Aqueous ink composition and inkjet ink

Abstract

To provide an aqueous ink composition capable of achieving both excellent discharge stability and storage stability over a long period of time even in an inkjet printhead having a nozzle having a stepped part having a smaller diameter toward the ink discharge direction.SOLUTION: There is provided an aqueous ink composition discharged from a nozzle having a stepped part having a smaller diameter toward the ink discharge direction, which comprises a pigment, a colloidal silica, more than 0.5 mass% of 1,2-hexanediol based on the total amount of the aqueous ink composition as a first solvent and 0.1 mass% or more and less than 4.0 mass% of an alcohol having a Hansen solubility parameter polarity term of 9 or less and a hydrogen bonding term of 17 or less based on the total amount of the aqueous ink composition as a second solvent.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an aqueous ink composition and an ink-jet ink containing the aqueous ink composition. [Background technology]

[0002] Inkjet recording is a method of recording in which ink droplets are ejected from extremely fine nozzles directly onto a recording material, causing them to adhere to the material, resulting in characters and images. This method has the advantages of low noise and good operability of the equipment used, as well as the ease of use, and it is easy to make it into a color printer. For this reason, in recent years, the application of inkjet recording has expanded not only to office and home output devices, but also to commercial and industrial printers. Inks for inkjet recording methods (inkjet inks) include solvent inks, UV inks, water-based inks, etc. Among them, the demand for water-based inks is increasing from the viewpoint of environmental friendliness, etc. (See, for example, Patent Document 1). On the other hand, in order to realize high-speed and high-quality printing required for industrial printers, there is an increasing demand for inkjet heads that use MEMS (Micro Electro Mechanical Systems) manufacturing technology, which allows for easy manufacturing of small, high-definition inkjet heads. There are several types of inkjet heads, depending on the type of ejection method, recording medium, etc. Therefore, it is necessary to select and use inkjet printing inks with characteristics that are suited to the type of ink ejection head to be used. The aqueous inkjet ink composition disclosed in Patent Document 2 is considered to be suitable for an inkjet recording method in which an inkjet head having a step between a portion where ink moves from a pressure chamber toward a nozzle and the nozzle is used, and ink filled in the pressure chamber is ejected through the nozzle and deposited on a recording medium. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2011-12226 A [Patent Document 2] JP 2018-177899 A Summary of the Invention [Problem to be solved by the invention]

[0004] The nozzle disclosed in Patent Document 2, which has a step portion with a smaller diameter toward the ink ejection direction, is structurally prone to accumulation of air bubbles and aggregates of relatively large pigments at the step portion. This makes it difficult to transmit the ejection pressure evenly to the ink composition, which makes it difficult to eject the ink stably. The water-based inkjet ink composition in Patent Document 2 contains a pigment dispersed by a dispersant resin, resin fine particles having a difference in SP value between the dispersant resin and the resin within a predetermined range, water, and a specific organic solvent in a predetermined amount or less, and the viscosity increase rate at 20°C when 20% by mass of the water contained is evaporated is within a predetermined range, which shows that the nozzle clogging is improved. However, there is still room for improvement from the viewpoint of maintaining the ejection stability of the ink and suppressing the occurrence of ejection distortion (print distortion).

[0005] An object of the present invention is to provide an aqueous ink composition that can suppress pigment aggregation and the generation of air bubbles, has little deposition at a stepped portion in a nozzle having a stepped portion whose aperture becomes smaller in the direction of ink ejection, has excellent ejection stability when printed by an inkjet printing method, can suppress the occurrence of ejection distortion (print distortion), and has excellent storage stability. As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by an aqueous ink composition containing colloidal silica and a plurality of specific solvents in given ranges, and have completed the present invention. [Means for solving the problem]

[0006] The present invention has the following aspects. [1] An aqueous ink composition that is ejected from a nozzle having a stepped portion whose aperture diameter decreases in the ink ejection direction, 1. An aqueous ink composition comprising: a pigment; colloidal silica; a first solvent, 1,2-hexanediol, in an amount of more than 0.5 mass% relative to the total amount of the aqueous ink composition; and a second solvent, an alcohol having a Hansen solubility parameter polarity term of 9 or less and a hydrogen bonding term of 17 or less, in an amount of 0.1 mass% or more and less than 4.0 mass% relative to the total amount of the aqueous ink composition. [2] The aqueous ink composition according to [1], further comprising a binder in an amount of 0.5 mass% or more based on the total amount of the aqueous ink composition. [3] The aqueous ink composition according to [1] or [2], wherein the pigment is a self-dispersing pigment. [4] The aqueous ink composition according to any one of [1] to [3], further comprising a surfactant having an HLB value of 15 or less. [5] An inkjet ink comprising the aqueous ink composition according to any one of [1] to [4]. Effect of the Invention

[0007] According to the present invention, it is possible to provide an aqueous ink composition that is capable of achieving both excellent ejection stability and storage stability over a long period of time, even in an inkjet head having a nozzle with a stepped portion whose aperture becomes smaller in the ink ejection direction. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a schematic cross-sectional view showing the structure of an inkjet head. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The present invention relates to an aqueous ink composition that is ejected from a nozzle having a stepped portion whose aperture becomes smaller in the ink ejection direction, the aqueous ink composition comprising a pigment, colloidal silica, and as a first solvent, 1,2-hexanediol at a concentration of more than 0.5 mass% relative to the total amount of the aqueous ink composition, and as a second solvent, an alcohol having a Hansen solubility parameter polarity term of 9 or less and a hydrogen bonding term of 17 or less in a range of 0.1 mass% or more and less than 4.0 mass% relative to the total amount of the aqueous ink composition. The aqueous ink composition of the present invention can suppress the aggregation of pigments and the generation of bubbles, and therefore, deposition at the stepped portion of a nozzle having a stepped portion with a smaller aperture in the ejection direction of the ink is reduced, and the ink composition has excellent ejection stability and storage stability when printed by an inkjet printing method, and these properties can be maintained for a long period of time, and the occurrence of ink ejection distortion (print distortion) can be suppressed. Each of the components of the aqueous ink composition of the present invention will now be described.

[0010] In the aqueous ink composition of the present invention, organic pigments and inorganic pigments that are commonly used in aqueous gravure inks or aqueous inkjet inks can be used as the pigment. The pigment may contain one or both of an organic pigment and an inorganic pigment. In addition, either a non-acid-treated pigment or an acid-treated pigment can be used. As the inorganic pigment, for example, iron oxide and carbon black produced by a contact method, a furnace method, a thermal method, or the like can be used. Examples of organic pigments that can be used include azo pigments (azo lake, insoluble azo pigment, condensed azo pigment, chelate azo pigment, etc.), polycyclic pigments (phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, etc.), lake pigments (basic dye type chelates, acid dye type chelates, etc.), nitro pigments, nitroso pigments, aniline black, and the like.

[0011] Examples of pigments (black pigments) that can be used in black inks include CI Pigment Black 1, 6, 7, 8, 10, 26, 27, and 28. Of these, CI Pigment Black 7 is preferred. Specific examples of black pigments include No. 2300, No. 2200B, No. 900, No. 960, No. 980, No. 33, No. 40, No. 45, No. 45L, No. 52, HCF88, MA7, MA8, MA100, etc. manufactured by Mitsubishi Chemical Corporation; Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. manufactured by Columbia Chemical Corporation; Regal 400R, Regal 330R, Regal 660R, Mogul L, Mogul 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc. manufactured by Cabot Corporation; and Color Black manufactured by Degussa. These include FW1, FW2, FW2V, FW18, FW200, S150, S160, S170, Printex 35, U, V, 1400U, Special Black 6, 5, 4, 4A, NIPEX150, NIPEX160, NIPEX170 and NIPEX180.

[0012] Specific examples of pigments (yellow pigments) that can be used in yellow inks include CI Pigment Yellow 1, 2, 12, 13, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 120, 128, 129, 138, 150, 151, 154, 155, 174, 180, and 185.

[0013] Specific examples of pigments (magenta pigments) that can be used in magenta inks include CI Pigment Red 5, 7, 12, 48(Ca), 48(Mn), 57(Ca), 57:1, 112, 122, 123, 146, 176, 184, 185, 202, 209, 269, 282, and the like; CI Pigment Violet 19, and the like.

[0014] Specific examples of pigments (cyan pigments) that can be used in cyan inks include CI Pigment Blue 1, 2, 3, 15, 15:3, 15:4, 15:6, 16, 22, 60, 63, and 66. Of these, CI Pigment Blue 15:3 is preferred.

[0015] Specific examples of pigments (white pigments) that can be used in white ink include alkaline earth metal sulfates, carbonates, finely powdered silicic acid, silicas such as synthetic silicates, calcium silicate, alumina, alumina hydrate, titanium oxide, zinc oxide, talc, clay, and the like.

[0016] From the viewpoint of obtaining an aqueous ink composition having excellent dispersion stability, it is preferable that a measure is taken for the pigment so that it can be stably dispersed in water and solvent components other than water.

[0017] For example, a dispersibility-imparting group (hydrophilic functional group and / or its salt) or an active species having a dispersibility-imparting group may be bonded (grafted) to the surface of the pigment directly or indirectly via an alkyl group, an alkyl ether group, an aryl group, etc. Such self-dispersing pigments can be produced, for example, by methods such as vacuum plasma treatment, oxidation treatment with hypohalous acid and / or a hypohalous acid salt, oxidation treatment with ozone, a wet oxidation method in which the pigment surface is oxidized with an oxidizing agent in water, or a method in which a carboxy group is bonded via a phenyl group by bonding p-aminobenzoic acid to the pigment surface. When using a self-dispersing pigment, the pigment dispersion resin described below is not required, so that foaming caused by the pigment dispersion resin can be suppressed, and it is easy to obtain an aqueous ink composition with excellent ejection stability. In addition, the amount of surfactant used can be reduced. Furthermore, since the increase in viscosity caused by the pigment dispersion resin can be suppressed, the pigment content can be increased, and it is easy to produce printed matter with high print density.

[0018] Commercially available products may be used as the self-dispersion pigment. Examples of commercially available self-dispersion pigments that are industrially produced include Microjet CW-1, BONJET BLACK CW-1, BONJET BLACK CW-1S, BONJET BLACK CW-2, BONJET BLACK CW-3 (all trade names; manufactured by Orient Chemical Industries Co., Ltd.), CAB-O-JET200, CAB-O-JET300 (all trade names; manufactured by CABOT Corporation), SENSIJET Black SDP100, SENSIJET Black SDP1000, SENSIJET Black SDP2000 (all trade names; manufactured by Sensient Technologies Co., Ltd.), etc.

[0019] As a means for stably dispersing the pigment, a pigment dispersing resin may be adsorbed onto the pigment surface, and the pigment may be dispersed and stabilized in water and solvent components other than water by steric hindrance or electrostatic repulsion (resin-dispersed pigment). The pigment dispersing resin, unlike the binder described below, exists in a state where it is adsorbed onto the pigment surface or covers a part or the whole of the pigment surface. When a pigment dispersing resin is used, the pigment dispersing resin corresponds to the solid content of the aqueous ink composition of the present invention.

[0020] Examples of pigment dispersing resins include polyvinyl alcohols, polyvinylpyrrolidones, acrylic resins such as acrylic acid-acrylic acid ester copolymers, styrene-acrylic resins such as styrene-acrylic acid copolymers, styrene-methacrylic acid copolymers, styrene-methacrylic acid-acrylic acid ester copolymers, styrene-α-methylstyrene-acrylic acid copolymers, and styrene-α-methylstyrene-acrylic acid-acrylic acid ester copolymers, aqueous resins of styrene-maleic acid copolymers, styrene-maleic anhydride copolymers, and vinylnaphthalene-acrylic acid copolymers, and salts of the above aqueous resins. Commercially available pigment dispersion resins can be used, such as the "Ajisper (registered trademark)" PB series manufactured by Ajinomoto Fine-Techno Co., Ltd., the "DISPER (registered trademark) BYK" series manufactured by BYK-Chemie, the "EFKA (registered trademark)" series manufactured by BASF, the "SOLSPERSE (registered trademark)" series manufactured by Lubrizol Japan, and the "TEGO (registered trademark) Dispers" series manufactured by Evonik. In addition, compounds exemplified as polymer (G) in WO2018 / 190139 can also be used as pigment dispersion resins. These pigment dispersion resins may be crosslinked with a crosslinking agent after pigment dispersion. When a pigment dispersing resin is used, the amount thereof is preferably in the range of 5 to 200% by mass, more preferably in the range of 5 to 100% by mass, and further preferably in the range of 20 to 60% by mass, based on the pigment.

[0021] The pigment content in the aqueous ink composition of the present invention is preferably 2% by mass or more, and more preferably 4% by mass or more, based on the total amount of the aqueous ink composition, from the viewpoint of ensuring sufficient print density. The pigment content is preferably 10% by mass or less, and more preferably 8% by mass or less, based on the total amount of the aqueous ink composition. From these viewpoints, the pigment content is preferably 2 to 10% by mass, and more preferably 4 to 8% by mass, based on the total amount of the aqueous ink composition. In the aqueous ink composition of the present invention, either the self-dispersed pigment or the resin-dispersed pigment described above can be used. Among them, when a self-dispersed pigment is used, the effects of the present invention, that is, the compatibility of ejection stability and storage stability over a long period of time and the suppression of ejection distortion (print distortion), can be more effectively achieved.

[0022] The aqueous ink composition of the present invention contains 1,2-hexanediol as a first solvent in an amount of more than 0.5% by mass based on the total amount of the aqueous ink composition. The content of 1,2-hexanediol is preferably in the range of 0.6 to 4.0% by mass, more preferably in the range of 0.8 to 2.5% by mass, based on the total amount of the aqueous ink composition.

[0023] The aqueous ink composition of the present invention is a solvent having a polarity term (δ) of the Hansen solubility parameter (HSP) as the second solvent. P ) is 9 or less, and the hydrogen bond term (δ H The aqueous ink composition preferably contains 0.1% by mass or more and less than 4.0% by mass of an alcohol having a molecular weight of 17 or less. The content of the second solvent is more preferably in the range of 0.3 to 3.5% by mass, and even more preferably in the range of 0.3 to 1.5% by mass, based on the total amount of the aqueous ink composition.

[0024] It is believed that the aqueous ink composition of the present invention, containing the specific first solvent and the specific second solvent within the above ranges, suppresses pigment aggregation and increases the wettability of the pigment surface, making it possible to reduce air bubbles on the pigment surface and facilitate the formation of an ink interface. Therefore, it is believed that there is less accumulation of pigment aggregates and air bubbles at the stepped portion of a nozzle with a stepped portion whose aperture becomes smaller in the ink ejection direction, and it is presumed that excellent ejection stability has been achieved.

[0025] Here, the Hansen solubility parameter is the solubility parameter (SP value: δ) introduced by Hildebrand, multiplied by the dispersion term δ D , polar term δ P , and the hydrogen bond term δ H This parameter takes into account the polarity of the substance and is expressed in three-dimensional space, and the following relationship holds: δ[(cal / cm 3 ) 0.5 ]=(δ D 2 +δ P 2 +δ H 2 ) 0.5 The dispersion term δ D , polar term δ P , and the hydrogen bond term δ HMany of these have been calculated by Hansen and subsequent researchers, and are listed, for example, in Polymer Handbook (4th edition) VII-698-711. In addition, Hansen solubility parameters for many solvents and resins have been investigated, and these solubility parameters are listed, for example, in Industrial Solvents Handbook (by Wesley L. Archer). They can also be calculated using Hansen Solubility Parameters in Practice (HSPiP) software.

[0026] The polarity term of the HSP of the second solvent is preferably from 3 to 9, and more preferably from 4 to 7. The hydrogen bond term of the HSP of the second solvent is preferably from 3 to 16, and even more preferably from 4 to 15. When the polarity term and hydrogen bond term of the HSP of the second solvent both satisfy the above-mentioned ranges, it is believed that the interaction with the first solvent is more effectively exerted, thereby further improving the wettability of the pigment surface in the aqueous ink composition of the present invention, and the dispersion stability and ejection stability are excellent. In other words, the first solvent facilitates the formation of an air-liquid interface, and the second solvent improves the wettability of the pigment surface. The combination of the first and second solvents enhances the effect of stabilizing ejection. The boiling point of the second solvent under atmospheric pressure is preferably 150°C or higher, and more preferably in the range of 150°C to 350°C, from the viewpoints of the ejection stability of the aqueous ink composition of the present invention and preventing drying of the inkjet ink in the ejection nozzles of the inkjet head. An example of an alcohol that satisfies the condition of the second solvent is 3-methoxy-3-methyl-1-butanol (δ P = 6.3, δ H = 12.9, boiling point 174 ° C), 3-methoxy-1-butanol (δ P = 5.4, δ H = 13.6, boiling point 158°C).

[0027] The water contained in the aqueous ink composition of the present invention is preferably pure water or ultrapure water from which impurities such as ionic impurities have been removed as much as possible, such as ion-exchanged water, ultrafiltered water, reverse osmosis water, distilled water, etc. The water may be sterilized by ultraviolet irradiation, addition of hydrogen peroxide, etc. From the viewpoints of the property of being able to dry quickly on the surface of the recording medium after printing (drying property) and ejection stability, the water content is preferably in the range of 30 to 90 mass % relative to the total amount of the aqueous ink composition, and more preferably in the range of 40 to 80 mass %.

[0028] The aqueous ink composition of the present invention may contain a solvent other than water, the first solvent, and the second solvent. The other solvent is preferably miscible with water, the first solvent, and the second solvent. The other solvent can have the role of adjusting the viscosity of the aqueous ink composition of the present invention, the role of further improving the ejection stability when printing by the inkjet printing method, the role of preventing the inkjet ink from drying in the ejection nozzle of the inkjet head, the role of adjusting the drying property described above, and the like.

[0029] Examples of other solvents include ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycols having a molecular weight of 2000 or less, propylene glycol, dipropylene glycol, tripropylene glycol, isopropylene glycol, isobutylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol, 1,4-butanediol, 1,3-butanediol, 3-methyl-1,3-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 2-methyl-2,4-pentanediol, 1,2-hexanediol, 1,6-hexanediol, mesoerythritol, pentaerythritol, glycerin, etc. Among them, solvents having a boiling point in the range of 180 to 300° C. are preferred, and propylene glycol or glycerin is more preferred. Furthermore, when propylene glycol and glycerin are used in combination as other solvents, aggregation and precipitation of the pigment in the aqueous ink composition are less likely to occur, and the viscosity can be adjusted to a desired level while maintaining dispersion stability, which is preferable from the standpoint of improving the ejection stability when printing by an inkjet printing method and of making it easier to improve the drying properties of the ink after printing. The content of the other solvent is preferably in the range of 0.1 to 35 mass %, and more preferably in the range of 15 to 35 mass %, based on the total amount of the aqueous ink composition.

[0030] The aqueous ink composition of the present invention contains colloidal silica. Colloidal silica is a colloid consisting of fine particles of inorganic oxide containing silicon with an average particle size of several hundreds of nm or less, and is also called colloidal silica, colloidal silicic acid, or silica sol. It has the characteristics of monodispersion of particle distribution and excellent stability. Colloidal silica contains silicon dioxide (including its hydrate) as the main component, and the composition of the particles is indefinite, and some colloidal silica forms siloxane bonds (-Si-O-, -Si-O-Si-) and is polymerized. The particle surface is porous, and is generally negatively charged in water. Colloidal silica may contain a small amount of an aluminate such as sodium aluminate or potassium aluminate. Colloidal silica may also contain inorganic salts such as sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, or organic salts such as tetramethylammonium hydroxide. These inorganic salts and organic salts act, for example, as colloid stabilizers. The dispersion medium for colloidal silica is not particularly limited, and may be any of water, the other solvents listed above, or a mixture thereof.

[0031] The method for producing colloidal silica is not particularly limited, and colloidal silica can be produced by known production methods such as aerosil synthesis by thermal decomposition of silicon tetrachloride, a production method using water glass as a raw material, and a liquid phase synthesis method by hydrolysis of alkoxide (see, for example, "Sen-i to Kogyo", Vol. 60, No. 7 (2004) p. 376).

[0032] The volume average particle diameter of the particles contained in the colloidal silica is preferably in the range of 60 to 150 nm, more preferably in the range of 65 to 100 nm. When the volume average particle diameter is within the above range, the aggregation of the pigments in the aqueous ink composition can be suppressed, the dispersion stability can be increased, and the ejection stability can be improved. In addition, damage (such as deformation due to erosion) caused by the aqueous ink composition to each member constituting the inkjet head, such as the base material, protective film, and liquid-repellent film, can be suppressed. The volume average particle diameter of colloidal silica can be determined by general measurements of dispersed particles, such as dynamic light scattering and laser diffraction. For example, in the case of dynamic light scattering, the colloidal silica, which is a dispersion, is diluted using a laser scattering type particle size measuring device, Nanotrac Wave II, manufactured by Microtrac, Inc., and the volume average particle diameter of the colloidal silica particles can be measured under the measurement conditions of a measurement time of 180 seconds and repeated measurements 3 times. The particle shape of the colloidal silica is not particularly limited, and may be any of spherical, elongated, needle-like, chain-like particles in which a plurality of primary particles are linked together, particles having a three-dimensional network structure, etc. From the viewpoint of further improving the ejection stability of the aqueous ink composition of the present invention, a spherical shape is preferred.

[0033] The colloidal silica may be one produced by the above-mentioned production method or a commercially available product. Examples of commercially available products include the "Snowtex (registered trademark)" series by Nissan Chemical Industries, Ltd., the "Syton" series by Remet, "Nalcoag-1060" and "Nalcoag-ID21-64" by Nalco Chem, the "Quartlon (registered trademark)" PL series by Fuso Chemical Industries, Ltd., the "Cataloid (registered trademark)" series by JGC Catalysts and Chemicals, the "Ludox (registered trademark)" series by WRGrace, and the "Adelite" series by ADEKA. These colloidal silicas may be used alone or in combination of two or more kinds. In addition, the pH of many commercially available colloidal silica dispersions is adjusted to be within the stable dispersion region of colloidal silica, and therefore the pH of the inkjet ink of the present invention described below is preferably determined taking into consideration the pH within the stable dispersion region of such colloidal silica.

[0034] The content of colloidal silica is preferably in the range of 0.1 to 5.0 mass %, more preferably 0.3 to 4.0 mass %, in terms of solid content in the colloidal silica, relative to the total amount of the aqueous ink composition. When the content of colloidal silica is within the above range, the ejection stability of the aqueous ink composition is improved, and the effect of erosion of head components such as the nozzle plate and ink flow path on deformation of the inkjet head can be suppressed.

[0035] The aqueous ink composition of the present invention may further contain a binder, and preferably contains a binder. The binder is used for the purpose of fixing the pigment in the aqueous ink composition to the recording medium. Unlike the pigment dispersion resin described above, the binder is not adsorbed to the surface of the pigment in the aqueous ink composition, and is present in the aqueous ink composition in a dispersed state in water, the first solvent, the second solvent, and other solvents, separately from the pigment and the pigment dispersion resin. Examples of binders include polyvinyl alcohol, gelatin, polyethylene oxide, polyvinylpyrrolidone, acrylic resins, urethane resins, olefin resins, modified polyolefin resins (such as acid-modified polypropylene), dextran, dextrin, carrageenan (κ, ι, λ, etc.), agar, pullulan, water-soluble polyvinyl butyral, hydroxyethyl cellulose, carboxymethyl cellulose, etc. These may be used alone or in combination of two or more.

[0036] As the binder, for example, a vinyl polymer (A1) having a structural unit derived from an aromatic vinyl monomer and a glass transition temperature (Tg) of 50 to 100°C, or a halogenated vinyl polymer (A2) having a Tg of 50 to 100°C can be used. The vinyl polymer (A1) may be a copolymer having a structural unit derived from an aromatic vinyl monomer and a structural unit derived from a (meth)acrylic monomer. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, and p-methylstyrene, and among these, styrene is preferred. Examples of the (meth)acrylic monomer include (meth)acrylic acid; and (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and octyl (meth)acrylate. The vinyl polymer (A1) is preferably a styrene-acrylic resin. The content of the structural units derived from aromatic vinyl monomers in the total amount of the vinyl polymer (A1) is preferably in the range of 50 to 99 mass%, more preferably 80 to 99 mass%, and the content of the structural units derived from (meth)acrylic monomers in the total amount of the vinyl polymer (A1) is preferably in the range of 1 to 50 mass%, more preferably 1 to 20 mass%.

[0037] Examples of the vinyl halide polymer (A2) include vinyl chloride (co)polymers, chlorinated polyolefins, and chlorinated rubbers, and vinyl chloride-acrylic polymers having structural units derived from vinyl chloride monomers and structural units derived from (meth)acrylic monomers are preferred. The (meth)acrylic monomers are as described above. The content of the structural units derived from vinyl chloride monomers in the total amount of the halogenated vinyl polymer (A2) is preferably in the range of 30 to 90 mass%, more preferably in the range of 50 to 80 mass%, and the content of the structural units derived from (meth)acrylic monomers in the total amount of the halogenated vinyl polymer (A2) is preferably in the range of 10 to 70 mass%, more preferably in the range of 20 to 50 mass%.

[0038] The vinyl polymer (A) can be produced by emulsion polymerization, solution polymerization, suspension polymerization, bulk polymerization, etc. A vinyl polymer (A) having a core-shell structure is more preferred. For example, a vinyl polymer having a core-shell structure in which a structural unit derived from an aromatic vinyl monomer or a structural unit derived from a vinyl chloride monomer is localized in the core portion and a structural unit derived from a (meth)acrylic monomer is localized in the shell portion can be mentioned. The vinyl polymer (A) having a core-shell structure can be produced, for example, by polymerizing a monomer component including a (meth)acrylic monomer to produce a polymer (x) constituting the shell, and then supplying an aromatic vinyl monomer or the like to a reaction vessel and polymerizing it within the particles of the polymer (x) to form a core portion.

[0039] Commercially available vinyl polymers (A) may be used. Examples include "JONCRYL PDX-7700", "JONCRYL PDX-7780", "JONCRYL 89-E", and "JONCRYL 89J" (all trade names (JONCRYL is a registered trademark), manufactured by BASF Japan Ltd.), "Hi-Loss X BE7503" (manufactured by Seiko PMC Co., Ltd.), "Vinyblan 745", and "Vinyblan 747" (manufactured by Nissin Chemical Industry Co., Ltd.).

[0040] Of the above-mentioned binders, styrene-acrylic resins are more preferred from the viewpoints of improving the ejection stability of the aqueous ink composition of the present invention and also improving the print density and image fastness of the resulting printed matter.

[0041] The weight average molecular weight (Mw) of the binder is preferably 100,000 to 1,000,000, and more preferably 300,000 to 750,000. The Mw of the binder is a value measured in terms of standard polystyrene by gel permeation chromatography (GPC). In detail, it was determined by a GPC-MALS method in which a multi-angle light scattering detector was connected to GPC under the measurement conditions described below. GPC main unit: Agilent Technologies "LC1100" series Column: Showa Denko Co., Ltd. "SHODEX SB806MHQ" Eluent: N / 3 phosphate buffer (pH 3) containing N / 10 sodium nitrate Flow rate: 1.0ml / min Detector 1: Wyatt Technology, multi-angle light scattering detector DAWN Detector 2: Showa Denko Co., Ltd., Differential refractive index detector RI-101

[0042] The acid value of the binder is preferably 50 mgKOH / g or less, more preferably in the range of 10 to 50 mgKOH / g, and even more preferably in the range of 10 to 30 mgKOH / g.

[0043] The volume average particle diameter of the binder is preferably 100 nm or less, and more preferably in the range of 40 to 60 nm. When the volume average particle diameter of the binder is within the above range, the ejection stability and storage stability of the aqueous ink composition of the present invention can be improved. The volume average particle diameter of the binder can be measured using a laser scattering type particle size measuring device, Nanotrac Wave II manufactured by Microtrac Corporation, by adjusting the dilution ratio so that the loading index value is 1, under the measurement conditions of a measurement time of 180 seconds and repeated measurement 3 times.

[0044] The content of the binder is preferably 0.5% by mass or more, more preferably 0.5 to 5.0% by mass, and even more preferably 0.5 to 3.0% by mass, based on the total amount of the aqueous ink composition. When the content of the binder is within the above range, the ejection stability and storage stability of the aqueous ink composition of the present invention are improved. In addition, the image fastness of the printed matter can be further improved, and even when water is dropped onto the printed matter or when the printed matter is rubbed with a cloth containing water, the aqueous ink tends to be excellent in water resistance without peeling off.

[0045] The aqueous ink composition of the present invention may further contain a surfactant. When the aqueous ink composition is contained, the aqueous ink composition discharged from the discharge port of the inkjet head is likely to wet and spread well on the surface after landing on the printed material, and the occurrence of printing defects is likely to be prevented. In addition, the surface tension of the aqueous ink composition is likely to be reduced, and the leveling property is likely to be improved. Examples of the surfactant include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants, with anionic surfactants and nonionic surfactants being preferred. Examples of the anionic surfactant include alkylbenzenesulfonates, alkylphenylsulfonates, alkylnaphthalenesulfonates, higher fatty acid salts, sulfates or sulfonates of higher fatty acid esters, sulfates or sulfonates of higher alcohol ethers, higher alkyl sulfosuccinates, polyoxyethylene alkyl ether carboxylates, sulfates, phosphates or alkyl phosphates of polyoxyethylene alkyl ethers, and the like.

[0046] Examples of the nonionic surfactant include glycol surfactants, silicone surfactants, fluorine surfactants, and acetylene diol surfactants. Examples of glycol surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylaryl ethers, polyoxyethylene fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene glycerin fatty acid esters, polyglycerin fatty acid esters, polyoxyethylene alkylamines, polyoxyethylene fatty acid amides, polyoxyethylene-polyoxypropylene block copolymers, and the like.

[0047] Examples of silicone surfactants include polysiloxane compounds, polyether-modified organosiloxanes, etc. Examples of commercially available silicone surfactants include the "BYK (registered trademark)" series manufactured by BYK Japan and the "KF" series manufactured by Shin-Etsu Chemical Co., Ltd. Examples of fluorosurfactants include perfluoroalkyl carboxylates, perfluoroalkyl sulfonates, oxyethylene perfluoroalkyl ethers, perfluoroalkyl phosphates, perfluoroalkyl betaines, perfluoroalkyl amine oxides, etc. Examples of commercially available fluorosurfactants include the "Megafac (registered trademark)" series manufactured by DIC Corporation, the "Ftergent (registered trademark)" series manufactured by Neos Corporation, and the "Novec (registered trademark)" series manufactured by Sumitomo 3M Limited.

[0048] The acetylene diol surfactant is a surfactant having a carbon-carbon triple bond in the molecule, and examples thereof include acetylene diol (having an acetylene bond and two hydroxyl groups in the same molecule) surfactants, and surfactants in which alkylene oxides such as ethylene oxide and propylene oxide are added to acetylene diol. Examples of such surfactants include alkylene oxide adducts of 2,4,7,9-tetramethyl-5-decyne-4,7-diol and 2,4,7,9-tetramethyl-5-decyne-4,7-diol, and alkylene oxide adducts of 2,4-dimethyl-5-decyne-4-ol and 2,4-dimethyl-5-decyne-4-ol. Commercially available acetylene diol surfactants include, for example, the "Surfynol (registered trademark)" series manufactured by Evonik or Nissin Chemical Industry Co., Ltd., the "Olfine (registered trademark)" series manufactured by Nissin Chemical Industry Co., Ltd., and the "Acetylenol (registered trademark)" series manufactured by Kawaken Fine Chemical Co., Ltd.

[0049] Among the above, from the viewpoint of easily preventing the occurrence of printing defects, acetylene diol surfactants are more preferable as the surfactant constituting the aqueous ink composition of the present invention. The surfactant constituting the aqueous ink composition of the present invention preferably has an HLB value of 15 or less, more preferably 10 or less. The HLB value of the surfactant is preferably 3 or more, more preferably 3.5 or more. When the HLB value of the surfactant is within the above range, the wettability of the pigment surface is increased, the effect of the combined use of the first solvent and the second solvent described above can be more effectively exhibited, the dispersion stability is improved, and the ejection stability and storage stability of the ink composition of the present invention are further improved. In the aqueous ink composition of the present invention, the HLB value of the surfactant is more preferably in the range of 3.5 to 10. Here, the "HLB (Hydrophilic-Lipophilic Balance) value" of a surfactant is a value defined by the following calculation formula (Griffin formula) according to the Griffin method. HLB value = 20 x [sum of chemical formula weights of hydrophilic parts] / molecular weight When a surfactant is contained, the content thereof is preferably in the range of 0.01 to 5 mass %, and more preferably in the range of 0.1 to 2 mass %, relative to the total amount of the aqueous ink composition. The surfactant may be used alone or in combination of two or more.

[0050] In addition to the above-mentioned components, the aqueous ink composition of the present invention may further contain other additives, such as a pH adjuster, a preservative, a polyolefin wax, an antioxidant, a viscosity adjuster, a wetting agent (a drying inhibitor), a penetrating agent, a chelating agent, a plasticizer, and an ultraviolet absorber, as necessary.

[0051] Examples of pH adjusters include potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium hydroxide, lithium hydroxide, potassium hydroxide, ammonia, diethanolamine, triethanolamine, triisopropanolamine, potassium carbonate, sodium carbonate, and sodium hydrogen carbonate. Examples of preservatives include sodium benzoate, sodium pentachlorophenol, sodium 2-pyridinethiol-1-oxide, sodium sorbate, sodium dehydroacetate, 1,2-dibenzisothiazolin-3-one, etc. Commercially available preservatives may be used, such as "ACTICIDE (registered trademark) B20" (trade name, manufactured by Thor Japan Co., Ltd.) and "Proxel (registered trademark) GXL" (trade name, manufactured by SC Johnson Co., Ltd.). Examples of polyolefin waxes include waxes or copolymers produced from olefins such as ethylene, propylene, and butylene, or derivatives thereof, such as polyethylene wax, polypropylene wax, polybutylene wax, and modified waxes thereof. Among them, oxidized polyethylene wax, which is obtained by oxidizing and modifying polyethylene wax, is preferred from the viewpoint of improving image fastness of printed matter. Commercially available polyolefin waxes may be used, such as the "AQUACER (registered trademark)" series (manufactured by BYK Japan). Examples of the antioxidant include phenol-based antioxidants, amine-based antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants. When other additives are further contained, the amount thereof is preferably in the range of 0.01 to 5 mass % relative to the total amount of the aqueous ink composition, and more preferably in the range of 0.02 to 3 mass %.

[0052] The method for preparing the aqueous ink composition of the present invention is not particularly limited, and the composition can be prepared by mixing the above-mentioned components. The above-mentioned components may be mixed all at once or sequentially. For example, the binder may be dissolved or dispersed in water or another solvent and then mixed. The pigment may be dispersed in water or another solvent together with a pigment dispersion resin and then mixed. When mixing, a dispersing machine such as a bead mill, an ultrasonic homogenizer, a high-pressure homogenizer, a paint shaker, a ball mill, a roll mill, a sand mill, a sand grinder, a dyno mill, a dispermat, an SC mill, or a nanomizer can be used.

[0053] The present invention is also an ink-jet ink comprising the aqueous ink composition of the present invention. The viscosity of the inkjet ink containing the aqueous ink composition of the present invention at 25°C is preferably 1 mPa·s or more, more preferably 2 mPa·s or more, from the viewpoints of excellent storage stability and ejection stability, and of reducing deviation of the landing position on the recording medium caused by deflection of the ink when used in an inkjet recording method, and of easily suppressing the occurrence of streaks in the printed matter. The viscosity is preferably 6 mPa·s or less, more preferably 5 mPa·s or less. The viscosity of the inkjet ink is a value measured, for example, using a cone-plate type rotational viscometer equivalent to an E-type viscometer under the following conditions. Measurement equipment: TVE-25 type viscometer (Toki Sangyo Co., Ltd., TVE-25L) Calibration standard solution: JS20 Measurement temperature: 25℃ Rotation speed: 10~100rpm Injection volume: 1200μL

[0054] The surface tension of the inkjet ink at 25° C. is preferably 20 mN / m or more, more preferably 25 mN / m or more. The surface tension is preferably 40 mN / m or less, more preferably 35 mN / m or less. If the surface tension is within this range, when used in an inkjet recording method, the ink has good wettability on the surface of the recording medium from the discharged droplets, and tends to have sufficient wetting and spreading after landing.

[0055] The pH of the inkjet ink at 25°C is preferably 7.0 to 11.0, and more preferably 7.5 to 10.5, from the viewpoints of improving the storage stability and ejection stability of the ink, improving the wetting spread, print density, and image fastness when printed on a recording medium, and suppressing deterioration of the components constituting the ink application or ejection device (ink ejection port, ink flow path, etc.).

[0056] An inkjet head to which the inkjet ink of the present invention can be applied will be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view showing the structure of an inkjet head 2 having a nozzle with a stepped portion in which the aperture becomes smaller toward the ink ejection direction. Note that in Fig. 1, the dimensions and ratios of each part are exaggerated for convenience and may differ from the actual dimensions. 1, the arrow indicates the movement direction of ink in the inkjet head 2. The inkjet head 2 includes a pressure chamber 21 and a piezoelectric element 23 that applies pressure to the pressure chamber 21 to eject an ink composition from a nozzle 22. In the pressure chamber 21, the piezoelectric element 23 is disposed at a position other than a position 24r facing an outlet 24 communicating with the nozzle 22. The position 24r in the pressure chamber 21 facing the outlet 24 communicating with the nozzle 22 means directly above the nozzle 22, and means the extension line 24b and the area surrounded by the extension line 24b if a line (shown by a dashed line in FIG. 1) is extended from the nozzle port 24a toward the top of the figure in Fig. 1. The piezoelectric element 23 being disposed in a location other than the facing position 24r means that at least a part of the piezoelectric element 23 is not located in at least a part of the position 24r facing this area.

[0057] The pressure chamber 21 has a retention portion 26 where the ink retains in a direction extending beyond the ink movement direction connecting a supply port 25 through which ink is supplied to the pressure chamber 21 and an outlet port 24 of the pressure chamber 21. The retention portion 26 is a portion formed during the process of mass-producing the inkjet head 2, and it is difficult to mass-produce an inkjet head equipped with a pressure chamber 21 without the retention portion 26. In the retention portion 26, the ink composition is likely to stagnate, and the dried ink (resin deposit) is also likely to accumulate. When air bubbles gather and create a space, the dried ink adheres to the wall surface.

[0058] On the other hand, the nozzles 22 are formed in a nozzle forming plate 20 made of silicon single crystal with a plane orientation (110). When the nozzle forming plate 20 is made of silicon crystal, the nozzles 22 are often formed by high-precision processing using a known etching process. Therefore, when the nozzle forming plate 20 made of silicon crystal is used, the nozzles can be formed at a higher density of 300 dpi or more than when the nozzles are formed by punching or the like. The nozzle density is more preferably 360 dpi or more. On the other hand, when the nozzle is formed by etching, a step 22a is likely to be formed in the flow path through which the ink passes between the pressure chamber 21 and the nozzle port 24a. Since the step 22a is formed by etching a silicon layer, it is difficult to form a nozzle forming plate 20 that eliminates the step 22a. The step 22a may be formed between the outlet 24 of the pressure chamber 21 and the nozzle port 24a, and for example, the step 22a is formed within a range of 20 μm to 100 μm in the direction from the nozzle port 24a toward the pressure chamber 21. In other words, in FIG. 1, the step 22a is formed by forming a portion where the diameter of the nozzle 22 differs in the thickness direction of the nozzle forming plate 20, but the step 22a is not necessarily formed in the nozzle forming plate 20, and may be formed so that a step is formed on the inner surface of the pressure chamber 21. In an inkjet head 2 having such a step 22a, air bubbles may adhere to and remain in the step 22a during initial filling of ink or cleaning. The air bubbles float up from the step 22a during recording and gather above the pressure chamber 21, where an air-liquid interface is generated and the ink dries, causing dried ink (resin deposits) to adhere to the inside of the inkjet head 2, particularly the step 22a and the stagnation portion 26. Since the aqueous ink composition of the present invention has excellent storage stability, even when the ink has the above-mentioned retention portion 26 and step 22a, the accumulation of dried ink in the inkjet head 2 can be reduced, and the ink composition has excellent ejection stability and can suppress the occurrence of ejection distortion (print distortion).

[0059] Although the water-based ink composition and ink-jet ink of the present invention have been described above, the present invention is not limited to the configurations of the above-described embodiments. For example, the aqueous ink composition and inkjet ink of the present invention may each have any other optional components in addition to the components of the above-mentioned embodiments, or may be substituted with any components that exhibit similar functions. EXAMPLES

[0060] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples. The compounds used in the examples are shown below. <Solvent> [First Solvent] 1,2HD: 1,2-Hexanediol (δ P = 6.6, δ H =17.1, boiling point 223℃) [Second Solvent] MMB: 3-methoxy-3-methyl-1-butanol (δ P = 6.3, δ H =12.9, boiling point 174℃) 3MB: 3-Methoxy-1-butanol (δ P = 5.4, δ H =13.6, boiling point 158℃) [Other Solvents] 1-Pr: 1-propanol (δ P = 6.8, δ H = 17.4, boiling point 97 °C) Gly: Glycerin (δ P = 11.3, δ H = 27.2, boiling point 290 °C) PG: Propylene glycol (δ P = 10.4, δ H = 21.3, boiling point 188 °C)

[0061] <Colloidal silica> 「Cataloid (registered trademark) SI-80P」 (volume average particle diameter 80 nm, 40 mass% dispersion, manufactured by Nichi-Kai Shokubai Co., Ltd.) <Binder> Styrene-acrylic resin (volume average particle diameter 50 nm, Mw 500000, acid value 20 mg KOH / g, manufactured by DIC Corporation) <Surfactant> SF420: 「Surfynol 420」 (trade name, manufactured by Evonik Corporation, HLB value = 4) SF465: 「Surfynol 465」 (trade name, manufactured by Evonik Corporation, HLB value = 10) <pH adjuster> TEA: Triethanolamine (manufactured by Nippon Shokubai Co., Ltd.) <Preservative> B20: 「ACTICIDE B20」 (trade name, manufactured by Thor Japan Co., Ltd.)

[0062] <Preparation of pigment dispersion> <Pigment dispersion A (black pigment)> 「Sensijet Black SDP100」 (product name, manufactured by Sensient Technologies Corporation, pigment concentration 15 mass%; self-dispersible pigment) was used as it was.

[0063] <Pigment dispersion B (black pigment)> As the black pigment, carbon black 「#960」 (trade name, C.I. Pigment Black 7) manufactured by Mitsubishi Chemical Corporation was prepared. 150 g of this black pigment, 50 g of pigment dispersion resin (styrene-acrylic acid copolymer), 75 g of propylene glycol, and 19.4 g of 34 mass% potassium hydroxide aqueous solution were charged into an intensive mixer (manufactured by Nippon Airich Co., Ltd.) with a capacity of 1 L, and kneaded for 25 minutes at a rotor peripheral speed of 2.94 m / s and a pan peripheral speed of 1 m / s. Next, 306 g of ion-exchanged water was gradually added to the kneaded material in the container of the intensive mixer while continuing to stir, and then 12 g of propylene glycol and 137.5 g of ion-exchanged water were further added and mixed to prepare pigment dispersion B with a pigment concentration of 20 mass%.

[0064] <Pigment Dispersion C (Cyan Pigment)> As a cyan pigment, "FASTOGEN BLUE SBG-SD" (product name, CI Pigment Blue 15:3) manufactured by DIC Corporation was prepared. Pigment dispersion C having a pigment concentration of 20 mass % was prepared in the same manner as in the preparation of pigment dispersion B, except that the cyan pigment was used instead of the black pigment.

[0065] 1. Preparation of Water-Based Ink Composition <Examples 1 to 8 and Comparative Examples 1 to 5> The pigment dispersion, the first solvent, the second solvent, other solvents, colloidal silica, a binder, a surfactant, a pH adjuster, a preservative and distilled water were mixed and stirred in the formulation ratios shown in Table 1, thereby obtaining aqueous ink compositions 1 to 13. The content of the pigment dispersion was an amount necessary to adjust the pigment concentration to 6.0% by mass relative to the total amount of the aqueous ink composition. For example, in Example 1, 40.0% by mass of Pigment Dispersion A was used, and the pigment concentration of the resulting aqueous ink composition 1 was set to 6.0% by mass. The content of the binder was adjusted so that the content (solid content) of the resin constituting the binder was the value shown in Table 1. The content of colloidal silica was adjusted so that the content of the colloidal silica itself was the value shown in Table 1. Distilled water was added so that the total content of all components constituting the aqueous ink composition was 100% by mass. The above contents are based on the total amount of the aqueous ink composition.

[0066] 2. Evaluation of Water-Based Ink Compositions 2-1. Distorted discharge Using the aqueous ink compositions obtained in each Example and Comparative Example, 100 solid patterns were continuously printed on photo paper using an inkjet printer (Epson Corporation, "PX-S270T") under an atmosphere of 40°C and 30% RH, and finally a nozzle check pattern was printed. The printing conditions were image resolution: 600 dpi x 600 dpi, printing mode: print quality: standard, and business plain paper was used as the paper. The obtained nozzle check pattern was visually observed to check for the presence or absence of print distortion, and ejection distortion was evaluated according to the following criteria. <Evaluation criteria> ○: No print distortion at all △: Print distortion is less than 5% of the total ×: Printing is missing or the printing is warped in more than 5% of the total area.

[0067] 2-2. Storage stability The aqueous ink compositions obtained in each of the Examples and Comparative Examples were filled into plastic bottles and stored in a thermostatic chamber at 60° C. for 4 weeks. The viscosity (v0) of the aqueous ink composition before storage in the thermostatic chamber and the viscosity (v1) of the aqueous ink composition after storage for 4 weeks were measured, and the rate of change was calculated, and the storage stability of the aqueous ink composition was evaluated according to the following criteria. The rate of change in viscosity was calculated according to the following formula. Rate of change (%) = 100 × (v1-v0) / v0 <Evaluation criteria> ○: Change rate is within ±10% △: Change rate is between ±10% and ±20% ×: Change rate exceeds ±20%

[0068] The above results are summarized in Table 1.

[0069] [Table 1]

[0070] As shown in the above examples, the aqueous ink composition of the present invention is capable of achieving both excellent storage stability and ejection stability. [Industrial Applicability]

[0071] The aqueous ink composition of the present invention has excellent storage stability and ejection stability, and is therefore particularly useful as an inkjet ink for industrial inkjet printers. [Explanation of symbols]

[0072] 2 Inkjet head 20 Nozzle forming plate 21 Pressure Chamber 22 Nozzle 22a Step 23 Piezoelectric element 24 Outlet 24a Nozzle mouth 24b extension line 24r Opposite position 25 Supply port 26 Retention part

Claims

1. An aqueous ink composition that is ejected from a nozzle having a stepped portion whose aperture diameter decreases in a direction in which the ink is ejected, 1. An aqueous ink composition comprising: a pigment; colloidal silica; a first solvent, 1,2-hexanediol, in an amount of more than 0.5% by mass relative to a total amount of the aqueous ink composition; and a second solvent, an alcohol having a Hansen solubility parameter polarity term of 9 or less and a hydrogen bonding term of 17 or less, in an amount of 0.1% by mass or more and less than 4.0% by mass relative to a total amount of the aqueous ink composition.

2. The aqueous ink composition according to claim 1 , further comprising a binder in an amount of 0.5% by mass or more based on the total amount of the aqueous ink composition.

3. The aqueous ink composition according to claim 1 or 2, wherein the pigment is a self-dispersing pigment.

4. The aqueous ink composition according to claim 1 or 2, further comprising a surfactant having an HLB value of 15 or less.

5. An ink-jet ink comprising the aqueous ink composition according to any one of claims 1 to 4.

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