JP2021025002A - Inkjet ink - Google Patents

Abstract

To provide an inkjet ink capable of being used for forming images of various colors and capable of forming images excellent in a color value on a recording medium while keeping color saturation in a desired range value.SOLUTION: An inkjet ink comprises a gallium-doped zinc oxide, a pigment, and an aqueous medium. A content of the gallium-doped zinc oxide is preferably 0.1 mass% or more and 0.4 mass% or less. The pigment is preferably a non-white pigment. The pigment is preferably carbon black.SELECTED DRAWING: Figure 1

Description

The present invention relates to an ink for an inkjet.

An image is formed on a recording medium by ejecting ink for inkjet from a recording head of an inkjet recording device. For example, Patent Document 1 describes an ink set for an inkjet for performing white recording. The ink contained in this ink set contains an inorganic oxide pigment and a dispersant.

JP-A-2007-223112

However, the inorganic oxide pigment contained in the ink contained in the inkjet ink set described in Patent Document 1 is often white. Therefore, the use of the inkjet ink set described in Patent Document 1 is limited to white recording.

The present invention has been made in view of the above problems, and an object of the present invention is to record an image having excellent brightness while maintaining saturation at a value within a desired range, which can be used for forming images of various colors. It is to provide an ink for an inkjet that can be formed on a medium.

The inkjet ink according to the present invention contains gallium-doped zinc oxide, a pigment, and an aqueous medium.

The inkjet ink according to the present invention can be used for forming images of various colors, and can form an image having excellent brightness on a recording medium while maintaining saturation within a desired range.

It is a graph which shows the measurement result of the reflectance of the image formed by using ink.

An embodiment of the present invention will be described. First, terms used in the present specification will be described.

Unless otherwise specified, the number average primary particle size is a value measured by the following method. Specifically, using a transmission electron microscope (TEM, "H-7100FA" manufactured by Hitachi High-Technologies Corporation), the particles to be measured (for example, particles of zinc gallium oxide) are observed at a magnification of 1000000 times, and 100 or more. Take a TEM photograph of the particle to be measured. From the obtained TEM photographs, TEM photographs of 100 particles to be measured are arbitrarily selected. For an arbitrarily selected TEM photograph, the equivalent circle diameter of the particle to be measured is measured using image analysis software (“WinROOF” manufactured by Mitani Shoji Co., Ltd.). The average number of measured circle-equivalent diameters is calculated and used as the average primary particle diameter of the particles to be measured.

_{Unless otherwise specified, the volume median diameter (D 50} ) of the particles to be measured (for example, pigment particles) is a laser diffraction / scattering type particle size distribution measuring device (“LA-950” manufactured by HORIBA, Ltd.). It is a value measured using.

Unless otherwise specified, the acid value is "JIS (Japanese Industrial Standard) K0070-1992 (test method for acid value, saponification value, ester value, iodine value, hydroxyl value, and unsaponified product of chemical products)". It is a value measured according to. The terms used in the present specification have been described above.

<Ink for Ink>
Next, the inkjet ink of the present embodiment (hereinafter referred to as ink) will be described. The ink of this embodiment is a water-based ink containing a water-based medium. The ink of this embodiment contains gallium-doped zinc oxide, a pigment, and an aqueous medium. Hereinafter, gallium-doped zinc oxide, pigments, and an aqueous medium will be described.

(Gallium-doped zinc oxide)
Gallium-doped zinc oxide is gallium-doped zinc oxide. The inclusion of gallium-doped zinc oxide in the ink provides the first and second advantages shown below.

The first advantage will be described. By including gallium-doped zinc oxide in the ink, the reflectance of the image formed by using the ink is increased. By increasing the reflectance of the image, it is possible to improve the brightness of the image while maintaining the saturation of the image within a desired range. In the present specification, maintaining the saturation of an image within a desired range means that the saturation of an image is maintained in comparison with the saturation of an image formed by using an ink that does not contain gallium-doped zinc oxide. It means that the saturation of an image formed by using an ink containing gallium-doped zinc oxide is the same degree of saturation, which is less than a desired value.

The second advantage will be described. In general, gallium-undoped zinc oxide and aluminum-doped zinc oxide (hereinafter, may be collectively referred to as non-gallium-doped zinc oxide) are white. Therefore, when non-gallium-doped zinc oxide is added to the ink, the saturation of the image formed by using the ink may change significantly. For this reason, the use of inks containing non-gallium-doped zinc oxide is limited to white recording. However, the ink of this embodiment contains gallium-doped zinc oxide rather than non-gallium-doped zinc oxide. Therefore, according to the ink of the present embodiment, it is possible to suppress a large change in the saturation of the formed image, and it is possible to maintain the saturation of the image within a desired range. Therefore, the ink of the present embodiment is not limited to white recording, and can be used to form images of various colors. The first and second advantages have been described above.

Whether or not zinc oxide is doped with gallium can be confirmed by, for example, elemental analysis. Gallium-doped zinc oxide contains at least zinc and gallium as metal components. The content of gallium in gallium-doped zinc oxide is not particularly limited. The gallium content in gallium-doped zinc oxide is preferably 0.1% or more and 10.0% or less with respect to the total number of zinc atoms and gallium atoms. When the gallium content in gallium-doped zinc oxide is 0.1% or more, the brightness can be further improved while maintaining the saturation of the image formed by using the ink within a desired range. When the gallium content in gallium-doped zinc oxide is 10.0% or less, gallium is less likely to precipitate from gallium-doped zinc oxide.

The gallium-doped zinc oxide may contain only zinc and gallium as metal components. Further, the gallium-doped zinc oxide may further contain a metal other than zinc and gallium as a metal component.

The surface of gallium-doped zinc oxide may be treated with a surface treatment agent. Examples of the surface treatment agent include resins, surfactants, and silane coupling agents.

The gallium-doped zinc oxide is, for example, in the form of particles. The number average primary particle size of the gallium-doped zinc oxide particles is preferably 10 nm or more and 200 nm or less, more preferably 10 nm or more and 50 nm or less, and further preferably 20 nm or more and 40 nm or less. When the number average primary particle size of the gallium-doped zinc oxide particles is within such a range, the brightness is improved while maintaining the saturation of the image formed by using the ink within the desired range. Can be made to. Further, when the number average primary particle diameter of the gallium-doped zinc oxide particles is within such a range, ink can be stably ejected from the nozzle of the inkjet recording device.

The content of gallium-doped zinc oxide is preferably 0.1% by mass or more and 10.0% by mass or less, and preferably 0.1% by mass or more and 2.0% by mass or less with respect to the mass of the ink. More preferably, it is 0.1% by mass or more and 0.4% by mass or less. When the content of gallium-doped zinc oxide is within such a range, the brightness can be improved while maintaining the saturation of the image formed by using the ink within a desired range. Further, when the content of gallium-doped zinc oxide is within such a range, ink can be stably ejected from the nozzle of the inkjet recording device.

(Pigment)
The pigment contained in the ink is not particularly limited. Examples of the pigment include black pigment, white pigment, yellow pigment, orange pigment, red pigment, blue pigment, and purple pigment. The ink may contain only one kind of pigment, or may contain two or more kinds of pigments.

Examples of the black pigment include C.I. I. Pigment Black 7 and carbon black can be mentioned. Examples of white pigments include zinc white (more specifically, zinc oxide not doped with gallium), titanium oxide, antimony white, zinc sulfide, barite powder, barium carbonate, clay, silica, white carbon, talc, and the like. Calcium carbonate, mica, kaolin, and alumina white can be mentioned. Examples of the yellow pigment include C.I. I. Pigment Yellow 74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193. Examples of the orange pigment include C.I. I. Pigment Orange 34, 36, 43, 61, 63, and 71. Examples of the red pigment include C.I. I. Pigment Red 122, and 202. Quinacridone magenta (PR122) may be used as the red pigment. Examples of the blue pigment include C.I. I. Pigment Blue 15 and 15: 3. Examples of the purple pigment include C.I. I. Pigment Violet 19, 23, and 33.

The ink is preferably a non-white ink. That is, the pigment contained in the ink is preferably a non-white pigment. The non-white pigment is a pigment other than the white pigment among the above examples of pigments. Examples of non-white pigments include black pigments, yellow pigments, orange pigments, red pigments, blue pigments, and purple pigments. As described in the second advantage, when non-gallium-doped zinc oxide is added to the ink, the saturation of the image formed by using the ink tends to change significantly. This tendency becomes more remarkable when the image is formed by using the non-white ink, as compared with the case where the image is formed by using the white ink. However, according to the ink of the present embodiment containing gallium-doped zinc oxide, even when an image is formed using a non-white ink containing a non-white pigment, the saturation of the formed image is desired. It can be maintained within the range. The pigment is preferably a black pigment, more preferably carbon black. The ink preferably does not contain zinc oxide, which is not doped with gallium. Further, it is preferable that the ink does not contain zinc oxide doped with a metal other than gallium (for example, aluminum).

The pigment is, for example, in the form of particles. _{The volume median diameter (D 50} ) of the pigment-containing particles (pigment particles) is preferably 70 nm or more and 130 nm or less.

The content of the pigment is preferably 5% by mass or more and 30% by mass or less, and more preferably 10% by mass or more and 15% by mass or less with respect to the mass of the ink. When the content of the pigment is 5% by mass or more, an image having a desired image density can be easily obtained. When the content of the pigment is 30% by mass or less, the fluidity of the ink and the permeability of the ink to the recording medium can be easily ensured.

(Aqueous medium)
The aqueous medium is a medium containing water as a main component. The aqueous medium may function as a solvent or a dispersion medium. Specific examples of the aqueous medium include water or a mixed solution of water and a polar solvent. Examples of polar solvents contained in aqueous media include methanol, ethanol, isopropyl alcohol, butanol, and methyl ethyl ketone. The content of water in the aqueous medium is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. The aqueous medium is preferably water, more preferably ion-exchanged water.

The content of the aqueous medium is preferably 5% by mass or more and 70% by mass or less, and more preferably 10% by mass or more and 60% by mass or less with respect to the mass of the ink. When the content of the aqueous medium is a value within such a range, an ink having an appropriate viscosity can be obtained.

(resin)
The ink may contain a resin, if necessary. If the ink contains a resin, the resin adheres to the surface of the pigment particles, and the dispersibility of the pigment particles in the ink can be enhanced. In the ink, at least a part of the resin is attached to the surface of the pigment particles. Further, in the ink, at least a part of the resin covers the surface of the pigment particles. However, a part of the resin may be dispersed in the ink without adhering to the pigment particles.

The resin is preferably anionic. Examples of the resin include a styrene-acrylic resin, a styrene-maleic acid copolymer, a styrene-maleic acid half ester copolymer, a vinylnaphthalene-acrylic acid copolymer, and a vinylnaphthalene-maleic acid copolymer. .. A styrene-acrylic resin is preferable as the resin because the pigment particles can be suitably dispersed in the ink.

The styrene-acrylic resin is a copolymer of a first monomer which is styrene and at least one second monomer selected from the group consisting of acrylic acid, methacrylic acid, acrylic acid ester, and methacrylic acid ester. In other words, the styrene-acrylic resin comprises a first repeating unit derived from styrene and at least one second repeating unit selected from the group consisting of acrylic acid, methacrylic acid, acrylic acid ester, and methacrylic acid ester. Have.

Examples of the styrene-acrylic resin include a copolymer of styrene, acrylic acid and acrylic acid alkyl ester, a copolymer of styrene, methacrylic acid, methacrylic acid alkyl ester and acrylic acid alkyl ester, and styrene and acrylic acid. Examples thereof include a copolymer, a copolymer of styrene and methacrylic acid, and a copolymer of styrene and methacrylic acid alkyl ester.

As the styrene-acrylic resin, a copolymer of styrene, methacrylic acid, methacrylic acid alkyl ester, and acrylic acid alkyl ester is preferable. As the styrene-acrylic resin, a copolymer of styrene, methacrylic acid, methyl methacrylate and butyl acrylate (more preferably n-butyl acrylate) is more preferable. The styrene-acrylic resin has a mass ratio of styrene, methacrylic acid, methyl methacrylate and butyl acrylate of 4: 1: 4: 1, styrene, methacrylic acid, methyl methacrylate and butyl acrylate (more preferably). , N-butyl acrylate) is more preferable.

The mass average molecular weight (Mw) of the styrene-acrylic resin is preferably 10,000 or more and 30,000 or less, and more preferably 15,000 or more and 25,000 or less. The acid value of the styrene-acrylic resin is preferably 50 mgKOH / g or more and 150 mgKOH / g or less, and more preferably 90 mgKOH / g or more and 110 mgKOH / g or less.

The content of the resin is preferably 15 parts by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the pigment. When the resin content is 15 parts by mass or more with respect to 100 parts by mass of the pigment, strike-through is unlikely to occur in the recording medium after image formation. When the content of the resin is 100 parts by mass or less with respect to 100 parts by mass of the pigment, an image having a desired image density can be formed on the recording medium.

(Surfactant)
The ink may contain a surfactant, if desired. If the ink contains a surfactant, the wettability of the ink with respect to the recording medium is improved. Examples of the surfactant include anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants. The surfactant contained in the ink is preferably a nonionic surfactant. The nonionic surfactant is preferably a surfactant having an acetylene glycol structure, and more preferably an acetylene diol ethylene oxide adduct. The HLB value of the surfactant is preferably 3 or more and 20 or less, more preferably 6 or more and 16 or less, and further preferably 8 or more and 10 or less. The HLB value of the surfactant is calculated, for example, by the Griffin method from the formula "HLB value = 20 × (sum of formulas of hydrophilic portions) / molecular weight". In order to improve the image density while suppressing the offset of the image, the content of the surfactant is preferably 0.1% by mass or more and 10.0% by mass or less with respect to the mass of the ink, and is 0. It is more preferably 1% by mass or more and 5.0% by mass or less, and further preferably 0.1% by mass or more and 2.0% by mass or less.

(Dissolution stabilizer)
The ink may contain a dissolution stabilizer, if desired. When the ink contains a dissolution stabilizer, the components contained in the ink are easily compatible with each other, and the dissolved state of the ink can be stabilized. Dissolution stabilizers include, for example, 2-pyrrolidone, N-methyl-2-pyrrolidone, and γ-butyrolactone. The dissolution stabilizer is preferably 2-pyrrolidone. The content of the dissolution stabilizer is preferably 0.1% by mass or more and 20.0% by mass or less, and more preferably 0.1% by mass or more and 10.0% by mass or less with respect to the mass of the ink. It is preferably 0.1% by mass or more and 1.0% by mass or less.

(Moisturizer)
The ink may contain a moisturizer, if desired. If the ink contains a moisturizer, the volatilization of liquid components from the ink can be suppressed. Moisturizers include, for example, polyalkylene glycols, alkylene glycols, and glycerin. Examples of polyalkylene glycols include polyethylene glycol and polypropylene glycol. Examples of alkylene glycols include 3-methyl-1,5-pentanediol, ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, trimethylene glycol (that is, 1,3-propanediol), and triethylene. Glycol, tripropylene glycol, 1,2,6-hexanetriol, thiodiglycol, 1,3-butanediol, and 1,5-pentanediol can be mentioned. The moisturizer is preferably at least one of alkylene glycols and glycerin, and more preferably at least one of 3-methyl-1,5-pentanediol and glycerin. The content of the moisturizer is preferably 2.0% by mass or more and 50.0% by mass or less, and more preferably 5.0% by mass or more and 40.0% by mass or less with respect to the mass of the ink. ..

(Penetrating agent)
The ink may contain a penetrant, if desired. If the ink contains a penetrant, the permeability of the ink to the recording medium is improved. Examples of the penetrant include 1,2-hexylene glycol, 1,2-octanediol, 2,4-diethyl-1,5-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, and the like. Examples thereof include triethylene glycol monobutyl ether and diethylene glycol monobutyl ether. The penetrant is preferably triethylene glycol monobutyl ether. The content of the penetrant is preferably 0.5% by mass or more and 20.0% by mass or less, and more preferably 0.5% by mass or more and 5.0% by mass or less with respect to the mass of the ink. ..

(Other ingredients)
If necessary, the ink may contain components other than those already described (more specifically, a viscosity modifier and the like).

<Ink manufacturing method>
Next, an example of the method for producing the ink of the present embodiment will be described.

(Preparation of pigment dispersion)
First, a pigment dispersion is prepared. Specifically, a resin, a pigment, and an aqueous medium are kneaded using a media-type disperser to obtain a pigment dispersion liquid containing a large number of pigment particles. Ink components (more specifically, at least one of a surfactant, a dissolution stabilizer, a moisturizer, a penetrant, and a viscosity modifier) added as needed are further added and kneaded. You may.

(Ink preparation)
Next, the obtained pigment dispersion, gallium-doped zinc oxide, and an aqueous medium are mixed using a stirrer. Ink components (more specifically, at least one of a surfactant, a dissolution stabilizer, a moisturizer, a penetrant, and a viscosity modifier) to be added as needed are further added and mixed. You may. The resulting mixture is filtered as needed. As a result, ink is obtained. The method for producing the ink of the present embodiment has been described above.

Next, examples of the present invention will be described. In the evaluation in which an error occurs, a considerable number of measured values in which the error is sufficiently small are obtained, and the arithmetic mean of the obtained measured values is used as the evaluation value.

[Ink preparation]
Inks (A-2) to (A-4) according to Examples and inks (B-1) according to Comparative Examples were prepared. The configurations of the inks (A-2) to (A-4) and (B-1) are shown in Table 1. The detailed configuration of "ink (B-1)" in Table 1 is shown in Table 2. The detailed configuration of the "pigment dispersion C" in Table 2 is shown in Table 3.

In Tables 1 to 3, "wt%" indicates mass%. For the preparation of inks (A-2) to (A-4), gallium-doped zinc oxide dispersion (“Pazette GK” manufactured by HakusuiTech Co., Ltd., gallium-doped zinc oxide concentration: 20% by mass, gallium-doped zinc oxide particles. Number average primary particle size: 30 nm, dispersion medium: mixed solution of water and butanol) was used. The "dispersion content" of "gallium-doped zinc oxide" in Table 1 indicates the content (unit: mass%) of the gallium-doped zinc oxide dispersion with respect to the mass of the ink. The "substantial content" of "gallium-doped zinc oxide" in Table 1 indicates the content of gallium-doped zinc oxide (unit: mass%) with respect to the mass of the ink. The substantial content of gallium-doped zinc oxide is determined by the formula "substantial content = content of gallium-doped zinc oxide dispersion x (20/100)" in consideration of the concentration of gallium-doped zinc oxide in Pazette GK. Calculated from.

<Preparation of resin SA>
First, the "resin SA" shown in Table 3 was synthesized. Specifically, a stirrer, a nitrogen introduction tube, a condenser (agitator), and a dropping funnel were set in a four-necked flask (capacity: 1000 mL). Next, 100 g of isopropyl alcohol and 300 g of methyl ethyl ketone were placed in a flask. While bubbling nitrogen into the contents of the flask, heating and refluxing was performed at 70 ° C.

Also, 40.0 g of styrene, 10.0 g of methacrylic acid, 40.0 g of methyl methacrylate, 10.0 g of n-butyl acrylate, and 0.4 g of azobisisobutyronitrile (AIBN, The polymerization initiator) was mixed to obtain a monomer solution. The monomer solution was added dropwise to the flask over 2 hours while being heated to reflux at 70 ° C. After the dropping, the mixture was heated under reflux at 70 ° C. for another 6 hours.

A methyl ethyl ketone solution containing 0.2 g of AIBN was added dropwise to the flask over 15 minutes. After the dropping, the mixture was heated under reflux at 70 ° C. for another 5 hours. In this way, resin SA (styrene-acrylic resin) was obtained. The mass average molecular weight (Mw) of the obtained resin SA was 20000, and the acid value was 100 mgKOH / g.

The mass average molecular weight of the resin SA was measured using gel filtration chromatography (“HLC-8020GPC” manufactured by Tosoh Corporation) under the following conditions.
Column: "TSKgel SuperMultipore HZ-H" manufactured by Tosoh Corporation (4.6 mm ID x 15 cm semi-micro column)
Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35 mL / min Sample injection volume: 10 μL
Measurement temperature: 40 ° C
Detector: IR detector

In addition, TSKgel standard polystyrene (7 kinds of F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000) manufactured by Tosoh Corporation, and n-propylbenzene. Was used to create a calibration curve.

The acid value of the resin SA is as described in "JIS (Japanese Industrial Standards) K0070-1992 (Test method for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponified product of chemical products)". Compliantly requested.

<Preparation of pigment dispersion C>
15.0 parts by mass of resin SA and 25.0 mass by mass on the vessel (capacity: 0.6 L) of a media type disperser (“Dyno (registered trademark) mill” manufactured by Willy et Bacoffen (WAB)) A portion of carbon black (“MA100” manufactured by Mitsubishi Chemical Corporation), 10.0 parts by mass of glycerin, and 50.0 parts by mass of water were added. Next, media (glass beads having a diameter of 0.5 mm) having a mass 1.5 times the mass of the contents of the vessel was added to the vessel. The contents of the vessel were kneaded using a media-type disperser while water-cooling the vessel under the conditions of a temperature of 10 ° C. and a peripheral speed of 8 m / sec. The contents of Vessel were kneaded until the volume median diameter (D ₅₀ ) of the pigment particles was in the range of 70 nm or more and 130 nm or less. In this way, the pigment dispersion liquid C was obtained.

<Preparation of ink (B-1)>
Next, an ink (B-1) was prepared. Specifically, 8.0 parts by mass of pigment dispersion C, 56.2 parts by mass of water, 7.7 parts by mass of glycerin, and 23.1 parts by mass of 3-methyl-1,5-pentanediol. , 3.0 parts by mass of triethylene glycol monobutyl ether, 1.5 parts by mass of 2-pyrrolidone, and 0.5 parts by mass of a surfactant (Nissin Chemical Industry Co., Ltd. "Surfinol (registered trademark) 440" , The acetylene glycol ethylene oxide adduct, HLB value: 8) was charged into the reaction vessel. The contents in the reaction vessel were stirred under the condition of a rotation speed of 400 rpm using a stirrer (“Three One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.). The obtained liquid was filtered using a filter (pore diameter: 5 μm). In this way, the ink (B-1) was obtained.

<Preparation of ink (A-2)>
A gallium-doped zinc oxide dispersion (“Pazette GK” manufactured by HakusuiTech Co., Ltd., 0.5 parts by mass) and 99.5 parts by mass of the ink (B-1) prepared in the above <Preparation of ink (B-1)>. Was put into the reaction vessel. The contents in the reaction vessel were stirred under the condition of a rotation speed of 400 rpm using a stirrer (“Three One Motor BL-600” manufactured by Shinto Kagaku Co., Ltd.). The obtained liquid was filtered using a filter (pore diameter: 5 μm). In this way, the ink (A-2) was obtained.

<Preparation of inks (A-3) and (A-4)>
The addition amounts of the gallium-doped zinc oxide dispersion and the ink (B-1) were changed so that the content of the gallium-doped zinc oxide dispersion and the content of the ink (B-1) were as shown in Table 1. Each of the inks (A-3) and (A-4) was prepared in the same manner as the ink (A-2) except for the above.

[Evaluation methods]
<Measurement of brightness and saturation>
By the bar coating method, inks (specifically, inks (A-2) to (A-4) and (B-1), respectively) were applied to plain paper for inkjet and dried. In this way, an image (whole surface solid image) was formed on the paper using ink. The brightness and saturation of the formed image were measured using a densitometer (“Fluorescence spectrophotometer FD-7” manufactured by Konica Minolta Co., Ltd.). Among the inks (A-2) to (A-4) and (B-1), as a representative example, the measurement results of the brightness and saturation of the inks (B-1) and (A-2) are shown in Table 4. Shown.

In Table 4, the "difference" of "brightness" indicates a value calculated from the formula "brightness difference = lightness of ink (A-2) -brightness of ink (B-1)". Further, in Table 4, the "difference" of "saturation" indicates a value calculated from the formula "saturation difference = saturation of ink (A-2) -saturation of ink (B-1)".

<Evaluation of brightness improvement>
The brightness of the ink (A-2) in the above formula "brightness difference = brightness of the ink (A-2) -brightness of the ink (B-1)" is determined by the brightness of the inks (A-3) and (A-4). The brightness difference between the inks (A-3) and (A-4) was calculated by the same method as the calculation of the brightness difference of the ink (A-2) except that the color was changed to. Based on the difference in brightness of each ink, gallium-doped zinc oxide-containing inks (A-2) to (A-4) were used as compared with gallium-doped zinc oxide-free ink (B-1) according to the following criteria. It was evaluated whether or not the brightness of the formed image was improved. The evaluation results are shown in Table 5. When the evaluation is A, it indicates that the brightness of the image formed by using each ink is improved as compared with the gallium-doped zinc oxide-free ink (B-1).

(Evaluation criteria for improving brightness)
Evaluation A: The difference in brightness is 0.50 or more.
Evaluation B: The difference in brightness is less than 0.50.

<Evaluation of saturation maintenance>
The saturation of the ink (A-2) in the above formula "saturation difference = saturation of the ink (A-2)-saturation of the ink (B-1)" is determined by the inks (A-3) and (A-4). The saturation difference between the inks (A-3) and (A-4) was calculated by the same method as the calculation of the saturation difference of the ink (A-2) except that the saturations of the inks (A-2) were changed. Based on the saturation difference of each ink, the gallium-doped zinc oxide-containing inks (A-2) to (A-4) were compared with the gallium-doped zinc oxide-free ink (B-1) according to the following criteria. It was evaluated whether the saturation of the image formed using the product was maintained within a desired range. The evaluation results are shown in Table 5. When the evaluation is A, the saturation of the image formed by each ink is maintained within a desired range as compared with the gallium-doped zinc oxide-free ink (B-1). Show that.

(Evaluation criteria for maintaining saturation)
Evaluation A: The saturation difference is less than 0.15.
Evaluation B: The saturation difference is 0.15 or more.

As is clear from Tables 4 and 5, the brightness of the image formed by using the gallium-doped zinc oxide-free ink (B-1) and the gallium-doped zinc oxide-containing inks (A-2) to ( The difference from the brightness of the image formed by using A-4) was 0.50 or more. Therefore, the gallium-doped zinc oxide-containing inks (A-2) to (A-4) are used as compared with the brightness of the image formed by using the gallium-doped zinc oxide-free ink (B-1). It is judged that the brightness of the formed image is improved.

As is clear from Tables 4 and 5, the saturation of the image formed by using the gallium-doped zinc oxide-free ink (B-1) and the gallium-doped zinc oxide-containing ink (A-2) to The difference from the saturation of the image formed using (A-4) was less than 0.15. Therefore, the gallium-doped zinc oxide-containing inks (A-2) to (A-4) are used to the same degree as the saturation of the image formed by using the gallium-doped zinc oxide-free ink (B-1). It is judged that the saturation of the image formed by using the ink was maintained. When the ink contains non-gallium-doped zinc oxide, the saturation of the ink tends to change significantly. However, since the inks (A-2) to (A-4) contained gallium-doped zinc oxide instead of non-gallium-doped zinc oxide, they were formed using the inks (A-2) to (A-4). It is determined that the saturation of the obtained image could be maintained within a desired range.

<Measurement of reflectance>
Using a densitometer (“Fluorescence spectroscopic densitometer FD-7” manufactured by Konica Minolta Co., Ltd.), measure the reflectance of an image formed with ink (A-2) in the above <Measurement of brightness and saturation>. did. In the same method, the reflectance of the image formed by using the ink (B-1) in the above <Measurement of brightness and saturation> was also measured. The reflectance of the image with respect to the light of each wavelength was measured while changing the wavelength of the light in the range of 380 nm or more and 730 nm or less. FIG. 1 shows the measurement results of the reflectance of the images formed by using the inks (A-2) and (B-1). The vertical axis in FIG. 1 indicates the reflectance of the image, and the horizontal axis indicates the wavelength (unit: nm) of the light irradiating the image. The solid line in FIG. 1 shows the measurement result of the reflectance of the image formed by using the ink (A-2). The broken line in FIG. 1 shows the measurement result of the reflectance of the image formed by using the ink (B-1). As is clear from FIG. 1, the reflectance of the image formed using the gallium-doped zinc oxide-containing ink (A-2) is higher than that of the gallium-doped zinc oxide-free ink (B-1). it was high. It is judged that the brightness of the image formed by using the gallium-doped zinc oxide-containing ink (A-2) is improved because of the high reflectance.

From the above, it is shown that the ink according to the present invention can be used for forming images of various colors, and can form an image having excellent brightness on a recording medium while maintaining saturation within a desired range. It was.

The ink according to the present invention can be used, for example, to form an image on a recording medium using an inkjet printer.

Claims (4)

An inkjet ink containing gallium-doped zinc oxide, a pigment, and an aqueous medium. The inkjet ink according to claim 1, wherein the content of the gallium-doped zinc oxide is 0.1% by mass or more and 0.4% by mass or less. The inkjet ink according to claim 1 or 2, wherein the pigment is a non-white pigment. The inkjet ink according to any one of claims 1 to 3, wherein the pigment is carbon black.

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