Water-based ink for inkjet printing

Abstract

To provide a water-based ink for inkjet recording having high storage stability and ease of maintenance, and an inkjet recording method using the water-based ink.SOLUTION: The present invention provides [1] a water-based ink for ink jet recording that contains polymer particles containing pigment, dispersed with a polymer dispersant (A), polymer particles containing wax, dispersed with a polymer dispersant (B), and organic solvent, where the constituent polymers of the polymer particles containing wax have an acid value of 160-200 mgKOH / g, and the wax has a melting point of 80°C or higher, and [2] an inkjet recording method that uses the water-based ink for recording to a low-absorbency recording medium.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. [Background technology]

[0002] Inkjet recording is a method of directly ejecting ink droplets from minute nozzles and depositing them on a recording medium to produce a recorded product with characters or 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 is non-contact with the recording medium. In recent years, inks that use pigments as colorants have become more widely used due to the weather resistance and water resistance of the recorded product. However, unlike dyes, pigment molecules cannot be uniformly dissolved in the ink vehicle, so it is necessary to maintain the pigment's dispersed state and improve the storage stability of the ink. Furthermore, pigment inks are prone to clogging the nozzles of the head, so maintenance is essential to clear this clogging, but if this cannot be done easily, the ease of maintenance becomes poor and becomes a practical problem. Maintainability refers to the ease of cleaning and wiping the head and nozzles to reduce inkjet head malfunctions caused by thickening and adhesion of the ink, and the fewer times wiping is required, the less effort is required to keep the nozzles in good condition.

[0003] Various proposals have been made to overcome the above-mentioned problems. For example, Patent Document 1 discloses an inkjet recording method that achieves both image transferability and robustness, which repeats the following steps: a step of applying a reaction liquid containing a viscosity-increasing component onto a transfer body; a step of applying an ink containing water, a colorant, and a wax, thereby forming a first image containing a liquid component containing water and a high-boiling-point water-soluble organic solvent and a solid component formed by mixing the reaction liquid and the ink; a step of contacting a porous body with the first image to form a second image from which part of the liquid component has been removed; and a transfer step of heating the second image to transfer it onto a recording medium, wherein the temperatures of the first and second images are controlled under specific conditions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-144736 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technology of Patent Document 1 does not provide satisfactory storage stability and is also insufficient in terms of maintainability. An object of the present invention is to provide a water-based ink for ink-jet printing that is excellent in storage stability and maintainability, and an ink-jet recording method that uses the water-based ink. [Means for solving the problem]

[0006] The present inventors have found that the above problems can be solved by incorporating wax particles, which are obtained by dispersing a high-melting-point wax in a polymer dispersant having a specific acid value, into a water-based ink. That is, the present invention provides the following [1] and [2]. [1] A water-based ink for inkjet printing, comprising polymer particles containing a pigment dispersed with a polymer dispersant (A), polymer particles containing a wax dispersed with a polymer dispersant (B), and an organic solvent, wherein the acid value of the polymer constituting the wax-containing polymer particles is 160 mgKOH / g or more and 200 mgKOH / g or less, and the melting point of the wax is 80°C or more. [2] An inkjet recording method for recording on a low-liquid-absorbent recording medium using the water-based ink according to [1] above. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a water-based ink for ink-jet printing that is excellent in storage stability and maintainability, and an ink-jet recording method that uses the water-based ink. 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, comprising polymer particles containing a pigment dispersed with a polymer dispersant (A), polymer particles containing a wax dispersed with a polymer dispersant (B), and an organic solvent, The acid value of the polymer constituting the wax-containing polymer particles is 160 mgKOH / g or more and 200 mgKOH / g or less, and the melting point of the wax is 80°C or more.

[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. "Low liquid absorption" is a concept that includes low liquid absorption and non-liquid absorption, and the amount of water absorption of the recording medium when the recording medium is in contact with pure water for 100 ms is 0 g / m 2 More than 10g / m 2 This means that:

[0010] The water-based ink for ink-jet printing of the present invention has excellent storage stability and maintainability, and although the reason for this is not clear, it is thought to be as follows. When inkjet recording is performed on a recording medium with low liquid absorption, a large amount of pigment remains on the surface of the recording medium after recording, making the pigment more likely to peel off from the surface of the recording medium. However, it is known that if a water-based ink contains wax, the friction resistance of the ink coating film when rubbed decreases. The wax is preferably dispersed by a polymer, and if the acid value of the polymer is 160 mgKOH / g or more, the wax particles or the pigment particles and the wax particles exhibit a high repulsive force in the water-based ink, suppressing aggregation and thus resulting in high storage stability for the water-based ink.Furthermore, if the acid value of the polymer dispersing the wax is 200 mgKOH / g or less, the decrease in the adsorption force of the polymer to the wax surface is suppressed, making it difficult for the polymer to detach from the wax surface, resulting in excellent storage stability. Furthermore, since the melting point of the wax is 80°C or higher, it is believed that even when the ink is stored at high temperatures for a long period of time, the molten wax does not promote aggregation of the pigment, thereby improving storage stability.

[0011] On the other hand, water-based inks for inkjet printing are exposed to strong drying conditions near the ejection nozzle, which can lead to the formation of aggregates near the air-liquid interface, resulting in nozzle clogging. It is known that nozzle clogging can be resolved by purging the ink and wiping it off with a rubber wiper after the nozzle is refilled with ink components. By increasing the acid value of the wax-dispersing polymer in the water-based ink to 160 mgKOH / g or higher, the water-based ink can penetrate more easily into the interface of aggregates formed near the ejection nozzle, facilitating the redispersion of the aggregates to their original dispersed state, which is believed to improve maintenance. Furthermore, by ensuring that the difference in acid value between the pigment-dispersing polymer and the wax-dispersing polymer is within 20 mgKOH / g, the difference in polarity at the interface between the wax and pigment aggregates is reduced, weakening the cohesive force of both, further improving maintenance.

[0012] <Pigments> The pigment used in the present invention may be either an inorganic pigment or an organic pigment. Examples of inorganic pigments include carbon black and metal oxides, and carbon black is preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of white inks include titanium dioxide, zinc oxide, silica, alumina, and metal oxides such as magnesium oxide. Examples of organic pigments include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments. The hue is not particularly limited, and achromatic pigments such as white, black, and gray can be used in achromatic inks, while chromatic pigments such as yellow, magenta, cyan, red, blue, orange, and green can be used in chromatic inks. The pigments can be used alone or in combination of two or more.

[0013] The pigment used in the present invention is used in the form of polymer particles containing the pigment dispersed with a polymer dispersant (A) from the viewpoint of improving storage stability and ease of maintenance. Here, "pigment-containing polymer particles" (hereinafter also referred to as "pigment-containing polymer particles") refers to particles in which the polymer dispersant (A) encapsulates the pigment, particles in which part of the pigment is exposed on the surface of particles consisting of the polymer dispersant (A) and the pigment, particles in which the polymer dispersant (A) is adsorbed to part of the pigment, and mixtures thereof. Of these, particles in which the polymer dispersant (A) encapsulates the pigment are more preferred.

[0014] [Pigment-containing polymer particles] The polymer dispersant (A) (hereinafter also referred to as "polymer (a)") that constitutes the pigment-containing polymer particles is not particularly limited as long as it has at least the ability to disperse pigments. Examples of the polymer (a) include vinyl resins obtained by addition polymerization of vinyl monomers, polyester resins, polyurethane resins, etc. Among these, vinyl resins are preferred from the viewpoints of pigment dispersion stability, storage stability, etc. The polymer (a) may be a non-crosslinked polymer, and may be suitably synthesized or a commercially available product. The pigment-containing polymer particles are more preferably crosslinked polymer particles containing a pigment made of a polymer (a) having a crosslinked structure (hereinafter also referred to as "pigment-containing crosslinked polymer particles"). The "polymer (a) having a crosslinked structure" means that the polymer dispersant (A) constituting the pigment-containing polymer particles is a polymer having structural units derived from the crosslinking agent in addition to structural units derived from the polymer (a).

[0015] When the polymer dispersant (A) does not have a crosslinked structure, the polymer (a) may be a water-soluble or water-insoluble polymer, but a water-insoluble polymer is more preferred. When the polymer dispersant (A) has a crosslinked structure, it inevitably becomes a water-insoluble polymer by having both the structural units of the polymer (a) and the structural units derived from the crosslinking agent. As used herein, the term "water-insoluble" refers to a polymer that has been dried at 105°C for 2 hours and allowed to reach a constant weight, and when the polymer is dissolved in 100 g of water at 25°C, the amount of dissolution is less than 10 g. In the case of an anionic polymer, the amount of dissolution is the amount of dissolution when the anionic groups of the polymer are 100% neutralized with sodium hydroxide.

[0016] [Polymer (a)] When the polymer (a) is a vinyl resin, the polymer (a) preferably contains (a-1) a structural unit derived from an ionic monomer, and more preferably further contains (a-2) a structural unit derived from a hydrophobic monomer and / or (a-3) a nonionic monomer.

[0017] [(a-1) Ionic Monomer] The (a-1) ionic monomer is preferably an anionic monomer, and examples thereof include carboxylic acid monomers and sulfonic acid monomers, with carboxylic acid monomers being more preferred. The carboxylic acid monomer may be at least one selected from (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, with (meth)acrylic acid being preferred. "(Meth)acrylic acid" refers to at least one selected from acrylic acid and methacrylic acid.

[0018] [(a-2) Hydrophobic Monomer] (a-2) The term "hydrophobic" in the hydrophobic monomer means that when the monomer is dissolved in 100 g of ion-exchanged water at 25° C. until saturation, the amount of the dissolved monomer is less than 10 g. Specific examples of the (a-2) hydrophobic monomer include alkyl(meth)acrylates and aromatic group-containing monomers. The alkyl(meth)acrylate preferably has an alkyl group having from 1 to 22 carbon atoms, and more preferably has an alkyl group having from 6 to 18 carbon atoms. Examples include methyl(meth)acrylate, ethyl(meth)acrylate, (iso)propyl(meth)acrylate, (iso- or tert-)butyl(meth)acrylate, (iso)amyl(meth)acrylate, cyclohexyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, (iso)octyl(meth)acrylate, (iso)decyl(meth)acrylate, (iso)dodecyl(meth)acrylate, and (iso)stearyl(meth)acrylate. It should be noted that "(iso or tert-)" and "(iso)" refer to both the case where these groups are present and the case where they are not present, and when these groups are not present, it means normal.

[0019] The aromatic group-containing monomer is preferably a vinyl monomer having an aromatic group with 6 to 22 carbon atoms, which may have a substituent containing a hetero atom, and more preferably at least one selected from a styrene-based monomer and an aromatic group-containing (meth)acrylate. The molecular weight of the aromatic group-containing monomer is preferably less than 500. As the styrene-based monomer, styrene, 2-methylstyrene, α-methylstyrene, vinyltoluene, and divinylbenzene are preferred, and styrene is more preferred. As the aromatic group-containing (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, etc. are preferred, and benzyl (meth)acrylate is more preferred. Among these, alkyl(meth)acrylates having an alkyl group with 1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, and more preferably 1 to 8 carbon atoms, and aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms, preferably 6 to 18 carbon atoms, are preferred, and one or more selected from alkyl(meth)acrylates having an alkyl group with 1 to 4 carbon atoms and aromatic group-containing monomers having an aromatic group with 6 to 12 carbon atoms are more preferred, and one or more selected from ethyl acrylate, styrene, and α-methylstyrene are even more preferred.

[0020] [(a-3) Nonionic Monomer] (a-3) The nonionic monomer is a monomer that has a high affinity with water or a water-soluble organic solvent, and is, for example, a monomer that contains a hydroxyl group or a polyalkylene glycol chain. Specific examples of the (a-3) component include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 3-hydroxypropyl (meth)acrylate; polyalkylene glycol (meth)acrylates such as polyethylene glycol (meth)acrylate (n=2 to 30, n represents the average number of moles of oxyalkylene groups added; the same applies below) and polypropylene glycol (n=2 to 30) (meth)acrylate; alkoxypolyalkylene glycol (meth)acrylates such as methoxypolyethylene glycol (n=1 to 30) (meth)acrylate; and phenoxy (ethylene glycol-propylene glycol copolymer) (n=1 to 30, where ethylene glycol: n=1 to 29) (meth)acrylate. 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.

[0021] (Content of each structural unit in polymer (a)) The content of the structural units derived from the components (a-1) to (a-3) in the polymer (a) is as follows, from the viewpoint of improving storage stability and ease of maintenance. The content of component (a-1) is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 12% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% 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 95% by mass or less, more preferably 90% by mass or less, even more preferably 88% by mass or less.

[0022] When the component (a-3) is contained, the content thereof is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less. The mass ratio of component (a-1) to component (a-2) [component (a-1) / component (a-2)] is preferably 0.2 or more, more preferably 0.25 or more, and is preferably 1 or less, more preferably 0.8 or less, and even more preferably 0.5 or less.

[0023] (Production of polymer (a)) The polymer (a) can be produced by copolymerizing a mixture of the above monomer components (a-1) to (a-3) by a known polymerization method, preferably a solution polymerization method. There are no limitations on the solvent used in the solution polymerization method, but polar solvents such as water, aliphatic alcohols, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred. During the polymerization, a polymerization initiator or a polymerization chain transfer agent can be used. Examples of the polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble azo polymerization initiators. Examples of the polymerization chain transfer agent include mercaptans. The polymerization temperature varies depending on the type of polymerization initiator, monomer, solvent, etc. used, but is preferably 30°C or higher, more preferably 50°C or higher, and is preferably 95°C or lower, more preferably 80°C or lower. The polymerization atmosphere is preferably a nitrogen gas atmosphere or an inert gas atmosphere. The polymer (a) is preferably neutralized with a neutralizing agent as described below.

[0024] When the polymer (a) does not have a crosslinked structure, its number average molecular weight is preferably 5,000 or more, more preferably 10,000 or more, and even more preferably 20,000 or more, from the viewpoint of improving storage stability and dispersibility of the pigment in an aqueous medium, and is preferably 80,000 or less, more preferably 60,000 or less, and even more preferably 40,000 or less. The number average molecular weight of the polymer is measured by the method described in the Examples.

[0025] [Production of pigment-containing polymer particles] The pigment-containing polymer particles can be efficiently produced as a pigment aqueous dispersion by a method including the following steps 1 and 2, and it is preferable to further carry out the following step 3. Step 1: A step of dispersing a pigment mixture containing a pigment, a polymer (a), an organic solvent, and water to obtain a dispersion. Step 2: A step of removing the organic solvent from the dispersion obtained in Step 1 to obtain an aqueous dispersion of pigment-containing polymer particles A (hereinafter also referred to as "pigment aqueous dispersion (i)"). Step 3: A step of adding a crosslinking agent to the pigment aqueous dispersion (i) obtained in Step 2 to crosslink the pigment-containing polymer particles, thereby obtaining an aqueous dispersion (I) of pigment-containing crosslinked polymer particles (hereinafter also referred to as "pigment aqueous dispersion (I)").

[0026] (Process 1) The pigment mixture in step 1 is preferably obtained by a method in which the polymer (a) is dissolved in an organic solvent, and the pigment, water, and, if necessary, a neutralizing agent, a surfactant, and the like are added to the obtained organic solvent solution and mixed to obtain an oil-in-water dispersion. Although there are no limitations on the organic solvent used in step 1, ketones, ethers, esters, aliphatic alcohols having from 1 to 3 carbon atoms, etc. are preferred, and from the viewpoint of improving the wettability to the pigment and the adsorption of polymer (a) to the pigment, ketones having from 4 to 8 carbon atoms are more preferred, and methyl ethyl ketone is even more preferred. When a vinyl resin is synthesized as polymer (a) by solution polymerization, the solvent used in the polymerization may be used as is.

[0027] When the polymer (a) has acid groups, it is preferable that at least a portion of the acid groups be neutralized with a neutralizing agent, which is believed to increase the charge repulsion force that occurs after neutralization, thereby suppressing aggregation of pigment particles in the aqueous ink and improving the dispersion stability of the pigment. When neutralizing, it is preferable to neutralize so that the pH is 7 or more and 11 or less. Examples of the neutralizing agent include bases such as sodium hydroxide, potassium hydroxide, ammonia, and various amines, with sodium hydroxide and ammonia being preferred. The polymer (a) may be neutralized in advance. From the viewpoint of improving dispersion stability and storage stability, the amount of neutralizing agent used is preferably 10 mol% or more, more preferably 15 mol% or more, even more preferably 20 mol% or more, and is preferably 150 mol% or less, more preferably 120 mol% or less, even more preferably 100 mol% or less. Here, the equivalent amount of the neutralizing agent used can be calculated by the following formula, where polymer (a) before neutralization is "polymer (a')". Equivalent amount of neutralizing agent used (mol %)=[{mass (g) of neutralizing agent added / equivalent amount of neutralizing agent} / [{acid value of polymer (a') (mg KOH / g) × mass (g) of polymer (a')} / (56 × 1,000)]] × 100

[0028] In the dispersion treatment in step 1, the pigment particles can be atomized to a desired particle size by main dispersion using shear stress alone. However, from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion. As a dispersing machine used for preliminary dispersion, a commonly used mixing and stirring device such as an anchor blade or a disperser blade can be used. Examples of means for applying shear stress used in this dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersers such as paint shakers and bead mills. Among these, it is preferable to use high-pressure homogenizers and bead mills 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 pigment aqueous 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.

[0029] (Process 2) The organic solvent can be removed by a known method in step 2. It is preferable that the organic solvent in the obtained pigment water dispersion (i) has been substantially removed, but it may remain as long as it does not impair the object of the present invention. Furthermore, in order to remove coarse particles and the like, it is preferable that the aqueous dispersion from which the organic solvent has been removed is further centrifuged, and then the liquid layer portion is filtered through a filter or the like, and the pigment aqueous dispersion (i) is obtained by passing through the filter or the like.

[0030] (Step 3) In step 3, which is optional, a crosslinking agent is added to the pigment aqueous dispersion (i) obtained in step 2, and reacted with some of the carboxy groups of the polymer (a) that constitutes the pigment-containing polymer particles to form a crosslinked structure in the surface layer of the pigment-containing polymer particles, thereby obtaining a pigment aqueous dispersion (I) of pigment-containing crosslinked polymer particles. This allows the pigment to have structural units derived from the crosslinking agent in addition to the structural units of the polymer (a), and the polymer obtained by crosslinking the polymer (a) is firmly adsorbed or fixed to the pigment surface, suppressing pigment aggregation. As a result, it is believed that the dispersion stability and storage stability of the resulting ink are further improved.

[0031] The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a compound having two or more glycidyl ether groups, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 4 carbon atoms. The epoxy equivalent of the crosslinking agent is preferably 90 or more, more preferably 100 or more, and preferably 300 or less, more preferably 200 or less. Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

[0032] From the viewpoint of improving storage stability, the degree of crosslinking in step 3 is preferably 8 mol % or more, more preferably 10 mol % or more, and is preferably 70 mol % or less, more preferably 50 mol % or less, in terms of the ratio of the molar equivalents of the crosslinkable functional groups of the crosslinking agent to the molar equivalents of the carboxy groups of polymer (a). When the polymer dispersant (A) has a crosslinked structure, it is difficult to measure its number average molecular weight. However, it is preferable that the number average molecular weight is larger than the number average molecular weight of the polymer (a) before the crosslinked structure is formed, from the viewpoint of improving the dispersibility of the pigment in an aqueous medium and the storage stability of the ink. From the viewpoint of crosslinking reaction efficiency, the temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 50° C. or higher, and preferably 90° C. or lower, more preferably 80° C. or lower.

[0033] The non-volatile component concentration (solid content concentration) of the resulting pigment water dispersion (I) is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less, from the viewpoint of improving dispersion stability. The solid content concentration is measured by the method described in the Examples. From the viewpoint of improving dispersion stability, the content of the pigment in the pigment water dispersion (I) is preferably 5% by mass or more, more preferably 8% by mass or more, and is preferably 40% by mass or less, more preferably 30% by mass or less. The mass ratio of the (crosslinked) polymer constituting the pigment-containing (crosslinked) polymer particles to the pigment [(crosslinked) polymer / pigment] in the pigment aqueous dispersion (I) is preferably 0.05 or more, more preferably 0.1 or more, and is preferably 1 or less, more preferably 0.5 or less. In the present invention, when calculating the mass ratio of the crosslinked polymer constituting the pigment-containing crosslinked polymer particles to the pigment in the pigment water dispersion (I), the mass of the constituent units derived from the crosslinking agent is also included.

[0034] From the viewpoint of improving the storage stability and maintainability of the ink, the acid value of the polymer constituting the pigment-containing (crosslinked) polymer particles before dispersion treatment (before crosslinking) is preferably 50 mgKOH / g or more, more preferably 80 mgKOH / g or more, even more preferably 100 mgKOH / g or more, and still more preferably 120 mgKOH / g or more, and is preferably 350 mgKOH / g or less, more preferably 300 mgKOH / g or less, and even more preferably 250 mgKOH / g or less. From the same viewpoints as above, the acid value of the polymer constituting the pigment-containing (crosslinked) polymer particles after dispersion treatment (crosslinking) is preferably 100 mgKOH / g or more, more preferably 120 mgKOH / g or more, even more preferably 140 mgKOH / g or more, still more preferably 160 mgKOH / g or more, still more preferably 170 mgKOH / g or more, and preferably 250 mgKOH / g or less, more preferably 230 mgKOH / g or less, still more preferably 200 mgKOH / g or less, still more preferably 190 mgKOH / g or less, still more preferably 180 mgKOH / g or less. The acid value of the polymer is measured by the method described in the Examples.

[0035] From the viewpoint of storage stability, etc., the average particle size of the pigment-containing (crosslinked) polymer particles is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and is preferably 400 nm or less, more preferably 300 nm or less, even more preferably 200 nm or less. The average particle size is measured by the method described in the Examples.

[0036] <Wax> In the present invention, the wax may be either a natural wax or a synthetic wax. Examples of natural waxes include petroleum waxes such as paraffin wax and microcrystalline wax; vegetable waxes such as carnauba wax, candelilla wax and rice wax; and animal waxes such as lanolin and beeswax. Examples of synthetic waxes include synthetic hydrocarbon waxes such as polyolefin wax and Fischer-Tropsch wax; silicone wax; and modified waxes such as paraffin wax derivatives, montan wax derivatives, and microcrystalline wax derivatives. Among these, polyolefin waxes containing an olefin monomer as the main component are preferred. The above waxes can be used alone or in combination of two or more.

[0037] From the viewpoint of improving the storage stability and maintainability of the ink, the melting point of the wax used in the present invention is 80°C or higher, preferably 100°C or higher, more preferably 105°C or higher, even more preferably 108°C or higher, and preferably 150°C or lower, more preferably 145°C or lower, even more preferably 140°C or lower. The melting point of the wax is measured by the method described in the Examples.

[0038] [Wax-containing polymer particles] The wax used in the present invention is used as polymer particles containing wax dispersed with a polymer dispersant (B) (hereinafter also referred to as "wax-containing polymer particles") from the viewpoint of improving the storage stability and maintainability of the ink. Here, "wax-containing polymer particles" refers to particles in which the polymer dispersant (B) encapsulates the wax, particles consisting of the polymer dispersant (B) and the wax with part of the wax exposed on the surface, particles in which the polymer dispersant (B) is adsorbed to part of the wax, and mixtures thereof. Among these, particles in which the polymer dispersant (B) and the wax with part of the wax exposed on the surface, or particles in which the polymer dispersant (B) is adsorbed to part of the wax are more preferred.

[0039] The polymer dispersant (B) (hereinafter also referred to as "polymer (b)") constituting the wax-containing polymer particles preferably has (b-1) a structural unit derived from a carboxylic acid monomer and (b-2) a structural unit derived from a hydrophobic monomer. Examples of the polymer (b) include vinyl resins obtained by addition polymerization of vinyl monomers, polyester resins, polyurethane resins, etc. Among these, vinyl resins are preferred from the viewpoints of pigment dispersion stability, storage stability, etc. The polymer (b) may be a non-crosslinked polymer, and may be an appropriately synthesized or commercially available product. The wax-containing polymer particles are more preferably crosslinked polymer particles containing wax (hereinafter also referred to as "wax-containing crosslinked polymer particles") in which wax is dispersed with a polymer dispersant (B) having a crosslinked structure. The "polymer dispersant (B) having a crosslinked structure" means that the polymer dispersant (B) is a polymer having structural units derived from a crosslinker in addition to the structural units of the aforementioned polymer (b).

[0040] [Polyolefin wax] Examples of the olefin-based monomer that is the main component of the polyolefin wax include linear olefins and cyclic olefins. However, those that are mainly composed of linear olefins having 2 to 6 carbon atoms are preferred, polyolefin waxes that are mainly composed of ethylene or propylene are more preferred, and polyethylene waxes that are mainly composed of ethylene are more preferred. Here, "mainly composed of ethylene or propylene" means that the content of ethylene or propylene relative to all components constituting the wax is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more. Oxidized polyolefin waxes can be obtained by 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 are included in the category of polyolefin waxes. That is, the polyolefin wax is preferably at least one selected from polyethylene wax, polypropylene wax, and oxidized polyethylene wax, and polyethylene wax is more preferred.

[0041] From the same viewpoints as above, the weight average molecular weight of the polyolefin wax is preferably 400 or more, more preferably 600 or more, and even more preferably 800 or more, and is preferably 10,000 or less, more preferably 8,000 or less, and even more preferably 6,000 or less. From the same viewpoint as above, the density of the polyolefin wax is preferably 0.890 g / cm 3 More preferably, 0.900 g / cm 3 More preferably, 0.910 g / cm 3 and preferably 0.985 g / cm 3 or less, more preferably 0.980 g / cm 3 More preferably, 0.975 g / cm or less 3 The following is the result. In the present invention, waxes other than the polyolefin wax may be contained within the range that does not impair the effects of the present invention. 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.

[0042] [Polymer (b)] The polymer (b) preferably contains at least (b-1) a structural unit derived from a carboxylic acid monomer and (b-2) a structural unit derived from a hydrophobic monomer, and may further contain (b-3) a structural unit derived from a nonionic monomer.

[0043] Specific examples and preferred examples of the (b-1) carboxylic acid monomer are the same as those of the (a-1) ionic monomer described in the section on polymer (a). That is, the (b-1) component is preferably at least one selected from acrylic acid and methacrylic acid. Specific examples and preferred examples of the hydrophobic monomer (b-2) are the same as those described in the section on polymer (a). That is, the component (b-2) is preferably one or more selected from alkyl(meth)acrylates having an alkyl group with 1 to 4 carbon atoms and aromatic group-containing monomers having an aromatic group with 6 to 12 carbon atoms, and more preferably one or more selected from ethyl acrylate, styrene, and α-methylstyrene. Specific examples and preferred examples of the (b-3) nonionic monomer are the same as the specific examples and preferred examples of the (a-3) nonionic monomer explained in the section on the polymer (a) above.

[0044] From the viewpoint of improving storage stability, the content of the structural units derived from components (b-1) to (b-3) in polymer (b) is the same as the content of the structural units derived from components (a-1) to (a-3) in polymer (a). That is, the content of the (b-1) component is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 12% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% 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 95% by mass or less, more preferably 90% by mass or less, even more preferably 88% by mass or less.

[0045] The polymer (b) can be produced in the same manner as in the above (production of polymer (a)). When polymer (b) has acid groups, it is preferable that at least a portion of the acid groups be neutralized with a neutralizing agent, which is believed to increase the charge repulsion force that occurs after neutralization, thereby suppressing aggregation of wax particles in the aqueous ink and improving the dispersion stability of the wax. The amount of neutralizing agent used is the same as described above (Production of polymer (a)), and is preferably 10 mol% or more, more preferably 15 mol% or more, even more preferably 20 mol% or more, and is preferably 150 mol% or less, more preferably 120 mol% or less, even more preferably 100 mol% or less.

[0046] [Production of wax-containing crosslinked polymer particles] Examples of methods for producing wax-containing crosslinked polymer particles include (i) a method in which wax is dispersed in a dispersion containing a (crosslinked) polymer obtained by copolymerizing a monomer mixture containing the aforementioned component (b-1), component (b-2), and, if necessary, component (b-3), and (ii) a method in which a crosslinking agent is added to a dispersion containing wax and an uncrosslinked polymer obtained by copolymerizing a monomer mixture containing the aforementioned component (b-1), component (b-2), and, if necessary, component (b-3), and a crosslinking treatment is performed. The dispersion treatment method and crosslinking treatment method are the same as those in the above-mentioned [Production of Pigment-Containing Polymer Particles], and specific examples and preferred examples are also basically the same. Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

[0047] Among the above manufacturing methods, from the viewpoint of improving the storage stability and maintainability of the ink, (ii) a method in which a crosslinking agent is added to a dispersion containing an uncrosslinked polymer obtained by copolymerizing a monomer mixture containing the above-mentioned components (b-1), (b-2), and, if necessary, (b-3), and a crosslinking treatment is performed is preferred. The reason for this is that in the step of dispersing the polyolefin wax, it is thought that an uncrosslinked polymer is more likely to be adsorbed more quickly and uniformly to the wax surface than a crosslinked polymer. By obtaining wax-containing crosslinked polymer particles using method (ii), it is possible to obtain particles containing structural units derived from the crosslinking agent in addition to the structural units of polymer (b). The crosslinking reaction of polymer (b) may be carried out in a system containing or not containing a pigment, but is preferably carried out in a system not containing a pigment, because if the crosslinking reaction is carried out in a system containing both an aqueous pigment dispersion and an aqueous dispersion of wax-containing polymer particles, the balance of electrostatic repulsive forces between dispersed particles in the system may change in the early stage of the crosslinking reaction, which may cause aggregation of the pigment in the aqueous pigment dispersion.

[0048] From the viewpoint of improving the storage stability and maintainability of the ink, the degree of crosslinking of the polymer (b) is preferably 1 mol % or more, more preferably 2 mol % or more, even more preferably 4 mol % or more, and is preferably 30 mol % or less, more preferably 25 mol % or less, even more preferably 20 mol % or less. The degree of crosslinking is the equivalent weight of crosslinking sites in all the monomers constituting the polymer, and is calculated by the following formula: Degree of crosslinking = [(molar equivalent of monomer forming crosslinked site) × (number of crosslinked sites in the monomer skeleton forming crosslinked site) / (molar equivalent of all monomers constituting the polymer)] × 100

[0049] From the viewpoint of improving the storage stability and maintainability of the ink, the acid value of the polymer before crosslinking that constitutes the wax-containing crosslinked polymer particles is preferably 50 mgKOH / g or more, more preferably 80 mgKOH / g or more, even more preferably 100 mgKOH / g or more, and even more preferably 120 mgKOH / g or more, and is preferably 350 mgKOH / g or less, more preferably 300 mgKOH / g or less, and even more preferably 250 mgKOH / g or less. From the same viewpoint as above, the acid value of the crosslinked polymer constituting the wax-containing crosslinked polymer particles is preferably 50 mgKOH / g or more, more preferably 80 mgKOH / g or more, even more preferably 100 mgKOH / g or more, and even more preferably 120 mgKOH / g or more, and is preferably 300 mgKOH / g or less, more preferably 250 mgKOH / g or less, and even more preferably 200 mgKOH / g or less.

[0050] It is preferable that the difference in acid value between the polymer constituting the pigment-containing polymer particles and the polymer constituting the wax-containing polymer particles is small. Specifically, the difference in acid value is more preferably within 20 mgKOH / g, even more preferably within 10 mgKOH / g, and even more preferably within 5 mgKOH / g, and it is most preferable that there is substantially no difference and that the acid values ​​are the same. That is, it is preferable that the polymer constituting the pigment-containing polymer particles and the polymer constituting the wax-containing polymer particles are the same. In the present invention, it is preferred that both the polymer constituting the pigment-containing polymer particles and the polymer constituting the wax-containing polymer particles are crosslinked.

[0051] The mass ratio of wax to polymer (b) in the wax-containing (crosslinked) polymer particles [wax / polymer (b)] is preferably 1 or more, more preferably 2 or more, even more preferably 4 or more, from the viewpoint of improving the storage stability of the resulting wax dispersion and ink, image fastness, etc., and is preferably 30 or less, more preferably 20 or less, even more preferably 15 or less. In the present invention, when calculating the mass ratio of the wax to the polymer (b) in the wax-containing crosslinked polymer particles, the mass of the constituent units derived from the crosslinking agent is also included.

[0052] From the viewpoint of improving the storage stability and maintainability of the ink, the average particle size of the wax-containing (crosslinked) polymer particles is preferably 20 nm or more, more preferably 40 nm or more, even more preferably 60 nm or more, and is preferably 250 nm or less, more preferably 200 nm or less, even more preferably 150 nm or less. The average particle size of the polymer particles is measured by the method described in the Examples.

[0053] <Organic solvents> The organic solvent used in the present invention mainly serves to impart wetting and spreading properties to the 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 an amount of 10 mL or more is preferred. From the viewpoint of improving the storage stability and maintainability of the ink, 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 a polyhydric alcohol and a polyhydric alcohol alkyl ether. 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.

[0054] 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, and one or more selected from dipropylene glycol monomethyl ether and diethylene glycol monobutyl ether are 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, dipropylene glycol monomethyl ether, and diethylene glycol monobutyl ether are 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.

[0055] The ink of the present invention may further contain a surfactant, if necessary, from the viewpoint of improving the wettability to the recording medium. Examples of surfactants include nonionic surfactants, silicone surfactants, and fluorine surfactants, with nonionic surfactants being more preferred. Examples of nonionic surfactants include acetylene glycol surfactants, polyoxyalkylene alkyl ether surfactants, polyhydric alcohol surfactants, fatty acid alkanolamides, etc. Among these, acetylene glycol surfactants and polyoxyalkylene alkyl ether surfactants are preferred, and for example, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, polyoxyethylene alkyl ether, etc. are more preferred. 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.

[0056] (Content of each component in the ink of the present invention) The content of each component in the ink of the present invention is as follows, from the viewpoint of improving the storage stability and maintainability of the ink.

[0057] (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 4% by mass or more, and is preferably 12% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less. The content of 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 5% by mass or more, and preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less.

[0058] (Wax content) The content of wax, particularly polyolefin wax, in the ink of the present invention is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, even more preferably 1% by mass or more, and preferably 4% by mass or less, more preferably 3% by mass or less, even more preferably 2.5% by mass or less. The content of wax-containing crosslinked polymer particles in the ink of the present invention is preferably 0.6% by mass or more, more preferably 1% by mass or more, even more preferably 1.5% by mass or more, and preferably 5% by mass or less, more preferably 4% by mass or less, even more preferably 3% by mass or less.

[0059] The mass ratio of pigment to wax (pigment / wax) in the water-based ink is preferably 1 or more, more preferably 2 or more, even more preferably 2.5 or more, and is preferably 6 or less, more preferably 5 or less, even more preferably 4 or less.

[0060] (Contents of polymer dispersants (A) and (B) in the ink of the present invention) The content of the polymer dispersant (A) in the ink of the present invention is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, even more preferably 0.7% by mass or more, and preferably 1.5% by mass or less, more preferably 1.2% by mass or less, even more preferably 1% by mass or less. The content of the polymer dispersant (B) in the ink of the present invention is preferably 0.1% by mass or more, more preferably 0.15% by mass or more, even more preferably 0.2% by mass or more, and preferably 1% by mass or less, more preferably 0.8% by mass or less, even more preferably 0.5% by mass or less. The mass ratio of the pigment to the total content of the polymer dispersants (A) and (B) in the water-based ink [pigment / polymer dispersant [(A)+(B)]] is preferably 2 or more, more preferably 3 or more, even more preferably 4 or more, and is preferably 12 or less, more preferably 10 or less, even more preferably 8 or less. In the present invention, when the polymer dispersants (A) and (B) in the water-based ink are crosslinked polymers, the mass ratio of the pigment to the total content of the polymer dispersants (A) and (B) is calculated by taking into account the mass of the structural units derived from the crosslinker in the polymer dispersants (A) and (B).

[0061] (Organic solvent content) The content of the organic solvent in the ink of the present invention is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less.

[0062] The content of the surfactant in the ink of the present invention is preferably 0.2% by mass or more, more preferably 0.4% by mass or more, even more preferably 0.6% by mass or more, and preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less. (Acetylenic glycol surfactant content) The content of the acetylene glycol surfactant in the water-based ink is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.2% by mass or more, from the viewpoint of improving the wettability of the ink to a recording medium, and is preferably 1.2% by mass or less, more preferably 1% by mass or less, and even more preferably 0.8% by mass or less, from the viewpoint of improving the ejection stability of the ink. (Content of other nonionic surfactants) The content of other nonionic surfactants in the water-based ink, such as polyoxyalkylene alkyl ether surfactants, is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, and is preferably 3% by mass or less, more preferably 2% by mass or less, even more preferably 1.5% by mass or less.

[0063] 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 75% by mass or less, even more preferably 70% by mass or less. The ink of the present invention may contain various additives as optional components depending on the application, such as preservatives, pH adjusters, viscosity adjusters, antifoaming agents, and rust inhibitors. In such cases, a portion of the water content may be replaced with the various additives.

[0064] (Physical properties of the ink of the present invention) From the viewpoint of improving the storage stability and maintainability of the ink, the average particle size of the polymer particles, including the pigment-containing polymer particles and wax-containing polymer particles, in the ink of the present invention is preferably 40 nm or more, more preferably 60 nm or more, even more preferably 80 nm or more, and is preferably 250 nm or less, more preferably 200 nm or less, even more preferably 150 nm or less. From the viewpoint of improving storage stability, the viscosity of the ink of the present invention at 32°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 5 mPa·s or more, and preferably 12 mPa·s or less, more preferably 9 mPa·s or less, even more preferably 7 mPa·s or less. The viscosity of the water-based ink can be measured using an E-type viscometer. From the viewpoint of improving storage stability, the pH of the ink of the present invention is preferably 7 or more, more preferably 7.5 or more, and even more preferably 8 or more. From the viewpoint of component resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less, and even more preferably 9.5 or less. The pH of the water-based ink is measured by the method described in the examples.

[0065] [Inkjet recording method] The ink jet recording method of the present invention is an ink jet recording method in which recording is performed on a low liquid-absorbent recording medium using the ink of the present invention. The ink of the present invention can be loaded into a known ink jet recording apparatus and ejected as ink droplets onto a recording medium to record an image or the like. Ink droplets are ejected in inkjet recording using a piezoelectric method, a thermal method, or an electrostatic method, with the piezoelectric method being preferred. In the piezoelectric method, multiple nozzles are each connected to a pressure chamber, and ink droplets are ejected from the nozzles by vibrating the walls of the pressure chambers with a piezoelectric element. The ink of the present invention can also be used to obtain high quality recordings on low-liquid-absorbency recording media by ink jet recording. Examples of low-liquid-absorbent recording media include low-liquid-absorbent coated paper, art paper, and non-liquid-absorbent resin films. Examples of coated paper include general-purpose glossy paper and multicolor foam glossy paper. Examples of resin films include transparent synthetic resin films, such as polyester, polyvinyl chloride, polyolefin, and nylon. These films may be biaxially oriented, uniaxially oriented, or unoriented. Among these, polyester films and oriented polypropylene films are preferred, and corona-discharge-treated polyethylene terephthalate (PET) films and corona-discharge-treated biaxially oriented polypropylene (OPP) films are more preferred. [Example]

[0066] In the following Production Examples, Preparation 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.

[0067] (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 guardcolumn 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 with a magnetic stirrer at 25°C for 10 hours, and filtering with a syringe filter (DISMIC-13HP, made of PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0068] (2) Measurement of the acid value of the polymer The acid value of the polymer was measured by dissolving the resin in a titration solvent consisting of a 2:1 mixture of toluene and acetone in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610) and titrating it 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. The acid value of the wax dispersion or pigment-containing polymer particle dispersion was measured by diluting 2 g of the dispersion with 50 g of ion-exchanged water, adding 3 ml of 0.1 N sodium hydroxide solution, slowly adding dropwise 0.1 N hydrochloric acid, and measuring the pH inflection points at two locations by potentiometric titration. The number of moles of acid calculated from the difference in the amount of 0.1 N hydrochloric acid added between the two points corresponds to the number of moles of carboxylic acid in the polymer, and this number of moles was converted to an acid value (mg KOH / g).

[0069] (3) Measurement of solids concentration of aqueous dispersion 10.0 g of sodium sulfate, brought to a constant weight in a desiccator, was weighed into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm), and approximately 1.0 g of the sample (aqueous dispersion) was added thereto and mixed. The mixture was then accurately weighed, maintained at 105°C for 2 hours to remove volatiles, and left in the desiccator for another 15 minutes before being weighed. The mass of the sample after volatiles removal was taken as the solid content and divided by the initial sample mass to obtain the solid content concentration.

[0070] (4) Measurement of the melting point of wax The melting point of the wax was measured using an apparatus 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 determined as the maximum peak temperature of melting, and this peak temperature was taken as the melting point.

[0071] (5) Measurement of the average particle size of polymer particles in pigment dispersion The particle size was measured by dynamic light scattering using a laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd., product name: ELS-8000), 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 integration times. The refractive index of water (1.333) was entered as the refractive index of the dispersion solvent. For the measurement sample, a water-based pigment dispersion was weighed into a screw tube (Maruem Co., Ltd., No. 5) and measured to a solids concentration of 2 x 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.

[0072] (6) Measurement of the average particle size of polymer particles in wax dispersion The average particle size (average dispersed particle size) of the polymer particles in the wax dispersion was measured using a Microtrac particle size analyzer UPA manufactured by Nikkiso Co., Ltd.

[0073] (7) pH measurement The pH of the ink at 20° C. was measured using a tabletop pH meter (manufactured by Horiba Ltd., product name: F-71) equipped with a pH electrode (manufactured by Horiba Ltd., product name: 6337-10D).

[0074] Production Example 1 (Preparation of Water-Insoluble Polymer (i)) A monomer mixture was prepared by mixing 29 parts of acrylic acid, 60 parts of styrene, and 11 parts of ethyl acrylate. 10 parts of MEK, 0.2 parts of 2-mercaptoethanol (a polymerization chain transfer agent), and 10% of the monomer mixture were placed in a reaction vessel and mixed, followed by thorough nitrogen gas replacement. Separately, a mixture of the remaining 90% of the monomer mixture, 0.2 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name: V-65) was placed in a dropping funnel, and the monomer mixture in the reaction vessel was heated to 65°C under a nitrogen atmosphere while being stirred, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 65°C from the end of the dropping, a solution of 0.1 parts of the polymerization initiator in 2 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and then at 70°C for 2 hours, and then dried under reduced pressure to obtain water-insoluble polymer (i) (number average molecular weight: 32,000, acid value: 226 mgKOH / g) used for water-insoluble polymer (a) and water-insoluble polymer (b). The results are shown in Table 1.

[0075] Production Examples 2 and 3 (Preparation of Water-Insoluble Polymers (ii) and (iii)) Water-insoluble polymers (ii) and (iii) were obtained in the same manner as in Production Example 1, except that the conditions in Production Example 1 were changed to those shown in Table 1. The results are shown in Table 1.

[0076] [Table 1]

[0077] Preparation Example I-1 (Preparation of Wax Dispersion W1) To 10 parts of the water-insoluble polymer (i) obtained in Production Example 1 as the polymer dispersant (B), 3.8 parts of a 5N aqueous sodium hydroxide solution (NaOH solid content: 16.9%) was added as a neutralizing agent for neutralizing the carboxyl groups of the polymer (i) to neutralize the polymer (i) (neutralizing agent use equivalent: 40 mol %). Further, 240 parts of ion-exchanged water was added, and polyethylene wax (HW110P: manufactured by Mitsui Chemicals, Inc., trade name: Hiwax 110P, melting point: 109°C, density: 0.920 g / cm) was added thereto. 3100 parts by mass of a polymer (weight average molecular weight: 1000, softening point: 113°C) was heated and dissolved at 85 to 95°C, and the resulting mixture was dispersed for 30 minutes using an ultrasonic homogenizer while being maintained at 90 to 95°C. The mixture was then cooled to room temperature and dispersed three times using a Microfluidizer (trade name, manufactured by Microfluidics) at a pressure of 200 MPa to obtain a dispersion. Ion-exchange water was added to the resulting dispersion, followed by 0.8 parts (corresponding to a crosslinking degree of 15 mol%) of an epoxy crosslinker (trimethylolpropane polyglycidyl ether, Nagase ChemteX Corporation, product name: Denacol EX-321, epoxy value: 140). The mixture was sealed and heated at 70°C for 5 hours with stirring to obtain Dispersion W1 of wax-containing crosslinked polymer particles (the mass ratio of the contents of Dispersion W1 was 20.0% wax, 2.2% polymer dispersant (B) (0.2% epoxy crosslinker-derived components in Dispersion W1), 0.14% sodium hydroxide, and the remainder was water). Average particle size: 88 nm, pH: 7.3, acid value of polymer dispersant (B): 171 mgKOH / g). The results are shown in Table 2.

[0078] Preparation Examples I-2 to I-10 (Preparation of Wax Dispersions W2 to W10) Wax dispersions W2 to W10 were obtained in the same manner as in Preparation Example I-1, except that the type of wax, the type of polymer dispersant (B), the amount of NaOH, and the amount of crosslinking agent were changed as shown in Table 2. The results are shown in Table 2. It should be noted that wax dispersions W6, W7, and W9 do not contain an epoxy crosslinking agent, and therefore the wax-containing polymer particles contained in wax dispersions W6, W7, and W9 do not have a crosslinked structure.

[0079] Details of the waxes used in the examples and comparative examples in Table 2 are as follows: HW110P: Hiwax 110P (polyethylene wax, melting point: 109°C, density: 0.920g / cm 3 , weight average molecular weight: 1000, manufactured by Mitsui Chemicals, Inc.) HW200P: High Wax 200P (polyethylene wax, melting point: 122°C, density: 0.970g / cm3 , weight average molecular weight: 2000, manufactured by Mitsui Chemicals, Inc.) HWNP055: Hiwax NP055 (Polypropylene wax, melting point: 136°C, density: 0.900g / cm 3 , weight average molecular weight: 7400, manufactured by Mitsui Chemicals, Inc.) TW131: TOWAX-131 (Carnauba wax, melting point: 109°C, density: 0.997g / cm 3 , low molecular weight mixture, manufactured by Toa Kasei Co., Ltd.) PW155: Paraffin wax 155 (Paraffin wax, melting point: 69°C, density: 0.927g / cm 3 , low molecular weight mixture, manufactured by Nippon Seiro Co., Ltd.)

[0080] [Table 2]

[0081] Preparation Example II-1 (Preparation of Pigment Dispersion P1) As the polymer dispersant (A), 40 parts of the water-insoluble polymer (i) obtained in Production Example 1 was mixed with 78.6 parts of MEK, and then 15.5 parts of a 5N aqueous sodium hydroxide solution (NaOH solid content: 16.9%) was added as a neutralizer to neutralize the carboxyl groups of the polymer (i) to neutralize the mixture (neutralizer equivalent: 40 mol%). Further, 500 parts of ion-exchanged water was added, and 300 parts of carbon black pigment (CB, CI Pigment Black 7, manufactured by Cabot Chemical Corporation, product name: Monarch 800) was added thereto, and the mixture was stirred for 60 minutes using a Disper mixer (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) at 20°C with the Disper blade rotating at 7000 rpm. The resulting mixture was subjected to a dispersion treatment using a Microfluidizer (trade name, manufactured by Microfluidics) at a pressure of 200 MPa for 10 passes to obtain a pigment dispersion (solid concentration: 22.6%, pigment: 20%, polymer: 2.6%). To the resulting pigment dispersion, 250 parts of ion-exchanged water was added and stirred. After stirring, MEK was completely removed at 60°C under reduced pressure. Further removal of water resulted in a solids concentration of 20% (pigment: 17.7%). After this, 3.4 parts of an epoxy crosslinker (trimethylolpropane polyglycidyl ether, Nagase ChemteX Corporation, trade name: Denacol EX-321, epoxy value: 140) (corresponding to a crosslinking degree of 15 mol%) was added. The container was sealed and heated at 70°C for 5 hours with stirring to obtain Dispersion P1, a pigment-containing crosslinked polymer particle dispersion (mass ratio of Dispersion P1: pigment: 17.7%, polymer dispersant (A): 2.5% (the amount of components derived from the epoxy crosslinker in Dispersion P1 is 0.2%), sodium hydroxide: 0.15%, and the remainder is water). Average particle size: 98 nm, pH: 7.3, acid value of polymer dispersant (A): 171 mgKOH / g). The results are shown in Table 3.

[0082] Preparation Examples II-2 to II-6 (Preparation of Pigment Dispersions P2 to P6) Pigment dispersions P2 to P6 were obtained in the same manner as in Preparation Example II-1, except that the type of polymer dispersant (A), the amount of NaOH, and the amount of crosslinking agent in Preparation Example II-1 were changed as shown in Table 3. The results are shown in Table 3. It should be noted that pigment dispersions P3, P4, and P6 do not contain an epoxy crosslinking agent, and therefore the pigment-containing polymer particles contained in pigment dispersions P3, P4, and P6 do not have a crosslinked structure.

[0083] [Table 3]

[0084] Example 1 (Production of Water-Based Ink 1 for Ink-Jet Printing) 10 parts of dispersion W1 of crosslinked polymer particles containing wax, 34 parts of dispersion P1 of crosslinked polymer particles containing pigment, 16.0 parts of propylene glycol, 6.0 parts of dipropylene glycol monomethyl ether, 2.0 parts of diethylene glycol monobutyl ether (all organic solvents manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., total amount 24.0 parts), 0.5 parts of an acetylene glycol-based nonionic surfactant (2,4,7,9-tetramethyl-5-decyne-4,7-diol, manufactured by Nissin Chemical Industry Co., Ltd., trade name: Surfynol 104), 0.5 parts of a nonionic surfactant (polyoxyethylene Aqueous ink 1 (solids concentration: 9.1%, pigment: 6%, polymer dispersant (A): 0.85%, wax: 2.0%, polymer dispersant (B): 0.22%) was obtained by adding 1.0 parts of ethylene alkyl ether (Emulgen 120, manufactured by Kao Corporation) (total amount of surfactants: 1.5 parts), 0.45 parts of triethanolamine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a pH adjuster to adjust the pH of the aqueous ink to 9.0, and ion-exchanged water to a total amount of 100 parts.

[0085] Examples 2 to 10 and Comparative Examples 1 to 4 (Production of Water-Based Inks 2 to 10 and 21 to 24) In the same manner as in Example 1, water-based inks 2 to 10 and 21 to 24 were obtained using the formulations shown in Table 4.

[0086] The storage stability and maintenance properties of the water-based inks obtained in the examples and comparative examples were evaluated by the following methods. The results are shown in Table 4.

[0087] (1) Evaluation of storage stability Each water-based ink was stored in an environment of 70°C for 30 days, and the rate of change in viscosity of each water-based ink before and after storage was evaluated. The ink viscosity at 32°C of each water-based ink was measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.) before and after storage, and the rate of change in viscosity was calculated using the following formula to evaluate storage stability. Viscosity change rate [= (ink viscosity after storage) / (ink viscosity before storage) x 100] The lower the rate of change in viscosity, the better the storage stability is judged to be, and a value of 110 or less is practically usable.

[0088] (2) Evaluation of maintainability A water-based ink was loaded into a print evaluation device (manufactured by Tritec Corporation) equipped with an inkjet head (Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezo type) in an environment with a temperature of 25±1°C and a relative humidity of 30±5%. The settings were head voltage: 26V, frequency: 10 kHz, ejection volume: 12 pL, head temperature: 32°C, resolution: 600 dpi, pre-ejection flushing: 200 shots, and negative pressure: -4.0 kPa. The recording medium was fixed to the conveying table under reduced pressure with its longitudinal direction aligned with the conveying direction. A print command was transferred to the print evaluation device, and a solid image was printed at 100% duty. The printer was then stopped for 30 minutes, allowing the print head to be exposed to the atmosphere. After 30 minutes, 5 mL of ink was purged from the inkjet head, wiped, and printing resumed. The nozzle recovery rate (%) was calculated using the following formula to evaluate maintainability. Nozzle recovery rate (%) = (number of normal nozzles / total number of nozzles) x 100 The higher the nozzle recovery rate (%), the better the nozzle recovery is judged to be, and a value of 90% or higher is practically usable.

[0089] [Table 4]

[0090] From Table 4, it can be seen that the water-based inks obtained in the examples have better storage stability and maintainability than the water-based inks obtained in the comparative examples.

Claims

1. A water-based ink for ink-jet printing, comprising polymer particles containing a pigment dispersed with a polymer dispersant (A), polymer particles containing a wax dispersed with a polymer dispersant (B), and an organic solvent, The water-based ink for ink-jet printing, wherein the acid value of the polymer constituting the wax-containing polymer particles is 160 mgKOH / g or more and 200 mgKOH / g or less, and the melting point of the wax is 80°C or more.

2. 2. The water-based ink for ink-jet printing according to claim 1, wherein the polymer constituting the wax-containing polymer particles is crosslinked.

3. 3. The water-based ink for ink-jet printing according to claim 1, wherein a difference in acid value between the polymer constituting the pigment-containing polymer particles and the polymer constituting the wax-containing polymer particles is 20 mgKOH / g or less.

4. 4. The water-based ink for ink-jet printing according to claim 1, wherein the acid value of the polymer constituting the pigment-containing polymer particles is from 100 mgKOH / g to 250 mgKOH / g.

5. 5. The water-based ink for ink-jet printing according to claim 1, wherein the polymer constituting the pigment-containing polymer particles is crosslinked.

6. 6. The water-based ink for ink-jet printing according to claim 1, wherein both the polymer constituting the pigment-containing polymer particles and the polymer constituting the wax-containing polymer particles are crosslinked.

7. 7. The water-based ink for ink-jet printing according to claim 1, wherein the wax is a polyolefin wax.

8. 8. The water-based ink for ink-jet printing according to claim 1, wherein the organic solvent contains a polyhydric alcohol and a polyhydric alcohol alkyl ether.

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

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