JP2023039649A - Ink for inkjet - Google Patents

Abstract

[Problem] To provide an inkjet ink capable of forming an image having a desired image density and excellent abrasion resistance. [Solution] The inkjet ink contains a pigment, specific resin particles, and an aqueous medium. The aqueous medium contains water, a first organic solvent, and a second organic solvent. The first organic solvent is at least one of 3-methyl-1,3-butanediol, ethylene glycol monomethyl ether, and dipropylene glycol. The surface tension of the second organic solvent is 25.0 mN/m or more and 35.0 mN/m or less. The specific resin particles contain a fluorine-containing (meth)acrylic resin. [Selected Figures] None

Description

The present invention relates to inkjet inks.

An inkjet ink used in an inkjet recording apparatus is required to have a desired image density and be capable of forming an image with excellent scratch resistance.

In response to such demands, for example, an inkjet ink containing pigment, wax, water-soluble solvent and water has been proposed (Patent Document 1). The inkjet ink described above is said to be capable of forming an image with high density and excellent abrasion resistance.

JP 2016-145313 A

However, even with the inkjet ink described in Patent Document 1, it is difficult to form an image that satisfies the image density and scratch resistance.

The present invention has been made in view of the above problems, and an object thereof is to provide an inkjet ink capable of forming an image having a desired image density and excellent scratch resistance.

The inkjet ink according to the present invention contains a pigment, specific resin particles, and an aqueous medium. The aqueous medium includes water, a first organic solvent, and a second organic solvent. The first organic solvent is at least one of 3-methyl-1,3-butanediol, ethylene glycol monomethyl ether and dipropylene glycol. The second organic solvent has a surface tension of 25.0 mN/m or more and 35.0 mN/m or less. The specific resin particles contain a fluorine-containing (meth)acrylic resin.

The inkjet ink according to the present invention can form an image having a desired image density and excellent scratch resistance.

Embodiments of the present invention will be described below. In this specification, the units of the dispersion term (dD), the polarization term (dP) and the hydrogen bonding term (dH) of the Hansen solubility parameters are all [MPa ^0.5 ].

The surface tension is a value measured at a liquid temperature of 25° C. according to the Wilhelmy method using a surface tensiometer (for example, “DY-300” manufactured by Kyowa Interface Science Co., Ltd.). Unless otherwise specified, the measured value of the volume median diameter (D ₅₀ ) is a value measured using a dynamic light scattering particle size distribution analyzer (eg, Malvern's "Zetasizer Nano ZS"). is.

In this specification, acryl and methacryl may be collectively referred to as "(meth)acryl". Each component described in this specification may be used alone or in combination of two or more.

<Inkjet ink>
An inkjet ink (hereinafter sometimes simply referred to as ink) according to an embodiment of the present invention will be described below. The ink of the present invention contains a pigment, specific resin particles, and an aqueous medium. The aqueous medium includes water, first organic solvent, and second organic solvent. The first organic solvent is at least one of 3-methyl-1,3-butanediol, ethylene glycol monomethyl ether and dipropylene glycol. The surface tension of the second organic solvent is 25.0 mN/m or more and 35.0 mN/m or less. The specific resin particles contain a fluorine-containing (meth)acrylic resin.

In the ink of the present invention, the first organic solvent is selected from the viewpoint of Hansen solubility parameter (HSP). First, the HSP will be explained. HSP is a value used to predict the solubility of a substance. HSP consists of the following three parameters (unit: MPa ^0.5 ).
Dispersion term (dD): Energy due to intermolecular dispersion force Polarization term (dP): Energy due to intermolecular dipole interaction Hydrogen bond term (dH): Energy due to intermolecular hydrogen bond

The three parameters that make up the HSP can be regarded as coordinates in a three-dimensional space (Hansen space). When two specific substances are arranged in the Hansen space, the closer the distance between the coordinates of the two substances, the closer the properties of the two substances tend to be.

A substance with unknown HSP can be measured for HSP by the following method. First, in a sealable container, 1 part by mass of a substance whose HSP is to be measured (hereinafter sometimes referred to as a target substance) and 49 masses of a solvent in which HSP is known (for example, a solvent with a literature value) Part and The substance of interest and solvent are then thoroughly mixed by handshaking the container. Next, the container is allowed to stand for 12 hours under normal temperature environment (23° C.). Next, the container is turned upside down and the bottom surface of the container is observed. If neither precipitates nor aggregates are present on the bottom of the container, the solvent is considered to have dissolved the target substance. This test is repeated with appropriate changes in solvent type. As a result, a combination of 10 types of solvents composed of a solvent that dissolves the target substance and a solvent that does not dissolve the target substance is determined. As for the combination of 10 kinds of solvents, it is desirable that approximately half (for example, 4 to 6 kinds) of the solvents are solvents that dissolve the target substance, and the remaining solvents are solvents that do not dissolve the target substance. A sphere called the Hansen sphere is then drawn in the Hansen space based on the test results for the ten solvents.

In this specification, the combination of 10 kinds of solvents includes the solvents shown in Table 1 below.

Describes how to draw a Hansen sphere. A sphere (Hansen sphere) that includes the coordinates of the solvent in which the target substance is dissolved and does not include the coordinates of the solvent in which the target substance is not dissolved is drawn in the Hansen space. The center coordinates of the drawn Hansen sphere indicate the HSP of the target substance. The size of the Hansen sphere varies depending on the type of target substance. Specifically, a target substance that dissolves in various solvents having different properties has a large Hansen sphere radius R ₀ . Conversely, the radius R ₀ of the Hansen sphere is small for a target substance that dissolves only in solvents with limited specific properties.

A specific method for predicting the solubility of two substances (for example, solvent X and solute Y) using HSP is described. First, two substances are each placed in the Hansen space based on the HSP. Then, the distance R _a between the coordinates of the two substances is calculated. The closer the distance R _a is, the easier it is for the two substances to dissolve in each other. The distance R _a between two substances can be calculated by the following formula (R).

In equation (R), dDx, dPx and dHx represent the dispersion term (dD), polarization term (dP) and hydrogen bonding term (dH) of solvent X, respectively. dDy, dPy and dHy denote the dispersion term (dD), polarization term (dP) and hydrogen bonding term (dH) of solute Y, respectively.

Applying this to the ink of the present invention, the distance R _a between the solvent and the pigment particles in the Hansen space is calculated by the following formula (R-1).

In formula (R-1), dDs, dPs and dHs represent the solvent dispersion term (dD), polarization term (dP) and hydrogen bonding term (dH), respectively. dDp, dPp and dHp indicate the dispersion term (dD), polarization term (dP) and hydrogen bonding term (dH) of the pigment particles, respectively.

Whether or not the solute Y dissolves in the solvent X is determined by whether or not the Hansen sphere of the solute Y includes the coordinates of the HSP of the solvent X in the Hansen space. Specifically, whether or not the solute Y dissolves in the solvent X is determined by the ratio (R _a /R ₀ ) of the distance R _a between the two substances to the radius R ₀ of the Hansen sphere of the solute Y. be. Hereinafter, the ratio (R _a /R ₀ ) is referred to as a relative energy difference (RED) (formula (r) below). When RED is less than 1, solute Y dissolves in solvent X because the coordinates of the HSPs of solvent X lie inside the Hansen sphere of solute Y. On the other hand, when RED is greater than 1, solute Y does not dissolve in solvent X because the HSP coordinates of solvent X lie outside the Hansen sphere of solute Y. Note that solute Y partially dissolves in solvent X when RED is exactly 1.
RED=R _a /R ₀ (r)

When solvent X is a mixed solvent, HSP of solvent X can be calculated by the following method. First, for each solvent constituting the solvent X, the product A of the dispersion term (dD) of the solvent and the mass ratio of the solvent (the ratio of the mass of the solvent to the mass of the solvent X) is calculated. Next, the sum B is calculated by summing the products A of each solvent. This sum B is defined as the dispersion term (dD) of the mixed solvent. The polarization term (dP) and the hydrogen bond term (dH) of the solvent X can also be calculated by the same method as the method of calculating the dispersion term (dD) of the solvent X. As a result, the HSP of solvent X, which is a mixed solvent, is calculated.

The ink of the present invention can form an image having a desired image density and excellent scratch resistance by having the above-described constitution. The reason is explained below. The ink of the present invention contains a first organic solvent (3-methyl-1,3-butanediol, ethylene glycol monomethyl ether or dipropylene glycol). The first organic solvent and the pigment have a Hansen Solubility Parameter RED of approximately 0.6 to 1.2, indicating relatively low compatibility. Therefore, the first organic solvent moderately reduces the dispersion stability of the pigment in the ink of the present invention. As a result, after the ink of the present invention lands on the recording medium, the pigment tends to aggregate on the surface of the recording medium. The pigment aggregated on the surface of the recording medium remains on the surface without penetrating into the interior of the recording medium. As a result, the ink of the present invention can form an image with sufficient image density even when an image with high image density is required.

On the other hand, when the surface of the recording medium rubs against another member (for example, another recording medium), the pigment remaining on the surface of the recording medium may adhere to another member. This phenomenon is likely to occur when the solvent remains on the surface of the recording medium. In contrast, the ink of the present invention contains a second organic solvent. The second organic solvent has a lower surface tension than water. Since the ink of the present invention contains the second organic solvent, the solvent can quickly permeate the inside of the recording medium after landing on the recording medium, and the pigment can mainly remain on the surface of the recording medium.

Further, the ink of the present invention contains specific resin particles containing a fluorine-containing (meth)acrylic resin. After the ink of the present invention lands on a recording medium, the specific resin particles stay on the surface of the recording medium and cover the pigment, thereby protecting the pigment. Fluorine-containing (meth)acrylic resins are resins based on (meth)acrylic resins. Therefore, the specific resin particles are excellent in dispersion stability. Fluorine-containing (meth)acrylic resin is a type of fluororesin. Therefore, the specific resin particles have a low coefficient of friction. As a result, the specific resin particles can reduce the coefficient of friction of the surface of the image formed by the ink of the present invention and efficiently protect the pigment adhering to the surface of the recording medium. As a result, the ink of the present invention can form an image with excellent scratch resistance. Each component of the ink of the present invention is described below.

[Pigment]
In the ink of the present invention, the pigment constitutes pigment particles together with, for example, a pigment-coating resin. Pigment particles are composed of, for example, a core containing a pigment and a pigment-coating resin that coats the core. The pigment coating resin exists, for example, dispersed in a solvent. From the viewpoint of improving the color density, hue, or stability of the ink of the present invention, the _D50 of the pigment particles is preferably 30 nm or more and 200 nm or less, more preferably 70 nm or more and 130 nm or less.

Pigments include, for example, yellow pigments, orange pigments, red pigments, blue pigments, purple pigments, and black pigments. Examples of yellow pigments include C.I. I. Pigment Yellow (74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193). Examples of orange pigments include C.I. I. Pigment Orange (34, 36, 43, 61, 63, and 71). Examples of red pigments include C.I. I. Pigment Reds (122 and 202). Examples of blue pigments include C.I. I. Pigment Blue (15, more specifically 15:3). Examples of purple pigments include C.I. I. Pigment Violet (19, 23, and 33). Examples of black pigments include C.I. I. Pigment Black (7) may be mentioned.

The content of the pigment in the ink of the present invention is preferably 3.0% by mass or more and 20.0% by mass or less, and preferably 6.0% by mass or more and 12.0% by mass or less. By setting the pigment content to 3.0% by mass or more, the ink of the present invention can more reliably form an image having a desired image density. By setting the content of the pigment to 20.0% by mass or less, the ejection stability of the ink of the present invention can be improved.

(pigment-coated resin)
Pigment coating resins are resins that are soluble in the inks of the present invention. A portion of the pigment coating resin is present, for example, on the surface of the pigment particles and improves the dispersibility of the pigment particles. A portion of the pigment-coating resin, for example, is present dissolved in the ink of the present invention. Styrene-acrylic resins are preferred as pigment coating resins. A styrene-acrylic resin is a polymer of monomers containing styrene and at least one of (meth)acrylic acid alkyl ester and (meth)acrylic acid.

In the ink of the present invention, the content of the pigment coating resin is preferably 1.0% by mass or more and 10.0% by mass or less, more preferably 3.0% by mass or more and 5.0% by mass or less.

[Specific resin particles]
The specific resin particles contain a fluorine-containing (meth)acrylic resin. The content of the fluorine-containing (meth)acrylic resin in the specific resin particles is preferably 90% by mass or more, more preferably 100% by mass. _D50 of the specific resin particles is preferably 20 μm or more and 300 μm or less.

(Fluorine-containing (meth)acrylic resin)
A fluorine-containing (meth)acrylic resin is a resin having a repeating unit derived from a fluorine-containing monomer and a repeating unit derived from a (meth)acrylic acid-based monomer. The fluorine-containing (meth)acrylic resin may further have a repeating unit (eg, styrene unit) derived from a monomer other than the fluorine-containing monomer and the (meth)acrylic acid-based monomer.

The fluorine-containing (meth)acrylic resin consists of a block ((meth)acrylic acid-based block) composed of repeating units derived from a (meth)acrylic acid-based monomer and a block composed of repeating units derived from a fluorine-containing monomer (fluorine-containing block ) is preferably a block copolymer having

((Meth)acrylic acid-based monomer)
Examples of (meth)acrylic acid-based monomers include (meth)acrylic acid and (meth)acrylic acid alkyl esters. (Meth)acrylic acid alkyl esters include, for example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate and butyl (meth)acrylate. As the (meth)acrylic acid-based monomer, methyl methacrylate, butyl acrylate, or acrylic acid is preferable.

In the fluorine-containing (meth)acrylic resin, the content ratio of repeating units derived from (meth)acrylic acid-based monomers to all repeating units is preferably 25.0% by mass or more and 75.0% by mass or less, and 40.0% by mass. % or more and 60.0 mass % or less is more preferable. By setting the content ratio of the repeating unit derived from the (meth)acrylic acid-based monomer to 25.0% by mass or more, the dispersion stability of the specific resin particles in the ink of the present invention can be improved. By setting the content ratio of the repeating unit derived from the (meth)acrylic acid-based monomer to 75.0% by mass or less, it is possible to sufficiently introduce the repeating unit derived from the fluorine-containing (meth)acrylic resin into the fluorine-containing (meth)acrylic resin. can. As a result, the scratch resistance of the image formed with the ink of the present invention can be further improved.

(Fluorine-containing monomer)
Fluorine-containing monomers include, for example, vinyl fluoride, vinylidene fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, bromotrifluoroethylene, pentafluoropropylene, and hexafluoropropylene. Preferred fluorine-containing monomers are vinylidene fluoride, tetrafluoroethylene and chlorotrifluoroethylene.

In the fluorine-containing (meth)acrylic resin, the content ratio of the repeating unit derived from the fluorine-containing monomer to the total repeating units is preferably 25.0% by mass or more and 75.0% by mass or less, and 40.0% by mass or more and 60.0% by mass. 0% by mass or less is more preferable. By setting the content ratio of the repeating unit derived from the fluorine-containing monomer to 25.0% by mass or more, the scratch resistance of the image formed by the ink of the present invention can be further improved. By setting the content ratio of the repeating unit derived from the fluorine-containing monomer to 75.0% by mass or less, the dispersion stability of the specific resin particles in the ink of the present invention can be improved.

As raw material monomers for the fluorine-containing (meth)acrylic resin, the following combination (I) is preferred, and combination (Ia) is more preferred.
Combination (I): vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, methyl (meth)acrylate, butyl (meth)acrylate and (meth)acrylic acid Combination (Ia): vinylidene fluoride, tetrafluoroethylene, Chlorotrifluoroethylene, methyl methacrylate, butyl acrylate and acrylic acid

The content of the specific resin particles in the ink of the present invention is preferably 2.0% by mass or more and 8.0% by mass or less, more preferably 4.0% by mass or more and 6.0% by mass or less. By setting the content ratio of the specific resin particles to 2.0% by mass or more, the scratch resistance of the image formed by the ink of the present invention can be further improved. By setting the content ratio of the specific resin particles to 8.0% by mass or less, the ejection stability of the ink of the present invention can be improved.

[Aqueous medium]
The aqueous medium includes water, first organic solvent and second organic solvent. Preferably, the aqueous medium further contains glycerin. The aqueous medium may further contain components other than water, the first organic solvent, the second organic solvent, and glycerin, in small amounts. In the aqueous medium, the total proportion of water, the first organic solvent, the second organic solvent and glycerin is preferably 90% by mass or more, more preferably 100% by mass.

(First organic solvent)
The first organic solvent is at least one of 3-methyl-1,3-butanediol, ethylene glycol monomethyl ether and dipropylene glycol. In the ink of the present invention, the content of the first organic solvent is 5.0% by mass or more and 40.0% by mass. % by mass or less is preferable, and 10.0% by mass or more and 30.0% by mass or less is more preferable. By setting the content of the first organic solvent to 5.0% by mass or more, the ink of the present invention can more reliably form an image having a desired image density. By setting the content of the first organic solvent to 40.0% by mass or less, the storage stability of the ink of the present invention can be improved.

(Second organic solvent)
The second organic solvent has a surface tension of 25.0 mN/m or more and 35.0 mN/m or less. The surface tension of the second organic solvent is preferably 25.0 mN/m or more and 30.0 mN/m or less. By setting the surface tension of the second organic solvent to 25.0 mN/m or more, the ejection stability of the ink of the present invention can be improved. By setting the surface tension of the second organic solvent to 35.0 mN/m or less, the ink of the present invention can form an image with excellent scratch resistance.

As the second organic solvent, for example, organic solvents listed in Table 2 below can be used. The second organic solvent is preferably 1,2-pentanediol, 1,2-hexanediol or 1,2-butanediol.

In the ink of the present invention, the content of the second organic solvent is preferably 3.0% by mass or more and 25.0% by mass or less, more preferably 7.0% by mass or more and 15.0% by mass or less. By setting the content of the second organic solvent to 3.0% by mass or more, the scratch resistance of the image formed by the ink of the present invention can be further improved. By setting the content of the second organic solvent to 25.0% by mass or less, the ejection stability of the ink of the present invention can be improved.

(glycerin)
In the ink of the present invention, glycerin functions as a humectant. The ink of the present invention can suppress the occurrence of nozzle clogging by containing glycerin. In the ink of the present invention, the content of glycerin is preferably 1.0% by mass or more and 10.0% by mass or less, more preferably 3.0% by mass or more and 7.0% by mass or less.

(water)
The content of water in the ink of the present invention is preferably 30.0% by mass or more and 75.0% by mass or less, more preferably 40.0% by mass or more and 60.0% by mass or less.

[Additive]
The ink of the present invention may optionally contain known additives (more specifically, for example, surfactants, antifoaming agents, humectants, dissolution stabilizers, drying inhibitors, antioxidants, viscosity modifiers). , a pH adjuster and an antifungal agent).

(Surfactant)
A surfactant improves the compatibility and dispersion stability of each component contained in the ink of the present invention. Surfactants include, for example, anionic surfactants, cationic surfactants and nonionic surfactants. A nonionic surfactant is preferred as the surfactant.

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

When the ink of the present invention contains a surfactant, the content of the surfactant in the ink of the present invention is preferably 0.1% by mass or more and 3.0% by mass or less, and 0.2% by mass or more and 1.5% by mass. 0% by mass or less is more preferable.

[Ink preparation method]
The ink of the present invention comprises, for example, a pigment dispersion liquid containing a pigment, an emulsion containing specific resin particles, water, a first organic solvent, a second organic solvent, and, if necessary, other It can be prepared by uniformly mixing ingredients (eg, glycerin and surfactant) with a stirrer. In the preparation of the ink of the present invention, foreign matter and coarse particles may be removed by uniformly mixing each component and filtering the mixture with a filter (for example, a filter with a pore size of 1 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 emulsion]
Emulsions (A) to (C) containing specific resin particles (a) to (c) containing a fluorine-containing (meth)acrylic resin were prepared.

Emulsion (A): "SIFCLEAR (registered trademark) F104" manufactured by E-Tech Co., Ltd. Solid concentration: 47% by mass, _D50 : 0.13 µm
Emulsion (B): “Lumiflon (registered trademark) FD1000” manufactured by AGC, solid content concentration: 40% by mass, D ₅₀ : 0.05 μm or more and 0.1 μm or less Emulsion (C): “Zeffle (registered trademark)” manufactured by Daikin SE-300A", solid content concentration: 51% by mass, D ₅₀ : 0.15 μm or more and 0.18 μm or less

The fluorine-containing (meth)acrylic resins contained in the specific resin particles (a) to (c) were all block copolymers having fluorine-containing blocks and (meth)acrylic acid blocks.

[Preparation of pigment dispersion]
Repeating units derived from methacrylic acid (MAA units), repeating units derived from methyl methacrylate (MMA units), repeating units derived from butyl acrylate (BA units), and repeating units derived from styrene (ST units ) to prepare a pigment coating resin (R-1). This pigment coating resin (R-1) had a mass average molecular weight (Mw) of 20,000 and an acid value of 100 mgKOH/g. The mass ratio of each repeating unit in this pigment coating resin (R-1) was "MAA unit: MMA unit: BA unit: ST unit = 6.5: 30.0: 30.0: 33.5". . This pigment coating resin (R-1) was mixed with an aqueous sodium hydroxide solution containing sodium hydroxide. The aqueous sodium hydroxide solution contained 1.05 times the amount of sodium hydroxide required for equivalent neutralization of the pigment coating resin (R-1). Thereby, the pigment coating resin (R-1) was neutralized with an equal amount (strictly 105% amount) of KOH. As a result, a pigment coating resin solution containing pigment coating resin (R-1) and water was obtained.

Black pigment ("MONARCH (registered trademark) 800" manufactured by Cabot Co., Ltd.), the above-mentioned pigment coating resin solution, Nissin Chemical Industry Co., Ltd. as a nonionic surfactant "OLFIN (registered trademark) E1010" (acetylene diol ethylene oxide adduct) and deionized water were put into a vessel. The amount of each raw material input is such that the composition of the contents of the vessel is 78.5% by mass of water, 6.0% by mass of pigment coating resin (R-1), 15.0% by mass of black pigment and 0.0% by mass of nonionic surfactant. The amount was set to 5% by mass. The content of "water" is the ion-exchanged water put into the vessel described above and the water contained in the pigment coating resin solution (more specifically, the water contained in the sodium hydroxide aqueous solution used to neutralize the pigment coating resin). water, and the water generated by the neutralization reaction of the pigment coating resin and sodium hydroxide).

Subsequently, the contents of the vessel were dispersed using zirconia beads (particle size: 0.5 mm) as media and a wet dispersing machine ("Nanograin Mill" manufactured by Asada Iron Works Co., Ltd.). The dispersion conditions were a temperature of 10° C., a peripheral speed of 8 m/sec, and a discharge rate of 300 g/min. Thus, a pigment dispersion (CB) was obtained.

Pigment dispersions (Y), (M) and (C) were prepared in the same manner as the pigment dispersion (CB), except that the types of pigments used were as follows.
Pigment dispersion (CB): black pigment ("MONARCH (registered trademark) 800" manufactured by Cabot Co., Ltd.)
Pigment dispersion liquid (Y): yellow pigment ("Palcohol Yellow D1115J" manufactured by BASF Corporation)
Pigment dispersion liquid (M): magenta pigment ("Magenta D4550" manufactured by BASF Corporation)
Pigment dispersion (C) Cyan pigment (manufactured by BASF Corporation "Heliogen (registered trademark) Blue D7088")

<Ink preparation>
Any of the pigment dispersions (CB), (Y), (M) and (C), any of the emulsions (A) to (C), and a surfactant so as to have the compositions shown in Tables 3 to 5 below. (“Polyflow (registered trademark) KL-850” manufactured by Kyoeisha Chemical Co., Ltd.), glycerin, a first organic solvent, a second organic solvent, another organic solvent, and water were put into a beaker. The amount of the pigment dispersion liquid added was such that the amount of pigment (either black pigment, yellow pigment, magenta pigment or cyan pigment) added in the mixture was the amount shown in Tables 3 to 5 below. The amount of the emulsion added was such that the amount of the specific resin particles (a) to (c) added in the mixture was the amount shown in Tables 3 to 5 below.

Using a stirrer ("Three One Motor (registered trademark) BL-600" manufactured by Sintokagaku Co., Ltd.), the contents of the beaker were stirred at a rotational speed of 400 rpm and uniformly mixed to obtain a mixed solution. Next, the mixed liquid was filtered using a filter (manufactured by Membrane Solutions LLC, material: PTFE, pore size: 1 μm) to remove foreign substances and coarse particles contained in the mixed liquid. Thus, inks of Examples 1-12 and Comparative Examples 1-5 were prepared.

The HSP and R ₀ of the pigment dispersion, the first organic solvent, and other organic solvents (diethylene glycol and 2,2-dimethyl-1,4-butanediol) used instead of the first organic solvent in Comparative Examples are shown in the table below. 6.

Table 7 below shows the surface tensions at 25° C. of the second organic solvent and other organic solvents (propylene glycol monomethyl ether and propylene glycol) used in place of the second organic solvent in Comparative Examples.

<Evaluation>
The image density and scratch resistance of the formed images were evaluated for the inks of Examples 1 to 12 and Comparative Examples 1 to 5 by the following methods. Further, the RED between the first organic solvent contained in each ink (or another organic solvent used in place of the first organic solvent) and the pigment was calculated by the method described in Examples. The results are shown in Tables 8-9 below.

[Evaluation machine]
In the following evaluations, an inkjet recording apparatus (prototype manufactured by Kyocera Document Solutions Co., Ltd.) equipped with a piezo recording head ("KJ4B-QA" manufactured by Kyocera Corporation) was used as an evaluation machine. The ink tank of the evaluation machine was filled with the evaluation object (any of the inks of Examples 1 to 12 and Comparative Examples 1 to 5). The evaluation machine was set so that the volume per ink droplet was 11.5 pL.

[Image Density]
Using an evaluation machine, a 2.3 cm × 11.5 cm solid image (100% print rate) was formed on copy paper (“Color Copy (registered trademark)” manufactured by Mondi, basis weight: 90 g/m ² , size: A4). formed. Next, the copy paper on which the image was formed was allowed to stand for 24 hours in a normal temperature and normal humidity environment with a temperature of 25° C. and a humidity of 60%. Next, the image density of the solid image was measured using a reflection densitometer (“FD-5” manufactured by Konica Minolta, Inc.). Specifically, the image density was measured at three randomly selected locations on the solid image, and the average value was used as the evaluation value. The measurement conditions were an observation light source of "D50", an illumination condition of "M2", a field of view of "2°", and a concentration status of "I". The image density was evaluated according to the following criteria depending on the ink color.

(Standard of image density)
Cyan ink (Example 12): "A (good)" for an image density of 0.93 or more, and "B (bad)" for an image density of less than 0.93
Magenta ink (Example 11): "A (good)" for an image density of 0.87 or more, and "B (bad)" for an image density of less than 0.87
Yellow ink (Example 10): "A (good)" for an image density of 1.08 or more, and "B (bad)" for an image density of less than 1.08
Black ink (Examples 1 to 9 and Comparative Examples 1 to 5): "A (good)" for image density of 1.10 or more, "B (bad)" for image density of less than 1.10

[Scratch resistance]
Using the evaluation machine, a solid image of 5 cm×4 cm (100% print rate) was formed on copy paper (“CC90” manufactured by Mondi, basis weight: 90 g/m ² , size: A4). An unprinted copy sheet (hereinafter sometimes referred to as evaluation sheet) was placed on the surface of the copy sheet on which the solid image was formed (the surface facing the solid image). Next, a rectangular parallelepiped weight (mass: 1 kg) with a bottom dimension of 5 cm×4 cm was placed on the evaluation sheet. The placement position of the weight was set to a position directly above the solid image. Next, the evaluation paper was moved horizontally by gripping both ends of the evaluation paper. In the horizontal movement, the weight was moved five times over the solid image. As a result, the solid image was rubbed with the evaluation paper under a load of 1 kg. After that, the image density of the area in contact with the solid image on the evaluation paper was measured using a reflection densitometer ("FD-9" manufactured by Konica Minolta, Inc.). The measurement conditions were an observation light source of "D50", an illumination condition of "M2", a field of view of "2°", and a concentration status of "I". In the measurement, the image density of the area of the evaluation paper that was not in contact with the solid image was taken as the background value. A value (FD) obtained by subtracting the background value from the measured image density was used as an evaluation value of scratch resistance. Scratch resistance was evaluated based on the following criteria.

(Abrasion resistance standard)
A (good): FD less than 0.02 B (poor): FD 0.02 or more

As shown in Tables 1-9, the inks of Examples 1-12 contained a pigment, specific resin particles, and an aqueous medium. The aqueous medium included water, a first organic solvent, and a second organic solvent. The first organic solvent was at least one of 3-methyl-1,3-butanediol, ethylene glycol monomethyl ether and dipropylene glycol. The surface tension of the second organic solvent was 25.0 mN/m or more and 35.0 mN/m or less. The specific resin particles contained a fluorine-containing (meth)acrylic resin. The inks of Examples 1 to 12 were able to form images having desired image densities and excellent abrasion resistance by having the above-described constitution.

On the other hand, since the ink of Comparative Example 1 did not contain the specific resin particles, the scratch resistance of the formed image was poor.

The ink of Comparative Example 2 contained diethylene glycol, an organic solvent with excessively low RED with the pigment, instead of the first organic solvent. In the ink of Comparative Example 2, the dispersion stability of the pigment is judged to be extremely high. Therefore, the ink of Comparative Example 2 could not aggregate the pigment on the surface of the recording medium after landing on the recording medium, resulting in poor image density.

The ink of Comparative Example 3 contained 2,2-dimethyl-1,4-butanediol, an organic solvent with excessively high RED with the pigment, instead of the first organic solvent. In the ink of Comparative Example 3, the dispersion stability of the pigment is judged to be extremely low. Therefore, in the ink of Comparative Example 3, after landing on the recording medium, the pigment excessively aggregated on the surface of the recording medium, and the scratch resistance of the formed image was poor.

The ink of Comparative Example 4 contained propylene glycol monomethyl ether, which is an organic solvent having a surface tension of less than 25.0 mN/m, instead of the second organic solvent. In the ink of Comparative Example 4, after landing on the recording medium, the solvent abruptly permeated into the recording medium, and along with this, the pigment also permeated into the recording medium. As a result, with the ink of Comparative Example 4, the image density of the formed image was poor.

The ink of Comparative Example 5 contained propylene glycol, which is an organic solvent having a surface tension of more than 35.0 mN/m, instead of the second organic solvent. With the ink of Comparative Example 5, the solvent did not sufficiently permeate the inside of the recording medium after landing on the recording medium, so the scratch resistance of the formed image was poor.

The inks of the invention can be used to form images.

Claims (5)

Containing a pigment, specific resin particles, and an aqueous medium,
The aqueous medium includes water, a first organic solvent, and a second organic solvent,
the first organic solvent is at least one of 3-methyl-1,3-butanediol, ethylene glycol monomethyl ether and dipropylene glycol;
The second organic solvent has a surface tension of 25.0 mN/m or more and 35.0 mN/m or less,
The inkjet ink, wherein the specific resin particles contain a fluorine-containing (meth)acrylic resin. The inkjet ink of claim 1, wherein the second organic solvent comprises 1,2-pentanediol, 1,2-hexanediol, or 1,2-butanediol. 3. The inkjet ink according to claim 1, wherein the content of the specific resin particles is 2.0% by mass or more and 8.0% by mass or less. The content of the first organic solvent is 5.0% by mass or more and 40.0% by mass or more. 4. The inkjet ink according to any one of claims 1 to 3, which is not more than % by mass. The content of the second organic solvent is 3.0% by mass or more.20. 5. The inkjet ink according to any one of claims 1 to 4, which is not more than % by mass.