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 enhancement of light resistance in azo pigments.SOLUTION: A method for producing a pigment composition includes a kneading step in which a mixture containing a pigment, a water-soluble inorganic salt, and an organic solvent is kneaded by a solvent salt milling process to provide a kneaded product. The pigment is an azo pigment containing organic impurities. The organic solvent is an organic solvent capable of dissolving the azo pigment in an amount of 0.005 mass% or more.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 an aqueous ink, and an inkjet recording method.

Background Art

[0002] In recent years, the market using pigments has expanded in various fields such as color filters and inks for inkjet. Along with this, pigments are required to have various reliabilities, such as not only high color characteristics but also stability, filterability when made into a coloring composition, and suitability when applied to a substrate. For the above-mentioned requirement for high color characteristics, it is effective to use a pigment with a refined primary particle size. As a method for refining the primary particle size of a pigment, for example, there is a method of mechanically kneading the pigment together with a water-soluble inorganic salt such as sodium chloride and a hydrophilic organic solvent using a kneading device such as a kneader. Such a method of kneading a mixture containing a pigment, a water-soluble inorganic salt, and a hydrophilic organic solvent is called salt milling or solvent salt milling. By washing the kneaded product obtained by the kneading step by the salt milling method with water, the water-soluble inorganic salt and the hydrophilic organic solvent are removed from the kneaded product, and a refined pigment can be obtained (see, for example, Patent Document 1). In this method, since the pulverization and crystal growth of the primary particles of the pigment occur in parallel, a pigment with a narrow particle size distribution and a small average particle size but a small surface area can be finally obtained.

[0003] On the other hand, the inkjet recording method enables the recording of high-definition images and has been rapidly developing in various fields. Among them, various studies have been conducted on the light resistance of the recorded matter. For example, Patent Document 2 discloses an ink composition using an insoluble azo pigment excellent in light resistance and the like.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] By using a highly crystalline pigment as a pigment as in the technique disclosed in Patent Document 2, it becomes possible to record an image excellent in light resistance. In particular, conventionally, there are pigments that are inferior in light resistance among pigments excellent in chroma, and it is known that highly crystalline pigments are useful as pigments that are excellent in chroma and light resistance.

[0006] Based on past findings, the present inventors attempted to produce an azo pigment excellent in light resistance. As a general method for improving the light resistance of a pigment, it is conceivable to increase the primary particle diameter of the pigment. This is presumably because even if the molecular structure of the pigment deteriorates due to light, there are many other molecular structures that affect color development, so the influence of deterioration is small. However, when the primary particle diameter of the pigment increases, the molecular structures that do not contribute to color development increase, so the color development property of the pigment decreases. Furthermore, as a result of the study by the present inventors, it was found that simply increasing the primary particle diameter of the pigment does not result in the desired light resistance.

[0007] In addition, when ink was prepared using a highly crystalline pigment as described in Patent Document 2 and evaluated, it was found that the desired light resistance could not be obtained. Therefore, there is a problem that it is difficult to improve the light resistance by the conventional method.

[0008] Therefore, an object of the present invention is to provide a method for producing a pigment composition capable of improving the light resistance in an azo pigment. 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

[0009] 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 to obtain a kneaded product, wherein the pigment is an azo pigment containing organic impurities, and the organic solvent is an organic solvent capable of dissolving 0.005% by mass or more of the azo pigment.

Effects of the Invention

[0010] According to the present invention, it is possible to provide a method for producing a pigment composition capable of improving lightfastness in an azo pigment. 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

[0011] Hereinafter, preferred embodiments will be given to explain the present invention in more detail. In the present invention, when the compound is a salt, although the salt dissociates into ions in the ink, for convenience, it is expressed as "containing a salt". In addition, the aqueous ink for inkjet may be simply referred to as "ink". Physical property values are values at room temperature (25°C) unless otherwise specified.

[0012] The present inventors considered that in azo pigments, the lightfastness is reduced due to another factor that does not appear in crystallinity, and various studies were attempted to produce azo pigments with improved lightfastness. As a result, the present inventors clarified that the organic impurities in azo pigments include organic impurities adhering to the pigment surface and organic impurities contained inside the pigment, and found that the organic impurities contained inside the azo pigment significantly reduce the lightfastness. This organic impurity also includes synthetic raw materials of azo pigments, by-products that unexpectedly occur during the synthesis reaction, and synergists added to control the particle size of the pigment. The present inventors speculate as follows about the reason why the organic impurities contained inside the azo pigment significantly reduce the lightfastness.

[0013] Organic colorants such as organic pigments and organic dyes absorb a part of white light, and the light of the wavelengths that cannot be absorbed is combined to express color. The absorption of light by the organic colorant is borne by the molecular structure contained in the colorant. The molecular structure absorbs light, converts its energy into electrons, and the molecular structure becomes an excited state. Then, this excited molecular structure gradually releases energy and returns to the original molecular structure. However, this excited state is an unstable state with a large amount of energy and is considered to be in a situation where it is likely to deteriorate as a molecule. If organic impurities are contained inside the azo pigment, it is considered that the energy release of the excited molecular structure is suppressed and the time in the excited molecular structure is prolonged. The inventors speculate that this increases the possibility of deterioration, and as a result, the light resistance of the azo pigment decreases.

[0014] Therefore, the inventors have come to the recognition that in order to improve the light resistance of azo pigments, it is necessary to remove the organic impurities contained inside the azo pigments. As a method for removing organic impurities inside azo pigments, it is common to wash the azo pigments with an aqueous solution, an organic solvent, or the like. Therefore, the inventors first attempted to wash the azo pigment containing organic impurities using an acidic and alkaline aqueous solution. However, it had no effect on the amount of organic impurities inside the azo pigment. Furthermore, the inventors identified that the organic impurities contained inside the azo pigment are the synthesis raw materials of the azo pigment, and performed washing using an organic solvent capable of dissolving the synthesis raw materials. However, it had no effect on the amount of organic impurities inside the azo pigment. From this, the inventors speculated that the organic impurities contained inside the azo pigment are incorporated into the crystal structure.

[0015] Therefore, the present inventors focused on the solvent-salt milling method as a technique for removing organic impurities incorporated inside azo pigments. As described above, the solvent-salt milling method involves kneading a mixture containing a pigment, a water-soluble inorganic salt, and an organic solvent, and it is presumed that the pulverization of primary particles of the pigment and crystal growth proceed simultaneously, thereby making it possible to produce a pigment with a uniform particle size distribution.

[0016] In addition, in the solvent-salt milling method, since a strong load is applied as kneading, it is possible to change the crystal form of pigments such as copper phthalocyanine blue. It was thought that by applying such a strong load, the organic impurities contained in the crystal structure of the azo pigment could be removed. Therefore, kneading was performed by the solvent-salt milling method using an azo pigment, a water-soluble inorganic salt, and diethylene glycol, which is generally used as an organic solvent. However, it had no effect on the amount of organic impurities inside the azo pigment.

[0017] The present inventors considered that a strong load alone was insufficient for removing the organic impurities incorporated in the crystal structure, and presumed that promoting dissolution and crystal growth in the solvent-salt milling method was the means of solution. Therefore, in addition to the azo pigment and the water-soluble inorganic salt, an organic solvent capable of dissolving 0.005 mass% or more of the azo pigment, in other words, an organic solvent having a solubility of 0.005 mass% or more with respect to the azo pigment, was used for kneading by the solvent-salt milling method. As a result, it was found that it was possible to reduce the organic impurities incorporated inside the azo pigment. In addition, when the light resistance was evaluated using the obtained pigment, it was confirmed that the light resistance was improved after the kneading treatment.

[0018] The solubility of an azo pigment in an organic solvent can be determined as follows. Prepare the azo pigment and the organic solvent to be determined. At 25°C, add the azo pigment to the organic solvent and stir for 30 minutes or more. Then, separate the azo pigment and the organic solvent, and confirm the mass absorption coefficient (absorbance ÷ mass of the added pigment) of the resulting solution. Thereafter, gradually increase the amount of the azo pigment added, confirm the mass absorption coefficient by the same operation, and when the mass absorption coefficient begins to decrease, the ratio (mass %) of the mass of the azo pigment to the mass of the organic solvent is determined to be the maximum amount that the organic solvent can dissolve.

[0019] <Method for producing a pigment composition> The production method of one embodiment of the present invention is 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 to obtain a kneaded product. The pigment is an azo pigment containing organic impurities. The organic solvent is an organic solvent capable of dissolving 0.005% by mass or more of the azo pigment. Hereinafter, the kneading step, each used material, etc. will be described in detail.

[0020] 〔Kneading step〕 In the kneading step in the production method of the pigment composition of the present embodiment, a mixture containing an azo pigment containing organic impurities, an organic solvent capable of dissolving 0.005% by mass or more of the azo pigment, and a water-soluble inorganic salt is kneaded by a 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 production method of the pigment composition of the present embodiment, a kneaded product containing a refined azo pigment, a water-soluble inorganic salt, and an organic solvent can be obtained by the kneading step by the solvent-salt milling method.

[0021] 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 and compressing 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 wing, blade, screw, and roll for stirring the material can be preferably used. Specific kneading devices include, for example, 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. As the planetary mixer, for example, Trimix (trade name) manufactured by Inoue Manufacturing Co., Ltd. can be mentioned. Also, as the continuous single-screw kneader, for example, Miracle KCK (trade name) manufactured by Asada Iron Works can be mentioned. Among the kneading devices listed above, it is preferable to use a planetary mixer.

[0022] Regarding the mixing ratio of the pigment, water-soluble inorganic salt, and organic solvent in the kneading step, it is preferably the following ratio based on the amount of the pigment used. In the kneading step, 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 mass ratio to the amount of the pigment used. Also, in the kneading step, the amount of the organic solvent used is preferably 0.5 times or more and 5.0 times or less, more preferably 0.8 times or more and 3.0 times or less, in terms of mass ratio to the amount of the pigment used.

[0023] The kneading time in the kneading step is not particularly limited, but it is preferably 1 hour or more and 10 hours or less, more preferably 2 hours or more and 8 hours or less. When the kneading time is 2 hours or more, it becomes easier to remove the organic impurities contained in the azo pigment and to improve the light resistance of the resulting pigment composition. On the other hand, when the kneading time is 8 hours or less, it becomes easier to suppress the aggregation of azo pigments, and it becomes easier to suppress the broadening of the particle size distribution of the pigment in the resulting pigment composition.

[0024] As a result of the studies by the present inventors, it has been found that in order to remove organic impurities in the crystal structure of azo pigments by the solvent-salt milling method, it is more preferable to apply the conditions described below during kneading.

[0025] (Shearing force) In the method for producing the pigment composition of the present embodiment, it is important to remove organic impurities from inside the azo pigment by the kneading step using the solvent-salt milling method. Therefore, as described above, applying a strong load to the azo pigment is one of the conditions. 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 from the viewpoint of easily applying a shearing force to the azo pigment itself, more preferably 15 s -1 or more, and even more preferably 20 s -1 or more.

[0026] (Temperature) Also, the temperature during kneading in the kneading step is preferably 20°C or higher and 130°C or lower, more preferably 25°C or higher and 120°C or lower, and even more preferably 45°C or higher and 90°C or lower. By setting the temperature during kneading to 45°C or higher, the azo pigment becomes more easily soluble in the organic solvent, making it easier to remove organic impurities from inside the azo pigment. On the other hand, by setting the temperature during kneading to 90°C or lower, it becomes easier to suppress the evaporation of the organic solvent, so it is easy to enhance the effect of removing organic impurities due to the solubility of the organic solvent in the azo pigment.

[0027] The temperature during kneading means the temperature of the kneaded material. The method for adjusting the temperature of the kneaded material is not particularly limited, but it is possible to use a solvent circulation device or the like for the kneading container. If kneading is performed at a temperature of 45°C or higher and 90°C or lower during kneading, the temperature change during kneading is not particularly limited, but in order to make the particle size distribution more uniform, it is preferable that the temperature change during kneading be within 5°C. Also, even when kneading is performed at 45°C or higher and 90°C or lower, it is assumed that the temperature may momentarily deviate from the range of 45°C or higher and 90°C or lower during kneading. In the present disclosure, even when the temperature during kneading in the kneading step temporarily deviates from the above range, if the time at the temperature outside the range is within 10% of the total steps of the kneading step, the kneading step is judged to have been performed at a temperature within the above range during kneading.

[0028] (Pigment) As the pigment used in the kneading step, an azo pigment containing organic impurities is used. Examples of the azo pigment include C.I. Pigment Yellow 3, C.I. Pigment Yellow 74, C.I. Pigment Yellow 93, C.I. Pigment Yellow 128, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, C.I. Pigment Yellow 205, C.I. Pigment Red 150, C.I. Pigment Red 170, C.I. Pigment Red 187, and C.I. Pigment Red 256. One kind of azo pigment may be used alone, or two or more kinds may be used in combination.

[0029] Among the above azo pigments, at least one selected from the group consisting of C.I. Pigment Yellow 74, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, and C.I. Pigment Red 150 is preferred. These azo pigments are likely to have improved lightfastness by removing the organic impurities contained in the azo pigments. In particular, C.I. Pigment Yellow 74 is more preferred because the improvement in lightfastness by removing organic impurities is remarkable. C.I. Pigment Yellow 74 has a monoazo structure and a nitro group that is ionic, so it is presumed that the molecular structure in the excited state is more unstable. Also, in the case of C.I. Pigment Yellow 74 containing the compound (o-acetetoacetanisidide) represented by the following chemical formula (1) as an organic impurity, the improvement in lightfastness by removing the compound represented by chemical formula (1) was remarkable. The compound represented by chemical formula (1) is a synthetic raw material of C.I. Pigment Yellow 74, and since their chemical structures are similar, it is presumed that it has the effect of lengthening the time that C.I. Pigment Yellow 74 is in the molecular structure in the excited state. As a result, it is considered that the compound represented by chemical formula (1) increases the possibility of C.I. Pigment Yellow 74 deteriorating, so it is presumed that the improvement in lightfastness by removing the compound represented by chemical formula (1) becomes remarkable.

[0030] TIFF2025092980000001.tif46170

[0031] For confirming the presence or absence of organic impurities in the azo pigment, a high-performance liquid chromatography mass spectrometer (HPLC / MS) can be used. Specifically, the azo pigment is added to a dimethyl sulfoxide (DMSO) solution and filtered, and the resulting liquid composition is measured by HPLC / MS. It is possible to determine the presence or absence of organic impurities based on whether there are peaks other than those derived from the pigment in the obtained chromatogram.

[0032] (organic solvent) The organic solvent used in the kneading process serves, as a first role, to wet the mixture of the pigment and the water-soluble inorganic salt and to make a dough (a mass formed by kneading) of appropriate firmness. As a result, since a strong load is likely to be applied to the kneaded material, the grinding effect can be increased and the refinement of the pigment can be promoted. Further, since a strong load is applied to the kneaded material, it plays a part in removing the organic impurities present inside the azo pigment.

[0033] As the organic solvent, organic solvents such as alcohols, glycols, ethers, and aprotic polar solvents are preferable. 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.

[0034] The second role of the organic solvent is to moderately dissolve the azo pigment. For this reason, the organic solvent needs to be an organic solvent that dissolves 0.005 mass% or more of the azo pigment at 25°C. However, if two or more organic solvents are mixed and the mixed organic solvent dissolves 0.005 mass% or more of the azo pigment, an organic solvent mixture of two or more may be used as needed. By using an organic solvent that can dissolve 0.005 mass% or more of the azo pigment as the organic solvent, in the kneading process, it is possible to promote the cycle of dissolution and crystal growth of the azo pigment and efficiently remove organic impurities inside the azo pigment.

[0035] The fact that the organic solvent can dissolve 0.005 mass% or more of the azo pigment means that the ratio (mass%) of the mass of the azo pigment dissolved in the organic solvent to the mass of the organic solvent is 0.005 mass% or more. For example, it means that 0.005 g or more of the azo pigment is dissolved in 100 g of the organic solvent. The solubility of the organic solvent with respect to the azo pigment can be determined by the method described above, and an organic solvent that dissolves the azo pigment more is preferred according to the determination method. As a result of the study by the present inventors, it was found that the organic solvent needs to dissolve 0.005 mass% or more of the azo pigment. When the solubility with respect to the azo pigment is less than 0.005 mass%, the ability to remove organic impurities in the crystal structure of the azo pigment is low, and it is difficult to improve the light resistance.

[0036] Also, it was found that using at least one of dimethyl sulfoxide and N-methylpyrrolidone as the organic solvent that can dissolve 0.005 mass% or more of the azo pigment is more preferable because the removal rate of organic impurities inside the azo pigment is high. The reason is presumably that the main factor is that dimethyl sulfoxide and N-methylpyrrolidone have high solubility in the azo pigment. In addition, it is presumably also partly due to the high affinity with the azo group inside the azo pigment due to being an aprotic polar solvent, and the fact that the organic solvent is less likely to evaporate even during solvent-assisted milling because it is a high-boiling solvent, and can stably enter the vicinity of the azo pigment.

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

[0038] Regarding the particle size of the water-soluble inorganic salt, in the volume-based particle size distribution, 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 a particularly fine pigment is desired, it is preferable that the water-soluble inorganic salt used as a grinding aid is also fine. Specifically, in the volume-based particle size distribution, 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 more preferably 20 μm or less for the water-soluble inorganic salt.

[0039] For D 50 and D 95 of the water-soluble inorganic salt, 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 for calculating D 50 and D 95 from the volume-based particle size distribution can be mentioned. For the water-soluble inorganic salt used in the examples described later, D 50 and D 95 were also determined by the above measurement method.

[0040] 〔Other Processes〕 Through the above kneading process, a kneaded product containing a pigment, 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 by passing the above pigment suspension through an ultrafiltration membrane or a dialysis membrane, or a method of separating with a high-pressure filter press, etc.

[0041] Through the process including the above filtration, etc., a wet cake of the pigment composition from which the water-soluble inorganic salt and the organic solvent have been separated is obtained. The obtained wet cake is preferably dried to a water content of 5% by mass or less in consideration of the growth of bacteria. As the drying method, for example, batch or continuous drying for dehydrating and / or desolventizing the wet cake by heating at 80 ° C or higher and 120 ° C or lower by a heat source installed in a dryer can be mentioned. Examples of the dryer include a box-type dryer, a band dryer, and a spray dryer.

[0042] <Method for producing pigment dispersion> It is possible to produce a pigment dispersion using the pigment composition obtained by the method for producing the pigment composition described above. The method for producing the pigment dispersion includes a dispersion step of mixing the pigment composition, a dispersant, and water obtained by the method for producing the pigment composition described above, and dispersing the pigment composition in water with the dispersant. As the pigment composition, it is preferable to use the pigment composition obtained after removing the water-soluble inorganic salt and the organic solvent from the kneaded product obtained in the method for producing the pigment composition described above. As the water, it is preferable to use deionized water such as ion-exchanged water or pure water.

[0043] 〔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-dispersing pigment having a hydrophilic group bonded to the particle surface of the pigment, etc. 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, etc. 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 these, it is more preferable to use a resin (resin having an anionic group) capable of stably dispersing the pigment in the dispersion medium by the action of the anionic group.

[0044] As the resin, a resin obtained by copolymerizing a polymerizable hydrophobic monomer and a polymerizable hydrophilic monomer, having a structural unit derived from the hydrophobic monomer, and a structural unit derived from a hydrophilic monomer having an anionic group 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, propyl acrylic acid, isopropyl acrylic acid, itaconic acid, and fumaric acid; hydrophilic monomers having a sulfonic acid group such as styrene sulfonic acid, sulfonic acid-2-propylacrylamide, acrylic acid-2-sulfonic acid ethyl, methacrylic acid-2-sulfonic acid ethyl, and butylacrylamide sulfonic acid; hydrophilic monomers having a phosphonic acid group such as methacrylic acid-2-phosphonic acid ethyl and acrylic acid-2-phosphonic acid ethyl; and the like. One or more of the hydrophilic monomers can be used.

[0045] The weight-average molecular weight of the resin used as a dispersant is preferably 1,000 or more and 30,000 or less, more preferably 3,000 or more and 20,000 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, 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.

[0046] When dispersing the 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.

[0047] The content (% by mass) of the pigment in the pigment dispersion is preferably 1.0% by mass or more and 50.0% by mass or less, more preferably 5.0% by mass or more and 30.0% by mass or less based on the total mass of the pigment dispersion.

[0048] 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.

[0049] <Method for manufacturing ink> It is possible to produce an aqueous ink using the pigment dispersion obtained by the above-described method for producing a pigment dispersion. The method for producing the aqueous ink includes a step of mixing a component containing the pigment dispersion obtained by the above-described method for producing a pigment dispersion and a water-soluble organic solvent. When preparing the ink in this step, as the above components, in addition to the aforementioned pigment dispersion and water-soluble organic solvent, water and other additives used as necessary can be blended to prepare the ink.

[0050] (Colorant) Since the ink contains the pigment dispersion obtained by the above-described method for producing a pigment dispersion, as a colorant, it contains the pigment composition obtained by the above-described method for producing a pigment composition. The content (mass%) of the pigment in the ink is preferably 0.1 mass% or more and 15.0 mass% or less, more preferably 1.0 mass% or more and 10.0 mass% or less, based on the total mass of the ink.

[0051] (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. Deionized water (ion-exchanged water) is preferably used as the water. The content (mass%) of water in the ink is preferably 40.0 mass% or more and 95.0 mass% or less, more preferably 50.0 mass% or more and 95.0 mass% or less, based on the total mass of the ink.

[0052] As the water-soluble organic solvent, any of those conventionally commonly used in 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, sulfur-containing compounds, and the like. 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.

[0053] (Other Additives) In the ink, in order to maintain moisture retention and the like, in addition to the above-described components, solid compounds having moisture retention properties such as urea, urea derivatives, trimethylolpropane, and trimethylolethane may be contained. The content (% by mass) of such a compound in the ink is preferably 0.1% by mass or more and 20.0% by mass or less, more preferably 3.0% by mass or more and 10.0% by mass or less, based on the total mass of the ink. Further, in the ink, various additives such as surfactants, pH adjusters, rust preventives, preservatives, fungicides, antioxidants, and anti-reducing agents may be contained as needed in addition to the above-described components.

[0054] <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 the 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.

[0055] <Ink Cartridge> When using the above ink in an inkjet recording method, an ink cartridge can be used. The ink cartridge includes the above ink and an ink storage unit for storing the ink. Furthermore, it may be an ink cartridge configured to have an ink storage unit and a recording head.

Example

[0056] Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples. However, the present invention is not limited to the following examples as long as the gist thereof is not exceeded. Regarding the amounts of components, "parts" and "%" are based on mass unless otherwise specified.

[0057] <Preparation of Pigment and Analysis of Organic Impurities> As pigments, C.I. Pigment Yellow 74 (two types: Pigment 1 and 2), C.I. Pigment Yellow 155 (Pigment 3), C.I. Pigment Yellow 180 (Pigment 4), and C.I. Pigment Red 150 (Pigment 5) were prepared. For each pigment, the presence or absence of organic impurities in the pigment was confirmed, and it was confirmed that all pigments contained organic impurities. The measurement of organic impurities in the pigment was determined by adding an excessive amount of the pigment to a dimethyl sulfoxide (DMSO) solution, filtering, and analyzing the resulting liquid composition by high performance liquid chromatography-mass spectrometry. The specific measurement method is shown below.

[0058] [HPLC Analysis Conditions] Column: Silica gel column (trade name "SunFire C18 Column", 2.1 mm × 150 mm, manufactured by Waters) Column temperature: 40 °C Flow rate: 0.2 mL / min Detector: Diode array (PDA) detector Detection range: 200 nm to 700 nm Analysis time: 45 minutes Gradient conditions of the mobile phase: Table 1 below Sample solution injection volume: 2 μL

[0059] TIFF2025092980000002.tif34170

[0060] [Mass spectrometry conditions] Regarding the peaks obtained in the HPLC analysis performed under the above conditions, mass spectra were measured under the following conditions. The m / z detected most strongly was measured for each of posi(ES+) and nega(ES-). Ionization method: Electrospray ionization method (ESI method) Capillary voltage: 3.5 kV Desolvation gas: 350 °C Ion source temperature: 120 °C Detector: 40 V, 200 - 1500 amu / 0.9 sec (for posi(ES+)) 40 V, 200 - 1500 amu / 0.9 sec (for nega(ES-)).

[0061] In the HPLC analysis performed under the above conditions, the peaks of the organic compound components obtained on the chromatogram at a detector wavelength of 254 nm were confirmed. When the MS chromatogram of the main peak (the one with the highest intensity) was confirmed, it was equivalent to the molecular weight of each pigment. On the other hand, for each pigment, peaks other than the main peak were also detected on the chromatogram at a detector wavelength of 254 nm. From this, it was determined that there are compounds other than those derived from the pigment. Also, in the case of pigment 2, a peak was detected only around the measurement time of 26 minutes on the chromatogram at a detector wavelength of 254 nm. When the MS chromatogram of this peak was confirmed, it was the molecular weight of chemical formula (1). From this, it was shown that pigment 2 contains only the impurity of chemical formula (1). The list of results is shown in Table 2.

[0062] TIFF2025092980000003.tif45170

[0063] [Confirmation of the solubility of organic solvents in pigments] As organic solvents, dimethyl sulfoxide, N-methylpyrrolidone, toluene, acetone, and diethylene glycol were prepared. Regarding these organic solvents, the solubility with respect to the above-described types of pigments was confirmed as follows.

[0064] (1) Solubility of organic solvents in C.I. Pigment Yellow 74 (Test Example 1-1: Dimethyl sulfoxide) To 100,000 parts of dimethyl sulfoxide at 25°C, 4 parts of C.I. Pigment Yellow 74 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of added pigment) of the obtained solution (filtrate) was confirmed. Subsequently, to 100,000 parts of dimethyl sulfoxide at 25°C, 5 parts of C.I. Pigment Yellow 74 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of added pigment) of the obtained solution (filtrate) was confirmed. The absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 4 parts. From the above results, it was determined that dimethyl sulfoxide can dissolve C.I. Pigment Yellow 74 at 0.005% or more.

[0065] (Test Example 1-2: N-methylpyrrolidone) A test was conducted in the same procedure as Test Example 1-1, except that the dimethyl sulfoxide used in Test Example 1-1 was changed to N-methylpyrrolidone. As a result, the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 4 parts. From the above results, it was determined that N-methylpyrrolidone can dissolve C.I. Pigment Yellow 74 at 0.005% or more.

[0066] (Test Example 1-3: Toluene) The test was conducted in the same procedure as in Test Example 1-1, except that the dimethyl sulfoxide used in Test Example 1-1 was changed to toluene. As a result, the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 5 parts did not change compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 4 parts. From the above results, it was determined that toluene can dissolve C.I. Pigment Yellow 74 at 0.005% or more.

[0067] (Test Example 1-4: Acetone) The test was conducted in the same procedure as in Test Example 1-1, except that the dimethyl sulfoxide used in Test Example 1-1 was changed to acetone. As a result, the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 5 parts decreased compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 4 parts. From the above results, it was determined that acetone cannot dissolve C.I. Pigment Yellow 74 at 0.005% or more.

[0068] (Test Example 1-5: Diethylene Glycol) The test was conducted in the same procedure as in Test Example 1-1, except that the dimethyl sulfoxide used in Test Example 1-1 was changed to diethylene glycol. As a result, the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 5 parts decreased compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 74 was 4 parts. From the above results, it was determined that diethylene glycol cannot dissolve C.I. Pigment Yellow 74 at 0.005% or more.

[0069] (2) Solubility of Organic Solvents in C.I. Pigment Yellow 155 (Test Example 2-1: Dimethyl Sulfoxide) To 100,000 parts of dimethyl sulfoxide at 25°C, 4 parts of C.I. Pigment Yellow 155 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of the added pigment) of the resulting solution (filtrate) was confirmed. Subsequently, to 100,000 parts of dimethyl sulfoxide at 25°C, 5 parts of C.I. Pigment Yellow 155 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of the added pigment) of the resulting solution (filtrate) was confirmed. The absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Yellow 155 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Yellow 155 was 4 parts. From the above results, it was determined that dimethyl sulfoxide can dissolve C.I. Pigment Yellow 155 at 0.005% or more.

[0070] (Test Example 2-2: N-Methylpyrrolidone) A test was conducted in the same procedure as Test Example 2-1, except that the dimethyl sulfoxide used in Test Example 2-1 was changed to N-methylpyrrolidone. As a result, the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Yellow 155 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Yellow 155 was 4 parts. From the above results, it was determined that N-methylpyrrolidone can dissolve C.I. Pigment Yellow 155 at 0.005% or more.

[0071] (Test Example 2-3: Diethylene Glycol) The test was conducted in the same procedure as Test Example 2-1, except that the dimethyl sulfoxide used in Test Example 2-1 was changed to diethylene glycol. As a result, the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 155 was 5 parts decreased compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 155 was 4 parts. From the above results, it was determined that diethylene glycol could not dissolve C.I. Pigment Yellow 155 by 0.005% or more.

[0072] (3) Solubility of organic solvents in C.I. Pigment Yellow 180 (Test Example 3-1: Dimethyl sulfoxide) To 100,000 parts of dimethyl sulfoxide at 25°C, 4 parts of C.I. Pigment Yellow 180 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of added pigment) of the obtained solution (filtrate) was confirmed. Subsequently, to 100,000 parts of dimethyl sulfoxide at 25°C, 5 parts of C.I. Pigment Yellow 180 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of added pigment) of the obtained solution (filtrate) was confirmed. The absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 180 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 180 was 4 parts. From the above results, it was determined that dimethyl sulfoxide could dissolve C.I. Pigment Yellow 180 by 0.005% or more.

[0073] (Test Example 3-2: N-Methylpyrrolidone) The test was conducted in the same procedure as in Test Example 3-1, except that the dimethyl sulfoxide used in Test Example 3-1 was changed to N-methylpyrrolidone. As a result, the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 180 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 180 was 4 parts. From the above results, it was determined that N-methylpyrrolidone can dissolve C.I. Pigment Yellow 180 at 0.005% or more.

[0074] (Test Example 3-3: Diethylene Glycol) The test was conducted in the same procedure as in Test Example 3-1, except that the dimethyl sulfoxide used in Test Example 3-1 was changed to diethylene glycol. As a result, the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 180 was 5 parts decreased compared to the absorption coefficient (absorbance ÷ mass of added pigment) obtained when the amount of C.I. Pigment Yellow 180 was 4 parts. From the above results, it was determined that diethylene glycol cannot dissolve C.I. Pigment Yellow 180 at 0.005% or more.

[0075] (4) Solubility of Organic Solvents in C.I. Pigment Red 150 (Test Example 4-1: Dimethyl Sulfoxide) To 100,000 parts of dimethyl sulfoxide at 25°C, 4 parts of C.I. Pigment Red 150 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of the added pigment) of the obtained solution (filtrate) was confirmed. Subsequently, to 100,000 parts of dimethyl sulfoxide at 25°C, 5 parts of C.I. Pigment Red 150 were added and stirred for 30 minutes. Then, the pigment and the organic solvent were separated by filtration, and the absorption coefficient (absorbance ÷ mass of the added pigment) of the obtained solution (filtrate) was confirmed. The absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Red 150 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Red 150 was 4 parts. From the above results, it was determined that dimethyl sulfoxide can dissolve C.I. Pigment Red 150 at 0.005% or more.

[0076] (Test Example 4-2: N-Methylpyrrolidone) The test was conducted in the same procedure as Test Example 4-1, except that the dimethyl sulfoxide used in Test Example 4-1 was changed to N-methylpyrrolidone. As a result, the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Red 150 was 5 parts showed no change compared to the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Red 150 was 4 parts. From the above results, it was determined that N-methylpyrrolidone can dissolve C.I. Pigment Red 150 at 0.005% or more.

[0077] (Test Example 4-3: Diethylene Glycol) The test was conducted in the same procedure as in Test Example 4-1, except that the dimethyl sulfoxide used in Test Example 4-1 was changed to diethylene glycol. As a result, the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Red 150 was 5 parts decreased as compared with the absorption coefficient (absorbance ÷ mass of the added pigment) obtained when the amount of C.I. Pigment Red 150 was 4 parts. From the above results, it was determined that diethylene glycol could not dissolve 0.005% or more of C.I. Pigment Red 150.

[0078] <Production of Pigment Composition> (Pigment Compositions 1 to 3, 5 to 13, 15 to 20, 22 to 27, and 29 to 34) Using the pigments shown in the upper row of Table 3 (Tables 3-1 to 3-5), each component (unit: part) shown in the middle row of Table 3 was mixed to obtain a mixture. As sodium chloride, those with D 50 of 250 μm and D 95 of 425 μm were used. The above mixture was kneaded for 8 hours under the kneading conditions shown in the lower row of Table 3 using a planetary mixer (trade name "Trimix", manufactured by Inoue Seisakusho; indicated as "1" in "Kneading Apparatus No." in the lower row of Table 3). In this way, Pigment Compositions 1 to 3, 5 to 13, 15 to 20, 22 to 27, and 29 to 34, which are kneaded products by the solvent salt milling method, were obtained.

[0079] (Pigment Composition 4) 1 part of Pigment 1, 10 parts of sodium chloride, and 2 parts of dimethyl sulfoxide were put into a kneader (trade name "PBV-03", manufactured by Irie Shokai; indicated as "2" in "Kneading Apparatus No." in the lower row of Table 3), and kneaded at a temperature of 60 °C and a shear rate of 20 s -1 for 8 hours. In this way, Pigment Composition 4, which is a kneaded product by the solvent salt milling method, was obtained.

[0080] (Pigment Compositions 14, 21, 28, and 35) 1 part of the pigment shown in the upper row of Tables 3-2 to 3-5 was added to a container containing 100 parts of ethanol, and after stirring for 1 hour, filtration and washing with water were repeated to obtain Pigment Compositions 14, 21, 28, and 35.

[0081] TIFF2025092980000004.tif89170

[0082] TIFF2025092980000005.tif88170

[0083] TIFF2025092980000006.tif89170

[0084] TIFF2025092980000007.tif90170

[0085] TIFF2025092980000008.tif89170

[0086] <Preparation of Pigment Dispersion Liquid> Using each of the obtained pigment compositions 1 to 35, pigment dispersion liquids 1 to 35 with numbers corresponding to the numbers of the pigment compositions were prepared. Specifically, after thoroughly washing, filtering, and drying the obtained pigment compositions 1 to 35, an aqueous resin solution was added to the obtained pigments, and dispersion treatment was performed at a treatment pressure of 200 MPa using a high-pressure homogenizer (trade name "Starburst", manufactured by Sugino Machine). As the resin in the aqueous resin solution, a styrene-acrylic acid copolymer (trade name "Joncryl 690", manufactured by BASF) with a weight average molecular weight of 16,500 and an acid value of 240 mgKOH / g was used. To this resin (styrene-acrylic acid copolymer), 0.9 equivalents of potassium hydroxide was added based on the acid value of the resin, and an aqueous resin solution with a resin content of 20.0% was used. After the dispersion treatment, an appropriate amount of ion-exchanged water was added to obtain pigment dispersion liquids 1 to 35 with a pigment content of 15.0%.

[0087] <Preparation of Ink> Using each of the obtained pigment dispersions 1 to 35, inks 1 to 35 corresponding to the numbers of the pigment dispersions were prepared. Specifically, using the pigment dispersions of the types (numbers) shown in Table 4 below, the following components containing the pigment dispersion 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 inks 1 to 35. Acetylenol E100 shown below is the trade name of a surfactant manufactured by Kawaken Fine Chemicals Co., Ltd. (Components of Ink) Pigment dispersion (any one of 1 to 35) 33.0 parts Glycerin 10.0 parts Triethylene glycol 7.0 parts Acetylenol E100 0.1 part Ion-exchanged water 49.9 parts

[0088] <Evaluation> Inks 1 to 35 were each filled into an ink cartridge and set in an inkjet recording apparatus (trade 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 1 drop of 30.4 ng of ink droplets 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, a solid image with a recording duty up to 140% was recorded on a recording medium (trade name "Canon Photo Paper Glossy Gold GL-101", manufactured by Canon). Table 4 shows each ink and the corresponding examples and comparative examples.

[0089] In the obtained solid image, using a weather resistance tester (trade name "Xenon Weather Meter X75SC", manufactured by Suga Test Instruments Co., Ltd.), the illuminance was 0.39 W / m 2Under the conditions of a black panel temperature of 60°C and a relative humidity of 70%, a light resistance test was conducted. When the initial (before the test) optical density (O.D. value) of each image was "0.5", the O.D. value after 300 hours (equivalent to 100 years) of the light resistance test was measured, and the ratio of the O.D. value after 300 hours (residual O.D. value) to the initial O.D. value was calculated and used as an evaluation index for light resistance. A spectrophotometer (trade name "Spectrolino", manufactured by Gretag Macbeth) was used to measure the O.D. value. The light resistance was evaluated according to the following evaluation criteria set for each main pigment. In this example, "A" and "B" were set as acceptable levels and "C" as an unacceptable level according to the following evaluation criteria. The evaluation results are shown in Table 4. (Evaluation criteria for Examples 1 to 10 and Comparative Examples 1 to 4) A: The residual O.D. value was 75% or more. B: The residual O.D. value was 50% or more and less than 75%. C: The residual O.D. value was less than 50%. (Evaluation criteria for Examples 11 to 15, Comparative Examples 5 and 6) A: The residual O.D. value was 85% or more. B: The residual O.D. value was 75% or more and less than 85%. C: The residual O.D. value was less than 75%. (Evaluation criteria for Examples 16 to 20, Comparative Examples 7 and 8) A: The residual O.D. value was 75% or more. B: The residual O.D. value was 50% or more and less than 75%. C: The residual O.D. value was less than 50%. (Evaluation criteria for Examples 21 to 25, Comparative Examples 9 and 10) A: The residual O.D. value was 75% or more. B: The residual O.D. value was 50% or more and less than 75%. C: The residual O.D. value was less than 50%.

[0090] TIFF2025092980000009.tif220170

[0091] Note that the disclosure of this embodiment includes the following methods. (Method 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 pigment is an azo pigment containing organic impurities, and the organic solvent is an organic solvent capable of dissolving 0.005% by mass or more of the azo pigment. A method for producing a pigment composition, characterized by the above. (Method 2) The method for producing a pigment composition according to Method 1, wherein the organic solvent contains at least one of dimethyl sulfoxide and N-methylpyrrolidone. (Method 3) The method for producing a pigment composition according to Method 1 or 2, wherein the temperature during kneading in the kneading step is 45°C or higher and 90°C or lower. (Method 4) The shear rate during kneading in the kneading step is 15 s -1 or higher. The method for producing a pigment composition according to any one of Methods 1 to 3. (Method 5) The method for producing a pigment composition according to any one of Methods 1 to 4, wherein the azo pigment is at least one selected from the group consisting of C.I. Pigment Yellow 74, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, and C.I. Pigment Red 150. (Method 6) The method for producing a pigment composition according to any one of Methods 1 to 5, wherein the azo pigment is C.I. Pigment Yellow 74. (Method 7) The method for producing a pigment composition according to any one of Methods 1 to 6, wherein the organic impurities contain a compound represented by the following chemical formula (1).

[0092] TIFF2025092980000010.tif46170

[0093] (Method 8) A method for producing a pigment dispersion, comprising mixing a pigment composition, a dispersant, and water obtained by the method for producing a pigment composition according to any one of Methods 1 to 7, and dispersing the pigment composition in the water with the dispersant. (Method 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 described in Method 8 and a component containing a water-soluble organic solvent. (Method 10) An inkjet recording method in which ink is ejected 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 described in 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 pigment is an azo pigment containing organic impurities, and the organic solvent is an organic solvent capable of dissolving 0.005% by mass or more of the azo pigment, characterized in that it is a method for producing a pigment composition.

2. The method for producing a pigment composition according to claim 1, wherein the organic solvent contains at least one of dimethyl sulfoxide and N-methylpyrrolidone.

3. The method for producing a pigment composition according to claim 1, wherein the temperature during kneading in the kneading step is 45°C or higher and 90°C or lower.

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

5. The method for producing a pigment composition according to claim 1, wherein the azo pigment is at least one selected from the group consisting of C.I. Pigment Yellow 74, C.I. Pigment Yellow 155, C.I. Pigment Yellow 180, and C.I. Pigment Red 150.

6. The method for producing a pigment composition according to claim 1, wherein the azo pigment is C.I. Pigment Yellow 74.

7. The method for producing a pigment composition according to claim 6, wherein the organic impurities contain a compound represented by the following chemical formula (1).

8. A method for producing a pigment dispersion liquid, comprising mixing a pigment composition, a dispersant, and water obtained by the method for producing a pigment composition according to any one of claims 1 to 7, and dispersing the pigment composition in the water by 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 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 claim 9.

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