Aqueous ink, ink cartridge, inkjet recording method, aqueous pigment dispersion, method for producing aqueous pigment dispersion, and method for producing aqueous ink

Abstract

To provide aqueous ink for inkjet containing C.I.pigment orange 43 which enables recording of an image having good discharge stability and good color development property.SOLUTION: Aqueous ink for inkjet contains a pigment, a resin dispersant, and an aqueous medium. The pigment contains C.I. pigment orange 43. The aqueous ink has an absorbance ratio represented by the following expression (1) of 0.95 or more and 1.15 or less. Absorbance of 50 mass% of an upper layer when the aqueous ink is subjected to centrifugal separation at 217,000 G for 30 minutes in 1.5 mass% of a pigment content is less than 0.20 at a wavelength 350 nm or more and 550 nm or less. Expression (1): absorbance ratio=absorbance of a peak in the vicinity of a wavelength 532 nm / absorbance of a peak in the vicinity of a wavelength of 496 nm.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Pigment inks for inkjet recording are generally based on four colors: the three basic primary colors of yellow, magenta, and cyan, plus black. In addition to these colors, intermediate color inks such as red, green, blue, orange, and violet are used to expand the color gamut. A wide variety of pigments are used in inkjet recording methods; for example, CI Pigment Orange 43, a pigment with a perinone skeleton, is used for inks with an orange hue.

[0003] As a pigment dispersion using CI Pigment Orange 43, for example, Patent Document 1 proposes an aqueous pigment dispersion that can be used to produce ink, in which coarse particles are reduced and the pigment dispersibility is excellent, and the storage stability is at a level that makes it unlikely for the physical properties to change over time.

[0004] Furthermore, Patent Document 2 discloses an inkjet recording method that provides good ink ejection stability and enables recording of images with good color development when ink containing CI Pigment Orange 43 is ejected from a recording head by the action of thermal energy. This inkjet recording method proposes the use of a water-based ink containing CI Pigment Orange 43, calcium, and a resin having an anionic group.

[0005] On the other hand, Patent Document 3 proposes a dispersion liquid for inkjet recording that has excellent storage stability, does not cause foaming or thickening, and has good ejection stability, and the absorbance of the separated liquid after centrifugation is in the range of 0.001 to 1.0 at a wavelength of 250 nm. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2018 / 168486 [Patent Document 2] Japanese Patent Application Publication No. 2019-155679 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-77177 Summary of the Invention [Problem to be solved by the invention]

[0007] The present inventors have investigated the aqueous pigment dispersion containing CI Pigment Orange 43 proposed in Patent Document 1. As a result, by micronizing CI Pigment Orange 43, it was found that the amount of coarse particles was indeed reduced and the dispersibility of the aqueous pigment dispersion was improved. However, on the other hand, it was found that when an aqueous ink prepared using the aqueous pigment dispersion was ejected from a recording head by the action of thermal energy, foreign matter was generated near the recording head's ejection orifices, resulting in insufficient ejection stability. Furthermore, even when calcium ions were added to the ink based on the aqueous ink proposed in Patent Document 2, the ejection stability was still insufficient.

[0008] Furthermore, the inventors of the present invention have examined the dispersion liquid proposed in Patent Document 3 and found that if the absorbance of the separated liquid after centrifugation is within the specified range, the resin content in the dispersion liquid is extremely low, and the ejection stability is also insufficient.

[0009] Therefore, an object of the present invention is to provide an aqueous inkjet ink containing CI Pigment Orange 43, which has good ejection stability and is capable of recording images with good color development. Another object of the present invention is to provide an ink cartridge and an inkjet recording method using this aqueous ink. Still another object of the present invention is to provide an aqueous pigment dispersion that can be used in producing the above-mentioned aqueous ink, a method for producing the aqueous pigment dispersion, and a method for producing an aqueous ink using the aqueous pigment dispersion obtained by the method for producing the aqueous pigment dispersion. [Means for solving the problem]

[0010] That is, according to the present invention, there is provided an aqueous inkjet ink containing a pigment, a resin dispersant, and an aqueous medium, wherein the pigment contains CI Pigment Orange 43, and the absorbance ratio represented by the following formula (1) is 0.95 or more and 1.15 or less: Absorbance ratio= Absorbance of the peak around 532 nm / Absorbance of the peak around 496 nm (1) The aqueous ink is characterized in that when the aqueous ink having a pigment content of 1.5% by mass is centrifuged at 217,000 G for 30 minutes, the absorbance of the upper 50% by mass of the ink is less than 0.20 in the wavelength range of 350 nm to 550 nm.

[0011] According to the present invention, there is also provided an ink cartridge comprising ink and an ink storage section for storing the ink, wherein the ink is the above-mentioned water-based ink.

[0012] According to the present invention, there is also provided an inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, wherein the ink is the above-mentioned aqueous ink.

[0013] Furthermore, according to the present invention, there is provided an aqueous pigment dispersion for use in producing an aqueous inkjet ink, the aqueous pigment dispersion containing a pigment, a resin dispersant, and an aqueous medium, wherein the pigment contains CI Pigment Orange 43, and the absorbance ratio represented by the following formula (1) is 0.95 or more and 1.15 or less: Absorbance ratio= Absorbance of the peak around 532 nm / Absorbance of the peak around 496 nm (1) The aqueous pigment dispersion is characterized in that when the aqueous pigment dispersion having a pigment content of 1.5% by mass is centrifuged at 217,000 G for 30 minutes, the absorbance of the upper 50% by mass of the dispersion is less than 0.20 in the wavelength range of 350 nm or more and 550 nm or less.

[0014] Further, according to the present invention, there is provided a method for producing an aqueous pigment dispersion for use in producing an aqueous inkjet ink, the method comprising: step A of mixing at least a raw pigment, an inorganic salt, and an organic solvent and performing solvent salt milling; step B of dispersing the mixture containing the pigment, resin dispersant, and aqueous medium obtained in step A to obtain a dispersion; and step C of ultrafiltration of the dispersion obtained in step B to obtain an aqueous pigment dispersion, wherein the raw pigment is a pigment having a BET specific surface area of ​​35 m 2 / g or less, and the aqueous pigment dispersion has an absorbance ratio represented by the following formula (1) of 0.95 or more and 1.15 or less, Absorbance ratio= Absorbance of the peak around 532 nm / Absorbance of the peak around 496 nm (1) The present invention provides a method for producing an aqueous pigment dispersion, characterized in that when the aqueous pigment dispersion having a pigment content of 1.5% by mass is centrifuged at 217,000 G for 30 minutes, the absorbance of the upper 50% by mass of the dispersion is less than 0.20 in the wavelength range of 350 nm to 550 nm.

[0015] The present invention also provides a method for producing an aqueous ink for inkjet use, the method comprising the step of mixing the aqueous pigment dispersion produced by the method for producing an aqueous pigment dispersion described above with other ink components. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide an aqueous inkjet ink containing CI Pigment Orange 43, which has good ejection stability and is capable of recording images with good color development. The present invention also provides an ink cartridge and an inkjet recording method using the aqueous ink. Furthermore, the present invention also provides an aqueous pigment dispersion that can be used in producing the above-mentioned aqueous ink, a method for producing the aqueous pigment dispersion, and a method for producing an aqueous ink using the aqueous pigment dispersion obtained by the method for producing the aqueous pigment dispersion. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a cross-sectional view schematically illustrating an embodiment of an ink cartridge of the present invention. [Figure 2] 1A and 1B are diagrams schematically illustrating an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, in which FIG. 1A is a perspective view of the main part of the inkjet recording apparatus, and FIG. 1B is a perspective view of a head cartridge. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in further detail below with reference to preferred embodiments. In this disclosure, unless otherwise specified, physical property values ​​are measured at room temperature (25°C), normal pressure (1 atmosphere = 101,325 Pa), and normal humidity (relative humidity 50%). Furthermore, unless otherwise specified, the term "unit" of a resin dispersant refers to a unit structure corresponding to one monomer. The terms "(meth)acrylic acid" and "(meth)acrylate" refer to "acrylic acid, methacrylic acid" and "acrylate, methacrylate," respectively. Aqueous pigment dispersions for inkjet printing and aqueous inkjet inks may be simply referred to as "pigment dispersions" and "ink," respectively.

[0019] The present inventors first investigated the reason why ink prepared using an aqueous pigment dispersion containing CI Pigment Orange 43 tends to exhibit insufficient ejection stability when ejected from a recording head by the action of thermal energy. As a result, they found that the main cause is the adhesion of foreign matter, primarily composed of CI Pigment Orange 43, to the flow paths of the recording head and the vicinity of the ejection orifices. When ink ejection stability is insufficient, the ejection direction of ink droplets changes, resulting in so-called ejection distortion, and unevenness in the recorded image.

[0020] One reason why foreign matter is likely to adhere is that CI Pigment Orange 43 is a pigment that is prone to crystal growth. CI Pigment Orange 43 is a highly planar molecule that is prone to π-π interactions. Furthermore, interactions between fused heterocyclic moieties and the high molecular symmetry that makes it easy for the molecules to pack together are thought to contribute to its tendency to crystal growth. CI Pigment Orange 43 is also known as CI Vat Orange 7 and is a compound classified as a vat dye. Therefore, it is possible that the high heat environment during ejection and the effects of the anionic resin dispersant may cause some of it to denature into a leuco-like form and dissolve in a monomolecular state.

[0021] Based on the above characteristics, it is speculated that CI Pigment Orange 43 promotes crystal growth and the adhesion and deposition of foreign matter through processes such as aggregation and fusion of pigment particles and precipitation and recrystallization of dissolved monomolecular compounds.

[0022] In the technology disclosed in Patent Document 1, coarse particles are reduced by solvent salt milling of CI Pigment Orange 43, but it has also been found that a large number of fine particles that are refined more than necessary are also generated. It has been found that these fine particles promote crystal growth and foreign matter formation, thereby reducing ejection stability.

[0023] Furthermore, in the technology disclosed in Patent Document 2, the orientation of calcium ions suppresses the decomposition of CI Pigment Orange 43, thereby preventing a decrease in ejection stability. However, it has been found that the effect of suppressing the decomposition of CI Pigment Orange 43 is limited when the formation of foreign matter based on fine particles is dominant as described above.

[0024] Furthermore, as in Patent Document 3, when the amount of CI Pigment Orange 43 fine particles is small and the resin content in the dispersion is extremely low, the dispersion stability of the pigment decreases and the frequency of contact between exposed pigment surfaces increases, promoting the formation of foreign matter. As a result, it was found that the ejection stability also becomes insufficient.

[0025] Based on the above findings, the present inventors have newly discovered that, to ensure the ejection stability of aqueous inks containing CI Pigment Orange 43, it is necessary to reduce the amount of pigment microparticles that cause foreign matter formation. They have also discovered that specifying the absorbance of the separated liquid after centrifugation under specific conditions in the wavelength range related to the absorption of pigment molecules and particles can determine the amount of microparticles. Specifically, for the aqueous ink of the present invention, the absorbance of the upper 50% by weight of the aqueous ink, when centrifuged at 217,000 G for 30 minutes with a pigment content of 1.5% by weight, is set to less than 0.20 in the wavelength range of 350 nm to 550 nm. If the absorbance in the wavelength range of 350 nm to 550 nm is 0.20 or higher, foreign matter formation becomes significant and ejection stability deteriorates. In this disclosure, the absorbance may also be referred to as the "absorbance after centrifugation."

[0026] Next, the present inventors investigated the requirements for achieving high color development in aqueous inks containing CI Pigment Orange 43. Color development requires not only high saturation but also a desirable hue. It was found that discussions based on pigment particle size, as defined in Patent Document 1, provide some correlation with saturation, but are insufficient with respect to hue.

[0027] As a result of our investigations, we have newly discovered that the absorption spectrum of the aqueous ink is appropriate for discussing color development, and that of the four absorption bands exhibited by CI Pigment Orange 43, it is necessary to focus particularly on the longest wavelength absorption band. The longest wavelength absorption band is significantly dependent on the interactions between pigment molecules. Therefore, if the absorption band is unnecessarily large, the color development efficiency will decrease due to large crystal size or aggregation, and the broadened spectral shape will lead to a decrease in saturation from the perspective of color purity. Furthermore, the hue will shift toward reddish. Conversely, if the longest wavelength absorption band is unnecessarily small, saturation will improve, but the two shorter wavelength absorption bands will also increase slightly, resulting in a significant shift in hue toward yellowish. This will undermine the original reddish-orange color characteristic of CI Pigment Orange 43. Furthermore, we have found that the formation of foreign matter also reduces ejection stability.

[0028] Thus, we found that the absorbance ratio, normalized based on the absorbance of the second-longest wavelength absorption band, is appropriate for relatively comparing the magnitude of the longest wavelength absorption band, which has a significant impact on color development and ejection stability. Specifically, in the aqueous ink of the present invention, the ratio (absorbance ratio) of the absorbance of the peak near 532 nm (the first peak from the longest wavelength) to the absorbance of the peak near 496 nm (the second peak from the longest wavelength) is set to 0.95 or more and 1.15 or less. If the absorbance ratio is less than 0.95, the saturation improves but the hue shifts to a yellowish hue. This impairs the original reddish-orange characteristic of CI Pigment Orange 43, and further reduces ejection stability due to the formation of foreign matter. Furthermore, if the absorbance ratio exceeds 1.15, the color development efficiency decreases due to large crystal size or aggregation, leading to a decrease in saturation and a reddish shift in hue.

[0029] It is also important that the aqueous pigment dispersion used in the production of aqueous inkjet inks has an absorbance of less than 0.20 after centrifugation. Specifically, when the aqueous pigment dispersion containing 1.5% by weight of pigment is centrifuged at 217,000 G for 30 minutes, it is important that the absorbance of the top 50% by weight of the dispersion is less than 0.20 in the wavelength range of 350 nm to 550 nm. Since the top 50% by weight of the sample is collected, particles approximately less than 30 nm in size are captured. The inventors have found that if such particles are treated as microparticles and the absorbance of these microparticles in the wavelength range of 350 nm to 550 nm is 0.20 or greater, the formation of foreign matter becomes significant and the ink ejection stability deteriorates. Furthermore, it is important that the aqueous pigment dispersion used in the production of aqueous inkjet inks has an absorbance ratio of 0.95 to 1.15, in order to improve color development. That is, in the aqueous pigment dispersion, the ratio (absorbance ratio) of the absorbance of the peak around 532 nm (the first peak from the long wavelength side) to the absorbance of the peak around 496 nm (the second peak from the long wavelength side) is set to 0.95 or more and 1.15 or less.

[0030] As described above, the present inventors have invented an aqueous ink containing CI Pigment Orange 43, which has good ejection stability and is capable of recording images with good color development. The present inventors have also invented an ink cartridge and an inkjet recording method using the aqueous ink. Furthermore, the present inventors have invented an aqueous pigment dispersion that can be used to produce the aqueous ink, a method for producing the aqueous pigment dispersion, and a method for producing an aqueous ink using the aqueous pigment dispersion obtained by the method for producing the aqueous pigment dispersion.

[0031] <Water-based pigment dispersion> The aqueous pigment dispersion of the present invention is an aqueous pigment dispersion containing a pigment, a resin dispersant, and an aqueous medium, and the pigment contains CI Pigment Orange 43.

[0032] (pigment) The aqueous pigment dispersion contains CI Pigment Orange 43 as a pigment. The aqueous pigment dispersion may contain only CI Pigment Orange 43 as a pigment, or may contain one or more other pigments. The content (mass %) of CI Pigment Orange 43 (solid content) in the aqueous pigment dispersion is preferably 1.0 mass % or more and 50.0 mass % or less, and more preferably 5.0 mass % or more and 30.0 mass % or less, based on the total mass of the aqueous pigment dispersion.

[0033] (Resin dispersant) The aqueous pigment dispersion contains a resin dispersant. The resin dispersant is a resin that disperses pigments, including CI Pigment Orange 43, in the aqueous pigment dispersion. The content (mass%) of the resin dispersant in the aqueous pigment dispersion is preferably 0.1% by mass or more and 10.0% by mass or less, and more preferably 0.5% by mass or more and 5.0% by mass or less, based on the total mass of the aqueous pigment dispersion. Furthermore, the content (mass%) of the resin dispersant in the aqueous pigment dispersion is preferably 0.05 to 0.4 times the mass of the pigment content. The aqueous pigment dispersion may contain one type of resin dispersant alone, or may contain two or more types of resin dispersants.

[0034] Examples of resin dispersants include acrylic resins and urethane resins. Among these, acrylic resins are preferred. As the acrylic resin, a copolymer having a unit derived from a styrene monomer and a unit derived from (meth)acrylic acid is more preferred, and the copolymer may further have a unit derived from a (meth)acrylic acid ester. More specifically, copolymers of a styrene monomer and (meth)acrylic acid, copolymers of a styrene monomer, a (meth)acrylic acid ester, and (meth)acrylic acid, and copolymers in which the acid groups of these copolymers have been neutralized with basic compounds such as potassium hydroxide and sodium hydroxide are even more preferred. These copolymers can be used in any form, including random copolymers, block copolymers, and graft copolymers.

[0035] A unit derived from (meth)acrylic acid is a hydrophilic unit having a carboxylic acid group, which is an anionic group. A unit derived from a styrene-based monomer or a unit derived from a (meth)acrylic acid ester is a hydrophobic unit. Examples of styrene-based monomers include styrene, α-methylstyrene, 2-methylstyrene, 3-methylstyrene, and 4-methylstyrene. One or more of the styrene-based monomers can be used. Examples of the (meth)acrylic acid ester include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and benzyl (meth)acrylate. The acrylic resin can include one or more of the (meth)acrylic acid esters.

[0036] The proportion (mass %) of units derived from styrene-based monomers relative to the total mass of the resin dispersant is more preferably 60 mass % or more and 77 mass % or less. When the proportion of units derived from styrene-based monomers in the resin dispersant is within the above range, the interaction with the π-plane of CI Pigment Orange 43 is improved, and adsorption of the resin dispersant is strengthened, thereby further improving ejection stability. Furthermore, when the proportion of units derived from styrene-based monomers in the resin dispersant is within the above range, pigment fine particles are effectively removed in the ultrafiltration treatment in step C of the production method for an aqueous pigment dispersion described below, thereby further improving ejection stability.

[0037] The acid value of the resin dispersant is preferably 150 mgKOH / g or more and 210 mgKOH / g or less, and more preferably 160 mgKOH / g or more and 200 mgKOH / g or less. When the acid value of the resin dispersant is 160 mgKOH / g or more, it becomes easier to remove pigment fine particles in the ultrafiltration process in step C of the production method of the aqueous pigment dispersion described below, and ejection stability tends to be further improved. On the other hand, when the acid value of the resin dispersant is 200 mgKOH / g or less, the color development of the recorded image tends to be further improved. In this specification, the acid value of the resin dispersant can be a value measured by a potentiometric titrator using a potassium hydroxide-ethanol titrant.

[0038] The weight average molecular weight (Mw) of the resin dispersant is preferably 1,000 or more and 30,000 or less, and more preferably 5,000 or more and 15,000 or less. In this specification, the weight average molecular weight of the resin dispersant can be a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0039] (aqueous medium) The aqueous pigment dispersion contains an aqueous medium containing at least water. The aqueous medium may be water alone, or an aqueous medium in which water is the main solvent and a protic or aprotic organic solvent is used in combination. The organic solvent is preferably one that is miscible or soluble with water in any ratio. In particular, it is preferable to use a homogeneous mixed solvent containing 50% by mass or more of water as the aqueous medium. As the water, it is preferable to use ion-exchanged water or pure water. The water content (mass %) in the aqueous pigment dispersion is preferably 50.0% by mass or more and 95.0% by mass or less, and more preferably 50.0% by mass or more and 90.0% by mass or less, based on the total mass of the aqueous pigment dispersion.

[0040] Protic organic solvents are organic solvents that have a hydrogen atom (acidic hydrogen atom) bonded to oxygen or nitrogen. Furthermore, aprotic organic solvents are organic solvents that do not have an acidic hydrogen atom. Examples of organic solvents include alcohols; alkylene glycols; polyalkylene glycols; glycol ethers; glycol ether esters; carboxylic acid amides; ketones; ketoalcohols; cyclic ethers; nitrogen-containing compounds; and sulfur-containing compounds. One of these organic solvents may be used alone, or two or more may be used in combination. When an organic solvent is used, it may be removed by post-treatment. For example, the organic solvent can be removed by reducing pressure or heating using an evaporator or the like.

[0041] (Characteristics of aqueous pigment dispersions) [Absorbance ratio] The aqueous pigment dispersion has an absorbance ratio represented by the following formula (1) of 0.95 or more and 1.15 or less. Absorbance ratio= Absorbance of the peak around 532 nm / Absorbance of the peak around 496 nm (1)

[0042] Of the four absorption bands exhibited by CI Pigment Orange 43, the longest absorption band peaks at approximately 532 nm, and the second longest absorption band peaks at approximately 496 nm. The absorbance ratio is calculated by diluting the aqueous pigment dispersion 10,000 times with ion-exchanged water and measuring the absorption spectrum in the wavelength range of 200 nm to 800 nm. The absorption spectrum can be measured using, for example, a spectrophotometer (trade name "U-3300", manufactured by Hitachi, Ltd.).

[0043] If the absorbance ratio exceeds 1.15, the coloring efficiency decreases due to large crystal size or aggregation, and the broadened spectral profile leads to a decrease in saturation from the perspective of color purity. Furthermore, the hue shifts toward reddish. If the absorbance ratio is less than 0.95, the saturation improves, but the two absorption bands on the shorter wavelength side also increase slightly, resulting in a significant shift in hue toward yellow. This impairs the original reddish-orange color characteristic of CI Pigment Orange 43. Furthermore, the formation of foreign matter also reduces ejection stability. Furthermore, the absorbance ratio is preferably between 1.00 and 1.12.

[0044] [Absorbance after centrifugation] The aqueous pigment dispersion has an absorbance of less than 0.20 in the wavelength range of 350 nm to 550 nm of the upper 50% by mass layer when the aqueous pigment dispersion with a pigment content of 1.5% is centrifuged at 217,000 G for 30 minutes. The absorbance is measured using a sample of the upper 50% by mass layer when a sample of the aqueous pigment dispersion prepared to have a pigment content of 1.5% by mass is centrifuged at 217,000 G for 30 minutes.

[0045] For centrifugation, for example, a centrifuge (trade name "Optima MAX-XP Ultracentrifuge," manufactured by Beckman Coulter) and a rotor (trade name "MLA-80," manufactured by Beckman Coulter) can be used. The S value and K factor were calculated from the rotor's maximum and minimum rotational radii, rotation speed, particle diameter and density, and solution density and viscosity. The centrifugation conditions were set at 56,000 rpm (217,000 G) for 30 minutes, assuming that 38 nm particles would settle. Furthermore, to collect 50% by mass of the upper layer of the sample, particles approximately smaller than 30 nm were captured. Such particles are considered microparticles. If the absorbance of these microparticles in the wavelength range of 350 nm to 550 nm is 0.20 or higher, the formation of foreign matter becomes significant and discharge stability decreases. Furthermore, the absorbance is preferably 0.19 or lower, and more preferably 0.17 or lower.

[0046] <Method for producing aqueous pigment dispersion> The method for producing an aqueous pigment dispersion of the present invention is a method for producing an aqueous pigment dispersion for use in producing aqueous inkjet inks. This method for producing an aqueous pigment dispersion includes Step A, which involves mixing at least a raw pigment, an inorganic salt, and an organic solvent, and performing solvent salt milling. This method for producing an aqueous pigment dispersion also includes Step B, which involves dispersing the mixture containing the pigment, resin dispersant, and aqueous medium obtained in Step A to obtain a dispersion, and Step C, which involves ultrafiltration of the dispersion obtained in Step B to obtain an aqueous pigment dispersion. A preferred aqueous pigment dispersion can be produced by the method for producing an aqueous pigment dispersion including Steps A, B, and C. Each step is described below.

[0047] [Step A: Solvent Salt Milling] In step A, at least a raw pigment, an inorganic salt, and an organic solvent are mixed and subjected to solvent salt milling. Solvent salt milling is a method in which pigment particles are mechanically ground using a kneader or the like in the presence of an inorganic salt (salt) and an organic solvent (solvent), and is a method that can efficiently reduce the size of pigment particles. The solvent salt milling method includes a kneading step in which a kneaded product containing the pigment, inorganic salt, and organic solvent is produced, and a subsequent step in which the inorganic salt and organic solvent are removed from the kneaded product.

[0048] (Raw pigment) The raw pigment (crude pigment) used in step A contains CI Pigment Orange 43. Specifically, in step A, coarse CI Pigment Orange 43, i.e., a pigment having a BET specific surface area of ​​35 m 2 The BET specific surface area of ​​the coarse CI Pigment Orange 43 is 15 m / g or less. 2 / g or more 35m 2 / g or less is preferable.

[0049] (inorganic salts) The inorganic salt, taking advantage of its high hardness, is used to pulverize the raw pigment during the kneading process and to make the raw pigment finer. Taking into consideration the handling during removal, it is preferable to use an inorganic salt that is water-soluble (a water-soluble inorganic salt). The water-soluble inorganic salt is not particularly limited as long as it dissolves in water. Specific examples include chlorides of alkali metals such as sodium chloride and potassium chloride, and chlorides of polyvalent metals such as zinc chloride and magnesium chloride. One type of water-soluble inorganic salt may be used alone, or two or more types may be used in combination.

[0050] Cumulative 50% particle size (D 50 ) is preferably 1 μm or more and 50 μm or less. 95 ) is preferably 80 μm or less. 50 and D 95 are the diameters of particles that are 50% and 95% of the total volume of the measured particles in the particle size integration curve, calculated from the small particle size side. 50 and D 95 can be measured, for example, using a particle size analyzer using dynamic light scattering.

[0051] The amount of inorganic salt used is preferably 3 parts by mass or more and 20 parts by mass or less, and more preferably 4 parts by mass or more and 10 parts by mass or less, per part by mass of raw pigment.

[0052] (organic solvent) The organic solvent is used in the kneading step to moisten the pigment particles and inorganic salt, thereby increasing the grinding effect and promoting the fineness of the pigment. The organic solvent is not particularly limited as long as it can achieve the above, but water-soluble organic solvents such as alcohols, glycols, and ethers are preferred. Of these, highly viscous water-soluble organic solvents such as ethylene glycol, diethylene glycol, and polyethylene glycol are more preferred from the viewpoint of improving the grinding effect. One of the organic solvents may be used alone, or two or more may be used in combination.

[0053] The amount of the organic solvent used is preferably 0.1 to 5 parts by mass, and more preferably 0.5 to 2.5 parts by mass, per part by mass of the raw pigment.

[0054] (Kneading process) The kneading step is a step in which the raw pigment is kneaded while being compressed under a load in the presence of the inorganic salt and the organic solvent. Examples of the apparatus used in the kneading step include a kneader, a roll mill, a ball mill, an attritor, a sand mill, and a planetary mixer. Among these, it is preferable to use a kneader.

[0055] The temperature during the kneading step is preferably 10° C. or higher and 60° C. or lower. By keeping the temperature during the kneading step at 60° C. or lower, the rate of crystal growth of CI Pigment Orange 43 is suppressed, facilitating pulverization. The time during the kneading step is preferably 2 hours or higher and 20 hours or lower, and more preferably 2 hours or higher and 8 hours or lower.

[0056] (post-process) The post-process is a process for removing inorganic salts and organic solvents from the kneaded product obtained in the kneading process. Specifically, when using a water-soluble inorganic salt, which is preferred as the inorganic salt, and a water-soluble organic solvent, which is preferred as the organic solvent, a method can be used in which water is added to the kneaded product in a predetermined ratio, and then the resulting slurry is filtered and washed. Ion-exchanged water or pure water is preferably used as the water. Examples of the filtration method include a method in which a suspension obtained by adding water to the kneaded product is passed through an ultrafiltration membrane or a dialysis membrane for separation, and a method in which the components are separated using a high-pressure filter press. By performing this process, a wet cake of the pigment composition can be obtained in which the contents of inorganic salts and organic solvents have been reduced or removed.

[0057] The obtained wet cake is preferably dried so that the water content (moisture content) is about 60% by mass or less. Examples of methods for removing water include batch or continuous drying, in which dehydration is performed by heating at 80°C to 120°C using a heat source installed in the dryer, and drying under reduced pressure. Specific examples of dryers include box dryers, band dryers, and spray dryers.

[0058] [Process B: Dispersion] In step B, the mixture containing the pigment obtained in step A, a resin dispersant, and an aqueous medium is subjected to a dispersion treatment to obtain a dispersion. Step B is a step (dispersion step) in which the pigment containing CI Pigment Orange 43 obtained in step A is dispersed in an aqueous medium. Examples of pigment dispersion methods include resin-dispersed pigments, self-dispersed pigments, resin-bonded pigments, and microencapsulated pigments, but resin-dispersed pigments are preferred. The resin dispersants and aqueous media used in the dispersion step can be the same as those contained in the aqueous pigment dispersions described above.

[0059] In the dispersion step, known dispersion methods such as media dispersion and medialess dispersion can be used. Examples of dispersers for media dispersion include paint shakers, bead mills, sand mills, ball mills, and roll mills. Examples of dispersers for medialess dispersion include ultrasonic homogenizers and high-pressure homogenizers. The dispersion step may be performed using one of the above dispersers alone, or two or more of them in combination.

[0060] The temperature of the dispersion step can be set as desired. Because the dispersion step is performed in an aqueous medium, it is preferably from 0°C to 100°C, and more preferably from 10°C to 80°C from the viewpoint of heat generation during the step and, when a media dispersion method is used, the reliability of the media. The time of the dispersion step can be adjusted depending on the device used, the concentration of the dispersion, and the like, and can be set as desired as long as the pigment is not over-dispersed.

[0061] A pre-dispersion step may be carried out to mix the pigment-containing components, wet them in an aqueous medium, and facilitate dispersion. The pre-dispersion step may utilize the dispersion methods and devices described above as being usable in the dispersion step.

[0062] [Step C: Ultrafiltration] In step C, the dispersion obtained in step B is subjected to ultrafiltration to obtain an aqueous pigment dispersion. Step C is a step (ultrafiltration step) in which the dispersion obtained in step B is purified using an ultrafiltration membrane with a specific particle cutoff diameter. More specifically, the purpose of step C is to remove fine pigment particles contained in the dispersion obtained in step B (aqueous pigment dispersion). Examples of filtration methods include cross-flow filtration and dead-end filtration. Cross-flow filtration is preferred because it allows particles deposited on the membrane surface to be scraped off by the shear force of a parallel flow, while maintaining fractionation performance over a long period of time.

[0063] The particle cutoff diameter of the ultrafiltration membrane is preferably 30 nm or less. By setting the particle cutoff diameter of the ultrafiltration membrane to 30 nm or less, it is possible to increase the ability to remove fine particles while reducing the percentage of pigment particles and resin dispersants of sizes that do not need to be removed that pass through the filtration membrane. Furthermore, although there is no direct correlation between the particle cutoff diameter and the molecular weight cutoff, when expressed as a molecular weight cutoff, it is preferable that it be 300 kDa or less. On the other hand, in order to prevent clogging of the filtration membrane and maintain the processing capacity of ultrafiltration, it is preferable that the molecular weight cutoff be 30 kDa or more.

[0064] The ultrafiltration membrane is not particularly limited as long as it has the desired particle size cutoff. Examples include polysulfone-based polymer membranes, aromatic ether-based polymer membranes, fluorine-based polymer membranes, olefin-based polymer membranes, cellulose-based membranes, (meth)acrylic polymer membranes, (meth)acrylonitrile-based polymer membranes, and vinyl alcohol-based polymer membranes. Among these, polysulfone-based polymer membranes are preferred.

[0065] The shape of the ultrafiltration membrane is not particularly limited as long as it can exhibit fractionation performance. Various shapes, such as hollow fiber, flat membrane, and tubular, can be used as the shape of the ultrafiltration membrane. Among these, hollow fiber membranes are preferred because they have a large effective filtration membrane area relative to their volume.

[0066] The type of device used for crossflow filtration is not particularly limited as long as it can control the concentration, linear velocity, filtration pressure, and other parameters of the pigment particles. For example, a crossflow filtration device can be used that combines a device that supplies water as a diluent to maintain a constant pigment particle concentration with a device that controls the linear velocity tangential to the ultrafiltration membrane and the pressure across the membrane. The linear velocity is not particularly limited as long as it can achieve separation performance. However, if the linear velocity is too fast, the pigment particles may be stressed, leading to aggregation or denaturation, while if the linear velocity is too slow, the throughput may be reduced. Regarding the filtration pressure, if the pressure is too high, the membrane may be easily clogged due to the rapid formation of a cake layer, while if the pressure is too low, the throughput may be reduced, leading to reduced productivity.

[0067] <Water-based inkjet ink> The aqueous pigment dispersion described above in detail can be used in various applications, such as paints for automobiles and building materials, printing inks such as offset inks, gravure inks, flexographic inks, and silkscreen inks, and inkjet inks. In particular, the aqueous pigment dispersion described above is preferably used in aqueous inkjet inks.

[0068] When the aqueous pigment dispersion is used in an inkjet ink, the ink can be prepared by adding components necessary for the inkjet ink to the aqueous pigment dispersion. The components added to the aqueous pigment dispersion preferably include a water-soluble organic solvent, water, and a surfactant. The ink may further contain, as needed, a resin such as an acrylic resin or a polyurethane resin for use as a binder, and other additives.

[0069] The aqueous ink contains the aforementioned pigment containing CI Pigment Orange 43, a resin dispersant, and an aqueous medium. The aqueous ink has an absorbance ratio expressed by the formula (1) of 0.95 to 1.15. Furthermore, when the aqueous ink containing 1.5% by weight of pigment is centrifuged at 217,000 G for 30 minutes, the absorbance of the upper 50% by weight of the ink is less than 0.20 in the wavelength range of 350 nm to 550 nm. The absorbance ratio of the aqueous ink can be measured in the same manner as the absorbance ratio of the aqueous pigment dispersion described above in the "Characteristics of the Aqueous Pigment Dispersion" section, except that an aqueous ink is used instead of the aqueous pigment dispersion and the aqueous ink is diluted 2,000 times with ion-exchanged water. The absorbance of the aqueous ink after centrifugation can be measured in the same manner as the absorbance of the aqueous pigment dispersion after centrifugation described above in the "Characteristics of the Aqueous Pigment Dispersion" section, except that an aqueous ink is used instead of the aqueous pigment dispersion. Furthermore, similar to the preferred range of the absorbance ratio of the aqueous pigment dispersion, the absorbance ratio of the ink is preferably 1.00 or more and 1.12 or less. Furthermore, similar to the preferred range of the absorbance ratio of the aqueous pigment dispersion after centrifugation, the absorbance of the ink after centrifugation is preferably 0.19 or less, and more preferably 0.17 or less.

[0070] The ink may contain only CI Pigment Orange 43 as the pigment, or may contain one or more other pigments. The ink may also contain one or more of the resin dispersants described above. Furthermore, the ink may contain only water as the aqueous medium, or may contain an aqueous medium that is a mixed solvent of water and one or more water-soluble organic solvents.

[0071] The content (% by mass) of CI Pigment Orange 43 in the aqueous ink is preferably 0.1% by mass to 15.0% by mass, and more preferably 1.0% by mass to 10.0% by mass, based on the total mass of the aqueous ink. The content (% by mass) of water in the aqueous ink is preferably 50.0% by mass to 95.0% by mass, and more preferably 50.0% by mass to 90.0% by mass, based on the total mass of the aqueous ink. The content (% by mass) of water-soluble organic solvent in the aqueous ink is preferably 3.0% by mass to 48.0% by mass, and more preferably 3.0% by mass to 25.0% by mass, based on the total mass of the aqueous ink.

[0072] Examples of water-soluble organic solvents that can be contained in the ink include monohydric or polyhydric alcohols, alkylene glycols, glycol ethers, nitrogen-containing polar compounds, and sulfur-containing polar compounds. These water-soluble organic solvents can be used alone or in combination. Other additives that can be contained in the ink include pH adjusters, preservatives, antifungals, antioxidants, antireducing agents, evaporation promoters, and chelating agents.

[0073] <Water-based ink manufacturing method> The method for producing an aqueous inkjet ink of the present invention includes a step of mixing an aqueous pigment dispersion and other ink components. The aqueous pigment dispersion is an aqueous pigment dispersion produced by the aforementioned "method for producing an aqueous pigment dispersion." Other ink components include water, water-soluble organic solvents, surfactants, resins, and the aforementioned "other additives." One or more of these may be used. The method for producing an aqueous ink may be carried out, for example, by adding the aqueous pigment dispersion and other ink components to a suitable container and stirring them. Conditions such as stirring speed, temperature, and time can be appropriately set according to the desired conditions. Other known production processes may also be combined.

[0074] <Ink cartridges> The ink cartridge of the present invention includes ink and an ink storage section that stores the ink. The ink stored in the ink storage section is the aqueous ink of the present invention described above. FIG. 1 is a cross-sectional view schematically illustrating one embodiment of the ink cartridge of the present invention. As shown in FIG. 1, the bottom of the ink cartridge is provided with an ink supply port 12 for supplying ink to the recording head. The interior of the ink cartridge serves as an ink storage section for storing the ink. The ink storage section is composed of an ink storage chamber 14 and an absorber storage chamber 16, which are connected to each other via a communication port 18. The absorber storage chamber 16 is also connected to the ink supply port 12. The ink storage chamber 14 stores liquid ink 20, and the absorber storage chamber 16 contains absorbers 22 and 24 that retain the ink in an impregnated state. The ink storage section may not have an ink storage chamber that stores liquid ink, but may instead use an absorber to hold all of the ink stored therein. Alternatively, the ink storage section may not have an absorber and may store all of the ink in a liquid state. Furthermore, the ink cartridge may be configured to have an ink storage section and a recording head.

[0075] <Inkjet recording method> The inkjet recording method of the present invention is a method of ejecting the above-described aqueous ink of the present invention from an inkjet recording head to record an image on a recording medium. Methods for ejecting the ink include a method of applying mechanical energy to the ink and a method of applying thermal energy to the ink. In the present invention, it is particularly preferable to employ a method of ejecting the ink by applying thermal energy to the ink. Other than using the ink of the present invention, the steps of the inkjet recording method may be any known method.

[0076] FIG. 2 is a diagram schematically illustrating an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main components of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge. The inkjet recording apparatus is provided with a transport means (not shown) for transporting a recording medium 32, and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 is equipped with recording heads 38 and 40, and is configured to accommodate an ink cartridge 42. While the head cartridge 36 is transported in the main scanning direction along the carriage shaft 34, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. An image is then recorded on the recording medium 32 by transporting the recording medium 32 in the sub-scanning direction by a transport means (not shown).

[0077] Any recording medium may be used. Recording media having ink absorption properties, such as recording media without a coating layer, such as plain paper, and recording media with a coating layer, such as glossy paper or matte paper, may be used. Recording media having low or no ink absorption properties, such as printing paper, coated paper, resin sheets, and resin films, may also be used. The ink of the present invention is suitable for applications in which the ink is directly applied to such recording media to record an image. [Example]

[0078] 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.

[0079] <Process A: Solvent Salt Milling> A kneader (trade name "PBV-0.3 type", manufactured by Irie Shokai) was charged with 40 parts of raw pigment, 200 parts of sodium chloride (average particle size 10 μm) as a water-soluble inorganic salt, and 40 parts of diethylene glycol as a water-soluble organic solvent. 2 CI Pigment Orange 43 (trade name "PV FAST ORANGE GRL", manufactured by Heubach) with a concentration of 1 / g was used. After adjusting the temperature inside the kneader to 40°C, kneading (solvent salt milling) was carried out for 4 hours to obtain a kneaded material containing the raw pigment, water-soluble inorganic salt, and water-soluble organic solvent.

[0080] Next, the kneaded material obtained above was transferred to a 2 L glass beaker, and 750 mL of 1% aqueous sulfuric acid solution was added and stirred to obtain a suspension containing dissolved sodium chloride and diethylene glycol. The suspension was centrifuged, and the sediment was collected in a 3 L beaker. 2.5 L of ion-exchanged water was added to the mixture, and the mixture was stirred and allowed to stand. The upper layer was discarded by decantation, and the remainder was centrifuged again to obtain a sediment. This washing procedure with ion-exchanged water was repeated two more times, and the collected sediment was then dried to obtain Pigment P1 with a solids content of 50% (water content of 50%).

[0081] Pigments P2 to P4, P6, and P7 were obtained in the same manner as in the production of pigment P1, except that the raw pigment was changed to CI Pigment Orange 43, the trade name, and the kneading time (h) were changed as shown in Table 1. Pigment P5 was used in the next step as a pigment with a solid content of 100% (water content of 0%) without undergoing solvent salt milling. ·PV FAST ORANGE GRL (CI Pigment Orange 43, manufactured by Heubach, BET specific surface area: 31m 2 / g) Orange A-76SP (CI Pigment Orange 43, manufactured by Arimoto Chemical Industry, BET specific surface area: 24 m 2 / g) PIGMENT ORANGE 4301 (CI Pigment Orange 43, manufactured by Sanyo Pigment, BET specific surface area: 43m 2 / g)

[0082] TIFF2025181714000001.tif60170

[0083] <Process B: Dispersion> [Synthesis of Resin Dispersant] Water-soluble resins B1 to B10, random copolymers with the compositions and properties shown in Table 2, were synthesized by polymerizing monomers using conventional methods to prepare resins for use as resin dispersants. For each resin, an appropriate amount of 10.0% potassium hydroxide aqueous solution was added to neutralize the anionic carboxylic acid groups, and then an appropriate amount of water was added to obtain a resin aqueous solution with a resin content of 20%. The resins were dissolved in tetrahydrofuran to prepare measurement samples. The acid values ​​of the resins were measured by potentiometric titration using an automatic potentiometric titrator (product name "AT510" manufactured by Kyoto Electronics Manufacturing Co., Ltd.) with a potassium hydroxide-ethanol titrant. The weight-average molecular weights of the resins, measured in terms of polystyrene, were all 10,000, as measured by GPC. The abbreviations for the monomers shown in Table 2 are as follows: St: Styrene αMSt: α-methylstyrene BA: n-butyl acrylate AA: Acrylic acid

[0084] TIFF2025181714000002.tif77170

[0085] [Dispersion] A mixture containing pigment, resin, and aqueous medium in the types and amounts shown in Table 3 was placed in a high-pressure homogenizer (product name "Starburst Mini", manufactured by Sugino Machine), and dispersion treatment was carried out at a pressure of 230 MPa for the number of passes shown in Table 3, thereby obtaining pigment dispersions D1 to D18. Note that one pass is defined as when the entire amount of the mixture is passed through the high-pressure homogenizer, which is the dispersion device, once.

[0086] TIFF2025181714000003.tif128170

[0087] <Step C: Ultrafiltration> [Ultrafiltration] Each of the pigment dispersions obtained by the dispersion process described above was subjected to ultrafiltration for the number of passes shown in Table 4. By adjusting the amount of ion-exchanged water replenishment, aqueous pigment dispersions D1 to D18 with a pigment content of 15% were obtained. For the ultrafiltration process, an ultrafiltration device was used, which included a container with a stirrer, a liquid feed pump, a hollow fiber filter (product name "S04-E070-05-N", molecular weight cutoff 70 kDa, manufactured by Repligen) equipped with a pressure gauge, and a line for replenishment of ion-exchanged water. Note that one pass is defined as when the amount of permeate recovered by ultrafiltration is equal to the amount of pigment dispersion used in the process.

[0088] [Absorbance ratio] For each aqueous pigment dispersion obtained above, the aqueous pigment dispersion was diluted 10,000 times with ion-exchanged water, and the absorption spectrum was measured in the wavelength range of 200 to 800 nm to calculate the absorbance ratio represented by the following formula (1). The absorption spectrum was measured using a spectrophotometer (trade name "U-3300", manufactured by Hitachi, Ltd.). Absorbance ratio= Absorbance of the peak around 532 nm / Absorbance of the peak around 496 nm (1)

[0089] [Absorbance after centrifugation] For each aqueous pigment dispersion obtained above, a sample was prepared by diluting the aqueous pigment dispersion to a pigment content of 1.5%. 6.00 g of this sample was poured into a centrifuge tube and centrifuged at 217,000 G for 30 minutes. After centrifugation, 50% of the upper layer (3.00 g) of the sample was collected and its absorption spectrum was measured in the wavelength range of 350 nm to 550 nm. A centrifuge (product name "Optima MAX-XP Ultracentrifuge", manufactured by Beckman Coulter) and a rotor (product name "MLA-80", manufactured by Beckman Coulter) were used for centrifugation. A spectrophotometer (product name "U-3300", manufactured by Hitachi, Ltd.) was used to measure the absorption spectrum.

[0090] TIFF2025181714000004.tif128170

[0091] <Ink Preparation> Each ink was prepared by mixing the components (unit: %) shown in Table 5 with the aqueous pigment dispersion of the type shown in the upper row of Table 5 (Tables 5-1 and 5-2), thoroughly stirring, and then filtering under pressure through a microfilter with a pore size of 2.5 μm. "Acetylenol E100" shown in Table 5 is the trade name of a nonionic surfactant (manufactured by Kawaken Fine Chemicals). The absorbance ratio of the prepared ink was measured in the same manner as for the aqueous pigment dispersion, except that ink was used instead of the aqueous pigment dispersion and that the ink was diluted 2,000 times with ion-exchanged water. The absorbance of the prepared ink after centrifugation was measured in the same manner as for the aqueous pigment dispersion, except that ink was used instead of the aqueous pigment dispersion. As a result, the absorbance ratio and absorbance after centrifugation of each ink were both similar to those of the aqueous pigment dispersion used in the ink (see Table 4).

[0092] TIFF2025181714000005.tif54170

[0093] TIFF2025181714000006.tif54170

[0094] <Evaluation> In the present invention, the evaluation criteria for each item shown below were such that "A" and "B" were acceptable levels and "C" was unacceptable. The evaluation results are shown in Table 6.

[0095] [Discharge stability] Each ink prepared was filled into an ink cartridge, and the following evaluations were performed using an inkjet recording device (trade name "PIXUS PRO-10S," manufactured by Canon) equipped with a recording head that ejects ink using thermal energy. With the inkjet recording device, an image recorded under conditions in which eight droplets of 3.8 ng of ink were applied to a unit area of ​​1 / 600 inch x 1 / 600 inch was defined as having a recording duty of 100%. Using the inkjet recording device, 300 A4-sized solid images were continuously recorded on a recording medium (plain paper, trade name "PB PAPER," manufactured by Canon) at a recording duty of 50%. The state of image distortion on the first and 300th sheets was visually confirmed, and the ink ejection stability was evaluated according to the evaluation criteria shown below. A: It was possible to record 300 images, and there was no distortion in the 300th image. B: It was possible to record 300 images, and the 300th image was distorted, but there were no white spots. C: It is possible to record 300 images, but the 300th image had some distortion and white spots.

[0096] [Color development] Using the inkjet recording device, a 200 mm x 200 mm solid image (recording duty 100%) was recorded on glossy paper (product name "Premium Glossy Paper 2", manufactured by Canon). After leaving the recorded solid image for one day, the saturation C of the solid image was measured using a spectrophotometer (product name "FD-7", manufactured by Konica Minolta) under the conditions of a light source D50 and a viewing angle of 2°. * The color development of the image was evaluated according to the following evaluation criteria. A: Saturation C * was 115 or more, and the hue angle h was less than 60°. B: Saturation C * was 110 or more but less than 115, or chroma C * was 115 or more, but the hue angle h was 60° or more. C: Saturation C * was less than 110.

[0097] TIFF2025181714000007.tif116170

Claims

1. An aqueous inkjet ink containing a pigment, a resin dispersant, and an aqueous medium, The pigment comprises C.I. Pigment Orange 43, The absorbance ratio represented by the following formula (1) is 0.95 or more and 1.15 or less, Absorbance ratio = Absorbance of peak near wavelength 532 nm / Absorbance of peak near wavelength 496 nm (1) The aqueous ink, characterized in that when the aqueous ink having a pigment content of 1.5% by mass is centrifuged at 217,000 G for 30 minutes, the absorbance of the upper 50% by mass of the ink is less than 0.20 in the wavelength range of 350 nm or more and 550 nm or less.

2. 2. The aqueous ink according to claim 1, wherein the acid value of the resin dispersant is 160 mgKOH / g or more and 200 mgKOH / g or less.

3. the resin dispersant is a copolymer having a unit derived from a styrene-based monomer and a unit derived from (meth)acrylic acid, The aqueous ink according to claim 1 , wherein the ratio (% by mass) of the units derived from the styrene-based monomer to the total mass of the resin dispersant is 60% by mass or more and 77% by mass or less.

4. An ink cartridge comprising ink and an ink storage section for storing the ink, 4. An ink cartridge, wherein the ink is the aqueous ink according to claim 1.

5. An inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, An ink-jet recording method, wherein the ink is the aqueous ink according to any one of claims 1 to 3.

6. An aqueous pigment dispersion for use in producing an aqueous inkjet ink, the aqueous pigment dispersion comprising a pigment, a resin dispersant, and an aqueous medium, The pigment comprises C.I. Pigment Orange 43, The absorbance ratio represented by the following formula (1) is 0.95 or more and 1.15 or less, Absorbance ratio= Absorbance of peak near wavelength 532 nm / Absorbance of peak near wavelength 496 nm (1) an aqueous pigment dispersion, wherein when the aqueous pigment dispersion having a pigment content of 1.5% by mass is centrifuged at 217,000 G for 30 minutes, the absorbance of the upper 50% by mass of the dispersion is less than 0.20 in a wavelength range of 350 nm or more and 550 nm or less.

7. 1. A method for producing an aqueous pigment dispersion for use in producing an aqueous inkjet ink, comprising: A step A of mixing at least a raw pigment, an inorganic salt, and an organic solvent and performing solvent salt milling; a step B of dispersing the mixture containing the pigment, the resin dispersant, and the aqueous medium obtained in the step A to obtain a dispersion; a step C of subjecting the dispersion obtained in the step B to an ultrafiltration treatment to obtain an aqueous pigment dispersion; and The raw pigment has a BET specific surface area of ​​35 m ２ / g or less C.I. Pigment Orange 43, the absorbance ratio of the aqueous pigment dispersion represented by the following formula (1) is 0.95 or more and 1.15 or less, Absorbance ratio= Absorbance of peak near wavelength 532 nm / Absorbance of peak near wavelength 496 nm (1) a method for producing an aqueous pigment dispersion, wherein when the aqueous pigment dispersion having a pigment content of 1.5% by mass is centrifuged at 217,000 G for 30 minutes, the absorbance of the upper 50% by mass of the dispersion is less than 0.20 in a wavelength range of 350 nm or more and 550 nm or less.

8. A method for producing a water-based inkjet ink, comprising: A method for producing an aqueous ink, comprising a step of mixing the aqueous pigment dispersion produced by the method for producing an aqueous pigment dispersion according to claim 7 with other ink components.

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