Quinacridone pigments, pigment dispersions and aqueous inkjet inks

Abstract

The problem to be solved by the present invention is to provide an innovative quinacridone pigment that maintains high chroma while exhibiting superior viscosity stability compared to conventional pigments in water-based ink applications. The quinacridone pigment of the present invention has a water vapor to nitrogen specific surface area ratio of 0.270 or higher and less than 0.430. The pigment dispersion of the present invention contains the above-described quinacridone pigment and a solvent. Furthermore, the water-based inkjet ink of the present invention contains the above-described pigment dispersion.

Description

Technical Field

[0001] This invention relates to quinacridone pigments, pigment dispersions containing quinacridone pigments, and water-based inkjet inks. Background Technology

[0002] Inkjet printing (IJ) is a printing method that guides ink into a fine nozzle and ejects it as a high-speed ink jet, controlling the printing process according to text or graphic signals. It is widely used in both industrial and household applications. IJ inks include solvent-based, UV-curable, water-based dye-based, and water-based pigment-based inks. In particular, water-based pigment IJ inks have advantages in terms of safety and environment compared to solvent-based, UV-curable, and water-based dye-based inks, and therefore their development is particularly anticipated from a sustainability perspective.

[0003] On the other hand, as IJ printing becomes increasingly faster and more precise, the requirements for pigments are also rising. One of the key performance requirements for pigments in IJ inks is dispersion stability, which can withstand complex ink designs. To improve dispersion stability, companies are focusing on pigment derivative processing, surface treatment, and particle size control, and are actively developing related technologies.

[0004] In addition to carbon black (CB) and azo pigments, pigments used in IJ printing also include condensed polycyclic organic pigments such as phthalocyanine and quinacridone pigments. When using condensed polycyclic organic pigments such as phthalocyanine and quinacridone pigments, which are generally not limited to IJ applications, flowability issues become significant. To improve flowability, research has been conducted on surface treatments of condensed polycyclic organic pigments. Methods include pigment derivative treatment, rosin treatment, surfactant treatment, resin-based dispersant treatment, and plasma treatment. However, even with these methods, depending on the intended use, the ink may sometimes become highly viscous, or the ink may thicken during storage (viscosity increase), leading to a decrease in flowability.

[0005] Among them, a method was proposed to obtain a β-type copper phthalocyanine pigment composition with good dispersibility and appropriate contact angle with water and diethylene glycol by using phthalocyanine pigments as condensed polycyclic organic pigments and specific phthalimide methyl copper phthalocyanine derivatives (Patent Document 1).

[0006] In addition, as a surface treatment method for pigments in water-dispersible pigments, the following method is known: hydrogen peroxide water containing phthalocyanine is placed in a reaction vessel, argon gas is introduced into the solution in the reaction vessel, the pigment is immersed in a water tank containing a specified amount of water, and ultrasonic waves are applied (Patent Document 2). Besides the above, there are many patents related to the hydrophilization and self-dispersion of pigment surfaces for carbon black (CB), but there are fewer patents related to the hydrophilization treatment of pigment surfaces for quinacridone pigments, and in particular, no patents have been found that address both dispersion stability and chroma in water-based ink applications.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2006-126440

[0010] Patent Document 2: Japanese Patent Application Publication No. 2003-201419 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] This invention provides an innovative quinacridone pigment that, through surface modification, appropriately controls its hydrophilic parameters, resulting in superior viscosity stability compared to conventional pigments in water-based ink applications, while maintaining high chroma. The high viscosity stability allows for use even under long-term storage conditions. Furthermore, the wider range of solvent types and ratios that can be used in inks increases the freedom of ink design, enabling the production of excellent water-based inks.

[0013] Methods for solving problems

[0014] The inventors conducted in-depth research to find the aforementioned innovative quinacridone pigment, and discovered that by modifying and hydrophilizing the pigment surface, its viscosity stability in inks was improved. The mechanism envisioned in this invention is as follows.

[0015] The processes that affect pigment dispersion in an aqueous system include: 1. pigment wetting; 2. dispersion stabilization (in this invention, a process of adsorbing dispersion resin onto the pigment surface). The phenomena occurring in processes 1 and 2 are considered as follows.

[0016] 1. By making the pigment surface hydrophilic, the wettability is improved (water-soluble solvents can easily enter the tiny gaps in the pigment particle aggregates), thereby reducing the cohesive force between particles and making them easier to break due to the mechanical force of the disperser.

[0017] 2. Furthermore, in the aqueous system where the dispersion resin is adsorbed onto the pigment, by making the surface of the hydrophobic quinacridone pigment hydrophilic, the interaction with the dispersion resin is strengthened, thereby achieving higher dispersion stability and viscosity stability.

[0018] On the other hand, this study shows that if the modification of the pigment surface becomes excessive, the hue (chroma) of the pigment deteriorates. The detailed mechanism is not yet clear, but it can be inferred that the modification of the pigment surface, that is, the change in the electronic and stereochemical structure of the pigment, will cause changes in the electronic state of the single molecule, changes in the intermolecular interactions, etc., thereby producing a change in hue.

[0019] In this invention, the balance between the improvement of dispersion stability and viscosity stability and hue change listed above was studied. As a result, by focusing on the specific surface area ratio of water vapor to nitrogen and controlling it within an appropriate range, a pigment with higher viscosity stability than before was successfully developed while suppressing hue change in a limited manner.

[0020] That is, the present invention relates to the following manner.

[0021] Item 1. A quinacridone pigment having a water vapor to nitrogen specific surface area ratio of 0.270 or more and less than 0.430.

[0022] Item 2. The quinacridone pigment as described in Item 1, which is a solid solution of CI Pigment Red 122, CI Pigment Violet 19, or CI Pigment Red 122 and CI Pigment Violet 19.

[0023] Item 3. A pigment dispersion comprising the quinacridone pigment and solvent described in Item 1 or 2.

[0024] Item 4. An aqueous inkjet (IJ) ink comprising the pigment dispersion described in Item 3.

[0025] Invention Effects

[0026] The quinacridone pigment of the present invention, when used particularly as a water-based ink, balances ink viscosity stability and printed color saturation. Furthermore, the quinacridone pigment of the present invention reduces the number of coarse particles and exhibits excellent particle size stability, thus improving the ejection performance of the ink. Therefore, the quinacridone pigment of the present invention is particularly useful in water-based ink applications. Detailed Implementation

[0027] The present invention will now be described in detail.

[0028] Quinacridone Pigment

[0029] The specific surface area ratio of water vapor to nitrogen in the quinacridone pigment of the present invention is 0.270 or more and less than 0.430. Preferably, the specific surface area ratio of water vapor to nitrogen is 0.280 or more and less than 0.420, more preferably 0.290 or more and less than 0.410, particularly preferably 0.290 or more and less than 0.370, and most preferably 0.300 or more and less than 0.370. Because the specific surface area ratio is within the above range, it is particularly advantageous when used as a water-based ink, as it balances the viscosity stability of the ink and the chroma of the printed matter. The specific surface area ratio of water vapor to nitrogen can be calculated by using a specific surface area / pore size distribution measuring device to measure the specific surface area of ​​water vapor and the specific surface area of ​​nitrogen in the pigment using water vapor and nitrogen gas, and then taking the ratio.

[0030] The specific surface area of ​​the water vapor mentioned above is typically 5–50 m². 2 / g, preferably 10-40m 2 / g, more preferably 15-35m 2 / g. Additionally, the specific surface area of ​​nitrogen mentioned above is typically 30–120 m². 2 / g, preferably 40-100m 2 / g, more preferably 60-90m 2 / g.

[0031] Examples of quinacridone pigments used in this invention include CI Pigment Red 122, CI Pigment Violet 19, CI Pigment Violet 42, CI Pigment Violet 55, CI Pigment Red 202, CI Pigment Red 206, CI Pigment Red 207, CI Pigment Red 209, CI Pigment Orange 48, and CI Pigment Orange 49. Solid solutions of these quinacridone pigments may also be used. From the perspective of easily obtaining the effects of this invention, solid solutions of CI Pigment Red 122, CI Pigment Violet 19, and CI Pigment Red 122 and CI Pigment Violet 19 are preferred. These quinacridone pigments can be used alone or in combination of two or more.

[0032] The primary particle size of the quinacridone pigment of the present invention is, for example, 50-500 nm, preferably 50-250 nm.

[0033] The quinacridone pigment of the present invention is suitable for use in inks for water-based applications, particularly as an aqueous pigment dispersion dispersed using a pigment dispersant or the like, and is suitable for use in water-based inks for water-based applications.

[0034] <Method for manufacturing quinacridone pigments>

[0035] The following illustrates an example of a method for manufacturing the quinacridone pigment of the present invention. The quinacridone pigment of the present invention is not limited to the quinacridone pigment manufactured by the following method, provided that the specific surface area ratio of water vapor to nitrogen is within the above-mentioned range.

[0036] The quinacridone pigment of the present invention can be manufactured, for example, through a pigment slurry manufacturing process, a pigment surface treatment process, and a post-treatment process.

[0037] Pigment slurry manufacturing process: adding quinacridone pigment as a raw material to a solvent and stirring to obtain pigment slurry.

[0038] Pigment surface treatment process: The process of adding iron or copper salts and hydrogen peroxide to pigment slurry, stirring, and treating the pigment surface.

[0039] Post-processing steps: filtering the reaction solution, drying and pulverizing the filtered material.

[0040] (Pigment paste manufacturing process)

[0041] Firstly, in the pigment paste manufacturing process, the quinacridone pigment used as a raw material can be any of the aforementioned pigment types, and can be a commercially available or conventionally known quinacridone pigment manufactured using conventional methods. The quinacridone pigment used as a raw material can be an untreated pigment, or it can be a quinacridone pigment whose surface has been treated with pigment derivatives such as quinacridone pigment sulfonic acid derivatives, amino-containing quinacridone pigment derivatives, phthalimide-methyl-containing quinacridone pigment derivatives, dispersants, surfactants, rosin, etc. Alternatively, after the pigment surface treatment process, other pigment particle surface treatments such as quinacridone pigment sulfonic acid derivatives, amino-containing quinacridone pigment derivatives, phthalimide-methyl-containing quinacridone pigment derivatives, dispersants, surfactants, rosin, etc., can be performed.

[0042] As a raw material, quinacridone pigments can be used, either quinacridone pigments that have undergone a pigmentation process to achieve uniform particle size and shape, or crude quinacridone pigments with irregular particle size and shape can be used, and then subjected to a pigmentation process after the pigment surface treatment process to achieve uniformity. The pigmentation process can be, for example, one or a combination of methods such as acidic paste method, acidic slurry method, dry grinding method, solvent method, and solvent grinding method.

[0043] As a solvent, water and / or organic solvents can be used. Among organic solvents, methanol, ethanol, n-propanol, 2-propanol, isobutanol, etc., can be used. From an economic point of view, water is preferred. Furthermore, the water used can be pure water or industrial water, and buffer solutions such as acetate buffer, phosphate buffer, citrate buffer, citrate-phosphate buffer, borate buffer, and tartrate buffer can also be used.

[0044] The amount of quinacridone pigment added as a raw material is preferably 1 to 30 parts by mass relative to 100 parts by mass of solvent. When the amount added is small, the productivity is low. When the amount added is large, the pigment slurry has high viscosity and requires too much energy for stirring. Therefore, it is more preferably 2 to 20 parts by mass, and particularly preferably 3 to 12 parts by mass.

[0045] The preferred temperature for the pigment slurry manufacturing process is 0°C to 120°C. Furthermore, the preferred temperature for the pigment surface treatment process is 0°C to 100°C. At low temperatures, the reaction rate of the pigment surface treatment slows down, while at high temperatures, the decomposition of hydrogen peroxide is promoted. Therefore, 10°C to 90°C is more preferred, and particularly 20°C to 80°C is especially preferred.

[0046] (Pigment surface treatment process)

[0047] Next, examples of iron or copper salts used in the pigment surface treatment process include ferric sulfate, ferric chloride, ferric fluoride, ferric bromide, ferric iodide, ferric nitrate, ferric phosphate, ferric borate, ferric carbonate, ferric acetate, copper sulfate, copper chloride, copper sulfide, and copper oxide. From an economic point of view, ferric sulfate, ferric chloride, and ferric nitrate are preferred. Divalent iron or copper can be used as the iron or copper. Furthermore, the iron and copper salts can be anhydrous or hydrated.

[0048] The iron salt is preferably added at 0.1 to 20% by mass relative to the quinacridone pigment used as a raw material. The iron salt acts as a catalyst for the pigment surface treatment reaction. Therefore, when the amount of iron salt added is small, the surface treatment reaction is slow, and when the amount added is excessive, it will promote the decomposition of hydrogen peroxide, which is economically disadvantageous. Therefore, 0.5 to 10% by mass is preferred.

[0049] The amount of hydrogen peroxide added relative to the quinacridone pigment in the raw material is preferably 1 to 40% by mass, more preferably 2 to 30% by mass, and particularly preferably 3 to 20% by mass. If the amount of hydrogen peroxide added is small, the surface treatment of the pigment becomes insufficient; on the other hand, if the amount is large, the surface modification of the pigment becomes excessive, and sometimes the hue (chroma) of the pigment deteriorates. Hydrogen peroxide can be added as hydrogen peroxide water diluted with water.

[0050] Ferric salts and hydrogen peroxide can be added to pigment slurry simultaneously or separately. When added simultaneously, if hydrogen peroxide and ferric salts are pre-mixed, the hydrogen peroxide will decompose and thus mix in the pigment slurry. When added separately, either the ferric salt or the hydrogen peroxide can be added first. Furthermore, hydrogen peroxide can be added dropwise or all at once.

[0051] The reaction time for the pigment surface treatment process is preferably 10 minutes to 2 hours. Since the treatment solution in the pigment surface treatment process is alkaline and iron ions precipitate, a pH of 1 to 7 is preferred.

[0052] (Post-processing steps)

[0053] The filtration, drying, and pulverizing processes in the post-processing steps can be carried out using methods commonly used in pigment manufacturing.

[0054] Pigment Dispersions

[0055] The pigment dispersion of the present invention contains the quinacridone pigment of the present invention described above and a solvent. The solvent can be any of an organic solvent, water, a water-soluble solvent, etc., preferably water and / or a water-soluble solvent. That is, from the perspective of easily exerting the effects of the quinacridone pigment of the present invention described above, the pigment dispersion of the present invention is preferably an aqueous pigment dispersion. Examples of water-soluble solvents include alcohol components, such as methanol, ethanol, isopropanol, and butanol. In addition to alcohol components, the water-soluble solvent may also contain glycols such as diethylene glycol, propylene glycol, and triethylene glycol, glycerol, and lower alkyl ethers of polyols.

[0056] The pigment dispersion of the present invention can be manufactured as follows: a high-concentration aqueous dispersion (pigment paste) of the quinacridone pigment of the present invention is prepared, diluted with water and / or a water-soluble solvent, and other additives are added as needed.

[0057] The method for dispersing the quinacridone pigment of the present invention in water and / or a water-soluble solvent to obtain a pigment paste is not particularly limited, but a known dispersion method is preferred. The dispersant used can be a known pigment dispersant dispersed in water, or a surfactant. As a pigment dispersant, an aqueous resin is preferred; preferred examples include polyvinyl alcohols, polyvinylpyrrolidones, polyurethane resins having anionic or cationic groups, and free radical copolymer resins having anionic or cationic groups. Examples of free radical copolymer resins having anionic or cationic groups include acrylic resins such as acrylic-acrylate copolymers, styrene-acrylic acid copolymers, styrene-methacrylic acid copolymers, styrene-methacrylic acid-acrylate copolymers, styrene-α-methylstyrene-acrylic acid copolymers, styrene-α-methylstyrene-acrylate-acrylate copolymers, styrene-maleic acid copolymers, styrene-maleic anhydride copolymers, vinylnaphthalene-acrylic acid copolymers, and salts of such aqueous resins.

[0058] Examples of compounds used as salts for forming copolymers include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, as well as compounds such as diethylamine, ammonia, ethylamine, triethylamine, propylamine, isopropylamine, dipropylamine, butylamine, isobutylamine, triethanolamine, diethanolamine, aminomethylpropanol, and morpholine. The amount of these compounds used to form the salt is preferably at least the neutralization equivalent of the copolymer. Alternatively, commercially available compounds can also be used as salts for forming the copolymer. Examples of commercially available compounds include the AJISPER PB series manufactured by Ajinomoto Fine-Techno Co., Ltd., the Disperbyk series and BYK- series manufactured by BYK-Chemie Japan Co., Ltd., and the EFKA series manufactured by BASF Japan Co., Ltd.

[0059] As a dispersion method in pigment dispersion, the following (1) to (3) can be shown, for example.

[0060] (1) A method for preparing a pigment paste by adding pigment to an aqueous medium containing a pigment dispersant and water, and then using a stirring / dispersing device to disperse the pigment in the aqueous medium.

[0061] (2) A method of preparing pigment paste by mixing pigments and pigment dispersants using a mixing mill such as a double roller or mixer, adding the resulting mixture to an aqueous medium containing water, and using a stirring / dispersing device.

[0062] (3) A method for preparing a pigment paste by adding pigment to a solution obtained by dissolving a pigment dispersant in an organic solvent that is compatible with water, such as methyl ethyl ketone or tetrahydrofuran, using a stirring / dispersing device to disperse the pigment in the organic solution, then using an aqueous medium for phase inversion emulsification, and finally distilling off the organic solvent.

[0063] There are no particular limitations on the type of mixing machine; examples include Henschel mixers, pressure kneaders, Banbury internal mixers, and planetary mixers. Similarly, there are no particular limitations on the type of stirring / dispersing device; examples include ultrasonic homogenizers, high-pressure homogenizers, paint vibrators, ball mills, roller mills, sand mixers, sand mills, Dinot mills, Dispermat mills, SC mills, and Nanomizers. One of these devices can be used alone, or two or more can be used in combination.

[0064] The amount of the quinacridone pigment of the present invention in the above-mentioned pigment paste is preferably 5 to 60% by mass, more preferably 10 to 50% by mass. When the amount of pigment is less than 5% by mass, the water-based ink prepared from the pigment paste will not be sufficiently colored, and there is a tendency to not obtain sufficient image density. On the contrary, when the amount of pigment is more than 60% by mass, the dispersion stability of the pigment in the pigment paste tends to decrease.

[0065] In addition, since coarse particles can cause nozzle clogging and deterioration of other image characteristics, it is preferable to remove coarse particles by centrifugation or filtration before or after ink preparation.

[0066] Following the dispersion process, impurity removal steps based on ion exchange and ultrafiltration can be performed, followed by post-treatment. Ion exchange removes ionic substances such as cations and anions (e.g., divalent metal ions), while ultrafiltration removes dissolved impurities (residues from pigment synthesis, excess components in the dispersion composition, resin not adsorbed onto organic pigments, and foreign matter). Ion exchange uses known ion exchange resins. Ultrafiltration uses known ultrafiltration membranes, which can be of either conventional or double-capacity-enhanced types.

[0067] <Water-based IJ ink>

[0068] The water-based ink of the present invention comprises the pigment dispersion of the present invention as described above. In addition to the pigment dispersion of the present invention, the water-based ink of the present invention may also contain, as needed, wetting agents (drying inhibitors), penetrants, surfactants, etc. Furthermore, as other additives, it may contain preservatives, viscosity modifiers, pH adjusters, chelating agents, plasticizers, antioxidants, ultraviolet absorbers, etc.

[0069] The wetting agent is added to prevent ink from drying. The content of the wetting agent in the ink for preventing drying is preferably 3-50% by mass. There are no particular limitations on the wetting agent used in this invention, but it is preferable to have miscibility with water and to provide an effect of preventing printhead clogging in inkjet printers. Examples include glycerol, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol with a molecular weight of less than 2000, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3-propanediol, isopropylene glycol, isobutylene glycol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, erythritol, pentaerythritol, etc. Among these, the inclusion of propylene glycol and 1,3-butanediol provides safety and exhibits excellent ink drying and ejection performance.

[0070] Penetrants are added to improve the permeability to the recording medium and adjust the dot diameter on the recording medium. Examples of penetrants include lower alcohols such as ethanol and isopropanol, ethylene oxide adducts of alkyl alcohols such as ethylene glycol hexyl ether and diethylene glycol butyl ether, and propylene oxide adducts of alkyl alcohols such as propylene glycol propyl ether.

[0071] Surfactants are added to adjust ink properties such as surface tension. Therefore, there are no particular limitations on the surfactants that can be added, including various anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants, among which anionic surfactants and nonionic surfactants are preferred.

[0072] Examples of anionic surfactants include alkylbenzene sulfonates, alkylphenyl sulfonates, alkylnaphthalene sulfonates, higher fatty acid salts, sulfate salts of higher fatty acid esters, sulfonates of higher fatty acid esters, sulfate salts and sulfonates of higher alcohol ethers, higher alkyl sulfosuccinates, polyoxyethylene alkyl ether carboxylates, polyoxyethylene alkyl ether sulfates, alkyl phosphates, and polyoxyethylene alkyl ether phosphates. Specific examples include dodecylbenzene sulfonate, isopropylnaphthalene sulfonate, monobutylphenylphenol monosulfonate, monobutylbiphenyl sulfonate, and dibutylphenylphenol disulfonate.

[0073] Examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, glycerol fatty acid esters, polyoxyethylene glycerol fatty acid esters, polyglycerol fatty acid esters, sucrose fatty acid esters, polyoxyethylene alkylamines, polyoxyethylene fatty acid amides, fatty acid alkanolamides, alkyl alkanolamides, acetylenol, ethoxyethylene adducts of acetylenol, and polyethylene glycol-polypropylene glycol block copolymers. Among these, polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene alkyl ether, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, fatty acid alkanolamides, acetylenol, ethoxyethylene adducts of acetylenol, and polyethylene glycol-polypropylene glycol block copolymers are preferred.

[0074] Other surfactants that can be used include silicone surfactants such as polysiloxane ethylene oxide adducts, fluorinated surfactants such as perfluoroalkyl carboxylates, perfluoroalkyl sulfonates, and perfluoroalkyl ethylene oxide ethers, and biosurfactants such as spirulisporic acid, rhamnolipids, and lysophosphatidylcholine.

[0075] These surfactants can be used alone or in combination of two or more. When surfactants are added, the amount added is preferably in the range of 0.001 to 2% by mass relative to the total mass of the ink, more preferably 0.001 to 1.5% by mass, and even more preferably 0.01 to 1% by mass. When the amount of surfactant added is less than 0.001% by mass, there is a tendency not to obtain the effect of adding surfactants, and when it is used more than 2% by mass, problems such as image bleeding are likely to occur.

[0076] Other additives such as preservatives, viscosity modifiers, pH adjusters, chelating agents, plasticizers, antioxidants, and UV absorbers may contain, in moderation, components commonly used for IJ applications.

[0077] There are no particular limitations on the physical properties of the ink. Considering the ejectibility of the ink as an IJ ink, at a test temperature of 25°C, the viscosity is preferably 1 to 10 (mPa·s), the surface tension is preferably 20 to 50 (mN / m), and the pigment concentration is preferably 1 to 10 by mass.

[0078] The water-based ink of the present invention is prepared by further adding a water-soluble solvent and / or water, an organic polymer compound containing anionic groups for adhesive purposes, etc., to the pigment dispersion of the present invention, and adding a wetting agent (drying inhibitor), penetrant, or other additives as needed according to the desired physical properties. After the ink is prepared, a centrifugal separation or filtration process may be added.

[0079] Example

[0080] The present invention will now be described in more detail using examples and comparative examples. In the following examples and comparative examples, unless otherwise specified, “%” means “mass %”.

[0081] In this embodiment, firstly, a quinacridone pigment was prepared, and the specific surface area ratio of water vapor to nitrogen was measured. Next, a pigment dispersion and IJ ink were prepared, and the printing performance and initial viscosity of the IJ ink were evaluated. Finally, to evaluate storage stability, viscosity was measured at 70°C for one week. Details are shown below. In this embodiment, the specific surface area ratio of water vapor to nitrogen is referred to as the "water vapor / nitrogen specific surface area ratio".

[0082] In this embodiment, the water vapor to nitrogen specific surface area ratio of the quinacridone pigment was determined using a specific surface area / pore size distribution measuring device (BELPREP-vacII, Microtrac BEL). After pretreatment at 100°C for 2 hours under reduced pressure, the pigment was installed in the device. The relative pressure was changed while adsorbing the adsorbate onto or desorbing it from the sample. The water vapor and nitrogen specific surface areas of the pigment were measured by using water vapor and nitrogen in the adsorbate. The ratio was then calculated to determine the water vapor / nitrogen specific surface area ratio.

[0083] [Preparation of Quinacridone Pigment: Example 1]

[0084] 200 parts (50 parts pigment component) of wet filter cake of CI pigment Red 122 (manufactured by DIC Corporation) and 800 parts of deionized water were added to a 2L stainless steel beaker and stirred for 30 minutes using a stainless steel horseshoe-shaped impeller at 150 rpm. 0.4 parts of ferric(II) sulfate heptahydrate (manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) were added, and the mixture was heated to 60°C. Next, 17.3 parts of 30% hydrogen peroxide solution (manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) were added, and the mixture was stirred at 60°C for 60 minutes. The slurry was then filtered using a suction filter, washed with warm water at 70°C, and dried in a blower-type constant-temperature desiccator for 24 hours. The resulting pigment block was pulverized to obtain quinacridone pigment. The water vapor / nitrogen specific surface area ratio was measured and found to be 0.290.

[0085] [Preparation of Quinacridone Pigment: Example 2]

[0086] The CI pigment Red 122 (manufactured by DIC Corporation) wet filter cake from Example 1 was replaced with CI pigment Violet 19 (manufactured by DIC Corporation) wet filter cake; 0.4 parts of ferric(II) sulfate heptahydrate (manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) was replaced with 0.5 parts; and 17.3 parts of 30% hydrogen peroxide water (manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) was replaced with 8.7 parts. Otherwise, the quinacridone pigment was obtained using the same method as in Example 1. The water vapor / nitrogen specific surface area ratio was 0.362.

[0087] [Preparation of Quinacridone Pigment: Example 3]

[0088] The wet filter cake of CI Pigment Red 122 (manufactured by DIC Corporation) in Example 1 was replaced with a wet filter cake of a solid solution (manufactured by DIC Corporation) of CI Pigment Red 122 and CI Pigment Violet 19 in a ratio of 8:2. Otherwise, the quinacridone pigment was obtained by the same method as in Example 1. The water vapor / nitrogen specific surface area ratio was 0.301.

[0089] [Preparation of Quinacridone Pigments: Comparative Example 1]

[0090] The wet filter cake of CI pigment Red 122 (manufactured by DIC Corporation) was dried in a forced-air constant-temperature dryer for 24 hours. The resulting pigment block was pulverized to obtain quinacridone pigment. The water vapor / nitrogen specific surface area ratio was measured and found to be 0.229.

[0091] [Preparation of Quinacridone Pigments: Comparative Example 2]

[0092] The wet filter cake of CI pigment Violet 19 (manufactured by DIC Corporation) was dried in a forced-air constant-temperature dryer for 24 hours. The resulting pigment block was pulverized to obtain quinacridone pigment. The water vapor / nitrogen specific surface area ratio was measured and found to be 0.266.

[0093] [Preparation of Quinacridone Pigments: Comparative Example 3]

[0094] The wet filter cake of CI pigment Red 122 (manufactured by DIC Corporation) in Example 1 was replaced with a wet filter cake of CI pigment Violet 19 (manufactured by DIC Corporation). The amount of ferric(II) sulfate heptahydrate (manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) was changed from 0.4 parts to 2.3 parts, and the amount of 30% hydrogen peroxide water (manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) was changed from 17.3 parts to 41.7 parts. All other steps were performed in the same manner to obtain the quinacridone pigment. The water vapor / nitrogen surface area ratio was 0.430.

[0095] [Preparation of Quinacridone Pigments: Comparative Example 4]

[0096] A wet filter cake of a solid solution (manufactured by DIC Corporation) containing CI pigment Red 122 and CI pigment Violet 19 in a ratio of 8:2 was dried in a forced-air constant-temperature dryer for 24 hours. The resulting pigment block was pulverized to obtain quinacridone pigment. The water vapor / nitrogen specific surface area ratio was measured and found to be 0.210.

[0097] [Preparation of Quinacridone Pigments: Comparative Example 5]

[0098] The 17.3 parts of 30% hydrogen peroxide water (manufactured by Fujifilm and Koh Genuine Chemicals Co., Ltd.) from Example 1 were set at 2.2 parts. Otherwise, the quinacridone pigment was obtained by the same method as in Example 1. The water vapor / nitrogen specific surface area ratio was measured and found to be 0.252.

[0099] [Preparation of Pigment Dispersions]

[0100] 10.0 parts by weight of the quinacridone pigment obtained above, 7 parts by weight of styrene-acrylic resin (manufactured by DIC Corporation) with an acid value of 170 mg KOH / g (3 parts by weight of resin component), 10 parts by weight of 5% potassium hydroxide aqueous solution, 73 parts by weight of ion-exchanged water, and 250 parts by weight of 0.5 mm zirconia beads were placed in a 100 mL wide-mouth polyethylene bottle and dispersed for 2 hours using a paint shaker (manufactured by Toyo Seiki Co., Ltd.).

[0101] Next, the obtained pigment dispersion was centrifuged at 6000G for 30 minutes using a high-speed centrifuge, and the supernatant was collected to obtain an aqueous pigment dispersion with a pigment concentration of 9.5% by mass.

[0102] [IJ Ink Production]

[0103] 42 parts by mass of the above-obtained waterborne pigment dispersion, 1 part by mass of surfactant (Surfynol 465), 15 parts by mass of glycerin as a humectant, 10 parts by mass of propylene glycol, and ion-exchanged water are mixed in such a way that the total mass is 100 to obtain waterborne IJ ink with a pigment concentration of 4% by mass.

[0104] [Print Evaluation (Saturation Change Rate)]

[0105] The IJ ink obtained above was filled into an inkjet printer PX-105 (manufactured by Seiko Epson Corporation) for printing 100% duty patterns. Canon photo paper / gloss gold GL-101A450 (manufactured by Canon Corporation) was used as the recording medium for printing, and the resulting sample was left to stand in a normal environment for 12 hours. After standing, a* and b* were measured using X-rite exact (manufactured by X-rite Corporation) under light source D50 / 2 and filter M3 conditions, and the chroma (c*) was calculated.

[0106] Using the same pigment wet filter cake as the original sample without surface treatment as a benchmark, the chroma change rate (%) of the printed matter based on surface treatment is calculated according to the following formula, and evaluated based on the following criteria.

[0107] Chroma Change Rate of Printed Matter (%) = {[(Chroma of printed matter with surface-treated pigments) - (Chroma of printed matter with untreated pigments)] / (Chroma of printed matter with untreated pigments)} × 100

[0108] ◎: 0% to -3%

[0109] ○: Less than -3% to -5%

[0110] ×: Less than -5%

[0111] Evaluation of ink viscosity stability (viscosity change rate)

[0112] The viscosity (initial viscosity) of the IJ ink, freshly manufactured using the above method, was measured at 20°C using an E-type viscometer.

[0113] Next, 10 mL of water-based ink was sealed in a glass container and allowed to stand at 70°C for one week. The viscosity of the IJ ink after standing was measured using an E-type viscometer.

[0114] The viscosity change rate (%) is calculated based on the following formula compared to the viscosity of the water-based ink immediately after manufacturing (initial viscosity), and the evaluation is based on the following benchmarks.

[0115] Viscosity change rate (%) = {[(viscosity of the above-mentioned water-based ink after standing) - (viscosity of the above-mentioned water-based ink immediately after manufacturing)] / (viscosity of the above-mentioned water-based ink immediately after manufacturing)} × 100

[0116] ◎: Less than ±20%

[0117] ○: ±20% or more but less than ±50%

[0118] ×: ±50% or more

[0119] [Table 1]

[0120] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Pigment types R.122 V.19 R.122 / V.19 R.122 V.19 V.19 R.122 / V.19 R.122 Water vapor / nitrogen specific surface area ratio 0.290 0.362 0.301 0.229 0.266 0.430 0.210 0.252 Chroma change rate ◎ ◎ ◎ ◎ ◎ × ◎ ◎ viscosity change rate ◎ ◎ ○ × × ◎ × ×

[0121] As shown in Table 1 above, the quinacridone pigments of Examples 1-3, with a water vapor to nitrogen specific surface area ratio of 0.270 or higher and less than 0.430, regardless of pigment type, all exhibited good results in both chroma change rate and ink viscosity change rate. On the other hand, the quinacridone pigments of Comparative Examples 1-5, with a water vapor to nitrogen specific surface area ratio outside the above range, regardless of pigment type, all exhibited poor results in either chroma change rate or ink viscosity change rate.

Claims

1. A quinacridone pigment having a water vapor to nitrogen specific surface area ratio of 0.280 or more and 0.370 or less, wherein the quinacridone pigment is a solid solution of CI Pigment Violet 19 or CI Pigment Red 122 and CI Pigment Violet 19.

2. The quinacridone pigment according to claim 1, wherein its water vapor specific surface area is 5-50 m². 2 / g.

3. A pigment dispersion comprising the quinacridone pigment and solvent as described in claim 1 or 2.

4. An aqueous inkjet ink comprising the pigment dispersion of claim 3.