Water-based inks for inkjet recording

Abstract

The present invention relates to a water-based ink for inkjet recording, which contains a pigment, water-insoluble cross-linked polymer particles, an organic solvent and water. The water-insoluble cross-linked polymer particles are composed of a polymer A having a carboxyl group and an acid value of 45 to 180 mgKOH / g, and contain 15 to 65 mol% of alkali metal ions and 55 to 145 mol% of ammonium ions relative to the total molar amount of the carboxyl groups of the polymer A present in the ink, and the total content of the alkali metal ions and the ammonium ions is 100 to 185 mol% relative to the total molar amount of the carboxyl groups of the polymer A.

Description

Technical Field

[0001] The present invention relates to a water-based ink for inkjet recording. Background Art

[0002] Inkjet recording is a method of recording characters, images, etc. by ejecting ink droplets from very fine nozzles directly onto the recording medium and making them adhere to the recording medium. This method has many advantages, such as easy full-colorization and low price, the ability to use ordinary paper as the recording medium, and no contact with the printed matter, so it has become widely popular.

[0003] In recent years, with the popularization of digital printing, inkjet recording is not limited to civilian printing, but has also been developed in commercial printing using low-absorbent coated paper and non-absorbent resin film, as well as in industrial printing.

[0004] In response to this, inks are required to have high storage stability and high water resistance to recording media with low liquid absorption or non-liquid absorption, and various technical proposals have been proposed.

[0005] For example, Japanese Patent Gazette No. 2018-28080 (Patent Document 1) discloses an aqueous ink for inkjet recording, which is an aqueous ink having excellent water resistance of recorded products and excellent storage stability of the ink while maintaining redispersibility, wherein the aqueous ink contains a pigment aqueous dispersion and a water-soluble organic solvent, the above-mentioned pigment aqueous dispersion contains water-insoluble cross-linked polymer particles containing a pigment, a water-soluble alkali compound and water, the water-insoluble cross-linked polymer constituting the particles has a carboxyl group and a cross-linked structure containing an ester group from a water-insoluble cross-linking agent, the saponification value of the water-insoluble cross-linked polymer is 170 to 300 mgKOH / g, and the water-soluble alkali compound contains a water-soluble amine compound.

[0006] In Japanese Patent Gazette No. 2019-14883 (Patent Document 2), a water-based ink is disclosed, which is a water-based ink that has excellent ejection stability and image fastness while maintaining printing concentration, and comprises water-insoluble cross-linked polymer particles containing pigments, a polymer emulsion and water, wherein the water-insoluble cross-linked polymer constituting the cross-linked polymer particles is a polymer obtained by cross-linking a polymer having a structural unit derived from a carboxylic acid monomer having an acid value of 200 mgKOH / g or more and a structural unit derived from a hydrophobic monomer using an epoxy compound, and the polymer constituting the polymer emulsion contains a structural unit derived from a carboxylic acid monomer and a structural unit derived from a hydrophobic monomer. Summary of the invention

[0007] The present invention relates to a water-based ink for inkjet recording, which contains a pigment, water-insoluble cross-linked polymer particles, an organic solvent and water.

[0008] The water-insoluble cross-linked polymer particles are composed of a polymer A having a carboxyl group, an acid value of 45 mgKOH / g or more and 180 mgKOH / g or less, and a cross-linked structure.

[0009] The ink contains 15 mol% to 65 mol% of alkali metal ions and 55 mol% to 145 mol% of ammonium ions relative to the total molar amount of carboxyl groups of the polymer A present in the ink.

[0010] The total content of the alkali metal ions and the ammonium ions is 100 mol% or more and 185 mol% or less relative to the total molar amount of the carboxyl groups of the polymer A. DETAILED DESCRIPTION

[0011] There is a high demand for improved productivity of aqueous inks for inkjet printing. However, there is a problem that inkjet printing cannot be ejected normally when restarting the printing after pausing the inkjet printing. Therefore, there is a demand for improved ejection stability during the next printing.

[0012] Although the aqueous inks described in Patent Documents 1 and 2 have achieved some degree of improvement, further improvements in storage stability, ejection stability, and water resistance of recorded products are required.

[0013] The present invention relates to an aqueous ink for inkjet printing which is excellent in storage stability and ejection stability and can produce a recorded product having excellent water resistance.

[0014] The present inventors have found that the above-mentioned technical problems can be solved by adjusting the content of sodium ions, ammonium ions, etc. relative to the carboxyl groups of the polymer in an aqueous ink containing a pigment, water-insoluble cross-linked polymer particles and an organic solvent.

[0015] That is, the present invention relates to an aqueous ink for inkjet printing, which contains a pigment, water-insoluble cross-linked polymer particles, an organic solvent and water.

[0016] The water-insoluble cross-linked polymer particles are composed of a polymer A having a carboxyl group, an acid value of 45 mgKOH / g or more and 180 mgKOH / g or less, and a cross-linked structure.

[0017] The ink contains 15 mol% to 65 mol% of alkali metal ions and 55 mol% to 145 mol% of ammonium ions relative to the total molar amount of carboxyl groups of the polymer A present in the ink.

[0018] The total content of the alkali metal ions and the ammonium ions is 100 mol% or more and 185 mol% or less relative to the total molar amount of the carboxyl groups of the polymer A.

[0019] According to the present invention, it is possible to provide an aqueous ink for inkjet printing which has excellent storage stability and ejection stability and can produce a recorded product having excellent water resistance.

[0020] [Water-based ink for inkjet printing]

[0021] The water-based ink for inkjet printing of the present invention contains a pigment, water-insoluble cross-linked polymer particles, an organic solvent and water (hereinafter also referred to as "the ink of the present invention"). The water-insoluble cross-linked polymer particles are composed of a polymer A, the polymer A has a carboxyl group, an acid value of 45 mgKOH / g or more and 180 mgKOH / g or less, and has a cross-linked structure, contains 15 mol% or more and 65 mol% or less of alkali metal ions, and contains 55 mol% or more and 145 mol% or less of ammonium ions relative to the total molar amount of the carboxyl groups of the polymer A present in the ink, and the total content of the alkali metal ions and the ammonium ions is 100 mol% or more and 185 mol% or less relative to the total molar amount of the carboxyl groups of the polymer A.

[0022] It should be noted that in this specification, "record" is a concept including printing and printing of recorded characters, images, etc., and "record product" is a concept including printed products and printed products on which characters, images, etc. are recorded.

[0023] "Aqueous" means that water accounts for the largest proportion of the media contained in the ink.

[0024] In addition, “low water absorption” is a concept that includes low liquid absorption and non-liquid absorption, which means that within 100 milliseconds of contact time between the recording medium and pure water, the water absorption of the recording medium is 0 g / m 2 Above and 10g / m 2 the following.

[0025] The aqueous ink for inkjet printing of the present invention is excellent in storage stability and ejection stability, and can provide a recorded product with excellent water resistance. Although the reason for this is not yet determined, the present inventors believe that the reason is as follows.

[0026] When the aqueous ink for inkjet recording is exposed to strong drying conditions near the jet nozzle, agglomerates are generated near the gas-liquid interface, thereby causing the problem of nozzle clogging. In the present invention, by making the water-insoluble polymer particles contained in the ink have a cross-linked structure and making the acid value of the polymer constituting the water-insoluble cross-linked polymer particles 45 mgKOH / g or more, even if exposed to strong drying conditions near the jet nozzle, high electrostatic repulsion between the polymer particles can be ensured, and the generation of agglomerates can be suppressed, thereby improving storage stability and jetting stability.

[0027] Furthermore, by ensuring that there is 15 mol % or more and 65 mol % or less of alkali metal ions in the system relative to the total amount of carboxyl groups in the polymer constituting the water-insoluble cross-linked polymer particles, even if agglomerates are generated, the ink can be restored to its original dispersed state when the ink is filled with ink components again, specifically, when the nozzle is purged, etc., due to the presence of alkali metal ions having a high affinity for water, thereby achieving excellent ejection stability.

[0028] Furthermore, by setting the acid value of the polymer constituting the water-insoluble cross-linked polymer particles to 180 mgKOH / g or less, the water resistance of the ink coating of the image formed on the low water-absorbent recording medium is improved. Furthermore, by setting the amount of alkali metal ions to the above-mentioned amount relative to the total amount of carboxyl groups present in the system, and further containing 55 mol% or more and 145 mol% or less of ammonium ions relative to the total amount of carboxyl groups present in the water-insoluble cross-linked polymer particles in the system, the time from image formation to the expression of water resistance can be shortened.

[0029] Moreover, by adjusting the total content of alkali metal ions and ammonium ions to 100 mol % or more and 185 mol % or less relative to the total amount of carboxyl groups of the water-insoluble cross-linked polymer particles present in the system, alkali (ammonium ions) that do not form neutralized salts are present in the ink, thereby promoting redispersion of agglomerates and improving ejection stability.

[0030] <Pigment>

[0031] The pigment used in the present invention may be any of an inorganic pigment and an organic pigment, and a lake pigment or a fluorescent pigment may also be used. In addition, these pigments may be used in combination with an extender pigment as required.

[0032] Specific examples of inorganic pigments include metal oxides such as carbon black, titanium oxide, iron oxide, red iron oxide, and chromium oxide, and pearlescent pigments. In particular, carbon black is preferred in black inks. Examples of carbon black include furnace black, pyrolytic lamp black, acetylene black, and channel black.

[0033] Specific examples of organic pigments include: azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelated azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, pyrene pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, dione pyrrolopyrrole pigments, benzimidazolone pigments, and Shihlin pigments.

[0034] There is no particular limitation on the color tone, and any of non-color pigments such as white, black, and gray, and color organic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used.

[0035] Specific examples of preferred organic pigments include products of one or more types selected from CI pigment yellow, CI pigment red, CI pigment orange, CI pigment violet, CI pigment blue, and CI pigment green.

[0036] Examples of the extender pigment include silicon dioxide, calcium carbonate, and talc.

[0037] The above pigments may be used alone or in combination of two or more.

[0038] The pigment is preferably dispersed in the aqueous ink via a water-insoluble cross-linked polymer having a cross-linked structure, wherein the water-insoluble cross-linked polymer is composed of a structure derived from a water-insoluble polymer having a carboxyl group and a structure derived from a cross-linking agent capable of reacting with the carboxyl group of the water-insoluble polymer.

[0039] As the dispersion form of the pigment, from the viewpoint of improving storage stability and ejection stability, the form of water-insoluble cross-linked polymer particles containing the pigment is preferred. In the present specification, water-insoluble cross-linked polymer particles containing the pigment (hereinafter also referred to as "cross-linked polymer particles containing the pigment") refer to particles in which a water-insoluble polymer having a cross-linked structure contains a pigment, particles in which a part of the pigment is exposed on the surface of the particles composed of the water-insoluble cross-linked polymer and the pigment, and particles in which the water-insoluble cross-linked polymer is adsorbed on a part of the pigment, or a mixture of these particles.

[0040] (Water-insoluble polymer having a carboxyl group)

[0041] The water-insoluble polymer having a carboxyl group (hereinafter also referred to as "polymer a") constituting the cross-linked polymer particles containing the pigment is a polymer having a pigment dispersing ability to disperse the pigment in an aqueous medium, and has a carboxyl group. Polymer a may be water-soluble or water-insoluble, and is preferably a water-insoluble polymer. Even if polymer a is a water-soluble polymer, it will become a water-insoluble polymer by cross-linking.

[0042] In the present invention, "water-insoluble" means that when a polymer dried at 105°C for 2 hours to a constant weight is dissolved in 100g of water at 25°C until saturation, the amount of the polymer dissolved is 10g or less. When the water-insoluble polymer is an anionic polymer, the amount of the polymer dissolved is the amount of the polymer dissolved when the anionic groups of the polymer are 100% neutralized with sodium hydroxide.

[0043] From the viewpoint of improving the storage stability and ejection stability of the ink of the present invention, the polymer a is preferably a vinyl polymer obtained by addition polymerization of a vinyl monomer.

[0044] The vinyl polymer contains a structural unit derived from a monomer having a carboxyl group (a-1), and is more preferably a vinyl polymer obtained by copolymerizing a monomer mixture A containing the component (a-1) and the hydrophobic monomer (a-2). The vinyl polymer contains a structural unit derived from the component (a-1) and a structural unit derived from the component (a-2).

[0045] The (a-1) monomer having a carboxyl group is preferably a carboxylic acid monomer, and more preferably at least one selected from acrylic acid and methacrylic acid.

[0046] Specific examples of the hydrophobic monomer (a-2) include compounds described in paragraphs

[0020] to

[0022] of JP-A-2018-83938. Among them, preferably, at least one selected from alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth)acrylate.

[0047] The polymer (a) may further contain (a-3) a nonionic monomer.

[0048] Specific examples of the component (a-3) include compounds described in paragraph

[0018] of Japanese Patent Application Publication No. 2018-83983. Among them, preferably one or more selected from methoxy polyethylene glycol (n=1 to 30) (meth)acrylate and polypropylene glycol (n=2 to 30) (meth)acrylate.

[0049] Based on the above viewpoints, polymer a is preferably a (meth)acryloyl-styrene resin, which has structural units derived from one or more selected from acrylic acid and methacrylic acid as the (a-1) component, and has structural units derived from one or more selected from (meth)acrylic acid alkyl esters, styrene, α-methylstyrene and benzyl (meth)acrylate (preferably one or more selected from styrene and α-methylstyrene) as the (a-2) component.

[0050] (Content of each component in monomer mixture A or each structural unit in polymer a)

[0051] From the viewpoint of improving the dispersion stability of the pigment and the storage stability and ejection stability of the ink of the present invention, the content of each component in the monomer mixture A when manufacturing the polymer a (the content calculated as the unneutralized amount, the same below) or the content of each structural unit in the polymer a is as described below.

[0052] The content of the component (a-1) is preferably 2% by mass or more, more preferably 5% by mass or more, further preferably 10% by mass or more, and is preferably 55% by mass or less, more preferably 50% by mass or less, further preferably 45% by mass or less.

[0053] The content of the component (a-2) is preferably 98% by mass or less, more preferably 95% by mass or less, further preferably 90% by mass or less, and is preferably 45% by mass or more, more preferably 50% by mass or more, further preferably 55% by mass or more.

[0054] When the component (a-3) is contained, the content thereof is preferably 5% by mass or more, more preferably 10% by mass or more, and is preferably 30% by mass or less, more preferably 25% by mass or less.

[0055] The mass ratio of the component (a-1) to the component (a-2) [(a-1) / (a-2)] is preferably 0.1 or more, more preferably 0.2 or more, further preferably 0.25 or more, and is preferably 1.2 or less, more preferably 1.0 or less, further preferably 0.8 or less.

[0056] In the present invention, the content of structural units derived from components (a-1) to (a-3) in polymer a can be determined by measurement, or the charging ratio of the raw material monomers containing components (a-1) to (a-3) when manufacturing polymer a can be used as the content of each component.

[0057] (Production of polymer a)

[0058] The polymer a can be produced by copolymerizing the monomer mixture A by a known polymerization method. As the polymerization method, a solution polymerization method is preferred.

[0059] The solvent used in the solution polymerization method is not limited, but preferably used is a polar solvent such as water, a lower aliphatic alcohol, a ketone such as methyl ethyl ketone, an ether, an ester, or the like.

[0060] During the polymerization, polymerization initiators such as azo compounds and persulfates and polymerization chain transfer agents such as thiols can be used. The polymerization temperature varies depending on the types of polymerization initiators, monomers, and solvents used, but is preferably 30° C. or higher, more preferably 50° C. or higher, and preferably 95° C. or lower, more preferably 80° C. or lower.

[0061] As the polymer a, a commercially available product may also be used. Examples of such commercially available products include BASF Japan Ltd. 690 and other styrene-acrylic resins, etc.

[0062] From the viewpoint of improving the dispersion stability of the pigment in the ink, the number average molecular weight of the polymer a is preferably 3,000 or more, more preferably 5,000 or more, further preferably 8,000 or more, and is preferably 150,000 or less, more preferably 100,000 or less, further preferably 50,000 or less.

[0063] Based on the same viewpoints as above, the acid value of polymer a is preferably 50 mgKOH / g or more, more preferably 80 mgKOH / g or more, and further preferably 100 mgKOH / g or more, and is preferably 500 mgKOH / g or less, more preferably 400 mgKOH / g or less, and further preferably 300 mgKOH / g or less.

[0064] The number average molecular weight and acid value of the polymer can be measured by the method described in Examples.

[0065] <Production of Pigment-Containing Crosslinked Polymer Particles>

[0066] From the viewpoint of efficient production, the water-insoluble crosslinked polymer particles containing a pigment (pigment-containing crosslinked polymer particles) are preferably produced as a pigment water dispersion by a method having the following steps I to III.

[0067] Step I: a step of dispersing a mixture containing polymer a, an organic solvent, a pigment and water (hereinafter also referred to as a "pigment mixture") to obtain a pigment dispersion of pigment-containing polymer particles (hereinafter also referred to as "pigment-containing polymer particles").

[0068] Step II: a step of removing the organic solvent from the pigment dispersion obtained in step I to obtain an aqueous dispersion of polymer particles containing a pigment (hereinafter also referred to as “pigment aqueous dispersion”).

[0069] Step III: a step of cross-linking the pigment aqueous dispersion obtained in step II with a cross-linking agent to obtain a pigment aqueous dispersion comprising "'pigment-containing cross-linked polymer particles' composed of a polymer A having a cross-linked structure and a pigment".

[0070] (Process I)

[0071] In step I, preferably, the polymer a is first dissolved in an organic solvent, and then a pigment, water, a neutralizing agent, and a surfactant added as needed are added to the obtained polymer solution and mixed to obtain an oil-in-water dispersion.

[0072] There is no limitation on the organic solvent for dissolving polymer a, and preferably aliphatic alcohols, ketones, ethers, esters, etc. having 1 to 3 carbon atoms. From the viewpoint of improving wettability to the pigment, solubility of polymer a, and adsorption of polymer a to the pigment, ketones having 4 to 8 carbon atoms are more preferred, methyl ethyl ketone and methyl isobutyl ketone are further preferred, and methyl ethyl ketone is further preferred. When the solution polymerization method is used to synthesize polymer a, the solvent used in the polymerization can be used directly.

[0073] (Neutralization of polymer a)

[0074] At least a part of the carboxyl groups of the polymer a is neutralized with a neutralizing agent. When neutralizing, it is preferred to neutralize to a pH of 6 or more and 10 or less.

[0075] As the neutralizing agent, an alkali metal hydroxide that generates alkali metal ions is used from the viewpoint of improving the storage stability and ejection stability of the obtained aqueous ink.

[0076] As the alkali metal ion, at least one selected from the group consisting of sodium ion and potassium ion is preferred. That is, as the alkali metal hydroxide, sodium hydroxide and potassium hydroxide are preferred.

[0077] Alternatively, the polymer a may be neutralized with alkali metal ions before step I.

[0078] From the viewpoint of fully and uniformly promoting neutralization, the neutralizing agent is preferably used in the form of an aqueous solution. The concentration of the neutralizing agent aqueous solution is preferably 3% by mass or more, more preferably 10% by mass or more, further preferably 15% by mass or more, and preferably 50% by mass or less, more preferably 25% by mass or less.

[0079] From the viewpoint of improving the storage stability and ejection stability of the obtained aqueous ink, the degree of neutralization of the polymer a in step I with alkali metal ions (hereinafter also referred to as "neutralization degree of polymer a") is preferably 7 mol% or more, more preferably 10 mol% or more, further preferably 13 mol% or more, and is preferably 30 mol% or less, more preferably 28 mol% or less, and further preferably 26 mol% or less, relative to the total molar amount of carboxyl groups in the polymer a.

[0080] In the present invention, the degree of neutralization of polymer a refers to the value obtained by dividing the number of moles of the neutralizing agent (alkali metal ions) in the pigment mixture by the number of moles of carboxyl groups of polymer a in the pigment mixture, that is, the value of "the number of moles of alkali metal ions in the pigment mixture / the number of moles of carboxyl groups of polymer a in the pigment mixture".

[0081] (Dispersion of pigment mixture)

[0082] In step I, the pigment mixture is subjected to a dispersion treatment to obtain a pigment dispersion of polymer particles containing a pigment. There is no particular limitation on the dispersion method used to obtain the pigment dispersion. Although the average particle size of the pigment particles can be micronized to a desired particle size only by the main dispersion, the following method is preferred: after applying a mechanical force to the pigment mixture for pre-dispersion, shear stress is applied for main dispersion, thereby controlling the average particle size of the pigment particles to a desired particle size.

[0083] The method of applying the mechanical force is not particularly limited, and a common mixing and stirring device such as an anchor blade and a dispersing blade can be used.

[0084] As the device for applying the shear stress of the main dispersion, a mixing device such as a roll mill and a kneader, a high-pressure homogenizer such as a microfluidizer (made by Microfluidics), a medium-type disperser such as a paint oscillator and a bead mill can be cited. Among them, a high-pressure homogenizer is preferred from the viewpoint of achieving a small particle size of the pigment.

[0085] When a high-pressure homogenizer is used, the pigment can be controlled to a desired particle size by controlling the processing pressure and the number of processing times. From the viewpoint of productivity and economic benefit, the processing pressure is preferably 80 MPa or more, more preferably 120 MPa or more, and preferably 300 MPa or less, more preferably 250 MPa or less. In addition, the number of processing times is preferably 3 or more, more preferably 5 or more, and preferably 30 or less, more preferably 25 or less.

[0086] (Process II)

[0087] In step II, an aqueous dispersion of polymer particles containing a pigment (aqueous pigment dispersion) can be obtained by removing the organic solvent from the pigment dispersion obtained in step I by a known method. It is preferred that the organic solvent in the obtained aqueous pigment dispersion is substantially removed, but it may remain as long as the purpose of the present invention is not impaired.

[0088] (Process III)

[0089] In step III, a cross-linking structure is formed on the surface of the pigment-containing polymer particles by chemically reacting with a part of the carboxyl groups of the polymer a constituting the pigment-containing polymer particles present in the pigment aqueous dispersion. That is, the polymer a constituting the pigment-containing polymer particles in the pigment dispersion is cross-linked by the cross-linking agent to form the polymer A having a cross-linked structure, thereby forming the pigment-containing cross-linked polymer particles.

[0090] (Crosslinking agent)

[0091] From the viewpoint of improving the storage stability and ejection stability of the obtained aqueous ink, the crosslinking agent used in the present invention is preferably a compound having two or more functional groups capable of reacting with the carboxyl groups of the polymer a in the molecule, and more preferably a polyfunctional epoxy compound.

[0092] The polyfunctional epoxy compound is preferably a compound having two or more epoxy groups in the molecule, more preferably a compound having a glycidyl ether group, and still more preferably a glycidyl ether compound of a polyol having a hydrocarbon group having 3 to 8 carbon atoms.

[0093] By using a glycidyl ether compound of a polyhydric alcohol, the crosslinked structure of the obtained water-insoluble crosslinked polymer particles can be made into a structure derived from the polyglycidyl ether compound of the polyhydric alcohol.

[0094] From the viewpoint of efficiently reacting with the carboxyl group to improve the storage stability of the pigment-containing cross-linked polymer particles, the number of epoxy groups in the polyfunctional epoxy compound is, per molecule, 2 or more, preferably 3 or more, and preferably 6 or less, and from the viewpoint of easy availability in the market, more preferably 4 or less.

[0095] Specific examples of the polyfunctional epoxy compound include polyglycidyl ethers such as polypropylene glycol diglycidyl ether, glycerol polyglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, and hydrogenated bisphenol A type diglycidyl ether.

[0096] Among them, at least one selected from the group consisting of trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, and pentaerythritol polyglycidyl ether is preferred.

[0097] The crosslinking reaction of polymer a is preferably carried out after the pigment is dispersed with polymer a. From the viewpoint of completion of the reaction and economic efficiency, the reaction temperature is preferably 50°C or higher, more preferably 60°C or higher, and preferably 95°C or lower, more preferably 85°C or lower.

[0098] The reaction time is preferably 0.5 hours or more, more preferably 1 hour or more, and is preferably 12 hours or less, more preferably 10 hours or less.

[0099] The degree of crosslinking of the polymer A constituting the water-insoluble crosslinked polymer particles is preferably 20 mol% or more, more preferably 25 mol% or more, further preferably 30 mol% or more, and is preferably 80 mol% or less, more preferably 70 mol% or less, further preferably 60 mol% or less.

[0100] The crosslinking degree of polymer A is the apparent crosslinking degree calculated from the acid value of the polymer and the epoxy group equivalent of the crosslinking agent. That is, the crosslinking degree is "the number of moles of epoxy groups of the crosslinking agent added to the pigment aqueous dispersion / the number of moles of carboxyl groups of polymer A in the pigment aqueous dispersion".

[0101] The acid value of polymer A obtained by crosslinking polymer a is 45 mgKOH / g or more, preferably 50 mgKOH / g or more, more preferably 55 mgKOH / g or more, further preferably 60 mgKOH / g or more, and is 180 mgKOH / g or less, preferably 170 mgKOH / g or less, more preferably 160 mgKOH / g or less, further preferably 150 mgKOH / g or less.

[0102] From the viewpoint of reducing coarse particles and improving the ejection stability of water-based ink, the average particle size of the pigment-containing cross-linked polymer particles in the pigment water dispersion is preferably greater than 60 nm, more preferably greater than 70 nm, further preferably greater than 80 nm, and is preferably less than 200 nm, more preferably less than 160 nm, further preferably less than 150 nm.

[0103] The average particle size of the pigment-containing cross-linked polymer particles in the aqueous ink is substantially the same as the average particle size in the aqueous pigment dispersion.

[0104] From the viewpoint of improving the dispersion stability of the pigment water dispersion and the viewpoint of facilitating the preparation of aqueous ink, the concentration of the non-volatile components of the obtained pigment water dispersion (solid component concentration) is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 30% by mass or less, more preferably 25% by mass or less.

[0105] It should be noted that the solid component concentration of the pigment aqueous dispersion is measured by the method described in Examples.

[0106] [Method for producing ink of the present invention]

[0107] The ink of the present invention can be efficiently produced by mixing the pigment aqueous dispersion containing the crosslinked polymer particles containing the pigment obtained in the above step III, aqueous ammonia, an organic solvent, water, and various additives such as a surfactant added as needed.

[0108] There is no particular limitation on the method of mixing the above-mentioned components.

[0109] (Alkali Metal Ions in the Ink of the Present Invention)

[0110] The ink of the present invention contains alkali metal ions in an amount corresponding to 15 mol % or more and 65 mol % based on the total molar amount of carboxyl groups of the water-insoluble crosslinked polymer A present in the ink of the present invention.

[0111] From the viewpoint of improving the storage stability and ejection stability of the obtained aqueous ink and obtaining a recorded product with excellent water resistance, the alkali metal ion content is preferably from 20 mol % to 60 mol %, more preferably from 25 mol % to 55 mol %, further preferably from 30 mol % to 55 mol %, and even more preferably from 35 mol % to 55 mol %, relative to the total molar amount of the carboxyl groups of the polymer A.

[0112] The alkali metal ion content in the ink of the present invention is calculated as the mole % relative to the total molar amount of the carboxyl groups of the cross-linked polymer A present in the ink of the present invention, based on the total amount of all these alkali metal ions present in the ink, in the presence of the alkali metal ions used in preparing the carboxyl groups of the polymer a before cross-linking and the alkali metal ions newly added when preparing the ink of the present invention.

[0113] (Ammonium ions in the ink of the present invention)

[0114] The ink of the present invention contains alkali metal ions, and preferably also contains ammonium ions. The ammonium ions are preferably ammonium ions derived from ammonia water.

[0115] From the viewpoint of improving the storage stability and ejection stability of the obtained water-based ink and obtaining a recorded product with excellent water resistance, the amount of ammonia water, in terms of ammonium ion content, is 55 mol% or more, preferably 60 mol% or more, more preferably 65 mol% or more, and further preferably 70 mol% or more, and is 145 mol% or less, preferably 140 mol% or less, more preferably 135 mol% or less, and further preferably 130 mol% or less, relative to the total molar amount of carboxyl groups of the water-insoluble cross-linked polymer particles present in the ink of the present invention.

[0116] Pigment-free polymer particles composed of acrylic resin, styrene-acrylic resin, etc. may be added to the ink of the present invention. In this case, the polymer may be neutralized with ammonium ions to adjust the content of ammonium ions to a range of 55 mol% to 145 mol% relative to the total molar amount of carboxyl groups of the polymer A present in the ink.

[0117] Based on the same viewpoints as above, in the ink of the present invention, the total content of alkali metal ions and ammonium ions is 100 mol% or more, preferably 105 mol% or more, more preferably 110 mol% or more, and further preferably 115 mol% or more, and is 185 mol% or less, preferably 180 mol% or less, more preferably 175 mol% or less, and further preferably 170 mol% or less, relative to the total molar amount of carboxyl groups of the polymer constituting the water-insoluble cross-linked polymer particles present in the ink of the present invention.

[0118] <Organic Solvent in the Ink of the Present Invention>

[0119] The organic solvent used in the ink of the present invention may be liquid or solid at 25°C, and is preferably a water-soluble organic solvent whose solubility is 10 ml or more when dissolved in 100 ml of water at 25°C.

[0120] From the viewpoint of improving the storage stability of the obtained aqueous ink, the boiling point of the organic solvent is preferably above 100°C, more preferably above 110°C, further preferably above 120°C, further preferably above 140°C, and is preferably below 260°C, more preferably below 250°C, further preferably below 240°C.

[0121] Examples of the water-soluble organic solvent include glycol ethers such as alkylene glycol ethers, polyols such as propylene glycol, and amide compounds, etc. Among them, alkylene glycol ethers are preferred, and monoalkylene glycol ethers are more preferred.

[0122] Examples of the monoalkylene glycol ether include at least one selected from ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol isobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol monobutyl ether.

[0123] Among them, from the same viewpoint as described above, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether and dipropylene glycol monomethyl ether are preferred, and dipropylene glycol monomethyl ether is more preferred.

[0124] <Other additives, etc.>

[0125] Examples of the surfactant include nonionic surfactants, anionic surfactants, amphoteric surfactants, etc. Among them, nonionic surfactants are preferred, and at least one selected from acetylene glycol surfactants and polyether-modified silicone surfactants is more preferred.

[0126] Examples of commercially available products of the acetylene glycol-based surfactants include Sufynol series manufactured by Air Products & Chemicals and ACETYLENOL series manufactured by Kawaken Fine Chemical Co., Ltd.

[0127] Specific examples of the polyether-modified silicone-based surfactant include KF series manufactured by Shin-Etsu Chemical Co., Ltd., SILFACE SAG manufactured by Nissin Chemical Industry Co., Ltd., and BYK series manufactured by BYK Chemie Japan Co., Ltd.

[0128] Examples of other additives used in the method for producing the ink of the present invention include color fixing aids, humectants, wetting agents, penetrants, viscosity modifiers, defoamers, preservatives, bactericides, rust preventives, and the like.

[0129] As the color fixing auxiliary agent, there can be mentioned an emulsion containing water-insoluble polymer particles. As the water-insoluble polymer particles, there can be mentioned particles of polycondensation resins such as polyurethane and polyester, and particles of vinyl resins such as (meth)acrylic resins, styrene resins, styrene-(meth)acrylic resins, butadiene resins, styrene-butadiene resins, vinyl chloride resins, vinyl acetate resins and acrylic silicone resins.

[0130] The contents of the components of the ink of the present invention obtained by the above-mentioned production method and the ink properties are as follows.

[0131] (Content of Pigment in the Ink of the Present Invention)

[0132] From the viewpoint of increasing the printing density of the ink of the present invention, the content of the pigment in the ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, and further preferably 4% by mass or more. In addition, from the viewpoint of improving ejection stability and storage stability, it is preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 8% by mass or less.

[0133] (Content of Water-Insoluble Cross-Linked Polymer in the Ink of the Present Invention)

[0134] The content of the water-insoluble cross-linked polymer in the ink of the present invention is preferably 2% by mass or more, more preferably 3% by mass or more, and further preferably 4% by mass or more, relative to the total amount of the ink of the present invention, and is preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 8% by mass or less.

[0135] (Content of the Pigment-Containing Cross-Linked Polymer Particles in the Ink of the Present Invention)

[0136] The content of the cross-linked polymer particles containing the pigment in the ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more, and is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 16% by mass or less, calculated as the total amount of the pigment and the water-insoluble cross-linked polymer, relative to the total amount of the ink of the present invention.

[0137] (Content of organic solvent in the ink of the present invention)

[0138] From the viewpoint of improving the ejection stability of the ink, the content of the organic solvent in the ink of the present invention is preferably 15% by mass or more, more preferably 20% by mass or more, and further preferably 22% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less, and further preferably 35% by mass or less, relative to the total amount of the ink of the present invention.

[0139] (Water Content in the Ink of the Present Invention)

[0140] From the viewpoint of improving ejection stability, the water content in the ink of the present invention is preferably 25% by mass or more, more preferably 30% by mass or more, further preferably 35% by mass or more, and is preferably 75% by mass or less, more preferably 70% by mass or less, further preferably 65% ​​by mass or less, and still further preferably 60% by mass or less, relative to the total amount of the ink of the present invention.

[0141] (Physical Properties of the Ink of the Present Invention)

[0142] From the viewpoint of improving the storage stability of the ink, the viscosity of the ink of the present invention at 32°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, further preferably 5 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 9 mPa·s or less, further preferably 7 mPa·s or less.

[0143] From the viewpoint of improving the storage stability of the ink, the pH value of the ink of the present invention is preferably 7 or more, more preferably 7.5 or more, and further preferably 7.9 or more. From the viewpoint of component resistance and skin irritation, it is preferably 10 or less, more preferably 9.5 or less, and further preferably 9.2 or less.

[0144] [Inkjet recording method]

[0145] The ink of the present invention can be loaded into a known inkjet recording device and ejected onto a recording medium in the form of ink droplets to obtain a recorded image or the like.

[0146] As inkjet recording devices, including thermal type and piezoelectric type, the ink of the present invention is more preferably used as an aqueous ink for piezoelectric type inkjet recording.

[0147] As recording media that can be used, there are highly absorbent plain paper, low absorbent coated paper and non-absorbent resin film. As coated paper, there are general glossy paper and multi-color foam glossy paper, etc.; as resin film, there are polyester film, polyvinyl chloride film, polypropylene film, polyethylene film, etc.

[0148] Example

[0149] In the following Examples and Comparative Examples, "parts" and "%" refer to "parts by mass" and "mass %" unless otherwise specified. It should be noted that the measurement methods of various physical properties and the like are as follows.

[0150] (1) Measurement of polymer number average molecular weight

[0151] Phosphoric acid and lithium bromide were dissolved in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively, as eluents, and gel chromatography [GPC apparatus (HLC-8320GPC) manufactured by Tosoh Corporation, chromatographic columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolum SuperAW-H) manufactured by Tosoh Corporation, flow rate: 0.5 mL / min] was used, and 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] were used as standard substances for measurement.

[0152] 0.1 g of the resin and 10 mL of the above eluent were mixed in a glass vial, stirred with a magnetic stirrer at 25° C. for 10 hours, and then filtered with a syringe filter (DISMIC-13HP PTFE 0.2 μm, manufactured by ADVANTEC Corporation). The obtained product was used as a test sample.

[0153] (2) Measurement of polymer acid value

[0154] The polymer was dissolved in a titration solvent of a mixture of toluene and acetone (2:1), and titrated with a 0.1N potassium hydroxide / ethanol solution using a potentiometric automatic titrator (manufactured by Kyoto Electronics Co., Ltd., electric burette, model: APB-610), and the inflection point on the titration curve was taken as the titration endpoint. The acid value was calculated based on the titration amount of the potassium hydroxide solution up to the endpoint.

[0155] (3) Measurement of solid component concentration of aqueous dispersion

[0156] After drying 5 g of the test sample at a drying temperature of 150°C and measurement mode 96 (monitoring time 2.5 minutes / amplitude 0.05%), the moisture content (%) of the test sample was measured using an infrared hygrometer "FD-230" (manufactured by KETT Scientific Laboratory Co., Ltd.), and the solid component concentration was calculated according to the following formula.

[0157] Solid component concentration (%) = 100 - water content of test sample (%)

[0158] Preparation Example 1 (Preparation of water-insoluble polymer a-1)

[0159] 31 parts of acrylic acid and 69 parts of styrene were mixed to prepare a monomer mixed solution. 10 parts of MEK, 0.2 parts of 2-mercaptoethanol (polymerization chain transfer agent) and 10% of the above monomer mixed solution were added to a reaction container and mixed, and the atmosphere was fully replaced with nitrogen.

[0160] On the other hand, a mixture of the remaining 90% of the monomer mixture, 0.13 parts of the above-mentioned polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of an azo radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65) was added to the dropping funnel, and the monomer mixture in the reaction container was heated to 65° C. while being stirred under a nitrogen atmosphere, and the mixture in the dropping funnel was added dropwise over 3 hours.

[0161] After 2 hours at 65°C from the completion of the dropwise addition, add a solution obtained by dissolving 0.1 part of the above-mentioned polymerization initiator in 2 parts of MEK, and then mature at 65°C for 2 hours and at 70°C for 2 hours. Thereafter, dry under reduced pressure to obtain a water-insoluble polymer a-1 (number average molecular weight: 19000, acid value: 241 mgKOH / g).

[0162] Preparation Examples 2 to 6 (Preparation of water-insoluble polymers a-2 to a-6)

[0163] Water-insoluble polymers a-2 to a-6 were prepared in the same manner as in Preparation Example 1 except that the amounts of the monomer components were changed as shown in Table 1. The results are shown in Table 1.

[0164] [Table 1]

[0165] Table 1

[0166]

[0167] Example 1 (Manufacturing of water-based ink)

[0168] (1) Preparation of pigment aqueous dispersion

[0169] 100 parts of the water-insoluble polymer a-1 obtained in Preparation Example 1 were mixed with 78.6 parts of MEK as a polymer, and 24.7 parts of a 5N sodium hydroxide aqueous solution (sodium hydroxide solid content: 16.9%) were added as sodium ions to neutralize the carboxyl groups of the polymer a-1, thereby neutralizing the polymer (neutralization degree: 24.7 mol %).

[0170] 800 parts of ion exchange water were added, and 100 parts of cyan pigment (PB15:3; CI pigment blue 15:3, manufactured by DIC Corporation, trade name: TGR-SD) were added thereto, and stirred for 60 minutes at a temperature of 20°C and a dispersing blade rotating at 7000 rpm using a disperser (manufactured by Asada Iron Works Co., Ltd., trade name UltraDisper). The resulting mixture was dispersed 10 times under a pressure of 200 MPa using a microfluidizer (manufactured by Microfluidics, trade name) to obtain a pigment dispersion (solid content concentration 18.1%, pigment concentration 9%).

[0171] (2) Crosslinking treatment of pigment aqueous dispersion

[0172] 250 parts of ion exchange water was added to the pigment dispersion obtained in the above (1), and after stirring, MEK was completely removed under reduced pressure and 60°C, and a part of water was further removed to adjust the solid content concentration to 20% (pigment concentration 10%). Then, 23.8 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Co., Ltd., trade name: DENACOL EX-321, epoxy value 140) was added as an epoxy crosslinking agent, and the lid was tightly closed. The mixture was heated at 70°C for 5 hours while being stirred with a stirrer to react with the carboxyl groups of the polymer, thereby obtaining a pigment aqueous dispersion of particles of a water-insoluble crosslinked acrylic polymer (the crosslinking structure of which is a structure derived from a polyglycidyl ether compound of trimethylolpropane, which is a polyol having a hydrocarbon group having 3 to 8 carbon atoms).

[0173] (3) Ammonium ion supplementation and ink preparation

[0174] After the pigment water dispersion was obtained by the above (2), it was left to stand for 5 hours, cooled to room temperature, and then 14.2 parts of 25% ammonia water as a supplementary neutralizer, 540 parts of dipropylene glycol monomethyl ether (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as an organic solvent, 15 parts of acetylene glycol nonionic surfactant (manufactured by Air Products & Chemicals, trade name: SURFYNOL 104, 2,4,7,9-tetramethyl-5-decyne-4,7-diol) were added, and ion exchange water was added so that the total amount became 2000 parts. The mixture was filtered using a 25 mL needleless syringe (manufactured by TERUMO Co., Ltd.) equipped with a 5 μm filter (cellulose acetate membrane, outer diameter: 2.5 cm, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to obtain an aqueous ink 1 (content of pigment-containing cross-linked polymer particles: 10%).

[0175] The water-insoluble crosslinked polymer particles of the obtained aqueous ink 1 had an acid value of 118 mgKOH / g, 40 mol % of the carboxyl groups of the polymer particles were neutralized with sodium ions, and contained ammonium ions in an amount corresponding to 80 mol % of the carboxyl groups of the polymer.

[0176] Examples 2 to 16, Comparative Examples 1 to 10 (Manufacturing of Water-Based Ink)

[0177] Except that the conditions in Example 1 were changed as shown in Table 2-1 and Table 3, the same operation as in Example 1 was carried out to obtain water-based inks 2 to 16 and 31 to 40. The details are shown in Table 2-1 and Table 3.

[0178] Embodiment 17

[0179] (1) Preparation of pigment aqueous dispersion

[0180] The same procedure as in Example 1(1) was carried out except that 100 parts of the water-insoluble polymer a-1 in Example 1(1) was replaced by 40 parts, thereby obtaining a pigment dispersion (solid content concentration: 13.4%, pigment concentration: 9.6%).

[0181] (2) Crosslinking treatment of pigment aqueous dispersion

[0182] 250 parts of ion exchange water were added to the pigment dispersion obtained in the above (1), and after stirring, MEK was completely removed under reduced pressure at 60° C., and a part of water was further removed to adjust the solid content concentration to 14% (pigment concentration 10%). Then, 9.52 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Co., Ltd., trade name: DENACOL EX-321, epoxy value 140) was added as an epoxy crosslinking agent, and the lid was tightly closed. The mixture was heated at 70° C. for 5 hours while being stirred with a stirrer to react with the carboxyl groups of the polymer, thereby obtaining a pigment aqueous dispersion of water-insoluble crosslinked acrylic polymer particles (acid value 118 mgKOH / g) neutralized with sodium ions having a crosslinked structure (wherein the crosslinked structure is a structure derived from a polyglycidyl ether compound of trimethylolpropane, which is a polyol having a hydrocarbon group having 3 to 8 carbon atoms).

[0183] (3) Preparation of pigment-free water-insoluble cross-linked polymer dispersion

[0184] 60 parts of the water-insoluble polymer a-1 obtained in Preparation Example 1 were mixed with 100 parts of ion-exchanged water, and then 14.2 parts of 25% ammonia water were added, and the mixture was stirred at 90°C for 3 hours in a glass bottle with a screw cap using a magnetic stirrer for self-emulsification, and then 14.28 parts of trimethylolpropane polyglycidyl ether were added and the lid was tightly closed, and the mixture was heated at 90°C for 3 hours while being stirred with a stirrer. After 3 hours, the temperature was lowered to room temperature to obtain a dispersion of water-insoluble crosslinked polymer particles (acid value 118 mgKOH / g) neutralized with ammonium ions and containing no pigment.

[0185] (4) Preparation of ink

[0186] After the pigment water dispersion obtained by the above (1) was left to stand for 5 hours, and after cooling to room temperature, the whole dispersion of water-insoluble polymer particles containing no pigment obtained in the above (3), 540 parts of dipropylene glycol monomethyl ether as an organic solvent, and 15 parts of acetylene glycol nonionic surfactant (SURFYNOL 104) were added, and ion exchange water was added to make the total amount 2000 parts, and then heated at 90° C. for 3 hours while stirring with a stirrer. After 3 hours, the mixture was cooled to room temperature and filtered using a 25 mL needleless syringe (manufactured by TERUMO Co., Ltd.) equipped with a 5 μm filter (cellulose acetate membrane, outer diameter: 2.5 cm, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), to obtain aqueous ink 17 (content of pigment-containing cross-linked polymer particles: 10%).

[0187] The water-insoluble crosslinked polymer of the obtained aqueous ink 17 had an acid value of 118 mgKOH / g, 40 mol % of the carboxyl groups of the polymer were neutralized with sodium ions, and contained ammonium ions in an amount corresponding to 80 mol % of the carboxyl groups of the polymer.

[0188] Examples 18 to 22

[0189] Aqueous inks 18 to 22 were obtained in the same manner as in Example 11 except that dipropylene glycol monomethyl ether used as the organic solvent was changed to an organic solvent shown in Table 2-2. The results are shown in Table 2-2.

[0190] The storage stability of the aqueous ink obtained above, the water resistance of the inkjet printed product, and the ejection stability during inkjet printing were evaluated by the following methods.

[0191] <Evaluation of Storage Stability of Water-Based Ink>

[0192] 40 g of the aqueous ink obtained above was added to a 50 ml screw-capped bottle, which was tightly covered and stored in a thermostatic bath set at 70° C. for 4 days.

[0193] The average particle size change rate before and after storage was calculated from the following formula to evaluate the storage stability.

[0194] Average particle size change rate (%) = (average particle size of polymer particles in ink after storage / initial average particle size of polymer particles in ink before storage) × 100

[0195] The closer the average particle size change rate is to 100%, the better the storage stability is.

[0196] <Evaluation of inkjet printing>

[0197] (1) Production of inkjet printed products

[0198] An image was formed on A4-size coated paper (manufactured by Oji Paper Co., Ltd., trade name: OK TopKote+) using aqueous ink by the following inkjet recording method.

[0199] (Inkjet recording method)

[0200] A printing evaluation device (manufactured by TRITEK Co., Ltd.) equipped with an inkjet recording head (manufactured by Kyocera Corporation, trade name: KJ4B-HD06MHG-STDV, piezoelectric type) was filled with aqueous ink under an environment of temperature 25±1°C and relative humidity 30±5%.

[0201] The recording head voltage was set to 26V, the driving frequency was set to 10kHz, the ejection droplet volume was set to 12pl, the recording head temperature was set to 32°C, the resolution was set to 600dpi, the number of flushing times before inkjet was set to 200 times, and the negative pressure was set to -4.0kPa. The recording medium was fixed on the conveying table by decompression in the same direction as the conveying direction of the length direction of the recording medium. The printing command was transmitted to the above-mentioned printing evaluation device, and a gradient image of 5% color gradation with a duty ratio (Duty) of 0 to 100% was printed to obtain a printed product.

[0202] (2) Evaluation of discharge stability

[0203] After the printed matter was produced according to the above (1), the printer was stopped for 30 minutes and the recording head was exposed to the atmosphere. After 30 minutes, the ink was purged from the inkjet recording head once, and the nozzle loss state when printing was restarted after wiping was observed. The nozzle recovery rate (%) was calculated according to the following formula to evaluate the ejection stability.

[0204] Nozzle recovery rate (%) = (number of normal spray nozzles / total number of nozzles) × 100

[0205] The larger the nozzle recovery rate (%) is, the better the nozzle recovery property is judged to be. When the value is 90% or more, it can be used in practice.

[0206] (3) Water resistance evaluation

[0207] After the print obtained in the above (1) was allowed to stand for 3 minutes, it was immersed in a stainless steel barrel containing 1000 mL of ion exchange water for 10 seconds, and a gradient image of 5% gradation with a duty cycle of 0 to 100% was flicked with a finger in sequence, and the duty cycle portion of the image where peeling was observed was recorded as the value of the water resistance evaluation.

[0208] The ink having a larger duty ratio without peeling is judged to have higher water resistance. When the water resistance evaluation value is 50% or more, it can be used in practical use.

[0209]

[0210] [Table 2-2]

[0211] Table 2-2

[0212] *1: Na relative to the total molar amount of carboxyl groups in the water-insoluble cross-linked polymer + Ions and NH4 + Total content of ions (mol %).

[0213]

[0214] It can be seen from Table 2-1, Table 2-2 and Table 3 that the water-based inks obtained in Examples 1 to 22 are superior in storage stability and ejection stability compared to the water-based inks obtained in Comparative Examples 1 to 10, and can provide recorded products with excellent water resistance.

[0215] [Possibility of Industrial Application]

[0216] The present invention provides an aqueous ink for inkjet printing that has excellent storage stability and ejection stability and can provide a recorded product with excellent water resistance. The aqueous ink for inkjet printing of the present invention is particularly useful for forming an image on a low water-absorbent recording medium by inkjet printing.

Claims

1. A water-based ink for inkjet recording, wherein: Contains pigment, water-insoluble cross-linked polymer particles, organic solvent and water. The water-insoluble cross-linked polymer particles are composed of a polymer A having a carboxyl group, an acid value of 45 mgKOH / g or more and 180 mgKOH / g or less, and a cross-linked structure. The ink contains 15 mol % to 65 mol % of alkali metal ions and 55 mol % to 145 mol % of ammonium ions relative to the total molar amount of carboxyl groups of the polymer A present in the ink. The total content of the alkali metal ions and the ammonium ions is 100 mol % or more and 185 mol % or less relative to the total molar amount of the carboxyl groups of the polymer A.

2. The aqueous ink for inkjet printing according to claim 1, wherein The alkali metal ions are one or more selected from the group consisting of sodium ions and potassium ions.

3. The aqueous ink for inkjet printing according to claim 1, wherein The crosslinked structure of the water-insoluble crosslinked polymer particles is a crosslinked structure derived from a polyglycidyl ether compound of a polyol having a hydrocarbon group having 3 to 8 carbon atoms.

4. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein The crosslinking degree of the polymer A having a crosslinked structure is 20 mol % or more and 80 mol % or less.

5. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein The boiling point of the organic solvent is 100° C. or higher and 260° C. or lower.

6. The aqueous ink for inkjet printing according to any one of claims 1 to 3, wherein The organic solvent contains glycol ether.

7. The aqueous ink for inkjet printing according to claim 6, wherein The glycol ether is one or more selected from ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol isobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol monobutyl ether.

8. The aqueous ink for inkjet printing according to claim 7, wherein The glycol ether is one or more selected from the group consisting of ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether and dipropylene glycol monomethyl ether.

9. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein The content of the organic solvent in the ink is 15% by mass or more and 40% by mass or less.

10. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein The present invention further contains at least one surfactant selected from the group consisting of acetylene glycol-based surfactants and polyether-modified silicone-based surfactants.

11. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein The content of the water in the ink is 25% by mass or more and 75% by mass or less.

12. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein The pigment is in the form of water-insoluble crosslinked polymer particles containing the pigment.

13. The aqueous ink for inkjet printing according to claim 12, wherein The polymer A having a cross-linked structure is obtained by cross-linking a water-insoluble polymer having a carboxyl group with a cross-linking agent.

14. The aqueous ink for inkjet printing according to claim 13, wherein The water-insoluble polymer having a carboxyl group is a vinyl polymer obtained by addition polymerization of a vinyl monomer.

15. The aqueous ink for inkjet printing according to claim 14, wherein The vinyl polymer is obtained by copolymerizing a monomer mixture A containing (a-1) a monomer having a carboxyl group and (a-2) a hydrophobic monomer.

16. The aqueous ink for inkjet printing according to claim 15, wherein The content of the component (a-1) in the monomer mixture A is 2% by mass or more and 55% by mass or less.

17. The aqueous ink for inkjet printing according to claim 15, wherein The content of the component (a-2) in the monomer mixture A is 45% by mass or more and 98% by mass or less.

18. The aqueous ink for inkjet printing according to claim 15, wherein The mass ratio (a-1) / (a-2) of the component (a-1) to the component (a-2) in the monomer mixture A is 0.1 or more and 1.2 or less.

19. The aqueous ink for inkjet printing according to claim 14, wherein The number average molecular weight of the vinyl polymer is 3,000 or more and 150,000 or less.

20. The aqueous ink for inkjet printing according to claim 14, wherein The acid value of the vinyl polymer is 50 mgKOH / g or more and 500 mgKOH / g or less.

21. The aqueous ink for inkjet printing according to claim 12, wherein The content of the pigment-containing water-insoluble cross-linked polymer particles in the ink is 3% by mass or more and 30% by mass or less.

22. The aqueous ink for inkjet printing according to claim 12, wherein The average particle size of the pigment-containing water-insoluble cross-linked polymer particles is 60 nm or more and 200 nm or less.

23. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein The content of the water-insoluble cross-linked polymer particles in the ink is 2% by mass or more and 15% by mass or less.

24. The water-based ink for inkjet printing according to any one of claims 1 to 3, wherein Also contains non-pigmented polymer particles.

25. The aqueous ink for inkjet printing according to claim 24, wherein The non-pigmented polymer particles are polymer particles neutralized with ammonium ions.

26. A method for producing a water-based ink for inkjet printing, comprising producing the water-based ink for inkjet printing according to any one of claims 1 to 25, wherein: Mixing a pigment aqueous dispersion containing water-insoluble crosslinked polymer particles containing a pigment obtained by the method having the following steps I to III, aqueous ammonia, an organic solvent, and water, Step I: a step of dispersing a mixture containing a water-insoluble polymer having a carboxyl group, an organic solvent, a pigment, and water to obtain a pigment dispersion of polymer particles containing a pigment; Step II: a step of removing the organic solvent from the pigment dispersion obtained in step I to obtain an aqueous dispersion of pigment-containing polymer particles; Step III: a step of cross-linking the pigment aqueous dispersion obtained in step II with a cross-linking agent to obtain a pigment aqueous dispersion comprising pigment-containing cross-linked polymer particles composed of the polymer A having a cross-linked structure and the pigment.

27. The method for producing a water-based ink for inkjet printing according to claim 26, wherein: The degree of neutralization of the water-insoluble polymer having a carboxyl group with the alkali metal ions in step I is 7 mol % or more and 30 mol % or less.

28. The method for producing a water-based ink for inkjet printing according to claim 26, wherein: Prior to step I, the water-insoluble polymer having a carboxyl group is a polymer that has been neutralized with an alkali metal ion in advance.

29. The method for producing a water-based ink for inkjet printing according to any one of claims 26 to 28, wherein: The ammonium ions in the ink come from ammonia water.

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