inkjet ink

Abstract

To improve the ejection stability of inkjet inks. [Solution] The inkjet ink contains a pigment, 3-methyl-1,5-pentanediol, triethylene glycol monobutyl ether, trimethylolpropane, a pigment dispersion resin, and water.

Description

Technical Field

[0005]

[0001] The present invention relates to ink for inkjet.

Background Art

[0002] In an inkjet recording apparatus, since the contact area between the ink and the atmosphere becomes large in the vicinity of the ejection surface where the nozzles in the recording head are arranged, the drying of the ink particularly tends to progress. Therefore, in the ink present in the vicinity of the ejection surface, the pigment tends to aggregate due to an increase in the concentration of the water-soluble organic solvent accompanying the progress of drying. As a result, nozzle clogging of the recording head tends to occur in the inkjet recording apparatus.

[0003] In Patent Documents 1 to 3, in order to suppress the progress of ink drying, techniques of blending a humectant into the ink are disclosed. Diethylene glycol-2-ethylhexyl ether or the like is blended as a humectant in the ink described in Patent Document 1. Glycerol or the like is blended as a humectant in the ink described in Patent Document 2. Glycerol and trimethylolpropane are blended as humectants in the ink described in Patent Document 3.

Prior Art Documents

Patent Documents

[0006] In view of the above circumstances, the object of the present invention is to improve the ejection stability of inkjet inks. [Means for solving the problem]

[0007] To achieve the above objective, an inkjet ink according to one embodiment of the present invention contains a pigment, 3-methyl-1,5-pentanediol, triethylene glycol monobutyl ether, trimethylolpropane, a pigment dispersion resin, and water.

[0008] Trimethylolpropane, a humectant incorporated into this inkjet ink, has a structure similar to glycerin, a well-known high-performance humectant, but with a longer carbon chain. Therefore, in this inkjet ink, trimethylolpropane exhibits humectant performance equivalent to glycerin while suppressing the evaporation of water-soluble organic solvents (3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether) through hydrophobic interactions with these solvents, which also have relatively long carbon chains. As a result, pigment aggregation is effectively suppressed even near the ejection surface, leading to excellent ejection stability.

[0009] In the above inkjet ink, it is preferable that the total content of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether is 20% by mass or more and 40% by mass or less.

[0010] The pigment dispersion resin may be crosslinked on the surface of the pigment. [Effects of the Invention]

[0011] As described above, the present invention can improve the ejection stability of inkjet inks. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will now be described. In this invention, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic," meaning that "(meth)acrylic" includes both "acrylic" and "methacrylic."

[0013] [Inkjet ink composition] (Schematic configuration) An inkjet ink according to one embodiment of the present invention (hereinafter also simply referred to as "ink") contains a pigment a, a water-soluble organic solvent b, a humectant c, a pigment dispersion resin d, and water. The ink according to this embodiment is a water-based ink that is ejected from the recording head of an inkjet recording device onto a recording medium to form an image on the recording medium.

[0014] The recording medium on which an image is formed using the ink according to this embodiment may be, for example, a permeable recording medium or a non-permeable recording medium. The ink according to this embodiment is suitable for forming images on a permeable recording medium which has excellent ink permeability. Examples of permeable recording media include printing paper and media made from fibers (for example, fabric). Examples of printing paper include plain paper, copy paper, recycled paper, thin paper, thick paper, and glossy paper.

[0015] In the ink according to this embodiment, 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether are used as the water-soluble organic solvent b, and trimethylolpropane is used as the humectant c. In the ink according to this embodiment, high discharge stability is obtained through the action of the water-soluble organic solvent b, humectant c, and pigment dispersion resin d. The details of each component of the ink according to this embodiment will be described below.

[0016] (Pigment a) In the ink according to this embodiment, pigment a can be, for example, a yellow pigment, an orange pigment, a red pigment, a blue pigment, a purple pigment, or a black pigment. Examples of yellow pigments include CI Pigment Yellow 74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, or 193. Examples of orange pigments include CI Pigment Orange 34, 36, 43, 61, 63, or 71. Examples of red pigments include CI Pigment Red 122 or 202. Examples of blue pigments include CI Pigment Blue 15 or 15:3. Examples of purple pigments include CI Pigment Violet 19, 23, or 33. Examples of black pigments include CI Pigment Black 7. In the ink according to this embodiment, the content of pigment a is preferably 3.0% by mass or more and 20.0% by mass or less, and more preferably 7.0% by mass or more and 12.0% by mass or less.

[0017] (Water-soluble organic solvent b) The ink according to this embodiment contains 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether as the water-soluble organic solvent b. In the ink according to this embodiment, from the viewpoint of obtaining high image density in the image formed on the recording medium, the total content of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether is 20% by mass or more. Furthermore, in the ink according to this embodiment, from the viewpoint of suppressing pigment aggregation, the total content of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether is 40% by mass or less.

[0018] (Moisturizer c) The ink according to this embodiment contains trimethylolpropane as a humectant c. Trimethylolpropane has a structure similar to glycerin, which is well known as a high-performance humectant, and can therefore exhibit the same level of moisturizing performance as glycerin. Furthermore, because trimethylolpropane has a longer carbon chain than glycerin, hydrophobic interactions occur between it and 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether, which constitute the water-soluble organic solvent b, which also have relatively long carbon chains. Therefore, in the ink according to this embodiment, the evaporation of the water-soluble organic solvent b is suppressed not only by its high water retention performance but also by the hydrophobic interactions it has with the water-soluble organic solvent b. As a result, in the inkjet ink according to this embodiment, aggregation of pigment a is effectively suppressed even when it is present near the ejection surface of the recording head, thus providing good ejection stability.

[0019] In the ink according to this embodiment, it is preferable that the trimethylolpropane content be 1.0% by mass or more in order to fully obtain the above-mentioned effects of trimethylolpropane. Furthermore, in the ink according to this embodiment, it is preferable that the trimethylolpropane content be 10% by mass or less in order to prevent the viscosity from deviating from the appropriate range.

[0020] (Pigment-dispersed resin d) The ink according to this embodiment is blended with a pigment dispersion resin d for enhancing the dispersibility of pigment a in a solvent. The pigment dispersion resin d is fine particles of a resin, and suppresses the aggregation of pigment a by adsorbing to the surface of pigment a. In the ink blended with the pigment dispersion resin d, pigment a and the pigment dispersion resin d together constitute a pigment dispersion. The pigment dispersion is composed of, for example, a core containing pigment a and a pigment dispersion resin d coating the core. In the pigment dispersion resin d, a part thereof may be dispersed in the solvent without adsorbing to the surface of pigment a.

[0021] The pigment dispersion resin d can be appropriately selected and used from known pigment dispersion resins. The pigment dispersion resin d may be a random polymer or a block polymer. Specific examples of the pigment dispersion resin d include polyester, polyurethane, (meth)acrylic acid, styrene-(meth)acrylic acid copolymer, styrene-maleic acid copolymer, styrene-maleic acid half ester copolymer, vinyl naphthalene-(meth)acrylic acid copolymer, or vinyl naphthalene-maleic acid copolymer. The styrene-(meth)acrylic acid copolymer is a resin containing units derived from styrene and units derived from (meth)acrylic acid, (meth)acrylic acid ester, or methacrylic acid ester. Examples of the styrene-(meth)acrylic acid copolymer include styrene-(meth)acrylic acid-(meth)acrylic acid alkyl ester copolymer, styrene-methacrylic acid-methacrylic acid alkyl ester-(meth)acrylic acid alkyl ester copolymer, styrene-(meth)acrylic acid copolymer, styrene-maleic acid-(meth)acrylic acid alkyl ester copolymer, styrene-methacrylic acid copolymer, or styrene-methacrylic acid alkyl ester copolymer. Among these pigment dispersion resins, a styrene-(meth)acrylic acid copolymer is preferable because it is easy to prepare and has an excellent dispersion effect on pigment a, a styrene-methacrylic acid-methacrylic acid alkyl ester-(meth)acrylic acid alkyl ester copolymer is more preferable, and a methacrylic acid-methyl methacrylate-(meth)acrylic acid butyl-styrene copolymer is even more preferable.

[0022] The neutralization rate of the pigment dispersion resin d is preferably 20% or more and 100% or less, more preferably 30% or more and 60% or less. The neutralization rate of the pigment dispersion resin d is defined as the percentage (100×(MB / MA)) of the actual amount of the basic compound (MB) used in the dispersion treatment step with respect to the theoretical amount (MA) of the basic compound required to completely neutralize the pigment dispersion resin d.

[0023] The acid value of the pigment dispersion resin d is preferably 50 mgKOH / g or more and 300 mgKOH / g or less, more preferably 75 mgKOH / g or more and 275 mgKOH / g or less. By setting the acid value of the pigment dispersion resin d to 50 mgKOH / g or more, crosslinking with the crosslinking agent described later can be easily performed. By setting the acid value of the pigment dispersion resin d to 300 mgKOH / g or less, high storage stability in the ink according to the present embodiment can be obtained. The acid value of the pigment dispersion resin d is obtained by measurement in accordance with JIS K 0070:1992 (Test methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products).

[0024] The mass average molecular weight (Mw) of the pigment dispersion resin d is preferably 3000 or more and 180000 or less, more preferably 5000 or more and 15000 or less. By setting the mass average molecular weight of the pigment dispersion resin d within this range, the efficiency of coating the surface of the pigment a is improved, and the pigment dispersion liquid and ink are easily maintained at an appropriate viscosity. The mass average molecular weight of the pigment dispersion resin d is obtained by measurement using gel permeation chromatography ("HLC-8020GPC" manufactured by Tosoh Corporation) under the following conditions. Column: "TSKgel SuperMultiporeHZ-H" (semi-micro column of 4.6 mm I.D.×15 cm) manufactured by Tosoh Corporation Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35 mL / min Sample injection volume: 10 μL Measurement temperature: 40°C Detector: IR detector The calibration curve is created by selecting seven types of TSKgel standard polystyrene manufactured by Tosoh Corporation—F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000—along with n-propylbenzene.

[0025] Furthermore, in the ink according to this embodiment, the pigment dispersion resin d may constitute a specific resin having a crosslinked structure formed by a crosslinking agent on the surface of the pigment a. In other words, in the ink according to this embodiment, the pigment dispersion may consist of pigment a and a specific resin. In the ink according to this embodiment, since the specific resin coating the pigment a has a crosslinked structure, the specific resin is constrained to the surface of the pigment a, thus preventing the specific resin from detaching from the surface of the pigment a. In the ink according to this embodiment, while maintaining the effect of the pigment dispersion resin d in suppressing the aggregation of pigment a, a strong restraining force on the surface of the pigment a by the specific resin is obtained, making it easier to maintain the dispersion state of the pigment dispersion.

[0026] In the ink according to this embodiment, the crosslinking rate of the specific resin is preferably 30% to 90%, and more preferably 40% to 70%. In the ink according to this embodiment, by keeping the crosslinking rate of the specific resin at 90% or less, the dispersibility of the pigment dispersion is well maintained, and the occurrence of nozzle clogging of the recording head can be suppressed. Furthermore, in the ink according to this embodiment, by setting the crosslinking rate of the specific resin to 30% or more, a good effect of suppressing the separation of pigment a from the surface can be obtained.

[0027] The crosslinking ratio is defined as the percentage of moles of repeating units having a crosslinked structure in a specific resin, where the total number of moles of repeating units derived from monomers having functional groups that react with the crosslinking agent is set to 100%. In this embodiment, the crosslinking ratio of the specific resin is the apparent crosslinking ratio calculated from the molar equivalents of carboxyl groups in the specific resin and the molar equivalents of crosslinking functional groups in the crosslinking agent, and is calculated using the following formula. Crosslinking rate (%) = 100 × (Molar equivalents of crosslinking functional groups of the crosslinking agent / Molar equivalents of carboxyl groups of the specific resin)

[0028] In this embodiment, the pigment-dispersing resin d constituting the specific resin is preferably a styrene-(meth)acrylic acid copolymer. The styrene-(meth)acrylic acid copolymer is a resin having styrene units, α-methylstyrene units, (meth)acrylic acid units, and nonionic units. In the styrene-(meth)acrylic acid copolymer, the content of styrene units relative to the total repeating units is preferably 1.0% by mass or more and 50.0% by mass or less, and more preferably 1.0% by mass or more and 30.0% by mass or less. In the styrene-(meth)acrylic acid copolymer, the content of α-styrene units relative to the total repeating units is preferably 1.0% by mass or more and 65.0% by mass or less, and more preferably 20.0% by mass or more and 65.0% by mass or less. In the styrene-(meth)acrylic acid copolymer, the content of repeating units derived from (meth)acrylic acid relative to the total repeating units is preferably 10.0% by mass or more and 40.0% by mass or less. In the styrene-(meth)acrylic acid copolymer, it is preferable that the content of repeating units derived from the nonionic monomer relative to the total repeating units is 1.0% by mass or more and 10.0% by mass or less. The nonionic monomer is preferably at least one selected from polyethylene glycol (meth)acrylate (n=1 to 9) and polypropylene glycol (meth)acrylate (n=1 to 9).

[0029] In this embodiment, the crosslinking agent used to crosslink the pigment dispersion resin d is preferably a crosslinking agent containing epoxy groups that react with the carboxyl groups of the pigment dispersion resin d. The epoxy group-containing crosslinking agent preferably has two or more epoxy groups in its molecule, and preferably three or more epoxy groups. Furthermore, the epoxy group-containing crosslinking agent preferably has a hydroxyl group. The epoxy group-containing crosslinking agent is preferably water-soluble, and its water solubility is preferably 80% or more (calculated as the mass of dissolved epoxy compound when 10 g of epoxy compound is mixed with 90 g of water), and more preferably 100%.

[0030] Examples of crosslinking agents that can be used to crosslink the pigment dispersion resin d in the ink according to this embodiment include sorbitol polyglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, and polyglycerol polyglycidyl ether. Examples of crosslinking agent products that can be used to crosslink the pigment dispersion resin d in the ink according to this embodiment include Denacol® EX-614B, Denacol® EX-313, Denacol® EX-421, Denacol® EX-512, and Denacol® EX-521, all of which are crosslinking agents manufactured by Nagase ChemteX Corporation.

[0031] In the ink according to this embodiment, the epoxy equivalent (g / eq.) of the crosslinking agent used to crosslink the pigment dispersion resin d is preferably 100 to 10000, and more preferably 120 to 250, from the viewpoint of efficiently reacting with the carboxyl groups of the pigment dispersion resin d and improving the dispersion stability of the pigment dispersion. If the amount of epoxy groups in the crosslinking agent is too high, crosslinking will not proceed efficiently and may instead act as water-soluble groups. Also, if the amount of epoxy groups in the crosslinking agent is too low, there will be insufficient crosslinking sites, which tends to result in insufficient suppression of the detachment of specific resins from the surface of pigment a. The mass-average molecular weight of the epoxy group-containing crosslinking agent is preferably 100 to 1500, from the viewpoint of ease of reaction and storage stability of the aqueous pigment dispersion.

[0032] In the ink according to this embodiment, the pigment dispersion content is preferably 5.0% by mass or more and 30.0% by mass or less, and more preferably 8.0% by mass or more and 15.0% by mass or less. In the ink according to this embodiment, by setting the pigment dispersion content to 5.0% by mass or more, it is easier to form an image with good image density on the recording medium. Furthermore, in the ink according to this embodiment, by keeping the pigment dispersion content at 30.0% by mass or less, it is easier to ensure sufficient fluidity as an ink.

[0033] In the ink according to this embodiment, the content of the specific resin is preferably 0.5% by mass or more and 10.0% by mass or less, and more preferably 2.0% by mass or more and 4.0% by mass or less. In the pigment dispersion, the content of the specific resin is preferably 10% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 30% by mass or less. Furthermore, in the pigment dispersion, the total content of pigment a and the specific resin is preferably 90% by mass or more, and more preferably 100% by mass. In the ink according to this embodiment, from the viewpoint of optimizing color density, hue, or stability, the median diameter by volume of the pigment dispersion containing the specific resin is preferably 30 nm or more and 200 nm or less, and more preferably 80 nm or more and 140 nm or less.

[0034] (water) In the ink according to this embodiment, for example, ion-exchanged water, purified water, or distilled water can be used as water. In the ink according to this embodiment, from the viewpoint of drying properties and ejection reliability, the water content is preferably 25.0% by mass or more and 80.0% by mass or less, and more preferably 35.0% by mass or more and 60.0% by mass or less.

[0035] (Other ingredients) The ink according to this embodiment may contain other components as needed. Surfactants can optimize the penetration (wetting) of the ink according to the present invention into the recording medium. Examples of surfactants that can be used in the ink according to this embodiment include anionic surfactants, cationic surfactants, and nonionic surfactants. Nonionic surfactants are preferred as surfactants. Examples of nonionic surfactants include polyoxyethylene dodecyl ether, polyoxyethylene hexadecyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and ethylene oxide adducts of acetylene glycol. Ethylene oxide adducts of acetylene glycol are preferred as nonionic surfactants.

[0036] Furthermore, in addition to surfactants, the ink according to this embodiment may contain various additives such as dissolving stabilizers, antioxidants, viscosity modifiers, pH adjusters, and antifungal agents, as needed.

[0037] [Method of manufacturing inkjet ink] (Introduction) An example of an ink manufacturing method according to this embodiment will be described below. In the following, an example in which the pigment dispersion resin d constitutes a specific resin having a crosslinked structure will be described. The ink manufacturing method according to this embodiment includes a dispersion process, a crosslinking process, a centrifugal process, and an ink preparation process. Note that the centrifugal process may be omitted in the ink manufacturing method according to this embodiment. The following describes each step of the ink manufacturing method according to this embodiment.

[0038] (Distributed processing process) In the dispersion process, a pigment dispersion (referred to as an "intermediate pigment dispersion" in this example to distinguish it from the pigment dispersion after crosslinking) is prepared by dispersing pigment a and pigment dispersion resin d in water. In the intermediate pigment dispersion, a pigment dispersion composed of pigment a and pigment dispersion resin d adsorbed on the surface of pigment a is dispersed in water. Examples of dispersion equipment used in the dispersion process include wet dispersion equipment such as a media-type disperser. It is preferable that the pigment dispersion resin d used in the dispersion process be neutralized in advance with a basic compound (e.g., KOH or NaON) to achieve a predetermined neutralization rate.

[0039] In the intermediate pigment dispersion, the content of pigment dispersion resin d is preferably 4.0% by mass or more and 25.0% by mass or less. In addition, the content of pigment a in the intermediate pigment dispersion is preferably 1.0% by mass or more and 30.0% by mass or less. In the dispersion process, an antifoaming agent may be added to the intermediate pigment dispersion. In the intermediate pigment dispersion, the content of the antifoaming agent is preferably 0.01% by mass or more and 0.1% by mass or less.

[0040] In the dispersion process, it is preferable to remove coarse particles from the intermediate pigment dispersion by filtration using a filter (for example, with a pore size of 5 μm).

[0041] (Crosslinking process) In the crosslinking process, an epoxy group-containing crosslinking agent is added to the intermediate pigment dispersion obtained in the dispersion process. This causes the carboxyl groups of the pigment dispersion resin d adsorbed on the surface of pigment a to react with the epoxy groups of the crosslinking agent, resulting in the crosslinking of the pigment dispersion resin d on the surface of pigment a, and obtaining the pigment dispersion according to this embodiment. In the crosslinking process, it is preferable to add the crosslinking agent to the intermediate pigment dispersion and then heat and stir the intermediate pigment dispersion. The heating temperature can be, for example, 50°C to 95°C. The heating time can be, for example, 30 minutes to 8 hours.

[0042] (Centrifugation process) In the centrifugal treatment process, the pigment dispersion obtained in the crosslinking treatment process is subjected to centrifugal treatment, and the supernatant liquid of the pigment dispersion after centrifugal treatment is replaced with water. This allows the components that were free in the water in the pigment dispersion to be removed in the centrifugal treatment process. For example, the centrifugal treatment conditions in the centrifugal treatment process can be a rotation speed of 10,000 rpm or more and 100,000 rpm or less, and a centrifugal treatment time of 12 hours or more and 48 hours or less.

[0043] (Ink preparation process) In the ink preparation step, a water-soluble organic solvent b, a humectant c, and water are added to the pigment dispersion obtained in the centrifugal treatment step. This yields the ink according to this embodiment. In the ink preparation step, other components besides the water-soluble organic solvent b, humectant c, and water (more specifically, at least one of surfactants, dissolution stabilizers, penetrating agents, viscosity modifiers, etc.) may be added as needed. In the ink preparation step, it is preferable to stir the obtained ink with a stirrer. In addition, in the ink preparation step, foreign matter and coarse particles may be removed from the obtained ink using a filter (for example, a filter with a pore size of 5 μm or less).

[0044] [Examples and Comparative Examples] Ink preparation and evaluation were carried out as examples and comparative examples of the present invention. Note that the following examples are merely examples of the present invention, and the configuration of the present invention is not limited to the configurations of these examples.

[0045] (Preparation of ink according to Example 1) • Synthesis of pigment-dispersed resin d First, a styrene-acrylic acid copolymer was synthesized as the pigment dispersion resin d. 100.0 g of isopropyl alcohol and 250.0 g of methyl ethyl ketone were placed in a four-necked flask (capacity: 1000 mL) equipped with a stirrer, nitrogen inlet tube, condenser, and dropping funnel. The mixed solution was supplied to the dropping funnel. The mixed solution contained 20.0 g of styrene, 40.0 g of α-methylstyrene, 5.0 g of ethylene glycol acrylate, 25.0 g of methacrylic acid, and 0.3 g of azobisisobutyronitrile (AIBN, polymerization initiator).

[0046] Nitrogen was introduced into the flask, and the mixture was heated under reflux at 70°C. While heated under reflux at 70°C, the aforementioned mixed solution was supplied into the flask from a dropping funnel over approximately 2 hours. After supplying the mixed solution, heating under reflux at 70°C was continued for another 6 hours. Then, the methyl ethyl ketone solution was supplied into the flask from a dropping funnel over 15 minutes. The methyl ethyl ketone solution contained 150.0 g of methyl ethyl ketone and 0.1 g of AIBN. After supplying the methyl ethyl ketone solution, heating under reflux at 70°C was continued for another 5 hours. Next, the contents of the reaction vessel were subjected to reduced pressure to remove the solvent. This yielded a styrene-acrylic acid copolymer.

[0047] • Distributed processing Next, an intermediate pigment dispersion was prepared as a preliminary step before crosslinking the styrene-acrylic acid dispersion resin. The styrene-acrylic acid dispersion resin was neutralized beforehand with potassium hydroxide and used as an aqueous solution with a neutralization rate of 60% and a solid content of 30%. 18.0 parts by mass of carbon black (Printex® 80, manufactured by Orion Engineered Carbons Co., Ltd.) as pigment a, 7.4 parts by mass (based on solid content) of the styrene-acrylic acid dispersion resin, 0.1 parts by mass of an antifoaming agent (SN Deformer 1340, manufactured by Sunopco Co., Ltd.), and ion-exchanged water were mixed to obtain a mixture of 100 parts by mass.

[0048] An intermediate pigment dispersion was obtained by dispersing the resulting mixture for 4 hours using a bead mill ("Dinomill" manufactured by Willy e Bakkofen). Zirconia beads (0.5 mm in diameter) were used as the media during the dispersion process. The media packing ratio in the bead mill vessel was set to 60% by volume. Furthermore, the processing temperature (chiller temperature) during the dispersion process was set to 10°C. After the dispersion process, the media was removed, and the obtained intermediate pigment dispersion was filtered through a 5 μm pore size filter to remove foreign matter and coarse particles.

[0049] • Cross-linking treatment A 1L three-necked flask equipped with a thermometer and stirring blades was used as the reaction vessel, and 100g of pigment dispersion was added to the reaction vessel. Next, while maintaining the internal temperature of the reaction vessel at 30°C using a water bath, the crosslinking agent was added to the reaction vessel and thoroughly stirred. Then, the contents of the reaction vessel were stirred at a speed of 150 rpm for 1 hour. Nagase ChemteX Corporation's "Denacol (registered trademark) EX-313" was used as the crosslinking agent, and the amount of crosslinking agent added was 0.98g. Next, while stirring at 250 rpm, the internal temperature of the reaction vessel was raised to 80°C at a heating rate of 0.5°C / min. While maintaining the internal temperature of the reaction vessel at 80°C, the contents of the reaction vessel were stirred at 250 rpm for 4 hours. Finally, the pigment dispersion was obtained by cooling the internal temperature of the reaction vessel to room temperature.

[0050] • Centrifugal processing The obtained pigment dispersion was transferred to a designated container, and the container was placed in a centrifugal adhesion measuring device (NS-C100, manufactured by NanoSeeds Co., Ltd.). The pigment dispersion in the container was subjected to centrifugation at a rotation speed of 50,000 rpm for 24 hours. After removing the supernatant from the container, an equal volume of ion-exchanged water was added to the container (displacement treatment). This removed components that had been released into the water from the pigment dispersion.

[0051] • Ink preparation Using a stirrer ("Three One Motor BL-600" manufactured by Shinto Kagaku Co., Ltd.) at a rotation speed of 400 rpm, a 50.0% by mass pigment dispersion, 5.0% by mass trimethylolpropane, 15.0% by mass 3-methyl-1,5-pentanediol, 15.0% by mass triethylene glycol monobutyl ether, 0.3% by mass nonionic surfactant ("Orphine E1004" manufactured by Nisshin Chemical Industry Co., Ltd.), and deionized water (remaining amount) were stirred to obtain ink. The obtained ink was filtered using a filter (pore size: 5 μm).

[0052] (Ink evaluation) The nozzle clogging of the inks used in the examples and comparative examples was evaluated. For the nozzle clogging evaluation, A4-sized inkjet matte paper (Seiko Epson Corporation's "Super Fine Paper") was used as the evaluation paper. An image forming apparatus (line head-equipped inkjet recording device, Kyocera Document Solutions test machine) was used as the evaluation machine. A 150mm x 200mm solid image was continuously printed on 100 sheets of evaluation paper using the evaluation machine.

[0053] Next, after purging the ink from the recording head of the evaluation machine, the recording head was cleaned by wiping the ejection surface. Then, a nozzle check pattern image was formed on evaluation paper using the evaluation machine to confirm that ink was being ejected from all nozzles. Finally, the recording head was cleaned again, and the evaluation machine was left undisturbed for 7 days with the recording head uncapped.

[0054] Next, after performing the recording head cleaning process again, a nozzle check pattern image was formed on the evaluation sheet using the evaluation machine. This nozzle check pattern image was examined, and the ratio of non-discharging nozzles to the total number of nozzles (7968) was calculated. The calculated ratio of non-discharging nozzles was used as the nozzle clogging evaluation value.

[0055] The evaluation was conducted according to the following A and B criteria for nozzle clogging. Ink with an A rating is considered acceptable, while ink with a B rating is considered unacceptable. A: Less than 10% B: 10% or more

[0056] (Examples 1-8) Table 1 shows the compositions of pigment a, water-soluble organic solvent b, and humectant c in the inks of Examples 1 to 8. Note that all values ​​in Table 1 are in "mass%". In Examples 2 to 8, the inks were prepared in the same manner as the ink preparation method for Example 1, so that the compositions of pigment a, water-soluble organic solvent b, and humectant c were as shown in Table 1.

[0057] [Table 1]

[0058] In Example 2, phthalocyanine blue (ECB-301, manufactured by Dainichi Seika Kogyo Co., Ltd.) was used as pigment a. In Example 3, quinacridone magenta (FASTROGEN® Super Magenta RGT, manufactured by DIC Corporation) was used as pigment a. In Example 4, monoazo yellow (Hansa® Brilliant Yellow 5GX01, manufactured by Clariant Co., Ltd.) was used as pigment a. In Examples 5-8, carbon black (Printex® 80, manufactured by Orion Engineered Carbons Co., Ltd.) was used as pigment a.

[0059] In all of Examples 1 to 8, 3-methyl-1,5-pentanediol (MPD) and triethylene glycol monobutyl ether (BTG) were used as the water-soluble organic solvent b, and trimethylolpropane (TMP) was used as the humectant c.

[0060] The inks from Examples 1 to 8 were evaluated for nozzle clogging. Table 2 shows the results of the nozzle clogging evaluation for the inks from Examples 1 to 8. All of the inks from Examples 1 to 8 passed the nozzle clogging evaluation.

[0061] [Table 2]

[0062] (Comparative Examples 1-3) Table 3 shows the compositions of pigment a, water-soluble organic solvent b, and humectant c of the inks related to Comparative Examples 1 to 3. Note that all values ​​in Table 3 are in "mass%". In Comparative Examples 1 to 3, the inks were prepared in the same manner as the ink preparation method for Example 1, so that the compositions of pigment a, water-soluble organic solvent b, and humectant c were as shown in Table 3.

[0063] [Table 3]

[0064] In Comparative Examples 1 to 3, carbon black (Printex® 80, manufactured by Orion Engineered Carbons Co., Ltd.) was used as pigment a. The ink in Comparative Example 1 differs from the ink in the above example in that glycerin (Gly) was used as humectant c instead of trimethylolpropane (TMP). The ink in Comparative Example 2 differs from the ink in the above example in that propylene glycol (PG) was used as water-soluble organic solvent b instead of 3-methyl-1,5-pentanediol (MPD). The ink in Comparative Example 3 differs from the ink in the above example in that diethylene glycol monoethyl ether (EG) was used as water-soluble organic solvent b instead of triethylene glycol monobutyl ether (BTG).

[0065] The inks related to Comparative Examples 1 to 3 were evaluated for nozzle clogging. Table 4 shows the results of the nozzle clogging evaluation for the inks related to Comparative Examples 1 to 3. All of the inks related to Comparative Examples 1 to 3 failed the nozzle clogging evaluation.

[0066] [Table 4]

Claims

1. Pigments and 3-methyl-1,5-pentanediol and Triethylene glycol monobutyl ether and Trimethylolpropane and Pigment dispersion resin and Water and, Inkjet ink containing [this ingredient].

2. An inkjet ink according to claim 1, The total content of 3-methyl-1,5-pentanediol and triethylene glycol monobutyl ether is 20% by mass or more and 40% by mass or less. Inkjet ink.

3. An inkjet ink according to claim 1 or 2, The pigment dispersion resin is crosslinked on the surface of the pigment. Inkjet ink.

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