JP7583575B2 - Water-based pigment dispersions - Google Patents

Description

The present invention relates to an aqueous pigment dispersion, a method for producing the aqueous pigment dispersion, and a water-in-oil emulsion ink that uses the aqueous pigment dispersion.

Inkjet recording is a method of directly ejecting ink droplets from minute nozzles and depositing them on a printing medium to obtain a printed matter on which characters and images are recorded. This method has the advantages of being easy to produce full color, being inexpensive, and not contacting the printed matter, so it is no longer limited to consumer printing, and in recent years it has begun to be applied to commercial and industrial printing fields.
Conventionally, water-based inks using pigments or dyes as colorants have been used as inks for inkjet recording. However, when water-based inks are used for printing on plain paper, the print density is high because the plain paper has high water absorption, but there is a problem that the water contained in the water-based ink causes the plain paper fibers to expand, resulting in curling and deformation of the printed matter.
In addition, in the fields of commercial and industrial printing, high-speed printing is required, and in order to meet this demand, printers that employ a line-head type inkjet recording method have been developed and oil-based pigment inks are used. When printing on plain paper using oil-based pigment inks, deformation of the printed matter can be reduced, but there is a problem that the ink penetrates into the interior of the printed matter, resulting in a low print density.
Therefore, the use of water-in-oil (W/O) emulsion inks has been investigated.
For example, Patent Document 1 discloses an inkjet emulsion ink that is a water-in-oil type emulsion ink consisting of an oil phase and an aqueous phase, the aqueous phase containing water and a colorant, the colorant being a modified pigment containing a polymer, for the purpose of providing an inkjet emulsion ink that can obtain a sufficient optical density of an image when recorded on plain paper.

JP 2015-189859 A

In the production of water-in-oil (W/O) emulsion inks, when a pigment is contained in the aqueous phase as a colorant, an aqueous pigment dispersion in which the pigment is dispersed in an aqueous medium may be used. Since the dispersion stability of the pigment in the aqueous pigment dispersion also affects the dispersion stability of the pigment in the aqueous phase of the water-in-oil emulsion ink, there is a demand for improving the performance of the aqueous pigment dispersion.
Furthermore, when a water-in-oil emulsion ink is used in an inkjet recording method, there is also a demand for improved ejection stability when continuously printing onto a print medium by the inkjet recording method (hereinafter also referred to as "continuous ejection stability").
However, although Patent Document 1 discloses a water-in-oil emulsion ink using a pigment dispersion in which a modified pigment containing a polymer is dispersed in water, it was found that when applied to a water-in-oil emulsion ink, it was not possible to obtain an aqueous pigment dispersion having excellent storage stability when stored for a long period of time at high temperatures, and the continuous ejection stability was insufficient.
An object of the present invention is to provide an aqueous pigment dispersion which, when used in a water-in-oil emulsion ink, has excellent storage stability and excellent continuous ejection stability in an inkjet recording system; a method for producing the aqueous pigment dispersion; and a water-in-oil emulsion ink which uses the aqueous pigment dispersion.

The present inventors have found that the above-mentioned problems can be solved by providing an aqueous pigment dispersion containing a pigment, a polymer dispersant, and water, in which the polymer dispersant has a crosslinked structure, the degree of crosslinking and the acid value of the polymer dispersant being within specific ranges, and the mass ratio of the pigment content to the polymer dispersant (A) content and the pigment content in the aqueous pigment dispersion being within specific ranges. That is, the present invention provides the following [1] to [3].
[1] An aqueous pigment dispersion containing a pigment, a polymer dispersant (A), and water,
The polymer dispersant (A) has a crosslinked structure, and the crosslinking degree of the polymer dispersant (A) is 30 mol % or more and 70 mol % or less,
The acid value of the polymer dispersant (A) is 70 mgKOH/g or more and 200 mgKOHg or less,
a mass ratio of the content of the pigment in the aqueous pigment dispersion to the content of the polymer dispersant (A) [pigment/polymer dispersant (A)] is 3.0 or more and 5.0 or less;
The content of the pigment in the aqueous pigment dispersion is 30% by mass or more.
[2] A method for producing the water-based pigment dispersion according to the above [1], comprising the following steps 1-1, 1, and 2:
Step 1-1: a step of neutralizing a portion of the acid groups of the polymer (a) with a neutralizing agent to obtain an aqueous dispersion of the polymer (a); Step 1: a step of mixing the aqueous dispersion of the polymer (a) obtained in step 1-1 with a pigment to obtain a pigment mixture, and then dispersing the pigment mixture to obtain an aqueous pigment dispersion (i) in which the pigment is dispersed in an aqueous medium by the polymer (a); Step 2: a step of adding a crosslinking agent to the aqueous pigment dispersion (i) obtained in step 1, and crosslinking the polymer (a) with the crosslinking agent to obtain an aqueous pigment dispersion in which the pigment is dispersed by the polymer dispersant (A). [3] A water-in-oil emulsion ink obtained by emulsifying the aqueous pigment dispersion described in [1] above in a water-insoluble organic solvent.

The present invention provides an aqueous pigment dispersion that, when used in a water-in-oil emulsion ink, has excellent storage stability and excellent continuous ejection stability in an inkjet recording system, a method for producing the aqueous pigment dispersion, and a water-in-oil emulsion ink that uses the aqueous pigment dispersion.

[Water-based pigment dispersion]
The aqueous pigment dispersion of the present invention contains a pigment, a polymer dispersant (A), and water, the polymer dispersant (A) has a crosslinked structure, the degree of crosslinking of the polymer dispersant (A) is 30 mol % or more and 70 mol % or less, the acid value of the polymer dispersant (A) is 70 mgKOH/g or more and 200 mgKOHg or less, the mass ratio of the content of the pigment to the content of the polymer dispersant (A) in the aqueous pigment dispersion [pigment/polymer dispersant (A)] is 3.0 or more and 5.0 or less, and the content of the pigment in the aqueous pigment dispersion is 30 mass % or more.
In the aqueous pigment dispersion of the present invention, the pigment is dispersed in an aqueous medium by the polymer dispersant (A). Here, "aqueous" means that water accounts for the largest proportion in the medium.
As the water in the aqueous medium, deionized water, ion-exchanged water or distilled water is preferably used.
The aqueous medium may further contain an organic solvent, examples of which include water-soluble organic solvents that dissolve in water, such as aliphatic alcohols having from 1 to 4 carbon atoms, such as methanol, ethanol, and 2-propanol, ketones having from 3 to 8 carbon atoms, such as acetone and methyl ethyl ketone, and ethers, such as tetrahydrofuran.
From the viewpoint of environmental friendliness, the water content in the aqueous medium is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, and still more preferably 95% by mass or more.

The aqueous pigment dispersion of the present invention, when used in a water-in-oil emulsion ink (hereinafter also referred to as "emulsion ink"), has excellent storage stability and excellent continuous ejection stability in an ink-jet recording system. The reason for this is not clear, but is thought to be as follows.
In the present invention, the polymer dispersant dispersing the pigment has a crosslinked structure, a degree of crosslinking in a specific range, an acid value in a specific range, and a mass ratio of the pigment to the polymer dispersant in a specific range, so that while having good pigment dispersibility in an aqueous medium, the crosslinked structure firmly adsorbs or fixes the pigment to the surface of the pigment, suppressing pigment aggregation in the aqueous phase, particularly depletion aggregation, and further suppressing swelling of the polymer. As a result, the viscosity of the aqueous pigment dispersion is reduced, and when used in an emulsion ink, the viscosity of the aqueous phase is also reduced, making it possible to obtain an emulsion ink in which a fine and stable emulsion is formed, and it is considered that the storage stability during long-term storage at high temperatures can be improved.
In addition, the pigment content in the aqueous pigment dispersion of the present invention is high, and the mass ratio of the pigment to the polymer dispersant is within a specific range. Therefore, when used in an emulsion ink, a fine and stable emulsion having an aqueous phase with a high pigment concentration can be formed. Normally, a recording device used in an inkjet recording method is provided with a filter to prevent the inclusion of foreign matter that would clog the nozzles or ink flow paths of the inkjet head, but in the present invention, a fine and stable emulsion can be formed, so that it is considered that clogging of the filter can be suppressed and continuous ejection stability can be improved.

<Pigments>
The pigment according to the present invention may be either an inorganic pigment or an organic pigment, and may be a lake pigment or a fluorescent pigment. If necessary, these pigments may be used in combination with an extender pigment.
Specific examples of inorganic pigments include carbon black, metal oxides such as titanium oxide, iron oxide, red iron oxide, and chromium oxide, pearlescent pigments, etc. In particular, carbon black is preferred for black ink.
The type of carbon black is not particularly limited, and examples thereof include furnace black, lamp black, acetylene black, and channel black.
The DBP oil absorption of the carbon black is preferably 20 mL/100 g or more and 200 mL/100 g or less from the viewpoints of improving the dispersion stability of the pigment and improving the storage stability and continuous ejection stability.
The DBP oil absorption refers to the oil absorption measured by the DBP method, specifically, a value based on ASTM D2414-65T.

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 chromatic pigment such as yellow, magenta, cyan, blue, red, orange, green, etc. 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 pigments can be used alone or in combination of two or more kinds.

<Polymer Dispersant (A)>
The polymer dispersant (A) according to the present invention contains an acid group and has a crosslinked structure, from the viewpoints of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous ejection stability.
Examples of the acid group of the polymer dispersant (A) include groups that exhibit acidity by dissociating and releasing hydrogen ions, such as a carboxy group (-COOM), a sulfonic acid group (-SO ₃ M), and a phosphate group (-OPO ₃ M ₂ ), or their dissociated ionic forms (-COO ^- , -SO ₃ ^- , -OPO ₃ ^2- , -OPO ₃ ^- M). In the above chemical formula, M represents a hydrogen atom, an alkali metal, ammonium, or an organic ammonium. Of these, the carboxy group (-COOM) is preferred from the same viewpoint as above.
From the viewpoints of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability, the polymer dispersant (A) is preferably a polymer obtained by crosslinking a non-crosslinked polymer with a crosslinking agent or a polymer having a self-crosslinking structure into which a crosslinkable group has been introduced, and more preferably a polymer obtained by crosslinking a non-crosslinked polymer (hereinafter also referred to as "polymer (a)") with a crosslinking agent.
When the polymer dispersant (A) is a polymer obtained by crosslinking a polymer (a) with a crosslinking agent, the polymer dispersant (A) has a structure derived from the crosslinking agent in addition to the constituent units derived from the monomers constituting the polymer (a).
Examples of the polymer dispersant (A) include polyester resins, polyurethane resins, vinyl resins, etc. Of these, the polymer dispersant (A) is preferably a vinyl resin from the same viewpoint as above.

The acid value of the polymer dispersant (A) is 70 mgKOH/g or more, preferably 75 mgKOH/g or more, more preferably 80 mgKOH/g or more, even more preferably 85 mgKOH/g or more, and even more preferably 90 mgKOH/g or more, and is 200 mgKOH/g or less, preferably 180 mgKOH/g or less, more preferably 160 mgKOH/g or less, and even more preferably 140 mgKOH/g or less, from the viewpoint of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability. If the acid value is in the above range, the amount of acid groups can be sufficient to ensure the dispersion stability of the pigment. It is also preferable from the viewpoint of the balance between the affinity of the polymer dispersant (A) with water and the interaction between the polymer dispersant (A) with the pigment.
The acid value of the polymer dispersant (A) can be measured by the method described in the Examples.
In addition, when the polymer dispersant (A) is a polymer obtained by crosslinking the above-mentioned polymer (a) with a crosslinking agent, the acid value of the polymer dispersant (A) can also be calculated by the following formula.
Acid value of polymer dispersant (A) (mg KOH/g)=[acid value of polymer (a) (mg KOH/g)]×[100−degree of crosslinking (mol %)]/100
Here, the acid value of the polymer (a) can be calculated from the mass ratio of the constituent monomers. The crosslinking degree is calculated from the equivalent of the acid group of the polymer (a) and the equivalent of the crosslinkable functional group of the crosslinking agent, and is expressed as "the molar equivalent number of the crosslinkable functional group of the crosslinking agent / the molar equivalent number of the acid group of the polymer (a)".

At least a part of the acid groups of the polymer dispersant (A) is preferably neutralized from the viewpoints of improving the dispersion stability of the pigment and improving the storage stability and continuous discharge stability.
Examples of the neutralizing agent used for neutralizing the polymer dispersant (A) include alkali metal hydroxides and ammonia.
The counter ion of the neutralized acid group of the polymer dispersant (A) is preferably an alkali metal cation, more preferably one or more selected from sodium ion (Na ⁺ ) and potassium ion (K ⁺ ), from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and continuous ejection stability.
Regarding the degree of neutralization of the polymer dispersant (A), a part of the acid groups of the polymer dispersant (A) may be neutralized, or all of the acid groups may be neutralized.

[Polymer (a)]
Examples of the polymer (a) include polyester resins, polyurethane resins, vinyl resins, etc. Among them, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and continuous discharge stability, the polymer (a) is preferably a vinyl resin, and more preferably a vinyl resin containing a constituent unit derived from a carboxyl group-containing monomer and a constituent unit derived from a hydrophobic monomer.

(Carboxy Group-Containing Monomer)
The carboxyl group-containing monomer is preferably used as a monomer component of the vinyl resin from the viewpoints of improving the dispersion stability of the pigment and improving the storage stability and continuous discharge stability.
Examples of the carboxy group-containing monomer include (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, and 2-methacryloyloxymethylsuccinic acid, with (meth)acrylic acid being preferred, and acrylic acid being more preferred.
In this specification, "(meth)acrylic acid" means at least one selected from acrylic acid and methacrylic acid.
The carboxyl group-containing monomers can be used alone or in combination of two or more kinds.

(Hydrophobic Monomer)
The hydrophobic monomer is preferably used as a monomer component of the vinyl resin from the viewpoints of improving the adsorption of the polymer to the pigment, improving the dispersion stability of the pigment, and improving the storage stability and continuous ejection stability.
In this specification, "hydrophobic" means that when a monomer is dissolved to saturation in 100 g of ion-exchanged water at 25° C., the amount of the dissolved hydrophobic monomer is less than 10 g. From the viewpoint of the adsorption of the polymer to the pigment, the amount of the dissolved hydrophobic monomer is preferably 5 g or less, and more preferably 1 g or less.
Examples of the hydrophobic monomer include aromatic group-containing monomers and (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol.
In this specification, the term "(meth)acrylate" refers to at least one selected from acrylate and methacrylate.
The hydrophobic monomers can be used alone or in combination of two or more kinds.

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, and more preferably at least one selected from a styrene-based monomer and 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, 2-methylstyrene, vinyltoluene, and divinylbenzene, 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, and phenoxyethyl (meth)acrylate, and more preferably benzyl (meth)acrylate.

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. For example, (meth)acrylates having a straight chain alkyl group such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, and stearyl (meth)acrylate; (meth)acrylates having a branched chain alkyl group such as isopropyl (meth)acrylate, isobutyl (meth)acrylate, tertiary butyl (meth)acrylate, isopentyl (meth)acrylate, isooctyl (meth)acrylate, isodecyl (meth)acrylate, isododecyl (meth)acrylate, isostearyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; and (meth)acrylates having an alicyclic alkyl group such as cyclohexyl (meth)acrylate.

In the present invention, a hydrophobic macromonomer may be used as the hydrophobic monomer constituting the vinyl resin.
The macromonomer is a compound having a number average molecular weight of 500 to 100,000 and having a polymerizable functional group at one end, and can be used as a monomer component of a vinyl resin from the viewpoint of the dispersion stability of the pigment. The polymerizable functional group present at one end is preferably an acryloyloxy group or a methacryloyloxy group, and more preferably a methacryloyloxy group.
The number average molecular weight of the macromonomer is preferably from 1,000 to 10,000. The number average molecular weight is measured by gel permeation chromatography using chloroform containing 1 mmol/L dodecyldimethylamine as a solvent and polystyrene as a standard substance.
As the macromonomer, from the viewpoint of dispersion stability of the pigment, aromatic group-containing monomer-based macromonomers and silicone-based macromonomers are preferred, and aromatic group-containing monomer-based macromonomers are more preferred.
Examples of the aromatic group-containing monomer constituting the aromatic group-containing monomer-based macromonomer include the aromatic group-containing monomers described above in relation to the hydrophobic monomer, and styrene and benzyl (meth)acrylate are preferred, with styrene being more preferred.
The macromonomers can be used alone or in combination of two or more kinds.
Specific examples of commercially available styrene-based macromonomers include AS-6(S), AN-6(S), and HS-6(S) (product names of Toagosei Co., Ltd.).
The silicone macromonomer may be, for example, an organopolysiloxane having a polymerizable functional group at one end.

From the viewpoint of improving the adsorption of the polymer to the pigment, improving the dispersion stability of the pigment, and improving the storage stability and continuous discharge stability, the hydrophobic monomer is preferably one or more selected from aromatic group-containing monomers, (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol, and aromatic group-containing monomer-based macromonomers, more preferably one or more selected from aromatic group-containing monomers and (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol, even more preferably an aromatic group-containing monomer, and even more preferably one or more selected from a styrene-based monomer and an aromatic group-containing (meth)acrylate.

(Nonionic Monomer)
From the viewpoint of dispersion stability of the pigment, a nonionic monomer can be used as a monomer component of the vinyl resin.
Examples of nonionic monomers 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; alkoxypolyalkylene glycol (meth)acrylates such as methoxypolyethylene glycol (n=1-30) (meth)acrylate; and phenoxy (ethylene glycol-propylene glycol copolymer) (n=1-30, ethylene glycol therein: n=1-29) (meth)acrylate. Among these, polypropylene glycol (n=2-30) (meth)acrylate, alkoxypolyalkylene glycol (meth)acrylate, and phenoxy (ethylene glycol/propylene glycol copolymer) (meth)acrylate are preferred.
The nonionic monomers can be used alone or in combination of two or more kinds.
Specific examples of commercially available nonionic monomers include NK Ester M-20G, 40G, 90G, 230G, and the like, manufactured by Shin-Nakamura Chemical Co., Ltd.; Blemmer PE-90, 200, 350, and the like, Blemmer PME-100, 200, 400, and the like, Blemmer PP-500, 800, 1000, and the like, Blemmer AP-150, 400, 550, and the like, Blemmer 50PEP-300, 50POEP-800B, 43PAPE-600B, and the like, manufactured by NOF Corporation.

As described above, the polymer (a) is preferably a vinyl resin containing one or more structural units derived from a carboxyl group-containing monomer selected from acrylic acid and methacrylic acid, and one or more structural units derived from a hydrophobic monomer selected from an aromatic group-containing monomer, a (meth)acrylate having a hydrocarbon group derived from an aliphatic alcohol, and an aromatic group-containing monomer-based macromonomer, and may further be a vinyl resin containing a structural unit derived from a nonionic monomer. However, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and continuous discharge stability, it is more preferably a vinyl resin containing one or more structural units derived from a carboxyl group-containing monomer selected from acrylic acid and methacrylic acid, and one or more structural units derived from a hydrophobic monomer selected from an aromatic group-containing monomer and a (meth)acrylate having a hydrocarbon group derived from an aliphatic alcohol, and even more preferably a vinyl resin containing one or more structural units derived from a carboxyl group-containing monomer selected from acrylic acid and methacrylic acid, and one or more structural units derived from a hydrophobic monomer selected from a styrene-based monomer and an aromatic group-containing (meth)acrylate.

(Content of structural units derived from each monomer in vinyl resin)
The content of the constituent units derived from each monomer in the vinyl resin is as follows, from the viewpoint of improving the dispersion stability of the pigment, and improving the storage stability and continuous discharge stability.
The content of structural units derived from carboxy group-containing monomers in the vinyl resin 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 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, and still more preferably 40% by mass or less.
The content of structural units derived from hydrophobic monomers in the vinyl resin is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, still more preferably 60% by mass or more, and is preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less.
The content of structural units derived from hydrophobic macromonomers in the vinyl resin is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, still more preferably 1% by mass or less, still more preferably substantially 0% by mass, and still more preferably 0% by mass.
The content of constituent units derived from nonionic monomers in the vinyl resin is preferably 10 mass% or less, more preferably 5 mass% or less, even more preferably 3 mass% or less, still more preferably 1 mass% or less, still more preferably substantially 0 mass%, and still more preferably 0 mass%.
The content of the constituent units derived from each monomer in the vinyl resin can be measured by analysis such as NMR. The preferred range of the content of each monomer in the raw material monomers during the production of the vinyl resin (content as the unneutralized amount) can be considered to be the same as the preferred range of the content of the constituent units derived from each monomer in the vinyl resin.

The total content of the constituent units derived from the carboxy group-containing monomer and the constituent units derived from the hydrophobic monomer in the vinyl resin is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, and is preferably 100% by mass or less, still more preferably substantially 100% by mass, and still more preferably 100% by mass.
The mass ratio of the content of constituent units derived from carboxy group-containing monomers to the content of constituent units derived from hydrophobic monomers in the vinyl resin [content of constituent units derived from carboxy group-containing monomers/content of constituent units derived from hydrophobic monomers] is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, still more preferably 0.4 or more, and is preferably 2 or less, more preferably 1.5 or less, even more preferably 1.0 or less, still more preferably 0.9 or less, still more preferably 0.7 or less, and still more preferably 0.5 or less.

Vinyl resins are produced by copolymerizing raw material monomers using known polymerization methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Among these polymerization methods, solution polymerization is preferred. There are no restrictions on the chain form of the polymerized monomers, and any polymerization form such as random, block, or graft may be used.

From the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and continuous ejection stability, the acid value of the polymer (a) is preferably 120 mgKOH/g or more, more preferably 140 mgKOH/g or more, even more preferably 160 mgKOH/g or more, still more preferably 180 mgKOH/g or more, still more preferably 200 mgKOH/g or more, still more preferably 220 mgKOH/g or more, and is preferably 320 mgKOH/g or less, more preferably 300 mgKOH/g or less, and still more preferably 280 mgKOH/g or less. If the acid value is within the above range, the amount of acid groups can be sufficient to ensure the dispersion stability of the pigment.
The acid value of the polymer (a) can be measured by the method described in the Examples. As described above, the acid value can be calculated from the mass ratio of the constituent monomers.

The weight average molecular weight of the polymer (a) is preferably 2,000 or more, more preferably 5,000 or more, even more preferably 7,000 or more, and is preferably 100,000 or less, more preferably 50,000 or less, even more preferably 30,000 or less, and still more preferably 20,000 or less. When the weight average molecular weight of the polymer (a) is within the above range, the adsorptivity to the pigment is sufficient, and dispersion stability can be exhibited.
The weight average molecular weight of the polymer (a) is measured by the method described in the Examples.

[Crosslinking Agent]
From the viewpoint of reactivity with the polymer (a), the water solubility (mass ratio) of the crosslinking agent is preferably 50% or less, more preferably 40% or less, and even more preferably 35% or less. The water solubility (mass ratio) refers to the solubility (%) when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of water at room temperature of 25° C.

The crosslinking agent is preferably a polyfunctional compound, and the crosslinkable functional group of the polyfunctional compound is preferably capable of reacting with the acid group of the polymer (a) to form a covalent bond. The covalent bond is preferably one or more selected from ester bond, thioester bond, amide bond, amino bond, ether bond, thioether bond, carbonyl bond, thiocarbonyl bond and sulfonyl bond, more preferably one or more selected from ester bond, thioester bond and amide bond. The crosslinkable functional group of the polyfunctional compound is not particularly limited as long as it can form the above-mentioned covalent bond.
[0113] From the viewpoints of improving the reactivity and the dispersion stability of the pigment to reduce the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous ejection stability, the molecular weight of the crosslinking agent is preferably 120 or more, more preferably 150 or more, and even more preferably 200 or more, and is preferably 2,000 or less, more preferably 1,500 or less, even more preferably 1,000 or less, still more preferably 500 or less, and even more preferably 300 or less.

The crosslinkable functional group is preferably one or more selected from epoxy group, isocyanate group, aziridino group, amino group and oxazoline group, more preferably epoxy group. The concept of epoxy group includes glycidyl group. That is, the crosslinking agent is preferably a multifunctional epoxy compound, and from the viewpoint of crosslinking efficiency, 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 even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 8 carbon atoms.
When the crosslinking agent is a polyfunctional epoxy compound, the epoxy equivalent of the crosslinking agent 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.
When the crosslinking agent is a polyfunctional epoxy compound, the number of epoxy groups in the crosslinking agent is 2 or more per molecule from the viewpoints of efficiently crosslinking, improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability, and is preferably 6 or less per molecule, and from the viewpoint of market availability, is more preferably 4 or less, even more preferably 3 or less, and still more preferably 2.

Specific examples of crosslinking agents include polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, diethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether. Among these, preferably, it is at least one selected from 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, diethylene glycol diglycidyl ether, trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, and neopentyl glycol diglycidyl ether, and more preferably, it is at least one selected from 1,6-hexanediol diglycidyl ether and trimethylolpropane polyglycidyl ether.

From the viewpoints of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability, the crosslinking degree of the polymer dispersant (A) is 30 mol % or more, preferably 35 mol % or more, more preferably 40 mol % or more, and even more preferably 45 mol % or more, and is 70 mol % or less, and preferably 65 mol % or less.
As described above, when the polymer dispersant (A) is a polymer obtained by crosslinking the polymer (a) with a crosslinking agent, the degree of crosslinking is expressed by "the number of molar equivalents of the crosslinkable functional groups of the crosslinking agent / the number of molar equivalents of the acid groups in the polymer (a)", as described above.
As described later, when a part of the acid groups of the polymer (a) is neutralized, the neutralized acid groups can also become crosslinking points. Therefore, when the sum of the neutralization degree and the crosslinking degree exceeds 100 mol %, the crosslinking degree is expressed as "the molar equivalent number of the crosslinkable functional group of the crosslinking agent/the molar equivalent number of the acid group of the polymer (a)" as described above, but the neutralization degree is a value obtained by subtracting the crosslinking degree from 100 mol %.

In the aqueous pigment dispersion of the present invention, the polymer dispersant (A) may be in a form in which it is adsorbed to the pigment, in which it contains the pigment (including a particle form in which the pigment is encapsulated and a particle form in which the pigment is uniformly dispersed), or in a form in which the pigment is not adsorbed. In the present invention, from the viewpoint of improving the dispersion stability of the pigment, the form of particles in which the polymer dispersant (A) contains the pigment (hereinafter also referred to as "pigment-containing crosslinked polymer particles") is preferred, and the form of particles in which the polymer dispersant (A) encapsulates the pigment is more preferred.

[Method of producing water-based pigment dispersion]
The water-based pigment dispersion of the present invention is preferably produced efficiently by a method including the following steps 1 and 2.
Step 1: A step of dispersing a pigment mixture containing a pigment, a polymer (a), and water to obtain an aqueous pigment dispersion (i) in which the pigment is dispersed in an aqueous medium by the polymer (a). Step 2: A step of adding a crosslinking agent to the aqueous pigment dispersion (i) obtained in step 1, and crosslinking the polymer (a) with the crosslinking agent to obtain an aqueous pigment dispersion in which the pigment is dispersed by the polymer dispersant (A).

(Step 1)
Specifically, step 1 is preferably carried out by a method (1) of dispersing a pigment mixture obtained by mixing a previously prepared aqueous dispersion of polymer (a) with a pigment, or a method (2) of dispersing a pigment mixture containing a pigment, polymer (a), an organic solvent, water, and, if necessary, a neutralizing agent, a surfactant, etc., to obtain a dispersion, and then removing the organic solvent from the dispersion, etc. Among these, method (1) is preferred from the viewpoint of improving storage stability and continuous discharge stability.

In the case of the method (1), it is preferable to have the following step 1-1 before the step 1.
Step 1-1: A step of neutralizing a part of the acid groups of the polymer (a) with a neutralizing agent to obtain an aqueous dispersion of the polymer (a) (hereinafter, also referred to as "aqueous polymer dispersion (D)"). Preferred neutralizing agents used for the neutralization in step 1-1 include the same as those exemplified for the polymer dispersant (A). From the viewpoint of promoting sufficient and uniform neutralization, the neutralizing agent is preferably used as an aqueous solution.
In step 1-1, from the viewpoints of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability, the degree of neutralization is preferably 20 mol % or more, more preferably 30 mol % or more, even more preferably 40 mol % or more, and is preferably 80 mol % or less, more preferably 70 mol % or less, even more preferably 60 mol % or less.
The degree of neutralization (mol %) can be calculated by the following formula.
Degree of neutralization (mol %)=[{weight (g) of neutralizing agent added/equivalent weight of neutralizing agent}/[{acid value (mg KOH/g) of polymer (a)×weight (g) of polymer (a)}/(56.1×1,000)]]×100

In the dispersion treatment in step 1, 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 means for applying shear stress used in the present dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersers such as paint shakers and bead mills. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment.
When the dispersion treatment is performed using a high-pressure homogenizer, the pigment can be controlled to have a desired particle size by controlling the treatment pressure and the number of passes, and the cumulant average particle size of the aqueous pigment dispersion described below can also be adjusted.
From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more and 300 MPa or less.
The number of passes is preferably 3 or more, more preferably 7 or more, from the viewpoint of reducing the particle size of the pigment, and is preferably 30 or less, more preferably 20 or less, from the viewpoint of the dispersion stability of the pigment.
When the pigment mixture contains an organic solvent, the organic solvent can be removed by a known method to obtain an aqueous pigment dispersion. It is preferable that the organic solvent in the obtained aqueous pigment dispersion is substantially removed, but it may remain as long as it does not impair the object of the present invention. The amount of the residual organic solvent is preferably 0.1% by mass or less, more preferably 0.01% by mass or less.

(Step 2)
In step 2, the polymer (a) that disperses the pigment in the aqueous pigment dispersion (i) obtained in step 1 is crosslinked with a crosslinking agent, thereby obtaining an aqueous pigment dispersion in which the pigment is dispersed in the polymer dispersant (A).
The preferred crosslinking agents and the degree of crosslinking are as described above.
The temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 50° C. or higher, and even more preferably 60° C. or higher, and is preferably 95° C. or lower, more preferably 85° C. or lower, and even more preferably 75° C. or lower.
From the viewpoints of completion of the crosslinking reaction and economic efficiency, the time for the crosslinking treatment is preferably 0.5 hours or more, more preferably 1 hour or more, even more preferably 3 hours or more, and is preferably 12 hours or less, more preferably 10 hours or less, even more preferably 8 hours or less.

The aqueous pigment dispersion of the present invention is preferably one in which pigment-containing polymer particles are dispersed in an aqueous medium. The form of the pigment-containing polymer particles is not particularly limited as long as the particles are formed by at least the pigment and the polymer dispersant (A), but as described above, the form of the particles in which the pigment is encapsulated in the polymer dispersant (A) is preferred.
The aqueous pigment dispersion of the present invention may contain 1% by mass or more and 10% by mass or less of glycerin, polyethylene glycol or the like as a moisturizing agent to prevent drying, and may also contain additives such as antifungal agents.
The above-mentioned additives may be added when the pigment is dispersed in the polymer (a), or may be added after the pigment is dispersed or after crosslinking.
The contents and physical properties of each component in the water-based pigment dispersion of the present invention are as follows.

The content of the pigment in the aqueous pigment dispersion of the present invention is 30% by mass or more, preferably 33% by mass or more, more preferably 35% by mass or more, from the viewpoint of facilitating adjustment of the viscosity of the aqueous phase component of the emulsion ink, improving storage stability and continuous ejection stability, and improving print density when used in an emulsion ink, and is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less, from the viewpoint of pigment dispersion stability.

The content of the polymer dispersant (A) in the aqueous pigment dispersion of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 7% by mass or more, and is preferably 18% by mass or less, more preferably 15% by mass or less, even more preferably 13% by mass or less, from the viewpoint of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability.

The mass ratio of the pigment content to the polymer dispersant (A) content in the aqueous pigment dispersion of the present invention [pigment/polymer dispersant (A)] is 3.0 or more, preferably 3.3 or more, more preferably 3.5 or more, even more preferably 3.7 or more, and 5.0 or less, preferably 4.7 or less, more preferably 4.5 or less, and even more preferably 4.3 or less, from the viewpoint of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability.

The water content in the aqueous pigment dispersion 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 60% by mass or less, more preferably 58% by mass or less, even more preferably 56% by mass or less, from the viewpoint of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability.

The non-volatile component concentration (solid content concentration) of the aqueous pigment dispersion of the present invention is preferably 37% by mass or more, more preferably 40% by mass or more, even more preferably 43% by mass or more, from the viewpoint of improving the dispersion stability of the pigment and facilitating the production of the emulsion ink, and is preferably 55% by mass or less, more preferably 50% by mass or less, even more preferably 47% by mass or less. The solid content concentration of the aqueous pigment dispersion is measured by the method described in the Examples.

The cumulant average particle diameter of the aqueous pigment dispersion of the present invention is preferably 50 nm or more, more preferably 60 nm or more, even more preferably 70 nm or more, even more preferably 80 nm or more, even more preferably 90 nm or more, and is preferably 200 nm or less, more preferably 150 nm or less, even more preferably 130 nm or less, even more preferably 100 nm or less, and even more preferably less than 100 nm, from the viewpoint of improving the dispersion stability of the pigment, reducing the viscosity of the aqueous pigment dispersion, and improving the storage stability and continuous discharge stability. The cumulant average particle diameter of the aqueous pigment dispersion is measured by the method described in the Examples.

From the viewpoint of facilitating adjustment of the viscosity of the aqueous phase component of the emulsion ink and improving storage stability and continuous ejection stability, the viscosity of the aqueous pigment dispersion of the present invention at 25°C is preferably 200 mPa·s or less, more preferably 150 mPa·s or less, even more preferably 100 mPa·s or less, even more preferably 50 mPa·s or less, even more preferably 30 mPa·s or less, and is preferably 5 mPa·s or more, more preferably 10 mPa·s or more, even more preferably 15 mPa·s or more, even more preferably 20 mPa·s or more. The viscosity of the aqueous pigment dispersion is measured by the method described in the Examples.

[Water-in-oil emulsion ink]
From the viewpoints of storage stability and continuous ejection stability, the water-based pigment dispersion of the present invention is preferably used in a water-in-oil emulsion ink.
The emulsion ink is preferably obtained by emulsifying the water-based pigment dispersion in a water-insoluble organic solvent, and the emulsion ink can be produced by mixing an aqueous phase component containing the water-based pigment dispersion with an oil phase component containing the water-insoluble organic solvent, and emulsifying the aqueous phase containing the water-based pigment dispersion.

(Aqueous Phase Components)
The aqueous phase component of the emulsion ink of the present invention may be the aqueous pigment dispersion as it is, or may contain other components other than the aqueous pigment dispersion as long as the effect of the present invention is not impaired. Such other components may include surfactants, water-soluble organic solvents, oil-in-water polymer emulsions, viscosity modifiers, defoamers, preservatives, antifungal agents, and rust inhibitors. Among these, surfactants and water-soluble organic solvents are preferably used. In addition, when preparing the aqueous phase component, ion-exchanged water, for example, may be further added to the aqueous pigment dispersion from the viewpoint of adjusting the content of components such as pigments in the ink to a desired range.

[Surfactant]
The surfactant is preferably a nonionic surfactant, more preferably one or more selected from an acetylene glycol surfactant, a polyoxyalkylene alkyl ether surfactant, a polyether-modified silicone surfactant, and a fluorine-based surfactant, even more preferably one selected from an acetylene glycol surfactant and a polyether-modified silicone surfactant, and still more preferably an acetylene glycol surfactant.

Examples of acetylene glycol surfactants include an ethylene oxide (hereinafter also referred to as "EO") adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, an EO adduct of 3,6-dimethyl-4-octyne-3,6-diol, an EO adduct of 2,5-dimethyl-3-hexyne-2,5-diol, an EO adduct of 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, an EO adduct of 3,5-dimethyl-1-hexyne-3-ol, and the like.
The average number of moles of EO added in the EO adduct used as the acetylene glycol surfactant is preferably 5 or more and 40 or less.
Commercially available acetylene glycol surfactants include the Surfynol (trade name) series manufactured by Nissin Chemical Industry Co., Ltd. and the Acetylenol (trade name) series manufactured by Kawaken Fine Chemicals Co., Ltd.

The polyether-modified silicone surfactant is preferably one having a structure in which the side chain and/or terminal hydrocarbon group of the silicone oil is replaced with a polyether group.As the polyether group, a polyethyleneoxy group, a polypropyleneoxy group, or a polyalkyleneoxy group in which an ethyleneoxy group (EO) and a propyleneoxy group (trimethyleneoxy group or propane-1,2-diyloxy group; PO) are added in a block or random manner is preferable, and a compound in which a polyether group is grafted onto a silicone main chain, a compound in which a silicone and a polyether group are bonded in a block form, etc. can be used.
Commercially available polyether-modified silicone surfactants include KF-353, KF-355A, KF-642, and the like manufactured by Shin-Etsu Chemical Co., Ltd.; Silface SAG005, and the like manufactured by Nissin Chemical Industry Co., Ltd.; FZ-2191, and the like manufactured by NUC Corporation; and BYK-348, and the like manufactured by BYK Japan K.K.

[Water-soluble organic solvent]
The water-soluble organic solvent refers to an organic solvent that, when dissolved in 100 mL of water at 25° C., dissolves in an amount of 10 mL or more.
The water-soluble organic solvent is preferably one that is commonly used in the aqueous phase of water-in-oil emulsion inks and is miscible with water in any ratio, and examples of such solvents include alkanols having from 1 to 4 carbon atoms, polyhydric alcohols, polyhydric alcohol alkyl ethers, ketones, cyclic ethers, nitrogen-containing heterocyclic compounds, amides, amines, sulfur-containing compounds, etc. Of these, one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers are preferred.
The boiling point of the water-soluble organic solvent is preferably 60° C. or higher, more preferably 90° C. or higher, and even more preferably 150° C. or higher, and is preferably 350° C. or lower, more preferably 330° C. or lower, and even more preferably 310° C. or lower.
The water-soluble organic solvent may be used alone or in combination of two or more kinds, so long as it is miscible with water in any ratio.

(Oily Phase Components)
The oil phase component of the emulsion ink of the present invention contains a water-insoluble organic solvent and preferably further contains an emulsifier, from the viewpoints of improving the dispersion stability of the pigment in the aqueous phase, thereby reducing the average particle size of the emulsion, and improving storage stability and continuous ejection stability.

[Non-water-soluble organic solvent]
The water-insoluble organic solvent according to the present invention refers to an organic solvent which, when dissolved in 100 mL of water at 25° C., dissolves in an amount of less than 10 mL.
As the non-water-soluble organic solvent, either a non-polar organic solvent or a polar organic solvent can be used. These can be used alone or in combination of two or more kinds as long as they form a single continuous phase.

Examples of non-polar organic solvents include aliphatic hydrocarbon solvents such as paraffin-based, isoparaffin-based, and naphthenic solvents, and petroleum-based hydrocarbon solvents such as alicyclic hydrocarbon solvents and aromatic hydrocarbon solvents.
The initial boiling point of the non-polar organic solvent is preferably 90° C. or higher, more preferably 150° C. or higher, and is preferably 300° C. or lower, more preferably 250° C. or lower.
Commercially available non-polar organic solvents include the Isopar (trade name) series and Exxol (trade name) series (both manufactured by Exxon Mobil Corporation); and the AF Solvent (trade name) series (manufactured by JXTG Nippon Oil & Energy Corporation).

Examples of the polar organic solvent include ester-based solvents and higher alcohol-based solvents.
The ester solvent is preferably a fatty acid ester solvent. Examples of the fatty acid ester solvent include fatty acid esters having 13 to 30 carbon atoms, such as isononyl isononanoate, isodecyl isononanoate, isotridecyl isononanoate, methyl laurate, isopropyl laurate, hexyl laurate, 2-ethylhexyl laurate, isopropyl myristate, isopropyl palmitate, isostearyl palmitate, methyl oleate, ethyl oleate, isopropyl oleate, isobutyl oleate, methyl linoleate, ethyl linoleate, isobutyl linoleate, butyl stearate, 2-ethylhexyl stearate, isopropyl isostearate, methyl soybean oil, isobutyl soybean oil, methyl tall oil, isobutyl tall oil, propylene glycol monocaprate, diisopropyl adipate, diisopropyl sebacate, diethyl sebacate, trimethylolpropane tri-2-ethylhexanoate, and glyceryl tri-2-ethylhexanoate.
Examples of higher alcohol solvents include higher alcohols having 12 to 20 carbon atoms, such as isomyristyl alcohol, isopalmityl alcohol, isostearyl alcohol, and oleyl alcohol.

The non-water-soluble organic solvent preferably contains a hydrocarbon solvent from the viewpoint of adjusting the viscosity of the emulsion ink and from the viewpoint of improving the storage stability and continuous ejection stability.
From the same viewpoints as above, the content of the hydrocarbon solvent in the water-insoluble organic solvent is preferably 50 mass% or more, more preferably 60 mass% or more, even more preferably 70 mass% or more, and preferably 100 mass% or less, even more preferably 90 mass% or less, and even more preferably 80 mass% or less.

〔emulsifier〕
There are no particular limitations on the emulsifier as long as it is contained in the oil phase and can emulsify the aqueous phase to form a water-in-oil emulsion, but lipophilic emulsifiers such as surfactants and polymer dispersants are preferred.
The lipophilic surfactant is preferably a nonionic surfactant having an HLB value of 6 or less. Examples of the nonionic surfactant include sorbitan fatty acid esters such as sorbitan monooleate, sorbitan sesquioleate, and sorbitan monostearate; polyoxyethylene sorbitan fatty acid esters such as polyoxyethylene sorbitan monooleate and polyoxyethylene sorbitan trioleate; glycerin fatty acid esters such as glycerol monostearate and glycerol monooleate; polyoxyethylene glycerin fatty acid esters such as polyoxyethylene sorbit fatty acid ester and polyoxyethylene glycerin vegetable oil fatty acid ester; sucrose fatty acid esters such as sucrose stearate, sucrose palmitate, sucrose oleate, sucrose erucate, and sucrose laurate; polyethylene glycol fatty acid esters; polyglycerin condensed ricinoleate; polyoxyethylene alkyl ethers; polyoxyethylene alkylphenyl ethers; polyoxyethylene alkylamines/fatty acid amides; and polyoxyethylene polyoxypropylene alkyl ethers.
The lipophilic surfactants can be used alone or in combination of two or more kinds.

Examples of lipophilic polymer dispersants include polyester polyimines, polyester polyamines, polyalkylol aminoamides and their salts, polyether polyamines, hydroxyl group-containing carboxylic acid esters, salts of long-chain polyaminoamides and high molecular weight acid esters, salts of high molecular weight polycarboxylic acids, salts of long-chain polyaminoamides and polar acid esters, high molecular weight unsaturated acid esters, modified polyurethanes, modified poly(meth)acrylates, polyether ester-type anionic surfactants, naphthalene sulfonate formalin condensate salts, polyoxyethylene alkyl phosphate esters, polyoxyethylene nonylphenyl ether, stearylamine acetate, and the like. Among these, one or more selected from polyester polyimines, polyester polyamines, polyalkylol aminoamides and their salts, and polyether polyamines are preferred, and polyester polyimines are more preferred. These may be used alone or in combination of two or more.

The polyester polyimine is preferably an amide or salt formed by reacting a polyalkyleneimine such as polyethyleneimine with a polyester having a carboxy group, such as a polymer of hydroxystearic acid or a polymer of a cyclic ester, and includes a dispersant having a graft structure in which two or more polyester chains are bonded as branches to a polyalkyleneimine chain as a backbone.
Commercially available lipophilic polymer dispersants include, for example, the Solsperse (trade name) series manufactured by Lubrizol Japan Co., Ltd.; the Efka (trade name) series manufactured by Efka Chemicals; the Disparlon (trade name) series manufactured by Kusumoto Chemicals Co., Ltd.; and the Ajisper (trade name) series manufactured by Ajinomoto Co., Inc.

The oil phase component of the emulsion ink of the present invention preferably does not contain a colorant such as a pigment, from the viewpoint of improving the dispersion stability of the pigment in the aqueous phase and improving storage stability and continuous ejection stability.

When the aqueous phase component of the emulsion ink of the present invention contains the above-mentioned surfactant, water-soluble organic solvent, or optional component, the aqueous pigment dispersion can be mixed with the surfactant, water-soluble organic solvent, or optional component as necessary, or the water content can be adjusted to prepare the aqueous phase component, and then the aqueous phase component and the oil phase component can be mixed and emulsified to prepare the emulsion ink.
The content of the pigment in the aqueous phase component is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more from the viewpoint of improving the print density, and is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less, from the viewpoint of reducing the ink viscosity when the solvent evaporates and improving the storage stability and continuous ejection stability.
From the viewpoint of forming a fine emulsion and improving storage stability, print density and continuous ejection stability, and from the viewpoint of using the emulsion ink for inkjet recording, the viscosity of the aqueous phase component at 25°C is preferably 200 mPa·s or less, more preferably 150 mPa·s or less, even more preferably 100 mPa·s or less, still more preferably 50 mPa·s or less, still more preferably 30 mPa·s or less, and is preferably 5 mPa·s or more, more preferably 10 mPa·s or more, even more preferably 15 mPa·s or more, and still more preferably 20 mPa·s or more.
The viscosity of the aqueous phase component is measured by the method described in the Examples.

If necessary, the emulsion ink of the present invention may further contain various additives commonly used in water-in-oil emulsion inks, such as a humectant, wetting agent, penetrating agent, dispersant, surfactant, viscosity modifier, defoamer, preservative, antifungal agent and rust inhibitor, and may further be subjected to filtration treatment using a filter or the like.
The contents and physical properties of each component in the emulsion ink of the present invention are as follows.

The pigment content in the emulsion ink is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and even more preferably 4% by mass or more, from the viewpoint of improving print density, and is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 7% by mass or less, from the viewpoint of reducing the ink viscosity upon solvent evaporation and improving storage stability and continuous ejection stability.

The mass ratio of the pigment content to the polymer dispersant (A) content in the emulsion ink [pigment/polymer dispersant (A)] is preferably 3.0 or more, more preferably 3.3 or more, more preferably 3.5 or more, even more preferably 3.7 or more, and is preferably 5.0 or less, preferably 4.7 or less, more preferably 4.5 or less, and even more preferably 4.3 or less, from the viewpoint of improving the dispersion stability of the pigment in the aqueous phase and improving the storage stability and continuous ejection stability.

The water content in the emulsion ink is preferably 35% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less, even more preferably 15% by mass or less, even more preferably 10% by mass or less, and even more preferably less than 10% by mass, from the viewpoint of improving storage stability and reducing deformation of printed matter, and is preferably 5% by mass or more, more preferably 7% by mass or more, from the viewpoint of improving continuous ejection stability.

The content of the water-insoluble organic solvent in the emulsion ink is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, even more preferably 60% by mass or more, and even more preferably 70% by mass or more, from the viewpoint of adjusting the viscosity of the emulsion ink, improving storage stability, and reducing deformation of the printed matter, and is preferably 90% by mass or less, more preferably 85% by mass or less, from the viewpoint of improving print density.

The content of the lipophilic emulsifier in the emulsion ink is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, from the viewpoint of forming a fine emulsion and improving storage stability and continuous ejection stability, and is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, from the viewpoint of adjusting the viscosity of the emulsion ink.

From the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and continuous ejection stability, the cumulant average particle size of the pigment particles in the aqueous phase of the emulsion ink is preferably 50 nm or more, more preferably 60 nm or more, even more preferably 70 nm or more, still more preferably 80 nm or more, still more preferably 90 nm or more, and is preferably 200 nm or less, more preferably 150 nm or less, even more preferably 130 nm or less, still more preferably 100 nm or less, and still more preferably less than 100 nm.
The cumulant average particle size of the pigment particles in the aqueous phase of the emulsion ink can be determined, for example, by subjecting the emulsion ink to centrifugal separation under conditions that allow the pigment particles in the aqueous phase of the emulsion ink to be maintained in a dispersed state, demulsifying the emulsion ink to recover the aqueous phase component, and measuring the cumulant average particle size of the pigment particles in the aqueous phase component by the method described in the examples.
The centrifugal separation conditions capable of maintaining the dispersion state of the pigment particles in the aqueous phase can be set by observing the dispersion state of the pigment particles before and after the centrifugation treatment under a microscope.

The viscosity of the emulsion ink at 25°C is preferably 2 mPa·s or more, more preferably 5 mPa·s or more, and even more preferably 7 mPa·s or more, and from the viewpoint of using the emulsion ink for inkjet recording, preferably 20 mPa·s or less, more preferably 17 mPa·s or less, and even more preferably 15 mPa·s or less.
The viscosity of the emulsion ink is measured in the same manner as the viscosity of the water-based pigment dispersion described in the Examples.

From the viewpoint of continuous ejection stability, the emulsion ink of the present invention is preferably used for inkjet recording. The emulsion ink can be loaded into a known inkjet recording device and ejected as ink droplets onto a printing medium to record an image or the like.
The printing medium to be used includes highly water-absorbent plain paper, low water-absorbent coated paper, and non-water-absorbent resin film. The coated paper includes general-purpose glossy paper, multicolor foam gloss paper, etc. The resin film is preferably at least one selected from polyester film, polyvinyl chloride film, polypropylene film, and polyethylene film. The resin film may be a corona-treated substrate.
Among these, plain paper is preferred from the viewpoints of print density and deformation of printed matter.

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

(1) Measurement of Acid Value of Polymer Dispersant (A) or Polymer (a) 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)].

(2) Measurement of Weight-Average Molecular Weight of Polymer (a) Using a solution of phosphoric acid and lithium bromide dissolved in N,N-dimethylformamide to concentrations of 60 mmol/L and 50 mmol/L, respectively, as an eluent, a weight-average molecular weight of polymer (a) was measured by gel permeation chromatography (GPC apparatus (model: HLC-8320GPC, manufactured by Tosoh Corporation), columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn Super AW-H, all trade names manufactured by Tosoh Corporation), flow rate: 0.5 mL/min) using a monodisperse polystyrene kit with known molecular weights [PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick B (F-100), F-200), F-100, F-200, F-300, F-400, F-500, F-600, F-700, F-800, F-900, F-1000, F-1000, F-2000, F-3000, F-400, F-500, F-600, F-700, F-800, F-900, F-10 ... C (F-288, F-40, F-4, A-5000, A-500), all of which are product names manufactured by Tosoh Corporation).
The measurement sample was prepared by mixing 0.1 g of polymer (a) 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 (product name: DISMIC-13HP, membrane filter material: hydrophilic PTFE, pore size 0.2 μm, manufactured by Advantec Co., Ltd.).

(3) Measurement of solid content concentration 10.0 g of sodium sulfate, which had been brought to a constant weight in a desiccator, was weighed out into a 30 mL polypropylene container (φ=40 mm, height=30 mm), and about 1.0 g of the sample was added thereto and mixed, after which it was accurately weighed, maintained at 105° C. for 2 hours to remove volatile matter, and then left in a desiccator at room temperature (25° C.) for 15 minutes, after which the mass was measured. The mass of the sample after removal of volatile matter was taken as the solid content, and divided by the mass of the sample added to obtain the solid content concentration.

(4) Measurement of cumulant average particle diameter Cumulant analysis was performed using a laser particle analysis system (model: 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. 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 solid concentration).

(5) Viscosity of Water-Based Pigment Dispersion and Water Phase Components The viscosity was measured at 25° C. using an E-type viscometer (model: TV-25, manufactured by Toki Sangyo Co., Ltd., using a standard cone rotor 1°34′×R24, rotation speed 50 rpm).

<Preparation of Polymer (a)>
Preparation Example 1
A raw material monomer was prepared by mixing 19.2 parts of acrylic acid, 70.8 parts of styrene, and 10.0 parts of α-methylstyrene. A reaction vessel was charged with 10 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), 0.3 parts of 2-mercaptoethanol (polymerization chain transfer agent), and 10% of the raw material monomer, and the atmosphere was thoroughly replaced with nitrogen gas.
Meanwhile, a mixture of the remaining raw material monomer (90%), 0.27 parts of the polymerization chain transfer agent, 40 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (product name: V-65, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 2,2'-azobis(2,4-dimethylvaleronitrile)) was placed in a dropping funnel, and the raw material monomer 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 stirring for 2 hours from the end of the dropping while maintaining the contents in the reaction vessel at 65°C, 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 polymer (a1) having a carboxy group. The weight average molecular weight and acid value are shown in Table 1.

Preparation Example 2 and Preparation Example 3
Solutions of polymers (a2) and (a3) having a carboxy group were obtained in the same manner as in Preparation Example 1, except that the raw material monomer compositions were changed as shown in Table 1. The weight average molecular weights and acid values are shown in Table 1.

<Production of polymer aqueous dispersion (D) (step 1-1)>
Production Example 1
The polymer solution obtained in Preparation Example 1 was dried under reduced pressure to obtain 80.0 parts of polymer (a1), and 316.0 parts of ion-exchanged water and 25.3 parts of 5N aqueous sodium hydroxide solution were added to neutralize the ratio of the number of moles of sodium hydroxide to the number of moles of carboxyl groups of polymer (a1) to 50 mol% (neutralization degree: 50 mol%). Then, the mixture was heated at 90°C for 5 hours while stirring at 150 rpm to obtain a polymer aqueous dispersion (D1) (solid concentration: 20%).

Production Example 2 and Production Example 3
Polymer aqueous dispersions (D2) and (D3) (solid concentration: 20% and degree of neutralization: 50 mol%) were obtained in the same manner as in Production Example 1, except that the type of polymer (a), the amount of the aqueous sodium hydroxide solution, and the amount of ion-exchanged water were changed as shown in Table 2.

<Water-based pigment dispersion (Step 1 and Step 2)>
Example 1-1
(Step 1)
400.0 parts of ion-exchanged water and 100.0 parts of black pigment (carbon black, trade name: MONARCH717, manufactured by Cabot Corporation, DBP oil absorption 53 mL/100 g) were added to 103.4 parts of the polymer aqueous dispersion (D2) obtained in Production Example 2, and the mixture was stirred for 60 minutes using a disper (manufactured by Asada Iron Works Co., Ltd., trade name: Ultra Disper) at 20° C. with a disper blade rotating at 7,000 rpm. The resulting mixture was subjected to 10 pass dispersion treatment at a pressure of 200 MPa using a microfluidizer (manufactured by Microfluidics, trade name) to obtain a dispersion treatment product.
The obtained dispersion was filtered with a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a 5 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) to remove coarse particles, and then an aqueous pigment dispersion (i-1) having a solid content concentration of 20% was obtained.
(Step 2)
100.0 parts (solids concentration 20%) of the obtained water-based pigment dispersion (i-1) was placed in a screw-capped glass bottle, and 0.62 parts of 1,6-hexanediol diglycidyl ether (product name: Denacol EX-212, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 151, insoluble in water) (hereinafter referred to as "EX-212") as a crosslinking agent was added so that 30 mol % of all carboxy groups in the polymer (a2) were crosslinked (crosslinking degree 30 mol %), and the bottle was sealed and heated at 70°C for 5 hours while stirring with a stirrer. After 5 hours, the temperature was lowered to room temperature, and the mixture was filtered using a 25 mL needleless syringe (Terumo Corporation) equipped with a 5 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm, Fujifilm Wako Pure Chemical Industries, Ltd.), and then concentrated under reduced pressure so that the pigment content in the resulting aqueous pigment dispersion was 36.0% by mass, to obtain an aqueous black pigment dispersion (hereinafter also referred to as "black pigment dispersion") Bk-I (solid content concentration: 44.2%). The pigment content, the mass ratio of the pigment to the polymer dispersant (A) [pigment/polymer dispersant (A)], the water content, the cumulant average particle size, and the viscosity are shown in Table 3.

Examples 1-2 to 1-5 and Comparative Examples 1-1 to 1-2
Black pigment dispersions Bk-II to Bk-V and Bk-CI to Bk-CII were obtained in the same manner as in Example 1-1, except that the type of polymer aqueous dispersion (D) or the amount of crosslinking agent was changed as shown in Table 3.

<Water-in-oil emulsion ink>
Example 2-1
A water-in-oil emulsion ink Bk-1 was obtained by the following procedure so that the pigment content in the resulting water-in-oil emulsion ink was 5.0%.
An aqueous phase component was prepared by mixing 13.9 parts of the black pigment dispersion Bk-I obtained in Example 1-1 and 0.6 parts of ion-exchanged water.
Separately, 58.0 parts of a hydrocarbon solvent (product name: AF Solvent No. 4 (petroleum-based hydrocarbon), manufactured by JXTG Nippon Oil & Energy Corporation) as a water-insoluble organic solvent, 22.5 parts of an ester solvent (product name: Exepar M-OL (methyl oleate), manufactured by Kao Corporation), and 5.0 parts of a sucrose erucate ester (product name: Ryoto Sugar Ester ER-290, manufactured by Mitsubishi Chemical Foods Corporation) as an emulsifier were mixed to prepare an oil phase component.
Using a homogenizer (product name: ULTLRA-TURRAX, model: T25 digital, shaft generator (model: S25N-25F), all manufactured by IKA Japan Co., Ltd.), the aqueous phase component was added dropwise while stirring the oil phase component at 5,000 rpm. Thereafter, the mixture was stirred at 12,000 rpm for 3 minutes to obtain water-in-oil emulsion ink Bk-1.

Examples 2-2 to 2-7 and Comparative Examples 2-1 to 2-4
Each water-in-oil emulsion ink shown in Table 4 was obtained in the same manner as in Example 2-1, except that the type or amount of each component constituting the aqueous phase component and the oil phase component was changed as shown in Table 4.

<Evaluation test of water-in-oil emulsion ink>
The emulsion inks obtained in the examples and comparative examples were evaluated for storage stability and continuous ejection stability. The results are shown in Table 4.

[Evaluation of storage stability]
The obtained emulsion ink was placed in a 100 mL screw vial and stored at 60° C. After standing, the emulsion ink was visually observed for separation of the oil and water phases and flocculated sediment, and evaluated according to the following criteria.
(Evaluation Criteria)
A: Even after storage for one week, there was no separation of the oil and water phases of the emulsion ink, and no flocculated sediment.
B: After one day of storage, there was no separation of the oil and water phases of the emulsion ink, and no flocculated sedimentation, but after one week of storage, there was separation of the oil and water phases of the emulsion ink, or there was flocculated sedimentation.
C: Immediately after preparation of the emulsion ink, there was no separation of the oil and water phases or no flocculated sediment, but after storage for 1 day, there was separation of the oil and water phases of the emulsion ink or no flocculated sediment.
D: Separation of the oil phase and the water phase or flocculation and sedimentation were observed immediately after preparation of the emulsion ink.

[Evaluation of Continuous Discharge Stability]
The obtained emulsion ink was filled into an inkjet printer "Business Printer PX-105" (manufactured by Seiko Epson Corporation), and a nozzle check pattern image in which 105 oblique lines corresponding to the diagonal portions of a 0.6 cm x 3.5 cm rectangle were arranged in parallel at 0.3 cm intervals was continuously printed on plain paper "Xerox Business 4200 Copy Paper" (manufactured by Fuji Xerox Co., Ltd.) at a printing speed of one sheet per 20 seconds under an environment of room temperature of 23°C and relative humidity of 50% RH. The occurrence of nozzle chipping was visually observed and evaluated according to the following criteria.
(Evaluation Criteria)
A: No nozzle chipping occurred even after printing 200 sheets.
B: Nozzle missing occurred between 100 and 199 printed sheets.
C: Nozzle chipping occurred between 10 and 99 printed sheets.
D: Nozzle chipping occurred up to the 9th printed sheet.

From Table 4, it can be seen that Examples 2-1 to 2-7 provide emulsion inks with superior storage stability and continuous ejection stability compared to Comparative Examples 2-1 to 2-4.

The aqueous pigment dispersion of the present invention makes it possible to obtain a water-in-oil emulsion ink that has excellent storage stability and continuous ejection stability. Furthermore, the water-in-oil emulsion ink of the present invention is useful for inkjet recording.

Claims (8)

An aqueous pigment dispersion comprising a pigment, a polymer dispersant (A), and water,
The polymer dispersant (A) has a crosslinked structure, and the crosslinking degree of the polymer dispersant (A) is 30 mol % or more and 70 mol % or less,
The acid value of the polymer dispersant (A) is 70 mgKOH/g or more and 200 mgKOHg or less,
a mass ratio of the content of the pigment in the aqueous pigment dispersion to the content of the polymer dispersant (A) [pigment/polymer dispersant (A)] is 3.0 or more and 5.0 or less;
the content of the pigment in the aqueous pigment dispersion is 30% by mass or more,
the polymer dispersant (A) is a polymer obtained by crosslinking a non-crosslinked polymer (a) with a crosslinking agent, the polymer (a) is a vinyl resin containing a structural unit derived from a carboxy group-containing monomer and a structural unit derived from a hydrophobic monomer, and the hydrophobic monomer is an aromatic group-containing monomer having a molecular weight of less than 500 . 2. The aqueous pigment dispersion according to claim 1 , wherein at least a portion of the acid groups of the polymer dispersant (A) are neutralized, and the counter ions of the neutralized acid groups are alkali metal cations. The aqueous pigment dispersion according to claim 1 or 2 , wherein the water content is from 30% by mass to 60% by mass. The aqueous pigment dispersion according to any one of claims 1 to 3 , which has a viscosity of 200 mPa·s or less. The aqueous pigment dispersion according to any one of claims 1 to 4 , which is used in a water-in-oil emulsion ink. A method for producing the water-based pigment dispersion according to any one of claims 1 to 5 , comprising the following steps 1-1, 1, and 2:
Step 1-1: A step of neutralizing a portion of the acid groups of the polymer (a) with a neutralizing agent to obtain an aqueous dispersion of the polymer (a). Step 1: A step of mixing the aqueous dispersion of the polymer (a) obtained in step 1-1 with a pigment to obtain a pigment mixture, and then dispersing the pigment mixture to obtain an aqueous pigment dispersion (i) in which the pigment is dispersed in an aqueous medium by the polymer (a). Step 2: A step of adding a crosslinking agent to the aqueous pigment dispersion (i) obtained in step 1, and crosslinking the polymer (a) with the crosslinking agent to obtain an aqueous pigment dispersion in which the pigment is dispersed by the polymer dispersant (A). 6. A water-in-oil emulsion ink comprising the aqueous pigment dispersion according to claim 1 emulsified in a non-water-soluble organic solvent. 8. The water-in-oil emulsion ink according to claim 7 , which is for ink-jet recording.