JP2024059567A - Water-based ink, ink set, ink-jet recording method, and ink-jet recording apparatus - Google Patents

Abstract

[Problem] To provide a water-based ink capable of recording images that combine high abrasion resistance and high color development. [Solution] A water-based ink for inkjet recording, which contains a coloring material and resin particles. The resin forming the resin particles contains a unit having an aromatic group, a unit having a cyano group, a unit derived from a (meth)acrylic acid ester having a branched alkyl group, and a unit represented by formula (1). In the formula, n is the number of repeating units, R1 is an alkyl group, E is the number of repeating units of an ethylene oxide group, and E≧1. TIFF2024059567000020.tif55170 [Selected Figure] None

Description

The present invention relates to a water-based ink, an ink set, an inkjet recording method, and an inkjet recording device.

In recent years, the use of inkjet recording devices has been increasing in the commercial and industrial printing fields. In these printing fields, the performance of the recorded matter must achieve high abrasion resistance and high color development compared to the so-called home printing field.

One method for achieving abrasion resistance is to use an ink containing resin particles. For example, Patent Document 1 proposes an aqueous ink containing specific resin particles and cyclic amides as a water-soluble organic solvent, with the aim of achieving both abrasion resistance and image clarity of the image. It is proposed to use, as the resin particles, resin particles having a core portion containing a cyano group-containing unit, and a shell portion containing an aromatic group-containing unit, an anionic group-containing unit, and a unit derived from a crosslinking agent, but not containing a cyano group-containing unit.

JP 2021-008604 A

The inventors of the present invention have investigated abrasion resistance using the information disclosed in Patent Document 1. However, they have found that there is room for improvement in order to obtain images that meet the recent demands for higher levels of abrasion resistance and color development.

Therefore, an object of the present invention is to provide an aqueous ink capable of recording images that combine high abrasion resistance with high color development. Another object of the present invention is to provide an ink set, an inkjet recording method, and an inkjet recording device that use this aqueous ink.

That is, according to the present invention, there is provided an aqueous ink for inkjet use that contains a colorant and resin particles, and is characterized in that the resin that forms the resin particles contains a unit having an aromatic group, a unit having a cyano group, a unit derived from a (meth)acrylic acid ester having a branched alkyl group, and a unit represented by the following general formula (1):

（前記一般式（１）中、ｎは繰り返し単位の数を表し、Ｒ_１はアルキル基を表し、Ｅはエチレンオキサイド基の繰り返し単位の数であり、Ｅ≧１である。）

(In the general formula (1), n represents the number of repeating units, _R1 represents an alkyl group, E represents the number of repeating units of an ethylene oxide group, and E≧1.)

According to the present invention, it is possible to provide an aqueous ink capable of recording images that have both high abrasion resistance and high color development. In addition, according to the present invention, it is possible to provide an ink set, an inkjet recording method, and an inkjet recording device that use this aqueous ink.

FIG. 1 is a schematic diagram illustrating an embodiment of an inkjet recording apparatus of the present invention. FIG. 2 is a perspective view showing an example of a liquid application device.

The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is present in the ink in the form of dissociation into ions, but for convenience, it is expressed as "containing a salt". In addition, the aqueous ink for inkjet printing and the reaction liquid may be simply referred to as "ink" and "reaction liquid". Unless otherwise specified, the physical property values are values at room temperature (25°C). When "(meth)acrylic acid" and "(meth)acrylate" are written, they mean "acrylic acid, methacrylic acid" and "acrylate, methacrylate", respectively. In the present invention, the "unit" constituting the resin means a repeating unit derived from one monomer.

The inventors have investigated resin particles that make up water-based inks capable of recording images that combine high abrasion resistance and high color development, both of which are required in the commercial and industrial printing fields, and have discovered the following:

It has become clear that the following configurations (a) and (b) are necessary to achieve high abrasion resistance. (a) The resin forming the resin particles contained in the ink contains a unit having an aromatic group. (b) The resin forming the resin particles contains a unit derived from a (meth)acrylic acid ester having a branched alkyl group.

The present inventors speculate that the mechanism by which high abrasion resistance is achieved by the above configurations (a) and (b) is as follows. Colorants usually have aromatic rings or alicyclic hydrocarbon moieties. First, the units having aromatic groups in the resin forming the resin particles have a rigid structure, and the aromatic rings or alicyclic hydrocarbon moieties of the colorant interact with the units having aromatic groups in the resin forming the resin particles. It is believed that the resin particles and the colorant are close to each other due to this interaction, which increases the hardness of the ink layer formed by the resin particles and the colorant on the image, thereby improving the abrasion resistance of the image. Compared to the case where the units having aromatic groups are not used in the resin forming the resin particles and the units having aromatic groups are replaced with linear units, the difference in the effect of improving abrasion resistance was clear.

Next, it was revealed that the use of resin particles formed from a resin containing units derived from a (meth)acrylic acid ester having a branched alkyl group results in higher abrasion resistance than the use of resin particles in which the branched alkyl group has been changed to a linear alkyl group. This is thought to be because the units derived from a (meth)acrylic acid ester having a branched alkyl group bond to the recording medium at multiple points due to the effect of the branched alkyl group, whereas units having a linear alkyl group bond to the recording medium at fewer points.

It has also become clear that the following configurations (c) and (d) are necessary to achieve high color development. (c) The resin forming the resin particles contains a unit having a cyano group. (d) The resin forming the resin particles contains a unit represented by general formula (1).

The present inventors speculate that the mechanism by which the above structures (c) and (d) achieve high color development is as follows. Colorants usually have polar sites in chromophores, etc. The present inventors have clarified that the interaction between the polar sites of the colorant and units having cyano groups as polar sites of the resin particles moderately disperses the state of the colorant, suppresses localization of the colorant on the image, and improves the color development efficiency of the colorant. Furthermore, they have clarified that the resin forming the resin particles has sulfonic acid groups in the units represented by general formula (1), which reduces the aggregation of resin particles in the ink and immediately after application of the ink to a recording medium, thereby suppressing the decrease in color development due to aggregation.

As described above, by using resin particles that satisfy the above configurations (a) to (d), it is possible to provide an aqueous ink that can record images that have both high abrasion resistance and high color development.

(ink)
The ink of the present invention is a water-based ink for inkjet use that contains resin particles. The resin forming the resin particles contains a unit having an aromatic group, a unit having a cyano group, a unit derived from a (meth)acrylic acid ester having a branched alkyl group, and a unit represented by general formula (1). Each component used in the ink will be described in detail below.

[Colorant]
The ink contains a coloring material. As the coloring material contained in the ink, a pigment or a dye can be used, and colored particles in which the pigment or dye is encapsulated in a resin particle can also be used. The ink can contain one or more types of coloring materials. The coloring material may be dissolved or dispersed in the aqueous ink, or may be encapsulated in a resin particle. Among them, a coloring material that is dispersed by the action of a carboxylic acid group is preferred. The content (mass %) of the coloring material in the ink is preferably 0.5% by mass or more and 15.0% by mass or less, and more preferably 1.0% by mass or more and 10.0% by mass or less, based on the total mass of the ink.

Specific examples of pigments include inorganic pigments such as carbon black and titanium oxide; and organic pigments such as azo, phthalocyanine, quinacridone, isoindolinone, imidazolone, diketopyrrolopyrrole, and dioxazine. One type of pigment may be used alone, or two or more types may be used in combination.

As the method of dispersing the pigment, it is possible to use resin-dispersed pigments using a resin as a dispersant, or self-dispersed pigments in which hydrophilic groups are bonded to the pigment particle surface. In addition, it is possible to use resin-bonded pigments in which an organic group containing a resin is chemically bonded to the pigment particle surface, or microcapsule pigments in which the pigment particle surface is coated with a resin or the like. It is also possible to use a combination of pigments with different dispersion methods among these. In the present invention, it is preferable to use resin-dispersed pigments in which a resin as a dispersant is physically adsorbed onto the pigment particle surface, or self-dispersed pigments in which hydrophilic groups are bonded to the pigment particle surface, rather than resin-bonded pigments or microcapsule pigments.

As a resin dispersant for dispersing a pigment in an aqueous medium, it is preferable to use one that can disperse the pigment in an aqueous medium by the action of anionic groups. As a resin dispersant, a resin having an anionic group can be used, and it is preferable to use a resin such as that described below, especially a water-soluble resin. Furthermore, a pigment that is dispersed by the action of a water-soluble resin having a carboxylic acid group is preferable. The content (mass%) of the pigment in the ink is preferably 0.3 to 10.0 times the content (mass%) of the resin dispersant in terms of mass ratio.

As the self-dispersing pigment, a pigment having an anionic group such as a carboxylic acid group, a sulfonic acid group, or a phosphonic acid group bonded to the surface of the pigment particles directly or via another atomic group (-R-) can be used. Among them, a pigment having a carboxylic acid group bonded to the surface of the pigment particles directly or via another atomic group (-R-) is preferred. The anionic group may be either an acid type or a salt type, and in the case of a salt type, it may be either a partially dissociated state or a completely dissociated state. In the case of the anionic group being a salt type, examples of the cation serving as the counter ion include an alkali metal cation, ammonium, and organic ammonium. Specific examples of the other atomic group (-R-) include a linear or branched alkylene group having 1 to 12 carbon atoms; an arylene group such as a phenylene group or a naphthylene group; a carbonyl group; an imino group; an amide group; a sulfonyl group; an ester group; and an ether group. In addition, a group formed by combining these groups may be used. When a self-dispersing pigment is used as a colorant, it is preferable to use one in which an anionic group (preferably a carboxylic acid group) is bonded to the particle surface of the pigment via another atomic group. In particular, a self-dispersing pigment in which -C ₆ H ₃ -(COOM) ₂ (phthalic acid structure) is bonded to the particle surface of the pigment is particularly preferable.

It is preferable to use a dye having an anionic group. Specific examples of dyes include azo, triphenylmethane, (aza)phthalocyanine, xanthene, and anthrapyridone dyes. A single type of dye may be used alone, or two or more types may be used in combination. The coloring material is preferably a pigment, and more preferably a resin-dispersed pigment or a self-dispersed pigment.

[Resin particles]
The ink contains resin particles. In this specification, the term "resin particles" refers to a resin that can be dispersed in an aqueous medium and exist in the aqueous medium in a state of having a particle size. Therefore, the resin particles exist in a dispersed state in the ink, that is, in a state of a resin emulsion. In addition, the resin particles may contain a coloring material (such as a dye, a pigment, or an invisible coloring material that develops color by fluorescence, etc.) or may not contain a coloring material. The resin particles may be of a single layer or may be formed of multiple layers. Among them, it is preferable that the resin particles are of a single layer, that is, that they do not have a core-shell structure.

Whether or not a certain resin is a "resin particle" can be determined according to the following method. First, prepare a liquid (resin solid content: 10% by mass) containing a resin neutralized with an alkali (sodium hydroxide, potassium hydroxide, etc.) equivalent to the acid value. Next, dilute the prepared liquid 10 times (volume basis) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by dynamic light scattering, if particles having a particle size are measured, the resin can be determined to be a "resin particle". A particle size analyzer (for example, product name "UPA-EX150", manufactured by Nikkiso) can be used as a particle size distribution measurement device using the dynamic light scattering method. The measurement conditions at this time can be, for example, SetZero: 30 seconds, number of measurements: 3 times, measurement time: 180 seconds, shape: spherical, refractive index: 1.59. Of course, the particle size distribution measurement device and measurement conditions used are not limited to those described above. The particle size is measured using neutralized resin in order to confirm that particles are formed even when the resin is sufficiently neutralized to make it more difficult to form particles. Even under these conditions, resins that have a particulate shape will remain in particulate form in the water-based ink.

The resin forming the resin particles contains a unit having an aromatic group, a unit having a cyano group, a unit derived from a (meth)acrylic acid ester having a branched alkyl group, and a unit represented by general formula (1). The proportion (mass %) of the resin containing the above four types of units in the resin forming the resin particles is preferably 50.00 mass % or more, and more preferably 100.00 mass %, based on the total mass of the resin. In other words, it is preferable that the resin particles are substantially formed only from the resin containing the above four types of units.

The resin forming the resin particles contains a unit having an aromatic group. As a monomer that becomes a unit having an aromatic group by polymerization, it is preferable that the monomer has an aromatic group and one polymerizable functional group such as an ethylenically unsaturated bond in the molecule, and an ethylenically unsaturated monomer having an aromatic group is more preferable. Therefore, it is more preferable that the unit having an aromatic group is a unit derived from an ethylenically unsaturated monomer having an aromatic group.

Examples of aromatic groups include phenyl, biphenyl, naphthyl, anthryl, carbazole, triazinyl, and pyrenyl groups, each of which may have a substituent. Examples of the substituent include alkyl groups and halogen atoms. Among these aromatic groups, unsubstituted phenyl groups and phenyl groups substituted with at least one type of substituent selected from the group consisting of alkyl groups and halogen atoms are preferred.

Examples of monomers that form units having an aromatic group include styrene, 4-methylstyrene, p-fluorostyrene, p-chlorostyrene, α-methylstyrene, 2-vinylnaphthalene, 9-vinylanthracene, 9-vinylcarbazole, phenyl(meth)acrylate, benzyl(meth)acrylate, 2-phenoxyethyl(meth)acrylate, 2,4-diamino-6-((meth)acryloyloxy)ethyl-1,3,5-triazine, 2-naphthyl(meth)acrylate, 9-anthryl(meth)acrylate, and (1-pyrenyl)methyl(meth)acrylate. These may be used alone or in combination of two or more. Of these, styrene, 4-methylstyrene, 2-chlorostyrene, p-fluorostyrene, and p-chlorostyrene are preferred.

Since it is possible to further improve the scratch resistance of the image, the unit having an aromatic group is preferably a unit represented by the following general formula (2). In general formula (2), R ₂ represents a substituent in the phenyl group, and y represents the number of the substituents. The alkyl group of R ₂ in general formula (2) can be a linear or branched alkyl group having about 1 to 6 carbon atoms. In addition, examples of the halogen atom include a fluorine atom, a chlorine atom, and a bromine atom. y is an integer of 0 to 5, preferably 0 or 1, and more preferably 0. When y is 0, the unit represented by general formula (2) has an unsubstituted phenyl group. Specifically, the unit represented by general formula (2) is more preferably a unit derived from at least one monomer selected from the group consisting of styrene, 4-methylstyrene, and 2-chlorostyrene.

（前記一般式（２）中、ｘは繰り返し単位であり、Ｒ_２はそれぞれ独立に、アルキル基、又はハロゲン原子を表し、ｙは０乃至５の整数である。）

(In the general formula (2), x is a repeating unit, each _R2 independently represents an alkyl group or a halogen atom, and y is an integer of 0 to 5.)

The proportion (mass %) of units having aromatic groups in the resin forming the resin particles is preferably 20.0 mass % or more and 50.0 mass % or less, and more preferably 30.0 mass % or more and 45.0 mass % or less, based on the total mass of the resin. When the above proportion (mass %) of units having aromatic groups is 20.0 mass % or more, the strength of the ink layer is appropriately increased, and the scratch resistance of the image can be further improved. On the other hand, when the above proportion (mass %) of units having aromatic groups is 50.0 mass % or less, the viscosity of the ink layer on the recording medium to which the ink is applied is appropriately maintained, and the scratch resistance of the image can be further improved.

The resin forming the resin particles contains a unit having a cyano group (also called a nitrile group). As a monomer that becomes a unit having a cyano group by polymerization, it is preferable that the monomer has a cyano group and one polymerizable functional group such as an ethylenically unsaturated bond in the molecule, and an ethylenically unsaturated monomer having a cyano group is more preferable. Therefore, it is more preferable that the unit having a cyano group is a unit derived from an ethylenically unsaturated monomer having a cyano group.

Examples of monomers that form a unit having a cyano group include acrylonitrile, methacrylonitrile, chloroacrylonitrile, and 2-cyanoethyl (meth)acrylate. These may be used alone or in combination of two or more. Acrylonitrile and methacrylonitrile are preferred because they can further enhance the color development of the image. In other words, the unit having a cyano group is preferably a unit derived from at least one monomer selected from the group consisting of acrylonitrile and methacrylonitrile. Note that the unit and monomer having both a cyano group and an aromatic group correspond to the above-mentioned unit having an aromatic group and the monomer that forms the unit, respectively.

The proportion (mass %) of units having a cyano group in the resin forming the resin particles is preferably 5.0 mass % or more and 50.0 mass % or less, and more preferably 10.0 mass % or more and 25.0 mass % or less, based on the total mass of the resin. When the above proportion (mass %) of units having a cyano group is 5.0 mass % or more, the effect of suppressing localization of the coloring material is easily produced, and the color development of the image can be further improved. On the other hand, when the above proportion (mass %) of units having a cyano group is 50.0 mass % or less, the localization of the coloring material caused by the coloring material being too densely packed around the resin particles can be suppressed, and the color development of the image can be further improved.

The resin forming the resin particles contains units derived from a (meth)acrylic acid ester having a branched alkyl group. The branched alkyl group preferably has 3 to 10 carbon atoms, more preferably 4 to 8 carbon atoms. Examples of branched alkyl groups include an isopropyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, and a 2-ethylhexyl group.

Examples of (meth)acrylic acid esters having a branched alkyl group include isobutyl (meth)acrylate, sec-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. These may be used alone or in combination of two or more. 2-ethylhexyl acrylate is preferred because it is possible to further improve the scratch resistance of the image, and the unit derived from a (meth)acrylic acid ester having a branched alkyl group is preferably a unit derived from 2-ethylhexyl acrylate. Note that a (meth)acrylic acid ester having both a branched alkyl group and an aromatic group corresponds to a monomer that forms a unit having an aromatic group as described above. Also, a (meth)acrylic acid ester having both a branched alkyl group and a cyano group corresponds to a monomer that forms a unit having a cyano group as described above.

The proportion (mass%) of units derived from (meth)acrylic acid esters having branched alkyl groups in the resin forming the resin particles is preferably 10.0% by mass or more and 50.0% by mass or less, based on the total mass of the resin. The above proportion (mass%) of units derived from (meth)acrylic acid esters having branched alkyl groups is more preferably 10.0% by mass or more and 40.0% by mass or less, and even more preferably 20.0% by mass or more and 35.0% by mass or less. When the above proportion (mass%) of units derived from (meth)acrylic acid esters having branched alkyl groups is 10.0% by mass or more, the adhesion of the resin particles to the recording medium is easily increased, and the scratch resistance of the image can be further improved. On the other hand, when the above proportion (mass%) of units derived from (meth)acrylic acid esters having branched alkyl groups is 50.0% by mass or less, the aggregation of the resin particles due to the entanglement of the alkyl groups is appropriately suppressed, and the color development of the image can be further improved.

The resin forming the resin particles contains a unit represented by the following general formula (1): In the following general formula (1), n represents the number of repeating units, _R1 represents an alkyl group, E represents the number of repeating units of an ethylene oxide group, and E≧1.

As a monomer that becomes a unit represented by general formula (1) by polymerization, a compound represented by the following general formula (3) can be used. One type of compound represented by general formula (3) may be used alone, or two or more types may be used in combination. _R1 and E in the following general formula (3) have the same meanings as those in general formula (1).

The compound represented by the general formula (3) has a vinyl group, which is a polymerizable functional group, bonded to the end of the molecule. The hydrophilic portion of the compound represented by the general formula (3) is an esterified portion (sulfuric acid ester; -O-SO ₃ ^- ) of polyoxyethylene (-(C ₂ H ₄ O) _E -) and sulfonic acid (-SO ₃ ^- ). The hydrophobic portion of the compound represented by the general formula (3) is an alkyl group represented by R _1. The hydrophilic portion and the hydrophobic portion are bonded via an ether group (-O-). A specific example of a monomer that can be the unit represented by the general formula (1) is a salt of a sulfate esterification reaction product of {a reaction product of 2-[(allyloxy)methyl]oxirane and an alkanol}·oxirane polyadduct) containing α-[1-(alkyloxy)-3-(allyloxy)propan-2-yl]-ω-hydroxypoly(oxyethylene) as the main component].

The alkyl group represented by _R1 in general formula (1) may be either a linear alkyl group or a branched alkyl group, and preferably has 9 to 14 carbon atoms. When the number of carbon atoms is 9 or more, the steric hindrance of the alkyl group easily alleviates the aggregation of the resin particles in the ink and immediately after application of the ink to a recording medium, and the color development of the image can be further improved. On the other hand, when the number of carbon atoms is 14 or less, the hydrophobicity of the alkyl group is appropriately suppressed, so that the aggregation of the resin particles is alleviated and the color development of the image can be further improved.

The number of repeating units of the ethylene oxide group represented by E in general formula (1) is preferably 5 or more and 30 or less. By having the number of repeating units of the ethylene oxide group of 5 or more, the hydrophilicity of the resin particles is appropriately maintained, which makes it possible to alleviate the aggregation of the resin particles and further enhance the color development of the image. On the other hand, by having the number of repeating units of the ethylene oxide group of 30 or less, it is easy to alleviate the aggregation of the resin particles in the ink and immediately after the ink is applied to the recording medium, which makes it possible to further enhance the color development of the image.

The proportion (mass %) of the unit represented by general formula (1) in the resin forming the resin particles is preferably 1.0 mass % or more and 10.0 mass % or less, and more preferably 2.0 mass % or more and 9.0 mass % or less, based on the total mass of the resin. When the proportion (mass %) of the unit represented by general formula (1) is 1.0 mass % or more, it is possible to alleviate the aggregation of the resin particles in the ink and immediately after the ink is applied to the recording medium, and to further improve the color development of the image. In addition, when the proportion (mass %) of the unit represented by general formula (1) is 10.0 mass % or less, the hydrophilicity of the resin particles is appropriately suppressed, and the scratch resistance of the image can be further improved.

The resin forming the resin particles may contain units (other units) other than the units described above, as long as the effects of the present invention are not impaired. Examples of other units include hydrophilic units and hydrophobic units other than the units described above.

The hydrophilic unit is a unit having a hydrophilic group such as an anionic group. Examples of the anionic group include a carboxylic acid group, a phenolic hydroxy group, and a phosphate ester group. As a monomer that becomes a unit having a hydrophilic group by polymerization, a monomer having an anionic group and one polymerizable functional group such as an ethylenically unsaturated bond in the molecule is preferable, and an ethylenically unsaturated monomer having an anionic group is more preferable. One type of such monomer may be used alone, or two or more types may be used in combination.

The proportion (mass %) of hydrophilic units as other units in the resin forming the resin particles is preferably 1.0 mass % or more and 15.0 mass % or less, and more preferably 2.0 mass % or more and 10.0 mass % or less, based on the total mass of the resin. When the above proportion (mass %) of hydrophilic units is 1.0 mass % or more, the ejection characteristics of the ink tend to be improved. On the other hand, when the above proportion (mass %) of units having an anionic group is 15.0 mass % or less, the hydrophilicity of the resin particles is appropriately suppressed, and the scratch resistance of the image tends to be improved.

Among the hydrophilic units, resin particles formed of a resin containing a unit having a carboxylic acid group are preferred. The unit having a carboxylic acid group is a unit formed by polymerizing a monomer having a carboxylic acid group. As a monomer having a carboxylic acid group, a monomer having a carboxylic acid group and one polymerizable functional group such as an ethylenically unsaturated bond in the molecule is preferred, and an ethylenically unsaturated monomer having a carboxylic acid group is more preferred. Therefore, it is more preferred that the unit having a carboxylic acid group is a unit derived from an ethylenically unsaturated monomer having a carboxylic acid group. The carboxylic acid group may be an anhydride or a salt. Examples of monomers forming a unit having a carboxylic acid group include acidic monomers having a carboxylic acid group such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid; anionic monomers such as anhydrides and salts of these acidic monomers; and the like. Examples of cations constituting the salt of an acidic monomer include ions of lithium, sodium, potassium, ammonium, and organic ammonium. The proportion (mass %) of units having carboxylic acid groups in the resin forming the resin particles is preferably 1.0 mass % or more and 10.0 mass % or less, and more preferably 2.0 mass % or more and 9.0 mass % or less, based on the total mass of the resin.

The hydrophobic unit is a unit that does not have a hydrophilic group such as an anionic group. The hydrophobic unit can be formed, for example, by polymerizing a hydrophobic monomer that does not have a hydrophilic group such as an anionic group. Specific examples of hydrophobic monomers include (meth)acrylic acid ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate. One type of hydrophobic monomer may be used alone, or two or more types may be used in combination.

The proportion (mass %) of hydrophobic units as other units in the resin forming the resin particles is preferably 1.0 mass % or more and 40.0 mass % or less, based on the total mass of the resin, and more preferably 2.0 mass % or more and 20.0 mass % or less.

The ratio W1 (mass%) of units having aromatic groups in the resin forming the resin particles and the ratio W2 (mass%) of units represented by general formula (1) preferably have a relationship of 4.0≦W1/W2≦40.0. It is even more preferable that the above W1 and W2 have a relationship of 6.5≦W1/W2≦10.0. When the value of W1/W2 is 4.0 times or more, it becomes easier to exhibit high abrasion resistance while maintaining high color development performance. On the other hand, when the value of W1/W2 is 40.0 times or less, it becomes easier to exhibit high color development while maintaining high abrasion resistance.

The content (mass %) of resin particles in the ink is preferably 1.0% by mass or more and 15.0% by mass or less, and more preferably 1.0% by mass or more and 12.0% by mass or less, based on the total mass of the ink. If the content of resin particles is 1.0% by mass or more, the scratch resistance of the image tends to be improved. On the other hand, if the content of resin particles is 15.0% by mass or less, the color development of the image tends to be improved.

The glass transition temperature (°C) of the resin particles is preferably 40°C or higher and 110°C or lower. When the glass transition temperature of the resin particles is 40°C or higher, the blocking resistance of the image is likely to be increased. On the other hand, when the glass transition temperature of the resin particles is 110°C or lower, the adhesion of the resin particles to the recording medium is strong, and the scratch resistance of the image is likely to be increased. The glass transition temperature of the resin particles is more preferably 45°C or higher and 90°C or lower, and even more preferably 50°C or higher and 80°C or lower. The glass transition temperature of the resin particles can be measured using a differential scanning calorimeter (for example, the product name "DSC Q1000" manufactured by TA Instruments, etc.).

The acid value of the resin forming the resin particles is preferably 5 mgKOH/g or more and 100 mgKOH/g or less. The weight average molecular weight of the resin forming the resin particles is preferably 1,000 or more and 2,000,000 or less. The volume average particle diameter of the resin particles measured by dynamic light scattering (50% cumulative particle diameter D50 based on volume) is preferably 50 nm or more and 500 nm or less, and more preferably 50 nm or more and 200 nm or less. The 50% cumulative particle diameter based on volume of the resin particles is the diameter of the particles that is 50% of the total volume of the measured particles when integrated from the small particle diameter side in the particle diameter integration curve. The volume average particle diameter of the resin particles can be measured using the dynamic light scattering particle size analyzer and measurement conditions described above.

[Method for producing resin particles]
The resin particles can be produced according to a conventionally known method, such as an emulsion polymerization method, a mini-emulsion polymerization method, a seed polymerization method, or a phase inversion emulsification method.

[Method for verifying resin particles]
The composition of the resin particles can be verified according to the following methods (i) and (ii). Below, a method for extracting resin particles from ink and analyzing and verifying them will be described, but resin particles extracted from an aqueous dispersion or the like can also be analyzed and verified in the same manner.

(i) Extraction of Resin Particles By density gradient centrifugation, resin particles can be separated and extracted from ink containing resin particles. Among density gradient centrifugation methods, density gradient sedimentation velocity method separates and extracts resin particles based on the difference in sedimentation coefficient of components. Also, among density gradient centrifugation methods, density gradient sedimentation equilibrium method separates and extracts resin particles based on the difference in density of components.

(ii) Analysis of Units (Monomers) Constituting Resin The resin particles to be used as a sample may be in the form of a dispersion liquid. Alternatively, the resin particles may be dried to form a film to be used as a sample. The sample resin is subjected to elemental analysis by a combustion method. Separately, the resin fractionated by an acid decomposition (addition of hydrofluoric acid) method or an alkali fusion method is pretreated, and then the inorganic components are quantitatively analyzed by inductively coupled plasma atomic emission spectrometry. Furthermore, the type and ratio of the units (monomers) constituting the resin can be known by nuclear magnetic resonance (NMR) spectroscopy.

[Water-soluble resin]
The ink may contain a water-soluble resin that is soluble in an aqueous medium. The content (mass %) of the water-soluble resin in the ink is preferably 0.1 mass % or more and 20.0 mass % or less, and more preferably 0.5 mass % or more and 15.0 mass % or less, based on the total mass of the ink.

Water-soluble resins can be added to inks (i) to stabilize the dispersion state of pigments, i.e., as resin dispersants or their assistants. They can also be added to inks (ii) to improve various properties of the images to be printed. Examples of the form of water-soluble resins include block copolymers, random copolymers, graft copolymers, and combinations of these.

[Composition of Water-Soluble Resin]
Examples of the water-soluble resin include acrylic resins, urethane resins, olefin resins, etc. Among them, acrylic resins and urethane resins are preferred, and acrylic resins composed of units derived from (meth)acrylic acid or (meth)acrylate are more preferred.

As the acrylic resin, one having a hydrophilic unit and a hydrophobic unit as constituent units is preferable. Among them, a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one of a monomer having an aromatic ring and a (meth)acrylic acid ester monomer is preferable. In particular, a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one of styrene and α-methylstyrene is preferable. These resins are easily interacted with pigments, and therefore can be suitably used as a resin dispersant for dispersing pigments.

The hydrophilic unit is a unit having a hydrophilic group such as an anionic group. The hydrophilic unit can be formed, for example, by polymerizing a hydrophilic monomer having a hydrophilic group. Specific examples of hydrophilic monomers having a hydrophilic group include acidic monomers having a carboxylic acid group such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid, and anionic monomers such as anhydrides and salts of these acidic monomers. Examples of cations constituting the salts of acidic monomers include ions of lithium, sodium, potassium, ammonium, and organic ammonium. The hydrophobic unit is a unit that does not have a hydrophilic group such as an anionic group. The hydrophobic unit can be formed, for example, by polymerizing a hydrophobic monomer that does not have a hydrophilic group such as an anionic group. Specific examples of hydrophobic monomers include monomers having an aromatic ring such as styrene, α-methylstyrene, and benzyl (meth)acrylate; and (meth)acrylic acid ester monomers such as methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

Urethane-based resins can be obtained, for example, by reacting polyisocyanate with polyol. They may also be obtained by further reacting with a chain extender. Examples of olefin-based resins include polyethylene and polypropylene.

[Properties of water-soluble resin]
In this specification, "a resin is water-soluble" means that when the resin is neutralized with an alkali equivalent to the acid value, the resin is present in an aqueous medium in a state in which no particles whose particle size can be measured by a dynamic light scattering method are formed. Whether or not a resin is water-soluble can be determined according to the following method. First, a liquid (resin solid content: 10 mass%) containing a resin neutralized with an alkali (sodium hydroxide, potassium hydroxide, etc.) equivalent to the acid value is prepared. Next, the prepared liquid is diluted 10 times (volume basis) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by a dynamic light scattering method, if no particles having a particle size are measured, the resin can be determined to be water-soluble. The measurement conditions at this time can be, for example, SetZero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds. In addition, as a particle size distribution measurement device, a particle size analyzer using a dynamic light scattering method (for example, the product name "UPA-EX150", manufactured by Nikkiso) can be used. Of course, the particle size distribution measurement device and measurement conditions used are not limited to those described above.

The acid value of the water-soluble resin is preferably 100 mgKOH/g or more and 250 mgKOH/g or less. The weight average molecular weight of the water-soluble resin is preferably 3,000 or more and 15,000 or less.

[Wax particles]
The ink may contain particles (wax particles) formed of wax. By using an ink containing wax particles, an image with further improved abrasion resistance can be recorded. In this specification, the wax may be a composition containing components other than wax, or the wax itself. The wax particles may be dispersed by a dispersant such as a surfactant or a resin. The wax may be used alone or in combination of two or more kinds. The content (mass %) of the wax particles in the ink is preferably 0.1 mass % or more and 10.0 mass % or less, and more preferably 1.0 mass % or more and 5.0 mass % or less, based on the total mass of the ink.

In the narrow sense, wax is an ester of a water-insoluble higher monohydric or dihydric alcohol and a fatty acid, and includes animal waxes and vegetable waxes, but does not include oils and fats. In the broad sense, wax includes high melting point fats, mineral waxes, petroleum waxes, and blends and modified products of various waxes. In the present invention, any wax in the broad sense can be used without particular restrictions. Wax in the broad sense can be classified into natural waxes, synthetic waxes, blends of these (blended waxes), and modified products of these (modified waxes).

Examples of natural waxes include animal waxes such as beeswax, spermaceti, and wool wax (lanolin); vegetable waxes such as wood wax, carnauba wax, sugarcane wax, palm wax, candelilla wax, and rice wax; mineral waxes such as montan wax; and petroleum waxes such as paraffin wax, microcrystalline wax, and petrolatum. Examples of synthetic waxes include hydrocarbon waxes such as Fischer-Tropsch wax and polyolefin wax (e.g., polyethylene wax and polypropylene wax). The blended wax is a mixture of the above-mentioned various waxes. The modified wax is a wax obtained by subjecting the above-mentioned various waxes to a modification treatment such as oxidation, hydrogenation, alcohol modification, acrylic modification, and urethane modification. One of the above waxes may be used alone, or two or more may be used in combination. The wax is preferably at least one selected from the group consisting of microcrystalline wax, Fischer-Tropsch wax, polyolefin wax, paraffin wax, and modified or blended products thereof. Among these, a blend of multiple types of wax is more preferable, and a blend of petroleum-based wax and synthetic wax is especially preferable.

The wax is preferably solid at room temperature (25°C). The melting point (°C) of the wax is preferably 40°C or higher and 120°C or lower, and more preferably 50°C or higher and 100°C or lower. The melting point of the wax can be measured in accordance with the test method described in 5.3.1 (melting point test method) of JIS K2235:1991 (petroleum wax). In the case of microcrystalline wax, petrolatum, and a mixture of multiple types of wax, the test method described in 5.3.2 can be used to measure more accurately. The melting point of the wax is easily affected by characteristics such as molecular weight (the higher the molecular weight, the higher the melting point), molecular structure (the higher the melting point if the chain is straight, and the lower the melting point if the chain is branched), crystallinity (the higher the crystallinity, the higher the melting point), and density (the higher the crystallinity, the higher the melting point). Therefore, by controlling these characteristics, it is possible to obtain a wax having a desired melting point. The melting point of the wax in the ink can be measured in accordance with the above test method, for example, after the ink is subjected to ultracentrifugation, the wax separated is washed and dried.

[Aqueous medium]
The ink is an aqueous ink containing at least water as an aqueous medium. The ink can contain water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. As the water, deionized water or ion-exchanged water is preferably used. The content (mass%) of water in the aqueous ink is preferably 50.0 mass% or more and 95.0 mass% or less based on the total mass of the ink. In addition, the content (mass%) of the water-soluble organic solvent in the aqueous ink is preferably 2.0 mass% or more and 40.0 mass% or less based on the total mass of the ink. As the water-soluble organic solvent, any of those usable for inkjet inks, such as alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing compounds, and sulfur-containing compounds, can be used. The water-soluble organic solvent may be used alone or in combination of two or more.

[Polyethylene glycol having 4 or more ethylene oxide groups]
The ink preferably contains, as a water-soluble organic solvent, polyethylene glycol (sometimes abbreviated as "PEG") having a number of ethylene oxide groups (n2; sometimes abbreviated as "EO number") of 4 or more. Furthermore, it is preferable that the number of ethylene oxide groups (sometimes abbreviated as "n1") of the unit represented by general formula (1) in the resin forming the resin particles and the EO number (n2) of the polyethylene glycol satisfy the relationship 0.2≦n1/n2≦5.0. By satisfying these conditions, the interaction between the resin particles and the polyethylene glycol is enhanced, and by these being present in the vicinity of each other, the polyethylene glycol promotes the film formation of the resin particles, and it is possible to further improve the scratch resistance of the image.

Normally, "water-soluble organic solvent" means a liquid, but in the present invention, polyethylene glycol, which is solid at 25°C (room temperature), is also included in the water-soluble organic solvent. Polyethylene glycol becomes solid as the number average molecular weight increases. For example, polyethylene glycol with a number average molecular weight of 600 is liquid at 25°C, and polyethylene glycol with a number average molecular weight of 1,000 is solid at 25°C. The content (mass %) of polyethylene glycol with 4 or more ethylene oxide groups in the ink is preferably 1.0 mass % or more and 20.0 mass % or less based on the total mass of the ink.

The number of ethylene oxide groups (n2) of the polyethylene glycol is preferably 4 or more and 45 or less, and more preferably 6 or more and 45 or less. By having the number of ethylene oxide groups (n2) of 4 or more and 45 or less, the interaction with the resin particles is enhanced, which enhances the film-forming promoting effect on the resin particles, and it is possible to further improve the scratch resistance of the image.

When the resin particles described above and polyethylene glycol having 4 or more ethylene oxide groups coexist, it is preferable to do as follows. That is, the alkyl group represented by R ₁ in the general formula (1) may be either a linear alkyl group or a branched alkyl group, and the number of carbon atoms is preferably 9 to 14. By having 9 or more carbon atoms, it is possible to suppress the resin particles from becoming excessively hydrophilic when interacting with polyethylene glycol, and it is possible to improve the scratch resistance of the image. On the other hand, by having 14 or less carbon atoms, the steric hindrance of the alkyl group is not significantly generated, so that interaction with polyethylene glycol is easily generated, and the film formation promoting action of the resin particles is generated, and it is possible to improve the scratch resistance of the image.

When the resin particles described above and polyethylene glycol having 4 or more ethylene oxide groups coexist, it is preferable to do as follows. That is, the number of repeating units of the ethylene oxide group represented by E in general formula (1) (n1) is preferably 5 or more and 30 or less. When the number of repeating units of the ethylene oxide group is 5 or more, interaction with polyethylene glycol is easily generated, and the film formation promoting effect of the resin particles is generated, and it is possible to increase the scratch resistance of the image. On the other hand, when the number of repeating units of the ethylene oxide group is 30 or less, it is possible to suppress the resin particles from becoming excessively hydrophilic when interacting with polyethylene glycol, and it is possible to increase the scratch resistance of the image.

When the resin particles described above and polyethylene glycol having 4 or more ethylene oxide groups coexist, it is preferable to do as follows. That is, the ratio (mass%) of the units represented by general formula (1) in the resin particles is preferably 1.0 mass% or more and 10.0 mass% or less based on the total mass of the resin. The above ratio is more preferably 2.0 mass% or more and 9.0 mass% or less. When the above ratio (mass%) of the units represented by general formula (1) is 1.0 mass% or more, the resin particles are more likely to interact with polyethylene glycol, and the scratch resistance of the image can be further improved. On the other hand, when the above ratio (mass%) of the units represented by general formula (1) is 10.0 mass% or less, the hydrophilicity of the resin particles is appropriately suppressed, and the scratch resistance of the image can be further improved.

When the resin particles described above and polyethylene glycol having 4 or more ethylene oxide groups coexist, it is preferable to do as follows. That is, the content (mass%) of the resin particles in the ink is preferably 0.3 to 5.0 times the content (mass%) of the polyethylene glycol having 4 or more ethylene oxide groups. When the mass ratio is 0.3 or more, the strength of the ink layer is less likely to decrease due to excess polyethylene glycol, and the scratch resistance of the image can be further improved. On the other hand, when the mass ratio is 5.0 or less, the moderate presence of polyethylene glycol makes it easier for the resin particles to promote film formation, and the scratch resistance of the image can be further improved.

[Water-soluble hydrocarbon compounds]
The water-soluble organic solvent contained in the ink preferably contains a specific water-soluble hydrocarbon compound. This water-soluble hydrocarbon compound is a compound having a hydrocarbon chain with three or more carbon atoms substituted with two or more hydrophilic groups selected from the group consisting of hydroxyl groups, amino groups, and anionic groups. However, the hydrocarbon chain may be interrupted by a sulfonyl group or an ether group. When the hydrocarbon chain has three or four carbon atoms, the hydrophilic group contains an anionic group, or the hydrocarbon chain is interrupted by a sulfonyl group.

In the present invention, a hydrocarbon compound that is dissolved in water at 25°C at the content of the compound in the ink is considered to be "water-soluble". In other words, the solubility in water at 25°C is greater than the content in the ink. The fact that the hydrocarbon chain is interrupted by a sulfonyl group or an ether group means that a sulfonyl group (-S(=O) ₂ -) or an ether group (-O-) is present in the middle of the hydrocarbon chain. Water-soluble hydrocarbon compounds have hydrogen-bonding groups such as hydroxyl groups, amino groups, anionic groups, sulfonyl groups, and ether groups. Therefore, by using an ink containing this hydrocarbon compound, cockling and curling of a recording medium on which an image is recorded can be suppressed. General hydrocarbon compounds having a hydrocarbon chain with a relatively small number of carbon atoms (3 or 4) tend to have a small molecular weight and a low vapor pressure. However, since the above water-soluble hydrocarbon compounds have anionic groups of hydrogen bonds or the hydrocarbon chain is interrupted by a sulfonyl group, they are difficult to evaporate due to intermolecular or intramolecular interactions, and remain between fibers to suppress cockling and curling. The content (mass %) of the water-soluble hydrocarbon compound in the ink is preferably 1.0 mass % or more and 20.0 mass % or less based on the total mass of the ink.

The number of carbon atoms in the hydrocarbon chain constituting the water-soluble hydrocarbon compound is preferably 3 to 50, and more preferably 3 to 10. Examples of anionic groups include sulfonic acid groups and carboxylic acid groups. Specific examples of water-soluble hydrocarbon compounds include alkanediols such as 1,5-pentanediol and 1,6-hexanediol; amino acids such as alanine, β-alanine, trimethylglycine, amidosulfuric acid (also known as sulfamic acid), aminomethanesulfonic acid, taurine (also known as 2-aminoethanesulfonic acid), carbamic acid, glycine, aspartic acid, glutamic acid, sulfanilic acid, or salts of the above-mentioned acids, phenylalanine, leucine, isoleucine, threonine, tryptophan, valine, methionine, lysine, and arginine; sulfonyl compounds such as bis(2-hydroxyethyl)sulfone; alkylene glycols such as triethylene glycol, tetraethylene glycol, tripropylene glycol, and polyethylene glycols having a number average molecular weight of about 200 to 1,000; and sugars such as sorbitol, D-sorbitol, xylitol, trehalose, fructose, and D(+)-xylose. The water-soluble hydrocarbon compounds may be used alone or in combination of two or more.

[Other ingredients]
In addition to the above components, the ink may contain various additives, such as antifoaming agents, surfactants, pH adjusters, viscosity adjusters, rust inhibitors, preservatives, antifungal agents, antioxidants, reduction inhibitors, etc. However, it is preferable that the ink does not contain a reactant used in the reaction liquid described below.

[Ink properties]
The ink is a water-based ink applied to an inkjet method. Therefore, from the viewpoint of reliability, it is preferable to appropriately control the physical property values. Specifically, the surface tension of the ink at 25° C. is preferably 20 mN/m or more and 60 mN/m or less. In addition, the viscosity of the ink at 25° C. is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the ink at 25° C. is preferably 7.0 or more and 9.5 or less, and more preferably 8.0 or more and 9.5 or less.

<Ink set>
The ink described above can be used as an ink set in combination with an aqueous reaction liquid (hereinafter sometimes referred to as "reaction liquid") that contains a reactant that reacts with the ink.

(Reaction solution)
[Reactants]
The reaction liquid reacts with the ink when it comes into contact with the ink, and aggregates the components in the ink (components having anionic groups, such as resins, surfactants, and self-dispersing pigments), and contains a reactant. The presence of the reactant can destabilize the state of the components having anionic groups in the ink when the ink and the reactant come into contact with each other on the recording medium, and promote the aggregation of the ink. Examples of the reactant include cationic components such as polyvalent metal ions and cationic resins, and organic acids. The reactant may be used alone or in combination of two or more.

Examples of polyvalent metal ions constituting the polyvalent metal salt include divalent metal ions such as Ca2 ⁺ , Cu2 ⁺ , Ni2 ⁺ , Mg2 ⁺ , Sr2 ⁺ , Ba2 ⁺ , and Zn2 ⁺ , and trivalent metal ions such as Fe3 ⁺ , Cr3 ⁺ , Y3 ⁺ , and Al3 ⁺ . In order to incorporate polyvalent metal ions into the reaction solution, a water-soluble polyvalent metal salt (which may be a hydrate) formed by combining a polyvalent metal ion with an anion can be used. Examples of the anion include inorganic anions such as Cl ⁻ , Br ⁻ , I ⁻ , ClO ⁻ , ClO ₂ ⁻ , ClO ₃ ⁻ , ClO ₄ ⁻ , NO ₂ ⁻ , NO ₃ ⁻ , SO ₄ ²⁻ , CO ₃ ²⁻ , HCO ₃ ⁻ , PO ₄ ³⁻ , HPO ₄ ²⁻ , and H ₂ PO ₄ ⁻ ; and organic anions such as HCOO ⁻ , (COO ⁻ ) ₂ , COOH(COO ⁻ ), CH ₃ COO ⁻ , CH ₃ CH(OH)COO ⁻ , C ₂ H ₄ (COO ⁻ ) ₂ , C ₆ H ₅ COO ⁻ , C ₆ H ₄ (COO ⁻ ) ₂ , and CH ₃ SO ₃ ^{− .} When a polyvalent metal ion is used as a reactant, the content (mass%) of the polyvalent metal salt in the reaction solution is preferably 1.0 mass% or more and 20.0 mass% or less based on the total mass of the reaction solution. In this specification, when the polyvalent metal salt is a hydrate, the "content (mass%) of the polyvalent metal salt in the reaction solution means the "content (mass%) of the anhydride of the polyvalent metal salt" excluding water as the hydrate.

The reaction liquid containing an organic acid has a buffering ability in the acidic range (less than pH 7.0, preferably pH 2.0 to 5.0), and thus efficiently converts the anionic groups of the components present in the ink into the acid form and causes them to aggregate. Examples of organic acids include monocarboxylic acids and their salts, such as formic acid, acetic acid, propionic acid, butyric acid, benzoic acid, glycolic acid, lactic acid, salicylic acid, pyrrole carboxylic acid, furan carboxylic acid, picolinic acid, nicotinic acid, thiophene carboxylic acid, levulinic acid, and coumaric acid; dicarboxylic acids and their salts and hydrogen salts, such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, itaconic acid, sebacic acid, phthalic acid, malic acid, and tartaric acid; tricarboxylic acids and their salts and hydrogen salts, such as citric acid and trimellitic acid; and tetracarboxylic acids and their salts and hydrogen salts, such as pyromellitic acid. When an organic acid is used as a reactant, the content (mass %) of the organic acid in the reaction solution is preferably 1.0 mass % or more and 50.0 mass % or less based on the total mass of the reaction solution.

Examples of cationic resins include resins having a primary to tertiary amine structure and resins having a quaternary ammonium salt structure. Specific examples include resins having structures such as vinylamine, allylamine, vinylimidazole, vinylpyridine, dimethylaminoethyl methacrylate, ethyleneimine, guanidine, diallyldimethylammonium chloride, and alkylamine-epichlorohydrin condensates. In order to increase the solubility in the reaction solution, the cationic resin can be used in combination with an acidic compound or the cationic resin can be subjected to a quaternization treatment. When a cationic resin is used as a reactant, the content (mass %) of the cationic resin in the reaction solution is preferably 0.1 mass % or more and 10.0 mass % or less based on the total mass of the reaction solution.

An ink set is preferably composed of a reaction liquid in which the reactant is a polyvalent metal ion, a coloring material dispersed by the action of a carboxylic acid group, and an ink containing resin particles formed of a resin containing a unit having a carboxylic acid group in addition to the four types of units described above. With an ink set of this composition, the scratch resistance of the image can be further improved and unevenness in the image can be suppressed. The polyvalent metal ions in the reaction liquid react with a component having a carboxylic acid group to strongly aggregate the target object. When the resin particles and coloring material in the ink both have carboxylic acid groups, the aggregation speeds of the coloring material and the resin particles tend to be equal when the ink comes into contact with the reaction liquid. As a result, only one of the resin particles and the coloring material aggregates first and does not exist separately from each other, but the resin particles and coloring material aggregate in a moderately mixed state, resulting in an image that shows even higher scratch resistance.

In addition, since the cyano group in the resin forming the resin particles is a highly polarized group, it easily interacts with the carboxylic acid group in the resin forming the resin particles and the coloring material. Because the cyano group covers the periphery of the carboxylic acid group due to this interaction, the aggregation rate due to the reaction between the polyvalent metal ion and the carboxylic acid group when the ink comes into contact with the reaction liquid is appropriately suppressed, and the aggregation proceeds slowly. This prevents the aggregation from proceeding excessively in some areas, and makes it difficult for the ink aggregates to be irregularly formed. In addition, the sulfonic acid group in the unit represented by general formula (1) in the resin forming the resin particles is more likely to undergo ionic dissociation than the carboxylic acid group and is less likely to react with the polyvalent metal ion, so it exhibits the effect of suppressing the growth of the ink aggregates. Therefore, by using an ink containing resin particles formed from a resin having a sulfonic acid group, it becomes difficult for the ink aggregates to be irregularly formed. As a result of these, it is possible to suppress unevenness in the image.

When an organic acid is used as a reactant instead of a polyvalent metal ion, the scratch resistance of the image is improved, but image unevenness is somewhat difficult to suppress. Also, when a cationic resin such as polyallylamine is used as a reactant instead of a polyvalent metal ion, image unevenness can be suppressed, but the effect of further improving the scratch resistance of the image is difficult to obtain. However, as long as the effect of the present invention is not impaired, polyvalent metal ions may be used in combination with cationic resins and organic acids.

[Aqueous medium]
The reaction liquid is an aqueous reaction liquid containing at least water as an aqueous medium. Examples of the aqueous medium used in the reaction liquid include the same aqueous medium as that which can be contained in the ink described above. The content (mass%) of the water-soluble organic solvent in the reaction liquid is preferably 1.0 mass% or more and 45.0 mass% or less based on the total mass of the reaction liquid. The water-soluble organic solvent preferably contains the specific water-soluble hydrocarbon compound described above. The content (mass%) of the water-soluble hydrocarbon compound in the reaction liquid is preferably 1.0 mass% or more and 20.0 mass% or less based on the total mass of the reaction liquid. In addition, the content (mass%) of water in the reaction liquid is preferably 50.0 mass% or more and 95.0 mass% or less based on the total mass of the reaction liquid.

[Other ingredients]
The reaction liquid may contain various other components as necessary. Examples of the other components include the same components as those that can be contained in the ink described above.

[Properties of reaction solution]
The reaction liquid is an aqueous reaction liquid applied to the inkjet method. Therefore, from the viewpoint of reliability, it is preferable to appropriately control the physical property values. Specifically, the surface tension of the reaction liquid at 25°C is preferably 20 mN/m or more and 60 mN/m or less. In addition, the viscosity of the reaction liquid at 25°C is preferably 1.0 mPa·s or more and 10.0 mPa·s or less. The pH of the reaction liquid at 25°C is preferably 5.0 or more and 9.5 or less, more preferably 6.0 or more and 9.0 or less.

<Inkjet recording method and inkjet recording apparatus>
An inkjet recording method according to one embodiment of the present invention and an inkjet recording apparatus suitable for use in the inkjet recording method will be described below with reference to the drawings. The inkjet recording method of this embodiment is a method for ejecting the above-mentioned ink from an inkjet recording head to record an image on a recording medium. The inkjet recording apparatus of this embodiment is an apparatus equipped with the above-mentioned ink and an inkjet recording head that ejects the ink.

Figure 1 is a schematic diagram showing an example of the general configuration of an inkjet recording device 100 of this embodiment. The inkjet recording device 100 is an inkjet recording device that records an image on a recording medium using ink and a reaction liquid containing a reactant that reacts with the ink. Here, an example will be described in which a reaction liquid is used together with the ink, but a reaction liquid does not have to be used. The X direction, Y direction, and Z direction respectively indicate the width direction (total length direction), depth direction, and height direction of the inkjet recording device. The recording medium is transported in the X direction.

The inkjet recording device 100 of the embodiment shown in FIG. 1 is configured with a recording unit 1000, a heating unit 2000, a fixing unit 3000, and a paper discharge unit 4000. In the recording unit 1000, various liquids are applied by a liquid application device 1200 to the recording medium 1100 conveyed from the paper feed device 1400 by a conveying member 1300. In the heating unit 2000, the liquid components of the image formed by the liquid applied to the recording medium 1100 are heated by the heating device 2100 to evaporate and dry. In the fixing unit 3000, the fixing member 3100 is brought into contact with the area including the image on the recording medium 1100 to heat the image and promote the fixing of the image to the recording medium 1100. Thereafter, the recording medium 1100 is conveyed by the conveying member 4100 of the paper discharge unit 4000 and is stacked and accommodated in the recording medium accommodation unit 4200. Here, an example is described in which the heating unit 2000 and the fixing unit 3000 are included, but depending on the printing conditions (such as the type of ink and printing medium, and printing speed), the heating unit and the fixing unit may be omitted. In the examples described below, printing was performed without using the heating unit 2000 and the fixing unit 3000.

Any recording medium may be used as the recording medium 1100. For example, a recording medium having ink absorption (permeability) such as a recording medium without a coating layer, such as plain paper, uncoated paper, or synthetic paper, or a recording medium having a coating layer, such as glossy paper or art paper, may be used. A recording medium having no ink absorption (permeability) such as a film or sheet formed of a resin material, such as polyvinyl chloride (PVC) or polyethylene terephthalate (PET), may also be used. The basis weight (g/m ² ) of the recording medium 1100 is preferably 30 g/m ² or more and 500 g/m ² or less, and more preferably 50 g/m ² or more and 450 g/m ² or less.

[Recording section]
The recording unit 1000 includes a liquid applying device 1200. The liquid applying device 1200 includes a reaction liquid applying device 1201 and an ink applying device 1202. The reaction liquid applying device 1201 shown in FIG. 1 is an example of a unit using an inkjet type ejection head. In addition, the reaction liquid applying device may be configured using a gravure coater, an offset coater, a die coater, a blade coater, or the like. The reaction liquid may be applied by the reaction liquid applying device 1201 either before or after the ink application as long as it can come into contact with the ink on the recording medium 1100. However, in order to record high-quality images on various recording media having different liquid absorption characteristics, it is preferable to apply the reaction liquid before the ink is applied. As the ink applying device 1202, an inkjet type ejection head (recording head) is used. The ejection method of the ejection head as the liquid applying device 1200 includes a method of ejecting liquid by generating film boiling in the liquid by an electrothermal converter to form bubbles, and a method of ejecting liquid by an electromechanical converter.

The liquid application device 1200 is a line head extending in the Y direction, with ejection ports arranged in a range that covers the image recording area of the widest usable recording medium. The ejection head has an ejection port surface (not shown) on its lower side (the recording medium 1100 side) where the ejection ports are formed, and the ejection port surface faces the recording medium 1100 at a very small distance of about a few millimeters.

A plurality of ink applicators 1202 may be provided to apply ink of each color to the recording medium 1100. For example, when recording images of each color using yellow ink, magenta ink, cyan ink, and black ink, four ink applicators 1202 ejecting the above four types of ink are arranged in the X direction. Hereinafter, the ink and reaction liquid may be collectively referred to as "liquid."

Figure 2 is a perspective view showing an example of a liquid application device. The liquid application device 1200 shown in Figure 2 is a line head, and multiple ejection element substrates 1203 each having an ejection port array are arranged in a straight line. Multiple ejection port arrays are arranged on the ejection element substrate 1203.

[Transport system]
As shown in FIG. 1, the recording unit 1000 is configured to include a liquid applying device 1200 and a conveying member 1300 that conveys the recording medium 1100. The liquid applying device 1200 applies a reaction liquid and ink to a desired position of the recording medium 1100 conveyed by the conveying member 1300. Each liquid receives an image signal of drawing data and applies the necessary reaction liquid and ink to each position. FIG. 1 shows the conveying member 1300 in the form of a conveying belt, but a spur, a conveying cylinder, or the like may be used as long as it has the function of conveying the recording medium 1100. In order to improve the conveying accuracy, a member that can fix the recording medium 1100 can be used as the conveying member 1300. Specifically, a method of providing a hole in the conveying member 1300 and fixing the recording medium 1100 by sucking from the back side, a method of forming the conveying member 1300 from an appropriate material and fixing the recording medium 1100 by electrostatic adsorption, etc. can be mentioned.

[Heating section]
As shown in Fig. 1, the heating section 2000 is configured to include a heating device 2100 and a transport member 2200. The recording medium 1100, to which the reaction liquid and ink have been applied and an image has been recorded, is heated by the heating device 2100 while being transported by the transport member 2200, thereby evaporating and drying the liquid components of the image. It is preferable to further include a drying step between the ink application step and the fixing step, in which the recording medium to which the ink has been applied is heated in a non-contact manner to dry the ink. By including such a drying step, deformation (cockling and curling) of the recording medium 1100 can be effectively suppressed.

The heating device 2100 may have any configuration as long as it can heat the recording medium 1100, and various conventional devices such as hot air dryers and heaters can be used. Among these, the use of non-contact heaters such as electric heating wires and infrared heaters is preferable from the standpoint of safety and energy efficiency. In addition, the drying efficiency can be easily improved by using a mechanism that incorporates a fan to spray heated gas onto the recording medium 1100 and blows hot air.

The method of heating may be from the side of the recording medium 1100 to which the reaction liquid and ink have been applied, from the back side, or from both sides. The transport member 2200 may be provided with a heating function. FIG. 1 shows the transport member 2200 in the form of a transport belt, but a spur, transport drum, or the like may also be used as long as it has the function of transporting the recording medium 1100.

[Fuser unit]
As shown in FIG. 1, the fixing unit 3000 conveys the recording medium 1100 by the conveying member 3200, and also brings the fixing member 3100 into contact with the recording medium 1100 under pressure, thereby heating the liquid such as the reaction liquid or ink applied to the recording medium 1100. This allows an image to be fixed on the recording medium 1100. From the recording medium 1100 on which an image has been recorded, the liquid components of the reaction liquid or ink permeate the recording medium 1100 or evaporate by passing through the heating unit 2000, and then the image is fixed in the fixing unit 3000, completing the image. By heating and pressurizing the recording medium 1100 while sandwiching it between the fixing member 3100 and the conveying member 3200, the image of the recording medium 1100 and the fixing member 3100 come into close contact with each other, and the image is fixed on the recording medium. When a liquid such as ink containing resin particles and coloring material is used, the resin particles are softened and formed into a film mainly by heating by the fixing unit 3000, and the coloring material can be bound onto the recording medium 1100.

Methods for heating the fixing member 3100 include a method in which a heat source such as a halogen heater is provided inside the roller that drives the fixing member 3100 as a fixing belt, and heating is performed. Also, a method in which a heat source such as an infrared heater is provided in a location separate from the fixing member 3100 and heating is performed can be used. Furthermore, these methods may be combined.

[Paper output section]
The recording medium 1100 after the image recording is accommodated in the paper discharge unit 4000 ( FIG. 1 ). Specifically, the recording medium 1100 on which the recording has been performed is transported by a transport member 4100, and is finally accommodated in a stacked state in the recording medium accommodation unit 4200. Two or more recording medium accommodation units 4200 may be provided in order to accommodate different recorded matters, respectively.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way to the following examples as long as it does not deviate from the gist of the invention. Unless otherwise specified, the terms "parts" and "%" used to describe the amounts of components are based on mass.

<Preparation of Pigment Dispersion>
(Pigment Dispersion 1)
1.5 g of 4-aminophthalic acid was added to a solution of 5.0 g of concentrated hydrochloric acid in 5.5 g of water at a temperature of 5°C. The container containing this solution was placed in an ice bath, and while stirring to maintain the temperature of the solution at 10°C or less, a solution of 0.9 g of sodium nitrite in 9.0 g of water at 5°C was added. After stirring for 15 minutes, 6.0 g of pigment (carbon black) was added, and further stirring was performed for 15 minutes to obtain a slurry. The obtained slurry was filtered with filter paper (standard filter paper No. 2, manufactured by Advantec), and the particles were thoroughly washed with water and dried in an oven at 110°C. Thereafter, sodium ions were replaced with potassium ions by an ion exchange method to obtain a self-dispersing pigment in which -C ₆ H ₃ -(COOK) ₂ groups were bonded to the particle surface of the pigment (carbon black). An appropriate amount of pure water was added to adjust the pigment content, and a pigment dispersion liquid 1 with a pigment (carbon black) content of 10.0% was obtained.

(Pigment Dispersion 2)
A styrene-ethyl acrylate-acrylic acid copolymer (resin 1) with an acid value of 150 mgKOH/g and a weight average molecular weight of 8,000 was prepared. 20.0 parts of resin 1 were neutralized with potassium hydroxide in an amount equimolar to the acid value, and then an appropriate amount of pure water was added to prepare an aqueous solution of resin 1 with a resin (solid content) content of 20.0%. 10.0 parts of pigment (C.I. Pigment Blue 15:3), 15.0 parts of the aqueous solution of resin 1, and 75.0 parts of pure water were mixed to obtain a mixture. The obtained mixture and 200 parts of zirconia beads with a diameter of 0.3 mm were placed in a batch-type vertical sand mill (manufactured by Imex) and dispersed for 5 hours while cooling with water. After removing coarse particles by centrifugation, the mixture was pressure-filtered with a cellulose acetate filter (manufactured by Advantec) with a pore size of 3.0 μm. In this manner, Pigment Dispersion Liquid 2 was prepared, which had a pigment (C.I. Pigment Blue 15:3) content of 10.0% and a resin dispersant (Resin 1) content of 3.0%.

(Pigment Dispersion 3)
The pigment was changed to a solid solution containing C.I. Pigment Violet 19 and C.I. Pigment Red 122. Except for this, a pigment dispersion 3 having a pigment (solid solution) content of 10.0% and a resin dispersant (resin 1) content of 3.0% was prepared in the same manner as in the pigment dispersion 2 described above.

(Pigment Dispersion 4)
The pigment was changed to C.I. Pigment Red 150. Except for this, a pigment dispersion 4 having a pigment (C.I. Pigment Red 150) content of 10.0% and a resin dispersant (Resin 1) content of 3.0% was prepared in the same manner as in the above-mentioned pigment dispersion 2.

(Pigment Dispersion 5)
The pigment was changed to C.I. Pigment Yellow 74. Apart from this, a pigment dispersion 5 having a pigment (C.I. Pigment Yellow 74) content of 10.0% and a resin dispersant (Resin 1) content of 3.0% was prepared in the same manner as in the above-mentioned pigment dispersion 2.

(Pigment Dispersion 6)
Except for the change of the pigment to carbon black, a pigment dispersion 6 having a pigment (carbon black) content of 10.0% and a resin dispersant (resin 1) content of 3.0% was prepared in the same manner as in the preparation of the pigment dispersion 2 described above.

<Preparation of Resin Particles>
(Monomers forming the unit represented by formula (1))
Monomers forming the unit represented by general formula (1) were prepared by a conventional method. The prepared monomers (compounds 1 to 10) forming the unit represented by general formula (1) are shown in Table 1. The monomer forming the unit represented by general formula (1) is a compound represented by the following general formula (3-1). R ₁ and E in general formula (3-1) have the same meanings as in general formula (1).

(Resin particles 1 to 44)
As monomers that become units having aromatic groups constituting the resin that forms the resin particles, styrene (St), 4-methylstyrene (4MSt), 2-chlorostyrene (2CSt), α-methylstyrene (αMSt), and 2-vinylnaphthalene (2VNA) were prepared. In addition, as monomers that become units having cyano groups constituting the resin that forms the resin particles, acrylonitrile (AN), methacrylonitrile (MAN), and 2-chloroacrylonitrile (2CAN) were prepared. Similarly, as (meth)acrylic acid esters having branched alkyl groups, 2-ethylhexyl acrylate (2EHA), 2-ethylhexyl methacrylate (2EHMA), isobutyl methacrylate (iBMA), and sec-butyl acrylate (sBA) were prepared. Furthermore, as monomers that become other units constituting the resin that forms the resin particles, ethyl methacrylate (EMA) and methacrylic acid (MAA) were prepared.

A reaction vessel equipped with a stirrer was set in a hot water bath. 1,178 parts of water was placed in the reaction vessel, and the internal temperature was maintained at 70°C. Monomers were mixed in the amounts (units: parts) shown in Table 2-1 to prepare a monomer mixture. In addition, 1.9 parts of potassium persulfate and 659 parts of water were mixed to prepare an aqueous solution of a polymerization initiator. The monomer mixture and the aqueous solution of a polymerization initiator were dropped into the reaction vessel in parallel over 60 minutes. After the dropwise addition was completed, the mixture was stirred for another 30 minutes to synthesize the resin particles shown in Table 2-2. Then, an appropriate amount of an 8 mol/L potassium hydroxide aqueous solution was added to the reaction vessel, and the pH of the liquid was adjusted to 8.5. The water content was adjusted to obtain an aqueous dispersion of each resin particle having a resin particle content of 40.0% and a volume average particle diameter (cumulative 50% particle diameter D50 based on volume) of 100 nm. The volume average particle diameter of the resin particles was measured using a dynamic light scattering particle size analyzer (product name "UPA-EX150", manufactured by Nikkiso) under the following conditions: SetZero: 30 seconds, number of measurements: 3, measurement time: 180 seconds, shape: spherical, refractive index: 1.59. The "W1/W2 value (times)" shown in Table 2-2 is the ratio of the proportion W1 (%) of units having aromatic groups to the proportion W2 (%) of units represented by general formula (1) based on the total mass of the resin.

(Resin Particles 45)
According to the description of "Preparation of Resin Particles 1" in Patent Document 1, an aqueous dispersion of resin particles 45 was obtained in which the resin particle content was 20.0% and the volume average particle diameter (cumulative 50% particle diameter based on volume) was 80 nm. The resin particles 45 are resin particles having a core-shell structure, and the resin forming the resin particles does not contain a unit derived from a (meth)acrylic acid ester having a branched alkyl group. In the resin forming the resin particles 45, the ratio W1 of units having aromatic groups was 59.1%, the ratio W2 of units represented by general formula (1) was 2.7%, and the value of W1/W2 was 21.9 times.

<Ink Preparation>
The components (unit: %) shown in the middle of Table 3 (Tables 3-1 to 3-6) were mixed and thoroughly stirred, and then pressure filtered with a cellulose acetate filter (manufactured by Advantec) having a pore size of 3.0 μm to prepare each ink. The pigment dispersion and the aqueous dispersion of resin particles were used with the numbers shown in the upper part of Tables 3-1 to 3-6. "Acetylenol E100" shown in Tables 3-1 to 3-6 is the trade name of a surfactant manufactured by Kawaken Fine Chemicals. The lower part of Tables 3-1 to 3-6 shows the ink characteristics, such as the pigment content A, the resin particle content B, and the PEG content C with an EO number of 4 or more (EO number ≧4) based on the total mass of the ink, as well as the values of A/B and B/C. Also shown are the EO numbers (n1 and n2) and n1/n2 of the resin particles and the PEG with an EO number of 4 or more (EO number ≧4) used in the ink.

<Preparation of reaction solution>
Each component (unit: %) shown in Table 4 was mixed and thoroughly stirred, and then pressure filtered through a cellulose acetate filter (manufactured by Advantec) with a pore size of 3.0 μm to prepare each reaction solution. "PAA-HCL-3L" shown in Table 4 is the trade name of polyallylamine manufactured by Nittobo Medical, and "Acetylenol E100" is the trade name of a surfactant manufactured by Kawaken Fine Chemical.

<Ink evaluation>
The inks shown in Table 5 were filled in the ink application device 1202 of the inkjet recording device 100 having the configuration shown in FIG. 1. Using the inkjet recording device 100, a 2 cm x 2 cm solid image with an ink recording duty of 100% was recorded on high-quality paper (product name "npi high-quality", manufactured by Nippon Paper Industries Co., Ltd., basis weight 157.0 g/ ^m2 ). In this example, an image recorded under the condition that one ink droplet of 3.0 ng is applied to a unit area of 1/1200 inch x 1/1200 inch is defined as having a recording duty of 100%. In this example, "A" and "B" were set as acceptable levels in the evaluation criteria for each item below, and "C" and "D" were set as unacceptable levels. The evaluation results are shown in Table 5.

(Abrasion resistance)
One minute after recording, the recorded solid image was subjected to a friction test under conditions of 3, 5, and 7 reciprocations at a load of 100 g using an abrasion resistance tester (manufactured by Imoto Manufacturing Co., Ltd.), which is a Gakushin type testing machine conforming to JIS L 0849. The solid image after the friction test was visually observed, and the abrasion resistance of the image was evaluated according to the following evaluation criteria.
A: The solid image was not scraped off even after seven round trips.
B: After five reciprocations, the solid image was not scraped off, but after seven reciprocations, the solid image was scraped off and the white background of the recording medium was visible.
C: After three reciprocations, the solid image was not scraped off, but after five reciprocations, the solid image was scraped off and the white background of the recording medium was visible.
D: After three round trips, the solid image was scraped off and the white background of the recording medium was visible.

(Color development)
The recorded solid image was left in an environment of 23° C. and 55% relative humidity for 24 hours. Thereafter, the optical density of the color component corresponding to the ink colorant was measured for the solid image using a spectrophotometer under conditions of light source: D50 and field of view: 2°, and the color development of the image was evaluated according to the following evaluation criteria. The optical density was measured using a spectrophotometer (trade name "Spectrolino", manufactured by Gretag Macbeth) for the black component when the colorant was carbon black, and for the color component (cyan, magenta, or yellow) when the colorant was a color pigment.
A: The optical density of the black component was 1.2 or more, or the optical density of the color component was 1.1 or more.
B: The optical density of the black component was 1.1 or more and less than 1.2, or the optical density of the color component was 1.0 or more and less than 1.1.
C: The optical density of the black component was less than 1.1, or the optical density of the color component was less than 1.0.

<Evaluation of Ink Set>
The reaction liquid and ink combinations shown in Table 6 were filled into the reaction liquid application device 1201 and the ink application device 1202 of the inkjet recording apparatus 100 having the configuration shown in Fig. 1. In this example, an image recorded under the condition that one ink droplet of 3.0 ng is applied to a unit area of 1/1,200 inch x 1/1,200 inch is defined as having a recording duty of 100%. In the present invention, in the evaluation criteria for each item below, "A" and "B" were set as acceptable levels, and "C" was set as an unacceptable level. The evaluation results are shown in Table 6.

(Abrasion resistance)
Using the inkjet recording device 100 having the above configuration, a 2 cm x 2 cm solid image was recorded on a recording medium, with the recording duty of the reaction liquid being 15% and the recording duty of the ink being 100%. As the recording medium, npi wood-free (manufactured by Nippon Paper Industries, basis weight 157.0 g/m ² ) was used. The recorded solid image was placed in an environment of 23°C temperature and 55% relative humidity for 24 hours. Thereafter, a friction test was performed on the recorded solid image using an abrasion resistance tester (manufactured by Imoto Seisakusho), which is a Gakushin type testing machine conforming to JIS L 0849, under the condition of 1 or 3 reciprocations with a load of 200 g. The solid image after the friction test was visually observed, and the abrasion resistance of the image was evaluated according to the evaluation criteria shown below.
A: The solid image was not scraped off even after five round trips.
B: After three reciprocations, the solid image was not scraped off, but after five reciprocations, the solid image was scraped off and the white background of the recording medium was visible.
C: After three round trips, the solid image was scraped off and the white background of the recording medium was visible.

(Suppression of image unevenness)
Using the inkjet recording device 100 having the above configuration, a 5 cm x 5 cm solid image was recorded on a recording medium, with the reaction liquid recording duty being 15% and the ink recording duty being 100%. The recorded solid image was read by a scanner (product name "DR-C225II", manufactured by Canon), the solid image was binarized, and the area ratio of the portion recorded according to the input data (the dark portion of the image) was calculated. The reading conditions were color and 600 dpi. The binarization process was performed by converting to 8-bit using the image processing software "ImageJ", and then setting the value of [(255 + Gtop) / 2] as the threshold value for the gradation value (Gtop) at the peak of the brightness histogram with white 255/black 0. From the area above the threshold thus obtained, the ratio (area ratio) was calculated, and the suppression of image unevenness was evaluated according to the evaluation criteria shown below.
A: The area ratio was 100.0%.
B: The area ratio was more than 99.0% and less than 100.0%.
C: The area ratio was 99.0% or less.

Claims (17)

A water-based inkjet ink containing a coloring material and resin particles,
an aqueous ink, characterized in that the resin forming the resin particles contains a unit having an aromatic group, a unit having a cyano group, a unit derived from a (meth)acrylic acid ester having a branched alkyl group, and a unit represented by the following general formula (1):

(In the general formula (1), n represents the number of repeating units, _R1 represents an alkyl group, E represents the number of repeating units of an ethylene oxide group, and E≧1.) The aqueous ink according to claim 1, wherein the proportion (mass %) of the units having the aromatic group in the resin forming the resin particles is 20.0 mass % or more and 50.0 mass % or less based on the total mass of the resin. The aqueous ink according to claim 1, wherein the proportion (mass %) of the units having the cyano group in the resin forming the resin particles is 5.0 mass % or more and 50.0 mass % or less based on the total mass of the resin. The aqueous ink according to claim 1, wherein the proportion (mass%) of the units derived from the (meth)acrylic acid ester having the branched alkyl group in the resin forming the resin particles is 10.0 mass% or more and 40.0 mass% or less based on the total mass of the resin. The aqueous ink according to claim 1, wherein the proportion (mass %) of the unit represented by the general formula (1) in the resin forming the resin particles is 1.0 mass % or more and 10.0 mass % or less based on the total mass of the resin. The aqueous ink according to claim 1, wherein the ratio W1 (mass%) of the units having the aromatic group in the resin forming the resin particles and the ratio W2 (mass%) of the units represented by the general formula (1) satisfy the relationship 4.0≦W1/W2≦40.0. The aqueous ink according to claim 1 , wherein the unit having an aromatic group is a unit represented by the following general formula (2):

(In the general formula (2), x is a repeating unit, each _R2 independently represents an alkyl group or a halogen atom, and y is an integer of 0 to 5.) The aqueous ink according to claim 1, wherein the unit having a cyano group is a unit derived from at least one monomer selected from the group consisting of acrylonitrile and methacrylonitrile. The aqueous ink according to claim 1, wherein the unit derived from a (meth)acrylic acid ester having a branched alkyl group is a unit derived from 2-ethylhexyl (meth)acrylate. The water-based ink according to claim 1 , wherein the alkyl group represented by R ₁ in the general formula (1) has 9 or more and 14 or less carbon atoms. The aqueous ink according to claim 1, wherein the number of repeating units of the ethylene oxide group represented by E in the general formula (1) is 5 or more and 30 or less. The aqueous ink according to claim 1, wherein the content (mass %) of the resin particles in the aqueous ink is 1.0 mass % or more and 15.0 mass % or less. the aqueous ink further contains a water-soluble organic solvent containing polyethylene glycol having 4 or more ethylene oxide groups;
2. The aqueous ink according to claim 1, wherein the number of ethylene oxide groups (n1) of the unit represented by general formula (1) in the resin forming the resin particles and the number of ethylene oxide groups (n2) of the polyethylene glycol satisfy the relationship 0.2≦n1/n2≦5.0. The aqueous ink according to claim 13, wherein the content (mass%) of the resin particles in the aqueous ink is 0.3 to 5.0 times the content (mass%) of the polyethylene glycol in the aqueous ink. An inkjet ink set having a combination of an aqueous ink and an aqueous reactive liquid containing a reactant that reacts with the aqueous ink,
The aqueous ink is the aqueous ink according to any one of claims 1 to 14,
The colorant is dispersed by the action of a carboxylic acid group,
the resin further comprises a unit having a carboxylic acid group,
The ink set, wherein the reactant is a polyvalent metal ion. An inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, comprising:
An ink-jet recording method, wherein the ink is a water-based ink according to claim 1 . An inkjet recording apparatus including an ink and an inkjet recording head that ejects the ink,
15. An ink jet recording apparatus, wherein the ink is a water-based ink according to claim 1.

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