JP2023092882A - Production method of aqueous pigment dispersion - Google Patents

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 The present invention relates to a method for producing a water-based pigment dispersion suitable for a water-based ink for inkjet recording, and a method for producing a water-based ink for inkjet recording.

The inkjet recording method is a recording method in which ink droplets are directly ejected from fine nozzles and deposited on a recording medium to obtain a recorded matter on which characters and images are recorded. This system has many advantages such as easy full-color printing, low cost, the ability to use plain paper as a recording medium, and no contact with the recording medium. In recent years, from the viewpoint of weather resistance and water resistance of printed matter, inks using pigments as coloring agents have been widely used.
In recent years, there has also been a movement to replace printing on low-absorption media with water-based inkjet, which used to be the mainstream of gravure printing using oil-based ink, from the perspective of reducing the environmental impact by not requiring plates. is attracting attention. Since the ink vehicle does not permeate the interior of the print medium, the pigment particles contained in the ink remain on the surface of the print medium. However, unlike oil-based inks, water-based inks cannot contain a large amount of a resin that imparts fixability, so they do not have sufficient adhesion and scratch resistance when printing on low-absorption media. Attempts have been made.

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 scratch resistance, wherein the polymer dispersant is a (meth)acrylic resin (A ) and a resin (B), wherein the resin (B) contains an acid-modified polyolefin resin.

JP 2019-119889 A

Water-based inks containing pigments have a problem of ejection stability compared to inks using dyes as colorants. Furthermore, ink containing resin particles in addition to pigments to improve fixability is exposed to strong shearing conditions in the vicinity of the ejection nozzle, and is likely to form aggregates near the nozzle edge, further reducing ejection stability. , and storage stability is also reduced.
The present invention provides a method for producing a water-based pigment dispersion and a water-based ink for inkjet recording that can improve the storage stability and ejection stability of an ink containing a resin for obtaining fixability, particularly an ink containing wax. Regarding.

As a method for obtaining a water-based ink for inkjet recording excellent in storage stability and ejection stability, the present inventors prepared a water-based pigment dispersion containing a pigment, wax, and a polymer dispersant at a temperature below the melting point of the wax. We have found that the above problems can be solved by providing a heat treatment step.

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, wax, and a polymer dispersant to obtain an aqueous dispersion Step (II): Adding a cross-linking agent to the aqueous dispersion obtained in the step (I), [2] A method for producing a water-based ink, comprising a step of mixing the water-based pigment dispersion obtained by the manufacturing method according to [1] with a water-soluble organic solvent.

INDUSTRIAL APPLICABILITY The present invention can provide a method for producing a water-based pigment dispersion suitable for a water-based ink having excellent storage stability and ejection stability, and a method for producing a water-based ink for inkjet recording using the same.

[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") comprises the following steps (I) and (II).
Step (I): A step of dispersing a pigment, wax, and a polymer dispersant to obtain an aqueous dispersion Step (II): Adding a cross-linking agent to the aqueous dispersion obtained in the step (I), A step of heat-treating below the melting point of the wax to obtain an aqueous pigment dispersion. In the present specification, the term "recording" is a concept including printing and printing for recording characters and images, and the term "recorded matter" is used. , printed matter on which characters and images are recorded, and printed matter.
By "aqueous" is meant that water is the largest proportion of the medium contained in the dispersion or ink.

According to the present invention, it is possible to obtain a water-based pigment dispersion suitable for a water-based ink for inkjet recording, which is excellent in storage stability and ejection stability. Although the reason is not clear, it is considered as follows.
Conventionally, in the method for producing a pigment dispersion, a cross-linking agent is added to the dispersion and heat treatment is performed at a temperature above the melting point of the wax. There is a problem that the free wax adsorbs to the pigment surface due to hydrophobic interaction and desorbs part of the polymeric dispersant distributed on the pigment surface before the heat treatment. As a result, it is considered that the storage stability of the pigment was lowered.
In contrast, in the production method of the present invention, in step (II), a cross-linking agent is added to the aqueous dispersion of the pigment, wax and polymer dispersant, and heat treatment is performed below the melting point of the wax to prevent the wax from melting. While preventing further, 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, and the dispersion stability is high despite the presence of wax. It is thought that it changes to a water-based pigment dispersion.
In addition, by cross-linking the polymer dispersant with a cross-linking agent, the polymer dispersant adheres more firmly to the surface of the pigment and wax, thereby further improving the dispersion stability of the aqueous pigment dispersion. Therefore, it is considered that the pigment dispersion obtained by the production method of the present invention can provide a water-based ink excellent in storage stability and ejection stability.

[Step (I)]
Step (I) is a step of dispersing a pigment, wax and polymer dispersant to obtain an aqueous dispersion.

In the present invention, prior to step (I), first, the pigment, wax, and polymer dispersant are mixed in an aqueous medium to obtain a mixture of pigment, wax, and polymer dispersant, that is, a mixing step. may

A mixing process can be performed by a conventional method. The mixing step may be performed by 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.

In step (I) of the present invention, the polymer dispersant may be neutralized. When neutralizing the polymeric dispersant, the neutralizing agent may be added in step (I) or during 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 dispersing treatment in step (I).

Although step (I) of the present invention may be carried out in a medium containing only water as a component, it is preferably carried out in an aqueous medium. The water-based medium means that the medium in which the pigment, wax, and polymer dispersant are dispersed in step (I) has a maximum proportion of water in terms of mass ratio. It may contain a solvent. As the organic solvent, aliphatic alcohols having 1 to 3 carbon atoms, ketones, ethers, esters, etc., improve the wettability to the pigment, the solubility of the polymer dispersant, and the adsorption of the polymer dispersant to the pigment. From the viewpoint of improvement, ketones having 4 to 8 carbon atoms are more preferable, methyl ethyl ketone and methyl isobutyl ketone are more preferable, and methyl ethyl ketone is still more preferable. Since these organic solvents do not remain in the water-based pigment dispersion of the present invention to be obtained, they preferably have a boiling point of 100° C. or less from the viewpoint that they can be easily removed after the intended purpose is achieved.

Dispersion treatment in step (I) means an operation to reduce solids such as pigments, waxes, and polymer dispersants to particle sizes suitable for inkjet water-based inks. The particle size suitable for inkjet water-based ink means a cumulant average particle size of 40 nm or more and 200 nm or less according to cumulant analysis described in Examples.
In the present invention, the dispersing treatment can be achieved by atomizing the mixture of the pigment, wax, and polymer dispersant to a desired particle size only by the main dispersion by shearing stress. It is preferred that the mixture is pre-dispersed and then main-dispersed.

In step (I), when pre-dispersing the mixture of pigment, wax, and polymer dispersant, a commonly used mixing stirring device such as a disper blade, or a conventional method such as an ultrasonic homogenizer can be used. However, an ultrasonic homogenizer is particularly preferable.
The pre-dispersion step may be performed by 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.

Examples of means for imparting shear stress for this dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as Microfluidizer (manufactured by Microfluidics), media-type dispersers such as paint shakers and bead mills. Commercially available media-type dispersers include Ultra Apex Mill (manufactured by Kotobuki Kogyo Co., Ltd.) and Pico Mill (manufactured by Asada Iron Works Co., Ltd.). A plurality of these devices can also be combined. Among these, a high-pressure homogenizer is preferably used from the viewpoint of reducing the particle size of the pigment.
When main dispersion is carried out using a high-pressure homogenizer, the pigment can be controlled to have a desired particle size by controlling the treatment pressure and the number of passes.
The treatment pressure is preferably 60 MPa or higher, more preferably 100 MPa or higher, still more preferably 130 MPa or higher, and preferably 250 MPa or lower, more preferably 230 MPa or lower, from the viewpoint of productivity and economy.
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.

Details of the pigments, waxes, and polymeric dispersants are provided below.

[Pigment]
The pigment used in the present invention may be either an inorganic pigment or an organic pigment.
Specific examples of inorganic pigments include metal oxides such as carbon black, titanium oxide, iron oxide, red iron oxide and chromium oxide, and pearl luster pigments. Carbon black is particularly preferred for black ink. Examples of carbon black include furnace black, thermal lamp black, acetylene black, channel black and the like.
Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; Examples include polycyclic pigments such as linone 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 be used.
A pigment can be used individually or in mixture of 2 or more types.

In the present invention, the pigment is obtained by cross-linking the pigment, wax, or polymer dispersant with a cross-linking agent from the viewpoint of the storage stability of the water-based pigment dispersion and the ejection stability of the water-based ink containing the water-based pigment dispersion. It is preferably contained as pigment-containing polymer particles (hereinafter also simply referred to as “pigment-containing polymer particles”).

〔wax〕
Wax is generally defined as an organic substance that is solid at room temperature and liquid when heated, but in the present invention, any wax such as polyolefin wax, ketone wax, amide wax, and paraffin wax can be used. Further, from the viewpoint of storage stability and ejection stability of the ink, the wax used in the present invention should be a wax having poor dispersibility in an aqueous medium, that is, a wax that does not disperse in an aqueous medium without a dispersant. preferable. Therefore, it is preferable that the wax of 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 is a polyolefin wax into which a monomer having a carboxy group or a carboxylate group is introduced by copolymerization, or a wax capable of generating a carboxy group by heating in water. It means a polyolefin wax into which an ester bond site or a carboxylic acid anhydride site has been introduced. However, waxes having a small amount of carboxyl groups in their structure, such as polyolefin waxes, ketone waxes, amide waxes, and oxidized waxes obtained by oxidizing paraffin waxes, can be used substantially without a dispersant. Since it does not disperse in an aqueous medium, it is not regarded as a wax modified by a compound having a carboxyl group in the present invention.

(polyolefin wax)
A polyolefin wax is a wax containing an olefin-based monomer as a main component, and specific examples thereof include polyethylene, polypropylene, ethylene-propylene copolymers, propylene-α-olefin copolymers, and the like. Either a block copolymer or a random copolymer can be used as the copolymer.
Examples of the olefin-based monomer, which is the main component of the polyolefin wax, include linear olefins, cyclic olefins, and the like, but those containing linear olefins having 2 to 6 carbon atoms as the main component are preferred, and polyethylene containing ethylene as the main component is preferred. Waxes are more preferred. Here, the phrase “mainly composed of ethylene” means that the content of ethylene is preferably 50% by mass or more, more preferably 65% by mass or more, and still more preferably 80% by mass, based on the total components constituting the wax. It means that it is more than
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, from the viewpoint of improving the storage stability and ejection stability of the water-based ink. be.

The oxidized polyolefin wax can be obtained by thermally decomposing a high-molecular-weight polyolefin polymer to a desired molecular weight while introducing an oxygen atom or the like into the molecule, and is included in the polyolefin wax. be done.
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., from the viewpoint of improving the storage stability and ejection stability of the water-based ink. It is below.

(Paraffin wax)
Paraffin wax is a solid wax mainly composed of saturated aliphatic hydrocarbons, and preferably has a melting point of 47°C to 69°C.

(modified wax)
The wax may be a modified wax. Modified waxes include, for example, polyolefins obtained by acid-modifying the above polyolefins with unsaturated carboxylic compounds.
The unsaturated carboxylic acid-based compound used for acid-modification of polyolefin includes at least one selected from unsaturated carboxylic acids, unsaturated carboxylic acid derivatives, and unsaturated carboxylic acid anhydrides. An unsaturated carboxylic acid means an unsaturated compound containing a carboxy group. Unsaturated carboxylic acid derivatives mean mono- or diesters, amides, imides and the like of unsaturated compounds containing a carboxy group. An unsaturated carboxylic acid anhydride means an acid anhydride of an unsaturated compound containing a carboxy group.
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, and fumaric acid. Dimethyl, 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, N,N-dimethylaminoethyl (meth)acrylate ) acrylates and the like. Among these, the anhydride of maleic acid is preferred.
Examples of the acid-modifying method include a method of copolymerizing an unsaturated carboxylic acid compound during synthesis of polyolefin, and a method of graft-modifying polyolefin.
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., from the viewpoint of improving the storage stability and ejection stability of the water-based ink. ℃ or less.

The weight-average molecular weight of the wax is preferably 300 or more, more preferably 500 or more, and still more preferably 1,000 or more, 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. 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).

The acid value of the wax is preferably 150 mgKOH/g or less, more preferably 100 mgKOH/g or less, still more preferably 20 mgKOH, 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. /g or less, more preferably 1 mgKOH/g or less, and even more preferably 0 mgKOH/g.
The acid value of wax can be measured by the method described in Examples.

Commercially available wax products include, for example, Hi-Wax (registered trademark) series manufactured by Mitsui Chemicals, Inc., Diacarna (registered trademark) series manufactured by Mitsubishi Chemical Corporation, and Paraffin Wax 155 and 150 manufactured by Nippon Seiro Co., Ltd. , 145, 140, 135, 130, 125, 120, and 115, Hitec E series and Hitec S series manufactured by Toho Chemical Co., Ltd., and the like.

[Polymer Dispersant]
From the viewpoint of dispersion stability of the pigment, the polymer dispersant used in the present invention preferably has an acid group, and at least a portion of the acid group is preferably neutralized with a neutralizing agent. It is believed that this increases the charge repulsive force that develops after neutralization, suppresses aggregation of the pigment particles in the aqueous pigment dispersion, suppresses thickening, and improves storage stability.
Examples of the acid group include groups exhibiting acidity by dissociating to release hydrogen ions, such as a carboxy group (--COOM), a sulfonic acid group (--SO ₃ M), and a phosphoric acid group (--OPO ₃ M ₂ ); or their dissociated ionic forms. Among these, a carboxy group (--COOM) is preferable from the viewpoint of improving the storage stability of water-based pigment dispersions and water-based inks, and the ejection stability of water-based inks.

The acid value of the polymer dispersant is preferably 50 mgKOH/g or more, more preferably 70 mgKOH/g or more, still more preferably 90 mgKOH/g or more, and even more preferably 150 mgKOH/g, from the viewpoint of improving the ejection stability of the water-based ink. 220 mgKOH/g or more, 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 to ensure the dispersion stability of the pigment and, as a result, the ejection stability of the water-based ink. It is also preferable from the viewpoint of the balance between the affinity between the polymer dispersant and the aqueous medium and the interaction between the polymer dispersant and the pigment and between the polymer dispersant and the wax.
The acid value of the polymer dispersant can be calculated from the mass ratio of the constituent monomers. It can also be obtained by a method of titration.

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.

The (a-1) carboxylic acid monomer includes one or more selected from (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, preferably (meth)acrylic acid. is. "(Meth)acrylic acid" means at least one selected from acrylic acid and methacrylic acid.

((a-2) hydrophobic monomer)
(a-2) The “hydrophobicity” of the hydrophobic monomer means that the amount of the monomer dissolved in 100 g of deionized water at 25° C. until saturation is less than 10 g.
(a-2) Specific examples of the hydrophobic monomer include those described in paragraphs [0020] to [0022] of JP-A-2018-83938. Among these, an alkyl (meth)acrylate having an alkyl group having 1 to 18 carbon atoms, particularly 1 to 10 carbon atoms, an aromatic group-containing monomer having an aromatic group having 6 to 22 carbon atoms, A macromonomer or the like having a polymerizable functional group is preferred, and more preferably one or more selected from styrene, α-methylstyrene and benzyl (meth)acrylate.

The macromonomer having a polymerizable functional group at one end has 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 the polymerizable functional group is an acryloyloxy group or a methacryloyloxy group. compound.
As the macromonomer, an aromatic group-containing monomer-based macromonomer is preferable, and examples of aromatic group-containing monomers constituting the macromonomer include the aromatic group-containing monomers described above.
Specific examples of commercially available styrenic macromonomers include AS-6(S), AN-6(S), HS-6(S) manufactured by Toagosei Co., Ltd., and the like.

((a-3) nonionic monomer)
The (a-3) nonionic monomer is a monomer having a high affinity with water and water-soluble organic solvents, such as a monomer containing a hydroxyl group or a polyalkylene glycol chain.
Specific examples of the component (a-3) include those described in paragraph [0018] of JP-A-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 is preferable.
For the above components (a-1) to (a-3), the monomer components contained in each component can be used singly or in combination of two or more.

The content of structural units derived from components (a-1) to (a-3) in the polymer dispersant is determined 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. ,It is as follows.
The content of component (a-1) is preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 12% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass. % or less, more preferably 35 mass % or less.
The content of component (a-2) is preferably 55% by mass or more, more preferably 60% by mass or more, still more preferably 65% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass. % or less, more preferably 88 mass % or less.

When component (a-3) is contained, its content is preferably 2% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, and preferably 45% by mass or less, It is more preferably 40% by mass or less, still more preferably 35% by mass or less.

The mass ratio of [(a-1) component/(a-2) component] is preferably 0.1 or more, more preferably 0.15 or more, still more preferably 0.25 or more, and preferably 3 Below, more preferably 2 or less, still more preferably 1 or less, and even more preferably 0.5 or less.
Further, 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, more preferably 0.1 or more, and preferably 1 or less, more preferably 0.8 or less, still more preferably 0.6 or less, and even more preferably 0.5 or less .

(Production of polymer dispersant)
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. A solution polymerization method is preferable as the polymerization method.
Solvents used in the solution polymerization method are not limited, 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.
A polymerization initiator and a polymerization chain transfer agent can be used in the polymerization.
Examples of polymerization initiators include azo compounds such as 2,2'-azobisisobutyronitrile and 2,2'-azobis(2,4-dimethylvaleronitrile), t-butyl peroxyoctoate, benzoyl peroxide, and the like. A known radical polymerization initiator such as an organic peroxide of can be used. The amount of the radical polymerization initiator is preferably 0.1 parts by weight or more, more preferably 0.5 parts by weight or more, and preferably 5 parts by weight or less, more preferably 2 parts by weight, per 100 parts by weight of the monomer mixture. It is below the department.
As the polymerization chain transfer agent, known chain transfer agents such as mercaptans such as octylmercaptan and 2-mercaptoethanol, and thiuram disulfides can be used.
Moreover, there is no restriction on the chain mode of the polymerization monomers, and any polymerization mode such as random, block, or graft may be used.

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

The number average molecular weight of the polymer dispersant is preferably 3,000 or more, more preferably 5,000 or more, and more preferably 5,000 or more, 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. It is preferably 10,000 or more, more preferably 30,000 or more, and from the viewpoint of the dispersion stability of the pigment, preferably 100,000 or less, more preferably 70,000 or less, still more preferably 60,000 It is below. When the number-average molecular weight of the polymer dispersant is within the above range, the adsorptive power to the pigment is sufficient and dispersion stability can be exhibited. The number average molecular weight can be measured by the method described in Examples.

〔Neutralizer〕
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 handling properties such as reducing skin irritation. From the viewpoint of suppressing corrosion, it is preferably 13 or less, more preferably 12 or less, and even more preferably 11 or less. Therefore, at least a portion of the carboxy groups of the polymeric dispersant are neutralized using a neutralizing agent.
Examples of the neutralizing agent include bases such as sodium hydroxide, potassium hydroxide, ammonia and various amines, preferably sodium hydroxide and ammonia.
Alternatively, the polymer dispersant may be neutralized in advance.
The neutralizing agent is preferably used as an aqueous neutralizing agent solution from the viewpoint of promoting neutralization sufficiently and uniformly. From the above viewpoint, the concentration of the neutralizing agent aqueous solution is preferably 3% by mass or more, more preferably 10% by mass or more, still more preferably 15% by mass or more, and preferably 50% by mass or less, more preferably It is 25% by mass or less.

The use equivalent (mol%) of the neutralizing agent in the polymer dispersant is preferably 10 mol% or more, more preferably 20 mol% or more, from the viewpoint of the dispersion stability of the polymer dispersant and the dispersion stability of the pigment. It is preferably 30 mol % or more, and preferably 90 mol % or less, more preferably 85 mol % or less, still more preferably 80 mol % or less.
Here, the equivalent amount (mol %) of the neutralizing agent used is the percentage obtained by dividing the molar equivalent number of the alkali metal compound by the molar equivalent number of the carboxy group of the polymer dispersant when an alkali metal compound is used as the neutralizing agent ( mol %), that is, the percentage (mol %) of "the number of molar equivalents of the alkali metal compound/the number of molar equivalents of the carboxy group of the polymer dispersant". In the present invention, the use equivalent of the neutralizing agent is calculated from the number of molar equivalents of the neutralizing agent used, so when the neutralizing agent is used in excess, the use equivalent of the neutralizing agent exceeds 100 mol %.

[Step (II)]
Step (II) is a step of adding a cross-linking agent to the aqueous dispersion obtained in step (I) and heat-treating it below the melting point of wax.

<Removal of organic solvent>
When 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 heat treatment. That is, step (II) is preferably carried out in a state that does not contain an organic solvent. The organic solvent is preferably substantially removed, but may remain as long as the object of the present invention is not impaired. When the organic solvent remains, the amount thereof is preferably 0.1% by mass or less, more preferably 0.01% by mass or less.

<Heat treatment>
In the present invention, from the viewpoint of uniformly dispersing the polymer dispersant on the surface of the pigment and the surface of the wax while preventing melting of the wax, the heat treatment temperature is equal to or lower than the melting point of the wax. The value obtained by subtracting the heat treatment temperature from the melting point of is preferably 5 ° C. or higher, more preferably 10 ° C. or higher, still more preferably 15 ° C. or higher, and still more preferably 19 ° C. or higher. is preferably 60° C. or lower, more preferably 50° C. or lower, and still more preferably 35° C. or lower, from the viewpoint of advancing to

In the present invention, the heat treatment temperature is preferably in the range of 45° C. or higher and 95° C. or lower. The temperature of the heat treatment is preferably 50° C. or higher from the viewpoint of promoting the cross-linking reaction and improving the ejection stability of the water-based ink. As a result, a water-based pigment dispersion is obtained in which the polymer dispersant is uniformly distributed on the surface of the pigment and the surface of the wax, and the pigment-containing polymer particles in which the polymer dispersant is crosslinked with a crosslinking agent are dispersed. Moreover, in the pigment-containing polymer particles, the wax may be crosslinked with a crosslinking agent.
From the viewpoint of improving the storage stability of water-based pigment dispersions and water-based inks, and the ejection stability of water-based inks, the pigment-containing polymer particles are obtained by partially neutralizing the acid groups of the polymer dispersant with a neutralizing agent. Further, it is preferable to have a crosslinked structure in which another part of the acid groups possessed by the polymer dispersant is crosslinked with a crosslinking agent.
The degree of progress of the cross-linking reaction can be confirmed by measuring the pH change of the aqueous pigment dispersion during the cross-linking reaction, and the cross-linking reaction can be considered to be completed when the pH no longer changes. It is desirable to carry out the heat treatment until the cross-linking reaction is completely completed.
The heat treatment time is preferably 0.5 hours or more, more preferably 1 hour or more, still more preferably 1.5 hours or more, and preferably 12 hours or less, from the viewpoint of completion of the cross-linking reaction and economy. It is more preferably 8 hours or less, still more preferably 6 hours or less.

In addition, from the viewpoint of enhancing the dispersion stability of pigments and waxes and suppressing the generation of aggregates in the heat treatment process, a state in which a surfactant is present in any of the mixing process, process (I), and process (II) You can do it with
As specific examples of surfactants, known anionic surfactants, nonionic surfactants, and amphoteric surfactants can be used, and among these, nonionic surfactants are preferred. These surfactants remain after addition even when the water-based pigment dispersion of the present invention is obtained. Cationic surfactants are not preferred because they reduce the dispersion stability of pigments and waxes.

Examples of nonionic surfactants include acetylene glycol-based surfactants and polyoxyethylene alkyl ether-based surfactants, with polyoxyethylene alkyl ether-based surfactants being preferred. Examples of commercially available polyoxyethylene alkyl ether-based surfactants include Emulgen 102KG, 103, 104P, 105, 106, 108, 120, 147, 150, and EMULGEN manufactured by Kao Corporation. 220, 350, 404, 420, 705, 707, 709, 1108, 4085, 2025G and the like, preferably Emulgen 420.

Further, in the heat treatment step, from the viewpoint of suppressing the generation of aggregates at the gas-liquid interface, any step of the mixing step, step (I), and step (II) is performed in a state in which an organic solvent having a high affinity with water is present. You can do it with Organic solvents having a high affinity for water include ethylene glycol, propylene glycol, diethylene glycol, and glycerin. Among them, 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 even when the water-based pigment dispersion of the present invention is obtained after being added.

[Crosslinking agent]
The cross-linking agent used in the present invention is selected from the viewpoint of efficiently reacting with the polymer dispersant in a medium mainly composed of water, and also improves the storage stability of the water-based pigment dispersion and the water-based ink, and the ejection stability of the water-based ink. From the viewpoint of improvement, the water solubility (mass ratio) of the cross-linking agent is preferably 50% or less, more preferably 40% or less, and still more preferably 35% or less. Here, the water solubility % (mass ratio) refers to the dissolution rate (%) when 10 parts by mass of the cross-linking agent is dissolved in 90 parts by mass of water at room temperature of 25°C.

The cross-linking 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, still more preferably a polyvalent compound having a hydrocarbon group having 3 to 4 carbon atoms. It is a polyglycidyl ether compound of alcohol.
The epoxy equivalent of the cross-linking agent is preferably 90 or more, more preferably 100 or more, still more preferably 110 or more, and preferably 300 or less, more preferably 200 or less, still more preferably 150 or less.
The number of epoxy groups in the cross-linking agent is 2 or more per molecule, and preferably 6 or less per molecule, from the viewpoint of efficiently reacting with acid groups to enhance the storage stability of the aqueous pigment dispersion. It is more preferably 4 or less, still more preferably 3 or less, from the viewpoint of sexuality.

Preferable examples of the cross-linking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

The degree of crosslinking is preferably 15 mol% or more, more preferably 20 mol% or more, and still more preferably 25 mol% or more, 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. and is preferably 70 mol % or less, more preferably 65 mol % or less, still more preferably 60 mol % or less.
Here, the degree of cross-linking is the apparent degree of cross-linking calculated from the equivalent weight of the acid value of the polymer dispersant and the cross-linking functional group of the cross-linking agent.
When the wax has an acid group, a cross-linked structure is also formed between the acid group of the wax and the cross-linking agent. From the viewpoint of improving the storage stability of and the ejection stability of a water-based ink, the ratio of the number of molar equivalents of the crosslinkable functional group of the crosslinking agent to the number of molar equivalents of the acid group of the polymer dispersant is used as an index of the degree of crosslinking.

The pigment dispersion of the present invention may contain 1% by mass or more and 10% by mass or less of glycerin, triethylene glycol, or the like as a moisturizing agent to prevent drying, and may contain additives such as antifungal agents. good too. The additive may be added when dispersing the pigment, or may be added after dispersing the pigment or after heat treatment.

The non-volatile component concentration (solid content concentration) of the pigment dispersion of the present invention is preferably 10% by mass or more from the viewpoint of improving the dispersion stability of the aqueous pigment dispersion and from the viewpoint of facilitating the production of the aqueous pigment dispersion. It is more preferably 15% by mass or more, preferably 40% by mass or less, and more preferably 35% by mass or less. The non-volatile component concentration (solid content concentration) is measured by the method described in Examples.

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, and still more preferably 13% by mass or more. It is 40% by mass or less, more preferably 30% by mass or less, and still more preferably 25% by mass or less.

The mass ratio of the pigment to the polymer dispersant (pigment/polymer dispersant) in the pigment dispersion of the present invention is preferably 0.3 or more, more preferably 1.5, from the viewpoint of improving the ejection stability of the water-based ink. It is 5 or more, more preferably 4 or more, and preferably 8 or less, more preferably 7 or less, and even more preferably 6.5 or less.

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, and still more preferably 2, from the viewpoint of improving the ejection stability of the water-based ink. It is 5 or more, and preferably 11 or less, more preferably 10 or less, and even more preferably 8 or less.

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, and even more preferably, from the viewpoint of improving the ejection stability of the aqueous ink. is 80 nm or more, and is preferably 200 nm or less, more preferably 150 nm or less, even more preferably 130 nm or less, even more preferably 120 nm or less, 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 Examples.
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. It is the same as the preferred embodiment of the diameter.

[Method for producing water-based ink for inkjet recording]
The method for producing a water-based ink for inkjet recording of the present invention (hereinafter also referred to as "the method for producing the ink of the present invention") comprises a step of mixing the pigment dispersion obtained by the production method of the present invention with a water-soluble organic solvent. have Moreover, in order to adjust the concentration, water may be added when mixing the pigment dispersion of the present invention and the water-soluble organic solvent. By containing 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. can be further improved.

In the method for producing the ink of the present invention, when the pigment dispersion of the present invention and the water-soluble organic solvent are mixed, if necessary, a moisturizing agent, a wetting agent, a penetrating agent, and a surface active agent which are usually used for water-based inks are added. Additives such as agents, viscosity modifiers, pH modifiers, antifoaming agents, preservatives, antifungal agents and antirust agents may be added.
Furthermore, filtration treatment with a filter or the like can be performed after mixing.

The content of each component of the ink of the present invention and the physical properties of the ink are as follows.

[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, one or more selected from polyhydric alcohols and polyhydric alcohol alkyl ethers are preferable, and 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 is preferably 240° C. or lower, more preferably 220° C. or lower, and still more preferably 200° C. or lower.

The ink obtained by the production method of the present invention may further contain an aqueous dispersion of polymer particles containing no pigment as a fixing aid from the viewpoint of improving adhesion and print density. When the water-based ink further contains polymer particles that do not contain pigment, the affinity between the polymer particles and the polymer dispersant provides a smooth coating film on the print medium, suppresses local aggregation of the pigment particles, Adhesion can be improved without lowering print density.
Polymers used as fixing aids include condensation polymers such as polyolefin resin, polyurethane resin and polyester resin; acrylic resin, styrene resin, styrene-acrylic resin, butadiene resin, styrene-butadiene resin, vinyl chloride resin. Vinyl polymers such as resins, vinyl acetate resins, and acrylic silicone resins can be used.
As for the aqueous dispersion of polymer particles containing no pigment, an appropriately synthesized one or a commercially available product may be used.

The amount of the aqueous dispersion of polymer particles containing no pigment in the ink obtained by the production method of the present invention is preferably 10 in terms of solid content from the viewpoint of improving the storage stability and ejection stability of the water-based ink. % by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less.

(Content of each component in 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.

(Pigment content)
The content of the pigment 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 still more preferably 3% by mass or more, from the viewpoint of print density, and From the viewpoint of lowering the viscosity of the ink when the solvent is volatilized and improving storage stability, the content is preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 12% by mass or less.

(Total content of pigment and polymer dispersant)
The total content of the pigment and the 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, from the viewpoint of improving the storage stability and ejection stability of the water-based ink. % by mass or more, more preferably 5% by mass or more, still more preferably 7% by mass or more, and preferably 20% by mass or less, more preferably 17% by mass or less, and even more preferably 15% by mass or less.

(Content of organic solvent)
The content of the 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, and still more preferably 20% by mass or more, from the same viewpoint as above. The content is preferably 45% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less.

(water content)
From the same viewpoint 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, it is 65% by mass or less.

(Physical properties of water-based ink)
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, and still more preferably 5 mPa·s or more, from the viewpoint of improving storage stability. And it is preferably 12 mPa·s or less, more preferably 9 mPa·s or less, and still more preferably 7 mPa·s or less.
The viscosity of the ink can be measured using an E-type viscometer.
The pH at 20° C. of the ink obtained by the production method of the present invention is preferably 7.0 or higher, more preferably 7.2 or higher, and still more preferably 7.5 or higher, from the viewpoint of storage stability. Also, from the viewpoint of member resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less, and even more preferably 9.5 or less.
The pH of water-based ink can be measured by a conventional method.

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. .
There are thermal type and piezo type ink jet recording apparatuses, and the ink of the present invention is more preferably used as an aqueous ink for piezo type ink jet recording.
Usable print media include highly water-absorbing plain paper, low water-absorbing coated paper, and non-water-absorbing resin film. Examples of coated paper include general-purpose glossy paper, multicolor foam gloss paper, and the like. The resin film is preferably at least one selected from polyester films, polyvinyl chloride films, polypropylene films, and polyethylene films. The resin film may be a corona-treated substrate.

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

(1) Measurement of number average molecular weight of polymer dispersant Gel permeation using a solution obtained by dissolving phosphoric acid and lithium bromide in N,N-dimethylformamide at concentrations of 60 mmol/L and 50 mmol/L, respectively, as an eluent. Chromatographic method [Tosoh Corporation GPC device (HLC-8320GPC), Tosoh Corporation columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolumn SuperAW-H), flow rate: 0.5 mL / min], as a standard substance Monodisperse polystyrene kit with known molecular weight [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].
A sample to be measured was obtained 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 passing it through a syringe filter (DISMIC-13HP, PTFE, 0.2 μm, Advantech Co., Ltd.) was used.

(2) Measurement of non-volatile component concentration (solid content concentration) of water-based pigment dispersion 10.0 g of sodium sulfate was weighed to a constant weight in a desiccator and placed in a 30 mL polypropylene container (φ = 40 mm, height = 30 mm). About 1.0 g of the sample was added to the mixture, mixed, accurately weighed, maintained at 105° C. for 2 hours to remove volatile matter, left in a desiccator for 15 minutes, and weighed. The solid content of the sample after removal of volatile matter was divided by the mass of the added sample to obtain the concentration of non-volatile matter.

(3) Measurement of acid value of polymer dispersant and wax According to JIS K0070 and JIS K5902, toluene and acetone ( 2:1, mass ratio) or trichlorobenzene and ethanol (2:1, mass ratio)), and titrated with 0.1N potassium hydroxide/ethanol solution by potentiometric titration. , the inflection point on the titration curve was taken as the end point. The acid value (mgKOH/g) of the polymer dispersant and wax was calculated from the amount of potassium hydroxide solution titrated to the end point.

(4) Measurement of melting point of wax The melting point of wax was measured using a measuring device conforming to JIS K 0064. Specifically, using a differential scanning calorimeter (Q20) manufactured by TA Instruments, the temperature of the sample was raised to 200°C, and then cooled to 0°C at a cooling rate of 10°C/min. Next, the temperature of the sample was increased at a temperature increase rate of 10°C/min, and the calorie was measured up to 200°C. Among the observed heat of fusion peaks, the temperature of the peak with the maximum peak area was defined as the maximum melting peak temperature, and the peak temperature was defined as the melting point.

(5) Measurement of average particle size of pigment-containing polymer particles diameter. The measurement conditions were a temperature of 25° C., an angle between the incident light and the detector of 90°, and 100 integration times, and the refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The measurement concentration was usually about 5×10 ^-3 %.

<Preparation of polymer dispersant>
Production Example 1 (Preparation of polymer dispersant a)
A monomer mixed solution was prepared by mixing 29 parts of acrylic acid, 60 parts of styrene and 1 part of ethyl acrylate. 10 parts of methyl ethyl ketone (MEK), 0.2 parts of 2-mercaptoethanol (polymerization chain transfer agent), and 10% of the above monomer mixture were added and mixed in a reaction vessel, and the mixture was sufficiently replaced with nitrogen gas.
In a dropping funnel, the remaining 90% of the monomer mixture, 0.2 parts of the polymerization chain transfer agent, 30 parts of MEK and an azo radical polymerization initiator (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65, 2 ,2′-azobis(2,4-dimethylvaleronitrile)) 1.1 part of the mixed solution was added, and the mixture in the reaction vessel was heated to 65° C. while stirring under a nitrogen atmosphere. The mixture was added dropwise over 3 hours. After 2 hours at 65°C from the end of the dropwise addition, a polymerization initiator solution prepared by dissolving 0.1 part of the polymerization initiator in 2 parts of MEK was added, further aged at 65°C for 2 hours and at 70°C for 2 hours, and then dried under reduced pressure. to obtain a polymer dispersant a (number average molecular weight: 32,000, acid value: 226 mgKOH/g).

Production Example 2 (Preparation of polymer dispersant b)
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) were mixed. A monomer mixture was prepared. 10 parts of MEK, 0.1 part of 2-mercaptoethanol (polymerization chain transfer agent), and 10% of the above-mentioned monomer mixture were added and mixed in a reaction vessel, and nitrogen gas was sufficiently replaced.
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 radical polymerization initiator (trade name: V-65) was added to a dropping funnel, Under a nitrogen atmosphere, the mixed liquid in the reaction vessel was heated to 77° C. while stirring, and the mixed liquid in the dropping funnel was added dropwise over 3 hours. After 0.5 hours had passed at 77°C from the end of 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 stirred at 77°C for 1 hour for aging. The polymerization initiator solution preparation, addition and aging were carried out five more times. Then, the reaction solution in the reaction vessel was maintained at 80° C. for 1 hour, and MEK was added to form a dispersion of polymer dispersant b (solid concentration: 40%, number average molecular weight: 53,000, acid value: 94 mgKOH/g). got

<Production of water-based pigment dispersion>
Example 1 (Production of water-based pigment dispersion D1)
(Mixing process)
In a reaction vessel equipped with anchor wings, 50 parts of the polymer dispersant a obtained in Production Example 1 and 5N aqueous sodium hydroxide solution ( NaOH solid content: 16.9%) 19.4 parts and 240 parts of ion-exchanged water were added, and 100 parts by mass of polyethylene wax (manufactured by Mitsui Chemicals, Inc., trade name: Hi-Wax 110P, melting point: 109 ° C.) was added. In addition, heat and melt at 85 to 95° C., add 300 parts of carbon black pigment (CI Pigment Black 7, manufactured by Cabot Chemical Co., trade name: Monarch 800), pigment, wax, polymer dispersion A mixture of agents was obtained.
(Step (I))
While maintaining the obtained mixture at 90 to 95 ° C., pre-dispersion treatment is performed for 90 minutes using an ultrasonic homogenizer, and then cooled to room temperature, 78.6 parts of MEK and 480 parts of ion-exchanged water are added, Furthermore, a microfluidizer (manufactured by Microfluidics, trade name) was subjected to 10 passes of dispersion treatment at a pressure of 200 MPa to form an aqueous dispersion containing a polymer dispersant a, a wax, and a pigment (solid content concentration: 27.0%). 6%, pigment content: 18.4%, wax content: 3.1%, polymeric dispersant content: 6.1%).
(Step (II))
After adding 250 parts of ion-exchanged water to the aqueous dispersion obtained in step (I) and stirring, MEK was completely removed at 60 ° C. under reduced pressure, and a part of the water was removed to reduce the solid content concentration to 20% (pigment content: 13.2%) and a dispersion in which the medium was replaced with water was obtained. After that, 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-substituted dispersion, and the mixture was tightly stoppered, and stirred with a stirrer to obtain a viscosity of 80%. ° C. for 3 hours to replace 55 mol% of the carboxy groups of the polymer dispersant a with glycidyl ester bonds, thereby containing pigments and waxes dispersed with the polymer dispersant a having an acid value of 76 mgKOH/g. Aqueous pigment dispersion D1 in which pigment-containing polymer particles are dispersed (solid concentration: 20.3%, pigment content: 13.2%, wax content: 2.2%, polymer dispersant content: 4.0%). 4%, cross-linking agent content: 0.5%, cumulant average particle diameter of pigment-containing polymer particles: 94 nm, pH: 8.0).

Example 2 (Production of water-based pigment dispersion D2)
In Example 1, 50 parts of the polymer dispersant a was replaced with 125 parts of the dispersion of the polymer dispersant b produced in Production Example 2 (solid content concentration: 40%), and 19.4 parts of a 5N sodium hydroxide aqueous solution was added to 15.5 parts. A polymer dispersant having an acid value of 62 mgKOH/g was prepared in the same manner as in Example 1 except that 78.6 parts of MEK was changed to 3.6 parts and 15.5 parts of trimethylolpropane polyglycidyl ether was changed to 4.1 parts. Aqueous pigment dispersion D2 in which pigment-containing polymer particles containing the pigment and wax dispersed in b were dispersed (solid concentration: 20.1%, pigment content: 13.2%, wax content: 2.2 %, polymer dispersant content: 4.4%, cross-linking agent content: 0.5%, cumulant average particle diameter of pigment-containing polymer particles: 102 nm, pH: 9.2).

Examples 3-6, Comparative Examples 1-2 (Production of water-based pigment dispersions D3-D6 and D21-D22)
In Example 1, in the same manner as in Example 1 except that each component and formulation were as shown in Table 1 or 2, water-based pigment dispersions D3 to D6 of Examples 3 to 6, and of Comparative Examples 1 and 2 Water-based pigment dispersions D21 to D22 were obtained. Results are shown in Tables 1 and 2.

Details of the waxes in Tables 1 and 2 are as follows.
HW110P: High Wax 110P (Polyethylene wax that is not a wax modified with a compound having a carboxyl group with a melting point of 109°C, acid value: 0 mgKOH/g, manufactured by Mitsui Chemicals, Inc., trade name)
DC30M: Diacarna 30M (maleic anhydride-modified polyolefin wax with a melting point of 76°C, acid value: 103 mgKOH/g, manufactured by Mitsubishi Chemical Corporation, trade name)
・ PW155: Paraffin wax 155 (paraffin wax that is not wax modified by 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)

Example 7 (Production of water-based pigment dispersion D7)
(Mixing process)
In a reaction vessel equipped with anchor wings, 50 parts of the polymer dispersant a obtained in Production Example 1 and 5N aqueous sodium hydroxide solution ( NaOH solid content: 16.9%) 19.4 parts and 240 parts of ion-exchanged water are added. Emulsion in which oxidized polyethylene wax, which is not wax modified by a compound having a carboxyl group, is emulsified with a polyoxyethylene alkyl ether surfactant, acid value: 1 mg KOH/g, wax content: 33.3%) 300 parts by mass ( 100 parts as wax), 300 parts of carbon black pigment (CI Pigment Black 7, manufactured by Cabot Chemical Co., trade name: Monarch 800) are added, 78.6 parts of MEK and 480 parts of ion-exchanged water are added, and the pigment, A mixture of wax and polymer dispersant was obtained.
(Step (I))
The resulting mixture was pre-dispersed by stirring using a disper (manufactured by Asada Iron Works Co., Ltd., trade name: Ultra Disper) for 1 hour at 20° C. under the condition of rotating the disper blade at 7,000 rpm. A mixture of a pre-dispersed pigment, an oxidized polyethylene wax emulsion, and a polymer dispersant is subjected to dispersion treatment for 10 passes at a pressure of 200 MPa using a microfluidizer (manufactured by Microfluidics, trade name) for main dispersion. An aqueous dispersion containing a pigment (solid concentration: 27.6%, pigment content: 18.4%, wax content: 3.1%, polymer dispersant content: 6.1%) was obtained.
(Step (II))
The same operation as in step (II) of Example 1 was performed, and water-based pigment dispersion D7 of Example 7 (solid content concentration: 20.1%, pigment content: 13.2%, wax content: 2.2 %, polymer dispersant content: 4.4%, cross-linking agent content: 0.3%, cumulant average particle size of pigment-containing polymer particles: 90 nm, pH: 8.0).

Example 8 (Production of water-based pigment dispersion D8)
A water-based pigment dispersion D8 of Example 8 was obtained in the same manner as in Example 7 with the formulation shown in Table 1.
In Table 1, HTS3121 is manufactured by Toho Chemical Co., Ltd., trade name: Hitec S-3121 (carboxylic acid-modified polyethylene wax emulsion with a melting point of 77° C., acid value: 140 mgKOH/g, wax content: 25.0%). .

Comparative Example 3 (Production of water-based pigment dispersion)
When heat treatment was performed 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.

Preparation Example 1 (Preparation of Wax Dispersion W1)
In a reaction vessel equipped with anchor wings, 10 parts of the polymer dispersant a obtained in Production Example 1 and a 5N aqueous sodium hydroxide solution (NaOH solid content: 16.9 %) 3.9 parts and 240 parts of ion-exchanged water are added, and 100 parts by mass of polyethylene wax (manufactured by Mitsui Chemicals, Inc., trade name: Hi-Wax 110P, melting point: 109°C) is added, and the mixture is heated to 85 to 95°C. was dissolved by heating. The resulting mixture was cooled to room temperature and subjected to 3-pass dispersion treatment with a microfluidizer at a pressure of 200 MPa to obtain a dispersion. To the obtained dispersion, 100 parts of ion-exchanged water is added, 3.1 parts of trimethylolpropane polyglycidyl ether (trade name: Denacol EX-321) is added, and the mixture is sealed and stirred at 80°C for 3 hours while stirring with a stirrer. By heating and heat-treating to replace 55 mol% of the carboxy groups of the polymer dispersant a with glycidyl ester bonds, wax dispersion W1 (solid concentration: 22.6%, wax content: 20%, polymer dispersion agent content: 2%, cross-linking agent content: 0.6%, cumulant average particle size: 88 nm, pH: 8.0, acid value of polymer: 76 mgKOH/g).

Preparation Example 2 (Preparation of Pigment Dispersion P1)
In a reaction vessel, 40 parts of the polymer dispersant a obtained in Production Example 1 and 78.6 parts of MEK are mixed, and further, as a neutralizing agent that neutralizes 40 mol% of the carboxy groups of the polymer dispersant a 15.5 parts of 5N aqueous sodium hydroxide solution (NaOH solid content: 16.9%) and 500 parts of ion-exchanged water are added, and carbon black pigment (C.I. Pigment Black 7, manufactured by Cabot Chemical Co., product Name: Monarch 800) (300 parts) was added, and pre-dispersed by stirring using a disper (trade name: Ultra Disper, manufactured by Asada Iron Works Co., Ltd.) at 20°C for 1 hour under the condition of rotating the disper blade at 7,000 rpm. The obtained mixture of the pigment and the polymer dispersant was subjected to 10 passes of dispersion treatment at a pressure of 200 MPa with a microfluidizer (manufactured by Microfluidics, trade name) to form an aqueous dispersion (solid concentration: 22.6%, pigment content: 20%, polymer dispersant content: 2.6%).
After adding 250 parts of ion-exchanged water to the resulting aqueous dispersion and stirring, MEK was completely removed at 60° C. under reduced pressure, and a part of the water was removed to reduce the solid content concentration to 20% (pigment-containing amount: 17.6%), 12.4 parts of trimethylolpropane polyglycidyl ether (trade name: Denacol EX-321) was added, and the mixture was tightly stoppered and heated at 80°C for 3 hours while stirring with a stirrer for heat treatment. Then, by substituting 55 mol% of the carboxy groups of the polymer dispersant a with glycidyl ester bonds, a dispersion P1 (solid content concentration: 20.7%, pigment content: 17.6%, polymer dispersant content: 2.4%, cross-linking agent content: 0.7%, cumulant average particle size: 94 nm, pH 8.0). .

Comparative Example 4 (Production of water-based pigment dispersion D24)
To 50 parts of the wax dispersion W1 obtained in Preparation Example 1, 170.5 parts of the pigment dispersion P1 obtained in Preparation Example 2 was added and stirred, and an aqueous pigment dispersion D24 of Comparative Example 4 (solid content 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).

<Preparation of water-based ink A>
For each of the water-based pigment dispersions obtained in Examples and Comparative Examples, the pigment content of the entire water-based ink was 6%, propylene glycol 16%, dipropylene glycol monomethyl ether 8%, diethylene glycol monobutyl ether 8%, nonionic surfactant agent (2,4,7,9-tetramethyl-5-decyne-4,7-diol: manufactured by Nissin Chemical Industry Co., Ltd., acetylene glycol-based nonionic surfactant) 1%, nonionic surfactant (Polyoxyethylene alkyl ether: manufactured by Kao Corporation, trade name: Emulgen 120) Each component was mixed to 1%, and triethanolamine was added as a pH adjuster so that the pH of the water-based ink was 9.0. In addition, ion-exchanged water was added and weighed so that the total amount became 100%. The mixture was stirred with a magnetic stirrer to mix well, and filtered with 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 a water-based ink A.

<Preparation of water-based ink B>
For each of the water-based pigment dispersions obtained in Examples and Comparative Examples, the pigment content of the entire water-based ink was 10%, dipropylene glycol 16%, glycerin 4%, 1,2-hexanediol 2%, nonionic surfactant Agent (2,4,7,9-tetramethyl-5-decyne-4,7-diol) 0.5%, nonionic surfactant (polyoxyethylene alkyl ether) 1% After mixing, pH adjustment and the like were performed in the same manner as in the preparation of water-based ink A, and water-based ink B was obtained.

Using the obtained water-based inks A and B, the following storage stability evaluation and ejection stability evaluation were performed. Results are shown in Tables 1 and 2.

(Storage stability evaluation)
Each water-based ink was stored in a 70° C. environment for 30 days, and the rate of change in viscosity of each water-based ink before and after storage was evaluated.
For each water-based ink before and after storage, the ink viscosity at 32 ° C. was measured 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 the storage stability.
It is judged that the lower the rate of change in viscosity, the better the storage stability.

(Discharge stability evaluation)
In an environment with a temperature of 25 ± 1 ° C and a relative humidity of 30 ± 5%, water-based ink is applied to an inkjet ejection evaluation device Jetepxet (manufactured by ImageXpert) equipped with an inkjet head (manufactured by Fuji Film Co., Ltd., "Samba G3L", piezo type). filled. A head voltage of 30 V, a frequency of 50 kHz, a push-pull drive waveform, an appropriate amount of ejection liquid of 2.5 pL, and a negative pressure of -4.0 kPa were set, and an ink ejection command was transferred to the ejection evaluation device. I checked the number of nozzles. Under the same conditions, continuous ejection was performed for 30 minutes, and after 30 minutes, the number of nozzles performing normal ejection was reconfirmed.
The continuous ejection rate (%) was calculated by the following formula, and the ejection stability was evaluated.
Continuous ejection rate (%) = (number of normal ejection nozzles after 30 minutes/number of initial normal ejection nozzles) x 100
It is judged that the higher the continuous discharge rate (%), the better the discharge stability.

From Tables 1 and 2, the water-based inks containing the water-based pigment dispersions obtained in the examples of the present invention have higher storage stability and ejection stability than the water-based inks containing the water-based pigment dispersions obtained in the comparative examples. It can be seen that it is superior in quality.
In particular, in Comparative Example 4, the heat treatment was not performed in a state where the pigment, wax, and polymer dispersant coexist, so the polymer dispersant distributed on the pigment surface and the polymer dispersant distributed on the wax surface were uniformly distributed. It is considered that the storage stability is poor because the rearrangement is not performed in such a manner as to do.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the water-based pigment dispersion suitable for the water-based ink for inkjet recording which is excellent in storage stability and discharge stability, and the manufacturing method of the water-based ink for inkjet-recording using the same can be provided. .

Claims (8)

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 cross-linking agent is added to the aqueous dispersion obtained in Step (I) to obtain the wax. Process of heat-treating below the melting point 2. The method for producing an aqueous pigment dispersion according to claim 1, wherein the wax is not a wax modified with a compound having a carboxyl group. 3. The method for producing an aqueous pigment dispersion according to claim 1, wherein the cross-linking agent is a polyhydric alcohol glycidyl ether compound having a hydrocarbon group having 3 to 8 carbon atoms. The method for producing a water-based pigment dispersion according to any one of claims 1 to 3, wherein the heat treatment is performed at a temperature lower than the melting point of the wax by 5°C or more. The method for producing a water-based pigment dispersion according to any one of claims 1 to 4, wherein the heat treatment is performed at 45°C or higher and 95°C or lower. The method for producing an aqueous pigment dispersion according to any one of claims 1 to 5, wherein the mass ratio of the pigment to the polymer dispersant (pigment/polymer dispersant) is 0.3 or more and 8 or less. The method for producing an aqueous pigment dispersion according to any one of claims 1 to 6, wherein the mass ratio of the pigment to the wax (pigment/wax) is 1 or more and 11 or less. 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 according to any one of claims 1 to 7 and a water-soluble organic solvent.