Water-based ink for inkjet printing

Abstract

To provide: an aqueous ink for inkjet recording, which has excellent storage stability and discharge stability and which, in recording on a low liquid absorbing recording medium, enables production of a recorded matter having excellent image fastness; and an inkjet recording method.SOLUTION: Provided are: [1] an aqueous ink for inkjet recording comprising pigment, cross-linked polymer particles containing a polyolefin wax, an aqueous organic solvent, and water, where a polymer constituting the cross-linked polymer particles is a polymer having a constituent unit derived from a carboxylic acid monomer and a constituent unit derived from a hydrophobic monomer; and [2] an inkjet recording method for recording on a low liquid absorbing recording medium using the aqueous ink.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based ink for ink-jet printing and an ink-jet recording method. [Background technology]

[0002] Inkjet recording is a method of directly ejecting ink droplets from minute nozzles and depositing them on a recording medium to produce a recorded product with characters or images. This method has become extremely popular due to its many advantages, including the ease and low cost of full color printing, the ability to use plain paper as the recording medium, and the fact that it is non-contact with the recording medium. In recent years, inks that use pigments as colorants have become more widely used due to the weather resistance and water resistance of the recorded product. However, unlike dyes, pigment molecules cannot be uniformly dissolved in the ink vehicle, so it is necessary to maintain the pigment's dispersed state, improve storage stability, and improve ejection stability during inkjet recording. Furthermore, in commercial printing and industrial printing, pigments are used to record product information on low-liquid-absorbency recording media such as coated paper and resin films, but since a large amount of pigment remains on the surface of the recording media, the pigment is prone to peeling, resulting in poor image fastness.

[0003] Various proposals have been made to improve the above problems. For example, Patent Document 1 discloses an inkjet recording method that achieves both image transferability and robustness, which repeats the following steps: a first image formation step in which a reaction liquid containing a viscosity-increasing component is applied onto a transfer body; and an ink containing water, a colorant, and a wax is applied, thereby forming a first image containing a liquid component containing water and a high-boiling-point water-soluble organic solvent and a solid component formed by mixing the reaction liquid and the ink; a second image formation step in which a porous body is brought into contact with the first image to remove some of the liquid component; and a transfer step in which the second image is heated and transferred onto a recording medium, and the temperature of the first and second images is controlled under specific conditions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-144736 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technology of Patent Document 1 is insufficient in terms of storage stability, ejection stability, and image fastness. An object of the present invention is to provide a water-based ink for inkjet printing that is excellent in storage stability and ejection stability and that can give recorded products with excellent image fastness even when recording on a low-liquid-absorbency recording medium, and an inkjet recording method using the water-based ink. [Means for solving the problem]

[0006] The present inventors have found that the above problems can be solved by incorporating crosslinked polymer particles, in which a polyolefin wax is incorporated into a specific crosslinked polymer, into the ink. That is, the present invention provides the following [1] and [2]. [1] A water-based ink for ink-jet printing, comprising a pigment, crosslinked polymer particles containing polyolefin wax, a water-soluble organic solvent, and water, A water-based ink for ink-jet printing, wherein the polymer constituting the crosslinked polymer particles is a polymer having a structural unit derived from a carboxylic acid monomer and a structural unit derived from a hydrophobic monomer. [2] An inkjet recording method for recording on a low-liquid-absorbent recording medium using the water-based ink according to [1] above. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a water-based ink for inkjet printing which is excellent in storage stability and ejection stability and which can give recorded products with excellent image fastness even when recording on a low-liquid-absorbing recording medium, and an inkjet recording method using the water-based ink. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Water-based ink for inkjet printing] The water-based ink for ink-jet printing of the present invention (hereinafter also referred to as "the ink of the present invention") is a water-based ink for ink-jet printing containing a pigment, crosslinked polymer particles containing polyolefin wax, a water-soluble organic solvent, and water, and the polymer constituting the crosslinked polymer particles is a polymer having a constituent unit derived from a carboxylic acid monomer and a constituent unit derived from a hydrophobic monomer.

[0009] In this specification, the term "water-based" means that water accounts for the largest proportion by mass of the medium in which the pigment is dispersed. Furthermore, "recording" is a concept that includes printing and printing out characters and images, and "recorded matter" is a concept that includes printed matter and printed matter on which characters and images are recorded. "Low liquid absorption" is a concept that includes low liquid absorption and non-liquid absorption, and the amount of water absorption of the recording medium when the recording medium is in contact with pure water for 100 ms is 0 g / m 2 More than 10g / m 2 This means that:

[0010] The ink of the present invention has excellent storage stability and ejection stability, and can provide recorded products with excellent image fastness even when recorded on a low-liquid-absorbency recording medium. The reason for this is not clear, but is thought to be as follows. A method is known in which a highly hydrophobic organic solvent such as glycol ether is blended into a water-based ink for a low-liquid-absorbency recording medium to improve fixability to the recording medium. However, when wax particles are blended into the water-based ink, the highly hydrophobic organic solvent reduces the dispersion stability and storage stability of the wax particles. In the ink of the present invention, the wax particles are contained in the form of crosslinked polymer particles in which polyolefin wax is contained (encapsulated) in crosslinked polymer particles. The wax particles contained in the crosslinked polymer particles exhibit high storage stability even in the presence of a highly hydrophobic organic solvent, and are thought to improve ejection stability during inkjet recording. Furthermore, it is believed that the inclusion of polyolefin wax in the ink of the present invention reduces the friction resistance of the ink coating formed on the surface of the recording medium, thereby improving the image fastness.

[0011] <Pigments> The pigment used in the present invention may be either an inorganic pigment or an organic pigment. Examples of inorganic pigments include carbon black and metal oxides, and carbon black is preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of white inks include titanium dioxide, zinc oxide, silica, alumina, and metal oxides such as magnesium oxide. Examples of organic pigments include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments. The hue is not particularly limited, and achromatic pigments such as white, black, and gray can be used in achromatic inks, while chromatic pigments such as yellow, magenta, cyan, red, blue, orange, and green can be used in chromatic inks. The pigments can be used alone or in combination of two or more.

[0012] Suitable forms of the pigment used in the ink of the present invention include (i) a pigment that can maintain a dispersed state without a dispersant, i.e., a self-dispersing pigment, (ii) pigment particles in which the pigment is dispersed with a surfactant, and (iii) polymer particles containing the pigment. Of these, the form of polymer particles containing the pigment is preferred from the viewpoints of storage stability, ejection stability, and image fastness. Here, "pigment-containing polymer particles" (hereinafter also referred to as "pigment-containing polymer particles") refers to particles in which a polymer encapsulates a pigment, particles consisting of a polymer and a pigment with part of the pigment exposed on the surface, particles in which a polymer is adsorbed to part of the pigment, and mixtures thereof. Of these, particles in which a polymer encapsulates a pigment are more preferred.

[0013] [Pigment-containing polymer particles] The polymer constituting the pigment-containing polymer particles (hereinafter also referred to as "polymer a") is not particularly limited as long as it has at least the ability to disperse the pigment. Examples of polymer a include vinyl resins obtained by addition polymerization of vinyl monomers, polyester resins, polyurethane resins, etc. Among these, vinyl resins are preferred from the viewpoints of pigment dispersion stability, storage stability, etc. Polymer a may be appropriately synthesized or a commercially available product. The pigment-containing polymer particles are more preferably pigment-containing crosslinked polymer particles (hereinafter also referred to as "pigment-containing crosslinked polymer particles") obtained by further crosslinking pigment-containing polymer particles with a crosslinking agent.

[0014] The polymer a before crosslinking may be a water-soluble polymer or a water-insoluble polymer, but a water-insoluble polymer is more preferred. Even if the polymer used is a water-soluble polymer, it will become a water-insoluble polymer if it is crosslinked. As used herein, the term "water-insoluble" refers to a polymer that has been dried at 105°C for 2 hours and allowed to reach a constant weight, and when the polymer is dissolved in 100 g of water at 25°C, the amount of dissolution is less than 10 g. In the case of an anionic polymer, the amount of dissolution is the amount of dissolution when the anionic groups of the polymer are 100% neutralized with sodium hydroxide.

[0015] [Polymer a] When polymer a is a vinyl resin, polymer a preferably contains (a-1) a structural unit derived from an ionic monomer, and more preferably further contains (a-2) a hydrophobic monomer and / or (a-3) a structural unit derived from a nonionic monomer.

[0016] [(a-1) Ionic Monomer] The (a-1) ionic monomer is preferably an anionic monomer, and examples thereof include carboxylic acid monomers and sulfonic acid monomers, with carboxylic acid monomers being more preferred. The carboxylic acid monomer may be at least one selected from (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, with (meth)acrylic acid being preferred. "(Meth)acrylic acid" refers to at least one selected from acrylic acid and methacrylic acid.

[0017] [(a-2) Hydrophobic Monomer] (a-2) The term "hydrophobic" in the hydrophobic monomer means that when the monomer is dissolved in 100 g of ion-exchanged water at 25° C. until saturation, the amount of the dissolved monomer is less than 10 g. Specific examples of the (a-2) hydrophobic monomer include those described in paragraphs

[0020] to

[0022] of JP 2018-83938 A. Among these, alkyl(meth)acrylates having an alkyl group with 1 to 18 carbon atoms, particularly 1 to 10 carbon atoms, aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end are preferred, with one or more selected from styrene, α-methylstyrene, and benzyl(meth)acrylate being more preferred.

[0018] The macromonomer having a polymerizable functional group at one end is a compound having a number average molecular weight of 500 or more and 100,000 or less, preferably 1,000 or more and 10,000 or less, and the polymerizable functional group can be an acryloyloxy group or a methacryloyloxy group. The macromonomer is preferably an aromatic group-containing monomer-based macromonomer, and examples of the aromatic group-containing monomer constituting the macromonomer include the aromatic group-containing monomers described above. Specific examples of commercially available styrene-based macromonomers include AS-6(S), AN-6(S), and HS-6(S) manufactured by Toagosei Co., Ltd.

[0019] [(a-3) Nonionic Monomer] (a-3) The nonionic monomer is a monomer that has a high affinity with water or a water-soluble organic solvent, and is, for example, a monomer that contains a hydroxyl group or a polyalkylene glycol chain. Specific examples of the component (a-3) include those described in paragraph

[0018] of JP-A No. 2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferred. The monomer components contained in each of the above components (a-1) to (a-3) can be used alone or in combination of two or more.

[0020] (Content of each structural unit in vinyl resin) The content of the structural units derived from the components (a-1) to (a-3) in the vinyl resin is as follows, from the viewpoint of improving storage stability, ejection stability, and image fastness. The content of component (a-1) is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 12% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less. The content of component (a-2) is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

[0021] When the component (a-3) is contained, the content thereof is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less. The mass ratio of component (a-1) to component (a-2) [component (a-1) / component (a-2)] is preferably 0.2 or more, more preferably 0.3 or more, and is preferably 1.2 or less, more preferably 0.8 or less, and even more preferably 0.5 or less.

[0022] (Production of polymer a) The polymer a can be produced by copolymerizing a mixture of the above monomer components (a-1) to (a-3) by a known polymerization method, preferably a solution polymerization method. There are no limitations on the solvent used in the solution polymerization method, but polar solvents such as water, aliphatic alcohols, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred. During the polymerization, a polymerization initiator or a polymerization chain transfer agent can be used. Examples of the polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble azo polymerization initiators. Examples of the polymerization chain transfer agent include mercaptans. The polymerization temperature varies depending on the type of polymerization initiator, monomer, solvent, etc. used, but is preferably 30°C or higher, more preferably 50°C or higher, and is preferably 95°C or lower, more preferably 80°C or lower. The polymerization atmosphere is preferably a nitrogen gas or inert gas atmosphere. The vinyl resin is preferably neutralized with a neutralizing agent as described below.

[0023] From the viewpoints of storage stability, ejection stability, and image fastness, the number average molecular weight of polymer a is preferably 5,000 or more, more preferably 10,000 or more, and even more preferably 20,000 or more, and is preferably 80,000 or less, more preferably 60,000 or less, and even more preferably 40,000 or less. The number average molecular weight of the polymer is measured by the method described in the Examples.

[0024] [Production of pigment-containing polymer particles] The pigment-containing polymer particles can be efficiently produced as a pigment aqueous dispersion by a method including the following steps 1 and 2. Step 1: A step of dispersing a pigment mixture containing a pigment, polymer a, an organic solvent, and water to obtain a dispersion. Step 2: A step of removing the organic solvent from the dispersion obtained in Step 1 to obtain an aqueous dispersion of pigment-containing polymer particles A (hereinafter also referred to as "pigment aqueous dispersion (i)"). When the polymer a is a vinyl resin, it is preferable to further carry out the following step 3. Step 3: A step of adding a crosslinking agent to the pigment aqueous dispersion (i) obtained in Step 2 to crosslink the pigment-containing polymer particles, thereby obtaining an aqueous dispersion (I) of pigment-containing crosslinked polymer particles (hereinafter also referred to as "pigment aqueous dispersion (I)"). The pigment-containing polymer particles according to the present invention also include the pigment-containing crosslinked polymer particles obtained in step 3.

[0025] (Process 1) The pigment mixture in step 1 is preferably obtained by a method in which polymer a is dissolved in an organic solvent, and then pigment, water, and, if necessary, a neutralizing agent, a surfactant, and the like are added to the obtained organic solvent solution and mixed to obtain an oil-in-water dispersion. Although there are no limitations on the organic solvent used in step 1, ketones, ethers, esters, aliphatic alcohols having from 1 to 3 carbon atoms, etc. are preferred, and from the viewpoint of improving the wettability to the pigment and the adsorption of polymer a to the pigment, ketones having from 4 to 8 carbon atoms are more preferred, and methyl ethyl ketone is even more preferred. When a vinyl resin is synthesized as polymer a by solution polymerization, the solvent used in the polymerization may be used as is.

[0026] When polymer a has acid groups, it is preferable that at least a portion of the acid groups be neutralized with a neutralizing agent, which is believed to increase the charge repulsion force that occurs after neutralization, thereby suppressing aggregation of pigment particles in aqueous inks and improving the dispersion stability of the pigment. When neutralizing, it is preferable to neutralize so that the pH is 7 or more and 11 or less. Examples of the neutralizing agent include bases such as sodium hydroxide, potassium hydroxide, ammonia, and various amines, with sodium hydroxide and ammonia being preferred. The polymer a may be neutralized in advance. From the viewpoint of improving dispersion stability and storage stability, the amount of neutralizing agent used is preferably 10 mol% or more, more preferably 15 mol% or more, even more preferably 20 mol% or more, and is preferably 150 mol% or less, more preferably 120 mol% or less, even more preferably 100 mol% or less. Here, the equivalent amount of the neutralizing agent used can be calculated by the following formula, where polymer a before neutralization is "polymer a'". Equivalent amount of neutralizing agent used (mol %)=[{weight (g) of neutralizing agent added / equivalent amount of neutralizing agent} / [{acid value of polymer a' (mg KOH / g) × weight (g) of polymer (B)} / (56 × 1,000)]] × 100

[0027] In the dispersion treatment in step 1, the pigment particles can be atomized to a desired particle size by main dispersion using shear stress alone. However, from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion. As a dispersing machine used for preliminary dispersion, a commonly used mixing and stirring device such as an anchor blade or a disperser blade can be used. Examples of means for applying shear stress used in this dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersers such as paint shakers and bead mills. Among these, it is preferable to use high-pressure homogenizers and bead mills from the viewpoint of reducing the particle size of the pigment. When the dispersion treatment is carried out using a high-pressure homogenizer, the average particle size of the pigment particles in the pigment aqueous dispersion can be adjusted by controlling the treatment pressure and the number of passes. From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more and 300 MPa or less, and the number of passes is preferably 3 or more and 30 or less.

[0028] (Process 2) The organic solvent can be removed by a known method in step 2. It is preferable that the organic solvent in the obtained pigment water dispersion (i) has been substantially removed, but it may remain as long as it does not impair the object of the present invention. Furthermore, in order to remove coarse particles and the like, it is preferable that the aqueous dispersion from which the organic solvent has been removed is further centrifuged, and then the liquid layer portion is filtered through a filter or the like, and the pigment aqueous dispersion (i) is obtained by passing through the filter or the like.

[0029] (Step 3) In step 3, which is optional, a crosslinking agent is added to the pigment aqueous dispersion (i) obtained in step 2 to crosslink some of the carboxy groups of polymer a constituting the pigment-containing polymer particles and form a crosslinked structure in the surface layer portion of the pigment-containing polymer particles, thereby obtaining a pigment aqueous dispersion (I) of pigment-containing crosslinked polymer particles. This allows the polymer formed by crosslinking polymer a to be firmly adsorbed or fixed to the pigment surface, suppressing pigment aggregation and, as a result, it is believed that the dispersion stability and storage stability of the resulting ink are further improved.

[0030] The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a compound having two or more glycidyl ether groups, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 4 carbon atoms. The epoxy equivalent of the crosslinking agent is preferably 90 or more, more preferably 100 or more, and preferably 300 or less, more preferably 200 or less. Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

[0031] From the viewpoint of improving storage stability, the degree of crosslinking in step 3 is preferably 8 mol % or more, more preferably 10 mol % or more, and is preferably 70 mol % or less, more preferably 50 mol % or less, in terms of the ratio of the molar equivalents of the crosslinkable functional groups of the crosslinking agent to the molar equivalents of the carboxyl groups of polymer a, from the viewpoint of improving storage stability, etc. From the viewpoint of crosslinking reaction efficiency, the temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 50° C. or higher, and preferably 90° C. or lower, more preferably 80° C. or lower.

[0032] The concentration of non-volatile components (solid content concentration) of the resulting pigment water dispersion (I) is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less, from the viewpoint of improving the dispersion stability of the pigment water dispersion. The solid content concentration is measured by the method described in the Examples. From the viewpoint of dispersion stability, the content of the pigment in the pigment water dispersion (I) is preferably 5% by mass or more, more preferably 8% by mass or more, and preferably 40% by mass or less, more preferably 30% by mass or less. The mass ratio of the (crosslinked) polymer constituting the pigment-containing (crosslinked) polymer particles in the pigment aqueous dispersion (I) to the pigment [(crosslinked) polymer / pigment] is preferably 0.1 or more, more preferably 0.5 or more, and is preferably 2 or less, more preferably 1.5 or less. From the viewpoint of dispersion stability, the average particle size of the pigment-containing (crosslinked) polymer particles is preferably 50 nm or more, more preferably 80 nm or more, and preferably 400 nm or less, more preferably 350 nm or less. The average particle size is measured by the method described in the Examples.

[0033] <Crosslinked polymer particles containing polyolefin wax> The ink of the present invention contains crosslinked polymer particles containing polyolefin wax (hereinafter also referred to as "wax-containing crosslinked polymer particles") from the viewpoint of improving the storage stability and ejection stability of the ink, and from the viewpoint of reducing the frictional resistance of the recording medium surface and improving the image fastness. The polymer constituting the wax-containing crosslinked polymer particles (hereinafter also referred to as "polymer b") has (b-1) a structural unit derived from a carboxylic acid monomer and (b-2) a structural unit derived from a hydrophobic monomer. The wax-containing crosslinked polymer particles are preferably used as an aqueous dispersion in which they are dispersed in an aqueous medium.

[0034] [Polyolefin wax] Polyolefin wax is a wax whose main component is an olefin-based monomer. From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the melting point of the polyolefin wax is preferably 100°C or higher, more preferably 105°C or higher, and even more preferably 108°C or higher, and is preferably 140°C or lower, more preferably 130°C or lower, and even more preferably 125°C or lower.

[0035] Examples of olefin monomers that are the main component of polyolefin wax include linear olefins and cyclic olefins, with those primarily composed of linear olefins having 2 to 6 carbon atoms being preferred, and polyethylene waxes primarily composed of ethylene being more preferred. Here, "primarily composed of ethylene" means that the ethylene content of the wax is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more, based on the total components constituting the wax. Oxidized polyolefin waxes can be obtained by adjusting a high molecular weight polyolefin polymer to a desired molecular weight by thermal decomposition or the like while introducing oxygen atoms or the like into the molecule, and are included in the category of polyolefin waxes. That is, the polyolefin wax is preferably at least one selected from polyethylene wax and oxidized polyethylene wax.

[0036] From the same viewpoints as above, the weight average molecular weight of the polyolefin wax is preferably 400 or more, more preferably 600 or more, and even more preferably 800 or more, and is preferably 10,000 or less, more preferably 6,000 or less, and even more preferably 4,000 or less. From the same viewpoint as above, the density of the polyolefin wax is preferably 0.890 g / cm 3 More preferably, 0.900 g / cm 3 More preferably, 0.910 g / cm 3 and preferably 0.985 g / cm 3 or less, more preferably 0.980 g / cm 3 More preferably, 0.975 g / cm or less 3 The following is the result. In the present invention, waxes other than the polyolefin wax may be contained within the range that does not impair the effects of the present invention. The content of the polyolefin wax in the total amount of wax is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and still more preferably 100% by mass.

[0037] The polyolefin wax is preferably used as a dispersion (emulsion) dispersed in an aqueous medium. There is no particular limitation on the method for producing the polyolefin wax, and for example, a method of mixing a polyolefin wax, other waxes used as needed, and a known surfactant to emulsify the mixture. The surfactant may be a nonionic surfactant, an anionic surfactant, or the like. Suitable examples of commercially available polyethylene waxes include the "Hiwax" series manufactured by Mitsui Chemicals, Inc. and the "Sunwax" series manufactured by Sanyo Chemical Industries, Ltd.

[0038] [Polymer b] Polymer b contains at least (b-1) a structural unit derived from a carboxylic acid monomer and (b-2) a structural unit derived from a hydrophobic monomer. Polymer b may further contain (b-3) a structural unit derived from a nonionic monomer, and / or (b-4) a structural unit derived from a polyfunctional monomer having two or more polymerizable double bonds.

[0039] Specific examples and preferred examples of the (b-1) carboxylic acid monomer are the same as the specific examples and preferred examples of the (a-1) ionic monomer explained in the section on polymer A. That is, the (b-1) component is preferably at least one selected from acrylic acid and methacrylic acid. Specific examples and preferred examples of the hydrophobic monomer (b-2) are the same as those described in the section on polymer A. That is, the component (b-2) is preferably one or more selected from styrene, α-methylstyrene, and benzyl (meth)acrylate, more preferably styrene. Specific examples and preferred examples of the (b-3) nonionic monomer are the same as the specific examples and preferred examples of the (a-3) nonionic monomer explained in the section for polymer a.

[0040] (b-4) A polyfunctional monomer having two or more polymerizable double bonds is used to make the polymer b a crosslinked polymer. Polymer b needs to be a crosslinked polymer from the viewpoint of improving discharge stability, image robustness, etc. However, if the raw material monomers for producing polymer b do not contain component (b-4), a separate crosslinking treatment using a crosslinking agent is required to make polymer b a crosslinked polymer. However, if the raw material monomers for producing polymer b contain component (b-4), polymer b can be made a crosslinked polymer during production of polymer b. Specific examples of the component (b-4) include diacrylate compounds such as polyethylene glycol diacrylate, triethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,6-butylene glycol diacrylate, 1,6-hexanediol diacrylate, 1,4-cyclohexanediol diacrylate, neopentyl glycol diacrylate, 1,9-nonanediol diacrylate, polypropylene glycol diacrylate, 2,2'-bis(4-acryloxypropyloxyphenyl)propane, 2,2'-bis(4-acryloxydiethoxyphenyl)propane, and trimethylolpropane triacrylate, as well as dimethacrylate compounds, triacrylate compounds, trimethacrylate compounds, tetraacrylate compounds, hexaacrylate compounds, methylenebisacrylamide, and divinylbenzene. Among these, 1,6-hexanediol diacrylate, 1,4-cyclohexanediol diacrylate, neopentyl glycol diacrylate, and trimethylolpropane triacrylate are preferred from the viewpoint of reactivity. The polymer b can be produced in the same manner as in the above (production of polymer a).

[0041] [Production of wax-containing crosslinked polymer particles] Examples of methods for producing wax-containing crosslinked polymer particles include (i) a method in which a dispersion containing a crosslinked polymer obtained by copolymerizing a monomer mixture containing the aforementioned component (b-1), component (b-2), and optionally component (b-3), and (b-4) a polyfunctional monomer, and a polyolefin wax are dispersed in the dispersion; and (ii) a method in which a crosslinking agent is added to a dispersion containing an uncrosslinked polymer obtained by copolymerizing a monomer mixture containing the aforementioned component (b-1), component (b-2), and optionally component (b-3), and a polyolefin wax, and a crosslinking treatment is performed. The dispersion treatment method and crosslinking treatment method are the same as those in the above-mentioned [Production of Pigment-Containing Polymer Particles], and specific examples and preferred examples are also basically the same. Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

[0042] Among the above manufacturing methods, from the viewpoint of improving the storage stability and ejection stability of the ink, (ii) a method of crosslinking by adding a crosslinking agent to a dispersion containing an uncrosslinked polymer obtained by copolymerizing a monomer mixture containing the above-mentioned components (b-1), (b-2), and, if necessary, (b-3) is preferred. The reason for this is that in the step of dispersing the polyolefin wax, it is thought that the uncrosslinked polymer is more likely to be adsorbed more quickly and uniformly on the wax surface than the crosslinked polymer. The crosslinking reaction of polymer b may be carried out in a system containing or not containing a pigment, but is preferably carried out in a system not containing a pigment, because if the crosslinking reaction is carried out in a system containing both an aqueous pigment dispersion and an aqueous dispersion of wax-containing polymer particles, the balance of electrostatic repulsive forces between dispersed particles in the system may change in the early stage of the crosslinking reaction, which may cause aggregation of the pigment in the aqueous pigment dispersion.

[0043] From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the degree of crosslinking of polymer b is preferably 0.4 mol % or more, more preferably 0.6 mol % or more, and even more preferably 0.8 mol % or more, and is preferably 30 mol % or less, more preferably 20 mol % or less, and even more preferably 15 mol % or less. When crosslinking is performed using the (b-4) component, the degree of crosslinking is preferably 0.4 mol% or more, more preferably 0.6 mol% or more, even more preferably 0.8 mol% or more, and preferably 15 mol% or less, more preferably 10 mol% or less, even more preferably 5 mol% or less. When crosslinking is performed using a crosslinking agent, the degree of crosslinking is preferably 0.6 mol% or more, more preferably 0.8 mol% or more, even more preferably 1 mol% or more, and preferably 30 mol% or less, more preferably 20 mol% or less, even more preferably 15 mol% or less. The degree of crosslinking is the equivalent weight of crosslinking sites in all the monomers constituting the polymer, and is calculated by the following formula: Degree of crosslinking = [(molar equivalent of monomer forming crosslinked site) × (number of crosslinked sites in the monomer skeleton forming crosslinked site) / (molar equivalent of all monomers constituting the polymer)] × 100

[0044] From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the acid value of the polymer before crosslinking that constitutes the wax-containing crosslinked polymer particles is preferably 10 mgKOH / g or more, more preferably 20 mgKOH / g or more, even more preferably 30 mgKOH / g or more, and still more preferably 40 mgKOH / g or more, and is preferably 300 mgKOH / g or less, more preferably 250 mgKOH / g or less, and even more preferably 200 mgKOH / g or less. From the same viewpoint as above, the acid value of the crosslinked polymer constituting the wax-containing crosslinked polymer particles is preferably 10 mgKOH / g or more, more preferably 20 mgKOH / g or more, even more preferably 30 mgKOH / g or more, and even more preferably 40 mgKOH / g or more, and is preferably 300 mgKOH / g or less, more preferably 250 mgKOH / g or less, and even more preferably 200 mgKOH / g or less.

[0045] From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the average particle size of the wax-containing crosslinked polymer particles in the ink of the present invention is preferably 20 nm or more, more preferably 40 nm or more, and even more preferably 60 nm or more, and is preferably 250 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less. The average particle size of the polymer particles is measured by the method described in the Examples.

[0046] <Water-soluble organic solvent> The water-soluble organic solvent used in the present invention mainly serves to impart wetting and spreading properties to the recording medium. The water-soluble organic solvent may be liquid or solid at 25°C, but when the organic solvent is dissolved in 100 mL of water at 25°C, the amount of dissolution is 10 mL or more. From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the boiling point of the water-soluble organic solvent is preferably 110°C or higher, more preferably 130°C or higher, and even more preferably 150°C or higher, and is preferably 250°C or lower, more preferably 240°C or lower, and even more preferably 235°C or lower. From the same viewpoint as above, the water-soluble organic solvent preferably contains a polyhydric alcohol and a polyhydric alcohol alkyl ether. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-hexanediol, 1,2-octanediol, 1,8-octanediol, 1,2-decanediol, 1,3-propanediol, 1,4-butanediol, 2-ethyl-1,3-hexanediol, and diethylene glycol. Among these, one or more selected from propylene glycol, 1,2-butanediol, 1,3-propanediol, 1,4-butanediol, etc. are preferred, with propylene glycol being more preferred.

[0047] Examples of polyhydric alcohol alkyl ethers include ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, and tripropylene glycol monomethyl ether. Among these, one or more selected from dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether are preferred. The water-soluble organic solvent may further contain organic solvents other than those mentioned above, within the range that does not impair the effects of the present invention.

[0048] The ink of the present invention may further contain a surfactant, if necessary, from the viewpoint of improving the wettability to the recording medium. Examples of surfactants include nonionic surfactants, silicone surfactants, and fluorine surfactants, with nonionic surfactants being more preferred. Examples of nonionic surfactants include polyoxyalkylene alkyl ether surfactants, acetylene glycol surfactants, polyhydric alcohol surfactants, fatty acid alkanolamides, etc. Among these, acetylene glycol surfactants are preferred, and for example, 2,4,7,9-tetramethyl-5-decyne-4,7-diol is more preferred. Examples of commercially available nonionic surfactants include the Surfynol series manufactured by Nissin Chemical Industry Co., Ltd. and Air Products & Chemicals, and the Acetylenol series manufactured by Kawaken Fine Chemicals Co., Ltd. The above surfactants can be used alone or in combination of two or more.

[0049] (Content of each component in the ink of the present invention) The content of each component in the ink of the present invention is as follows, from the viewpoint of improving the storage stability, ejection stability and image fastness of the ink.

[0050] (Pigment content) 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, even more preferably 2.5% by mass or more, and preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less. The content of pigment-containing polymer particles in the ink of the present invention is preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 6% by mass or more, and preferably 18% by mass or less, more preferably 16% by mass or less, even more preferably 14% by mass or less, and still more preferably 12% by mass or less.

[0051] (Polyolefin wax content) The content of polyolefin wax in the ink of the present invention is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, even more preferably 0.8% by mass or more, and preferably 2.5% by mass or less, more preferably 2.2% by mass or less, even more preferably 2.0% by mass or less. The content of wax-containing crosslinked polymer particles in the ink of the present invention is preferably 0.8% by mass or more, more preferably 1% by mass or more, even more preferably 1.5% by mass or more, and preferably 6% by mass or less, more preferably 5% by mass or less, even more preferably 4% by mass or less, and even more preferably 3% by mass or less.

[0052] (Water-soluble organic solvent content) The content of the water-soluble organic solvent in the 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 is preferably 40% by mass or less, more preferably 35% by mass or less, even more preferably 30% by mass or less.

[0053] The content of the surfactant in the ink of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and is preferably 3% by mass or less, more preferably 2.5% by mass or less, even more preferably 2% by mass or less. The water content in the ink of the present invention is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less. The ink of the present invention may contain various additives as optional components depending on the application, such as preservatives, pH adjusters, viscosity adjusters, antifoaming agents, and rust inhibitors. In such cases, a portion of the water content may be replaced with the various additives.

[0054] (Physical properties of the ink of the present invention) From the viewpoint of improving storage stability and image density, 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, even more preferably 5 mPa s or more, and preferably 12 mPa s or less, more preferably 9 mPa s or less, even more preferably 7 mPa s or less. The viscosity of the water-based ink can be measured using an E-type viscometer. From the viewpoint of improving storage stability and image density, the pH of the ink of the present invention is preferably 7.0 or higher, more preferably 7.2 or higher, and even more preferably 7.5 or higher. From the viewpoint of component resistance and skin irritation, the pH is preferably 11 or lower, more preferably 10 or lower, and even more preferably 9.5 or lower. The pH of the water-based ink can be measured by a conventional method.

[0055] [Inkjet recording method] The ink jet recording method of the present invention is an ink jet recording method in which recording is performed on a low liquid-absorbent recording medium using the ink of the present invention. The ink of the present invention can be loaded into a known ink jet recording device such as a piezo type, and ejected as ink droplets onto a recording medium to record an image or the like. The ink of the present invention can provide a recorded product having excellent image fastness even when ink-jet recording is performed on a recording medium having low liquid absorption. Examples of low-liquid-absorbent recording media include low-liquid-absorbent coated paper, art paper, and non-liquid-absorbent resin films. Examples of coated paper include general-purpose glossy paper and multicolor form glossy paper. Examples of resin films include transparent synthetic resin films, such as polyester, polyvinyl chloride, polyolefin, and nylon. These films may be biaxially oriented, uniaxially oriented, or unoriented. Among these, polyester films and oriented polypropylene films are preferred, and corona-discharge-treated polyethylene terephthalate (PET) films and corona-discharge-treated biaxially oriented polypropylene (OPP) films are more preferred. [Example]

[0056] In the following Production Examples, Preparation Examples, Examples and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The methods for measuring each physical property are as follows.

[0057] (1) Measurement of polymer number average molecular weight The measurements were performed using gel permeation chromatography (GPC) on a Tosoh GPC system (HLC-8320GPC) with Tosoh columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn Super AW-H) at a flow rate of 0.5 mL / min, using N,N-dimethylformamide containing phosphate and lithium bromide at concentrations of 60 mmol / L and 50 mmol / L, respectively, as the eluent. The measurements were performed using a monodisperse polystyrene kit (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), both manufactured by Tosoh) with known molecular weights as standards. The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25°C for 10 hours, and filtering with a syringe filter (DISMIC-13HP, made of PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0058] (2) Measurement of the acid value of the polymer The resin was dissolved in a titration solvent of toluene and acetone (2:1) in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with a 0.1 N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was taken as the endpoint. The acid value (mg KOH / g) was calculated from the titration volume of the potassium hydroxide solution up to the endpoint.

[0059] (3) Measurement of the average particle size of polymer particles in aqueous dispersion The particle size was measured by dynamic light scattering using a laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd., product name: ELS-8000), and calculated by cumulant analysis. The measurement conditions were a temperature of 25°C, an angle between the incident light and the detector of 90°, and 100 integration times. The refractive index of water (1.333) was entered as the refractive index of the dispersion solvent. For the measurement sample, a water-based pigment dispersion was weighed into a screw tube (Maruem Co., Ltd., No. 5) and measured to a solids concentration of 2 x 10 -4 Water was added to the mixture so that the concentration reached 5% by mass, and the mixture was stirred at 25°C for 1 hour using a magnetic stirrer.

[0060] (4) Measurement of solid concentration 10.0 g of sodium sulfate, brought to a constant weight in a desiccator, was weighed into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm), and approximately 1.0 g of the sample was added and mixed. The mixture was then accurately weighed and maintained at 105°C for 2 hours to remove volatiles. The mixture was then left in the desiccator for another 15 minutes and the mass was measured. The mass of the sample after devolatilization was taken as the solid content and divided by the mass of the added sample to obtain the solid content concentration.

[0061] (5) pH measurement The pH of the ink at 20° C. was measured using a tabletop pH meter (manufactured by Horiba Ltd., product name: F-71) equipped with a pH electrode (manufactured by Horiba Ltd., product name: 6337-10D).

[0062] Production Example 1 (Production of Water-Insoluble Polymer a) A monomer mixture was prepared by mixing 8 parts of acrylic acid, 91 parts of styrene, and 1 part of 1,6-hexanediol diacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a polyfunctional monomer. 10 parts of methyl ethyl ketone (MEK), 0.2 parts of 2-mercaptoethanol (a polymerization chain transfer agent), and 10% of the monomer mixture were placed in a reaction vessel and mixed, followed by thorough nitrogen gas replacement. Meanwhile, a mixture of the remaining 90% of the monomer mixture, 0.2 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name: V-65) was placed in a dropping funnel, and the monomer mixture in the reaction vessel was heated to 65°C under a nitrogen atmosphere while being stirred, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 65°C from the end of the dropping, a solution of 0.1 parts of the polymerization initiator in 2 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and then at 70°C for 2 hours, followed by drying under reduced pressure to obtain a water-insoluble polymer a having a crosslinked structure with 1,6-hexanediol diacrylate (number average molecular weight: 36,000, degree of crosslinking: 0.89 mol%, acid value: 62 mgKOH / g).

[0063] Production Examples 2 to 6 (Production of Water-Insoluble Polymers b to f) Water-insoluble polymers b to f were obtained in the same manner as in Production Example 1, except that the amounts of the monomer components in Production Example 1 were changed as shown in Table 1. The results are shown in Table 1. It should be noted that the water-insoluble polymers d to f do not contain 1,6-hexanediol diacrylate as a monomer component and therefore do not have a crosslinked structure.

[0064] [Table 1]

[0065] Preparation Example I-1 (Preparation of Wax Dispersion A) 10 parts of the water-insoluble polymer a obtained in Production Example 1 was neutralized by adding 2.2 parts of a 5N aqueous sodium hydroxide solution (solid sodium hydroxide content: 16.9%) as sodium ions to neutralize the carboxyl groups of the polymer a (neutralization degree: 82 mol%). Further, 240 parts of ion-exchanged water was added, and polyethylene wax with a melting point of 109°C (HW110P: manufactured by Mitsui Chemicals, Inc., trade name: Hiwax 110P, melting point: 109°C, density: 0.920 g / cm) was added thereto. 3 100 parts by weight of wax-containing crosslinked polymer particles (weight average molecular weight: 1000, softening point: 113°C) were dissolved by heating at 85-95°C. The resulting mixture was dispersed for 30 minutes using an ultrasonic homogenizer while maintaining the temperature at 90-95°C. After cooling to room temperature, the mixture was dispersed three times using a Microfluidizer (Microfluidics, product name) at a pressure of 200 MPa to obtain a dispersion. Ion-exchange water was added to the resulting dispersion to obtain Dispersion A of wax-containing crosslinked polymer particles (solids concentration: 22%, wax: 20%, polymer: 0.2%, average particle size: 124 nm, pH 7.4).

[0066] Preparation Examples I-2 to I-6 (Preparation of Wax Dispersions B to F) Wax dispersions B to F were prepared in the same manner as in Preparation Example I-1, except that the type of water-insoluble polymer and the amount of aqueous sodium hydroxide solution were changed as shown in Table 2. The results are shown in Table 2. It should be noted that wax dispersions D to F use water-insoluble polymers d to f that do not have a crosslinked structure, and therefore the wax-containing polymer particles contained in wax dispersions D to F do not have a crosslinked structure.

[0067] [Table 2]

[0068] Preparation Example I-7 (Preparation of Wax Dispersion G) To the wax dispersion D obtained in Preparation Example I-4, 0.4 parts (corresponding to a crosslinking degree of 2.60 mol%) of an epoxy crosslinker (1,6-hexanediol diglycidyl ether, manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-212, epoxy equivalent 150) and 55.1 parts of ion-exchanged water were added, the mixture was sealed, and the mixture was heated at 70°C for 5 hours while stirring with a stirrer to react with the carboxy groups of polymer d, thereby obtaining dispersion G of wax-containing crosslinked polymer particles (wax: 18%, average particle size 113 nm, pH 8.4).

[0069] Preparation Example I-8 (Preparation of Wax Dispersion H) To the wax dispersion E obtained in Preparation Example I-5, 2.5 parts of an epoxy crosslinker (trade name: Denacol EX-212) (corresponding to a crosslinking degree of 13.2 mol%) and 53.1 parts of ion-exchanged water were added, the container was sealed, and the mixture was heated at 70°C for 5 hours while stirring with a stirrer to obtain a dispersion H of crosslinked polymer particles containing wax (wax: 18%, average particle size 82 nm, pH 7.8).

[0070] Preparation Example I-9 (Preparation of Wax Dispersion I) In Preparation Example I-8, 2.5 parts of the epoxy crosslinking agent (trade name: Denacol EX-212) was replaced with 6.8 parts (corresponding to a crosslinking degree of 13.2 mol%) of a carbodiimide crosslinking agent (manufactured by Nisshinbo Chemical Inc., trade name: Carbodilite V-10, NCN equivalent: 410), and 53.1 parts of ion-exchanged water was replaced with 48.7 parts. A dispersion I of wax-containing crosslinked polymer particles (wax: 18%, average particle size 90 nm, pH 7.8) was obtained in the same manner as in Preparation Example I-8.

[0071] Preparation Example I-10 (Preparation of Wax Dispersion J) A dispersion J of wax-containing crosslinked polymer particles (wax: 18%, average particle size 105 nm, pH 7.8) was obtained in the same manner as in Preparation Example I-8, except that in Preparation Example I-8, 2.5 parts of the epoxy crosslinker (trade name: Denacol EX-212) was replaced with 3.7 parts (corresponding to a crosslinking degree of 13.2 mol%) of an oxazoline crosslinker (manufactured by Nippon Shokubai Co., Ltd., trade name: Epocross WS-700, oxazoline value 220) and 53.1 parts of ion-exchanged water was replaced with 51.8 parts.

[0072] Preparation Example I-11 (Preparation of Wax Dispersion K) In Preparation Example I-1, instead of the polyethylene wax having a melting point of 109°C (trade name: Hiwax 110P), a polyethylene wax having a melting point of 122°C (manufactured by Mitsui Chemicals, Inc., trade name: Hiwax 200P, melting point: 122°C, density: 0.970 g / cm 3 A dispersion K of wax-containing crosslinked polymer particles (degree of crosslinking: 0.89 mol%) (wax: 18%, average particle size: 109 nm, pH 7.8) was obtained in the same manner as in Preparation Example I-1, except that the weight average molecular weight was 2000 and the softening point was 130°C.

[0073] <Pigment water dispersion> Preparation Example II-1 (Preparation of Pigment Water Dispersion) 100 parts of the water-insoluble polymer e obtained in Production Example 5 was mixed with 78.6 parts of MEK, and 24.7 parts of a 5N aqueous sodium hydroxide solution (solid sodium hydroxide content: 16.9%) was added to neutralize the carboxyl groups of the water-insoluble polymer e (neutralization degree: 25 mol%). Further, 800 parts of ion-exchanged water was added, and 100 parts of carbon black pigment (CI Pigment Black 7, manufactured by Cabot Chemical Corporation, product name: Monarch 800) was added thereto, and the mixture was stirred for 60 minutes using a Disper mixer (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) at 20°C with the Disper blade rotating at 7000 rpm. The resulting mixture was subjected to a dispersion treatment using a Microfluidizer (trade name, manufactured by Microfluidics) at a pressure of 200 MPa for 10 passes to obtain a pigment dispersion (solid concentration: 18.1%, pigment: 9%, polymer: 9%). To the resulting pigment dispersion, 250 parts of ion-exchanged water was added and stirred. After that, the MEK was completely removed under reduced pressure at 60°C, and then some of the water was removed to a solids concentration of 20% (pigment: 10%). After that, 23.8 parts (corresponding to a crosslinking degree of 13.5 mol%) of an epoxy crosslinker (trimethylolpropane polyglycidyl ether, manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-321, epoxy value 140) was added, the mixture was sealed, and heated at 70°C for 5 hours while stirring with a stirrer to obtain a pigment aqueous dispersion of pigment-containing crosslinked polymer particles (solids concentration: 20%, pigment: 10%, polymer: 10%, average particle size: 94 nm).

[0074] Example 1 (Production of Water-Based Ink) 50 parts of the pigment aqueous dispersion obtained in Preparation Example II-1, 9 parts of wax dispersion A obtained in Preparation Example I-1 (wax: 20%), 12 parts of propylene glycol, 12 parts of dipropylene glycol monomethyl ether (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 0.5 parts of a nonionic surfactant (a 50% solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol in propylene glycol, manufactured by Nissin Chemical Industry Co., Ltd., trade name: Surfynol 104PG50), and 16.5 parts of ion-exchanged water were added (total 100 parts) to obtain aqueous ink 1 (solid concentration: 12%, pigment: 5%, pigment dispersion polymer: 5.0%, wax 1.8%, wax dispersion polymer: 0.2%).

[0075] Examples 2 to 8 and Comparative Examples 1 to 4 (Production of Water-Based Inks) In the same manner as in Example 1, water-based inks 2 to 8 and 21 to 24 were obtained using the formulations shown in Table 3.

[0076] The storage stability, ejection stability, and image fastness of the resulting water-based ink were evaluated by the following methods. The results are shown in Table 3.

[0077] (1) Evaluation of storage stability Each water-based ink was stored in an environment of 70°C for 28 days, and the ink viscosity at 32°C was measured for each water-based ink before and after storage using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.). The rate of change in viscosity (%) was calculated using the following formula to evaluate storage stability. Viscosity change rate (%) = (ink viscosity after storage / ink viscosity before storage) x 100 The lower the rate of change in viscosity, the better the storage stability is judged to be, and a value of 105 or less is practically usable.

[0078] (2) Evaluation of ejection stability Using a water-based ink, an image was printed on A4 size coated paper (manufactured by Oji Paper Co., Ltd., trade name: OK Topcoat+) by the following inkjet recording method. (inkjet recording method) In an environment with a temperature of 25±1°C and a relative humidity of 30±5%, a printing evaluation device (manufactured by Tritec Corporation) equipped with an inkjet recording head (manufactured by Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezo type) was filled with water-based ink. The settings were head voltage 26V, drive frequency 10kHz, ejection liquid volume 12pl, head temperature 32°C, resolution 600dpi, pre-ejection flushing count 200 shots, and negative pressure -4.0kPa. The recording medium was fixed to the conveying table under reduced pressure with the longitudinal direction of the recording medium aligned with the conveying direction. A printing command was transferred to the printing evaluation device, and a solid image was printed at 100% duty. The print evaluation device was then stopped for 30 minutes, and the recording head was exposed to the atmosphere. After 30 minutes had passed, the ink was purged from the recording head, wiped, and printing resumed. The state of nozzle chipping was observed, and the nozzle recovery rate (%) was calculated using the following formula to evaluate the ejection stability. Nozzle recovery rate (%) = (number of normal nozzles / total number of nozzles) x 100 The higher the nozzle recovery rate (%), the better the nozzle recovery is judged to be, and a value of 90% or higher is practically usable.

[0079] (3) Evaluation of image robustness A solid image was printed at a duty factor of 100% using the inkjet recording method described in (2) above. After leaving the printed paper stationary for 24 hours, a 2 x 2 cm piece of unprinted coated paper was placed on the printed surface and rubbed back and forth 10 times with a load of 10 N using a Gakushin-type abrasion tester (manufactured by Daiei Scientific Instruments Manufacturing Co., Ltd., product name: RT-300). The density of the transferred ink was measured using a spectrophotometer (manufactured by Sakata Inx Engineering Corporation, product name: Spectro-Eye). The lower the density of the transferred ink, the better the image fastness is judged to be, and a value of 0.2 or less is practically usable.

[0080] [Table 3]

[0081] Table 3 shows that the water-based inks obtained in the examples have superior storage stability and ejection stability compared to the water-based inks obtained in the comparative examples, and can produce printed materials with excellent image fastness.

Claims

1. A water-based ink for ink-jet printing, comprising a pigment, crosslinked polymer particles containing polyolefin wax, a water-soluble organic solvent, and water, the polymer constituting the crosslinked polymer particles is a polymer having a structural unit derived from a carboxylic acid monomer and a structural unit derived from a hydrophobic monomer, A water-based ink for ink-jet printing, wherein the hydrophobic monomer is at least one selected from the group consisting of cyclohexyl (meth)acrylate, (iso)dodecyl (meth)acrylate, and (iso)stearyl (meth)acrylate.

2. 2. The water-based ink for ink-jet printing according to claim 1, wherein the polyolefin wax has a melting point of 100[deg.] C. or higher.

3. 3. The water-based ink for ink-jet printing according to claim 1, wherein the polyolefin wax is at least one wax selected from the group consisting of polyethylene wax and oxidized polyethylene wax.

4. 4. The water-based ink for ink-jet printing according to claim 1, wherein the carboxylic acid monomer is at least one selected from the group consisting of acrylic acid and methacrylic acid.

5. 5. The water-based ink for ink-jet printing according to claim 1, wherein the hydrophobic monomer is cyclohexyl(meth)acrylate.

6. 6. The water-based ink for ink-jet printing according to claim 1, wherein the polymer constituting the crosslinked polymer particles further comprises a constituent unit derived from a polyfunctional monomer having two or more polymerizable double bonds.

7. 7. The water-based ink for ink-jet printing according to claim 1, wherein the water-soluble organic solvent is at least one selected from the group consisting of dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether.

8. 8. The water-based ink for ink-jet printing according to claim 1, wherein the pigment is in the form of polymer particles containing the pigment.

9. An inkjet recording method for recording on a low liquid-absorbent recording medium using the water-based ink according to any one of claims 1 to 8.

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