Pigment dispersion, inkjet ink, method for producing a pigment dispersion, and method for producing an inkjet ink

Abstract

This invention provides a technology for inkjet inks that offers high storage stability and suppresses nozzle clogging in the recording head. [Solution] The pigment dispersion contains an aqueous medium and a pigment dispersion dispersed in the aqueous medium. The pigment dispersion contains a pigment and a specific resin. The specific resin has a crosslinked structure in which a pigment dispersion resin containing a copolymer of a plurality of specific monomers is crosslinked by a crosslinking agent. The plurality of specific monomers contain 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid. The copolymer has an acid value of 170 mg KOH / g to 230 mg KOH / g, a neutralization rate of 40% to 100%, and a mass-average molecular weight of 3000 to 18000. The crosslinking agent contains a plurality of epoxy groups and at least one hydroxyl group in one molecule and has a water solubility of 80% by mass or more. The crosslinking rate of the specific resin is 30% to 70%.

Description

Technical Field

[0001] The present invention relates to a pigment dispersion, an ink for inkjet, a method for producing a pigment dispersion, and a method for producing an ink for inkjet.

Background Art

[0002] For inkjet recording devices, for example, aqueous inks containing pigments and an aqueous medium are used. In such inks, if aggregation or sedimentation of the pigment easily occurs, it becomes difficult to ensure storage stability. On the other hand, Patent Documents 1 to 4 disclose techniques for blending a pigment dispersion resin for enhancing the dispersibility of the pigment in the ink.

[0003] The ink according to Patent Document 1 is copolymerized with 8 to 98% by weight of cyclohexyl acrylate and / or benzyl acrylate, 1 to 31% by weight of methacrylic acid, and 1 to 25% by weight of acrylic acid as main components, and a pigment dispersion resin having an acid value of 10 to 200 mgKOH / g is blended.

[0004] The ink according to Patent Document 2 is blended with a pigment dispersion resin containing 10 to 96% by weight of cyclohexyl methacrylate, 1 to 90% by weight of acrylic acid units, and 1 to 25% by weight of styrene units.

[0005] The ink according to Patent Document 3 contains a pigment dispersion resin in which polymer particles coating the pigment are crosslinked by a crosslinking agent, and also contains a water-insoluble polymer containing a structural unit derived from (meth)acrylic acid and a structural unit derived from (meth)acrylate.

[0006] The ink according to Patent Document 4 contains a pigment dispersion resin having a structure in which a copolymer having a structural unit derived from one or more components selected from styrene monomers, aromatic group-containing acrylates, and styrene macromonomers and a structural unit derived from one or more components selected from acrylic acid and methacrylic acid is crosslinked.

Prior Art Documents

[0007] [Patent Document 1] Patent No. 5532809 [Patent Document 2] International Publication No. 2012 / 118078 [Patent Document 3] Patent No. 7265353 [Patent Document 4] Patent No. 7141791 [Overview of the project] [Problems that the invention aims to solve]

[0008] In inkjet printers, the contact area between the ink and the atmosphere is larger near the recording head, which accelerates ink drying. As a result, the concentration of water-soluble organic solvents increases in the ink near the recording head, making it easier for the pigment dispersion resin to detach from the surface of the pigment. When this leads to pigment aggregation in the ink near the recording head, nozzle clogging of the recording head becomes more likely.

[0009] In this regard, in the inks described in Patent Documents 1 and 2, the pigment dispersion resin is not constrained by the pigment, making it easy for the pigment dispersion resin to detach from the surface of the pigment near the recording head. Furthermore, in the ink described in Patent Document 3, the influence of the water-insoluble polymer makes it easy for an excessive increase in viscosity to occur near the recording head, which makes it easy for the nozzles of the recording head to become clogged.

[0010] Furthermore, in the inks described in Patent Document 4, while the aggregation of pigments can be suppressed when pigments such as Pigment Black 7 or Pigment Blue 15:3 are used, the aggregation of pigments is less likely to be suppressed when quinacridone pigments such as Pigment Red 122 or azo pigments such as Pigment Yellow 74 are used.

[0011] In view of the above circumstances, the object of the present invention is to provide an inkjet ink that can achieve high storage stability and suppress the occurrence of nozzle clogging of the recording head. [Means for solving the problem]

[0012] To achieve the above objective, a pigment dispersion according to one embodiment of the present invention contains an aqueous medium and a pigment dispersion dispersed in the aqueous medium. The above-mentioned pigment dispersion comprises a pigment and a specific resin. The above-mentioned specific resin has a crosslinked structure in which a pigment dispersion resin containing a copolymer of multiple specific monomers is crosslinked with a crosslinking agent. The above-mentioned multiple specific monomers include 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid. In the copolymer of the above-mentioned multiple specific monomers, the acid value is between 170 mg KOH / g and 230 mg KOH / g, the neutralization rate is between 40% and 100%, and the mass-average molecular weight is between 3000 and 18000. The above crosslinking agent contains multiple epoxy groups and at least one hydroxyl group within a single molecule and has a water solubility of 80% by mass or more. The crosslinking rate of the specified resin is between 30% and 70%.

[0013] In this pigment dispersion, specific resins are less likely to detach from the surface of the pigment in the pigment dispersion, thus maintaining the dispersion state of the pigment dispersion in the aqueous medium. Therefore, inkjet inks using this pigment dispersion achieve high storage stability and suppress the occurrence of nozzle clogging in the recording head.

[0014] The above pigment dispersion resin may further contain a styrene-(meth)acrylic acid copolymer. The above-mentioned styrene-(meth)acrylic acid copolymer may be a copolymer of α-methylstyrene, styrene, (meth)acrylic acid, and a nonionic monomer. The above-mentioned plurality of specific monomers may further contain (meth)acrylate esters other than cyclohexyl (meth)acrylate.

[0015] The ink for inkjet according to one embodiment of the present invention contains the above-mentioned pigment dispersion liquid.

[0016] The method for preparing a pigment dispersion liquid according to one embodiment of the present invention includes a dispersion treatment step of dispersing a pigment and a pigment dispersion resin in an aqueous medium to prepare an intermediate pigment dispersion liquid, and adding a crosslinking agent to the intermediate pigment dispersion liquid to crosslink the pigment dispersion resin on the surface of the pigment to form a specific resin, thereby preparing a pigment dispersion liquid. The above-mentioned pigment dispersion resin contains a copolymer of a plurality of specific monomers. The above-mentioned plurality of specific monomers contain 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid. In the copolymer of the above-mentioned plurality of specific monomers, the acid value is 170 mgKOH / g or more and 230 mgKOH / g or less, the neutralization rate is 40% or more and 100% or less, and the mass average molecular weight is 3000 or more and 18000 or less. The above-mentioned crosslinking agent contains a plurality of epoxy groups and at least one hydroxy group in one molecule and has a water solubility rate of 80% or more. The crosslinking rate of the above-mentioned specific resin is 30% or more and 70% or less.

[0017] The method may further include a centrifugation treatment step of applying centrifugation treatment to the pigment dispersion liquid after the above-mentioned crosslinking treatment step.

[0018] The neutralization rate of the above-mentioned pigment dispersion resin may be 40% or more and 100% or less.

[0019] A method for manufacturing an ink for inkjet according to an aspect of the present invention includes a dispersion treatment step of dispersing a pigment and a pigment dispersion resin in an aqueous medium to prepare an intermediate pigment dispersion liquid, a crosslinking treatment step of adding a crosslinking agent to the intermediate pigment dispersion liquid and crosslinking the pigment dispersion resin on the surface of the pigment to form a specific resin, thereby preparing a pigment dispersion liquid, and an ink preparation step of preparing an ink for inkjet by adding a water-soluble organic solvent and water to the pigment dispersion liquid. The pigment dispersion resin contains a copolymer of a plurality of specific monomers. The plurality of specific monomers include 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid. In the copolymer of the plurality of specific monomers, the acid value is 170 mgKOH / g or more and 230 mgKOH / g or less, the neutralization rate is 40% or more and 100% or less, and the mass average molecular weight is 3000 or more and 18000 or less. The crosslinking agent contains a plurality of epoxy groups and at least one hydroxy group in one molecule and has a water solubility rate of 80% or more. The crosslinking rate of the specific resin is 30% or more and 70% or less. [Advantages of the Invention]

[0020] As described above, the present invention can provide a technique capable of obtaining high storage stability and suppressing the occurrence of nozzle clogging in a recording head in an ink for inkjet. [Embodiments for Carrying Out the Invention]

[0021] Embodiments of the present invention will be described. In the present invention, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic", that is, "(meth)acrylic" includes both "acrylic" and "methacrylic".

[0022] [Configuration of Ink for Inkjet] (Schematic Configuration) The inkjet ink according to this embodiment (hereinafter also simply referred to as "ink") contains an aqueous medium and a pigment dispersion dispersed in the aqueous medium. The pigment dispersion contains a pigment and a specific resin that coats the pigment.

[0023] In the ink according to this embodiment, a specific resin has a crosslinked structure in which a pigment dispersion resin, which has the effect of suppressing pigment aggregation in an aqueous medium by adsorption to the surface of the pigment, is crosslinked with a crosslinking agent. In the ink according to this embodiment, because the specific resin coating the pigment has a crosslinked structure, the specific resin is constrained to the surface of the pigment, so it is possible to prevent the specific resin from detaching from the surface of the pigment. In the ink according to this embodiment, while maintaining the effect of suppressing pigment aggregation by the pigment dispersion resin, a strong restraining force on the surface of the pigment by the specific resin is obtained, so the dispersion state of the pigment dispersion in the aqueous medium is easily maintained.

[0024] The ink according to this embodiment contains a copolymer of a plurality of specific monomers as the pigment dispersion resin. The plurality of specific monomers (total 100% by mass) include 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid, and may further contain (meth)acrylic acid esters other than cyclohexyl (meth)acrylate. Furthermore, the copolymer of the plurality of specific monomers has an acid value of 170 mg KOH / g to 230 mg KOH / g, a neutralization rate of 40% to 100%, and a mass-average molecular weight (Mw) of 3000 to 18000.

[0025] In the ink according to this embodiment, the pigment dispersion resin may contain a resin other than the copolymer of the above-mentioned plurality of specific monomers, for example, a styrene-(meth)acrylic acid copolymer. In other words, in the ink according to this embodiment, the specific resin may have a crosslinked structure in which the copolymer of the plurality of specific monomers and the styrene-(meth)acrylic acid copolymer are crosslinked by a crosslinking agent. As a result, the ink according to this embodiment can more effectively suppress pigment aggregation in an aqueous medium. The styrene-(meth)acrylic acid copolymer is preferably a copolymer of α-methylstyrene, styrene, (meth)acrylic acid, and a nonionic monomer.

[0026] In the ink according to this embodiment, the crosslinking agent used for crosslinking the pigment dispersion resin is an epoxy group-containing crosslinking agent that contains multiple epoxy groups and at least one hydroxyl group in one molecule and has a water solubility of 80% by mass or more. The water solubility (mass%) of the crosslinking agent refers to the percentage of the mass of the crosslinking agent dissolved in water when 10 g of the crosslinking agent is mixed with 90 g of water.

[0027] In the ink according to this embodiment, the crosslinking rate of the specific resin is 30% or more and 70% or less. The crosslinking rate is defined as the percentage of moles of repeating units having a crosslinked structure in the specific resin, when the total number of moles of repeating units derived from monomers having functional groups that react with the crosslinking agent is taken as 100%. The crosslinking rate of the specific resin according to this embodiment is the apparent crosslinking rate calculated from the molar equivalents of the carboxyl groups of the specific resin and the molar equivalents of the crosslinking functional groups of the crosslinking agent, and is calculated by the following formula. Crosslinking rate (%) = 100 × (Molar equivalents of crosslinking functional groups of the crosslinking agent / Molar equivalents of carboxyl groups of the specific resin)

[0028] The ink according to this embodiment is not particularly limited in its use, but can be used, for example, to form images on permeable or impermeable recording media. In particular, the ink according to this embodiment is suitable for forming images on permeable recording media. Any recording media with high ink permeability can be used as a permeable recording media, such as printing paper or a medium made from fibers (e.g., fabric). Examples of printing paper include plain paper, copy paper, recycled paper, thin paper, thick paper, and glossy paper.

[0029] The ink according to this embodiment, by having the above-described configuration, can achieve high storage stability and suppress the occurrence of nozzle clogging in the recording head. The reason for this is presumed to be as follows.

[0030] The ink according to this embodiment contains a pigment dispersion comprising a pigment and a specific resin. The pigment, on its own, is a component with low dispersibility in aqueous media. On the other hand, the specific resin is a component with excellent affinity for aqueous media. The pigment dispersion exhibits excellent dispersibility in aqueous media by containing the specific resin together with the pigment. Furthermore, by configuring the specific resin with the monomer composition described above, even better dispersibility of the pigment dispersion in aqueous media can be obtained. Moreover, by keeping the crosslinking rate of the specific resin at 70% or less, good dispersibility of the pigment dispersion in aqueous media can be maintained. As a result, the ink according to this embodiment can obtain high storage stability and suppress the occurrence of nozzle clogging of the recording head.

[0031] Furthermore, the specific resin is crosslinked with a crosslinking agent containing multiple epoxy groups, and has a crosslinked structure that is less susceptible to the effects of water-soluble organic solvents. Therefore, even when the ink dries and the concentration of water-soluble organic solvents increases, the specific resin is less likely to detach from the surface of the pigment. As a result, in the ink according to this embodiment, the detachment of the specific resin from the pigment and the aggregation of the exposed pigment can be suppressed. By setting the crosslinking rate of the specific resin to 30% or more, the effect of suppressing detachment from the surface of the pigment can be obtained effectively. Therefore, in the ink according to this embodiment, the effect of the specific resin in improving dispersibility in aqueous media is easily maintained.

[0032] (Pigment dispersion) The pigment dispersion contains a pigment and a specific resin. The pigment dispersion is composed, for example, of a core containing a pigment and a specific resin coating the core. In the pigment dispersion, the total content of the pigment and the specific resin is preferably 90% by mass or more, and more preferably 100% by mass. In the ink according to this embodiment, from the viewpoint of optimizing color density, hue, or stability, the median diameter of the volume of the pigment dispersion is preferably 30 nm to 200 nm, and more preferably 80 nm to 140 nm.

[0033] In the ink according to this embodiment, the pigment dispersion content is preferably 5.0% by mass or more and 30.0% by mass or less, and more preferably 8.0% by mass or more and 15.0% by mass or less. In the ink according to this embodiment, by setting the pigment dispersion content to 5.0% by mass or more, it is easier to form an image with good image density on the recording medium. Furthermore, in the ink according to this embodiment, by keeping the pigment dispersion content at 30.0% by mass or less, it is easier to ensure sufficient fluidity as an ink.

[0034] (Pigment) Examples of pigments usable in the ink according to this embodiment include yellow pigment, orange pigment, red pigment, blue pigment, purple pigment, and black pigment. Examples of yellow pigments include CI Pigment Yellow (74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193). Examples of orange pigments include CI Pigment Orange (34, 36, 43, 61, 63, and 71). Examples of red pigments include CI Pigment Red (122 and 202). Examples of blue pigments include CI Pigment Blue (15, more specifically 15:3). Examples of purple pigments include CI Pigment Violet (19, 23, and 33). Examples of black pigments include CI Pigment Black (7).

[0035] In the ink according to this embodiment, the pigment content is preferably 3.0% by mass or more and 20.0% by mass or less, and more preferably 7.0% by mass or more and 12.0% by mass or less. In the ink according to this embodiment, the pigment content in the pigment dispersion is preferably 50% by mass or more and 90% by mass or less, and more preferably 70% by mass or more and 80% by mass or less.

[0036] (Specific resin) In the ink according to this embodiment, the mass-average molecular weight of the copolymer of multiple specific monomers constituting the specific resin is 3,000 or more and 18,000 or less. In the ink according to this embodiment, by setting the mass-average molecular weight of the copolymer of multiple specific monomers constituting the specific resin within this range, the efficiency of coating the surface of the pigment with the specific resin is improved, and the pigment dispersion and ink are more easily maintained at an appropriate viscosity.

[0037] In the ink according to this embodiment, the crosslinking rate of the specific resin is preferably 30% to 70%, and more preferably 40% to 60%. In the ink according to this embodiment, setting the crosslinking rate of the specific resin to 30% or more makes it easier to suppress the detachment of the specific resin from the surface of the pigment. Furthermore, in the ink according to this embodiment, setting the crosslinking rate of the specific resin to 70% or less makes it easier to maintain the effect of suppressing pigment aggregation in the aqueous medium by the copolymer of multiple specific monomers before crosslinking.

[0038] In the ink according to this embodiment, the content of the specific resin is preferably 0.5% by mass or more and 10.0% by mass or less, and more preferably 2.0% by mass or more and 4.0% by mass or less. In the pigment dispersion of the ink according to this embodiment, the content of the specific resin is preferably 10% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 30% by mass or less.

[0039] (Crosslinking agent) In the ink according to this embodiment, the number of epoxy groups in the molecule of the crosslinking agent used to crosslink the pigment dispersion resin is preferably two or more, and more preferably three or more. This makes it easier to obtain a sufficient crosslinking rate in a specific resin. Furthermore, in the ink according to this embodiment, the number of hydroxyl groups in the molecule of the crosslinking agent used to crosslink the pigment dispersion resin is preferably one or more, and more preferably two or more. This improves the affinity of the specific resin with the aqueous medium, and can more effectively suppress the occurrence of nozzle clogging of the recording head. Furthermore, in the ink according to this embodiment, the water solubility of the crosslinking agent used to crosslink the pigment dispersion resin is preferably 80% or more, and more preferably 100%.

[0040] Examples of crosslinking agents that can be used to crosslink the pigment dispersion resin in the ink according to this embodiment include sorbitol polyglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, and polyglycerol polyglycidyl ether. Examples of crosslinking agent products that can be used to crosslink the pigment dispersion resin in the ink according to this embodiment include Denacol® EX-614B, Denacol® EX-313, Denacol® EX-421, Denacol® EX-512, and Denacol® EX-521, all of which are crosslinking agents manufactured by Nagase ChemteX Corporation.

[0041] In the ink according to this embodiment, the epoxy equivalent (g / eq.) of the crosslinking agent used for crosslinking the pigment dispersion resin is preferably 100 to 10000, and more preferably 120 to 250, from the viewpoint of efficiently reacting with the carboxyl groups of the pigment dispersion resin and improving the dispersion stability of the pigment dispersion. If the amount of epoxy groups in the crosslinking agent is too high, crosslinking will not proceed efficiently and may even act as water-soluble groups. On the other hand, if the amount of epoxy groups in the crosslinking agent is too low, there will be insufficient crosslinking sites, which tends to result in insufficient suppression of the detachment of specific resins from the surface of the pigment.

[0042] In the ink according to this embodiment, from the viewpoint of ease of reaction and dispersion stability of the pigment dispersion, it is preferable that the mass-average molecular weight of the crosslinking agent used for crosslinking the pigment dispersion resin is 100 or more and 1500 or less.

[0043] (aqueous medium) The aqueous medium contained in the ink according to this embodiment is a water-containing medium. The aqueous medium may function as a solvent or as a dispersion medium. Specific examples of the aqueous medium include an aqueous medium containing water and a water-soluble organic solvent. In the ink according to this embodiment, the water content is preferably 25.0% by mass or more and 80.0% by mass or less, and more preferably 35.0% by mass or more and 60.0% by mass or less.

[0044] Examples of water-soluble organic solvents that can be used with the ink according to this embodiment include glycol compounds, triol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycols, and dimethyl sulfoxides.

[0045] Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,2-pentanediol, 1,5-pentanediol, 1,2-octanediol, 1,8-octanediol, 3-methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, and 2-ethyl-1,2-hexanediol. Preferably, the glycol compound is ethylene glycol, diethylene glycol, 2-ethyl-1,2-hexanediol, 3-methyl-1,5-pentanediol, 1,3-propanediol, 1,5-pentanediol, or propylene glycol.

[0046] Examples of triol compounds include glycerin and 1,2,3-butanetriol.

[0047] Examples of glycol ether compounds include diethyl diglycol, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, and propylene glycol monomethyl ether. Triethylene glycol monobutyl ether is preferred as the glycol ether compound.

[0048] Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone. 2-pyrrolidone is preferred as the lactam compound.

[0049] Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0050] Examples of acetate compounds include diethylene glycol monoethyl ether acetate.

[0051] In the ink according to this embodiment, it is preferable to use at least one of glycol compounds, triol compounds, glycol ether compounds, and lactam compounds as the water-soluble organic solvent, and it is more preferable to use a mixed solvent that is a mixture of the following combinations (α) to (δ). Combination (α): Ethylene glycol and diethyl diglycol Combination (β): Glycerin, triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol, and 2-pyrrolidone Combination (γ): Triethylene glycol monobutyl ether, 2-pyrrolidone, 1,3-propanediol, and 1,5-pentanediol Combination (δ): 3-methyl-1,5-pentanediol, 1,2,3-butanetriol, 2-ethyl-1,2-hexanediol, and propylene glycol

[0052] In the ink according to this embodiment, the content of the water-soluble organic solvent is preferably 15.0% by mass or more and 50.0% by mass or less, and more preferably 30.0% by mass or more and 40.0% by mass or less.

[0053] (Surfactants) The ink according to this embodiment preferably further contains a surfactant. The surfactant has the effect of increasing the penetration (wetting) of the ink into the recording medium. Examples of surfactants include anionic surfactants, cationic surfactants, and nonionic surfactants. Nonionic surfactants are preferred as the surfactant.

[0054] Examples of nonionic surfactants include polyoxyethylene dodecyl ether, polyoxyethylene hexadecyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and ethylene oxide adducts of acetylene glycol. Ethylene oxide adducts of acetylene glycol are preferred as nonionic surfactants.

[0055] In the ink according to this embodiment, the surfactant content is preferably 0.05% by mass or more and 3.0% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less.

[0056] (Other ingredients) The ink according to this embodiment may contain other components as needed. For example, the ink according to this embodiment may contain various additives such as dissolving stabilizers, humectants, penetrating agents, drying inhibitors, antioxidants, viscosity modifiers, pH adjusters, and antifungal agents.

[0057] [Composition of Pigment Dispersion] The pigment dispersion according to this embodiment is used to prepare the ink according to this embodiment. The pigment dispersion according to this embodiment contains an aqueous medium and a pigment dispersion dispersed in the aqueous medium. The aqueous medium of the pigment dispersion according to this embodiment is typically water. Furthermore, the pigment dispersion of the pigment dispersion according to this embodiment has the same configuration as the pigment dispersion of the ink according to this embodiment described above.

[0058] [Method of manufacturing inkjet ink] (Introduction) An example of an ink manufacturing method according to this embodiment will be described. The ink manufacturing method according to this embodiment includes a dispersion process, a crosslinking process, a centrifugal process, and an ink preparation process. Note that the centrifugal process may be omitted in the ink manufacturing method according to this embodiment. The following describes each step of the ink manufacturing method according to this embodiment.

[0059] (Distributed processing process) In the dispersion process, an intermediate pigment dispersion is prepared by dispersing the pigment and pigment dispersion resin in an aqueous medium, thereby preparing a preliminary pigment dispersion for the stage before crosslinking the pigment dispersion resin. In the intermediate pigment dispersion, a pigment dispersion composed of the pigment and the pigment dispersion resin adsorbed on the surface of the pigment is dispersed in water. Examples of dispersion equipment used in the dispersion process include wet dispersion equipment such as a media-type disperser.

[0060] It is preferable that the pigment dispersion resin used in the dispersion process be pre-neutralized with a basic compound (e.g., KOH or NaON) so that the neutralization rate is between 40% and 100%. The neutralization rate of the pigment dispersion resin is obtained as the percentage of the amount of basic compound used in the dispersion process (100 × (MB / MA)) relative to the theoretical value MA, where MA is the theoretical value of the amount of basic compound required to completely neutralize the pigment dispersion resin, and MB is the actual amount of basic compound used in the dispersion process.

[0061] In the intermediate pigment dispersion, the pigment dispersion resin content is preferably 4.0% by mass or more and 25.0% by mass or less. Furthermore, the pigment content is preferably 1.0% by mass or more and 30.0% by mass or less. In the dispersion process, an antifoaming agent may be added to the intermediate pigment dispersion. In the intermediate pigment dispersion, the antifoaming agent content is preferably 0.01% by mass or more and 0.1% by mass or less.

[0062] In the dispersion process, it is preferable to remove coarse particles from the intermediate pigment dispersion by filtration using a filter (for example, with a pore size of 5 μm).

[0063] (Crosslinking process) In the crosslinking process, a crosslinking agent is added to the intermediate pigment dispersion obtained in the dispersion process. This causes the carboxyl groups of the pigment dispersion resin adsorbed on the surface of the pigment to react with the epoxy groups of the crosslinking agent, resulting in the crosslinking of the pigment dispersion resin on the surface of the pigment and obtaining the pigment dispersion according to this embodiment. In the crosslinking process, it is preferable to add the crosslinking agent to the intermediate pigment dispersion and then heat and stir the intermediate pigment dispersion. The heating temperature can be, for example, 50°C to 95°C. The heating time can be, for example, 30 minutes to 8 hours.

[0064] (Centrifugation process) In the centrifugal treatment process, the pigment dispersion obtained in the crosslinking treatment process is subjected to centrifugal treatment, and the supernatant liquid of the pigment dispersion after centrifugal treatment is replaced with water. This allows the components that were free in the water in the pigment dispersion to be removed in the centrifugal treatment process. For example, the centrifugal treatment conditions in the centrifugal treatment process can be a rotation speed of 10,000 rpm or more and 100,000 rpm or less, and a centrifugal treatment time of 12 hours or more and 48 hours or less.

[0065] (Ink preparation process) In the ink preparation step, a water-soluble organic solvent and water are added to the pigment dispersion obtained in the centrifugal treatment step. This yields the ink according to this embodiment. In the ink preparation step, other components besides the water-soluble organic solvent and water (more specifically, at least one of surfactants, dissolving stabilizers, humectants, penetrating agents, viscosity modifiers, etc.) may be added as needed. In the ink preparation step, it is preferable to stir the obtained ink with a stirrer. In addition, in the ink preparation step, foreign matter and coarse particles may be removed from the obtained ink using a filter (for example, a filter with a pore size of 5 μm or less).

[0066] [Examples and Comparative Examples] Ink preparation and evaluation were performed as examples and comparative examples of the present invention.

[0067] (Ink preparation) The inks used in the examples and comparative examples were prepared as follows.

[0068] • Synthesis of copolymers of multiple specific monomers A four-necked flask (capacity: 1000 mL) equipped with a stirrer, nitrogen inlet tube, condenser, and dropping funnel contained 100.0 g of isopropyl alcohol and 250.0 g of methyl ethyl ketone. Next, a mixed solution containing several specific monomers and azobisisobutyronitrile (polymerization initiator) was placed in the dropping funnel.

[0069] Nitrogen was introduced into the flask, and the mixture was heated under reflux at 70°C. While heated under reflux at 70°C, the aforementioned mixed solution was supplied into the flask from a dropping funnel over approximately 2 hours. After supplying the mixed solution, heating under reflux at 70°C was continued for another 6 hours. Then, the methyl ethyl ketone solution was supplied into the flask from a dropping funnel over 15 minutes. The methyl ethyl ketone solution contained 150.0 g of methyl ethyl ketone and 0.1 g of azobisisobutyronitrile. After supplying the methyl ethyl ketone solution, heating under reflux at 70°C was continued for another 5 hours. Finally, the contents of the reaction vessel were subjected to reduced pressure to remove the solvent, thereby obtaining the pigment dispersion resin.

[0070] Pigment dispersion resins s1 to s12 were synthesized according to the above procedure. Table 1 shows the specific monomers m1 to m4 used in the synthesis of pigment dispersion resins s1 to s12. Specific monomer m1 is cyclohexyl (meth)acrylate. Specific monomer m2 is (meth)acrylic acid. Specific monomer m3 is a (meth)acrylic acid ester other than cyclohexyl (meth)acrylate. Specific monomer m4 is α-methylstyrene and styrene. Table 1 also shows the amount (mass%) of each specific monomer blended in pigment dispersion resins s1 to s12. Furthermore, Table 2 shows the acid value, mass-average molecular weight, and neutralization rate for pigment dispersion resins s1 to s12.

[0071] [Table 1]

[0072] [Table 2]

[0073] The mass-average molecular weight of the pigment-dispersed resin was measured using gel filtration chromatography (HLC-8020GPC, manufactured by Tosoh Corporation) under the following conditions. Column: TSKgel SuperMultiporeHZ-H manufactured by Tosoh Corporation (semi-micro column with an I.D. of 4.6 mm and a length of 15 cm) Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35mL / min Sample injection volume: 10 μL Measurement temperature: 40℃ Detector: IR detector The calibration curve was created by selecting seven types of TSKgel standard polystyrene manufactured by Tosoh Corporation—F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000—along with n-propylbenzene.

[0074] Furthermore, the acid value of the pigment dispersion resin was measured in accordance with JIS K 0070:1992 (Test methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products).

[0075] Preparation of intermediate pigment dispersion 15.0 parts by mass of pigment, pigment dispersion resin, 0.1 parts by mass of defoaming agent, and ion-exchanged water were mixed to obtain 100 parts by mass of mixture. "SN Deformer 1340" manufactured by Sunopco Corporation was used as the defoaming agent. The obtained mixture was subjected to a dispersion treatment for 4 hours using a bead mill ("Dinomill" manufactured by Willy E. Bakkofen) to obtain an intermediate pigment dispersion. Zirconia beads (0.5 mm in diameter) were used as the media during the dispersion treatment. The media packing ratio in the bead mill vessel was set to 60% by volume. Furthermore, the treatment temperature (chiller temperature) was set to 10°C. After the dispersion treatment, the media was removed, and the obtained intermediate pigment dispersion was filtered through a 5 μm pore size filter to remove foreign matter and coarse particles.

[0076] The following pigments p1 to p4 were used in the examples and comparative examples. p1: FAST YELLOW 7413 (Sanyo Pigment Co., Ltd.) p2: FASTOGEN SUPER MAGENTA JM03 (DIC Corporation) p3: FASTOGEN SUPER MAGENTA JM04 (DIC Corporation) p4: Printex80 (Orion Engineered Carbon)

[0077] • Cross-linking treatment A 1L three-necked flask equipped with a thermometer and stirring blades was used as the reaction vessel, and 100g of the intermediate pigment dispersion was added to the reaction vessel. Next, while maintaining the internal temperature of the reaction vessel at 30°C using a water bath, the crosslinking agent was added to the reaction vessel and stirred thoroughly. Then, the contents of the reaction vessel were stirred at a speed of 150 rpm for 1 hour. Next, the crosslinking agent was added to the reaction vessel, and while stirring at 250 rpm, the internal temperature of the reaction vessel was raised to 75°C at a heating rate of 0.5°C / min. While maintaining the internal temperature of the reaction vessel at 75°C, the contents of the reaction vessel were stirred at 250 rpm for 3 hours. Finally, the pigment dispersion was obtained by cooling the internal temperature of the reaction vessel to room temperature.

[0078] The following crosslinking agents c1 to c7 were used in the examples and comparative examples. c1: Denacol (registered trademark) EX-614B c2: Denacol (registered trademark) EX-313 c3: Denacol (registered trademark) EX-421 c4: Denacol (registered trademark) EX-512 c5: Denacol (registered trademark) EX-521 c6: Denacol (registered trademark) EX-612 c7: Denacol (registered trademark) EX-145

[0079] • Ink preparation (formulation q1) Using a stirrer (Shinto Kagaku Co., Ltd. "Three One Motor BL-600"), a 60.0% by mass pigment dispersion, 6.0% by mass glycerin, 12.0% by mass triethylene glycol monobutyl ether, 12.0% by mass 3-methyl-1,5-pentanediol, 2.0% by mass 2-pyrrolidone, 0.3% by mass nonionic surfactant (Nisshin Kagaku Kogyo Co., Ltd. "Orphine E1004"), and deionized water (remaining amount) were stirred at a rotation speed of 400 rpm. The resulting liquid was filtered using a filter (pore size: 5 μm).

[0080] • Ink preparation (formulation q2) Using a stirrer (Shinto Kagaku Co., Ltd. "Three One Motor BL-600"), a 60.0% by mass dispersion, 10.0% by mass triethylene glycol monobutyl ether, 2.0% by mass 2-pyrrolidone, 10.0% by mass 1,3-propanediol, 12.5% ​​by mass 1,5-pentanediol, 0.3% by mass nonionic surfactant (Nisshin Chemical Industry Co., Ltd. "Orphine E1004"), and deionized water (remaining amount) were stirred at a rotation speed of 400 rpm. The resulting liquid was filtered using a filter (pore size: 5 μm).

[0081] • Ink preparation (formulation q3) Using a stirrer (Shinto Kagaku Co., Ltd. "Three One Motor BL-600"), a 60.0% by mass dispersion, 15.0% by mass propylene glycol, 15.0% by mass 3-methyl-1,5-pentanediol, 0.5% by mass nonionic surfactant (Nisshin Chemical Industry Co., Ltd. "Surfinol 440"), and deionized water (remaining amount) were stirred at a rotation speed of 400 rpm. The resulting liquid was filtered using a filter (pore size: 5 μm).

[0082] (Ink evaluation) In the examples and comparative examples, the storage stability and nozzle clogging of the ink were evaluated.

[0083] • Method for evaluating storage stability Approximately 30g of each ink was placed in a 50mL container, and the containers containing each ink were dried by placing them in a constant temperature oven set to 60°C. In both the examples and comparative examples, the drying rate, calculated using the following formula, was adjusted to 35%. Drying rate (%) = 100 - 100 × (Weight of ink after drying / Weight of ink before drying)

[0084] The particle size of the pigment dispersion before and after drying was measured using a Zetasizer (Malvern). The particle size change rate was calculated using the following formula, with the initial particle size D1 before drying and the dried particle size D2 after drying being used as the evaluation value for storage stability. Particle size change rate (%) = 100 × ((D1 - D2) / D1)

[0085] The evaluation values ​​were assessed according to the following A-C criteria. For storage stability, inks with an A rating are considered acceptable, while inks with B and C ratings are considered unacceptable. A: -5% to less than +5% B: Less than -10% or less than -5% or more, or 5% or more but less than 10% C: -10% or less, or 10% or more.

[0086] • Method for evaluating nozzle clogging For the nozzle clogging evaluation, A4-sized inkjet matte paper (Seiko Epson Corporation's "Super Fine Paper") was used as the evaluation paper. An image forming apparatus (line head-equipped inkjet recording device, Kyocera Document Solutions test machine) was used as the evaluation machine. 100 solid images measuring 150mm x 200mm were continuously printed on the evaluation paper using the evaluation machine.

[0087] Next, after purging the ink from the recording head of the evaluation machine, the recording head was cleaned by wiping the ejection surface. Then, a nozzle check pattern image was formed on evaluation paper using the evaluation machine to confirm that ink was being ejected from all nozzles. Finally, the recording head was cleaned again, and the evaluation machine was left undisturbed for 7 days with the recording head uncapped.

[0088] Next, after performing the recording head cleaning process again, a nozzle check pattern image was formed on the evaluation sheet using the evaluation machine. This nozzle check pattern image was examined, and the ratio of non-discharging nozzles to the total number of nozzles (7968) was calculated. The calculated ratio of non-discharging nozzles was used as the nozzle clogging evaluation value.

[0089] The evaluation was conducted according to the following A and B criteria for nozzle clogging. Ink with an A rating is considered acceptable, while ink with a B rating is considered unacceptable. A: Less than 10% B: 10% or more

[0090] (Examples 1-6) In Examples 1 to 6, the ink was prepared by the method described above, with the type of pigment, the type and content of the pigment dispersion resin, the type and amount of the crosslinking agent, the crosslinking rate of the specific resin, and the ink formulation as shown in Table 3.

[0091] [Table 3]

[0092] The inks from Examples 1 to 6 were evaluated for storage stability and nozzle clogging. Table 4 shows the evaluation results for storage stability and nozzle clogging of the inks from Examples 1 to 6. All of Examples 1 to 6 passed the evaluations for both storage stability and nozzle clogging.

[0093] [Table 4]

[0094] In Comparative Examples 1 to 11, inks were prepared using the method described above, with the type of pigment, the type and content of the pigment dispersion resin, the type and amount of the crosslinking agent, the crosslinking rate of the specific resin, and the ink formulation as shown in Table 3. The inks in Comparative Examples 1 to 11 differed in composition from those in the above examples. Specifically, the inks in Comparative Examples 1 to 3 differ from the inks in the above examples in that they do not use cyclohexyl (meth)acrylate as the specific monomer.

[0095] The ink according to Comparative Example 4 differs from the ink according to the above example in that the acid value of the pigment dispersion resin is less than 170 mgKOH / g. The ink according to Comparative Example 5 differs from the ink according to the above example in that the (meth)acrylic acid content is more than 30% by mass and the acid value of the pigment dispersion resin is more than 230 mgKOH / g. The ink according to Comparative Example 6 differs from the ink according to the above example in that the neutralization rate of the pigment dispersion resin is less than 40%. The ink according to Comparative Example 7 differs from the ink according to the above example in that the mass-average molecular weight of the pigment dispersion resin is more than 18000.

[0096] The ink according to Comparative Example 8 differs from the ink according to the above example in that the crosslinking agent used to crosslink the pigment dispersion resin has one epoxy group per molecule. The ink according to Comparative Example 9 differs from the ink according to the above example in that the water solubility of the crosslinking agent used to crosslink the pigment dispersion resin is less than 80% by mass. The ink according to Comparative Example 10 differs from the ink according to the above example in that the crosslinking rate of the specific resin is less than 30%. The ink according to Comparative Example 11 differs from the ink according to the above example in that the crosslinking rate of the specific resin is more than 70%.

[0097] [Table 5]

[0098] The inks related to Comparative Examples 1 to 11 were evaluated for storage stability and nozzle clogging. Table 6 shows the evaluation results for storage stability and nozzle clogging for the inks related to Comparative Examples 1 to 11. The inks related to Comparative Examples 1 to 5 and 7 to 11 failed at least one of the storage stability and nozzle clogging evaluations. In addition, the ink related to Comparative Example 6 could not be evaluated for storage stability and nozzle clogging because the pigment dispersion was not sufficiently dispersed in the aqueous medium.

[0099] [Table 6]

Claims

1. It contains an aqueous medium and a pigment dispersion dispersed in the aqueous medium, The aforementioned pigment dispersion comprises a pigment and a specific resin. The aforementioned specific resin has a crosslinked structure in which a pigment dispersion resin containing a copolymer of multiple specific monomers is crosslinked with a crosslinking agent. The aforementioned plurality of specific monomers include 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid. In the copolymer of the aforementioned plurality of specific monomers, the acid value is 170 mg KOH / g or more and 230 mg KOH / g or less, the neutralization rate is 40% or more and 100% or less, and the mass-average molecular weight is 3000 or more and 18000 or less. The crosslinking agent contains a plurality of epoxy groups and at least one hydroxyl group in one molecule, and has a water solubility of 80% by mass or more. The crosslinking ratio of the specified resin is 30% or more and 70% or less. Pigment dispersion.

2. A pigment dispersion according to claim 1, The pigment dispersion resin further comprises a styrene-(meth)acrylic acid copolymer. Pigment dispersion.

3. A pigment dispersion according to claim 2, The styrene-(meth)acrylic acid copolymer is a copolymer of α-methylstyrene, styrene, (meth)acrylic acid, and a nonionic monomer. Pigment dispersion.

4. A pigment dispersion according to claim 1 or 2, The plurality of specific monomers further comprises (meth)acrylic acid esters other than cyclohexyl (meth)acrylate. Pigment dispersion.

5. Contains the pigment dispersion according to claim 1 or 2 Inkjet ink.

6. A dispersion process for preparing an intermediate pigment dispersion by dispersing a pigment and a pigment dispersion resin in an aqueous medium, The process includes a crosslinking step of preparing a pigment dispersion by adding a crosslinking agent to the intermediate pigment dispersion and crosslinking the pigment dispersion resin on the surface of the pigment to form a specific resin. The pigment dispersion resin comprises a copolymer of a plurality of specific monomers, The aforementioned plurality of specific monomers include 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid. In the copolymer of the aforementioned plurality of specific monomers, the acid value is 170 mg KOH / g or more and 230 mg KOH / g or less, the neutralization rate is 40% or more and 100% or less, and the mass-average molecular weight is 3000 or more and 18000 or less. The crosslinking agent contains a plurality of epoxy groups and at least one hydroxyl group in one molecule, and has a water solubility of 80% by mass or more. The crosslinking ratio of the specified resin is 30% or more and 70% or less. A method for producing a pigment dispersion.

7. A method for producing a pigment dispersion according to claim 6, The process further includes a centrifugal treatment step in which the pigment dispersion liquid after the crosslinking treatment step is subjected to centrifugal treatment. A method for producing a pigment dispersion.

8. A method for producing a pigment dispersion according to claim 6 or 7, The neutralization rate of the pigment dispersion resin is 40% or more and 100% or less. A method for producing a pigment dispersion.

9. A dispersion process for preparing an intermediate pigment dispersion by dispersing a pigment and a pigment dispersion resin in an aqueous medium, A crosslinking process is performed to prepare a pigment dispersion by adding a crosslinking agent to the intermediate pigment dispersion and crosslinking the pigment dispersion resin on the surface of the pigment to form a specific resin. The process includes an ink preparation step of preparing an inkjet ink by adding a water-soluble organic solvent and water to the aforementioned pigment dispersion, The pigment dispersion resin comprises a copolymer of a plurality of specific monomers, The aforementioned plurality of specific monomers include 60% to 80% by mass of cyclohexyl (meth)acrylate and 22% to 30% by mass of (meth)acrylic acid. In the copolymer of the aforementioned plurality of specific monomers, the acid value is 170 mg KOH / g or more and 230 mg KOH / g or less, the neutralization rate is 40% or more and 100% or less, and the mass-average molecular weight is 3000 or more and 18000 or less. The crosslinking agent contains a plurality of epoxy groups and at least one hydroxyl group in one molecule, and has a water solubility of 80% by mass or more. The crosslinking ratio of the specified resin is 30% or more and 70% or less. A method for manufacturing inkjet ink.

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