Pigment dispersion and ink composition

Abstract

A pigment dispersion and an ink composition, the pigment dispersion containing water, a pigment, and a polymeric dispersant having a cross-linked structure, the polymeric dispersant having a cross-linked structure including a structural unit represented by formula (1) and a structural unit represented by formula (2). In the formula, R1 represents a hydrogen atom or an alkyl group having 1-4 carbon atoms, R2 represents an alkylene group having 2-5 carbon atoms, R3 represents an aromatic group, X1 and X2 each independently represent-O-or-NH-, m represents an integer of 2 or more, R4 represents a hydrogen atom or an alkyl group having 1-4 carbon atoms, R5 represents an alkyl group having 8 or more carbon atoms, and X3 represents-O-or-NH-.

Description

Technical Field

[0001] This invention relates to a pigment dispersion and an ink composition. Background Technology

[0002] In the past, various studies have been conducted on pigment dispersions used in ink compositions.

[0003] For example, Patent Document 1 discloses an aqueous pigment dispersion that achieves a higher level of both pigment dispersibility and dispersion stability. The aqueous pigment dispersion contains an aqueous medium, a pigment, and a polymeric dispersant. The polymeric dispersant contains at least a (meth)acrylate structural unit having a benzene ring, an alkyl (meth)acrylate structural unit having 12 or more carbon atoms, and a structural unit having an ionic group. In the polymeric dispersant, the content of the (meth)acrylate structural unit having a benzene ring is 20-60% by mass, and the content of the (meth)acrylate structural unit having alkyl (12 or more carbon atoms) is 10-40% by mass. The polymeric dispersant forms a cross-linked structure in the aqueous pigment dispersion.

[0004] Previous technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2016-069487 Summary of the Invention

[0007] The technical problem to be solved by the invention

[0008] However, there are times when it is required to further improve the long-term stability of pigment dispersions.

[0009] One embodiment of the present invention aims to solve the problem of providing a pigment dispersion and ink composition with excellent stability over time.

[0010] means for solving technical problems

[0011] The present invention includes the following methods.

[0012] <1> A pigment dispersion comprising water, pigment, and a polymeric dispersant containing a cross-linked structure.

[0013] The polymeric dispersant containing the cross-linked structure also contains the structural unit represented by the following formula (1) and the structural unit represented by the following formula (2).

[0014] [Chemical Formula 1]

[0015]

[0016] In equation (1), R1 Indicates an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0017] R 2 Indicates alkylene groups having 2 to 5 carbon atoms.

[0018] R 3 Indicates an aromatic group,

[0019] X 1 and X 2 They can be represented independently as -O- or -NH-.

[0020] m represents an integer greater than or equal to 2.

[0021] In equation (2),

[0022] R 4 Indicates an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0023] R 5 Indicates an alkyl group having 8 or more carbon atoms.

[0024] X 3 It represents -O- or -NH-.

[0025] <2> According to the pigment dispersion described in <1>, wherein,

[0026] The polymeric dispersant containing the cross-linked structure also contains structural units derived from monomers containing acid groups, with a hydrophilicity-to-hydrophobicity ratio of 8 or more as expressed in the following formula.

[0027] The ratio of closeness to unfamiliarity

[0028] = (mass ratio of the structural unit represented by equation (2) in the total mass of the polymeric dispersant containing the cross-linked structure × R in equation (2)) 2 (Number of carbon atoms of the alkylene group) / (Mass ratio of structural units derived from monomers containing acid groups to the total mass of the polymeric dispersant containing cross-linked structures)

[0029] <3> The pigment dispersion according to <1> or <2>, wherein,

[0030] In polymeric dispersants containing cross-linked structures, the Ar valence, expressed by the following formula, is 0.4 mmol / g to 2.9 mmol / g.

[0031] Ar price

[0032] = (the number of mmol of structural units represented by formula (1) per 1g of polymeric dispersant containing cross-linked structures) × (the number of aromatic rings in the structural units represented by formula (1))

[0033] <4> The pigment dispersion according to any one of <1> to <3>, wherein,

[0034] In equation (1), R 2 It represents an alkylene group with 2 carbon atoms.

[0035] <5> The water-based coating according to any one of <1> to <4>, wherein,

[0036] A cross-linked polymeric dispersant is formed by cross-linking an uncross-linked polymeric dispersant.

[0037] The uncrosslinked polymeric dispersant contains the structural unit represented by formula (1), the structural unit represented by formula (2), and acid groups, of which 10% to 90% of the total number of acid groups are neutralized.

[0038] <6> An ink composition comprising any one of <1> to <5> pigment dispersions.

[0039] Invention Effects

[0040] According to one embodiment of the present invention, a pigment dispersion and ink composition with excellent stability over time can be provided. Detailed Implementation

[0041] In this specification, the numerical range indicated by “~” refers to the range encompassed by taking the values ​​before and after “~” as the minimum and maximum values, respectively.

[0042] In the numerical ranges described in this specification, the upper or lower limit of a certain numerical range can be replaced with the upper or lower limit of other numerical ranges described in different periods. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of a certain numerical range can be replaced with the value shown in the embodiment.

[0043] In this specification, when a composition contains multiple substances corresponding to each component, unless otherwise stated, the amount of each component in the composition represents the total amount of the multiple substances present in the composition.

[0044] In this specification, a combination of two or more preferred methods is a more preferred method.

[0045] In this specification, the term "process" includes not only independent processes, but also processes that can be clearly distinguished from other processes, as long as they achieve the intended purpose of the process.

[0046] In this specification, "(meth)acrylate" is a concept that includes both acrylate and methacrylate. Furthermore, "(meth)acrylic acid" is a concept that includes both acrylic acid and methacrylic acid.

[0047] In this invention, when the elements are described in a singular form, unless otherwise expressly stated, the existence of multiple elements is not excluded as long as it does not create a technical contradiction.

[0048] [Pigment dispersion]

[0049] The pigment dispersion of the present invention is as follows: it contains water, pigment and a polymeric dispersant containing a crosslinked structure, wherein the polymeric dispersant containing the crosslinked structure further contains a structural unit represented by the following formula (1) (hereinafter also referred to as "formula (1) unit") and a structural unit represented by the following formula (2) (hereinafter also referred to as "formula (2) unit").

[0050] [Chemical Formula 2]

[0051]

[0052] In equation (1), R 1 Indicates an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0053] R 2 Indicates alkylene groups having 2 to 5 carbon atoms.

[0054] R 3 Indicates an aromatic group,

[0055] X 1 and X 2 They can be represented independently as -O- or -NH-.

[0056] m represents an integer greater than or equal to 2.

[0057] In equation (2),

[0058] R 4 Indicates an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0059] R 5 Indicates an alkyl group having 8 or more carbon atoms.

[0060] X 3 It represents -O- or -NH-.

[0061] The pigment dispersion of the present invention exhibits excellent stability over time.

[0062] Therefore, ink compositions containing the pigment dispersions of the present invention (e.g., ink compositions made by adding other components to the pigment dispersions of the present invention) also exhibit excellent stability over time.

[0063] The reason for the excellent long-term stability of the pigment dispersion of the present invention is speculated as follows.

[0064] It is believed that in the pigment dispersion of the present invention, the pigment is dispersed in a state in which the polymeric dispersant is adsorbed onto the pigment surface. It is believed that the polymeric dispersant will disperse R in the unit of formula (1). 3 The aromatic groups represented act as adsorption groups and are adsorbed onto the pigment.

[0065] The polymeric dispersant contains a cross-linked structure. Therefore, it is believed that the polymeric dispersant can be inhibited from detaching from the pigment surface, thereby inhibiting the reduction in the dispersion stability of the pigment caused by the aforementioned detachment (i.e., the reduction in the time stability of the pigment dispersion).

[0066] Furthermore, in polymeric dispersants, m in formula (1) is an integer greater than or equal to 2. That is, the polymeric dispersant contains "-R 2 -X 2 - The repeating number of the structure (e.g., alkoxy structure) is more than 2 in the unit of formula (1). It is believed that this increases the affinity of the polymeric dispersant for water, and as a result, the phenomenon of the polymeric dispersant agglomerating into random coils in aqueous pigment dispersions is suppressed. As a result, it is believed that the polymeric dispersant readily absorbs the R in the unit of formula (1). 3 The aromatic groups represented are adsorbed onto the pigment as adsorption groups, thereby improving the dispersion stability of the pigment (i.e., the time stability of the pigment dispersion).

[0067] Moreover, in polymeric dispersants, R in formula (2) 5 It is an alkyl group with 8 or more carbon atoms. That is, the polymeric dispersant contains an alkyl group (R0) with 8 or more carbon atoms as a hydrophobic group. 5 The unit of formula (2) is used. Therefore, it is believed that the overall hydrophilicity of the polymeric dispersant can be suppressed from becoming too high, which in turn can suppress the decrease in the dispersion stability of the pigment (i.e., the decrease in the time stability of the pigment dispersion).

[0068] As described above, it is believed that in the pigment dispersion of the present invention, the cross-linking structure in the polymeric dispersant, the unit of formula (1), and the unit of formula (2) complement each other, thereby achieving excellent stability over time.

[0069] <Water>

[0070] The pigment dispersion of the present invention contains water.

[0071] The water content relative to the total amount of pigment dispersion is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, and even more preferably 70% by mass or more.

[0072] The upper limit of water content relative to the total amount of pigment dispersion, for example, 98% by mass, 95% by mass, 90% by mass, etc.

[0073] <Pigment>

[0074] The pigment dispersion of the present invention contains at least one pigment.

[0075] As a pigment, there are no particular restrictions; it can be either an organic pigment or an inorganic pigment.

[0076] As for pigments, examples include those described in publicly available literature such as Seishiro Itō's "Encyclopedia of Pigments" (published in 2000), W. Herbst and K. Hunger's "Industrial Organic Pigments," Japanese Patent Application Publication Nos. 2002-12607, 2002-188025, 2003-26978, 2003-342503, and 2015-193729.

[0077] As pigments, for example, the following can be listed:

[0078] Polycyclic pigments including azo lake pigments, azo pigments, phthalocyanine pigments, perylene pigments, violet ketone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, diketopyrrolopyrrole pigments, thioindolinone pigments, isoindolineone pigments, and quinolineone pigments;

[0079] Organic pigments such as nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments;

[0080] Inorganic pigments such as titanium dioxide, iron oxide, and carbon black;

[0081] wait.

[0082] As pigments, azo pigments, phthalocyanine pigments, anthraquinone pigments, quinacridone pigments, or carbon black pigments are preferred.

[0083] Regarding pigments, one may also refer to publicly known documents such as Japanese Patent No. 5404669.

[0084] From the perspective of hue, examples of pigments include cyan, magenta, yellow, black, and white pigments.

[0085] The pigment content relative to the total amount of pigment dispersion is preferably 1% to 40% by mass, more preferably 3% to 30% by mass, even more preferably 5% to 25% by mass, and even more preferably 10% to 20% by mass.

[0086] <Polymer dispersants containing cross-linked structures>

[0087] The pigment dispersion of the present invention contains at least one polymeric dispersant comprising a cross-linked structure.

[0088] The polymeric dispersant containing the cross-linked structure also contains the unit of formula (1) (i.e., the structural unit represented by formula (1)) and the unit of formula (2) (i.e., the structural unit represented by formula (2)).

[0089] (The structural unit represented by equation (1))

[0090] The polymeric dispersant containing a cross-linked structure contains at least one unit of formula (1) (i.e., the structural unit represented by formula (1)).

[0091] [Chemical Formula 3]

[0092]

[0093] In equation (1), R 1 Indicates an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0094] R 2 Indicates alkylene groups having 2 to 5 carbon atoms.

[0095] R 3 Indicates an aromatic group,

[0096] X 1 and X 2 They can be represented independently as -O- or -NH-.

[0097] m represents an integer greater than or equal to 2.

[0098] In equation (1), R 1 It represents an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0099] As R 1 Preferably, it is an alkyl group having 1 to 3 carbon atoms, more preferably a hydrogen atom, methyl or ethyl, and even more preferably a hydrogen atom or methyl.

[0100] In equation (1), R 2 Indicates alkylene groups having 2 to 5 carbon atoms.

[0101] As R 2 Preferably, it is an alkylene group having 2 to 4 carbon atoms, more preferably an alkylene group having 2 or 3 carbon atoms, and even more preferably an alkylene group having 2 carbon atoms.

[0102] In equation (1), R 3 It represents an aromatic group.

[0103] R 3 The aromatic group represented contains at least one aromatic ring.

[0104] R 3 The aromatic ring in the aromatic group can be a monocyclic or polycyclic ring (e.g., fused ring).

[0105] R 3 The aromatic group represented may include a group formed by a single ring linkage (e.g., biphenyl).

[0106] R 3 The aromatic group represented may contain heteroatoms (e.g., oxygen atoms).

[0107] R 3 The aromatic ring in the aromatic group may have substituents.

[0108] Examples of substituents for aromatic rings include straight-chain or branched hydrocarbon groups (e.g., straight-chain or branched alkyl groups, straight-chain or branched alkenyl groups, straight-chain or branched alkynyl groups, etc.) and halogen atoms with 1 to 30 carbon atoms (more preferably 1 to 20).

[0109] R 3 The aromatic group represented preferably has 4 to 30 carbon atoms, more preferably 6 to 30, even more preferably 6 to 25, and even more preferably 6 to 20.

[0110] As R 3 The aromatic group represented is preferably phenyl, naphthyl, furanyl, biphenyl or alkylphenyl with 7 to 30 carbon atoms (preferably 7 to 25 carbon atoms, more preferably 7 to 20 carbon atoms).

[0111] In equation (1), X 1 and X 2 They can be represented independently as -O- or -NH-.

[0112] X 1 and X 2 The preferred option is -O-.

[0113] In equation (1), m represents an integer greater than 2.

[0114] m is preferably an integer from 2 to 30, more preferably an integer from 2 to 20, and even more preferably an integer from 2 to 10.

[0115] The content of the unit of formula (1) relative to the total amount of the polymeric dispersant containing the cross-linked structure is preferably 3% to 80% by mass, more preferably 5% to 70% by mass, even more preferably 10% to 60% by mass, and even more preferably 10% to 50% by mass.

[0116] The content of the unit of formula (1) relative to the total amount of the portion excluding the cross-linked structure from the polymeric dispersant containing the cross-linked structure is preferably 3% to 80% by mass, more preferably 5% to 70% by mass, even more preferably 10% to 60% by mass, and even more preferably 10% to 50% by mass.

[0117] The unit of formula (1) can be formed by the polymerization of monomers formed by the unit of formula (1).

[0118] The monomer used to form the unit in formula (1) is the following monomer (1M).

[0119] [Chemical Formula 4]

[0120]

[0121] In monomer (1M), R 1 R 2 R 3 X 1 X 2 -O-, -NH- and m are respectively related to R in equation (1) 1 R 2 R 3 X 1 X 2 -O-, -NH-, and m have the same meaning.

[0122] For a specific example of the monomer (1M), please refer to the example of the monomer for forming the unit of formula (1) in the embodiment described later.

[0123] The ratio (mass%) of the amount of monomer (1M) in the manufacture of polymeric dispersant to the total amount of all monomers corresponds to the content (mass%) of the unit of formula (1) relative to the total amount of polymeric dispersant mentioned above.

[0124] (The structural unit shown in equation (2))

[0125] The polymeric dispersant containing a cross-linked structure contains at least one unit of formula (2) (i.e., the structural unit represented by formula (2)).

[0126] [Chemical Formula 5]

[0127]

[0128] In equation (2),

[0129] R 4 Indicates an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0130] R 5 Indicates an alkyl group having 8 or more carbon atoms.

[0131] X 3 It represents -O- or -NH-.

[0132] In equation (2), R 4 It represents an alkyl group having 1 to 4 hydrogen atoms or carbon atoms.

[0133] As R 4 Preferably, it is an alkyl group having 1 to 3 carbon atoms, more preferably a hydrogen atom, methyl or ethyl, and even more preferably a hydrogen atom or methyl.

[0134] In equation (2), R 5 It refers to an alkyl group with 8 or more carbon atoms.

[0135] As R 5 Preferably, it is an alkyl group with 8 to 50 carbon atoms, more preferably an alkyl group with 8 to 40 carbon atoms, even more preferably an alkyl group with 8 to 30 carbon atoms, even more preferably an alkyl group with 10 to 30 carbon atoms, and even more preferably an alkyl group with 12 to 25 carbon atoms.

[0136] In equation (2), X 3 It represents -O- or -NH-.

[0137] X 3 The preferred option is -O-.

[0138] The content of the unit of formula (2) relative to the total amount of the polymeric dispersant containing the cross-linked structure is preferably 3% to 80% by mass, more preferably 5% to 70% by mass, even more preferably 10% to 60% by mass, and even more preferably 10% to 50% by mass.

[0139] The content of the unit of formula (2) relative to the total amount of the portion excluding the cross-linked structure from the polymeric dispersant containing the cross-linked structure is preferably 3% to 80% by mass, more preferably 5% to 70% by mass, even more preferably 10% to 60% by mass, and even more preferably 10% to 50% by mass.

[0140] The unit of equation (2) can be formed by the polymerization of monomers formed by the unit of equation (2).

[0141] The monomer used to form the unit in formula (2) is the following monomer (2M).

[0142] [Chemical Formula 6]

[0143]

[0144] In monomer (2M), R 4 R 5 and X 3respectively with R in equation (2) 4 R 5 and X 3 They have the same meaning.

[0145] For a specific example of the monomer (2M), please refer to the example of the monomer for forming the unit of formula (2) in the embodiment described later.

[0146] The ratio (mass%) of the amount of monomer (2M) in the manufacture of the polymeric dispersant to the total amount of all monomers corresponds to the content (mass%) of the unit of formula (2) relative to the total amount of the polymeric dispersant.

[0147] (Cross-linked structure)

[0148] The polymeric dispersant containing a cross-linked structure in the pigment dispersion of the present invention can be formed by cross-linking an uncross-linked polymeric dispersant (i.e., a polymeric dispersant that does not contain a cross-linked structure before cross-linking).

[0149] The polymeric dispersant containing a crosslinked structure is preferably a reaction product of an uncrosslinked polymeric dispersant containing an acid group (e.g., a carboxyl group) and a crosslinking agent. In this case, the crosslinked structure is formed by the reaction of the acid group with a crosslinking group (e.g., an epoxy group in an epoxy compound) in the crosslinking agent (e.g., an acid-epoxy reaction).

[0150] In this invention, the acid group is a concept that includes both unneutralized acid groups (e.g., carboxyl groups) and neutralized acid groups (e.g., salts of carboxyl groups (e.g., -C(=O)ONa)).

[0151] The discussion on neutralization will follow later.

[0152] The pigment dispersion of the present invention is preferably manufactured as follows: a dispersion (hereinafter referred to as "uncrosslinked dispersion") is prepared by dispersing a pigment using an uncrosslinked polymeric dispersant containing acid groups; the obtained uncrosslinked dispersion and a crosslinking agent are mixed to allow the acid groups in the uncrosslinked polymeric dispersant in the uncrosslinked dispersion to react with the crosslinking agent, thereby producing the pigment dispersion of the present invention. Through this reaction, the acid groups in the uncrosslinked polymeric dispersant react with the crosslinking agent to form a crosslinked structure. It is believed that the formation reaction of this crosslinked structure occurs while the uncrosslinked polymeric dispersant is adsorbed on the pigment surface. Therefore, it is believed that by forming a crosslinked structure, the effect of the polymeric dispersant not easily detaching from the pigment surface can be obtained.

[0153] -Crosslinking agent-

[0154] As a crosslinking agent, a compound having two or more reaction sites with an uncrosslinked polymeric dispersant (e.g., an uncrosslinked polymeric dispersant containing a carboxyl group) is preferred.

[0155] The preferred combination of crosslinking agent and non-crosslinked polymeric dispersant is a combination of a compound having two or more epoxy groups (i.e., an epoxy compound with two or more functions) as a crosslinking agent and a non-crosslinked polymeric dispersant containing at least one of a carboxyl group and its salt as a non-crosslinked polymeric dispersant.

[0156] In the above combination, a cross-linked structure is formed by reacting at least one of the carboxyl groups and their salts in an uncross-linked polymeric dispersant with an epoxy group in a compound having two or more epoxy groups to form a cross-linked structure, thereby forming a polymeric dispersant containing a cross-linked structure.

[0157] The formation of this cross-linked structure is preferably carried out after the pigment has been dispersed using an uncross-linked polymeric dispersant containing at least one of a carboxyl group and its salt.

[0158] Specific examples of epoxy compounds with two or more functions that are preferred as crosslinking agents include, for example, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, trimethylolpropane triglycidyl ether, etc.

[0159] Preferably, it is polyethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, or trimethylolpropane triglycidyl ether.

[0160] As a crosslinking agent, commercially available products can also be used.

[0161] As commercially available products, for example, Denacol EX-321, EX-821, EX-830, EX-850, EX-851 (manufactured by Nagase ChemteX Corporation) can be used.

[0162] From the viewpoint of the crosslinking reaction rate and / or the stability of the dispersion after crosslinking, the molar ratio of the crosslinking site (e.g., epoxy group) in the crosslinking agent to the crosslinked site (e.g., carboxyl group or its salt) in the uncrosslinked polymeric dispersion [crosslinking site (e.g., epoxy group) in the crosslinking agent : crosslinked site (e.g., carboxyl group or its salt) in the uncrosslinked polymeric dispersion] is preferably 1:1.1 to 1:10, more preferably 1:1.1 to 1:5, and even more preferably 1:1.1 to 1:3.

[0163] (Uncrosslinked polymeric dispersant)

[0164] Uncrosslinked polymeric dispersants (i.e., polymeric dispersants before the formation of crosslinked structures) may also contain units of formula (1) and formula (2), just like polymeric dispersants containing crosslinked structures. Units of formula (1) and formula (2) do not change due to the formation reaction of crosslinked structures and are maintained in their original state in polymeric dispersants containing crosslinked structures.

[0165] The preferred range of the content of the unit of formula (1) relative to the total amount of the uncrosslinked polymeric dispersant is the same as the preferred range of the content of the unit of formula (1) relative to the total amount of the portion of the polymeric dispersant containing the crosslinked structure excluding the crosslinked structure.

[0166] The preferred range of the content of the unit of formula (2) relative to the total amount of the uncrosslinked polymeric dispersant is the same as the preferred range of the content of the unit of formula (2) relative to the total amount of the portion of the polymeric dispersant containing the crosslinked structure excluding the crosslinked structure.

[0167] Uncrosslinked polymeric dispersants may also contain acid groups.

[0168] Cross-linked structures are formed through the reaction of the acid group with a cross-linking agent (e.g., acid-epoxy reaction).

[0169] A portion of the total number of acid groups in uncrosslinked polymeric dispersants may also remain in polymeric dispersants containing crosslinked structures.

[0170] As mentioned above, the acid groups contained in the uncrosslinked polymeric dispersant can be neutralized acid groups or unneutralized acid groups.

[0171] The preferred acid group to be neutralized is the -C(=O)OM group (where M represents an alkali metal, ammonium ion, or organic cation).

[0172] Examples of organic cations include alkylammonium cations with 1 to 10 carbon atoms, hydroxyl-substituted alkylammonium cations with 1 to 10 carbon atoms, carboxyl-substituted alkylammonium cations with 2 to 10 carbon atoms, and organic cations having 2 to 10 alkylimine units with 2 to 4 carbon atoms.

[0173] As the unneutralized acid group, the preferred option is a carboxyl group (-C(=O)OH group).

[0174] The neutralization of acid groups can be achieved by reacting a base (hereinafter also referred to as a "neutralizing base") with an uncrosslinked polymeric dispersant containing acid groups.

[0175] Examples of neutralizing bases include: alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; ammonia; organic amines such as dimethylethanolamine and diisopropylethylamine; etc.

[0176] -Neutralization Rate-

[0177] In uncrosslinked polymeric dispersants, preferably 10% to 90% of the total number of acid groups contained therein are neutralized (i.e., formed as salts of acid groups).

[0178] In other words, in the uncrosslinked polymeric dispersant, the proportion of neutralized acid groups in the total number of neutralized and unneutralized acid groups (i.e., the number %; hereinafter also referred to as the "neutralization rate (%)") is preferably 10% to 90%.

[0179] When the neutralization rate (%) is 10% to 90%, cross-linked structures are more likely to form.

[0180] The neutralization rate (%) is preferably 20% to 80%, more preferably 20% to 60%.

[0181] The neutralization rate (%) was determined by neutralization titration.

[0182] -acid group unit-

[0183] Both the preferred non-crosslinked polymeric dispersants and the polymeric dispersants containing crosslinked structures contain structural units derived from monomers containing acid groups (hereinafter also referred to as "acid group units").

[0184] Here, structural units derived from monomers containing acid groups refer to structural units formed from monomers containing acid groups as raw materials.

[0185] In structural units (i.e., acid units) derived from monomers containing acid groups, the acid groups can remain in their original state or disappear. For example, after monomers containing acid groups polymerize to form structural units, the acid groups in these structural units can disappear through cross-linking reactions.

[0186] As a monomer containing an acid group, a compound containing both an acid group and a polymerizable group is preferred.

[0187] As a polymerizable group, it is more preferably a group containing an olefinic double bond, and even more preferably (meth)acryloyl, allyl, styrene or vinyl.

[0188] As monomers containing acid groups, examples include (meth)acrylic acid, β-carboxyethyl acrylate, fumaric acid, itaconic acid, maleic acid, and crotonic acid. Among these, (meth)acrylic acid is particularly preferred.

[0189] The content of acidic units relative to the total amount of uncrosslinked polymeric dispersant or polymeric dispersant containing crosslinked structures is preferably 10% to 70% by mass, more preferably 15% to 65% by mass, and even more preferably 20% to 60% by mass.

[0190] -Ratio of water affinity to hydrophilicity-

[0191] When the polymeric dispersant containing the cross-linked structure includes units of formula (1), formula (2), and acidic units, the hydrophilicity-hydrophobicity ratio, expressed by the following formula, is preferably 8 or more. This further improves the long-term stability of the pigment dispersion.

[0192] The ratio of closeness to unfamiliarity

[0193] = (mass ratio of the unit in equation (2) to the total mass of the polymeric dispersant containing the cross-linked structure × R in equation (2)) 2 (Number of carbon atoms of the alkylene group) / (Mass ratio of the acid group unit to the total mass of the polymeric dispersant containing the cross-linked structure)

[0194] From the viewpoint of further improving the long-term stability of pigment dispersions and ensuring the affinity of polymeric dispersants containing cross-linked structures for water, the hydrophilicity-hydrophobicity ratio is preferably 8 or more and 60 or less, more preferably 10 or more and 50 or less, and even more preferably 12 or more and 40 or less.

[0195] -Ar price-

[0196] In cross-linked polymeric dispersants containing cross-linked structures, from the viewpoint of further improving the long-term stability of pigment dispersions, the Ar valence expressed by the following formula is preferably 0.3 mmol / g to 3.1 mmol / g, more preferably 0.4 mmol / g to 2.9 mmol / g.

[0197] Ar price

[0198] = (the number of mmol of Equation (1) units per 1g of polymeric dispersant containing cross-linked structures) × (the number of aromatic rings in Equation (1) units)

[0199] Here, regarding the number of aromatic rings in the unit of equation (1), n ​​fused rings are counted as n (here, n is an integer greater than 2).

[0200] -Other structural units-

[0201] Uncrosslinked polymeric dispersants and polymeric dispersants containing crosslinked structures may each contain structural units other than the units of formula (1), formula (2) and acid units (hereinafter also referred to as other structural units).

[0202] The proportion of the total of the unit of formula (1), the unit of formula (2) and the acid group unit in the total amount of the uncrosslinked polymeric dispersant or the polymeric dispersant containing the crosslinked structure is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0203] Other structural units that can be listed are those derived from vinyl monomers.

[0204] Examples of vinyl monomers include, for instance, methyl methacrylate.

[0205] Other structural units in uncrosslinked polymeric dispersants may also remain in polymeric dispersants containing crosslinked structures.

[0206] -Weight-average molecular weight (Mw) of uncrosslinked polymeric dispersants-

[0207] There are no particular limitations on the weight-average molecular weight (Mw) of the uncrosslinked polymeric dispersant, but from the viewpoint of pigment dispersibility, it is preferably 3,000 to 200,000, more preferably 5,000 to 150,000, and even more preferably 10,000 to 100,000.

[0208] In this invention, the weight-average molecular weight (Mw) was determined by gel permeation chromatography (GPC). An HLC-8220 GPC (manufactured by Tosoh Corporation) was used, with three columns (TSKgeL SuperHZM-H, TSKgeL SuperHZ4000, and TSKgel SuperHZ2000, all manufactured by Tosoh Corporation) connected in series. THF (tetrahydrofuran) was used as the eluent. The sample concentration was set to 0.45% by mass, the flow rate to 0.35 ml / min, the sample injection volume to 10 μl, and the measurement temperature to 40°C. A differential refractive index detector was used. Calibration curves were prepared using eight samples manufactured by Tosoh Corporation: "TSK standard, polystyrene": "F-40", "F-20", "F-4", "F-1", "A-5000", "A-2500", "A-1000", and "n-propylbenzene".

[0209] In the pigment dispersion of the present invention, the ratio of pigment content to content of polymeric dispersant containing cross-linking structure, by mass, is preferably 1:0.04 to 1:3, more preferably 1:0.05 to 1:1, and even more preferably 1:0.05 to 1:0.5.

[0210] In the aforementioned uncrosslinked dispersion, the ratio of pigment content to uncrosslinked polymeric dispersant content, by mass, is preferably 1:0.04 to 1:3, more preferably 1:0.05 to 1:1, and even more preferably 1:0.05 to 1:0.5.

[0211] Uncrosslinked polymeric dispersants can be manufactured by known methods of copolymerizing the monomers used to form each structural unit (i.e., monomers for forming units of formula (1), monomers for forming units of formula (2), monomers containing acid groups, etc.).

[0212] <Other Ingredients>

[0213] The pigment dispersion of the present invention may contain other components besides those described above.

[0214] Other components that may be contained in an ink composition can be listed.

[0215] Other components include, for example, water-soluble organic solvents.

[0216] In this invention, "water solubility" refers to the property of being able to dissolve more than 1g in 100g of water at 25°C.

[0217] Specific examples of water-soluble organic solvents are as follows.

[0218] • Alcohols (e.g., methanol, ethanol, propanol, isopropanol, butanol, isobutanol, sec-butanol, tert-butanol, pentanol, hexanol, cyclohexanol, benzyl alcohol, etc.)

[0219] • Polyols (e.g., ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butanediol, hexanediol, pentanediol, glycerol, hexanediol, thiodiethylene glycol, 2-methylpropanediol, etc.).

[0220] • Polyol ethers (e.g., ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, tripropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol dimethyl ether, ethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, ethylene glycol monophenyl ether, propylene glycol monophenyl ether, etc.).

[0221] The total amount of water, pigment and polymeric dispersant containing cross-linking structure in the pigment dispersion of the present invention is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0222] <An example of a method for manufacturing pigment dispersions (method A)>

[0223] The following shows an example of a method for manufacturing the pigment dispersion of the present invention (hereinafter referred to as Method A).

[0224] Method A includes the following steps:

[0225] To manufacture the above-mentioned unit of formula (1), the above-mentioned unit of formula (2), and acid-containing polymers containing acid groups;

[0226] At least a portion of the total number of acid groups in the aforementioned acid-containing polymer is neutralized using an alkali, thereby obtaining an uncrosslinked polymeric dispersant.

[0227] The pigment is dispersed in water using the above-mentioned uncrosslinked polymeric dispersant, thereby obtaining an uncrosslinked dispersion containing water, pigment and the above-mentioned uncrosslinked polymeric dispersant;

[0228] The uncrosslinked polymeric dispersant in the above-mentioned uncrosslinked dispersion is reacted with a crosslinking agent to obtain a pigment dispersion containing water, pigment, and a polymeric dispersant containing a crosslinked structure.

[0229] The neutralization of acid-containing polymers for obtaining uncrosslinked polymeric dispersants and the preferred methods thereof are described above.

[0230] The process of obtaining the uncrosslinked dispersion can be carried out using conventional pigment dispersion methods.

[0231] Examples of dispersers used for pigment dispersion include ball mills, roller mills, sand mills, bead mills, and nano-pulverizers.

[0232] The reaction between the uncrosslinked polymeric dispersant and the crosslinking agent (i.e., the formation of the crosslinked structure) in the process of obtaining the pigment dispersion and its preferred manner are as described above.

[0233] [Ink Composition]

[0234] The ink composition of the present invention contains the aforementioned pigment dispersion of the present invention.

[0235] That is, the ink composition of the present invention contains the components of the pigment dispersion of the present invention as described above.

[0236] The ink composition of the present invention can be manufactured by adding certain components to the aforementioned pigment dispersion of the present invention.

[0237] Furthermore, the ink composition of the present invention can be the pigment dispersion of the present invention itself.

[0238] That is, the ink composition of the present invention can be a composition that satisfies the necessary conditions of the pigment dispersion of the present invention as described above.

[0239] The ink composition of the present invention is preferably an inkjet ink.

[0240] For information on inkjet recording methods in this situation, please refer to the descriptions in publicly available literature.

[0241] <Water>

[0242] The ink composition of the present invention contains water.

[0243] The water content relative to the total amount of the ink composition is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and even more preferably 60% by mass or more.

[0244] The upper limit of water content relative to the total amount of the ink composition is, for example, 90% by mass, 80% by mass, etc.

[0245] <Pigment>

[0246] The ink composition of the present invention contains at least one pigment.

[0247] Preferred methods for pigments are shown in the section on "pigment dispersions".

[0248] The pigment content relative to the total amount of the ink composition is preferably 1% to 30% by mass, more preferably 1.5% to 20% by mass, even more preferably 2% to 15% by mass, and even more preferably 2% to 10% by mass.

[0249] <Polymer dispersants containing cross-linked structures>

[0250] The ink composition of the present invention contains at least one polymeric dispersant comprising a cross-linked structure.

[0251] Preferred methods for polymeric dispersants containing cross-linked structures are shown in the section on "pigment dispersions".

[0252] The ratio of pigment content to content of polymeric dispersant containing cross-linked structure, by mass, is preferably 1:0.04 to 1:3, more preferably 1:0.05 to 1:1, and even more preferably 1:0.05 to 1:0.5.

[0253] <Water-soluble organic solvents>

[0254] The ink composition of the present invention preferably contains a water-soluble organic solvent.

[0255] Therefore, when the ink composition of the present invention is used as inkjet ink, the ejection performance from the inkjet head is improved.

[0256] Specific examples of water-soluble organic solvents are shown in the section on “pigment dispersions”.

[0257] The content of water-soluble organic solvent relative to the total amount of ink composition is preferably 5% to 40% by mass, more preferably 10% to 30% by mass, and even more preferably 15% to 30% by mass.

[0258] <Urea>

[0259] The ink composition of the present invention preferably contains urea.

[0260] Urea functions as a solid wetting agent.

[0261] The urea content relative to the total amount of the ink composition is preferably 1% to 10% by mass, more preferably 2% to 9% by mass, and even more preferably 3% to 8% by mass.

[0262] <Other Ingredients>

[0263] The ink composition of the present invention may contain other ingredients as needed.

[0264] Other ingredients may include, for example, surfactants, resin particles, water-soluble resins, silicone compounds, UV absorbers, antioxidants, anti-fading agents, conductive salts, and alkaline compounds (pH adjusters).

[0265] For information on the components that may be contained in the ink composition, please refer to publicly available documents such as Japanese Patent Application Publication No. 2015-193729 and Japanese Patent Application Publication No. 2016-069487.

[0266] <Physical Properties>

[0267] The pH (25°C) of the ink composition is preferably 7 to 10, more preferably 7.5 to 9.5.

[0268] pH is measured using a pH meter at 25°C, for example, using a pH meter (model "HM-31") manufactured by DKK-TOA CORPORATION.

[0269] The viscosity (30°C) of the ink composition is preferably 0.5 mPa·s to 30 mPa·s, more preferably 2 mPa·s to 20 mPa·s, even more preferably 2 mPa·s to 15 mPa·s, and even more preferably 3 mPa·s to 10 mPa·s.

[0270] Viscosity is the value measured using a viscometer at 25°C. Viscosity is measured using a viscometer at 25°C, for example, using a TV-22 type viscometer manufactured by Toki Sangyo Co., Ltd.

[0271] The surface tension (at 25°C) of the ink composition is preferably 60 mN / m or less, more preferably 20 mN / m to 50 mN / m, and even more preferably 30 mN / m to 45 mN / m.

[0272] Surface tension is measured at 25°C using a surface tension meter, for example, by using the plate method with an automated surface tension meter (product name "CBVP-Z") manufactured by Kyowa Interface Science Co., Ltd.

[0273] Example

[0274] The present invention will be described in more detail below through embodiments, but the present invention is not limited to the following embodiments as long as it does not depart from its spirit.

[0275] Hereinafter, polymeric dispersants will sometimes be referred to simply as "dispersants".

[0276] <Synthesis of Uncrosslinked Dispersants>

[0277] As non-crosslinked dispersants, dispersants A1 to A28 and comparative dispersants C1 to C3, as shown in Table 1, were synthesized.

[0278] (Synthesis of dispersant A1)

[0279] 153.5 parts by weight of tripropylene glycol monomethyl ether (product name "MFTG", manufactured by Nippon Nyukazai Co., LTd.) as a reaction solvent were added to a reaction vessel equipped with a gas inlet pipe, thermometer, condenser and stirrer, and then nitrogen gas was replaced in the reaction vessel.

[0280] Next, the reaction vessel was heated to 85°C, and a mixture of 76.8 parts by mass of MFTG as a reaction solvent, 16 parts by mass of nonylphenoxy pentapropylene glycol acrylate (NPh(PO)5A) as a monomer for unit formation of formula (1), 28 parts by mass of octadecyl methacrylate (STMA) as a monomer for unit formation of formula (2), 54 parts by mass of methacrylic acid (MAA) as a monomer for forming acid-containing units, 2 parts by mass of methyl methacrylate (MMA) as another monomer, and 3 parts by mass of V-601 (manufactured by FUJIFILM Wako Pure Chemical Corporation) as a polymerization initiator was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition was completed, the reaction was further carried out at 85°C for 4 hours, thereby obtaining a solution containing polymer.

[0281] After cooling the resulting solution to room temperature, 251 parts by mass of a 1 mol / L NaOH aqueous solution was added as a neutralizing base, thereby neutralizing 40% of the total number of carboxyl groups in the polymer (i.e., neutralization at a neutralization rate of 40%). Water was then added to the neutralized solution to bring the dispersant A1 content to 5.6% by mass, thus obtaining a solution of dispersant A1.

[0282] Table 1 shows the types and amounts (parts by mass) of monomers used to form the uncrosslinked dispersant, the neutralizing base used to neutralize the carboxyl groups in the uncrosslinked dispersant, the neutralization rate (%) of the carboxyl groups in the uncrosslinked dispersant, the weight-average molecular weight (Mw) of the uncrosslinked dispersant, the hydrophilicity-hydrophobicity ratio of the crosslinked dispersant, and the Ar valence (mmol / g) of the crosslinked dispersant.

[0283] The methods for determining Mw, calculating the hydrophilicity-hydrophobicity ratio, and calculating the Ar valence are as described above.

[0284] In the classification of monomers in Table 1, “Formula (1)”, “Formula (2)”, “acid group” and “other” refer to monomers for forming units of Formula (1), monomers for forming units of Formula (2), monomers for forming units containing acid groups and other monomers, respectively.

[0285] (Synthesis of dispersants A2-A28, and comparative dispersants C1-C3)

[0286] The composition (type and amount) of the monomers and the neutralizing base were changed as shown in Table 1 below. Otherwise, dispersants A2 to A28 and comparative dispersants C1 to C3 as non-crosslinked dispersants were synthesized in the same manner as dispersant A1.

[0287] [Example 1]

[0288] <Preparation of Uncrosslinked Dispersions>

[0289] After pre-dispersing the mixture of the following components to make it homogeneous, a bead mill (bead diameter: Zirconia beads were dispersed for 3 hours. This yielded an uncrosslinked dispersion of magenta pigment dispersed in dispersant A1.

[0290] -Composition of the uncrosslinked dispersion-

[0291] Pigment Red 122 (Magenta Pigment)

[0292] …20.4 parts by weight

[0293] • Solution of dispersant A1 (non-crosslinked dispersant)

[0294] …35.8 parts by weight

[0295] ·MFTG

[0296] …6.1 parts by weight

[0297] ·water

[0298] …73.7 parts by weight

[0299] <Preparation of crosslinked dispersions>

[0300] Next, the mixture of the following components is reacted at 70°C for 6 hours and then cooled to 25°C, thereby causing the dispersant A1 (uncrosslinked polymeric dispersant) in the uncrosslinked dispersion to be crosslinked by a crosslinking agent, thereby obtaining a crosslinked dispersion containing water, pigment and a polymeric dispersant containing a crosslinked structure (hereinafter also referred to as "crosslinked dispersant").

[0301] -composition-

[0302] Uncrosslinked dispersions

[0303] …136.0 parts by weight

[0304] • "Denacol EX-321" (manufactured by Nagase ChemteX Corporation) (trimethylolpropane polyglycidyl ether; crosslinking agent)

[0305] …3.0 parts by weight

[0306] <Preparation of Pigment Dispersions as Targets>

[0307] Next, the crosslinked dispersion was ultrafiltered at a flow rate of 600 mL per minute through an ultrafiltration apparatus (cross-flow ultrafiltration (UF), manufactured by Sartorius) equipped with a polyethersulfone (PESU) membrane (micropore size: 0.1 μm). The liquid temperature was adjusted to 25°C, and ultrafiltration was performed 10 times, with each cycle being equal to one volume of the added liquid. Then, deionized water was added to bring the pigment concentration to 15% by mass.

[0308] Through the above, a pigment dispersion containing water, pigment and crosslinking dispersant (i.e., a polymeric dispersant containing a crosslinking structure) as the target substance was obtained (hereinafter referred to as "pigment dispersion A").

[0309] <Preparation of Ink Compositions>

[0310] The following mixture is stirred and mixed until homogeneous to obtain an ink composition.

[0311] The ink composition described below contains 5 parts by weight of pigment and 1.5 parts by weight of dispersant.

[0312] -Composition of the ink composition-

[0313] Pigment dispersion A

[0314] …33.3 parts by weight

[0315] Propylene glycol (water-soluble organic solvent)

[0316] …20.0 parts by weight

[0317] Hexanediol (a water-soluble organic solvent)

[0318] …3.4 parts by weight

[0319] ·Urea

[0320] …5.0 parts by weight

[0321] ·water

[0322] …38.3 parts by weight

[0323] <Evaluation of the time-dependent stability of the ink composition>

[0324] The ink composition (30 mL) was filled into a sample vial and sealed. This ink composition will be referred to below as "ink before storage".

[0325] Next, the ink before heating was heated to 80°C and stored at 80°C for 24 hours. The ink after being stored at 80°C for 24 hours will be referred to as "stored ink".

[0326] The average particle size of the ink before and after storage was measured using a Nanotrac wave II (manufactured by MicrotracBEL Corp.), and the particle size change rate was calculated according to the following formula.

[0327] Particle size change rate (%)

[0328] = [(Average particle size in the ink after storage) - (Average particle size in the ink before storage)) / (Average particle size in the ink before storage)] × 100

[0329] Based on the obtained particle size change rate, the time stability of the ink composition was determined using the following evaluation criteria.

[0330] In the following evaluation criteria, the highest grade for the stability of the ink composition over time is "A".

[0331] -Evaluation criteria for stability over time-

[0332] A: Particle size change rate is less than 10%.

[0333] B: Particle size variation rate is 10% or more but less than 25%.

[0334] C: Particle size variation rate is 25% or more but less than 50%.

[0335] D: Particle size variation rate is above 50%

[0336] [Examples 2-30, Comparative Examples 1-3]

[0337] The types of non-crosslinked dispersants and pigments were changed as shown in Table 1. Otherwise, the same operations as in Example 1 were performed.

[0338] The results are shown in Table 1.

[0339]

[0340] Regarding the pigments in Table 1, PR122, PY150, and PB15:3 refer to Pigment Red 122, Pigment Yellow 150, and Pigment Blue 15:3, respectively.

[0341] The abbreviations and structures of the monomers in Table 1 are as follows.

[0342] NPh(PO)5A: Nonylphenoxy pentapropylene glycol acrylate

[0343] NPh(EO)8A: Nonylphenoxy octaethylene glycol acrylate

[0344] OPhPh(EO)2A: o-Phenylphenoxyethylene glycol acrylate

[0345] Ph(EO)2A: Phenoxy diethylene glycol acrylate

[0346] Npt(EO)2A: Naphthoxydiethylene glycol methacrylate

[0347] Fu(EO)2A: Furan diethylene glycol acrylate

[0348] Ph(EO)4A: Phenoxytetraethylene glycol acrylate

[0349] NPh(EO)4A: Nonylphenoxy tetraethylene glycol acrylate

[0350] PhEMA: Phenoyl methacrylate

[0351] BzMA: Benzyl methacrylate

[0352] STMA: Octadecyl methacrylate

[0353] LMA: Lauryl methacrylate

[0354] VA: Docosyl acrylate

[0355] HA: Hexyl acrylate

[0356] MAA: Methacrylic acid

[0357] AA: Acrylic acid

[0358] MMA: Methyl methacrylate

[0359] [Chemical Formula 7]

[0360]

[0361] Regarding the neutralizing bases in Table 1, DMEA refers to dimethylethanolamine, and DIPEA refers to diisopropylethylamine.

[0362] As shown in Table 1, the inks of any of the embodiments exhibit good stability over time.

[0363] In any embodiment, the crosslinking dispersant in the pigment dispersion is an integer where m in formula (1) is 2 or more and R in formula (2) is used. 5 It is a dispersant containing alkyl groups with 8 or more carbon atoms.

[0364] The results for the comparative examples are as follows, relative to the embodiments.

[0365] Comparative Example 1 is a comparative example in which m in Equation (1) is changed to 1 (more specifically, the unit in Equation (1) is changed to a PhEMA unit).

[0366] Comparative Example 2 is a comparative example in which m in Equation (1) is changed to 0 (more specifically, the element in Equation (1) is changed to a BzMA element).

[0367] Comparative Example 3 is to replace R in equation (2) 5 A comparative example in which the number of carbon atoms of the alkyl group is changed to 6 (more specifically, the unit of formula (2) is changed to HA unit).

[0368] In any of the Comparative Examples 1 to 3, the stability of the ink over time decreased.

[0369] The entire contents of the invention of Japanese Patent Application No. 2023-059325, filed on March 31, 2023, are incorporated herein by reference. All documents, patent applications, and technical standards described in this specification are incorporated herein by reference to the same extent as those specifically described and separately incorporated herein by reference.

Claims

1. A pigment dispersion containing water, a pigment, and a high-molecular dispersant containing a crosslinked structure, the high-molecular dispersant containing a crosslinked structure further contains a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), [Chemical Formula 1] In formula (1), R 1 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, R 2 represents an alkylene group having 2 to 5 carbon atoms, R 3 represents an aromatic group, X 1 and X 2 each independently represents -O- or -NH-, m represents an integer of 2 or more, in formula (2), R 4 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, R 5 represents an alkyl group having 8 or more carbon atoms, X 3 represents -O- or -NH-.

2. The pigment dispersion according to claim 1, wherein the high-molecular dispersant containing a crosslinked structure further contains a structural unit derived from a monomer containing an acid group, and a hydrophilic-hydrophobic ratio represented by the following formula is 8 or more, Hydrophilic-hydrophobic ratio = (mass ratio of the structural unit represented by the formula (2) in the total mass of the high polymer dispersant containing a cross-linking structure x R 2 the number of carbon atoms of the alkylene group represented by the formula (2)) / (mass ratio of the structural unit derived from the monomer containing an acid group in the total mass of the high polymer dispersant containing a cross-linking structure).

3. The pigment dispersion according to claim 1, wherein in the high-molecular dispersant containing a crosslinked structure, an Ar valence number represented by the following formula is 0.4 mmol / g to 2.9 mmol / g, Ar valence number = (mmol number of the structural unit represented by the formula (1) in the high-molecular dispersant containing a crosslinked structure per 1 g) x (number of aromatic rings in the structural unit represented by the formula (1)).

4. The pigment dispersion according to claim 1, wherein 5. The pigment dispersion according to claim 1, wherein In the formula (1), the R 2 represents an alkylene group having 2 carbon atoms. the high-molecular dispersant containing a crosslinked structure is formed by crosslinking an uncrosslinked high-molecular dispersant, the uncrosslinked high-molecular dispersant contains the structural unit represented by the formula (1), the structural unit represented by the formula (2), and an acid group, and 10% to 90% of the total number of the acid groups contained is neutralized.

6. An ink composition containing the pigment dispersion according to any one of claims 1 to 5. ​

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