Aqueous ink, ink cartridge and ink jet recording method

Abstract

To provide an aqueous ink for ink jet recording or the like with superior ejection stability and storage stability and the capability of recording an image with high chromaticity in a yellow region.SOLUTION: An aqueous ink contains a pigment (C.I. Pigment Yellow 101) and a resin dispersant for the pigment. The resin dispersant has a unit with an aromatic group and a unit with an anionic group.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based ink, an ink cartridge, and an inkjet recording method. [Background technology]

[0002] One of the needs in the printing industry is the production of eye-catching, vividly colored recorded materials. For example, posters, POP displays, and food and beverage packaging must be recorded in vivid colors to attract customers' attention. Images in the bright yellow range are particularly eye-catching. Furthermore, recording images in the bright yellow range can significantly expand the color gamut that can be expressed. Therefore, there is a strong demand in the printing industry for recording images in the bright yellow range. Fluorescent colors can be used to meet these demands.

[0003] Currently, offset printing is the mainstream method for recording fluorescent color images. However, to achieve vivid color development, it is common to perform two or more overprints. As a result, productivity has not always been sufficiently high.

[0004] In the case of digital recording using electrophotography, it is possible to record highly colored fluorescent images using liquid toner, but electrophotography has limitations on the recording media that can be used, making it difficult to apply it to textile recording, large formats, or thick materials, for example.

[0005] On the other hand, inkjet printing can be applied to a variety of recording media, taking advantage of the fact that the print head that ejects the ink does not come into contact with the recording medium (non-contact). However, because the ink needs to be ejected stably from tiny nozzles, the ink's physical properties are easily limited. In particular, many of the materials that determine the performance of ink, such as colorants and resins, are solid. Even if you try to add a sufficient amount of fluorescent colorant to the ink to achieve the color development of the image, limitations arise for the reasons mentioned above.

[0006] To address the above-mentioned issues, a fluorescent inkjet ink has been proposed that contains resin particles, a core of which contains a fluorescent colorant and a polymer and a shell made of a polymer having hydrophilic groups formed on the outside, a water-soluble organic solvent, and water (see Patent Document 1). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-26964 Summary of the Invention [Problem to be solved by the invention]

[0008] The present inventors have investigated the water-based ink proposed in Patent Document 1. As a result, they have found that it is difficult to achieve both ejection stability and storage stability, as well as the ability to print images in the yellow region with high color development.

[0009] Therefore, an object of the present invention is to provide a water-based inkjet ink that has excellent ejection stability and storage stability and is capable of recording images with high color development in the yellow region, an ink cartridge using the water-based ink, and an inkjet recording method. [Means for solving the problem]

[0010] The above object is achieved by the present invention, which is described below. Specifically, the aqueous ink of the present invention is an aqueous ink for inkjet use, containing a pigment and a resin dispersant for the pigment, wherein the pigment is CI Pigment Yellow 101, and the resin dispersant contains a unit having an aromatic group and a unit having an anionic group. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a water-based inkjet ink that has excellent ejection stability and storage stability and is capable of recording images having high color development in the yellow region, an ink cartridge using the water-based ink, and an inkjet recording method. [Brief explanation of the drawings]

[0012] [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

[0013] The present invention will be described in further detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is present in the ink dissociated into ions, but for convenience it will be expressed as "containing a salt." "CI" is an abbreviation for "color index." A resin "unit" refers to the smallest repeating unit constituting the resin, and refers to a structure formed by (co)polymerization of one monomer. Furthermore, aqueous inkjet inks are sometimes simply referred to as "ink." Unless otherwise specified, physical property values ​​are values ​​at room temperature (25°C).

[0014] The present inventors investigated the composition of an ink that has excellent ejection stability and storage stability and can record images with high color development in the yellow region. First, they examined the color development of images recorded with ink containing a conventionally commonly used yellow pigment for yellow ink, one of the basic colors commonly used in the inkjet field. As a result, they were unable to record images with high color development in the yellow region. This is because the pigment contained in the basic yellow ink (e.g., CI Pigment Yellow 74) is not a fluorescent colorant. To record images with high color development in the yellow region, as described above, it is necessary to incorporate a fluorescent colorant into the ink. Here, a fluorescent colorant is a colorant that reflects light with wavelengths in the visible light region under daylight (sunlight) and absorbs daylight to emit (fluoresce) light with wavelengths in the visible light region. When an image recorded with ink containing a colorant that is not a fluorescent colorant was measured, the maximum reflected light intensity at the maximum absorption wavelength within the wavelength range of 400 to 700 nm was less than 100%. On the other hand, when measuring the color of an image printed with ink containing a fluorescent coloring material, the maximum reflected light intensity is 100% or more because the fluorescence is added to the reflected light. In other words, printing with ink containing a fluorescent coloring material produces an image with high color development. Fluorescent coloring materials are broadly classified into four types: fluorescent dyes, resin particles dyed with fluorescent dyes, plastic-type fluorescent pigments, and organic pigment-type fluorescent pigments (organic fluorescent pigments). Therefore, the present inventors investigated what kind of coloring material should be added to create an ink with excellent ejection stability and storage stability.

[0015] First, the inventors attempted to add a fluorescent dye (fluorescent dye), a fluorescent coloring material, to the ink. However, they found that increasing the amount of fluorescent dye added to obtain high image color development improved the saturation of the image, but significantly reduced the brightness, making it impossible to obtain high image color development. This is thought to be due to concentration quenching, which is unique to fluorescent materials. "Concentration quenching" refers to the phenomenon in which the fluorescence intensity decreases when the concentration of the fluorescent coloring material exceeds a certain level.

[0016] Next, we attempted to add resin particles dyed with a fluorescent dye to the ink. We found that by dyeing the resin particles with a fluorescent dye, the fluorescent dye was fixed to the resin particles, thereby suppressing the decrease in image brightness. In particular, we found that using resin particles dyed with a fluorescent dye through strong interaction enabled us to record images in the yellow region, which has high color development, thereby achieving high image color development and ejection stability. However, we found that the fluorescent dye detached from the resin particles during ink storage, resulting in poor storage stability.

[0017] Furthermore, we attempted to add a plastic-type fluorescent pigment to the ink. Plastic-type fluorescent pigments are made by dyeing resin with a fluorescent dye, then pulverizing it and processing it into fine particles with a particle size on the order of microns. As a result, we found that even though the plastic-type fluorescent pigment was pulverized, its particle size was large and its dispersibility was low, making it difficult to apply to aqueous inkjet inks.

[0018] Finally, we attempted to add an organic fluorescent pigment to the ink. Organic fluorescent pigments refer to pigments containing organic molecules that emit fluorescence. Examples of organic fluorescent pigments with a yellow hue include CI Pigment Yellow 101 and 9,10-dianilinoanthracene. The inventors evaluated various ink properties depending on the type of resin dispersant. As a result, we found that the use of CI Pigment Yellow 101 and a resin dispersant containing a specific unit allows for the recording of images with excellent ejection stability and storage stability and high color development in the yellow region. The inventors speculate as follows about the mechanism by which inks containing the pigment and pigment resin dispersant described above improve ejection stability, storage stability, and image color development.

[0019] CI Pigment Yellow 101 (hereinafter simply referred to as the pigment) has a structure containing a naphthalene skeleton within its molecule. The naphthalene skeleton is a structure in which two benzene rings share one carbon-carbon bond. Therefore, the naphthalene skeleton portion of the pigment particle surface interacts with the π electrons of the aromatic group of the resin dispersant (hereinafter simply referred to as the resin), allowing the resin to strongly adsorb to the pigment. In this case, electrostatic repulsion occurs due to the anionic groups of the resin adsorbed to the pigment particle surface, suppressing pigment aggregation. Furthermore, as mentioned above, the resin strongly adsorbs to the pigment. This not only suppresses pigment aggregation near the ejection orifice, but also prevents the resin from detaching from the pigment during ink storage, thereby improving ejection stability and storage stability. Furthermore, the electrostatic repulsion due to the anionic groups of the resin maintains sufficient distance between pigment particles even on the recording medium. As a result, even when the pigment content is increased, concentration quenching does not occur, and the color development of the image can be improved.

[0020] 9,10-dianilinoanthracene, an organic fluorescent pigment like CI Pigment Yellow 101, has an anthracene skeleton within its molecule, which creates strong π-π interactions between molecules. This makes the pigment more prone to aggregation than CI Pigment Yellow 101. As a result, even when dispersed with the resin mentioned above, aggregation of the pigment on the recording medium cannot be prevented, causing the pigment particles to become closer together and resulting in concentration quenching. This results in poor image color development.

[0021] If the resin does not contain a unit having an aromatic group, the resin easily detaches from the pigment particle surface. As a result, electrostatic repulsion due to the anionic groups of the resin is not obtained, the pigment cannot be stably dispersed, and ejection stability and storage stability are not achieved. In addition, the distance between pigment particles becomes short on the recording medium, causing concentration quenching. As a result, high image color development is not achieved. If the resin does not contain a unit having an anionic group, electrostatic repulsion does not occur between the resin adsorbed on the pigment particle surface. As a result, the pigment cannot be stably dispersed, and ejection stability and storage stability are not achieved. In addition, the distance between pigment particles becomes short on the recording medium, causing concentration quenching. As a result, high image color development is not achieved.

[0022] <Water-based ink> The ink of the present invention is a water-based inkjet ink containing a pigment and a specific resin dispersant. The components constituting the ink of the present invention and the physical properties of the ink will be described in detail below. Hereinafter, the terms "(meth)acrylic acid," "(meth)acrylate," and "(meth)acryloyl" refer to "acrylic acid, methacrylic acid," "acrylate, methacrylate," and "acryloyl, methacryloyl," respectively.

[0023] (CI Pigment Yellow 101) The ink contains CI Pigment Yellow 101 as a colorant (pigment). The pigment content (mass %) in the aqueous ink is preferably 0.5% to 15.0% by mass, based on the total mass of the ink. If the pigment content is less than 0.5% by mass, the pigment content is too low, and the image may not have sufficient color development. If the pigment content is more than 15.0% by mass, the pigment content is too high, and clogging may occur near the ejection orifices of the recording head, and sufficient ejection stability may not be achieved.

[0024] The pigment crystallite size is preferably 30 nm or more. The pigment crystallite size can be determined by determining the half-width of the diffraction peak from an X-ray diffraction (XRD) spectrum and using the Scherrer equation (L = Kλ / β cosθ, where L is the crystallite size [nm], K is the Scherrer constant, λ is the X-ray wavelength [nm], β is the full width at half maximum, and θ is the Bragg angle [rad]). The XRD spectrum can be measured using an X-ray diffractometer. If the pigment crystallite size is less than 30 nm, the intensity of the fluorescence emitted by the pigment decreases, the brightness of the recorded image decreases, and the image color development may not be sufficient. The pigment crystallite size is preferably 100 nm or less.

[0025] The aspect ratio of the pigment is preferably 3.0 or less. The aspect ratio of the pigment is the average ratio of the major axis to the minor axis of a pigment particle approximated to an ellipse (major axis / minor axis). If the aspect ratio of the pigment exceeds 3.0, the shape of the pigment particles is rod-like, so the pigment particles are oriented in the direction of their long sides on the recording medium, resulting in a short distance between the particles. As a result, concentration quenching occurs, the brightness of the recorded image decreases, and the color development of the image may be insufficient. The aspect ratio of the pigment is preferably 1.1 or more. If the aspect ratio of the pigment is less than 1.1, the shape of the pigment particles is close to spherical, so the pigment particles tend to pack tightly on the recording medium, making it difficult to space them apart. As a result, concentration quenching occurs, the brightness of the recorded image decreases, and the color development of the image may be insufficient. The aspect ratio of the pigment can be calculated by averaging the measurements of 100 or more pigment particles measured using a scanning electron microscope.

[0026] Pigment volume-based cumulative 50% particle size (D 50(hereinafter also referred to as the average particle diameter) is preferably 200 nm or less. The volume-based cumulative 50% particle diameter of a pigment is the diameter of the particle that is 50% of the total volume of the measured particles when accumulated from the smallest particle diameter side in a particle diameter accumulation curve. If the average particle diameter of the pigment exceeds 200 nm, the color development of the image may be insufficient. The average particle diameter of the pigment is preferably 40 nm or more. A particle size analyzer using a dynamic light scattering method (for example, the product name "UPA-EX150" manufactured by Nikkiso) can be used as a device for measuring the average particle diameter of the pigment. The measurement conditions in this case can be, for example, Set Zero: 30 seconds, number of measurements: 3, and measurement time: 180 seconds. Of course, the particle size distribution measurement device and measurement conditions used are not limited to those described above.

[0027] The span value S of the pigment is preferably 1.0 or more and 3.0 or less. The span value S of the pigment is calculated by dividing the cumulative 10% particle diameter (D 10 ), cumulative 50% particle size (D 50 ), cumulative 90% particle size (D 90 ) and measure S=(D 90 -D 10 ) / D 50 It can be calculated using the formula: The span value represents the width of the particle size distribution; the smaller the span value, the narrower the particle size distribution; the larger the span value, the wider the particle size distribution. If the pigment span value is less than 1.0, the pigment particle size distribution is narrow and the pigment particles are uniform in size, which tends to result in close spacing between pigment particles on the recording medium. As a result, concentration quenching occurs, the brightness of the recorded image decreases, and the image color development may not be sufficient. If the pigment span value is more than 3.0, the pigment particle size distribution is wide and there are many coarse particles, which may lead to clogging near the nozzles of the recording head and insufficient ejection stability.

[0028] The content (mass %) of the resin dispersant in the ink is preferably 0.05 to 2.00 times the mass ratio of the content (mass %) of the pigment. If the mass ratio is less than 0.05, the amount of resin dispersant is too small, which may result in insufficient electrostatic repulsion and inability to stably disperse the pigment. As a result, sufficient ejection stability, storage stability, and image color development may not be achieved. If the mass ratio is more than 2.00, the amount of resin dispersant is too high, which may increase the viscosity of the ink and result in insufficient ejection stability.

[0029] Any known dispersion method can be used to disperse the pigment. Examples of dispersion methods include media dispersion methods such as ball mills, sand mills, roll mills, bead mills, and paint shakers; and medialess dispersion methods such as ultrasonic homogenizers and high-pressure homogenizers. Among these, bead mills, sand mills, and high-pressure homogenizers are preferred because they facilitate controlling the pigment crystallite size, aspect ratio, and span value within preferred ranges. On the other hand, paint shakers and ultrasonic homogenizers make it difficult to control the pigment crystallite size, aspect ratio, and span value within preferred ranges, even when conditions such as processing time are changed. When using a bead mill or sand mill, it is preferable to use beads with a diameter of 0.07 mm or more and 0.39 mm or less in order to adjust the aspect ratio within the above range.

[0030] (Resin dispersant) The ink contains a resin (resin dispersant) containing a unit having an aromatic group and a unit having an anionic group. That is, the pigment of the ink is a resin-dispersed pigment. The content (mass %) of the resin dispersant in the ink is preferably 0.025% by mass or more and 30.0% by mass or less, and more preferably 0.1% by mass or more and 20.0% by mass or less, based on the total mass of the ink. Of these, 0.1% by mass or more and 15.0% by mass or less is particularly preferred.

[0031] Examples of the resin form include block copolymers, random copolymers, graft copolymers, and combinations thereof. The resin may be a water-soluble resin that can be dissolved in an aqueous medium, or may be resin particles that can be dispersed in an aqueous medium. The resin particles do not need to encapsulate a colorant. It is particularly preferred that the resin used as a resin dispersant be a water-soluble resin.

[0032] As used herein, "a resin is water-soluble" means that when the resin is neutralized with an alkali equivalent to its acid value, it exists in an aqueous medium in a state in which it does not form particles whose particle size can be measured by dynamic light scattering. Whether a resin is water-soluble or not can be determined according to the following method. First, a liquid (resin solids content: 10% by mass) containing a resin neutralized with an alkali (sodium hydroxide, potassium hydroxide, etc.) equivalent to the acid value is prepared. Next, the prepared liquid is diluted 10 times (by volume) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by dynamic light scattering, if no particles having the particle size are measured, the resin can be determined to be water-soluble.

[0033] Examples of resins used as resin dispersants include acrylic resins, urethane resins, and urea resins. Among these, acrylic resins are preferred. Resins used as resin dispersants contain at least a unit having an aromatic group and a unit having an anionic group. In the case of acrylic resins, the monomer having an aromatic unit preferably has one polymerizable functional group, such as an ethylenically unsaturated bond, in the molecule. Specific examples include styrene, α-methylstyrene, and benzyl (meth)acrylate. Examples of anionic groups include a carboxylic acid group, a phenolic hydroxy group, and a phosphate ester group. Of these, a carboxylic acid group is preferred. Examples of monomers having a carboxylic acid group preferably have one polymerizable functional group, such as an ethylenically unsaturated bond, in the molecule. Examples include monomers such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid, as well as salts or anhydrides thereof. Examples of cations constituting salts include ions of lithium, sodium, potassium, ammonium, and organic ammonium. It is preferred that the monomer having a carboxylic acid group does not have an aromatic group or a cyano group. The molecular weight of the monomer having a carboxylic acid group is preferably 300 or less, and more preferably 200 or less. Of these, (meth)acrylic acid is preferred. The resin may also contain hydrophilic or hydrophobic units other than those mentioned above in order to improve adsorption to the pigment particle surface and the storage stability of the ink.

[0034] The hydrophilic unit is a unit having a hydrophilic group such as a hydroxy group or an ethylene oxide group. The hydrophilic unit can be formed, for example, by polymerizing a hydrophilic monomer having a hydrophilic group. Examples of the hydrophilic monomer include 2-hydroxyethyl (meth)acrylate and (poly)ethylene glycol (meth)acrylate. The hydrophobic unit is a unit not having a hydrophilic group such as an anionic group, a hydroxy group or an ethylene oxide group. The hydrophobic unit can be formed, for example, by polymerizing a hydrophobic monomer not having the above-mentioned hydrophilic group. Examples of the hydrophobic monomer include (meth)acrylic acid ester monomers such as ethyl (meth)acrylate, methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

[0035] The acid value (mgKOH / g) of the resin (resin dispersant) is preferably 50 mgKOH / g or more and 200 mgKOH / g or less. If the acid value of the resin dispersant is less than 50 mgKOH / g, the anionic groups are too few, which may result in insufficient electrostatic repulsion and inability to stably disperse the pigment. As a result, sufficient ejection stability, storage stability, and image color development may be insufficient. If the acid value of the resin dispersant is more than 200 mgKOH / g, the resin dispersant is less likely to adsorb to the particle surface of hydrophobic pigments, which may result in insufficient pigment dispersion. As a result, sufficient ejection stability, storage stability, and image color development may be insufficient.

[0036] The weight-average molecular weight Mw of the resin (resin dispersant) is preferably 1,000 or more and 30,000 or less. If the weight-average molecular weight of the resin dispersant is less than 1,000, the size of the resin dispersant is too small, and even if the resin adsorbs to the surface of the pigment particles, sufficient repulsion due to steric hindrance may not be obtained, and the pigment may not be stably dispersed. As a result, sufficient ejection stability, storage stability, and image color development may not be obtained. If the weight-average molecular weight of the resin dispersant is more than 30,000, the viscosity of the ink may increase. As a result, sufficient ejection stability may not be obtained.

[0037] As long as the ejection stability, storage stability, and color development of the image are not impaired, other resins than the resin containing a unit having a carboxylic acid group and a unit having an aromatic group may be added to the ink. The content (mass %) of other resins in the ink is preferably 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink.

[0038] The physical properties of the resin (resin dispersant) can be measured as follows. By analyzing the resin in the ink using a high-temperature gas chromatography / mass spectrometer (high-temperature GC / MS), the type of units that make up the resin can be confirmed. In addition, nuclear magnetic resonance ( 13 Quantitative analysis using C-NMR or Fourier transform infrared spectroscopy (FT-IR) can confirm the molecular weight and type of monomers that make up each unit. To increase accuracy, when analyzing the resin in the ink, it is preferable to use the sediment and supernatant obtained by centrifuging the ink rather than using the ink itself. For example, the ink can be centrifuged at 75,000 rpm, and an excess of acid (such as hydrochloric acid) can be added to the supernatant, after which the precipitated resin can be dried and used.

[0039] (Compounds represented by general formula (1) and compounds represented by general formula (2)) The ink preferably contains at least one of a compound represented by general formula (1) and a compound represented by general formula (2). That is, the ink preferably contains at least one compound selected from the group consisting of a compound represented by general formula (1) and a compound represented by general formula (2). The above compound forms hydrogen bonds with the hydroxyl group portion of the pigment, allowing it to exist near the surface of the pigment particles. This suppresses pigment aggregation and maintains sufficient distance between pigment particles on the recording medium. As a result, concentration quenching can be suppressed, further improving the color development of the image.

[0040] The content (mass %) of the compound represented by general formula (1) in the ink is preferably 0.01% by mass or more and 5.0% by mass or less, based on the total mass of the ink. In general formula (1), R1, R2, R3, and R4 each independently represent a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. Examples of the alkyl group having 1 to 5 carbon atoms include linear or branched alkyl groups, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, and n-pentyl. It is particularly preferred that the combinations of R1 and R3, and R2 and R4, respectively, are i-butyl and methyl. R5 and R6 each independently represent an alkylene group having 1 to 5 carbon atoms. Therefore, R5O and R6O are alkylene oxide groups. It is particularly preferred that R5 and R6 are ethylene groups, i.e., R5O and R6O are ethylene oxide groups. m and n are each independently an integer of 0 or greater. Either m or n may be 0, or both may be 0. m+n is preferably 25 or less. In particular, m+n is preferably 10.

[0041] The content (mass %) of the compound represented by general formula (2) in the ink is preferably 0.1 mass % or more and 50.0 mass % or less, based on the total mass of the ink. In general formula (2), R7 is an alkylene group having 2 to 6 carbon atoms. When R7 is an alkylene group having 2 carbon atoms, p is preferably an integer of 2 or more and preferably an integer of 25 or less. When R7 is other than an alkylene group having 2 carbon atoms, p is preferably an integer of 1 or more and preferably an integer of 25 or less. When R7 is an ethylene group, the compound represented by general formula (2) can be a low molecular weight compound such as diethylene glycol or triethylene glycol, or even a polyethylene glycol with a molecular weight of about 1,000. Of these, triethylene glycol is preferred.

[0042] As a result of investigations by the present inventors, it was found that a compound in which R7 is an alkylene group having two carbon atoms and p is 1 (ethylene glycol) does not sufficiently improve the color development of images. This is thought to be because the molecular chain is short and aggregation of pigment particles cannot be sufficiently suppressed. Other specific examples of compounds represented by general formula (2) include propylene glycol, butylene glycol, 1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, and 1,6-hexanediol. Multiple compounds listed above may be used.

[0043] [ka]

[0044] (In general formula (1), R1, R2, R3, and R4 each independently represent a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. R5 and R6 each independently represent an alkylene group having 1 to 5 carbon atoms. m and n each independently represent an integer of 0 or greater.)

[0045] [ka]

[0046] (In general formula (2), R7 is an alkylene group having 2 to 6 carbon atoms. When R7 is an alkylene group having 2 carbon atoms, p is an integer of 2 or more; otherwise, p is an integer of 1 or more.)

[0047] (Compound represented by general formula (3)) The ink preferably contains a compound represented by general formula (3). This compound has a molecular structure similar to the molecular skeleton (naphthalene skeleton) of the pigment CI Pigment Yellow 101, and thus can reside near the pigment particle surface due to hydrophobic interactions. This suppresses pigment aggregation and maintains sufficient distance between pigments on the recording medium. As a result, concentration quenching can be efficiently suppressed. Furthermore, CI Pigment Yellow 101 absorbs the fluorescence emitted by the compound represented by general formula (3), thereby improving the intensity of the fluorescence emitted by CI Pigment Yellow 101. This improves the brightness of the recorded image and further enhances the color development of the image. A compound in which R8 in general formula (3) is a hydrogen atom is 2-naphthol, and a compound in which R8 is a formyl group is 2-hydroxy-1-naphthaldehyde.

[0048] The content of the compound represented by general formula (3) in the ink is preferably 5 ppm or more and 5,000 ppm or less, based on the pigment content. The content of the compound represented by general formula (3) in the ink is a value calculated using the following formula. If the content of the compound represented by general formula (3) in the ink is less than 5 ppm, the amount of the compound represented by general formula (3) in the ink is too small, and the above-mentioned effects are not fully exhibited, and sufficient color development of the image may not be obtained. If the content of the compound represented by general formula (3) in the ink is more than 5,000 ppm, the compound represented by general formula (3) may adhere to the vicinity of the ejection orifice of the recording head, causing clogging and resulting in insufficient ejection stability. (Content of compound represented by general formula (3) in ink, ppm) = [(Content of compound represented by general formula (3), % by mass) / (Content of pigment, % by mass)] × 1,000,000

[0049] [ka]

[0050] (In general formula (3), R8 is a hydrogen atom or a formyl group.)

[0051] (aqueous medium) The ink is an aqueous ink containing water as the aqueous medium. The ink may contain an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. Deionized water (ion-exchanged water) is preferably used as the water. The water content (mass %) in the ink is preferably 50.0 mass % or more and 95.0 mass % or less, based on the total mass of the ink.

[0052] The water-soluble organic solvent is not particularly limited as long as it is water-soluble (preferably, one that dissolves in water at 25°C in any proportion). Specifically, other alcohols, other alkylene glycols, glycol ethers, nitrogen-containing polar compounds, sulfur-containing polar compounds, etc. can be used. The content (mass %) of the water-soluble organic solvent in the ink is preferably 3.0% to 50.0% by mass, and more preferably 10.0% to 40.0% by mass, based on the total mass of the ink. If the content (mass %) of the water-soluble organic solvent is less than 3.0% by mass, the ink may solidify in the inkjet recording device, resulting in insufficient solidification resistance. If the content (mass %) of the water-soluble organic solvent is more than 50.0% by mass, the viscosity of the ink may become too high, reducing fluidity and potentially resulting in poor ink supply.

[0053] (Other additives) In addition to the additives described above, the ink may contain various additives, such as other surfactants, pH adjusters, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, evaporation promoters, and chelating agents, as needed. Among these, it is preferable for the ink to contain a surfactant. The content (mass %) of the surfactant in the ink is preferably 0.1% by mass or more and 5.0% by mass or less, and more preferably 0.1% by mass or more and 2.0% by mass or less, based on the total mass of the ink. Examples of surfactants include anionic surfactants, cationic surfactants, and nonionic surfactants. Among these, nonionic surfactants, which have low affinity with pigments, are preferred because they are used to adjust various physical properties of the ink.

[0054] (Ink properties) Since the ink is an aqueous ink used in inkjet printing, it is preferable to appropriately control its physical properties. Specifically, the surface tension of the ink at 25°C, measured by the plate method, is preferably 20 mN / m or more and 60 mN / m or less, and more preferably 25 mN / m or more and 45 mN / m or less. The viscosity of the ink at 25°C is preferably 1.0 mPa·s or more and 10.0 mPa·s or less, and more preferably 1.0 mPa·s or more and 5.0 mPa·s or less. The pH of the ink at 25°C is preferably 7.0 or more and 10.0 or less. The pH of the ink can be measured using a common pH meter equipped with a glass electrode or the like.

[0055] <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.

[0056] <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.

[0057] 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). [Example]

[0058] 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.

[0059] <Analysis of pigments> (particle size, span value) Pigment volume-based cumulative 50% particle size (D 50) was measured using a particle size measuring device using the dynamic light scattering method (trade name "Nanotrac WaveII-EX150", manufactured by Microtrac Bell). The measurement conditions were: Set Zero: 30 seconds, number of measurements: 3, measurement time: 120 seconds, shape: aspherical, refractive index: 1.51, mono-disperse mode: on. Here, the volume-based cumulative 50% particle size is the diameter of the particle that is 50% of the total volume of the measured particles when integrated from the small particle size side in the particle size integration curve. In addition, the volume-based cumulative 10% particle size (D 10 ), cumulative 90% particle size (D 90 ) and measure S=(D 90 -D 10 ) / D 50 The span value S was calculated based on the formula:

[0060] (crystallite size) The pigment dispersion was diluted with ion-exchanged water to a pigment content of approximately 1.5% by mass, and then ultracentrifuged at 25°C, 57,000 rpm, and 30 minutes to collect the precipitate. The precipitate was allowed to dry at 25°C for one day and then powdered using an agate mortar and pestle. The XRD spectrum was measured using a multipurpose X-ray diffractometer (trade name "Empyrean" manufactured by Malvern Panalytical). The measurement conditions were: detector: PIXcel3D, emission current: 40 mA, light source: CuKα, divergence slit: fixed, 0.5°, tube voltage: 45 kV, scan angle: 5°-60°, scan step size: 0.04°. The crystallite size of the highest and second highest intensity diffraction peaks in the obtained spectrum was calculated using analysis software (trade name "HighScore Plus" manufactured by Malvern Panalytical). The average of these values ​​was then used to determine the crystallite size of the pigment.

[0061] (aspect ratio) A 1 μL drop of pigment dispersion diluted 10,000 times with pure water was placed on a silicon substrate whose surface had been hydrophilized by corona discharge treatment and then dried. SEM images of 100 pigment particles were obtained using a scanning electron microscope. The resulting SEM images were thresholded to obtain a binary image with a white background and black pigment particles. The major and minor diameters of the pigment particles were measured by elliptical approximation of the black areas representing the pigment particles. The aspect ratio was calculated by averaging the major / minor diameter ratios of the 100 pigment particles. ImageJ (developed by Wayne Rasband, NIH) was used for thresholding and elliptical approximation of the SEM images. When the above pigment analysis was performed on ink, the same values ​​were obtained as when using the pigment dispersion.

[0062] <Analysis of Resin Dispersants> (Weight average molecular weight) The weight-average molecular weight (Mw) of the resin used as a resin dispersant was measured as a polystyrene-equivalent value by gel permeation chromatography (GPC). Specifically, the resin was dissolved in a solvent (tetrahydrofuran) at 25°C for 24 hours. The resulting solution was filtered through a membrane filter to obtain a sample solution. The sample solution was adjusted so that the concentration of components soluble in the solvent was approximately 0.3%. Using this sample solution, the weight-average molecular weight of the resin was measured under the following conditions. Apparatus: Molecular weight measurement apparatus (trade name "Acquity Advanced Polymer Chromatography", manufactured by Waters) Column: "Shodex Column GPC KF-806M, 8 x 300 mm" (product name, manufactured by Showa Denko) connected in series (four columns) Eluent: tetrahydrofuran ·Flow rate: 1.0mL / min Sample injection volume: 0.100 mL Oven temperature: 40℃ Detector: Refractive index (RI) detector (trade name: Waters 2414 Refractive Index Detector, manufactured by Waters)

[0063] The weight-average molecular weight was calculated using a molecular weight calibration curve prepared using standard polystyrene reagents, PS-1 and PS-2 (trade names, manufactured by Polymer Laboratories).

[0064] (acid number) The acid value of the resin used as a resin dispersant was measured by titration according to JIS K-0070. 0.5 to 2.0 g of resin was precisely weighed and used as the measurement sample. The sample was placed in a 50.0 mL beaker, and 25.0 mL of a mixture of tetrahydrofuran and ethanol (volume ratio = 2:1) was added to dissolve the sample. Potentiometric titration was performed using a 0.1 mol / L ethanol solution of potassium hydroxide as the titrant, and the amount of titrant used was recorded as S (mL). A blank sample was also titrated in the same manner, and the amount of ethanol solution of potassium hydroxide used was recorded as B (mL). An automatic potentiometric titrator (product name "AT-510", manufactured by Kyoto Electronics Manufacturing Co., Ltd.) was used as the measuring device. The acid value was calculated from the obtained S and B using the following formula: f is the factor (titer) of the ethanol solution of potassium hydroxide, and M (g) is the precisely weighed value of the sample. Acid value [mgKOH / g]=(SB)×f×5.61 / M

[0065] <Synthesis of resin dispersant> (Resin Dispersant 1-14) Each resin (resin dispersant) was synthesized by a standard method using each monomer (unit: parts) listed in Table 1 and 200.0 parts of propylene glycol monomethyl ether acetate as a solvent. 20.0 parts of each resin was neutralized with potassium hydroxide in an amount equimolar to its acid value, and an appropriate amount of ion-exchanged water was added to prepare a liquid containing each resin dispersant with a resin dispersant content of 20.0%.

[0066] The abbreviations of the monomers in Table 1 respectively represent St: styrene, BzMA: benzyl methacrylate, AA: acrylic acid, MAA: methacrylic acid, nBA: n-butyl acrylate, and HEMA: 2-hydroxyethyl methacrylate.

[0067] [Table 1]

[0068] (Resin Dispersant 15) A liquid containing Resin Dispersant 15 was prepared in the same manner as Resin Dispersants 1 to 14, except that 50.0 parts of styrene, 30.0 parts of n-butyl acrylate, and 20.0 parts of Blemmer PME1000 were used. "Blemmer PME1000" is the trade name for methoxypolyethylene glycol methacrylate manufactured by NOF Corporation. Resin Dispersant 15 had a weight-average molecular weight of 15,000 and an acid value of 0 mgKOH / g.

[0069] <Preparation of pigment dispersion> (Pigment dispersions 1-11, 16-39, 44-46, 54-56) The components shown on the left side of Table 2 were mixed and dispersed using a batch-type vertical sand mill (manufactured by Imex) filled with zirconia beads under the dispersion conditions shown on the right side of Table 2. The mixture was centrifuged to remove coarse particles, and then pressure filtered through a microfilter (manufactured by Fujifilm) with a pore size of 3.0 μm to obtain each pigment dispersion.

[0070] [Table 2]

[0071] (Pigment dispersions 12-15, 47, 48) The components shown on the left side of Table 3 were mixed and pre-dispersed (rotation speed: 10,000 rpm) using a dispersing / emulsifying machine (product name "Clearmix", manufactured by M Technique) for the time shown in Table 3. Dispersion processing was then carried out using a high-pressure homogenizer (product name "Starburst", manufactured by Sugino Machine) under the dispersion conditions shown on the right side of Table 3. The mixture was centrifuged to remove coarse particles, and pressure filtered through a microfilter with a pore size of 3.0 μm (manufactured by Fujifilm) to obtain each pigment dispersion.

[0072] [Table 3]

[0073] (Pigment dispersions 40, 41, 49, 50) The components shown on the left side of Table 4 were mixed and dispersed using an ultrasonic homogenizer (product name "UH-300", manufactured by SMT) under the dispersion conditions shown in Table 4. Coarse particles were removed by centrifugation, and the mixture was pressure filtered through a microfilter with a pore size of 3.0 μm (manufactured by Fujifilm) to obtain each pigment dispersion.

[0074] [Table 4]

[0075] (Pigment dispersions 42, 43, 51, 52) The components shown on the left side of Table 5 were mixed and dispersed using a paint shaker under the conditions shown in Table 5. The mixture was centrifuged to remove coarse particles, and then pressure filtered through a microfilter (manufactured by Fujifilm) with a pore size of 3.0 μm to obtain each pigment dispersion.

[0076] [Table 5]

[0077] (Pigment Dispersion 53) A flask was charged with 45.0 parts of a pigment (CI Pigment Yellow 101) and 12.0 parts of sulfanilic acid and mixed. The flask was placed in a water bath at 70°C, and an aqueous solution of potassium nitrite (6.3 parts of potassium nitrite dissolved in 223.1 parts of ion-exchanged water) was added with stirring to obtain a slurry. Hydrochloric acid was added to adjust the pH of the slurry to 2.0, and the mixture was stirred at 70°C for 1 hour. The resulting slurry was dried to obtain a dispersion containing a self-dispersing pigment in which -CH-SOK groups were bonded to the pigment particle surface. 450 parts of the resulting dispersion was diluted to 2,250 parts with ion-exchanged water and then concentrated by ultrafiltration to 450 parts. The pigment dispersion was purified by repeating the dilution and concentration steps until the electrical conductivity of the filtrate reached 50 μS / cm or less, and finally concentrated to 300 parts. An ultrafiltration membrane (product name "OS300C11", molecular weight cutoff 300K, manufactured by Nippon Pall) was used for ultrafiltration. Next, the mixture was centrifuged at 5,000 rpm for 15 minutes using a centrifuge (product name "CR-21G", manufactured by Hitachi Koki Co., Ltd.) to remove coarse particles, and then a 4 mol / L aqueous potassium hydroxide solution was added to adjust the pH to 11.0. An appropriate amount of ion-exchanged water was added to obtain Pigment Dispersion 53 with a pigment content of 20.0%.

[0078] (Pigment Dispersion 57) A flask was charged with 10.0 parts of pigment (CI Pigment Yellow 101), 60.0 parts of dimethyl sulfoxide, and 30.0 parts of a 25.0% aqueous solution of tetramethylammonium hydroxide, and mixed to prepare a liquid containing the pigment dissolved therein. While vigorously stirring this liquid, an aqueous citric acid solution prepared by mixing 70.0 parts of ion-exchanged water and 25.0 parts of citric acid was added dropwise at a constant rate over two hours to obtain a pigment suspension. The resulting suspension was filtered under reduced pressure to obtain a pigment wet cake. The pigment wet cake was then added to 5,000 parts of ion-exchanged water and stirred in a dispersing / emulsifying machine (product name "Clearmix," manufactured by M Technique) at 6,000 rpm for 1.5 minutes, after which it was filtered under reduced pressure to recover the pigment wet cake. The above stirring and filtering procedures were repeated four times to wash the pigment, yielding a pigment wet cake. This pigment wet cake was then dried under reduced pressure to obtain a pigment powder. 20.0 parts of pigment powder, 30.0 parts of a liquid containing resin dispersant 1, and 50.0 parts of ion-exchange water were mixed, and the mixture was dispersed for 5.0 hours using a batch-type vertical sand mill (manufactured by Imex) filled with 200 parts of zirconia beads with a diameter of 0.10 mm. The mixture was centrifuged to remove coarse particles, and then pressure-filtered through a 3.0 μm pore-size microfilter (manufactured by Fujifilm) to obtain pigment dispersion 57, which had a pigment content of 20.0% and a resin (resin dispersant) content of 6.0%.

[0079] (Pigment Dispersion 58) Pigment dispersion 58, having a pigment content of 20.0% and a resin (resin dispersant) content of 6.0%, was obtained in the same manner as in pigment dispersion 57, except that the amount of pigment used was changed to 5.0 parts and the amount of dimethyl sulfoxide used was changed to 65.0 parts.

[0080] (Pigment Dispersion 59) 20.0 parts of a plastic-type yellow fluorescent pigment (product name "FZ-5005" manufactured by Shinroihi), 30.0 parts of a liquid containing resin dispersant 1, and 50.0 parts of ion-exchange water were mixed. Dispersion was carried out for 5.0 hours using a batch-type vertical sand mill (manufactured by Imex) filled with 200 parts of zirconia beads with a diameter of 0.10 mm. The mixture was centrifuged to remove coarse particles, and then pressure-filtered through a 3.0 μm pore-size microfilter (manufactured by Fujifilm) to obtain pigment dispersion 59, which had a pigment content of 20.0% and a resin (resin dispersant) content of 6.0%.

[0081] The physical properties of each of the pigment dispersions prepared above are summarized in Table 6.

[0082] [Table 6]

[0083] <Ink Preparation> Each ink was prepared by mixing the components shown in the upper rows of Tables 7 to 10 (unit: %, except that the unit for 2-naphthol and 2-hydroxy-1-naphthaldehyde is ppm), thoroughly stirring, and then filtering under pressure through a microfilter (manufactured by Fujifilm) with a pore size of 3.0 μm. Ion-exchange water was added in an amount that would make the total of the components 100.0%.

[0084] The lower rows of Tables 7 to 10 show ink properties. In Tables 7 to 10, the content (ppm) of the compound represented by general formula (3) based on the pigment is calculated using the formula 1,000,000 × A / P (A and P are both in %). For example, the ink of Example 1 has an A (%) of 0.01% (100 ppm) and a P (%) of 5.0%, so the column for the content (ppm) of the compound represented by general formula (3) based on the pigment in Table 7 is entered as "2,000." In the tables, "Acetylenol E100" and "Acetylenol E60" are trade names of nonionic surfactants manufactured by Kawaken Fine Chemicals, and are compounds represented by general formula (1). The number next to polyethylene glycol (PEG) is the number-average molecular weight of the polyethylene glycol.

[0085] (Comparative Example 13) The ink of Comparative Example 13 was prepared according to the description in "Synthesis Example 4" of Patent Document 1. Specifically, 50.0 parts of CI Pigment Yellow 101 were mixed with 10.0 parts of polyvinyl butyral and 150.0 parts of ethyl acetate, dispersed using a sand grinder filled with 0.5 nm zirconia beads at a volume ratio of 50%, and centrifuged to obtain a dispersion. 12.5 parts of the dispersion, 4.0 parts of polyvinyl butyral, and 36.0 parts of ethyl acetate were placed in a flask equipped with a stirrer and a nitrogen reflux device and stirred under a nitrogen atmosphere. 90.0 parts of an aqueous solution containing 1.9 parts of sodium lauryl sulfate were added dropwise and stirred, followed by ultrasonic treatment for 300 seconds using an ultrasonic disperser (product name "UH-150" manufactured by SMT Co., Ltd.) to emulsify the components. The ethyl acetate was then removed under reduced pressure. The resulting compound was dissolved in 1.5 parts of potassium persulfate, heated to 80°C, and reacted for 7 hours while a mixture of 1.5 parts of styrene and 1.5 parts of 2-hydroxyethyl methacrylate was added dropwise. 20.0 parts of ethylene glycol and an appropriate amount of ion-exchanged water were added to the resulting dispersion to obtain an ink with a pigment content of 2.5%. The B / P ratio was 3.10.

[0086] (Comparative Example 14) In accordance with the description of "Example 10" in JP 2021-8598 A, an ink of Comparative Example 14 containing resin particles 10 and water-soluble resin 1 described below was prepared. The resin particles are core-shell type resin particles and are dyed with a basic dye (two types of fluorescent dyes) having a coumarin skeleton. The two types of fluorescent dyes are CI Disperse Yellow 82 and CI Solvent Yellow 160:1 (80:20 (mass ratio)). The core portion is composed of units derived from styrene and acrylonitrile, respectively. The shell portion is composed of units derived from styrene, methacrylic acid, ethylene glycol dimethacrylate, and ethylene glycol diglycidyl ether (trade name "Denacol EX-810", manufactured by Nagase ChemteX). The water-soluble resin is a resin composed of units derived from styrene, n-butyl acrylate, and methacrylic acid, respectively.

[0087] [Table 7]

[0088] [Table 8]

[0089] [Table 9]

[0090] [Table 10]

[0091] <Evaluation> Each ink prepared was filled into an ink cartridge and set in an inkjet recording device (trade name "PIXUS Pro-10", manufactured by Canon) equipped with a recording head that ejects ink using thermal energy. In this example, an image recorded under conditions in which eight droplets of 3.8 ng ± 10% ink were applied to a unit area of ​​1 / 600 inch x 1 / 600 inch was defined as having a recording duty of 100%. The recording environment was a temperature of 25°C and a relative humidity of 55%. In the present invention, the following evaluation criteria for each item were used: "A" and "B" were considered acceptable levels, and "C" was considered unacceptable. The evaluation results are shown in Table 11.

[0092] (Color development) Using the inkjet recording device described above, an image containing the following gradation pattern was recorded on a recording medium (glossy paper, product name "Canon Photo Paper - Fine Grain Gloss Luster", manufactured by Canon). The gradation pattern was composed of multiple 2cm x 2cm solid images in which the amount of ink applied was gradually changed under the condition that a maximum of six drops of ink were applied to a unit area of ​​1 / 600 inch x 1 / 600 inch. After the image was allowed to dry for one day, a spectrophotometer (product name "X-Rite eXact" (M1 light source), manufactured by X-Rite) was used to measure the maximum reflected light intensity, chroma (C * ), and lightness (L * ) was measured. * , and L * is based on the color difference display method specified by CIE. The color development of the image was evaluated according to the following evaluation criteria. However, lightness was evaluated using the value at a saturation of 50, and if the maximum saturation did not reach 50, the data obtained by measuring the color of the gradation pattern was extrapolated and the calculated lightness value obtained was used. A: The maximum reflected light intensity is 100% or more, and the maximum saturation is C * is 70 or more and brightness L * is 80 or more or maximum saturation C * is 65 or more and lightness L * was 85 or higher. B: The maximum reflected light intensity is 100% or more, and the maximum saturation is C * is 65 or more and less than 70 and brightness L * was between 80 and 85. C: Maximum reflected light intensity is less than 100%, maximum saturation C * is less than 65, and brightness L * or less than 80.

[0093] (Discharge stability) Using the inkjet recording device, 19 cm x 26 cm solid images were recorded on 10 sheets of recording medium (plain paper, product name "GF-500", manufactured by Canon) at a recording duty of 100%. The solid images on the fifth and tenth sheets were visually inspected and the ejection stability was evaluated according to the following evaluation criteria. A: No white streaks or blurring was observed on the fifth sheet, and no white streaks or blurring was observed on the tenth sheet (or only a small amount was observed). B: No white streaks or blurring was observed on the fifth sheet, but white streaks and blurring were observed on the tenth sheet. C: White streaks and blurring were observed on the fifth sheet.

[0094] (Storage stability) Each ink was diluted 3,000-fold with ion-exchange water, and the absorption spectrum was measured to determine the maximum absorbance A0 in the wavelength range of 380 to 700 nm. The absorption spectrum was measured using a spectrophotometer (product name "U-3300" manufactured by Hitachi, Ltd.). The viscosity V0 was measured using an E-type viscometer (product name "RE80-L" manufactured by Toki Sangyo Co., Ltd.) equipped with a rotor (1°34' x R24) and placed in a thermostatic bath set at 25°C, through which antifreeze solution was circulated via a tube. The ink was then placed in a sealed container and stored in an oven at 70°C for 14 days. After returning to 25°C, the absorption spectrum and viscosity were measured using the same procedures as above, and the maximum absorbance A1 and viscosity V1 in the wavelength range of 380 to 700 nm were measured. The rate of change in absorbance and viscosity before and after storage was calculated, and the storage stability of the ink was evaluated according to the following criteria. A: The rate of change in maximum absorbance was less than 2% and the rate of increase in viscosity was less than 3%. B: The rate of change in maximum absorbance was 2% or more but less than 5%, and the rate of increase in viscosity was less than 5%, or the rate of change in maximum absorbance was less than 2%, and the rate of increase in viscosity was 3% or more but less than 5%. C: At least one of the rate of change in maximum absorbance and the rate of increase in viscosity was 5% or more.

[0095] [Table 11]

[0096] The evaluation results for image color development and ejection stability of Examples 47 and 49 were both "B", but were relatively inferior compared to Examples 40, 41, 48, and 50, which had similar evaluation results. Furthermore, the evaluation results for Example 70 were both "B", but were relatively inferior in all evaluation items compared to Examples 58, 61, 62, and 66, which had similar evaluation results.

Claims

1. A water-based inkjet ink containing a pigment and a resin dispersant for the pigment, The pigment is C.I. Pigment Yellow 101, The aqueous ink, wherein the resin dispersant contains a unit having an aromatic group and a unit having an anionic group.

2. 2. The aqueous ink according to claim 1, wherein the pigment has a crystallite size (nm) of 30 nm or more.

3. 2. The aqueous ink according to claim 1, wherein the pigment has a crystallite size (nm) of 100 nm or less.

4. 2. The aqueous ink according to claim 1, wherein the aspect ratio of the pigment is 3.0 or less.

5. 2. The aqueous ink according to claim 1, wherein the aspect ratio of the pigment is 1.1 or more.

6. 2. The aqueous ink according to claim 1, wherein the span value of the pigment is 1.0 or more and 3.0 or less.

7. 2. The aqueous ink according to claim 1, wherein the pigment has a volume-based cumulative 50% particle size of 200 nm or less.

8. 2. The aqueous ink according to claim 1, wherein the pigment has a volume-based cumulative 50% particle size of 40 nm or more.

9. 2. The aqueous ink according to claim 1, wherein the content (% by mass) of the pigment in the aqueous ink is 0.5% by mass or more and 15.0% by mass or less, based on the total mass of the ink.

10. The aqueous ink according to claim 1, further comprising at least one compound selected from the group consisting of compounds represented by the following general formula (1) and compounds represented by the following general formula (2): 【Chemistry 1】 (In general formula (1), R １ , R ２ , R ３ , and R ４ are each independently a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. ５ , and R ６ are each independently an alkylene group having 1 to 5 carbon atoms; and m and n are each independently an integer of 0 or greater. 【Chemistry 2】 (In general formula (2), R ７ is an alkylene group having 2 to 6 carbon atoms. ７ When is an alkylene group having 2 carbon atoms, p is an integer of 2 or more, and in other cases, p is an integer of 1 or more.

11. The water-based ink according to claim 1, further comprising a compound represented by general formula (3): 【Transformation 3】 (In general formula (3), R ８ is a hydrogen atom or a formyl group.

12. 12. The aqueous ink according to claim 11, wherein the content of the compound represented by general formula (3) is 5 ppm or more and 5,000 ppm or less based on the content of the pigment.

13. 2. The aqueous ink according to claim 1, wherein the acid value (mgKOH / g) of the resin dispersant is 50 mgKOH / g or more and 200 mgKOH / g or less.

14. 2. The aqueous ink according to claim 1, wherein the weight average molecular weight of the resin dispersant is 1,000 or more and 30,000 or less.

15. The aqueous ink according to claim 1 , wherein the content (mass %) of the resin dispersant is 0.05 to 2.00 times the content (mass %) of the pigment in terms of mass ratio.

16. An aqueous ink as described in claim 1, wherein the resin dispersant is at least one selected from the group consisting of acrylic resins, urethane resins, and urea resins.

17. An aqueous ink as described in claim 1, wherein the unit having an aromatic group is at least one selected from the group consisting of a unit derived from styrene, a unit derived from α-methylstyrene, and a unit derived from benzyl (meth)acrylate.

18. An aqueous ink as described in claim 1, wherein the anionic group is at least one selected from the group consisting of a carboxylic acid group, a phenolic hydroxy group, and a phosphate ester group.

19. An ink cartridge comprising ink and an ink storage section for storing the ink, 19. An ink cartridge, wherein the ink is the aqueous ink according to claim 1.

20. 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 18.

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