Water-based ink, ink cartridge, and inkjet recording method

Abstract

To provide aqueous ink for inkjet which is excellent in ink storage stability and image clarity of an image.SOLUTION: Aqueous ink for inkjet contains a pigment, a resin and an additive, wherein the resin contains a unit having a carboxylic acid and a unit having an aromatic group, the additive is at least one additive selected from the group consisting of a butane dicarboxylic acid, a pentane dicarboxylic acid, a pentane tricarboxylic acid, a hexane dicarboxylic acid, a hexane tricarboxylic acid, a heptane tricarboxylic acid, an octane tricarboxylic acid and a nonane tricarboxylic acid, and a content (ppm) of the additive is 180 ppm or more and 1,200 ppm or less based on the total ink mass.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an aqueous ink, an ink cartridge, and an inkjet recording method.

Background Art

[0002] In recent years, as an ink used in an inkjet recording method, a pigment ink containing a pigment has been widely used because it is easy to enhance fastness. In fields such as photographs and graphic arts, it is required to be able to record a higher-definition and more glossily excellent image. In such applications, a high level of storage stability of the ink is required so that an image can be stably recorded even when an inkjet recording apparatus is used for a long period of time. Also, generally, due to the fact that the pigment in the pigment ink is a particle, there is a problem that the imageability of an image recorded on a recording medium having gloss on the surface (so-called glossy papers) is likely to be lower than that of a dye ink in which the coloring material is a dye. "Imageability" indicates the sharpness of an image when an image is projected onto the surface of an image, and when the imageability is low, the image looks blurred, and when the imageability is high, the image looks clear.

[0003] So far, techniques for improving various inkjet suitability have been studied depending on materials added to the pigment ink. For example, a pigment ink containing an additive having a specific functional group has been proposed (see Patent Documents 1 and 2).

Prior Art Documents

Patent Documents

[0004] <00​​​​​​​​​​​​​​​​​The present inventors investigated the storage stability and image rendering quality of pigment inks using additives proposed in Patent Documents 1 and 2. As a result, it was found that it is difficult to achieve both storage stability and image rendering quality with conventional pigment ink compositions.

[0006] Therefore, the object of the present invention is to provide an aqueous inkjet ink with excellent storage stability and image rendering properties, an ink cartridge using the aqueous ink, and an inkjet recording method. [Means for solving the problem]

[0007] The above objective is achieved by the present invention as follows: The ink according to the present invention is an aqueous inkjet ink containing a pigment, a resin, and an additive, wherein the resin includes units having a carboxylic acid group and units having an aromatic group, and the additive is 2,4-butanedicarboxylic acid, 2,3-butanedicarboxylic acid, 2-methyl-2,3-butanedicarboxylic acid, 2,2,3,3-tetramethylbutanedicarboxylic acid, 3-methyl-1,3-butanedicarboxylic acid The additive is characterized by being at least one selected from the group consisting of pentanedicarboxylic acid, pentanetricarboxylic acid, hexanedicarboxylic acid, hexanetricarboxylic acid, heptanetricarboxylic acid, octantricarboxylic acid, and nonanetricarboxylic acid, and the content (ppm) of the additive is 180 ppm or more and 1,200 ppm or less based on the total mass of the ink. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an aqueous inkjet ink with excellent storage stability and image rendering properties, an ink cartridge using the aqueous ink, and an inkjet recording method. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic cross-sectional view showing one embodiment of the ink cartridge of the present invention. [Figure 2]This figure schematically shows 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 part of the inkjet recording apparatus and (b) is a perspective view of the head cartridge. [Modes for carrying out the invention]

[0010] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when a compound is a salt, the salt exists in the ink dissociated as an ion, but for convenience, it will be expressed as "contains a salt." "Molecular chain length of alkyl chain" refers to the number of carbon atoms that form the longest chain structure in the molecule, excluding carbon atoms that constitute substituents such as carboxylic acid groups. A "unit" of a resin refers to the smallest repeating unit that makes up the resin, and refers to a structure formed by the (co)polymerization of a single monomer. Also, water-based ink for inkjet printers may be simply referred to as "ink." Unless otherwise specified, physical properties are values ​​at room temperature (25°C).

[0011] The inventors have conducted various studies on improving the storage stability and image quality of ink by incorporating resins and additives into the ink. As a result, they have found that using a resin containing units having carboxylic acid groups and units having aromatic groups, and a specific additive, improves the storage stability and image quality of the ink. This additive has the following characteristics: The additive is a compound having an alkyl chain molecular length of 4 to 9 units and 2 or 3 carboxylic acid groups. Specifically, it is a compound selected from the group consisting of butanedicarboxylic acid, pentanedicarboxylic acid, pentanetricarboxylic acid, hexanedicarboxylic acid, hexanetricarboxylic acid, heptanetricarboxylic acid, octantricarboxylic acid, and nonanetricarboxylic acid. In other words, the additive used in the ink of the present invention is an alkanedicarboxylic acid or alkanetricarboxylic acid in which the alkyl chain molecular length and the number of carboxylic acid groups are within a specific range. The inventors hypothesize the following mechanism by which the storage stability and image quality of the ink are improved by an ink containing the above-described resin and additive.

[0012] To improve the storage stability of inks containing resin, it is necessary to adsorb the resin onto the surface of pigment particles and keep it in their vicinity. This requires a hydrophobic portion that interacts with the pigment particle surface to promote adsorption, and a hydrophilic portion that has affinity for aqueous media, spreading the resin from the pigment particle surface into the aqueous media and obtaining repulsive force due to steric hindrance. In resins having units with carboxylic acid groups and units with aromatic groups, the aromatic group exhibits hydrophobicity and has good affinity for the pigment particle surface, which is inherently hydrophobic. Therefore, the resin can adsorb onto the pigment particle surface through the action of at least one of van der Waals forces, π-π interactions, and hydrophobic interactions. Furthermore, the carboxylic acid group exhibits hydrophilicity and has affinity for water, the main component of aqueous media, so it is attracted to the aqueous media and spreads sterically within the aqueous media, obtaining repulsive force due to steric hindrance. This suppresses pigment aggregation and the resulting increase in pigment particle size.

[0013] However, when the ink is applied to the recording medium and an image is formed, the ink becomes concentrated and pigment aggregation begins due to the penetration and evaporation of the aqueous medium in the ink into the recording medium. At this time, the distance between pigment particles becomes very small, and the resin adsorbed on the surface of the pigment particles can interact with nearby pigment particles, so the repulsive force due to steric hindrance disappears and pigment aggregation is promoted. As a result, an uneven distribution of pigment occurs on the surface of the image, and the image quality is lost. In other words, resins containing units with carboxylic acid groups and units with aromatic groups have a positive effect on improving the storage stability of the ink, but tend to reduce the image quality.

[0014] To improve image quality, it is considered necessary for the pigment to be deposited evenly on the recording medium when the ink is applied and an image is formed. On the recording medium, the aqueous medium in the ink permeates and evaporates, causing the ink to concentrate, the pigment to aggregate, and the image to be formed. When the above-mentioned alkanedicarboxylic acid and alkanetricarboxylic acid are included in the ink as additives, the presence of hydrophobic alkyl chains allows them to adhere to the surface of the pigment particles and the resin having units with aromatic groups. As a result, during the process of ink concentration and pigment aggregation, the above-mentioned additives can be present near the pigment particles, and the rapid aggregation of the pigment can be suppressed by electrostatic repulsion caused by the carboxylic acid groups. In other words, the pigment is evenly distributed on the recording medium when the image is formed, improving the image quality.

[0015] On the other hand, when the additive is an alkane monocarboxylic acid, there are fewer carboxylic acid groups relative to the alkyl chain, resulting in low solubility in aqueous media. Therefore, the additive adsorbs to the surface of the pigment particles and remains on the surface of the pigment particles. As a result, during the process of ink concentration and pigment aggregation, the additive adsorbed on the surface of the pigment particles alone cannot sufficiently suppress the aggregation of the resin on the surface of the pigment particles. Therefore, the rapid aggregation of pigments when an image is formed cannot be suppressed, and image clarity cannot be obtained. Furthermore, when the additive has four or more carboxylic acid groups, the entire molecule of the additive exhibits high hydrophilicity. Therefore, even if the additive is adsorbed to the surface of the pigment particles, it is easily detached. In addition, due to its high hydrophilicity, the additive has high solubility in aqueous media, and during the process of ink concentration and pigment aggregation, it may penetrate into the recording medium along with the aqueous medium, or it may exist locally on the surface of the ink droplet due to the evaporation of the aqueous medium. As a result, the additive cannot remain in the vicinity of the pigment particles when an image is formed, and image clarity cannot be obtained.

[0016] Furthermore, even if the additive is an alkanedicarboxylic acid or alkanetricarboxylic acid, if the molecular chain length of the alkyl chain is between 1 and 3, there are more hydrophilic carboxylic acid groups than hydrophobic alkyl chains, resulting in high hydrophilicity for the additive molecule as a whole. As a result, the additive cannot remain near the pigment particles when an image is formed, and image mapping quality cannot be obtained. Also, if the molecular chain length of the alkyl chain is 10 or more, the additive adheres well to the surface of the pigment particles, but there are fewer carboxylic acid groups than alkyl chains. Therefore, the additive remains on the surface of the pigment particles, and the rapid aggregation of the pigment when an image is formed cannot be suppressed, resulting in image mapping quality cannot be obtained.

[0017] The content (ppm) of the additive in the ink needs to be 180 ppm or more and 1,200 ppm or less based on the total mass of the ink. When the content of the additive is less than 180 ppm, the effect of suppressing the aggregation of the pigment after the ink is applied to the recording medium is small, and the image reproducibility cannot be obtained. Also, when the content of the additive exceeds 1,200 ppm, the amount of the additive remaining near the pigment particles increases, inhibiting the adsorption of the resin to the surface of the pigment particles. As a result, the dispersion state of the pigment becomes unstable, and the storage stability of the ink cannot be obtained.

[0018] That is, in the present invention, it is important that the additive has a good balance of hydrophobicity and hydrophilicity and that the content of the additive is within an appropriate range in consideration of the adsorption to the pigment. That is, the storage stability of the ink is improved by the above resin, and the aggregation of the pigment after the ink is applied to the recording medium is suppressed by the balance of the hydrophilicity and hydrophobicity of the above additive, thereby obtaining the storage stability of the ink and the image reproducibility at the levels required in recent years.

[0019] <Water-based ink> The ink of the present invention is an aqueous ink for inkjet containing a pigment, a specific resin, and a specific additive. Hereinafter, the components constituting the ink of the present invention, the physical properties of the ink, etc. will be described in detail.

[0020] (Pigment) The ink contains a pigment as a coloring material. The content (mass%) of the pigment in the ink is preferably 0.1 mass% or more and 15.0 mass% or less, more preferably 1.0 mass% or more and 10.0 mass% or less based on the total mass of the ink.

[0021] Specific examples of the pigment include inorganic pigments such as carbon black and titanium oxide; organic pigments such as azo, phthalocyanine, quinacridone, isoindolinone, imidazolone, diketopyrrolopyrrole, and dioxazine. Among them, pigments having a quinacridone skeleton, pigments having a diketopyrrolopyrrole skeleton, and pigments having a perylene skeleton are preferred. Pigments having these skeletons tend to aggregate, but the additive can effectively suppress the aggregation of the pigments and improve the image mapping property. When not using pigments having these skeletons, the aggregation force between the pigments to be suppressed by the additive is small, so the additive interacts with the resin more than with the pigments, inhibits the adsorption of the resin to the pigments, and the storage stability of the ink may be slightly reduced.

[0022] Examples of the quinacridone pigment include C.I. Pigment Violet 19 (unsubstituted quinacridone), C.I. Pigment Red 122 (2,9-dimethylquinacridone), C.I. Pigment Red 202 (2,9-dichloroquinacridone), etc. As the quinacridone pigment, a quinacridone solid solution pigment formed of two or more kinds of quinacridone pigments can also be used. Among them, a solid solution pigment of C.I. Pigment Red 202 and C.I. Pigment Violet 19; a solid solution pigment of C.I. Pigment Red 122 and C.I. Pigment Violet 19; are preferred. Examples of the diketopyrrolopyrrole pigment include C.I. Pigment Orange: 71, 73, etc.; C.I. Pigment Red: 254, 255, 264, etc. Examples of the perylene pigment include C.I. Pigment Orange 43, C.I. Pigment Red: 149, 179, etc.

[0023] The content (% by mass) of the resin in the ink is preferably 0.15 times or more and 0.30 times or less in terms of the mass ratio to the content (% by mass) of the pigment. When the mass ratio is less than 0.15 times, the dispersion state of the pigment becomes unstable, and the storage stability of the ink may not be sufficiently obtained. Also, when the mass ratio exceeds 0.30 times, it is considered that excess resin may aggregate, so the storage stability of the ink may not be sufficiently obtained.

[0024] As for the dispersion method of the pigment, resin-dispersed pigments using a resin (resin dispersant) as a dispersant, and self-dispersing pigments in which hydrophilic groups are bonded to the surface of the pigment particles can be used. In addition, resin-bonded pigments in which organic groups containing resin are chemically bonded to the surface of the pigment particles, and microcapsule pigments in which the surface of the pigment particles is coated with a resin or the like can be used. Among these, resin-dispersed pigments in which the pigment is dispersed by a resin dispersant are preferred. It is preferable that the resin used as the pigment dispersant is not crosslinked. Furthermore, pigments with different dispersion methods described above may be combined.

[0025] (resin) The ink contains a resin comprising units having carboxylic acid groups and units having aromatic groups. The resin content (mass%) 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.

[0026] Resins can be incorporated into inks for purposes such as (i) stabilizing the dispersion state of pigments, i.e., as a resin dispersant or auxiliary for pigments, and (ii) improving various properties of the recorded image. Examples of resin forms include block copolymers, random copolymers, graft copolymers, and combinations thereof. The resin may be a water-soluble resin that can dissolve in an aqueous medium, or it may be resin particles dispersed in an aqueous medium. Among these, the resin is preferably a water-soluble resin. The resin particles do not need to contain colorants.

[0027] In this specification, "water-soluble resin" means that when the resin is neutralized with an alkali equivalent to its acid value, it exists in an aqueous medium without forming particles whose particle size can be measured by dynamic light scattering. Whether or not a resin is water-soluble can be determined according to the following method. First, prepare a liquid containing the resin (resin solids content: 10% by mass) neutralized with an alkali equivalent to its acid value (sodium hydroxide, potassium hydroxide, etc.). Next, prepare a sample solution by diluting the prepared liquid 10 times (by volume) with pure water. Then, when the particle size of the resin in the sample solution is measured by dynamic light scattering, if no particles with a particle size are measured, the resin can be determined to be water-soluble. The measurement conditions in this case can be, for example, SetZero: 30 seconds, Number of measurements: 3, Measurement time: 180 seconds. As a particle size distribution analyzer, a particle size analyzer using dynamic light scattering (for example, product name "UPA-EX150", manufactured by Nikkiso) can be used. Of course, the particle size distribution analyzer and measurement conditions used are not limited to those described above.

[0028] The acid value of the resin is preferably 80 mg KOH / g or more and 250 mg KOH / g or less, more preferably 110 mg KOH / g or more and 160 mg KOH / g or less, and particularly preferably 110 mg KOH / g or more and 145 mg KOH / g or less. The acid value of the resin can be measured by the method based on JIS K-0070.

[0029] The weight-average molecular weight of the resin is preferably between 1,000 and 30,000, and more preferably between 6,000 and 15,000. The weight-average molecular weight of the resin is a polystyrene-converted value measured by gel permeation chromatography (GPC). GPC measurement can be performed as follows: Add the resin to the eluent and allow it to stand at 25°C for 24 hours to prepare a sample solution with a resin content (mass%) of 0.5% by mass. After pressure filtration of the sample solution through a solvent-resistant membrane filter (pore size: 0.45 μm), GPC measurement is performed. A molecular weight calibration curve can be used to calculate the molecular weight.

[0030] The degree of dispersion (Mw / Mn), defined as the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the resin, is preferably 3.4 or higher, and more preferably 3.4 to 5.0. Generally, the surface of pigment particles is not uniform, and there are parts with different physical shapes and chemical structures. Therefore, it is preferable that the resin that stably disperses the pigment also has properties that can accommodate the differences in surface conditions described above. If the degree of dispersion is less than 3.4, it means that the resin has little variation in molecular weight. As a result, parts of the pigment particle surface are likely to occur where the resin does not interact well, and the storage stability of the ink may not be sufficiently obtained. If the degree of dispersion is greater than 5.0, it means that the resin has a large variation in molecular weight. In this case, although the large degree of dispersion can accommodate the variation in the state of the pigment particle surface, there is an increase in low-molecular-weight resin that is easily adsorbed and desorbed from the pigment particle surface, and it is easy for it to detach from the pigment particle surface, so the storage stability of the ink may not be sufficiently obtained. The number-average molecular weight of the resin can be measured under the same conditions as the weight-average molecular weight described above.

[0031] Examples of resins include acrylic resins, urethane resins, and urea resins. Among these, acrylic resins are preferred. The resin contains at least units having carboxylic acid groups and units having aromatic groups. In the case of acrylic resins, examples of monomers having carboxylic acid groups include monomers such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid, as well as their salts or anhydrides. Examples of cations constituting the salts include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. From the viewpoint of compatibility between the resin and the additive, (meth)acrylic acid is preferred. Specific examples of monomers having aromatic groups include styrene, α-methylstyrene, and benzyl (meth)acrylate. Furthermore, the resin may also contain hydrophilic units and hydrophobic units other than those mentioned above in order to improve the adsorption of pigments to the particle surface and the storage stability of the ink.

[0032] Hydrophilic units are units that have hydrophilic groups such as hydroxyl groups and ethylene oxide groups. Hydrophilic units can be formed, for example, by polymerizing hydrophilic monomers that have hydrophilic groups. Examples of hydrophilic monomers include 2-hydroxyethyl (meth)acrylate and (poly)ethylene glycol (meth)acrylate. Hydrophobic units are units that do not have hydrophilic groups such as anionic groups, hydroxyl groups, and ethylene oxide groups. Hydrophobic units can be formed, for example, by polymerizing hydrophobic monomers that do not have the above-mentioned hydrophilic groups. Examples of hydrophobic monomers include (meth)acrylate ester monomers such as ethyl (meth)acrylate, methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

[0033] Furthermore, other resins besides those containing carboxylic acid groups and aromatic groups may be added to the ink, provided that the storage stability of the ink and the image quality are not impaired. 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.

[0034] (Additives) The ink contains at least one additive selected from the group consisting of butanedicarboxylic acid, pentanedicarboxylic acid, pentanetricarboxylic acid, hexanedicarboxylic acid, hexanetricarboxylic acid, heptanetricarboxylic acid, octantricarboxylic acid, and nonanetricarboxylic acid. The alkanedicarboxylic acid and alkanetricarboxylic acid may be in acid form or salt form. Examples of cations constituting the salt include ions such as lithium, sodium, potassium, ammonium, and organic ammonium. The content (ppm) of the additive in the ink must be 180 ppm or more and 1,200 ppm or less, based on the total mass of the ink, and preferably 200 ppm or more and 500 ppm or less. Furthermore, the content (mass%) of the additive in the ink is preferably 3.0% by mass or more and 20.0% by mass or less, and more preferably 5.0% by mass or more and 10.0% by mass or less, based on the resin content (mass%). Furthermore, the content (mass%) of the additive in the ink is preferably 0.6% by mass or more and 4.0% by mass or less, and more preferably 1.0% by mass or more and 3.0% by mass or less, based on the pigment content (mass%).

[0035] As the additive, at least one of the above-mentioned alkanedicarboxylic acids or alkanetricarboxylic acids is used. Examples of butanedicarboxylic acids include 1,4-butanedicarboxylic acid (adipic acid), 2,4-butanedicarboxylic acid (2-methylglutaric acid), and 2,3-butanedicarboxylic acid. Examples of pentanedicarboxylic acids include 1,4-pentanedicarboxylic acid, 2,4-pentanedicarboxylic acid, and 2-methyl-2,4-pentanedicarboxylic acid (2,2,4-trimethylglutaric acid). Examples of pentanetricarboxylic acids include 1,3,6-pentanetricarboxylic acid, 2,3,5-pentanetricarboxylic acid, and 3-methyl-1,3,4-pentanetricarboxylic acid. Examples of hexanedicarboxylic acids include 2,5-hexanedicarboxylic acid (2,5-dimethyladipic acid). Examples of hexanetricarboxylic acids include 1,3,5-hexanetricarboxylic acid, 1,4,5-hexanetricarboxylic acid, 3-methyl-2,3,5-hexanetricarboxylic acid, 5-methyl-1,3,5-hexanetricarboxylic acid, 5-methyl-1,4,5-hexanetricarboxylic acid, 3-methyl-1,3,6-hexanetricarboxylic acid, and 3,5-dimethyl-1,3,5-hexanetricarboxylic acid. Examples of heptanetricarboxylic acids include 1,4,6-heptanetricarboxylic acid, 2,4,6-heptanetricarboxylic acid, and 2,4-dimethyl-2,4,6-heptanetricarboxylic acid. Examples of octantricarboxylic acids include 1,2,8-octantricarbon. Examples of nonanetricarboxylic acids include 2,5,8-nonanetricarboxylic acid.

[0036] From the viewpoint of image mapping quality, the alkyl chain of the additive is preferably short because it increases the density of carboxylic acid groups and more effectively suppresses aggregation. Also, from the viewpoint of storage stability of the ink, the alkyl chain of the additive is preferably long because it is easier to obtain dispersion stabilization effect due to the repulsive force caused by steric hindrance. From the viewpoint of balancing these two, the molecular chain length of the alkyl chain is preferably 6, that is, hexanedicarboxylic acid or hexanetricarboxylic acid. Among hexanetricarboxylic acids, it is particularly preferable to have a structure in which three acrylic acid groups are polymerized, and it is also preferable to have one methacrylic acid group. Specifically, examples include 1,3,5-hexanetricarboxylic acid, 1,4,5-hexanetricarboxylic acid, 3-methyl-2,3,5-hexanetricarboxylic acid, 5-methyl-1,3,5-hexanetricarboxylic acid, 5-methyl-1,4,5-hexanetricarboxylic acid, and 3-methyl-1,3,6-hexanetricarboxylic acid.

[0037] (aqueous medium) The ink is an aqueous ink containing water as an aqueous medium. The ink may contain an aqueous medium which is water or a mixed solvent of water and a water-soluble organic solvent. It is preferable to use deionized water (ion-exchanged water) as the water. The water content (mass%) in the ink is preferably 50.0% by mass or more and 95.0% by mass or less, based on the total mass of the ink.

[0038] There are no particular restrictions on the water-soluble organic solvent, as long as it is water-soluble (preferably, soluble in water at 25°C in any proportion). Specifically, monohydric or polyhydric alcohols, 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 5.0% by mass or more and 90.0% by mass or less, and more preferably 10.0% by mass or more and 50.0% by mass or less, based on the total mass of the ink.

[0039] (Other additives) In addition to the additives mentioned above, the ink may contain various other additives as needed, such as surfactants, pH adjusters, rust inhibitors, preservatives, fungicides, antioxidants, reduction inhibitors, evaporation accelerators, and chelating agents. Among these, it is preferable that the ink contains a surfactant. The surfactant content (mass%) in the ink is preferably 0.1% to 5.0% by mass, and more preferably 0.1% to 2.0% by mass, based on the total mass of the ink. Examples of surfactants include anionic surfactants, cationic surfactants, and nonionic surfactants.

[0040] (Ink preparation) During resin synthesis, unreacted monomers and polymerization initiators commonly used in resin synthesis may be mixed into the ink along with the resin. To reduce the influence of polymerization initiators and unreacted monomers on the properties of the ink, the ink may be purified during preparation. Purification methods include ultrafiltration and precipitation. In the case of purification by ultrafiltration, it is preferable to use an ultrafiltration membrane with a molecular weight cutoff of 10 kDa to 80 kDa, and more preferably 10 kDa to 70 kDa, from the viewpoint of ease of removal of polymerization initiators and unreacted monomers. In the case of purification by precipitation, the resin (or a liquid containing the resin) is added to a solvent such as methanol or hexane to precipitate the resin, and the resin is recovered as a precipitate. From the viewpoint of the water solubility of the resin, methanol is preferred as the solvent used for purification by precipitation. The amount of solvent used should be an excess amount that can precipitate the resin, for example, preferably 5 to 20 times the mass ratio of the resin.

[0041] (Ink properties) Since the ink of the present invention is an ink for use in inkjet systems, it is preferable to appropriately control its physical properties. The surface tension of the ink at 25°C is preferably 10 mN / m to 60 mN / m, and more preferably 20 mN / m to 40 mN / m. The viscosity of the ink at 25°C is preferably 1.0 mPa·s to 10.0 mPa·s. The pH of the ink at 25°C is preferably 5.0 to 10.0, and more preferably 6.0 to 8.5.

[0042] <Ink Cartridge> The ink cartridge of the present invention comprises ink and an ink storage section for storing this ink. The ink stored in this ink storage section is the aqueous ink of the present invention as described above. Figure 1 is a schematic cross-sectional view showing one embodiment of the ink cartridge of the present invention. As shown in Figure 1, an ink supply port 12 for supplying ink to the recording head is provided on the bottom surface of the ink cartridge. The inside of the ink cartridge is an ink storage section for storing ink. The ink storage section consists of an ink storage chamber 14 and an absorbent storage chamber 16, which are in communication with each other via a communication port 18. The absorbent storage chamber 16 is also in communication with the ink supply port 12. Liquid ink 20 is stored in the ink storage chamber 14, and absorbent materials 22 and 24 that hold the ink in an impregnated state are stored in the absorbent storage chamber 16. The ink storage section may not have an ink storage chamber for storing liquid ink, and the entire amount of ink to be stored may be held by an absorbent. Alternatively, the ink storage section may not have an absorbent, and the entire amount of ink may be stored in a liquid state. Furthermore, the ink cartridge may be configured to include an ink storage section and a recording head.

[0043] <Inkjet recording method> The inkjet recording method of the present invention is a method of recording an image on a recording medium by ejecting the aqueous ink of the present invention described above from an inkjet recording head. Methods for ejecting the ink include methods that impart mechanical energy to the ink and methods that impart thermal energy to the ink. In the present invention, it is particularly preferable to employ a method that imparts thermal energy to the ink to eject it. Aside from using the ink of the present invention, the steps of the inkjet recording method may be those of known sources. In the present invention, it is not necessary to perform steps to apply heat or pressure to the image or to irradiate it with active energy rays such as ultraviolet light after the ink has been applied.

[0044] Figure 2 is a schematic diagram showing 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 part of the inkjet recording apparatus and (b) is a perspective view of the head cartridge. The inkjet recording apparatus is provided with a transport means (not shown) for transporting the recording medium 32 and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 comprises recording heads 38 and 40 and is configured to hold an ink cartridge 42. While the head cartridge 36 is transported along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, the recording medium 32 is transported in the sub-scanning direction by the transport means (not shown), and an image is recorded on the recording medium 32. [Examples]

[0045] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples unless it exceeds the gist of the invention. Unless otherwise specified, amounts of components indicated in "parts" and "%" are based on mass.

[0046] <Analysis of resins> (Weight-average molecular weight and number-average molecular weight) The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of the resin were measured as polystyrene equivalent values ​​obtained by gel permeation chromatography (GPC). Specifically, the following conditions were used. • Measuring device: Molecular weight analyzer (product name "Waters ALLIANCE e2695", manufactured by Waters) • Columns: Asahipak "GF-1G 7B" (guard column), "GF-310 HQ", and "GF-510 HQ" (product names, manufactured by Showa Denko) connected in series. • Eluent: 20 mmol / L lithium bromide solution with N,N-dimethylformamide ·Flow rate: 0.6mL / min • Sample injection volume: 0.100 mL Oven temperature: 40℃ • Detector: Differential refractive index (RI) detector (product name "Wyatt Optilab rex refractive index detector", manufactured by Wyatt Technology)

[0047] For calculating the molecular weight, a molecular weight calibration curve created using a molecular weight standard (product name "EasiCal Type PS-2 Polystyrene," manufactured by Agilent Technology) was used.

[0048] (Acid value) The acid value of the resin was measured by titration according to JIS K-0070. 0.5 to 2.0 g of resin was accurately weighed and used as the sample for measurement. 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. A 0.1 mol / L potassium hydroxide ethanol solution was used as the titrant, and titration was performed by potentiometric titration, with the amount of titrant used being S (mL). A blank sample without any material was also titrated in the same manner, and the amount of titrant used in this titration was defined as B (mL). An automatic titrator (product name "COM-2500", manufactured by Hiranuma Sangyo Co., Ltd.) was used as the measuring device. From the obtained S and B, the acid value was calculated using the following formula: f is the factor (titer) of the potassium hydroxide ethanol solution, and M (g) is the accurate weight of the sample. Acid value [mgKOH / g]=(SB)×f×5.61 / M

[0049] <Synthesis of resins> 200.0 parts of solvent (methyl ethyl ketone) were placed in a flask equipped with a stirrer, nitrogen inlet tube, reflux condenser, and thermometer, and the temperature was raised to 78°C under a nitrogen atmosphere while stirring. A polymerization initiator solution was prepared by dissolving the amount of polymerization initiator (2,2'-azobis(2-methylbutyronitrile)) shown in Table 1 in 20.0 parts of methyl ethyl ketone. The monomers and polymerization initiator solutions shown in Table 1 were added dropwise to the flask over 2 hours while maintaining the temperature at 78°C. The system was stirred for 4 hours while maintaining the temperature at 78°C to synthesize each resin. All resins synthesized in this study are water-soluble resins. Subsequently, all resins except resin 4 underwent the purification treatment (precipitation or ultrafiltration) shown in Table 1.

[0050] The ultrafiltration purification process was carried out according to the following procedure. After synthesizing the resins, potassium hydroxide in an amount equal to 0.9 times the acid value of the resin (on a molar basis) and an appropriate amount of deionized water were added to the flask, and methyl ethyl ketone was removed under reduced pressure. Subsequently, ultrafiltration was performed using a diafiltration method. As the ultrafiltration membrane, a modified polyethersulfone hollow fiber module with a fractional molecular weight cutoff of 70 kDa or 10 kDa (trade name "MicroKross", manufactured by Spectrum Laboratories) was used. Then, an appropriate amount of deionized water was added to obtain liquids containing each resin with a resin content of 20.0%.

[0051] The purification process by precipitation was carried out according to the following procedure. After synthesizing the resin, methyl ethyl ketone was removed under reduced pressure to isolate the resin, which was then added to methanol in an amount 10 times its mass to precipitate the resin. The precipitate was recovered and dried. Subsequently, potassium hydroxide in an amount 0.9 times the acid value of the resin (on a molar basis) and an appropriate amount of deionized water were added to obtain a liquid containing each resin with a resin content of 20.0%.

[0052] In Table 1, the abbreviations for monomers are as follows: St: styrene, αMSt: α-methylstyrene, nBA: n-butyl acrylate, CHMA: cyclohexyl methacrylate, AA: acrylic acid, MMA: methyl methacrylate, HEMA: 2-hydroxyethyl methacrylate. Table 1 also shows the weight-average molecular weight (Mw), number-average molecular weight (Mn), dispersion (weight-average molecular weight / number-average molecular weight), and acid value of the resins.

[0053] [Table 1]

[0054] (Synthesis of urethane resin) A urethane resin was synthesized as follows, in accordance with the method for synthesizing water-soluble urethane resin described in Japanese Patent Publication No. 2021-008563. 35.3 parts of isophorone diisocyanate, 50.4 parts of polypropylene glycol (number average molecular weight: 2,000), 12.1 parts of dimethylolpropionic acid, and 200.0 parts of methyl ethyl ketone were mixed to obtain a mixture. The mixture obtained above was placed in a flask equipped with a stirrer, nitrogen inlet tube, reflux condenser, and thermometer, and reacted at 80°C for 6 hours. Subsequently, 2.2 parts of dimethylolpropionic acid, 0.60 parts of ethylenediamine as a chain extender, and 100.0 parts of methyl ethyl ketone were added. The remaining percentage of isocyanate groups was confirmed by FT-IR, and the reaction was continued until it reached 15%, to obtain a reaction solution. After cooling the reaction solution to 40°C, an appropriate amount of deionized water was added, and potassium hydroxide in an amount equal to 1.0 times the acid value of the resin (molar basis) was added while stirring at high speed with a homomixer. Methyl ethyl ketone was removed under reduced pressure, and an appropriate amount of deionized water was added to obtain a water-soluble liquid containing urethane resin with a urethane resin content of 20.0%. The acid value of the urethane resin was 60 mgKOH / g, and the weight-average molecular weight was 15,000.

[0055] <Preparation of Pigment Dispersion> (Pigment dispersions 1-14, 16-25, 27-34) The components shown on the left side of Table 2 were mixed and dispersed using a high-pressure homogenizer (product name "Starburst", manufactured by Sugino Machine) at a processing pressure of 150 MPa and 10 passes. Afterward, coarse particles were removed by centrifugation at 10,000 rpm for 10 minutes. After pressure filtration through a 1.0 μm pore size polypropylene filter (manufactured by F-Tech), an appropriate amount of deionized water was added as needed to obtain each pigment dispersion. The solid solution pigment of pigment dispersion 3 is a solid solution pigment of CI Pigment Red 202 and CI Pigment Violet 19. In Table 2, the "Resin Content" column indicates the resin content, including units with carboxylic acid groups and units with aromatic groups. Upon examination of the prepared pigment dispersion 32, the pigments were found to have aggregated. Therefore, the "Characteristics" column in Table 2 is marked with "-".

[0056] (Pigment dispersion 15) A commercially available pigment dispersion (product name "Cab-O-Jet 400", manufactured by Cabot) was heated to concentrate it to half its original volume, thereby preparing a concentrated solution with a self-dispersing pigment content of 30.0%. The above-mentioned commercially available pigment dispersion is a self-dispersing pigment in which anionic groups are bonded to the surface of pigment (carbon black) particles, dispersed in water, with a pigment content of 15.0%. 50.0 parts of the obtained concentrated solution, 15.0 parts of a liquid containing resin 4, and 35.0 parts of ion-exchanged water were mixed to obtain pigment dispersion 15. The pigment content in pigment dispersion 15 was 15.0%, and the resin content was 3.0%.

[0057] (Pigment dispersion 26) Pigment dispersion 26 was obtained using the same procedure as for the preparation of pigment dispersion 1, except that equal parts of the liquid containing resin 6 and the liquid containing resin 7 were mixed. The pigment content in pigment dispersion 26 was 15.0%, and the resin content was 3.0%. The degree of resin dispersion measured for the sample in which equal parts of the liquid containing resin 6 and the liquid containing resin 7 were mixed was 5.0.

[0058] (Pigment dispersion 35) A pigment dispersion 35 was obtained using the same procedure as for the preparation of the pigment dispersion 15, except that a liquid containing resin 1 was used. The pigment content in the pigment dispersion 35 was 15.0%, and the resin content was 3.0%.

[0059] [Table 2]

[0060] <Ink preparation> Each of the following components was mixed, thoroughly stirred, and then pressure filtered through a 1.2 μm pore size microfilter (Sartorius) to prepare each ink. The types of additives used in the preparation of the inks are shown in Table 3. Acetylenel E100 (trade name) is a nonionic surfactant manufactured by Kawaken Fine Chemicals. • Pigment dispersions of the types shown in Tables 4 and 5: 20.0% • Types of additives shown in Tables 4 and 5: Usage amounts (ppm) shown in Tables 4 and 5 Glycerin: 5.0% Triethylene glycol: 10.0% Acetyleneol E100: 0.1% • 1,2-Hexanediol: 2.0% • Ion-exchanged water: The amount remaining when the total amount of components reaches 100.0%

[0061] [Table 3]

[0062] <Rating> Each ink obtained above was evaluated for the following items. In this invention, "AA," "A," and "B" were defined as acceptable levels, and "C" as an unacceptable level in the evaluation criteria for each item below. Comparative Example 1 was marked "Unacceptable" in the evaluation result column because pigment aggregation occurred during ink preparation. The evaluation results are shown on the right side of Tables 4 and 5. In Tables 4 and 5, "Resin Content" refers to the content of resins containing units having carboxylic acid groups and units having aromatic groups.

[0063] (Storage stability) The particle size of the pigment in the prepared ink was measured (referred to as "particle size before storage"). Each ink was placed in a sealed container and stored at 80°C for 4 days. After the ink was returned to 25°C, the particle size of the pigment was measured again (referred to as "particle size after storage"). The particle size of the pigment was measured using a dynamic light scattering particle size analyzer (product name "UPA-EX150", manufactured by Nikkiso), and was the volume-based cumulative 50% particle size (D 50 The particle size difference was calculated based on the formula "particle size difference" (nm) = "particle size after storage" - "particle size before storage," and the storage stability of the ink was evaluated according to the evaluation criteria shown below. AA: The difference in ink particle size was 10 nm or less. A: The difference in ink particle size was greater than 10 nm and less than or equal to 20 nm. B: The difference in ink particle size was greater than 20 nm and less than or equal to 30 nm. C: The difference in ink particle size exceeded 30 nm.

[0064] (Mapping property) Each ink was filled into its own ink cartridge and mounted in an inkjet recording device (product name "PIXUS Pro 9500", manufactured by Canon) that ejects ink from the recording head using thermal energy. In this embodiment, the recording duty cycle of a solid image recorded under the condition that 28 ng of ink is applied to a unit area of ​​1 / 600 inch x 1 / 600 inch is defined as 100%. Using the above inkjet recording device, a solid image with a recording duty cycle of 100% was recorded on the entire surface of an A4-sized recording medium (photo paper, product name "Canon Photo Paper Gloss Gold GL-101", manufactured by Canon). After drying the image at 25°C for 24 hours, two fluorescent lamps placed in parallel at a distance of 10 cm apart were used as observation light sources, and the fluorescent light was shone on the image at a 45-degree angle from a distance of 2 m (illumination angle 45 degrees). The shape of a fluorescent lamp was projected onto an image, and the projected shape of the fluorescent lamp was visually confirmed from a 45-degree angle (observation angle 45 degrees). The image quality was then evaluated according to the evaluation criteria shown below. A: The boundary between the two projected fluorescent lights was discernible, and no blurring was observed at the edge. B: The boundary between the two projected fluorescent lights was discernible, but a slight blurring was observed at the edge. C: I couldn't see the boundary between the two projected fluorescent lights.

[0065] [Table 4]

[0066] [Table 5]

[0067] The evaluation results for the inks in Examples 32 and 40 both showed storage stability and image quality at "A". However, comparing Examples 32 and 40, Example 40 showed superior storage stability and image quality.

[0068] This embodiment includes the following configurations and methods.

[0069] [Configuration 1] An aqueous inkjet ink containing pigments, resins, and additives, The resin comprises a unit having a carboxylic acid group and a unit having an aromatic group. The aforementioned additive is at least one selected from the group consisting of butanedicarboxylic acid, pentanedicarboxylic acid, pentanetricarboxylic acid, hexanedicarboxylic acid, hexanetricarboxylic acid, heptanetricarboxylic acid, octantricarboxylic acid, and nonanetricarboxylic acid. A water-based ink characterized in that the content (ppm) of the aforementioned additive is 180 ppm or more and 1,200 ppm or less, based on the total mass of the ink.

[0070] [Configuration 2] The aqueous ink according to configuration 1, wherein the additive is hexanetricarboxylic acid.

[0071] [Configuration 3] The aqueous ink according to configuration 2, wherein the additive is at least one selected from the group consisting of 1,3,5-hexanetricarboxylic acid, 1,4,5-hexanetricarboxylic acid, 3-methyl-2,3,5-hexanetricarboxylic acid, 5-methyl-1,3,5-hexanetricarboxylic acid, 5-methyl-1,4,5-hexanetricarboxylic acid, and 3-methyl-1,3,6-hexanetricarboxylic acid.

[0072] [Structure 4] The aqueous ink according to any one of the three claims, wherein the pigment has a quinacridone skeleton, a diketopyrrolopyrrole skeleton, or a perylene skeleton.

[0073] [Composition 5] The aqueous ink according to any one of the configurations 1 to 4, wherein the resin content (mass%) is 0.15 times or more and 0.30 times or less in mass ratio to the pigment content (mass%).

[0074] [Composition 6] The aqueous ink according to any one of the configurations 1 to 5, wherein the content (mass%) of the additive is 3.0% by mass or more and 20.0% by mass or less, based on the content (mass%) of the resin.

[0075] [Composition 7] The aqueous ink according to any one of the configurations 1 to 6, wherein the content (mass%) of the additive is 0.6% by mass or more and 4.0% by mass or less, based on the content (mass%) of the pigment.

[0076] [Structure 8] The aqueous ink according to any one of the configurations 1 to 7, wherein the weight-average molecular weight of the resin is 6,000 or more and 15,000 or less.

[0077] [Composition 9] The aqueous ink according to any one of the configurations 1 to 8, wherein the degree of dispersion (Mw / Mn), defined by the ratio of the weight-average molecular weight (Mw) and the number-average molecular weight (Mn) of the resin, is 3.4 or higher.

[0078] [Configuration 10] The aqueous ink according to any one of the configurations 1 to 9, wherein the degree of dispersion (Mw / Mn), defined by the ratio of the weight-average molecular weight (Mw) and the number-average molecular weight (Mn) of the resin, is 3.4 or more and 5.0 or less.

[0079] [Composition 11] The aqueous ink according to any one of the configurations 1 to 10, wherein the acid value (mgKOH / g) of the resin is 110 mgKOH / g or more and 160 mgKOH / g or less.

[0080] [Composition 12] The aqueous ink according to any one of the configurations 1 to 11, wherein the resin is a resin dispersant for dispersing the pigment.

[0081] [Composition 13] An aqueous ink according to any one of the configurations 1 to 12, wherein the content (ppm) of the additive is 200 ppm or more and 500 ppm or less, based on the total mass of the ink.

[0082] [Composition 14] An aqueous inkjet ink containing pigments, resins, and additives, The resin comprises a unit having a carboxylic acid group and a unit having an aromatic group. The additive is a compound having an alkyl chain molecular length of 4 to 9 and 2 or 3 carboxylic acid groups. A water-based ink characterized in that the content (ppm) of the aforementioned additive is 180 ppm or more and 1,200 ppm or less, based on the total mass of the ink.

[0083] [Composition 15] An ink cartridge comprising ink and an ink storage section for storing the ink, An ink cartridge characterized in that the ink is the water-based ink described in any one of items 1 to 14 of the configuration.

[0084] [Composition 16] An inkjet recording method that records an image on a recording medium by ejecting ink from an inkjet recording head, An inkjet recording method characterized in that the ink is an aqueous ink according to any one of the items 1 to 14.

Claims

1. An aqueous inkjet ink containing pigments, resins, and additives, The resin comprises a unit having a carboxylic acid group and a unit having an aromatic group. The additive is at least one selected from the group consisting of 2,4-butanedicarboxylic acid, 2,3-butanedicarboxylic acid, 2-methyl-2,3-butanedicarboxylic acid, 2,2,3,3-tetramethylbutanedicarboxylic acid, 3-methyl-1,3-butanedicarboxylic acid, pentanedicarboxylic acid, pentanetricarboxylic acid, hexanedicarboxylic acid, hexanetricarboxylic acid, heptanetricarboxylic acid, octantricarboxylic acid, and nonanetricarboxylic acid. A water-based ink characterized in that the content (ppm) of the aforementioned additive is 180 ppm or more and 1,200 ppm or less, based on the total mass of the ink.

2. The aqueous ink according to claim 1, wherein the additive is hexanetricarboxylic acid.

3. The aqueous ink according to claim 2, wherein the additive is at least one selected from the group consisting of 1,3,5-hexanetricarboxylic acid, 1,4,5-hexanetricarboxylic acid, 3-methyl-2,3,5-hexanetricarboxylic acid, 5-methyl-1,3,5-hexanetricarboxylic acid, 5-methyl-1,4,5-hexanetricarboxylic acid, and 3-methyl-1,3,6-hexanetricarboxylic acid.

4. The aqueous ink according to claim 1, wherein the pigment has a quinacridone skeleton, a diketopyrrolopyrrole skeleton, or a perylene skeleton.

5. The aqueous ink according to claim 1, wherein the resin content (mass%) is 0.15 times or more and 0.30 times or less in mass ratio to the pigment content (mass%).

6. The aqueous ink according to claim 1, wherein the content (mass%) of the additive is 3.0% by mass or more and 20.0% by mass or less, based on the content (mass%) of the resin.

7. The aqueous ink according to claim 1, wherein the content (mass%) of the additive is 0.6% by mass or more and 4.0% by mass or less, based on the content (mass%) of the pigment.

8. The aqueous ink according to claim 1, wherein the weight-average molecular weight of the resin is 6,000 or more and 15,000 or less.

9. The aqueous ink according to claim 1, wherein the degree of dispersion (Mw / Mn), defined by the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the resin, is 3.4 or greater.

10. The aqueous ink according to claim 1, wherein the degree of dispersion (Mw / Mn), defined by the ratio of the weight-average molecular weight (Mw) and the number-average molecular weight (Mn) of the resin, is 3.4 or more and 5.0 or less.

11. The aqueous ink according to claim 1, wherein the acid value (mgKOH / g) of the resin is 110 mgKOH / g or more and 160 mgKOH / g or less.

12. The aqueous ink according to claim 1, wherein the resin is a resin dispersant for dispersing the pigment.

13. The aqueous ink according to claim 1, wherein the content (ppm) of the additive is 200 ppm or more and 500 ppm or less, based on the total mass of the ink.

14. The aqueous ink according to claim 1, wherein the resin content (mass%) is 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the aqueous ink.

15. The aqueous ink according to claim 1, wherein the resin content (mass%) is 0.42% by mass or more and 0.93% by mass or less, based on the total mass of the aqueous ink.

16. The water-based ink according to claim 1, wherein the resin is an acrylic resin.

17. The aqueous ink according to claim 1, wherein the unit having a carboxylic acid group is a unit derived from (meth)acrylic acid.

18. The aqueous ink according to claim 1, wherein the aromatic group-containing unit is at least one selected from the group consisting of units derived from styrene and units derived from α-methylstyrene.

19. The aqueous ink according to claim 1, wherein the resin further comprises a unit derived from (meth)acrylic acid ester.

20. An aqueous inkjet ink containing pigments, resins, and additives, The resin comprises a unit having a carboxylic acid group and a unit having an aromatic group. The additive is a compound having an alkyl chain molecular length of 4 to 9 and having 2 or 3 carboxylic acid groups, and when the alkyl chain molecular length is 4, it is at least one selected from the group consisting of 2,4-butanedicarboxylic acid, 2,3-butanedicarboxylic acid, 2-methyl-2,3-butanedicarboxylic acid, 2,2,3,3-tetramethylbutanedicarboxylic acid, and 3-methyl-1,3-butanedicarboxylic acid. A water-based ink characterized in that the content (ppm) of the aforementioned additive is 180 ppm or more and 1,200 ppm or less, based on the total mass of the ink.

21. An ink cartridge comprising ink and an ink storage section for storing the ink, An ink cartridge characterized in that the ink is the water-based ink described in any one of claims 1 to 20.

22. An inkjet recording method that records an image on a recording medium by ejecting ink from an inkjet recording head, An inkjet recording method characterized in that the ink is an aqueous ink according to any one of claims 1 to 20.

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