Water based ink for inkjet recording

Abstract

To provide water-based ink excellent in discharge stability and dryability and capable of obtaining a recorded matter excellent in image fastness even in inkjet recording to a low absorbent recording medium.SOLUTION: Water-based ink for inkjet recording includes organic color, polyethylene wax having a melting point of 110°C or more, an organic solvent, and water. The organic solvent includes 1,2-propane diol (a) and one or more kinds of solvents (b) selected from ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether. The weighted average boiling point of the solvent (b) is 160°C or more; the total content of the solvent (b) in the ink is 7 mass% or more; the weighted average boiling point of the organic solvent is 190°C or less; and the content of the organic solvent in the ink is 30 mass% or less.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Inkjet recording is a method of directly ejecting ink droplets from fine nozzles and depositing them on a recording medium to obtain a recorded matter on which characters or images are recorded. This method has become extremely popular due to its many advantages, including the ease and low cost of producing full-color images, the ability to use plain paper as a recording medium, and the fact that the recording medium is not in contact with the ink. In recent years, inks that use pigments as colorants have come to be widely used in terms of the weather resistance and water resistance of recorded matter. However, unlike dyes, pigments cannot be uniformly dissolved in the ink vehicle, and therefore it is necessary to maintain the pigment's dispersed state and improve the ejection stability during inkjet recording.

[0003] For example, Patent Document 1 discloses an aqueous ink that contains a pigment, polypropylene glycol, an organic solvent, a wax emulsion, and water, and that has excellent storage stability, ensures intermittent ejection stability in inkjet printing, and is capable of suppressing roller transfer of the pigment even when a recording medium with low liquid absorption is used. The aqueous ink contains a wax emulsion having a melting point of 108°C or higher. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-58962 A Summary of the Invention [Problem to be solved by the invention]

[0005] When recording on a low-absorbency recording medium such as coated paper using the inkjet recording method, it is important to dry the ink quickly, so a drying operation is generally performed after recording. During this process, the vicinity of the recording head is exposed to strong drying conditions, so when ink ejection is temporarily stopped and the ink around the ejection nozzle hole dries, aggregated and viscous substances are generated in the vicinity, which makes it easy for nozzle chipping and ejection distortion to occur. In recent years, from the viewpoints of improving recording speed and simplifying the ink drying mechanism, there is a demand for ink with high drying properties. Furthermore, in commercial printing and industrial printing, product information is recorded on a low-absorbency recording medium using a chromatic organic pigment, but since a large amount of the pigment remains on the surface of the recording medium, the pigment is easily peeled off, and image fastness is reduced. Conventional water-based inks do not provide satisfactory image fastness on low-absorbency recording media, and there has been a demand for water-based inks of higher quality. Although the aqueous ink of Patent Document 1 has been improved to some extent in terms of storage stability and intermittent ejection stability, there is still room for improvement, and improvement in image fastness has also been desired. An object of the present invention is to provide a water-based ink that is excellent in ejection stability and drying properties, and is capable of producing recorded matter having excellent image fastness even when used for inkjet recording on a low-liquid-absorbent recording medium. [Means for solving the problem]

[0006] The present inventors have found that the above problems can be solved by blending, as ink components, a specific polyethylene wax and two specific organic solvents in specific amounts. That is, the present invention provides a water-based ink for ink-jet printing, which contains an organic pigment, a polyethylene wax having a melting point of 110° C. or higher, an organic solvent, and water, the organic solvent comprises 1,2-propanediol (a) and one or more solvents (b) selected from ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether; The weighted average boiling point of the solvent (b) is 160°C or higher, and the total content of the solvent (b) in the ink is 7% by mass or higher, and the weighted average boiling point of the organic solvent is 190°C or lower, and the content of the organic solvent in the ink is 30% by mass or lower (hereinafter also referred to as the "ink of the present invention"). Effect of the Invention

[0007] According to the present invention, it is possible to provide a water-based ink that is excellent in ejection stability and drying properties, and is capable of producing recorded matter with excellent image fastness even in ink jet recording on a low liquid-absorbent recording medium. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Water-based ink for inkjet printing] The water-based ink for ink-jet recording of the present invention is a water-based ink for ink-jet recording, which contains an organic pigment, a polyethylene wax having a melting point of 110° C. or higher, an organic solvent, and water, the organic solvent comprises 1,2-propanediol (a) and one or more solvents (b) selected from ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether; The weighted average boiling point of the solvent (b) is 160°C or higher, and the total content of the solvent (b) in the ink is 7% by mass or more, the weighted average boiling point of the organic solvent is 190°C or lower, and the content of the organic solvent in the ink is 30% by mass or less. In this specification, the term "water-based" means that water accounts for the largest proportion by mass of the medium in which the pigment is dispersed. In addition, "recording" is a concept that includes printing and printing out characters and images, and "recorded matter" is a concept that includes printed matter and printed matter on which characters and images are recorded. "Low liquid absorption" is a concept including low liquid absorption and non-liquid absorption, and the amount of water absorption of the recording medium when the recording medium is in contact with pure water for 100 ms is 0 g / m 2 More than 10g / m 2 This means that:

[0009] The ink of the present invention is excellent in ejection stability and drying property, and can obtain a recorded matter having excellent image fastness even in ink jet recording on a recording medium having low liquid absorption. Although the reason for this is not clear, it is thought to be as follows. As a means for improving the drying property of the ink, it has been proposed to lower the boiling point of the organic solvent contained in the ink or to add glycol ether, which has high permeability to the recording medium. However, if a highly hydrophobic solvent is used, the dispersion stability of the ink decreases and the aggregation of pigments and the like over time is promoted. In particular, the vicinity of the ejection nozzle is exposed to strong drying, so the aggregation in the vicinity of the nozzle is further promoted and the ejection stability is reduced. The ink of the present invention contains 1,2-propanediol (a) (boiling point: 188°C) and a specific glycol ether (b), and the weighted average boiling point of the glycol ether (b) is set to 160°C or higher and the total content of the solvent (b) is set to 7 mass% or more. By doing so, while the organic solvent in the ink exerts its moisturizing effect in the ejection nozzle, the specific glycol ether (b) volatilizes preferentially and 1,2-propanediol (a) remains effectively and preferentially in the nozzle. This makes it possible to suppress aggregation in the ink and is believed to achieve both drying properties and ejection stability. Furthermore, when polyethylene wax is contained in a water-based ink, the frictional resistance of the ink coating is reduced and image fastness can be improved, but in recent high-speed recording, high-temperature drying is performed after recording, which causes the problem of melting of the polyethylene wax. However, since the melting point of the polyethylene wax used in the ink of the present invention is 110°C or higher, melting of the polyethylene wax can be prevented even during the high-temperature drying process of the ink, and it is believed that image fastness is improved.

[0010] <Organic pigments> In the present invention, an organic pigment (hereinafter, also simply referred to as "pigment") is used. Specific examples of 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. The hue is not particularly limited, and any chromatic pigment such as yellow, magenta, cyan, blue, red, orange, green, etc. can be used. Specific examples of preferred 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. The above pigments can be used alone or in combination of two or more kinds.

[0011] Examples of the form of the pigment include (i) a pigment that can maintain a dispersed state without a dispersant, i.e., a self-dispersing pigment, (ii) a pigment particle formed by dispersing the pigment with a surfactant, and (iii) a polymer particle containing the pigment. Among these, from the viewpoint of improving the ejection stability, image fastness, etc., the form of "polymer particles containing a pigment" is preferred, and the form of "water-insoluble polymer particles containing a pigment" is more preferred. In this specification, "polymer particles containing a pigment" (hereinafter also referred to as "pigment-containing polymer particles") means particles in which a polymer encapsulates a pigment, particles consisting of a polymer and a pigment, with part of the pigment exposed on the surface, particles in which a polymer is adsorbed to part of the pigment, and mixtures thereof. Of these, particles in which a polymer encapsulates a pigment are more preferred.

[0012] (Pigment-containing polymer particles) The polymer constituting the pigment-containing polymer particles (hereinafter also referred to as "polymer a") is not particularly limited as long as it is a polymer having the pigment dispersibility to disperse the pigment in an aqueous medium containing water as the main component. The polymer a is preferably a water-insoluble polymer. As used herein, the term "water-insoluble polymer" refers to a polymer that dissolves in an amount of less than 10 g when the polymer is dried at 105° C. for 2 hours and reaches a constant weight, and then dissolved in 100 g of water at 25° C. When the polymer is an anionic polymer, the amount of dissolution is the amount of dissolution when 100% of the anionic groups of the polymer are neutralized with sodium hydroxide.

[0013] (Polymer a) Polymer a may have any structure. From the viewpoint of improving the ejection stability, image fastness, etc. of the ink of the present invention, however, a vinyl-based polymer obtained by addition polymerization of a vinyl monomer such as a vinyl compound, a vinylidene compound, or a vinylene compound is preferred. As the vinyl polymer, (a-1) a polymer containing a structural unit derived from an ionic monomer is preferable, and (a-2) a copolymer having a structural unit derived from an ionic monomer and a structural unit derived from a hydrophobic monomer is more preferable.

[0014] [(a-1) Ionic Monomer] As the (a-1) ionic monomer, an anionic monomer is preferred from the viewpoint of improving the dispersion stability of the pigment. Examples of the anionic monomer include carboxylic acid monomers, sulfonic acid monomers, phosphoric acid monomers, etc. Examples of the carboxylic acid monomer include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, etc. Among these, carboxylic acid monomers are more preferred, and one or more selected from acrylic acid and methacrylic acid are even more preferred.

[0015] [(a-2) Hydrophobic Monomer] From the viewpoint of improving the dispersion stability of the pigment, it is preferable to use the hydrophobic monomer (a-2) as an additional monomer component in addition to the component (a-1). Specific examples of the (a-2) component include those described in paragraphs

[0020] to

[0022] of JP 2018-83938 A. Among these, one or more selected from alkyl (meth)acrylates having an alkyl group having 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth)acrylate are preferred.

[0016] [(a-3) Nonionic Monomer] The nonionic monomer (a-3) can be used from the viewpoint of further improving the dispersion stability of the pigment. The component (a-3) is a monomer that has a high affinity for water or a water-soluble organic solvent, such as a monomer that contains a hydroxyl group or a polyalkylene glycol chain. Specific examples of the (a-3) component include those described in paragraph

[0018] of JP 2018-83938 A. Among these, one or more selected from methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferred. The monomer components contained in each of the above components (a-1) to (a-3) can be used alone or in combination of two or more kinds.

[0017] (Content of each structural unit in polymer a) The contents of the structural units derived from the components (a-1) to (a-3) in the polymer a are as follows, from the viewpoint of improving the dispersion stability of the pigment and improving the ejection stability and image fastness of the ink of the present invention. The content of the (a-1) component is preferably 5 mass% or more, more preferably 10 mass% or more, even more preferably 15 mass% or more, and is preferably 45 mass% or less, more preferably 40 mass% or less, even more preferably 35 mass% or less. The content of the (a-2) component is preferably 40 mass% or more, more preferably 45 mass% or more, even more preferably 50 mass% or more, and preferably 90 mass% or less, more preferably 85 mass% or less, even more preferably 80 mass% or less. When the component (a-3) is contained, the content thereof is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less. The mass ratio of the component (a-1) to the component (a-2) [(a-1) / (a-2)] is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.25 or more, and is preferably 1.2 or less, more preferably 1.0 or less, even more preferably 0.8 or less. In the present invention, the content of the structural units derived from the components (a-1) to (a-3) in the polymer a can be determined by measurement, or can be substituted by the charging ratio of the raw material monomers including the components (a-1) to (a-3) during the production of the polymer a.

[0018] (Production of polymer a) The polymer a can be produced by copolymerizing a monomer mixture containing the components (a-1) to (a-3) by a known polymerization method, preferably a solution polymerization method. The solvent used in the solution polymerization method is not limited, but polar solvents such as water, aliphatic alcohols, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred. In the polymerization, a polymerization initiator and a polymerization chain transfer agent can be used. Examples of the polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble azo polymerization initiators, and examples of the polymerization chain transfer agent include mercaptans. The polymerization temperature varies depending on the types of polymerization initiator, monomer, solvent, etc. used, but is preferably 30° C. or higher, more preferably 50° C. or higher, and is preferably 95° C. or lower, more preferably 80° C. or lower. The polymerization atmosphere is preferably a nitrogen gas or inert gas atmosphere. It is preferable that the polymer a is neutralized with a neutralizing agent as described below.

[0019] From the viewpoint of improving the dispersion stability of the pigment dispersed in the polymer, the number average molecular weight of polymer a is preferably 10,000 or more, more preferably 15,000 or more, and is preferably 100,000 or less, more preferably 50,000 or less. From the same viewpoint as above, the acid value of polymer a is preferably 80 mgKOH / g or more, more preferably 120 mgKOH / g or more, and preferably 300 mgKOH / g or less, more preferably 280 mgKOH / g or less. The number average molecular weight and acid value of the polymer can be measured by the method described in the Examples.

[0020] Polymer a may be a commercially available product as long as it has a structural unit derived from component (a-1) and a structural unit derived from component (a-2). Commercially available examples of vinyl polymers include styrene-acrylic resins such as the Joncryl series manufactured by BASF.

[0021] [Preparation of pigment-containing polymer particles] From the viewpoint of efficient production, the pigment-containing polymer particles are preferably produced as pigment dispersion A in which the pigment-containing polymer particles are dispersed in an aqueous medium by a method including the following step I and, if necessary, step II: Step I: A step of dissolving polymer a in a solvent to obtain a solution of polymer a, and then adding and mixing a pigment and, if necessary, a neutralizing agent, a surfactant, etc., to obtain a pigment dispersion A consisting of a pigment mixture. In order to efficiently incorporate the pigment into the polymer particles, it is preferable that the solvent contains an organic solvent in Step I. When the solvent contains an organic solvent, in addition to Step I, the method may further include the following Step II. Step II: A step of removing the organic solvent from the pigment mixture obtained in step I to obtain a pigment dispersion A. In addition to the above steps I and II, it is preferable to further carry out the following step III. Step III: A step of mixing the pigment dispersion A obtained in step I or step II with a crosslinking agent a to obtain a pigment dispersion of crosslinked pigment-containing crosslinked polymer particles. In this specification, pigment dispersion A refers to both a pigment dispersion in which pigment-containing polymer particles are dispersed in an aqueous medium, and a pigment dispersion in which pigment-containing crosslinked polymer particles are dispersed in an aqueous medium.

[0022] (Process I) In step I, the solvent in which polymer a is dissolved is not limited, but from the viewpoints of wettability to the pigment, solubility of polymer a, and adsorption to the pigment, it is preferable to use one or more selected from water, aliphatic alcohols having from 1 to 3 carbon atoms, ketones, ethers, esters, etc. When polymer a is synthesized by a solution polymerization method, the solvent used in the polymerization may be used as it is. When the polymer a is an anionic polymer, it is preferable to neutralize the anionic groups using a neutralizing agent so that the pH is 7 or more and 11 or less. Examples of the neutralizing agent include bases such as sodium hydroxide, potassium hydroxide, ammonia, and various amines. The polymer a may also be neutralized in advance. From the viewpoint of improving the storage stability of the ink of the present invention, the degree of neutralization of the anionic groups of polymer a, expressed as the ratio of the molar equivalents of the neutralizing agent to the molar equivalents of the anionic groups of polymer a, is preferably 20 mol % or more, more preferably 25 mol % or more, even more preferably 30 mol % or more, and is preferably 200 mol % or less, more preferably 100 mol % or less, even more preferably 80 mol % or less.

[0023] In step I, it is preferable to apply mechanical force to the obtained pigment mixture to perform a dispersion treatment. There are no particular limitations on the method for applying mechanical force, but examples of the method include the method described in paragraph

[0032] of JP2018-83938A. In the dispersion treatment, the pigment particles can be atomized to a desired particle size only by main dispersion using shear stress. However, from the viewpoint of obtaining a uniform pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion. As a dispersing machine used for preliminary dispersion, a commonly used mixing and stirring device such as an anchor blade or a dispersing blade can be used. Examples of dispersing machines used in the dispersion include kneading machines such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersing machines such as paint shakers and bead mills. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When a dispersion treatment is performed using a high-pressure homogenizer, the average particle size of the pigment particles in the pigment dispersion can be adjusted by controlling the treatment pressure and the number of passes. From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more and 300 MPa or less, and the number of passes is preferably 3 to 30.

[0024] (Process II) Step II is an optional step. It is preferable that the organic solvent in the obtained pigment dispersion A is substantially removed, but it may remain in an amount of, for example, about 0.1 mass % or less as long as the effects of the present invention are not impaired. From the viewpoint of improving storage stability and enabling high-definition recording, the average particle size of the pigment-containing polymer particles in Pigment Dispersion A is preferably 50 nm or more, more preferably 100 nm or more, even more preferably 120 nm or more, and is preferably 350 nm or less, more preferably 320 nm or less, even more preferably 300 nm or less. Even when crosslinking treatment is carried out in the below-described step III, the average particle size of the pigment-containing crosslinked polymer particles in the resulting pigment dispersion A is the same as the average particle size of the pigment-containing polymer particles. The average particle size is measured by the method described in the Examples.

[0025] (Process III) In step III, a part of the carboxyl groups of the polymer a constituting the pigment-containing polymer particles is crosslinked to form a crosslinked structure in a part or all of the surface layer of the pigment-containing polymer particles, thereby converting the pigment-containing polymer particles into pigment-containing crosslinked polymer particles. This crosslinking treatment is believed to suppress swelling and shrinkage of the polymer a, which adversely affects the dispersion stability of the pigment, and to further improve the ejection stability and drying property of the ink of the present invention, and the image fastness of the resulting recorded matter.

[0026] When the polymer a is an anionic polymer, the crosslinking agent a is preferably a compound having a functional group capable of reacting with an anionic group, and more preferably a compound having 2 to 6 such functional groups in the molecule. Suitable examples of the crosslinking agent a include compounds having two or more epoxy groups, oxazoline groups, or isocyanate groups in the molecule. Among these, compounds that are water-insoluble with a water solubility of 50% by mass or less, preferably 40% by mass or less, and have 2 to 4 epoxy groups in the molecule are preferred, and one or more selected from trimethylolpropane polyglycidyl ether and pentaerythritol polyglycidyl ether are more preferred, and trimethylolpropane polyglycidyl ether (water solubility of 27% by mass) is even more preferred. Here, the "water solubility" refers to the solubility (mass%) when 10 parts by mass of the crosslinking agent are dissolved in 90 parts by mass of ion-exchanged water at 25°C.

[0027] From the viewpoint of improving ejection stability, image fastness, etc., the crosslinking rate of the pigment-containing polymer particles in step III is preferably 10 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and is preferably 80 mol % or less, more preferably 70 mol % or less, even more preferably 65 mol % or less, expressed as the ratio of the molar equivalent number of the crosslinkable functional groups of the crosslinking agent to the molar equivalent number of the carboxy groups of polymer a. From the viewpoints of completion of the crosslinking reaction and economic efficiency, the temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 55° C. or higher, and is preferably 95° C. or lower, more preferably 80° C. or lower. From the viewpoint of improving the dispersion stability of the pigment dispersion, the solid content concentration of the obtained pigment dispersion A is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less. The solids concentration of the pigment dispersion is measured by the method described in the examples.

[0028] <Polyethylene wax with a melting point of 110°C or higher> From the viewpoint of improving the ejection stability, image fastness, etc., the polyethylene wax used in the present invention has a melting point of 110° C. or higher, preferably 115° C. or higher, more preferably 120° C. or higher, even more preferably 125° C. or higher, and preferably 160° C. or lower, more preferably 150° C. or lower, even more preferably 145° C. or lower.

[0029] The polyethylene wax is a wax containing ethylene as a main component. Here, "containing ethylene as a main component" means that the ethylene content of 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 polyethylene wax can be obtained by adjusting a high molecular weight polyethylene to a desired molecular weight by thermal decomposition or the like while introducing oxygen atoms or the like into the molecule, and can be used as the polyethylene wax. The content of the polyethylene wax 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 still more preferably 100% by mass.

[0030] The polyethylene wax is preferably used as an emulsion. There is no particular limitation on the method for producing the wax emulsion, and for example, a method of mixing polyethylene wax, other waxes used as necessary, and a known surfactant and emulsifying them can be mentioned. As the surfactant, a nonionic surfactant, an anionic surfactant, etc. can be used. Examples of nonionic surfactants include ethylene oxide adducts of higher alcohols, ethylene oxide adducts of alkylated phenols, etc. Examples of anionic surfactants include sulfate ester salts and phosphate ester salts based on ethylene oxide adducts of higher alcohols, and alkylated benzenesulfonate salts. The polyethylene wax is preferably a nonionic polyethylene wax emulsion from the viewpoint of improving the ejection stability, image fastness, and the like.

[0031] From the viewpoint of improving the dispersion stability of the wax, the ejection stability of the ink, the fastness of the image, etc., the average particle size of the polyethylene wax in the ink is preferably 300 nm or less, more preferably 200 nm or less, even more preferably 100 nm or less, and is preferably 10 nm or more, more preferably 30 nm or more. The average particle size of the polyethylene wax can be measured by dynamic light scattering, for example, by the method described in the Examples using a Microtrack particle size analyzer UPA manufactured by Nikkiso Co., Ltd. Suitable examples of commercially available wax emulsions include the Hi-Tec E series manufactured by Toho Chemical Industry Co., Ltd., the AQUACER series manufactured by BYK, the Cellosol series manufactured by Chukyo Yushi Co., Ltd., and the Chemipearl series manufactured by Mitsui Chemicals, Inc.

[0032] <Organic solvent> From the viewpoint of improving the ejection stability, drying property, and image fastness of the ink of the present invention, the organic solvent used in the present invention contains 1,2-propanediol (a) (boiling point: 188°C) and one or more solvents (b) selected from ethylene glycol monobutyl ether (boiling point: 171°C), propylene glycol monomethyl ether (boiling point: 121°C), propylene glycol monobutyl ether (boiling point: 170°C), and dipropylene glycol monomethyl ether (boiling point: 187°C), wherein the weighted average boiling point of the solvent (b) is 160°C or higher and the total content of the solvent (b) in the ink is 7 mass% or higher, the weighted average boiling point of the organic solvent is 190°C or lower, and the content of the organic solvent in the ink is 30 mass% or lower.

[0033] From the same viewpoints as above, the weighted average boiling point of the solvent (b) is preferably 162°C or more, more preferably 165°C or more, even more preferably 168°C or more, and preferably 187°C or less, more preferably 185°C or less, even more preferably 180°C or less, and still more preferably 176°C or less. The total content of the solvent (b) in the ink is preferably 8% by mass or more, and preferably 20% by mass or less, more preferably 18% by mass or less, and even more preferably 16% by mass or less.

[0034] The organic solvent may contain other organic solvents in addition to those described above, as long as the effects of the present invention are not impaired. Examples of the other organic solvents that may be used include water-soluble organic solvents such as polyhydric alcohols and polyhydric alcohol alkyl ethers. From the same viewpoints as above, the weighted average boiling point of all organic solvents including 1,2-propanediol (a) and solvent (b) is preferably 189°C or lower, more preferably 188°C or lower, even more preferably 187°C or lower, and is preferably 168°C or higher, more preferably 169°C or higher, even more preferably 170°C or higher. The total content (total amount) of all organic solvents in the ink is preferably 25% by mass or less, more preferably 20% by mass or less, and is preferably 8% by mass or more.

[0035] The ink of the present invention may further contain a surfactant, if necessary. Examples of the surfactant include nonionic surfactants, silicone-based surfactants, and fluorine-based surfactants, with nonionic surfactants being more preferred. Examples of nonionic surfactants include polyoxyalkylene alkyl ether surfactants, acetylene glycol surfactants, polyhydric alcohol surfactants, fatty acid alkanolamides, etc. Among these, acetylene glycol surfactants are preferred, and for example, 2,4,7,9-tetramethyl-5-decyne-4,7-diol is more preferred. Examples of commercially available nonionic surfactants include the Surfynol series manufactured by Nissin Chemical Industry Co., Ltd. and Air Products & Chemicals, and the Acetylenol series manufactured by Kawaken Fine Chemicals Co., Ltd. The above surfactants can be used alone or in combination of two or more.

[0036] <Method of manufacturing water-based ink> The ink of the present invention can be obtained by mixing the pigment dispersion of the above-mentioned pigment-containing polymer particles, polyethylene wax having a melting point of 110°C or higher, an organic solvent, water, and, as necessary, various additives commonly used in inks, such as a humectant, wetting agent, penetrating agent, surfactant, viscosity modifier, defoamer, preservative, antifungal agent, and rust inhibitor.

[0037] <Content of each component in water-based ink> The contents of the various components in the ink of the present invention are as follows, from the viewpoint of improving the ejection stability, drying properties, and image fastness of the ink of the present invention.

[0038] (Pigment content) The content of the pigment in the ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 2.5% by mass or more, and preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less. The content of the pigment-containing polymer particles in the ink of the present invention is preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 6% by mass or more, and preferably 18% by mass or less, more preferably 16% by mass or less, even more preferably 14% by mass or less, and even more preferably 12% by mass or less.

[0039] (Polyethylene wax content with melting point of 110°C or higher) The content of polyethylene wax having a melting point of 110°C or higher in the ink of the present invention is preferably 0.5 mass% or more, more preferably 0.6 mass% or more, even more preferably 0.8 mass% or more, and preferably 1.3 mass% or less, more preferably 1.2 mass% or less, even more preferably 1.1 mass% or less.

[0040] (Organic solvent content) The content of the organic solvent in the ink of the present invention is 30% by mass or less, preferably 25% by mass or less, more preferably 20% by mass or less, and preferably 8% by mass or more. (Water content) The water content in the ink of the present invention is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less.

[0041] <Water-based ink properties> From the viewpoint of improving storage stability and image density, 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, even more preferably 7 mPa s or less. The viscosity of the water-based ink can be measured using an E-type viscometer. From the viewpoint of improving storage stability and image density, the pH of the ink of the present invention is preferably 7.0 or more, more preferably 7.2 or more, and even more preferably 7.5 or more. From the viewpoint of member resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less, and even more preferably 9.5 or less. The pH of the water-based ink can be measured by a conventional method.

[0042] The ink of the present invention can be loaded into a known ink jet recording device such as a piezoelectric type, and ejected as ink droplets onto a recording medium to record an image or the like. The ink of the present invention can provide a recorded product having excellent image fastness even in ink jet recording on a low liquid absorbing recording medium. Examples of low-absorbency recording media include low-absorbency coated paper, art paper, and non-absorbent resin films. Examples of coated paper include general-purpose glossy paper and multi-color form glossy paper. Examples of the resin film include transparent synthetic resin films, such as polyester, polyvinyl chloride, polyolefin, and nylon films. These films may be biaxially oriented, uniaxially oriented, or non-oriented films. Among these, polyester films and oriented 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

[0043] 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.

[0044] (1) Measurement of polymer number average molecular weight The measurement was performed by gel permeation chromatography (GPC apparatus (HLC-8320GPC) manufactured by Tosoh Corporation, columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolum Super AW-H) manufactured by Tosoh Corporation, flow rate: 0.5mL / min) using a solution of phosphoric acid and lithium bromide dissolved in N,N-dimethylformamide to a concentration of 60mmol / L and 50mmol / L, respectively, as an eluent, and using a monodisperse polystyrene kit with known molecular weight (PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick C (F-288, F-40, F-4, A-5000, A-500), manufactured by Tosoh Corporation) as a standard substance. 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 with a syringe filter (DISMIC-13HP, PTFE, 0.2 μm, Advantec Co., Ltd.).

[0045] (2) Measurement of solids concentration of pigment dispersion 10.0 g of sodium sulfate that had been kept at a constant weight in a desiccator was weighed out into a 30 mL ointment container, and approximately 1.0 g of the sample was added and mixed, then accurately weighed, kept at 105°C for 2 hours to remove volatile matter, and left in the desiccator for a further 15 minutes, after which the mass was measured. The mass of the sample after volatile matter removal was taken as the solid content, and was divided by the initial mass of the sample to obtain the solid content concentration.

[0046] (3) Average particle size of pigment dispersion The average particle size of the pigment dispersion was measured by dynamic light scattering using a laser particle analysis system (Otsuka Electronics Co., Ltd., product name: ELS-8000), 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 entered as the refractive index of the dispersion solvent. For the measurement sample, the pigment dispersion was weighed into a screw tube (Maruemu Co., Ltd. No. 5) and the solids concentration was 2×10 -4 Water was added so as to obtain the desired mass %, and the mixture was stirred at 25° C. for 1 hour using a magnetic stirrer.

[0047] (4) Measurement of the acid value of the polymer The resin was dissolved in a titration solvent of toluene and acetone (2:1) in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with 0.1N potassium hydroxide / ethanol solution by potentiometric titration, with the inflection point on the titration curve as the end point. The acid value (mgKOH / g) was calculated from the titration amount of potassium hydroxide solution up to the end point.

[0048] (5) Measurement of the melting point of wax The melting point of the wax was measured using a measuring device conforming to JIS K 0064. Specifically, using a differential scanning calorimeter (Q20, manufactured by TA Instruments), the sample was heated to 200°C and cooled from that temperature to 0°C at a rate of 10°C / min. Next, the sample was heated at a rate of 10°C / min, and the amount of heat was measured up to 200°C. Among the observed heat of fusion peaks, the temperature of the peak with the largest peak area was determined as the maximum peak temperature of melting, and this peak temperature was determined as the melting point.

[0049] (6) Measurement of average particle size of wax emulsion The average particle size (average dispersed particle size) of the wax emulsion was measured using a Microtrack particle size analyzer UPA manufactured by Nikkiso Co., Ltd.

[0050] Production Example 1 (Production of Polymer (a)) A monomer mixture was prepared by mixing 31 parts of acrylic acid and 69 parts of styrene. 10 parts of methyl ethyl ketone (MEK), 0.2 parts of a polymerization chain transfer agent (2-mercaptoethanol), and 10% of the monomer mixture were mixed in a reaction vessel, and the atmosphere was thoroughly replaced with nitrogen gas. Meanwhile, a mixture of the remaining 90% of the monomer mixture, 0.13 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of a radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name: V-65) was placed in a dropping funnel, and the monomer mixture in the reaction vessel was heated to 65°C while stirring under a nitrogen atmosphere, and the mixture in the dropping funnel was dropped over 3 hours. After 2 hours at 65°C from the end of the dropping, a solution in which 0.1 parts of the polymerization initiator was dissolved in 2 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and 70°C for 2 hours, and then dried under reduced pressure to obtain polymer (a) (number average molecular weight: 19000, acid value: 240mgKOH / g).

[0051] Example 1 (Preparation of Water-Based Ink 1) (1) Manufacturing of pigment dispersions 100 parts of the polymer (a) obtained in Production Example 1 was mixed with 78.6 parts of MEK, and 41.2 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide 16.9%) was added as a neutralizing agent to neutralize (neutralization degree: 40 mol%). 800 parts of ion-exchanged water was further added, and 100 parts of a cyan pigment (CI Pigment Blue 15:3, manufactured by DIC Corporation, product name: TGR-SD) was added thereto, and the mixture was stirred for 60 minutes at 20°C using a Disper (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) with the Disper blade rotating at 7000 rpm. The resulting mixture was dispersed 10 times using a Microfluidizer (manufactured by Microfluidics, product name) at a pressure of 200 MPa. To the obtained dispersion, 250 parts of ion-exchanged water was added and stirred. After that, MEK was completely removed at 60°C under reduced pressure. Further, some of the water was removed until the pigment concentration reached 10%, after which 35.7 parts of an epoxy crosslinker (trimethylolpropane polyglycidyl ether, Nagase ChemteX Corporation, product name: Denacol EX-321, epoxy equivalent 140) was added, the container was sealed, and the container was heated at 70°C for 5 hours while stirring with a stirrer. The container was then cooled to room temperature to obtain a pigment dispersion (solids concentration: 24%) containing crosslinked polymer particles (crosslinking rate 60 mol%) containing the pigment.

[0052] (2) Manufacture of Water-Based Ink 1 50 parts of the obtained pigment dispersion, 2.85 parts of a polyethylene wax emulsion (manufactured by Toho Chemical Industry Co., Ltd., product name: Hi-Tec E-6500, melting point (mp): 140°C, solid content concentration: 35% nonionic), 19 parts of 1,2-propanediol (manufactured by FUJIFILM Wako Pure Chemical Industries Co., Ltd.) as an organic solvent (a), 8 parts of ethylene glycol monobutyl ether (manufactured by FUJIFILM Wako Pure Chemical Industries Co., Ltd.) as an organic solvent (b), and 0.5 parts of an acetylene glycol surfactant (2,4,7,9-tetramethyl-5-decyne-4,7-diol, manufactured by Air Products and Chemicals, product name: Surfynol 104) were mixed, and ion exchanged water was added so that the total amount became 100 parts. The solution was filtered using a 25 mL needleless syringe (Terumo Corporation) equipped with a 5 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm, Fujifilm Wako Pure Chemical Industries, Ltd.) to obtain water-based ink 1 (solid concentration: 13%, pigment-containing polymer particles: 12% (pigment: 5%, polymer: 7%), wax: 1%).

[0053] Examples 2 to 12 and Comparative Examples 1 to 9 (Production of Water-Based Inks 2 to 12, 21 to 29) Water-based inks 2 to 12 and 21 to 29 were obtained in the same manner as in Example 1, except that the conditions in Example 1 were changed to those shown in Table 1. The results are shown in Table 1. The Hi-Tech E-8237 used in Comparative Example 8 is a polyethylene wax emulsion (melting point: 106° C., average particle size: 80 nm, solid content: 35%) manufactured by Toho Chemical Industry Co., Ltd. The numerical values ​​for the ink compositions in Table 2 are the effective amounts (solid contents).

[0054] The water-based inks obtained in the examples and comparative examples were evaluated for ejection stability, drying property, and image fastness by the following methods. The results are shown in Table 1.

[0055] (1) Evaluation of discharge stability Using a water-based ink, an image was printed on A4 size coated paper (manufactured by Oji Paper Co., Ltd., product name: OK Topcoat+) by the following inkjet recording method. (Inkjet recording method) In an environment of temperature 25±1°C and relative humidity 30±5%, a water-based ink was filled into a print evaluation device (manufactured by Tritec Co., Ltd.) equipped with an inkjet recording head (manufactured by Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezo type). The head voltage was set to 26V, the driving frequency to 10kHz, the appropriate amount of ejected liquid to 12pL, the head temperature to 32°C, the resolution to 600dpi, the number of flushings before ejection to 200, and the negative pressure to -4.0kPa. The recording medium was fixed to the conveying table under reduced pressure so that the longitudinal direction of the recording medium was the same as the conveying direction. A print command was transferred to the print evaluation device, and a solid image with a duty of 100% was printed. After that, the printing evaluation device was stopped for 30 minutes and the recording head was exposed to the atmosphere. After 30 minutes had passed, the ink was purged from the recording head once, and the state of nozzle chipping when printing was resumed after wiping was observed, and the nozzle recovery rate (%) was calculated using the following formula to evaluate the ejection stability. Nozzle recovery rate (%) = (number of normal nozzles / total number of nozzles) x 100 The higher the nozzle recovery rate (%), the better the nozzle recovery is judged to be, and a value of 85% or higher is practically usable.

[0056] (2) Evaluation of drying properties Using the inkjet recording method described above in (1), a gradation image was printed in 5% increments with a duty ranging from 0 to 100%. After that, the printed paper was left to stand for 3 seconds, and then an unprinted coated paper was placed on the printed surface, and a load of 30 N was applied for 15 seconds, and the transfer of the ink to the unprinted coated paper was visually observed. The duty value at which no transfer occurs was confirmed. The higher the duty value at which no transfer occurs, the better the drying property is judged to be. A duty value of 70% or more is practically usable.

[0057] (3) Evaluation of image robustness A solid image with a duty of 100% was printed by the inkjet recording method described above in (1). Immediately after printing, the printed paper was dried for 3 seconds with a short-wavelength infrared heater (Heraeus, product name: ZKB1200 / 340G x 9, heater effective length 340 mm, output 1200 W). After leaving the dried printed paper stationary for 24 hours, a non-printed coated paper cut to 2 x 2 cm was placed on the printed surface using a Gakushin-type abrasion tester (Daiei Scientific Instruments Manufacturing Co., Ltd., product name: RT-300), and the paper was run back and forth 10 times with a load of 10 N, and the density of the transferred ink was measured using a spectrophotometer (Sakata Inx Engineering Co., Ltd., product name: Spectro-Eye). The lower the density of the transferred ink, the better the image fastness is judged to be, and a value of 0.2 or less is practically usable.

[0058] [Table 1]

[0059] From Table 1, it can be seen that the water-based inks obtained in Examples 1 to 12 have superior ejection stability and drying properties compared to the water-based inks obtained in Comparative Examples 1 to 9, and can produce printed matter with excellent image fastness.

Claims

1. A water-based ink for ink-jet printing, comprising an organic pigment, a polyethylene wax having a melting point of 110° C. or higher, an organic solvent, and water, the organic solvent comprises 1,2-propanediol (a) and one or more solvents (b) selected from ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether; the weighted average boiling point of the solvent (b) is 160° C. or higher, and the total content of the solvent (b) in the ink is 7% by mass or higher; the weighted average boiling point of the organic solvent is 190° C. or less, and the content of the organic solvent in the ink is 30% by mass or less.

2. 2. The water-based ink for ink-jet printing according to claim 1, wherein the content of the organic solvent in the ink is 25% by mass or less.

3. 3. The water-based ink for ink-jet printing according to claim 1, wherein the polyethylene wax has an average particle size of 10 nm or more and 300 nm or less.

4. 4. The water-based ink for ink-jet printing according to claim 1, wherein the content of the polyethylene wax having a melting point of 110° C. or higher in the ink is 0.5% by mass or more and 1.3% by mass or less.

5. 5. The water-based ink for ink-jet printing according to claim 1, wherein the organic pigment is in the form of polymer particles containing the organic pigment.

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