JP7628440B2 - Water-based inkjet inks and pigment complexes - Google Patents

Description

The present invention relates to an aqueous inkjet ink, a pigment complex, a dispersion copolymer, and a method for producing the dispersion.

Inkjet recording is a method of ejecting highly fluid inkjet ink as droplets from a fine nozzle to record an image on a recording medium placed opposite the nozzle. This method has rapidly spread in recent years because it allows for low-noise, high-speed printing. Inks used in this type of inkjet recording method include aqueous inks that contain water as the main solvent, ultraviolet-curable inks (UV inks) that contain a high content of polymerizable monomers as the main component, and hot-melt inks (solid inks) that contain a high content of wax as the main component, as well as so-called non-aqueous inks that contain a non-aqueous solvent as the main solvent.

Water-based inkjet inks are used in a wide range of fields due to their safety and drying properties. In order to ensure stable ejection, water-based inkjet inks that use pigments must be finely dispersed in the ink to improve pigment dispersibility and long-term dispersion stability.

Patent Document 1 proposes an aqueous pigment ink that uses, as a dispersant, a copolymer obtained by polymerizing a monomer having an oxyalkylene group with acrylic acid, methacrylic acid, or a salt thereof, or allylsulfonic acid, methacrylic sulfonic acid, or a salt thereof, in order to prevent clogging in the head of an inkjet printer and improve the storage stability of the ink.
Patent Document 2 proposes an aqueous pigment dispersion for inkjet recording in which the particle diameter of particles in the pigment dispersion is small and which has excellent storage stability, the dispersion containing a yellow azo pigment, an aromatic group-containing polymer, and an alkali metal hydroxide, in which the aromatic group-containing polymer is crosslinked.
Patent Document 3 proposes a dispersion containing colorant particles, a basic compound, a crosslinking agent, and water as a colorant dispersion that has excellent dispersion stability and printability even in the presence of a solvent with high permeability and a solvent with a high boiling point.

Japanese Patent Application Publication No. 09-183926 JP 2018-109095 A JP 2019-038890 A

Water-based inks (1) have a high surface tension and tend to be difficult to wet depending on the substrate, and (2) tend to have poor on-machine stability because water gradually evaporates even at room temperature. For example, in the ejection head of an inkjet recording device, when the ink is exposed to the atmosphere at the nozzle, the water in the ink evaporates and is locally reduced, which can reduce the pigment dispersion stability of the ink and cause ejection defects.
Although there is a method for improving the above problem (1) by adding a water-soluble low-polarity solvent to the ink, this tends to further exacerbate the above problem (2), because the appropriate dispersion form differs between water, which has high polarity, and a low-polarity solvent.

The aqueous ink disclosed in Patent Document 1 tends to have insufficient pigment dispersibility at an early stage. Also, the dispersant disclosed in Patent Document 1 is likely to dissolve in the solvent in a system using water and a low polarity solvent, and may not fully exhibit its function.
In Patent Documents 2 and 3, the dispersant is crosslinked to suppress elution of the dispersant into a solvent and thereby improve storage stability. However, the adsorptivity of the pigment dispersant to the pigment and the electrostatic repulsion of the pigment dispersant are insufficient, and there is room for further improvement in storage stability.

One object of the present invention is to provide an aqueous inkjet ink with excellent storage stability.

One aspect of the present invention is an aqueous ink-jet ink comprising a pigment, a dispersant, a water-soluble solvent, and water, wherein the dispersant is a copolymer of a monomer mixture comprising a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and a vinyl monomer (D) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group, and has a crosslinked structure.
Another aspect of the present invention is a pigment complex comprising: a dispersant having a crosslinked structure, which is a copolymer of a monomer mixture including (A) a vinyl monomer containing a carboxyl group, (B) a vinyl monomer containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, (C) a nonionic vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and (D) a vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group; and a pigment.

Yet another aspect of the present invention is a copolymer for dispersion, comprising a copolymer of a monomer mixture including (A) a vinyl monomer containing a carboxyl group, (B) a vinyl monomer containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, (C) a nonionic vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and (D) a vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group.
Yet another aspect of the present invention is a method for producing a dispersion, comprising: synthesizing a copolymer using a monomer mixture containing (A) a vinyl monomer containing a carboxyl group; (B) a vinyl monomer containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group; (C) a nonionic vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more; and (D) a vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group; dispersing a dispersoid using the copolymer; and crosslinking the copolymer in the presence of the dispersoid.

According to one embodiment of the present invention, it is possible to provide an aqueous inkjet ink with excellent storage stability.

The present invention will be described below using one embodiment. The examples in the following embodiment do not limit the present invention.

(Water-based inkjet ink)
The aqueous inkjet ink according to one embodiment comprises a pigment, a dispersant, a water-soluble solvent, and water, and is characterized in that the dispersant is a copolymer of a monomer mixture including a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and a vinyl monomer (D) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group, and has a crosslinked structure.
According to this embodiment, it is possible to provide an aqueous inkjet ink having excellent storage stability. Moreover, this aqueous inkjet ink can maintain good storage stability even when a low-polarity solvent is used. Furthermore, by using a low-polarity solvent, this aqueous inkjet ink can print on a non-permeable substrate with good image quality.

Hereinafter, the aqueous inkjet ink will be simply referred to as "aqueous ink" or "ink", and the components forming the copolymer will be respectively referred to as monomer (A), monomer (B), monomer (C), and monomer (D). Furthermore, the polyoxyalkylene chain can be formed by addition polymerization of alkylene oxide, and the number of moles added represents the number of repetitions of alkylene oxide forming the polyoxyalkylene chain in the monomer, and the number of moles added of alkylene oxide will be simply referred to as "AO addition mole number".
In this disclosure, (meth)acrylic acid collectively refers to methacrylic acid, acrylic acid, or a combination thereof, (meth)acrylate collectively refers to methacrylate, acrylate, or a combination thereof, and (meth)acrylamide collectively refers to methacrylamide, acrylamide, or a combination thereof.

By using the dispersant according to one embodiment, it is possible to provide an aqueous ink that has high initial dispersibility as well as excellent long-term storage stability. Since the dispersant has a crosslinked structure, even if the ink contains a low-polarity solvent, the crosslinked structure suppresses the dispersant from being detached from the pigment, and excellent storage stability can be obtained over a long period of time.
The detailed principle is explained below, but the present invention is not limited to this theory.
In one embodiment, the dispersant is a copolymer having a crosslinked structure, and therefore the dispersant that coats the pigment is crosslinked in a copolymer-copolymer or copolymer-pigment surface due to the crosslinked structure, so that even if a low-polarity solvent is contained, it is possible to prevent the dispersant from being detached from the pigment surface. This makes it possible to prevent the state of the ink from changing even when the ink is stored for a long period of time, and to maintain good jettability.
The carboxy group derived from the monomer (A) acts as a base point for the crosslinking reaction. The carboxy group has excellent reactivity and can react with polyfunctional compounds such as polyfunctional epoxy, oxazoline, and aziridine to form a crosslinked structure.
The β-diketone group and/or biphenyl group derived from monomer (B) strongly interacts with the pigment surface through π-π interactions, hydrogen bonds, etc., and can increase the adsorption force of the dispersant to the pigment. The improvement in the adsorption force to the pigment is also useful for the formation of a crosslinked structure between the copolymer and the pigment surface.
The steric repulsion effect of the long polyoxyalkylene chains derived from monomers (C) and (D) can suppress cross-linking between pigments. The sulfo group derived from monomer (D) is less likely to participate in a cross-linked structure than the carboxy group derived from monomer (A), so that the sulfo group derived from monomer (D) is oriented outward in the state of the dispersant adsorbed on the pigment, thereby improving the pigment dispersibility by electrostatic repulsion.

The ink preferably contains a pigment and a dispersant. In the ink, the pigment and the dispersant may form a pigment complex. In the pigment complex, the dispersant is preferably oriented on the surface of the pigment, the dispersant is preferably attached to the surface of the pigment, and the pigment surface is preferably partially or entirely covered with the dispersant.
The dispersant preferably has a crosslinked structure. When a pigment complex is formed by the pigment and the dispersant, the pigment complex preferably has a crosslinked structure.
In the dispersant or pigment complex, the crosslinked structure is preferably crosslinked with a carboxy group derived from the monomer (A) of the dispersant as the base point. The crosslinked structure of the dispersant may be crosslinked between multiple carboxy groups present in one molecule of the copolymer, or may be crosslinked between multiple copolymers with a carboxy group as the base point. For example, it is preferable that the copolymers are crosslinked with each other in a state where the copolymer is attached to the pigment to form the dispersant. This makes it possible to further strengthen the state in which the dispersant covers the pigment and prevent the dispersant from being detached from the pigment.
In the pigment complex, a crosslinked structure may be present between the copolymer and the pigment. Some pigments have groups having crosslinking reactivity, such as carboxyl groups, on the pigment surface. In such pigments, a crosslinked structure can be formed between the pigment and the copolymer. For example, it is preferable that the pigment-copolymer bond is crosslinked in a state where the copolymer is attached to the pigment. This makes it possible to more firmly coat the pigment with the dispersant and prevent the dispersant from being detached from the pigment.
By combining the above-mentioned crosslinking between copolymers and crosslinking between the pigment and the copolymer, the dispersant can more firmly coat the pigment, making it possible to more effectively prevent the dispersant from being detached from the pigment.

Pigments that can be used include organic pigments such as azo pigments, phthalocyanine pigments, polycyclic pigments, and dye lake pigments, as well as inorganic pigments such as carbon black and metal oxides. Azo pigments include soluble azo lake pigments, insoluble azo pigments, and condensed azo pigments. Phthalocyanine pigments include metal phthalocyanine pigments and metal-free phthalocyanine pigments. Polycyclic pigments include quinacridone pigments, perylene pigments, perinone pigments, isoindoline pigments, isoindolinone pigments, dioxazine pigments, thioindigo pigments, anthraquinone pigments, quinophthalone pigments, metal complex pigments, and diketopyrrolopyrrole (DPP). Carbon blacks include furnace carbon black, lamp black, acetylene black, and channel black. Metal oxides include titanium oxide and zinc oxide.

The above-mentioned pigments may be surface-treated. In particular, pigments whose surfaces have been subjected to an oxidation treatment are more preferred because they have more crosslinking base points on the pigment surface.
The average particle size (volume basis) of the pigment is preferably 300 nm or less, and more preferably 200 nm or less, from the viewpoints of jetting ability and dispersion stability.

The above pigments may be used alone or in combination of two or more kinds.
The blending amount of the pigment may be appropriately adjusted, but is preferably from 1 to 30% by mass, more preferably from 2 to 20% by mass, and even more preferably from 5 to 15% by mass, based on the total amount of the ink.

As the dispersant, a copolymer of a monomer mixture containing at least the following components and having a crosslinked structure can be used.
A vinyl monomer containing a carboxy group (A).
A vinyl monomer (B) containing at least one member selected from the group consisting of a β-dicarbonyl group and a biphenyl group.
A nonionic vinyl monomer (C) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added thereto.
A vinyl monomer (D) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added, and a sulfo group.

The copolymer that forms the dispersant will now be described.
In the copolymer, the vinyl monomer can preferably be a compound containing an ethylenically unsaturated bond. Examples of the vinyl monomer include vinyl acetate, vinyl chloride, allyl compounds, ethylene, (meth)acrylic acid, (meth)acrylate, (meth)acrylamide, acrylonitrile, and derivatives thereof. Various functional groups can be introduced into these compounds and used as each monomer.

Monomer (A) is a vinyl monomer containing a carboxy group.
By forming a copolymer using a vinyl monomer containing a carboxy group, a copolymer is formed with other monomer components, and carboxy groups serving as base points of crosslinked structures are introduced in a well-balanced manner into the copolymer. The carboxy groups introduced into the copolymer can react with various polyfunctional compounds to form crosslinked structures.

As the monomer (A), a compound containing a carboxy group and an ethylenically unsaturated bond can be preferably used. In the monomer (A), it is sufficient that one carboxy group is contained in one molecule, and two or more carboxy groups may be contained.

In the copolymer, the carboxy group introduced by the monomer (A) is preferably located near the main chain from the viewpoint of crosslinking reactivity. For example, it is preferable that the carboxy group is directly introduced to a carbon atom of the main chain. Examples of such monomer (A) include methacrylic acid, acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, etc. Among them, acrylic acid, methacrylic acid, or a combination thereof is preferable.

In the case where a carboxy group -COOH is introduced to the carbon atom C of the main chain of the monomer (A) via a linker L, it is preferable that the linker L has a group having at least one selected from the group consisting of a linear or branched chain hydrocarbon group, an alicyclic hydrocarbon group, an aromatic hydrocarbon group, an ether chain, and an ester chain, or a group having a combination of these. The total number of carbon atoms in the linker L is preferably 1 to 12, and more preferably 1 to 8.

Examples of the monomer (A) in which a carboxy group -COOH is introduced to the carbon atom C of the main chain via a linker L include β-carboxyethyl (meth)acrylate, 4-[2-(methacryloyloxy)ethoxy]-4-oxo-2-butenoic acid, 2-acryloyloxyethyl succinic acid, 2-acryloyloxyethyl hexahydrophthalic acid, 2-acryloyloxypropyl phthalic acid, 2-acryloyloxypropyl hexahydrophthalic acid, methacryloyloxymethyl succinic acid, methacryloyloxyethyl succinic acid, methacryloyloxyethyl phthalic acid, methacryloyloxyethyl hexahydrophthalic acid, methacryloyloxypropyl phthalic acid, and methacryloyloxypropyl hexahydrophthalic acid.
In the copolymer, the monomer (A) may be used alone or in combination of two or more kinds.

The carboxy group derived from the monomer (A) may be neutralized from the viewpoint of improving electrostatic repulsion. The carboxy group derived from the monomer (A) can be neutralized using a water-soluble base compound as a neutralizing agent. From the viewpoint of the stability of the dispersant having a carboxy group, the water-soluble base compound is preferably a monovalent one. The monovalent water-soluble base compound neutralizes the carboxy group to exhibit an electrostatic repulsion effect, and can further suppress aggregation of pigments.
Examples of the water-soluble basic compound include inorganic bases such as ammonium hydroxide, sodium hydroxide, and potassium hydroxide; and amines such as aminomethylpropanol, aminoethylpropanol, dimethylethanolamine, triethylamine, diethylethanolamine, dimethylaminopropanol, and triethanolamine.

The degree of neutralization of the carboxyl groups derived from the monomer (A) is preferably from 40 to 90 mol %.
The degree of neutralization of the carboxyl groups derived from the monomer (A) is preferably 40 mol % or more, and more preferably 50 mol % or more, which can increase the electrostatic repulsion in the dispersant and maintain good initial dispersibility.
The degree of neutralization of the carboxy group derived from the monomer (A) is preferably 90 mol % or less, more preferably 80 mol % or less, and even more preferably 60 mol % or less, which can increase the reactivity of the carboxy group with the polyfunctional compound or prevent the deactivation of the polyfunctional compound, thereby further increasing the crosslinking reactivity.
In addition, a degree of neutralization of 100% indicates that an equivalent or greater amount of a water-soluble basic compound has been added relative to the molar equivalent number of the carboxy groups derived from monomer (A), but some of the carboxy groups derived from monomer (A) remain unneutralized and can participate in a crosslinked structure.

In this disclosure, "degree of neutralization of carboxy groups" refers to the ratio (mol %) of the number of molar equivalents of water-soluble base compounds to the number of molar equivalents of carboxy groups before neutralization.

Monomer (B) is a vinyl monomer containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group.
Monomer (B) may be a vinyl monomer containing a β-dicarbonyl group, a vinyl monomer containing a biphenyl group, or a vinyl monomer containing a β-dicarbonyl group and a biphenyl group.
The β-dicarbonyl group introduced into the copolymer, which originates from a vinyl monomer containing a β-dicarbonyl group, is adsorbed to the polar group on the pigment surface through hydrogen bonding, thereby exhibiting pigment affinity.
The biphenyl group introduced into the copolymer, which originates from a vinyl monomer containing a biphenyl group, interacts with and is adsorbed to the π electrons on the pigment surface, thereby exhibiting pigment affinity.
In this way, by introducing a portion exhibiting pigment affinity into the copolymer, the pigment dispersibility of the dispersant formed by the copolymer can be improved, and in particular, the pigment can be finely dispersed in the ink at the initial stage, which is useful for applying the ink components more uniformly to the printed matter.
From the viewpoint of water resistance of the printed matter, it is preferable to use a vinyl monomer containing a biphenyl group. By introducing a biphenyl group into the dispersant, when the printed matter is rubbed in a moist state, the dispersant is less likely to dissolve in water, and high water rub resistance can be obtained.

As the vinyl monomer containing a β-dicarbonyl group, a compound containing a β-dicarbonyl group and an ethylenically unsaturated bond can be preferably used.
The β-dicarbonyl group can be a β-diketone group (-C(=O)-C-C(=O)-), a β-keto acid ester group (-C(=O)-C-C(=O)OR, where R is a hydrocarbon group), or a combination thereof. Examples of the β-diketone group include an acetoacetyl group and a propionoacetyl group, and examples of the β-keto acid ester group include an acetoacetoxy group and a propionoacetoxy group.
In the vinyl monomer containing a β-dicarbonyl group, it is sufficient that one molecule contains one β-dicarbonyl group, but two or more β-dicarbonyl groups may be contained.
As the vinyl monomer containing a β-dicarbonyl group, a compound in which a β-dicarbonyl group has been introduced into (meth)acrylic acid, (meth)acrylate, (meth)acrylamide, an allyl compound, ethylene or a derivative thereof can be used. Among them, a (meth)acrylate having a β-dicarbonyl group is preferred. This makes it possible to provide a copolymer in which the main chain is a (meth)acrylic skeleton.

Examples of vinyl monomers containing a β-dicarbonyl group include acetoacetoxyalkyl (meth)acrylates such as acetoacetoxyethyl (meth)acrylate, acetoacetoxypropyl (meth)acrylate, and acetoacetoxybutyl (meth)acrylate; ethylene glycol monoacetoacetate mono(meth)acrylate, 2,3-di(acetoacetoxy)propyl (meth)acrylate, and 2,4-hexadione (meth)acrylate; allyl acetoacetate; vinyl acetoacetate; and acetoacetoxyalkyl (meth)acrylamides such as acetoacetoxyethyl (meth)acrylamide.

An example of a commercially available vinyl monomer containing a β-dicarbonyl group is "ethylene glycol monoacetoacetate monomethacrylate" manufactured by Tokyo Chemical Industry Co., Ltd.

As the vinyl monomer containing a biphenyl group, a compound containing a biphenyl group and an ethylenically unsaturated bond can be preferably used.
The biphenyl group may be substituted or unsubstituted, but is preferably unsubstituted. The bonding site of the biphenyl group may be any of the o-position, m-position, and p-position, but is preferably the o-position.
In the vinyl monomer containing a biphenyl group, it is sufficient that one biphenyl group is contained in one molecule, but two or more biphenyl groups may be contained.
As the vinyl monomer containing a biphenyl group, a compound in which a biphenyl group is introduced into (meth)acrylic acid, (meth)acrylate, (meth)acrylamide, an allyl compound, ethylene or a derivative thereof can be used. Among them, a (meth)acrylate having a biphenyl group is preferable. This makes it possible to provide a copolymer in which the main chain is a (meth)acrylic skeleton.

Examples of vinyl monomers containing a biphenyl group include biphenyl (meth)acrylate and ethoxylated o-phenylphenol (meth)acrylate.

Biphenyl (meth)acrylate can be synthesized by an esterification reaction between biphenyl alcohol and (meth)acrylic acid.
The ethoxylated o-phenylphenol (meth)acrylate can be synthesized by an esterification reaction between a polyethylene glycol-modified biphenyl alcohol having an added mole number of ethylene oxide of 13 or less and (meth)acrylic acid. For example, it may be an esterification product of a biphenyl alcohol modified with ethylene glycol and (meth)acrylic acid.

An example of a commercially available vinyl monomer containing a biphenyl group is "A-LEN-10" (product name) manufactured by Shin-Nakamura Chemical Co., Ltd.

As the monomer (B), a vinyl monomer containing the above-mentioned β-dicarbonyl and biphenyl groups in combination may be used.
In the copolymer, the monomer (B) may be used alone or in combination of two or more kinds.
When higher on-machine stability is required, it is preferable to select an appropriate monomer (B) depending on the type of pigment. For example, a vinyl monomer containing a β-dicarbonyl group is preferable for a quinacridone pigment, and a vinyl monomer containing a β-dicarbonyl group and a vinyl monomer containing a biphenyl group are preferably used in combination for a phthalocyanine pigment.

Monomer (C) is a nonionic vinyl monomer containing a polyoxyalkylene chain with 15 or more moles of alkylene oxide (AO) added.

In the polyoxyalkylene chain, the number of moles of AO added is preferably 15 or more, more preferably 18 or more, and even more preferably 20 or more. This makes it possible to better maintain dispersion stability in the aqueous ink not only when the ink contains a large amount of water, but also when the water content has evaporated and decreased.
In the polyoxyalkylene chain, the number of moles of AO added is preferably not more than 50, more preferably not more than 45, and even more preferably not more than 40. This makes it possible to prevent the dispersant itself from thickening in the aqueous ink when the amount of water blended is small or when the amount of water has evaporated and become small, thereby preventing an increase in the viscosity of the entire ink.
In the monomer (C), the number of moles of AO added to the polyoxyalkylene chain is preferably 15-50, more preferably 18-45, and even more preferably 20-35.

As the nonionic vinyl monomer containing a polyoxyalkylene chain having an AO addition mole number of 15 or more, a compound containing a polyoxyalkylene chain having an AO addition mole number of 15 or more and an ethylenically unsaturated bond can be preferably used.
As the polyoxyalkylene chain, a linking chain in which one or more types of alkylene oxide groups are continuously linked can be used.
The alkylene oxide group preferably has 1 to 10 carbon atoms, more preferably has 1 to 8 carbon atoms, even more preferably has 2 to 4 carbon atoms, and still more preferably has 2 or 3 carbon atoms.
Examples of the alkylene oxide group include a methylene oxide group, an ethylene oxide group, a propylene oxide group, a butylene oxide group, etc. Among these, an ethylene oxide group, a propylene oxide group, or a combination thereof is preferred, and an ethylene oxide group is more preferred.
Specifically, the monomer (C) preferably has a polyethylene glycol chain, a polypropylene glycol chain, a polyethylene propylene glycol chain, or the like, with the AO addition mole number being 15 or more, and more preferably a polyethylene glycol chain, which can further increase the hydrophilicity of the dispersant in the aqueous ink and further improve the dispersion stability.

The molecular weight of monomer (C) is preferably 800 to 3000, more preferably 1000 to 2000.

The monomer (C) is preferably nonionic, and specifically, it is preferred that no ionic group is introduced. For example, the terminal of the polyalkylene glycol chain of the monomer (C) may be a hydroxyl group, or a nonionic functional group may be introduced into the hydroxyl group. Specifically, a hydrocarbon group may be introduced into the hydroxyl group at the terminal of the polyalkylene glycol chain.
The hydrocarbon group is preferably an alkyl group having 1 to 20 carbon atoms, or an aryl group which may be substituted with an alkyl group having 1 to 10 carbon atoms, and more preferably an alkyl group having 1 to 10 carbon atoms.
The alkyl group having 1 to 10 carbon atoms is preferably a linear or branched alkyl group having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, and more preferably 1 to 4 carbon atoms. Examples of the alkyl group include a methyl group, an ethyl group, a trimethyl group, a propyl group, an n-butyl group, a tert-butyl group, and a sec-butyl group, and preferably a methyl group or an ethyl group.

Monomer (C) may contain one polyoxyalkylene chain having 15 or more moles of AO added in one molecule, and may contain two or more polyoxyalkylene chains having 15 or more moles of AO added in one molecule.
As the monomer (C), a compound in which a polyoxyalkylene chain having an AO addition mole number of 15 or more is introduced into (meth)acrylic acid, (meth)acrylate, (meth)acrylamide, or a derivative thereof can be used, which makes it possible to provide a copolymer in which the main chain is a (meth)acrylic skeleton.
Examples of the polyalkylene glycol-modified (meth)acrylate include ethers of (meth)acrylic acid and polyalkylene glycol, and polyalkylene glycol-modified (meth)acrylates. The polyalkylene glycol-modified (meth)acrylates can be obtained, for example, by reacting polyalkylene glycol with a (meth)acrylate having a functional group that serves as a starting point for an isocyanate group or the like introduced therein.

Specific examples of monomer (C) include polyethylene glycol mono(meth)acrylate, methoxypolyethylene glycol mono(meth)acrylate, ethoxypolyethylene glycol mono(meth)acrylate, octoxypolyethylene glycol mono(meth)acrylate, stearoxypolyethylene glycol mono(meth)acrylate; polyethylene glycol modified 2-isocyanatoethyl(meth)acrylate, polypropylene glycol mono(meth)acrylate, polyethylene glycol-propylene glycol-mono(meth)acrylate, polyethylene glycol-trimethylene glycol-mono(meth)acrylate; polyethylene glycol-allyl ether, methoxypolyethylene glycol-allyl ether, polyethylene glycol-polypropylene glycol-allyl ether, polypropylene glycol-allyl ether, polyethylene glycol-diallyl ether, polypropylene glycol-diallyl ether; methoxypolyethylene glycol acrylamide, etc.

Examples of commercially available products of the monomer (C) include "ADEKA REASOAP ER-20" manufactured by ADEKA CORPORATION, "BLEMMER PME 1000" manufactured by NOF CORPORATION, NK ESTER M-230G (product name) manufactured by Shin-Nakamura Chemical Co., Ltd., "BLEMMER PME-4000" manufactured by NOF CORPORATION, and "LIGHT ESTER 041MA" manufactured by Kyoeisha Chemical Co., Ltd. (all of which are product names).
In the copolymer, the monomer (C) may be used alone or in combination of two or more kinds.

Monomer (D) is a vinyl monomer containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide (AO) added, and a sulfo group.
In the dispersant, the sulfo group derived from the monomer (D) exhibits a high electrostatic repulsion effect, and high dispersibility of the pigment can be obtained even when a low-polarity solvent is contained. On the other hand, if sufficient repulsion is obtained only with the sulfo group derived from the monomer (D), the repulsive effect between the sulfo groups derived from the monomer (D) in the molecule becomes strong, which may prevent the units derived from the monomer (A) and the units derived from the monomer (B) from adsorbing to the pigment. Therefore, by introducing a nonionic polyoxyalkylene chain derived from the monomer (C), the repulsive effect between the sulfo groups derived from the monomer (D) can be reduced to some extent.

In the polyoxyalkylene chain, the number of moles of AO added is preferably 15 or more, more preferably 18 or more, and even more preferably 20 or more. This makes it possible to better maintain dispersion stability in the aqueous ink not only when the ink contains a large amount of water, but also when the water content has evaporated and decreased.
In the polyoxyalkylene chain, the number of moles of AO added is preferably not more than 50, more preferably not more than 45, and even more preferably not more than 40. This makes it possible to prevent the dispersant itself from thickening in the aqueous ink when the amount of water blended is small or when the amount of water has evaporated and become small, thereby preventing an increase in the viscosity of the entire ink.
In the monomer (D), the number of moles of AO added to the polyoxyalkylene chain is preferably 15-50, more preferably 18-45, and even more preferably 20-40.

As the vinyl monomer containing a polyoxyalkylene chain having an AO addition mole number of 15 or more, a compound containing a polyoxyalkylene chain having an AO addition mole number of 15 or more and an ethylenically unsaturated bond can be preferably used.
As the polyoxyalkylene chain, a linking chain in which one or more types of alkylene oxide groups are continuously linked can be used.
The alkylene oxide group preferably has 1 to 10 carbon atoms, more preferably has 1 to 8 carbon atoms, further preferably has 2 to 4 carbon atoms, and even more preferably has 2 or 3 carbon atoms. Details of the preferred alkylene oxide group and polyethylene glycol chain are as described above in regard to Monomer (C).

The molecular weight of monomer (D) is preferably 1,000 to 4,000, and more preferably 1,200 to 3,000.

Monomer (D) is preferably one in which a sulfo group has been introduced to the hydroxy group at the end of the polyalkylene glycol chain. By introducing a sulfo group to the end of the polyoxyalkylene chain, it becomes less likely to react with the crosslinking reactive compound, and the electrostatic repulsion effect is more likely to be expressed in the dispersant even after the crosslinking reaction.

The sulfo group derived from monomer (D) has a lower crosslinking reactivity than the carboxy group derived from monomer (A). Since the sulfo group derived from monomer (D) remains in the dispersant without participating in the crosslinking reaction, it exhibits a high electrostatic repulsion effect, and high dispersibility of the pigment can be obtained even when a low-polarity solvent is contained.
The sulfo group may form a salt with a counter ion such as NH ₄ , Na, K, Li, or an organic amine.

As a method for introducing a sulfo group into the monomer (D), there is a method in which a monomer having a polyoxyalkylene chain with an AO addition mole number of 15 or more is treated with a chemical modifying agent.
Examples of chemical modifying agents for introducing a sulfo group include sulfamic acid, trichloroethyl sulfate, and phenyl sulfate.

Monomer (D) may contain one polyoxyalkylene chain having 15 or more moles of AO added in one molecule, and may contain two or more polyoxyalkylene chains having 15 or more moles of AO added in one molecule.
As the monomer (D), a compound in which a polyoxyalkylene chain having an AO addition mole number of 15 or more is introduced into (meth)acrylic acid, (meth)acrylate, (meth)acrylamide, or a derivative thereof can be used, which makes it possible to provide a copolymer in which the main chain is a (meth)acrylic skeleton.
For example, in an ether of (meth)acrylic acid and a polyalkylene glycol, or a (meth)acrylate modified with a polyalkylene glycol, a compound in which a sulfo group has been introduced into the hydroxy group of the polyalkylene glycol may be mentioned.

Specific examples of monomer (D) include ammonium sulfonate of an aliphatic polyethylene glycol glyceryl ether having a vinyl group, ammonium sulfonate of an aromatic polyethylene glycol glyceryl ether having a vinyl group, ammonium sulfonate of a polyethylene glycol-α-methylbenzyl ether having a 1-propenyl group, ammonium sulfonate of a polyethylene glycol-nonylphenyl ether having a 1-propenyl group, and ammonium sulfonate of a polyethylene glycol-modified 2-isocyanatoethyl (meth)acrylate.
As a specific example of the monomer (D), a monomer in which a terminal of a polyalkylene glycol chain is chemically modified to a hydroxyl group to introduce a sulfo group can be used from among the specific examples of the monomer (C) described above.

Commercially available products of monomer (D) include, for example, "ADEKA REASOAP SR series, SE series" (manufactured by ADEKA CORPORATION), "AQUALON AR series, BC series, KH series" (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and the like (all trade names).
In the copolymer, the monomer (D) may be used alone or in combination of two or more kinds.

The copolymer for forming the dispersant may be formed using, in addition to the above-mentioned components, other monomers. Hereinafter, the other monomers may also be simply referred to as monomer (E).
A hydrophobic monomer can be used as the monomer (E), which allows the dispersant formed by the copolymer to have a well-balanced composition of hydrophilic and hydrophobic moieties, thereby further improving the dispersion stability.
Examples of the hydrophobic monomer include (meth)acrylates and aromatic ring-containing monomers, and preferably (meth)acrylates.

Specific examples of other monomers (E) include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate (e.g., n- or isopropyl (meth)acrylate), butyl (meth)acrylate (e.g., n-, iso, sec- or tert-butyl (meth)acrylate), pentyl (meth)acrylate (e.g., n-, iso, or neopentyl (meth)acrylate), 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate (e.g., n- or isooctyl (meth)acrylate), decyl (meth)acrylate (e.g., n- or isodecyl (meth)acrylate), dodecyl (meth)acrylate (e.g., n- or isododecyl (meth)acrylate), stearyl (meth)acrylate (e.g., n- or isostearyl (meth)acrylate), cyclohexyl (meth)acrylate, and benzyl (meth)acrylate.
Further, specific examples of the other monomer (E) include styrene-based monomers such as styrene and α-methylstyrene; vinyl ether-based polymers such as vinyl acetate, vinyl benzoate, and butyl vinyl ether; maleic acid esters; fumaric acid esters; acrylonitrile; methacrylonitrile; and α-olefins.
In the copolymer, the monomer (E) may be used alone or in combination of two or more kinds.

It is preferable to design the dispersant so that the outermost layer of the dispersant is an ionic polyoxyalkylene chain derived from monomer (D) when the dispersant is adsorbed on the pigment surface, since electrostatic repulsion is easily caused by the sulfo group. Therefore, it is more preferable that the polyoxyalkylene chain derived from monomer (D) is equal to or longer than the polyoxyalkylene chain derived from monomer (C).
In the copolymer, the ratio of the number of moles of AO added to the polyoxyalkylene chain of monomer (C) to the number of moles of AO added to the polyoxyalkylene chain of monomer (D) is preferably C:D=1:0.8 to 1:2, more preferably 1:0.8 to 1:1.5.
The ratio of the number of moles added, "C:D", is preferably 1:0.8 or more, more preferably 1:1 or more, and even more preferably 1:1.2 or more, based on the number of moles added of AO in monomer (D).
The ratio of the number of moles added, "C:D", is preferably 1:2 or less, and more preferably 1:1.5 or less, based on the number of moles added of AO in the monomer (D).
Within the above range, the initial pigment dispersibility of the dispersant can be further improved, and the scratch resistance of the printed matter can be further improved.

The number of moles of AO added to the polyoxyalkylene chain of monomer (C) and the number of moles of AO added to the polyoxyalkylene chain of monomer (D) are each preferably independently 40 or less. When at least one of the number of moles of AO added to the polyoxyalkylene chain of monomer (C) and the number of moles of AO added to the polyoxyalkylene chain of monomer (D) is 40 or less, and more preferably when both are 40 or less, it is preferable that the ratio of the above-mentioned added moles "C:D" is 1:0.8 to 1:2, particularly 1:1 to 1:1.5.

In the copolymer, the units derived from monomer (C) and the units derived from monomer (D) have a molar ratio "C:D" of preferably 1:3 to 1:0.3, more preferably 1:2 to 1:0.8, and even more preferably 1:1.5 to 1:0.9. This can improve the pigment dispersibility in the initial state and the long-term dispersion stability, particularly the dispersion stability after water evaporation due to leaving the head standing, etc.

As components forming the copolymer, monomer (A) is preferably 10 to 50 mol % relative to the total amount of the copolymer, monomer (B) is preferably 20 to 70 mol %, monomer (C) is preferably 10 to 30 mol %, and monomer (D) is preferably 10 to 30 mol %. Furthermore, when another monomer (E) is used in the synthesis of the copolymer, monomer (E) is preferably 10 to 30 mol % relative to the total amount of the copolymer.

The dispersant formed by the copolymer of the above-mentioned monomer mixture can contain units (A) having a carboxy group, units (B) containing one or more selected from the group consisting of a β-dicarbonyl group and a biphenyl group, nonionic units (C) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added, units (D) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added and a sulfo group, and other units (E) which are selectively introduced.
As an example of this copolymer, it is preferable that the main chain has a (meth)acrylic skeleton.

For this copolymer, the respective components are preferably blended in the following proportions.
The content of the units derived from the monomer (A) is preferably 10 to 50 mol%, more preferably 20 to 40 mol%, and even more preferably 20 to 30 mol% based on the total amount of the copolymer. When the content of the units derived from the monomer (A) is 10 mol% or more, the dispersant can contain a crosslinked structure at a sufficient ratio. When the content of the units derived from the monomer (A) is 50 mol% or less, the water solubility of the dispersant can be prevented from becoming high, and the initial dispersibility can be further improved.
The unit derived from the monomer (B) is preferably from 20 to 70 mol %, more preferably from 30 to 60 mol %, and even more preferably from 30 to 50 mol %, based on the total amount of the copolymer.
The unit derived from the monomer (C) preferably accounts for 10 to 30 mol %, more preferably 10 to 20 mol %, based on the total amount of the copolymer.
The unit derived from the monomer (D) preferably accounts for 10 to 30 mol %, more preferably 10 to 20 mol %, based on the total amount of the copolymer.
The unit derived from the monomer (E) preferably accounts for 10 to 30 mol %, more preferably 10 to 20 mol %, based on the total amount of the copolymer.
Here, the amount of each unit can be calculated from the total amount of monomers added to the synthesis system.

The total amount of the units derived from monomer (C) and the units derived from monomer (D) is preferably 10 to 80 mol %, more preferably 20 to 50 mol %, and even more preferably 20 to 40 mol %, based on the total amount of the copolymer.

The ink contains the above-mentioned pigment and dispersant, and the dispersant is a copolymer having a crosslinked structure. In the ink, the pigment and dispersant may form a pigment complex, and the pigment complex may have a crosslinked structure. Such a crosslinked structure strengthens the bonds between the copolymers, making it possible to suppress the dissolution of the dispersant into the solvent. In addition, the dispersant coats the pigment more firmly, preventing the dispersant from being detached from the pigment, and therefore suppressing the dissolution of the dispersant into the solvent.
The crosslinked structure of the dispersant can be obtained, for example, by adding a crosslinking agent to a pigment in a state in which the copolymer is adhered to the pigment, and carrying out a crosslinking reaction using the carboxy group derived from the monomer (A) of the copolymer as the base point.
The crosslinking agent may be a polyfunctional compound, such as a compound having two or more functional groups reactive with a carboxy group.

The polyfunctional compound preferably has 2 to 6 reactive groups in one molecule, more preferably 3 to 5 reactive groups, and even more preferably 4 or 5 reactive groups.
When the polyfunctional compound has two or more reactive groups in one molecule, more preferably three or more reactive groups, and even more preferably four or more reactive groups in one molecule, the density of the three-dimensional network structure formed by the crosslinked structure of the dispersant is increased, making it possible to better prevent the dissolution of the dispersant into the solvent.
In the polyfunctional compound, the reactive group may be any group that is reactive with the carboxy group derived from the monomer (A) of the copolymer, but it is preferable that the polyfunctional compound has an epoxy group, more specifically a glycidyl group, as the reactive group.
Since the polyfunctional compound is used in the crosslinking reaction with the copolymer in an aqueous solvent, it is preferable that the polyfunctional compound exhibits hydrophilicity. For example, it is preferable that a hydrophilic group such as a hydroxyl group is introduced into the polyfunctional compound.

Examples of polyfunctional compounds include epoxy compounds, oxazoline compounds, aziridine compounds, carbodiimide compounds, etc. These can be used alone or in combination of two or more.

Examples of the epoxy compounds include bifunctional epoxy compounds such as ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, and 1,6-hexanediol diglycidyl ether; trifunctional epoxy compounds such as glycerol triglycidyl ether; tetrafunctional epoxy compounds such as pentaerythritol polyglycidyl ether; and polyfunctional epoxy compounds optionally having a hydroxy group such as glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitol polyglycidyl ether, and trimethylolpropane polyglycidyl ether.
Among these, polyfunctional epoxy compounds having a hydroxy group are preferred, and polyglycerol polyglycidyl ether is more preferred.

Commercially available epoxy compounds include, for example, "Denacol EX-201, EX-211, EX-212, EX-313, EX-314, EX-321, EX-411, EX-421, EX-512, EX-521, EX-611, EX-612, EX-614, EX-614B" and "EX-200, EX-800, EX-900 series" manufactured by Nagase ChemteX Corporation; and "Epolite 40E, 100E, 200E, 400E, 70P, 200P, 400P, 1500NP, 1600, 80MF" manufactured by Kyoeisha Chemical Co., Ltd. (all trade names).

As the oxazoline-based compound, for example, a polymer having an oxazoline group is preferable. As the polymer having an oxazoline group, a polymer obtained by polymerizing a monomer component containing an addition-polymerizable oxazoline as an essential component is preferable. Examples of the addition-polymerizable oxazoline group-containing monomer include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline.

Examples of commercially available oxazoline compounds that can be used include "Epocross WS-300, 500, 700," "Epocross K-1010, 1020, 1030E," and "Epocross K-2010, 2020, 2030" manufactured by Nippon Shokubai Co., Ltd. (all trade names).

The aziridine compound is a compound having an aziridine group in the molecule, and it is preferable to use a polyfunctional aziridine compound having two or more aziridine groups in the molecule.
For example, 2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate], 4,4-bis(ethyleneiminocarbonylamino)diphenylmethane, etc. can be used.
As commercially available aziridine compounds, for example, "PZ-33" and "DZ-22E" from the Chemitite series manufactured by Nippon Shokubai Co., Ltd. can be used (all trade names).

The carbodiimide compound is a compound having a carbodiimide group represented by "-N=C=N-" in the molecule.
For example, cyclic carbodiimides, isocyanate-terminated carbodiimides, dicyclohexylcarbodiimide, diisopropylcarbodiimide, amino group-containing carbodiimides, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, Nt-butyl-N-ethylcarbodiimide, di-t-butylcarbodiimide, and the like can be used.

Examples of commercially available carbodiimide compounds that can be used include "V-02B" and "V-04K" from the Carbodilite series manufactured by Nisshin Chemical Corporation (both are product names).

When forming a crosslinked structure using the carboxy group derived from monomer (A) as the base point, it is preferable to use an epoxy-based compound as the crosslinking agent, and among these, an epoxy-based compound having 2 to 4 epoxy groups is preferred, and an epoxy-based compound having a hydrophilic group in addition to 2 to 4 epoxy groups is even more preferred.

In the dispersant, the molar equivalent of the reactive group of the crosslinking agent is preferably 10 to 50 mol% relative to the molar equivalent of the carboxy group derived from monomer (A). For example, the equivalent of the carboxy group is calculated from the amount of copolymer added to the reaction system, and a crosslinking agent is added whose equivalent of the reactive group satisfies the above range relative to this equivalent of the carboxy group.

The dispersant may be used in an amount of 0.1 to 10, more preferably 0.1 to 5, and even more preferably 0.2 to 3, based on the solid content of the pigment.
The dispersant is contained in an amount of preferably 0.5 to 20% by mass, more preferably 1 to 10% by mass, and even more preferably 1 to 5% by mass, in terms of solid content, relative to the total amount of the ink.
Here, the content of the dispersant is the total amount of the mass of the copolymer in the monomer mixture and the mass of the crosslinking agent.

The amount of the crosslinking agent is preferably 0.005 part by mass or more, and more preferably 0.02 part by mass or more, per part by mass of the copolymer before crosslinking.
On the other hand, the amount of the crosslinking agent is preferably 0.5 parts by mass or less, and more preferably 0.1 parts by mass or less, per part by mass of the copolymer before crosslinking.
In addition to the dispersant according to one embodiment, other dispersants may be contained in the ink as long as the effects of the present invention are not impaired.

The synthesis method of the dispersant is described below. Note that the dispersant according to one embodiment is not limited to the one synthesized by the synthesis method below, and any dispersant having the above-mentioned characteristics can be used.

An example of a method for synthesizing a dispersant includes synthesizing a copolymer using a monomer mixture containing at least a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and a vinyl monomer (D) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group, and crosslinking the copolymer. Preferably, the copolymer is crosslinked in the presence of a pigment. The monomer mixture may optionally contain another monomer (E).
This makes it possible to provide a dispersant having excellent dispersion stability.

Specifically, the copolymer can be synthesized by mixing monomer (A), monomer (B), monomer (C), monomer (D), and optionally added monomer (E) all at once or in portions, and allowing the reaction to proceed in a solvent as necessary.

The mixing ratio of each monomer in the monomer mixture for obtaining the copolymer may be appropriately adjusted and mixed so as to obtain a preferable molar ratio in the above-mentioned copolymer.
The polymerization may be any of random copolymerization, block copolymerization, and graft copolymerization. For example, a random copolymer can be obtained by polymerizing a raw material mixture containing each monomer component in a lump. In addition, a block copolymer can be obtained by a method of polymerizing one type of monomer first and then polymerizing another monomer to this polymer, a method of polymerizing one type of monomer first and block polymerizing the obtained multiple types of polymers, a method of controlling the polymerization reaction by living polymerization, etc. A graft copolymer can be obtained by a method of polymerizing a main chain copolymer using one or more types of monomers, introducing a reactive group into this copolymer, polymerizing another monomer from this reactive group, or a method of introducing a polymer polymerized using another monomer from this reactive group.

The polymerization can be carried out according to a solution polymerization method, a bulk polymerization method, a suspension polymerization method, an emulsion polymerization method, etc., and is preferably a solution polymerization method.
Examples of the polymerization solvent used in the solution polymerization include aliphatic alcohols having 1 to 3 carbon atoms, such as ethanol and propanol, ketones, such as acetone and methyl ethyl ketone, esters, such as ethyl acetate, etc. It is preferable that the polymerization solvent is capable of dissolving the monomer components and is removable after polymerization, and therefore it is preferable to use a volatile highly polar solvent as the polymerization solvent.

In the polymerization, various additives such as a radical polymerization initiator, a polymerization chain transfer agent, and a RAFT agent can be used.
As the radical polymerization initiator, for example, azo compounds such as 2,2'-azobisisobutyronitrile (AIBN), 2,2'-azobis(2,4-dimethylvaleronitrile), dimethyl-2,2'-azobisbutyrate, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(1-cyclohexanecarbonitrile) etc. can be preferably used. In addition, as the radical polymerization initiator, organic peroxides such as t-butylperoxyoctoate, di-t-butylperoxide, dibenzoyloxide etc. can be used.
The amount of the radical polymerization initiator added is preferably 0.1 to 5 parts by mass based on 100 parts by mass of the monomer.

Examples of polymerization chain transfer agents include mercaptans such as octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-tetradecyl mercaptan, and 2-mercaptoethanol; xanthogen disulfides such as dimethyl xanthogen disulfide and diisopropyl xanthogen disulfide; thiuram disulfides such as tetramethyl thiuram disulfide and tetrabutyl thiuram disulfide; halogens such as carbon tetrachloride and ethylene bromide. Examples of such compounds include unsaturated cyclic hydrocarbons such as fluorinated hydrocarbons, pentaphenylethane, and the like; unsaturated cyclic hydrocarbon compounds such as acrolein, methacrolein, allyl alcohol, 2-ethylhexyl thioglycolate, turbinolene, α-terpinene, γ-terpinene, dipentene, α-methylstyrene dimer, 9,10-dihydroanthracene, 1,4-dihydronaphthalene, indene, and 1,4-cyclohexadiene; and unsaturated heterocyclic compounds such as 2,5-dihydrofuran.

The polymerization conditions can be appropriately adjusted depending on the monomers used, additives such as a radical polymerization initiator, and the polymerization solvent.
In general, the polymerization temperature is preferably 30 to 120° C., more preferably 50 to 100° C. The polymerization time is preferably 10 minutes to 20 hours, more preferably 1 hour to 10 hours. The reaction atmosphere is preferably a non-oxidizing atmosphere, more preferably an inert atmosphere, specifically, a nitrogen atmosphere, an argon atmosphere, or the like.
In the case of bulk polymerization, the reaction may be carried out in the same manner as in solution polymerization, except that the reaction is carried out without a solvent, and the polymerization conditions may be the same as in solution polymerization.

The weight average molecular weight (Mw) of the dispersant before crosslinking is preferably about 5,000 to 100,000, more preferably 5,000 to 50,000. The number average molecular weight (Mn) of the dispersant is preferably about 3,000 to 100,000, more preferably 4,000 to 50,000. The molecular weight distribution Mw/Mn of the dispersant is preferably 1.0 to 2.0.
Here, the weight average molecular weight and the number average molecular weight are both values determined by the GPC method and converted into standard polystyrene.

Next, a method for crosslinking the obtained copolymer will be described. The crosslinking reaction is preferably carried out in the presence of the copolymer and the pigment. This forms a crosslinked structure with the copolymer attached to the pigment, which can further increase the adsorption of the dispersant to the pigment.
In one method, the obtained copolymer and the pigment are added to an aqueous solvent, mixed and dispersed to obtain a pre-pigment dispersion, and then a multifunctional compound is added as a crosslinking agent to the pre-pigment dispersion to carry out a crosslinking reaction. The multifunctional compound may be any of those mentioned above.
Specifically, a mixture containing a pigment, a copolymer, water, and a water-soluble solvent is dispersed using a dispersing machine such as a bead mill to obtain a pre-pigment dispersion, and then a polyfunctional compound, a water-soluble solvent for dilution, and water are added to the pre-pigment dispersion to obtain a pigment dispersion. The pre-pigment dispersion is preferably dispersed so that the average particle size of the pigment is about 10 to 200 nm. The obtained pigment dispersion may be filtered using a filter or the like. Various additives may also be added as appropriate.

The crosslinking reaction is preferably carried out under heating, and the temperature of the crosslinking reaction is preferably 50 to 70°C.
The time for the crosslinking reaction is not particularly limited, but may be from 5 to 20 hours.
In order to accelerate the crosslinking reaction, a reaction accelerator may be used, such as a tertiary amine, boric acid, a boric acid ester, an organic metal compound, or imidazole.

The resulting crosslinking reaction product may be used as is as an ink, or may be provided as an ink by adding a dilution solvent, or may be provided as an ink by removing the solvent used in the crosslinking reaction and adding an ink solvent.

Next, a method for neutralizing the carboxy group derived from the monomer (A) will be described. The step of neutralizing the carboxy group derived from the monomer (A) is preferably carried out by adding a neutralizing agent after obtaining a copolymer from a monomer mixture. The step of neutralizing the carboxy group derived from the monomer (A) is preferably carried out by adding a neutralizing agent before adding a crosslinking agent to carry out a crosslinking reaction.
In one method, the obtained copolymer and pigment are added to an aqueous solvent, and then a neutralizing agent is added, followed by mixing and dispersing to obtain a pre-pigment dispersion, and then a polyfunctional compound is added as a crosslinking agent to the pre-pigment dispersion to carry out a crosslinking reaction. The neutralizing agent and polyfunctional compound may be any of those described above.

The aqueous ink preferably contains water as an aqueous solvent, and the main solvent may be water.
The water is not particularly limited, but is preferably one that contains as few ionic components as possible. In particular, from the viewpoint of the pigment dispersion stability of the ink, it is preferable that the content of polyvalent metal ions such as calcium is small. As the water, for example, ion-exchanged water, distilled water, ultrapure water, etc. may be used.
From the viewpoint of adjusting the ink viscosity, the water content is preferably from 20% by mass to 90% by mass, more preferably from 30% by mass to 80% by mass, and even more preferably from 40% by mass to 70% by mass, based on the total amount of the ink.

The aqueous ink may contain a water-soluble solvent. The water-soluble solvent is preferably compatible with water. As the water-soluble solvent, an organic compound that is liquid at room temperature and can be dissolved or mixed in water may be used from the viewpoint of wettability and moisture retention, and it is preferable to use a water-soluble solvent that can be uniformly mixed with the same volume of water at 20° C. under 1 atmosphere.
The boiling point of the water-soluble solvent is preferably 170 to 250° C. When the boiling point of the water-soluble solvent is 170° C. or higher, more preferably 200° C. or higher, the evaporation of the water-soluble solvent from the ink can be suppressed, and the on-press stability can be further improved. When the boiling point of the water-soluble solvent is 250° C. or lower, the drying property of the printed matter can be further improved.

Examples of the water-soluble solvent include glycols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,2-propanediol, 1,3-propanediol, dipropylene glycol, tripropylene glycol, polypropylene glycol, 1,4-butanediol, 1,6-hexanediol, and 1,2-hexanediol; glycerins such as glycerin, diglycerin, triglycerin, and polyglycerin; acetins such as monoacetin and diacetin; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, and diethylene glycol monomethyl ether. Glycol ethers such as ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, and dipropylene glycol monomethyl ether; 1-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, β-thiodiglycol, and sulfolane can be used.
These may be used alone or in combination of two or more kinds so long as they form a single phase.

As the water-soluble solvent, it is preferable to use a solvent having an SP value calculated from the Fedors formula of less than 13 (cal/cm ³ ) ^1/2 . This can increase the wettability of the ink to the substrate, particularly to a non-permeable substrate, and prevent ink repellency and unevenness on the substrate. It can also further increase the wettability to the pigment and dispersant, and further improve the on-machine stability. From the same viewpoint, an alcohol-based solvent can be preferably used as the water-soluble solvent.
Among these, alcohol-based solvents having an SP value of less than 13 (cal/cm ³ ) ^1/2 are likely to solvate with the polyoxyalkylene chain contained in the dispersant structure, and are capable of maintaining good dispersion stability.
Examples of alcohol-based solvents having an SP value of less than 13 (cal/cm ³ ) ^1/2 include 1,2-butanediol, 1,2-hexanediol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monobenzyl ether, dipropylene glycol monomethyl ether, etc. These may be used alone or in combination of two or more in the aqueous ink. Furthermore, other water-soluble solvents may be used in combination with these alcohol-based solvents.

From the viewpoint of moisture retention, the composition may contain a water-soluble solvent having an SP value of 13 (cal/cm ³ ) ^1/2 or more. Examples of water-soluble solvents having an SP value of 13 (cal/cm ³ ) ^1/2 or more include ethylene glycol, diethylene glycol, dipropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, and 1,6-hexanediol.

From the viewpoints of wettability, moisturizing effect, viscosity adjustment, and the like, the water-soluble solvent may be contained in an amount of 1 to 80% by mass, more preferably 10 to 60% by mass, and even more preferably 20 to 50% by mass, based on the total amount of the ink.
The alcohol-based solvent having an SP value of less than 13 (cal/cm ³ ) ^1/2 can be contained in an amount of 1 to 50% by mass, more preferably 10 to 30% by mass, and even more preferably 10 to 20% by mass, based on the total amount of the ink.

In addition to the above-mentioned components, the water-based ink may optionally contain various additives such as fixing resins, pH adjusters, defoamers, wetting agents (moisturizers), surface tension adjusters (penetrating agents), preservatives, fixing agents, and antioxidants.

Examples of pH adjusters include inorganic bases such as ammonium hydroxide, sodium hydroxide, and potassium hydroxide; acids such as sulfuric acid, nitric acid, and acetic acid; and amines such as triethanolamine, aminomethylpropanol, aminoethylpropanol, dimethylethanolamine, triethylamine, diethylethanolamine, and dimethylaminopropanol.
The content of the pH adjuster is preferably 0.1 to 5% by mass, and more preferably 0.1 to 1% by mass, based on the total amount of the ink.
By maintaining the pH of the ink at 7 or more, the sulfo group of the monomer (D) is neutralized and the monomer (D) has an electric charge, which prevents the pigment from aggregating due to electrostatic repulsion and ensures stability. Therefore, the pH of the ink is preferably 7 or more, and more preferably 7.5 or more.
As described above, when a basic compound is used as a pH adjuster, the basic compound may inhibit the crosslinking reaction, so that these additives are preferably added after the crosslinking reaction.

The surfactant can be used as a defoaming agent, a wetting agent (moisturizing agent), a surface tension adjusting agent (penetrating agent), and the like.
The surfactant is not particularly limited, but examples thereof include acetylene glycol surfactants, silicone surfactants, polyoxyethylene alkylphenols, polyoxyethylene alkyl ethers, polyoxyethylene fatty acid esters, etc. An example of the acetylene glycol surfactant is "Surfynol 465" (trade name) manufactured by Nissin Chemical Industry Co., Ltd. An example of the silicone surfactant is "Silface SAG002" (trade name) manufactured by Nissin Chemical Industry Co., Ltd.
The amount of the surfactant is preferably 0.1 to 10% by mass, and more preferably 0.1 to 5% by mass, based on the total amount of the ink.

The suitable range of viscosity of aqueous ink varies depending on the nozzle diameter of the ejection head of the inkjet recording system and the ejection environment, but generally, it is preferably 1 to 30 mPa·s at 23°C, and more preferably 3 to 15 mPa·s. The viscosity of aqueous ink can be measured using a rotational viscometer.

The aqueous inkjet ink according to an embodiment may be printed on an untreated substrate, or may be printed on a substrate treated with a pretreatment agent. In particular, when a non-permeable substrate is used as the substrate, it is preferable to treat the substrate with a pretreatment agent, since the aqueous inkjet ink is unlikely to permeate the substrate.
The pretreatment agent preferably contains, for example, an aqueous solvent together with a surfactant, a flocculant, inorganic particles, or a combination thereof. More preferably, the pretreatment agent contains an aqueous solvent and a surfactant and/or a flocculant. The pretreatment agent may also contain a binder resin to fix the flocculant to the substrate.

After the ink is applied to the substrate, a step of post-treating the substrate to form an overcoat layer may be further provided. The method of post-treating the substrate can be performed by applying a post-treatment agent to the substrate. As the post-treatment agent, for example, a post-treatment liquid containing a resin capable of forming a film and an aqueous or oil-based solvent can be used.

An example of a method for producing a printed matter will be described below.
The method for producing a printed matter may include, for example, forming an image on a substrate using an aqueous inkjet ink. The aqueous inkjet ink may be any of the aqueous inkjet inks described above. The method may also include treating the substrate with a pretreatment agent prior to forming an image using the aqueous inkjet ink.

An image can be formed on a substrate using an aqueous inkjet ink by inkjet printing, which allows easy and flexible image formation on demand without contacting the substrate.
The inkjet printing method is not particularly limited and may be any method such as a piezoelectric method, an electrostatic method, or a thermal method. For example, ink can be ejected from an inkjet head based on a digital signal, and the ejected ink droplets can be attached to a substrate.

The substrate to which the aqueous ink is applied is not particularly limited, and examples thereof include printing paper such as plain paper, coated paper, and special paper, cloth, inorganic sheets, films, and OHP sheets, as well as adhesive sheets having an adhesive layer on the back surface of the substrate made of any of these.
Here, plain paper is paper on which no ink receiving layer or film layer is formed on the normal paper. Examples of plain paper include fine paper, medium quality paper, PPC paper, wood paper, recycled paper, etc.

As coated paper, inkjet coated paper such as matte paper, glossy paper, and semi-glossy paper, and so-called coated printing paper can be preferably used. Here, coated printing paper is printing paper that has been used conventionally in letterpress printing, offset printing, gravure printing, etc., and is printing paper in which a coating layer is provided on the surface of fine or medium-quality paper with a paint containing an inorganic pigment such as clay or calcium carbonate and a binder such as starch. Coated printing paper is classified into lightly coated paper, fine lightweight coated paper, medium lightweight coated paper, fine coated paper, medium coated paper, art paper, cast coated paper, etc., depending on the amount of paint applied and the coating method.

The substrate may be either a permeable substrate or a non-permeable substrate. The aqueous ink according to one embodiment can prevent the dissolution of the dispersant into the solvent even when a low-polarity solvent is used, so by including a low-polarity solvent in the aqueous ink, printing on a non-permeable substrate can be performed without ink repellency or unevenness.

Other Embodiments
According to another embodiment of the present invention, there can be provided a pigment complex comprising a dispersant having a crosslinked structure, which is a copolymer of a monomer mixture including a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and a vinyl monomer (D) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group, and a pigment. Details of the dispersant and the pigment are as described above.
In this embodiment, the crosslinked structure may be a copolymer-copolymer or copolymer-pigment surface crosslink. The crosslinked structure of this pigment complex can prevent the dispersant from being detached from the pigment surface even when a dispersion or ink contains a low-polarity solvent.

According to yet another embodiment of the present invention, it is possible to provide a copolymer for dispersion, which includes a copolymer of a monomer mixture including a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and a vinyl monomer (D) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group. Details of the copolymer are as described above.
In this embodiment, in the copolymer of the dispersion copolymer, the carboxyl group derived from the vinyl monomer (A) becomes the crosslinking base point. For example, by adding a crosslinking agent in a state where the dispersion copolymer and the dispersoid are mixed, a crosslinked structure can be formed on the copolymer-copolymer or copolymer-dispersoid surface. In this way, the dispersion dispersed by the dispersion copolymer can prevent the dispersant from being detached from the dispersoid surface due to the crosslinked structure, even if it contains a low polarity solvent. Examples of the dispersoid include pigments, resin particles, silica particles, glass particles, other ceramic particles, etc. This composition preferably contains water as the main solvent of the dispersion medium, and may contain a water-soluble organic solvent.

According to still another embodiment of the present invention, there can be provided a method for producing a dispersion, comprising synthesizing a copolymer using a monomer mixture containing a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and a vinyl monomer (D) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group, dispersing a dispersoid using the copolymer, and crosslinking the copolymer in the presence of the dispersoid. The details of the copolymer are as described above.
In this embodiment, a crosslinked structure can be formed on the surface of the copolymer-copolymer or copolymer-dispersoid by dispersing the dispersoid using the copolymer and crosslinking the copolymer in the presence of the dispersoid. The crosslinked dispersion can prevent the dispersant from being detached from the dispersoid surface due to the crosslinked structure, even when it contains a low polarity solvent. Examples of the dispersoid include pigments, resin particles, silica particles, glass particles, other ceramic particles, etc. The main solvent of the dispersion medium is preferably water, and may contain a water-soluble organic solvent.
The above-mentioned method for producing a dispersion can be preferably applied to the production of a pigment dispersion by crosslinking a copolymer in the presence of a pigment. According to this method for producing a pigment dispersion, for example, the above-mentioned aqueous inkjet ink can be preferably produced.

The present invention will be described in detail below with reference to examples. The present invention is not limited to the following examples. In each table, "-" indicates that no additive was added.

"Synthesis of Monomers"
(Monomer 1)
Polyethylene glycol 1540 was dissolved in dichloromethane and stirred in a flask at 40° C. Karenz AOI was dissolved in dichloromethane and added to the flask over 1 hour using a dropping funnel. After that, the mixture was heated and stirred for 8 hours, and the product was separated by column chromatography.
The product was further dissolved in dichloromethane, sulfamic acid was added, and the mixture was stirred at 120° C. for 3 hours.
The products were separated by column chromatography to give Monomer 1.
The obtained Monomer 1 has a structure in which a polyethylene glycol chain is introduced to the NCO group of 2-isocyanatoethyl acrylate with an addition mole number of 34, and a sulfo group is oriented at the end of the polyethylene glycol chain.

(Monomer 2)
Polyethylene glycol 1000 was dissolved in dichloromethane and stirred in a flask at 40° C. Karenz AOI was dissolved in dichloromethane and added to the flask over 1 hour using a dropping funnel. After that, the mixture was heated and stirred for 8 hours, and the product was separated by column chromatography.
The product obtained above was dissolved in acetone, and Jones reagent was added to the solution over 1 hour while cooling with ice. Then, an excess of 2-propanol was added, and the solution was filtered by suction to volatilize the acetone. Finally, the product was washed to obtain Monomer 2.
The obtained Monomer 2 has a structure in which a polyethylene glycol chain is introduced to the NCO group of 2-isocyanatoethyl acrylate with an addition mole number of 23, and a carboxy group is oriented at the end of the polyethylene glycol chain.

The components used are as follows:
Polyethylene glycol 1540 (product name): EO mole number 34, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.
Polyethylene glycol 1000 (product name): EO mole number 23, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.
2-Isocyanatoethyl acrylate: "Karenz AOI" (product name) manufactured by Showa Denko K.K.
Dichloromethane: Fujifilm Wako Pure Chemical Industries, Ltd.
N,N-Dimethylformamide: Fujifilm Wako Pure Chemical Industries, Ltd.
EO represents ethylene oxide. The same applies below.

"Examples 1 to 13, Comparative Example 1"
For Examples 1 to 13 and Comparative Example 1, the formulations of the copolymers used are shown in Table 1, and the ink formulations are shown in Tables 2 and 3.
(1) Synthesis of Copolymer 1 The raw materials shown in Table 1 were placed in a flask, and after bubbling with nitrogen, the mixture was heated and stirred at 70° C. for 6 hours under a nitrogen atmosphere. The solvent was removed using an evaporator to obtain Copolymer 1 having a resin content of 100% by mass.
The monomer ratio of the copolymer was calculated from the charged amounts of the monomers.

(2) Preparation of ink The pigment, copolymer, water, and neutralizer shown in the dispersion formulation in Tables 2 and 3 (1) were placed in a polypropylene (PP) bottle and dispersed in a rocking mill to obtain a pigment dispersion. The rocking mill conditions were φ0.5 mm zirconia beads, 60 Hz, and 3 hours. Copolymer 1 was used as the copolymer.
After filtering off the beads with a #120 mesh, the dispersion was transferred to a glass bottle, and the crosslinking agent shown in (1) Dispersion Formulation was added and stirred with a magnetic stirrer to cause a crosslinking reaction, thereby obtaining a crosslinked pigment dispersion. The stirring conditions were 65° C., 16 hours, and 600 rpm.
After filtering the crosslinked pigment dispersion through a #355 mesh, the solvent, surfactant, and water shown in (2) Ink formulation were added, and the mixture was filtered through a 3 μm filter to obtain an ink.

"Examples 14 to 20"
In Examples 14 to 20, aqueous inks were prepared using Copolymers 2 to 8, respectively. The formulations of Copolymers 2 to 8 and the ink formulations of Examples 14 to 20 are shown in Table 4.
Copolymers 2 to 8 having a resin content of 100 mass % were synthesized in the same manner as in the synthesis of Copolymer 1, except that the copolymer formulation was changed to the formulation shown in the table. Inks of Examples 14 to 20 were prepared in the same manner as in Example 1, except that the ink formulation was changed to the formulation shown in the table using Copolymers 2 to 8.

"Comparative Examples 2 to 6"
In Comparative Examples 2 to 6, aqueous inks were prepared using copolymers 9 to 13, respectively. The formulations of copolymers 9 to 13 and the ink formulations of Comparative Examples 2 to 6 are shown in Table 5.
Copolymers 9 to 13 having a resin content of 100% were synthesized in the same manner as in the synthesis of Copolymer 1, except that the copolymer formulation was changed to the formulation shown in the table. Inks of Comparative Examples 2 to 6 were prepared in the same manner as in Example 1, except that the ink formulation was changed to the formulation shown in the table using Copolymers 9 to 13.
In each comparative example, when determining the monomer ratio of the copolymer from the charged amount of the monomer, the reference monomers (B') and (D') were included in the monomers (B) and (D), respectively, and the ratio of the number of moles of EO chains added (D)/(C) was also calculated in the same manner.

The components used are as follows:
"Copolymer"
Monomer (A);
Acrylic acid: Fujifilm Wako Pure Chemical Industries, Ltd.
Methacrylic acid: Fujifilm Wako Pure Chemical Industries, Ltd.
2-Acryloyloxyethyl-succinic acid: "Light Acrylate HOA-MS (N)" (product name), manufactured by Kyoeisha Chemical Co., Ltd.
Monomers (B) and (B');
AAEM: ethylene glycol monoacetoacetate monomethacrylate, a monomer having a β-dicarbonyl group, manufactured by Tokyo Chemical Industry Co., Ltd.
A-LEN-10 (product name): ethoxylated phenylphenol acrylate, a monomer having a biphenyl group, manufactured by Shin-Nakamura Chemical Co., Ltd.
Benzyl methacrylate: a monomer having a phenyl group, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

Monomer (C);
Blemmer PME400 (trade name): methoxypolyethylene glycol methacrylate, n=9; nonionic, manufactured by NOF Corporation.
Blemmer PME1000 (trade name): methoxypolyethylene glycol methacrylate, n=23; nonionic, manufactured by NOF Corporation.
Light Ester 041MA (trade name): methoxypolyethylene glycol methacrylate, n=30; nonionic, manufactured by Kyoeisha Chemical Co., Ltd.
Monomers (D) and (D');
ADEKA REASOAP SR-10 (product name): n=10, terminal group: —SO ₃ NH ₄ , manufactured by ADEKA Corporation.
ADEKA REASOAP SR-20 (product name): n=20, terminal group: —SO ₃ NH ₄ , manufactured by ADEKA Corporation.
Monomer 1: n=34 moles, terminal group: —SO ₃ NH ₄ , synthesized according to the above synthesis recipe.
Monomer 2: n=23 mol, terminal group: —COONH ₄ , synthesized according to the above synthesis recipe.

Initiator: AIBN (2,2'-Azobis(isobutyronitrile)): manufactured by Tokyo Chemical Industry Co., Ltd.
Chain transfer agent: lauryl mercaptan: manufactured by Tokyo Chemical Industry Co., Ltd.
2-Propanol: manufactured by Tokyo Chemical Industry Co., Ltd.
n indicates the number of moles of ethylene oxide (EO) added to the polyethylene glycol chain. The same applies in the table.

(ink)
Pigments;
Carbon black "MA8" (product name): manufactured by Mitsubishi Chemical Corporation.
Copper phthalocyanine pigment: "Chromofine 4927" (product name), manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.
Quinacridone pigment: "FASTOGEN SUPER MAGENTA R" (product name), manufactured by DIC Corporation.
Condensed disazo pigment: "Hansa Brilliant Yellow 5GX" (product name), manufactured by Clariant.
Carbon black "#45" (product name): manufactured by Mitsubishi Chemical Corporation.
Crosslinking agent;
Denacol EX-521 (product name): a tetrafunctional or higher epoxy compound, manufactured by Nagase ChemteX Corporation.
Denacol EX-411 (product name): a tri- to tetra-functional epoxy compound, manufactured by Nagase ChemteX Corporation.
Epolite 200E (product name): a bifunctional epoxy compound, manufactured by Nagase ChemteX Corporation.
Surfactants;
Silface SAG002 (product name): silicone-based surfactant, manufactured by Nissin Chemical Industry Co., Ltd.
The neutralizing agent and the solvent are available from Tokyo Chemical Industry Co., Ltd. or Fujifilm Wako Pure Chemical Industries, Ltd.

The SP values of the solvents are as follows:
Diethylene glycol: 14.97 (cal/cm ³ ) ^1/2 .
1,2-Hexanediol: 11.80 (cal/cm ³ ) ^1/2 .
Diethylene glycol monobutyl ether: 10.51 (cal/cm ³ ) ^1/2 .

The degree of neutralization shown in the table was calculated from the molar equivalents of the neutralizing agent relative to the molar equivalents of the carboxyl groups derived from monomer (A) before crosslinking.

"Evaluation Method"
(1) Evaluation of storage stability First, the viscosity of the ink was measured immediately after the ink was prepared. Next, the ink was placed in a sealed container and left in a 70°C environment for 4 weeks. After that, the viscosity of the ink was measured, and the viscosity change rate was calculated according to the following formula. Then, the storage stability was evaluated according to the following criteria.
The ink viscosity was measured at 23° C. using an Anton Paar rheometer "MCR302" (cone angle 1°, diameter 50 mm).
Viscosity change rate: [(viscosity after 4 weeks)-(initial viscosity))/(initial viscosity)] x 100 (%)
AA: The absolute value of the viscosity change rate is 5% or less.
A: The absolute value of the viscosity change rate is more than 5% and 10% or less.
B: The absolute value of the viscosity change rate is more than 10% and 20% or less.
C: The absolute value of the viscosity change rate exceeds 20%, or the ink gels and cannot be measured.

(2) Evaluation of Printed Matter Printed matters were produced using the obtained inks under the following conditions.
Printer: Shuttle printer with piezo printhead with 300 npi resolution.
Printing substrate: PVC substrate, Avery Dennison MPI3002P WPE.
Printing conditions: Resolution: 1200 x 600 dpi, Image: 30 cm x 30 cm solid image.
The image quality of the resulting prints was evaluated according to the following criteria.
A: A uniform solid layer is formed.
B: Unevenness occurs.
C: Poor ejection occurs and printing cannot be performed correctly.

As shown in each table, the ink of each example contained an appropriate copolymer, the copolymer was crosslinked, and the storage stability was good. It can also be seen that the ink of each example is suitable for printing on PVC substrates.

Through Examples 1 to 4, it can be seen that the inks of Examples 1, 2, and 4, which contain a low-polarity solvent, can maintain good storage stability, similar to Example 3, which contains only a polar solvent. Furthermore, since Examples 1, 2, and 4 contain a low-polarity solvent, the wettability to the non-permeable substrate is improved, and the printed image is good. Since Example 3 does not contain a low-polarity solvent, the wettability to the non-permeable substrate is insufficient, and unevenness occurs in the printed image.
Examples 5 to 8 show that good results can be obtained with a variety of pigments.

Among Examples 1, 9, and 10, it is considered that a stronger crosslinked structure was formed in Example 1, in which the crosslinking agent had higher water solubility and the number of functional groups was larger, and therefore Example 1 had better storage stability.
The influence of the neutralization degree of the carboxy group of the monomer (A) can be confirmed through Examples 1, and 11 to 13. The neutralization degree of the carboxy group is preferably 40 to 90 mol %, and it is considered that a low neutralization degree leads to insufficient electrostatic repulsion and reduced initial dispersibility, whereas a high neutralization degree leads to reduced reactivity with the crosslinking agent, or the crosslinking agent is deactivated and a sufficient crosslinking reaction does not occur, resulting in reduced storage stability.

It can be seen from Examples 14 to 20 that good results can be obtained with dispersants using various copolymers.
The storage stability was better in the examples in which the monomer (A) was acrylic acid or methacrylic acid in Examples 1, 14, and 15. This is believed to be because the carboxyl group located away from the main skeleton of the copolymer is less likely to cause crosslinking between the pigment and the copolymer.
In Examples 1 and 17 to 20, the storage stability was better when the number of moles of the polyoxyalkylene chain of the monomer (D) was larger than the number of moles of the polyoxyalkylene chain of the monomer (C). This is considered to be because when the polyoxyalkylene chain of the monomer (C) is larger than the polyoxyalkylene chain of the monomer (D), the sulfo group at the end of the polyoxyalkylene chain of the monomer (D) is buried, resulting in a decrease in electrostatic repulsion.

Comparative Example 1 is an example in which a crosslinking agent was not used, and the storage stability was reduced. This is believed to be because the copolymer was not crosslinked, and therefore the dispersant, i.e., the non-crosslinked copolymer, was dissolved in the solvent containing the low polarity solvent, causing aggregation of the pigment.
Comparative Example 2 is an example in which a monomer with poor adsorption to the pigment was used in the synthesis of the copolymer, and the storage stability was reduced. This is thought to be because the initial dispersibility was deteriorated and crosslinking between the pigment and the copolymer was difficult to occur.
Comparative Examples 3 and 4 are examples using a monomer having a polyoxyalkylene group with a small number of added moles, and the storage stability was reduced. This is thought to be because the initial dispersibility was reduced and the repulsive force between the pigments was reduced, causing crosslinking between copolymers that adsorb to different pigments, resulting in aggregation of the pigments.
Comparative Example 5 is an example in which a monomer having a carboxy group, instead of a sulfo group, at the end of the polyoxyalkylene chain was used, and the storage stability was reduced. This is believed to be because the presence of a reactive carboxy group in the outermost layer of the copolymer adsorbed on the pigment surface made crosslinking between copolymers of different pigments more likely to occur.
Comparative Example 6 is an example in which monomer (C) was not used, and the storage stability was reduced. This is believed to be because the sulfo groups at the ends of the polyoxyalkylene chains of monomer (D) repelled each other, preventing the copolymer from adsorbing to the pigment, and the initial dispersibility was significantly reduced. In addition, the physical properties of the ink changed at room temperature due to the solvent containing the low polarity solvent, causing ejection failure.

Claims (9)

A pigment, a dispersant, a water-soluble solvent, and water,
The dispersant is
(A) a vinyl monomer containing a carboxyl group;
(B) a vinyl monomer containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group;
The aqueous inkjet ink has a crosslinked structure and is a copolymer of a monomer mixture including: a nonionic vinyl-based monomer (C) having a polyoxyalkylene chain with an added mole number of alkylene oxide of 15 or more; and a vinyl-based monomer (D) having a polyoxyalkylene chain with an added mole number of alkylene oxide of 15 or more and a sulfo group. The aqueous inkjet ink according to claim 1, wherein the degree of neutralization of the carboxyl group of the vinyl monomer (A) is 40 to 90 mol %. The aqueous inkjet ink according to claim 1 or 2, wherein the vinyl monomer (A) contains at least one of acrylic acid and methacrylic acid. The aqueous inkjet ink according to any one of claims 1 to 3, wherein the ratio of the number of moles of alkylene oxide added to the polyoxyalkylene chain of the vinyl monomer (C) to the number of moles of alkylene oxide added to the polyoxyalkylene chain of the vinyl monomer (D) satisfies (C):(D) = 1:0.8 to 1:2. The aqueous ink-jet ink according to claim 1 , wherein the water-soluble solvent comprises an alcohol-based solvent having an SP value of less than 13 (cal/cm ³ ) ^1/2 . The aqueous inkjet ink according to any one of claims 1 to 5, wherein the pigment and the dispersant form a pigment complex, and the pigment complex has a crosslinked structure. A pigment complex comprising a dispersant having a crosslinked structure, which is a copolymer of a monomer mixture including a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added, and a vinyl monomer (D) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added and a sulfo group, and a pigment. A copolymer for dispersion comprising a copolymer of a monomer mixture containing a vinyl monomer (A) containing a carboxyl group, a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added, and a vinyl monomer (D) containing a polyoxyalkylene chain having 15 or more moles of alkylene oxide added and a sulfo group. A method for producing a dispersion, comprising: synthesizing a copolymer using a monomer mixture containing (A) a vinyl monomer containing a carboxyl group, (B) a vinyl monomer containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group, (C) a nonionic vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more, and (D) a vinyl monomer containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group; dispersing a dispersoid using the copolymer; and crosslinking the copolymer in the presence of the dispersoid.