JP7740985B2 - Method for producing aqueous pigment dispersion - Google Patents

Description

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

Inkjet recording is a recording method in which ink droplets are ejected directly from minute nozzles and deposited on a recording medium to produce printed matter with characters and images recorded on it. This method has become extremely popular due to its many advantages, including ease of full-color printing, low cost, the ability to use plain paper as the recording medium, and non-contact with the recording medium. In particular, from the perspective of weather resistance and water resistance of the recorded matter, the use of pigments as colorants has become mainstream.
On the other hand, pigment water dispersions and water-based pigment inks have the problem of poor storage stability due to the presence of coarse particles derived from the pigment or polymer, since pigment particles are dispersed in a liquid.
Since inkjet recording is suitable for recording a wide variety of products in small quantities, the range of recording media to which it is applicable is expanding. With this expansion of the range of applications, improved solvent resistance is required not only for printing on highly water-absorbent recording media such as plain paper, but also for commercial printing using low-water-absorbent recording media such as coated paper and non-water-absorbent resin films.
Furthermore, in particular with non-water-absorbent recording media, the water-based ink does not penetrate into the interior of the recording medium, so the pigment particles contained in the water-based ink remain on the surface of the recording medium, resulting in a problem of poor abrasion resistance of the resulting recorded matter.

Various proposals have been made to improve the properties of aqueous pigment dispersions and water-based pigment inks.
For example, Patent Document 1 discloses an aqueous dispersion that can be used in a water-based ink for inkjet recording, and that is excellent in improving print density and glossiness and suppressing bronzing. The aqueous dispersion contains crosslinked core-shell polymer particles containing a pigment, in which both the core and the shell are crosslinked and the pigment and the crosslinked core-shell polymer have a specific weight ratio.
Patent Document 2 discloses a method for producing a colored microparticle dispersion used in a water-based ink for inkjet printing, which can provide images having excellent abrasion resistance and gloss even on a low-water-absorbency recording medium, by removing an organic solvent from Dispersion 1 containing a pigment, a polymerizable surfactant, water, and an organic solvent to obtain Dispersion 2, and then emulsion-polymerizing Dispersion 2 and a polymerizable monomer to obtain a colored microparticle dispersion.

JP 2008-88427 A JP 2016-27150 A

The pigment aqueous dispersions disclosed in Patent Documents 1 and 2 have room for improvement in terms of storage stability, solvent resistance and abrasion resistance of the resulting images.
An object of the present invention is to provide a method for producing a pigment aqueous dispersion and a method for producing a water-based ink, which are capable of producing recorded materials that have excellent storage stability, solvent resistance, and abrasion resistance.

The present inventors have found that the above-mentioned problems can be solved by preparing in advance a specific pigment pre-dispersion and a pre-emulsion in which a radical polymerizable monomer is dispersed with a polymer, and then mixing the two to perform emulsion polymerization in the production of a pigment aqueous dispersion.
That is, the present invention provides the following [1] and [2].
[1] A method for producing a pigment aqueous dispersion, comprising the following steps 1 to 3:
[0023] [0024] [0025] [0026] [0026] [0027] [0028] [0029] [0030] [0029] [0031] [0029] [0032] [0029] [0033] [0029] [0034] [0035] [0036] [0037] [0038] [0039] [0040] [0039] [0041] [0039] [0042] [0039] [0043] [0039] [0044] [0039] [0045] [0046] [0047] [0048] [0049] [0050] [0051] [0052] [0053] [0054] [0055] [0056] [0057] [0058] [0059] [0060] [0061] [0062] [0063] [0064] [0065] [0066] [0067] [0070] [0071] [0072] [0073] [0074] [0075] [0076] [0077] [0078] [0080] [0081] [0082] [0083] [0084] [0085] [0086] [0087] [0088] [0089] [0090] [0091] [0092] [0093] [0094] [0095] [0096] [0097] [0098] [0099] [0099] [

The present invention provides a method for producing a pigment aqueous dispersion and a method for producing a water-based ink that can produce printed materials with excellent storage stability, solvent resistance, and abrasion resistance.

[Method for producing aqueous pigment dispersion]
The method for producing a pigment water dispersion of the present invention is characterized by comprising the following steps 1 to 3:
Step 1: mixing and dispersing a pigment, an anionic polymer (P) having an acid value of 100 mgKOH/g or more and 300 mgKOH/g or less, and an aqueous medium to obtain a pigment pre-dispersion containing pigment particles having an average particle size of 160 nm or less; Step 2: mixing a radically polymerizable monomer with an aqueous dispersion of anionic polymer (Q) to obtain a pre-emulsion of the radically polymerizable monomer; Step 3: mixing the pigment pre-dispersion obtained in Step 1 with the pre-emulsion of the radically polymerizable monomer obtained in Step 2, adding a radical polymerization initiator, and emulsion-polymerizing the radically polymerizable monomer to obtain an aqueous dispersion of core-shell polymer particles containing the pigment. The pigment aqueous dispersion obtained by the method of the present invention can be used in aqueous inks for flexographic printing, gravure printing, and inkjet recording, and is particularly preferably used in aqueous inks for inkjet recording.
In this specification, "aqueous" means that water accounts for the largest proportion of the medium in which the pigment is dispersed. Furthermore, "recording" is a concept that includes printing or printing characters or images.

The pigment water dispersion and water-based ink obtained by the method of the present invention have excellent storage stability, and when ink-jet recording is performed on a recording medium, recorded materials having excellent solvent resistance and abrasion resistance can be obtained. The reason for this is not clear, but is thought to be as follows.
In the pigment pre-dispersion according to the present invention, the pigment is dispersed using an anionic polymer (P) with a high acid value, and the pigment is coated with the anionic polymer (P). When a pre-emulsion in which a radically polymerizable monomer is dispersed with an anionic polymer (Q) is added in this state and emulsion polymerization is carried out, polymerization of the radically polymerizable monomer supplied from the pre-emulsion proceeds between the pigment and the anionic polymer (P) that has coated the pigment, resulting in pigment particles whose cores are coated with a polymer formed by emulsion polymerization of the radically polymerizable monomer, and polymer particles containing the pigment having a core-shell structure in which the cores are coated with the anionic polymer (P). As a result, the outermost shells of the polymer particles encapsulating the pigment are coated with the high-acid-value anionic polymer (P) that has a high charge repulsion force, and the pigment is coated with the polymer formed by emulsion polymerization of the radically polymerizable monomer, which is thought to improve the storage stability of the resulting aqueous pigment dispersion.
Furthermore, in the method of the present invention, the pigment is coated with a polymer formed by emulsion polymerization of a radically polymerizable monomer, which is thought to improve film-forming properties after recording on a recording medium, and to provide excellent solvent resistance and abrasion resistance.

<Step 1>
Step 1 is a step of mixing and dispersing a pigment, an anionic polymer (P) having an acid value of 100 mgKOH/g or more and 300 mgKOH/g or less, and an aqueous medium to obtain a pigment pre-dispersion containing pigment particles having an average particle size of 160 nm or less.

[Pigments]
The pigment used in the present invention may be either an inorganic pigment or an organic pigment.
Examples of inorganic pigments include carbon black and metal oxides, and carbon black is preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of white inks include titanium dioxide, zinc oxide, silica, alumina, and metal oxides such as magnesium oxide.
Examples of organic pigments include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments.
The hue is not particularly limited, and achromatic pigments such as white, black, and gray can be used in achromatic inks, while chromatic pigments such as yellow, magenta, cyan, red, blue, orange, and green can be used in chromatic inks.
The pigments may be used alone or in combination of two or more.

[Anionic polymer (P)]
The anionic polymer (P) used in the present invention (hereinafter also simply referred to as "polymer (P)") is an anionic polymer having an acid value of 100 mgKOH/g or more and 300 mgKOH/g or less, and has pigment dispersibility.
Examples of the polymer (P) include vinyl polymers obtained by addition polymerization of vinyl monomers, polyesters, polyurethanes, etc. Among these, vinyl polymers obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds) are preferred from the viewpoints of pigment dispersion stability, storage stability, etc. The polymer (P) may be suitably synthesized or a commercially available product.

The polymer (P) may be either a water-soluble polymer or a water-insoluble polymer, but from the viewpoint of the storage stability of the resulting pigment aqueous dispersion and the aqueous ink using the same, a water-insoluble polymer is more preferred. Here, the term "water-insoluble polymer" refers to a polymer that, when dried at 105°C for 2 hours and allowed to reach a constant weight, dissolves in 100 g of water at 25°C in an amount of less than 10 g. The amount dissolved is the amount dissolved when the anionic groups of the polymer are 100% neutralized with sodium hydroxide.
The polymer (P) is preferably a vinyl polymer obtained by copolymerizing a monomer mixture containing (a-1) an anionic monomer and (a-2) a hydrophobic monomer, and the vinyl polymer has structural units derived from the component (a-1) and structural units derived from the component (a-2).

[(a-1) Anionic Monomer]
Examples of the anionic monomer (a-1) include carboxylic acid monomers and sulfonic acid monomers, with carboxylic acid monomers being more preferred.
The carboxylic acid monomer may be at least one selected from (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, and is preferably (meth)acrylic acid. "(Meth)acrylic acid" refers to at least one selected from acrylic acid and methacrylic acid.

[(a-2) Hydrophobic Monomer]
The term "hydrophobic" in (a-2) hydrophobic monomer means that when the monomer is dissolved in 100 g of ion-exchanged water at 25° C. until saturation, the amount of the dissolved monomer is less than 10 g.
Specific examples of the hydrophobic monomer (a-2) include those described in paragraphs [0020] to [0022] of JP-A-2018-83938. Among these, preferred are one or more selected from alkyl (meth)acrylates having an alkyl group having 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, and aromatic group-containing monomers having an aromatic group having 6 to 22 carbon atoms, preferably 6 to 18 carbon atoms, and more preferred are one or more selected from ethyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, styrene, α-methylstyrene, and benzyl (meth)acrylate.

[(a-3) Nonionic Monomer]
The polymer (P) may further contain a structural unit derived from a nonionic monomer (a-3). The nonionic monomer (a-3) is a monomer having high affinity for water or a water-soluble organic solvent, such as a monomer containing a hydroxyl group or a polyalkylene glycol chain.
Specific examples of the component (a-3) include those described in paragraph [0018] of JP-A No. 2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferred.
The monomer components contained in each of the above components (a-1) to (a-3) can be used alone or in combination of two or more.

(Content of each structural unit in polymer (P))
The contents of the structural units derived from the components (a-1) to (a-3) in the polymer (P) are as follows, from the viewpoint of improving the storage stability of the resulting pigment water dispersion and the water-based ink using the same, and the solvent resistance and abrasion resistance of the resulting recorded matter.
The content of structural units derived from component (a-1) 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 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less.
The content of structural units derived from component (a-2) is preferably 45% by mass or more, more preferably 50% by mass or more, and even more preferably 55% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less.
When a structural unit derived from the component (a-3) is contained, the content thereof is preferably 10 mass % or less.
The mass ratio of the component (a-1) to the component (a-2) [component (a-1)/component (a-2)] is preferably 0.05 or more, more preferably 0.1 or more, even more preferably 0.2 or more, and is preferably 0.8 or less, more preferably 0.6 or less, even more preferably 0.5 or less.

(Production of Polymer (P))
The polymer (P) can be produced by copolymerizing a mixture of the monomer components (a-1) to (a-3) by a known polymerization method, preferably a solution polymerization method.
There are no limitations on the solvent used in the solution polymerization method, but polar solvents such as water, aliphatic alcohols having 1 to 3 carbon atoms, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred.
During the polymerization, known polymerization initiators and polymerization chain transfer agents can be used. As the polymerization initiator, a water-soluble azo compound such as 4,4'-azobis(4-cyanovaleric acid) is preferred, and as the polymerization chain transfer agent, a mercaptan compound such as 3-mercaptopropionic acid is preferred.
Although preferred polymerization conditions vary depending on the type of polymerization initiator, the polymerization temperature is preferably 50 to 90°C, and the polymerization time is preferably 1 to 10 hours. The polymerization atmosphere is preferably a nitrogen gas atmosphere or an inert gas atmosphere such as argon.

From the viewpoint of improving the storage stability of the resulting pigment water dispersion and the water-based ink using the same, and the solvent resistance and abrasion resistance of the resulting recorded matter, the weight-average molecular weight of the polymer (P) is preferably 3,000 or more, more preferably 8,000 or more, even more preferably 10,000 or more, still more preferably 12,000 or more, and still more preferably 15,000 or more, and is preferably 100,000 or less, more preferably 80,000 or less, and still more preferably 60,000 or less.
From the viewpoint of improving the storage stability of the resulting pigment water dispersion and the water-based ink using the same, and the solvent resistance and abrasion resistance of the resulting recorded matter, the acid value of the polymer (P) is 100 mgKOH/g or more, preferably 120 mgKOH/g or more, more preferably 130 mgKOH/g or more, even more preferably 140 mgKOH/g or more, still more preferably 220 mgKOH/g or more, and is 300 mgKOH/g or less, preferably 280 mgKOH/g or less, more preferably 260 mgKOH/g or less, and even more preferably 250 mgKOH/g or less.
The weight average molecular weight and acid value of the polymer (P) are measured by the methods described in the examples.

(Preparation of Pigment Pre-Dispersion)
In step 1, a pigment, a polymer (P), and an aqueous medium are mixed and dispersed to obtain a pigment pre-dispersion containing pigment particles having an average particle size of 160 nm or less.
The pigment and the polymer (P) are as described above.
The aqueous medium used here is not particularly limited, but one containing water as the main component is preferred. An organic solvent may be further added to the aqueous medium from the viewpoint of improving the wettability of the pigment and the adsorption of the polymer (P) to the pigment. As the organic solvent, aliphatic alcohols having from 1 to 3 carbon atoms, ketones having from 3 to 8 carbon atoms, ethers, esters, etc. are preferred, ketones having from 4 to 8 carbon atoms are more preferred, and methyl ethyl ketone is even more preferred. When a vinyl polymer is synthesized as the polymer (P) by solution polymerization, the solvent used in the polymerization may be used as is.
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, and even more preferably 90% by mass or more.
When an organic solvent is added, it is preferable to remove the organic solvent by a known method before carrying out step 3.

It is preferable to neutralize at least a portion of the acid groups of the polymer (P) using a neutralizing agent, which is thought to increase the charge repulsion force that occurs after neutralization, thereby suppressing aggregation of pigment particles in the pigment water dispersion and water-based ink, and improving the dispersion stability of the pigment.
In the case of neutralization, it is preferable to neutralize so that the pH is 7 or more and 11 or less.
Examples of the neutralizing agent for the polymer (P) include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, ammonia, and water-soluble amines, and are preferably at least one selected from alkali metal hydroxides and ammonia, more preferably alkali metal hydroxides, and even more preferably sodium hydroxide.
The polymer (P) may be neutralized in advance.
From the viewpoint of improving dispersion stability, the amount of the neutralizing agent used relative to the polymer (P) is preferably 10 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 120 mol% or less, even more preferably 100 mol% or less.
Here, the equivalent amount of the neutralizing agent used can be calculated by the following formula, where the polymer (P) before neutralization is "polymer (P')".
Equivalent amount of neutralizing agent used (mol %)=[{weight (g) of neutralizing agent added/equivalent amount of neutralizing agent}/[{acid value of polymer (P') (mg KOH/g)×weight (g) of polymer (P)}/(56×1,000)]]×100

In the dispersion treatment in step 1, the pigment particles can be atomized to a desired particle size by only 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 disperser blade can be used.
Examples of means for applying shear stress used in this 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 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.

The resulting pigment pre-dispersion is one in which pigment particles are dispersed in water as the main medium. Here, the form of the pigment particles is not particularly limited, as long as composite particles are formed from at least the pigment and the polymer (P). For example, particle forms in which the pigment is encapsulated in the polymer (P), particle forms in which the pigment is uniformly dispersed in the polymer (P), particle forms in which the pigment is exposed on the surface of the polymer (P), etc. are included.
The mass ratio of the pigment to the total amount of the pigment and the polymer (P) in the pigment pre-dispersion [pigment/(pigment+polymer (P))] is preferably 0.82 or more, more preferably 0.83 or more, even more preferably 0.84 or more, and still more preferably 0.90 or more, from the viewpoint of firmly bonding the shell portion and the core portion and enhancing the solvent resistance of the pigment aqueous dispersion, and is preferably 0.99 or less, more preferably 0.98 or less, and even more preferably 0.97 or less.

From the viewpoint of dispersion stability of pigment particles, the solids concentration of the pigment pre-dispersion is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 18% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less.
The average particle size of the pigment particles in the pigment pre-dispersion is 160 nm or less, preferably 140 nm or less, more preferably 120 nm or less, even more preferably 110 nm or less, and is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, from the viewpoint of improving the storage stability of the resulting pigment aqueous dispersion and the aqueous ink using the same, and the solvent resistance and abrasion resistance of the resulting recorded matter.
The solid content concentration and average particle size of the pigment pre-dispersion are measured by the method described in the examples.

<Step 2>
Step 2 is a step of mixing a radical polymerizable monomer with an aqueous dispersion of anionic polymer (Q) to obtain a pre-emulsion of the radical polymerizable monomer.
In step 2, a radically polymerizable monomer is dispersed in an anionic polymer (Q) in advance to obtain a pre-emulsion, and then in step 3, this is mixed with the pigment pre-dispersion obtained in step 1, and a radical polymerization initiator is added to carry out emulsion polymerization without using a surfactant, thereby efficiently obtaining an aqueous dispersion of core-shell polymer particles containing a pigment.

[Radical polymerizable monomer]
The radically polymerizable monomer is a monomer containing a radically polymerizable group having an unsaturated double bond, such as one or more groups selected from the group consisting of vinyl, allyl, acryloyl, and methacryloyl groups.
From the viewpoint of the storage stability of the resulting pigment aqueous dispersion, the radically polymerizable monomer is preferably a hydrophobic monomer. Such a hydrophobic radically polymerizable monomer has a hydrophobic group and a group having a radically polymerizable unsaturated double bond in its structure.
The hydrophobic radical polymerizable monomer may be at least one selected from (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol and aromatic group-containing monomers.

The (meth)acrylate having a hydrocarbon group derived from an aliphatic alcohol is preferably a (meth)acrylate having an alkyl group having 1 to 8 carbon atoms, preferably 1 to 6 carbon atoms, more preferably a (meth)acrylate having a straight-chain alkyl group such as methyl (meth)acrylate, ethyl (meth)acrylate, or butyl (meth)acrylate; a (meth)acrylate having a branched-chain alkyl group such as 2-ethylhexyl (meth)acrylate; or a (meth)acrylate having an alicyclic alkyl group such as cyclohexyl (meth)acrylate, and even more preferably one or more selected from methyl (meth)acrylate, ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate.
The aromatic group-containing monomer is preferably a vinyl monomer having an aromatic group having from 6 to 22 carbon atoms, more preferably at least one selected from styrene, 2-methylstyrene, benzyl (meth)acrylate, and phenoxyethyl (meth)acrylate, and even more preferably at least one selected from styrene and benzyl (meth)acrylate.
From the above viewpoints, the radical polymerizable monomer is preferably at least one selected from (meth)acrylates having an alkyl group with 1 to 8 carbon atoms and vinyl monomers having an aromatic group with 6 to 22 carbon atoms.
The radical polymerizable monomer may be used alone or in combination of two or more of the above monomers.

From the viewpoint of improving the storage stability of the resulting pigment water dispersion and the water-based ink using it, and the solvent resistance and abrasion resistance of the resulting recorded material, the amount of radically polymerizable monomer in the pre-emulsion of step 2 is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less.

[Anionic polymer (Q)]
As the anionic polymer (Q) (hereinafter also referred to as "polymer (Q)"), the same one as the anionic polymer (P) used in step 1 can be used.
That is, the polymer (Q) is preferably a vinyl polymer.
The vinyl polymer preferably contains (b-1) a structural unit derived from an anionic monomer and (b-2) a structural unit derived from a hydrophobic monomer, and may further contain (b-3) a structural unit derived from a nonionic monomer.
Specific examples, suitable examples, and the contents and ratios of the structural units of the components (b-1) to (b-3) are the same as the specific examples, suitable examples, and the contents and ratios of the structural units of the components (a-1) to (a-3) described in the section for polymer (P).
The preferred weight average molecular weight and acid value of the polymer (Q) are also the same as those explained in the section for the polymer (P).

The vinyl polymer can be produced by the same method as that for producing the polymer (P).
When polymer (Q) is produced by solution polymerization, the pH tends to be acidic due to the presence of carboxy groups on the surface of polymer (Q) particles, which tends to cause an increase in viscosity and aggregation. Therefore, it is preferable to neutralize the polymer (Q) using a neutralizing agent.
The neutralizing agent for the polymer (Q) is preferably one or more selected from alkali metal hydroxides, water-soluble amines, and ammonia. That is, it is preferable that the polymer (Q) is neutralized with one or more selected from alkali metal hydroxides, water-soluble amines, and ammonia. From the viewpoint of solvent resistance, the neutralizing agent is more preferably ammonia or a water-soluble amine, and even more preferably ammonia.
The amount of neutralizing agent added is determined appropriately so that the pH of the polymer emulsion falls within the range of 7.5 to 9.5, preferably 7.5 to 8.5.
From the viewpoint of improving the storage stability of the resulting pigment water dispersion and the water-based ink using the same, the amount of the neutralizing agent used relative to the polymer (Q) is preferably 10 mol % or more, more preferably 15 mol % or more, even more preferably 20 mol % or more, and is preferably 120 mol % or less, more preferably 100 mol % or less, even more preferably 90 mol % or less.
From the viewpoint of enhancing the storage stability of the resulting pigment water dispersion and the water-based ink using the same, the difference between the acid value of the polymer (P) in step 1 and the acid value of the polymer (Q) in step 2 [acid value of polymer (P) - acid value of polymer (Q)] is preferably 170 mgKOH/g or less, more preferably 100 mgKOH/g or less, even more preferably 30 mgKOH/g or less, and even more preferably 0 mgKOH/g, i.e., the acid values of the polymer (P) and the polymer (Q) are the same.
Furthermore, from the viewpoint of enhancing the storage stability of the resulting pigment water dispersion and the water-based ink using the same, it is even more preferable that the polymer (P) and the polymer (Q) are the same.

<Step 3>
Step 3 is a step of mixing the pigment pre-dispersion obtained in Step 1 with the pre-emulsion of the radical polymerizable monomer obtained in Step 2, adding a radical polymerization initiator, and emulsion-polymerizing the radical polymerizable monomer to obtain an aqueous dispersion of pigment-containing core-shell polymer particles.

[Radical Polymerization Initiator]
As the radical polymerization initiator (hereinafter also simply referred to as "polymerization initiator"), those used in ordinary emulsion polymerization can be used. Specific examples include persulfates such as potassium persulfate and ammonium persulfate; inorganic peroxides such as hydrogen peroxide; organic peroxides such as t-butyl hydroperoxide; azo polymerization initiators; and redox polymerization initiators. Among these, water-soluble azo polymerization initiators are preferred from the viewpoints of promoting the polymerization of the radically polymerizable monomer and improving the storage stability of the resulting pigment aqueous dispersion and the water-based ink using the same, as well as the solvent resistance and abrasion resistance of the resulting recorded matter.

Examples of the water-soluble azo polymerization initiator include anionic azo polymerization initiators and nonionic azo polymerization initiators such as hydroxyl group-containing azoamide compounds, with anionic azo polymerization initiators being more preferred.
Examples of the anionic azo polymerization initiator include azobiscarboxylic acids having from 8 to 16 carbon atoms, such as 1,1'-azobis(cyclohexane-1-carboxylic acid), azobiscyanocarboxylic acids having from 8 to 16 carbon atoms, such as 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(4-cyanovaleric acid), and 4,4'-azobis(2-cyanopentanoic acid), and carboxyl group-containing azo compounds such as salts thereof. Among these, carboxyl group-containing azo compounds are preferred, and at least one selected from azobiscyanocarboxylic acids having from 10 to 14 carbon atoms and salts thereof is more preferred, with 4,4'-azobis(4-cyanovaleric acid) and salts thereof being even more preferred.

In step 3, a polymerization chain transfer agent such as a mercaptan or a xanthogen may be used within a range that does not impair the effects of the present invention.
The amount of the polymerization initiator is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, even more preferably 1 part by mass or more, and is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, even more preferably 4 parts by mass or less, and still more preferably 3 parts by mass or less, relative to 100 parts by mass of the radical polymerizable monomer.

[Emulsion Polymerization]
The emulsion polymerization in step 3 can be carried out by mixing the pigment pre-dispersion obtained in step 1 with the pre-emulsion of the radical polymerizable monomer obtained in step 2, and adding the polymerization initiator, without adding a surfactant.
During emulsion polymerization, water may be added as a dispersion medium as needed, and organic solvents such as alcohols and ketones may also be added as needed. The proportion of water in the entire dispersion medium is preferably 50% by mass or more, more preferably 65% by mass or more, and even more preferably 75% by mass or more.
The conditions for emulsion polymerization are not particularly limited, and the polymerization can be carried out by a conventional method.
The polymerization temperature for emulsion polymerization varies depending on the type of polymerization initiator, but is preferably 55°C or higher, more preferably 70°C or higher, from the viewpoint of reactivity, and is preferably 90°C or lower, more preferably 85°C or lower, from the viewpoint of the molecular weight distribution of the resulting polymer.
The polymerization atmosphere is preferably an inert gas atmosphere such as nitrogen or argon.
After the emulsion polymerization, it is preferable to age the mixture for 0.5 hours or more while maintaining the temperature at the polymerization temperature.

The mass ratio of radically polymerizable monomer to pigment during emulsion polymerization [radically polymerizable monomer/pigment] is preferably 1.0 or more, more preferably 1.3 or more, even more preferably 1.5 or more, and is preferably 2.5 or less, more preferably 1.9 or less, even more preferably 1.7 or less, from the viewpoint of improving the storage stability and abrasion resistance of the resulting pigment aqueous dispersion and the water-based ink using it.

[Pigment-containing core-shell polymer particles]
It is believed that the core of the pigment-containing core-shell polymer particles obtained by step 3 has a structure in which the polymer obtained by emulsion polymerization of the radically polymerizable monomer in the pre-emulsion contains (encloses) the pigment, and the shell has a structure in which the core is coated with the anionic polymer (P) in the pigment pre-dispersion.
The glass transition temperature (Tg) of the core polymer constituting the pigment-containing core-shell polymer particles is preferably 40°C or higher, more preferably 50°C or higher, even more preferably 60°C or higher, and is preferably 120°C or lower, more preferably 100°C or lower, even more preferably 90°C or lower, and still more preferably 80°C or lower.

The glass transition temperature can be calculated from the mass ratio of the monomers in each polymer portion according to the Fox equation below.
1/Tg=(W ₁ /Tg ₁ )+(W ₂ /Tg ₂ )+...+(W _m /Tg _m )
W ₁ +W ₂ +...W _m =1
In the Fox formula, Tg is the glass transition temperature of the polymer, Tg ₁ , Tg ₂ , ..., Tg _m are the glass transition temperatures of each polymerized monomer, and the unit of temperature is K. Furthermore, W ₁ , W ₂ , ..., W _m represent the mass ratio of each polymerized monomer.
As the glass transition temperature of each polymerizable monomer in the Fox equation, for example, the value described in Polymer Handbook Third Edition (Wiley-Interscience 1989) can be used.

The average particle size of the pigment-containing core-shell polymer particles is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 90 nm or more, from the viewpoint of improving the storage stability of the resulting pigment water dispersion and the water-based ink using the same, and the solvent resistance and abrasion resistance of the resulting recorded matter, and is preferably 200 nm or less, more preferably 160 nm or less, even more preferably 130 nm or less.
The average particle size of the polymer particles is measured by the method described in the examples.

<Step 4>
Step 4 is a step of crosslinking the pigment-containing core-shell polymer particles obtained in step 3 with an epoxy crosslinking agent to obtain pigment-containing core-shell crosslinked polymer particles.
Step 4 is an optional step, but it involves crosslinking some of the anionic groups of the polymer constituting the pigment-containing core-shell polymer particles to form a crosslinked structure in the shell portion of the core-shell polymer particles. This is thought to enable the polymer (Q), which is thought to be adsorbed on the surface of the core-shell polymer particles after completion of step 3, to be firmly integrated with the core-shell polymer particles, thereby suppressing aggregation of the pigment particles. As a result, it is thought that the storage stability of the resulting aqueous dispersion and water-based ink is further improved, and the solvent resistance and abrasion resistance of the resulting recorded matter are also further improved.

[Epoxy crosslinking agent]
As the epoxy crosslinking agent, a polyfunctional epoxy compound having two or more epoxy groups in the molecule is preferred, a compound having two or more glycidyl ether groups is more preferred, and a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having from 3 to 4 carbon atoms is even more preferred.
The epoxy equivalent (g/eq) of the epoxy crosslinking agent is preferably 90 or more, more preferably 100 or more, and preferably 300 or less, more preferably 200 or less.
Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

From the viewpoint of improving the solvent resistance and abrasion resistance of the resulting recorded matter, the degree of crosslinking of the shell polymer in step 4 is preferably 10 mol% or more, more preferably 20 mol% or more, even more preferably 30 mol% or more, and is preferably 70 mol% or less, more preferably 60 mol% or less, even more preferably 50 mol% or less.
Here, the degree of crosslinking is an apparent degree of crosslinking calculated from "the number of molar equivalents of epoxy groups in the epoxy crosslinking agent/the number of molar equivalents of anionic groups (carboxy groups) in the polymer before crosslinking."
From the viewpoint of crosslinking reaction efficiency, the temperature of the crosslinking treatment is preferably 50° C. or higher, more preferably 60° C. or higher, and preferably 95° C. or lower, more preferably 92° C. or lower.

(Content of each component in aqueous dispersion of pigment-containing core-shell polymer particles (pigment aqueous dispersion))
The contents of the various components in the pigment water dispersion obtained by the method of the present invention are as follows, from the viewpoint of improving the storage stability of the resulting pigment water dispersion and the water-based ink using the same, and the solvent resistance and abrasion resistance of the resulting recorded matter.

The solids concentration of the pigment water dispersion is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less, even more preferably 30% by mass or less.
The content of the pigment in the pigment water dispersion is preferably 4% by mass or more, more preferably 6% by mass or more, even more preferably 7% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less.
The content of the (crosslinked) polymer in the pigment water dispersion is preferably 6% by mass or more, more preferably 10% by mass or more, more preferably 12% by mass or more, and is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less.

[Method of manufacturing water-based ink]
The method for producing the water-based ink of the present invention is a method in which an organic solvent is added to the pigment aqueous dispersion obtained by the method of the present invention.
From the viewpoint of improving the storage stability of the water-based ink, the organic solvent preferably has a boiling point of 110° C. or higher, preferably 150° C. or higher, and preferably 240° C. or lower, more preferably 220° C. or lower. Specific examples of organic solvents include polyhydric alcohols, polyhydric alcohol alkyl 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.
Examples of polyhydric alcohols include 1,2-alkanediols such as ethylene glycol, propylene glycol, and 1,2-butanediol, diethylene glycol, dipropylene glycol, 1,3-propanediol, and 1,4-butanediol.
Specific examples of polyhydric alcohol alkyl ethers include ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monoisobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monoisobutyl ether, and diethylene glycol monobutyl ether.

In the method for producing the water-based ink of the present invention, in addition to water and surfactants, other additives such as humectants and viscosity modifiers may be mixed as needed.
Examples of surfactants include nonionic surfactants, anionic surfactants, and amphoteric surfactants, with nonionic surfactants being preferred, and polyether-modified silicone surfactants, acetylene glycol surfactants, and the like being more preferred.
Specific examples of polyether-modified silicone surfactants include PEG-9 dimethicone, PEG-9PEG-9 dimethicone, PEG-9 methyl ether dimethicone, PEG-10 dimethicone, and PEG-11 methyl ether dimethicone.
Examples of the acetylene glycol surfactant include acetylene glycols having 8 to 22 carbon atoms and ethylene adducts of the acetylene glycols, and 2,4,7,9-tetramethyl-5-decyne-4,7-diol and its ethylene oxide adduct are preferred.

(Content of each component in water-based ink)
The content of each component in the water-based ink obtained by the method of the present invention is as follows, from the viewpoint of improving storage stability, solvent resistance and abrasion resistance of the resulting recorded matter.
(Pigment content)
The content of the pigment in the water-based ink is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% 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.
(Content of (crosslinked) polymer particles containing pigment)
The content of the pigment-containing (crosslinked) polymer particles in the water-based ink, in terms of the total of the pigment and the (crosslinked) polymer, is preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 6% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less.
The mass ratio (pigment/polymer) of the pigment to the polymer (the total amount of the polymer (P), the polymer (Q), and the polymer produced by polymerization of the radical polymerizable monomer) in the water-based ink 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.

(Organic solvent content)
The content of the organic solvent in the water-based ink is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less.
(Water content)
The water content in the water-based ink is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% 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.

(Inkjet recording method)
Examples of recording media on which inkjet recording or the like can be performed using the water-based ink according to the present invention include low-water-absorbency recording papers such as plain paper and coated paper, and non-water-absorbency resin films such as PET film and polypropylene film. The water-based ink according to the present invention has excellent storage stability, and can provide recorded matter that is excellent in solvent resistance and abrasion resistance even when recording on low-water-absorbency or non-water-absorbency recording media.

In the following preparation examples, manufacturing examples, working examples, and comparative examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The methods for measuring each physical property are as follows:

(1) Measurement of polymer weight-average molecular weight (Mw) Measurement was performed by gel permeation chromatography (GPC apparatus (HLC-8320GPC) manufactured by Tosoh Corporation, columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn Super AW-H) manufactured by Tosoh Corporation, flow rate: 0.5 mL/min) using a solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide to concentrations of 60 mmol/L and 50 mmol/L, respectively, as an eluent, and using monodisperse polystyrene kits with known molecular weights (PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick C (F-288, F-40, F-4, A-5000, A-500) manufactured by Tosoh Corporation) as standards.
The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring at 25° C. for 10 hours, and filtering through a syringe filter (Advantec Co., Ltd., DISMIC-13HP, PTFE, 0.2 μm).

(2) Measurement of Acid Value of Polymer According to the neutralization titration method described in JIS K0070-1992, the acid value of the polymer was measured by changing the measurement solvent from a mixed solvent of ethanol and ether to a mixed solvent of acetone and toluene [acetone:toluene=1:1 (volume ratio)].

(3) Measurement of average particle size of pigment particles in pigment pre-dispersion and core-shell polymer particles containing pigment in pigment aqueous dispersion. Using a laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd., product name: ELS-8000), particle size was measured by dynamic light scattering, and calculated by cumulant analysis.
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 sample was prepared by weighing the dispersion into a screw tube (Maruem Corporation, No. 5), adding water so that the solids concentration was 2 × ^10-4 mass%, and stirring at 25°C for 1 hour.

(4) Measurement of solid content concentration of dispersion 10.0 g of sodium sulfate, brought to a constant weight in a desiccator, was weighed into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm), and approximately 1.0 g of the sample was added thereto and mixed, then weighed, and maintained at 105°C for 2 hours to remove volatiles. The mixture was then left in the desiccator for another 15 minutes, and the mass was measured. The mass of the sample after volatiles removal was taken as the solid content, and divided by the mass of the added sample to obtain the solid content concentration.

Preparation Example 1-1 (Preparation of Anionic Polymer P1)
A monomer mixture was prepared by mixing 31 parts of acrylic acid and 69 parts of styrene. 5 parts of methyl ethyl ketone (MEK), 0.25 parts of 3-mercaptopropionic acid (a polymerization chain transfer agent), and 10% of the monomer mixture were placed in a reaction vessel and mixed, followed by thorough nitrogen gas replacement.
Meanwhile, a dropping funnel was charged with a mixed solution of the remaining 90% of the monomer mixed solution, 2.25 parts of the polymerization chain transfer agent, 75 parts of MEK, and 1.5 parts of an azo-based radical polymerization initiator (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-501, 4,4'-azobis(4-cyanovaleric acid)), and under a nitrogen atmosphere, the monomer mixed solution in the reaction vessel was heated to 77°C with stirring, and the mixed solution in the dropping funnel was added dropwise over 5 hours. After completion of the dropping, a solution in which 0.5 parts of the polymerization initiator was dissolved in 5 parts of MEK was added, and the reaction was further carried out at 77°C for 2 hours to obtain a solution of anionic polymer P1 (acid value: 240 mgKOH/g, weight average molecular weight: 13,900).
The results are shown in Table 1.

Preparation Examples 1-2 to 1-5 (Preparation of Anionic Polymers P2 to P5)
Solutions of anionic polymers P2 to P5 were obtained in the same manner as in Preparation Example 1-1, except that the amounts of acrylic acid, styrene, and 3-mercaptopropionic acid in Preparation Example 1-1 were changed as shown in Table 1. The results are shown in Table 1.

Preparation Example 2-1 (Preparation of Water Dispersion P1-D1 of Anionic Polymer P1)
The solution of anionic polymer P1 obtained in Preparation Example 1 was dried under reduced pressure, and 80 parts of the solution was added with 255 parts of ion-exchanged water and 64.8 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide solids content: 16.9%) as an alkali metal compound (neutralizing agent), and neutralized so that the ratio of the number of moles of sodium hydroxide to the number of moles of carboxy groups in the anionic polymer P1 was 80 mol% (neutralization degree: 80 mol%). The resulting aqueous solution was heated at 90°C for 5 hours while stirring at 150 rpm, to obtain an aqueous dispersion P1-D1 of anionic polymer P1 (solids concentration: 20%). The results are shown in Table 2.

Preparation Examples 2-2 to 2-3, 2-6 to 2-9 (Preparation of Water Dispersions of Anionic Polymers P1 to P5 (P1-D2, P1-D3, P2-D1 to P5-D1))
Aqueous dispersions of anionic polymers P1-D2, P1-D3, and P2-D1 to P5-D1 were obtained in the same manner as in Preparation Example 2-1, except that the type of anionic polymer, the amount of aqueous sodium hydroxide solution, and the amount of ion-exchanged water were changed as shown in Table 2. The results are shown in Table 2.

Preparation Example 2-4 (Preparation of Water Dispersion P1-D4 of Anionic Polymer P1)
The anionic polymer P1 solution obtained in Preparation Example 1 was dried under reduced pressure, and 80 parts of the solution was mixed with 311 parts of ion-exchanged water and 9.3 parts of an aqueous ammonia solution (ammonia concentration: 25%) as a neutralizing agent, and neutralized so that the ratio of the number of moles of ammonia to the number of moles of carboxy groups in the water-insoluble polymer (P1) was 40 mol% (neutralization degree: 40 mol%). The resulting aqueous solution was heated at 90°C for 5 hours while stirring at 150 rpm, to obtain an aqueous dispersion P1-D4 of anionic polymer P1 (solids concentration: 20%). The results are shown in Table 2.

Preparation Example 2-5 (Preparation of Water Dispersion P1-D5 of Anionic Polymer P1)
The anionic polymer P1 solution obtained in Preparation Example 1 was dried under reduced pressure, and 80 parts of the solution was mixed with 311 parts of ion-exchanged water and 17.8 parts of triethanolamine as a neutralizing agent to neutralize the solution so that the ratio of the number of moles of triethanolamine to the number of moles of carboxy groups in the water-insoluble polymer (P1) was 35 mol% (degree of neutralization: 35 mol%). The resulting aqueous solution was heated at 90°C for 5 hours while stirring at 150 rpm to obtain an aqueous dispersion P1-D5 of anionic polymer P1 (solids concentration: 20%). The results are shown in Table 2.

Production Example 1-1 (Step 1: Production of Pigment Pre-Dispersion i-1)
26.3 parts of the aqueous dispersion P1-D1 of the anionic polymer P1 obtained in Preparation Example 2-1, 538 parts of ion-exchanged water, 47 parts of MEK, and 100 parts of a cyan pigment (PB15:3, manufactured by DIC Corporation, C.I. Pigment Blue 15:3) were mixed and stirred for 60 minutes using a Disper (Ultra Disper manufactured by Asada Iron Works Co., Ltd.) at 20 ° C. with a Disper blade rotating at 7,000 rpm. The resulting mixture was subjected to 10-pass dispersion treatment using a Microfluidizer (manufactured by Microfluidics, trade name) at a pressure of 200 MPa, to obtain a dispersion treatment product.
From the obtained dispersion, MEK was distilled off under reduced pressure, and further, a portion of the water was removed. The resulting mixture was filtered using a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a 5 μm filter (acetyl cellulose membrane, manufactured by Fujifilm Corporation) to remove coarse particles, thereby obtaining pigment pre-dispersion i-1 containing pigment particles with a solids concentration of 22%.
The mass ratio of the pigment in the pigment particles [pigment/(pigment+anionic polymer (P))] was 0.95, and the average particle size of the pigment particles was 90 nm. The results are shown in Table 3.

Production Examples 1-2 to 1-7 (Step 1: Production of Pigment Pre-Dispersions i-2 to i-7)
Pigment pre-dispersions i-2 to i-7 containing pigment particles were obtained in the same manner as in Production Example 1-1, except that the type and amount of the aqueous dispersion of anionic polymer and the amounts of ion-exchanged water and MEK were changed as shown in Table 3. The results are shown in Table 3.

Production Example 1-8 (Step 1: Production of Pigment Pre-Dispersion i-8)
11.1 parts of a reactive surfactant (KH-05, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., trade name: Aqualon KH-05, ether sulfate ester salt, active ingredient concentration: 100%), 483 parts of ion-exchanged water, 42 parts of MEK, and 100 parts of a cyan pigment (PB15:3, C.I. Pigment Blue 15:3) were mixed, and using a Disper (manufactured by Asada Iron Works Co., Ltd., trade name: Ultra Disper), in the same manner as in Production Example 1-1, a pigment pre-dispersion i-8 (solids concentration: 22%) containing pigment particles dispersed with the reactive surfactant was obtained.
The mass ratio of the pigment in the pigment particles [pigment/(pigment+reactive surfactant)] was 0.90, and the average particle size of the pigment particles was 105 nm. The results are shown in Table 3.

Production Example 1-9 (Step 1: Production of Pigment Pre-Dispersion i-9)
26.3 parts of the aqueous dispersion P1-D1 of the anionic polymer P1 obtained in Preparation Example 2-1, 335 parts of ion-exchanged water, 335 parts of MEK, and 100 parts of a cyan pigment (PB 15:3, C.I. Pigment Blue 15:3) were mixed and stirred for 60 minutes using a Disper (Ultra Disper, manufactured by Asada Iron Works Co., Ltd.) at 20 ° C. with the Disper blade rotating at 7,000 rpm. The resulting mixture was subjected to a single-pass dispersion treatment using a Microfluidizer (trade name, manufactured by Microfluidics) at a pressure of 50 MPa, to obtain a dispersion treatment product.
From the obtained dispersion, MEK was distilled off under reduced pressure, and further, a portion of the water was removed. The resulting mixture was filtered using a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a 5 μm filter (acetyl cellulose membrane, manufactured by FUJIFILM Corporation) to remove coarse particles, thereby obtaining pigment pre-dispersion i-9 containing pigment particles with a solids concentration of 22%.
The mass ratio of the pigment in the pigment particles [pigment/(pigment+anionic polymer (P))] was 0.95, and the average particle size of the pigment particles was 188 nm. The results are shown in Table 3.

Production Example 2-1 (Step 2: Production of Pre-emulsion E-1)
Into a plastic container, 15.3 parts of the aqueous dispersion P1-D1 of the anionic polymer obtained in Preparation Example 2-1 as the anionic polymer (Q) (acid value of anionic polymer P1: 240 mg KOH/g, degree of neutralization: 80 mol%, neutralizing agent: NaOH), 14.3 parts of cyclohexyl methacrylate as radical polymerizable monomers, 6.1 parts of methyl methacrylate, and 15.3 parts of ion-exchanged water were mixed and shaken to obtain pre-emulsion E-1. The results are shown in Table 4.

Production Examples 2-2 to 2-12 (Step 2: Production of Pre-emulsions E-2 to E-12)
Pre-emulsions E-2 to E-12 were obtained in the same manner as in Production Example 2-1, except that the anionic polymer (Q), the aqueous dispersion of an anionic polymer, and the radical polymerizable monomer were changed as shown in Table 4. The results are shown in Table 4.

Production Example 2-13 (Step 2: Production of Pre-emulsion E-13)
Pre-emulsion E-13 was obtained in the same manner as in Production Example 2-1, except that 0.6 parts of a reactive surfactant (trade name: Aqualon KH-05) instead of the anionic polymer, 30.0 parts of ion-exchanged water, 14.3 parts of cyclohexyl methacrylate, and 6.1 parts of methyl methacrylate were mixed. The results are shown in Table 4.

Production Example 2-14 (Step 2: Production of Pre-emulsion E-14)
Pre-emulsion E-14 was obtained in the same manner as in Production Example 2-1, except that 14.3 parts of cyclohexyl methacrylate, 6.1 parts of methyl methacrylate, and 30.6 parts of ion-exchanged water were mixed without using an anionic polymer. However, as soon as stirring was stopped, the mixture separated into two phases, and a stable pre-emulsion could not be obtained. The results are shown in Table 4.

Production Example 3-1 (Step 3: Production of aqueous dispersion I-1 of pigment-containing core-shell polymer particles)
60.3 parts of the pigment pre-dispersion i-1 obtained in Production Example 1-1 and 51.0 parts of the pre-emulsion E-1 obtained in Production Example 2-1 were placed in a separable flask and stirred, and then the inside of the flask was thoroughly purged with nitrogen gas.
Separately, 0.4 parts of 4,4'-azobis(4-cyanovaleric acid) (radical polymerization initiator), 4.2 parts of ion-exchanged water, and 0.2 parts of 25% aqueous ammonia solution were mixed and stirred in a glass container to obtain an aqueous solution of the radical polymerization initiator.
4.8 parts of the obtained aqueous radical polymerization initiator solution was added to the dropping funnel, and the mixture in the separable flask was heated to 80°C under a nitrogen atmosphere while being stirred, and the aqueous radical polymerization initiator solution in the dropping funnel was added dropwise over 3 hours to emulsion-polymerize the radical polymerizable monomer in pre-emulsion E-1.
After completion of the dropwise addition of the aqueous radical polymerization initiator solution, the mixture was aged at 80°C for 1 hour, cooled to 25°C, and then filtered through a 5 µm filter (acetyl cellulose membrane, manufactured by Fujifilm Corporation). The solid content was adjusted with ion-exchanged water to 25% (pigment: 8.6%, polymer: 16.4%), thereby obtaining an aqueous dispersion I-1 of pigment-containing core-shell polymer particles.

The resulting pigment-containing core-shell polymer particles had a core structure in which a polymer obtained by emulsion polymerization of the radically polymerizable monomer in pre-emulsion E-1 contained (enclosed) the pigment, and a shell structure in which the core was coated with the anionic polymer in the pigment pre-dispersion. The glass transition temperature (Tg) of the core polymer constituting the pigment-containing polymer particles was 72°C. The results are shown in Table 5.
Here, the Tg of the core polymer was calculated using the above-mentioned FOX formula from the Tg when each of the radical polymerizable monomers in pre-emulsion E-1 was homopolymerized to form a homopolymer and the composition ratio of the radical polymerizable monomers.
In the following production examples, the Tg of the core polymer was calculated in the same manner as above.

Production Examples 3-2 to 3-18, Comparative Production Examples 3-1 to 3-2 (Step 3: Production of aqueous dispersions I-2 to I-18, CI-1 and CI-2 of pigment-containing core-shell polymer particles)
Aqueous dispersions I-2 to I-18 and CI-1 to CI-2 of pigment-containing core-shell polymer particles were obtained in the same manner as in Production Example 3-1, except that the type and amount of pre-emulsion in Production Example 3-1 were changed as shown in Table 5 or Table 6. The results are shown in Tables 5 and 6.

Production Example 3-19 (Step 4: Production of aqueous dispersion I-19 of pigment-containing core-shell crosslinked polymer particles)
To the aqueous dispersion I-3 of pigment-containing core-shell type polymer particles obtained in Production Example 3-3, 0.86 parts (an amount equivalent to 40 mol% of epoxy groups relative to the total number of moles of carboxy groups in the anionic polymer (P) and the anionic polymer (Q)) of 1,6-hexanediol diglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-212L, epoxy equivalent: 135) was added, and the mixture was then heated to 90°C and reacted for 2 hours. After cooling to 25°C, the mixture was filtered through a 5 μm filter and the solids concentration was adjusted to 25% with ion-exchanged water to obtain an aqueous dispersion I-19 of pigment-containing core-shell type crosslinked polymer particles.
In these core-shell crosslinked polymer particles, only the shell portion was crosslinked (crosslinking degree: 40 mol%), and the Tg of the core polymer was 72°C. Note that the Tg and composition ratio of the components forming the crosslinked structure were not substituted into the FOX formula. The results are shown in Table 5.

Comparative Production Example 3-3 (Production of aqueous dispersion CI-3 of pigment-containing polymer particles)
A separable flask was charged with 63.7 parts of the pigment pre-dispersion i-8 dispersed with a reactive surfactant obtained in Production Example 1-8 and 51.0 parts of the pre-emulsion E-10 obtained in Production Example 2-10, and the mixture was stirred, followed by thorough nitrogen gas replacement.
Thereafter, in the same manner as in Comparative Production Example 3-1, an aqueous solution of a radical initiator was added dropwise, the radical polymerizable monomer was emulsion-polymerized, and the mixture was aged and filtered, and the solid content was adjusted to 25% with ion-exchanged water to obtain an aqueous dispersion CI-3 of pigment-containing polymer particles.
The polymer particles containing this pigment were not core-shell type polymer particles, and the Tg of the polymer constituting the particles was 72° C. The results are shown in Table 6.

Comparative Production Example 3-4 (Production of aqueous dispersion CI-4 of pigment-containing core-shell polymer particles)
67.4 parts of pigment pre-dispersion i-3 obtained in Production Example 1-3 and 51.0 parts of pre-emulsion E-13 obtained in Production Example 2-13 were placed in a separable flask and stirred, and then the inside of the flask was thoroughly purged with nitrogen gas.
Thereafter, in the same manner as in Comparative Production Example 3-1, an aqueous solution of a radical initiator was added dropwise, the radical polymerizable monomer was emulsion-polymerized, and the mixture was aged and filtered, and the solid content was adjusted to 25% with ion-exchanged water to obtain an aqueous dispersion CI-4 of pigment-containing core-shell polymer particles.
The Tg of the core polymer of the core-shell polymer particles was 72°C.
The results are shown in Table 6.

Comparative Production Example 3-5 (Production of aqueous dispersion CI-5 of pigment-containing core-shell polymer particles)
Into a separable flask, 67.4 parts of the pigment pre-dispersion i-3 obtained in Production Example 1-3 was filtered through a 5 μm filter, and the solid content was adjusted to 25% with ion-exchanged water to obtain an aqueous dispersion of pigment-containing polymer particles CI-5. The results are shown in Table 6.
Comparative Example 3-5 is an example in which step 3 is not included.

Comparative Example 3-6
Production Example 3-1 (Production of aqueous dispersion CI-6 of pigment-containing core-shell polymer particles)
60.3 parts of the pigment pre-dispersion i-1 obtained in Production Example 1-1 and 51.0 parts of the pre-emulsion E-14 obtained in Production Example 2-14 were placed in a separable flask and stirred, and then the inside of the flask was thoroughly purged with nitrogen gas.
Thereafter, an aqueous solution of a radical initiator was added dropwise in the same manner as in Comparative Production Example 3-1. After completion of the addition, the mixture was aged at 80°C for 1 hour. After the temperature was lowered to 25°C, the contents were checked and it was found that the unreacted monomer and the aqueous pigment dispersion had separated, and polymerization had not progressed.
This example shows that emulsion polymerization does not proceed unless the anionic polymer (Q) is used in producing the pre-emulsion.

Water-based inks were prepared using the pigment aqueous dispersions obtained in the Production Examples using the following method, and their storage stability, solvent resistance (ethanol), and abrasion resistance were evaluated. The results are shown in Tables 5 and 6.

<Preparation of Water-Based Ink>
To a 100 mL screw tube were added 46.6 parts of the aqueous dispersion I-1 of core-shell polymer particles containing the pigment obtained in Production Example 3-1, 4.0 parts of diethylene glycol monoisobutyl ether (iBDG), 20.0 parts of propylene glycol, 0.7 parts of a polyether-modified silicone surfactant (PEG-11 methyl ether dimethicone, manufactured by Shin-Etsu Chemical Co., Ltd., product name: KF-6011), and 28.7 parts of ion-exchanged water, and the mixture was stirred at room temperature for 15 minutes to obtain a mixed solution. The mixture was then filtered through a filter with a pore size of 5.0 μm to obtain Water-based Ink 1 (pigment content: 4.0%).
For water-based inks 2 to 19 and 21 to 25, water-based inks were prepared in the same manner as above, except that the amount of the water dispersion of pigment-containing (core-shell type) polymer particles and the amount of ion-exchanged water were changed so that the pigment content in the water-based ink was 4.0%.
For the aqueous dispersion CI-6, no water-based ink was prepared.

(Method for evaluating storage stability)
The water-based inks 1 to 19 and 21 to 25 obtained above were stored in a thermostatic chamber at 70°C for 7 days, and then the average particle size was measured. The rate of change in average particle size after storage at 70°C for 7 days was calculated using the following formula (rounded down to the nearest whole number), and the storage stability was evaluated according to the following evaluation criteria.
Average particle size change rate (%) = [(average particle size after storage/average particle size before storage) - 1] x 100
(Evaluation criteria)
5: The absolute value of the rate of change in average particle size is less than 5%.
4: The absolute value of the rate of change in average particle size is 5% or more and less than 15%.
3: The absolute value of the rate of change in average particle size is 15% or more and less than 30%.
2: The absolute value of the rate of change in average particle size is 30% or more and less than 50%.
1: The absolute value of the rate of change in average particle size is 50% or more, or the water-based ink has lost fluidity and is not in a state where the average particle size can be measured.
If the evaluation result is 3 or higher, there is no problem in practical use.

(Printing evaluation on resin film (PVC))
The aqueous inks 1-19 and 21-25 obtained above were filled into an inkjet printer (manufactured by Ricoh Company, Ltd., product name: IPSiO GX 2500, piezoelectric type), and an A4 solid image was printed on polyvinyl chloride (PVC) film (product name: 3M ^TM Scotchcal ^TM Graphic Film IJ40-10R (white glossy)). The print was then dried for 5 minutes in a dryer at 80°C and left to stand for 1 day in an environmental chamber at a room temperature of 25°C and a relative humidity of 50%, yielding a print for evaluation. The resulting print was evaluated for solvent resistance (ethanol) and abrasion resistance using the following methods.

(Method for evaluating solvent (ethanol) resistance)
A cotton ball (AATCC CROCKMETER SQUARES) moistened with a 70% aqueous ethanol solution was attached to a Gakushin tester (manufactured by Daiei Scientific Instruments Co., Ltd., product name: RT-300), and a rubbing test was performed with 100 strokes back and forth under a load of 200 g. The rubbed area of the printed matter was captured as an image and binarized using Image J software at 8 bits with a threshold of 90. The percentage of the area where color remained after rubbing (color retention rate) was calculated using the following formula, and ethanol resistance was evaluated according to the following evaluation criteria.
Color retention rate (%) = [number of points counted as black/(number of points counted as black + number of points counted as white)] x 100
(Evaluation criteria)
8: Color retention is 90% or more.
7: Color retention rate is 80% or more and less than 90%.
6: Color retention rate is 70% or more and less than 80%.
5: Color retention rate is 60% or more and less than 70%.
4: Color retention rate is 50% or more and less than 60%.
3: Color retention rate is 40% or more and less than 50%.
2: Color retention rate is 20% or more and less than 40%.
1: Color retention rate is less than 20%.
If the evaluation result is 4 or more, there is no problem in practical use, but 6 or more is preferable.

(Method for evaluating scratch resistance)
The evaluation prints obtained above were subjected to a rub test using an AB-201 Sutherland type Ink Labo Tester (manufactured by Tester Sangyo Co., Ltd.). A 900 g load was applied to a cellulose nonwoven fabric (manufactured by Asahi Kasei Corporation, product name: BEMCOT M-3) as a friction material using the bottom surface of a weight (bottom surface area 50 cm ² ) and rubbed 10 times (back and forth). The condition of the printed surface of the rubbed print was visually inspected and evaluated according to the following evaluation criteria.
(Evaluation criteria)
4: No scratches were found on the printed surface, and there was no change in the surface condition of the printed surface before and after rubbing.
3: Scratches were observed on the printed surface, but the surface of the printed medium was not exposed.
2: Scratches were observed on the printed surface, and part of the surface of the printed medium was exposed.
1: The entire print medium in the abraded area was exposed.
If the evaluation result is 3 or more, there is no problem in practical use, but 4 or more is preferable.

Tables 5 and 6 show that the water-based inks 1 to 19 of the examples have superior storage stability and can produce printed materials with excellent solvent resistance and abrasion resistance compared to the water-based inks 21 to 25 of the comparative examples.

Claims (8)

A method for producing a pigment aqueous dispersion, comprising the following steps 1 to 3:
Step 1: A step of mixing and dispersing a pigment, an anionic polymer (P) having an acid value of 100 mgKOH/g or more and 300 mgKOH/g or less, and an aqueous medium to obtain a pigment pre-dispersion containing pigment particles having an average particle size of 160 nm or less. Step 2: A step of mixing a radically polymerizable monomer with an aqueous dispersion of anionic polymer (Q) to obtain a pre-emulsion of the radically polymerizable monomer. Step 3: A step of mixing the pigment pre-dispersion obtained in Step 1 with the pre-emulsion of the radically polymerizable monomer obtained in Step 2, adding a radical polymerization initiator, and emulsion-polymerizing the radically polymerizable monomer to obtain an aqueous dispersion of core-shell polymer particles containing the pigment. The method for producing a pigment aqueous dispersion according to claim 1, wherein the weight-average molecular weight of the anionic polymer (P) is 3,000 or more and 100,000 or less. The method for producing a pigment aqueous dispersion according to claim 1 or 2, wherein the mass ratio of the pigment to the total amount of the pigment and the anionic polymer (P) [pigment/(pigment + anionic polymer (P)] is 0.82 or more and 0.99 or less. The method for producing a pigment aqueous dispersion according to any one of claims 1 to 3, wherein the radical polymerizable monomer is at least one selected from the group consisting of (meth)acrylates having an alkyl group with 1 to 8 carbon atoms and vinyl monomers having an aromatic group with 6 to 22 carbon atoms. The method for producing a pigment aqueous dispersion according to any one of claims 1 to 4, wherein the anionic polymer (Q) is neutralized with one or more selected from the group consisting of alkali metal hydroxides, water-soluble amines, and ammonia. The method for producing a pigment aqueous dispersion according to any one of claims 1 to 5, wherein the glass transition temperature (Tg) of the core polymer constituting the pigment-containing core-shell polymer particles is 40°C or higher and 120°C or lower. The method for producing the pigment water dispersion according to any one of claims 1 to 6, further comprising the following step 4:
Step 4: A step of crosslinking the pigment-containing core-shell polymer particles obtained in Step 3 with an epoxy crosslinking agent to obtain pigment-containing core-shell crosslinked polymer particles. A method for producing a water-based ink, comprising adding an organic solvent to the pigment aqueous dispersion obtained by the method described in any one of claims 1 to 7.