Method for producing aqueous pigment dispersions

Abstract

The present invention provides a method for producing an aqueous pigment dispersion that contains azo pigments, which are highly pigmented pigments with low thermal stability, and provides an aqueous inkjet recording ink that achieves both excellent ejection stability and excellent storage stability. [Solution] A method for producing an aqueous pigment dispersion containing pigment-containing polymer particles comprising an azo pigment and a polymer dispersant, comprising the step of heat-treating a mixture of the azo pigment dispersed in the polymer dispersant and a crosslinking agent at a heat treatment temperature X [°C] of 30°C or more and 60°C or less, wherein the heat treatment time Y [hours] is 1 [hours] or more calculated by formula (1). Y1 = 2^(8 - X / 10) (1)

Description

Technical Field

[0001] The present invention relates to a method for producing an aqueous pigment dispersion and a method for producing an aqueous ink for inkjet recording using the aqueous pigment dispersion obtained by the production method.

Background Art

[0002] The inkjet recording method is a recording method in which ink droplets are directly ejected from fine nozzles and adhered to a printing medium to obtain a printed matter on which characters and images are recorded. This method is easy and inexpensive to achieve full-color printing and has the advantage of being non-contact with the printed material. Therefore, it is not only used for consumer-oriented civilian printing, but in recent years, it has begun to be applied to commercial printing and industrial printing fields that were previously carried out by analog printing such as offset printing and gravure printing. The advantages of the development of inkjet printing in the fields of commercial printing and industrial printing are that it does not require a printing plate like analog printing, so it can handle on-demand printing such as small quantity production and variable printing. And from the viewpoints of reducing the environmental load and improving the weather resistance and water resistance of printed materials, aqueous inks using pigments as colorants have become the mainstream.

[0003] For example, Patent Document 1 aims to provide a method for producing an aqueous pigment dispersion that is excellent in ejection reliability and the bending resistance of printed materials when used in an aqueous ink, and an inkjet recording ink containing the aqueous pigment dispersion. A method for producing an aqueous pigment dispersion (I) containing a pigment and a polymer, which comprises a step of heat-treating a mixture of a pigment aqueous dispersion (A), an emulsion of a polymer (B), and a crosslinking agent (C) at 45°C or higher and 100°C or lower, and the mass ratio of the pigment to the total amount of the polymer (pigment / total polymer amount) is 0.48 or higher and 2.15 or lower. A method for producing an aqueous pigment dispersion (I) is disclosed.

[0004] Patent Document 2 discloses a recording method that does not involve color mixing when two or more inks of different hues are superimposed to record an image. The method includes the steps of ejecting a first ink from a first inkjet head onto a recording medium, and ejecting a second ink of a different hue from the first ink from a second inkjet head onto an area on the recording medium to which the first ink has been applied. The recording method satisfies the conditions "0≦γ2(10)-γ1(T)≦3.0" and "0≦γ2(10)-γ1(10)" when the time from when the first ink is ejected from the first inkjet head until when the second ink is ejected from the second inkjet head is T milliseconds, the dynamic surface tension of the first ink at T milliseconds is γ1(T)mN / m, the dynamic surface tension of the first ink at 10 milliseconds is γ1(10)mN / m, and the dynamic surface tension of the second ink at 10 milliseconds is γ2(10)mN / m. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-119789 [Patent Document 2] International Publication No. 2023 / 084968 [Overview of the project] [Problems that the invention aims to solve]

[0006] To expand the applications of inkjet printing, high color rendering on the substrate is required. Azo pigments are commonly used as highly color-developing pigments in yellow and magenta inks. However, when the present inventors investigated azo pigments, they found that the inkjet recording ink containing the aqueous pigment dispersion (I) in Patent Document 1 and the ink set described in Patent Document 2 had room for improvement in achieving both ejection stability and storage stability. The present invention aims to provide a method for producing an aqueous pigment dispersion that, despite containing azo pigments, which are highly pigmented pigments with low thermal stability, provides an inkjet water-based inkjet ink that achieves both excellent ejection stability and excellent storage stability when used in an inkjet water-based inkjet ink. [Means for solving the problem]

[0007] The inventors have found that the above problem can be solved in the manufacturing process of an aqueous pigment dispersion containing pigment-containing polymer particles including an azo pigment and a polymer dispersant by crosslinking the azo pigment dispersed with the polymer dispersant using a crosslinking agent under specific conditions. In other words, the present invention relates to a method for producing an aqueous pigment dispersion containing pigment-containing polymer particles comprising an azo pigment and a polymer dispersant, comprising the step of heat-treating a mixture of the azo pigment dispersed in the polymer dispersant and a crosslinking agent at a heat treatment temperature X [°C] of 30°C to 60°C, wherein the heat treatment time Y [hours] is 1 [hours] or more, calculated by formula (1). Y1 = 2^(8 - X / 10) (1)

[0008] In other words, the present invention relates to the following [1] to [9]. [1] A method for producing an aqueous pigment dispersion containing pigment-containing polymer particles comprising an azo pigment and a polymer dispersant, The process involves heat-treating a mixture of the azo pigment dispersed in the polymer dispersant and a crosslinking agent at a heat treatment temperature X [°C] of 30°C to 60°C. A method for producing an aqueous pigment dispersion, wherein the heat treatment time Y [hours] is equal to or greater than the time Y1 [hours] calculated by equation (1). Y1 = 2^(8 - X / 10) (1) [2] The method for producing an aqueous pigment dispersion according to [1], wherein the azo pigment is CI Pigment Yellow 74. [3] A method for producing an aqueous pigment dispersion according to [1] or [2], wherein the crosslinking agent is a polyglycidyl ether of a polyhydric alcohol having 3 to 8 carbon atoms. [4] A method for producing an aqueous pigment dispersion according to any one of [1] to [3], wherein the mass ratio of the pigment to the polymer dispersant (pigment / polymer dispersant) is 0.3 or more and 8 or less. [5] A method for producing an aqueous pigment dispersion according to any one of [1] to [4], wherein the heat treatment time Y [hours] is less than or equal to the time Y2 [hours] calculated by formula (2). Y² = {2^(8-X / 10)} × 2 (2) [6] A method for producing an aqueous pigment dispersion according to any one of [1] to [5], wherein the polymer dispersant is a vinyl resin. [7] A method for producing an aqueous pigment dispersion according to any one of [1] to [6], wherein the acid value of the polymer dispersant is 50 mg KOH / g or more and 300 mg KOH / g or less. [8] A method for producing an aqueous pigment dispersion according to any one of [1] to [7], wherein the ratio of the number of molar equivalents of the crosslinking agent to the number of molar equivalents of the polymer dispersant is 20% or more and 50% or less. A method for producing an aqueous ink for inkjet recording, comprising the step of mixing an aqueous pigment dispersion obtained by the manufacturing method described in any of [9][1] to [8] with a water-soluble organic solvent. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method for producing an aqueous pigment dispersion that, despite containing azo pigments, which are highly pigmented pigments with low thermal stability, provides an inkjet water-based inkjet ink that achieves both excellent intermittent ejection stability and excellent storage stability when used in an inkjet water-based inkjet ink. [Modes for carrying out the invention]

[0010] [Method for producing aqueous pigment dispersions] A method for producing an aqueous pigment dispersion according to one aspect of the present invention is a method for producing an aqueous pigment dispersion containing pigment-containing polymer particles comprising an azo pigment and a polymer dispersant, comprising the step of heat-treating a mixture of the azo pigment dispersed in the polymer dispersant and a crosslinking agent at a heat treatment temperature X [°C] of 30°C or more and 60°C or less. In the heat treatment step, the heat treatment time Y [hours] is equal to or greater than the time Y1 [hours] calculated by formula (1). Y1 = 2^(8 - X / 10) (1)

[0011] The definitions of various terms used in this specification are shown below. "Aqueous-based" means that in the medium in which the pigment is dispersed, water accounts for the largest proportion by mass. "Record" is a concept that includes printing and marking, which record text and images. "(Meth)acrylic acid" means at least one selected from the group consisting of acrylic acid and methacrylic acid. "(Meth)acrylate" means at least one selected from the group consisting of acrylates and methacrylates.

[0012] The aqueous pigment dispersion of the present invention provides an aqueous pigment dispersion that achieves both excellent intermittent dispensing stability and excellent storage stability, despite containing azo pigments, which are highly pigmented pigments with low thermal stability. The reason for this is not entirely clear, but we believe it is as follows. Inkjet water-based inks, prepared by adding an organic solvent to an aqueous pigment dispersion of water-insoluble polymer particles containing pigments, swell due to the added organic solvent. In particular, at high temperatures such as in summer, the mobility of the water-insoluble polymer particles increases, leading to increased contact between the swollen water-insoluble polymer particles. This causes the water-insoluble polymer particles to aggregate rapidly, reducing the storage stability of the inkjet water-based ink. Therefore, to suppress the aggregation of water-insoluble polymer particles, a method is known in which a crosslinking agent is added to the water-insoluble polymer particles during the manufacturing process of the aqueous pigment dispersion and heat treatment is performed to crosslink the pigment-dispersed water-insoluble polymer particles, thereby improving the storage stability of the inkjet water-based ink. However, when the pigment is an azo pigment, we found that crosslinking the pigment dispersion containing the azo pigment improves storage stability but worsens intermittent ejection stability. This is thought to be because azo pigments have low thermal stability, and the heat treatment during crosslinking causes the azo pigment to decompose. When the water-based inkjet recording ink containing the decomposed azo pigment is left in the inkjet head for a long period of time, the water in the ink evaporates, and during the process of ink concentration, the azo pigment and its decomposition products precipitate, causing nozzle clogging and other problems. In contrast, the present invention's manufacturing method suppresses the decomposition of azo pigments and prevents deterioration of intermittent discharge stability by setting the heat treatment temperature to 60°C or lower. Furthermore, by setting the heat treatment temperature to 30°C or higher and the heat treatment time to be longer than the time calculated from the heat treatment temperature, it is possible to sufficiently advance the crosslinking reaction of the pigment dispersion containing the azo pigment and provide an aqueous pigment dispersion with good storage stability.

[0013] <Azo pigments> The types of azo pigments used in the present invention are not particularly limited. Examples of azo pigments include condensed disazo pigments such as C.I. Pigment Yellow 93, C.I. Pigment Yellow 94, C.I. Pigment Yellow 95, C.I. Pigment Yellow 128, C.I. Pigment Yellow 166, C.I. Pigment Orange 34, C.I. Pigment Orange 13, C.I. Pigment Orange 31, Pigment Red 144, Pigment Red 166, Pigment Red 220, C.I. Pigment Red 221, C.I. Pigment Red 242, Pigment Red 248, C.I. Pigment Red 262, C.I. Pigment Brown 23; disazo pigments such as C.I. Pigment Yellow 13, C.I. Pigment Yellow 83, C.I. Pigment Yellow 188; and monoazo pigments such as C.I. Pigment Red 187, C.I. Pigment Red 170, C.I. Pigment Yellow 74,C.I. Pigment Yellow 150, C.I. Pigment Red 48, C.I. Pigment Red 53, C.I. Pigment Orange 64, C.I. Pigment Red 247, C.I. Pigment Red 31, C.I. Pigment Red 146, C.I. Pigment Red 147, C.I. Pigment Red 150, C.I. Pigment Red 266, C.I. Pigment Red 269, etc. Among these, monoazo pigments are preferred, C.I. Pigment Yellow 74 and C.I. Pigment Red 150 are more preferred, and C.I. Pigment Yellow 74 is even more preferred. The above pigments can be used alone or in combination of two or more.

[0014] <Polymer dispersant> The polymer dispersant used in the present invention is a resin that disperses the above azo pigments, and may be either a water-soluble resin or a water-insoluble resin, and is preferably a water-insoluble resin. Here, regarding the "water solubility" and "water insolubility" of the resin, when the resin dried at 105°C for 2 hours until a constant weight is dissolved in 100 g of water at 25°C until saturation, if the dissolved amount exceeds 10 g, it is judged as "water soluble", and if it is 10 g or less, it is judged as "water insoluble". Further, as described later, when the polymer dispersant has an anionic group and the anionic group is neutralized with a neutralizing agent, the dissolved amount measured under the condition that the mass ratio of the polymer dispersant to the neutralizing agent is the same as that in the aqueous ink according to the present invention and the neutralizing agent is present is used for judgment.

[0015] Examples of the polymer dispersant include vinyl resins obtained by addition polymerization of vinyl monomers (vinyl compounds, vinylidene compounds, vinylene compounds); condensation resins such as polyester resins and polyurethane resins. The polymer dispersant may be appropriately synthesized or a commercially available product may be used. Among these, from the viewpoint of providing an aqueous ink for inkjet recording that achieves both excellent intermittent ejection stability and excellent storage stability, a vinyl resin (hereinafter also referred to as "vinyl resin (I)") is preferable.

[0016] From the viewpoint of providing an aqueous ink for inkjet recording that achieves both excellent intermittent ejection stability and excellent storage stability, it is preferable that the vinyl resin (I) contains a structural unit derived from an anionic group-containing monomer. In the present specification, the "anionic group" refers to an anionic group or a group that can be ionized to become an anionic group. Examples of the anionic group include a carboxy group (-COOM), a sulfonic acid group (-SO3M), and a phosphoric acid group (-OPO3M2). Among these, from the viewpoint of reacting with the reactive group of the crosslinking agent, a carboxy group is preferable. In the above chemical formula, M represents a hydrogen atom, an alkali metal, ammonium, or organic ammonium. Examples of vinyl resin (I) include homopolymers of anionic group-containing monomers, copolymers of anionic group-containing monomers and hydrophobic monomers, and copolymers of anionic group-containing monomers, hydrophobic monomers and nonionic monomers. If vinyl resin (I) is a copolymer, it may be a random copolymer, a block copolymer, an alternating copolymer, or a graft copolymer. In this specification, "hydrophobic" in the context of a hydrophobic monomer means that when the monomer is dissolved in 100 g of deionized water at 25°C until saturated, the amount of dissolved monomer is less than 10 g. Nonionic monomers are monomers that have a high affinity for water and water-soluble organic solvents, and are examples of monomers that contain hydroxyl groups or polyalkylene glycol chains.

[0017] Examples of anionic group-containing monomers include carboxyl group-containing monomers, sulfonic acid group-containing monomers, and phosphate group-containing monomers. Among these, carboxyl group-containing monomers are preferred, and (meth)acrylic acid is more preferred.

[0018] Examples of hydrophobic monomers include (meth)acrylates having hydrocarbon groups derived from aliphatic alcohols with 1 to 22 carbon atoms; styrene monomers; aromatic group-containing monomers such as aromatic group-containing (meth)acrylates; and styrene macromonomers. The molecular weight of aromatic group-containing monomers, preferably styrene monomers, is preferably less than 500. Styrene macromonomers are compounds with a number average molecular weight of 500 to 100,000 that have a polymerizable functional group at one end. Among these, hydrophobic monomers are preferably styrene monomers and styrene macromonomers, more preferably one or more selected from the group consisting of styrene, α-methylstyrene, 2-methylstyrene, and styrene macromonomers, and even more preferably one or more selected from the group consisting of styrene and styrene macromonomers. Hydrophobic monomers can be used individually or in combination of two or more.

[0019] Examples of nonionic monomers include polyalkylene glycol mono(meth)acrylates such as polyethylene glycol mono(meth)acrylate; and alkoxy polyalkylene glycol mono(meth)acrylates such as methoxy polyethylene glycol mono(meth)acrylate and octoxy polyethylene glycol mono(meth)acrylate. Nonionic monomers can be used individually or in combination of two or more.

[0020] When the vinyl resin (I) is a copolymer of an anionic group-containing monomer and a hydrophobic monomer, or a copolymer of an anionic group-containing monomer, a hydrophobic monomer, and a nonionic monomer, the content of constituent units derived from each monomer component in the vinyl resin (I) is as follows: The content of structural units derived from anionic group-containing monomers in the vinyl resin (I) is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less, from the viewpoint of providing an inkjet recording water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. The content of constituent units derived from hydrophobic monomers in the vinyl resin (I) is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 55% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less, from the viewpoint of providing an inkjet recording water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. When the vinyl resin (I) contains structural units derived from nonionic monomers, the content of structural units derived from nonionic monomers in the vinyl resin is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less, from the viewpoint of providing an inkjet recording water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. Vinyl resin (I) can be obtained, for example, by addition polymerization of raw material monomers, including anionic group-containing monomers and hydrophobic monomers, and optionally nonionic monomers, using known methods.

[0021] The acid value of the polymer dispersant is preferably 50 mg KOH / g or more, more preferably 70 mg KOH / g or more, even more preferably 90 mg KOH / g or more, and preferably 300 mg KOH / g or less, more preferably 270 mg KOH / g or less, and even more preferably 250 mg KOH / g or less, from the viewpoint of providing an inkjet water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. If the acid value is within the above range, the amount of acid groups and their neutralized acid groups is sufficient, and the dispersion stability of the pigment is ensured. This is also preferable from the viewpoint of balancing the affinity between the polymer dispersant and the water-based medium and the interaction between the polymer dispersant and the pigment. The acid value of a polymer dispersant can be calculated from the mass ratio of its constituent monomers. Alternatively, it can be determined by titrating the polymer dispersant after dissolving or swelling it in an organic solvent such as methyl ethyl ketone.

[0022] 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, and preferably 100,000 or less, more preferably 70,000 or less, and even more preferably 60,000 or less, from the viewpoint of dispersion stability of the azo pigment. The number-average molecular weight of the polymer dispersant is measured by the method described in the examples.

[0023] Examples of commercially available polymer dispersants include polyacrylic acid such as "Aron AC-10SL" (manufactured by Toagosei Co., Ltd.) and styrene / acrylic resins such as "Joncryl 67", "Joncryl 611", "Joncryl 678", "Joncryl 680", "Joncryl 690", and "Joncryl 819" (all manufactured by BASF Japan Ltd.).

[0024] The method for producing the aqueous pigment dispersion of the present invention may include a step of dispersing the above azo pigment in an aqueous medium with the above polymer dispersant. The form of particles containing azo pigment dispersed in a polymer dispersant (hereinafter also referred to as "azo pigment-containing resin particles") is not particularly limited, and it is sufficient that the particles are formed by at least the pigment and the pigment-dispersing resin, and are particles formed by the adsorption of the polymer dispersant onto the azo pigment in an aqueous medium. Examples of the form of azo pigment-containing resin particles include particles in which the azo pigment is encapsulated in the polymer dispersant, particles in which the azo pigment is uniformly dispersed in the polymer dispersant, and particles in which the azo pigment is exposed on the surface of the azo pigment-dispersing resin particles, and mixtures thereof are also included. Dispersion of azo pigments using polymer dispersants can be obtained as an aqueous dispersion by dispersing the azo pigment, polymer dispersant, and optionally a neutralizing agent, surfactant, etc.

[0025] In the process of dispersing azo pigments in an aqueous medium using a polymer dispersant, the aqueous medium refers to a dispersion medium in which water accounts for the largest proportion by mass, excluding the azo pigment, polymer dispersant, and neutralizing agent. The aqueous medium may contain an organic solvent from the viewpoint of improving the wettability of the pigment and the adsorption of the polymer dispersant to the pigment. Suitable examples of organic solvents include alcohols having 1 to 3 carbon atoms and ketones having 3 to 6 carbon atoms. The dispersion treatment may involve using shear stress alone to finely atomize the azo pigment particles to the desired particle size. However, from the viewpoint of obtaining a uniform aqueous dispersion of azo pigment-containing resin particles, it is preferable to pre-disperse a mixture of azo pigment, polymer dispersant, etc., before performing the final dispersion. For pre-dispersion, commonly used mixing and stirring devices such as anchor blades and disperser blades can be used as dispersers. Means of applying shear stress for this dispersion include, for example, 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 a high-pressure homogenizer from the viewpoint of reducing the particle size of the pigment. When performing dispersion processing using a high-pressure homogenizer, the average particle size of azo pigment-containing resin particles in the aqueous dispersion of azo pigment-containing resin particles can be adjusted by controlling the processing pressure and the number of passes. From the viewpoint of productivity and economic efficiency, the processing pressure is preferably 60 MPa to 300 MPa, and the number of passes is preferably 3 to 30.

[0026] In the step of dispersing an azo pigment in an aqueous medium using a polymer dispersant, if the polymer dispersant has carboxyl groups, at least a portion of the carboxyl groups are neutralized with a neutralizing agent. The neutralizing agent used in the step of dispersing an azo pigment in an aqueous medium using a polymer dispersant is preferably sodium hydroxide. The equivalent amount (mol%) of the neutralizing agent used is preferably 10 mol% or more, more preferably 20 mol% or more, even more preferably 30 mol% or more, and preferably 90 mol% or less, more preferably 80 mol% or less, and even more preferably 70 mol% or less, from the viewpoint of the dispersion stability of the (meth)acrylic resin emulsion and the dispersion stability of the azo pigment. The amount of neutralizing agent used (mol%) can be calculated using the following formula. When the amount of neutralizing agent used is 100 mol% or less, it is synonymous with the degree of neutralization. Amount of neutralizing agent used (mol%) = [{Mass of neutralizing agent added (g) / Equivalent amount of neutralizing agent} / [{Weighted average acid value of resin constituting the polymer dispersant (mgKOH / g) × Mass of resin constituting the polymer dispersant (g)} / (56 × 1000)] × 100

[0027] Preferably, the organic solvent in the resulting aqueous dispersion of azo pigment-containing resin particles is substantially removed, but it may remain as long as it does not impair the objective of the present invention.

[0028] The concentration of nonvolatile components (solids) in the aqueous dispersion of azo pigment-containing resin particles is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, from the viewpoint of suitably carrying out the reaction between the azo pigment-containing resin particles and the crosslinking agent in the heat treatment step described later. The solid content concentration of the aqueous dispersion of azo pigment-containing resin particles is measured by the method described in the examples.

[0029] The mass ratio of pigment to polymer dispersant in azo pigment-containing resin particles [pigment / polymer dispersant] is preferably 0.3 or higher, more preferably 1 or higher, even more preferably 1.5 or higher, and preferably 8 or lower, more preferably 5 or lower, and even more preferably 3 or lower, from the viewpoint of providing an inkjet recording water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability.

[0030] The average particle size of the azo pigment-containing resin particles is preferably 30 nm or larger, more preferably 50 nm or larger, even more preferably 70 nm or larger, and preferably 300 nm or smaller, more preferably 200 nm or smaller, and even more preferably 150 nm or smaller, from the viewpoint of providing an inkjet water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. The average particle size of the azo pigment-containing resin particles in the aqueous dispersion is measured by the method described in the examples.

[0031] <Crosslinking agent> The crosslinking agent used in the present invention crosslinks the polymer dispersant contained in the azo pigment-containing resin particles by the method described later. The crosslinking agent has two or more reactive groups in its molecule, and the reactive group is preferably one or more selected from the group consisting of a carbodiimide group, an oxazoline group, an epoxy group, an isocyanate group, an azilidino group, and an amino group. Note that the concept of an epoxy group includes a glycidyl group. Among these, from the viewpoint of providing an inkjet water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability, the reactive group is more preferably one or more selected from the group consisting of a carbodiimide group, an oxazoline group, and an epoxy group, even more preferably one or more selected from the group consisting of a carbodiimide group and an oxazoline group, and even more preferably an epoxy group. That is, the crosslinking agent is preferably one or more selected from the group consisting of a polyfunctional carbodiimide compound, a polyfunctional oxazoline compound, and a polyfunctional epoxy compound, and more preferably a polyfunctional epoxy compound. The reactive group equivalent of the crosslinking agent is preferably 70 or higher, more preferably 90 or higher, even more preferably 100 or higher, and preferably 300 or lower, more preferably 250 or lower, and even more preferably 200 or lower, from the viewpoint of providing an inkjet water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. Reactive group equivalent refers to the mass of the crosslinking agent per mole of reactive groups.

[0032] (Polyfunctional epoxy compound) A polyfunctional epoxy compound is a compound that has two or more epoxy groups in its molecule. As the polyfunctional epoxy compound, compounds containing two or more glycidyl ether groups in the molecule are preferred, polyglycidyl ethers of polyhydric alcohols are more preferred, and polyglycidyl ethers of polyhydric alcohols having 3 to 8 carbon atoms are even more preferred.

[0033] Examples of polyfunctional epoxy compounds include polypropylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, and hydrogenated bisphenol A type diglycidyl ether. Among these, the polyfunctional epoxy compound is preferably one or more selected from trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether, and diethylene glycol diglycidyl ether, and more preferably trimethylolpropane polyglycidyl ether.

[0034] (Polyfunctional carbodiimide compounds) A polyfunctional carbodiimide compound is a compound having two or more carbodiimide groups in its molecule. A polymer containing two or more carbodiimide groups (hereinafter also referred to as a "carbodiimide group-containing polymer") is preferred as the polyfunctional carbodiimide compound. Carbodiimide group-containing polymers are preferably obtained, for example, by encapsulating the terminal isocyanate groups of a condensation product obtained by a decarboxylation condensation reaction of diisocyanates in the presence of a carbodiimide catalyst with hydrophilic groups.

[0035] Examples of diisocyanates used in the decarboxylation reaction include aliphatic diisocyanates such as hexamethylene diisocyanate (HDI), decamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate; 4,4'-dicyclohexylmethane diisocyanate (H12MDI), isophorone diisocyanate (IPDI), 2,5- or 2,6-norbornane diisocyanate, hydrogenated xylylene diisocyanate (H6XDI), hydrogenated tolylene diisocyanate, and 2,4-bis-(8 Examples include alicyclic diisocyanates such as -isocyanate octyl)-1,3-dioctylcyclobutane (OCDI); aromatic aliphatic diisocyanates such as m- or p-xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI); and aromatic diisocyanates such as 2,4,6-triisopropylphenyl diisocyanate (TIDI), 4,4'- or 2',4-diphenylmethane diisocyanate (MDI) and 2,4- or 2,6-tolylene diisocyanate (TDI).

[0036] The compound used to encapsulate the terminal isocyanate group of the condensation product is a compound having a functional group that can react with the isocyanate group, and examples include polyethylene glycol monomethyl ether and polypropylene glycol monomethyl ether.

[0037] The carbodiimide group equivalent of the carbodiimide group-containing polymer is preferably 100 or more, more preferably 170 or more, even more preferably 200 or more, and preferably 500 or less, more preferably 400 or less, and even more preferably 300 or less, from the viewpoint of providing an inkjet water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. Note that the carbodiimide group equivalent refers to the mass of the carbodiimide group-containing polymer per mole of carbodiimide groups. Examples of commercially available polymers containing carbodiimide groups include Carbodilite E-02, Carbodilite E-05, and Carbodilite E-07S (all manufactured by Nisshinbo Chemical Co., Ltd., trade names).

[0038] (Polyfunctional oxazoline compounds) A polyfunctional oxazoline compound is a compound having two or more oxazoline groups in its molecule. A polymer containing two or more oxazoline groups (hereinafter also referred to as an "oxazoline group-containing polymer") is preferred as the polyfunctional oxazoline compound. From the viewpoint of increasing reactivity, the number-average molecular weight of the oxazoline group-containing polymer is preferably 1,000 or more, more preferably 5,000 or more, even more preferably 10,000 or more, and preferably 100,000 or less, more preferably 50,000 or less, and even more preferably 30,000 or less. Examples of oxazoline group-containing polymers include polymers with an acrylic backbone, polymers with a styrene / acrylic backbone, polymers with a styrene backbone, and polymers with an acrylonitrile / styrene backbone.

[0039] The oxazoline group equivalent of the oxazoline group-containing polymer is preferably 100 or more, more preferably 170 or more, even more preferably 200 or more, and preferably 500 or less, more preferably 400 or less, and even more preferably 300 or less, from the viewpoint of providing an inkjet water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. Note that the oxazoline group equivalent refers to the mass of the oxazoline group-containing polymer per mole of oxazoline groups. Examples of commercially available oxazoline group-containing polymers include the "Epocross WS" series, such as "Epocross WS-300," "Epocross WS-500," and "Epocross WS-700" (all manufactured by Nippon Shokubai Co., Ltd., and are water-soluble).

[0040] [Heat treatment process] The present invention provides a method for producing an aqueous pigment dispersion, comprising the step of heat-treating a mixture of the azo pigment (azo pigment-containing resin particles) dispersed in the polymer dispersant and the crosslinking agent at a heat treatment temperature X [°C] of 30°C to 60°C. The heat treatment temperature X [°C] is preferably 35°C or higher, more preferably 40°C or higher, preferably 55°C or lower, and more preferably 50°C or lower. During the heat treatment process, the reactive groups in the polymer dispersant react with the reactive groups in the crosslinking agent (hereinafter also referred to as the "crosslinking reaction"), and the polymer dispersant in the azo pigment-containing resin particles is crosslinked by the crosslinking agent. In the present invention, the azo pigment-containing resin particles in which the polymer dispersant has been crosslinked by the crosslinking agent are pigment-containing polymer particles. The heat treatment time Y [hours] is equal to or greater than the time Y1 [hours] calculated by equation (1), from the viewpoint of the crosslinking reaction rate. Y1 = 2^(8 - X / 10) (1)

[0041] Furthermore, it is preferable that the heat treatment time Y [hours] is less than or equal to the time Y2 [hours] calculated by equation (2). Y² = {2^(8-X / 10)} × 2 (2) If the heat treatment time Y [hours] is less than or equal to the time Y2 [hours] calculated by equation (2), the decomposition of the azo pigment can be further suppressed, and an aqueous pigment dispersion can be produced that provides an aqueous inkjet recording ink with superior intermittent ejection stability.

[0042] When mixing azo pigment-containing resin particles with a crosslinking agent, it is preferable to add the crosslinking agent to an aqueous dispersion of azo pigment-containing resin particles. In this case, the ratio of the number of molar equivalents of the crosslinking agent to the number of molar equivalents of the polymer dispersant in the azo pigment-containing resin particles is preferably 20 mol% or more, more preferably 23 mol% or more, even more preferably 26 mol% or more, and preferably 70 mol% or less, more preferably 76 mol% or less, and even more preferably 72 mol% or less, resulting in an aqueous inkjet recording ink that achieves both excellent intermittent ejection stability and excellent storage stability. A carboxyl group is preferred as the reactive group for the polymer dispersant. Furthermore, one or more groups selected from epoxy groups, carbodiimide groups, and oxazoline groups are preferred as the reactive group for the crosslinking agent, with epoxy groups being more preferred.

[0043] Furthermore, the amount of crosslinking agent added to 100 parts by mass of polymer dispersant contained in the azo pigment-containing resin particles is preferably 0.2 parts by mass or more, more preferably 0.3 parts by mass or more, even more preferably 0.4 parts by mass or more, and preferably 1.5 parts by mass or less, more preferably 1.45 parts by mass or less, and even more preferably 1.4 parts by mass or less.

[0044] In this case, if the reactive group in the polymer dispersant is a carboxyl group, the completion of the crosslinking reaction by the heat treatment step can be confirmed by measuring the pH of the reaction system. At the start of the reaction, the pH of the reaction system is acidic due to the carboxyl group in the polymer dispersant. Subsequently, as the reaction progresses, the carboxyl group decreases due to the reaction with the reactive group in the crosslinking agent, so the pH of the reaction system gradually increases, and when it becomes constant, the crosslinking reaction can be considered complete. Here, if the change in the pH of the reaction system measured at 10-minute intervals is 0.1 or less, it is determined that the pH has become constant.

[0045] A method for producing an aqueous pigment dispersion according to another aspect of the present invention comprises the step of heat-treating a mixture of the azo pigment (azo pigment-containing resin particles) dispersed in the polymer dispersant and the crosslinking agent at a heat treatment temperature X [°C] of 30°C to 60°C, wherein the heat treatment time Y [hours] is 3 [hours] or more calculated by formula (3) and 4 [hours] or less calculated by formula (4). Y3 = -0.6X + 40 (3) Y4 = -1.2X + 80 (4)

[0046] The average particle size of the pigment-containing polymer particles is preferably 30 nm or larger, more preferably 50 nm or larger, even more preferably 70 nm or larger, and preferably 300 nm or smaller, more preferably 200 nm or smaller, and even more preferably 150 nm or smaller, from the viewpoint of reducing coarse particles and providing an inkjet water-based ink that achieves both excellent intermittent ejection stability and excellent storage stability. The average particle size of the pigment-containing polymer particles is measured by the method described in the examples.

[0047] The content of pigment-containing polymer particles in the aqueous pigment dispersion 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 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less.

[0048] [Manufacturing method for water-based ink for inkjet recording] A method for producing an aqueous ink for inkjet recording according to one aspect of the present invention comprises the step of mixing the aqueous pigment dispersion produced above with a water-soluble organic solvent. Here, "water-soluble" in "water-soluble organic solvent" refers to the property of being able to be mixed with water in any proportion. There are no particular restrictions on the method of mixing the aqueous pigment dispersion with the water-soluble organic solvent.

[0049] (Water-soluble organic solvent) Examples of water-soluble organic solvents include polyhydric alcohols, polyhydric alcohol alkyl ethers, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds. Among these, the organic solvent is preferably one or more selected from polyhydric alcohols, more preferably one or more selected from the group consisting of ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-heptanediol, 1,2-hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, glycerin, trimethylolpropane, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, and diethylene glycol diethyl ether, and even more preferably one or more selected from the group consisting of dipropylene glycol, glycerin, and 1,2-hexanediol.

[0050] From the viewpoint of improving the intermittent ejection stability and storage stability of the ink, the content of the organic solvent in the ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less.

[0051] (Surfactants) The water-based inkjet recording ink of the present invention preferably contains a surfactant from the viewpoint of improving the intermittent ejection stability and storage stability of the ink. In the step of mixing the water-based pigment dispersion and the water-soluble organic solvent, a mixture of the water-soluble organic solvent and the surfactant may be mixed into the water-based pigment dispersion, or the surfactant may be mixed into a mixture of the water-soluble pigment dispersion and the water-soluble organic solvent, or a mixture of the water-based pigment dispersion and the surfactant may be mixed into the water-based pigment dispersion.

[0052] Examples of surfactants include nonionic surfactants, anionic surfactants, and amphoteric surfactants, but nonionic surfactants are preferred, and one or more selected from the group consisting of acetylene glycol-based surfactants and polyoxyalkylene alkyl ether-based surfactants are more preferred. Preferred acetylene glycol-based surfactants include acetylene glycol having 8 to 22 carbon atoms and ethylene oxide adducts of the acetylene glycol, with 2,4,7,9-tetramethyl-5-decine-4,7-diol or its ethylene oxide adduct being more preferred. A specific example of a polyoxyalkylene alkyl ether surfactant is polyoxyethylene lauryl ether.

[0053] From the viewpoint of improving the intermittent ejection stability and storage stability of the ink, the surfactant content in the ink of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.4% by mass or more, and preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less.

[0054] The water-based inkjet recording ink of the present invention may further contain, if necessary, various additives commonly used in water-based inkjet recording inks, such as humectants, wetting agents, penetrating agents, dispersants, viscosity modifiers, defoamers, preservatives, fungicides, and rust inhibitors. (Physical properties of water-based inks) The viscosity of the water-based inkjet recording ink of the present invention at 25°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 4 mPa·s or more, and preferably 12 mPa·s or less, more preferably 9 mPa·s or less, and even more preferably 7 mPa·s or less. The pH of the water-based inkjet recording ink of the present invention is preferably 7.0 or higher, more preferably 7.5 or higher, and even more preferably 8.0 or higher. Furthermore, from the viewpoint of material resistance and skin irritation, the pH is preferably 11 or lower, more preferably 10.5 or lower, and even more preferably 10 or lower.

[0055] The content of pigment-containing polymer particles in the water-based inkjet recording ink of the present invention is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less, from the viewpoint of improving the intermittent ejection stability and storage stability of the ink.

[0056] The pigment content in the water-based inkjet recording ink of the present invention is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, from the viewpoint of image density, and preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, from the viewpoint of improving the intermittent ejection stability and storage stability of the ink.

[0057] The average particle size of the pigment-containing polymer particles in the aqueous inkjet recording ink of the present invention is preferably the same as the average particle size of the pigment-containing polymer particles in the aqueous pigment dispersion of the present invention. Therefore, the preferred configuration of the average particle size of the pigment-containing polymer particles in the aqueous inkjet recording ink of the present invention is also preferably the same as the preferred configuration of the pigment-containing polymer particles in the aqueous pigment dispersion of the present invention.

[0058] (wax) The water-based ink for inkjet recording of the present invention may contain wax from the viewpoint of improving long-term ejection reliability and image fastness of recorded materials. The melting point of the wax is preferably 85°C or higher, more preferably 90°C or higher, even more preferably 95°C or higher, even more preferably 100°C or higher, even more preferably 110°C or higher, and preferably 150°C or lower, more preferably 145°C or lower, and even more preferably 140°C or lower, from the viewpoint of providing an inkjet recording water-based ink that improves long-term ejection reliability and image fastness of the recorded material. Examples of waxes include polyolefin waxes, which are mainly composed of olefin monomers; paraffin waxes, which consist of a mixture of chain-type saturated hydrocarbons with 20 to 30 carbon atoms; and synthetic waxes such as sazole waxes. Among these, one or more selected from polyolefin waxes and paraffin waxes are preferred, with polyolefin waxes being more preferred.

[0059] Examples of olefin monomers used in polyolefin waxes include chain olefins and cyclic olefins, but those mainly composed of chain olefins having 2 to 6 carbon atoms are preferred, and polyolefin waxes (polyethylene waxes) mainly composed of ethylene are more preferred. Here, "mainly composed of ethylene" means that the ethylene content relative to the total components constituting the wax is preferably 50% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 80% by mass or more. Oxidized polyolefin waxes can be obtained by introducing oxygen atoms and other elements into the molecule of a high molecular weight polyolefin polymer while adjusting it to a desired molecular weight through thermal decomposition or other means, and can be used as polyolefin waxes. The content of polyolefin-based 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 even more preferably 100% by mass.

[0060] The wax is preferably in the form of particles dispersed in an aqueous medium, i.e., a wax emulsion. There are no particular restrictions on the method for producing the wax emulsion. For example, one method involves mixing a polyolefin wax with other waxes used as needed and emulsifying it with a known surfactant. In the present invention, it is preferable that the wax is particles dispersed in an aqueous medium with one or more nonionic surfactants selected from nonionic surfactants and anionic surfactants. Examples of nonionic surfactants include ethylene oxide adducts of higher alcohols and ethylene oxide adducts of alkylated phenols. Examples of anionic surfactants include sulfate ester salts and phosphate ester salts based on ethylene oxide adducts of higher alcohols, and alkylated benzenesulfonates. Among these, the preferred solution is a nonionic wax emulsion obtained by emulsifying wax with a nonionic surfactant, or an anionic wax emulsion obtained by emulsifying wax with an anionic surfactant, as it provides an inkjet recording water-based ink that improves long-term ejection reliability and image fastness of the recorded material. More preferably, it is a nonionic wax emulsion obtained by emulsifying wax with a nonionic surfactant. That is, the wax is preferably dispersed with one or more surfactants selected from nonionic surfactants and anionic surfactants, and more preferably dispersed with a nonionic surfactant.

[0061] The average particle size of the wax emulsion is preferably 500 nm or less, more preferably 300 nm or less, even more preferably 100 nm or less, and preferably 10 nm or more, and preferably 30 nm or more, as this improves the dispersion stability of the wax emulsion and provides an inkjet recording water-based ink that improves long-term ejection reliability and image fastness of the recorded material. The average particle size of a wax emulsion can be measured by known dynamic light scattering methods. The average particle size of the wax emulsion in the aqueous ink after its preparation is substantially the same as the average particle size of the wax emulsion prepared before the aqueous ink was prepared. Suitable examples of commercially available wax emulsions include polyethylene wax emulsions such as Hi-Tec E-6500, E-6400, and E-8237 from Toho Chemical Industry Co., Ltd., AQUACER 507, 513, 515, 526, 531, 533, 537, 539, 552, and 1547 from BYK Corporation, Cellosol 428, H620, 686, 524, Trasol CN, Polylon L-787, and L-788 from Chukyo Oil & Fat Co., Ltd., and Chemipearl W900 and W4005 from Mitsui Chemicals, Inc.

[0062] The wax content in the ink of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.5% by mass or more, even more preferably 0.8% by mass or more, and preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, and even more preferably 1.5% by mass or less.

[0063] The ink of the present invention may further contain, if necessary, various additives commonly used in water-based inkjet inks, such as humectants, wetting agents, penetrating agents, dispersants, viscosity modifiers, defoamers, preservatives, fungicides, and rust inhibitors. [Examples]

[0064] In the following preparation examples, embodiments, and comparative examples, "parts" and "%" refer to "parts by mass" and "mass%" respectively, unless otherwise specified. Furthermore, the various physical properties of the aqueous dispersions obtained in the preparation example, the example, and the comparative example were measured and evaluated by the following method.

[0065] (1) Measurement of the number-average molecular weight of polymer dispersants The solutions prepared by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol / L and 50 mmol / L, respectively, were used as eluents. The results were measured using gel permeation chromatography (GPC instrument (HLC-8320GPC) manufactured by Tosoh Corporation, columns manufactured by Tosoh Corporation (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn SuperAW-H), flow rate: 0.5 mL / min) as standard substances, using monodisperse polystyrene kits with known molecular weights (PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick C (F-288, F-40, F-4, A-5000, A-500), manufactured by Tosoh Corporation). The measurement sample was prepared by mixing 0.1 g of polymer dispersant with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25°C for 10 hours, and filtering through a syringe filter (DISMIC-13HP, PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

[0066] (2) Measurement of non-volatile component concentration (solid content concentration) Approximately 10 g of sodium sulfate, which had been stabilized in a desiccator, was accurately weighed into a 30 mL polypropylene container (φ=40 mm, height=30 mm). Approximately 1 g of the sample was added and mixed, then accurately weighed again. The mixture was maintained at 105°C for 2 hours to remove volatile components, and then left in the desiccator for another 15 minutes before being accurately weighed again. The mass of the sample after removal of volatile components was taken as the solid content, and the concentration of non-volatile components (solid content concentration) was obtained by dividing it by the mass of the added sample.

[0067] (3) Measurement of the acid value of polymers The resin was dissolved in a titration solvent of toluene and acetone (2:1 volume ratio) in a potentiometric automatic titrator (manufactured by Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with a 0.1N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was used as the endpoint. The acid value (mgKOH / g) was calculated from the amount of potassium hydroxide solution titrated to the endpoint.

[0068] (4) pH measurement The pH at 20°C was measured using a benchtop pH meter "F-71" (manufactured by Horiba, Ltd.) equipped with a pH electrode "6337-10D" (manufactured by Horiba, Ltd.).

[0069] (5) Measurement of the average particle size of pigment-containing polymer particles The average particle size of the pigment dispersion was measured by dynamic light scattering using a laser particle analysis system (ELS-8000, manufactured by Otsuka Electronics Co., Ltd.) and calculated by cumulant analysis. The measurement conditions were a temperature of 25°C, an angle of 90° between the incident light and the detector, and 100 integration cycles. The refractive index of water (1.333) was input as the refractive index of the dispersion solvent. For the measurement sample, pigment-containing polymer particles were weighed into a screw tube (No. 5, manufactured by Maruemu Co., Ltd.) and the solid content concentration was 2 × 10⁻⁶. -4 Water was added to the solution to a mass percentage, and the mixture was stirred using a magnetic stirrer at 25°C for 1 hour.

[0070] (6) Measurement of the viscosity of water-based ink The viscosity of water-based inks at 25°C was measured using an E-type viscometer (TV-25, manufactured by Toki Sangyo Co., Ltd., with a standard cone rotor of 1°34'×R24 and a rotation speed of 50 rpm).

[0071] (Synthesis of polymer dispersants) Synthesis Example 1 (Synthesis of polymer dispersant p1) 16 parts of methacrylic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 44 parts of styrene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent), 30 parts of styrene macromonomer "AS-6S" (manufactured by Toagosei Co., Ltd., number average molecular weight 6,000, solids content 50%) (15 parts of active ingredient), and 25 parts of methoxypolyethylene glycol methacrylate "Bremmer PME-200" (manufactured by NOF Corporation) were mixed to prepare 115 parts of monomer mixture. In a reaction vessel, 18 parts of methyl ethyl ketone, 0.03 parts of 2-mercaptoethanol as a chain transfer agent, and 10% (11.5 parts) of the monomer mixture were added and mixed, and the vessel was thoroughly purged with nitrogen gas. Meanwhile, a mixture of the remaining 90% (103.5 parts) of the monomer mixture, 0.27 parts of the chain transfer agent, 42 parts of methyl ethyl ketone, and 3 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a polymerization initiator was placed in a dropping funnel. Under a nitrogen atmosphere, the mixture in the reaction vessel was heated to 75°C while stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours at 75°C following the completion of the dropwise addition, a solution of 3 parts of the polymerization initiator dissolved in 5 parts of methyl ethyl ketone was added, and the mixture was aged for a further 2 hours at 75°C, followed by 2 hours at 80°C. After cooling to room temperature, 50 parts of methyl ethyl ketone were added to obtain a solution of polymer dispersant p1 (number average molecular weight: 53,000, acid value: 104 mgKOH / g), which is a water-insoluble polymer. The solid content concentration of the polymer dispersant p1 solution was 45%.

[0072] Synthesis Example 2 (Synthesis of polymer dispersant p2) 31 parts acrylic acid and 69 parts styrene were mixed to prepare 100 parts monomer mixture. 10 parts methyl ethyl ketone, 0.2 parts 2-mercaptoethanol as a polymerization chain transfer agent, and 10% (10 parts) of the monomer mixture were added to the reaction vessel and mixed, followed by thorough nitrogen gas purging. Separately, a dropping funnel was filled with a mixture of the remaining 90% (90 parts) of the monomer mixture, 0.13 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a radical polymerization initiator. Under a nitrogen atmosphere, the monomer mixture in the reaction vessel was heated to 65°C while stirring, and the mixture in the dropping funnel was added dropwise over 3 hours. After 2 hours had elapsed since the end of the dropwise addition while maintaining the temperature at 65°C, a solution of 0.1 parts of the polymerization initiator dissolved in 2 parts of methyl ethyl ketone was added, and the mixture was further aged at 65°C for 2 hours and then at 70°C for 2 hours. After cooling to room temperature, 50 parts of methyl ethyl ketone were added to obtain a solution of polymer dispersant p2 (number average molecular weight: 32,000, acid value: 240 mg KOH / g), which is a water-insoluble polymer. The solid content concentration of the polymer dispersant p2 solution was 45%.

[0073] (Manufacturing of aqueous pigment dispersions) Example 1-1 (Preparation of aqueous pigment dispersion 1) (Step 1: Preparation of an aqueous dispersion of azo pigment-containing polymer particles) 95.2 parts of the polymer dispersant p1 solution obtained in Synthesis Example 1 were mixed with 53.9 parts of methyl ethyl ketone. 13.0 parts of 5N sodium hydroxide aqueous solution, 0.5 parts of 25% aqueous ammonia, and 341 parts of deionized water were added as neutralizing agents. Furthermore, 100 parts of CI Pigment Yellow 74 (Sanyo Pigment Co., Ltd., Fast Yellow 7414) were added as an azo pigment to obtain a pigment mixture. The degree of neutralization of the polymer dispersant p1 was 68.8 mol%. The obtained pigment mixture was mixed using a disperser blade at 7,000 rpm and 20°C for 1 hour, and then subjected to a 15-pass dispersion treatment at a pressure of 180 MPa using a microfluidizer "High-Pressure Homogenizer M-140K" (Microfluidics Inc.) to obtain a dispersion. The entire volume of the obtained dispersion was placed in a 2L round-bottom flask, and deionized water was added to bring the solid content to 15%. Then, using a rotary distillation apparatus (Tokyo Rikakikai Co., Ltd., rotary evaporator: N-1000S), the dispersion was heated in a 32°C bath at a rotation speed of 50 r / min and maintained at a pressure of 0.09 MPa (abs) for 3 hours to remove methyl ethyl ketone. Next, the bath was adjusted to 50°C, and the pressure was reduced to 0.07 MPa (abs) to concentrate the solution until the solid content reached 25% to obtain a concentrate. The obtained concentrate was placed in a 500 mL angle rotor and centrifuged at 3,660 r / min for 20 minutes using a centrifuge (Hitachi Koki Co., Ltd., high-speed refrigerated centrifuge: himac CR22G, set temperature 20°C) to obtain the liquid phase. The liquid phase was then collected and filtered through a filter ("Minisart syringe filter" manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, yielding an aqueous dispersion of azo pigment-containing polymer particles P-1 (solid content concentration: 25%). (Step 2: Production of aqueous pigment dispersion) To 100 parts of the obtained aqueous dispersion P-1 of azo pigment-containing polymer particles, 0.54 parts of trimethylolpropane polyglycidyl ether "Denacol EX321L" (manufactured by Nagase ChemteX Corporation) and 15.23 parts of deionized water were added as crosslinking agents, and the mixture was heated at 40°C for 24 hours while stirring. At this time, the crosslinking treatment was carried out with an amount of crosslinking agent that was sufficient to react with 30% of the total number of carboxyl groups contained in the polymer dispersant p1. Next, after cooling to room temperature, the liquid phase was collected and filtered through a filter "Minisart syringe filter" (manufactured by Sartorius, pore size: 5 μm, material: cellulose acetate) to remove coarse particles, and aqueous pigment dispersion 1 (solid content: 22.0%, pigment content: 15.3%, crosslinked polymer dispersant content: 6.7%) was obtained as an aqueous dispersion of pigment-containing crosslinked polymer particles (acid value: 75 mg KOH / g). The average particle size of the pigment-containing polymer particles in aqueous pigment dispersion 1 was 105 nm.

[0074] Examples 1-2 (Preparation of aqueous pigment dispersion 2) Except for using 17.5 parts of a 5N sodium hydroxide aqueous solution in step 1 to neutralize the polymer dispersant p2 to 40.1 mol%, and using 1.24 parts of trimethylolpropane polyglycidyl ether "Denacol EX321L" (manufactured by Nagase ChemteX Corporation), an aqueous pigment dispersion 2 (solids concentration: 22.0%, pigment content: 15.2%, crosslinked polymer dispersant content: 6.8%) was obtained as an aqueous dispersion of pigment-containing crosslinked polymer particles (acid value: 175 mg KOH / g) in the same manner as in Example 1-1. The average particle size of the pigment-containing polymer particles in aqueous pigment dispersion 2 was 104 nm.

[0075] Examples 1-3 to 1-8 and Comparative Examples 1-1 to 1-5 (Production of aqueous pigment dispersions 3-8 and 11-15) Aqueous pigment dispersions 3-8 and 11-15 of Examples 1-3 to 1-8 and Comparative Examples 1-1 to 1-5 were obtained in the same manner as in Example 1-1, except that the heating temperature and heating time were changed as shown in Table 1. The average particle size of the pigment-containing polymer particles in aqueous pigment dispersions 3-8 and 11-15 is shown in Table 1.

[0076] Examples 1-9 (Preparation of aqueous pigment dispersion 9) Aqueous pigment dispersion 9 of Example 1-9 was obtained in the same manner as in Example 1-1, except that the azo pigment was changed to CI Pigment Red 150 (manufactured by Toyo Color Co., Ltd., LIONOGEN RED LX10231) as shown in Table 1. The average particle size of the pigment-containing polymer particles in aqueous pigment dispersion 9 is shown in Table 1.

[0077] Examples 1-10 (Preparation of aqueous pigment dispersion 10) Aqueous pigment dispersion 10 of Example 1-10 was obtained in the same manner as in Example 1-1, except that the amount of crosslinking agent added was changed as shown in Table 1. The average particle size of the pigment-containing polymer particles in the aqueous pigment dispersion 10 is shown in Table 1.

[0078] [Table 1]

[0079] (Manufacturing of water-based inks for inkjet recording) Examples 2-1 to 2-10 and Comparative Examples 2-1 to 2-5 Aqueous pigment dispersions 1-10 and 11-15, each containing 10% pigment in the inkjet water-based ink, were prepared. These dispersions were mixed with the inkjet water-based ink containing 5% dipropylene glycol, 5% glycerin, 5% 1,2-hexanediol, and 0.5% nonionic surfactant (2,4,7,9-tetramethyl-5-decine-4,7-diol). Triethanolamine was added as a pH adjuster to achieve a pH of 9.5, and deionized water was added to bring the total volume to 100%. The mixture was stirred thoroughly with a magnetic stirrer and filtered using a 25 mL needleless syringe fitted with a 1.2 μm filter (acetylcellulose membrane, outer diameter: 2.5 cm, manufactured by Fujifilm Corporation) to obtain the inkjet water-based ink. The viscosity of the inkjet water-based ink is shown in Table 2.

[0080] (Storage stability evaluation) Water-based inkjet inks were stored at 60°C for 60 days, and the percentage change in viscosity of each water-based ink before and after storage was evaluated. The viscosity of water-based inkjet recording inks before and after storage was measured at 25°C using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.), and the percentage change in viscosity was calculated using the following formula to evaluate storage stability. The results are shown in Table 2. A lower percentage change in viscosity indicates better storage stability, and a value of 110% or less indicates that the ink is usable for practical purposes. Viscosity change rate (%) [= (Ink viscosity after storage) / (Ink viscosity before storage) × 100]

[0081] (Evaluation of intermittent discharge stability) In an environment with a temperature of 25±1℃ and a relative humidity of 30±5%, an inkjet ejection evaluation device (manufactured by Seiko Epson Corporation) equipped with an inkjet head "S800-A1" (piezoelectric type) was filled with the inkjet recording inks of the examples and comparative examples. The head voltage was set to 37V, the frequency to 25kHz, the drive waveform to be push-pull type, and the appropriate amount of ejected liquid to 7.5pL. An ink ejection command was sent to the ejection evaluation device, and the initial number of normal ejected nozzles when the ink was being ejected normally was confirmed. Under the same conditions, the printer was stopped for 15 minutes and the inkjet head was exposed to the atmosphere. After 15 minutes, an ink ejection command with the same ejection conditions as above was sent to the ejection evaluation device, and the number of normal ejected nozzles after the 15-minute gap was observed. The intermittent ejection performance (%) was calculated using the following formula, and the intermittent ejection stability was evaluated. The higher the intermittent ejection performance (%), the better the intermittent ejection stability is judged to be, and if the intermittent ejection performance is 80% or higher, it can be used in practical applications. The results are shown in Table 2. Intermittent dispensing performance (%) = (Number of normal dispensing nozzles after a 15-minute interruption / Initial number of normal dispensing nozzles) × 100

[0082] [Table 2]

[0083] Table 2 shows that the water-based inkjet recording inks of the examples exhibit excellent storage stability and ejection stability. On the other hand, the aqueous inkjet inks of Comparative Examples 2-1 and 2-2, which used aqueous pigment dispersions 11 and 12 obtained by performing the heat treatment process at 70°C, higher than 60°C, exhibited poor intermittent ejection stability. This is thought to be because the azo pigment decomposed due to the excessively high temperature of the heat treatment process used to manufacture the aqueous pigment dispersions 11 and 12. Furthermore, the water-based inkjet inks of Comparative Examples 2-3 and 2-4, which used water-based pigment dispersions 13 and 14 obtained by performing the heat treatment process for a shorter time than the time Y1 calculated by formula (1), exhibited poor storage stability. This is thought to be because the heat treatment process in the production of water-based pigment dispersions 13 and 14 was too short, resulting in insufficient crosslinking of the polymer dispersant, and the pigment-containing polymer particles aggregated during storage of the water-based inkjet ink. Furthermore, the water-based inkjet inks used in Comparative Examples 2-5, which utilized an aqueous pigment dispersion 15 obtained by performing the heat treatment process at a temperature lower than 30°C (20°C), exhibited poor storage stability. This is thought to be because the low heat treatment temperature during the production of the aqueous pigment dispersion 15 prevented sufficient crosslinking of the polymer dispersant, leading to aggregation of pigment-containing polymer particles during storage of the water-based inkjet ink.

Claims

1. A method for producing an aqueous pigment dispersion containing pigment-containing polymer particles comprising an azo pigment and a polymer dispersant, The process involves heat-treating a mixture of the azo pigment dispersed in the polymer dispersant and a crosslinking agent at a heat treatment temperature X [°C] of 30°C to 60°C. A method for producing an aqueous pigment dispersion, wherein the heat treatment time Y [hours] is equal to or greater than the time Y1 [hours] calculated by formula (1). Y1=2^(8-X / 10) (1)

2. A method for producing an aqueous pigment dispersion according to claim 1, wherein the azo pigment is C.I. Pigment Yellow 74.

3. The method for producing an aqueous pigment dispersion according to claim 1, wherein the crosslinking agent is a polyglycidyl ether of a polyhydric alcohol having 3 to 8 carbon atoms.

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

5. A method for producing an aqueous pigment dispersion according to claim 1, wherein the heat treatment time Y [hours] is less than or equal to the time Y2 [hours] calculated by formula (2). Y2={2^(8-X / 10)}×2 (2)

6. The method for producing an aqueous pigment dispersion according to claim 1, wherein the polymer dispersant is a vinyl resin.

7. A method for producing an aqueous pigment dispersion according to claim 1, wherein the acid value of the polymer dispersant is 50 mg KOH / g or more and 300 mg KOH / g or less.

8. A method for producing an aqueous pigment dispersion according to claim 1, wherein the ratio of the number of molar equivalents of the crosslinking agent to the number of molar equivalents of the polymer dispersant is 20% or more and 50% or less.

9. A method for producing an aqueous ink for inkjet recording, comprising the step of mixing an aqueous pigment dispersion obtained by the manufacturing method described in any one of claims 1 to 8 with a water-soluble organic solvent.

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