JP2024020793A - inkjet ink - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inkjet ink capable of suppressing the occurrence of nozzle clogging.

SOLUTION: There is provided an inkjet ink containing an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles contain a pigment and a specific resin. The specific resin is a reaction product of an oxazoline group-containing crosslinking agent and a neutralized product of a base resin. The base resin has an acid value of 60 mgKOH/g or more and 200 mgKOH/g or less. The neutralized product of the base resin has a neutralization rate_X of 45% or more and 92% or less. The specific resin has a crosslinking rate of 45% or more and 82% or less. The specific resin has an un-neutralized acid value of 15 mgKOH/g or more. The specific resin has a neutralized acid value of 30 mgKOH/g or more and 150 mgKOH/g or less. In the inkjet ink, the solid content ratio in the supernatant obtained by centrifuging at 1,050,000 G for 3 hours is 2.0 mass% or less.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to an inkjet ink.

An aqueous inkjet ink containing a pigment and an aqueous medium is sometimes used in an inkjet recording device. Inkjet inks are required to be able to suppress the occurrence of nozzle clogging in the recording head of an inkjet recording apparatus.

In response to such demands, for example, an inkjet ink using crosslinked polymer particles containing a colorant has been proposed (Patent Document 1). The above-mentioned inkjet ink is said to have excellent storage stability.

Japanese Patent Application Publication No. 2008-150535

However, even with the inkjet ink described in Patent Document 1, the occurrence of nozzle clogging cannot be sufficiently suppressed.

An object of the present invention has been made in view of the above problems, and is to provide an inkjet ink that can suppress the occurrence of nozzle clogging.

The inkjet ink according to the present invention contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles include a pigment and a specific resin. The specific resin is a reaction product of an oxazoline group-containing crosslinking agent and a neutralized base resin. The acid value of the base resin is 60 mgKOH/g or more and 200 mgKOH/g or less. The neutralization rate _X of the neutralized product of the base resin is 45% or more and 92% or less. The crosslinking rate of the specific resin is 45% or more and 82% or less. The unneutralized acid value of the specific resin represented by the following formula (1) is 15 mgKOH/g or more. The neutralized acid value of the specific resin expressed by the following formula (2) is 30 mgKOH/g or more and 150 mgKOH/g or less. In the inkjet ink, the solid content in the supernatant obtained by centrifugation at 1,050,000 G for 3 hours is 2.0% by mass or less.
Unneutralized acid value = base resin acid value x (100% - neutralization rate _x )...(1)
Neutralized acid value = acid value of base resin x neutralization rate _X ... (2)

The inkjet ink according to the present invention can suppress the occurrence of nozzle clogging.

Embodiments of the present invention will be described below. In addition, in the following, the measured value of the volume median diameter (D ₅₀ ) is measured using a dynamic light scattering particle size distribution analyzer ("Zetasizer Nano ZS" manufactured by Malvern) unless otherwise specified. is the value given.

In this specification, the acid value can be determined according to the method described in JIS (Japanese Industrial Standard) K0070:1992.

In this specification, acrylic and methacrylic may be collectively referred to as "(meth)acrylic".

<Inkjet ink>
Hereinafter, an inkjet ink (hereinafter sometimes simply referred to as ink) according to an embodiment of the present invention will be described. The ink of the present invention contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles contain a pigment and a specific resin. The specific resin is a reaction product of an oxazoline group-containing crosslinking agent and a neutralized product of the base resin. The acid value of the base resin is 60 mgKOH/g or more and 200 mgKOH/g or less. The neutralization rate _X of the neutralized base resin is 45% or more and 92% or less. The crosslinking rate of the specific resin is 45% or more and 82% or less. The unneutralized acid value of the specific resin represented by the following formula (1) is 15 mgKOH/g or more. The neutralized acid value of the specific resin represented by the following formula (2) is 30 mgKOH/g or more and 150 mgKOH/g or less. In the ink, the solid content in the supernatant liquid obtained by centrifuging at 1,050,000 G for 3 hours is 2.0% by mass or less.
Unneutralized acid value = base resin acid value x (100% - neutralization rate _x )...(1)
Neutralized acid value = acid value of base resin x neutralization rate _X ... (2)

In this specification, the crosslinking rate refers to the crosslinking rate in a specific resin, when the total number of functional groups (groups that can react with a crosslinking agent) contained in the raw material of the specific resin (neutralized product of the base resin) is 100%. Shows the ratio of the number of functional groups forming the structure. The oxazoline group-containing crosslinking agent used in the present invention reacts with unneutralized acid groups (eg, -COOH) and does not react with neutralized acid groups (eg, -COOK and -COONa). Therefore, "the total number of functional groups contained in the raw material of the specific resin" corresponds to the total number of unneutralized acid groups contained in the neutralized product of the base resin. The crosslinking rate can be calculated using the following formula (3).
Crosslinking rate [%] = 100 x number of molar equivalents of crosslinking functional groups of crosslinking agent [mmol] / number of molar equivalents of unneutralized acid groups of neutralized product of base resin [mmol]... (3)

The use of the ink of the present invention is not particularly limited, but it can be used, for example, to form an image on a permeable recording medium or a non-permeable recording medium. The ink of the present invention is suitable for forming images on permeable recording media. Permeable recording media have excellent ink permeability. Examples of permeable recording media include printing paper and media made from fibers (eg, cloth). Examples of printing paper include plain paper, copy paper, recycled paper, thin paper, thick paper, and glossy paper.

By having the above-described structure, the ink of the present invention can suppress the occurrence of nozzle clogging. The reason is inferred as follows. Solid content contained in the supernatant liquid of ink may cause nozzle clogging in the recording head of an inkjet recording apparatus. Since the ink of the present invention contains a small amount of solid content in the supernatant liquid, it is possible to suppress the occurrence of nozzle clogging caused by the above-mentioned solid content.

Further, the ink of the present invention contains pigment particles containing a pigment and a specific resin. Pigments alone have low dispersion stability in aqueous media. On the other hand, the specific resin is derived from a neutralized product of a base resin having an appropriately high acid value, and thus has excellent affinity with an aqueous medium. The pigment particles exhibit excellent dispersion stability in an aqueous medium by containing a specific resin together with the pigment. Since the specific resin is appropriately crosslinked with the oxazoline group-containing crosslinking agent, it is difficult for the specific resin to be detached from the pigment particles. Thereby, in the ink of the present invention, the specific resin is desorbed from the pigment particles, and the exposed pigment can be prevented from aggregating. Here, the oxazoline group-containing crosslinking agent has excellent reactivity and can easily and reliably form the above-mentioned crosslinked structure. Further, the oxazoline group-containing crosslinking agent can form a highly hydrophilic crosslinked structure. Therefore, by using an oxazoline group-containing crosslinking agent as a crosslinking agent, appropriate hydrophilicity can be imparted to the specific resin. Furthermore, the oxazoline group-containing crosslinking agent has lower corrosivity than other crosslinking agents (for example, epoxy crosslinking agents), and is less likely to corrode the nozzles of the recording head of an inkjet recording device. As a result, the ink of the present invention can suppress the occurrence of nozzle clogging.

Here, the neutralized product of the base resin includes unneutralized acid groups and neutralized acid groups. Unneutralized acid groups have reactivity with the oxazoline group-containing crosslinking agent, but have lower hydrophilicity than neutralized acid groups. Neutralized acid groups have no reactivity with the oxazoline group-containing crosslinking agent, but have higher hydrophilicity than unneutralized acid groups. In the ink of the present invention, the neutralization _rate Sufficient hydrophilicity can be imparted to a specific resin while introducing a crosslinked structure. As a result, the ink of the present invention is more likely to exhibit the effect of nozzle clogging.

The ink of the present invention will be explained in more detail below. In addition, each component demonstrated below may be used individually by 1 type, and may be used in combination of 2 or more types.

[Pigment particles]
The pigment particles contain a pigment and a specific resin. Pigment particles are composed of, for example, a core containing a pigment and a specific resin covering the core. The total content of the pigment and the specific resin in the pigment particles is preferably 90% by mass or more, and more preferably 100% by mass.

From the viewpoint of optimizing the color density, hue, or stability of the ink of the present invention, the volume median diameter of the pigment particles is preferably 30 nm or more and 200 nm or less, and more preferably 80 nm or more and 130 nm or less.

In the ink of the present invention, the content of pigment particles is preferably 5.0% by mass or more and 30.0% by mass or less, more preferably 8.0% by mass or more and 15.0% by mass or less. By setting the content of pigment particles to 5.0% by mass or more, the ink of the present invention can form an image having a desired image density. Further, by controlling the content of pigment particles to 30.0% by mass or less, the fluidity of the ink of the present invention can be optimized.

(pigment)
Examples of pigments include yellow pigments, orange pigments, red pigments, blue pigments, violet pigments, and black pigments. Examples of yellow pigments include C.I. I. pigment yellow (74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193). Examples of orange pigments include C.I. I. Pigment Orange (34, 36, 43, 61, 63, and 71). Examples of red pigments include C.I. I. Pigment Red (122 and 202). Examples of blue pigments include C.I. I. Pigment Blue (15, more specifically 15:3). Examples of the purple pigment include C.I. I. pigment violet (19, 23, and 33). Examples of black pigments include C.I. I. Pigment Black (7).

In the ink of the present invention, the content of the pigment is preferably 3.0 parts by mass or more and 20.0 parts by mass, more preferably 7.0 parts by mass or more and 12.0 parts by mass or less. The content ratio of the pigment in the pigment particles is preferably 50% by mass or more and 90% by mass or less, more preferably 70% by mass or more and 80% by mass or less.

(Specified resin)
The specific resin coats the pigment in, for example, pigment particles. The specific resin is a reaction product of an oxazoline group-containing crosslinking agent and a neutralized product of the base resin. The crosslinking rate of the specific resin is 45% or more and 82% or less, preferably 60% or more and 75% or less. By setting the crosslinking rate of the specific resin to 45% or more, it is possible to suppress the specific resin from being desorbed from the pigment particles. By setting the crosslinking rate of the specific resin to 82% or less, excellent dispersion stability in an aqueous medium can be imparted to the pigment particles.

The unneutralized acid value of the specific resin is 15 mgKOH/g or more, preferably 20 mgKOH/g or more and 90 mgKOH/g or less, and more preferably 25 mgKOH/g or more and 60 mgKOH/g or less. By setting the unneutralized acid value of the specific resin to 15 mgKOH/g or more, it becomes easy to sufficiently introduce a crosslinked structure into the specific resin. By setting the unneutralized acid value of the specific resin to 90 mgKOH/g or less, excellent dispersion stability in an aqueous medium can be imparted to the pigment particles.

The neutralized acid value of the specific resin is 30 mgKOH/g or more and 150 mgKOH/g or less, preferably 30 mgKOH/g or more and 130 mgKOH/g or less, and more preferably 35 mgKOH/g or more and 70 mgKOH/g or less. By setting the neutralized acid value of the specific resin to 30 mgKOH/g or more, even better dispersion stability in an aqueous medium can be imparted to the pigment particles. By setting the neutralized acid value of the specific resin to 150 mgKOH/g or less, a crosslinked structure can be sufficiently introduced into the specific resin.

In the ink of the present invention, the content of the specific resin is preferably 0.5 parts by mass or more and 10.0 parts by mass, more preferably 2.0 parts by mass or more and 4.0 parts by mass or less. The content of the specific resin in the pigment particles is preferably 10% by mass or more and 50% by mass or less, more preferably 20% by mass or more and 30% by mass or less.

The specific resin has, for example, an unopened oxazoline group and an amide ester group. It is preferable that the specific resin has a repeating unit (1) represented by the following general formula (A) and a repeating unit (2) represented by the following general formula (B). Repeating unit (1) has an unopened oxazoline group. The repeating unit (2) has an amide ester group.

In the general formula (A), R ¹ represents a hydrogen atom or an alkyl group having 1 or more and 10 or less carbon atoms which may be substituted with a phenyl group. In general formula (B), R ² represents a hydrogen atom or an alkyl group having 1 or more and 10 or less carbon atoms which may be substituted with a phenyl group. * represents a bonding site with an atom contained in the base resin.

In general formula (A) or (B), examples of the alkyl group having 1 to 10 carbon atoms represented by R ¹ or R ² include methyl group, ethyl group, n-propyl group, isopropyl group, n -butyl group, sec-butyl group, tert-butyl group, n-pentyl group, isopentyl group, neopentyl group, 1,2-dimethylpropyl group, linear or branched hexyl group, linear or branched Examples include a branched heptyl group, a linear or branched octyl group, a linear or branched nonyl group, and a linear or branched decyl group.

In general formula (A), R ¹ preferably represents a hydrogen atom, a methyl group, an ethyl group, or an isopropyl group. In general formula (B), R ² preferably represents a hydrogen atom, a methyl group, an ethyl group, or an isopropyl group.

(Base resin)
The acid value of the base resin is 60 mgKOH/g or more and 200 mgKOH/g or less, preferably 75 mgKOH/g or more and 120 mgKOH/g or less. By setting the acid value of the base resin to 60 mgKOH/g or more, sufficient hydrophilicity can be imparted to the specific resin while sufficiently introducing a crosslinked structure into the specific resin. By setting the acid value of the base resin to 200 mgKOH/g or less, it is possible to suppress aggregation of pigment particles during storage of the ink of the present invention.

The neutralization rate _X of the neutralized base resin is 45% or more and 92% or less, preferably 55% or more and 85% or less, and more preferably 55% or more and 70% or less. By setting the neutralization rate _X to 45% or more, sufficient hydrophilicity can be imparted to the specific resin. By setting the neutralization rate _X to 92% or less, a crosslinked structure can be sufficiently introduced into the specific resin.

It is preferable that the neutralized product of the base resin contains an alkali metal atom. That is, the neutralized base resin is preferably neutralized with a neutralizing agent containing an alkali metal atom. Alkali metal atoms do not volatilize from the ink of the present invention even when the ink of the present invention is exposed to dry conditions. Therefore, since the neutralized product of the base resin is neutralized with a neutralizing agent containing an alkali metal atom, the neutralized state (hydrophilicity) of the specific resin is maintained even when the ink of the present invention is exposed to a dry state. be done. As the alkali metal atom, a potassium atom or a sodium atom is preferable. As the neutralizing agent, a hydroxide containing an alkali metal atom is preferred, and NaOH or KOH is more preferred.

The base resin has a functional group that can react with an oxazoline group. Examples of the functional group include a carboxy group, an aromatic thiol group, and a phenol group. As the functional group, a carboxy group is preferable. That is, the base resin is preferably a carboxyl group-containing resin. The carboxyl group-containing resin has, for example, a repeating unit derived from a carboxyl group-containing monomer. Examples of the carboxy group-containing monomer include (meth)acrylic acid, polyvalent carboxylic acid, polyvalent carboxylic acid derivatives, and unsaturated carboxylic acids.

The base resin may be a random polymer or a block polymer. Preferably, the base resin is a random polymer.

Examples of the base resin include polyester resin, urethane resin, (meth)acrylic resin, styrene-(meth)acrylic resin, styrene-maleic acid copolymer, styrene-maleic acid half ester copolymer, and vinylnaphthalene-(meth)acrylic resin. ) acrylic acid copolymers and vinylnaphthalene-maleic acid copolymers. As the base resin, styrene-(meth)acrylic resin is preferred.

(Styrene-(meth)acrylic resin)
Styrene-(meth)acrylic resin is a resin having styrene units and repeating units derived from (meth)acrylic acid alkyl ester. It is preferable that the styrene-(meth)acrylic resin further has a repeating unit derived from (meth)acrylic acid. Examples of the (meth)acrylic acid alkyl ester include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and n-butyl (meth)acrylate.

In the styrene-(meth)acrylic resin, the content ratio of styrene units to all repeating units is preferably 20.0% by mass or more and 80.0% by mass or less, more preferably 30.0% by mass or more and 50.0% by mass or less. preferable.

In the styrene-(meth)acrylic resin, the content ratio of repeating units derived from (meth)acrylic acid to all repeating units is preferably 4.0% by mass or more and 40.0% by mass or less, and 7.0% by mass or more. More preferably, it is 28.0% by mass or less. By setting the content of repeating units derived from (meth)acrylic acid to 4.0% by mass or more and 40.0% by mass or less, it is possible to impart an appropriate acid value to the styrene-(meth)acrylic resin, and also to A moderate amount of cross-linked structure can be introduced into the structure.

In the styrene-(meth)acrylic resin, the content ratio of repeating units derived from (meth)acrylic acid alkyl ester to all repeating units is preferably 15.0% by mass or more and 50.0% by mass or less, and 25.0% by mass. % or more and 35.0% by mass or less is more preferable.

As raw material monomers for the styrene-(meth)acrylic resin, the following combinations (I), (II), or (III) are preferred.
Combination (I): Styrene, n-butyl acrylate, and methacrylic acid Combination (II): Styrene, n-butyl acrylate, methacrylic acid, and methyl methacrylate Combination (III): Styrene, methyl methacrylate, and acrylic acid

(Oxazoline group-containing crosslinking agent)
The oxazoline group-containing crosslinking agent is, for example, a resin having a plurality of oxazoline groups in its molecule. The oxazoline group-containing crosslinking agent has, for example, the above-mentioned repeating unit (1).

The oxazoline group-containing crosslinking agent may further have a repeating unit (3) derived from a vinyl compound. Examples of the vinyl compound include ethylene, propylene, butadiene, vinyl chloride, (meth)acrylic acid, (meth)acrylic acid ester (preferably (meth)acrylic acid alkyl ester), acrylonitrile, and styrene. It is preferable that the oxazoline group-containing crosslinking agent further contains a repeating unit derived from an alkyl (meth)acrylate ester or a repeating unit derived from styrene.

The oxazoline group-containing crosslinking agent is, for example, a (meth)acrylic resin or a styrene-(meth)acrylic resin having a plurality of oxazoline groups in the molecule.

The oxazoline group-containing crosslinking agent is preferably water-soluble. Since the oxazoline group-containing crosslinking agent is water-soluble, the dispersion stability of pigment particles can be improved. Here, if a uniform and transparent aqueous solution is obtained when 10 g of the oxazoline group-containing crosslinking agent is dissolved in 90 g of water at a temperature of 25° C., it can be determined that the oxazoline group-containing crosslinking agent is water-soluble.

Examples of the oxazoline group-containing crosslinking agent include "Epocross (registered trademark) WS-300" and "Epocross (registered trademark) WS-700" manufactured by Nippon Shokubai Co., Ltd. "Epocross (registered trademark) WS-300" contains a copolymer of 2-vinyl-2-oxazoline and methyl methacrylate (oxazoline group-containing (meth)acrylic resin). The mass ratio of the monomers constituting this copolymer is (2-vinyl-2-oxazoline):(methyl methacrylate)=9:1. "Epocross (registered trademark) WS-700" contains a copolymer of 2-vinyl-2-oxazoline, methyl methacrylate, and butyl acrylate (oxazoline group-containing (meth)acrylic resin). The mass ratio of the monomers constituting the copolymer is (2-vinyl-2-oxazoline):(methyl methacrylate):(butyl acrylate)=5:4:1. 2-vinyl-2-oxazoline is a compound represented by the following chemical formula (A-1).

The amount of oxazoline groups in the oxazoline group-containing crosslinking agent is preferably 3 mmol/g or more and 10 mmol/g or less. The weight average molecular weight of the oxazoline group-containing crosslinking agent is preferably 1,000 or more and 15,000 or less.

[Solid content in supernatant liquid]
In the ink of the present invention, the solid content in the supernatant obtained by centrifugation at 1,050,000 G for 3 hours is 2.0% by mass or less. The content of the above-mentioned solid content is preferably 1.0% by mass or less, more preferably 0.5% by mass or less. The above-mentioned solid content is generated, for example, when a part of the specific resin contained in the pigment particles is detached and released into the aqueous medium during the production and storage of the ink of the present invention. The above-mentioned solid content may cause nozzle clogging in the recording head of an inkjet recording apparatus. Therefore, it is preferable that the above-mentioned solid content is as small as possible.

The above solid content can be measured by the method described in Examples or a method based thereon.

[Aqueous medium]
The aqueous medium contained in the ink of the present invention is a medium containing water. The aqueous medium may function as a solvent or a dispersion medium. Specific examples of the aqueous medium include water and a water-soluble organic solvent.

(water)
In the ink of the present invention, the water content is preferably 25.0% by mass or more and 80.0% by mass or less, more preferably 35.0% by mass or more and 60.0% by mass or less.

(Water-soluble organic solvent)
Examples of water-soluble organic solvents include glycol compounds, triol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycol, and dimethyl sulfoxide.

Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,2-pentanediol, 1,5-pentanediol, 1,2-octanediol, 1,8-octanediol, 3- Examples include methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, and 2-ethyl-1,2-hexanediol. As the glycol compound, ethylene glycol, diethylene glycol, 2-ethyl-1,2-hexanediol, 3-methyl-1,5-pentanediol, 1,3-propanediol, 1,5-pentanediol or propylene glycol is preferred.

Examples of triol compounds include glycerin and 1,2,3-butanetriol.

Examples of glycol ether compounds include diethyl diglycol, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl Ethers, triethylene glycol monobutyl ether and propylene glycol monomethyl ether may be mentioned. As the glycol ether compound, triethylene glycol monobutyl ether is preferred.

Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone. As the lactam compound, 2-pyrrolidone is preferred.

Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

Examples of the acetate compound include diethylene glycol monoethyl ether acetate.

The water-soluble organic solvent is preferably a glycol compound, a triol compound, a glycol ether compound, or a lactam compound, and more preferably a mixed solvent of the following combinations (i) to (iii).
Combination (i): Ethylene glycol and diethyl diglycol Combination (ii): Glycerin, triethylene glycol monobutyl ether, 3-methyl-1,5-pentanediol and 2-pyrrolidone Combination (iii): Triethylene glycol monobutyl ether, 2- Pyrrolidone, 1,3-propanediol and 1,5-pentanediol

The content ratio of the water-soluble organic solvent in the ink of the present invention is preferably 10.0% by mass or more and 50.0% by mass or less, more preferably 30.0% by mass or more and 40.0% by mass or less.

[Surfactant]
Preferably, the ink of the present invention further contains a surfactant. The surfactant can optimize the permeability (wettability) of the ink of the present invention to the recording medium. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. As the surfactant, nonionic surfactants are preferred.

Examples of nonionic surfactants include polyoxyethylene dodecyl ether, polyoxyethylene hexadecyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and ethylene oxide of acetylene glycol. Examples include adducts. As the nonionic surfactant, an ethylene oxide adduct of acetylene glycol is preferred.

The content of the surfactant in the ink of the present invention is preferably 0.05% by mass or more and 3.0% by mass or less, more preferably 0.1% by mass or more and 0.5% by mass or less.

[Other ingredients]
The ink of the present invention may further contain known additives (for example, a dissolution stabilizer, a drying inhibitor, an antioxidant, a viscosity modifier, a pH adjuster, and a fungicide) as necessary.

[Ink manufacturing method]
Next, an example of a method for manufacturing the ink of the present invention will be described. The method for producing the ink of the present invention includes, for example, a dispersion treatment step in which a pigment particle dispersion is prepared by dispersing a neutralized product of pigment and base resin in water, and an oxazoline group-containing crosslinking agent is added to the pigment particle dispersion. The method includes a crosslinking step of adding an aqueous medium to the pigment particle dispersion after the crosslinking step, and an addition step of preparing an ink by adding an aqueous medium to the pigment particle dispersion liquid after the crosslinking step. The ink manufacturing method of the present invention preferably further includes a centrifugation step of replacing the supernatant with an aqueous medium after centrifuging the pigment particle dispersion after the crosslinking step.

(Pigment treatment process)
In this step, a pigment particle dispersion is prepared by dispersing the pigment and the neutralized base resin in water. Examples of the dispersion device used in the dispersion process include wet-type dispersion devices such as a media-type dispersion machine.

It is preferable that the base resin used in this step is partially neutralized in advance with a basic compound (for example, KOH or NaON). The specific neutralization rate _X of the base resin is as described above. Note that the neutralization rate _X is the theoretical value of the amount of basic compound required to completely neutralize the base resin, M _A is the actual amount of the basic compound used in this process, and M _B is the actual amount of the basic compound used in this process. Sometimes, the percentage of the usage amount M _B with respect to the theoretical value M _A (100×M _B /M _A ) is indicated.

In this step, the content of the base resin in the solution subjected to the dispersion treatment is, for example, 5.0% by mass or more and 25.0% by mass or less. The content of the pigment in the solution to be subjected to the dispersion treatment is, for example, 1.0% by mass or more and 10.0% by mass or less. In this step, the solution to be subjected to the dispersion treatment preferably further contains an antifoaming agent. The content of the antifoaming agent in the solution to be subjected to the dispersion treatment is, for example, 0.01% by mass or more and 0.1% by mass or less.

In this step, coarse particles are preferably removed from the obtained pigment particle dispersion by filtration using a filter (eg, 5 μm in pore size).

(Crosslinking process)
In this step, an oxazoline group-containing crosslinking agent is added to the pigment particle dispersion. As a result, the neutralized product of the base resin contained in the pigment particle dispersion and the oxazoline group-containing crosslinking agent react, and the neutralized product of the base resin is crosslinked. As a result, a specific resin is produced which is a reaction product of a neutralized product of an oxazoline group-containing crosslinking agent and a base resin. In this step, it is preferable to heat the pigment particle dispersion while stirring after adding the oxazoline group-containing crosslinking agent. The heating temperature can be, for example, 70°C or higher and 95°C or lower. The heating time can be, for example, 30 minutes or more and 2 hours or less.

(Centrifugal process)
In this step, after the pigment particle dispersion after the crosslinking step is centrifuged, the supernatant liquid is replaced with an aqueous medium. Thereby, free components contained in the aqueous medium of the pigment particle dispersion can be removed. The centrifugation conditions can be, for example, a rotation speed of 10,000 rpm or more and 100,000 rpm or less, and a centrifugation time of 12 hours or more and 48 hours or less.

(Addition process)
In this step, an aqueous medium is added to the pigment particle dispersion after the crosslinking step (if the centrifugation step is performed, the pigment particle dispersion after the centrifugation step). In this way, ink can be obtained. In addition, in this step, other components (more specifically, at least one of a surfactant, a dissolution stabilizer, a humectant, a penetrant, and a viscosity modifier) are further added as necessary. Good too. In this step, after adding the aqueous medium, it is preferable to stir the resulting mixed solution with a stirrer. The obtained ink may be filtered to remove foreign matter and coarse particles (for example, a filter with a pore size of 5 μm or less).

Examples of the present invention will be described below. However, the present invention is not limited to the following examples.

[Preparation of resin (R-1)]
100.0 parts by mass of isopropyl alcohol and 250.0 parts by mass of methyl ethyl ketone were charged into a four-necked flask equipped with a stirrer, a nitrogen introduction tube, a condenser, and a dropping funnel. Separately, 70.0 parts by mass of styrene, 20.0 parts by mass of n-butyl acrylate, 10.0 parts by mass of methacrylic acid, and 0.3 parts by mass of azobisisobutyronitrile (AIBN, polymerized (initiator) to prepare a monomer solution. Further, 150.0 parts by mass of methyl ethyl ketone and 0.1 parts by mass of AIBN were mixed to prepare a methyl ethyl ketone solution.

Next, nitrogen gas was introduced into the four-neck flask described above to create a nitrogen atmosphere. Next, while heating and refluxing the contents of the four-necked flask at 70° C., the entire amount of the monomer solution was supplied into the four-necked flask over 2 hours using a dropping funnel. After supplying the monomer solution, the contents of the four-necked flask were further heated to reflux at 70°C for 6 hours. Next, while heating and refluxing the contents of the four-necked flask at 70° C., the entire amount of the methyl ethyl ketone solution was supplied into the four-necked flask over 15 minutes using a dropping funnel. After supplying the methyl ethyl ketone solution, the contents of the four-necked flask were further heated to reflux at 70° C. for 5 hours. As a result, a resin solution containing resin (R-1), which is a styrene-(meth)acrylic resin, was obtained. Resin (R-1) was isolated by distilling off methyl ethyl ketone and isopropyl alcohol from the resin solution.

[Preparation of resins (R-2) to (R-5)]
In preparing the monomer solution, resins (R-2) to (R- 5) was prepared.

(Measurement of acid value)
Based on the method described in JIS (Japanese Industrial Standards) K0070:1992 (testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value, and unsaponifiable substances of chemical products), resin (R-1) The acid value (acid value of the base resin) of ~(R-5) was measured. The measurement results are shown in Table 1 below.

The abbreviations used in Table 1 below are explained.
MMA: Methyl methacrylate CA: Cyclohexyl acrylate

<Preparation of ink>
Inks of Examples 1 to 6 and Comparative Examples 1 to 15 shown in Table 2 below were prepared by the following method. First, the pigments and crosslinking agents used in the examples are shown below.

(pigment)
Pigment (BK): Carbon black (“Printex (registered trademark) 80” manufactured by Orion Engineered Carbons Co., Ltd.)
Pigment (C): Phthalocyanine blue (“ECB-301” manufactured by Dainichiseika Kagyo Co., Ltd.)
Pigment (M): Quinacridone Magenta (“Ink Jet Magenta ES” manufactured by Clariant Co., Ltd.)
Pigment (Y): Monoazo yellow (“Hansa (registered trademark) Brilliant Yellow 5GX01” manufactured by Clariant Co., Ltd.)

(Crosslinking agent)
Crosslinking agent (WS-300): Aqueous solution containing oxazoline group-containing (meth)acrylic resin ("Epocross (registered trademark) WS-300" manufactured by Nippon Shokubai Co., Ltd.), 10% by mass of active ingredient, oxazoline group per 1 g of active ingredient Amount: 7.7 mmol
Crosslinking agent (WS-500): Aqueous solution containing oxazoline group-containing (meth)acrylic resin ("Epocross (registered trademark) WS-500" manufactured by Nippon Shokubai Co., Ltd.), active ingredient 39% by mass, oxazoline group per 1 g of active ingredient Amount: 4.5 mmol
Crosslinking agent (WS-700): Aqueous solution containing oxazoline group-containing (meth)acrylic resin ("Epocross (registered trademark) WS-700" manufactured by Nippon Shokubai Co., Ltd.), active ingredient 25% by mass, oxazoline group per 1 g of active ingredient Amount: 4.5 mmol
Crosslinking agent (K-2010E): Emulsion containing oxazoline group-containing styrene-(meth)acrylic resin ("Epocross (registered trademark) K-2010E" manufactured by Nippon Shokubai Co., Ltd.), 40% by mass of active ingredient, per 1 g of active ingredient Oxazoline base amount 1.8 mmol
Crosslinking agent (EX-810): Ethylene glycol diglycidyl ether (“Denacol (registered trademark) EX-810” manufactured by Nagase ChemteX Co., Ltd.), active ingredient 100% by mass, epoxy equivalent 113 g/eq

[Example 1]
(neutralization treatment)
30 parts by mass of resin (R-1), potassium hydroxide, and water were mixed to prepare an aqueous resin solution having a concentration of a neutralized product of resin (R-1) of 30% by mass. The amount of potassium hydroxide added was an amount corresponding to 60% by mass of the neutralization equivalent of the resin (R-1) (neutralization rate _X of the neutralized product of the resin (R-1): 60%). The amount of water added was such that the mass of the prepared aqueous resin solution was 100 parts by mass.

(Distributed processing)
15.0 parts by mass of pigment (BK-1) (Orion Engineered Carbons Co., Ltd. "Printex (registered trademark) 80", carbon black) and 15.0 parts by mass of the above-mentioned resin aqueous solution (of resin (R-1)). (containing 4.5 parts by mass of neutralized product), 0.1 part by mass of an antifoaming agent ("SN Deformer 1340" manufactured by San Nopco Co., Ltd., amide wax surfactant), and ion-exchanged water to form a mixture. I got it. The amount of ion-exchanged water added was such that the mixture would be 100 parts by mass.

The above mixture was subjected to a dispersion treatment for 4 hours using a media type dispersion machine ("Dyno Mill" manufactured by Willy & Bacchofen (WAB)). In the dispersion treatment, zirconia beads with a diameter of 0.5 mm were used as the media. The filling rate of the media was 60% by volume based on the capacity of the vessel. The treatment temperature (chiller temperature) in the dispersion treatment was set at 10°C. After the dispersion treatment, the media was removed from the contents of the media type dispersion machine to obtain a pigment particle dispersion. Next, foreign matter and coarse particles were removed by filtering the pigment particle dispersion through a filter with a pore size of 5 μm.

(Crosslinking treatment)
A three-necked flask equipped with a thermometer and stirring blade was used as a reaction vessel. 100.0 parts by mass of the filtered pigment particle dispersion was charged into a reaction vessel. The temperature of the contents of the reaction vessel was maintained at 30°C using a water bath. Next, 1.90 parts by mass of a crosslinking agent (WS-300) (containing 0.19 parts by mass of an oxazoline group-containing crosslinking agent) was charged into the reaction vessel. Next, while stirring the contents of the reaction vessel at 250 rpm, the temperature of the contents of the reaction vessel was raised to 80°C at a temperature increase rate of 0.5°C/min. Next, the contents of the reaction vessel were stirred at 250 rpm for 1 hour while maintaining the temperature of the contents of the reaction vessel at 80°C. This caused the contents of the reaction vessel to react. Through the reaction, the resin (R-1) was crosslinked with the crosslinking agent (WS-300) to form a specific resin. Next, the contents of the reaction vessel were allowed to cool until the temperature reached room temperature. Thereby, a pigment particle dispersion liquid after crosslinking treatment was obtained.

The crosslinking rate of the specific resin was calculated by the following method. For convenience, it was assumed that 1 part by mass = 1 g. First, the number of molar equivalents of the crosslinking functional group of the crosslinking agent was calculated. The number of molar equivalents of the crosslinking functional group of the crosslinking agent is "amount of crosslinking agent (WS-300) used (1.90 parts by mass)" and "concentration of active ingredient of crosslinking agent (WS-300) (10% by mass)" , and "amount of oxazoline groups (7.7 mmol) per 1 g of active ingredient of crosslinking agent (WS-300)". The number of molar equivalents of the crosslinking functional group of the crosslinking agent was 1.46 mmol.

Next, the number of molar equivalents of unneutralized carboxy groups in the base resin was calculated. The number of molar equivalents of unneutralized carboxyl groups in the base resin is "amount used (4.5 parts by mass) of base resin (resin (R-1))", "carboxylic acid of base resin (resin (R-1))" It was determined by multiplying the base equivalent (1.16 mmol/g) by the value obtained by subtracting the neutralization rate _X (60%) of the base resin after neutralization from 100% (40%). "Carboxy group equivalent (1.16 mmol/g) of base resin (resin (R-1))" is calculated by subtracting "acid value (65 mgKOH/g) of base resin (resin (R-1))" from "mole of KOH. It was calculated by dividing by "mass (56.1 g/mol)". The number of molar equivalents of unneutralized carboxy groups in the base resin was 2.09 mmol.

Next, the crosslinking rate of the specific resin was calculated by applying the value determined above to the following mathematical formula (3). In Example 1, the crosslinking rate of the specific resin was 70%.
Crosslinking rate [%] = 100 x number of molar equivalents of crosslinking functional groups of crosslinking agent [mmol] / number of molar equivalents of unneutralized acid groups of neutralized product of base resin [mmol]... (3)

(centrifugal treatment)
The pigment particle dispersion after the crosslinking treatment was transferred to a container, and this container was placed in a centrifugal adhesion measurement device ("NS-C100" manufactured by Nano Seeds Co., Ltd.). Using the centrifugal adhesion measuring device described above, the pigment particle dispersion after the above-mentioned crosslinking treatment was centrifuged at a rotational speed of 50,000 rpm for 24 hours. After centrifugation, the supernatant was removed from the container, and then ion-exchanged water of the same volume as the removed supernatant was added to the container. In this way, free components were removed from the aqueous medium of the pigment particle dispersion after crosslinking treatment.

(Mixing process)
In a container, 60.0 parts by mass of the pigment particle dispersion after centrifugation (approximately 9 parts by mass of pigment, approximately 2.7 parts by mass of resin), 20.0 parts by mass of ethylene glycol, and 15.0 parts by mass of diethyldiglycol. , 0.3 parts by mass of a nonionic surfactant ("Olfine (registered trademark) E1004" manufactured by Nissin Chemical Industry Co., Ltd.), and 4.7 parts by mass of ion-exchanged water were added (Formulation A). Using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Kagaku Co., Ltd.), the contents of the above-mentioned container were stirred at a rotational speed of 400 rpm to obtain a mixture. The obtained liquid mixture was filtered using a filter (pore size: 5 μm). In this way, the ink of Example 1 was obtained.

[Examples 2 to 6 and Comparative Examples 1 to 10]
Type of resin and neutralization rate _X (amount of potassium hydroxide used) used in neutralization treatment, type of pigment used in dispersion treatment, amount of neutralized base resin, and crosslinking agent used in crosslinking treatment By the same method as the ink preparation of Example 1, except that the type and amount of and the prescription in the mixing process (any of the formulations A to C in Table 3 below) were changed as shown in Table 2 below. Inks of Examples 2 to 6 and Comparative Examples 1 to 10 were prepared.

[Comparative Examples 11 to 15]
Next, a portion of the ink of Comparative Example 2 prepared above was taken out. The ink of Comparative Example 2 was subjected to filtration treatment (ultrafiltration) to remove free resin. As the dialysate used for the filtration treatment, an aqueous medium having the same composition as the aqueous medium contained in the above-mentioned ink was used. In the filtration treatment, a hollow fiber filter (nominal molecular weight cut off: 50,000) was used for 2 hours under conditions of a filtration internal pressure of 0.05 MPa and a filtration flow rate of 200 g/min. The ink after the filtration treatment was designated as the ink of Comparative Example 11. Similarly, a portion of the inks of Comparative Examples 5 and 8 to 10 were taken out, and each ink was subjected to the same filtration treatment as in Comparative Example 11 to remove free resin. The filtered inks were used as inks of Comparative Examples 12 to 15, respectively.

In Table 2 below, "parts" indicate parts by mass. "Parts" of resin in the dispersion treatment refers to parts by mass of the neutralized product (solid content) of the base resin contained in the aqueous resin solution used. "Y" in filtration treatment indicates that filtration treatment was performed. "N" in the filtration process indicates that the filtration process was not performed. In Table 3 below, "surfactant" refers to a nonionic surfactant ("Olfine (registered trademark) E1004" manufactured by Nissin Chemical Industry Co., Ltd.).

(Measurement of solid content in supernatant liquid)
The inks of Examples 1 to 6 and Comparative Examples 1 to 15 were centrifuged by the following method. Then, the content ratio of solids contained in the obtained supernatant liquid was measured.

First, an object to be measured (any of the inks of Examples 1 to 6 and Comparative Examples 1 to 15) was placed in an ultracentrifuge ("himac (registered trademark) CS150FNX" manufactured by Eppendorf Highmac Technologies Co., Ltd.), rotor: S140AT. ) for 3 hours at a rotation speed of 140,000 rpm (1,050,000 G). This precipitated the pigment particles contained in the measurement target.

Next, 30 μL of the supernatant contained in the measurement target after centrifugation was transferred to an aluminum container for thermal mass measurement. Next, the mass (A) of 30 μL of the supernatant was measured. Next, thermal mass spectrometry was performed using a thermal mass spectrometer ("TG/DTA7200" manufactured by Hitachi High-Tech Science Co., Ltd.) according to the scheme shown in Table 4 below. Then, the mass loss (B) of the measurement target was measured at a temperature of 200°C to 500°C. The mass loss (B) was regarded as the mass of solid content (mainly free resin) in the supernatant liquid. The content of solids in the supernatant liquid was calculated according to the following formula. The measurement results are shown in Table 5 below.
Solid content content in supernatant liquid [mass%] = 100 x B/A

Table 5 below shows the acid value of the base resin, the crosslinking rate of the specific resin calculated by the method described above, the neutralization rate _X of the specific resin (same value as the neutralization rate _X of the base resin), and the unneutralized acid value of the specific resin. and the neutralized acid value of the specific resin.

<Evaluation>
Each of the inks of Examples 1 to 6 and Comparative Examples 1 to 15 was evaluated for nozzle clogging and corrosivity using the following methods. Note that the evaluation was performed under an environment of a temperature of 25° C. and a humidity of 60% RH unless otherwise specified. The evaluation results are shown in Table 6 below.

[Nozzle clogging]
In the evaluation of nozzle clogging, an inkjet recording device equipped with a line head (testing device manufactured by Kyocera Document Solutions Co., Ltd.) was used as an evaluation device. The inks of Examples 1 to 6 and Comparative Examples 1 to 15 to be evaluated were the black ink tank (Example 1, Comparative Examples 3, 9, and 14) and the cyan ink tank (Example 3) of the evaluation machine. , Comparative Examples 1 and 7), magenta ink tank (Examples 2, 5, Comparative Examples 4, 8, 10, 13 and 15) or yellow ink tank (Examples 4, 6, Comparative Examples 2, 5, 6) , 11 and 12).

Solid images (150 mm x 200 mm) were continuously formed on 100 sheets of matte paper ("Super Fine Paper" manufactured by Seiko Epson Corporation) using an evaluation machine. Next, a purge process was performed to purge ink from the recording head of the evaluation machine. Next, a wiping process was performed in which the ink ejection surface of the recording head of the evaluation machine was wiped with a cleaning wiper. Next, a nozzle check pattern was formed on the above-mentioned matte paper using an evaluation machine. As a result, it was confirmed that ink was ejected from all nozzles (7968 nozzles) in all evaluation targets. That is, the number of nozzles in which nozzle clogging occurred was zero. Next, the recording head of the evaluation machine was subjected to purge processing and wipe processing. Next, the evaluation machine was left standing for 7 days without a cap attached to the recording head of the evaluation machine. Next, the above-mentioned purge process and wipe process were performed on the recording head of the evaluation machine. Next, a nozzle check pattern was formed on the above-mentioned matte paper using an evaluation machine. Then, the formed nozzle check pattern was observed, and the number of nozzles in which nozzle clogging occurred was confirmed. The ratio of the number of nozzles in which nozzle clogging occurred to the total number of nozzles in the recording head of the evaluation machine was defined as the nozzle clogging evaluation value. Nozzle clogging was determined according to the following criteria.

(criteria for nozzle clogging)
A (Good): Evaluation value is less than 10% B (Bad): Evaluation value is 10% or more

[Corrosiveness]
The corrosivity of the evaluation target (any of the inks of Examples 1 to 6 and Comparative Examples 1 to 15) was evaluated using a SUS plate (3 cm long x 4 cm wide x 0.3 mm thick). First, the SUS plate and the evaluation target were placed in a glass bottle. At this time, a sufficient amount of the evaluation object was added so that the entire SUS plate was immersed in the evaluation object. Next, the SUS plate was taken out from the glass bottle and washed with ion-exchanged water. Next, the SUS plate was dried for 24 hours in a constant temperature bath set at 60°C. Then, the mass (initial mass) of the SUS plate was measured. Next, the SUS plate and the evaluation object were put into the glass bottle again. At this time, a sufficient amount of the evaluation object was added so that the entire SUS plate was immersed in the evaluation object. Next, the lid of the glass bottle was closed. Next, the glass bottle was placed in a constant temperature bath set at 60° C. and subjected to constant temperature treatment for 3 months. After the constant temperature treatment, the SUS plate was taken out from the glass bottle and washed with ion-exchanged water. Next, the SUS plate was dried for 24 hours in a constant temperature bath set at 60°C. Then, the mass of the SUS plate (mass after treatment) was measured. The rate of change in mass of the SUS plate before and after constant temperature treatment was measured according to the following formula. In addition, the surface of the SUS plate was observed under a microscope to confirm the presence or absence of corrosion. In Table 6 below, the case where corrosion could be confirmed by microscopic observation was marked as "NG", and the case where it could not be confirmed was marked as "OK". Corrosivity was determined according to the following criteria.
Mass change rate [%] = 100 x (mass after treatment - initial mass) / initial mass

(corrosion standard)
A (good): The absolute value of the rate of change in mass was less than 5%, and no corrosion was confirmed by microscopic observation.
B (Poor): The absolute value of the rate of change in mass was 5% or more, or corrosion was confirmed by microscopic observation.

As shown in Tables 1-6, the inks of Examples 1-6 contained an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles contained a pigment and a specific resin. The specific resin was a reaction product of an oxazoline group-containing crosslinking agent and a neutralized product of the base resin. The acid value of the base resin was 60 mgKOH/g or more and 200 mgKOH/g or less. The neutralization rate _X of the neutralized base resin was 45% or more and 92% or less. The crosslinking rate of the specific resin was 45% or more and 82% or less. The unneutralized acid value of the specific resin represented by the above-mentioned formula (1) was 15 mgKOH/g or more. The neutralized acid value of the specific resin represented by the above-mentioned formula (2) was 30 mgKOH/g or more and 150 mgKOH/g or less. In the inks of Examples 1 to 6, the solid content in the supernatant obtained by centrifugation at 1,050,000 G for 3 hours was 2.0% by mass or less. The inks of Examples 1 to 6 were able to suppress the occurrence of nozzle clogging. Furthermore, the inks of Examples 1 to 6 had low corrosivity.

On the other hand, in the ink of Comparative Example 1, the crosslinking rate of the specific resin was less than 45%. If the crosslinking rate of the specific resin is too low, it is determined that the specific resin is likely to be easily desorbed from the pigment particles. As a result, the ink of Comparative Example 1 could not suppress the occurrence of nozzle clogging.

In the ink of Comparative Example 2, the unneutralized acid value of the specific resin was less than 15 mgKOH/g. If the unneutralized acid value of the specific resin is too low, it is determined that a sufficient amount of crosslinked structure cannot be introduced into the specific resin. As a result, it is determined that the specific resin is likely to be easily desorbed from the pigment particles, similar to when the specific resin has a low crosslinking rate. Further, in the ink of Comparative Example 2, the solid content in the supernatant liquid was more than 2.0% by mass. As a result, the ink of Comparative Example 2 could not suppress the occurrence of nozzle clogging.

The ink of Comparative Example 3 used an epoxy crosslinking agent instead of an oxazoline group-containing crosslinking agent for crosslinking the specific resin. Ink using an epoxy crosslinking agent is judged to have residual chlorine, which is inevitably included in the epoxy crosslinking agent. As a result, it was determined that the ink of Comparative Example 3 corroded the nozzles of the recording head of the inkjet recording apparatus. Therefore, it is determined that the ink of Comparative Example 3 will eventually cause nozzle clogging if used for a long period of time.

In the ink of Comparative Example 4, the crosslinking rate of the specific resin was over 82%. When the crosslinking rate of the specific resin is too high, it is determined that the hydrophilicity of the specific resin decreases and the dispersion stability of the pigment particles decreases. As a result, the ink of Comparative Example 4 could not suppress the occurrence of nozzle clogging.

The ink of Comparative Example 5 had a specific resin neutralization rate _x of over 92%. If the neutralization rate _x of the specific resin is too high, it is determined that a sufficient amount of crosslinked structure cannot be introduced into the specific resin. As a result, it is determined that the specific resin is likely to be easily desorbed from the pigment particles, similar to when the specific resin has a low crosslinking rate. Further, in the ink of Comparative Example 5, the unneutralized acid value of the specific resin was less than 15 mgKOH/g. Furthermore, in the ink of Comparative Example 5, the solid content in the supernatant liquid was more than 2.0% by mass. As a result, the ink of Comparative Example 5 could not suppress the occurrence of nozzle clogging.

In the inks of Comparative Examples 6 and 7, the solid content in the supernatant liquid was more than 2.0% by mass. As a result, the inks of Comparative Examples 6 and 7 were unable to suppress the occurrence of nozzle clogging.

In the ink of Comparative Example 8, the acid value of the base resin was less than 60 mgKOH/g. If the acid value of the base resin is too low, it is determined that a sufficient amount of crosslinked structure cannot be introduced into the specific resin. As a result, it is determined that the specific resin is likely to be easily desorbed from the pigment particles, similar to when the specific resin has a low crosslinking rate. Further, in the ink of Comparative Example 8, the neutralized acid value of the specific resin was less than 30 mgKOH/g. Furthermore, in the ink of Comparative Example 8, the solid content in the supernatant liquid was more than 2.0% by mass. As a result, the ink of Comparative Example 8 could not suppress the occurrence of nozzle clogging.

In the inks of Comparative Examples 9 and 10, the base resin had an acid value of more than 200 mgKOH/g. If the acid value of the base resin is too high, it is determined that aggregation of pigment particles occurs during storage of the ink. Further, in the ink of Comparative Example 9, the neutralization rate _X of the specific resin was less than 45%. If the neutralization rate _X of the specific resin is too low, it is determined that the hydrophilicity of the specific resin decreases and the dispersion stability of the pigment particles decreases. Furthermore, in the ink of Comparative Example 10, the neutralized acid value of the specific resin was over 150 mgKOH/g. If the neutralized acid value of the specific resin is too high, it is determined that a sufficient amount of crosslinked structure cannot be introduced into the specific resin. As a result, it is determined that the specific resin is likely to be easily desorbed from the pigment particles, similar to when the specific resin has a low crosslinking rate. Furthermore, in the inks of Comparative Examples 9 and 10, the solid content in the supernatant liquid was more than 2.0% by mass. As a result, the inks of Comparative Examples 9 and 10 were unable to suppress the occurrence of nozzle clogging.

The inks of Comparative Examples 11 to 15 are comparative examples in which the solid content in the supernatant liquid was excluded from Comparative Examples 2, 5, and 8 to 10. The inks of Comparative Examples 11 to 15 have improved the solid content in the supernatant liquid to some extent, but the acid value of the base resin, the neutralization rate _X of the neutralized product of the base resin, and the unneutralized acid value of the specific resin Or, because the neutralizing acid value did not meet the conditions, the occurrence of nozzle clogging could not be suppressed.

The ink of the present invention can be used to form images.

Claims (4)

containing an aqueous medium and pigment particles dispersed in the aqueous medium,
The pigment particles include a pigment and a specific resin,
The specific resin is a reaction product of an oxazoline group-containing crosslinking agent and a neutralized product of the base resin,
The acid value of the base resin is 60 mgKOH/g or more and 200 mgKOH/g or less,
The neutralization rate _X of the neutralized product of the base resin is 45% or more and 92% or less,
The crosslinking rate of the specific resin is 45% or more and 82% or less,
The unneutralized acid value of the specific resin represented by the following formula (1) is 15 mgKOH/g or more,
The neutralized acid value of the specific resin expressed by the following formula (2) is 30 mgKOH/g or more and 150 mgKOH/g or less,
An inkjet ink, wherein the solid content in a supernatant obtained by centrifugation at 1,050,000 G for 3 hours is 2.0% by mass or less.
Unneutralized acid value = base resin acid value x (100% - neutralization rate _x )...(1)
Neutralized acid value = acid value of base resin x neutralization rate _X ... (2) The inkjet ink according to claim 1, wherein the pigment particles have a volume median diameter of 80 nm or more and 130 nm or less. The inkjet ink according to claim 1 or 2, wherein the base resin includes styrene-(meth)acrylic resin. The inkjet ink according to claim 1 or 2, wherein the neutralized base resin contains an alkali metal atom.