means that when the polymer is mixed with water, (i) the insoluble portion can be visually confirmed, (ii) even if the insoluble portion is too small to be visually confirmed, the Tyndall phenomenon can be observed by observation with laser light or normal light, or (iii) the average particle size can be observed by measurement with the laser particle analysis system described in the examples.

When polymer (a) or polymer (b) is crosslinked, the degree of crosslinking is, from the viewpoint of improving the storage stability and redispersibility of the water-based ink, preferably 20 mol % or more, more preferably 30 mol % or more, even more preferably 35 mol % or more, and is preferably 80 mol % or less, more preferably 78 mol % or less, even more preferably 75 mol % or less.
Here, the degree of crosslinking (mol %) of polymer (a) or polymer (b) is calculated by "(number of moles of epoxy groups derived from epoxy compound (a) or (b) in the ink of the present invention/number of moles of carboxy groups derived from polymer (a) or (b) in the ink of the present invention) x 100".
Incidentally, the "number of moles of carboxy groups derived from polymer (a) or (b)" includes the number of moles of an emulsifier used in producing the polymer by emulsion polymerization that has a carboxy group, in addition to the carboxy group-containing vinyl monomer that is component (a-1) or (b-1) above.

[Method of manufacturing water-based ink]
The ink of the present invention can be efficiently produced by a method including the following steps 1 to 4.
Step 1: A step of neutralizing at least a portion of the carboxy groups of the water-insoluble polymer (a) with an alkali metal compound to obtain an aqueous dispersion of the water-insoluble polymer (a). Step 2: A step of dispersing the aqueous dispersion of the water-insoluble polymer (a) obtained in Step 1 and a pigment to obtain an aqueous pigment dispersion (i) in which the pigment is dispersed in the water-insoluble polymer (a). Step 3: A step of adding a polyfunctional epoxy compound to the aqueous pigment dispersion (i) obtained in Step 2, and crosslinking the aqueous pigment dispersion (i) to obtain an aqueous pigment dispersion (I). Step 4: A step of mixing the aqueous pigment dispersion (I) obtained in Step 3, the crosslinked polymer emulsion (B), an organic solvent, and water to obtain an aqueous ink.

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

(Step 2)
In step 2, the aqueous dispersion of the water-insoluble polymer (a) obtained in step 1 and a pigment are subjected to a dispersion treatment to obtain an aqueous pigment dispersion (i) in which the pigment is dispersed in the water-insoluble polymer (a).
In the dispersion treatment in step 2, the pigment particles can be atomized to a desired particle size only by main dispersion using shear stress. However, from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion.
As a dispersing machine used for preliminary dispersion, a commonly used mixing and stirring device such as an anchor blade or a dispersing blade can be used.

Examples of dispersing machines used in the dispersion include kneading machines such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersing machines such as paint shakers and bead mills. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment.
When the dispersion treatment is carried out using a high-pressure homogenizer, the average particle size of the pigment particles in the pigment aqueous dispersion (i) can be adjusted by controlling the treatment pressure and the number of passes.
From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more and 300 MPa or less, and the number of passes is preferably 3 or more and 30 or less.

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

The pigment ratio x', represented by the following formula (1), in the pigment water dispersion (i) is, from the viewpoint of improving the redispersibility of the water-based ink and the image density, preferably 0.81 or more, more preferably 0.84 or more, even more preferably 0.88 or more, and is preferably 0.99 or less, more preferably 0.98 or less, even more preferably 0.97 or less.
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)

The cumulant average particle size of the pigment water dispersion (i) is, from the viewpoints of reducing coarse particles and improving storage stability and image density, preferably 50 nm or more, more preferably 60 nm or more, even more preferably 70 nm or more, and is preferably 200 nm or less, more preferably 160 nm or less, even more preferably 150 nm or less.
The cumulant average particle size is measured by the method described in the Examples.

(Step 3)
In step 3, a polyfunctional epoxy compound is added to the pigment water dispersion (i) obtained in step 2, and crosslinking treatment is carried out to obtain a water-based pigment dispersion (I).
In step 3, the polymer (a) dispersing the pigment in the pigment aqueous dispersion (i) is crosslinked with the epoxy compound to form a crosslinked polymer, thereby obtaining an aqueous pigment dispersion (I) in a form in which crosslinked polymer particles (A) containing the pigment are dispersed in an aqueous medium.
The preferred epoxy compound and the degree of crosslinking are as described above.

From the viewpoints of completion of the crosslinking reaction and economy, the temperature of the crosslinking treatment is preferably 50°C or higher, more preferably 70°C or higher, and preferably 95°C or lower, more preferably 92°C or lower. From the same viewpoints as above, the time of the crosslinking treatment is preferably 0.5 hours or higher, more preferably 1 hour or higher, and preferably 10 hours or lower, more preferably 6 hours or lower.

The non-volatile component concentration (solid content concentration) of the resulting aqueous pigment dispersion (I) is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less, from the viewpoint of improving dispersion stability and facilitating the production of the ink.

The pigment ratio x of the aqueous pigment dispersion (I) represented by the following formula (2) is preferably 0.80 or more, more preferably 0.84 or more, even more preferably 0.88 or more, particularly preferably 0.90 or more, and is preferably 0.95 or less, more preferably 0.94 or less, even more preferably 0.93 or less.
Pigment ratio x of the aqueous pigment dispersion (I)=[mass of pigment in the aqueous pigment dispersion (I)/(mass of pigment in the aqueous pigment dispersion (I)+mass of components derived from polymer (a) in the aqueous pigment dispersion (I)+mass of components derived from epoxy compound (a) in the aqueous pigment dispersion (I)] (2)
When the pigment ratio x of the aqueous pigment dispersion (I) is within the above range, when the aqueous pigment dispersion (I) is used in an aqueous ink, the resulting ink can have both high redispersibility and high image density.

The cumulant average particle size of the water-based pigment dispersion (I) is preferably 50 nm or more, more preferably 60 nm or more, and even more preferably 70 nm or more, from the viewpoint of reducing coarse particles and improving storage stability and image density, and is preferably 200 nm or less, more preferably 160 nm or less, and even more preferably 150 nm or less.

(Step 4)
In step 4, the aqueous pigment dispersion (I) obtained in step 3 (aqueous dispersion of the pigment-containing crosslinked polymer particles (A)), the crosslinked polymer emulsion (B), an organic solvent, and water are mixed to obtain an aqueous ink.
The crosslinked polymer emulsion (B) is formed by crosslinking the polymer (b) with the epoxy compound (b). From the viewpoint of improving the storage stability and redispersibility of the water-based ink, the acid value of the polymer after crosslinking in the crosslinked polymer emulsion (B) is preferably 8 mgKOH/g or more, more preferably 10 mgKOH/g or more, even more preferably 12 mgKOH/g or more, and is preferably 70 mgKOH/g or less, more preferably 50 mgKOH/g or less, even more preferably 30 mgKOH/g or less.
From the viewpoints of reducing coarse particles and improving the storage stability and redispersibility of the water-based ink, the cumulant average particle size of the crosslinked polymer emulsion (B) is preferably 40 nm or more, more preferably 60 nm or more, and even more preferably 80 nm or more, and is preferably 200 nm or less, more preferably 160 nm or less, and even more preferably 120 nm or less.

The organic solvent used is preferably a water-soluble organic solvent which can be mixed with water in any ratio.
Examples of the water-soluble organic solvent include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, sulfur-containing compounds, etc. Among these, from the viewpoint of improving the image density of the water-based ink, one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers are preferred.
As the polyhydric alcohol, polyhydric alcohols having a boiling point of 250° C. or higher, such as glycerin, triethylene glycol, and tripropylene glycol, are more preferred, and a combination of glycerin and triethylene glycol is even more preferred.

[Water-based ink]
In addition to the above-mentioned components, the ink of the present invention may further contain, as necessary, various additives commonly used in inks, such as a humectant, wetting agent, penetrating agent, surfactant, viscosity modifier, defoamer, preservative, antifungal agent and rust inhibitor.
The content of each component in the ink of the present invention is as follows, from the viewpoints of maintaining the storage stability of the water-based ink, improving redispersibility and image density, and imparting the required ink performance.

(Pigment content)
The content of the pigment in the ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 2.5% by mass or more, and is preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less.

From the viewpoint of improving the redispersibility and image density of the water-based ink, the pigment ratio r of the water-based ink, represented by the following formula (3), is preferably 0.4 or more, more preferably 0.5 or more, even more preferably 0.55 or more, particularly preferably 0.65 or more, and is preferably 0.95 or less, more preferably 0.9 or less, even more preferably 0.85 or less, particularly preferably 0.75 or less.
Pigment ratio r=[mass of pigment in water-based ink/[mass of pigment in water-based ink+mass of cross-linked polymer (A) in water-based ink+mass of cross-linked polymer emulsion (B) in water-based ink]] (3)

(Content of Crosslinked Polymer Particles (A))
From the viewpoint of achieving a high image density of the water-based ink, the content of the crosslinked polymer particles (A) in the ink of the present invention, expressed as solids in the water-based ink, is preferably 2% by mass or more, more preferably 3% by mass or more, even more preferably 4% by mass or more, and is preferably 18% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less, and still more preferably 8% by mass or less.

(Content of Crosslinked Polymer Emulsion (B))
From the viewpoint of improving image density, the content of the crosslinked polymer emulsion (B) in the ink of the present invention, expressed as the solid content in the water-based ink, is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, even more preferably 1% by mass or more, and is preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 6% by mass or less, and particularly preferably 4% by mass or less.
(Total Amount Ratio of Polymer (a) and Epoxy Compound (a))
The total quantitative ratio of the polymer (a) and the epoxy compound (a) in the ink of the present invention, represented by the following formula (4), is preferably 0.15 or more, more preferably 0.18 or more, even more preferably 0.2 or more, and is preferably 0.65 or less, more preferably 0.5 or less, even more preferably 0.4 or less.
Total amount ratio of polymer (a) to epoxy compound (a)=[[mass of polymer (a)+mass of epoxy compound (a)]/[mass of polymer (a)+mass of epoxy compound (a)+solid content mass of crosslinked polymer emulsion (B)]] (4)
In formula (4), the mass of each component means the mass in the water-based ink.

(Organic Solvent Content)
From the viewpoint of ensuring the redispersibility of the water-based ink, the content of the organic solvent in the ink of the present invention, in particular the water-soluble organic solvent, 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 ink of the present invention is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less.

(Water-based ink properties)
From the viewpoint of improving storage stability and image density, the viscosity of the ink of the present invention 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 is 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 ink of the present invention 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.

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.

(1) Measurement of the weight average molecular weight of the polymer Using a solution of phosphoric acid and lithium bromide dissolved in N,N-dimethylformamide to a concentration of 60 mmol/L and 50 mmol/L, respectively, as an eluent, the weight average molecular weight was measured by gel permeation chromatography (GPC apparatus (HLC-8320GPC) manufactured by Tosoh Corporation, columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn Super AW-H), flow rate: 0.5 mL/min) 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), manufactured by 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 the mixture with a magnetic stirrer at 25° C. for 10 hours, and filtering the mixture with a syringe filter (DISMIC-13HP PTFE 0.2 μm, manufactured by Advantec Co., Ltd.).

(2) Measurement of Acid Value of Polymer The measurement was performed in accordance with 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)].

(3) Measurement of cumulant average particle diameter 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 diameter. The measurement conditions were a temperature of 25°C, an angle between the incident light and the detector of 90°, and 100 cumulative measurements, and 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 x 10 ^-3 % (converted into solids concentration).

(4) Measurement of solid content concentration of pigment dispersion Using an infrared moisture meter (FD-230, manufactured by Kett Electric Laboratory Ltd.), 5 g of a measurement sample was dried under conditions of 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 by the following formula.
Solid content (mass%)=100-water content (mass%)

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, a dropping funnel was charged with a mixture of the remainder of the raw material monomer mixture, 0.27 parts of the polymerization chain transfer agent, 40 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (V-501; 4,4'-azobis(4-cyanovaleric acid) manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 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 had elapsed 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 at 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 240 mgKOH/g).

Production Example 1-1 (Production of polymer aqueous dispersion (step 1))
271 parts of ion-exchanged water and 48.6 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 (a) 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 (a) was 60 mol% (neutralization degree 60 mol%). The obtained aqueous solution was heated at 90°C for 5 hours while stirring at 150 rpm to obtain a polymer aqueous dispersion (solid content concentration 20%).

Production Example 2-1 (Production of Pigment Water Dispersion)
(Step 2: Production of Pigment Water Dispersion (i-1))
352 parts of ion-exchanged water and 100 parts of magenta pigment (PV19: Pigment Violet 19, manufactured by Dainichiseika Color & Chemicals Co., Ltd.) were added to 26 parts of the polymer aqueous dispersion obtained in Production Example 1-1, and the mixture was stirred for 60 minutes using a disper (Ultra Disper, manufactured by Asada Iron Works Co., Ltd.) at 20° C. with a disper blade rotating at 7,000 rpm. The resulting mixture was subjected to 10 passes of 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 with a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a filter having a pore size of 5 μm (manufactured by Sartorius Stedim Biotech, Mini Sart Syringe Filter, filter diameter: 26 mm, material: cellulose acetate) to remove coarse particles, and then ion-exchanged water was added to the dispersion to a solids concentration of 20%, thereby obtaining a pigment aqueous dispersion (i-1).
In the obtained pigment water dispersion (i-1), the pigment ratio x' represented by the above formula (1) was 0.95, and the cumulant average particle size measured one hour after the preparation of the pigment water dispersion (i-1) was 118 nm.

(Step 3: Production of Water-Based Pigment Dispersion (A1))
100 parts (solid content 20%) of the pigment water dispersion (i-1) obtained in the above step 1 was placed in a screw-capped glass bottle, and 0.35 parts of diethylene glycol diglycidyl ether (Denacol EX-216L, manufactured by Nagase ChemteX Corporation, [C/O] ratio: q(a)=3.5) was added as an epoxy compound so that 55 mol % of all carboxy groups in the polymer (a) in the pigment water dispersion (i-1) were crosslinked (degree of crosslinking: 55 mol %). The bottle was then sealed and heated at 90° C. for 5 hours while stirring with a stirrer. After 5 hours had elapsed, the temperature was lowered to room temperature, and the mixture was filtered using a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a filter having a pore size of 5 μm (manufactured by Sartorius Stedim Biotech, Mini Sart Syringe Filter, filter diameter: 26 mm, material: cellulose acetate), and then ion exchanged water was added to adjust the solid content to 20%, thereby obtaining an aqueous dispersion of pigment-containing crosslinked polymer particles (A1) (aqueous pigment dispersion A1).
In the obtained water-based pigment dispersion A1, the pigment ratio x represented by the above formula (2) was 0.94, and the cumulant average particle size was 132 nm.

Production Examples 2-2 to 2-6 and Comparative Production Examples 2-1 to 2-2
Water-based pigment dispersions (A2) to (A6) and (AC1) to (AC2) were obtained in the same manner as in Production Example 2-1, except that the type and amount of the epoxy compound in Production Example 2-1 was changed as shown in Table 1. The results are shown in Table 1.
Table 1 shows the [C/O] ratio: q(a) calculated from the compound name listed in the catalog for the epoxy compound used, the epoxy equivalent, and the cumulant average particle size.

Details of the epoxy compounds shown in Table 1 are as follows.
EX-216L: Nagase ChemteX Corporation, Denacol EX-216L: cyclohexanedimethanol diglycidyl ether, [C/O] ratio: q(a)=3.5, epoxy equivalent: 150, water solubility: 0%
EX-212L: Nagase ChemteX Corporation, Denacol EX-212L: 1,6-hexanediol diglycidyl ether, [C/O] ratio: q(a)=3.0, epoxy equivalent: 150, water solubility: 0%
EX-850L: Nagase ChemteX Corporation, Denacol EX-850L, diethylene glycol diglycidyl ether, [C/O] ratio: q(b)=2.0, epoxy equivalent: 145, water solubility: 100%
EX-252: Nagase ChemteX Corporation, Denacol EX-252, hydrogenated bisphenol A diglycidyl ether, [C/O] ratio: q(b) = 5.3, epoxy equivalent: 213, water solubility: 0%

Production Examples 3-1 to 3-2, and Comparative Production Examples 3-1 to 3-3 (Production of Polymer Emulsions (B1) to (B2), (BC1) to (BC3))
(1) Into a 500 mL glass beaker, 144.5 parts of methyl methacrylate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 50 parts of 2-ethylhexyl acrylate (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 5.5 parts of methacrylic acid (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 18.5 parts of an aqueous solution of polyoxyethylene (18) lauryl ether sodium sulfate (manufactured by Kao Corporation, Latemul E-118B, solid content 26%), 96 parts of ion-exchanged water, and 0.4 parts of potassium persulfate were charged and stirred (300 rpm) with a magnetic stirrer to obtain a monomer emulsion.
A 1000 ml separable flask equipped with a dropping funnel was charged with 4.6 parts of LATEMURU E-118B, 186 parts of ion-exchanged water, and 0.08 parts of potassium persulfate, and nitrogen gas replacement was thoroughly performed. Under a nitrogen atmosphere, the temperature in the separable flask was raised to 80° C. while stirring (200 rpm) with a stirring blade, and the monomer emulsion charged in the dropping funnel was dropped over 3 hours and reacted. Ion-exchanged water was added to this reaction liquid to make the effective content 20%, thereby obtaining a polymer emulsion (B) (weight average molecular weight: 100,000, cumulant average particle size: 100 nm, acid value 36 mg KOH/g) before crosslinking.

(2) 100 parts of the polymer emulsion (B) (solid content 20%) obtained in (1) above was placed in a screw-capped glass bottle, and 1 part of an epoxy compound (Denacol EX-850L, Denacol EX-216L, or Denacol EX-212L) and 4.2 parts of ion-exchanged water were added so that 55 mol % of the total carboxy groups of the polymer contained in the polymer emulsion were crosslinked (crosslinking degree 55 mol %), and the bottle was sealed and heated at 90°C for 5 hours while stirring with a stirrer. After 5 hours, the temperature was lowered to room temperature and the solution was filtered using a 25 mL needleless syringe (Terumo Corporation) equipped with a 5 μm pore size filter (Mini Sart Syringe Filter, Sartorius Stedim Biotech, filter diameter: 26 mm, material: cellulose acetate), and ion-exchanged water was added to adjust the solids concentration to 20%, yielding polymer emulsions (B1) to (B2) and (BC1) to (BC3). The results are shown in Table 2.

Details of the epoxy compounds listed in Table 2 are as follows.
EX-214L: Nagase ChemteX Corporation, Denacol EX-214L, 1,4-butanediol diglycidyl ether, [C/O] ratio: q(b)=2.5, epoxy equivalent: 120, water solubility: 100%
EX-622: Nagase ChemteX Corporation, Denacol EX-622, sorbitol polyglycidyl ether, [C/O] ratio: q(b)=1.5, epoxy equivalent: 191, water solubility: 0%

Example 1
(Step 4: Preparation of Water-Based Ink)
A mixture of 30.0 parts of the aqueous pigment dispersion (A2) of the crosslinked polymer particles (A2) obtained in Production Example 2-2 (pigment content in the aqueous ink: 5.6 parts), 10.0 parts of the polymer emulsion (B1) obtained in Production Example 3-1, 5 parts of glycerin as an organic solvent, and 5 parts of diethylene glycol monobutyl ether (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 5 parts of 1.2-hexanediol (manufactured by Tokyo Chemical Industry Co., Ltd., reagent), and 0.5 parts of a nonionic surfactant (product name: Acetylenol E100, manufactured by Kawaken Fine Chemicals Co., Ltd., 10 mole ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol) was added with ion-exchanged water to a total amount of 100 parts, and the mixture was thoroughly mixed by stirring for 2 hours with a magnetic stirrer, and then filtered through a 1.2 μm pore size filter (Sartorius Stedim The mixture was filtered using a 25 mL needleless syringe equipped with a Minisart Syringe Filter (manufactured by Biotech, filter diameter: 26 mm, material: cellulose acetate) to obtain Water-based Ink 1.

Examples 2 to 8 and Comparative Examples 1 to 6
Water-based inks 2 to 8 and C1 to C6 were obtained in the same manner as in Example 1, except that the conditions in Example 1 were changed to those shown in Table 3.

Example 9
Water-based ink 9 was obtained in the same manner as in Example 1, except that 5 parts of glycerin and 10 parts of triethylene glycol were used instead of 5 parts of glycerin, 5 parts of diethylene glycol monobutyl ether (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., reagent), and 5 parts of 1,2-hexanediol (manufactured by Tokyo Chemical Industry Co., Ltd., reagent) as the organic solvent in Example 1.

The water-based inks obtained in the examples and comparative examples were used to evaluate the redispersibility of the ink and the image density when printed on plain paper using the following methods. The results are shown in Table 3.

(Redispersibility of Water-Based Ink)
Using a micropipette, 10 μL of the water-based ink was dropped onto the center of the bottom of a screw tube No. 5 (manufactured by Maruemu Co., Ltd.), and the tube was left uncovered in an environment of 40° C. and 25% RH for 24 hours to evaporate and dry the water-based ink.
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 the redispersibility of the water-based ink was evaluated according to the following evaluation criteria.
(Evaluation criteria)
◯: The solid matter was uniformly redispersed.
Δ: The solid matter was redispersed, but some residue remained.
×: The solid matter was not redispersed.
Higher redispersibility is preferable, but in practical use, a rating of "△" or better is sufficient.

(Image density on plain paper prints)
An inkjet printer (Seiko Epson Corporation, model number: EW-M-660FT, piezoelectric type) was filled with each of the water-based inks 1 to 12 and the water-based inks C1 to C8 of the comparative examples, and a solid image was printed on plain paper (Xerox Corporation, product name: XEROX 4200) under the printing conditions of [paper type: plain paper, print quality: standard] at a room temperature of 23° C. and a relative humidity of 50%, to obtain a plain paper print.
The image density of the plain paper print was measured using a spectrophotometer (manufactured by X-Rite, product number: Spectro Eye) under the following measurement conditions: observation light source D65, observation field of view 2 degrees, and density standard DIN 16536. The numerical values of the color density components of each color were read. Measurements were taken at different locations, and the average value of a total of 10 points was calculated. The higher the numerical value, the higher the image density.
Although a high image density is preferable, in practical use, there is no problem if the image density is 0.90 or more for color inks (organic pigments).

From Table 3, it can be seen that the water-based inks obtained in Examples 1 to 9 have a high level of balance between redispersibility and image density compared to the water-based inks obtained in Comparative Examples 1 to 6.

The water-based ink of the present invention has high redispersibility and can produce recorded material with high image density even when inkjet recorded on plain paper.

Claims (9)

A water-based ink comprising a crosslinked polymer particle (A) containing a pigment, a crosslinked polymer emulsion (B), and water,
the polymer constituting the pigment-containing crosslinked polymer particle (A) is obtained by crosslinking a polymer (a) having a structural unit derived from a carboxyl group-containing vinyl monomer and a structural unit derived from a hydrophobic vinyl monomer with an epoxy compound (a);
the polymer constituting the crosslinked polymer emulsion (B) is obtained by crosslinking a polymer (b) having a structural unit derived from a carboxyl group-containing vinyl monomer and a structural unit derived from a hydrophobic vinyl monomer with an epoxy compound (b);
The ratio q(a) of [number of carbon atoms/number of oxygen atoms] of the epoxy compound (a) is 2.7 or more and 3.8 or less;
A water-based ink, comprising an epoxy compound (b) having a ratio q(b) of [number of carbon atoms/number of oxygen atoms] of 1.6 or more and less than 2.7. The water-based ink according to claim 1, in which the difference in the ratio of [number of carbon atoms/number of oxygen atoms] between epoxy compounds (a) and (b), [q(a)-q(b)], is 0.6 or more and 2.5 or less. The water-based ink according to claim 1 or 2, wherein the epoxy compound (a) is at least one selected from cyclohexanedimethanol diglycidyl ether, 1.6-hexanediol diglycidyl ether, neopentyl glycol diglycidyl ether, resorcinol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether. The water-based ink according to any one of claims 1 to 3, wherein the epoxy compound (b) is one or more selected from diethylene glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, triethylene glycol diglycidyl ether, pentaerythritol polyglycidyl ether, glycerin diglycidyl ether, and glycerol polyglycidyl ether. The water-based ink according to any one of claims 1 to 4, wherein the degree of crosslinking of polymer (a) is 20 mol% or more and 80 mol% or less. 6. The water-based ink according to claim 1, wherein the pigment-containing crosslinked polymer particles (A) have a pigment ratio x represented by the following formula (2) of 0.80 or more and 0.95 or less:
Pigment ratio x=[mass of pigment in pigment water dispersion (I)/(mass of pigment in pigment water dispersion (I)+mass of component derived from polymer (a) in pigment water dispersion (I)+mass of component derived from epoxy compound (a) in pigment water dispersion (I)] (2) The water-based ink according to any one of claims 1 to 6, wherein the acid value of the polymer (a) constituting the pigment-containing crosslinked polymer particles (A) is 180 mg KOH/g or more and 300 mg KOH/g or more. The water-based ink according to any one of claims 1 to 7, which is for inkjet recording. The method for producing the water-based ink according to any one of claims 1 to 8, comprising mixing an aqueous dispersion of crosslinked polymer particles (A) containing a pigment, a polymer emulsion (B), an organic solvent, and water.