Inkjet inks, inkjet recording methods, ink sets, ink media sets, and printing media

Abstract

To provide an inkjet ink that excels in stability and secondary color image quality, and a printed material including the inkjet ink.SOLUTION: An inkjet ink is used in a printing method that includes a drying step between printing using first ink and printing using second ink. The ink contains a pigment, a water-soluble organic solvent, an organic compound, and water. The water-soluble organic solvent has a specific logP value. The organic compound has a specific interfacial tension against PET. The organic compound includes at least one of a compound of formula (1) and a compound of formula (2). (In formula (1), R1 is a phenyl group, a naphthyl group or a benzyl group, and n1 is an integer of 1-10). (In formula (2), R2 is a linear or branched C7-C12 hydrocarbon group).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to inkjet ink, inkjet recording method, ink set, ink media set, and printing media. [Background technology]

[0002] Inkjet printing, one of the most representative color recording methods, works by generating tiny droplets of ink and depositing them onto printing media such as paper. In recent years, demand for inks that can print on a variety of media has increased in commercial and industrial applications.

[0003] In the commercial sector, such as catalogs and brochures, printing media with lower ink absorption than plain paper, such as coated paper and art paper (hereinafter also referred to as "low-ink-absorbing media"), are often used. In the industrial sector, such as outdoor signage and flexible food packaging, non-ink-absorbing printing media, such as polyvinyl chloride film, PET (polyethylene terephthalate) film, and polyolefin film (hereinafter also referred to as "non-ink-absorbing media"), are often used.

[0004] For media that do not absorb ink well or do not absorb ink at all, development has progressed on solvent-based inks using organic solvents as the main solvent, and curable inks containing polymerizable monomers. However, these inks have many safety concerns regarding the natural environment and human health. Therefore, in recent years, the development of water-based inks using water as the main solvent has become increasingly popular.

[0005] Water-based inks generally have poor wetting properties on media that do not absorb ink well or do not absorb ink at all. To improve wettability, it is necessary to add a large amount of organic solvent. However, inks with a high solvent content suffer from reduced drying properties on the aforementioned media. Therefore, methods have been employed to lower surface tension by using silicone-based surfactants instead of large amounts of organic solvents (Patent Document 3). While low surface tension inks improve wettability on the above-mentioned media, when performing high-speed printing, ink repellency occurs, as described later, resulting in a decrease in image quality.

[0006] In order to increase productivity, inkjet printing systems are increasingly required to be able to perform high-speed, full-color printing (Patent Document 4). To meet this requirement, the use of line-head type inkjet printers is suitable, and in that case, ink is required that has excellent wetting and spreading properties on the media, dries quickly, has high print density, and produces high-quality printed materials with minimal color bleeding. Furthermore, in order to perform high-speed printing with a line-head type, a method of incorporating a drying process between inkjet heads can be considered (Patent Documents 5 and 6).

[0007] However, when printing in color, if the ink is allowed to dry completely between the print heads, the ink will sit on top of the dried ink film, causing the ink to be repelled and resulting in a decrease in image quality. Furthermore, trying to avoid ink repelling requires adjustments such as reducing the drying strength, which necessitates adjustments to the printing system, making it not a simple system to use. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 5504890 [Patent Document 2] Japanese Patent Publication No. 2020-125382 [Patent Document 3] Japanese Patent Publication No. 2020-55943 [Patent Document 4] Japanese Patent Application Publication No. 10-309803 [Patent Document 5] Special publication 2013-514904 [Patent Document 6] Japanese Patent Publication No. 2022-119849 [Overview of the project] [Problems that the invention aims to solve]

[0009] The present invention provides an ink suitable for use in inkjet systems when inter-head drying is employed. Specifically, the objective is to provide an inkjet ink that exhibits excellent wettability and dryness on the media and on the dried ink film, even when inter-head drying is performed during high-speed full-color printing with an inkjet printer, without ink repellency, and furthermore, an ink set containing the inkjet ink, an ink media set containing the inkjet ink or the ink set and a printing media, and a printing media to which each of the inkjet inks of the inkjet ink or the ink set is attached. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to solve the above problems and have developed an ink for use in a printing method having a drying step between printing with a first ink and printing with a second ink, comprising a first ink application step in which the first ink is ejected from a first inkjet head and attached to a recording medium, After the first ink application step, a first drying step is performed to dry the recording medium, After the first drying step, a second ink application step is performed in which a second ink is ejected from a second inkjet head and attached to the recording medium, The process includes a second drying step, which dries the recording medium after the second ink application step, The aforementioned ink contains a pigment, a water-soluble organic solvent, an organic compound, and water. The logP value of the water-soluble organic solvent is greater than -5.42 and less than 1.25, and the interfacial tension of the organic compound with PET is 15.5 mN / m or less. An inkjet ink in which the total content of the water-soluble organic solvent in the total mass of the ink is 1 to 20% by mass and the total content of the organic compound is 0.01% to 1% by mass, and which satisfies at least one of the following conditions (A) and (B). The present inventors have found that this ink can solve the above problems and have completed the present invention. (A) The organic compound contains at least one compound selected from the group consisting of compounds represented by the following formula (1).

[0011]

Chemical formula

[0012] (In formula (1), R 1 represents a phenyl group, a naphthyl group or a benzyl group, and n1 represents an integer of 1 to 10.) (B) The organic compound contains at least one compound selected from the group consisting of compounds represented by the following formula (2).

[0013]

Chemical formula

[0014] (In formula (2), R 2 represents a linear or branched C7-C12 hydrocarbon group.)

[0015] That is, the present invention relates to the following 1) to 10). 1) An ink used in a printing method having a drying step between printing with a first ink and printing with a second ink, the first ink being discharged from a first inkjet head and adhered to a recording medium in a first ink adhesion step, After the first ink adhesion step, a first drying step of drying the recording medium, After the first drying step, a second ink adhesion step of discharging a second ink from a second inkjet head and adhering it to the recording medium, After the second ink adhesion step, a second drying step of drying the recording medium, and The aforementioned ink contains a pigment, a water-soluble organic solvent, an organic compound, and water. The logP value of the water-soluble organic solvent is greater than -5.42 and less than 1.25, and the interfacial tension of the organic compound with PET is 15.5 mN / m or less. An inkjet ink wherein the total content of the water-soluble organic solvent in the total mass of the ink is 1 to 20% by mass, and the total content of the organic compound is 0.01% to 1% by mass, and satisfies at least one of the following conditions (A) or (B). (A) The organic compound comprises at least one compound selected from the group consisting of compounds represented by the following formula (1).

[0016] [ka]

[0017] (In formula (1), R 1 (where n1 represents a phenyl group, naphthyl group, or benzyl group, and n1 represents an integer from 1 to 10.) (B) The organic compound comprises at least one compound selected from the group consisting of compounds represented by the following formula (2).

[0018] [ka]

[0019] (In formula (2), R 2 (This represents a linear or branched C7-C12 hydrocarbon group.) 2) The inkjet ink according to 1), wherein the water-soluble organic solvent comprises at least one compound selected from the group consisting of C4-C5 alkanediols, compounds represented by the following formula (3), and compounds represented by the following formula (4).

[0020] [ka]

[0021] (In formula (3), R 3 R represents a hydrogen atom or a methyl group. In formula (4), R 4 (where n2 represents a linear or branched C1-C4 hydrocarbon group, and n2 represents 1 or 2.) 3) An inkjet recording method that prints onto a recording medium by ejecting droplets of inkjet ink described in 1) or 2) from an inkjet head. 4) The inkjet recording method according to 3), wherein the recording medium is an ink-poorly absorbent or ink-non-absorbent printing medium. 5) The inkjet recording method according to 3), wherein the first inkjet head and the second inkjet head are each inkjet heads including a circulation mechanism. 6) The inkjet recording method according to any one of items 3) to 5), wherein the drying step includes drying by infrared irradiation. 7) The inkjet recording method according to 6), wherein the drying step in 6) further includes drying with a hot air heater. 8) The inkjet recording method according to 6) or 7), wherein after applying ink to the recording medium, drying is performed by infrared irradiation for 0.001 to 2.0 seconds. 9) The inkjet recording method according to any one of items 6) to 8), wherein the output of the infrared irradiation is 2400W or less and 250V or less, and the infrared irradiation time is 0.5 seconds to 3 seconds. 10) An inkjet ink set comprising the inkjet ink described in 1) or 2) and another inkjet ink different from the said inkjet ink. [Effects of the Invention]

[0022] The present invention provides an inkjet ink with excellent compatibility, wettability of printed materials, and ejection properties. [Modes for carrying out the invention]

[0023] The following describes in detail specific embodiments to which the present invention is applied. In this specification, "CI" means "Color Index." Furthermore, in this specification, the terms "alkylene," "propylene," and "alkyl" are used to encompass both linear and branched structures unless otherwise specified.

[0024] <Inkjet ink> The inkjet ink according to this embodiment (hereinafter also simply referred to as "ink") is an ink used in a printing method having a drying step between printing with a first ink and printing with a second ink, and comprises a first ink application step in which the first ink is ejected from a first inkjet head and attached to a recording medium, After the first ink application step, a first drying step is performed to dry the recording medium, After the first drying step, a second ink application step is performed in which a second ink is ejected from a second inkjet head and attached to the recording medium, The process includes a second drying step, which dries the recording medium after the second ink application step, The aforementioned ink contains a pigment, a water-soluble organic solvent, an organic compound, and water. The logP value of the water-soluble organic solvent is greater than -5.42 and less than 1.25, and the interfacial tension of the organic compound with PET is 15.5 mN / m or less. The total content of the water-soluble organic solvent in the total mass of the ink is 1 to 20% by mass, and the total content of the organic compound is 0.01% to 1% by mass, and the inkjet ink satisfies at least one of the above conditions (A) and (B). The components contained in the ink according to this embodiment will be described in detail below. Each of the components described below may be used individually or in combination of two or more. It is preferable that only one of the above conditions (A) and (B) is satisfied. As stated above,

[0025] <Pigments> Examples of the above-mentioned pigments include inorganic pigments, organic pigments, extender pigments, and hollow particles.

[0026] Examples of inorganic pigments include carbon black, metal oxides, metal hydroxides, metal sulfides, metal ferrocyanides, and metal chlorides.

[0027] In this embodiment, if the ink is black ink and the colorant is an inorganic pigment, the inorganic pigment contained in the black ink is preferably carbon black such as thermal black, acetylene black, oil furnace black, gas furnace black, lamp black, gas black, or channel black. Specific examples of carbon black include, for example, the Raven series from Columbia Carbon; the Monarch series, Regal series, and Mogul series from Cabot; the HiBlack series, ColorBlack series, Printex series, SpecialBlack series, and Nerox series from Orion Engineered Carbons; and the MA series, MCF series, No. 25, No. 33, No. 40, No. 47, No. 52, No. 900, and No. 2300 from Mitsubishi Chemical Corporation.

[0028] In this embodiment, if the ink is a white ink and the colorant is an inorganic pigment, examples of inorganic pigments contained in the white ink include oxides, nitrides, or oxidized nitrides of metals such as zinc, silicon, aluminum, titanium, strontium, and zirconium; inorganic compounds such as glass and silica; and among these, titanium dioxide and zinc oxide are preferred.

[0029] Examples of organic pigments include azo, disazo, phthalocyanine, quinacridone, isoindolinone, dioxazine, perylene, perinone, thioindigo, antholaquinone, and quinophthalone.

[0030] Examples of organic pigments include, for instance, yellow pigments such as CIPigment 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, 213; and CIPigment Red Red pigments 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, 269, 272; Blue pigments such as CIPigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 22, 25, 60, 66, 80; Violet pigments such as CIPigment Violet 19, 23, 29, 37, 38, 50; Orange pigments such as CIPigment Orange 13, 16, 43, 68, 69, 71, 73; Green Examples include green pigments such as 7, 36, and 54; and black pigments such as CIPigment Black 1. Among these, CIPigment Blue 15:4 is preferred.

[0031] Examples of extender pigments include silica, calcium carbonate, talc, clay, barium sulfate, and white carbon. Extender pigments are often used in combination with other colorants.

[0032] As hollow particles, known hollow particles described in, for example, U.S. Patent No. 4,880,465, Japanese Patent Publication No. 3,562,754, Japanese Patent Publication No. 6,026,234, Japanese Patent Publication No. 5,459,460, Japanese Patent Application Publication No. 2003-268694, Japanese Patent Publication No. 4902,216, etc., can be used, and it is particularly preferable to use them as a white pigment.

[0033] In addition to the pigments mentioned above, other dyes may also be included. Other dyes mentioned above include, for example, disperse dyes, solvent dyes, direct dyes, acid dyes, and reactive dyes.

[0034] As for disperse dyes, dyes selected from CIDispers, for example, are preferred. Specific examples include, for instance, yellow dyes such as CIDispers Yellow 9, 23, 33, 42, 49, 54, 58, 60, 64, 66, 71, 76, 79, 83, 86, 90, 93, 99, 114, 116, 119, 122, 126, 149, 160, 163, 165, 180, 183, 186, 198, 200, 211, 224, 226, 227, 231, 237; red dyes such as CIDispers Red 60, 73, 88, 91, 92, 111, 127, 131, 143, 145, 146, 152, 153, 154, 167, 179, 191, 192, 206, 221, 258, 283; and CIDispers Orange Examples include orange dyes such as 9, 25, 29, 30, 31, 32, 37, 38, 42, 44, 45, 53, 54, 55, 56, 61, 71, 73, 76, 80, 96, and 97; violet dyes such as CIDispers Violet 25, 27, 28, 54, 57, 60, 73, 77, 79, and 79:1; and blue dyes such as CIDispers Blue 27, 56, 60, 79:1, 87, 143, 165, 165:1, 165:2, 181, 185, 197, 202, 225, 257, 266, 267, 281, 341, 353, 354, 358, 364, 365, and 368.

[0035] The average particle size of the above pigment is preferably 30 to 300 nm, and more preferably 50 to 250 nm. In this specification, the average particle size refers to the average particle size measured using the laser light scattering method.

[0036] The pigment content is preferably 1 to 30% by mass, more preferably 1 to 10% by mass, and even more preferably 2 to 8% by mass, based on the total mass of the ink according to this embodiment.

[0037] If the above inkjet ink contains multiple types of colorants, the mixing ratio of each colorant can be arbitrarily set according to the purpose. Furthermore, if the ink contains other dyes in addition to the colorants, the mixing ratio of the total amount of colorants to the total amount of other dyes can also be set to any arbitrary ratio.

[0038] <Water-soluble organic solvents> The above-mentioned water-soluble organic solvent is not particularly limited as long as its logP value is greater than -5.42 and less than 1.25, but it is preferably between -1.164 and 1.188. In this specification, "logP value" refers to the octanol / water partition coefficient and can be expressed as the "ClogP" value obtained, for example, by calculating using ChemDraw Professional ver. 16.0 from Perkin Elmer. Examples of the above-mentioned water-soluble organic solvents include 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 3-methoxy-3-methyl-1-butanol, 3-methoxy-1-butanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, and diethylene glycol monobutyl ether, with 1,4-butanediol, 1,5-pentanediol, 3-methoxy-3-methyl-1-butanol, 3-methoxy-1-butanol, ethylene glycol monobutyl ether, and diethylene glycol monobutyl ether being preferred. The content of the water-soluble organic solvent is 1 to 20% by mass, more preferably 1 to 15% by mass, even more preferably 1 to 10% by mass, and particularly preferably 3 to 7% by mass, relative to the total mass of the ink according to this embodiment. The water-soluble organic solvent used in this invention is a compound with a balanced combination of hydrophilicity and hydrophobicity, and is soluble in both water and organic compounds contained in the ink. Due to this property, it is possible to stably dissolve organic compounds even in inks where water accounts for the majority of the total mass of the ink, and it is believed that the ink can be prevented from undergoing phase separation.

[0039] It is preferable that the above water-soluble organic solvent contains at least one compound selected from the group consisting of C4-C5 alkanediols, compounds represented by formula (3), and compounds represented by formula (4).

[0040] In equation (3) above, R 3 R represents a hydrogen atom or a methyl group. In formula (4), R 4 represents a linear or branched C1-C4 hydrocarbon group, and n2 represents 1 or 2.

[0041] <Organic compounds> The above-mentioned organic compound is not particularly limited as long as it is a compound other than the above-mentioned pigment and the above-mentioned water-soluble organic solvent, and has an interfacial tension with respect to PET of 15.5 mN / m or less, preferably 14.00 mN / m or less, and more preferably 11.00 mN / m or less. In this specification, "interfacial tension with respect to PET" means the interfacial tension at 25°C between a PET film and a mixture of an organic compound (1 part by mass), 1,4-butanediol (9 parts by mass), and purified water (90 parts by mass) when the mixture is brought into contact with the PET film, and can be calculated using Young's formula from the contact angle of the mixture with respect to the PET film. As the PET film, for example, PET film (E5102) manufactured by Toyobo Co., Ltd. is used. Examples of the organic compound having an interfacial tension against PET of 15.5 mN / m or less include glycol ethers such as ethylene glycol monophenyl ether, diethylene glycol monophenyl ether, triethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, diethylene glycol monobenzyl ether, triethylene glycol monobenzyl ether, monooxyethylene β-naphthyl ether, dioxyethylene β-naphthyl ether, and trioxyethylene β-naphthyl ether (preferably mono-, di-, or triethylene glycol aryl ether); alkanediols such as 2-methyl-2-propyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethylhexane-1,3-diol, 1,2-heptanediol, 1,2-octanediol, 1,2-nonanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, and 1,10-decanediol (preferably C7-C10 terminal alkanediol); etc. Mono-, di-, or triethylene glycol aryl ether and C7-C10 terminal alkanediol are preferred, and diethylene glycol monobenzyl ether, diethylene glycol monophenyl ether, and 1,8-octanediol are more preferred. The organic compound handled in the present invention has a property of being likely to be oriented at the interface between the ink and the printing medium after the ink lands on the printing medium. Due to this property, it is considered that excellent wettability is exhibited by reducing the interfacial tension generated at the interface between the ink and the printing medium and promoting the wet spreading of the ink on the printing medium.

[0042] In the total mass of the ink, the total content of the organic compound is 0.01% to 1% by mass, and it is preferable that the total content of the organic compound is 0.01% to 0.4% by mass.

[0043] In the above formula (1), R 1 represents a phenyl group, a naphthyl group or a benzyl group, and n1 represents an integer of 1 to 10.

[0044] In the above formula (2), R2 This represents a linear or branched C7-C12 hydrocarbon group. Examples of linear or branched C7-C12 hydrocarbon groups include n-heptylene, 1-methylhexylene, 2-methylhexylene, 3-methylhexylene, 1,5-dimethylpentylene, 2,4-dimethylpentylene, n-octylene, 1-methylheptylene, 2-methylheptylene, 3-methylheptylene, 4-methylheptylene, 1,6-dimethylhexylene, and 2,5-dimethylhexylene. Examples include silene group, 2-ethylpentylene group, 2-ethyl-1-propylpropylene group, n-nonylene group, 1-methyloctylene group, 2-methyloctylene group, 3-methyloctylene group, 4-methyloctylene group, 1,7-dimethylheptylene group, 2,6-dimethylheptylene group, 3,5-dimethylheptylene group, 2-butyl-2-ethylpropylene group, n-decylene group, n-undecylene group, n-dodecylene group, etc. It is preferable that the group be a linear or branched C7-C9 hydrocarbon group.

[0045] <Water> As for the water, water with a low content of impurities such as metal ions is preferred, i.e., ion-exchanged water, distilled water, etc. The water content is 60 to 90% by mass, and preferably 70 to 90% by mass, relative to the total mass of the ink according to this embodiment.

[0046] The above ink is used in a printing method that has a drying step between printing with the first ink and printing with the second ink, and comprises a first ink application step in which the first ink is ejected from the first inkjet head and attached to the recording medium, After the first ink application step, a first drying step is performed to dry the recording medium, After the first drying step, a second ink application step is performed in which a second ink is ejected from a second inkjet head and attached to the recording medium, The process includes a second drying step, which dries the recording medium after the second ink application step.

[0047] The above ink is used in at least one of the first ink and the second ink. It is also preferable to use the above ink in both the first ink and the second ink.

[0048] The above drying process involves heating and drying the image between printing with the first ink and printing with the second ink. Heat drying using a heat source is useful in the drying process. As a heating means, one that can uniformly heat the recording surface is preferred. For example, hot air drying by blowing hot air onto the printing surface, drum drying by warming the drum roller in contact with the recording medium, and other methods such as nichrome wire heaters, halogen heaters, ceramic heaters, and carbon heaters can be appropriately selected depending on the purpose, and are not limited to these. The first drying step described above is a step of drying the recording medium after the first ink application step, and the second drying step described above is a step of drying the recording medium after the second ink application step, preferably including drying by infrared irradiation. It is also preferable that the drying step further includes drying by a hot air heater.

[0049] It is also preferable that, after applying the ink to the recording medium, drying is performed by infrared irradiation for 0.001 to 2.0 seconds, and that the output of the infrared irradiation is 2400W or less and 250V or less, and the infrared irradiation time is 0.5 to 3 seconds.

[0050] The first inkjet head mentioned above is an inkjet head containing the first ink. The second inkjet head mentioned above is an inkjet head containing the second ink.

[0051] The recording medium described above is not particularly limited as long as it is a substance that can be colored by the ink. Examples of recording media include paper, film, fibers and cloth (cellulose, nylon, wool, etc.), leather, and substrates for color filters. The printing medium is not particularly limited, but it is preferable that it is a printing medium that is poorly absorbent or non-absorbent of ink. Examples of poorly absorbent recording media include recording media used in gravure printing and offset printing, art paper, coated paper, matte paper, and cast paper. Examples of non-absorbent recording media include PET (polyethylene terephthalate) film, PP (polypropylene) film, vinyl chloride sheet, metal, glass, and rubber.

[0052] The ink according to this embodiment may optionally contain ink preparation agents such as dispersants, organic solvents other than the above-mentioned water-soluble organic solvents, surfactants, fungicides, preservatives, pH adjusters, chelating reagents, rust inhibitors, defoamers, water-soluble ultraviolet absorbers, antioxidants, and resin emulsions. The content of each ink preparation agent can be arbitrarily set according to the intended use of the ink.

[0053] Examples of dispersants include copolymers composed of at least two monomers selected from monomers such as styrene and its derivatives; vinylnaphthalene and its derivatives; aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylic acids; (meth)acrylic acid and its derivatives; maleic acid and its derivatives; itaconic acid and its derivatives; fahric acid and its derivatives; vinyl acetate, vinyl alcohol, vinylpyrrolidone, acrylamide, and their derivatives (preferably at least one of which is a hydrophilic monomer). Examples of hydrophilic monomers include monomers that retain a carboxyl group after polymerization, such as acrylic acid and methacrylic acid.

[0054] Examples of such copolymers include 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, and styrene-maleic acid copolymer. Among these, styrene-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymer, (meth)acrylic acid ester-(meth)acrylic acid copolymer, and polyethylene glycol (meth)acrylate-(meth)acrylic acid copolymer are preferred, styrene-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymer, and (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. Examples of copolymer types include block copolymers, random copolymers, and graft copolymers. These copolymers may also be in the form of salts. In this specification, the term "(meth)acrylic" is used to include both "acrylic" and "methacrylic." The same applies to "(meth)acrylate," etc.

[0055] Dispersants can be synthesized or obtained commercially. Examples of dispersants obtained by synthesis include the AB block polymer disclosed in International Publication No. 2013 / 115071. The monomer constituting the A block of the AB block polymer disclosed in International Publication No. 2013 / 115071 is at least one monomer selected from (meth)acrylic acid and linear or branched C4 alkyl (meth)acrylate, with at least one monomer selected from methacrylic acid and n-butyl methacrylate being preferred, and more preferably a combination of these two monomers. The monomer constituting the B block of the AB block polymer disclosed in International Publication No. 2013 / 115071 is at least one monomer selected from benzyl methacrylate and benzyl acrylate, with benzyl methacrylate being preferred. Specific examples of AB block polymers include the block copolymers disclosed in Synthesis Examples 3 to 8 of International Publication No. 2013 / 115071.

[0056] Examples of commercially available dispersants include Joncyrl 62, 67, 68, 678, and 687 (styrene-acrylic copolymers manufactured by BASF); Aron AC-10SL (polyacrylic acid manufactured by Toagosei Co., Ltd.); and BYKJET 9151, 9152, 9170, and 9171 (wetting dispersants manufactured by Bic Chemie Japan Co., Ltd.).

[0057] The mass-average molecular weight (MW) of the dispersant is preferably 3,000 to 50,000, and more preferably 7,000 to 25,000. The mass-average molecular weight of the dispersant can be measured by gel permuration chromatography (GPC). Specifically, the measurement can be performed using an HLC-8320GPC (manufactured by Tosoh Corporation) as the GPC instrument, two TSK gel Super Multipore HZ-H columns (manufactured by Tosoh Corporation, inner diameter 4.6 mm × 15 cm), tetrahydrofuran as the eluent, and TSK Standard (manufactured by Tosoh Corporation) as the standard sample.

[0058] The acid value of the dispersant is preferably 50 to 300 mg KOH / g, more preferably 80 to 275 mg KOH / g, and even more preferably 80 to 250 mg KOH / g.

[0059] The dispersant can be used in a mixed state with the colorant. Alternatively, the dispersant can be used to coat part or all of the surface of the colorant. Or, both of these states may be used in combination.

[0060] When the ink according to this embodiment contains a dispersant, the ratio of the total mass of the dispersant to the total mass of the colorant is preferably 0.01 to 1.0, more preferably 0.05 to 0.6, and even more preferably 0.1 to 0.5.

[0061] Examples of surface tension modifiers include 1,2-heptanediol, 1,2-octanediol, 1,2-nonanediol, 1,2-decanediol, 1,2-undecanediol, 1,2-dodecanediol, diethylene glycol monohexyl ether, diethylene glycol monoheptyl ether, ethylene glycol monohexyl ether, and ethylene glycol monoheptyl ether, with 1,2-nonanediol, diethylene glycol monohexyl ether, and ethylene glycol monohexyl ether being preferred. These surface tension modifiers are all highly hydrophobic compounds and have the property of easily orienting the ink surface after the ink has landed on the printing medium. This property is thought to lower the surface tension of the ink and promote the wetting and spreading of the ink on the printing medium, thereby exhibiting excellent wettability.

[0062] Examples of surfactants include anionic, nonionic, silicone, and fluorine-based surfactants. Among these, surfactants selected from silicone-based and fluorine-based surfactants are preferred, and silicone-based surfactants are more preferred from the viewpoint of safety for living organisms and the environment.

[0063] Examples of anionic surfactants include alkyl sulfocarboxylates, α-olefin sulfonates, polyoxyethylene alkyl ether acetates, polyoxyethylene alkyl ether sulfates, N-acyl amino acids or their salts, N-acyl methyl taurates, alkyl sulfates, polyoxyalkyl ether sulfates, alkyl sulfates, polyoxyethylene alkyl ether phosphates, rosinic acid soaps, castor oil sulfates, lauryl alcohol sulfates, alkylphenol type phosphates, alkyl type phosphates, alkylaryl sulfonates, diethyl sulfosulfate, diethylhexyl sulfosulfate, and dioctyl sulfosulfate.

[0064] Nonionic surfactants include, for example, ether-based surfactants such as polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene oleyl ether, polyoxyethylene lauryl ether, polyoxyethylene alkyl ether, and polyoxyethylene distyrenated phenyl ether (e.g., Emulgen A-60, A-90, A-500 manufactured by Kao Corporation); polyoxyethylene oleate ester, polyoxyethylene distearate ester, sorbitan laurate, and sorbitan monostearate. Examples include ester-based compounds such as sorbitan monooleate, sorbitan sesquioleate, polyoxyethylene monooleate, and polyoxyethylene stearate; acetylene glycol (or acetylene alcohol)-based compounds such as 2,4,7,9-tetramethyl-5-decine-4,7-diol, 3,6-dimethyl-4-octin-3,6-diol, and 3,5-dimethyl-1-hexyn-3-ol (for example, Surfinol 104, 104PG50, 82, 420, 440, 465, 485; Olfin STG; etc., manufactured by Evonik Japan Co., Ltd.); and polyglycol ether-based compounds.

[0065] Examples of silicone-based surfactants include polyether-modified siloxanes and polyether-modified polydimethylsiloxanes. Examples include Dynol 960 and 980 from Air Products Inc.; Silface SAG001, SAG002, SAG003, SAG005, SAG503A, SAG008, SAG009, and SAG010 from Nisshin Chemical Industry Co., Ltd.; BYK-345, 347, 348, 349, 3450 (also known as BYKLPX 23289), 3451 (also known as BYKLPX 23347), 3455, LP-X23288, and LP G20726 from BIC Chemie Japan Inc.; and TEGO® Twin 4000, TEGO® Wet KL 245, 250, 260, 265, 270, and 280 from Evonic Tego Chemie Inc.

[0066] Examples of fluorinated surfactants include perfluoroalkyl sulfonic acid compounds, perfluoroalkyl carboxylic acid compounds, perfluoroalkyl phosphate ester compounds, perfluoroalkyl ethylene oxide adducts, and polyoxyalkylene ether polymer compounds having perfluoroalkyl ether groups in their side chains. Examples of commercially available products include Capstone FS-30 and FS-31 manufactured by Chemors.

[0067] Examples of antifungal agents include sodium dehydroacetate, sodium benzoate, sodium pyridinethion-1-oxide, ethyl p-hydroxybenzoate, 1,2-benzisothiazolin-3-one, and their salts.

[0068] Examples of preservatives include compounds such as organic sulfur, organic nitrogen sulfur, organic halogen, haloaryl sulfone, iodopropagyl, haloalkylthio, nitrile, pyridine, 8-oxyquinoline, benzothiazole, isothiazolin, dithiol, pyridine oxide, nitropropane, organotin, phenol, quaternary ammonium salt, triazine, thiazine, anilide, adamantane, dithiocarbamate, brominated indanone, benzyl bromacetate, and inorganic salts. Specific examples of organic halogen compounds include, for example, sodium pentachlorophenol. Specific examples of pyridine oxide compounds include, for example, sodium 2-pyridinethiol-1-oxide. Specific examples of isothiazolin compounds include, for example, 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, and 2-methyl-4-isothiazolin-3-one calcium chloride. Specific examples of other preservatives and fungicides include, for example, anhydrous sodium acetate, sodium sorbate, sodium benzoate, and Arch Chemical's trade names Proxel GXL(S), Proxel LV, and Proxel XL-2(S).

[0069] Examples of pH adjusting agents include alkanolamines such as diethanolamine, triethanolamine, and N-methyldiethanolamine; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; ammonium hydroxide (ammonia water); 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.

[0070] Examples of chelating reagents include disodium ethylenediaminetetraacetate, sodium nitrilotriacetate, sodium hydroxyethylethylenediaminetriacetate, sodium diethylenetriaminepentaacetate, and sodium uracildiacetate.

[0071] Examples of rust inhibitors include acidic sulfites, sodium thiosulfate, ammonium thioglycolate, diisopropylammonium nitride, pentaerythritol tetranitrate, and dicyclohexylammonium nitride.

[0072] Examples of defoaming agents include silicone-based, silica mineral oil-based, olefin-based, and acetylene-based compounds. Examples of commercially available defoaming agents include Surfinol DF37, DF58, DF110D, DF220, MD-20, and Olfin SK-14, manufactured by Shin-Etsu Chemical Co., Ltd.

[0073] Examples of water-soluble ultraviolet absorbers include sulfonated benzophenone compounds, benzotriazol compounds, salicylic acid compounds, cinnamic acid compounds, and triazine compounds.

[0074] As antioxidants, for example, various organic and metal complex-based colorfastness inhibitors can be used. Examples of organic colorfastness inhibitors include hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines, amines, indanes, chromans, alkoxyanilines, and heterocyclic compounds.

[0075] The ink according to this embodiment may contain a resin emulsion to improve the ink's fixation to the printing medium. The inclusion of a resin emulsion in the ink according to this embodiment tends to improve the image fastness of the printed image on the printing medium, such as water resistance, scratch resistance, and alcohol resistance. At least one of polymer emulsions and wax emulsions is preferred as the resin emulsion.

[0076] Examples of polymer emulsions include emulsions containing urethane-based, polyester-based, acrylic-based, vinyl acetate-based, vinyl chloride-based, styrene-acrylic-based, acrylic-silicone-based, and styrene-butadiene-based polymers. Among these, emulsions of polymers selected from urethane-based, acrylic-based, and styrene-butadiene-based polymers are preferred.

[0077] Examples of commercially available polymer emulsions include: Superflex 420, 470, 890 (urethane resin emulsions manufactured by Daiichi Kogyo Seiyaku Co., Ltd.); Hydran HW-350, HW-178, HW-163, HW-171, AP-20, AP-30, WLS-201, WLS-210 (urethane resin emulsions manufactured by DIC Corporation); 0569, 0850Z, 2108 (styrene-butadiene resin emulsions manufactured by JSR Corporation); AE980, AE981A, AE982, AE986B, AE104 (acrylic resin emulsions manufactured by E-Tech Co., Ltd.); NeoCryl A-1105, A-1125, A-1127 (acrylic resin emulsions manufactured by DSM Coating Resin Co., Ltd.); NeoCryl A-655 (DSM Coating Examples include styrene-acrylic resin emulsion manufactured by Resin Corporation.

[0078] As the wax emulsion, an emulsion in which natural wax or synthetic wax is dispersed in an aqueous medium can be used.

[0079] Examples of natural wax emulsions include emulsions of various waxes such as: petroleum-based waxes like paraffin wax and microcrystalline wax; lignite-based waxes like montane wax; plant-based waxes like carnauba wax and candelilla wax; and animal and plant-based waxes like beeswax and lanolin.

[0080] Examples of synthetic wax emulsions include polyalkylene wax (preferably poly C2-C4 alkylene wax), oxidized polyalkylene wax (preferably oxidized poly C2-C4 alkylene wax), and paraffin wax emulsions. Among these, emulsions of waxes selected from polyethylene wax, polypropylene wax, oxidized polyethylene wax, oxidized polypropylene wax, and paraffin wax are preferred.

[0081] The average particle size of the wax is preferably 50 nm to 5 μm, and more preferably 100 nm to 1 μm, in order to prevent clogging of the inkjet head.

[0082] Examples of commercially available wax emulsions include CERAFLOUR 925, 929, 950, 991, AQUACER 498, 515, 526, 531, 537, 539, 552, 1547, AQUAMAT 208, 263, 272; MINERPOL 221 (all manufactured by Big Chemie Japan Co., Ltd.); Mitsui High Wax NL100, NL200, NL500, 4202E, 1105A, 2203A, NP550, NP055, NP505 (all manufactured by Mitsui Chemicals, Inc.); KUE-100, 11 (all manufactured by Sanyo Chemical Industries, Ltd.); HYTEC P-5300, E-6500, 9015, 6400 (all manufactured by Toho Chemical Industries, Ltd.).

[0083] When the ink according to this embodiment contains a resin emulsion, its solid content is preferably 1 to 20% by mass, and more preferably 3 to 15% by mass. A solid content of 1% by mass or more of the resin emulsion tends to result in good adhesion to the printing medium. Furthermore, a solid content of 20% by mass or less of the resin emulsion tends to result in good ink discharge and storage stability.

[0084] The method for preparing the ink according to this embodiment is not particularly limited, and known preparation methods can be employed. For example, one method is to prepare a dispersion containing a colorant and a dispersant, and then add water, organic compounds such as compounds a to c mentioned above, and an ink preparation agent as needed to this dispersion and mix.

[0085] Methods for preparing the dispersion include, for example, phase inversion emulsification, acid precipitation, interfacial polymerization, in-situ polymerization, liquid curing coating, coacervation (phase separation), liquid drying, melt-dispersion-cooling, air suspension coating, and spray drying. Among these, phase inversion emulsification, acid precipitation, and interfacial polymerization are preferred, with phase inversion emulsification being more preferred.

[0086] When preparing a dispersion by the phase inversion emulsification method, for example, a dispersant is dissolved in an organic solvent such as 2-butanone, and an aqueous solution of a neutralizing agent is added to prepare an emulsion. A coloring agent is added to the resulting emulsion and the dispersion treatment is performed. By removing the organic solvent and some of the water from the liquid obtained in this way under reduced pressure, the desired dispersion can be obtained.

[0087] Dispersion can be performed using a sand mill (bead mill), roll mill, ball mill, paint shaker, ultrasonic disperser, microfluidizer, etc. For example, when using a sand mill, beads with a particle size of about 0.01 to 1 mm can be used, and the bead packing ratio can be appropriately set to perform the dispersion. The particle size of the particles contained in the dispersion can be made uniform by performing operations such as filtration and centrifugation on the dispersion obtained in the above manner. If foaming occurs during the preparation of the dispersion, a very small amount of a known antifoaming agent such as a silicone-based or acetylene glycol-based agent can be added.

[0088] In this embodiment, it is preferable that the ink contains a low amount of inorganic impurities such as metal cation chlorides (e.g., sodium chloride) and metal sulfates (e.g., sodium sulfate). Such inorganic impurities are often found in commercially available colorants. The guideline for the inorganic impurity content is approximately 1% by mass or less relative to the total mass of the colorant, and the lower limit is below the detection limit of analytical instruments, i.e., ideally 0% by mass. Methods for obtaining a colorant with low levels of inorganic impurities include, for example, using a reverse osmosis membrane; suspending and stirring the solid colorant in a mixed solvent of C1-C4 alcohol such as methanol and water, filtering and separating the colorant, and drying it; and exchanging and adsorbing inorganic impurities with an ion exchange resin.

[0089] The ink according to this embodiment is preferably microfiltered. A membrane filter, glass filter paper, or the like can be used for microfiltration. The pore size of the filter when performing microfiltration is usually 0.5 to 20 μm, preferably 0.5 to 10 μm.

[0090] The ink according to this embodiment exhibits excellent storage stability, redispersibility, color development, and saturation. Furthermore, printed images recorded using the ink according to this embodiment exhibit excellent fastness in various aspects, such as lightfastness, heat resistance, and oxidation gas resistance (e.g., ozone gas resistance). In addition, the ink according to this embodiment exhibits excellent image formation performance with minimal coating unevenness during image formation.

[0091] Examples of ink sets according to this embodiment include sets of the inks according to this embodiment described above, or sets that include the inks according to this embodiment described above and other inks different from those inks. The other inks are not particularly limited as long as they have a different composition from the inks according to this embodiment, but it is preferable that they have a different hue from the inks according to this embodiment.

[0092] Furthermore, the printing media according to this embodiment has at least one selected from the group consisting of the ink or inkjet ink included in the ink set according to this embodiment described above attached to it.

[0093] Furthermore, the ink media set according to this embodiment includes the ink or ink set according to this embodiment described above, and a printing medium.

[0094] The printing media is not particularly limited, but ink-poorly absorbent or ink-non-absorbent printing media are preferred, and ink-non-absorbent printing media are more preferred. Examples of ink-poorly absorbent printing media include plain paper without an ink-receiving layer, media used in gravure printing and offset printing, art paper, coated paper, matte paper, and cast paper. Examples of ink-non-absorbent printing media include PET (polyethylene terephthalate) film, PP (polypropylene) film, vinyl chloride sheet, glass, and rubber.

[0095] The inkjet recording method according to this embodiment records ink droplets of the above-described ink embodiment onto a printing medium by ejecting them from an inkjet head. There are no particular restrictions on the ink nozzles of the inkjet printer that eject the ink, and they can be appropriately selected according to the purpose. Furthermore, the inkjet recording method is not particularly limited as long as it includes a first ink adhesion step of ejecting the first ink from a first inkjet head and adhering it to a recording medium; a first drying step of drying the recording medium after the first ink adhesion step; a second ink adhesion step of ejecting a second ink from a second inkjet head and adhering it to the recording medium after the first drying step; and a second drying step of drying the recording medium after the second ink adhesion step. For example, it may further include any step selected from the group consisting of the first ink adhesion step, the second ink adhesion step, and a third ink adhesion step different from the first and second inks, either before the first ink adhesion step or after the second drying step. Furthermore, the first inkjet head and the second inkjet head may each be used individually or in multiple quantities. The third ink mentioned above is not limited in any way, as long as it is different from the first and second inks mentioned above. It may be used individually or in combination with others. Furthermore, the number of inks and drying steps can be increased to match the number of colors. For example, four-color printing can include the first to fourth inks and the first to fourth drying steps. Similarly, eight-color printing can include the first to eighth inks and the first to eighth drying steps.

[0096] The inkjet recording method according to this embodiment also includes methods for improving image quality by ejecting a large number of inks with a low colorant content in a small volume; methods for improving image quality by using multiple inks with substantially the same hue but different colorant content in the ink; and methods for improving the fixation of colorants to printing media by using a colorless transparent ink and an ink containing a colorant in combination.

[0097] Any known inkjet recording method can be used. Examples include charge control methods, drop-on-demand methods (also called pressure pulse methods), acoustic inkjet methods, and thermal inkjet methods. The inkjet recording method may be either a multi-pass method or a single-pass method (one-pass printing method). In industrial inkjet printers, single-pass printing using a line-head type inkjet printer is also preferred for the purpose of increasing the printing speed. In recent years, circulating heads that have a mechanism to prevent ink drying near the nozzle by circulating the ink to the vicinity of the nozzle (i.e., a circulation mechanism) have been actively developed. The ink according to this embodiment can be suitably used in such circulating heads, and it is preferable that at least one of the first inkjet head and the second inkjet head is a circulating head, and it is more preferable that the first inkjet head and the second inkjet head each include a circulation mechanism.

[0098] When recording to a printable medium, for example, an ink-containing container (ink tank) is loaded into the designated position in the inkjet printer, and the data is recorded to the printable medium using the printing method described above. Furthermore, full-color printing can be achieved by loading containers containing each color of ink into the designated positions in the inkjet printer and recording the data to the printable medium using the printing method described above.

[0099] Furthermore, when using printing media that does not have an ink-receiving layer, surface modification treatment is also preferably performed. Examples of surface modification treatments include corona discharge treatment, plasma treatment, and flame treatment. It is generally known that the effect of surface modification treatment decreases over time. For this reason, it is preferable to perform the surface modification treatment process and the inkjet recording process consecutively, and it is even more preferable to perform the surface modification treatment process immediately before the inkjet recording process.

[0100] For all the matters mentioned above, a combination of desirable items is more desirable, and a combination of more desirable items is even more desirable. The same applies to combinations of desirable items and more desirable items, and combinations of more desirable items and even more desirable items, and so on. [Examples]

[0101] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the examples, unless otherwise specified, "parts" means parts by mass and "%" means mass percent. When it was necessary to quantify the pigment solids content in the dispersion, it was determined by the dry weight method using the MS-70 manufactured by A&D Co., Ltd. The pigment solids content is a converted value calculated from the total amount of solids, with only the pigment solids content being considered.

[0102] [Preparation Example 1] Preparation of Cyanide Dispersion Joncyrl 68 (BASF, mass-average molecular weight: 13000) (9 parts) and triethanolamine (6 parts) were dissolved in deionized water (75 parts) and stirred for 1 hour. To the resulting solution, CIPigment Blue 15:4 (Dainichi Seika Kogyo Co., Ltd., Chromofine Blue 4851) (30 parts) was added, and the mixture was dispersed in a sand grinder at 1500 rpm for 15 hours. Deionized water (40 parts) was added dropwise to the resulting liquid, and the dispersion beads were removed by filtration to obtain a cyanide dispersion with a pigment solid content of 18.7%. [Preparation Example 2] Preparation of Yellow Dispersion Joncyrl 68 (BASF, mass-average molecular weight: 13000) (6.9 parts) and triethanolamine (4.6 parts) were dissolved in deionized water (78.5 parts) and stirred for 1 hour. To the resulting solution, CIPigment Yellow 74 (Clarant, HANSA Yellow 5GX01) (30 parts) was added and the mixture was dispersed in a sand grinder at 1500 rpm for 15 hours. Deionized water (40 parts) was added dropwise to the resulting liquid, and the dispersion beads were removed by filtration to obtain a yellow dispersion with a pigment solid content of 18.7%.

[0103] [Ink A~N]: Ink preparation. After mixing the components listed in Tables 1 and 2 below, the mixtures were filtered through a 3 μm membrane filter to obtain inks A through N for evaluation testing. The redispersibility and stability of each ink were then evaluated according to the following criteria. The abbreviations in Table 1 below represent the following: Yellow dispersion: Pigment dispersion obtained in Preparation Example 1 Cyanide dispersion: Pigment dispersion obtained in Preparation Example 2 Brownon BN3: (Nonionic surfactant, manufactured by Aoki Oil & Fat Industry Co., Ltd.) Surfinol 420 (nonionic surfactant, manufactured by Nisshin Chemical Industry Co., Ltd.) BYK-349: (Silicone-based surfactant, manufactured by Bic Chemie Japan Co., Ltd.) BYK-3450: (Silicone-based surfactant, manufactured by Bic Chemie Japan Co., Ltd.) Mowinyl 6899D @ 46% (Acrylic resin emulsion, manufactured by Japan Coating Resin Co., Ltd., solids content 46%) AQ-515@35%: AQUACER 515 (Polyethylene wax emulsion, manufactured by BIC Chemie Japan Co., Ltd., solids content 35%)

[0104] [Table 1]

[0105] <Rating> [Secondary color image quality evaluation] The secondary color image quality was evaluated under the following conditions. A KJ4B-1200 (manufactured by Kyocera Corporation) was used as the inkjet head. For both the first and second inks, a solid image of 3 pL, 1200 dpi x 1200 dpi was printed. The stage heater temperature was set so that the substrate temperature during printing was 32°C. A ZKB1200 / 340G (Heraeus) (1200W, 230V) far-infrared (IR) heater was used for drying. PET film (Toyobo Co., Ltd., E5102) was used as the substrate. After printing the first ink, the substrate was placed in the drying unit after 0.5 seconds and dried at the time and power output specified in the example table. Then, 20 seconds later, the second ink was printed so that the second ink was printed on top of the first ink. After printing, the substrate was placed in the drying unit after 0.5 seconds and dried at the time and power output specified in the example table. After the second drying process was completed, the substrate printed with the first and second inks was observed to evaluate the secondary color image quality. The quality of the solid print areas (secondary color) was evaluated by visual inspection and measurement of graininess values ​​using a handheld image quality analyzer (PIAS-II) (Quality Engineering Associates (QEA), Inc.). Inks A through J all showed excellent stability. -Evaluation Criteria- A: The image quality is uniform and the solid print is consistent. Graininess less than 1.3 B: The image quality is almost uniform, with solid print. Graininess: less than 1.3-1.4 C: Image quality shows some unevenness. Graininess: 1.4-1.5 D: Vertical streaks and unevenness are observed in the image quality. The paint is not applied evenly. Graininess 1.5 or higher [Stability evaluation] The inks of Examples 1-6 and Comparative Examples 1-3 were stored at 60°C for 48 hours. After 48 hours, they were left to stand at room temperature for at least 1 hour. The viscosity of the inks before and after storage was measured using a Toki Sangyo Co., Ltd. E-type viscometer at 32°C and 100 rpm. The viscosity change rate before and after storage was calculated. The evaluation criteria were set on the following four levels. The results are shown in Table 3. A: Viscosity change rate is less than 3% compared to before storage. B: Viscosity change rate is less than 8% compared to before storage. C: Viscosity change rate is less than 10% compared to before storage. D: Viscosity change rate is 10% or more compared to before storage. The change in surface tension was similarly measured at 25 degrees Celsius using a DY-300 (manufactured by Kyowa Interfacial Chemical). The change in surface tension before and after storage was calculated. The evaluation criteria were set to the following five levels. A: The change in surface tension compared to before storage is less than 0 to |0.5|mN / m. B: The change in surface tension compared to before storage is 0.5 mN / m or more, and less than 1 mN / m. C: Compared to before storage, the change in surface tension is |1|mN / m or more and |2|mN / m or less. D: The change in surface tension is 2 mN / m or more compared to before storage.

[0106] [Table 2]

[0107] From the results in Tables 1 and 2 above, it can be seen that the ink in the example is superior to the ink in the comparative example in terms of stability and secondary color image quality. [Industrial applicability]

[0108] The ink of the present invention is extremely useful as an ink for various recording purposes, particularly for inkjet recording, due to its excellent stability and secondary color image quality.

Claims

1. An ink used in a printing method having a drying step between printing with a first ink and printing with a second ink, comprising a first ink application step in which the first ink is ejected from a first inkjet head and attached to a recording medium, After the first ink application step, a first drying step is performed to dry the recording medium, After the first drying step, a second ink application step is performed in which a second ink is ejected from a second inkjet head and attached to the recording medium, The process includes a second drying step, which dries the recording medium after the second ink application step, The aforementioned ink contains a pigment, a water-soluble organic solvent, an organic compound, and water. The logP value of the water-soluble organic solvent is greater than -5.42 and less than 1.25, and the interfacial tension of the organic compound with PET is 15.5 mN / m or less. The water-soluble organic solvent comprises at least one compound selected from the group consisting of C4-C5 alkanediols, compounds represented by the following formula (3), and compounds represented by the following formula (4). 【Chemistry 1】 (In formula (3), R 3 represents a hydrogen atom or a methyl group. In formula (4), R 4 represents a linear or branched C1-C4 hydrocarbon group, and n2 represents 1 or 2.) An inkjet ink wherein the total content of the water-soluble organic solvent in the total mass of the ink is 1 to 20% by mass, and the total content of the organic compound is 0.01% to 1% by mass, and satisfies at least one of the following conditions (A) or (B). (A) The organic compound comprises at least one compound selected from the group consisting of compounds represented by the following formula (1). 【Chemistry 2】 (In formula (1), R １ (where n1 represents a phenyl group, naphthyl group, or benzyl group, and n1 represents an integer from 1 to 10.) (B) The organic compound comprises at least one compound selected from the group consisting of compounds represented by the following formula (2). 【Transformation 3】 (In formula (2), R ２ (This represents a linear or branched C7-C12 hydrocarbon group.)

2. An inkjet recording method that prints onto a recording medium by ejecting droplets of inkjet ink described in claim 1 from an inkjet head.

3. The inkjet recording method according to claim 2, wherein the recording medium is an ink-poorly absorbent or ink-non-absorbent printing medium.

4. The inkjet recording method according to claim 2, wherein the first inkjet head and the second inkjet head are each inkjet heads including a circulation mechanism.

5. The inkjet recording method according to claim 2, wherein the drying step includes drying by infrared irradiation.

6. The inkjet recording method according to claim 5, wherein the drying step in claim 5 further includes drying by a hot air heater.

7. The inkjet recording method according to claim 5, wherein after applying ink to a recording medium, drying is performed by infrared irradiation for 0.001 to 2.0 seconds.

8. The inkjet recording method according to claim 5, wherein the output of the infrared irradiation is 2400W or less and 250V or less, and the infrared irradiation time is 0.5 seconds to 3 seconds.

9. An inkjet ink set comprising the inkjet ink described in claim 1 and another inkjet ink different from the said inkjet ink.

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