Nanocellulose and dispersion thereof

Abstract

To provide a nanocellulose that does not contain an N-oxyl compound and exhibits excellent dispersion stability in a dispersion medium.SOLUTION: Provided is a nanocellulose which is an oxide of a cellulose raw material by hypochlorous acid or a salt thereof, has an average fiber width of 1 nm to 200 nm, substantially does not contain an N-oxyl compound, and has a zeta potential of -30 mV or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to nanocellulose and a dispersion thereof. More specifically, the present invention relates to nanocellulose obtained by defibrating oxidized cellulose obtained by oxidizing a cellulosic raw material with an oxidizing agent, and a nanocellulose dispersion containing the same. [Background technology]

[0002] Various techniques have been proposed for producing nanocellulose materials such as cellulose nanofibers (hereinafter also referred to as CNF) by oxidizing various cellulosic raw materials with an oxidizing agent and then micronizing the resulting oxidized cellulose (see, for example, Patent Documents 1 and 2).

[0003] Patent Document 1 discloses a method for oxidizing a cellulosic raw material using hypochlorous acid or its salts as an oxidizing agent under high-concentration conditions of 14 to 43% available chlorine in the reaction system, and then defibrating and nano-sizing the resulting oxidized cellulose. Patent Document 2 discloses a method for oxidizing a cellulosic raw material using hypochlorous acid or its salts as an oxidizing agent, setting the available chlorine concentration in the reaction system to 6 to 14% available chlorine, and adjusting the pH to 5.0 to 14.0, and then defibrating and nano-sizing the resulting oxidized cellulose. These techniques perform the oxidation process without using N-oxyl compounds such as 2,2,6,6-tetramethyl-1-piperidine-N-oxyl radical (TEMPO) as a catalyst, so that N-oxyl compounds do not remain in the cellulose fibers. This makes it possible to produce nanocellulose materials while reducing their impact on the environment. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2018 / 230354 [Patent Document 2] International Publication No. 2020 / 027307 Summary of the Invention [Problem to be solved by the invention]

[0005] Nanocellulose materials are sometimes used in a dispersed state in a dispersion medium such as water or an organic solvent to facilitate mixing with other materials (e.g., resins). Nanocellulose materials are also sometimes used in a slurry state by mixing with inorganic particles such as pigments and a dispersion medium. In this case, the nanocellulose material is required to exhibit good dispersion stability in the dispersion medium.

[0006] The present invention was made in consideration of the above circumstances, and its main object is to provide nanocellulose that does not contain N-oxyl compounds in cellulose fibers and has excellent dispersion stability in a dispersion medium. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides the following means. [1] Nanocellulose that is an oxide of a cellulose-based raw material using hypochlorous acid or its salt, has an average fiber width of 1 nm or more and 200 nm or less, is substantially free of N-oxyl compounds, and has a zeta potential of -30 mV or less. [2] Nanocellulose of [1] having an average fiber width of 1 nm or more and 5 nm or less. [3] Nanocellulose according to [1] or [2], having an aspect ratio of 20 or more and 150 or less. [4] Nanocellulose according to any one of [1] to [3], which has a light transmittance of 95% or more in a mixed liquid obtained by mixing with water to a solid content concentration of 0.1% by mass. [5] Nanocellulose, which is an oxidation product of a cellulose-based raw material using hypochlorous acid or its salt, does not contain N-oxyl compounds, and has an average fiber width of 1 nm or more and 5 nm or less. [6] Nanocellulose, which is an oxidation product of a cellulose-based raw material using hypochlorous acid or its salt, has an average fiber width of 1 nm or more and 200 nm or less, does not contain N-oxyl compounds, and has an aspect ratio of 20 or more and 150 or less. [7] Nanocellulose, which is an oxide of a cellulose-based raw material using hypochlorous acid or its salt, has an average fiber width of 1 nm or more and 200 nm or less, does not contain N-oxyl compounds, and has a light transmittance of 95% or more when mixed with water to a solids concentration of 0.1% by mass. [8] A nanocellulose dispersion in which any one of the nanocelluloses [1] to [7] is dispersed in a dispersion medium. [Effects of the Invention]

[0008] According to the present invention, nanocellulose with excellent dispersion stability in a dispersion medium can be obtained. Furthermore, since it does not contain N-oxyl compounds, its impact on the environment can be reduced. DETAILED DESCRIPTION OF THE INVENTION

[0009] Nanocellulose The nanocellulose of the present disclosure (hereinafter also referred to as "the present nanocellulose") is a fibrous nanocellulose obtained by defibrating oxidized cellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof. The oxidized cellulose can also be said to be an oxide of a cellulosic raw material with hypochlorous acid or a salt thereof. The present nanocellulose will be described in detail below. "Nanocellulose" is a fibrous cellulose obtained by micronizing oxidized fibrous cellulose, and is therefore also called "microfine cellulose fiber."

[0010] Nanocellulose is substantially free of N-oxyl compounds because the cellulosic raw material is oxidized with hypochlorous acid or its salt. Here, in this specification, "substantially free of N-oxyl compounds" means that the nanocellulose does not contain any N-oxyl compounds, or the content of N-oxyl compounds is 2.0 mass ppm or less, preferably 1.0 mass ppm or less, relative to the total amount of nanocellulose. Also, when the content of N-oxyl compounds is increased from the cellulosic raw material by preferably 2.0 mass ppm or less, more preferably 1.0 mass ppm or less, it also means "substantially free of N-oxyl compounds." By being substantially free of N-oxyl compounds, it is possible to prevent N-oxyl compounds, which are of concern for their impact on the environment and human body, from remaining in the nanocellulose. The content of N-oxyl compounds can be measured by known means. Known means include a method using a trace total nitrogen analyzer. Specifically, the nitrogen component derived from N-oxyl compounds in nanocellulose can be measured as the amount of nitrogen using a trace total nitrogen analyzer (for example, Mitsubishi Chemical Analytech Co., Ltd., model TN-2100H, etc.).

[0011] First embodiment Fiber Width The average fiber width of the nanocellulose is 1 to 200 nm. When the average fiber width exceeds 200 nm, the proportion of coarse nanocellulose is high, and when nanocellulose is dispersed in a dispersion medium to form a nanocellulose dispersion, there is a tendency for significant nanocellulose precipitation and a decrease in quality. Furthermore, because the quality of the nanocellulose dispersion is not uniform, when it is made into a slurry further containing solid particles such as pigments (hereinafter also referred to as a "nanocellulose-containing slurry"), the viscosity of the slurry tends to be unstable and the handleability and coatability tend to be reduced. From this perspective, the average particle diameter of the nanocellulose is preferably 50 nm or less, more preferably 10 nm or less, and particularly preferably 5 nm or less. Furthermore, when the average fiber width is less than 1 nm, the nanocellulose becomes similar to the state of a single cellulose molecule, and the quality of the nanocellulose tends to be non-uniform. When it is made into a slurry, the viscosity stability, handleability, and coatability tend to be reduced. For this reason, the average fiber width is preferably 1.2 nm or more, more preferably 1.5 nm or more.

[0012] In particular, when the average fiber width of the present nanocellulose is 1 to 5 nm, it is preferable because it not only has dispersion stability in the dispersion medium, but also has good viscosity stability, handleability, and coatability when made into a nanocellulose-containing slurry.

[0013] Aspect Ratio In this nanocellulose, the aspect ratio (average fiber length / average fiber width), which is the ratio of the average fiber width to the average fiber length, is preferably 20 to 150. When the aspect ratio is 150 or less, a uniform and dense network of fine cellulose is easily formed in the dispersion medium, resulting in a stable structure and improved dispersion stability. Furthermore, when a nanocellulose-containing slurry is formed, a uniform and dense network of solid particles and fine cellulose is easily formed, suppressing solid particle aggregation and improving dispersion stability. Furthermore, aggregation between solid particles and nanocellulose, or between nanocellulose particles themselves, can be suppressed, improving the handleability of the slurry and suppressing processing irregularities. From these perspectives, the aspect ratio is more preferably 145 or less, even more preferably 130 or less, even more preferably 120 or less, and even more preferably 100 or less.

[0014] On the other hand, if the aspect ratio is too low, i.e., if the nanocellulose is shaped like a thick rod rather than a long, thin fiber, it becomes difficult to form a network, aggregation occurs due to uneven distribution, and the viscosity stability of the slurry tends to decrease. Furthermore, the viscosity of the slurry increases, which tends to decrease handling properties and result in poor slurry coatability. Therefore, the aspect ratio is more preferably 30 or more, even more preferably 35 or more, and even more preferably 40 or more.

[0015] The average fiber width and average fiber length were calculated by mixing nanocellulose with water to 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 these average fiber widths and lengths. While the image processing conditions are arbitrary, differences in calculated values may occur even for the same image depending on the image processing conditions. The range of difference in values depending on the image processing conditions is preferably within ±100 nm for average fiber length. The range of difference in values depending on the conditions is preferably within ±10 nm for average fiber width. More detailed measurement methods follow the methods described in the Examples below.

[0016] The present nanocellulose preferably has a structure in which at least two of the hydroxyl groups of the glucopyranose ring constituting the cellulose are oxidized, more specifically, the second and third hydroxyl groups of the glucopyranose ring are oxidized and a carboxyl group is introduced. Furthermore, it is preferable that the hydroxyl group at the sixth position of the glucopyranose ring in the present nanocellulose is not oxidized and remains as a hydroxyl group. Note that the position of the carboxyl group in the glucopyranose ring of nanocellulose is determined by the solid 13 It can be analyzed by C-NMR spectroscopy. The above solid 13 The presence of an oxidized structure can be determined by observing peaks corresponding to the carboxy groups at the second and third positions of the glucopyranose ring in a C-NMR spectrum. In this case, the peaks corresponding to the carboxy groups at the second and third positions can be observed as broad peaks in the range of 165 ppm to 185 ppm. The broad peak here can be determined by the peak area ratio. That is, a baseline is drawn around the peak in the range of 165 ppm to 185 ppm in the NMR spectrum, the total area value is calculated, and then the area value is vertically divided at the peak top to calculate the ratio of the two peak area values (large area value / small area value). If the ratio of the peak area values is 1.2 or more, it can be said that the peak is broad. The presence or absence of the 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 more, it can be determined that a broad peak is present. The ratio L' / L may be 0.2 or more, 0.3 or more, 0.4 or more, or even 0.5 or more. There is no particular upper limit to the ratio L' / L, but it is usually 3.0 or less, and may be 2.0 or less, or 1.0 or less.

[0017] In addition, the structure of the above-mentioned glucopyranose ring of this nanocellulose can also be determined by analysis in accordance with the method described in Sustainable Chem. Eng. 2020, 8, 48, 17800-17806.

[0018] [Zeta potential] In a preferred embodiment of the present disclosure, the nanocellulose has a zeta potential of -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, and when made into a slurry, it can achieve excellent viscosity stability, handleability, and coatability. From the perspective of dispersion stability, there is no particular lower limit for the zeta potential. However, 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 and exhibit excellent coatability.

[0019] From the above viewpoint, the zeta potential of the present 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 on an aqueous cellulose dispersion prepared by mixing nanocellulose and water to a nanocellulose concentration of 0.1% by mass at a pH of 8.0 and 20°C. Specifically, it can be measured according to the conditions described in the examples below.

[0020] [Light transmittance] A nanocellulose dispersion in which this nanocellulose is dispersed in a dispersion medium exhibits little light scattering by cellulose fibers and can exhibit high light transmittance. Specifically, in one preferred embodiment, this nanocellulose has a light transmittance of 95% or more in a mixture mixed with water to a solids concentration of 0.1% by mass. Therefore, this nanocellulose and nanocellulose dispersions containing it can be widely used in applications requiring transparency and are useful. 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 measured at a wavelength of 660 nm using a spectrophotometer.

[0021] [Method of manufacturing nanocellulose] Next, we will explain the method for producing this nanocellulose. This nanocellulose can be produced by a method including step A, in which a cellulosic raw material is oxidized with hypochlorous acid or a salt thereof to obtain oxidized cellulose, and step B, in which the oxidized cellulose is defibrated. Since "oxidized cellulose" is oxidized fibrous cellulose, it is also called "oxidized cellulose fiber."

[0022] (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 thereof include pulp, natural cellulose, regenerated cellulose, and fine cellulose obtained by depolymerizing cellulose through mechanical processing. Commercially available cellulosic raw materials, such as crystalline cellulose derived from pulp, can be used as they are. Alternatively, unused biomass containing a large amount of cellulose components, such as soybean pulp refuse or soybean hulls, can also be used as the raw material. Furthermore, the cellulosic raw material may be pre-treated with an alkali of an appropriate concentration in order to facilitate the penetration of the oxidizing agent used into the raw pulp.

[0023] 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.

[0024] 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. The available chlorine concentration of hypochlorous acid or a salt thereof in the reaction solution is preferably 6 to 43% by mass, more preferably 7 to 43% by mass, even more preferably 10 to 43% by mass, and even more preferably 14 to 43% by mass. When the available chlorine concentration of the reaction solution is within the above range, the amount of carboxy groups in the oxidized cellulose can be sufficiently increased, making it easy to defibrate the oxidized cellulose when obtaining nanocellulose.

[0025] From the viewpoint of efficiently and sufficiently increasing the amount of carboxy groups in the oxidized cellulose, the available chlorine concentration of the reaction solution is more preferably 15% 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 any 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.

[0026] The available chlorine concentration of hypochlorous acid or its salts is defined as follows: Hypochlorous acid is a weak acid that exists as an aqueous solution, and hypochlorite is a compound in which the hydrogen of hypochlorous acid is replaced by another cation. For example, sodium hypochlorite, a hypochlorite, exists in a solvent (preferably in an aqueous solution), so the concentration is measured as the amount of available chlorine in the solution, not the concentration of sodium hypochlorite. Here, with regard to the available chlorine of sodium hypochlorite, the oxidizing power of the divalent oxygen atom generated by the decomposition of sodium hypochlorite is equivalent to two atomic equivalents of monovalent chlorine, so the bonded chlorine atoms of sodium hypochlorite (NaClO) have the same oxidizing power as two atoms of unbonded chlorine (Cl2), and the available chlorine = 2 × (chlorine in NaClO). The specific measurement procedure is as follows: First, the sample is precisely weighed, and water, potassium iodide, and acetic acid are added and left to stand. The liberated iodine is titrated with a sodium thiosulfate solution using a starch aqueous solution as an indicator to measure the available chlorine concentration.

[0027] 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 6.0 or higher, even more preferably 7.0 or higher, and even more preferably 8.0 or higher. The upper limit of the pH of the reaction system is more preferably 13.5 or lower, even more preferably 13.0 or lower. The pH range of the reaction system is more preferably 7.0 to 14.0, and even more preferably 8.0 to 13.5.

[0028] 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.

[0029] 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: 6% by mass to 43% by mass) include concentrating a sodium hypochlorite aqueous solution having a lower effective chlorine concentration than the target concentration, diluting a sodium hypochlorite aqueous solution having a higher effective chlorine concentration 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 is easy to adjust the effective chlorine concentration.

[0030] 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.

[0031] To efficiently promote the oxidation reaction of the cellulosic raw material, it is preferable to stir the mixture of the cellulosic raw material and the aqueous sodium hypochlorite solution during the oxidation reaction. Examples of stirring methods include a magnetic stirrer, a stirring rod, a stirrer with stirring blades (Three-One Motor), 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 stirring blades, and disperser-type mixers are preferred, as they allow the oxidation reaction of the cellulosic raw material to proceed smoothly and make it easy to adjust the degree of polymerization of oxidized cellulose to a predetermined value or less. Methods using a stirrer with stirring blades are particularly preferred. When using a stirrer with stirring blades, devices equipped with known stirring blades such as propeller blades, paddle blades, and turbine blades can be used. Furthermore, when using a stirrer with stirring blades, stirring is preferably performed at a rotation speed of 50 to 300 rpm.

[0032] The reaction temperature in the oxidation reaction is preferably 15°C to 100°C, and more preferably 20°C to 90°C. During the reaction, the pH of the reaction system decreases as carboxyl 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 to adjust the pH of the reaction system to the above-mentioned preferred range. The reaction time of 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 to be 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.

[0033] The fiber width and zeta potential of nanocellulose can be adjusted to the desired values by adjusting the reaction time, reaction temperature, stirring conditions, etc. of the oxidation reaction. Specifically, as the reaction time is extended and / or the reaction temperature is increased, oxidation of the surface of the 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 oxidation reaction time, reaction temperature, and stirring conditions (e.g., by extending the reaction time) to promote oxidation (i.e., to increase the degree of oxidation).

[0034] The carboxyl group amount of the oxidized cellulose obtained by the above oxidation reaction is preferably 0.30 to 2.0 mmol / g. When the carboxyl group amount of the oxidized cellulose is 0.30 mmol / g or more, the defibration ability of the oxidized cellulose can be sufficiently high, and nanocellulose with a uniform fiber width can be obtained. This allows the quality of the nanocellulose-containing slurry to be uniform, and the viscosity stability, handleability, and coatability of the slurry can be improved. On the other hand, when the carboxyl group amount is 2.0 mmol / g or less, excessive decomposition of cellulose during the defibration treatment can be suppressed, and nanocellulose with a low proportion of particulate cellulose and uniform quality can be obtained. From this perspective, the carboxyl group amount of the oxidized cellulose is 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 carboxyl group amount may be 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. A preferred range of the carboxyl group amount can be determined by appropriately combining the above-mentioned upper and lower limits. The carboxyl group amount of the present oxidized cellulose is more preferably 0.35 to 2.0 mmol / g, even more preferably 0.35 to 1.5 mmol / g, still 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.

[0035] The amount of carboxyl groups (mmol / g) in oxidized cellulose can be 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, by adding 0.1 M aqueous hydrochloric acid to an aqueous solution containing oxidized cellulose to adjust the pH to 2.5, then adding 0.05 N aqueous sodium hydroxide dropwise and measuring the electrical conductivity until the pH reaches 11. Amount of carboxyl group = a (ml) x 0.05 / mass of oxidized cellulose (g)

[0036] The solution containing oxidized cellulose obtained by the above reaction can be subjected to a known isolation treatment such as filtration, and further purified as necessary to obtain oxidized cellulose as an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof. Furthermore, prior to the isolation treatment such as filtration, in order to improve the filterability and yield of the isolation treatment, an acid can be added to the solution containing oxidized cellulose, for example, to adjust the pH to 4.0 or less, thereby converting at least a portion of the carboxy groups produced by oxidation into salt forms (-COO - X + :X + indicates cations such as sodium and lithium) to proton type (-COO - H + ) can be used. In the infrared absorption spectrum, the proton type has an absorption peak at 1720 cm -1 Nearby, the salt type is 1600cm -1 They can be distinguished from each other by the presence of peaks in the vicinity. The solution containing oxidized cellulose obtained by the above reaction may be subjected to defibration treatment as is.

[0037] When the pH of a solution containing oxidized cellulose is adjusted to 4.0 or less for the isolation treatment, in order to improve the handling when the solution is used for the subsequent defibration treatment, for example, a base is added to adjust the pH to 6.0 or more, and at least a portion of the carboxyl groups are converted to a salt form (-COO - X + :X + indicates a cation such as sodium or lithium). Furthermore, a solution containing oxidized cellulose may be converted into a composition containing oxidized cellulose by, for example, replacing the solvent. In a composition containing oxidized cellulose, for example, the pH may be adjusted to alkaline conditions of 10 or higher, and at least a portion of the carboxy groups may be converted into a salt form (-COO - X + :X + refers to cations such as sodium, lithium, etc.

[0038] The method for producing oxidized cellulose may further include a step of mixing the obtained oxidized cellulose with a compound having a modifying group in order to control the physical properties of the oxidized cellulose. The compound having a modifying group is not particularly limited, as long as it has a modifying group capable of forming an ionic or covalent bond with a carboxyl group or hydroxyl group in the oxidized cellulose. Examples of compounds having a modifying group capable of forming an ionic bond include primary amines, secondary amines, tertiary amines, quaternary ammonium compounds, and phosphonium compounds. Examples of compounds having a modifying group capable of forming a covalent bond include alcohols, isocyanate compounds, and epoxy compounds. As described above, oxidized cellulose includes salt forms, proton forms, and forms modified with modifying groups. Nanocellulose obtained from this oxidized cellulose also includes salt forms, proton forms, and forms modified with modifying groups.

[0039] (Process B: Defibration processing) This nanocellulose can be obtained by defibrating the oxidized cellulose obtained above into nanoparticles. Methods for defibrating oxidized cellulose include weak stirring using a magnetic stirrer or the like, and mechanical defibration. Mechanical defibration of oxidized cellulose is preferred because it allows for sufficient defibration of oxidized cellulose and can shorten the defibration time. Here, nanocellulose (also called nanocellulose) is a general term for cellulose that has been nanosized, and includes cellulose nanofibers, cellulose nanocrystals, etc.

[0040] 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-flow opposing collision-type disperser, beater, disk refiner, conical refiner, double-disc refiner, grinder, single-shaft or multi-shaft kneader, planetary stirrer, vibration stirrer, etc. Nanocellulose can be produced by treating oxidized cellulose with one of these devices alone or in combination of two or more types, preferably in a dispersion medium, to nanosize the oxidized cellulose.

[0041] For defibration of oxidized cellulose, a method using an ultra-high-pressure homogenizer is preferably used, as it allows for the efficient production of nanocellulose with a more advanced defibration process. 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 times, more preferably three or more times. 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, when using the above-mentioned oxidized cellulose, uniform nanocellulose can be obtained even when the defibration treatment is performed under mild defibration conditions.

[0042] 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.

[0043] 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.

[0044] Using an organic solvent as a dispersion medium during the defibration process facilitates the isolation of oxidized cellulose and the nanocellulose obtained by defibrating it. Furthermore, because nanocellulose is obtained dispersed in an organic solvent, it can be easily mixed with resins that dissolve in organic solvents and their raw material monomers. Nanocellulose dispersions, in which the nanocellulose obtained by defibration is dispersed in a dispersion medium of water and / or organic solvent, can be used for mixing with various components such as resins, rubbers, and solid particles.

[0045] The nanocellulose and nanocellulose dispersions containing it described above can be used in a variety of applications. Specifically, for example, they can be mixed with various materials (e.g., resins, fibers, rubber, etc.) as reinforcing materials, or used as thickeners or dispersants in various applications (e.g., foods, cosmetics, medical products, paints, inks, etc.). Nanocellulose dispersions can also be formed into membranes and used as various sheets or films. The fields in which this nanocellulose and nanocellulose dispersions containing it can be applied are not particularly limited, and they can be used in the manufacture of products in various fields, such as automotive components, machine parts, electrical appliances, electronic devices, cosmetics, medical products, building materials, daily necessities, and stationery. Furthermore, when nanocellulose and nanocellulose dispersions containing it are used as additives to slurries containing inorganic particles such as pigments, they are advantageous in that they can improve the viscosity stability, handleability, and coating performance of the slurry.

[0046] Second embodiment In a preferred embodiment of the present disclosure, the nanocellulose is nanocellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or its salts (also referred to as nanocellulose, an oxide of a cellulosic raw material with hypochlorous acid or its salts), does not contain N-oxyl compounds, and has an average fiber width of 1 nm to 5 nm. When the average fiber width of the nanocellulose is 1 to 5 nm, the zeta potential of the nanocellulose is preferably -25 mV or less, and more preferably -30 mV or less, in order to improve the dispersion stability of the nanocellulose and improve the handleability when made into a slurry. Note that the explanation of the first embodiment above can be used for details of the method for producing the nanocellulose.

[0047] Third embodiment In a preferred embodiment of the present disclosure, the nanocellulose is nanocellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or its salt (also referred to as nanocellulose, an oxide of a cellulosic raw material with hypochlorous acid or its salt), does not contain N-oxyl compounds, and has an aspect ratio of 20 to 150. When the aspect ratio of the nanocellulose is 20 to 150, the zeta potential of the nanocellulose is preferably -25 mV or less, and more preferably -30 mV or less, in order to improve the dispersion stability of the nanocellulose and improve the handleability when made into a slurry. Note that the explanation of the first embodiment above can be used for details of the method for producing the nanocellulose. [Example]

[0048] 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".

[0049] (1) Production of oxidized cellulose and nanocellulose [Production Example 1] As a cellulosic raw material, softwood pulp (Sigma-Aldrich NIST RM 8495, bleached kraft pulp) was cut into 5 mm square pieces with scissors and processed at 25,000 rpm for 1 minute in a Wonder Blender WB-1 manufactured by Osaka Chemical Co., Ltd. to mechanically defibrate the material into a flocculent state. 350 g of sodium hypochlorite pentahydrate crystals with an available chlorine concentration of 42% by mass were placed in a beaker, and pure water was added and stirred to obtain a sodium hypochlorite aqueous solution with an available chlorine concentration of 21% by mass. 35% by mass of hydrochloric acid was added thereto and stirred to obtain an aqueous solution with a pH of 11.0. This sodium hypochlorite aqueous solution was heated to 30°C in a constant temperature water bath while stirring at 200 rpm using a propeller-type stirring blade in a Shinto Scientific mixer (Three-One Motor, BL600), and then 50 g of the mechanically defibrated softwood kraft pulp (carboxyl group content: 0.05 mmol / g) was added. After the cellulosic raw material was supplied, the mixture was kept at 30°C in the same thermostatic water bath. The pH during the reaction was adjusted to 11.0 by adding 48% by mass of sodium hydroxide, and the mixture was stirred at 200 rpm for 20 minutes using a propeller-type stirring blade in the above-mentioned mixer to carry out the oxidation reaction. After the reaction was completed, the product was subjected to solid-liquid separation by suction filtration using a PTFE membrane filter with a mesh size of 0.1 μm, and the resulting oxidized cellulose was washed with pure water. The carboxyl group content of the filtered product (oxidized cellulose) after washing was measured and found to be 0.37 mmol / g. Next, pure water was added to the oxidized cellulose to create a 5% dispersion, which was then processed at 200 MPa for 10 passes using a Sugino Machine ultra-high-pressure homogenizer, Starburst Lab HJP-25005, to obtain CNF aqueous dispersion A as a nanocellulose dispersion. Note that in the ultra-high-pressure homogenizer, the oxidized cellulose aqueous dispersion is circulated through the ultra-high-pressure defibration section built into the homogenizer to promote defibration. One pass through the ultra-high-pressure defibration section is called one pass. In addition, the nitrogen content derived from N-oxyl compounds in the oxidized cellulose was measured using a trace total nitrogen analyzer (manufactured by Mitsubishi Chemical Analytech Co., Ltd., device name: TN-2100H), and the increase from the raw pulp was calculated, resulting in a value of less than 1 ppm.

[0050] The available chlorine concentration in the aqueous sodium hypochlorite solution was measured by the following method. (Measurement of available chlorine concentration in sodium hypochlorite solution) 0.582 g of an aqueous solution of sodium hypochlorite pentahydrate crystals in 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 bottle was immediately sealed and left in a dark place for 15 minutes. After leaving it for 15 minutes, the liberated iodine was titrated with 0.1 mol / L sodium thiosulfate solution (indicator: starch TS), and the titer was found to be 34.55 ml. A blank test was performed separately to correct for this, and since 1 ml of 0.1 mol / L sodium thiosulfate solution corresponds to 3.545 mg Cl, the available chlorine concentration in the sodium hypochlorite aqueous solution was found to be 21% by mass.

[0051] The amount of carboxy groups in the oxidized cellulose was measured by the following method. (Measurement of Carboxy Group Amount) 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 carboxyl 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, in which the change in electrical conductivity was gradual. Amount of carboxyl group = a (ml) x 0.05 / mass of oxidized cellulose (g)

[0052] [Production Example 2] CNF aqueous dispersion B was obtained by treating under the same conditions as in Production Example 1, except that the reaction time in the oxidation reaction was 30 minutes. [Production Example 3] CNF aqueous dispersion C was obtained by treating under the same conditions as in Production Example 1, except that the reaction time for the oxidation reaction was 120 minutes. [Production Example 4] CNF aqueous dispersion D was obtained by treating under the same conditions as in Production Example 1, except that the reaction time for the oxidation reaction was 360 minutes. [Production Example 5] CNF aqueous dispersion E was obtained by treating under the same conditions as in Production Example 1, except that the reaction time for the oxidation reaction was 480 minutes. [Production Example 6] CNF aqueous dispersion F was obtained by treating under the same conditions as in Production Example 1, except that the reaction temperature in the oxidation reaction was changed from 30°C to 40°C and the reaction time was changed to 120 minutes. [Production Example 7] CNF aqueous dispersion G was obtained by treating under the same conditions as in Production Example 1, except that the reaction temperature in the oxidation reaction was changed from 30°C to 50°C and the reaction time was changed to 120 minutes. [Production Example 8] CNF aqueous dispersion H was obtained by treating under the same conditions as in Production Example 1, except that the reaction temperature in the oxidation reaction was changed from 30°C to 40°C and the reaction time was changed to 480 minutes. [Production Example 9] CNF aqueous dispersion I was obtained by treating under the same conditions as in Production Example 1, except that the reaction temperature in the oxidation reaction was changed from 30°C to 20°C and the reaction time was changed to 120 minutes. [Production Example 10] CNF aqueous dispersion J was obtained by treating under the same conditions as in Production Example 1, except that the reaction time for the oxidation reaction was 15 minutes. [Production Example 11] CNF aqueous dispersion K was obtained by treating under the same conditions as in Production Example 1, except that the reaction temperature in the oxidation reaction was changed from 30°C to 15°C and the reaction time was changed to 120 minutes.

[0053] The oxidized cellulose obtained in each production example was freeze-dried, and then the solid of the sample was left at 23°C and 50% RH for 24 hours or more. 13 As a result of measuring C-NMR, it was confirmed that both compounds have a structure in which the hydroxyl groups at the second and third positions of the glucopyranose ring have been oxidized and carboxyl groups have been introduced. 13 The measurement conditions for C-NMR are as follows: (1) Sample tube: Zirconia tube (4 mm diameter) (2) Magnetic field strength: 9.4T (1H resonance frequency: 400MHz) (3) MAS rotation speed: 15 kHz (4) Pulse sequence: CPMAS method (5) Contact time: 3 ms (6) Waiting time: 5 seconds (7) Accumulation count: 10,000 to 15,000 times (8) Measuring device: JNM ECA-400 (manufactured by JEOL Ltd.) Furthermore, the fact that the oxidized cellulose obtained in each production example has a structure in which the hydroxyl groups at the second and third positions of the glucopyranose ring have been oxidized to introduce carboxyl groups was also confirmed by the results of measuring model molecules of the oxidized cellulose as samples. In addition, regarding the sixth place, solid cellulosic raw materials 13 C-NMR and solid state oxidized cellulose 13 Since no change was observed in the spectral data with C-NMR, it was determined that the hydroxyl group at position 6 was not oxidized and remained as a hydroxyl group in the oxidized cellulose.

[0054] [Comparative Production Example 1] As a cellulosic raw material, softwood pulp (Sigma-Aldrich NIST RM 8495, bleached kraft pulp) was cut into 5 mm square pieces with scissors and processed at 25,000 rpm for 1 minute in a Wonder Blender WB-1 manufactured by Osaka Chemical Co., Ltd. to mechanically defibrate the material into a flocculent state. 30.0 g of sodium hypochlorite pentahydrate crystals with an available chlorine concentration of 43% by mass were placed in a 100 ml beaker, and purified water and 35% by mass hydrochloric acid were added and stirred to obtain an aqueous solution with an available chlorine concentration of 21% by mass and a pH of 11.0. This aqueous sodium hypochlorite solution was heated to 30°C in a thermostatic water bath while stirring with a stirrer, and then 0.35 g of the mechanically defibrated softwood kraft pulp was added. After the cellulosic raw material was added, the mixture was kept at 30°C in the same thermostatic water bath, and 48% by mass of sodium hydroxide was added to maintain a pH of 11.0. The mixture was then stirred for 30 minutes with a stirrer. The product was then subjected to solid-liquid separation by suction filtration using a PTFE membrane filter with a 0.1 μm mesh size. The resulting filter cake was washed with pure water. The carboxyl group content of the washed filter cake (oxidized cellulose) was measured and found to be 0.42 mmol / g. The obtained oxidized cellulose was dispersed in pure water to prepare a 5% dispersion, which was then defibrated for 10 minutes using a Hielscher ultrasonic homogenizer "UP-400S" under conditions of CYCLE = 0.5 and AMPLITUDE = 50 to obtain CNF aqueous dispersion P. In the ultrasonic homogenizer, the ultrasonic oscillator was immersed in the oxidized cellulose aqueous dispersion placed in a container, and defibration was promoted by the ultrasonic waves emitted from the ultrasonic oscillator.

[0055] [Comparative Production Example 2] As a cellulosic raw material, softwood pulp (Sigma-Aldrich NIST RM 8495, bleached kraft pulp) was cut into 5 mm square pieces with scissors and processed at 25,000 rpm for 1 minute in a Wonder Blender WB-1 manufactured by Osaka Chemical Co., Ltd. to mechanically defibrate the material into a flocculent state. 30.3 g of sodium hypochlorite pentahydrate crystals with an effective chlorine concentration of 42% by mass were placed in a beaker, and pure water was added and stirred to adjust the effective chlorine concentration to 14% by mass. 35% by mass of hydrochloric acid was added and stirred to obtain an aqueous solution with a pH of 9.0. This aqueous sodium hypochlorite solution was heated to 30°C in a constant temperature water bath while stirring with a stirrer, and then 0.35 g of the mechanically defibrated softwood kraft pulp was added. After the cellulosic raw material was added, the mixture was kept at 30°C in the same thermostatic water bath. The pH during the reaction was adjusted to 9.0 by adding 48% by mass of sodium hydroxide, and the mixture was stirred with a stirrer for 30 minutes to carry out the oxidation reaction. After the reaction was completed, the product was subjected to solid-liquid separation by suction filtration using a PTFE mesh filter with a mesh size of 0.1 μm. The resulting filter cake was washed with pure water. The carboxyl group content of the washed filter cake (oxidized cellulose) was measured and found to be 1.12 mmol / g. Next, pure water was added to the oxidized cellulose to prepare a 5% dispersion, which was then defibrated using an ultrasonic homogenizer under the same conditions as in Comparative Production Example 1, to obtain CNF aqueous dispersion Q.

[0056] [Comparative Production Example 3] As a cellulose-based raw material, softwood pulp (Sigma-Aldrich NIST RM 8495, bleached kraft pulp) was cut with scissors into 5 mm squares and mechanically defibrated into a cotton-like state by processing at 25,000 rpm for 1 minute in a Wonder Blender WB-1 manufactured by Osaka Chemical Co., Ltd. The mechanically defibrated cellulose fibers were dispersed in sufficient water and suction-filtered using a PTFE mesh filter with 0.1 μm openings to obtain a wet powder. The above wet powder (moisture 80% by mass, 20 g in dry powder equivalent) was placed in a container, and then ozone at a concentration of 200 g / m 3 60 L of ozone-oxygen mixed gas was added and the mixture was shaken at 25°C for 2 minutes. After leaving it to stand for 6 hours, the ozone and other substances were removed from the container, and the oxidized cellulose was removed and washed with pure water by suction filtration using a 0.1 μm PTFE mesh filter. Pure water was added to the resulting oxidized cellulose to prepare a 2% by mass dispersion, and sodium hydroxide was added to make a 0.3% by mass sodium hydroxide solution. After stirring for 5 minutes, the mixture was left to stand at 25°C for 30 minutes. Next, it was washed with pure water by suction filtration using a 0.1 μm PTFE mesh filter. The carboxyl group content of the oxidized cellulose after washing was measured and found to be 0.43 mmol / g. Pure water was added to the oxidized cellulose to prepare a 5% dispersion, which was then processed in a Sugino Machine ultra-high pressure homogenizer, Starburst Lab HJP-25005, at 200 MPa and 10 passes to obtain CNF aqueous dispersion R.

[0057] Comparative Production Example 4 As a cellulosic raw material, softwood pulp (Sigma-Aldrich NIST RM 8495, bleached kraft pulp) was cut into 5 mm square pieces with scissors and processed at 25,000 rpm for 1 minute in a Wonder Blender WB-1 manufactured by Osaka Chemical Co., Ltd. to mechanically defibrate the material into a flocculent state. 4.92 g of sodium periodate was placed in a beaker, and purified water was added to make an aqueous solution (total volume 600 ml). This aqueous sodium periodate solution was heated to 55°C in a constant temperature water bath while being stirred at 200 rpm using a propeller-type stirring blade in a mixer (Three-One Motor, BL600) manufactured by Shinto Scientific Co., Ltd., and then 6 g of the mechanically defibrated softwood kraft pulp was added. After the cellulosic raw material was supplied, the mixture was stirred under the same conditions for 3 hours using a stirrer while maintaining the temperature at 55°C in the same thermostatic water bath. After the reaction was completed, the product was subjected to solid-liquid separation by suction filtration using a PTFE membrane filter with a mesh size of 0.1 μm, and then washed with pure water. The product obtained above was then added to a 1M aqueous acetic acid solution containing sodium chlorite, and the mixture was stirred at 25°C for 48 hours under the same stirring conditions as above. After the reaction was completed, the product was subjected to solid-liquid separation by suction filtration using a PTFE membrane filter with a mesh size of 0.1 μm, and then washed with pure water. The amount of carboxyl groups in the oxidized cellulose after washing was measured and found to be 1.72 mmol / g. The resulting oxidized cellulose was mixed with purified water to prepare a 5% dispersion, which was then adjusted to pH 7.5 with aqueous sodium hydroxide and washed with water. The resulting dispersion was processed in a Sugino Machine ultra-high-pressure homogenizer, Starburst Lab HJP-25005, at 200 MPa and 10 passes to obtain CNF aqueous dispersion S.

[0058] Comparative Example 5 As a cellulosic raw material, softwood pulp (Sigma-Aldrich NIST RM 8495, bleached kraft pulp) was cut into 5 mm square pieces with scissors and processed at 25,000 rpm for 1 minute in a Wonder Blender WB-1 manufactured by Osaka Chemical Co., Ltd. to mechanically defibrate the material into a flocculent state. 0.016 g of TEMPO and 0.1 g of sodium bromide were placed in a beaker, and purified water was added and stirred to form an aqueous solution, to which 1.0 g of the mechanically defibrated softwood kraft pulp was added. The above aqueous solution was heated to 25 ° C. in a thermostatic water bath while stirring with a stirrer, and then 0.1M sodium hydroxide was added and stirred to obtain an aqueous solution of pH 10.0. 2.58g of an aqueous solution of sodium hypochlorite with an effective chlorine concentration of 13.2% by mass was added thereto, and while keeping the temperature at 25 ° C. in the same thermostatic water bath, the pH during the reaction was adjusted to 10.0 by adding 0.1M sodium hydroxide, and the solution was stirred with a stirrer for 120 minutes. After the reaction was completed, the product was subjected to solid-liquid separation by suction filtration using a 0.1 μm PTFE membrane filter, and the resulting filter cake was washed with pure water. The carboxyl group content of the washed filter cake (oxidized cellulose) was measured and found to be 1.55 mmol / g. Pure water was added to the resulting filter cake to prepare a 5% dispersion, which was then adjusted to pH 7.5 with aqueous sodium hydroxide and washed with water. The resulting dispersion was processed using a Sugino Machine Co., Ltd. ultra-high-pressure homogenizer, Starburst Lab HJP-25005, at 200 MPa and 10 passes to obtain CNF aqueous dispersion T. The nitrogen content derived from N-oxyl compounds in the oxidized cellulose was measured as the nitrogen content under the same conditions as in Production Example 1, and the increase from the raw pulp was calculated to be 5 ppm.

[0059] [First Example] [Examples 1-1 to 1-9, Comparative Examples 1-1 to 1-5]: Study on Zeta Potential The following evaluations were carried out using the CNF aqueous dispersions obtained in Production Examples 1 to 9 and Comparative Production Examples 1 to 5, and the relationship between the zeta potential of nanocellulose and dispersion stability was investigated. The evaluation results are shown in Table 1.

[0060] [Zeta potential measurement] Pure water was added to each of the CNF aqueous dispersions A to I, P to T obtained above to dilute them so that the nanocellulose concentration was 0.1%. After dilution, a 0.05 mol / L aqueous solution of sodium hydroxide was added to the CNF aqueous dispersion to adjust the pH to 8.0, and the zeta potential was measured at 20°C using a zeta potential meter (ELSZ-1000) manufactured by Otsuka Electronics Co., Ltd. [Measurement of average fiber width] Pure water was added to each of the CNF aqueous dispersions A-I, P-T obtained above to adjust the nanocellulose concentration in the CNF aqueous dispersion to 5 ppm. After adjusting the concentration, the CNF aqueous dispersion was air-dried on a mica substrate, and the nanocellulose morphology was observed using an Oxford Asylum MFP-3D Infinity scanning probe microscope in AC mode. The average fiber width was calculated using the software provided with the MFP-3D Infinity for 50 or more fibers, with the number-average fiber width (nm) calculated as the cross-sectional height of the morphology image = fiber width.

[0061] [Light transmittance measurement] Each of the CNF aqueous dispersions A to I and P to T obtained above was placed in a 10 mm thick quartz cell, and the light transmittance at a wavelength of 660 nm was measured using a spectrophotometer (JASCO V-550). [Stability of CNF aqueous dispersion] Pure water was added to each of the CNF aqueous dispersions A to I, P to T to dilute them so that the nanocellulose concentration in the CNF aqueous dispersion was 0.1% by mass, and the dispersion was then stored at 25°C for 4 weeks. The solids concentration in the supernatant was measured immediately after dilution and after 4 weeks of storage, and the dispersion ratio was calculated using the following formula. The dispersion stability was also evaluated according to the following criteria. The solids concentration was calculated from the change in mass before and after drying at 105°C. Dispersion rate (%) = (solid concentration after 4 weeks / solid concentration immediately after dilution) x 100 ◎: Dispersion rate is less than 95% ○: Dispersion rate is 90% or more but less than 95% △: Dispersion rate is 85% or more but less than 90% ×: Dispersion rate is less than 85%

[0062] [Slurry Viscosity Stability] Aqueous slurries (50 g) containing 5% by mass of titanium dioxide (R-820, manufactured by Ishihara Sangyo Co., Ltd.) and each of the CNF aqueous dispersions A to I, P to T were prepared by varying the amount of nanocellulose added so that the initial viscosity (viscosity immediately after preparation) was the same for each example (300 mPa·s). A Thinky Mixer "Awatori Rentaro ARE-310" (mix mode, revolution: 2000 rpm, rotation: 800 rpm, 20 minutes) was used to mix the aqueous slurries. The viscosity was measured immediately after preparation (initial viscosity) and after standing for one week, and the viscosity change rate was calculated using the following formula, and the viscosity stability of the aqueous slurry was judged according to the following criteria. Viscosity change rate (%) = (N2 / N1) x 100 (In the formula, N1 is the initial viscosity of the slurry, and N2 is the viscosity of the slurry after the sample has been left standing for one week after preparation.) ◎: Viscosity change rate is less than 105% ○: Viscosity change rate is 105% or more but less than 110% △: Viscosity change rate is 110% or more but less than 115% ×: Viscosity change rate is 115% or more The samples were left to stand indoors at 23±2°C. The initial viscosity of the slurry and the viscosity after standing for one week were measured by stirring with a spatula at a speed that did not allow bubbles to form, and then measuring the viscosity at 25°C, 100 rpm (shear rate 200 s) using a Toki Sangyo E-type viscometer (TV-22). -1 ) was measured under the conditions.

[0063] [Slurry handling] Water was added to each of the CNF aqueous dispersions A-I, P-T so that the aluminum silicate powder and nanocellulose were 5% by mass and 0.5% by mass, respectively, and the mixture was stirred to prepare a processing solution. After lightly stirring the processing solution with a spatula, the solution was scooped up and tilted, and dripping was visually observed. The slurry handleability was evaluated according to the following criteria. ⊚: Dripping occurred immediately after tilting. ○: Dripping occurred 5 seconds after tilting. △: Dripping occurred 10 seconds after tilting. ×: No dripping occurred even after 15 seconds.

[0064] [Surface condition after slurry coating (coatability)] Water was added to each of the CNF aqueous dispersions A to I, P to T so that the aluminum silicate powder and nanocellulose were 5% by mass and 0.5% by mass, respectively, and the mixture was stirred to prepare a processing solution. 2 The processing solution was applied to woven fabric (100% polyester, 100 mm x 100 mm) so that the solution was applied, and then dried. Ten pieces of coated woven fabric were visually inspected for uneven coating (processing irregularities) and evaluated according to the following criteria. ◎: No unevenness in processing was visible on any of the 10 sheets. ○: No unevenness in processing was observed on 8 to 9 sheets. △: No unevenness in processing was observed on 4 to 7 sheets. ×: No unevenness in processing was observed on 1 to 3 sheets, or unevenness in processing was observed on all 10 sheets.

[0065] [Table 1]

[0066] In Table 1, cases where no N-oxyl compound was used during the oxidation treatment of the cellulosic raw material (i.e., cases where the CNF dispersion contains substantially no N-oxyl compound) are represented by "X", and cases where an N-oxyl compound was used (i.e., cases where the CNF dispersion contains an N-oxyl compound) are represented by "○" (the same applies to Tables 2 and 3 below).

[0067] As shown in Table 1, when comparing Examples 1-1 to 1-9, in which nanocellulose was produced by oxidation treatment with hypochlorite, with Comparative Examples 1-1 and 1-2, Examples 1-1 to 1-9, which had a zeta potential of -30 mV or less, showed better slurry properties than Comparative Examples 1-1 and 1-2, which had zeta potentials of -17.9 mV and -21.7 mV, respectively. Specifically, the CNF aqueous dispersions of Examples 1-1 to 1-9 had high dispersion stability of nanocellulose. Furthermore, the slurries obtained in Examples 1-1 to 1-9 had a good balance of viscosity stability, handleability, and coatability. In particular, Examples 1-1 to 1-7 were rated as "◎" or "◯" for all of the viscosity stability, handleability, and coatability of the slurries, demonstrating excellent slurry properties. Furthermore, the results for Examples 1-1, 1-2, and 1-9, which have similar zeta potentials, revealed that Examples 1-1 and 1-2, which have an average fiber width of 5 nm or less, exhibited superior slurry properties compared to Example 1-9, which has an average nanocellulose fiber width of 5.3 nm. In contrast, in Comparative Examples 1-1 to 1-4, the dispersion stability was evaluated as "x", and the slurry properties were also inferior to those of Examples 1-1 to 1-9. In Comparative Example 1-5, the dispersion stability was good, but the slurry properties were all evaluated as "x".

[0068] [Second Example] [Examples 2-1 to 2-9, Comparative Examples 2-1 to 2-5]: Investigation of average fiber width The same evaluations as in Example 1 were carried out using CNF aqueous dispersions A to H, J, and P to T obtained in Production Examples 1 to 8 and 10 and Comparative Production Examples 1 to 5, and the relationship between the average fiber width of nanocellulose and dispersion stability was investigated. The evaluation results are shown in Table 2.

[0069] [Table 2]

[0070] According to Table 2, when comparing Examples 2-1 to 2-9, in which nanocellulose was produced by oxidation treatment with hypochlorite, with Comparative Examples 2-1 and 2-2, Examples 2-1 to 2-9, in which the average fiber diameter was 5.0 nm or less, showed better slurry properties than Comparative Examples 2-1 and 2-2, in which the average fiber diameter was 5.3 nm and 5.2 nm, respectively. Furthermore, from the results of Examples 2-1 and 2-9, which have similar average fiber widths, it was found that Example 2-1, in which the zeta potential of the nanocellulose is -30 mV or less, exhibits better slurry properties than Example 2-9, in which the zeta potential is -28.5 mV.

[0071] [Third Example] [Examples 3-1 to 3-9, Comparative Examples 3-1 to 3-5]: Consideration of aspect ratio Using the CNF aqueous dispersions A to H, K, and P to T obtained in Production Examples 1 to 8 and 11 and Comparative Production Examples 1 to 5, the average fiber length and average fiber width of nanocellulose were measured to calculate the aspect ratio, and evaluations similar to those in Example 1 were performed to investigate the relationship between the aspect ratio of nanocellulose and dispersion stability. The average fiber length and average fiber width were measured using the following procedure. [Measurement of average fiber length and average fiber width] Pure water was added to each of the CNF aqueous dispersions A-H, K, P-T obtained above to adjust the nanocellulose concentration in the CNF aqueous dispersion to 5 ppm. The CNF aqueous dispersions after concentration adjustment were air-dried on a mica substrate and the nanocellulose morphology was observed using an Oxford Asylum MFP-3D Infinity scanning probe microscope in AC mode. The images were binarized and analyzed using the image processing software Image J. The number-average fiber length was calculated for 100 fibers as fiber length = perimeter / 2. The number-average fiber width (nm) was calculated for 50 or more fibers using the software included with the MFP-3D Infinity, where the cross-sectional height of the morphology image was equal to the fiber width. The aspect ratio was also calculated from the ratio (average fiber length / average fiber width). The evaluation results are shown in Table 3.

[0072] [Table 3]

[0073] According to Table 3, when comparing Examples 3-1 to 3-9, in which nanocellulose was produced by oxidation treatment with hypochlorite, with Comparative Examples 3-1 and 3-2, Examples 3-1 to 3-9, which had aspect ratios of 150 or less, showed better slurry properties than Comparative Examples 3-1 and 3-2, which had aspect ratios of 183 and 165, respectively. Furthermore, from the results of Examples 3-2 and 3-9, which have similar aspect ratios, it was found that Example 3-2, whose zeta potential is -30 mV or less, exhibits better slurry properties than Example 3-9, whose nanocellulose zeta potential is -28.6 mV.

Claims

1. Nanocellulose is an oxide of a cellulose-based raw material using hypochlorous acid or a salt thereof, and has an average fiber width of 1 nm or more and 200 nm or less, Substantially free of N-oxyl compounds, Nanocellulose having a zeta potential of -30 mV or less.

2. The nanocellulose of claim 1, wherein the average fiber width is 1 nm or more and 5 nm or less.

3. The nanocellulose according to claim 1 or 2, having an aspect ratio of 20 or more and 150 or less.

4. The nanocellulose according to any one of claims 1 to 3, wherein the light transmittance in a mixed liquid obtained by mixing with water to a solid content concentration of 0.1% by mass is 95% or more.

5. Nanocellulose is an oxide of a cellulosic raw material with hypochlorous acid or its salt, Does not contain N-oxyl compounds, Nanocellulose having an average fiber width of 1 nm or more and 5 nm or less.

6. Nanocellulose is an oxide of a cellulose-based raw material using hypochlorous acid or a salt thereof, and has an average fiber width of 1 nm or more and 200 nm or less, Does not contain N-oxyl compounds, Nanocellulose having an aspect ratio of 20 or more and 150 or less.

7. Nanocellulose is an oxide of a cellulose-based raw material using hypochlorous acid or a salt thereof, and has an average fiber width of 1 nm or more and 200 nm or less, Does not contain N-oxyl compounds, Nanocellulose having a light transmittance of 95% or more in a mixed liquid obtained by mixing with water to a solid content concentration of 0.1% by mass.

8. A nanocellulose dispersion in which the nanocellulose according to any one of claims 1 to 7 is dispersed in a dispersion medium.

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