Method for producing pigment composition, method for producing pigment dispersion, method for producing aqueous ink, and inkjet recording method

Abstract

To provide a method for producing a pigment composition which enables preparation of a pigment ink with superior bronze resistance.SOLUTION: A method for producing a pigment composition includes a kneading step of kneading a mixture containing a pigment, a water-soluble inorganic salt, an organic solvent, and a water-insoluble fluorescent brightener by the solvent salt milling technique to provide a kneaded product.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a pigment composition, a method for producing a pigment dispersion, a method for producing a water-based ink, and an inkjet recording method. [Background technology]

[0002] Inkjet recording methods can be easily made full color, and can provide high-definition images comparable to those of silver halide photography or offset printing at a relatively low cost. Inkjet recording methods generally use cyan, magenta, yellow, and black inks, but in some cases, special color inks such as blue, green, and red are also used to further expand the color gamut.

[0003] Copper phthalocyanine pigments are widely used as coloring materials for cyan inks because of their high coloring power. Similarly, dioxazine violet pigments are widely used as coloring materials for blue inks. A major problem when recording an image using an aqueous ink containing a pigment as a coloring material (hereinafter, sometimes referred to as "pigment ink"), is that the image appears reddish or yellowish due to the so-called bronzing phenomenon. This phenomenon is particularly noticeable on recording media such as glossy paper, which has a smooth surface and on which pigments are likely to be present.

[0004] As a solution to such problems, for example, Patent Document 1 discloses an ink containing a copper phthalocyanine pigment and a urethane resin. Also, Patent Document 2 discloses an inkjet recording method in which a colorless ink containing a styrene-acrylic acid copolymer is ejected onto an image recorded with an ink containing a copper phthalocyanine pigment.

[0005] On the one hand, Patent Document 3 discloses a coating composition for forming a high-chroma coating film in which a fluorescent whitening agent is added to a coating composition containing a blue-based organic pigment such as copper phthalocyanine blue and dioxazine violet for the purpose of improving the chroma of the coating film. Further, Patent Document 4 discloses an aqueous pigment ink composition for inkjet containing, in addition to a phthalocyanine-based blue pigment, a colored resin dyed with a fluorescent whitening agent and / or a fluorescent yellow dye for the purpose of suppressing the greenish tinge and redness in dark colors peculiar to pigments.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, as a result of the study by the present inventors, it has been clarified that it is difficult to achieve high-level bronze resistance by using any of the methods disclosed in Patent Documents 1 to 4.

[0008] For example, the pigment ink described in Patent Document 1 attempts to suppress the bronze phenomenon by adding a urethane resin in an amount equal to or more than a certain amount to a copper phthalocyanine pigment. However, it has been found that the above pigment ink shows an effect of suppressing the bronze phenomenon when the pigment content is low, but shows almost no effect when the pigment content is high.

[0009] In addition, the method described in Patent Document 2 attempts to suppress the bronze phenomenon by ejecting a colorless ink containing a styrene-acrylic acid copolymer onto an image recorded with an ink containing a copper phthalocyanine pigment. However, since this method requires the use of a colorless ink in addition to ordinary ink, there are problems such as complication of recording control. Moreover, when an image is recorded on a highly permeable recording medium such as art paper, it has been found that the resin in the colorless ink easily penetrates into the interior of the recording medium, making it difficult to sufficiently suppress the bronze phenomenon.

[0010] In addition, Patent Document 3 mentions that the chroma of a coating film is improved by an oil-based coating composition for forming a coating film to which a water-insoluble fluorescent brightening agent is added in addition to pigments such as copper phthalocyanine blue and dioxazine violet. When a pigment ink was prepared by dispersing the water-insoluble fluorescent brightening agent disclosed in Patent Document 3 together with the pigment in an aqueous medium using a resin with reference to this coating composition, it was found that the suppression of the bronze phenomenon was insufficient.

[0011] Furthermore, Patent Document 4 mentions that an aqueous pigment ink composition containing a phthalocyanine-based blue pigment and a colored resin dyed with a fluorescent brightening agent and / or a fluorescent yellow dye suppresses the turbidity and redness in dark colors peculiar to the pigment. However, it has been found that even when such a colored resin is used, the suppression of the bronze phenomenon is insufficient.

[0012] Therefore, an object of the present invention is to provide a method for producing a pigment composition capable of producing a pigment ink excellent in bronze resistance. Another object of the present invention is to provide a method for producing a pigment dispersion, a method for producing an aqueous ink, and an inkjet recording method using the pigment composition obtained by the above production method.

Means for Solving the Problems

[0013] That is, according to the present invention, there is provided a method for producing a pigment composition including a kneading step of kneading a mixture containing a pigment, a water-soluble inorganic salt, and an organic solvent by a solvent-salt milling method, wherein the mixture further contains a water-insoluble optical brightening agent.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a method for producing a pigment composition capable of producing a pigment composition capable of preparing a pigment ink excellent in bronze resistance. Further, according to the present invention, it is possible to provide a method for producing a pigment dispersion, a method for producing an aqueous ink, and an inkjet recording method using the pigment composition obtained by the above production method.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, the present invention will be described in more detail with reference to preferred embodiments. In the present invention, when a compound is a salt, although the salt is dissociated into ions in the ink, for convenience, it is expressed as "containing a salt". Further, an aqueous ink for inkjet may be simply referred to as "ink". Physical property values are values at room temperature (25°C) unless otherwise specified.

[0016] As a result of studies by the present inventors, by using a pigment composition obtained through kneading a mixture containing a pigment, a water-soluble inorganic salt, an organic solvent, and a water-insoluble optical brightening agent by a solvent-salt milling method, it was found that an aqueous ink excellent in bronze resistance can be obtained. It is considered that the action of the optical brightening agent improves the bronze resistance of the ink. The reason is presumably that, for example, with respect to the red bronze light derived from a copper phthalocyanine pigment, the blue fluorescence generated from the excited optical brightening agent undergoes additive color mixing to approach white, making it difficult to recognize.

[0017] When the inventors added a water-soluble optical brightening agent directly to the pigment ink in order to incorporate the optical brightening agent into the pigment ink, they were unable to improve the bronze resistance. This is presumably because the following phenomena occur when an image is recorded on a recording medium by an inkjet recording method using a pigment ink containing a water-soluble optical brightening agent. That is, when recording, the water-soluble optical brightening agent easily penetrates into the recording medium and cannot sufficiently absorb the ultraviolet rays necessary for excitation. Also, the fluorescence generated is difficult to visually recognize because it is blocked by the pigment layer that exists relatively above on the recording medium.

[0018] Therefore, a water-insoluble optical brightening agent was used, and this water-insoluble optical brightening agent was dispersed in an aqueous medium using a resin dispersant together with a pigment to prepare a pigment ink. As a result, an improvement in bronze resistance was confirmed, but it was not at the level desired by the inventors. Although the water-insoluble optical brightening agent is considered to remain on the recording medium, it is not always present in the vicinity of the pigment, and it is presumed that absorption and reduction of excitation light and fluorescence occur due to shielding by the pigment. Alternatively, in the dispersed particles of the optical brightening agent, it is also presumed that self-absorption and concentration quenching occur due to the local presence of a large amount of the optical brightening agent, resulting in a decrease in fluorescence.

[0019] Similarly, when a pigment ink containing a resin particle in which an optical brightening agent is dyed and encapsulated is used to suppress the localization of the optical brightening agent, the bronze resistance was not at the level desired by the inventors. It is presumed that the optical brightening agent is easily affected by shielding by the pigment because it exists as resin particles separate from the pigment, and is also affected by the absorption of excitation light and fluorescence by the resin particle component.

[0020] From the above results, the inventors came to the idea that retaining the optical brightener on the pigment surface is important for improving bronzing resistance. Therefore, first, the pigment and the water-insoluble optical brightener were mixed and stirred in an organic solvent. After washing this with water and dispersing it in an aqueous medium, a pigment ink was prepared, but the bronzing resistance was not at the level desired by the inventors. It is presumed that the optical brightener could not be retained on the pigment surface by a simple stirring operation.

[0021] Next, the inventors focused on the solvent-salt milling method. The solvent-salt milling method is generally a technique for kneading a mixture containing a pigment, a water-soluble inorganic salt, and an organic solvent. The pigment is crushed by the water-soluble inorganic salt, creating an energetically unstable crushed surface. The crushed surfaces of the finely crushed pigments adsorb to each other and crystalize, thereby adjusting the particle diameter of the pigment. The inventors thought that if the strong adsorptive force of the crushed surface of the pigment created by the solvent-salt milling method was utilized, it would be possible to retain the optical brightener on the pigment surface.

[0022] Therefore, the inventors blended a pigment, a water-soluble inorganic salt, a water-insoluble optical brightener, and an organic solvent, kneaded them by the solvent-salt milling method to produce a kneaded product, and prepared an ink using the pigment composition obtained after washing this kneaded product with water. Then, when the bronzing resistance was evaluated using that ink, a significant improvement in bronzing resistance was confirmed. As described above, by using the pigment composition obtained through a kneading step of kneading a mixture containing a pigment, a water-insoluble optical brightener, a water-soluble inorganic salt, and an organic solvent by the solvent-salt milling method, it is possible to prepare a pigment ink having excellent bronzing resistance.

[0023] <Method for manufacturing a pigment composition> The manufacturing method according to an embodiment of the present invention is a method for manufacturing a pigment composition including a kneading step of kneading a mixture containing a pigment, a water-soluble inorganic salt, and an organic solvent by the solvent-salt milling method to obtain a kneaded product. The above mixture further contains a water-insoluble optical brightener. Hereinafter, the kneading step, each material used, etc. will be described in detail.

[0024] [Kneading Step] In the kneading step in the method for producing the pigment composition of the present embodiment, a mixture containing a pigment, a water-insoluble fluorescent brightening agent, a water-soluble inorganic salt, and an organic solvent is kneaded by the solvent-salt milling method to obtain a kneaded product. The solvent-salt milling method is a method of kneading a mixture containing a pigment, a water-soluble inorganic salt, and an organic solvent while applying a load to the mixture to compress it using a kneading device. In the method for producing the pigment composition of the present embodiment, a kneaded product containing a refined pigment, a water-insoluble fluorescent brightening agent, a water-soluble inorganic salt, and an organic solvent can be obtained by the kneading step by the solvent-salt milling method.

[0025] As the kneading device, for example, batch-type and continuous-type, as well as normal-pressure type, pressure-type, and reduced-pressure type kneading devices can be used, and a device that kneads while applying a load to the contents to compress them can be preferably used. Also, a kneading device equipped with a material input section such as a kneading kettle and a hopper, and a stirring section such as a stirring blade, stirring vane, blade, screw, and roll for stirring the material can be preferably used. Specific examples of the kneading device include kneading devices such as a kneader, a roll mill, a ball mill, an attritor, a sand mill, a planetary mixer, and a continuous single-screw kneader. Examples of the planetary mixer include Trimix (trade name) manufactured by Inoue Manufacturing Co., Ltd. Examples of the continuous single-screw kneader include Miracle KCK (trade name) manufactured by Asada Iron Works. Among the kneading devices listed above, it is preferable to use a planetary mixer.

[0026] In the method for producing the pigment composition of the present embodiment, in order to adsorb the water-insoluble fluorescent brightening agent onto the pigment surface, it is important to create a crushed surface of the pigment during kneading. Therefore, kneading is performed by the solvent-salt milling method that applies a strong load to the pigment. When specifically expressing this "applying a load", it can be expressed by the "shearing rate". The shearing rate during kneading in the kneading step is preferably 10 s -1 or more, and 15 s -1More preferably, it is as described above. The shearing rate is 15 s -1 By performing kneading at the above rate or higher, sufficient shearing force is obtained, and the adsorption amount of the water-insoluble fluorescent whitening agent on the pigment surface increases, making it easier to enhance the bronze resistance. From this perspective, the shearing rate is preferably 20 s -1 or higher, and more preferably so.

[0027] The temperature during kneading in the kneading process is preferably 0°C or higher and 130°C or lower, more preferably 10°C or higher and 120°C or lower, and even more preferably 20°C or higher and 90°C or lower.

[0028] Also, the kneading time in the kneading process is preferably 1 hour or longer and 10 hours or shorter, and more preferably 2 hours or longer and 8 hours or shorter. When the kneading time is 2 hours or longer, the adsorption amount of the water-insoluble fluorescent whitening agent on the pigment surface is likely to be sufficient, and the bronze resistance is likely to be improved.

[0029] In the kneading process, the amount of the water-insoluble optical brightening agent used is preferably 0.2% by mass or more and 8.0% by mass or less, more preferably 0.5% by mass or more and 5.0% by mass or less, based on the amount of the pigment used. When the amount of the water-insoluble optical brightening agent used is 0.5% by mass or more based on the amount of the pigment used, it becomes easier to ensure the amount of the water-insoluble optical brightening agent adsorbed on the pigment surface and to enhance the bronze resistance. From this viewpoint, the ratio of the amount of the water-insoluble optical brightening agent used based on the amount of the pigment used is more preferably 1.0% by mass or more. On the other hand, when the amount of the water-insoluble optical brightening agent used is 5.0% by mass or less based on the amount of the pigment used, self-absorption and concentration quenching are less likely to occur, so that a decrease in fluorescence caused by these effects is less likely to occur, and the bronze resistance is more likely to be enhanced. Further, in this case, most of the optical brightening agent used is adsorbed on the pigment surface, and the amount of the optical brightening agent that cannot be adsorbed on the pigment surface is suppressed, so that the storage stability and ejection stability of the ink are also likely to be enhanced. From these viewpoints, the ratio of the amount of the water-insoluble optical brightening agent used based on the amount of the pigment used is more preferably 4.0% by mass or less. The ratio (% by mass) of the amount of the water-insoluble optical brightening agent used based on the amount of the pigment used as described above is determined by {mass of the water-insoluble optical brightening agent used / mass of the pigment used} × 100 in the kneading process.

[0030] Regarding the mixing ratio of the pigment, water-soluble inorganic salt, and organic solvent in the kneading process, it is preferably the following ratio based on the amount of the pigment used. In the kneading process, the amount of the water-soluble inorganic salt used is preferably 3.0 times or more and 20.0 times or less, more preferably 5.0 times or more and 10.0 times or less, in terms of the mass ratio to the amount of the pigment used. Also, in the kneading process, the amount of the organic solvent used is preferably 0.5 times or more and 5.0 times or less, more preferably 0.7 times or more and 3.0 times or less, in terms of the mass ratio to the amount of the pigment used.

[0031] (Pigment) Examples of pigments used in the kneading process include carbon black and organic pigments. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of organic pigments include azo pigments, copper phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, imidazolone pigments, diketopyrrolopyrrole pigments, dioxazine pigments, perinone pigments, and indanthrone pigments. One of these pigments may be used alone, or two or more thereof may be used in combination.

[0032] Among these pigments, copper phthalocyanine pigments and dioxazine violet pigments are frequently used in cyan inks and blue inks due to their high coloring power, etc., while strong bronze phenomena in red and yellow are likely to be observed. Therefore, applying a water-insoluble optical brightener to these pigments is preferable in terms of whitening the bronze light by the blue fluorescence generated from the water-insoluble optical brightener. Specifically, it is preferable to use C.I. Pigment Blue 15:3 as the copper phthalocyanine pigment and C.I. Pigment Violet 23 as the dioxazine violet pigment. That is, the pigment used in the kneading process is preferably at least one selected from the group consisting of C.I. Pigment Blue 15:3 and C.I. Pigment Violet 23.

[0033] (Water-insoluble optical brightener) The water-insoluble optical brightener used in the kneading process is not particularly limited as long as it is an optical brightener having a solubility in water of 0.2 mass% or less at 25°C (an optical brightener of 0.2 g / 99.8 g-H2O or less), and it can be arbitrarily selected from those commonly available on the market and used.

[0034] Examples of the water-insoluble fluorescent brighteners include stilbene-based fluorescent brighteners, distyryl biphenyl-based fluorescent brighteners, coumarin-based fluorescent brighteners, naphthalimide-based fluorescent brighteners, pyrene-based fluorescent brighteners, and azole-based fluorescent brighteners. These water-insoluble fluorescent brighteners may be used alone or in combination of two or more kinds.

[0035] Among the water-insoluble fluorescent brighteners listed above, the azole-based fluorescent brighteners are further classified into oxazole-based, thiazole-based, imidazole-based, triazole-based, pyrazole-based, benzoxazole-based, and bisbenzoxazole-based ones. Since these azole-based fluorescent brighteners have a heterocyclic ring containing a nitrogen atom (nitrogen-containing heterocyclic ring) in their structures, they have a high affinity for the copper phthalocyanine pigments and dioxazine violet pigments that are suitable as the above-described pigments. From this viewpoint, it is preferable that the water-insoluble fluorescent brightener is at least one selected from azole-based fluorescent brighteners.

[0036] Specific examples of the azole-based fluorescent brighteners include C.I. Fluorescent Brightener 41, 45, 70, 121, 133, 135, 181, 184, 185, 189, 190, 216, 229, 236, 316, 317, 354, 367, 368, 369, 386, 389, 393, and OB-2. Examples of the stilbene-based fluorescent brighteners include C.I. Fluorescent Brightener 199. Examples of the coumarin-based fluorescent brighteners include C.I. Fluorescent Brightener 140.

[0037] (Water-soluble inorganic salt) The water-soluble inorganic salts used in the kneading process utilize their high hardness to crush pigments in the kneading process, contributing to the refinement of the primary particles of the pigments. The water-soluble inorganic salts are not particularly limited as long as they are inorganic salts soluble in water. Specific examples of water-soluble inorganic salts include, for example, sodium chloride, potassium chloride, sodium sulfate, zinc chloride, calcium chloride, and magnesium chloride, as well as mixtures of two or more of them. Among these, it is preferable to use sodium chloride in terms of cost.

[0038] As for the particle size of the water-soluble inorganic salts, in the particle size distribution based on volume, the cumulative 50% particle size (median particle size; D 50 ) is preferably 1 μm or more and 250 μm or less, and the cumulative 95% particle size (D 95 ) is preferably 500 μm or less. When particularly fine pigments are desired, it is preferable that the water-soluble inorganic salts used as grinding aids are also fine. Specifically, in the particle size distribution based on volume, the cumulative 50% particle size (D 50 ) is preferably 1 μm or more and 10 μm or less, and the cumulative 95% particle size (D 95 ) is preferably 20 μm or less for the water-soluble inorganic salts.

[0039] For D 50 and D 95 of the water-soluble inorganic salts, values measured using an optical microscope can be adopted. Specifically, the particle sizes of 500 water-soluble inorganic salt particles are measured using an optical microscope, and a method of calculating D 50 and D 95 from the particle size distribution based on volume can be mentioned. For the water-soluble inorganic salts used in the examples described later, D 50 and D 95 were also determined by the above measurement method.

[0040] (Organic solvent) The organic solvent used in the kneading process serves, as a first role, to moisten a mixture containing a pigment, a water-insoluble fluorescent brightening agent, and a water-soluble inorganic salt, and to make a dough (a lump formed by kneading) of appropriate firmness. As a result, in the solvent-salt milling method, a strong load is likely to be applied to the kneaded material, increasing the grinding effect and playing a part in generating a crushed surface of the pigment.

[0041] The organic solvent is not particularly limited, but organic solvents such as alcohols, glycols, ethers, and aprotic polar solvents are preferred. Specifically, for example, 2-(methoxymethoxy)ethanol, 2-butoxyethanol, 2-(isopentyloxy)ethanol, 2-(hexyloxy)ethanol, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol, triethylene glycol monomethyl ether, liquid polyethylene glycol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, dipropylene glycol, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, low molecular weight polypropylene glycol, aniline, pyridine, tetrahydrofuran, dioxane, methanol, ethanol, isopropanol, n-propanol, isobutanol, n-butanol, ethylene glycol, propylene glycol, propylene glycol monomethyl ether acetate, ethyl acetate, isopropyl acetate, acetone, methyl ethyl ketone, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone can be mentioned. One of the above water-soluble organic solvents may be used alone, or two or more thereof may be mixed and used as necessary.

[0042] In addition, in the kneading process, the organic solvent preferably plays a second role of dissolving the water-insoluble optical brightener. Therefore, the organic solvent used in the kneading process preferably contains a first organic solvent capable of dissolving the water-insoluble optical brightener, and in the kneading process, it is more preferable to use the first organic solvent capable of dissolving the water-insoluble optical brightener for a part of the organic solvent. In the kneading process, by dissolving the water-insoluble optical brightener using the first organic solvent capable of dissolving the water-insoluble optical brightener, the water-insoluble optical brightener can be efficiently adsorbed onto the pigment. Therefore, it is possible to further improve the bronze resistance.

[0043] As the first organic solvent, it is more preferable to use an organic solvent that completely dissolves the water-insoluble optical brightener, but an organic solvent capable of dissolving 0.4% by mass or more of the water-insoluble optical brightener at 25°C can be suitably used as the first organic solvent.

[0044] The ratio of the amount of the first organic solvent used to the total amount of the organic solvents used in the kneading process is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 5% by mass or more based on the total amount of the organic solvents used. Also, the above ratio of the amount of the first organic solvent used is preferably 100% by mass or less, more preferably 50% by mass or less, and even more preferably 20% by mass or less.

[0045] For example, when an azole-based optical brightener is used as the water-insoluble optical brightener, since nitrogen-containing solvents such as N-methylpyrrolidone and N,N-dimethylformamide have high solubility, they can be suitably used as the above first organic solvents. Therefore, the organic solvent preferably contains at least one selected from the group consisting of N-methylpyrrolidone and N,N-dimethylformamide, and it is more preferable to use at least one of them for a part of the organic solvent.

[0046] (Other materials) When producing a kneaded product in the kneading process, in addition to a mixture containing a pigment, a water-insoluble fluorescent brightening agent, a water-soluble inorganic salt, and an organic solvent, a dye derivative or rosin may be added for the purpose of adjusting crystal growth and crystal dislocation of the pigment. As the dye derivative to be added, a dye derivative having the same structure as the pigment as the parent body is preferable, but a dye derivative having a different structure may also be used. Examples of the substituent of the dye derivative include a hydroxyl group, a carboxy group, a carbamoyl group, a sulfonic acid group, a sulfonamide group, and a phthalimidomethyl group. In addition, organic dyes generally include light yellow aromatic polycyclic compounds such as naphthalene-based and anthraquinone-based compounds that are not generally called dyes. In particular, a dye derivative having a basic group can be preferably used because it has a large dispersion effect on the pigment. These can be used alone or in combination of two or more. Examples of rosins include rosin, hydrogenated rosin, disproportionated rosin, polymerized rosin, and esters of these with polyhydric alcohols such as glycerin, and rosin amine. These can be used alone or in combination of two or more.

[0047] 〔Other processes〕 By the above kneading process, a kneaded product containing a pigment, a water-insoluble fluorescent brightening agent, a water-soluble inorganic salt, an organic solvent, etc. can be obtained. In the method for producing the pigment composition of the present embodiment, it is preferable to perform a step of removing the water-soluble inorganic salt and the organic solvent from the kneaded product obtained by the above kneaded process. As the method, for example, after putting the kneaded product into water at a predetermined ratio with respect to the kneaded product to obtain a pigment suspension (slurry), the water-soluble inorganic salt and the organic solvent can be removed from the kneaded product by filtering and washing this pigment suspension. The filtration method is not particularly limited, but it is preferable to adopt a method of separating the above pigment suspension by passing it through an ultrafiltration membrane or a dialysis membrane, or a method of separating it with a high-pressure filter press.

[0048] Through the steps including the above-mentioned filtration, etc., a wet cake of a pigment composition from which water-soluble inorganic salts and organic solvents have been separated is obtained. Considering the growth of bacteria, it is preferable to dry the obtained wet cake so that the water content is 30% by mass or less. Examples of the drying method include batch or continuous drying in which dehydration and / or desolvation of the wet cake is performed by heating at 80°C or higher and 120°C or lower using a heat source installed in a dryer. Examples of the dryer include a box dryer, a band dryer, and a spray dryer.

[0049] <Method for producing pigment dispersion> Using the pigment composition obtained by the above-described method for producing a pigment composition, it is possible to produce a pigment dispersion. The method for producing the pigment dispersion includes a dispersion step of mixing the pigment composition obtained by the above-described method for producing a pigment composition, a dispersant, and water, and dispersing the pigment composition in water with the dispersant. As the pigment composition, it is preferable to use a pigment composition containing a pigment and a water-insoluble fluorescent brightening agent obtained by removing water-soluble inorganic salts and organic solvents from the kneaded product obtained in the above-described method for producing a pigment composition. As the water, it is preferable to use deionized water such as ion-exchanged water or pure water.

[0050] 〔Dispersion step〕 As a dispersion method for dispersing a pigment in water as a dispersion medium, for example, a resin-dispersed pigment using a resin as a dispersant, a self-dispersed pigment having a hydrophilic group bonded to the particle surface of the pigment, or the like can be used. Further, a resin-bonded pigment in which an organic group containing a resin is chemically bonded to the particle surface of the pigment, a microcapsule pigment in which the particle surface of the pigment is coated with a resin or the like, or the like can be used. In order to disperse the above-described pigment composition in water as a dispersion medium, it is preferable to use a resin (resin dispersant) or a surfactant as a dispersant. Among them, it is more preferable to use a resin (resin having an anionic group) capable of stably dispersing the pigment composition in the dispersion medium by the action of an anionic group.

[0051] As the resin, a resin having a structural unit derived from a hydrophobic monomer and a structural unit derived from a hydrophilic monomer having an anionic group, which is obtained by copolymerizing a polymerizable hydrophobic monomer and a polymerizable hydrophilic monomer, is preferable. Examples of the hydrophobic monomer include styrene, α-methylstyrene, n-butyl acrylate, n-hexyl acrylate, benzyl methacrylate, and the like. One or more of the hydrophobic monomers can be used. Examples of the hydrophilic monomer include hydrophilic monomers having a carboxy group such as acrylic acid, methacrylic acid, crotonic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, and fumaric acid; hydrophilic monomers having a sulfonic acid group such as styrene sulfonic acid, sulfonic acid-2-propylacrylamide, 2-sulfonic acid ethyl acrylate, 2-sulfonic acid ethyl methacrylate, and butylacrylamide sulfonic acid; hydrophilic monomers having a phosphonic acid group such as 2-phosphonic acid ethyl methacrylate and 2-phosphonic acid ethyl acrylate; and the like. One or more of the hydrophilic monomers can be used.

[0052] The weight average molecular weight of the resin used as the dispersant is preferably 1,000 or more and 30,000 or less, and more preferably 3,000 or more and 15,000 or less. The weight average molecular weight of the resin can be a value in terms of standard polystyrene measured using gel permeation chromatography (GPC). The acid value (mgKOH / g) of the resin is preferably 40 mgKOH / g or more and 300 mgKOH / g or less, and more preferably 100 mgKOH / g or more and 250 mgKOH / g or less. The acid value of the resin can be a value measured by a potentiometric titration apparatus using a potassium hydroxide-methanol titrant. The amount of the resin used is preferably 10% by mass or more and 50% by mass or less based on the amount of the pigment used.

[0053] When dispersing a pigment composition in a dispersion medium, a dispersing device can be used. Examples of the dispersing device include an ultrasonic homogenizer, a high-pressure homogenizer, a paint shaker, a ball mill, a sand mill, a sand grinder, a dyno mill, a dispermat, an SC mill, a spike mill, a nanomizer, an agitator mill, and a planetary mill.

[0054] The content (mass%) of the pigment in the pigment dispersion is preferably 1.0 mass% or more and 50.0 mass% or less, more preferably 5.0 mass% or more and 30.0 mass% or less, based on the total mass of the pigment dispersion. The content (mass) of the water-insoluble optical brightener in the pigment dispersion is preferably 0.2 mass% or more and 8.0 mass% or less, more preferably 0.5 mass% or more and 5.0 mass% or less, based on the content (mass) of the pigment in the pigment dispersion. This ratio is even more preferably 1.0 mass% or more and 4.0 mass% or less.

[0055] The pigment composition and the pigment dispersion containing the same can be suitably used for any application that requires a coloring function. Examples of such applications include paints, printing inks, colored molded articles, toners for electrostatic charge image development, color filters for liquid crystal display devices, and inks for inkjet. Among these, aqueous inks for inkjet are preferred.

[0056] <Method for manufacturing ink> It is possible to manufacture an aqueous ink using the pigment dispersion obtained by the method for manufacturing the pigment dispersion described above. The method for manufacturing the aqueous ink includes a step of mixing the pigment dispersion obtained by the method for manufacturing the pigment dispersion described above and components containing a water-soluble organic solvent. When preparing the ink in this step, in addition to the aforementioned pigment dispersion and water-soluble organic solvent, water and other additives that are used as needed can be blended as the above components to prepare the ink.

[0057] (Colorant) Since the ink contains the pigment dispersion obtained by the above-described method for producing a pigment dispersion, it contains, as a coloring material, the pigment composition obtained by the above-described method for producing a pigment composition. The content (% by mass) of the pigment in the ink is preferably 0.1% by mass or more and 15.0% by mass or less, more preferably 1.0% by mass or more and 10.0% by mass or less, based on the total mass of the ink. The content (mass) of the water-insoluble optical brightening agent in the ink is preferably 0.2% by mass or more and 8.0% by mass or less, more preferably 0.5% by mass or more and 5.0% by mass or less, based on the content (% by mass) of the pigment in the ink. This ratio is more preferably 1.0% by mass or more and 4.0% by mass or less.

[0058] (aqueous medium) The ink is an aqueous ink containing at least water as an aqueous medium. As the aqueous medium for the ink, water or a mixed solvent of water and a water-soluble organic solvent can be used. As the water, deionized water (ion-exchanged water) is preferably used. The content (% by mass) of water in the ink is preferably 40.0% by mass or more and 95.0% by mass or less, more preferably 50.0% by mass or more and 95.0% by mass or less, based on the total mass of the ink.

[0059] As the water-soluble organic solvent, any of those conventionally generally used for inks for inkjet can be used. Examples of the water-soluble organic solvent include alkyl alcohols having 1 to 4 carbon atoms, amides, ketones, ketoalcohols, ethers, polyalkylene glycols, glycols, alkylene glycols having 2 to 6 carbon atoms in the alkylene group, polyhydric alcohols, alkyl ether acetates, alkyl ethers of polyhydric alcohols, nitrogen-containing compounds, and sulfur-containing compounds. These water-soluble organic solvents can be used alone or in combination of two or more as needed. The content (% by mass) of the water-soluble organic solvent in the ink is preferably 3.0% by mass or more and 50.0% by mass or less, more preferably 3.0% by mass or more and 40.0% by mass or less, based on the total mass of the ink.

[0060] (Other additives) In addition to the above-described components, the ink may contain solid compounds at normal temperature having moisture retention properties, such as urea, urea derivatives, trimethylolpropane, and trimethylolethane, in order to maintain moisture retention properties and the like. The content (mass %) of such a compound in the ink is preferably 0.1 mass % or more and 20.0 mass % or less, more preferably 3.0 mass % or more and 10.0 mass % or less, based on the total mass of the ink. Further, in addition to the above-described components, the ink may contain various additives such as surfactants, pH adjusters, rust preventives, preservatives, fungicides, antioxidants, anti-reduction agents, and ultraviolet absorbers, as necessary.

[0061] <Inkjet recording method> The ink obtained by the above-described method for producing an ink is preferably used in an inkjet recording method. The inkjet recording method is an inkjet recording method in which an aqueous ink obtained by the above-described method for producing an aqueous ink is ejected from an inkjet recording head and an image is recorded on a recording medium. Examples of the method of ejecting the ink include a method of ejecting the ink by applying mechanical energy to the ink and a method of ejecting the ink by applying thermal energy to the ink.

[0062] <Ink cartridge> When the above-described ink is used in an inkjet recording method, an ink cartridge can be used. The ink cartridge includes the above-described ink and an ink storage unit for storing the ink. Furthermore, an ink cartridge configured to have an ink storage unit and a recording head may be used.

Examples

[0063] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples as long as the gist thereof is not exceeded. Unless otherwise specified, "parts" and "%" described with respect to the amounts of the components are based on mass.

[0064] <Manufacture of Pigment Composition> (Pigment) The following pigments were prepared. · C.I. Pigment Blue 15:3 (hereinafter also referred to as "PB15:3"). · C.I. Pigment Violet 23 (hereinafter also referred to as "PV23"). · C.I. Pigment Blue 60 (hereinafter also referred to as "PB60").

[0065] (Fluorescent Whitening Agent) The following fluorescent whitening agents were prepared. · C.I. Fluorescent Brightener 135 (hereinafter also referred to as "FB135"): An azole-based fluorescent whitening agent which is a water-insoluble fluorescent whitening agent · C.I. Fluorescent Brightener 184 (hereinafter also referred to as "FB184"): An azole-based fluorescent whitening agent which is a water-insoluble fluorescent whitening agent · C.I. Fluorescent Brightener 369 (hereinafter also referred to as "FB369"): An azole-based fluorescent whitening agent which is a water-insoluble fluorescent whitening agent · C.I. Fluorescent Brightener 199 (hereinafter also referred to as "FB199"): A stilbene-based fluorescent whitening agent which is a water-insoluble fluorescent whitening agent · C.I. Fluorescent Brightener 140 (hereinafter also referred to as "FB140"): A coumarin-based fluorescent whitening agent which is a water-insoluble fluorescent whitening agent · C.I. Fluorescent Brightener 351 (hereinafter also referred to as "FB351"): A water-soluble fluorescent whitening agent

[0066] (Water-soluble Inorganic Salt) The following water-soluble inorganic salts were prepared. · NaCl: D 50 is 20 μm, D 95 is 50 μm of sodium chloride · Na2SO4: D 50 is 35 μm, D 95 is 80 μm of sodium sulfate

[0067] (Organic Solvent) The following organic solvents were prepared. N-methylpyrrolidone, N,N-dimethylformamide, and dimethyl sulfoxide correspond to the first organic solvent for any water-insoluble fluorescent brightening agent to be used and are organic solvents with high solubility. On the other hand, diethylene glycol does not correspond to the first organic solvent for any water-insoluble fluorescent brightening agent to be used and is an organic solvent with low solubility. · N-methylpyrrolidone (hereinafter also referred to as "NMP"). · N,N-dimethylformamide (hereinafter also referred to as "DMF"). · Dimethyl sulfoxide (hereinafter also referred to as "DMSO"). · Diethylene glycol (hereinafter also referred to as "DEG").

[0068] <Manufacture of Pigment Composition> (Pigment Compositions 1 to 27) A mixture prepared by blending each component (unit: part) shown in the upper row of Table 1 (Tables 1-1 to 1-3) was kneaded using the kneading apparatus and kneading conditions shown in the middle row of Table 1 to obtain a kneaded product by the solvent-salt milling method. The obtained kneaded product was thoroughly washed with water, filtered, and dried to remove water-soluble inorganic salts and organic solvents from the kneaded product, thereby producing Pigment Compositions 1 to 27. "○" in the kneading apparatus column in the middle row of Table 1 represents the kneading apparatus used. Trimix is the trade name "Trimix TX-15" of a planetary mixer manufactured by Inoue Seisakusho. For the kneader, the trade name "PBV-03" manufactured by Irie Shokai was used. "Fluorescent Brightening Agent / Pigment (%)" shown in the lower row of Table 1 represents the ratio (%) of the amount (part) of the water-insoluble fluorescent brightening agent used to the amount (part) of the pigment used.

[0069] TIFF2025098316000001.tif147170

[0070] TIFF2025098316000002.tif145170

[0071] TIFF2025098316000003.tif145170

[0072] (Pigment Composition 28) 100 parts of C.I. Pigment Blue 15:3 and 2.5 parts of C.I. Fluorescent Brightener 135 were mixed, and the mixture was added to a container containing 10 parts of N-methylpyrrolidone and 190 parts of diethylene glycol, and stirred for 8 hours. Thereafter, the mixture was thoroughly washed with water, filtered, and dried to remove the organic solvent, thereby producing Pigment Composition 28.

[0073] <Preparation of Aqueous Resin Solution> Resins 1 and 2, which are water-soluble random copolymers, were synthesized according to the following procedure. After putting 200.0 parts of isopropanol into a flask equipped with a stirrer, a nitrogen inlet tube, a reflux condenser, and a thermometer, the temperature was raised to 85°C with stirring under a nitrogen atmosphere. The monomer mixtures shown in Table 2 and the polymerization initiator were each added dropwise into the flask over 2 hours while maintaining the temperature at 80°C. The internal temperature was maintained at 80°C and stirred for 4 hours to synthesize each resin. After adding 0.9 equivalents of potassium hydroxide and an appropriate amount of ion-exchanged water with respect to the acid value of the resin, isopropanol was removed under reduced pressure to obtain Aqueous Resin Solutions 1 and 2, which are alkaline aqueous solutions of Resins 1 and 2. The content of the resin (solid content) in Aqueous Resin Solutions 1 and 2 was 20.0% in both cases. The meanings of the abbreviations in Table 2 are shown below. St: Styrene nBA: n-Butyl acrylate AA: Acrylic acid Polymerization initiator: A solution prepared by dissolving 5.0 parts of the product named "Perkadox L-W75(LS)" (manufactured by Kayaku Akzo, dibenzoyl peroxide, purity 75%) in 10.0 parts of isopropanol

[0074] TIFF2025098316000004.tif41170

[0075] <Preparation of Pigment Dispersion> (Pigment Dispersions 1 to 27, 29 to 32) The respective components (unit: parts) shown in the component column of Table 3 were mixed, and using a high-pressure homogenizer (trade name "Starburst", manufactured by Sugino Machine), dispersion treatment was carried out at a treatment pressure of 200 MPa. Thereafter, an appropriate amount of ion-exchanged water was added to obtain Pigment Dispersions 1 to 27 and 29 to 32 in which the pigment content was 15.0%. As the water-insoluble fluorescent brightening agent in Pigment Composition 31, C.I. Fluorescent Brightener 184, which is an azole-based fluorescent brightening agent, was used.

[0076] (Pigment Dispersion 28) The respective components (unit: parts) shown in the component column of Table 3 were mixed, placed in a batch vertical sand mill (manufactured by Aimex), filled with 150.0 parts of zirconia beads having a diameter of 0.3 mm, and dispersion treatment was carried out for 5 hours while cooling with water. Thereafter, an appropriate amount of ion-exchanged water was added to obtain Pigment Dispersion 28 in which the pigment content was 15.0%.

[0077] TIFF2025098316000005.tif218170

[0078] <Preparation of Resin Particle Dispersion> Into a reaction vessel equipped with a reflux tube, 49.0 parts of ion-exchanged water, 2.5 parts of ammonium lauryl sulfate, 40.0 parts of methyl acrylate, 5.0 parts of acrylonitrile, 3.0 parts of methacrylonitrile, and 0.5 part of potassium persulfate were placed. While stirring this mixed solution, the temperature was raised to 80 °C under a nitrogen stream, and after continuing the reaction for 3 hours while maintaining that temperature, it was cooled to room temperature to complete the reaction, obtaining an emulsion polymerization liquid having a viscosity of 35 cP and a resin (solid content) content of 48.0%. To 100 parts of the obtained emulsion polymerization liquid, a mixture of 5 parts of C.I. Fluorescent Brightener 363 (Uvitex BAC) as a fluorescent brightening agent, 2 parts of sodium β-naphthalenesulfonate, and 88 parts of ion-exchanged water was added at room temperature. The temperature was raised to 90 °C over 2 hours, the temperature was maintained for 1 hour, and then it was cooled to room temperature. Thereafter, pressure filtration was carried out using a filter with a pore size of 1.2 μm (trade name "HDCII", manufactured by Paul), and further diluted with ion-exchanged water to prepare a resin particle dispersion having a total solid content of 10% and a fluorescent brightening agent content rate of 9.4%.

[0079] <Preparation of Ink> (Inks 1 to 34) The respective components shown in Table 4 (unit: part) were mixed, stirred well and dispersed, and then pressure filtration was performed using a microfilter with a pore size of 3.0 μm (manufactured by Fujifilm) to prepare each ink. "Gly" and "TEG" shown in Table 4 represent glycerin and triethylene glycol, respectively. Also, "AE100" shown in Table 1 is the product name "Acetylenol E100" of a surfactant manufactured by Kawaken Fine Chemicals. For the "water-soluble fluorescent brightener" in Ink 33, C.I. Fluorescent Brightener 351 was used. For the "resin particle dispersion" in Ink 34, the resin particle dispersion prepared as described above was used.

[0080] TIFF2025098316000006.tif224170

[0081] <Evaluation> Inks 1 to 34 were each filled into an ink cartridge and set in an inkjet recording apparatus (product name "PIXUS PRO-10", manufactured by Canon) equipped with a recording head that discharges ink by thermal energy. The resolution of this inkjet recording apparatus is 2400 dpi × 1200 dpi. Then, an image recorded under the condition of applying 8 ink droplets of 3.8 ng to a unit area of 1 / 600 inch × 1 / 600 inch is defined as having a recording duty of 100%. Using the above inkjet recording apparatus, solid images with recording duties of 50% and 100% were recorded on a recording medium (glossy paper; product name "Canon Photo Paper Gloss Gold GL-101", manufactured by Canon).

[0082] (Bronze Resistance) For the obtained solid images, the bronze resistance was evaluated according to the following evaluation criteria. In this example, according to the following evaluation criteria, "A" and "B" are acceptable levels, and "C" and "D" are unacceptable levels. The results are shown in Table 5. A: In the solid image with a recording duty of 100%, a slight bronze phenomenon was confirmed. B: Bronze phenomenon was slightly confirmed in the solid images with both recording duties of 50% and 100%. C: Bronze phenomenon was confirmed in the solid images with both recording duties of 50% and 100%. D: Obvious bronze phenomenon was confirmed in the solid images with both recording duties of 50% and 100%.

[0083] TIFF2025098316000007.tif121170

[0084] In addition, the disclosure of this embodiment includes the following methods. (Method 1) A method for manufacturing a pigment composition including a kneading step of kneading a mixture including a pigment, a water-soluble inorganic salt, and an organic solvent by a solvent-salt milling method to obtain a kneaded product, The method for manufacturing a pigment composition, characterized in that the mixture further includes a water-insoluble optical brightening agent. (Method 2) The method for manufacturing a pigment composition according to Method 1, wherein the shear rate during kneading in the kneading step is 15 s -1 or more. (Method 3) The method for manufacturing a pigment composition according to Method 1 or 2, wherein the usage amount of the water-insoluble optical brightening agent in the kneading step is 0.5% by mass or more and 5.0% by mass or less based on the usage amount of the pigment. (Method 4) The method for manufacturing a pigment composition according to any one of Methods 1 to 3, wherein the pigment is at least one selected from the group consisting of C.I. Pigment Blue 15:3 and C.I. Pigment Violet 23. (Method 5) The method for manufacturing a pigment composition according to any one of Methods 1 to 4, wherein the water-insoluble optical brightening agent is at least one selected from azole-based optical brightening agents. (Method 6) The method for manufacturing a pigment composition according to any one of Methods 1 to 5, wherein the organic solvent includes a first organic solvent capable of dissolving the water-insoluble optical brightening agent. (Method 7) The method for manufacturing a pigment composition according to any one of Methods 1 to 6, wherein the organic solvent includes at least one selected from the group consisting of N-methylpyrrolidone and N,N-dimethylformamide. (Method 8) A method for producing a pigment dispersion liquid, comprising a dispersion step of mixing a pigment composition obtained by the method for producing a pigment composition according to any one of Items 1 to 7, a dispersant, and water, and dispersing the pigment composition in the water by the dispersant. (Method 9) A method for producing an aqueous ink, comprising a step of mixing a pigment dispersion liquid obtained by the method for producing a pigment dispersion liquid according to Method 8 and a component containing a water-soluble organic solvent. (Method 10) An inkjet recording method of ejecting ink from an inkjet recording head and recording an image on a recording medium, wherein the ink is an aqueous ink obtained by the method for producing an aqueous ink according to Method 9.

Claims

1. A method for producing a pigment composition, comprising a kneading step of kneading a mixture containing a pigment, a water-soluble inorganic salt, and an organic solvent by a solvent-salt milling method to obtain a kneaded product, wherein the mixture further contains a water-insoluble optical brightening agent. A method for producing a pigment composition, characterized by this.

2. The shear rate during kneading in the kneading step is 15 s －１ or more, and the method for producing a pigment composition according to claim 1.

3. The method for producing a pigment composition according to claim 1, wherein the amount of the water-insoluble optical brightening agent used in the kneading step is 0.5% by mass or more and 5.0% by mass or less based on the amount of the pigment used.

4. The method for producing a pigment composition according to claim 1, wherein the pigment is at least one selected from the group consisting of C.I. Pigment Blue 15:3 and C.I. Pigment Violet 23.

5. The method for producing a pigment composition according to claim 1, wherein the water-insoluble optical brightening agent is at least one selected from azole-based optical brightening agents.

6. The method for producing a pigment composition according to claim 1, wherein the organic solvent contains a first organic solvent capable of dissolving the water-insoluble optical brightening agent.

7. The method for producing a pigment composition according to claim 1, wherein the organic solvent contains at least one selected from the group consisting of N-methylpyrrolidone and N,N-dimethylformamide.

8. A method for producing a pigment dispersion, comprising a dispersion step of mixing a pigment composition obtained by the method for producing a pigment composition according to any one of claims 1 to 7, a dispersant, and water, and dispersing the pigment composition in the water with the dispersant.

9. A method for producing an aqueous ink, comprising a step of mixing a pigment dispersion obtained by the method for producing a pigment dispersion according to claim 8 and a component containing a water-soluble organic solvent.

10. An inkjet recording method of ejecting an ink from an inkjet recording head to record an image on a recording medium, wherein the ink is an aqueous ink obtained by the method for producing an aqueous ink according to claim 9. An inkjet recording method, characterized by this.

Images