Method for producing aqueous pigment dispersion

Abstract

To provide a production method of an aqueous pigment dispersion suitable for an aqueous ink excellent in storage stability and discharge stability, and a production method of an aqueous ink for ink jet recording made from the aqueous pigment dispersion.SOLUTION: [1] A production method of an aqueous pigment dispersion comprises a step (I) of dispersing a pigment, a wax and a polymer dispersant to obtain an aqueous dispersion, and a step (II) of adding a cross-linking agent to the resulting aqueous dispersion and heat treating the mixture at a temperature equal to or less than the melting point of the wax. [2] A production method of an aqueous ink for ink jet recording comprises a step of mixing the aqueous pigment dispersion and a water-soluble organic solvent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a water-based pigment dispersion suitable for use in a water-based ink for ink-jet printing, and a method for producing a water-based ink for ink-jet printing. [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 ease of full color production, low cost, the ability to use plain paper as the recording medium, and 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. Recently, particularly, there has been a growing interest in replacing gravure printing using oil-based inks with aqueous inkjet printing on low-liquid-absorbency media, which has traditionally been the mainstream method, with water-based inkjet printing, due to the reduced environmental impact of not requiring printing plates. Because the ink vehicle does not penetrate into low-liquid-absorbency media, pigment particles contained in the ink remain on the surface of the printing medium. However, because water-based inks cannot be formulated with large amounts of resins that impart fixation properties like oil-based inks, they lack sufficient adhesion and abrasion resistance when printed on low-liquid-absorbency media. Various attempts have been made to improve these issues.

[0003] For example, Patent Document 1 discloses an aqueous pigment dispersion in which a pigment is dispersed with a polymer dispersant for the purpose of improving fixability and abrasion resistance, the polymer dispersant containing a (meth)acrylic resin (A) and a resin (B), and the resin (B) containing an acid-modified polyolefin resin. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-119889 Summary of the Invention [Problem to be solved by the invention]

[0005] Water-based inks containing pigments have problems with ejection stability compared to inks that use dyes as colorants. Furthermore, inks that contain resin particles in addition to pigments to improve fixation are exposed to strong shear conditions near the ejection nozzle, which makes them prone to forming aggregates near the nozzle edge, further reducing ejection stability and storage stability. The present invention relates to an aqueous pigment dispersion and a method for producing an aqueous ink for ink-jet printing, which can improve the storage stability and ejection stability of an ink blended with a resin to improve fixability, particularly an ink blended with a wax. [Means for solving the problem]

[0006] The present inventors have found that the above-mentioned problems can be solved by providing a step of heat-treating an aqueous pigment dispersion containing a pigment, a wax, and a polymer dispersant at a temperature equal to or lower than the melting point of the wax when preparing the aqueous pigment dispersion, as a method for obtaining an aqueous ink for inkjet printing having excellent storage stability and ejection stability.

[0007] That is, the present invention relates to the following [1] and [2]. [1] A method for producing an aqueous pigment dispersion, comprising the following steps (I) and (II): Step (I): A step of dispersing a pigment, a wax, and a polymer dispersant to obtain an aqueous dispersion. Step (II): A step of adding a crosslinking agent to the aqueous dispersion obtained in step (I) and heat-treating the mixture at a temperature equal to or lower than the melting point of the wax. [2] A method for producing a water-based ink, comprising the step of mixing the water-based pigment dispersion obtained by the production method described in [1] above with a water-soluble organic solvent. [Effects of the Invention]

[0008] The present invention provides a method for producing a water-based pigment dispersion suitable for use in water-based inks that have excellent storage stability and ejection stability, and a method for producing a water-based ink for ink-jet printing using the same. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Method for producing aqueous pigment dispersion] The method for producing the water-based pigment dispersion of the present invention (hereinafter also simply referred to as "pigment dispersion of the present invention") includes the following steps (I) and (II). Step (I): A step of dispersing a pigment, a wax, and a polymer dispersant to obtain an aqueous dispersion. Step (II): A step of adding a crosslinking agent to the aqueous dispersion obtained in Step (I), and heat-treating the mixture at a temperature equal to or lower than the melting point of the wax to obtain an aqueous pigment dispersion. In this specification, "recording" is a concept that includes printing and printing characters or images, and "recorded matter" is a concept that includes printed matter and printed matter on which characters or images are recorded. By "water-based" it is meant that water constitutes the majority of the medium contained in the dispersion or ink.

[0010] According to the present invention, it is possible to obtain a water-based pigment dispersion that is excellent in storage stability and ejection stability and is suitable for use in a water-based ink for ink-jet printing. The reason for this is not clear, but is thought to be as follows. In conventional pigment dispersion manufacturing methods, a crosslinking agent is added to the dispersion and then heat-treated at a temperature equal to or higher than the melting point of the wax, which results in a problem that the wax melts partially or entirely, and the molten wax, which does not maintain its particle shape, adsorbs onto the pigment surface due to hydrophobic interactions, causing a portion of the polymer dispersant that was distributed on the pigment surface before the heat-treatment to be desorbed. As a result, it is believed that the storage stability of the pigment is reduced. In contrast, in the production method of the present invention, in step (II), a crosslinking agent is added to an aqueous dispersion of pigment, wax, and polymer dispersant, and the mixture is heat-treated at a temperature equal to or lower than the melting point of the wax. This prevents the wax from melting, and furthermore, it is thought that the polymer dispersant distributed on the pigment surface and the polymer dispersant distributed on the wax surface are rearranged so as to be uniformly distributed, resulting in an aqueous pigment dispersion that has high dispersion stability despite containing wax. Furthermore, by crosslinking the polymer dispersant with a crosslinking agent, the polymer dispersant adheres more firmly to the surfaces of the pigment and wax, thereby further improving the dispersion stability of the aqueous pigment dispersion. Therefore, it is believed that the pigment dispersion obtained by the production method of the present invention can provide an aqueous ink with excellent storage stability and ejection stability.

[0011] [Process (I)] Step (I) is a step in which a pigment, a wax, and a polymer dispersant are dispersed to obtain an aqueous dispersion.

[0012] In the present invention, prior to step (I), a step of first mixing the pigment, wax, and polymer dispersant in an aqueous medium to obtain a mixture of the pigment, wax, and polymer dispersant, i.e., a mixing step, may be included.

[0013] The mixing step can be carried out by a conventional method. The mixing step may be carried out under heating. The heating temperature is preferably 80°C or higher, more preferably 85°C or higher, and preferably 98°C or lower, more preferably 95°C or lower.

[0014] In step (I) of the present invention, the polymer dispersant may be neutralized. When the polymer dispersant is neutralized, the neutralizing agent may be added in step (I) or in the mixing step. In the present invention, the neutralizing agent for neutralizing the polymer dispersant is preferably added in the mixing step from the viewpoint of efficiently progressing the dispersion treatment in step (I).

[0015] Step (I) of the present invention may be carried out in a medium containing only water, but is preferably carried out in an aqueous medium. By aqueous medium, we mean a medium in which the pigment, wax, and polymer dispersant are dispersed in step (I), with water accounting for the largest proportion by mass. The medium may contain an organic solvent as a component other than water. As organic solvents, aliphatic alcohols, ketones, ethers, esters, etc. having 1 to 3 carbon atoms are preferred from the viewpoint of improving the pigment wettability, the solubility of the polymer dispersant, and the adsorption of the polymer dispersant to the pigment. Ketones having 4 to 8 carbon atoms are more preferred, with methyl ethyl ketone and methyl isobutyl ketone being even more preferred, and methyl ethyl ketone being even more preferred. These organic solvents preferably have a boiling point of 100°C or less, so that they do not remain in the resulting aqueous pigment dispersion of the present invention and can be easily removed after achieving their intended purpose.

[0016] The dispersion treatment in step (I) refers to an operation of reducing solids such as pigments, waxes, and polymer dispersants to particle sizes suitable for use in aqueous inkjet inks, where the particle sizes are 40 nm to 200 nm in terms of cumulant average particle size determined by cumulant analysis as described in the Examples. In the dispersion treatment of the present invention, the mixture of pigment, wax, and polymer dispersant can be atomized to a desired particle size by only main dispersion using shear stress. However, from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the mixture and then further main dispersion.

[0017] In step (I), when pre-dispersing the mixture of pigment, wax, and polymer dispersant, a commonly used mixing and stirring device such as a disperser blade, or a conventional method such as an ultrasonic homogenizer can be used, but an ultrasonic homogenizer is preferred. The pre-dispersion step may be carried out under heating. The heating temperature is preferably 80°C or higher, more preferably 85°C or higher, and is preferably 98°C or lower, more preferably 95°C or lower.

[0018] Examples of means for applying shear stress for this dispersion include kneading machines such as roll mills and kneaders, high-pressure homogenizers such as Microfluidizer (manufactured by Microfluidics), and media-type dispersers such as paint shakers and bead mills. Commercially available media-type dispersers include Ultra Apex Mill (manufactured by Kotobuki Industries Co., Ltd.) and Picomill (manufactured by Asada Iron Works Co., Ltd.). A combination of these devices can also be used. Of these, it is preferable to use a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When the main dispersion is carried out using a high-pressure homogenizer, the pigment particle size can be controlled to a desired size by controlling the processing pressure and the number of passes. From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more, more preferably 100 MPa or more, even more preferably 130 MPa or more, and is preferably 250 MPa or less, more preferably 230 MPa or less. The number of passes is preferably 3 or more, more preferably 8 or more, and is preferably 20 or less, more preferably 15 or less.

[0019] The pigments, waxes, and polymeric dispersants are described in detail below.

[0020] [Pigments] The pigment used in the present invention may be either an inorganic pigment or an organic pigment. Specific examples of inorganic pigments include carbon black, metal oxides such as titanium oxide, iron oxide, red iron oxide, and chromium oxide, and pearlescent pigments. Carbon black is particularly preferred for black inks. Examples of carbon black include furnace black, thermal lamp black, acetylene black, and channel black. Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and threne pigments. The hue is not particularly limited, and achromatic pigments such as white, black, and gray, and chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can all be used. The pigments can be used alone or in combination of two or more.

[0021] In the present invention, from the viewpoint of the storage stability of the aqueous pigment dispersion and the ejection stability of an aqueous ink containing the aqueous pigment dispersion, the pigment is preferably contained as pigment-containing polymer particles (hereinafter also simply referred to as "pigment-containing polymer particles") obtained by crosslinking the pigment, wax, and polymer dispersant with a crosslinking agent.

[0022] 〔wax〕 Generally, a wax is defined as an organic substance that is solid at room temperature and becomes liquid upon heating. However, in the present invention, any wax, such as polyolefin wax, ketone wax, amide wax, or paraffin wax, can be used. Furthermore, from the viewpoint of ink storage stability and ejection stability, the wax used in the present invention is preferably a wax that is poorly dispersible in aqueous media, i.e., a wax that does not disperse in aqueous media without a dispersant. Therefore, it is preferable that the wax used in the present invention is not a wax modified with a compound having a carboxy group. A wax modified with a compound having a carboxy group refers to a polyolefin wax into which a carboxy group or a carboxylic acid ester group has been introduced by copolymerizing a monomer having such a group, or a polyolefin wax into which an ester bond moiety or a carboxylic acid anhydride moiety capable of generating a carboxy group upon heating in water has been introduced. However, waxes that contain a small amount of carboxy groups in their structure, such as oxidized waxes obtained by oxidizing polyolefin wax, ketone wax, amide wax, or paraffin wax, are not substantially dispersible in aqueous media without a dispersant, and therefore are not considered to be waxes modified with a compound having a carboxy group in the present invention.

[0023] (Polyolefin wax) Polyolefin wax is a wax whose main component is an olefin-based monomer, and specific examples thereof include polyethylene, polypropylene, ethylene-propylene copolymer, propylene-α-olefin copolymer, etc. The copolymer may be either a block copolymer or a random copolymer. Examples of olefin monomers that are the main component of polyolefin wax include linear olefins and cyclic olefins, with those primarily composed of linear olefins having 2 to 6 carbon atoms being preferred, and polyethylene waxes primarily composed of ethylene being more preferred. Here, "primarily composed of ethylene" means that the ethylene content of the wax is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more, based on the total components constituting the wax. From the viewpoint of improving the storage stability and ejection stability of the water-based ink, the melting point of the polyolefin wax is preferably 100°C or higher, more preferably 105°C or higher, and preferably 140°C or lower, more preferably 130°C or lower.

[0024] 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. From the viewpoint of improving the storage stability and ejection stability of the water-based ink, the melting point of the oxidized polyolefin wax is preferably 120°C or higher, more preferably 125°C or higher, and preferably 160°C or lower, more preferably 150°C or lower.

[0025] (paraffin wax) Paraffin wax is a solid wax whose main component is saturated aliphatic hydrocarbon, and preferably has a melting point of 47°C to 69°C.

[0026] (modified wax) The wax may be a modified wax, such as a polyolefin obtained by acid-modifying the above-mentioned polyolefin with an unsaturated carboxylic acid compound. The unsaturated carboxylic acid compound used for acid modification of polyolefins may be at least one selected from unsaturated carboxylic acids, unsaturated carboxylic acid derivatives, and unsaturated carboxylic acid anhydrides. The unsaturated carboxylic acid refers to an unsaturated compound containing a carboxy group. The unsaturated carboxylic acid derivative refers to a mono- or diester, amide, imide, or the like of an unsaturated compound containing a carboxy group. The unsaturated carboxylic acid anhydride refers to an acid anhydride of an unsaturated compound containing a carboxy group. Examples of unsaturated carboxylic acid compounds include fumaric acid, maleic acid, itaconic acid, citraconic acid, aconitic acid, nadic acid and their anhydrides, methyl fumarate, ethyl fumarate, propyl fumarate, butyl fumarate, dimethyl fumarate, diethyl fumarate, dipropyl fumarate, dibutyl fumarate, methyl maleate, ethyl maleate, propyl maleate, butyl maleate, dimethyl maleate, diethyl maleate, dipropyl maleate, dibutyl maleate, maleimide, N-phenylmaleimide, (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, cyclohexyl (meth)acrylate, n-lauryl (meth)acrylate, benzyl (meth)acrylate, and N,N-dimethylaminoethyl (meth)acrylate. Among these, maleic anhydride is preferred. Examples of the acid modification method include a method in which an unsaturated carboxylic acid compound is used for copolymerization during the synthesis of the polyolefin, and a method in which the polyolefin is graft-modified. From the viewpoint of improving the storage stability and ejection stability of the water-based ink, the melting point of the acid-modified polyolefin is preferably 50°C or higher, more preferably 60°C or higher, and preferably 100°C or lower, more preferably 90°C or lower.

[0027] From the viewpoint of improving the storage stability of the aqueous pigment dispersion and the aqueous ink, and the ejection stability of the aqueous ink, the weight-average molecular weight of the wax is preferably 300 or more, more preferably 500 or more, and even more preferably 1,000 or more, and is preferably 10,000 or less, more preferably 8,000 or less. The weight-average molecular weight is a value measured by gel permeation chromatography (standard substance: polystyrene).

[0028] From the viewpoint of improving the storage stability of the aqueous pigment dispersion and the aqueous ink, and the ejection stability of the aqueous ink, the acid value of the wax is preferably 150 mgKOH / g or less, more preferably 100 mgKOH / g or less, even more preferably 20 mgKOH / g or less, still more preferably 1 mgKOH / g or less, and still more preferably 0 mgKOH / g. The acid value of the wax can be measured by the method described in the Examples.

[0029] Examples of commercially available waxes include the Hiwax (registered trademark) series manufactured by Mitsui Chemicals, Inc., the Diacarna (registered trademark) series manufactured by Mitsubishi Chemical Corporation, Paraffin Wax 155, 150, 145, 140, 135, 130, 125, 120, and 115 manufactured by Nippon Seiro Co., Ltd., and the Hi-Tec E series and Hi-Tec S series manufactured by Toho Chemical Industry Co., Ltd.

[0030] [Polymer dispersant] From the viewpoint of the dispersion stability of the pigment, the polymer dispersant used in the present invention preferably has acid groups, and it is preferable that at least a part of the acid groups be neutralized with a neutralizer. This is thought to increase the charge repulsion force that occurs after neutralization, suppress aggregation of pigment particles in the aqueous pigment dispersion, suppress thickening, and improve storage stability. Examples of the acid group include groups that exhibit acidity by dissociating and releasing a hydrogen ion, such as a carboxy group (-COOM), a sulfonic acid group (-SO3M), and a phosphate group (-OPO3M2), or their dissociated ionic forms. Of these, the carboxy group (-COOM) is preferred from the viewpoint of improving the storage stability of the water-based pigment dispersion and water-based ink, as well as the ejection stability of the water-based ink.

[0031] From the viewpoint of improving the jetting stability of the aqueous ink, the acid value of the polymer dispersant is preferably 50 mgKOH / g or more, more preferably 70 mgKOH / g or more, even more preferably 90 mgKOH / g or more, even more preferably 150 mgKOH / g or more, and even more preferably 220 mgKOH / g or more, and is preferably 300 mgKOH / g or less, more preferably 270 mgKOH / g or less, and even more preferably 250 mgKOH / g or less. When the acid value is within the above range, the amount of acid groups and their neutralized acid groups is sufficient, ensuring the dispersion stability of the pigment and, as a result, the jetting stability of the aqueous ink. This range is also preferable in terms of the balance between the affinity of the polymer dispersant to the aqueous medium and the interactions between the polymer dispersant and the pigment and between the polymer dispersant and the wax. The acid value of a polymer dispersant can be calculated from the mass ratio of the constituent monomers, but it can also be determined by dissolving or swelling the polymer dispersant in an appropriate organic solvent (e.g., MEK, acetone, toluene, THF, trichlorobenzene) and titrating it.

[0032] The polymer dispersant used in the present invention contains at least (a-1) a structural unit derived from a carboxylic acid monomer and (a-2) a structural unit derived from a hydrophobic monomer. The polymer dispersant may further contain (a-3) a structural unit derived from a nonionic monomer.

[0033] The (a-1) 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.

[0034] ((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 those described in paragraphs

[0020] to

[0022] of JP 2018-83938 A. Among these, alkyl(meth)acrylates having an alkyl group with 1 to 18 carbon atoms, particularly 1 to 10 carbon atoms, aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end are preferred, with one or more selected from styrene, α-methylstyrene, and benzyl(meth)acrylate being more preferred.

[0035] Examples of the macromonomer having a polymerizable functional group at one end include compounds having a number average molecular weight of 500 or more and 100,000 or less, preferably 1,000 or more and 10,000 or less, and in which the polymerizable functional group is an acryloyloxy group or a methacryloyloxy group. The macromonomer is preferably an aromatic group-containing monomer-based macromonomer, and examples of the aromatic group-containing monomer constituting the macromonomer include the aromatic group-containing monomers described above. Specific examples of commercially available styrene-based macromonomers include AS-6(S), AN-6(S), and HS-6(S) manufactured by Toagosei Co., Ltd.

[0036] ((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 component (a-3) include those described in paragraph

[0018] of JP-A No. 2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferred. The monomer components contained in each of the above components (a-1) to (a-3) can be used alone or in combination of two or more.

[0037] The content of the structural units derived from components (a-1) to (a-3) in the polymer dispersant is as follows, from the viewpoint of improving the storage stability of the water-based pigment dispersion and the water-based ink, and the ejection stability of the water-based ink. 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 55% by mass or more, more preferably 60% by mass or more, even more preferably 65% ​​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.

[0038] 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.

[0039] The mass ratio of [(a-1) component / (a-2) component] is preferably 0.1 or more, more preferably 0.15 or more, even more preferably 0.25 or more, and is preferably 3 or less, more preferably 2 or less, even more preferably 1 or less, and still more preferably 0.5 or less. Furthermore, when the (a-3) component is contained, the mass ratio of [(a-1) component / [(a-2) component+(a-3) component]] is preferably 0.03 or more, more preferably 0.05 or more, even more preferably 0.1 or more, and is preferably 1 or less, more preferably 0.8 or less, even more preferably 0.6 or less, and still more preferably 0.5 or less.

[0040] (Production of polymer dispersants) The polymer dispersant can be produced by copolymerizing a mixture of the above monomer components (a-1) to (a-3) by a known polymerization method, preferably solution polymerization. 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. As the polymerization initiator, known radical polymerization initiators can be used, such as azo compounds such as 2,2'-azobisisobutyronitrile and 2,2'-azobis(2,4-dimethylvaleronitrile), and organic peroxides such as t-butyl peroxyoctoate and benzoyl peroxide. The amount of the radical polymerization initiator is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and preferably 5 parts by mass or less, more preferably 2 parts by mass or less, per 100 parts by mass of the monomer mixture. As the polymerization chain transfer agent, known chain transfer agents such as mercaptans such as octyl mercaptan and 2-mercaptoethanol, and thiuram disulfides can be used. There is no limitation on the chain form of the polymerized monomers, and any of the polymerization forms such as random, block, and graft may be used.

[0041] Preferred polymerization conditions vary depending on the types of polymerization initiator, monomer, and solvent used, but typically the polymerization temperature is preferably 30° C. or higher, more preferably 50° C. or higher, and preferably 95° C. or lower, more preferably 80° C. or lower. The polymerization time is preferably 1 hour or longer, more preferably 2 hours or longer, and preferably 20 hours or shorter, more preferably 10 hours or shorter. The polymerization atmosphere is preferably a nitrogen gas atmosphere or an inert gas atmosphere such as argon. After the polymerization reaction is completed, the produced polymer can be isolated from the reaction solution by a known method such as reprecipitation, solvent distillation, etc. The obtained polymer can also be purified by removing unreacted monomers and the like by reprecipitation, membrane separation, chromatography, extraction, etc.

[0042] The number-average molecular weight of the polymer dispersant is preferably 3,000 or more, more preferably 5,000 or more, even more preferably 10,000 or more, and even more preferably 30,000 or more, from the viewpoint of improving the storage stability of the aqueous pigment dispersion and the aqueous ink, and the ejection stability of the aqueous ink. From the viewpoint of pigment dispersion stability, the number-average molecular weight is preferably 100,000 or less, more preferably 70,000 or less, and even more preferably 60,000 or less. When the number-average molecular weight of the polymer dispersant is within the above range, the polymer dispersant has sufficient adsorption power to the pigment and can exhibit dispersion stability. The number-average molecular weight can be measured by the method described in the examples.

[0043] [Neutralizing agent] The pH of the aqueous pigment dispersion produced by the production method of the present invention is preferably 5.5 or higher, more preferably 6 or higher, from the viewpoint of ease of handling, such as reducing skin irritation, and is preferably 13 or lower, more preferably 12 or lower, and even more preferably 11 or lower, from the viewpoint of inhibiting corrosion of components. Therefore, at least a portion of the carboxy groups of the polymer dispersant are neutralized with a neutralizing agent. 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 dispersant may also be neutralized in advance. From the viewpoint of promoting sufficient and uniform neutralization, the neutralizing agent is preferably used as an aqueous solution of the neutralizing agent. From the viewpoint described above, the concentration of the aqueous solution of the neutralizing agent is preferably 3% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, and is preferably 50% by mass or less, and more preferably 25% by mass or less.

[0044] From the viewpoint of the dispersion stability of the polymer dispersant and the dispersion stability of the pigment, the use equivalent (mol %) of the neutralizing agent for the polymer dispersant is preferably 10 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and is preferably 90 mol % or less, more preferably 85 mol % or less, even more preferably 80 mol % or less. Here, the term "usage equivalent (mol %) of neutralizing agent" refers to the percentage (mol %) obtained by dividing the molar equivalents of the alkali metal compound by the molar equivalents of the carboxyl groups in the polymer dispersant when an alkali metal compound is used as the neutralizing agent, i.e., the percentage (mol %) of "molar equivalents of alkali metal compound / molar equivalents of the carboxyl groups in the polymer dispersant." In the present invention, the usage equivalent of the neutralizing agent is calculated from the molar equivalents of the neutralizing agent used, so if an excess of the neutralizing agent is used, the usage equivalent of the neutralizing agent will exceed 100 mol %.

[0045] [Step (II)] Step (II) is a step in which a crosslinking agent is added to the aqueous dispersion obtained in step (I), and the mixture is heat-treated at a temperature below the melting point of the wax.

[0046] <Removal of organic solvents> If the aqueous dispersion obtained in step (I) contains an organic solvent, the organic solvent is removed from the aqueous dispersion obtained in step (I) by a known method before the heat treatment. That is, step (II) is preferably performed in an organic solvent-free state. It is preferable that the organic solvent is substantially removed, but it may remain as long as it does not impair the object of the present invention. If the organic solvent remains, its amount is preferably 0.1% by mass or less, more preferably 0.01% by mass or less.

[0047] <Heat treatment> In the present invention, from the viewpoint of uniformly dispersing the polymer dispersant on the pigment surface and the wax surface while preventing the wax from melting, the heat treatment temperature is not higher than the melting point of the wax, and the difference between the wax melting point and the heat treatment temperature, i.e., the value obtained by subtracting the heat treatment temperature from the wax melting point, is preferably 5°C or higher, more preferably 10°C or higher, even more preferably 15°C or higher, and still more preferably 19°C or higher. From the viewpoint of efficiently progressing the crosslinking reaction by heat treatment, the temperature is preferably not higher than 60°C, more preferably not higher than 50°C, and even more preferably not higher than 35°C.

[0048] In the present invention, the heat treatment temperature is preferably in the range of 45°C or higher and 95°C or lower. From the viewpoint of accelerating the crosslinking reaction and improving the ejection stability of the water-based ink, the heat treatment temperature is preferably 50°C or higher. This results in an aqueous pigment dispersion in which the polymer dispersant is uniformly distributed on the pigment surface and the wax surface, and further in which pigment-containing polymer particles in which the polymer dispersant has been crosslinked with a crosslinking agent are dispersed. Furthermore, in the pigment-containing polymer particles, the wax may be crosslinked with a crosslinking agent. From the viewpoint of improving the storage stability of the aqueous pigment dispersion and the aqueous ink, as well as the ejection stability of the aqueous ink, it is preferred that the pigment-containing polymer particles have a crosslinked structure in which some of the acid groups of the polymer dispersant are neutralized with a neutralizing agent, and another part of the acid groups of the polymer dispersant are crosslinked with a crosslinking agent. The progress of the crosslinking reaction can be confirmed by measuring the change in pH of the aqueous pigment dispersion during the crosslinking reaction, and the crosslinking reaction can be considered complete when no further change in pH occurs. It is desirable to continue the heat treatment until the crosslinking reaction is completely completed. From the viewpoints of completion of the crosslinking reaction and economy, the heat treatment time is preferably 0.5 hours or more, more preferably 1 hour or more, even more preferably 1.5 hours or more, and is preferably 12 hours or less, more preferably 8 hours or less, even more preferably 6 hours or less.

[0049] Furthermore, from the viewpoint of increasing the dispersion stability of the pigment and wax and suppressing the generation of aggregates in the heat treatment step, any of the mixing step, step (I), and step (II) may be carried out in the presence of a surfactant. Specific examples of surfactants that can be used include known anionic surfactants, nonionic surfactants, and amphoteric surfactants, with nonionic surfactants being preferred. These surfactants remain after addition even when the aqueous pigment dispersion of the present invention is obtained. Cationic surfactants are not preferred because they reduce the dispersion stability of the pigment and wax.

[0050] As nonionic surfactant, acetylene glycol surfactant and polyoxyethylene alkyl ether surfactant can be mentioned, and polyoxyethylene alkyl ether surfactant is preferred.As the commercially available product of polyoxyethylene alkyl ether surfactant, for example, Emulgen 102KG, 103, 104P, 105, 106, 108, 120, 147, 150, 220, 350, 404, 420, 705, 707, 709, 1108, 4085, 2025G etc. manufactured by Kao Corporation can be mentioned, and preferably Emulgen 420.

[0051] Furthermore, in order to suppress the generation of aggregates at the gas-liquid interface in the heat treatment step, any of the mixing step, step (I), and step (II) may be carried out in the presence of an organic solvent having a high affinity for water. Examples of organic solvents having a high affinity for water include ethylene glycol, propylene glycol, diethylene glycol, and glycerin. Among these, glycerin, which has a high affinity for water and a high boiling point, is preferred. These organic solvents having a high affinity for water remain after addition even when the aqueous pigment dispersion of the present invention is obtained.

[0052] [Crosslinking agent] The crosslinking agent used in the present invention preferably has a water solubility (mass ratio) of 50% or less, more preferably 40% or less, and even more preferably 35% or less, from the viewpoint of efficiently reacting with the polymer dispersant in a medium mainly composed of water, and from the viewpoint of improving the storage stability of the aqueous pigment dispersion and the aqueous ink, and the ejection stability of the aqueous ink. Here, the water solubility (mass ratio) refers to the solubility (%) when 10 parts by mass of the crosslinking agent is dissolved in 90 parts by mass of water at room temperature (25°C).

[0053] 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 from 3 to 4 carbon atoms. The epoxy equivalent of the crosslinking agent is preferably 90 or more, more preferably 100 or more, even more preferably 110 or more, and preferably 300 or less, more preferably 200 or less, even more preferably 150 or less. The number of epoxy groups in the crosslinking agent is 2 or more per molecule from the viewpoint of efficiently reacting with the acid groups to enhance the storage stability of the aqueous pigment dispersion, and is preferably 6 or less per molecule, and from the viewpoint of market availability, is more preferably 4 or less, and even more preferably 3 or less.

[0054] 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.

[0055] From the viewpoint of improving the storage stability of the aqueous pigment dispersion and the aqueous ink, and the ejection stability of the aqueous ink, the degree of crosslinking is preferably 15 mol% or more, more preferably 20 mol% or more, even more preferably 25 mol% or more, and is preferably 70 mol% or less, more preferably 65 mol% or less, even more preferably 60 mol% or less. Here, the degree of crosslinking is an apparent degree of crosslinking calculated from the acid value of the polymer dispersant and the equivalent weight of the crosslinkable functional group of the crosslinking agent. When the wax has acid groups, a crosslinked structure is also formed between the acid groups of the wax and the crosslinking agent. In the present invention, from the viewpoint of controlling the degree of crosslinking of the polymer dispersant and improving the storage stability of the water-based pigment dispersion and the water-based ink, as well as the ejection stability of the water-based ink, the ratio of the molar equivalents of the crosslinkable functional groups of the crosslinking agent to the molar equivalents of the acid groups of the polymer dispersant is used as an index of the degree of crosslinking.

[0056] The pigment dispersion of the present invention may contain 1% by mass to 10% by mass of a moisturizing agent such as glycerin or triethylene glycol to prevent drying, or may contain additives such as an antifungal agent. The additives may be added when dispersing the pigment, or after dispersing the pigment or after heat treatment.

[0057] The nonvolatile component concentration (solid content concentration) of the pigment dispersion of the present invention is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less, from the viewpoint of improving the dispersion stability of the aqueous pigment dispersion and facilitating the production of the aqueous pigment dispersion. The nonvolatile component concentration (solid content concentration) is measured by the method described in the examples.

[0058] From the viewpoint of dispersion stability, the content of the pigment in the pigment dispersion of the present invention is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 13% by mass or more, and is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less.

[0059] From the viewpoint of improving the ejection stability of the water-based ink, the mass ratio of the pigment to the polymer dispersant in the pigment dispersion of the present invention (pigment / polymer dispersant) is preferably 0.3 or more, more preferably 1.5 or more, even more preferably 4 or more, and is preferably 8 or less, more preferably 7 or less, even more preferably 6.5 or less.

[0060] From the viewpoint of improving the ejection stability of the water-based ink, the mass ratio of the pigment to the wax (pigment / wax) in the pigment dispersion of the present invention is preferably 1 or more, more preferably 2 or more, even more preferably 2.5 or more, and is preferably 11 or less, more preferably 10 or less, even more preferably 8 or less.

[0061] From the viewpoint of improving the ejection stability of the water-based ink, the cumulant average particle size of the pigment-containing polymer particles is preferably 40 nm or more, more preferably 50 nm or more, even more preferably 60 nm or more, still more preferably 70 nm or more, still more preferably 80 nm or more, and is preferably 200 nm or less, more preferably 150 nm or less, even more preferably 130 nm or less, still more preferably 120 nm or less, and still more preferably 110 nm or less. The cumulant average particle size of the pigment-containing polymer particles is measured by the method described in the examples. Furthermore, the cumulant average particle size of the pigment-containing polymer particles in the water-based ink is the same as the cumulant average particle size in the water-based pigment dispersion, and the preferred embodiment of the cumulant average particle size is the same as the preferred embodiment of the cumulant average particle size in the water-based pigment dispersion.

[0062] [Method of manufacturing water-based ink for inkjet printing] The method for producing a water-based ink for inkjet printing of the present invention (hereinafter also referred to as the "method for producing the ink of the present invention") includes a step of mixing the pigment dispersion obtained by the production method of the present invention with a water-soluble organic solvent. Furthermore, in order to adjust the concentration, water may be further added when mixing the pigment dispersion of the present invention with the water-soluble organic solvent. By including the pigment dispersion obtained by the production method of the present invention, the ink of the present invention can improve the storage stability of the water-based ink, and by including a water-soluble organic solvent, the storage stability of the water-based ink can be further improved.

[0063] In the method for producing an ink of the present invention, when the pigment dispersion of the present invention is mixed with a water-soluble organic solvent, various additives commonly used in aqueous inks, such as a humectant, wetting agent, penetrant, surfactant, viscosity adjuster, pH adjuster, antifoaming agent, preservative, antifungal agent, and antirust agent, may be further added, if necessary. Furthermore, after mixing, filtration treatment using a filter or the like can be carried out.

[0064] The content of each component of the ink of the present invention and the ink properties are as follows.

[0065] [Water-soluble organic solvent] Examples of the water-soluble organic solvent include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, sulfur-containing compounds, etc. Among these, one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers are preferred, and one or more selected from ethylene glycol, propylene glycol, 1,2-hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol, polyethylene glycol, glycerin, trimethylolpropane, ethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and diethylene glycol diethyl ether are more preferred, and propylene glycol is even more preferred. The water-soluble organic solvent preferably contains one or more organic solvents having a boiling point of 90° C. or higher. The weighted average boiling point of the organic solvent is preferably 150° C. or higher, more preferably 180° C. or higher, and preferably 240° C. or lower, more preferably 220° C. or lower, and even more preferably 200° C. or lower.

[0066] The ink obtained by the production method of the present invention may further contain an aqueous dispersion of pigment-free polymer particles as a fixing aid, from the viewpoint of improving adhesion, print density, etc. When the aqueous ink further contains pigment-free polymer particles, a smooth coating film is obtained on the printing medium due to the affinity between the polymer particles and the polymer dispersant, and local aggregation of the pigment particles is suppressed, thereby improving adhesion without reducing print density. Examples of polymers that can be used as fixing aids include condensation polymers such as polyolefin resins, polyurethane resins, and polyester resins; and vinyl polymers such as acrylic resins, styrene resins, styrene-acrylic resins, butadiene resins, styrene-butadiene resins, vinyl chloride resins, vinyl acetate resins, and acrylic silicone resins. The aqueous dispersion of polymer particles not containing a pigment may be suitably synthesized or may be a commercially available product.

[0067] The amount of the aqueous dispersion of pigment-free polymer particles in the ink obtained by the production method of the present invention is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less, in terms of solids, from the viewpoint of improving the storage stability and ejection stability of the aqueous ink.

[0068] (Content of each component in the ink) The content of each component in the ink obtained by the production method of the present invention is as follows, from the viewpoint of improving the storage stability, ejection stability, etc. of the ink.

[0069] (Pigment content) The pigment content in the ink obtained by the production method of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, from the viewpoint of print density, and is preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 12% by mass or less, from the viewpoint of reducing the ink viscosity when the solvent evaporates and improving storage stability.

[0070] (Total content of pigment and polymer dispersant) From the viewpoint of improving the storage stability and ejection stability of the water-based ink, the total content of the pigment and polymer dispersant in the ink obtained by the production method 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 still more preferably 7% by mass or more, and is preferably 20% by mass or less, more preferably 17% by mass or less, and even more preferably 15% by mass or less.

[0071] (organic solvent content) From the same viewpoints as above, the content of organic solvent in the ink obtained by the production method 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 preferably 45% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less.

[0072] (Water content) From the same viewpoints as above, the water content in the ink obtained by the production method of the present invention is preferably 45% by mass or more, more preferably 50% by mass or more, and preferably 70% by mass or less, more preferably 65% ​​by mass or less.

[0073] (Water-based ink properties) From the viewpoint of improving storage stability, the viscosity at 32°C of the ink obtained by the production method of the present invention is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 5 mPa·s or more, and is 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 ink can be measured using an E-type viscometer. From the viewpoint of storage stability, the pH of the ink obtained by the production method of the present invention at 20° C. is preferably 7.0 or higher, more preferably 7.2 or higher, and even more preferably 7.5 or higher. From the viewpoints of component resistance and skin irritation, the pH is preferably 11 or lower, more preferably 10 or lower, and even more preferably 9.5 or lower. The pH of the water-based ink can be measured by a conventional method.

[0074] The ink obtained by the production method of the present invention can be suitably used as an ink for inkjet recording, and can be loaded into a known inkjet recording apparatus and ejected as ink droplets onto a printing medium to record an image or the like. Ink jet recording apparatuses include thermal and piezo types, and the ink of the present invention is more preferably used as a water-based ink for piezo type ink jet printing. Usable printing media include highly absorbent plain paper, low-absorbent coated paper, and non-absorbent resin film. Examples of coated paper include general-purpose glossy paper and multicolor foam gloss paper. The resin film is preferably at least one selected from polyester film, polyvinyl chloride film, polypropylene film, and polyethylene film. The resin film may be a corona-treated substrate. [Example]

[0075] In the following Preparation Examples, Examples and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The various physical properties of the aqueous dispersions obtained in the Preparation Examples, Examples, and Comparative Examples were measured and evaluated by the following methods.

[0076] (1) Measurement of the number average molecular weight of polymer dispersants The measurements were performed using gel permeation chromatography (GPC) using a Tosoh GPC system (HLC-8320GPC) and Tosoh columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guard column SuperAW-H) at a flow rate of 0.5 mL / min, with N,N-dimethylformamide containing 60 mmol / L phosphoric acid and 50 mmol / L lithium bromide as the eluent. The measurements were performed using a Tosoh 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)) with known molecular weights as standards. The measurement sample was prepared by mixing 0.1 g of the polymer dispersant with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25°C for 10 hours, and filtering with a syringe filter (DISMIC-13HP, PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0077] (2) Measurement of non-volatile component concentration (solid content concentration) of water-based pigment 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 was added and mixed. The mixture was then accurately weighed and maintained at 105°C for 2 hours to remove volatiles. The mixture was then left in the desiccator for a further 15 minutes, and the mass was measured. The mass of the sample after volatiles removal was taken as the solid content, and divided by the mass of the added sample to determine the non-volatile component concentration.

[0078] (3) Measurement of the acid value of polymer dispersants and waxes In accordance with JIS K0070 and JIS K5902, an automatic potentiometric titrator (motorized burette, model number: APB-610) manufactured by Kyoto Electronics Manufacturing Co., Ltd. was used to titrate the samples with a 0.1 N potassium hydroxide / ethanol solution by potentiometric titration, using a titration solvent consisting of a mixture of toluene and acetone (2:1, mass ratio) or a mixture of trichlorobenzene and ethanol (2:1, mass ratio). The inflection point on the titration curve was taken as the endpoint. The acid values ​​(mg KOH / g) of the polymer dispersant and wax were calculated from the titration volume of the potassium hydroxide solution up to the endpoint.

[0079] (4) Measurement of the melting point of wax The melting point of the wax was measured using a measuring device conforming to JIS K 0064. Specifically, using a differential scanning calorimeter (Q20) manufactured by TA Instruments, the sample was heated to 200°C and then cooled to 0°C at a rate of 10°C / min. The sample was then heated at a rate of 10°C / min, and the calorific value was measured up to 200°C. The temperature of the peak with the largest peak area among the observed heat of fusion peaks was taken as the maximum peak temperature of melting, and this peak temperature was taken as the melting point.

[0080] (5) Measurement of the average particle size of pigment-containing polymer particles The cumulant average particle size measured using Otsuka Electronics Co., Ltd.'s laser particle analysis system "ELS-8000" (cumulant analysis) was used as the average particle size of the pigment-containing polymer particles. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 integration times. The refractive index of water (1.333) was entered as the refractive index of the dispersion solvent. The measurement concentration was usually 5 x 10 -3 It was done at about %.

[0081] <Preparation of polymer dispersant> Production Example 1 (Preparation of polymer dispersant a) A monomer mixture was prepared by mixing 29 parts of acrylic acid, 60 parts of styrene, and 1 part of ethyl acrylate. A reaction vessel was charged with 10 parts of methyl ethyl ketone (MEK), 0.2 parts of 2-mercaptoethanol (a polymerization chain transfer agent), and 10% of the monomer mixture, and the mixture was thoroughly purged with nitrogen gas. 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 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65, 2,2'-azobis(2,4-dimethylvaleronitrile)) was placed in a dropping funnel. The mixture in the reaction vessel was heated to 65°C under a nitrogen atmosphere while stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 65°C from the end of the dropping, a polymerization initiator solution prepared by dissolving 0.1 parts of the polymerization initiator in 2 parts of MEK was added. 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 polymer dispersant a (number average molecular weight: 32,000, acid value: 226 mgKOH / g).

[0082] Production Example 2 (Preparation of Polymer Dispersant b) A monomer mixture was prepared by mixing 14 parts of methacrylic acid, 30 parts of styrene, and 21 parts of styrene macromer (manufactured by Toagosei Co., Ltd., trade name: AS-6(S), active ingredient concentration: 50% by mass, number average molecular weight: 6,000). 10 parts of MEK, 0.1 parts of 2-mercaptoethanol (polymerization chain transfer agent), and 10% of the monomer mixture were placed in a reaction vessel and mixed, followed by thorough nitrogen gas replacement. A mixture of the remaining 90% of the monomer mixture, 0.1 parts of the polymerization chain transfer agent, 20 parts of MEK, and 0.7 parts of an azo-based radical polymerization initiator (trade name: V-65) was placed in a dropping funnel. Under a nitrogen atmosphere, the mixture in the reaction vessel was heated to 77°C while stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After 0.5 hours at 77°C from the end of the dropping, a polymerization initiator solution prepared by dissolving 0.6 parts of the polymerization initiator in 5 parts of MEK was added, and the mixture was aged by stirring at 77°C for 1 hour. This process of preparing, adding, and aging the polymerization initiator solution was repeated five more times. The reaction solution in the reaction vessel was then maintained at 80°C for 1 hour, and MEK was added to obtain a dispersion of polymer dispersant b (solids concentration: 40%, number average molecular weight: 53,000, acid value: 94 mgKOH / g).

[0083] <Production of Water-Based Pigment Dispersion> Example 1 (Production of Water-Based Pigment Dispersion D1) (Mixing process) A reaction vessel equipped with anchor blades was charged with 50 parts of polymer dispersant a obtained in Production Example 1, 19.4 parts of a 5N aqueous sodium hydroxide solution (NaOH solids content: 16.9%) as a neutralizing agent to neutralize 40 mol% of the carboxy groups in polymer dispersant a, and 240 parts of ion-exchanged water. 100 parts by mass of polyethylene wax (manufactured by Mitsui Chemicals, Inc., trade name: Hiwax 110P, melting point: 109°C) was added thereto and heated to 85 to 95°C until dissolved. 300 parts of carbon black pigment (CI Pigment Black 7, manufactured by Cabot Chemical Company, trade name: Monarch 800) was added thereto, yielding a mixture of pigment, wax, and polymer dispersant. (Process (I)) The resulting mixture was maintained at 90-95°C and subjected to a preliminary dispersion treatment for 90 minutes using an ultrasonic homogenizer. After cooling to room temperature, 78.6 parts of MEK and 480 parts of ion-exchanged water were added, and the mixture was further dispersed using a Microfluidizer (trade name, manufactured by Microfluidics) for 10 passes at a pressure of 200 MPa to obtain a water-based dispersion containing polymer dispersant a, wax, and pigment (solid concentration: 27.6%, pigment content: 18.4%, wax content: 3.1%, polymer dispersant content: 6.1%). (Step (II)) 250 parts of ion-exchanged water was added to the aqueous dispersion obtained in step (I) and stirred. After stirring, the MEK was completely removed at 60°C under reduced pressure, and then some of the water was removed to give a dispersion with a solids concentration of 20% (pigment content: 13.2%) and in which the medium was replaced with water. Subsequently, 15.5 parts of trimethylolpropane polyglycidyl ether (manufactured by Nagase ChemteX Corporation, trade name: Denacol EX-321, epoxy value: 140) was added to the water-displaced dispersion, and the mixture was sealed and heated at 80°C for 3 hours while stirring with a stirrer. This heat treatment substituted 55 mol% of the carboxy groups of polymer dispersant a with glycidyl ester bonds, thereby obtaining aqueous pigment dispersion D1 (solids concentration: 20.3%, pigment content: 13.2%, wax content: 2.2%, polymer dispersant content: 4.4%, crosslinker content: 0.5%, cumulant average particle size of pigment-containing polymer particles: 94 nm, pH: 8.0), in which pigment-containing polymer particles containing a pigment dispersed in polymer dispersant a with an acid value of 76 mg KOH / g and wax were dispersed.

[0084] Example 2 (Preparation of Water-Based Pigment Dispersion D2) In Example 1, 50 parts of polymer dispersant a was replaced with 125 parts of the dispersion of polymer dispersant b produced in Production Example 2 (solid content concentration 40%), 19.4 parts of 5N sodium hydroxide aqueous solution was replaced with 15.5 parts, 78.6 parts of MEK was replaced with 3.6 parts, and 15.5 parts of trimethylolpropane polyglycidyl ether was replaced with 4.1 parts. The same procedure as in Example 1 was used to obtain a water-based pigment dispersion D2 (solid content concentration: 20.1%, pigment content: 13.2%, wax content: 2.2%, polymer dispersant content: 4.4%, crosslinker content: 0.5%, cumulant average particle size of pigment-containing polymer particles: 102 nm, pH: 9.2).

[0085] Examples 3 to 6 and Comparative Examples 1 to 2 (Production of Water-Based Pigment Dispersions D3 to D6 and D21 to D22) Water-based pigment dispersions D3 to D6 of Examples 3 to 6 and water-based pigment dispersions D21 to D22 of Comparative Examples 1 and 2 were obtained in the same manner as in Example 1, except that the components and formulations in Example 1 were changed as shown in Tables 1 and 2. The results are shown in Tables 1 and 2.

[0086] The wax details in Tables 1 and 2 are as follows: HW110P: Hiwax 110P (a polyethylene wax not modified with a compound containing a carboxyl group, melting point 109°C, acid value: 0 mg KOH / g, manufactured by Mitsui Chemicals, Inc., product name) DC30M: Diacarna 30M (maleic anhydride modified polyolefin wax with a melting point of 76°C, acid value: 103 mg KOH / g, manufactured by Mitsubishi Chemical Corporation, trade name) PW155: Paraffin wax 155 (a paraffin wax that is not modified with a compound having a carboxyl group, with a melting point of 69°C, acid value: 0 mg KOH / g, manufactured by Nippon Seiro Co., Ltd., trade name)

[0087] Example 7 (Preparation of Water-Based Pigment Dispersion D7) (Mixing process) A reaction vessel equipped with an anchor blade was charged with 50 parts of the polymer dispersant a obtained in Production Example 1, 19.4 parts of a 5N aqueous sodium hydroxide solution (NaOH solids content: 16.9%) as a neutralizing agent to neutralize 40 mol% of the carboxy groups of polymer dispersant a, and 240 parts of ion-exchanged water. 300 parts by mass (100 parts as wax) of an aqueous polyethylene wax emulsion (HTE6500: manufactured by Toho Chemical Industry Co., Ltd., trade name: Hitec E-6500, an emulsion of oxidized polyethylene wax (not a wax modified with a compound having carboxy groups, having a melting point of 140°C) emulsified with a polyoxyethylene alkyl ether surfactant, acid value: 1 mg KOH / g, wax content: 33.3%) and 300 parts of a carbon black pigment (CI Pigment Black 7, manufactured by Cabot Chemical Co., trade name: Monarch 800), and 78.6 parts of MEK and 480 parts of ion-exchanged water were then added to obtain a mixture of pigment, wax, and polymer dispersant. (Process (I)) The resulting mixture was pre-dispersed by stirring for 1 hour using a Disper (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) at 20°C with the Disper blade rotating at 7,000 rpm. The pre-dispersed mixture of pigment, oxidized polyethylene wax emulsion, and polymer dispersant was then fully dispersed by 10 passes using a Microfluidizer (manufactured by Microfluidics, product name) at a pressure of 200 MPa, yielding an aqueous dispersion containing polymer dispersant, wax, and pigment (solids concentration: 27.6%, pigment content: 18.4%, wax content: 3.1%, polymer dispersant content: 6.1%). (Step (II)) The same operation as in step (II) of Example 1 was carried out to obtain aqueous pigment dispersion D7 of Example 7 (solid concentration: 20.1%, pigment content: 13.2%, wax content: 2.2%, polymer dispersant content: 4.4%, crosslinker content: 0.3%, cumulant average particle size of pigment-containing polymer particles: 90 nm, pH: 8.0).

[0088] Example 8 (Preparation of Water-Based Pigment Dispersion D8) In the same manner as in Example 7, a water-based pigment dispersion D8 of Example 8 was obtained using the formulation shown in Table 1. In Table 1, HTS3121 is manufactured by Toho Chemical Industry Co., Ltd. and has the trade name Hitec S-3121 (a carboxylic acid-modified polyethylene wax emulsion with a melting point of 77°C, an acid value of 140 mgKOH / g, and a wax content of 25.0%).

[0089] Comparative Example 3 (Production of Water-Based Pigment Dispersion) When heat treatment was carried out under the conditions shown in Table 2 in the same manner as in Example 8, the system gelled and no water-based pigment dispersion was obtained.

[0090] Preparation Example 1 (Preparation of Wax Dispersion W1) A reaction vessel equipped with anchor blades was charged with 10 parts of polymer dispersant a obtained in Production Example 1, 3.9 parts of a 5N aqueous sodium hydroxide solution (NaOH solid content: 16.9%) as a neutralizer for the carboxyl groups of polymer dispersant a, and 240 parts of ion-exchanged water, to which 100 parts by mass of polyethylene wax (manufactured by Mitsui Chemicals, Inc., product name: Hiwax 110P, melting point: 109°C) was added and dissolved by heating at 85 to 95°C. The resulting mixture was cooled to room temperature and subjected to a dispersion treatment using a microfluidizer at a pressure of 200 MPa for three passes to obtain a dispersion. To the resulting dispersion, 100 parts of ion-exchanged water was added, and 3.1 parts of trimethylolpropane polyglycidyl ether (trade name: Denacol EX-321) was added. The mixture was then sealed and heated at 80°C for 3 hours while stirring with a stirrer. This resulted in heat treatment, replacing 55 mol% of the carboxy groups in polymer dispersant a with glycidyl ester bonds, to obtain wax dispersion W1 (solid concentration: 22.6%, wax content: 20%, polymer dispersant content: 2%, crosslinker content: 0.6%, cumulant average particle size: 88 nm, pH: 8.0, polymer acid value: 76 mg KOH / g).

[0091] Preparation Example 2 (Preparation of Pigment Dispersion P1) A reaction vessel was charged with 40 parts of polymer dispersant a obtained in Production Example 1 and 78.6 parts of MEK, and then mixed. 15.5 parts of a 5N aqueous sodium hydroxide solution (NaOH solids content: 16.9%) was added as a neutralizer to neutralize 40 mol% of the carboxyl groups in polymer dispersant a, and 500 parts of ion-exchanged water were added. 300 parts of carbon black pigment (CI Pigment Black 7, manufactured by Cabot Chemical Co., Ltd., product name: Monarch 800) was added, and the mixture was pre-dispersed using a Disper (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) at 20°C and 7,000 rpm for 1 hour. The resulting pigment and polymer dispersant mixture was then subjected to 10 passes in a Microfluidizer (manufactured by Microfluidics, product name) at 200 MPa for final dispersion, yielding an aqueous dispersion (solids concentration: 22.6%, pigment content: 20%, polymer dispersant content: 2.6%). 250 parts of ion-exchanged water was added to the resulting aqueous dispersion and stirred. After stirring, the MEK was completely removed at 60°C under reduced pressure. Some of the water was further removed to a solids concentration of 20% (pigment content: 17.6%). 12.4 parts of trimethylolpropane polyglycidyl ether (trade name: Denacol EX-321) was then added, the container was sealed, and the mixture was heated to 80°C for 3 hours while stirring with a stirrer. This resulted in heat treatment, substituting 55 mol% of the carboxy groups of polymer dispersant a with glycidyl ester bonds, to obtain a dispersion P1 (solids concentration: 20.7%, pigment content: 17.6%, polymer dispersant content: 2.4%, crosslinker content: 0.7%, cumulant average particle size: 94 nm, pH 8.0) of crosslinked polymer particles containing a pigment dispersed in a polymer dispersant with an acid value of 76 mgKOH / g.

[0092] Comparative Example 4 (Production of Water-Based Pigment Dispersion D24) 170.5 parts of the pigment dispersion P1 obtained in Preparation Example 2 was added to 50 parts of the wax dispersion W1 obtained in Preparation Example 1 and stirred to obtain an aqueous pigment dispersion D24 of Comparative Example 4 (solid concentration: 21.1%, pigment content: 13.6%, wax content: 4.5%, polymer dispersant content: 2.3%, crosslinker content: 0.6%, cumulant average particle size: 94 nm, pH: 8.0).

[0093] <Preparation of Water-Based Ink A> For each aqueous pigment dispersion obtained in the Examples and Comparative Examples, the components were mixed so that the pigment content of the total aqueous ink was 6%, propylene glycol 16%, dipropylene glycol monomethyl ether 8%, diethylene glycol monobutyl ether 8%, nonionic surfactant (2,4,7,9-tetramethyl-5-decyne-4,7-diol: Nissin Chemical Industry Co., Ltd., an acetylene glycol-based nonionic surfactant) 1%, and nonionic surfactant (polyoxyethylene alkyl ether: Kao Corporation, product name: Emulgen 120) 1%. Triethanolamine was added as a pH adjuster to adjust the pH of the aqueous ink to 9.0, and ion-exchanged water was added to bring the total volume to 100%. The mixture was stirred thoroughly with a magnetic stirrer and filtered through a 25 mL needleless syringe equipped with a 1.2 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm, Fujifilm Corporation) to obtain Water-Based Ink A.

[0094] <Preparation of Water-Based Ink B> For each of the aqueous pigment dispersions obtained in the Examples and Comparative Examples, the components were mixed so that the pigment content relative to the total aqueous ink was 10%, dipropylene glycol 16%, glycerin 4%, 1,2-hexanediol 2%, nonionic surfactant (2,4,7,9-tetramethyl-5-decyne-4,7-diol) 0.5%, and nonionic surfactant (polyoxyethylene alkyl ether) 1%, and then the pH was adjusted in the same manner as in the preparation of aqueous ink A, to obtain aqueous ink B.

[0095] The following storage stability and ejection stability evaluations were carried out using the resulting water-based inks A and B. The results are shown in Tables 1 and 2.

[0096] (Storage stability evaluation) 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 before and after storage using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.), and the rate of change in viscosity (%) [= (ink viscosity after storage) / (ink viscosity before storage) × 100] was calculated to evaluate storage stability. 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.

[0097] (Evaluation of ejection stability) In an environment with a temperature of 25±1°C and a relative humidity of 30±5%, a Jetepxet (ImageXpert) inkjet ejection evaluation device equipped with an inkjet head (Fujifilm Corporation's "Samba G3L," piezo type) was filled with water-based inks. The head voltage, frequency, and negative pressure were set to 30 V, 50 kHz, a push-pull drive waveform, a suitable ejection volume of 2.5 pL, and a negative pressure of -4.0 kPa. An ink ejection command was sent to the ejection evaluation device, and the number of nozzles ejecting each water-based ink correctly was counted. Continuous ejection was performed under the same conditions for 30 minutes, and the number of nozzles ejecting correctly after 30 minutes was reconfirmed. The continuous ejection rate (%) was calculated using the following formula to evaluate the ejection stability. Continuous discharge rate (%) = (number of nozzles discharging normally after 30 minutes / number of nozzles discharging normally at the beginning) x 100 The higher the continuous discharge rate (%), the better the discharge stability is judged to be, and a value of 90% or more is practically usable.

[0098] [Table 1]

[0099] [Table 2]

[0100] Tables 1 and 2 show that the water-based inks containing the water-based pigment dispersions obtained in the examples of the present invention have better storage stability and ejection stability than the water-based inks containing the water-based pigment dispersions obtained in the comparative examples. In particular, in Comparative Example 4, heat treatment was not performed in a state in which the pigment, wax, and polymer dispersant coexisted, and therefore the polymer dispersant distributed on the pigment surface and the polymer dispersant distributed on the wax surface were not rearranged so as to be uniformly distributed, which is thought to result in poor storage stability. [Industrial Applicability]

[0101] According to the present invention, it is possible to provide a method for producing a water-based pigment dispersion suitable for a water-based ink for ink-jet printing that has excellent storage stability and ejection stability, and a method for producing a water-based ink for ink-jet printing using the same.

Claims

1. A method for producing an aqueous pigment dispersion, comprising the following steps (I) and (II): the crosslinking agent is a glycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having from 3 to 8 carbon atoms, The wax is a polyolefin wax, a ketone wax, an amide wax, or a paraffin wax, which is not a wax modified with a compound having a carboxy group, The polymer dispersant described below comprises (a-1) one or more structural units derived from a carboxylic acid monomer selected from (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid; A method for producing an aqueous pigment dispersion, comprising at least one structural unit derived from a hydrophobic monomer (a-2) selected from the group consisting of an alkyl(meth)acrylate having an alkyl group having from 1 to 18 carbon atoms, an aromatic group-containing monomer having an aromatic group having from 6 to 22 carbon atoms, benzyl(meth)acrylate, and a macromonomer having a polymerizable functional group at one end. Step (I): A step of dispersing a pigment, a wax, and a polymer dispersant to obtain an aqueous dispersion Step (II): A step of adding a crosslinking agent to the aqueous dispersion obtained in step (I) and heat-treating the mixture at a temperature equal to or lower than the melting point of the wax.

2. The method for producing an aqueous pigment dispersion according to claim 1, wherein the hydrophobic monomer of the (a-2) hydrophobic monomer-derived structural unit is one or more selected from styrene, α-methylstyrene, ethyl acrylate, styrene macromonomer, and benzyl (meth)acrylate.

3. The method for producing an aqueous pigment dispersion according to claim 1 or 2, wherein the heat treatment is carried out at a temperature that is 5°C or more lower than the melting point of the wax.

4. The method for producing an aqueous pigment dispersion according to any one of claims 1 to 3, wherein the heat treatment is carried out at a temperature of 45°C or higher and 95°C or lower.

5. 5. The method for producing an aqueous pigment dispersion according to claim 1, wherein a mass ratio of the pigment to the polymer dispersant (pigment / polymer dispersant) is 0.3 or more and 8 or less.

6. The method for producing an aqueous pigment dispersion according to any one of claims 1 to 5, wherein a mass ratio of the pigment to the wax (pigment / wax) is 1 or more and 11 or less.

7. A method for producing a water-based ink for inkjet recording, comprising a step of mixing the water-based pigment dispersion obtained by the production method described in any one of claims 1 to 6 with a water-soluble organic solvent.

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