WO2020066468A1

WO2020066468A1 - Pigment dispersion, inkjet ink composition and ink set obtained using said pigment dispersion, and image formed using said inkjet ink composition, and method for forming said image

Info

Publication number: WO2020066468A1
Application number: PCT/JP2019/034322
Authority: WO
Inventors: 祥平片岡
Original assignee: 富士フイルム株式会社
Priority date: 2018-09-26
Filing date: 2019-09-02
Publication date: 2020-04-02
Also published as: JP2022001610A

Abstract

Provided are: a pigment dispersion which, through use thereof in an ink, can adequately increase the color density of a formed image part; an inkjet ink composition and an ink set obtained using the dispersion; and an image formed using the inkjet ink composition, and an image formation method. This pigment dispersion contains an aqueous medium, a pigment, and a pigment dispersant comprising a polymer having an anionic group, the pKa (DMSO) of the polymer before neutralization of the pigment dispersant being 14 or greater. The inkjet ink composition and the ink set use the pigment dispersion. The image is formed and the image formation method forms an image using the inkjet ink composition.

Description

Pigment dispersion, inkjet ink composition and ink set using the pigment dispersion, image formed using the inkjet ink composition, and method for forming the same

The present invention relates to a pigment dispersion, an ink-jet ink composition and an ink set using the pigment dispersion, an image formed using the ink-jet ink composition, and a method for forming the image.

One of the image recording methods for forming an image on a recording medium such as paper based on an image data signal is an ink jet method. 2. Description of the Related Art Conventionally, a printer that is an ink jet recording apparatus generally uses three colors of red, blue, and yellow or four colors of ink obtained by adding black to white paper as a recording medium. However, in recent years, there has been an increasing demand for image formation on a recording medium using a colored recording medium. In this case, white ink is used as the ink in addition to the above-described colored ink. When an image is formed on a transparent or light-colored recording medium, a white ink is applied in advance to a region of the recording medium to which the colored ink is to be applied (so-called white pressing) to reduce the sharpness of the formed image. Enhancing is done. White ink applied to such a colored recording medium or a transparent or light-colored recording medium is required to have a high concealing property.

As a white pigment which is a coloring material for white ink, a metal oxide is generally used. In order to realize a high concealing property by the white pigment composed of the metal oxide, it is necessary to increase the particle size of the white pigment to some extent. For example, titanium oxide, which is a white pigment having high shielding properties, exhibits the highest shielding effect near an average primary particle diameter of 250 nm. However, since metal oxides generally have a large specific gravity, white ink in which titanium oxide particles having a large average primary particle diameter of about 250 nm are dispersed tends to settle, and once the pigment has settled, it is likely to be redispersed. There is a problem that is difficult. In the ink jet method, ink is ejected from an extremely fine ejection port. Therefore, when particles settle in the ink tank, clogging of the nozzle or ink mist due to solidification of the ink near the nozzle ejection port, nozzle missing, etc. There is a possibility that a flight bend or the like may occur.

On the other hand, by using metal oxide particles having a small particle diameter as the white pigment, sedimentation of the pigment particles in the ink composition can be suppressed to some extent. For example, Patent Literatures 1 and 2 disclose an ink composition containing metal oxide particles having a small average particle diameter and a polyvalent metal ion or a cationic polymer as an aggregating agent for aggregating the particles. According to Patent Documents 1 and 2, these ink compositions suppress the sedimentation of particles, and also enhance the light-shielding properties of a film formed using this composition.

WO 2016/175743 JP 2016-137482 A

In the ink compositions described in Patent Literatures 1 and 2, a polyvalent cation as a coagulant and a white pigment such as titanium oxide having a small particle diameter coexist in the ink composition. When a coating film is formed using this ink composition, the concentration of the polyvalent cation increases due to volatilization of the solvent in the coating film, and aggregation of the white pigment is induced to form an image area. However, agglomeration of the pigment utilizing the change in the concentration of the ink occurs even when the ink composition is dried at or near the discharge port of the nozzle to increase the concentration. In that case, the ejection stability becomes insufficient.

There is also known a method of forming an image portion on a base material without including a flocculant in the ink composition. For example, an ink composition in which a pigment is dispersed is provided on a substrate having a layer provided with a coagulant (for example, an acidic undercoat layer), and the ink composition is coagulated on the substrate by the action of the coagulant. It is known.

In general, a binder resin is added to an ink composition using a pigment as a coloring material in order to impart abrasion resistance or the like to a formed image area. As the binder resin, resin fine particles capable of realizing low viscosity even when the solid content concentration is high are frequently used from the viewpoint of enhancing the characteristics such as storage stability and ejection stability of the ink. These resin fine particles also coagulate together with the pigment by the action of the coagulant, and the mechanical properties of the image area are enhanced.

As described in Patent Documents 1 and 2, when a white pigment having a smaller particle size is used, it is advantageous for improving the dispersion stability of the pigment. On the other hand, the concealability of the formed image portion tends to decrease.

The hiding power is also affected by the cohesiveness of the binder particles. That is, when the white pigment and the binder particles are aggregated at one time, the pigment concentration per unit volume of the image area decreases, and as a result, the concealing property (color density) also decreases.

In the above description, the description has been made focusing on the white pigment. However, the improvement of the color density of the colored pigment is also an issue.

The present invention has been made in view of the above circumstances, and a pigment dispersion capable of sufficiently increasing the color density of a formed image portion when used in an ink, and an ink-jet ink composition using the dispersion And an ink set, and an image formed using the inkjet ink composition and an image forming method.

The above object of the present invention has been solved by the following means.

<1>

A pigment dispersion containing a pigment dispersant (A) comprising a polymer having an anionic group, a pigment and an aqueous medium, wherein the pKa of the polymer before neutralization of the pigment dispersant (A) in a DMSO solution is: 14. A pigment dispersion, which is 14 or more.

<2>

The pigment dispersion according to <1>, wherein the anionic group is a sulfonamide salt type group or an imide salt type group.

<3>

The pigment dispersion according to <2>, wherein the sulfonamide salt type group is represented by the following general formula (I) or (II).

In the above formula, R ¹ represents a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms, R ² represents a hydrocarbon group having 1 to 20 carbon atoms, and Ar ¹ and Ar ² each independently represent a carbon group having 6 to 10 carbon atoms. 20 represents an arylene group or a heteroarylene group; L ¹ and L ² each independently represent a single bond or a divalent linking group; and M ⁺ represents an alkali metal ion or an ammonium ion. * Indicates a linking site with the polymer.

<4>

The pigment dispersion according to <3>, wherein the pigment dispersant (A) includes a structural unit derived from a vinyl monomer having a sulfonamide salt type group represented by the general formula (I).

<5>

The pigment dispersion according to any one of <1> to <4>, wherein the pigment contains titanium oxide having an average particle size of 1 to 100 nm.

<6>

The pigment dispersion according to any one of <1> to <5>, containing binder particles.

<7>

An inkjet ink composition comprising the pigment dispersion according to any one of <1> to <6>.

<8>

An ink set comprising the inkjet ink composition according to <7> and a treating agent for coagulating the ink composition.

<9>

The ink set according to <8>, wherein the treatment agent contains an acidic compound.

<10>

An image formed by the inkjet ink composition according to <7>.

<11>

<7> An image forming method, comprising a step of applying the inkjet ink composition to a recording medium to which a treatment agent for aggregating the inkjet ink composition according to <7> is applied.

In this specification, the term “compound” includes not only the compound itself but also its salt and its ion. In addition, a structure in which a part of the structure is changed is included as long as an intended effect is not impaired.

In addition, as for the compound which is not specified to be substituted or unsubstituted, a compound having an arbitrary substituent is included as long as a desired effect is not impaired. The same applies to a substituent, a linking group, a ring structure and the like (hereinafter, referred to as a substituent and the like).

In the present specification, when there are a plurality of substituents and the like represented by specific symbols, or when simultaneously defining a plurality of substituents and the like, each substituent and the like may be the same as each other, unless otherwise specified. It may be different. This holds true for the definition of the number of substituents and the like. When a plurality of substituents and the like are close to each other (particularly adjacent to each other), they may be connected to each other to form a ring unless otherwise specified.

In the present invention, when a polymer has a plurality of repeating units represented by the same chemical structure, each repeating unit present in the polymer may be the same or different. This is the same for each group forming the repeating unit.

Further, when the number of carbon atoms of the group is limited, the number of carbon atoms of the group means the total number of carbon atoms including the substituent unless otherwise specified.

In the present invention, when a group can form a non-cyclic skeleton and a cyclic skeleton, this group includes a non-cyclic skeleton group and a cyclic skeleton group unless otherwise specified. For example, the alkyl group includes a linear alkyl group, a branched alkyl group, and a cyclic (cyclo) alkyl group. When the group can form a cyclic skeleton, the lower limit of the number of atoms of the group forming the cyclic skeleton is 3 or more, preferably 5 or more, irrespective of the lower limit of the number of atoms specifically described for this group.

In this specification, “(meth) acrylate” is used to mean both acrylate and methacrylate. The same applies to “(meth) acrylic acid”, “(meth) acrylamide” and “(meth) acryloyl group”.

In this specification, the term “vinyl monomer” is used in a broader sense than usual. That is, other _"CH 2 = CH-,"_"CH 2 = CR ^B -" or "-CH = CR ^B -" or "-CH = CH -" sequential ^{(R B} substituent) represented by In the present invention, a monomer having a polymerizable group is also included in the vinyl monomer.

In this specification, a numerical range represented by using “to” means a range including numerical values described before and after “to” as a lower limit and an upper limit.

In this specification, the mass average molecular weight and the number average molecular weight were measured by GPC. More specifically, using HLC-8220GPC (manufactured by Tosoh Corporation), three columns of TSKgeL Super HZ2000, TSKgeL Super HZ4000 and TSKgeL Super HZ-H (manufactured by Tosoh Corporation, 4.6 mm × 15 cm) are connected in series as columns. And THF (tetrahydrofuran) was used as an eluent. The sample concentration was 0.3% by mass, the flow rate was 0.35 ml / min, the sample injection amount was 10 μL, the measurement temperature was 40 ° C., and an IR detector was used as a detector. The calibration curve is “Standard sample TSK standard, polystyrene” manufactured by Tosoh Corporation: “F-80”, “F-20”, “F-4”, “F-2”, “A-5000”, “A -1000 ".

The pigment dispersion, the inkjet ink composition and the ink set of the present invention can be applied to an inkjet recording method to form an image portion, thereby increasing the color density of the formed image portion.

In the image of the present invention, the color density of the image portion is sufficiently increased.

According to the image forming method of the present invention, the color density of the formed image portion can be sufficiently increased.

[Pigment dispersion]

The pigment dispersion of the present invention (hereinafter, also simply referred to as “pigment dispersion”) is a pigment dispersant (A) composed of a polymer having an anionic group (hereinafter, also referred to as “pigment dispersant (A)”). In the state before the pigment dispersant (A) is neutralized (polymer before neutralization), the pKa in the DMSO solution at 25 ° C. is 14 or more.

The pigment dispersion of the present invention has excellent dispersion stability without sedimentation even when the ink is stored for a long period of time, and can further enhance the color density of the formed image. The reason for this is not necessarily clear, but is presumed as follows.

In the pigment dispersion of the present invention, the pigment dispersant (A) is effective as a dispersant for dispersing a pigment as a coloring material in an aqueous medium due to charge repulsion between anionic groups of the pigment dispersant (A). Functioning. In addition, in the pigment dispersant (A), the charge repulsion is rapidly weakened by the action of the coagulant (coagulation inducing component described later), and the coagulation of the pigment occurs quickly. For this reason, when a film (image area) is formed using the inkjet ink composition using this dispersion, the pigment dispersant (A) has a higher priority (selective) than the aggregation of the binder that is usually compounded. It is thought that the pigment concentration in the image area can be further increased by aggregating quickly. This is because the pigment dispersant (A) has an anionic group which is different from a dissociative group (an acidic group such as a carboxylic acid group, a sulfonic acid group, and a phosphoric acid group) which the binder has for affinity with an aqueous medium. Is considered to be more easily and quickly protonated.

<Pigment dispersant (A)>

The pigment dispersant (A) is a polymer that functions as a pigment dispersant in the pigment dispersion of the present invention.

(PKa of polymer before neutralization)

The pKa (acid dissociation constant; hereinafter simply referred to as “pKa (DMSO)”) of the polymer before neutralization of the pigment dispersant (A) in a DMSO (dimethyl sulfoxide) solution is 14 or more.

In the present specification, the term “polymer before neutralization of the pigment dispersant (A)” refers to a polymer (a) in which all counter cations in the anionic group of the pigment dispersant (A) are hydrogen atoms (hereinafter simply referred to as “polymer”). "Polymer (a)").

The pKa (DMSO) is a pKa in a DMSO solution at 25 ° C. and can be measured by a method described in Examples described later. Specifically, pKa (DMSO) can be measured by titrating a 1% by mass DMSO solution at 25 ° C. with a 0.1 mol% (concentration) aqueous potassium hydroxide solution. In order to convert all the counter cations in the anionic group in the pigment dispersant (A) into a form having a hydrogen atom, for example, a 1 mol% aqueous hydrochloric acid solution is dissolved after dissolving the pigment dispersant (A) in DMSO. Can be processed.

The pKa (DMSO) is preferably 15 or more, and more preferably 16 or more, from the viewpoint of increasing the color density of an image. In addition, when used as an inkjet ink composition, from the viewpoint of suppressing hydrolysis of the silicone resin film at the nozzle discharge portion (hereinafter referred to as “liquid-repellent film resistance”), it is preferably 20 or less, more preferably 19 or less. It is preferably 18 or less, more preferably 17 or less.

When the polymer (a) has a plurality of pKas, the pKa (DMSO) means the largest pKa that the polymer (a) has. At least the largest pKa of the polymer (a) satisfies the above-mentioned pKa (DMSO), whereby the aggregation of the pigment by the aggregating agent can be quickly caused.

Further, when the polymer (a) does not have a clear pKa (for example, since the polymer (a) has a plurality of pKas and the values of the plurality of pKas are close to each other, a broad neutralization curve is obtained when the polymer (a) is neutralized. In the case of drawing), the pKa (DMSO) of the polymer (a) means the end point of the neutralization curve.

(Anionic group)

The pigment dispersant (A) is a polymer that functions as a pigment dispersant in the pigment dispersion of the present invention by having an anionic group in the polymer.

The anionic group contained in the pigment dispersant (A) means a salt-type group in which a group contributing to the polymer (a) exhibiting the above-mentioned pKa (DMSO) is neutralized. Of the cation. In the pigment dispersion of the present invention, it is preferable that part or all of the counter cation of the anionic group contained in the pigment dispersant (A) is dissociated.

The counter cation may be monovalent or divalent or higher, but is preferably a monovalent counter cation, more preferably an alkali metal ion or an ammonium ion, and further preferably an alkali metal ion.

Examples of the alkali metal ion include a lithium ion, a sodium ion, a potassium ion, and a cesium ion, and a sodium ion or a potassium ion is preferable.

Examples of the ammonium ion include an ammonium ion (NH ₄ ⁺ ), an alkyl group (the number of carbon atoms is preferably 1 to 10, more preferably 1 to 6, and still more preferably 1 to 3) and an aryl group (the number of carbon atoms is 1 to 3). , 6 to 20, preferably 6 to 18, more preferably 6 to 12), and a quaternary ammonium ion substituted with a nitrogen atom. Preferably, a tetramethylammonium ion, a triethylammonium ion, and a dimethyl 2-hydroxyethylammonium ion are more preferable.

The anionic group is not particularly limited as long as the polymer (a) satisfies the above-mentioned pKa (DMSO). And a benzimidazolium salt type group. Among these, a sulfonamide salt type group or an imide salt type group is preferable from the viewpoint of liquid repellent film resistance, and a sulfonamide salt type group is more preferable from the viewpoint of visibility.

The sulfonamide salt-type group preferably has a structure represented by the following general formula (I) or (II). In the case where the polymer has a sulfonamide salt type group in the polymer side chain (preferably, the case where a chain polymerization type polymer described later has a sulfonamide salt type group in the side chain), the structure represented by the general formula (I) In the case where the polymer has a sulfonamide salt type group in the polymer main chain (preferably, the case where the sequential polymerization type polymer described later has a sulfonamide salt type group in the main chain), the general formula ( It preferably has a structure represented by II).

In the above formula, R ¹ represents a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms, R ² represents a hydrocarbon group having 1 to 20 carbon atoms, and Ar ¹ and Ar ² each independently represent a carbon group having 6 to 10 carbon atoms. 20 represents an arylene group or a heteroarylene group; L ¹ and L ² each independently represent a single bond or a divalent linking group; and M ⁺ represents an alkali metal ion or an ammonium ion. * Indicates a linking site with the polymer.

The hydrocarbon group having 1 to 10 carbon atoms for R ¹ is preferably an alkyl group (preferably 1 to 6 carbon atoms, more preferably 1 to 3) and an alkenyl group (preferably 2 to 6 carbon atoms). And an alkyl group having 1 to 10 carbon atoms is more preferable.

R ¹ is preferably a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, more preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, further preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and more preferably a hydrogen atom. Particularly preferred.

As the hydrocarbon group having 1 to 20 carbon atoms for R ² , an alkyl group (preferably having 1 to 10 carbon atoms, more preferably 1 to 6) and an alkenyl group (preferably having 2 to 6 carbon atoms) are preferable. And an alkyl group having 1 to 20 carbon atoms is more preferable.

R ² is preferably a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, more preferably a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and even more preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.

The arylene group having 6 to 20 carbon atoms in Ar ¹ and Ar ² preferably has 6 to 12 carbon atoms.

As the heteroarylene group for Ar ¹ and Ar ² , a 5-membered aromatic heterocycle, a fused heterocycle containing a 5-membered aromatic heterocycle, a 6-membered aromatic heterocycle, or a 6-membered aromatic A group consisting of a condensed heterocycle including an aromatic heterocycle is preferred. In the heteroarylene group, these aromatic heterocycles or condensed heterocycles may have a substituent (for example, a substituent Z described later, among which an alkyl group is preferable, and an alkyl group having 1 to 3 carbon atoms is preferable. Is more preferable.). Further, the number of ring-constituting atoms of the heteroarylene group is preferably 5 to 20, more preferably 5 to 13. In the heteroarylene group, the ring-constituting atom preferably has at least one of an oxygen atom, a nitrogen atom and a sulfur atom.

Examples of the 5-membered aromatic hetero ring and the fused hetero ring including the 5-membered aromatic hetero ring include a pyrrole ring, an imidazole ring, a pyrazole ring, an oxazole ring, a thiazole ring, an isoxazole ring, Examples include an isothiazole ring, a triazole ring, an oxadiazole ring, a thiodiazole ring, a furan ring, a thiophene ring, a benzimidazole ring, a benzoxazole ring, a benzothiazole ring, and an indazole ring.

Examples of the 6-membered aromatic hetero ring and the fused hetero ring including the 6-membered aromatic hetero ring include a pyridine ring, a pyrimidine ring, a pyrazine ring, a pyridazine ring, a triazine ring, a quinoline ring, and an isoquinoline. Ring, quinoxaline ring, phthalazine ring, cinnoline ring, and quinazoline ring.

The heteroarylene group in Ar ¹ and Ar ² is preferably dibenzofuran-diyl, benzofuran-diyl, benzothiophen-diyl, or dibenzothiophen-diyl, and more preferably dibenzofuran-diyl.

Ar ¹ is preferably an arylene group having 6 to 20 carbon atoms, more preferably an arylene group having 6 to 12 carbon atoms, and further preferably phenylene.

Ar ² is preferably an arylene group having 6 to 12 carbon atoms or a heteroarylene group having 5 to 20 ring atoms, more preferably phenylene, naphthylene, benzofuran-diyl, dibenzofuran-diyl, benzothiophen-diyl, and dibenzothiophen-diyl. Preferably, phenylene or dibenzofuran-diyl is more preferred.

The divalent linking group in L ¹ and L ² is an alkylene group (preferably having 1 to 6 carbon atoms, more preferably 1 to 3), an ether bond, an ester bond or an amide bond, or a group (bond) of these groups. And a group obtained by combining two or more of the above.

Examples of the group obtained by combining two or more kinds of the above alkylene group, ether bond, ester bond or amide bond include ether-alkylene-amide and ether-alkylene-ester.

L ¹ is preferably an ester bond, an amide bond, or an ether-alkylene-amide.

L ² is preferably a single bond.

As the alkali metal ion and ammonium ion for M ⁺ , the description of the alkali metal ion and ammonium ion described in the above anionic group can be preferably applied.

The pigment dispersant (A) may have one kind of the above-described anionic group, or may have two or more kinds of the anionic group.

The content of the anionic group in the pigment dispersant (A) is preferably 0.01 to 6.0 mmol / g, more preferably 0.02 to 5.0 mmol / g, and more preferably 0.03 to 4.5 mmol / g. / G is more preferred.

The pigment dispersant (A) has an anionic group (for example, a carboxylate group, a sulfonate group, or a phosphate group) having a pKa (DMSO) of less than 14, as long as the effects of the present invention are not impaired. You may.

In the pigment dispersant (A), the ratio of the anionic group contained in the pigment dispersant (A) to the total amount of the groups exhibiting pKa (DMSO) in the polymer (a) (that is, the pigment dispersant (A) From the viewpoint of dispersion stability, the practical lower limit of the neutralization ratio of the anionic group therein is 5 mol% or more, preferably 7 mol% or more, more preferably 10 mol% or more.

The base number of the pigment dispersant (A) is preferably from 0.01 to 4.0 mmol / g, more preferably from 0.02 to 3.0 mmol / g, and more preferably from 0.02 to 3.0 mmol / g, from the viewpoint of compatibility between dispersion stability and color density. ~ 2.5 mmol / g is more preferred.

The base number can be measured by dissolving 0.1 g of the pigment dispersant (A) in DMSO to form a 1% by mass solution, and then titrating the solution at 25 ° C. using a 0.1 mol% hydrochloric acid aqueous solution.

The acid value of the pigment dispersant (A) is determined by dissolving 0.1 g of the pigment dispersant (A) in DMSO to form a 1% by mass solution, and then titrating the solution at 25 ° C. using a 0.1 mol% aqueous sodium hydroxide solution. Can be measured.

The total amount of anionic groups in the pigment dispersant (A) is represented by the sum of the above-mentioned base value and acid value, and the neutralization ratio is represented by the following formula.

Neutralization rate = base number / (base number + acid number)

The pigment dispersant (A) preferably has an adsorptive group for the pigment in addition to the anionic group, in that it functions as a pigment dispersant in the pigment dispersion of the present invention. Since the adsorptivity varies depending on the surface of the pigment, the structure cannot be specified as an adsorptive group.For example, an alkyl group, an aryl group, a heteroaryl group, an ether group, a carbonate group, a hydroxyl group, an amide group, a carbonyl group And a phosphate group.

(Structure of the pigment dispersant (A))

The pigment dispersant (A) may be a chain polymerization type polymer composed of a structural unit derived from a vinyl monomer, or may be a sequential polymerization type (that is, a polyaddition type or polycondensation type) polymer.

Examples of the structure in which the pigment dispersant (A) has an anionic group include a structure in which at least one of structural units derived from a vinyl monomer has an anionic group, a monomer constituting a sequential polymerization type polymer (at least bifunctional or more) Or a compound having at least any one of the bonding parts of the monomers constituting the sequential polymerization type polymer has an anionic group.

Among these, from the viewpoint of color density, at least one of a structure in which at least one of the structural units derived from a vinyl monomer has an anionic group, or a monomer (at least a bifunctional or more compound) constituting a sequentially polymerizable polymer Is preferably a structure having an anionic group, and from the viewpoint of storage stability of the resin, a structure having at least one of the vinyl monomer-derived structural units has an anionic group is more preferable. Among them, the pigment dispersant (A) preferably contains a structural unit derived from a vinyl monomer having a sulfonamide salt type group represented by the above general formula (I) from the viewpoint of liquid repellent film resistance.

Hereinafter, the structural unit of the pigment dispersant (A) will be specifically described with reference to a monomer-derived structural unit before neutralization.

When the pigment dispersant (A) is a chain-polymerizable polymer composed of a vinyl monomer-derived structural unit, it preferably has the following monomer-derived structural unit.

Examples of the structural unit derived from a vinyl monomer having an anionic group (hereinafter, referred to as “structural unit (a1)”) include N- (4-sulfamoylphenyl) (meth) acrylamide, N- [4- (N′-ethylsulfamoyl) phenyl] (meth) acrylate, maleimide, 4-hydroxystyrene, N- [2- (4-sulfamoylphenoxy) ethyl] (meth) acrylamide and N- (4-hydroxyphenyl) And (meth) acrylamide-derived structural units.

As a structural unit derived from a vinyl monomer other than the vinyl monomer having an anionic group (hereinafter, referred to as “structural unit (a2)”), the alkyl group has 1 to 20, preferably 1 to 18, carbon atoms. Preferably 1 to 12 alkyl (meth) acrylates, more specifically methyl (meth) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate Alkyl (meth) acrylates such as isobutyl (meth) acrylate, t-butyl (meth) acrylate, hexyl (meth) acrylate, ethylhexyl (meth) acrylate, and octadecyl (meth) acrylate; hydroxymethyl (meth) acrylate, 2-hydroxy Ethyl (meth) acryle Hydroxyalkyl (meth) acrylates such as 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, hydroxypentyl (meth) acrylate, and hydroxyhexyl (meth) acrylate; dimethylaminoethyl (meth) acrylate; Alkylaminoalkyl (meth) acrylates such as diethylaminoethyl (meth) acrylate and t-butylaminoethyl (meth) acrylate; aromatic ring-containing (meth) acrylates such as benzyl (meth) acrylate and phenoxyethyl (meth) acrylate; styrene; Styrenes such as α-methylstyrene and chlorostyrene, cyclopropyl (meth) acrylate, cyclobutyl (meth) acrylate, cyclopentyl (meth) acrylate, cyclohexyl (Meth) acrylate, cycloheptyl (meth) acrylate, cyclooctyl (meth) acrylate, cyclononyl (meth) acrylate, cyclodecyl (meth) acrylate, isobornyl (meth) acrylate, norbornyl (meth) acrylate, adamantyl (meth) acrylate, diamine Alicyclic (meth) acrylates such as cyclopentanyl (meth) acrylate and dicyclopentenyloxyethyl (meth) acrylate; N-hydroxymethyl (meth) acrylamide, N-hydroxyethyl (meth) acrylamide, N-hydroxybutyl ( N-hydroxyalkyl (meth) acrylamides such as meth) acrylamide; N-methoxymethyl (meth) acrylamide, N-ethoxymethyl (meth) acrylamide, N- (n-, iso ) N-alkoxyalkyl (meth) acrylamides such as butoxymethyl (meth) acrylamide, N-methoxyethyl (meth) acrylamide, N-ethoxyethyl (meth) acrylamide, N- (n-, iso) butoxyethyl (meth) acrylamide And a structural unit derived from a monomer selected from (meth) acrylonitrile, (meth) acrylate, and maleimide.

The pigment dispersant (A) may have one or more structural units derived from vinyl monomers other than those described above, as long as the effects of the present invention are not impaired.

In the pigment dispersant (A), the structural unit (a1) may be one type or two or more types, and the structural unit (a2) may be one type or two or more types.

The total content of the structural unit (a1) in the pigment dispersant (A) is not particularly limited, but is preferably 5 to 95% by mass, more preferably 5 to 90% by mass, and still more preferably 10 to 80% by mass.

The total content of the structural unit (a2) in the pigment dispersant (A) is not particularly limited, but is preferably from 5 to 95% by mass, more preferably from 10 to 95% by mass, and still more preferably from 20 to 90% by mass.

As the pigment dispersant (A) which is a chain polymerization type polymer, a commonly used method can be used. For example, radical polymerization can be mentioned, and as a polymerization initiator, a reaction terminator, a solvent and the like, an agent usually used in combination with a monomer, an oligomer and the like to be used can be appropriately used. In addition, a commercially available polymer can also be used as the polymer.

The polymerization initiator is not particularly limited, and may be an inorganic persulfate (eg, potassium persulfate, sodium persulfate, ammonium persulfate, etc.), an azo-based initiator (eg, 2,2′-azobis (2-amidinopropane) ) Dihydrochloride, 2,2′-azobis [2-methyl-N- (2-hydroxyethyl) -propionamide], 4,4′-azobis (4-cyanovaleric acid), etc.), organic peroxides ( For example, t-butyl peroxypivalate, t-butyl hydroperoxide, disuccinic peroxide and the like, or a salt thereof can be used. These can be used alone or in combination of two or more. Among them, it is preferable to use an azo initiator or an organic peroxide.

The amount of the polymerization initiator used in the present invention is usually 0.01 to 5 parts by mass, preferably 0.2 to 2 parts by mass, based on 100 parts by mass of all the monomers.

The pigment dispersant (A) which is a chain polymerization type polymer may have a polymerization initiator residue on at least one of both terminals.

The pigment dispersant (A), which is a chain polymerizable polymer, may be a block polymer or a random polymer.

As the chain transfer agent, known compounds such as carbon tetrahalide, styrene dimers, (meth) acrylate dimers, mercaptans, and sulfides can be used. Among them, dimers of styrenes and mercaptans described in JP-A-5-17510 can be suitably used.

The anionic group in the pigment dispersant (A) can be introduced by neutralizing the corresponding group, but the introduction may be performed before, during or after the polymerization.

When the above neutralization is performed before the polymerization, the pigment dispersant (A) can be obtained by performing the polymerization using a neutralization type monomer of the monomer described in the above structural unit, and during or after the polymerization. In the case of performing the above-mentioned neutralization, the pigment dispersant (A) can be obtained by neutralizing the polymer (a) obtained by polymerizing the monomers mentioned in the above structural units.

Neutralization can be performed by a conventional method. For example, a method of adding a solution of a basic substance such as potassium hydroxide to an object to be neutralized (either a monomer, an oligomer or a polymer, preferably a solution thereof). Is mentioned. The counter cation of the anionic group, the neutralization ratio, and the like of the pigment dispersant (A) can be adjusted by the neutralization conditions.

From the viewpoint of inhibiting hydrolysis, the neutralization is preferably performed after the polymerization.

When the pigment dispersant (A) is a sequential polymerization type polymer, the polymer (a) is preferably a polysulfonamide or a polyamide. In the case where the polymer structure has both a sulfonamide bond and an amide bond as a repeating unit, it is classified as polysulfonamide in the present specification.

The polysulfonamide preferably has a structural unit represented by the above general formula (II), and can be synthesized, for example, with reference to a method for synthesizing a polysulfonamide described in WO2017 / 061561.

The polyamide can be synthesized by a conventional method.

The anionic group in the pigment dispersant (A), which is a sequentially polymerizable polymer, can also be neutralized according to the neutralization method described for the chain polymer. From the viewpoint of inhibiting hydrolysis, the neutralization is preferably performed after the polymerization of the polymer (a).

The terminal structures of the pigment dispersant (A) and the polymer (a) are not particularly limited, and the presence or absence of other structural units, the type of substrate used during synthesis, or the type of quenching agent (reaction terminator) during synthesis. Is not uniquely determined. The terminal structure is, for example, a hydrogen atom, a hydroxy group, a halogen atom, an ethylenically unsaturated group, an alkyl group, an aromatic heterocyclic group (preferably a thiophene ring) or an aromatic hydrocarbon group (preferably a benzene ring). ).

The mass average molecular weight (Mw) of the pigment dispersant (A) is preferably from 3,000 to 100,000, more preferably from 4,000 to 50,000, and more preferably from 5,000 to 25,000 from the viewpoint of dispersion stability. Is more preferred.

The molecular weight distribution of the pigment dispersant (A), that is, the ratio (Mw / Mn) of the mass average molecular weight (Mw) to the number average molecular weight (Mn) is preferably from 1.0 to 6.0, and more preferably from 1.2 to 5 0.0 is more preferable, and 1.5 to 4.5 is more preferable.

The pigment dispersion of the present invention may contain one kind of the pigment dispersant (A), or may contain two or more kinds thereof.

<Pigment>

The pigment used in the present invention is not particularly limited, and includes special pigments in addition to ordinary organic pigments and inorganic pigments.

Examples of the organic pigment include an azo pigment, a polycyclic pigment, a lake pigment, a nitro pigment, a nitroso pigment, and aniline black. Among these, azo pigments or polycyclic pigments are preferred.

Examples of the azo pigment include an azo lake, an insoluble azo pigment, a condensed azo pigment, and a chelate azo pigment.

Examples of the polycyclic pigment include phthalocyanine pigment, perylene pigment, perinone pigment, anthraquinone pigment, quinacridone pigment, dioxazine pigment, diketopyrrolopyrrole pigment, indigo pigment, thioindigo pigment, isoindolinone pigment, and quinophthalone pigment. Can be

Examples of the lake pigment include a basic dye type lake pigment and an acidic dye type lake pigment.

Inorganic pigments include synthetic inorganic pigments and natural mineral pigments.

Examples of the synthetic inorganic pigment include metal oxides such as titanium oxide, iron oxide, zinc oxide, antimony oxide and zirconium oxide; metal sulfides such as zinc sulfide and cadmium sulfide; metal hydroxides such as aluminum hydroxide; Carbonates and sulfates of alkaline earth metals such as calcium and barium sulfate, barium yellow (barium chromate), cadmium red, chrome yellow (graphite), and furnace carbon black, lamp black, acetylene black, channel black, etc. Carbon black.

Natural inorganic pigments include, for example, loess, ultramarine and navy blue.

Examples of the special pigment include a fluorescent pigment, a metal powder pigment, a hollow resin pigment, and a polymer pigment.

Examples of the fluorescent pigment include, for example, an organic fluorescent pigment obtained by fixing a fluorescent dye in a polymer in the form of a solid solution and then pulverizing (refining), zinc sulfide, zinc silicate, cadmium sulfide, calcium tungstate and cadmium tungstate. And the like, and an inorganic fluorescent pigment obtained by heat-treating the same.

Examples of the metal powder pigment include metal powder pigments such as cobalt, iron, chromium, copper, zinc, lead, titanium, vanadium, manganese, and nickel.

As the pigment used in the present invention, an organic pigment, an inorganic pigment, and a metal powder pigment are preferable, and a white pigment, an azo raw material, and a polycyclic pigment are more preferable.

Examples of the white pigment include C.I. I. Pigment White 1 (basic lead carbonate), 4 (zinc oxide), 5 (mixture of zinc sulfide and barium sulfate), 6 (titanium oxide), 6: 1 (titanium oxide containing other metal oxides), 7 (Zinc sulfide), 18 (calcium carbonate), 19 (clay), 20 (titanium mica), 21 (barium sulfate), 22 (natural barium sulfate), 23 (gloss white), 24 (alumina white), 25 (gypsum) ), 26 (magnesium oxide / silicon oxide), 27 (silica), 28 (anhydrous calcium silicate) and the like.

As the yellow pigment, C.I. I. Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 167, 172, 180 and the like.

Specific examples of the pigment that can be used in the present invention include the pigments described in paragraphs 0142 to 0145 of JP-A-2007-10071.

In the pigment dispersion of the present invention, the pigment dispersant (A) is an aqueous medium of an inorganic pigment having a large specific gravity (for example, a white pigment such as titanium oxide, barium sulfate, and zinc oxide) having an average particle diameter of 1 to 100 nm. Suitable as a dispersant in it. An inorganic pigment having a large specific gravity is excellent in dispersion stability, but if the increase in particle size due to aggregation is not sufficient, the concealing property tends to be poor. When the pigment dispersion of the present invention contains such an inorganic pigment having a small average particle size, the aggregation of the inorganic pigment can be promoted, so that the shielding property can be enhanced.

The white pigment contained in the pigment dispersion of the present invention is more preferably titanium oxide. The particle shape of the titanium oxide is not particularly limited, and may be any shape such as a granular shape and a needle shape.

The crystal structure of titanium oxide is not particularly limited, and may be any of rutile type (tetragonal), anatase type (tetragonal), and brookite type (orthogonal).

Titanium oxide can be produced by a gas phase method or a liquid phase method.

Titanium oxide is preferably surface-treated from the viewpoint of obtaining good dispersibility. The surface treatment only needs to be at least partially or entirely treated. The surface treatment of titanium oxide is not particularly limited. For example, surface treatment with at least one of inorganic substances such as alumina, silica, zinc oxide, zirconia, and magnesium oxide, and titanium coupling agent, silane coupling agent, and silicone oil Surface treatment with an organic substance. From the viewpoint of improving the hiding power, the titanium oxide used as the white pigment is preferably a titanium oxide surface-treated with at least one of silica, alumina and an organic substance.

The average primary particle size of the pigment in the pigment dispersion of the present invention (hereinafter, also simply referred to as “average particle size”) is not particularly limited, but from 1 to 200 nm in terms of color reproducibility, droplet ejection characteristics, and light resistance. Is preferably 1 to 150 nm, more preferably 1 to 100 nm. A pigment having a large specific gravity and easily settling in water (for example, the above-mentioned inorganic pigment having a large specific gravity) has an average particle diameter of preferably 1 to 100 nm, more preferably 10 to 80 nm, and more preferably 20 to 80 to suppress the sedimentation of the pigment. 60 nm is more preferred.

The particle size distribution of the pigment in the pigment dispersion of the present invention is not particularly limited, and may be either a wide particle size distribution or a monodisperse particle size distribution. Further, two or more pigments having a monodisperse particle size distribution may be used in combination.

The average primary particle size of the pigment can be measured by the following method.

The average primary particle diameter is determined by directly photographing titanium oxide at a magnification of 10,000 to 100,000 by electron microscopy (JEM-1200FX, manufactured by JEOL Ltd.), and observing the particle size from the photographed image. , And the arithmetic average value obtained by measuring the size of 1,000 pieces is adopted.

Here, the particle diameter is calculated as a circle equivalent diameter, that is, a diameter when the photographed particles are assumed to be circles having the same area.

For example, when the particles have an elliptical shape, diameters such as vertical, horizontal, and oblique angles are measured, the average value thereof is calculated, and the diameter converted into a circle may be calculated.

The pigment contained in the pigment dispersion of the present invention may be one type or a mixture of two or more types.

The content of the pigment in the pigment dispersion of the present invention is preferably 1 to 20% by mass, more preferably 1 to 15% by mass.

<Aqueous medium>

The aqueous medium used in the present invention is an aqueous medium containing at least water, and may contain a water-soluble organic solvent as needed.

Preferred examples of the water used in the present invention include water containing no ionic impurities such as ion-exchanged water and distilled water. The water-soluble organic solvent used in the present invention may be one kind or two or more kinds.

The content of the aqueous medium in the pigment dispersion is appropriately selected according to the purpose, but is usually preferably from 10 to 95% by mass, more preferably from 10 to 80% by mass, and more preferably from 20 to 70% by mass. More preferably, it is mass%. The content of water in the aqueous medium is appropriately selected according to the purpose, but is usually preferably from 10 to 100% by mass, more preferably from 30 to 100% by mass, and more preferably from 50 to 100% by mass. More preferably, it is mass%.

－ Water-soluble organic solvents －

As the above-mentioned water-soluble organic solvent, a solvent which can obtain the effect of preventing drying, wetting or promoting penetration is preferable. Here, the term “prevention of drying” means that the ink is prevented from adhering and drying to the ink discharge port of the ejection nozzle to form an aggregate and clogging. For preventing drying and wetting, a water-soluble organic solvent having a lower vapor pressure than water is preferable. Further, the water-soluble organic solvent can be used as a penetration enhancer for enhancing the permeability of the ink into paper.

Examples of the water-soluble organic solvent include, for example, alkanediols (polyhydric alcohols) such as glycerin, 1,2,6-hexanetriol, trimethylolpropane, ethylene glycol and propylene glycol; sugar alcohols; ethanol, methanol, C 1-4 alkyl alcohols such as butanol, propanol and isopropanol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono -N-propyl ether, ethylene glycol mono-iso-propyl ether, diethylene glyco Rumono-iso-propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, diethylene glycol mono-t-butyl ether, triethylene glycol monoethyl ether, 1-methyl-1-methoxybutanol, propylene glycol monomethyl ether Propylene glycol monoethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-iso-propyl ether, dipropylene glycol, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, Dipropylene glycol mono-n-propyl ether, dipropylene glycol mono-iso Propyl ether, glycol ethers such as tripropylene glycol monomethyl ether, and the like.

Polyhydric alcohols are useful for the purpose of preventing drying and wetting, for example, glycerin, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3-butanediol, , 3-butanediol and the like.

For the purpose of promoting penetration, a polyol compound is preferred, and an aliphatic diol is preferred. Examples of the aliphatic diol include 2-ethyl-2-methyl-1,3-propanediol, 3,3-dimethyl-1,2-butanediol, 2,2-diethyl-1,3-propanediol, -Ethyl-1,3-hexanediol, 2,2,4-trimethyl-1,3-pentanediol and the like. Of these, 2-ethyl-1,3-hexanediol and 2,2,4-trimethyl-1,3-pentanediol are preferred examples.

<Binder particles>

The pigment dispersion of the present invention also preferably contains binder particles.

The binder particles, unlike the pigment dispersant (A), do not have a function as a dispersant in the pigment dispersion, and are polymer particles that function as a binder when forming an image. That is, the binder particles do not contain a pigment inside the particles.

The binder particles mean particles having a low aggregation rate with respect to the pigment dispersant (A) when treated with a treatment agent described below. Examples of the binder particles include, for example, binder particles applied for improving image quality in an inkjet ink composition.

The binder particles are preferably dispersed in an aqueous medium, and are more preferably self-dispersing binder particles. Here, the self-dispersing binder particles refer to fine particles made of a water-insoluble polymer that can be dispersed in an aqueous medium due to a functional group (particularly, an acidic group or a salt thereof) of the binder particles themselves.

Here, the dispersed state refers to an emulsified state (emulsion) in which a water-insoluble polymer is dispersed in a liquid state in an aqueous medium, and a dispersed state (suspension) in which a water-insoluble polymer is dispersed in a solid state in an aqueous medium. It includes both states.

Further, “water-insoluble” means that the amount of water dissolved in 100 parts by mass of water (25 ° C.) is 5.0 parts by mass or less.

The polymer constituting the binder particles preferably has an acid group having a pKa (DMSO) of less than 14, more preferably has a carboxylic acid group, a sulfonic acid group or a phosphoric acid group, and has a carboxylic acid group. More preferred.

The content of the acid group having a pKa (DMSO) less than 14 in the polymer constituting the binder particles is preferably 0.05 to 3.0 mmol / g, more preferably 0.07 to 2.0 mmol / g, and 0 to 2.0 mmol / g. More preferably, it is from 1 to 1.5 mmol / g.

The polymer constituting the binder particles may have a small amount of an acid group having a pKa (DMSO) of 14 or more as long as the effects of the present invention are not impaired. The content of the acid group having a pKa (DMSO) of 14 or more in the polymer constituting the binder particles is preferably 0 to 0.5 mmol / g, more preferably 0 to 0.3 mmol / g, and 0 to 0.1 mmol. / G is more preferred. The polymer constituting the binder particles preferably does not have an acid group having a pKa (DMSO) of 14 or more.

These acid groups are preferably neutralized salt-type groups, and the counter cation in this salt-type group is not particularly limited. For example, the counter cation in the anionic group of the pigment dispersant (A) may be used. The description of the cation can be preferably applied.

The neutralization rate of these acid groups suppresses the neutralization reaction between the acid groups of the binder particles and the anionic groups of the pigment dispersant (A), and the anionic group in the pigment dispersant (A) is reduced. From the viewpoint that the group returns to the corresponding acid and suppresses the pigment dispersant (A) from being easily aggregated in the pigment dispersion, the practical lower limit is 95 mol% or more, and preferably 97 mol% or more, It is more preferably at least 99 mol%, further preferably at least 99.5 mol%.

Examples of the structural unit constituting the binder particles include those described in JP-A-2001-181549 and JP-A-2002-88294.

Specific examples of the structural unit constituting the binder particles include alkyl (meth) acrylates having 1 to 20, preferably 1 to 15, and more preferably 1 to 12 carbon atoms in the alkyl group, and more specifically, methyl (meth) acrylate. A) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, t-butyl (meth) acrylate, hexyl (meth) acrylate Alkyl (meth) acrylates, such as ethylhexyl (meth) acrylate; hydroxymethyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, hydroxype Hydroxyalkyl (meth) acrylates such as tyl (meth) acrylate and hydroxyhexyl (meth) acrylate; alkylaminoalkyl such as dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth) acrylate, and t-butylaminoethyl (meth) acrylate (Meth) acrylates; aromatic ring-containing (meth) acrylates such as benzyl (meth) acrylate and phenoxyethyl (meth) acrylate; styrenes such as styrene, α-methylstyrene, chlorostyrene, cyclopropyl (meth) acrylate, and cyclobutyl ( (Meth) acrylate, cyclopentyl (meth) acrylate, cyclohexyl (meth) acrylate, cycloheptyl (meth) acrylate, cyclooctyl (meth) acrylate, cyclono (Meth) acrylate, cyclodecyl (meth) acrylate, isobornyl (meth) acrylate, norbornyl (meth) acrylate, adamantyl (meth) acrylate, dicyclopentanyl (meth) acrylate, dicyclopentenyloxyethyl (meth) acrylate, etc. Alicyclic (meth) acrylate; N-hydroxyalkyl (meth) acrylamide such as N-hydroxymethyl (meth) acrylamide, N-hydroxyethyl (meth) acrylamide, N-hydroxybutyl (meth) acrylamide; N-methoxymethyl ( (Meth) acrylamide, N-ethoxymethyl (meth) acrylamide, N- (n-, iso) butoxymethyl (meth) acrylamide, N-methoxyethyl (meth) acrylamide, N-ethoxyethyl (methyl (T) Structural units derived from monomers selected from N-alkoxyalkyl (meth) acrylamides such as acrylamide and N- (n-, iso) butoxyethyl (meth) acrylamide, (meth) acrylonitrile, (meth) acrylate and the like. . In addition, as long as the effect of the present invention is not impaired, it may have a structural unit other than the above.

The binder particles may have one type of these structural units, or may have two or more types of these structural units.

In the polymer constituting the binder particles, the content of these structural units is preferably from 5 to 100% by mass, more preferably from 10 to 100% by mass, and still more preferably from 20 to 100% by mass.

The weight average molecular weight (Mw) of the polymer constituting the binder particles is preferably 10,000 or more, more preferably 10,000 to 1,000,000, and further preferably 20,000 to 800,000. By setting the mass average molecular weight within the above range, the color density of an image can be further improved. The mass average molecular weight is measured using a gel permeation chromatograph (GPC) by a method described in Examples described later.

The polymer constituting the binder particles may be a block copolymer or a random copolymer.

When binder particles are contained in the pigment dispersion of the present invention, the binder particles may be of one type or a mixture of two or more types.

The content of the binder particles in the pigment dispersion of the present invention is preferably 1 to 20% by mass, more preferably 1 to 15% by mass.

In the aqueous ink composition of the present invention, the particle size of the binder particles used in the present invention is preferably 1 to 400 nm, more preferably 1 to 300 nm, and more preferably 1 to 200 nm, from the viewpoint of ink dischargeability. More preferably, it is more preferably from 1 to 150 nm, even more preferably from 50 to 120 nm.

The above particle size of the binder particles means a volume average particle size. This volume average particle size can be measured by the method described in Examples described later.

The polymer constituting the binder particles can be synthesized by a conventional method. For example, the method of synthesizing the polymer in the pigment dispersant (A) can be applied.

After a step of adding an aqueous solution containing a basic substance to a mixture of a polymer constituting the binder particles and an organic solvent dissolving or dispersing the polymer (mixing and hydration steps), a step of removing the organic solvent (solvent removal) Step E) and phase inversion emulsification to produce a dispersion of binder particles.

The organic solvent needs to be capable of dissolving or dispersing the polymer constituting the binder particles. In addition, the organic solvent preferably has a certain affinity for water. Specifically, those having a solubility in water at 20 ° C. of 10 to 50% by mass or less are preferable.

Preferred examples of the organic solvent include a water-soluble organic solvent. Of these, isopropanol, acetone and methyl ethyl ketone are preferred, and methyl ethyl ketone is particularly preferred. The organic solvent may be used alone or in combination of two or more.

The basic substance is used for neutralizing the acid group of the binder particles.

The method of removing the organic solvent in the production process of the binder particle dispersion is not particularly limited, and the organic solvent can be removed by a known method such as distillation under reduced pressure.

<Other components>

The pigment dispersion of the present invention may further contain, if necessary, a drying inhibitor (swelling agent), a coloring inhibitor, a penetration enhancer, an ultraviolet absorber, a preservative, a rust inhibitor, a defoaming agent, a viscosity modifier, It may contain additives such as a regulator and a chelating agent.

In the pigment dispersion of the present invention, the blending amount of the pigment dispersant (A) with respect to the pigment is determined based on 100 parts by mass of the pigment with respect to the pigment dispersant (A) from the viewpoint of the dispersibility of the pigment, the ink coloring property, and the dispersion stability. Is preferably 10 to 90 parts by mass, more preferably 20 to 70 parts by mass, and particularly preferably 30 to 50 parts by mass.

When the blending amount of the pigment dispersant (A) with respect to the pigment is within the above range, the pigment is coated with an appropriate amount of the pigment dispersant (A), and a pigment dispersion having a small particle size and excellent stability over time is obtained. It is preferable because it tends to be easily obtained.

The pigment dispersion of the present invention can be obtained, for example, by dispersing a mixture containing a pigment, a pigment dispersant (A), an aqueous solvent, and the like using a disperser. When dispersing, it is preferable to use crushed beads such as zirconia beads. When the pigment dispersion is prepared using the crushed beads, the prepared pigment dispersion is filtered using a mesh filter cloth or the like to remove the crushed beads. When the pigment dispersion of the present invention contains binder particles, a pigment dispersion containing at least a pigment and a pigment dispersant (A), a dispersion of binder particles, and, if necessary, an aqueous solvent are mixed. It is preferable to prepare the pigment dispersion of the present invention containing binder particles. Further, in the pigment dispersion of the present invention, additives described as the above other components may be dispersed.

The liquid property of the pigment dispersion of the present invention is preferably, for example, one having a pH (25 ° C.) of 4.5 to 10.

[Inkjet ink composition]

The inkjet ink composition of the present invention (hereinafter, also simply referred to as “ink composition”) may be the pigment dispersion of the present invention itself, but is usually prepared using the pigment dispersion of the present invention as a raw material. More specifically, it is preferable to prepare the ink composition of the present invention by mixing at least the pigment dispersion of the present invention and an aqueous medium (hereinafter, referred to as an aqueous medium (b)). The ink composition of the present invention may contain, if necessary, a surfactant, a drying inhibitor (swelling agent), a coloring inhibitor, a penetration enhancer, an ultraviolet absorber, a preservative, a rust inhibitor, a defoamer, and a viscosity. Additives such as a regulator, a pH regulator, and a chelating agent may be mixed. The mixing method is not particularly limited, and a commonly used mixing method can be appropriately selected to obtain the aqueous ink composition of the present invention.

<Surfactant>

The ink composition of the present invention may contain a surfactant as a surface tension modifier.

As the surfactant, any of an anionic surfactant, a cationic surfactant, an amphoteric surfactant, a nonionic surfactant, and a betaine surfactant can be used.

Specific examples of the anionic surfactant include, for example, sodium dodecylbenzenesulfonate, sodium lauryl sulfate, sodium alkyldiphenyletherdisulfonate, sodium alkylnaphthalenesulfonate, sodium dialkylsulfosuccinate, sodium stearate, potassium oleate, sodium dioctyl Sulfosuccinate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkyl phenyl ether sulfate, sodium dialkyl sulfosuccinate, sodium stearate, sodium oleate, t-octylphenoxyethoxy polyethoxyethyl Sodium sulfate and the like, and one or more of these can be selected. Rukoto can.

Specific examples of the nonionic surfactant include, for example, acetylene diol derivatives such as acetylene diol ethylene oxide adduct, polyoxyethylene lauryl ether, polyoxyethylene octyl phenyl ether, polyoxyethylene oleyl phenyl ether, and polyoxyethylene nonyl. Examples include phenyl ether, block copolymers of polyoxyethylene and polyoxypropylene, t-octylphenoxyethyl polyethoxyethanol, nonylphenoxyethyl polyethoxyethanol, and the like, and one or more of these can be selected.

Examples of the cationic surfactant include a tetraalkylammonium salt, an alkylamine salt, a benzalkonium salt, an alkylpyridium salt, an imidazolium salt, and the like. Specifically, for example, dihydroxyethylstearylamine, 2-heptadecenyl -Hydroxyethylimidazoline, lauryldimethylbenzylammonium chloride, cetylpyridinium chloride, stearamidomethylpyridium chloride and the like.

Among these surfactants, a nonionic surfactant is preferred from the viewpoint of stability, and an acetylene diol derivative is more preferred.

When the aqueous ink composition of the present invention is used in an ink jet recording method, it is preferable to adjust the amount of the surfactant so that the surface tension of the aqueous ink composition is 20 to 60 mN / m from the viewpoint of ink ejection properties. More preferably, the amount is 20 to 45 mN / m, and further preferably, the amount is 25 to 40 mN / m.

The surface tension of the aqueous ink composition is measured at a temperature of 25 ° C. using an Automatic Surface Tensiometer CBVP-Z (manufactured by Kyowa Interface Science Co., Ltd.).

The content of the surfactant in the ink composition is preferably an amount that allows the ink composition to fall within the above range of the surface tension. More specifically, the content of the surfactant in the ink composition is preferably 0.1% by mass or more, more preferably 0.1 to 10% by mass, and still more preferably 0.2 to 3% by mass. It is.

The viscosity of the aqueous ink composition of the present invention is not particularly limited, but the viscosity at 25 ° C. is preferably from 1.2 mPa · s to 15.0 mPa · s, more preferably from 2 mPa · s to 13 mPa · s. s, more preferably 2.5 mPa · s or more and less than 10 mPa · s.

The pH (25 ° C.) of the aqueous ink composition of the present invention is preferably pH 6 to 11 from the viewpoint of dispersion stability. When an ink set described below is used, it is preferable that the ink composition is rapidly aggregated by contact with a treating agent containing an aggregation component such as an acidic compound.

[Ink set]

The ink set of the present invention includes at least a part made of the inkjet ink composition (containing a pigment) of the present invention, and a treating agent that comes into contact with the ink composition to aggregate the ink composition. Further, the ink set of the present invention may include a maintenance liquid used for removing the ink composition (for example, dried and solidified ink solids) attached to the inkjet recording head.

An image having excellent image quality can be formed by forming an image using the inkjet ink composition of the present invention and the treatment agent.

Hereinafter, the processing agents constituting the ink set will be described.

<Treatment agent>

The treatment agent constituting the ink set of the present invention contains an aggregation-inducing component (hereinafter, also referred to as “aggregation component” for short) that aggregates the ink composition when it comes into contact with the ink composition of the present invention. Examples of the aggregation component include components selected from an acidic compound, a polyvalent metal salt, and a cationic polymer, and the aggregation component is preferably an acidic compound. The treating agent may contain other components as necessary, in addition to the agglomerated components.

The treating agent constituting the ink set of the present invention is usually in the form of an aqueous solution.

－ Acidic compounds －

The acidic compound is capable of coagulating (fixing) the ink composition by coming into contact with the ink composition on the recording medium, and functions as a fixing agent. For example, if a treatment agent containing an acidic compound is applied to a recording medium (preferably, coated paper) and the ink composition is dropped on the recording medium, the ink composition can be aggregated, An object can be immobilized on a recording medium.

Examples of the acidic compound include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, polyacrylic acid, acetic acid, glycolic acid, malonic acid, malic acid, maleic acid, ascorbic acid, succinic acid, glutaric acid, fumaric acid, phthalic acid, and citric acid. Acid, tartaric acid, lactic acid, sulfonic acid, orthophosphoric acid, metaphosphoric acid, pyrrolidonecarboxylic acid, pyronecarboxylic acid, pyrrolecarboxylic acid, furancarboxylic acid, pyridinecarboxylic acid, coumaric acid, thiophenecarboxylic acid, nicotinic acid, oxalic acid, acetic acid and Benzoic acid is mentioned. From the viewpoint of achieving both volatilization suppression and solubility in a solvent, the acidic compound is preferably an acid having a molecular weight of 35 to 1,000, more preferably an acid having a molecular weight of 50 to 500, and still more preferably an acid having a molecular weight of 50 to 200. Further, as the pKa (in H ₂ O, 25 ° C.), an acid of −10 to 7 is preferable, an acid of 1 to 7 is more preferable, and an acid of 1 to 7 is more preferable, from the viewpoint of preventing ink bleeding and achieving photocurability. Particularly preferred are acids of 5 or less.

pKa (in H ₂ O, 25 ° C.) is a value calculated by Advanced Chemistry Development (ACD / Labs) Software V11.02 (1994-2014 ACD / Labs) or a literature value (for example, J. Phys. Chem. A 201115). , 6641-6645, etc.) can be used.

Among these, acidic compounds having high water solubility are preferred. Further, from the viewpoint of fixing the whole ink by reacting with the ink composition, a trivalent or less acidic compound is preferable, and a divalent or trivalent acidic compound is more preferable.

As the treating agent, one acidic compound may be used alone, or two or more acidic compounds may be used in combination.

When the treating agent is an aqueous solution containing an acidic compound, the pH (25 ° C.) of the treating agent is preferably 0.1 to 6.8, more preferably 0.1 to 6.0, and More preferably, it is from 0.1 to 5.0.

When the treating agent contains an acidic compound as an aggregating component, the content of the acidic compound in the treating agent is preferably 40% by mass or less, more preferably 0.01 to 40% by mass, and more preferably 0.01 to 35% by mass. Is more preferably, and particularly preferably 0.05 to 30% by mass. By setting the content of the acidic compound in the treatment agent within the above range, the components in the ink composition can be more efficiently fixed.

When the treating agent contains an acidic compound as an aggregating component, the amount of the treating agent applied to the recording medium is not particularly limited as long as the amount is sufficient to coagulate the ink composition, but if the ink composition is easily fixed. In view of the above, it is preferable to apply the treating agent so that the applied amount of the acidic compound is 0.5 g / m ² to 4.0 g / m ^2, and 0.9 g / m ² to 3.75 g / m ² . It is preferable to apply a treating agent so that

－ Polyvalent metal salt －

As the treatment agent, a form containing one or more polyvalent metal salts as an aggregation component is also preferable. By including a polyvalent metal salt as an aggregation component, high-speed aggregation can be improved. Examples of the polyvalent metal salt include salts of alkaline earth metals belonging to Group 2 of the periodic table (for example, magnesium and calcium), salts of transition metals belonging to Group 3 of the periodic table (for example, lanthanum), and salts of the 13th group of the periodic table. Salts of cations from the genus (eg, aluminum) and lanthanides (eg, neodymium) can be mentioned. As metal salts, carboxylate (formic acid, acetic acid, benzoate, etc.), nitrate, chloride, and thiocyanate are preferable. Among them, preferred are calcium salts or magnesium salts of carboxylic acids (such as formic acid, acetic acid, and benzoate), calcium salts or magnesium salts of nitric acid, calcium chloride, magnesium chloride, and calcium salts or magnesium salts of thiocyanic acid.

When the treating agent contains a polyvalent metal salt as an aggregating component, the content of the polyvalent metal salt in the treating agent is preferably 0.01 to 10% by mass, more preferably 0.01 to 10% by mass, from the viewpoint of the aggregating effect. To 7% by mass, more preferably 0.05 to 6% by mass.

－ Cationic polymer －

It is also preferable that the treating agent contains one or more cationic polymers as an aggregation component. The cationic polymer includes a homopolymer of a cationic monomer having a primary to tertiary amino group or a quaternary ammonium group as a cationic group, or a copolymer of the cationic monomer and a non-cationic monomer. Those obtained as a polymer or a condensation polymer are preferred. As the cationic polymer, any of a water-soluble polymer and a water-dispersible latex particle may be used.

Preferred specific examples of the cationic polymer include poly (vinylpyridine) salt, polyalkylaminoethyl acrylate, polyalkylaminoethyl methacrylate, poly (vinylimidazole), polyethyleneimine, polybiguanide, polyguanide, and polyallylamine and derivatives thereof. Cationic polymers can be mentioned.

The weight average molecular weight of the cationic polymer is preferably smaller from the viewpoint of the viscosity of the treating agent. When the treating agent is applied to the recording medium by the ink jet system, the range is preferably from 1,000 to 500,000, more preferably from 1,500 to 200,000, and further preferably from 2,000 to 100,000. Range. When the mass average molecular weight is 1,000 or more, it is advantageous from the viewpoint of aggregation speed, and when it is 500,000 or less, it is advantageous in terms of ejection reliability. However, when the treatment agent is applied to the recording medium by a method other than the inkjet method, this is not always the case.

When the treating agent contains a cationic polymer as an aggregating component, the content of the cationic polymer in the treating agent is preferably from 0.01 to 50% by mass, more preferably from 0.01 to 30% by mass, from the viewpoint of the aggregating effect. %, More preferably in the range of 0.05 to 20% by mass.

[Image forming method]

The image forming method of the present invention includes a step of applying the inkjet ink composition of the present invention on a recording medium to which a processing agent has been applied.

As the treatment agent, the description of the treatment agent in the ink set of the present invention is preferably applied.

The image forming method of the present invention preferably includes an ink applying step of forming an image by applying an inkjet ink composition of the present invention containing a pigment onto a recording medium to which a processing agent has been applied by an inkjet method. A treating agent applying step of applying an agent onto a recording medium, and an ink applying step of applying an inkjet ink composition of the present invention containing a pigment by an inkjet method to form an image on the recording medium after the treating agent applying step. It is more preferred to include

<Recording medium>

The recording medium is not particularly limited, and may be a permeable recording medium that is a paper medium or a low permeable recording medium represented by coated paper (coated paper), and may be a non-permeable recording medium such as plastic, metal, or glass. It is also preferably a recording medium. The aqueous ink composition of the present invention can be dried quickly even when an image portion is formed on a low-permeability or non-permeability recording medium, and forms a desired image at high speed and with high accuracy. be able to.

In the present invention, the “low-permeability recording medium” means a recording medium having a water absorption coefficient Ka of 0.05 to 0.5 mL / m ² · ms ^1/2 . Further, in the present invention, the “non-permeable recording medium” means a recording medium having a water absorption coefficient Ka of less than 0.05 mL / m ² · ms ^1/2 .

The water absorption coefficient Ka has the same meaning as that described in JAPAN TAPPI Paper and Pulp Test Method No. 51: 2000 (issued by Japan Society of Paper and Pulp Technology). Calculated from the difference in the amount of water transferred between the contact time of 100 ms and the contact time of 900 ms.

As the permeable recording medium or the low permeable recording medium, commercially available ones can be used.For example, Oji Paper's "OK Prince Fine", Nippon Paper's "Shiraoi", and Fine paper (A) such as "New NPI Fine" manufactured by Nippon Paper Co., Ltd., high quality coated paper such as "Silver Diamond" manufactured by Nippon Paper, "OK Everlight Coat" manufactured by Oji Paper, and Nippon Paper Fine coated paper such as "Aurora S", lightweight coated paper (A3) such as "OK Coat L" manufactured by Oji Paper Co., Ltd. and "Aurora L" manufactured by Nippon Paper Co., Ltd .; "OK Top Coat +" manufactured by Oji Paper Co., Ltd. And coated papers (A2, B2) such as "Aurora Coat" manufactured by Nippon Paper Co., Ltd., and art papers (A1) such as "OK Kinto +" manufactured by Oji Paper Co., Ltd. and "Torishi Art" manufactured by Mitsubishi Paper Mills, etc. Is mentioned.

It is also possible to use various kinds of photographic paper for ink jet recording.

Among these recording media, so-called coated paper (coated paper) used for general offset printing and the like is preferable. Coated paper is a paper in which a coating material is applied to the surface of high quality paper or neutral paper or the like, which is mainly cellulose and has not been subjected to surface treatment, and is generally provided with a coating layer. Coated paper is likely to cause quality problems, such as gloss and abrasion resistance of the image, in image formation by ordinary aqueous inkjet, but when using the ink composition or ink set, uneven gloss is suppressed. Thus, an image having good gloss and scratch resistance can be obtained. In particular, it is preferable to use coated paper having base paper and a coat layer containing kaolin and / or calcium bicarbonate. More specifically, art paper, coated paper, lightweight coated paper or lightly coated paper is more preferred.

The non-permeable base material is not particularly limited, but a resin base material is preferred. The resin substrate is not particularly limited, and includes, for example, a substrate formed by molding a thermoplastic resin into a sheet. The resin substrate preferably contains polyester such as polyethylene, polypropylene, and polyethylene terephthalate, nylon, or polyimide.

The resin base material may be a transparent resin base material or a colored resin base material, and at least a part thereof may be subjected to a metal deposition treatment or the like.

The shape of the resin substrate is not particularly limited. The resin substrate is usually a sheet-shaped resin substrate, and is more preferably a sheet-shaped resin substrate capable of forming a roll by winding from the viewpoint of productivity of a recording medium.

The thickness of the resin substrate is preferably from 10 μm to 200 μm, more preferably from 10 μm to 100 μm.

The resin substrate may be surface-treated from the viewpoint of improving the surface energy.

Examples of the surface treatment include, but are not limited to, corona treatment, plasma treatment, flame treatment, heat treatment, abrasion treatment, light irradiation treatment (UV treatment), and flame treatment.

The corona treatment can be performed using, for example, a corona master (PS-10S, manufactured by Shinko Electric Meter Co., Ltd.).

Conditions for the corona treatment may be appropriately selected depending on the case, such as the type of the resin base material and the composition of the ink. As an example, the following processing conditions are given.

-Processing voltage: 10 to 15.6 kV

・ Processing speed: 30 to 100 mm / s

<Treatment agent application step>

In the treatment agent applying step, the treatment agent is applied on a recording medium. The treating agent is usually applied on a recording medium in the form of an aqueous solution. The application of the treatment agent onto the recording medium can be performed by any known method for applying a liquid without particular limitation, and any method such as spray application, application using an application roller, application using an inkjet method, and immersion can be selected. it can.

Specifically, for example, a size press method represented by a horizontal size press method, a roll coater method, a calendar size press method, and the like; a size press method represented by an air knife coater method; Knife coater method; transfer roll coater method such as gate roll coater method, roll coater method represented by direct roll coater method, reverse roll coater method, squeeze roll coater method; bill blade coater method, short dwell coater method; Blade coater method represented by a stream coater method; bar coater method represented by a rod bar coater method; bar coater method represented by a rod bar coater method; cast coater method; gravure coater method; Coater method; die coater method; brush coater method; and the like transfer method.

Further, a method in which the amount of application is controlled and applied by using an application device having a liquid amount limiting member, such as the application device described in JP-A-10-230201, may be used.

The area to which the treatment agent is applied may be an entire area applied to the entire recording medium or a partial area applied to an area to which ink is applied in the ink applying step. In the present invention, from the viewpoint of uniformly adjusting the application amount of the treatment liquid, uniformly recording fine lines and fine image portions, and suppressing density unevenness such as image unevenness, a recording medium is applied by using an application roller or the like. Is preferably applied to the entire image forming surface.

As a method of applying the treatment agent while controlling the applied amount within the above range, for example, a method using an anilox roller may be mentioned. The anilox roller is a roller in which the surface of the roller on which the ceramic has been sprayed is processed by a laser to have a shape such as a pyramid shape, a diagonal line, and a turtle shape. The treatment liquid enters into the concave portion provided on the roller surface, is transferred when it comes into contact with the paper surface, and is applied at an application amount controlled by the concave portion of the anilox roller.

<Ink application step>

In the ink application step, the ink composition is applied on a recording medium by an inkjet method.

In image formation by an ink jet method, an ink composition is ejected onto a recording medium by applying energy to form an image portion. As a preferred inkjet recording method for the present invention, the method described in paragraphs 0093 to 0105 of JP-A-2003-306623 can be applied.

The inkjet method is not particularly limited, and is a known method, for example, a charge control method for discharging ink using electrostatic attraction, a drop-on-demand method using a vibration pressure of a piezo element (pressure pulse method), Either an acoustic ink jet method in which an electric signal is converted into an acoustic beam and the ink is ejected by irradiating the ink using radiation pressure, or a thermal ink jet method in which the ink is heated to form bubbles and the generated pressure is used. You may.

The ink jet head used in the ink jet system may be either an on-demand system or a continuous system. Further, there are no particular restrictions on the ink nozzles and the like used when recording is performed by the above-described inkjet method, and they can be appropriately selected according to the purpose.

In addition, the inkjet method includes a method of ejecting a large number of low-density inks called photo inks in a small volume, a method of improving image quality using a plurality of inks having substantially the same hue and different densities, and a method of colorless and transparent ink. The method used is included.

In addition, as an inkjet system, a shuttle system that uses a short serial head and performs recording while scanning the head in the width direction of the recording medium, and a line head in which recording elements are arranged corresponding to the entire area of one side of the recording medium. And a line method using the In the line system, an image can be recorded on the entire surface of the recording medium by scanning the recording medium in a direction orthogonal to the arrangement direction of the recording elements, and a transport system such as a carriage for scanning a short head is not required. Further, complicated scanning control between the movement of the carriage and the recording medium is not required, and only the recording medium moves, so that a higher recording speed can be realized as compared with the shuttle method.

There is no particular limitation on the order in which the processing agent application step and the ink application step are performed, but from the viewpoint of image quality, it is preferable that the ink application step is performed after the processing agent application step. That is, the ink applying step is preferably a step of applying the inkjet ink composition of the present invention onto the recording medium to which the treating agent has been applied.

From the viewpoint of forming a high-definition print, the droplet amount of the ink composition discharged by the inkjet method is preferably from 1.5 to 3.0 pL, more preferably from 1.5 to 2.5 pL. The amount of the ink composition to be ejected can be adjusted by appropriately adjusting the ejection conditions.

<Ink drying process>

The image forming method of the present invention may include, if necessary, an ink drying step of drying and removing a solvent (for example, water, the above-described aqueous medium, etc.) in the aqueous ink composition applied on the recording medium. Good. The ink drying step is not particularly limited as long as at least a part of the ink solvent can be removed, and a commonly used method can be applied.

<Heat fixing process>

The image forming method of the present invention preferably includes a heat fixing step after the ink drying step, if necessary. By performing the heat fixing process, the image on the recording medium is fixed, and the resistance to scratching of the image can be further improved. As the heat fixing step, for example, the heat fixing steps described in paragraphs <0112> to <0120> of JP-A-2010-221415 can be employed.

<Ink removal process>

The ink jet recording method using the ink jet ink composition of the present invention is an ink for removing an aqueous ink composition (for example, an ink solid that has solidified by drying) attached to an ink jet recording head with a maintenance liquid, if necessary. A removing step may be included. For details of the maintenance liquid and ink removing step, the maintenance liquid and ink removing step described in International Publication No. 2013/180074 can be preferably applied.

Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples. Unless otherwise specified, “parts” and “%” representing compositions are based on mass. Room temperature is 25 ° C.

[Synthesis of polymer, preparation of pigment dispersant solution composed of polymer]

(1) Synthesis of polymer P-1

100 g of methyl ethyl ketone was added to a 500 mL four-necked flask, and heated to 78 ° C. while stirring with a stirring blade to replace with nitrogen. In this solvent, a solution prepared by dissolving 50 g of N- (4-sulfamoylphenyl) acrylamide, 10 g of styrene, 20 g of benzyl methacrylate, 10 g of methyl methacrylate and 0.8 g of mercaptopropionic acid in 100 g of methyl ethyl ketone, and perbutyl O (product) Name, perester, manufactured by Nippon Oil & Fats Co., Ltd.) and a mixed solution of 1.0 g of methyl ethyl ketone and 20 g of methyl ethyl ketone were separately dropped over 3 hours using a dropping pump. After completion of the dropwise addition, the mixture was heated and stirred at 78 ° C. for 3 hours and cooled to room temperature. Reprecipitation was performed with hexane / ethyl acetate = 80/20, and the obtained precipitate was dried to obtain 82 g (yield: 82%) of a white solid polymer p-1. The weight average molecular weight of the polymer p-1 was 15,000.

The obtained polymer p-1 was dissolved in propylene glycol monomethyl ether to obtain a 30% by mass polymer solution, and 20.3 g of a 50% by mass aqueous solution of potassium hydroxide was added to neutralize the polymer. Pigment Dispersant Solution No. P-1 P-1 was prepared.

(2) Synthesis of polymers P-2 to P-14 and PR-1

Polymers p-2 to p-7, p-11, p-12 and pR-1 before neutralization having the following structure were synthesized in the same manner as in the method for synthesizing polymer P-1, and these were neutralized. The neutralized polymers P-2 to P-7, P-11, P-12 and PR-1 were synthesized to prepare respective pigment dispersant solutions.

Polymers P-8 to P-10, P-13 and P-14 were prepared according to the method for synthesizing polysulfonamide described in WO 2017/061561, before the neutralization of polymers p-8 to p-10 and p-14. -13 and p-14 were synthesized, and the polymers P-8 to P-10, P-13 and P-neutralized after neutralization were synthesized in the same manner as the method for neutralizing the polymer p-1 in the method for synthesizing the polymer P-1. No. 14 was synthesized, and each pigment dispersant solution was prepared.

(Method of measuring pKa (DMSO))

After dissolving 0.1 g of the polymer for measurement in DMSO to form a 1% by mass solution, titration was performed at 25 ° C. using an aqueous 0.1 mol% (concentration) potassium hydroxide solution to measure pKa (DMSO). did.

Here, the polymer for measurement is a polymer before the neutralization with an aqueous potassium hydroxide solution or the like (the above-mentioned polymer p-1 or the like).

The structure of each polymer synthesized above is described below. The physical properties of the polymer before each neutralization are collectively described in Table 1 below.

In the following structural formula, “M” means a hydrogen atom or a potassium ion. In the case of a polymer before neutralization (for example, polymer p-1), M means a hydrogen atom, and in the case of a polymer after neutralization (for example, polymer P-1), M is potassium depending on the neutralization rate. Ion, or potassium ion and hydrogen atom.

In addition, the numbers of the respective structural units of the following polymers represent mass ratios. “*” Shown in each structural unit indicates a linking site to be incorporated into the polymer.

[Example]

1. Preparation of white ink composition

(1) Preparation of titanium oxide dispersion

A titanium oxide mixture having the following composition was placed in a 2 ml container, and a dispersion operation was performed for 5 hours using Delta Mixer Se-08 (manufactured by TAITEC). The obtained dispersion was filtered using an 80 μm mesh filter cloth to prepare a titanium oxide dispersion.

In this dispersion, the content of titanium oxide was 45% by mass, the average particle size of titanium oxide was 50 nm, and the content of the pigment dispersant was 0.11% by mass.

(Composition of titanium oxide mixture)

0.6 g of fine particle titanium oxide (trade name: TTO-55 (C), manufactured by Ishihara Sangyo Co., Ltd.)

0.6 g of 0.3 mm diameter zirconia beads (manufactured by Nikkato)

0.5 g of pigment dispersant solution

0.6 g of water

(2) Preparation of binder dispersion

A polymer having a mass mixing ratio of monomer components of methyl methacrylate: isobornyl methacrylate: methacrylic acid = 70/20/10 and a weight average molecular weight of 30,000 was dissolved in methyl ethyl ketone. To this solution, a 50% by mass aqueous solution of potassium hydroxide containing potassium hydroxide in an equimolar amount to the methacrylic acid component in the polymer was added for neutralization. After water was added to the resulting solution and mixed, the organic solvent was distilled off, phase inversion emulsification was performed, and a binder (polymer) dispersion having a solid content of 20% by mass and a neutralization ratio of 100 mol% was prepared. .

The volume average particle diameter of the binder particles in the obtained binder dispersion was 5 nm. The volume average particle size was measured with a Microtrac UPA EX-150 (manufactured by Nikkiso Co., Ltd.).

(3) Preparation of white ink composition

The white ink compositions W-1 to W-15 having a titanium oxide content of 10% by mass, a pigment dispersant content of 2.5% by mass, and a binder particle content of 5% by mass according to the composition shown in Table 2 below. , CW-11 and cW-12 were prepared.

(Preparation of white ink composition)

0.5 g of titanium oxide dispersion

0.45 g of binder dispersion

1.3 g of water

0.03 g of Emar 2F-30 (manufactured by Kao Chemical Company)

2. Preparation of PY74 ink composition

Except that Pigment Yellow 74 was used in place of the fine particle titanium oxide as the pigment in the titanium oxide mixture, the PY74 ink composition Y-1 having the composition shown in Table 3 below was prepared in the same manner as in the preparation of the white ink composition. ~ Y-3 and cY-11 were prepared.

The average particle size of Pigment Yellow 74 in the PY74 ink composition was 100 nm.

3. Test example

For each of the ink compositions prepared above, the color density (hiding power) was evaluated according to the following test. The results are shown in Tables 2 and 3 below.

Test Example 1: Color density

(1) Preparation of solid coated sample

On a polyester film (manufactured by Toyobo Co., Ltd., trade name: A4300), a pretreatment aqueous solution containing 0.1% by mass of a pretreatment additive described in Table 2 or 3 described below was adjusted to 10 g / m ^2. And a 100 μm bar coater, and placed on a hot plate at 60 ° C. with the coated side facing up, and dried with a dryer for 15 minutes to produce a pretreated polyester film. On the prepared pretreated polyester film, apply the ink composition prepared above so that the thickness of the coating film after drying is 10 μm, and dry with a dryer for 30 minutes to obtain a solid-coated sample of the ink composition. Produced. However, the applied ink composition is an ink composition within one hour after preparation.

(2) Evaluation of color density

The L value of the solid sample prepared above was measured using a spectrophotometer (trade name: X-Rite 938, manufactured by X-Rite), and the color density was evaluated according to the following evaluation criteria.

(Evaluation criteria)

7: L value is 70 or more

6: L value is 65 or more and less than 70

5: L value is 60 or more and less than 65

4: L value is 55 or more and less than 60

3: L value is 50 or more and less than 55

2: L value is 40 or more and less than 50

1: L value is less than 40

As shown in Table 2 above, the comparative white ink compositions cW-11 and cW-12 containing no pigment dispersant (A) had low color densities and poor concealing properties. In contrast, all of the ink compositions W-1 to W-15 containing the pigment dispersant (A) had high color density and excellent shielding properties.

Further, as shown in Table 3 above, the comparative PY ink composition cY-11 containing no pigment dispersant (A) had a low color density and a poor concealing property. On the other hand, the PY ink compositions Y-1 to Y-3 containing the pigment dispersant (A) all had high color density and excellent concealability.

[Reference example]

The ink composition prepared above was evaluated for liquid-repellent film resistance according to the following test. The results are shown in Table 4 below.

Test Example 2: Liquid repellent film resistance

A 100 μm-thick silicone resin film was formed on a glass substrate by baking at 250 ° C. using KS-700 (trade name, release agent, manufactured by Shin-Etsu Silicone Co., Ltd.) to prepare a sample substrate. The prepared sample substrate was immersed in the ink composition prepared above at 60 ° C. for 72 hours. After washing the ink composition adhering to the immersed sample substrate with water, the sample substrate is dried under reduced pressure at 120 ° C. for 3 hours, and a silicone resin film remaining on the sample substrate (hereinafter referred to as “residual film”). Was measured for thickness.

The thickness of the remaining film was applied to the following evaluation criteria, and the liquid-repellent film resistance was evaluated.

(Evaluation criteria)

3: The thickness of the remaining film is 80 μm or more

2: The thickness of the remaining film is 50 μm or more and less than 80 μm

1: The thickness of the remaining film is less than 50 μm

As shown in Table 4 above, all of the white ink compositions W-1 to W-15 containing the pigment dispersant (A) were excellent in liquid repellent film resistance. That is, when the pKa (DMSO) of the polymer before neutralization of the pigment dispersant (A) is 20 or less, the ink composition of the present invention can be used in addition to high color density when forming an image by an inkjet method. It is considered that the hydrolysis of the silicone resin film applied to the discharge port of the nozzle can also be suppressed.

Claims

A pigment dispersion containing a pigment dispersant (A) comprising a polymer having an anionic group, a pigment and an aqueous medium, wherein the pKa of the polymer before neutralization of the pigment dispersant (A) in a DMSO solution is: 14. A pigment dispersion, which is 14 or more.
The pigment dispersion according to claim 1, wherein the anionic group is a sulfonamide salt type group or an imide salt type group.
The pigment dispersion according to claim 2, wherein the sulfonamide salt type group is represented by the following general formula (I) or (II).

In the above formula, R 1 represents a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms, R 2 represents a hydrocarbon group having 1 to 20 carbon atoms, and Ar 1 and Ar 2 each independently represent a carbon group having 6 to 10 carbon atoms. 20 represents an arylene group or a heteroarylene group; L 1 and L 2 each independently represent a single bond or a divalent linking group; and M + represents an alkali metal ion or an ammonium ion. * Indicates a linking site with the polymer.
The pigment dispersion according to claim 3, wherein the pigment dispersant (A) includes a structural unit derived from a vinyl monomer having a sulfonamide salt type group represented by the general formula (I).
The pigment dispersion according to any one of claims 1 to 4, wherein the pigment contains titanium oxide having an average particle size of 1 to 100 nm.
The pigment dispersion according to any one of claims 1 to 5, comprising a binder particle.
An inkjet ink composition comprising the pigment dispersion according to any one of claims 1 to 6.
An ink set comprising the inkjet ink composition according to claim 7 and a treating agent for coagulating the ink composition.
The ink set according to claim 8, wherein the treating agent contains an acidic compound.
An image formed by the inkjet ink composition according to claim 7.
An image forming method, comprising: applying the inkjet ink composition to a recording medium to which a treatment agent for aggregating the inkjet ink composition according to claim 7 has been applied.