Water-based white ink for inkjet recording.

Abstract

This invention provides a water-based white ink for inkjet recording that exhibits minimal change in whiteness and good ejection stability even when printing is paused and left unattended for a long period of time while using a circulation device. [Solution] An inkjet water-based white ink containing an inorganic pigment, a polymer dispersant, and crosslinked polymer particles, wherein the polymer dispersant contains a vinyl polymer comprising at least one selected from constituent units derived from acrylic acid and constituent units derived from sulfonic acid having vinyl groups, and the crosslinked polymer particles comprise constituent units derived from polymerizable monomers containing carboxyl groups, and at least two of these carboxyl groups are crosslinked by a structure derived from the crosslinking agent.

Description

Technical Field

[0001] The present invention relates to an aqueous white ink for inkjet recording.

Background Art

[0002] The inkjet recording method is a recording method in which ink droplets are directly ejected from very fine nozzles onto a recording medium and adhered to obtain a printed matter on which characters and images are recorded. Since the inkjet recording method does not use a plate unlike conventional recording methods, it is expected to have a wide range of application fields as on-demand printing that can handle small quantities and multiple varieties. In particular, in recent years, there has been an increasing demand for printing on low-absorbency recording media such as coated paper and polymer films that are not white paper, rather than on conventional white paper.

[0003] For example, Patent Document 1 discloses an inkjet recording method for obtaining good ejection properties by using an aqueous ink in which titanium oxide can be easily redispersed by simple stirring even when it precipitates, and the ink physical properties can be quickly restored by excellent antifoaming or defoaming properties. An inkjet recording method for recording with an inkjet recording apparatus using an aqueous ink containing rutile-type titanium oxide and a polymer dispersant, wherein the polymer dispersant contains 72% by mass or more of structural units derived from at least one anionic group-containing monomer selected from acrylic acid, methacrylic acid, maleic acid, and vinyl monomers containing a sulfonic acid group, the weight average molecular weight of the polymer dispersant is 3,000 or more and 50,000 or less, the content of the polymer dispersant is 1% by mass or more and 7% by mass or less with respect to the titanium oxide, the recording apparatus has a dispersion means for dispersing the titanium oxide in the aqueous ink, and the method includes a step of redispersing the aqueous ink by the dispersion means and a step of ejecting the redispersed aqueous ink in Step 1 onto a recording medium for recording. Patent Document 2 discloses a pigment dispersion that, after long-term storage, exhibits minimal viscosity changes and is less prone to generating coarse particles, and is suitable for use with inkjet printers. The dispersion comprises an inorganic pigment (A) and a polymer (B) having one or more monomer units having anionic groups, wherein the content of the polymer (B) is greater than 7.0 parts by mass and 45 parts by mass or less per 100 parts by mass of the inorganic pigment (A), and the anionic group of the monomer unit having anionic groups is one or more selected from carboxyl groups and sulfonic acid groups. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-75302 [Patent Document 2] Japanese Patent Publication No. 2020-84072 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, the water-based ink described in Patent Document 1 had room for improvement in suppressing changes in whiteness when printing was stopped and left for a long period of time in a printing apparatus with a circulation device. Furthermore, the white ink composition containing a pigment dispersion described in Patent Document 2 also had room for improvement in suppressing changes in whiteness and ejection stability when printing was stopped and left for a long period of time in a printing apparatus with a circulation device. The present invention aims to provide an inkjet water-based white ink that can suppress changes in whiteness and achieve good ejection stability even when printing is stopped and the device is left unattended for a long period of time in a printing apparatus having a circulation mechanism. [Means for solving the problem]

[0006] In other words, the present invention provides the following [1]. [1] A water-based white ink for inkjet recording, comprising an inorganic pigment, a polymer dispersant, and crosslinked polymer particles, The polymer dispersant contains a vinyl polymer comprising at least one selected from structural units derived from acrylic acid and structural units derived from sulfonic acid having a vinyl group. The crosslinked polymer particles include structural units derived from polymerizable monomers containing carboxyl groups, and at least two of these carboxyl groups are crosslinked by a structure derived from a crosslinking agent. Water-based white ink for inkjet recording. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an inkjet water-based white ink that can suppress changes in whiteness and achieve good ejection stability even when printing is stopped and the device is left unattended for a long period of time in a printing apparatus having a circulation device. [Modes for carrying out the invention]

[0008] [Water-based white ink for inkjet recording] The water-based inkjet recording ink of the present invention (hereinafter also simply referred to as "ink") contains an inorganic pigment, a polymer dispersant, and crosslinked polymer particles. Here, the polymer dispersant contains a vinyl polymer comprising at least one selected from constituent units derived from acrylic acid and constituent units derived from sulfonic acid having vinyl groups, and the crosslinked polymer particles contain constituent units derived from carboxyl groups, with at least two carboxyl groups being crosslinked by the crosslinking agent.

[0009] The definitions of various terms used in this specification are shown below. "Aqueous system" means that water accounts for the largest proportion by mass in the medium used to disperse the inorganic pigment. "Record" is a concept that includes printing and printing of text and images, while "record material" is a concept that includes printed materials and printed objects on which text and images are recorded. "Low liquid absorption" is a concept that includes both low liquid absorption and non-liquid absorption, where the amount of water absorbed by the recording medium during a 100 m second contact time with pure water is 0 g / m³. 2 More than 10g / m 2 This means the following: "(Meth)acrylic acid" means at least one selected from the group consisting of acrylic acid and methacrylic acid. "(Meth)acrylate" means at least one selected from the group consisting of acrylates and methacrylates.

[0010] According to the present invention, it is possible to provide an inkjet water-based white ink that can suppress changes in whiteness and achieve good ejection stability even when printing is stopped and the device is left for a long period of time in a printing apparatus having a circulation device. The reason for this is not entirely clear, but it is thought to be as follows. In the ink of the present invention, the inorganic pigment forms a pigment dispersion dispersed by a vinyl polymer containing at least one selected from constituent units derived from acrylic acid and constituent units derived from sulfonic acid having vinyl groups. Since the pigment dispersion has a high density of charge derived from acrylic acid and / or sulfonic acid having vinyl groups, the charge repulsion between the pigment dispersions is large, making aggregation of the pigment and pigment dispersion less likely, and even if aggregation occurs, it is thought to have excellent redispersibility. Furthermore, because crosslinked polymer particles contain a crosslinked structure, the mobility of the polymer on the particle surface is restricted compared to polymer particles without a crosslinked structure. Therefore, fusion between crosslinked polymer particles is suppressed, and interaction with pigments and pigment dispersions is considered to be very small. As a result, neither the pigment dispersion nor the cross-linked polymer particles interfere with each other during ink circulation, and the ink has relatively high fluidity. Therefore, clogging due to aggregation or thickening is less likely to occur in the circulation channel and near the nozzles of the inkjet head, and thus the ink of the present invention is considered to have excellent ejection stability. Furthermore, the fact that clogging is less likely to occur in the circulation channel and near the nozzles of the inkjet head suggests that the concentration change of the pigment dispersion in the ejected ink is small, and therefore, even when printing is stopped and left for a long time, changes in the whiteness of the resulting image can be suppressed.

[0011] <Inorganic pigments> The inorganic pigment contained in the ink of the present invention is a metal compound that can be used as a white ink, and preferably in the form of particles of the metal compound. Examples of inorganic pigments include metal oxides, barium sulfate, calcium carbonate, etc. Examples of metal oxides include titanium dioxide (hereinafter referred to as "titanium oxide"), barium titanate, zirconium oxide, zinc oxide, silicon dioxide (silica), aluminum oxide (alumina), magnesium oxide, etc. Among these, titanium oxide is preferred from the viewpoint of exhibiting high whiteness in refractive index and from the viewpoint of reproducing the hue of conventional printing inks.

[0012] The inorganic pigment is preferably a coated pigment having a first coating layer on the surface of metal compound particles, which is made of one or more materials selected from the group consisting of silica and alumina. When the inorganic pigment is, for example, titanium dioxide, it has photocatalytic activity that decomposes organic matter. Therefore, it is preferable to form a first coating layer on the surface of the titanium dioxide particles by surface-treating the surface of the titanium dioxide particles with an inorganic oxide such as silica, alumina, zinc, magnesium, or zirconium. Titanium dioxide with the first coating layer formed on it has improved affinity to solvents during the manufacture of inks, etc. In particular, when the first coating layer contains alumina, the dispersibility of the inorganic pigment in the solvent during the manufacture of inks, etc., is improved by appropriately setting the pH when dispersing during ink production. Furthermore, when the first coating layer includes silica treatment, the isoelectric point of the particle surface becomes lower, making it easier for polymer dispersants to coat the surface of the inorganic pigment, and improving the dispersibility of the inorganic pigment by polymer dispersants. Also, when the inorganic pigment is, for example, barium sulfate or calcium carbonate, it is preferable to form a first coating layer on the surface of the inorganic pigment by surface-treating it with an inorganic oxide such as silica or zinc oxide in order to improve the affinity with other components during the preparation of the pigment dispersion.

[0013] It is preferable to form a second coating layer on the first coating layer using an organic compound such as a polyol for the inorganic pigment. This can suppress drying aggregation when the inorganic pigment is once powdered, and thus improve the dispersibility when preparing a composition such as ink.

[0014] The particle shape of the inorganic pigment includes granular, needle-like, etc. and is not particularly limited. However, from the viewpoint of obtaining an ink that suppresses changes in whiteness, the arithmetic average of the major axis of the primary particles is preferably 50 nm or more, more preferably 100 nm or more, still more preferably 150 nm or more, and preferably 450 nm or less, more preferably 400 nm or less, still more preferably 350 nm or less. The average primary particle diameter of the inorganic pigment is measured by the method described in the examples.

[0015] (Titanium Oxide) Titanium oxide as an inorganic pigment preferably has a rutile-type (tetragonal) crystal structure among titanium oxides having rutile-type (tetragonal), anatase-type (tetragonal), and brookite-type (orthorhombic) crystal structures, from the viewpoints of crystal stability, hiding power, and availability. (Hereinafter, also referred to as "rutile-type titanium oxide").

[0016] Examples of commercially available products of rutile-type titanium oxide include the product names: Typepeke R, CR, PF series manufactured by Ishihara Sangyo Co., Ltd., the product names: R series manufactured by Sakai Chemical Industry Co., Ltd., the product names: JR, MT series manufactured by Tayca Corporation, the product names: KURONOS KR series manufactured by Titanium Industry Co., Ltd., the product names: TR series manufactured by Fuji Titanium Industry Co., Ltd., and the like.

[0017] From the perspective of obtaining an ink with excellent whiteness, the content of the inorganic pigment in the ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 8% by mass or more, even more preferably 9% by mass or more, and is preferably 14% by mass or less, more preferably 13% by mass or less, still more preferably 12% by mass or less, and even more preferably 11% by mass or less.

[0018] <Polymer dispersant> The polymer dispersant contained in the ink of the present invention contains a vinyl polymer composed of at least one selected from two types of structural units: a structural unit derived from acrylic acid and a structural unit derived from a sulfonic acid having a vinyl group.

[0019] 〔Structural unit derived from sulfonic acid having a vinyl group〕 Examples of the structural unit derived from a sulfonic acid having a vinyl group include a structural unit derived from an alkenyl sulfonic acid, a structural unit derived from styrene sulfonic acid, a structural unit derived from an ester of acrylic acid and a hydroxyalkyl sulfonic acid, a structural unit derived from an amide of acrylic acid and an aminoalkyl sulfonic acid, etc. From the perspective of suppressing changes in whiteness and obtaining an ink with good ejection stability, it is preferably a structural unit derived from an amide of acrylic acid and an aminoalkyl sulfonic acid.

[0020] From the perspective of suppressing changes in whiteness and obtaining an ink with good ejection stability, the number of carbon atoms of the alkenyl sulfonic acid is preferably 2 or more, and is preferably 6 or less, more preferably 3 or less. The alkenyl group of the alkenyl sulfonic acid may be a straight-chain alkenyl group or a branched-chain alkenyl group. Further, from the perspective of the reactivity during the polymerization reaction, the carbon-carbon double bond of the alkenyl sulfonic acid is preferably present at the terminal.

[0021] Styrene sulfonic acid may be substituted or unsubstituted. If styrene sulfonic acid has substituents, examples of substituents include halogen atoms such as fluorine and chlorine, C1-C4 alkyl groups, and C1-C4 alkoxy groups.

[0022] In esters of acrylic acid and hydroxyalkyl sulfonic acid, and amides of acrylic acid and aminoalkyl sulfonic acid, the number of carbon atoms in the alkylene group is preferably 1 or more, preferably 6 or less, and more preferably 4 or less, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability. The alkylene group may be a straight-chain alkylene group or a branched-chain alkylene group, but from the viewpoint of availability, a branched-chain alkylene group is preferred.

[0023] From the viewpoint of availability and economic efficiency, the constituent units derived from sulfonic acid having a vinyl group are preferably vinyl sulfonic acid, allyl sulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid, and more preferably 2-acrylamido-2-methylpropanesulfonic acid.

[0024] The vinyl polymer dispersant preferably contains both constituent units derived from acrylic acid and constituent units derived from sulfonic acid having vinyl groups, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability. A vinyl polymer means a polymer consisting of constituent units derived from monomers substantially having vinyl groups (CH2=CH-). While constituent units derived from other reactive unsaturated groups such as methacrylic acid may be included to the extent that they do not impair the effects of the present invention, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, the constituent units derived from monomers having vinyl groups are preferably 95% by mass or more, more preferably 99% by mass or more, and even more preferably 100% by mass.

[0025] In the vinyl polymer dispersant, the total content of constituent units derived from acrylic acid and constituent units derived from sulfonic acid having vinyl groups is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 99% by mass or more, and more preferably 100% by mass, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability. Note that the content of constituent units derived from acrylic acid and constituent units derived from sulfonic acid having a vinyl group in the polymer dispersant does not include the content of constituent units derived from polymerization initiators and chain polymerization agents. The constituent units derived from acrylic acid and constituent units derived from sulfonic acid having a vinyl group may also be constituent units derived from salts such as ammonium salts and alkali metal salts of these acids. The content of constituent units derived from acrylic acid in the polymer dispersant is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 100% by mass or less, more preferably 95% by mass or less, even more preferably 90% by mass or less, even more preferably 80% by mass or less, and even more preferably 70% by mass or less, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability. The content of constituent units derived from sulfonic acid having vinyl groups in the polymer dispersant is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, and even more preferably 30% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability.

[0026] The vinyl polymer dispersant may contain structural units other than those derived from acrylic acid and those derived from sulfonic acid having a vinyl group. Examples of such structural units include those derived from methacrylic acid and those derived from nonionic monomers. Examples of nonionic monomers include esters of acrylic acid and (poly)alkylene glycol, such as 2-hydroxyethyl acrylate, polypropylene glycol acrylate, and polyethylene glycol acrylate.

[0027] From the viewpoint of obtaining an ink that suppresses changes in whiteness, the content of the polymer dispersant in the ink of the present invention is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and even more preferably 0.20% by mass or more, and from the viewpoint of obtaining an ink with good ejection stability, it is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.3% by mass or less.

[0028] (Manufacturing of polymer dispersants) Polymer dispersants can be produced by homopolymerizing or copolymerizing acrylic acid or its salts and / or vinyl group-containing sulfonic acid or its salts by known methods. Solution polymerization is preferred as the polymerization method. Examples of acrylic acid salts and vinyl group-containing sulfonic acid salts include those that form salts with countercations derived from basic compounds described later, and preferably sodium or ammonium salts of acrylic acid and sodium or ammonium salts of vinyl group-containing sulfonic acid. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as water, aliphatic alcohols, ketones, ethers, and esters are preferred, and water is more preferred from an economic standpoint. Polymerization initiators and polymerization chain transfer agents can be used during polymerization. Examples of polymerization initiators include persulfates such as ammonium persulfate and potassium persulfate; hypophosphates; and azo compounds such as water-soluble azo polymerization initiators and polymer azo polymerization initiators. Examples of polymerization chain transfer agents include thiols and mercaptans. The polymerization temperature varies depending on the type of polymerization initiator, monomer, and solvent used, but from the viewpoint of promoting the polymerization reaction, it is preferably 50°C or higher, more preferably 60°C or higher, and from the viewpoint of safety, it is preferably 95°C or lower, more preferably 85°C or lower. The polymer dispersant is preferably neutralized with a basic compound, as described later.

[0029] (Physical properties of polymer dispersants) The weight-average molecular weight of the polymer dispersant is preferably 5,000 or more, more preferably 7,000 or more, even more preferably 9,000 or more, and preferably 50,000 or less, more preferably 30,000 or less, even more preferably 20,000 or less, and even more preferably 15,000 or less, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability. The weight-average molecular weight of the polymer is measured by the method described in the examples.

[0030] The acid value of the polymer dispersant is preferably 100 mg KOH / g or more, more preferably 200 mg KOH / g or more, even more preferably 300 mg KOH / g or more, and preferably 1000 mg KOH / g or less, more preferably 900 mg KOH / g or less, and even more preferably 800 mg KOH / g or less, from the viewpoint of dispersing and stabilizing the inorganic pigment in the ink. The acid value of a polymer dispersant is calculated from the mass ratio of monomers containing carboxyl groups in the monomer raw materials that make up the polymer dispersant. In other words, the acid value of a polymer dispersant does not include the acid value derived from sulfonic acid groups.

[0031] (Pigment-containing polymer particles) The inorganic pigment used in the ink of the present invention is held in a dispersed state within the ink by a polymer dispersant. Preferably, the inorganic pigment and polymer dispersant in the ink of the present invention are polymer particles containing the inorganic pigment (hereinafter also referred to as "pigment-containing polymer particles"). Pigment-containing polymer particles refer to particles in which at least a portion of the polymer dispersant is in contact with the inorganic pigment, and the inorganic pigment can be dispersed in the ink. This includes particles in which the polymer dispersant encapsulates the inorganic pigment, particles in which a portion of the inorganic pigment is exposed on the surface of particles composed of the polymer dispersant and inorganic pigment, particles in which the polymer dispersant is adsorbed onto a portion of the inorganic pigment, and mixtures thereof.

[0032] From the viewpoint of obtaining an ink that suppresses changes in whiteness, the mass ratio of polymer dispersant to inorganic pigment (polymer dispersant / inorganic pigment) in the pigment-containing polymer particles is preferably 0.010 or higher, more preferably 0.015 or higher, even more preferably 0.02 or higher, and preferably 0.50 or lower, more preferably 0.30 or lower, and even more preferably 0.10 or lower.

[0033] From the viewpoint of obtaining an ink with excellent whiteness, the content of pigment-containing polymer particles in the ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and even more preferably 9% by mass or more, and preferably 14% by mass or less, more preferably 13% by mass or less, even more preferably 12% by mass or less, and even more preferably 11% by mass or less.

[0034] (Manufacturing of pigment-containing polymer particles) Pigment-containing polymer particles can be efficiently produced by a method that includes a step of dispersing a pigment mixture containing an inorganic pigment, a polymer dispersant, and water to obtain an aqueous dispersion of pigment-containing polymer particles.

[0035] It is preferable that at least a portion of the acidic groups of the polymer dispersant are neutralized using a basic compound. This is thought to increase the charge repulsion force that emerges after neutralization, thereby suppressing the aggregation of pigment-containing polymer particles in the ink and improving the dispersion stability of the pigment-containing polymer particles. When neutralizing a polymer dispersant, it is preferable to neutralize it so that the pH of the aqueous dispersion of pigment-containing polymer particles is between 7 and 11. Examples of basic compounds include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, ammonia, and various amines, with sodium hydroxide and ammonia being preferred. Furthermore, the polymer dispersant may be neutralized beforehand. From the viewpoint of improving the dispersion stability of the polymer dispersant in the ink, the equivalent amount of basic compound used is preferably 10 mol% or more, more preferably 50 mol% or more, even more preferably 80 mol% or more, and preferably 150 mol% or less, more preferably 120 mol% or less, and even more preferably 100 mol% or less.

[0036] In step 1, it is preferable to perform preliminary dispersion as needed before the final dispersion. For pre-dispersion, commonly used mixing and stirring devices such as anchor blades and disperser blades can be used as dispersers. Examples of dispersers used for 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, high-pressure homogenizers and bead mills are preferred from the viewpoint of reducing the particle size of the pigment.

[0037] In the dispersion process, if the medium contains an organic solvent, the organic solvent can be removed by known methods. Preferably, the organic solvent in the resulting aqueous dispersion of pigment-containing polymer particles is substantially removed, but it may remain as long as it does not impair the objective of the present invention. Furthermore, in order to remove coarse particles and the like, it is preferable to further centrifuge the obtained aqueous dispersion and then filter the liquid layer to obtain an aqueous dispersion of pigment-containing polymer particles.

[0038] The concentration of nonvolatile components (solids content) in the resulting aqueous dispersion of pigment-containing polymer particles is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, from the viewpoint of increasing the freedom of ink formulation, and preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 50% by mass or less, from the viewpoint of improving the dispersion stability of the aqueous dispersion of pigment-containing polymer particles. The solid content concentration is measured by the method described in the examples. The average particle size of the pigment-containing polymer particles is preferably 100 nm or more, more preferably 200 nm or more, and even more preferably 250 nm or more, from the viewpoint of productivity of aqueous dispersions of pigment-containing polymer particles, and preferably 400 nm or less, more preferably 350 nm or less, and even more preferably 330 nm or less, from the viewpoint of dispersion stability of pigment-containing polymer particles. The average particle size of the pigment-containing polymer particles is measured by the method described in the examples.

[0039] <Cross-linked polymer particles> The crosslinked polymer particles contained in the ink of the present invention are polymers comprising structural units derived from polymerizable monomers containing carboxyl groups, wherein at least two carboxyl groups of the polymer are crosslinked by a crosslinking agent. From the viewpoint of forming a crosslinked structure by reacting with polyfunctional epoxy compounds or polyfunctional oxazoline groups, and from the viewpoint of obtaining an ink with good discharge stability, it is preferable that the crosslinked polymer particles are particles in which at least two carboxyl groups of the acrylic polymer (A) described later are crosslinked by a crosslinking agent.

[0040] [Acrylic polymer (A)] From the viewpoint of productivity, availability, and economics, the acrylic polymer (A) preferably contains constituent units derived from (meth)acrylic acid (a-1) that react with polyfunctional epoxy compounds or polyfunctional oxazoline groups to form a crosslinked structure.

[0041] (Meth)acrylic acid (a-1) (constituent units derived from this compound) (Meth)acrylic acid (a-1) is one or more selected from the group consisting of acrylic acid and methacrylic acid, and acrylic acid is preferred from the viewpoint of ease of crosslinking of the acrylic polymer (A).

[0042] The content of constituent units derived from (meth)acrylic acid (a-1) in the acrylic polymer (A) is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, of the total constituent units of the acrylic polymer (A), from the viewpoint of forming a crosslinked structure by reacting with a polyfunctional epoxy compound, and from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability. Furthermore, from the same viewpoint as above, it is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less.

[0043] (Constituent units derived from hydrophobic monomer (a-2)) From the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability, the acrylic polymer (A) preferably further contains constituent units derived from the hydrophobic monomer (a-2). From the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, as well as from the viewpoint of availability and economy, the hydrophobic monomer (a-2) is more preferably a (meth)acrylate having a linear or branched alkyl group or an alicyclic alkyl group having 5 to 22 carbon atoms, and even more preferably a (meth)acrylate having an alicyclic alkyl group.

[0044] As the (meth)acrylate having an alicyclic alkyl group with 5 to 22 carbon atoms, a (meth)acrylate having a cycloalkyl group or an isobonyl group is preferred, an acrylate having a cycloalkyl group or an isobonyl group is more preferred, a cycloalkyl acrylate or an isobonyl acrylate is even more preferred, and a cycloalkyl acrylate is even more preferred. Specifically, from the viewpoint of suppressing changes in whiteness and obtaining ink with good ejection stability, as well as from the viewpoint of availability and economic efficiency, cyclohexyl (meth)acrylate and isobonyl (meth)acrylate are preferred, cyclohexyl acrylate and isobonyl acrylate are more preferred, and cyclohexyl acrylate is even more preferred. Examples of linear or branched alkyl groups having 5 to 22 carbon atoms include the n-octyl group, lauryl group, stearyl group, and 2-ethylhexyl group. Specifically, examples include n-octyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. From the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability, n-octyl (meth)acrylate is preferred, and n-octyl acrylate is more preferred.

[0045] As for the hydrophobic monomer (a-2), from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, as well as from the viewpoint of availability and economy, one or more selected from n-octyl (meth)acrylate, isobonyl (meth)acrylate, and cyclohexyl (meth)acrylate are preferred, one or more selected from n-octyl acrylate, isobonyl acrylate, and cyclohexyl acrylate are more preferred, one or more selected from isobonyl acrylate and cyclohexyl acrylate are even more preferred, and cyclohexyl acrylate is even more preferred.

[0046] From the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability, the content of constituent units derived from hydrophobic monomer (a-2) in the acrylic polymer (A) is preferably 40% by mass or more, more preferably 45% by mass or more, and even more preferably 50% by mass or more, and from the same viewpoint as above, preferably 70% by mass or less, more preferably 65% ​​by mass or less, and even more preferably 60% by mass or less.

[0047] (Constituent units derived from other monomers (a-3)) The acrylic polymer (A) may contain constituent units derived from other monomers (a-3) other than (a-1) and (a-2), as long as they do not impede the effects of the present invention. Examples of other monomers (a-3) include ionic monomers other than (a-1), hydrophobic monomers such as alkyl (meth)acrylates having 1 to 4 carbon atoms, and nonionic monomers. From the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, hydrophobic monomers of alkyl (meth)acrylates having 1 to 4 carbon atoms are preferred.

[0048] From the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability, the content of constituent units derived from other monomers (a-3) in the acrylic polymer (A) is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, and from the same viewpoint as above, preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less.

[0049] (Manufacturing of acrylic polymer (A)) The acrylic polymer (A) may be one that has been synthesized as appropriate, or a commercially available product may be used. Acrylic polymer (A) can be produced by copolymerizing raw material monomers containing monomers (a-1) to (a-3) using a known polymerization method. Solution polymerization is preferred as the polymerization method. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as aliphatic alcohols, ketones, ethers, and esters are preferred, and one or more selected from methanol, ethanol, acetone, and methyl ethyl ketone are more preferred. During polymerization, polymerization initiators and polymerization chain transfer agents can be used. Examples of polymerization initiators and polymerization chain transfer agents include those listed in "(Production of Polymer Dispersants)" above. The polymerization temperature varies depending on the type of polymerization initiator, monomer, and solvent used, but from the viewpoint of promoting the polymerization reaction, it is preferably 50°C or higher, more preferably 60°C or higher, and from the viewpoint of safety, it is preferably 95°C or lower, more preferably 85°C or lower.

[0050] (Physical properties of acrylic polymer (A)) The acid value of the acrylic polymer (A) is preferably 100 mg KOH / g or more, more preferably 150 mg KOH / g or more, and even more preferably 180 mg KOH / g or more, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, and also preferably 300 mg KOH / g or less, more preferably 250 mg KOH / g or less, and even more preferably 220 mg KOH / g or less. The weight-average molecular weight of the acrylic polymer (A) is preferably 5,000 or more, more preferably 8,000 or more, and even more preferably 10,000 or more, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, and also preferably 100,000 or less, more preferably 50,000 or less, and even more preferably 30,000 or less, from the same viewpoint as above. The acid value and weight-average molecular weight of acrylic polymer (A) are measured by the method described in the examples. Alternatively, the acid value of acrylic polymer (A) may be calculated from the mass ratio of the monomers constituting acrylic polymer (A).

[0051] [Crosslinking agent] The crosslinking agent for crosslinking at least two carboxyl groups of crosslinked polymer particles is a compound having at least two reactive groups that react with carboxyl groups such as epoxy groups and oxazoline groups to form covalent bonds, and is preferably a compound having one or more reactive groups selected from epoxy groups and oxazoline groups. From the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, the crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule and a polyfunctional oxazoline compound having two or more oxazoline groups in the molecule, and more preferably a polyfunctional epoxy compound.

[0052] (Polyfunctional epoxy compound) The polyfunctional epoxy compound is a polyfunctional epoxy compound having at least two epoxy groups in its molecule, and from the viewpoint of suppressing changes in whiteness and obtaining an ink with excellent discharge stability, it is preferably a polyglycidyl ether compound of a polyhydric alcohol having hydrocarbon groups with 3 to 8 carbon atoms, more preferably one or more selected from the group consisting of trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether, and diethylene glycol diglycidyl ether, and even more preferably 1,6-hexanediol diglycidyl ether. The epoxy group equivalent of the polyfunctional epoxy compound is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and even more preferably 130 or more, and preferably 300 or less, more preferably 200 or less, and even more preferably 170 or less, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability.

[0053] (Polyfunctional oxazoline compounds) A polyfunctional oxazoline compound is a compound having at least two oxazoline groups in its molecule. A polymer containing at least two oxazoline groups (hereinafter also referred to as an "oxazoline group-containing polymer") is preferred as the polyfunctional oxazoline compound. The number-average molecular weight of the oxazoline group-containing polymer is preferably 1,000 or more, more preferably 5,000 or more, even more preferably 10,000 or more, and preferably 100,000 or less, more preferably 50,000 or less, and even more preferably 30,000 or less, from the viewpoint of increasing reactivity with the carboxyl group of the acrylic polymer (A). Examples of oxazoline group-containing polymers include polymers with an acrylic backbone, polymers with a styrene / acrylic backbone, polymers with a styrene backbone, and polymers with an acrylonitrile / styrene backbone.

[0054] The oxazoline group equivalent of the oxazoline group-containing polymer is preferably 100 or more, more preferably 170 or more, even more preferably 200 or more, and preferably 500 or less, more preferably 400 or less, and even more preferably 300 or less, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability. Note that the oxazoline group equivalent refers to the mass of the oxazoline group-containing polymer per mole of oxazoline groups. Examples of commercially available oxazoline group-containing polymers include the "Epocross WS" series, such as "Epocross WS-300," "Epocross WS-500," and "Epocross WS-700" (all manufactured by Nippon Shokubai Co., Ltd., and are water-soluble).

[0055] The content of crosslinked polymer particles in the ink of the present invention is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 3% by mass or more, from the viewpoint of suppressing changes in whiteness, and preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 8% by mass or less, from the viewpoint of obtaining an ink with good discharge stability.

[0056] (Manufacturing of cross-linked polymer particles) Crosslinked polymer particles can be efficiently produced by a method comprising the following steps 1 and 2. Crosslinked polymer particles produced by the method comprising steps 1 and 2 have a crosslinked structure on the surface of the crosslinked polymer particles, which consists of a structure derived from carboxyl groups in the acrylic polymer (A) and a structure derived from the crosslinking agent. Step 1: A step to obtain an aqueous dispersion of acrylic polymer (A) by neutralizing at least a portion of the carboxyl groups of acrylic polymer (A) with an alkali metal compound. Step 2: A process to obtain an aqueous dispersion of crosslinked polymer particles by adding a crosslinking agent to the aqueous dispersion of acrylic polymer (A) obtained in Step 1 and reacting the carboxyl groups in acrylic polymer (A) with the reactive groups in the crosslinking agent.

[0057] (Process 1) Preferably, at least a portion of the carboxyl groups in the acrylic polymer (A) are neutralized using an alkali metal compound. This is thought to increase the charge repulsion force that emerges after neutralization, suppressing aggregation of crosslinked polymer particles in the ink and improving the dispersion stability of the crosslinked polymer particles. In step 1, neutralization is preferably performed so that the pH is between 5 and 10. Examples of alkali metal compounds include one or more selected from alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; alkali metal salts of carbonic acid such as disodium carbonate, sodium bicarbonate, and dipotassium carbonate; and alkali metal salts of boric acid such as sodium borate. Among these, from the viewpoint of availability and economic efficiency, alkali metal hydroxides are preferred, more preferably one or more selected from sodium hydroxide and potassium hydroxide, and even more preferably sodium hydroxide.

[0058] The degree of neutralization of the acrylic polymer (A) is preferably 25 mol% or more, more preferably 30 mol% or more, and even more preferably 35 mol% or more, from the viewpoint of ensuring the dispersion stability of the acrylic polymer (A), and from the viewpoint of the productivity of crosslinked polymer particles, it is preferably 55 mol% or less, more preferably 50 mol% or less, and even more preferably 45 mol% or less. The degree of neutralization (mol%) is calculated using the following formula. Degree of neutralization (mol %) = [Number of moles of alkali metal compound / Number of moles of carboxyl groups in acrylic polymer (A)] × 100 In this invention, if an excess of alkali metal compound is used compared to the number of moles of carboxyl groups in the acrylic polymer (A), the degree of neutralization may exceed 100 mol%.

[0059] (Process 2) In step 2, the temperature at which the carboxyl groups in the acrylic polymer (A) react with the reactive groups in the crosslinking agent is preferably 50°C or higher, more preferably 70°C or higher, and preferably 100°C or lower, and more preferably 95°C or lower, from the viewpoint of completing the crosslinking reaction and economic efficiency. Also, from the same viewpoint as above, the crosslinking treatment time is preferably 1 hour or more, more preferably 3 hours or more, and preferably 10 hours or less, and more preferably 8 hours or less.

[0060] The amount of crosslinking agent used is such that, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, the degree of crosslinking of the resulting crosslinked polymer particles is preferably 10 mol% or more, more preferably 20 mol% or more, and even more preferably 30 mol% or more. Furthermore, from the same viewpoint, the amount of crosslinking agent used is such that the degree of crosslinking of the resulting crosslinked polymer particles is preferably 90 mol% or less, more preferably 80 mol% or less, and even more preferably 70 mol% or less. In step 2, the degree of crosslinking (mol%) of the crosslinked polymer particles can be calculated as the ratio of the number of moles of the functional group of the crosslinking agent to the number of moles of the carboxyl group of the polymer before crosslinking, and the theoretical degree of crosslinking calculated by the following formula may be used. However, if the theoretical degree of crosslinking exceeds 100 mol%, the degree of crosslinking shall be set to 100 mol%. Degree of crosslinking of crosslinked polymer particles (mol%) = [Number of moles of functional groups of the crosslinking agent reacted with acrylic polymer (A) × 100 / Number of moles of carboxyl groups present in acrylic polymer (A)] Specifically, you can calculate it using the following formula. The degree of crosslinking of crosslinked polymer particles (mol%) = [[Amount of crosslinking agent (g) / Equivalent amount of functional groups of the crosslinking agent (g / eq.)] × 100 / [Number of moles of carboxyl groups in 1g of acrylic polymer (A) (mol / g) × Amount of acrylic polymer (A) (g)]]

[0061] In the ink of the present invention, the mass ratio of the content of crosslinked polymer particles to the total content of inorganic pigment and polymer dispersant (crosslinked polymer particles / (total content of inorganic pigment and polymer dispersant)) is preferably 0.15 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and even more preferably 0.4 or more, from the viewpoint of suppressing changes in whiteness, and from the viewpoint of obtaining an ink that suppresses changes in whiteness and has good ejection stability, it is preferably 1.2 or less, more preferably 1.0 or less, even more preferably 0.8 or less, and even more preferably 0.6 or less.

[0062] (Physical properties of cross-linked polymer particles) From the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, the acid value of the crosslinked polymer particles is preferably 40 mg KOH / g or more, more preferably 75 mg KOH / g or more, even more preferably 100 mg KOH / g or more, and from the same viewpoint as above, preferably 270 mg KOH / g or less, more preferably 200 mg KOH / g or less, and even more preferably 160 mg KOH / g or less. In the present invention, the acid value of the crosslinked polymer particles is the acid value of the polymer constituting the crosslinked polymer particles, and is calculated from the acid value of the acrylic polymer (A) before crosslinking, the degree of crosslinking, and the amount of crosslinking agent added.

[0063] The average particle size of the crosslinked polymer particles is preferably 20 nm or more, more preferably 25 nm or more, and even more preferably 30 nm or more, from the viewpoint of productivity of the crosslinked polymer particles, and preferably 100 nm or less, more preferably 90 nm or less, and even more preferably 80 nm or less, from the viewpoint of dispersion stability of the crosslinked polymer particles. The average particle size of the crosslinked polymer particles is measured by the method described in the examples.

[0064] <Water-soluble organic solvents> From the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability, the ink of the present invention preferably contains a water-soluble organic solvent with a boiling point of 100°C or higher and 300°C or lower. The water-soluble organic solvent may be a liquid or a solid at room temperature (25°C). In the present invention, a water-soluble organic solvent refers to an organic solvent in which the amount of solvent dissolved when the organic solvent is dissolved in 100 mL of water at 25°C is 10 mL or more. The boiling point of the water-soluble organic solvent is preferably 110°C or higher, more preferably 130°C or higher, even more preferably 150°C or higher, and even more preferably 170°C or higher, and preferably 290°C or lower, more preferably 270°C or lower, and even more preferably 250°C or lower, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability. Here, boiling point refers to the standard boiling point (boiling point at 1 atmosphere), and when two or more water-soluble organic solvents are used, the weighted average value is used, weighted by the content (mass%) of each water-soluble organic solvent.

[0065] Examples of water-soluble organic solvents include glycol ethers, polyhydric alcohols, nitrogen-containing heterocyclic compounds such as 2-pyrrolidone, and alkanolamines. However, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, one or more selected from glycol ethers and polyhydric alcohols are preferred. As the glycol ether, alkylene glycol monoalkyl ethers and alkylene glycol dialkyl ethers are preferred, and alkylene glycol monoalkyl ethers are more preferred, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability. The number of carbon atoms in the alkyl group of the glycol ether is 1 or more, preferably 2 or more, more preferably 3 or more, and preferably 6 or less, more preferably 5 or less, and even more preferably 4 or less. The alkyl group may be linear or branched.

[0066] Suitable examples of alkylene glycol monoalkyl ethers include ethylene glycol monoisopropyl ether, ethylene glycol monopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monoisobutyl ether, and diethylene glycol monobutyl ether. However, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability, one or more selected from ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, and diethylene glycol monoisobutyl ether are preferred, with diethylene glycol monoisobutyl ether being more preferred.

[0067] As for the polyhydric alcohol, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability, one or more selected from propylene glycol, diethylene glycol, alkanediols having 2 to 6 carbon atoms such as 1,2-hexanediol, glycerin, and polypropylene glycol with a molecular weight of 500 to 1000 are preferred, with propylene glycol being more preferred.

[0068] In the ink of the present invention, the mass ratio of glycol ether content to polyhydric alcohol content (glycol ether / polyhydric alcohol) is preferably 0.15 or higher, more preferably 0.18 or higher, even more preferably 0.20 or higher, and preferably 0.50 or lower, more preferably 0.45 or lower, even more preferably 0.40 or lower, even more preferably 0.35 or lower, and even more preferably 0.30 or lower, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good ejection stability.

[0069] In the present invention, other organic solvents other than water-soluble organic solvents having a boiling point of 100°C to 300°C may be included, as long as the effects of the present invention are not impaired. Examples of other organic solvents include monohydric alcohols such as ethanol, isopropyl alcohol, and n-propyl alcohol.

[0070] 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 even more preferably 20% 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, and even more preferably 30% by mass or less, from the viewpoint of suppressing changes in whiteness and obtaining an ink with good discharge stability.

[0071] <Water> The inkjet ink of the present invention contains water. As the water used in the inkjet ink according to the present invention, pure water or ion-exchanged water is preferred from the viewpoint of preventing the contamination of unintended substances.

[0072] The water content in the ink of the present invention is preferably 45% by mass or more, more preferably 50% by mass or more, and even more preferably 55% by mass or more, from the viewpoint of obtaining an ink that suppresses changes in whiteness, and from the viewpoint of obtaining an ink that suppresses changes in whiteness and has good ejection stability, and from the viewpoint of increasing the freedom of ink formulation, it is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less.

[0073] <Surfactants> From the viewpoint of obtaining an ink with excellent whiteness, the ink of the present invention preferably contains a surfactant. As for the surfactant, nonionic surfactants are preferred from the viewpoint of obtaining an ink with excellent whiteness, one or more selected from acetylene glycol-based surfactants and silicone-based surfactants are more preferred, and using acetylene glycol-based surfactants and silicone-based surfactants in combination is even more preferred.

[0074] (Acetylene glycol-based surfactant) Examples of acetylene glycol-based surfactants include acetylene diols such as 2,4,7,9-tetramethyl-5-decine-4,7-diol, 3,6-dimethyl-4-octin-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, and 2,4-dimethyl-5-hexyn-3-ol, as well as their ethylene oxide adducts. The sum of the average number of moles of ethylene oxy groups (EO) added to the ethylene oxide adduct (n) is preferably 0 or greater, preferably 20 or less, and more preferably 10 or less, from the viewpoint of obtaining an ink with excellent whiteness. The HLB (hydrophilic-lipophilic balance) value of the acetylene glycol-based surfactant is preferably 1 or higher, more preferably 2 or higher, and even more preferably 3 or higher, from the viewpoint of solubility in aqueous inks, and preferably 6 or lower, more preferably 5 or lower, and even more preferably 4 or lower, from the viewpoint of improving the adhesion of the coating film formed by the ink. Examples of commercially available acetylene glycol-based surfactants include the "Surfinol" series and "Orfin" series from Nisshin Chemical Industry Co., Ltd., and the "Acetylenel" series from Kawaken Fine Chemical Co., Ltd.

[0075] (Silicone-based surfactant) Examples of silicone-based surfactants include dimethylpolysiloxane, polyether-modified silicone, amino-modified silicone, and carboxy-modified silicone. However, from the viewpoint of obtaining an ink with excellent whiteness, polyether-modified silicone is preferred. The HLB (hydrophilic-lipophilic balance) value of the polyether-modified silicone surfactant is preferably 8 or higher, more preferably 10 or higher, and even more preferably 13 or higher, from the viewpoint of solubility in aqueous inks. In the present invention, the HLB value is the HLB value determined by the Griffin method. Specific examples of polyether-modified silicone surfactants include the KF series manufactured by Shin-Etsu Chemical Co., Ltd., the Silface SAG series manufactured by Nisshin Chemical Industry Co., Ltd., and the BYK series manufactured by BIC Chemie Japan Co., Ltd. Among these, the KF series manufactured by Shin-Etsu Chemical Co., Ltd. is preferred.

[0076] [Ink manufacturing method] The ink of the present invention can be obtained by mixing an inorganic pigment, a polymer dispersant, crosslinked polymer particles, an organic solvent, water, and, if necessary, various additives commonly used in inks, such as humectants, wetting agents, penetrating agents, surfactants, viscosity modifiers, defoamers, preservatives, antifungal agents, and rust inhibitors. In this case, the inorganic pigment and polymer dispersant may be used in the form of pigment-containing polymer particles.

[0077] (Physical properties of water-based inkjet inks) 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 4 mPa·s or more, and preferably 11 mPa·s or less, more preferably 8 mPa·s or less, and even more preferably 6 mPa·s or less, from the viewpoint of improving ejection stability and image density. The viscosity of water-based inks can be measured using an E-type viscometer.

[0078] From the viewpoint of storage stability and improving 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. Furthermore, from the viewpoint of material 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 the method described in the examples.

[0079] The ink of the present invention can be loaded into an inkjet recording device having a known circulation device such as a piezo, thermal, or electrostatic type, and ejected as ink droplets onto a recording medium to record characters, images, and the like.

[0080] In printing using the ink of the present invention, not only highly absorbent plain paper but also low-absorbent recording media can be used as printing substrates. Examples of low-absorbent recording media include low-absorbent coated paper and non-absorbent polymer films. Examples of low-absorbent coated papers include general-purpose glossy paper and multi-color foam gloss paper. Examples of polymer films include transparent synthetic polymer films, such as polyester, polyvinyl chloride, polyolefin, and nylon films. These films may be biaxially oriented, uniaxially oriented, or unoriented. Among these, polyester films and stretched polypropylene films are preferred from the viewpoint of improving the adhesion of the coating film formed by the ink, and corona discharge treated polyethylene terephthalate (PET) films and corona discharge treated biaxially oriented polypropylene (OPP) films are more preferred. [Examples]

[0081] In the following manufacturing examples, embodiments, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" unless otherwise specified. The measurement methods for each physical property are as follows.

[0082] (1) Measurement of the weight-average molecular weight of polymer dispersants and acrylic polymers The molecules were measured using gel permeation chromatography (using a Tosoh Corporation GPC instrument (HLC-8320GPC), Tosoh Corporation columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolum Super AW-H), and a flow rate of 0.5 mL / min) with known molecular weight monodisperse polystyrene kits (PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick C (F-288, F-40, F-4, A-5000, A-500), Tosoh Corporation) as standard substances. The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the above eluent in a glass vial, stirring with a magnetic stirrer at 25°C for 10 hours, and filtering through a syringe filter (DISMIC-13HP, made of PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0083] (2) Measurement of the average primary particle size of inorganic pigments (titanium dioxide) The average primary particle diameter of inorganic pigments (titanium dioxide) was determined by extracting 500 primary particles of inorganic pigment (titanium dioxide) using a transmission electron microscope "JEM-2100" (manufactured by JEOL Ltd.) and measuring their particle diameters. The average of these measurements was then calculated to obtain the arithmetic mean particle diameter. If the inorganic pigment (titanium dioxide) had both a major and minor axis, the major axis was used for the calculation.

[0084] (3) Measurement of the average particle size of pigment-containing polymer particles and crosslinked polymer particles Cumulant analysis was performed using the laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.) to measure the average particle size. The concentration of the particles to be measured was 5 × 10⁻⁶. -3 A dispersion of pigment-containing polymer particles or cross-linked polymer particles, diluted with water to a mass percent (converted to solid content concentration), was used. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 cumulative measurements. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent, and the resulting cumulant average particle size was taken as the average particle size of the pigment-containing polymer particles and cross-linked polymer particles.

[0085] (4) Measurement of solid content concentration of aqueous solutions of polymer dispersants and aqueous dispersions of acrylic polymers Using an infrared moisture meter "FD-230" (manufactured by Kett Scientific Research Institute Co., Ltd.), 5g of the sample was dried at a drying temperature of 150°C and measurement mode 96 (monitoring time 2.5 minutes / variation range 0.05%). The moisture content (%) of the sample was then measured, and the solid content concentration was calculated using the following formula. Solid content concentration (%) = 100 - Moisture content of the sample (%)

[0086] (5) Measurement of the solid content concentration of aqueous dispersions of pigment-containing polymer particles and crosslinked polymer particles 10.0 g of sodium sulfate, which had been stabilized in a desiccator, was accurately weighed into a 30 mL polypropylene container (40 mm in diameter, 30 mm in height). Approximately 1.0 g of the aqueous dispersion sample was added, mixed, and then accurately weighed. The mixture was maintained at 105°C for 2 hours to remove volatile components, and then left in the desiccator for another 15 minutes. The mass was then accurately weighed. The mass of the sample after removal of volatile components was taken as the solid content, and the solid content concentration (%) was obtained by dividing it by the mass of the added aqueous dispersion sample.

[0087] (6) Measurement of the acid value of acrylic polymers A solution of polymer dissolved in a titration solvent of toluene and acetone (volume ratio 2:1) was titrated with a 0.1N potassium hydroxide / ethanol solution using a potentiometric automatic titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610) by potentiometric titration. The inflection point on the titration curve was defined as the endpoint, and the acid value (mgKOH / g) was calculated from the amount of potassium hydroxide solution titrated to the endpoint.

[0088] (7) Measurement of pH of aqueous solution of polymer dispersant, aqueous dispersion of pigment-containing polymer particles, and inkjet ink Using a benchtop pH meter (Horiba, Ltd. "F-71") equipped with a pH electrode (Horiba, Ltd. "6337-10D"), the pH of an aqueous solution of polymer dispersant, an aqueous dispersion of pigment-containing polymer particles, and inkjet ink were measured at 25°C.

[0089] Manufacturing Example I-1 (Manufacturing of Polymer Dispersant P1) 45 g of deionized water was placed in a 500 mL glass reaction vessel equipped with a dropping funnel, and the temperature was raised to 75°C under a nitrogen atmosphere. Next, under a nitrogen gas atmosphere, three solutions were simultaneously and gradually added dropwise to the reaction vessel over a period of 3 hours: 60.0 g of acrylic acid (manufactured by Tokyo Chemical Industry Co., Ltd.) and 80.0 g of a 50% aqueous solution of sodium 2-acrylamido-2-methylpropanesulfonate (manufactured by Tokyo Chemical Industry Co., Ltd.) as dropwise solution 1; 4.7 g of a 15% aqueous solution of sodium persulfate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as dropwise solution 2; and 22.6 g of a 30% aqueous solution of sodium hypophosphite (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as dropwise solution 3. After the dropwise addition was complete, the mixture was allowed to mature at 75°C for 1 hour. Subsequently, the solution was cooled to 40°C, and 69.4 g of a 48% sodium hydroxide aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 14.1 g of deionized water were added to adjust the solid content to 40% and the degree of neutralization to 100 mol%, thereby obtaining an aqueous solution of polymer dispersant P1. The weight-average molecular weight (Mw) of polymer dispersant P1 is shown in Table 1.

[0090] Manufacturing Example I-2 (Manufacturing of Polymer Dispersant P2) An aqueous solution of polymer dispersant P2 with a solid content of 40% and a degree of neutralization of 100 mol% was obtained in the same manner as in Production Example 1, except that the conditions shown in Table 1 were changed. The weight-average molecular weight (Mw) of polymer dispersant P2 is shown in Table 1.

[0091] [Table 1]

[0092] Manufacturing Example I-3 (Manufacturing of Polymer Dispersant P3) According to Production Example 2 of Japanese Patent Publication No. 2017-75302, an aqueous solution (solid content concentration 40%, degree of neutralization 50 mol%) of polymer dispersant P3 containing constituent units derived from methacrylic acid and constituent units derived from methoxypolyethylene glycol monomethacrylate was obtained. The weight-average molecular weight (Mw) of polymer dispersant P3 was 24,000.

[0093] Manufacturing Example IA-1 (Production of aqueous dispersion D1 of pigment-containing polymer particles) In a 2L plastic container, 19.2g of an aqueous solution of polymer dispersant P1, 300.0g of titanium dioxide (manufactured by Ishihara Sangyo Co., Ltd., CI Pigment White 6, product name: Typake CR-50, rutile type, Al treated, average primary particle size 250nm), and 450.0g of deionized water were added. 1000g of 1.5mm zirconia beads were then added, and the mixture was dispersed at 250rpm for 8 hours using a tabletop pot mill stand (AS ONE Corporation). The zirconia beads were removed using a metal mesh, and the solid content concentration was adjusted with deionized water to obtain an aqueous dispersion of pigment-containing polymer particles D1 (solid content concentration 40%, titanium dioxide concentration 39%, polymer dispersant P1 concentration 1%). Table 2 shows the pH of the aqueous dispersion of pigment-containing polymer particles D1 and the average particle size of the pigment-containing polymer particles in the aqueous dispersion of pigment-containing polymer particles D1.

[0094] Manufacturing Example IA-2 (Production of Aqueous Dispersion D2 of Pigment-Containing Polymer Particles) An aqueous dispersion of pigment-containing polymer particles D2 (solids content 40%, titanium dioxide concentration 39%, polymer dispersant P2 concentration 1%) was obtained in the same manner as in Production Example IA-1, except that the aqueous solution of polymer dispersant P1 was replaced with an aqueous solution of polymer dispersant P2. The pH of the aqueous dispersion of pigment-containing polymer particles D2 and the average particle size of the pigment-containing polymer particles in the aqueous dispersion of pigment-containing polymer particles D2 are shown in Table 2.

[0095] Manufacturing Example IA-3 (Production of Aqueous Dispersion D3 of Pigment-Containing Polymer Particles) 40.0 g of polyacrylic acid-based dispersant (manufactured by Toagosei Co., Ltd., Aron A-10SL, Mw5000, acid value 779 mg KOH / g, solid content concentration 40%) and 35.9 g of deionized water were added to a 250 mL plastic container, and the container was cooled in an ice bath. While cooling in the ice bath, the solution was stirred at 100 rpm, and 26.3 g of 5N sodium hydroxide aqueous solution was slowly added, followed by the addition of deionized water to adjust the solid content concentration to 20% and the degree of neutralization to 50 mol%, thereby obtaining a neutralized aqueous solution of the polyacrylic acid-based dispersant. In a 2L plastic container, 38.4g of the neutralized aqueous solution of the polyacrylic acid-based dispersant prepared above, 300.0g of titanium dioxide (manufactured by Ishihara Sangyo Co., Ltd., CI Pigment White 6, product name: Typeque CR-50, rutile type, Al treated, average primary particle size 250nm), and 430.8g of deionized water were added. 1000g of 1.5mm zirconia beads were then added, and the mixture was dispersed at 250rpm for 8 hours using a tabletop pot mill stand (AS ONE Corporation). The zirconia beads were removed using a metal mesh, and the solid content concentration was adjusted to 40% with deionized water to obtain aqueous dispersion D3 of pigment-containing polymer particles. Table 2 shows the pH of aqueous dispersion D3 of pigment-containing polymer particles and the average particle size of the pigment-containing polymer particles in aqueous dispersion D3.

[0096] Manufacturing Example IA-4 (Production of Aqueous Dispersion D4 of Pigment-Containing Polymer Particles) An aqueous dispersion of pigment-containing polymer particles D4 (solids concentration 40%, titanium dioxide concentration 39%, polymer dispersant P3 concentration 1%) was obtained in the same manner as in Production Example IA-1, except that the aqueous solution of polymer dispersant P1 was replaced with an aqueous solution of polymer dispersant P3. Table 2 shows the pH of the aqueous dispersion of pigment-containing polymer particles D4 and the average particle size of the pigment-containing polymer particles in the aqueous dispersion of pigment-containing polymer particles D4.

[0097] [Table 2]

[0098] <Production of aqueous dispersions of crosslinked polymer particles> (Manufacturing of acrylic polymer (A)) Manufacturing Example II-1 (Manufacturing of Acrylic Polymer A1) A monomer mixture was prepared by mixing 25.7 parts of acrylic acid, 55.0 parts of cyclohexyl acrylate, 19.3 parts of butyl acrylate, and 100 parts of methyl ethyl ketone (hereinafter referred to as "MEK"). Furthermore, a polymerization initiator solution was prepared by mixing 1.1 parts of 2,2'-azobis-(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65) and 12.7 parts of MEK as polymerization initiators. In a reaction vessel equipped with a stirrer, reflux condenser, and two dropping funnels, 10% of the monomer mixture was added as an initial charge, and the vessel was thoroughly purged with nitrogen gas. In one dropping funnel, a mixture of the remaining 90% of the monomer mixture and 1.2 parts of 2-mercaptoethanol as a polymerization chain transfer agent was prepared. In the other dropping funnel, 80% of the polymerization initiator solution was added. Under a nitrogen atmosphere, the contents of the reaction vessel were stirred while the temperature was raised to 65°C. The monomer mixture and polymerization initiator solution from two dropping funnels were then continuously added to the reaction vessel over 3 hours. After the addition was complete, the reaction was carried out at 65°C for 2 hours. Subsequently, the remaining 20% ​​of the polymerization initiator solution was added, and the reaction was carried out again at 65°C for 2 hours. After raising the temperature to 70°C, the reaction was carried out for another 2 hours. After cooling to room temperature, MEK was removed by vacuum drying to obtain acrylic polymer A1 (acid value 200 mg KOH / g, weight-average molecular weight 21,000).

[0099] Manufacturing Examples II-2 and II-3 (Manufacturing of Acrylic Polymers A2 and A3) Acrylic polymers A2 and A3 were produced in the same manner as in Production Example II-1, except that the raw material monomers were changed as shown in Table 3. The acid value and weight-average molecular weight (Mw) of acrylic polymers A2 and A3 are shown in Table 3.

[0100] [Table 3]

[0101] (Production of aqueous dispersions of crosslinked polymer particles) Manufacturing Example III-1 (Production of EM1 aqueous dispersion of cross-linked polymer particles) (Process 1) 35.0 parts of acrylic polymer A1 were dissolved in 65.0 parts of MEK, and 11.9 g of 5N sodium hydroxide aqueous solution (16.9% sodium hydroxide solids, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration) was added to achieve a neutralization degree of 40 mol% of acrylic polymer A1. Subsequently, 200.0 parts of deionized water were added over 1 hour. After the addition was complete, the MEK and deionized water were removed using an evaporator until the solids content was 20% by mass, obtaining an aqueous dispersion of acrylic polymer A1. (Process 2) To 100.0 parts of the aqueous dispersion of acrylic polymer A1 obtained in step 1, 3.9 parts of 1,6-hexanediol diglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-212, epoxy equivalent 150) were added as a crosslinking agent so that the degree of crosslinking with respect to the carboxyl groups was 35 mol%, and then 20.4 g of deionized water was added and mixed. The mixture was then heated at 90°C for 5 hours with stirring to react the carboxyl groups in acrylic polymer A1 with the epoxy groups in 1,6-hexanediol diglycidyl ether. The mixture was then cooled to 25°C and filtered through a 5 μm pore size filter (acetylcellulose membrane, outer diameter 2.5 cm, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to obtain an aqueous dispersion of crosslinked polymer particles EM1 with a solid content concentration of 20% by mass. The average particle size of the crosslinked polymer particles in the aqueous dispersion EM1 was 38 nm.

[0102] Manufacturing Examples III-2 and III-3 (Production of Aqueous Dispersions EM2 and EM3 of Crosslinked Polymer Particles) Aqueous dispersions of crosslinked polymer particles EM2 and EM3 were prepared in the same manner as in Production Example III-1, except that acrylic polymer (A) was changed to the acrylic polymer shown in Table 4. The degree of crosslinking and the average particle size of the crosslinked polymer particles in the aqueous dispersions EM2 and EM3 are shown in Table 4.

[0103] Manufacturing Example III-4 (Production of EM4, an aqueous dispersion of acrylic polymer particles) Except for omitting step 2, an aqueous dispersion of acrylic polymer A1, EM4, was prepared in the same manner as in Production Example III-1. The average particle size of acrylic polymer A1 in the aqueous dispersion EM4 of acrylic polymer A1 is shown in Table 4.

[0104] [Table 4]

[0105] Manufacturing Example III-5 (Manufacturing of EM5, an aqueous dispersion of polymer particles) A aqueous dispersion of styrene-acrylic acid copolymer particles, EM5 (solid content concentration 20% by mass), was obtained according to the method described in paragraph

[0066] of Japanese Patent Publication No. 2020-84072, "Preparation of Joncryl690 NaOH aqueous solution". The styrene-acrylic acid copolymer particles in the aqueous dispersion of styrene-acrylic acid copolymer particles, EM5, do not have a crosslinking structure.

[0106] Example 1 (Manufacturing of water-based white ink 1 for inkjet recording) 25.5g of aqueous dispersion D1 of pigment-containing polymer particles (solid content concentration 40%), 25.0g of aqueous dispersion EM1 of crosslinked polymer particles (solid content concentration 20%), 5.0g of diethylene glycol monoisobutyl ether (manufactured by Nippon Emulsifier Co., Ltd.), 20.0g of propylene glycol, 1.0g of acetylene glycol-based surfactant (manufactured by Kawaken Fine Chemical Co., Ltd., product name: Surfinol 104PG50, 2,4,7,9-tetramethyl-5-decine-4,7-diol, HLB 3.0), propylene glycol solution with an effective content of 50%), 0.3g of silicone-based surfactant (manufactured by Shin-Etsu Chemical Co., Ltd., product name: KF-6011, HLB 14.5), and 1N sodium hydroxide aqueous solution (manufactured by Fujifilm Wako Pure Chemical Corporation). A volumetric analysis solution was prepared by mixing an amount of water that resulted in an ink pH of 8.5. Deionized water was then added to bring the total volume to 100 parts. The mixture was filtered through a membrane filter (Sartorius, product name: Minisart syringe filter, pore size: 5.0 μm, material: cellulose acetate) to obtain water-based white ink for inkjet recording 1 (solid content 15.2%, titanium dioxide 10%, polymer 5.2% (polymer dispersant P1 0.2%, cross-linked polymer particles 5.0%)). The viscosity of water-based white ink for inkjet recording 1 was 4.6 mPa·s.

[0107] Examples 2-8 and Comparative Examples 1-3 (Production of Examples 2-8 and C1-C3) In the same procedure as in Example 1, except that the formulation of each component was changed as shown in Table 5, water-based white inks 2-8 and C1-C3 for inkjet recording were obtained.

[0108] The change in whiteness was evaluated using the water-based white inkjet inks 1-8 and C1-C3 obtained in the above examples and comparative examples, according to the method shown in [1] below. Furthermore, the nozzle normality rate was evaluated according to the method shown in [2] below. The results are shown in Table 5.

[0109] [1] Evaluation of changes in whiteness Under conditions of 25±1℃ temperature and 30±5% relative humidity, an inkjet ejection evaluation system (ImageXpert "JetXpert") equipped with an inkjet recording head (Fujifilm Corporation "Samba G3L", piezo type) and a circulation device (Megnajet Corporation "CIMS II") was filled with various water-based inkjet recording inks. The system was set to a head voltage of 30V, a frequency of 30kHz, a push-pull drive waveform, and an appropriate ejection liquid volume of 2.5pL. The recording medium was fixed to the transport table under reduced pressure with an A4-sized polyester film (Futamura Chemical Co., Ltd., FE-2001) oriented so that its longitudinal direction and transport direction were the same. An ink ejection command was transferred to the inkjet ejection evaluation system, and a solid image 1 with 100% duty cycle was printed. With the printed surface of the final printed polyester film facing upwards, the transmittance density of the obtained solid image 1 was measured using a portable monochrome transmittance densitometer (Ihara Electronics Industry Co., Ltd., T5 Plus). After printing solid color image 1, the ink flow rate in the inkjet head's circulation channel was set to 30 mL / min, and the circulation system was left running for 60 hours. Solid color image 2 was printed under the same conditions as solid color image 1, and the transmittance density of solid color image 2 was measured under the same conditions as the transmittance density of solid color image 1. From the transmittance densities of solid color image 1 and solid color image 2, the change in whiteness of the solid color image before and after 60 hours of ink circulation (transmittance density of solid color image 1 - transmittance density of solid color image 2) was calculated. The results are shown in Table 5. A smaller change in whiteness indicates better suppression of whiteness change, and a value of 0.15 or less is considered practically usable.

[0110] [2] Evaluation of nozzle normality rate Under conditions of 25±1℃ temperature and 30±5% relative humidity, the inkjet ejection evaluation device used for "evaluation of changes in whiteness" was filled with each inkjet recording water-based ink. The ink flow rate in the inkjet head circulation channel was set to 30 mL / min, and the circulation device was left running for 60 hours. After that, the head voltage was set to 30V, the frequency to 30kHz, the drive waveform to be a push-pull type, and the appropriate amount of ejected liquid to 2.5 pL. An ink ejection command was sent to the inkjet ejection evaluation device, and the number of nozzles that ejected each inkjet recording water-based ink normally was confirmed. The nozzle normality rate (%) was calculated using the formula below, and the ejection stability was evaluated. The results are shown in Table 5. The total number of nozzles in the head is 2048. A higher nozzle normality rate (%) indicates better ejection stability, and a value of 60% or less is considered to be practically usable. Nozzle Normality Rate (%) = (Number of Normal Discharge Nozzles / Total Number of Nozzles in the Head 2048) × 100

[0111] [Table 5]

[0112] Table 5 shows that, compared to the water-based white inks C1-C3 obtained in the examples, the ink whiteness of the water-based white inks 1-8 obtained in the comparative examples was suppressed even when printing was paused and left for a long period of time when using a circulation device, resulting in good ejection stability. In Comparative Example 1, the polymer dispersant has methacrylic acid-derived structural units instead of acrylic acid-derived structural units, which is presumed to have reduced redispersibility of the pigment-containing polymer particles and a lower nozzle normality rate. In Comparative Examples 2 and 3, because the polymer particles are not crosslinked, when printing is paused and left for a long time while using the circulation device, aggregates tend to form between the polymer particles and the pigment-containing polymer particles. These aggregates clog the circulation channel and the vicinity of the inkjet head nozzles, which is presumed to have reduced whiteness.

Claims

1. A water-based white ink for inkjet recording, containing an inorganic pigment, a polymer dispersant, and crosslinked polymer particles, The polymer dispersant contains a vinyl polymer comprising at least one selected from structural units derived from acrylic acid and structural units derived from sulfonic acid having a vinyl group. The crosslinked polymer particles include structural units derived from polymerizable monomers containing carboxyl groups, and at least two of these carboxyl groups are crosslinked by a structure derived from a crosslinking agent. Water-based white ink for inkjet recording.

2. The aqueous white ink according to claim 1, wherein the total content of constituent units derived from acrylic acid and constituent units derived from sulfonic acid having a vinyl group in the vinyl polymer is 90% by mass or more.

3. The aqueous white ink according to claim 1 or 2, wherein the vinyl polymer comprises both a structural unit derived from acrylic acid and a structural unit derived from sulfonic acid having a vinyl group.

4. The aqueous white ink according to claim 1 or 2, wherein the crosslinking agent has one or more functional groups selected from epoxy groups and oxazoline groups.

5. The aqueous white ink according to claim 1 or 2, wherein the degree of crosslinking of the crosslinked polymer particles is 10 mol% or more and 90 mol% or less.

6. The aqueous white ink according to claim 1 or 2, wherein the crosslinked polymer particles include constituent units derived from a hydrophobic monomer (a-2).

7. The aqueous white ink according to claim 6, wherein the constituent unit derived from the hydrophobic monomer (a-2) includes a constituent unit derived from a (meth)acrylate having an alicyclic alkyl group.

8. The aqueous white ink according to claim 1 or 2, wherein the mass ratio of the content of the crosslinked polymer particles to the total content of the inorganic pigment and the polymer dispersant (crosslinked polymer particles / total content of inorganic pigment and polymer dispersant) is 0.01 or more and 10 or less.

9. The aqueous white ink according to claim 1 or 2, wherein the inorganic pigment is titanium dioxide.

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