Water-based pigment dispersion

Abstract

This invention relates to an aqueous pigment dispersion that can produce an aqueous ink with excellent storage stability and redispersibility, and to an aqueous ink containing the aqueous pigment dispersion. [Solution] An aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium by a crosslinked polymer (A), wherein the crosslinked polymer (A) comprises a structure derived from a water-dispersible polymer (A') having a carboxyl group and a structure derived from a polyfunctional epoxy compound, and the polyfunctional epoxy compound comprises a high water-soluble epoxy compound (S) with a specific water solubility and a low water-soluble epoxy compound (I) with a specific water solubility.

Description

Technical Field

[0001] The present invention relates to an aqueous pigment dispersion and an aqueous ink containing the aqueous pigment dispersion.

Background Art

[0002] The inkjet recording method is a recording method in which ink droplets are directly ejected from a fine nozzle and adhered to a recording medium to obtain a recording in which characters and images are recorded. This method has many advantages such as being easy and inexpensive to full-colorize, being able to use plain paper as a recording medium, and being non-contact with the recording medium, so it has become extremely popular. In particular, from the viewpoints of the weather resistance and water resistance of the recording, those using a pigment as a colorant have become mainstream.

[0003] For example, in Patent Document 1, when incorporated into ink, while maintaining excellent storage stability of the ink, in the inkjet recording method, particularly suppressing the occurrence of coking in the heater portion of the thermal head and improving the continuous ejection stability. For the purpose of providing a pigment aqueous dispersion or the like that can be used, a pigment aqueous dispersion in which an organic pigment A is dispersed with a crosslinked polymer, wherein the organic pigment A is one or more selected from disazo pyrazolone-based pigments, perinone-based pigments, dioxazine-based pigments, and copper phthalocyanine-based pigments, and the crosslinked polymer is a polymer crosslinked with 1,4-butanediol diglycidyl ether as a crosslinking agent, and an inkjet recording aqueous ink containing the pigment aqueous dispersion and a solvent are described.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Water-based inks, including water-based inks for inkjet recording, are sometimes used after being stored for a certain period of time after manufacturing, so they require excellent storage stability. Furthermore, water-based inkjet inks lose fluidity as the water-based medium evaporates, causing them to adhere to the nozzles and clog them. To remove the adhered ink by redispersing it, water-based inkjet inks are required to have the property of redispersing back into the water-based medium after the water-based medium has evaporated (redispersibility).

[0006] The present invention relates to an aqueous pigment dispersion that can produce an aqueous ink with excellent storage stability and redispersibility, and to an aqueous ink containing the aqueous pigment dispersion. [Means for solving the problem]

[0007] The present inventors have found that the above problems can be solved by a crosslinked polymer obtained by crosslinking a water-dispersible polymer having a carboxyl group with a water-soluble polyfunctional epoxy crosslinking agent of a specific water solubility and a water-insoluble polyfunctional epoxy crosslinking agent of a specific water solubility, and by an aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium.

[0008] In other words, the present invention relates to the following [1] to [3]. [1] An aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium with a crosslinked polymer (A), The crosslinked polymer (A) comprises a structure derived from a water-dispersible polymer (A') having a carboxyl group and a structure derived from a polyfunctional epoxy compound. The polyfunctional epoxy compound comprises a highly water-soluble epoxy compound (S) and a low water-soluble epoxy compound (I). When 10 parts by mass of a highly water-soluble epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 70% by mass or more and 100% by mass or less. An aqueous pigment dispersion wherein the water solubility of 10 parts by mass of a low water-soluble epoxy compound (I) is added to 90 parts by mass of deionized water at 25°C, and the water solubility is 0% by mass or more and 30% by mass or less. [2] A water-based ink comprising the aqueous pigment dispersion described in [1] above and a water-soluble organic solvent. [3] A method for producing an aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium with a crosslinked polymer (A), comprising the following step I: Step I: A step to obtain an aqueous pigment dispersion containing a pigment, the crosslinked polymer (A), and an aqueous medium by reacting the carboxyl groups of a water-dispersible polymer (A') having carboxyl groups with a polyfunctional epoxy compound to form a crosslinked polymer (A). The polyfunctional epoxy compound comprises a highly water-soluble epoxy compound (S) and a low water-soluble epoxy compound (I). When 10 parts by mass of a highly water-soluble epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 70% by mass or more and 100% by mass or less. When 10 parts by mass of a low water-soluble epoxy compound (I) is added to 90 parts by mass of deionized water, the water solubility is 0% by mass or more and 30% by mass or less. A method for producing an aqueous pigment dispersion. [Effects of the Invention]

[0009] The present invention provides an aqueous pigment dispersion that can produce an aqueous ink with excellent storage stability and redispersibility, and an aqueous ink containing the aqueous pigment dispersion. [Modes for carrying out the invention]

[0010] [Water-based pigment dispersion] The aqueous pigment dispersion of the present invention comprises a pigment dispersed in an aqueous medium by a crosslinked polymer (A), wherein the crosslinked polymer (A) includes a structure derived from a water-dispersible polymer (A') having a carboxyl group (hereinafter also simply referred to as "water-dispersible polymer (A')") and a structure derived from a polyfunctional epoxy compound, wherein the polyfunctional epoxy compound includes a highly water-soluble epoxy compound (S) and a low water-soluble epoxy compound (I), wherein the water solubility when 10 parts by mass of the highly water-soluble epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C is 70% by mass or more and 100% by mass or less, and the water solubility when 10 parts by mass of the low water-soluble epoxy compound (I) is added to 90 parts by mass of deionized water at 25°C is 0% by mass or more and 30% by mass or less. The "water solubility" in "When 10 parts by mass of high water solubility epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 70% by mass or more and 100% by mass or less" specifically refers to the water solubility obtained by the measurement method described in the examples. Similarly, the "water solubility" in "When 10 parts by mass of low water solubility epoxy compound (I) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 0% by mass or more and 30% by mass or less" specifically refers to the water solubility obtained by the measurement method described in the examples.

[0011] In this specification, "aqueous system" means a medium in which the pigment is dispersed, in which water constitutes the largest proportion. Furthermore, "recording" is a concept that includes printing and inscription, which record characters and images. The aqueous ink containing the aqueous pigment dispersion of the present invention is preferable for use as an aqueous ink for inkjet recording because it exhibits excellent redispersibility.

[0012] The aqueous pigment dispersion and water-soluble organic solvent contained in the present invention exhibit excellent storage stability and redispersibility. The reason for this is not entirely clear, but it is thought to be as follows. The aqueous pigment dispersion of the present invention contains a pigment dispersed in a crosslinked polymer (A), and the crosslinked polymer (A) includes a structure derived from a water-dispersible polymer (A'), a structure derived from a highly water-soluble polyfunctional epoxy crosslinking agent, and a structure derived from a low water-soluble polyfunctional epoxy crosslinking agent. The low water-soluble polyfunctional epoxy crosslinking agent is highly hydrophobic and orients near the hydrophobic pigment, contributing to the crosslinking of the water-dispersible polymer (A') near the pigment, thereby efficiently suppressing detachment from the pigment due to polymer swelling, resulting in a stable aqueous pigment dispersion. On the other hand, the highly water-soluble polyfunctional epoxy crosslinking agent is easily orientable in water, and contributes to the crosslinking of the water-dispersible polymer (A') present near the interface between the water-dispersible polymer (A') adsorbed on the pigment and water, so that the outermost shell of the pigment-containing polymer particles in the aqueous pigment dispersion becomes a hydrophilic crosslinked structure. Therefore, even if aggregates are generated near the inkjet nozzle, they are easily redispersed when aqueous ink is supplied again. Based on the above, the aqueous ink containing the aqueous pigment dispersion and water-soluble organic solvent of the present invention is considered to have excellent storage stability and redispersibility.

[0013] [Pigment] The pigment may be either an inorganic pigment or an organic pigment. Examples of inorganic pigments include carbon black and metal oxides. In black ink, carbon black is preferred. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. In white ink, examples of metal oxides include titanium dioxide, zinc oxide, silica, alumina, magnesium oxide, etc. 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. In achromatic ink, achromatic pigments such as white, black, and gray can be used, and in chromatic ink, chromatic pigments such as yellow, magenta, cyan, red, blue, orange, and green can be used. The pigment can be used alone or in combination of two or more. In the present invention, the pigment is dispersed by a crosslinked polymer (A) and contained in the form of particles in an aqueous medium.

[0014] [Crosslinked Polymer (A)] The crosslinked polymer contains a structure derived from a water-dispersible polymer (A') having a carboxy group (hereinafter also simply referred to as "water-dispersible polymer (A')") and a structure derived from a polyfunctional epoxy compound. The crosslinked polymer (A) can be used alone or in combination of two or more.

[0015] The aqueous pigment dispersion of the present invention is obtained by dispersing a pigment in an aqueous medium with a crosslinked polymer (A). In the aqueous pigment dispersion of the present invention and the forms of the pigment and the crosslinked polymer (A) in the aqueous ink containing the aqueous pigment dispersion, they are common to both. For example, there are a state where the crosslinked polymer (A) is adsorbed on the pigment and a state where the crosslinked polymer (A) contains the pigment in a pigment inclusion (capsule) state. Among these, from the viewpoints of the storage stability and redispersibility of the aqueous ink, the form of crosslinked polymer particles containing the pigment (hereinafter, also referred to as "pigment-containing crosslinked polymer particles") is preferable, and the form of pigment inclusion crosslinked polymer particles in which the crosslinked polymer (A) encloses the pigment is more preferable.

[0016] <Water-dispersible polymer (A') having a carboxy group> The water-dispersible polymer (A') having a carboxy group has a function as a pigment dispersant that exhibits a pigment dispersing action. In the present invention, the "water-dispersible polymer" means a polymer that can be dispersed in water or an aqueous medium at room temperature (25°C).

[0017] Examples of the water-dispersible polymer (A') include at least one selected from polyester, polyurethane, and vinyl polymers. From the viewpoints of the storage stability and redispersibility of the aqueous ink, vinyl polymers obtained by addition polymerization of vinyl monomers are preferable. Examples of the vinyl monomer include vinyl compounds, vinylidene compounds, and vinylene compounds.

[0018] From the viewpoints of the storage stability and redispersibility of the aqueous ink, the water-dispersible polymer (A') is preferably a vinyl polymer containing a structural unit derived from a carboxy group-containing monomer, and more preferably a vinyl polymer containing a structural unit derived from a carboxy group-containing monomer and a structural unit derived from a hydrophobic monomer. When the water-dispersible polymer (A') contains a structural unit derived from a carboxy group-containing monomer and a structural unit derived from a hydrophobic monomer, it is preferably obtained by copolymerizing a raw material monomer containing a carboxy group-containing monomer and a hydrophobic monomer.

[0019] (Carboxy group-containing monomer) Preferred carboxyl group-containing monomers include carboxylic acid monomers. Examples of carboxylic acid monomers include one or more selected from the group consisting of unsaturated monocarboxylic acids such as (meth)acrylic acid and crotonic acid; and unsaturated dicarboxylic acids such as itaconic acid, maleic acid, fumaric acid, citraconic acid, and α,β-unsaturated dicarboxylic acids. Among these, (meth)acrylic acid is preferred as the carboxyl group-containing monomer from the viewpoint of storage stability and redispersibility of water-based inks, and acrylic acid is more preferred. The unsaturated dicarboxylic acid may also be an anhydride. In this specification, "(meth)acrylic," as in (meth)acrylic acid, is a concept that includes both acrylic and methacrylic.

[0020] (Hydrophobic monomer) In this invention, "hydrophobic monomer" means a monomer in which the amount dissolved when dissolved in 100g of ion-exchanged water at 25°C until saturated is less than 10g. Specific examples of hydrophobic monomers include alkyl (meth)acrylates and aromatic group-containing monomers described in paragraphs

[0020] to

[0022] of Japanese Patent Publication No. 2018-83938. Among these, from the viewpoint of storage stability and redispersibility of water-based inks, the hydrophobic monomer is preferably at least one selected from alkyl (meth)acrylates having an alkyl group having 1 to 18 carbon atoms, more preferably 1 to 10 carbon atoms, and aromatic group-containing monomers having an aromatic group having 6 to 22 carbon atoms, more preferably 6 to 18 carbon atoms. More preferably at least one selected from ethyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, styrene, α-methylstyrene, and benzyl (meth)acrylate. Even more preferably one or more selected from styrene, α-methylstyrene, and benzyl (meth)acrylate. Even more preferably at least one selected from styrene and α-methylstyrene. Styrene is even more preferred.

[0021] The raw material monomers for the water-dispersible polymer (A') can be used individually or in combination of two or more types.

[0022] ≪Content of each constituent unit in the water-dispersible polymer (A')≫ The content of constituent units derived from carboxyl group-containing monomers in the water-dispersible polymer (A') is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and preferably 55% by mass or less, more preferably 45% by mass or less, and even more preferably 35% by mass or less, from the viewpoint of storage stability and redispersibility of the water-based ink. The content of hydrophobic monomer-derived constituent units in the water-dispersible polymer (A') is preferably 45% by mass or more, more preferably 55% by mass or more, even more preferably 65% ​​by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 75% by mass or less, from the viewpoint of storage stability and redispersibility of the water-based ink. The total content of constituent units derived from carboxyl group-containing monomers and constituent units derived from hydrophobic monomers in the water-dispersible polymer (A') is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and 100% by mass or less, and preferably 100% by mass, from the viewpoint of storage stability and redispersibility of the water-based ink. The mass ratio [carboxyl group-containing monomer / hydrophobic monomer] of the content of constituent units derived from carboxyl group-containing monomers to the content of constituent units derived from hydrophobic monomers in the water-dispersible polymer (A') is preferably 0.1 or higher, more preferably 0.2 or higher, even more preferably 0.3 or higher, and preferably 1.2 or lower, more preferably 1.0 or lower, and even more preferably 0.7 or lower, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0023] ≪Manufacturing of water-dispersible polymer (A')≫ A water-dispersible polymer (A') can be produced, for example, by copolymerizing raw material monomers containing a carboxyl group monomer and a hydrophobic monomer using a known polymerization method. Solution polymerization is preferred as the polymerization method. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as water, aliphatic alcohols with 1 to 3 carbon atoms, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, etc. are more preferred. During polymerization, known polymerization initiators can be used, and if necessary, known polymerization chain transfer agents may also be used. Preferred polymerization initiators are azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile) and 4,4'-azobis(4-cyanovaleric acid), and preferred polymerization chain transfer agents are mercaptan compounds such as 2-mercaptoethanol and 3-mercaptopropionic acid. The preferred polymerization conditions vary depending on the type of polymerization initiator, but the polymerization temperature is preferably between 50°C and 90°C, and the polymerization time is preferably between 1 hour and 10 hours. Furthermore, the polymerization atmosphere is preferably an inert gas atmosphere such as a nitrogen gas atmosphere or argon.

[0024] ≪Acid value of water-dispersible polymer (A')≫ The acid value of the water-dispersible polymer (A') is preferably 100 mg KOH / g or more, more preferably 150 mg KOH / g or more, even more preferably 200 mg KOH / g or more, and preferably 400 mg KOH / g or less, more preferably 350 mg KOH / g or less, and even more preferably 300 mg KOH / g or less. If the acid value of the water-dispersible polymer (A') is within the above range, the amount of carboxyl groups crosslinked by the polyfunctional epoxy compound and the amount of carboxyl groups neutralized can be ensured, and the dispersion stability of the pigment dispersed by the crosslinked polymer (A) can be guaranteed. The acid value of the water-dispersible polymer (A') can be determined by neutralization titration, as described in the examples. Alternatively, it can be calculated from the mass ratio of the monomers constituting the water-dispersible polymer. Furthermore, the acid value of the water-dispersible polymer (A') can also be calculated from the acid value of the cross-linked polymer (A), which will be described later, and the degree of cross-linking of the cross-linked polymer (A), which will also be described later, using the following formula. The acid value (mgKOH / g) of a water-dispersible polymer = 100 × (acid value of crosslinked polymer (A)) / (100 - (degree of crosslinking of crosslinked polymer (A))) Here, the unit of the acid value of the crosslinked polymer (A) is "mgKOH / g", and the unit of the degree of crosslinking of the crosslinked polymer is "mol%". The acid value of crosslinked polymer (A) can be calculated from the degree of crosslinking of crosslinked polymer (A) using the following formula, and represents the amount of acid groups of water-dispersible polymer (A') that remain without participating in the formation of the crosslinked structure through reaction with the polyfunctional epoxy compound. Acid value of crosslinked polymer (A) (mgKOH / g) = (Acid value of water-dispersible polymer (A')) × (100 - (Degree of crosslinking of crosslinked polymer (A))) / 100

[0025] Weight-average molecular weight of water-dispersible polymers The weight-average molecular weight of the water-dispersible polymer is preferably 1,000 or more, more preferably 5,000 or more, even more preferably 10,000 or more, and preferably 50,000 or less, more preferably 30,000 or less, and even more preferably 20,000 or less, from the viewpoint of storage stability and redispersibility of the water-based ink. The weight-average molecular weight of the water-dispersible polymer is measured by the method described in the examples.

[0026] <Polyfunctional epoxy compounds> Polyfunctional epoxy compounds include high water soluble epoxy compounds (S) and low water soluble epoxy compounds (I). The term "polyfunctional" in polyfunctional epoxy compounds means having two or more epoxy groups. When 10 parts by mass of the high water-soluble epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 70% by mass or more and 100% by mass or less. When 10 parts by mass of the low water-soluble epoxy compound (I) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 0% by mass or more and 30% by mass or less. The water solubility of the high water-soluble epoxy compound (S) and the low water-soluble epoxy compound (I) are specifically the values ​​calculated by the method described in the examples. Furthermore, the high water-soluble epoxy compound (S) may be used alone or in combination of two or more types. When two or more high water-soluble epoxy compounds (S) are used in combination, the water solubility of the high water-soluble epoxy compound (S) shall refer to the water solubility of the mixture of the two or more high water-soluble epoxy compounds (S). The same applies to the water solubility of the low water-soluble epoxy compound (I).

[0027] (High water solubility epoxy compound (S)) The water solubility of the highly water-soluble epoxy compound (S) is 70% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 93% by mass or more, even more preferably 97% by mass or more, and 100% by mass or less, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0028] The number of epoxy groups in a highly water-soluble epoxy compound (S) is 2 or more, preferably 7 or less, more preferably 5 or less, and even more preferably 3 or less, from the viewpoint of storage stability, redispersibility, and availability of the water-based ink. Similarly, the number of glycidyl ether groups in a highly water-soluble epoxy compound (S) is 2 or more, preferably 7 or less, more preferably 5 or less, and even more preferably 3 or less, from the same viewpoint. When using a combination of highly water-soluble epoxy compounds (S) with different numbers of epoxy groups, the "number of epoxy groups in a highly water-soluble epoxy compound (S)" refers to a weighted average value. The same applies to the number of glycidyl ether groups.

[0029] The epoxy equivalent of the highly water-soluble epoxy compound (S) is preferably 70 g / eq or more, more preferably 80 g / eq or more, even more preferably 90 g / eq or more, and preferably 300 g / eq or less, more preferably 250 g / eq or less, and even more preferably 200 g / eq or less, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0030] Specific examples of highly water-soluble epoxy compounds (S) include sorbitol polyglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, aliphatic epoxy compounds, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, and polypropylene glycol diglycidyl ether. Among these, glycerol polyglycidyl ether, polyethylene glycol diglycidyl ether, and 1,4-butanediol diglycidyl ether are preferred from the viewpoint of storage stability and redispersibility of water-based inks.

[0031] Examples of commercially available high water-soluble epoxy compounds (S) include the following (all manufactured by Nagase ChemteX Corporation): Denacol EX-614 (sorbitol polyglycidyl ether (tetrafunctional), epoxy equivalent: 167 g / eq, water solubility: 78% by mass) Denacol EX-614B (sorbitol polyglycidyl ether (tetrafunctional), epoxy equivalent: 173 g / eq, water solubility: 94% by mass) Denacol EX-313 (glycerol polyglycidyl ether (mixture of bifunctional and trifunctional compounds), epoxy equivalent: 141 g / eq, water solubility: 99% by mass) Denacol EX-421 (Diglycerol polyglycidyl ether (trifunctional), epoxy equivalent: 159 g / eq, water solubility: 88% by mass) Denacol EX-512 (Polyglycerol polyglycidyl ether (tetrafunctional), epoxy equivalent: 168 g / eq, water solubility: 100% by mass) Denacol EX-521 (Polyglycerol polyglycidyl ether (pentafunctional), epoxy equivalent: 183 g / eq, water solubility: 100% by mass) Denacol EX-1610 (aliphatic epoxy compound, epoxy equivalent: 170 g / eq, water solubility: 100% by mass) Denacol EX-810 (ethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 113 g / eq, water solubility: 100% by mass) Denacol EX-810P (ethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 95 g / eq, water solubility: 100% by mass) Denacol EX-811 (ethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 132 g / eq, water solubility: 98% by mass) Denacol EX-850 (polyethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 122 g / eq, water solubility: 100% by mass) Denacol EX-851 (polyethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 150 g / eq, water solubility: 99% by mass) Denacol EX-821 (polyethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 185 g / eq, water solubility: 100% by mass) Denacol EX-830 (polyethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 268 g / eq, water solubility: 100% by mass) Denacol EX-832 (polyethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 284 g / eq, water solubility: 100% by mass) Denacol EX-841 (polyethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 372 g / eq, water solubility: 100% by mass) Denacol EX-861 (polyethylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 551 g / eq, water solubility: 100% by mass) Denacol EX-214L (1,4-butanediol diglycidyl ether (bifunctional), epoxy equivalent: 115 g / eq, water solubility: 100% by mass) Denacol EX-920 (polypropylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 176 g / eq, water solubility: 100% by mass)

[0032] (Low water solubility epoxy compound (I)) The water solubility of the low water-soluble epoxy compound (I) is 0% by mass or more, preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, even more preferably 20% by mass or more, and 30% by mass or less, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0033] The number of epoxy groups in low water-soluble epoxy compound (I) is 2 or more, preferably 7 or less, more preferably 5 or less, even more preferably 3.5 or less, and still more preferably 3 or less, from the viewpoint of storage stability and redispersibility of water-based inks. Similarly, the number of glycidyl ether groups in low water-soluble epoxy compound (I) is 2 or more, preferably 7 or less, more preferably 5 or less, even more preferably 4 or less, even more preferably 3.5 or less, and still more preferably 3 or less. When using a combination of low water-soluble epoxy compounds (I) with different numbers of epoxy groups, the "number of epoxy groups in low water-soluble epoxy compound (I)" refers to a weighted average value. The same applies to the number of glycidyl ether groups.

[0034] The epoxy equivalent of the low water-soluble epoxy compound (I) is preferably 90 g / eq or more, more preferably 100 g / eq or more, even more preferably 110 g / eq or more, and preferably 300 g / eq or less, more preferably 250 g / eq or less, and even more preferably 200 g / eq or less, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0035] Specific examples of low water-soluble epoxy compounds (I) include sorbitol polyglycidyl ether, trimethylolpropane polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, hydrogenated bisphenol A diglycidyl ether, phthalate diglycidyl ester, and polypropylene glycol diglycidyl ether. Among these, sorbitol polyglycidyl ether, trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether are preferred from the viewpoint of storage stability and redispersibility of water-based inks, and trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether are more preferred.

[0036] Examples of commercially available low water-soluble epoxy compounds (I) include the following (all manufactured by Nagase ChemteX Corporation): Denacol EX-622 (Sorbitol polyglycidyl ether (tetrafunctional), epoxy equivalent: 191 g / eq, water solubility: 0% by mass) Denacol EX-321 (Trimethylolpropane polyglycidyl ether (mixture of bifunctional and trifunctional compounds), epoxy equivalent: 140 g / eq, water solubility: 27% by mass) Denacol EX-321L (Trimethylolpropane polyglycidyl ether (mixture of bifunctional and trifunctional compounds), epoxy equivalent: 130 g / eq, water solubility: 0% by mass) Denacol EX-201 (resorcinol diglycidyl ether (bifunctional), epoxy equivalent: 117 g / eq, water solubility: 0% by mass) Denacol EX-201-IM (resorcinol diglycidyl ether (bifunctional), epoxy equivalent: 120 g / eq, water solubility: 0% by mass) Denacol EX-211 (Neopentyl glycol diglycidyl ether (bifunctional), epoxy equivalent: 138 g / eq, water solubility: 0% by mass) Denacol EX-212 (1,6-Hexanediol diglycidyl ether (bifunctional), epoxy equivalent: 151 g / eq, water solubility: 0% by mass) Denacol EX-212L (1,6-Hexanediol diglycidyl ether (bifunctional), epoxy equivalent: 135 g / eq, water solubility: 0% by mass) Denacol EX-216L (Cyclohexanedimethanol diglycidyl ether, epoxy equivalent: 150 g / eq, water solubility: 0% by mass) Denacol EX-252 (Hydrogenated Bisphenol A Diglycidyl Ether (Bifunctional), Epoxy Equivalent: 213 g / eq, Water Soluble: 0% by mass) Denacol EX-721 (Diglycidyl phthalate (bifunctional), epoxy equivalent: 154 g / eq, water solubility: 0% by mass) Denacol EX-931 (polypropylene glycol diglycidyl ether (bifunctional), epoxy equivalent: 471 g / eq, water solubility: 0% by mass) Denacol EX-991L ((bifunctional), epoxy equivalent: 450 g / eq, water solubility: 0% by mass)

[0037] ≪Degree of cross-linking of cross-linked polymer (A)≫ The degree of crosslinking of the crosslinked polymer (A) (the "total degree of crosslinking" as described in Table 1 below) is preferably 35 mol% or more, more preferably 55 mol% or more, even more preferably 75 mol% or more, even more preferably 85 mol% or more, and preferably 98 mol% or less, more preferably 95 mol% or less, from the viewpoint of storage stability and redispersibility of the water-based ink. Here, the degree of crosslinking of the crosslinked polymer (A) refers to the ratio of the number of molar equivalents of epoxy groups in the polyfunctional epoxy compound to the number of molar equivalents of carboxyl groups in the water-dispersible polymer (A').

[0038] The degree of crosslinking (SR) of the crosslinked polymer (A) with a highly water-soluble epoxy compound (S) is preferably 30 mol% or more, more preferably 55 mol% or more, even more preferably 65 mol% or more, and preferably 95 mol% or less, more preferably 90 mol% or less, and more preferably 85 mol% or less, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0039] The degree of crosslinking (IR) of the crosslinked polymer (A) with the low water-soluble epoxy compound (I) is preferably 2 mol% or more, more preferably 5 mol% or more, even more preferably 8 mol% or more, and preferably 55 mol% or less, more preferably 35 mol% or less, and more preferably 25 mol% or less, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0040] From the viewpoint of storage stability and redispersibility of water-based inks, the degree of crosslinking (SR) and the degree of crosslinking (IR) preferably satisfy the following formula (1), more preferably satisfy the following formula (2), even more preferably satisfy the following formula (3), and even more preferably satisfy the following formula (4). 0.45 ≤ SR / (SR+IR) < 1.0 (1) 0.65 ≤ SR / (SR+IR) < 1.0 (2) 0.70 ≤ SR / (SR+IR) < 1.0 (3) 0.75 ≤ SR / (SR+IR) < 1.0 (4)

[0041] [Method for producing aqueous pigment dispersions] The present invention provides a method for producing an aqueous pigment dispersion, comprising the following step I. Step I: A step to obtain an aqueous pigment dispersion containing a pigment, the crosslinked polymer (A), and an aqueous medium by reacting the carboxyl groups of a water-dispersible polymer (A') having carboxyl groups with a polyfunctional epoxy compound to form a crosslinked polymer (A). The polyfunctional epoxy compound comprises a highly water-soluble epoxy compound (S) and a low water-soluble epoxy compound (I). When 10 parts by mass of a highly water-soluble epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 70% by mass or more and 100% by mass or less. When 10 parts by mass of low water-soluble epoxy compound (I) is added to 90 parts by mass of deionized water, the water solubility is 0% by mass or more and 30% by mass or less.

[0042] The method for producing the aqueous pigment dispersion of the present invention preferably includes steps 1 and 3 below, and more preferably includes steps 1 to 3 below. Step 3 corresponds to step (I) above. Step 1: A process to obtain a pigment dispersion by applying shear stress to a pigment mixture containing a pigment and an aqueous dispersion of a carboxyl group-containing aqueous polymer (A') (hereinafter also referred to as "polymer aqueous dispersion (D)") and dispersing it. Step 2: A step to obtain a pigment aqueous dispersion (e) by filtering the pigment dispersion to remove coarse particles. Step 3: A step to obtain an aqueous pigment dispersion (hereinafter also referred to as "aqueous pigment dispersion (E)") containing a pigment, a crosslinked polymer (A), and an aqueous medium by reacting the carboxyl groups of the water-dispersible polymer (A') in the aqueous pigment dispersion (e) obtained in Step 2 with a polyfunctional epoxy compound to form a crosslinked polymer (A).

[0043] [Process 1] Step 1 is a step of dispersing a pigment in an aqueous medium using a water-dispersible polymer (A') having a carboxyl group as a dispersant to obtain a pigment dispersion. In step 1, the aqueous medium may contain an organic solvent from the viewpoint of improving the wettability of the pigment and the adsorption of the water-dispersible polymer (A') having a carboxyl group to the pigment. Suitable examples of organic solvents include alcohols having 1 to 3 carbon atoms and ketones having 3 to 6 carbon atoms. In step 1, the dispersion treatment may be carried out solely by shear stress to finely atomize the pigment particles to the desired particle size. However, from the viewpoint of obtaining a uniform pigment dispersion, it is preferable to pre-disperse the pigment mixture before further dispersion. For pre-dispersion, commonly used mixing and stirring devices such as anchor blades and disperser blades can be used as dispersers. Means of applying shear stress for this dispersion include, for example, kneaders such as roll mills and kneaders; high-pressure homogenizers such as microfluidizers; and media-type dispersers such as paint shakers and bead mills. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When performing dispersion processing using a high-pressure homogenizer, the average particle size of pigment particles in the pigment dispersion can be adjusted by controlling the processing pressure and the number of passes. From the viewpoint of productivity and economic efficiency, the processing pressure is preferably 60 MPa to 300 MPa, and the number of passes is preferably 3 to 30.

[0044] In step 1, it is preferable that at least a portion of the carboxyl groups of the water-dispersible polymer (A') are neutralized with at least an alkali metal hydroxide as a neutralizing agent. That is, at least a portion of the carboxyl groups of the structure derived from the water-dispersible polymer (A') that constitutes the crosslinked polymer (A) become alkali metal salts, and as a result, at least a portion of the carboxyl groups of the crosslinked polymer (A) become alkali metal salts. The neutralization of the water-dispersible polymer (A') having a carboxyl group is preferably carried out by dispersing the water-dispersible polymer (A') in water to obtain a polymer aqueous dispersion (D) and mixing in a neutralizing agent at that time. Furthermore, the concentration of nonvolatile components (solid content) of the polymer aqueous dispersion (D) is preferably 5% by mass or more, more preferably 10% by mass or more, and also preferably 25% by mass or less, more preferably 20% by mass or less, from the viewpoint of facilitating the production of the pigment dispersion. The solid content concentration of the polymer aqueous dispersion (D) is measured by the method described in the examples.

[0045] From the viewpoint of promoting sufficient and uniform neutralization, it is preferable to use the neutralizing agent as an aqueous solution. Furthermore, the neutralizing agent may contain compounds other than alkali metal hydroxides. Specific examples of compounds other than alkali metal hydroxides include ammonia and amines. If the compound other than alkali metal hydroxide is ammonia or an amine, it is preferable that the ammonia or amine is removed in one of the steps of the operation for producing the aqueous pigment dispersion of the present invention.

[0046] The degree of neutralization of the carboxyl groups of the water-dispersible polymer (A') is preferably 25 mol% or more, more preferably 30 mol% or more, even more preferably 35 mol% or more, and preferably 70 mol% or less, more preferably 55 mol% or less, and even more preferably 45 mol% or less, from the viewpoint of storage stability and redispersibility of the water-based ink. Here, the degree of neutralization is the value obtained by dividing the molar equivalents of the neutralizing agent by the molar equivalents of the carboxyl groups of the water-dispersible polymer (A'), i.e., the percentage of "molar equivalents of the neutralizing agent / molar equivalents of the carboxyl groups of the water-dispersible polymer (A')". Normally, the degree of neutralization does not exceed 100 mol%, but in this invention, since it is calculated from the molar equivalents of the neutralizing agent, it will exceed 100 mol% if an excess of the neutralizing agent is used. Furthermore, when using a neutralizing agent with two or more valent properties, the "molar equivalent of the neutralizing agent" shall be considered synonymous with the functional group equivalent.

[0047] [Process 2] In step 2, the pigment dispersion is filtered to remove coarse particles to obtain a pigment aqueous dispersion (e). In step 2, it is preferable that the organic solvent in the obtained pigment aqueous dispersion (e) is substantially removed, but it may remain as long as it does not impair the purpose of the present invention. The organic solvent can be removed, for example, by distillation under reduced pressure.

[0048] The pigment aqueous dispersion (e) obtained by step 2 is obtained by dispersing the pigment in an aqueous medium with a carboxyl group-containing water-dispersible polymer (A'). Here, there are no particular restrictions on the form in which the pigment and the carboxyl group-containing water-dispersible polymer (A') exist in the pigment aqueous dispersion (e), as long as particles are formed by at least the pigment and the carboxyl group-containing water-dispersible polymer (A'). For example, this includes particle forms in which the pigment is encapsulated in the carboxyl group-containing water-dispersible polymer (A'), particle forms in which the pigment is uniformly dispersed in the carboxyl group-containing water-dispersible polymer (A'), particle forms in which the pigment is exposed on the surface of the carboxyl group-containing water-dispersible polymer (A') particles, and mixtures thereof.

[0049] The concentration of nonvolatile components (solids content) of the pigment aqueous dispersion (e) obtained in step 2 is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, from the viewpoint of improving the dispersion stability of the pigment in the pigment aqueous dispersion (e) and from the viewpoint of facilitating the preparation of water-based inks. The solid content concentration of the pigment aqueous dispersion (e) is measured by the method described in the examples.

[0050] The pigment content in the pigment aqueous dispersion (e) is preferably 4% by mass or more, more preferably 8% by mass or more, and even more preferably 12% by mass or more, from the viewpoint of facilitating the preparation of water-based inks, and from the viewpoint of improving the dispersion stability of the pigment dispersion (e), it is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less.

[0051] The content of the water-dispersible polymer (A') having a carboxyl group in the pigment aqueous dispersion (e) is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, from the viewpoint of improving the dispersion stability of the pigment dispersion (e), and preferably 14% by mass or less, more preferably 10% by mass or less, and even more preferably 7% by mass or less, from the viewpoint of facilitating the preparation of water-based inks and improving the dispersion stability of the pigment dispersion (e).

[0052] In the pigment aqueous dispersion (e), the mass ratio of the pigment to the total mass of the pigment and the water-dispersible polymer (A') having a carboxyl group [pigment / pigment + water-dispersible polymer (A') having a carboxyl group] is preferably 0.3 or higher, more preferably 0.5 or higher, even more preferably 0.7 or higher, and preferably 0.9 or lower, more preferably 0.85 or lower, and even more preferably 0.8 or lower, from the viewpoint of storage stability and redispersibility of the water-based ink.

[0053] [Step 3] Step 3 is a step in which the carboxyl groups of the water-dispersible polymer (A') in the pigment aqueous dispersion (e) obtained in Step 2 are reacted with a polyfunctional epoxy compound to obtain an aqueous pigment dispersion (E) in which the pigment is dispersed in an aqueous medium by a crosslinked polymer (A). The reaction between the carboxyl groups of the water-dispersible polymer (A') and the polyfunctional epoxy compound forms a crosslinked polymer (A) containing a crosslinked structure.

[0054] The reaction between the carboxyl group of the water-dispersible polymer (A') and the polyfunctional epoxy compound may be carried out in a single step or in multiple steps. When carried out in multiple steps, the order in which the polyfunctional epoxy compounds react with the carboxyl group of the water-dispersible polymer (A') is not restricted. For example, the carboxyl group of the water-dispersible polymer (A') may be reacted with one high-water-soluble epoxy compound (S) and one or two low-water-soluble epoxy compounds (I) in a single step or in separate steps. When reacting in separate steps, from the viewpoint of storage stability and redispersibility of the water-based ink, it is preferable to react the carboxyl group of the water-dispersible polymer (A') with one low-water-soluble epoxy compound (I) first, and then further react the carboxyl group of the water-dispersible polymer (A') with one high-water-soluble epoxy compound (S). The reaction between the carboxyl group of the water-dispersible polymer (A') and the polyfunctional epoxy compound is preferably carried out in stages from the viewpoint of storage stability and redispersibility of the water-based ink, but is preferable to carry out the reaction all at once from the viewpoint of productivity.

[0055] The reaction temperature between the carboxyl group of the water-dispersible polymer (A') and the polyfunctional epoxy compound is preferably 60°C or higher, more preferably 75°C or higher, and preferably 110°C or lower, more preferably 100°C or lower, from the viewpoint of completing the reaction and economic efficiency. Similarly, the reaction time is preferably 0.5 hours or higher, more preferably 1 hour or higher, and preferably 15 hours or lower, more preferably 8 hours or lower, from the same viewpoint. When the reaction between the carboxyl group of the water-dispersible polymer (A') and the polyfunctional epoxy compound is carried out in multiple steps, the reaction temperature and reaction time for each step are the same as described above.

[0056] The concentration of nonvolatile components (solids content) of the aqueous pigment dispersion (E) obtained in step 3 is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, from the viewpoint of improving the dispersion stability of the pigment-containing crosslinked polymer particles in the aqueous pigment dispersion (E) and from the viewpoint of facilitating the preparation of the aqueous ink.

[0057] The pigment content in the aqueous pigment dispersion (E) is preferably 3% by mass or more, more preferably 7% by mass or more, and even more preferably 10% by mass or more, from the viewpoint of facilitating the preparation of aqueous inks, and from the viewpoint of improving the dispersion stability of the aqueous pigment dispersion (E), it is preferably 28% by mass or less, more preferably 22% by mass or less, and even more preferably 18% by mass or less.

[0058] The content of the crosslinked polymer (A) in the aqueous pigment dispersion (E) is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, from the viewpoint of improving the dispersion stability of the aqueous pigment dispersion (E), and preferably 18% by mass or less, more preferably 14% by mass or less, and even more preferably 9% by mass or less, from the viewpoint of facilitating the preparation of the aqueous ink and improving the dispersion stability of the aqueous pigment dispersion (E).

[0059] In the aqueous pigment dispersion (E), the mass ratio of pigment to the total mass of pigment and crosslinked polymer [pigment / pigment + crosslinked polymer (A)] is preferably 0.40 or higher, more preferably 0.50 or higher, even more preferably 0.60 or higher, and preferably 0.95 or lower, more preferably 0.85 or lower, even more preferably 0.80 or lower, and even more preferably 0.75 or lower, from the viewpoint of storage stability and redispersibility of the aqueous ink.

[0060] The average particle size of the particles in the aqueous pigment dispersion (E) is preferably 60 nm or larger, more preferably 80 nm or larger, even more preferably 100 nm or larger, and also preferably 160 nm or smaller, more preferably 140 nm or smaller, and even more preferably 125 nm or smaller, from the viewpoint of reducing coarse particles and improving the storage stability and redispersibility of the aqueous ink. The average particle size of the particles in the aqueous pigment dispersion (E) is measured by the method described in the examples.

[0061] [Water-based ink] The aqueous ink of the present invention contains the aqueous pigment dispersion of the present invention. That is, the aqueous ink of the present invention contains the components contained in the aqueous pigment dispersion of the present invention. The water-based ink of the present invention preferably contains an organic solvent, and more preferably contains a water-soluble organic solvent, from the viewpoint of improving discharge performance. A "water-soluble organic solvent" refers to an organic solvent whose dissolution amount is 10 mL or more when dissolved in 100 mL of water at 25°C. Specific examples of water-soluble organic solvents include methyl glycol, methyl diglycol, methyl triglycol, methyl polyglycol, isopropyl glycol, isopropyl diglycol, butyl glycol, butyl diglycol, butyl triglycol, isobutyl glycol, isobutyl diglycol, allyl glycol, benzyl diglycol, methylpropylene glycol, methylpropylene diglycol, methylpropylene triglycol, propylpropylene glycol, dimethyl glycol, dimethyl diglycol, dimethyl triglycol, diethyl diglycol, methyl ethyl diglycol, and the like.

[0062] The water-based ink of the present invention preferably contains a surfactant from the viewpoint of improving dispensing performance. Examples of surfactants include nonionic surfactants, anionic surfactants, and amphoteric surfactants, but nonionic surfactants are preferred, at least one selected from acetylene glycol-based surfactants and silicone-based surfactants is more preferred, and acetylene glycol-based surfactants are even more preferred. Examples of acetylene glycol-based surfactants include acetylene diols such as 2,4,7,9-tetramethyl-5-decine-4,7-diol, 3,6-dimethyl-4-octin-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, and 2,4-dimethyl-5-hexyn-3-ol, as well as ethylene oxide adducts of these acetylene diols. Examples of commercially available acetylene glycol-based surfactants include the "Surfinol" series and "Orfin" series from Nisshin Chemical Industry Co., Ltd., and the "Acetinol" series from Kawaken Fine Chemical Co., Ltd. As the silicone-based surfactant, polyether-modified silicone is preferred. Examples of commercially available silicone-based surfactants include the KF series from Shin-Etsu Chemical Co., Ltd. (KF-353, KF-355A, KF-642, KF-6011, etc.), the Silface SAG series from Nisshin Chemical Industry Co., Ltd., and the BYK series from Big Chemie Japan Co., Ltd.

[0063] The water-based ink of the present invention may further contain, if necessary, various additives commonly used in water-based inks, such as humectants, wetting agents, penetrating agents, dispersants, viscosity modifiers, defoamers, preservatives, fungicides, and rust inhibitors.

[0064] The pigment content in the aqueous ink of the present invention is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 5% by mass or more, and preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, from the viewpoint of storage stability and redispersibility of the aqueous ink.

[0065] The total content of pigment and crosslinked polymer in the aqueous ink of the present invention is preferably 2% by mass or more, more preferably 3% by mass or more, even more preferably 5% by mass or more, and preferably 28% by mass or less, more preferably 23% by mass or less, and even more preferably 18% by mass or less, from the viewpoint of storage stability and redispersibility of the aqueous ink.

[0066] In the aqueous ink of the present invention, from the viewpoint of storage stability and redispersibility of the aqueous ink, the mass ratio of the pigment content to the total content of the pigment and crosslinked polymer (A) (pigment / (pigment + crosslinked polymer (A))) is preferably 0.40 or more, more preferably 0.50 or more, even more preferably 0.60 or more, and preferably 0.95 or less, more preferably 0.85 or less, even more preferably 0.80 or less, and even more preferably 0.75 or less.

[0067] The content of the water-soluble organic solvent in the water-based ink of the present invention is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less, from the viewpoint of storage stability and redispersibility of the water-based ink. From the viewpoint of storage stability and redispersibility of the water-based ink, the surfactant content in the water-based ink of the present invention is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.3% by mass or more, and preferably 5% by mass or less, preferably 3% by mass or less, and even more preferably 2% by mass or less.

[0068] The water content in the aqueous ink of the present invention is preferably 50% by mass or more, more preferably 55% by mass or more, even more preferably 60% by mass or more, and preferably 85% by mass or less, preferably 80% by mass or less, and even more preferably 75% by mass or less, from the viewpoint of storage stability and redispersibility of the aqueous ink.

[0069] [Method for manufacturing water-based ink] The aqueous ink of the present invention can be prepared by adding an organic solvent or surfactant to the aqueous pigment dispersion of the present invention as needed, dispersing it by a known method, and filtering it as needed to remove coarse particles. In the production of the aqueous ink of the present invention, if an alkali metal hydroxide is further included in addition to the aforementioned aqueous pigment dispersion, the alkali metal hydroxide added to the aqueous pigment dispersion is added to the alkali metal hydroxide that neutralizes the carboxyl groups of the crosslinked polymer contained in the aqueous pigment dispersion.

[0070] [Inkjet recording] The water-based ink of the present invention can be loaded into a known inkjet recording device and ejected as ink droplets onto a recording medium to record characters or images. Inkjet recording devices include thermal and piezo types, but the thermal type is preferred. In other words, the water-based ink of the present invention is preferably used for thermal inkjet recording. Plain paper can be used as the recording medium. Alternatively, low-absorbent recording paper such as coated paper, or non-absorbent resin films such as polyethylene terephthalate (PET) film or polypropylene film can also be used. Furthermore, "low water absorption" and "non-water absorption" refer to the amount of water absorbed by the recording medium during a 100 m second contact time with pure water being 10 g / m³. 2 This means that the water absorption capacity is 10 g / m². 2 Ultra-absorbent recording media are highly absorbent, and ordinary paper is classified as highly absorbent. [Examples]

[0071] In the following manufacturing examples, embodiments, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" unless otherwise specified. The measurement or calculation methods for each physical property are as follows.

[0072] (1) Measurement of the weight-average molecular weight of the water-dispersible polymer (A') The results were obtained by gel permeation chromatography. The measurement conditions are shown below. GPC device: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolum Super AW-H" manufactured by Tosoh Corporation. Eluent: A solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively. Flow rate: 0.5mL / min Standard materials: Monodisperse polystyrene kits with known molecular weights "PStQuick B (F-550, F-80, F-10, F-1, A-1000)" and "PStQuick C (F-288, F-40, F-4, A-5000, A-500)" (all manufactured by Tosoh Corporation) Measurement sample: 0.1 g of water-dispersible polymer (A') was mixed with 10 mL of eluent in a glass vial, stirred with a magnetic stirrer at 25°C for 10 hours, and the solution was filtered through a syringe filter "DISMIC-13HP" (material: polytetrafluoroethylene (PTFE), pore size: 0.2 μm, manufactured by ADVANTEC).

[0073] (2) Measurement of the acid value of the water-dispersible polymer (A') The measurement was performed according to the neutralization titration method described in JIS K0070-1992, except that the measurement solvent was changed from a mixed solvent of ethanol and ether to a mixed solvent of acetone and toluene [acetone:toluene = 1:1 (volume ratio)].

[0074] (3) Water solubility of polyfunctional epoxy compounds 90 parts by mass of deionized water and 10 parts by mass of a polyfunctional epoxy compound were added to a glass tube (25 mmφ × 250 mmh) at 25°C, and the glass tube was left to stand for 1 hour in a constant temperature bath adjusted to 25°C. Next, the glass tube was shaken at 1500 rpm for 1 minute using a small vibrator (AS ONE Corporation, product name: Test Tube Mixer TRIO (High Type) HM-1N), and then left to stand again in the constant temperature bath for 10 minutes. Then, the undissolved material was collected, dried, and weighed, and the water solubility (mass%) was calculated using the following formula. Water solubility (mass %) = 100 × (10 - parts by mass of undissolved material) / 10

[0075] (4) Measurement of solid content concentration of polymer aqueous dispersion (D), pigment aqueous dispersion (e), and aqueous pigment dispersion (E) 10.0 g of sodium sulfate, which had been stabilized in a desiccator, was weighed into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm). Approximately 1.0 g of the sample was added and mixed, and then accurately weighed. This mixture was maintained at 105°C for 2 hours to remove volatile components, and then left in the desiccator for another 15 minutes. The mass was then measured. The mass of the sample after removal of volatile components was taken as the solid content, and the solid content concentration was obtained by dividing it by the mass of the added sample.

[0076] (5) Measurement of the average particle size of pigment-containing crosslinked polymer particles in aqueous pigment dispersion (E) and aqueous ink. 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 of 90° between the incident light and the detector, and 100 cumulative measurements. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The measured concentration was 5 × 10⁻¹⁶. -3 The measurement was performed in mass percent (converted to solid content concentration), and the resulting cumulant average particle size was taken as the average particle size of the pigment-containing crosslinked polymer particles.

[0077] [Preparation of water-dispersible polymer (A')] Preparation Example 1 (Preparation of water-dispersible polymer (A'1)) A monomer mixture was prepared by mixing 31 parts acrylic acid and 69 parts styrene. 5 parts methyl ethyl ketone (hereinafter referred to as "MEK"), 0.25 parts 3-mercaptopropionic acid as a polymerization chain transfer agent, and 10% by mass of the monomer mixture were added to the reaction vessel and mixed, followed by thorough nitrogen gas purging. Meanwhile, a mixture of the remaining 90% by mass of the monomer mixture, 2.0 parts of polymerization chain transfer agent, 75 parts of MEK, and 1.5 parts of 4,4'-azobis(4-cyanovaleric acid) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-501") as an azo radical polymerization initiator was placed in a dropping funnel. Under a nitrogen atmosphere, the monomer mixture in the reaction vessel was heated to 77°C while stirring, and the mixture in the dropping funnel was added dropwise over 5 hours. After the dropwise addition was complete, a solution of 0.5 parts of polymerization initiator dissolved in 5 parts of MEK was added, and the reaction was continued at 77°C for another 2 hours to obtain a solution of the water-dispersible polymer (A'1). The acid value of the water-dispersible polymer (A'1) was 240 mgKOH / g, and the weight-average molecular weight was 16,900.

[0078] [Production of polymer aqueous dispersion (D)] Manufacturing Example 1 (Manufacturing of Polymer Aqueous Dispersion (D1)) A solution of the water-dispersible polymer (A'1) was dried under reduced pressure to obtain 80 parts of the water-dispersible polymer (A'1) having carboxyl groups. 421 parts of deionized water and 32.4 parts of a 5N sodium hydroxide aqueous solution (16.9% sodium hydroxide solids) as an alkali metal compound (neutralizing agent) were added, and the mixture was neutralized to a ratio of 40 mol% (degree of neutralization 40 mol%) of moles of sodium hydroxide to the number of moles of carboxyl groups in the water-dispersible polymer (A'1). The resulting mixture was heated at 90°C for 5 hours while stirring at 150 rpm, and then deionized water was added to obtain a polymer aqueous dispersion (D1) (solids concentration 15%).

[0079] [Production of aqueous pigment dispersion (E)] Example 1 (Production of aqueous pigment dispersion (E1) and aqueous ink 1) [Manufacturing of aqueous pigment dispersion (E1)] (Process 1) 190 parts of polymer aqueous dispersion (D1) (solid content concentration 15%) were mixed with 45 parts of MEK and 268 parts of deionized water. Then, 90 parts of magenta pigment (PR150: manufactured by Fuji Pigment Co., Ltd., trade name FUJI FAST CARMINE 522-1D) were added to obtain a pigment mixture. The pigment mixture was mixed using a disperser blade at 7000 rpm and 20°C for 1 hour. Subsequently, dispersion treatment was performed using an Ultra Apex Mill (Hiroshima Metal & Machinery Co., Ltd., product name: UAM-05) filled with 0.05 mm diameter zirconia beads (Nikkatoh Co., Ltd., YTZ balls) at a packing rate of 85% under conditions of a stirring blade peripheral speed of 10 m / s and a circulation flow rate of 200 mL / min, using a 10-pass operation method. After that, further dispersion treatment was performed using a microfluidizer (Microfluidics, high-pressure homogenizer "M-140K") at a pressure of 180 MPa for 10 passes to obtain the pigment dispersion.

[0080] (Process 2) MEK was removed from the pigment dispersion under reduced pressure at 60°C, and some of the water was further removed. After centrifugation, the liquid phase was filtered through a membrane filter (Sartorius "Minisart Syringe Filter", pore size: 5 μm, material: cellulose acetate) to remove coarse particles, and a pigment aqueous dispersion (e-1) (solid content concentration 20%) was obtained, in which pigment-containing polymer particles were dispersed in an aqueous medium.

[0081] (Step 3) 100 parts of a pigment aqueous dispersion (e-1) (solid content concentration 20%) were placed in a screw-top glass bottle, and 2.32 parts (degree of crosslinking: 80 mol%) of glycerol polyglycidyl ether (Denacol EX-313, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 141 g / eq, water solubility 99%, (hereinafter, "EX-313" and "Denacol" may be omitted. The same applies to other "Denacol" series products)) as a high water soluble epoxy compound (S), 0.29 parts (degree of crosslinking: 10 mol%) of trimethylolpropane polyglycidyl ether (Denacol EX-321, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 140 g / eq, water solubility 27%) as a low water soluble epoxy compound (I), and 10.4 parts of deionized water were added. The bottle was then tightly sealed and heated at 90°C for 5 hours while stirring with a stirrer. The mixture was then cooled to room temperature and filtered through a membrane filter (Sartorius "Minisart Syringe Filter", pore size: 5 μm, material: cellulose acetate) to remove coarse particles, obtaining an aqueous pigment dispersion (E1) (solid content concentration 20%) in which pigment-containing crosslinked polymer particles were dispersed in an aqueous medium. The average particle size of the aqueous pigment dispersion (E1) is shown in Table 1. The degree of crosslinking is a calculated value assuming that all epoxy groups of the polyfunctional epoxy compound reacted with the carboxyl groups of the uncrosslinked water-dispersible polymer (A'1). It was determined from the mass and epoxy equivalent of the polyfunctional epoxy compound used, and the mass and acid value of the uncrosslinked water-dispersible polymer (A'1). The same method was used for determining the degree of crosslinking below.

[0082] [Manufacturing of water-based ink 1] Using the aqueous pigment dispersion (E1), an aqueous ink was produced with a total content of each component at 100 parts, as shown below. Aqueous pigment dispersion (E1): Amount such that the pigment (magenta pigment) content in the aqueous ink is 10 parts. Water-soluble organic solvent (butyl diglycol): 20 parts Nonionic surfactant (manufactured by Kawaken Fine Chemicals Co., Ltd., acetylenol E100, ethylene oxide 10mol adduct of 2,4,7,9-tetramethyl-5-decine-4,7-diol): 0.5 parts Deionized water: Remaining amount Next, the mixture was filtered using a 25 mL needleless syringe fitted with a 1.2 μm pore size filter (Sartorius "Minisart Syringe Filter", material: cellulose acetate) to obtain aqueous ink 1.

[0083] Examples 2, 4-7, 9-12, Comparative Examples 1, 2 (Production of aqueous pigment dispersions (E2), (E4)-(E7), (E9)-(E12), (EC1), (EC2), aqueous inks 2, 4-7, 9-12, C1, C2) Using the aqueous pigment dispersions listed in Table 1, aqueous pigment dispersions (E2), (E4) to (E7), (E9) to (E12), (EC1), (EC2), aqueous inks 2, 4 to 7, 9 to 12, C1, and C2 were manufactured in the same manner as in Example 1.

[0084] Example 3 (Production of aqueous pigment dispersion (E3) and aqueous ink 3) Aqueous pigment dispersion (E3) and aqueous ink 3 were manufactured in the same manner as in Example 1, except that step 3 was performed as described below. (Step 3) 100 parts of the pigment aqueous dispersion (e-1) (solids concentration 20%) were placed in a screw-top glass bottle, 0.57 parts of EX-321 (degree of crosslinking: 20 mol%) as a low water-soluble epoxy compound (I) and 10.4 parts of deionized water were added, the bottle was sealed tightly, and heated at 90°C for 5 hours while stirring with a stirrer. After that, the temperature was lowered to room temperature, 2.03 parts of EX-313 (degree of crosslinking: 70 mol%) as a high water-soluble epoxy compound (S) were added, the bottle was sealed tightly, and heated at 90°C for 5 hours while stirring with a stirrer. The mixture was filtered through a membrane filter (Sartorius "Minisart Syringe Filter", pore size: 5 μm, material: cellulose acetate) to remove coarse particles, and an aqueous pigment dispersion (E3) (solids concentration 20%) was obtained, in which pigment-containing crosslinked polymer particles were dispersed in an aqueous medium.

[0085] Example 8 (Production of aqueous pigment dispersion (E8) and aqueous ink 8) Aqueous pigment dispersion (E8) and aqueous ink 8 were manufactured in the same manner as in Example 1, except that step 3 was performed as described below. (Step 3) Transfer 100 parts of the pigment aqueous dispersion (e-1) (solid content concentration 20%) to a screw-top glass bottle, add 2.03 parts of EX-313 as the high water-soluble epoxy compound (S) (degree of crosslinking: 70 mol%), 0.29 parts of EX-321 as the low water-soluble epoxy compound (I) (degree of crosslinking: 10 mol%), and 1,6-hexanediol diglycidyl ether (Denacol EX-212L manufactured by Nagase ChemteX Corporation, epoxy equivalent: 135) as the low water-soluble epoxy compound (I). 0.28 parts (10 mol%) of g / eq, 0% water solubility, and 10.4 parts of deionized water were added, the mixture was sealed tightly, and heated at 90°C for 5 hours while stirring with a stirrer. After that, it was cooled to room temperature and filtered through a membrane filter (Sartorius "Minisart Syringe Filter", pore size: 5 μm, material: cellulose acetate) to remove coarse particles, obtaining an aqueous pigment dispersion (E8) (solids content 20%) in which pigment-containing crosslinked polymer particles were dispersed in an aqueous medium.

[0086] Comparative Example 3 (Production of aqueous pigment dispersion (EC3) and aqueous ink C3) Aqueous pigment dispersion (EC3) and aqueous ink C3 were manufactured in the same manner as in Example 1, except that step 3 was performed as described below. (Step 3) 100 parts of the pigment aqueous dispersion (e-1) (solid content concentration 20%) were placed in a screw-top glass bottle, 1.01 parts of EX-321 (degree of crosslinking: 35 mol%) as a low water-soluble epoxy compound (I) and 10.0 parts of deionized water were added, the bottle was tightly sealed, and heated at 90°C for 5 hours while stirring with a stirrer. Subsequently, the mixture was cooled to room temperature, and 1.53 parts (degree of crosslinking: 35 mol%) of hydrogenated bisphenol A diglycidyl ether (Denacol EX-252, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 213 g / eq, water solubility: 0%) was added as a low water-soluble epoxy compound (I). The mixture was then sealed tightly and heated at 90°C for 5 hours while stirring with a stirrer. The mixture was filtered through a membrane filter (Minisart syringe filter, manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, and an aqueous pigment dispersion (EC3) (solid content concentration 20%) was obtained, in which pigment-containing crosslinked polymer particles were dispersed in an aqueous medium.

[0087] [evaluation] Test 1 (Storage Stability Evaluation) Teng of each water-based ink was sealed in a screw-cap tube (No. 5, 20mL capacity) manufactured by Maruemu Co., Ltd., and left to stand for one week in a constant temperature room set at 70°C. The average particle size was then measured, and the average particle size increase rate was calculated using the following formula. A smaller average particle size increase rate indicates suppressed aggregation and superior storage stability. Average particle size increase rate (%) = [(Average particle size of pigment-containing cross-linked polymer particles in the water-based ink after storage - Average particle size of pigment-containing cross-linked polymer particles in the water-based pigment dispersion (E) used in the production of the water-based ink) / Average particle size of pigment-containing cross-linked polymer particles in the water-based pigment dispersion (E) used in the production of the water-based ink] × 100

[0088] Test 2 (Evaluation of redistribution) The total volume of each water-based ink was set to 100 parts, and 0.1 parts were added to a screw-cap tube (No. 5, 20 mL capacity) manufactured by Maruemu Co., Ltd., while measuring the weighed volume (W1). The tube was left unsealed and dried for 24 hours at 40°C and 25% Rh. Then, 10 parts of deionized water were added, and the mixture was shaken vigorously up and down 10 times. After standing for 1 day, the supernatant liquid was weighed and diluted appropriately (dilution ratio D1) so that the absorbance at a wavelength of 569 nm was approximately 0.8. The absorbance was then measured, and the absorbance value at a wavelength of 569 nm (Ab1) was determined. Furthermore, the absorbance without the redispersion test was determined as follows. 0.1 parts of each water-based ink were weighed (W2) and added to a screw tube (No. 5) manufactured by Maruemu Co., Ltd., and 10 parts of deionized water were added and stirred. After that, the ink was appropriately diluted (dilution ratio D2) so that the absorbance at a wavelength of 569 nm was approximately 0.8, and the absorbance was measured to determine the absorbance value (Ab2) at a wavelength of 569 nm. The redistribution rate was measured using the following formula. Redispersion rate (%)=100×(Ab1×D1 / W1) / (Ab2×D2 / W2) Measurement conditions for absorption spectra and absorbance Equipment: Spectrophotometer (Hitachi Ltd. "U-3900") Measurement environment: Room temperature and pressure Measurement wavelength: 300nm~800nm Measured concentration: The concentration at which the absorbance at 590 nm is approximately 0.8.

[0089] [Table 1]

[0090] The details of the functional epoxy compounds are shown below. All of the commercially available products listed are manufactured by Nagase ChemteX Corporation. [High water solubility epoxy compound (S)] Denacol EX-313: Glycerol polyglycidyl ether, epoxy equivalent 141 g / eq, water solubility: 99% Denacol EX-850: Polyethylene glycol diglycidyl ether, epoxy equivalent 122 g / eq, water solubility: 100% Denacol EX-214L: 1,4-butanediol diglycidyl ether, epoxy equivalent 115g / eq, water solubility: 100% [Low water solubility epoxy compounds (I)] Denacol EX-321: Trimethylolpropane polyglycidyl ether, epoxy equivalent 140 g / eq, water solubility: 27% Denacol EX-252: Hydrogenated bisphenol A diglycidyl ether, epoxy equivalent 213 g / eq, water solubility 0% Denacol EX-212L: 1,6-Hexanediol diglycidyl ether, epoxy equivalent 135 g / eq, water solubility: 0% Denacol EX-216L: Cyclohexanedimethanol diglycidyl ether, epoxy equivalent 150 g / eq, water solubility: 0% Denacol EX-622: Sorbitol polyglycidyl ether, epoxy equivalent 191 g / eq, water solubility: 0%

[0091] Table 4 shows that the water-based inks of the present invention exhibit excellent storage stability and redispersibility (Examples 1-12). In contrast, the water-based ink produced without using the low-water-soluble epoxy compound (I) (Comparative Example 1) has poor storage stability. Furthermore, the water-based inks produced without using the high-water-soluble epoxy compound (S) (Comparative Examples 2 and 3) have poor redispersibility.

Claims

1. An aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium with a crosslinked polymer (A), The crosslinked polymer (A) comprises a structure derived from a water-dispersible polymer (A') having a carboxyl group and a structure derived from a polyfunctional epoxy compound. The polyfunctional epoxy compound comprises a highly water-soluble epoxy compound (S) and a low water-soluble epoxy compound (I). When 10 parts by mass of a highly water-soluble epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 70% by mass or more and 100% by mass or less. An aqueous pigment dispersion wherein the water solubility of 10 parts by mass of a low water-soluble epoxy compound (I) is added to 90 parts by mass of deionized water at 25°C, and the water solubility is 0% by mass or more and 30% by mass or less.

2. The aqueous pigment dispersion according to claim 1, wherein the acid value of the aqueous dispersible polymer (A') having a carboxyl group is 100 mg KOH / g or more and 300 mg KOH / g or less.

3. The aqueous pigment dispersion according to claim 1 or 2, wherein the degree of crosslinking of the crosslinked polymer (A) is 35 mol% or more and 95 mol% or less, based on the ratio of the number of mol equivalents of epoxy groups of the polyfunctional epoxy compound to the number of mol equivalents of carboxyl groups of the aqueous dispersible polymer (A') having carboxyl groups.

4. The aqueous pigment dispersion according to claim 1 or 2, wherein the ratio of the mass of the pigment to the total mass of the pigment and the crosslinked polymer (A) (pigment / (pigment + crosslinked polymer (A))) is 0.60 or more and 0.80 or less.

5. The aqueous pigment dispersion according to claim 1 or 2, wherein the degree of crosslinking (SR) of the crosslinked polymer (A) with a high water-soluble epoxy compound (S) and the degree of crosslinking (IR) of the crosslinked polymer (A) with a low water-soluble epoxy compound (I) satisfy the following formula. 0.45≦SR / (SR+IR)<1.0

6. The aqueous pigment dispersion according to claim 1 or 2, wherein the low water solubility epoxy compound (I) has 2 to 4 epoxy groups.

7. The aqueous pigment dispersion according to claim 1 or 2, wherein the low water solubility epoxy compound (I) is at least one selected from sorbitol polyglycidyl ether, trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether.

8. A water-based ink containing the aqueous pigment dispersion described in claim 1 or 2, and a water-soluble organic solvent.

9. A method for producing an aqueous pigment dispersion in which a pigment is dispersed in an aqueous medium with a crosslinked polymer (A), comprising the following step I: Step I: A step to obtain an aqueous pigment dispersion containing a pigment, the crosslinked polymer (A), and an aqueous medium by reacting the carboxyl groups of a water-dispersible polymer (A') having carboxyl groups with a polyfunctional epoxy compound to form a crosslinked polymer (A). The polyfunctional epoxy compound comprises a highly water-soluble epoxy compound (S) and a low water-soluble epoxy compound (I). When 10 parts by mass of a highly water-soluble epoxy compound (S) is added to 90 parts by mass of deionized water at 25°C, the water solubility is 70% by mass or more and 100% by mass or less. When 10 parts by mass of a low water-soluble epoxy compound (I) is added to 90 parts by mass of deionized water, the water solubility is 0% by mass or more and 30% by mass or less. A method for producing an aqueous pigment dispersion.

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