Water-based ink for inkjet recording

Abstract

To provide a water-based ink for inkjet recording that is superior in storage stability and discharge stability, is also superior in the wetting and spreading of ink dots even when recording on a low-absorbency recording medium, and can yield a recorded material having superior image fastness, and an inkjet recording method.SOLUTION: A water-based ink for inkjet recording contains a pigment, an organic solvent (C-1) with a water solubility of 20 mass% or less, water-insoluble polymer particles containing wax, and water. An inkjet recording method uses the water-based ink to record on a low-absorbency recording medium.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aqueous ink for inkjet recording and an inkjet recording method. [Background technology]

[0002] Inkjet recording is a recording method that uses extremely fine nozzles to directly eject ink droplets onto a recording medium, thereby obtaining a recorded material with text and 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 personal and office applications to include commercial and industrial printing, and inkjet printing is required to print on a variety of recording media. Furthermore, the demand for water-based inks is increasing in order to achieve higher printing speeds, higher image quality, and reduced environmental impact.

[0003] Therefore, various proposals have been made to address these demands. For example, Patent Document 1 discloses an inkjet recording method for which the first and second images are temperature-controlled under specific conditions, with the aim of providing an inkjet recording method that achieves both image transferability and robustness. This method involves repeatedly performing the following steps: applying a reaction solution containing a high-viscosity component to a transfer body; applying an ink containing water, a colorant, and a wax; forming a first image containing a liquid component containing water and a high-boiling point water-soluble organic solvent, and a solid component formed by mixing the reaction solution and the ink; forming a second image in which a porous body is brought into contact with the first image and a portion of the liquid component is removed; and transferring the second image to a recording medium by heating it. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-144736 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In commercial and industrial printing, pigments are required as colorants from the standpoint of weather resistance and water resistance of the recorded material. However, unlike dyes, pigments cannot uniformly dissolve pigment molecules in the ink vehicle. Therefore, it is necessary to maintain a good dispersion state of the pigment to improve the storage stability of the ink and the ejection performance during inkjet recording. In particular, there is a need to improve the ejection stability (hereinafter also simply referred to as "ejection stability") when inkjet recording is resumed after a long period of inactivity. Furthermore, when recording on low-liquid-absorption recording media, ink dots that land on the recording media tend not to spread sufficiently, leading to a decrease in the image uniformity of the recorded material. Therefore, there is a need to improve the wetting and spreading properties of ink dots on the recording media. Furthermore, when pigments are used to record on low-liquid-absorbent recording media such as coated paper or resin film, a large amount of pigment remains on the surface of the recording medium, making it prone to peeling and resulting in poor image durability. However, it was found that the technology described in Patent Document 1 had insufficient wetting and spreading properties of the ink dots. Thus, the increasing demands of recent years for improved ink storage stability and ejection stability, improved ink dot wetting and spreading properties when recording on low-liquid-absorption recording media, and improved image robustness of the resulting recordings have not yet been fully met. The present invention aims to provide an inkjet water-based ink and an inkjet recording method that exhibit excellent storage stability and ejection stability, and that can produce recorded materials with excellent ink dot wetting and spreading properties, as well as excellent image fastness, even when recording on low-liquid-absorption recording media. [Means for solving the problem]

[0006] The inventors have found that the above problem can be solved by incorporating a low-water-soluble organic solvent, whose solubility in water is below a certain value, and water-insoluble polymer particles containing wax into a water-based ink containing pigment. In other words, the present invention provides the following [1] and [2]. [1] An inkjet water-based ink containing a pigment, an organic solvent (C-1) with a water solubility of 20% by mass or less, water-insoluble polymer particles containing wax, and water. [2] An inkjet recording method for recording on a low liquid absorption recording medium using the aqueous ink described in [1] above. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an inkjet water-based ink and an inkjet recording method that can produce recorded materials with excellent storage stability and ejection stability, as well as excellent ink dot wetting and spreading properties and excellent image fastness, even when recording on low liquid absorption recording media. [Modes for carrying out the invention]

[0008] [Water-based ink for inkjet recording] The water-based inkjet recording ink of the present invention (hereinafter also referred to as "the ink of the present invention" or "ink") contains a pigment, an organic solvent (C-1) (hereinafter also referred to as "organic solvent (C-1)") having a solubility in water of 20% by mass or less, water-insoluble polymer particles containing wax, and water. 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 at room temperature (25°C) and becomes liquid when heated. Furthermore, "record" is a concept that includes printing and printing of text and images, while "record material" is a concept that includes printed materials and printed objects on which text and images are recorded. "Low liquid absorbency" is a concept that includes low liquid absorbency and non-liquid absorbency, and means that the water absorption amount of the recording medium at 100 milliseconds of the contact time between the recording medium and pure water is 0 g / m 2 or more and 10 g / m 2 or less.

[0009] According to the present invention, it is possible to obtain a recorded matter excellent in storage stability and ejection stability, and also excellent in the wet spreading property of ink dots and excellent in image fastness even in recording on a low liquid absorbency recording medium. Although the reason is not clear, it is considered as follows. Generally, it is known that a low liquid absorbency recording medium has a smooth surface but a low surface free energy, so that an aqueous ink is difficult to wet and spread. The ink of the present invention contains a low water-soluble organic solvent having a solubility in water of 20% by mass or less. After the ink droplets are ejected from the inkjet head nozzle and land on the recording medium, the organic solvent has a low affinity for water, so it quickly migrates to the surface of the ink dots on the recording medium. As a result, since the surface tension of the ink dots decreases quickly, even on a low liquid absorbency recording medium with a low surface free energy, it is considered that the recording medium can be quickly wetted and the wet spreading property of the ink dots can be improved. Furthermore, the ink of the present invention contains a wax that can reduce the frictional resistance of the ink coating film during rubbing in the form of water-insoluble polymer particles containing the wax. Even in an aqueous ink containing a low water-soluble organic solvent, the dispersion stability of the wax can be improved. Even when the ink is stored for a long time or the ink is held in the inkjet nozzle for a long time due to a long pause in inkjet recording, the aggregation of the wax can be suppressed. As a result, it is considered that the storage stability and ejection stability can be improved while improving the image fastness.

[0010] <Pigment> Examples of pigments include white pigments such as titanium oxide; black pigments such as carbon black; and colored pigments such as azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments. The pigments can be used alone or in combination of two or more.

[0011] Examples of the form of the pigment in the ink of the present invention include a form dispersed with a polymer dispersant or a surfactant as a dispersant, or a form of a self-dispersing pigment dispersed without using a dispersant. The polymer dispersant may be either a water-soluble polymer or a water-insoluble polymer. Here, for the "water-soluble" and "water-insoluble" of the polymer, 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 exceeds 10 g, it is determined as "water-soluble", and if it is 10 g or less, it is determined as "water-insoluble". Also, as described later, when the polymer has an anionic group, the dissolved amount is the dissolved amount when the anionic group of the polymer is 100% neutralized with sodium hydroxide.

[0012] Specifically, as the form of the pigment in the ink of the present invention, from the viewpoint of improving the dispersion stability of the pigment and thus improving the storage stability and ejection stability, (A) a form in which a water-soluble polymer is adsorbed on the pigment surface, (B) a form in which a water-soluble surfactant or a water-dispersible surfactant is adsorbed on the pigment surface, (C) a form of a self-dispersing pigment in which a hydrophilic functional group is chemically or physically introduced on the pigment surface and dispersed without using a polymer dispersant and a surfactant, (D) a form in which the pigment is coated with a water-insoluble polymer, etc. are preferably mentioned. Among these, the form of the pigment in the ink of the present invention is preferably one or more forms selected from the group consisting of the forms of (C) and (D), and more preferably the form of (D), from the same viewpoint as above.

[0013] (D) As a form in which the pigment is coated with a water-insoluble polymer (hereinafter also referred to as "water-insoluble polymer (a)" or "polymer (a)"), particles of water-insoluble polymer (a) containing the pigment (hereinafter also referred to as "pigment-containing polymer particles") are preferred. The forms of pigment-containing polymer particles include a form in which the water-insoluble polymer encapsulates the pigment, a form in which the pigment is uniformly dispersed in the water-insoluble polymer, a form in which the pigment is exposed from the surface of the water-insoluble polymer particles, a form in which the water-insoluble polymer is adsorbed onto the pigment, and mixtures thereof.

[0014] (Water-insoluble polymer (a)) Examples of water-insoluble polymers (a) constituting pigment-containing polymer particles include vinyl polymers, polyester polymers, and polyurethane polymers. Among these, polymer (a) is preferably a vinyl polymer obtained by addition polymerization of vinyl monomers, from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and discharge stability. More preferably, it is a vinyl polymer containing (a-1) constituent units derived from ionic vinyl monomers, and even more preferably, it is a vinyl polymer containing (a-1) constituent units derived from ionic vinyl monomers and (a-2) constituent units derived from hydrophobic vinyl monomers.

[0015] [(a-1) Ionic vinyl monomer] (a-1) The ionic vinyl monomer is preferably an anionic vinyl monomer having an anionic group, from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and discharge stability. In the present invention, "anionic group" refers to an anionic group or a group that can be ionized to become an anionic group. Examples of anionic groups include carboxyl groups (-COOM), sulfonic acid groups (-SO3M), and phosphate groups (-OPO3M2). In the above chemical formulas, M represents a hydrogen atom, an alkali metal, an ammonium compound, or an organic ammonium compound. Examples of anionic vinyl monomers include carboxyl group-containing vinyl monomers, sulfonic acid group-containing vinyl monomers, and phosphate group-containing vinyl monomers. Examples of carboxyl group-containing vinyl monomers include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid. Among these, (a-1) the ionic vinyl monomer is more preferably a carboxyl group-containing vinyl monomer, and even more preferably one or more selected from the group consisting of acrylic acid and methacrylic acid.

[0016] [(a-2) Hydrophobic vinyl monomer] (a-2) The term "hydrophobic" in "hydrophobic vinyl monomer" means that when the monomer is dissolved in 100g of deionized water at 25°C until saturated, the amount dissolved is less than 10g. (a-2) Specific examples of hydrophobic vinyl monomers include those described in paragraphs

[0020] to

[0022] of Japanese Patent Publication No. 2018-83938. Among these, 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; 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 is more preferred; one or more selected from the group consisting of alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth)acrylate is even more preferred; and 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 preferred. In this specification, "(meth)acrylate" means either acrylate or methacrylate, or both.

[0017] [(a-3) Nonionic vinyl monomer] Polymer (a) may further contain, in addition to structural units derived from ionic vinyl monomer (a-1) and hydrophobic vinyl monomer (a-2), structural units derived from nonionic monomer (a-3) from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and discharge stability. (a-3) Nonionic vinyl monomers are monomers that have a high affinity for water and water-soluble organic solvents, and are, for example, monomers that contain hydroxyl groups or polyalkylene glycol chains. (a-3) Specific examples of nonionic vinyl monomers 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 (m=1~30) (meth)acrylate and polypropylene glycol (m=2~30) (meth)acrylate are preferred. Here, m represents the average number of moles added of oxyalkylene groups. 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.

[0018] The content of constituent units derived from each monomer component in polymer (a) is as follows, from the viewpoint of improving the dispersion stability of the pigment and thereby improving storage stability and dispensing stability. The content of constituent units derived from component (a-1) in polymer (a) is preferably 5% by mass or more, more preferably 7% by mass or more, even more preferably 10% by mass or more, and preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. The content of constituent units derived from component (a-2) in polymer (a) is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, and even more preferably 70% by mass or more, and preferably 95% by mass or less, more preferably 93% by mass or less, and even more preferably 90% by mass or less. When polymer (a) contains structural units derived from component (a-3), the content of structural units derived from component (a) in polymer (a) is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and even more preferably 5% by mass or less. The total content of constituent units derived from component (a-1) and component (a-2) in polymer (a) is preferably 60% by mass or more, more preferably 75% by mass or more, even more preferably 90% by mass or more, and even more preferably 100% by mass. The mass ratio [(a-1) component / (a-2) component] of the constituent units derived from component (a) in polymer (a) to the constituent units derived from component (a-2) is preferably 0.05 or more, more preferably 0.1 or more, even more preferably 0.15 or more, and preferably 1.3 or less, more preferably 1 or less, even more preferably 0.8 or less, even more preferably 0.5 or less, and even more preferably 0.3 or less.

[0019] The number-average molecular weight of polymer (a) is preferably 8,000 or more, more preferably 12,000 or more, even more preferably 16,000 or more, and preferably 80,000 or less, more preferably 60,000 or less, even more preferably 40,000 or less, and even more preferably 30,000 or less. The number-average molecular weight of polymer (a) is measured by the method described in the examples.

[0020] 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, even more preferably 90 mg KOH / g or more, even more preferably 100 mg KOH / g or more, and preferably 350 mg KOH / g or less, more preferably 300 mg KOH / g or less, and even more preferably 250 mg KOH / g or less, from the viewpoint of improving storage stability and discharge stability. The acid value of polymer (a) is measured by the method described in the examples.

[0021] When the form of the pigment in the ink of the present invention is form (D), it is preferable that the water-insoluble polymer (a) is a water-insoluble crosslinked polymer having a crosslinked structure obtained by crosslinking with a crosslinking agent (hereinafter also referred to as "crosslinked polymer (a')"). That is, it is preferable that the form of the pigment in the ink of the present invention is in the form of particles of a water-insoluble crosslinked polymer containing the pigment (hereinafter also referred to as "pigment-containing crosslinked polymer particles"). The pigment-containing crosslinked polymer particles consist of the pigment and the crosslinked polymer (a'), and it is preferable that the crosslinked polymer (a') is obtained by crosslinking polymer (a) with a crosslinking agent. That is, it is considered that the crosslinked polymer (a') constituting the pigment-containing crosslinked polymer particles has a three-dimensional structure due to components derived from the crosslinking agent, where polymer (a) has a two-dimensional structure which may have branched chains. In the present invention, it is considered that the crosslinked polymer (a') is strongly adsorbed or fixed to the pigment surface by this three-dimensional structure, the aggregation of the pigment in the ink is suppressed, and the swelling of the crosslinked polymer is also suppressed, thereby further improving the storage stability and discharge stability of the ink, and further improving the image fastness of the recorded material. In the present invention, the pigment in the ink is in the form of pigment-containing crosslinked polymer particles, and the crosslinked polymer (a') constituting the pigment-containing crosslinked polymer particles is obtained by crosslinking polymer (a) with a crosslinking agent. In this case, polymer (a) is preferably a vinyl polymer containing constituent units derived from ionic vinyl monomer (a-1) and constituent units derived from hydrophobic vinyl monomer (a-2).

[0022] When polymer (a) is a vinyl polymer containing structural units derived from an ionic vinyl monomer (a-1) and a hydrophobic vinyl monomer (a-2), and the ionic group of the ionic vinyl monomer (a-1) is a carboxyl group, the crosslinking agent is preferably a compound having two or more epoxy groups in the molecule, from the viewpoint of further improving storage stability and discharge stability. 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 (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. The epoxy equivalent of the compound having the epoxy group 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 group of polymer (a) in a water-based medium. The epoxy group-containing compounds can be used individually or in combination of two or more.

[0023] Specific examples of the 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.

[0024] 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 storage stability and discharge stability, and from the viewpoint of improving image fastness. The crosslinking ratio is calculated using the following formula. Crosslinking ratio = [(Amount of crosslinking agent (g) / Epoxy equivalent of crosslinking agent (g / eq)] / [(Acid value of polymer (a) (mgKOH / g) / (56.1 × 1000)) × Amount of polymer (a)]

[0025] (Manufacturing of pigment-containing polymer particles and pigment-containing crosslinked polymer particles) When the form of the pigment in the ink of the present invention is form (D), it is preferable that the pigment is formulated as an aqueous dispersion of pigment-containing polymer particles or pigment-containing crosslinked polymer particles (hereinafter referred to as "pigment-containing (crosslinked) polymer particles"). Aqueous dispersions of pigment-containing (crosslinked) polymer particles can be efficiently produced by a method that includes a step of dispersing a pigment mixture containing a pigment, polymer (a), and water (hereinafter also referred to as "step 1"). An aqueous dispersion of pigment-containing crosslinked polymer particles can be efficiently produced by a method that includes obtaining an aqueous dispersion of pigment-containing polymer particles in step 1, and then adding a crosslinking agent to the aqueous dispersion of pigment-containing polymer particles obtained in step 1, thereby crosslinking polymer (a) with the crosslinking agent (hereinafter also referred to as "step 2"). Polymer (a) may be synthesized as appropriate, similar to polymer (b) described later, or a commercially available product may be used. When synthesizing polymer (b), the same polymerization method and polymerization conditions as those described later for polymer (b) can be used.

[0026] Step 1 is preferably carried out by, specifically, a method of dispersing a pigment mixture obtained by mixing a pre-prepared aqueous dispersion of polymer (a) with a pigment (1); or a method of dispersing a pigment mixture containing a pigment, polymer (a), organic solvent, water, and optionally a neutralizing agent, surfactant, etc., to obtain a dispersion, and then removing the organic solvent from the dispersion by a known method (2). For example, in method (1), it is preferable to obtain the aqueous dispersion of polymer (a) by neutralizing some of the ionic groups of polymer (a) with a neutralizing agent. Here, preferred neutralizing agents used for neutralization are the same as those exemplified in polymer (b) described later. Furthermore, the preferred range for the equivalent amount of the neutralizing agent used in polymer (a) is the same as the preferred range in polymer (b). From the viewpoint of promoting sufficient and uniform neutralization, the neutralizing agent is preferably used as an aqueous solution. The dispersion process in step 1 can be carried out by known methods using kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers (manufactured by Microfluidics), paint shakers, bead mills, and other media-type dispersers.

[0027] In step 2, a polymer (a) constituting the pigment-containing polymer particles contained in the aqueous dispersion obtained in step 1 is crosslinked with a crosslinking agent to obtain an aqueous dispersion of pigment-containing crosslinked polymer particles. The preferred crosslinking agent and crosslinking ratio are as described above. The temperature for the crosslinking treatment is preferably 40°C or higher, more preferably 50°C or higher, even more preferably 60°C or higher, and preferably 95°C or lower, more preferably 85°C or lower, and even more preferably 75°C or lower. The duration of the crosslinking treatment is preferably 0.5 hours or more, more preferably 1 hour or more, even more preferably 3 hours or more, and preferably 12 hours or less, more preferably 10 hours or less, and even more preferably 8 hours or less, from the viewpoint of completion of the crosslinking reaction and economic efficiency.

[0028] The acid value of pigment-containing (crosslinked) polymer particles is preferably 50 mg KOH / g or more, more preferably 60 mg KOH / g or more, even more preferably 70 mg KOH / g or more, and preferably 200 mg KOH / g or less, more preferably 150 mg KOH / g or less, and even more preferably 110 mg KOH / g or less, from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and discharge stability. The acid value of pigment-containing (crosslinked) polymer particles is the acid value of the polymer having a crosslinked structure if the polymer constituting the particles has a crosslinked structure, and is the acid value of the polymer used for pigment dispersion if the polymer constituting the particles does not have a crosslinked structure. The acid value of pigment-containing (crosslinked) polymer particles is measured by the method described in the examples.

[0029] In an aqueous dispersion of pigment-containing (crosslinked) polymer particles, the average particle size of the pigment-containing (crosslinked) polymer particles is preferably 60 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and even more preferably 90 nm or more, and preferably 200 nm or less, more preferably 170 nm or less, and even more preferably 130 nm or less, from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and discharge stability. The average particle size of the pigment-containing (crosslinked) polymer particles in the aqueous dispersion is measured by the method described in the examples. Furthermore, it is preferable that the pigment-containing (crosslinked) polymer particles do not swell or shrink, nor do they aggregate with other particles. It is also preferable that the average particle size of the pigment-containing (crosslinked) polymer particles in the ink of the present invention is the same as the average particle size in the aqueous dispersion. In other words, the preferred embodiment of the average particle size of the pigment-containing (crosslinked) polymer particles in the ink of the present invention is the same as the preferred embodiment of the average particle size of the pigment-containing (crosslinked) polymer particles in the aqueous dispersion.

[0030] <Organic solvent (C-1)> The ink of the present invention contains an organic solvent (C-1) having a solubility in water of 20% by mass or less. The organic solvent (C-1) is thought to rapidly reduce the surface tension of ink dots on a low-liquid-absorbent recording medium, thereby contributing to improved wettability of the ink dots. The solubility of the organic solvent (C-1) in water is calculated according to the following formula based on the total mass of the organic solvent added when the process of adding the organic solvent in small amounts to 100 g of deionized water at 20°C and stirring is repeated until phase separation can be visually confirmed, as described in the examples. Solubility in water (mass %) = (Total mass of organic solvent added until phase separation is visually observed (g) / Mass of water (g)) × 100 The solubility of the organic solvent (C-1) in water is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, even more preferably 2% by mass or more, even more preferably 3% by mass or more, even more preferably 4% by mass or more, even more preferably 5% by mass or more, and preferably 19% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and even more preferably 8% by mass or less, from the viewpoint of improving storage stability and discharge stability, and improving the wettability of ink dots on the recording medium.

[0031] As the organic solvent (C-1), an alkylene glycol ether represented by the following formula (1) is preferred from the viewpoint of improving storage stability and ejection stability, and from the viewpoint of improving the wettability and spreadability of ink dots on the recording medium. R 1 -O-(AO) n -R 2 (1) [In formula (1), R 1 R represents a hydrocarbon group with 3 or more carbon atoms. 2 [where represents a hydrogen atom or a hydrocarbon group having 3 or more carbon atoms, AO represents an alkylene oxy group having 2 or 3 carbon atoms, and n represents the average number of moles of AO added, which is between 1 and 3.]

[0032] In equation (1), R 1is preferably at least one selected from the group consisting of an alkyl group having 3 to 12 carbon atoms, a phenyl group optionally substituted with an alkyl group having 1 to 3 carbon atoms, and a benzyl group, from the viewpoints of improving storage stability and ejection stability, and improving the wet spreading property of ink dots on a recording medium; more preferably at least one selected from the group consisting of an alkyl group having 3 to 10 carbon atoms, a phenyl group optionally substituted with an alkyl group having 1 to 3 carbon atoms, and a benzyl group; still more preferably at least one selected from the group consisting of an alkyl group having 3 to 10 carbon atoms, a phenyl group, and a benzyl group; even more preferably an alkyl group having 3 to 10 carbon atoms; and even more preferably an alkyl group having 3 to 6 carbon atoms. R 1 When R is an alkyl group, it may be either linear or branched. In formula (1), R 2 is preferably at least one selected from the group consisting of a hydrogen atom and an alkyl group having 3 to 12 carbon atoms, from the viewpoints of improving storage stability and ejection stability, and improving the wet spreading property of ink dots on a recording medium; more preferably at least one selected from the group consisting of a hydrogen atom and an alkyl group having 3 to 10 carbon atoms; still more preferably at least one selected from the group consisting of a hydrogen atom and an alkyl group having 3 to 8 carbon atoms; even more preferably at least one selected from the group consisting of a hydrogen atom and an alkyl group having 3 to 6 carbon atoms; and even more preferably a hydrogen atom. R 2 When R is an alkyl group, it may be either linear or branched. In formula (1), AO is preferably at least one selected from the group consisting of an ethyleneoxy group and a propyleneoxy group (trimethyleneoxy group or propane-1,2-diyl oxy group), from the viewpoints of improving storage stability and ejection stability, and improving the wet spreading property of ink dots on a recording medium; more preferably a propyleneoxy group. In formula (1), n is preferably a number of 1 to 2, more preferably 1, from the viewpoints of improving storage stability and ejection stability, and improving the wet spreading property of ink dots on a recording medium.

[0033] As for the organic solvent (C-1), from the viewpoint of improving storage stability and discharge stability, and from the viewpoint of improving the wettability and spreadability of ink dots on the recording medium, one or more selected from the group consisting of ethylene glycol-based glycol ethers, propylene glycol-based glycol ethers, and alkylene glycol-based dialkyl ethers are preferred. The organic solvent (C-1) may be used individually or in combination of two or more types.

[0034] Specific examples of ethylene glycol-based glycol ethers include monoethylene glycol-based glycol ethers such as ethylene glycol monohexyl ether (solubility in water 1.0% by mass), ethylene glycol mono-2-ethylhexyl ether (solubility in water 0.2% by mass), ethylene glycol monophenyl ether (solubility in water 2.7% by mass), and ethylene glycol monobenzyl ether (solubility in water 0.4% by mass); and diethylene glycol-based glycol ethers such as diethylene glycol monohexyl ether (solubility in water 1.7% by mass), diethylene glycol mono-2-ethylhexyl ether (solubility in water 0.3% by mass), and diethylene glycol monophenyl ether (solubility in water 3.4% by mass).

[0035] Specific examples of propylene glycol-based glycol ethers include monopropylene glycol-based glycol ethers such as propylene glycol monobutyl ether (solubility in water 6.4% by mass) and propylene glycol monophenyl ether (solubility in water 0.2% by mass); dipropylene glycol-based glycol ethers such as dipropylene glycol monopropyl ether (solubility in water 19.0% by mass) and dipropylene glycol monobutyl ether (solubility in water 3.0% by mass); and tripropylene glycol-based glycol ethers such as tripropylene glycol monobutyl ether (solubility in water 0.4% by mass).

[0036] Alkylene glycol-based dialkyl ethers are preferably represented by the following formula (1-1). R 11 -O-(AO) n1 -R 12 (1-1) [In formula (1-1), R 11 and R 12 Each of the following independently represents a hydrocarbon group having 3 or more carbon atoms, AO represents an alkylene oxy group having 2 or 3 carbon atoms, and n1 represents the average number of moles of AO added, which is between 1 and 3. Specific examples of alkylene glycol-based dialkyl ethers include diethylene glycol dibutyl ether (solubility in water 0.3% by mass) and others.

[0037] The boiling point of the organic solvent (C-1) is preferably 120°C or higher, more preferably 140°C or higher, even more preferably 160°C or higher, and preferably 250°C or lower, more preferably 240°C or lower, even more preferably 235°C or lower, even more preferably 220°C or lower, and even more preferably 200°C or lower, from the viewpoint of improving storage stability and ejection stability, and improving the wettability and spreadability of ink dots on the recording medium. When using two or more organic solvents (C-1), the boiling point values ​​of the organic solvents (C-1) shall be the weighted average value weighted by the content (mass%) of each organic solvent.

[0038] Among these, the organic solvent (C-1) is preferably one or more selected from the group consisting of ethylene glycol ethers and propylene glycol ethers, from the viewpoint of improving storage stability and discharge stability, and improving the wettability and spreadability of ink dots on the recording medium, more preferably one or more selected from the group consisting of monoethylene glycol ethers, diethylene glycol ethers, monopropylene glycol ethers, and dipropylene glycol ethers, and even more preferably propylene glycol monobutyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, ethylene glycol It is one or more selected from the group consisting of monohexyl ether and ethylene glycol mono-2-ethylhexyl ether, more preferably one or more selected from the group consisting of propylene glycol monobutyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether and ethylene glycol monohexyl ether, more preferably one or more selected from the group consisting of propylene glycol monobutyl ether, dipropylene glycol monopropyl ether and dipropylene glycol monobutyl ether, and even more preferably one or more selected from the group consisting of propylene glycol monobutyl ether and dipropylene glycol monopropyl ether.

[0039] <Water-soluble organic solvent (C-2)> The ink of the present invention preferably further contains, in addition to the organic solvent (C-1), a water-soluble organic solvent (C-2) (hereinafter also simply referred to as "water-soluble organic solvent (C-2)") having a solubility in water of more than 20% by mass. The water-soluble organic solvent (C-2) is thought to uniformly dissolve the organic solvent (C-1) in the ink, improving storage stability and ejection stability, and contributing to further improvement in the wettability and spreadability of ink dots on the recording medium. The solubility of water-soluble organic solvent (C-2) in water can be determined using the same method as for calculating the solubility of organic solvent (C-1) in water. The water-soluble organic solvent (C-2) may be a liquid or a solid at 25°C.

[0040] As the water-soluble organic solvent (C-2), one or more selected from the group consisting of diols, polyhydric alcohols of trivalent or higher, and alkylene glycol ethers, other than the organic solvent (C-1), are preferred. 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 alkylene glycol 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.

[0041] Among these, the water-soluble organic solvent (C-2) is preferably one or more selected from the group consisting of diols other than organic solvent (C-1) and alkylene glycol ethers, and 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. From the viewpoint of improving storage stability, ejection stability, and the wettability and spreadability of ink dots on recording media in a balanced manner, the water-soluble organic solvent (C-2) preferably contains a diol, more preferably one or more selected from the group consisting of propylene glycol, 1,2-butanediol, 1,3-propanediol, 1,4-butanediol, and diethylene glycol, and even more preferably contains propylene glycol. From the viewpoint of improving the wettability and spreadability of ink dots on the recording medium, the water-soluble organic solvent (C-2) preferably contains an alkylene glycol ether, more preferably one or more selected from the group consisting of diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, and dipropylene glycol monomethyl ether, and even more preferably diethylene glycol monoisopropyl ether.

[0042] <Water-insoluble polymer particles containing wax> The ink of the present invention contains water-insoluble polymer particles containing wax, from the viewpoint of improving the storage stability and ejection stability of the ink, and from the viewpoint of improving image fastness by reducing the frictional resistance of the ink coating film when rubbed. In the present invention, "water-insoluble polymer particles containing wax" means particles in which wax is dispersed by a water-insoluble polymer (b). Preferred forms of water-insoluble polymer particles containing wax include particles in which the water-insoluble polymer (b) encapsulates the wax, particles in which a portion of the wax is exposed on the surface of particles composed of water-insoluble polymer (b) and wax, particles in which the water-insoluble polymer (b) is adsorbed onto a portion of the wax, and mixtures thereof. Among these, the form of particles in which a portion of the wax is exposed on the surface of particles composed of water-insoluble polymer (b) and wax, or the form of particles in which the water-insoluble polymer (b) is adsorbed onto a portion of the wax, is more preferred. Hereinafter, water-insoluble polymer particles containing wax will also be simply referred to as "wax-containing polymer particles." In this invention, the "water insolubility" of the polymer is as described above.

[0043] (wax) The wax 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. Waxes can be used individually or in combination of two or more types.

[0044] The melting point of the wax is preferably 80°C or higher, more preferably 100°C or higher, even more preferably 105°C or higher, even more preferably 108°C or higher, even more preferably 115°C or higher, and preferably 150°C or lower, more preferably 145°C or lower, even more preferably 140°C or lower, and even more preferably 130°C or lower, from the viewpoint of improving the image robustness of the recorded material. The melting point of the wax is measured by the method described in the examples.

[0045] Among these, polyolefin waxes mainly composed of olefin monomers are preferred from the viewpoint of improving the storage stability and ejection stability of the ink, improving the wettability and spreadability of the ink dots, and improving the image fastness of the recorded material.

[0046] Examples of olefin monomers that are the main components of polyolefin wax include chain olefins and cyclic olefins, but those mainly composed of chain olefins having 2 to 6 carbon atoms are preferred, polyolefin wax mainly composed of ethylene or propylene is more preferred, and polyethylene wax mainly composed of ethylene is even more preferred. Here, "primarily composed of ethylene or propylene" means that the content of ethylene or propylene relative to the total components constituting the wax is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more. Oxidized polyolefin waxes can be obtained by introducing oxygen atoms and other elements into the molecule of a high molecular weight polyolefin polymer while adjusting it to a desired molecular weight through thermal decomposition or other means, and are included in the category of polyolefin waxes. In other words, the polyolefin wax is preferably one or more selected from the group consisting of polyethylene wax, polypropylene wax, and oxidized polyethylene wax, with polyethylene wax being more preferred.

[0047] From the same viewpoint as above, the weight-average molecular weight of the polyolefin wax is preferably 400 or more, more preferably 600 or more, even more preferably 800 or more, and preferably 10,000 or less, more preferably 8,000 or less, and even more preferably 6,000 or less. From the same viewpoint as above, the density of the polyolefin wax is preferably 0.890 g / cm³. 3 More preferably 0.900 g / cm³ 3 More preferably 0.910 g / cm³ 3 The above applies, and preferably 0.985 g / cm³. 3 More preferably, 0.980 g / cm³ 3 More preferably, 0.975 g / cm³ 3 The following applies: In the present invention, other waxes besides polyolefin waxes may be included, as long as they do not impair the effects of the present invention. The polyolefin wax content in the total amount of wax is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 100% by mass.

[0048] Polyolefin wax may also be used as a dispersion (emulsion) in an aqueous medium. There are no particular restrictions on the method of producing polyolefin wax; for example, one method involves mixing polyolefin wax with other waxes as needed and emulsifying it with a known surfactant. Nonionic surfactants, anionic surfactants, etc., can be used as surfactants. Suitable examples of commercially available polyethylene waxes include the "High Wax" series manufactured by Mitsui Chemicals, Inc. and the "Sun Wax" series manufactured by Sanyo Chemical Industries, Ltd.

[0049] (Water-insoluble polymer (b)) The water-insoluble polymer (b) (hereinafter also referred to as "polymer (b)") is preferably an anionic polymer having an anionic group. In the present invention, the "anionic group" is as described above. Among these, from the viewpoint of improving the storage stability and ejection stability of the ink, the anionic group is preferably a carboxyl group. Examples of polymer backbone for polymer (b) include vinyl polymers obtained by addition polymerization of vinyl monomers, polyester polymers, polyurethane polymers, etc. Among these, the polymer backbone for polymer (b) is preferably a vinyl polymer from the viewpoint of improving the storage stability and discharge stability of the ink. Polymer (b) is preferably a vinyl polymer containing (b-1) a carboxyl group-containing vinyl monomer and (b-2) a hydrophobic vinyl monomer, and may further contain (b-3) a nonionic monomer.

[0050] [(b-1) Carboxylate-containing vinyl monomer] (b-1) Carboxylic group-containing vinyl monomers are thought to improve the dispersion stability of wax-containing polymer particles through their charge repulsion, thereby contributing to improved storage stability and ejection stability of the ink. (b-1) Specific examples and preferred examples of carboxyl group-containing vinyl monomers are the same as the specific examples and preferred examples of ionic vinyl monomers (a-1) described in the section on polymer (a). That is, (b-1) the carboxyl group-containing vinyl monomer is preferably one or more selected from the group consisting of acrylic acid and methacrylic acid, from the viewpoint of improving storage stability and discharge stability.

[0051] [(b-2) Hydrophobic vinyl monomer] (b-2) Hydrophobic vinyl monomers are thought to enhance the interaction between polymer (b) and wax, thereby improving the dispersion stability of wax-containing polymer particles and contributing to improved storage stability and ejection stability of the ink. (b-2) The hydrophobic vinyl monomer is 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; more preferably one or more selected from the group consisting of alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth)acrylate; even more preferably one or more selected from the group consisting of alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, styrene, and benzyl (meth)acrylate; and even more preferably one or more selected from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, stearyl (meth)acrylate, styrene, and benzyl (meth)acrylate.

[0052] [(b-3) Nonionic vinyl monomer] (b-3) Nonionic vinyl monomers are monomers that have functional groups or sites with high affinity for water and water-soluble organic solvents, and are thought to improve the dispersion stability of wax-containing polymer particles, thereby contributing to improved storage stability and ejection stability of inks. (b-3) Examples of nonionic vinyl monomers include monomers containing hydroxyl groups or polyalkylene glycol chains. (b-3) Specific examples of nonionic vinyl monomers include those described in paragraph

[0018] of Japanese Patent Application Publication No. 2018-83938, similar to polymer (a). Among these, (b-3) nonionic vinyl monomer is preferably one or more selected from the group consisting of polyethylene glycol (m=1~30) (meth)acrylate, polypropylene glycol (m=2~30) (meth)acrylate, and methoxypolyethylene glycol (m=1~30) (meth)acrylate. Here, m represents the average number of moles of alkylene oxide added. The above components (b-1) to (b-3) can each be used by using one monomer component alone or in combination of two or more.

[0053] The content of each monomer-derived constituent unit in polymer (b) is as follows, from the viewpoint of improving the dispersion stability of wax-containing polymer particles and improving the storage stability and ejection stability of the ink. The content of constituent units derived from component (b-1) in polymer (b) is preferably 4% by mass or more, more preferably 8% by mass or more, even more preferably 12% by mass or more, and preferably 28% by mass or less, more preferably 24% by mass or less, and even more preferably 20% by mass or less. The content of constituent units derived from component (b-2) in polymer (b) is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 75% by mass or more, and preferably 96% by mass or less, more preferably 92% by mass or less, and even more preferably 88% by mass or less. The content of constituent units derived from component (b-3) in polymer (b) is preferably 40% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, even more preferably 2% by mass or less, and even more preferably 0% by mass. The total content of constituent units derived from component (b-1) and component (b-2) in polymer (b) is preferably 60% by mass or more, more preferably 75% by mass or more, even more preferably 90% by mass or more, and even more preferably 100% by mass. The mass ratio [(b-1) component / (b-2) component] of the constituent units derived from component (b-1) in polymer (b) to the constituent units derived from component (b-2) is preferably 0.05 or more, more preferably 0.1 or more, even more preferably 0.15 or more, and preferably 1.3 or less, more preferably 1 or less, even more preferably 0.8 or less, even more preferably 0.5 or less, and even more preferably 0.3 or less.

[0054] The number-average molecular weight of polymer (b) is preferably 8,000 or more, more preferably 12,000 or more, even more preferably 16,000 or more, and preferably 80,000 or less, more preferably 60,000 or less, even more preferably 40,000 or less, and even more preferably 30,000 or less. The number-average molecular weight of polymer (b) is measured by the method described in the examples.

[0055] 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, even more preferably 90 mg KOH / g or more, even more preferably 100 mg KOH / g or more, and preferably 350 mg KOH / g or less, more preferably 300 mg KOH / g or less, and even more preferably 250 mg KOH / g or less, from the viewpoint of improving storage stability and discharge stability. The acid value of polymer (b) is measured by the method described in the examples.

[0056] If polymer (b) is an anionic polymer having anionic groups, it is preferable that at least a portion of the anionic groups are neutralized using a neutralizing agent. This increases the charge repulsion force of the anionic groups that emerge after neutralization, which suppresses aggregation of wax-containing polymer particles in the ink of the present invention and improves dispersion stability, and as a result, improves storage stability and discharge stability. Examples of neutralizing agents include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, ammonia, and various organic amines, with alkali metal hydroxides being preferred. Alternatively, polymer (b) may be neutralized beforehand. The equivalent amount of neutralizing agent used is preferably 30 mol% or more, more preferably 40 mol% or more, even more preferably 50 mol% or more, and even more preferably 60 mol% or more, and preferably 150 mol% or less, more preferably 100 mol% or less, even more preferably 90 mol% or less, and even more preferably 85 mol% or less, from the viewpoint of ensuring the dispersion stability of the wax-containing polymer particles. Here, the equivalent amount of neutralizing agent used can be determined by the following formula. If the equivalent amount of neutralizing agent used is 100 mol% or less, it is equivalent to the degree of neutralization. If the equivalent amount of neutralizing agent used in the following formula exceeds 100 mol%, it means that the neutralizing agent is in excess of the carboxyl groups of polymer (b), and in this case, the degree of neutralization of polymer (b) is considered to be 100 mol%. Amount of neutralizing agent used (mol%) = [{Mass of neutralizing agent added (g) / Equivalent amount of neutralizing agent (g / mol)} / [{Acid value of polymer (b) before neutralization (mgKOH / g) × Mass of polymer (b) before neutralization (g)} / (56.1 × 1,000)]] × 100 In this invention, the calculation of the equivalent amount of neutralizing agent used does not include the amount of neutralizing agent added that has a boiling point of 100°C or lower at atmospheric pressure.

[0057] Polymer (b) may be synthesized as appropriate, or a commercially available product may be used. Polymer (b) is synthesized by copolymerizing raw material monomers, for example, (b-1) carboxyl group-containing vinyl monomer, (b-2) hydrophobic vinyl monomer, and optionally (b-3) nonionic vinyl monomer, using known polymerization methods such as solution polymerization, bulk polymerization, suspension polymerization, and emulsion polymerization. Among these polymerization methods, solution polymerization is preferred. There are no restrictions on the solvent used in solution polymerization, but polar solvents such as water, lower aliphatic alcohols, ketones such as methyl ethyl ketone, ethers, and esters are preferred. During polymerization, polymerization initiators such as azo compounds and persulfates, and polymerization chain transfer agents such as mercaptans can be used. The polymerization temperature varies depending on the type of polymerization initiator, monomer, and solvent used, but is preferably 30°C or higher, more preferably 50°C or higher, and preferably 100°C or lower, more preferably 90°C or lower.

[0058] (Manufacturing of wax-containing polymer particles) Methods for producing wax-containing polymer particles include dispersing a mixture containing polymer (b) and wax to obtain an aqueous dispersion of wax-containing polymer particles. Specifically, a method of dispersing a mixture obtained by mixing a pre-prepared aqueous dispersion of polymer (b) with wax is preferred. Dispersion processing can be carried out by known methods using kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers (manufactured by Microfluidics), paint shakers, bead mills, and other media-type dispersers.

[0059] The average particle size of the wax-containing polymer particles in the aqueous dispersion of wax-containing polymer particles is preferably 20 nm or more, more preferably 40 nm or more, even more preferably 80 nm or more, and preferably 250 nm or less, more preferably 200 nm or less, even more preferably 150 nm or less, and even more preferably 130 nm or less, from the viewpoint of improving storage stability and discharge stability. The average particle size of the wax-containing polymer particles is measured by the method described in the examples. Furthermore, it is preferable that the wax-containing polymer particles do not swell or shrink, nor do they aggregate with other particles. The average particle size of the wax-containing polymer particles in the ink of the present invention is preferably the same as the average particle size in the aqueous dispersion. The preferred embodiment of the average particle size of the wax-containing polymer particles in the ink of the present invention is the same as the preferred embodiment of the average particle size of the wax-containing polymer particles in the aqueous dispersion.

[0060] <Water> The 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.

[0061] The ink of the present invention may further contain a surfactant from the viewpoint of improving wettability to the recording medium. A nonionic surfactant is preferred as the surfactant. Examples of nonionic surfactants include acetylene glycol-based surfactants, polyoxyalkylene alkyl ether-type surfactants, polyhydric alcohol-type surfactants, fatty acid alkanolamides, silicone-based nonionic surfactants, and fluorine-based nonionic surfactants. Among these, one or more selected from the group consisting of acetylene glycol-based surfactants and polyoxyalkylene alkyl ether-type surfactants are preferred, and more preferably, one or more selected from the group consisting of 2,4,7,9-tetramethyl-5-decine-4,7-diol and its ethylene oxide adduct, and polyoxyethylene alkyl ethers. Commercially available nonionic surfactants include the "Surfinol" series from Nisshin Chemical Industry Co., Ltd. and Air Products & Chemicals, the "Acetyleneol" series from Kawaken Fine Chemical Co., Ltd., and the "Emulgen" series from Kao Corporation. Surfactants can be used individually or in combination of two or more types.

[0062] The ink of the present invention may contain various additives as optional components, depending on its application, such as pH adjusters, viscosity adjusters, defoamers, preservatives, and rust inhibitors. In this case, a portion of the water content may be replaced with these additives.

[0063] (Content of each component in the ink of the present invention) From the viewpoint of improving image density, the pigment content in the 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 improving storage stability and ejection stability, it is preferably 12% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less. The mass ratio of the pigment content to the total content of the pigment and polymer (a) in the ink of the present invention [pigment / (pigment + polymer (a))] is preferably 0.3 or higher, more preferably 0.5 or higher, even more preferably 0.7 or higher, and preferably 0.9 or lower, more preferably 0.85 or lower, and even more preferably 0.8 or lower, from the viewpoint of improving the dispersion stability of the pigment and improving storage stability and ejection stability. The content of pigment-containing (crosslinked) polymer particles in the 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 improving image density, and preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less, from the viewpoint of improving storage stability and ejection stability.

[0064] The content of the organic solvent (C-1) in the ink of the present invention is preferably 0.3% by mass or more, more preferably 1% by mass or more, even more preferably 2% by mass or more, even more preferably 3% by mass or more, and even more preferably 4% by mass or more, from the viewpoint of improving the wettability and spreadability of ink dots on the recording medium, and preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 8% by mass or less, and even more preferably 6% by mass or less, from the viewpoint of improving storage stability and ejection stability. The content of the water-soluble organic solvent (C-2) in the ink of the present invention is preferably 1% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, even more preferably 15% by mass or more, and preferably 30% by mass or less, more preferably 27% by mass or less, even more preferably 25% by mass or less, and even more preferably 23% by mass or less, from the viewpoint of uniformly dissolving the organic solvent (C-1) in the ink, improving the storage stability and discharge stability of the ink, and improving the wettability of ink dots on the recording medium.

[0065] The mass ratio of organic solvent (C-1) to the total content of organic solvent (C-1) and water-soluble organic solvent (C-2) in the ink of the present invention [organic solvent (C-1) / [organic solvent (C-1) + water-soluble organic solvent (C-2)] is preferably 0.01 or higher, more preferably 0.05 or higher, even more preferably 0.07 or higher, even more preferably 0.1 or higher, and even more preferably 0.15 or higher, from the viewpoint of improving the wettability of ink dots, and preferably 0.7 or lower, more preferably 0.5 or lower, and even more preferably 0.3 or lower, from the viewpoint of improving storage stability and discharge stability.

[0066] The wax content in the ink of the present invention is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, even more preferably 0.6% by mass or more, and even more preferably 0.8% by mass or more, from the viewpoint of improving the image fastness of the recorded material, and from the viewpoint of improving the storage stability and ejection stability of the ink, and improving the wettability of the ink dots, it is preferably 4% by mass or less, more preferably 3% by mass or less, even more preferably 2.5% by mass or less, and even more preferably 2% by mass or less. The mass ratio of the wax content to the polymer (b) content in the ink of the present invention [wax / polymer(b)] is preferably 0.5 or more, more preferably 1 or more, and even more preferably 3 or more, from the viewpoint of improving image fastness by exhibiting the effect of wax to reduce frictional resistance when the ink coating formed on the recording material is rubbed, and from the viewpoint of improving the dispersion stability of the wax-containing polymer particles, improving storage stability and ejection stability, and also improving the wettability of the ink, it is preferably 15 or less, more preferably 10 or less, even more preferably 8 or less, and even more preferably 6 or less. The content of wax-containing polymer particles in the ink of the present invention is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 1.1% by mass or more, and preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 5% by mass or less, even more preferably 3% by mass or less, even more preferably 2% by mass or less, and even more preferably 1.5% by mass or less, from the viewpoint of achieving both the storage stability and ejection stability of the ink and the image fastness of the recorded material.

[0067] The mass ratio of the wax content to the organic solvent (C-1) content in the ink of the present invention [wax / organic solvent (C-1)] is preferably 0.05 or more, more preferably 0.1 or more, and even more preferably 0.15 or more, from the viewpoint of improving the image fastness of the recorded material, and from the viewpoint of improving the storage stability and ejection stability of the ink, and improving the wettability of the ink, is preferably 1 or less, more preferably 0.8 or less, even more preferably 0.6 or less, and even more preferably 0.4 or less. The mass ratio [wax / pigment] of the wax content to the pigment content in the ink of the present invention is preferably 0.1 or higher, more preferably 0.15 or higher, and even more preferably 0.2 or higher, from the viewpoint of improving the storage stability and ejection stability of the ink, improving the wettability and spreadability of the ink, and improving the image fastness of the recorded material, and from the same viewpoint as above, preferably 1 or lower, more preferably 0.75 or lower, and even more preferably 0.5 or lower.

[0068] The surfactant content in the ink of the present invention is preferably 0.2% by mass or more, more preferably 0.4% by mass or more, even more preferably 0.6% by mass or more, and preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less. The water content in the 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.

[0069] The viscosity of the ink of the present invention at 32°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 5 mPa·s or more, and preferably 12 mPa·s or less, more preferably 9 mPa·s or less, and even more preferably 7 mPa·s or less, from the viewpoint of improving ejection stability. The viscosity of the water-based ink can be measured using an E-type viscometer. The pH of the ink of the present invention at 20°C is preferably 7 or higher, more preferably 7.5 or higher, and even more preferably 8 or higher, from the viewpoint of improving ejection stability. 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 of the water-based ink can be measured by conventional methods.

[0070] [Inkjet recording method] The ink of the present invention is preferably used in an inkjet recording method in which the ink is loaded into a known inkjet recording device such as a piezo type and ejected as ink droplets onto a recording medium to record images or the like. The ink of the present invention can be used with both low-liquid-absorbent recording media and high-liquid-absorbent recording media, but from the viewpoint of ink wetting spreadability and image fastness, it is preferable to use it in an inkjet recording method that records on a low-liquid-absorbent recording media. The ink of the present invention can produce recordings with excellent ink dot wetting and spreading properties, as well as excellent image fastness, even when used for inkjet recording on low-liquid-absorption recording media. Examples of low-liquid-absorbent recording media include low-liquid-absorbent coated paper, art paper, and non-liquid-absorbent resin films. Examples of coated paper include general-purpose glossy paper and multi-color foam gloss paper. Examples of resin films include transparent synthetic resin films, such as polyester, polyvinyl chloride, polyolefin, and nylon films. These films may be biaxially oriented, uniaxially oriented, or unoriented. Among these, polyester films and stretched polypropylene films are preferred, and corona-discharge treated polyethylene terephthalate (PET) films and corona-discharge treated biaxially oriented polypropylene (OPP) films are more preferred. [Examples]

[0071] 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.

[0072] (1) Measurement of the number-average molecular weight of polymers The results were obtained by gel permeation chromatography. The measurement conditions are shown below. GPC device: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolum 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: 0.5mL / 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) The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25°C for 10 hours, and filtering the solution through a syringe filter (DISMIC-13HP, made of PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0073] (2) Measurement of the acid value of polymers The polymer was dissolved in a titration solvent of toluene and acetone (2:1) using a potentiometric automatic titrator (manufactured by Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with a 0.1N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was used as the endpoint. The acid value (mgKOH / g) was calculated from the amount of potassium hydroxide / ethanol solution titrated to the endpoint.

[0074] (3) Measurement of the acid value of pigment-containing (crosslinked) polymer particles in an aqueous dispersion 2 g of an aqueous dispersion of pigment-containing (crosslinked) polymer particles was diluted with 50 g of deionized water, and 3 mL of a 0.1 N sodium hydroxide / ethanol solution was added. 0.1 N hydrochloric acid was then gradually added dropwise, and the pH inflection points were measured at two locations by potentiometric titration. The number of moles of acid was then calculated from the difference in the amount of 0.1 N hydrochloric acid added between the two points. The acid value of the polymer was calculated from this number of moles of acid, the solid content of the pigment-containing (crosslinked) polymer particles in the aqueous dispersion, and the ratio of pigment and polymer constituting the particles. This was defined as the acid value (mgKOH / g) of the pigment-containing (crosslinked) polymer particles in the aqueous dispersion.

[0075] (4) Measurement of the average particle size of pigment-containing (crosslinked) polymer particles in aqueous dispersions Using a laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd., product name: ELS-8000), particle size was measured by dynamic light scattering 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 solvent. For the measurement sample, an aqueous pigment dispersion was weighed into a screw tube (Maruemu Co., Ltd., No. 5) and the solid content concentration was 2 × 10⁻⁶. -4 Water was added to the solution to a mass percentage, and the mixture was stirred using a magnetic stirrer at 25°C for 1 hour.

[0076] (5) Measurement of the melting point of wax The melting point of the wax was measured using an apparatus conforming to JIS K 0064. Specifically, a differential scanning calorimeter (Q20, manufactured by T.A. Instruments) was used to heat the sample to 200°C, and then cool it down to 0°C at a rate of 10°C / min. Next, the sample was heated again at a 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.

[0077] (6) Measurement of the average particle size of wax-containing polymer particles in an aqueous dispersion The average particle size (average dispersed particle size) of polymer particles in the wax dispersion was measured using a Microtrac particle size analyzer (UPA, manufactured by Nikkiso Co., Ltd.).

[0078] (7) Measurement of solid content concentration 10.0 g of sodium sulfate, which had been stabilized 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. The mixture was then accurately weighed and maintained at 105°C for 2 hours to remove volatile components. After standing in the desiccator for another 15 minutes, 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 was obtained by dividing it by the mass of the added sample.

[0079] (8) Measurement of the solubility of organic solvents in water 100g of deionized water was placed in a container, and organic solvents were added in small amounts at 20°C while stirring. This process was repeated until phase separation could be visually confirmed, and the solubility of various organic solvents in water was calculated using the following formula. Solubility in water (mass %) = (Total mass of organic solvent added until phase separation is visually observed (g) / Mass of water (g)) × 100 If no phase separation is observed when 100g of organic solvent is added to 100g of water, the solubility of the organic solvent in water is stated as "100% by mass or more".

[0080] Manufacturing Example 1 (Manufacturing of water-insoluble polymer (i)) A monomer mixture was prepared by mixing 14 parts acrylic acid, 11 parts ethyl acrylate, and 75 parts styrene. In a reaction vessel, 10 parts methyl ethyl ketone (hereinafter referred to as "MEK"), 0.2 parts 2-mercaptoethanol (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 remaining 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 an azo radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65) 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 (i) (number average molecular weight: 24,000, acid value: 109 mgKOH / g).

[0081] Manufacturing Examples 2 and 3 (Manufacturing of water-insoluble polymers (ii) and (iii)) In Production Example 1, water-insoluble polymers (ii) and (iii) were obtained in the same manner as in Production Example 1, except that the monomer composition was changed as shown in Table 1. The number-average molecular weight and acid value of water-insoluble polymers (ii) and (iii) are shown in Table 1.

[0082] [Table 1]

[0083] Preparation Example I (Preparation of aqueous dispersion P1 of pigment-containing crosslinked polymer particles) (Preparation of aqueous polymer dispersions) As polymer (a), 32 parts of water-insoluble polymer (i) obtained in Production Example 1 were mixed with 202 parts of ion-exchanged water. Further, 9.4 parts of 5N sodium hydroxide aqueous solution (sodium hydroxide solids content 16.9% by mass) were added to neutralize the mixture so that the ratio of moles of sodium hydroxide to moles of carboxyl groups in polymer (a) was 65 mol% (degree of neutralization 65 mol%). The mixture was heated to 90°C using a warm bath and stirred for 1 hour to completely disperse the polymer in water. After cooling to room temperature (25°C), an aqueous dispersion of the polymer was obtained. (Process 1) To the aqueous dispersion of the polymer obtained above, 100 parts of carbon black pigment (CB, CI Pigment Black 7, manufactured by Cabot Chemical, trade name: Monarch 800) were added, and the mixture was stirred for 3 hours at 20°C with a disperser (manufactured by Asada Iron Works Co., Ltd., trade name: 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 (manufactured by 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 (manufactured by Hitachi Koki Co., Ltd., trade name: 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-containing polymer particles (solid content concentration: 25% by mass). (Process 2) 100 parts of the aqueous dispersion of pigment-containing polymer particles obtained in step 1 were placed in a screw-top glass bottle, 26 parts of deionized water were added, and 0.2 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-321, epoxy equivalent: 140) was added as a crosslinking agent. The bottle was then sealed tightly and heated at 70°C for 5 hours while stirring with a stirrer. At this time, the crosslinking treatment was performed with an amount of crosslinking agent that could react with 15% of the total number of carboxyl groups contained in polymer (a) (crosslinking rate: 15 mol%). After 5 hours, the dispersion was cooled to room temperature (25°C), filtered through the 5 μm filter, and an aqueous dispersion P1 of pigment-containing crosslinked polymer particles (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, average particle size: 105 nm).

[0084] Preparation Example II-1 (Preparation of aqueous dispersion W1 of wax-containing polymer particles) To prepare polymer (b), 20 parts of water-insoluble polymer (i) obtained in Production Example 1 were neutralized by adding 5.9 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 (i) (equivalent amount of neutralizing agent used: 65 mol%). Furthermore, 240 parts of deionized water are added, and polyethylene wax (manufactured by Mitsui Chemicals, Inc., product name: High Wax 200P, melting point: 122℃, density: 0.970 g / cm³) is added to it. 3 (Weight-average molecular weight: 2,000) 80 parts were heated and dissolved at 85-95°C to obtain a mixture. Next, the obtained mixture was dispersed for 30 minutes using an ultrasonic homogenizer while being maintained at 90-95°C. After cooling to room temperature, it was dispersed in three passes at a pressure of 200 MPa using a microfluidizer (Microfluidics, trade name) to obtain a dispersion. Deionized water was added to the obtained dispersion to obtain an aqueous dispersion of wax-containing polymer particles W1 (solid content concentration: 20% by mass, wax content: 16% by mass, polymer (b) content: 4% by mass, average particle size: 90 nm).

[0085] Preparation Examples II-2 to II-7 (Preparation of aqueous dispersions W2 to W7 of wax-containing polymer particles) In Preparation Example II-1, aqueous dispersions of wax-containing polymer particles W2-7 were obtained in the same manner as in Preparation Example II-1, except that the formulation composition during the preparation of the dispersion was changed as shown in Table 2. The waxes shown in Table 2 are listed below. • HW200P: Polyolefin wax, manufactured by Mitsui Chemicals, Inc., product name: High Wax 200P • HW110P: Polyolefin wax, manufactured by Mitsui Chemicals, Inc., product name: High Wax 110P • TW-131: Carnauba wax, manufactured by Toagosei Co., Ltd., product name: TOWAX-131

[0086] [Table 2]

[0087] Example 1 22.8 parts of aqueous dispersion P1 of pigment-containing crosslinked polymer particles obtained in Preparation Example I, and W1 of aqueous dispersion of wax-containing polymer particles obtained in Preparation Example II-1. 6.3 parts of propylene glycol monobutyl ether (manufactured by Nippon Emulsifier Co., Ltd.) (hereinafter referred to as "BFG"), 5.0 parts of propylene glycol (manufactured by AGC Inc.) (hereinafter referred to as "PG"), 0.8 parts (in its original form) of acetylene glycol-based surfactant (manufactured by Nisshin Chemical Industry Co., Ltd., product name: Surfinol 104PG50, PG (50% by mass) solution) and 0.5 parts of polyoxyethylene lauryl ether (manufactured by Kao Corporation, product name: Emulgen 120) were added as nonionic surfactants. Deionized water was then added and mixed to a total volume of 100 parts. The resulting mixture was filtered using a 25 mL needleless syringe fitted with a 1.2 μm filter (acetylcellulose membrane, outer diameter 2.5 cm, manufactured by Fujifilm Corporation) to remove coarse particles and obtain water-based ink 1.

[0088] Examples 2-19 and Comparative Examples 1 and 2 Aqueous inks for Examples 2-19 and Comparative Examples 1 and 2 were obtained in the same manner as in Example 1, except that the formulation composition was changed as shown in Table 3. The components shown in Table 3 are described below. [Self-dispersing pigments] • CAB-O-JET300: Manufactured by Cabot Corporation [Organic solvents] [Organic solvent (C-1)] • BFG: Propylene glycol monobutyl ether, manufactured by Nippon Emulsifier Co., Ltd. • PFDG: Dipropylene glycol monopropyl ether, manufactured by Nippon Emulsifier Co., Ltd. BFDG: Dipropylene glycol monobutyl ether, manufactured by Nippon Emulsifier Co., Ltd. • HeG: Ethylene glycol monohexyl ether, manufactured by Nippon Emulsifier Co., Ltd. • EHG: Ethylene glycol mono-2-ethylhexyl ether, manufactured by Nippon Emulsifier Co., Ltd. The solubility of organic solvent (C-1) in water is shown in Table 2. [Water-soluble organic solvent (C-2)] • PG: Propylene glycol, manufactured by AGC Inc. iPDG: Diethylene glycol monoisopropyl ether, manufactured by Nippon Emulsifier Co., Ltd. • BDG: Diethylene glycol monobutyl ether, manufactured by Nippon Emulsifier Co., Ltd.

[0089] The obtained water-based inks were evaluated for storage stability, ejection stability, image fastness, and ink dot wetting properties using the following method. The results are shown in Table 3.

[0090] <Evaluation of storage stability> Each water-based ink was stored at 70°C for 28 days. The viscosity of each water-based ink at 32°C was measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.) before and after storage, and the viscosity change rate was calculated using the following formula to evaluate storage stability. Viscosity change rate (%) = [(Ink viscosity after storage) / (Ink viscosity before storage)] × 100 A lower viscosity change rate indicates better storage stability. A viscosity change rate of 140% or less is sufficient for practical use.

[0091] <Evaluation of discharge stability> Using various water-based inks, images were formed on A4-sized coated paper (manufactured by Oji Paper Co., Ltd., product name: OK Topcoat+) using the following inkjet recording methods. (Inkjet recording method) In an environment with a temperature of 25±1℃ and a relative humidity of 30±5%, a printing evaluation device (manufactured by Trytech Co., Ltd.) equipped with an inkjet head (Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezo type) was filled with water-based ink. The head voltage was set to 26V, frequency to 10kHz, appropriate amount of ejected liquid to 12pL, head temperature to 32℃, resolution to 600dpi, number of pre-ejection flushes to 200, and negative pressure to -4.0kPa. The recording medium was fixed to the transport table under reduced pressure with its longitudinal direction and transport direction being the same. A print command was transferred to the printing evaluation device, and a solid image with a duty cycle of 100% was printed. Subsequently, the printing evaluation device was stopped for 30 minutes, and the inkjet head was exposed to the atmosphere. After 30 minutes, the ink was purged from the inkjet head, wiped, and printing was resumed. The nozzle defect status was observed, and the nozzle recovery rate (%) was calculated using the following formula to evaluate the ejection stability. Nozzle recovery rate (%) = (Number of normal dispensing nozzles / Total number of nozzles) × 100 A higher nozzle recovery rate (%) indicates better dispensing stability. A nozzle recovery rate (%) of 90% or higher is considered practically usable.

[0092] <Evaluation of image robustness> A solid image with a 100% duty cycle was printed using the same method as the inkjet recording method described above for ejection stability, and a recorded material was obtained. After the obtained recorded material was left to stand for 24 hours, a JSPS-type abrasion tester (manufactured by Daiei Kagaku Seiki Seisakusho Co., Ltd., product name: RT-300) was used to check the image density of the ink transferred to the unprinted coated paper by placing a 2 x 2 cm piece of unprinted coated paper (OK Topcoat+) on the printed surface and running it back and forth 10 times with a load of 10 N. A lower transferred image density indicates better image fastness. If the transferred image density is 0.2 or less, it can be used for practical purposes.

[0093] <Evaluation of ink dot wetting properties> In an environment with a temperature of 25±1℃ and a relative humidity of 30±5%, an inkjet printer (ImageXpert, product name: Jetxpert) equipped with an inkjet head (FUJIFILM Dimatix, product name: Samba G3L, piezo type) was filled with each water-based ink. After adjusting the head voltage to 30V, frequency to 20kHz, push-pull drive waveform, and ink ejection volume to 2.0pL, an image was printed on coated paper (OK Topcoat+) at a resolution of 1200dpi to achieve an image density of 6%. The obtained recordings were observed under an optical microscope, and the diameter of the ink dots was measured and recorded as the dot diameter. To print evenly on paper at a resolution of 1200 dpi, a dot diameter of 30 μm is theoretically required. However, due to the impact accuracy inherent in the inkjet head, a dot diameter of 30 μm may cause streaks (white spots) and other issues. Therefore, we determined that a dot diameter of 31 μm or larger is practically usable in terms of dot wetting and spreading properties.

[0094] [Table 3]

[0095] Table 3 shows that the aqueous ink of the example has a larger dot diameter and superior ink dot wetting spread compared to the aqueous ink of Comparative Example 1, and superior ejection stability compared to the aqueous ink of Comparative Example 2. From this, it can be seen that, according to the present invention, it is possible to obtain a recording that has excellent storage stability and ejection stability, as well as excellent ink dot wetting spread and image fastness even when recording on a low liquid absorption recording medium.

Claims

1. A water-based ink for ink-jet printing, comprising a pigment, an organic solvent (C-1) having a solubility in water of 20% by mass or less, water-insoluble polymer particles containing a wax, and water.

2. 2. The water-based ink for ink-jet printing according to claim 1, wherein the wax has a melting point of 80[deg.] C. or higher.

3. 2. The water-based ink for ink-jet printing according to claim 1, wherein the water-insoluble polymer (b) constituting the wax-containing water-insoluble polymer particles is a vinyl polymer containing (b-1) a constituent unit derived from a carboxy group-containing vinyl monomer and (b-2) a constituent unit derived from a hydrophobic vinyl monomer.

4. 4. The water-based ink for ink-jet printing according to claim 3, wherein the carboxy group-containing vinyl monomer (b-1) is at least one monomer selected from the group consisting of acrylic acid and methacrylic acid.

5. The water-based ink for ink-jet printing according to claim 3, wherein the hydrophobic vinyl monomer (b-2) is at least one selected from the group consisting of alkyl (meth)acrylates having an alkyl group having from 1 to 22 carbon atoms, styrene, and benzyl (meth)acrylate.

6. 2. The water-based ink for ink-jet printing according to claim 1, wherein the organic solvent (C-1) is at least one selected from the group consisting of ethylene glycol-based glycol ethers, propylene glycol-based glycol ethers, and alkylene glycol-based dialkyl ethers.

7. 2. The water-based ink for ink-jet printing according to claim 1, further comprising a water-soluble organic solvent (C-2) having a solubility in water of more than 20% by mass.

8. The mass ratio of the content of the wax to the content of the organic solvent (C-1) [wax / organic solvent (C-1)] is 0.05 or more and 1 or less. The water-based ink for ink-jet printing according to claim 1.

9. An ink-jet recording method, comprising recording on a low liquid-absorbent recording medium using the water-based ink according to any one of claims 1 to 8.