Water-based ink, ink cartridge, inkjet recording method, aqueous resin particle dispersion, and method for producing aqueous resin particle dispersion.

Abstract

This invention provides a water-based inkjet ink that can record images with good resolution and has good storage stability. [Solution] An aqueous inkjet ink containing an aqueous medium and resin particles. The resin forming the resin particles is a copolymer having units derived from monomers having acid groups. The resin particles contain ammonium ions of 2 μmol / g to 60 μmol / g inside the resin particles. Furthermore, the amount of surface acid groups (μmol / g) of the resin particles is 100 μmol / g to 400 μmol / g.

Description

[Technical Field]

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

[0002] Resin particles are used as binders in water-based inks and paints. It is desirable that the properties of the water-based resin particle dispersion remain unchanged from its manufacture to the preparation of inks and paints using the dispersion. Furthermore, a key characteristic of the resin particles contained in the inks and paints is good image quality.

[0003] For example, Patent Document 1 proposes a method for producing a polymer emulsion (aqueous resin particle dispersion) that improves the storage stability, clogging reliability, and ejection stability of an ink composition. Furthermore, Patent Document 2 proposes an aqueous ink for inkjet printers having a pH within a specific range to extend the lifespan of inkjet recording heads. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2011-1557 [Patent Document 2] Special Publication No. 2021-535239 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The present inventors investigated the method for producing an aqueous resin particle dispersion proposed in Patent Document 1. As a result, it was found that it is not possible to maintain the storage stability of the obtained aqueous resin particle dispersion while achieving the high level of image clarity required in recent years when used in an aqueous ink. Here, image clarity refers to the characteristic that indicates the sharpness of the image of an object when it is projected onto the surface of an image. If the image clarity is low, the image will appear blurry, and if the image clarity is high, the image will appear sharp. Furthermore, storage stability refers to the property that the physical properties of the aqueous resin particle dispersion or aqueous ink do not change easily under the usage environment and logistics environment.

[0006] On the other hand, it was found that the image quality of the resulting image is insufficient because the ink proposed in Patent Document 2 does not contain an aqueous resin particle dispersion.

[0007] Therefore, an object of the present invention is to provide an aqueous inkjet ink capable of recording images with good image quality and good storage stability, an ink cartridge using the aqueous ink, and an inkjet recording method. Another object of the present invention is to provide an aqueous resin particle dispersion that can be prepared to record an aqueous inkjet ink capable of recording images with good image quality and has good storage stability, and a method for producing the aqueous resin particle dispersion. [Means for solving the problem]

[0008] In other words, the present invention provides an aqueous ink for inkjet use containing an aqueous medium and resin particles, wherein the resin forming the resin particles is a copolymer having units derived from monomers having acid groups, the resin particles contain ammonium ions of 2 μmol / g to 60 μmol / g inside the resin particles, and the amount of surface acid groups (μmol / g) of the resin particles is 100 μmol / g to 400 μmol / g. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an aqueous inkjet ink capable of recording images with good image quality and good storage stability, an ink cartridge using the aqueous ink, and an inkjet recording method. Furthermore, according to the present invention, it is possible to prepare an aqueous inkjet ink capable of recording images with good image quality, as well as an aqueous resin particle dispersion with good storage stability, and a method for producing the aqueous resin particle dispersion. [Brief explanation of the drawing]

[0010] [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]

[0011] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when a compound is a salt, the salt exists in the ink dissociated into ions, but for convenience, it will be expressed as "contains a salt." Also, water-based ink for inkjet printers may be simply referred to as "ink." Unless otherwise specified, physical properties are given at room temperature (25°C), normal pressure (1 atmosphere = 101,325 Pa), and normal humidity (50% relative humidity). Furthermore, with respect to resins, "unit" means the unit structure corresponding to one monomer unless otherwise specified. When "(meth)acrylic acid," "(meth)acrylate," and "(meth)acryloyl" are written, they represent "acrylic acid, methacrylic acid," "acrylate, methacrylate," and "acryloyl, methacryloyl," respectively.

[0012] The inventors investigated an aqueous ink for inkjet printers that can record images with good resolution while maintaining storage stability. Furthermore, the inventors investigated an aqueous resin particle dispersion to improve the resolution of images recorded with an aqueous ink prepared using that aqueous resin particle dispersion, while maintaining the storage stability of the aqueous resin particle dispersion. To ensure good storage stability for aqueous inks and aqueous resin particle dispersions, it is important that the resin particles maintain a stable dispersion state in the aqueous medium. In aqueous inks and aqueous resin particle dispersions, dispersion stability due to charge repulsion caused by the dissociation of acid groups (groups that may have anionic properties when dissociated) on the surface of the resin particles and the constraints resulting from intermolecular interactions within the resin particles is useful. Hereinafter, the state in which the acid group has dissociated and has anionic properties will be referred to as anionic group. On the other hand, to obtain images with good resolution, it is useful that when the ink is formed on the recording medium, the molecules within the film-formed resin particles move to form a flat film.

[0013] As a result of our investigations, we found that resin particles with good dispersion stability can be obtained by having units derived from monomers with acidic groups in the resin that form the resin particles, and by including a specific amount of ammonium ions inside the resin particles. It is thought that the presence of anionic groups derived from acidic groups and a specific amount of ammonium ions inside the resin particles, and the formation of ammonium salts by these, generates cohesive forces due to ionic bonding inside the resin particles, resulting in good dispersion stability of the resin particles. Furthermore, it was found that dispersion stability due to charge repulsion can be maintained on the surface of the resin particles by controlling the amount of anionic groups derived from acidic groups on the surface of the resin particles.

[0014] Furthermore, when recording images using ink, the ammonium salts inside the resin particles are bonded ionically between anionic groups derived from acidic groups and ammonium ions, allowing for easy rearrangement of intermolecular ionic bonds. As a result, when the ink forms a film on the recording medium, molecules are easily formed between and within the resin particles, making it easier to form a flat film and thus enabling the acquisition of images with good resolution.

[0015] Therefore, a copolymer having units derived from an acid group-containing monomer is used as the resin that forms the resin particles. The resin particles contain 2 µmol / g or more and 60 µmol / g or less of ammonium ions inside thereof, and the amount of surface acid groups of the resin particles is set to 100 µmol / g or more and 400 µmol / g or less. With this configuration, it is possible to record an image with excellent image clarity while maintaining good storage stability of aqueous ink for inkjet. Furthermore, by using the aqueous resin particle dispersion after maintaining good storage stability of the aqueous resin particle dispersion, it becomes possible to prepare an aqueous ink for inkjet that can record an image with excellent image clarity.

[0016] <Aqueous resin particle dispersion> The aqueous resin particle dispersion of the present invention contains an aqueous medium and resin particles. The resin forming the resin particles is a copolymer having units derived from an acid group-containing monomer. Further, the resin particles contain a specific amount of ammonium ions inside thereof. Hereinafter, the components constituting the aqueous resin particle dispersion of the present invention and the physical properties of the aqueous resin particle dispersion will be described in detail.

[0017] >(Resin particles) In the aqueous resin particle dispersion, the resin particles exist in a state dispersed in an aqueous medium, that is, in the form of a resin emulsion. The content of the resin particles (solid content) in the aqueous resin particle dispersion is preferably 10% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 50% by mass or less, based on the total mass of the aqueous resin particle dispersion. The above-mentioned "resin particles (solid content)" may include, in addition to resin particles formed from the monomers constituting the resin, solid content components such as emulsifiers and polymerization initiators used in the synthesis of the resin.

[0018] As used herein, the term "resin particles" refers to a resin that is dispersed in an aqueous medium and can exist in the aqueous medium in a state having a particle diameter. Whether or not a given resin is a "resin particle" can be determined according to the method described below. First, a sample is prepared by diluting a liquid containing the resin to be determined with pure water so that the resin content is about 1.0% by mass. When the particle diameter of the resin in the sample is measured by a dynamic light scattering method, if particles having a particle diameter are detected, the resin is determined to be "resin particles" (that is, a "water-dispersible resin"). On the other hand, if no particles having a particle diameter are detected, the resin is determined not to be "resin particles" (that is, it is a "water-soluble resin"). The measurement conditions in this case can be, for example, as follows. [Measurement Conditions] SetZero: 30 seconds Number of measurements: 3 times Measurement time: 120 seconds Shape: True sphere Refractive index: 1.5 Density: 1.0

[0019] As a particle size distribution analyzer based on the dynamic light scattering method, a particle size analyzer (for example, the product name "Nanotrac WAVEII-Q", manufactured by Microtrac MRB) can be used. Of course, the particle size distribution analyzer and measurement conditions to be used are not limited to those described above.

[0020] [Amount of Ammonium Ions Inside Resin Particles] The resin particles are formed from a copolymer (resin) having units derived from monomers with acidic groups, and contain ammonium ions of 2 μmol / g to 60 μmol / g inside the resin particles. If the amount of ammonium ions inside the resin particles is less than 2 μmol / g, there are fewer ionic bonds between the ammonium ions and acidic groups (anionic groups) inside the resin particles, resulting in weaker cohesive force inside the resin particles and reduced storage stability. On the other hand, if the amount of ammonium ions inside the resin particles is greater than 60 μmol / g, the cohesive force inside the resin particles is strong, making it difficult for film formation between the resin particles to occur when recording an image using ink containing resin particles. As a result, the smoothness of the image surface is poor, and the image quality is reduced. From the viewpoint of further improving the storage stability and image quality of the aqueous resin particle dispersion, it is preferable that the amount of ammonium ions inside the resin particles be 5 μmol / g to 50 μmol / g. As ammonium ions, ammonia water, ammonium chloride, and ammonium salts such as ammonium sulfate can be used.

[0021] The amount of ammonium ions (μmol / g) inside the resin particles can be measured by measuring the ammonium ion concentration using an ion chromatograph or the like. By storing an aqueous resin particle dispersion at a temperature above the glass transition temperature of the resin particles but below 100°C (preferably 96°C or lower) for one month or more, it is possible to dissolve the ammonium ions incorporated inside the resin particles into the aqueous medium. The difference in the amount of ammonium ions contained in the aqueous medium before and after storage of the aqueous resin particle dispersion can be determined as the amount of ammonium ions (μmol / g) contained inside the resin particles. The amount of ammonium ions inside the resin particles can be controlled by the timing of adding the ammonium salt during resin particle synthesis, the pH adjustment method during resin particle synthesis, and heat treatment. The amount of ammonium ions inside the resin particles includes both ammonium ions existing in the form of ammonium ions and ammonium ions forming ammonium salts inside the resin particles.

[0022] [Monomers containing acidic groups] The resin forming the resin particles is a copolymer having units derived from monomers having acidic groups. These units derived from monomers having acidic groups can be formed, for example, by polymerizing the monomers having acidic groups. Within the resin particles, the acidic groups (anionic groups) are necessary for improving the storage stability of the aqueous resin particle dispersion and the image quality of the image obtained by recording the ink containing the resin particles, through interaction with the ammonium ions mentioned above.

[0023] Examples of monomers having acidic groups include monomers having carboxylic acid groups, monomers having sulfonic acid groups, and monomers having phosphoric acid groups. These monomers are polymerizable monomers that have polymerizable reactive groups such as ethylenically unsaturated bonds in addition to acidic groups within their molecules. Specifically, monomers having acidic groups and (meth)acryloyloxy groups, monomers having acidic groups and allyl groups, and monomers having acidic groups and vinyl groups are preferred.

[0024] Examples of monomers having a carboxylic acid group include acrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconic acid. Examples of monomers having a sulfonic acid group include styrene sulfonic acid, (meth)acrylate 2-sulfoethyl, and polymerizable surfactants having a sulfonic acid group. Examples of polymerizable surfactants having a sulfonic acid group include polyoxyethylene-1-(allyloxymethyl)alkyl ether sulfate ammonium salt and alkylallyl sulfosuccinate. Examples of monomers having a phosphate group include (meth)acryloyloxyethyl acid phosphate, phosphate 2-((meth)acryloyloxy)ethyl, and phosphate ester type polymerizable surfactants. In addition, anionic monomers such as anhydrides and salts of monomers having an acid group can be used. Examples of salts include alkali metal salts such as lithium, sodium, and potassium, ammonium salts, and organic ammonium salts. One or more monomers having an acid group can be used.

[0025] [Types and ratios of acid groups] The above-mentioned monomers having acidic groups preferably include monomers having carboxylic acid groups and monomers having sulfonic acid groups. That is, it is preferable that the resin forming the resin particles contains both units derived from monomers having carboxylic acid groups and units derived from monomers having sulfonic acid groups. Carboxylic acid groups readily form ionic bonds with ammonium ions inside the resin particles, and sulfonic acid groups readily exert electrostatic repulsion on the surface of the resin particles. Therefore, having both carboxylic acid groups and sulfonic acid groups in the resin forming the resin particles makes it easier to further improve the storage stability of the aqueous resin particle dispersion. In order to effectively exert this effect, the ratio of the molar amount of carboxylic acid groups to the molar amount of sulfonic acid groups (molar ratio of carboxylic acid groups / sulfonic acid groups) is preferably 8 or more and 22 or less, and more preferably 10 or more and 20 or less. When the molar ratio of carboxylic acid groups / sulfonic acid groups is 10 or more, the number of carboxylic acid groups inside the resin particles becomes relatively large, the cohesive force inside the resin particles tends to increase appropriately, and the storage stability of the aqueous resin particle dispersion tends to be better. On the other hand, when the molar ratio of carboxylic acid groups to sulfonic acid groups is 20 or less, the amount of carboxylic acid groups is kept relatively moderate. As a result, the cohesive force within the resin particles is moderately suppressed, and when an image is recorded using an ink containing resin particles, the resin particles tend to form a film, resulting in good surface smoothness of the image and further improving image clarity.

[0026] [Other monomers] The resin forming the resin particles may contain units derived from monomers having acidic groups, as well as units derived from monomers other than those having acidic groups (hereinafter sometimes referred to as "other monomers"). Other monomers that can copolymerize with monomers having acidic groups can be used. Other monomers are polymerizable monomers that have polymerizable reactive groups, such as ethylenically unsaturated bonds, within their molecules. Examples of polymerizable reactive groups include (meth)acryloyloxy groups, allyl groups, and vinyl groups.

[0027] Other monomers include, for example, (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, cyclohexyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic vinyls such as styrene, 4-methylstyrene, and 1-vinylnaphthalene; and acrylonitrile and methacrylonitrile. One or more of these can be used.

[0028] [Amount of surface acid groups in resin particles] The amount of surface acid groups (μmol / g) of resin particles is between 100 μmol / g and 400 μmol / g. "Amount of surface acid groups (μmol / g) of resin particles" is expressed as the amount of acid groups (μmol) present on the surface of a resin particle per unit mass (g) of resin particles. Therefore, when all acid groups on the surface of the resin particle are dissociated, the amount of surface acid groups can also be expressed as the amount of anionic groups on the surface of the resin particle. If the amount of surface acid groups of resin particles is less than 100 μmol / g, the affinity of the resin particles to aqueous media is low, making aggregation between the resin particles more likely and potentially reducing the storage stability of the aqueous resin particle dispersion. On the other hand, if the amount of surface acid groups of resin particles exceeds 400 μmol / g, the affinity of the resin particles to aqueous media is too high, potentially reducing the drying speed after recording an image with ink containing the aqueous resin particle dispersion, impairing the smoothness of the image surface, and reducing image quality.

[0029] From the viewpoint of further improving the storage stability and image rendering quality of the aqueous resin particle dispersion, it is even more preferable that the surface acid group content (μmol / g) of the resin particles be 250 μmol / g or more and 350 μmol / g or less. The surface acid group content (μmol / g) of the resin particles can be adjusted by the amount of monomer having acid groups in the resin particles, the particle size, and the method of producing the resin particles. The surface acid group content (μmol / g) of the resin particles can be measured by colloidal titration using a potential difference at a pH of 12 or higher, using a methyl glycol chitosan solution as the titration reagent. To achieve a pH of 12 or higher, a monovalent inorganic hydroxide aqueous solution is added to the measurement sample of the aqueous resin particle dispersion to adjust the pH to 12, and by confirming that the pH remains 12 or higher after colloidal titration, reproducible measurements are possible.

[0030] [Dissociation rate of surface acid groups of resin particles at pH 9] Preferably, 80% or more of the acid groups on the surface of the resin particles dissociate and become anionic at pH 9, and more preferably 84% to 92% dissociate and become anionic at pH 9. This value is the ratio of the amount of dissociated acid groups at pH 9, measured by the same method with the sample at pH 9, to the amount of surface acid groups (i.e., the total amount of acid groups present on the surface of the resin particles) measured by colloidal titration using potential difference at pH 12 or higher. This ratio is sometimes referred to as the dissociation rate of surface acid groups of resin particles at pH 9. Here, pH 9 represents a pH of 8.5 or higher and less than pH 9.5, and by adding a pH buffer that can be adjusted to this range to the sample, it is possible to measure without pH change before and after colloidal titration. When the above dissociation rate is 80% or higher, the acid groups of the resin particles dissociate sufficiently, charge repulsion becomes sufficient, and dispersion stability tends to be further enhanced. On the other hand, if the dissociation rate is 92% or less, the electrostatic repulsion of the resin particles is moderately suppressed, making it easier for film formation between the resin particles to proceed when recording an image, resulting in better surface smoothness of the image and further improving image clarity. The dissociation rate of the surface acid groups of the resin particles at pH 9 can be adjusted by the method of manufacturing the resin particles.

[0031] [Particle size of resin particles] Cumulative 50% particle size (D) of the volume-based particle size distribution of resin particles 50 The D of the resin particles is preferably 40 nm to 230 nm, and more preferably 50 nm to 200 nm. 50 When the diameter is 50 nm or greater, the surface area per unit mass of the resin particles becomes relatively smaller, and the probability of contact between resin particles decreases, which suppresses aggregation between resin particles and further enhances the storage stability of aqueous resin particle dispersions. On the other hand, the D of the resin particles 50 When the D of the resin particles is 200 nm or less, after recording an image using ink containing resin particles, the irregularities caused by the resin particles in the image are suppressed, and the image quality tends to be even better. 50 This can be prepared by the method for producing an aqueous resin particle dispersion, as described later. D of the resin particles 50 This can be measured by dynamic light scattering under the same conditions as the method used to determine whether or not a particle is a "resin particle" as described above.

[0032] [Glass transition temperature (Tg) of resin particles] The glass transition temperature (Tg) of the resin particles is preferably 45°C to 85°C, and more preferably 50°C to 80°C. When the Tg of the resin particles is 50°C or higher, the softening of the resin under storage conditions of the aqueous resin particle dispersion is suppressed, and coalescence between resin particles is less likely to occur, thus further improving the storage stability of the aqueous resin particle dispersion. On the other hand, when the Tg of the resin particles is 80°C or lower, the resin particles tend to form a film when recording an image using an ink containing resin particles, thus further improving the image quality. The Tg of the resin particles can be adjusted by the type, amount, and molecular weight of the monomer used in the synthesis of the resin particles. The Tg of the resin particles is measured using differential scanning calorimetry (DSC) with dried resin particle powder.

[0033] (aqueous medium) The aqueous resin particle dispersion contains an aqueous medium. It is preferable to use the same aqueous medium as that used for the ink described later, and it is more preferable to use deionized water or ion-exchanged water.

[0034] (Other ingredients) Aqueous resin particle dispersions may contain other components besides resin particles and an aqueous medium, as needed. Examples of other components include surfactants and pH adjusters.

[0035] (Method for manufacturing resin particles) Any known method can be used as a method for producing resin particles, as long as it satisfies the configuration of the present invention described above. Specifically, resin particles can be produced by methods such as emulsion polymerization, pre-emulsion polymerization, and seed polymerization. For example, resin particles can be synthesized by mixing monomers that form the units of the resin that make up the resin particles and performing emulsion polymerization, thereby obtaining an aqueous resin particle dispersion. Emulsion polymerization is preferably carried out in an aqueous liquid medium.

[0036] A preferred method for producing an aqueous resin particle dispersion includes the following three steps: the first, second, and third steps. By performing the first, second, and third steps in order, the aqueous resin particle dispersion of the present invention can be easily obtained. • First step: A step in which a monomer component containing a monomer having an acid group is emulsion polymerized in the presence of an ammonium salt to obtain a first aqueous dispersion containing resin particles. • Second step: A step to obtain a second aqueous dispersion by adjusting the pH of the first aqueous dispersion obtained in the first step to between 6.5 and 9.5. • Third step: After the second step, the second aqueous dispersion is subjected to heat treatment at a temperature above the glass transition temperature of the resin particles for no more than 24 hours.

[0037] In the first step, the monomer component may be a monomer having an acid group as described above, and in addition, other monomers as described above may also be used. As for the ammonium salt, as described above, aqueous ammonia, ammonium chloride, and ammonium sulfate can be used. Emulsion polymerization is preferably carried out in an aqueous liquid medium as described above. Polymerization initiators and surfactants can be used during emulsion polymerization.

[0038] In the second step, if the pH of the first aqueous dispersion obtained in the first step is adjusted to less than 6.5, the dissociation of the anionic groups of the resin particles tends to be insufficient, making it difficult to improve the storage stability of the aqueous resin particle dispersion. On the other hand, if the pH is greater than 9.5, the electrostatic repulsion of the resin particles becomes too high, making it difficult for film formation between the resin particles to proceed when recording an image using ink, impairing the smoothness of the image surface and resulting in poor image quality. pH adjustment is preferably performed by adding a monovalent inorganic hydroxide, and a compound with pH buffering properties may also be used in combination.

[0039] If the heat treatment temperature in the third step is below the glass transition temperature (Tg) of the resin particles, the resin cannot undergo molecular motion, resulting in a lower dissociation rate and thus impairing the dispersion stability of the aqueous resin particle dispersion. Furthermore, if the heat treatment time at or above the Tg of the resin particles exceeds 24 hours, ammonium from inside the resin particles dissolves, reducing the ionic bonding between ammonium and acid groups within the resin particles. This weakens the cohesive force inside the resin particles, impairing the storage stability of the aqueous resin particle dispersion. The heat treatment temperature in the third step is preferably below 100°C, and more preferably below 96°C.

[0040] By sequentially performing the first, second, and third steps described above, an appropriate amount of ammonium ions are retained inside the resin particles, and the anionic groups dissociate on the surface of the resin particles. This improves the storage stability of the aqueous resin particle dispersion and the image quality of images recorded with aqueous ink prepared using the aqueous resin particle dispersion.

[0041] <Water-based ink> The aqueous inkjet ink of the present invention contains an aqueous medium and resin particles. This ink can be prepared using the aforementioned aqueous resin particle dispersion. By using the aqueous resin particle dispersion, the aforementioned resin particles can be incorporated into the ink. From the viewpoint of recording images with excellent image quality, the ink preferably contains the aforementioned aqueous resin particle dispersion (resin particles and aqueous medium), a pigment, and a water-soluble organic solvent, and more preferably contains a surfactant in addition to these components.

[0042] (Resin particles) The ink contains the aforementioned resin particles. The resin particles exist in the ink in a dispersed state, i.e., in the form of particles. The content (mass%) of resin particles in the aqueous ink is preferably 2.0% by mass or more and 20.0% by mass or less, and more preferably 5.0% by mass or more and 15.0% by mass or less, based on the total mass of the ink. If the resin particle content in the ink is 5.0% by mass or more, there will be a sufficient amount of resin when the ink forms a film, and the image quality will likely be even better. On the other hand, if the resin particle content in the ink is 15.0% by mass or less, the viscosity of the ink will be suppressed, and the dispersion stability of the resin particles in the ink will likely be better.

[0043] Furthermore, the content (mass%) of resin particles in the water-based ink is preferably 0.5 to 7.0 times the pigment content (mass%), and more preferably 1.0 to 5.0 times. When the resin particle content is 1.0 times or more the pigment content in the ink, there is a sufficient amount of resin when the ink forms a film, and the image quality tends to be even better. Also, when the resin particle content is 5.0 times or less the pigment content in the ink, the viscosity of the ink is suppressed, and the dispersion stability of the resin particles in the ink tends to be good. Note that the particle size (D) of the resin particles contained in the ink 50The amount of ammonium ions inside, the amount of surface acid groups, the dissociation rate of surface acid groups at pH 9, and the glass transition temperature (Tg) can be measured using resin particles extracted from the ink in the same manner as described above for the aqueous resin particle dispersion.

[0044] (Pigment) The ink preferably contains a pigment as a colorant. The pigment content (by mass) in the ink is preferably 0.1% by mass or more and 15.0% by mass or less, and more preferably 1.0% by mass or more and 10.0% by mass or less, based on the total mass of the ink.

[0045] Specific examples of pigments include inorganic pigments and organic pigments. Examples of inorganic pigments include carbon black and titanium dioxide. Examples of organic pigments include azo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, imidazolon pigments, diketopyrrolopyrrole pigments, dioxazine pigments, and perinone pigments. Pigments may be used individually or in combination of two or more.

[0046] Examples of pigment dispersion methods include 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. 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 resin or the like can be used. Among these, resin-dispersed pigments dispersed by a resin dispersant physically adsorbed onto the surface of the pigment particles, and self-dispersing pigments in which anionic groups are bonded directly to the surface of the pigment particles or via other atomic groups are preferred.

[0047] For dispersing pigments in an aqueous medium, it is preferable to use a resin that can disperse pigments in an aqueous medium through the action of anionic groups, and more preferably a water-soluble resin having anionic groups. Examples of resin dispersants include acrylic resins and urethane resins. Among these, acrylic resins are preferred, and acrylic resins having hydrophilic units and hydrophobic units as constituent units are even more preferred. In particular, acrylic resins having hydrophilic units derived from (meth)acrylic acid and hydrophobic units derived from monomers having aliphatic or aromatic groups are preferred.

[0048] Hydrophilic units are units having hydrophilic groups such as anionic groups, hydroxyl groups, and ethylene oxide groups. Hydrophilic units can be formed, for example, by polymerizing monomers having hydrophilic groups. Specific examples of monomers having hydrophilic groups include acidic monomers having carboxylic acid groups such as (meth)acrylic acid; anionic monomers such as anhydrides and salts of these acidic monomers; monomers having hydroxyl groups such as 2-hydroxyethyl (meth)acrylate; monomers having ethylene oxide groups such as methoxypolyethylene glycol (meth)acrylate; and so on. Cationic ions that constitute salts of acidic monomers include lithium, sodium, potassium, ammonium, and organic ammonium ions. One or more types of monomers having hydrophilic groups can be used.

[0049] A hydrophobic unit is a unit that does 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 hydrophilic groups. Specific examples of hydrophobic monomers include monomers having aromatic groups such as styrene, α-methylstyrene, and benzyl (meth)acrylate; and monomers having aliphatic groups such as ethyl (meth)acrylate, methyl (meth)acrylate, (iso-)propyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. One or more hydrophobic monomers may be used.

[0050] Self-dispersing pigments include those in which anionic groups are directly or via other atomic groups bonded to the surface of the pigment particles. Examples of anionic groups include carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, and phosphonic acid groups. Examples of counterions to the anionic groups include hydrogen atoms, alkali metals, ammonium, and cations such as organic ammonium. The other atomic groups are groups that function as spacers between the surface of the pigment particles and the anionic groups, and their molecular weight is preferably 1,000 or less. Examples of other atomic groups include alkylene groups with about 1 to 6 carbon atoms, arylene groups such as phenylene groups and naphthylene groups, ester groups, imino groups, amide groups, sulfonyl groups, and ether groups. Alternatively, groups may be combinations of these groups.

[0051] (aqueous medium) The ink is an aqueous ink containing at least 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. Deionized water or ion-exchanged water is preferred as the water. The water content (mass%) in the ink is preferably 40.0% by mass or more and 95.0% by mass or less, and more preferably 50.0% by mass or more and 90.0% by mass or less, based on the total mass of the ink. The water-soluble organic solvent content (mass%) in the ink is preferably 2.0% by mass or more and 45.0% by mass or less, and more preferably 3.0% by mass or more and 40.0% by mass or less, based on the total mass of the ink.

[0052] (Water-soluble organic solvent) Examples of water-soluble organic solvents include polyhydric alcohols, polyhydric alcohol alkyl ethers, polyhydric alcohol aryl ethers, nitrogen-containing heterocyclic compounds, amides, amines, sulfur-containing compounds, propylene carbonate, and ethylene carbonate. Water-soluble organic solvents may be used individually or in combination of two or more.

[0053] Examples of polyhydric alcohols include ethylene glycol, 1,3-butanediol, 1,5-pentanediol, 3-methyl-1,3-butanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, glycerin, 1,2,6-hexanetriol, 1,2,4-butanetriol, 1,2,3-butanetriol, and petriol.

[0054] Examples of polyhydric alcohol alkyl ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, and propylene glycol monoethyl ether. Examples of polyhydric alcohol aryl ethers include ethylene glycol monophenyl ether and ethylene glycol monobenzyl ether.

[0055] Examples of nitrogen-containing heterocyclic compounds include N-methyl-2-pyrrolidone, N-hydroxyethyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethylimidazolidinone, and ε-caprolactam. Examples of amides include formamide, N-methylformamide, and N,N-dimethylformamide. Examples of amines include monoethanolamine, diethanolamine, triethanolamine, monoethylamine, diethylamine, and triethylamine. Examples of sulfur-containing compounds include dimethyl sulfoxide, sulfolane, thiodiethanol, and thiodiglycol.

[0056] Of the various water-soluble organic solvents listed above, one or more types may be used. Among the water-soluble organic solvents listed above, one or more of the following are preferred because they offer excellent solubility and prevent poor spray characteristics due to water evaporation. Specifically, glycerin, ethylene glycol, 1,3-butanediol, 1,5-pentanediol, 3-methyl-1,3-butanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, 1,2,4-butanetriol, 1,2,6-hexanetriol, thiodiglycol, 2-pyrrolidone, N-methyl-2-pyrrolidone, and N-hydroxyethyl-2-pyrrolidone are preferred. Among these, glycerin, diethylene glycol, 1,3-butanediol, 1,5-pentanediol, 3-methyl-1,3-butanediol, 1,6-hexanediol, 2-pyrrolidone, and N-methyl-2-pyrrolidone are more preferred.

[0057] (Surfactants) The ink may contain surfactants. Examples of surfactants include anionic surfactants, cationic surfactants, nonionic surfactants, fluorinated surfactants, and amphoteric surfactants. One of these may be used alone, or two or more may be used in combination. Among these, anionic surfactants and nonionic surfactants are preferred.

[0058] Examples of anionic surfactants include alkylallyl sulfonates, alkylnaphthalene sulfonates, alkyl phosphates, alkyl sulfates, alkyl sulfonates, alkyl ether sulfates, alkyl sulfosuccinates, alkyl ester sulfates, alkylbenzene sulfonates, alkyldiphenyl ether disulfonates, alkylaryl ether phosphates, alkylaryl ether sulfates, alkylaryl ether ester sulfates, olefin sulfonates, alkaneolefin sulfonates, polyoxyethylene alkyl ether phosphates, polyoxyethylene alkyl ether sulfates, ether carboxylates, sulfosuccinates, α-sulfo fatty acid esters, fatty acid salts, condensates of higher fatty acids and amino acids, and naphthenates. Among these, polyoxyethylene alkyl ether acetates and dialkyl sulfosuccinates are preferred.

[0059] Examples of nonionic surfactants include acetylene glycol surfactants, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, and polyoxyethylene sorbitan fatty acid esters.

[0060] (Other ingredients) In addition to the components mentioned above, the ink may contain other components as needed. Examples of other components include pH adjusters, preservatives and antifungal agents, rust inhibitors, antioxidants, UV absorbers, oxygen absorbers, and light stabilizers.

[0061] As for pH adjusting agents, there are no particular restrictions as long as they can adjust the pH of the ink to 7 or higher, and any substance can be used depending on the purpose. Examples of pH adjusting agents include amines such as diethanolamine and triethanolamine; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; ammonium hydroxide, quaternary ammonium hydroxide, and quaternary phosphonium hydroxide; alkali metal carbonates such as lithium carbonate, sodium carbonate, and potassium carbonate; and the like.

[0062] Examples of preservatives and fungicides include 1,2-benzisothiazolin-3-one, sodium dehydroacetate, sodium sorbate, sodium 2-pyridinethiol-1-oxide, sodium benzoate, and sodium pentachlorophenol.

[0063] Examples of rust inhibitors include acidic sulfites, sodium thiosulfate, ammonium thiodiglycolate, diisopropylammonium nitrite, pentaerythritol tetranitrate, and dicyclohexylammonium nitrite.

[0064] Examples of antioxidants include phenolic antioxidants (including hindered phenolic antioxidants), amine antioxidants, sulfuric antioxidants, and phosphorus-based antioxidants. Examples of phenolic antioxidants (including hindered phenolic antioxidants) include butylated hydroxyanisole, 2,6-di-tert-butyl-4-ethylphenol, stearyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-butylidenebis(3-methyl-6-tert-butylphenol), and 3,9-bis[1,1-dimethyl Examples include til-2-[β-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]2,4,8,10-tetraixaspiro[5,5]undecane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, and tetrakis[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane. Examples of amine-based antioxidants include phenyl-β-naphthylamine, α-naphthylamine, N,N'-di-sec-butyl-p-phenylenediamine, phenothiazine, N,N'-diphenyl-p-phenylenediamine, 2,6-di-tert-butyl-p-cresol, 2,6-di-tert-butylphenol, 2,4-dimethyl-6-tert-butylphenol, butylhydroxyanisole, and 2,2'-methylenebis(4- Examples include methyl-6-tert-butylphenol), 4,4'-butylidenebis(3-methyl-6-tert-butylphenol), 4,4'-thiobis(3-methyl-6-tert-butylphenol), tetrakis[methylene-3(3,5-di-tert-butyl-4-dihydroquinone)propionate]methane, and 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane.Examples of sulfur-based antioxidants include dilauryl 3,3'-thiodipropionate, distearyl thiodipropionate, lauryl stearyl thiodipropionate, dimyristyl 3,3'-thiodipropionate, distearyl β,β'-thiodipropionate, 2-mercaptobenzimidazole, and dilauryl sulfide. Examples of phosphorus-based antioxidants include triphenylphosphite, octadecylphosphite, triisodecylphosphite, trilauryl trithiophosphite, and trinonylphenylphosphite.

[0065] Examples of UV absorbers include benzophenone-based UV absorbers, benzotriazole-based UV absorbers, salicylate-based UV absorbers, cyanoacrylate-based UV absorbers, and nickel complex salt-based UV absorbers. Examples of benzophenone-based UV absorbers include 2-hydroxy-4-n-octoxybenzophenone, 2-hydroxy-4-n-dodecyloxybenzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, and 2,2',4,4'-tetrahydroxybenzophenone. Examples of benzotriazole-based UV absorbers include 2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-4'-octoxyphenyl)benzotriazole, and 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole. Examples of salicylate-based UV absorbers include phenyl salicylate, p-tert-butylphenyl salicylate, and p-octylphenyl salicylate. Examples of cyanoacrylate-based UV absorbers include ethyl-2-cyano-3,3'-diphenyl acrylate, methyl-2-cyano-3-methyl-3-(p-methoxyphenyl) acrylate, and butyl-2-cyano-3-methyl-3-(p-methoxyphenyl) acrylate. Examples of nickel complex salt-based UV absorbers include nickel bis(octylphenyl) sulfide, 2,2'-thiobis(4-tert-octylferate)-n-butylamine nickel(II), 2,2'-thiobis(4-tert-octylferate)-2-ethylhexylamine nickel(II), and 2,2'-thiobis(4-tert-octylferate)triethanolamine nickel(II).

[0066] (Physical properties of ink) The ink of the present invention is an aqueous ink for use in inkjet systems. Therefore, from the viewpoint of reliability, it is preferable to appropriately control its physical properties. The viscosity of the ink at 25°C is preferably 1.0 mPa·s to 10.0 mPa·s, and more preferably 2.0 mPa·s to 7.0 mPa·s. The surface tension of the ink at 25°C is preferably 10 mN / m to 60 mN / m, more preferably 20 mN / m to 60 mN / m, and even more preferably 30 mN / m to 50 mN / m. The pH of the ink at 25°C is preferably 6.0 to 10.0, more preferably 6.5 to 10.0, and even more preferably 7.0 to 9.5. When the pH of the ink is 7.0 or higher, the dissociation rate of resin particles and pigments tends to increase, and charge repulsion is more easily exerted, thus further improving dispersion stability. On the other hand, if the ink's pH is 9.5 or lower, the electrostatic repulsion between resin particles is moderately suppressed when the ink forms a film on the recording medium, which reduces surface irregularities and further improves image quality.

[0067] <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.

[0068] <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 origin.

[0069] 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.

[0070] Any recording medium can be used as the recording medium to be recorded using the ink of the present invention. Recording media with ink absorption properties can be used, such as recording media without a coating layer, such as plain paper, and recording media with a coating layer, such as glossy paper or matte paper. In addition, recording media with low ink absorption or no ink absorption properties, such as printing paper, coated paper, resin sheets, and resin films, can be used. The ink of the present invention can be suitably used for applications in which images are recorded by directly applying ink to such recording media. [Examples]

[0071] 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.

[0072] <Method for measuring physical properties> The measurement methods for each physical property are shown below. The measured physical properties are listed in Table 1 (Tables 1-1 to 1-9).

[0073] (Glass transition temperature (Tg) of resin) Solid or powdered resin was prepared as the measurement sample, and the displacement temperature measured using differential scanning calorimetry (DSC) (product name "DSC2500", manufactured by TA instruments) in accordance with JIS K 6240 was defined as the glass transition temperature (Tg).

[0074] (Determination of whether the sample is a particle or not, particle size) The liquid containing the sample was diluted with pure water to obtain a measurement sample with a sample content of approximately 1.0%. Then, the particle size (D) of the resin particles in the measurement sample was measured using a particle size distribution analyzer. 50 The particle size distribution was measured. The measurement conditions are as follows. A particle size analyzer using the dynamic light scattering method (product name "NanoTrac WAVE II-Q", manufactured by MicroTrac-Bel) was used as the particle size distribution analyzer. If particles with a particle size were measured using the above method, the sample was determined to be "particles" ("aqueous dispersion"), and if particles with a particle size were not measured, the sample was determined not to be "particles" ("aqueous solution"). [Measurement conditions] SetZero: 30 seconds Number of measurements: 3 Measurement time: 120 seconds Shape: true spherical Refractive index: 1.5 Density: 1.0

[0075] (Amount of surface acid groups in resin particles) For samples obtained by adjusting a liquid containing resin particles (aqueous resin particle dispersion) to pH 12, the amount of acid groups (μmol / g) present on the surface of the resin particles per unit mass was measured by colloidal titration using potentiometry. For colloidal titration, a potentiometric automatic titrator (product name "AT510", manufactured by Kyoto Electronics Manufacturing Co., Ltd.) equipped with a flow potentiometry titration unit (product name "PCD-500", manufactured by Kyoto Electronics Manufacturing Co., Ltd.) was used. A 0.005 mol / L methyl glycol chitosan solution was used as the titration reagent.

[0076] (Dissociation rate of surface acid groups in resin particles at pH 9) For a sample obtained by adjusting a liquid containing resin particles (aqueous resin particle dispersion) to pH 9, the amount of acid groups dissociated in an anionic form on the surface of the resin particles at pH 9 was measured by colloidal titration in the same manner as the measurement of the amount of surface acid groups of the resin particles. The ratio of the amount of acid groups dissociated in an anionic form at pH 9 to the amount of surface acid groups of the resin particles at pH 12 was defined as the dissociation rate (%) of surface acid groups of the resin particles at pH 9.

[0077] (Amount of ammonium ions inside resin particles) For each resin particle dispersion, a resin particle dispersion before storage and a resin particle dispersion stored for one month at a temperature of glass transition temperature (Tg) of the resin particles + 5°C were prepared. Each was treated at 80,000 rpm for 30 minutes using an ultracentrifuge (trade name "Optima MAX", manufactured by Beckman Coulter) to precipitate the resin component, and the obtained supernatant was used as a sample. Ammonium ions (NH4 + ) concentration was measured using ion chromatography (trade name "Dionex ICS-6000", manufactured by Thermo Fisher SCIENTIFIC). NH4 obtained from samples before and after storage of the resin particle dispersion + The difference in concentration was determined as the amount (μmol / g) of ammonium ions present inside the resin particles.

[0078] <Production of Resin Particle Dispersions E-1 to E-55> [First Step] 235.0 parts of deionized water were placed in a flask equipped with a dropping funnel, stirrer, nitrogen inlet tube, thermometer, and reflux condenser. A mixture was obtained by mixing the components of the type and amount (in parts) shown in the "First Step" column of Table 1 (Tables 1-1 to 1-9) and placed in the dropping funnel. 14% of the total volume of the mixture was added dropwise into the flask, and the temperature was raised to 65°C while slowly blowing in nitrogen gas. Polymerization was started by adding 25.0 parts of 4.0% potassium persulfate (KPS) aqueous solution as a polymerization initiator. After the reaction heat from polymerization subsided, the temperature in the flask was raised to 70°C, and the remaining mixture and 90.0 parts of 0.05% potassium persulfate aqueous solution were added dropwise over 140 minutes. After the dropwise addition was complete, the temperature was maintained at the same temperature for 120 minutes. In this way, a first aqueous dispersion containing resin particles was obtained by emulsion polymerization.

[0079] [Second process] To the first aqueous dispersion obtained in the first step described above, a pH adjusting agent or buffer of the type shown in the "Second Step" column of Table 1 was added to adjust the pH to the value shown in the same column to obtain a second aqueous dispersion.

[0080] [3rd step] The second aqueous dispersion after the second step was left to stand at the temperature and holding time shown in the "Third Step" column of Table 1. The resin particle (solid content) content was then adjusted to 40%, and the mixture was filtered through a 1 μm filter to obtain aqueous resin particle dispersions E-1 to E-55.

[0081] The meanings of the abbreviations in Table 1 are as follows. St: Styrene • 2EHA: 2-ethylhexyl acrylate • MMA: Methyl methacrylate BA: Butyl acrylate BMA; Butyl methacrylate • CHMA: Cyclohexyl methacrylate AN: Acrylonitrile • MAA: Methacrylic acid AA: Acrylic acid • Light Ester P-1M: A brand name for 2-methacryloyloxyethyl acid phosphate manufactured by Kyoeisha Chemical Co., Ltd. • KH-10: Product name "Aqualon KH-10" (manufactured by Daiichi Kogyo Seiyaku; polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate ammonium salt, an anionic surfactant) • Latemul PD-420: A brand name of a non-reactive nonionic surfactant (polyoxyalkylene alkenyl ether) manufactured by Kao Corporation. SDS: Sodium dodecyl sulfate • KOH: 8 mol / L potassium hydroxide aqueous solution • NaOH: 10 mol / L sodium hydroxide aqueous solution • NH3aq: 28% aqueous ammonia • Buffer solution: 7.5% sodium bicarbonate aqueous solution

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[0091] <Manufacturing of resin particle dispersion E-56> The process up to the "second step" was carried out in the same manner as described above for resin particle dispersions E-1 to E-55. After that, the resin particle (solid content) content was adjusted to 40%, and the mixture was filtered through a 1 μm filter to obtain resin particle dispersion E-56.

[0092] <Manufacturing of resin particle dispersion E-57> The "first step" was performed in the same manner as described above for resin particle dispersions E-1 to E-55. Then, while maintaining the liquid temperature at 80°C, 10.0 parts of a 4.0% potassium persulfate (KPS) aqueous solution were added, and the temperature was maintained for 60 minutes. This process was repeated three times. After that, potassium hydroxide was added to adjust the pH to 8, and the resin particle (solid content) was adjusted to 40%. The mixture was then filtered through a 1 μm filter to obtain resin particle dispersion E-57.

[0093] <Preparation of Pigment Dispersions P-1 to P-3> A batch-type vertical sand mill (manufactured by AIMEX) filled with 200 zirconia beads with a diameter of 0.3 mm was used to disperse a mixture of 10.0 parts of the pigments of the types shown in Table 2, 20.0 parts of a resin-containing liquid, and 70.0 parts of deionized water for a predetermined time. The abbreviations for the pigment types shown in Table 2 are as follows: "NIPex90" is the trade name for carbon black manufactured by Orion Engineered Carbons, "PB15:3" is CI Pigment Blue 15:3, and "PR122" is CI Pigment Red 122. As the resin-containing liquid, an aqueous solution with a resin content of 30.0% was used, obtained by dissolving a water-soluble resin in water containing potassium hydroxide equimolar to its acid value. The water-soluble resin used was a styrene-ethyl acrylate-acrylic acid copolymer with an acid value of 167 mgKOH / g and a weight-average molecular weight of 10,000. Subsequently, the mixture was centrifuged to remove coarse particles, then pressure filtered through a 3.0 μm pore size microfilter (manufactured by Fujifilm), and an appropriate amount of deionized water was added to adjust the pigment dispersion content. In this way, pigment dispersions P-1 to P-3 were obtained, each containing 10.0% pigment and 6.0% water-soluble resin (resin dispersant). The types of pigments used in the preparation of pigment dispersions P-1 to P-3, and the cumulative 50% particle size (D) of the volume-based particle size distribution of the obtained pigment dispersions were determined. 50) are shown in Table 2.

[0094] <Preparation of Pigment Dispersion P-4> A solution of 5.0g concentrated hydrochloric acid dissolved in 5.5g water was cooled to 5°C, and 1.6g of 4-aminophthalic acid was added to it. The container of this solution was placed in an ice bath and stirred to maintain the solution temperature below 10°C, while a solution obtained by dissolving 1.8g of sodium nitrite in 9.0g of ion-exchanged water at 5°C was added. After stirring for 15 minutes, 6.0g of carbon black (product name "NIPex90", manufactured by Orion Engineered Carbons) was added while stirring. The mixture was stirred for another 15 minutes to obtain a slurry. The obtained slurry was filtered through filter paper (product name "Standard Filter Paper No. 2", manufactured by Advantec), the particles were thoroughly washed with water, and the mixture was dried in an oven at 110°C. Subsequently, sodium ions were replaced with potassium ions by ion exchange to obtain a pigment dispersion P-4 containing a self-dispersing pigment in which two -C6H3-(COOK) groups were bonded to the surface of carbon black particles, with a self-dispersing pigment content of 10.0%. The type of pigment used in the preparation of pigment dispersion P-4 and the D of the obtained pigment dispersion P-4 are described below. 50 This is shown in Table 2.

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[0096] <Ink preparation> (Examples 1-60, Comparative Examples 1-12) Each ink was prepared by mixing the components listed below, adjusting the pH to the levels shown in Table 3 (Tables 3-1 and 3-2) using 8 mol / L potassium hydroxide, stirring thoroughly, and then pressure filtering through a 2.5 μm pore size microfilter. Of the components listed below, "Acetylenel E100" is the trade name of a nonionic surfactant (manufactured by Kawaken Fine Chemicals). Table 3 shows the types of resin particle dispersions and pigment dispersions used in the inks, as well as the content of resin particles and pigments in the inks. The physical properties of the resin particles in the ink (glass transition temperature of the resin, particle size of the resin particles, amount of surface acid groups of the resin particles, dissociation rate of surface acid groups of the resin particles at pH 9, and amount of ammonium ions inside the resin particles) are the same as those of the resin particle dispersions used. • Resin particle dispersion: The amount of resin particles (%) of the type shown in Table 3 that corresponds to the value shown in Table 3. • Pigment dispersion: The amount of pigment content (%) shown in Table 3 for each type shown in Table 3. Glycerin: 5.0% Triethylene glycol: 10.0% • Acetyleneol E100: 0.1% • 8 mol / L potassium hydroxide: appropriate amount • Ion-exchanged water: Remaining volume (%) when the total amount of components reaches 100.0%

[0097] <Rating> In this disclosure, "A" and "B" are considered acceptable levels, and "C" is considered an unacceptable level, based on the evaluation criteria for each item shown below. Furthermore, in the evaluation of mapping properties shown below, if a clear difference was observed even within the same evaluation criteria, "AA" was used to indicate a relatively superior result among "A"s, and "B-" was used to indicate a relatively inferior result among "B"s. The evaluation results are shown in Table 3.

[0098] (Storage stability) Each manufactured resin particle dispersion was placed in a sealed polytetrafluoroethylene container and stored at 60°C for 3 months. The viscosity η1 (mPa·s) of the resin particle dispersion before storage and the viscosity η2 (mPa·s) of the resin particle dispersion after storage were measured. The rate of change P (%) of the viscosity of the resin particle dispersion was calculated according to the formula P = {(η2-η1) / η1} × 100 (%). Viscosity was measured using a rotational viscometer (product name "E-type viscometer", manufactured by Toki Sangyo Co., Ltd.) at 25°C, rotor 48' × R24, and rotation speed 50 rpm. The storage stability of the resin particle dispersion was then evaluated according to the evaluation criteria below. A large value of the rate of change P indicates that the dispersion state of the resin particles is unstable, and the viscosity of the resin particle dispersion increases due to an increase in particle size and aggregation of resin particles, thus indicating low storage stability of the resin particle dispersion. A: The rate of change P was less than 5%. B: The rate of change P was between 5% and 10%. C: The rate of change P was 10% or more. Furthermore, the storage stability of each prepared ink was evaluated using the same method, except that ink was used instead of the resin particle dispersion. As a result, the evaluation results for the storage stability of each ink were the same as the evaluation results for the resin particle dispersion used in each ink.

[0099] (Mapping property) Each prepared ink was filled into an ink cartridge. The ink cartridges filled with ink were set in an inkjet recording device (product name "PIXUS iP3100", manufactured by Canon) equipped with a recording head that ejects ink by the action of thermal energy and a heating and fixing unit. In this embodiment, the recording duty cycle of a solid image recorded under the condition that one drop of ink with a concentration of 5 pL per drop is applied to a unit area of ​​1 / 1,200 inch x 1 / 1,200 inch is defined as 100%. Using the above inkjet recording device with the heating and fixing unit temperature set to 80°C, a 200 mm x 200 mm solid image (recording duty cycle 100%) was recorded on a recording medium having a coating layer (product name "Aurora Coat", manufactured by Nippon Paper Industries). After leaving the recorded solid image for 24 hours, two fluorescent lamps placed in parallel at a distance of 10 cm apart were used to illuminate the image at a 45-degree angle from a distance of 2 m (illumination angle 45 degrees). The shape of the fluorescent lamp projected onto the image was visually confirmed from a 45-degree angle (observation angle 45 degrees), and the image quality was 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 blurring was observed at the edge. C: I couldn't see the boundary between the two projected fluorescent lights.

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Claims

1. An aqueous inkjet ink containing an aqueous medium and resin particles, The resin forming the resin particles is a copolymer having units derived from monomers having acid groups. The resin particles contain ammonium ions of 2 μmol / g to 60 μmol / g within the resin particles. An aqueous ink characterized in that the amount of surface acid groups (μmol / g) of the resin particles is 100 μmol / g or more and 400 μmol / g or less.

2. The aqueous ink according to claim 1, wherein the amount of ammonium ions inside the resin particles is 5 μmol / g or more and 50 μmol / g or less.

3. The monomer having an acid group includes a monomer having a carboxylic acid group and a monomer having a sulfonic acid group. The aqueous ink according to claim 1, wherein the ratio of the molar amount of the carboxylic acid group to the molar amount of the sulfonic acid group (molar ratio of carboxylic acid group to sulfonic acid group) is 10 or more and 20 or less.

4. The aqueous ink according to claim 1, wherein the glass transition temperature of the resin particles is 50°C or more and 80°C or less.

5. The aqueous ink according to claim 1, wherein 80% or more of the acid groups on the surface of the resin particles dissociate at pH 9 and become anionic.

6. The aqueous ink according to claim 1, wherein the amount of surface acid groups (μmol / g) of the resin particles is 250 μmol / g or more and 350 μmol / g or less, and 84% to 92% of the acid groups on the surface of the resin particles dissociate at pH 9 to become anionic.

7. The cumulative 50% particle size (D) of the volume-based particle size distribution of the aforementioned resin particles ５０ The aqueous ink according to claim 1, wherein the wavelength is 50 nm or more and 200 nm or less.

8. The aqueous ink according to claim 1, wherein the content of the resin particles in the aqueous ink is 5.0% by mass or more and 15.0% by mass or less, based on the total mass of the ink.

9. The aqueous ink according to claim 1, comprising a pigment and a water-soluble organic solvent.

10. The aqueous ink according to claim 9, wherein the content (mass%) of the resin particles in the aqueous ink is 1.0 to 5.0 times the mass ratio of the content (mass%) of the pigment.

11. The aqueous ink according to claim 1, wherein the pH is 7.0 or higher and 9.5 or lower.

12. 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 11.

13. 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 the aqueous ink described in any one of claims 1 to 11.

14. An aqueous resin particle dispersion liquid used in the manufacture of an aqueous inkjet ink, comprising an aqueous medium and resin particles, The resin forming the resin particles is a copolymer having units derived from monomers having acid groups. The resin particles contain ammonium ions of 2 μmol / g to 60 μmol / g within the resin particles. An aqueous resin particle dispersion characterized in that the amount of surface acid groups (μmol / g) of the resin particles is 100 μmol / g or more and 400 μmol / g or less.

15. A method for producing an aqueous resin particle dispersion for use in the manufacture of water-based inkjet inks, A first step involves emulsion polymerization of a monomer component containing an acid group monomer in the presence of an ammonium salt to obtain a first aqueous dispersion containing resin particles, A second step involves adjusting the pH of the first aqueous dispersion obtained in the first step to 6.5 or higher and 9.5 or lower to obtain a second aqueous dispersion. A third step is to heat-treat the second aqueous dispersion after the second step, at a temperature above the glass transition temperature of the resin particles for no more than 24 hours. A method for producing an aqueous resin particle dispersion characterized by containing [a certain substance].

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