Water-based ink for inkjet printing

Abstract

To provide a water-based ink for inkjet recording that excels in water repellency to a recording head including a silicon member or a glass member as well as long-term discharge reliability in inkjet recording, and can give a recorded material having excellent image fastness, and an inkjet recording method using the same.SOLUTION: The present invention provides [1] a water-based ink for inkjet recording that contains a pigment, pigment-free polymer particles, a wax, an organic solvent, a silicate compound and water, with the content of the silicate compound being 2.5-250 mass ppm, and [2] an inkjet recording method that uses the water-based ink to discharge the ink from a recording head including a nozzle plate member composed of at least one selected from silicon and silicon oxide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based ink for ink-jet printing and an ink-jet recording method using the same. [Background technology]

[0002] Inkjet recording is a method of directly ejecting ink droplets from minute nozzles onto a recording medium to produce a recorded product with characters and images. This method has become extremely popular due to its many advantages, including the ease and low cost of full color printing, the ability to use plain paper as the recording medium, and the fact that it does not contact the recording medium. In the commercial printing field, in addition to printing on conventional highly water-absorbent recording media such as plain paper and copy paper, there is a demand for printing on low-water-absorbent coated paper such as offset coated paper. In recent years, recording heads with dramatically improved ejection accuracy have come into use by using MEMS (Micro Electro Mechanical Systems) technology to process nozzle plates and other components made of silicon or glass materials.

[0003] Water-based pigment inks with good light resistance and water resistance are commonly used as colorants in inkjet printers. However, when such water-based pigment inks are filled into inkjet recording heads that use silicon or glass members and used or left for a long period of time, silicon and other materials in contact with the ink are eluted, reducing the water repellency of the nozzle plate and other components, which can lead to a decrease in the printer's ejection accuracy and a decrease in image quality. Various proposals have been made to improve the above problems. For example, Patent Document 1 proposes an inkjet ink composition containing a water-soluble alkali metal silicate, self-dispersing polymer particles, and a pigment, as an ink composition that has excellent dispersion stability and is inhibited from reducing the liquid repellency of inkjet recording head members. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-057754 Summary of the Invention [Problem to be solved by the invention]

[0005] The ink composition of Patent Document 1 deteriorates in ejection properties due to deterioration of the ink over time, and the image fastness of the resulting prints is insufficient. An object of the present invention is to provide a water-based ink for inkjet printing which has excellent water repellency against a recording head using a silicon member or a glass member, excellent long-term ejection reliability in inkjet recording, and which can give recorded matter with excellent image fastness, and an inkjet recording method using the same. [Means for solving the problem]

[0006] The present inventors have found that the above-mentioned problems can be solved by adding wax and a specific amount of silicate compound to a water-based ink for inkjet printing that contains a pigment, polymer particles, wax, an organic solvent, and a silicate compound. That is, the present invention provides the following [1] and [2]. [1] A water-based ink for inkjet printing containing a pigment, polymer particles not containing a pigment, wax, an organic solvent, a silicic acid compound, and water, wherein the content of the silicic acid compound in the ink is 2.5 ppm by mass or more and 250 ppm by mass or less. [2] An ink-jet recording method, comprising using the water-based ink for ink-jet recording according to [1] above and ejecting the ink from a recording head using one or more materials selected from silicon and silicon oxide as a nozzle plate member. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a water-based ink for ink-jet printing which is excellent in water repellency to a recording head using a silicon member or a glass member, is excellent in long-term ejection reliability in ink-jet recording, and can give recorded matter which is excellent in image fastness, and an ink-jet recording method using the same. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Water-based ink for inkjet printing] The water-based ink for ink-jet printing of the present invention (hereinafter also referred to as "the ink of the present invention") is a water-based ink for ink-jet printing containing a pigment, pigment-free polymer particles, wax, an organic solvent, a silicic acid compound, and water, characterized in that the content of the silicic acid compound in the ink is 2.5 ppm by mass or more and 250 ppm by mass or less.

[0009] 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. Furthermore, "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.

[0010] The ink of the present invention has excellent water repellency against recording heads using silicon or glass members, long-term ejection reliability in ink-jet recording, and can produce recorded products with excellent image fastness. The reasons for this are not clear, but are thought to be as follows. In recording heads in which nozzle plates using silicon or glass materials are processed using MEMS technology, silicate ions are usually eluted from the silicon, etc. However, the ink of the present invention contains silicate compounds such as colloidal silica and sodium silicate, which inhibits the elution of silicate ions from the glass or silicon, and is therefore thought to be able to inhibit the reduction in surface water repellency due to corrosion of the nozzle plate even when in contact with ink for a long period of time. Another method for improving the image robustness of recorded materials is to incorporate wax into the ink. By incorporating wax into the ink, it is possible to reduce the frictional resistance of the surface of the recorded material, but because the highly hydrophobic wax is dispersed in the ink, it acts as a nucleus, accelerating the recrystallization and Ostwald ripening of silicate compounds and resulting in the significant generation of coarse particles, which creates a new problem of deteriorating long-term ejection reliability. Therefore, by incorporating wax into the ink of the present invention and setting the content of the silicic acid compound in the ink to between 2.5 ppm and 250 ppm by mass, it is possible to suppress elution of the nozzle plate member into the ink and the generation of coarse particles due to crystallization of the silicic acid compound, thereby ensuring long-term ejection reliability. Furthermore, because this specific amount of silicic acid compound can localize the wax on the surface of the recorded material, it is possible to significantly reduce the frictional resistance of the ink coating compared to when no silicic acid compound is present or when the amount is below a certain specific amount, and it is possible to significantly improve the image robustness of the resulting recorded material.

[0011] <Pigments> The pigment used in the present invention may be either an organic pigment or an inorganic pigment. 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. The hue is not particularly limited, and any chromatic pigment such as yellow, magenta, cyan, blue, red, orange, or green 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. Examples of inorganic pigments include metal oxides such as titanium oxide and zinc oxide, metal salts such as calcium carbonate and calcium sulfate, and carbon black, with carbon black being preferred. The above pigments can be used alone or in combination of two or more.

[0012] The form of the pigment can be (i) a pigment that can maintain a dispersed state without a dispersant, i.e., a self-dispersing pigment, (ii) a pigment particle form in which the pigment is dispersed with a surfactant, or (iii) a polymer particle form containing the pigment. Of these, from the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded materials, the form of "polymer particles containing the pigment" is preferred, and from the same viewpoint, the form of "crosslinked polymer particles containing the pigment" described below is more preferred. In this specification, the form of polymer particles containing a pigment means particles in which the polymer encapsulates the pigment, particles consisting of a polymer and a pigment with part of the pigment exposed on the surface, particles in which the polymer is adsorbed to part of the pigment, and mixtures thereof.

[0013] (Pigment-containing polymer particles) The pigment-containing polymer particles are composed of the above-mentioned pigment and a polymer (hereinafter also referred to as "polymer (a)") that has the function of dispersing the pigment in a medium whose main component is water. There are no particular limitations on the polymer (a) as long as it is a polymer that has the function of dispersing the pigment in an aqueous medium whose main component is water. The polymer (a) is preferably a water-insoluble polymer. As used herein, the term "water-insoluble polymer" refers to a polymer that, when dried at 105°C for 2 hours and allowed to reach a constant weight, dissolves in 100 g of water at 25°C in an amount of less than 10 g. When the polymer is an anionic polymer, the amount of dissolution is the amount of dissolution when 100 mol % of the anionic groups of the polymer are neutralized with sodium hydroxide.

[0014] (Polymer (a)) The polymer (a) may have any structure, but from the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded matter, a vinyl 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 preferred, and (a-2) a copolymer polymer having a structural unit derived from an ionic monomer and a structural unit derived from a hydrophobic monomer is more preferred.

[0015] [(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 a carboxylic acid monomer, a sulfonic acid monomer, a phosphoric acid monomer, etc. Examples of the carboxylic acid monomer include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid. Among these, carboxylic acid monomers are more preferred, and one or more selected from acrylic acid and methacrylic acid are more preferred, with acrylic acid being more preferred.

[0016] [(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 a further 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, and styrene is more preferred.

[0017] [(a-3) Nonionic Monomer] (a-3) The nonionic monomer 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 with 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 component (a-3) include those described in paragraph

[0018] of JP-A No. 2018-83938. 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.

[0018] From the above viewpoints, the polymer (a) preferably has an (a-1) component of at least one selected from acrylic acid and methacrylic acid, and an (a-2) component of at least one selected from styrene and α-methylstyrene, and is more preferably a styrene-(meth)acrylic acid copolymer.

[0019] [Content of each structural unit in polymer (a)] The content of each component in the monomer mixture during the production of polymer (a) (content as unneutralized amount; the same applies hereinafter) or the content of the structural unit derived from each component in polymer (a) is as follows, from the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded matter. The content of component (a-1) is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less. The content of component (a-2) is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

[0020] When the component (a-3) is contained, the content of the component (a-3) is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and still more preferably 5% by mass or less. The mass ratio of [component (a-1) / component (a-2)] is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.35 or more, still more preferably 0.38 or more, and is preferably 2 or less, more preferably 1.5 or less, even more preferably 1 or less, still more preferably 0.8 or less, and particularly preferably 0.6 or less.

[0021] [Production of polymer (a)] The polymer (a) is produced by copolymerizing a monomer mixture containing the components (a-1) and (a-2), and optionally the component (a-3), by a known polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. Among these polymerization methods, the solution polymerization method is preferred.

[0022] The acid value of polymer (a) is derived from the carboxy group, and from the viewpoint of improving the long-term ejection reliability and storage stability of the ink of the present invention, the acid value is preferably 180 mgKOH / g or more, more preferably 200 mgKOH / g or more, even more preferably 220 mgKOH / g or more, and preferably 320 mgKOH / g or less, more preferably 300 mgKOH / g or less, even more preferably 280 mgKOH / g or less. The acid value of the polymer (a) can be measured by the method described in the Examples, or can be calculated from the mass ratio of the constituent monomers. From the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded materials, the number average molecular weight of the polymer (a) is preferably 4,000 or more, more preferably 6,000 or more, even more preferably 8,000 or more, and is preferably 80,000 or less, more preferably 50,000 or less, even more preferably 30,000 or less. The number average molecular weight is measured by the method described in the examples.

[0023] [Neutralization] At least a portion of the carboxy groups of the polymer (a) is preferably neutralized with an alkali metal compound or the like. Examples of the alkali metal compound include one or more compounds selected from alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; alkali metal salts of carbonate such as disodium carbonate, sodium hydrogen carbonate, and dipotassium carbonate; and alkali metal salts of boric acid such as sodium borate. Among these, alkali metal hydroxides are preferred, sodium hydroxide and potassium hydroxide are more preferred, and sodium hydroxide is even more preferred.

[0024] From the viewpoint of ensuring the dispersion stability of the pigment, the degree of neutralization of the polymer (a) is preferably 15 mol% or more, more preferably 20 mol% or more, even more preferably 30 mol% or more, and is preferably 150 mol% or less, more preferably 100 mol% or less, even more preferably 80 mol% or less. Here, the degree of neutralization (mol %) is calculated by the following formula. Degree of neutralization (mol %) = [number of moles of alkali metal compound / number of moles of carboxyl groups in polymer (a)] × 100 In the present invention, when the alkali metal compound is used in excess of the number of moles of carboxy groups in the polymer (a), the degree of neutralization may exceed 100 mol %.

[0025] [Crosslinking of polymer (a)] The polymer (a) is preferably crosslinked using a crosslinking agent to form a crosslinked structure. That is, in the present invention, the pigment is preferably a crosslinked polymer particle containing the pigment. In this case, the polymer constituting the crosslinked polymer particle containing the pigment is composed of a component derived from the polymer (a) and a component derived from the crosslinking agent. This crosslinking treatment converts the two-dimensional structure of the polymer (a) into a three-dimensional structure, and the polymer is firmly adsorbed or fixed to the pigment surface, suppressing aggregation of the pigment in the ink of the present invention and further suppressing swelling of the polymer. This is thought to improve the long-term ejection reliability and storage stability of the ink of the present invention, as well as improving the color development and abrasion resistance of the recorded material and improving image fastness. The crosslinking agent is preferably a compound having two or more epoxy groups in the molecule. The crosslinking agent may be water-soluble or water-insoluble, but from the viewpoint of more efficiently crosslinking with the carboxyl group of the polymer (a) in a water-based medium, its water solubility is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Here, the water solubility (% by mass) refers to the solubility (% by mass) of the crosslinking agent when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of water at room temperature (25°C). The water solubility (% by mass) can be specifically measured by the method described in the Examples.

[0026] The compound having two or more epoxy groups in the molecule is preferably a compound having two or more glycidyl ether 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. The epoxy equivalent of the compound having two or more epoxy groups in the molecule is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and is preferably 300 or less, more preferably 200 or less, even more preferably 170 or less, from the viewpoint of more efficiently crosslinking with the carboxyl group of the polymer (a) in a medium mainly composed of water.

[0027] Specific examples of the compound having two or more epoxy groups in the molecule include one or more selected from polyglycidyl ethers such as cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether. Among these, one or more selected from cyclohexanedimethanol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether are preferred.

[0028] (Production of pigment-containing crosslinked polymer particles) The pigment-containing crosslinked polymer particles can be efficiently produced by a method including the following steps 1 to 3. Step 1: Neutralizing at least a portion of the carboxy groups of the polymer (a) with an alkali metal compound to obtain an aqueous dispersion of the polymer (a). Step 2: A step of dispersing the aqueous dispersion of polymer (a) obtained in Step 1 and a pigment to obtain an aqueous pigment dispersion of polymer particles containing the pigment dispersed in polymer (a). Step 3: A step of adding an epoxy compound to the pigment aqueous dispersion obtained in Step 2 and subjecting it to a crosslinking treatment to obtain an aqueous dispersion of crosslinked polymer particles containing the pigment.

[0029] The neutralization in step 1 is preferably carried out so that the pH is 7 or more and 11 or less. The degree of neutralization of the alkali metal compound used for neutralization and the polymer (a) is as described above. In the dispersion treatment in step 2, the pigment particles can be atomized to the desired particle size by main dispersion using shear stress alone, but from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further perform main dispersion. As a disperser used for pre-dispersion, a commonly used mixing and stirring device such as an anchor blade or a disperser blade can be used. Examples of dispersing machines used for this dispersion include kneading machines such as a roll mill and a kneader, high-pressure homogenizers such as a microfluidizer, and media-type dispersing machines such as a paint shaker and a bead mill. Among these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When the dispersion treatment is carried out using a high-pressure homogenizer, the average particle size of the pigment particles in the aqueous 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 or more and 30 or less.

[0030] In step 3, the polymer (a) dispersing the pigment in the pigment aqueous dispersion is crosslinked by the epoxy compound to form a crosslinked polymer, thereby obtaining an aqueous dispersion in which particles of the crosslinked polymer containing the pigment are dispersed in an aqueous medium. The preferred epoxy compounds are as described above. From the viewpoints of completion of the crosslinking reaction and economy, the temperature of the crosslinking treatment is preferably 50° C. or higher, more preferably 70° C. or higher, and preferably 95° C. or lower, more preferably 92° C. or lower. From the same viewpoints as above, the time of the crosslinking treatment is preferably 0.5 hours or higher, more preferably 1 hour or higher, and preferably 10 hours or lower, more preferably 6 hours or lower.

[0031] The acid value of the polymer constituting the pigment-containing crosslinked polymer particles is preferably 90 mgKOH / g or more, more preferably 95 mgKOH / g or more, even more preferably 180 mgKOH / g or more, and is preferably 200 mgKOH / g or less, more preferably 210 mgKOH / g or less, even more preferably 160 mgKOH / g or less.

[0032] From the viewpoint of facilitating the preparation of the ink of the present invention, the concentration of non-volatile components (solid content concentration) of the resulting pigment aqueous dispersion is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 30% by mass or less, more preferably 25% by mass or less. The solid content concentration of the pigment water dispersion is measured by the method described in the examples.

[0033] From the viewpoint of improving the color development of recorded matter, the content of the pigment in the resulting pigment aqueous dispersion is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 18% by mass or less.

[0034] From the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded matter, the crosslinking rate of polymer (a) is preferably 20 mol % or more, more preferably 30 mol % or more, even more preferably 40 mol % or more, and is preferably 80 mol % or less, more preferably 70 mol % or less, even more preferably 60 mol % or less. Here, the crosslinking rate (mol %) is calculated by (mol number of epoxy groups in the epoxy compound / mol number of carboxy groups in the polymer (a))×100.

[0035] The average particle size of the crosslinked polymer particles containing the pigment in the pigment aqueous dispersion is preferably 60 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and is preferably 200 nm or less, more preferably 160 nm or less, even more preferably 120 nm or less, from the viewpoints of reducing coarse particles and improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded images. The average particle size is measured by the method described in the examples. The average particle size of the crosslinked polymer particles containing the pigment in the aqueous ink after preparation of the aqueous ink is substantially the same as the average particle size in the pigment aqueous dispersion.

[0036] <Pigment-free polymer particles> The ink of the present invention contains polymer particles that do not contain pigment, from the viewpoints of improving the fixability of the ink of the present invention to a recording medium, achieving high levels of both color development and abrasion resistance of the recorded material, and improving the image fastness of the recorded material. Examples of the polymer constituting the pigment-free polymer particles (hereinafter also referred to as "polymer (b)") include (meth)acrylic resins, styrene resins, urethane resins, polyester resins, butadiene resins, vinyl chloride resins, etc. Among these, from the viewpoint of improving the hiding power of the ink of the present invention and the blocking resistance of the recorded matter, (meth)acrylic resins are preferred, and styrene-(meth)acrylic resins are more preferred. The polymer particles not containing a pigment are preferably used as an aqueous dispersion in which they are dispersed in water. The polymer (b) may be an appropriately synthesized product or a commercially available product. Furthermore, from the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of the recorded matter, it is preferable that the polymer (b) has a crosslinked structure crosslinked with a crosslinking agent, i.e., is a crosslinked polymer particle containing no pigment. The crosslinked polymer particle containing no pigment is composed of a component derived from the polymer (b) and a component derived from the crosslinking agent.

[0037] [Polymer (b)] The (meth)acrylic resin as the polymer (b) preferably has (b-1) a structural unit derived from a carboxy group-containing vinyl monomer and (b-2) a structural unit derived from a hydrophobic vinyl monomer. Examples of the component (b-1) include the same carboxylic acid monomers as those of the component (a-1). Among them, from the viewpoint of improving fixability to a recording medium and improving image fastness of a recorded product, one or more selected from acrylic acid and methacrylic acid are preferred, and acrylic acid is more preferred. As the component (b-2), the same alkyl (meth)acrylates and aromatic group-containing monomers as those of the component (a-2) are preferably used. Among them, styrene-based monomers are preferred, and one or more selected from styrene and α-methylstyrene are preferred, with styrene being more preferred. The monomer components contained in the above components (b-1) and (b-2) can be used alone or in combination of two or more.

[0038] [Content of each structural unit in polymer (b)] The content of the structural units derived from the components (b-1) and (b-2) in the polymer (b) is as follows, from the viewpoint of improving fixability to a recording medium and improving image fastness of the recorded material. The content of the (b-1) component is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and is preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less. The content of the (b-2) component is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

[0039] The polymer (b) can be produced by copolymerizing a monomer mixture containing the components (b-1) and (b-2) by a known solution polymerization method or the like. The methods for producing, neutralizing, and crosslinking polymer (b) are the same as those for producing, neutralizing, and crosslinking polymer (a) described above, and therefore will not be described here.

[0040] From the viewpoint of improving fixability to a recording medium and improving image fastness of a recorded product, the acid value of polymer (b) is preferably 180 mgKOH / g or more, more preferably 200 mgKOH / g or more, even more preferably 220 mgKOH / g or more, and is preferably 320 mgKOH / g or less, more preferably 300 mgKOH / g or less, even more preferably 280 mgKOH / g or less. The number average molecular weight of polymer (b) is preferably 4,000 or more, more preferably 6,000 or more, even more preferably 8,000 or more, and preferably 80,000 or less, more preferably 50,000 or less, even more preferably 30,000 or less. The acid value and number average molecular weight of the polymer (b) can be measured in the same manner as in the case of the polymer (a).

[0041] The average particle size of the pigment-free polymer particles is preferably 60 nm or more, more preferably 80 nm or more, and even more preferably 100 nm or more, from the viewpoint of improving fixation to a recording medium and improving image fastness of the recorded material, and is preferably 200 nm or less, more preferably 160 nm or less, and even more preferably 150 nm or less. The average particle size of the polymer particles not containing a pigment is measured by the method described in the Examples. The average particle size of the pigment-free polymer particles in the ink of the present invention after preparation of the ink of the present invention is substantially the same as the average particle size of the pigment-free polymer particles prepared before preparation of the water-based ink.

[0042] Polymer (a) and polymer (b) may be the same or different, i.e., polymer (a) and polymer (b) may have different compositions (structures), or may be the same polymer including the composition (structure) and differ only in the presence or absence of a pigment. Commercially available polymers (a) and (b) may also be used. Examples of commercially available dispersions of polymers (a) and (b) that can be used include Neocryl A1127 (anionic self-crosslinking waterborne acrylic resin) manufactured by DSM Neo Resins, Joncryl 390 (acrylic resin) manufactured by BASF, Joncryl PDX-7775 (styrene-acrylic resin) manufactured by BASF, and Vinyblan 700 (vinyl chloride-acrylic resin) manufactured by Nissin Chemical Industry Co., Ltd. When the polymer particles containing a pigment and the polymer particles not containing a pigment are both crosslinked polymer particles containing a pigment and crosslinked polymer particles not containing a pigment, it is preferable that the crosslinking agent is the same. Furthermore, when the polymer particles containing a pigment and the polymer particles not containing a pigment are both crosslinked polymer particles containing a pigment and crosslinked polymer particles not containing a pigment, it is preferable that the polymer (a) and the polymer (b) are the same, and further that the crosslinking agent is also the same.

[0043] <Wax> The wax used in the present invention is used from the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of the recorded matter. From the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded matter, the melting point of the wax is preferably 85°C or higher, more preferably 90°C or higher, even more preferably 95°C or higher, still more preferably 100°C or higher, and even more preferably 110°C or higher, and is preferably 150°C or lower, more preferably 145°C or lower, and even more preferably 140°C or lower. Examples of waxes include polyolefin waxes containing an olefin monomer as a main component, paraffin waxes consisting of a mixture of chain saturated hydrocarbons having 20 to 30 carbon atoms, and synthetic waxes such as Sasol wax. Among these, one or more selected from polyolefin waxes and paraffin waxes are preferred, and polyolefin waxes are more preferred.

[0044] Examples of olefin monomers used in polyolefin waxes include linear olefins and cyclic olefins, with those primarily composed of linear olefins having 2 to 6 carbon atoms being preferred, and polyolefin waxes (polyethylene waxes) primarily composed of ethylene being more preferred. Here, "primarily composed of ethylene" means that the ethylene content of 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, based on the total components constituting the wax. Oxidized polyolefin wax can be obtained by adjusting a high molecular weight polyolefin polymer 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 a polyolefin wax. The content of the polyolefin 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.

[0045] The wax is preferably in the form of particles dispersed in an aqueous medium, that is, in the form of a wax emulsion. There are no particular limitations on the method for producing the wax emulsion, and examples thereof include a method in which a polyolefin wax, other waxes that are used as needed, and a known surfactant are mixed and emulsified. In the present invention, the wax is preferably in the form of particles dispersed in an aqueous medium using one or more surfactants selected from nonionic surfactants and anionic surfactants. Examples of nonionic surfactants include ethylene oxide adducts of higher alcohols and ethylene oxide adducts of alkylated phenols. Examples of anionic surfactants include sulfates and phosphates based on ethylene oxide adducts of higher alcohols, and alkylated benzenesulfonates. Among these, from the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded matter, preferred are nonionic wax emulsions in which wax is emulsified with a nonionic surfactant or anionic wax emulsions in which wax is emulsified with an anionic surfactant, and more preferred are nonionic wax emulsions in which wax is emulsified with a nonionic surfactant. That is, the wax is preferably dispersed with one or more surfactants selected from nonionic surfactants and anionic surfactants, and more preferably dispersed with a nonionic surfactant.

[0046] The average particle size of the wax emulsion is preferably 500 nm or less, more preferably 300 nm or less, and even more preferably 100 nm or less, from the viewpoint of improving the dispersion stability of the wax emulsion and improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded matter, and is preferably 10 nm or more, and preferably 30 nm or more. The average particle size of the wax emulsion can be measured by dynamic light scattering, for example, using a Microtrac particle size analyzer UPA manufactured by Nikkiso Co., Ltd., according to the method described in the Examples. The average particle size of the wax emulsion in the water-based ink after preparation of the water-based ink is substantially the same as the average particle size of the wax emulsion prepared before preparation of the water-based ink. Suitable examples of commercially available wax emulsions include polyethylene wax emulsions such as Hitec E-6500, E-6400, and E-8237 manufactured by Toho Chemical Industry Co., Ltd.; AQUACER 507, 513, 515, 526, 531, 533, 537, 539, 552, and 1547 manufactured by BYK; Cellosol 428, H620, 686, 524, Trasol CN, Polylon L-787, and L-788 manufactured by Chukyo Yushi Co., Ltd.; and Chemipearl W900 and W4005 manufactured by Mitsui Chemicals, Inc.

[0047] <Organic solvents> The organic solvent used in the present invention mainly serves to improve the storage stability and maintenance properties of the ink of the present invention, and to impart wetting and spreading properties to the ink of the present invention on a recording medium. The organic solvent may be liquid or solid at 25° C., but a water-soluble organic solvent that dissolves in 100 mL of water at 25° C. in a volume of 10 mL or more is preferred. From the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded materials, the boiling point of the water-soluble organic solvent is preferably 110°C or higher, more preferably 130°C or higher, even more preferably 150°C or higher, and is preferably 250°C or lower, more preferably 240°C or lower, even more preferably 235°C or lower. From the same viewpoint as above, the water-soluble organic solvent preferably contains at least one solvent selected from glycol-based solvents and glycol ether-based solvents. Examples of glycol solvents include ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-hexanediol, 1,2-octanediol, 1,8-octanediol, 1,2-decanediol, 1,3-propanediol, 1,4-butanediol, 2-ethyl-1,3-hexanediol, diethylene glycol, dipropylene glycol, and glycerin. Among these, one or more selected from propylene glycol, 1,2-butanediol, 1,3-propanediol, and 1,4-butanediol are preferred, with propylene glycol being more preferred.

[0048] Examples of glycol ether solvents include ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, and tripropylene glycol monomethyl ether. Among these, one or more selected from dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether are preferred, and diethylene glycol monoisopropyl ether is more preferred. In the present invention, it is preferable to contain one or more glycol solvents and one or more glycol ether solvents. In this case, preferred examples of the glycol solvent are one or more selected from propylene glycol, 1,2-butanediol, 1,3-propanediol, and 1,4-butanediol, and propylene glycol is more preferred. Preferred examples of the glycol ether solvent are one or more selected from dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether, and diethylene glycol monoisopropyl ether is more preferred. The organic solvent may further contain organic solvents other than those mentioned above, within the range that does not impair the effects of the present invention. The organic solvent used herein preferably contains one or more organic solvents having a boiling point of 90°C or higher, and preferably has a weighted average boiling point of 250°C or lower. The weighted average boiling point of the organic solvent is preferably 150°C or higher, more preferably 180°C or higher, and is preferably 240°C or lower, more preferably 220°C or lower, and even more preferably 200°C or lower. The total organic solvent content in the ink is preferably 35% by mass or less, more preferably 30% by mass or less, and is preferably 20% by mass or more.

[0049] <Silicate compounds> The ink of the present invention contains a silicate compound from the viewpoint of suppressing the elution of silicate ions from glass or silicon and suppressing the decrease in surface water repellency that accompanies corrosion of the nozzle plate when it comes into contact with the ink for a long period of time. The silicic acid compound may be selected from a wide range of silicic acids and silicates. As the silicic acid, silicic anhydride is preferred, and colloidal silica is more preferred. As the silicate, alkali metal salts and alkaline earth metal salts of silicic acid are preferred, and alkali metal salts are more preferred. Among them, the silicic acid compound is preferably one or more selected from sodium silicate, potassium silicate, calcium silicate, magnesium silicate, and silicic anhydride. Of these silicate compounds, one or more selected from sodium silicate and potassium silicate are more preferred from the viewpoint of improving the stability over time of the ink of the present invention during storage by suppressing recrystallization and Ostwald ripening of the silicate compound and suppressing the generation of coarse particles, and sodium silicate is even more preferred from the viewpoint of the long-term ejection reliability of the ink of the present invention and suppressing a decrease in the water repellency of the nozzle plate surface when the ink of the present invention comes into contact with it. As the sodium silicate, industrially used sodium silicate can be used, and its standard is specified in JIS K 1408. Any of No. 1 sodium silicate, No. 2 sodium silicate, and No. 3 sodium silicate in solution, and solid sodium metasilicate can be used. Of these, from the viewpoint of blending stability with the ink, it is preferable to use No. 1 sodium silicate, No. 2 sodium silicate, and No. 3 sodium silicate in solution. Furthermore, from the viewpoint of improving the long-term jetting reliability of the ink of the present invention and from the viewpoint of preventing a decrease in the water repellency of the nozzle plate surface when it comes into contact with the ink of the present invention, it is more preferable to use No. 3 sodium silicate. These industrially used sodium silicates may be added as they are when blended into the ink, or may be diluted with water before blending.

[0050] <Surfactant> The ink of the present invention may contain a surfactant other than the surfactant that disperses the wax. Examples of surfactants that may be contained in addition to the surfactant that disperses the wax include nonionic surfactants, silicone surfactants, and fluorine 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, polyoxyalkylene alkyl ether surfactants and acetylene glycol surfactants are preferred, and for example, it is more preferred to use 2,4,7,9-tetramethyl-5-decyne-4,7-diol in combination with a polyoxyalkylene alkyl ether surfactant. Examples of commercially available nonionic surfactants include the "Surfynol" series manufactured by Nissin Chemical Industry Co., Ltd. and Air Products & Chemicals, the "Acetylenol" series manufactured by Kawaken Fine Chemicals Co., Ltd., and the "Emulgen" series manufactured by Kao Corporation. The above surfactants can be used alone or in combination of two or more.

[0051] <Ink manufacturing method> The ink of the present invention can be obtained by mixing the above-mentioned pigment, pigment-free polymer particles, wax, an organic solvent, a silicate compound, water, and, if necessary, various additives commonly used in inks, such as a humectant, wetting agent, penetrant, surfactant, viscosity modifier, antifoaming agent, preservative, antifungal agent, and antirust agent.

[0052] <Content of each ink component> The content of each component in the ink of the present invention is as follows, from the viewpoint of improving the long-term ejection reliability and drying properties of the ink of the present invention, as well as the image fastness of the recorded material.

[0053] (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 3% by mass or more, even more preferably 4% by mass or more, and is preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less, and even more preferably 7% by mass or less.

[0054] The content of non-pigmented polymer particles in the 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 15% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less.

[0055] (Wax content) The wax content in the ink of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.5% by mass or more, still more preferably 0.8% by mass or more, and preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, and still more preferably 1.5% by mass or less.

[0056] (Organic solvent content) The content of organic solvent in the ink of the present invention is preferably 35% by mass or less, more preferably 30% by mass or less, even more preferably 28% by mass or less, and is preferably 15% by mass or more, more preferably 18% by mass or more, even more preferably 20% by mass or more.

[0057] The content of the silicic acid compound in the ink of the present invention is 2.5 ppm by mass or more, preferably 3 ppm by mass or more, more preferably 3.5 ppm by mass or more, even more preferably 4 ppm by mass or more, still more preferably 30 ppm by mass or more, and even more preferably 50 ppm by mass or more, from the viewpoint of suppressing a decrease in the water repellency of the nozzle plate surface upon contact with the ink of the present invention. Furthermore, from the viewpoint of suppressing recrystallization and Ostwald ripening of the silicic acid compound and suppressing the generation of coarse particles, thereby improving the stability over time of the ink of the present invention during storage and improving the long-term jetting reliability of the ink of the present invention, the content is 250 ppm by mass or less, preferably 230 ppm by mass or less, more preferably 200 ppm by mass or less, even more preferably 170 ppm by mass or less, still more preferably 150 ppm by mass or less, still more preferably 120 ppm by mass or less, and even more preferably 100 ppm by mass or less.

[0058] (mass ratio of wax to silicate compound in ink) The mass ratio of wax to silicate compound in the ink of the present invention [wax / silicic acid compound] is preferably 10 or more, more preferably 25 or more, even more preferably 50 or more, and still more preferably 100 or more, from the viewpoint of suppressing a decrease in the water repellency of the nozzle plate, and is preferably 5000 or less, more preferably 1000 or less, even more preferably 700 or less, and still more preferably 500 or less, from the viewpoint of ensuring the image fastness of the recorded matter.

[0059] 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 is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less.

[0060] <Ink properties> From the viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded images, 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 4 mPa s or more, and preferably 10 mPa s or less, more preferably 7 mPa s or less, even more preferably 6 mPa s or less. In the present invention, the viscosity of the ink can be measured using an E-type viscometer. The pH of the 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 viewpoint of improving the long-term ejection reliability of the ink of the present invention and the image fastness of recorded images. Furthermore, from the viewpoint of preventing a decrease in the water repellency of the nozzle plate upon contact with the ink of the present invention, the pH is preferably 10 or lower, more preferably 9.5 or lower, and even more preferably 9.0 or lower. The pH of the water-based ink can be measured by a conventional method.

[0061] [Inkjet recording method] The water-based ink for ink-jet printing of the present invention is characterized in that the ink is ejected from a recording head using one or more materials selected from silicon and silicon oxide as a nozzle plate member. Nozzle plates made of silicon and silicon oxide have a water-repellent film formed on the surface of the nozzle plate, thereby improving the ink repellency of the nozzle plate surface. As a recording head that uses silicon or the like as a nozzle plate member, commercially available recording heads can be used, such as Samba G3L and Samba G5L manufactured by Fujifilm Dimatix Corporation, and S3200, S800, I3200, I1600, and D3000 manufactured by Seiko Epson Corporation.

[0062] The ink of the present invention can be loaded into a known inkjet recording device such as a piezoelectric type and ejected as ink droplets onto a recording medium to obtain a recorded product. As the inkjet recording medium, highly water-absorbent plain paper, low water-absorbent coated paper, and non-water-absorbent resin film can be used. Examples of plain paper include "4200" (manufactured by Fuji Xerox Co., Ltd.) and "NPi Form NEXT-IJ" (manufactured by Nippon Paper Industries Co., Ltd.). Examples of coated paper include the general-purpose glossy paper "OK ​​Top Coat Plus" (manufactured by Oji Paper Co., Ltd.), Multicolor Foam Gloss Paper (manufactured by Oji Paper Co., Ltd.), UPM Finesse Gloss (manufactured by UPM), UPM Finesse Matt (manufactured by UPM), TerraPress Silk (manufactured by Stora Enso), and LumiArt (manufactured by Stora Enso). Examples of the resin film include transparent synthetic resin films, such as films of polyester, polyvinyl chloride, polyolefin, nylon, etc. Among these, polyester film and oriented polypropylene film are preferred, and those that have been subjected to corona discharge treatment are more preferred.

[0063] Even when the ink of the present invention is used in a recording head equipped with a nozzle plate using a silicon or glass member, it is possible to achieve both long-term ejection reliability by suppressing the elution of the silicon member and the like and the occurrence of agglomerated foreign matter in the ink, and high image robustness in the resulting recorded matter. [Example]

[0064] In the following Preparation Examples, 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.

[0065] <Measurement> (1) Measurement of polymer number average molecular weight The measurements were performed using gel permeation chromatography (GPC) on a Tosoh GPC system (HLC-8320GPC) with Tosoh columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guard column Super AW-H) at a flow rate of 0.5 mL / min, using N,N-dimethylformamide containing phosphate and lithium bromide at concentrations of 60 mmol / L and 50 mmol / L, respectively, as the eluent. The measurements were performed using a monodisperse polystyrene kit (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), both manufactured by Tosoh) with known molecular weights as standards. The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring at 25° C. for 10 hours, and filtering through a syringe filter (DISMIC-13HP, made of PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0066] (2) Measurement of the solids concentration of pigment dispersions 10.0 g of sodium sulfate, which 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. The mixture was then accurately weighed, kept at 105°C for 2 hours to remove volatiles, and left in the desiccator for a further 15 minutes before the mass was measured. The mass of the sample after volatile matter removal was taken as the solid content, and divided by the initial mass of the sample to obtain the solid content concentration.

[0067] (3) Average particle size of pigment-containing polymer particles and pigment-free polymer particles Using a laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd., product name: ELS-8000), the average particle size of the polymer particles was measured from the aqueous dispersion of the polymer particles by dynamic light scattering, and calculated by cumulant analysis. The measurement conditions were a temperature of 25°C, an angle between the incident light and the detector of 90°, and 100 accumulations, and the refractive index of water (1.333) was input as the refractive index of the dispersion solvent. For the measurement sample, the aqueous dispersion of the polymer particles was weighed into a screw tube (manufactured by Maruemu Co., Ltd., No. 5), and the solids concentration was 2 × 10 -4 Water was added to the mixture so that the concentration reached 5% by mass, and the mixture was stirred at 25°C for 1 hour using a magnetic stirrer.

[0068] (4) Measurement of polymer acid value The resin was dissolved in a titration solvent consisting of a mixture of toluene and acetone (2:1 by mass) in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with a 0.1 N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was taken as the endpoint. The acid value (mg KOH / g) was calculated from the titration volume of the potassium hydroxide solution up to the endpoint.

[0069] (5) Measurement of wax melting point 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 then cooled to 0°C at a rate of 10°C / min. The sample was then heated at a rate of 10°C / min, and the calorific value was measured up to 200°C. The temperature of the peak with the largest peak area among the observed heat of fusion peaks was taken as the maximum peak temperature of melting, and this peak temperature was taken as the melting point.

[0070] (6) Measurement of the average particle size of wax emulsion The average particle size (average dispersed particle size) of the wax emulsion was measured using a Microtrac particle size analyzer UPA manufactured by Nikkiso Co., Ltd.

[0071] (7) Measurement of water solubility of crosslinking agent At room temperature (25°C), 90 parts by mass of ion-exchanged water and 10 parts by mass of crosslinker (W1) were added to a glass tube (25 mmφ×250 mmh), and the glass tube was left to stand for 1 hour in a thermostatic bath adjusted to a water temperature of 25°C. The glass tube was then vigorously shaken for 1 minute and then left to stand again in the thermostatic bath for 12 hours. The undissolved matter that separated from the water and precipitated or floated was then collected, dried for 6 hours at 40°C under a gauge pressure of -0.08 MPa, and then weighed (W2). The water solubility (mass%) was calculated using the following formula (1): Water solubility (mass%)={(W1-W2) / W1}×100 (1)

[0072] (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.).

[0073] Preparation Example 1 (Preparation of Polymer (a)) A monomer mixture was prepared by mixing 31 parts of acrylic acid and 69 parts of styrene. 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 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. The monomer mixture in the reaction vessel was heated to 65°C under a nitrogen atmosphere while 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, followed by drying under reduced pressure to obtain polymer (a) (number average molecular weight: 12,000, acid value: 240 mgKOH / g).

[0074] Production Example I-1 (Production of pigment-containing crosslinked polymer particles) (Process 1) 32 parts of the polymer (a) obtained in Preparation Example 1 was mixed with 202 parts of ion-exchanged water, and 12.8 parts of a 5N aqueous sodium hydroxide solution (solid sodium hydroxide content: 16.9%) was added to neutralize the mixture so that the ratio of the number of moles of sodium hydroxide to the number of moles of carboxyl groups in the polymer (a) was 40% (neutralization degree: 40 mol%). The mixture was heated to 90°C using a warm bath and stirred for 1 hour to completely disperse the polymer in water, thereby obtaining a polymer dispersion. (Process 2) The polymer dispersion obtained in step 1 was cooled to room temperature (25°C) and then 100 parts of a cyan pigment (CI Pigment Blue 15:3, manufactured by DIC Corporation, product name: TGR-SD) was added. The mixture was stirred for 3 hours at 20°C using a Disper (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) with the Disper blade rotating at 6,000 rpm. Next, 124 parts of ion-exchanged water was added, and the mixture was dispersed 15 times using a Microfluidizer (manufactured by Microfluidics, product name: Microfluidics) at a pressure of 150 MPa. The resulting dispersion was placed in a 500 mL angle rotor and centrifuged at 3,660 rpm for 20 minutes using a high-speed refrigerated centrifuge (manufactured by Hitachi Koki Co., Ltd., product name: Himac CR22G, set temperature: 20°C). The liquid layer was then recovered and filtered through a 5 μm membrane filter (manufactured by Sartorius, product name: Minisart) to obtain a pigment aqueous dispersion. The solids concentration of the pigment aqueous dispersion was 25%. (Step 3) 100 parts of the aqueous pigment dispersion was placed in a screw-cap glass bottle, 32 parts of ion-exchanged water was added, and 1.8 parts of trimethylolpropane polyglycidyl ether (Nagase ChemteX Corporation, trade name: Denacol EX-321LT, epoxy equivalent: 140, water solubility: 27%) was added as a crosslinker. The bottle was sealed and heated at 70°C for 5 hours with stirring. The crosslinking was carried out using an amount of crosslinker sufficient to react with 50% of the total carboxyl groups in the polymer (crosslinking rate: 50 mol%). After 5 hours, the dispersion was cooled to room temperature (25°C) and filtered using a 25 mL needleless syringe (Terumo Corporation) equipped with a 5 μm filter to obtain an aqueous dispersion of pigment-containing crosslinked polymer particles (acid value: 120 mg KOH / g) (solids concentration: 20%, pigment: 14.2%, polymer: 5.8%, average particle size: 105 nm).

[0075] Production Example I-2 (Production of aqueous dispersion of pigment-free crosslinked polymer particles) 15.3 parts of the polymer (a) obtained in Preparation Example 1 was mixed with 63.5 parts of ion-exchanged water, and 6.2 parts of a 5N aqueous sodium hydroxide solution (solid content: 16.9%) was further added to neutralize the mixture so that the ratio of the number of moles of sodium hydroxide to the number of moles of carboxyl groups in the polymer was 40% (neutralization degree: 40 mol%). The mixture was heated to 90°C using a warm bath and stirred for 1 hour to completely disperse the polymer in water, thereby obtaining a polymer dispersion. After cooling this polymer dispersion to room temperature, 4.6 parts of trimethylolpropane polyglycidyl ether (Nagase ChemteX Corporation, trade name: Denacol EX-321LT, epoxy equivalent: 140, water solubility: 27%) was added as a crosslinking agent, and the mixture was sealed and heated at 90°C for 1.5 hours while stirring with a stirrer. Crosslinking was performed using an amount of crosslinking agent sufficient to react with 50% of the total number of carboxyl groups contained in the polymer (crosslinking rate: 50 mol%). The polymer dispersion was then cooled to room temperature (25°C) and filtered using a 25 mL needleless syringe (Terumo Corporation) equipped with a 5 μm membrane filter (trade name: Minisart) to obtain an aqueous dispersion of pigment-free crosslinked polymer particles (solids concentration: 20%).

[0076] Example 1 (Production of Water-Based Ink) 28.2 parts of an aqueous dispersion of pigment-containing crosslinked polymer particles obtained in Production Example I-1 (PB15:3, solids concentration: 20%) (breakdown: 4.0 parts pigment, 1.6 parts crosslinked polymer, 22.6 parts ion-exchanged water), 25 parts of an aqueous dispersion 1 of pigment-free crosslinked polymer particles obtained in Production Example I-2 (solids concentration: 20%) (breakdown: 5.0 parts crosslinked polymer, 20.0 parts ion-exchanged water), polyethylene wax emulsion (Toho Chemical Industry Co., Ltd., Hi-Tec E-6500 Wax emulsion dispersed with nonionic surfactant, melting point 140°C, particle size 80nm, solid content 35%) 0.29 parts (breakdown: wax 0.10 parts, ion-exchanged water 0.19 parts), propylene glycol (PG) 20.0 parts, diethylene glycol monoisopropyl ether (iPDG, manufactured by Nippon Nyukazai Co., Ltd.) 5.0 parts, Surfynol 104PG-50 (manufactured by Nissin Chemical Industry Co., Ltd., acetylene glycol active content 50%) 2.0 parts, Emulgen 120 (manufactured by Kao Corporation, ethylene oxide adduct of lauryl alcohol) 0.5 parts, No. 3 sodium silicate (manufactured by Fuji Chemical Co., Ltd., Kao 0.0197 parts of aqueous sodium hydroxide solution (active ingredient 38% by mass) and an appropriate amount of 1N aqueous sodium hydroxide solution were mixed to adjust the ink pH to 8.5, and ion-exchanged water was added to make the total amount 100 parts. The mixture was filtered through a 25 mL needleless syringe (manufactured by Terumo Corporation) equipped with a 5 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to obtain water-based ink II-1 (pigment-containing crosslinked polymer particles: 5.6% (of which pigment 4.0%), pigment-free crosslinked polymer particles: 5.0%, wax 0.1%, silicon compound content 75 ppm).

[0077] Examples 2 to 10 and Comparative Examples 1 to 3 (Production of Water-Based Inks) Water-based inks II-1 to II-10 and II-11 to II-13 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 organic solvents and waxes in Table 1 are as follows: (organic solvent) PG: Propylene glycol iPDG: Diethylene glycol monoisopropyl ether, manufactured by Nippon Nyukazai Co., Ltd. (wax) Hi-Tech E-6500: Polyethylene wax emulsion. Wax emulsion dispersed with a nonionic surfactant. Manufactured by Toho Chemical Industry Co., Ltd., melting point 140°C, particle size 80 nm, solids concentration 35%. Hi-Tech E-8237: Polyethylene wax emulsion. Wax emulsion dispersed with a nonionic surfactant. Manufactured by Toho Chemical Industry Co., Ltd., melting point 106°C, average particle size 80nm, solids concentration 35%. AQUACER 537: Anionic emulsion of modified paraffin wax, manufactured by BYK, melting point 110°C, average particle size 61nm, solid content 30%

[0078] <Evaluation> Using the water-based inks obtained in the examples and comparative examples, the water repellency of the silicon nozzle plate used in the MEMS processed recording head, the long-term ejection reliability of the MEMS processed recording head, and the image robustness of the recorded material were evaluated by the following methods. The results are shown in Table 1.

[0079] (1) Water repellency evaluation of silicon nozzle plates A Samba G3L inkjet recording head manufactured by Fujifilm Dimatex Corporation, which is a recording head using a silicon oxide nozzle plate, was used as a test specimen, and the contact angle of water on the water-repellent film was measured as follows to evaluate the effect of the ink composition on the liquid repellency of the water-repellent film. 300 mL of the aqueous ink compositions obtained in the examples and comparative examples were weighed into 500 mL wide-mouth bottles (500 mL i-Boy wide-mouth bottles (manufactured by AS ONE Corporation)). The test pieces were then immersed in the ink compositions, and the containers were sealed and then left to stand in a thermostatic chamber set at 50°C for 28 days. The test pieces were then removed and washed with ion-exchanged water, and the water contact angle of the water-repellent film surface of the nozzle plate was measured. The water contact angle was measured using ion-exchanged water and a contact angle measuring device (DM-500, manufactured by Kyowa Interface Science Co., Ltd.) in an environment of 25°C and a relative humidity of 50% according to a conventional method. Regarding the water repellency of the silicon nozzle plate, if the contact angle is 80° or more, there is no practical problem, and if the contact angle is 90° or more, it can be used preferably. (2) Evaluation of long-term discharge reliability The aqueous inks obtained in the examples and comparative examples were placed in glass containers, sealed, and placed in a 50°C thermostatic chamber for 28 days as an accelerated test, followed by an additional 24 hours at room temperature. The inks were then loaded into a Fujifilm Dimatix Samba G3L inkjet recording head and ejected using a jetXpert inkjet liquid observation device (imageXpert) at a temperature of 25±1°C and a relative humidity of 30±5% to confirm their flight behavior. The ejection conditions were a single-pulse standard waveform, with the voltage adjusted to achieve a droplet volume of 2.4 pL, and the inks were continuously ejected for 30 minutes. After that, a drop located approximately 0.2 mm from the nozzle and a drop located approximately 0.6 mm from the nozzle were simultaneously photographed using a strobe, and the angle of the line connecting the two drops, which were approximately 0.4 mm apart, was measured to see if it deviated from the perpendicular (90 degrees) 1,000 times for each nozzle, and the standard deviation σ was calculated to be used as an index of ejection direction disturbance. The standard deviation σ was taken as the average value for 30 nozzles for one ink. If the two drops are ejected completely perpendicular from the nozzle plate, the standard deviation σ = 0. On the other hand, if the positions of the first and second drops deviate to the left or right of the perpendicular line from the nozzle plate, the value of the standard deviation σ will increase, and it will be evaluated as indicating that ejection direction disturbance has occurred. If the standard deviation σ is less than 15 mrad, there is no problem in practical use, but if it is less than 10 mrad, it can be used preferably, and if it is less than 5 mrad, it can be used more preferably.If it is 15 mrad or more, when the gap between the recording head and the paper surface is 1 mm, there is a high probability that the ink landing position will be shifted by 15 μm or more, so it is not practical to use.

[0080] (3) Evaluation of image robustness of recorded materials (3-1) Creation of Records In an environment of a temperature of 25±1°C and a relative humidity of 30±5%, the water-based inks obtained in the examples and comparative examples were filled into a printing evaluation device (manufactured by Altec Corporation) equipped with a Samba G3L inkjet recording head manufactured by Fujifilm Dimatix. The recording medium was coated paper "OK ​​Topcoat+" (Oji Paper Co., Ltd., product name, water absorption 4.9 g / m 2 ) was fixed to the conveyance table of the printing evaluation device under reduced pressure so that the longitudinal direction of the recording medium and the conveyance direction were aligned in the same direction. The ejection conditions were a single-pulse standard waveform, voltage adjusted to a droplet volume of 2.4 pL, a drive frequency of 40 kHz, a resolution of 1200 x 1200 dpi, and a 2 cm square solid image (image area for dot diameter measurement) printed using a one-pass method with a duty of 100%. Immediately after printing, the image was dried for 3 seconds using a short-wavelength infrared heater (Heraeus, product name: ZKB1200 / 340G x 9, heater effective length 340 mm, output 1200 W), resulting in a recorded product using the water-based ink of each example and comparative example. (3-2) Evaluation method for image robustness of recorded materials After leaving the recorded matter to stand for 24 hours, a piece of unprinted coated paper cut to 12 x 2 cm was placed on the printed surface of the recorded matter, and the coated paper was reciprocated 10 times at a load of 10 N using a Gakushin-type abrasion tester (manufactured by Daiei Scientific Instruments Manufacturing Co., Ltd., product name: RT-300). Thereafter, the concentration of the ink transferred to the coated paper was measured using a spectrophotometer (manufactured by Sakata Inx Engineering Corporation, product name: Spectro-Eye). The lower the density of the ink transferred to the coated paper, the better the image fastness is judged to be, and a numerical value of 0.3 or less is practically usable.

[0081] [Table 1]

[0082] Table 1 shows that the water-based inks obtained in Examples 1 to 10 are superior in maintaining the water repellency of the nozzle plate surface and in long-term ejection reliability in inkjet recording compared to the water-based inks obtained in Comparative Examples 1 to 3, and are capable of producing recorded materials with excellent image robustness.

Claims

1. A water-based ink for ink-jet printing, comprising a pigment, pigment-free polymer particles, a wax, an organic solvent, a silicate compound, and water, the content of the silicic acid compound in the ink is 2.5 ppm by mass or more and 250 ppm by mass or less, The silicate compound is a silicate. Water-based ink for inkjet printing.

2. 2. The water-based ink for ink-jet printing according to claim 1, wherein the silicate compound is at least one selected from the group consisting of sodium silicate, potassium silicate, calcium silicate, and magnesium silicate.

3. 3. The water-based ink for ink-jet printing according to claim 1, wherein the wax is at least one selected from the group consisting of polyolefin waxes and paraffin waxes.

4. 4. The water-based ink for ink-jet printing according to claim 1, wherein the wax has a melting point of 85° C. or higher.

5. 5. The water-based ink for ink-jet printing according to claim 1, wherein the wax is in the form of particles that are dispersed in the aqueous medium.

6. 6. The water-based ink for ink-jet printing according to claim 5, wherein the wax is in the form of particles dispersed in an aqueous medium using at least one surfactant selected from the group consisting of a nonionic surfactant and an anionic surfactant.

7. 7. The water-based ink for ink-jet printing according to claim 1, wherein the content of the wax in the ink is from 0.1% by mass to 5% by mass.

8. 8. The water-based ink for ink-jet printing according to claim 1, wherein the organic solvent comprises at least one solvent selected from the group consisting of glycol-based solvents and glycol ether-based solvents.

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

10. An ink-jet recording method comprising: ejecting the water-based ink for ink-jet printing according to any one of claims 1 to 9 from a recording head using at least one material selected from silicon and silicon oxide as a nozzle plate member.

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