Pigment dispersions and water-based inks

Abstract

This invention provides a pigment dispersion with good storage stability, using a halogen-free pigment as a coloring agent, as an alternative to CI Pigment Green 7, a representative green pigment. [Solution] A pigment dispersion containing a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, and water. The first pigment is a pigment that does not have halogen elements as substituents, and is at least one selected from the group consisting of phthalocyanine pigments and cyanide pigments having an anthraquinone skeleton. The second pigment is a pigment that does not have halogen elements as substituents, and is at least one selected from the group consisting of a yellow pigment having a monoazo skeleton, a yellow pigment having a disazo skeleton, a yellow pigment having a benzimidazolon skeleton, and a yellow pigment having an isoindoline skeleton. The first dispersant and the second dispersant are copolymers having units derived from styrene monomers and units derived from (meth)acrylic acid monomers.

Description

[Technical Field]

[0001] This invention relates to a pigment dispersion and an aqueous ink. [Background technology]

[0002] Among printing inks, colorants used in green ink include the halogenated copper phthalocyanine pigment CI Pigment Green 7. Colorants used in blue ink include the halogenated dioxane pigment CI Pigment Violet 23.

[0003] In addition to the copper phthalocyanine and dioxane halogenated pigments mentioned above, pigments such as azo pigments may contain polychlorinated biphenyls (PCBs), which are residues or decomposition products of the raw materials used in their synthesis. Since PCBs are chemical substances that do not easily decompose in the natural environment, they are subject to regulations. For this reason, in order to reduce harmful substances such as PCBs remaining in pigments, synthesis reaction conditions that are less likely to generate impurities and methods for removing impurities from pigments have been proposed (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2022-534336 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in recent years, there has been a trend to refrain from using halogenated pigments, such as CI Pigment Green 7, which have halogen atoms such as chlorine atoms as substituents in their molecules. Therefore, there is a need for the development of water-based inks that use pigments as colorants to replace CI Pigment Green 7, as well as green pigment dispersions for manufacturing such water-based inks.

[0006] Therefore, an object of the present invention is to provide a pigment dispersion having good storage stability, which uses as a colorant a halogen-free pigment that can replace CI Pigment Green 7, a typical green pigment. Another object of the present invention is to provide a water-based ink having excellent ejection stability, which uses as a colorant a halogen-free pigment that can replace CI Pigment Green 7, a typical green pigment. [Means for solving the problem]

[0007] That is, according to the present invention, there is provided a pigment dispersion comprising a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, and water, wherein the first pigment is a pigment that does not have a halogen element as a substituent and is at least one selected from the group consisting of phthalocyanine pigments and cyan pigments having an anthraquinone skeleton, the second pigment is a pigment that does not have a halogen element as a substituent and is at least one selected from the group consisting of yellow pigments having a monoazo skeleton, yellow pigments having a disazo skeleton, yellow pigments having a benzimidazolone skeleton, and yellow pigments having an isoindoline skeleton, and the first dispersant and the second dispersant are copolymers having units derived from a styrene-based monomer and units derived from a (meth)acrylic acid-based monomer.

[0008] Furthermore, the present invention provides an aqueous ink comprising a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, a water-soluble organic solvent, and water, wherein the first pigment is a pigment that does not have halogen elements as substituents and is selected from the group consisting of phthalocyanine pigments and cyan pigments having an anthraquinone skeleton; the second pigment is a pigment that does not have halogen elements as substituents and is selected from the group consisting of yellow pigments having a monoazo skeleton, yellow pigments having a disazo skeleton, yellow pigments having a benzimidazolon skeleton, and yellow pigments having an isoindoline skeleton; and the first dispersant and the second dispersant are copolymers having units derived from styrene monomers and units derived from (meth)acrylic acid monomers. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a pigment dispersion with good storage stability using a halogen-free pigment as a coloring agent, as an alternative to CI Pigment Green 7, a typical green pigment. Furthermore, according to the present invention, it is possible to provide an aqueous ink with excellent discharge stability using a halogen-free pigment as a coloring agent, as an alternative to CI Pigment Green 7, a typical green pigment. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when a compound is a salt, the salt exists in the ink dissociated into ions, but for convenience, it will be expressed as "contains a salt." Also, in this specification, water-based ink may be simply referred to as "ink." Unless otherwise specified, physical properties are values ​​at room temperature (25°C) and normal pressure (1 atm). When "(meth)acrylic acid" or "(meth)acrylate" is written, it means "acrylic acid, methacrylic acid" and "acrylate, methacrylate," respectively.

[0011] The inventors investigated obtaining a pigment dispersion using a halogen-free pigment as a substitute for CI Pigment Green 7 as a colorant. Specifically, they attempted to investigate pigment dispersions in which cyan and yellow pigments, which do not have halogen elements as substituents, are dispersed separately or simultaneously in a liquid medium. However, it was found that in the case of dispersions in which colorants are mixed, the hue may be unclear or color separation may occur. In particular, depending on the type of dispersant used, the interaction between individual pigments and the dispersant may prevent obtaining the desired particle size and number of coarse particles (i.e., number of coarse particles), or the pigment dispersion may thicken or the particle size may coarseen during long-term storage.

[0012] Therefore, the inventors investigated combinations of pigments that exhibit vivid hues and are less prone to color separation, as well as combinations of dispersants that are less likely to cause thickening or coarsening of particle size during dispersion. As a result, it was found that it is necessary to combine the following first and second pigments, and to use the following copolymers as the first dispersant for dispersing the first pigment and the second dispersant for dispersing the second pigment. By incorporating these components into a pigment dispersion, it is possible to obtain a pigment dispersion (green pigment dispersion) that exhibits a green hue and has good storage stability. The first pigment is a pigment that does not have a halogen element as a substituent, and is at least one selected from the group consisting of phthalocyanine pigments and cyanide pigments having an anthraquinone skeleton. The second pigment is also a pigment that does not have a halogen element as a substituent. Furthermore, the second pigment is at least one selected from the group consisting of a yellow pigment having a monoazo skeleton, a yellow pigment having a disazo skeleton, a yellow pigment having a benzimidazolon skeleton, and a yellow pigment having an isoindoline skeleton. The first and second dispersants are copolymers having units derived from styrene monomers and units derived from (meth)acrylic acid monomers.

[0013] <Pigment dispersion> The pigment dispersion of the present invention contains a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, and water. The components and physical properties of the pigment dispersion will be described below.

[0014] (pigment) The pigment dispersion contains a first pigment and a second pigment. The first pigment is a pigment that does not have a halogen element as a substituent, and is selected from the group consisting of phthalocyanine pigments and cyanide pigments having an anthraquinone skeleton. The second pigment is also a pigment that does not have a halogen element as a substituent. Of these pigments, the second pigment is selected from the group consisting of a yellow pigment having a monoazo skeleton, a yellow pigment having a disazo skeleton, a yellow pigment having a benzimidazolon skeleton, and a yellow pigment having an isoindoline skeleton. At least two types of pigments, the first pigment and the second pigment, are used in the pigment dispersion. Multiple types of each of the first and second pigments may be used.

[0015] Examples of phthalocyanine pigments that do not have halogen elements as substituents include CI Pigment Blue 15:3 and CI Pigment Blue 16. Examples of cyanide pigments having anthraquinone skeletons that do not have halogen elements as substituents include CI Pigment Blue 60.

[0016] Examples of yellow pigments having a monoazo skeleton without halogen elements as substituents include CI Pigment Yellow 74 and CI Pigment Yellow 151. Examples of yellow pigments having a disazo skeleton without halogen elements as substituents include CI Pigment Yellow 155 and CI Pigment Yellow 180. Examples of yellow pigments having a benzimidazolone skeleton without halogen elements as substituents include CI Pigment Yellow 180. Examples of yellow pigments having an isoindoline skeleton without halogen elements as substituents include CI Pigment Yellow 185.

[0017] The first pigment is preferably at least one selected from the group consisting of CI Pigment Blue 15:3, CI Pigment Blue 16, and CI Pigment Blue 60. These pigments are halogen-free pigments that do not contain halogen elements in their molecules.

[0018] The second pigment is preferably at least one selected from the group consisting of CI Pigment Yellow 74, CI Pigment Yellow 151, CI Pigment Yellow 155, CI Pigment Yellow 180, and CI Pigment Yellow 185. These pigments are halogen-free pigments that do not contain halogen elements in their molecules.

[0019] By combining the preferred examples of the first pigment and the preferred examples of the second pigment described above, it becomes easier to obtain a pigment dispersion with a clearer hue and less color separation. Furthermore, by using a combination of the preferred examples of the first pigment and the preferred examples of the second pigment, and further by using a specific dispersant described later, it is possible to further reduce the likelihood of thickening and coarsening of particle size when preparing the pigment dispersion.

[0020] From the viewpoint of dispersibility and storage stability, it is preferable to use the first pigment and the second pigment whose average particle diameter (cumulative 50% particle diameter (D50) of the volume-based particle size distribution) is adjusted to about 100 nm or more and 200 nm or less using a salt milling method or the like.

[0021] The cumulative 50% particle size (D50) of the volume-based particle size distribution of the pigments (first pigment and second pigment) in the pigment dispersion is preferably 100 nm to 200 nm, and more preferably 100 nm to 160 nm. Furthermore, the cumulative 90% particle size (D90) of the volume-based particle size distribution of the pigments in the pigment dispersion is preferably 200 nm to 350 nm, and more preferably 250 nm to 320 nm. By using the preferred combination of the first and second pigments described above, it is easy to keep the D50 and D90 of the pigments in the pigment dispersion within the above ranges. In this specification, the cumulative 50% particle size (D50) and cumulative 90% particle size (D90) of the volume-based particle size distribution of the pigments are values ​​measured using a dynamic light scattering particle size distribution analyzer.

[0022] Furthermore, the number of particles with a particle size of 0.5 μm or more and less than 5.0 μm per 15% by mass of pigment, based on the total mass of the pigment dispersion liquid (i.e., the number of coarse particles), is 1.0 × 10⁻⁶. 10 Preferably, the number is 1.0 × 10 9 It is even more preferable that the number of particles is less than or equal to 10. By using the preferred combination of the first and second pigments described above, it is easier to keep the number of coarse particles of pigment in the pigment dispersion within the above range. In this specification, the above number of coarse particles is determined by measuring the number of particles with a particle size of 0.5 μm or more and less than 5.0 μm in a sample liquid obtained by diluting the pigment dispersion with water to any ratio using a particle counting particle size distribution analyzer, and calculating the value per 15% by mass of pigment from the measured value.

[0023] The ratio of the first pigment to the second pigment in the pigment dispersion can be adjusted according to the target green hue. Preferably, the ratio of the first pigment to the second pigment in the pigment dispersion is in the range of 90:10 to 20:80 by mass ratio, and more preferably in the range of 80:20 to 30:70.

[0024] The content (mass %) of the pigment (total of the first pigment and the second pigment) in the pigment dispersion is preferably 1 mass % or more and 30 mass % or less, and more preferably 5 mass % or more and 20 mass % or less, based on the total mass of the pigment dispersion.

[0025] (dispersant) The pigment dispersion contains a first dispersant for dispersing a first pigment and a second dispersant for dispersing a second pigment. Hereinafter, the first dispersant and the second dispersant may be collectively referred to simply as "dispersants." Dispersants are components for dispersing pigments in pigment dispersions or inks.

[0026] Both the first and second dispersants are copolymers having units derived from styrene monomers and units derived from (meth)acrylic acid monomers. Therefore, the first and second dispersants may be the same or different. From the viewpoint of obtaining a pigment dispersion with even better storage stability, it is preferable that the first and second dispersants are the same dispersant. In this case, the pigment dispersion may contain one or more dispersants common to both the first and second dispersants.

[0027] Examples of styrene-based monomers include styrene, α-methylstyrene, 4-methylstyrene, and 2-chlorostyrene. One of these styrene-based monomers may be used alone, or two or more may be used in combination. In this specification, the term "(meth)acrylic acid-based monomer" encompasses acrylic acid, methacrylic acid, and salts thereof. One of these (meth)acrylic acid-based monomers may be used alone, or two or more may be used in combination. Examples of salts of (meth)acrylic acid include alkali metal salts and ammonium salts. Examples of alkali metal salts include sodium salts and potassium salts.

[0028] The copolymer used as a dispersant may contain, in addition to the styrene-based monomer and the (meth)acrylic acid-based monomer, units derived from other polymerizable monomers copolymerizable therewith. Examples of other polymerizable monomers include (meth)acrylic acid esters; acrylamide, methacrylamide, and derivatives thereof; acrylonitrile, methacrylonitrile, and derivatives thereof; and unsaturated carboxylic acid monomers other than (meth)acrylic acid. Among these, (meth)acrylic acid esters are preferred. Examples of (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination.

[0029] The copolymer used as the dispersant is preferably a copolymer of a styrene-based monomer and a (meth)acrylic acid-based monomer, or a copolymer of a styrene-based monomer, a (meth)acrylic acid ester, and a (meth)acrylic acid-based monomer. Furthermore, the acid group (carboxylic acid group) in the copolymer used as the dispersant may be in the form of a salt neutralized with a basic compound such as potassium hydroxide. By using these copolymers as dispersants, pigment aggregation and sedimentation are less likely to occur, and a pigment dispersion with even better storage stability is more likely to be obtained. The copolymer may be in the form of a random copolymer, a block copolymer, or a graft copolymer.

[0030] In the copolymer, the mass ratio of the units derived from the (meth)acrylic acid-based monomer to the units derived from the styrene-based monomer is preferably 0.1 or more and 1.0 or less, more preferably 0.2 or more and 0.4 or less. When the mass ratio of the units derived from the (meth)acrylic acid-based monomer to the units derived from the styrene-based monomer is within the above range, the continuous ejection stability of an ink prepared using a pigment dispersion containing this copolymer as a dispersant can be improved. Specifically, even when this ink is continuously ejected from an inkjet recording head, deposition of deposits in the ink flow path of the recording head and in the vicinity of the ejection orifices, kogation, and the like are less likely to occur, thereby maintaining stable ejection performance. Therefore, the pigment dispersion is useful as a pigment dispersion for preparing aqueous inkjet inks. Furthermore, the pigment dispersion is particularly useful as a pigment dispersion for preparing aqueous inkjet inks that are ejected from a recording head (so-called thermal-type recording head) by the action of thermal energy.

[0031] The above mass ratio can be determined by the content of units derived from acrylic acid monomers in the copolymer (mass%) / content of units derived from styrene monomers in the copolymer (mass%). Alternatively, for convenience, it can also be determined by the amount of acrylic acid monomer used when synthesizing the copolymer (parts by mass or mass%) of the total monomer / amount of styrene monomer used when synthesizing the copolymer (parts by mass or mass%) of the total monomer.

[0032] The acid value of the copolymer used as a dispersant is preferably 50 mg KOH / g or more and 300 mg KOH / g or less. By using a copolymer with an acid value within the above range as a dispersant, it is possible to prepare an inkjet ink that is less prone to ejection defects, even when ejected from a thermal type recording head. In this specification, the acid value of the copolymer can be taken as a value measured by a potentiometric titrator using a potassium hydroxide-ethanol titration solution.

[0033] The weight-average molecular weight of the copolymer used as a dispersant is preferably 1,000 to 30,000, and more preferably 3,000 to 15,000. In this specification, the weight-average molecular weight of the copolymer can be measured as a value equivalent to standard polystyrene, measured by gel permeation chromatography (GPC).

[0034] The content (mass%) of the dispersant (total of the first and second dispersants) in the pigment dispersion is preferably 0.2% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less, based on the total mass of the pigment dispersion. Furthermore, the content (mass%) of the dispersant (total of the first and second dispersants) in the pigment dispersion is preferably 0.2 times or more and 0.5 times or less in mass ratio to the content (mass%) of the pigment (total of the first and second pigments).

[0035] (water) The pigment dispersion is an aqueous pigment dispersion containing water as a dispersion medium. Deionized water or ion-exchanged water is preferably used as the water. The water content (mass%) in the pigment dispersion is preferably 70% to 90% by mass, based on the total mass of the pigment dispersion.

[0036] (Other ingredients) In addition to the above components, the pigment dispersion may further contain other components as needed, such as resins other than the dispersant, surfactants, waxes, low surface tension organic solvents, wetting agents, penetrating agents, antifoaming agents, preservatives, viscosity adjusters, pH adjusters, chelating agents, plasticizers, antioxidants, and ultraviolet absorbers.

[0037] (Method of manufacturing pigment dispersion liquid) The pigment dispersion can be produced, for example, by mixing the above-mentioned components and then dispersing them using a dispersing device. Specifically, the pigment dispersion can be produced by a dispersing method that can appropriately control dispersion conditions such as dispersion time, peripheral speed, and, if necessary, the type and particle size of the media used. Examples of dispersing devices include roll mills, bead mills, paint shakers, sand mills, agitator mills, nanomizers, homogenizers, microfluidizers, ultimizers, high-shear homomixers, and ultrasonic dispersers.

[0038] Alternatively, a pigment dispersion can be obtained by mixing a first pigment dispersion containing a first pigment, a first dispersant, and water with a second pigment dispersion containing a second pigment, a second dispersant, and water. In this case, the first pigment, the first dispersant, and water can be mixed in advance and subjected to a dispersion treatment to prepare a first pigment dispersion in which the first pigment is dispersed in water by the first dispersant. Similarly, the second pigment, the second dispersant, and water can be mixed in advance and subjected to a dispersion treatment to prepare a second pigment dispersion in which the second pigment is dispersed in water by the second dispersant.

[0039] (Applications of pigment dispersions) Pigment dispersions can be suitably used in any application requiring coloring functionality. Examples of such applications include paints, printing inks, colored molded products, toners for electrostatic image development, color filters for liquid crystal display devices, and water-based inks for inkjet printers. Among these, pigment dispersions are particularly preferred for use in water-based inkjet printers. By using a pigment dispersion, the water-based inks described below can be easily prepared.

[0040] <Water-based ink> The aqueous ink of the present invention contains a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, a water-soluble organic solvent, and water. By incorporating these components into the ink, it is possible to prepare an aqueous ink exhibiting a green hue (aqueous green pigment ink). The first pigment, the second pigment, the first dispersant, and the second dispersant are all the same as those described in the above-mentioned description of the pigment dispersion. From the viewpoint of easily preparing the above-mentioned aqueous ink, it is preferable that the aqueous ink contains the above-mentioned pigment dispersion.

[0041] The content (mass%) of pigment (total of the first pigment and the second pigment) in the ink is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less, based on the total mass of the ink. The content (mass%) of dispersant (total of the first dispersant and the second dispersant) in the ink is preferably 0.02% by mass or more and 7.5% by mass or less, and more preferably 0.2% by mass or more and 5% by mass or less, based on the total mass of the ink.

[0042] (aqueous medium) The ink is an aqueous ink containing water and a water-soluble organic solvent as an aqueous medium. The water contained in the ink is preferably deionized water or ion-exchanged water. The water content (mass %) in the ink is preferably 50% to 95% by mass, and more preferably 50% to 90% by mass, based on the total mass of the ink. The water content (mass %) in the ink is preferably 3% to 48% by mass, and more preferably 3% to 25% by mass, based on the total mass of the ink.

[0043] Examples of the water-soluble organic solvent include glycol ethers such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, and triethylene glycol monobutyl ether; alkanols having 1 to 4 carbon atoms such as methanol, ethanol, propanol, isopropanol, n-butanol, sec-butanol, isobutanol, and t-butanol; carboxylic acid amides such as N,N-dimethylformamide and N,N-dimethylacetamide; ketones or ketoalcohols such as acetone, methyl ethyl ketone, and 2-methyl-2-hydroxypentan-4-one; cyclic ethers such as tetrahydrofuran and dioxane; Examples of suitable water-soluble organic solvents include glycols such as ethanol, triethylene glycol, tetraethylene glycol, propylene glycol, and butylene glycol; polyethylene glycols having an average molecular weight of 200 to 2,000, more specifically, number-average molecular weights of 200, 400, 600, 1,000, and 2,000; acetylene glycol derivatives; polyhydric alcohols such as glycerin, 1,3-butanediol, 1,5-pentanediol, 1,2-hexanediol, 1,6-hexanediol, and 1,2,6-hexanetriol; heterocycles such as 2-pyrrolidone, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, and N-methylmorpholine; and sulfur-containing compounds such as thiodiglycol and dimethyl sulfoxide. These water-soluble organic solvents may be used singly or in combination.

[0044] (Other ingredients) Furthermore, the ink may contain various additives in addition to the above-mentioned components, as needed, such as surfactants, pH adjusters, rust inhibitors, preservatives, fungicides, antioxidants, and reduction inhibitors.

[0045] (Water-based ink manufacturing method) When preparing water-based ink, it is preferable to use the aforementioned pigment dispersion. More specifically, it is preferable to prepare the ink by adding a water-soluble organic solvent or water (ion-exchanged water or deionized water) to the pigment dispersion, stirring, and then filtering. It is also possible to prepare this ink without using the aforementioned pigment dispersion. Specifically, this method uses the first and second pigment dispersions described above, and the ink can also be prepared by mixing the first pigment dispersion, the second pigment dispersion, a water-soluble organic solvent, and water, stirring, and then filtering. [Example]

[0046] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as the gist of the invention is not exceeded. The terms "parts" and "%" used to describe the amounts of components are based on mass unless otherwise specified.

[0047] <Methods for measuring physical properties> (Viscosity of pigment dispersion) The viscosity (mPa·s) of the pigment dispersion was measured at 25°C using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.).

[0048] (D50 and D90 of the volume-based particle size distribution of pigment) A sample liquid was prepared by diluting a pigment dispersion 10,000 times (by mass) with pure water. The volume-based particle size distribution of the pigment in the prepared sample liquid was measured using a dynamic light scattering particle size distribution analyzer (product name "NanoTrack UPA-EX150," manufactured by Nikkiso). The cumulative 50% particle size (D50(nm)) and cumulative 90% particle size (D90(nm)) of this particle size distribution were then determined. The measurement conditions are shown below. [Measurement conditions] Setzero:30s • Measurement time: 120s • Number of measurements: 3 • Particle refractive index: 1.8 • Solvent: Water • Filter: Standard • Sensitivity: Standard

[0049] (Number of particles with a particle size of 0.5 μm or more and less than 5.0 μm) A sample liquid was prepared by diluting a pigment dispersion 10,000 times (by mass) with pure water. The number of particles with a diameter of 0.5 μm or more and less than 5.0 μm in the prepared sample liquid was measured using a particle counting particle size analyzer (product name "AccuSizer780APS", manufactured by Particle Sizing System). The measurement conditions are shown below. From the measured values, the number of particles with a diameter of 0.5 μm or more and less than 5.0 μm per 15% of pigment, relative to the total mass of the pigment dispersion, was determined. [Measurement conditions] • Measurement time: 60s • Number of channels: 128 ·Flow rate: 60mL / min • Sample loop volume: 1 mL • First stage dilution ratio: 30x • Second stage dilution ratio: 40x

[0050] <Manufacturing of Pigment Dispersion> (Pigment dispersion 1) CI Pigment Blue 15:3 (PB15:3) was used as the first pigment, and CI Pigment Yellow 74 (PY74) was used as the second pigment. A mixture was obtained by mixing 12.0 parts of the first pigment (PB15:3), 3.0 parts of the second pigment (PY74), 7.5 parts of a dispersant, and 77.5 parts of deionized water. As the dispersant, a resin was used that was obtained by neutralizing a random copolymer (acid value 120 mg KOH / g, weight-average molecular weight 8,000) obtained by polymerizing 73 parts of styrene, 10 parts of butyl acrylate, and 17 parts of acrylic acid according to a conventional method with an aqueous potassium hydroxide solution. The above mixture was charged into a batch-type vertical sand mill (manufactured by AIMEX) packed with 85 parts of 0.3 mm zirconia beads. After dispersion treatment for 3 hours while cooling with water, the mixture was centrifuged to remove undispersed material containing coarse particles. Furthermore, the mixture was pressure-filtered using a 3.0 μm pore size microfilter (manufactured by Fujifilm) to prepare pigment dispersion 1, which contained 10.0% pigment (solids) and 3.0% resin (polymer).

[0051] (Pigment dispersion 2-15) Pigment dispersions 2 to 15 were obtained in the same manner as the above-described pigment dispersion 1, except that the combinations of the first pigment and the second pigment shown in Table 1 were used instead of the combination of the first pigment and the second pigment used in the preparation of pigment dispersion 1.

[0052] (Pigment Dispersion 16) A mixture was prepared by mixing 15.0 parts of CI Pigment Blue 15:3 (PB15:3) as the first pigment, 7.5 parts of a dispersant, and 77.5 parts of ion-exchanged water. The dispersant used was a resin obtained by neutralizing a random copolymer (acid value 120 mgKOH / g, weight-average molecular weight 8,000) prepared by conventional polymerization of 73 parts of styrene, 10 parts of butyl acrylate, and 17 parts of acrylic acid with an aqueous potassium hydroxide solution. The mixture was placed in a batch-type vertical sand mill (manufactured by Imex) packed with 85 parts of 0.3 mm zirconia beads. After 3 hours of dispersion treatment with water cooling, the mixture was centrifuged to remove undispersed material, including coarse particles. The mixture was then pressure-filtered through a 3.0 μm pore-size microfilter (manufactured by Fujifilm) to prepare a first pigment dispersion with a pigment content (solids) of 10.0% and a resin (copolymer) content of 3.0%. Next, a second pigment dispersion was prepared in the same manner as the first pigment dispersion, except that CI Pigment Yellow 74 (PY74) was used as the second pigment instead of the first pigment (PB15:3) used in preparing the first pigment dispersion.The first pigment dispersion and the second pigment dispersion were then mixed in a mass ratio of first pigment dispersion:second pigment dispersion = 4:1 to prepare pigment dispersion 16, which had a pigment content (solid content) of 10.0% and a resin (copolymer) content of 3.0%.

[0053] (Pigment Dispersion 17) A random copolymer (acid value 120 mg KOH / g, weight-average molecular weight 8,000) was obtained by polymerizing 63 parts benzyl methacrylate, 20 parts butyl acrylate, and 17 parts acrylic acid according to a conventional method, and neutralized with an aqueous potassium hydroxide solution to obtain a resin. Pigment dispersion 17 was prepared in the same manner as pigment dispersion 1 described above, except that this resin was used as a dispersant.

[0054] (Pigment Dispersion 18) Pigment dispersion 18 was prepared in the same manner as the first pigment dispersion in pigment dispersion 16, except that CI pigment green 7 (PG7) was used instead of CI pigment blue 15:3 (PB15:3).

[0055] The obtained pigment dispersions 1-18 were pigment dispersions exhibiting a green hue (green pigment dispersions). The physical properties and evaluation results of the obtained pigment dispersions are shown in Table 1. The meaning of the abbreviations in Table 1 is explained below. PB15:3: CI Pigment Blue 15:3 PB16: CI Pigment Blue 16 PB60: CI Pigment Blue 60 PG7: CI Pigment Green 7 PY74: CI Pigment Yellow 74 PY151: CI Pigment Yellow 151 PY155: CI Pigment Yellow 155 PY180: CI Pigment Yellow 180 PY185: CI Pigment Yellow 185

[0056] <Evaluation of pigment dispersion> (Storage stability) The pigment dispersion was placed in a sealed container and stored in a 60°C constant temperature bath for 60 days. After cooling to room temperature, the viscosity (mPa·s), the D50 (nm) and D90 (nm) values ​​of the pigment, and the number of particles with a particle size between 0.5 μm and 5.0 μm were measured. These values ​​were then compared to the physical properties before storage, and the storage stability of the pigment dispersion was evaluated according to the evaluation criteria shown below. The results are shown in Table 1. A: For all physical properties, including the viscosity of the pigment dispersion, the D50 and D90 of the pigment, and the number of particles with a particle size of 0.5 μm or more and less than 5.0 μm, the rate of change from the physical property value before storage was 10% or less. C: The rate of change in at least one of the physical properties of the pigment dispersion liquid, the pigment D50, the pigment D90, and the number of particles with a particle diameter of 0.5 μm or more and less than 5.0 μm, based on the physical property value before storage, was more than 10%.

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[0058] <Ink preparation> (Ink 1) The following ingredients were mixed and thoroughly stirred, then pressure filtered through a fluoropore filter with a pore size of 2.5 μm to obtain ink 1 (water-based green pigment ink 1). Among the following ingredients, "Acetylenel E100" is the trade name of a nonionic acetylene glycol-based surfactant (manufactured by Kawaken Fine Chemicals). Also, "Proxel GXL(S)" is the trade name of a preservative (manufactured by Arch Chemicals). (Ink 1) • Pigment dispersion 1 30.0 parts Glycerin 10.0 parts 1,2-Hexanediol 5.0 parts Polyethylene glycol (number average molecular weight 1,000) 3.0 parts • Acetyleneol E100 0.5 part • Proxel GXL (S) 0.2 parts • Ion-exchanged water 51.3 parts

[0059] (Ink 2-18) Inks 2 to 18 (water-based green pigment inks 2 to 18) were obtained in the same manner as Ink 1 above, except that pigment dispersion 1 was replaced with the pigment dispersions shown in Table 2.

[0060] <Ink evaluation> (Continuous discharge stability) Each ink cartridge was filled with ink and then placed in an inkjet printer (product name "PIXUS iP3100", manufactured by Canon) that ejects ink from the recording head using thermal energy. After continuously recording solid images on 200 sheets of A4 paper, a nozzle check pattern was recorded. The recorded nozzle check pattern was visually inspected, and the continuous ejection stability was evaluated according to the evaluation criteria shown below. The results are shown in Table 2. A: There was no disturbance in the nozzle check pattern and printing was successful. B: Slight disturbance was observed in the nozzle check pattern. C: Non-ejection and irregularities were clearly observed in the nozzle check pattern, and normal printing was not possible.

[0061] (deposit) Each ink cartridge was filled with ink and set in an inkjet recording device (product name "PIXUS iP3100", manufactured by Canon) that ejects ink from the recording head using thermal energy. After continuously recording solid images on 200 sheets of A4-sized paper, the recording head was removed and the heater and the inside of the nozzle were observed under an optical microscope. The deposits were then evaluated according to the following evaluation criteria. The results are shown in Table 2. A: The deposits were barely visible. C: The deposits were visible throughout the area.

[0062] TIFF2026044393000002.tif128170

[0063] The disclosure of this embodiment includes the following configuration. (Configuration 1) A pigment dispersion containing a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, and water, the first pigment is a pigment that does not have a halogen element as a substituent, and is at least one pigment selected from the group consisting of a phthalocyanine pigment and a cyan pigment having an anthraquinone skeleton; the second pigment is a pigment that does not have a halogen element as a substituent, and is at least one pigment selected from the group consisting of a yellow pigment having a monoazo skeleton, a yellow pigment having a disazo skeleton, a yellow pigment having a benzimidazolone skeleton, and a yellow pigment having an isoindoline skeleton; A pigment dispersion liquid, wherein the first dispersant and the second dispersant are copolymers having a unit derived from a styrene-based monomer and a unit derived from a (meth)acrylic acid-based monomer. (Configuration 2) The first pigment is at least one selected from the group consisting of CI Pigment Blue 15:3, CI Pigment Blue 16, and CI Pigment Blue 60, 2. The pigment dispersion according to claim 1, wherein the second pigment is at least one selected from the group consisting of CI Pigment Yellow 74, CI Pigment Yellow 151, CI Pigment Yellow 155, CI Pigment Yellow 180, and CI Pigment Yellow 185. (Configuration 3) The pigment dispersion according to Configuration 1 or 2, wherein the mass ratio of the units derived from the (meth)acrylic acid-based monomer to the units derived from the styrene-based monomer in the copolymer is 0.2 or more and 0.4 or less. (Configuration 4) A pigment dispersion liquid according to any one of Configurations 1 to 3, used in an aqueous ink for inkjet printers. (Configuration 5) A water-based ink containing a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, a water-soluble organic solvent, and water, the first pigment is a pigment that does not have a halogen element as a substituent, and is at least one pigment selected from the group consisting of a phthalocyanine pigment and a cyan pigment having an anthraquinone skeleton; the second pigment is a pigment that does not have a halogen element as a substituent, and is at least one pigment selected from the group consisting of a yellow pigment having a monoazo skeleton, a yellow pigment having a disazo skeleton, a yellow pigment having a benzimidazolone skeleton, and a yellow pigment having an isoindoline skeleton; The aqueous ink is characterized in that the first dispersant and the second dispersant are copolymers having a unit derived from a styrene-based monomer and a unit derived from a (meth)acrylic acid-based monomer. (Configuration 6) The first pigment is at least one selected from the group consisting of CI Pigment Blue 15:3, CI Pigment Blue 16, and CI Pigment Blue 60, 6. The aqueous ink according to claim 5, wherein the second pigment is at least one selected from the group consisting of CI Pigment Yellow 74, CI Pigment Yellow 151, CI Pigment Yellow 155, CI Pigment Yellow 180, and CI Pigment Yellow 185. (Configuration 7) The aqueous ink according to Configuration 5 or 6, wherein the mass ratio of the units derived from the (meth)acrylic acid-based monomer to the units derived from the styrene-based monomer in the copolymer is 0.2 or more and 0.4 or less. (Configuration 8) The aqueous ink according to any one of Configurations 5 to 7, which is an aqueous ink for inkjet use.

Claims

1. A pigment dispersion comprising a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, and water, The first pigment is a pigment that does not have a halogen element as a substituent, and is at least one selected from the group consisting of phthalocyanine pigments and cyanide pigments having an anthraquinone skeleton. The second pigment is a pigment that does not have a halogen element as a substituent, and is at least one selected from the group consisting of a yellow pigment having a monoazo skeleton, a yellow pigment having a disazo skeleton, a yellow pigment having a benzimidazolone skeleton, and a yellow pigment having an isoindoline skeleton. A pigment dispersion characterized in that the first dispersant and the second dispersant are copolymers having units derived from styrene monomers and units derived from (meth)acrylic acid monomers.

2. The first pigment is at least one selected from the group consisting of C.I. Pigment Blue 15:3, C.I. Pigment Blue 16, and C.I. Pigment Blue 60. The pigment dispersion according to claim 1, wherein the second pigment is at least one selected from the group consisting of C.I. Pigment Yellow 74, C.I. Pigment Yellow 151, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, and C.I. Pigment Yellow 185.

3. The pigment dispersion according to claim 1, wherein the mass ratio of units derived from the (meth)acrylic acid monomer to units derived from the styrene monomer in the copolymer is 0.2 or more and 0.4 or less.

4. The pigment dispersion according to claim 1, which is used in an aqueous inkjet ink.

5. An aqueous ink comprising a first pigment, a second pigment, a first dispersant for dispersing the first pigment, a second dispersant for dispersing the second pigment, a water-soluble organic solvent, and water, The first pigment is a pigment that does not have a halogen element as a substituent, and is at least one selected from the group consisting of phthalocyanine pigments and cyanide pigments having an anthraquinone skeleton. The second pigment is a pigment that does not have a halogen element as a substituent, and is at least one selected from the group consisting of a yellow pigment having a monoazo skeleton, a yellow pigment having a disazo skeleton, a yellow pigment having a benzimidazolone skeleton, and a yellow pigment having an isoindoline skeleton. The aqueous ink is characterized in that the first dispersant and the second dispersant are copolymers having a unit derived from a styrene-based monomer and a unit derived from a (meth)acrylic acid-based monomer.

6. The first pigment is at least one selected from the group consisting of C.I. Pigment Blue 15:3, C.I. Pigment Blue 16, and C.I. Pigment Blue 60.

6. The aqueous ink according to claim 5, wherein the second pigment is at least one selected from the group consisting of C.I. Pigment Yellow 74, C.I. Pigment Yellow 151, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, and C.I. Pigment Yellow 185.

7. The aqueous ink according to claim 5 , wherein the mass ratio of the units derived from the (meth)acrylic acid-based monomer to the units derived from the styrene-based monomer in the copolymer is 0.2 or more and 0.4 or less.

8. The water-based ink according to claim 5, which is a water-based ink for inkjet printing.

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