Water-based ink set for inkjet recording

Abstract

The present invention relates to providing a water-based ink set for inkjet printing that can produce a recorded product in which bleeding of characters is suppressed even when characters printed with black ink and a background printed with color ink are printed overlapping each other, regardless of the printing order of the black ink and the color ink, and in which solid color areas are uniformly wetted and spread to provide good solid filling, a method for producing the same, and a printing method using the water-based ink set for inkjet printing. [Solution] A water-based ink set for inkjet recording comprising at least one black ink and one color ink, each inkjet ink containing a pigment, a poorly water-soluble resin, a nonionic surfactant, an organic solvent, and water, wherein the content of organic solvents having a boiling point exceeding 250°C at 1 atmospheric pressure relative to the total amount of inkjet ink in each inkjet ink is 5% by mass or less, and where the total solids content in each black ink is X% by mass and the total solids content in each color ink is Y% by mass, the values ​​of X and Y are both 0.1 or more and 5.0 or less, and the total content (% by mass) of nonionic surfactants in each black ink is less than the total content (% by mass) of nonionic surfactants in each color ink.

Description

[Technical Field]

[0001] The present invention relates to a water-based ink set for ink-jet printing, a method for producing the same, and a printing method using the water-based ink set for ink-jet printing. [Background technology]

[0002] Inkjet recording is a method of printing characters and images by ejecting ink droplets directly from minute nozzles onto a recording medium. This method has many advantages, including the ease of full-color printing, low cost, the ability to use plain paper as a recording medium, and non-contact printing. In recent years, the spread of digital printing has led to its use not only in consumer printing but also in commercial and industrial printing using low-absorbency coated paper, etc., and there is an increasing demand for inkjet inks with the aims of increasing printing speed, improving image quality, and reducing the environmental impact. In order to meet such demands, various inkjet inks have been proposed, on the premise that they satisfy basic performance requirements such as ejection stability and storage stability.

[0003] For example, Patent Document 1 aims to provide an inkjet recording method that can obtain good images without unevenness or color mixing without thermally deforming the medium when used to print on a low-liquid-absorbent recording medium, and discloses an ink set used in this recording method in which inks are ejected in order of decreasing static surface tension. Furthermore, Patent Document 2 discloses an ink set in which the solids concentration of the black ink is 2.5% by weight or more higher than the solids concentration of each color ink, with the aim of providing an ink set for inkjet recording that does not cause color bleeding even on low-liquid-absorbent coated printing paper. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-222146 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-111752 Summary of the Invention [Problem to be solved by the invention]

[0005] In inkjet recording systems, characters are sometimes printed in black ink, and then solid color inks are printed so that they overlap the characters. The reason for printing in layers is to make it difficult to identify printing defects caused by misalignment of the color ink droplets due to misalignment of the recording head or uneven transport due to bending of the mechanically transported recording medium. However, to the inventor's knowledge, there is no inkjet ink set that can print characters without bleeding and background colors evenly, regardless of the printing order of the black and color inks. For example, it has been found that the ink set described in Patent Document 1 has a problem in that if the black ink is printed after the color inks, the characters printed with the black ink bleed. Also, it has been found that the ink set for inkjet recording described in Patent Document 2 has a problem in that the characters printed with the black ink bleed, regardless of the printing order.

[0006] The present invention relates to providing a water-based ink set for inkjet printing that can produce a recorded product in which bleeding of characters is suppressed even when characters printed with black ink and a background printed with color ink are printed overlapping each other, regardless of the printing order of the black ink and the color ink, and in which solid color areas are uniformly wetted and spread to provide good solid filling, a method for producing the same, and a printing method using the water-based ink set for inkjet printing. [Means for solving the problem]

[0007] The present invention relates to the following [1]. [1] A water-based ink set for inkjet recording comprising at least one black ink and one color ink, each inkjet ink comprising a pigment, a poorly water-soluble resin, a nonionic surfactant, an organic solvent, and water, wherein the content of organic solvents having a boiling point of higher than 250°C at 1 atmospheric pressure relative to the total amount of inkjet ink in each inkjet ink is 5% by mass or less, and where the total solid content in each black ink is X% by mass and the total solid content in each color ink is Y% by mass, the values ​​of X and Y are both 0.1 or more and 5.0 or less, and the total content (% by mass) of nonionic surfactants in each black ink is less than the total content (% by mass) of nonionic surfactants in each color ink. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a water-based ink set for inkjet printing, which is capable of producing a recorded matter in which bleeding of characters is suppressed even when characters printed with black ink and a background printed with color ink are printed overlapping each other, regardless of the printing order of the black ink and the color ink, and in which solid color areas are uniformly wetted and spread to provide good solid filling, as well as a method for producing the same, and a printing method using the water-based ink set for inkjet printing. DETAILED DESCRIPTION OF THE INVENTION

[0009] As a result of extensive research into the above-mentioned problems, the present inventors have newly discovered that the above-mentioned problems can be solved by limiting the content of a specific organic solvent to a specific amount or less, making the total solids content (% by mass) in each black ink a specific amount greater than the total solids content (% by mass) in each color ink, and making the total content (% by mass) of nonionic surfactants in each black ink less than the total content (% by mass) of nonionic surfactants in each color ink. Although the mechanism behind this is unclear, the following reasons are thought to be the cause. When black ink is printed first and then color inks are printed, the amount of highly wettable nonionic surfactant is low, making it less likely to spread. Furthermore, because the black ink printed first has a high solids content, its concentrated viscosity increases as it dries, solidifying it on the recording medium before the color inks are ejected and land on the recording medium, making it less likely to bleed. Furthermore, poorly water-soluble resins and pigments dispersed in resins are hydrophobic, so they adsorb hydrophobic nonionic surfactants, which contribute to wetting. Therefore, the surfactant of the color ink printed later is adsorbed by the resin contained in large amounts in the black ink forming the previously printed characters, minimizing the wetting and spreading of the color ink on the black ink, thereby preventing bleeding of the characters. On the other hand, because the color inks contain large amounts of nonionic surfactants, they are thought to be able to spread evenly and achieve good solid coverage in background areas where black ink is not printed. Next, when printing color inks and then black ink, the color ink printed first contains a large amount of nonionic surfactant, allowing for uniform wetting and spreading, resulting in good solid coverage. However, because the color ink contains more nonionic surfactant than the black ink, its dynamic surface tension decreases quickly, creating a surface tension difference between it and the black ink printed later, which contains less nonionic surfactant. This creates the risk that the color ink, with its lower surface tension, will erode the black ink, with its higher surface tension. However, because the black ink in this ink set has a high total solids content, the hydrophobic particle components in the black ink adsorb the nonionic surfactant in overlapping areas and at the boundary between the text and background, minimizing erosion of the color ink. Furthermore, this increases the concentrated viscosity, allowing the black ink to quickly solidify on the recording medium. These combined factors are thought to suppress bleeding in the text areas. Furthermore, by keeping the content of organic solvents with a boiling point of greater than 250°C at 1 atmosphere relative to the total amount of ink to 5% by mass or less, it is possible to prevent a significant decrease in the drying properties of the ink, and it is believed that the above-mentioned effects will be more likely to be achieved even in recording methods with high printing speeds, such as one-pass printing.

[0010] [Water-based ink set for inkjet printing] The water-based ink set for inkjet printing (hereinafter also simply referred to as "ink set") of the present invention comprises two or more inkjet inks, and comprises at least one black ink and one color ink. The color inks may comprise one or more known color inks such as yellow ink, magenta ink, cyan ink, green ink, orange ink, violet ink, and white ink. Preferred embodiments include ink sets comprising black ink, yellow ink, magenta ink, and cyan ink, and ink sets comprising green ink, orange ink, violet ink, and / or white ink. Furthermore, the ink set may also comprise, in addition to the black ink and the color inks, a pretreatment agent effective for improving image quality and a post-treatment agent effective for protecting the ink coating.

[0011] [Inkjet ink] The ink-jet ink of the present invention (hereinafter also simply referred to as "ink") contains a pigment, a poorly water-soluble resin, a nonionic surfactant, an organic solvent, and water. An inkjet ink is an ink used in inkjet printing, and the inkjet ink of the present invention is a water-based inkjet ink in which water accounts for the largest proportion by mass of the medium it contains.

[0012] The definitions of various terms used in this specification are shown below. "Printing" is a concept that includes printing and printing out characters and images, and "printed matter" is a concept that includes printed matter and printed out matter on which characters and images are recorded. "(Meth)acrylic acid" means at least one selected from the group consisting of acrylic acid and methacrylic acid. The term "(meth)acrylate" refers to at least one selected from the group consisting of acrylate and methacrylate.

[0013] <Pigments> The pigment contained in the ink-jet ink of 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, magnesium oxide, and other metal oxides. 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 pigments can be used alone or in combination of two or more. In this patent, black ink refers to ink containing carbon black as a pigment, and color ink refers to ink containing a pigment other than carbon black.

[0014] Suitable forms of the pigment include a pigment that can maintain a dispersed state without a dispersant, i.e., a self-dispersing pigment, pigment particles in which the pigment is dispersed with a low-molecular-weight or high-molecular-weight surfactant, and resin particles containing the pigment. Among these, the form of resin particles containing the pigment is preferred from the viewpoint of the dispersion stability and fixability of the pigment. Furthermore, the resin of the resin particles containing the pigment may be either a non-crosslinked resin or a crosslinked resin. Here, "resin particles containing a pigment" (hereinafter also referred to as "pigment-containing resin particles") means particles in which a pigment is encapsulated in a resin, particles made of a resin and a pigment with part of the pigment exposed on the surface, particles in which a resin is adsorbed to part of the pigment, or a mixture thereof. Of these, particles in which a pigment is encapsulated in a resin are more preferred.

[0015] [Pigment-containing resin particles] The pigment-dispersing resin constituting the pigment-containing resin particles is sufficient as long as it has at least the pigment dispersibility to disperse the pigment in an aqueous medium, and may be either water-soluble or poorly water-soluble, but is preferably a poorly water-soluble resin. Here, "poorly water-soluble" means that when a resin that has been dried at 105°C for 2 hours and reached a constant weight is dissolved in 100 g of water at 25°C until saturation is reached, the amount of dissolution is 10 g or less. When the poorly water-soluble resin is an anionic resin, the amount of dissolution is the amount of dissolution when the anionic groups of the resin are 100% neutralized with sodium hydroxide. When the pigment dispersing resin constituting the pigment-containing resin particles is poorly water-soluble, it may be a resin having the same structural units and physical properties as the poorly water-soluble resin described below, or may be a resin having different structural units or physical properties.

[0016] [Pigment Dispersion Resin] Examples of the pigment dispersion resin include vinyl resins, polyester resins, polyurethane resins, etc. Among these, vinyl resins obtained by addition polymerization of vinyl monomers are preferred from the viewpoint of storage stability and ejection stability of the inkjet ink. Such vinyl resins preferably contain (a-1) structural units derived from hydrophilic ionic monomers and (a-2) structural units derived from hydrophobic monomers, and may further contain (a-3) structural units derived from hydrophilic nonionic monomers. The "hydrophobic" of hydrophobic monomers refers to their property of excluding water molecules and being incompatible with water. The "hydrophilic" of hydrophilic ionic monomers and hydrophilic nonionic monomers refers to their property of forming weak bonds with water molecules through electrostatic interactions, hydrogen bonds, etc., and exhibiting compatibility with water.

[0017] [(a-1) Hydrophilic ionic monomer] (a-1) The hydrophilic ionic monomer is preferably an anionic monomer, and examples thereof include a carboxylic acid monomer and a sulfonic acid monomer, with a carboxylic acid monomer being more preferred. The carboxylic acid monomer may be at least one selected from acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, with at least one selected from acrylic acid and methacrylic acid being more preferred.

[0018] [(a-2) Hydrophobic Monomer] 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.

[0019] 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.

[0020] [(a-3) Hydrophilic Nonionic Monomer] (a-3) The hydrophilic nonionic monomer is a monomer that has a high affinity for 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.

[0021] (Content of each structural unit in pigment dispersion resin) The content of the structural units derived from components (a-1) to (a-3) in the pigment dispersion resin is as follows, from the viewpoint of the dispersion stability of the pigment, and the storage stability and ejection stability of the inkjet ink. The content of component (a-1) is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and is preferably 50% by mass or less, more preferably 45% by mass or less, even more preferably 40% by mass or less. The content of component (a-2) is preferably 50% by mass or more, more preferably 55% by mass or more, even more preferably 60% by mass or more, and preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less. When the component (a-3) is contained, the content of the component (a-3) is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and still more preferably 5% by mass or less.

[0022] The mass ratio of [(a-1) component / (a-2) component] is preferably 0.4 or more, more preferably 0.5 or more, even more preferably 0.6 or more, and is preferably 2 or less, more preferably 1.5 or less, even more preferably 1 or more. In the present invention, the content of the structural units derived from the components (a-1) to (a-3) in the pigment dispersion resin can be determined by measurement, or can be substituted by the charging ratio of the raw material monomers containing the components (a-1) to (a-3) during the production of the pigment dispersion resin.

[0023] (Manufacturing of pigment dispersion resins) The pigment dispersing resin can be produced by copolymerizing the mixture of the above-mentioned monomers 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, lower aliphatic alcohols, ketones such as methyl ethyl ketone, ethers, and esters are preferred. During polymerization, a polymerization initiator such as an azo compound or a persulfate, or a polymerization chain transfer agent such as a mercaptan, can be used. The polymerization temperature varies depending on the type of polymerization initiator, monomer, and solvent used, but is preferably 30°C or higher, more preferably 50°C or higher, and preferably 95°C or lower, more preferably 80°C or lower. The pigment dispersing resin is preferably neutralized with an alkali metal compound as described below.

[0024] From the viewpoint of pigment dispersion stability, the weight average molecular weight of the pigment dispersing resin is preferably 4,000 or more, more preferably 6,000 or more, even more preferably 8,000 or more, and still more preferably 10,000 or more, and is preferably 150,000 or less, more preferably 100,000 or less, even more preferably 80,000 or less, still more preferably 50,000 or less, still more preferably 30,000 or less, and still more preferably 20,000 or less. From the same viewpoints as above, the acid value of the pigment dispersion resin is preferably 50 mgKOH / g or more, more preferably 90 mgKOH / g or more, even more preferably 100 mgKOH / g or more, even more preferably 180 mgKOH / g or more, still more preferably 200 mgKOH / g or more, still more preferably 220 mgKOH / g or more, and is preferably 400 mgKOH / g or less, more preferably 320 mgKOH / g or less, still more preferably 300 mgKOH / g or less, and still more preferably 280 mgKOH / g or less. The weight average molecular weight and acid value of the pigment dispersing resin can be measured by the method described in the examples.

[0025] <Production of pigment-containing resin particles> The pigment-containing resin particles can be efficiently produced by a method including the following steps I and II. Step I: A step of neutralizing at least a portion of the carboxy groups of a pigment dispersing resin with an alkali metal compound to obtain an aqueous dispersion of the pigment dispersing resin. Step II: A step of dispersing the aqueous dispersion of the pigment dispersing resin obtained in Step I and a pigment to obtain an aqueous pigment dispersion of resin particles containing the pigment dispersed in the pigment dispersing resin.

[0026] Furthermore, the production of pigment-containing resin particles may further include, if necessary, a step III in which the pigment aqueous dispersion obtained in the step II is crosslinked with a crosslinking agent.

[0027] (Process I) It is preferable that at least a portion of the carboxyl groups of the pigment dispersion resin be neutralized with an alkali metal compound, which is believed to increase the charge repulsion force that occurs after neutralization, thereby suppressing aggregation of pigment particles in the inkjet ink and improving the dispersion stability of the pigment. The neutralization in step I is preferably carried out so that the pH is 7 or more and 11 or less. Examples of the alkali metal compound include one or more selected from alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide, alkali metal carbonates such as disodium carbonate, sodium hydrogen carbonate, and dipotassium carbonate, and alkali metal borate salts such as sodium borate, etc. Among these, from the viewpoints of availability and economy, alkali metal hydroxides are preferred, more preferably one or more selected from sodium hydroxide and potassium hydroxide, and even more preferably sodium hydroxide.

[0028] The degree of neutralization of the pigment dispersing resin is preferably 20 mol% or more, more preferably 25 mol% or more, and even more preferably 30 mol% or more, from the viewpoint of ensuring the dispersion stability of the pigment, and is preferably 60 mol% or less, more preferably 55 mol% or less, and even more preferably 50 mol% or less, from the viewpoint of crosslinking of the pigment dispersing resin with the crosslinking agent. Here, the degree of neutralization (mol %) is calculated by the following formula. Degree of neutralization (mol %) = [number of moles of alkali metal compound / number of moles of carboxyl groups in pigment dispersing resin] x 100 In the present invention, when the alkali metal compound is used in excess of the number of moles of carboxy groups in the pigment dispersing resin, the degree of neutralization may exceed 100 mol %.

[0029] (Process II) In the dispersion treatment in Step II, the pigment particles can be atomized to the desired particle size by main dispersion using shear stress alone, but 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 disperser used for pre-dispersion, a commonly used mixing and stirring device such as an anchor blade or a disperser blade can be used. Examples of dispersing machines used for this dispersion include kneading machines such as a roll mill and a kneader, high-pressure homogenizers such as a microfluidizer, and media-type dispersing machines such as a paint shaker and a bead mill. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When the dispersion treatment is carried out using a high-pressure homogenizer, the average particle size of the pigment-containing resin 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.

[0030] (Process III) In step III, the pigment dispersing resin in which the pigment is dispersed is crosslinked by a crosslinking agent to form a crosslinked resin, and an aqueous pigment dispersion can be obtained in which particles of the crosslinked resin containing the pigment are dispersed in an aqueous medium.

[0031] The crosslinking agent used in step III is preferably an epoxy compound, more preferably a compound having two or more epoxy groups in the molecule. The compounds having two or more epoxy groups in the molecule are similar to the polyfunctional epoxy compounds described below. The preferred range of the epoxy equivalent of the compounds having two or more epoxy groups in the molecule is also similar to the polyfunctional epoxy compounds described below.

[0032] When the pigment dispersion resin is crosslinked with a crosslinking agent, the degree of crosslinking of the particles of the crosslinked pigment dispersion resin is preferably 35 mol% or more, more preferably 40 mol% or more, and even more preferably 45 mol% or more, from the viewpoints of the storage stability and ejection stability of the inkjet ink, and from the same viewpoints as above, is preferably 65 mol% or less, more preferably 60 mol% or less, and even more preferably 55 mol% or less.

[0033] Here, in step III, the degree of crosslinking (mol %) of the crosslinked pigment-dispersing resin particles may be the theoretical degree of crosslinking calculated by the following formula: However, when the theoretical degree of crosslinking exceeds 100 mol %, the degree of crosslinking is taken to be 100 mol %. Degree of crosslinking of crosslinked pigment dispersing resin particles (mol %) = [amount of crosslinking agent added / {functional group equivalent of crosslinking agent (g / eq.) × number of moles of carboxylic acid of pigment dispersing resin contained in pigment-containing resin particles}] × 100

[0034] From the viewpoint of dispersion stability, the non-volatile component concentration (solid content concentration) of the resulting aqueous dispersion of pigment-containing resin particles is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less.

[0035] From the viewpoint of dispersion stability, the pigment content in the aqueous dispersion of pigment-containing resin particles is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 12% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 18% by mass or less.

[0036] The average particle size of the pigment-containing resin particles in the aqueous dispersion of pigment-containing resin particles is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and is preferably 350 nm or less, more preferably 250 nm or less, even more preferably 150 nm or less, from the viewpoint of reducing coarse particles and improving the ejection stability of the inkjet ink.

[0037] The average particle size of the pigment-containing resin particles in the inkjet ink is substantially the same as the average particle size of the pigment-containing resin particles in the aqueous dispersion. The solid content concentration and average particle size of the aqueous dispersion of pigment-containing resin particles are measured by the method described in the examples.

[0038] The content of the pigment in the inkjet ink is preferably 2% by mass or more, more preferably 3% by mass or more, and even more preferably 3.5% by mass or more, from the viewpoint of storage stability and ejection stability of the inkjet ink, and is preferably 8% by mass or less, more preferably 6% by mass or less, and even more preferably 5% by mass or less, from the viewpoint of dispersion stability.

[0039] When the pigment is in the form of pigment-containing resin particles, the content of the pigment-containing resin particles in the inkjet ink is preferably 3% by mass or more, more preferably 4% by mass or more, even more preferably 4.5% by mass or more, and still more preferably 5% by mass or more, from the viewpoint of the storage stability and ejection stability of the inkjet ink, and is preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 7% by mass or less, and still more preferably 6% by mass or less, from the viewpoint of dispersion stability.

[0040] <Poorly water-soluble resin> The poorly water-soluble resin is preferably in the form of poorly water-soluble resin particles. Examples of poorly water-soluble resins include vinyl resins, polyester resins, and polyurethane resins. Among these, from the viewpoint of storage stability and ejection stability of the inkjet ink, vinyl resins obtained by addition polymerization of vinyl monomers are preferred, and acrylic resins are more preferred. The water-insoluble resin preferably has a pigment content of 0.5% by mass or less, more preferably 0.1% by mass or less, even more preferably 0.01% by mass or less, and even more preferably substantially no pigment is contained, i.e., 0% by mass. Such vinyl resins preferably contain (b-1) structural units derived from hydrophilic ionic monomers and (b-2) structural units derived from hydrophobic monomers, and may further contain (b-3) structural units derived from hydrophilic nonionic monomers.

[0041] [(b-1) Hydrophilic ionic monomer] (b-1) The hydrophilic ionic monomer is preferably an anionic monomer, and examples thereof include a carboxylic acid monomer and a sulfonic acid monomer, with a carboxylic acid monomer being more preferred. The carboxylic acid monomer may be at least one selected from acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, with at least one selected from acrylic acid and methacrylic acid being more preferred.

[0042] [(b-2) Hydrophobic Monomer] The poorly water-soluble resin preferably contains one or more hydrophobic monomers selected from the group consisting of alkyl (meth)acrylates, cycloalkyl (meth)acrylates, aromatic group-containing (meth)acrylates, and aromatic group-containing monomers. The alkyl (meth)acrylates are preferably alkyl (meth)acrylates having an alkyl group with 1 to 22 carbon atoms. The cycloalkyl (meth)acrylates are preferably cycloalkyl (meth)acrylates having a cycloalkyl group with preferably 4 to 12, more preferably 5 to 8, carbon atoms. The aromatic group-containing (meth)acrylates are preferably aromatic group-containing (meth)acrylates having 6 to 22 carbon atoms. The aromatic group-containing monomers are preferably aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms. Among these, one or more selected from cycloalkyl (meth)acrylates, styrene, α-methylstyrene, and benzyl (meth)acrylate are more preferred, with cycloalkyl (meth)acrylates being even more preferred. Suitable examples of cycloalkyl (meth)acrylates include one or more selected from cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, and cycloheptyl (meth)acrylate. Among these, one or more selected from cyclopentyl acrylate, cyclohexyl acrylate, and cycloheptyl acrylate are preferred, with cyclohexyl acrylate being more preferred. The content of structural units derived from cycloalkyl (meth)acrylate in the poorly water-soluble resin is preferably 30% by mass or more, more preferably 35% by mass or more, even more preferably 40% by mass or more, and even more preferably 45% by mass or more, and preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less, and even more preferably 75% by mass or less.

[0043] [(b-3) Hydrophilic Nonionic Monomer] (b-3) Hydrophilic nonionic monomers are monomers that have high affinity with water and water-soluble organic solvents, and are, for example, monomers that contain a hydroxyl group or a polyalkylene glycol chain. Specific examples of the component (b-3) include those described in paragraph

[0018] of JP-A-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 (b-1) to (b-3) can be used alone or in combination of two or more.

[0044] (Production of poorly water-soluble resin particles) The poorly water-soluble resin particles may be appropriately synthesized or commercially available. As a preferred embodiment of the poorly water-soluble resin particles, an embodiment in which the poorly water-soluble resin is an acrylic resin will be described below. The acrylic resin can be produced by copolymerizing raw material monomers including the monomers (b-1) to (b-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 aliphatic alcohols, ketones, ethers, and esters are preferred, and methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred. During 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 azo compounds such as water-soluble azo polymerization initiators and polymeric azo polymerization initiators. Examples of the polymerization chain transfer agent include thiols and 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.

[0045] The poorly water-soluble resin particles are preferably used as an aqueous dispersion in which they are dispersed in an aqueous medium, and may contain a dispersant such as a surfactant, if necessary.

[0046] (Physical properties of acrylic resin) From the viewpoint of obtaining high-quality recorded matter on a low-liquid-absorbency recording medium, the acid value of the acrylic resin in the poorly water-soluble resin particles is preferably 100 mgKOH / g or more, more preferably 110 mgKOH / g or more, even more preferably 120 mgKOH / g or more, still more preferably 150 mgKOH / g or more, and even more preferably 190 mgKOH / g or more, and from the same viewpoint as above, it is preferably 350 mgKOH / g or less, more preferably 330 mgKOH / g or less, even more preferably 310 mgKOH / g or less, and even more preferably 290 mgKOH / g or less.

[0047] The weight-average molecular weight of the acrylic resin for the poorly water-soluble resin particles is preferably 5,000 or more, more preferably 8,000 or more, and even more preferably 10,000 or more, from the viewpoint of obtaining high-quality recorded matter on a low-liquid-absorbing recording medium, and from the same viewpoint as above, is preferably 100,000 or less, more preferably 50,000 or less, and even more preferably 30,000 or less.

[0048] The acid value of the acrylic resin in the poorly water-soluble resin particles is a value calculated from the mass ratio of the constituent monomers. The weight average molecular weight of the acrylic resin is measured by the method described in the examples.

[0049] In the production of poorly water-soluble resin particles, the obtained aqueous dispersion may be treated with a crosslinking agent to form crosslinked resin particles. The crosslinking agent used is preferably an epoxy compound, more preferably a compound having two or more epoxy groups in the molecule. The compounds having two or more epoxy groups in the molecule are similar to the polyfunctional epoxy compounds described below. The preferred range of the epoxy equivalent of the compounds having two or more epoxy groups in the molecule is also similar to the polyfunctional epoxy compounds described below.

[0050] (Multifunctional epoxy compound) The polyfunctional epoxy compound is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having from 3 to 8 carbon atoms, even more preferably one or more compounds 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 trimethylolpropane polyglycidyl ether. The epoxy group equivalent weight of the polyfunctional epoxy compound is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and is preferably 300 or less, more preferably 200 or less, even more preferably 170 or less.

[0051] (Production of crosslinked resin particles) The crosslinked resin particles can be efficiently produced by a method including the following steps 1 and 2. The crosslinked resin particles produced by the method including steps 1 and 2 have, on the surface of the crosslinked resin particles, a crosslinked structure consisting of a structure derived from a carboxy group in the poorly water-soluble resin particles and a structure derived from a polyfunctional epoxy compound. Step 1: A step of neutralizing at least a portion of the carboxy groups of the poorly water-soluble resin particles with an alkali metal compound to obtain an aqueous dispersion of the poorly water-soluble resin particles. Step 2: A step of adding a polyfunctional epoxy compound to the aqueous dispersion of poorly water-soluble resin particles obtained in Step 1, and reacting the carboxyl groups in the poorly water-soluble resin particles with the epoxy groups in the polyfunctional epoxy compound to obtain an aqueous dispersion of crosslinked resin particles.

[0052] (Process 1) It is preferable that at least a portion of the carboxyl groups of the poorly water-soluble resin particles be neutralized with an alkali metal compound, which is believed to increase the charge repulsion force that occurs after neutralization, thereby suppressing aggregation of the crosslinked resin particles in the ink and improving the dispersion stability of the crosslinked resin particles. The neutralization in step 1 is preferably carried out so that the pH is 7 or more and 11 or less. Examples of the alkali metal compound include one or more selected from alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide, alkali metal carbonates such as disodium carbonate, sodium hydrogen carbonate, and dipotassium carbonate, and alkali metal borate salts such as sodium borate, etc. Among these, from the viewpoints of availability and economy, alkali metal hydroxides are preferred, more preferably one or more selected from sodium hydroxide and potassium hydroxide, and even more preferably sodium hydroxide.

[0053] The degree of neutralization of the poorly water-soluble resin particles is preferably 15 mol% or more, more preferably 20 mol% or more, and even more preferably 25 mol% or more from the viewpoint of ensuring the dispersion stability of the poorly water-soluble resin particles, and is preferably 50 mol% or less, more preferably 40 mol% or less, and even more preferably 35 mol% or less from the viewpoint of productivity of the crosslinked resin particles. Here, the degree of neutralization (mol %) is calculated by the following formula. Degree of neutralization (mol %) = [number of moles of alkali metal compound / number of moles of carboxyl groups of poorly water-soluble resin particles] x 100 In the present invention, when the alkali metal compound is used in excess of the number of moles of carboxy groups in the poorly water-soluble resin particles, the degree of neutralization may exceed 100 mol %.

[0054] (Process 2) In step 2, the temperature at which the carboxyl groups in the poorly water-soluble resin particles react with the epoxy groups in the polyfunctional epoxy compound is, from the viewpoints of completion of the crosslinking reaction and economy, preferably 50° C. or higher, more preferably 70° C. or higher, and preferably 95° C. or lower, more preferably 90° C. or lower. From the same viewpoints as above, the time for the crosslinking treatment is preferably 1 hour or longer, more preferably 3 hours or longer, and preferably 10 hours or shorter, more preferably 8 hours or shorter.

[0055] The amount of polyfunctional epoxy compound used is an amount that will result in a crosslinking degree of the resulting crosslinked resin particles of preferably 10 mol% or more, more preferably 15 mol% or more, even more preferably 20 mol% or more, and even more preferably 30 mol% or more, from the viewpoint of obtaining high-quality recorded matter on a low-liquid-absorbing recording medium, and from the same viewpoint as above, the amount is an amount that will result in a crosslinking degree of the resulting crosslinked resin particles of preferably 80 mol% or less, more preferably 75 mol% or less, even more preferably 70 mol% or less, and even more preferably 65 mol% or less. In step 2, the crosslinking degree (mol %) of the crosslinked resin particles may be the theoretical crosslinking degree calculated by the following formula, which is taken as the crosslinking degree of the pigment-containing polymer. However, when the theoretical crosslinking degree exceeds 100 mol %, the crosslinking degree is taken as 100 mol %. Degree of crosslinking of crosslinked resin particles (mol %)=[amount of polyfunctional epoxy compound added / {functional group equivalent of polyfunctional epoxy compound (g / eq.) × number of moles of carboxylic acid of poorly water-soluble resin particles contained in crosslinked resin particles}] × 100

[0056] From the viewpoint of obtaining high-quality recorded matter on a low-liquid-absorbency recording medium, the acid value of the crosslinked resin particles is preferably 35 mgKOH / g or more, more preferably 40 mgKOH / g or more, and even more preferably 50 mgKOH / g or more, and from the same viewpoint as above, it is preferably 220 mgKOH / g or less, more preferably 190 mgKOH / g or less, even more preferably 155 mgKOH / g or less, even more preferably 130 mgKOH / g or less, even more preferably 110 mgKOH / g or less, even more preferably 100 mgKOH / g or less, even more preferably 80 mgKOH / g or less, and even more preferably 70 mgKOH / g or less. In the present invention, the acid value of the crosslinked resin particles is the acid value of the resin constituting the crosslinked resin particles, and is calculated from the acid value of the poorly water-soluble resin particles before crosslinking, the degree of crosslinking, and the amount of polyfunctional epoxy compound added.

[0057] From the viewpoint of obtaining high-quality recorded images on a recording medium with low liquid absorption, the content of the poorly water-soluble resin particles in the inkjet ink is preferably 2.5% by mass or more, more preferably 3.0% by mass or more, even more preferably 3.5% by mass or more, and even more preferably 4.0% by mass or more, and from the same viewpoint as above, it is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less. When the inkjet ink is black, from the viewpoint of obtaining high-quality recorded matter on a low-absorbency recording medium, the content is preferably 2.5% by mass or more, more preferably 3.0% by mass or more, even more preferably 3.5% by mass or more, still more preferably 4.0% by mass or more, and even more preferably 5% by mass or more, and from the same viewpoint as above, the content is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 7% by mass or less. When the inkjet ink is a color ink, from the viewpoint of obtaining high-quality recorded matter on a low-absorbency recording medium, the content is preferably 2.5% by mass or more, more preferably 3.0% by mass or more, even more preferably 3.5% by mass or more, and even more preferably 4.0% by mass or more, and from the same viewpoint as above, the content is preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 7% by mass or less, even more preferably 6% by mass or less, and even more preferably 5% by mass or less.

[0058] The total solids content in the inkjet ink is not particularly limited as long as it satisfies the following relationship, but from the viewpoint of obtaining high-quality recorded matter on a low-liquid-absorbency recording medium, it is preferably 8% by mass or more, more preferably 9% by mass or more, and even more preferably 10% by mass or more, and from the same viewpoint as above, it is preferably 16% by mass or less, more preferably 15% by mass or less, and even more preferably 14% by mass or less.

[0059] From the viewpoints of suppressing character bleeding and enabling good solid coverage, the total solid content of the inkjet inks should be such that the values ​​of X and Y, where X is the total solid content of each black ink and Y is the total solid content of each color ink, are 0.1 to 5.0, preferably 0.2 or more, more preferably 0.5 or more, even more preferably 1.0 or more, and even more preferably 1.5 or more. From the same viewpoints as above, the values ​​should be 4.0 or less, more preferably 3.5 or less, even more preferably 3.0 or less, and even more preferably 2.5 or less. In embodiments using multiple black inks or color inks, the above relationship should be satisfied between each inkjet. For example, in an ink set having a total solid content of X% by mass in the black ink, a total solid content of Y1% by mass in the yellow ink, a total solid content of Y2% by mass in the magenta ink, and a total solid content of Y3% by mass in the cyan ink, the values ​​of X-Y1, X-Y2, and X-Y3 should all be within the above ranges.

[0060] <Nonionic surfactants> Examples of nonionic surfactants contained in the inkjet ink of the present invention include acetylene glycol surfactants, ether surfactants, silicone surfactants, and fluorine surfactants. Among these, one or more surfactants selected from acetylene glycol surfactants, ether surfactants, and silicone surfactants are preferred, with acetylene glycol surfactants being most preferred. The nonionic surfactants can be used alone or in combination of two or more.

[0061] Acetylene glycol surfactants, from the viewpoints of wettability to recording media and defoaming properties, include acetylene diols such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, and 2,4-dimethyl-5-hexyn-3-ol, and ethylene oxide adducts of these acetylene diols. Examples of commercially available acetylene glycol surfactants include the "Surfynol" series and the "Olfine" series manufactured by Nissin Chemical Industry Co., Ltd.

[0062] As the ether surfactant, a polyoxyalkylene alkyl ether surfactant is preferred. Examples of commercially available ether surfactants include the "Emulgen (registered trademark)" series manufactured by Kao Corporation.

[0063] As the silicone surfactant, polyether-modified silicone is preferred. Examples of commercially available silicone surfactants include the KF series (KF-353, KF-355A, KF-642, KF-6011, etc.) manufactured by Shin-Etsu Chemical Co., Ltd., the Silface SAG series (SAG-005, SAG-008, etc.) manufactured by Nissin Chemical Industry Co., Ltd., the DOWSIL series (FZ-2123, etc.) manufactured by Dow-Toray Industries, Inc., and the BYK series manufactured by BYK Japan K.K.

[0064] The total content of nonionic surfactants in the inkjet ink is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.5% by mass or more, from the viewpoints of improving the continuous ejection properties of the ink and obtaining good images free of unevenness and color mixing, and from the same viewpoints as above, is preferably 5.0% by mass or less, more preferably 4.0% by mass or less, even more preferably 3.0% by mass or less, and even more preferably 2.5% by mass or less. When the inkjet ink is black, the total content of nonionic surfactants in the inkjet ink is, from the viewpoint of improving the continuous ejection properties of the ink and obtaining good images free of unevenness and color mixing, preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.5% by mass or more, still more preferably 0.7% by mass or more, and even more preferably 1.0% by mass or more, and from the same viewpoints as above, is preferably 5.0% by mass or less, more preferably 4.0% by mass or less, even more preferably 3.0% by mass or less, still more preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less. When the inkjet ink is a color ink, the total content of nonionic surfactants in the inkjet ink is, from the viewpoint of improving the continuous ejection properties of the ink and obtaining good images free of unevenness and color mixing, preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.5% by mass or more, still more preferably 0.7% by mass or more, still more preferably 1.0% by mass or more, and still more preferably 1.5% by mass or more, and from the same viewpoints as above, preferably 5.0% by mass or less, more preferably 4.0% by mass or less, even more preferably 3.0% by mass or less, still more preferably 2.5% by mass or less, and still more preferably 2.0% by mass or less.

[0065] In the water-based ink set for inkjet printing of the present invention, as described above, the total content (% by mass) of nonionic surfactants in each black ink is less than the total content (% by mass) of nonionic surfactants in each color ink. For example, if the total content of nonionic surfactants in each black ink is x% by mass and the total content of nonionic surfactants in each color ink is y% by mass, the value of yx is preferably 0.01 or more, more preferably 0.05 or more, even more preferably 0.10 or more, even more preferably 0.15 or more, even more preferably 0.20 or more, even more preferably 0.25 or more, and even more preferably 0.30 or more, from the viewpoints of suppressing character bleeding and enabling good solid filling. Also, from the same viewpoints as above, it is preferably 1.00 or less, more preferably 0.80 or less, and even more preferably 0.50 or less. In an embodiment in which a plurality of black inks or color inks are used, the above relationship is satisfied between each inkjet. For example, in an ink set embodiment in which the total content of nonionic surfactants in the black ink is x mass %, the total content of nonionic surfactants in the yellow ink is y1 mass %, the total content of nonionic surfactants in the magenta ink is y2 mass %, and the total content of nonionic surfactants in the cyan ink is y3 mass %, the values ​​of y1-x, y2-x, and y3-x all satisfy the relationship of being within the above ranges.

[0066] <Organic solvents> Examples of organic solvents contained in the inkjet ink of the present invention include glycol ethers, polyhydric alcohols, nitrogen-containing heterocyclic compounds such as 2-pyrrolidone, alkanolamines, etc. Among these, from the viewpoints of the spreadability and permeability of the inkjet ink onto a recording medium, storage stability, and ejection stability, one or more organic solvents selected from the group consisting of glycol ethers and polyhydric alcohols are preferred. The water-soluble organic solvents can be used alone or in combination of two or more.

[0067] Examples of glycol ethers include alkylene glycol monoalkyl ethers such as monoalkylene glycol monoalkyl ethers, dialkylene glycol monoalkyl ethers, and trialkylene glycol monoalkyl ethers; and alkylene glycol dialkyl ethers such as monoalkylene glycol dialkyl ethers and dialkylene glycol dialkyl ethers. The alkylene oxide group of the glycol ether may be at least one selected from the group consisting of an ethylene oxide group and a propylene oxide group, with an ethylene oxide group being more preferred. The glycol ether preferably has at least one hydrocarbon group having 2 to 8 carbon atoms.

[0068] As the glycol ether, glycol ethers having an octanol-water partition coefficient logP value (hereinafter also simply referred to as "logP") of 0 or more are preferred from the viewpoint of the storage stability and ejection stability of the inkjet ink. Examples of glycol ethers having an octanol-water partition coefficient logP value of 0 or more include alkylene glycol monoalkyl ethers. Among these, one or more selected from the group consisting of ethylene glycol monoisopropyl ether (logP: 0.23), diethylene glycol monoisopropyl ether (logP: 0.07), ethylene glycol monoallyl ether (logP: 0.26), ethylene glycol monobutyl ether (logP: 0.81), diethylene glycol monoisobutyl ether (logP: 0.64), diethylene glycol mono-n-butyl ether (logP: 0.67), diethylene glycol ethyl methyl ether (logP: 0.12), diethylene glycol diethyl ether (logP: 0.45), dipropylene glycol monomethyl ether (logP: 0.05), propylene glycol monopropyl ether (logP: 0.71), propylene glycol monobutyl ether (logP: 1.13), dipropylene glycol monopropyl ether (logP: 0.88), and dipropylene glycol monobutyl ether (logP: 1.29) are preferred, and dipropylene glycol monomethyl ether is more preferred.

[0069] From the viewpoint of the storage stability and ejection stability of the inkjet ink of the present invention, the content of glycol ether in the inkjet ink of the present invention is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 4% by mass or more, still more preferably 5% by mass or more, and even more preferably 7% by mass or more, and from the same viewpoint as above, it is preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 10% by mass or less.

[0070] From the viewpoint of storage stability and ejection stability of the inkjet ink, the polyhydric alcohol is preferably one or more selected from the group consisting of alkanediols having 2 to 6 carbon atoms, such as propylene glycol (1,2-propanediol) and 1,2-hexanediol, and diethylene glycol, and propylene glycol is more preferred.

[0071] From the viewpoint of the storage stability and ejection stability of the inkjet ink of the present invention, the content of polyhydric alcohol in the inkjet ink of the present invention is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 4% by mass or more, still more preferably 5% by mass or more, and even more preferably 7% by mass or more, and from the same viewpoint as above, it is preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 10% by mass or less.

[0072] The content of the water-soluble organic solvent in the inkjet ink of the present invention is preferably 10% by mass or more, more preferably 20% by mass or more, from the viewpoints of the storage stability and ejection stability of the inkjet ink, and from the same viewpoints as above, is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less.

[0073] Furthermore, from the viewpoint of suppressing character bleeding and enabling good solid filling, it is preferable that the content of organic solvents having a boiling point of above 250°C at 1 atmospheric pressure be small. The content of organic solvents having a boiling point of above 250°C at 1 atmospheric pressure relative to the total amount of inkjet ink is 5% by mass or less, preferably 4% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, even more preferably 1% by mass or less, still more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, and may be 0% by mass. Examples of organic solvents having a boiling point of above 250°C at 1 atmospheric pressure include glycerin, triethanolamine, triethylene glycol monobutyl ether, tripropylene glycol monobutyl ether, diethylene glycol monohexyl ether, diethylene glycol mono-2-ethylhexyl ether, diethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, diethylene glycol monobenzyl ether, and polyethylene glycol monomethyl ether.

[0074] <Water> The ink-jet ink of the present invention contains water. The water used in the inkjet ink according to the present invention is preferably pure water or ion-exchanged water, from the viewpoint of preventing the inclusion of unintended substances.

[0075] The water content in the inkjet ink is preferably 50% by mass or more, more preferably 55% by mass or more, and even more preferably 60% by mass or more, from the viewpoints of storage stability and ejection stability of the inkjet ink, and from the same viewpoints as above, is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less.

[0076] The ink-jet ink of the present invention may contain various additives commonly used in ink-jet inks, such as humectants, wetting agents, viscosity modifiers, antifoaming agents, preservatives, antifungal agents, rust inhibitors, and neutralizing agents.

[0077] [Inkjet ink manufacturing method] The inkjet ink of the present invention can be efficiently produced by blending the above-mentioned pigment, poorly water-soluble resin, nonionic surfactant, organic solvent, and water, and, if necessary, other surfactants, other additives, etc., and mixing the blend. There are no particular restrictions on the method for mixing them, but stirring is preferred. The amount of each component blended in the method for producing the inkjet ink of the present invention can be considered as the content of each component in the inkjet ink of the present invention.

[0078] (Physical properties of inkjet ink) The viscosity of the inkjet ink of the present invention at 32°C is preferably 2 mPa s or more, more preferably 3 mPa s or more, and even more preferably 3.5 mPa s or more, from the viewpoint of obtaining high-quality recorded matter on a low-absorbency recording medium, and from the same viewpoint as above, is preferably 10 mPa s or less, more preferably 8 mPa s or less, and even more preferably 6 mPa s or less.

[0079] The pH of the inkjet 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 viewpoints of component resistance and skin irritation, the pH is preferably 11.0 or lower, more preferably 10.0 or lower, and even more preferably 9.0 or lower. The viscosity and pH of the inkjet ink at 32°C can be measured by a conventional method.

[0080] [Ink set manufacturing method] The ink set of the present invention can be prepared by loading the above-described inkjet inks into a known inkjet printing device.

[0081] [Ink set printing method] The ink set of the present invention can be loaded into a known inkjet printing device and ejected as ink droplets onto a recording medium such as coated paper or film to print an image, etc. The ink droplet ejection method can be any of a piezoelectric, thermal, or electrostatic method. The ink set of the present invention is preferably used in an inkjet recording device that performs recording by scanning a recording head once relative to the recording medium.

[0082] As recording media for use in printing with the ink set of the present invention, low-absorbency coated paper, particularly gloss coated paper, matte coated paper, and low-absorbency printing substrates such as non-absorbent resin films can also be used. In the case of a printing substrate, the term "low liquid absorption" is a concept that includes both low liquid absorption and non-liquid absorption, and refers to a resin film substrate with a water absorption of 0 g / m when the resin film substrate is in contact with pure water for 100 msec. 2 More than 10g / m 2 This means that: Examples of the resin film include transparent synthetic resin films, such as polyester films such as polyethylene terephthalate films, vinyl chloride films, polyolefin films such as polypropylene films and polyethylene films, and polyamide films such as nylon films. These resin films may be oriented films such as biaxially oriented films and uniaxially oriented films, or non-oriented films. Among these, the ink set of the present invention can obtain high-quality recorded images on gloss coated paper and matte coated paper, and therefore, the printing substrate is preferably one or more selected from gloss coated paper and matte coated paper. [Example]

[0083] The present invention will be specifically explained below with reference to examples. Note that the following examples are merely illustrative of the present invention and are not intended to be limiting in any way. Note that "normal pressure" refers to a state in which pressure is not applied or reduced, and "normal temperature" refers to 25°C. In the production examples, examples, and comparative examples, "parts" and "%" refer to "parts by mass" and "% by mass" unless otherwise specified. Note that the methods for measuring and calculating each physical property are as follows.

[0084] (1) Measurement of the weight average molecular weight of the resin The measurement was performed by gel permeation chromatography under the following conditions. GPC equipment: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolumn Super AW-H" manufactured by Tosoh Corporation Eluent: N,N-dimethylformamide dissolved with phosphoric acid and lithium bromide at concentrations of 60mmol / L and 50mmol / L, respectively. Flow rate: 0.5mL / min Standard material: Monodisperse polystyrene kit with known molecular weight [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) Measurement sample: 0.1 g of resin was mixed with 10 mL of the eluent in a glass vial, stirred with a magnetic stirrer at 25°C for 10 hours, and filtered with a syringe filter "DISMIC-13HP" (PTFE, 0.2 μm, Advantec Co., Ltd.).

[0085] (2) Measurement of the average particle size of pigment-containing resin particles and crosslinked resin particles Cumulant analysis was performed using a laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.), and the obtained cumulant average particle size was taken as the average particle size of the pigment-containing acrylic resin particles. The measurement sample had a particle concentration of 5 × 10 -3 The dispersion was diluted with water to a concentration of 0.05% (solids concentration equivalent). The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 integrations. The refractive index of water (1.333) was entered as the refractive index of the dispersion solvent, and the obtained cumulant average particle size was taken as the average particle size of the pigment-containing polymer particles.

[0086] (3) Measurement of the acid value of acrylic resin and pigment dispersion resin 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.

[0087] (4) Calculation of the degree of crosslinking of crosslinked resin particles The degree of crosslinking of the crosslinked resin particles was calculated from the mass and epoxy equivalent of the crosslinking agent and the mass and acid value of the acrylic resin, assuming that all of the epoxy groups in the crosslinking agent had reacted with the carboxy groups of the acrylic resin.

[0088] (5) Calculation of the acid value of crosslinked resin particles The acid value of the crosslinked resin particles was calculated from the acid value of the acrylic resin or acrylic vinyl resin before crosslinking and the degree of crosslinking and mass of the crosslinked resin particles.

[0089] (6) Measurement of solids concentration of aqueous dispersion of crosslinked resin particles and aqueous dispersion of pigment-containing resin particles 10.0 g of sodium sulfate was weighed out precisely into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm) and kept constant in a desiccator. Approximately 1.0 g of the sample was added to the container, mixed, and then weighed. The mixture was kept at 105°C for 2 hours to remove volatiles, and then left in the desiccator for another 15 minutes, after which the mass was weighed precisely. The mass of the sample after volatile matter removal was taken as the solid content, and the solid content concentration was determined by dividing the mass by the mass of the added sample.

[0090] (7) Measurement of pH of inkjet ink The pH of the inkjet ink at 25° C. was measured using a tabletop pH meter (Horiba Ltd., "F-71") equipped with a pH electrode (Horiba Ltd., "6337-10D").

[0091] (8) Measurement of wax melting point The melting point of the wax was measured using a measuring device conforming to JIS K 0064. Specifically, using a differential scanning calorimeter (Q20, manufactured by TA Instruments), the sample was heated to 200°C and then cooled to 0°C at a rate of 10°C / min. The sample was then heated at a rate of 10°C / min, and the calorific value was measured up to 200°C. The temperature of the peak with the largest peak area among the observed heat of fusion peaks was taken as the maximum peak temperature of melting, and this peak temperature was taken as the melting point.

[0092] (9) Measurement of the average particle size of wax particles in a wax emulsion The average particle size of wax particles in the wax emulsion was measured using a Nikkiso Microtrac particle size analyzer UPA at a laser wavelength of 780 nm, a laser output of 3 mW, and a sample concentration of 5 × 10 -3The median value of the volume average particle size distribution (D 50 ) was taken as the average particle size of the wax particles in the wax emulsion.

[0093] <Production of poorly water-soluble resin particles (aqueous dispersion of crosslinked resin particles)> (Production of acrylic resin (A)) Manufacturing Example I-1: Manufacturing of acrylic resin A1 A monomer mixture was prepared by mixing 25.7 parts of acrylic acid, 55.0 parts of cyclohexyl acrylate, 55.0 parts of butyl acrylate, and 100 parts of methyl ethyl ketone (hereinafter referred to as "MEK"). Also, 1.1 parts of 2,2'-azobis-(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65) as a polymerization initiator and 12.7 parts of MEK were mixed to prepare a polymerization initiator solution. A reaction vessel equipped with a stirrer, a reflux condenser, and two dropping funnels was initially charged with 10% of the monomer mixture, and the inside of 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, and in the other dropping funnel, 80% of the polymerization initiator solution was added. Under a nitrogen atmosphere, the contents of the reaction vessel were heated to 65°C while stirring, and the mixture and polymerization initiator solution in the two dropping funnels were each continuously added dropwise to the reaction vessel over 3 hours. After the dropwise addition was completed, the reaction was carried out at 65°C for 2 hours. The remaining 20% ​​of the polymerization initiator solution was then added, and the reaction was carried out for an additional 2 hours at 65°C. After the temperature was raised to 70°C, the reaction was carried out for an additional 2 hours. After cooling to room temperature, the MEK was removed by drying under reduced pressure, and acrylic resin A1 (acid value: 200 mgKOH / g, weight average molecular weight: 21,000) was obtained.

[0094] Manufacturing Example I-2: Manufacturing of acrylic resin A2 Acrylic resin A2 was produced in the same manner as in Production Example I-1, except that the monomer mixture was 25.7 parts acrylic acid, 74.3 parts styrene, and 100 parts methyl ethyl ketone (hereinafter referred to as "MEK"). The acid value of the resulting acrylic resin A2 was 200 mgKOH / g, and the weight-average molecular weight was 20,000.

[0095] (Production of aqueous dispersion of crosslinked resin particles) Production Example I-3: Production of aqueous dispersion 1 of crosslinked resin particles (Process 1) 40.0 parts of acrylic resin A1 was dissolved in 60.0 parts of MEK, and 10.1 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide solids content 16.9%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration) was added thereto so that the degree of neutralization of acrylic resin A1 was 30 mol %. Then, 200.0 parts of water was added over 1 hour, and after the addition was completed, the MEK was removed using an evaporator, and ion-exchanged water was added so that the solids concentration was 20 mass %, obtaining an aqueous dispersion of acrylic resin A1. (Process 2) To 200.0 parts of the aqueous dispersion of acrylic resin A1 obtained in step 1, 12.0 parts of trimethylolpropane polyglycidyl ether (Nagase ChemteX Corporation, product name: Denacol EX-321LT, epoxy equivalent: 140) as a crosslinking agent was added and mixed. The mixture was then heated to 80°C for 5 hours with stirring to react the carboxyl groups in acrylic resin A1 with the epoxy groups in the trimethylolpropane polyglycidyl ether. The mixture was then cooled to 25°C and filtered through a 5 μm pore size filter (acetyl cellulose membrane, outer diameter: 2.5 cm, Fujifilm Wako Pure Chemical Industries, Ltd.). Ion-exchange water was added to a solids concentration of 20% by mass to obtain aqueous dispersion 1 of crosslinked resin particles. The crosslinked resin particles in aqueous dispersion 1 of crosslinked resin particles had a degree of crosslinking of 60 mol%, an acid value of 60.4 mg KOH / g, and an average particle size of 99 nm.

[0096] Production Example I-4: Production of aqueous dispersion 2 of crosslinked resin particles Aqueous dispersion 2 of crosslinked resin particles was produced in the same manner as in Production Example I-3, except that acrylic resin A1 was changed to acrylic resin A2. The degree of crosslinking of the obtained aqueous dispersion 2 was 60 mol%, the acid value was 61.5 mgKOH / g, and the average particle size was 105 nm.

[0097] <Production of aqueous dispersion of pigment-containing resin particles> (Production of acrylic resin B1) Manufacture of acrylic resin B1 Acrylic resin B1 was produced in the same manner as in Production Example I-1, except that the monomer mixture solution was changed to 30.8 parts of acrylic acid, 69.2 parts of styrene, and 100 parts of methyl ethyl ketone (hereinafter referred to as "MEK"). The acid value of the obtained acrylic resin B1 was 240 mgKOH / g, and the weight average molecular weight was 14,000.

[0098] (Production of aqueous dispersion of pigment-containing resin particles) Production Example II-1: Production of aqueous dispersion 1 of pigment-containing resin particles (Process I) 25 parts of the obtained acrylic resin B1 was dissolved in 78.6 parts of MEK, and 10.1 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide solids content 16.9%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration) was added thereto so that the degree of neutralization of the acrylic resin B1 was 40 mol %, to obtain an aqueous dispersion of neutralized acrylic resin B1.

[0099] (Process II) To the aqueous dispersion of acrylic resin B1 obtained in step I, 400.0 parts of ion-exchanged water and 100.0 parts of carbon black pigment (CI Pigment Black 7, manufactured by Cabot Corporation, trade name: Monarch 717) were added, and the mixture was stirred for 60 minutes using a Disper (manufactured by Asada Iron Works Co., Ltd., trade name: Ultra Disper) at 20°C with the Disper blade rotating at 7000 rpm. The mixture was then subjected to 15 passes of dispersion treatment at a pressure of 150 MPa using a Microfluidizer (Microfluidics Corporation, high-pressure homogenizer, product name: M-140K) to obtain a dispersion. 250.0 parts of ion-exchanged water was added to the obtained dispersion, and after stirring, the MEK was completely removed at 60°C under reduced pressure, and then some of the water was removed. The liquid phase of the obtained dispersion was recovered using a centrifuge and filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain an aqueous dispersion (solids concentration 25% by mass) in which the pigment was dispersed in acrylic resin B1.

[0100] (Process III) 100.0 parts of the aqueous dispersion of the pigment obtained in step II dispersed in acrylic resin B1 was placed in a screw-capped glass bottle, 31.0 parts of ion-exchanged water was added, and 1.5 parts of trimethylolpropane polyglycidyl ether (Nagase ChemteX Corporation, trade name: Denacol EX-321LT, epoxy equivalent: 140) was added as a crosslinking agent to a degree of crosslinking of 50 mol%, and the bottle was sealed and heated at 70 ° C for 5 hours while stirring with a stirrer. The dispersion was then cooled to room temperature (25 ° C) and filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain aqueous dispersion 1 of pigment-containing resin particles (solids concentration 20 mass%, carbon black 15.7%, pigment dispersing resin 4.3%, average particle size of pigment-containing resin particles 107 nm).

[0101] Production Example II-2: Production of aqueous dispersion 2 of pigment-containing resin particles A water-based dispersion 2 of pigment-containing resin particles (solid content concentration 20 mass%, cyan pigment 15.7%, pigment dispersing resin 4.3%, average particle size of pigment-containing resin particles 110 nm) was obtained in the same manner as in Production Example II-1, except that the pigment used in Step II was changed to a cyan pigment (manufactured by DIC Corporation, trade name: Fastogen Blue FA5380, Pigment Blue 15:3).

[0102] [Ink set manufacturing] Example 1 (Black ink manufacturing) 30.0 parts of aqueous dispersion 1 of crosslinked resin particles (solid content concentration 20% by mass) (breakdown: resin 6.0 parts, water 24.0 parts), 30.0 parts of aqueous dispersion 1 of pigment-containing resin particles (solid content concentration 20% by mass) (breakdown: carbon black 4.7 parts, pigment dispersion resin B1 1.3 parts, water 24.1 parts), propylene glycol (manufactured by AGC Corporation) 10.0 parts, dipropylene glycol monomethyl ether (manufactured by Nippon Nyukazai Co., Ltd.) 10.0 parts, Surfynol 104PG-50 (manufactured by Nissin Chemical Industry Co., Ltd., 50% propylene glycol solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol) 1.0 part, Emulgen (registered trademark) 120 (manufactured by Kao Corporation, ethylene oxide adduct of lauryl alcohol, active ingredient 100%) 0.8 parts, KF-6011 (manufactured by Shin-Etsu Chemical Co., Ltd., polyether-modified silicone 0.2 parts of hydroxybenzoates (active ingredient 100%) and 2-dimethylaminoethanol (a reagent manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed together to adjust the ink pH to 8.5, and ion-exchanged water was added to make the total amount 100 parts. The mixture was then filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain inkjet ink 1.

[0103] (Manufacturing cyan ink (color ink)) Inkjet ink 2 was obtained in the same manner as in the production of the black ink, except that aqueous dispersion 1 of pigment-containing resin particles was changed to aqueous dispersion 2 of pigment-containing resin particles. In addition, inkjet ink 1, which is a black ink, and inkjet ink 2, which is a cyan ink, were used as an ink set and subjected to printing evaluation.

[0104] Examples 2 to 15 and Comparative Examples 1 to 4 (Production of Inkjet Inks 3 to 34) In the same manner as in Example 1, water-based inkjet inks 3 to 34 were prepared according to the formulations shown in Tables 1 and 2, thereby obtaining ink sets for the respective Examples and Comparative Examples. The amounts of each component in Tables 1 and 2 are the actual amounts blended.

[0105] The waxes in Tables 1 and 2 are as follows: Polyethylene wax: Toho Chemical Industry Co., Ltd., product name: Hi-Tec E-6500, nonionic polyethylene wax emulsion, melting point: 140°C, average particle size: 60 nm, solid content: 35% by mass

[0106] [Ink set evaluation] <Measurement of line width of black ink in overprinted area> Inkjet ink 1 and inkjet ink 2 were loaded into separate inkjet recording heads in a print evaluation device (manufactured by Tritec Corporation) equipped with an inkjet recording head (Kyocera Corporation, "KJ4B-HD06MHG-STDV," piezo-type line head) at a temperature of 25±1°C and a relative humidity of 30±5%. The print conditions were set to a head voltage of 26V, a drive frequency of 20kHz, a head temperature of 32°C, a droplet volume of 7pl, a resolution of 600 × 600 dpi, 200 pre-discharge flushes, and a negative pressure of -4.0 kPa. Matte coated paper (manufactured by GRAFICAS Y FORMULARIOS, trade name: Fitnes Matt) was used, and the print medium was fixed to a transfer table under reduced pressure with its longitudinal direction aligned with the transport direction. A print command was transferred to the print evaluation device, and a 3cm square print pattern was printed in cyan ink at 100% duty using a single pass, overlapping a 20-dot line of black ink. A print was obtained. The width of the black line at the overlapping printed portion was measured using a handheld image evaluation system "PIAS-II" manufactured by Quality Engineering Associates Inc. The smaller the line width, the less bleeding there is and the more readable the print will be. Since the line width in the non-overprinted areas was 950 μm, there will be no problems in practical use if it is between 950 μm and 1010 μm, preferably 990 μm or less, and more preferably 970 μm or less.

[0107] <Measurement of OD value of color ink in solid print area> The optical density (hereinafter referred to as "OD value") of the solid color ink print area where no black ink was printed on the printed matter obtained in the above <Measurement of black ink line width in overprinted area> was measured using an X-Rite compact spectrophotometer "eXact Standard" under the conditions of light source D65, viewing angle 2°, and STATUS E. If the ink does not wet and spread on the paper, streaks and uneven color will occur during solid printing, resulting in a lower OD value. Streaks and uneven color will look uneven and be judged as low quality, so a high OD value is desirable. An OD value of 1.55 or higher will not pose any problems in practical use, preferably 1.65 or higher, and more preferably 1.80 or higher.

[0108] The ink sets of Examples 2 to 15 and Comparative Examples 1 to 4 shown in Tables 1 and 2 were also evaluated in the same manner. The ink on the left side of the ink set shown in each example was set in the printing machine so that it was printed first. The results are shown in Tables 1 and 2.

[0109] [Table 1]

[0110] [Table 2]

[0111] As shown in Tables 1 and 2, the ink sets of Examples 1 to 15 all suppressed bleeding of characters and provided good solid printing. Furthermore, a comparison between Example 1 and Example 14 shows that the results were good regardless of the printing order of the black ink and color ink. [Industrial Applicability]

[0112] The water-based ink set for ink-jet recording of the present invention can be suitably used in ink-jet printing.

Claims

1. a water-based ink set for inkjet printing comprising at least one black ink and one color ink, each inkjet ink comprising a pigment, a poorly water-soluble resin, a nonionic surfactant, an organic solvent, and water, wherein the content of the organic solvent having a boiling point of higher than 250°C at 1 atmospheric pressure relative to the total amount of the inkjet ink in each inkjet ink is 5% by mass or less, and wherein the value of X-Y, where the total solid content in each black ink is X% by mass and the total solid content in each color ink is Y% by mass, is both 0.1 or more and 5.0 or less, and the total content (% by mass) of the nonionic surfactant in each black ink is less than the total content (% by mass) of the nonionic surfactant in each color ink.

2. 2. The water-based ink set for ink-jet printing according to claim 1, wherein the pigment is in the form of resin particles containing the pigment, and the acid value of the pigment dispersion resin constituting the resin particles is 100 mgKOH / g or more and 400 mgKOH / g or less.

3. 2. The water-based ink set for ink-jet recording according to claim 1, wherein the poorly water-soluble resin is an acrylic resin.

4. 2. The water-based ink set for ink-jet printing according to claim 1, wherein the poorly water-soluble resin contains one or more hydrophobic monomers selected from the group consisting of cycloalkyl (meth)acrylates.

5. 2. The water-based ink set for ink-jet printing according to claim 1, wherein the nonionic surfactant comprises an acetylene glycol surfactant.

6. The water-based ink set for inkjet printing according to claim 1, wherein the value of y - x, where the total content of the nonionic surfactant in each black ink is x% by mass and the total content of the nonionic surfactant in each color ink is y% by mass, is 0.01 or more and 1.00 or less.

7. The water-based ink set for ink-jet printing according to claim 1 , wherein the organic solvent comprises a glycol ether.

8. 2. The water-based ink set for ink-jet printing according to claim 1, which is used in an ink-jet printing apparatus that performs printing by scanning the print head once relative to the print medium.

9. A water-based ink set for inkjet recording according to claim 1, which is used for printing on low-absorbency coated paper or non-absorbent film.

10. Use of an ink set for inkjet recording, comprising at least one black ink and one color ink, each ink containing a pigment, a poorly water-soluble resin, a nonionic surfactant, an organic solvent, and water, wherein the content of organic solvents having a boiling point of over 250°C at 1 atmosphere relative to the total amount of ink in each ink is 5% by mass or less, and where the total solids content in each black ink is X% by mass and the total solids content in each color ink is Y% by mass, the value of X-Y is between 0.1 and 5.0, and the total content (% by mass) of nonionic surfactant in each black ink is less than the total content (% by mass) of nonionic surfactant in each color ink.

11. The use described in claim 10, wherein the pigment is in the form of resin particles containing the pigment, and the acid value of the pigment dispersion resin constituting the resin particles is 100 mg KOH / g or more and 400 mg KOH / g or less.

12. The use according to claim 10, wherein the recording head is used in an inkjet recording device that performs recording by scanning the recording medium once relative to the recording medium.

13. The use according to claim 10, used for printing on low-absorbency coated paper or non-absorbent film.

14. A printing method, comprising printing using an ink set according to any one of claims 1 to 9.

15. A printing device that prints using an ink set described in any one of claims 1 to 9.

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