JP2024018161A - Inkjet ink and method for producing inkjet ink - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inkjet ink excellent in discharge stability, storage stability, and fixability to an impermeable recording medium and capable of forming an image having a desired image density.

SOLUTION: The inkjet ink contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles have a core and a coating layer covering the core. The core contains acidic carbon black and a carbodiimide group-containing polymer. The coating layer contains a carboxy group-containing vinyl polymer. In the pigment particles, the ratio (M₂/M₁) of the total mass M₂ of the carbodiimide group-containing polymer and the carboxy group-containing vinyl polymer to the mass M₁ of the acidic carbon black is 1.40 or more and 3.10 or less.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to an inkjet ink and a method for producing an inkjet ink.

An aqueous inkjet ink containing a pigment and an aqueous medium is sometimes used in an inkjet recording device. Inkjet inks are required to have excellent ejection stability and storage stability, and to be able to form images with desired image density. Furthermore, inkjet recording apparatuses sometimes form images on non-permeable recording media such as OHP (overhead projector) sheets. Typical inkjet inks tend to have low fixability to non-penetrating recording media. Therefore, when forming an image on a non-permeable recording medium, the inkjet ink is required to have excellent fixing properties to the non-permeable recording medium.

In response to such demands, for example, an inkjet ink containing resin particles, a surfactant having a specific structure, and water has been proposed (Patent Document 1). The above-mentioned inkjet ink is said to have excellent fixing properties on non-penetrating recording media.

Japanese Patent Application Publication No. 2015-30224

However, the inkjet ink described in Patent Document 1 does not satisfy all of the ejection stability, storage stability, image density of the formed image, and fixability to non-permeable recording media.

An object of the present invention has been made in view of the above-mentioned problems, and provides an inkjet that has excellent ejection stability, storage stability, and fixing property to non-permeable recording media, and is capable of forming images having a desired image density. The objective is to provide ink for

The inkjet ink according to the present invention contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles have a core and a coating layer covering the core. The core includes acidic carbon black and a carbodiimide group-containing polymer. The coating layer includes a carboxyl group-containing vinyl polymer. In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black is 1.40 or more and 3.10 or less. It is. The inkjet ink according to the present invention does not contain the free resin dispersed in the aqueous medium or contains the free resin, and the inkjet ink contains the free resin dispersed in the aqueous medium, and The ratio of free resin is more than 0.00% by mass and not more than 3.00% by mass.

The method for producing an inkjet ink according to the present invention includes a core forming step in which a core is obtained by wet-pulverizing acidic carbon black and a carbodiimide group-containing polymer, and a kneaded product by melt-kneading the core and a carboxy group-containing vinyl polymer. and a pigment particle dispersion preparation step of obtaining a dispersion containing pigment particles by wet-pulverizing the kneaded material in an aqueous medium. In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black is 1.40 or more and 3.10 or less. It is.

The inkjet ink and the method for producing the inkjet ink according to the present invention have excellent ejection stability, storage stability, and fixing properties on non-permeable recording media, and are capable of forming an image having a desired image density. We can provide ink.

Embodiments of the present invention will be described below. In addition, in the following, the measured value of the volume median diameter (D ₅₀ ) is measured using a dynamic light scattering particle size distribution analyzer ("Zetasizer Nano ZS" manufactured by Malvern) unless otherwise specified. is the value given.

The acid value can be determined according to the method described in JIS (Japanese Industrial Standard) K0070:1992.

Unless otherwise specified, the measured value of the glass transition point (Tg) is measured using a differential scanning calorimeter (for example, "DSC-60" manufactured by Shimadzu Corporation) according to "JIS (Japanese Industrial Standard) K7121-2012". This is the value measured according to the following. In the endothermic curve measured with a differential scanning calorimeter (vertical axis: heat flow (DSC signal), horizontal axis: temperature, heating rate 5°C/min), the temperature at the inflection point caused by the glass transition (in detail, the temperature at the base The temperature at the intersection of the extrapolated line of the line and the extrapolated line of the falling line corresponds to Tg (glass transition point).

In this specification, acrylic and methacrylic may be collectively referred to as "(meth)acrylic".

<First embodiment: Inkjet ink>
Hereinafter, an inkjet ink (hereinafter sometimes simply referred to as ink) according to a first embodiment of the present invention will be described. The ink of the present invention contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles have a core and a coating layer covering the core. The core includes acidic carbon black and a carbodiimide group-containing polymer. The coating layer contains a carboxy group-containing vinyl polymer. In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black is 1.40 or more and 3.10 or less. The ink of the present invention does not contain a free resin dispersed in an aqueous medium or contains a free resin, and the ratio of the free resin to the total amount of the carbodiimide group-containing polymer, the carboxyl group-containing vinyl polymer, and the free resin is 0.00 mass. % to 3.00% by mass or less.

The use of the ink of the present invention is not particularly limited, but it can be used, for example, to form an image on a permeable recording medium or a non-permeable recording medium. The ink of the present invention is suitable for forming images on non-permeable recording media. Permeable recording media have excellent ink permeability. Examples of permeable recording media include printing paper and media made from fibers (eg, cloth). Examples of printing paper include plain paper, copy paper, recycled paper, thin paper, thick paper, and glossy paper.

Non-permeable recording media have poorer ink permeability than permeable recording media. In a non-permeable recording medium, the absorption amount of the aqueous medium is, for example, 1.0 g/m ² or less. Examples of non-permeable recording media include recording media made of resin, recording media made of metal, and recording media made of glass. Examples of resin-made recording media include resin sheets and resin films. The resin contained in the resin recording medium is preferably a thermoplastic resin. Specific examples of the resin include polyethylene, polyvinyl chloride, and polyethylene terephthalate (PET). An example of the resin recording medium is a PET sheet. When an image is formed on a resin recording medium using the ink of the present invention, the surface (printed surface) of the recording medium is preferably subjected to corona treatment.

By having the above-described structure, the ink of the present invention has excellent ejection stability, storage stability, and fixability to a non-permeable recording medium, and can form an image having a desired image density. The reason is inferred as follows. The ink of the present invention contains pigment particles containing acidic carbon black, a carbodiimide group-containing polymer, and a carboxyl group-containing vinyl polymer. Pigment particles are composed of a core and a covering layer. Since the coating layer contains a carboxyl group-containing vinyl polymer, which is a resin with excellent hydrophilicity, it has excellent affinity with an aqueous medium. Since the pigment particles are provided with a coating layer that has excellent affinity with the aqueous medium, they can be stably dispersed in the ink.

Two types of resins (a carbodiimide group-containing polymer and a carboxyl group-containing vinyl polymer) contained in the pigment particles function as binder resins when forming an image with the ink of the present invention, and bind acidic carbon black attached to the recording medium. Protect. The ink of the present invention has a ratio (M ₂ /M ₁ ) of the mass _{M 2} of the two types of resins to the mass M 1 of the acidic carbon black, and is non-penetrating because it contains a sufficient amount of binder resin. Excellent fixing properties on non-standard recording media. On the other hand, if the ink contains an excessive amount of binder resin, the amount of pigment in the ink may be relatively reduced, and the image density of the formed image may be reduced. On the other hand, in the ink of the present invention, the ratio (M ₂ /M ₁ ) is kept below a certain level and an excessive amount of binder resin is not included, so that an image having a desired image density can be formed.

On the other hand, known inks containing pigment particles containing a pigment and a binder resin tend to have insufficient storage stability and ejection stability. This is because the above-mentioned known inks tend to contain a large amount of free resin that is dispersed in the aqueous medium. The free resin is generated when the binder resin contained in the pigment particles is detached from the pigment particles and released into the aqueous medium, and causes a decrease in the storage stability and ejection stability of the ink. On the other hand, the ink of the present invention does not contain free resin, or even if it contains free resin, the content thereof is very small. Therefore, the ink of the present invention can suppress deterioration in storage stability and ejection stability caused by a large amount of free resin.

Further, when the ink is stored for a long time or exposed to dry conditions, the binder resin may be detached from the pigment particles, and the amount of free resin may increase. In particular, when the ink is exposed to dry conditions, the concentration of organic solvent increases as water evaporates from the aqueous medium of the ink. This changes the dispersibility of the binder resin in the aqueous medium, making it easier for the binder resin to detach from the pigment particles and become liberated in the aqueous medium. In contrast, in the ink of the present invention, in the pigment particles, a covalent bond is formed between the acid group of the acidic carbon black and the carbodiimide group of the carbodiimide group-containing polymer, and the covalent bond is formed between the acid group of the acidic carbon black and the carbodiimide group of the carbodiimide group-containing polymer. A covalent bond is formed between the carboxy group of the carboxyl group-containing vinyl polymer. Thereby, in the pigment particles, the acidic carbon black and the two types of resins are stably bonded to each other. Therefore, even if the ink of the present invention is exposed to long-term storage or dry conditions, the two types of resins are difficult to separate from the pigment particles. As a result, the ink of the present invention has excellent storage stability and ejection stability.

The ink of the present invention will be explained in more detail below. In addition, each component demonstrated below may be used individually by 1 type, and may be used in combination of 2 or more types.

[Pigment particles]
The pigment particles have a core and a coating layer covering the core. The core includes acidic carbon black and a carbodiimide group-containing polymer. The coating layer contains a carboxy group-containing vinyl polymer. The total content of acidic carbon black, carbodiimide group-containing polymer, and carboxyl group-containing vinyl polymer in the pigment particles is preferably 90% by mass or more, and more preferably 100% by mass.

From the viewpoint of optimizing the color density, hue, or stability of the ink of the present invention, the volume median diameter of the pigment particles is preferably 30 nm or more and 200 nm or less, and more preferably 70 nm or more and 130 nm or less.

In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black is 1.40 or more and 3.10 or less, and 1 It is preferably .60 or more and 2.20 or less. By setting the ratio (M ₂ /M ₁ ) to 1.40 or more, the ink of the present invention can optimize its fixability to non-permeable recording media. By setting the ratio (M ₂ /M ₁ ) to 3.10 or less, the ink of the present invention can form an image having a desired image density.

In the ink of the present invention, the content of pigment particles is preferably 3.0% by mass or more and 20.0% by mass or less, more preferably 7.0% by mass or more and 12.0% by mass or less. By setting the content of pigment particles to 3.0% by mass or more, the ink of the present invention can more easily form an image having a desired image density. By controlling the content of pigment particles to 20.0% by mass or less, the fluidity of the ink of the present invention can be optimized.

(core)
The core is composed of, for example, acidic carbon black coated with a carbodiimide group-containing polymer. In the core, a portion of the acidic carbon black and a portion of the carbodiimide group-containing polymer are covalently bonded. In the core, the content of the carbodiimide group-containing polymer based on 100 parts by mass of acidic carbon black is preferably 3 parts by mass or more and 30 parts by mass or less, and more preferably 5 parts by mass or more and 15 parts by mass or less. In the core, the total content of acidic carbon black and carbodiimide group-containing polymer is preferably 90% by mass or more, more preferably 100% by mass.

(acidic carbon black)
Acidic carbon black is a black pigment. Commercially available acidic carbon blacks include, for example, "MA100" manufactured by Mitsubishi Chemical Corporation, "MA14" manufactured by Mitsubishi Chemical Corporation, "MA77" manufactured by Mitsubishi Chemical Corporation, "Special Black 350" manufactured by Degussa Corporation, and "Special Black 350" manufactured by Degussa Corporation. "Special Black 100", Degussa "Special Black 250", Degussa "Special Black 5", Degussa "Special Black 550", Degussa "Special Black 6", and Cabot "REGAL" Trademark) 400R".

The acidity of acidic carbon black can be indicated by the pH of a dispersion liquid when acidic carbon black is dispersed in water to prepare a dispersion liquid having a solid content concentration of 3% by mass. The pH of the above dispersion liquid is preferably 2.0 or more and 5.0 or less, more preferably 3.0 or more and 4.0 or less.

(Carbodiimide group-containing polymer)
The carbodiimide group-containing polymer is a resin having a carbodiimide group (-N=C=N-). The carbodiimide group-containing polymer plays a role in binding the acidic carbon black and the carboxy group-containing vinyl polymer in the pigment particles. Furthermore, carbodiimide group-containing polymers are suitable for the ink of the present invention in that they are highly stable resins.

The number of carbodiimide groups that the carbodiimide group-containing polymer has in the molecule is, for example, 2 or more and 30 or less. Examples of the carbodiimide group-containing polymer include aromatic polycarbodiimide, alicyclic polycarbodiimide, and aliphatic polycarbodiimide. Examples of the aromatic polycarbodiimide include poly(4,4'-diphenylmethanecarbodiimide), poly(p-phenylenecarbodiimide), poly(m-phenylenecarbodiimide), poly(diisopropylphenylcarbodiimide), and poly(triisopropylphenylcarbodiimide). ). Examples of alicyclic polycarbodiimides include poly(dicyclohexylmethanecarbodiimide). Examples of aliphatic polycarbodiimides include poly(diisopropylcarbodiimide). Commercially available carbodiimide group-containing polymers include, for example, "Carbodilite (registered trademark) E-01", "Carbodilite (registered trademark) E-02", and "Carbodilite (registered trademark) V-02" manufactured by Nisshinbo Co., Ltd. Examples include "Carbodilite (registered trademark) V-04" and "Carbodilite (registered trademark) V-10."

(covering layer)
A part of the carboxy group-containing vinyl polymer contained in the coating layer and a part of the carbodiimide group-containing polymer contained in the core are covalently bonded. In the coating layer, the content of the carboxy group-containing vinyl polymer is preferably 90% by mass or more, more preferably 100% by mass.

In the pigment particles, the mass of the coating layer relative to 100 parts by mass of the core is preferably 100 parts by mass or more and 400 parts by mass or less, more preferably 150 parts by mass or more and 250 parts by mass or less. By setting the mass of the coating layer to 100 parts by mass or more, the dispersion stability of the pigment particles can be optimized. By controlling the mass of the coating layer to 400 parts by mass or less, the ink of the present invention can more easily form an image having a desired image density.

(Carboxy group-containing vinyl polymer)
A carboxyl group-containing vinyl polymer has, for example, a repeating unit derived from a carboxyl group-containing vinyl monomer. Examples of the carboxyl group-containing vinyl monomer include (meth)acrylic acid. As a specific carboxy group-containing vinyl polymer, (meth)acrylic resin or styrene-(meth)acrylic resin is preferable.

A (meth)acrylic resin as a carboxyl group-containing vinyl polymer has, for example, a repeating unit derived from a (meth)acrylic acid alkyl ester and a repeating unit derived from (meth)acrylic acid. It is preferable to further include a repeating unit derived from an aryl ester. Styrene-(meth)acrylic resin as a carboxyl group-containing vinyl polymer has a styrene unit, a repeating unit derived from an alkyl (meth)acrylate ester, and a repeating unit derived from (meth)acrylic acid, It is preferable to further include a repeating unit derived from aryl meth)acrylate or hydroxyalkyl ester (meth)acrylate. Examples of the (meth)acrylic acid alkyl ester include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and n-butyl (meth)acrylate. Examples of the (meth)acrylic acid aryl ester include phenyl (meth)acrylate and benzyl (meth)acrylate. Examples of the (meth)acrylic acid hydroxyalkyl ester include 2-hydroxyethyl (meth)acrylate.

In the carboxy group-containing vinyl polymer, the content ratio of styrene units to all repeating units is preferably 15.0% by mass or more and 50.0% by mass or less, more preferably 20.0% by mass or more and 40.0% by mass or less.

In the carboxy group-containing vinyl polymer, the content ratio of repeating units derived from (meth)acrylic acid to all repeating units is preferably 4.0% by mass or more and 25.0% by mass or less, and 7.0% by mass or more and 16.0% by mass or less. More preferably, it is 0% by mass or less. By controlling the content of repeating units derived from (meth)acrylic acid to 4.0% by mass or more and 25.0% by mass or less, appropriate hydrophilicity can be imparted to the coating layer.

In the carboxy group-containing vinyl polymer, the content ratio of repeating units derived from (meth)acrylic acid alkyl ester to all repeating units is preferably 5.0% by mass or more and 50.0% by mass or less, and 25.0% by mass or more. More preferably, it is 40.0% by mass or less.

In the carboxy group-containing vinyl polymer, the content ratio of repeating units derived from (meth)acrylic acid aryl ester to all repeating units is preferably 15.0% by mass or more and 70.0% by mass or less, and 25.0% by mass or more. More preferably, it is 40.0% by mass or less.

In the carboxy group-containing vinyl polymer, the content ratio of repeating units derived from (meth)acrylic acid hydroxyalkyl ester to all repeating units is preferably 1.0% by mass or more and 15.0% by mass or less, and 3.0% by mass. The content is more preferably 7.0% by mass or less.

As raw material monomers for the carboxyl group-containing vinyl polymer, the following combinations (I) to (V) are preferred.
Combination (I): Methyl methacrylate, styrene, n-butyl acrylate, acrylic acid and benzyl methacrylate Combination (II): Styrene, n-butyl acrylate, methacrylic acid, benzyl acrylate and benzyl methacrylate Combination (III) : Methyl methacrylate, styrene, n-butyl acrylate, acrylic acid and benzyl acrylate Combination (IV): Methyl methacrylate, acrylic acid, benzyl acrylate and benzyl methacrylate Combination (V): Methyl methacrylate, styrene, acrylic n-butyl acid, methacrylic acid, 2-hydroxyethyl methacrylate and benzyl acrylate

The acid value of the carboxyl group-containing vinyl polymer is preferably 60 mgKOH/g or more and 120 mgKOH/g or less, and preferably 80 mgKOH/g or more and 100 mgKOH/g or less. By setting the acid value of the carboxyl group-containing vinyl polymer to 60 mgKOH/g or more and 120 mgKOH/g or less, appropriate hydrophilicity can be imparted to the coating layer.

The glass transition point of the carboxyl group-containing vinyl polymer is preferably 40°C or higher and 70°C or lower, and preferably 50°C or higher and 60°C or lower. By setting the glass transition point of the carboxyl group-containing vinyl polymer to 40° C. or higher, the storage stability of the ink of the present invention can be further optimized. By setting the glass transition point of the carboxyl group-containing vinyl polymer to 70° C. or lower, the ink of the present invention can further optimize the fixability to non-permeable recording media.

[Free resin]
In the ink of the present invention, the free resin is a resin that exists in a dispersed state (in a state in which no pigment particles are formed) in an aqueous medium. The free resin is generated when a part of the carbodiimide group-containing polymer or carboxyl group-containing vinyl polymer contained in the pigment particles is detached from the pigment particles and released into the aqueous medium during the production and storage of the ink of the present invention. . Therefore, the type of free resin is the same as the above-mentioned carbodiimide group-containing polymer or carboxyl group-containing vinyl polymer.

Free resin has an undesirable effect on the ejection stability and storage stability of the ink. Therefore, it is desirable that the ink of the present invention does not contain free resin. However, the ink of the present invention usually inevitably contains a small amount of free resin. The ink of the present invention can exhibit excellent ejection stability and storage stability by minimizing the ratio of free resin.

When the ink of the present invention contains a free resin, in the ink of the present invention, the ratio of the free resin to the total amount of the carbodiimide group-containing polymer, the carboxyl group-containing vinyl polymer, and the free resin is more than 0.00% by mass and 3.00% by mass. The content is not more than 0.00% by mass and not more than 2.50% by mass, more preferably more than 0.00% by mass and not more than 2.20% by mass. By controlling the ratio of free resin to more than 0.00% by mass and 3.00% by mass or less, the ejection stability and storage stability of the ink of the present invention can be optimized.

[Aqueous medium]
The aqueous medium contained in the ink of the present invention is a medium containing water. The aqueous medium may function as a solvent or a dispersion medium. Specific examples of the aqueous medium include water and a water-soluble organic solvent.

(water)
In the ink of the present invention, the water content is preferably 30.0% by mass or more and 85.0% by mass or less, more preferably 60.0% by mass or more and 75.0% by mass or less.

Examples of water-soluble organic solvents include glycol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycol, glycerin, and dimethyl sulfoxide.

Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,2-pentanediol, 1,5-pentanediol, 1,2-octanediol, 1,8-octanediol, 3- Mention may be made of methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol and tetraethylene glycol.

Examples of glycol ether compounds include diethylene glycol diethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl Ethers, triethylene glycol monobutyl ether and propylene glycol monomethyl ether may be mentioned.

Examples of lactam compounds include 2-pyrrolidone and N-methyl-2-pyrrolidone.

Examples of nitrogen-containing compounds include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

Examples of the acetate compound include diethylene glycol monoethyl ether acetate.

The water-soluble organic solvent is preferably a glycol compound, a glycol ether compound, or glycerin, and more preferably ethylene glycol, propylene glycol, triethylene glycol monobutyl ether, or glycerin.

The content ratio of the water-soluble organic solvent in the ink of the present invention is preferably 10.0% by mass or more and 50.0% by mass or less, more preferably 20.0% by mass or more and 30.0% by mass or less.

[Surfactant]
Preferably, the ink of the present invention further contains a surfactant. Surfactants can optimize the wettability of the ink of the invention on the recording medium. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. As the surfactant, nonionic surfactants are preferred.

Examples of nonionic surfactants include polyoxyethylene dodecyl ether, polyoxyethylene hexadecyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and ethylene oxide of acetylene glycol. Examples include adducts. As the nonionic surfactant, an ethylene oxide adduct of acetylene glycol is preferred.

The content of the surfactant in the ink of the present invention is preferably 0.05% by mass or more and 3.0% by mass or less, more preferably 0.1% by mass or more and 1.0% by mass or less.

[Other ingredients]
The ink of the present invention may further contain known additives (for example, a dissolution stabilizer, a drying inhibitor, an antioxidant, a viscosity modifier, a pH adjuster, and a fungicide) as necessary.

<Second embodiment: Method for manufacturing inkjet ink>
Hereinafter, a method for manufacturing an inkjet ink (hereinafter sometimes simply referred to as an ink manufacturing method) according to a second embodiment of the present invention will be described. The method for producing ink of the present invention includes a core forming step in which a core is obtained by wet-pulverizing acidic carbon black and a carbodiimide group-containing polymer, and a kneading step in which a kneaded product is obtained by melt-kneading the core and a carboxy group-containing vinyl polymer. and a pigment particle dispersion preparation step of obtaining a dispersion containing pigment particles by wet-pulverizing the kneaded material in an aqueous medium. In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black is 1.40 or more and 3.10 or less.

The method for producing ink of the present invention includes, after the pigment particle dispersion preparation step, a centrifugation step of centrifuging the dispersion, and after the centrifugation step, a replacement step of isolating the precipitate from the dispersion and dispersing it in another aqueous medium. It is preferable to further include. The ink manufacturing method of the present invention can manufacture the ink according to the first embodiment. The details of each step will be explained below.

Note that the details of the acidic carbon black, the carbodiimide group-containing polymer, the carboxyl group-containing vinyl polymer, and the aqueous medium used in the second embodiment can be the same as those in the first embodiment, and therefore redundant explanation will be omitted.

[Core formation process]
In this step, for example, a solution containing acidic carbon black and a carbodiimide group-containing polymer is wet-pulverized to coat the acidic carbon black with the carbodiimide group-containing polymer to obtain a core. In this step, the amount of the carbodiimide group-containing polymer to be used with respect to 100 parts by weight of acidic carbon black is preferably 3 parts by weight or more and 30 parts by weight or less, more preferably 5 parts by weight or more and 15 parts by weight or less. The solution subjected to wet pulverization in this step contains, for example, water and a surfactant in addition to the acidic carbon black and the carbodiimide group-containing polymer.

Examples of devices used for wet pulverization include ball mills, high-speed rotary pulverizers, jet mills, bead mills, ultrasonic homogenizers, and high-pressure homogenizers.

[Kneading process]
In this step, the core and the carboxyl group-containing vinyl polymer are kneaded to obtain a kneaded product in which the core is coated with the carboxyl group-containing vinyl polymer. In addition, in this step, due to the heat during kneading, part of the acidic carbon black and part of the carbodiimide group-containing polymer are covalently bonded, and a part of the carboxy group-containing vinyl polymer and the carbodiimide group-containing polymer are bonded together. Some of them are covalently bonded.

In this step, the amount of the carboxyl group-containing vinyl polymer used per 100 parts by weight of the core is preferably 100 parts by weight or more and 400 parts by weight or less, more preferably 150 parts by weight or more and 250 parts by weight or less.

In this step, it is preferable to use a twin screw extruder as the kneading device. In this step, the kneading temperature is preferably 100°C or more and 150°C or less.

[Pigment particle dispersion preparation process]
In this step, the kneaded material is wet-pulverized in an aqueous medium to make the kneaded material fine and form pigment particles, and at the same time, the pigment particles are dispersed in the aqueous medium. This results in a dispersion containing pigment particles.

Examples of devices used for wet pulverization include ball mills, high-speed rotary pulverizers, jet mills, bead mills, ultrasonic homogenizers, and high-pressure homogenizers.

In this step, it is preferable that the kneaded product is coarsely pulverized in advance (for example, to a set particle size of 3 to 10 μm), and the obtained coarsely pulverized product is wet-pulverized in an aqueous medium. Moreover, in this step, it is preferable to add a base to the solution containing the kneaded material and the aqueous medium before wet pulverization to neutralize the carboxy group-containing vinyl polymer contained in the kneaded material. Examples of the base include sodium hydroxide. The amount of the base added can be 0.4 times or more and 1.2 times or less relative to the equivalent amount of the base that can completely neutralize the carboxyl group-containing vinyl polymer contained in the kneaded material.

In this step, the kneaded material is preferably wet-pulverized in an aqueous medium in the presence of a dispersant. As the dispersant, nonionic dispersants are preferred. The amount of the dispersant added is preferably 0.1 parts by mass or more and 2.0 parts by mass or less, based on a total of 100 parts by mass of the dispersant, kneaded material, and aqueous medium.

The content of the wet-pulverized material in the dispersion obtained by this step is, for example, 10% by mass or more and 30% by mass or less.

[Centrifugation process]
In this step, the dispersion is centrifuged after the pigment particle dispersion preparation step. This causes the pigment particles contained in the dispersion to precipitate. In this step, the free resin dispersed in the aqueous medium does not easily settle even if centrifuged, and most of it remains as a supernatant. Therefore, in this step, the free resin dispersed in the aqueous medium and the pigment particles can be separated.

The centrifugation conditions in this step are, for example, a rotation speed of 10,000 rpm or more and 200,000 rpm or less, and a centrifugation time of 12 hours or more and 48 hours or less.

[Replacement process]
In this step, the precipitate is isolated from the dispersion and dispersed in another aqueous medium. Through the centrifugation step and the displacement step, a dispersion liquid from which most of the free resin dispersed in the aqueous medium has been removed can be obtained. Thereby, the proportion of free resin contained in the manufactured ink can be reduced. Water is preferred as the aqueous medium used in this step.

Through the above steps, a dispersion containing pigment particles and an aqueous medium is obtained. The dispersion liquid may be used as an ink as it is, or may be used as an ink after performing a mixing process by adding an organic solvent, a surfactant, etc., if necessary. Furthermore, foreign matter and coarse particles may be removed from the above-mentioned dispersion using a filter (for example, a filter with a pore size of 5 μm or less).

Examples of the present invention will be described below. However, the present invention is not limited to the following examples.

[Preparation of core]
(Core (C-1))
120 parts by mass of acidic carbon black ("Carbon Black #970" manufactured by Mitsubishi Chemical Corporation) and a polymer dispersion containing a carbodiimide group-containing polymer ("Carbodilite (registered trademark) V-02" manufactured by Nisshinbo Chemical Corporation), solid content 10 parts by mass of nonionic surfactant ("Olfine (registered trademark) E1004" manufactured by Nissin Chemical Industry Co., Ltd.), and 469 parts by mass of ion-exchanged water were mixed. Obtained. The resulting mixed solution was subjected to coarse pulverization treatment at 10,000 rpm for 30 minutes using a high-speed emulsifying and dispersing machine ("Homomixer Mark II" manufactured by Primix Co., Ltd.).

The mixed liquid after the coarse pulverization treatment was put into a vessel of a media-type wet dispersion machine ("Dyno Mill" manufactured by Willy & Backoffen (WAB)). Next, the vessel was filled with media (zirconia beads with a diameter of 1.0 mm). The filling amount of the media was 70% by volume based on the capacity of the vessel. Next, the mixed liquid was subjected to a wet dispersion treatment using the above-mentioned media type wet dispersion machine under the conditions of a discharge rate of 300 g/min and a number of passes of 1 time. Thereby, a dispersion liquid was obtained. Next, the dispersion was filtered using a Buchner funnel to obtain a filtrate. Next, the filtrate was washed by dropping ion-exchanged water onto the filtrate on the Buchner funnel. At this time, the filtrate (ion-exchanged water after washing) was collected and its electrical conductivity was measured, and washing was repeated until the electrical conductivity of the filtrate became 5 μS/cm or less. Next, the washed filtrate was crushed using a crusher ("Sample Mill SK-M2" manufactured by Kyoritsu Riko Co., Ltd.). Thereby, a core (C-1) was obtained.

(Core (C-2) to (C-6))
Cores (C-2) to (C-6) were prepared in the same manner as in the preparation of core (C-1), except that the types and amounts of each component were changed as shown in Table 1 below. In Table 1 below, "(R)" indicates a registered trademark.

Details of the carbon black and polymer dispersion used to prepare the core are shown below.

#970: Acidic carbon black (“Carbon Black #970” manufactured by Mitsubishi Chemical Corporation)
MOGUL (R) L: Acidic carbon black (“MOGUL (registered trademark) L” manufactured by Cabot)
MA600: Neutral carbon black (“MA600” manufactured by Mitsubishi Chemical Corporation)
#1000: Acidic carbon black (“Carbon Black #1000” manufactured by Mitsubishi Chemical Corporation)
V-02: Solution containing polycarbodiimide resin to which a hydrophilic segment has been added ("Carbodilite (registered trademark) V-02" manufactured by Nisshinbo Chemical Co., Ltd.), solid content concentration 40%
V-10: Solution containing polycarbodiimide resin to which a hydrophilic segment has been added ("Carbodilite (registered trademark) V-10" manufactured by Nisshinbo Chemical Co., Ltd.), solid content concentration 40%
V-04: Solution containing polycarbodiimide resin to which a hydrophilic segment has been added ("Carbodilite (registered trademark) V-04" manufactured by Nisshinbo Chemical Co., Ltd.), solid content concentration 40%

[Preparation of resin]
(Resin (R-1))
100.0 parts by mass of isopropyl alcohol and 250.0 parts by mass of methyl ethyl ketone were charged into a four-necked flask equipped with a stirrer, a nitrogen introduction tube, a condenser, and a dropping funnel. Separately, 20.0 parts by mass of methyl methacrylate, 25.0 parts by mass of styrene, 12.0 parts by mass of n-butyl acrylate, 8.0 parts by mass of acrylic acid, and 35.0 parts by mass. benzyl methacrylate and 0.3 parts by mass of azobisisobutyronitrile (AIBN, polymerization initiator) were mixed to prepare a monomer solution. Further, 150.0 parts by mass of methyl ethyl ketone and 0.1 parts by mass of AIBN were mixed to prepare a methyl ethyl ketone solution.

Next, nitrogen gas was introduced into the four-neck flask described above to create a nitrogen atmosphere. Next, while heating and refluxing the contents of the four-necked flask at 70° C., the entire amount of the monomer solution was supplied into the four-necked flask over 2 hours using a dropping funnel. After supplying the monomer solution, the contents of the four-necked flask were further heated to reflux at 70°C for 6 hours. Next, while heating and refluxing the contents of the four-necked flask at 70° C., the entire amount of the methyl ethyl ketone solution was supplied into the four-necked flask over 15 minutes using a dropping funnel. After supplying the methyl ethyl ketone solution, the contents of the four-necked flask were further heated to reflux at 70°C for 5 hours. As a result, a resin solution containing resin (R-1), which is a styrene-(meth)acrylic resin, was obtained. Resin (R-1), which is a styrene-(meth)acrylic resin, was isolated by distilling off methyl ethyl ketone and isopropyl alcohol from the resin solution.

(Measurement of glass transition point)
The glass transition point (Tg) of the resin (R-1) was measured using a differential scanning calorimeter ("DSC-60" manufactured by Shimadzu Corporation) in accordance with "JIS (Japanese Industrial Standards) K7121-2012". The temperature increase rate was 5° C./min. The measured glass transition point of the resin (R-1) was 56°C.

(Measurement of acid value)
Based on the method described in JIS (Japanese Industrial Standards) K0070:1992 (testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value, and unsaponifiable substances of chemical products), resin (R-1) The acid value was measured. The measured acid value of the resin (R-1) was 62 mgKOH/g.

(Resin (R-2) to (R-5))
Resins (R-2) to (R-5) were synthesized in the same manner as in the synthesis of resin (R-1), except that the composition of the monomer solution used was changed as shown in Table 2 below. Note that resins (R-2) to (R-3) and (R-5) were styrene-(meth)acrylic resins. Resin (R-4) was a (meth)acrylic resin. Resins (R-1) to (R-5) were all vinyl polymers containing carboxyl groups. Table 2 below also shows the glass transition point (Tg) and acid value of each resin. In Table 2 below, 2-HE methacrylate indicates 2-hydroxyethyl methacrylate.

<Preparation of ink>
Inks of Examples 1 to 5 and Comparative Examples 1 to 5 shown in Table 3 below were prepared by the following method.

[Example 1]
(kneading process)
Using an FM mixer ("FM-10B" manufactured by Nippon Coke Industry Co., Ltd.), 1000 parts by mass of resin (R-1) and 500 parts by mass of core (C-1) were mixed at a rotation speed of 2000 rpm for 4 hours. Premixed for a minute to obtain a mixture.

The resulting mixture was melt-kneaded using a twin-screw extruder ("PCM-30" manufactured by Ikegai Co., Ltd.) at a melt-kneading temperature (cylinder temperature) of 120° C., a rotation speed of 150 rpm, and a processing speed of 100 g/min. The obtained melt-kneaded product was coarsely pulverized to a particle size of 2 mm or less using a pulverizer (“Rotoplex (registered trademark)” manufactured by Hosokawa Micron Co., Ltd.) to obtain a coarsely pulverized product. Next, the coarsely pulverized material was further pulverized using a pulverizer ("Turbo Mill RS type" manufactured by Freund Turbo Co., Ltd.) under conditions of a set particle size of 4 μm to obtain a pulverized material.

(Pigment particle dispersion preparation process)
Mixing 20.0 parts by mass of the above-mentioned pulverized material, 0.5 parts by mass of a nonionic dispersant ("DISPERBYK (registered trademark) 193" manufactured by BYK Chemie Japan Co., Ltd.), an aqueous NaOH solution, and ion-exchanged water, A mixed solution was obtained. The amount of NaOH aqueous solution used was equal to the amount of NaOH aqueous solution required to completely neutralize the resin (R-1) contained in the pulverized product. The amount of ion-exchanged water used was such that the total amount of the mixed liquid was 100 parts by mass. Using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Kagaku Co., Ltd.), the above-mentioned liquid mixture was sufficiently stirred at a rotation speed of 400 rpm.

Next, the above-mentioned liquid mixture was introduced into a vessel of a media-type wet disperser ("Dyno Mill" manufactured by Willy & Backoffen (WAB)). Further, the vessel was filled with media (zirconia beads with a diameter of 1.0 mm). The filling amount of the media was 70% by volume based on the capacity of the vessel. Thereafter, the mixed liquid was wet-pulverized using the above-mentioned media-type wet dispersion machine under the conditions of a discharge rate of 300 g/min and a number of passes of 1 time. Next, the mixed liquid after wet pulverization was subjected to high-pressure dispersion treatment (pressure conditions: 190 MPa) using a high-pressure dispersion device ("Nano Veita (registered trademark) C-ES" manufactured by Yoshida Kikai Kogyo Co., Ltd.). Thereby, a first dispersion containing pigment particles was obtained. The pigment particles had a core and a coating layer covering the core. The core included acidic carbon black and a polymer containing carbodiimide groups. The covering layer contained a carboxy group-containing vinyl polymer.

(Centrifugation process and replacement process)
The mixed solution after the high-pressure dispersion treatment was transferred to a container, and this container was placed in a centrifugal adhesion measuring device ("NS-C100" manufactured by Nano Seeds Co., Ltd.). Using the centrifugal adhesion measuring device described above, the dispersion was centrifuged at a rotational speed of 50,000 rpm for 24 hours to precipitate the pigment particles (centrifugation step). After centrifugation, the supernatant was removed from the container. Thereafter, the same volume of ion-exchanged water as the removed supernatant liquid was added to the container to redisperse the pigment particles (replacement step). In this way, the aqueous medium of the first dispersion was replaced with ion-exchanged water to obtain a second dispersion.

(Mixing process)
A container contains 50.0 parts by mass of the second dispersion, 10.0 parts by mass of glycerin, 5.0 parts by mass of triethylene glycol monobutyl ether, and 0.3 parts by mass of a nonionic surfactant (Nissin Chemical). "Olfine (registered trademark) E1004" manufactured by Kogyo Co., Ltd.) and ion-exchanged water were added to obtain a mixed solution (formulation A). The amount of ion-exchanged water used was such that the total amount of the mixed liquid was 100 parts by mass. The above-mentioned liquid mixture was stirred using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Kagaku Co., Ltd.) at a rotation speed of 400 rpm. The mixed solution after stirring was filtered using a filter (pore size: 5 μm). In this way, the ink of Example 1 was obtained.

[Examples 2-3 and Comparative Examples 1-2]
The ink was prepared in the same manner as in Example 1, except that the types and amounts of the core and resin in the kneading step and the amount of NaOH used in the pigment particle dispersion preparation step were changed as shown in Tables 3 and 4 below. Inks of Examples 2 and 3 and Comparative Examples 1 and 2 were prepared. In Comparative Example 2, the above-mentioned acidic carbon black ("Carbon Black #970" manufactured by Mitsubishi Chemical Corporation) was used as it was instead of the core.

In addition, in Tables 3 and 4 below, "NaOH [equivalent ratio]" refers to the amount of NaOH aqueous solution required to completely neutralize the resin contained in the pulverized material as X, and the amount of NaOH aqueous solution actually used as Y. The ratio (Y/X) when doing so is shown. For example, if the amount X of the NaOH aqueous solution required to completely neutralize the resin contained in the pulverized material is 100 g, and the amount Y of the NaOH aqueous solution actually used is 50 g, the NaOH [equivalent ratio] will be 0.5. .

[Examples 4-5 and Comparative Examples 3-5]
Inks of Examples 4 to 5 and Comparative Examples 3 to 5 were prepared in the same manner as the ink of Example 1 except for the following changes. In preparing the inks of Examples 4 to 5 and Comparative Examples 3 to 5, the types and amounts of the core and resin in the kneading step and the amount of NaOH used in the dispersion step were changed as shown in Tables 3 and 4 below. Further, in preparing the inks of Examples 4 to 5 and Comparative Examples 3 to 5, the mixing step was performed as shown below.

(Mixing process)
In a container, 50.0 parts by mass of the second dispersion liquid, 10.0 parts by mass of propylene glycol, 10.0 parts by mass of glycerin, 5.0 parts by mass of triethylene glycol monobutyl ether, and 0.5 parts by mass. A nonionic surfactant ("Surfynol (registered trademark) 440" manufactured by Nissin Chemical Industry Co., Ltd.) and ion-exchanged water were added to obtain a mixed solution (formulation B). The amount of ion-exchanged water used was such that the total amount of the mixed liquid was 100 parts by mass. The above-mentioned liquid mixture was stirred using a stirrer ("Three-One Motor BL-600" manufactured by Shinto Kagaku Co., Ltd.) at a rotation speed of 400 rpm. The mixed solution after stirring was filtered using a filter (pore size: 5 μm). As a result, inks of Examples 4 to 5 and Comparative Examples 3 to 5 were obtained.

[Measurement of ratio (M ₂ /M ₁ ) and free resin]
The "ratio (M ₂ /M ₁ )" and "ratio of free resin" were measured for each ink by the following method. Here, "ratio ( _M2 / _M1 )" is the ratio of the total mass _M2 of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass _M1 of the pigment in the pigment particles. The "ratio of free resin" refers to the ratio of free resin to the total amount of the carbodiimide group-containing polymer, the carboxyl group-containing vinyl polymer, and the free resin in the ink. The measurement results are also shown in Tables 3 and 4 below.

First, the object to be measured (any of the inks of Examples 1 to 5 and Comparative Examples 1 to 5) was placed in a vacuum oven. At this time, the input amount of the measurement target was defined as the mass W [g]. Next, a vacuum oven was set at a temperature of 150° C. and a pressure of 0.67 Pa to evaporate the aqueous medium from the measurement target. Thereby, the first solid content was obtained. The amount M _A of resin contained in the first solid content (total amount of carbodiimide group-containing polymer and carboxyl group-containing vinyl polymer contained in the pigment particles and free resin) was measured by gel filtration chromatography (GPC).

Separately, the mass W [g] to be measured was transferred to a container, and this container was placed in a centrifugal adhesion measuring device ("NS-C100" manufactured by Nano Seeds Co., Ltd.). Using the centrifugal adhesion measuring device described above, the measurement target described above was centrifuged at a rotational speed of 50,000 rpm for 24 hours. After centrifugation, the supernatant was collected from the container. The collected supernatant liquid was placed in a vacuum oven. Next, a vacuum oven was set at a temperature of 150° C. and a pressure of 0.67 Pa to evaporate the aqueous medium from the supernatant liquid. As a result, a second solid content was obtained. The amount of resin M _B (mass of free resin) contained in the second solid content was measured by gel filtration chromatography (GPC).

The amount M ₁ of resin contained in the first solid content, the amount M ₂ of resin contained in the second solid content, and the mass of the pigment were applied to the following formula. Thereby, the ratio (M ₂ /M ₁ ) of the pigment particles was calculated. Note that the mass of the pigment was calculated by multiplying the mass W [g] of the measurement target by the content ratio of the pigment in the measurement target.
Ratio (M ₂ /M ₁ ) = (Amount of resin contained in the first solid content M _A - Amount of resin contained in the second solid content M _B ) / Mass of pigment

The amount M _A of resin contained in the first solid content and the amount M _B of resin contained in the second solid content were applied to the following formula. Thereby, the ratio of free resin was calculated.
Ratio of free resin = amount of resin contained in the second solid content M _B / amount of resin contained in the first solid content M _A

The GPC conditions used to measure the ratio (M ₂ /M ₁ ) and free resin are shown below.

(GPC conditions)
Measuring device: “HLC-8020GPC” manufactured by Tosoh Corporation
Column: Ultra-high performance semi-micro SEC column (“TSKgel SuperMultipore HZ-H” manufactured by Tosoh Corporation, packing material: styrene-divinylbenzene resin, column size: inner diameter 4.6 mm x length 15 cm, filler particle diameter: 6 μm)
Number of columns: 3 Eluent: Tetrahydrofuran Eluent flow rate: 0.35 mL/min Sample solution amount: 10 μL
Column temperature: 40℃
Detector: RI (refractive index) detector Calibration curve: Monodisperse polystyrene standard samples manufactured by Tosoh Corporation (F-40, F-20, F-4, F-1, A-5000, A-2500 and A- 1000) and n-propylbenzene.

[Comparative example 6]
Resin (R-6) was synthesized in the same manner as resin (R-1) except for the following changes. In the synthesis of resin (R-6), acrylic acid was not used in the preparation of the monomer solution, but instead an equivalent amount of methyl methacrylate was used. Resin (R-6) was a vinyl polymer containing no carboxy groups. The ink of Comparative Example 6 was prepared in the same manner as the ink of Example 1 except that the type of resin in the kneading step was changed to resin (R-6). However, when resin (R-6) was used in the kneading step, the pigment particles were not dispersed but aggregated in the pigment particle dispersion preparation step. As a result, the first dispersion containing pigment particles could not be obtained, and the ink of Comparative Example 6 could not be prepared. Vinyl polymers that do not contain carboxy groups do not covalently bond to polymers that contain carbodiimide groups. Therefore, it is determined that the vinyl polymer contained in the pigment particles described above was easily desorbed and caused aggregation.

[Comparative Example 7]
The ink of Comparative Example 7 was prepared in the same manner as the ink of Example 1 except that resin (R-1) was not used in the kneading step. However, when the resin (R-1) was not used in the kneading step, the pigment particles were not dispersed but aggregated in the pigment particle dispersion preparation step. As a result, the first dispersion containing pigment particles could not be obtained, and the ink of Comparative Example 7 could not be prepared. Pigment particles not using a carboxyl group-containing vinyl polymer are judged to have low dispersion stability and are likely to agglomerate.

<Evaluation>
Regarding the inks of Examples 1 to 5 and Comparative Examples 1 to 5, the image density of the formed images, ejection stability (occurrence of nozzle clogging and landing accuracy), and fixation on non-permeable recording media were determined by the following methods. The properties and storage stability (long-term storage stability, short-term storage stability, and _D50 change rate when drying) were evaluated. Note that the evaluation was performed under an environment of a temperature of 25° C. and a humidity of 60% RH unless otherwise specified. The evaluation results are shown in Tables 5 and 6 below.

[Evaluation machine]
In the evaluation, an inkjet recording device equipped with a line type recording head (prototype manufactured by Kyocera Document Solutions Co., Ltd.) was used as an evaluation machine. The evaluation targets (inks of Examples 1 to 5 and Comparative Examples 1 to 5) were filled into the black ink tank of the evaluation machine. The ejection conditions of the evaluation machine were set so that the volume per drop of ink ejected from the recording head of the evaluation machine was 11 pL.

[Image density]
Evaluation of image density was performed under an environment of a temperature of 25° C. and a humidity of 50% RH. As the recording medium, A4 size plain paper ("Color Copy (registered trademark)" manufactured by Mondi, basis weight 90 g/m ² ) was used. A solid image (size: 10 cm x 10 cm, printing rate: 100%) was formed on a recording medium using an evaluation machine. The image density (ID) of the formed solid image was measured using a reflection densitometer ("RD-19" manufactured by X-Rite). Specifically, image densities at 10 randomly selected locations in the solid image were measured using the above-mentioned reflection densitometer, and the arithmetic mean value of the image densities at 10 locations was taken as the evaluation value of image density. Image density was determined according to the following criteria.

(Image density standard)
Good (A): Evaluation value is 1.20 or more Bad (B): Evaluation value is less than 1.20

[Nozzle clogging]
In the evaluation of nozzle clogging, A4 size plain paper ("Color Copy (registered trademark)" manufactured by Mondi, basis weight 90 g/m ² ) was used as the recording medium. Solid images of 150 mm x 200 mm were continuously printed on 100 recording media using an evaluation machine. Next, the evaluation target was purged from the recording head of the evaluation machine. Next, the ink ejection surface of the recording head of the evaluation machine was wiped to clean the recording head. Hereinafter, the operation of cleaning the recording head by purging and wiping may be referred to as a cleaning process. Next, a nozzle check pattern image was formed on the recording medium using an evaluation machine. As a result, no matter which evaluation target was used, the evaluation target was ejected from all nozzles (the number of non-ejecting nozzles was 0). Next, the recording head was subjected to cleaning treatment again. Next, the evaluation machine was left standing for 7 days without a cap attached to the recording head. Next, the recording head was subjected to cleaning treatment again. Next, a nozzle check pattern image (evaluation image) was formed on the recording medium using an evaluation machine. The evaluation images were checked, and the ratio of the number of non-ejecting nozzles to all the nozzles (7968) was calculated. The calculated ratio of the number of non-ejecting nozzles was taken as the nozzle clogging evaluation value. Nozzle clogging was determined according to the following criteria.

(criteria for nozzle clogging)
A (Pass): Evaluation value is less than 10% B (Fail): Evaluation value is 10% or more

[Impact accuracy]
In the evaluation of landing accuracy, A4 size inkjet matte paper ("Super Fine Paper" manufactured by Seiko Epson Corporation) was used as the recording medium. A solid image of 150 mm x 200 mm was continuously printed on 5000 recording media using an evaluation machine. Next, a cleaning process was performed on the recording head of the evaluation machine.

Next, a stripe image formed by a plurality of parallel thin lines was formed on the recording medium using an evaluation machine. In forming the striped image, the line width of the thin line was set to 1 pixel, and the interval between adjacent thin lines (interline pitch) was set to 3 pixels. Next, the stripe image formed on the recording medium was read using a microscope. Specifically, the distance A between a specific thin line a and a thin line b located 16 pixels away from the thin line a was measured at 204 locations. Note that three other thin lines existed between the thin line a and the thin line b. The variation (3σ) of the measured interval A was calculated using image processing software (manufactured by Kyocera Document Solutions Co., Ltd.). The calculated dispersion (3σ) of the interval A was taken as the evaluation value of the landing accuracy. The impact accuracy was determined according to the following criteria.

(Standard for impact accuracy)
A (good): 3σ is less than 15 B (bad): 3σ is 15 or more

If the ink to be evaluated passes the test in terms of nozzle clogging and has good landing accuracy, it is determined that the ejection stability of the ink is good. In other cases, the ejection stability of the ink is determined to be poor.

[Fixability]
Evaluation of fixability was performed at a temperature of 25° C. and a humidity of 60% RH. In the fixation evaluation, as a non-permeable recording medium, an A4 size OHP sheet ("Espet (registered trademark) E5200" manufactured by Toyobo Co., Ltd.), a biaxially stretched polyester film whose surface was corona treated, and a thick 0.10 mm) was used. A solid image (size: 2 cm x 2 cm, printing rate: 100%) was formed on a recording medium using an evaluation machine. Next, the above recording medium was dried at 60° C. for 2 hours using a dryer. Next, the solid image was fixed on the recording medium described above at a fixing temperature of 160° C. using a fixing device. Note that the fixing device was a fixing device that was installed in a multifunction device (“TASKalfa 2551ci” manufactured by Kyocera Document Solutions Co., Ltd.) and was removed.

Next, a rubbing test was conducted on the above-mentioned recording medium. Specifically, a metal weight (cylindrical shape with a diameter of 50 mm, mass of 1 kg) whose bottom surface was covered with a fabric was placed on the solid image formed on the above-mentioned recording medium. Then, the solid image was rubbed back and forth 10 times using a weight with a load of 1 kg. Next, the solid image was visually observed to confirm whether or not the solid image had been peeled off from the OHP sheet. Fixability was evaluated according to the following criteria.

(Standards for fixability)
A (good): Peeling of the solid image was not observed.
B (Poor): Peeling of the solid image was confirmed.

[Long-term storage stability]
Evaluation of long-term storage stability was performed at a temperature of 25° C. and a humidity of 50% RH. Using a vibrating viscometer (“VM-200T” manufactured by Nittetsu Hokkaido Control System Co., Ltd.), the viscosity (initial viscosity V ₁ ) was measured. Next, about 30 g of the evaluation target was put into a container with a volume of 50 mL, and the container was sealed. The above-mentioned container was placed in a thermostat set at an internal temperature of 60° C. and kept warm for 3 months (long-term storage). After that, the above-mentioned container was taken out from the thermostat, and then the above-mentioned container was left at room temperature for 3 hours. Thereafter, the object to be measured was taken out from the above-mentioned container, and the viscosity (post-treatment viscosity V ₂ ) was measured using the above-mentioned vibratory viscometer. The viscosity change rate [%] was calculated based on the measured initial viscosity V ₁ and post-treatment viscosity V ₂ (the following formula). The calculated viscosity change rate was used as an evaluation value of the long-term storage stability of the ink to be measured. Long-term storage stability was evaluated according to the following criteria.
Viscosity change rate [%] = 100 x (V ₁ - V ₂ )/V ₁

(Standards for long-term storage stability)
A (good): The absolute value of the viscosity change rate is 5% or less B (poor): The absolute value of the viscosity change rate is more than 5% and 10% or less C (especially poor): The absolute value of the viscosity change rate is more than 10%

[Short-term storage stability]
Short-term storage stability was evaluated using the same method as the long-term storage stability evaluation, except that the storage period of the container containing the evaluation target was changed to 2 weeks (short-term storage) in a thermostat (inner temperature 60°C). The viscosity change rate [%] was calculated as an evaluation value. Short-term storage stability was evaluated according to the following criteria.

(Standards for short-term storage stability)
A (good): The absolute value of the viscosity change rate is 5% or less B (poor): The absolute value of the viscosity change rate is more than 5% and 10% or less C (especially poor): The absolute value of the viscosity change rate is more than 10%

[ _D50 change rate during drying]
Regarding the evaluation target (specifically, any of the inks of Examples 1 to 5 and Comparative Examples 1 to 5), the volume was measured using a dynamic light scattering particle size distribution analyzer ("Zetasizer Nano ZS" manufactured by Malvern). The median diameter (initial D ₅₀ ) was determined. Next, about 30 g of the evaluation target was put into a container with a volume of 50 mL. At this time, the container was not sealed, and the evaluation target was allowed to come into contact with the atmosphere. Next, the above-mentioned container was placed in a constant temperature bath at 60° C. (drying treatment). The drying process was carried out until the water contained in the measurement target evaporated and the mass of the measurement target decreased by 35% by mass (when the mass of the measurement target before drying was 100 parts by mass, mass is 65 parts by mass). Next, the volume median diameter (D ₅₀ after drying) of the object to be measured after the drying treatment was determined using the above-mentioned dynamic light scattering particle size distribution measuring device. The D ₅₀ change rate during drying was calculated using the following formula. The drying _D50 change rate was determined according to the following criteria.
Drying _D50 change rate [%] = 100 x (Drying _D50 - Initial _D50 ) / Initial _D50

(Standard for _D50 change rate during drying)
A (Good): The absolute value of the _D50 change rate when drying is 10% or less. B (Poor): The absolute value of the _D50 change rate when drying is over 10%.

If the long-term storage stability, short-term storage stability, and drying _D50 change rate of the ink are all good, the ink is judged to have good storage stability. Otherwise, the storage stability of the ink is determined to be poor.

As shown in Tables 1-6, the inks of Examples 1-5 contained an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles had a core and a coating layer covering the core. The core included acidic carbon black and a polymer containing carbodiimide groups. The covering layer contained a carboxy group-containing vinyl polymer. In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black was 1.40 or more and 3.10 or less. The inks of Examples 1 to 5 contained a free resin, and the ratio of the free resin to the total amount of the carbodiimide group-containing polymer, the carboxyl group-containing vinyl polymer, and the free resin was more than 0.00% by mass and 3.00% by mass or less. Ta. The inks of Examples 1 to 5 were excellent in ejection stability, storage stability, and fixability to non-permeable recording media, and were able to form images with desired image density.

On the other hand, in the ink of Comparative Example 1, neutral carbon black was used instead of acidic carbon black in the pigment particles. Neutral carbon black is a pigment that does not react with carbodiimide group-containing polymers. Therefore, in the ink of Comparative Example 1, the carbon black and the carbodiimide group-containing polymer are not covalently bonded in the core of the pigment particles, and the carbodiimide group-containing polymer and the carboxy group-containing vinyl polymer are easily detached from the pigment particles during storage. be judged. As a result, the ink of Comparative Example 1 had poor long-term storage stability and dry _D50 change rate.

In the ink of Comparative Example 2, the carbodiimide group-containing polymer, which is a component that plays a role in binding the acidic carbon black and the carboxy group-containing vinyl polymer, was not used in the pigment particles. Therefore, in the ink of Comparative Example 2, the acidic carbon black and the carboxyl group-containing vinyl polymer are not stably bonded to each other in the core of the pigment particles, and the carboxyl group-containing vinyl polymer is easily detached from the pigment particles during storage. Then it will be judged. As a result, the ink of Comparative Example 2 caused nozzle clogging and had poor long-term storage stability, short-term storage stability, and dry _D50 change rate.

The ink of Comparative Example 3 had a ratio (M ₂ /M ₁ ) of more than 3.10 in pigment particles. It is determined that the ink of Comparative Example 3 lacks the amount of acidic carbon black in the pigment particles. As a result, with the ink of Comparative Example 3, the image density of the formed image was poor.

In the ink of Comparative Example 4, the ratio (M ₂ /M ₁ ) of pigment particles was less than 1.40. It is determined that the ink of Comparative Example 4 lacks the amount of binder resin. As a result, the ink of Comparative Example 4 had poor fixability to non-penetrating recording media.

The ink of Comparative Example 5 had a free resin ratio of more than 3.00% by mass. The ink of Comparative Example 5 contained an excessive amount of free resin, and therefore had poor landing accuracy, as well as poor long-term storage stability, short-term storage stability, and dry D ₅₀ change rate.

The ink of the present invention can be used to form images. The ink manufacturing method of the present invention can provide ink.

Claims (6)

containing an aqueous medium and pigment particles dispersed in the aqueous medium,
The pigment particles have a core and a coating layer covering the core,
The core includes acidic carbon black and a carbodiimide group-containing polymer,
The coating layer includes a carboxyl group-containing vinyl polymer,
In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black is 1.40 or more and 3.10 or less. and
does not contain the free resin dispersed in the aqueous medium, or contains the free resin, and the ratio of the free resin to the total amount of the carbodiimide group-containing polymer, the carboxyl group-containing vinyl polymer, and the free resin is 0.00 mass % to 3.00% by mass or less. The inkjet ink according to claim 1, wherein the carboxy group-containing vinyl polymer has an acid value of 60 mgKOH/g or more and 120 mgKOH/g or less. The inkjet ink according to claim 1 or 2, wherein the carboxyl group-containing vinyl polymer has a glass transition point of 40°C or more and 70°C or less. The inkjet ink according to claim 1 or 2, wherein the carboxyl group-containing vinyl polymer includes a styrene-(meth)acrylic resin or a (meth)acrylic resin. A core forming step in which a core is obtained by wet-pulverizing acidic carbon black and a carbodiimide group-containing polymer;
A kneading step of obtaining a kneaded product by melt-kneading the core and the carboxyl group-containing vinyl polymer;
A pigment particle dispersion preparation step of obtaining a dispersion containing pigment particles by wet-pulverizing the kneaded material in an aqueous medium,
In the pigment particles, the ratio (M ₂ /M ₁ ) of the total mass M ₂ of the carbodiimide group-containing polymer and the carboxyl group-containing vinyl polymer to the mass M ₁ of the acidic carbon black is 1.40 or more and 3.10 or less. A method for producing an inkjet ink. After the pigment particle dispersion preparation step, a centrifugation step of centrifuging the dispersion;
The method for producing an inkjet ink according to claim 5, further comprising a replacement step of isolating a precipitate from the dispersion liquid and dispersing it in another aqueous medium after the centrifugation step.