Aqueous ink for inkjet printing

Abstract

To provide a water-based ink for inkjet printing excellent in discharge stability and refilling property.SOLUTION: A water-based ink for ink-jet printing on a low-liquid absorbing printing substrate, which comprises a pigment, an organic solvent (A), an organic solvent (B), a surfactant (C) having an HLB value of not less than 1.0 and not more than 6.0, and water, the organic solvent (A) is a glycol ether having a boiling point of not lower than 200 °C and not higher than 280 °C and an octanol-water partition coefficient logP of not less than 0.75, the organic solvent (B) is a polyhydric alcohol having a boiling point of not lower than 210 °C and not higher than 280 °C and an octanol-water partition coefficient logP of less than 0.75, and a distance Ra between Hansen solubility parameters (HSP) of the organic solvent (A) and the organic solvent (B) is not less than 6.0 and not more than 12.5.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based ink for ink-jet printing. [Background technology]

[0002] Inkjet printing involves ejecting ink droplets from minute nozzles and depositing them directly onto a printing substrate to produce printed matter bearing characters and images. This printing method has become extremely popular due to its numerous advantages, including the ease and cost-effectiveness of producing full-color prints, the ability to use a variety of printing substrates, including plain paper, label paper, and plastic film, and the fact that it is non-contact with the printing substrate. In particular, inks that use pigments as colorants are becoming mainstream, due to their superior weather resistance and water resistance.

[0003] For example, Patent Document 1 discloses an inkjet ink containing a pigment, acrylic resin particles, a surfactant, trimethylglycine, and water, with the aim of providing a water-based ink for inkjet recording that is excellent in fixability, image quality, and maintainability, and that can be fixed on coated paper by drying at a low temperature in a short time, wherein the glass transition temperature of the acrylic resin particles is 25°C or higher and lower than 50°C, and the static surface tension of the water-based ink for inkjet recording is 27 mN / m or less. Furthermore, Patent Document 2 discloses an image forming method for the purpose of providing an image forming method that has high image density and excellent fixability and ejection stability, and that includes an ink applying step of applying ink containing an organic solvent, a resin, and a colorant to a recording medium, and a drying step of drying the ink applied to the recording medium, wherein the glass transition temperature of the resin is 60°C or higher and 100°C or lower, the drying temperature in the drying step is −20°C or higher and +30°C or lower than the glass transition temperature of the resin, and the drying time in the drying step is 1.0 second or higher and 5.0 seconds or lower. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-156108 [Patent Document 2] Japanese Patent Application Publication No. 2020-146880 Summary of the Invention [Problem to be solved by the invention]

[0005] In commercial and industrial printing using inkjet recording methods, large ink droplets of 10 pL or more are often used to improve design, but large droplet printing uses a large amount of ink, making the refillability of the ink an issue. Furthermore, in industrial printing, low-absorbency printing substrates such as low-absorbency coated paper and non-absorbent resin films are used as printing substrates. However, since low-absorbency printing substrates are hydrophobic, hydrophobic organic solvents or surfactants may be blended in the ink to improve image quality. However, in the case of inks blended with such hydrophobic organic solvents, the dispersion stability of the pigment may decrease when the ink is concentrated in the inkjet nozzle, which may result in ink ejection defects. Therefore, in order to improve the productivity of printed materials, improvement of ejection stability is required. However, the inkjet ink described in Patent Document 1 leaves room for improvement in terms of refillability of the inkjet ink. Furthermore, the ink described in Patent Document 2 leaves room for improvement in terms of ink ejection stability. An object of the present invention is to provide a water-based ink for ink-jet printing that is excellent in ejection stability and refillability, and an ink-jet printing method that uses the water-based ink for ink-jet printing. [Means for solving the problem]

[0006] The present inventors have discovered that the above-mentioned problems can be solved by providing a water-based ink for inkjet printing on a low-absorbency printing substrate, the ink comprising a pigment, organic solvent (A), organic solvent (B), surfactant (C) having an HLB value of 1.0 or more and 6.0 or less, and water, wherein the organic solvent (A) is a glycol ether having a boiling point of 200°C or more and 280°C or less and an octanol-water partition coefficient logP (hereinafter also simply referred to as "logP") of 0.75 or more, and the organic solvent (B) is a polyhydric alcohol having a boiling point of 210°C or more and 280°C or less and an octanol-water partition coefficient logP of less than 0.75, and the Hansen solubility parameter (HSP) distance Ra between the organic solvent (A) and the organic solvent (B) is 6.0 or more and 12.5 or less. That is, the present invention provides the following [1] and [2]. [1] A water-based ink for inkjet printing on a low-absorbency printing substrate, the ink contains a pigment, an organic solvent (A), an organic solvent (B), a surfactant (C) having an HLB value of 1.0 or more and 6.0 or less, and water; the organic solvent (A) is a glycol ether having a boiling point of 200°C or higher and 280°C or lower and an octanol-water partition coefficient logP of 0.75 or higher; the organic solvent (B) is a polyhydric alcohol having a boiling point of 210°C or higher and 280°C or lower and an octanol-water partition coefficient logP of less than 0.75; the HSP distance Ra between the organic solvent (A) and the organic solvent (B) is 6.0 or more and 12.5 or less; Water-based ink for inkjet printing. [2] An inkjet printing method, comprising printing on a low-liquid-absorbent printing substrate using the water-based ink for inkjet printing described in [1] above. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a water-based ink for ink-jet printing that is excellent in ejection stability and refillability, and an ink-jet printing method that uses the water-based ink for ink-jet printing. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Water-based ink for inkjet printing] The water-based ink for inkjet printing of the present invention (hereinafter also simply referred to as "water-based ink") is a water-based ink for inkjet printing on a low-liquid-absorbency printing substrate, and the ink contains a pigment, organic solvent (A), organic solvent (B), surfactant (C) having an HLB value of 1.0 or more and 6.0 or less, and water, wherein the organic solvent (A) is a glycol ether having a boiling point of 200°C or more and 280°C or less and an octanol-water partition coefficient logP of 0.75 or more, and the organic solvent (B) is a polyhydric alcohol having a boiling point of 210°C or more and 280°C or less and an octanol-water partition coefficient logP of less than 0.75, and the Hansen solubility parameter (HSP) distance Ra between the organic solvents (A) and (B) is 6.0 or more and 12.5 or less. In the present invention, the boiling point means the boiling point at normal pressure (101.33 kPa).

[0009] According to the present invention, it is possible to provide a water-based ink for ink-jet printing that is excellent in ejection stability and refillability. The reason for this is not entirely clear, but is thought to be as follows. The aqueous ink of the present invention contains an organic solvent (A) with a logP of 0.75 or greater and relatively high hydrophobicity, and an organic solvent (B) with a logP of less than 0.75 and a relatively low hydrophobicity or hydrophilicity, with an HSP distance from the organic solvent (A) of 6.0 to 12.5. Therefore, it is believed that the organic solvent (A) undergoes phase separation in the aqueous ink. Generally, a hydrophobic material such as a silicone tube is used to connect the ink tank and the ejection head. The organic solvent (A) has high wettability to the hydrophobic material, which is believed to improve the refillability of the aqueous ink. Furthermore, the surfactant (C) contained in the aqueous ink of the present invention has a low HLB value and is hydrophobic, which is believed to further improve the refillability of the aqueous ink. Furthermore, it is believed that the organic solvent (A) undergoes phase separation in the aqueous ink inside the inkjet nozzle, forming a layer of organic solvent (A) at the gas-liquid interface of the nozzle. Since the pigment dispersion does not enter this phase-separated layer of organic solvent (A), it is believed that aggregation of the pigment dispersion inside the nozzle is suppressed. Furthermore, because the organic solvents (A) and (B) have high boiling points, it is believed that drying of the ink inside the inkjet nozzle is also suppressed. For this reason, it is believed that the aqueous ink of the present invention has excellent ejection stability.

[0010] The definitions of various terms used in this specification are shown below. "Containing component X" also means "composed of component X." "Aqueous" means that water accounts for the largest proportion by mass of the medium. The term "water-soluble organic solvent" refers to an organic solvent that dissolves in 100 mL of water at 25°C in an amount of 5 mL or more. The term "low liquid absorption" in the context of low liquid absorption printing substrates refers to a concept that encompasses both low liquid absorption and non-liquid absorption, and refers to the amount of water absorption of the printing substrate when the printing substrate is in contact with pure water for 100 ms. 2 More than 10g / m 2 The water absorption amount can be measured using an automatic scanning absorption meter (for example, "KM500win" manufactured by Kumagai Riki Kogyo Co., Ltd.) as the amount transferred when pure water is in contact for 100 ms under conditions of 23°C and a relative humidity of 50%. "Printing" is a concept that includes printing and printing out characters and images, and "printed matter" is a concept that includes printed matter and printed out matter on which characters and images are recorded.

[0011] <Pigments> The pigment used in the present invention may be either an inorganic pigment or an organic pigment, and lake pigments and fluorescent pigments may also be used. If necessary, these pigments may also be used in combination with extender pigments. Specific examples of inorganic pigments include carbon black, metal oxides such as titanium oxide, iron oxide, red iron oxide, and chromium oxide, and pearlescent pigments. Carbon black is particularly preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and threne pigments. In the achromatic ink, achromatic pigments such as white, black, and gray can be used, while in the chromatic ink, chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used. Specific examples of preferred organic pigments include one or more product numbers selected from CI Pigment Yellow, CI Pigment Red, CI Pigment Orange, CI Pigment Violet, CI Pigment Blue, and CI Pigment Green. Examples of extender pigments include silica, calcium carbonate, and talc. The above pigments can be used alone or in combination of two or more.

[0012] From the viewpoint of print density, the content of the pigment in the water-based ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and is preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 6% by mass or less.

[0013] (pigment dispersant) In the aqueous ink of the present invention, the pigment is dispersed in the medium using a pigment dispersant. Examples of the pigment in the aqueous ink of the present invention include a form in which the pigment is dispersed using a resin (hereinafter also referred to as a "pigment dispersing resin") or a surfactant as the pigment dispersant. Among these, the form of the pigment in the aqueous ink of the present invention is preferably a form in which the pigment is dispersed using a pigment dispersing resin, and more preferably a form in which the pigment is contained in resin particles (hereinafter also referred to as "pigment-containing resin particles"). The form of the pigment-containing resin particles is not particularly limited, as long as they are formed from at least a pigment and a pigment dispersing resin, and are particles in which the pigment dispersing resin is adsorbed onto the pigment in the aqueous ink. Examples of the form of the pigment-containing resin particles include a form in which the pigment is encapsulated in the pigment dispersing resin, a form in which the pigment is uniformly dispersed in the pigment dispersing resin, a form in which the pigment is exposed on the surface of the pigment dispersing resin particles, and mixtures thereof.

[0014] (pigment dispersion resin) The pigment dispersing resin may be either a water-soluble resin or a water-insoluble resin. Here, regarding the "water-soluble" and "water-insoluble" of a resin, when a resin that has reached a constant weight after drying at 105°C for 2 hours is dissolved in 100 g of water at 25°C until saturation is reached, if the dissolved amount exceeds 10 g it is judged to be "water-soluble," and if it is 10 g or less it is judged to be "water-insoluble." Furthermore, as described below, if the pigment dispersion resin has anionic groups that are further neutralized with a neutralizer, the solubility is judged from the dissolved amount measured in the presence of a neutralizer such that the mass ratio of the pigment dispersion resin to the neutralizer is the same as that in the water-based ink of the present invention.

[0015] Examples of pigment dispersion resins include vinyl resins obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds); and condensation resins such as polyester resins and polyurethane resins. The pigment dispersion resin may be an appropriately synthesized product or a commercially available product. Among these, the pigment dispersion resin is preferably one or more selected from the group consisting of vinyl resins, from the viewpoint of obtaining an ink with excellent ejection stability and refillability.

[0016] The pigment dispersion resin may have a crosslinked structure. In this case, the pigment dispersion resin preferably has a structure including a resin having a linear two-dimensional structure that may have branched chains and a component derived from a crosslinking agent. It is believed that such a crosslinked structure is formed by a resin having a linear two-dimensional structure that may have branched chains being converted into a three-dimensional structure by a component derived from a crosslinking agent. Examples of resins having a linear two-dimensional structure that may have branched chains include vinyl resins obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds); condensation resins such as polyester resins and polyurethane resins; and the vinyl resins described below are preferred. The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having from 3 to 8 carbon atoms, even more preferably one or more compounds selected from the group consisting of trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether, and diethylene glycol diglycidyl ether, and even more preferably trimethylolpropane polyglycidyl ether. When the crosslinking agent is a polyfunctional epoxy compound, the epoxy group equivalent weight of the crosslinking agent 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, even more preferably 150 or less.

[0017] [Vinyl resin] From the viewpoint of improving the dispersion stability of the pigment, the vinyl resin used as the pigment dispersing resin preferably contains a structural unit derived from an anionic group-containing monomer. In this specification, the term "anionic group" refers to an anionic group or a group that can be ionized to become an anionic group. Examples of anionic groups include a carboxy group (-COOM), a sulfonic acid group (-SO3M), and a phosphate group (-OPO3M2). In the above chemical formula, M represents a hydrogen atom, an alkali metal, ammonium, or an organic ammonium. Examples of vinyl resins include homopolymers of anionic group-containing monomers, copolymers of anionic group-containing monomers and hydrophobic monomers, and copolymers of anionic group-containing monomers, hydrophobic monomers, and nonionic monomers. Here, the term "hydrophobic" in the context of a hydrophobic monomer means that when the monomer is dissolved in 100 g of ion-exchanged water at 25° C. until saturation, the amount of dissolution is less than 10 g. The nonionic monomer is a monomer that has a high affinity for water, such as a monomer that contains a hydroxy group or a polyalkylene glycol chain. When the vinyl resin is a copolymer, it may be any of a random copolymer, a block copolymer, an alternating copolymer, and a graft copolymer.

[0018] Examples of the anionic group-containing monomer include a carboxy group-containing monomer, a sulfonic acid group-containing monomer, and a phosphoric acid group-containing monomer. Among these, the carboxy group-containing monomer is preferred, and (meth)acrylic acid is more preferred. Examples of hydrophobic monomers include (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms; aromatic group-containing monomers such as styrene-based monomers and aromatic group-containing (meth)acrylates; and styrene-based macromonomers. The molecular weight of the aromatic group-containing monomer, preferably the styrene-based monomer, is preferably less than 500. The styrene-based macromonomer is a compound having a polymerizable functional group at one end and a number-average molecular weight of from 500 to 100,000. Among these, the hydrophobic monomer is preferably a (meth)acrylate or styrene-based monomer having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms, more preferably one or more selected from the group consisting of alkyl (meth)acrylates having from 3 to 8 carbon atoms, styrene, α-methylstyrene, 2-methylstyrene, vinyltoluene, and divinylbenzene, and even more preferably one or more selected from the group consisting of butyl acrylate, cyclohexyl acrylate, and styrene. Examples of nonionic monomers include polyalkylene glycol mono(meth)acrylates such as polyethylene glycol mono(meth)acrylate; and alkoxypolyalkylene glycol mono(meth)acrylates such as methoxypolyethylene glycol mono(meth)acrylate and octoxypolyethylene glycol mono(meth)acrylate. The term "(meth)acrylic acid" refers to at least one selected from the group consisting of acrylic acid and methacrylic acid, and the term "(meth)acrylate" refers to at least one selected from the group consisting of acrylate and methacrylate. Moreover, each of the monomers of the vinyl resin may be used alone or in combination of two or more.

[0019] When the vinyl resin is a copolymer, the vinyl resin preferably contains one or more structural units derived from anionic group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, one or more structural units derived from hydrophobic monomers selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms, aromatic group-containing monomers, and styrene-based macromers, and one or more structural units derived from nonionic monomers selected from the group consisting of polyalkylene glycol mono(meth)acrylates and alkoxypolyalkylene glycol mono(meth)acrylates, and more preferably one or more structural units derived from anionic group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid. and structural units derived from one or more hydrophobic monomers selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms and aromatic group-containing monomers, and structural units derived from one or more nonionic monomers selected from the group consisting of polyalkylene glycol mono(meth)acrylates and alkoxypolyalkylene glycol mono(meth)acrylates, and more preferably structural units derived from one or more anionic group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, and structural units derived from one or more hydrophobic monomers selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms and aromatic group-containing monomers.

[0020] When the vinyl resin is a copolymer of an anionic group-containing monomer and a hydrophobic monomer, the content of the constituent units derived from each monomer component in all the constituent units of the vinyl resin is as follows. The content of structural units derived from anionic group-containing monomers in all structural units of the vinyl resin is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of improving the dispersion stability of the pigment and obtaining an ink that is excellent in ejection stability and refillability, and from the same viewpoints as above, is preferably 45% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. The content of structural units derived from hydrophobic monomers in all structural units of the vinyl resin is preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 65% ​​by mass or more, from the viewpoint of improving the dispersion stability of the pigment and obtaining an ink that is excellent in ejection stability and refillability, and from the same viewpoints as above, is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less. The vinyl resin can be obtained, for example, by addition polymerization of raw material monomers including an anionic group-containing monomer, a hydrophobic monomer, or a nonionic monomer by a known method.

[0021] From the viewpoint of improving the dispersion stability of the pigment, the acid value of the vinyl resin is preferably 40 mgKOH / g or more, more preferably 50 mgKOH / g or more, and even more preferably 60 mgKOH / g or more. From the same viewpoint as above, it is preferably 800 mgKOH / g or less, more preferably 500 mgKOH / g or less, and even more preferably 300 mgKOH / g or less. The acid value of the vinyl resin can be determined by the method described in the Examples, but it can also be calculated from the mass ratio of the constituent monomers. Furthermore, the acid value of a vinyl resin having a crosslinked structure can also be calculated using the following formula. Acid value of vinyl resin with crosslinked structure (mg KOH / g) = [Acid value of vinyl resin before crosslinking (mg KOH / g) x [(100 - crosslinking rate (mol%)) / 100]

[0022] The weight-average molecular weight of the vinyl resin is preferably 5,000 or more, more preferably 8,000 or more, and even more preferably 10,000 or more, from the viewpoints of pigment dispersion stability and obtaining an ink with excellent ejection stability and refillability, and from the same viewpoints as above, is preferably 100,000 or less, more preferably 50,000 or less, and even more preferably 30,000 or less. The weight-average molecular weight of the vinyl resin can be measured by the method described in the examples.

[0023] Commercially available vinyl resins include, for example, polyacrylic acids such as "Aron AC-10SL" (manufactured by Toagosei Co., Ltd.); and styrene / acrylic resins such as "Joncryl 67," "Joncryl 611," "Joncryl 678," "Joncryl 680," "Joncryl 690," and "Joncryl 819" (all manufactured by BASF Japan Ltd.).

[0024] When the pigment in the water-based ink of the present invention is in the form of pigment-containing resin particles, the pigment-containing resin particles are preferably blended into the water-based ink as an aqueous dispersion (hereinafter also referred to as "pigment aqueous dispersion") obtained by dispersing the pigment, pigment dispersing resin, and, if necessary, a neutralizing agent, a surfactant, and the like, by a known method. A preferred method for producing a pigment aqueous dispersion is to disperse a pigment mixture containing a pigment, a pigment dispersing resin, an organic solvent, and water to obtain a dispersion, and then remove the organic solvent from the obtained dispersion. If necessary, a crosslinking agent may be further added to the obtained pigment aqueous dispersion to crosslink the pigment dispersing resin. Specific examples of such methods include those described in paragraphs

[0022] to

[0026] of JP-A-2022-104084. When the pigment is blended in the aqueous ink as a pigment aqueous dispersion, the average particle size of the pigment-containing resin particles in the aqueous dispersion is preferably 30 nm or more, more preferably 50 nm or more, and even more preferably 70 nm or more, from the viewpoints of dispersion stability of the pigment and of obtaining an ink with excellent ejection stability and refillability, and is preferably 250 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less, from the same viewpoints as above. The average particle size of the pigment-containing resin particles in the pigment aqueous dispersion can be measured by the method described in the Examples.

[0025] When the pigment is in the form of pigment-containing resin particles, the mass ratio of the pigment content to the total content of the pigment and pigment dispersant [pigment / (pigment+pigment dispersant)] is preferably 0.4 or more, more preferably 0.5 or more, and even more preferably 0.6 or more, from the viewpoint of improving the dispersion stability of the pigment, and is preferably 0.9 or less, more preferably 0.8 or less, and even more preferably 0.75 or less, from the same viewpoint as above. When the pigment dispersant is a pigment dispersant resin that has been crosslinked with a crosslinking agent, the content of the pigment dispersant in the ink of the present invention is the total content of the pigment dispersant resin before crosslinking and the crosslinking agent.

[0026] <Organic solvents> The water-based ink of the present invention contains an organic solvent (A) and an organic solvent (B). The distance Ra of the Hansen solubility parameters (HSP) between the organic solvent (A) and the organic solvent (B) (hereinafter also referred to as "HSP distance Ra") is 6.0 or more, preferably 6.3 or more, more preferably 6.6 or more, and is 12.5 or less, preferably 12.0 or less, more preferably 11.0 or less, and even more preferably 10.5 or less. The Hansen Solubility Parameter (HSP) is a value expressed by δ using the following formula (unit: MPa) 1 / 2 ) δ=(δD 2 +δP 2 +δH 2 ) 1 / 2 In the above formula, δD represents the dispersion term, δP represents the polar term, and δH represents the hydrogen bond term (all in units of MPa 1 / 2 ). The dispersion term δD, polarity term δP, and hydrogen bond term δH are values ​​calculated using HSPiP (Hansen Solubility Parameter in Practice). When the HSPs of organic solvent (A) and organic solvent (B) are plotted as coordinates (δD, δP, δH) in Hansen space, the closer the coordinates of organic solvent (A) and organic solvent (B), the more easily organic solvent (A) and organic solvent (B) dissolve in each other. The HSP distance Ra is an index of whether the coordinates of organic solvent (A) and organic solvent (B) are close to each other, and is calculated by the method described in the examples.

[0027] (Organic solvent (A)) The organic solvent (A) is a glycol ether having a boiling point of 200° C. or higher and 280° C. or lower and an octanol-water partition coefficient log P of 0.75 or higher. The boiling point of the organic solvent (A) is preferably 205°C or higher, more preferably 210°C or higher, from the viewpoint of obtaining an ink having excellent ejection stability and refillability, and from the same viewpoint, is preferably 278°C or lower, more preferably 276°C or lower. The octanol-water partition coefficient logP of the organic solvent (A) is preferably 0.80 or more, more preferably 0.85 or more, from the viewpoint of obtaining an ink having excellent ejection stability and refillability, and from the same viewpoint, is preferably 2.50 or less, more preferably 2.20 or less, and even more preferably 2.00 or less.

[0028] Examples of the organic solvent (A) include monoaliphatic ethers of alkylene glycols and monoaromatic ethers of alkylene glycols.

[0029] Examples of organic solvents (A) include dipropylene glycol monopropyl ether (boiling point: 212°C, logP: 0.88, δD: 15.6, δP: 6.1, δH: 11.0), dipropylene glycol monobutyl ether (boiling point: 231°C, logP: 1.29, δD: 15.7, δP: 6.5, δH: 10.0), ethylene glycol monobenzyl ether (boiling point: 253°C, logP: 1.30, δD: 17.7, δP: 5.8, δH: 10.2), and diethylene glycol monohexyl ether (boiling point: 259°C, logP: 1. 49, δD:16.2, δP:5.6, δH:9.4), ethylene glycol monohexyl ether (boiling point: 258°C, logP:1.65, δD:16.2, δP:5.5, δH:10.7), tripropylene glycol monobutyl ether (boiling point: 274°C, logP:1.90, δD:16.0, δP:6.7, δH:7.0), diethylene glycol mono-2-ethylhexyl ether (boiling point: 272°C, logP:2.31, δD:16.2, δP:5.6, δH:9.4), ethylene glycol mono-2-ethylhexyl Examples include monoaliphatic ethers of alkylene glycol such as ethylene glycol monophenyl ether (boiling point: 229°C, logP: 2.46, δD: 16.0, δP: 4.1, δH: 10.5); and monoaromatic ethers of alkylene glycol such as ethylene glycol monophenyl ether (boiling point: 245°C, logP: 1.39) and propylene glycol monophenyl ether (boiling point: 243°C, logP: 1.71, δD: 18.3, δP: 6.3, δH: 11.1). From the viewpoint of obtaining an ink that is excellent in ejection stability and refilling, preferred are dipropylene glycol monophenyl ethers. The ethylene glycol monopropyl ether is at least one selected from glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monohexyl ether, and ethylene glycol monophenyl ether, and more preferably at least one selected from dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether, and ethylene glycol monohexyl ether. The organic solvent (A) may be used alone or in combination of two or more.

[0030] From the viewpoint of obtaining an ink having excellent ejection stability and refillability, the content of the organic solvent (A) is preferably 0.1% by mass or more, more preferably 1% by mass or more, and even more preferably 3% by mass or more, and from the same viewpoint, it is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 13% by mass or less.

[0031] (Organic solvent (B)) The organic solvent (B) is a polyhydric alcohol having a boiling point of 210° C. or higher and 280° C. or lower and an octanol-water partition coefficient log P of less than 0.75. The boiling point of the organic solvent (B) is preferably 212°C or higher, more preferably 213°C or higher, from the viewpoint of obtaining an ink having excellent ejection stability and refillability, and from the same viewpoint, is preferably 270°C or lower, more preferably 260°C or lower. The octanol-water partition coefficient logP of the organic solvent (B) is preferably 0.70 or less, more preferably 0.60 or less, from the viewpoint of obtaining an ink that is excellent in ejection stability and refillability, and from the same viewpoint, is preferably −2.00 or more, more preferably −1.50 or more, and even more preferably −1.00 or more.

[0032] Examples of the organic solvent (B) include polyalkylene glycols and alkanediols, and the organic solvent (B) is selected so that the distance Ra of the Hansen solubility parameter (HSP) between the organic solvent (B) and the organic solvent (A) is 6.0 or more and 12.5 or less.

[0033] Examples of the organic solvent (B) include polyalkylene glycols such as diethylene glycol (boiling point: 245°C, logP: -1.98, δD: 16.1, δP: 12.0, δH: 19.0), triethylene glycol (boiling point: 276°C, logP: -1.48, δD: 16.0, δP: 12.5, δH: 18.6), and tripropylene glycol (boiling point: 273°C, logP: -0.38, δD: 16.6, δP: 9.1, δH: 14.1), 2-methyl-1,3-propanediol (214°C, logP: -0.74, δD: 17.2, δP: 8.6, δH: 21.2), and 1,2-hexanediol ( Examples of alkanediols include 2-methyl-1,3-propanediol, 1,2-hexanediol, 1,5-pentanediol (224°C, logP: 0.58, δD: 16.7, δP: 7.1, δH: 17.5), 1,5-pentanediol (242°C, logP: -0.49, δD: 17.0, δP: 8.9, δH: 18.9), and 1,6-hexanediol (250°C, logP: 0.00, δD: 15.7, δP: 8.4, δH: 17.8), and from the viewpoint of obtaining an ink that has excellent ejection stability and refillability, preferred is one or more selected from 2-methyl-1,3-propanediol, 1,2-hexanediol, 1,5-pentanediol, and 1,6-hexanediol. The organic solvent (B) may be used alone or in combination of two or more.

[0034] From the viewpoint of obtaining an ink having excellent ejection stability and refillability, the content of the organic solvent (B) is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 4% by mass or more, and from the same viewpoint, it is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 13% by mass or less.

[0035] (Organic solvent (D)) The water-based ink of the present invention preferably contains an organic solvent (D) other than the organic solvents (A) and (B). The organic solvent (D) can be appropriately selected depending on the purpose. Examples of the organic solvent (D) include water-soluble organic solvents such as polyhydric alcohols, glycol ethers, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, from the viewpoint of obtaining an ink with excellent ejection stability and refillability, the water-based ink preferably contains at least one organic solvent (D) selected from polyhydric alcohols and glycol ethers.

[0036] Examples of polyhydric alcohols include alkanediols such as ethylene glycol (boiling point: 197°C), propylene glycol (boiling point: 188°C), 1,2-butanediol (boiling point: 207°C), 1,3-butanediol (boiling point: 207°C), 1,2-pentanediol (boiling point: 206°C), 3-methyl-1,3-butanediol (boiling point: 204°C), and 2-methyl-2,4-pentanediol (boiling point: 197°C). Examples of glycol ethers include ethylene glycol dimethyl ether (boiling point: 82°C), propylene glycol monopropyl ether (boiling point: 150°C), diethylene glycol dimethyl ether (boiling point: 162°C), dipropylene glycol monomethyl ether (boiling point: 187°C), diethylene glycol monoisopropyl ether (boiling point: 207°C), tripropylene glycol monomethyl ether (boiling point: 242°C, δD: 16.3, δP: 5.9, δH: 8.3), triethylene glycol monomethyl ether (boiling point: 242°C, δD: 16.3, δP: 5.9, δH: 8.3), and triethylene glycol monomethyl ether (boiling point: 242°C, δD: 16.3, δP: 5.9, δH: 8.3). Examples include ethylene glycol monobutyl ether (boiling point: 271°C, δD: 16.2, δP: 6.1, δH: 9.1), diethylene glycol monoisobutyl ether (boiling point: 220°C, δD: 0.50, δD: 16.1, δP: 6.0, δH: 9.9), diethylene glycol monobutyl ether (boiling point: 231°C, δD: 0.67, δD: 16.3, δP: 6.2, δH: 10.5), and triethylene glycol dimethyl ether (boiling point: 216°C, δD: 16.1, δP: 5.8, δH: 6.8). Among these, the organic solvent (D) is preferably a polyhydric alcohol, more preferably an alkanediol, and even more preferably propylene glycol.

[0037] From the viewpoint of obtaining an ink having excellent ejection stability and refillability, the content of the organic solvent (D) is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, and from the same viewpoint, it is preferably 30% by mass or less, more preferably 28% by mass or less, and even more preferably 25% by mass or less.

[0038] <Surfactant (C)> The water-based ink of the present invention contains a surfactant (C) having an HLB value (Griffin method) of 1.0 or more and 6.0 or less. From the viewpoint of reducing the solubility in water and obtaining an ink that is excellent in ejection stability and refillability, the HLB value of the surfactant (C) is preferably 2.0 or more, more preferably 2.5 or more, even more preferably 3.0 or more, and is preferably 5.5 or less, more preferably 5.0 or less, even more preferably 4.5 or less.

[0039] The surfactant (C) preferably contains at least one selected from an acetylene-based surfactant and a polyether-modified silicone-based surfactant, and more preferably is an acetylene-based surfactant or a polyether-modified silicone-based surfactant.

[0040] (acetylene surfactant) Examples of the acetylene surfactant include acetylene glycols having 8 to 22 carbon atoms and ethylene oxide adducts of the acetylene glycols. One or more selected from 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, or 3,5-dimethyl-1-hexyne-3-ol, 2,4-dimethyl-5-hexyne-3-ol, and ethylene oxide adducts thereof are preferred, one or more selected from 2,4,7,9-tetramethyl-5-decyne-4,7-diol and ethylene oxide adducts thereof are more preferred, and an ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol is even more preferred. Commercially available products of ethylene oxide adducts of 2,4,7,9-tetramethyl-5-decyne-4,7-diol include Surfynol 104-PG50 (HLB value 4.0) and Surfynol SE (HLB value 6.0) manufactured by Nissin Chemical Industry Co., Ltd.

[0041] (Polyether-modified silicone surfactant) Examples of polyether-modified silicone surfactants include compounds in which polyether groups are grafted onto a silicone main chain, and compounds in which polyether groups are bonded in block form to both ends of a silicone main chain. Examples of polyether groups include polyethyleneoxy groups, polypropyleneoxy groups, and polyalkyleneoxy groups in which ethyleneoxy groups and propyleneoxy groups (trimethyleneoxy groups or propane-1,2-diyloxy groups) are added in block form or randomly.

[0042] Specific examples of polyether-modified silicone surfactants include PEG-3 dimethicone (HLB value 4.5), PEG-9 methyl ether dimethicone (HLB value 4.5), PEG-10 dimethicone (HLB value 4.5), PEG / PPG-20 / 22 butyl ether dimethicone (HLB value 7.0), PEG-9 polydimethylsiloxyethyl dimethicone (HLB value 4.0), cetyl PEG / PPG-10 / 1 dimethicone (HLB value 3.5), and lauryl PEG-9 polydimethylsiloxyethyl dimethicone (HLB value 3.0). Examples of commercially available polyether-modified silicone surfactants include KF-6015 (HLB value 4.5), KF-6017 (HLB value 4.5), KF-6017P (HLB value 4.5), KF-6028 (HLB value 4.0), KF-6028P (HLB value 4.0), and KF-6048 (HLB value 3.5), all manufactured by Shin-Etsu Chemical Co., Ltd.; TEGO (registered trademark) Wet 270 (HLB value 3.0) and TEGO (registered trademark) Wet 280 (HLB value 3.5), all manufactured by EVONIK; and BYK345 (HLB value 3.5) and BYK349 ​​(HLB value 4.0), all manufactured by BYK Japan.

[0043] From the viewpoint of obtaining an ink having excellent ejection stability and refillability, the content of surfactant (C) is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more, and from the same viewpoint, it is preferably 1.5% by mass or less, more preferably 1.2% by mass or less, and even more preferably 1.0% by mass or less.

[0044] In addition to the surfactant (C), the water-based ink of the present invention may further contain one or more surfactants selected from the group consisting of acetylene-based surfactants and polyether-modified silicone-based surfactants, each having an HLB value of more than 6.0.

[0045] Examples of acetylene surfactants having an HLB value of more than 6.0 include ethylene oxide adducts of acetylene glycols having 8 to 22 carbon atoms, which have a different number of moles of ethylene oxide added than the ethylene oxide adducts of acetylene glycols having 8 to 22 carbon atoms listed as surfactant (C) above. Examples of commercially available acetylene surfactants having an HLB value of more than 6.0 include Surfynol 440 (HLB value 8.0), Surfynol 2502 (HLB value 8.0), Dynol 604 (HLB value 8.0), Dynol 607 (HLB value 8.0), Surfynol 465 (HLB value 13.0), and Surfynol 485 (HLB value 17.0), all manufactured by Nissin Chemical Industry Co., Ltd.; and Acetylenol E81 (HLB value 13.9) and Acetylenol E200 (HLB value 16.4), all manufactured by Kawaken Fine Chemicals Co., Ltd.

[0046] Furthermore, examples of polyether-modified silicone surfactants having an HLB value of more than 6.0 include PEG-9 dimethicone (HLB value 10.0) and PEG-11 methyl ether dimethicone (HLB value 14.5). Examples of commercially available polyether-modified silicone surfactants having an HLB value of more than 6.0 include KF-6012 (HLB value 7.0), KF-6004 (HLB value 9.0), KF-6011 (HLB value 14.5), KF-6011P (HLB value 14.5), and KF-6043 (HLB value 14.5), all manufactured by Shin-Etsu Chemical Co., Ltd.; Silface (registered trademark) SAG005 (HLB value 7.0), Silface (registered trademark) SAG008 (HLB value 7.0), Silface (registered trademark) SAG002 (HLB value 12.0), and Silface (registered trademark) SAG503A (HLB value 11.0), all manufactured by Nissin Chemical Industry Co., Ltd.; and ABIL (registered trademark) Care XL 80 MB (HLB value 11.0), all manufactured by EVONIK.

[0047] From the viewpoint of obtaining an ink that has excellent ejection stability and refillability, the content of surfactants having an HLB value of more than 6.0 is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, and from the same viewpoint, it is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1.5% by mass or less.

[0048] <Fixing resin> The water-based ink of the present invention may further contain a fixing resin. From the viewpoint of improving abrasion resistance and water resistance, the fixing resin preferably has a carboxyl group (-COOM). The carboxyl group is a group that exhibits acidity by dissociating and releasing a hydrogen ion, or a group in the dissociated ionic form (-COOM). - In the above chemical formula, M represents a hydrogen atom, an alkali metal, ammonium, or an organic ammonium, as in the anionic group described above.

[0049] From the viewpoint of improving abrasion resistance and water resistance, the acid value of the fixing resin is preferably 3 mgKOH / g or more, more preferably 5 mgKOH / g or more, and even more preferably 10 mgKOH / g or more, and from the same viewpoint as above, it is preferably 30 mgKOH / g or less, more preferably 25 mgKOH / g or less, and even more preferably 20 mgKOH / g or less. The acid value of the fixing resin can be determined by the method described in the examples, but it can also be determined by calculation from the mass ratio of the constituent monomers.

[0050] Examples of fixing resins include vinyl resins such as (meth)acrylic resins, styrene resins, styrene / (meth)acrylic resins, butadiene resins, styrene / butadiene resins, vinyl chloride resins, vinyl acetate resins, and acrylic silicone resins; polyurethane resins; and polyester resins. Note that "(meth)acrylic" refers to acrylic or methacrylic. Furthermore, when the fixing resin is a copolymer, it may be any of a random copolymer, a block copolymer, an alternating copolymer, and a graft copolymer. Among these, from the viewpoint of improving abrasion resistance and water resistance, the fixing resin is preferably one or more selected from the group consisting of vinyl resins having carboxy groups, polyurethane resins having carboxy groups, and polyester resins having carboxy groups, and more preferably vinyl resins having carboxy groups.

[0051] The vinyl resin having a carboxy group used in the present invention preferably contains a structural unit derived from a carboxy group-containing monomer and a structural unit derived from a hydrophobic monomer. The carboxy group-containing monomer is preferably (meth)acrylic acid. Preferred examples of the hydrophobic monomer include those exemplified for the vinyl resin used as the pigment dispersant. Among these, the hydrophobic monomer is preferably at least one selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms, aromatic group-containing monomers, and styrene macromers, more preferably at least one selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms and aromatic group-containing monomers, even more preferably (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms, still more preferably (meth)acrylates having an alkyl group having from 1 to 12 carbon atoms, and still more preferably (meth)acrylates having an alkyl group having from 1 to 8 carbon atoms. Each of the monomers of the vinyl resin having a carboxy group can be used alone or in combination of two or more.

[0052] The vinyl resin having a carboxy group preferably contains structural units derived from one or more carboxy group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, and structural units derived from one or more hydrophobic monomers selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms, aromatic group-containing monomers, and styrene-based macromers, more preferably contains structural units derived from one or more carboxy group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid, and structural units derived from one or more hydrophobic monomers selected from the group consisting of (meth)acrylates having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms and aromatic group-containing monomers, and even more preferably contains structural units derived from one or more carboxy group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid. The (meth)acrylic resin is a (meth)acrylic resin containing structural units derived from a carboxy group-containing monomer and structural units derived from a (meth)acrylate having a hydrocarbon group derived from an aliphatic alcohol having from 1 to 22 carbon atoms, and even more preferably a (meth)acrylic resin containing structural units derived from one or more carboxy group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid and structural units derived from a (meth)acrylate having an alkyl group having from 1 to 12 carbon atoms, and even more preferably a (meth)acrylic resin containing structural units derived from one or more carboxy group-containing monomers selected from the group consisting of acrylic acid and methacrylic acid and structural units derived from a (meth)acrylate having an alkyl group having from 1 to 8 carbon atoms, and even more preferably a copolymer of methacrylic acid, methyl methacrylate, and 2-ethylhexyl acrylate.

[0053] When the vinyl resin having a carboxy group is a copolymer of a carboxy group-containing monomer and a hydrophobic monomer, the content of the constituent units derived from each monomer component in the vinyl resin is as follows. The content of structural units derived from carboxyl group-containing monomers in the vinyl resin having a carboxyl group is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, from the viewpoint of improving abrasion resistance and water resistance, and from the same viewpoint as above, is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. The content of structural units derived from hydrophobic monomers in the vinyl resin having a carboxy group is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, from the viewpoint of improving abrasion resistance and water resistance, and from the same viewpoint as above, is preferably 99.5% by mass or less, more preferably 99% by mass or less, and even more preferably 98% by mass or less.

[0054] The weight-average molecular weight of the vinyl resin having a carboxy group is preferably 5,000 or more, more preferably 10,000 or more, even more preferably 30,000 or more, still more preferably 50,000 or more, and even more preferably 100,000 or more from the viewpoint of improving abrasion resistance and water resistance, and is preferably 1,500,000 or less, more preferably 1,000,000 or less, and even more preferably 800,000 or less from the same viewpoint as above. The weight-average molecular weight can be measured by the method described in the Examples. The vinyl resin having a carboxy group is produced by polymerizing raw material monomers by a known polymerization method, such as emulsion polymerization or suspension polymerization, with emulsion polymerization being more preferred.

[0055] The polyurethane resin having a carboxy group used in the present invention is preferably a polyaddition product of an organic compound (polyol) component having two or more alcoholic hydroxyl groups in the molecule, including a dialkanolcarboxylic acid, and a polyisocyanate component. Examples of dialkanolcarboxylic acids include dimethylolbutanoic acid, dimethylolpropionic acid, and salts thereof. Among these, dimethylolpropionic acid is preferred.

[0056] The polyol component is not particularly limited as long as it is a compound having two or more alcoholic hydroxyl groups in the molecule, and examples thereof include polycarbonate-based polyols, polyester-based polyols, and polyether-based polyols. Examples of the polyisocyanate component include chain aliphatic diisocyanates such as tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, dodecamethylene diisocyanate, trimethylhexamethylene diisocyanate, and lysine diisocyanate; aliphatic diisocyanates having a cyclic structure such as isophorone diisocyanate, hydrogenated xylylene diisocyanate, and dicyclohexylmethane 4,4'-diisocyanate; aliphatic diisocyanates having an aromatic ring such as xylylene diisocyanate and tetramethylxylylene diisocyanate; aromatic diisocyanates such as tolylene diisocyanate and diphenylmethane diisocyanate; and modified products of these diisocyanates (carbodiimide-, uretdione-, and uretoimine-containing modified products, etc.).

[0057] From the viewpoint of improving abrasion resistance and water resistance, the polyurethane resin having a carboxy group is more preferably a polyaddition product of dimethylolpropionic acid and a polycarbonate polyol as the polyol component and dicyclohexylmethane 4,4'-diisocyanate as the polyisocyanate component.

[0058] Examples of solvents used in the polyaddition reaction include acetone, methyl ethyl ketone, tetrahydrofuran, dioxane, ethyl acetate, toluene, and xylene. In the polyaddition reaction, a chain extender or a reaction terminator may be used in combination, if necessary. The use of a chain extender can further increase the molecular weight. Examples of chain extenders include polyols and polyamines, and examples of reaction terminators include monoalcohols and monoamines.

[0059] In terms of dispersion stability in aqueous inks, it is preferable that at least a portion of the carboxy groups of the vinyl resin having a carboxy group, the polyurethane resin having a carboxy group, and the polyester resin having a carboxy group be neutralized with a neutralizing agent. Examples of the neutralizing agent include alkylamines such as butylamine and triethylamine; alkanolamines such as monoethanolamine, diethanolamine and triethanolamine; and inorganic bases such as morpholine, ammonia and sodium hydroxide.

[0060] Commercially available dispersions of vinyl resin particles having carboxy groups that do not contain pigments include, for example, acrylic resins such as "Neocryl A-1127" (trade name, anionic self-crosslinking aqueous acrylic resin, manufactured by DSM Coating Resins), "Joncryl 390," "Joncryl 7100," "Joncryl 7600," "Joncryl 537J," "Joncryl PDX-7164," "Joncryl 538J," and "Joncryl 780" (trade names, manufactured by BASF Japan Ltd.); styrene / butadiene resins such as "SR-100" and "SR102" (all trade names, manufactured by Nippon A&L Inc.); and vinyl chloride resins such as "Vinyblan 700" and "Vinyblan 701" (trade names, manufactured by Nissin Chemical Industry Co., Ltd.). Commercially available dispersions of polyurethane resin particles having carboxy groups that do not contain pigments include, for example, "NeoRez R-9603" (trade name, manufactured by DSM Coating Resins) and "WBR-2018" and "WBR-2000U" (trade names, manufactured by Taisei Fine Chemical Co., Ltd.). Commercially available dispersions of carboxyl-containing polyester resin particles that do not contain pigment include, for example, "ELITEL KA-5034," "ELITEL KA-5071S," "ELITEL KZA-1734," "ELITEL KZA-6034," "ELITEL KZA-1449," "ELITEL KZA-0134," and "ELITEL KZA-3556" (all of which are product names manufactured by Unitika Ltd.).

[0061] The fixing resin is preferably used as resin particles that do not contain a pigment, and from the viewpoint of improving the productivity of the water-based ink, it is preferably blended in the water-based ink as an aqueous dispersion of resin particles that do not contain a pigment. The fixing resin may be suitably synthesized or may be a commercially available product. When the fixing resin is formulated as an aqueous dispersion of pigment-free resin particles, the average particle size of the pigment-free resin particles in the aqueous dispersion is preferably 30 nm or more, more preferably 50 nm or more, and even more preferably 70 nm or more, from the viewpoint of ink storage stability, and is preferably 250 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less, from the same viewpoint as above. The average particle size of the pigment-free resin particles in the aqueous dispersion is measured by the method described in the Examples.

[0062] When the water-based ink of the present invention contains a fixing resin, the content of the fixing resin in the water-based ink 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 obtaining an ink with excellent ejection stability and refillability, and from the same viewpoint as above, is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less. When the pigment in the water-based ink of the present invention is in a form dispersed by a pigment dispersing resin, the mass ratio of the content of the fixing resin to the total content of the pigment dispersing resin and the fixing resin in the water-based ink of the present invention [fixing resin / (pigment dispersing resin+fixing resin)] is preferably 0.10 or more, more preferably 0.20 or more, even more preferably 0.30 or more, and is preferably 1.0 or less, more preferably 0.7 or less, even more preferably 0.5 or less.

[0063] <Water> The water-based ink of the present invention contains water. The water used in the water-based ink according to the present invention is preferably pure water or ion-exchanged water, from the viewpoint of preventing the inclusion of unintended substances.

[0064] From the viewpoint of obtaining an ink with excellent ejection stability and refillability, the water content in the water-based ink is preferably 40% by mass or more, more preferably 45% by mass or more, and even more preferably 50% by mass or more, and from the same viewpoint as above, the water content is preferably 70% by mass or less, more preferably 65% ​​by mass or less, and even more preferably 60% by mass or less.

[0065] The water-based ink of the present invention may further contain various additives, such as a humectant, wetting agent, wetting / penetrating agent, viscosity adjuster, antifoaming agent, preservative, antifungal agent, and antirust agent, if necessary.

[0066] The water-based ink of the present invention can be obtained by mixing and stirring a pigment, organic solvent (A), organic solvent (B), and water, and, if necessary, a fixing resin, a surfactant, a neutralizing agent, additives, etc. When the pigment is in the form of pigment-containing resin particles, as described above, the pigment, pigment dispersing resin, and, if necessary, a neutralizing agent, a surfactant, and the like are dispersed by a known method to obtain an aqueous dispersion of the pigment-containing resin particles, and then the resulting dispersion is preferably blended into the water-based ink.

[0067] (Physical properties of water-based inks for inkjet printing) The viscosity of the water-based 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 3.5 mPa·s or more, and preferably 12 mPa·s or less, more preferably 9 mPa·s or less, even more preferably 7 mPa·s or less. The viscosity of the water-based ink can be measured using an E-type viscometer. The pH of the water-based ink of the present invention is preferably 7.0 or higher, more preferably 7.2 or higher, and even more preferably 7.5 or higher. From the viewpoints of component 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 a conventional method.

[0068] [Inkjet printing method] The inkjet printing method of the present invention is a method of printing on a low liquid-absorbent printing substrate using the above-mentioned water-based ink. In the inkjet printing method of the present invention, the method for ejecting the water-based ink is preferably a piezo type from the viewpoint of ejection properties.

[0069] (Low liquid absorption printing base material) Examples of low-liquid-absorbent printing substrates that can be used in the ink-jet printing method of the present invention include low-liquid-absorbent coated paper and resin films. Examples of low-liquid-absorbency coated paper include general-purpose glossy paper and multicolor foam glossy paper. Examples of the resin film include films made of synthetic resins. Examples of such synthetic resins include polyolefin resins such as polyethylene resins and polypropylene resins; polyester resins such as polyethylene terephthalate resins; and polyvinyl chloride resins. The resin film may be a biaxially stretched film, a uniaxially stretched film, or a non-stretched film. Among these, the low-liquid-absorbent printing substrate is preferably a printing substrate made of a synthetic resin, and more preferably a low-liquid-absorbent printing substrate made of one or more synthetic resins selected from the group consisting of polyethylene resin, polypropylene resin, and polyethylene terephthalate resin. [Example]

[0070] In the following Production Examples, Examples, and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The methods for measuring each physical property are as follows. Pressure is expressed as absolute pressure.

[0071] (1) Measurement of the acid value of resin The resin was dissolved in a titration solvent (toluene:acetone = 2:1 (volume ratio)) mixed with toluene and acetone in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with 0.1 N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was taken as the endpoint. The acid value (mgKOH / g) was calculated from the titration volume of the potassium hydroxide solution up to the endpoint.

[0072] (2) Measurement of the weight average molecular weight of the resin The content was determined by gel permeation chromatography. The measurement sample was prepared by mixing 0.1 g of resin with 10 mL of the eluent described below in a glass vial, stirring with a magnetic stirrer at 25°C for 10 hours, and filtering with a syringe filter (Advantec Co., Ltd. "DISMIC-13HP" pore size: 0.2 μm, material: PTFE). The measurement conditions are shown below. GPC equipment: Tosoh Corporation "HLC-8320GPC" Columns: "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolumn Super AW-H" manufactured by Tosoh Corporation Eluent: N,N-dimethylformamide dissolved with phosphoric acid and lithium bromide at concentrations of 60mmol / L and 50mmol / L, respectively. Flow rate: 0.5mL / min Standard material: Monodisperse polystyrene kits with known molecular weights, manufactured by Tosoh Corporation: "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)"

[0073] (3) Measurement of solids concentration of aqueous pigment dispersion and aqueous dispersion of fixing resin particles Approximately 10 g of sodium sulfate, brought to a constant weight in a desiccator, was weighed out into a 30 mL ointment container, and approximately 1 g of the sample was added and mixed, then weighed out and kept at 105°C for 2 hours to remove volatiles.The mixture was then left in the desiccator for a further 15 minutes, and the mass was then weighed out. The mass of the sample after volatile matter removal was taken as the solid content, and divided by the mass of the added sample to obtain the solid content concentration (%).

[0074] (4) Calculation of the octanol-water partition coefficient logP of organic solvents The log P of each organic solvent was calculated using ChemDraw Professional ver. 22.2.0.3300 (PerkinElmer).

[0075] (5) Calculation of the HSP distance Ra between organic solvent (A) and organic solvent (B) The HSP distance Ra was calculated using the following formula: (Ra) 2 = 4(δD B -δD A ) 2 +(δP B -δP A ) 2 +(δH B -δH A ) 2 In the above formula, δD A , δP A , and δH A respectively indicate δD, δP, and δH of the organic solvent (A), and δD B , δP B , and δH B respectively represent ΔD, ΔP, and ΔH of the organic solvent (B).

[0076] (6) Calculation of HLB value of surfactant The HLB value indicates the affinity of a surfactant for water and oil, and was calculated by the Griffin method using the following formula: In the following formula, examples of the "hydrophilic group contained in the surfactant" include a hydroxyl group and an ethyleneoxy group. HLB = 20 × [(molecular weight of hydrophilic group contained in surfactant) / (molecular weight of surfactant)]

[0077] (7) Measurement of viscosity of water-based ink The viscosity of the water-based ink at 32°C was measured using an E-type viscometer ("TV-25" manufactured by Toki Sangyo Co., Ltd., standard cone rotor 1°34'×R24, rotation speed 50 rpm).

[0078] (8) Measurement of pH of water-based ink The pH of the water-based ink at 25°C was measured using a tabletop pH meter ("F-71" manufactured by Horiba Ltd.) equipped with a pH electrode ("6337-10D" manufactured by Horiba Ltd.).

[0079] Production Example 1 (Production of Pigment Water Dispersion 1) (1) Synthesis of pigment dispersing resin D1 A monomer mixture was prepared by mixing 31 parts of acrylic acid and 69 parts of styrene. A reaction vessel was charged with 10 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), 0.2 parts of 2-mercaptoethanol (a polymerization chain transfer agent), and 10% of the monomer mixture, and the mixture was thoroughly purged with nitrogen gas. Meanwhile, a mixture of the remaining 90% of the monomer mixture, 0.2 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of an azo-based 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. The monomer mixture in the reaction vessel was heated to 65°C under a nitrogen atmosphere while being stirred, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 65°C from the end of the dropping, a solution of 0.1 parts of the polymerization initiator in 2 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and then at 70°C for 2 hours. The mixture was then dried under reduced pressure to partially remove the MEK, yielding an MEK solution (solids concentration 45%) of pigment dispersion resin D1 (acid value: 240 mgKOH / g, weight-average molecular weight: 13,900). 17.0 parts of the obtained MEK solution of pigment dispersion resin D1 was diluted with 4.9 parts MEK to a solids concentration of 35%. Next, 3.1 parts of a 5N NaOH aqueous solution was added so that the degree of neutralization of the carboxyl groups of pigment dispersion resin D1 was 40 mol%, and the mixture was stirred at 25°C. 75.0 parts of water was then added over 1 hour to cause phase inversion emulsification of pigment dispersion resin D1. After the addition was completed, the MEK was distilled off using an evaporator to obtain an aqueous dispersion of pigment dispersion resin D1 (solids concentration 25%).

[0080] (2) Preparation of Pigment Water Dispersion 1 To 32.97 parts of an aqueous dispersion of pigment dispersion resin D1 (solid content concentration 25%), 2.10 parts of MEK and 17.25 parts of ion-exchanged water were added, and 18.77 parts of a cyan pigment (manufactured by DIC Corporation, trade name: Fastogen Blue CA5380 Pigment Blue 15:3) was further added to obtain a pigment mixture. The resulting pigment mixture was mixed using a disper blade at 7000 rpm and 20°C for 1 hour, and then further dispersed using a Microfluidizer (high-pressure homogenizer, manufactured by Microfluidics, product name: M-140K) for 10 passes at a pressure of 180 MPa to obtain a pigment dispersion. From the resulting pigment dispersion, MEK was removed under reduced pressure at 60°C, and then some of the water was removed and centrifuged. The liquid phase was then filtered through a membrane filter (manufactured by Sartorius, trade name: Minisart Syringe Filter, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, yielding Pigment Water Dispersion 1 (solid concentration 22%, pigment: 15.4%, resin: 6.6%). The average particle size of the pigment-containing resin particles in Pigment Water Dispersion 1 was 105.3 nm.

[0081] Production Example 2 (Production of Pigment Water Dispersion 2) (1) Synthesis of pigment dispersing resin D2 A monomer mixture was prepared by mixing 26 parts of acrylic acid, 19 parts of butyl acrylate, and 55 parts of cyclohexyl acrylate. 10 parts of MEK, 0.2 parts of 2-mercaptoethanol (a polymerization chain transfer agent), and 10% of the monomer mixture were placed in a reaction vessel and mixed, followed by thorough nitrogen gas replacement. Meanwhile, a dropping funnel was charged with a mixture of the remaining 90% of the monomer mixture, 0.2 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65). Under a nitrogen atmosphere, the monomer mixture in the reaction vessel was heated to 65°C with stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 65°C from the end of the dropping, a solution of 0.1 parts of the polymerization initiator in 2 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and then at 70°C for 2 hours. The mixture was then dried under reduced pressure to partially remove the MEK, yielding a MEK solution (solids concentration: 45%) of pigment dispersion resin D2 (acid value: 200 mgKOH / g, weight-average molecular weight: 16,100). 17.0 parts of the obtained MEK solution of pigment dispersion resin D2 was diluted with 4.9 parts MEK to a solids concentration of 35%. Next, 3.1 parts of a 5N aqueous solution of NaOH was added so that the degree of neutralization of the carboxyl groups of pigment dispersion resin D2 was 40 mol%, and the mixture was stirred at 25°C. 75.0 parts of water was then added over 1 hour to cause phase inversion emulsification of pigment dispersion resin D2. After the addition was completed, the MEK was distilled off using an evaporator to obtain an aqueous dispersion of pigment dispersion resin D2 (solids concentration 25%).

[0082] (2) Preparation of Pigment Water Dispersion 2 To 32.97 parts of an aqueous dispersion of pigment dispersion resin D2 (solid content concentration 25%), 2.10 parts of MEK and 17.25 parts of ion-exchanged water were added, and 18.77 parts of a cyan pigment (manufactured by DIC Corporation, trade name: Fastogen Blue CA5380 Pigment Blue 15:3) was further added to obtain a pigment mixture. The resulting pigment mixture was mixed using a disper blade at 7000 rpm and 20°C for 1 hour, and then further dispersed using a Microfluidizer (high-pressure homogenizer, manufactured by Microfluidics, product name: M-140K) for 10 passes at a pressure of 180 MPa to obtain a pigment dispersion. From the resulting pigment dispersion, MEK was removed under reduced pressure at 60°C, and then some of the water was removed and centrifuged. The liquid phase was then filtered through a membrane filter (manufactured by Sartorius, trade name: Minisart Syringe Filter, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, yielding Pigment Water Dispersion 2 (solids concentration 22%, pigment: 15.4%, resin: 6.6%). The average particle size of the pigment-containing resin particles in Pigment Water Dispersion 2 was 103.7 nm.

[0083] Production Example 3 (Production of Water Dispersion of Fixing Resin Particles) In a reaction vessel equipped with a dropping funnel, the monomers shown in "Initially charged monomer emulsion" in Table 1, LATEMULL E-118B (polyoxyethylene alkyl ether sodium sulfate, manufactured by Kao Corporation) as a surfactant, potassium persulfate (Fujifilm Wako Pure Chemical Industries, Ltd.) as a polymerization initiator, and ion-exchanged water were mixed and purged with nitrogen gas to obtain an initially charged monomer emulsion. Also, the monomers, surfactant, polymerization initiator, and ion-exchanged water shown in "Dropped monomer emulsion" in Table 1 were mixed to obtain a dropped monomer emulsion, and then the dropped monomer emulsion was placed in the dropping funnel and purged with nitrogen gas. Under a nitrogen atmosphere, the initial monomer emulsion in the reaction vessel was heated from room temperature to 80°C over 30 minutes while stirring. While maintaining the temperature at 80°C, the monomer emulsion in the dropping funnel was gradually added dropwise to the reaction vessel over 3 hours. After the addition was complete, the mixture was stirred for 1 hour while maintaining the temperature inside the reaction vessel. The mixture was then filtered through a 200-mesh filter to obtain an aqueous dispersion of fixing resin particles (acid value: 16 mg KOH / g, weight-average molecular weight: 750,000, solids concentration: 44.1%).

[0084] [Table 1]

[0085] Example 1 (Preparation of Water-Based Ink I1) To obtain the ink composition (total 100 parts) shown in Table 2, 7.1 parts (solids content) of Pigment Water Dispersion 1 obtained in Production Example 1 (solids content: 22%, pigment: 15.4%, polymer: 6.6%, acid value of pigment dispersion resin: 240 mgKOH / g), 4.0 parts (solids content) of an aqueous dispersion of fixing resin particles (solids content: 44.1%), 5.0 parts (solids content) of dipropylene glycol monopropyl ether (reagent, Fujifilm Wako Pure Chemical Industries, Ltd.), 5.0 parts (reagent, Fujifilm Wako Pure Chemical Industries, Ltd.), 1,5-pentanediol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd.), acetylene glycol monopropyl ether (reagent, Fujifilm Wako Pure Chemical Industries, Ltd.), 1,5-pentanediol ... Aqueous ink I1 was obtained by adding 0.5 parts of a polyether-based surfactant ("Surfynol 104PG50" manufactured by Nissin Chemical Industry Co., Ltd., active content 50%), 1.0 parts of a polyether-modified silicone-based surfactant ("KF-6011" manufactured by Shin-Etsu Chemical Co., Ltd., active content 100%), 20.0 parts of propylene glycol (reagent, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 57.4 parts of ion-exchanged water, thoroughly stirring the mixture, and filtering it through a membrane filter ("Minisart Syringe Filter" manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate).

[0086] Examples 2 to 12 and Comparative Examples 1 to 4 Water-based inks I2 to I12 and IC1 to IC4 were obtained in the same manner as in Example 1, except that the ink compositions in Example 1 were changed to those shown in Table 2.

[0087] The notations in Table 2 have the following meanings: PFDG: Dipropylene glycol monopropyl ether (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 212°C, logP: 0.88, δD: 15.6, δP: 6.1, δH: 11.0) BFDG: Dipropylene glycol monobutyl ether (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 231°C, log P: 1.29, δD: 15.7, δP: 6.5, δH: 10.0) HeDG: diethylene glycol monohexyl ether (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 259°C, logP: 1.49, δD: 16.2, δP: 5.6, δH: 9.4) BFTG: Tripropylene glycol monobutyl ether (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 274°C, log P: 1.90, δD: 16.0, δP: 6.7, δH: 7.0) BDG: Diethylene glycol monobutyl ether (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 231°C, logP: 0.67, δD: 16.3, δP: 6.2, δH: 10.5) MPD: 2-methyl-1,3-propanediol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 214°C, logP: -0.74, δD: 17.2, δP: 8.6, δH: 21.2) 1,2-HD: 1,2-hexanediol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 224°C, logP: 0.58, δD: 16.7, δP: 7.1, δH: 17.5) 1,5-PD: 1,5-pentanediol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 242°C, logP: -0.49, δD: 17.0, δP: 8.9, δH: 18.9) 1,6-HD: 1,6-hexanediol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 250°C, logP: 0.00, δD: 15.7, δP: 8.4, δH: 17.8) MBD: 3-methyl-1,3-butanediol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 204°C, logP: -0.38, δD: 16.8, δP: 8.1, δH: 156.8) DPG: dipropylene glycol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point: 230°C, log P: -0.56, δD: 16.7, δP: 8.2, δH: 15.5) TEGO WET 280: Polyether-modified silicone surfactant (EVONIK, "TEGO (registered trademark) Wet 280", HLB value 3.5) 104PG50: Acetylene glycol surfactant (manufactured by Nissin Chemical Industry Co., Ltd., "Surfynol 104-PG50", HLB value 4.0) SE: Acetylene glycol surfactant (Nissin Chemical Industry Co., Ltd., "Surfynol SE," HLB value 6.0) SAG005: Polyether-modified silicone surfactant (manufactured by Nissin Chemical Industry Co., Ltd., "Silface SAG005", HLB value 7.0) KF-6011: Polyether-modified silicone surfactant (Shin-Etsu Chemical Co., Ltd., HLB value 14.5) PG: Propylene glycol (reagent, Fujifilm Wako Pure Chemical Industries, Ltd., boiling point 188°C)

[0088] The inks obtained in the examples and comparative examples were evaluated according to the following (1) to (3).

[0089] (1) Discharge stability (discharge recovery rate) In an environment with a temperature of 32°C, an inkjet printing evaluation device (manufactured by Tritec Corporation) equipped with a print head (manufactured by Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezoelectric type) was filled with the water-based inks obtained in the examples and comparative examples. The print head voltage was set to 26 V, drive frequency to 20 kHz, ejected droplet volume to 5 pL, print head temperature to 32°C, and print head resolution to 600 dpi. Ten 20 mm x 20 mm solid images were printed at 100% duty on a polyethylene terephthalate film ("Lumirror #25-T60" manufactured by Toray Industries, Inc.) (hereinafter referred to as "PET") as a low-absorbency printing substrate heated to 40°C, and then the film was left unprinted for 10 minutes. Thereafter, from the state of the solid print obtained by printing one 20 mm x 20 mm solid image, the ratio of the ejection area of ​​the solid print immediately after being left for 10 minutes (i.e., the solid print obtained by an instruction to print one sheet immediately after being left for 10 minutes) to the ejection area of ​​the solid print immediately before being left for 10 minutes (i.e., the 10th print of the solid print obtained by an instruction to print 10 sheets) was calculated (ejection recovery rate (%) according to the following formula), and the ejection stability was evaluated. Discharge recovery rate (%) = (discharge area of ​​solid print immediately after leaving for 10 minutes / discharge area of ​​solid print immediately before leaving for 10 minutes) × 100 The higher the ejection recovery rate (%), the better the ejection stability of the ink, and an ejection recovery rate of 85% or higher is practically acceptable. The results are shown in Table 2. (Evaluation criteria) A: Discharge recovery rate is 95% or more B: Discharge recovery rate is 90% or more but less than 95% C: Discharge recovery rate is 85% or more but less than 90% D: Discharge recovery rate is 80% or more but less than 85% E: Discharge recovery rate is less than 80%

[0090] (2) Refillability In an environment with a temperature of 32°C, an inkjet printing evaluation device (manufactured by Tritec Corporation) equipped with a print head (manufactured by Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezoelectric type) was filled with the water-based inks obtained in the Examples and Comparative Examples. The print head voltage was set to 26 V, drive frequency to 20 kHz, droplet volume to 12 pL, print head temperature to 32°C, and print head resolution to 600 dpi. Ten 20 mm x 20 mm solid images were printed at 100% duty at 10-second intervals on a polyethylene terephthalate film ("Lumirror #25-T60" manufactured by Toray Industries, Inc.) (hereinafter referred to as "PET") as a low-absorbency printing substrate heated to 40°C. The tenth print was evaluated. If the refillability is good, sufficient ink is supplied to the ejection head within the set range, resulting in a solid image as designed, but if the refillability is poor, the ink supply to the ejection head is insufficient, making it impossible to obtain a solid image as designed. The solid image was visually observed, and the refillability was evaluated according to the following criteria. A rating of C or higher indicates no practical problems. The results are shown in Table 2. (Evaluation criteria) A: A solid image measuring 20cm x 20cm was obtained. B: Less than 20cm x 20cm Only solid images of 20cm x 18cm or more can be obtained. C: Less than 20cm x 18cm Only solid images of 20cm x 16cm or more can be obtained. D: Less than 20cm x 16cm Only solid images of 20cm x 10cm or more can be obtained. E: Only solid images of less than 20cm x 10cm can be obtained.

[0091] [Table 2-1]

[0092] [Table 2-2]

[0093] From Table 2, it can be seen that the water-based inks obtained in the examples of the present invention are excellent in ejection stability and refillability. On the other hand, Comparative Example 1 shows that when diethylene glycol monobutyl ether, an organic solvent with a logP of less than 0.75, is used instead of the organic solvent (A) in the aqueous ink, the ejection stability and refillability are poor. Comparative Example 2 shows that when 3-methyl-1,3-butanediol, an organic solvent with a boiling point of less than 210°C, is used instead of the organic solvent (B) in the aqueous ink, the ejection stability is poor. Comparative Example 3 shows that when dipropylene glycol, an organic solvent (B) with an HSP distance Ra from dipropylene glycol monopropyl ether, the organic solvent (A), is used, the ejection stability and refillability are poor. As can be seen from Comparative Example 3, some organic solvents (B) no longer qualify as organic solvents (B) when combined with organic solvent (A). Comparative Example 4 shows that when SAG005, a surfactant with an HLB value greater than 6.0, is used instead of the surfactant (C) in the aqueous ink, the refillability is poor. Furthermore, if the boiling point of organic solvent (A) or organic solvent (B) exceeds 280° C., the drying property after printing will be poor, causing color mixing and preventing normal printing. Furthermore, if the HLB of the surfactant is less than 1.0, it is thought that the solubility in the water-based ink will be poor, causing aggregation and resulting in poor ejection stability. [Industrial Applicability]

[0094] The present invention provides a water-based ink for inkjet printing that has excellent ejection stability and refillability. Therefore, the water-based ink for inkjet printing of the present invention can be suitably used for printing on low-liquid-absorbent printing substrates for which water-based inks for inkjet printing have conventionally been used.

Claims

1. A water-based ink for inkjet printing on a low-liquid-absorbent printing substrate, comprising: the ink contains a pigment, an organic solvent (A), an organic solvent (B), a surfactant (C) having an HLB value of 1.0 or more and 6.0 or less, and water; the organic solvent (A) is a glycol ether having a boiling point of 200°C or higher and 280°C or lower and an octanol-water partition coefficient log P of 0.75 or higher; the organic solvent (B) is a polyhydric alcohol having a boiling point of 210°C or higher and 280°C or lower and an octanol-water partition coefficient log P of less than 0.75; the distance Ra of the Hansen solubility parameters (HSP) between the organic solvent (A) and the organic solvent (B) is 6.0 or more and 12.5 or less; Water-based ink for inkjet printing.

2. 2. The water-based ink for ink-jet printing according to claim 1, wherein the organic solvent (A) is at least one selected from the group consisting of dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monohexyl ether, and ethylene glycol monophenyl ether.

3. 2. The water-based ink for ink-jet printing according to claim 1, wherein the content of the organic solvent (A) is from 0.1% by mass to 20% by mass.

4. 2. The water-based ink for ink-jet printing according to claim 1, wherein the organic solvent (B) is at least one selected from the group consisting of 2-methyl-1,3-propanediol, 1,2-hexanediol, 1,5-pentanediol, and 1,6-hexanediol.

5. 2. The water-based ink for ink-jet printing according to claim 1, wherein the content of the organic solvent (B) is from 1% by mass to 20% by mass.

6. The water-based ink for ink-jet printing according to claim 1 , further comprising an organic solvent (D) other than the organic solvent (A) and the organic solvent (B).

7. 7. The water-based ink for ink-jet printing according to claim 6, wherein the organic solvent (D) is at least one selected from the group consisting of polyhydric alcohols and glycol ethers.

8. 2. The water-based ink for ink-jet printing according to claim 1, wherein the content of the surfactant (C) having an HLB value of 1.0 or more and 6.0 or less is 0.01% by mass or more and 1.5% by mass or less.

9. The water-based ink for ink-jet printing according to claim 1, further comprising at least one surfactant selected from the group consisting of an acetylene-based surfactant and a polyether-modified silicone-based surfactant, each having an HLB value of more than 6.

0.

10. 2. The water-based ink for ink-jet printing according to claim 1, further comprising a fixing resin, wherein the fixing resin is at least one resin selected from the group consisting of a vinyl resin having a carboxy group, a polyurethane resin having a carboxy group, and a polyester resin having a carboxy group.

11. 2. The water-based ink for ink-jet printing according to claim 1, wherein the pigment is in the form of pigment-containing resin particles.

12. An inkjet printing method, comprising printing on a low liquid-absorbent printing substrate using the water-based ink for inkjet printing according to any one of claims 1 to 11.

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