Aqueous ink composition, method for producing same and recording method

Abstract

The present invention addresses the problem of providing: an aqueous ink composition which is capable of achieving a good balance among fastness, adhesion and optical density of a coating film; and the like. The problem is able to be solved by means of an aqueous ink composition which contains nanocellulose, wherein the nanocellulose contains an oxide of a cellulose starting material obtained by means of hypochlorous acid or a salt thereof, but does not substantially contain an N-oxyl compound.

Description

Water-based ink composition, its manufacturing method and recording method

[0001] The present invention relates to an aqueous ink composition, a method for producing the same, and a recording method.

[0002] In recent years, aqueous ink compositions (hereinafter sometimes referred to as "aqueous inks") have been replacing organic solvent-based inks in a wide range of fields. This is due to their safety and ability to reduce environmental impact, as well as the need to prevent organic solvents from contaminating food and other products when using inks for laminating flexible packaging, and the demand for reduced odor aqueous inks that can be used in closed spaces such as offices.

[0003] In recent years, research has been conducted into the use of plant fibers as a component of aqueous inks. Because plant fibers leave almost no ash behind when burned, problems such as ash disposal in incinerators and landfill disposal do not arise. In particular, research has been conducted into the use of nanocellulose, which is plant fiber defibrated to the nano level. Nanocellulose is cellulose that has been uniformly pulverized (nanofiberized) to the nano level, and is known to be added to pigment compositions to impart good rheological properties (see, for example, Patent Document 1).

[0004] Aqueous ink compositions are known that ensure excellent dispersion stability even when using colorants with high specific gravity by incorporating nanocellulose into the aqueous ink (see, for example, Patent Document 2). The cellulose fibers contained in the aqueous ink composition of Patent Document 2 are cellulose fibers with a number-average fiber diameter of 2 to 150 nm, and the cellulose has a cellulose type I crystal structure, and the hydroxyl group at the C6 position of each glucose unit in the cellulose molecule has been selectively oxidized and modified to form either an aldehyde group, a ketone group, or a carboxy group. The carboxy group content is 0.6 to 2.0 mmol / g, and the total content of aldehyde and ketone groups measured by the semicarbazide method is 0.3 mmol / g or less. The aqueous ink composition of Patent Document 2 is said to have excellent dispersion stability over a long period of time.

[0005] Patent Document 3 discloses an aqueous pigment dispersion comprising at least a pigment, an anionic group-containing organic polymer compound, and bionanofibers having an average diameter of 1 nm or more and 100 nm or less and an aspect ratio of 100 or more. The aqueous pigment dispersion of Patent Document 3 is said to produce images with excellent optical density and abrasion resistance.

[0006] JP-T-2013-527876A JP-A-2013-181167A WO 2015 / 166808

[0007] The microfibrillated cellulose used in the pigment-containing composition of Patent Document 1 is produced by treating cellulosic fibers with an enzyme. Because an enzyme is used, there is a problem in efficiently and stably supplying the microfibrillated cellulose.

[0008] The cellulose fibers contained in the aqueous ink composition of Patent Document 2 are obtained by oxidation using a co-oxidant in the presence of an N-oxyl compound such as TEMPO, and are then subjected to reduction, purification, and micronization treatments. The cellulose fibers obtained in this manner have low production efficiency, which makes it difficult to efficiently obtain an aqueous ink composition using them.

[0009] The bionanofibers contained in the aqueous pigment dispersion of Patent Document 3 are specifically nanocellulose obtained by mechanical defibration, and since the production of nanocellulose requires energy, there is a problem in that an aqueous ink composition cannot be obtained efficiently.

[0010] Furthermore, the ink composition is required to have functionality such as robustness, adhesion, and optical density in the coating film obtained from the composition.

[0011] The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide an aqueous ink composition that can achieve a good balance between the fastness, adhesion, and optical density of a coating film, and an efficient method for producing the same.

[0012] As a result of intensive research aimed at solving the above problems, the inventors discovered that by using a specific nanocellulose, it is possible to efficiently obtain an aqueous ink composition that can achieve both robustness, adhesion, and optical density of the coating film, and thus completed the present invention. Specifically, the present invention provides the following means.

[0013] [1] An aqueous ink composition containing nanocellulose, wherein the nanocellulose comprises an oxide of a cellulose-based raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds. [2] The aqueous ink composition according to [1], further comprising an anionic group-containing organic polymer compound. [3] The aqueous ink composition according to [1] or [2], wherein at least a portion of the carboxy groups of the nanocellulose are modified with at least one selected from the group consisting of metals, ammonia, amines, and quaternary ammonium. [4] The aqueous ink composition according to any one of [1] to [3], which is used for inkjet recording. [5] A recording method comprising a step of ejecting the aqueous ink composition according to any one of [1] to [3] onto a recording member by an inkjet system. [6] A method for producing an aqueous ink composition containing nanocellulose, comprising the step of defibrating the oxidized cellulose by stirring a mixture containing oxidized cellulose and materials other than the nanocellulose of the aqueous ink composition to obtain an aqueous ink composition containing nanocellulose, wherein the oxidized cellulose comprises an oxide of a cellulose-based raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds. [7] A method for producing an aqueous ink composition containing nanocellulose, comprising the step of defibrating the oxidized cellulose by stirring the oxidized cellulose and continuously mixing with materials other than the nanocellulose of the aqueous ink composition to obtain an aqueous ink composition containing nanocellulose, wherein the oxidized cellulose comprises an oxide of a cellulose-based raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds. [8] The method according to [6] or [7], wherein the material further comprises an anionic group-containing organic polymer compound. [9] The production method according to any one of [6] to [8], wherein at least a portion of the carboxy groups of the oxidized cellulose are modified with at least one selected from the group consisting of metals, ammonia, amines, and quaternary ammonium.

[0014] According to the present invention, it is possible to provide an aqueous ink composition that can achieve a good balance between coating film fastness, adhesion, and optical density, and an efficient method for producing the same.

[0015] Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present invention.

[0016] [Aqueous Ink Composition] The aqueous ink composition of the present invention contains nanocellulose. The nanocellulose contained in the aqueous ink composition of the present invention contains an oxide of a cellulose-based raw material (oxidized nanocellulose) produced by hypochlorous acid or its salt, and is substantially free of N-oxyl compounds. The aqueous ink composition of the present invention has color development (optical density) and abrasion resistance for images formed with the aqueous ink composition. The mechanism is presumed to be as follows, but is not limited to the following. The glucose units that make up cellulose adopt a chair conformation, so that hydroxyl groups are aligned horizontally relative to the glucopyranose ring, and only C and H are arranged vertically. As a result, cellulose is hydrophobic in the vertical direction and hydrophilic in the horizontal direction. Nano-sizing increases the surface area, further enhancing this tendency. Therefore, in aqueous ink compositions, the nanocellulose-formed complex with pigment particles and the anionic group-containing organic polymer compound described below reduces the interfacial tension between components such as the pigment particles and the anionic group-containing organic polymer compound, controls the charge on the pigment particle surface, and imparts a repulsive force capable of overcoming the van der Waals forces between pigment particles, achieving a high level of balance essential for stable pigment dispersion. Therefore, the pigment particles can be fixed without unnecessary penetration into the printed material, resulting in high optical density and preventing adjacent ink colors from mixing in multicolor printing. Furthermore, the nanocellulose contained in the present invention has excellent binder properties and provides excellent abrasion resistance, such as coating film robustness and adhesion. Furthermore, the nanocellulose of the present invention can be easily obtained by defibrating oxidized cellulose oxidized with hypochlorous acid or its salt. Nano-sized nanocellulose can be obtained efficiently without requiring a large energy load, such as with a high-pressure homogenizer. Therefore, the aqueous ink composition of the present invention can also be obtained efficiently.

[0017] <Nanocellulose> Nanocellulose in the present invention is nano-sized oxidized cellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or its salt. Here, the oxidized cellulose can also be said to be an oxide of the cellulosic raw material. Therefore, nanocellulose in the present invention includes an oxide of the cellulosic raw material with hypochlorous acid or its salt. Note that the main component of plants is cellulose, and bundles of cellulose molecules are called cellulose microfibrils. The cellulose in cellulosic raw materials is also contained in the form of cellulose microfibrils. Nanocellulose in the present invention is a general term for nano-sized cellulose, including fine cellulose fibers and cellulose nanocrystals, as well as modified forms of these (details of the modified forms will be described later). Fine cellulose fibers are also called cellulose nanofibers (also referred to as CNF).

[0018] The aqueous ink composition of the present invention contains nanocellulose, but it can also be produced by blending oxidized cellulose that has been defibrated and nanosized, or by using oxidized cellulose as a raw material during the preparation of the aqueous ink composition and nanoizing the oxidized cellulose during the preparation. Thus, the nanocellulose in the aqueous ink composition has been nanosized at an appropriate time.

[0019] In the production of nanocellulose in the present invention, N-oxyl compounds such as TEMPO are not used in the process of oxidizing cellulosic raw materials with hypochlorous acid or its salts. Therefore, the nanocellulose in the present invention is substantially free of N-oxyl compounds. Therefore, nanocellulose has a high level of safety as the impact of N-oxyl compounds on the environment and human body is sufficiently reduced, making it suitable for applications such as cosmetics, foods, and medical products. Here, in this specification, nanocellulose (or oxidized cellulose) being "substantially free of N-oxyl compounds" means that the nanocellulose contains no N-oxyl compounds at all, or that the content of N-oxyl compounds is 2.0 ppm by mass or less, preferably 1.0 ppm by mass or less, relative to the total amount of nanocellulose. Furthermore, when the content of N-oxyl compounds is increased from the cellulosic raw material by preferably 2.0 ppm by mass or less, more preferably 1.0 ppm by mass or less, this also means that the nanocellulose is "substantially free of N-oxyl compounds." The content of N-oxyl compounds can be measured by known means. A known method is to use a trace total nitrogen analyzer (for example, manufactured by Nitto Seiko Analytech Co., Ltd., device name: TN-2100H).

[0020] The oxidized cellulose has excellent defibration properties. In particular, the oxidized cellulose can be uniformly pulverized even when defibrated under mild conditions, demonstrating excellent defibration properties. Furthermore, when mixed with the nanocellulose to form a slurry, the slurry viscosity remains stable over time and is easy to handle.

[0021] (Carboxy group amount) The carboxy group amount of nanocellulose and oxidized cellulose is preferably 0.20 to 2.0 mmol / g. When the carboxy group amount is 0.20 mmol / g or more, it is possible to impart sufficient easy defibration properties to the oxidized cellulose. This makes it possible to obtain a nanocellulose-containing slurry of uniform quality even when defibration treatment is performed under mild conditions, and improves the viscosity stability and handleability of the slurry. On the other hand, when the carboxy group amount is 2.0 mmol / g or less, excessive decomposition of cellulose during defibration treatment can be suppressed, and nanocellulose of uniform quality with a low proportion of particulate cellulose can be obtained. This is thought to improve dispersibility and increase the strength and water resistance of coating films containing nanocellulose. From this perspective, the amount of carboxy groups in nanocellulose and oxidized cellulose is more preferably 0.30 mmol / g or more, even more preferably 0.35 mmol / g or more, even more preferably 0.40 mmol / g or more, even more preferably 0.42 mmol / g or more, even more preferably 0.50 mmol / g or more, even more preferably more than 0.50 mmol / g, and even more preferably 0.55 mmol / g or more. The upper limit of the carboxy group amount may be less than 2.0 mmol / g, 1.5 mmol / g or less, 1.2 mmol / g or less, 1.0 mmol / g or less, or 0.9 mmol / g or less. The preferred range of the carboxy group amount can be determined by appropriately combining the above-mentioned upper and lower limits. The amount of carboxy groups in nanocellulose is more preferably 0.30 mmol / g or more and less than 2.0 mmol / g, even more preferably 0.35 to 2.0 mmol / g, even more preferably 0.35 to 1.5 mmol / g, even more preferably 0.40 to 1.5 mmol / g, even more preferably 0.50 to 1.2 mmol / g, even more preferably more than 0.50 to 1.2 mmol / g, and even more preferably 0.55 to 1.0 mmol / g.

[0022] The amount of carboxy groups (mmol / g) was calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of weak acid, where the change in electrical conductivity is gradual, after adding 0.1 M aqueous hydrochloric acid to an aqueous solution of oxidized cellulose mixed with water to adjust the pH to 2.5, and then adding 0.05 N aqueous sodium hydroxide dropwise and measuring the electrical conductivity until the pH reaches 11.0. Details are given in accordance with the method described in the Examples below. The amount of carboxy groups can be adjusted by changing the reaction time, reaction temperature, pH of the reaction solution, etc. of the oxidation reaction. Amount of carboxy groups = a (ml) x 0.05 / mass of oxidized cellulose (g)

[0023] The above-mentioned oxidized cellulose can be obtained, for example, by oxidizing a cellulosic raw material under conditions in which the available chlorine concentration of hypochlorous acid or a salt thereof in the reaction system is relatively high (e.g., 6% to 43% by mass). The oxidized cellulose of the present invention can also be produced by appropriately controlling the reaction conditions, such as the available chlorine concentration, the pH during the reaction, and the reaction temperature. The oxidized cellulose thus obtained preferably has a structure in which at least two of the hydroxyl groups on the glucopyranose ring constituting the cellulose are oxidized, and more specifically, preferably has a structure in which the hydroxyl groups at the second and third positions on the glucopyranose ring are oxidized and dicarboxy groups are introduced. Furthermore, it is preferable that the hydroxyl group at the sixth position on the glucopyranose ring in the oxidized cellulose is not oxidized and remains as a hydroxyl group. The position of the carboxyl group on the glucopyranose ring of oxidized cellulose is determined by the solid state. 13 It can be analyzed by C-NMR spectrum.

[0024] Rayon has the same chemical structure as cellulose, and its oxide (rayon oxide) is water-soluble. １３By performing C-NMR measurement, a carbon peak assigned to the carboxy group is observed at 165 to 185 ppm. In one embodiment of the oxidized cellulose or nanocellulose used in the present invention, obtained by oxidizing a cellulosic raw material with hypochlorous acid or its salt, two signals appear in this chemical shift range. Furthermore, by solution two-dimensional NMR measurement, it can be determined that the carboxy groups are introduced at the 2- and 3-positions.

[0025] Oxidized cellulose or nanocellulose solid obtained by oxidizing cellulosic raw materials with hypochlorous acid or its salts １３ In C-NMR, when the amount of carboxyl groups introduced is large, two signals appear at 165 to 185 ppm, and when the amount of carboxyl groups introduced is small, a very broad signal may appear. As can be seen from the results for oxidized rayon, the signals of the carboxyl group carbons introduced at the 2nd and 3rd positions are close to each other, and the signals are not clearly visible in solid state analysis with low resolution. １３ In C-NMR, the separation of the two signals is insufficient. Therefore, when the amount of carboxyl group introduced is small, a broad signal is observed. １３ In the C-NMR spectrum, the introduction of carboxy groups at the 2nd and 3rd positions can be confirmed by evaluating the broadening of the peak appearing at 165 to 185 ppm. １３A baseline is drawn around the peak in the range of 165 ppm to 185 ppm in a C-NMR spectrum to determine the overall area value, and then the area value is vertically divided at the peak top to determine the ratio of the two peak area values ​​(larger area value / smaller area value). If this peak area value ratio is 1.2 or greater, the peak is considered to be broad. The presence or absence of a broad peak can be determined by the ratio of the length L of the baseline in the range of 165 ppm to 185 ppm to the length L' of the perpendicular line from the peak top to the baseline. That is, if the ratio L' / L is 0.1 or greater, it can be determined that a broad peak is present. The ratio L' / L may be 0.2 or greater, 0.3 or greater, 0.4 or greater, or 0.5 or greater. The upper limit of the ratio L' / L is not particularly limited, but it is usually 3.0 or less, or may be 2.0 or less, or 1.0 or less. The structure of the glucopyranose ring can also be determined by analysis according to the method described in Sustainable Chem. Eng. 2020, 8, 48, 17800-17806.

[0026] The nanocellulose of the present invention is a collection of individual fibers. When the nanocellulose of the present invention contains carboxylated nanocellulose, it is sufficient that it contains at least one carboxylated nanocellulose, and it is preferable that the carboxylated nanocellulose is the main component. Here, carboxylated nanocellulose being the main component means that the proportion of carboxylated nanocellulose in the total amount of nanocellulose exceeds 50% by mass, preferably exceeds 70% by mass, and more preferably exceeds 80% by mass. The upper limit of the above proportion is 100% by mass, but it may also be 98% by mass or 95% by mass.

[0027] The average fiber length of nanocellulose is preferably 50 to 2000 nm, more preferably 100 to 1000 nm, even more preferably 100 to 700 nm, even more preferably 100 to 500 nm, and even more preferably 100 to 400 nm. If the average fiber length exceeds 2000 nm, the slurry will become significantly thickened and difficult to handle. Also, if the average fiber length is less than 50 nm, the viscosity characteristic of CNF will be difficult to achieve.

[0028] The average fiber width of nanocellulose is preferably 1 to 200 nm, more preferably 1 to 15.0 nm, more preferably 1 to 10 nm, and even more preferably 1 to 5 nm. If the average fiber width is less than 1 nm, the strength improvement of the resin containing nanocellulose is unlikely to be achieved. Furthermore, if the average fiber width is greater than 200 nm, the ink composition may experience reduced ejection stability during inkjet printing. The average fiber width and average fiber length were calculated by mixing nanocellulose with water to achieve a nanocellulose concentration of approximately 1 to 10 ppm, air-drying the resulting diluted cellulose aqueous dispersion on a mica substrate, observing the shape of the nanocellulose using a scanning probe microscope, randomly selecting any number of fibers from the obtained image, and calculating the cross-sectional height of the shape image = fiber width and the perimeter divided by 2 = fiber length. Image processing software can be used to calculate the average fiber width and average fiber length. While the image processing conditions are arbitrary, differences in the calculated values ​​may occur even for the same image depending on the conditions. The range of difference in the value due to the conditions is preferably within ±100 nm for the average fiber length, and within ±10 nm for the average fiber width.

[0029] The nanocellulose of the present invention contains a carboxyl group, and the carboxyl group may be in the H type (—COOH) or in the salt type (—COO - X + :X +is a cation that forms a salt form), or may be modified by reacting the carboxy groups with other compounds to form covalent bonds. Thus, the aqueous ink composition of the present invention includes an embodiment containing nanocellulose in which at least a portion of the carboxy groups of the nanocellulose have been modified with a metal, ammonia, an amine, or a quaternary ammonium. The aqueous ink composition of the present invention may contain a metal, ammonia, an amine, or a quaternary ammonium added as needed during its production process. This results in at least a portion of the carboxy groups of the nanocellulose being modified with the metal, ammonia, amine, or quaternary ammonium. Furthermore, nanocellulose that has been previously modified with a metal, ammonia, amine, or quaternary ammonium may be used during the production of the aqueous ink composition of the present invention.

[0030] As used herein, "modified nanocellulose" refers to the interaction of the carboxyl groups of nanocellulose with a metal, ammonia, an amine, or a quaternary ammonium salt. The interaction is not limited to ionic or covalent bonds, as long as it is a chemical bond that interacts with the carboxyl groups. It is believed that the interaction of a metal, ammonia, an amine, or a quaternary ammonium salt compound with the carboxyl groups on the surface of nanocellulose modifies the nanocellulose, improving its affinity for components in the aqueous ink composition (e.g., pigments, anionic group-containing organic polymer compounds, etc.). Furthermore, nanocellulose modified with a metal, ammonia, an amine, or a quaternary ammonium salt compound also functions as a surfactant. This allows the use of a surfactant to be omitted in the process of dispersing the anionic group-containing organic polymer compound. The absence of a surfactant prevents foaming during drying of the resulting aqueous ink composition, which is advantageous in terms of workability.

[0031] The metal that modifies nanocellulose is not particularly limited, and examples include sodium, potassium, magnesium, lithium, silver, gold, copper, zinc, etc. When used in aqueous inkjet recording inks, potassium salts are preferred because they improve ejection stability.

[0032] The amine used to modify nanocellulose is not particularly limited and may be primary, secondary, or tertiary. The number of carbon atoms in the hydrocarbon group or aromatic group bonded to the nitrogen atom of the amine or quaternary ammonium salt compound (when two or more hydrocarbon groups or aromatic groups are bonded to the nitrogen atom, the total number of carbon atoms) is not particularly limited and may be selected from between 1 and 100 carbon atoms. From the viewpoint of miscibility, which imparts sufficient hydrophobicity to nanocellulose and makes it easy to blend with the resin component, the number of carbon atoms is preferably 3 or more, more preferably 5 or more.

[0033] In addition to hydrocarbon groups and aromatic groups, propylene oxide / ethylene oxide (PO / EO) copolymer moieties and the like can also be used as organic groups possessed by amines. These may be introduced into nanocellulose alone or in combination of two or more. From the viewpoint of improving pigment dispersibility, amines having PO / EO copolymer moieties are preferred. Amines having PO / EO copolymer moieties can be prepared according to known methods, and reference can be made to JP-A-3-181448 for the production method. Commercially available amines having PO / EO copolymer moieties are also suitable for use. Specific examples include Jeffamine M-600, Jeffamine M-1000, Jeffamine M-2005, and Jeffamine M-2070 manufactured by HUNTSMAN. These may be used alone or in combination of two or more.

[0034] The quaternary ammonium salt compound that modifies nanocellulose is not particularly limited. Specific examples of the quaternary ammonium salt compound include quaternary ammonium hydroxides such as tetrabutylammonium hydroxide, quaternary ammonium chlorides such as tetrabutylammonium chloride, quaternary ammonium bromides such as tetrabutylammonium bromide, and quaternary ammonium iodides such as tetrabutylammonium iodide.

[0035] <Method for producing nanocellulose> The nanocellulose of the present invention can be produced, for example, by a method including step A of oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof to obtain oxidized cellulose, and step B of defibrating the oxidized cellulose as needed.

[0036] (Step A: Production of oxidized cellulose) The cellulosic raw material is not particularly limited as long as it is a material primarily composed of cellulose, and examples include pulp, natural cellulose, regenerated cellulose, and fine cellulose obtained by depolymerizing cellulose through mechanical treatment. As the cellulosic raw material, commercially available products such as crystalline cellulose made from pulp can be used as is. Alternatively, unused biomass containing a large amount of cellulose components, such as soybean pulp refuse or soybean hulls, may also be used as the raw material. Furthermore, in order to facilitate the penetration of the oxidizing agent used into the raw pulp, the cellulosic raw material may be treated in advance with an alkali of an appropriate concentration.

[0037] Examples of hypochlorous acid or a salt thereof used for oxidizing a cellulosic raw material include hypochlorous acid water, sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, and ammonium hypochlorite. Among these, sodium hypochlorite is preferred from the viewpoint of ease of handling.

[0038] Methods for producing oxidized cellulose by oxidation of a cellulosic raw material include mixing the cellulosic raw material with a reaction solution containing hypochlorous acid or a salt thereof. Water is preferred as the solvent contained in the reaction solution, as it is easy to handle and unlikely to cause side reactions. While the amount of hypochlorous acid or a salt thereof used in the oxidation is not particularly limited, it is preferable to use hypochlorous acid or a salt thereof with an effective chlorine concentration of 6% by mass or more and 43% by mass or less. By using hypochlorous acid or a salt thereof with an effective chlorine concentration of 6% by mass or more and 43% by mass or less, the amount of carboxy groups in the oxidized cellulose can be sufficiently increased, allowing sufficient micronization to proceed, and eliminating the need for mechanical fiberization after the oxidation reaction. Furthermore, the effective chlorine concentration of hypochlorous acid or a salt thereof in the reaction solution (reaction system) is preferably in the range of 6 to 43% by mass.

[0039] From the viewpoint of sufficiently increasing the amount of carboxy groups in the oxidized cellulose, the available chlorine concentration of the reaction solution is more preferably 14% by mass or more, even more preferably 18% by mass or more, and even more preferably 20% by mass or more. Furthermore, from the viewpoint of suppressing excessive decomposition of cellulose during defibration, the available chlorine concentration of the reaction solution is more preferably 40% by mass or less, and even more preferably 38% by mass or less. The range of the available chlorine concentration of the reaction solution can be an appropriate combination of the above-mentioned lower and upper limits. The range of the available chlorine concentration is more preferably 16 to 43% by mass, and even more preferably 18 to 40% by mass.

[0040] From the viewpoints of reducing the cost of producing oxidized cellulose and improving productivity, such as by facilitating the availability and handling of hypochlorous acid or its salts, it is preferable to keep the effective chlorine concentration low. From this viewpoint, the upper limit of the effective chlorine concentration is preferably less than 14% by mass, more preferably 13% by mass or less, even more preferably 12% by mass or less, and still more preferably 11% by mass or less. From the viewpoints of more smoothly proceeding with the micronization of oxidized cellulose and improving productivity, the range of the effective chlorine concentration is preferably from 6% by mass to less than 14% by mass, more preferably from 7% by mass to less than 14% by mass, even more preferably from 7% by mass to 13% by mass, and still more preferably from 8% by mass to 13% by mass.

[0041] The effective chlorine concentration of hypochlorous acid or its salts is defined as follows: Hypochlorous acid is a weak acid present in an aqueous solution, and hypochlorite is a compound in which the hydrogen of hypochlorous acid is replaced by another cation. For example, sodium hypochlorite, which is a hypochlorite, exists in a solvent (preferably in an aqueous solution), so the concentration is measured as the amount of effective chlorine in the solution, not as the concentration of sodium hypochlorite. Here, with regard to the effective chlorine of sodium hypochlorite, the oxidizing power of the divalent oxygen atom generated by the decomposition of sodium hypochlorite is equivalent to two atoms of monovalent chlorine, so the bonded chlorine atoms of sodium hypochlorite (NaClO) have the same oxidizing power as two atoms of unbonded chlorine (Cl), and effective chlorine = 2 × (chlorine in NaClO). The specific measurement procedure is as follows: First, accurately weigh the sample, add water, potassium iodide, and acetic acid, and leave it, and then titrate the liberated iodine with sodium thiosulfate solution using starch aqueous solution as an indicator to measure the effective chlorine concentration.

[0042] The oxidation reaction of the cellulosic raw material with hypochlorous acid or a salt thereof is preferably carried out while adjusting the pH to the range of 5.0 to 14.0. Within this range, the oxidation reaction of the cellulosic raw material can be sufficiently progressed, and the amount of carboxy groups in the oxidized cellulose can be sufficiently increased. This facilitates defibration of the oxidized cellulose. The pH of the reaction system is more preferably 7.0 or higher, even more preferably 8.0 or higher, even more preferably 8.5 or higher, even more preferably 9.0 or higher, and even more preferably 9.5 or higher. There is no particular upper limit to the pH of the reaction system, but it is preferably 14.5 or lower, more preferably 14.0 or lower, even more preferably 13.0 or lower, even more preferably 12.5 or lower, even more preferably 12.0 or lower, and even more preferably 11.5 or lower. The pH range of the reaction system is more preferably 7.0 to 14.0, even more preferably 8.0 to 13.5, and even more preferably 8.5 to 13.0.

[0043] During the reaction, the pH of the reaction system decreases as carboxy groups are generated in the cellulosic raw material by the oxidation reaction. Therefore, from the viewpoint of efficiently progressing the oxidation reaction, it is preferable to add an alkaline agent (e.g., sodium hydroxide) or an acid (e.g., hydrochloric acid) to the reaction system and carry out the oxidation reaction while adjusting the pH of the reaction system.

[0044] Hereinafter, the method for producing oxidized cellulose will be further explained using as an example the case where sodium hypochlorite is used as hypochlorous acid or a salt thereof.

[0045] When oxidizing a cellulosic raw material using sodium hypochlorite, the reaction liquid is preferably a sodium hypochlorite aqueous solution. Methods for adjusting the effective chlorine concentration of a sodium hypochlorite aqueous solution to a target concentration (for example, target concentration: in the range of 6% by mass to 43% by mass) include concentrating a sodium hypochlorite aqueous solution having an effective chlorine concentration lower than the target concentration, diluting a sodium hypochlorite aqueous solution having an effective chlorine concentration higher than the target concentration, and dissolving sodium hypochlorite crystals (for example, sodium hypochlorite pentahydrate) in a solvent. Among these, the method of diluting a sodium hypochlorite aqueous solution or dissolving sodium hypochlorite crystals in a solvent to adjust the effective chlorine concentration as an oxidizing agent is preferred because it causes less self-decomposition (i.e., less reduction in effective chlorine concentration) and makes it easy to adjust the effective chlorine concentration.

[0046] The method for mixing the cellulosic raw material with the aqueous sodium hypochlorite solution is not particularly limited, but from the viewpoint of ease of operation, it is preferable to add the cellulosic raw material to the aqueous sodium hypochlorite solution and mix them.

[0047] To efficiently promote the oxidation reaction of the cellulosic raw material, it is preferable to carry out the oxidation reaction while stirring the mixture of the cellulosic raw material and the aqueous sodium hypochlorite solution. Examples of stirring methods include a stirrer with agitating blades, a homomixer, a disperser-type mixer, a homogenizer, and external circulation stirring. Among these, methods using one or more of shear-type stirrers such as homomixers and homogenizers, stirrers with agitating blades, and disperser-type mixers are preferred, as they allow the oxidation reaction of the cellulosic raw material to proceed smoothly and make it easier to adjust the degree of polymerization of the oxidized cellulose to a predetermined value or less. Methods using a stirrer with agitating blades are particularly preferred. When a stirrer with agitating blades is used, devices equipped with known agitating blades such as propeller blades, paddle blades, turbine blades, swept blades, anchor blades, gate blades, Max Blend blades, Full Zone blades, helical ribbon blades, and screw blades (e.g., with a draft tube) can be used. Furthermore, when a stirrer with agitating blades is used, stirring is preferably carried out at a rotation speed of 50 to 300 rpm. Furthermore, a multi-screw kneader such as a single-screw kneader or a twin-screw kneader can also be used.

[0048] The reaction temperature in the oxidation reaction is preferably 15°C to 100°C, and more preferably 20°C to 90°C. The reaction time for the oxidation reaction can be set according to the degree of progress of the oxidation, but is preferably about 15 minutes to 50 hours. When the pH of the reaction system is set to 10 or higher, it is preferable to set the reaction temperature to 30°C or higher and / or the reaction time to 30 minutes or longer.

[0049] The pressure under which the reaction is carried out is not particularly limited, but is typically in the range of atmospheric pressure to 1.0 MPaG (gauge pressure, the same applies hereinafter). Here, atmospheric pressure refers to a pressure equivalent to atmospheric pressure. By carrying out the oxidation under pressure, the amount of hypochlorous acid or a salt thereof used can be reduced, and oxidized cellulose tends to be produced more efficiently. From the standpoint of efficiency, the pressure is preferably 0.1 MPaG to 1.0 MPaG. In this case, the available chlorine concentration of hypochlorous acid or a salt thereof may be greater than 0% by mass and not more than 43% by mass, and from the standpoint of increasing efficiency, it is preferably 0.1% by mass to 20% by mass, more preferably 1.0% by mass to 15% by mass, and even more preferably 1.0% by mass to 10% by mass.

[0050] In the production of oxidized cellulose, after the oxidization of the cellulosic raw material, a treatment to terminate the oxidation reaction may be carried out. The treatment to terminate the oxidation reaction is not particularly limited, but examples thereof include a method of adding an acid or a metal catalyst. Another preferred example is a method of reducing hypochlorous acid or a salt thereof. Specific examples of treatment to terminate the oxidation reaction include a method of adding a reducing agent such as sodium sulfite. The amount of reducing agent added may be adjusted appropriately depending on the amount of hypochlorous acid or a salt thereof (available chlorine concentration).

[0051] The solution containing oxidized cellulose obtained by the above reaction can be subjected to known isolation processes such as centrifugation and filtration, and further purified as necessary to obtain oxidized cellulose as an oxide of a cellulosic raw material with hypochlorous acid or a salt thereof. Furthermore, in the isolation process, some or all of the carboxy groups in the oxidized cellulose may be converted to the H type (—COOH) by, for example, adjusting the pH of the solution containing oxidized cellulose to 4 or less. Furthermore, the solution containing oxidized cellulose obtained by the above reaction may be used as is, or may be subjected to the subsequent defibration step, for example.

[0052] (Reduction Treatment Step) After the oxidation treatment, the cellulose fiber may be reduced with a reducing agent, if necessary. This reduces at least a portion of the aldehyde groups and ketone groups, returning them to hydroxyl groups. Carboxy groups are not reduced. The total content of carbonyl groups (aldehyde groups and ketone groups) in the oxidized cellulose obtained by this reduction, calculated by the semicarbazide method described below, is not particularly limited, and may usually be 0.3 mmol / g or less, or may be 0.1 mmol / g or less.

[0053] The total content of carbonyl groups (aldehyde and ketone groups) using the semicarbazide method is measured, for example, as follows. First, exactly 50 ml of a 3 g / L aqueous solution of semicarbazide hydrochloride adjusted to pH 5 with a phosphate buffer is added to a dried sample, the container is sealed, and the mixture is shaken for two days. Next, exactly 10 ml of this solution is placed in a 100 ml beaker, to which 25 ml of 5 N sulfuric acid and 5 ml of 0.05 N potassium iodate aqueous solution are added, and the mixture is stirred for 10 minutes. Then, 10 ml of a 5% aqueous potassium iodide solution is added, and the mixture is immediately titrated with a 0.1 N sodium thiosulfate solution using an automatic titrator. The amount of carbonyl groups in the sample can be calculated from the titration amount and other data using the following formula: Note that semicarbazide reacts with aldehyde and ketone groups to form Schiff bases (imines), but does not react with carboxy groups. Therefore, the above measurement is thought to enable the quantification of only the amount of carbonyl groups.

[0054] The reducing agent used in the reduction reaction can be a commonly used one, such as borates, sulfites, etc. Use of these reducing agents can prevent coloration.

[0055] The above-mentioned borates are a general term for borates and boronates. In the present invention, borates refer to salts composed of an anion derived from boric acid (B(OH)3) and a monovalent metal ion. In the present invention, borates are salts represented by M3[BO3], M2[HBO3], M[H2BO3], M2[R-BO2], or M[R-BO2H] (M is a monovalent metal ion, R is a monovalent hydrocarbon group). These borates may exist in the composition of the present invention as a solid, in an ionized state, or in a state reacted with the functional groups of nanocellulose.

[0056] Specific examples of the borate salts include sodium borate, lithium borate, potassium borate, rubidium borate, cesium borate, sodium boronate, potassium boronate, rubidium boronate, cesium boronate, etc. The amount of the borate salts may be in the range of 1 to 30% by mass relative to the bone dry mass of oxidized cellulose.

[0057] The sulfites refer to sulfites (M2SO3: M is a monovalent cation site), bisulfites (MHSO3: M is a monovalent cation site), pyrosulfites (M2SO5 or M'S2O5: M is a monovalent cation site, M' is a divalent cation site), hyposulfites (M2SO4 or M'S2O4: M is a monovalent cation site, M' is a divalent cation site), or hydrates thereof. M includes alkali metal ions and ammonium ions, and M' includes alkaline earth metal ions. Specific examples of sulfites include sodium bisulfite, potassium bisulfite, ammonium bisulfite, sodium sulfite, potassium sulfite, ammonium sulfite, sodium hyposulfite, potassium hyposulfite, calcium hyposulfite, sodium pyrosulfite, potassium pyrosulfite, magnesium pyrosulfite, and calcium pyrosulfite. The amount of the sulfites is preferably 0.1 to 15% by mass, more preferably 1 to 15% by mass, even more preferably 1.0 to 12% by mass, and even more preferably 3.0 to 10% by mass, based on the bone dry mass of oxidized cellulose.

[0058] The reduction temperature may be about 10 to 90°C from the viewpoints of reduction efficiency and suppression of fiber degradation. The pH during reduction may be adjusted appropriately depending on the reducing agent used, and may generally be in the range of pH 2 to 12. The reaction time for the reduction reaction may be appropriately set depending on the degree of progress of reduction, and is not particularly limited, but may generally be in the range of 0.5 to 6 hours.

[0059] The oxidized cellulose is preferably in the form of a dispersion. The dispersion referred to here is a suspension containing oxidized cellulose. The dispersion may contain the solvent used during oxidation. Alternatively, a dispersion medium may be added as appropriate to form the dispersion. When the oxidized cellulose is in the form of a dispersion, it is easier to handle and tends to be more easily pulverized. When the oxidized cellulose of the present invention is in the form of a dispersion, the amount of oxidized cellulose is typically in the range of 0.1% to 95% by mass, preferably 1% to 50% by mass, and more preferably 1% to 30% by mass, when the total amount of the dispersion is taken as 100% by mass.

[0060] Oxidized cellulose includes fibrous cellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or its salts. Oxidized cellulose is also called oxidized cellulose fiber. That is, oxidized cellulose includes an oxidation product of a cellulosic raw material with hypochlorous acid or its salts. Cellulose is the main component of plants, and bundles of cellulose molecules are called cellulose microfibrils. The cellulose in cellulosic raw materials is also contained in the form of cellulose microfibrils.

[0061] (Step B: Defibration Treatment) Nanocellulose can be obtained by defibrating the oxidized cellulose obtained above into nano-sized particles as needed. Methods for defibrating oxidized cellulose include weak stirring using a magnetic stirrer or mechanical defibration.

[0062] Examples of mechanical defibration methods include methods using various mixing and stirring devices such as a screw mixer, paddle mixer, disperser mixer, turbine mixer, homomixer under high-speed rotation, high-pressure homogenizer, ultra-high-pressure homogenizer, double-cylinder homogenizer, ultrasonic homogenizer, water current opposing collision type disperser, beater, disk refiner, conical refiner, double-disc refiner, grinder, single-shaft or multi-shaft kneader, planetary stirrer, vibration stirrer, etc. These devices can be used alone or in combination of two or more types to treat oxidized cellulose, preferably in a dispersion medium, to nanosize the oxidized cellulose and produce nanocellulose.

[0063] For defibration of oxidized cellulose, a method using an ultra-high-pressure homogenizer is preferably used, since it allows the production of nanocellulose with a more advanced defibration. When defibration treatment using an ultra-high-pressure homogenizer is applied, the pressure during the defibration treatment is preferably 100 MPa or more, more preferably 120 MPa or more, and even more preferably 150 MPa or more. The number of defibration treatments is not particularly limited, but from the viewpoint of sufficiently progressing defibration, it is preferably two or more, more preferably three or more. Furthermore, the above-mentioned oxidized cellulose can be sufficiently defibrated by mild stirring using a planetary stirrer or a vibration stirrer. Examples of vibration stirrers include a vortex mixer (touch mixer). In other words, with the above-mentioned oxidized cellulose, homogenized nanocellulose can be obtained even when the defibration treatment is performed under mild defibration conditions.

[0064] The defibration treatment is preferably carried out in a state where the oxidized cellulose is mixed with a dispersion medium. There are no particular restrictions on the dispersion medium, and it can be selected appropriately depending on the purpose. Specific examples of dispersion mediums include water, alcohols, ethers, ketones, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. As the solvent, one of these may be used alone, or two or more may be used in combination.

[0065] Among the dispersion media, examples of alcohols include methanol, ethanol, isopropanol, isobutanol, sec-butyl alcohol, tert-butyl alcohol, methyl cellosolve, ethylene glycol, and glycerin. Examples of ethers include ethylene glycol dimethyl ether, 1,4-dioxane, and tetrahydrofuran. Examples of ketones include acetone and methyl ethyl ketone.

[0066] The nanocellulose in the present invention preferably satisfies the following zeta potential and light transmittance.

[0067] (Zeta Potential) The zeta potential of the nanocellulose of the present invention is preferably -30 mV or less. When the zeta potential is -30 mV or less (i.e., an absolute value of 30 mV or more), sufficient repulsion between microfibrils is obtained, making it easier to produce nanocellulose with a high surface charge density during mechanical defibration. This improves the dispersion stability of the nanocellulose, resulting in excellent viscosity stability and ease of handling when made into a slurry. The upper limit of the zeta potential is not particularly limited, but is usually -100 mV or less. When the zeta potential is -100 mV or more (i.e., an absolute value of 100 mV or less), oxidative scission in the fiber direction as oxidation progresses tends to be suppressed, making it possible to obtain nanocellulose of uniform size, which increases the stability and dispersibility of nanocellulose in water, and the resulting aqueous ink composition contains nanocellulose uniformly.

[0068] From the above viewpoint, the zeta potential of nanocellulose is preferably -35 mV or less, more preferably -40 mV or less, and even more preferably -50 mV or less. Furthermore, the lower limit of the zeta potential is preferably -90 mV or more, more preferably -85 mV or more, even more preferably -80 mV or more, even more preferably -77 mV or more, even more preferably -70 mV or more, and even more preferably -65 mV or more. The range of the zeta potential can be appropriately combined with the above-mentioned lower and upper limits. The zeta potential is preferably -90 mV or more to -30 mV or less, more preferably -85 mV or more to -30 mV or less, even more preferably -80 mV or more to -30 mV or less, even more preferably -77 mV or more to -30 mV or less, even more preferably -70 mV or more to -30 mV or less, even more preferably -65 mV or more to -35 mV or less. In this specification, the zeta potential is a value measured at pH 8.0 and 20°C for a cellulose aqueous dispersion prepared by mixing nanocellulose and water to a nanocellulose concentration of 0.1% by mass. Specifically, it can be measured according to the conditions described below. [Zeta potential measurement] Pure water is added to a dispersion containing nanocellulose to dilute it so that the nanocellulose concentration becomes 0.1%. After dilution, a 0.05 mol / L aqueous solution of sodium hydroxide is added to the aqueous nanocellulose dispersion to adjust the pH to 8.0, and the zeta potential is measured at 20°C using a zeta potential meter (ELSZ-1000) manufactured by Otsuka Electronics Co., Ltd.

[0069] (Light transmittance) The nanocellulose dispersion of the present invention, in which nanocellulose is dispersed in a dispersion medium, has little light scattering by cellulose fibers and can exhibit high light transmittance. Specifically, the nanocellulose of the present invention preferably has a light transmittance of 95% or more in a mixed solution mixed with water to a solids concentration of 0.1% by mass. The light transmittance is more preferably 96% or more, even more preferably 97% or more, and even more preferably 99% or more. Note that the light transmittance is a value at a wavelength of 660 nm measured using a spectrophotometer. The light transmittance can also be measured using an aqueous dispersion containing nanocellulose. Specifically, it can be measured according to the conditions described in the examples below.

[0070] The zeta potential and light transmittance can be controlled by oxidation using hypochlorous acid or a salt thereof, particularly by adjusting the reaction time, reaction temperature, stirring conditions, etc. of the oxidation reaction. Specifically, as the reaction time and / or reaction temperature is increased, oxidation of the surface of cellulose microfibrils in the cellulosic raw material progresses, and the repulsion between fibrils due to electrostatic repulsion and osmotic pressure increases, tending to result in a smaller average fiber width. Furthermore, the zeta potential tends to be increased by setting one or more of the reaction time, reaction temperature, and stirring conditions of the oxidation (for example, by increasing the reaction time) to promote oxidation (i.e., to increase the degree of oxidation).

[0071] As described above, the nanocellulose of the present invention is obtained by obtaining oxidized cellulose using hypochlorous acid or a salt thereof and then defibrating it. Here, the degree of polymerization of the oxidized cellulose used in the present invention is preferably 600 or less. When the degree of polymerization of oxidized cellulose exceeds 600, defibration tends to require a large amount of energy, making it difficult to achieve sufficient defibration properties, resulting in decreased pigment dispersibility and ultimately a decrease in optical density. Furthermore, when the degree of polymerization of oxidized cellulose exceeds 600, a large amount of oxidized cellulose is insufficiently defibrated. Therefore, when this oxidized cellulose is micronized and dispersed in a dispersion medium, light scattering and other phenomena can occur, resulting in decreased transparency. Furthermore, the size of the resulting nanocellulose tends to vary, resulting in uneven quality. This can increase the viscosity of ink compositions containing nanocellulose and reduce the ejection stability during inkjet printing. From the perspective of defibration properties, no lower limit for the degree of polymerization of oxidized cellulose is specifically set. However, when the degree of polymerization of oxidized cellulose is less than 50, the proportion of particulate cellulose rather than fibrous cellulose increases, potentially reducing the scratch resistance of the resulting coating. From the above viewpoints, the degree of polymerization of the oxidized cellulose is preferably in the range of 50 or more and 600 or less.

[0072] The degree of polymerization of oxidized cellulose is more preferably 580 or less, even more preferably 560 or less, still more preferably 550 or less, even more preferably 500 or less, even more preferably 450 or less, and even more preferably 400 or less. From the viewpoint of improving the viscosity stability and coatability of the slurry, the lower limit of the degree of polymerization is more preferably 60 or more, even more preferably 70 or more, even more preferably 80 or more, even more preferably 90 or more, even more preferably 100 or more, even more preferably 110 or more, and particularly preferably 120 or more. A preferred range of the degree of polymerization can be determined by appropriately combining the above-mentioned upper and lower limits. The degree of polymerization of chemically modified cellulose is more preferably 60 to 600, even more preferably 70 to 600, even more preferably 80 to 600, even more preferably 80 to 550, even more preferably 80 to 500, even more preferably 80 to 450, and particularly preferably 80 to 400.

[0073] The degree of polymerization of oxidized cellulose can be adjusted by changing the reaction time, reaction temperature, pH, and available chlorine concentration of hypochlorous acid or its salt during the oxidation reaction. Specifically, since increasing the degree of oxidation tends to decrease the degree of polymerization, methods for decreasing the degree of polymerization include increasing the oxidation reaction time and / or reaction temperature. Alternatively, the degree of polymerization of oxidized cellulose can be adjusted by changing the stirring conditions of the reaction system during the oxidation reaction. For example, under conditions where the reaction system is sufficiently homogenized using a stirring blade or the like, the oxidation reaction proceeds smoothly and the degree of polymerization tends to decrease. On the other hand, under conditions where the reaction system is likely to be insufficiently stirred, such as stirring with a stirrer, the reaction tends to become non-uniform, making it difficult to sufficiently reduce the degree of polymerization of oxidized cellulose. Furthermore, the degree of polymerization of oxidized cellulose tends to vary depending on the selection of raw cellulose. Therefore, the degree of polymerization of oxidized cellulose can also be adjusted by selecting the appropriate cellulosic raw material. In this specification, the degree of polymerization of oxidized cellulose is the average degree of polymerization (viscosity-average degree of polymerization) measured by a viscosity method. Details are given in accordance with the method described in the Examples below.

[0074] <Solvent> The aqueous ink composition of the present invention contains an aqueous medium such as a water-soluble solvent and / or water as the solvent. These may be water alone or a mixed solvent consisting of water and a water-soluble solvent. Examples of water-soluble solvents include ketones such as acetone, methyl ethyl ketone, methyl butyl ketone, and methyl isobutyl ketone; alcohols such as methanol, ethanol, 2-propanol, 2-methyl-1-propanol, 1-butanol, and 2-methoxyethanol; ethers such as tetrahydrofuran, 1,4-dioxane, and 1,2-dimethoxyethane; and amides such as 2-pyrrolidone, dimethylformamide, and N-methylpyrrolidone. These may be used alone or in combination of two or more. Among these, ketones having 3 to 6 carbon atoms and alcohols having 1 to 5 carbon atoms are preferred. In addition to the above-mentioned water-soluble solvents, other examples include glycols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, polyethylene glycol, and polypropylene glycol; diols such as butanediol, pentanediol, hexanediol, and diols of the same group; glycol esters such as propylene glycol laurate; glycol ethers such as diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, and diethylene glycol monohexyl ether, propylene glycol ether, dipropylene glycol ether, and cellosolves containing triethylene glycol ether; sulfolane; lactones such as γ-butyrolactone; lactams such as N-(2-hydroxyethyl)pyrrolidone; glycerin and its derivatives, and polyoxyethylene benzyl alcohol ether. Among these, polyhydric alcohols such as glycols and diols having a high boiling point, low volatility, and high surface tension are preferred, and glycols such as diethylene glycol and triethylene glycol are more preferred. These water-soluble organic solvents can be used alone or in combination.

[0075] <Pigment> The ink composition of the present invention contains at least one pigment selected from known and commonly used organic pigments or inorganic pigments. Furthermore, the pigment in the present invention may be either an untreated pigment or a treated pigment. The treated pigment also includes so-called self-dispersing pigments. Self-dispersing pigments are produced, for example, by subjecting a pigment to a physical or chemical treatment to bond (graft) hydrophilic groups to the pigment surface.

[0076] When printing on plain paper, yellow ink, cyan ink, magenta ink, black ink, etc. are used alone or in combination as an ink set. There are no particular limitations on the pigments used, and pigments typically used for aqueous inks can be used. Specifically, known inorganic or organic pigments that are dispersible in water or water-soluble organic solvents can be used. Examples of inorganic pigments include iron oxide and carbon black produced by known methods such as the contact method, furnace method, and thermal method. Examples of organic pigments that can be used include azo pigments (including azo lakes, insoluble azo pigments, condensed azo pigments, and chelate azo pigments), polycyclic pigments (e.g., phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments), dye chelates (e.g., basic dye chelates, acid dye chelates), nitro pigments, nitroso pigments, and aniline black.

[0077] Examples of pigments used in black ink include carbon black, such as No. 2300, No. 2200B, No. 900, No. 960, No. 980, No. 33, No. 40, No. 45, No. 45L, and No. 45L manufactured by Mitsubishi Chemical Corporation. 52, HCF88, MA7, MA8, MA100, etc. manufactured by Columbia Corporation; Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. manufactured by Cabot Corporation; Regal 400R, Regal 330R, Regal 660R, Mogul L, Mogul 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc. manufactured by Degussa Corporation; Color Black, manufactured by Degussa Corporation; Examples of suitable polyesters include FW1, FW2, FW2V, FW18, FW200, S150, S160, S170, Printex 35, U, V, 1400U, Special Black 6, 5, 4, 4A, NIPEX150, NIPEX160, NIPEX170, and NIPEX180.

[0078] Examples of pigments used in yellow inks include C.I. Pigment Yellow 1, 2, 12, 13, 14, 16, 17, 73, 74, 75, 83, 93, 95, 97, 98, 109, 110, 114, 120, 128, 129, 138, 150, 151, 154, 155, 174, 180, and 185.

[0079] Examples of pigments used in magenta inks include C.I. Pigment Red 5, 7, 12, 48(Ca), 48(Mn), 57(Ca), 57:1, 112, 122, 123, 146, 168, 176, 184, 185, 202, and 209.

[0080] Examples of pigments used in cyan ink include C.I. Pigment Blue 1, 2, 3, 15, 15:3, 15:4, 16, 22, 60, 63, and 66.

[0081] The self-dispersion pigment may be, for example, a commercially available product. Examples of commercially available products include "CAB-O-JET200," "CAB-O-JET250C," "CAB-O-JET260M," "CAB-O-JET270Y," "CAB-O-JET300," "CAB-O-JET400," "CAB-O-JET450C," "CAB-O-JET465M," and "CAB-O-JET470Y" manufactured by Cabot Specialty Chemicals Corporation; "BONJET BLACK CW-2" and "BONJET BLACK CW-3" manufactured by Orient Chemical Industry Co., Ltd.; and "LIOJET WD BLACK 002C" manufactured by Toyo Ink Mfg. Co., Ltd. Alternatively, commercially available pigment dispersions in which self-dispersing pigments are dispersed in aqueous media can be used. Examples include "SENSIJJET Black SDP100," "SENSIJJET Black SDP1000," "SENSIJJET Black SDP2000," "Sensijet Ultra Yellow PY74," "Sensijet Ultra Magenta PR122," "Sensijet Ultra Cyan PB15:4," and "Sensijet Ultra K," all manufactured by Sensient Colors, Inc. of the United States. These self-dispersing pigments may be used alone or in combination.

[0082] In aqueous inks, particularly those for inkjet printing, the pigment content (by mass) may be in the range of 0.5 to 30% of the total amount of the aqueous ink. The pigment particle size is preferably 1 μm or less, more preferably 10 nm to 150 nm, and even more preferably 50 nm to 120 nm.

[0083] The content of nanocellulose used in the present invention is preferably in the range of 1 / 100 to 50 / 100, more preferably 1 / 100 to 30 / 100, and even more preferably 3 / 100 to 30 / 100, in terms of the ratio (nanocellulose / pigment) of the aqueous ink by mass, based on the weight of the nanocellulose used. By being in the above range, the color development and abrasion resistance of the formed image tend to be exhibited without impairing suitability for various printing methods.

[0084] When the aqueous ink composition is intended for inkjet printing, the nanocellulose content is preferably 0.001 to 3% by mass, more preferably 0.01 to 2% by mass, and even more preferably 0.1 to 1.8% by mass, based on the total mass of the ink composition. A content of 0.001% or more makes it easier to ensure optical density and abrasion resistance, while a content of 3% or less tends to suppress increases in ink viscosity and provide excellent printability.

[0085] <Dispersant> The ink composition of the present invention contains at least the nanocellulose described above, but may also contain other dispersants. Anionic group-containing organic polymer compounds are suitable as dispersants. The anionic group-containing organic polymer compounds are primarily used to disperse pigments, but because they are polymers, they also function as binders. By including a binder component, the functionality of the coating film obtained from the ink composition, such as robustness, adhesion, and optical density, can be further improved. Known anionic group-containing organic polymer compounds can be used. For detailed embodiments of the anionic group-containing organic polymer compound, see International Publication No. 2015 / 166808.

[0086] Examples of anionic group-containing organic polymeric compounds include organic polymeric compounds containing carboxyl, sulfonic acid, or phosphoric acid groups. Examples of such anionic group-containing organic polymeric compounds include polyvinyl resins having anionic groups, polyester resins having anionic groups, amino resins having anionic groups, acrylic resins having anionic groups, epoxy resins having anionic groups, polyurethane resins having anionic groups, polyether resins having anionic groups, polyamide resins having anionic groups, unsaturated polyester resins having anionic groups, phenolic resins having anionic groups, silicone resins having anionic groups, fluorine-containing polymeric compounds having anionic groups, and polysaccharide derivatives having anionic groups. Furthermore, acrylic resins having anionic groups and polyurethane resins having anionic groups are preferred because of their abundant raw materials, ease of design, and excellent pigment dispersion properties. The anionic group-containing organic polymeric compounds used in the present invention can be used alone or in combination of two or more types. Combining two or more types may sometimes balance the various properties of the aqueous ink.

[0087] (Acrylic resin having anionic group) Specific examples of the acrylic resin having anionic group include a resin made of a copolymer of a monomer having an anionic group such as (meth)acrylic acid and another anionic group-containing organic polymer compound copolymerizable therewith. In the present invention, (meth)acrylic acid is a general term for acrylic acid and methacrylic acid. The same applies to various esters of (meth)acrylic acid.

[0088] Preferred examples of the monomer for the anionic group-containing organic polymer compound include alkylstyrenes such as styrene, α-methylstyrene, β-methylstyrene, 2,4-dimethylstyrene, α-ethylstyrene, α-butylstyrene, and α-hexylstyrene, halogenated styrenes such as 4-chlorostyrene, 3-chlorostyrene, and 3-bromostyrene, styrene monomers such as 3-nitrostyrene, 4-methoxystyrene, and vinyltoluene, and (meth)acrylic acid ester monomers having a benzene ring such as benzyl (meth)acrylate, phenyl (meth)acrylate, phenylethyl (meth)acrylate, phenylpropyl (meth)acrylate, and phenoxyethyl (meth)acrylate. Among these, styrene monomers such as styrene, α-methylstyrene, and tert-butylstyrene are particularly preferred.

[0089] The copolymer relating to the acrylic resin having an anionic group in the present invention preferably contains a polymerization unit of (meth)acrylic acid and other copolymerizable polymerization units as essential polymerization units, and may be a binary copolymer thereof or a ternary or higher multi-component copolymer further containing other copolymerizable polymerization units.

[0090] Examples of the monomer of the anionic group-containing organic polymer compound include acrylic acid esters and methacrylic acid esters such as methyl acrylate, methyl methacrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, sec-butyl acrylate, tert-butyl acrylate, 2-ethylbutyl acrylate, 1,3-dimethylbutyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, ethyl methacrylate, n-butyl methacrylate, 2-methylbutyl methacrylate, pentyl methacrylate, heptyl methacrylate, and nonyl methacrylate; 3-ethoxypropyl acrylate, 3-ethoxybutyl acrylate, dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate; Examples of suitable monomers include acrylic acid ester derivatives and methacrylic acid ester derivatives such as 2-hydroxybutyl acrylate, ethyl-α-(hydroxymethyl)acrylate, dimethylaminoethyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate; aryl acrylate esters and aralkyl acrylate esters such as phenyl acrylate, benzyl acrylate, phenylethyl acrylate, and phenylethyl methacrylate; monoacrylic acid esters or monomethacrylic acid esters of polyhydric alcohols such as diethylene glycol, triethylene glycol, polyethylene glycol, glycerin, and bisphenol A; dialkyl maleates such as dimethyl maleate and diethyl maleate; and vinyl acetate. These monomers may be used alone or in combination.

[0091] The copolymer relating to the acrylic resin having an anionic group used in the present invention may be a linear copolymer containing only polymerization units of a monoanionic group-containing organic polymer compound, or may be a copolymer containing a partially crosslinked portion obtained by copolymerizing a very small amount of various crosslinkable anionic group-containing organic polymer compounds.

[0092] Examples of such a monomer for an anionic group-containing organic polymer compound having crosslinkability include glycidyl (meth)acrylate, and poly(meth)acrylates of polyhydric alcohols such as divinylbenzene, ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, poly(oxyethyleneoxypropylene) glycol di(meth)acrylate, and tri(meth)acrylate of an alkylene oxide adduct of glycerin.

[0093] In the present invention, the reaction rates of the monomers used are considered to be approximately the same, and the charging ratio of each monomer is regarded as the content ratio of the polymerized units of each monomer converted into mass. The copolymer related to the acrylic resin having an anionic group in the present invention can be synthesized by various known reaction methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. In this case, known and commonly used polymerization initiators, chain transfer agents (polymerization degree regulators), surfactants, and antifoaming agents can also be used in combination.

[0094] In the present invention, the weight-average molecular weight of the acrylic resin having an anionic group is preferably within the range of 5,000 to 20,000. For example, when the styrene-acrylic acid copolymer is used, its weight-average molecular weight is preferably within the range of 5,000 to 20,000, and more preferably within the range of 5,000 to 18,000. Of these, a range of 5,500 to 15,000 is particularly preferred. The weight-average molecular weight here is a value measured by GPC (gel permeation chromatography) and is a value converted into the molecular weight of polystyrene used as a standard substance.

[0095] When the anionic group-containing organic polymer compound used in the present invention is a styrene-acrylic acid copolymer, it has carboxy groups derived from acrylic acid monomers and methacrylic acid monomers, and its acid value is preferably 50 to 220 (mgKOH / g), and more preferably 60 to 200 (mgKOH / g). When the acid value is 220 (mgKOH / g) or less, pigment aggregation tends to be less likely to occur. The acid value in this specification is a value measured in accordance with the Japanese Industrial Standards "K0070:1992. Testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value, and unsaponifiable matter of chemical products," and is the amount (mg) of potassium hydroxide required to completely neutralize 1 g of resin.

[0096] (Polyurethane-based resin having an anionic group) Specific examples of the polyurethane-based resin having an anionic group used in the present invention include urethane resins obtained by reacting a polyol having an anionic group such as a carboxy group or a sulfonic acid group with a polyisocyanate, and, if necessary, a general-purpose polyol not having an anionic group or a chain extender.

[0097] Examples of polyols having a carboxy group used in the present invention include esters obtained by reacting polyhydric alcohols with polybasic acid anhydrides, and dihydroxyalkanoic acids such as 2,2-dimethylollactic acid, 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, and 2,2-dimethylolvaleric acid. Preferred compounds include 2,2-dimethylolpropionic acid and 2,2-dimethylolbutanoic acid. Examples of polyols having a sulfonic acid group include polyester polyols obtained by reacting dicarboxylic acids such as 5-sulfoisophthalic acid, sulfoterephthalic acid, 4-sulfophthalic acid, and 5[4-sulfophenoxy]isophthalic acid, or salts thereof, with the low-molecular-weight polyols described above.

[0098] Examples of the diisocyanate used in the present invention include aliphatic diisocyanate compounds such as hexamethylene diisocyanate and 2,2,4-trimethylhexamethylene diisocyanate; alicyclic diisocyanate compounds such as isophorone diisocyanate, hydrogenated xylylene diisocyanate and 4,4-cyclohexylmethane diisocyanate; aromatic aliphatic diisocyanate compounds such as xylylene diisocyanate and tetramethylxylene diisocyanate; and aromatic diisocyanates such as toluylene diisocyanate and phenylmethane diisocyanate.

[0099] Examples of general-purpose polyols without an anionic group include polyester polyols, polyether polyols, polyhydroxy polycarbonates, polyhydroxy polyacetals, polyhydroxy polyacrylates, polyhydroxy polyester amides, and polyhydroxy polythioethers. Among these, polyester polyols, polyether polyols, and polyhydroxy polycarbonates are preferred. These polyols may be reacted alone or in combination with several other polyols. In addition to the polyols, a low-molecular-weight diol may be used in combination, as appropriate, for the purpose of adjusting the film hardness of printed materials. Examples include ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,2-propanediol, 1,3-propanediol, and 1,4-butanediol.

[0100] The chain extender used in the present invention may be one or more of diols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,9-nonanediol, 1,4-bis(β-hydroxyethoxy)benzene, 1,4-cyclohexanediol, and xylylene glycol; and diamines such as ethylenediamine, propylenediamine, xylylenediamine, isophoronediamine, 4,4′-diaminodiphenylmethane, tolylenediamine, and 4,4′-diaminodicyclohexylmethane.

[0101] The polyurethane resin can be produced by reacting the polyol with the polyisocyanate in the absence of a solvent or in the presence of an organic solvent, followed by neutralization with a basic compound or the like (described later) to form an anionic group-containing urethane resin, which is then mixed with an aqueous medium to make it water-soluble, and optionally mixed with a chain extender to cause a reaction.

[0102] The reaction between the polyol and the polyisocyanate is preferably carried out in such a manner that the equivalent ratio of the isocyanate groups of the polyisocyanate to the hydroxyl groups of the polyol is in the range of 0.8 to 2.5, and more preferably in the range of 0.9 to 1.5.

[0103] In the present invention, the weight-average molecular weight of the polyurethane resin having an anionic group is preferably within the range of 5,000 to 500,000, more preferably 10,000 to 200,000, and particularly preferably 15,000 to 100,000. Here, the weight-average molecular weight is a value measured by GPC (gel permeation chromatography) and converted into the molecular weight of polystyrene used as a standard substance.

[0104] The polyurethane resin preferably has an acid value in the range of 2 to 200 (mgKOH / g), and an acid value in the range of 2 to 100 (mgKOH / g) is preferred in terms of improving the aqueous dispersion stability of the polyurethane resin.

[0105] (Neutralizing Agent for Acrylic Resin Having Anionic Groups and Polyurethane Resin Having Anionic Groups) In the present invention, the acrylic resin having anionic groups and the polyurethane resin having anionic groups are preferably neutralized with a basic compound before use. Known basic compounds can be used, including hydroxides of alkali metals such as potassium and sodium; carbonates of alkali metals such as potassium and sodium; carbonates of alkaline earth metals such as calcium and barium; inorganic basic compounds such as ammonium hydroxide, and amine or quaternary ammonium salt compounds used in modifying nanocellulose. Among these, alkali metal hydroxides such as potassium hydroxide, sodium hydroxide, and lithium hydroxide contribute to lowering the viscosity of aqueous pigment dispersions and are preferred in terms of ejection stability when used in aqueous inkjet recording inks, with potassium hydroxide being particularly preferred. Furthermore, using the same basic compound as that used in modifying nanocellulose may improve storage stability.

[0106] (Polysaccharide Derivative Having Anionic Group) The ink composition of the present invention may contain a polysaccharide derivative having an anionic group. The polysaccharide derivative having an anionic group is not particularly limited as long as it is a polyanionic polysaccharide. Specific examples include natural polysaccharides such as hyaluronic acid, alginic acid, pectin, and polygalacturonic acid; carboxyalkyl polysaccharides such as carboxymethyl pullulan, carboxymethyl chitin, carboxymethyl chitosan, carboxymethyl mannan, carboxymethyl starch, carboxymethyl dextran, carboxymethyl cellulose, carboxyethyl cellulose, and carboxymethyl pullulan; oxidized polysaccharides such as oxidized cellulose and oxidized starch; and polysaccharides containing sulfate groups such as chondroitin sulfate, dermatan sulfate, heparin, and heparan sulfate. Among these, carboxymethyl cellulose and hyaluronic acid are preferred, with carboxymethyl cellulose being particularly preferred. The weight-average molecular weight of the starting polysaccharide is not particularly limited, but is generally in the range of 50,000 to 1,000,000, and may be in the range of 50,000 to 500,000.

[0107] The polysaccharide derivative having an anionic group in the present invention preferably forms a salt with a cation. The cation that forms a salt with the polysaccharide derivative having an anionic group is not particularly limited, but examples include protons and metal ions, specifically metal ions such as sodium, potassium, lithium, calcium, and magnesium, and organic cations such as organic ammonium. Among these, sodium carboxymethylcellulose salt and ammonium carboxymethylcellulose salt are preferred because they are easily available and easily achieve the desired effect, and ammonium carboxymethylcellulose salt is particularly preferred because of its significant viscosity-reducing effect. These polysaccharide derivatives having an anionic group can be used alone or in combination of two or more.

[0108] The degree of etherification (also referred to as the degree of substitution (DS)) and degree of neutralization of the polysaccharide derivative having an anionic group used in the present invention, particularly the sodium salt or ammonium salt of carboxymethylcellulose, are not particularly limited, but the viscosity of a 1% by mass aqueous solution measured with a Brookfield viscometer at a rotor speed of 60 rpm and 25°C is usually in the range of 300 to 7000 mPa·s, and may be in the range of 500 to 5000 mPa·s. Here, the "degree of etherification" refers to the average number of hydroxyl groups to which carboxymethyl groups are ether-linked, out of the three hydroxyl groups contained in anhydroglucose, which is a constituent unit of cellulose, in carboxymethylcellulose. Therefore, the value is theoretically between 0 and 3.

[0109] The amount of the polysaccharide derivative having anionic groups to be blended is not particularly limited, and an appropriate amount is blended depending on the purpose described above. For example, when the purpose is to improve the color development (optical density) of the formed image or the scratch resistance of the image, the mass parts of the polysaccharide derivative having anionic groups per 1 part by mass of nanocellulose may be 5 or less, 3 or less, or 2 or less.

[0110] Commercially available products can be used as sodium carboxymethylcellulose and ammonium carboxymethylcellulose. For example, the Cellogen series sold by Daiichi Kogyo Seiyaku Co., Ltd., the CMC Daicel series sold by Daicel FineChem Co., Ltd., and the Sunrose F series sold by Nippon Paper Chemicals Co., Ltd. can be used. Examples of ammonium carboxymethylcellulose include the DN series sold by Daicel FineChem Co., Ltd.

[0111] The content (by mass) of the anionic group-containing organic polymer compound used in the present invention is preferably 0.1 to 10 mass %, more preferably 0.3 to 5%, and even more preferably 0.5 to 2% of the total mass of the aqueous pigment dispersion, particularly for inkjet printing. A content of 0.1% or more makes it easier to ensure abrasion resistance, while a content of 10% or less tends to suppress an increase in the viscosity of the ink and result in excellent printability.

[0112] Of course, commercially available anionic group-containing organic polymer compounds can also be used, such as the Ajisper PB series manufactured by Ajinomoto Fine-Techno Co., Ltd., the DISPERBYK series and BYK series manufactured by BYK, and the Efka series manufactured by BASF.

[0113] <Other Components> The aqueous ink composition of the present invention may be diluted with a water-soluble solvent at any timing as needed, or a wetting agent (drying inhibitor), a penetrating agent, a surfactant, and other additives, so-called known and commonly used additives, may be added. By adding such additives, the aqueous ink composition of the present invention can be used in a variety of applications, such as in the fields of coatings for automobiles and building materials, printing inks such as offset inks, gravure inks, flexographic inks, and silkscreen inks, and inks for inkjet recording. After preparation of the ink, a centrifugation or filtration step may be added to remove coarse particles.

[0114] (Humectant) A humectant is added to prevent the ink from drying. The content of the humectant in the ink for the purpose of preventing drying is preferably 3 to 50% by mass. The humectant used in the present invention is not particularly limited, and examples thereof include glycerin, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycols having a molecular weight of 2000 or less, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3-propylene glycol, isopropylene glycol, isobutylene glycol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, mesoerythritol, and pentaerythritol. In particular, the inclusion of propylene glycol or 1,3-butyl glycol ensures safety and exhibits excellent effects on ink drying properties and ejection performance in inkjet recording applications.

[0115] (Penetrant) A penetrant is added for the purpose of improving the penetrability into the recording medium and adjusting the dot diameter on the recording medium. Examples of penetrants include lower alcohols such as ethanol and isopropyl alcohol, ethylene oxide adducts of alkyl alcohols such as ethylene glycol hexyl ether and diethylene glycol butyl ether, and propylene oxide adducts of alkyl alcohols such as propylene glycol propyl ether.

[0116] (Surfactant) A surfactant is added to adjust ink properties such as surface tension, etc. The surfactant that can be added for this purpose is not particularly limited, and examples thereof include various anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, etc., and of these, anionic surfactants and nonionic surfactants are preferred.

[0117] Examples of anionic surfactants include alkylbenzenesulfonates, alkylphenylsulfonates, alkylnaphthalenesulfonates, higher fatty acid salts, sulfate ester salts of higher fatty acid esters, sulfonates of higher fatty acid esters, sulfate ester salts and sulfonates of higher alcohol ethers, higher alkyl sulfosuccinates, polyoxyethylene alkyl ether carboxylates, polyoxyethylene alkyl ether sulfates, alkyl phosphates, polyoxyethylene alkyl ether phosphates, and the like. Specific examples of these include dodecylbenzenesulfonate, isopropylnaphthalenesulfonate, monobutylphenylphenol monosulfonate, monobutylbiphenylsulfonate, and dibutylphenylphenol disulfonate.

[0118] Examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, glycerin fatty acid esters, polyoxyethylene glycerin fatty acid esters, polyglycerin fatty acid esters, sucrose fatty acid esters, polyoxyethylene alkylamines, polyoxyethylene fatty acid amides, fatty acid alkylolamides, alkylalkanolamides, acetylene glycol, oxyethylene adducts of acetylene glycol, and polyethylene glycol polypropylene glycol block copolymers. Of these, polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene alkyl ethers, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, fatty acid alkylolamides, acetylene glycol, oxyethylene adducts of acetylene glycol, and polyethylene glycol polypropylene glycol block copolymers are preferred.

[0119] Other surfactants that can be used include silicone surfactants such as polysiloxane oxyethylene adducts; fluorine-based surfactants such as perfluoroalkyl carboxylates, perfluoroalkyl sulfonates, and oxyethylene perfluoroalkyl ethers; and biosurfactants such as spiculisporic acid, rhamnolipid, and lysolecithin.

[0120] These surfactants can be used alone or in combination of two or more. When a surfactant is added, the amount added is not limited, but is usually in the range of 0.001 to 2% by mass based on the total mass of the ink.

[0121] If necessary, other additives such as preservatives, viscosity adjusters, pH adjusters, chelating agents, plasticizers, antioxidants, and ultraviolet absorbers may be added.

[0122] (Recording Material) The aqueous ink composition of the present invention exhibits high optical density and abrasion resistance, particularly on plain paper. It can also be used on other absorbent recording materials. Examples of recording materials include, but are not limited to, plain paper, (lightly) coated paper, fabric, inkjet paper, inkjet glossy paper, cardboard, and wood.

[0123] [Recording Method] The recording method of the present invention comprises a step of ejecting the above-described aqueous ink composition onto a recording medium by an inkjet method. The details of the inkjet method are not particularly limited, and any known method may be used.

[0124] [Method for Producing Aqueous Ink Composition] The aqueous ink composition of the present invention can be produced by any known method without any particular limitations. Nanocellulose and other additives can be added at any time, regardless of whether they are added before or after the media-less dispersion process described below. In producing the ink composition of the present invention, not only nanocellulose but also oxidized cellulose before nano-dispersion can be used. Examples of such methods include (1) a method in which an aqueous pigment dispersion containing at least the pigment of the present invention and nanocellulose and other additives are prepared by media-less dispersion, optionally with the addition of other additives; and (2) a method in which a high-concentration aqueous dispersion of the pigment (pigment paste) is prepared in advance, and then the dispersion is diluted with an aqueous medium while simultaneously adding nanocellulose and other additives as required.

[0125] (1) Media-less dispersion of aqueous pigment dispersion In this specification, (1) media-less dispersion specifically refers to ultrasonic dispersion, and dispersion methods using a high-speed disc impeller, colloid mill, roll mill, high-pressure homogenizer, nanomizer, ultimizer, etc., but ultrasonic dispersion is preferred in consideration of productivity and contamination (incorporation of foreign matter and contamination) due to wear of media. The present invention will be described in detail below using an example in which ultrasonic dispersion is used.

[0126] Before ultrasonic dispersion, the pigment and aqueous medium may be mixed and stirred as needed. The viscosity range at this time is usually 0.1 to 100 mPa·s to ensure fluidity. The pigment concentration at this time is not particularly limited, but may be in the range of 1 to 30% by mass. The ultrasonic irradiation conditions are not particularly limited, and ultrasonic irradiation can be performed at an output of 100 to 3000 W and a frequency of 15 to 40 kHz.

[0127] The ultrasonic irradiation time should be long enough to substantially uniformly disperse the pigment particles, nanocellulose, etc. in the aqueous pigment dispersion. The amount of power applied is usually 5 to 100 W / g relative to the mass of the pigment contained in the dispersion.

[0128] After ultrasonic irradiation of the aqueous pigment dispersion is completed, further dispersion may be performed as necessary. Dispersion and ultrasonic irradiation may also be repeated. Dispersion equipment that can be used in this dispersion step may be any of various known types of equipment, and is not particularly limited. Examples include media dispersion equipment such as a sand mill, bead mill, pebble mill, ball mill, pearl mill, basket mill, attritor, dyno mill, bore mill, viscomill, mortar mill, SC mill, dry mill, and paint conditioner, and medialess dispersion equipment such as a high-speed disc impeller, colloid mill, high-pressure homogenizer, nanomizer, and ultimizer.

[0129] The temperature of the aqueous pigment dispersion to be subjected to ultrasonic irradiation is not particularly limited, but it is preferable to irradiate the aqueous pigment dispersion with ultrasonic waves while controlling the temperature to be between the solidification point and 70°C.

[0130] (2) Method via High-Concentration Aqueous Pigment Dispersion (Pigment Paste) There are no particular limitations on the method for preparing a pigment paste in advance, and any known dispersion method can be used.

[0131] Examples of methods for preparing this pigment paste include the following (i) to (iii): (i) a method of adding a pigment to an aqueous medium containing a pigment dispersant and water, and then dispersing the pigment in the aqueous medium using a stirring and dispersing device to prepare a pigment paste; (ii) a method of kneading a pigment and a pigment dispersant using a kneader such as a two-roll mill or a mixer, adding the resulting kneaded mixture to an aqueous medium containing water, and preparing a pigment paste using a stirring and dispersing device; and (iii) a method of dissolving a pigment dispersant in a water-compatible organic solvent such as methyl ethyl ketone or tetrahydrofuran, adding the pigment to the resulting solution, dispersing the pigment in the organic solution using a stirring and dispersing device, subsequently carrying out phase inversion emulsification using an aqueous medium, and then distilling off the organic solvent to prepare a pigment paste.

[0132] The kneading machine is not particularly limited, and examples thereof include a Henschel mixer, a pressure kneader, a Banbury mixer, a planetary mixer, etc. The stirring / dispersing device is also not particularly limited, and examples thereof include an ultrasonic homogenizer, a high-pressure homogenizer, a paint shaker, a ball mill, a roll mill, a sand mill, a sand grinder, a Dyno Mill, a Dispermat, an SC mill, a Nanomizer, etc. One of these may be used alone, or two or more types of devices may be used in combination.

[0133] The amount of the pigment in the pigment paste may be 5 to 60% by mass.

[0134] Residual coarse particles can cause deterioration of various image characteristics, so it is preferable to remove the coarse particles as appropriate by centrifugation, filtration, or the like before or after preparing the ink.

[0135] After the dispersion step, an impurity removal step using ion exchange treatment or ultrafiltration treatment may be performed, followed by post-treatment. Ion exchange treatment can remove ionic substances such as cations and anions (divalent metal ions, etc.), and ultrafiltration treatment can remove dissolved impurities (residual substances from pigment synthesis, excess components in the dispersion composition, resin not adsorbed to the organic pigment, contaminants, etc.). A known ion exchange resin is used for the ion exchange treatment.

[0136] The nanocellulose included in the present invention is derived from oxidized cellulose, and the oxidized cellulose is easily defibrated. The aqueous ink composition of the present invention can also be made into an aqueous ink composition containing nanocellulose by blending the oxidized cellulose with other ingredients of the aqueous ink composition and appropriately defibrating and nano-sizing it.

[0137] As described above, oxidized cellulose can be used in the production of an aqueous ink composition. During production, the oxidized cellulose is defibrated in the composition by a dispersion or kneading operation to form nanocellulose. Specifically, nanocellulose can be obtained by blending the oxidized cellulose with materials other than the oxidized cellulose in the aqueous ink composition and defibrating the mixture by stirring, such as a dispersion or kneading operation, or by the user defibrating the oxidized cellulose themselves to form nanocellulose. The stirring can be performed by the method described above (Step B: Defibration Treatment).

[0138] One aspect of the present invention is a method for producing a binder composition using oxidized cellulose, which contains an oxidation product of a cellulose-based raw material with hypochlorous acid or a salt thereof and is substantially free of N-oxyl compounds. Specifically, the method is a method for producing an aqueous ink composition containing nanocellulose, which includes a step of defibrating the oxidized cellulose by stirring a mixture containing oxidized cellulose and the materials of the aqueous ink composition other than nanocellulose to obtain the aqueous ink composition. Another aspect of the present invention is a method for producing an aqueous ink composition containing nanocellulose, which includes a step of defibrating the oxidized cellulose by stirring the oxidized cellulose and subsequently mixing the materials of the aqueous ink composition other than nanocellulose to obtain the aqueous ink composition. The nanocellulose, oxidized cellulose, and aqueous ink composition are as described above. The materials of the aqueous ink composition other than nanocellulose refer to any material other than nanocellulose that can be contained in the aqueous ink composition, including, but not limited to, the pigments, dispersants, and solvents described above. Furthermore, in this specification, "continuously mixing materials" means that the oxidized cellulose is refined by stirring and the materials are added in succession. Specific embodiments in which stirring and addition are carried out in succession include, for example, an embodiment in which stirring and micronizing oxidized cellulose and adding the above-mentioned materials are carried out in a one-pot operation; an embodiment in which the above-mentioned materials are added simultaneously while stirring the oxidized cellulose; and the like, but are not limited to these.

[0139] In the production method of the present invention, when stirring oxidized cellulose to at least partially pulverize it, any operation that disperses the components that make up the nanocellulose-containing composition can be used without particular limitations. For example, any velocity field and velocity fluctuation of any intensity; collision with inclusions or obstacles; ultrasound; pressure loading; etc. can be utilized. A submerged disperser can be suitably used for such dispersion operations. Therefore, in one embodiment of the production method of the present invention, stirring is performed using a submerged disperser. The submerged disperser is not particularly limited, and examples include methods using a homomixer, magnetic stirrer, stirring rod, a stirrer with stirring blades, a disperser-type mixer, a homogenizer, an external circulation stirrer, a planetary stirrer, a vibration stirrer, an ultrasonic disperser, etc. In addition to the above-mentioned devices, examples of the submerged disperser include a rotary shear type agitator, a colloid mill, a roll mill, a pressure homogenizer, a container-driven mill, a media stirring mill, etc. Furthermore, a kneader can be used as the submerged disperser. A rotary shear agitator is a device that disperses materials to be stirred by passing them through the gap between a rotating blade and an outer cylinder, and disperses them by shear flow in the gap and strong speed fluctuations between the front and rear. A colloid mill is a device that disperses materials by shear flow in the gap between a rotating disk and a fixed disk. A roll mill disperses materials by shear force and compression force utilizing the gap between multiple rotating rolls. A pressure homogenizer is used as a disperser that ejects slurries and the like from fine holes at high pressure, and is also called a pressure injection disperser. A preferred pressure homogenizer is a high-pressure homogenizer. A high-pressure homogenizer is one that can eject slurries at a pressure of, for example, 10 MPa or more, preferably 100 MPa or more. Examples of high-pressure homogenizers include counter-impingement high-pressure homogenizers such as microfluidizers and wet jet mills. Vessel-driven mills are devices that disperse materials by the collision and friction of media such as balls in a vessel, and specific examples include rotary mills, vibration mills, and planetary mills. Media-agitated mills are devices that use media such as balls or beads to disperse materials by the impact and shear forces of the media, and specific examples include attritors and bead mills (sand mills).A kneader is a device that wets powders and the like with a liquid (also known as kneading or kneading). Specific examples include a twin-arm kneader (a device that disperses materials using two mixing blades in two semi-cylindrical containers); a Banbury mixer (a device that disperses materials under pressure in a closed system); and extrusion-type kneaders such as a screw extruder, co-kneader, and extruder. These devices may be used alone or in combination of two or more. While the oxidized cellulose can be refined by stirring using such devices, stirring may be continued until the components of the nanocellulose-containing composition are homogenized or emulsified. This allows the nanocellulose to be uniformly dispersed in the nanocellulose-containing composition, and also allows the nanocellulose-containing composition to be obtained as an emulsion.

[0140] The aqueous ink composition containing nanocellulose obtained by the production method using oxidized cellulose can be recovered by appropriate post-treatment and used as an aqueous ink composition. The solvent used in the polymerization reaction of the anionic group-containing organic polymer compound may or may not be removed. The resulting product may also be filtered and washed to produce an aqueous ink composition.

[0141] The presence of nanocellulose in the aqueous ink composition of the present invention can be determined by the physical properties of the aqueous ink composition. That is, this can be determined by whether the composition has the function provided by nanocellulose, for example, whether the aqueous ink composition has a slurry state or a viscous state. Specifically, when compared with a composition that does not contain nanocellulose, it can be determined that the composition obtained by the manufacturing method of the present invention contains nanocellulose because it is in a slurry state, thickens, or does not precipitate. Furthermore, when compared with a composition containing nanocellulose produced in accordance with WO 2018 / 230354, for example, the composition obtained by the manufacturing method of the present invention can be determined to contain nanocellulose because it has a slurry state and viscosity equivalent to the composition.

[0142] Whether the nanocellulose-containing composition obtained by the production method of the present invention contains nanocellulose can also be determined by observing it with a transmission phase contrast optical microscope and determining whether the oxidized cellulose used remains coarse (whether the original oxidized cellulose is maintained).

[0143] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass".

[0144] Various physical properties of oxidized cellulose or nanocellulose were measured by the following methods.

[0145] [Measurement of Average Fiber Length and Average Fiber Width] Pure water was added to the nanocellulose dispersion, and the nanocellulose concentration in the nanocellulose aqueous dispersion was adjusted to 5 ppm. After adjusting the concentration, the nanocellulose aqueous dispersion was air-dried on a mica substrate, and the nanocellulose shape was observed in AC mode using an Oxford Asylum scanning probe microscope "MFP-3D Infinity." The obtained images were binarized and analyzed using the image processing software "ImageJ." For 100 or more fibers, the number-average fiber length was calculated as fiber length = "perimeter" ÷ 2. For 50 or more fibers, the number-average fiber width [nm] was calculated using the software included with the "MFP-3D Infinity." The cross-sectional height of the shape image was calculated as fiber width.

[0146] [Measurement of Light Transmittance] The dispersion containing nanocellulose was placed in a quartz cell with a thickness of 10 mm, and the light transmittance at a wavelength of 660 nm was measured using a spectrophotometer (JASCO V-550).

[0147] [Measurement of Viscosity-Average Degree of Polymerization] Oxidized cellulose was added to an aqueous solution of sodium borohydride adjusted to pH 10 and reduced at 25°C for 5 hours. The amount of sodium borohydride was 0.1 g per 1 g of oxidized cellulose. After reduction, solid-liquid separation was performed by suction filtration and washing with water. The resulting oxidized cellulose was freeze-dried. 0.04 g of dried oxidized cellulose was added to 10 ml of pure water and stirred for 2 minutes, followed by dissolution with 10 ml of 1 M copper ethylenediamine solution. The flow time of the blank solution and the cellulose solution were then measured at 25°C using a capillary viscometer. The relative viscosity (ηr), specific viscosity (ηsp), and intrinsic viscosity ([η]) were calculated sequentially from the flow time of the blank solution (t0), the flow time of the cellulose solution (t), and the oxidized cellulose concentration (c [g / ml]) according to the following equations. The degree of polymerization (DP) of the oxidized cellulose was then calculated using the viscosity measurement equation. ηr=η / η0=t / t0 ηsp=ηr−1 [η]=ηsp / (100×c(1+0.28ηsp)) DP=175×[η]

[0148] [Measurement of Carboxy Group Amount in Oxidized Cellulose] To 60 ml of an oxidized cellulose aqueous dispersion in which the oxidized cellulose concentration had been adjusted to 0.5% by mass, 0.1 M aqueous hydrochloric acid was added to adjust the pH to 2.5, and then 0.05 N aqueous sodium hydroxide solution was added dropwise, and the electrical conductivity was measured until the pH reached 11.0. The amount of carboxy groups (mmol / g) was calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity was gradual: Amount of Carboxy Groups = a (ml) × 0.05 / Mass of Oxidized Cellulose (g).

[0149] Production Example 1: Production of Oxidized Cellulose A 350 g of sodium hypochlorite pentahydrate crystals with an available chlorine concentration of 42% by mass were placed in a beaker, and purified water was added and stirred to adjust the available chlorine concentration to 21% by mass. 35% by mass hydrochloric acid was added thereto and stirred to obtain an aqueous solution with a pH of 11. The sodium hypochlorite aqueous solution was heated to 30°C in a constant-temperature water bath while being stirred at 200 rpm using a propeller-type stirring blade in a stirrer (Three-One Motor, BL600) manufactured by Shinto Scientific Co., Ltd., and 50 g of powdered pulp (VP-1) manufactured by TDI Corporation, a cellulosic raw material, was then added. After supplying the cellulosic raw material, the mixture was kept at 30°C in the same constant-temperature water bath, and the pH during the reaction was adjusted to 11 by adding 48% by mass sodium hydroxide. The reaction was continued for 30 minutes under the same stirring conditions. Thereafter, pure water was added to dilute the solution two-fold, and sodium hydroxide was added to adjust the pH to 13, slowing down the oxidation reaction and obtaining a cellulose oxide dispersed in water. Then, hydrochloric acid was added to convert the carboxyl groups of the cellulose oxide into salts (-COO － Na ＋ ) to proton type (-COO － H ＋ ) to obtain an aqueous dispersion with a pH of 2.5. In this example, the pH was controlled using a pH controller (Tokyo Glass Instruments Co., Ltd., FD-02). The obtained aqueous dispersion with a pH of 2.5 was subjected to solid-liquid separation and washing. Specifically, the supernatant was removed by centrifugation (1000 G, 10 minutes) and decantation, and an amount of pure water equivalent to the amount removed was added, followed by thorough stirring with a spoon to make it homogenous. This procedure was repeated six times, and finally the above centrifugation and decantation were carried out to obtain oxidized cellulose. Thereafter, sodium hydroxide was added in an amount approximately equimolar to the amount of carboxy groups introduced, and the carboxylic acid groups were converted to the proton type (-COO － H ＋ ) to salt form (-COO － Na ＋) and adjusted to pH 7.5. The temperature of the resulting aqueous dispersion slurry was raised to 30°C, and sodium borohydride was added in an amount of 0.2 mmol / g relative to the cellulose-based raw material, followed by a reduction treatment by reaction for 2 hours to obtain oxidized cellulose A. The content concentration of this oxidized cellulose was 10 mass%, and the degree of polymerization (viscosity average degree of polymerization) of oxidized cellulose A was 90, and the amount of carboxy groups was 0.65 mmol / g.

[0150] The effective chlorine concentration in the sodium hypochlorite aqueous solution was measured by the following method. (Measurement of effective chlorine concentration in sodium hypochlorite aqueous solution) 0.582 g of an aqueous solution prepared by adding sodium hypochlorite pentahydrate crystals to pure water was precisely weighed, 50 ml of pure water was added, 2 g of potassium iodide and 10 ml of acetic acid were added, and the container was immediately sealed and left in the dark for 15 minutes. After leaving the container for 15 minutes, the liberated iodine was titrated with 0.1 mol / L sodium thiosulfate solution (indicator: starch test solution), and the titration amount was 34.55 ml. A blank test was separately performed to correct for this, and 1 ml of 0.1 mol / L sodium thiosulfate solution corresponded to 3.545 mg Cl, so the effective chlorine concentration in the sodium hypochlorite aqueous solution was 21% by mass.

[0151] [Production Example 2: Production of oxidized cellulose B] Sodium hydroxide was replaced with potassium hydroxide, and the carboxylic acid group was converted to a proton type (—COO － H ＋ ) to salt form (-COO － K ＋ Oxidized cellulose B was obtained by carrying out the same process as in Production Example 1, except that the oxidized cellulose content was 10% by mass.

[0152] [Production Example 3: Production of oxidized cellulose C] Sodium hydroxide was replaced with JEFFAMINE 2070 (PO / EO modified amine, Mw 2000, PO / EO = 29 / 6 molar ratio), and the carboxylic acid group was converted to a proton type (-COO － H ＋ Oxidized cellulose C was obtained by the same treatment as in Production Example 1, except that oxidized cellulose C was converted from cellulose nitrate to an amine-modified cellulose. The concentration of oxidized cellulose in this oxidized cellulose was 10% by mass.

[0153] The nanocellulose I-2SX used in Comparative Examples 1 and 2 is nanocellulose obtained by pulverization by TEMPO oxidation. The degree of polymerization (viscosity average degree of polymerization) of I-2SX was 461, and the amount of carboxyl groups was 2.0 mmol / g.

[0154] <Anionic Group-Containing Organic Polymer Compound> The following was used as the anionic group-containing organic polymer compound. (Anionic Group-Containing Acrylic Resin) Resin A was prepared using a monomer composition ratio of styrene / methacrylic acid / acrylic acid = 77 / 13 / 10 (mass ratio), with a weight average molecular weight of 8,800, an acid value of 150 mgKOH / g, and a glass transition point of 107°C. 50 parts of methyl ethyl ketone (hereinafter abbreviated as MEK), 50 parts of the resin A, 87.4 parts of ion-exchanged water, and 22 g of a 34% by mass aqueous potassium hydroxide (KOH) solution were added and thoroughly stirred to obtain a resin solution. MEK was removed from this resin solution under reduced pressure conditions of a water bath temperature of 45°C and 40 hPa, and the resulting resin solution had a resin solids content of 20%, which was designated as a solution (SA-1) of anionic group-containing acrylic resin.

[0155] (Polyurethane-based resin having anionic groups) 64.2 parts by mass of methyl ethyl ketone was added to a nitrogen-substituted vessel equipped with a thermometer, a nitrogen gas inlet tube, and a stirrer, and 18.4 parts by mass of 2,2-dimethylolpropionic acid and 33.9 parts by mass of isophorone diisocyanate were mixed in the methyl ethyl ketone and reacted at 80°C for 4 hours. After 4 hours, an additional 38.2 parts by mass of methyl ethyl ketone was added, and the mixture was cooled to below 60°C. Then, 140.1 parts by mass of polyether polyol ("PTMG2000" polytetramethylene glycol, number average molecular weight 1000, manufactured by Mitsubishi Chemical Corporation) and 0.01 parts by mass of dibutyltin dilaurate (hereinafter referred to as DBTDL) were added, and the reaction was continued at 80°C. After confirming that the weight average molecular weight of the reactant had reached the range of 20,000 to 50,000, the reaction was terminated by adding 1.3 parts by mass of methanol. Next, 41.6 parts by mass of methyl ethyl ketone was added to obtain a solution of urethane resin in an organic solvent. 15.1 parts by mass of a 50% by mass aqueous potassium hydroxide solution was added to the organic solvent solution of urethane resin to partially or completely neutralize the carboxy groups of the urethane resin. 848.5 parts by mass of water was then added and thoroughly stirred to obtain a mixture containing urethane resin, methyl ethyl ketone, and water, with the urethane resin dispersed or dissolved in the water. After aging the mixture for approximately 2 hours, 0.07 parts by mass of Surfynol 440 (manufactured by Air Products, an ethylene oxide adduct of acetylene glycol, nonvolatile content 100% by mass) was added to the mixture and stirred for approximately 20 minutes to obtain a mixture, which was then distilled under reduced pressure of approximately 1 to 50 kPa. After confirming that 144 parts by mass of methyl ethyl ketone had been removed from the mixture, 0.03 parts by mass of Surfynol 440 (manufactured by Air Products) was added under reduced pressure, and the distillation under reduced pressure was continued. After confirming that 147 parts by mass of water contained in the mixture had been removed, the vacuum distillation was terminated. Water was then added to adjust the nonvolatile content to 20% by mass of the resin solids, which was used as a solution of a polyurethane resin having an anionic group (UR-1).

[0156] (Polysaccharide Derivative Having Anionic Group) Commercially available DN-800H (manufactured by Daicel FineChem Co., Ltd.): carboxymethylcellulose ammonium was used.

[0157] <Pigments and the like> The following commercially available pigments were used: #960 (manufactured by Mitsubishi Chemical Corporation): carbon black; FASTOGEN Blue TGR (manufactured by DIC Corporation): CI Pigment Blue 15:3; FASTOGEN Super Magenta RY (manufactured by DIC Corporation): CI Pigment Red 122; Fast Yellow 7413 (manufactured by Sanyo Pigment Co., Ltd.): CI Pigment Yellow 74

[0158] Example 1: Preparation of aqueous ink composition: To a metal beaker, 60 parts of "#960" (carbon black manufactured by Mitsubishi Chemical Corporation), 14 parts of a solution of an anionic group-containing organic polymer compound (SA-1), 20 parts of triethylene glycol, and 20 parts of 2-pyrrolidone were added. The pH was adjusted to between 9 and 10.5 with 34% potassium hydroxide, and pure water was added to a total volume of 300 parts. The mixture was then stirred and mixed for 3 minutes at 120 RPM using a Three-One motor. The mixture was then ultrasonically dispersed for 10 minutes using an ultrasonic disperser (Hielscher UP200St, operating frequency: 26 kHz, operating power: 160 W). Thirty parts of oxidized cellulose A (10% by mass) weighed in a separate container were added with pure water to adjust the oxidized cellulose concentration to 1% by mass, and the mixture was then defibrated using a homomixer (10,000 rpm, 10 minutes) to obtain nanocellulose A with an average fiber length of 200 nm and an average fiber width of 3 nm. A portion of the resulting aqueous dispersion of nanocellulose A was removed and pure water was added to prepare an aqueous dispersion with a solids concentration of 0.1% by mass. The light transmittance (660 nm) was 97%, confirming that the oxidized cellulose had been nanosized. The aqueous dispersion of nanocellulose A was added to the mixture after ultrasonic dispersion, resulting in 200 parts of the mixture (2 parts in terms of the solids content of nanocellulose A). Two parts of Surfynol 440 were added and manually stirred. Pure water was added to bring the total volume to 1,000 parts, and the mixture was ultrasonically dispersed for 25 minutes using an ultrasonic disperser. The resulting liquid was filtered through a 1.2 μm membrane filter to obtain the aqueous ink composition of Example 1.

[0159] Examples 2 to 10 Aqueous ink compositions were obtained in the same manner as in Example 1, except that the type of pigment used, the type and amount of anionic group-containing organic polymer compound, and the type and amount of nanocellulose were changed to the formulations shown in Table 1. In Table 1, nanocelluloses A to C are derived from oxidized celluloses A to C, respectively.

[0160] Example 11 To a metal beaker were added 20 parts of oxidized cellulose A (10% by mass), 60 parts of "#960" (carbon black manufactured by Mitsubishi Chemical Corporation), 14 parts of a solution of an anionic group-containing organic polymer compound (SA-1), 20 parts of triethylene glycol, and 20 parts of 2-pyrrolidone. The pH was adjusted to between 9 and 10.5 with 34% potassium hydroxide, and pure water was added to bring the total volume to 300 parts to obtain a mixture. This mixture was stirred and mixed for 3 minutes at 120 RPM using a Three-One motor. It was then ultrasonically dispersed for 10 minutes using an ultrasonic disperser (UP200St manufactured by Hielscher, operating frequency: 26 kHz, operating power: 160 W). To the ultrasonically dispersed mixture, 2 parts of Surfynol 440 were added and manually stirred. Pure water was then added to bring the total volume to 1,000 parts, and the mixture was ultrasonically dispersed for 25 minutes using the ultrasonic disperser. The resulting solution was filtered through a 1.2 μm membrane filter to obtain the aqueous ink composition of Example 11. The nano-sized oxidized cellulose was confirmed by the following procedure. An aqueous dispersion was prepared using 20 parts of oxidized cellulose A (10% by mass) and pure water to a total volume of 300 parts. That is, the concentration of oxidized cellulose A in this aqueous dispersion was adjusted to the same concentration as that of oxidized cellulose A in the above mixture. The mixture was stirred and mixed for 3 minutes at 120 RPM using a Three-One motor. It was then ultrasonically dispersed for 10 minutes using an ultrasonic disperser (Hielscher UP200St, operating frequency: 26 kHz, operating power: 160 W). The dispersed dispersion was confirmed to be nanocellulose A with an average fiber length of 200 nm and an average fiber width of 3 nm. Furthermore, when a portion of the aqueous dispersion of nanocellulose A was taken and pure water was added to prepare an aqueous dispersion with a solids concentration of 0.1% by mass, the light transmittance (660 nm) was 97%, confirming that the oxidized cellulose had been nano-sized.

[0161] Comparative Example 1: To a metal beaker, 60 parts of "#960" (carbon black manufactured by Mitsubishi Chemical Corporation), 14 parts of a solution of an anionic group-containing organic polymer compound (SA-1), 20 parts of triethylene glycol, and 20 parts of 2-pyrrolidone were added, and the pH was adjusted to between 9 and 10.5 with 34% potassium hydroxide. Pure water was added to bring the total volume to 300 parts, and the mixture was stirred and mixed for 3 minutes at 120 RPM using a Three-One motor. Subsequently, the mixture was ultrasonically dispersed for 10 minutes using an ultrasonic disperser (Hielscher UP200St, operating frequency: 26 kHz, operating power: 160 W). Pure water was added to 30 parts of I-2SX weighed in a separate container, and the content concentration was adjusted to 1% by mass. An aqueous dispersion of I-2SX was added to the mixture after ultrasonic dispersion to obtain 200 parts of a mixture (2 parts in terms of the solids content of I-2SX), to which 2 parts of Surfynol 440 was added and manually stirred, and then pure water was added so that the total amount became 1000 parts, and the mixture was ultrasonically dispersed for 25 minutes using an ultrasonic disperser. The resulting liquid was filtered through a 1.2 μm membrane filter to obtain the aqueous ink composition of Comparative Example 1.

[0162] Comparative Example 2 A water-based ink composition was obtained in the same manner as in Comparative Example 1, except that the amounts of the anionic group-containing organic polymer compound and I-2SX added were changed to the formulations shown in Table 1.

[0163] Table 1 shows the amounts of the aqueous ink compositions used and the evaluation results of their physical properties.

[0164]

[0165] The physical properties of the aqueous ink composition were evaluated as follows.

[0166] <Measurement of Ink pH> Measurement was carried out using an MM-60R (manufactured by DKK-TOA Corporation) (ink temperature 25° C.).

[0167] <Inkjet (IJ) Suitability> With regard to ejection properties and printing performance, the aqueous inks of the Examples and Comparative Examples were loaded into an inkjet recording device (ENVY4500 manufactured by Hewlett-Packard) equipped with a thermal inkjet nozzle in a constant temperature and humidity room (room temperature 25°C, humidity 50%). Subsequently, a test printing pattern (with text, ruled lines, and solid areas) was printed continuously for 10 minutes on PPC paper as the recording material. After 10 minutes of continuous printing, the paper surface was visually observed by three panelists, and the printed pattern was evaluated for chipping or bleeding according to the following criteria: A: Three panelists did not notice any chipping or bleeding in the printed pattern. B: Only one panelist noticed chipping or bleeding in the printed pattern (the other two panelists did not notice any chipping or bleeding in the printed pattern). C: Two or more panelists noticed chipping or bleeding in the printed pattern.

[0168] <Measurement of Optical Density (OD) Value> Using an inkjet recording device equipped with the above-described thermal inkjet nozzle, a test printing pattern (solid area) was printed on PPC paper as a recording material using the aqueous inks of the Examples and Comparative Examples to prepare evaluation samples. The optical density (OD value) at three points in the evaluation sample was measured using a SpectroEye spectrophotometer manufactured by X-Rite (light source: D50, viewing angle: 2°, ANSI-T), and the average value was calculated. The optical density (OD value) was evaluated according to the following evaluation criteria. Optical Density (OD Value) Evaluation Criteria A: The optical density (OD value) was 1.40 or more. B: The optical density (OD value) was 1.35 or more and less than 1.40. C: The optical density (OD value) was 1.30 or more and less than 1.35. D: The optical density (OD value) was less than 1.30.

[0169] (Abrasion Resistance Test) To examine the ink durability and adhesion, the prepared ink was applied to glossy paper using a wire bar #3. After air drying for 24 hours, the applied surface was rubbed with a 45R abrader wrapped with PPC paper for abrasion using a Gakushin-type abrasion tester (manufactured by Daiei Scientific Instruments Co., Ltd.) under conditions of 200 g or 500 g load and 10 abrasions. Five panelists then visually evaluated the condition of the applied surface and rated it according to the following criteria: 5: No scratches at both 200 g and 500 g loads, very good; 4: No scratches at 200 g load, but slight scratches at 500 g load; 3: Slight scratches at 200 g load, but no practical problems; 2: Noticeable scratches at 200 g load; 1: Peeling of the coating at 200 g load.

[0170] According to the present invention, it is possible to provide an aqueous pigment dispersion and an aqueous ink composition that have excellent pigment dispersibility and that can easily and inexpensively form a coating film that is excellent in fastness, adhesion, color density, etc.

Claims

1. 1. An aqueous ink composition comprising nanocellulose, The nanocellulose comprises an oxide of a cellulosic raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds; Aqueous ink compositions.

2. An aqueous ink composition comprising nanocellulose, The nanocellulose has a structure in which the second and third hydroxyl groups of the glucopyranose ring are oxidized to introduce carboxyl groups, Aqueous ink compositions.

3. Further comprising an anionic group-containing organic polymer compound; The aqueous ink composition according to claim 1 or 2.

4. At least a portion of the carboxyl groups of the nanocellulose are modified with at least one selected from the group consisting of metals, ammonia, amines, and quaternary ammonium. The aqueous ink composition according to any one of claims 1 to 3.

5. For inkjet recording, The aqueous ink composition according to any one of claims 1 to 4.

6. A recording method comprising a step of ejecting the aqueous ink composition according to any one of claims 1 to 4 onto a recording medium by an inkjet system.

7. A method for producing an aqueous ink composition containing nanocellulose, comprising the steps of: The method includes a step of defibrating the oxidized cellulose by stirring a mixture containing oxidized cellulose and materials other than nanocellulose of the aqueous ink composition to obtain an aqueous ink composition containing nanocellulose, The oxidized cellulose comprises an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds. Manufacturing method.

8. A method for producing an aqueous ink composition containing nanocellulose, comprising the steps of: The method includes a step of defibrating the oxidized cellulose by stirring a mixture containing oxidized cellulose and materials other than nanocellulose of the aqueous ink composition to obtain an aqueous ink composition containing nanocellulose, The oxidized cellulose has a structure in which the second and third hydroxyl groups of the glucopyranose ring are oxidized and carboxyl groups are introduced. Manufacturing method.

9. A method for producing an aqueous ink composition containing nanocellulose, comprising the steps of: The method includes a step of defibrating the oxidized cellulose by stirring the oxidized cellulose and continuously mixing the materials other than nanocellulose of the aqueous ink composition to obtain an aqueous ink composition containing nanocellulose, The oxidized cellulose comprises an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds. Manufacturing method.

10. A method for producing an aqueous ink composition containing nanocellulose, comprising the steps of: The method includes a step of defibrating the oxidized cellulose by stirring the oxidized cellulose and continuously mixing the materials other than nanocellulose of the aqueous ink composition to obtain an aqueous ink composition containing nanocellulose, The oxidized cellulose has a structure in which the second and third hydroxyl groups of the glucopyranose ring are oxidized and carboxyl groups are introduced. Manufacturing method.

11. The material further comprises an anionic group-containing organic polymer compound. The method according to any one of claims 7 to 10.

12. At least a portion of the carboxy groups of the oxidized cellulose are modified with at least one selected from the group consisting of metals, ammonia, amines, and quaternary ammonium. The method according to any one of claims 7 to 11.