Aqueous ink, ink cartridge and inkjet recording method

Abstract

To provide an aqueous ink for inkjet capable of recording an image high in an optical density, and unlikely to cause a discharge failure of an ink even when images with a larger ink consumption are consecutively recorded.SOLUTION: An aqueous ink for inkjet includes a first pigment, a second pigment and a surfactant. The first pigment and the second pigment are each self-dispersible carbon black with an organic group including an anionic group bonded onto a particle surface of carbon black. A cumulative 50% particle size (nm) in a volume basis of the first pigment is 1.3 times or more in a proportion to a cumulative 50% particle size (nm) in a volume basis of the second pigment. The cumulative 50% particle size in a volume basis of the second pigment is 60 nm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based ink, an ink cartridge, and an inkjet recording method. [Background technology]

[0002] In recent years, inkjet recording methods have been adopted not only for recording documents for home and office use, but also for recording commercial and industrial images, etc. In particular, there has been an increasing need for applications in which highly colored images are continuously recorded on recording media such as plain paper and coated paper using inkjet recording devices capable of recording on large-sized recording media such as A3, and various inks suitable for such applications have been developed.

[0003] Inks used to record images on recording media such as plain paper and coated paper are required to have high cohesiveness, to be resistant to penetration in the thickness direction of the recording medium, and to be capable of recording high-quality images with high optical density. To meet these requirements, aqueous inks containing, as a colorant, self-dispersible carbon black, in which anionic functional groups are directly bonded to the particle surface of carbon black, have been widely used. For example, inks using two types of self-dispersible carbon black with different dibutyl phthalate (DBP) oil absorption have been proposed to record images with high optical density (Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-021099 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-003767 Summary of the Invention [Problem to be solved by the invention]

[0005] The present inventors investigated the inks proposed in Patent Documents 1 and 2 in order to continuously print images with high optical density on recording media such as plain paper and coated paper. As a result, they found that when these inks are used to continuously print images that consume a lot of ink, such as solid images, ejection failure occurs, making the image more likely to have uneven density or blurred images. When they investigated the cause of this ejection failure, they found that air bubbles that had become trapped in the ink flow path of the print head made the ink more likely to fail to eject.

[0006] Therefore, an object of the present invention is to provide a water-based inkjet ink that is less likely to fail to eject ink even when continuously recording images that consume a large amount of ink, and that is capable of recording images with high optical density. Another object of the present invention is to provide an ink cartridge and an inkjet recording method that use this water-based ink. [Means for solving the problem]

[0007] That is, according to the present invention, there is provided an aqueous inkjet ink containing a first pigment, a second pigment, and a surfactant, wherein the first pigment and the second pigment are both self-dispersing carbon black in which organic groups containing anionic groups are bonded to the particle surfaces of the carbon black, and the volume-based cumulative 50% particle diameter (nm) of the first pigment is 1.3 or more times the volume-based cumulative 50% particle diameter (nm) of the second pigment, and the volume-based cumulative 50% particle diameter of the second pigment is 60 nm or less. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a water-based inkjet ink that is less likely to fail to eject ink even when continuously recording images that consume a large amount of ink, and that is capable of recording images with high optical density. Furthermore, according to the present invention, it is possible to provide an ink cartridge and an inkjet recording method that use this water-based ink. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view schematically illustrating an embodiment of an ink cartridge of the present invention. [Figure 2] 1A and 1B are diagrams schematically illustrating an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, in which FIG. 1A is a perspective view of the main part of the inkjet recording apparatus, and FIG. 1B is a perspective view of a head cartridge. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in further detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is present in the ink as dissociated ions, but for convenience, it will be expressed as "containing a salt." Furthermore, aqueous inkjet inks may be simply referred to as "ink." Physical property values ​​are values ​​at room temperature (25°C) unless otherwise specified.

[0011] The inventors speculated as follows about the reason why air bubbles mixed into the ink flow path of a print head are likely to cause ink non-ejection when continuously printing images that consume a large amount of ink, such as solid images. When continuously printing images that consume a large amount of ink, the ink flow rate in the ink flow path of the print head increases, and multiple air bubbles tend to accumulate in areas of the ink flow path where air bubbles are likely to accumulate. Furthermore, they found that air bubbles tend to exist stably in ink containing compounds such as surfactants, whose molecules contain both hydrophobic and hydrophilic moieties. This is because the hydrophobic moiety of the surfactant is oriented toward the air inside the highly hydrophobic bubbles, while the hydrophilic moiety of the surfactant is oriented toward the ink, thereby increasing the stability of the film at the bubble interface. It is believed that non-ejection occurs when the air bubbles reach the flow path that connects to the individual ejection ports of the print head.

[0012] Next, the inventors investigated inks that did not contain surfactants. However, they found that the ink had insufficient wettability to the print head, which resulted in deviations in the ink ejection direction and reduced ejection accuracy. This suggests that it is also necessary to reduce the bubble stability of inks that contain surfactants.

[0013] The present inventors further investigated the particle size of carbon black in order to reduce the stability of bubbles in surfactant-containing inks and improve the ejection stability of the ink. As a result, they discovered that by satisfying the following requirements (i) to (iv), the stability of bubbles is reduced, ejection accuracy is less likely to decrease, and ejection stability is improved, even when images that consume a large amount of ink are continuously recorded, and thus the present invention was achieved. (i) A water-based inkjet ink containing a first pigment, a second pigment, and a surfactant. (ii) Both the first pigment and the second pigment are self-dispersing carbon blacks in which organic groups containing anionic groups are bonded to the particle surfaces of the carbon black. (iii) The volume-based cumulative 50% particle size (nm) of the first pigment is 1.3 times or more the volume-based cumulative 50% particle size (nm) of the second pigment. (iv) The volume-based cumulative 50% particle size of the second pigment is 60 nm or less.

[0014] The inventors speculate that the mechanism by which the above-described effects are achieved is as follows. When multiple bubbles approach each other in the ink, they push against each other, forming a film that traps the ink between the bubbles. Over time, ink is expelled from the film, gradually thinning it. Eventually, the film becomes unsustainable and breaks due to the impact of the film breaking. However, as mentioned above, the presence of a surfactant increases the stability of the film, making it more difficult to break. If highly hydrophobic carbon black is present in the film formed by the bubbles, the surfactant adsorbs to the hydrophobic portions of the carbon black particle surface, reducing the film's stability and making it more susceptible to bubble breakage. While carbon black particles with a relatively large particle size exhibit bubble-breaking properties when the film is thick, they are unable to penetrate into the film as it thins over time. Therefore, by further incorporating carbon black with a relatively small particle size (average particle size of 60 nm or less), these carbon black particles can penetrate into the thinned film, improving bubble-breaking efficiency. As a result, even when images that consume a large amount of ink, such as solid images, are continuously recorded, it is believed that the ejection accuracy is less likely to decrease and ejection stability is improved.

[0015] <Water-based ink> The ink of the present invention is an aqueous inkjet ink containing a first pigment, a second pigment, and a surfactant. Both the first pigment and the second pigment are self-dispersing carbon black particles in which an organic group containing an anionic group is bonded to the surface of the carbon black particles. The volume-based cumulative 50% particle diameter (nm) of the first pigment is 1.3 times or more the volume-based cumulative 50% particle diameter (nm) of the second pigment, and the volume-based cumulative 50% particle diameter of the second pigment is 60 nm or less. The components constituting the ink and the physical properties of the ink are described in detail below.

[0016] (Self-dispersing carbon black) The ink contains a first pigment and a second pigment. Both the first pigment and the second pigment are self-dispersing carbon black particles having an organic group containing an anionic group bonded to the surface of the carbon black particles. Hereinafter, when there is no need to distinguish between the pigments, they may be described without distinction. When there is a condition that does not require distinction, it is preferable that both pigments satisfy that condition.

[0017] [Organic group containing an anionic group] The organic group bonded to the carbon black particle surface is an anionic group or a group in which an anionic group is bonded to another atomic group (hereinafter, also referred to simply as a "functional group"). Examples of the anionic group include a carboxylic acid group, a sulfonic acid group, a phosphate group, and a phosphonic acid group. Among these, the anionic group is preferably a carboxylic acid group. Furthermore, it is preferable that both the first pigment and the second pigment are self-dispersible carbon blacks in which an organic group containing a carboxylic acid group is bonded to the carbon black particle surface. If the types of anionic groups in the first pigment and the second pigment are different, the difference in the aggregability between the first pigment and the second pigment may become large. This may result in pigment distribution within the image, and the effect of improving the optical density may be slightly reduced. Furthermore, if the anionic group is a group other than a carboxylic acid group, the aggregability of the first pigment and the second pigment may be reduced, and the effect of improving the optical density of the image may be slightly reduced.

[0018] When the anionic group forms a salt, at least one proton of the anionic group is substituted with a cation (counter ion). Examples of counter ions include alkali metal ions, ammonium ions, and organic ammonium ions. Examples of alkali metal ions include lithium, sodium, and potassium ions. Examples of organic ammonium ions include cations of aliphatic amines such as mono- to tri-alkylamines; and cations of aliphatic alcohol amines such as mono- to trialkanolamines. Among these, the counter ion is preferably a potassium ion or an ammonium ion. It is also preferable that the counter ions of the first and second pigments are both potassium ions or ammonium ions, and that the counter ions of the first and second pigments are the same. Using different types of counter ions can significantly increase the difference in the coagulation properties of the first pigment and the second pigment. This can result in pigment distribution within the image, slightly reducing the optical density improvement effect. Furthermore, if the counter ion is an ion other than potassium ions or ammonium ions, the storage stability of the ink may be slightly reduced.

[0019] Examples of other atomic groups include alkylene groups such as methylene, ethylene, and propylene; arylene groups such as phenyl, naphthyl, anthracenyl, phenanthrenyl, and biphenyl; heteroarylene groups such as pyridylene, imidazolylene, pyrazolylene, pyridinylene, thienylene, and thiazolylene; carbonyl groups; ester groups such as carboxylic acid ester groups, sulfonic acid ester groups, phosphate ester groups, and phosphonic acid ester groups; imino groups; amide groups; sulfonyl groups; ether groups; and combinations of these groups are also possible.

[0020] It is preferable to use a self-dispersing pigment in which anionic groups are bonded to the pigment particle surface via other atomic groups, rather than a self-dispersing pigment in which anionic groups are bonded directly to the pigment (carbon black) particle surface. Because the particle surface of carbon black is in various states, the anionic groups bonded directly to the particle surface also have diverse properties, which can result in the generation of anionic groups in a state that is easily oxidized, making the self-dispersing carbon black itself more susceptible to oxidation. For this reason, it is preferable to use a self-dispersing carbon black in which anionic groups are bonded to the carbon black particle surface via other atomic groups. In particular, it is preferable to use a self-dispersing pigment in which a structure derived from phthalic acid [—CH—(COOM) (M is the counter ion described above)] is bonded to the pigment particle surface. It is particularly preferable that both the first pigment and the second pigment are self-dispersing pigments in which a structure derived from phthalic acid is bonded to the pigment particle surface.

[0021] [Types of carbon black] Examples of carbon black include furnace black, lamp black, acetylene black, and channel black.

[0022] [Average particle diameter] The volume-based cumulative 50% particle diameter (nm) of the first pigment is at least 1.3 times, preferably 1.5 to 3.0 times, and particularly preferably 1.5 to 2.5 times, of the volume-based cumulative 50% particle diameter (nm) of the second pigment. A ratio of 1.3 or more is believed to efficiently achieve both the first pigment's ability to break down bubbles in the thick film formed by air bubbles and the second pigment's ability to break down the film that thins over time. In other words, a ratio of 1.3 or more can prevent ink misfiring due to trapped air bubbles, even when continuously recording images such as solid images, which consume a lot of ink.

[0023] The volume-based cumulative 50% particle size (average particle size) of the first pigment is preferably 80 nm or more. If the average particle size of the first pigment is less than 80 nm, the amount of the first pigment that penetrates in the thickness direction of the recording medium increases, which may reduce the effect of improving optical density. Furthermore, the average particle size of the first pigment is preferably 150 nm or less, and more preferably 130 nm or less. If the average particle size of the first pigment is more than 150 nm, the effect of improving the storage stability of the ink may be reduced.

[0024] The second pigment has a volume-based cumulative 50% particle size (average particle size) of 60 nm or less. When the average particle size of the second pigment is 60 nm or less, carbon black with an average particle size of 60 nm or less penetrates into the film of air bubbles that thin over time, allowing for efficient bubble destruction. Even when continuously recording images, such as solid images, which consume a large amount of ink, ink ejection failures due to trapped air bubbles can be suppressed. The average particle size of the second pigment is preferably 40 nm or more, and more preferably 45 nm or more. When the average particle size of the second pigment is less than 40 nm, the amount of the second pigment that penetrates in the thickness direction of the recording medium increases, which may reduce the effect of improving optical density.

[0025] In this specification, the term "average particle size" simply refers to the "volume-based cumulative 50% particle size (D50)." The "volume-based cumulative 50% particle size" is the diameter of the particle that is 50% of the total volume of the measured particles when calculated from the smallest particle size in a particle size integration curve measured by dynamic light scattering. The average particle size of the pigment in the ink can be determined, for example, as follows. First, the particle size distribution of the pigment in the ink is measured using a particle size measurement device. The particle size measurement device used in this case can be a device based on a dynamic light scattering method, a disk centrifuge method, a scanning mobility measurement method, or the like. Alternatively, the particle size distribution can be measured by image processing of images observed with a transmission electron microscope or a scanning electron microscope. If the particle size distribution of the pigment measured using the ink has two or more maximum particle sizes and the ratio of these maximum particle sizes is 1.3 or more, it can be determined that the ratio requirement defined in the present invention is met, and the effects of the present invention described above can be achieved.

[0026] [Amount of anionic groups] The anionic group amount (mmol / g) of the pigment is a physical property that can be used as an index representing the amount of anionic groups in the self-dispersing carbon black, and can be measured by colloid titration. The anionic group amount of the first pigment is preferably 0.10 mmol / g or more and 0.18 mmol / g or less, and more preferably 0.13 mmol / g or more and 0.18 mmol / g or less. If the anionic group amount of the first pigment is less than 0.10 mmol / g, the effect of improving the storage stability of the ink may be reduced. On the other hand, if the anionic group amount of the first pigment is more than 0.18 mmol / g, the aggregation tendency of the first pigment may be reduced, and the effect of improving the optical density may be slightly reduced.

[0027] The amount of anionic groups in the second pigment is preferably 0.27 mmol / g or less. If the amount of anionic groups in the second pigment exceeds 0.27 mmol / g, much of the carbon black particle surface will be covered with organic groups containing anionic groups, reducing the proportion of "hydrophobic portions" on the particle surface where no anionic groups are present. This makes it difficult for surfactants to adsorb, and the effect of suppressing non-ejection may be reduced when continuously recording images such as solid images that require a large amount of ink. The amount of anionic groups in the second pigment is preferably 0.18 mmol / g or more, and more preferably 0.20 mmol / g or more. If the amount of anionic groups in the second pigment is less than 0.18 mmol / g, the effect of improving the storage stability of the ink may be reduced.

[0028] The ratio of the anionic group content (mmol / g) of the second pigment to the anionic group content (mmol / g) of the first pigment is preferably 1.1 to 2.0 times, more preferably 1.1 to 1.8 times. Because the second pigment has smaller particles than the first pigment, it penetrates relatively easily in the thickness direction of the recording medium. It is presumed that if the agglomeration property of the first pigment is relatively high, the penetration of the second pigment in the thickness direction of the recording medium is suppressed by the sealing effect of the aggregates of the first pigment formed earlier, thereby increasing the optical density of the image. Here, if the ratio is less than 1.1 times, the agglomeration property of the second pigment becomes equal to or greater than that of the first pigment, reducing the sealing effect of the aggregates of the first pigment and slightly reducing the effect of improving the optical density. On the other hand, if the ratio is more than 2.0 times, the difference between the agglomeration property of the first pigment and the agglomeration property of the second pigment becomes too large, which may result in pigment distribution within the image and slightly reducing the effect of improving the optical density.

[0029] [Content] The content (% by mass) of each pigment in the ink is preferably 0.10% to 10.00% by mass, based on the total mass of the ink. The content (% by mass) of the first pigment in the ink is preferably 1.00% to 8.00% by mass, based on the total mass of the ink, and more preferably 2.00% to 5.00% by mass. The content (% by mass) of the second pigment in the ink is preferably 0.10% to 5.00% by mass, based on the total mass of the ink, and more preferably 0.50% to 1.80% by mass.

[0030] The mass ratio of the first pigment content (mass %) in the ink to the second pigment content (mass %) is preferably 1.5 to 5.0 times, and more preferably 2.0 to 4.0 times. If the mass ratio is less than 1.5 times, the sealing effect of the first pigment aggregates may be reduced. This may increase the penetration of the second pigment in the thickness direction of the recording medium, slightly reducing the effect of improving the optical density of the image. On the other hand, if the mass ratio is more than 5.0 times, fewer second pigment particles can penetrate into the film of air bubbles that thins over time. This may reduce the effect of preventing ejection failures when continuously recording images, such as solid images, which consume a lot of ink.

[0031] [DBP oil absorption] The DBP oil absorption of the carbon black is preferably 30 mL / 100 g or more and 200 mL / 100 g or less. The DBP oil absorption of the carbon black of the first pigment is preferably 100 mL / 100 g or more, more preferably 110 mL / 100 g or more, and preferably 180 mL / 100 g or less. In particular, if the DBP oil absorption of the carbon black of the first pigment is less than 100 mL / 100 g, the surface reflection of the image tends to increase, and the effect of improving optical density may be slightly reduced. The DBP oil absorption of the carbon black of the second pigment is preferably 95 mL / 100 g or less, more preferably 90 mL / 100 g or less, and particularly preferably 40 mL / 100 g or more. The DBP oil absorption of the carbon black can be measured by a method conforming to JIS K 6221 and ASTM D 2414. These methods involve adding dibutyl phthalate dropwise to 100 g of carbon black while stirring, and measuring the amount of dibutyl phthalate added when the torque reaches its maximum.

[0032] (surfactant) The ink contains a surfactant. A nonionic surfactant or an anionic surfactant can be used as the surfactant. The use of a cationic surfactant may slightly reduce the storage stability of the ink. The surfactant is preferably a nonionic surfactant. The use of an anionic surfactant may facilitate stabilization of bubbles that occur in the ink, which may reduce the effect of suppressing non-ejection when continuously recording images, such as solid images, which consume a large amount of ink.

[0033] Examples of nonionic surfactants include acetylene glycol surfactants, fluorine-based surfactants, silicone-based surfactants, and polyoxyalkylene alkyl ether surfactants. Among these, polyoxyethylene alkyl ethers are preferred as nonionic surfactants. The use of polyoxyethylene alkyl ethers can improve the ink's wettability with respect to the components of the recording head without unnecessarily lowering the ink's surface tension. This can further improve the ink ejection accuracy while maintaining a high optical density of the image.

[0034] The HLB value of the polyoxyethylene alkyl ether determined by the Griffin method is preferably 17.0 or less. If the HLB value of the polyoxyethylene alkyl ether exceeds 17.0, the hydrophilic group may be too long, making it difficult for the hydrophobic group to adsorb to the surface of carbon black particles. As a result, when images such as solid images that consume a large amount of ink are continuously recorded, the effect of suppressing ink ejection failure due to bubbles introduced may be reduced. The HLB value of the polyoxyethylene alkyl ether determined by the Griffin method is preferably 11.0 or more. If the HLB value of the polyoxyethylene alkyl ether is less than 11.0, the solubility in ink may be low, and the storage stability of the ink may be slightly reduced.

[0035] The HLB value can be calculated by the following formula (1). The HLB value determined by the Griffin method is a physical property value that indicates the degree of hydrophilicity or lipophilicity of a surfactant, and takes a value from 0.0 to 20.0. The smaller the HLB value, the higher the lipophilicity, and the larger the HLB value, the higher the hydrophilicity. HLB value = 20 × (formula weight of hydrophilic group of surfactant / molecular weight of surfactant) (1)

[0036] The polyoxyethylene alkyl ether has, for example, a structure represented by the following formula (2). RO-(CH2CH2O) n -H···(2) (In formula (2), R represents a hydrocarbon group, and n represents a natural number.)

[0037] In formula (2), the number of carbon atoms in the hydrocarbon group represented by R is usually within the range in which the polyoxyethylene alkyl ether exhibits surfactant properties. In formula (2), the number of carbon atoms in the hydrocarbon group represented by R is preferably 12 to 22. In formula (2), examples of the hydrocarbon group represented by R include alkyl groups such as lauryl group (12), cetyl group (16), stearyl group (18), and behenyl group (22), and alkenyl groups such as oleyl group (18) (the number in parentheses represents the number of carbon atoms in the hydrocarbon group). In formula (2), n is a natural number and represents the number of repeating ethylene oxide groups. The value of n is determined from the structure of R and the HLB value. In formula (2), n is preferably 5 to 30.

[0038] The content (% by mass) of the surfactant in the ink is preferably 0.05% by mass or more and 2.0% by mass or less, and more preferably 0.05% by mass or more and 1.0% by mass or less, based on the total mass of the ink. Different types of surfactants may be used in combination.

[0039] (Water-soluble resin) If the ink contains a water-soluble resin, it is preferable not to include too much of it. Water-soluble resins tend to adsorb to the particle surfaces of self-dispersed carbon black. Therefore, adding a large amount of water-soluble resin to the ink may reduce the coagulation of the self-dispersed carbon black, slightly reducing the effect of improving the optical density of the image. Furthermore, adding a large amount of water-soluble resin to the ink may inhibit the adsorption of surfactants to the hydrophobic portions of the particle surfaces of the self-dispersed carbon black. Therefore, when continuously recording images, such as solid images, which consume a large amount of ink, the effect of suppressing ink ejection failure due to trapped air bubbles may be reduced. If the ink contains a water-soluble resin, the content (mass %) of the water-soluble resin in the ink is preferably 0.10 mass % or less, based on the total mass of the ink. It is particularly preferable that the ink does not contain a water-soluble resin.

[0040] Resins are usually made water-soluble by neutralization with a neutralizing agent such as a hydroxide of an alkali metal (such as lithium, sodium, or potassium) or aqueous ammonia. In this specification, "water-soluble resin" means that a sample obtained by neutralizing the resin with an alkali equivalent to its acid value does not form particles whose particle size can be measured by dynamic light scattering. In addition, in this specification, "does not contain a water-soluble resin" means that a trace amount of water-soluble resin may be contained within a range that does not affect the effect.

[0041] (aqueous medium) The ink typically contains water or a mixed solvent of water and a water-soluble organic solvent as an aqueous medium. Deionized water or ion-exchanged water is preferably used as the water. Any of the known water-soluble organic solvents commonly used in inkjet inks can be used. Examples of water-soluble organic solvents include monohydric or polyhydric alcohols, alkylene glycols having an alkylene group with approximately 1 to 4 carbon atoms, polyethylene glycols having a number-average molecular weight of approximately 200 to 2,000, glycol ethers, and nitrogen-containing compounds.

[0042] (Other ingredients) In addition to the above components, the ink may contain, if necessary, organic compounds that are solid at room temperature, such as trimethylolethane and trimethylolpropane, and nitrogen-containing compounds, such as urea and ethyleneurea. In addition to the above components, the ink may further contain, if necessary, various additives, such as a pH adjuster, a rust inhibitor, a preservative, an anti-mold agent, an antioxidant, an anti-reducing agent, an evaporation accelerator, and a chelating agent.

[0043] (Ink properties) The static surface tension of the ink at 25°C, measured by the plate method, is preferably 28 mN / m or more and 45 mN / m or less. The dynamic surface tension of the ink at 25°C after a 10 ms lifespan, measured by the maximum bubble pressure method, is preferably 45 mN / m or more, and more preferably 50 mN / m or more. Furthermore, the dynamic surface tension of the ink at 25°C after a 10 ms lifespan, measured by the maximum bubble pressure method, is preferably 65 mN / m or less, and more preferably 60 mN / m or less.

[0044] <Ink cartridges> The ink cartridge of the present invention includes ink and an ink storage section that stores the ink. The ink stored in the ink storage section is the aqueous ink of the present invention described above. FIG. 1 is a cross-sectional view schematically illustrating one embodiment of the ink cartridge of the present invention. As shown in FIG. 1, the bottom of the ink cartridge is provided with an ink supply port 12 for supplying ink to the recording head. The interior of the ink cartridge serves as an ink storage section for storing the ink. The ink storage section is composed of an ink storage chamber 14 and an absorber storage chamber 16, which are connected to each other via a communication port 18. The absorber storage chamber 16 is also connected to the ink supply port 12. The ink storage chamber 14 stores liquid ink 20, and the absorber storage chamber 16 contains absorbers 22 and 24 that retain the ink in an impregnated state. The ink storage section may not have an ink storage chamber that stores liquid ink, but may instead use an absorber to hold all of the ink stored therein. Alternatively, the ink storage section may not have an absorber and may store all of the ink in a liquid state. Furthermore, the ink cartridge may be configured to have an ink storage section and a recording head.

[0045] <Inkjet recording method> The inkjet recording method of the present invention is a method of ejecting the above-described aqueous ink of the present invention from an inkjet recording head to record an image on a recording medium. Methods for ejecting the ink include a method of applying mechanical energy to the ink and a method of applying thermal energy to the ink. In the present invention, it is particularly preferable to employ a method of ejecting the ink by applying thermal energy to the ink. Other than using the ink of the present invention, the steps of the inkjet recording method may be any known method.

[0046] FIG. 2 is a diagram schematically illustrating an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main components of the inkjet recording apparatus, and (b) is a perspective view of a head cartridge. The inkjet recording apparatus is provided with a transport means (not shown) for transporting a recording medium 32, and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 is equipped with recording heads 38 and 40, and is configured to accommodate an ink cartridge 42. While the head cartridge 36 is transported in the main scanning direction along the carriage shaft 34, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. An image is then recorded on the recording medium 32 by transporting the recording medium 32 in the sub-scanning direction by a transport means (not shown). [Example]

[0047] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as the gist of the invention is not exceeded. The terms "parts" and "%" used to describe the amounts of components are based on mass unless otherwise specified.

[0048] <Methods for measuring physical properties> (Average particle size of pigment (cumulative 50% particle size of volume distribution (D50))) Each pigment dispersion was diluted with pure water, and the average particle size (volume-based cumulative 50% particle size (D50 (nm)) of the pigment was measured using a particle size analyzer that uses dynamic light scattering. The particle size distribution analyzer used was a Nanotrac UPA-EX150 (manufactured by Microtrac Bell). The measurement conditions were: Set Zero: 30 seconds, number of measurements: 3, measurement time: 120 seconds, shape: aspherical, refractive index: 1.80.

[0049] (Amount of anionic groups in pigment) The amount of anionic groups in the pigment was measured by colloidal titration using potential difference using an automatic potentiometric titrator (AT-510, Kyoto Electronics Manufacturing Co., Ltd.) equipped with a streaming potential titration unit (PCD-500). 5 mmol / L methyl glycol chitosan was used as the titration reagent.

[0050] <Production of water-soluble resin> A four-neck flask equipped with a stirrer, reflux condenser, and nitrogen gas inlet tube was prepared. 200.0 parts of ethylene glycol monobutyl ether was placed in the four-neck flask and stirred under a nitrogen gas atmosphere while the temperature was raised to 130°C. 65.0 parts of styrene, 20.0 parts of butyl acrylate, 15.0 parts of acrylic acid, and 4.0 parts of t-butyl peroxide were added dropwise over 3 hours. After aging for 2 hours, the ethylene glycol monobutyl ether was distilled off under reduced pressure to obtain a resin. To the obtained resin, potassium hydroxide in an amount equimolar to the acid value of the resin and an appropriate amount of pure water were added, and the mixture was heated to 80°C to dissolve. This yielded an aqueous solution of water-soluble resin 1 with a water-soluble resin (solid content) content of 20.0%.

[0051] <Characteristics of carbon black (pigment)> The types of carbon black prepared were shown in Table 1. The properties of the carbon black are shown in Table 1.

[0052] TIFF2025146695000001.tif117170

[0053] <Preparation of pigment dispersion> (Pigment dispersions 1-9, 13, 19-26, 28, 29, 31) A solution prepared by dissolving 5.0 g of concentrated hydrochloric acid in 5.5 g of pure water was cooled to 5°C, and the type and amount (mmol) of treatment agent shown in Table 2 was added. The container containing this solution was placed in an ice bath, and while stirring to maintain the solution temperature below 10°C, a solution prepared by dissolving 1.8 g of sodium nitrite in 9.0 g of 5°C ion-exchanged water was added. After stirring for 15 minutes, 6.0 g of the pigment type shown in Table 2 was added under stirring, and the mixture was stirred for another 15 minutes to obtain a slurry. The resulting slurry was filtered through filter paper (trade name "Standard Filter Paper No. 2" manufactured by Advantec), after which the particles were thoroughly washed with water and dried in an oven at 110°C. If necessary, counterions were substituted by ion exchange to create counterions to the anionic groups shown in Table 2. An appropriate amount of pure water was then added to obtain each pigment dispersion.

[0054] (Pigment Dispersion 10) A self-dispersing pigment was prepared by surface oxidation of Pigment 1 using ozone gas, following the description of Example 3 in JP-A-2003-535949. Specifically, Pigment 1 was first pre-dispersed in ion-exchanged water, followed by 2.5 hours of ozone treatment. Next, potassium hydroxide was added to adjust the pH of the mixture to approximately 7, while the mixture was circulated for 2 hours using a liquid-liquid collision disperser. An appropriate amount of ion-exchanged water was then added to obtain Pigment Dispersion 10.

[0055] (Pigment Dispersion 11) 20 g of Pigment 1, 4.4 mmol of ((4-aminobenzoylamino)-methane-1,1-diyl)bisphosphonic acid sodium salt, 7.5 mmol of nitric acid, and 200 g of water were mixed. The mixture was stirred at 6,000 rpm at room temperature using a Silverson mixer. After 30 minutes, 7.5 mmol of sodium nitrite dissolved in a small amount of pure water was slowly added to the resulting mixture and mixed. The mixture reached a temperature of 60°C and was allowed to react at this temperature for 1 hour. The pH of the mixture was adjusted to 10 using a 1.0 mol / L potassium hydroxide aqueous solution. After 30 minutes, 20 mL of pure water was added, and the mixture was desalted and low molecular weight substances were removed using a Spectrum membrane. An appropriate amount of pure water was added to obtain Pigment Dispersion 11.

[0056] (Pigment Dispersion 12) A solution of 25.0 g of Pigment 1 and 10.8 mmol of sulfanilic acid dissolved in hot water was mixed and stirred until the solution temperature reached 30°C. 28.2 mmol of concentrated hydrochloric acid was added, and a solution of 10.6 mmol of sodium nitrite dissolved in a small amount of water was added over 1 hour. Purified water was added and stirred, and then an aqueous potassium hydroxide solution was added to adjust the pH to 9.0. The water was evaporated in an oven until the pigment content was 10.0%, and then the mixture was filtered through a 1.2 μm filter to obtain Pigment Dispersion 12.

[0057] (Pigment Dispersion 14) 20 g of Pigment 4, 2.6 mmol of ((4-aminobenzoylamino)-methane-1,1-diyl)bisphosphonic acid sodium salt, 4.5 mmol of nitric acid, and 200 g of water were mixed. The mixture was stirred at 6,000 rpm at room temperature using a Silverson mixer. After 30 minutes, 4.5 mmol of sodium nitrite dissolved in a small amount of pure water was slowly added to the resulting mixture and mixed. This brought the mixture temperature to 60°C, and the mixture was allowed to react at this temperature for 1 hour. The pH of the mixture was adjusted to 10 using a 1.0 mol / L aqueous potassium hydroxide solution. After 30 minutes, 20 mL of pure water was added, and the mixture was desalted and low molecular weight substances were removed using a Spectrum membrane. An appropriate amount of pure water was added to obtain Pigment Dispersion 14.

[0058] (Pigment Dispersion 15) A mixture was obtained by mixing 10.0 parts of pigment 1, 15.0 parts of an aqueous solution of water-soluble resin 1, and 75.0 parts of pure water. The resulting mixture was placed in a sand grinder and dispersed for 1 hour. After centrifuging to remove coarse particles, the mixture was pressure-filtered using a microfilter (manufactured by Fujifilm) with a pore size of 3.0 μm. An appropriate amount of pure water was added to adjust the pigment content, yielding pigment dispersion 15, which contained 10.0% pigment and 3.0% resin dispersant (water-soluble resin 1).

[0059] (Pigment Dispersion 16) Pigment dispersion 16 was obtained in the same manner as in the preparation of "Pigment dispersion B" described in JP-A No. 2002-003767 (Patent Document 2), except that pigment 6 was used.

[0060] (Pigment Dispersion 17) Pigment dispersion 17 was obtained in the same manner as in the preparation method of "Pigment dispersion 1" described in JP-A-2018-021099 (Patent Document 1), except that Pigment 7 was used.

[0061] (Pigment Dispersion 18) Pigment dispersion 18 was obtained in the same manner as in the preparation method of "Pigment dispersion 1" described in JP-A-2018-149804, except that Pigment 1 was used.

[0062] (Pigment Dispersion 27) Pigment Dispersion Liquid 27 was obtained in the same manner as in the case of Pigment Dispersion Liquid 10, except that Pigment 8 was used instead of Pigment 1.

[0063] (Pigment dispersion 30) Pigment dispersion 30 was obtained in the same manner as in the case of pigment dispersion 15, except that pigment 8 was used instead of pigment 1.

[0064] (Pigment Dispersion 32) Pigment dispersion 32 was obtained in the same manner as in the preparation of "pigment dispersion A" described in JP-A No. 2002-003767 (Patent Document 2), except that pigment 12 was used.

[0065] (Pigment Dispersion 33) Pigment dispersion 33 was obtained in the same manner as in the preparation method of "Pigment dispersion 7" described in JP-A-2018-021099 (Patent Document 1), except that pigment 13 was used.

[0066] (Pigment Dispersion 34) Pigment dispersion 34 was obtained in the same manner as in the preparation of "Pigment dispersion 11" described in JP-A-2018-149804, except that pigment 14 was used.

[0067] (Characteristics of pigment dispersion) The properties of the pigment dispersion are shown in Table 2.

[0068] TIFF2025146695000002.tif247170

[0069] <Preparing surfactant> The types of surfactants shown in Table 3 were prepared. The properties of the surfactants are shown in Table 3. The HLB values ​​were calculated using the Griffin method described above. "Surfactant 8" and "Surfactant 9" are surfactants manufactured by Kawaken Fine Chemicals, and the other surfactants are surfactants manufactured by Nikko Chemicals.

[0070] TIFF2025146695000003.tif88170

[0071] <Ink Preparation> The components (unit: mass %) shown in the upper rows of Tables 4-1 to 4-5 were mixed, thoroughly stirred, and then pressure filtered through a 3.0 μm pore size microfilter (manufactured by Fujifilm) to prepare each ink. The properties of each ink prepared are shown in the lower rows of Tables 4-1 to 4-5.

[0072] TIFF2025146695000004.tif228170

[0073] TIFF2025146695000005.tif228170

[0074] TIFF2025146695000006.tif228170

[0075] TIFF2025146695000007.tif246170

[0076] TIFF2025146695000008.tif255166

[0077] <Evaluation> The prepared inks were evaluated as follows. In the present invention, "A" and "B" were considered acceptable levels and "C" was considered unacceptable in the evaluation criteria for each evaluation item shown below. The evaluation results are shown in Table 5.

[0078] (optical density) Each ink cartridge was filled with the prepared ink and set in an inkjet recording device (trade name "PIXUS PRO-10S", manufactured by Canon) equipped with a recording head that ejects ink using thermal energy. In this example, an image recorded under conditions in which eight ink droplets, each with a mass of 3.5 ng, are deposited in a unit area of ​​1 / 600 inch x 1 / 600 inch, is defined as having a recording duty of 100%. The recording environment was a temperature of 25°C and a relative humidity of 55%. Using the inkjet recording device described above, a solid image was recorded on the entire surface of A4-sized plain paper (trade name "CS-064", manufactured by Canon) with a recording duty of 50%. One day after recording, the optical density of the image was measured using a spectrophotometer (trade name "FD-7", manufactured by Konica Minolta), and the optical density of the image was evaluated according to the following evaluation criteria. A: The optical density was 1.2 or more. B: The optical density was 1.1 or more and less than 1.2. C: The optical density was less than 1.1.

[0079] (Discharge stability) Under the same conditions as the evaluation of "optical density" described above, 10 solid images with a print duty of 100% were printed continuously on the entire surface of A4-sized plain paper (product name "CS-064," manufactured by Canon). Then, using the 768 ejection ports of the print head, a nozzle check pattern was printed on the A4-sized plain paper (product name "CS-064," manufactured by Canon) using the "PIXUS PRO-10S." The number of ejection ports that failed to eject ink was then counted, and the ink ejection stability was evaluated according to the following evaluation criteria. A: There were no ejection ports where ejection failure occurred. B: The number of non-discharge ports was 1 to 3. C: Four or more ejection ports were found to be non-ejectable.

[0080] (Discharge accuracy) Under the same conditions as the evaluation of "optical density" described above, five solid images were printed on the entire surface of A4-sized plain paper (product name "CS-064," manufactured by Canon) with a print duty of 50%. Then, a nozzle check pattern was printed on the A4-sized plain paper (product name "CS-064," manufactured by Canon) using a PIXUS PRO-10S. The resulting nozzle check pattern was visually inspected, and the condition of the print head's ejection ports was observed under a microscope. The ink ejection accuracy was evaluated according to the following evaluation criteria: A: No ink droplets adhered to the vicinity of the ejection port, and there was no disturbance in the nozzle check pattern. B: Ink droplets adhered near the ejection port, but the nozzle check pattern was not disturbed. C: Ink droplets adhered near the ejection port, and the nozzle check pattern was also disturbed.

[0081] TIFF2025146695000009.tif158170

Claims

1. 1. A water-based inkjet ink containing a first pigment, a second pigment, and a surfactant, the first pigment and the second pigment are both self-dispersible carbon blacks in which an organic group containing an anionic group is bonded to the surface of a carbon black particle, a volume-based cumulative 50% particle diameter (nm) of the first pigment is 1.3 times or more the volume-based cumulative 50% particle diameter (nm) of the second pigment, and the volume-based cumulative 50% particle diameter of the second pigment is 60 nm or less.

2. The aqueous ink according to claim 1 , wherein the first pigment has a volume-based cumulative 50% particle size of 80 nm or more.

3. 2. The aqueous ink according to claim 1, wherein the amount (mmol / g) of the anionic group in the second pigment is 1.1 to 2.0 times the amount (mmol / g) of the anionic group in the first pigment.

4. 2. The aqueous ink according to claim 1, wherein the amount (mmol / g) of anionic groups in the second pigment is 0.27 mmol / g or less.

5. The aqueous ink according to claim 1 , wherein the content (mass %) of the first pigment is 1.5 to 5.0 times the content (mass %) of the second pigment in terms of mass ratio.

6. The aqueous ink according to claim 1 , wherein the anionic groups of the first pigment and the second pigment are both carboxylic acid groups.

7. the anionic group has a counter ion, 2. The aqueous ink according to claim 1, wherein the counter ion is a potassium ion or an ammonium ion, and the counter ions of the first pigment and the second pigment are the same type.

8. 2. The aqueous ink according to claim 1, wherein the surfactant is a nonionic surfactant.

9. 9. The aqueous ink according to claim 8, wherein the nonionic surfactant is a polyoxyethylene alkyl ether.

10. 10. The aqueous ink according to claim 9, wherein the HLB value of the polyoxyethylene alkyl ether determined by the Griffin method is 17.0 or less.

11. The water-based ink according to claim 1, which does not contain a water-soluble resin.

12. An ink cartridge comprising ink and an ink storage section for storing the ink, 12. An ink cartridge, wherein the ink is the aqueous ink according to claim 1.

13. An inkjet recording method for recording an image on a recording medium by ejecting ink from an inkjet recording head, An ink-jet recording method, wherein the ink is the aqueous ink according to any one of claims 1 to 11.

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