Water-based ink set for inkjet recording.

Abstract

The present invention relates to an inkjet water-based ink set for recording, a method for manufacturing the same, and a printing method using the inkjet water-based ink set, which can produce a record in which bleeding of characters is suppressed even when characters printed with black ink and backgrounds printed with color ink overlap, regardless of the printing order of black ink and color ink, and in which solid color areas are uniformly wetted and spread, resulting in good solid coverage. [Solution] An inkjet water-based ink set for inkjet recording comprising at least one black ink and one color ink, wherein each inkjet ink contains a pigment, a water-insoluble resin, a nonionic surfactant, an organic solvent, and water, and the content of the organic solvent, which has a boiling point of more than 250°C at 1 atmosphere, in each inkjet ink is 5% by mass or less relative to the total amount of inkjet ink, and the values ​​of X and Y, where X is the total solid content in each black ink and Y is the total solid content in each color ink, are 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.

Description

Technical Field

[0001] The present invention relates to an aqueous ink set for inkjet recording, a method for manufacturing the same, and a printing method using the aqueous ink set for inkjet recording.

Background Art

[0002] The inkjet recording method is a recording method in which ink droplets are directly ejected from fine nozzles and adhered to a recording medium to obtain characters and images. This method has many advantages such as easy full-colorization, low cost, usability of plain paper as a recording medium, and non-contact with the printed matter, and thus has been remarkably popularized. In recent years, due to the spread of digital printing, it has been increasingly used not only for consumer printing but also for commercial printing and industrial printing using low-absorbency coated paper and the like. The demand for inkjet inks is increasing more and more for the purpose of increasing printing speed, improving image quality, and further reducing environmental impact. In order to meet such demands, various inkjet inks have been proposed on the premise of satisfying basic performances such as ejection stability and storage stability.

[0003] For example, Patent Document 1 aims to provide an inkjet recording method capable of obtaining a good image without unevenness or color mixing without thermally deforming the low-absorbency recording medium when used for printing on the low-absorbency recording medium. As an ink set used in the recording method, an ink set that ejects inks in order from the ink having a high static surface tension is disclosed. Further, Patent Document 2 aims to provide an inkjet recording ink set that does not cause color bleeding even on coated paper for low-absorbency printing, and discloses an ink set in which the solid content concentration of black ink is 2.5% by weight or more higher than the solid content concentration of each color ink.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] In inkjet printing, text is sometimes printed with black ink, and then a solid color ink is printed over the text. The reason for this overlapping printing is to make it difficult to detect printing defects caused by misalignment of the landing positions of each color due to improper adjustment of the recording head or uneven transport caused by bending of the mechanically transported recording media. However, to the inventor's knowledge, no inkjet ink set exists that can print text without blurring and with an even background color, regardless of the printing order of black and color inks. For example, it was found that the ink set described in Patent Document 1 has a problem in that if the black ink is printed after the color ink, the characters printed with black ink will bleed. Also, it was found that the inkjet recording ink set described in Patent Document 2 has a problem in that the characters printed with black ink will bleed regardless of the printing order.

[0006] The present invention relates to an inkjet water-based ink set for recording, a method for manufacturing the same, and a printing method using the inkjet water-based ink set, which can produce a record in which bleeding of characters is suppressed even when characters printed with black ink and backgrounds printed with color ink overlap, regardless of the printing order of black ink and color ink, and in which solid color areas are uniformly wetted and spread, resulting in good solid coverage. [Means for solving the problem]

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

[0008] According to the present invention, regardless of the printing order of black ink and color ink, even when characters printed with black ink and backgrounds printed with color ink overlap, bleeding of characters is suppressed, and the solid color areas are uniformly wetted and spread, resulting in a record that provides good solid coverage. The present invention also provides an inkjet water-based ink set for recording, a method for manufacturing the same, and a printing method using the inkjet water-based ink set for recording. [Modes for carrying out the invention]

[0009] As a result of diligent research by the present inventors on the above problems, we have newly discovered that the above problems can be solved by setting the content of a specific organic solvent to below a specific amount, setting the total solid content (mass%) in each black ink to a specific amount greater than the total solid content (mass%) in each color ink, and setting the total content (mass%) of nonionic surfactants in each black ink to be less than the total content (mass%) of nonionic surfactants in each color ink. The mechanism is not clear, but the following reasons are possible. When printing black ink first and then color ink, the amount of highly wettable nonionic surfactant is small, resulting in less wetting and spreading. Furthermore, because the black ink printed first has a high solid content, the viscosity increases due to drying and it becomes fixed on the recording medium before the color ink is ejected and lands on the recording medium, thus reducing bleeding. In addition, water-insoluble resins and pigments dispersed in resins are hydrophobic, so they adsorb hydrophobic nonionic surfactants that easily contribute to wetting. Therefore, the resins abundant in the black ink that forms the characters printed first adsorb the surfactants of the later-printed color ink, minimizing the wetting and spreading of the color ink on the black ink and thus preventing bleeding of the characters. On the other hand, since the color ink contains many nonionic surfactants, it is thought that it enables uniform wetting and good solid filling in the background areas where black ink is not printed. Next, when printing color ink followed by black ink, the color ink printed first contains a large amount of nonionic surfactant, allowing for uniform wetting and good coverage. On the other hand, because the color ink contains more nonionic surfactant than the black ink, its dynamic surface tension decreases rapidly, creating a surface tension difference between it and the black ink, which has less nonionic surfactant and is printed later. Therefore, there is a risk that the color ink with low surface tension will erode the black ink with high surface tension. However, in this ink set, the total solid content of the black ink is high, so the hydrophobic particulate components in the black ink adsorb the nonionic surfactant in overlapping areas and areas at the boundary between text and background colors, minimizing the erosion of the color ink. Furthermore, the increase in concentrated viscosity causes the black ink to quickly fix on the recording medium. It is believed that bleeding of the text is suppressed due to these combined factors. Furthermore, by limiting the content of organic solvents with a boiling point exceeding 250°C at 1 atmosphere to 5% by mass or less relative to the total ink volume, it is possible to prevent a significant decrease in the ink's drying properties, and it is believed that the above effects will be more easily achieved even in recording methods with high printing speeds, such as one-pass printing.

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

[0011] [Inkjet ink] The inkjet ink of the present invention (hereinafter also simply referred to as "ink") contains a pigment, a water-poorly soluble resin, a nonionic surfactant, an organic solvent, and water. 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 in 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 marking up text and images, while "printed material" is a concept that includes printed materials and markings on which text and images are recorded. "(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.

[0013] <Pigments> The pigment contained in the inkjet ink of the present invention may be either an inorganic pigment or an organic pigment. Examples of the inorganic pigment include carbon black, metal oxides, etc. In black ink, carbon black is preferred. Examples of carbon black include furnace black, lamp black, acetylene black, channel black, etc. In white ink, metal oxides such as titanium dioxide, zinc oxide, silica, alumina, magnesium oxide, etc. can be mentioned. Examples of the organic pigment include azo pigment, diazo pigment, phthalocyanine pigment, quinacridone pigment, isoindolinone pigment, dioxazine pigment, perylene pigment, perinone pigment, thioindigo pigment, anthraquinone pigment, quinophthalone pigment, etc. The above-mentioned pigments can be used alone or in combination of two or more. Note that the black ink in this patent refers to ink containing carbon black as a pigment, and the color ink refers to ink containing pigments other than carbon black.

[0014] Preferred forms of the pigment include a pigment that can maintain a dispersed state without a dispersant, that is, a self-dispersing type pigment form, a form of pigment particles dispersed with a low molecular weight or high molecular weight surfactant, and a form of resin particles containing a pigment. Among these, from the viewpoints of dispersion stability and fixing property of the pigment, the form of resin particles containing a pigment is preferred. Also, the resin of the resin particles containing a pigment may be an uncrosslinked resin or a crosslinked resin. Here, "resin particles containing a pigment" (hereinafter also referred to as "pigment-containing resin particles") means particles in which the resin contains the pigment, particles in which a part of the pigment is exposed on the surface of the particles composed of the resin and the pigment, particles in which the resin is adsorbed on a part of the pigment, or a mixture thereof. Among these, particles in which the resin contains the pigment are more preferred.

[0015] 〔Pigment-containing resin particles〕 The resin for pigment dispersion that constitutes the pigment-containing resin particles only needs to be a resin having at least the ability to disperse pigments in an aqueous medium, and it may be water-soluble or water-insoluble, but is preferably a water-insoluble resin. Here, "water-insoluble" means that when a resin dried at 105°C for 2 hours until a constant weight is reached is dissolved in 100 g of water at 25°C until saturation, the dissolved amount is 10 g or less. When the water-insoluble resin is an anionic resin, the dissolved amount is the dissolved amount when the anionic group of the resin is 100% neutralized with sodium hydroxide. When the resin for pigment dispersion that constitutes the pigment-containing resin particles is water-insoluble, it may be a resin having the same constitutional units and physical properties as the water-insoluble resin described below, or a resin having different constitutional units or physical properties.

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

[0017] [(a-1) Hydrophilic Ionic Monomer] (a-1) As the hydrophilic ionic monomer, anionic monomers are preferred, and examples include carboxylic acid monomers, sulfonic acid monomers, etc., and carboxylic acid monomers are more preferred. Examples of carboxylic acid monomers include one or more selected from acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, but one or more selected from acrylic acid and methacrylic acid are more preferred.

[0018] [(a-2) Hydrophobic monomers] (a-2) Specific examples of hydrophobic monomers include those described in paragraphs

[0020] to

[0022] of Japanese Patent Publication No. 2018-83938. 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, macromonomers having a polymerizable functional group at one end are preferred, and one or more selected from styrene, α-methylstyrene, and benzyl (meth)acrylate are more preferred.

[0019] Macromonomers having a polymerizable functional group at one end are compounds with a number-average molecular weight of 500 to 100,000, preferably 1,000 to 10,000, and examples of polymerizable functional groups include acryloyloxy groups or methacryloyloxy groups. As macromonomers, aromatic group-containing monomer-based macromonomers are preferred, and examples of aromatic group-containing monomers that constitute them include the aforementioned aromatic group-containing monomers. 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 monomers] (a-3) Hydrophilic nonionic monomers are monomers that have a high affinity for water and water-soluble organic solvents, such as monomers containing hydroxyl groups or polyalkylene glycol chains. (a-3) Specific examples of the component include those described in paragraph

[0018] of Japanese Patent Publication No. 2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n=1~30) (meth)acrylate and polypropylene glycol (n=2~30) (meth)acrylate are preferred. The above components (a-1) to (a-3) can each be used by using the monomer components contained in each component individually or by mixing two or more of them.

[0021] (Content of each constituent unit in the pigment dispersion resin) The content of constituent units derived from components (a-1) to (a-3) in the pigment dispersion resin is as follows, from the viewpoint of pigment dispersion stability and the storage stability and ejection stability of 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 preferably 50% by mass or less, more preferably 45% by mass or less, and 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, and even more preferably 80% by mass or less. If component (a-3) is present, the content of 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 even 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 preferably 2 or less, more preferably 1.5 or less, and even more preferably 1 or less. In the present invention, the content of constituent units derived from components (a-1) to (a-3) in the pigment dispersion resin can be determined by measurement, or it can be substituted by the charging ratio of raw material monomers containing components (a-1) to (a-3) during the production of the pigment dispersion resin.

[0023] (Manufacturing of resins for pigment dispersion) The pigment dispersion resin can be produced by copolymerizing the monomer mixture using a known polymerization method. Solution polymerization is preferred as the polymerization method. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as water, lower aliphatic alcohols, ketones such as methyl ethyl ketone, ethers, and esters are preferred. During polymerization, polymerization initiators such as azo compounds and persulfates, and polymerization chain transfer agents such as mercaptans 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 resin for pigment dispersion is preferably neutralized with an alkali metal compound, as described later.

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

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

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

[0027] (Process I) It is preferable that at least a portion of the carboxyl groups in the pigment dispersion resin are neutralized using an alkali metal compound. This is thought to increase the charge repulsion force that emerges after neutralization, thereby suppressing the aggregation of pigment particles in inkjet inks and improving the dispersion stability of the pigment. In step I, neutralization is preferably performed so that the pH is between 7 and 11. 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.

[0028] The degree of neutralization of the pigment dispersion 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 preferably 60 mol% or less, more preferably 55 mol% or less, and even more preferably 50 mol% or less, from the viewpoint of crosslinking the pigment dispersion resin with a crosslinking agent. 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 pigment dispersion resin] × 100 In this invention, if an excess of alkali metal compound is used compared to the number of moles of carboxyl groups in the pigment dispersion resin, the degree of neutralization may exceed 100 mol%.

[0029] (Process II) In step II, the dispersion treatment can be performed solely by shear stress to finely atomize the pigment particles to the desired particle size. However, from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture before further pre-dispersion. For pre-dispersion, commonly used mixing and stirring devices such as anchor blades and disperser blades can be used. Dispersion machines used for this dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersants such as paint shakers and bead mills. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When performing dispersion processing using a high-pressure homogenizer, the average particle size of pigment-containing resin particles in the aqueous pigment dispersion can be adjusted by controlling the processing pressure and the number of passes. From the viewpoint of productivity and economic efficiency, the processing pressure is preferably 60 MPa to 300 MPa, and the number of passes is preferably 3 to 30.

[0030] (Process III) In step III, the pigment dispersion resin containing the pigment is crosslinked with a crosslinking agent to form a crosslinked resin, and a pigment aqueous 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, and more preferably a compound having two or more epoxy groups in its molecule. For compounds having two or more epoxy groups in the molecule, the same applies as for the polyfunctional epoxy compounds described later. Furthermore, the preferred range of epoxy equivalents for compounds having two or more epoxy groups in the molecule is also the same as for the polyfunctional epoxy compounds described later.

[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 viewpoint of storage stability and ejection stability of the inkjet ink, and also preferably 65 mol% or less, more preferably 60 mol% or less, and even more preferably 55 mol% or less, from the same viewpoint as above.

[0033] Here, in step III, the degree of crosslinking (mol%) of the crosslinked pigment dispersion resin particles may be the theoretical degree of crosslinking calculated by the following formula. However, if the theoretical degree of crosslinking exceeds 100 mol%, the degree of crosslinking shall be 100 mol%. The degree of crosslinking (mol%) of particles of crosslinked pigment dispersion resin = [Amount of crosslinking agent added / {Equivalent amount of functional groups of crosslinking agent (g / eq.) × Number of moles of carboxylic acid in the pigment dispersion resin contained in the pigment-containing resin particles}] × 100

[0034] From the viewpoint of dispersion stability, the concentration of nonvolatile components (solids content) in the aqueous dispersion of pigment-containing resin particles obtained 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 preferably 25% by mass or less, more preferably 20% by mass or less, and 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 larger, more preferably 70 nm or larger, even more preferably 80 nm or larger, and preferably 350 nm or smaller, more preferably 250 nm or smaller, and even more preferably 150 nm or smaller, from the viewpoint of reducing coarse particles and improving the ejection stability of inkjet ink.

[0037] The average particle size of pigment-containing resin particles in inkjet ink is substantially the same as the average particle size of pigment-containing resin particles in an 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 pigment content 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 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 resin particles containing the pigment, 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 even more preferably 5% by mass or more, from the viewpoint of storage stability and ejection stability of the inkjet ink, and preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 7% by mass or less, and even more preferably 6% by mass or less, from the viewpoint of dispersion stability.

[0040] <Water-soluble resin> As for water-insoluble resins, the form of water-insoluble resin particles is preferred. Examples of water-insoluble resins include vinyl resins, polyester resins, and polyurethane resins. Among these, from the viewpoint of storage stability and ejection stability of inkjet inks, vinyl resins obtained by addition polymerization of vinyl monomers are preferred, and acrylic resins are more preferred. The water-soluble resin preferably contains 0.5% by mass or less of pigment, more preferably 0.1% by mass or less, even more preferably 0.01% by mass or less, and most preferably substantially free of pigment, i.e., 0% by mass. Such vinyl resins preferably contain (b-1) constituent units derived from hydrophilic ionic monomers and (b-2) constituent units derived from hydrophobic monomers, and may further contain (b-3) constituent units derived from hydrophilic nonionic monomers.

[0041] [(b-1) Hydrophilic ionic monomers] (b-1) As hydrophilic ionic monomers, anionic monomers are preferred, including carboxylic acid monomers and sulfonic acid monomers, with carboxylic acid monomers being more preferred. Examples of carboxylic acid monomers include one or more selected from acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, but one or more selected from acrylic acid and methacrylic acid are more preferred.

[0042] [(b-2) Hydrophobic monomers] The water-insoluble 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. As alkyl (meth)acrylates, alkyl (meth)acrylates having an alkyl group with 1 to 22 carbon atoms are preferred; as cycloalkyl (meth)acrylates, cycloalkyl (meth)acrylates having a cycloalkyl group preferably with 4 to 12 carbon atoms, more preferably 5 to 8 carbon atoms are preferred; as aromatic group-containing (meth)acrylates, aromatic group-containing (meth)acrylates having 6 to 22 carbon atoms are preferred. As aromatic group-containing monomers, aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms are preferred. Among these, one or more selected from cycloalkyl (meth)acrylates, styrene, α-methylstyrene, and benzyl (meth)acrylates are more preferred, and cycloalkyl (meth)acrylates are even more preferred. Preferred 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 constituent units derived from cycloalkyl (meth)acrylate in the water-insoluble resin is preferably 30% by mass or more, more preferably 35% by mass or more, even more preferably 40% by mass or more, 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 monomers] (b-3) Hydrophilic nonionic monomers are monomers that have a high affinity for water and water-soluble organic solvents, such as monomers containing hydroxyl groups or polyalkylene glycol chains. (b-3) Specific examples of the component include those described in paragraph

[0018] of Japanese Patent Publication No. 2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n=1~30) (meth)acrylate and polypropylene glycol (n=2~30) (meth)acrylate are preferred. The above components (b-1) to (b-3) can each be used by using the monomer components contained in each component individually or by mixing two or more of them.

[0044] (Manufacturing of water-poorly soluble resin particles) The water-poorly soluble resin particles may be synthesized as appropriate or commercially available. A preferred embodiment of the water-poorly soluble resin particles, in which the water-poorly soluble resin is acrylic resin, is described below. Acrylic resin can be produced by copolymerizing raw material monomers, including the monomers (b-1) to (b-3), etc., 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 methanol, ethanol, acetone, and methyl ethyl ketone are more preferred. 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; 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 is preferably 30°C or higher, more preferably 50°C or higher, and more preferably 95°C or lower, and more preferably 80°C or lower.

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

[0046] (Physical properties of acrylic resin) The acid value of the acrylic resin relating to the water-poorly soluble resin particles is preferably 100 mg KOH / g or more, more preferably 110 mg KOH / g or more, even more preferably 120 mg KOH / g or more, even more preferably 150 mg KOH / g or more, and even more preferably 190 mg KOH / g or more, and from the same viewpoint as above, preferably 350 mg KOH / g or less, more preferably 330 mg KOH / g or less, even more preferably 310 mg KOH / g or less, and even more preferably 290 mg KOH / g or less.

[0047] The weight-average molecular weight of the acrylic resin relating to the water-poorly 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 recordings in low-liquid-absorbent recording media, and from the same viewpoint as above, 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 relating to the poorly water-soluble resin particles is calculated from the mass ratio of the constituent monomers. Furthermore, the weight-average molecular weight of the acrylic resin is measured by the method described in the examples.

[0049] Furthermore, in the production of water-insoluble 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, and more preferably a compound having two or more epoxy groups in its molecule. For compounds having two or more epoxy groups in the molecule, the same applies as for the polyfunctional epoxy compounds described later. Furthermore, the preferred range of epoxy equivalents for compounds having two or more epoxy groups in the molecule is also the same as for the polyfunctional epoxy compounds described later.

[0050] (Polyfunctional 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 hydrocarbon groups with 3 to 8 carbon atoms, even 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 trimethylolpropane polyglycidyl 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 preferably 300 or less, more preferably 200 or less, and even more preferably 170 or less.

[0051] (Manufacturing of cross-linked resin particles) Crosslinked resin particles can be efficiently manufactured by a method comprising the following steps 1 and 2. Crosslinked resin particles manufactured by the method comprising steps 1 and 2 have a crosslinked structure on the surface of the crosslinked resin particles, consisting of a structure derived from carboxyl groups in the water-insoluble resin particles and a structure derived from a polyfunctional epoxy compound. Step 1: A step to obtain an aqueous dispersion of water-insoluble resin particles by neutralizing at least a portion of the carboxyl groups of the water-insoluble resin particles with an alkali metal compound. Step 2: A step to obtain an aqueous dispersion of crosslinked resin particles by adding a polyfunctional epoxy compound to the aqueous dispersion of water-insoluble resin particles obtained in Step 1, and reacting the carboxyl groups in the water-insoluble resin particles with the epoxy groups in the polyfunctional epoxy compound.

[0052] (Process 1) It is preferable that at least a portion of the carboxyl groups of the poorly water-soluble resin particles are neutralized using an alkali metal compound. This is thought to increase the charge repulsion force that emerges after neutralization, thereby suppressing aggregation of crosslinked resin particles in the ink and improving the dispersion stability of the crosslinked resin particles. In step 1, neutralization is preferably performed so that the pH is between 7 and 11. 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.

[0053] The degree of neutralization of the water-insoluble 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 water-insoluble resin particles, and from the viewpoint of the productivity of the crosslinked resin particles, it is preferably 50 mol% or less, more preferably 40 mol% or less, and even more preferably 35 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 water-insoluble resin particles] × 100 In this invention, if an excess of alkali metal compound is used compared to the number of moles of carboxyl groups in the water-insoluble 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 water-insoluble resin particles react with the epoxy groups in the polyfunctional epoxy compound is preferably 50°C or higher, more preferably 70°C or higher, and preferably 95°C or lower, and more preferably 90°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.

[0055] The amount of polyfunctional epoxy compound used is such that, from the viewpoint of obtaining high-quality recordings on low-liquid-absorption recording media, the degree of crosslinking of the resulting crosslinked resin particles is 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. Furthermore, from the same viewpoint as above, the amount is such that the degree of crosslinking of the resulting crosslinked resin particles is 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 degree of crosslinking (mol%) of the crosslinked resin particles may be the theoretical degree of crosslinking calculated by the following formula, which may be used as the degree of crosslinking of the pigment-containing polymer. However, if the theoretical degree of crosslinking exceeds 100 mol%, the degree of crosslinking shall be 100 mol%. Degree of crosslinking of crosslinked resin particles (mol%) = [Amount of polyfunctional epoxy compound added / {Equivalent amount of functional groups of polyfunctional epoxy compound (g / eq.) × Number of moles of carboxylic acid in water-poorly soluble resin particles contained in crosslinked resin particles}] × 100

[0056] From the viewpoint of obtaining high-quality recordings in low-liquid-absorbent recording media, the acid value of the crosslinked resin particles is preferably 35 mg KOH / g or more, more preferably 40 mg KOH / g or more, even more preferably 50 mg KOH / g or more, and from the same viewpoint as above, preferably 220 mg KOH / g or less, more preferably 190 mg KOH / g or less, even more preferably 155 mg KOH / g or less, even more preferably 130 mg KOH / g or less, even more preferably 110 mg KOH / g or less, even more preferably 100 mg KOH / g or less, even more preferably 80 mg KOH / g or less, and even more preferably 70 mg KOH / 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 water-poorly 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 recordings on low-liquid-absorbent recording media, the content of water-insoluble 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, 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 recordings on a low-liquid-absorbent recording medium, the amount 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, even more preferably 4.0% by mass or more, and even more preferably 5% by mass or more. Furthermore, 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 7% by mass or less. When the inkjet ink is color, from the viewpoint of obtaining high-quality recordings on a low-liquid-absorbent recording medium, the amount 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. Furthermore, from the same viewpoint as above, it 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 solid content in inkjet ink is not limited as long as it satisfies the following relationship, but from the viewpoint of obtaining high-quality recordings on low-liquid-absorbent recording media, 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 viewpoint of suppressing ink bleeding and enabling good solid filling, the total solid content in inkjet inks is such that the values ​​of X and Y, where X is the total solid content in each black ink and Y is the total solid content in each color ink, are all between 0.1 and 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. Furthermore, from the same viewpoint as above, preferably 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 and color inks, the above relationship is satisfied between each inkjet. For example, in an ink set embodiment having a total solid content of X in black ink, Y1 in yellow ink, Y2 in magenta ink, and Y3 in cyan ink, the values ​​of X-Y1, X-Y2, and X-Y3 all satisfy the relationship within the above range.

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

[0061] Examples of acetylene glycol-based surfactants include acetylene diols such as 2,4,7,9-tetramethyl-5-decine-4,7-diol, 3,6-dimethyl-4-octin-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, and 2,4-dimethyl-5-hexyn-3-ol, as well as ethylene oxide adducts of these acetylene diols, from the viewpoint of wettability to recording media and defoaming properties. Examples of commercially available acetylene glycol-based surfactants include the "Surfinol" series and "Orfin" series manufactured by Nisshin Chemical Industry Co., Ltd.

[0062] As the ether-based surfactant, polyoxyalkylene alkyl ether type surfactants are preferred. Examples of commercially available ether-based surfactants include the "Emulgen®" series manufactured by Kao Corporation.

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

[0064] The total content of nonionic surfactants in 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 viewpoint of improving the continuous ejection of ink and obtaining good images without unevenness or color mixing, and also 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 surfactant in the inkjet ink is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.5% by mass or more, even more preferably 0.7% by mass or more, and even more preferably 1.0% by mass or more, and from the same viewpoint 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, even more preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less. When the inkjet ink is colored, the total content of nonionic surfactant in the inkjet ink is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.5% by mass or more, even more preferably 0.7% by mass or more, even more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more, and from the same viewpoint 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, even more preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less.

[0065] In the water-based inkjet recording ink set of the present invention, as described above, the total content (mass%) of nonionic surfactant in each black ink is less than the total content (mass%) of nonionic surfactant in each color ink. For example, if the total content of nonionic surfactant in each black ink is x mass%, and the total content of nonionic surfactant in each color ink is y mass%, then 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, and from the same viewpoint as above, preferably 1.00 or less, more preferably 0.80 or less, and even more preferably 0.50 or less. In embodiments using multiple black inks and color inks, the above relationship is satisfied between each inkjet. For example, in an embodiment of an ink set comprising a total nonionic surfactant content of x mass% in black ink, y1 mass% in yellow ink, y2 mass% in magenta ink, and y3 mass% in cyan ink, the values ​​of y1-x, y2-x, and y3-x all satisfy the relationship within the above range.

[0066] <organic solvents> The organic solvents contained in the inkjet ink of the present invention include glycol ethers, polyhydric alcohols, nitrogen-containing heterocyclic compounds such as 2-pyrrolidone, and alkanolamines. Among these, one or more selected from the group consisting of glycol ethers and polyhydric alcohols are preferred from the viewpoint of the spreadability and penetration of the inkjet ink onto the recording medium, storage stability, and ejection stability. Water-soluble organic solvents can be used individually 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 can be one or more selected from the group consisting of ethylene oxide group and propylene oxide group, with the ethylene oxide group being more preferred. Glycol ethers preferably have at least one hydrocarbon group having 2 to 8 carbon atoms.

[0068] As for glycol ethers, those having an octanol-water partition coefficient logP value (hereinafter also simply referred to as "logP") of 0 or greater are preferred from the viewpoint of storage stability and ejection stability of inkjet inks. Examples of glycol ethers having an octanol-water partition coefficient logP value of 0 or greater 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, with dipropylene glycol monomethyl ether being more preferred.

[0069] The glycol ether content 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, even more preferably 5% by mass or more, and even more preferably 7% by mass or more, and from the same viewpoint as above, 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 inkjet inks, one or more polyhydric alcohols selected from the group consisting of propylene glycol (1,2-propanediol), 1,2-hexanediol, and other alkanediols having 2 to 6 carbon atoms, and diethylene glycol are preferred, with propylene glycol being more preferred.

[0071] The polyhydric alcohol content 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, even more preferably 5% by mass or more, and even more preferably 7% by mass or more, and from the same viewpoint as above, 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 viewpoint of storage stability and ejection stability of the inkjet ink, and, from the same viewpoint as above, 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 ink bleeding and enabling good filling, it is preferable to have a low content of organic solvents with a boiling point of more than 250°C at 1 atmosphere. The content of organic solvents with a boiling point of more than 250°C at 1 atmosphere 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, even 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 with a boiling point of more than 250°C at 1 atmosphere 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, polyethylene glycol monomethyl ether, and the like.

[0074] <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.

[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 viewpoint of storage stability and ejection stability of the inkjet ink, and from the same viewpoint as above, preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less.

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

[0077] [Inkjet ink manufacturing method] The inkjet ink of the present invention can be efficiently manufactured by blending the above-mentioned pigment, water-insoluble resin, nonionic surfactant, organic solvent, and water, and optionally other surfactants, other additives, etc., and mixing the mixture. There are no particular restrictions on the method of mixing, but stirring is preferred. The amount of each component blended in the inkjet ink manufacturing method 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 inks) 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 recordings on low-liquid-absorbent recording media, and also preferably 10 mPa·s or less, more preferably 8 mPa·s or less, and even more preferably 6 mPa·s or less, from the same viewpoint as above.

[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. Furthermore, from the viewpoint of material 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 inkjet ink at 32°C can be measured by conventional methods.

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

[0081] [Ink Set Printing Method] The ink set of the present invention can be loaded into a known inkjet printing apparatus and ejected as ink droplets onto a recording medium such as coated paper or film to print images, etc. Any of the following methods can be used for ejecting the ink droplets: piezoelectric, thermal, or electrostatic. The ink set of the present invention is preferably used in an inkjet recording apparatus that records by performing a single relative scan of the recording medium with the recording head.

[0082] As recording media used in printing with the ink set of the present invention, low-absorbent coated paper, particularly gloss coated paper, matte coated paper, and low-absorbent printing substrates such as non-absorbent resin films can also be used. In printing substrates, "low liquid absorption" is a concept that includes both low liquid absorption and non-liquid absorption, where the amount of water absorbed by the resin film substrate during a 100 m / s contact time with pure water is 0 g / m². 2 More than 10g / m 2 This means the following: Examples of resin films include transparent synthetic resin films, such as polyester films like polyethylene terephthalate film; vinyl chloride film; polyolefin films like polypropylene film and polyethylene film; and polyamide films like nylon film. These resin films may be stretched films such as biaxially oriented films and uniaxially oriented films, or unstretched films. Among these, the ink set of the present invention is preferable because it can produce high-quality records on gloss-coated paper and matte-coated paper; therefore, one or more types of paper selected from gloss-coated paper and matte-coated paper are preferred as the printing substrate. [Examples]

[0083] The present invention will be specifically described below with reference to examples and other relevant information. The following examples are merely illustrative of the present invention and do not imply any limitations. "Normal pressure" refers to a state without pressurization or depressurization, and "room temperature" refers to 25°C. In the manufacturing examples, examples, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" unless otherwise specified. The measurement and calculation methods for each physical property are as follows.

[0084] (1) Measurement of the weight-average molecular weight of the resin The results were obtained by gel permeation chromatography. The measurement conditions are shown below. GPC device: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolumn Super AW-H" manufactured by Tosoh Corporation. Eluent: A solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively. Flow rate: 0.5mL / min Standard material: Monodisperse polystyrene kits 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)] (all manufactured by 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 through a syringe filter "DISMIC-13HP" (PTFE, 0.2 μm, manufactured by 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 the laser particle analysis system "ELS-8000" (manufactured by Otsuka Electronics Co., Ltd.), and the resulting average cumulant particle size was used as the average particle size of the pigment-containing acrylic resin particles. The measurement sample contained particles with a concentration of 5 × 10⁻⁶ -3 A dispersion solution diluted with water to a concentration of % (converted to solid content) 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.

[0086] (3) Measurement of the acid value of acrylic resin and pigment-dispersed resin The resin was dissolved in a titration solvent of toluene and acetone (2:1) using a potentiometric automatic titrator (manufactured by Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610). The titration was performed using potentiometric titration with a 0.1N potassium hydroxide / ethanol solution, and the inflection point on the titration curve was defined as the endpoint. The acid value (mgKOH / g) was calculated from the amount of potassium hydroxide solution titrated 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 epoxy groups in the crosslinking agent reacted with the carboxyl 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 determined from the acid value of the acrylic resin or acrylic resin vinyl-based resin before crosslinking, and the degree of crosslinking and mass of the crosslinked resin particles.

[0089] (6) Measurement of solid content concentration of aqueous dispersions of crosslinked resin particles and aqueous dispersions of pigment-containing resin particles 10.0 g of sodium sulfate, which had been stabilized in a desiccator, was accurately weighed into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm). Approximately 1.0 g of the 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 before the mass was accurately weighed. The mass of the sample after removing volatile components was used as the solid content, and the solid content concentration was obtained by dividing it by the mass of the added sample.

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

[0091] (8) Measurement of the melting point of wax The melting point of the wax was determined using a measuring device compliant with JIS K 0064. Specifically, a differential scanning calorimeter (Q20, manufactured by T.A. Instruments) was used to heat the sample to 200°C, and then cool it down to 0°C at a rate of 10°C / min. Next, the sample was heated again at a rate of 10°C / min, and the heat quantity was measured up to 200°C. Among the observed heat of fusion peaks, the temperature of the peak with the largest peak area was defined as the maximum peak temperature of melting, and this peak temperature was defined as the melting point.

[0092] (9) Measurement of the average particle size of wax particles in wax emulsion The average particle size of wax particles in the wax emulsion was determined using a Microtrac particle size analyzer UPA manufactured by Nikkiso Co., Ltd., with a laser wavelength of 780 nm, laser power of 3 mW, and a sample concentration of 5 × 10⁻¹⁶. -3The median of the volume-average particle size distribution (D) was obtained by inputting the refractive index of water (1.333) as the mass percent, injecting approximately 5 mL of the sample into a cell, inputting 1 as the refractive index of the resin, and then inputting 1. 50 The average particle size of the wax particles in the wax emulsion was defined as ).

[0093] <Manufacturing of water-poorly soluble resin particles (aqueous dispersion of cross-linked resin particles)> (Manufacturing 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"). 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) with 12.7 parts of MEK as a polymerization initiator. 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 mixture from two dropping funnels and the polymerization initiator solution were then continuously added dropwise 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 resin A1 (acid value: 200 mg KOH / g, weight-average molecular weight: 21,000).

[0094] Manufacturing Example I-2: Manufacturing of Acrylic Resin A2 Acrylic resin A2 was prepared in the same manner as in Production Example I-1, except that the monomer mixture consisted of 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 obtained acrylic resin A2 was 200 mgKOH / g, and the weight-average molecular weight was 20,000.

[0095] (Manufacturing of aqueous dispersions of cross-linked resin particles) Manufacturing Example I-3: Production of Aqueous Dispersion 1 of Crosslinked Resin Particles (Process 1) 40.0 parts of acrylic resin A1 were dissolved in 60.0 parts of MEK, and 10.1 parts of a 5N sodium hydroxide aqueous solution (16.9% sodium hydroxide solids, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration) were added to achieve a neutralization degree of acrylic resin A1 of 30 mol%. Then, 200.0 parts of water were added over 1 hour. After the addition was complete, the MEK was removed using an evaporator, and deionized water was added to obtain an aqueous dispersion of acrylic resin A1 with a solid content concentration of 20% by mass. (Process 2) 200.0 parts of the aqueous dispersion of acrylic resin A1 obtained in step 1 were mixed with 12.0 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-321LT, epoxy equivalent: 140) as a crosslinking agent. The mixture was then heated at 80°C for 5 hours with stirring to react the carboxyl groups in acrylic resin A1 with the epoxy groups in trimethylolpropane polyglycidyl 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.). Deionized water was added to achieve a solid content concentration of 20% by mass to obtain aqueous dispersion of crosslinked resin particles 1. The degree of crosslinking of the crosslinked resin particles in aqueous dispersion of crosslinked resin particles 1 was 60 mol%, the acid value was 60.4 mg KOH / g, and the average particle size was 99 nm.

[0096] Manufacturing Example I-4: Production of Aqueous Dispersion 2 of Crosslinked Resin Particles Aqueous dispersion 2 of crosslinked resin particles was prepared in the same manner as in production example I-3, except that acrylic resin A1 was replaced with acrylic resin A2. The resulting aqueous dispersion 2 had a degree of crosslinking of 60 mol%, an acid value of 61.5 mg KOH / g, and an average particle size of 105 nm.

[0097] <Manufacturing of aqueous dispersion of pigment-containing resin particles> (Manufacturing of acrylic resin B1) Manufacturing of acrylic resin B1 Acrylic resin B1 was prepared in the same manner as in Production Example I-1, except that the monomer mixture solution consisted of 30.8 parts acrylic acid, 69.2 parts styrene, and 100 parts 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 dispersions of pigment-containing resin particles) Manufacturing Example II-1: Production of Aqueous Dispersion 1 of Pigment-Containing Resin Particles (Process I) 25 parts of the obtained acrylic resin B1 were dissolved in 78.6 parts of MEK, and 10.1 parts of a 5N sodium hydroxide aqueous solution (16.9% sodium hydroxide solids, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration) were added to achieve a neutralization degree of 40 mol% of acrylic resin B1, thereby obtaining 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 deionized water and 100.0 parts of carbon black pigment (CI Pigment Black 7, manufactured by Cabot, trade name: Monarch 717) were added, and the mixture was stirred for 60 minutes at 20°C with a disperser (manufactured by Asada Iron Works Co., Ltd., trade name: Ultra Disperser) rotating the disperser blades at 7000 rpm. Next, the mixture was subjected to a 15-pass dispersion treatment at a pressure of 150 MPa using a microfluidizer (Microfluidics, high-pressure homogenizer, product name: M-140K) to obtain a dispersion. 250.0 parts of deionized water were added to the obtained dispersion, and after stirring, MEK was completely removed under reduced pressure at 60°C, and some of the water was further 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 (solid content concentration 25% by mass) in which the pigment was dispersed in acrylic resin B1.

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

[0101] Manufacturing Example II-2: Production of Aqueous Dispersion 2 of Pigment-Containing Resin Particles Aqueous dispersion of pigment-containing resin particles 2 (solid content concentration 20% by mass, cyan pigment 15.7%, pigment dispersion 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 cyan pigment (manufactured by DIC Corporation, product name: Fastogen Blue FA5380, Pigment Blue 15:3).

[0102] [Manufacturing of ink sets] Example 1 (Manufacturing of black ink) 30.0 parts of crosslinked resin particle aqueous dispersion 1 (solid content concentration 20% by mass) (breakdown: resin 6.0 parts, water 24.0 parts), 30.0 parts of pigment-containing resin particle aqueous dispersion 1 (solid content concentration 20% by mass) (breakdown: carbon black 4.7 parts, pigment-dispersing resin B1 1.3 parts, water 24.1 parts), 10.0 parts of propylene glycol (manufactured by AGC Inc.), 10.0 parts of dipropylene glycol monomethyl ether (manufactured by Nippon Emulsifier Co., Ltd.), 1.0 part of Surfinol 104PG-50 (manufactured by Nisshin Chemical Industry Co., Ltd., 50% propylene glycol solution of 2,4,7,9-tetramethyl-5-decine-4,7-diol), 0.8 parts of Emulgen (registered trademark) 120 (manufactured by Kao Corporation, ethylene oxide adduct of lauryl alcohol, 100% effective content), KF-6011 (manufactured by Shin-Etsu Chemical Co., Ltd., polyether-modified silicone) 0.2 parts of 100% of the active ingredient and 2-dimethylaminoethanol (reagent manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed to achieve an ink pH of 8.5. Deionized water was then added to bring the total volume to 100 parts, and the mixture was filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain inkjet ink 1.

[0103] (Manufacturing of cyan ink (color ink)) Inkjet ink 2 was obtained in the same manner as the production of black ink, except that the aqueous dispersion of pigment-containing resin particles 1 was changed to an aqueous dispersion of pigment-containing resin particles 2. Furthermore, inkjet ink 1 (black ink) and inkjet ink 2 (cyan ink) were evaluated as an ink set.

[0104] Examples 2-15, Comparative Examples 1-4 (Manufacturing of inkjet inks 3-34) 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, and ink sets for each example and comparative example were obtained. The amounts of each component in Tables 1 and 2 are the actual formulation amounts.

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

[0106] [Ink set review] <Measurement of line width of black ink in overprinted areas> In an environment with a temperature of 25±1℃ and a relative humidity of 30±5%, inkjet ink 1 and inkjet ink 2 were loaded into different inkjet recording heads of a printing evaluation device (manufactured by Trytech Co., Ltd.) equipped with an inkjet recording head (Kyocera Corporation, "KJ4B-HD06MHG-STDV", piezo line head). The head voltage was set to 26V, the drive frequency to 20kHz, the head temperature to 32℃, the ejection droplet volume to 7pl, the resolution to 600×600dpi, the number of pre-ejection flushing cycles to 200, and the negative pressure to -4.0kPa. Matte coated paper (GRAFICAS Y FORMULARIOS, product name: Fitnes Matt) was used, and the print medium was fixed to the transport table under reduced pressure with the longitudinal direction of the print medium and the transport direction being the same. A print command was transferred to the printing evaluation device, and a print pattern with a 3cm square area and a duty cycle of 100% was printed in a single pass using cyan ink, overlapping a line of 20 dots of black ink, to obtain a printed material. The width of the black lines in areas where the printed parts overlapped was measured using the "PIAS-II" handheld image evaluation system manufactured by Quality Engineering Associates Inc. The smaller the line width, the less bleeding occurs, resulting in a more readable printed material. Since the line width in areas without overlapping printing was 950 μm, a line width of 950 μm to 1010 μm is acceptable for practical use, preferably 990 μm or less, and more preferably 970 μm or less.

[0107] <Measurement of OD value of color ink in solid print areas> The optical density (hereinafter referred to as "OD value") of the unprinted color ink solid areas of the printed material obtained in the above-mentioned "Measurement of Black Ink Line Width in Overprinted Areas" was measured using the X-Rite eXact Standard small spectrophotometer under the conditions of light source D65, field of view 2°, and STATUS E. If the ink does not spread evenly across the paper, streaks and uneven coloring occur during solid printing, leading to a decrease in the OD value. Since streaks and uneven coloring result in an uneven appearance and are judged as low image quality, a high OD value is desirable. An OD value of 1.55 or higher is acceptable for practical use, preferably 1.65 or higher, and more preferably 1.80 or higher.

[0108] The ink sets for Examples 2-15 and Comparative Examples 1-4, shown in Tables 1 and 2, were evaluated in the same manner. For each example, the ink on the left side of the ink set was set in the printer to be 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 character bleeding and provided good solid filling. Furthermore, a comparison between Example 1 and Example 14 shows that the results were good regardless of the printing order of the black and color inks. [Industrial applicability]

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

Claims

1. An inkjet water-based ink set for inkjet recording, comprising at least one black ink and one color ink, wherein each inkjet ink contains a pigment, a water-insoluble resin, a nonionic surfactant, an organic solvent, and water, and the content of the organic solvent, which has a boiling point of more than 250°C at 1 atmosphere, in each inkjet ink is 5% by mass or less relative to the total amount of inkjet ink, and 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 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. The water-based inkjet recording ink set according to claim 1, wherein the form of the pigment is in the form of resin particles containing the pigment, and the acid value of the pigment-dispersing resin constituting the resin particles is 100 mg KOH / g or more and 400 mg KOH / g or less.

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

4. The water-soluble resin comprises one or more hydrophobic monomers selected from the group consisting of cycloalkyl (meth)acrylates, as described in claim 1.

5. The water-based ink set for inkjet recording according to claim 1, wherein the nonionic surfactant includes an acetylene glycol-based surfactant.

6. The water-based ink set for inkjet recording according to claim 1, wherein 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%, and the value of y-x is 0.01 or more and 1.00 or less in both cases.

7. The aqueous ink set for inkjet recording according to claim 1, wherein the organic solvent contains a glycol ether.

8. The water-based inkjet recording ink set according to claim 1, for use in an inkjet recording device that performs recording by scanning the recording head with respect to the recording medium once.

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