JP2024014311A - Pigment dispersion and inkjet ink - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide pigment dispersions that suppress nozzle clogging when contained in inkjet inks.

SOLUTION: A pigment dispersion contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particle includes a core and a coating layer coating the core. The core contains pigments. The coating layer contains a crosslinked resin. The crosslinked resin has a main chain structure derived from a predetermined resin and a crosslinked structure derived from a crosslinking agent. The predetermined resin is a resin that can react with the crosslinking agent. The predetermined resin has a mass-average molecular weight of 4000 or more and 40000 or less. The crosslinking agent has at least two reactive functional groups capable of reacting with a group possessed by the predetermined resin, and a structure represented by formula (1). In the formula, R¹ represents H or a methyl group, n represents a number of 4 or more and 13 or less, and "*" represents a bond.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a pigment dispersion and an inkjet ink.

Inkjet ink is ejected from the nozzles of a recording head included in an inkjet recording apparatus. Inkjet inks are required to have ejection stability from nozzles. In response to such demands, Patent Document 1 describes an inkjet ink containing a coloring material, resin particles, a crosslinking agent, and an aqueous medium. The resin particles have units derived from a predetermined compound. The crosslinking agent has a ketone group derived from a predetermined compound of the resin particles and a functional group that forms a crosslinked structure through a dehydration reaction. In Examples of Patent Document 1, adipic acid dihydrazide is described as a crosslinking agent.

Japanese Patent Application Publication No. 2013-14745

However, due to the formation of a crosslinked structure due to the dehydration reaction, the hydrophilicity of the resin particles may be reduced, and the dispersibility of the coloring material in an aqueous medium may be reduced. Decreased dispersibility of the coloring material may cause the coloring material to aggregate and cause nozzle clogging. Therefore, the inkjet ink described in Patent Document 1 is insufficient in suppressing the occurrence of nozzle clogging.

The present invention has been made in view of the above problems, and an object thereof is to provide a pigment dispersion that can suppress the occurrence of nozzle clogging when contained in an inkjet ink. Another object of the present invention is to provide an inkjet ink that can suppress the occurrence of nozzle clogging.

The pigment dispersion according to the present invention contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles include a core and a coating layer covering the core. The core contains a pigment. The coating layer contains a crosslinked resin. The crosslinked resin has a main chain structure derived from a predetermined resin and a crosslinked structure derived from a crosslinking agent. The predetermined resin is a resin that can react with the crosslinking agent. The predetermined resin has a mass average molecular weight of 4,000 or more and 40,000 or less. The crosslinking agent has at least two reactive functional groups capable of reacting with a group of the predetermined resin, and a structure represented by formula (1).

In the formula (1), R ¹ represents a hydrogen atom or a methyl group, n represents a number of 4 or more and 13 or less, and * represents a bond.

The inkjet ink according to the present invention contains the pigment dispersion described above.

The pigment dispersion of the present invention can suppress the occurrence of nozzle clogging when contained in an inkjet ink. The inkjet ink according to the present invention can suppress the occurrence of nozzle clogging.

Embodiments of the present invention will be described below. First, terms used in this specification will be explained. Note that the evaluation results regarding particles (values indicating shape, physical properties, etc.) are the number average of values measured for a considerable number of particles, unless otherwise specified. In addition, unless otherwise specified, the measured value of the volume median diameter (D ₅₀ ) of the particles was measured using a dynamic light scattering particle size distribution analyzer (“Zetasizer nano-ZS” manufactured by Malvern). It is a value. Further, unless otherwise specified, viscosity means viscosity at 25°C. Unless otherwise specified, the measured value of viscosity is a value measured in accordance with the method described in "JIS Z8803:2011 (method for measuring liquid viscosity)". The measured value of mass average molecular weight (Mw) is a value measured using gel permeation chromatography unless otherwise specified. Further, the acid value is a value measured according to "JIS (Japanese Industrial Standard) K0070-1992" unless otherwise specified. Furthermore, acrylic and methacrylic may be collectively referred to as "(meth)acrylic". Unless otherwise specified, the "main component" of a material means the component that is most present in the material on a mass basis. Each of the components described below may be used alone or in combination of two or more. The terms used in this specification have been explained above.

[First embodiment: Pigment dispersion]
The pigment dispersion according to the first embodiment of the present invention will be described below. The pigment dispersion of the first embodiment is, for example, contained in an inkjet ink (hereinafter sometimes referred to as ink) and ejected from a nozzle of a recording head included in an inkjet recording apparatus.

The pigment dispersion of the first embodiment contains an aqueous medium and pigment particles. The pigment particles are dispersed in an aqueous medium. The pigment particles include a core and a covering layer. The covering layer covers the core. The core contains pigment. The coating layer contains a crosslinked resin. The crosslinked resin has a main chain structure and a crosslinked structure. The main chain structure is derived from a predetermined resin. The crosslinked structure originates from the crosslinking agent. The predetermined resin is a resin that can react with a crosslinking agent. The predetermined resin has a mass average molecular weight of 4,000 or more and 40,000 or less. The crosslinking agent has at least two reactive functional groups and a structure represented by formula (1). The reactive functional group is capable of reacting with a group possessed by a predetermined resin. In formula (1), R ¹ represents a hydrogen atom or a methyl group, n represents a number from 4 to 13, and * represents a bond. Hereinafter, "the structure represented by formula (1)" may be referred to as "structure (1)". Furthermore, in this specification, the "predetermined resin" is defined as "a resin capable of reacting with a crosslinking agent."

By having the above structure, the pigment dispersion of the first embodiment can suppress the occurrence of nozzle clogging when contained in ink. The reason is inferred as follows.

A pigment is a component that has low dispersibility in an aqueous medium when used alone. Therefore, in order to disperse the pigment in an aqueous medium, for example, a resin having a hydrophilic group may be adsorbed onto the pigment. However, the adsorption power of the resin to the pigment is relatively weak, and the resin tends to be easily released from the pigment. In particular, when water evaporates due to drying of the ink in the nozzle and the hydrophobicity of the aqueous medium contained in the ink increases, the resin tends to be liberated from the pigment. Therefore, a crosslinked resin is formed by crosslinking the resin adsorbed to the pigment using a crosslinking agent. In this way, pigment particles are formed that include a core containing a pigment and a coating layer containing a crosslinked resin. Since the pigment is crosslinked with a crosslinking agent, the crosslinked resin is difficult to separate from the pigment. Therefore, it is possible to suppress the release of the crosslinked resin from the pigment and the aggregation of the exposed pigment. As a result, the pigment dispersion of the first embodiment can suppress the occurrence of nozzle clogging when contained in ink.

However, when at least a part of the hydrophilic groups (for example, groups possessed by a given resin, more specifically carboxy groups, etc.) possessed by the resin before crosslinking are crosslinked, the hydrophilicity of the resin decreases, and the pigment becomes less resistant to aqueous media. Particle dispersibility may be reduced. Therefore, in the first embodiment, the crosslinking agent, and eventually the crosslinked structure of the crosslinked resin, includes the structure (1). Since the ether bond (-O-) in structure (1) imparts hydrophilicity to the crosslinked resin, the pigment particles are suitably dispersed in the aqueous medium.

Furthermore, in the first embodiment, n in formula (1) is a number of 4 or more and 13 or less. When n is a number of 4 or more, a sufficient number of ether bonds in structure (1) can be ensured, and sufficient hydrophilicity can be imparted to the crosslinked resin. On the other hand, when n is a number exceeding 13, the ether bonds in structure (1) tend to form intramolecular hydrogen bonds with each other, and the hydrophilicity of the crosslinked resin tends to decrease. When n is a number of 13 or less, the ether bonds in structure (1) are unlikely to form intramolecular hydrogen bonds with each other, and sufficient hydrophilicity is imparted to the crosslinked resin.

As described above, since sufficient hydrophilicity is imparted to the crosslinked resin, the pigment particles provided with the coat layer containing the crosslinked resin are suitably dispersed in the aqueous medium. Therefore, the pigment dispersion of the first embodiment can suppress the occurrence of nozzle clogging when contained in ink.

In addition, as described above, since sufficient hydrophilicity is imparted to the crosslinked resin, the pigment particles provided with the coat layer containing the crosslinked resin are easily resistant to purged ink when the recording head is cleaned. redistribute to. Due to redispersion, ink adhering to the nozzle surface of the recording head is easily cleaned. As a result, the pigment dispersion of the first embodiment can suppress the occurrence of nozzle clogging when contained in ink.

Furthermore, the crosslinked resin has a main chain structure derived from a predetermined resin. The predetermined resin has a mass average molecular weight of 4,000 or more and 40,000 or less. If the weight average molecular weight of the predetermined resin is within this range, nozzle clogging can be further suppressed when the pigment dispersion of the first embodiment is contained in the ink.

The reason why the pigment dispersion of the first embodiment can suppress the occurrence of nozzle clogging when contained in ink has been explained above. The pigment dispersion of the first embodiment will be described in more detail below.

<Pigment particles>
In the pigment dispersion of the first embodiment, a large number of pigment particles are dispersed in an aqueous medium. The pigment particles include a core and a coating layer covering the core. The entire surface of the core may be covered with a coating layer. Further, a part of the surface of the core may be covered with a coating layer.

The core contains pigment. Preferably, the core contains a pigment as a main component. The pigment content in the core is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. The core may contain only pigment. Moreover, in addition to the pigment, the core may further contain other components (more specifically, a part of the crosslinked resin, other components described below, etc.).

The coating layer contains a crosslinked resin. Preferably, the coating layer contains a crosslinked resin as a main component. The content of the crosslinked resin in the coating layer is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. The coating layer may contain only the crosslinked resin. In addition to the crosslinked resin, the coating layer may further contain other components (more specifically, some pigments, additives described below, etc.).

The ratio Wr/Wp of the mass Wr of the crosslinked resin to the mass Wp of the pigment is preferably 0.1 or more and 1.0 or less. The total content of the pigment and crosslinked resin in the pigment particles is preferably 90% by mass or more, more preferably 100% by mass. The content of pigment particles in the pigment dispersion is preferably 10% by mass or more and 30% by mass or less. The content of the pigment in the pigment dispersion is preferably 5% by mass or more and 20% by mass or less. The content of the crosslinked resin in the pigment dispersion is preferably 2% by mass or more and 10% by mass or less.

From the viewpoint of optimizing the color density, hue, and stability of the ink when the pigment dispersion is contained in the ink, the volume median diameter of the pigment particles in the pigment dispersion is preferably 30 nm or more and 200 nm or less, More preferably, the thickness is 70 nm or more and 130 nm or less.

(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 and 15:3. As the purple pigment, for example, C.I. I. Pigment Violet 19, 23, and 33. Examples of black pigments include C.I. I. Pigment Black 7 and carbon black.

(Crosslinked resin)
The crosslinked resin has a main chain structure and a crosslinked structure. For example, a relatively highly hydrophobic main chain structure is located on the pigment side, and a relatively highly hydrophilic crosslinked structure is located on the aqueous medium side. With such a positional relationship, the pigment particles are suitably dispersed in the aqueous medium. The crosslinked resin functions as a dispersant that disperses pigment particles in an aqueous medium.

The main chain structure is derived from a predetermined resin. The crosslinked structure originates from the crosslinking agent. A crosslinked resin is formed by crosslinking a predetermined resin with a crosslinking agent. More specifically, a crosslinked resin is formed by a crosslinking reaction between at least two reactive functional groups of the crosslinking agent and a group of a predetermined resin.

(Crosslinking agent)
The crosslinking agent has at least two reactive functional groups and structure (1) in one molecule. As already mentioned, the structure (1) that the crosslinking agent has is represented by the following formula (1).

In formula (1), R ¹ represents a hydrogen atom or a methyl group, n represents a number (for example, an integer) of 4 or more and 13 or less, and * represents a bond. When R ¹ represents a hydrogen atom, structure (1) is a polyethylene glycol structure composed of repeating units derived from ethylene glycol. When R ¹ represents a methyl group, structure (1) is a polypropylene glycol structure composed of repeating units derived from propylene glycol. The number represented by n corresponds to the average degree of polymerization of repeating units derived from ethylene glycol or repeating units derived from propylene glycol. In order to suppress the occurrence of nozzle clogging, n preferably represents a number of 5 or more and 10 or less, and more preferably a number of 7 or more and 9 or less. * specifically represents a bond bonded to an atom constituting the crosslinking agent.

The reactive functional group possessed by the crosslinking agent is capable of reacting with the group possessed by a predetermined resin. For example, the reactive functional group that the crosslinking agent has can react with the carboxyl group that the carboxyl group-containing resin has. Examples of the reactive functional group include a glycidyl group, an oxazoline group, a carbodiimide group, and an isocyanate group. As the reactive functional group, a glycidyl group, an oxazoline group, or a carbodiimide group is preferable.

When the reactive functional group of the crosslinking agent is a glycidyl group, the epoxy equivalent of the crosslinking agent is preferably 150 g/eq or more and 500 g/eq.

When the reactive functional group of the crosslinking agent is a carbodiimide group, the chemical formula weight of the crosslinking agent per mole of carbodiimide group is preferably 400 g/eq or more and 700 g/eq or less.

The at least two reactive functional groups may be the same group or different groups. The number of reactive functional groups that the crosslinking agent has may be, for example, two or three.

An example of the crosslinking agent is a crosslinking agent represented by formula (2) (hereinafter sometimes referred to as crosslinking agent (2)).

In formula (2), R ^1A represents a hydrogen atom or a methyl group, n ^A represents a number from 4 to 13, and R ^2r and R ^3r each represent a reactive functional group. R ^2r and R ^3r may be the same group or different groups. It is preferable that R ^2r and R ^3r each independently represent a glycidyl group, an oxazoline group, or a carbodiimide group. More preferably, R ^2r and R ^3r each represent a glycidyl group.

Commercially available products that can be used as the crosslinking agent (2) include, for example, Denacol (registered trademark) EX-821, Denacol (registered trademark) EX-830, and Denacol (registered trademark) EX-841 manufactured by Nagase ChemteX Corporation. , and Epolite (registered trademark) 400E and Epolite (registered trademark) 400P manufactured by Kyoeisha Chemical Co., Ltd.

Another example of the crosslinking agent is a crosslinking agent having a repeating unit represented by formula (3) and at least two repeating units represented by formula (4). Hereinafter, "the repeating unit represented by formula (3)" and "the repeating unit represented by formula (4)" will be referred to as "repeat unit (3)" and "repeat unit (4)", respectively. There is.

In formula (3), R ^1B represents a hydrogen atom or a methyl group, n ^B represents a number from 4 to 13, and R ⁴ represents a hydrogen atom or a methyl group. In formula (4), R ⁵ represents a hydrogen atom or a methyl group, and R ^6r represents a reactive functional group.

In formula (3), R ^1B preferably represents a hydrogen atom. n ^B preferably represents a number of 8 or more and 10 or less, and particularly preferably represents 9. Preferably, R ⁴ represents a hydrogen atom. In formula (4), R ⁵ preferably represents a hydrogen atom. R ^6r preferably represents a glycidyl group, an oxazoline group, or a carbodiimide group, and more preferably an oxazoline group.

A specific example of the crosslinking agent having the repeating unit (3) and at least two repeating units (4) is a copolymer of 2-vinyl-2-oxazoline and methoxypolyethylene glycol acrylate. As the methoxypolyethylene glycol acrylate, a commercially available product (for example, "Light Acrylate 130A" manufactured by Kyoeisha Chemical Co., Ltd.) may be used.

Still other examples of crosslinking agents include polycarbodiimide-containing crosslinking agents. As the polycarbodiimide-containing crosslinking agent, a commercially available product (for example, "Carbodilite (registered trademark) V-02" manufactured by Nisshinbo Chemical Co., Ltd.) may be used.

Preferably, the crosslinking agent is water-soluble. If the crosslinking agent is water-soluble, the crosslinking reaction with the predetermined resin will proceed efficiently in an aqueous medium. In this specification, whether or not a crosslinking agent is water-soluble can be confirmed by mixing 10 g of a crosslinking agent and 90 g of water at a temperature of 25°C. If the crosslinking agent dissolves in water to give a homogeneous and transparent solution, it is determined to be water-soluble.

(Crosslinked structure)
By subjecting a predetermined resin to a crosslinking reaction using a crosslinking agent, a crosslinked structure derived from the crosslinking agent can be introduced into the crosslinked resin. The crosslinked structure includes at least structure (1). The crosslinked structure further includes a structure derived from a reactive functional group. Structures derived from reactive functional groups include, for example, groups represented by formulas (20), (30), and (40).

In formula (20), * ²⁰ represents a bond that is bonded to an atom that constitutes a crosslinked structure, and * ^MC2 represents a bond that is bonded to an atom that constitutes a main chain structure. In formula (30), * ³⁰ represents a bond that is bonded to an atom that constitutes a crosslinked structure, and * ^MC3 represents a bond that is bonded to an atom that constitutes a main chain structure. In formula (40), * ⁴⁰ and * ⁴¹ each represent a bond that is bonded to an atom that constitutes a crosslinked structure, and * ^MC4 represents a bond that is bonded to an atom that constitutes a main chain structure. The atoms constituting the crosslinked structure to which the bonds represented by * ²⁰ , * ³⁰ , * ⁴⁰ , and * ⁴¹ are bonded are, for example, the oxygen atom to which * in formula (1) is bonded, and the oxygen atom in formula (2A) described below. This is an oxygen atom to which X ^2r and X ^3r are bonded, or a carbon atom to which X ^6r in formula (4A) described below is bonded.

When the reactive functional group of the crosslinking agent is a glycidyl group, the crosslinked structure includes a group represented by formula (20). When the reactive functional group of the crosslinking agent is an oxazoline group, the crosslinked structure includes a group represented by formula (30). When the reactive functional group of the crosslinking agent is a carbodiimide group, the crosslinked structure includes a group represented by formula (40).

When the already mentioned crosslinking agent (2) is used, the crosslinked structure includes a structure represented by formula (2A). R ^1A and n ^A in formula (2A) have the same meanings as R ^1A and n ^A in formula (2). X ^2r and X ^3r in formula (2A) each independently represent a group represented by formula (20), (30), or (40).

When a crosslinking agent having a repeating unit (3) and at least two repeating units (4) is used, the crosslinking structure includes a repeating unit (3) and at least two repeating units represented by formula (4A). include. R ⁵ in formula (4A) has the same meaning as R ⁵ in formula (4). X ^6r in formula (4A) represents a group represented by formula (20), (30), or (40).

The crosslinking rate of the crosslinking agent is preferably 30% or more and 100%, more preferably 50% or more and 80%. When the crosslinking rate of the crosslinking agent is within this range, the pigment particles are suitably dispersed in the aqueous medium, and the pigment particles are suitably redispersed in the purge ink. When the predetermined resin is a carboxyl group-containing resin, the crosslinking rate of the crosslinking agent is calculated using the formula: “Crosslinking rate = 100 x number of molar equivalents of reactive functional groups of the crosslinking agent/mole equivalent of carboxyl groups of the carboxyl group-containing resin. It is calculated based on the formula: number=100×number of molar equivalents of reactive functional groups of crosslinking agent/[(mass of carboxy group-containing resin×acid value of carboxy group-containing resin/1000)/formula weight of KOH].

(Specified resin)
The predetermined resin is a pre-crosslinked resin before being crosslinked with a crosslinking agent. As already stated, in this specification, the "predetermined resin" is defined as a "resin capable of reacting with a crosslinking agent."

As already mentioned, the mass average molecular weight of the predetermined resin is 4,000 or more and 40,000 or less. The mass average molecular weight of the predetermined resin is preferably 5,000 or more and 30,000 or less. If the mass average molecular weight of the predetermined resin is within this range, the core can be efficiently covered with the coating layer. Further, the viscosity of the pigment dispersion and, by extension, the viscosity of the ink containing the pigment dispersion can be optimized.

The predetermined resin has a group capable of reacting with the reactive functional group of the crosslinking agent. Examples of the group possessed by the predetermined resin, which is a group capable of reacting with a reactive functional group, include a carboxy group, a hydroxy group, a primary amino group, and a secondary amino group. The group that the predetermined resin has is preferably a carboxy group.

Examples of the predetermined resin include carboxyl group-containing resins, polyester resins, urethane resins, and acrylic urethane resins. The predetermined resin that can react with the crosslinking agent is preferably a carboxyl group-containing resin. The carboxyl group-containing resin is a resin having a carboxyl group. The carboxyl group-containing resin will be explained below.

The acid value of the carboxyl group-containing resin is preferably 30 mgKOH/g or more and 250 mgKOH/g or less, more preferably 60 mgKOH/g or more and 220 mgKOH/g or less, and 130 mgKOH/g or more and 170 mgKOH/g or less. More preferred. When the acid value of the carboxyl group-containing resin is within this range, the pigment particles are suitably dispersed in the aqueous medium.

In order to suitably disperse pigment particles in an aqueous medium, the carboxyl group-containing resin is preferably anionic. Examples of the carboxyl group-containing resin include styrene acrylic resin, styrene-maleic acid copolymer, styrene-(meth)acrylic acid-maleic acid copolymer, and styrene-(meth)acrylic acid-(meth)acrylic acid ester. Examples include maleic acid copolymers, styrene-maleic acid half ester copolymers, vinylnaphthalene-(meth)acrylic acid copolymers, and vinylnaphthalene-maleic acid copolymers. The carboxyl group-containing resin may be a random polymer or a block polymer.

As the carboxyl group-containing resin, styrene acrylic resin is preferred. When the carboxyl group-containing resin is a styrene acrylic resin, the pigment particles are suitably dispersed in the aqueous medium. Furthermore, pigment particles can be easily produced. Styrene acrylic resin has at least a repeating unit derived from styrene and a repeating unit derived from (meth)acrylic acid as repeating units. It is preferable that the styrene acrylic resin further has a repeating unit derived from a (meth)acrylic acid ester as a repeating unit.

The repeating unit derived from styrene is represented by formula (50). Hereinafter, "the repeating unit represented by formula (50)" may be referred to as "repeating unit (50)".

In the styrene acrylic resin, the content ratio of repeating units derived from styrene to all repeating units is preferably 10.0% by mass or more and 90.0% by mass or less, and 60.0% by mass or more and 80.0% by mass or less. More preferred. The higher the content of repeating units derived from styrene, the higher the hydrophobicity of the main chain structure. The highly hydrophobic main chain structure is suitably adsorbed to the highly hydrophobic pigment.

The repeating unit derived from (meth)acrylic acid is represented by formula (51). In formula (51), R ⁵¹ represents a hydrogen atom or a methyl group. Hereinafter, "the repeating unit represented by formula (51)" may be referred to as "repeating unit (51)".

In the styrene acrylic resin, the content ratio of repeating units derived from (meth)acrylic acid to all repeating units is preferably 5.0% by mass or more and 40.0% by mass or less, and 10.0% by mass or more and 15.0% by mass or less. It is more preferably less than % by mass. If the content ratio of repeating units derived from (meth)acrylic acid is within this range, an appropriate acid value can be imparted to the styrene acrylic resin, and a crosslinked structure can be appropriately introduced into the crosslinked resin.

Examples of repeating units derived from (meth)acrylic esters include repeating units derived from alkyl (meth)acrylates. The repeating unit derived from alkyl (meth)acrylate is represented by formula (52), for example. In formula (52), R ⁵² represents a hydrogen atom or a methyl group. R ⁵³ represents an alkyl group. Hereinafter, the "repeat unit represented by formula (52)" may be referred to as "repeat unit (52)."

In formula (52), the alkyl group represented by R ⁵³ is linear or branched and unsubstituted unless otherwise specified. The alkyl group represented by R ⁵³ is preferably an alkyl group having 1 to 8 carbon atoms, more preferably an alkyl group having 1 to 4 carbon atoms, and is a methyl group or n-butyl group. More preferably.

In the styrene acrylic resin, the content ratio of repeating units derived from (meth)acrylic acid ester to all repeating units is preferably 10.0% by mass or more and 30.0% by mass or less, and 15.0% by mass or more and 20.0% by mass or less. More preferably, it is 0% by mass or less.

The styrene acrylic resin preferably has repeating units derived from monomers shown in combination (I) or (II) below. It is more preferable that the styrene acrylic resin has only repeating units derived from the monomers shown in combination (I) or (II) as repeating units.
Combination (I): Styrene, methacrylic acid, and acrylic acid alkyl ester Combination (II): Styrene, acrylic acid, and methacrylic acid alkyl ester

As combination (I), the following combination (Ia) is preferred, and combination (Ib) is more preferred.
Combination (Ia): Styrene, methacrylic acid, and butyl acrylate Combination (Ib): Styrene, methacrylic acid, and n-butyl acrylate

As combination (II), the following combination (IIa) is preferable.
Combination (IIa): styrene, acrylic acid, and methyl methacrylate

Styrene acrylic resin can be synthesized, for example, by adding a monomer or prepolymer capable of synthesizing styrene acrylic resin and a polymerization initiator to a predetermined solvent, and heating and refluxing the mixture at a predetermined temperature.

(main chain structure)
By crosslinking a predetermined resin with a crosslinking agent, a main chain structure derived from the predetermined resin can be introduced into the crosslinked resin. When the predetermined resin is a carboxyl group-containing resin, by crosslinking the carboxyl group-containing resin with a crosslinking agent, a main chain structure derived from the carboxyl group-containing resin can be introduced into the crosslinked resin. For example, the carboxy group of the repeating unit (51) reacts with a crosslinking agent to form a repeating unit represented by formula (51A) (hereinafter sometimes referred to as repeating unit (51A)). In this case, the main chain structure has a repeating unit (51A).

R ^51A in formula (51A) has the same meaning as R ⁵¹ in formula (51). R ⁵⁴ in formula (51A) represents -O-* ^CL or -* ^CL . * ^CL represents a bond that is bonded to an atom that constitutes a crosslinked structure. When the reactive functional group of the crosslinking agent is a glycidyl group or an oxazoline group, R ⁵⁴ represents -O-* ^CL . When the reactive functional group of the crosslinking agent is a carbodiimide group, R ⁵⁴ represents -* ^CL . The atoms constituting the crosslinked structure to which the bond represented by * ^CL is bonded are, for example, the carbon atom to which * ^MC2 in formula (20) is bonded, the carbon atom to which *MC3 in formula (30) is bonded, or the carbon atom to which * ^MC3 in formula (30) is bonded, or * ^MC4 in 40) is the bonding nitrogen atom.

When the carboxyl group-containing resin is a styrene acrylic resin, the main chain structure has at least repeating units (50) and (51A). The main chain structure may further have one or both of repeating unit (51) and formula (52). The main chain structure may have only one type of repeating unit (51) or may have two types. The main chain structure may have only one type of repeating unit (51A), or may have two or more types. The main chain structure may have only one type of repeating unit (52), or may have two or more types.

<Aqueous medium for pigment dispersion>
The aqueous medium contained in the pigment dispersion of the first embodiment is a medium containing water. Hereinafter, the "aqueous medium contained in the pigment dispersion" may be referred to as "aqueous medium (D)". The aqueous medium (D) may function as a solvent or a dispersion medium.

The aqueous medium (D) contains at least water. Examples of water include ion exchange water and pure water.

In addition to water, the aqueous medium (D) may further contain a water-soluble organic solvent. The water-soluble organic solvent will be described later in the second embodiment. The water-soluble organic solvent contained in the aqueous medium (D) is preferably a glycol compound, and more preferably 1,2-pentanediol. When the aqueous medium (D) further contains a water-soluble organic solvent, the content of the water-soluble organic solvent in the aqueous medium (D) is preferably more than 0% by mass and 15% by mass or less. However, the aqueous medium (D) does not need to contain a water-soluble organic solvent.

The content of the aqueous medium (D) in the pigment dispersion is preferably 20% by mass or more and 80% by mass or less, more preferably 60% by mass or more and 80% by mass or less.

<Other ingredients>
The pigment dispersion of the first embodiment may further contain known additives, if necessary. Known additives include, for example, antifoaming agents, neutralizing agents, dissolution stabilizers, humectants, penetrants, anti-drying agents, antioxidants, viscosity modifiers, pH modifiers, and antifungal agents. . Examples of the neutralizing agent include basic compounds, more specifically potassium hydroxide and sodium hydroxide. Note that since the pigment particles can be sufficiently dispersed by the crosslinked resin, it is not necessary to contain a dispersant other than the crosslinked resin (for example, a polymer dispersant other than the crosslinked resin, and a surfactant).

<Method for producing pigment dispersion>
Next, an example of a method for manufacturing the pigment dispersion of the first embodiment will be described. The method for producing a pigment dispersion includes, for example, a dispersion step and a crosslinking step. The method for producing a pigment dispersion may further include a centrifugation step, if necessary.

(Dispersion process)
In the dispersion step, the pigment and a predetermined resin before crosslinking are dispersed in an aqueous medium. In this way, the first dispersion before crosslinking is obtained in this step. Examples of the dispersion device used in the dispersion process include wet-type dispersion devices such as a media-type dispersion machine. By changing the particle size of the media used in the media-type dispersion machine (for example, the diameter of beads), the degree of dispersion of the pigment particles, the amount of resin liberated in the pigment dispersion, and the particle size of the pigment particles can be adjusted. For example, the smaller the particle size of the media, the smaller the particle size of the pigment particles tends to be.

When the predetermined resin used in this step is a carboxyl group-containing resin, the carboxyl group-containing resin is preferably neutralized with a neutralizing agent in advance. In this case, it is preferable not to completely neutralize the carboxyl group-containing resin, but to partially neutralize it. Neutralization facilitates the dissolution of the carboxyl group-containing resin into the aqueous medium. As the neutralizing agent, a basic compound is preferable, an alkali metal hydroxide is more preferable, and potassium hydroxide or sodium hydroxide is even more preferable, since it is difficult to evaporate by drying and can further suppress the occurrence of nozzle clogging.

The neutralization rate of the carboxyl group-containing resin is preferably 50% or more and 90% or less. The neutralization rate (unit: %) is calculated from the formula: "Neutralization rate = 100 x number of molar equivalents of neutralizing agent/number of molar equivalents of carboxyl groups in carboxyl group-containing resin". If the neutralization rate is 50% or more, the carboxyl group-containing resin will be easily dissolved in the aqueous medium. If the neutralization rate is 90% or less, the viscosity of the first pigment dispersion can be optimized. Moreover, the hydrophilicity of the carboxyl group-containing resin becomes moderately low, and the carboxyl group-containing resin is sufficiently adsorbed to the pigment.

The neutralized acid value of the carboxyl group-containing resin is preferably 30 mgKOH/g or more and 150 mgKOH/g or less. The unneutralized acid value of the carboxyl group-containing resin is preferably 15 mgKOH/g or more. The neutralized acid value of the carboxyl group-containing resin is the acid value of the portion of the carboxyl group-containing resin that has been neutralized by a neutralizing agent. The neutralized acid value is calculated from the formula "neutralized acid value = acid value of carboxyl group-containing resin before neutralization x neutralization rate/100". The unneutralized acid value of the carboxyl group-containing resin is the acid value of the portion of the carboxyl group-containing resin that remains without being neutralized. The unneutralized acid value is calculated from the formula "unneutralized acid value = acid value of carboxyl group-containing resin before neutralization x (100-neutralization rate)/100".

It is preferable that the aqueous medium used for the dispersion treatment further contains an antifoaming agent. The content of the antifoaming agent in the pigment dispersion is, for example, 0.01% by mass or more and 0.1% by mass or less.

It is preferable to filter the obtained first pigment dispersion using a filter (eg, 5 μm in pore size) to remove coarse particles.

(Crosslinking process)
In the crosslinking step, a predetermined resin contained in the first pigment dispersion and a crosslinking agent are subjected to a crosslinking reaction to form a crosslinked resin. In this way, a crosslinked second pigment dispersion is obtained in this step. In this step, for example, after adding the crosslinking agent, the first pigment dispersion is heated while being stirred. The heating temperature is, for example, 70°C or higher and 95°C or lower. The heating time is, for example, 30 minutes or more and 5 hours or less. If the centrifugation step described below is not performed, the second pigment dispersion obtained in this step becomes the pigment dispersion of the first embodiment.

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

[Second embodiment: ink]
The ink according to the second embodiment of the present invention will be described below. The ink of the second embodiment contains at least the pigment dispersion of the first embodiment. The ink of the second embodiment may further contain an aqueous medium different from the aqueous medium (D). Hereinafter, "aqueous medium different from aqueous medium (D)" may be referred to as "additional aqueous medium". Furthermore, the ink of the second embodiment may further contain a surfactant.

Since the ink of the second embodiment contains at least the pigment dispersion of the first embodiment, the ink of the second embodiment can suppress the occurrence of nozzle clogging for the same reason as described in the first embodiment. The content of the pigment dispersion in the ink is preferably 30% by mass or more and 90% by mass or less, more preferably 50% by mass or more and 70% by mass or less.

The content of pigment particles in the ink is preferably 4% by mass or more and 15% by mass or less. If the content ratio of pigment particles in the ink is within this range, an image having a desired image density can be easily obtained. Further, it is easy to ensure sufficient permeability of the ink into the recording medium. The pigment content in the ink is preferably 3% by mass or more and 10% by mass or less. The content of the crosslinked resin in the ink is preferably 1% by mass or more and 5% by mass or less. The preferred range of the volume median diameter of the pigment particles in the ink is the same as the preferred range of the volume median diameter of the pigment particles in the pigment dispersion described in the first embodiment.

<Aqueous medium for ink>
The ink of the second embodiment contains an aqueous medium. Hereinafter, the "aqueous medium contained in the ink" may be referred to as "aqueous medium (I)". Aqueous medium (I) contains water and a water-soluble organic solvent. The aqueous medium (I) may function as a solvent or a dispersion medium. Aqueous medium (I) is composed of aqueous medium (D) and an additional aqueous medium. The content ratio of the water-soluble organic solvent in the aqueous medium (I) is preferably higher than the content ratio of the water-soluble organic solvent in the aqueous medium (D). The content of the water-soluble organic solvent in the aqueous medium (I) is preferably more than 15% by mass and 50% by mass or less.

Examples of water-soluble organic solvents include glycol compounds, triol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, γ-butyrolactone, thiodiglycol, and dimethyl sulfoxide.

Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,3-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1 , 2-octanediol, 1,8-octanediol, 3-methyl-1,3-butanediol, 3-methyl-1,2-pentanediol, 3-methyl-1,5-pentanediol, 2,4- Diethyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, diethylene glycol, dipropylene glycol, trimethylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, 2-ethyl- Examples include 1,2-hexanediol and thiodiglycol. As the glycol compound, 1,2-pentanediol, ethylene glycol, 3-methyl-1,5-pentanediol, 1,3-propanediol, and 1,5-pentanediol are preferred.

Examples of triol compounds include glycerin, 1,2,3-butanetriol, and 1,2,6-hexanetriol. Glycerin is preferred as the triol compound.

Examples of glycol ether compounds include diethylene glycol diethyl ether, diethylene glycol monoethyl ether, 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 ether, triethylene glycol monobutyl ether, and propylene glycol monomethyl ether are mentioned. As the glycol ether compound, diethylene glycol diethyl ether and triethylene glycol monobutyl ether are 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.

As the aqueous medium (I), a mixed solvent containing the following combinations (i), (ii), or (iii) is preferred.

Combination (i): Water, 1,2-pentanediol, ethylene glycol, and diethylene glycol diethyl ether Combination (ii): Water, 1,2-pentanediol, glycerin, 3-methyl-1,5-pentanediol, triethylene Glycol monobutyl ether, and 2-pyrrolidone Combination (iii): water, 1,2-pentanediol, triethylene glycol monobutyl ether, 2-pyrrolidone, 1,3-propanediol, and 1,5-pentanediol

Glycol compounds and triol compounds, for example, function as humectants. The total content of glycol compounds and triol compounds in the ink is preferably 2% by mass or more and 40% by mass or less, more preferably 10% by mass or more and 40% by mass or less.

The content of the aqueous medium (I) in the ink is preferably 30% by mass or more and 95% by mass or less, more preferably 70% by mass or more and 90% by mass or less.

<Surfactant>
The surfactant optimizes the permeability (wettability) of the ink 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 acetylene glycol surfactants. Examples of commercially available acetylene glycol surfactants include "Olfine (registered trademark) E1004" manufactured by Nissin Chemical Industries, Ltd. "Olfine (registered trademark) E1004" contains an ethylene oxide adduct of acetylene diol. In order to optimize the image density of the formed image while suppressing the offset of the formed image, the content of the surfactant in the ink is preferably 0.05% by mass or more and 2.00% by mass or less.

<Other ingredients>
The ink of the second embodiment may further contain known additives, if necessary. Examples of known additives that can be used in the ink are the same as the examples of known additives that can be used in the pigment dispersion described in the first embodiment.

<Ink manufacturing method>
Next, an example of a method for manufacturing the ink of the second embodiment will be described. The ink manufacturing method includes, for example, a mixing step. In the mixing step, the pigment dispersion of the first embodiment and an additional aqueous medium are mixed to obtain the ink of the second embodiment. In this step, other components (more specifically, at least one selected from the group consisting of surfactants and known additives) may be further added as necessary. Mixing is performed, for example, using a stirrer. It is preferable that the obtained ink of the second embodiment is filtered through a filter (for example, with a pore size of 5 μm) to remove coarse particles.

<How to use ink>
Next, an example of a method of using the ink of the second embodiment will be described. The ink of the second embodiment is ejected from a nozzle of a recording head included in an inkjet recording apparatus onto a printing surface of a recording medium. Examples of the recording medium include permeable recording media and non-permeable recording media. Permeable recording media have high permeability to water-based inks. Examples of the permeable recording medium include printing paper and those processed using fibers (for example, cloth). Examples of printing paper include plain paper, copy paper, recycled paper, thin paper, thick paper, and glossy paper. Non-permeable recording media have lower permeability to aqueous inks than permeable recording media. For example, the absorption amount of an aqueous medium of a non-permeable recording medium is 1.0 g/m ² or less. Examples of non-permeable recording media include recording media made of resin, recording media made of metal, and recording media made of glass. Examples of the resin recording medium include a resin sheet and a resin film. The resin contained in the resin recording medium is preferably a thermoplastic resin, and more preferably polyethylene, polyvinyl chloride, or polyethylene terephthalate (PET). An example of a recording medium made of resin is an OHP (overhead projector) sheet. The printing surface of the resin-made recording medium may be subjected to corona treatment.

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

[Preparation of resin]
<Preparation of resin (R1)>
A mixed solvent was prepared by mixing 100.0 g of isopropyl alcohol and 250.0 g of methyl ethyl ketone. Separately, 70.0 g of styrene, 20.0 g of n-butyl acrylate, 10.0 g of methacrylic acid, and 0.3 g of azobisisobutyronitrile (AIBN, polymerization initiator) were mixed, A monomer solution was prepared. Separately, 150.0 g of methyl ethyl ketone and 0.1 g of AIBN were mixed to prepare a methyl ethyl ketone solution.

The above mixed solvent was charged into a four-necked flask equipped with a stirrer, a nitrogen introduction tube, a condenser, and a dropping funnel. Next, nitrogen gas was introduced into the flask to create a nitrogen atmosphere. Next, while heating the contents of the flask at 70° C. under reflux, the entire amount of the monomer solution was dropped into the flask over 2 hours using a dropping funnel. After feeding the monomer solution, the flask contents were heated to reflux at 70° C. for an additional 6 hours. Next, while heating the contents of the flask under reflux at 70° C., the entire amount of the above methyl ethyl ketone solution was supplied into the flask over 15 minutes using a dropping funnel. After feeding the methyl ethyl ketone solution, the flask contents were heated to reflux at 70° C. for an additional 5 hours. Thereby, a resin solution containing resin (R1), which is a styrene acrylic resin, was obtained. The resin (R1) was isolated by distilling off the solvent from the resin solution under reduced pressure.

<Preparation of resin (R2)>
Resin (R2), which is a styrene acrylic resin, was prepared in the same manner as in the preparation of resin (R1), except that the following points were changed. In preparing the monomer solution, the addition of AIBN was changed from 0.3g to 0.5g. Further, in preparing the monomer solution, 2.0 g of 2-cyano-2-propylbenzodithioate (polymerization initiator) was further mixed.

<Preparation of resin (R3)>
Resin (R3), which is a styrene acrylic resin, was prepared in the same manner as in the preparation of resin (R1), except that the following points were changed. In preparing the monomer solution, the amount of AIBN added was changed from 0.3 g to 0.6 g. Further, in preparing the monomer solution, 2.4 g of 2-cyano-2-propylbenzodithioate (polymerization initiator) was further mixed.

<Preparation of resin (R4)>
Resin (R4), which is a styrene acrylic resin, was prepared in the same manner as in the preparation of resin (R1), except that the following points were changed. In preparing the monomer solution, the amount of AIBN added was changed from 0.3 g to 0.2 g. Further, the dropping time of the monomer solution was changed from 2 hours to 4 hours.

<Preparation of resin (R5)>
Resin (R5), which is a styrene acrylic resin, was prepared in the same manner as in the preparation of resin (R1), except for the following changes. Instead of the monomer solution used in the preparation of resin (R1), a mixed solution of 70.0 g of styrene, 15.0 g of acrylic acid, 15.0 g of methyl methacrylate, and 0.3 g of AIBN was used. It was used.

[Measurement of mass average molecular weight]
The mass average molecular weights (Mw) of the resins (R1) to (R5) were measured by gel filtration chromatography (GPC) under the following conditions. The measurement results are shown in Table 1 below.

<GPC conditions>
Measuring device: “HLC-8020GPC” manufactured by Tosoh Corporation
Column: Ultra-high performance semi-micro SEC column (“TSKgel SuperMultipore HZ-H” manufactured by Tosoh Corporation, packing material: styrene-divinylbenzene resin, column size: inner diameter 4.6 mm x length 15 cm, filler particle diameter: 6 μm)
Number of columns: 3 Eluent: Tetrahydrofuran Eluent flow rate: 0.35 mL/min Sample solution amount: 10 μL
Column temperature: 40℃
Detector: RI (refractive index) detector Calibration curve: Monodisperse polystyrene standard samples manufactured by Tosoh Corporation (F-40, F-20, F-4, F-1, A-5000, A-2500 and A- 1000) and n-propylbenzene.

[Measurement of acid value]
Resins (R1) to ( The acid value of R5) was measured. The measurement results are shown in Table 1 below.

[Preparation of pigment dispersion]
Next, pigment dispersions (DA-1) to (DA-11) and (DA-14) to (DA-16) according to Examples, and pigment dispersions (DB-1) to (DB -3) was prepared. The pigments, pre-crosslinking resins, and crosslinking agents used in the preparation of these pigment dispersions are shown in Tables 2 and 3 below. As described later in Tables 4 and 5, the same pigment dispersion (DA-1) as ink (IA-1) was used in inks (IA-12) and (IA-13). Therefore, in order to make it easier to understand the correspondence with the ink numbers, the pigment dispersion numbers (DA-12) and (DA-13) were left blank.

The meanings of the terms in Tables 2 and 3 are as follows.
Gly: Glycidyl group OZ: Oxazoline group CDI: Carbodiimide group EG: Repeating unit derived from ethylene glycol (Structure (1) when R ¹ represents a hydrogen atom)
PG: Repeating unit derived from propylene glycol (Structure (1) when R ¹ represents a methyl group)
Content rate: Content rate of pigment with respect to the mass of the pigment dispersion Mw: Mass average molecular weight

<Pigment>
The pigments shown in Tables 2 and 3 are as follows.
Pigment (Px): Carbon black (“Printex (registered trademark) 80” manufactured by Orion Engineered Carbons Co., Ltd.)
Pigment (ECB-301): Phthalocyanine blue (“ECB-301” manufactured by Dainichiseika Kagyo Co., Ltd.)
Pigment (5GX01): Monoazo yellow (“Hansa (registered trademark) Brilliant Yellow 5GX01” manufactured by Clariant Co., Ltd.)
Pigment (ES): Quinacridone Magenta (“Ink Jet Magenta ES” manufactured by Clariant Co., Ltd.)

<Crosslinking agent>
The crosslinking agents shown in Tables 2 and 3 are as follows. The type and number of reactive functional groups of each crosslinking agent, as well as the type and degree of polymerization of repeating units in structure (1), are already shown in Tables 2 and 3.
Crosslinking agent (EX-821): Polyethylene glycol diglycidyl ether (“Denacol (registered trademark) EX-821” manufactured by Nagase ChemteX Co., Ltd., epoxy equivalent 185 g/eq)
Crosslinking agent (EX-830): Polyethylene glycol diglycidyl ether (“Denacol (registered trademark) EX-830” manufactured by Nagase ChemteX Co., Ltd., epoxy equivalent 268 g/eq)
Crosslinking agent (EX-841): Polyethylene glycol diglycidyl ether (“Denacol (registered trademark) EX-841” manufactured by Nagase ChemteX Co., Ltd., epoxy equivalent 372 g/eq)
Crosslinking agent (EX-850): Polyethylene glycol diglycidyl ether (“Denacol (registered trademark) EX-850” manufactured by Nagase ChemteX Co., Ltd., epoxy equivalent 122 g/eq)
Crosslinking agent (EX-861): Polyethylene glycol diglycidyl ether (“Denacol (registered trademark) EX-861” manufactured by Nagase ChemteX Co., Ltd., epoxy equivalent weight 551 g/eq)
Crosslinking agent (400E): Polyethylene glycol diglycidyl ether (“Epolite (registered trademark) 400E” manufactured by Kyoeisha Chemical Co., Ltd., epoxy equivalent (median value) 277 g/eq)
Crosslinking agent (400P): Polypropylene glycol diglycidyl ether (“Epolite (registered trademark) 400P” manufactured by Kyoeisha Chemical Co., Ltd., epoxy equivalent (median value) 315 g/eq)
Crosslinking agent (V-02): Polycarbodiimide-containing crosslinking agent ("Carbodilite (registered trademark) V-02" manufactured by Nisshinbo Chemical Co., Ltd., chemical formula amount per mole of carbodiimide group: 590, nonvolatile content: 40% by mass)
Crosslinking agent (CL1): Copolymer of 2-vinyl-2-oxazoline and methoxypolyethylene glycol acrylate (“Light Acrylate 130A” manufactured by Kyoeisha Chemical Co., Ltd.) Crosslinking agent (CL2): 2-vinyl-2-oxazoline and methoxy Copolymer with polyethylene glycol acrylate (“Light Acrylate 130A” manufactured by Kyoeisha Chemical Co., Ltd.)

<Preparation of pigment dispersion (DA-1)>
(neutralization treatment)
Resin (R1), which is a pre-crosslinked resin, potassium hydroxide, and water were mixed to prepare an aqueous resin solution having a resin concentration 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 (R1). That is, the neutralization rate of the resin (R1) was 60%.

(Distributed processing)
15.0 parts by mass of pigment (Px), 20.0 parts by mass of the resin aqueous solution obtained by the above neutralization treatment (containing 6.0 parts by mass of resin), and 5.0 parts by mass of 1.2-pentanediol. , 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 were mixed to obtain a mixture. The amount of ion-exchanged water added was such that the mixture would be 100.0 parts by mass.

The above mixture was subjected to a dispersion treatment for 4 hours using a media type dispersion machine ("Dyno (registered trademark) 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 process, the media was removed from the contents of the vessel of the media type dispersion machine. The contents of the vessel were filtered through a filter with a pore size of 5 μm to remove foreign matter and coarse particles. In this way, a first pigment dispersion before crosslinking treatment was obtained.

(Crosslinking treatment)
A three-necked flask equipped with a thermometer and stirring blade was used as a reaction vessel. 100.0 g of the first pigment dispersion was charged into the reaction vessel. The temperature of the contents of the reaction vessel was maintained at 30°C using a water bath. Next, 2.7 g of crosslinking agent (EX-830) was charged into the reaction vessel. The contents of the reaction vessel were then stirred at 150 rpm for 1 hour. 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 3 hours 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 (R1) was crosslinked with the crosslinking agent (EX-830) to form a crosslinked resin. Next, the contents of the reaction vessel were allowed to cool until the temperature reached room temperature. Thereby, a second pigment dispersion after crosslinking treatment was obtained.

(centrifugal treatment)
The second pigment dispersion was transferred to a container, and this container was placed in a centrifugal adhesion measuring device ("NS-C100" manufactured by Nano Seeds Co., Ltd.). The second pigment dispersion was centrifuged for 24 hours at a rotation speed of 50,000 rpm using a centrifugal adhesion measuring device. 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 (eg, free resin) were removed from the aqueous medium contained in the second pigment dispersion. In this way, a pigment dispersion (DA-1) was obtained.

<Preparation of pigment dispersions (DA-2) to (DA-11), (DA-14) to (DA-16), and (DB-1) to (DB-3)>
Except that the type of pre-crosslinking resin used in the neutralization treatment, the type of pigment used in the dispersion treatment, and the type of crosslinking agent used in the crosslinking treatment were set as shown in Tables 2 and 3. Pigment dispersions (DA-2) to (DA-11), (DA-14) to (DA-16), and (DB-1) to (DB-3) was prepared.

[Preparation of ink]
Next, inks (IA-1) to (IA-16) and (IB-1) to (IB-3) were prepared. The pigment dispersions used to prepare these inks and their ink formulations are shown in Tables 4 and 5 below. The ink prescriptions shown in the ink prescription columns of Tables 4 and 5 are shown in Table 6, which will be described later.

<Preparation of ink (IA-1)>
According to the ink formulation (P1) shown in Table 6, 60.0 parts by mass of pigment dispersion (DA-1), 20.0 parts by mass of ethylene glycol, 15.0 parts by mass of diethylene glycol diethyl ether, and a nonionic surfactant ( 0.3 parts by mass of "OLFINE (registered trademark) E1004" manufactured by Nissin Chemical Industry Co., Ltd. and the remaining amount of ion-exchanged water were placed in a container. The contents of the container were stirred at a rotation speed of 400 rpm using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Kagaku Co., Ltd.) to obtain a mixed solution. Note that the remaining amount, which is the amount of ion-exchanged water added, means the amount of the resulting mixed liquid that is 100.0 parts by mass. The obtained liquid mixture was filtered using a filter with a pore size of 5 μm. In this way, ink (IA-1) was obtained.

<Preparation of inks (IA-2) to (IA-16) and (IB-1) to (IB-3)>
Inks (IA-2)-( IA-16) and (IB-1) to (IB-3) were prepared.

[Evaluation of nozzle clogging]
Nozzle clogging was evaluated for each of the inks (IA-1) to (IA-16) and (IB-1) to (IB-3) by the following method. As an evaluation device, an inkjet recording device (testing device manufactured by Kyocera Document Solutions Co., Ltd.) equipped with a line-type recording head was used. As the paper, A4 size inkjet matte paper ("Super Fine Paper" manufactured by Seiko Epson Corporation) was used. The evaluation environment was an environment with a temperature of 28° C. and a humidity of 30% RH. The recording head was filled with the ink to be evaluated. The amount of ink ejected per pixel was set to be 10.5 pL.

A solid image (150 mm x 200 mm) was continuously printed on 100 sheets of paper 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 image was printed on the 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. After standing still for 7 days, the recording head of the evaluation machine was again subjected to purge processing and wipe processing. Next, a nozzle check pattern image was printed on the paper using an evaluation machine. Then, the printed nozzle check pattern image 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 taken as the evaluation value of nozzle clogging. Whether the occurrence of nozzle clogging was suppressed was determined according to the following criteria. The evaluation values and judgment results are shown in Tables 4 and 5.

(Standards for suppressing the occurrence of nozzle clogging)
A (good): Evaluation value is less than 10%.
B (Poor): Evaluation value is 10% or more.

The meanings of the terms in Table 6 are as follows. "Olfine E1004" refers to "Olfine (registered trademark) E1004" manufactured by Nissin Chemical Industry Co., Ltd. "Water" refers to ion-exchanged water. The "remaining amount" indicates the amount of the mixed liquid obtained in the preparation of the ink to be 100.0 parts by mass. "Part" indicates part by mass.

As shown in Table 3, the crosslinked structure of the crosslinked resin contained in the pigment particles in the pigment dispersion (DB-1) was formed by a crosslinking agent having a degree of polymerization of less than 4. That is, n in formula (1) was a number less than 4. The crosslinked structure of the crosslinked resin contained in the pigment particles in the pigment dispersion (DB-2) was formed by a crosslinking agent having a degree of polymerization of more than 13. That is, n in formula (1) was a number exceeding 13. The main chain structure of the crosslinked resin contained in the pigment particles in the pigment dispersion (DB-3) was formed of a predetermined resin (more specifically, styrene acrylic resin) having a mass average molecular weight of less than 4000. Therefore, as shown in Table 5, the inks (IB-1) to (IB-3) containing any of the pigment dispersions (DB-1) to (DB-3) have a nozzle clogging rating of B. (Poor), and the occurrence of nozzle clogging could not be suppressed.

On the other hand, as shown in Tables 2 and 3, the crosslinked structure of the crosslinked resin contained in the pigment particles in pigment dispersions (DA-1) to (DA-11) and (DA-14) to (DA-16) is It was formed using a crosslinking agent having a degree of polymerization of 4 or more and 13 or less. That is, n in formula (1) was a number of 4 or more and 13 or less. The main chain structure of the crosslinked resin contained in the pigment particles in the pigment dispersions (DA-1) to (DA-11) and (DA-14) to (DA-16) has a mass average molecular weight of 4000 to 40000. It was formed from a certain predetermined resin (more specifically, styrene acrylic resin). Therefore, as shown in Tables 4 and 5, the ink (IA-1) containing any of the pigment dispersions (DA-1) to (DA-11) and (DA-14) to (DA-16) ) to (IA-16) were evaluated as A (good) for nozzle clogging, and the occurrence of nozzle clogging could be suppressed.

From the above, the pigment dispersion according to the present invention, including pigment dispersions (DA-1) to (DA-11) and (DA-14) to (DA-16), when contained in an ink, It is judged that the occurrence of nozzle clogging can be suppressed. Further, it is determined that the inks according to the present invention including inks (IA-1) to (IA-16) can suppress the occurrence of nozzle clogging.

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

Claims (7)

containing an aqueous medium and pigment particles dispersed in the aqueous medium,
The pigment particles include a core and a coating layer covering the core,
the core contains a pigment,
The coating layer contains a crosslinked resin,
The crosslinked resin has a main chain structure derived from a predetermined resin and a crosslinked structure derived from a crosslinking agent, and the predetermined resin is a resin capable of reacting with the crosslinking agent,
The predetermined resin has a mass average molecular weight of 4,000 or more and 40,000 or less,
The crosslinking agent is a pigment dispersion having at least two reactive functional groups capable of reacting with a group of the predetermined resin, and a structure represented by formula (1).

(In the above formula (1), R ¹ represents a hydrogen atom or a methyl group, n represents a number from 4 to 13, and * represents a bond.) The pigment dispersion according to claim 1, wherein the crosslinking agent is represented by formula (2).

(In the formula (2), R ^1A represents a hydrogen atom or a methyl group, n ^A represents a number from 4 to 13, and R ^2r and R ^3r each represent the reactive functional group. ) The pigment dispersion according to claim 1, wherein the crosslinking agent has a repeating unit represented by formula (3) and at least two repeating units represented by formula (4).

(In the above formula (3), R ^1B represents a hydrogen atom or a methyl group, n ^B represents a number from 4 to 13, R ⁴ represents a hydrogen atom or a methyl group,
In the formula (4), R ⁵ represents a hydrogen atom or a methyl group, and R ^6r represents the reactive functional group. ) The pigment dispersion according to any one of claims 1 to 3, wherein the reactive functional group is a glycidyl group, an oxazoline group, or a carbodiimide group. The pigment dispersion according to any one of claims 1 to 3, wherein the predetermined resin is a carboxyl group-containing resin, and the group possessed by the predetermined resin is a carboxyl group. The pigment dispersion according to claim 5, wherein the carboxyl group-containing resin is a styrene acrylic resin. An inkjet ink containing the pigment dispersion according to any one of claims 1 to 3.