JP2025042501A - Pigment dispersions and inks - Google Patents

Abstract

To provide a pigment dispersion and ink which are excellent in temporal stability.SOLUTION: There are provided: a pigment dispersion which contains water, a pigment and a polymer dispersion agent, wherein the polymer dispersion agent includes a structural unit represented by a following formula (1) and a structural unit represented by a following formula (2), and Ar valence that is the mmol number of an aromatic ring in 1 g of the polymer dispersion agent is 3.0 mmol/g or less; and ink. R1 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, R2 represents an alkylene group having 2 to 5 carbon atoms, R3 represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, X1 and X2 each dependently represent -O- or -NH-, m represents an integer of 2 or more, R4 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, R5 represents an alkyl group having 14 or more carbon atoms, and X3 represents -O- or -NH-.SELECTED DRAWING: None

Description

This disclosure relates to pigment dispersions and inks.

Conventionally, various studies have been conducted on pigment dispersions used in inks.
For example, Patent Document 1 listed below discloses a water-based ink that is excellent in print density, water resistance, marker resistance, and abrasion resistance, and that uses an aqueous dispersion in which polymer particles of a water-insoluble polymer having an acid value of 60 to 200 KOHmg/g and in which 30 to 80 mol % of acid groups have been neutralized with a base are mixed with carbon black having a pH of 1 to 6.

Furthermore, the following Patent Document 2 discloses the following aqueous dispersion as an aqueous dispersion that, when used in an inkjet ink, gives images with the requisite optical density and chromaticity required in emerging inkjet application fields.
The aqueous dispersion system disclosed in Patent Document 2 is
an aqueous dispersion comprising: solid particles; a structured vinyl polymer dispersant comprising at least one hydrophilic segment and at least one hydrophobic segment, said hydrophilic segment comprising at least one crosslinkable moiety; and a crosslinker insoluble in the aqueous dispersion, said crosslinkable moiety being crosslinked with said crosslinker, said structured vinyl polymer dispersant not comprising polyethylene glycol monomethacrylate and polypropylene glycol monomethacrylate as monomers, said crosslinked hydrophilic segments forming a matrix around said solid particles, said matrix being resistant to changes in aqueous ink vehicle composition and to temperature changes.
It is an aqueous dispersion.

Furthermore, the following Patent Document 3 discloses that an optimal combination of the monomer structure, acid value, and neutralizing agent of a polymer dispersant has been investigated, and a particulate colorant having excellent image fastness is stably dispersed in an aqueous medium, thereby providing an ink for inkjet printers having excellent ejection stability.
The ink for inkjet printers described in Patent Document 3 listed below comprises at least a water-insoluble colorant, a polymer, and an aqueous medium, and is characterized in that the polymer has a specific monomer composition and a specific acid value and is neutralized with a specific alkali.

JP 2003-231831 A JP 2016-026256 A JP 2005-320402 A

However, there are cases where it is required to further improve the stability over time of the pigment dispersion.
An object of one embodiment of the present disclosure is to provide a pigment dispersion and ink that are excellent in stability over time.

The present disclosure includes the following aspects.
<1> A coating composition comprising water, a pigment, and a polymer dispersant,
The polymer dispersant contains a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2),
The Ar value, which is the number of mmoles of aromatic rings in 1 g of the polymer dispersant, is 3.0 mmol/g or less.
Pigment dispersion.

In formula (1),
R ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R2 represents an alkylene group having 2 to 5 carbon atoms;
^R3 represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms;
^X1 and ^X2 each independently represent -O- or -NH-;
m represents an integer of 2 or more.
In formula (2),
^R4 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R5 represents an alkyl group having 14 or more carbon atoms;
^X3 represents -O- or -NH-.

<2> The pigment dispersion according to <1>, wherein in formula (2), R ⁵ represents an alkyl group having 16 to 22 carbon atoms.
<3> The pigment dispersion according to <1> or <2>, wherein in formula (1), m represents an integer of 2 to 4.
<4> The pigment dispersion according to any one of <1> to <3>, wherein the polymer dispersant has a crosslinked structure.
<5> The polymer dispersant containing a crosslinked structure is formed by crosslinking an uncrosslinked polymer dispersant,
The pigment dispersion according to <4>, wherein the polymer dispersant having a crosslinked structure has a crosslinking value represented by the following formula of 0.8 mmol/g or more.
Crosslinking value [mmol/g]
= Acid value of uncrosslinked polymer dispersant [mmol/g] + Base value of uncrosslinked polymer dispersant [mmol/g] - Acid value of polymer dispersant containing crosslinked structure [mmol/g] - Base value of polymer dispersant containing crosslinked structure [mmol/g]

<6> The polymer dispersant containing a crosslinked structure is formed by crosslinking an uncrosslinked polymer dispersant,
The uncrosslinked polymer dispersant contains a structural unit represented by formula (1), a structural unit represented by formula (2), and an acid group, and 10% to 90% of the total number of the acid groups contained therein are neutralized.
<4> or <5>,
<7> An ink containing the pigment dispersion according to any one of <1> to <6>.
<8> The ink according to <7>, further comprising an organic solvent having a ClogP value of −0.60 or more.
<9> The ink according to <8>, wherein the content of the organic solvent having a ClogP value of −0.60 or more is 3.0% by mass to 10.0% by mass relative to the total amount of the ink.

According to one embodiment of the present disclosure, a pigment dispersion and ink having excellent stability over time are provided.

In this specification, the numerical range indicated using "to" means a range that includes the numerical values before and after "to" as the minimum and maximum values, respectively.
In the numerical ranges described in this specification, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in a certain numerical range. In addition, in the numerical ranges described in this specification, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples.
In this specification, the amount of each component in a composition means the total amount of the multiple substances present in the composition when multiple substances corresponding to each component are present in the composition, unless otherwise specified.
As used herein, a combination of two or more preferred aspects is a more preferred aspect.
In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.
In this specification, the term "(meth)acrylate" is a concept that encompasses both acrylate and methacrylate, and the term "(meth)acrylic" is a concept that encompasses both acrylic and methacrylic.

[Pigment Dispersion]
The pigment dispersion of the present disclosure contains water, a pigment, and a polymer dispersant, the polymer dispersant containing a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), and the Ar value, which is the number of mmoles of aromatic rings in 1 g of the polymer dispersant, is 3.0 mmol/g or less.

In formula (1),
R ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R2 represents an alkylene group having 2 to 5 carbon atoms;
^R3 represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms;
^X1 and ^X2 each independently represent -O- or -NH-;
m represents an integer of 2 or more.
In formula (2),
^R4 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R5 represents an alkyl group having 14 or more carbon atoms;
^X3 represents -O- or -NH-.

The pigment dispersion of the present disclosure has excellent stability over time.
Therefore, inks containing the pigment dispersion of the present disclosure (for example, inks prepared by adding other components to the pigment dispersion of the present disclosure) also have excellent stability over time.

The reason why the pigment dispersion of the present disclosure has excellent stability over time is presumed to be as follows.
In order to enhance the stability over time of an aqueous (i.e., water-containing) pigment dispersion, it is considered important to enhance the adsorption ability of the dispersant to the pigment in the pigment dispersion and to prevent the dispersant from being desorbed from the pigment.
The polymer dispersant in the pigment dispersion of the present disclosure is believed to be excellent in these performances.
That is, the polymer dispersant contains a structural unit represented by formula (1) (i.e., a structural unit containing ^{an R2X2} chain (e.g., an ethyleneoxy (EO) chain)) and a structural unit represented by formula (2) (i.e., a structural unit containing ^R5 which is a long-chain alkyl group). In the polymer dispersant, ^R5 which is an alkyl group having 14 or more carbon atoms is considered to exhibit adsorption ability to the pigment through hydrophobic interaction with the pigment, and ^{the R2X2} chain is considered to exhibit a function of ensuring the water compatibility of the polymer dispersant (i.e., suppressing threading (i.e., entanglement) of the polymer chain). Due to these functions, the polymer dispersant is considered to enhance the dispersion stability of the pigment.
However, there are cases where it is required to further improve the dispersion stability of the pigment.
Therefore, by setting the Ar value of the above polymer dispersant to 3.0 mmol/g or less, threading (i.e., entanglement) of the polymer chains due to ππ interactions of the aromatic rings can be suppressed, and as a result, the dispersion stability of the pigment can be further improved.
For the reasons described above, it is believed that the pigment dispersion of the present disclosure has excellent stability over time.

<Water>
The pigment dispersion of the present disclosure contains water.
The content of water relative to the total amount of the 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 still more preferably 70% by mass or more.
The upper limit of the water content relative to the total amount of the pigment dispersion is, for example, 98 mass %, 95 mass %, 90 mass %, etc.

<Pigments>
The pigment dispersion of the present disclosure contains at least one pigment.
There is no particular limitation on the pigment, and it may be either an organic pigment or an inorganic pigment.
Examples of the pigment include pigments described in known documents such as "Encyclopedia of Pigments" edited by Seijiro Ito (published in 2000), "Industrial Organic Pigments" by W. Herbst and K. Hunger, JP-A Nos. 2002-12607, 2002-188025, 2003-26978, 2003-342503, and 2015-193729.

As pigments,
For example, polycyclic pigments such as azo lake pigments, azo pigments, phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, diketopyrrolopyrrole pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments;
Organic pigments such as nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments;
Inorganic pigments such as titanium oxide, iron oxide, and carbon black;
etc.
The pigment is preferably an azo pigment, a phthalocyanine pigment, an anthraquinone pigment, a quinacridone pigment, or a carbon black pigment.
For the pigment, the descriptions in known documents such as Japanese Patent No. 5404669 may be appropriately referred to.

In terms of hue, examples of pigments include cyan pigments, magenta pigments, yellow pigments, black pigments, white pigments, etc.

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

<Polymer Dispersant>
The pigment dispersion of the present disclosure contains at least one polymeric dispersant.
The polymer dispersant contains a structural unit represented by formula (1) and a structural unit represented by formula (2), and the Ar value, which is the number of mmoles of aromatic rings in 1 g of the polymer dispersant, is 3.0 mmol/g or less.

(Structural unit represented by formula (1))
The polymer dispersant contains at least one structural unit represented by formula (1) (hereinafter also referred to as "formula (1) unit").

In formula (1),
R ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R2 represents an alkylene group having 2 to 5 carbon atoms;
^R3 represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms;
^X1 and ^X2 each independently represent -O- or -NH-;
m represents an integer of 2 or more.

In formula (1), R ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.
R ¹ is preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, more preferably a hydrogen atom, a methyl group, or an ethyl group, and even more preferably a hydrogen atom or a methyl group.

In formula (1), R ² represents an alkylene group having 2 to 5 carbon atoms.
^R2 is preferably 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.

In formula (1), R ³ represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms.
^R1 is preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, more preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, still more preferably a hydrogen atom, a methyl group, or an ethyl group, and still more preferably a hydrogen atom or a methyl group.

In formula (1), ^X1 and ^X2 each independently represent -O- or -NH-.
^X1 and ^X2 are preferably —O—.

In formula (1), m represents an integer of 2 or more.
When m in formula (1) is an integer of 2 or more, the water compatibility of the polymer dispersant is increased, and as a result, threading (i.e., entanglement) of the polymer chains of the polymer dispersant is suppressed, which results in increased dispersion stability of the pigment in the pigment dispersion and increased stability over time of the pigment dispersion.
m is preferably an integer of 2 to 30, more preferably an integer of 2 to 20, even more preferably an integer of 2 to 10, even more preferably an integer of 2 to 8, even more preferably an integer of 2 to 6, and even more preferably an integer of 2 to 4.

The content of the formula (1) unit relative to the total amount of the polymer dispersant containing a crosslinked structure is preferably 3% by mass to 80% by mass, more preferably 5% by mass to 70% by mass, even more preferably 10% by mass to 60% by mass, and even more preferably 10% by mass to 50% by mass.

The content of the formula (1) unit relative to the total amount of the polymer dispersant containing a crosslinked structure, excluding the crosslinked structure, is preferably 3% by mass to 80% by mass, more preferably 5% by mass to 70% by mass, even more preferably 10% by mass to 60% by mass, and even more preferably 10% by mass to 50% by mass.

The formula (1) unit can be formed by polymerization of a monomer for forming the formula (1) unit.
The monomer for forming the unit of formula (1) is the following monomer (1M).

In the monomer (1M), R ¹ , R ² , R ³ , X ¹ , X ² , --O--, --NH--, and m are respectively defined as R ¹ , R ² , R ³ , X ¹ , X ² , --O--, --NH--, and m in the formula (1).
For specific examples of the monomer (1M), see the examples of the monomer for forming the unit of formula (1) in the Examples section below.
The ratio (mass %) of the amount of monomer (1M) charged to the amount of all monomers charged when producing a polymer dispersant corresponds to the content (mass %) of the unit of formula (1) relative to the total amount of the polymer dispersant, as described above.

(Structural unit represented by formula (2))
The polymer dispersant containing a crosslinked structure contains at least one unit of formula (2) (i.e., a structural unit represented by formula (2)).

In formula (2),
^R4 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R5 represents an alkyl group having 14 or more carbon atoms;
^X3 represents -O- or -NH-.

In formula (2), R ⁴ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.
R ⁴ is preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, more preferably a hydrogen atom, a methyl group, or an ethyl group, and even more preferably a hydrogen atom or a methyl group.

In formula (2), R ⁵ represents an alkyl group having 14 or more carbon atoms.
When the number of carbon atoms in the alkyl group represented by ^R5 is 14 or more, the hydrophobic interaction between R5 in the polymer dispersant and the pigment (i.e., the adsorption ability to the pigment) is strengthened, thereby improving the dispersion stability of the pigment in the pigment dispersion and improving the stability over time of the pigment dispersion.
^R5 is preferably an alkyl group having 16 to 30 carbon atoms, more preferably an alkyl group having 16 to 26 carbon atoms, and further preferably an alkyl group having 16 to 22 carbon atoms.

In formula (2), ^X3 represents -O- or -NH-.
^X3 is preferably --O--.

The content of the formula (2) unit relative to the total amount of the polymer dispersant containing a crosslinked structure is preferably 3% by mass to 80% by mass, more preferably 5% by mass to 70% by mass, even more preferably 10% by mass to 60% by mass, and even more preferably 10% by mass to 50% by mass.

The content of the unit of formula (2) relative to the total amount of the polymer dispersant containing a crosslinked structure, excluding the crosslinked structure, is preferably 3% by mass to 80% by mass, more preferably 5% by mass to 70% by mass, even more preferably 10% by mass to 60% by mass, and even more preferably 10% by mass to 50% by mass.

The formula (2) unit can be formed by polymerization of a monomer for forming the formula (2) unit.
The monomer for forming the unit of formula (2) is the following monomer (2M).

In the monomer (2M), R ⁴ , R ⁵ and X ³ have the same definitions as R ⁴ , R ⁵ and X ³ in the formula (2), respectively.
For specific examples of the monomer (2M), see the examples of the monomer for forming the unit of formula (2) in the Examples section below.
The ratio (mass %) of the amount of monomer (2M) charged to the amount of all monomers charged when producing the polymer dispersant corresponds to the content (mass %) of the unit of formula (2) relative to the total amount of the polymer dispersant, as described above.

(Ar value)
The polymeric dispersant may or may not contain an aromatic ring.
The Ar value, which is the number of mmoles of aromatic rings in 1 g of the polymer dispersant, is 3.0 mmol/g or less.
This suppresses threading (i.e., entanglement) of polymer chains caused by ππ interactions of aromatic rings, thereby improving the dispersion stability of the pigment in the pigment dispersion and improving the stability over time of the pigment dispersion.
The Ar value may be 0 mmol/g.
Here, the number of mmoles of aromatic rings means the number of mmoles per single ring of the aromatic ring. For example, one condensed naphthalene ring is counted as two aromatic rings.
The Ar value is preferably 2.0 mmol/g or less, more preferably 1.0 mmol/g or less.

(Cross-linked structure)
The polymer dispersant preferably contains a crosslinked structure.
This suppresses the detachment of the polymer dispersant adsorbed to the pigment, thereby improving the dispersion stability of the pigment, and as a result, the stability over time of the pigment dispersion is further improved.

The polymeric dispersant containing a crosslinked structure can be formed by crosslinking an uncrosslinked polymeric dispersant (i.e., a polymeric dispersant that does not contain a crosslinked structure before being crosslinked).
The polymeric dispersant containing a crosslinked structure is preferably a reaction product of an uncrosslinked polymeric dispersant containing an acid group (e.g., a carboxy group) and a crosslinking agent. In this case, the crosslinked structure is a structure formed by a reaction (e.g., an acid-epoxy reaction) between the acid group and a crosslinking agent (e.g., an epoxy group in an epoxy compound).

In the present disclosure, the term "acid group" refers to a concept that encompasses both an unneutralized acid group (e.g., a carboxy group) and a neutralized acid group (e.g., a salt of a carboxy group (e.g., -C(=O)ONa)).
Neutralization will be described later.

The pigment dispersion of the present disclosure can be produced by dispersing a pigment with an uncrosslinked polymer dispersant containing an acid group to produce a dispersion (hereinafter referred to as "uncrosslinked dispersion"), mixing the obtained uncrosslinked dispersion with a crosslinking agent, and reacting the acid groups in the uncrosslinked polymer dispersant in the uncrosslinked dispersion with the crosslinking agent. This reaction causes the acid groups in the uncrosslinked polymer dispersant to react with the crosslinking agent to form a crosslinked structure. It is believed that the reaction to form this crosslinked structure proceeds with the uncrosslinked polymer dispersant adsorbed to the surface of the pigment. For this reason, it is believed that the formation of the crosslinked structure has the effect of making it difficult for the polymer dispersant to detach from the surface of the pigment.

- Crosslinking agent -
The crosslinking agent is preferably a compound having two or more reactive sites with the uncrosslinked polymeric dispersant (for example, an uncrosslinked polymeric dispersant containing a carboxy group).
A preferred combination of the crosslinking agent and the uncrosslinked polymer dispersing agent is
a compound having two or more epoxy groups (i.e., a di- or higher-functional epoxy compound) as a crosslinking agent;
and an uncrosslinked polymer dispersant containing at least one of a carboxy group and a salt thereof, as an uncrosslinked polymer dispersant.
In the above combination, a crosslinked structure is formed by reaction between the carboxy group and/or a salt thereof in the uncrosslinked polymeric dispersant containing the carboxy group and/or a salt thereof and the epoxy group in the compound having two or more epoxy groups, thereby forming a polymeric dispersant containing a crosslinked structure.
The formation of such a crosslinked structure is preferably carried out after dispersing the pigment with a non-crosslinked polymeric dispersant containing at least one of a carboxy group and a salt thereof.

Specific examples of difunctional or higher functional epoxy compounds, which are preferred embodiments of the crosslinking agent, include 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, and trimethylolpropane triglycidyl ether.
Among these, polyethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, or trimethylolpropane triglycidyl ether is preferred.

As the crosslinking agent, commercially available products can also be used.
Commercially available products that can be used include, for example, Denacol EX-321, EX-821, EX-830, EX-850, and EX-851 (manufactured by Nagase ChemteX Corporation).

The molar ratio of the crosslinking site in the crosslinking agent (e.g., epoxy group) to the crosslinked site in the uncrosslinked polymer dispersant (e.g., carboxy group or its salt) [crosslinking site in the crosslinking agent (e.g., epoxy group):crosslinked site in the uncrosslinked polymer dispersant (e.g., carboxy group or its salt)] 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, from the viewpoint of the crosslinking reaction rate and/or the stability of the dispersion after crosslinking.

(Uncrosslinked polymer dispersant)
An uncrosslinked polymeric dispersant (i.e., a polymeric dispersant before a crosslinked structure is formed) may contain a unit of formula (1) and a unit of formula (2) in the same manner as a polymeric dispersant containing a crosslinked structure. The unit of formula (1) and the unit of formula (2) are not changed by the reaction for forming the crosslinked structure and may be maintained as they are in the polymeric dispersant containing a crosslinked structure.

The preferred range of the content of the unit of formula (1) relative to the total amount of the uncrosslinked polymer dispersant is the same as the preferred range of the content of the unit of formula (1) relative to the total amount of the polymer dispersant containing a crosslinked structure, excluding the crosslinked structure.
The preferred range of the content of the unit of formula (2) relative to the total amount of the uncrosslinked polymer dispersant is the same as the preferred range of the content of the unit of formula (2) relative to the total amount of the polymer dispersant containing a crosslinked structure, excluding the crosslinked structure.

The uncrosslinked polymeric dispersant may further include acid groups.
A crosslinked structure is formed by the reaction of the acid group with a crosslinking agent (eg, an acid-epoxy reaction).
A part of the total number of acid groups in the uncrosslinked polymeric dispersant may remain in the polymeric dispersant containing a crosslinked structure.

As described above, the acid group contained in the uncrosslinked polymer dispersant may be a neutralized acid group or a non-neutralized acid group.
The neutralized acid group is preferably a -C(=O)OM group (wherein M represents an alkali metal, an ammonium ion, or an organic cation).
Examples of the organic cation include an alkylammonium cation having 1 to 10 carbon atoms, a hydroxy-substituted alkylammonium cation having 1 to 10 carbon atoms, a carboxy-substituted alkylammonium cation having 2 to 10 carbon atoms, and an organic cation having 2 to 10 alkyleneimine units having 2 to 4 carbon atoms.
The unneutralized acid group is preferably a carboxy group (-C(=O)OH group).

The acid groups can be neutralized by reacting an uncrosslinked polymeric dispersant containing acid groups with a base (hereinafter also referred to as a "neutralizing base").
Examples of the neutralizing base include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; ammonia; and organic amines such as dimethylethanolamine and diisopropylethylamine.

-Neutralization rate-
In the uncrosslinked polymeric dispersant, it is preferred that 10% to 90% of the total number of acid groups contained therein are neutralized (ie, are salts of acid groups).
In other words, in the uncrosslinked polymer dispersant, the proportion of neutralized acid groups in the total number of neutralized acid groups and non-neutralized acid groups (i.e., % by number; hereinafter, also referred to as "neutralization rate (%)") is preferably 10% to 90%.
When the neutralization rate (%) is between 10% and 90%, a crosslinked structure is more likely to be formed.
The neutralization rate (%) is preferably 20% to 80%, and more preferably 20% to 60%.
The neutralization rate (%) is determined by neutralization titration.

-Acid group unit-
Both the uncrosslinked polymeric dispersant and the polymeric dispersant containing a crosslinked structure preferably contain a structural unit derived from a monomer containing an acid group (hereinafter also referred to as an "acid group unit").
Here, the structural unit derived from a monomer containing an acid group means a structural unit formed using a monomer containing an acid group as a raw material.
In the structural unit derived from the monomer containing an acid group (i.e., the acid group unit), the acid group may remain as it is or may disappear. For example, after the monomer containing an acid group is polymerized to form a structural unit, the acid group in this structural unit may disappear due to a crosslinking structure forming reaction.

The monomer containing an acid group is preferably a compound containing an acid group and a polymerizable group.
As the polymerizable group, a group containing an ethylenic double bond is more preferable, and a (meth)acryloyl group, an allyl group, a styryl group, or a vinyl group is further preferable.

Specific examples of monomers containing an acid group include (meth)acrylic acid, β-carboxyethyl acrylate, fumaric acid, itaconic acid, maleic acid, and crotonic acid. Of these, (meth)acrylic acid is particularly preferred.

The content of acid group units relative to the total amount of the uncrosslinked polymer dispersant or the polymer dispersant containing a crosslinked structure is preferably 10% by mass to 70% by mass, more preferably 15% by mass to 65% by mass, and even more preferably 20% by mass to 60% by mass.

-Hydrophilic/hydrophobic ratio-
When the polymer dispersant containing a crosslinked structure contains the unit of formula (1), the unit of formula (2), and an acid group unit, the hydrophilic/hydrophobic ratio represented by the following formula is preferably at least 8. This further improves the stability over time of the pigment dispersion.

Hydrophilic/hydrophobic ratio=(mass ratio of the units of formula (2) in the total mass of the polymer dispersant containing a crosslinked structure×number of carbon atoms in the alkylene group represented by ^R2 in formula (2))/(mass ratio of the acid group units in the total mass of the polymer dispersant containing a crosslinked structure)

From the viewpoint of further improving the stability over time of the pigment dispersion and ensuring the affinity of the polymer dispersant containing a crosslinked structure to water, the hydrophilic/hydrophobic 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.

(Crosslinking value)
The polymer dispersant containing the above-mentioned crosslinked structure has a crosslinking value represented by the following formula of preferably 0.7 mmol/g or more, more preferably 0.8 mmol/g or more, and even more preferably 0.9 mmol/g or more.
Crosslinking value [mmol/g]
= Acid value of uncrosslinked polymer dispersant [mmol/g] + Base value of uncrosslinked polymer dispersant [mmol/g] - Acid value of polymer dispersant containing crosslinked structure [mmol/g] - Base value of polymer dispersant containing crosslinked structure [mmol/g]

Where:
The acid value of an uncrosslinked polymer dispersant is the number of mmoles of unneutralized acid groups (e.g., —COOH groups) contained in 1 g of the uncrosslinked polymer dispersant,
The base number of an uncrosslinked polymeric dispersant is the number of mmoles of neutralized acid groups (e.g., —COONa groups) contained in 1 g of the uncrosslinked polymeric dispersant,
The acid value of a polymer dispersant containing a crosslinked structure is the number of mmoles of unneutralized acid groups (e.g., —COOH groups) contained in 1 g of the polymer dispersant containing a crosslinked structure,
The base number of a polymer dispersant containing a crosslinked structure is the number of mmoles of neutralized acid groups (for example, --COONa groups) contained in 1 g of the polymer dispersant containing a crosslinked structure.
Both the acid value and the base value can be determined by neutralization titration.

The upper limit of the crosslinking value of a polymer dispersant containing a crosslinked structure is, for example, 2.4 mmol/g.

-Other structural units-
Each of the uncrosslinked polymeric dispersant and the polymeric dispersant containing a crosslinked structure may contain structural units other than the unit of formula (1), the unit of formula (2), and the acid group unit (hereinafter also referred to as other structural units).
However, the total proportion of the formula (1) unit, the formula (2) unit, and the acid group unit in the total amount of the uncrosslinked polymeric dispersant or the polymeric dispersant containing a crosslinked structure is preferably 70 mass% or more, more preferably 80 mass% or more, and even more preferably 90 mass% or more.

Other structural units include structural units derived from vinyl monomers.
An example of the vinyl monomer is methyl (meth)acrylate.
Other structural units in the uncrosslinked polymer dispersant may remain in the polymer dispersant containing a crosslinked structure.

--Weight average molecular weight (Mw) of uncrosslinked polymer dispersant--
The weight average molecular weight (Mw) of the uncrosslinked polymer dispersant is not particularly limited, 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.

In this disclosure, the weight average molecular weight (Mw) is measured by gel permeation chromatography (GPC). GPC is performed using HLC-8220GPC (manufactured by Tosoh Corporation), three columns (TSKgel SuperHZM-H, TSKgel SuperHZ4000, and TSKgel SuperHZ2000, all of which are product names manufactured by Tosoh Corporation) connected in series, and THF (tetrahydrofuran) is used as the eluent. The conditions are a sample concentration of 0.45% by mass, a flow rate of 0.35 ml/min, a sample injection amount of 10 μl, a measurement temperature of 40° C., and a differential refractive index detector. Additionally, the calibration curve will be created from eight samples of "Standard sample TSK standard, polystyrene" manufactured by Tosoh Corporation: "F-40", "F-20", "F-4", "F-1", "A-5000", "A-2500", "A-1000", and "n-propylbenzene".

In the pigment dispersion of the present disclosure, the ratio of the pigment content to the polymer dispersant content containing a crosslinked structure 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, by mass.

In the above-mentioned uncrosslinked dispersion, the ratio of the pigment content to the uncrosslinked polymer dispersant content 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, by mass.

The uncrosslinked polymer dispersant can be produced by a known method of copolymerizing each monomer for forming each structural unit (i.e., a monomer for forming a unit of formula (1), a monomer for forming a unit of formula (2), a monomer containing an acid group, etc.).

<Other ingredients>
The pigment dispersion of the present disclosure may contain other components in addition to the above-mentioned components.
The other components include components that can be contained in the ink.
Examples of other components include water-soluble organic solvents.

In this disclosure, "water-soluble" means the property of dissolving at least 1 g in 100 g of water at 25°C.

Specific examples of water-soluble organic solvents are the same as the water-soluble organic solvents that can be contained in the ink.

The total amount of water, pigment, and polymer dispersant containing a crosslinked structure in the total amount of the pigment dispersion of the present disclosure is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

<Example of a method for producing a pigment dispersion (Production method A)>
An example of a method for producing the pigment dispersion of the present disclosure (hereinafter referred to as Production Method A) is shown below.
The manufacturing method A is
A step of producing an acid group-containing polymer containing the above formula (1) unit, the above formula (2) unit, and an acid group;
a step of neutralizing at least a portion of the total number of acid groups in the produced acid group-containing polymer with a base to obtain an uncrosslinked polymer dispersant;
a step of dispersing a pigment in water with the uncrosslinked polymer dispersant to obtain an uncrosslinked dispersion containing water, a pigment, and the uncrosslinked polymer dispersant;
a step of reacting the uncrosslinked polymer dispersant in the uncrosslinked dispersion with a crosslinking agent to obtain a pigment dispersion containing water, a pigment, and a polymer dispersant having a crosslinked structure;
Includes.

The neutralization of the acid group-containing polymer to obtain the uncrosslinked polymer dispersant and the preferred embodiments thereof are as described above.
The step of obtaining the uncrosslinked dispersion can be carried out using a conventional pigment dispersion technique.
Examples of dispersing machines used for dispersing the pigment include a ball mill, a roll mill, a sand mill, a bead mill, and a Nanomizer.
The reaction between the uncrosslinked polymer dispersant and the crosslinking agent (i.e., the formation of a crosslinked structure) in the step of obtaining a pigment dispersion and the preferred aspects thereof are as described above.

〔ink〕
The inks of the present disclosure include the pigment dispersions of the present disclosure described above.
That is, the inks of the present disclosure contain the pigment dispersion components of the present disclosure described above.
The ink of the present disclosure can be produced by adding any component to the pigment dispersion of the present disclosure described above.
The ink of the present disclosure may also be the pigment dispersion of the present disclosure itself.
That is, the ink of the present disclosure may satisfy the requirements of the pigment dispersion of the present disclosure described above.

The inks of the present disclosure are preferably inkjet inks.
Regarding the ink-jet recording method in this case, the descriptions in known documents can be appropriately referred to.

<Water>
The inks of the present disclosure contain water.
The water content relative to the total amount of the ink is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and still more preferably 60% by mass or more.
The upper limit of the water content relative to the total amount of the ink is, for example, 90% by mass, 80% by mass, etc.

<Pigments>
The inks of the present disclosure contain at least one pigment.
The preferred embodiment of the pigment is as described in the section "Pigment Dispersion."
The content of the pigment relative to the total amount of the ink is preferably 1% by mass to 30% by mass, more preferably 1.5% by mass to 20% by mass, even more preferably 2% by mass to 15% by mass, and still more preferably 2% by mass to 10% by mass.

<Polymer Dispersant>
The inks of the present disclosure contain at least one polymeric dispersant.
The preferred embodiments of the polymer dispersant are as described in the section "Pigment Dispersion."

The ratio of the pigment content to the polymer dispersant content 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, by mass.

<Water-soluble organic solvent>
The ink of the present disclosure preferably contains a water-soluble organic solvent.
This improves the ejection properties from an inkjet head when the ink of the present disclosure is used as an inkjet ink.
Specific examples of the water-soluble organic solvent are as shown in the section "Pigment dispersion."
The content of the water-soluble organic solvent in the total amount of the ink is preferably 5% by mass to 40% by mass, more preferably 10% by mass to 30% by mass, and even more preferably 15% by mass to 30% by mass.

<Organic Solvent with ClogP Value of -0.60 or More (Organic Solvent A)>
The ink of the present disclosure preferably contains at least one organic solvent having a ClogP value of −0.60 or more (hereinafter also referred to as “organic solvent A”).
Organic solvent A is an organic solvent having a higher hydrophobicity than organic solvent B (an organic solvent having a ClogP value of less than −0.60) described below.
The organic solvent A is a component that contributes to improving the image quality.

In the present disclosure, ClogP is a value indicating the degree of hydrophobicity.
The higher the ClogP, the more hydrophobic the compound.
The ClogP value can be determined from a chemical structural formula. In the present disclosure, ClogP refers to a value calculated using ChemDraw (registered trademark) Professional (ver. 20.1.1.125) (manufactured by PerkinElmer).

As organic solvent A, among organic solvents having a ClogP value of −0.60 or more, organic solvents having 6 or more carbon atoms are more preferred, organic solvents having 6 to 10 carbon atoms are even more preferred, and glycol ethers having 6 to 10 carbon atoms are even more preferred.
The ClogP value of organic solvent A is preferably in the range of −0.60 to 2.70, more preferably in the range of −0.40 to 1.90.

Examples of the organic solvent A include:
Ethylene glycol ethers such as ethylene glycol monoethyl ether (ClogP value: -0.22), ethylene glycol monopropyl ether (ClogP value: 0.31), ethylene glycol monoisopropyl ether (ClogP value: 0.09), ethylene glycol monobutyl ether (ClogP value: 0.84), ethylene glycol monoisobutyl ether (ClogP value: 0.71), ethylene glycol t-butyl ether (ClogP value: 0.49), and ethylene glycol monohexyl ether (EGmHE) (ClogP value: 1.90);
Diethylene glycol ethers such as diethylene glycol monoethyl ether (DEGmEE) (ClogP value: −0.39), diethylene glycol monoisopropyl ether (ClogP value: −0.08), diethylene glycol monobutyl ether (ClogP value: 0.67), diethylene glycol monoisobutyl ether (ClogP value: 0.54), and diethylene glycol monohexyl ether (ClogP value: 1.72);
Triethylene glycol ethers such as triethylene glycol monobutyl ether (Clog P value: 0.49), triethylene glycol monohexyl ether (Clog P value: 1.55), triethylene glycol dimethyl ether (Clog P value: -0.43), and triethylene glycol butyl methyl ether (Clog P value: 1.01);
Propylene glycol ethers such as propylene glycol monomethyl ether (ClogP value: −0.30), propylene glycol monoethyl ether (ClogP value: 0.09), propylene glycol monopropyl ether (ClogP value: 0.62), propylene glycol monobutyl ether (ClogP value: 1.15), and propylene glycol dimethyl ether (ClogP value: 0.23);
dipropylene glycol ethers such as dipropylene glycol monomethyl ether (ClogP value: 0.09), dipropylene glycol monopropyl ether (ClogP value: 0.75), dipropylene glycol monobutyl ether (ClogP value: 1.28), and dipropylene glycol monohexyl ether (ClogP value: 2.34);
Tripropylene glycol ethers such as tripropylene glycol monomethyl ether (ClogP value: −0.03) and tripropylene glycol monobutyl ether (ClogP value: 1.42);
Alkanediols such as 1,2-butanediol (ClogP value: −0.53), 1,2-pentanediol (ClogP value: 0), and 1,2-hexanediol (ClogP value: 0.53);
etc.

Organic solvent A is
Ethylene glycol monohexyl ether (EGmHE),
Diethylene glycol monoethyl ether (DEGmEE),
Diethylene glycol monoisopropyl ether,
diethylene glycol monobutyl ether,
diethylene glycol monoisobutyl ether,
propylene glycol monomethyl ether,
propylene glycol monoethyl ether,
propylene glycol monopropyl ether,
propylene glycol monobutyl ether,
Dipropylene glycol monomethyl ether Tripropylene glycol monomethyl ether (TPGmME)
1,2-butanediol,
It is preferable that the diol contains at least one selected from the group consisting of 1,2-pentanediol and 1,2-hexanediol.

Organic solvent A preferably contains a glycol ether having a CLogP value of -0.50 to 2.00, more preferably contains a glycol ether having a CLogP value of -0.40 to 1.90, and even more preferably contains at least one selected from the group consisting of EGmHE (1.90), DEGmEE (-0.39), and TPGmME (-0.03).

The content of organic solvent A relative to the total amount of the ink is preferably 2.0% by mass to 11.0% by mass, and more preferably 3.0% by mass to 10.0% by mass.
When the content of organic solvent A is 2.0% by mass or more, the image quality is further improved.
When the content of organic solvent A is 11.0% by mass or less, the temporal stability of the ink is further improved.

<Organic Solvent Having a ClogP Value of Less than −0.60 (Organic Solvent B)>
The ink of the present disclosure contains at least one organic solvent having a ClogP value of less than −0.60 (hereinafter also referred to as “organic solvent B”).
Organic solvent B in the present disclosure is an organic solvent having a ClogP value of less than −0.60.
Organic solvent B is an organic solvent that is less hydrophobic (that is, more hydrophilic) than organic solvent A (an organic solvent having a ClogP value of −0.60 or more).
Organic solvent B is a component for mixing organic solvent A in the aqueous ink, thereby further improving the image quality.

As organic solvent B, among organic solvents having a ClogP value of less than -0.60, organic solvents having 6 or less carbon atoms are preferred, organic solvents having 5 or less carbon atoms are more preferred, and organic solvents having 4 or less carbon atoms are even more preferred. The preferred range of the ClogP value of organic solvent B is -1.60 to -0.60, and the more preferred range is -1.40 to -0.70.

Examples of the organic solvent B include
Propylene glycol (hereinafter also referred to as "PG"; ClogP value = -1.06),
Glycerin (hereinafter also referred to as "GL"; ClogP value = -1.54),
1,3-butanediol (hereinafter referred to as "1,3-BD"; ClogP value = -0.72),
Triethylene glycol (hereinafter referred to as "TEG"; ClogP value = -1.48),
2-pyrrolidone (ClogP value = -0.97),
etc.

The content of organic solvent B relative to the total amount of the ink is preferably 2.0% by mass to 30.0% by mass, and more preferably 10.0% by mass to 30.0% by mass.
When the content of organic solvent B is 2.0% by mass or more, the image quality is further improved.
When the content of organic solvent A is 11.0% by mass or less, the temporal stability of the ink is further improved.

<S(A)/S(B)>
If the content (mass %) of organic solvent A relative to the total amount of ink is "S(A)" and the content (mass %) of organic solvent B relative to the total amount of ink is "S(B)," it is preferable that S(A) and S(B) satisfy the formula ("S(A)/S(B)>0.25").
That is, it is preferable that S(A)/S(B) is greater than 0.25.
This further improves the image quality.
From the viewpoint of improving image quality, the upper limit of S(A)/S(B) is preferably 0.50, and more preferably 0.45.

<Urea>
The inks of the present disclosure preferably contain urea.
The urea functions as a solid wetting agent.
The content of urea relative to the total amount of the ink is preferably 1% by mass to 10% by mass, more preferably 2% by mass to 9% by mass, and even more preferably 3% by mass to 8% by mass.

<Other ingredients>
The inks of the present disclosure may optionally contain other ingredients.
Other components may include additives such as surfactants, resin particles, water-soluble resins, silicon compounds, UV absorbers, antioxidants, anti-fading agents, conductive salts, and basic compounds (pH adjusters).
For components that can be contained in the ink, reference can be made to publicly known documents such as JP-A-2015-193729 and JP-A-2016-069487.

<Physical Properties>
The pH of the ink (at 25° C.) is preferably from 7 to 10, and more preferably from 7.5 to 9.5.
The pH is measured at 25° C. using a pH meter, for example, a pH meter manufactured by Toa DKK Corporation (model number "HM-31").

The viscosity of the ink (30° C.) is preferably from 0.5 mPa·s to 30 mPa·s, more preferably from 2 mPa·s to 20 mPa·s, more preferably from 2 mPa·s to 15 mPa·s, and even more preferably from 3 mPa·s to 10 mPa·s.
The viscosity is a value measured using a viscometer at 25° C. The viscosity is measured using a viscometer at 25° C., for example, using a TV-22 type viscometer manufactured by Toki Sangyo Co., Ltd.

The surface tension of the ink (25° C.) 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.
The surface tension is measured at 25° C. using a surface tensiometer, for example, an automatic surface tensiometer (product name "CBVP-Z") manufactured by Kyowa Interface Science Co., Ltd., by the plate method.

<Recording media>
In image recording using the ink of the present disclosure, an image is recorded by applying the ink onto a recording medium.
The recording medium is not particularly limited, but examples thereof include so-called coated paper used in general offset printing.
Coated paper is a type of paper in which a coating layer is formed by applying a coating material to the surface of cellulose-based fine paper or neutral paper that is generally not surface-treated.

The coated paper may be any paper that is generally available on the market.
For example, the coated paper may be a general-purpose coated paper for printing, and specific examples include coated papers (A2, B2) such as "OK Topcoat+" manufactured by Oji Paper Co., Ltd., "Aurora Coat" and "Yurite" manufactured by Nippon Paper Industries Co., Ltd., and art papers (A1) such as "Tokubishi Art" manufactured by Mitsubishi Paper Mills.

The recording medium may be a low water-absorbent recording medium or a non-water-absorbent recording medium.
In the present disclosure, a low water-absorbent recording medium refers to one having a water absorption coefficient Ka of 0.05 mL/ ^m2 ·ms1 ^{/2 to} 0.5 mL/m2·ms1 ^/2 , preferably 0.1 mL/ ^m2 ·ms1 ^{/ 2} to 0.4 mL/m2·ms1 ^/2 , and more preferably 0.2 mL/ ^m2 ·ms1 ^/2 to 0.3 mL/ ^m2 ·ms1 ^/2 .
Moreover, a non-water-absorbent recording medium refers to one having a water absorption coefficient Ka of less than 0.05 mL/m ² ·ms ^1/2 .
The water absorption coefficient Ka is the same as that described in JAPAN TAPPI Paper and Pulp Testing Method No. 51:2000 (published by the Paper and Pulp Technology Association), and specifically, the absorption coefficient Ka is calculated from the difference in the amount of water transferred between a contact time of 100 ms and a contact time of 900 ms using an automatic scanning liquid absorption meter KM500Win (manufactured by Kumagai Riki Co., Ltd.).

The non-absorbent recording medium is preferably a resin substrate, for example a substrate made of a thermoplastic resin formed into a sheet.
The resin substrate preferably comprises polypropylene, polyethylene terephthalate, nylon, polyethylene, or polyimide.
The resin substrate may be a transparent resin substrate or a colored resin substrate, and at least a part of the substrate may be subjected to a metal deposition treatment or the like.
The shape of the resin substrate is not particularly limited, but a sheet-like resin substrate is preferable, and from the viewpoint of productivity of printed matter, a sheet-like resin substrate that can be wound up to form a roll is more preferable.

<Transport speed of recording medium>
In the image recording method using the ink of the present disclosure, an image may be recorded by applying the ink onto a recording medium that is being transported.
In this case, the conveying speed of the recording medium is preferably 100 m/min or more and 300 m/min or less.
When the conveying speed of the recording medium is 100 m/min or more, image recording can be performed at a higher speed.
When the conveying speed of the recording medium is 300 m/min or less, the graininess of the image, the resolution of the image, or the abrasion resistance of the image can be further improved.
The conveying speed of the recording medium is preferably 100 m/min to 250 m/min, more preferably 150 m/min to 200 m/min.

<Ink application conditions>
In an image recording mode in which ink is applied onto a transported recording medium, the ink is preferably applied onto the transported recording medium using an inkjet method under conditions in which the resolution in a direction perpendicular to the transport direction of the recording medium is 1200 dpi or more.
In this specification, dpi is an abbreviation for dots per inch.

As the inkjet method, a known method can be applied.
Inkjet methods include, for example, a charge control method that uses electrostatic attraction to eject ink, a drop-on-demand method (pressure pulse method) that uses the vibration pressure of a piezoelectric element, an acoustic inkjet method in which an electrical signal is converted into an acoustic beam and irradiated onto the ink, ejecting the ink using radiation pressure, and a thermal inkjet (Bubble Jet (registered trademark)) method in which ink is heated to form bubbles, and the resulting pressure is utilized.

As an inkjet head for use in the inkjet method, a line head in which recording elements are arranged to cover the entire area of one side of the substrate is preferred.

In the inkjet method using a line head, a pattern can be formed over the entire surface of a substrate by scanning the substrate in a direction intersecting the arrangement direction of the recording elements, eliminating the need for a transport system such as a carriage for scanning a short head.
Furthermore, with the inkjet method using a line head, there is no need for complex scanning control of the carriage movement and the substrate; only the substrate moves, which allows for faster printing speeds than with the shuttle method that scans a short head.

The amount of ink droplets ejected from the inkjet head is preferably 1 pL (picoliter) to 100 pL, more preferably 1.5 pL to 80 pL, and even more preferably 1.8 pL to 50 pL.

In applying the ink, the resolution in the direction perpendicular to the conveying direction of the recording medium is 1200 dpi or more.
The above resolution contributes to the high quality of the resulting images.
The resolution is preferably 1200 dpi or more and 2400 dpi or less.

Hereinafter, the present disclosure will be described more specifically with reference to examples. However, the present disclosure is not limited to the following examples as long as it does not depart from the gist of the disclosure.
Hereinafter, the polymer dispersant may be simply referred to as the "dispersant".

Example 1
<Synthesis of Uncrosslinked Polymer Dispersant>
A reaction vessel equipped with a gas inlet tube, a thermometer, a condenser, and a stirrer was charged with 153.5 parts by mass of tripropylene glycol monomethyl ether (product name "MFTG", manufactured by Nippon Nyukazai Co., Ltd.) as a reaction solvent, and then the atmosphere inside the reaction vessel was replaced with nitrogen gas.
Next, the inside of the reaction vessel is heated to 85° C., and
76.8 parts by mass of MFTG as a reaction solvent,
24.0 parts by mass of methoxydiethylene glycol methacrylate (Me(EO)2MA) as a monomer for forming units of formula (1),
39.0 parts by mass of stearyl methacrylate (C18MA) as a monomer for forming units of formula (2),
37.0 parts by mass of methacrylic acid (MAA) as a monomer for forming an acid group-containing unit, and
A mixture of 3.0 parts of V-601 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a polymerization initiator was dropped over 3 hours to carry out a polymerization reaction, and after the end of the dropwise addition, the mixture was further reacted at 85 ° C. for 3 hours to complete the polymerization. The obtained solution was cooled to room temperature, and then an aqueous NaOH solution was added thereto as an aqueous solution of a neutralizing base to neutralize 40 mol % of the carboxyl groups in the polymer (i.e., neutralization was performed at a neutralization rate of 40%). Water was added to the neutralized solution so that the uncrosslinked polymer dispersant was 7.5 mass %, and a 7.5 mass % solution of the uncrosslinked polymer dispersant was obtained.

Table 1 shows the type and amount of each monomer used to synthesize the uncrosslinked polymer dispersant (including n in formula (1) and the carbon number of ^R5 in formula (2)), the weight average molecular weight (Mw) of the uncrosslinked polymer dispersant, the Ar value of the uncrosslinked polymer dispersant, the acid value of the uncrosslinked polymer dispersant before neutralization, the type of neutralizing base, and the neutralization rate (%) of the uncrosslinked polymer dispersant.
Some of this information is also shown in Table 2 below.

<Production of Uncrosslinked Dispersion>
A mixture of the following composition was preliminarily dispersed to a uniform state, and then dispersed for 3 hours using a bead mill (bead diameter: 0.1 mmφ, zirconia beads), to obtain an uncrosslinked dispersion in which the magenta pigment was dispersed by the uncrosslinked polymer dispersant.

--Composition of uncrosslinked dispersion--
・Pigment Red 122 (magenta pigment)
... 15 parts by weight, 7.5% by weight solution of uncrosslinked polymer dispersant ... 60 parts by weight, MFTG
… 4.5 parts by weight Water … the balance that makes the total 100 parts by weight

<Preparation of Crosslinked Dispersion>
Next, a mixture having the following composition was reacted at 70°C for 6 hours and then cooled to 25°C, whereby the uncrosslinked polymer dispersant in the uncrosslinked dispersion was crosslinked by the crosslinking agent to obtain a crosslinked dispersion containing water, a pigment, and a polymer dispersant having a crosslinked structure (hereinafter also referred to as "crosslinked dispersant").

-composition-
Uncrosslinked dispersion: 100 parts by mass "Denacol EX-321" (manufactured by Nagase ChemteX Corporation) (trimethylolpropane polyglycidyl ether; crosslinking agent)
… 1.1 parts by mass

By neutralization titration, the acid value (mmol/g) (A) of the polymer dispersant before crosslinking (i.e., uncrosslinked polymer dispersant), the base value (mmol/g) (B) of the polymer dispersant before crosslinking, the acid value (mmol/g) (C) of the polymer dispersant after crosslinking, and the base value (mmol/g) (C) of the polymer dispersant after crosslinking were each measured, and the crosslinking value (mmol/g) (= A + B-C-D) was calculated based on the obtained results.
The results are shown in Table 1.

<Production of Pigment Dispersion as Target Product>
Next, the crosslinked dispersion was ultrafiltered by passing it through an ultrafiltration device (cross-flow type ultrafilter (UF), manufactured by Sartorius) equipped with a polyethersulfone (PESU) membrane (micropore size: 0.1 μm) at a flow rate of 600 mL per minute. At this time, the liquid temperature was adjusted to 25° C., and ultrafiltration was performed 10 times, with one time being the volumetric magnification of the charged liquid. Thereafter, ion-exchanged water was added so that the pigment concentration was 15% by mass.
As a result of the above, a pigment dispersion containing water, a pigment, and a polymer dispersant having a crosslinked structure (hereinafter referred to as "pigment dispersion A") was obtained as the target product.

<Ink Preparation>
The above pigment dispersion A was mixed with other components to obtain an ink having the following composition.
The following ink contains 5 parts by weight of pigment and 1.5 parts by weight of dispersant.

- Ink composition -
Pigment Red 122 ("PR-122" in Table 2) (a magenta pigment that is a quinacridone pigment)
... 5 parts by mass of crosslinked pigment dispersion resin B1 (i.e., pigment dispersion resin containing a crosslinked structure)
… 1.5 parts by mass of ethylene glycol monohexyl ether ("EGmHE" in the table; an organic solvent with a ClogP value of -0.60 or more)
… 8.0 parts by mass of propylene glycol (an organic solvent having a ClogP value of less than −0.60)
… 20.0 parts by mass BYK-345 (a silicone surfactant manufactured by BYK Corporation)
... 0.4 parts by mass of acrylic resin particles (solid content (acrylic resin particles) in acrylic resin particle dispersion "Neocryl A-1105" manufactured by DSM)
… 1.7 parts by weight Water … The balance that makes the total 100 parts by weight

<Evaluation of ink stability over time>
The ink stability over time was evaluated as follows.
A storage test was performed in which the ink was placed in a sealed container made of Teflon (registered trademark) and left for one day in a thermostatic chamber at 80° C. The viscosity of the ink was measured before and after the storage test, and the rate of change in viscosity of the ink was calculated based on the following formula, and the stability of the ink over time was evaluated according to the following evaluation criteria.
The results are shown in Table 2.
Ink viscosity change rate=(ink viscosity after storage test)/(ink viscosity before storage test)
The viscosity of the ink was measured at a temperature of 30° C. using a VISCOMETER TV-22 (manufactured by Toki Sangyo Co., Ltd.).
The closer the ink viscosity change rate is to 1.0, the higher the ink stability over time. Therefore, in the following evaluation criteria, the most excellent ink stability over time is ranked 5.

-Evaluation criteria for ink stability over time-
5: The viscosity change rate was less than 1.1.
4: The rate of change in viscosity was 1.1 or more and less than 1.5.
3: The rate of change in viscosity was 1.5 or more and less than 2.0.
2: The rate of change in viscosity was 2.0 or more and less than 2.5.
1: The viscosity change rate was 2.5 or more.

<Image quality>
(Image recording)
A printer head (GELJET (registered trademark) GX5000) manufactured by Ricoh Co., Ltd. was prepared, and the ink prepared above was loaded into a storage tank connected to the printer head.
Next, the printer head was fixed and positioned so that the direction of the line head in which the nozzles were arranged was inclined at 75.7 degrees with respect to a direction perpendicular to the direction of movement of the stage.
A recording medium (OK Topcoat+ manufactured by Oji Paper Co., Ltd.) was prepared, and while moving this recording medium at a constant speed in the moving direction of the stage, ink was discharged from the nozzles of the printer head to apply ink to an area of 140 mm x 50 mm on the recording medium. The ink was applied under the conditions of an ink droplet volume of 2 pL, a discharge frequency of 25.5 kHz, and a resolution of 1200 dpi x 1200 dpi.
Immediately after application of the ink, the recording medium to which the ink was applied was placed on a hot plate at 70°C with the non-inked side (i.e., the side opposite to the side to which the ink was applied) in contact with the hot plate, and the ink on the recording medium was dried for 10 seconds with hot air at 120°C using a dryer.
In this manner, an image was recorded on the recording medium, and an image recording was obtained.

(Image quality evaluation)
According to the above image recording operation, an image was recorded on the recording medium with a recording duty of 80%. The graininess (presence or absence of graininess) of the resulting image was visually observed, and the image quality was evaluated according to the following evaluation criteria.
The results are shown in Table 2.
In the following evaluation criteria, the most excellent image quality is ranked 5.

- Image quality evaluation criteria -
5: No graininess is observed when the image is observed from a distance of 5 cm.
4: When the image is observed from a distance of 5 cm, graininess is noticeable, but when observed from a distance of 10 cm, no graininess is noticeable.
3: Graininess is observed when the image is observed from a distance of 10 cm, but no graininess is observed when observed from a distance of 15 cm.
2: When the image is observed from a distance of 15 cm, graininess is noticeable, but when observed from a distance of 20 cm, no graininess is noticeable.
1: Graininess is observed even when the image is observed from a distance of 20 cm.

[Examples 2 to 25, Comparative Examples 1 and 2]
The same operations as in Example 1 were performed except that the combinations of the type and amount of monomer used to synthesize the polymer dispersant, the type and neutralization rate of the neutralizing base, the crosslinking value, the type of pigment, and the type or content relative to the total amount of the ink of the organic solvent having a ClogP value of −0.60 or more were changed as shown in Table 1.
The evaluation results are shown in Table 2.

In Examples 1 to 25 and Comparative Examples 1 and 2, the amount of the polymerization initiator was appropriately adjusted so that the weight average molecular weight (Mw) of the uncrosslinked polymer dispersant would be the value shown in Table 1.

For the pigments in Tables 1 and 2, PR122, PY74, and PB15:3 refer to Pigment Red 122, Pigment Yellow 74, and Pigment Blue 15:3, respectively.
For the neutralizing bases in Tables 1 and 2, DIPEA means diisopropylethylamine and NH3 means ammonia.
The structures of the monomers in Table 1 are as follows:
In Table 1, AA is acrylic acid and MAA is methacrylic acid (the structures are omitted).

As shown in Tables 1 and 2, the ink stability over time was good in all of the Examples.
In all of the Examples, a dispersant in which m in formula (1) is an integer of 2 or more, ^R5 in formula (2) is an alkyl group having 14 or more carbon atoms, and the Ar value is 3.0 mmol/g or less was used as the crosslinked dispersant in the pigment dispersion.

In comparison with the examples, the results of the comparative examples were as follows.
Comparative Example 1 is an example in which the Ar value of the polymer dispersant was changed to more than 3.0 mmol/g.
Comparative Example 2 is an example in which the number of carbon atoms in the alkyl group represented by R ⁵ in formula (2) was changed to 13.
In both Comparative Examples 1 and 2, the ink stability over time decreased.

From the results of Examples 1, 5, 6, and 22, it can be seen that when R ⁵ in formula (2) is an alkyl group having 16 to 22 carbon atoms (Examples 1, 5, and 6), the ink stability over time is further improved.

The results of Examples 1 to 4 show that when m in formula (1) is an integer between 2 and 4 (Examples 1 and 2), the ink has improved stability over time.

The results of Examples 1 and 23 show that when the crosslinking value of the polymer dispersant is 0.8 mmol/g or more (Example 1), the ink stability over time is further improved.

The results of Examples 1, 15, and 24 show that when the content of the organic solvent having a ClogP value of −0.60 or more is 3.0% by mass to 10.0% by mass relative to the total amount of the ink (Examples 1 and 15), the ink stability over time and the image quality are particularly excellent.

Claims (9)

Contains water, a pigment, and a polymeric dispersant;
The polymer dispersant contains a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2),
The Ar value, which is the number of mmoles of aromatic rings in 1 g of the polymer dispersant, is 3.0 mmol/g or less.
Pigment dispersion.

In formula (1),
R ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R2 represents an alkylene group having 2 to 5 carbon atoms;
^R3 represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms;
^X1 and ^X2 each independently represent -O- or -NH-;
m represents an integer of 2 or more.
In formula (2),
^R4 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms;
^R5 represents an alkyl group having 14 or more carbon atoms;
^X3 represents -O- or -NH-. The pigment dispersion according to claim 1, wherein in the formula (2), R ⁵ represents an alkyl group having 16 to 22 carbon atoms. The pigment dispersion according to claim 1, wherein in formula (1), m represents an integer of 2 to 4. The pigment dispersion according to claim 1, wherein the polymer dispersant includes a crosslinked structure. The polymer dispersant containing the crosslinked structure is formed by crosslinking an uncrosslinked polymer dispersant,
The pigment dispersion according to claim 4 , wherein the polymer dispersant containing the crosslinked structure has a crosslinking value represented by the following formula of 0.8 mmol/g or more.
Crosslinking value [mmol/g]
= Acid value of the uncrosslinked polymer dispersant [mmol/g] + Base value of the uncrosslinked polymer dispersant [mmol/g] - Acid value of the polymer dispersant containing the crosslinked structure [mmol/g] - Base value of the polymer dispersant containing the crosslinked structure [mmol/g] The polymer dispersant containing the crosslinked structure is formed by crosslinking an uncrosslinked polymer dispersant,
The uncrosslinked polymer dispersant contains a structural unit represented by the formula (1), a structural unit represented by the formula (2), and an acid group, and 10% to 90% of the total number of the acid groups contained therein are neutralized.
The pigment dispersion according to claim 4. An ink containing the pigment dispersion according to any one of claims 1 to 6. The ink according to claim 7 further contains an organic solvent having a ClogP value of -0.60 or more. 9. The ink according to claim 8, wherein the content of the organic solvent having a ClogP value of −0.60 or more is 3.0% by mass to 10.0% by mass relative to the total amount of the ink.