Water-based ink for inkjet recording.

Abstract

To provide a water-based ink for inkjet recording having excellent long-term discharge reliability and discharge recovery properties in inkjet recording while suppressing degradation of water repellency of an inkjet head using a silicon member or a silicon oxide member and to provide an inkjet recording method using the same.SOLUTION: There is provided [1] a water-based ink for inkjet recording which contains a polymer particles containing carbon black, an organic solvent, a silicic acid compound and water, wherein the content of the silicic acid compound is 1 mass ppm or more and 450 mass ppm or less in the ink. There is provided [2] an inkjet recording method which discharges the ink from an inkjet head using one or more selected from silicon and a silicon oxide for a nozzle plate member using the water-based ink for inkjet recording of [1].SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Inkjet recording is a recording method that directly ejects ink droplets from fine nozzles and adheres them to a recording medium to obtain a recorded material with text and images. This method has become extremely popular because it is easy and inexpensive to implement in full color, can use plain paper as a recording medium, and is contactless to the recording medium. In the commercial printing sector, in addition to printing on conventional high-absorbency recording media such as plain paper and copy paper, there is a growing demand for printing on low-absorbency recording media such as offset coated paper. In recent years, inkjet heads that dramatically improve ejection accuracy by using MEMS (Micro Electro Mechanical Systems) technology to process nozzle plates and other components using silicon materials or silicon oxide materials have come into use.

[0003] Inkjet printers commonly use water-based pigment inks as colorants, which have good lightfastness and water resistance. However, when such water-based pigment inks are filled into inkjet heads that use silicone or silicon oxide components and used or left for extended periods, the silicone and other components in contact with the ink may dissolve, leading to deterioration of components such as nozzle plates. This can reduce the water repellency of the inkjet head, decrease the printer's ejection accuracy, and ultimately result in a decline in image quality. Various proposals have been made to address the above problems. For example, Patent Document 1 describes an aqueous ink comprising a colorant, a resin, water glass, and an organic solvent, with the aim of providing an aqueous ink that improves the ejection stability and clogging resistance from nozzles and pen tips without reducing the fixability, scratch resistance, water resistance, and marker resistance of the formed image, wherein the colorant is a self-dispersing carbon black having anionic hydrophilic groups on its surface. Furthermore, Patent Document 2 describes an ink composition containing water, a colorant, a water-soluble organic solvent, a surfactant, and a water-soluble silicate in an amount of 0.0001% by mass or more and 0.5% by mass or less based on the total mass, with the aim of providing an ink composition that has good ink ejection reliability and can suppress a decrease in the water repellency of the inkjet head. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2003-342501 [Patent Document 2] Japanese Patent Publication No. 2011-63725 [Overview of the project] [Problems that the invention aims to solve]

[0005] The water-based ink in Reference 1 was insufficient in terms of nozzle discharge recovery after printing stopped due to drying. Furthermore, it was insufficient in terms of long-term discharge reliability because discharge performance deteriorated over time due to the degradation of the ink. Furthermore, the ink composition described in Reference 2, when using carbon black as the black ink, did not adequately suppress the decrease in water repellency of the inkjet head and was insufficient in terms of long-term ejection reliability. The present invention aims to provide an aqueous ink for inkjet recording that suppresses the decrease in water repellency of inkjet heads using silicon components or silicon oxide components, while also exhibiting excellent long-term ejection reliability and ejection recovery properties in inkjet recording, and an inkjet recording method using the same. [Means for solving the problem]

[0006] The inventors have found that by including a specific amount of silicate compound in an inkjet recording water-based ink containing polymer particles containing carbon black, an organic solvent, and a silicate compound, the above-mentioned problems can be solved due to the excellent water repellency to inkjet heads using silicon components or silicon oxide components. In other words, the present invention provides the following [1] and [2]. [1] An inkjet water-based ink for recording, comprising polymer particles containing carbon black, an organic solvent, a silicate compound, and water, wherein the silicate compound is contained in an amount of 1 ppm by mass or more and 450 ppm by mass or less in the ink. [2] An inkjet recording method comprising using the water-based inkjet recording ink described in [1] above, and ejecting the ink from an inkjet head using one or more selected from silicon and silicon oxide as the nozzle plate member. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an aqueous ink for inkjet recording that suppresses the decrease in water repellency of inkjet heads using silicon members or silicon oxide members, while also exhibiting excellent long-term ejection reliability and ejection recovery properties in inkjet recording, as well as an inkjet recording method using the same. [Modes for carrying out the invention]

[0008] [Water-based ink for inkjet recording] The present invention provides an aqueous inkjet recording ink (hereinafter also referred to as "the present invention ink") which contains polymer particles containing carbon black, an organic solvent, a silicate compound, and water, characterized in that the silicate compound is contained in an amount of 1 ppm by mass or more and 450 ppm by mass or less in the ink.

[0009] In this specification, "aqueous system" means a medium in which water accounts for the largest proportion by mass of the carbon black dispersion medium. Furthermore, "record" is a concept that includes printing and printing of text and images, while "record material" is a concept that includes printed materials and printed objects on which text and images are recorded.

[0010] The ink of this invention suppresses the decrease in water repellency of inkjet heads using silicon or silicon oxide components, while also exhibiting excellent long-term ejection reliability and ejection recovery in inkjet recording. The reason for this is not entirely clear, but it is thought to be as follows. In inkjet heads processed using MEMS technology, such as nozzle plates using silicon or silicon oxide materials, silicate ions are normally released from silicon, etc. However, since the ink of the present invention contains silicate compounds such as sodium silicate and colloidal silica, it is believed that the release of silicate ions from silicon and silicon oxide can be suppressed, and even after long-term contact with the ink, the decrease in water repellency of the inkjet head due to corrosion of the nozzle plate can be suppressed. On the other hand, in general inkjet recording inks that use carbon black as a colorant, the strong material-capturing power of carbon black means that even if silicate compounds are included in the ink, the balance of silicate compounds in the ink is disrupted over time. As a result, the effect of suppressing the decrease in water repellency of the inkjet head mentioned above cannot be achieved, and the long-term ejection reliability and ejection recovery of the inkjet ink cannot be achieved. The ink of the present invention uses polymer particles containing carbon black, and since the carbon black is at least partially coated with the polymer, the ability of the carbon black to be captured by a silicate compound or the like can be within an appropriate range, so it is considered that the balance of the content of the silicate compound in the ink can be appropriately maintained. Therefore, the effect of suppressing the decrease in water repellency of the above inkjet head can be exhibited, and it is considered that the ink of the present invention can improve long-term ejection reliability and ejection recovery. Further, by using polymer particles containing carbon black, the carbon black can be stably dispersed in the ink, so it is considered that the generation of coarse particles due to the aggregation of carbon black is controlled, contributing to further improvement of long-term ejection reliability and ejection recovery in inkjet recording.

[0011] <Polymer particles containing carbon black> The polymer particles containing carbon black are composed of carbon black and a polymer (hereinafter also referred to as "polymer (a)") having a function of dispersing the carbon black in a medium mainly composed of water. Polymer (a) is not particularly limited as long as it is a polymer having a function of dispersing carbon black in an aqueous medium mainly composed of water. Polymer (a) is preferably a water-insoluble polymer. In this specification, the "water-insoluble polymer" means a polymer whose amount of dissolution is less than 10 g when a polymer dried at 105 °C for 2 hours to reach a constant weight is 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 polymer (a) is neutralized with sodium hydroxide to the same degree of neutralization as the degree of neutralization of the polymer constituting the polymer particles containing carbon black in the state where the ink of the present invention is contained. When confirming whether it is water-insoluble, if it exhibits a dispersed state, the dispersion is sedimented by centrifugation and judged by the amount of dissolution in the aqueous phase part. Also, even if the dispersed state cannot be visually confirmed and it appears transparent, if the particle size is measured by the same procedure as the measurement of the particle size of the polymer particles described in the examples, it is judged to be in a dispersed state. The polymer particles containing carbon black are composed of carbon black and a polymer (hereinafter also referred to as "polymer (a)") having a function of dispersing the carbon black in a medium mainly composed of water. Polymer (a) is not particularly limited as long as it is a polymer having a function of dispersing carbon black in an aqueous medium mainly composed of water.

[0012] (Carbon black) In the ink of the present invention, the carbon black contained in the polymer particles containing carbon black can be a known carbon black such as channel black, furnace black, acetylene black, thermal black, ketjen black, etc. Among these, from the viewpoints of long-term ejection reliability and ejection recovery property, one or more selected from channel black and furnace black are preferable, and channel black is more preferable. Carbon black can be used alone or in combination of two or more. Specific examples of carbon black include Regal400R, 660R, Monarch717, 800, 880, 900, 1100 (manufactured by Cabot Corporation), ColorBlack FW1, FW18, S160, S170, Nipex180IQ, 170IQ, 160IQ, Printex55, 70, 80, 90, L6, U, V, 150T (manufactured by Orion Engineered Carbons), No.45, 47, 900, 2200B, 2300, 2600, 990, 980, 970, 960, 950, 850, MCF-88, MA8, 600, 100 (manufactured by Mitsubishi Chemical Corporation), etc. However, it is not limited thereto.

[0013] The specific surface area of carbon black is preferably 100 m 2 , 2 / g or more and 450 m 2 / g or less. In the present invention, when the specific surface area of the carbon black used is 100 m 2 / g or more and 45 m 2 / g or less, the carbon black captures a predetermined amount of the silicate compound dissolved in the ink, so that the concentration of the silicate compound in the ink can be maintained below the concentration at which recrystallization does not occur, and it is considered that the long-term ejection reliability of the ink of the present invention can be enhanced and the ejection recovery property can be improved. The specific surface area of carbon black is more preferably 110 m 2 / g or more, still more preferably 180 m 2 / g or more, even more preferably 230 m2 / g or more. Further, the specific surface area of the carbon black is more preferably 400 m 2 / g or less, still more preferably 340 m 2 / g or less, even more preferably 280 m 2 / g or less. The specific surface area of the carbon black is measured by the method described in the Examples.

[0014] The pH of the carbon black is preferably 3.0 or more and 9.5 or less. In the present invention, when the pH of the carbon black used is 3.0 or more and 9.5 or less, it is considered that the affinity between the carbon black and the polymer (a) in the polymer particles containing the carbon black can be enhanced, and the dispersion stability of the polymer particles containing the carbon black in the aqueous medium is improved, and it is considered that the long-term discharge stability and discharge recovery of the ink of the present invention can be improved. In addition, carbon black has an acid component, and it is known that hydrogen ions are continuously supplied into the ink from the surface of the carbon black during ink storage. The supply power of these hydrogen ions becomes more prominent as the pH of the carbon black is lower. Since the hydrogen ions supplied from the surface of the carbon black are present in the ink, hydrolysis of silicon such as the nozzle plate can be suppressed, and thus reduction of the water repellency of the inkjet head can be further suppressed. From the viewpoint of suppressing the reduction of the water repellency of the inkjet head and improving the long-term discharge reliability and discharge recovery, the pH of the carbon black is preferably 9.5 or less, more preferably 8.5 or less, still more preferably pH 7.0 or less, and even more preferably pH 5.5 or less. [[ID=##**##]]The pH of the carbon black is measured by the method described in the Examples.

[0015] In this specification, the form of polymer particles containing carbon black means particles in which the polymer encapsulates carbon black, particles in which a portion of the carbon black is exposed on the surface of particles consisting of polymer and carbon black, particles in which a portion of the polymer is adsorbed onto the carbon black, and mixtures thereof. In the ink of the present invention, the polymer particles containing carbon black are preferably "crosslinked polymer particles containing carbon black" as described below, from the viewpoint of improving long-term ejection reliability and ejection recovery.

[0016] (polymer (a)) Polymer (a) can be of any composition, but from the viewpoint of improving the long-term ejection reliability and ejection recovery of the ink of the present invention, vinyl polymers obtained by addition polymerization of vinyl monomers such as vinyl compounds, vinylidene compounds, and vinylene compounds are preferred. As vinyl polymers, polymers containing structural units derived from (a-1) ionic monomers are preferred, and copolymer polymers having structural units derived from (a-1) ionic monomers and (a-2) hydrophobic monomers are more preferred.

[0017] [(a-1) Ionic monomers] (a-1) As the ionic monomer (hereinafter also referred to as "component (a-1)"), anionic monomers are preferred from the viewpoint of improving the dispersion stability of carbon black. Examples of anionic monomers include carboxylic acid monomers, sulfonic acid monomers, and phosphate monomers. Examples of carboxylic acid monomers include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid. Among these, carboxylic acid monomers are more preferred, one or more selected from acrylic acid and methacrylic acid are preferred, and acrylic acid is more preferred.

[0018] [(a-2) Hydrophobic monomers] (a-2) Hydrophobic monomers (hereinafter also referred to as "component (a-2)") are preferable to be used as further monomer components in addition to component (a-1) from the viewpoint of improving the dispersion stability of carbon black. (a-2) Specific examples of the component include those described in paragraphs

[0020] to

[0022] of Japanese Patent Publication No. 2018-83938. Among these, one or more selected from alkyl (meth)acrylate having an alkyl group with 1 to 22 carbon atoms, styrene, α-methylstyrene, and benzyl (meth)acrylate are preferred, with styrene being more preferred.

[0019] [(a-3) Nonionic monomers] (a-3) Nonionic monomers (hereinafter also referred to as "component (a-3)") can be used to further improve the dispersion stability of carbon black. (a-3) Component is a monomer with high affinity for water and water-soluble organic solvents, such as a monomer containing a hydroxyl group or a polyalkylene glycol chain. (a-3) Specific examples of the component include those described in paragraph

[0018] of Japanese Patent Publication No. 2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n=1~30) (meth)acrylate and polypropylene glycol (n=2~30) (meth)acrylate are preferred. Here, n represents the average number of moles added of oxyalkylene groups. The above components (a-1) to (a-3) can each be used by using the monomer components contained in each component individually or by mixing two or more of them.

[0020] From the above viewpoint, it is preferable that polymer (a) has one or more components selected from acrylic acid and methacrylic acid as component (a-1), and one or more components selected from styrene and α-methylstyrene as component (a-2), and more preferably that it is a styrene-(meth)acrylic acid copolymer.

[0021] [Content of each constituent unit in polymer (a)] From the viewpoint of improving the long-term ejection reliability and ejection recovery performance of the ink of the present invention, 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 constituent units derived from each component in polymer (a) is as follows. The content of component (a-1) is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 40% 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, and even more preferably 75% by mass or less.

[0022] If component (a-3) is present, the content of 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 even more preferably 5% by mass or less. The mass ratio of [(a-1) component / (a-2) component] is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.35 or more, even more preferably 0.38 or more, and preferably 2 or less, more preferably 1.5 or less, even more preferably 1 or less, even more preferably 0.8 or less, and particularly preferably 0.6 or less.

[0023] [Production of polymer (a)] Polymer (a) is produced by copolymerizing a monomer mixture containing component (a-1) and component (a-2), and optionally further component (a-3), using known polymerization methods such as solution polymerization, bulk polymerization, suspension polymerization, and emulsion polymerization. Among these polymerization methods, solution polymerization is preferred.

[0024] The acid value of polymer (a) is derived from the carboxyl group, but 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 160 mg KOH / g or more, more preferably 180 mg KOH / g or more, even more preferably 200 mg KOH / g or more, and preferably 300 mg KOH / g or less, more preferably 280 mg KOH / g or less, and even more preferably 260 mg KOH / g or less. The acid value of polymer (a) can be measured by the method described in the examples. It can also be calculated from the mass ratio of the constituent monomers. The number-average molecular weight of polymer (a) 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, and even more preferably 30,000 or less, from the viewpoint of improving the long-term ejection reliability and ejection recovery performance of the ink of the present invention. The number-average molecular weight is measured by the method described in the examples.

[0025] [Neutralization] Preferably, at least a portion of the carboxyl groups of polymer (a) are neutralized with an alkali metal compound or the like. Examples of alkali metal compounds include one or more selected from alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; alkali metal salts of carbonic acid such as sodium carbonate, sodium bicarbonate, and potassium 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.

[0026] The degree of neutralization of polymer (a) is preferably 15 mol% or more, more preferably 20 mol% or more, even more preferably 30 mol% or more, and preferably 150 mol% or less, more preferably 100 mol% or less, and even more preferably 80 mol% or less, from the viewpoint of ensuring the dispersion stability of carbon black. In the present invention, when the degree of neutralization of polymer (a) is 100 mol% or less, the degree of neutralization is synonymous with the equivalent amount of neutralizing agent used, and the equivalent amount (mol%) of the neutralizing agent used for polymer (a) is calculated by the following formula. The equivalent amount (mol%) of neutralizing agent used for polymer (a) = [[Mass of neutralizing agent added to neutralize polymer (a) (g) / Equivalent amount of neutralizing agent] / [Acid value of polymer (a) (mgKOH / g) × Mass of polymer (a) (g) / (56 × 1000)]] × 100 In this invention, if the amount of neutralizing agent used is in excess of the number of moles of carboxyl groups in polymer (a), it means that the amount of neutralizing agent is in excess of the carboxyl groups in polymer (a), and in this case, the degree of neutralization is considered to be 100 mol%.

[0027] [Cross-linked structure of polymer (a)] It is preferable that polymer (a) is crosslinked using a crosslinking agent to form a crosslinked structure. In other words, in the present invention, it is preferable that the polymer particles containing carbon black have a crosslinked structure, that is, it is preferable that they are crosslinked polymer particles containing carbon black. The crosslinked polymer constituting the carbon black-containing crosslinked polymer particles is preferably a crosslinked polymer (A) composed of a component derived from polymer (a) and a component derived from a crosslinking agent. The crosslinked polymer (A) constituting the carbon black-containing crosslinked polymer particles (hereinafter also simply referred to as "crosslinked polymer (A)") has a three-dimensional structure, which is firmly adsorbed or fixed to the carbon black surface. Therefore, aggregation of carbon black in the ink of the present invention is suppressed, and further swelling of the polymer is suppressed, which is thought to improve the long-term ejection reliability and storage stability of the ink of the present invention. The crosslinking agent is preferably a compound having two or more epoxy groups in its molecule (hereinafter also referred to as "epoxy compound"). The crosslinking agent may be water-soluble or water-insoluble, but from the viewpoint of more efficiently crosslinking with the carboxyl groups of polymer (a) in a water-based medium, its water solubility is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Here, water solubility (mass%) refers to the solubility (mass%) of the crosslinking agent when 10 parts by mass of the crosslinking agent are dissolved in 90 parts by mass of water at room temperature (25°C). Specifically, the water solubility (mass%) can be measured by the method described in the examples.

[0028] Compounds having two or more epoxy groups in the molecule are preferably compounds having two or more glycidyl ether groups in the molecule, and more preferably polyglycidyl ether compounds of polyhydric alcohols having hydrocarbon groups with 3 to 8 carbon atoms. The epoxy equivalent of a compound having two or more epoxy groups in its molecule is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and preferably 300 or less, more preferably 200 or less, and even more preferably 170 or less, from the viewpoint of more efficiently crosslinking with the carboxyl groups of polymer (a) in a water-based medium.

[0029] Specific examples of compounds having two or more epoxy groups in their 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 type 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.

[0030] The acid value of the crosslinked polymer (A) is preferably 90 mg KOH / g or more, more preferably 95 mg KOH / g or more, even more preferably 100 mg KOH / g or more, and preferably 200 mg KOH / g or less, more preferably 180 mg KOH / g or less, and even more preferably 160 mg KOH / g or less, from the viewpoint of improving the long-term ejection reliability and storage stability of the ink of the present invention. The degree of neutralization of the crosslinked polymer (A) is preferably 20 mol% or more, more preferably 40 mol% or more, even more preferably 60 mol% or more, and preferably 150 mol% or less, more preferably 120 mol% or less, and even more preferably 100 mol% or less, from the viewpoint of ensuring the dispersion stability of carbon black. The acid value of cross-linked polymer (A) can be determined from the charging ratio based on the acid value of polymer (a).

[0031] (Manufacturing of polymer particles containing carbon black) Polymer particles containing carbon black can be efficiently manufactured by a method comprising the following steps 1 and 2. Step 1: A step to obtain an aqueous dispersion of polymer (a) by neutralizing at least a portion of the carboxyl groups of polymer (a) with an alkali metal compound. Step 2: A step to obtain an aqueous dispersion of polymer particles containing carbon black by dispersing the aqueous dispersion of polymer (a) obtained in Step 1 with carbon black. Furthermore, if the polymer particles containing carbon black are cross-linked polymer particles containing carbon black, the cross-linked polymer particles containing carbon black can be efficiently manufactured by a method that includes the following step 3 in addition to steps 1 and 2 described above. Step 3: A process to obtain an aqueous dispersion of crosslinked polymer particles containing carbon black by adding a crosslinking agent to the aqueous dispersion of carbon black-containing polymer particles obtained in Step 2 and performing a crosslinking treatment.

[0032] In step 1, neutralization is preferably carried out so that the pH is between 7 and 11. The degree of neutralization of the alkali metal compound and polymer (a) used for neutralization is as described above. In step 2, the dispersion process can be performed by primary dispersion using shear stress alone to finely atomize the carbon black particles to the desired particle size. However, from the viewpoint of obtaining a uniform aqueous dispersion of carbon black, it is preferable to perform the dispersion process in two or more stages, such as first pre-dispersing a mixture containing carbon black and polymer (a), and then performing the primary dispersion. As the disperser used for the preliminary dispersion, commonly used mixing and stirring devices such as anchor blades and disperser blades can be used. Dispersion machines used for this dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersants 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 carbon black. When performing dispersion processing using a high-pressure homogenizer, the average particle size of carbon black particles in the carbon black aqueous dispersion can be adjusted by controlling the processing pressure and the number of passes. From the viewpoint of productivity and economic efficiency, the processing pressure is preferably 60 MPa to 300 MPa, and the number of passes is preferably 3 to 30.

[0033] In step 3, the polymer (a) in which the carbon black is dispersed in the carbon black aqueous dispersion is crosslinked with a crosslinking agent to form a crosslinked polymer (A) having a three-dimensional structure, and an aqueous dispersion can be obtained in which particles of the crosslinked polymer (A) containing carbon black are dispersed in an aqueous medium. The preferred crosslinking agent is as described above. From the viewpoint of ensuring the completion of the crosslinking reaction and economic efficiency, 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. Similarly, from the same viewpoint, the duration of the crosslinking treatment is preferably 0.5 hours or more, more preferably 1 hour or more, and preferably 10 hours or less, more preferably 6 hours or less.

[0034] The acid value of polymer (a) constituting the polymer particles containing carbon black is preferably 160 mg KOH / g or more, more preferably 180 mg KOH / g or more, even more preferably 200 mg KOH / g or more, and preferably 300 mg KOH / g or less, more preferably 280 mg KOH / g or less, and even more preferably 260 mg KOH / g or less.

[0035] The acid value of the crosslinked polymer (A) constituting the crosslinked polymer particles containing carbon black is preferably 90 mg KOH / g or more, more preferably 95 mg KOH / g or more, even more preferably 100 mg KOH / g or more, and preferably 200 mg KOH / g or less, more preferably 180 mg KOH / g or less, and even more preferably 160 mg KOH / g or less.

[0036] The concentration of nonvolatile components (solids concentration) of the resulting aqueous dispersion of carbon black-containing polymer particles or aqueous dispersion of carbon black-containing crosslinked polymer particles is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, from the viewpoint of facilitating the preparation of the ink of the present invention.

[0037] The carbon black content in the resulting aqueous dispersion of carbon black-containing polymer particles or aqueous dispersion of carbon black-containing crosslinked polymer particles is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 18% by mass or less, from the viewpoint of improving the color development of the recorded material.

[0038] The crosslinking ratio of the crosslinked polymer (A) is preferably 20 mol% or more, more preferably 30 mol% or more, even more preferably 40 mol% or more, and preferably 80 mol% or less, more preferably 70 mol% or less, and even more preferably 60 mol% or less, from the viewpoint of improving the long-term ejection reliability and ejection recovery of the ink of the present invention. When an epoxy compound is used as a crosslinking agent, the crosslinking ratio (mol%) of the crosslinked polymer (A) is calculated using the following formula. Crosslinking ratio (mol%) of crosslinked polymer (A) = [Molar equivalents of epoxy groups in the epoxy compound used in the preparation of crosslinked polymer (A)] / [Molar equivalents of carboxyl groups in polymer (a) used in the preparation of crosslinked polymer (A)] × 100 When the crosslinking ratio of the crosslinked polymer (A) is within the range of the preferred crosslinking ratio described above, a three-dimensional structure is appropriately formed, and it exhibits water insolubility regardless of the type of polymer (a) or the type of crosslinking agent.

[0039] The average particle size of carbon black-containing polymer particles in an aqueous dispersion of carbon black-containing polymer particles, or the average particle size of carbon black-containing crosslinked polymer particles in an aqueous dispersion of carbon black-containing crosslinked polymer particles, is preferably 60 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and also preferably 200 nm or less, more preferably 160 nm or less, and even more preferably 120 nm or less, from the viewpoint of improving the long-term ejection reliability and ejection recovery performance of the ink of the present invention using the aqueous dispersion. The average particle size is measured by the method described in the examples. Furthermore, the average particle size of carbon black-containing polymer particles in the aqueous ink after preparation, or the average particle size of carbon black-containing crosslinked polymer particles, is substantially the same as the average particle size in the aqueous dispersion of carbon black-containing crosslinked polymer particles.

[0040] <Organic solvents> The organic solvent used in the ink of the present invention primarily serves to improve the long-term ejection reliability and ejection recovery of the ink. The organic solvent may be a liquid or a solid at 25°C, but a water-soluble organic solvent is preferred, such that when dissolved in 100 mL of water at 25°C, the amount dissolved is 10 mL or more. From the viewpoint of improving the long-term ejection reliability and ejection recovery performance of the ink of the present invention, the boiling point of the water-soluble organic solvent is preferably 90°C or higher, more preferably 110°C or higher, even more preferably 130°C or higher, and even more preferably 150°C or higher, and preferably 250°C or lower, more preferably 240°C or lower, and even more preferably 235°C or lower. From the same viewpoint as above, it is preferable that the water-soluble organic solvent contains one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers. Examples of polyhydric alcohols 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.

[0041] Examples of polyhydric alcohol alkyl ethers 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, with diethylene glycol monoisopropyl ether being more preferred. Among the above organic solvents, one or more selected from propylene glycol, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether are preferred, and one or more selected from propylene glycol and diethylene glycol monoisopropyl ether are more preferred. The organic solvent may further contain other organic solvents as long as they do not impair the effects of the present invention. The organic solvent used here preferably contains one or more water-soluble organic solvents with a boiling point of 90°C or higher, and more preferably contains two or more water-soluble organic solvents with a boiling point of 90°C or higher. When the organic solvent contains two or more water-soluble organic solvents, the weighted average of the boiling points of the organic solvents is preferably 150°C or higher, more preferably 180°C or higher, and preferably 250°C or lower, more preferably 240°C or lower, even more preferably 220°C or lower, and even more preferably 200°C or lower. When two or more water-soluble organic solvents are used as the organic solvent, the weighted average of the boiling points of the organic solvents is a weighted average weighted by the content (mass%) of each water-soluble organic solvent.

[0042] <Silicate compounds> The ink of the present invention contains a silicate compound to suppress the elution of silicate ions from silicon and silicon oxide, to suppress the decrease in water repellency of the inkjet head, and to improve the long-term ejection reliability and ejection recovery of the ink of the present invention. The silicate compound can be broadly selected from silica and silicates, but it is particularly preferable that it be one or more selected from silicates of silica with alkali metals or alkaline earth metals, such as sodium silicate, potassium silicate, calcium silicate, and magnesium silicate, and anhydrous silica (silica). As for the silicate, alkali metal salts of silica, known as water glass, are preferred. Colloidal silica is preferred as the anhydrous silicic acid (silica). Among these silicate compounds, alkali metal salts of one or more silicates selected from sodium silicate and potassium silicate are more preferable from the viewpoint of improving the long-term stability of the ink of the present invention when stored and improving the long-term ejection reliability and ejection recovery of the ink of the present invention. Furthermore, sodium silicate is even more preferable from the viewpoint of suppressing the decrease in water repellency of the inkjet head and improving the long-term ejection reliability and ejection recovery of the ink of the present invention. When the alkali metal salt is a sodium or potassium salt, the structural formula for the alkali metal salt of silica is given by X₂O·nSiO₂ (where X represents sodium or potassium, and n represents the molar ratio). In the ink of the present invention, the mass ratio of the SiO2 content to the X2O content in the alkali metal salt of silicate [(SiO2 (mass%)) / (X2O (mass%))] is preferably 1.0 or higher, more preferably 1.5 or higher, even more preferably 2.0 or higher, and even more preferably 3.0 or higher, from the viewpoint of suppressing a decrease in the water repellency of the inkjet head and improving the long-term ejection reliability of the ink of the present invention, and similarly, preferably 5.0 or lower, more preferably 4.0 or lower, and even more preferably 3.5 or lower. Furthermore, when sodium silicate and potassium silicate are included as alkali metal salts of silicate, the SiO2 content and X2O content in the alkali metal salt of silicate are the sum of the respective SiO2 and X2O content contained in sodium silicate and potassium silicate.

[0043] As for sodium silicate, industrially used sodium silicate is preferred. The standard for sodium silicate is specified in the Japan Inorganic Chemicals Association's collective standard (sodium silicate (sodium sodium silicate)), and it can be used in any form: sodium silicate No. 1, sodium silicate No. 2, and sodium silicate No. 3 in solution state, or sodium metasilicate in solid state. Among these, from the viewpoint of consistency in ink formulation, it is preferable to use any of sodium silicate No. 1, sodium silicate No. 2, or sodium silicate No. 3 in aqueous solution state. Furthermore, sodium silicate No. 3 is more preferable from the viewpoint of suppressing the decrease in water repellency of the inkjet head and improving the long-term ejection reliability of the ink of the present invention.

[0044] <Pigment-free polymer particles> From the viewpoint of improving the fixation of the ink to recording media and imparting image robustness to recorded materials, while maintaining the long-term ejection reliability and ejection recovery properties of the ink of the present invention, it is preferable that the ink of the present invention further contains polymer particles that do not contain pigment. Examples of polymers constituting polymer particles that do not contain pigments (hereinafter also referred to as "polymer (b)") include (meth)acrylic resins, styrene resins, styrene-(meth)acrylic resins, urethane resins, polyester resins, butadiene resins, and vinyl chloride resins. Among these, from the viewpoint of improving the fixation of the ink of the present invention, one or more selected from (meth)acrylic resins and styrene-(meth)acrylic resins are preferred, and styrene-(meth)acrylic resins are more preferred. Polymer particles that do not contain pigments are preferably used as an aqueous dispersion in which they are dispersed in water. Polymer (b) may be synthesized as appropriate, or a commercially available product may be used. Furthermore, polymer (b) preferably has a crosslinked structure from the viewpoint of improving the long-term ejection reliability of the ink of the present invention. That is, the pigment-free polymer particles are preferably pigment-free crosslinked polymer particles. These pigment-free crosslinked polymer particles are preferably crosslinked polymer (B) consisting of components derived from polymer (b) and components derived from the crosslinking agent.

[0045] [Polymer (b)] The (meth)acrylic resin as polymer (b) preferably has (b-1) constituent units derived from a carboxyl group-containing vinyl monomer (hereinafter also referred to as "(b-1) component") and (b-2) constituent units derived from a hydrophobic vinyl monomer (hereinafter also referred to as "(b-2) component". Component (b-1) can be a carboxylic acid monomer similar to that of component (a-1) above. Among these, one or more selected from acrylic acid and methacrylic acid are preferred, with acrylic acid being more preferred, from the viewpoint of improving the image fastness of the recorded material while maintaining the long-term ejection reliability and ejection recovery of the ink of the present invention. Component (b-2) is preferably an alkyl (meth)acrylate or an aromatic group-containing monomer, similar to component (a-2) above. Vinyl chloride monomer can also be used. Among these, styrene monomers are preferred, with one or more selected from styrene and α-methylstyrene being preferred, and styrene being more preferred. The above components (b-1) and (b-2) can be used by using the monomer components contained in each component individually or by mixing two or more of them.

[0046] [Content of each constituent unit in polymer (b)] The content of constituent units derived from components (b-1) and (b-2) in polymer (b) is as follows, from the viewpoint of improving the image robustness of the recorded material while maintaining the long-term ejection reliability and ejection recovery of the ink of the present invention. The content of component (b-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, and even more preferably 50% by mass or less. The content of component (b-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, and even more preferably 75% by mass or less.

[0047] Polymer (b) can be produced by copolymerizing a monomer mixture containing component (b-1), component (b-2), etc., using a known solution polymerization method or the like. The method for producing polymer (b), the neutralization method, and the crosslinking treatment method are the same as those described above for polymer (a), so their description is omitted.

[0048] The acid value of polymer (b) is preferably 180 mg KOH / g or more, more preferably 200 mg KOH / g or more, even more preferably 220 mg KOH / g or more, and preferably 320 mg KOH / g or less, more preferably 300 mg KOH / g or less, and even more preferably 280 mg KOH / g or less, from the viewpoint of improving the image robustness of the recorded material while maintaining the long-term ejection reliability and ejection recovery performance of the ink of the present invention. 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, and even more preferably 30,000 or less. The acid value and number-average molecular weight of polymer (b) can be measured in the same manner as for polymer (a).

[0049] The acid value of the crosslinked polymer (B) is preferably 90 mg KOH / g or more, more preferably 95 mg KOH / g or more, even more preferably 100 mg KOH / g or more, and preferably 200 mg KOH / g or less, more preferably 180 mg KOH / g or less, and even more preferably 160 mg KOH / g or less, while maintaining the long-term ejection reliability and ejection recovery of the ink of the present invention, and improving the fixation of the ink of the present invention to the recording medium and imparting image robustness to the recorded material. The acid value of cross-linked polymer (B) can be calculated from the charge ratio based on the value of polymer (b).

[0050] The crosslinking ratio of the crosslinked polymer (B) is preferably 20 mol% or more, more preferably 30 mol% or more, even more preferably 40 mol% or more, and preferably 80 mol% or less, more preferably 70 mol% or less, and even more preferably 60 mol% or less, from the viewpoint of maintaining the long-term ejection reliability and ejection recovery of the ink of the present invention, while improving the fixation of the ink of the present invention to the recording medium and imparting image robustness to the recorded material. When an epoxy compound is used as a crosslinking agent, the crosslinking ratio (mol%) of the crosslinked polymer (B) is calculated using the following formula. Crosslinking ratio (mol%) of crosslinked polymer (B) = [Molar equivalents of epoxy groups in the epoxy compound used in the preparation of crosslinked polymer (B)] / [Molar equivalents of carboxyl groups in polymer (b) used in the preparation of crosslinked polymer (B)] × 100

[0051] The average particle size of the pigment-free polymer particles is preferably 20 nm or more, more preferably 30 nm or more, even more preferably 40 nm or more, and also preferably 150 nm or less, more preferably 100 nm or less, and even more preferably 70 nm or less, from the viewpoint of improving the image fastness of the recorded material while maintaining the long-term ejection reliability and ejection recovery performance of the ink of the present invention. The average particle size of the pigment-free polymer particles is measured by the method described in the examples. The average particle size of pigment-free polymer particles in the ink of the present invention after ink preparation is substantially the same as the average particle size of pigment-free polymer particles prepared before water-based ink preparation.

[0052] Polymer (a) and polymer (b) may be the same or different. That is, polymer (a) and polymer (b) may have different compositions (structures), or they may be the same polymer including their composition (structure), differing only in the presence or absence of pigment. Polymers (a) and (b) can also be commercially available. Examples of commercially available dispersions of polymers (a) and (b) that can be used include Neocryl A1127 (anionic self-crosslinked aqueous acrylic resin) from DSM Neo Resins, Joncryl 390 etc. (acrylic resin) from BASF, Joncryl PDX-7775 etc. (styrene-acrylic resin), and Vinibran 700 etc. (vinyl chloride-acrylic resin) from Nisshin Chemical Industry Co., Ltd. When both polymer particles containing carbon black and polymer particles without pigment are crosslinked polymer particles, it is preferable that the crosslinking agent-derived components among the components constituting the crosslinked polymer are the same. Furthermore, when both polymer particles containing carbon black and polymer particles without pigment are crosslinked polymer particles, it is preferable that polymer component (a) and polymer component (b) among the components constituting the crosslinked polymer are the same, and that the component derived from the crosslinking agent among the components constituting the crosslinked polymer is also the same. If the carbon black constituting the ink of the present invention is a crosslinked polymer particle containing carbon black, and the polymer particles that do not contain pigment are crosslinked polymer particles that do not contain pigment, and if polymer (a) component and polymer (b) component of the crosslinked polymer are the same, and the crosslinking agent-derived component of the crosslinked polymer is also the same, then not only is image fastness improved, but long-term ejection reliability and ejection recovery are also significantly improved. The details of the reason for this are unclear, but it is thought that when the type of polymer is the same, the coating of carbon black by the polymer becomes more stable, and this makes it easier to maintain a constant amount of silicate compound in the ink.

[0053] <wax> The ink of the present invention may contain a wax separately from the viewpoint of improving the image fastness of the recorded material. As for the wax, from the viewpoint of improving the image fastness of the recorded material while maintaining the long-term ejection reliability and ejection recovery of the ink of the present invention, its melting point is preferably 95°C or higher, more preferably 100°C or higher, even more preferably 105°C or higher, and 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, which are mainly composed of olefin monomers; petroleum-based paraffin waxes, which consist of a mixture of chain-type saturated hydrocarbons with 20 to 30 carbon atoms; and synthetic waxes such as Fischer-Tropsch wax. Among these, one or more selected from polyolefin waxes and paraffin waxes are preferred. The wax is preferably contained in the ink as wax particles, which are wax dispersed in an aqueous medium.

[0054] <Surfactants> The ink of the present invention may contain a surfactant. The surfactants that may be contained in the ink of the present invention refer to surfactants other than those introduced from wax emulsions or resin emulsions when the ink of the present invention contains a wax emulsion or resin emulsion. Examples of nonionic surfactants include polyoxyalkylene alkyl ether type surfactants, acetylene glycol-based surfactants, polyhydric alcohol-type surfactants, and fatty acid alkanolamides. Among these, polyoxyalkylene alkyl ether type surfactants and acetylene glycol-based surfactants are preferred, and for example, the combined use of 2,4,7,9-tetramethyl-5-decine-4,7-diol and a polyoxyalkylene alkyl ether type surfactant is more preferred. Examples of commercially available nonionic surfactants include the "Surfinol" series from Nisshin Chemical Industry Co., Ltd. and Air Products & Chemicals, the "Acetyleneol" series from Kawaken Fine Chemical Co., Ltd., and the "Emulgen" series from Kao Corporation. The above-mentioned surfactants can be used individually or in combination of two or more.

[0055] <Method for manufacturing the ink of the present invention> The ink of the present invention can be obtained by mixing polymer particles containing carbon black, an organic solvent, a silicate compound, water, and optionally polymer particles that do not contain pigment, wax, and various additives commonly used in inks, such as humectants, wetting agents, penetrating agents, surfactants, viscosity modifiers, defoamers, preservatives, fungicides, and rust inhibitors.

[0056] <Content of each component in the present invention ink> The content of each component in the ink of the present invention is as follows, from the viewpoint of suppressing a decrease in the water repellency of the inkjet head and improving the long-term ejection reliability and ejection recovery of the ink of the present invention.

[0057] (Carbon black content) The carbon black content 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, and even more preferably 8% by mass or less. The content of polymer particles containing carbon black 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 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.

[0058] The silicate compound content in the ink of the present invention is 1 ppm by mass or more, preferably 3 ppm by mass or more, more preferably 5 ppm by mass or more, even more preferably 10 ppm by mass or more, and even more preferably 25 ppm by mass or more, from the viewpoint of suppressing a decrease in the water repellency of the inkjet head. Furthermore, from the viewpoint of improving the long-term ejection reliability of the ink of the present invention, the silicate compound content in the ink is 450 ppm by mass or less, preferably 300 ppm by mass or less, more preferably 200 ppm by mass or less, even more preferably 150 ppm by mass or less, even more preferably 100 ppm by mass or less, even more preferably 75 ppm by mass or less, and even more preferably 50 ppm by mass or less.

[0059] (Organic solvent content) The content of the organic solvent in the ink of the present invention is preferably 15% by mass or more, more preferably 18% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of suppressing a decrease in the water repellency of the inkjet head and improving long-term ejection reliability and ejection recovery. Similarly, it is preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 28% by mass or less.

[0060] When polymer particles that do not contain pigment are included in the ink of the present invention, their content is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less.

[0061] (The ratio of the silicate compound content in the ink to the product of the specific surface area of ​​carbon black and the carbon black content in the ink.) In the ink of the present invention, the ratio of the content of silicate compounds in the ink to the product of the specific surface area of ​​carbon black and the content of carbon black in the ink is [(content of silicate compounds (mass ppm)) / (specific surface area of ​​carbon black (m²)]. 2 The ratio [(g) × carbon black content (mass%) / 100] is preferably 0.1 or more and 30 or less. In the aforementioned ratio, the product of the specific surface area of ​​carbon black in the ink and the carbon black content is an indicator of how much silicate compound the carbon black present in the ink of the present invention can capture. In other words, the aforementioned ratio is an indicator of how much silicate compound is contained in the ink relative to the ability of the carbon black in the ink to capture the silicate compound dissolved in the ink. The larger the ratio, the more likely it is that the silicate compound dissolved in the ink will be in excess, leading to recrystallization and the formation of coarse particles, which will negatively affect long-term ejection reliability. Conversely, if the ratio is too small, it indicates that there is insufficient amount of silicate compound in the ink to maintain the water repellency of the inkjet head. The above ratio is more preferably 0.3 or higher, even more preferably 1 or higher, even more preferably 1.5 or higher, and even more preferably 2 or higher, from the viewpoint of suppressing a decrease in the water repellency of the inkjet head and improving long-term ejection reliability and ejection recovery performance. Furthermore, from the viewpoint of suppressing the aggregation of particles containing carbon black and improving long-term ejection reliability, the ratio is more preferably 15 or lower, even more preferably 10 or lower, even more preferably 7 or lower, even more preferably 5 or lower, even more preferably 4 or lower, and even more preferably 3 or lower.

[0062] 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 80% by mass or less, more preferably 70% by mass or less.

[0063] <Ink properties> 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, and even more preferably 6 mPa·s or less, from the viewpoint of improving the long-term ejection reliability and ejection recovery performance of the ink of the present invention. In the present invention, the viscosity of the ink can be measured using an E-type viscometer. From the viewpoint of improving the long-term ejection reliability and ejection recovery performance of the ink of the present invention, the pH of the ink is preferably 7.0 or higher, more preferably 7.2 or higher, and even more preferably 7.5 or higher. Furthermore, from the viewpoint of suppressing the decrease in water repellency of the inkjet head, 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 the method described in the examples.

[0064] [Inkjet recording method] The inkjet recording method of the present invention is characterized by using the water-based inkjet recording ink of the present invention and ejecting ink from an inkjet head that uses one or more selected from silicon and silicon oxide as the nozzle plate member. Nozzle plates using silicon and silicon oxide as materials improve ink repellency on the nozzle plate surface by forming a water-repellent film on the nozzle plate surface. Examples of inkjet heads that use silicon or the like as a nozzle plate component include the Samba G3L and Samba G5L from Fujifilm Dimatix, and the S3200, S800, I3200, I1600, and D3000 from Seiko Epson.

[0065] The ink of the present invention can be loaded into a known inkjet recording device such as a piezo-type device and ejected as ink droplets onto a recording medium to obtain a recorded material. As the inkjet recording medium, a highly absorbent plain paper, a low absorbent coated paper, and a non-absorbent resin film can be used. Examples of standard paper include "4200" (manufactured by Fuji Xerox Co., Ltd.) and "NPi Form NEXT-IJ" (manufactured by Nippon Paper Industries Ltd.). Examples of coated papers include the general-purpose glossy paper "OK ​​Topcoat Plus" (manufactured by Oji Paper Co., Ltd.), multi-color 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 resin films include transparent synthetic resin films, such as polyester, polyvinyl chloride, polyolefin, and nylon films. Among these, polyester films and stretched polypropylene films are preferred, and those treated with corona discharge are more preferred.

[0066] Even when used with an inkjet head equipped with a nozzle plate or the like that uses a silicon component or a silicon oxide component, the ink of the present invention suppresses the elution of the silicon component and the generation of aggregated foreign matter in the ink, thereby suppressing the decrease in the water repellency of the inkjet head and improving long-term ejection reliability and ejection recovery performance. [Examples]

[0067] In the following preparation examples, manufacturing examples, examples, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" unless otherwise specified. The measurement methods for each physical property are as follows.

[0068] <Measurement> (1) Measurement of the number-average molecular weight of polymers The solutions prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively, were used as eluents. The measurements were performed using gel permeation chromatography (GPC instrument (HLC-8320GPC) manufactured by Tosoh Corporation, columns manufactured by Tosoh Corporation (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn Super AW-H), flow rate: 0.5 mL / min) and monodisperse polystyrene kits with known molecular weights as standard substances (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). 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.).

[0069] (2) Measurement of the solid content concentration of aqueous dispersions of polymer particles containing carbon black and polymer particles that do not contain pigments. 10.0 g of sodium sulfate, which had been stabilized in a desiccator, was weighed into a 30 mL ointment container. Approximately 1.0 g of the sample was added and mixed, then accurately weighed. The mixture was kept at 105°C for 2 hours to remove volatile components, and then left in the desiccator for another 15 minutes before the mass was measured. The mass of the sample after removing volatile components was taken as the solid content, and the solid content concentration was obtained by dividing it by the mass of the initial sample.

[0070] (3) Measurement of the average particle size of polymer particles containing carbon black and polymer particles that do not contain pigments. 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 an 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 of 90° between the incident light and the detector, and 100 integration cycles. 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 (No. 5, manufactured by Maruemu Co., Ltd.), and the solid content concentration was 2 × 10⁻⁶. -4 Water was added to the solution to a mass percentage, and the mixture was stirred using a magnetic stirrer at 25°C for 1 hour.

[0071] (4) Measurement of the acid value of polymers The resin was dissolved in a titration solvent of toluene and acetone (2:1) using a potentiometric automatic titrator (manufactured by Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610). The titration was performed using potentiometric titration with a 0.1N potassium hydroxide / ethanol solution, and the inflection point on the titration curve was defined as the endpoint. The acid value (mgKOH / g) was calculated from the amount of potassium hydroxide solution titrated to the endpoint.

[0072] (5) Measurement of the pH of carbon black In accordance with JIS K 5101-17-2:2004, a mixture of carbon black and distilled water was measured using a glass electrode pH meter. The measurement temperature was 23°C ± 1°C.

[0073] (6) Measurement of the specific surface area of ​​carbon black In accordance with ASTM D 6556, the amount of nitrogen adsorption was calculated using the BET formula.

[0074] (7) Measurement of the water solubility of the crosslinking agent 90 parts by mass of deionized water and 10 parts by mass of crosslinking agent (W1) were added to a glass tube (25 mmφ × 250 mmh) at room temperature of 25°C, and the glass tube was left to stand for 1 hour in a constant temperature bath adjusted to a water temperature of 25°C. Next, the glass tube was shaken vigorously for 1 minute, and then left to stand again in the constant temperature bath for 12 hours. Then, the undissolved material that separated from the water and settled or floated was collected, dried for 6 hours at 40°C and a gauge pressure of -0.08 MPa, and weighed (W2). The water solubility (mass%) was calculated using the following formula (1). Water solubility (mass%)={(W1-W2) / W1}×100 (1)

[0075] (8) Measurement of pH of water-based ink The pH of water-based ink at 25°C was measured using a benchtop pH meter "F-71" (manufactured by Horiba, Ltd.) equipped with a pH electrode "6337-10D" (manufactured by Horiba, Ltd.).

[0076] Preparation Example 1 (Preparation of Polymer (a1)) A monomer mixture was prepared by mixing 31 parts of acrylic acid (reagent manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 69 parts of styrene (reagent manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). In a reaction vessel, 10 parts of methyl ethyl ketone (MEK, reagent manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 0.2 parts of 2-mercaptoethanol (polymerization chain transfer agent, reagent manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 10% of the monomer mixture were added and mixed, and the mixture was thoroughly purged with nitrogen gas. Meanwhile, the remaining 90% of the monomer mixture, 0.13 parts of 2-mercaptoethanol, 30 parts of MEK, and 1.1 parts of 2,2'-azobis-(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name: V-65 polymerization initiator) were placed in a dropping funnel. Under a nitrogen atmosphere, the contents of the reaction vessel were stirred and the temperature was raised to 65°C, and the contents of the dropping funnel were added dropwise over 3 hours. The reaction vessel was maintained at 65°C for 2 hours after the completion of the dropwise addition from the dropping funnel. Then, a solution of 0.1 parts of the polymerization initiator dissolved in 2 parts of MEK was added, and the mixture was maintained at 65°C for another 2 hours, followed by aging at 70°C for another 2 hours, after which it was dried under reduced pressure to obtain polymer (a1) (non-crosslinked) (number average molecular weight: 12000, acid value: 240 mgKOH / g).

[0077] Manufacturing Example 1 (Production of an aqueous dispersion of polymer particles containing carbon black (the polymer constituting the particles = cross-linked polymer (A1))) (Process 1) 25 parts of polymer (a1) obtained in Preparation Example 1 were mixed with 78.6 parts of MEK, and then 10.2 parts of 5N sodium hydroxide aqueous solution (16.9% sodium hydroxide solids, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for volumetric titration) were added to neutralize the mixture so that the ratio of moles of sodium hydroxide to moles of carboxyl groups of the polymer was 40% (degree of neutralization 40%). Then 400 parts of deionized water were added, and 100 parts of carbon black pigment (CI Pigment Black 7, manufactured by Orion Engineered Carbons, trade name: NIPex 180IQ) were added to the mixture. The mixture was stirred for 60 minutes at 20°C with the disperser blades rotating at 7000 rpm using a disperser (manufactured by Asada Iron Works Co., Ltd., trade name: Ultra Disperser). (Process 2) Next, the mixture was dispersed in 15 passes at a pressure of 150 MPa using a microfluidizer (Microfluidics, trade name). 250 parts of deionized water were added to the resulting dispersion, and after stirring, MEK was completely removed under reduced pressure at 60°C, and some of the water was further removed. The liquid phase of the resulting dispersion was recovered using a centrifuge and filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain an aqueous dispersion of polymer particles containing carbon black. At this time, the solid content concentration of the aqueous dispersion of polymer particles containing carbon black was 25%. (Step 3) 100 parts of an aqueous dispersion of polymer particles containing carbon black were placed in a screw-top glass bottle, 31 parts of deionized water were added, and 1.5 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-321LT, epoxy equivalent: 140, water solubility 27%) were added. The bottle was then sealed tightly and heated at 70°C for 5 hours while stirring with a stirrer (crosslinking rate: 50 mol%). After 5 hours, the dispersion was cooled to room temperature (25°C) and filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain aqueous dispersion I-1 of polymer particles containing carbon black (polymer = crosslinked polymer (A1), acid value 120 mg KOH / g, degree of neutralization 80%) (solid content concentration: 20%, carbon black 15.1%, polymer 4.9%, average particle size 107 nm).

[0078] Manufacturing Examples 2-6 (Production of aqueous dispersions of polymer particles containing carbon black (polymer = cross-linked polymer (A1))) In Production Example 1, aqueous dispersions of polymer particles containing carbon black, I-2 to I-6, were obtained using the same procedure as in Production Example 1, except that the carbon black was changed to the one listed in Table 1. The carbon blacks used in manufacturing examples 1-6 are listed in Table 1 as follows: • NIPex180IQ (manufactured by Orion Engineered Carbons, pH 4.5, specific surface area 260 m²) 2 / g, channel black) • NIPex160IQ (manufactured by Orion Engineered Carbons, pH 4.5, specific surface area 180m²) 2 / g, channel black) • COLOUR BLACK FW 1 (manufactured by Orion Engineered Carbons, pH 3.5, specific surface area 320m²) 2 / g, channel black) • MA8 (manufactured by Mitsubishi Chemical Corporation, pH 3.0, specific surface area 120 m²) 2 ( / g, Furnace Black) • Monarch 717 (manufactured by Cabot, pH 8.5, specific surface area 183 m²) 2 ( / g, Furnace Black) • Printex55 (manufactured by Orion Engineered Carbons, pH 9.5, specific surface area 110 m²) 2 ( / g, Furnace Black)

[0079] Manufacturing Example 7 (Production of an aqueous dispersion of polymer particles containing carbon black (polymer = polymer(a1) (non-crosslinked))) In the procedure of Production Example 1, steps (1) and (2) were performed, but step (3) was omitted. In step (1), 25 parts of polymer (a1) obtained in Preparation Example 1 were increased to 33.3 parts, and 10.2 parts of 5N sodium hydroxide aqueous solution were added to 20.2 parts so that the ratio of moles of sodium hydroxide to moles of carboxyl groups of the polymer was 80% (degree of neutralization 80%), thereby obtaining an aqueous dispersion of polymer particles containing carbon black, I-7 (polymer = polymer (a1) (non-crosslinked), acid value 240 mg KOH / g, degree of neutralization 80%) (solid content concentration: 20%, carbon black 15.1%, polymer 4.9%, average particle size 105 nm).

[0080] [Table 1]

[0081] Manufacturing Example 8 (Production of an aqueous dispersion of polymer particles without pigments) 15.3 parts of polymer (a1) obtained in Preparation Example 1 were mixed with 63.5 parts of deionized water, and then 6.2 parts of 5N sodium hydroxide aqueous solution (solids content: 16.9%) were added to neutralize the mixture so that the ratio of moles of sodium hydroxide to moles of carboxyl groups of the polymer was 40% (degree of neutralization: 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 the polymer dispersion to room temperature, 4.6 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-321LT, epoxy equivalent: 140, water solubility 27%) were added, the container was sealed, and heated at 90°C for 1.5 hours while stirring with a stirrer (crosslinking rate 50 mol%). Subsequently, the polymer dispersion was cooled to room temperature (25°C) and filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain aqueous dispersion II-1 (solid content concentration: 20%, average particle size 51 nm) of pigment-free polymer particles (crosslinked polymer, acid value 120 mg KOH / g, degree of neutralization 80%).

[0082] Example 1 (Manufacturing of water-based ink) 28.2 parts of aqueous dispersion I-1 of polymer particles containing carbon black obtained in Production Example 1 (solid content concentration: 20% by mass) (composition: carbon black 4.2 parts, crosslinked polymer A1 1.4 parts, ion-exchanged water 22.6 parts), 25 parts of aqueous dispersion II-1 of pigment-free polymer particles obtained in Production Example 8 (solid content concentration: 20% by mass) (composition: polymer 5.0 parts, ion-exchanged water 20.0 parts), propylene glycol (PG, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) 20 parts of reagents, 5 parts of diethylene glycol monoisopropyl ether (iPDG, manufactured by Nippon Emulsifier Co., Ltd.), 2 parts of Surfinol 104PG-50 (manufactured by Nisshin Chemical Industry Co., Ltd., 50% propylene glycol solution of 2,4,7,9-tetramethyl-5-decine-4,7-diol) (1 part of 2,4,7,9-tetramethyl-5-decine-4,7-diol solids), 0.5 parts of Emulgen 120 (manufactured by Kao Corporation, ethylene oxide adduct of lauryl alcohol, 100% effective content), 0.0079 parts of sodium silicate No. 3 (manufactured by Fuji Chemical Co., Ltd., aqueous sodium silicate solution, 38% by mass effective content, (SiO2 (mass%)) / (Na2O (mass%))=3.2) (0.003 parts of sodium silicate effective content), and 1N aqueous sodium hydroxide solution (manufactured by Fujifilm Wako Pure Chemical Corporation). A suitable amount of (for volumetric analysis) was mixed to achieve an ink pH of 8.5, and then deionized water was added to bring the total volume to 100 parts. The mixture was then filtered through a cellulose acetate membrane filter with a pore size of 5 μm to obtain aqueous ink III-1 (polymer particles containing carbon black: 5.6% by mass (of which carbon black is 4.0% by mass, polymer particles without pigment are 5.0% by mass, and silicate compounds are 30 ppm by mass)).

[0083] Examples 2-11 and Comparative Examples 1-3 (Manufacturing of water-based inks) In Example 1, water-based inks III-2 to III-11 and III-12 to III-14 were obtained in the same manner as in Example 1, except that the conditions shown in Table 2 were changed. The results are shown in Table 2. In Comparative Example 1, a modified carbon black aqueous dispersion (CAB-O-JET200: trade name, manufactured by Cabot Corporation, 20% solids content, polymer-free, average particle size 130 nm) was used instead of polymer particles containing carbon black.

[0084] <Rating> Using the water-based inks obtained in the examples and comparative examples, the water repellency of an inkjet head using a silicon oxide nozzle plate was evaluated, as well as the long-term ejection reliability and ejection recovery performance of the water-based ink in the inkjet head, using the method described below. The results are shown in Table 2.

[0085] (1) Evaluation of water repellency of inkjet heads using silicon oxide nozzle plates As an inkjet head using a silicon oxide nozzle plate, a Samba G3L inkjet head manufactured by Fujifilm Dimatix Corporation was used as a test specimen. The contact angle of water on the water-repellent film was measured as follows, and the effect of the ink composition on the liquid-repellent properties of the water-repellent film was evaluated. 300 ml each of the water-based inks obtained in the examples and comparative examples were measured and placed into 500 ml wide-mouth bottles (iBoy wide-mouth bottle 500 ml (manufactured by AS ONE Corporation)). The test pieces were then immersed in the water-based ink, the containers were sealed tightly, and the test pieces were left standing in a constant temperature bath set to 50°C for 28 days. After that, the test pieces were removed, washed with deionized water, and the water contact angle on the water-repellent film surface of the nozzle plate was measured. Deionized water was used to measure the water contact angle, and the measurement was performed using a contact angle measuring device (Kyowa Interface Science Co., Ltd., DM-500) in an environment of 25°C and 50% relative humidity by a conventional method. Regarding the water repellency of the silicone nozzle plate, there are no practical problems if the contact angle is 80° or more, and it can be used suitably if the contact angle is 90° or more. (2) Evaluation of long-term discharge reliability The aqueous inks obtained in the examples and comparative examples were placed in glass containers, sealed tightly, and left to stand in a constant temperature bath at 50°C for 28 days, followed by a further 24 hours of standing at room temperature. The inks were then loaded into a Fujifilm Dimatix Samba G3L inkjet head, and ejection tests were conducted using a jetXpert inkjet liquid monitoring device (imageXpert) in an environment of 25±1°C and 30±5% relative humidity to confirm the ejection behavior. The ejection conditions involved using a single-pulse standard waveform, adjusting the voltage to achieve a droplet volume of 2.4 pL, and continuous ejection for 30 minutes. Subsequently, droplet 1, located approximately 0.2 mm from the nozzle, and droplet 2, located approximately 0.6 mm from the nozzle, were simultaneously photographed using a strobe light. The angle of the straight line connecting droplets 1 and 2, separated by approximately 0.4 mm, was measured 1000 times per nozzle to determine the deviation from the vertical (90 degrees). The standard deviation σ was calculated as an indicator of ejection direction disturbance. The standard deviation σ was the average value of 30 nozzles for each ink. If the two droplets are ejected perfectly perpendicularly from the nozzle plate, the standard deviation σ = 0. On the other hand, if the positions of droplet 1 and droplet 2 are shifted to the left or right relative to the perpendicular from the nozzle plate, the value of the standard deviation σ increases, and it is evaluated that there is an ejection direction disturbance. While a standard deviation σ of less than 15 mrad is acceptable for practical use, it is preferable to use it if it is less than 10 mrad, and even more preferable if it is less than 5 mrad.

[0086] (3) Evaluation of discharge recovery In an environment with a temperature of 25±1℃ and a relative humidity of 30±5%, a printing evaluation device (manufactured by Altech Co., Ltd.) equipped with a Fujifilm Dimatix Samba G3L inkjet head was filled with the water-based inks obtained in the examples and comparative examples. As a recording medium, coated paper "OK ​​Topcoat+" (manufactured by Oji Paper Co., Ltd., product name, water absorption capacity 4.9 g / m²) is used. 2 The recording medium was fixed to the transport table of the printing evaluation apparatus under reduced pressure so that its longitudinal direction and transport direction were in the same orientation. The ejection conditions were set to a single-pulse standard waveform, with the voltage adjusted to achieve a droplet volume of 2.4 pL, a drive frequency of 20 kHz, and a resolution of 1200 × 1200 dpi. A solid 2 cm square image with 100% duty cycle was printed in a single pass. The printer was then stopped for 10 minutes to expose the inkjet head to the atmosphere. After 10 minutes, the printing substrate was changed to a new one, and printing was resumed under the conditions of no flushing and 20 kHz. The same image as printed before the inkjet head was exposed to the atmosphere was printed, and the ejection recovery rate (%) was calculated based on observation of the obtained printout according to the following criteria to evaluate the ejection stability. Discharge recovery rate (%) = (Discharge area of ​​solid print after 10 minutes of exposure to air / Discharge area of ​​solid print before 10 minutes of exposure to air) × 100 A higher discharge recovery rate (%) indicates better discharge stability.

[0087] [Table 2]

[0088] Table 2 shows that the water-based inks obtained in Examples 1 to 11 exhibit superior suppression of the decrease in water repellency of the inkjet head, as well as excellent long-term ejection reliability and ejection recovery in inkjet recording, compared to the water-based inks obtained in Comparative Examples 1 to 3.

Claims

1. An inkjet water-based ink containing polymer particles containing carbon black, an organic solvent, a silicate compound, and water, The content of the silicate compound in the ink is 1 ppm by mass or more and 450 ppm by mass or less. The pH of the carbon black is 3.0 or higher and 9.5 or lower, and the specific surface area of ​​the carbon black is 100 m² / g or higher and 450 m² / g or lower. Water-based ink for inkjet recording.

2. The water-based inkjet recording ink according to claim 1, wherein the polymer particles containing carbon black are cross-linked polymer particles containing carbon black.

3. The water-based inkjet recording ink according to claim 2, wherein the acid value of the crosslinked polymer constituting the crosslinked polymer particles containing carbon black is 90 mg KOH / g or more and 200 mg KOH / g or less.

4. The aqueous inkjet recording ink according to claim 1, wherein the organic solvent is one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers.

5. The water-based ink for inkjet recording according to claim 1, further containing polymer particles that do not contain pigment.

6. The ratio of the silicate compound content in the ink to the product of the specific surface area of ​​carbon black and the carbon black content in the ink [(silicite compound content (mass ppm)) / {specific surface area of ​​carbon black (m²)} 2 The water-based inkjet recording ink according to claim 1, wherein the ratio of [(amount per gram) × carbon black content (mass%)] / 100 is 0.1 or more and 30 or less.

7. An inkjet recording method comprising using an inkjet water-based inkjet recording ink according to any one of claims 1 to 6, and ejecting the ink from an inkjet head using one or more selected from silicon and silicon oxide as the nozzle plate member.

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