Aqueous ink composition, aqueous ink set, and inkjet recording method

Abstract

To provide an aqueous ink composition and an inkjet recording method which achieve excellent storage stability with reduced erosion of a nozzle plate.SOLUTION: An aqueous ink composition comprises a colorant, water, an ester solvent, and a water-soluble organic solvent, wherein the content of the ester solvent in the aqueous ink composition is 0.05 mass% or more and less than 2.0 mass%.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Among various color recording methods, the recording method using an inkjet printer, one of the most representative methods, uses an inkjet head to generate small droplets of ink, which are then deposited on a recording material such as paper to perform recording. Advances in inkjet technology have led to inkjet recording methods being used in the field of high-definition printing, which previously had been achieved using silver halide photography and offset printing. Furthermore, in recent years, demand for inkjet recording in industrial applications has increased, leading to a demand for printing on a variety of recording media, including poorly absorbent substrates such as coated paper.

[0003] The inkjet head is equipped with a nozzle plate made of silicon and silicon oxide and having a plurality of ejection ports. The nozzle plate has a water-repellent film on its surface to maintain ink ejection properties, making it difficult for the ink composition ejected from the nozzles to adhere to the periphery of the nozzles.

[0004] Inkjet printers often use water-based inks that use water-soluble organic solvents to improve the ink's wettability and permeability to recording media and to improve the ink's moisture retention, as described in Patent Documents 1 to 6. However, when such inks are filled into inkjet heads over a long period of time, there are problems such as deterioration of the ink's water repellency due to contact with the ink, and reduced ejection reliability.

[0005] Furthermore, pigments are often used as coloring materials from the viewpoint of lightfastness and water resistance, but the dispersion stability is impaired by water-soluble organic solvents, which can lead to aggregation of pigment particles.

[0006] For these reasons, there is a strong market demand for water-based inks for ink-jet printing that contain a water-soluble organic solvent but maintain the water repellency of the nozzle plate and have good stability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-262272 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-56832 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-101517 [Patent Document 4] Japanese Patent Application Laid-Open No. 2016-13697 [Patent Document 5] Japanese Patent Application Laid-Open No. 2017-114129 [Patent Document 6] Japanese Patent Application Publication No. 2018-202870 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a water-based ink composition that is excellent in storage stability while suppressing erosion of a nozzle plate, and a recording medium to which the water-based ink composition is adhered. [Means for solving the problem]

[0009] As a result of extensive research into solving the above problems, the present inventors have found that the above problems can be solved by the inventions described in 1) to 8) below, and have completed the present invention.

[0010] 1) 1. A water-based ink composition comprising a colorant, water, an ester-based solvent, and a water-soluble organic solvent, wherein the content of the ester-based solvent in the water-based ink composition is 0.05% by mass or more and less than 2.0% by mass. 2) 1), wherein the ester solvent is an ester solvent having a total carbon number of 5 or more and 12 or less. 3) 1), wherein the ester solvent includes at least one selected from the group consisting of a lactate ester and a citrate ester. 4) 3) The water-based ink composition according to 3), wherein the lactate ester includes at least one selected from the group consisting of ethyl lactate and butyl lactate. 5) 3) The water-based ink composition according to 3), wherein the citric acid ester includes triethyl citrate. 6) An ink set comprising the water-based ink composition according to 1). 7) An inkjet recording method comprising a step of ejecting droplets of the water-based ink composition according to any one of 1) to 4) above and depositing them onto a printing medium to form an image. 8) 7) The inkjet recording method according to 7), wherein the printing medium comprises at least one selected from the group consisting of cardboard, liner paper, coated board, and coated paper. [Effects of the Invention]

[0011] The present invention provides a water-based ink composition that exhibits good storage stability while suppressing erosion of a nozzle plate, and also provides a recording medium to which the water-based ink composition is attached. DETAILED DESCRIPTION OF THE INVENTION

[0012] In this specification, "CI" is an abbreviation for Color Index. In addition, in this specification, "%" and "parts" are all written on a mass basis unless otherwise specified. "(Meth)acrylic" is used to mean both "acrylic" and "methacrylic". The same applies to "(meth)acrylate" and the like. In addition, a water-based ink composition may be abbreviated as "ink".

[0013] [Water-based ink composition] The water-based ink composition of the present invention contains a colorant, water, an ester-based solvent, and a water-soluble organic solvent, and the content of the ester-based solvent in the water-based ink composition is 0.05% by mass or more and less than 2.0% by mass.

[0014] [Colorant] The ink contains a colorant. Examples of the colorant that can be used include pigments, disperse dyes, and solvent dyes. These colorants may be used alone or in combination of two or more. Furthermore, for the purpose of adjusting the hue, two or more colorants with different hues may be used in combination. From the viewpoint of image fastness, such as lightfastness and waterfastness, of the recorded image, the colorant is preferably a pigment, and the pigment is preferably an inorganic pigment, an organic pigment, or an extender pigment.

[0015] Examples of the inorganic pigment include carbon black, white pigment, metal oxides, hydroxides, sulfides, ferrocyanides, and metal chlorides. Preferred black pigments contained in the black ink include carbon blacks such as furnace black, lamp black, acetylene black, and channel black.Specific examples of carbon black include Raven 760 ULTRA, Raven 780 ULTRA, Raven 790 ULTRA, Raven 1060 ULTRA, Raven 1080 ULTRA, Raven 1170, Raven 1190 ULTRA II, Raven 1200, Raven 1250, Raven 1255, Raven 1500, Raven 2000, Raven 2500ULTRA, Raven 3500, Raven 5000 ULTRA II, Raven 5250, Raven 5750, and Raven 7000 (manufactured by Columbia Carbon Corporation); Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, and Regal 1330R, Regal 1400R, Regal 1660R, Mogul L (manufactured by Cabot); Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Printex 140V, SpecIal Black 4, SpecIal Black 4A, SpecIal Black 5, Special Black No. 6, Nerox 305, Nerox 505, Nerox 510, Nerox 600, Nerox 605, NIPex 180IQ, NIPex 170IQ, NIPex 160IQ, NIPex 150IQ (manufactured by Orion Engineered Carbons); MA7, MA8, MA100, MA600, MCF-88, No. 25, No. 33, No. 40, No. 47, No. 52, No. 900, No. 2300 (manufactured by Mitsubishi Chemical Corporation), and the like.

[0016] The white pigment is not particularly limited as long as it is visible to the human eye as white, and examples thereof include white inorganic pigments such as titanium oxide, zinc oxide, zinc sulfide, antimony oxide, and zirconium oxide. Specific examples include CI Pigment White 1 (basic lead carbonate), CI Pigment White 4 (zinc oxide), CI Pigment White 5 (mixture of zinc sulfide and barium sulfate), CI Pigment White 6 (titanium oxide), CI Pigment White 6:1 (titanium oxide containing other metal oxides), CI Pigment White 7 (zinc sulfide), CI Pigment White 18 (calcium carbonate), CI Pigment White 19 (clay), CI Pigment White 20 (titanium mica), CI Pigment White 21 (barium sulfate), CI Pigment White 22 (natural barium sulfate), CI Pigment White 23 (gloss white), CI Pigment White 24 (alumina white), CI Pigment White 25 (gypsum), CI Pigment White 26 (magnesium oxide and silicon oxide), CI Pigment White 27 (silica), and CI Pigment White 28 (anhydrous calcium silicate).

[0017] The titanium oxide may be surface-treated with silica or alumina (hereinafter, sometimes referred to as surface-treated titanium oxide). Surface-treated titanium oxide is preferred because of its excellent color development and hiding power. Furthermore, the rutile crystal type of the titanium oxide is preferred. Specific examples of titanium oxide products include Tipaque (registered trademark) R-580, R-670, R-780, R-850, R-855, and CR-60 (all manufactured by Ishihara Sangyo Kaisha, Ltd.); JR-301, JR-403, JR-405, JR-804, JR-806, JR-600A, and JR-800 (all manufactured by Teika Corporation).

[0018] The white pigment may also be preferably used in the form of a dispersion (slurry) in which it is dispersed. A specific example of such a dispersion (slurry) product is TF-5760 WHITE (D2B) (titanium oxide slurry manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., titanium oxide solid content concentration: 60%, average particle size: 300 nm).

[0019] The white pigment in a solid state can also be dispersed using a dispersant to form a dispersion.

[0020] Examples of the organic pigment include azo, disazo, phthalocyanine, quinacridone, isoindolinone, dioxazine, perylene, perinone, thioindigo, anthraquinone, and quinophthalone. Specific examples of the organic pigment include yellows such as CI Pigment Yellow 1, 2, 3, 12, 13, 14, 16, 17, 24, 55, 73, 74, 75, 83, 93, 94, 95, 97, 98, 108, 114, 128, 129, 138, 139, 150, 151, 154, 155, 180, 185, 193, 199, 202, and 213; and CI Pigment Red. Reds such as 5, 7, 12, 48, 48:1, 57, 88, 112, 122, 123, 146, 149, 150, 166, 168, 177, 178, 179, 184, 185, 202, 206, 207, 254, 255, 257, 260, 264, and 272; Blues such as CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 22, 25, 60, 66, and 80; Violets such as CI Pigment Violet 19, 23, 29, 37, 38, and 50; Oranges such as CI Pigment Orange 13, 16, 68, 69, 71, and 73; and Greens such as CI Pigment Violet 19, 23, 29, 37, 38, and 50. Examples of organic pigments include green pigments such as CI Pigment Black 1, CI Pigment Black 2, and CI Pigment Black 3.

[0021] Examples of the extender pigment include silica, calcium carbonate, talc, clay, barium sulfate, and white carbon.

[0022] As the coloring material contained in the ink, it is preferable to use any one selected from the group consisting of carbon black as the inorganic pigment and CI Pigment Blue 15:3, 15:4, CI Pigment Yellow 74, 155, CI Pigment Red 122, 150, and CI Violet 19 as the organic pigment, and it is more preferable to use carbon black as the inorganic pigment and any one selected from the group consisting of CI Pigment Blue 15:4, CI Pigment Yellow 155, CI Pigment Red 122, 150, and CI Pigment Violet 19 as the organic pigment.

[0023] Specific examples of the disperse dye include CI Disperse Yellow 3, 4, 5, 7, 9, 13, 24, 30, 33, 34, 42, 44, 49, 50, 51, 54, 56, 58, 60, 63, 64, 66, 68, 71, 74, 76, 79, 82, 83, 85, 86, 88, 90, 91, 93, 98, 99, 100, 104, 114, 116, 118, 119, 122, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 1 6, 135, 140, 141, 149, 160, 162, 163, 164, 165, 179, 180, 184:1, 182, 183, 184, 186, 192, 198, 199, 201, 202, 204, 210, 211, 215, 216, 218, 224, 231, 232, 241, etc.; CI Disperse Orange 1, 3, 5, 7, 11, 13, 17, 20, 21, 25, 26, 29, 30, 31, 32, 33, 37, 38, 42, 43, 44, 45, 47, 48, 50, 53, 54, 55, 56, 57, 58, 59, 61, 66, 76, 78, 80, 89, 90, 91, 93, 96, 97, 119, 127, 130, 139, 142, etc.; CI Disperse Red 1, 4, 5, 7, 11, 12, 13, 15, 17, 27, 43, 44, 50, 52, 53, 54, 55, 56, 58, 59, 60, 65, 72, 73, 74, 75, 76, 78, 81, 82, 86, 88, 90, 91, 93, 96, 103, 105, 106, 107, 108, 110, 111, 113, 117, 118, 121, 122, 126, 127, 128, 131, 132, 134, 135, 137, 143, 145, 146, 151, 152, 153, 154, 157 , 159, 164, 167, 169, 177, 179, 181, 183, 184, 185, 188, 189, 190, 191, 192, 200, 201, 202, 203, 205, 206, 207, 210, 221, 224, 225, 227, 229, 239, 240, 257, 258, 277, 278, 279, 281, 288, 289, 298, 302, 303, 310, 311, 312, 320, 324, 328, 343, 362, 364, etc.; CI Disperse Violet 1,4,8,17,23,26,27,28,31,33,35,36,38,40,43,46,48,50,51,52,56,57,59,61,63,69,77 etc;CIDisperse Green 6:1, 9, etc.; CI Disperse Brown 1, 2, 4, 9, 13, 19, 26, 27, etc.; CI Disperse Blue 3, 7, 9, 14, 16, 19, 20, 26, 27, 35, 43, 44, 54, 55, 56, 58, 60, 62, 64, 71, 72, 73, 75, 79, 81, 82, 83, 87, 91, 93, 94, 95, 96, 102, 106, 108, 112, 113, 115, 118, 120, 122, 125, 128, 130, 139, 141, 142, 143, 146, 148, 149, 153, 154, 158, 16 5, 167, 171, 173, 174, 176, 181, 183, 185, 186, 187, 189, 197, 198, 200, 201, 205, 207, 211, 214, 224, 225, 257, 259, 267, 268, 270, 284, 285, 287, 288, 291, 293, 295, 297, 301, 315, 330, 332, 333, 334, 343, 359, 360, etc.; CI Disperse Black 1, 3, 10, 24, etc.; etc.

[0024] Specific examples of the solvent dyes include CI Solvent Yellow 2, 6, 14, 16, 21, 25, 29, 30, 33, 51, 56, 77, 80, 82, 88, 89, 93, 116, 150, 160, 1, 163, 179, etc.; CI Solvent Orange 1, 2, 14, 45, 60, etc.; CI Solvent Red 1, 3, 7, 8, 9, 18, 19, 23, 24, 25, 27, 49, 100, 109, 121, 122, 125, 127, 130, 132, 135, 218, 225, 230, etc.; CI Solvent Violet 13, 31, etc.; CI Solvent Green 3, etc.; CI Solvent Brown 3, 5, etc.; CI Solvent Blue. 2, 11, 14, 24, 25, 35, 36, 38, 48, 55, 59, 63, 67, 68, 70, 73, 83, 105, 111, 132, etc.: CI Solvent Black 3, 5, 7, 23, 27, 28, 29, 34, etc.: etc.

[0025] The content of the coloring materials other than the white pigment relative to the total mass of the ink is typically 0.5% to 30.0% by mass, preferably 1.0% to 10.0% by mass, more preferably 2.0% to 7.0% by mass, even more preferably 3.0% to 6.5% by mass, and particularly preferably 3.5% to 6.0% by mass. The content of the white pigment is typically 1.0% to 30% by mass, preferably 1.0% to 20% by mass, more preferably 2.0% to 20% by mass, and most preferably 5.0% to 20% by mass.

[0026] By setting the content of the coloring material relative to the total mass of the ink within the above range, it is possible to further improve the storage stability and printing stability of the ink, as well as the color development, abrasion resistance, and dot diameter of the printed matter.

[0027] When the pigment is used as the coloring material, one type of pigment can be used, or two or more types of pigments can be used in combination. Furthermore, for the purpose of adjusting the hue, two or more types of pigments with different hues can be used in combination. When two or more types of pigments are used in combination, the blending ratio between the pigments can be adjusted depending on the purpose.

[0028] [Ester-based solvents] The ink preferably contains an ester solvent, and the total number of carbon atoms in the ester solvent is preferably 5 or more and 12 or less.

[0029] The "ester solvent having a total carbon number of 5 or more and 12 or less" is not particularly limited, but examples thereof include lactic acid esters such as ethyl lactate, butyl lactate, and hexyl lactate; citric acid esters such as triethyl citrate; and acetate esters such as butyl acetate, isobutyl acetate, and amyl acetate. Among these, lactic acid esters and citric acid esters are preferred, and ethyl lactate, butyl lactate, and triethyl citrate are particularly preferred. The ink preferably contains at least one selected from the group consisting of ethyl lactate and butyl lactate. The citrate ester preferably contains triethyl citrate, and more preferably consists solely of triethyl citrate.

[0030] The ink may contain one or more of the ester solvents, and the ink may contain the ester solvents to suppress corrosion of the nozzle plate and maintain water repellency.

[0031] The content of the ester solvent in the total mass of the ink is 0.05% by mass or more and less than 2.0% by mass, preferably 0.05% by mass or more and 1.5% by mass or less, more preferably 0.05% by mass or more and 1.2% by mass or less, even more preferably 0.08% by mass or more and 1.2% by mass or less, particularly preferably 0.1% by mass or more and 1.2% by mass or less, especially preferably 0.4% by mass or more and 1.2% by mass or less, extremely preferably 0.7% by mass or more and 1.2% by mass or less, and most preferably 0.8% by mass or more and 1.1% by mass or less.

[0032] [Water-soluble organic solvent] The ink contains a water-soluble organic solvent. The water-soluble organic solvent is not particularly limited as long as it is a solvent other than the above-mentioned "ester-based solvents" and has a carbon atom in the molecule, but specific examples include C1-C6 alkanols such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, sec-butanol, and tert-butanol; carboxylic acid amides such as N,N-dimethylformamide and N,N-dimethylacetamide; lactams such as 2-pyrrolidone, N-methyl-2-pyrrolidone, and N-methylpyrrolidin-2-one; 1,3-dimethylimidazolidine-2-one; cyclic ureas such as acetone, 2-methyl-2-hydroxypentan-4-one, 1,3-dimethylhexahydropyrimid-2-one; ketones or keto alcohols such as acetone, 2-methyl-2-hydroxypentan-4-one, ethylene carbonate; cyclic ethers such as tetrahydrofuran, dioxane; ethylene glycol, diethylene glycol, propylene glycol, 1,2-butylene glycol, 1,4-butylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol (preferably having a molecular weight of 400, 80 mono-, oligo-, or polyalkylene glycols or thioglycols having C2-C6 alkylene units, such as propylene glycol, thiodiglycol, and dithiodiglycol; C3-C9 polyols (triols), such as glycerin, diglycerin, hexane-1,2,6-triol, and trimethylolpropane; glycol ethers, such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoallyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-2-ethylhexyl ether (2-ethylhexyldiglycol), diethylene glycol monobutyl ether (butyl carbitol), triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether;Examples of suitable water-soluble organic solvents include C5-C9 alkanediols such as 1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,2-octanediol, 1,6-hexanediol, 2-methyl-2,4-pentanediol, 2-ethyl-1,3-hexanediol, and 2,4-diethyl-1,5-pentanediol; γ-butyrolactone; and dimethyl sulfoxide. These water-soluble organic solvents may be used alone or in combination of two or more.

[0033] The water-soluble organic solvent is preferably isopropanol, propylene glycol, 2-pyrrolidone, N-methyl-2-pyrrolidone, glycerin, triethylene glycol, ethylene glycol monoallyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol mono-2-ethylhexyl ether, propylene glycol monopropyl ether, 1,2-pentanediol, 1,2-hexanediol, 1,2-octanediol, 2-methyl-2,4-pentanediol, 2-ethyl-1,3-hexanediol, or 2,4-diethyl-1,5-pentanediol, and preferably contains at least one selected from the group consisting of glycerin, triethylene glycol, propylene glycol, diethylene glycol mono-2-ethylhexyl ether, 1,2-hexanediol, and 1,2-octanediol, and more preferably contains 1,2-hexanediol or diethylene glycol mono-2-ethylhexyl ether. When the ink contains 1,2-hexanediol and diethylene glycol mono-2-ethylhexyl ether, where X is the mass content of 1,2-hexanediol in the ink and Y is the mass content of diethylene glycol mono-2-ethylhexyl ether, the value calculated by X / Y is preferably 0.1 to 10, more preferably 0.25 to 5, more preferably 0.25 to 4, and even more preferably 0.4 to 2.5.

[0034] Furthermore, the total content of the ester solvent and the water-soluble organic solvent combined is preferably 1% by mass or more and 10.0% by mass or less, more preferably 1% by mass or more and 8.0% by mass or less, even more preferably more than 1.5% by mass and 5.0% by mass or less, particularly preferably 2.0% by mass or more and 4.5% by mass or less, and extremely preferably 2.5% by mass or less and 4.0% by mass or less.

[0035] By keeping the total amount of water-soluble organic solvents contained in the ink within the above range, the amount of organic solvents that diffuse into the atmosphere during printing can be reduced, thereby reducing the burden on the human body and the environment.

[0036] [water] The ink contains water. The water preferably contains little impurities such as metal ions, i.e., ion-exchanged water, distilled water, etc. The water content is typically 50% to 90% by mass, preferably 60% to 90% by mass, more preferably 70% to 90% by mass, and particularly preferably 75% to 90% by mass, relative to the total mass of the ink according to this embodiment.

[0037] In addition to the above components, the ink may contain, as necessary, ink preparation agents such as surfactants, viscosity adjusters, antifoaming agents, antifungal agents, preservatives, pH adjusters, chelating agents, rust inhibitors, water-soluble ultraviolet absorbers, water-soluble polymer compounds, dispersants, resin emulsions, waxes, sugar alcohols as water-soluble solid moisturizing agents, and antioxidants.

[0038] [Surfactants] The ink preferably contains a surfactant for the purpose of adjusting the surface tension, etc. Examples of the surfactant include known surfactants such as anionic surfactants, cationic surfactants, nonionic surfactants, amphoteric surfactants, silicon-based surfactants, and fluorine-based surfactants.

[0039] Examples of the anionic surfactants include alkyl sulfocarboxylates, α-olefin sulfonates, polyoxyethylene alkyl ether acetates, polyoxyethylene alkyl ether sulfates, N-acylamino acids or salts thereof, N-acylmethyltaurines, alkyl sulfates, polyoxyalkyl ether sulfates, alkyl sulfates, polyoxyethylene alkyl ether phosphates, rosin acid soaps, castor oil sulfates, lauryl alcohol sulfates, alkylphenol phosphates, alkyl phosphates, alkylaryl sulfonates, diethyl sulfosuccinates, diethylhexyl sulfosuccinates, and dioctyl sulfosuccinates. Specific examples of commercially available surfactants include Hitenol TMLA-10, LA-12, and LA-16, and Neohitenol TMECL-30S and ECL-45, all manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.

[0040] Examples of the cationic surfactant include 2-vinylpyridine derivatives and poly(4-vinylpyridine) derivatives.

[0041] Examples of the nonionic surfactants include ether surfactants such as polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene oleyl ether, polyoxyethylene lauryl ether, and polyoxyethylene alkyl ether; polyoxyethylene oleate, polyoxyethylene distearate, sorbitan laurate, sorbitan monostearate, sorbitan monooleate, sorbitan sesquioleate, and polyoxyethylene monooleate. ester-based surfactants such as polyoxyethylene stearate; acetylene glycol (alcohol)-based surfactants such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, and 3,5-dimethyl-1-hexyn-3-ol; Surfynol™ 104, 104PG50, 105PG50, 82, 420, 440, 465, and 485, all manufactured by Nissin Chemical Industry Co., Ltd.; Olfine™ STG; and polyglycol ether-based surfactants (for example, TergItol™ 15-S-7 manufactured by SIGMA-ALDRICH).

[0042] Examples of the amphoteric surfactant include lauryl dimethylaminoacetic acid betaine, 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine, coconut oil fatty acid amidopropyl dimethylaminoacetic acid betaine, polyoctyl polyaminoethyl glycine, and imidazoline derivatives.

[0043] Examples of the silicone surfactant include polyether-modified siloxane and polyether-modified polydimethylsiloxane. Specific examples include BYK-306, BYK-307, BYK-333, BYK-341, BYK-345, BYK-346, BYK-347, BYK-348, BYK-349, BYK-3420, BYK-3450, BYK-3455, and BYK-348. 0, BYK-3565, BYK-3760 (manufactured by BYK-Chemie), KF-351A, KF-352A, KF-353, KF-354L, KF-355A, KF-615A, KF-945, KF-640, KF-642, KF-643, KF-6020, X-22-4515, KF-6011, KF-6012, KF-6015, KF-6017 (manufactured by Shin-Etsu Chemical Co., Ltd.), and the like.

[0044] Examples of the fluorine-based surfactant include perfluoroalkyl sulfonic acid compounds, perfluoroalkyl carbonic acid compounds, perfluoroalkyl phosphate ester compounds, perfluoroalkyl ethylene oxide compounds, and polyoxyethylene ether polymer compounds having perfluoroalkyl ether groups on the side chains. Specific examples include Zonyl TBS, FSP, FSA, FSN-100, FSN, FSO-100, FSO, FS-300 (manufactured by DuPont); Capstone FS-30, FS-31, FS-3100 (Chemours); PF-151N, PF-154N (Omnova); F-114, F-410, F-444, EXP.TF-2066, EXP.TF-2148, EXP.TF-2149, F-430, F-477, F-552, F-553, F-554, F-555, F-556, F-5 Examples include F-57, F-558, F-559, F-561, F-562, R-40, R-41, RS-72-K, RS-75, RS-76-E, RS-76-NS, RS-77, EXP.TF-1540, EXP.TF-1760 (manufactured by DIC Corporation); BYK-340, BYK-3440, BYK-3441, etc. (manufactured by BYK Japan).

[0045] Among the above surfactants, anionic, nonionic and silicone surfactants are preferred, and nonionic and silicone surfactants are particularly preferred. These surfactants may be used alone or in combination of two or more. When the ink contains a surfactant, the total content of the surfactant is usually 0.1% to 5% by mass, preferably 0.2% to 4% by mass, more preferably 0.2% to 3% by mass, and even more preferably 0.2% to 1% by mass, relative to the total mass of the ink. By using a surfactant within the above range in the ink composition, it is possible to improve the penetration into the substrate and the dispersion stability of the colorant in the ink.

[0046] The ink may further contain a viscosity modifier. The viscosity range of ink that can be ejected from an industrial inkjet printer is usually determined based on the specifications of the printer head installed in the printer. For this reason, it may be important to add a viscosity modifier to the ink to adjust the viscosity to an appropriate range. The viscosity modifier is not particularly limited as long as it is a substance that can adjust the viscosity of the ink, and known substances can be used. Specific examples include water-soluble polymer compounds.

[0047] The viscosity range of the ink at 25°C is typically from 2.0 mPa·s to 10 mPa·s, preferably from 2.0 mPa·s to 9.0 mPa·s, more preferably from 3.0 mPa·s to 9.0 mPa·s, and most preferably from 4.0 mPa·s to 9.0 mPa·s. The viscosity of the ink composition can be measured using a commercially available E-type viscometer, such as the TR-100E, TV-100E, TP-200E, and RE-215 viscometers manufactured by Toki Sangyo Co., Ltd.

[0048] Examples of the defoaming agent include silicone-based, silica mineral oil-based, olefin-based, acetylene-based, etc. Commercially available defoaming agents include Surfynol DF37, DF58, DF110D, DF220, MD-20, Olfine SK-14 (manufactured by Shin-Etsu Chemical Co., Ltd.), BYK-1719, BYK-1724, BYK-1770 (manufactured by BYK Japan), etc.

[0049] Examples of the antifungal agent include sodium dehydroacetate, sodium benzoate, sodium pyridinethione-1-oxide, p-hydroxybenzoic acid ethyl ester, 1,2-benzisothiazolin-3-one and salts thereof.

[0050] Examples of the preservative include organic sulfur compounds, organic nitrogen sulfur compounds, organic halogen compounds, haloarylsulfone compounds, iodopropargyl compounds, haloalkylthio compounds, nitrile compounds, pyridine compounds, 8-oxyquinoline compounds, benzothiazole compounds, isothiazolin compounds, dithiols, pyridine oxide compounds, nitropropane compounds, organic tin compounds, phenol compounds, quaternary ammonium salt compounds, triazine compounds, thiazine compounds, anilides, adamantane compounds, dithiocarbamates, brominated indanone compounds, benzyl bromoacetate compounds, and inorganic salt compounds. Specific examples of organic halogen compounds include sodium pentachlorophenol, specific examples of pyridine oxide compounds include sodium 2-pyridinethiol-1-oxide, and specific examples of isothiazoline compounds include 1,2-benzisothiazolin-3-one, 2-n-octyl-4-isothiazolin-3-one, 5-chloro-2-methyl-4-isothiazolin-3-one, 5-chloro-2-methyl-4-isothiazolin-3-one magnesium chloride, 5-chloro-2-methyl-4-isothiazolin-3-one calcium chloride, 2-methyl-4-isothiazolin-3-one calcium chloride, etc. Specific examples of other antiseptics and fungicides include anhydrous sodium acetate, sodium sorbate, or sodium benzoate, and products manufactured by Arch Chemical Co. under the trade names Proxel GXL(S) and Proxel XL-2(S).

[0051] Examples of the pH adjuster include alkanolamines such as diethanolamine, triethanolamine, N-methyldiethanolamine, and 2-amino-2-methyl-1-propanol; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; ammonium hydroxide (aqueous ammonia); alkali metal carbonates such as lithium carbonate, sodium carbonate, sodium bicarbonate, and potassium carbonate; alkali metal salts of organic acids such as sodium silicate and potassium acetate; and inorganic bases such as disodium phosphate.

[0052] Examples of the chelating agent include disodium ethylenediaminetetraacetate, sodium nitrilotriacetate, sodium hydroxyethylethylenediaminetriacetate, sodium diethylenetriaminepentaacetate, and sodium uracildiacetate.

[0053] Examples of the rust inhibitor include acid sulfite, sodium thiosulfate, ammonium thioglycolate, diisopropylammonium nitrite, pentaerythritol tetranitrate, and dicyclohexylammonium nitrite.

[0054] Examples of the water-soluble ultraviolet absorber include sulfonated benzophenone compounds, benzotriazole compounds, salicylic acid compounds, cinnamic acid compounds, and triazine compounds.

[0055] Examples of the water-soluble polymer compound include polyethylene glycol, polyvinyl alcohol, cellulose derivatives, polysaccharides, polyamines or polyimines, and neutralized aqueous solutions of styrene-acrylic copolymers.

[0056] The dispersant is not particularly limited, and known dispersants can be used. Examples of known dispersants include copolymers composed of at least two monomers (preferably, at least one of which is a hydrophilic monomer) selected from the group consisting of styrene and its derivatives; vinylnaphthalene and its derivatives; aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylic acids; acrylic acid and its derivatives; maleic acid and its derivatives; itaconic acid and its derivatives; faric acid and its derivatives; vinyl acetate, vinyl alcohol, vinylpyrrolidone, acrylamide, and derivatives thereof. Examples of such copolymers include styrene-(meth)acrylic acid copolymers, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymers, (meth)acrylic acid ester-(meth)acrylic acid copolymers, polyethylene glycol (meth)acrylate-(meth)acrylic acid copolymers, and styrene-maleic acid copolymers.

[0057] Among these, styrene-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymer, (meth)acrylic acid ester-(meth)acrylic acid copolymer, polyethylene glycol (meth)acrylate-(meth)acrylic acid copolymer are preferred; styrene-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymer, (meth)acrylic acid ester-(meth)acrylic acid copolymer are more preferred; (meth)acrylic acid ester-(meth)acrylic acid copolymer is even more preferred; and methacrylic acid ester-methacrylic acid copolymer is particularly preferred.

[0058] The types of copolymers include, for example, block copolymers, random copolymers, graft copolymers, and / or salts thereof, with block copolymers being preferred.

[0059] Dispersants can be synthesized or commercially available. Specific examples of commercially available dispersants include styrene-acrylic copolymers such as JONCRYL 62, 67, 68, 678, and 687, all manufactured by BASF; Movinyl S-100A (a modified vinyl acetate copolymer manufactured by Hoechst Chemicals); and JURIMER AT-210 (a polyacrylic acid ester copolymer manufactured by Nippon Junyaku Co., Ltd.). A preferred example of the block copolymer obtained by synthesis is the AB block polymer disclosed in WO 2013 / 115071 Gazette. These dispersants may be used alone or in combination of two or more.

[0060] The resin emulsion is not particularly limited as long as it is different from the dispersant, and examples thereof include urethane resins, polyester resins, acrylic resins, vinyl acetate resins, vinyl chloride resins, acrylic styrene resins, and acrylic silicone resins. Specific examples of resin emulsions include Superflex 126, 150, 170, 210, 420, 470, 820, 830, 860, and 890 (urethane-based resin emulsions, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.); Hydran HW-350, HW-178, HW-163, HW-171, AP-20, AP-30, AP-40F, WLS-201, and WLS-210 (urethane-based resin emulsions, manufactured by DIC Corporation); and 0569, 0850Z, and 2108 (styrene-butadiene-based resin emulsions). Examples of suitable resin emulsions include AE980, AE981A, AE982, AE986B, and AE104 (acrylic resin emulsions, manufactured by E-Tech Co., Ltd.), Saivinol SK-200 (acrylic resin emulsion, manufactured by Saiden Chemical Co., Ltd.), and Boncoat 4001 and Boncoat 5454 (acrylic resin emulsions, manufactured by DIC Corporation). When the ink composition contains the resin emulsion, the solids content of the resin emulsion in the ink composition is typically 0.1% to 10.0% by mass, preferably 0.3% to 9.0% by mass, more preferably 0.5% to 8.0% by mass, even more preferably 1.0% to 8.0% by mass, and extremely preferably 1.5% to 8.0% by mass.

[0061] The wax agent is preferably a wax emulsion, and more preferably an aqueous wax emulsion. Natural waxes and synthetic waxes can be used as the wax agent. Examples of natural waxes include emulsions of petroleum-based waxes such as paraffin wax and microcrystalline wax; lignite-based waxes such as montan wax; plant-based waxes such as carnauba wax and candelilla wax; and plant-based waxes such as beeswax and lanolin dispersed in an aqueous medium. Examples of synthetic waxes include polyalkylene waxes (preferably poly C2-C4 alkylene wax), oxidized polyalkylene waxes (preferably poly C2-C4 alkylene wax), and paraffin wax. Among these, one or more waxes selected from polyethylene wax, polypropylene wax, oxidized polyethylene wax, oxidized polypropylene wax, and paraffin wax are preferred, with oxidized polyethylene wax being more preferred. The average particle size of the wax is 50 nm to 5 μm, preferably 100 nm to 1 μm, to prevent clogging of the inkjet head. Commercially available wax emulsions include, for example, AQUACER 515 (acid value: 5) manufactured by BYK Japan and HYTEC E-6500 (acid value: 10 to 20) manufactured by Toho Chemical Industry Co., Ltd. When the ink composition contains the wax, the solid content of the wax agent in the ink composition is typically 0.1 to 5% by mass, preferably 0.2 to 3% by mass, more preferably 0.5 to 3% by mass, even more preferably 0.5 to 2% by mass, and extremely preferably 0.5 to 1.5% by mass.

[0062] The sugar alcohol used as the water-soluble solid moisturizing agent refers to a compound formed by reducing a sugar compound (e.g., aldose and ketose). Sugar alcohols have many hydroxyl groups and therefore have excellent solubility in water. Sugar alcohols have a melting point of 25°C or higher. Therefore, sugar alcohols are solid at room temperature. The melting point of sugar alcohols is usually 25°C or higher and 270°C or lower, preferably 50°C or higher and 250°C or lower, more preferably 50°C or higher and 200°C or lower, even more preferably 50°C or higher and 170°C or lower, and extremely preferably 50°C or higher and 150°C or lower. By using a sugar alcohol having a melting point in this range, an image having excellent fixability at room temperature, drying property, abrasion resistance, and transfer prevention properties can be obtained. Examples of sugar alcohols include threitol (melting point 88 to 90°C), erythritol (melting point 121°C), arabitol (melting point 103°C), ribitol (melting point 102°C), xylitol (melting point 92 to 96°C), sorbitol (glucitol) (melting point 95°C), mannitol (melting point 164 to 169°C), iditol (melting point 75 to 79°C), galactitol (melting point 186 to 191°C), allitol (melting point 150 to 154°C), maltitol (melting point 145°C), isomaltitol (melting point 98 to 103°C), lactitol (melting point 146°C), and pentaerythritol (melting point 190 to 263°C). These can be used alone or in combination of two or more. Among these, sorbitol (glucitol) is particularly preferred.

[0063] Furthermore, the sugar alcohol may contain unreduced glucose (grape sugar) as an impurity.

[0064] The content of the sugar alcohol relative to the total mass of the ink is typically 1.0% by mass or more and 15% by mass or less, preferably 2.0% by mass or more and 15% by mass or less, more preferably 2.0% by mass or more and 12% by mass or less, even more preferably 3.0% by mass or more and 10% by mass or less, particularly preferably 4.0% by mass or more and 9.0% by mass or less, and most preferably 4.0% by mass or more and 8.0% by mass or less. When the total mass of the ester solvents contained in the ink is E1 and the total mass of the sugar alcohols is S1, the value calculated by S1 / E1 is preferably 1 to 100, more preferably 5 to 80, even more preferably 5 to 30, particularly preferably 5 to 20, especially preferably 5 or more and less than 15, and extremely preferably 6 or more and 10 or less. By containing the sugar alcohol within the above range, it is possible to achieve a good balance between redispersibility, drying property on the substrate, and prevention of transfer.

[0065] The antioxidant may be, for example, various organic and metal complex anti-fading agents, such as hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines, amines, indanes, chromans, alkoxyanilines, or heterocycles.

[0066] The ink can be used in various printing applications, such as writing instruments, various printing applications, information printing, and textile printing, and is particularly preferably used in inkjet printing.

[0067] The ink can be prepared by a known production method, for example, by adding an ester solvent, a water-soluble organic solvent, water, and, if necessary, an ink preparation agent to an aqueous dispersion prepared from a colorant, a dispersant, and water, and mixing the mixture to prepare the ink.

[0068] The ink can be prepared using a conventionally known apparatus, such as a ball mill, a sand mill, an attritor, a basket mill, a roll mill, a stirrer, etc. During preparation, it is preferable to remove coarse particles using a membrane filter, a mesh filter, etc.

[0069] The present invention also includes an ink set containing at least the above ink. Examples of the ink set include an ink set containing two or more of the above inks, an ink set containing one or more of the above inks and one or more of other inks other than the above inks, etc. The other inks other than the above inks are not particularly limited as long as they have a different composition from the above inks, but it is preferable that they have a different hue from the above inks.

[0070] The ink and ink set are preferably used in inkjet printing.

[0071] The present invention also includes an inkjet recording method including a step of ejecting droplets of the ink and depositing them on a printing medium to form an image.

[0072] When the ink is used for inkjet recording, it is preferable to remove aggregates and the like by filtering the ink. Filtering can be performed by any known method. For example, suction filtration can be performed using filter paper such as glass filter paper GC-50 (retention particle diameter 0.5 μm, manufactured by Advantec) or glass filter paper GA-100 (retention particle diameter 1.0 μm, manufactured by Advantec).

[0073] The printing media refers to media onto which the ink can be attached, and can be broadly divided into those that have ink absorption and those that do not. Examples of printing media that have ink absorption include inkjet paper, inkjet film, glossy paper, corrugated board, and liner paper contained in corrugated board. Specific examples of liner paper include K-liner, C-liner, and lightweight liner paper manufactured by Oji Materia Co., Ltd., Nippon Paper Industries Co., Ltd., and Daio Paper Co., Ltd.

[0074] Examples of poorly absorbent printing media include various types of paper, such as coated paper, lightweight coated paper, lightly coated paper, coated cardboard, and art paper, used in applications such as gravure printing and offset printing, as well as cast-coated paper used in label printing. These poorly absorbent printing media are formed by applying a coating agent to untreated high-quality paper or the like to provide a coating layer to impart whiteness, etc. Specific examples include OK Top Coat+, Mirror Coat Platinum (manufactured by Oji Paper Co., Ltd.), Aurora Coat (manufactured by Nippon Paper Industries Co., Ltd.), Pearl Coat (manufactured by Mitsubishi Paper Mills, Ltd.), Mari Coat (manufactured by Hokuetsu Corporation), and Raicho Coat (manufactured by Chuetsu Pulp Industries Co., Ltd.).

[0075] The ink can be applied to any printing medium, but is particularly suitable for use with liner paper and low-absorbency printing media.

[0076] When using such printing media that are poor at absorbing ink, it is preferable to subject the printing media to a surface modification treatment in order to improve the fixation of coloring materials.

[0077] The surface modification method is preferably at least one treatment selected from corona discharge treatment, plasma treatment, and flame treatment. These treatments can be performed by known methods. It is generally known that the effects of surface modification based on these treatments weaken over time. Therefore, after surface modification of poorly absorbent media, it is preferable to perform inkjet recording promptly. Surface modification of poorly absorbent media can be carried out by appropriately adjusting the number of treatments, treatment time, applied voltage, etc. to obtain the desired effect. The state of surface modification can be confirmed by measuring the contact angle using a known method.

[0078] The ink exhibits excellent wetting and spreading properties on poorly absorbent media, enabling high-density printed images to be obtained even on liner paper. The ink is extremely useful for various printing applications, particularly inkjet printing.

[0079] The inkjet recording method is a method of recording by ejecting droplets of each ink of the ink set from an inkjet printer in response to a recording signal and depositing them on a printing medium. There are no particular restrictions on the ink nozzles, etc. used during recording, and they can be appropriately selected depending on the purpose.

[0080] Inkjet printers can use any known recording method, such as a charge control method that uses electrostatic attraction to eject ink, a drop-on-demand method (pressure pulse method) that uses the vibration pressure of a piezoelectric element, an acoustic inkjet method that converts an electric signal into an acoustic beam and irradiates the ink, using the resulting radiation pressure to eject the ink, or a thermal inkjet method that heats the ink to form bubbles and uses the resulting pressure. The ink jet recording method of the present invention also includes a method in which the fixation of coloring materials is improved by using colorless and transparent ink.

[0081] Industrial inkjet printers are preferably configured as line-head inkjet printers for the purpose of achieving high printing speeds, and single-pass printing is also preferred. The ink described above can produce good printed matter even under such printing conditions.

[0082] The inkjet recording method is a method of recording using an inkjet printer having a container containing the ink, and is preferably an inkjet recording method using as the printing medium at least one selected from the group consisting of cardboard, liner paper, coated board, and coated paper, with cardboard, liner paper, and coated board being particularly preferred. A recording method in which the printing medium is a poorly ink-absorbent medium is also preferred.

[0083] Unless otherwise specified, for all of the above items, combinations of preferred items are more preferred, and combinations of more preferred items are even more preferred. The same applies to combinations of preferred items and more preferred items, and combinations of more preferred items and even more preferred items. Furthermore, unless otherwise specified, all of the above-mentioned components can be used alone or in combination of two or more types.

[0084] The ink of the present invention has excellent water repellency on a nozzle plate, little erosion of the nozzle plate, excellent storage stability, and excellent ejection reliability. Images recorded with the ink have excellent color development and saturation, as well as excellent fastness properties such as heat abrasion resistance, transferability, water resistance, light resistance, heat resistance, resistance to oxidizing gases (e.g., ozone gas), and abrasion resistance. Furthermore, the ink has excellent drying properties during image formation, little coating unevenness or bleeding, and a large dot diameter, resulting in excellent image formability.

[0085] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples in any way. The content of the coloring material in the total mass of the dispersion was calculated as a converted value of only the content of the coloring material by the dry weight method using MS-70 manufactured by A&D Co., Ltd.

[0086] [Preparation Example 1]: Preparation of black dispersion A block copolymer was prepared as described in Synthesis Example 3 of WO 2013 / 115071. The resulting block copolymer (6 parts) was dissolved in 20 parts of methyl ethyl ketone to obtain a homogeneous solution. A mixture of sodium hydroxide (0.45 parts) and water (53.55 parts) was added to the resulting solution, followed by 20 parts of carbon black (Nerox 605, manufactured by ORION ENGINEERED CARBONS). The mixture was dispersed in a sand grinder at 1500 rpm for 15 hours to obtain a liquid. Water (100 parts) was added to the resulting solution, and the resulting solution was filtered through a GA-100 glass filter to remove aggregates and obtain a filtrate. The resulting filtrate was subjected to vacuum distillation of the methyl ethyl ketone and a portion of the water in the filtrate, resulting in a pigment content of 12.0%. This yielded a black dispersion (Kdp).

[0087] [Preparation Example 2]: Preparation of cyan dispersion A cyan dispersion (Cdp) with a pigment content of 12.0% was obtained in the same manner as in Preparation Example 1, except that CI Pigment Blue 15:4 (manufactured by Dainichiseika Color & Chemicals Co., Ltd., Chromofine Blue 4851) was used instead of the carbon black used in Preparation Example 1.

[0088] [Preparation Example 3]: Preparation of magenta dispersion A magenta dispersion (Mdp) with a pigment content of 12.0% was obtained in the same manner as in Preparation Example 1, except that CI Pigment Magenta 122 (Clariant Inkjet Magenta E02VP2621) was used instead of the carbon black used in Preparation Example 1.

[0089] [Preparation Example 4]: Preparation of yellow dispersion A yellow dispersion (Ydp) having a pigment content of 12.0% was obtained in the same manner as in Preparation Example 1, except that CI Pigment Yellow 155 (Clariant Ink Jet Yellow 4G VP2532) was used instead of the carbon black used in Preparation Example 1.

[0090] [Preparation Example 5]: Preparation of resin emulsion. By following Preparation Example 4 of WO 2015 / 147192, a resin emulsion with an acid value of 6 mg KOH / g and a solids content of 25% was prepared. This is designated Resin 1.

[0091] [Examples 1 to 9 and Comparative Examples 1 and 2: Preparation Examples of Ink Compositions] Each of the components shown in Tables 1 and 2 below was mixed and then filtered through a 3 μm membrane filter to obtain each of the ink compositions for evaluation tests.

[0092] The abbreviations in Tables 1 and 2 below represent the following: Kdp: Black dispersion obtained in Preparation Example 1. Cdp: black dispersion obtained in Preparation Example 2. Mdp: black dispersion obtained in Preparation Example 3. Ydp: Black dispersion obtained in Preparation Example 4. 1,2HD: 1,2-hexanediol. EHDG: diethylene glycol mono-2-ethylhexyl ether. EL: ethyl lactate. BL: butyl lactate. TEC: Triethyl citrate. AMP: 2-amino-2-methyl-1-propanol. SF440: Surfynol 440 (manufactured by Nissin Chemical Industry Co., Ltd.) Resin 1: Resin emulsion obtained in Preparation Example 5. Wax: Hi-Tec E6314 (wax emulsion manufactured by Toho Chemical Industry Co., Ltd., solid content 35%) JC678: Joncryl 678 (BASF) neutralized with triethanolamine in an aqueous solution (25% solids) Water: Ion-exchanged water.

[0093] [Changes in water repellency of nozzle plate over time] A 0.5 cm square silicon nozzle plate was immersed in the ink composition of each Example and Comparative Example and stored in an environment of 60°C for one week to obtain a test piece. The contact angle of water on the obtained test piece was measured using a contact angle meter. The contact angle was measured by the sessile drop method using a DMI-501Hi manufactured by Kyowa Interface Science Co., Ltd. Specifically, a 2 μL droplet of water was dropped onto the test piece, and the contact angle was measured 1 second after the drop at 25°C. The obtained value was rounded to the nearest whole number and evaluated according to the following five-level evaluation criteria A to E. A higher contact angle indicates less loss of water repellency and less erosion of the silicon plate. The evaluation results for each example and comparative example are shown in Tables 1 and 2 below. [Evaluation criteria] A: 90° or more B: 85° or more and less than 90° C: 70° or more and less than 85° D: Less than 70° E: Measurement impossible due to ink aggregation

[0094] [Storage stability] The ink compositions of each Example and Comparative Example were diluted 300 times with ion-exchanged water, and the median diameter (D50) was measured using a particle size distribution analyzer (nanoSAQLA manufactured by Otsuka Electronics Co., Ltd.). The ink compositions of each Example and Comparative Example were then stored in an environment of 60°C for one week. The median diameter after storage was measured using the same procedure as before storage, and the rate of change before and after storage was evaluated. The obtained change rates were rounded off to the nearest whole number and evaluated according to the following four-level evaluation criteria, A to D. The evaluation results for each of the examples and comparative examples are shown in Tables 1 and 2 below. [Evaluation criteria] A: Change rate less than 30% B: Change rate 30% or more but less than 40% C: Change rate 40% or more but less than 50% D: Measurement impossible due to ink aggregation

[0095] [Table 1]

[0096] [Table 2]

[0097] From the results in Tables 1 and 2 above, it was found that the example inks had both the effect of maintaining the water repellency of the nozzle plate and high storage stability. [Industrial Applicability]

[0098] As described above, the aqueous ink composition of the present invention causes little corrosion to a water-repellent film and has good storage stability, making the ink of the present invention extremely useful as an ink for ink-jet recording.

Claims

1. 1. A water-based ink composition comprising a colorant, water, an ester-based solvent, and a water-soluble organic solvent, wherein the content of the ester-based solvent in the water-based ink composition is 0.05% by mass or more and less than 2.0% by mass.

2. The water-based ink composition according to claim 1 , wherein the ester solvent has a total carbon number of 5 or more and 12 or less.

3. The water-based ink composition according to claim 1 , wherein the ester-based solvent includes at least one selected from the group consisting of a lactate ester and a citrate ester.

4. The water-based ink composition according to claim 3 , wherein the lactate ester comprises at least one selected from the group consisting of ethyl lactate and butyl lactate.

5. The water-based ink composition of claim 3 , wherein the citrate ester comprises triethyl citrate.

6. An ink set comprising the water-based ink composition according to claim 1.

7. An inkjet recording method comprising the step of ejecting droplets of the water-based ink composition according to claim 1 onto a printing medium to form an image.

8. The inkjet recording method according to claim 7, wherein the print medium comprises at least one selected from the group consisting of corrugated cardboard, liner paper, coated cardboard, and coated paper.

Images