Inkjet recording water-based ink

Abstract

To provide an inkjet recording water-based ink capable of producing recorded materials that have superior storage stability and have superior image density even in recording on low-liquid-absorption recording media, a method for producing the water-based ink, and an inkjet recording method using the water-based ink.SOLUTION: An inkjet recording water-based ink is provided that comprises pigment particles (A) dispersed with a water-insoluble polymer (a), wax particles (B) dispersed with a surfactant (I) and a water-insoluble polymer (b), a water-soluble organic solvent (C), and water, wherein the surfactant (I) is at least one selected from the group consisting of nonionic surfactants and anionic surfactants. Also provided are a method for producing the water-based ink, and an inkjet recording method using the water-based ink.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an aqueous ink for inkjet recording, a method for producing the aqueous ink, and an inkjet recording method using the aqueous ink.

Background Art

[0002] The inkjet recording method is a recording method in which ink droplets are directly ejected from very fine nozzles onto a recording medium and adhered to obtain a recorded matter on which characters and images are formed. This method has many advantages such as being easy to achieve full color, being inexpensive, being able to use plain paper as a recording medium, and being non-contact with the recording medium, so it has become extremely popular. In recent years, due to the spread of digital printing, it has been increasingly used not only in consumer printing but also in commercial printing and industrial printing using a variety of low-absorbency coated papers. The demand for aqueous inks is increasing more and more for the purpose of increasing the printing speed, improving the image quality, and reducing the environmental impact. Therefore, various proposals have been made to meet such requirements.

[0003] For example, Patent Document 1 discloses an inkjet recording method for achieving both image transferability and fastness. The method includes a step of applying a reaction liquid containing a high-viscosity component onto a transfer body, and a step of applying an ink containing water and a coloring material. By passing through these steps, a first image forming step is performed, which includes a liquid component containing water and a high-boiling water-soluble organic solvent, and a solid component insoluble in the liquid component formed by mixing the reaction liquid and the ink. Then, a second image is formed by bringing a porous body into contact with the first image to remove a part of the liquid component contained in the first image. A transfer step of heating the second image and transferring it onto a recording medium is repeatedly performed. An inkjet recording method for temperature control of the first image and the second image under specific conditions is disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Here, generally, when an aqueous ink is applied to a low-absorbency recording medium by an inkjet recording method, a large amount of pigment remains on the surface of the recording medium after recording, so the pigment is likely to peel off from the surface of the ink coating film. Therefore, it has been proposed to improve the image fastness by reducing the frictional resistance of the ink coating film during rubbing by incorporating wax into the aqueous ink. On the other hand, wax has high hydrophobicity and tends to aggregate in aqueous ink, and it is necessary to improve the dispersion stability of wax and the storage stability of the ink. Patent Document 1 discloses that an ink containing water and a coloring material further contains wax particles, and in the examples, an ink containing a wax particle dispersion using a resin as a wax dispersant is disclosed, but it has been found that the storage stability of the ink is not satisfactory and the image density of the obtained recorded matter is also insufficient. An object of the present invention is to provide an aqueous ink for inkjet recording, a method for producing the aqueous ink, and an inkjet recording method using the aqueous ink, which can obtain a recorded matter having excellent storage stability and excellent image density even in recording on a low-absorbency recording medium.

Means for Solving the Problems

[0006] The present inventors have found that the above problems can be solved by incorporating pigment particles dispersed with a water-insoluble polymer and wax particles dispersed with a specific surfactant and a water-insoluble polymer into an aqueous ink. That is, the present invention relates to the following [1] to [3]. [1] An aqueous ink for inkjet recording, containing pigment particles (A) dispersed with a water-insoluble polymer (a), wax particles (B) dispersed with a surfactant (I) and a water-insoluble polymer (b), a water-soluble organic solvent (C), and water, An aqueous ink for inkjet recording, wherein the surfactant (I) is at least one selected from the group consisting of a nonionic surfactant and an anionic surfactant. [2] An inkjet recording method of recording on a recording medium using the aqueous ink according to [1]. [3] A method for producing the aqueous ink for inkjet recording according to [1], including a step of mixing an aqueous dispersion of the pigment particles (A), an aqueous dispersion of the wax particles (B), and the water-soluble organic solvent (C).

Effects of the Invention

[0007] According to the present invention, it is possible to provide an aqueous ink for inkjet recording, a method for producing the aqueous ink, and an inkjet recording method using the aqueous ink, which are excellent in storage stability and can obtain a recorded matter excellent in image density even in recording on a low liquid-absorbing recording medium.

Modes for Carrying Out the Invention

[0008] [Aqueous Ink for Inkjet Recording] The aqueous ink for inkjet recording of the present invention (hereinafter, also simply referred to as "aqueous ink" or "ink") contains pigment particles (A) dispersed with a water-insoluble polymer (a), wax particles (B) dispersed with a surfactant (I) and a water-insoluble polymer (b), a water-soluble organic solvent (C), and water, and the surfactant (I) is at least one selected from the group consisting of a nonionic surfactant and an anionic surfactant. In the present invention, "aqueous" means that water occupies the largest mass ratio in the medium. In the present invention, "wax" means an organic substance that is solid or semi-solid at room temperature (25°C) and becomes liquid by heating. Here, "the wax is semi-solid" means that the wax deforms and flows when a force is applied, but can maintain a certain shape when no force is applied to the wax. Further, the temperature at which the wax becomes liquid by heating, that is, the so-called melting point of the wax, exists in a temperature range of 45°C or higher. In addition, the term "low liquid absorption" is a concept that includes low liquid absorption and non-liquid absorption, and the amount of water absorption of the recording medium when the recording medium is in contact with pure water for 100 ms is 0 g / m 2 More than 10g / m 2 "Highly absorbent" means that the water absorption of the recording medium is 10 g / m2 or less when the recording medium is in contact with pure water for 100 ms. 2 The water absorption amount can be measured using an automatic scanning absorptivity meter (for example, KM500win manufactured by Kumagai Riki Kogyo Co., Ltd.) as the transferred amount in a contact time of 100 msec with pure water under conditions of 23° C. and relative humidity of 50%.

[0009] According to the present invention, it is possible to obtain a recorded matter having excellent storage stability of the water-based ink and excellent image density even when recorded on a low liquid-absorbent recording medium. The reason for this is not clear, but is thought to be as follows. In the present invention, the wax is contained in the aqueous ink in the form of wax particles dispersed with one or more surfactants selected from the group consisting of nonionic surfactants and anionic surfactants and a water-insoluble polymer. Wax particles in this form can suppress the detachment of the surfactant from the wax surface even in an aqueous ink containing a water-soluble organic solvent, and it is considered that the dispersion stability of the wax particles can be improved and the storage stability can be improved by the steric repulsion of the hydrophilic group of the nonionic surfactant or the electrostatic repulsion of the anionic group of the anionic surfactant and the steric repulsion between the water-insoluble polymers. Even when the effect of improving the dispersion stability of wax particles by the water-insoluble polymer is not sufficiently exerted due to insufficient adsorption of the water-insoluble polymer onto the wax particles, it is considered that coarsening of the wax particles can be suppressed because the effect of improving the dispersion stability of the wax particles can be supplemented by a low-molecular surfactant. Further, since both the pigment particles and the wax particles are dispersed by the water-insoluble polymer, the surface state of the wax particles and the surface state of the pigment particles can be made similar, thereby suppressing heteroaggregation. As a result, the storage stability can be improved, and even in recording on a low liquid-absorbing recording medium, an ink coating film having good smoothness can be formed, and irregular reflection can be suppressed, so that a recorded matter having a high image density can be obtained.

[0010] <Pigment particles (A) dispersed with a water-insoluble polymer (a)> The aqueous ink of the present invention contains pigment particles (A) (hereinafter also referred to as "pigment particles (A)") dispersed with a water-insoluble polymer (a) from the viewpoint of improving the dispersion stability of the pigment, improving the storage stability and the image density. The form of the pigment particles (A) in the aqueous ink of the present invention only needs to be composed of a pigment and a water-insoluble polymer (a), and includes a form in which the water-insoluble polymer (a) encapsulates (capsules) the pigment, a form in which the pigment is uniformly dispersed in the water-insoluble polymer (a), a form in which the pigment is exposed from the surface of the particles of the water-insoluble polymer (a), a form in which the water-insoluble polymer (a) is adsorbed on the pigment, and mixtures thereof.

[0011] (Pigment) The pigment constituting the pigment particles (A) may be either an inorganic pigment or an organic pigment, and lake pigments and fluorescent pigments can also be used. Further, if necessary, they can be used in combination with extender pigments. Examples of inorganic pigments include metal oxides such as carbon black, titanium oxide, iron oxide, red iron oxide, chromium oxide, and nacreous pigments. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and threne pigments. In achromatic inks, achromatic pigments such as white, black, and gray can be used, and in chromatic inks, chromatic organic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used. Specific examples of preferred organic pigments include one or more product numbers selected from the group consisting of C.I. Pigment Yellow, C.I. Pigment Red, C.I. Pigment Orange, C.I. Pigment Violet, C.I. Pigment Blue, and C.I. Pigment Green. Examples of extender pigments include silica, calcium carbonate, and talc. The pigments can be used alone or in combination of two or more.

[0012] (Water-insoluble polymer (a)) Regarding the "water-insolubility" of the water-insoluble polymer (a) (hereinafter also referred to as "polymer (a)") constituting the pigment particles (A), when a resin dried at 105°C for 2 hours until a constant weight is dissolved in 100 g of water at 25°C until saturation, if the dissolved amount is 10 g or less, it is judged as "water-insoluble". Further, when the polymer (a) has an anionic group and the anionic group is neutralized with a neutralizing agent as described below, it is judged based on the dissolved amount measured under the condition that the neutralizing agent is present under the condition that the mass ratio of the polymer (a) to the neutralizing agent is the same as that in the aqueous ink of the present invention. Polymer (a) may be used alone or in combination of two or more.

[0013] Examples of the polymer (a) include vinyl polymers obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds), polyesters, and polyurethanes. Among these, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, the polymer (a) is preferably a vinyl polymer obtained by addition polymerization of vinyl monomers, and more preferably a vinyl polymer having an anionic group. Here, the "anionic group" refers to an anionic group or a group that can be ionized to become an anionic group. Examples of the anionic group include a carboxy group (-COOM), a sulfonic acid group (-SO 3 M), and a phosphoric acid group (-OPO 3 M 2 ). In the above chemical formula, M represents a hydrogen atom, an alkali metal, ammonium, or an organic ammonium. Among these, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, the anionic group is preferably a carboxy group. That is, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, the polymer (a) is more preferably a vinyl polymer containing a structural unit derived from a carboxy group-containing vinyl monomer (a-1), and even more preferably a vinyl polymer containing a structural unit derived from a carboxy group-containing vinyl monomer (a-1) and a structural unit derived from a hydrophobic vinyl monomer (a-2).

[0014] Examples of the carboxy group-containing vinyl monomer (a-1) preferably include one or more selected from the group consisting of acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid. Among these, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, the carboxy group-containing vinyl monomer (a-1) is more preferably one or more selected from the group consisting of acrylic acid and methacrylic acid.

[0015] The "hydrophobicity" of the hydrophobic vinyl monomer (a-2) means that when the monomer is dissolved until saturated in 100 g of ion-exchanged water at 25°C, the dissolved amount is less than 10 g. Specific examples of the hydrophobic vinyl monomer (a-2) include those described in paragraphs

[0020] to

[0022] of JP-A-2018-83938. Among these, as the hydrophobic vinyl monomer (a-2), from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms, aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end are preferably mentioned, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms and aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms are more preferably mentioned, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth) acrylate are further preferably mentioned, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms, styrene, and benzyl (meth) acrylate are even more preferably mentioned, and one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms and styrene are even more preferably mentioned. In this specification, "(meth) acrylate" means at least one selected from the group consisting of acrylate and methacrylate.

[0016] In addition to the structural units derived from the carboxy group-containing vinyl monomer (a-1) and the structural units derived from the hydrophobic vinyl monomer (a-2), the polymer (a) may further contain structural units derived from the nonionic vinyl monomer (a-3) from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density. The nonionic vinyl monomer (a-3) is a monomer having a high affinity for water and water-soluble organic solvents, and is, for example, a monomer containing a hydroxyl group or a polyalkylene glycol chain. Examples of the nonionic vinyl monomer (a-3) include those described in paragraph

[0018] of JP-A-2018-83938. Among these, one or more selected from the group consisting of methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferable. Here, n represents the average number of moles of alkylene oxide added. As the above components (a-1) to (a-3), the monomer components contained in each component can be used alone or in combination of two or more.

[0017] The content of the structural unit derived from the carboxy group-containing vinyl monomer (a-1) in the polymer (a) is preferably 5% by mass or more, more preferably 7% by mass or more, still more preferably 10% by mass or more, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, and is preferably 40% by mass or less, more preferably 30% by mass or less, still more preferably 20% by mass or less, from the same viewpoint as above. The content of the structural unit derived from the hydrophobic vinyl monomer (a-2) in the polymer (a) is preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, and is preferably 95% by mass or less, more preferably 93% by mass or less, still more preferably 90% by mass or less, from the same viewpoint as above. The polymer (a) can be obtained, for example, by addition polymerization of a raw material monomer containing a carboxy group-containing vinyl monomer (a-1), a hydrophobic vinyl monomer (a-2), or a nonionic vinyl monomer (a-3) by a known method.

[0018] In the present invention, the polymer (a) constituting the pigment particles (A) is preferably crosslinked with a crosslinking agent from the viewpoints of improving the dispersion stability of the pigment, and improving the storage stability and image density. The polymer (a) crosslinked with a crosslinking agent (hereinafter also referred to as "crosslinked polymer (a)") is considered to have a three-dimensional structure formed by a constituent component derived from the crosslinking agent in a polymer (a) having a two-dimensional structure which may have branched chains. In the present invention, due to this three-dimensional structure, the crosslinked polymer (a) is firmly adsorbed or immobilized on the pigment surface, aggregation of the pigment in the aqueous ink is suppressed, and furthermore, swelling of the crosslinked polymer (a) is also suppressed. Therefore, it is considered that the storage stability and image density of the aqueous ink can be further improved. When the polymer (a) is crosslinked with a crosslinking agent, the polymer (a) (crosslinked polymer (a)) is preferably a polymer obtained by crosslinking a vinyl polymer containing a structural unit derived from the above-mentioned carboxy group-containing vinyl monomer (a-1) and a structural unit derived from a hydrophobic vinyl monomer (a-2) with a crosslinking agent from the viewpoints of improving the dispersion stability of the pigment, and improving the image density and storage stability.

[0019] (Crosslinking agent) The crosslinking agent is a compound having two or more crosslinkable functional groups in the molecule, and preferably a compound having two or more epoxy groups in the molecule (hereinafter also referred to as "epoxy group-containing compound") from the viewpoints of improving the dispersion stability of the pigment, and improving the storage stability and image density. The crosslinking agent may be water-soluble or water-insoluble, but from the viewpoint of more efficiently causing a crosslinking reaction with the carboxy group of the polymer (a) in a medium mainly composed of water, its water solubility rate is preferably 50% by mass or less, more preferably 40% by mass or less, still more preferably 35% by mass or less. Here, the water solubility rate (% by mass) refers to the dissolution rate (% by mass) of the crosslinking agent when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of water at room temperature of 25°C. The epoxy group-containing compound is preferably a compound having two or more glycidyl ether groups in the molecule, and more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group with 3 to 8 carbon atoms. From the viewpoint of more efficiently causing a crosslinking reaction with the carboxy groups of the polymer (a) in a medium mainly composed of water, the epoxy equivalent of the epoxy group-containing compound is preferably 90 or more, more preferably 100 or more, still more preferably 110 or more, and is preferably 300 or less, more preferably 200 or less, still more preferably 170 or less.

[0020] The epoxy group-containing compound can be used alone or in combination of two or more. Specific examples of the epoxy group-containing compound include polyglycidyl ethers such as polypropylene glycol diglycidyl ether, glycerin polyglycidyl ether, polyglycerin polyglycidyl ether, trimethylolpropane polyglycidyl ether, 1,6-hexanediol diglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, and hydrogenated bisphenol A type diglycidyl ether. Among these, one or more selected from the group consisting of 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether are preferable.

[0021] From the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, the crosslinking rate of the crosslinked polymer (a) is preferably 3 mol% or more, more preferably 5 mol% or more, still more preferably 10 mol% or more, and is preferably 30 mol% or less, more preferably 25 mol% or less, still more preferably 20 mol% or less. Here, the crosslinking rate is calculated by the following formula from the acid value of the polymer (a) before crosslinking and the equivalent of the crosslinkable functional group of the crosslinking agent. Crosslinking rate = [(amount of crosslinking agent (g) / equivalent of crosslinkable functional group of crosslinking agent (g / eq))] / [((acid value of polymer (a) (mgKOH / g) / (56.1 × 1000)) × amount of polymer (a))]

[0022] The number average molecular weight of polymer (a) is preferably 5,000 or more, more preferably 7,000 or more, still more preferably 10,000 or more, and preferably 100,000 or less, more preferably 50,000 or less, still more preferably 30,000 or less, from the viewpoints of improving the dispersion stability of the pigment and improving the storage stability and image density. The number average molecular weight is measured by the method described in the examples.

[0023] The acid value of polymer (a) is preferably 60 mgKOH / g or more, more preferably 70 mgKOH / g or more, still more preferably 80 mgKOH / g or more, even more preferably 90 mgKOH / g or more, and preferably 800 mgKOH / g or less, more preferably 500 mgKOH / g or less, still more preferably 300 mgKOH / g or less, even more preferably 200 mgKOH / g or less, even more preferably 150 mgKOH / g or less, even more preferably 130 mgKOH / g or less, from the viewpoints of improving the dispersion stability of the pigment and improving the storage stability and image density. The acid value of polymer (a) can be determined by the method described in the examples, but can also be calculated from the mass ratio of the constituent monomers. Further, when polymer (a) is crosslinked with a crosslinking agent, the acid value of the crosslinked polymer (a), that is, the acid value of the crosslinked polymer (a) can also be calculated by the following formula. Acid value of crosslinked polymer (a) (mgKOH / g) = [Acid value of polymer (a) (mgKOH / g) × ((100 - crosslinking rate (mol%)) / 100)] The crosslinking rate (mol%) is a value calculated from the acid value of polymer (a) before crosslinking and the equivalent of the crosslinkable functional group of the crosslinking agent, as described above.

[0024] (Production of pigment particles (A)) In the present invention, the pigment particles (A) are preferably incorporated into the aqueous ink in the form of an aqueous dispersion (hereinafter also referred to as "pigment aqueous dispersion") dispersed in an aqueous medium. The aqueous dispersion of the pigment particles (A) can be obtained by dispersing a pigment, polymer (a), and, if necessary, a neutralizing agent or the like by a known method. As a method for producing the aqueous dispersion of the pigment particles (A), a method of subjecting an aqueous dispersion of a pigment and a polymer (a) to a dispersion treatment is preferably mentioned. Further, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, it is preferable to further add a crosslinking agent to the obtained aqueous dispersion of the pigment particles (A) and crosslink the polymer (a) with the crosslinking agent. When the pigment particles (A) are incorporated into an aqueous ink as an aqueous dispersion, the average particle diameter of the pigment particles (A) in the aqueous dispersion is preferably 30 nm or more, more preferably 50 nm or more, still more preferably 70 nm or more, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, and, from the same viewpoint as above, preferably 600 nm or less, more preferably 400 nm or less, still more preferably 300 nm or less, even more preferably 200 nm or less. The average particle diameter of the pigment particles (A) in the aqueous dispersion can be measured by the method described in the examples. The pigment particles (A) contained in the aqueous ink of the present invention are preferably those in which swelling, shrinkage, and aggregation between the particles hardly occur. In this case, it is considered that the average particle diameter of the pigment particles (A) in the aqueous ink of the present invention is the same as the average particle diameter of the pigment particles (A) in the aqueous dispersion. From this viewpoint, the preferred embodiment of the average particle diameter of the pigment particles (A) in the aqueous ink of the present invention is the same as the preferred embodiment of the average particle diameter of the pigment particles (A) in the aqueous dispersion. The average particle diameter of the pigment particles (A) in the aqueous ink of the present invention is also measured by the same method as the method described in the examples.

[0025] <Wax particles (B) dispersed with surfactant (I) and water-insoluble polymer (b)> From the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, the aqueous ink of the present invention contains wax particles (B) (hereinafter also referred to as "wax particles (B)") dispersed with surfactant (I) and water-insoluble polymer (b). The wax particles (B) are particles composed of a wax (w), a surfactant (I), and a water-insoluble polymer (b), and the wax (w) is dispersed with the surfactant (I) and the water-insoluble polymer (b).

[0026] (Wax (w)) The wax (w) that constitutes the wax particles (B) (hereinafter also referred to as "wax (w)") may be either natural wax or synthetic wax. Examples of natural waxes include petroleum waxes such as paraffin wax and microcrystalline wax; vegetable waxes such as carnauba wax, candelilla wax, and rice wax; and animal waxes such as lanolin and beeswax. Examples of synthetic waxes include synthetic hydrocarbon waxes such as polyolefin wax and Fischer-Tropsch wax; silicone waxes; and modified waxes such as paraffin wax derivatives, montan wax derivatives, and microcrystalline wax derivatives. Among these, from the viewpoints of improving storage stability and image density, polyolefin wax mainly composed of olefin monomers is preferable. The wax (w) can be used alone or in combination of two or more.

[0027] From the viewpoints of improving storage stability and the maintainability of the inkjet recording apparatus, the melting point of the wax (w) is preferably 80°C or higher, more preferably 90°C or higher, still more preferably 100°C or higher, even more preferably 110°C or higher, and even more preferably 115°C or higher, and is preferably 150°C or lower, more preferably 140°C or lower, and still more preferably 130°C or lower. The melting point of the wax (w) is measured by the method described in the examples.

[0028] (Surfactant (I)) The surfactant (I) is one or more selected from the group consisting of nonionic surfactants and anionic surfactants. The surfactant (I) can be used alone or in combination of two or more.

[0029] As the nonionic surfactant, for example, one or more selected from the group consisting of polyethylene glycol type surfactants, polyhydric alcohol type surfactants, and fatty acid alkanolamides are preferably mentioned, one or more selected from the group consisting of polyethylene glycol type surfactants and polyhydric alcohol type surfactants are more preferable, and polyethylene glycol type surfactants are even more preferable. Examples of the polyethylene glycol type surfactant include polyoxyethylene alkyl ether, polyoxyethylene alkenyl ether, polyoxyethylene aryl ether, polyoxyethylene polyoxypropylene alkyl ether, polyoxyethylene fatty acid ester, polyoxyethylene hydrogenated castor oil, and polyoxyethylene sorbitan fatty acid ester. Among these, the nonionic surfactant is more preferably one or more selected from the group consisting of polyoxyethylene alkyl ether, polyoxyethylene alkenyl ether, and polyoxyethylene aryl ether.

[0030] The alkyl group in the polyoxyethylene alkyl ether preferably has 8 to 22 carbon atoms, more preferably 10 to 20 carbon atoms, and even more preferably 12 to 18 carbon atoms. Examples of the polyoxyethylene alkyl ether preferably include one or more selected from the group consisting of polyoxyethylene lauryl ether, polyoxyethylene myristyl ether, polyoxyethylene cetyl ether, and polyoxyethylene stearyl ether. The alkenyl group in the polyoxyethylene alkenyl ether preferably has 8 to 22 carbon atoms, more preferably 10 to 20 carbon atoms, and even more preferably 12 to 18 carbon atoms. Examples of the polyoxyethylene alkenyl ether preferably include polyoxyethylene oleyl ether. Examples of the polyoxyethylene aryl ether preferably include one or more selected from the group consisting of polyoxyethylene distyrenated phenyl ether and polyoxyethylene tribenzyl phenyl ether. The average number of moles of ethylene oxide added in polyoxyethylene alkyl ethers, polyoxyethylene aryl ethers, and polyoxyethylene alkenyl ethers is preferably 6 or more, more preferably 8 or more, still more preferably 10 or more, from the viewpoint of improving the dispersion stability of the wax, and improving the storage stability and image density. From the same viewpoint as above, it is preferably 20 or less, more preferably 16 or less, still more preferably 14 or less.

[0031] Examples of the anionic surfactant preferably include one or more selected from the group consisting of polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl ether carboxylates, polyoxyethylene alkyl ether phosphates, saturated fatty acid salts, and unsaturated fatty acid salts. The counter ion of the anionic group of the anionic surfactant is preferably one or more selected from the group consisting of alkali metal ions, ammonium ions (NH 4 + ), and organic ammonium ions. Suitable examples of the organic ammonium ion include ammonium ions of alkylamines and ammonium ions of alcoholamines. Among these, the organic ammonium ion is preferably an ammonium ion of an alcoholamine, more preferably an ammonium ion of ethanolamine, and still more preferably an ammonium ion of diethylethanolamine. Examples of the anionic surfactant preferably include one or more selected from the group consisting of sodium stearate, potassium stearate, diethylethanolamine stearate, triethanolamine stearate, sodium laurate, potassium laurate, diethylethanolamine laurate, triethanolamine stearate, sodium oleate, potassium oleate, diethylethanolamine oleate, and triethanolamine stearate. More preferably, one or more selected from the group consisting of diethylethanolamine oleate and potassium oleate are more preferred, and diethylethanolamine oleate is still more preferred.

[0032] Among these, the surfactant (I) is preferably a nonionic surfactant, more preferably a polyethylene glycol type surfactant, and still more preferably one or more selected from the group consisting of polyoxyethylene alkyl ethers, polyoxyethylene alkenyl ethers, and polyoxyethylene aryl ethers, from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density. Even more preferably, it is one or more selected from the group consisting of polyoxyethylene alkyl ethers and polyoxyethylene alkenyl ethers, from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density.

[0033] (Water-insoluble polymer (b)) Regarding the "water-insolubility" of the water-insoluble polymer (b) (hereinafter also referred to as "polymer (b)") constituting the wax particles (B), when the resin dried at 105°C for 2 hours until a constant weight is dissolved in 100 g of water at 25°C until saturation is reached, if the dissolved amount is 10 g or less, it is judged as "water-insoluble". Further, when the polymer (b) has an anionic group and the anionic group is neutralized with a neutralizing agent as described below, it is judged by the dissolved amount measured under the condition that the neutralizing agent is present under the condition that the mass ratio of the polymer (b) to the neutralizing agent is the same as that in the aqueous ink of the present invention. The polymer (b) may be used alone or in combination of two or more.

[0034] Examples of the polymer (b) include vinyl polymers obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds), polyesters, and polyurethanes. Among these, the polymer (b) is preferably a vinyl polymer obtained by addition polymerization of vinyl monomers, and more preferably a vinyl polymer having an anionic group, from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density. Here, the "anionic group" means an anion group or a group that can be ionized to become an anion group, as described above. Examples of the anionic group include a carboxy group (-COOM), a sulfonic acid group (-SO3 M), phosphate group (-OPO 3 M 2 ) and the like. In the above chemical formula, M represents a hydrogen atom, an alkali metal, ammonium, or an organic ammonium. Among these, from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, the anionic group is preferably a carboxy group. That is, the polymer (b) is more preferably a vinyl polymer containing a structural unit derived from a carboxy group-containing vinyl monomer (b-1) from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, and even more preferably a vinyl polymer containing a structural unit derived from a carboxy group-containing vinyl monomer (b-1) and a structural unit derived from a hydrophobic vinyl monomer (b-2).

[0035] The carboxy group-containing vinyl monomer (b-1) is preferably at least one selected from the group consisting of, for example, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid. Among these, the carboxy group-containing vinyl monomer (b-1) is more preferably at least one selected from the group consisting of acrylic acid and methacrylic acid from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density.

[0036] The "hydrophobicity" of the hydrophobic vinyl monomer (b-2) means that when the monomer is dissolved until saturated in 100 g of ion-exchanged water at 25 °C, the dissolved amount is less than 10 g. Specific examples of the hydrophobic vinyl monomer (b-2) include those described in paragraphs

[0020] to

[0022] of JP-A-2018-83938. Among them, as the hydrophobic vinyl monomer (b-2), from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms, aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end are preferably mentioned, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms and aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms are more preferable, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth) acrylate are still more preferable, one or more selected from the group consisting of alkyl (meth) acrylates having an alkyl group with 1 to 22 carbon atoms, styrene, and benzyl (meth) acrylate are even more preferable, one or more selected from the group consisting of methyl (meth) acrylate, ethyl (meth) acrylate, stearyl (meth) acrylate, styrene, α-methylstyrene, and benzyl (meth) acrylate are even more preferable, and one or more selected from the group consisting of styrene and benzyl (meth) acrylate are even more preferable. In the present specification, "(meth) acrylate" means at least one selected from the group consisting of acrylate and methacrylate.

[0037] In addition to the structural units derived from the carboxy group-containing vinyl monomer (b-1) and the hydrophobic vinyl monomer (b-2), the polymer (b) may further contain a structural unit derived from a nonionic vinyl monomer (b-3) from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density. The nonionic vinyl monomer (b-3) is a monomer having a high affinity for water or a water-soluble organic solvent, and is, for example, a monomer containing a hydroxyl group or a polyalkylene glycol chain. Examples of the nonionic vinyl monomer (b-3) include those described in paragraph

[0018] of JP-A-2018-83938. Among them, one or more selected from the group consisting of methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferable. Here, n represents the average number of moles of alkylene oxide added. As the above components (b-1) to (b-3), the monomer components contained in each component can be used alone or in admixture of two or more.

[0038] From the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, the content of the structural unit derived from the carboxy group-containing vinyl monomer (b-1) in the polymer (b) is preferably 5% by mass or more, more preferably 7% by mass or more, still more preferably 10% by mass or more, and from the same viewpoint as above, preferably 40% by mass or less, more preferably 30% by mass or less, still more preferably 20% by mass or less. From the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, the content of the structural unit derived from the hydrophobic vinyl monomer (b-2) in the polymer (b) is preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, and from the same viewpoint as above, preferably 95% by mass or less, more preferably 93% by mass or less, still more preferably 90% by mass or less. The polymer (b) can be obtained, for example, by addition polymerization of a raw material monomer containing a carboxy group-containing vinyl monomer (b-1), a hydrophobic vinyl monomer (b-2), or a nonionic vinyl monomer (b-3) by a known method.

[0039] In the present invention, the polymer (b) may be an uncrosslinked polymer, but from the viewpoint of improving the dispersion stability of the wax and improving the image density and storage stability, those crosslinked with a crosslinking agent are preferred. The polymer (b) crosslinked with a crosslinking agent (hereinafter also referred to as "crosslinked polymer (b)") is considered to have a three-dimensional structure by the constituent components derived from the crosslinking agent, with a polymer (b) having a two-dimensional structure that may have branched chains. In the present invention, due to this three-dimensional structure, the crosslinked polymer (b) is firmly adsorbed or immobilized on the wax surface, aggregation of the wax in the aqueous ink is suppressed, and further swelling of the crosslinked polymer (b) is suppressed. Therefore, it is considered that the storage stability and image density of the aqueous ink can be further improved. When the polymer (b) is crosslinked with a crosslinking agent, the polymer (b) (crosslinked polymer (b)) is preferably a polymer obtained by crosslinking a vinyl polymer containing a structural unit derived from the above-mentioned carboxy group-containing vinyl monomer (b-1) and a structural unit derived from a hydrophobic vinyl monomer (b-2) with a crosslinking agent, from the viewpoint of improving the dispersion stability of the wax and improving the image density and storage stability.

[0040] (Crosslinking agent) As the crosslinking agent, those similar to the crosslinking agent used in the above-mentioned crosslinked polymer (a) are preferred. Among them, from the viewpoint of further improving the storage stability and image density, the crosslinking agent is preferably the above-mentioned epoxy group-containing compound, more preferably a compound having two or more glycidyl ether groups in the molecule, and still more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group with 3 to 8 carbon atoms, and even more preferably one or more selected from the group consisting of 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

[0041] The crosslinking rate of the crosslinked polymer (b) is preferably 3 mol% or more, more preferably 5 mol% or more, still more preferably 10 mol% or more, from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, and is preferably 30 mol% or less, more preferably 25 mol% or less, still more preferably 20 mol% or less. Here, the crosslinking rate is calculated by the following formula from the acid value of the polymer (b) before crosslinking and the equivalent of the crosslinkable functional group of the crosslinking agent. Crosslinking rate = [(amount of crosslinking agent (g) / equivalent of crosslinkable functional group of crosslinking agent (g / eq))] / [((acid value of polymer (b) (mgKOH / g) / (56.1 × 1000)) × amount of polymer (b))]

[0042] The number average molecular weight of the polymer (b) is preferably 5,000 or more, more preferably 7,000 or more, still more preferably 10,000 or more, from the viewpoint of improving the dispersion stability of the wax and improving the storage stability and image density, and is preferably 100,000 or less, more preferably 50,000 or less, still more preferably 30,000 or less. The number average molecular weight is measured by the method described in the examples.

[0043] The acid value of the polymer (b) is preferably 60 mgKOH / g or more, more preferably 70 mgKOH / g or more, still more preferably 80 mgKOH / g or more, even more preferably 90 mgKOH / g or more, from the viewpoint of improving the dispersion stability of the pigment and improving the storage stability and image density, and from the same viewpoint as above, is preferably 800 mgKOH / g or less, more preferably 500 mgKOH / g or less, still more preferably 300 mgKOH / g or less, even more preferably 200 mgKOH / g or less, even more preferably 150 mgKOH / g or less, even more preferably 130 mgKOH / g or less. The acid value of the polymer (b) can be determined by the method described in the examples, but can also be calculated from the mass ratio of the constituent monomers. Further, the acid value when the polymer (b) is crosslinked with a crosslinking agent, that is, the acid value of the crosslinked polymer (b), can also be calculated by the following formula. Acid value (mgKOH / g) of the crosslinked polymer (b) = [Acid value (mgKOH / g) of the polymer (b) × ((100 - Crosslinking rate (mol%)) / 100) The crosslinking rate (mol%) is a value calculated from the acid value of the polymer (b) before crosslinking and the equivalent weight of the crosslinkable functional group of the crosslinking agent, as described above.

[0044] (Production of wax particles (B)) In the present invention, the wax particles (B) are preferably incorporated into the aqueous ink in the form of an aqueous dispersion dispersed in an aqueous medium. As a preferred example of the method for producing the aqueous dispersion of the wax particles (B), a method in which the polymer (b), the surfactant (I), the wax (w), and, if necessary, a neutralizing agent are mechanically stressed using a dispersing device in the presence of an aqueous medium to be atomized can be mentioned. The average particle size of the wax particles (B) in the aqueous dispersion is preferably 30 nm or more, more preferably 50 nm or more, still more preferably 70 nm or more, and preferably 200 nm or less, more preferably 150 nm or less, still more preferably 100 nm or less. The average particle size of the wax particles (B) in the aqueous dispersion is measured by the method described in the examples. The wax particles (B) contained in the aqueous ink of the present invention are preferably those in which swelling, shrinkage, and aggregation between the particles hardly occur. In this case, it is considered that the average particle size of the wax particles (B) in the aqueous ink of the present invention is the same as the average particle size of the wax particles (B) in the aqueous dispersion. From this viewpoint, the preferred embodiment of the average particle size of the wax particles (B) in the aqueous ink of the present invention is the same as the preferred embodiment of the average particle size of the wax particles (B) in the aqueous dispersion. The average particle size of the wax particles (B) in the aqueous ink of the present invention is also measured by the same method as the method described in the examples.

[0045] <Water-soluble organic solvent (C)> The aqueous ink of the present invention contains a water-soluble organic solvent (C) from the viewpoint of improving the image density and storage stability. The water-soluble organic solvent (C) can be used alone or in combination of two or more. In the present invention, the "water-soluble organic solvent" refers to an organic solvent whose dissolution amount is 10 mL or more when the organic solvent is dissolved in 100 mL of water at 25°C. The boiling point of the water-soluble organic solvent (C) under atmospheric pressure is preferably 150°C or higher, more preferably 160°C or higher, still more preferably 170°C or higher, and preferably 350°C or lower, more preferably 300°C or lower, still more preferably 250°C or lower. When two or more water-soluble organic solvents are used in combination as the water-soluble organic solvent (C), the boiling point of the water-soluble organic solvent (C) is the weighted average value weighted by the content (% by mass) of each water-soluble organic solvent.

[0046] Examples of the water-soluble organic solvent (C) include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, from the viewpoint of improving image density and storage stability, it is preferably at least one selected from the group consisting of polyhydric alcohols and polyhydric alcohol alkyl ethers. A plurality of polyhydric alcohols included in the concept of polyhydric alcohols can be mixed and used. Similarly, a plurality of polyhydric alcohol alkyl ethers included in the concept of polyhydric alcohol alkyl ethers can be mixed and used.

[0047] As the polyhydric alcohol, at least one selected from the group consisting of diols and polyhydric alcohols having three or more hydroxyl groups is preferably mentioned. Examples of the diol include ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-hexanediol, 1,2-octanediol, 1,8-octanediol, 1,2-decanediol, 1,3-propanediol, 1,4-butanediol, 2-ethyl-1,3-hexanediol, diethylene glycol, and dipropylene glycol. Examples of the polyhydric alcohol having three or more hydroxyl groups include glycerin, trimethylolpropane, and pentaerythritol.

[0048] Examples of the polyhydric alcohol alkyl ether include ethylene glycol monoalkyl ethers such as ethylene glycol monoethyl ether and ethylene glycol monoisopropyl ether; diethylene glycol monoalkyl ethers such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether; tetraethylene glycol monoalkyl ethers such as tetraethylene glycol monomethyl ether; propylene glycol monoalkyl ethers such as propylene glycol monomethyl ether and propylene glycol monoethyl ether; dipropylene glycol monoalkyl ethers such as dipropylene glycol monomethyl ether; and tripropylene glycol monoalkyl ethers such as tripropylene glycol monomethyl ether, i.e., alkylene glycol-based monoalkyl ethers.

[0049] Among these, the water-soluble organic solvent (C) is more preferably at least one selected from the group consisting of propylene glycol, 1,2-butanediol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, and dipropylene glycol monomethyl ether. The water-soluble organic solvent (C) may further contain water-soluble organic solvents other than the above as long as the effects of the present invention are not inhibited.

[0050] <Water> The aqueous ink of the present invention contains water. As the water used in the ink of the present invention, pure water or ion-exchanged water is preferable from the viewpoint of preventing the contamination of unintended substances.

[0051] In addition to the above components, the aqueous ink of the present invention may further contain various additives such as surfactants, fixing aids, humectants, wetting agents, penetrants, defoamers, preservatives, antifungal agents, and rust inhibitors, which are usually used as required.

[0052] (Surfactant (II)) In addition to the aforementioned surfactant (I), the aqueous ink of the present invention preferably further contains a surfactant (II). The surfactant (II) is considered to have the effect of appropriately spreading the ink on a low liquid-absorbing recording medium and increasing the image density due to its high adsorption property to the ink-gas-liquid interface. The surfactant (II) can be used alone or in combination of two or more. The surfactant (II) is preferably at least one selected from the group consisting of acetylene-based surfactants and silicone-based surfactants.

[0053] Examples of the acetylene-based surfactant preferably include 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyne-3-ol, 2,4-dimethyl-5-hexyne-3-ol, and ethylene oxide (hereinafter also referred to as "EO") adducts thereof. Among these, more preferably, it is at least one selected from the group consisting of 2,4,7,9-tetramethyl-5-decyne-4,7-diol and EO adducts of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, and still more preferably 2,4,7,9-tetramethyl-5-decyne-4,7-diol.

[0054] Examples of the silicone-based surfactant preferably include polyether-modified silicone-based surfactants. As the polyether group of the polyether-modified silicone-based surfactant, for example, a polyalkyleneoxy group in which a polyethyleneoxy group, a polypropyleneoxy group, an ethyleneoxy group, and a propyleneoxy group (trimethyleneoxy group or propane-1,2-diyl group) are added in a block or random manner is suitable, and a compound in which a polyether group is grafted to the silicone main chain or a compound in which polyether groups are block-bonded to both ends of the silicone main chain can be used.

[0055] Examples of commercially available acetylene-based surfactants include "Surfynol 104" (2,4,7,9-tetramethyl-5-decyne-4,7-diol, average number of moles of ethylene oxide added: 0, HLB: 3.0), "Surfynol 104E" (50% ethylene glycol diluted product of 2,4,7,9-tetramethyl-5-decyne-4,7-diol), "Surfynol 104PG-50" (50% propylene glycol diluted product of 2,4,7,9-tetramethyl-5-decyne-4,7-diol), "Surfynol 420" (ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (average number of moles of ethylene oxide added: 1), HLB: 4.7), "Surfynol 440" (ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (average number of moles of ethylene oxide added: 3.5), HLB: 8 (catalog value)), "Surfynol 465" (ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (average number of moles of ethylene oxide added: 10), HLB: 13.1), and "Surfynol 485" (ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (average number of moles of ethylene oxide added: 30), HLB: 17.3) manufactured by Nissin Chemical Industry Co., Ltd. Examples of commercially available silicone-based surfactants include the "KF" series manufactured by Shin-Etsu Chemical Co., Ltd., "Silface SAG005" manufactured by Nissin Chemical Industry Co., Ltd., and "BYK-348" manufactured by BYK-Chemie Japan Co., Ltd.

[0056] [Method for producing an aqueous ink for inkjet recording] The method for producing the aqueous ink of the present invention preferably includes a step of mixing the above-described aqueous dispersion of pigment particles (A), the above-described aqueous dispersion of wax particles (B), and the water-soluble organic solvent (C). In the method for producing the aqueous ink of the present invention, the above-described various additives can be further added as needed, and filtration treatment can be performed using a filter or the like. In the method for producing the aqueous ink of the present invention, the mixing of the aqueous dispersion of pigment particles (A), the aqueous dispersion of wax particles (B), the water-soluble organic solvent (C), and the various additives is carried out by a conventional method.

[0057] (Content of each component in the aqueous ink for inkjet recording) From the viewpoint of image density, the content of the pigment particles (A) in the aqueous ink of the present invention is preferably 2% by mass or more, more preferably 3% by mass or more, still more preferably 4% by mass or more, and from the viewpoint of improving storage stability, it is preferably 16% by mass or less, more preferably 11% by mass or less, still more preferably 7% by mass or less. From the viewpoint of image density, the content of the pigment in the aqueous ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, still more preferably 3% by mass or more, and from the viewpoint of improving storage stability, it is preferably 15% by mass or less, more preferably 10% by mass or less, still more preferably 8% by mass or less. The mass ratio of the content of the pigment to the total content of the pigment and the polymer (a) in the aqueous ink of the present invention [pigment / (pigment + polymer (a))] is preferably 0.4 or more, more preferably 0.5 or more, still more preferably 0.6 or more, even more preferably 0.7 or more from the viewpoint of image density, and preferably 0.9 or less, more preferably 0.8 or less from the viewpoint of improving storage stability. When the polymer (a) is crosslinked with a crosslinking agent, the content of the polymer (a) in the aqueous ink of the present invention is the total content of the polymer (a) before crosslinking and the crosslinking agent.

[0058] From the viewpoint of improving image fastness, the content of the wax particles (B) in the aqueous ink of the present invention is preferably 0.3% by mass or more, more preferably 0.7% by mass or more, still more preferably 1% by mass or more, and from the viewpoint of improving storage stability, it is preferably 7% by mass or less, more preferably 5% by mass or less, still more preferably 3% by mass or less, even more preferably 2% by mass or less. From the viewpoint of improving image fastness, the content of wax (w) in the aqueous ink of the present invention is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, still more preferably 0.8% by mass or more. From the viewpoint of improving storage stability, it is preferably 3% by mass or less, more preferably 2.5% by mass or less, still more preferably 2.0% by mass or less, even more preferably 1.5% by mass or less, and even more preferably 1.0% by mass or less.

[0059] In the present invention, the mass ratio [surfactant (I) / polymer (b)] of the content of surfactant (I) to the content of polymer (b) constituting wax particles (B) is preferably 0.20 or more, more preferably 0.30 or more, still more preferably 0.40 or more, and even more preferably 0.50 or more from the viewpoint of improving image density. From the viewpoint of improving storage stability, it is preferably 1.80 or less, more preferably 1.50 or less, still more preferably 1.00 or less, and even more preferably 0.80 or less. When the polymer (b) is crosslinked with a crosslinking agent, the content of the polymer (b) in the aqueous ink of the present invention is the total content of the polymer (b) before crosslinking and the crosslinking agent. In the present invention, the mass ratio [surfactant (I) / wax (w)] of the content of surfactant (I) to the content of wax (w) constituting wax particles (B) is preferably 0.10 or more, more preferably 0.15 or more, still more preferably 0.20 or more from the viewpoint of improving image density. From the viewpoint of improving storage stability, it is preferably 1.00 or less, more preferably 0.80 or less, still more preferably 0.60 or less, and even more preferably 0.40 or less. In the present invention, the mass ratio [polymer (b) / wax (w)] of the content of polymer (b) to the content of wax (w) constituting wax particles (B) is preferably 0.10 or more, more preferably 0.20 or more, still more preferably 0.30 or more from the viewpoints of improving storage stability and image density. From the viewpoint of improving image fastness, it is preferably 1.00 or less, more preferably 0.95 or less, still more preferably 0.90 or less.

[0060] The mass ratio of the content of the wax particles (B) to the total content of the pigment particles (A) and the wax particles (B) in the aqueous ink of the present invention [wax particles (B) / (pigment particles (A) + wax particles (B))] is preferably 0.05 or more, more preferably 0.08 or more, still more preferably 0.10 or more from the viewpoint of improving the image fastness, and preferably 0.40 or less, more preferably 0.35 or less, still more preferably 0.30 or less from the viewpoint of improving the storage stability.

[0061] The content of the water-soluble organic solvent (C) in the aqueous ink of the present invention is preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 15% by mass or more from the viewpoints of improving the storage stability and the image density, and preferably 35% by mass or less, more preferably 30% by mass or less, still more preferably 25% by mass or less from the same viewpoints as above.

[0062] The content of the surfactant (II) in the aqueous ink of the present invention is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, still more preferably 0.5% by mass or more from the viewpoint of improving the image density, and preferably 3% by mass or less, more preferably 1% by mass or less, still more preferably 1% by mass or less from the same viewpoints as above.

[0063] The content of water in the aqueous ink of the present invention is preferably 40% by mass or more, more preferably 50% by mass or more, still more preferably 60% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, still more preferably 75% by mass or less.

[0064] (Physical properties of the aqueous ink for inkjet recording) The viscosity of the aqueous ink of the present invention at 32 °C is preferably 1 mPa·s or more, more preferably 2 mPa·s or more, still more preferably 3 mPa·s or more, and preferably 10 mPa·s or less, more preferably 7 mPa·s or less, still more preferably 5 mPa·s or less. The viscosity is measured by the method described in the examples. The pH of the aqueous ink of the present invention is preferably 7.0 or higher, more preferably 7.2 or higher, still more preferably 7.5 or higher. Further, from the viewpoints of member resistance and skin irritation, the pH is preferably 11 or lower, more preferably 10 or lower, still more preferably 9.5 or lower. The pH is measured by the method described in the examples.

[0065] [Inkjet recording method] The inkjet recording method using the aqueous ink of the present invention (hereinafter, also referred to as "the inkjet recording method of the present invention") is a method of recording on a recording medium using the above-described aqueous ink. In the inkjet recording method of the present invention, from the viewpoint of ejection property, the piezo type is preferable as the method of ejecting the aqueous ink.

[0066] Examples of the recording medium used in the inkjet recording method of the present invention include high-absorbency plain paper, low-absorbency coated paper, and resin films. Among these, from the viewpoint of obtaining a recording having high image density while having excellent image fastness, the low-absorbency recording medium is preferable for the aqueous ink of the present invention. Examples of the coated paper include general-purpose glossy paper and multicolor foam gloss paper. Examples of the resin film include films using synthetic resins. Examples of such synthetic resins include polyolefin resins such as polyethylene resin and polypropylene resin; polyester resins such as polyethylene terephthalate resin; and polyvinyl chloride resin. The resin film may be a biaxially stretched film, a uniaxially stretched film, or an unstretched film, and may also be a film subjected to corona discharge treatment. Among these, the recording medium used in the inkjet recording method of the present invention is preferably a low-absorbency recording medium.

Examples

[0067] In the following preparation examples, production examples, examples, and comparative examples, "parts" means "parts by mass" unless otherwise specified. The measurement methods of each physical property, etc. are as follows.

[0068] (1) Measurement of the number average molecular weight of the polymer It was determined by gel permeation chromatography. The measurement conditions are shown below. GPC apparatus: "HLC-8320GPC" manufactured by Tosoh Corporation Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", "TSKgel guardcolumn Super AW-H" manufactured by Tosoh Corporation Eluent: A solution prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively Flow rate: 0.5 mL / min Standard substances: Monodisperse polystyrene kits with known molecular weights "PStQuick B (F-550, F-80, F-10, F-1, A-1000)", "PStQuick C (F-288, F-40, F-4, A-5000, A-500)" (manufactured by Tosoh Corporation) Measurement sample: 0.1 g of the polymer was mixed with 10 mL of the above eluent in a glass vial, stirred with a magnetic stirrer at 25°C for 10 hours, and filtered through a syringe filter (membrane filter material: hydrophilic PTFE, pore size: 0.2 μm, "DISMIC-13HP" manufactured by Advantec Co., Ltd.).

[0069] (2) Measurement of the acid value of the polymer The resin was dissolved in a titration solvent (toluene:acetone = 2:1 (volume ratio)) of toluene and acetone in an automatic potentiometric titrator (manufactured by Kyoto Electronics Industry Co., Ltd., motor-driven burette, model number: APB-610), and titrated with a 0.1 N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was taken as the end point. The acid value (mgKOH / g) was calculated from the titration volume up to the end point of the potassium hydroxide solution.

[0070] (3) Measurement of the average particle size of pigment particles (A) in the aqueous dispersion and the average particle size of wax particles (B) in the aqueous dispersion Using a laser particle analysis system ("ELS-8000" manufactured by Otsuka Electronics Co., Ltd.), the particle size was measured by the dynamic light scattering method and calculated by cumulant method analysis. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 integration times. The refractive index of water (1.333) was input as the refractive index of the dispersion medium. For the measurement sample, an aqueous dispersion of pigment particles (A) or an aqueous dispersion of wax particles (B) was weighed into a screw tube (manufactured by Maruemu Co., Ltd., No. 5), and water was added so that the solid content concentration became 2×10 -4 mass%, and the mixture was stirred at 25°C for 1 hour using a magnetic stirrer.

[0071] (4) Measurement of solid content concentration Weighed 10.0 g of sodium sulfate that had been constant-weighted in a desiccator into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm), added about 1.0 g of the sample and mixed them, then weighed accurately, maintained at 105°C for 2 hours to remove the volatile matter, and after leaving it in the desiccator for 15 minutes, measured the mass. The mass of the sample after removing the volatile matter was taken as the solid content, and divided by the mass of the added sample to obtain the solid content concentration (mass%).

[0072] (5) Measurement of melting point of wax (w) The melting point of wax (w) was measured using an apparatus compliant with JIS K 0064:1992. Specifically, using a differential scanning calorimeter (manufactured by TA Instruments, model: Q20), the sample was heated up to 200°C, and then cooled from that temperature to 0°C at a cooling rate of 10°C / min. Next, the sample was heated at a heating rate of 10°C / min and the heat quantity was measured up to 200°C. Among the observed melting heat peaks, the temperature of the peak with the largest peak area was taken as the maximum peak temperature of melting, and this peak temperature was taken as the melting point.

[0073] (6) Measurement of viscosity of aqueous ink Using an E-type viscometer ("TV-25" manufactured by Toki Sangyo Co., Ltd., standard cone rotor 1°34’×R24, rotation speed 50 rpm), the viscosity of the aqueous ink at 32°C was measured.

[0074] (7) Measurement of the pH of the aqueous ink Using a desktop pH meter (manufactured by Horiba, Ltd., "F-71") with a pH electrode (manufactured by Horiba, Ltd., "6337-10D"), the pH of the aqueous ink at 25 °C was measured.

[0075] Production Example 1 (Production of water-insoluble polymer (P1)) 14 parts of acrylic acid, 11 parts of ethyl acrylate, and 75 parts of styrene were mixed to prepare a monomer mixture. In a reaction vessel, 10 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), 0.2 part of 2-mercaptoethanol as a polymerization chain transfer agent, and 10% by mass of the monomer mixture were added and mixed, and sufficient nitrogen gas substitution was performed. On the other hand, in a dropping funnel, the remaining monomer mixture (90% by mass of the monomer mixture), 0.2 part of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Corporation, "V-65") as an azo radical polymerization initiator were mixed. While stirring the monomer mixture in the reaction vessel under a nitrogen atmosphere, the temperature was raised to 65 °C, and the mixture in the dropping funnel was dropped over 3 hours. After 2 hours had elapsed at 65 °C from the end of the dropping, a solution prepared by dissolving 0.1 part of the polymerization initiator in 2 parts of MEK was added, and after aging at 65 °C for 2 hours and at 70 °C for 2 hours, drying was performed under reduced pressure to obtain a water-insoluble polymer (P1) (number average molecular weight: 24,000, acid value: 109 mgKOH / g).

[0076] Production Examples 2 and 3 (Production of water-insoluble polymers (P2) and (P3)) In Production Example 1, water-insoluble polymers (P2) and (P3) were obtained in the same manner as in Production Example 1, except that the monomer composition shown in Table 1 was changed. The physical properties of each water-insoluble polymer are shown in Table 1.

[0077]

Table 1

[0078] Preparation Example I-1 (Production of Aqueous Dispersion of Pigment Particles (A’1)) 32 parts of the water-insoluble polymer (P1) obtained in Production Example 1 was mixed with 202 parts of ion-exchanged water, and further 9.4 parts of 5N aqueous sodium hydroxide solution (sodium hydroxide solid content: 16.9% by mass) was added. Neutralization was carried out so that the ratio of the number of moles of sodium hydroxide to the number of moles of carboxy groups of the water-insoluble polymer (P1) was 65 mol% (neutralization degree: 65 mol%). After heating to 90 °C using a warm bath and stirring for 1 hour to disperse the water-insoluble polymer (P1) in water, it was cooled to room temperature (25 °C) to obtain an aqueous dispersion of the water-insoluble polymer (P1). To the aqueous dispersion of the water-insoluble polymer (P1) obtained above as the polymer (a), 100 parts of carbon black pigment (CB, C.I. Pigment Black 7, "Monarch 800" manufactured by Cabot Chemical Co., Ltd.) was added, and using a disper (Ultra Disper manufactured by Asada Iron Works Co., Ltd.), stirring was carried out at 20 °C under the condition that the disper blade was rotated at 6,000 rpm for 3 hours. Next, 124 parts of ion-exchanged water was added, and dispersion treatment was carried out at a pressure of 150 MPa for 15 passes using a microfluidizer (manufactured by Microfluidics, trade name). The obtained dispersion was put into a 500 mL angle rotor, centrifuged at 3,660 rpm for 20 minutes using a high-speed cooling centrifuge ("himac CR22G" manufactured by Hitachi Koki Co., Ltd., set temperature 20 °C), and then the liquid phase part was recovered and filtered through a 5 μm membrane filter made of acetyl cellulose to obtain an aqueous dispersion of pigment particles (A1) (solid content concentration: 25% by mass). 100 parts of the aqueous dispersion of the pigment particles (A1) obtained above was taken into a glass bottle with a screw cap, 26 parts of ion-exchanged water was added, and 0.2 part of trimethylolpropane polyglycidyl ether ("Denacol EX-321" manufactured by Nagase ChemteX Corporation, epoxy equivalent: 140) (corresponding to a crosslinking rate of 15 mol%) was added as a crosslinking agent, sealed, and heated at 70 °C for 5 hours while stirring with a stirrer. After cooling to room temperature (25 °C), it was filtered through the 5 μm filter to obtain an aqueous dispersion of pigment particles (A’1) (acid value: 87 mgKOH / g) (solid content concentration: 20% by mass, pigment content: 15.1% by mass, crosslinked polymer (a) content: 4.9% by mass (component derived from the crosslinking agent is 0.2% by mass), average particle diameter: 105 nm).

[0079] Preparation Example II-1 (Preparation of Aqueous Dispersion of Wax Particles (B’1)) To 30 parts of the water-insoluble polymer (P1) obtained in Production Example 1 as the polymer (b), 8.9 parts of a 5N aqueous sodium hydroxide solution (NaOH solid content: 16.9% by mass) was added as a neutralizing agent to neutralize the carboxy groups of the polymer (P1) (equivalent amount of neutralizing agent used: 65 mol%). Further, 300 parts of ion-exchanged water and 20 parts of polyoxyethylene oleyl ether (carbon number of alkyl group: 18, average number of moles of ethylene oxide added: 12) as surfactant (I) were added. Into this, 80 parts of polyethylene wax (manufactured by Mitsui Chemicals, Inc., "Hi-Wax 200P", melting point: 122 °C, density: 0.970 g / cm 3 , weight average molecular weight: 2,000) (hereinafter referred to as "HW200P") was added and heated and dissolved at 85 to 95 °C. While maintaining the obtained mixture at 90 to 95 °C, it was subjected to dispersion treatment for 30 minutes using an ultrasonic homogenizer, then cooled to room temperature, and subjected to 3-pass dispersion treatment at a pressure of 200 MPa with a microfluidizer (manufactured by Microfluidics, trade name) to obtain an aqueous dispersion of wax particles (B1). To the aqueous dispersion of wax particles (B1) obtained above, 260 parts of ion-exchanged water was further added, and 1.2 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, "Denacol EX-321", epoxy equivalent: 140) (corresponding to a crosslinking rate of 15 mol%) was added as a crosslinking agent, sealed, and heated at 70 °C for 5 hours while stirring with a stirrer to obtain an aqueous dispersion of wax particles (B’1) (solid content concentration: 20% by mass, wax (w) content: 11.4% by mass, surfactant (I) content: 2.9% by mass, crosslinked polymer (b) content: 5.7% by mass (component derived from crosslinking agent: 0.2% by mass), average particle diameter: 78 nm).

[0080] Preparation Example II-2 and Comparative Preparation Example II-1 (Preparation of Aqueous Dispersion of Wax Particles (B2) and Aqueous Dispersion of Wax Particles (BC1)) In Preparation Example II-1, in the same manner except that the composition of the aqueous dispersion of wax particles (B) was changed as shown in Table 2 and the crosslinking treatment with a crosslinking agent was not performed, an aqueous dispersion of wax particles (B2) and an aqueous dispersion of wax particles (BC1) were obtained respectively.

[0081] Preparation Examples II-3 to II-12 and Comparative Preparation Example II-2 (Preparation of aqueous dispersions of wax particles (B'3) to (B'12) and an aqueous dispersion of wax particles (B'C2)) In Preparation Example II-1, in the same manner except that the composition of the aqueous dispersion of wax particles (B) was changed as shown in Table 2, aqueous dispersions of each wax particle (B) were obtained. The wax (w) and surfactant (I) other than the components used in Preparation Example II-1 shown in Table 2 are shown below. HW110P: "Hiwax 110P" manufactured by Mitsui Chemicals, Inc., polyolefin wax, melting point: 109 °C, density: 0.920 g / cm 3 , weight average molecular weight: 1,000 TW-131: "TOWAX-131" manufactured by Toagosei Co., Ltd., carnauba wax, melting point: 109 °C Polyoxyethylene stearyl ether: number of carbon atoms in the alkyl group 18, average number of moles of ethylene oxide added 12 Diethyl ethanolamine oleate: number of carbon atoms in the alkyl group 18

[0082]

Table 2

[0083] Example 1 26.5 parts of the aqueous dispersion of the pigment particles (A1’) obtained in Production Example I-1, 8.2 parts of the aqueous dispersion of the wax particles (B’1) obtained in Production Example II-1, 20 parts of propylene glycol (manufactured by AGC Inc.) (hereinafter referred to as “PG”) as the water-soluble organic solvent (C), 1.2 parts of an acetylene-based surfactant (Surfinol 104-PG50 (50% diluted product of 2,4,7,9-tetramethyl-5-decyn-4,7-diol with propylene glycol) manufactured by Nissin Chemical Industry Co., Ltd.) as the surfactant (II) (hereinafter referred to as “Surfinol 104-PG50”), and ion-exchanged water were added and stirred so that the total amount became 100 parts, and then filtered through a membrane filter (Mini Sartorius Syringe Filter manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate) to obtain aqueous ink 1 (viscosity at 32°C: 3.4 mPa·s, pH: 7.9).

[0084] Examples 2 to 15 and Comparative Examples 1 to 2 In Example 1, except that the composition shown in Table 3 was changed, each aqueous ink was obtained in the same manner. In Example 15, diethylene glycol monoisopropyl ether (manufactured by Nippon Emulsion Co., Ltd.) (hereinafter referred to as “iPDG”) was used as the water-soluble organic solvent (C) as shown in Table 3. Evaluation method

[0085] [Evaluation] Using each aqueous ink obtained in the examples and comparative examples, evaluation was carried out according to the following (1) and (2). The results are shown in Table 1.

[0086] (1) Evaluation of image density On A4-sized coated paper (OK Top Coat + manufactured by Oji Paper Co., Ltd., water absorption: 4.9 g / m 2 ), using each aqueous ink of the examples and comparative examples, a recording was made by the following inkjet recording method and evaluated. In an environment of temperature 25 ± 1°C and relative humidity 30 ± 5%, each aqueous ink was filled into a printing evaluation apparatus (manufactured by Trytec Co., Ltd.) equipped with an inkjet head (KJ4B-HD06MHG-STDV, piezo type, manufactured by Kyocera Corporation). The head voltage was set to 26 V, the frequency to 10 kHz, the appropriate ejection liquid volume to 12 pL, the head temperature to 25 °C, the resolution to 600 dpi, and the negative pressure to -4.0 kPa. The recording medium was fixed to the transport table under reduced pressure in the direction where the longitudinal direction and the transport direction of the recording medium were the same. A printing command was transferred to the printing evaluation device to form an image with a Duty of 100%. Immediately after image formation, the image formed on the recording medium was dried for 1 minute in a constant-temperature dryer (manufactured by Yamato Scientific Co., Ltd., model: DVS402) with the set temperature of 70 °C to obtain a recorded matter. After the obtained recorded matter was left standing at room temperature for 24 hours, the image densities at five arbitrary locations were measured using a spectrocolorimeter (X-Rite eXact manufactured by X-Rite) under the conditions of light source D50, viewing angle 2°, CIE color system, and filter T. The average value of the image densities at the five locations was calculated as the image density of the recorded matter. It is judged that the larger the numerical value of the image density, the better.

[0087] (2) Evaluation of storage stability Each aqueous ink of the examples and comparative examples was stored in a sealed container under a constant temperature of 70 °C, taken out after 28 days, and the average particle size after storage was measured. The average particle size change rate after storage at 70 °C for 28 days was calculated by the following formula (where the decimal part is rounded down). It is judged that the lower the average particle size change rate, the better the storage stability. Average particle size change rate (%) = [Average particle size after storage / Average particle size before storage] × 100

[0088]

Table 3

[0089] From Table 3, it can be seen that the aqueous inks of Examples 1 to 15 are superior in image density and storage stability compared to Comparative Examples 1 and 2.

Claims

1. An aqueous ink for inkjet recording, containing pigment particles (A) dispersed by a water-insoluble polymer (a), wax particles (B) dispersed by a surfactant (I) and a water-insoluble polymer (b), a water-soluble organic solvent (C), and water, wherein the surfactant (I) is at least one selected from the group consisting of a nonionic surfactant and an anionic surfactant.

2. The aqueous ink for inkjet recording according to claim 1, wherein the nonionic surfactant is at least one selected from the group consisting of polyoxyethylene alkyl ether, polyoxyethylene alkenyl ether, and polyoxyethylene aryl ether.

3. The aqueous ink for inkjet recording according to claim 1 or 2, wherein the water-insoluble polymer (b) is a vinyl polymer containing a structural unit derived from a carboxy group-containing vinyl monomer (b-1) and a structural unit derived from a hydrophobic vinyl monomer (b-2).

4. The aqueous ink for inkjet recording according to claim 3, wherein the water-insoluble polymer (b) is a polymer obtained by crosslinking the vinyl polymer with a crosslinking agent.

5. The aqueous ink for inkjet recording according to claim 3 or 4, wherein the carboxy group-containing vinyl monomer (b-1) is at least one selected from the group consisting of acrylic acid and methacrylic acid.

6. The aqueous ink for inkjet recording according to any one of claims 3 to 5, wherein the hydrophobic vinyl monomer (b-2) is at least one selected from the group consisting of alkyl (meth)acrylate having an alkyl group with 1 to 22 carbon atoms, styrene, and benzyl (meth)acrylate.

7. The aqueous ink for inkjet recording according to any one of claims 1 to 6, wherein the mass ratio [surfactant (I) / water-insoluble polymer (b)] of the content of the surfactant (I) to the content of the water-insoluble polymer (b) is 0.20 or more and 1.80 or less.

8. The aqueous ink for inkjet recording according to any one of claims 1 to 7, wherein the mass ratio [surfactant (I) / wax (w)] of the content of the surfactant (I) to the content of the wax (w) constituting the wax particles (B) is 0.10 or more and 1.00 or less.

9. The mass ratio [water-insoluble polymer (b) / wax (w)] of the content of the water-insoluble polymer (b) to the content of the wax (w) constituting the wax particles (B) is 0.10 or more and 1.00 or less. The aqueous ink for inkjet recording according to any one of claims 1 to 8.

10. The melting point of the wax (w) constituting the wax particles (B) is 80°C or higher. The aqueous ink for inkjet recording according to any one of claims 1 to 9.

11. An inkjet recording method of recording on a recording medium using the aqueous ink according to any one of claims 1 to 10.

12. A method for producing an aqueous ink for inkjet recording according to any one of claims 1 to 10, including a step of mixing the aqueous dispersion of the pigment particles (A), the aqueous dispersion of the wax particles (B), and the water-soluble organic solvent (C).

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