JP7842726B2 - Water-based ink for inkjet recording. - Google Patents

Description

This invention relates to an aqueous ink for inkjet recording, a method for manufacturing the aqueous ink, and an inkjet recording method using the aqueous ink.

Inkjet recording is a recording method that uses extremely fine nozzles to directly eject ink droplets onto a recording medium, thereby creating a recorded object with characters or images. This method has become widespread due to its numerous advantages, including ease of full-color printing, low cost, the ability to use plain paper as the recording medium, and non-contact operation.
In recent years, with the spread of digital printing, its use has expanded beyond consumer printing to include commercial and industrial printing using a wide variety of low-absorbent coated papers. This has led to a growing demand for water-based inks, driven by the need for faster printing speeds, higher image quality, and reduced environmental impact. To address these demands, various proposals have been put forward.

For example, Patent Document 1 discloses an inkjet recording method that achieves both image transferability and robustness. This method involves repeatedly performing the following steps: a first image formation step, which includes applying a reaction solution containing a high-viscosity component to a transfer body; applying an ink containing water and a colorant; forming a liquid component containing water and a high-boiling-point water-soluble organic solvent, and a solid component insoluble in the liquid component formed by mixing the reaction solution and the ink; a second image, which is formed by contacting a porous body with the first image to remove a portion of the liquid component contained in the first image; and a transfer step, which involves heating the second image and transferring it onto a recording medium. The method also includes temperature control of the first and second images under specific conditions.

Japanese Patent Publication No. 2017-144736

Generally, when water-based ink is applied to a low-liquid-absorbent recording medium using an inkjet recording method, a large amount of pigment remains on the surface of the recording medium after recording, making it easy for the pigment to peel off from the ink coating surface. Therefore, it has been proposed to include wax in the water-based ink to reduce the frictional resistance of the ink coating during rubbing and improve image fastness.
On the other hand, wax is highly hydrophobic and tends to aggregate in water-based inks, so it is necessary to improve the dispersion stability of the wax and thereby improve the storage stability of the ink.
Patent Document 1 discloses an ink containing water and a colorant that further contains wax particles, and in the examples, an ink is disclosed that is formulated with a wax particle dispersion using a resin as a wax dispersant. However, it has been found that the storage stability of the ink is not satisfactory, and the image density of the resulting recording is also insufficient.
The present invention aims to provide an aqueous ink for inkjet recording, a method for manufacturing the aqueous ink, and an inkjet recording method using the aqueous ink, which can produce recordings that have excellent storage stability and excellent image density even when recorded on low liquid absorption recording media.

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

According to the present invention, it is possible to provide an aqueous ink for inkjet recording, a method for manufacturing the aqueous ink, and an inkjet recording method using the aqueous ink, which can produce recordings with excellent storage stability and excellent image density even when recorded on low-liquid-absorption recording media.

[Water-based ink for inkjet recording]
The water-based inkjet recording ink of the present invention (hereinafter also simply referred to as "water-based ink" or "ink") contains pigment particles (A) dispersed in a water-insoluble polymer (a), wax particles (B) dispersed in a surfactant (I) and a water-insoluble polymer (b), a water-soluble organic solvent (C), and water, wherein the surfactant (I) is one or more selected from the group consisting of nonionic surfactants and anionic surfactants.
In this invention, "aqueous system" means that water accounts for the largest proportion by mass in the medium.
In this invention, "wax" refers to an organic substance that is solid or semi-solid at room temperature (25°C) and becomes liquid upon heating. Here, "semi-solid wax" means that the wax deforms and flows when force is applied, but can maintain a certain shape when no force is applied. Furthermore, the temperature at which the wax becomes liquid upon heating, the so-called melting point of wax, is in the temperature range of 45°C or higher.
Furthermore, "low liquid absorption" is a concept that includes both low liquid absorption and non-liquid absorption, and means that the amount of water absorbed by the recording medium during a 100 m second contact time with pure water is 0 g/ ^m² or more and less than 10 g/ ^m² . "High liquid absorption" refers to recording media that absorb 10 g/m² or ^more during a 100 m second contact time with pure water, and this amount of water absorption can be measured as the amount of transfer during a 100 m second contact time with pure water using an automatic scanning liquid absorber (for example, KM500win manufactured by Kumagai Riki Kogyo Co., Ltd.) under conditions of 23°C and 50% relative humidity.

According to the present invention, it is possible to obtain a recording material that exhibits excellent storage stability of water-based inks and also has excellent image density even when recorded on a low-liquid-absorption recording medium. The reason for this is not entirely clear, but it is thought to be as follows.
In this invention, wax is incorporated into an 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 desorption of the surfactant from the wax surface even in aqueous inks containing water-soluble organic solvents. Furthermore, it is believed that the dispersion stability of the wax particles can be improved and storage stability improved by the steric repulsion of the hydrophilic groups of the nonionic surfactant or the electrostatic repulsion of the anionic groups of the anionic surfactant and the steric repulsion between the water-insoluble polymers.
Furthermore, even if the water-insoluble polymer does not adequately improve the dispersion stability of the wax particles due to insufficient adsorption of the water-insoluble polymer to the wax particles, the low molecular weight surfactant can compensate for this, thereby suppressing the coarsening of the wax particles. Moreover, since both the pigment particles and the wax particles are dispersed in a 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, storage stability can be improved, and even when recording on low-liquid-absorption recording media, an ink coating film with good smoothness can be formed, and diffuse reflection can be suppressed, thus making it possible to obtain recordings with high image density.

<Pigment particles (A) dispersed in a water-insoluble polymer (a)>
The water-based ink of the present invention contains pigment particles (A) dispersed in a water-insoluble polymer (a) (hereinafter also referred to as "pigment particles (A)"), from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and image density.
The form of the pigment particles (A) in the water-based ink of the present invention may be any form that consists of a pigment and a water-insoluble polymer (a), and includes a form in which the water-insoluble polymer (a) encapsulates 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 water-insoluble polymer (a) particles, a form in which the water-insoluble polymer (a) is adsorbed onto the pigment, and mixtures thereof.

(Pigment)
The pigments constituting the pigment particles (A) may be either inorganic or organic pigments, and lake pigments and fluorescent pigments may also be used. Furthermore, these may be used in combination with extender pigments as needed.
Examples of inorganic pigments include carbon black, titanium dioxide, iron oxide, red iron oxide, metal oxides such as chromium oxide, and pearlescent 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; and 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 slene pigments.
For achromatic inks, achromatic pigments such as white, black, and gray can be used, while for 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 products from each product code 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.
Pigments can be used individually or in mixtures of two or more types.

(Water-insoluble polymer (a))
The "water insolubility" of the water-insoluble polymer (a) (hereinafter also referred to as "polymer (a)") constituting the pigment particles (A) is determined to be "water-insoluble" when the resin, which has been dried at 105°C for 2 hours and reached a constant weight, is dissolved in 100g of water at 25°C until saturation is reached, and the amount dissolved is 10g or less. Furthermore, if polymer (a) has anionic groups as described later, and these anionic groups are neutralized with a neutralizing agent, the determination is made by measuring the amount dissolved under conditions in which the neutralizing agent is present under conditions in which the mass ratio of polymer (a) to the neutralizing agent is the same as that in the water-based ink of the present invention.
Polymer (a) may be used alone or in combination of two or more types.

Examples of polymer (a) include vinyl polymers obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds), polyesters, and polyurethanes. Among these, polymer (a) is preferably a vinyl polymer obtained by addition polymerization of vinyl monomers, and more preferably a vinyl polymer having anionic groups, from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and image density.
Here, "anionic group" refers to an anionic group or a group that can be ionized to become an anionic group. Examples of anionic groups include a carboxyl group (-COOM), a sulfonic acid group ( _-SO3M ), and a phosphate group ( _-OPO3M2 ). In the above chemical formula, M represents a hydrogen atom, an alkali metal, an ammonium, _or an organic ammonium. Among these, from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and image density, the anionic group is preferably a carboxyl group. That is, polymer (a) is more preferably a vinyl polymer containing a structural unit derived from a carboxyl group-containing vinyl monomer (a-1), and even more preferably a vinyl polymer containing a structural unit derived from a carboxyl group-containing vinyl monomer (a-1) and a structural unit derived from a hydrophobic vinyl monomer (a-2), from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and image density.

The carboxyl group-containing vinyl monomer (a-1) is preferably 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, the carboxyl group-containing vinyl monomer (a-1) is more preferably one or more selected from the group consisting of acrylic acid and methacrylic acid, from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and image density.

The term "hydrophobic" in hydrophobic vinyl monomer (a-2) means that when the monomer is dissolved in 100 g of deionized water at 25°C until saturated, the amount dissolved is less than 10 g.
Specific examples of hydrophobic vinyl monomers (a-2) include those described in paragraphs [0020] to [0022] of Japanese Patent Application Publication No. 2018-83938. Among these, as the hydrophobic vinyl monomer (a-2), from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and image density, one or more selected from the group consisting of alkyl (meth)acrylate having an alkyl group having 1 to 22 carbon atoms, aromatic group-containing monomer having an aromatic group having 6 to 22 carbon atoms, and macromonomer having a polymerizable functional group at one end is preferred. More preferably, one or more selected from the group consisting of alkyl (meth)acrylate having an alkyl group having 1 to 22 carbon atoms and aromatic group-containing monomer having an aromatic group having 6 to 22 carbon atoms is preferred. Even more preferably, one or more selected from the group consisting of alkyl (meth)acrylate having an alkyl group having 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth)acrylate is preferred. Even more preferably, one or more selected from the group consisting of alkyl (meth)acrylate having an alkyl group having 1 to 22 carbon atoms, styrene, and benzyl (meth)acrylate is preferred. Even more preferably, one or more selected from the group consisting of alkyl (meth)acrylate having an alkyl group having 1 to 22 carbon atoms and styrene is preferred.
In this specification, "(meth)acrylate" means at least one selected from the group consisting of acrylates and methacrylates.

Polymer (a) may further contain, in addition to structural units derived from carboxyl group-containing vinyl monomer (a-1) and hydrophobic vinyl monomer (a-2), structural units derived from nonionic vinyl monomer (a-3) from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and image density.
Nonionic vinyl monomers (a-3) are monomers that have high affinity for water and water-soluble organic solvents, and are monomers that contain hydroxyl groups or polyalkylene glycol chains, for example.
Examples of nonionic vinyl monomers (a-3) include those described in paragraph [0018] of Japanese Patent Application Publication No. 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 preferred. Here, n represents the average number of moles of alkylene oxide added.
The above components (a-1) to (a-3) can each be used by using one monomer component alone or in combination of two or more.

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

In the present invention, the polymer (a) constituting the pigment particles (A) is preferably crosslinked with a crosslinking agent, from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and image density. The polymer (a) crosslinked with a crosslinking agent (hereinafter also referred to as "crosslinked polymer (a)") is considered to be a two-dimensional polymer (a) which may have branched chains, and which has a three-dimensional structure due to components derived from the crosslinking agent. In the present invention, this three-dimensional structure allows the crosslinked polymer (a) to be firmly adsorbed or fixed to the pigment surface, suppressing the aggregation of the pigment in the water-based ink, and further suppressing the swelling of the crosslinked polymer (a), so that the storage stability and image density of the water-based ink can be further improved.
If polymer (a) is crosslinked with a crosslinking agent, it is preferable that polymer (a) (crosslinked polymer (a)) is a polymer obtained by crosslinking a vinyl polymer containing the aforementioned carboxyl group-containing vinyl monomer (a-1) and the hydrophobic vinyl monomer (a-2) with a crosslinking agent, from the viewpoint of improving the dispersion stability of the pigment and improving image density and storage stability.

(Crosslinking agent)
The crosslinking agent is a compound having two or more crosslinkable functional groups in its molecule, and from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and image density, it is preferably a compound having two or more epoxy groups in its molecule (hereinafter also referred to as "epoxy group-containing compound").
The crosslinking agent may be water-soluble or water-insoluble, but from the viewpoint of more efficiently crosslinking with the carboxyl groups of polymer (a) in a water-based medium, its water solubility is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Here, water solubility (mass%) refers to the solubility (mass%) of the crosslinking agent when 10 parts by mass of the crosslinking agent are 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 its molecule, and more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group with 3 to 8 carbon atoms.
The epoxy equivalent of the epoxy group-containing compound is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and preferably 300 or less, more preferably 200 or less, and even more preferably 170 or less, from the viewpoint of more efficiently crosslinking with the carboxyl groups of polymer (a) in a water-based medium.

Epoxy group-containing compounds can be used individually or in combination of two or more.
Specific examples of epoxy group-containing compounds 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 preferred.

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

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

The acid value of polymer (a) is preferably 60 mg KOH/g or more, more preferably 70 mg KOH/g or more, even more preferably 80 mg KOH/g or more, and even more preferably 90 mg KOH/g or more, and from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and image density, and also preferably 800 mg KOH/g or less, more preferably 500 mg KOH/g or less, even more preferably 300 mg KOH/g or less, even more preferably 200 mg KOH/g or less, even more preferably 150 mg KOH/g or less, and even more preferably 130 mg KOH/g or less. The acid value of polymer (a) can be determined by the method described in the examples, but it can also be calculated from the mass ratio of the constituent monomers. Furthermore, the acid value when polymer (a) is crosslinked with a crosslinking agent, i.e., the acid value of crosslinked polymer (a), can also be calculated using the following formula.
Acid value of crosslinked polymer (a) (mgKOH/g) = [Acid value of polymer (a) (mgKOH/g) × [(100 - Crosslinking rate (mol%)) / 100]
As mentioned above, the crosslinking rate (mol%) is a value calculated from the acid value of polymer (a) before crosslinking and the equivalent amount of crosslinkable functional groups of the crosslinking agent.

(Manufacturing of pigment particles (A))
In the present invention, it is preferable that the pigment particles (A) are dispersed in an aqueous medium and incorporated into the aqueous ink in the form of an aqueous dispersion (hereinafter also referred to as "pigment aqueous dispersion").
An aqueous dispersion of pigment particles (A) can be obtained by dispersing the pigment, polymer (a), and optionally a neutralizing agent, etc., using a known method.
A preferred method for producing an aqueous dispersion of pigment particles (A) is to disperse an aqueous dispersion of pigment and polymer (a). Furthermore, from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and image density, it is preferable to further add a crosslinking agent to the obtained aqueous dispersion of pigment particles (A) and crosslink the polymer (a) with the crosslinking agent.
When pigment particles (A) are incorporated into an aqueous ink as an aqueous dispersion, the average particle size of the pigment particles (A) in the aqueous dispersion is preferably 30 nm or more, more preferably 50 nm or more, and even more preferably 70 nm or more, from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and image density, and also preferably 600 nm or less, more preferably 400 nm or less, even more preferably 300 nm or less, and even more preferably 200 nm or less, from the same viewpoint as above. The average particle size 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 that are less prone to swelling, shrinking, or aggregation between particles. In this case, the average particle size of the pigment particles (A) in the aqueous ink of the present invention is considered to be the same as the average particle size of the pigment particles (A) in the aqueous dispersion. From this viewpoint, the preferred embodiment of the average particle size of the pigment particles (A) in the aqueous ink of the present invention is the same as the preferred embodiment of the average particle size of the pigment particles (A) in the aqueous dispersion. The average particle size of the pigment particles (A) in the aqueous ink of the present invention is also measured by the same method as described in the examples.

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

(Wax (w))
The wax (w) constituting the wax particles (B) (hereinafter also referred to as "wax (w)") may be either a natural wax or a synthetic wax.
Examples of natural waxes include petroleum-based waxes such as paraffin wax and microcrystalline wax; plant-based waxes such as carnauba wax, candelilla wax, and rice wax; and animal-based 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, polyolefin wax, which mainly consists of olefin monomers, is preferred from the viewpoint of improving storage stability and image density.
Wax (w) can be used individually or in combination of two or more types.

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

(Surfactant (I))
Surfactant (I) is one or more selected from the group consisting of nonionic surfactants and anionic surfactants.
Surfactants (I) can be used individually or in combination of two or more types.

Examples of nonionic surfactants include one or more selected from the group consisting of polyethylene glycol-type surfactants, polyhydric alcohol-type surfactants, and fatty acid alkanolamides, more preferably one or more selected from the group consisting of polyethylene glycol-type surfactants and polyhydric alcohol-type surfactants, and even more preferably polyethylene glycol-type surfactants.
Examples of polyethylene glycol-type surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkenyl ethers, polyoxyethylene aryl ethers, polyoxyethylene polyoxypropylene alkyl ethers, polyoxyethylene fatty acid esters, polyoxyethylene hydrogenated castor oil, and polyoxyethylene sorbitan fatty acid esters.
Among these, the nonionic surfactant is more preferably one or more selected from the group consisting of polyoxyethylene alkyl ethers, polyoxyethylene alkenyl ethers, and polyoxyethylene aryl ethers.

The alkyl group in polyoxyethylene alkyl ethers 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 polyoxyethylene alkyl ethers 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 polyoxyethylene alkenyl ethers preferably has 8 to 22 carbon atoms, more preferably 10 to 20 carbon atoms, and even more preferably 12 to 18 carbon atoms. A preferred example of polyoxyethylene alkenyl ether is polyoxyethylene oleyl ether.
Examples of polyoxyethylene aryl ethers include one or more selected from the group consisting of polyoxyethylene distyleninated phenyl ether and polyoxyethylene tripenzylphenyl ether.
The average number of moles of ethylene oxide added to polyoxyethylene alkyl ethers, polyoxyethylene aryl ethers, and polyoxyethylene alkenyl ethers is preferably 6 or more, more preferably 8 or more, and even more preferably 10 or more, from the viewpoint of improving the dispersion stability of the wax and improving storage stability and image density, and also preferably 20 or less, more preferably 16 or less, and even more preferably 14 or less, from the same viewpoint as above.

Examples of preferred anionic surfactants 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 counterion of ^the anionic group of an anionic surfactant is preferably one or more selected from the group consisting of alkali metal ions, ammonium ions ( _NH₄⁺ ), and organic ammonium ions. Suitable examples of organic ammonium ions include alkylamine ammonium ions and alcoholamine ammonium ions. Among these, the organic ammonium ion is preferably an alcoholamine ammonium ion, more preferably an ethanolamine ammonium ion, and even more preferably an ethylethanolamine ammonium ion.
Examples of anionic surfactants 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; one or more selected from the group consisting of diethylethanolamine oleate and potassium oleate are more preferred, and diethylethanolamine oleate is even more preferred.

Among these, surfactant (I) is preferably a nonionic surfactant, more preferably a polyethylene glycol-type surfactant, even more preferably one or more selected from the group consisting of polyoxyethylene alkyl ethers, polyoxyethylene alkenyl ethers, and polyoxyethylene aryl ethers, and even more preferably one or more selected from the group consisting of polyoxyethylene alkyl ethers and polyoxyethylene alkenyl ethers.

(Water-insoluble polymer (b))
The "water insolubility" of the water-insoluble polymer (b) (hereinafter also referred to as "polymer (b)") constituting the wax particles (B) is determined to be "water-insoluble" when the resin, which has been dried at 105°C for 2 hours and reached a constant weight, is dissolved in 100g of water at 25°C until saturation is reached, and the amount dissolved is 10g or less. Furthermore, if polymer (b) has anionic groups as described later, and these anionic groups are neutralized with a neutralizing agent, the determination is made by the amount dissolved when the neutralizing agent is present under conditions where the mass ratio of polymer (b) to the neutralizing agent is the same as that in the water-based ink of the present invention.
Polymer (b) may be used alone or in combination of two or more types.

Examples of polymer (b) include vinyl polymers obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds), polyesters, and polyurethanes. Among these, polymer (b) is preferably a vinyl polymer obtained by addition polymerization of vinyl monomers, and more preferably a vinyl polymer having anionic groups, from the viewpoint of improving the dispersion stability of the wax and improving storage stability and image density.
Here, "anionic group" refers to an anionic group, or a group that can be ionized to become an anionic group, as described above. Examples of anionic groups include a carboxyl group (-COOM), a sulfonic acid group ( _-SO3M ), and _a phosphate group ( _-OPO3M2 ). 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 thereby improving storage stability and image density, the anionic group is preferably a carboxyl group. That is, polymer (b) is more preferably a vinyl polymer containing a structural unit derived from a carboxyl group-containing vinyl monomer (b-1), and even more preferably a vinyl polymer containing a structural unit derived from a carboxyl group-containing vinyl monomer (b-1) and a structural unit derived from a hydrophobic vinyl monomer (b-2), from the viewpoint of improving the dispersion stability of the wax and thereby improving storage stability and image density.

The carboxyl group-containing vinyl monomer (b-1) is preferably 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, the carboxyl group-containing vinyl monomer (b-1) is more preferably one or more selected from the group consisting of acrylic acid and methacrylic acid, from the viewpoint of improving the dispersion stability of the wax and thereby improving storage stability and image density.

The term "hydrophobic" in hydrophobic vinyl monomer (b-2) means that when the monomer is dissolved in 100 g of deionized water at 25°C until saturated, the amount dissolved is less than 10 g.
Specific examples of hydrophobic vinyl monomers (b-2) include those described in paragraphs [0020] to [0022] of Japanese Patent Application Publication No. 2018-83938. Among these, as hydrophobic vinyl monomers (b-2), from the viewpoint of improving the dispersion stability of the wax and improving storage stability and image density, one or more selected from the group consisting of alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, aromatic group-containing monomers having an aromatic group having 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end is preferred, and more preferably one or more selected from the group consisting of alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, and aromatic group-containing monomers having an aromatic group having 6 to 22 carbon atoms, and alkyl One or more selected from the group consisting of methyl(meth)acrylate, styrene, α-methylstyrene, and benzyl(meth)acrylate is more preferable; one or more selected from the group consisting of alkyl(meth)acrylate having an alkyl group having 1 to 22 carbon atoms, styrene, and benzyl(meth)acrylate is 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 is even more preferable; and one or more selected from the group consisting of styrene and benzyl(meth)acrylate is even more preferable.
In this specification, "(meth)acrylate" means at least one selected from the group consisting of acrylates and methacrylates.

Polymer (b) may further contain, in addition to structural units derived from carboxyl group-containing vinyl monomer (b-1) and hydrophobic vinyl monomer (b-2), structural units derived from nonionic vinyl monomer (b-3) from the viewpoint of improving the dispersion stability of the wax and thereby improving storage stability and image density.
Nonionic vinyl monomers (b-3) are monomers that have a high affinity for water and water-soluble organic solvents, and are monomers that contain hydroxyl groups or polyalkylene glycol chains, for example.
Examples of nonionic vinyl monomers (b-3) include those described in paragraph [0018] of Japanese Patent Application Publication No. 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 preferred. Here, n represents the average number of moles of alkylene oxide added.
The above components (b-1) to (b-3) can each be used by using the monomer components contained in each component individually or by mixing two or more of them.

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

In the present invention, polymer (b) may be a non-crosslinked polymer, but from the viewpoint of improving the dispersion stability of the wax and improving image density and storage stability, it is preferable that it be crosslinked with a crosslinking agent. A polymer (b) crosslinked with a crosslinking agent (hereinafter also referred to as "crosslinked polymer (b)") is thought to have a three-dimensional structure due to components derived from the crosslinking agent, where polymer (b) may have a two-dimensional structure with branched chains. In the present invention, this three-dimensional structure allows the crosslinked polymer (b) to be firmly adsorbed or fixed to the wax surface, suppressing the aggregation of wax in the water-based ink, and further suppressing the swelling of the crosslinked polymer (b), so it is thought that the storage stability and image density of the water-based ink can be further improved.
If polymer (b) is crosslinked with a crosslinking agent, it is preferable that polymer (b) (crosslinked polymer (b)) is a polymer obtained by crosslinking a vinyl polymer containing the aforementioned carboxyl group-containing vinyl monomer (b-1) and the hydrophobic vinyl monomer (b-2) with a crosslinking agent, from the viewpoint of improving the dispersion stability of the wax and improving image density and storage stability.

(Crosslinking agent)
As the crosslinking agent, the same type as the crosslinking agent used in the aforementioned crosslinked polymer (a) is preferred. Among these, the crosslinking agent is preferably the epoxy group-containing compound described above, more preferably a compound having two or more glycidyl ether groups in the molecule, even more preferably a polyglycidyl ether compound of a polyhydric alcohol having hydrocarbon groups 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.

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

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

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

(Manufacturing of wax particles (B))
In the present invention, it is preferable that the wax particles (B) are incorporated into the aqueous ink in the form of an aqueous dispersion in which they are dispersed in an aqueous medium.
A suitable example of a method for producing an aqueous dispersion of wax particles (B) is a method in which a polymer (b), a surfactant (I), a wax (w), and a neutralizing agent, if necessary, are atomized by applying mechanical stress using a dispersion device in the presence of an aqueous medium.
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, even more preferably 70 nm or more, and preferably 200 nm or less, more preferably 150 nm or less, and even 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 that are less prone to swelling, shrinking, or aggregation between particles. In this case, the average particle size of the wax particles (B) in the aqueous ink of the present invention is considered to be 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 described in the examples.

<Water-soluble organic solvent (C)>
The water-based ink of the present invention contains a water-soluble organic solvent (C) from the viewpoint of improving image density and storage stability. The water-soluble organic solvent (C) can be used alone or in combination of two or more types.
In the present invention, "water-soluble organic solvent" means an organic solvent in which the amount of solvent dissolved when dissolved in 100 mL of water at 25°C is 10 mL or more.
The boiling point of the water-soluble organic solvent (C) at atmospheric pressure is preferably 150°C or higher, more preferably 160°C or higher, even more preferably 170°C or higher, and preferably 350°C or lower, more preferably 300°C or lower, and even 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 (mass%) of each water-soluble organic solvent.

Examples of water-soluble organic solvents (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 preferable to use one or more selected from the group consisting of polyhydric alcohols and polyhydric alcohol alkyl ethers.
Polyhydric alcohols can be used by mixing multiple substances included in the concept of polyhydric alcohols. Similarly, multiple substances included in the concept of polyhydric alcohol alkyl ethers can also be used by mixing them.

Preferably, the polyhydric alcohol is one or more selected from the group consisting of diols and polyhydric alcohols of three or higher valencies.
Examples of diols 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 polyhydric alcohols with a valency of 3 or higher include glycerin, trimethylolpropane, and pentaerythritol.

Examples of polyhydric alcohol alkyl ethers 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 alkylene glycol monoalkyl ethers such as tripropylene glycol monomethyl ether.

Among these, the water-soluble organic solvent (C) is more preferably one or more 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 other water-soluble organic solvents as long as they do not impair the effects of the present invention.

<Water>
The water-based 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 preferred from the viewpoint of preventing the contamination of unintended substances.

In addition to the above components, the water-based ink of the present invention may further contain various additives, such as surfactants, fixing agents, humectants, wetting agents, penetrating agents, defoaming agents, preservatives, antifungal agents, and rust inhibitors, as needed.

(Surfactant (II))
The aqueous ink of the present invention preferably further contains surfactant (II) in addition to surfactant (I) as described above. Surfactant (II) is thought to have the effect of appropriately spreading the ink on a low-liquid-absorbent recording medium and increasing image density due to its high adsorption to the ink-gas-liquid interface. Surfactant (II) can be used alone or in combination of two or more types.
The surfactant (II) is preferably one or more selected from the group consisting of acetylene-based surfactants and silicone-based surfactants.

Preferred acetylene-based surfactants include 2,4,7,9-tetramethyl-5-decine-4,7-diol, 2,5,8,11-tetramethyl-6-dodecine-5,8-diol, 3,6-dimethyl-4-octin-3,6-diol, 3,5-dimethyl-1-hexyn-3-ol, 2,4-dimethyl-5-hexyn-3-ol, and their ethylene oxide (hereinafter also referred to as "EO") adducts. Among these, more preferably are one or more selected from the group consisting of 2,4,7,9-tetramethyl-5-decine-4,7-diol and EO adducts of 2,4,7,9-tetramethyl-5-decine-4,7-diol, and even more preferably 2,4,7,9-tetramethyl-5-decine-4,7-diol.

As silicone-based surfactants, polyether-modified silicone-based surfactants are preferred. Suitable polyether groups in polyether-modified silicone-based surfactants include, for example, polyethylene oxy groups, polypropylene oxy groups, and polyalkylene oxy groups in which ethylene oxy groups and propylene oxy groups (trimethylene oxy groups or propane-1,2-diyl oxy groups) are added in a block-like or random manner. Compounds in which polyether groups are grafted onto a silicone main chain, or compounds in which polyether groups are bonded in a block-like manner to both ends of a silicone main chain, can be used.

Examples of commercially available acetylene-based surfactants include, for example, Nisshin Chemical Industry Co., Ltd.'s "Surfinol 104" (2,4,7,9-tetramethyl-5-decine-4,7-diol, average EO addition moles: 0, HLB: 3.0), "Surfinol 104E" (50% ethylene glycol dilution of 2,4,7,9-tetramethyl-5-decine-4,7-diol), "Surfinol 104PG-50" (50% propylene glycol dilution of 2,4,7,9-tetramethyl-5-decine-4,7-diol), and "Surfinol 420" (2,4,7,9-tetramethyl-5-decine-4,7 Examples include: "EO adduct of 2,4,7,9-tetramethyl-5-decine-4,7-diol (average number of moles of EO added: 1), HLB: 4.7", "Surfinol 440" (EO adduct of 2,4,7,9-tetramethyl-5-decine-4,7-diol (average number of moles of EO added: 3.5), HLB: 8 (catalog value)), "Surfinol 465" (EO adduct of 2,4,7,9-tetramethyl-5-decine-4,7-diol (average number of moles of EO added: 10), HLB: 13.1), and "Surfinol 485" (EO adduct of 2,4,7,9-tetramethyl-5-decine-4,7-diol (average number of moles of EO added: 30), HLB: 17.3).
Examples of commercially available silicone-based surfactants include the "KF" series from Shin-Etsu Chemical Co., Ltd., "Sylface SAG005" from Nisshin Chemical Industry Co., Ltd., and "BYK-348" from Big Chemie Japan Co., Ltd.

[Method for manufacturing water-based ink for inkjet recording]
A preferred method for producing the aqueous ink of the present invention includes a step of mixing an aqueous dispersion of the aforementioned pigment particles (A), an aqueous dispersion of the aforementioned wax particles (B), and a water-soluble organic solvent (C). In the method for producing the aqueous ink of the present invention, the aforementioned various additives may be added as needed, and filtration may be performed using a filter or the like.
In the method for producing the water-based 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 various additives is carried out by conventional methods.

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

The content of 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, and even more preferably 1% by mass or more, from the viewpoint of improving image fastness, and preferably 7% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, and even more preferably 2% by mass or less, from the viewpoint of improving storage stability.
The wax (w) content in the aqueous ink of the present invention is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, and even more preferably 0.8% by mass or more, from the viewpoint of improving image fastness, and from the viewpoint of improving storage stability, it is preferably 3% by mass or less, more preferably 2.5% by mass or less, even 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.

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

The mass ratio of the content of wax particles (B) to the total content of pigment particles (A) and 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 higher, more preferably 0.08 or higher, and even more preferably 0.10 or higher, from the viewpoint of improving image fastness, and preferably 0.40 or lower, more preferably 0.35 or lower, and even more preferably 0.30 or lower, from the viewpoint of improving storage stability.

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, and even more preferably 15% by mass or more, from the viewpoint of improving storage stability and image density. Furthermore, from the same viewpoint, it is preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less.

From the viewpoint of improving image density, the content of 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, and even more preferably 0.5% by mass or more. Furthermore, from the same viewpoint, it is preferably 3% by mass or less, more preferably 1% by mass or less, and even more preferably 1% by mass or less.

The water content in the water-based ink of the present invention is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, and even more preferably 75% by mass or less.

(Physical properties of water-based inks 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, even more preferably 3 mPa·s or more, and preferably 10 mPa·s or less, more preferably 7 mPa·s or less, and even 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, and even more preferably 7.5 or higher. Furthermore, from the viewpoint of material resistance and skin irritation, the pH is preferably 11 or lower, more preferably 10 or lower, and even more preferably 9.5 or lower. The pH is measured by the method described in the examples.

[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 aforementioned aqueous ink. In the inkjet recording method of the present invention, a piezo type is preferred as the method for ejecting the aqueous ink from the viewpoint of ejection performance.

Recording media used in the inkjet recording method of the present invention include highly absorbent plain paper, low absorbent coated paper, and resin film. Among these, the water-based ink of the present invention is preferred for low absorbent recording media from the viewpoint of obtaining a recording with high image density while having excellent image fastness.
Examples of coated paper include general-purpose glossy paper and multi-color foam glossy paper.
Examples of resin films include films made from 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 oriented film, a uniaxially oriented film, or an unoriented film, and may also be a film that has undergone corona discharge treatment.
Among these, a low liquid absorption recording medium is preferred for use in the inkjet recording method of the present invention.

In the following preparation examples, manufacturing examples, examples, and comparative examples, "parts" refers to "parts by mass" unless otherwise specified. The measurement methods for each physical property are as follows.

(1) Measurement of the number-average molecular weight of the polymer was determined by gel permeation chromatography. The measurement conditions are shown below.
GPC device: Tosoh Corporation "HLC-8320GPC"
Columns: Tosoh Corporation's "TSKgel Super AWM-H", "TSKgel Super AW3000", and "TSKgel guardcolumn Super AW-H"
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 materials: Monodisperse polystyrene kits with known molecular weights: "PStQuick B (F-550, F-80, F-10, F-1, A-1000)" and "PStQuick C (F-288, F-40, F-4, A-5000, A-500)" (all manufactured by Tosoh Corporation)
Measurement sample: 0.1 g of polymer was mixed with 10 mL of the 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.).

(2) Measurement of the acid value of polymers The resin was dissolved in a titration solvent (toluene:acetone = 2:1 (volume ratio)) mixed with toluene and acetone in a potentiometric automatic titrator (manufactured by Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with a 0.1 N potassium hydroxide/ethanol solution by potentiometric titration, with the inflection point on the titration curve as the endpoint. The acid value (mgKOH/g) was calculated from the amount of potassium hydroxide solution titrated to the endpoint.

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

(4) Measurement of Solid Content Concentration 10.0 g of sodium sulfate, which had been weighed to a constant weight in a desiccator, was weighed into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm). Approximately 1.0 g of the sample was added and mixed, then accurately weighed, maintained at 105°C for 2 hours to remove volatile components, and then left in a desiccator for 15 minutes before the mass was measured. The mass of the sample after removal of volatile components was taken as the solid content, and the solid content concentration (mass%) was obtained by dividing it by the mass of the added sample.

(5) Measurement of the melting point of wax (w) The melting point of wax (w) was measured using an apparatus in accordance with JIS K 0064:1992. Specifically, a differential scanning calorimeter (TA Instruments, model: Q20) was used to heat the sample to 200°C, and then 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 heat of fusion peaks, the temperature of the peak with the largest peak area was defined as the maximum peak temperature of melting, and this peak temperature was defined as the melting point.

(6) Measurement of the viscosity of water-based ink The viscosity of water-based ink at 32°C was measured using an E-type viscometer (TV-25 manufactured by Toki Sangyo Co., Ltd., standard cone rotor 1°34'×R24, rotation speed 50 rpm).

(7) Measurement of pH of water-based ink The pH of water-based ink at 25°C was measured using a benchtop pH meter (F-71, manufactured by Horiba, Ltd.) equipped with a pH electrode (6337-10D, manufactured by Horiba, Ltd.).

Manufacturing Example 1 (Manufacturing of water-insoluble polymer (P1))
A monomer mixture was prepared by mixing 14 parts of acrylic acid, 11 parts of ethyl acrylate, and 75 parts of styrene. In a reaction vessel, 10 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), 0.2 parts of 2-mercaptoethanol as a polymerization chain transfer agent, and 10% by mass of the monomer mixture were added and mixed, and the mixture was thoroughly purged with nitrogen gas.
Meanwhile, a mixture of the remainder of the monomer mixture (90% by mass of the monomer mixture), 0.2 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as an azo radical polymerization initiator was placed in a dropping funnel. Under a nitrogen atmosphere, the monomer mixture in the reaction vessel was heated to 65°C while stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 65°C following the completion of the dropwise addition, a solution of 0.1 parts of the polymerization initiator dissolved in 2 parts of MEK was added, and the mixture was aged for a further 2 hours at 65°C and 2 hours at 70°C before being dried under reduced pressure to obtain a water-insoluble polymer (P1) (number average molecular weight: 24,000, acid value: 109 mgKOH/g).

Manufacturing Examples 2 and 3 (Manufacturing 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 was changed as shown in Table 1. The physical properties of each water-insoluble polymer are shown in Table 1.

Preparation Example I-1 (Preparation of an aqueous dispersion of pigment particles (A'1))
32 parts of the water-insoluble polymer (P1) obtained in Production Example 1 were mixed with 202 parts of ion-exchanged water. Further, 9.4 parts of a 5N sodium hydroxide aqueous solution (16.9% by mass of sodium hydroxide solids) were added to neutralize the mixture so that the ratio of moles of sodium hydroxide to moles of carboxyl groups in the water-insoluble polymer (P1) was 65 mol% (degree of neutralization 65 mol%). The mixture was then heated to 90°C using a warm bath and stirred for 1 hour to disperse the water-insoluble polymer (P1) in water. After cooling to room temperature (25°C), an aqueous dispersion of the water-insoluble polymer (P1) was obtained.
To an aqueous dispersion of the water-insoluble polymer (P1) obtained above as polymer (a), 100 parts of carbon black pigment (CB, C.I. Pigment Black 7, Cabot Chemicals "Monarch 800") were added, and the mixture was stirred for 3 hours at 20°C with a disperser (Asada Iron Works Co., Ltd. "Ultra Disperser") rotating the disperser blades at 6,000 rpm. Next, 124 parts of deionized water were added, and the mixture was dispersed in 15 passes at a pressure of 150 MPa using a microfluidizer (Microfluidics, trade name). The obtained dispersion was placed in a 500 mL angle rotor and centrifuged at 3,660 rpm for 20 minutes using a high-speed refrigerated centrifuge (Hitachi Koki Co., Ltd. "himac CR22G", set temperature 20°C). The liquid phase was then collected and filtered through a 5 μm acetylcellulose membrane filter to obtain an aqueous dispersion of pigment particles (A1) (solid content concentration: 25% by mass).
100 parts of the aqueous dispersion of pigment particles (A1) obtained above were placed in a screw-top glass bottle, 26 parts of deionized water were added, and 0.2 parts of trimethylolpropane polyglycidyl ether (Denacol EX-321, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 140) (corresponding to a crosslinking rate of 15 mol%) were added as a crosslinking agent. The bottle was then tightly sealed, heated at 70°C for 5 hours while stirring with a stirrer, then cooled to room temperature (25°C), filtered through the 5 μm filter, and an aqueous dispersion of pigment particles (A'1) (acid value: 87 mg KOH/g) was obtained (solid content concentration: 20% by mass, pigment content: 15.1% by mass, crosslinked polymer (a) content: 4.9% by mass (components derived from the crosslinking agent: 0.2% by mass), average particle size: 105 nm).

Preparation Example II-1 (Preparation of an aqueous dispersion of wax particles (B'1))
As polymer (b), 30 parts of the water-insoluble polymer (P1) obtained in Production Example 1 were neutralized by adding 8.9 parts of a 5N sodium hydroxide aqueous solution (NaOH solids content: 16.9% by mass) as a neutralizing agent to neutralize the carboxyl groups of polymer (P1) (equivalent amount of neutralizing agent used: 65 mol%). Furthermore, 300 parts of deionized water and 20 parts of polyoxyethylene oleyl ether (alkyl group with 18 carbon atoms, average number of added ethylene oxide moles of 12) as surfactant (I) were added, and 80 parts of polyethylene wax (Mitsui Chemicals, Inc.'s "High Wax 200P", melting point: 122°C, density: 0.970 g/ ^cm³ , weight-average molecular weight: 2,000) (hereinafter referred to as "HW200P") were heated and dissolved at 85-95°C. The resulting mixture was dispersed for 30 minutes using an ultrasonic homogenizer while maintaining the temperature at 90-95°C, then cooled to room temperature, and subjected to a three-pass dispersion treatment at a pressure of 200 MPa using a microfluidizer (Microfluidics, Inc., product name) to obtain an aqueous dispersion of wax particles (B1).
To the aqueous dispersion of wax particles (B1) obtained above, 260 parts of deionized water were added, and 1.2 parts of trimethylolpropane polyglycidyl ether (Denacol EX-321, manufactured by Nagase ChemteX Corporation, epoxy equivalent: 140) (corresponding to a crosslinking rate of 15 mol%) were added as a crosslinking agent. The container was then sealed tightly 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: 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 (components derived from the crosslinking agent: 0.2% by mass), average particle size: 78 nm).

Preparation Example II-2 and Comparative Preparation Example II-1 (Preparation of aqueous dispersions of wax particles (B2) and wax particles (BC1))
In Preparation Example II-1, as shown in Table 2, the formulation composition for the preparation of the aqueous dispersion of wax particles (B) was changed, and the crosslinking treatment with a crosslinking agent was omitted. In the same manner, aqueous dispersions of wax particles (B2) and wax particles (BC1) were obtained, respectively.

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 aqueous dispersion of wax particles (B'C2))
In Preparation Example II-1, aqueous dispersions of each wax particle (B) were obtained in the same manner as in the previous example, except that the formulation composition for the preparation of the aqueous dispersion of wax particles (B) was changed as shown in Table 2.
The waxes (w) and surfactants (I) other than those used in Preparation Example II-1 shown in Table 2 are listed below.
HW110P: "High Wax 110P" manufactured by Mitsui Chemicals, Inc., polyolefin wax, melting point: 109°C, density: 0.920 g/ ^cm³ , weight-average molecular weight: 1,000
TW-131: "TOWAX-131" manufactured by Toagosei Co., Ltd., carnauba wax, melting point: 109°C
Polyoxyethylene stearyl ether: 18 carbon atoms in the alkyl group, average number of moles of ethylene oxide added: 12
Diethylethanolamine oleate: 18 carbon atoms in the alkyl group.

Example 1
26.5 parts of an aqueous dispersion of pigment particles (A1') obtained in Production Example I-1, 8.2 parts of an aqueous dispersion of 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 a water-soluble organic solvent (C), 1.2 parts of an acetylene-based surfactant (Nisshin Chemical Industry Co., Ltd. "Surfinol 104-PG50" (50% diluted propylene glycol product of 2,4,7,9-tetramethyl-5-decine-4,7-diol)) (hereinafter referred to as "Surfinol 104-PG50") as a surfactant (II), and deionized water were added and stirred to a total volume of 100 parts. The mixture was then filtered through a membrane filter (Sartorius "Minisart Syringe Filter", pore size: 5 μm, material: cellulose acetate) to obtain water-based ink 1 (viscosity at 32°C: 3.4 mPa·s, pH: 7.9).

Examples 2-15 and Comparative Examples 1-2
In Example 1, the same procedure was followed except that the composition was changed as shown in Table 3 to obtain each water-based ink. In Example 15, diethylene glycol monoisopropyl ether (manufactured by Nippon Emulsifier Co., Ltd.) (hereinafter referred to as "iPDG") was used as the water-soluble organic solvent (C) as shown in Table 3.
Evaluation method

[evaluation]
Each aqueous ink obtained in the examples and comparative examples was evaluated according to (1) and (2) below. The results are shown in Table 1.

(1) Evaluation of Image Density Recorded materials were prepared and evaluated using the following inkjet recording methods on A4 size coated paper (OK Topcoat+ manufactured by Oji Paper Co., Ltd., water absorption: 4.9 g/ ^m² ) as a low liquid absorption recording medium, using the aqueous inks of the examples and comparative examples.
In an environment with a temperature of 25±1°C and a relative humidity of 30±5%, a printing evaluation device (manufactured by Trytech Co., Ltd.) equipped with an inkjet head (Kyocera Corporation's "KJ4B-HD06MHG-STDV", piezo type) was filled with each water-based ink.
The head voltage was set to 26V, the frequency to 10kHz, the appropriate amount of discharge liquid to 12pL, the head temperature to 25°C, the resolution to 600dpi, and the negative pressure to -4.0kPa. The recording medium was fixed to the transport table under reduced pressure, with the longitudinal direction of the recording medium and the transport direction being the same. A print command was transferred to the print evaluation device, and an image with a duty cycle of 100% was formed. 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 a set temperature of 70°C to obtain a recorded material.
After the obtained recordings were left to stand at room temperature for 24 hours, the image density at five arbitrary locations was measured using a spectrophotometer (X-Rite eXact, manufactured by X-Rite Corporation) under the conditions of light source D50, field of view 2°, CIE color system, and filter T. The average of the five image densities was calculated as the image density of the recording. A higher image density value indicates a better quality.

(2) Evaluation of storage stability Each of the aqueous inks in the examples and comparative examples was stored in a sealed container in a constant temperature room at 70°C, and after 28 days, it was removed and the average particle size after storage was measured. The average particle size change rate after 28 days of storage at 70°C was calculated using the following formula (however, decimal places were rounded down). A lower average particle size change rate indicates better storage stability.
Average particle size change rate (%) = [Average particle size after storage / Average particle size before storage] × 100

Table 3 shows that the aqueous inks in Examples 1 to 15 exhibit superior image density and storage stability compared to Comparative Examples 1 and 2.

Claims (10)

An inkjet water-based ink comprising pigment particles (A) dispersed in a water-insoluble polymer (a), wax particles (B) dispersed in a surfactant (I) and a water-insoluble polymer (b), a water-soluble organic solvent (C), and water,
The surfactant (I) is one or more selected from the group consisting of nonionic surfactants and anionic surfactants .
The water-insoluble polymer (b) is a vinyl polymer comprising a structural unit derived from a carboxyl group-containing vinyl monomer (b-1) and a structural unit derived from a hydrophobic vinyl monomer (b-2),
The carboxyl group-containing vinyl monomer (b-1) is one or more selected from the group consisting of acrylic acid and methacrylic acid.
An inkjet water-based ink for recording , wherein the hydrophobic vinyl monomer (b-2) comprises one or more selected from the group consisting of styrene and benzyl (meth)acrylate . The water-based inkjet recording ink according to claim 1, wherein the nonionic surfactant is one or more selected from the group consisting of polyoxyethylene alkyl ethers, polyoxyethylene alkenyl ethers, and polyoxyethylene aryl ethers. The water-insoluble polymer (b) is a polymer obtained by crosslinking the vinyl polymer with a crosslinking agent, as described in claim 1 . The aqueous inkjet recording ink according to claim 1, wherein the hydrophobic vinyl monomer (b-2) further comprises an alkyl (meth)acrylate having an alkyl group having 1 to 22 carbon atoms. The water-based inkjet recording ink according to claim 1, wherein the mass ratio of the content of the surfactant (I) to the content of the water-insoluble polymer (b) [surfactant (I) / water-insoluble polymer (b)] is 0.20 or more and 1.80 or less. The water-based inkjet recording ink according to claim 1, wherein the mass ratio of the surfactant (I) content to the wax (w) content constituting the wax particles (B) [surfactant (I) / wax (w)] is 0.10 or more and 1.00 or less. The water-based inkjet recording ink according to claim 1, wherein the mass ratio of the water-insoluble polymer (b) content to the wax (w) content constituting the wax particles (B) [water-insoluble polymer (b) / wax (w)] is 0.10 or more and 1.00 or less. The water-based inkjet recording ink according to claim 1, wherein the melting point of the wax (w) constituting the wax particles (B) is 80°C or higher. An inkjet recording method comprising recording on a recording medium using an inkjet-based aqueous ink for inkjet recording described in any one of claims 1 to 8 . A method for producing an inkjet water-based ink according to any one of claims 1 to 8 , comprising the step of mixing an aqueous dispersion of pigment particles (A), an aqueous dispersion of wax particles (B), and a water-soluble organic solvent (C).