Aqueous ink, ink cartridge, and inkjet recording method

Abstract

The present invention relates to an aqueous ink, an ink cartridge, and an inkjet recording method. An aqueous ink for inkjet that can record an image excellent in color development by suppressing a bronzing phenomenon derived from carbon black is provided. The aqueous ink for inkjet contains: self-dispersible carbon black; and resin particles formed of an acrylic resin. The DBP oil absorption of the carbon black is 120 mL / 100 g or more, the glass transition temperature of the acrylic resin particles is 30° C. or more, and the mass ratio of the content (mass %) of the resin particles to the content (mass %) of the carbon black is 0.10 times or more and 2.0 times or less.

Description

Technical Field

[0001] The present invention relates to an aqueous ink, an ink cartridge and an inkjet recording method. Background Art

[0002] The number of opportunities for the use of inkjet recording methods for printing business documents including text and diagrams on recording media such as plain paper has been increasing in recent years. In such uses, aqueous inks that can record images that are excellent in color development and fastness are required. When recording images that are excellent in characteristics such as color development, pigment inks that use pigments as coloring materials are often used. In addition, in black inks, carbon black is mainly used as a pigment. The improvement of the color development of images to be recorded with inks using carbon black as a pigment is a particularly important issue, and therefore various studies have been conducted to date.

[0003] For example, an aqueous ink for inkjet containing self-dispersible carbon black having high DBP oil absorption and large structure is proposed (Japanese Patent Application Laid-Open No. 2017-136846). In addition, a black ink for inkjet containing carbon black, resin particles having a blue dye, and resin particles having a fluorescent whitening agent is proposed (Japanese Patent Application Laid-Open No. 2019-143095). It is said that the use of black ink improves the color development of the image as follows: the blue fluorescence of the fluorescent whitening agent and the yellow reddish bronze light derived from the carbon black undergo additive color mixing to approach white, so the bronze phenomenon is suppressed.

[0004] The inventors of the present invention studied the aqueous ink proposed in Japanese Patent Application Laid-Open No. 2017-136846. As a result, the inventors found that although an image with a somewhat satisfactory color development can be recorded with this ink, the color development improvement effect is not necessarily sufficient due to the bronze phenomenon originating from the carbon black present on the surface of the recording medium. In addition, the inventors of the present invention also studied the black ink proposed in Japanese Patent Application Laid-Open No. 2019-143095. As a result, the inventors revealed that when a self-dispersible pigment of carbon black having a large structure is used, the bronze phenomenon cannot be sufficiently suppressed, and thus the color development improvement effect becomes insufficient. Summary of the invention

[0005] Therefore, an object of the present invention is to provide an aqueous ink for inkjet that can record an image excellent in color development by suppressing the bronzing phenomenon derived from carbon black. Another object of the present invention is to provide an ink cartridge and an inkjet recording method each using the aqueous ink.

[0006] That is, according to the present invention, there is provided an aqueous ink for inkjet comprising self-dispersible carbon black and resin particles formed from an acrylic resin, wherein the DBP oil absorption of the carbon black is 120 mL / 100 g or more, the glass transition temperature of the acrylic resin particles is 30° C. or more, and the mass ratio of the content (mass %) of the resin particles to the content (mass %) of the carbon black is 0.10 times or more and 2.0 times or less.

[0007] Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a cross-sectional view schematically showing one embodiment of an ink cartridge according to the present invention.

[0009] Figure 2A and Figure 2B is a diagram schematically showing an example of an inkjet recording apparatus to be used for the inkjet recording method of the present invention, wherein Figure 2A is a perspective view of a main part of an inkjet recording apparatus and Figure 2B A perspective view of the head box. DETAILED DESCRIPTION

[0010] The present invention is described in more detail below by way of preferred embodiments. In the present invention, when the ink contains a salt, although the salt is dissociated into ions in the ink, it is referred to as "containing a salt" for convenience. In addition, the aqueous ink for inkjet can be simply recorded as "ink". Unless otherwise specified, the physical property values ​​are values ​​at room temperature (25°C).

[0011] The bronzing phenomenon that occurs in images recorded with ink containing carbon black has been a problem so far, and the treatment of this problem has been studied mainly in glossy images recorded on recording media having surface gloss such as glossy paper. The inventors of the present invention have conducted research and found that the bronzing phenomenon also occurs in non-glossy images recorded on recording media having no surface gloss such as plain paper, and that this phenomenon serves as a cause of reduced color development of the image.

[0012] The bronzing phenomenon is a phenomenon in which the color of reflected light appears different from its original color due to the fact that the refractive index on the surface of the pigment particles present on the recording medium has wavelength dependence. Therefore, it is considered that as the exposure amount of the surface of the pigment particles on the surface of the recording medium becomes larger, the bronzing phenomenon appears more significantly.

[0013] The inventors of the present invention have studied inks that can record images with high optical density and excellent color development by suppressing the bronzing phenomenon derived from carbon black. As a result, the inventors have found the combined use of a self-dispersible pigment of carbon black having a DBP oil absorption of 120 mL / 100 g or more and resin particles formed of an acrylic resin, the glass transition temperature of the particles being 30° C. or more. In addition, the inventors have found that it is effective to set the mass ratio of the content (mass %) of the resin particles to the content (mass %) of the carbon black to 0.10 times or more and 2.0 times or less. Thus, the inventors have arrived at the present invention.

[0014] Carbon black is dispersed in an aqueous medium in a state where several to dozens of primary particles are arranged in a row to form a cluster structure. The size of the structure of carbon black can be grasped by means of the DBP oil absorption of carbon black. Specifically, the higher the value of the DBP oil absorption means that the carbon black has a larger structure, and therefore has a more massive structure. The use of a self-dispersible pigment of carbon black with a large structure can improve the color development of the image. It is believed that this is because after the ink is imparted to the recording medium, the aggregation of carbon black is promoted by the evaporation of liquid components such as water in the ink, and the aggregated carbon black easily remains on the surface of the recording medium. However, the inventors conducted research and found that when carbon black easily remains on the surface of the recording medium, the bronze phenomenon derived from carbon black significantly occurs to impart a red to yellow hue to the image and inhibit the improvement of its color development. In view of the above, the inventors of the present invention studied how to reduce the amount of pigment exposed on the surface of the pigment layer formed on the surface of the recording medium.

[0015] As a result of the study, the inventors of the present invention have found that the occurrence of the bronze phenomenon originating from carbon black can be suppressed by adding a specific amount of acrylic resin particles having a glass transition temperature of a certain value or more to the ink. The inventors of the present invention speculate that the mechanism of the above situation is as follows. The presence of the granular resin in the pigment layer formed on the surface of the recording medium increases the thickness of the pigment layer and reduces the density of the pigment in the pigment layer. It is speculated that as a result of the above situation, the amount of pigment exposed on the surface of the pigment layer is reduced to suppress the bronze phenomenon. It is believed that the acquisition of such an effect as described above requires the presence of granular acrylic resin particles in the pigment layer. For this purpose, the glass transition temperature (Tg) of the resin particles needs to be 30°C or more. It is estimated that the temperature of the general recording environment is at most 30°C. Therefore, in the present invention, the glass transition temperature of the resin particles is specified to be 30°C or more so that the resin particles maintain their particle shape in the recording environment. When the glass transition temperature of the resin particles formed by the acrylic resin is lower than 30°C, the resin particles will hardly remain in the pigment layer while maintaining their particle state. Therefore, the bronze phenomenon cannot be suppressed. In addition, the resin particles need to be formed by an acrylic resin. When resin particles formed of a resin other than an acrylic resin, such as wax resin particles, are used, the bronzing phenomenon cannot be suppressed, and thus the color development property of an image is reduced.

[0016] The mass ratio of the content (mass %) of the resin particles to the content (mass %) of the carbon black needs to be 0.10 times or more and 2.0 times or less. When the mass ratio is less than 0.10 times, the bronze phenomenon originating from the carbon black cannot be suppressed, so the color development of the image is reduced. Meanwhile, when the mass ratio is greater than 2.0 times, although the bronze phenomenon can be suppressed, the color development of the image is reduced due to the influence of the color tone of the resin particles.

[0017] The DBP oil absorption of carbon black is 120 mL / 100 g or more. When the DBP oil absorption of carbon black is less than 120 mL / 100 g, its aggregation is weak, so the color development property of the image to be recorded cannot be improved.

[0018] <Water-based ink>

[0019] The ink of the present invention is a water-based ink for inkjet containing self-dispersible carbon black and resin particles formed of an acrylic resin. The DBP oil absorption of the carbon black is 120 mL / 100 g or more and the glass transition temperature (Tg) of the resin particles is 30° C. or more. In addition, the mass ratio of the content (mass %) of the resin particles to the content (mass %) of the carbon black is 0.10 times or more and 2.0 times or less. The components used to form the ink are described below.

[0020] (pigment)

[0021] The ink contains self-dispersible carbon black. Any carbon black can be used as the carbon black, as long as the carbon black can be used for inkjet ink. Examples of carbon black can include furnace black, lamp black, acetylene black, channel black and thermal black. These carbon blacks can be used alone or in combination of two or more. For purposes such as coloring, dyes and the like can be further introduced into the ink.

[0022] Self-dispersible carbon black is a self-dispersible pigment in which a hydrophilic group such as anionic group is directly or via other atomic groups bonded to the particle surface of carbon black. After applying the ink containing the self-dispersible pigment to the recording medium, due to the evaporation of its liquid component such as water, viscosity increase and state changes such as association and aggregation of the pigment are easy to occur. Therefore, the use of self-dispersible carbon black can provide an ink that can record images with high optical density and excellent color development. At the same time, when resin-dispersible carbon black is used instead of self-dispersible carbon black, the color development of the image to be recorded cannot be improved.

[0023] The DBP oil absorption of carbon black is 120 mL / 100 g or more, preferably 130 mL / 100 g or more. Although the upper limit of the DBP oil absorption is not particularly limited, the DBP oil absorption is preferably 200 mL / 100 g or less, more preferably 180 mL / 100 g or less. The DBP (dibutyl phthalate) oil absorption of carbon black can be measured according to ASTM D-2414. The DBP oil absorption of carbon black is correlated with the structure of carbon black. The DBP oil absorption of the carbon black used in each of the following examples is a value measured according to ASTM D-2414.

[0024] Examples of anionic groups that are directly or via other atomic groups bonded to the particle surface of carbon black may include carboxyl, sulfonic, phosphoric and phosphonic acid groups. These anionic groups may form salts. When anionic groups form salts, at least one proton of each group is replaced by a cation. Examples of cations may include alkali metal ions, ammonium ions and organic ammonium ions. Examples of alkali metal ions may include lithium ions, sodium ions and potassium ions. Examples of organic ammonium ions may include cations of the following substances: aliphatic amines, such as monoalkylamines to trialkylamines; and aliphatic alcohol amines, such as monoalkanolamines to trialkanolamines; and salts thereof. The anionic group is preferably an alkali metal salt type or ammonium salt type such as a sodium salt type or a potassium salt type, and more preferably an alkali metal salt type such as a sodium salt type or a potassium salt type.

[0025] The anionic group may be directly bonded to the particle surface of the carbon black or may be bonded thereto via other atomic groups (-R-). Examples of other atomic groups (-R-) may include: alkylene groups, such as methylene, ethylene or propylene; arylene groups, such as phenylene, naphthylene, anthrylene, phenanthrene or biphenylene; heteroarylene groups, such as pyridylene groups, imidazolylene, pyrazolyl, pyridinylene groups, thienylene or thiazolylene groups; carbonyl groups; ester groups, such as carboxylate groups, sulfonate groups, phosphate groups or phosphonate groups; imino groups; amide groups; sulfonyl groups; and ether groups. In addition, groups obtained by combining these groups may also be used. In order to obtain high color development, the anionic group bonded to the particle surface of the carbon black directly or via other atomic groups is preferably a carboxylic acid group. In order to obtain higher color development, it is more preferred to use a self-dispersible pigment in which the carboxylic acid group is bonded to the particle surface of the carbon black via other atomic groups.

[0026] The content (mass %) of carbon black in the ink is preferably 1.0 mass % or more and 10.0 mass % or less relative to the total mass of the ink. In addition, the volume-based cumulative 50% particle size (D50) of the carbon black is preferably 50 nm or more and 150 nm or less. As used herein, the simple term "average particle size" means "volume-based cumulative 50% particle size (D50)". "Volume-based cumulative 50% particle size (D50)" can be measured using a particle size distribution measurement device based on a dynamic light scattering method. In addition, the BET specific surface area of ​​the carbon black is preferably 200 m 2 / g and above 400m 2 / g or less, more preferably 220m 2 / g and above and 300m 2 The BET specific surface area of ​​carbon black can be measured in accordance with JIS Z 8830:2013 (ISO 9277:2010).

[0027] (resin particles)

[0028] The ink contains resin particles formed by an acrylic resin. The term "resin particles" means particles formed by a resin and having a particle size that can exist in a dispersed state in an aqueous medium. Resin particles can be produced according to known methods. Examples of methods for producing resin particles may include emulsion polymerization, pre-emulsion polymerization, seed polymerization, and phase inversion emulsification. The resin particles preferably exist in an independent manner without being integrated with the self-dispersible pigment. That is, it is not necessary to adopt the form described below: the resin particles encapsulate the self-dispersible pigment; or the resin particles are attached to the self-dispersible pigment to disperse the pigment. Although the self-dispersible pigment can be dispersed by itself without the assistance of a resin, the situation in which a part of the resin particles is attached to the self-dispersible pigment is not excluded.

[0029] Whether a resin is a "resin particle" can be judged according to the following method. First, a liquid (resin solid content: 10% by mass) containing a resin neutralized with an alkali (e.g., sodium hydroxide or potassium hydroxide) equivalent to the acid value of the resin is prepared. Next, the prepared liquid is diluted ten times (based on volume) with pure water to prepare a sample solution. Then, the particle size of the resin in the sample solution is measured by a dynamic light scattering method. In this case, when particles with a particle size are observed, the resin can be judged as a "resin particle". A particle size analyzer (e.g., product name "UPA-EX150", manufactured by Nikkiso Co., Ltd.) or the like can be used as a particle size distribution measurement device based on a dynamic light scattering method. The measurement conditions at this time can be set to, for example, as follows: SetZero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds. The particle size distribution measurement device and measurement conditions to be used are of course not limited to the aforementioned. The purpose of measuring the particle size with neutralized particles is to confirm that particles are formed even when the resin is fully neutralized to reach a state where it is more difficult to form particles.

[0030] The resin particles need to be formed of an acrylic resin. The glass transition temperature (Tg) of the resin particles is 30°C or more. The glass transition temperature of the resin particles is preferably 150°C or less, more preferably 100°C or less. The glass transition temperature of the resin particles can be measured, for example, using a differential scanning calorimeter. In the present invention, the glass transition temperature of the resin particles is a value measured for the resin particles themselves taken out of the ink. The temperature cycle during the measurement is preferably set to the following conditions: the temperature is increased from 25°C to 200°C at 10°C / min; the temperature is decreased from 200°C to -50°C at 5°C / min; and the temperature is increased from -50°C to 200°C at 10°C / min.

[0031] Relative to the total mass of the ink, the content (mass %) of the resin particles in the ink is preferably 1.0 mass % or more and 10.0 mass % or less, more preferably 1.5 mass % or more and 8.0 mass % or less. In addition, the mass ratio of the content (mass %) of the resin particles in the ink to the content (mass %) of the carbon black is 0.10 times or more and 2.0 times or less. The mass ratio is preferably 0.20 times or more and 1.0 times or less. In particular, the mass ratio is more preferably 0.30 times or more and 0.80 times or less. When the mass ratio is less than 0.20 times, the bronze phenomenon suppression effect may be reduced to some extent and the color development of the image may also be reduced to some extent. At the same time, when the mass ratio is greater than 1.0 times, although the bronze phenomenon can be suppressed, the color development of the image may be reduced to some extent due to the influence of the hue of the resin particles. Relative to the total mass of the ink, the total content (mass %) of the pigment and the resin particles in the ink is preferably 2.0 mass % or more and 20.0 mass % or less, more preferably 2.0 mass % or more and 10.0 mass % or less. In particular, the total content is particularly preferably 2.0% by mass or more and 7.5% by mass or less.

[0032] The cumulative 50% particle size (D50) based on volume of the resin particles in the ink is preferably 50 nm or more and 300 nm or less, more preferably 50 nm or more and 250 nm or less, and particularly preferably 50 nm or more and 230 nm or less. When the cumulative 50% particle size of the resin particles in the ink is less than 50 nm, the pigment layer to be formed becomes thinner. Therefore, the bronze phenomenon suppression effect may be reduced to some extent and the color development of the image may also be reduced to some extent. At the same time, when the cumulative 50% particle size of the resin particles in the ink is greater than 300 nm, although the bronze phenomenon can be suppressed, the effect of improving the color development of the image may be reduced to some extent due to the influence of light scattering.

[0033] The ratio of the cumulative 50% particle size based on volume of the resin particles to the cumulative 50% particle size based on volume of the carbon black is preferably 0.50 times or more and 2.0 times or less. When the value of the above ratio deviates from the predetermined range, the pigment layer to be formed becomes thinner. Therefore, the bronzing phenomenon suppression effect may be reduced to some extent and the color development property of the image may also be reduced to some extent.

[0034] The acid value of the acrylic resin forming the resin particles is preferably 5 mg KOH / g or more and 100 mg KOH / g or less, more preferably 5 mg KOH / g or more and 30 mg KOH / g or less. In addition, the weight average molecular weight of the acrylic resin forming the resin particles is preferably 1,000 or more and 2,000,000 or less. The density of the anionic groups of the resin particles (the amount of anionic groups in moles per unit surface area) is preferably 1 μmol / m 2Above and 500 μmol / m 2 Below, more preferably 1 μmol / m 2 Above 50 μmol / m 2 the following.

[0035] The acrylic resin forming the resin particles preferably has a hydrophilic unit and a hydrophobic unit as its constituent units. The term "unit" of the resin as used herein refers to a unit structure derived from one monomer. A resin having the following units is preferred: a hydrophilic unit derived from (meth) acrylic acid; and a hydrophobic unit derived from at least one of a (meth) acrylate monomer or a monomer having an aromatic ring.

[0036] The hydrophilic unit is a unit having a hydrophilic group such as an anionic group. The hydrophilic unit can be formed by, for example, polymerizing a hydrophilic monomer having a hydrophilic group. Specific examples of hydrophilic monomers having a hydrophilic group may include: acidic monomers each having a carboxylic acid group, such as (meth) acrylic acid, itaconic acid, maleic acid, and fumaric acid; and anionic monomers, such as anhydrides and salts of these acidic monomers. Examples of cations forming salts may include ions of the following substances: alkali metals, such as lithium, sodium, and potassium; ammonium; and organic ammonium. Among them, ions of alkali metals such as potassium are preferred.

[0037] The hydrophobic unit is a unit that does not have any hydrophilic groups such as anionic groups. The hydrophobic unit can be formed by, for example, polymerizing a hydrophobic monomer that does not have any hydrophilic groups such as anionic groups. Specific examples of the hydrophobic monomer may include: (meth)acrylate monomers such as ethyl (meth)acrylate, methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; and monomers each having an aromatic ring, such as styrene, α-methylstyrene, and benzyl (meth)acrylate.

[0038] The resin particles preferably have a cross-linked structure. The resin particles having a cross-linked structure tend to remain in the pigment layer while maintaining their particle state to a greater extent. Therefore, the bronze phenomenon can be further suppressed. In order to introduce a cross-linked structure into the resin particles, a monomer having two or more polymerizable functional groups such as ethylenically unsaturated bonds in the molecule can be used. Specific examples of monomers having two or more polymerizable functional groups in the molecule can include: diene compounds, such as butadiene and isoprene; multifunctional (meth) acrylates, such as 1,4-butanediol di(meth) acrylate, (poly)ethylene glycol di(meth) acrylate and trimethylolpropane tri(meth) acrylate; and divinylbenzene. When using resin particles that do not have any cross-linked structure, the bronze phenomenon suppression effect may be reduced to some extent and the color development of the image may also be reduced to some extent.

[0039] The acrylic resin forming the resin particles preferably has a unit derived from a reactive surfactant. The dispersion state of the resin particles formed from the acrylic resin having a unit derived from a reactive surfactant is further stabilized by the repulsion caused by steric hindrance. Therefore, the use of resin particles formed from an acrylic resin having a unit derived from a reactive surfactant can improve the ejection stability of the ink. In contrast, the use of resin particles formed from an acrylic resin not having any unit derived from a reactive surfactant can reduce the ejection stability of the ink to some extent.

[0040] As the reactive surfactant, it is preferred to use the following compounds: compounds in which a polymerizable functional group such as a (meth)acryloyl group, a maleoyl group, a vinyl group or an allyl group is bonded to the inside or the end of a molecule including a hydrophilic part and a hydrophobic part. Examples of the hydrophilic part may include polyalkylene oxide chains such as ethylene oxide chains and propylene oxide chains. In addition, examples of the hydrophobic part may include structures such as alkyl groups, aryl groups and combinations thereof.

[0041] (Salt formed by the combination of a monovalent cation and an anion)

[0042] The ink preferably further comprises a salt. The term "salt" means a compound formed by the combination of a monovalent cation and an anion. The introduction of a salt into the ink can promote the aggregation of its pigment in the recording medium, thus further improving the color development of the image.

[0043] Examples of monovalent cations may include alkali metal ions, ammonium ions, and organic ammonium ions. Examples of alkali metal ions may include lithium ions, sodium ions, and potassium ions. Examples of organic ammonium ions may include cations of: alkylamines each having 1 or more and 3 or less carbon atoms, such as methylamine and ethylamine; and alkanolamines each having 1 or more and 4 or less carbon atoms, such as monoethanolamine, diethanolamine, and triethanolamine. Among them, alkali metal ions are preferred and potassium ions are particularly preferred.

[0044] Examples of anions may include Cl - Br - ,I - , ClO - 、ClO2 - 、ClO3 - 、ClO4 - 、NO2 - 、NO3 - 、SO4 2- 、CO3 2- 、HCO3 - HCOO - ,(COO - )2.COOH(COO- )、CH3COO - 、C2H4(COO - 2. C6H5COO - 、C6H4(COO - 2. PO4 3- 、HPO4 2- and H2PO4 - .

[0045] When a monovalent cation is represented by M, examples of salts formed by the combination of a cation and an anion may include MCl, MBr, MI, MClO, MClO2, MClO3, MClO4, MNO2, MNO3, M2SO4, M2CO3, MHCO3, HCOOM, (COOM)2, COOH (COOM), CH3COOM, C2H4 (COOM)2, C6H5COOM, C6H4 (COOM)2, M3PO4, M2HPO4, and MH2PO4. Among them, potassium chloride, sodium acetate, sodium benzoate, potassium benzoate, ammonium benzoate, trisodium citrate, potassium phthalate, ammonium phthalate, and the like are preferred, and potassium phthalate is particularly preferred. Compared with any other salt, the inclusion of potassium phthalate can improve the jetting stability of the ink.

[0046] The content (mass %) of the salt in the ink is preferably 0.05 mass % or more and 1.0 mass % or less, and more preferably 0.1 mass % or more and 0.5 mass % or less, relative to the total mass of the ink.

[0047] (Aqueous medium)

[0048] The ink is an aqueous ink containing at least water as an aqueous medium. As an aqueous medium, a water-soluble organic solvent can be further introduced into the ink. As water, deionized water or ion-exchanged water is preferably used. Relative to the total mass of the ink, the content (mass %) of water in the ink is preferably 10.0 mass % or more and 90.0 mass % or less, more preferably 50.0 mass % or more and 90.0 mass % or less. In addition, as a water-soluble organic solvent, any one commonly used in ink can be used. Examples thereof may include alcohols, (poly) alkylene glycols, glycol ethers, nitrogen-containing compounds and sulfur-containing compounds. Relative to the total mass of the ink, the content (mass %) of the water-soluble organic solvent in the ink is preferably 3.0 mass % or more and 50.0 mass % or less.

[0049] (Other additives)

[0050] In addition to the above components, the ink may also contain a water-soluble organic compound that is solid at room temperature (25°C), for example, a polyol such as trimethylolpropane or trimethylolethane, or a urea derivative such as urea or ethylene urea, as required. In addition, the ink may contain various additives, for example, a water-soluble resin such as an acrylic resin or a polyurethane resin, a surfactant, a pH adjuster, a rust inhibitor, a preservative, an antifungal agent, an antioxidant, an anti-reducing agent, an evaporation promoter, and a chelating agent, as required. The content (mass %) of any such additive in the ink is preferably 0.05% by mass or more and 10.0% by mass or less, more preferably 0.2% by mass or more and 5.0% by mass or less, relative to the total mass of the ink. However, when a water-soluble acrylic resin is used, it is preferred not to set its content to a very large value. The content (mass %) of the water-soluble acrylic resin in the ink is preferably 0.1% by mass or more and 1.5% by mass or less, more preferably 0.1% by mass or more and 1.1% by mass or less, relative to the total mass of the ink. In addition, the water-soluble polyurethane resin is effective in improving the scratch resistance of the image. The content (mass %) of the water-soluble polyurethane resin in the ink is preferably 0.1 mass % or more and 5.0 mass % or less, more preferably 0.1 mass % or more and 2.0 mass % or less, relative to the total mass of the ink.

[0051] (Physical Properties of Ink)

[0052] The dynamic surface tension of the ink when the life time is 10ms (milliseconds) is preferably 40mN / m or more, more preferably 45mN / m or more. When the dynamic surface tension of the ink when the life time is 10ms is 40mN / m or more, the penetration of the ink along the thickness direction of the recording medium is moderately suppressed, so that an image with better color development can be recorded. The dynamic surface tension of the ink when the life time is 10ms is preferably 50mN / m or less. The dynamic surface tension of the ink can be easily controlled by appropriately setting the type and content of the surfactant and the water-soluble organic solvent to be introduced into the ink. In order to set the dynamic surface tension of the ink when the life time is 10ms to 40mN / m or more, it is preferred not to use a water-soluble organic solvent with a low surface tension, or, if used, not to set its content to a larger value.

[0053] The dynamic surface tension of the ink when the life time is 10ms can be measured by the maximum bubble pressure method. The maximum bubble pressure method is a method comprising: measuring the maximum pressure required to discharge the bubble generated at the front end of a probe (thin tube) immersed in the liquid to be measured; and finding the surface tension of the liquid from the maximum pressure. The life time is the time from the time point when a new bubble surface is generated at the front end of the probe to the time point when the bubble reaches the maximum bubble pressure (the time point when the radius of curvature of the bubble and the radius of the front end portion of the probe become equal to each other) in the maximum bubble pressure method. Herein, the dynamic surface tension of the ink is a value measured at 25°C.

[0054] The viscosity of the ink at 25° C. is preferably 1.0 mPa·s or more and 10.0 mPa·s or less, more preferably 1.0 mPa·s or more and 5.0 mPa·s or less. The static surface tension of the ink at 25° C. is preferably 30 mN / m or more and 45 mN / m or less. The pH of the ink at 25° C. is preferably 5.0 or more and 10.0 or less.

[0055] <Ink Cartridges>

[0056] The ink cartridge of the present invention includes ink and an ink storage portion configured to store the ink. In addition, the ink stored in the ink storage portion is the above-mentioned aqueous ink of the present invention. Figure 1 Schematically shows a cross-sectional view of an embodiment of an ink cartridge according to the present invention. Figure 1 As shown in , an ink supply port 12 for supplying ink to a recording head is provided on the bottom surface of the ink cartridge. The interior of the ink cartridge is an ink storage portion for storing ink. The ink storage portion includes an ink storage chamber 14 and an absorber storage chamber 16, and the chambers are connected to each other through a communication port 18. In addition, the absorber storage chamber 16 is connected to the ink supply port 12. Liquid ink 20 is stored in the ink storage chamber 14, and absorbers 22 and 24, each of which holds ink in a state impregnated with ink, are stored in the absorber storage chamber 16. The ink storage portion may be in a form that does not have any ink storage chamber configured to store liquid ink and is configured to hold the total amount of ink to be stored with an absorber. In addition, the ink storage portion may be in a form that does not have any absorber and is configured to store the total amount of ink in a liquid state. In addition, an ink cartridge configured to include an ink storage portion and a recording head may be adopted.

[0057] <Inkjet Recording Method>

[0058] The inkjet recording method of the present invention is a method comprising ejecting the above-mentioned aqueous ink of the present invention from the recording head of the inkjet system to record an image on a recording medium. The system for ejecting ink is, for example, a system for imparting mechanical energy to the ink or a system for imparting thermal energy to the ink. In the present invention, it is particularly preferred to adopt a system for imparting thermal energy to the ink to eject the ink. In addition to using the ink of the present invention, the process of the inkjet recording method can be a known process. Even in a simple device configuration without any drying unit, etc., when the ink of the present invention is used, the bronze phenomenon originating from its carbon black is suppressed, so that an image excellent in color development can be recorded. Therefore, in the inkjet recording method of the present invention, there is no need to perform a process of drying the image with heat or wind.

[0059] Figure 2A and Figure 2B is a diagram schematically showing an example of an inkjet recording apparatus to be used for the inkjet recording method of the present invention, wherein Figure 2A is a perspective view of a main part of an inkjet recording apparatus and Figure 2B is a perspective view of the head box. Figure 2A The inkjet recording apparatus of this embodiment shown in includes a serial type recording head. However, the inkjet recording apparatus used in the inkjet recording method of the present invention is not limited to Figure 2A The inkjet recording apparatus of the embodiment shown in FIG. 1 may be an inkjet recording apparatus including a line type recording head. Figure 2A In the inkjet recording apparatus shown in , a conveying unit (not shown) and a carriage shaft 34 configured to convey a recording medium 32 are provided. A head box 36 can be mounted on the carriage shaft 34. The head box 36 includes recording heads 38 and 40 and is formed so as to set an ink cartridge 42 therein. When the head box 36 is conveyed along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, the recording medium 32 is conveyed in the sub-scanning direction by means of a conveying unit (not shown). Thus, an image is recorded on the recording medium 32. In the present invention, an inkjet recording apparatus including a serial recording head is preferably used. In addition, any recording medium can be used as a recording medium on which recording is to be performed with the ink of the present invention, and a permeable paper-based recording medium such as plain paper or a recording medium including a coating (glossy paper or coated paper) is preferably used. Among them, it is particularly preferred to use a recording medium such as plain paper that does not have any coating.

[0060] Example

[0061] The present invention is described in more detail below with reference to Examples and Comparative Examples, but the present invention is not limited in any way by the following Examples as long as the gist thereof is not exceeded. In the description of the component amounts, "parts" and "%" are based on mass unless otherwise specified.

[0062] The average particle size of the pigment (D P , cumulative 50% particle diameter based on volume) was measured using a dynamic light scattering type particle size distribution measuring apparatus (product name: "UPA-EX150", manufactured by Nikkiso Co., Ltd.) The measurement conditions at this time were set as follows: Set Zero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds.

[0063] <Preparation of Pigment Dispersion>

[0064] (Pigment dispersions 1 to 4, 6 to 8 and 10)

[0065] The solution obtained by dissolving 5.0g of concentrated hydrochloric acid in 5.5g of water was brought to a state cooled to 5°C, and then 1.6g of the treatment agent shown in Table 1 was added thereto in this state. The container containing the solution was placed in an ice bath. While the temperature of the solution was kept below 10°C by stirring the solution, a solution obtained by dissolving 1.8g of sodium nitrite in 9.0g of 5°C ion exchange water was added thereto. After the mixture was stirred for 15 minutes, 6.0g of carbon black having the characteristics shown in Table 1 was added thereto under stirring, and the whole was further stirred for 15 minutes to provide a slurry. After filtering the resulting slurry with filter paper (product name: "Standard Filter Paper No. 2", manufactured by Advantec Co., Ltd.), the particles were fully washed with water and dried in an oven at 110°C. After the counter ions were replaced from sodium ions to potassium ions by an ion exchange method, an appropriate amount of ion exchange water was added to the resultant to adjust the content of the pigment. Thus, each pigment dispersion liquid having a pigment content of 10.0% was obtained. The average particle size D of the pigment P The size (nm) and the structure of the functional groups on the particle surface of the pigment are shown in Table 1.

[0066] (Pigment dispersion 5)

[0067] Carbon black (specific surface area: 260m 2 / g, DBP oil absorption: 140mL / 100g) was added to ion exchange water and the mixture was stirred thoroughly. An appropriate amount of sodium hypochlorite (effective chlorine concentration: 4%) was added dropwise to the mixture, and the whole was stirred at 100°C for 10 hours to react. After the reaction was completed, the resultant was purified by ultrafiltration and its pH was adjusted to 7.5 using potassium hydroxide. In addition, an appropriate amount of ion exchange water was added to the resultant to adjust the content of the pigment. Thus, a pigment dispersion 5 having a pigment content of 10.0% was obtained. The average particle size D of the pigment P The pigment is a self-dispersible pigment in which -COOK groups are bonded to the particle surface of carbon black.

[0068] (Pigment dispersion 9)

[0069] By mixing 15.0 parts of carbon black (specific surface area: 260 m 2 / g, DBP oil absorption: 140 mL / 100 g), 30.0 parts of an aqueous solution of a resin dispersant and 55.0 parts of ion exchange water are mixed to provide a mixture. As an aqueous solution of a resin dispersant, an aqueous solution having a resin content of 20.0% is used by dissolving a styrene / acrylic acid copolymer as a water-soluble resin in ion exchange water using sodium hydroxide in a molar amount equal to the acid value. The composition (molar) ratio between styrene and acrylic acid of the styrene / acrylic acid copolymer is 33:67, the weight average molecular weight is 10,000 and the acid value is 200 mg KOH / g. The resulting mixture is loaded into a sand mill and dispersed for 1 hour. After removing coarse particles by centrifugation, pressure filtration is performed with a microfilter (manufactured by FUJIFILM Corporation) having a pore size of 3.0 μm. Thereafter, an appropriate amount of ion exchange water is added to the filtrate to adjust the content of the pigment. Thus, a pigment dispersion 9 having a pigment content of 10.0% and a resin dispersant content of 6.0% is obtained. The average particle size D of the pigment P It is 115nm.

[0070] (Pigment dispersion 11)

[0071] Into a glass container having a filling rate of 50% of zirconium oxide beads each having a diameter of 0.3 mm, 15.0 parts of carbon black (product name: "MONARCH 1100" (manufactured by Cabot Corporation)), 30.0 parts of a 25.0% aqueous solution of a resin dispersant, and 50.0 parts of ion exchange water were charged. As the aqueous solution of the resin dispersant, an aqueous solution obtained by neutralizing a styrene / acrylic acid copolymer (product name: "JONCRYL 690", manufactured by BASF Corporation) as a water-soluble resin with potassium hydroxide at a molar ratio of 0.85 times relative to the acid group was used. The weight average molecular weight of the styrene / acrylic acid copolymer was 16,500 and the acid value was 240 mg KOH / g. The contents of the above glass container were mixed for 15 hours with a simple disperser (product name: "DAS200-K", manufactured by LAU Corporation) to disperse the carbon black. The resultant was centrifuged at a rotation number of 5,000 rpm for 30 minutes to remove the aggregated components. Thereafter, an appropriate amount of ion exchange water was added to the residue to adjust the content of the pigment. Thus, a pigment dispersion 11 having a pigment content of 15.0% and a resin dispersant content of 7.5% was obtained. The average particle size D of the pigment P It is 80nm.

[0072] Table 1: Preparation conditions and characteristics of pigment dispersions

[0073]

[0074] <Preparation of resin particles>

[0075] (Measurement conditions of physical properties)

[0076] The glass transition temperature of the resin particles is measured according to the following procedure. First, a product obtained by heating a dispersion of the resin particles to 60°C to dry and solidify the dispersion is sealed in an aluminum container to provide a sample. Next, the glass transition temperature (°C) is measured with a differential scanning calorimeter (product name: "DSC Q1000", manufactured by TA Instruments Corporation) as follows: the temperature of the sample is increased to 200°C at 10°C / min; the temperature is decreased to -50°C at 5°C / min; and the temperature is increased to 200°C at 10°C / min. In addition, the average particle size (D R , cumulative 50% particle diameter based on volume) was measured with a dynamic light scattering type particle size distribution measuring apparatus (product name: "UPA-EX150", manufactured by Nikkiso Co., Ltd.). The measurement conditions at this time were set as follows: Set Zero: 30 seconds, number of measurements: 3 times, and measurement time: 180 seconds.

[0077] (Resin particles 1 to 16 and 18)

[0078] The amount of ion exchange water shown in Table 2 and 0.1 part of potassium persulfate are mixed under a nitrogen atmosphere to provide a solution. An emulsion of the monomers shown in Table 2 in the type and amount is added dropwise to the solution, and the mixture is subjected to a polymerization reaction at 80° C. under stirring. After cooling to 25° C., potassium hydroxide in a molar amount equal to the acid value of the resin and an appropriate amount of ion exchange water are added thereto to provide a dispersion of resin particles having a content of 20.0% of the resin particles. The properties of the resin particles in the resulting dispersion are shown in Table 2. The abbreviations of the monomers in Table 2 have the following meanings: MMA: methyl methacrylate; BMA: n-butyl methacrylate; EMA: ethyl methacrylate; MAA: methacrylic acid; AA: acrylic acid; BDDMA: 1,4-butanediol dimethacrylate; and KH-05: reactive surfactant (product name: "AQUALON KH-05", manufactured by DKS Co., Ltd.). The reactive surfactant (product name: "AQUALON KH-05", manufactured by DKS Co., Ltd.) is a surfactant having a structure obtained by introducing an allyl group (polymerizable functional group) into the basic skeleton of a polyoxyethylene alkyl ether sulfate.

[0079] (resin particles 17)

[0080] A dispersion of resin particles 17 having a content of 20.0% of resin particles was obtained by using wax (resin particles) which is a copolymer of 1-octene and maleic anhydride. The glass transition temperature Tg of the resin particles 17 was 80° C. and the average particle diameter was 170 nm. The properties of the resin particles in the obtained dispersion are shown in Table 2.

[0081] (resin particles 19)

[0082] In 40.0 parts of ethyl acetate, 2.0 parts of CI solvent blue 70 (product name: "Orasol Blue 855", manufactured by BASF Corporation) and 8.0 parts of styrene-acrylic acid copolymer (product name: "JONCRYL 611", manufactured by BASF Corporation) are dissolved to provide a solution. The resulting solution is added to a solution obtained by dispersing 0.15 parts of sodium lauryl sulfate in 90.0 parts of ion exchange water, and the mixture is stirred. The mixture is emulsified for 10 minutes with an amplitude of 50% using an ultrasonic homogenizer (product name: "Advanced Digital Sonifier 250DA", manufactured by Branson Corporation) to provide an emulsion. Ethyl acetate is distilled off from the resulting emulsion with an evaporator. After standing and cooling, pressure filtration is performed with a filter having a pore size of 1.2 μm (product name: "HDCII", manufactured by Pall Corporation), and then an appropriate amount of ion exchange water is added to the filtrate. Thus, a dispersion of resin particles 19 having a content of 10.0% of resin particles is obtained. The encapsulation rate of the blue dye in the resin particles 19 was 20.0%. Table 2 shows the properties of the resin particles in the obtained dispersion.

[0083] (resin particles 20)

[0084] A dispersion of resin particles 20 was obtained in the same manner as in the case of the above-mentioned resin particles 19, except that CI fluorescent whitening agent 184 (product name: "Uvitex OB", manufactured by BASF Corporation) was used instead of CI Solvent Blue 70. The content of the resin particles in the obtained dispersion was 10.0% and the encapsulation rate of the fluorescent whitening agent (CI fluorescent whitening agent 184) in the resin particles 20 was 20.0%. The properties of the resin particles in the obtained dispersion are shown in Table 2.

[0085] Table 2: Synthesis conditions and characteristics of resin particles

[0086]

[0087] <Preparation of ink>

[0088] The components (unit: %) shown in the upper section of Table 3 are mixed and the mixture is stirred thoroughly to disperse. Thereafter, pressure filtration is performed with a polypropylene filter (manufactured by Pall Corporation) having a pore size of 2.5 μm to prepare each ink. The numerical value attached to the term "polyethylene glycol" in Table 3 is the number average molecular weight and the term "ACETYLENOL E100" is the product name of a nonionic surfactant manufactured by Kawaken Fine Chemicals Co., Ltd. The characteristics of each prepared ink are shown in the lower section of Table 3. Regarding the dynamic surface tension of each ink, the value at a life time of 10 ms is measured using a dynamic surface tension measuring device based on the maximum bubble pressure method (product name: "bubble pressure tensiometer BP2", manufactured by KRUSS Corporation).

[0089]

[0090] <Evaluation>

[0091] The following items were evaluated by using the prepared ink. An inkjet recording device (product name: "GX6030", manufactured by Canon Inc.) including a recording head for ejecting liquid by the action of thermal energy was used for image recording. In this embodiment, the recording task of a solid image recorded under the following conditions was defined as 100%: two ink droplets each having a mass of 11.7ng±10% per drop were given to a unit area of ​​1 / 600 inch×1 / 600 inch in size. In the present invention, in the evaluation criteria of the following items, levels "AA", "A" and "B" were defined as acceptable levels, while level "C" was defined as an unacceptable level. The evaluation results are shown in Table 4.

[0092] (Color rendering)

[0093] Solid images each having a recording duty of 100% and each having a size of 2 cm×2 cm were recorded on 3 recording media (plain paper, product name: "CS-064F A4", manufactured by Canon Inc.). After 1 day had passed, the optical density of the solid image was measured with a fluorescence spectrophotometer (product name: "FD-7", manufactured by Konica Minolta, Inc.) under the conditions of illumination of M1 (D50), observation light source of D50, and field of view of 2°. Then, the average value of the optical density of the images recorded on the 3 recording media was calculated, and the color development of the image was evaluated according to the following evaluation criteria.

[0094] AA: The average value of the optical density is 1.46 or more.

[0095] A: The average value of the optical density is 1.43 or more and less than 1.46.

[0096] B: The average value of the optical density is 1.40 or more and less than 1.43.

[0097] C: The average value of the optical density is less than 1.40.

[0098] (Bronze resistance)

[0099] 10 kinds of solid images each having a size of 2 cm×2 cm, in which the recording duty was changed from 10% to 100% in increments of 10%, were recorded on a recording medium (plain paper, product name: "CS-064F A4", manufactured by Canon Inc.). After 1 day had passed, the image showed a red to yellow hue, that is, the recording duty of the solid image where the bronze phenomenon occurred, and then the evaluation of the bronzing resistance of each image was performed according to the following evaluation criteria. In general, as the recording duty becomes higher, the amount of reflected light from the image increases, and thus the bronze phenomenon is more likely to occur. In other words, a higher recording duty where the bronze phenomenon occurs means that the image is more excellent in bronzing resistance.

[0100] A: No bronze phenomenon occurs regardless of the recording task of the solid image.

[0101] B: A bronze phenomenon occurs in solid images where the recording duties are each 70% or more.

[0102] C: Bronzing occurs in solid images where the recording duties are each less than 70%.

[0103] (Jet stability)

[0104] Solid images each having a recording duty of 100% and each having a size of 2 cm×2 cm were recorded on 10 sheets of recording medium (plain paper, product name: "CS-064F A4", manufactured by Canon Inc.). Thereafter, a nozzle check pattern of GX6030 was recorded on the same recording medium. Next, a solid image was recorded on 3,000 sheets under the same conditions, and then a nozzle check pattern was recorded again. The nozzle check pattern after 10 sheets of recording and the nozzle check pattern after 3,000 sheets of recording were compared with each other, and then the ejection stability of each ink was evaluated according to the following evaluation criteria.

[0105] A: Each nozzle check pattern after recording 10 sheets and after recording 3,000 sheets was recorded normally.

[0106] B: Although the nozzle check pattern after 10-sheet recording was recorded normally, there was noise in the nozzle check pattern after 3,000-sheet recording.

[0107] C: Noise is present in each nozzle check pattern after recording 10 sheets and after recording 3,000 sheets.

[0108] Table 4: Evaluation results

[0109]

[0110] According to the present invention, an aqueous ink for inkjet that can record an image excellent in color development by suppressing a bronzing phenomenon derived from carbon black can be provided. In addition, according to the present invention, an ink cartridge and an inkjet recording method each using the aqueous ink can be provided.

[0111] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments.The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims

1. A water-based ink for inkjet, comprising self-dispersible carbon black and resin particles formed of an acrylic resin, characterized in that: The DBP oil absorption of the carbon black is 120 mL / 100 g or more, The resin particles have (i) a structure cross-linked by units derived from a multifunctional (meth)acrylate and (ii) a glass transition temperature of 30° C. or higher, The mass ratio of the content of the resin particles in mass % to the content of the carbon black in mass % is 0.10 times or more and 2.0 times or less, and The carbon black is a self-dispersible pigment in which anionic groups are bonded to the particle surfaces of the carbon black via other atomic groups. 2 . The aqueous ink according to claim 1 , wherein a mass ratio of a content in mass % of the resin particles to a content in mass % of the carbon black is 0.20 times or more and 1.0 times or less. 3 . The aqueous ink according to claim 1 , wherein the carbon black is a self-dispersible pigment in which a carboxylic acid group is bonded to a particle surface of the carbon black via other atomic groups. 4 . The aqueous ink according to claim 1 , wherein a cumulative 50% particle size on a volume basis of the resin particles is 50 nm or more and 300 nm or less. 5 . The aqueous ink according to claim 1 , wherein a ratio of a cumulative 50% particle size based on volume of the resin particles to a cumulative 50% particle size based on volume of the carbon black is 0.50 times or more and 2.0 times or less. The aqueous ink according to claim 1 , wherein the acrylic resin has a unit derived from a reactive surfactant. 7 . The aqueous ink according to claim 1 , wherein the dynamic surface tension of the aqueous ink at a life time of 10 ms is 40 mN / m or more. The aqueous ink according to claim 1 , further comprising potassium phthalate.

9. An ink cartridge comprising ink and an ink storage portion configured to store the ink, It is characterized in that The ink comprises the aqueous ink according to any one of claims 1 to 8.

10. An inkjet recording method comprising ejecting ink from a recording head of an inkjet system to record an image on a recording medium, It is characterized in that The ink comprises the aqueous ink according to any one of claims 1 to 8.

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