WO2024162297A1 - Ink composition, ink set, recording medium, and inkjet recording method - Google Patents

Abstract

Provided is an ink composition that contains a titanium oxide, a polymer dispersant and a flocculant, and in which the content of the titanium oxide is at least 1 mass% and below 30 mass%. Also provided is an ink set comprising the ink composition, a recording medium with the ink composition adhering thereto, and an inkjet recording method using the ink composition.

Description

Ink composition, ink set, recording medium, and inkjet recording method

The present invention relates to an ink composition, an ink set, a recording medium, and an inkjet recording method.

The printing method using an inkjet printer (inkjet printing method) is a method in which ink is sprayed from a nozzle and deposited on a recording medium. Unlike conventional printing methods, the inkjet printing method does not use plates, and is therefore expected to be used in a wide range of fields as an on-demand printing method that can handle small quantities of a wide variety of products.

In recent years, there has been an increasing demand for printing on non-white recording media such as corrugated cardboard, paperboard, and resin film, rather than on plain paper and other white recording media. In the field of packaging containers, mainly made of corrugated cardboard, flexographic printing has generally been used, but in recent years, from the perspective of increasing added value, there has been growing interest in inkjet printing, which is capable of reproducing high-quality designs.

When printing on a recording medium that is not white, white ink may be used to express the color white. Also, in order to hide the color of the recording medium itself and increase visibility, a white base may be formed using white ink, and then a color image may be formed using color inks.

Titanium oxide is known as a white pigment contained in white ink. However, titanium oxide has a higher specific gravity than organic pigments, and the particle size tends to be relatively large in order to achieve sufficient whiteness. For this reason, if titanium oxide is used in low-viscosity inkjet recording ink, the titanium oxide may settle or aggregate during storage of the ink, which may adversely affect the ejection properties of the ink. If titanium oxide settles or aggregates during storage of the ink, it is necessary to redisperse the ink using a stirrer before installing it in the inkjet printer, but even using a stirrer may not be enough to completely break down the aggregates. In addition to when the ink is stored, if the flow of ink stops in the ink flow path due to a pause in printing, etc., titanium oxide may settle or aggregate in the ink flow path, which may adversely affect the ejection properties of the ink.

JP 2017-75302 A JP 2020-111021 A

The present invention aims to provide an ink composition that has excellent dispersion stability and inkjet ejection properties of titanium oxide and is capable of producing printed matter with excellent color development, an ink set that includes the ink composition, a recording medium to which the ink composition is applied, and an inkjet recording method that uses the ink composition.

Specific means for solving the above problems include the following embodiments.
＜1＞
Contains titanium oxide, a polymer dispersant, and a flocculant,
The ink composition has a titanium oxide content of 1% by mass or more and less than 30% by mass.
＜2＞
The ink composition according to <1>, further comprising water.
＜3＞
The ink composition according to <1> or <2>, wherein the polymer dispersant has an acid value and/or an amine value that satisfies the formula (acid value)-(amine value)≦50 mgKOH/g.
＜4＞
The ink composition according to any one of <1> to <3>, wherein the amine value of the polymer dispersant is from 0 mgKOH/g to 130 mgKOH/g.
＜5＞
The ink composition according to any one of <1> to <4>, wherein the aggregating agent contains a polyvalent metal salt or a cationic resin.
＜6＞
The flocculant comprises a polyvalent metal salt,
The ink composition according to <5>, wherein the content of the polyvalent metal salt is 1% by mass or more and 70% by mass or less with respect to the content of the titanium oxide.
＜７＞
The ink composition according to <6>, wherein the polyvalent metal salt is at least one salt selected from the group consisting of calcium salts and magnesium salts.
＜8＞
the flocculant contains a cationic resin having a constituent unit derived from a vinyl monomer having a quaternary ammonium salt group and having a mass average molecular weight of 1,500 or more and 25,000 or less;
The ink composition according to <5>, wherein the content of the cationic resin is 0.5% by mass or more and 18% by mass or less with respect to the content of the titanium oxide.
＜9＞
The ink composition according to any one of items <1> to <8>, further comprising water.
＜10＞
The ink composition according to any one of <1> to <9>, further comprising an organic solvent.
<11>
<10> An ink set comprising the ink composition according to any one of <1> to <10>, and a white ink composition containing a white colorant and an anionic dispersant.
＜12＞
<10> An ink set comprising the ink composition according to any one of <1> to <10>, and a non-white ink composition containing a non-white colorant and an anionic dispersant.
＜13＞
A recording medium to which the ink composition according to any one of <1> to <10> or each ink composition included in the ink set according to <11> or <12> is adhered.
＜14＞
The recording medium according to <13>, wherein the recording medium is a permeable recording medium.
＜15＞
The recording medium according to <14>, wherein the permeable recording medium is a liner paper for cardboard.
＜16＞
An inkjet recording method, comprising ejecting the ink composition according to any one of <1> to <10> or each ink composition included in the ink set according to <11> or <12> from an inkjet head and depositing the ink composition on a recording medium.
＜１７＞
The inkjet recording method according to <16>, wherein the recording medium is liner paper.
＜18＞
The inkjet recording method according to <16> or <17>, wherein the inkjet head has a circulation mechanism.

The present invention provides an ink composition that has excellent dispersion stability of titanium oxide and excellent inkjet ejection properties, and is capable of producing printed matter with excellent color development, an ink set that includes the ink composition, a recording medium to which the ink composition is applied, and an inkjet recording method that uses the ink composition.

Specific embodiments to which the present invention is applied are described in detail below. In this specification, the term "(meth)acrylic acid" means both "acrylic acid" and "methacrylic acid". The same applies to terms such as "(meth)acrylate" and "(meth)acrylamide".

<Ink composition>
The ink composition according to the present embodiment contains titanium oxide, a polymeric dispersant, and an aggregating agent, and the content of titanium oxide is 1% by mass or more and less than 30% by mass. The ink composition according to the present embodiment is excellent in dispersion stability of titanium oxide and inkjet dischargeability. In addition, by applying the ink composition according to the present embodiment and an anionic white ink composition to the same area on a recording medium, a printed matter having excellent white color development can be obtained. Furthermore, by applying the ink composition according to the present embodiment on a recording medium to form a base, and then applying an anionic non-white ink composition on the base, a printed matter having excellent color development can be obtained. The reason why these effects are achieved is unclear, but is presumed to be as follows. That is, it is considered that the polymeric dispersant is adsorbed on the surface of the titanium oxide contained in the ink composition according to the present embodiment, thereby narrowing the particle size distribution width of the titanium oxide, and improving the dispersion stability and inkjet dischargeability of the ink composition. In addition, it is considered that the aggregating agent contained in the ink composition according to the present embodiment causes the anionic components contained in the anionic white ink composition or the non-white ink composition to aggregate, thereby reducing the fluidity of the ink and obtaining a printed matter having excellent color development.

The components contained in the ink composition according to this embodiment are described in detail below. Each of the following components may be used alone or in combination of two or more.

<Titanium oxide>
The ink composition according to the present embodiment contains titanium oxide. The crystal structure of titanium oxide includes rutile type (tetragonal crystal), anatase type (tetragonal crystal), and brookite type (orthorhombic crystal), and from the viewpoints of crystal stability, hiding power, and availability, it is preferable to use rutile type titanium oxide. In addition, titanium oxide can be produced by a gas phase method or a liquid phase method, and titanium oxide produced by a gas phase method is preferable because it is easy to obtain titanium oxide with high crystallinity.

Titanium oxide has the ability to decompose organic matter due to photocatalytic activity. For this reason, it is preferable to treat the surface of titanium oxide particles with alumina from the viewpoint of sealing the photocatalytic activity and improving the wettability of titanium oxide during dispersion. Furthermore, it is more preferable to treat the surface of titanium oxide particles with alumina and silica from the viewpoint of adjusting the acid-base state of the surface of titanium oxide particles and improving durability. In other words, titanium oxide that has been surface-treated with alumina treatment or alumina-silica treatment is preferable. In addition to the above, examples of surface treatment of titanium oxide with inorganic substances include a method of surface treatment by coating with inorganic hydrates containing zinc, magnesium, zirconium, etc. Surface-treated titanium oxide can be fired at 800°C to 1000°C to improve the fluidity and dispersibility of the secondary particle size without promoting sintering between particles.

The particle shape of titanium oxide is not particularly limited, and examples include granular and needle-like shapes. From the viewpoint of whiteness, the average primary particle diameter of titanium oxide is preferably 100 nm or more, more preferably 150 nm or more, and even more preferably 200 nm or more. From the viewpoint of redispersibility, the average primary particle diameter of titanium oxide is preferably 600 nm or less, more preferably 500 nm or less, and even more preferably 400 nm or less.

The content of titanium oxide in the ink composition according to this embodiment is 1% by mass or more and less than 30% by mass, and from the viewpoint of whiteness and dispersion stability, it is preferably 2% by mass or more and 25% by mass or less, more preferably 5% by mass or more and 20% by mass or less, even more preferably 7% by mass or more and 18% by mass or less, and particularly preferably 8% by mass or more and 16% by mass or less.

<Polymer Dispersant>
The ink composition according to this embodiment contains a polymer dispersant. The polymer dispersant is not particularly limited as long as it is a polymer having a mass average molecular weight of 2500 or more that can disperse titanium oxide.

The polymer dispersant may have an acid value. The acid value of the polymer dispersant is preferably 0 mgKOH/g or more and 120 mgKOH/g or less, more preferably 0 mgKOH/g or more and 20 mgKOH/g or less, and even more preferably 0 mgKOH/g or more and 10 mgKOH/g or less. The acid value of the polymer dispersant can be calculated in accordance with JIS K0070.

The polymer dispersant may have an amine value. The amine value of the polymer dispersant is preferably 0 mgKOH/g or more and 130 mgKOH/g or less, more preferably 10 mgKOH/g or more and 120 mgKOH/g or less, even more preferably 20 mgKOH/g or more and 75 mgKOH/g or less, and particularly preferably 40 mgKOH/g or more and 60 mgKOH/g or less. The amine value of the polymer dispersant can be measured by potentiometric titration in accordance with JIS K7237.

The polymer dispersant preferably has an acid value and/or amine value that meets the condition "(acid value) - (amine value) ≦ 50 mgKOH/g". The value expressed by (acid value) - (amine value) is more preferably 10 mgKOH/g or less, even more preferably -100 mgKOH/g or more and 5 mgKOH/g or less, particularly preferably -80 mgKOH/g or more and 0 mgKOH/g or less, especially preferably -70 mgKOH/g or more and less than -8 mgKOH/g, extremely preferably -60 mgKOH/g or more and less than -30 mgKOH/g, and most preferably -55 mgKOH/g or more and less than -45 mgKOH/g.

In the ink composition according to this embodiment, it is preferable that the titanium oxide content is 2% by mass or more and 25% by mass or less, and the value represented by the (acid value)-(amine value) of the polymer dispersant is 50 mgKOH/g or less, it is more preferable that the titanium oxide content is 2% by mass or more and 5% by mass or less, and the value represented by the (acid value)-(amine value) of the polymer dispersant is 10 mgKOH/g or less, it is even more preferable that the titanium oxide content is 2% by mass or more and 25% by mass or less, and the value represented by the (acid value)-(amine value) of the polymer dispersant is -100 mgKOH/g or more and 5 mgKOH/g or less, it is even more preferable that the titanium oxide content is 5% by mass or more and 20 .... It is particularly preferable that the titanium oxide content is 7% by mass or more and 18% by mass or less, and the value of the polymer dispersant expressed as (acid value)-(amine value) is -70 mgKOH/g or more and less than -8 mgKOH/g, it is extremely preferable that the titanium oxide content is 8% by mass or more and 16% by mass or less, and the value of the polymer dispersant expressed as (acid value)-(amine value) is -60 mgKOH/g or more and less than -30 mgKOH/g, and it is most preferable that the titanium oxide content is 8% by mass or more and 16% by mass or less, and the value of the polymer dispersant expressed as (acid value)-(amine value) is -55 mgKOH/g or more and -45 mgKOH/g or less.

Polymer dispersants are available as commercially available products. Examples of commercially available polymer dispersants include the DISPERBYK series manufactured by BYK Japan Co., Ltd. and the Joncryl series manufactured by BASF.

The content of the polymer dispersant in the ink composition according to this embodiment is preferably 0.03% by mass or more and 5% by mass or less, more preferably 0.1% by mass or more and 4% by mass or less, even more preferably 0.15% by mass or more and 3% by mass or less, and particularly preferably 0.2% by mass or more and 2.5% by mass or less.

<Flocculant>
The ink composition according to the present embodiment contains an aggregating agent. The aggregating agent is not particularly limited as long as it can aggregate the anionic components. Examples of the aggregating agent include polyvalent metal salts and cationic resins.

[Polyvalent metal salt]
The polyvalent metal salt is composed of a divalent or higher polyvalent metal ion and at least one acidic compound selected from an organic acidic compound and an inorganic acidic compound that binds to the polyvalent metal ion, and is soluble in water. The polyvalent metal salt may be one that is generated in the ink composition by adding a hydroxide of the polyvalent metal ion and an organic acidic compound to the ink composition. The polyvalent metal salt may be an anhydrous salt or a hydrate.

Examples of polyvalent metal ions include calcium ions, magnesium ions, aluminum ions, titanium ions, iron (II) ions, iron (III) ions, cobalt ions, nickel ions, copper ions, zinc ions, barium ions, and strontium ions. Among these polyvalent metal ions, calcium ions, magnesium ions, nickel ions, zinc ions, and aluminum ions are preferred from the viewpoint of aggregation rate.

The organic acidic compound is not particularly limited as long as it is an organic compound having at least one acidic group. Examples of the acidic group include a phosphate group, a phosphonic acid group, a phosphinic acid group, a sulfate group, a sulfonic acid group, a sulfinic acid group, and a carboxy group. Among these acidic groups, from the viewpoint of aggregation rate, the phosphate group and the carboxy group are preferred, and the carboxy group is more preferred. Examples of organic compounds having a carboxy group (organic carboxylic acid) include polyacrylic acid, acetic acid, glycolic acid, propionic acid, lactic acid (preferably DL-lactic acid), isobutyric acid, L-aspartic acid, gluconic acid, oxalic acid, malonic acid, malic acid (preferably DL-malic acid), maleic acid, ascorbic acid, succinic acid, glutaric acid, fumaric acid, citric acid, tartaric acid, phthalic acid, 4-methylphthalic acid, lactic acid, D-isoascorbic acid, itaconic acid, levulinic acid, mandelic acid, 1,4-cyclohexanedicarboxylic acid, tannic acid, 3-ethoxypropionic acid, 2,2-bis(hydroxymethyl)propionic acid, and derivatives thereof. Among these organic acidic compounds, 2,2-bis(hydroxymethyl)propionic acid, 1,4-cyclohexanedicarboxylic acid, L-aspartic acid, and gluconic acid are preferred because of their excellent color development properties, and 2,2-bis(hydroxymethyl)propionic acid and 1,4-cyclohexanedicarboxylic acid are more preferred because of their excellent inkjet ejection properties and storage stability. In other words, as a polyvalent metal salt composed of a polyvalent metal ion and an organic acidic compound, an organic acid salt generated from a hydroxide of a polyvalent metal ion (e.g., calcium hydroxide) and 2,2-bis(hydroxymethyl)propionic acid, 1,4-cyclohexanedicarboxylic acid, L-aspartic acid, or gluconic acid is preferred.

Examples of inorganic acidic compounds include hydrochloric acid, bromic acid, nitric acid, and sulfuric acid. Among these inorganic acidic compounds, nitric acid is preferred because it is less corrosive to the metal components that make up the inkjet head and printer. In other words, as a polyvalent metal salt composed of a polyvalent metal ion and an inorganic acidic compound, a nitrate of a polyvalent metal (e.g., calcium) is preferred.

When the ink composition according to this embodiment contains a polyvalent metal salt as an aggregating agent, the content of the polyvalent metal salt relative to the titanium oxide content is preferably 1% by mass or more and 70% by mass or less, more preferably 5% by mass or more and 50% by mass or less, even more preferably 8% by mass or more and 45% by mass or less, particularly preferably 10% by mass or more and less than 40% by mass, and extremely preferably more than 10% by mass or more and 30% by mass or less.

Furthermore, when the ink composition according to this embodiment contains a polyvalent metal salt as an aggregating agent, the content of the polyvalent metal salt in the ink composition is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.5% by mass or more and 10% by mass or less, even more preferably 0.7% by mass or more and 7% by mass or less, particularly preferably 0.9% by mass or more and 6% by mass or less, especially preferably 1.2% by mass or more and less than 6% by mass, extremely preferably 1.5% by mass or more and less than 5% by mass, and most preferably 2% by mass or more and 4% by mass or less.

[Cationic resin]
Examples of the cationic resin include those having a constituent unit derived from a vinyl monomer having a quaternary ammonium salt group and having a mass average molecular weight of from 1500 to 25000. From the viewpoints of the dispersion stability and inkjet dischargeability of titanium oxide, the mass average molecular weight of the cationic resin is preferably from 1500 to 23000, more preferably from 3000 to 22000, and even more preferably from 5000 to 22000.

Examples of vinyl monomers having a quaternary ammonium base include diallyldimethylammonium salt, methacryloyloxyethyltrimethylammonium salt, and methacryloyloxyethyldimethylbenzylammonium salt.

The proportion of constituent units derived from vinyl monomers having a quaternary ammonium base in the cationic resin is preferably 30 mol% or more, more preferably 45 mol% or more, and even more preferably 80 mol% or more.

When the ink composition according to this embodiment contains a cationic resin as an aggregating agent, the content of the cationic resin relative to the titanium oxide content is preferably 0.5% by mass or more and 18% by mass or less, more preferably 1% by mass or more and 16% by mass or less, even more preferably 2% by mass or more and 14% by mass or less, particularly preferably 3% by mass or more and 12% by mass or less, and extremely preferably 4% by mass or more and 10% by mass or less.

In addition, when the ink composition according to this embodiment contains a cationic resin as an aggregating agent, the content of the cationic resin in the ink composition is preferably 0.1% by mass or more and 4.5% by mass or less, more preferably 0.15% by mass or more and 3.5% by mass or less, even more preferably 0.2% by mass or more and 3% by mass or less, particularly preferably 0.3% by mass or more and 2.5% by mass or less, especially preferably 0.35% by mass or more and 2% by mass or less, extremely preferably 0.4% by mass or more and 1.8% by mass or less, and most preferably 0.5% by mass or more and 1.5% by mass or less.

<Water>
The ink composition according to this embodiment may contain water. As the water, water containing a small amount of impurities such as metal ions, that is, ion-exchanged water, distilled water, or the like, is preferable.

When the ink composition according to this embodiment contains water, the water content in the ink composition is preferably 55% by mass or more and 90% by mass or less, and more preferably 60% by mass or more and 85% by mass or less.

<Ink Preparation Agents>
The ink composition according to this embodiment may further contain ink preparation agents in addition to the above-mentioned components, such as a resin emulsion, an organic solvent, a viscosity adjuster, a surfactant, a preservative, an antifungal agent, a pH adjuster, a chelating agent, an antirust agent, a water-soluble ultraviolet absorber, an antioxidant, and an antifoaming agent.

[Resin emulsion]
The resin emulsion is different from the above-mentioned polymer dispersant, and preferably has an acid value of less than 50 mgKOH/g. By including a resin emulsion having an acid value of less than 50 mgKOH/g in the ink composition, the viscosity of the ink composition can be adjusted to a suitable range, and there is a tendency that a printed image with extremely little graininess can be realized. As the resin emulsion, at least one selected from a polymer emulsion and a wax emulsion is preferable.

The method for preparing the resin emulsion is not particularly limited. Examples include a method in which the resin is mechanically finely divided in an aqueous medium and dispersed; a method in which the resin emulsion is prepared by emulsion polymerization, dispersion polymerization, suspension polymerization, or the like. Emulsion polymerization can be carried out using an emulsifier or in a soap-free manner. An example of a method for preparing the resin emulsion is the method disclosed in Production Example 1 of JP-A-2000-336292. The resin content of the resin emulsion is preferably 20% by mass or more and 50% by mass or less.

Examples of polymer emulsions include emulsions containing urethane-based, polyester-based, acrylic-based, vinyl acetate-based, vinyl chloride-based, styrene-acrylic-based, acrylic-silicone-based, and styrene-butadiene-based polymers. Among these, emulsions of polymers selected from urethane-based, acrylic-based, and styrene-butadiene-based are preferred, with acrylic polymer emulsions being more preferred.

Commercially available polymer emulsions include, for example, urethane-based polymer emulsions such as U-coat UX-320 (acid value: 10 mg KOH/g) manufactured by Sanyo Chemical Industries, Ltd., and WBR-016U (acid value: 7 mg KOH/g) and WBR-2101 (acid value: 10 mg KOH/g) manufactured by Taisei Fine Chemical Co., Ltd.; polyester-based polymer emulsions such as Vylonal MD-1480 (acid value: 3 mg KOH/g), Vylonal MD-1985 (acid value: 2 mg KOH/g), and Vylonal MD-2000 (acid value: 2 mg KOH/g) manufactured by Toyobo Co., Ltd.; vinyl acetate-based polymer emulsions such as Vinyblan 715 (acid value: 8 mg KOH/g) and Vinyblan 985 (acid value: 5 mg KOH/g) manufactured by Nissin Chemical Industry Co., Ltd.; and the like.

As the wax emulsion, a water-based wax emulsion is preferred. As the wax, natural wax and synthetic wax can be used. As the natural wax, for example, petroleum wax such as paraffin wax and microcrystalline wax; lignite wax such as montan wax; vegetable wax such as carnauba wax and candelilla wax; animal and vegetable wax such as beeswax and lanolin; etc. are listed. As the synthetic wax, for example, polyalkylene wax (preferably poly C2-C4 alkylene wax), oxidized polyalkylene wax (preferably poly C2-C4 alkylene wax), paraffin wax, etc. are listed. Among these synthetic waxes, at least one wax selected from polyethylene wax, polypropylene wax, oxidized polyethylene wax, oxidized polypropylene wax, and paraffin wax is preferred, and oxidized polyethylene wax is more preferred. In addition, the average particle size of the wax is preferably 50 nm or more and 5 μm or less, more preferably 100 nm or more and 1 μm or less, in order to prevent clogging of the inkjet head.

Commercially available wax emulsions include, for example, AQUACER 515 (acid value: 5 mg KOH/g) manufactured by BYK Japan Co., Ltd., and HYTEC E-6500 (acid value: 10 to 20 mg KOH/g) manufactured by Toho Chemical Industry Co., Ltd.

When the ink composition according to this embodiment contains a resin emulsion, the solid content of the resin emulsion in the ink composition is preferably 0.2% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less.

[Organic solvent]
The ink composition according to this embodiment may contain an organic solvent in order to adjust the penetrability into a recording medium, the viscosity, drying property, defoaming property, etc. of the ink composition. Examples of the organic solvent include carboxylic acid amides such as N,N-dimethylformamide and N,N-dimethylacetamide; lactams such as 2-pyrrolidone, N-methyl-2-pyrrolidone, and N-methylpyrrolidin-2-one; cyclic ureas such as 1,3-dimethylimidazolidin-2-one and 1,3-dimethylhexahydropyrimid-2-one; ketones, ketoalcohols, and carbonates such as acetone, 2-methyl-2-hydroxypentan-4-one, and ethylene carbonate; cyclic ethers such as tetrahydrofuran and dioxane; ethylene glycol, diethylene glycol, 1,2-propanediol, 1,3-propanediol, and the like. oligo- or polyalkylene glycols or thioglycols having a C2-C6 alkylene unit, such as butyl diol, 1,2-butylene glycol, 1,4-butylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, thiodiglycol, and dithiodiglycol; C3-C9 polyols (triols), such as glycerin, diglycerin, hexane-1,2,6-triol, and trimethylolpropane; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoallyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether (butyl carbitol), diethylene glycol monobutyl ether, diethylene glycol monopentyl ether, diethylene glycol monohexyl ether, diethylene glycol monophenyl ether, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol glycol ethers such as ethylene glycol monomethyl ether (preferably glycol ethers selected from the group consisting of C3-C10 mono-, di-, or triethylene glycol ethers, and C4-C13 mono-, di-, or tripropylene glycol ethers); C3-C9 alkanediols such as 1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,6-hexanediol, 2-methyl-2,4-pentanediol, 2-ethyl-1,3-hexanediol, and 2,4-diethyl-1,5-pentanediol; γ-butyrolactone, dimethyl sulfoxide, and the like. Among these, triethylene glycol, propylene glycol, glycerin, 1,4-butylene glycol, and 1,2-hexanediol are preferred, and triethylene glycol and glycerin are more preferred.

When the ink composition according to this embodiment contains an organic solvent, the content of the organic solvent in the ink composition is preferably 0.1% by mass or more and 40% by mass or less, more preferably 0.2% by mass or more and 35% by mass or less, even more preferably 1% by mass or more and 30% by mass or less, and particularly preferably 2% by mass or more and 20% by mass or less.

[Viscosity modifier]
The ink composition according to the present embodiment may contain a viscosity modifier. In industrial inkjet printers, the viscosity range of the ink composition that can be ejected is usually determined based on the specifications of the inkjet head mounted thereon. For this reason, it is preferable to add a viscosity modifier to the ink composition to adjust the viscosity to an appropriate range. The viscosity modifier is not particularly limited as long as it is a substance that can adjust the viscosity of the ink composition, except for the above-mentioned polymer dispersant, and known substances can be used. Specific examples thereof include organic solvents, hydrophilic resins, resin emulsions, and the like.

[Surfactant]
The ink composition according to this embodiment may contain a surfactant. Examples of the surfactant include anionic, cationic, nonionic, silicone, and fluorine-based surfactants. Among these, cationic surfactants and nonionic surfactants are preferred.

Examples of anionic surfactants include alkyl sulfocarboxylates, α-olefin sulfonates, polyoxyethylene alkyl ether acetates, polyoxyethylene alkyl ether sulfates, N-acylamino acids or salts thereof, N-acylmethyltaurines, alkyl sulfates polyoxyalkyl ether sulfates, alkyl sulfates polyoxyethylene alkyl ether phosphates, rosin acid soaps, castor oil sulfates, lauryl alcohol sulfates, alkylphenol phosphates, alkyl phosphates, alkylaryl sulfonates, diethyl sulfosuccinates, diethylhexyl sulfosuccinates, dioctyl sulfosuccinates, etc.

Examples of cationic surfactants include 2-vinylpyridine derivatives and poly-4-vinylpyridine derivatives.

Nonionic surfactants include, for example, ether-based surfactants such as polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene alkyl ethers having 4 to 18 carbon atoms (for example, polyoxyethylene butyl ether, polyoxyethylene lauryl ether, polyoxyethylene oleyl ether, polyoxyethylene 2-ethylhexyl ether, etc.), polyoxyethylene (di)styrenated phenyl ether (for example, Emulgen A-60, A-90, A-500 manufactured by Kao Corporation; DSP-9, DSP-12.5, TSP-7.5, KTSP-16, TSP-50 manufactured by Aoki Oil Industries Co., Ltd.); Examples of such esters include ethylene oleate, polyoxyethylene distearate, sorbitan laurate, sorbitan monostearate, sorbitan monooleate, sorbitan sesquioleate, polyoxyethylene monooleate, and polyoxyethylene stearate; acetylene glycols (or acetylene alcohols) such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, and 3,5-dimethyl-1-hexyne-3-ol (for example, Surfynol 420, 440, 465, and 485, Olfin STG, E1004, and E1040, all manufactured by Nissin Chemical Industry Co., Ltd.); and polyglycol ethers.

Examples of silicone surfactants include polyether-modified siloxane, polyether-modified polydimethylsiloxane, etc. Specific examples of commercially available products include Dynol 960 and 980 manufactured by Air Products Co., Ltd.; Silface SAG001, SAG002, SAG003, SAG005, SAG503A, SAG008, SAG009, and SAG010 manufactured by Nissin Chemical Industry Co., Ltd.; BYK-345, 347, 348, 349, 3450, 3451, and 3455 manufactured by BYK Additives & Instruments Co., Ltd.; and TEGO Twin 4000, TEGO Wet KL 245, 250, 260, 265, 270, and 280 manufactured by Evonik Tego Chemie Co., Ltd.

Examples of fluorine-based surfactants include perfluoroalkyl sulfonic acid compounds, perfluoroalkyl carboxylic acid compounds, perfluoroalkyl phosphate ester compounds, perfluoroalkyl ethylene oxide adducts, and polyoxyalkylene ether polymer compounds having perfluoroalkyl ether groups on the side chains. Specific examples of commercially available products include Capstone FS-30 and FS-31 manufactured by Chemours.

[Preservatives]
Examples of preservatives include compounds such as organic sulfur compounds, organic nitrogen sulfur compounds, organic halogen compounds, haloarylsulfone compounds, iodopropargyl compounds, haloalkylthio compounds, nitrile compounds, pyridine compounds, 8-oxyquinolines, benzothiazole compounds, isothiazolinone compounds, dithiols, pyridine oxide compounds, nitropropane compounds, organic tin compounds, phenol compounds, quaternary ammonium salt compounds, triazine compounds, thiazine compounds, anilides, adamantane compounds, dithiocarbamates, brominated indanone compounds, benzyl bromoacetate compounds, inorganic salt compounds, etc. Specific examples of commercially available products include Proxel GXL(S) and XL-2(S) manufactured by Arch Chemical Co., Ltd.

[Mold-proofing agent]
Examples of antifungal agents include sodium dehydroacetate, sodium benzoate, sodium pyridinethione-1-oxide, p-hydroxybenzoic acid ethyl ester, 1,2-benzisothiazolin-3-one, and salts thereof.

[pH adjuster]
Examples of pH adjusters include alkanolamines such as diethanolamine, triethanolamine, and N-methyldiethanolamine; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; ammonium hydroxide (aqueous ammonia); alkali metal carbonates such as lithium carbonate, sodium carbonate, and potassium carbonate; alkali metal salts of organic acids such as potassium acetate; and inorganic bases such as sodium silicate and disodium phosphate.

[Chelating Agent]
Examples of the chelating agent include disodium ethylenediaminetetraacetate, sodium nitrilotriacetate, sodium hydroxyethylethylenediaminetriacetate, sodium diethylenetriaminepentaacetate, sodium uracildiacetate, α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin.

[Rust inhibitor]
Examples of the rust inhibitor include acidic sulfite, sodium thiosulfate, ammonium thioglycolate, diisopropylammonium nitrite, pentaerythritol tetranitrate, and dicyclohexylammonium nitrite.

[Water-soluble UV absorber]
Examples of the water-soluble ultraviolet absorbent include sulfonated benzophenone compounds, benzotriazole compounds, salicylic acid compounds, cinnamic acid compounds, and triazine compounds.

[Antioxidants]
As the antioxidant, for example, various organic and metal complex-based discoloration inhibitors can be used. Examples of organic discoloration inhibitors include hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines, amines, indanes, chromans, alkoxyanilines, ascorbic acid, isoascorbic acid, chlorogenic acid, sulfur dioxide, catechins, dibutylhydroxytoluene, tocopherol, butylhydroxyanisole, and other compounds.

[Antifoaming agent]
Examples of the defoaming agent include acetylene-based defoaming agents such as Surfynol 104 series (104A, 104E, 104H, 104PA, 104PG-50), Surfynol DF110D, Surfynol AD01, and Surfynol MD-20 (all manufactured by Nissin Chemical Industry Co., Ltd.); DF-58 (manufactured by Nissin Chemical Industry Co., Ltd.), BYK-017, BYK-018, BYK-019, BYK-021, BYK-022, BYK-023, BYK-024, BYK-025, BYK-028, BYK-044, BYK-092, BYK-1610, BYK-1611 , BYK-1615, BYK-1617, BYK-1650, BYK1679, BYK-1719, BYK-1723, BYK-1724, BYK-1730, BYK-17 Silicone-based defoamers such as BYK-70, BYK-1781, BYK-1786, and BYK-1789 (all manufactured by BYK Corporation); mineral oil-based defoamers such as BYK-035, BYK-037, BYK-038, and BYK-1630 (all manufactured by BYK Corporation); higher alcohol derivatives such as Bisform CS, Bisform ECC, and Bisform TDI-1 (all manufactured by Nisshin Chemical Laboratory Co., Ltd.); and fatty acid derivatives such as Bisform TS-10 (manufactured by Nisshin Chemical Laboratory Co., Ltd.). As the defoamer, acetylene-based or silicone-based defoamers are preferred, and Surfynol DF-58 is more preferred.

<Method of preparing ink composition>
The method for preparing the ink composition according to this embodiment is not particularly limited, and may employ known preparation methods using a ball mill, a sand mill, an attritor, a basket mill, a roll mill, etc. Examples of the preparation method include a method of preparing an aqueous dispersion containing titanium oxide and a polymer dispersant, and adding water, a polyvalent metal salt, and, if necessary, an ink preparation agent to the aqueous dispersion and mixing them; a method of preparing an aqueous dispersion containing titanium oxide, a polymer dispersant, and a cationic resin, and adding water and, if necessary, an ink preparation agent to the aqueous dispersion and mixing them; and the like.

The ink composition according to this embodiment is preferably subjected to precision filtration using a membrane filter, glass filter paper, or the like. When performing precision filtration, a membrane filter, glass filter paper, or the like can be used. The pore size of the filter, etc., used when performing precision filtration is usually 0.5 μm or more and 20 μm or less, preferably 0.5 μm or more and 10 μm or less.

The pH of the ink composition according to this embodiment at 25°C is usually 5 or more and 11 or less, preferably 6 or more and 9 or less. The surface tension of the ink composition according to this embodiment at 25°C is usually 20 mN/m or more and 60 mN/m or less, preferably 25 mN/m or more and 50 mN/m or less. The viscosity of the ink composition according to this embodiment at 25°C is usually 2 mPa·s or more and 30 mPa·s or less, preferably 3 mPa·s or more and 15 mPa·s or less. The pH, surface tension, and viscosity of the ink composition can be appropriately adjusted with a pH adjuster, a surfactant, an organic solvent, etc.

The ink composition according to this embodiment has excellent dispersion stability of titanium oxide and inkjet ejection properties, and also has excellent storage stability, redispersibility, and circulation stability of the ink composition. The ink composition according to this embodiment has little coating unevenness during image formation and has excellent image forming properties. The image recorded using the ink composition according to this embodiment has a large dot diameter, a good dot shape (e.g., the coffee stain phenomenon is suppressed), little graininess, good hue and color development, and excellent fastness properties such as abrasion resistance, heat abrasion resistance, light resistance, heat resistance, and oxidation gas resistance (e.g., ozone gas resistance). The image recorded using an ink set including the ink composition according to this embodiment has little inter-color bleeding, little graininess in the printed parts of secondary or higher colors, and excellent solid uniformity in the printed parts of secondary or higher colors.

The ink composition according to this embodiment can be used in various printing processes. For example, it is suitable for writing instruments, various printing processes, information printing, textile printing, etc., and is particularly preferably used in inkjet printing.

<Ink set>
A first aspect of the ink set according to this embodiment includes the ink composition according to this embodiment described above, and a white ink composition containing a white colorant and an anionic dispersant. A second aspect of the ink set according to this embodiment includes the ink composition according to this embodiment described above, and a non-white ink composition containing a non-white colorant and an anionic dispersant.

<White Ink Composition>
The white ink composition contains a white colorant and an anionic dispersant. Each component contained in the white ink composition will be described in detail below. Each of the following components may be used alone or in combination of two or more.

[White color material]
The white ink composition contains a white coloring material. The white coloring material may be titanium oxide or a white coloring material other than titanium oxide.

It is preferable that the white ink composition be such that the L ^* value of a coating obtained by coating the white ink composition is at least 68. An example of a coating for measuring the L ^* value is a coating obtained by coating 50 μL of the white ink composition onto liner paper (K liner, manufactured by Daio Paper Corporation, weighing: 170 g/m ² , brown liner with an L ^* value of 63) described below using a bar coater #8.

The content of the white colorant in the white ink composition is preferably 1% by mass or more and 30% by mass or less, more preferably 2% by mass or more and 25% by mass or less, even more preferably 5% by mass or more and 20% by mass or less, and particularly preferably 7% by mass or more and 18% by mass or less.

[Anionic dispersant]
The white ink composition contains an anionic dispersant. Examples of the anionic dispersant include copolymers composed of at least two types of monomers (preferably at least one of which is a hydrophilic monomer) selected from monomers such as styrene and its derivatives; vinyl naphthalene and its derivatives; aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylic acids; (meth)acrylic acid and its derivatives; maleic acid and its derivatives; itaconic acid and its derivatives; fumaric acid and its derivatives; vinyl acetate, vinyl alcohol, vinyl pyrrolidone, acrylamide, and their derivatives. Examples of such copolymers include styrene-(meth)acrylic acid copolymers, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymers, (meth)acrylic acid ester-(meth)acrylic acid copolymers, polyethylene glycol (meth)acrylate-(meth)acrylic acid copolymers, and styrene-maleic acid copolymers. Types of copolymers include block copolymers, random copolymers, and graft copolymers. These copolymers may be in the form of a salt.

The mass average molecular weight of the anionic dispersant is preferably 2,500 or more and 100,000 or less, and more preferably 7,000 or more and 25,000 or less. The acid value of the anionic dispersant is preferably 50 mgKOH/g or more and 300 mgKOH/g or less, more preferably 80 mgKOH/g or more and 275 mgKOH/g or less, and even more preferably 80 mgKOH/g or more and 250 mgKOH/g or less.

Preferred anionic dispersants include the following two types of anionic dispersants:

(First Anionic Dispersant (Ac))
The first anionic dispersant (Ac) is polyacrylic acid; polymethacrylic acid; or a copolymer of acrylic acid or methacrylic acid with at least two monomers selected from maleic acid and a sulfonic acid group-containing vinyl monomer. As the sulfonic acid group-containing vinyl monomer, an unsaturated sulfonic acid monomer is preferable, and examples thereof include vinyl sulfonic acid, styrene sulfonic acid, and 2-acrylamido-2-methylpropane sulfonic acid. These sulfonic acid group-containing vinyl monomers may form a salt. Examples of the salt include a sodium salt and a potassium salt.

The mass average molecular weight of the first anionic dispersant (Ac) is preferably 2,500 or more and 90,000 or less, and more preferably 4,000 or more and 60,000 or less.

The anionic group contained in the first anionic dispersant (Ac) may be neutralized with a base. Examples of bases include organic amines such as ammonia, dimethylaminoethanol, and triethylamine; and alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide. Among these, sodium hydroxide and potassium hydroxide are preferred from the viewpoint of dispersion stability. From the viewpoint of dispersion stability, the degree of neutralization of the first anionic dispersant (Ac) is preferably 30 mol% or more and 150 mol% or less. The degree of neutralization when neutralized with the theoretical equivalent of the acid value of the first anionic dispersant (Ac) is taken as 100%.

The first anionic dispersant (Ac) may be a commercially available product or may be synthesized. Commercially available polymethacrylic acid includes, for example, polymethacrylic acid manufactured by Sigma-Aldrich. Commercially available polyacrylic acid includes, for example, polyacrylic acid manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.; Aron T-50 (mass average molecular weight: 6000), Aron A-10SL (mass average molecular weight: 5000), Aron A-30SL (mass average molecular weight: 6000) manufactured by Toa Gosei Co., Ltd.; Aqualic DL and Aqualic YS manufactured by Nippon Shokubai Co., Ltd.; and the like. Commercially available sulfonic acid group-containing polyacrylic acid includes, for example, Aron A-6012 (mass average molecular weight: 10000) manufactured by Toa Gosei Co., Ltd.; Aqualic GL and Aqualic LS manufactured by Nippon Shokubai Co., Ltd.

When the white ink composition contains a first anionic dispersant (Ac), its content is preferably 0.3% by mass or more and 20% by mass or less, more preferably 0.3% by mass or more and 10% by mass or less, even more preferably 0.3% by mass or more and 5% by mass or less, and particularly preferably 0.3% by mass or more and less than 1.0% by mass, relative to the content of the white colorant.

Second Anionic Dispersant (StAc)
The second anionic dispersant (StAc) has, for example, 35% by mass or more and 85% by mass or less of constitutional units derived from at least one monomer selected from styrene and α-methylstyrene, and 8% by mass or more and 40% by mass or less of constitutional units derived from (meth)acrylic acid, and at least a part of the anionic groups is neutralized, and the degree of neutralization is 40 mol% or more and 150 mol% or less. From the viewpoint of dispersibility, the proportion of constitutional units derived from at least one monomer selected from styrene and α-methylstyrene is preferably 38% by mass or more and 78% by mass or less, and more preferably 38% by mass or more and 75% by mass or less. From the viewpoint of dispersibility, the proportion of constitutional units derived from (meth)acrylic acid is preferably 10% by mass or more and 35% by mass or less, and more preferably 10% by mass or more and 32% by mass or less.

The second anionic dispersant (StAc) may have structural units derived from other monomers. Examples of other monomers include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth), t-butyl (meth)acrylate, isoamyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cetyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, ethyl ... linear or branched alkyl (meth)acrylates such as uryl (meth)acrylate, tridecyl (meth)acrylate, isomyristyl (meth)acrylate, stearyl (meth)acrylate, and isostearyl (meth)acrylate; cyclic alkyl (meth)acrylates such as cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentenyl (meth)acrylate, and isobornyl (meth)acrylate; tetrahydrofuran (meth)acrylate, tridecyl (meth)acrylate, isomyristyl (meth)acrylate, tridecyl (meth)acrylate, isomyristyl (meth)acrylate, and isostearyl (meth)acrylate; (Meth)acrylates having a heterocyclic ring such as furfuryl (meth)acrylate and 3-methyl-3-oxetanyl (meth)acrylate; (meth)acrylates having an aromatic ring such as benzyl (meth)acrylate and phenoxyethyl (meth)acrylate; 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-methoxypropyl (meth)acrylate, diethylene glycol monomethyl ether (meth)acrylate, diethylene glycol monoethyl ether (meth)acrylate, diethylene glycol mono-2-ethylhexyl ether (meth)acrylate, dipropylene glycol monomethyl ether (meth)acrylate, triethylene glycol monomethyl ether (meth)acrylate, triethylene glycol monoethyl ether (meth)acrylate, tripropylene glycol monomethyl ether (meth)acrylate, tetraethylene glycol monomethyl ether (meth)acrylate, polyethylene glycol monomethyl ether (meth)acrylate (poly)alkylene glycol monoalkyl ether (meth)acrylates such as polypropylene glycol monomethyl ether (meth)acrylate, polyethylene glycol monolauryl ether (meth)acrylate, polyethylene glycol monostearyl ether (meth)acrylate, and octoxypolyethylene glycol-polypropylene glycol (meth)acrylate; phenoxyethyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxytetraethylene glycol (meth)acrylate, phenoxyhexaethylene glycol (meth)acrylate, phenoxypolyethylene glycol (meth)acrylate, paracumylphenoxyethyl (meth)acrylate, paracumylphenoxyethylene glycol (meth)acrylate, paracumylphenoxypolyethylene glycol (meth)acrylate, nonylphenoxypolyethylene glycol (meth)acrylate, nonylphenoxypolypropylene glycol (meth)acrylate, nonylphenoxypoly Examples of such compounds include (poly)alkylene glycol (meth)acrylates having an aromatic ring, such as (ethylene glycol-propylene glycol) (meth)acrylate; fluoroalkyl (meth)acrylates, such as trifluoroethyl (meth)acrylate, octafluoropentyl (meth)acrylate, perfluorooctylethyl (meth)acrylate, and tetrafluoropropyl (meth)acrylate; silicone macromers, such as (meth)acryloxy-modified polydimethylsiloxane; N-substituted (meth)acrylamides, such as (meth)acrylamide, dimethyl (meth)acrylamide, N,N-diethyl (meth)acrylamide, N-isopropyl (meth)acrylamide, diacetone (meth)acrylamide, and acryloylmorpholine; nitriles, such as (meth)acrylonitrile; vinyl ethers, such as ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, and isobutyl vinyl ether; and fatty acid vinyls, such as vinyl acetate and vinyl propionate.

The second anionic dispersant (StAc) may be a copolymer of polyalkylene glycol (meth)acrylate and (meth)acrylic acid. Examples of polyalkylene glycol (meth)acrylate include methoxypolyethylene glycol mono(meth)acrylate, octoxypolyethylene glycol-polypropylene glycol mono(meth)acrylate, lauroxypolyethylene glycol mono(meth)acrylate, stearoxypolyethylene glycol mono(meth)acrylate, phenoxypolyethylene glycol mono(meth)acrylate, phenoxypolyethylene glycol-polypropylene glycol mono(meth)acrylate, nonylphenoxypolypropylene glycol mono(meth)acrylate, nonylphenoxypoly(ethylene glycol-propylene glycol) mono(meth)acrylate, etc.

When the white ink composition contains a second anionic dispersant (StAc), its content is preferably from 0.5% to 18% by mass, more preferably from 0.5% to 10% by mass, even more preferably from 0.5% to 5% by mass, and particularly preferably from 0.5% to 2.0% by mass, relative to the content of the white colorant.

(Method of preparing anionic dispersants)
The anionic dispersant can be prepared by copolymerizing a monomer mixture by a known polymerization method. Among the known polymerization methods, a solution polymerization method is preferred from the viewpoint of controlling the molecular weight.

Solvents used in the solution polymerization method include water; aliphatic alcohols having 1 to 3 carbon atoms, ketones having 3 to 8 carbon atoms, esters such as ethyl acetate, and mixed solvents of one or more of these with water; etc. Among these, water is preferred.

As the polymerization initiator used in the solution polymerization method, persulfates are preferred, and ammonium persulfates are more preferred. From the viewpoint of the molecular weight distribution of the resulting dispersant, the amount of polymerization initiator used is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.1 parts by mass or more, per 100 parts by mass of the total amount of monomers. Furthermore, the amount of polymerization initiator used is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 2 parts by mass or less, per 100 parts by mass of the total amount of monomers.

As chain transfer agents used in the solution polymerization method, mercaptans are preferred, and 2-mercaptoethanol is more preferred. From the viewpoint of molecular weight distribution of the resulting dispersant, the amount of chain transfer agent used is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.8 parts by mass or more, per 100 parts by mass of the total amount of monomers. Furthermore, the amount of chain transfer agent used is preferably 8 parts by mass or less, more preferably 6 parts by mass or less, and even more preferably 4 parts by mass or less, per 100 parts by mass of the total amount of monomers.

Preferred polymerization conditions vary depending on the type of polymerization initiator, etc. The polymerization temperature is preferably 50°C or higher and 90°C or lower, and the polymerization time is preferably 1 hour or higher and 20 hours or lower. When persulfate is used as the polymerization initiator, the polymerization temperature is preferably 70°C or higher, more preferably 75°C or higher, from the viewpoint of reactivity. When persulfate is used as the polymerization initiator, the polymerization temperature is preferably 85°C or lower, more preferably 83°C or lower, from the viewpoint of the molecular weight distribution of the resulting dispersant. The polymerization atmosphere is preferably an inert gas atmosphere such as nitrogen gas or argon gas.

After the polymerization reaction is completed, the anionic dispersant can be isolated from the reaction solution by known methods such as reprecipitation and solvent distillation. After isolating the anionic dispersant, it is preferable to remove unreacted monomers, etc., by reprecipitation, membrane separation, chromatography, extraction, etc.

The anionic dispersant obtained has constituent units derived from anionic group-containing monomers, and can be ionized by neutralizing the anionic groups to make it water-soluble. Neutralizing agents used for neutralization include ammonia; organic amines such as ethylamine, diethylamine, trimethylamine, triethylamine, and triethanolamine; and alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide. Among these neutralizing agents, from the viewpoint of redispersibility, alkali metal hydroxides are preferred, and sodium hydroxide is more preferred.

[water]
The white ink composition may contain water. The water is preferably water with a low content of impurities such as metal ions, i.e., ion-exchanged water, distilled water, or the like.

If the white ink composition contains water, the water content in the white ink composition is preferably 55% by mass or more and 90% by mass or less, and more preferably 60% by mass or more and 85% by mass or less.

[Other ingredients]
The white ink composition may further contain an ink preparation agent in addition to the above-mentioned components. Examples of the ink preparation agent include those exemplified in the ink composition according to the present embodiment described above. It is preferable that the white ink composition does not contain the above-mentioned aggregating agent.

[Method of preparing white ink composition]
The method for preparing the white ink composition is not particularly limited, and any known preparation method can be used. Examples of the preparation method include a method in which an aqueous dispersion containing a white colorant and an anionic dispersant is prepared, and water and, if necessary, an ink preparation agent are added to the aqueous dispersion and mixed.

As with the ink composition according to the present embodiment described above, it is preferable to filter the white ink composition using a membrane filter, glass filter paper, or the like.

<Non-white ink composition>
The non-white ink composition contains a non-white colorant and an anionic dispersant. Each component contained in the non-white ink composition will be described in detail below. Each of the following components may be used alone or in combination of two or more.

[Non-white coloring materials]
The non-white ink composition contains a non-white coloring material. Examples of the non-white coloring material include pigments, disperse dyes, and solvent dyes. Among these, pigments are preferred from the viewpoint of image fastness such as light fastness and water fastness of the recorded image. Examples of the pigment include inorganic pigments, organic pigments, and extender pigments.

Examples of inorganic pigments include carbon black, metal oxides, metal hydroxides, metal sulfides, metal ferrocyanides, and metal chlorides.

When a black ink composition is used, the inorganic pigment is preferably carbon black such as furnace black, lamp black, acetylene black, channel black, etc. Commercially available carbon black products include, for example, Raven 760 ULTRA, Raven 780 ULTRA, Raven 790 ULTRA, Raven 1060 ULTRA, Raven 1080 ULTRA, Raven 1170, Raven 1190 ULTRA II, Raven 1200, Raven 1250, Raven 1255, Raven 1500, Raven 2000, Raven 2500U ULTRA, Raven 3500, Raven 5000 ULTRA II, Raven 5250, Raven 5750, Raven 7000 (all manufactured by Columbia Carbon); Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, Regal 1330R, Regal 1400R, Regal 1500R, Regal 1600R, Regal 1700R, Regal 1800R, Regal 1900R, Regal 2000R, Regal 2100R, Regal 2200R, Regal 2300R, Regal 2400R, Regal 2500R, Regal 2600R, Regal 2700R, Regal 2800R, Regal 2900R, Regal 3000R, Regal 3100R, Regal 3200R, Regal 3300R, Regal 3400R, Regal 3500R, Regal 3600R, Regal 3700R, Regal 3800R, Regal 39 ... gal 1660R, Mogul L (manufactured by Cabot); Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Printex 140V, Special Black 4, Special Black 4A, Special Black 5, Special Black 6, Nerox 305, Nerox 505, Nerox 510, Nerox 600, Nerox 605, NIPex 180IQ, NIPex 170IQ, NIPex 160IQ, NIPex 150IQ (all manufactured by Orion Engineered Carbons); MA7, MA8, MA100, MA600, MCF-88, No. 25, No. 33, No. 40, No. 47, No. 52, No. 900, No. 2300 (all manufactured by Mitsubishi Chemical Corporation); and the like.

Examples of organic pigments include various pigments such as azo, disazo, phthalocyanine, quinacridone, isoindolinone, dioxazine, perylene, perinone, thioindigo, anthraquinone, and quinophthalone. Specific examples of organic pigments include yellow pigments such as C.I. Pigment Yellow 1, 2, 3, 12, 13, 14, 16, 17, 24, 55, 73, 74, 75, 83, 93, 94, 95, 97, 98, 108, 114, 128, 129, 138, 139, 150, 151, 154, 155, 180, 185, 193, 199, 202, and 213; C.I. Pigment Red 5, 7, 12, 48, 48:1, 57, 88, 112, 122, 123, 146, 149, 150, 166, 168, 177, 178, 179, 184, 185, 202, 206, 207, 254, 255, 257, 260, 264, 272 and other red pigments; C.I. Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 22, 25, 60, 66, 80 and other blue pigments; C.I. Pigment Violet 19, 23, 29, 37, 38, 50 and other violet pigments; C.I. Orange pigments such as C.I. Pigment Orange 13, 16, 68, 69, 71, and 73; green pigments such as C.I. Pigment Green 7, 36, and 54; and black pigments such as C.I. Pigment Black 1.

Examples of extender pigments include silica, calcium carbonate, talc, clay, barium sulfate, and white carbon.

The non-white colorant preferably contains a pigment, and more preferably consists of a pigment. As the pigment, it is preferable to use at least one selected from the group consisting of carbon black; C.I. Pigment Blue 15:3, 15:4; C.I. Pigment Yellow 74, 155; C.I. Pigment Red 122, 150; and C.I. Pigment Violet 19; and it is preferable to use at least one selected from the group consisting of carbon black; C.I. Pigment Blue 15:4; C.I. Pigment Yellow 155; C.I. Pigment Red 122, 150; and C.I. It is more preferable to use at least one selected from the group consisting of Pigment Violet 19.

Specific examples of disperse dyes include C.I. Disperse Yellow 3, 4, 5, 7, 9, 13, 24, 30, 33, 34, 42, 44, 49, 50, 51, 54, 56, 58, 60, 63, 64, 66, 68, 71, 74, 76, 79, 82, 83, 85, 86, 88, 90, 91, 93, 98, 99, 100, 104, 114, 116, 118, 119, 1 Yellow dyes such as C.I. 22, 124, 126, 135, 140, 141, 149, 160, 162, 163, 164, 165, 179, 180, 184:1, 182, 183, 184, 186, 192, 198, 199, 201, 202, 204, 210, 211, 215, 216, 218, 224, 231, 232, 241; Orange dyes such as Disperse Orange 1, 3, 5, 7, 11, 13, 17, 20, 21, 25, 26, 29, 30, 31, 32, 33, 37, 38, 42, 43, 44, 45, 47, 48, 50, 53, 54, 55, 56, 57, 58, 59, 61, 66, 76, 78, 80, 89, 90, 91, 93, 96, 97, 119, 127, 130, 139, 142; Disperse Red 1, 4, 5, 7, 11, 12, 13, 15, 17, 27, 43, 44, 50, 52, 53, 54, 55, 56, 58, 59, 60, 65, 72, 73, 74, 75, 76, 78, 81, 82, 86, 88, 90, 91, 93, 96, 103, 105, 106, 107, 108, 110, 111, 113, 117, 118, 121, 122, 126, 127, 128, 131, 132, 134, 135, 137, 143, 145, 146, 151, 152, 15 Red dyes such as C.I.3, 154, 157, 159, 164, 167, 169, 177, 179, 181, 183, 184, 185, 188, 189, 190, 191, 192, 200, 201, 202, 203, 205, 206, 207, 210, 221, 224, 225, 227, 229, 239, 240, 257, 258, 277, 278, 279, 281, 288, 289, 298, 302, 303, 310, 311, 312, 320, 324, 328, 343, 362, 364; Violet dyes such as C.I. Disperse Violet 1, 4, 8, 17, 23, 26, 27, 28, 31, 33, 35, 36, 38, 40, 43, 46, 48, 50, 51, 52, 56, 57, 59, 61, 63, 69, 77; green dyes such as C.I. Disperse Green 6:1, 9; brown dyes such as C.I. Disperse Brown 1, 2, 4, 9, 13, 19, 26, 27; Disperse Blue 3, 7, 9, 14, 16, 19, 20, 26, 27, 35, 43, 44, 54, 55, 56, 58, 60, 62, 64, 71, 72, 73, 75, 79, 81, 82, 83, 87, 91, 93, 94, 95, 96, 102, 106 , 108, 112, 113, 115, 118, 120, 122, 125, 128, 130, 139, 141, 142, 143, 146, 148, 149, 153, 1 54, 158, 165, 167, 171, 173, 174, 176, 181, 183, 185, 186, 187, 189, 197, 198, 200, 201, 205, 207, 211, 214, 224, 225, 257, 259, 267, 268, 270, 284, 285, 287, 288, 291, 293, 295, 297, 301, 315, 330, 332, 333, 334, 343, 359, 360 and other blue dyes; C.I. Disperse Black 1, 3, 10, 24 and other black dyes; and the like.

Specific examples of solvent dyes include yellow dyes such as C.I. Solvent Yellow 2, 6, 14, 16, 21, 25, 29, 30, 33, 51, 56, 77, 80, 82, 88, 89, 93, 116, 150, 160:1, 163, 179, etc.; orange dyes such as C.I. Solvent Orange 1, 2, 14, 45, 60, etc.; red dyes such as C.I. Solvent Red 1, 3, 7, 8, 9, 18, 19, 23, 24, 25, 27, 49, 100, 109, 121, 122, 125, 127, 130, 132, 135, 218, 225, 230, etc.; Violet dyes such as C.I. Solvent Violet 13 and 31; green dyes such as C.I. Solvent Green 3; brown dyes such as C.I. Solvent Brown 3 and 5; blue dyes such as C.I. Solvent Blue 2, 11, 14, 24, 25, 35, 36, 38, 48, 55, 59, 63, 67, 68, 70, 73, 83, 105, 111, and 132; black dyes such as C.I. Solvent Black 3, 5, 7, 23, 27, 28, 29, and 34.

It is preferable that the L ^* value of a coated product of the non-white ink composition is less than 68. An example of a coated product for measuring the L ^* value is a coated product obtained by applying 50 μL of the non-white ink composition to a liner paper (K liner, manufactured by Daio Paper Corporation, weighing: 170 g/m ² , L ^* value: 63, brown liner) described below using a bar coater #8.

The content of the non-white colorant in the non-white ink composition is preferably 0.5% by mass or more and 30.0% by mass or less, more preferably 1.0% by mass or more and 10.0% by mass or less, even more preferably 2.0% by mass or more and 7.0% by mass or less, and particularly preferably 2.0% by mass or more and 6.0% by mass or less.

[Anionic dispersant]
The non-white ink composition contains an anionic dispersant. Examples of the anionic dispersant include copolymers composed of at least two types of monomers (preferably at least one of which is a hydrophilic monomer) selected from monomers such as styrene and its derivatives, vinyl naphthalene and its derivatives, aliphatic alcohol esters of α,β-ethylenically unsaturated carboxylic acids, (meth)acrylic acid and its derivatives, maleic acid and its derivatives, itaconic acid and its derivatives, fumaric acid and its derivatives, vinyl acetate, vinyl alcohol, vinyl pyrrolidone, acrylamide, and their derivatives. Examples of such copolymers include styrene-(meth)acrylic acid copolymers, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymers, (meth)acrylic acid ester-(meth)acrylic acid copolymers, polyethylene glycol (meth)acrylate-(meth)acrylic acid copolymers, and styrene-maleic acid copolymers. Among these, styrene-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymer, (meth)acrylic acid ester-(meth)acrylic acid copolymer, and polyethylene glycol (meth)acrylate-(meth)acrylic acid copolymer are preferred, styrene-(meth)acrylic acid copolymer, styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymer, and (meth)acrylic acid ester-(meth)acrylic acid copolymer are more preferred, (meth)acrylic acid ester-(meth)acrylic acid copolymer is even more preferred, and methacrylic acid ester-methacrylic acid copolymer is particularly preferred. Examples of the copolymer include block copolymers, random copolymers, and graft copolymers. These copolymers may be in the form of a salt.

Anionic dispersants can be obtained commercially or synthesized.

Commercially available anionic dispersants include, for example, Joncyrl 62 (mass average molecular weight: 13,000, acid value: 213 mg KOH/g), 67 (mass average molecular weight: 12,500, acid value: 213 mg KOH/g), 68 (mass average molecular weight: 13,000, acid value: 200 mg KOH/g), 586 (mass average molecular weight: 4,600, acid value: 108 mg KOH/g), 611 (mass average molecular weight: 8,100, acid value: 53 mg KOH/g), and 678 (mass average molecular weight: 8,600, acid value: 2 15mgKOH/g), 679 (mass average molecular weight: 9000, acid value: 200mgKOH/g), 680 (mass average molecular weight: 4500, acid value: 215mgKOH/g), 682 (mass average molecular weight: 1700, acid value: 238mgKOH/g), 683 (mass average molecular weight: 8000, acid value: 165mgK OH/g), 690 (mass average molecular weight: 16,500, acid value: 240 mgKOH/g), 693 (mass average molecular weight: 6,000, acid value: 205 mgKOH/g), 819 (mass average molecular weight: 14,500, acid value: 7 5mgKOH/g); Hirose manufactured by Seiko PMC Co., Ltd. NS-2092 (mass average molecular weight: 9000, acid value: 200mgKOH/g), RS-1191 (mass average molecular weight: 6500, acid value: 280mgKOH/g), VS-1047 (mass average molecular weight: 10000, acid value: 240mgKO H/g), BS-1291 (mass average molecular weight: 10,000, acid value: 210 mgKOH/g), ZS-1417 (mass average molecular weight: 14,000, acid value: 156 mgKOH/g), US-1071 (mass average molecular weight: 1 Examples of styrene-acrylic copolymers include styrene-acrylic copolymers such as ...

A preferred example of an anionic dispersant obtained by synthesis is the A-B block copolymer disclosed in WO 2013/115071.

The mass average molecular weight of the anionic dispersant is preferably less than 50,000, more preferably from 3,000 to less than 50,000, and even more preferably from 7,000 to less than 25,000, from the viewpoints of storage stability of the dispersion liquid of the non-white colorant, improvement of ejection properties, and durability of the recorded image. The acid value of the anionic dispersant is preferably from 50 mgKOH/g to 300 mgKOH/g, more preferably from 60 mgKOH/g to 275 mgKOH/g, and even more preferably from 70 mgKOH/g to 250 mgKOH/g.

The anionic dispersant can be used in a state where it is mixed with a non-white colorant. It can also be used in a state where the surface of the non-white colorant is partially or entirely covered with the dispersant. Alternatively, both of these states can be used in combination.

Anionic dispersants have salt-forming groups. Salt-forming groups include carboxy, hydroxy, sulfo, phosphate, and amino groups. Salt-forming groups can be introduced into anionic dispersants by polymerizing a monomer mixture that contains a salt-forming group-containing monomer.

The non-white ink composition is preferably prepared by preparing a dispersion containing a non-white colorant and an anionic dispersant, and then mixing it with other components. One example of a method for preparing a dispersion is the phase inversion emulsification method. For example, an anionic dispersant is dissolved in an organic solvent such as methyl ethyl ketone, and an aqueous solution of a neutralizing agent is added to prepare an emulsion. A non-white colorant is added to the obtained emulsion, and a dispersion process is performed. The organic solvent and some of the water are distilled off under reduced pressure from the liquid obtained in this manner, to obtain the desired dispersion.

[water]
The non-white ink composition may contain water. The water is preferably water with a low content of impurities such as metal ions, i.e., ion-exchanged water, distilled water, or the like.

When the non-white ink composition contains water, the water content in the non-white ink composition is preferably 55% by mass or more and 90% by mass or less, and more preferably 60% by mass or more and 85% by mass or less.

[Other ingredients]
In addition to the above components, the non-white ink composition may further contain titanium oxide, ink preparation agents, etc. Examples of titanium oxide and ink preparation agents include those exemplified in the ink composition according to the present embodiment described above. It is preferable that the non-white ink composition does not contain the above-mentioned aggregating agent.

[Method of preparing non-white ink composition]
The method for preparing the non-white ink composition is not particularly limited, and any known preparation method can be used. Examples of the preparation method include a method in which an aqueous dispersion containing a non-white colorant and an anionic dispersant is prepared, and water and, if necessary, an ink preparation agent are added to the aqueous dispersion and mixed.

As with the ink composition according to the present embodiment described above, it is preferable to precisely filter the non-white ink composition using a membrane filter, glass filter paper, or the like.

Inkjet recording method and recording medium
The inkjet recording method according to this embodiment includes ejecting the ink composition according to this embodiment or each ink composition included in the ink set according to this embodiment from an inkjet head and depositing it onto a recording medium. The recording medium according to this embodiment has the ink composition according to this embodiment or each ink composition included in the ink set according to this embodiment deposited thereon, and can be obtained by the inkjet recording method according to this embodiment.

　Any known inkjet printing method can be used. Specific examples of inkjet printing methods include, for example, a charge control method, a drop-on-demand (pressure pulse) method, an acoustic inkjet method, and a thermal inkjet method, with the drop-on-demand method being preferred. Inkjet printing methods also include a method of improving image quality by ejecting a large number of ink compositions with a small content of coloring material in a small volume; a method of improving image quality by using multiple ink compositions with substantially the same hue but different concentrations of coloring material in the ink composition; a method of improving the fixation of coloring material by using a colorless and transparent ink composition; and the like.

As the inkjet head, either one having an ink circulation mechanism or one not having an ink circulation mechanism can be used. From the viewpoint of suppressing the precipitation of titanium oxide, it is preferable to use an inkjet head having a circulation mechanism inside the head, that is, an inkjet head having a circulation mechanism. An inkjet head having a circulation mechanism is, for example, an inkjet head having a plurality of droplet ejection elements, a common flow path communicating with each of the plurality of droplet ejection elements via a supply path, and a common circulation path communicating with each of the plurality of droplet ejection elements via a return path, and is equipped with an ink circulation device in which an ink composition is supplied from the common flow path to the plurality of droplet ejection elements and circulates through the common circulation path. The circulation flow rate of the ink composition is not particularly limited, but is preferably 10 mL/min or more and 1000 mL/min or less, and more preferably 20 mL/min or more and 500 mL/min or less. As a mechanism of an industrial inkjet printer, a line head type in which inkjet heads are arranged side by side is known. In the case of a line head type, the above-mentioned preferable circulation flow rate refers to the circulation flow rate for each inkjet head. Examples of inkjet heads equipped with a circulation mechanism include the SambaGL3 made by Fujifilm Corporation, the S series made by Canon Inc., and the KJ4B-EX1200 print head made by Kyocera Corporation.

When each ink composition included in the ink set according to the present embodiment described above is applied to a recording medium, the ejection order of each ink composition is appropriately set according to the composition of the ink set.

When the ink set includes the ink composition according to the present embodiment described above and a white ink composition containing a white colorant and an anionic dispersant, the order in which the two are ejected is not particularly limited. By applying both ink compositions so that they overlap in the same area of the recording medium, a printed matter with excellent white color development can be obtained. From the viewpoint of further increasing the whiteness, it is preferable that the ink composition according to the present embodiment is ejected before the white ink composition containing a white colorant and an anionic dispersant.

When the ink set includes the ink composition according to the present embodiment described above and a non-white ink composition containing a non-white colorant and an anionic dispersant, it is preferable that the ink composition according to the present embodiment is ejected before the non-white ink composition containing a non-white colorant and an anionic dispersant. By applying the ink composition according to the present embodiment onto a recording medium to form a base, and then applying the non-white ink composition onto the base, a printed material with excellent color development can be obtained.

The recording medium is not particularly limited as long as it is a material to which the ink composition can adhere, and examples of the recording medium include paper, film, cans, leather, cloth, and fiber. Recording media are broadly divided into non-permeable recording media such as films and cans into which the ink composition does not permeate, and permeable recording media into which the ink composition permeates, with permeable recording media being preferred. Examples of permeable recording media include plain paper, inkjet paper, art paper, coated paper, matte paper, cast paper, and liner paper.

Among these, liner paper is preferred as a recording medium. Liner paper is cardboard liner paper described in Japanese Industrial Standard JIS P3902:2011 "Corrugated cardboard liners" and is made primarily from recycled paper and kraft pulp. In this embodiment, in addition to the liner paper itself, the liner paper after it has been processed into cardboard is also the subject of recording. Also, in this embodiment, the liner paper on the surface after it has been processed and formed into a cardboard box is also the subject of recording.

The liner paper may be non-white liner paper. As mentioned above, liner paper is often made from recycled paper and is often not white. Such non-white liner paper tends to have poor color development in the recorded image formed on its surface, but the inkjet recording method according to this embodiment makes it possible to form a recorded image with excellent color development.

The surface of the liner paper may be provided with a coating layer or may be subjected to a surface treatment. Examples of surface treatments include water-repellent treatment, anti-fouling treatment, and coloring treatment. In particular, if the surface of the liner paper is treated with water-repellent treatment, when a water-based ink composition is applied to the surface, the ink composition is repelled, and unevenness in the image is likely to occur. By using the ink composition according to the present embodiment described above, it is possible to form a good image even on liner paper that has been treated with water-repellent treatment as described above. The water-repellent treatment can be performed by applying a water-repellent agent, and there are commercially available products such as cardboard that contain water-repellent liner paper.

For all of the above, combinations of preferred items are more preferred, and combinations of more preferred items are even more preferred. The same applies to combinations of preferred items and more preferred items, combinations of more preferred items and even more preferred items, etc.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. In the examples, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass." The "water" used in the examples is "ion-exchanged water."

[Method for measuring acid value]
The acid value (mgKOH/g) was measured in accordance with the potentiometric method of JIS K0070, and the obtained value was converted into 100% nonvolatile content.

[Method for measuring amine value]
The amine value (mgKOH/g) was measured according to the potentiometric titration method of JIS K7237, and the obtained value was converted into 100% nonvolatile content.

[Method of measuring particle size]
The particle size was measured using a dynamic light scattering particle size distribution analyzer nanotrac WAVE II manufactured by Microtrac Bell Co., Ltd. The titanium oxide concentration was adjusted with water to 0.075%, and the measurement was performed. The median diameter (D50, number average particle diameter) value at this time was taken as the average particle diameter of the particles in the dispersion.

[Method for measuring dynamic surface tension with a lifespan of 0.1 sec using the maximum bubble pressure method]
The measurement was performed at room temperature using a bubble pressure type dynamic surface tensiometer SITA t60 manufactured by Eiko Seiki Co., Ltd., and the value at which the bubble lifetime was about 10 Hz was taken as the dynamic surface tension at a lifetime of 0.1 sec according to the maximum bubble pressure method.

<Preparation Examples 1 to 6: Preparation of titanium oxide dispersions Dp1-1 to Dp1-6>
The components shown in Table 1 below were charged and dispersed in a sand grinder using zirconia beads with a particle size of 0.3 mm at 1500 rpm for 3 hours. The resulting dispersion was filtered through a glass fiber filter (GC-50, manufactured by Advantec Corporation) to obtain titanium oxide dispersions Dp1-1 to Dp1-6.

The numerical value in each component column in Table 1 indicates the amount (parts) of the component added, and the blank spaces mean that the component is not used. The abbreviations in Table 1 have the following meanings.
・CR-50: TIPAQUE CR-50 (manufactured by Ishihara Sangyo Kaisha, Ltd.)
・ BYK2018: DISPERBYK-2018 (manufactured by BYK Japan Co., Ltd., non-volatile content: 52%, acid value: 0 mgKOH / g, amine value: 26 mgKOH / g, acid value calculated based on 100% non-volatile content: 0 mgKOH / g, amine value calculated based on 100% non-volatile content: 50 mgKOH / g)
・BYK2055: DISPERBYK-2055 (manufactured by BYK Japan Co., Ltd., non-volatile content: 100%, acid value: 0 mgKOH/g, amine value: 40 mgKOH/g)
PVP: Polyvinyl alcohol K-30 (manufactured by Nippon Shokubai Co., Ltd., non-volatile content: 100%, acid value: 0 mg KOH/g, amine value: 0 mg KOH/g)
BYK182: DISPERBYK-182 (manufactured by BYK Japan Co., Ltd., non-volatile content: 43%, acid value: 0 mgKOH/g, amine value: 13 mgKOH/g, acid value calculated based on 100% non-volatile content: 0 mgKOH/g, amine value calculated based on 100% non-volatile content: 30 mgKOH/g)
・ BYK2012: DISPERBYK-2012 (manufactured by BYK Japan Co., Ltd., non-volatile content: 40%, acid value: 4 mg KOH / g, amine value: 7 mg KOH / g, acid value calculated based on 100% non-volatile content: 10 mg KOH / g, amine value calculated based on 100% non-volatile content: 18 mg KOH / g)
BYK180: DISPERBYK-180 (manufactured by BYK Japan Co., Ltd., non-volatile content: 81%, acid value: 94 mg KOH/g, amine value: 94 mg KOH/g, acid value calculated based on 100% non-volatile content: 116 mg KOH/g, amine value calculated based on 100% non-volatile content: 116 mg KOH/g)
DF-58: Surfynol DF-58 (manufactured by Nissin Chemical Industry Co., Ltd.)

<Examples 1 to 15: Preparation of Inks A1-1 to A1-15>
The components shown in Tables 2 and 3 were thoroughly mixed and filtered through a mixed cellulose ester filter having a pore size of 5 μm, and then degassed using a vacuum pump to obtain inks A1-1 to A1-15. The numerical value in the column for each component in Tables 2 and 3 indicates the amount (parts) of the component added, and a blank space means that the component was not used.

<Evaluation>
Using each of the inks prepared in Examples 1 to 15, evaluation of dispersion stability, inkjet dischargeability, and color development properties were carried out as described below. The results are shown in Table 4 below.

[Dispersion stability evaluation: rate of change in particle size]
Each of the inks prepared in Examples 1 to 15 was allowed to stand in a thermostatic chamber at 60° C. for one week, after which the rate of change from the initial particle size of titanium oxide was calculated and the dispersion stability was evaluated according to the following evaluation criteria: Ratings A, B, and C indicate good dispersion stability, and rating D indicates poor dispersion stability.
-Evaluation criteria-
A: D50 change rate is 20% or less. B: D50 change rate is more than 20% and less than 40%. C: D50 change rate is more than 40% and less than 50%. D: D50 change rate is more than 50%.

[Evaluation of inkjet ejection properties]
Each of the inks prepared in Examples 1 to 15 was ejected onto HEIKO cut paper color wood-free black (manufactured by Shimojima Co., Ltd.) using a printer equipped with an inkjet head KJ4B-YH (600 dpi x 600 dpi) manufactured by Kyocera Corporation at a droplet size of 12 pL and a speed of 25 m/min to perform inkjet recording, thereby obtaining an initial printed image. Inkjet recording was performed so as to obtain a solid image with a duty of 100%. The nozzle portion of the inkjet head was left uncapped for 3 minutes immediately after printing the initial printed image, and inkjet recording was performed again to obtain a printed image after 3 minutes. The area of the initial printed image and the area of the printed image after 3 minutes were then measured, and the inkjet dischargeability was evaluated according to the following evaluation criteria. Evaluations A, B, and C indicate good inkjet dischargeability, and evaluation D indicates poor inkjet dischargeability.
-Evaluation criteria-
A: The area of the printed image after 3 minutes is 80% or more compared to the initial printed image. B: The area of the printed image after 3 minutes is 70% or more but less than 80% compared to the initial printed image. C: The area of the printed image after 3 minutes is 60% or more but less than 70% compared to the initial printed image. D: The area of the printed image after 3 minutes is less than 60% compared to the initial printed image.

[Color development evaluation]
C.I. Pigment Red 122 (18.0 parts), Joncyrl 63J (neutralized dispersion of Joncyrl 67, non-volatile content: 30%, manufactured by BASF) (18.0 parts), BYK-1770 (manufactured by BYK) (0.1 parts), and water (63.9 parts) were mixed, and dispersed in a sand grinder for 15 hours under conditions of 1500 rpm using zirconia beads having a particle diameter of 0.3 mm, to obtain a magenta dispersion. The obtained magenta dispersion (27.78 parts), 1,4-butanediol (5.00 parts), propylene glycol (25.00 parts), Surfynol 485 (manufactured by Nissin Chemical Industry Co., Ltd.) (0.10 parts), and water (42.12 parts) were mixed to prepare a magenta ink.

50 μL of each of the inks prepared in Examples 1 to 15 was applied to a liner paper (K liner, manufactured by Daio Paper Co., Ltd., brown liner with basis weight: 170 g/m ² and L ^* value: 63) using a bar coater #8, and then dried in a thermostatic chamber at 120° C. for 15 minutes. 50 μL of magenta ink was applied to the removed print under the same conditions so as to create areas that overlap with each of the inks of Examples 1 to 15 and areas that do not, to prepare test pieces. The magenta OD value of the areas that were coated so as to overlap with the inks of Examples 1 to 15 was compared with the magenta OD value of the areas that were coated so as not to overlap with the inks of Examples 1 to 15, and the color development was evaluated according to the following evaluation criteria. Evaluations A, B, and C indicate good color development, and evaluation D indicates poor color development.
-Evaluation criteria-
A: The rate of change in magenta OD value is greater than 25%. B: The rate of change in magenta OD value is 10% or more and 25% or less. C: The rate of change in magenta OD value is 5% or more and less than 10%. D: The rate of change in magenta OD value is less than 5%.

As shown in Table 4, inks A1-1 to A1-15 of Examples 1 to 15 had good dispersion stability, inkjet ejection properties, and color development properties.

<Preparation Example 7: Preparation of anionic dispersant aqueous solution Ac-2>
DL-453 (polyacrylic acid, manufactured by Nippon Shokubai Co., Ltd., mass average molecular weight: 50,000, non-volatile content: 35%) was used as an anionic dispersant aqueous solution Ac-2 without pH adjustment. The pH of the anionic dispersant aqueous solution Ac-2 was 8.4. In addition, when the dynamic surface tension of an aqueous solution with a non-volatile content of 10% was measured by the maximum bubble pressure method at a life of 0.1 sec, it was 72 mN/m.

<Preparation Example 8: Preparation of anionic dispersant aqueous solution Ac-3>
YS-100 (polyacrylic acid, manufactured by Nippon Shokubai Co., Ltd., mass average molecular weight: 5000, non-volatile content: 45%) was used as it is as anionic dispersant aqueous solution Ac-3 without pH adjustment. The pH of the anionic dispersant aqueous solution Ac-3 was 8.4. In addition, when the dynamic surface tension of an aqueous solution with a non-volatile content of 10% was measured by the maximum bubble pressure method with a life of 0.1 sec, it was 73 mN/m.

<Preparation Examples 9 to 10: Preparation of anionic dispersant aqueous solutions StAc-4 to StAc-5>
Methyl ethyl ketone (80 parts) was placed in a reaction vessel, and the vessel was heated to 80°C while nitrogen gas was injected into the vessel. At the same temperature, a mixture of each component shown in Table 5 below was added dropwise over 2 hours to proceed with the reaction. After the dropwise addition, a solution of azobisisobutyronitrile (0.8 parts) dissolved in methyl ethyl ketone (20 parts) was added, and the reaction was continued for another 3 hours at 80°C. Next, methyl ethyl ketone was completely distilled off under reduced pressure to obtain solid anionic dispersants StAc-4 to StAc-5. The dynamic surface tension of a 10% aqueous solution of anionic dispersants StAc-4 to StAc-5 at a life of 0.1 sec measured by the maximum bubble pressure method is also shown in Table 5 below.

The numerical value in each component column in Table 5 indicates the amount (parts) of the component added, and the blank spaces mean that the component is not used. The abbreviations in Table 5 have the following meanings.
St: styrene; α-MeSt: α-methylstyrene; MMA: methyl methacrylate; BA: n-butyl acrylate; CA: carbitol acrylate; AA: acrylic acid; MAA: methacrylic acid

The obtained anionic dispersants StAc-4 to StAc-5, neutralizer, and water were charged in the amounts (units: parts) shown in Table 6 below, heated to 80°C, and stirred for 2 hours to obtain aqueous anionic dispersant solutions StAc-4 to StAc-5 (non-volatile content: 20%).

<Preparation Examples 11 to 16: Preparation of titanium oxide dispersions Dp3-1 to Dp3-6>
The components shown in Table 7 below were charged and dispersed in a sand grinder using zirconia beads with a particle size of 0.3 mm at 1500 rpm for 3 hours. The resulting dispersion was filtered through a glass fiber filter (GC-50, manufactured by Advantec Corporation) to obtain titanium oxide dispersions Dp3-1 to Dp3-6.

The numerical value in each component column in Table 7 indicates the amount (parts) of the component added, and the blank spaces mean that the component was not used. The abbreviations in Table 7 have the following meanings.
・R960: Ti-Pure R960 (manufactured by Chemours)

<Preparation Examples 17 to 22: Preparation of Inks B1 to B6>
The components shown in Table 8 below were thoroughly mixed and filtered through a mixed cellulose ester filter with a pore size of 5 μm, and then degassed using a vacuum pump to obtain inks B1 to B6. The numerical value in the column for each component in Table 8 indicates the amount (parts) of the component added.

<Examples 16 to 31>
A liner paper (K liner, Daio Paper Co., Ltd., brown liner with basis weight: 170 g/ ^m2 , L ^* value: 63) was coated with 50 μL of the first color ink shown in Table 9 below using bar coater #8, and then dried for 15 minutes in a thermostatic bath at 120° C. The removed print was coated with the second color ink shown in Table 9 below under the same conditions, so as to create areas that overlapped with the first color ink and areas that did not, to prepare a test piece.

<Evaluation>
The test pieces obtained in Examples 16 to 31 were evaluated for whiteness, scratch resistance, and water resistance as described below. The results are shown in Table 9 below.

[Whiteness Evaluation]
The whiteness was evaluated by measuring the L ^* value of the test piece. The colorimeter used was an eXact manufactured by X-Rite, and the L ^* value in the CIE/L ^* a ^* b ^* color system was measured. The colorimetric conditions were an observation light source of D65, an observation field of view of 2°, and a density of Status E. Each test piece was subjected to color measurement five times, and the whiteness was evaluated according to the following evaluation criteria, with the average value being the measurement result. Evaluations A, B, and C indicate good whiteness, and evaluation D indicates poor whiteness.
-Evaluation criteria-
A: L ^* value is 85 or more B: L ^* value is 80 or more and less than 85 C: L ^* value is 70 or more and less than 80 D: L ^* value is less than 70

[Abrasion resistance evaluation]
The abrasion resistance of each test piece was evaluated using a No. 428 Gakushin abrasion tester (friction tester type II) manufactured by Yasuda Seiki Seisakusho Co., Ltd. The image portion was rubbed 40 times with a load of 500 g applied to the test piece, and the degree of deterioration of the recorded image was evaluated for abrasion resistance according to the following evaluation criteria.
-Evaluation criteria-
A: Almost no scratches were found on the recorded image.
B: Slight scratches were observed on the recorded image.
D: The scratches on the recorded image were very large.

[Water resistance evaluation]
The test piece was rubbed back and forth over a width of 1 cm using a cotton swab moistened with ion-exchanged water, and the number of times the swab was rubbed back and forth until the ink at the rubbed area was completely peeled off was measured. Measurements were performed at a total of five locations within the same coating film, and the average number of times the swab was rubbed back and forth was used as the measurement result, and the water resistance was evaluated according to the following evaluation criteria.
-Evaluation criteria-
A: The ink did not come off even after rubbing five or more times.
B: The ink did not come off even after rubbing 1 to 4 times.
D: The ink was removed after just one rub.

As shown in Table 9, by combining inks A1-1 to A1-10 of Examples 1 to 10 with inks B1 to B6 of Preparation Examples 17 to 22, it was possible to obtain recorded images with excellent whiteness, abrasion resistance, and water resistance.

<Preparation Examples 23 to 25: Preparation of Aqueous Cationic Resin Solutions B-1 to B-3>
In a 200 mL three-neck flask equipped with a stirrer and a thermometer, the components shown in Table 10 below, except for ammonium persulfate, were charged while cooling with ice water. After heating to 70°C, ammonium persulfate was added to proceed with the reaction. After completion of the reaction, the nonvolatile content was measured, and water was added so that the nonvolatile content was 20%, to obtain aqueous cationic resin solutions B-1 to B-3. The numerical value in the column for each component in Table 10 indicates the amount (parts) of the component added, and a blank cell means that the component was not used.

<Preparation Examples 26 to 38: Preparation of titanium oxide dispersions Dp2-1 to Dp2-13>
The components shown in Tables 11 and 12 below were charged and dispersed in a sand grinder using zirconia beads with a particle size of 0.3 mm at 1500 rpm for 3 hours. The resulting dispersion was filtered through a glass fiber filter (GC-50, manufactured by Advantec Corporation) to obtain titanium oxide dispersions Dp2-1 to Dp2-13.

The numerical value in each component column in Tables 11 and 12 indicates the amount (parts) of the component added, and the blank spaces mean that the component is not used. The abbreviations in Tables 11 and 12 have the following meanings.
・R960: Ti-Pure R960 (manufactured by Chemours)
・ BYK2018: DISPERBYK-2018 (manufactured by BYK Japan Co., Ltd., non-volatile content: 52%, acid value: 0 mgKOH / g, amine value: 26 mgKOH / g, acid value calculated based on 100% non-volatile content: 0 mgKOH / g, amine value calculated based on 100% non-volatile content: 50 mgKOH / g)
・BYK2055: DISPERBYK-2055 (manufactured by BYK Japan Co., Ltd., non-volatile content: 100%, acid value: 0 mgKOH/g, amine value: 40 mgKOH/g)
・ BYK2012: DISPERBYK-2012 (manufactured by BYK Japan Co., Ltd., non-volatile content: 40%, acid value: 4 mg KOH / g, amine value: 7 mg KOH / g, acid value calculated based on 100% non-volatile content: 10 mg KOH / g, amine value calculated based on 100% non-volatile content: 18 mg KOH / g)
PVP: Polyvinyl alcohol K-30 (manufactured by Nippon Shokubai Co., Ltd., non-volatile content: 100%, acid value: 0 mg KOH/g, amine value: 0 mg KOH/g)
BYK182: DISPERBYK-182 (manufactured by BYK Japan Co., Ltd., non-volatile content: 43%, acid value: 0 mgKOH/g, amine value: 13 mgKOH/g, acid value calculated based on 100% non-volatile content: 0 mgKOH/g, amine value calculated based on 100% non-volatile content: 30 mgKOH/g)
BYK180: DISPERBYK-180 (manufactured by BYK Japan Co., Ltd., non-volatile content: 81%, acid value: 94 mg KOH/g, amine value: 94 mg KOH/g, acid value calculated based on 100% non-volatile content: 116 mg KOH/g, amine value calculated based on 100% non-volatile content: 116 mg KOH/g)
・ BYK2015: DISPERBYK-2015 (manufactured by BYK Japan Co., Ltd., non-volatile content: 40%, acid value: 10 mgKOH/g, amine value: 0 mgKOH/g, acid value calculated based on 100% non-volatile content: 25 mgKOH/g, amine value calculated based on 100% non-volatile content: 0 mgKOH/g)
GXL(S): Proxel GXL(S) (Arch Chemicals)
DF-58: Surfynol DF-58 (manufactured by Nissin Chemical Industry Co., Ltd.)

<Examples 32 to 46: Preparation of Inks A2-1 to A2-15>
The components shown in Tables 13 and 14 were thoroughly mixed and filtered through a mixed cellulose ester filter having a pore size of 5 μm, and then degassed using a vacuum pump to obtain inks A2-1 to A2-15. The numerical values in the columns for each component in Tables 13 and 14 indicate the amount (parts) of the component added.

<Evaluation>
Using each of the inks prepared in Examples 32 to 46, evaluation of dispersion stability, evaluation of inkjet dischargeability, and evaluation of color development were carried out in the same manner as in Examples 1 to 15. In addition, evaluation of dispersibility and whiteness were carried out as described below. The results are shown in Table 15 below.

[Dispersibility evaluation: initial particle size]
The initial particle size (D50) of titanium oxide was measured, and the dispersibility was evaluated according to the following evaluation criteria: Ratings A, B, and C indicate good dispersibility, and rating D indicates poor dispersibility.
-Evaluation criteria-
A: D50 is 300 nm or less. B: D50 is more than 300 nm and less than 400 nm. C: D50 is more than 400 nm and less than 450 nm. D: D50 is greater than 450 nm.

[Whiteness Evaluation]
50 μL of each of the inks prepared in Examples 1 to 15 was applied to a liner paper (K liner, manufactured by Daio Paper Co., Ltd., basis weight: 170 g/m ² , brown liner with L ^* value: 63) using a bar coater #8, and then dried in a thermostatic chamber at 120° C. for 15 minutes. The L ^* value of the removed print was measured to evaluate the whiteness. The colorimeter used was eXact manufactured by X-Rite, and the L ^* value in the CIE/L ^* a ^* b ^* color system was measured. The colorimetric conditions at this time were an observation light source of D65, an observation field of view of 2°, and a density of Status E. The colorimetric measurements were performed five times for each print, and the average value was used as the measurement result to evaluate the whiteness according to the following evaluation criteria. Evaluations A, B, and C indicate good whiteness, and evaluation D indicates poor whiteness.
-Evaluation criteria-
A: L ^* value is 85 or more B: L ^* value is 80 or more and less than 85 C: L ^* value is 70 or more and less than 80 D: L ^* value is less than 70

As shown in Table 15, inks A2-1 to A2-15 of Examples 32 to 46 had good dispersibility, dispersion stability, inkjet dischargeability, whiteness, and color development.

<Examples 47 to 61>
A liner paper (K liner, Daio Paper Co., Ltd., brown liner with basis weight: 170 g/ ^m2 , L ^* value: 63) was coated with 50 μL of the first color ink shown in Table 16 below using bar coater #8, and then dried for 15 minutes in a thermostatic bath at 120° C. The removed print was coated with the second color ink shown in Table 16 below under the same conditions, so as to create areas that overlapped with the first color ink and areas that did not, to prepare a test piece.

<Evaluation>
The test pieces obtained in Examples 47 to 61 were evaluated for whiteness, scratch resistance, and water resistance as described below. The results are shown in Table 16 below.

[Whiteness Evaluation]
The whiteness was evaluated by measuring the L ^* value of the test piece. The colorimeter used was an eXact manufactured by X-Rite, and the L ^* value in the CIE/L ^* a ^* b ^* color system was measured. The colorimetric conditions were an observation light source of D65, an observation field of view of 2°, and a density of Status E. Each test piece was subjected to color measurement five times, and the whiteness was evaluated according to the following evaluation criteria, with the average value being the measurement result. Evaluations A, B, and C indicate good whiteness, and evaluation D indicates poor whiteness.
-Evaluation criteria-
A: L ^* value is 85 or more B: L ^* value is 80 or more and less than 85 C: L ^* value is 70 or more and less than 80 D: L ^* value is less than 70

[Abrasion resistance evaluation]
The abrasion resistance of each test piece was evaluated using a No. 428 Gakushin abrasion tester (friction tester type II) manufactured by Yasuda Seiki Seisakusho Co., Ltd. With a load of 500 g applied to the test piece, the uncoated liner paper and the image portion of the test piece were rubbed together 40 times, and then the L ^* value of the uncoated liner paper was measured and the abrasion resistance was evaluated according to the following evaluation criteria.
-Evaluation criteria-
A: The rate of change in the L ^* value of uncoated liner paper is 1% or less. B: The rate of change in the L ^* value of uncoated liner paper is more than 1% and less than 3%. C: The rate of change in the L ^* value of uncoated liner paper is more than 3% and less than 5%. D: The rate of change in the L ^* value of uncoated liner paper is greater than 5%.

[Water resistance evaluation]
The test piece was rubbed back and forth over a width of 1 cm using a cotton swab moistened with ion-exchanged water, and the number of times the swab was rubbed back and forth until the ink at the rubbed area was completely peeled off was measured. Measurements were performed at a total of five locations within the same coating film, and the average number of times the swab was rubbed back and forth was used as the measurement result, and the water resistance was evaluated according to the following evaluation criteria.
-Evaluation criteria-
A: The ink did not come off even after rubbing five or more times.
B: The ink did not come off even after rubbing 3 to 4 times.
C: The ink was not peeled off even after rubbing once or twice.
D: The ink was removed after just one rub.

As shown in Table 16, by combining inks A2-1 to A2-15 of Examples 32 to 46 with inks B1 to B6 of Preparation Examples 17 to 22, it was possible to obtain recorded images with excellent whiteness, abrasion resistance, and water resistance.

Claims (18)

Contains titanium oxide, a polymer dispersant, and a flocculant,
The ink composition has a titanium oxide content of 1% by mass or more and less than 30% by mass. The ink composition according to claim 1, further comprising water. The ink composition according to claim 1, wherein the polymer dispersant has an acid value and/or an amine value that satisfies (acid value) - (amine value) ≦ 50 mg KOH/g. The ink composition according to claim 3, wherein the amine value of the polymer dispersant is from 0 mg KOH/g to 130 mg KOH/g. The ink composition according to claim 1, wherein the coagulant comprises a polyvalent metal salt or a cationic resin. The flocculant comprises a polyvalent metal salt,
The ink composition according to claim 5 , wherein the content of the polyvalent metal salt is from 1% by mass to 70% by mass with respect to the content of the titanium oxide. The ink composition according to claim 6, wherein the polyvalent metal salt is at least one salt selected from calcium salts and magnesium salts. the flocculant contains a cationic resin having a constituent unit derived from a vinyl monomer having a quaternary ammonium salt group and having a mass average molecular weight of 1,500 or more and 25,000 or less;
The ink composition according to claim 5 , wherein the content of the cationic resin is 0.5% by mass or more and 18% by mass or less with respect to the content of the titanium oxide. The ink composition according to claim 1, further comprising water. The ink composition according to claim 1, further comprising an organic solvent. An ink set comprising the ink composition according to claim 1 and a white ink composition containing a white colorant and an anionic dispersant. An ink set comprising the ink composition according to claim 1 and a non-white ink composition containing a non-white colorant and an anionic dispersant. A recording medium to which the ink composition according to any one of claims 1 to 10 or each of the ink compositions included in the ink set according to claim 11 or 12 has been adhered. The recording medium according to claim 13, wherein the recording medium is a permeable recording medium. The recording medium according to claim 14, wherein the permeable recording medium is a liner paper for corrugated cardboard. An inkjet recording method comprising ejecting the ink composition according to any one of claims 1 to 10, or each ink composition included in the ink set according to claim 11 or 12, from an inkjet head and depositing it onto a recording medium. The inkjet recording method according to claim 16, wherein the recording medium is liner paper. The inkjet recording method according to claim 16, wherein the inkjet head has a circulation mechanism.