JP7537989B2 - Water-based ink for inkjet printing - Google Patents

Description

The present invention relates to a water-based ink for inkjet printing and an inkjet printing method.

Inkjet recording is a method of printing characters and images by ejecting ink droplets from extremely fine nozzles directly onto a recording medium. This method has become extremely popular due to its many advantages, including the ease of full-color printing, low cost, the ability to use plain paper as a recording medium, and no contact with the printed material.
Meanwhile, in the commercial printing field, in addition to printing on conventional highly absorbent print media such as plain paper and copy paper, there is a demand for printing on low-absorbency coated paper such as offset coated paper, and in the industrial printing field there is a demand for printing on non-absorbent resin films.
Quinacridone pigments, which have good lightfastness, are often used in magenta inkjet inks used in these printing processes, but quinacridone pigments also have various problems, and a technique for using quinacridone pigments in combination with azo pigments is known.

For example, Patent Document 1 discloses a method for improving abrasion resistance using an ink that contains a quinacridone pigment and an azo pigment as colorants, and further contains a polymer having a monomer selected from a methacrylic acid derivative and an acrylic acid derivative as a constituent, a wax, and water.
Other known techniques include adding a highly hydrophobic solvent such as glycol ether to allow ink droplets to properly wet and spread on a printing medium with low ink absorption, and incorporating a resin binder component or wax component into the ink to improve abrasion resistance.

JP 2018-58936 A

However, the technology of Patent Document 1 is insufficient in terms of dot wetting and spreading properties on low-liquid-absorbent printing media, storage stability, and abrasion resistance is also unsatisfactory.
An object of the present invention is to provide a water-based ink for inkjet printing which has excellent storage stability and which is capable of producing printed matter which has excellent color development, wetting and spreading of ink dots onto a printing medium, and a highly balanced abrasion resistance, even when printed on a low-water-absorbency printing medium, and to provide an inkjet printing method which uses the water-based ink.

The present inventors have found that the above-mentioned problems can be solved by selecting and combining C.I. Pigment Violet 19 as a quinacridone pigment and C.I. Pigment Red 150 as an azo pigment from among many pigments, and using a glycol ether compound having an alkyl group with 1 to 3 carbon atoms and a glycol ether compound having an alkyl group with 4 carbon atoms as an organic solvent.
That is, the present invention provides the following [1] and [2].
[1] A water-based ink for ink-jet printing, comprising: crosslinked polymer particles A containing a pigment; polymer particles B not containing a pigment; wax C; an organic solvent D; and water,
The pigment contains C.I. Pigment Violet 19 and C.I. Pigment Red 150, and the organic solvent D contains a glycol ether compound D1 having an alkyl group having 1 to 3 carbon atoms and a glycol ether compound D2 having an alkyl group having 4 carbon atoms;
Water-based ink for inkjet printing.
[2] An inkjet printing method for printing on a low water-absorbent print medium using the water-based ink according to [1] above.

The present invention provides a water-based ink for inkjet printing that has excellent storage stability and can produce printed matter that has excellent color development, ink dot wetting and spreading properties on the print medium, and a highly balanced abrasion resistance, even when printed on a low-water-absorbency print medium, and an inkjet printing method that uses the water-based ink.

[Water-based ink for inkjet printing]
The water-based ink for ink-jet printing of the present invention is a water-based ink for ink-jet printing comprising: crosslinked polymer particles A containing a pigment; polymer particles B not containing a pigment; wax C; an organic solvent D; and water,
The pigment contains C.I. Pigment Violet 19 (hereinafter also referred to as "PV19") and C.I. Pigment Red 150 (hereinafter also referred to as "PR150"), and the organic solvent D contains a glycol ether compound D1 having an alkyl group having 1 to 3 carbon atoms and a glycol ether compound D2 having an alkyl group having 4 carbon atoms.
Here, "water-based" means that water accounts for the largest proportion of the medium contained in the ink.

According to the present invention, it is possible to obtain a printed matter having excellent storage stability, and even when printed on a low water-absorbent print medium, it is possible to obtain a highly balanced printout having excellent color development, wetting and spreading of ink dots on the print medium, and abrasion resistance. The reason for this is unclear, but is thought to be as follows.
PV19, which is a quinacridone pigment, is characterized by a high hydrophilicity of the pigment surface compared to PR122, which is a representative quinacridone pigment. In addition, quinacridone pigments are generally known to have a high pigment coagulation tendency in water compared to azo pigments, making stabilization difficult, and when a quinacridone pigment and an azo pigment are used in combination, it is a task to stabilize both of them to the same degree. In the present invention, by specifically selecting and combining PV19 and PR150, the storage stability is surprisingly improved specifically in a water-based ink containing a hydrophobic glycol ether compound, polymer particles that do not contain a pigment as a binder component, and wax. This is because the surface of PV19, which has a relatively hydrophilic surface among quinacridone pigments, is stabilized by a crosslinked polymer, and the stabilization of the pigment surface in water can be achieved to a degree close to that of PR150, whose surface is stabilized by a crosslinked polymer, and hetero-aggregation between different pigments and with the binder component can be effectively suppressed.
In addition, by using, as the glycol ether compound, glycol ether compound D1 having an alkyl group with 1 to 3 carbon atoms and glycol ether compound D2 having an alkyl group with 4 carbon atoms in combination, it is possible to greatly improve the affinity between the aqueous ink and low water-absorbent printing media, etc., without deteriorating the stability of the aqueous ink, and it is believed that the color development, wetting and spreading properties of the ink dots, and abrasion resistance are also improved.

<Pigment-containing crosslinked polymer particles A>
The form of the crosslinked polymer particle A containing the pigment in the present invention may be a form in which the crosslinked polymer encompasses the pigment, a form in which a part of the pigment is exposed on the surface of a particle consisting of the crosslinked polymer and the pigment, a form in which the crosslinked polymer is adsorbed to a part of the pigment, or a mixed form of these. Among these, the form in which the crosslinked polymer encompasses the pigment is preferred from the viewpoint of improving the storage stability and color development of the water-based ink, the wet spreadability and abrasion resistance of the ink dots.

[Pigments]
In the present invention, since one of the objects is to improve the color development of a quinacridone pigment, the pigment contains at least PV19 which is a quinacridone pigment and PR150 which is a type of azo pigment. Therefore, it is preferable that the crosslinked polymer particles A containing a pigment contain crosslinked polymer particles containing PV19 and crosslinked polymer particles containing PR150.
Here, PV19 is 5,12-dihydroquino[2,3-b]acridine-7,14-dione and PR150 is 3-hydroxy-4-[2-methoxy-5-(phenylcarbamoyl)phenylazo]-2-naphthalenecarboxamide.
In the present invention, pigments other than PV19 and PR150 can be contained within a range that does not impair the objects and effects of the present invention.
As the other pigments, organic pigments are preferable, and examples thereof include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments other than PV19 and PR150; 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.
As for the hue, organic pigments of chromatic colors such as magenta, red, and orange are preferable.

From the viewpoint of improving the storage stability, color development, wet spreadability, and abrasion resistance of the water-based ink, the pigment is dispersed in the water-based ink in the form of crosslinked polymer particles containing the pigment obtained by crosslinking polymer particles in which a polymer having a carboxy group (hereinafter also referred to as "polymer (a)") contains the pigment, with a crosslinking agent such as an epoxy compound.
The polymer (a) 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 a laser particle analysis system described in the examples.

[Polymer (a)]
The polymer (a) preferably has (a-1) a structural unit derived from a carboxy group-containing vinyl monomer (hereinafter also referred to as "component (a-1)") and (a-2) a structural unit derived from a hydrophobic vinyl monomer (hereinafter also referred to as "component (a-2)"). The polymer (a) may further contain (a-3) a structural unit derived from a nonionic monomer, 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 water-based ink of the present invention, as a source for realizing dispersion stability of the pigment.
The component (a-1) may be a carboxylic acid monomer.
The carboxylic acid 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 viewpoint of improving the dispersion stability of the pigment.
Here, "hydrophobic" means that when a monomer is dissolved in 100 g of ion-exchanged water at 25° C. until saturation, the amount of the monomer that dissolves is less than 10 g.
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 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, and stearyl (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.

As the aromatic group-containing monomer, a vinyl monomer having an aromatic group with 6 to 22 carbon atoms which may have a substituent containing a hetero atom is preferred, and a styrene-based monomer or an aromatic group-containing (meth)acrylate is more preferred.
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.

[(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.
As the (a-3) nonionic monomer, polypropylene glycol (n=2 to 30) (meth)acrylate and phenoxy (ethylene glycol-propylene glycol copolymer) (meth)acrylate are preferable, and examples thereof include polypropylene glycol (n=2 to 30) (meth)acrylate.
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-(meth)acrylic acid copolymer.

[Content of each structural unit in polymer (a)]
The content of each component in the monomer mixture during production of polymer (a) (content as unneutralized amount; the same applies hereinafter) or the content of the constitutional units derived from each component in polymer (a) is as follows, from the viewpoint of improving the storage stability and color development of the water-based ink, the wetting spreadability and abrasion resistance of the ink dots.
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.

The acid value of the polymer (a) is derived from the carboxy group, and from the viewpoint of improving the storage stability, etc. of the water-based ink, the acid value is 180 mgKOH/g or more, preferably 200 mgKOH/g or more, more preferably 220 mgKOH/g or more, and 320 mgKOH/g or less, preferably 300 mgKOH/g or less, more preferably 280 mgKOH/g or less.
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.
From the viewpoint of improving the storage stability and the like of the water-based ink, the number 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.
The number average molecular weight is measured by the method described in the Examples.

[Neutralization]
At least a portion of the carboxy groups of the polymer (a) is preferably neutralized with an alkali metal compound or the like.
Examples of the alkali metal compound include one or more compounds selected from alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; alkali metal salts of carbonates such as disodium carbonate, sodium hydrogen carbonate, and dipotassium carbonate; and alkali metal salts of boric acid such as sodium borate.
Among these, alkali metal hydroxides are preferred, sodium hydroxide and potassium hydroxide are more preferred, and sodium hydroxide is even more preferred.

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 150 mol % or less, more preferably 100 mol % or less, even more preferably 80 mol % or less.
The degree of neutralization (mol %) is calculated by the following formula.
Degree of neutralization (mol %)=[moles of alkali metal compound/moles of carboxyl groups of polymer (a)]×100
In the present invention, when the alkali metal compound is used in an amount greater than the number of moles of carboxy groups in the polymer (a), the degree of neutralization may exceed 100 mol %.

[Crosslinking of polymer (a)]
The polymer (a) is crosslinked with a crosslinking agent such as an epoxy compound to form a crosslinked structure. This crosslinking process changes the two-dimensional structure of the polymer (a) to a three-dimensional structure, and the polymer is firmly adsorbed or fixed to the pigment surface, suppressing the aggregation of the pigment in the water-based ink and further suppressing the swelling of the polymer, which is believed to improve the storage stability, color development, wet spreadability of the ink dots, and abrasion resistance.
The epoxy compound may be water-soluble or water-insoluble, but from the viewpoint of more efficiently crosslinking with the carboxy group of the polymer (a) in a medium mainly composed of water, its water solubility is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Here, the water solubility (mass%) refers to the solubility (mass%) when 10 parts by mass of the epoxy compound is dissolved in 90 parts by mass 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 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.

Specific examples of the epoxy compound include one or more selected from polyglycidyl ethers such as cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether.
Among these, at least one selected from cyclohexanedimethanol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether is preferred.

(Preparation of Pigment-Containing Crosslinked Polymer Particles A)
The pigment-containing crosslinked polymer particles A can be efficiently produced by a method including the following steps 1 to 3.
Step 1: A step of neutralizing at least a portion of the carboxy groups of the polymer (a) with an alkali metal compound to obtain an aqueous dispersion of the polymer (a). Step 2: A step of dispersing the aqueous dispersion of the polymer (a) obtained in step 1 and a pigment to obtain an aqueous dispersion of a pigment of polymer particles containing the pigment dispersed in the polymer (a). Step 3: A step of adding an epoxy compound to the aqueous pigment dispersion obtained in step 2, and performing a crosslinking treatment to obtain an aqueous dispersion of a pigment of crosslinked polymer particles A containing the pigment.

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.
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, but from the viewpoint of obtaining a uniform pigment aqueous dispersion, it is preferable to pre-disperse the pigment mixture and then further perform main dispersion. As a disperser used for pre-dispersion, a commonly used mixing and stirring device such as an anchor blade or a disperser 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 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.

In step 3, the polymer (a) dispersing the pigment in the pigment aqueous dispersion is crosslinked by the epoxy compound to form a crosslinked polymer, thereby obtaining a pigment aqueous dispersion in which particles A of the crosslinked polymer containing the pigment are dispersed in an aqueous medium.
The preferred epoxy compounds are as described above.
From the viewpoints of completion of the crosslinking reaction and economic efficiency, the temperature of the crosslinking treatment is preferably 50° C. or higher, more preferably 70° C. or higher, and preferably 95° C. or lower, more preferably 92° C. or lower. From the same viewpoints as above, the time of the crosslinking treatment is preferably 0.5 hours or higher, more preferably 1 hour or higher, and preferably 10 hours or lower, more preferably 6 hours or lower.

The acid value of the polymer constituting the pigment-containing crosslinked polymer particles A is preferably 80 mgKOH/g or more, more preferably 88 mgKOH/g or more, even more preferably 92 mgKOH/g or more, and is preferably 220 mgKOH/g or less, more preferably 210 mgKOH/g or less, even more preferably 195 mgKOH/g or less.

From the viewpoint of facilitating the preparation of an aqueous ink, the non-volatile component concentration (solids concentration) of the resulting pigment water dispersion is 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 pigment water dispersion is measured by the method described in the examples.

From the viewpoint of improving image density, the pigment content in the solid content of the obtained pigment aqueous dispersion is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 65% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less.
From the viewpoint of improving image density, the content of the pigment in the obtained pigment water dispersion is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 18% by mass or less.

From the viewpoint of improving storage stability, color development, wet spreadability of ink dots, and abrasion resistance, the crosslinking rate of the polymer (a) is preferably 20 mol % or more, more preferably 30 mol % or more, and even more preferably 40 mol % or more, and is preferably 80 mol % or less, more preferably 70 mol % or less, and even more preferably 60 mol % or less.
Here, the crosslinking rate (mol %) is calculated by "(mol number of epoxy groups in the epoxy compound/mol number of carboxy groups in the polymer (a))×100".

The average particle size of the crosslinked polymer particles A containing the pigment in the pigment water dispersion is preferably 60 nm or more, more preferably 80 nm or more, and even more preferably 100 nm or more, from the viewpoints of reducing coarse particles and improving storage stability, color development, wet spreadability of ink dots, and abrasion resistance, and is preferably 200 nm or less, more preferably 160 nm or less, and even more preferably 150 nm or less. The average particle size is measured by the method described in the Examples.
The average particle size of the crosslinked polymer particles A containing the pigment in the water-based ink after preparation of the water-based ink is substantially the same as the average particle size in the pigment water dispersion.

<Polymer particles B not containing pigment>
The water-based ink of the present invention contains polymer particles B (hereinafter also referred to as “polymer particles B”) that do not contain a pigment, from the viewpoint of achieving high levels of fixation to a print medium, color development, and abrasion resistance.
Examples of the polymer constituting the polymer particles B (hereinafter also referred to as "polymer (b)") include (meth)acrylic resins, styrene resins, urethane resins, polyester resins, butadiene resins, vinyl chloride resins, etc. Among these, from the viewpoint of improving the concealing properties and blocking resistance of the ink, (meth)acrylic resins are preferred, and styrene-(meth)acrylic resins are more preferred.
It is preferable to use the polymer particles B in the form of an aqueous dispersion in which the polymer particles B are dispersed in water. As the polymer (b), a suitably synthesized product or a commercially available product may be used.
The polymer (b) preferably has a crosslinked structure crosslinked with a crosslinking agent such as an epoxy compound.

[Polymer (b)]
The (meth)acrylic resin as the polymer (b) preferably has (b-1) a structural unit derived from a carboxy group-containing vinyl monomer and (b-2) a structural unit derived from a hydrophobic vinyl monomer.
Examples of the component (b-1) include the same carboxylic acid monomers as those of the component (a-1). Among them, from the viewpoint of improving fixability, one or more selected from acrylic acid and methacrylic acid are preferred.
Preferred examples of the (b-2) component include alkyl (meth)acrylates and aromatic group-containing monomers similar to those of the (a-2) component, and among these, styrene-based monomers are preferred, and one or more selected from styrene and α-methylstyrene are preferred.
The monomer components contained in each of the above components (b-1) and (b-2) can be used alone or in combination of two or more.

[Content of each structural unit in polymer (b)]
The content of the structural units derived from the components (b-1) and (b-2) in the polymer (b) is as follows, from the viewpoint of improving fixability to a print medium.
The content of the (b-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 (b-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.

The polymer (b) can be produced by copolymerizing a monomer mixture containing the components (b-1) and (b-2) by a known solution polymerization method or the like.
The production method, neutralization method, and crosslinking method of the polymer (b) are the same as those of the polymer (a) described above, and therefore the description thereof will be omitted.

From the viewpoint of improving fixation, the acid value of the polymer (b) is 180 mgKOH/g or more, preferably 200 mgKOH/g or more, more preferably 220 mgKOH/g or more, and is 320 mgKOH/g or less, preferably 300 mgKOH/g or less, more preferably 280 mgKOH/g or less.
The number average molecular weight of polymer b is preferably 4,000 or more, more preferably 6,000 or more, even more preferably 8,000 or more, and preferably 80,000 or less, more preferably 50,000 or less, even more preferably 30,000 or less.
The acid value and number average molecular weight of the polymer (b) can be measured in the same manner as in the case of the polymer (a).

From the viewpoint of improving fixability, the average particle size of the polymer particles B in the water-based ink is preferably 60 nm or more, more preferably 80 nm or more, and even more preferably 100 nm or more, and is preferably 200 nm or less, more preferably 160 nm or less, and even more preferably 150 nm or less.
The average particle size of the polymer particles B is measured by the method described in the Examples.

Polymer (a) and polymer (b) may be the same or different, that is, polymer (a) and polymer (b) may have different compositions (structures), or may be the same polymer including the composition (structure) and differ only in the presence or absence of a pigment.
The polymer (a) and the polymer (b) may be commercially available products.
Examples of commercially available dispersions of the polymers (a) and (b) that can be used include Neocryl A1127 (anionic self-crosslinking water-based acrylic resin) manufactured by DSM Neo Resins, Joncryl 390 (acrylic resin) and Joncryl PDX-7775 (styrene-acrylic resin) manufactured by BASF, and Vinyblan 700 (vinyl chloride-acrylic resin) manufactured by Nissin Chemical Industry Co., Ltd.

<Wax C>
From the viewpoint of improving the storage stability and abrasion resistance of the ink, the wax C used in the present invention preferably has a melting point of 110° C. or higher, more preferably 115° C. or higher, even more preferably 120° C. or higher, and preferably 150° C. or lower, more preferably 145° C. or lower, even more preferably 140° C. or lower.
Examples of wax C include polyolefin waxes containing an olefin monomer as a main component, petroleum paraffin waxes consisting of a mixture of linear saturated hydrocarbons having 20 to 30 carbon atoms, and synthetic waxes such as Sasol wax.
Among these, one or more waxes selected from polyolefin waxes and paraffin waxes are preferred, and polyolefin waxes are more preferred.

Olefin monomers used in polyolefin waxes include linear olefins and cyclic olefins, with those primarily composed of linear olefins having 2 to 6 carbon atoms being preferred, and polyolefin waxes (polyethylene waxes) primarily composed of ethylene being more preferred. Here, "primarily composed of ethylene" means that the ethylene content is preferably 50% by mass or more, more preferably 65% by mass or more, and even more preferably 80% by mass or more, based on the total components constituting the wax.
Oxidized polyolefin wax can be obtained by adjusting a high molecular weight polyolefin polymer to a desired molecular weight by thermal decomposition or the like while introducing oxygen atoms or the like into the molecule, and can be used as a polyolefin wax.
The content of the polyolefin wax in the total amount of wax is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and still more preferably 100% by mass.

It is preferable to use wax C as an emulsion. There is no particular limitation on the method for producing the wax emulsion, and for example, a method of mixing a polyolefin wax, other waxes used as necessary, and a known surfactant and emulsifying them can be mentioned. As the surfactant, a nonionic surfactant, an anionic surfactant, etc. can be used.
Examples of nonionic surfactants include ethylene oxide adducts of higher alcohols, ethylene oxide adducts of alkylated phenols, etc. Examples of anionic surfactants include sulfate ester salts and phosphate ester salts based on ethylene oxide adducts of higher alcohols, and alkylated benzenesulfonate salts.
Among these, from the viewpoint of improving the storage stability of the ink, anionic wax emulsions in which wax is emulsified with an anionic surfactant are more preferred.

From the viewpoint of improving the dispersion stability of wax C, the storage stability of the ink, and the abrasion resistance, the average particle size of wax C in the ink is preferably 500 nm or less, more preferably 300 nm or less, and even more preferably 100 nm or less, and is preferably 10 nm or more, and preferably 30 nm or more.
The average particle size of Wax C can be measured by dynamic light scattering, for example, by the method described in the Examples using a Microtrack particle size analyzer UPA manufactured by Nikkiso Co., Ltd.
Suitable examples of commercially available wax emulsions include polyethylene wax emulsions such as AQUACER 507, 513, 515, 526, 531, 533, 537, 539, 552, and 1547 manufactured by BYK Corporation, Cellosol 428, Cellosol H620, 686, 524, Trasol CN, Polylon L-787, and L-788 manufactured by Chukyo Yushi Co., Ltd., and Chemipearl W900 and W4005 manufactured by Mitsui Chemicals, Inc.

<Organic Solvent D>
The organic solvent D used in the present invention contains a glycol ether compound D1 having an alkyl group having 1 to 3 carbon atoms and a glycol ether compound D2 having an alkyl group having 4 carbon atoms, from the viewpoint of improving the storage stability and color development of the water-based ink, and the wetting and spreading properties and abrasion resistance of the ink dots.
The glycol ether compound D1 and the glycol ether compound D2 may be a liquid or a solid at 25° C., but are preferably water-soluble organic solvents that dissolve in 100 mL of water at 25° C. in an amount of 10 mL or more.
The boiling point of organic solvent D is preferably 100° C. or higher, more preferably 130° C. or higher, even more preferably 150° C. or higher, still more preferably 170° C. or higher, and is 250° C. or lower, preferably 245° C. or lower, more preferably 240° C. or lower, and even more preferably 235° C. or lower.

Suitable examples of the glycol ether compound D1 include one or more selected from ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monopropyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, etc. Among these, one or more selected from diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, and dipropylene glycol monomethyl ether are preferred, and one or more selected from diethylene glycol monoisopropyl ether, dipropylene glycol monomethyl ether, and propylene glycol monomethyl ether are more preferred.

Suitable examples of the glycol ether compound D2 include one or more selected from ethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, triethylene glycol monoisobutyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether, and the like.
Among these, one or more selected from ethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, propylene glycol monobutyl ether, and dipropylene glycol monobutyl ether are preferred, and one or more selected from ethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, and propylene glycol monobutyl ether are more preferred.
From the viewpoint of improving the storage stability of the water-based ink and the wetting and spreading properties of the ink dots, it is preferable that the organic solvent D further contains propylene glycol.
The organic solvent D may further contain organic solvents other than those mentioned above, within the range that does not impair the effects of the present invention.

The water-based ink of the present invention may further contain a surfactant, if necessary.
Examples of the surfactant include nonionic surfactants, silicone-based surfactants, and fluorine-based surfactants, with nonionic surfactants being more preferred.
Examples of the nonionic surfactant include polyoxyalkylene alkyl ether surfactants, acetylene glycol surfactants, polyhydric alcohol surfactants, and fatty acid alkanolamides.
Examples of commercially available nonionic surfactants include the Surfynol series manufactured by Nissin Chemical Industry Co., Ltd. and Air Products & Chemicals, the Acetylenol series manufactured by Kawaken Fine Chemicals Co., Ltd., and Emulgen 120 manufactured by Kao Corporation.
The above surfactants may be used alone or in combination of two or more.

(Content of each component in the water-based ink of the present invention)
The content of each component in the water-based ink of the present invention is as follows, from the viewpoint of improving storage stability, color development, wetting and spreading properties of ink dots, and abrasion resistance.
(Pigment content)
The content of PV19 in the water-based ink 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 12% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less.
The content of PR150 in the water-based ink is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.25% by mass or more, and is preferably 1.2% by mass or less, more preferably 1.0% by mass or less, even more preferably 0.8% by mass or less.
The mass ratio of PR150 to PV19 (PR150/PV19) is preferably 0.05 or more, more preferably 0.07 or more, even more preferably 0.09 or more, still more preferably 0.1 or more, and is preferably 1.2 or less, more preferably 1.1 or less, even more preferably 1.0 or less.

The content of the crosslinked polymer particles containing PV19 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 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less.
The mass ratio of polymer to PV19 in the crosslinked polymer particles containing PV19 [polymer/PV19] is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and preferably 2 or less, more preferably 1 or less, even more preferably 0.8 or less.

The content of the crosslinked polymer particles containing PR150 in the water-based ink is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% 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.
The mass ratio of the polymer to PR150 in the crosslinked polymer particles containing PR150 [polymer/PR150] is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and is preferably 2 or less, more preferably 1 or less, even more preferably 0.8 or less.

The content of polymer particles B not containing a pigment in the water-based ink is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and is preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less.
(Content of Wax C)
The content of wax C in the water-based ink is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and is preferably 1.2% by mass or less, more preferably 1.0% by mass or less, even more preferably 0.8% by mass or less.

(Content of organic solvent D)
The content of organic solvent D1 in the water-based ink is preferably 4% by mass or more, more preferably 6% by mass or more, even more preferably 8% by mass or more, and is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less.
The content of the organic solvent D2 in the water-based ink is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 2% by mass or more, and is preferably 12% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less.
The mass ratio of the organic solvent D2 to the organic solvent D1 in the water-based ink [D2/D1] is preferably 0.5 or more, more preferably 0.8 or more, even more preferably 1.0 or more, and is preferably 3 or less, more preferably 2.5 or less, even more preferably 2 or less.
The content of propylene glycol in the water-based ink is preferably 4% by mass or more, more preferably 6% by mass or more, even more preferably 8% by mass or more, and is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less.

The content of the surfactant in the water-based ink is 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.5% by mass or more, and is 3% by mass or less, preferably 2.5% by mass or less, and more preferably 2% by mass or less.
The water content in the water-based ink is preferably more than 50% by mass, more preferably 55% by mass or more, even more preferably 57% by mass or more, and is preferably 80% by mass or less, more preferably 75% by mass or less, even more preferably 70% by mass or less.
The water-based ink of the present invention may contain various additives such as preservatives, pH adjusters, viscosity adjusters, defoamers, and rust inhibitors as optional components depending on the application. In such cases, a portion of the water content may be replaced with the various additives.

(Physical Properties of the Water-Based Ink of the Present Invention)
From the viewpoints of improving storage stability, ejection properties, spreading of dots on the surface of a printing medium, and the like, the viscosity of the water-based ink at 32°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 5 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 9 mPa·s or less, even more preferably 7 mPa·s or less.
The pH of the water-based ink is preferably 7.0 or more, more preferably 7.2 or more, and even more preferably 7.5 or more, from the viewpoint of improving storage stability, etc. Also, 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.

[Inkjet printing method]
The inkjet printing method of the present invention is a method for printing on a low water-absorbent print medium using the water-based ink of the present invention.
Here, the term "low water absorption" in printing media is a concept that includes low liquid absorption and non-liquid absorption, and means that the amount of water absorption of the printing substrate when the printing substrate is in contact with pure water for 100 ms is 0 g/ ^m2 or more and 10 g/ ^m2 or less.

Low water-absorbency print media include low water-absorbency coated paper, art paper, and resin film.
Examples of coated paper include general-purpose glossy paper and multi-color form glossy paper.
Examples of the resin film include transparent synthetic resin films, such as polyester films, vinyl chloride films, polypropylene films, polyethylene films, and nylon films. These films may be biaxially stretched films, uniaxially stretched films, or non-stretched films. Among these, polyester films and stretched polypropylene films are preferred, and corona-discharge-treated polyethylene terephthalate (PET) films and corona-discharge-treated biaxially stretched polypropylene (OPP) films are more preferred.

In the present invention, either a serial head type or a line head type print head can be used, but the line head type is preferred from the viewpoint of enabling high speed printing. The line head type is a type in which the head is fixed, the print medium is moved in the transport direction, and ink droplets are ejected from the nozzle openings of the head in conjunction with this movement and adhered to the print medium, and images, etc. can be printed in a single pass.
There are three types of ink droplet ejection methods: piezo, thermal, and electrostatic, with the piezo method being the most preferable. In the piezo method, a large number of nozzles are each connected to a pressure chamber, and ink droplets are ejected from the nozzles by vibrating the walls of the pressure chambers with a piezo element.

In the following preparation examples, manufacturing examples, examples, and comparative examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. Measurements of each physical property were performed using the following methods.

(1) Measurement of the number average molecular weight of a polymer Using a solution of phosphoric acid (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., special grade reagent) and lithium bromide (manufactured by Tokyo Chemical Industry Co., Ltd., reagent) dissolved in N,N-dimethylformamide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., for high performance liquid chromatography) to concentrations of 60 mmol/L and 50 mmol/L, respectively, as an eluent, the measurement was performed by gel chromatography method [GPC apparatus (HLC-8120GPC) manufactured by Tosoh Corporation, columns (TSK-GEL, α-M × 2 columns) manufactured by Tosoh Corporation, flow rate: 1 mL/min] using monodisperse polystyrene with a known molecular weight as a standard substance.

(2) Measurement of average particle size of crosslinked polymer particles A and polymer particles B containing pigment Measurement was performed by cumulant analysis using a laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.). 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 measurement was performed at a concentration of 5 x ^10-3 % by mass (converted into solid content concentration), and the obtained cumulant average particle size was taken as the average particle size of the polymer particles.

(3) Measurement of acid value of polymer 2 g of polymer before crosslinking or 2 g of pigment water dispersion after crosslinking was diluted with 50 g of ion-exchanged water, and 3 mL of 0.1 N sodium hydroxide solution was added. 0.1 N hydrochloric acid was gradually added dropwise thereto, and the pH inflection points were measured at two points. The number of moles of acid calculated from the difference in the amount of 0.1 N hydrochloric acid added between the two points corresponds to the number of moles of carboxylic acid in the polymer, and this number of moles was converted into the acid value.

(4) Measurement of solid content concentration of pigment water dispersion, etc. 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, and then accurately weighed, and the mixture was kept at 105° C. for 2 hours to remove volatile matter, and further left in the desiccator for 15 minutes, and the mass was measured. The mass of the sample after removing the volatile matter was taken as the solid content, and was divided by the mass of the sample added to obtain the solid content concentration.
(6) Measurement of Viscosity of Water-Based Ink The viscosity of the water-based ink at 32° C. was measured using an E-type viscometer “TV-25” (manufactured by Toki Sangyo Co., Ltd., using a standard cone rotor 1°34′×R24, rotation speed 50 rpm).
(7) Measurement of pH of Water-Based Ink The pH of the water-based ink at 25° C. was measured using a tabletop pH meter “F-71” (manufactured by Horiba, Ltd.) equipped with a pH electrode “6337-10D” (manufactured by Horiba, Ltd.).

(8) Water absorption amount of the recording medium when the contact time between the recording medium and pure water is 100 ms Using an automatic scanning liquid absorption meter (KM500win, manufactured by Kumagai Riki Kogyo Co., Ltd.), the amount of transfer when the recording medium was in contact with pure water for 100 ms was measured under conditions of 23° C. and a relative humidity of 50%, and this was taken as the water absorption amount for 100 ms.

Preparation Example 1 (Preparation of Polymer (a))
A monomer mixture was prepared by mixing 31 parts of acrylic acid and 69 parts of styrene. 10 parts of MEK, 0.2 parts of 2-mercaptoethanol (polymerization chain transfer agent), and 10% of the monomer mixture were placed in a reaction vessel and mixed, and the atmosphere was thoroughly replaced with nitrogen gas.
Meanwhile, a mixture of the remaining 90% of the monomer mixture, 0.13 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile), manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., product name: V-65) was placed in a dropping funnel, and the monomer mixture in the reaction vessel was heated to 65°C while being stirred 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 0.1 parts of the polymerization initiator was dissolved in 2 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and at 70°C for 2 hours, and then dried under reduced pressure to obtain a polymer (a) (number average molecular weight: 19,000, acid value: 240 mgKOH/g).

Production Example 1 (Production of Pigment Water Dispersion 1)
(Step I)
32 parts of the polymer (a) obtained in Preparation Example 1 was mixed with 202 parts of ion-exchanged water, and 12.8 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide solid content: 16.9%) was further added to neutralize the mixture so that the ratio of the number of moles of sodium hydroxide to the number of moles of carboxy groups in the polymer (a) became 40% (neutralization degree: 40 mol%). The mixture was heated to 90°C using a warm bath and stirred for 1 hour to completely disperse the polymer in water, thereby obtaining a polymer dispersion.
(Step II)
After cooling the polymer dispersion obtained in step I to room temperature (25°C), 100 parts of magenta pigment (C.I. Pigment Violet 19, Clariant, product name: Inkjet Magenta E5B02) was added thereto, and the mixture was stirred for 3 hours at 20°C using a disper (Asada Iron Works, product name: Ultra Disper) with the disper blade rotating at 6,000 rpm. Next, 124 parts of ion-exchanged water was added, and the mixture was dispersed 15 times using a microfluidizer (Microfluidics, product name) at a pressure of 150 MPa. The obtained dispersion was placed in a 500 mL angle rotor and centrifuged at 3,660 rpm for 20 minutes using a high-speed cooling centrifuge (Hitachi Koki, product name: himac CR22G, set temperature 20°C), and the liquid layer was collected and filtered with a 5 μm membrane filter (Sartorius, product name: Minisart) to obtain a pigment aqueous dispersion. At this time, the solid content of the pigment aqueous dispersion was 25%.
(Step III)
100 parts of the pigment aqueous dispersion was taken in a screw-top glass bottle, 32 parts of ion-exchanged water was added, and 1.8 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, product name: Denacol EX-321LT, epoxy equivalent: 140, water solubility rate 27%) was added as a water-insoluble crosslinking agent, and the bottle was sealed and heated at 70°C for 5 hours while stirring with a stirrer. At this time, crosslinking treatment was performed with an amount of crosslinking agent of an epoxy amount capable of reacting with 50% of the total number of carboxyl groups contained in the polymer (crosslinking rate: 50%). After 5 hours, the dispersion was cooled to room temperature (25°C) and filtered with a 25mL needleless syringe (manufactured by Terumo Corporation) equipped with the 5 μm filter to obtain a pigment aqueous dispersion 1 (solid concentration: 20%, pigment 14.2%, polymer 5.8%, average particle size 125 nm) of pigment-containing crosslinked polymer particles (acid value 120 mgKOH/g).

Production Examples 2 to 4, Comparative Production Examples 1 to 3 (Preparation of Pigment Water Dispersions 2 to 4, 11 to 13)
Pigment water dispersions 2 to 4 and 11 to 13 of pigment-containing crosslinked polymer particles were obtained in the same manner as in Production Example 1, except that the conditions were changed as shown in Table 1. The results are shown in Table 1.
The details of the pigments in Table 1 are as follows.
PV19: C.I. Pigment Violet 19, manufactured by Clariant, product name: Inkjet Magenta E5B02
PR150: C.I. Pigment Red 150, manufactured by Toyo Color Co., Ltd., product name: LIONOGEN RED LX10231
PR122: C.I. Pigment Red 122, manufactured by Dainichiseika Chemicals Co., Ltd., product name: Chromofine Red 6111T

Production Example 6 (Preparation of Water Dispersion of Polymer Particles B)
15.3 parts of the polymer A obtained in Preparation Example 1 was mixed with 63.5 parts of ion-exchanged water, and 6.2 parts of a 5N aqueous sodium hydroxide solution (solid content: 16.9%) was further added to neutralize the mixture so that the ratio of the number of moles of sodium hydroxide to the number of moles of carboxyl groups in the polymer became 40% (degree of neutralization: 40 mol%). The mixture was heated to 90°C using a warm bath and stirred for 1 hour to completely disperse the polymer in water, thereby obtaining a polymer dispersion.
After cooling this polymer dispersion to room temperature, 4.6 parts of trimethylolpropane polyglycidyl ether (Denacol EX-321LT, Nagase Chemtex Corporation, epoxy equivalent: 140) was added as a crosslinking agent, sealed, and heated at 90°C for 1.5 hours while stirring with a stirrer. At this time, crosslinking treatment was performed with an amount of crosslinking agent of an epoxy amount capable of reacting with 50% of the total number of carboxyl groups contained in the polymer (crosslinking rate 50%). Thereafter, the polymer dispersion was cooled to room temperature (25°C) and filtered with a 25mL needleless syringe (Terumo Corporation) equipped with a 5μm membrane filter (trade name: Minisart) to obtain an aqueous dispersion of crosslinked polymer particles B (solid concentration: 20%).

Example 1
33.3 parts (pigment 4.7%, polymer 1.9%) of pigment aqueous dispersion 1 obtained in Production Example 1 (PV19, solid content: 20%), 3.7 parts (pigment 0.5%, polymer 0.2%) of pigment aqueous dispersion 4 obtained in Production Example 4 (PR150, solid content: 20%), 22.5 parts (solid content: 20%) of polymer particles B obtained in Production Example 6, 1.7 parts of polyethylene wax emulsion (product name: AQUACER 507, manufactured by BYK Corporation, anionic emulsion of oxidized high-density polyethylene wax, melting point 130° C., average particle size 50 nm, pH 9.7, solid content: 30%), propylene glycol [0113] Water-based ink 1 was obtained by mixing 10.0 parts of ethyl acetate, 12.0 parts of diethylene glycol monoisopropyl ether (iPDG, manufactured by Nippon Nyukazai Co., Ltd.), 4.0 parts of diethylene glycol monoisobutyl ether (iBDG, manufactured by Nippon Nyukazai Co., Ltd.), 2.0 parts of Surfynol 104PG-50 (manufactured by Nissin Chemical Industry Co., Ltd., acetylene glycol active content 50%), 0.5 parts of Emulgen 120 (manufactured by Kao Corporation, ethylene oxide adduct of lauryl alcohol), and a 1N aqueous sodium hydroxide solution in appropriate amounts so that the ink pH was 9.0, and then adding ion exchanged water so that the total amount was 100 parts.

Examples 2 to 9 and Comparative Examples 1 to 8
In the same manner as in Example 1, water-based inks 2 to 9 of the examples and water-based inks 11 to 18 of the comparative examples were obtained according to the formulations shown in Table 2.

The notations in Table 2 are as follows, and the content values indicate the effective amount (solid content).
[Glycol ether compound D1]
iPDG: diethylene glycol monoisopropyl ether MFDG: dipropylene glycol monomethyl ether [glycol ether compound D2]
iBDG: Diethylene glycol monoisobutyl ether BG: Ethylene glycol monobutyl ether BFG: Propylene glycol monobutyl ether [other glycol ether compounds]
HG: Ethylene glycol monohexyl ether

The resulting water-based inks 1 to 9 and 11 to 18 were used to evaluate the storage stability, color development, and abrasion resistance of the inks using the following methods. The results are shown in Table 2.

<Evaluation of storage stability>
The prepared inks were placed in sealed containers and subjected to a storage test in a constant temperature room at 70°C. After one week, the inks were removed and their viscosities were measured (using an E-type viscometer), and the viscosity change rate was calculated using the following formula (rounded down to the nearest whole number). If the viscosity change rate obtained was within ±10%, it was deemed to be practically usable, and if it was within ±5%, it was deemed to be particularly excellent.
Viscosity change rate (%)=100−[(viscosity after storage)/(viscosity before storage)]×100

<Preparation of printed matter>
The water-based inks obtained in the Examples and Comparative Examples were filled into a print evaluation device (manufactured by Tritec Corporation) equipped with an inkjet recording head (Kyocera Corporation, "KJ4B-HD06MHG-STDV", piezoelectric line head) in an environment of 25±1°C temperature and 30±5% relative humidity. The following conditions were set: head voltage 26V, driving frequency 10kHz, head temperature 32°C, ejection droplet volume 12pl, resolution 600x600dpi, number of pre-ejection flushings 200 shots, and negative pressure -4.0kPa. Coated paper "OK Topcoat+" (manufactured by Oji Paper Co., Ltd., product name, water absorption 4.9g/ ^m2 ) was used as a low water-absorbency printing medium, and the printing medium was fixed to a conveying table under reduced pressure with the longitudinal direction and conveying direction the same. A print command was transferred to the print evaluation device, and a print pattern having a 2 cm square image with a duty of 30% (image area for measuring dot diameter) and a 2 cm square solid image with a duty of 100% (image area for gamut measurement and abrasion resistance evaluation) was printed using a one-pass method, and a print was obtained using the low water absorbency printing medium.

<Evaluation of Color Development>
The solid image area of the print was measured using an optical densitometer "SpectroEye" (manufactured by X-Rite) in measurement mode (D65 light source, observation field of view 2°, white reference Abs, no filter) to measure the coordinate axes L ^* , a ^* , b ^* in the CIELAB color space.
Regarding the color development of the magenta ink, a ^* is preferably 73 or more, more preferably 74 or more, and even more preferably 75 or more, and b ^* is preferably 3 or less, more preferably -10 or more and 2 or less, and even more preferably -8 or more and 1 or less. When ^a* and b ^* are within the above ranges, it can be said that the color development is excellent.

<Evaluation of ink dot diameter>
The ink dot diameter of the printed matter at a duty of 30% was measured using a handheld image evaluation system "PIAS-II" manufactured by Quality Engineering Associates Inc. under conditions of a threshold of 50%, a minimum dot diameter of 40 μm, and a maximum dot diameter of 500 μm.
The larger the dot diameter, the more the graininess of the printed image can be reduced, and the more the streaks can be reduced when printing solid images. A dot diameter of 65 μm or more is considered to be excellent, and a dot diameter of 68 μm or more is even better.

<Evaluation of Scratch Resistance>
The obtained print was dried for 1 minute in a thermostatic chamber set at 60°C, and a rub test was performed on the solid image area (duty 100%) of the print using an AB-201 Sutherland type ink lab tester (manufactured by Tester Sangyo Co., Ltd.). A load of 900 g was applied to "OK Topcoat+" coated paper using the bottom surface of a weight (bottom surface area ⁵⁰ cm2) as a friction material, and the print was rubbed 50 times (reciprocating). The condition of the printed surface of the rubbed print was visually confirmed and evaluated according to the following evaluation criteria.
(Evaluation Criteria)
◯: Scratches on the surface of the print can be visually confirmed, but the surface of the print medium is not exposed and it can be used for practical purposes.
x: Scratches on the surface of the printed matter were visually confirmed, and part of the surface of the printed medium was exposed, which was problematic.

From Table 2, it can be seen that the water-based inks of the Examples have excellent storage stability, color development, wetting and spreading of ink dots onto the print medium, and abrasion resistance, and are highly balanced, compared to the water-based inks of the Comparative Examples.

Claims (10)

1. A water-based ink for ink-jet printing comprising: crosslinked polymer particles A containing a pigment; polymer particles B not containing a pigment; wax C; an organic solvent D; and water,
The pigment contains C.I. Pigment Violet 19 and C.I. Pigment Red 150;
the organic solvent D contains a glycol ether compound D1 having an alkyl group having 1 to 3 carbon atoms and a glycol ether compound D2 having an alkyl group having 4 carbon atoms;
Water-based ink for inkjet printing. The water-based ink according to claim 1, wherein the pigment-containing crosslinked polymer particles A include crosslinked polymer particles containing C.I. Pigment Violet 19 and crosslinked polymer particles containing C.I. Pigment Red 150. The water-based ink according to claim 1 or 2, wherein the acid value of the polymer constituting the pigment-containing crosslinked polymer particles A is 80 mg KOH/g or more and 220 mg KOH/g or less. The water-based ink according to any one of claims 1 to 3, wherein the polymer constituting the polymer particles B is a (meth)acrylic resin. The water-based ink according to any one of claims 1 to 4, wherein the mass ratio (PR150/PV19) of C.I. Pigment Red 150 (PR150) to C.I. Pigment Violet 19 (PV19) is 0.05 or more and 1.2 or less. The water-based ink according to any one of claims 1 to 5, wherein wax C is a polyolefin wax having a melting point of 110°C or higher and 150°C or lower. 7. The water-based ink according to claim 1, wherein the organic solvent D further contains propylene glycol. The water-based ink according to any one of claims 1 to 7, wherein the glycol ether compound D1 having an alkyl group with 1 to 3 carbon atoms is one or more selected from diethylene glycol monoisopropyl ether, dipropylene glycol monomethyl ether, and propylene glycol monomethyl ether. The water-based ink according to any one of claims 1 to 8, wherein the glycol ether compound D2 having an alkyl group with 4 carbon atoms is at least one selected from ethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, and propylene glycol monobutyl ether. An inkjet printing method for printing on a low water-absorbent print medium using the water-based ink according to any one of claims 1 to 9.