Black ink composition, ink set, and image forming method   

This application is based on and claims priority under 35 USC 119 from Japanese patent Application No. 2010-217823 filed on Sep. 28, 2010, the disclosure of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a black ink composition containing carbon black, an ink set including the composition, and an image forming method.

2. Description of the Related Art

Inkjet recording methods allow recording of high-quality images on various recording media by ejecting droplets of inks from a number of nozzles provided at an inkjet head. Therefore, inkjet recording methods are widely used.

Pigments are widely used as a coloring agent, which is a component of an ink. In inks, pigments are used in a state of being dispersed in a medium such as water. When pigments are used in the dispersed state, important factors include the diameter of the dispersed particles, stability after dispersing, size uniformity, ejectability from ejection nozzles, and image density. Various studies are carried out on techniques for improving such factors.

Further, in some cases, recording on plain sheet provides only insufficient quality in terms of rubbing resistance of the formed image (fixability), resolution, and the like, as well as image density. There is a tendency for the insufficiency in quality to become more conspicuous when the inkjet recording speed is increased. Specifically, compatibility with high-speed recording in a single-pass manner rather than a shuttle-scan manner is requested in terms of the rubbing resistance and resolution of the formed image. In the single-pass recording, recording can be carried out by a single head operation.

Meanwhile, in addition to the image density, generation of unevenness in image has to be considered from the viewpoint of image quality because unevenness in image, such as streak-shaped unevenness, may be generated when an image is recorded on plain paper or the like. The generation of unevenness is significant when high-speed inkjet recording is carried out. Specifically, streak-shaped unevenness is more likely to be generated in an image when the image is recorded at high speeds in a single-pass manner, which enables recording with one operation of a head, as compared to a shuttle-scan manner. The unevenness is particularly remarkable when an image of an intermediate tone of black (i.e., gray tone) is formed.

When high-speed recording is carried out in a single-pass manner, an inkjet head having a width that is equal to the width of a recording medium is used. However, in such a case, unevenness in hue tends to be generated in the width direction owing to a variation in ejection amounts of ink along the width direction. The unevenness in hue is particularly significant when an image of intermediate tone of black (i.e., gray tone), which is achromatic, is formed.

Among pigments used in inks for inkjet recording, carbon black is widely used as a black pigment. A technique in which a pigment other than carbon black, such as a cyan pigment, is used together with carbon black (hereinafter abbreviated to CB) is also known.

Specifically, a pigment dispersion aqueous recording liquid is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-285344. This pigment dispersion aqueous recording liquid contains carbon black (CB) and a pigment other than CB, such as a cyan pigment, together with a urethane resin. Further, an aqueous ink for inkjet recording that contains polymer particles containing carbon black is disclosed in, for example, JP-A No. 2009-144007.

Regarding an ink containing carbon black (CB), a neutral black ink or black ink set which contains carbon black as well as a cyan pigment and a magenta pigment is disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-213505, or JP-A No. 2003-55592), which enables improvements in tone of an yellowish or brownish image, and provides excellent color reproducibility.

Furthermore, a black ink is disclosed (see, for example, JP-A No. 2009-132766), which contains a dispersion of surface-treated black pigments capable of being dispersed in water even in the absence of a dispersant, a magenta pigment-containing polymer particle dispersion, a cyan pigment-containing polymer particle dispersion, a yellow pigment-containing polymer particle dispersion, and resin emulsion. It is disclosed that the black ink is excellent in glossiness of image, ejection property, and the like.

On the other hand, a pigment, being hard powder, has a problem in that the pigment may easily damage a nozzle surface when an ink is wiped off for maintenance of a nozzle for ejecting the ink. In particular, when carbon black, from among various pigments, is contained in an ink, the ink tends to damage a nozzle surface as compared with a case in which a color ink containing an organic pigment is used.

SUMMARY

Techniques of using carbon black in combination with a pigment other than a black pigment, such as a cyan pigment or magenta pigment, have been proposed, such as the techniques described in the pieces of related art described above. However, when carbon black is simply mixed with a pigment of another color, it is only expected that color reproducibility is improved to a certain degree, and it is difficult to prevent streak-shaped unevenness, unevenness in hue, and the like, which are generated in high-speed recording in single-pass manner.

Use of an ink containing carbon black tends to exacerbate nozzle damage, as compared to cases in which inks containing other organic pigments are used. The nozzle damage may be suppressed by decreasing the content ratio of carbon black. However, a simple reduction in the content ratio of carbon black may make it impossible to obtain a desired black density, may shift the color obtained when, for example, a gray tone is desired, and may provide only a yellowish hue.

When the amounts of ink components, such as polymer particles, other than pigment are increased, the proportion of carbon black in an image is relatively decreased, as a result of which the shielding ratio is decreased, and a change in hue such as yellow tinge tends to occur. Accordingly, it has not been possible to obtain a desired black color or an intermediate tone, such as gray, with less yellow tinge, while alleviating streak-shaped unevenness in an image and improving maintenance properties.

The present invention has been made in view of the above, and provides a black ink composition and an ink set, each of which enables provision of an image having black color with less yellow tinge or an intermediate tone (for example, gray) with less yellow tinge, and suppresses streak-shaped unevenness in an image, unevenness in hue in an image, and generation of nozzle damage during maintenance. The present invention further provides an image forming method by which generation of streak-shaped unevenness and generation of unevenness in hue are prevented, and an image of black color or an intermediate tone (for example, gray) with less yellow tinge can be obtained, while controlling damage to a nozzle surface at a low level.

Aspects of the invention include the following.

<1> A black ink composition, comprising:

carbon black and a first water-insoluble resin that covers at least a part of a surface of the carbon black;

a cyan pigment and a second water-insoluble resin that covers at least a part of a surface the cyan pigment;

a magenta pigment and a third water-insoluble resin that covers at least a part of a surface of the magenta pigment;

water-insoluble resin particles; and

water,

wherein a content ratio of the carbon black is from 1.0 to 2.0% by mass with respect to a total mass of the composition, and a total amount of pigments is from 1.8 to 3.5% by mass with respect to the total mass of the composition.

<2> The black ink composition according to <1>, wherein a content of the water-insoluble resin particles is larger than a total amount of pigments, at least a part of surfaces of which are coated with water-insoluble resins, and the water-insoluble resins that cover the at least a part of surfaces of the pigments.

<3> The black ink composition according to <2>, wherein a ratio of the content of the water-insoluble resin particles to the total amount of pigments, at least a part of surfaces of which are coated with water-insoluble resins, and the water-insoluble resins that cover the at least a part of surfaces of the pigments, is more than 1.0 but not more than 4.0.

<4> The black ink composition according to <1>, wherein the water-insoluble resin particles comprise self-dispersing polymer particles.

<5> The black ink composition according to <1>, wherein the water-insoluble resin particles have a Tg of 100° C. or higher.

<6> The black ink composition according to <1>, further comprising a solid humectant.

<7> The black ink composition according to <6>, wherein the humectant is selected from urea, a urea derivative, or a mixture thereof.

<8> The black ink composition according to <1>, further comprising a yellow pigment and a fourth water-insoluble resin that covers at least a part of a surface of the yellow pigment.

<9> An ink set, comprising:

the black ink composition according to <1>; and

a treatment liquid comprising an aggregation component which is capable of causing formation of an aggregate when contacting the black ink composition.

<10> An image forming method, comprising:

applying the black ink composition according to <1> to a recording medium by an inkjet method; and

applying a treatment liquid to the recording medium, the treatment liquid comprising an aggregation component which is capable of causing formation of an aggregate when contacting the black ink composition.

<11> The image forming method according to <10>, wherein the applying of the black ink composition comprises applying the black ink composition by a piezoelectric inkjet method.

<12> The image forming method according to <11>, further comprising heating an image formed through the applying of the black ink composition and the applying of the treatment liquid, to fix the image on the recording medium.

According to the invention, a black ink composition and ink set are provided, each of which enables provision of an image having black color with less yellow tinge or an intermediate tone (for example, gray) with less yellow tinge, and suppresses streak-shaped unevenness in an image, unevenness in hue in an image, and generation of nozzle damage during maintenance.

Furthermore, according to the invention, an image forming method is provided by which generation of streak-shaped unevenness and generation of unevenness in hue are prevented, and an image of black color or an intermediate tone (for example, gray) with less yellow tinge can be obtained, while controlling damage to a nozzle surface at a low level.

DETAILED DESCRIPTION

Hereinbelow, a black ink composition of the invention, and an ink set including the black composition and an image forming method in which the black ink composition is used are described in detail.

As used herein, the scope of “maintenance” includes maintaining a recording head that ejects an inkjet recording ink composition and the ejection performance thereof in an initial state or in a state close to the initial state, and further encompasses having the recording head serviced and maintained in a better state by cleaning the recording head.

<Black Ink Composition>

A black ink composition of the invention (which may be referred to as “ink composition” hereinbelow) is an aqueous composition including at least:

carbon black and a first water-insoluble resin that covers at least a part of the surface of the carbon black;

a cyan pigment and a second water-insoluble resin that covers at least a part of the surface of the cyan pigment;

a magenta pigment and a third water-insoluble resin that covers at least a part of the surface of the magenta pigment;

water-insoluble resin particles; and

water,

and in which the content ratio of the carbon black (excluding the first water-insoluble resin, the same applies hereinafter) is from 1.0% by mass to 2.0% by mass with respect to the total amount of the composition, and the total amount of pigments (excluding coating resins such as the water-insoluble resins, the same applies hereinafter) is from 1.8% by mass to 3.5% by mass with respect to the total amount of the composition.

The black ink composition of the invention may further include other components such as an organic solvent or a surfactant, if necessary.

In the invention, the black ink composition, which includes carbon black (which may be abbreviated to “CB”) as a black colorant for forming a black ink, has a configuration in which:

the ink composition includes a cyan pigment and a magenta pigment together with the CB;

at least a part of the surfaces of the particles of each of the CB, cyan pigment, and magenta pigment is coated with a water-insoluble resin;

the content ratio of the carbon black, which has high tendency to damage the surface of a head nozzle, is decreased, and resin particles, which have low tendency to damage the surface of a head nozzle, are included; and

the amounts of pigments (excluding coating resins such as water-insoluble resins) are adjusted such that the total mass of pigments is at a certain ratio relative to the total amount of the ink.

Due to this configuration, generation of damage to the head nozzle surface during maintenance is regulated to be low, and when, for example, a black image or an image of an intermediate tone such as gray is formed at a high speed in a single-pass manner, generation of streak-shaped unevenness or hue unevenness in an image is prevented, and an achromatic image in which a change in color from a black or intermediate tone image is suppressed can be obtained.

When, as described above, the amount of carbon black (CB) contained in the ink composition is decreased in order to prevent generation of streak-shaped unevenness and damage to the surface of a head nozzle, but is still capable of maintaining a black density and the color reproducibility of an achromatic color, and the ink composition further includes a resin-coated cyan pigment and a resin-coated magenta pigment in amounts such that the total pigment amount falls within a certain range, a color change (yellowing) caused by the decrease in the CB amount and generation of striped-shape unevenness in an image are prevented, and damage to the surface of a head nozzle, which tends to occur when the head nozzle is wiped during maintenance, is prevented. The hue unevenness caused during printing in a single-pass manner is caused by slight fluctuation in ejection from an inkjet head, and may be prevented by adjusting the CB amount and the total amount of pigments to be within the respective specific ranges such that a change in the ejection amount (i.e., image dot ratio) does not lead to a change in hue. Furthermore, generation of damage on a nozzle surface is further suppressed by coating the CB, the cyan pigment, and the magenta pigment with at least one water-insoluble resin and including water-insoluble resin particles, in addition to reducing the amount of CB.

(Resin-Coated Carbon Black)

The black ink composition of the invention includes at least one carbon black at least a part of the surface of which is coated with a water-insoluble resin (which may be referred to as “resin-coated CB” or “resin-coated carbon black” hereinbelow). The resin-coated CB is a particle obtained by coating a part or all of the particle surface of carbon black (CB) with a water-insoluble resin, and is included in a dispersed state in the ink composition.

—Carbon Black—

Examples of carbon blacks include a carbon black produced by a known method such as a contact method, a furnace method, or a thermal method. Specific examples thereof include furnace black, thermal lamp black, acetylene black, and channel black.

Specific examples of carbon blacks include, but are not limited to, RAVEN 7000, RAVEN 5750, RAVEN 5250, RAVEN 5000 ULTRA II, RAVEN 3500, RAVEN 2000, RAVEN 1500, RAVEN 1250, RAVEN 1200, RAVEN 1190 ULTRA II, RAVEN 1170, RAVEN 1255, RAVEN 1080, RAVEN 1060, RAVEN 700 (all of which are trade names, manufactured by Columbian Chemicals Company), REGAL 1400R, REGAL 1330R, REGAL 1660R, MOGUL L, BLACK PEARLS L, MONARCH 700, MONARCH 800, MONARCH 880, MONARCH 900, MONARCH 1000, MONARCH 1100, MONARCH 1300, MONARCH 1400 (all of which are trade names, manufactured by Cabot Corporation), COLOR BLACK FW1, COLOR BLACK FW2, COLOR BLACK FW2V, COLOR BLACK 18, COLOR BLACK FW200, COLOR BLACK S 150, COLOR BLACK S 160, COLOR BLACK S170, PRINTEX 35, PRINTEX U, PRINTEX V, PRINTEX 140U, PRINTEX 140V, SPECIAL BLACK 6, SPECIAL BLACK 5, SPECIAL BLACK 4A, SPECIAL BLACK 4 (all of which are trade names, manufactured by Evonik-Degussa GmbH), and No. 25, No. 33, No. 40, No. 45, No. 47, No. 52, No. 900, No. 2200B, No. 2300, MCF-88, MA600, MA7, MA8, MA100 (all of which are trade names, manufactured by Mitsubishi Chemical Corporation).

—Water-Insoluble Resin—

Examples of the water-insoluble resin with which the carbon black (CB) is coated include [1] a polymer including a repeating unit (a) represented by the following Formula (1) and a repeating unit (b) having an ionic group, and [2] a polymer including a structural unit derived from a salt-forming-group-containing monomer (c) and at least one structural unit derived from a styrenic macromonomer (d) and/or a hydrophobic monomer (e).

As used herein, “insoluble” means that, when a polymer is mixed with an aqueous medium at 25° C., the amount of polymer that dissolves in the aqueous medium is 10% by mass or less with respect to the total amount of polymer mixed with the aqueous medium.

[1] Polymer containing repeating unit (a) represented by Formula (1) and repeating unit (b) having ionic group

The polymer contains at least one repeating unit represented by Formula (1) and at least one repeating unit having an ionic group, and may further contain, if necessary, another structural unit such as a hydrophobic repeating unit other than the repeating unit represented by Formula (1) or a hydrophilic repeating unit having a non-ionic functional group.

<Repeating Unit (a) Represented by Formula (1)>

In Formula (1), R1 represents a hydrogen atom, a methyl group, or a halogen atom; L1 represents *—COO—, *—OCO—, *—CONR2—, *—O—, or a substituted or unsubstituted phenylene group, R2 represents a hydrogen atom or an alkyl group having from 1 to 10 carbon atoms, and “*” shown in the group represented by L1 indicates a position at which L1 is connected to the main chain; L2 represents a single bond or a divalent linking group; and Ar represents a monovalent aromatic ring group derived from an aromatic ring.

In Formula (1), R1 represents a hydrogen atom, a methyl group, or a halogen atom, and preferably represents a methyl group.

L1 represents *—COO—, *—COO—, *—CONR2—, *—O—, or a substituted or unsubstituted phenylene group. When L1 represents a phenylene group, the phenylene group is preferably an unsubstituted phenylene group. R2 represents a hydrogen atom or an alkyl group having from 1 to 10 carbon atoms.

L2 represents a single bond or a divalent linking group. The divalent linking group is preferably a linking group having from 1 to 30 carbon atoms, more preferably a linking group having from 1 to 25 carbon atoms, even more preferably a linking group having from 1 to 20 carbon atoms, and particularly preferably a linking group having from 1 to 15 carbon atoms.

In particular, examples of most preferable linking groups include an alkyleneoxy group having from 1 to 25 carbon atoms (more preferably, having from 1 to 10 carbon atoms), an imino group (—NH—), a sulfamoyl group, a divalent linking group containing an alkylene group, such as an alkylene group having from 1 to 20 carbon atoms (more preferably, having from 1 to 15 carbon atoms) or an ethylene oxide group [—(CH2CH2O)n—, n=1 to 6], and a group obtained by combining two or more types of linking group selected from the above.

Ar represents a monovalent group derived from an aromatic ring.

The aromatic ring of the monovalent group represented by Ar is not particularly limited, and examples thereof include a benzene ring, a condensed aromatic ring having 8 or more carbon atoms, an aromatic ring having a hetero ring condensed thereto, and an aromatic ring obtained by condensing at least two benzene rings.

The term “condensed aromatic ring having 8 or more carbon atoms” described above refers to an aromatic ring which has 8 or more carbon atoms, and of which rings are composed of two or more benzene rings condensed with each other, or of at least one type of aromatic ring and at least one alicyclic hydrocarbon condensed with the at least one type of aromatic ring. Specific examples thereof include naphthalene, anthracene, fluorene, phenathrene, and acenaphthene.

The term “aromatic ring condensed with a heterocycle” described above refers to a compound that includes at least one aromatic compound free of a heteroatom (preferably, a benzene ring) and at least one heteroatom-containing cyclic compound condensed with the at least one aromatic compound. Here, the heteroatom-containing cyclic compound is preferably a 5-membered ring or a 6-membered ring. The heteroatom is preferably a nitrogen atom, an oxygen atom, or a sulfur atom. The heteroatom-containing cyclic compound may have plural heteroatoms. In this case, the heteroatoms may be the same as or different from each other.

Specific examples of hetero ring having an aromatic ring condensed thereto include phthalimide, acridone, carbazole, benzoxazole, and benzothiazole.

Specific examples of a monomer for forming a repeating unit represented by Formula (1) may include vinyl monomers such as (meth)acrylates, (meth)acrylamides, styrenes, and vinyl esters.

In the present invention, a hydrophobic structural unit having an aromatic ring bonded to an atom in the main chain via a linking group has a structure in which an aromatic ring is bonded to an atom in the main chain of the water-insoluble resin via a linking group, and does not directly binds to an atom in the main chain of the water-insoluble resin. Therefore, a suitable distance is maintained between the hydrophobic aromatic ring and a hydrophilic structural unit, and thus, an interaction between the water-insoluble resin and the pigment is enhanced, resulting in strong adsorption and further improvement of the dispersibility.

Specific examples of a monomer for forming a repeating unit represented by Formula (1) include, but are not limited to, the following monomers.

Ar in the repeating unit (a) represented by Formula (1) is preferably a monovalent group derived from benzyl (meth)acrylate, phenoxyethyl acrylate, phenoxyethyl methacrylate, acridone, or phthalimide, from the viewpoint of the dispersion stability of the coated pigment.

The repeating units may be used singly, or in combination of two or more thereof.

The content ratio of the repeating unit represented by Formula (1) in a polymer is preferably from 5 to 25% by mass, and more preferably from 10 to 18% by mass, with respect to the total mass of the polymer. When the content ratio is 5% by mass or higher, generation of image defects such as white spots may be significantly prevented. When the content ratio is 25% by mass or less, problems in production suitability which may be caused by decrease in solubility of the polymer in a polymerization reaction solution (e.g., methyl ethyl ketone) hardly arise.

<Other Hydrophobic Repeating Unit>

The polymer [1] preferably further contains, as a hydrophobic structural unit, a hydrophobic repeating unit other than the repeating unit represented by Formula (1). Examples of a hydrophobic repeating unit other than the repeating unit represented by Formula (1) include a structural unit derived from a vinyl monomer such as a (meth)acrylate-containing compound, a (meth)acrylamide-containing compound, a styrenic compound, or a vinyl ester, each of which is not a hydrophilic structural unit (for example, each of which has no hydrophilic functional group); and a hydrophobic structural unit having an aromatic ring which is bonded to an atom of the main chain thereof via a linking group. The structural units may be used alone, or in combination of two or more thereof. As a result of the incorporation of a repeating unit having properties that are intermediate between the properties of a repeating unit represented by Formula (1) and the properties of the hydrophilic repeating unit described below, the dispersion properties and the dispersion stability of the resultant pigment dispersion can further be improved.

Examples of the (meth)acrylate-containing compound include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and hexyl (meth)acrylate. In particular, an alkyl (meth)acrylate having an alkyl moiety having from 1 to 5 carbon atoms is preferable. Specifically, examples of preferable (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate, and methyl (meth)acrylate and ethyl (meth)acrylate are particularly preferable.

Examples of the (meth)acrylamide-containing compound include N-cyclohexyl (meth)acrylamide, N-(2-methoxyethyl) (meth)acrylamide, N,N-diallyl (meth)acrylamide, and N-allyl (meth)acrylamide.

Examples of the styrenic compound include styrene, methylstyrene, dimethylstyrene, trimethylstyrene, ethylstyrene, isopropylstyrene, n-butylstyrene, tert-butylstyrene, methoxystyrene, butoxystyrene, acetoxystyrene, chlorostyrene, dichlorostyrene, bromostyrene, chloromethyl styrene, hydroxystyrene protected with a group (e.g., t-Boc) capable of deprotection using an acidic substance, hydrosystyrene, methyl vinylbenzoate, α-methylstyrene, and vinylnaphthalene. Among these, styrene and α-methylstyrene are preferable.

Examples of vinyl esters include vinyl acetate, vinyl chloroacetate, vinyl propionate, vinyl butyrate, vinyl methoxyacetate, and vinyl benzoate. Among these, vinyl acetate is preferable.

<Repeating Unit (b) Having Ionic Group>

Examples of repeating units having an ionic group include a repeating unit derived from a monomer having an ionic group such as a carboxyl group, a sulfo group, or a phsphonate group. More specifically, the monomer having an ionic group may be a vinyl monomer having an ionic functional group, such as (meth)acrylic acid, a (meth)acrylate-containing compound, a (meth)acrylamide-containing compound, or a vinyl ester. The repeating unit having an ionic group can be introduced into the polymer by, for example, polymerization of a corresponding monomer or introduction of an ionic group into the polymer chain after polymerization.

From among the above repeating units having an ionic group, a repeating unit derived from acrylic acid or a repeating unit derived from methacrylic acid is preferable. Polymer [1] preferably includes a structural unit derived from acrylic acid or a structural unit derived from methacrylic acid or both.

It is preferable that the content of the repeating unit (b) having an ionic group in polymer [1] is 15% by mass or lower with respect to the entire mass of the polymer, and that the repeating units having an ionic group in polymer [1] include a structural unit (repeating unit) derived from (meth)acrylic acid.

When the content of the repeating unit (b) having an ionic group is 15% by mass or lower with respect to the entire mass of the polymer, excellent dispersion stability can be realized. In particular, the ratio of repeating units (b) having an ionic group is more preferably from 5% by mass to 15% by mass, and still more preferably from 7% by mass to 13% by mass, relative to the entire mass of the polymer, from the viewpoint of dispersion stability.

The polymer [1] is capable of being stably present in an aqueous ink composition, and mitigates adhesion and deposition of aggregates onto an inkjet head or the like, and has excellent properties with respect to the removability of adhered aggregates. From these viewpoints, the polymer [1] may further contain a hydrophobic structural unit other than the repeating unit represented by Formula (1) and/or a hydrophilic structural unit other than the repeating unit (b) having an ionic group.

<Hydrophilic Structural Unit>

The hydrophilic structural unit other than the repeating unit (b) having an ionic group may be, for example, a repeating unit derived from a monomer having a nonionic hydrophilic group, examples of which include a vinyl monomer having a nonionic hydrophilic group such as a (meth)acrylate-containing compound having a nonionic hydrophilic functional group, a (meth)acrylamide-containing compound having a nonionic hydrophilic functional group, or a vinyl ester having a nonionic hydrophilic functional group.

Examples of the nonionic hydrophilic functional group include a hydroxyl group, an amino group, an amido group of which the nitrogen atom is unsubstituted, and an alkylene oxide such as polyethylene oxide or polypropylene oxide, which are described below.

The monomer for forming a hydrophilic repeating unit having a nonionic hydrophilic group is not particularly limited, as long as the monomer has a functional group capable of forming a polymer, such as an ethylenic unsaturated double bond, and a nonionic hydrophilic functional group. The monomer for forming a hydrophilic repeating unit having a nonionic hydrophilic group may be selected from known monomers. Preferable examples thereof include hydroxyethyl (meth)acrylate, hydroxybutyl (meth)acrylate, (meth)acrylamide, aminoethyl acrylate, aminopropyl acrylate, and a (meth)acrylate that contains an alkylene oxide polymer.

The hydrophilic repeating unit having a nonionic hydrophilic group can be formed by, for example, polymerization of a corresponding monomer or introduction of a nonionic hydrophilic functional group into the polymer chain after polymerization.

The hydrophilic repeating unit having a nonionic hydrophilic group is more preferably a hydrophilic structural unit having an alkylene oxide structure. The alkylene portion of the alkylene oxide structure is preferably an alkylene having from 1 to 6 carbon atoms, more preferably an alkylene having from 2 to 6 carbon atoms, and particularly preferably an alkylene having from 2 to 4 carbon atoms, from the viewpoint of hydrophilicity. The polymerization degree of the alkylene oxide structure is preferably from 1 to 120, more preferably from 1 to 60, and particularly preferably from 1 to 30.

It is also preferable that the hydrophilic repeating unit having a nonionic hydrophilic group is a hydrophilic repeating unit containing a hydroxyl group. The number of hydroxyl groups in the repeating unit is not particularly limited, and is preferably from 1 to 4, more preferably from 1 to 3, and particularly preferably from 1 to 2, from the viewpoints of the hydrophilicity of the water-insoluble resin and the compatibility with solvent or other monomers at the time of polymerization.

A suitable ratio between hydrophilic repeating units and hydrophobic repeating units (including a repeated structure represented by the above Formula (1)) in polymer [1] depends on the strength of the hydrophilicity/hydrophobicity of the individual repeating units. In polymer [1], the ratio of hydrophilic repeating units is preferably 15% by mass or lower relative to the entire mass of the water-insoluble resin (polymer [1]). Here, the ratio of hydrophobic repeating units is preferably higher than 80% by mass relative to the entire mass of the water-insoluble resin, and more preferably 85% by mass or higher relative to the entire mass of the water-insoluble resin.

When the content of hydrophilic repeating units is 15% by mass or less, the amount of components that singly dissolves in an aqueous medium is suppressed, various performances such as dispersing of pigment is excellent, and favorable ink ejection properties are achieved at the time of inkjet recording.

The content ratio of hydrophilic repeating units is preferably from more than 0% by mass to 15% by mass, more preferably from 2 to 15% by mass, still more preferably from 5 to 15% by mass, and particularly preferably from 8 to 12% by mass, relative to the entire mass of the water-insoluble resin.

The content of aromatic rings in the water-insoluble resin is preferably 27% by mass or lower, more preferably 25% by mass or lower, and still more preferably 20% by mass or lower, with respect to the entire mass of the water-insoluble resin. In particular, the content of aromatic rings in the water-insoluble resin is further preferably from 15 to 20% by mass, and still further preferably from 17 to 20% by mass, with respect to the entire mass of the water-insoluble resin. A content ratio of aromatic rings within the above range provides improved resistance against rubbing.

Specific examples of polymer [1] include, but are not limited to, those described below. Here, “Mw” represents weight average molecular weight.

Phenoxyethyl acrylate/methyl methacrylate/acrylic acid copolymer (50/45/5 [mass %])

Phenoxyethyl acrylate/benzyl methacrylate/isobutyl methacrylate/methacrylic acid copolymer (30/35/29/6 [mass %])

Phenoxyethyl methacrylate/isobutyl methacrylate/methacrylic acid copolymer (50/44/6 [mass %])

Phenoxyethyl acrylate/methyl methacrylate/ethyl acrylate/acrylic acid copolymer (30/55/10/5 [mass %])

Benzyl methacrylate/methyl methacrylate/methacrylic acid copolymer (60/30/10 [mass %])

(M-25/M-27) mixture/ethyl methacrylate/methacrylic acid copolymer (15/75/10 [molar ratio], Mw: 49,400, acid value: 65.2 mgKOH/g)

(M-25)/ethyl methacrylate/methacrylic acid copolymer (18/69/13 [molar ratio], Mw: 41,600, acid value: 84.7 mgKOH/g)

(M-28/M-29) mixture/ethyl methacrylate/methacrylic acid copolymer (15/85/10 [molar ratio], Mw: 38,600, acid value: 65.2 mgKOH/g)

(M-28)/ethyl methacrylate/methacrylic acid copolymer (20/73/7 [molar ratio], Mw: 45,300, acid value: 45.6 mgKOH/g)

[2] Polymer containing structural unit derived from salt-forming group-containing monomer (c), structural unit derived from styrene macromer (d), and/or structural unit derived from hydrophobic monomer (e)

Polymer [2] is a water-insoluble polymer that is preferable from the viewpoint of imparting ejection stability and cleanability. Polymer [2] is more preferably a water-insoluble graft polymer containing a structural unit derived from the styrenic macromer (d). It is preferable that the water-insoluble graft polymer has, at a main chain thereof, a polymer including a structural unit derived from the salt-forming-group-containing monomer (c) and a structural unit derived from the hydrophobic monomer (e), and has, at a side chain thereof, a structural unit derived from the styrenic macromer (d). The water-insoluble polymer is preferably a water-insoluble vinyl polymer obtained by copolymerization of a monomer mixture including the salt-forming-group-containing monomer (c) (hereinafter sometimes referred to as “(c) component”), the styrenic macromer (d) (hereinafter sometimes referred to as “(d) component”), and/or the hydrophobic monomer (e) (hereinafter sometimes referred to as “(e) component”). This monomer mixture is hereinafter sometimes referred to as “monomer mixture”.

(Salt-Forming-Group-Containing Monomer(c))

The salt-forming-group-containing monomer (c) is used with a view to, for example, increasing the dispersion stability of the dispersion obtained. Examples of the salt-forming group include a carboxyl group, a sulfonic acid group, a phosphoric acid group, an amino group, and an ammonium group. Examples of the (c) component include cationic monomers and anionic monomers, specific examples of which include those described in column 7, line 24 to column 8, line 29 on page 5 of JP-A No. 9-286939.

Representative examples of the cationic monomers include unsaturated amino group-containing monomers and unsaturated ammonium salt-containing monomers. From among these monomers, N,N-dimethylaminoethyl (meth)acrylate or N—(N′,N′-dimethylaminopropyl)(meth)acrylamide is preferable.

Representative examples of the anionic monomers include unsaturated carboxylic acid monomers, unsaturated sulfonic acid monomers, and unsaturated phosphoric acid monomers.

Examples of unsaturated carboxylic acid monomers include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, and 2-methacryloyloxymethyl succinic acid.

Examples of unsaturated sulfonic acid monomers include styrenesulfonic acid, 2-acrylamide-2-methylpropanesulfonic acid, 3-sulfopropyl (meth)acrylate, and bis-(3-sulfopropyl)-itaconate.

Examples of unsaturated phosphoric acid monomers include vinyl phosphonic acid, vinyl phosphate, bis(methacryloxyethyl)phosphate, diphenyl-2-acryloyloxyethyl phosphate, diphenyl-2-methacryloyloxyethyl phosphate, and dibutyl-2-acryloyloxyethyl phosphate.

Among the anionic monomers, an unsaturated carboxylic acid monomer is preferable, and acrylic acid or methacrylic acid is more preferable, from the viewpoints of dispersion stability, ejection property, and the like.

(Styrenic Macromer (d))

The styrenic macromer (d) (hereinafter sometimes simply referred to as “macromer”) is used with a view to increasing the affinity for a colorant (particularly, a pigment), thereby increasing the dispersion stability of water-insoluble polymer particles containing a pigment. The styrenic macromer (d) may be, for example, a macromer that is a monomer having a number average molecular weight of from 500 to 100,000, more preferably from 1,000 to 10,000, and having a polymerizable functional group, such as an unsaturated group, at one terminal thereof. The macromer as the (d) component is preferably capable of forming a hydrophobic graft chain, from the viewpoint of increasing the affinity for a pigment.

The number-average molecular weight of the (d) component may be obtained by gel permeation chromatography in which polystyrene is used as a standard substance, and tetrahydrofuran containing 50 mmol/L of acetic acid is used as a solvent.

The term “styrenic macromer” refers to a macromer including a structural unit derived from a styrene-containing monomer such as styrene, α-methylstyrene, or vinyltoluene. From among styrene-containing monomers, styrene is preferable. The styrenic macromer may be, for example, a styrene homopolymer having a polymerizable functional group at one terminal thereof, or a copolymer of styrene and at least one other monomer wherein the copolymer has a polymerizable functional group at one terminal thereof. The polymerizable functional group that is present at one terminal of the macromer is preferably an acryloyloxy group or a methacryloyloxy group. A water-insoluble graft polymer including a structural unit derived from a styrenic macromer can be obtained by copolymerization using the styrenic macromer as a copolymerization component.

The content of structural units derived from styrenic monomers in the styrene macromer is preferably 60% by mass or higher, more preferably 70% by mass or higher, and particularly preferably 90% by mass or higher, from the viewpoint of pigment dispersion property.

The styrene macromer may be a commercially-available product, examples of which include AS-6, AS-6S, AN-6, AN-6S, HS-6, and HS-6S (all of which are trade names, manufactured by To a Gosei Co., Ltd.).

(Hydrophobic Monomer (e))

A hydrophobic monomer (e) may be used from the viewpoints of, for example, improving the dispersion stability of the water-resistant colorant and reducing the amount of free polymers. Examples thereof include an alkyl (meth)acrylate, an alkyl (meth)acrylamide, an aromatic-ring-containing monomer (a monomer that includes an aromatic ring), and a monomer capable of forming a repeating unit represented by the Formula (1)-A or (2) below and compounds thereof.

Examples of the alkyl (meth)acrylate include (meth)acrylic esters having an alkyl group having from 1 to 22 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, (iso)propyl (meth)acrylate, (iso or tertiary-)butyl (meth)acrylate, (iso)amyl (meth)acrylate, cyclohexyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, (iso)octyl (meth)acrylate, (iso)decyl (meth)acrylate, (iso)dodecyl (meth)acrylate, and (iso)stearyl(meth)acrylate.

Examples of the alkyl (meth)acrylamide include (meth)acrylamides having an alkyl group having from 1 to 22 carbon atoms, such as methyl (meth)acrylamide, dimethyl (meth)acrylamide, diethyl (meth)acrylamide, dibutyl (meth)acrylamide, t-butyl (meth)acrylamide, octyl (meth)acrylamide, and dodecyl (meth)acrylamide.

Examples of the aromatic-ring-containing monomer include styrenic monomers such as styrene, 2-methylstyrene, and vinyltoluene; aryl esters of (meth)acrylic acid such as benzyl (meth)acrylate and phenoxyethyl (meth)acrylate, and vinyl monomers having an aromatic hydrocarbon group having from 6 to 22 carbon atoms, such as ethyl vinyl benzene, 4-vinyl biphenyl, 1,1-diphenylethylene, vinyl naphthalene, and chlorostyrene.

The expression “(iso or tertiary-)” as used herein means iso- or tertiary- or normal-. The expression “(iso)” as used herein means iso- or normal-. Furthermore, the scope of “(meth)acrylate” includes both of acrylate and methacrylate.

In Formula (1)-A or (2), R1 represents a hydrogen atom or a substituent. One of R2 to R5 represents a single bond connected to W, and the others of R2 to R5 each independently represent a hydrogen atom or a substituent. J represents *—CO—, *—COO—, *—CONR10—, *—COO—, a methylene group, a phenylene group, or *—C6H4CO—. R10 represents a hydrogen atom, an alkyl group, an aryl group, or an aralkyl group. W represents a single bond or a divalent linking group. A1 represents a heterocyclic group. Q1 represents an atomic group necessary for forming a ring together with the carbon atoms. The * sign represents a site connected to the main chain.

Examples of substituents that R1 to R5 may represent include monovalent substituents. Examples of the monovalent substituents (hereinafter, referred to as substituent Z) include an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, a heterocyclyloxy group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group, an acylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfonylamino group, a sulfamoyl group, a carbamoyl group, an alkylthio group, an arylthio group, a heterocyclylthio group, a sulfonyl group, a sulfinyl group, a ureido group, a phosphoric acid amide group, a hydroxyl group, a mercapto group, a halogen atom, a cyano group, a sulfo group, a carboxyl group, an oxo group, a nitro group, a hydroxamic acid group, a sulfino group, a hydrazino group, an imino group, a heterocyclic group, a silyl group, and a silyloxy group. These groups may themselves be substituted by one or more substituents selected from the above substituents Z.

Among the above, R1 preferably represents a hydrogen atom, an alkyl group (an alkyl group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 10 carbon atoms, such as methyl, ethyl, iso-propyl, tert-butyl, n-octyl, n-decyl, or n-hexadecyl), or an aryl group (an aryl group having preferably from 6 to 30 carbon atoms, more preferably from 6 to 20 carbon atoms, and particularly preferably from 6 to 12 carbon atoms, such as phenyl, p-methylphenyl, naphthyl, or anthranyl). R1 more preferably represents a hydrogen atom or an alkyl group.

One of R2 to R5 represents a single bond connected to W, and the others of R2 to R5 each independently represent preferably a hydrogen atom, an alkyl group (an alkyl group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 10 carbon atoms, such as methyl, ethyl, iso-propyl, tert-butyl, n-octyl, n-decyl, or n-hexadecyl), an aryl group (an aryl group having preferably from 6 to 30 carbon atoms, more preferably from 6 to 20 carbon atoms, and particularly preferably from 6 to 12 carbon atoms, such as phenyl, p-methylphenyl, naphthyl, or anthranyl), an amino group (an amino group having preferably from 0 to 30 carbon atoms, more preferably from 0 to 20 carbon atoms, and particularly preferably from 0 to 10 carbon atoms, such as amino, methylamino, dimethylamino, diethylamino, dibenzylamino, diphenylamino, or ditolylamino), an alkoxy group (an alkoxy group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 10 carbon atoms, such as methoxy, ethoxy, butoxy, or 2-ethylhexyloxy), an aryloxy group (an aryloxy group having preferably from 6 to 30 carbon atoms, more preferably from 6 to 20 carbon atoms, and particularly preferably from 6 to 12 carbon atoms, such as phenyloxy, 1-naphthyloxy, or 2-naphthyloxy), an acyl group (an acyl group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 12 carbon atoms, such as acetyl, benzoyl, formyl, or pivaloyl), an alkoxycarbonyl group (an alkoxycarbonyl group having preferably from 2 to 30 carbon atoms, more preferably from 2 to 20 carbon atoms, and particularly preferably from 2 to 12 carbon atoms, such as methoxycarbonyl or ethoxycarbonyl), an aryloxycarbonyl group (an aryloxycarbonyl group having preferably from 7 to 30 carbon atoms, more preferably from 7 to 20 carbon atoms, and particularly preferably from 7 to 12 carbon atoms, such as phenyloxycarbonyl), an acyloxy group (an acyloxy group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 10 carbon atoms, such as acetoxy or benzoyloxy), an acylamino group (an acylamino group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 10 carbon atoms, such as acetylamino or benzoylamino), an alkoxycarbonylamino group (an alkoxycarbonylamino group having preferably from 2 to 30 carbon atoms, more preferably from 2 to 20 carbon atoms, and particularly preferably from 2 to 12 carbon atoms, such as methoxycarbonylamino), an aryloxycarbonylamino group (an aryloxycarbonylamino group having preferably from 7 to 30 carbon atoms, more preferably from 7 to 20 carbon atoms, and particularly preferably from 7 to 12 carbon atoms, such as phenyloxycarbonylamino), a sulfonylamino group (a sulfonylamino group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 12 carbon atoms, such as methanesulfonylamino or benzenesulfonylamino), a carbamoyl group (a carbamoyl group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 12 carbon atoms, such as carbamoyl, methylcarbamoyl, diethylcarbamoyl, or phenylcarbamoyl), a sulfonyl group (a sulfonyl group having preferably from 1 to 30 carbon atoms, more preferably from 1 to 20 carbon atoms, and particularly preferably from 1 to 12 carbon atoms, such as mesyl or tosyl), a hydroxyl group, a halogen atom (for example, a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom; more preferably, a fluorine atom), a cyano group, a carboxyl group, a nitro group, or a heterocyclic group;

further more preferably a hydrogen atom, an alkyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, an acyl group, an acylamino group, a sulfonylamino group, a carbamoyl group, a sulfonyl group, a hydroxyl group, a halogen atom, or a cyano group, and still further preferably a hydrogen atom, an acyl group, a hydroxyl group, a halogen atom, or a cyano group.

In Formulae (1)-A and (2), J preferably represents *—CO—, *—CONR10—, a phenylene group, or *—C6H4CO—, and more preferably *—C6H4CO—. R10 represents a hydrogen atom, an alkyl group, an aryl group, or an aralkyl group, and preferably represents a hydrogen atom, an alkyl group, or an aryl group. Preferable definitions of the alkyl group and the aryl group are the same as the preferable definitions of the alkyl group and the aryl group described in the explanation of the substituents Z, respectively.

In Formulae (1)-A and (2), W represents a single bond or a divalent linking group.

Examples of the divalent linking group include an imino group, a liner, branched, or cyclic alkylene group (having preferably from 1 to 30 carbon atoms, more preferably from 1 to 12 carbon atoms, and further more preferably from 1 to 4 carbon atoms, examples of which include a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, a hexylene group, an octylene group, and a decylene group), an aralkylene group (having preferably from 7 to 30 carbon atoms, and more preferably from 7 to 13 carbon atoms, examples of which include a benzylidene group and a cinnamylidene group), an arylene group (having preferably from 6 to 30 carbon atoms, and more preferably from 6 to 15 carbon atoms, examples of which include a phenylene group, a cumenylene group, a mesitylene group, a tolylene group, and a xylylene group), *—(CR11R12)nNHCONH—, and *—(CR11R12)nCONH—, in which * indicates a site connected to the main chain, and R11 and R12 each independently represent a hydrogen atom or a substituent, preferably a hydrogen atom, an alkyl group, a halogen atom, or a hydroxyl group, more preferably a hydrogen atom or an alkyl group, and further more preferably a hydrogen atom. When plural R11\'s and R12\'s are present, plural R11\'s may be the same as or different from each other, and plural R12\'s may be the same as or different from each other. Furthermore, n represents a positive integer, preferably an integer of from 1 to 10, and more preferably an integer of from 2 to 5. Among those described above, *—(CR11R12)nNHCONH—, *—(CR11R12)nCONH—, or an imino group is preferable, and an imino group is more preferable.

W preferably represents a single bond, an alkylene group, or an arylene group, and more preferably represents a single bond or an alkylene group. W is even more preferably a single bond. W may further have a substituent, and the definition and specifics of the substituent are the same as those of the substituents Z described above. W may represent a group formed from a combination of at least two of the divalent linking groups described above. It is preferable that W has an ether bond therein.

In Formula (1)-A, A1 represents a heterocyclic group. The “heterocyclic group” or “heterocyclyl group” refers to a monovalent group obtained by removing one hydrogen atom from a heterocyclic compound.

The heterocyclic group represented by A1 is preferably a heterocyclic group capable of forming a colorant (especially, a pigment). The presence of a heterocyclic group having a high affinity for a pigment due to a Van-der-Waals interaction provides the polymer with favorable adsorptivity to pigment, whereby a stable dispersion is obtained.

A heterocyclic compound for forming the heterocyclic group is preferably a compound having at least one hydrogen bonding group in a molecule thereof, and examples thereof include thiophene, furan, xanthene, pyrrole, imidazole, isoindoline, isoindolinone, benzimidazolone, indole, quinoline, carbazole, acridine, acridone, quinacridon, anthraquinone, phthalimide, quinaldine, and quinophthalone. Of these, benzimidazolone, indole, quinoline, carbazole, acridine, acridone, anthraquinone, and phthalimide are particularly preferable.

The heterocyclic group is particularly preferably a heterocyclic group that is similar to the pigment to be used. Specifically, it is particularly preferable to use at least one selected from acridone and anthraquinone when a quinacridon pigment is used. In such a case, adsorption between the water-insoluble polymer and the colorant is strong, and detachment of the polymer from the colorant is suppressed regardless of the kind or amount of solvent used as the ink solvent.

In Formula (2), Q1 represents an atomic group necessary for forming a ring together with the carbon atoms (specifically, the two carbon atoms of —C═C—). The atomic group may consist of carbon, nitrogen, oxygen, silicon, phosphorus, and/or sulfur, preferably carbon, nitrogen, oxygen, and/or sulfur, more preferably carbon, nitrogen, and/or oxygen, and even more preferably carbon and/or nitrogen. Q1 which is formed by the atomic group may be saturated or unsaturated. In a case in which Q1 is capable of being substituted, Q1 may have a substituent. In this case, the definition and specifics of the substituent are the same as the definition and specifics of the substituent Z above.

In Formula (2), examples of the cyclic structure group connected to W (a cyclic structure group constituted by the aryl group having R2 to R5 and Q1) include a cyclic structure group which is represented by any one of the following Formulae (i) to (vi), and which may have a substituent. In Formulae (i) to (vi), the * sign represents a site connected to W. Among them, a cyclic structure group which is represented by the following Formula (i), (ii), or (iii), and which may have a substituent are preferable, and a cyclic structure group represented by the following Formula (i) which may have a substituent is more preferable.

The structural unit (repeating unit) represented by Formula (2) is preferably a repeating unit represented by the following Formula (3).

In Formula (3), R6 to R9 each independently represent a hydrogen atom or a substituent. R1 to R5, J, and W have the same definitions and preferable definitions as R1 to R5, J, and W in Formula (2), respectively.

When any of R6 to R9 represents a substituent, examples of the substituent include those described in the explanation of the substituents Z. R6 to R9 each independently represent preferably a hydrogen atom, an alkyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group, an acylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfonylamino group, a carbamoyl group, a sulfonyl group, a hydroxyl group, a halogen atom, a cyano group, a carboxyl group, a nitro group, or a heterocyclic group; more preferably a hydrogen atom, an alkyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, an acyl group, an acylamino group, a sulfonylamino group, a carbamoyl group, a sulfonyl group, a hydroxyl group, a halogen atom, or a cyano group; furthermore preferably a hydrogen atom, an acyl group, a hydroxyl group, a halogen atom, or a cyano group; and particularly preferably a hydrogen atom.

In the repeating unit represented by Formula (3), the combination of substituents is preferably the following combination (a), (b), (c), or (d), more preferably a combination (b), (c), or (d), even more preferably combination (c) or (d), and particularly preferably combination (d).

Combination (a): J represents *—CO—, *—CONR10—, a phenylene group, or *—C6H4CO—, in which R10 represents a hydrogen atom, an alkyl group, or an aryl group. W represents a single bond, an imino group, an alkylene group, or an arylene group. R1 represents a hydrogen atom, an alkyl group, or an aryl group. R2 to R5 each independently represent a single bond, a hydrogen atom, an alkyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group, acylamino group, alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfonylamino group, a carbamoyl group, a sulfonyl group, a hydroxyl group, a halogen atom, a cyano group, a carboxyl group, a nitro group, or a heterocyclic group, provided that one of R2 to R5 represents a single bond connected to W. R6 to R9 each independently represent a hydrogen atom, an alkyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group, an acylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, a sulfonylamino group, a carbamoyl group, a sulfonyl group, a hydroxyl group, a halogen atom, a cyano group, a carboxyl group, a nitro group, or a heterocyclic group.

Combination (b): J represents *—C6H4CO—, *—CONR10—, or a phenylene group, and R10 represents a hydrogen atom or an alkyl group. W represents an imino group, a single bond, or an arylene group. R1 represents a hydrogen atom or an aryl group. R2 to R5 each independently represent a hydrogen atom, an alkyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, an acyl group, an acylamino group, a sulfonylamino group, a carbamoyl group, a sulfonyl group, a hydroxyl group, a halogen atom, or a cyano group, provided that one of R2 to R5 represents a single bond connected to W. R6 to R9 each independently represent a hydrogen atom, an alkyl group, an aryl group, an amino group, an alkoxy group, an aryloxy group, an acyl group, an acylamino group, a sulfonylamino group, a carbamoyl group, a sulfonyl group, a hydroxyl group, a halogen atom, or a cyano group.

Combination (c): J represents *—C6H4CO— or *—CONR10—, and R10 represents a hydrogen atom. W represents an imino group or a single bond. R1 represents a hydrogen atom or an aryl group. R2 to R5 each independently represent a hydrogen atom, an acyl group, a hydroxyl group, a halogen atom, or a cyano group, provided that one of R2 to R5 represents a single bond connected to W. R6 to R9 each independently represent a hydrogen atom, an acyl group, a hydroxyl group, a halogen atom, or a cyano group.

Combination (d): J represents *—C6H4CO—. W represents an imino group. R1 represents a hydrogen atom or an aryl group. R2 to R5 each independently represent a hydrogen atom, an acyl group, a hydroxyl group, a halogen atom, or a cyano group, provided that one of R2 to R5 represents a single bond connected to W. R6 to R9 each represent a hydrogen atom.

Specific examples of the repeating unit represented by Formula (1)-A include, but are not limited to, those shown below.

Specific examples of the repeating unit represented by Formula (2) include, but are not limited to, those shown below

The (e) component is preferably an aromatic-ring-containing monomer, or a monomer capable of forming a heterocycle-containing repeating unit when used for formation of the water-insoluble polymer, from the viewpoints of improving the dispersibility of a colorant (especially, a pigment) and reducing the amount of free polymer. Among them, the (e) component is more preferably a monomer capable of forming a heterocycle-containing repeating unit when used for the formation of the water-insoluble polymer described above (this monomer is hereinafter referred to as “(e-1) component”), from the viewpoints of, for example, improving the dispersibility of a colorant (especially, a pigment) or reducing the amount of free polymer when used in the black ink composition of the present invention.

The content of the (e-1) component in the (e) component is preferably from 10% by mass to 100% by mass, and more preferably from 20% by mass to 80% by mass, with respect to the total mass of the (e) component, from the viewpoints of, for example, reducing the amount of free polymer, and improving print density and rubbing resistance.

The aromatic-ring-containing monomer is preferably a styrenic monomer, which is hereinafter referred to as “(e-2) component”, and is more preferably styrene or 2-methylstyrene. The content of the (e-2) component in the (e) component is preferably from 10% by mass to 100% by mass, and more preferably from 20% by mass to 80% by mass, with respect to the total mass of the (e) component, from the viewpoints of, for example, improvement in the print density and rubbing resistance.

From the viewpoint of, for example, improvement in the colorant dispersibility, the aromatic-ring-containing monomer is preferable as the (e) component. In particular, an aryl ester of (meth)acrylic acid, which is hereinafter referred to as “(e-3) component”, is preferable, and a (meth)acrylate having an arylalkyl group having from 7 to 22 carbon atoms, preferably from 7 to 18 carbon atoms, and more preferably from 7 to 12 carbon atoms, or a (meth)acrylate having an aryl group having from 6 to 22 carbon atoms, preferably from 6 to 18 carbon atoms, and more preferably from 6 to 12 carbon atoms, is more preferable. Preferable specific examples of such a monomer include benzyl (meth)acrylate and phenoxyethyl (meth)acrylate. The content of the (e-2) component in the (e) component is preferably from 10% by mass to 100% by mass, and more preferably from 20% by mass to 80% by mass, with respect to the total mass of the (e) component, from the viewpoints of improvement in the colorant dispersibility and the like.

The (e) components may be used alone, or in combination of two or more thereof. It is preferable to use a combination of the (e-1) component and the (e-2) component, a combination of the (e-2) component and the (e-3) component, or a combination of the (e-1) component and the (e-3) component. From the viewpoint of reducing free polymer, it is preferable to use a combination of the (e-1) component and the (e-2) component or a combination of the (e-2) component and the (e-3) component, and most preferable to use a combination of the (e-1) component and the (e-2) component.

In the present invention, it is preferable that the monomer mixture containing the above-described (c), (d), and (e) components further contains a hydroxyl group-containing monomer (f) (hereinafter also referred to as an “(f) component”).

The (f) component increases dispersion stability. Examples of the (0 component include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, polyethylene glycol (n=2 to 30; n represents an average addition mol number of oxyalkylene groups, and the same applies hereinbelow) (meth)acrylate, propylene glycol (n=2 to 30) (meth)acrylate, and poly(ethylene glycol (n=1 to 15)-propylene glycol (n=1 to 15)) (meth)acrylate. Of these, 2-hydroxyethyl (meth)acrylate, polyethylene glycol monomethacrylate, and polypropylene glycol methacry late are preferable.

The monomer mixture may include a monomer (g) represented by the following Formula (A). The monomer (g) is hereinafter sometimes referred to as “(g) component”.

CH2═C(R3)COO(R4O)pR5  Formula (A)

In Formula (A), R3 represents a hydrogen atom or an alkyl group having from 1 to 5 carbon atoms; R4 represents a divalent hydrocarbon group which has 1 to 30 carbon atoms and which may have a hetero atom; R5 represents a monovalent hydrocarbon group which has 1 to 30 carbon atoms and which may have a hetero atom; and p represents an average addition mol number and represents a number of from 1 to 60, preferably from 1 to 30.

The (g) component increases ejection stability of an ink composition, and exhibits excellent effects on preventing misdirection of ink droplets even in continuous printing, and the like.

Examples of the hetero atom which may be included in R4 or R5 in Formula (A) include a nitrogen atom, an oxygen atom, a halogen atom, and a sulfur atom.

Representative examples of the group represented by R4 include an aromatic group having from 6 to 30 carbon atoms, a heterocyclic group having from 3 to 30 carbon atoms, and an alkylene group having from 1 to 30 carbon atoms, each of which may have a substituent. Representative examples of the group represented by R5 include an aromatic group having from 6 to 30 carbon atoms, and a heterocyclic group having from 3 to 30 carbon atoms, each of which may have a substituent. R4 may represent a combination of two or more of these groups, and, similarly, R5 may represent a combination of two or more of these groups. Examples of the substituent include an aromatic group, a heterocyclic group, an alkyl group, a halogen atom, and an amino group.

Preferable examples of the group represented by R4 include: a phenylene group that may have a substituent having from 1 to 24 carbon atoms; an aliphatic alkylene group having from 1 to 30 carbon atoms, and more preferably having from 1 to 20 carbon atoms; an alkylene group which has at least one aromatic ring bonded thereto and which has from 7 to 30 carbon atoms (including the carbon atoms of the at least one aromatic ring), and an alkylene group which has at least one heterocycle bonded thereto and which has from 4 to 30 carbon atoms (including the carbon atoms of the at least one heterocycle). Particularly preferable specific examples of the R4O group include an oxyethylene group, an oxy(iso)propylene group, an oxytetramethylene group, an oxyheptamethylene group, an oxyhexamethylene group, an oxyalkylene group which is composed of one or more types of oxyalkylene selected from the above, and which has from 2 to 7 carbon atoms, and an oxyphenylene group.

Specific examples of R5 include a phenyl group, an aliphatic alkyl group having from 1 to 30 carbon atoms (preferably an aliphatic alkyl group which has from 1 to 20 carbon atoms and which may be branched), an aromatic-ring-containing alkyl group having from 7 to 30 carbon atoms, and a heterocycle-containing alkyl group having from 4 to 30 carbon atoms. Examples of more preferable R5 include an alkyl group having from 1 to 12 carbon atoms, such as a methyl group, an ethyl group, an iso-propyl group, a propyl group, an iso-butyl group, a butyl group, an iso-pentyl group, a pentyl group, an iso-hexyl group, or a hexyl group; and a phenyl group.

Specific examples of the (g) component include methoxypolyethyleneglycol(p in the above Formula (A) being from 1 to 30) (meth)acrylate, methoxypolytetramethyleneglycol(p being from 1 to 30) (meth)acrylate, ethoxypolyethyleneglycol(p being from 1 to 30) (meth)acrylate, (iso)propoxypolyethyleneglycol(p being from 1 to 30) (meth)acrylate, butoxypolyethyleneglycol(p being from 1 to 30) (meth)acrylate, octoxypolyethyleneglycol(p being from 1 to 30) (meth)acrylate, methoxypolypropyleneglycol(p being from 1 to 30) (meth)acrylate, and methoxypoly(ethyleneglycol-co-propyleneglycol) (meth)acrylate (wherein p is from 1 to 30 and the number of ethyleneglycol units is from 1 to 29). From among the above, methoxypolyethyleneglycol(p being from 1 to 30) (meth)acrylate is preferable.

Specific examples of the (f) and (g) components include polyfunctional acrylate monomers (NK-ESTER) M-40G, 90G, and 230G (trade names, manufactured by Shin-Nakamura Chemical Co., Ltd.), and BLEMMER series products including PE-90, 200, and 350, PME-100, 200, 400, and 1000, PP-1000, PP-500, PP-800, AP-150, AP-400, AP-550, AP-800, 50PEP-300, 50POEP-800B, and 43PAPE-600B (trade names, manufactured by NOF CORPORATION).

With respect to each of the (a) to (g) components, the component may each be used singly, or in combination of two or more thereof.

The contents of the (c) to (e) components in a monomer mixture are as described below.