JP2024058036A - Inkjet ink, inkjet recording method, ink set, ink media set, and print medium - Google Patents

Abstract

[Problem] To provide an inkjet ink having excellent stability and secondary color image quality, and a printed matter using the inkjet ink. [Solution] The ink is used in a printing method having a drying step between printing using a first ink and printing using a second ink, the ink containing a pigment, a water-soluble organic solvent, an organic compound, and water, the water-soluble organic solvent having a specific log P value, the organic compound having a specific interfacial tension with PET, and the organic compound containing at least one of a compound represented by the following formula (1) and a compound represented by the following formula (2): TIFF2024058036000011.tif22155 (In formula (1), R1 represents a phenyl group, a naphthyl group, or a benzyl group, and n1 represents an integer from 1 to 10.) TIFF2024058036000012.tif15153 (In formula (2), R2 represents a linear or branched C7-C12 hydrocarbon group.) [Selected diagram] None

Description

The present invention relates to an inkjet ink, an inkjet recording method, an ink set, an ink media set, and a printing medium.

The inkjet recording method, one of the most representative color recording methods, generates small droplets of ink and deposits them on a print medium such as paper to record. In recent years, demand has increased for commercial and industrial applications, and there is a demand for inks that can be used to print on a variety of print media.

In commercial fields such as catalogs and pamphlets, printing media with lower ink absorption than plain paper (hereinafter also referred to as "low-ink-absorbency media"), such as coated paper and art paper, are often used. In industrial fields such as outdoor signage and soft food packaging, printing media that do not absorb ink (hereinafter also referred to as "non-ink-absorbency media"), such as polyvinyl chloride film, PET (polyethylene terephthalate) film, and polyolefin film, are often used.

For poorly ink-absorbent media and non-ink-absorbent media, development has been progressing on solvent inks that use organic solvents as the main solvent, and curable inks that contain polymerizable monomers. However, these inks have many safety issues for the natural environment and the human body. Therefore, in recent years, there has been active development of water-based inks that use water as the main solvent.

Water-based inks generally have the property of being difficult to wet and spread on poorly or non-absorbent media. To address this issue, it is necessary to add a large amount of organic solvent to improve wettability. However, inks with a large amount of solvent have the problem of poor drying properties on the above media.
Therefore, a method has been adopted in which a silicon-based surfactant or the like is used to reduce the surface tension instead of a large amount of organic solvent (Patent Document 3). While low surface tension inks improve the wetting of the above-mentioned media, there is a problem that ink repellency occurs during high-speed printing, as described below, resulting in a deterioration in image quality.

In order to increase productivity, there is an increasing demand for inkjet printing systems to be able to perform full-color printing at high speeds (Patent Document 4). To meet this demand, the use of a line-head type inkjet printer is suitable, and in this case, inks are required that have excellent wetting and spreading properties on the media, quick drying, high print density, and high-quality prints with little inter-color bleeding. Furthermore, in order to print at high speeds using the line-head type, a method of high-speed printing that includes a drying process between inkjet heads can be considered (Patent Documents 5 and 6).

However, when printing in color, if the ink is allowed to dry completely between the heads, the ink will sit on top of the dried ink film, causing the ink to be repelled, resulting in a problem of reduced image quality. In addition, to avoid ink repelling, adjustments such as lowering the drying strength are required, which requires adjustments to the printing system and does not result in a simple system.

Japanese Patent No. 5504890 JP 2020-125382 A JP 2020-55943 A JP 10-309803 A Patent Publication No. 2013-514904 JP 2022-119849 A

The present invention provides an ink suitable for use with inter-head drying in an inkjet system. In other words, the objective of the present invention is to provide an inkjet ink that does not repel ink even when inter-head drying is performed during high-speed full-color printing with an inkjet printer, and that has excellent wettability and dryness on the media and on the dried ink film, as well as an ink set containing the inkjet ink, an ink media set containing the inkjet ink or the ink set and a printing medium, and a printing medium to which the inkjet ink or each inkjet ink of the ink set is adhered.

As a result of intensive research by the inventors to solve the above problems, they have discovered that a printing method includes a drying step between printing using a first ink and printing using a second ink, the printing method including a first ink deposition step of ejecting the first ink from a first inkjet head and depositing the first ink on a recording medium;
a first drying step of drying the recording medium after the first ink applying step;
a second ink applying step of ejecting a second ink from a second inkjet head and applying the second ink to a recording medium after the first drying step;
a second drying step of drying the recording medium after the second ink applying step,
the ink contains a pigment, a water-soluble organic solvent, an organic compound, and water;
the log P 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 respect to PET is 15.5 mN/m or less;
The inventors have found that an ink-jet ink, in which the total content of the water-soluble organic solvent is 1 to 20% by mass and the total content of the organic compound is 0.01% to 1% by mass, based on the total mass of the ink, and which satisfies at least one of the following conditions (A) and (B), can solve the above-mentioned problems, and have completed the present invention.
(A) The organic compound includes at least one compound selected from the group consisting of compounds represented by the following formula (1):

(In formula (1), ^R1 represents a phenyl group, a naphthyl group, or a benzyl group, and n1 represents an integer of 1 to 10.)
(B) The organic compound includes at least one compound selected from the group consisting of compounds represented by the following formula (2):

(In formula (2), ^R2 represents a linear or branched C7-C12 hydrocarbon group.)

That is, the present invention relates to the following 1) to 10).
1)
The ink is used in a printing method having a drying step between printing using a first ink and printing using a second ink, the method comprising: a first ink deposition step of ejecting the first ink from a first inkjet head and depositing the first ink on a recording medium;
a first drying step of drying the recording medium after the first ink applying step;
a second ink applying step of ejecting a second ink from a second inkjet head and applying the second ink to a recording medium after the first drying step;
a second drying step of drying the recording medium after the second ink applying step,
the ink contains a pigment, a water-soluble organic solvent, an organic compound, and water;
the log P 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 respect to PET is 15.5 mN/m or less;
An ink-jet ink, wherein a total content of the water-soluble organic solvent is 1 to 20% by mass, and a total content of the organic compound is 0.01% to 1% by mass, based on the total mass of the ink, and the ink satisfies at least one of the following conditions (A) and (B):
(A) The organic compound includes at least one compound selected from the group consisting of compounds represented by the following formula (1):

(In formula (1), ^R1 represents a phenyl group, a naphthyl group, or a benzyl group, and n1 represents an integer of 1 to 10.)
(B) The organic compound includes at least one compound selected from the group consisting of compounds represented by the following formula (2):

(In formula (2), ^R2 represents a linear or branched C7-C12 hydrocarbon group.)
2)
The ink-jet ink according to 1), wherein the water-soluble organic solvent contains at least one compound selected from the group consisting of a C4-C5 alkanediol, a compound represented by the following formula (3), and a compound represented by the following formula (4):

(In formula (3), ^R3 represents a hydrogen atom or a methyl group. In formula (4), ^R4 represents a linear or branched C1-C4 hydrocarbon group, and n2 represents 1 or 2.)
3)
An inkjet recording method comprising ejecting droplets of the inkjet ink according to 1) or 2) from an inkjet head to print on a recording medium.
4)
3) The ink jet recording method according to 3), wherein the recording medium is a printing medium that is poorly ink-absorbent or non-ink-absorbent.
5)
The inkjet recording method according to 3), wherein the first inkjet head and the second inkjet head each include a circulation mechanism.
6)
The inkjet recording method according to any one of 3) to 5), wherein the drying step includes drying by infrared radiation.
7)
6) The ink jet recording method according to 6), wherein the drying step further comprises drying with a hot air heater.
8)
The inkjet recording method according to 6) or 7), wherein after the ink is applied to the recording medium, it is dried by infrared irradiation for 0.001 to 2.0 seconds.
9)
The inkjet recording method according to any one of 6) to 8), wherein the output of the infrared radiation is 2400 W or less and 250 V or less, and the infrared radiation irradiation time is 0.5 seconds to 3 seconds.
10)
An ink-jet ink set comprising the ink-jet ink according to 1) or 2) and another ink-jet ink different from the ink-jet ink.

The present invention provides an inkjet ink with excellent compatibility, wettability and ejection properties.

Hereinafter, specific embodiments to which the present invention is applied will be described in detail.
In this specification, "C.I." means "Color Index." In addition, in this specification, the terms "alkylene,""propylene," and "alkyl" are used to mean both linear and branched structures unless otherwise specified.

<Inkjet ink>
The inkjet ink according to this embodiment (hereinafter, simply referred to as "ink") is an ink used in a printing method having a drying step between printing using a first ink and printing using a second ink, and the printing method includes a first ink deposition step of ejecting the first ink from a first inkjet head and depositing it on a recording medium,
a first drying step of drying the recording medium after the first ink applying step;
a second ink applying step of ejecting a second ink from a second inkjet head and applying the second ink to a recording medium after the first drying step;
a second drying step of drying the recording medium after the second ink applying step,
the ink contains a pigment, a water-soluble organic solvent, an organic compound, and water;
the log P 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 respect to PET is 15.5 mN/m or less;
The inkjet ink has a total content of the water-soluble organic solvents of 1 to 20% by mass and a total content of the organic compounds of 0.01% to 1% by mass, based on the total mass of the ink, and 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. Note that each of the components described below may be used alone or in combination of two or more types. It is also preferable that only one of the above conditions (A) and (B) is satisfied.
As of,

<Pigments>
Examples of the pigment include inorganic pigments, organic pigments, extender pigments, hollow particles, and the like.

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

When the ink according to the present embodiment is a 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, channel black, etc. Specific examples of carbon black include the Raven series manufactured by Columbia Carbon; the Monarch series, Regal series, and Mogul series manufactured by Cabot Corporation; the HiBlack series, ColorBlack series, Printex series, SpecialBlack series, and Nerox series manufactured by Orion Engineered Carbons; and the MA series, MCF series, No. 25, No. 33, No. 40, No. 47, No. 52, No. 900, and No. 2300 manufactured by Mitsubishi Chemical Corporation.

When the ink according to this embodiment is a white ink and the colorant is an inorganic pigment, examples of the inorganic pigment contained in the white ink include oxides, nitrides, and oxynitrides of metals such as zinc, silicon, aluminum, titanium, strontium, and zirconium; inorganic compounds such as glass and silica; and the like. Among these, titanium dioxide and zinc oxide are preferred.

Examples of organic pigments include azo, disazo, phthalocyanine, quinacridone, isoindolinone, dioxazine, perylene, perinone, thioindigo, anthraquinone, and quinophthalone.

Specific examples of organic pigments include yellow pigments such as C.I. 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; C.I. red pigments such as C.I. Pigment Red 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 C.I. Pigment 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 C.I. Pigment Violet 19, 23, 29, 37, 38, 50; Examples of suitable pigments include orange pigments such as C.I. Pigment Orange 13, 16, 43, 68, 69, 71, and 73; green pigments such as C.I. Pigment Green 7, 36, and 54; and black pigments such as C.I. Pigment Black 1. Of these, C.I. Pigment Blue 15:4 is preferred.

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.

As hollow particles, for example, known hollow particles described in U.S. Pat. No. 4,880,465, Japanese Patent No. 3,562,754, Japanese Patent No. 6,026,234, Japanese Patent No. 5,459,460, JP-A-2003-268694, Japanese Patent No. 4,902,216, etc. can be used, and they are particularly preferably used as white pigments.

In addition to the above pigments, other dyes may also be included.
Examples of the other dyes include disperse dyes, solvent dyes, direct dyes, acid dyes, and reactive dyes.

As the disperse dye, for example, a dye selected from C.I. Dispers is preferable. Specific examples thereof include yellow dyes such as C.I. Dispers 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, and 237; C.I. Red dyes such as C.I. Dispers Red 60, 73, 88, 91, 92, 111, 127, 131, 143, 145, 146, 152, 153, 154, 167, 179, 191, 192, 206, 221, 258, 283; Orange dyes such as C.I. Dispers Orange 9, 25, 29, 30, 31, 32, 37, 38, 42, 44, 45, 53, 54, 55, 56, 61, 71, 73, 76, 80, 96, 97; Violet dyes such as C.I. Dispers Violet 25, 27, 28, 54, 57, 60, 73, 77, 79, 79:1; Blue dyes such as Dispers 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; and the like.

The average particle size of the 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 of the particles measured using a laser light scattering method.

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

When the inkjet ink contains multiple types of the colorants, the mixing ratio of each colorant can be set arbitrarily depending on the purpose. Furthermore, when the inkjet ink contains other dyes in addition to the colorants, the mixing ratio of the total amount of the colorants to the total amount of the other dyes can be set arbitrarily.

<Water-soluble organic solvent>
The water-soluble organic solvent is not particularly limited as long as the log P value is greater than -5.42 and less than 1.25, and is preferably -1.164 to 1.188. In this specification, the "log P value" refers to the octanol/water partition coefficient, and can be expressed by the "Clog P" value obtained by calculation using, for example, ChemDraw Professional ver. 16.0 manufactured by Perkin Elmer. Examples of the water-soluble organic solvent 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. Of these, 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 are 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 the present invention is a compound with a good balance between hydrophilicity and hydrophobicity, and has the property of being soluble in both the water and the organic compounds contained in the ink. This property makes it possible to stably dissolve the organic compounds even in inks in which water accounts for the majority of the total mass of the ink, and is thought to be able to prevent phase separation of the ink.

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

In the above formula (3), ^R3 represents a hydrogen atom or a methyl group, and in the formula (4), ^R4 represents a linear or branched C1-C4 hydrocarbon group, and n2 represents 1 or 2.

<Organic Compounds>
The organic compound is not particularly limited as long as it is a compound other than the pigment and the water-soluble organic solvent and has an interfacial tension with 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, the "interfacial tension with PET" refers to the interfacial tension at 25°C between a mixed liquid of an organic compound (1 part by mass), 1,4-butanediol (9 parts by mass), and purified water (90 parts by mass) and brought into contact with a PET film, and can be calculated from the contact angle of the mixed liquid with the PET film using Young's equation. As the PET film, for example, a PET film (E5102) manufactured by Toyobo Co., Ltd. is used.
Examples of organic compounds having an interfacial tension with PET of 15.5 mN/m or less include glycol ethers (preferably mono-, di-, or triethylene glycol aryl 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; 2-methyl-2-propyl-1,3-propanediol; Examples of suitable organic compounds include alkanediols (preferably C7-C10 alkanediols at both ends) such as 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; among which mono-, di-, or triethylene glycol aryl ether and C7-C10 alkanediols at both ends are preferred, and diethylene glycol monobenzyl ether, diethylene glycol monophenyl ether, and 1,8-octanediol are more preferred. The organic compound used in the present invention has a property that the ink is easily oriented at the interface between the ink and the printing medium after landing on the printing medium. This property is believed to reduce the interfacial tension generated at the interface between the ink and the printing medium and promote the wetting and spreading of the ink on the printing medium, thereby exhibiting excellent wettability.

It is preferable that the total content of the organic compounds in the total mass of the ink is 0.01% to 1% by mass, and that the total content of the organic compounds is 0.01% to 0.4% by mass.

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.

In the above formula (2), ^R2 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, 2,5-dimethylhexylene, Examples of the alkylene group include a silylene group, a 2-ethylpentylene group, a 2-ethyl-1-propylpropylene group, an n-nonylene group, a 1-methyloctylene group, a 2-methyloctylene group, a 3-methyloctylene group, a 4-methyloctylene group, a 1,7-dimethylheptylene group, a 2,6-dimethylheptylene group, a 3,5-dimethylheptylene group, a 2-butyl-2-ethylpropylene group, an n-decylene group, an n-undecylene group, and an n-dodecylene group.
A linear or branched C7-C9 hydrocarbon group is preferred.

<Water>
The water is preferably water with a low content of impurities such as metal ions, 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.

The ink is used in a printing method having a drying step between printing using a first ink and printing using a second ink, the printing method including a first ink deposition step of ejecting the first ink from a first inkjet head and depositing the first ink on a recording medium;
a first drying step of drying the recording medium after the first ink applying step;
a second ink applying step of ejecting a second ink from a second inkjet head and applying the second ink to a recording medium after the first drying step;
and a second drying step of drying the recording medium after the second ink applying step.

The ink is used for at least one of the first ink and the second ink. It is also preferable to use the ink for both the first ink and the second ink.

The drying step is a step of heating and drying the image between printing using the first ink and printing using the second ink. In the drying step, heat drying using a heat source is useful. As the heating means, one capable of uniformly heating the recording surface is preferable. For example, hot air drying by blowing hot air (hot air) onto the printing surface, drum drying by heating a drum roller in contact with the recording medium, and other heaters such as nichrome wire heaters, halogen heaters, ceramic heaters, and carbon heaters can be appropriately selected according to the purpose, but are not limited thereto.
The first drying step is a step of drying the recording medium after the first ink application step, and the second drying step is a step of drying the recording medium after the second ink application step, and preferably includes drying by infrared irradiation. It is also preferable that the drying step further includes drying by a hot air heater.

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

The first inkjet head is an inkjet head containing the first ink. The second inkjet head is an inkjet head containing the second ink.

There are no particular limitations on the recording medium, so long as it is a material that can be colored by the ink. Examples of recording media include paper, film, fibers and cloth (cellulose, nylon, wool, etc.), leather, and color filter substrates. There are no particular limitations on the printing media, but it is preferable that the printing media is poorly ink-absorbent or non-ink-absorbent. Examples of recording media that are poorly ink-absorbent include recording media used in gravure printing and offset printing, art paper, coated paper, matte paper, cast paper, and the like. Examples of non-ink-absorbent recording media include PET (polyethylene terephthalate) film, PP (polypropylene) film, polyvinyl chloride sheet, metal, glass, rubber, and the like.

The ink according to this embodiment may contain ink preparation agents such as dispersants, organic solvents other than the above-mentioned water-soluble organic solvents, surfactants, antifungal agents, preservatives, pH adjusters, chelating agents, rust inhibitors, defoamers, water-soluble UV absorbers, antioxidants, and resin emulsions, as necessary. The content of each ink preparation agent can be set as desired depending on the application of the ink, etc.

Examples of dispersants include copolymers composed of at least two types of monomers (preferably at least one of which is a hydrophilic monomer) selected from the following monomers: 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; faric acid and its derivatives; vinyl acetate, vinyl alcohol, vinylpyrrolidone, acrylamide, and their derivatives. Examples of hydrophilic monomers include monomers that leave a carboxy group after polymerization, such as acrylic acid and methacrylic acid.

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. Among these, styrene-(meth)acrylic acid copolymers, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymers, (meth)acrylic acid ester-(meth)acrylic acid copolymers, and polyethylene glycol (meth)acrylate-(meth)acrylic acid copolymers are preferred, styrene-(meth)acrylic acid copolymers, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymers, and (meth)acrylic acid ester-(meth)acrylic acid copolymers are more preferred, (meth)acrylic acid ester-(meth)acrylic acid copolymers are even more preferred, and methacrylic acid ester-methacrylic acid copolymers are particularly preferred. Examples of types of copolymers include block copolymers, random copolymers, and graft copolymers. These copolymers may be in the form of a salt. In this specification, the term "(meth)acrylic" is used to mean both "acrylic" and "methacrylic". The same applies to "(meth)acrylate", etc.

Dispersants can be synthesized or commercially available.
Examples of dispersants obtained by synthesis include the A-B block polymer disclosed in WO 2013/115071. The monomer constituting the A block of the A-B block polymer disclosed in WO 2013/115071 is at least one monomer selected from (meth)acrylic acid and linear or branched C4 alkyl (meth)acrylate, preferably at least one monomer selected from methacrylic acid and n-butyl methacrylate, and more preferably these two monomers are used in combination. In addition, the monomer constituting the B block of the A-B block polymer disclosed in WO 2013/115071 is at least one monomer selected from benzyl methacrylate and benzyl acrylate, preferably benzyl methacrylate. Specific examples of the A-B block polymer include the block copolymers disclosed in Synthesis Examples 3 to 8 of WO 2013/115071.

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 Toa Gosei Co., Ltd.); and BYKJET 9151, 9152, 9170, and 9171 (wetting dispersants manufactured by BYK Japan KK).

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 permeation chromatography (GPC). Specifically, the measurement can be performed using a GPC device HLC-8320GPC (manufactured by Tosoh Corporation), two columns TSK gel Super Multipore HZ-H (manufactured by Tosoh Corporation, inner diameter 4.6 mm x 15 cm), tetrahydrofuran as the eluent, and TSK Standard (manufactured by Tosoh Corporation) as the standard sample.

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.

The dispersant can be used in a state where it is mixed with the colorant. It can also be used in a state where the surface of the colorant is partially or entirely covered with the dispersant. Alternatively, both of these states can be used in combination.

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.

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. All of these surface tension modifiers are highly hydrophobic compounds, and have the property of easily orienting on the ink surface after the ink lands on the printing medium. It is believed that this property reduces the surface tension of the ink and promotes the wetting and spreading of the ink on the printing medium, thereby exhibiting excellent wettability.

Examples of surfactants include anionic, nonionic, silicone, and fluorine-based surfactants. Among these, surfactants selected from silicone and fluorine-based surfactants are preferred, and silicone-based surfactants are more preferred from the standpoint of safety to living organisms and the environment.

Examples of 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, dioctyl sulfosuccinates, etc.

Examples of nonionic surfactants include 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 (for example, Emulgen A-60, A-90, and A-500 manufactured by Kao Corporation); polyoxyethylene oleate, polyoxyethylene distearate, sorbitan laurate, and sorbitan monostearate. esters such as sorbitan monooleate, sorbitan sesquioleate, polyoxyethylene monooleate, and polyoxyethylene stearate; acetylene glycols (or acetylene alcohols) 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-hexyne-3-ol (for example, Surfynol 104, 104PG 50, 82, 420, 440, 465, and 485; Olfin STG; and the like, all manufactured by Evonik Japan Co., Ltd.); and polyglycol ethers.

Examples of silicone surfactants include polyether-modified siloxane, polyether-modified polydimethylsiloxane, etc. Examples of such products include Dynol 960 and 980 manufactured by Air Products Co., Ltd.; Silface SAG001, SAG002, SAG003, SAG005, SAG503A, SAG008, SAG009, and SAG010 manufactured by Nissin 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 manufactured by BYK Japan Co., Ltd.; and TEGO (registered trademark) Twin 4000 and TEGO (registered trademark) Wet KL manufactured by EvonikTego Chemie Co., Ltd. 245, 250, 260, 265, 270, 280; etc.

Examples of fluorine-based surfactants include perfluoroalkyl sulfonic acid compounds, perfluoroalkyl carboxylic acid compounds, perfluoroalkyl phosphate compounds, perfluoroalkyl ethylene oxide adducts, and polyoxyalkylene ether polymer compounds having perfluoroalkyl ether groups on the side chains. Commercially available products include Capstone FS-30 and FS-31 manufactured by Chemours.

Examples of antifungal agents include sodium dehydroacetate, sodium benzoate, sodium pyridinethione-1-oxide, p-hydroxybenzoic acid ethyl ester, 1,2-benzisothiazolin-3-one and its salts.

Examples of preservatives include organic sulfur, organic nitrogen sulfur, organic halogen, haloarylsulfone, iodopropargyl, haloalkylthio, nitrile, pyridine, 8-oxyquinoline, benzothiazole, isothiazolinone, dithiol, pyridine oxide, nitropropane, organic tin, phenol, quaternary ammonium salt, triazine, thiazine, anilide, adamantane, dithiocarbamate, brominated indanone, benzyl bromoacetate, 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. 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, sodium benzoate, and Arch Chemical's products under the trade names Proxel GXL (S), Proxel LV, and Proxel XL-2 (S).

Examples of pH adjusters include alkanolamines such as diethanolamine, triethanolamine, and N-methyldiethanolamine; 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.

Examples of chelating agents include disodium ethylenediaminetetraacetate, sodium nitrilotriacetate, sodium hydroxyethylethylenediaminetriacetate, sodium diethylenetriaminepentaacetate, and sodium uracildiacetate.

Examples of rust inhibitors include acid sulfite, sodium thiosulfate, ammonium thioglycolate, diisopropylammonium nitrite, pentaerythritol tetranitrate, and dicyclohexylammonium nitrite.

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

Examples of water-soluble UV absorbers include sulfonated benzophenone compounds, benzotriazole compounds, salicylic acid compounds, cinnamic acid compounds, and triazine compounds.

As the antioxidant, for example, various organic and metal complex-based anti-fading agents can be used. Organic anti-fading agents include hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines, amines, indanes, chromans, alkoxyanilines, heterocycles, etc.

The ink according to this embodiment may contain a resin emulsion for the purpose of improving the fixation of the ink to the print medium. When the ink according to this embodiment contains a resin emulsion, the image fastness, such as water resistance, abrasion resistance, and alcohol resistance, of the image printed on the print medium tends to be improved. As the resin emulsion, at least one type selected from a polymer emulsion and a wax emulsion is preferable.

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 are preferred.

Commercially available polymer emulsions include, for example, Superflex 420, 470, 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, WLS-201, and WLS-210 (urethane-based resin emulsions manufactured by DIC Corporation); 0569, 0850Z, and 2108 (styrene-butadiene-based resin emulsions manufactured by JSR Corporation); AE980, AE981A, AE982, AE986B, and AE104 (acrylic resin emulsions manufactured by E-Tech Co., Ltd.); and NeoCryl A-1105, A-1125, and A-1127 (DSM Coating Co., Ltd.). Examples include NeoCryl A-655 (a styrene-acrylic resin emulsion manufactured by DSM Coating Resin); and NeoCryl A-655 (a styrene-acrylic resin emulsion manufactured by DSM Coating Resin).

As a wax emulsion, an emulsion in which natural wax or synthetic wax is dispersed in an aqueous medium can be used.

Examples of natural wax emulsions include emulsions of various waxes, such as petroleum-based waxes such as paraffin wax and microcrystalline wax; lignite-based waxes such as montan wax; vegetable waxes such as carnauba wax and candelilla wax; and animal and vegetable waxes such as beeswax and lanolin.

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

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

Commercially available wax emulsions include, for example, CERAFLOUR 925, 929, 950, 991, AQUACER 498, 515, 526, 531, 537, 539, 552, 1547, AQUAMAT 208, 263, 272; MINERPOL 221 (all manufactured by BYK Japan Co., Ltd.); Mitsui Hiwax 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 Examples include P-5300, E-6500, 9015, and 6400 (all manufactured by Toho Chemical Industry Co., Ltd.).

When the ink according to this embodiment contains a resin emulsion, the solid content is preferably 1 to 20% by mass, and more preferably 3 to 15% by mass. By making the solid content of the resin emulsion 1% by mass or more, the ink tends to exhibit good adhesion to the printing media. Furthermore, by making the solid content of the resin emulsion 20% by mass or less, the ink tends to have good ejection properties and storage stability.

The method for preparing the ink according to this embodiment is not particularly limited, and any known preparation method can be used. One example is a method in which a dispersion liquid containing a colorant and a dispersant is prepared, and water, an organic compound such as the above compounds a to c, and, if necessary, an ink preparation agent are added to this dispersion liquid and mixed.

Methods for preparing the dispersion include, for example, phase inversion emulsification, acid precipitation, interfacial polymerization, in-situ polymerization, liquid hardening 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.

When preparing a dispersion by phase inversion emulsification, 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 colorant is added to the resulting emulsion, and a dispersion process is carried out. The organic solvent and a portion of the water are distilled off under reduced pressure from the liquid thus obtained, to obtain the desired dispersion.

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 rate can be appropriately set to perform dispersion. The particle size of the particles contained in the dispersion can be made uniform by subjecting the dispersion obtained as described above to filtering, centrifuging, etc. If foaming occurs during preparation of the dispersion, a very small amount of a known silicone-based, acetylene glycol-based, or other defoaming agent can be added.

The ink according to this embodiment preferably has a low content of inorganic impurities such as chlorides of metal cations (e.g., sodium chloride) and metal sulfates (e.g., sodium sulfate). Such inorganic impurities are often contained in commercially available colorants. The content of inorganic impurities is approximately 1% by mass or less of the total mass of the colorant, and the lower limit is ideally below the detection limit of analytical equipment, i.e., 0% by mass. Methods for obtaining colorants with low inorganic impurities include, for example, a method using a reverse osmosis membrane; a method in which a solid colorant is suspended and stirred in a mixed solvent of C1-C4 alcohol such as methanol and water, and the colored body is filtered and separated and dried; a method in which inorganic impurities are exchanged and adsorbed with an ion exchange resin; and the like.

The ink according to this embodiment is preferably microfiltered. For microfiltration, a membrane filter, glass filter paper, or the like can be used. The pore size of the filter used for microfiltration is usually 0.5 to 20 μm, and preferably 0.5 to 10 μm.

The ink according to this embodiment also has excellent storage stability, redispersibility, color development, and saturation. Furthermore, printed images recorded using the ink according to this embodiment also have excellent fastness properties such as light resistance, heat resistance, and oxidation gas resistance (e.g., ozone gas resistance). Furthermore, the ink according to this embodiment has little coating unevenness during image formation, and has excellent image forming properties.

The ink set according to this embodiment may be a set of the inks according to this embodiment described above, or a set including the ink according to this embodiment described above and another ink different from the ink. The other ink is not particularly limited as long as it has a different composition from the ink according to this embodiment, but it is preferable that the other ink has a different hue from the ink according to this embodiment.

The print media according to this embodiment is coated with at least one selected from the group consisting of the inks or inkjet inks included in the ink set according to this embodiment.

The ink media set according to this embodiment includes the ink or ink set according to this embodiment described above and a printing medium.

The printing media is not particularly limited, but is preferably poorly ink-absorbent or non-ink-absorbent printing media, and is more preferably non-ink-absorbent printing media. Examples of poorly ink-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, cast paper, etc. Examples of non-ink-absorbent printing media include PET (polyethylene terephthalate) film, PP (polypropylene) film, polyvinyl chloride sheet, glass, rubber, etc.

The inkjet recording method according to the present embodiment is a method for ejecting droplets of the ink according to the present embodiment from an inkjet head to record on a print medium. There is no particular limitation on the ink nozzles of the inkjet printer that eject the ink, and they can be appropriately selected according to the purpose. The inkjet recording method is not particularly limited as long as it includes a first ink deposition step of ejecting the first ink from a first inkjet head to deposit it on a recording medium, a first drying step of drying the recording medium after the first ink deposition step, a second ink deposition step of ejecting the second ink from a second inkjet head to deposit it on the recording medium after the first drying step, and a second drying step of drying the recording medium after the second ink deposition step. For example, the inkjet recording method may further include any step selected from the group consisting of a first ink deposition step, a second ink deposition step, and a third ink deposition step different from the first and second inks before the first ink deposition step or after the second drying step.
Moreover, the first ink-jet head and the second ink-jet head may each be one or more.
The third ink is not particularly limited as long as it is different from the first ink and the second ink, and one of the inks may be used alone, or a combination of a plurality of inks may be used.
In addition, the number of inks and drying steps can be increased according to the number of colors. For example, when printing in four colors, the first to fourth inks and the first to fourth drying steps can be included. When printing in eight colors, the first to eighth inks and the first to eighth drying steps can be included in the same manner.

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

As the inkjet recording method, a known method can be adopted. Examples of such methods include, for example, a charge control method, a drop-on-demand method (also called a pressure pulse method), an acoustic inkjet method, and a thermal inkjet method. In addition, 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 preferably performed for the purpose of increasing the printing speed. In addition, in recent years, circulation heads having a mechanism (i.e., a circulation mechanism) that prevents the ink from drying near the nozzle by circulating the ink to the vicinity of the nozzle have been actively developed. The ink according to this embodiment is also suitably used in such a circulation head, and it is preferable that at least one of the first inkjet head and the second inkjet head is a circulation head, and it is more preferable that the first inkjet head and the second inkjet head are each an inkjet head including a circulation mechanism.

When recording on print media, for example, a container containing ink (ink tank) is loaded into a predetermined position of an inkjet printer, and recording is performed on the print media using the printing method described above. Note that full-color printing can also be achieved by loading containers containing ink of each color into a predetermined position of an inkjet printer, and recording on the print media using the printing method described above.

When using a printing medium that does not have an ink-receiving layer, it is also preferable to carry out a surface modification treatment. Examples of surface modification treatments include corona discharge treatment, plasma treatment, and frame treatment. It is generally known that the effect of surface modification treatment decreases over time. For this reason, it is preferable to carry out the surface modification treatment step and the inkjet recording step consecutively, and it is even more preferable to carry out the surface modification treatment step immediately before the inkjet recording step.

For all of the above, 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, more preferred items and even more preferred items, etc.

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 % by mass. When quantitative determination of the pigment solid content in the dispersion was required, it was determined by the dry weight method using an MS-70 manufactured by A&D Co., Ltd. The pigment solid content is a converted value calculated from the total amount of solids, taking only the pigment solid content.

[Preparation Example 1] Preparation of cyan dispersion Joncyrl 68 (manufactured by BASF, mass average molecular weight: 13000) (9 parts) and triethanolamine (6 parts) were dissolved in ion-exchanged water (75 parts) and stirred for 1 hour. C.I. Pigment Blue 15:4 (manufactured by Dainichiseika Color & Chemicals Co., Ltd., Chromofine Blue 4851) (30 parts) was added to the obtained solution, and dispersion treatment was performed in a sand grinder under the condition of 1500 rpm for 15 hours. Ion-exchanged water (40 parts) was dropped into the obtained liquid, and the dispersion beads were removed by filtration to obtain a cyan 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 ion-exchanged water (78.5 parts) and stirred for 1 hour. C.I. Pigment Yellow 74 (Clariant, HANSA Yellow 5GX01) (30 parts) was added to the obtained solution, and dispersion treatment was performed in a sand grinder under the condition of 1500 rpm for 15 hours. Ion-exchanged water (40 parts) was dropped into the obtained liquid, and the dispersion beads were removed by filtration to obtain a yellow dispersion with a pigment solid content of 18.7%.

[Inks A to N]: Preparation of inks.
The components shown in Tables 1 and 2 below were mixed and then filtered through a 3 μm membrane filter to obtain Inks A to N for evaluation tests. The redispersibility and stability of each of the obtained inks were evaluated according to the following evaluation criteria.
The abbreviations in Table 1 below represent the following:
Yellow dispersion: Pigment dispersion obtained in Preparation Example 1 Cyan dispersion: Pigment dispersion obtained in Preparation Example 2 Brownon BN3: (nonionic surfactant, manufactured by Aoki Oil Industries Co., Ltd.)
Surfynol 420 (nonionic surfactant, manufactured by Nissin Chemical Industry Co., Ltd.)
BYK-349: (Silicone surfactant, manufactured by BYK Japan Co., Ltd.)
BYK-3450: (Silicone surfactant, manufactured by BYK Japan Co., Ltd.)
Mowinyl 6899D @ 46% (acrylic resin emulsion, Japan Coating Resin Co., Ltd., solid content 46%)
AQ-515@35%: AQUACER 515 (polyethylene wax emulsion, manufactured by BYK Japan Co., Ltd., solid content 35%)

<Evaluation>
[Secondary color image quality evaluation]
The secondary color image quality was evaluated under the following conditions: KJ4B-1200 (manufactured by Kyocera Corporation) was used as the inkjet head.
Both the first ink and the second ink were used to print a solid image of 3 pL and 1200 dpi x 1200 dpi. The temperature of the stage heater was set so that the substrate temperature during printing was 32°C. The far-infrared (IR) heater for drying was ZKB1200/340G (Heraeus) (1200W, 230V). PET film (Toyobo, E5102) was used as the substrate. After the first ink printing, the substrate entered the drying unit in 0.5 seconds and dried at the time and output described in the example table. After that, the second ink printing was performed 20 seconds later so that the second ink was printed on the first ink. After printing, the substrate entered the drying unit in 0.5 seconds and dried at the time and output described in the example table. After the second drying process was completed, the substrate on which the first ink and the second ink were printed was observed to evaluate the secondary color image quality. The solid print (secondary color image quality) was evaluated by visual inspection and by measuring the graininess using a handheld image quality analyzer (PIAS-II) (Quality Engineering Associates (QEA), Inc.). All of Ink A to Ink J showed excellent stability.
-Evaluation criteria-
A: The image quality is uniform and solidly printed with no unevenness. Graininess less than 1.3 B: The image quality is almost uniform and solidly printed. Graininess 1.3 to less than 1.4 C: The image quality is slightly uneven. Graininess 1.4 to less than 1.5 D: Vertical streaks or unevenness are observed in the image quality. The image is not evenly coated. Graininess 1.5 or more [Stability evaluation]
The inks of Examples 1 to 6 and Comparative Examples 1 to 3 were stored at 60°C for 48 hours. After 48 hours, the inks were left to stand at room temperature for at least 1 hour. The inks before and after storage were measured at 32°C and 100 rpm using a Toki Sangyo Co., Ltd. E-type viscometer. The rate of change in viscosity before and after storage was calculated. The evaluation criteria were the following four levels. The results are shown in Table 3.
A: The viscosity change rate compared to before storage is less than 3%.
B: The viscosity change rate compared to before storage is less than 8%.
C: The rate of change in viscosity compared to before storage is less than 10%.
D: The viscosity change rate compared to before storage is 10% or more.
The change in surface tension was also measured at 25°C using DY-300 (manufactured by Kyowa Interface Science). The change in surface tension before and after storage was calculated. The evaluation was based on the following five-point scale.
A: Compared to before storage, the change in surface tension is 0 to less than |0.5| mN/m. B: Compared to before storage, the change in surface tension 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 less than |2| mN/m. D: Compared to before storage, the change in surface tension is |2| mN/m or more.

The results in Tables 1 and 2 above show that the inks of the examples have superior stability and secondary color image quality compared to the inks of the comparative examples.

The ink of the present invention has excellent stability and secondary color image quality, making it extremely useful for various recording applications, particularly as an inkjet recording ink.

Claims (10)

The ink is used in a printing method having a drying step between printing using a first ink and printing using a second ink, the method comprising: a first ink deposition step of ejecting the first ink from a first inkjet head and depositing the first ink on a recording medium;
a first drying step of drying the recording medium after the first ink applying step;
a second ink applying step of ejecting a second ink from a second inkjet head and applying the second ink to a recording medium after the first drying step;
a second drying step of drying the recording medium after the second ink applying step,
the ink contains a pigment, a water-soluble organic solvent, an organic compound, and water;
the log P 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 respect to PET is 15.5 mN/m or less;
An ink-jet ink, wherein a total content of the water-soluble organic solvent is 1 to 20% by mass, and a total content of the organic compound is 0.01% to 1% by mass, based on the total mass of the ink, and the ink satisfies at least one of the following conditions (A) and (B):
(A) The organic compound includes at least one compound selected from the group consisting of compounds represented by the following formula (1):

(In formula (1), ^R1 represents a phenyl group, a naphthyl group, or a benzyl group, and n1 represents an integer of 1 to 10.)
(B) The organic compound includes at least one compound selected from the group consisting of compounds represented by the following formula (2):

(In formula (2), ^R2 represents a linear or branched C7-C12 hydrocarbon group.) 2. The ink-jet ink according to claim 1, wherein the water-soluble organic solvent contains at least one compound selected from the group consisting of a C4-C5 alkanediol, a compound represented by the following formula (3), and a compound represented by the following formula (4):

(In formula (3), ^R3 represents a hydrogen atom or a methyl group. In formula (4), ^R4 represents a linear or branched C1-C4 hydrocarbon group, and n2 represents 1 or 2.) An inkjet recording method in which droplets of the inkjet ink according to claim 1 or 2 are ejected from an inkjet head to print on a recording medium. The inkjet recording method according to claim 3, wherein the recording medium is a printing medium that is poorly ink-absorbent or non-ink-absorbent. The inkjet recording method according to claim 3, wherein the first inkjet head and the second inkjet head each include a circulation mechanism. The inkjet recording method according to claim 3, wherein the drying step includes drying by infrared irradiation. The inkjet recording method according to claim 6, wherein the drying step further includes drying with a hot air heater. The inkjet recording method according to claim 6, wherein after the ink is applied to the recording medium, it is dried by infrared irradiation for 0.001 to 2.0 seconds. The inkjet recording method according to claim 6, wherein the output of the infrared radiation is 2400 W or less and 250 V or less, and the infrared radiation exposure time is 0.5 to 3 seconds. An inkjet ink set comprising the inkjet ink according to claim 1 or 2 and another inkjet ink different from the inkjet ink.