Method for producing oxidized cellulose and nanocellulose

Abstract

An object of the present invention is to stably and efficiently provide oxidized cellulose having excellent defibration properties.SOLUTION: The problem can be solved by a method for producing oxidized cellulose containing an oxidized product of a cellulose-based raw material by hypochlorous acid or a salt thereof and substantially not containing an N-oxyl compound and having a degree of polymerization of 600 or less, the method including a step of obtaining oxidized cellulose by oxidizing the cellulose-based raw material using hypochlorous acid or a salt thereof, wherein a viscosity of a slurry of the cellulose-based raw material having the same concentration as that at the time of the oxidation is in a range of 1000 Pa·s or less at 30°C. or 40°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing oxidized cellulose and nanocellulose. [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 cellulose-based 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 the production of oxidized cellulose fibers by oxidizing a cellulosic raw material using hypochlorous acid or its salts as an oxidizing agent under high-concentration conditions of 14 to 43% by mass of available chlorine in the reaction system. Patent Document 2 discloses the production of oxidized cellulose fibers by 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% by mass, and adjusting the pH to 5.0 to 14.0. These techniques perform the oxidation process without using N-oxyl compounds such as 2,2,6,6-tetramethyl-1-piperidine-N-oxy 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] <Problem to be solved by the first invention (first problem)> Patent Documents 1 and 2 disclose specific examples of producing nanocellulose materials by micronizing oxidized cellulose, in which nanocellulose materials are obtained through a mechanical defibration process using an ultrasonic homogenizer. However, this process leaves room for further improvement in terms of the energy required for defibration. In the production of nanocellulose materials, from the perspective of production costs, there is a demand for oxidized cellulose that is easily defibrated, even under mild processing conditions. Furthermore, to consistently produce micronized cellulose fibers or to obtain highly transparent nanocellulose materials with minimal light scattering in dispersion media, the oxidized cellulose in its pre-defibration state must have good defibration properties.

[0006] Patent Documents 1 and 2 specifically describe how oxidized cellulose and nanocellulose materials can be obtained by using several hundred milligrams of cellulosic raw material and reacting it with hypochlorous acid or a salt thereof, with an available chlorine concentration of 6 to 43% by mass in the reaction system. However, these methods are limited in the amount of the target oxidized cellulose and nanocellulose material, and there is a need for a method that can stably and efficiently supply oxidized cellulose fibers with excellent defibration properties.

[0007] The present invention was made in view of the above circumstances, and its main object is to provide oxidized cellulose with excellent defibrillation properties in a stable and efficient manner.

[0008] <Problem to be solved by the second invention (second problem)> Patent Documents 1 and 2 disclose specific examples of obtaining oxidized cellulose by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof, in which the cellulosic raw material is oxidized with hypochlorous acid or a salt thereof, and the resulting cellulose oxide is subjected to solid-liquid separation by filtration to obtain oxidized cellulose. However, in the methods for producing oxidized cellulose, there is room for further improvement in the yield of the oxidized cellulose obtained with the above-mentioned solid-liquid separation process.

[0009] The present invention was made in view of the above circumstances, and a primary object of the present invention is to provide a method for producing oxidized cellulose in high yield. [Means for solving the problem]

[0010] <Means for solving the first problem> As a result of extensive research, the present inventors have discovered that oxidized cellulose with excellent defibrillation properties can be stably and efficiently produced by oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof while controlling the viscosity of a slurry of the cellulosic raw material at the same concentration as that used during oxidation to a range of 1000 Pa s or less, and have thus completed the present invention.

[0011] <Means for solving the second problem> As a result of extensive research, the present inventors have discovered that oxidized cellulose can be obtained in high yield by a method comprising the steps of subjecting an oxide dispersion containing a cellulose-based oxide and a dispersing medium to solid-liquid separation to obtain oxidized cellulose, wherein the pH of the oxide dispersion is 4.0 or less and the oxide dispersion is substantially free of N-oxyl compounds, or by further comprising the step of oxidizing a cellulose-based raw material with a predetermined amount of hypochlorous acid or a salt thereof to obtain the cellulose-based oxide, wherein the pH of the oxide dispersion is 4.0 or less, and have completed the present invention.

[0012] Specifically, the present invention is as follows. [1] A method for producing oxidized cellulose, which comprises oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof, is substantially free of N-oxyl compounds, and has a degree of polymerization of 600 or less, The method includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof, the viscosity of a slurry of the cellulosic raw material having the same concentration as that used for the oxidation is in the range of 1000 Pa s or less when measured using a viscometer equipped with an SPP rotor at a rotation speed of 100 rpm and at 30°C or 40°C; Manufacturing method. [2] The production method according to [1], wherein the concentration of the cellulosic raw material is 35 mass% or less based on the total amount of the reaction mixture. [3] The manufacturing method according to [1] or [2], wherein the concentration of the cellulosic raw material is more than 6.5% by mass based on the total amount of the reaction mixture. [4] The method according to any one of [1] to [3], wherein the available chlorine concentration in the reaction system of the hypochlorous acid or a salt thereof is 6% by mass or more and 43% by mass or less. [5] The method according to any one of [1] to [3], wherein the available chlorine concentration in the reaction system of the hypochlorous acid or a salt thereof is less than 14 mass %. [6] The method according to any one of [1] to [5], wherein the oxidation reaction temperature is 30°C or higher. [7] The method according to any one of [1] to [6], wherein the reaction time for the oxidation is 2 hours or more. [8] The production method according to any one of [1] to [7], wherein the pH of the reaction system is less than 11. [9] The method according to any one of [1] to [8], wherein the viscosity is in the range of 30 Pa·s or less.

[10] [1] to [9], after the oxidation step in the manufacturing method according to any one of [1] to [9], a step of obtaining nanocellulose by defibrating Method for producing nanocellulose.

[11] a step of subjecting an oxide dispersion containing a cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain oxidized cellulose; the pH of the oxide dispersion is 4.0 or less, and the oxide dispersion is substantially free of N-oxyl compounds; Method for producing oxidized cellulose.

[12] The method according to

[11] , further comprising the step of oxidizing a cellulose-based raw material with hypochlorous acid or a salt thereof to obtain the cellulose-based oxide.

[13] a step of oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof in a mass ratio relative to the cellulosic raw material of 0.2 or more to obtain a cellulosic oxide; and a step of subjecting the oxide dispersion containing the cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain oxidized cellulose, The pH of the oxide dispersion is 4.0 or less. Method for producing oxidized cellulose.

[14] The method according to

[12] or

[13] , further comprising a step of treating the oxide dispersion with hypochlorous acid or a salt thereof.

[15] The method according to any one of

[11] to

[14] , further comprising the step of adding an acid and / or a cation exchange resin to prepare the oxide dispersion having a pH of 4.0 or less.

[16] The method according to any one of

[11] to

[15] , further comprising the step of adding a base to adjust the pH of the oxidized cellulose dispersion containing the oxidized cellulose and a dispersion medium to greater than 4.0.

[17] The method according to any one of

[11] to

[16] , wherein the pH of the oxide dispersion is 2.5 or less.

[18] The method according to any one of

[11] to

[17] , wherein the step of obtaining the oxidized cellulose is a step of performing solid-liquid separation by filtering the oxide dispersion.

[19] The method according to any one of

[11] to

[18] , further comprising a step of washing the oxide dispersion or the oxidized cellulose with an acidic washing solution.

[20] The method according to any one of

[11] to

[19] , wherein the degree of polymerization of the oxidized cellulose is 600 or less. [twenty one] Oxidized cellulose obtained by the production method according to any one of

[11] to

[20] . [twenty two]

[11] to

[20] , and defibrating the oxidized cellulose obtained by the production method to obtain nanocellulose. Method for producing nanocellulose. [twenty three] Nanocellulose obtained by the manufacturing method described in

[22] . [Effects of the Invention]

[0013] <Effects of the first invention> According to the production method of the present invention, oxidized cellulose with excellent defibration properties can be obtained stably and efficiently. In particular, the oxidized cellulose according to the present invention can be uniformly finely divided even when subjected to defibration treatment under mild conditions, and has excellent easy defibration properties.

[0014] <Effects of the second invention> According to the method for producing oxidized cellulose of the present invention, oxidized cellulose can be obtained in high yield. In particular, the production method of the present invention is excellent in production efficiency because it can suppress clogging when performing solid-liquid separation by filtration. [Brief explanation of the drawings]

[0015] [Figure 1] 10 is a flowchart showing an example of a manufacturing method of the second invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The first invention focuses primarily on obtaining oxidized cellulose by oxidizing a cellulosic raw material. The second invention focuses primarily on post-treating the oxidized cellulose obtained by oxidizing a cellulosic raw material. The first and second inventions will be described separately below, but specific aspects of the first invention may be referenced in the second invention, and specific aspects of the second invention may be referenced in the first invention. The first and second inventions may also be combined to form a method comprising a step of oxidizing a cellulosic raw material to obtain oxidized cellulose and a step of post-treating the oxidized cellulose.

[0017] <<First embodiment of the invention>> <Method of producing oxidized cellulose> The production method of the present invention is a method for producing oxidized cellulose that contains an oxidation of a cellulosic raw material with hypochlorous acid or a salt thereof, is substantially free of N-oxyl compounds, and has a degree of polymerization of 600 or less. The production method of the present invention includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof. Furthermore, the viscosity of a slurry of the cellulosic raw material at the same concentration as that used during the oxidation is in the range of 1000 Pa s or less when measured using a viscometer equipped with an SPP rotor at a rotation speed of 100 rpm and at 30°C or 40°C (hereinafter also referred to as "measurement condition A").

[0018] The above-described manufacturing method of the present invention can also be rephrased as follows. A method for producing oxidized cellulose, which comprises oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof, is substantially free of N-oxyl compounds, and has a degree of polymerization of 600 or less, The method includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material in a reaction system containing hypochlorous acid or a salt thereof and the cellulosic raw material, the reaction system contains the cellulose raw material at a predetermined concentration, The predetermined concentration is a concentration in which a slurry consisting only of the cellulose-based raw material and water is prepared, and the viscosity of the slurry is measured under measurement condition A to be 1000 Pa·s or less.

[0019] The above-mentioned production method of the present invention can also be rephrased as follows: The "initial viscosity of the reaction system" below refers to the viscosity of the reaction system at the start of the reaction. A method for producing oxidized cellulose, which comprises oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof, is substantially free of N-oxyl compounds, and has a degree of polymerization of 600 or less, The method includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material in a reaction system containing hypochlorous acid or a salt thereof and the cellulosic raw material, The production method, wherein the initial viscosity of the reaction system under measurement condition A is 1000 Pa·s or less.

[0020] The start of the reaction is preferably within 10 minutes, more preferably within 5 minutes, and even more preferably within 1 minute after the components constituting the reaction system have been added to the system. In one specific embodiment, the start of the reaction is preferably within 10 minutes, more preferably within 5 minutes, and even more preferably within 1 minute after the cellulosic raw material has been added to the system containing hypochlorous acid or a salt thereof.

[0021]

[0003] Conventional methods for producing oxidized cellulose have the problem that the amount of starting cellulose supplied to the oxidation reaction is small, limiting the amount of oxidized cellulose obtained and making it difficult to improve the productivity of the desired oxidized cellulose. One method for increasing the productivity of oxidized cellulose is to increase the scale of the reaction and the concentration of the cellulosic starting material in the reaction system. As a result of studies by the present inventors, it has become clear that in order to obtain oxidized cellulose with excellent defibration properties, it is desirable to supply a sufficient amount of hypochlorous acid or a salt thereof to the surface of the starting cellulose, and that increasing the amount of starting cellulose results in portions of the starting cellulose where hypochlorous acid or a salt thereof cannot sufficiently act, making it difficult to obtain the desired oxidized cellulose satisfactorily. The present inventors discovered that by controlling the viscosity of the reaction system, specifically by preparing a slurry consisting of only the cellulosic raw material and water at the same concentration as that used for oxidation (i.e., the concentration of the cellulosic raw material at the time of charging for the oxidation reaction) and measuring the viscosity (hereinafter also referred to as slurry viscosity) of the slurry at a cellulosic raw material concentration such that it is 1000 Pa s or less at 30°C or 40°C, hypochlorous acid or a salt thereof becomes sufficiently uniform in the reaction system, enabling the stable and efficient production of oxidized cellulose with excellent defibration ability, and thus completed the present invention. Furthermore, by using a cellulosic raw material concentration in the reaction system such that the viscosity is 1000 Pa s or less, the reaction system can be made uniform using a stirrer or kneader, as described below. The reaction system in the present invention refers to a mixture of constituent components (including the dispersion medium) during the oxidation reaction (hereinafter also referred to as a reaction mixture).

[0022] In the present invention, the slurry viscosity or the initial viscosity of the reaction system is preferably 30 Pa·s or less, more preferably 20 Pa·s or less, even more preferably 10 Pa·s or less, and still more preferably 5 Pa·s or less, from the viewpoint of facilitating stirring and kneading operations. The lower limit of the above-mentioned slurry viscosity or the initial viscosity of the reaction system is preferably as low as possible from the viewpoint of making the reaction system uniform. The lower limit is not particularly limited, but may be more than 0 Pa·s, 0.01 Pa·s or more, 0.1 Pa·s or more, or 0.3 Pa·s or more. The range of the slurry viscosity or the initial viscosity of the reaction system may be, for example, more than 0 Pa·s and not more than 30 Pa·s, 0.01 Pa·s or more and not more than 20 Pa·s, 0.1 Pa·s or more and not more than 10 Pa·s, or 0.3 Pa·s or more and not more than 5 Pa·s.

[0023] Examples of methods for controlling the slurry viscosity or the initial viscosity of the reaction system include adjusting the concentration of the cellulosic raw material or the temperature. Specifically, the slurry viscosity or the initial viscosity of the reaction system is controlled to increase as the concentration of the cellulosic raw material increases. Furthermore, the slurry viscosity or the initial viscosity of the reaction system is controlled to increase as the temperature during oxidation increases.

[0024] The above-mentioned slurry viscosity is measured using a slurry of a cellulosic raw material. The viscosity measured from the above-mentioned slurry reproduces the initial viscosity of the above-mentioned reaction system (a mixture of components during the oxidation reaction). The above-mentioned slurry viscosity is measured by preparing a slurry of the cellulosic raw material to have the same concentration as that during oxidation, heating it to 30°C or 40°C, and stirring it at 100 rpm using a viscometer equipped with an SPP rotor. Specifically, the initial viscosity of the reaction system can be measured by the method described in the Examples.

[0025] As the oxidation of the cellulosic raw material progresses and oxidized cellulose is produced, the viscosity of the reaction system tends to decrease. The viscosity of the reaction system tends to be highest at the start of the reaction. The above-mentioned slurry viscosity can be said to be equivalent to the viscosity of the reaction system at the start of the reaction (i.e., the initial viscosity of the reaction system).

[0026] The cellulosic raw material used in the present invention is not particularly limited as long as it is a material mainly composed of cellulose, and examples thereof 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. In addition, 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. Cotton and sea squirts can also be used as the raw material.

[0027] The type of pulp is not particularly limited, and examples thereof include coniferous trees, broad-leaved trees other than coniferous trees, bamboo, straw, bagasse, hemp, kenaf, etc. These pulps may be used alone or in combination of two or more. 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. As the pulp, for example, mechanical pulp, chemical-mechanical pulp, semi-chemical pulp, and chemical pulp (kraft pulp, sulfite pulp, alkaline pulp) can be used.

[0028] In the production method of the present invention, fine cellulose obtained by mechanically or chemically treating cellulose may be used as the cellulosic raw material. Pulp powder is a suitable example of fine cellulose. Use of pulp powder promotes further pulverization, tending to efficiently obtain nanocellulose. The particle size of the pulp powder is typically in the range of 1 to 1,000 μm, preferably in the range of 1 to 500 μm, and more preferably in the range of 1 to 100 μm. The particle size referred to here is the average particle size, and refers to the value at which the volume accumulation distribution is 50% when the particle size distribution is expressed as a volume accumulation distribution using a laser scattering method as the measurement principle.

[0029] The range of crystallinity of the cellulose-based raw material is not limited as long as it is within the range that allows nanocellulose to be obtained, and is usually in the range of 10 to 90%. The crystallinity is preferably in the range of 20 to 80%, and more preferably in the range of 30 to 70%. The above crystallinity is determined based on the solid state of the cellulosic raw material. 13 The degree of crystallinity can be calculated from the ratio of the crystalline portion to the amorphous portion by C-NMR measurement. Specifically, the degree of crystallinity can be calculated by the method described in the Examples.

[0030] 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.

[0031] One method for producing oxidized cellulose by oxidation of a cellulosic raw material is to mix the cellulosic raw material with a reaction solution containing hypochlorous acid or a salt thereof. The solvent contained in the reaction solution is preferably water, as it is easy to handle and is less likely to cause side reactions. In the oxidation, the amount of hypochlorous acid or its salt used is not particularly limited, but it is preferable to use hypochlorous acid or its salt with an available chlorine concentration of 6% by mass or more and 43% by mass or less. By using hypochlorous acid or its salt with an available 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 for sufficient pulverization, and making it possible to omit the mechanical fiberization treatment that follows the oxidation reaction. The available chlorine concentration of hypochlorous acid or a salt thereof in the reaction liquid (reaction system) is also preferably in the range of 6 to 43 mass %.

[0032] From the viewpoint of smoothly proceeding with the micronization of oxidized cellulose, the lower limit of the available chlorine concentration is more preferably 7% by mass or more, even more preferably 8% by mass or more, even more preferably 8.5% by mass or more, and even more preferably 9% by mass or more. Furthermore, from the viewpoint of suppressing excessive decomposition of cellulose, 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 7 to 43% by mass, and even more preferably 8 to 43% by mass.

[0033] From the viewpoints of reducing the cost of producing oxidized cellulose and improving productivity by facilitating the availability and handling of hypochlorous acid or its salts, it is preferable to keep the available chlorine concentration low. From this viewpoint, the upper limit of the available 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 even 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 available chlorine concentration is preferably from 6% to less than 14% by mass, more preferably from 7% to less than 14% by mass, even more preferably from 7% to 13% by mass, and even more preferably from 8% to 13% by mass.

[0034] 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.

[0035] The reaction in the production method of the present invention may or may not be adjusted to any desired pH range, but it is recommended to adjust the pH to 5.0 or higher. Within this range, the oxidation reaction of the cellulosic raw material can proceed sufficiently, the amount of carboxy groups in the oxidized cellulose is sufficiently increased, and pulverization by stirring tends to proceed easily. 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 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.

[0036] Increasing the scale of the reaction and the concentration of the cellulosic raw material increases the viscosity of the reaction system, which tends to make it difficult to adequately supply hypochlorous acid or its salt to the surface of the raw cellulose. Furthermore, as mentioned above, it is preferable to maintain the available chlorine concentration low from the viewpoints of reducing the cost of oxidized cellulose production and improving productivity, such as by making hypochlorous acid or its salt easier to obtain and handle. On the other hand, maintaining the available chlorine concentration low, for example, to less than 14% by mass, can result in insufficient progress of the oxidation reaction and a decrease in the defibration ability of the oxidized cellulose. To enhance the oxidizing effect of hypochlorous acid or its salt on the surface of the raw cellulose while maintaining a low available chlorine concentration, the pH of the reaction system is preferably less than 11, more preferably 10.7 or less, and even more preferably 10.5 or less. The lower limit of the pH of the reaction system is not particularly limited, but is usually 5.0 or higher, preferably 6.0 or higher, more preferably 7.0 or higher, even more preferably 8.0 or higher, even more preferably 9.0 or higher, and even more preferably greater than 9.0. The pH range of the reaction system may be determined by appropriately combining the above upper and lower limits. The pH of the reaction system is preferably 5.0 or more and less than 11, more preferably 6.0 or more and less than 11, even more preferably 7.0 or more and less than 11, still more preferably 8.0 or more and less than 11, still more preferably 8.0 or more and less than 10.7, even more preferably 9.0 or more and less than 10.7, still more preferably 9.0 or more and less than 10.5, and even more preferably more than 9.0 and less than 10.5.

[0037] 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 and carry out the oxidation reaction while adjusting the pH of the reaction system.

[0038] 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.

[0039] 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, a 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 is easy to adjust the effective chlorine concentration.

[0040] 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.

[0041] 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 stirrer with a stirring blade, 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 a stirring blade, and a disperser-type mixer 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 the oxidized cellulose to a predetermined value or less. Methods using a stirrer with a stirring blade are particularly preferred. When using a stirrer with a stirring blade, devices equipped with known stirring blades such as propeller blades, paddle blades, turbine blades, swept-back 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 using a stirrer with a stirring blade, stirring is preferably performed at a rotation speed of 50 to 1,000 rpm. Furthermore, a multi-screw kneader such as a single-screw kneader or a twin-screw kneader can also be used.

[0042] The reaction temperature in the oxidation reaction is usually in the range of 15°C to 100°C. From the viewpoint of further accelerating the progress of the oxidation reaction, the reaction temperature is preferably 30°C or higher, more preferably above 30°C, even more preferably 31°C or higher, and even more preferably 35°C or higher. The higher the reaction temperature, the higher the viscosity tends to be, and the homogeneity of the reaction system tends to decrease. From the viewpoint of increasing the homogeneity of the reaction system and improving productivity, the reaction temperature is preferably 60°C or lower, more preferably 55°C or lower, and even more preferably 40°C or lower. The reaction temperature here refers to the temperature measured by measuring the temperature of the reaction mixture.

[0043] The reaction time for the oxidation reaction can be set according to the degree of progress of the oxidation, but is usually about 15 minutes to 50 hours. From the viewpoint of further accelerating the progress of the oxidation reaction, the reaction time is preferably 2 hours or more, more preferably more than 2 hours, and even more preferably 3 hours or more. The upper limit of the reaction time is not particularly limited, but is preferably 20 hours or less, more preferably 15 hours or less, and even more preferably 12 hours or less.

[0044] The concentration of the cellulosic raw material is preferably 35% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, and even more preferably 10% by mass or less, relative to the total amount of the reaction mixture at the start of the oxidation reaction (i.e., the total amount of the reaction system), from the viewpoint of improving workability by setting the slurry viscosity or the initial viscosity of the reaction system in the range of 1000 Pa·s or less and facilitating stirring during the oxidation reaction. The lower limit of the concentration of the cellulosic raw material is usually 0.1% by mass or more, and from the viewpoint of improving productivity, it is preferably more than 6.5% by mass, more preferably 6.6% by mass or more, even more preferably 6.8% by mass or more, and even more preferably 7% by mass or more. The concentration of the cellulosic raw material is preferably in the range of more than 6.5% by mass and not more than 35% by mass, more preferably in the range of more than 6.5% by mass and not more than 20% by mass, even more preferably in the range of more than 6.5% by mass and not more than 15% by mass, and even more preferably in the range of more than 6.5% by mass and not more than 10% by mass. The concentration of the cellulosic raw material during the oxidation reaction referred to here is the concentration of the cellulosic raw material at the time of charging.

[0045] The pressure under which the reaction is carried out is not particularly limited, but is usually in the range of normal pressure to 1.0 MPaG (gauge pressure, the same applies hereinafter). Here, normal pressure means a pressure equal to atmospheric pressure. By carrying out oxidation under pressure, the amount of hypochlorous acid or a salt thereof used can be reduced, tending to enable more efficient production of oxidized cellulose. From the viewpoint of efficiency, the pressure is preferably 0.1 MPaG or more and 1.0 MPaG or less. In this case, the available chlorine concentration of hypochlorous acid or a salt thereof may be more than 0 mass% and 43 mass% or less, and from the viewpoint of increasing efficiency, it is preferably 0.1 mass% or more and 20 mass% or less, more preferably 1.0 mass% or more and 15 mass% or less, and even more preferably 1.0 mass% or more and 10 mass% or less.

[0046] In the production of oxidized cellulose, after the oxidation of the cellulosic raw material, a treatment to terminate the oxidation reaction may be carried out. That is, the process may further include a step of treating the hypochlorous acid or a salt thereof used in the oxidation step (hereinafter also referred to as a "treatment step"). The method for treating the hypochlorous acid or a salt thereof is not particularly limited, and may involve ultraviolet irradiation, autolysis under high temperature conditions, or the like, but a preferred example is a method of reducing hypochlorous acid or a salt thereof. Specific examples include a method of adding a reducing agent such as sulfites, sulfamic acid or a salt thereof, thiosulfate, hydrogen peroxide, oxalic acid or a salt thereof, formic acid or a salt thereof, or hypophosphite, or a method of adding a decomposition catalyst such as nickel oxide. Examples of the sulfites include sulfites, bisulfites, pyrosulfites, hyposulfites, etc., which may be hydrates. Specific examples of the sulfites include sodium bisulfite, potassium bisulfite, ammonium bisulfite, calcium bisulfite, sodium sulfite, potassium sulfite, ammonium sulfite, zinc sulfite, ammonium sulfite, sodium hyposulfite, potassium hyposulfite, calcium hyposulfite, sodium pyrosulfite, potassium pyrosulfite, magnesium pyrosulfite, calcium pyrosulfite, ammonium pyrosulfite, etc., and among these, sodium sulfite is preferred. Among the above sulfamic acids and salts thereof, sulfamates are preferred, and specific examples of sulfamates include sodium sulfamate, potassium sulfamate, calcium sulfamate, and nickel sulfamate. Specific examples of the thiosulfates include sodium thiosulfate, potassium thiosulfate, and ammonium thiosulfate. Specific examples of the oxalate include sodium oxalate and potassium oxalate. Specific examples of the formate salt include sodium formate and potassium formate. Specific examples of hypophosphites include sodium hypophosphite. These reducing agents may be used alone or in combination of two or more. The amount of reducing agent added may be adjusted appropriately depending on the amount of hypochlorous acid or its salt (effective chlorine concentration). In addition to the method of treating with hypochlorous acid or its salt, a method of adding an acid or a metal catalyst to stop the reaction of oxidizing the cellulosic raw material may also be used.

[0047] 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. Alternatively, the solution containing oxidized cellulose obtained by the above reaction can be directly subjected to the next step.

[0048] [Oxidized cellulose] In the production method of the present invention, a cellulosic raw material is oxidized using hypochlorous acid or a salt thereof, thereby obtaining oxidized cellulose. The oxidized cellulose is preferably in the form of a slurry. The term "slurry" as used herein refers to a suspension containing oxidized cellulose. The slurry may contain the solvent used during oxidation. Alternatively, a dispersion medium may be added as needed to form a slurry. When the oxidized cellulose is in the form of a slurry, it is easy to handle and tends to be more easily pulverized. When the oxidized cellulose of the present invention is in the form of a slurry, 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 slurry is taken as 100% by mass.

[0049] The oxidized cellulose of the present invention includes fibrous cellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof. The oxidized cellulose of the present invention is also referred to as oxidized cellulose fiber. That is, the oxidized cellulose of the present invention includes an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof. Note that the main component of plants is cellulose, and bundles of cellulose molecules are called cellulose microfibrils. The cellulose in the cellulosic raw material is also contained in the form of cellulose microfibrils.

[0050] (Degree of polymerization) The degree of polymerization of the oxidized cellulose in the present invention is 600 or less. When the degree of polymerization of oxidized cellulose exceeds 600, a large amount of energy tends to be required for defibration, and sufficient easy defibration properties tend not to be exhibited. When the degree of polymerization of oxidized cellulose is 600 or less, it tends to be pulverized under mild conditions and can be pulverized by ordinary stirring or kneading, and nanocellulose tends to be obtained efficiently. From the viewpoint of easy defibration properties, no lower limit for the degree of polymerization of the oxidized cellulose is particularly set. However, when the degree of polymerization of the oxidized cellulose is less than 30, the proportion of particulate cellulose rather than fibrous cellulose increases, and the quality of the slurry containing the oxidized cellulose becomes non-uniform and the viscosity becomes unstable. From the above viewpoints, the degree of polymerization of the oxidized cellulose is preferably 30 to 600.

[0051] The degree of polymerization 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 of the slurry, the lower limit of the degree of polymerization is more preferably 50 or more, even more preferably 60 or more, even more preferably 80 or more, even more preferably 90 or more, even more preferably 100 or more, and particularly preferably 110 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 oxidized cellulose is more preferably 50 to 600, even more preferably 60 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.

[0052] The degree of polymerization of oxidized cellulose can be set to 600 or less by setting the slurry viscosity or the initial viscosity of the reaction system to 1000 Pa·s or less, which allows hypochlorous acid or its salts to become sufficiently uniform within the reaction system. The degree of polymerization of oxidized cellulose can be adjusted, for example, by changing the reaction time, reaction temperature, pH, and available chlorine concentration of hypochlorous acid or a salt thereof during the oxidation reaction. Specifically, since the degree of polymerization tends to decrease as the degree of oxidation increases, methods for decreasing the degree of polymerization include, for example, increasing the reaction time and / or reaction temperature of the oxidation. Furthermore, the degree of polymerization of oxidized cellulose can be adjusted by 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. For this reason, the degree of polymerization of oxidized cellulose can also be adjusted by selecting the cellulosic raw material. Note that, 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. Specifically, the degree of polymerization of oxidized cellulose can be measured by the method described in the Examples.

[0053] (carboxyl group amount) The amount of carboxy groups in the oxidized cellulose is preferably 0.30 to 2.0 mmol / g. A carboxy group amount of 0.30 mmol / g or more can impart sufficient defibrability to the oxidized cellulose. This allows for micronization under mild conditions, and tends to be achieved by ordinary stirring or kneading. On the other hand, a carboxy group amount of 2.0 mmol / g or less can prevent excessive decomposition of the oxidized cellulose when blended with other components, resulting in nanocellulose of uniform quality with a low proportion of particulate cellulose. From this perspective, the amount of carboxy groups in 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 greater than 0.50 mmol / g, and even more preferably 0.55 mmol / g or more. The upper limit of the carboxyl group content is more preferably 1.5 mmol / g or less, even more preferably 1.2 mmol / g or less, even more preferably 1.0 mmol / g or less, and even more preferably 0.9 mmol / g. A preferred range of the carboxyl group content can be determined by appropriately combining the above-mentioned upper and lower limits. The carboxyl group content of the oxidized cellulose is 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.

[0054] The amount of carboxy groups (mmol / g) in oxidized cellulose 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 is gradual, after which 0.1 mol / L hydrochloric acid aqueous solution is added to an aqueous solution of oxidized cellulose mixed with water to adjust the pH to 2.5, 0.05 N sodium hydroxide aqueous solution is added dropwise, and the electrical conductivity is measured until the pH reaches 11.0. The amount of carboxy groups in oxidized cellulose can be adjusted by changing the reaction time, reaction temperature, pH of the reaction solution, etc. of the oxidation reaction. Amount of carboxyl group = a (ml) x 0.05 / mass of oxidized cellulose (g)

[0055] Specifically, the amount of carboxy groups can be measured according to the following procedure. 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 above 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.

[0056] (light transmittance) The oxidized cellulose of the present invention is preferably a nanocellulose aqueous dispersion obtained by defibrating a 0.1% by mass aqueous dispersion of the oxidized cellulose in a planetary centrifugal mixer at a revolution speed of 2000 rpm and a rotation speed of 800 rpm for 10 minutes, and the optical transmittance of the resulting nanocellulose aqueous dispersion is preferably 60% or higher. The optical transmittance of this nanocellulose aqueous dispersion is more preferably 70% or higher, even more preferably 75% or higher, and even more preferably 80% or higher. Note that the optical transmittance is measured at a wavelength of 660 nm using a spectrophotometer.

[0057] The reasons why the oxidized cellulose of the present invention has excellent defibration properties (especially easy defibration properties) and provides a high-quality slurry are not clear, but are thought to be roughly as follows: Defibration proceeds by severing hydrogen bonds between cellulose microfibrils. In oxidation treatment using hypochlorous acid or a salt thereof, the degree of polymerization of microfibrils decreases as oxidation progresses (i.e., the cellulose molecular chains become shorter). In the present embodiment, the oxidation treatment reduces the number of hydrogen bonds that must be severed by defibration in each microfibril, and furthermore, the amount of carboxyl groups increases as oxidation progresses, which is thought to strengthen the repulsive force between microfibrils and improve the defibration properties of oxidized cellulose.

[0058] The oxidized cellulose of the present invention is obtained by oxidation using hypochlorous acid or a salt thereof. The oxidized cellulose thus obtained preferably has a structure in which at least two of the hydroxyl groups on the glucopyranose ring that constitutes the cellulose have been oxidized, more specifically, the hydroxyl groups at the second and third positions on the glucopyranose ring have been oxidized and a carboxyl group has been introduced. Furthermore, it is preferable that the hydroxyl group at the sixth position on the glucopyranose ring in the nanocellulose or oxidized cellulose is not oxidized and remains as a hydroxyl group. The position of the carboxyl group on the glucopyranose ring can be determined by the solution NMR spectrum using oxidized rayon as a model molecule and the solid NMR spectrum of oxidized cellulose. 13 It can be analyzed by comparing C-NMR spectra.

[0059] Rayon has the same chemical structure as cellulose, and its oxide (rayon oxide) is water-soluble. 13 C-NMR measurements reveal a carbon peak attributable to carboxy groups at 165 to 185 ppm. In one embodiment of the oxidized cellulose or nanocellulose used in the present invention, obtained by oxidizing raw cellulose with hypochlorous acid or its salt, two signals appear within this chemical shift range. Furthermore, solution two-dimensional NMR measurements reveal that carboxy groups have been introduced at the 2- and 3-positions.

[0060] Oxidized cellulose or nanocellulose solid obtained by oxidizing raw cellulose 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 this is difficult to achieve with low-resolution solid state spectroscopy. 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 peaks appearing at 165 to 185 ppm. That is, solid 13 A baseline is drawn around the peak in the range of 165 ppm to 185 ppm in the 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 ​​(large area value / small area value). If the ratio of the peak area values ​​is 1.2 or more, the peak can be said to be 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. 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.

[0061] The oxidized cellulose of the present invention is prepared without the need for N-oxyl compounds such as TEMPO. Therefore, the oxidized cellulose and nanocellulose of the present invention are substantially free of N-oxyl compounds. Here, in the first invention, the expression "substantially free of N-oxyl compounds" in oxidized cellulose or nanocellulose means that the oxidized cellulose or 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 oxidized cellulose or nanocellulose. Furthermore, "substantially free of N-oxyl compounds" also refers to cases where the content of N-oxyl compounds, as an increase from the cellulosic raw material, is preferably 2.0 ppm by mass or less, more preferably 1.0 ppm by mass or less. By being substantially free of N-oxyl compounds, it is possible to prevent N-oxyl compounds, which are of concern due to their impact on the environment and human body, from remaining in the oxidized cellulose or nanocellulose. The content of N-oxyl compounds can be measured by known means. Known means include methods using a trace total nitrogen analyzer. Specifically, the nitrogen component derived from N-oxyl compounds in oxidized cellulose or nanocellulose can be measured as the amount of nitrogen using a trace total nitrogen analyzer (e.g., Mitsubishi Chemical Analytech Co., Ltd., model TN-2100H, etc.).

[0062] [Nanocellulose] The oxidized cellulose of the present invention may be micronized to form nanocellulose. One aspect of the present invention is a method for producing nanocellulose, comprising the step of obtaining nanocellulose by defibrating the oxidized cellulose obtained by the production method of the present invention. That is, the method for producing nanocellulose of the present invention comprises the steps of obtaining oxidized cellulose by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof, and obtaining nanocellulose by defibrating the oxidized cellulose, wherein the oxidized cellulose comprises an oxide of a cellulosic raw material with hypochlorous acid or a salt thereof, is substantially free of N-oxyl compounds, has a degree of polymerization of 600 or less, and has a slurry viscosity or initial viscosity of the reaction system of 1000 Pa s or less. The oxidized cellulose of the present invention may also be used by blending it with other components. That is, by blending it with other components without micronizing it and then appropriately stirring the oxidized cellulose by stirring, etc., a nanocellulose-containing composition containing nanocellulose and at least one other component can be obtained. Furthermore, the oxidized cellulose of the present invention can also be micronized by the user themselves at the time of use to produce nanocellulose.

[0063] Nanocellulose in the present invention refers to oxidized cellulose obtained by the production method of the present invention, which has been defibrated and refined. Nanocellulose includes fine cellulose fibers.

[0064] The average fiber length of the nanocellulose in the present invention is preferably 50 nm or more and 800 nm or less. When the average fiber length is 50 nm or more, the quality of the nanocellulose tends to be more uniform. From the viewpoint of making the quality more uniform, the lower limit of the average fiber length is more preferably 100 nm or more, and even more preferably 150 nm or more. When the average fiber length is 800 nm or less, the proportion of coarse cellulose fibers tends to be reduced and the occurrence of precipitation of nanocellulose tends to be suppressed. From the viewpoint of further suppressing the occurrence of precipitation, the upper limit of the average fiber length is more preferably 700 nm or less, and even more preferably 600 nm or less. From the viewpoint of further improving the quality of nanocellulose, the average fiber length is more preferably 50 nm or more and 700 nm or less, even more preferably 100 nm or more and 700 nm or less, and even more preferably 100 nm or more and 600 nm or less.

[0065] The average fiber width of the nanocellulose in the present invention is preferably 1 nm or more and 100 nm or less. When the average fiber width is 1 nm or more, the quality of the nanocellulose tends to be more uniform. From the viewpoint of making the quality more uniform, the lower limit of the average fiber width is more preferably 2 nm or more, and even more preferably 3 nm or more. When the average fiber width is 100 nm or less, the proportion of coarse nanocellulose tends to be reduced and the occurrence of nanocellulose precipitation tends to be suppressed. From the viewpoint of further suppressing the occurrence of precipitation, the average fiber width is more preferably 50 nm or less, and even more preferably 30 nm or less. From the viewpoint of further improving the quality of nanocellulose, the average fiber width is more preferably 2 nm or more and 50 nm or less, and even more preferably 3 nm or more and 30 nm or less.

[0066] In the nanocellulose of the present invention, 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 or more and 200 or less. An aspect ratio of 200 or less tends to result in uniform dispersion of nanocellulose and improved quality. From this perspective, the aspect ratio is more preferably 190 or less, and even more preferably 180 or less. On the other hand, if the aspect ratio is too low, i.e., if the nanocellulose is shaped like thick rods rather than long, thin fibers, aggregation occurs due to uneven distribution, and the quality of the nanocellulose tends to deteriorate. Therefore, the aspect ratio is preferably 20 or more, more preferably 30 or more, and even more preferably 40 or more.

[0067] 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 conditions. The range of difference in values ​​between conditions is preferably within ±100 nm for average fiber length. The range of difference in values ​​between conditions is preferably within ±10 nm for average fiber width. More detailed measurement methods follow the methods described in the Examples below.

[0068] When measuring various physical properties of nanocellulose in the present invention, the nanocellulose may be used as the measurement sample, the nanocellulose-containing composition may be used as the measurement sample, or the nanocellulose after separating the nanocellulose from other components (compounds) from the nanocellulose-containing composition may be used as the measurement sample.

[0069] In one aspect, the nanocellulose of the present invention can be characterized by the average fiber width, average fiber length, or aspect ratio, as described above, but in other aspects, it may have a predetermined zeta potential or light transmittance.

[0070] (zeta potential) In one embodiment of the present disclosure, the nanocellulose of the present invention preferably 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. This improves the dispersion stability of the nanocellulose, and when made into a slurry, it can achieve excellent viscosity stability and handleability. 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, and nanocellulose of uniform size tends to be obtained. The zeta potential tends to be increased by, for example, setting one or more of the reaction time, reaction temperature, and stirring conditions of the oxidation to the side where oxidation is further advanced (i.e., the side where oxidation degree is increased) (for example, by lengthening the reaction time). In addition, the zeta potential can be suitably controlled by performing oxidation using hypochlorous acid or its salt.

[0071] From the above viewpoint, the zeta potential of the nanocellulose in the present invention is more preferably -35 mV or less, even more preferably -40 mV or less, and even more preferably -50 mV or less. 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, and even more preferably -77 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 and -35 mV or less, more preferably -85 mV or more and -40 mV or less, and even more preferably -80 mV or more and -50 mV or less. Note that the zeta potential in this specification is a value measured at pH 8.0 and 20°C for a cellulose aqueous dispersion obtained by mixing the nanocellulose of the present invention with water to a nanocellulose concentration of 0.1% by mass.

[0072] Specifically, the zeta potential can be measured according to the following method. Pure water is added to the nanocellulose to dilute it to a nanocellulose concentration of approximately 0.1%. After dilution, a 0.05 mol / L aqueous solution of sodium hydroxide is added to the nanocellulose aqueous dispersion to adjust the pH to approximately 8.0, and the zeta potential is measured at 20°C using, for example, a zeta potential meter (ELSZ-1000) manufactured by Otsuka Electronics Co., Ltd.

[0073] (light transmittance) The nanocellulose dispersion of the present invention, in which nanocellulose is dispersed in a dispersion medium, exhibits little light scattering by cellulose fibers and exhibits high light transmittance. Specifically, in a preferred embodiment, the nanocellulose of the present invention has a light transmittance of 95% or more in a mixed solution obtained by mixing 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. The light transmittance is a value measured at a wavelength of 660 nm using a spectrophotometer.

[0074] The light transmittance can be measured, for example, by placing an aqueous dispersion of nanocellulose in a 10 mm thick quartz cell and using a spectrophotometer (JASCO V-550).

[0075] The nanocellulose of the present invention is an aggregate of individual fibers. When a carboxyl group is introduced into the nanocellulose of the present invention, it is sufficient that it contains at least one carboxylated nanocellulose (also referred to as carboxylated CNF), and it is preferable that the carboxylated nanocellulose is the main component. Here, "carboxylated CNF is the main component" means that the proportion of carboxylated CNF in the total amount of fine cellulose 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.

[0076] The method for defibrating oxidized cellulose is not particularly limited as long as it is a procedure that can disperse nanocellulose. Here, nanocellulose is a general term for finely divided cellulose, and includes cellulose nanofibers, cellulose nanocrystals, etc.

[0077] For example, defibration can be achieved by using a velocity field and velocity fluctuation of any intensity, collision with inclusions or obstacles, ultrasonic waves, pressure loading, etc. A submerged disperser can be suitably used for such a dispersion operation. The submerged disperser is not particularly limited, and examples thereof include methods using a homomixer, a magnetic stirrer, a 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 stirrer, 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 mixer is a device that disperses materials by passing them through the gap between the rotor and the outer cylinder, and disperses them by shear flow in the gap and strong speed fluctuations back and forth. A colloid mill is a device that disperses particles by shear flow in the gap between a rotating disk and a fixed disk, while a roll mill disperses particles by shear and compression forces that utilize the gap between multiple rotating rolls. A pressure homogenizer is used as a disperser that ejects a slurry or 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 a homogenizer capable of ejecting a slurry 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 used to wet powders and other materials with a liquid (also known as kneading or kneading).Specific examples include twin-arm kneaders (devices that disperse materials using two mixing blades inside two semi-cylindrical containers); Banbury mixers (devices that disperse materials under pressure in a closed system); and extrusion-type kneaders such as screw extruders, co-kneaders, and extruders.

[0078] Examples of the defibration method include methods using various mixing or 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 disk refiner, grinder, single-shaft or multi-shaft kneader, planetary stirrer, and vibration stirrer.

[0079] The devices used for defibration can be used alone or in combination of two or more types.

[0080] Oxidized cellulose may be defibrated using, for example, an ultra-high pressure homogenizer, which allows for the production of more defibrated nanocellulose. When defibration is performed using an ultra-high pressure homogenizer, 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 times the defibration treatment is performed is not particularly limited, but from the viewpoint of ensuring sufficient progress of defibration, it is preferably two or more times, more preferably three or more times.

[0081] Oxidized cellulose has excellent defibration properties, so it can be sufficiently defibrated even when mild agitation using, for example, a planetary centrifugal mixer or a vibration mixer is used as a defibration method, and uniform nanocellulose can be obtained.

[0082] A planetary centrifugal mixer is a device that mixes materials in a container by rotating and revolving the container into which the materials are placed. A planetary centrifugal mixer performs mixing without using stirring blades, allowing for gentler mixing. The revolution speed and rotation speed during mixing by a planetary centrifugal mixer can be set as appropriate; for example, the revolution speed can be set to 400 to 3000 rpm, and the rotation speed can be set to 200 to 1500 rpm. When using a planetary centrifugal mixer, from the perspective of ensuring gentle mixing while ensuring uniformity of quality, it is preferable to perform the defibration treatment under conditions of a revolution speed of 1200 to 2500 rpm and a rotation speed of 600 to 1000 rpm, stirring for 3 to 15 minutes. The revolution speed is more preferably 1500 to 2300 rpm, and the rotation speed is more preferably 700 to 950 rpm. When the present oxidized cellulose fibers are defibrated using a planetary centrifugal mixer, the concentration of the oxidized cellulose aqueous dispersion used as the raw material is, for example, 0.01 to 1.0% by mass, and preferably 0.1 to 0.5% by mass.

[0083] An example of a vibration-type agitator is a vortex mixer (touch mixer). In a vortex mixer, agitation is performed by forming a vortex in the liquid material in a container. A vibration-type agitator such as a vortex mixer performs agitation without using a stirring blade, thereby achieving gentler agitation. Furthermore, a vibration-type agitator such as a vortex mixer can achieve gentle agitation with simple equipment, making it advantageous in terms of production equipment and production costs. The rotation speed of the vortex mixer is, for example, 600 to 3,000 rpm, and the defibration treatment is preferably performed under stirring conditions of 3 to 15 minutes. When the present oxidized cellulose fibers are defibrated using a vortex mixer, the concentration of the oxidized cellulose aqueous dispersion used as the raw material is, for example, 0.01 to 1.0% by mass, and preferably 0.1 to 0.5% by mass.

[0084] 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.

[0085] 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.

[0086] 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 to mix with various components such as resins, rubbers, and solid particles.

[0087] The oxidized cellulose and nanocellulose obtained by the production method of the present invention can be used in a variety of applications. Specifically, for example, they may be used as various materials (e.g., resins, fibers, rubber, etc.) or in various applications (e.g., foods, cosmetics, medical products, paints, inks, etc.). Nanocellulose-containing compositions can also be formed into membranes and used as various sheets or films. The fields in which nanocellulose-containing compositions can be used 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, stationery, etc.

[0088] <<Embodiment for Carrying Out the Second Invention>> <Method of producing oxidized cellulose> The method for producing oxidized cellulose of the present invention includes a step of obtaining oxidized cellulose by solid-liquid separation of an oxide dispersion containing a cellulose-based oxide and a dispersion medium (hereinafter also referred to as the "separation step"). The pH of the oxide dispersion is 4.0 or less. Furthermore, the oxide dispersion is substantially free of N-oxyl compounds, or the method further includes a step of oxidizing a cellulose-based raw material with a predetermined amount of hypochlorous acid or a salt thereof to obtain the cellulose-based oxide.

[0089] Here, in the second invention, the meaning of "substantially free of N-oxyl compounds" is as explained in relation to the above <<Form for carrying out the first invention>> (however, "oxidized cellulose or nanocellulose" should be read as "oxide dispersion").

[0090] The method for producing oxidized cellulose of the present invention allows for the production of oxidized cellulose in high yield. The reason for this is presumed to be (but is not limited to) the following: In conventional production methods, when a cellulose-based oxide obtained by oxidizing a cellulose-based raw material with hypochlorous acid or a salt thereof is dispersed in a dispersion medium and subjected to solid-liquid separation, the cellulose-based oxide is micronized in the dispersion medium, and a portion of the cellulose-based oxide transfers from the solid phase to the liquid phase, resulting in a low yield of oxidized cellulose. In particular, when solid-liquid separation is performed by filtration, clogging of the filter cloth can occur, making the solid-liquid separation itself difficult. On the other hand, in the method for producing oxidized cellulose of the present invention, the pH of the oxide dispersion is 4.0 or less, which prevents the cellulose-based oxide from being micronized in the dispersion medium and improves the yield of oxidized cellulose recovered by solid-liquid separation. In particular, when solid-liquid separation is performed by filtration, clogging of the filter cloth does not occur, improving the operability of solid-liquid separation and making it easy to wash the oxidized cellulose on the filter cloth afterwards.

[0091] Hereinafter, the manufacturing method of the present invention will be described in detail for each step of an example shown in the flowchart of FIG. 1, but the present invention is not limited to this, and various modifications are possible within the scope of the gist of the method.

[0092] [Oxidation process] The production method of the present invention may include a step of oxidizing a cellulosic raw material to obtain a cellulosic oxide to be used in the separation step (hereinafter referred to as the "oxidation step"). In the oxidation step, an oxidizing agent can be used to oxidize the cellulosic raw material, and it is particularly preferable to use hypochlorous acid or a salt thereof.

[0093] By oxidizing with hypochlorous acid or its salt, a cellulose-based oxide can be obtained without using an N-oxyl compound such as 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (TEMPO) as an oxidizing agent.

[0094] Specific examples of hypochlorous acid or a salt thereof are as explained in relation to the above <<First Mode for Carrying Out the Invention>>.

[0095] Specific examples of the cellulosic raw material are as explained in relation to the above <<First Mode for Carrying Out the Invention>>.

[0096] In the second aspect of the present invention, the cellulose-based oxide refers to a material mainly composed of cellulose containing carboxyl groups. In the separation step described below, it is preferable to use the cellulose-based oxide obtained in the oxidation step, but there is no particular limitation. In relation to the second invention, the oxide dispersion means a dispersion containing a cellulose-based oxide and a dispersion medium, and having a pH of 4.0 or less. In the context of the second invention, oxidized cellulose refers to a component of a material primarily composed of carboxyl groups and cellulose that can be present as a solid phase in a dispersion. For example, it is a polymerized component of cellulose that does not dissolve in the dispersion medium and is extracted as a solid phase due to its degree of polymerization being equal to or greater than a predetermined value or its water solubility being less than a predetermined value. The pH in this specification can be measured using a pH meter equipped with a pH electrode, and the pH range can also be controlled using a pH controller equipped with a pH electrode.

[0097] A method for obtaining a cellulose oxide by oxidation of a cellulose raw material includes mixing the cellulose raw material with a reaction solution containing hypochlorous acid or a salt thereof. The solvent contained in the reaction solution is preferably water, because it is easy to handle and does not easily cause side reactions.

[0098] In the oxidation step, the mass ratio of hypochlorous acid or a salt thereof to the cellulosic raw material is not particularly limited, but is preferably 0.2 or more, more preferably 0.5 or more, and even more preferably 1.0 or more. Within this range, the amount of carboxy groups in the resulting cellulose oxide and oxidized cellulose can be sufficiently increased, and sufficient pulverization tends to proceed during defibration, as described below. The upper limit of this mass ratio is not particularly limited, but is preferably 20 or less, more preferably 10 or less, and even more preferably 5.0 or less. The range of the mass ratio can be determined by appropriately combining the upper and lower limits, and may be, for example, 0.2 to 20, 0.5 to 10, or 1.0 to 5.0.

[0099] Oxidized cellulose can be obtained, for example, by oxidizing a cellulosic raw material under the above-mentioned mass ratio conditions, followed by a separation step described below, or by appropriately controlling the above-mentioned mass ratio, reaction conditions such as pH during the reaction, reaction temperature, etc. The structure of the oxidized cellulose thus obtained (oxidation state of hydroxyl groups) is as explained above in relation to <<First Mode for Carrying Out the Invention>>.

[0100] In the oxidation step, the pH may be adjusted or not adjusted, and the pH range may be set as desired. The preferred pH value and the method for adjusting the pH are as described above in relation to the "First Embodiment of the Invention."

[0101] [Processing process] The production method of the present invention may further include a step of treating the hypochlorous acid or a salt thereof used in the oxidation step (hereinafter also referred to as a "treatment step"). By including the treatment step, the reaction of oxidizing the cellulosic raw material can be stopped. Specific examples of the treatment step are as described above in relation to <<First Embodiment of the Invention>>.

[0102] [Protonation process] The production method of the present invention may further include a step of adding an acid to prepare an oxide dispersion having a pH of 4.0 or less (hereinafter also referred to as a "protonation step") in order to prepare an oxide dispersion to be used in the separation step. Here, the cellulose oxide contained in the oxide dispersion contains carboxyl groups, and the protonation step is a step for converting at least a portion of the carboxyl groups from the salt form (-COO- X+: X+ represents a cation such as sodium) to the proton form (-COO-H+).

[0103] The acid used in the protonation step is not particularly limited as long as it can prepare an oxide dispersion having a pH of 4.0 or less, and examples thereof include inorganic acids and organic acids. Among these, inorganic acids, particularly hydrochloric acid, are preferred from the viewpoint of ease of handling. A cation exchange resin may also be used in the protonation step.

[0104] As the cation exchange resin, either a strong acid ion exchange resin or a weak acid ion exchange resin can be used as long as the counter ion is H+, and among these, a strong acid ion exchange resin is preferred. Examples of strong acid ion exchange resins and weak acid ion exchange resins include styrene-based resins or acrylic-based resins into which sulfonic acid groups or carboxy groups have been introduced. The shape of the cation exchange resin is not particularly limited, and various shapes such as fine particles (granules), membranes, and fibers can be used. Among these, granular shapes are preferred from the viewpoints of efficiently desalting the carboxylated cellulose nanofiber salt and facilitating separation after desalting. Commercially available cation exchange resins can be used. Commercially available products include, for example, Amberjet 1020, 1024, 1060, and 1220 (all manufactured by Organo Corporation), Amberlite IR-200C and IR-120B (all manufactured by Tokyo Organic Chemical Industry Co., Ltd.), Lewatit SP112 and Lewatit S100 (all manufactured by Bayer), GELCK08P (manufactured by Mitsubishi Chemical Corporation), and Dowex 50W-X8 (manufactured by The Dow Chemical Company). After protonation using the cation exchange resin, the cation exchange resin may be removed by filtration using a metal mesh or the like.

[0105] The dispersion medium used in the protonation step is not particularly limited, and it is preferable to use the solvent contained in the acid as is. Depending on the purpose, a dispersion medium not contained in the acid can be used in combination as appropriate, or the solvent contained in the acid can be replaced with another dispersion medium. Specific examples of dispersion mediums include those described as dispersion liquids used in the defibration treatment in the above-mentioned <<First Embodiment of the Invention>>.

[0106] The pH of the oxide dispersion is preferably 4.0 or less, more preferably 3.0 or less, and even more preferably 2.5 or less. In this case, the lower limit of the pH of the oxide dispersion is not particularly limited, and is usually 1.0 or more, preferably 1.5 or more, and more preferably 2.0 or more. The pH range of the oxide dispersion may be an appropriate combination of the above upper and lower limits, and may be, for example, 1.0 to 4.0, 1.5 to 3.0, or 2.0 to 2.5.

[0107] When a protonation step is included, the protonation step may be carried out before the separation step, preferably after the oxidation step, but may also be carried out simultaneously with part of the oxidation step.

[0108] The cellulose-based oxide may be further purified before, during, or after the protonation step, as necessary, before being used in the separation step. Alternatively, the solution containing the cellulose-based oxide obtained in the oxidation step may be directly subjected to the separation step.

[0109] [Separation process] The method for producing oxidized cellulose of the present invention includes a step of subjecting an oxide dispersion containing a cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain oxidized cellulose (separation step), wherein the pH of the oxide dispersion is 4.0 or less.

[0110] The method for solid-liquid separation of the oxide dispersion is not particularly limited, but examples include methods in which the liquid phase is removed by known isolation processes such as centrifugation or filtration, and the oxidized cellulose contained in the solid phase is obtained. Of these, solid-liquid separation by filtering the oxide dispersion is preferred from the standpoint of operability.

[0111] The pH of the oxide dispersion in the separation step is the same as the pH of the oxide dispersion prepared in the protonation step, and the specific value is as described above.

[0112] The dispersion medium used in the separation step is the same as the dispersion medium used in the protonation step, and it is preferable to use the dispersion medium used there as is. Specific examples of the dispersion medium are also the same as those used in the protonation step.

[0113] [Cleaning process] The method for producing oxidized cellulose of the present invention may further include a step of washing the oxide dispersion or oxidized cellulose with an acidic washing solution (hereinafter also referred to as the "washing step").

[0114] The acidic washing solution used in the washing step may be a solution containing the acid and dispersion medium used in the protonation step, but is not particularly limited.

[0115] The pH of the acidic cleaning solution is the same as that of the oxide dispersion described above, and the specific value is as described above.

[0116] When a washing step is included, the washing step may be performed before, after, or simultaneously with the separation step, and the separation step and washing step may be repeated as described later in the Examples.

[0117] [Chlorination process] The production method of the present invention can further include a step of adding a base to adjust the pH of the oxidized cellulose dispersion containing the oxidized cellulose obtained in the separation step and a dispersion medium to above 4.0 (hereinafter also referred to as the "chlorination step", or "neutralization step"). Here, the oxidized cellulose contained in the oxidized cellulose dispersion contains carboxyl groups, and the chloride step is a step for converting at least a portion of these carboxyl groups from the proton form (-COO-H+) to the salt form (-COO- X+: X+ represents a cation such as sodium or lithium).

[0118] The base used in the salification step is not particularly limited as long as it can adjust the pH of the oxidized cellulose dispersion to greater than 4.0, and examples thereof include inorganic bases and organic bases. Of these, inorganic bases, particularly sodium hydroxide, are preferred from the standpoint of ease of handling. Furthermore, an amine can also be used as the base, and the amine may be a primary amine, a secondary amine, a tertiary amine, or a quaternary amine.

[0119] The pH of the oxidized cellulose dispersion is preferably 5.0 or higher, more preferably 6.0 or higher, and even more preferably 7.0 or higher. The upper limit of the pH is not particularly limited, but is preferably 14.5 or lower, more preferably 14.0 or lower, even more preferably 12.0 or lower, still more preferably 10.0 or lower, even more preferably 9.0 or lower, and particularly preferably 8.0 or lower. The pH range can be determined by appropriately combining the above upper and lower limits, and may be, for example, 5.0 or higher to 14.5 or lower, 5.0 or higher to 14.0 or lower, 6.0 or higher to 12.0 or lower, 6.0 or higher to 10.0 or lower, 7.0 or higher to 9.0 or lower, or 7.0 or higher to 8.0 or lower.

[0120] There are no particular limitations on the dispersion medium used in the salification step, and it is preferable to use the solvent contained in the base as is. Depending on the purpose, a dispersion medium not contained in the base can be used in combination, or the solvent contained in the base can be replaced with another dispersion medium. Specific examples of dispersion media are the same as those used in the protonation step. However, water and / or organic solvents are preferred because they facilitate the isolation of nanocellulose in the defibration step described below.

[0121] When the salification step is included, the salification step may be performed after the separation step, and may be performed before, after, or simultaneously with the washing step.

[0122] <Oxidized cellulose> The oxidized cellulose of the present invention is oxidized cellulose obtained by the production method of the present invention. Specifically, the oxidized cellulose is derived from the oxide dispersion used in the separation step and is a component extracted as a solid phase by solid-liquid separation.

[0123] The oxidized cellulose is preferably in the form of a slurry. The term "slurry" used here refers to a suspension containing oxidized cellulose. The slurry may contain the dispersion medium used in the separation step. Alternatively, the slurry may be formed by adding an appropriate dispersion medium. When the oxidized cellulose is in the form of a slurry, it is easy to handle and tends to be more easily pulverized.

[0124] Oxidized cellulose includes fibrous cellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof. The oxidized cellulose of the present invention is also referred to as oxidized cellulose fiber. That is, the oxidized cellulose of the present invention includes an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof.

[0125] The preferred degree of polymerization of oxidized cellulose, the method for preparing it, and the method for measuring it are as explained in relation to the above <<First Mode for Carrying Out the Invention>>.

[0126] The preferred light transmittance of the nanocellulose aqueous dispersion obtained by defibrating an aqueous dispersion of oxidized cellulose at a concentration of 0.1% by mass in a planetary centrifugal mixer at a revolution speed of 2000 rpm and a rotation speed of 800 rpm for 10 minutes is as explained in relation to the above <<First embodiment of the invention>>.

[0127] The oxidized cellulose of the present invention may also be used by blending it with other components. That is, by blending it with other components without micronizing it and then appropriately stirring the oxidized cellulose by stirring, etc., a nanocellulose-containing composition containing nanocellulose and at least one other component can be obtained. Furthermore, the oxidized cellulose of the present invention can also be micronized by the user themselves at the time of use to produce nanocellulose.

[0128] <Method of manufacturing nanocellulose> The method for producing nanocellulose of the present invention includes a step of defibrating oxidized cellulose obtained by the method for producing oxidized cellulose of the present invention to obtain nanocellulose.

[0129] The defibration method is not particularly limited as long as it is a method that can micronize oxidized cellulose, but is preferably carried out in a state where the oxidized cellulose is mixed with a dispersion medium. Specific defibration methods are as explained in relation to the "First Mode for Carrying Out the Invention" above.

[0130] The dispersion medium used in the defibration step is the same as the dispersion medium used in the salification step, and it is preferable to use the dispersion medium used there as is. Specific examples of the dispersion medium are the same as those used in the protonation step. However, water and / or organic solvents are preferred because this makes it easier to isolate nanocellulose. Furthermore, since nanocellulose dispersed in an organic solvent is obtained, it is easy to mix with resins that dissolve in organic solvents and their raw material monomers. The nanocellulose dispersion obtained by dispersing the nanocellulose obtained by defibration in a dispersion medium of water and / or organic solvent can be used to mix with various components such as resins, rubber, and solid particles.

[0131] <Nanocellulose> The nanocellulose of the present invention is nanocellulose obtained by the nanocellulose production method of the present invention, is derived from the oxidized cellulose of the present invention, and refers to the oxidized cellulose that has been defibrated and refined. In this specification, nanocellulose is a general term for cellulose that has been refined, and includes fine cellulose fibers and cellulose nanocrystals. Fine cellulose fibers are also called cellulose nanofibers (also called CNF).

[0132] The average fiber length, average fiber width, measurement methods therefor, and aspect ratio (average fiber length / average fiber width) of nanocellulose are as explained in relation to the above <<First Embodiment of the Invention>>.

[0133] It is preferable that the light transmittance of the aqueous dispersion of nanocellulose is 60% or more. The light transmittance of this aqueous dispersion of nanocellulose is more preferably 70% or more, even more preferably 75% or more, and even more preferably 80% or more. Specific methods for measuring light transmittance will be explained in the examples below.

[0134] The uses of the oxidized cellulose and nanocellulose obtained by the production method of the present invention are as explained in relation to the above <<First Mode for Carrying Out the Invention>>.

[0135] <<Combination of the first invention and the second invention>> The first and second inventions may be combined to form a method comprising a step of oxidizing a cellulosic raw material to obtain oxidized cellulose and a step of post-treating the oxidized cellulose. The following method is an example, but is not limited to this, and specific aspects of the first invention and the second invention described above can be combined as appropriate.

[0136] [Example of combination] a step of obtaining a first oxidized cellulose by oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof (wherein the viscosity of a slurry of the cellulosic raw material at the same concentration as that used in the oxidation is in the range of 1000 Pa s or less when measured using a viscometer equipped with an SPP rotor at a rotation speed of 100 rpm at 30°C or 40°C [part of the first invention]); a step of subjecting an oxide dispersion containing the first oxidized cellulose and a dispersion medium to solid-liquid separation to obtain a second oxidized cellulose (wherein the pH of the oxide dispersion is 4.0 or less) [part of the second invention]; A method for producing oxidized cellulose, which is substantially free of N-oxyl compounds and has a degree of polymerization of 600 or less, comprising:

[0137] The terms "first oxidized cellulose" and "second oxidized cellulose" in the "Second Invention Section" correspond to the terms "cellulose-based oxide" and "oxidized cellulose" in the "Embodiments for Carrying Out the Second Invention" above, respectively. [Example]

[0138] 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". Various physical properties were measured as follows.

[0139] [Measurement of viscosity average degree of polymerization of oxidized cellulose] Oxidized cellulose was added to a sodium borohydride solution 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 fiber. After reduction, solid-liquid separation was performed by suction filtration, followed by washing with water. The resulting oxidized cellulose fiber was freeze-dried. 0.04 g of dried oxidized cellulose fiber was added to 10 ml of purified water and stirred for 2 minutes. 10 ml of 1 mol / L copper ethylenediamine solution was then added to dissolve the fiber. The flow times 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 times of the blank solution (t0), the flow times of the cellulose solution (t), and the concentration of oxidized cellulose fiber (c [g / ml]) using the following equations. The degree of polymerization (DP) of the oxidized cellulose fiber was then calculated using the following viscometric equation. ηr=η / η0=t / t0 ηsp=ηr-1 [η]=ηsp / (100×c(1+0.28ηsp)) DP=175×[η]

[0140] [Measurement of average fiber width and average fiber length] Pure water was added to the aqueous dispersion of nanocellulose to adjust the oxidized CNF concentration in the aqueous dispersion to 5 ppm. After adjusting the concentration, the CNF aqueous dispersion was allowed to air dry on a mica substrate, and the shape of the oxidized CNF was observed using an Oxford Asylum MFP-3D Infinity scanning probe microscope in AC mode. The average fiber length was calculated by binarizing the images obtained using the image processing software "ImageJ." For 100 or more fibers, the average fiber length was calculated as fiber length = "perimeter" ÷ 2. 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 shape image = fiber width.

[0141] [Measurement of viscosity of reaction system] First, slurries consisting of only the cellulosic raw material and water were prepared. Specifically, using the powdered pulp (VP-1) used in the Examples and Comparative Examples, slurries with concentrations of 7%, 15%, and 20% by mass were prepared, which were the same as the concentrations of the cellulosic raw material in the reaction systems of the Examples and Comparative Examples. The viscosities of these slurries were measured at 30°C or 40°C. The viscosity of the slurry was measured using a viscometer (RE-85U, manufactured by Toki Sangyo Co., Ltd.) equipped with a scrolled parallel plate (SPP) rotor at 100 rpm (equivalent to a shear rate of 200 s-1). The SPP rotor used was a spiral grooved parallel plate rotor system manufactured by Toki Sangyo Co., Ltd., with a rotor diameter of φ19.4. The viscosity was measured after the cellulosic raw material and water had been mixed. The viscosities of the 7 mass%, 15 mass%, and 20 mass% slurries at 30°C were 1.00 Pa·s or less, 1.91 Pa·s, and 9.11 Pa·s, respectively. The viscosities of the 7 mass%, 15 mass%, and 20 mass% slurries at 40°C were 1.00 Pa·s or less, 2.32 Pa·s, and 11.91 Pa·s, respectively.

[0142] [Light transmittance of nanocellulose] Pure water was added to nanocellulose to prepare an aqueous dispersion with a CNF concentration of 0.1% by mass. This aqueous dispersion was placed in a 10 mm thick quartz cell and measured with a spectrophotometer (JASCO V-550) at a wavelength of 660 nm, which was taken as the light transmittance.

[0143] [Crystallization degree of cellulose-based raw materials] Crystallinity was measured by solid-state chromatography of freeze-dried cellulosic materials. 13 C-NMR measurements were performed, and the degree of crystallinity was calculated from the peak at the fourth carbon position of cellulose (hereinafter also referred to as C4). The C4 peak appears in the range of approximately 80 to 95 ppm, with peaks from the crystalline portion (higher ppm side, approximately 85 to 95 ppm) and the amorphous portion (lower ppm side) overlapping, and the area of ​​each peak was divided by the vertical division method (crystalline portion: SC, amorphous portion: SA). The degree of crystallinity was calculated using the following formula. Crystallinity=SC / (SC+SA)×100 solid 13 C-NMR measurements were performed under the following conditions. Equipment: JNM-ECA, JEOL Frequency: 15kHz Measurement method CP / MAS method Wait time: 5 seconds Accumulation count: 10,000 times

[0144] <Examples and Comparative Examples of the First Invention> Example 1A In a 2 L baffled jacketed glass vessel, 780 g of sodium hypochlorite pentahydrate crystals with an effective chlorine concentration of 42% by mass were placed, and pure water was added and stirred to adjust the effective chlorine concentration to 21% by mass. 35% by mass hydrochloric acid was added and stirred to obtain a sodium hypochlorite aqueous solution with a pH of 11. The sodium hypochlorite aqueous solution was stirred at 300 rpm using a Shinto Scientific mixer (Three-One Motor, BL600) with three swept-back blades while circulating 30°C water through the jacket to heat the solution to 30°C. Then, 118 g of powdered pulp (VP-1, crystallinity: 40%) from TDI was added as a cellulosic raw material. After the cellulosic raw material was supplied, the temperature was maintained at 30°C, and the pH during the reaction was adjusted to 11 by adding 25% by mass of sodium hydroxide. The reaction was then stirred for 2 hours under the same conditions using a stirrer. No problems were observed with the stirring of the reaction system. Since the initial concentration of the cellulosic raw material in the slurry during the reaction was 7% by mass, the viscosity of the reaction system was determined to be 1.00 Pa·s or less based on the viscosity measured in accordance with the procedure described above in [Viscosity measurement of the reaction system]. After the reaction was complete, the oxidized cellulose was recovered by repeating centrifugation (1000G, 10 minutes), decantation, and adding pure water equivalent to the amount of the removed liquid. Water was added to adjust the oxidized cellulose concentration to 1%, and the mixture was defibrated in a homomixer at 10,000 rpm for 10 minutes to obtain an aqueous dispersion of nanocellulose. Analysis of the aqueous dispersion revealed nanocellulose with an average fiber length of 165 nm and an average fiber width of 3.2 nm. The degree of polymerization of the oxidized cellulose was 96.

[0145] 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.

[0146] 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.)

[0147] Example 2A The conditions were the same as in Example 1A, except that the amount of powdered pulp was changed to 275 g. Since the initial concentration of the cellulose-based raw material in the slurry during the reaction was 15 mass%, the viscosity of the reaction system was set to 1.91 Pa s based on the viscosity measured in accordance with the above [Viscosity measurement of the reaction system]. No problems were observed with the stirring of the reaction system, and analysis of the aqueous dispersion of nanocellulose obtained by defibration under the same conditions revealed that it was nanocellulose with an average fiber length of 168 nm and an average fiber width of 3.4 nm. The degree of polymerization of the oxidized cellulose was 105.

[0148] Example 3A In a 4 L baffled jacketed glass vessel, 780 g of sodium hypochlorite pentahydrate crystals with an effective chlorine concentration of 42% by mass were placed, and pure water was added and stirred to adjust the effective chlorine concentration to 13% by mass. 35% by mass hydrochloric acid was added and stirred to obtain a sodium hypochlorite aqueous solution with a pH of 10. The sodium hypochlorite aqueous solution was stirred at 300 rpm using a Shinto Scientific mixer (Three-One Motor, BL600) with three swept-back blades while circulating 40°C water through the jacket to heat the solution to 40°C. Then, 190 g of powdered pulp (VP-1, crystallinity: 40%) from TDI was added as a cellulosic raw material. After the cellulosic raw material was supplied, the temperature was maintained at 40°C, and the pH during the reaction was adjusted to 10 by adding 25% by mass of sodium hydroxide. The reaction was then stirred for 4 hours under the same conditions using a stirrer. No problems were observed with the stirring of the reaction system. Since the initial concentration of the cellulosic raw material in the slurry during the reaction was 7% by mass, the viscosity of the reaction system was determined to be 1.00 Pa s or less based on the viscosity measured in accordance with the procedure described above in [Viscosity measurement of the reaction system]. After the reaction was complete, the oxidized cellulose was recovered by repeating centrifugation (1000G, 10 minutes), decantation, and adding pure water equivalent to the amount of the removed liquid. Water was added to adjust the oxidized cellulose concentration to 1%, and the mixture was defibrated in a homomixer at 10,000 rpm for 10 minutes to obtain an aqueous dispersion of nanocellulose. Analysis of the aqueous dispersion revealed nanocellulose with an average fiber length of 174 nm and an average fiber width of 4.2 nm. The degree of polymerization of the oxidized cellulose was 101.

[0149] Example 4A The conditions were the same as in Example 3A, except that the amount of powdered pulp was changed to 445 g. Since the initial concentration of the cellulosic raw material in the slurry during the reaction was 15 mass%, the viscosity of the reaction system was determined to be 2.32 Pa s based on the viscosity measured in accordance with the above "Viscosity measurement of the reaction system." No problems were observed with stirring of the reaction system, and analysis of the aqueous dispersion of nanocellulose obtained by defibration under the same conditions as in Example 3A revealed that the nanocellulose had an average fiber length of 181 nm and an average fiber width of 4.5 nm. The degree of polymerization of the oxidized cellulose was 114.

[0150] Example 5A The same procedure as in Example 1A was repeated, except that the concentration of the cellulosic raw material in the reaction system was 20% by mass relative to the total amount of the reaction system. The viscosity of the reaction system measured according to the above "Viscosity measurement of reaction system" was 9.11 Pa s. Although the progress of the oxidation reaction could be confirmed because the viscosity gradually decreased, it was difficult to stir the reaction system.

[0151] Example 6A The same procedure as in Example 3A was repeated, except that the concentration of the cellulosic raw material in the reaction system was 20% by mass relative to the total amount of the reaction system. The viscosity of the reaction system, measured according to the above-mentioned "Viscosity measurement of the reaction system," was 11.91 Pa s. Although the gradual decrease in viscosity confirmed the progress of the oxidation reaction, it was difficult to stir the reaction system.

[0152] [Comparative Example 1A] When the viscosity exceeded 1000 Pa·s, the mixture could not be stirred even with a kneader, and the oxidation reaction could not be carried out.

[0153] <Examples and Comparative Examples of the Second Invention> Example 1B (oxidation process) 350 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 21% by mass. 35% by mass of hydrochloric acid was added thereto and stirred to obtain a sodium hypochlorite 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 Shinto Scientific mixer (Three-One Motor, BL600). Then, 50 g of powdered pulp (VP-1, crystallinity: 40%) from TDI Corporation was added as a cellulosic raw material. After supplying the cellulosic raw material, the temperature was maintained at 30°C in the constant-temperature water bath, and the pH of the reaction system was adjusted to 11 by adding 48% by mass of sodium hydroxide. The mixture was then stirred for 2 hours in the mixer. The mixture was then diluted 2-fold with pure water, and sodium hydroxide was added to adjust the pH to 13, slowing the oxidation reaction and obtaining a cellulosic oxide dispersed in water.

[0154] (Processing process) An aqueous solution of sodium sulfite was added to the resulting cellulose oxide dispersed in water to reduce the remaining excess sodium hypochlorite.

[0155] (Protonation step) Then, hydrochloric acid was added to convert the carboxyl groups of the cellulose oxide from the salt form (-COO-Na+) to the proton form (-COO-H+), and an aqueous dispersion with a pH of 2.5 was obtained. In this example, the pH was controlled using a pH controller (Tokyo Glass Instruments Co., Ltd., FD-02).

[0156] (Separation process, washing process) The resulting aqueous dispersion (pH 2.5) was subjected to solid-liquid separation and washing. Specifically, the supernatant was removed by centrifugation (1000G, 10 minutes) and decantation, and an amount of pure water equivalent to the amount removed was added and thoroughly stirred with a spoon to homogenize. This procedure was repeated six times, and finally, the same centrifugation and decantation were performed to obtain oxidized cellulose. The mass yield of oxidized cellulose (amount of oxidized cellulose / amount of raw cellulose × 100) was 63%.

[0157] (chlorination process) Subsequently, sodium hydroxide was added in an amount approximately equimolar to the amount of carboxyl groups introduced, converting the carboxylic acid groups from the proton form (-COO-H+) to the salt form (-COO-Na+), yielding an aqueous dispersion with a pH of 7.5. The concentration of oxidized cellulose in this aqueous dispersion was 12% by mass. The degree of polymerization of the oxidized cellulose was 90.

[0158] (defibration process) Pure water was added to the aqueous dispersion at pH 7.5 to adjust the oxidized cellulose concentration to 1% by mass, and then the dispersion was defibrated in a homomixer (10,000 rpm, 10 minutes) to obtain nanocellulose with an average fiber length of 200 nm and an average fiber width of 3 nm. Pure water was added to the resulting CNF to prepare an aqueous dispersion with a solids concentration of 0.1% by mass, and the light transmittance (660 nm) was 97%. A higher light transmittance indicates better defibration ability.

[0159] Compared to Comparative Example 1B described below, the light transmittance of the CNF was higher, and one of the reasons for this is presumably that the protonation process removed components that interfere with the defibration of oxidized cellulose in the defibration process.

[0160] Example 2B Oxidized cellulose and nanocellulose were obtained in the same manner as in Example 1B, except that in the protonation step, instead of obtaining an aqueous dispersion with a pH of 2.5, hydrochloric acid was added to obtain an aqueous dispersion with a pH of 3.5, and in the salification step, instead of obtaining an aqueous dispersion with a pH of 7.5, sodium hydroxide was added in an amount approximately equimolar to the amount of carboxy groups introduced, to obtain an aqueous dispersion with a pH of 7.3.

[0161] The mass yield of the obtained oxidized cellulose was 46%. The content concentration of oxidized cellulose after the salification step was 11 mass%, and the degree of polymerization of the oxidized cellulose was 95. The obtained nanocellulose had an average fiber length of 210 nm and an average fiber width of 3 nm, and the light transmittance of its aqueous dispersion at a solid content concentration of 0.1% by mass was 95%.

[0162] Example 3B Instead of repeating six times the procedure of removing the supernatant by centrifugation (1000 G, 10 minutes) and decantation, adding an amount of pure water equivalent to the amount removed, and thoroughly stirring with a spoon to make the mixture homogenous, solid-liquid separation was carried out by pressure filtration (0.2 MPa, filter cloth air permeability 0.3 cc / cm2 / sec), the liquid phase, filtrate, was removed, and the separated solid phase was washed with pure water of pH 6.8. In addition, in the salification step, instead of obtaining an aqueous dispersion with a pH of 7.5, an amount of sodium hydroxide approximately equimolar to the amount of carboxy groups introduced was added, and an aqueous dispersion with a pH of 7.4 was obtained. Except for these, oxidized cellulose and nanocellulose were obtained in the same manner as in Example 1B.

[0163] During pressure filtration, the filter cloth did not clog, and it was possible to subsequently use the filter cloth to wash with pure water at pH 6.8. A slight cloudiness was observed in the filtrate after washing with pure water. Here, the cloudiness was only partial, and it is presumed that washing with pure water at pH 6.8 dissociated the carboxylic acid groups from the proton form (-COO-H+) to the salt form (-COO-Na+), causing some of the cellulose oxide to redisperse from the separated solid phase into the liquid phase, resulting in the cloudiness of the filtrate.

[0164] The mass yield of the obtained oxidized cellulose was 67%. The content concentration of oxidized cellulose after the salification step was 12 mass%, and the degree of polymerization of the oxidized cellulose was 92. The obtained nanocellulose had an average fiber length of 190 nm and an average fiber width of 3 nm, and the light transmittance of its aqueous dispersion at a solid content concentration of 0.1% by mass was 95%.

[0165] Example 4B Oxidized cellulose and nanocellulose were obtained in the same manner as in Example 3B, except that instead of washing the separated solid phase with pure water of pH 6.8, the separated solid phase was washed with water adjusted to pH 2.5 by adding hydrochloric acid to pure water.

[0166] Similar to the results of Example 3B, the filter cloth did not become clogged during pressure filtration, and it was possible to subsequently wash the filter cloth with pure water. However, no cloudiness was observed in the filtrate after washing.

[0167] The mass yield of the obtained oxidized cellulose was 69%. The content concentration of oxidized cellulose after the salification step was 12 mass%, and the degree of polymerization of the oxidized cellulose was 91. The obtained nanocellulose had an average fiber length of 185 nm and an average fiber width of 3 nm, and the light transmittance of an aqueous dispersion of the nanocellulose at a solid content concentration of 0.1% by mass was 95%.

[0168] Compared to Example 3B, the mass yield of oxidized cellulose was higher, and this is presumably because washing the separated solid phase with water adjusted to pH 2.5 prevented the carboxylic acid groups from dissociating from the proton form (-COO-H+) to the salt form (-COO-Na+), thereby preventing a portion of the cellulose-based oxide from redispersing from the separated solid phase into the liquid phase. Furthermore, from the perspective of wastewater treatment of the filtrate, it is preferable for the filtrate to be free of cloudiness.

[0169] Example 5B A jacketed glass vessel was charged with 500 g of sodium hypochlorite solution (pH 12.6, available chlorine concentration 12% by mass) and heated to 30°C while stirring at 300 rpm using a Shinto Scientific agitator (Three-One Motor, BL600) with three swept-back blades. Then, 40 g of powdered pulp (KC Flock W-100GK, crystallinity: 38%) from Nippon Paper Industries Co., Ltd. was added as the cellulosic raw material. After the cellulosic raw material was added, the vessel was stirred at 30°C until the pH dropped to 10.3. The pH was then adjusted to 10.3 by adding 25% by mass of aqueous sodium hydroxide. The vessel was stirred under the same conditions for a total of 7 hours after the cellulosic raw material was added. After the reaction, the redox potential was monitored, and the remaining sodium hypochlorite was inactivated by adding aqueous hydrogen peroxide. Hydrochloric acid was then added to convert the carboxyl groups of the oxidized cellulose from the salt form (-COO-Na+) to the proton form (-COO-H+), yielding an aqueous dispersion at pH 2.5. Solid-liquid separation was performed by pressure filtration at 0.2 MPa, followed by washing with aqueous hydrochloric acid at pH 2.5. Sodium hydroxide was added to the resulting proton-form oxidized cellulose to convert the carboxylic acid groups from the proton form (-COO-H+) back to the salt form (-COO-Na+), yielding an aqueous dispersion of salt-form oxidized cellulose at pH 6.8. The amount of carboxyl groups was measured to be 0.73 mmol / g, and the degree of polymerization was 100.

[0170] Example 6B Oxidized cellulose and nanocellulose were obtained in the same manner as in Example 1B, except that ashless cotton filter paper (Advantec Toyo, crystallinity 59%) was used as the cellulosic raw material. The mass yield of the obtained oxidized cellulose was 65%. The content concentration of oxidized cellulose after the salification step was 10 mass%, and the degree of polymerization of the oxidized cellulose was 90. The obtained nanocellulose had an average fiber length of 160 nm and an average fiber width of 10 nm, and the light transmittance of an aqueous dispersion of the nanocellulose at a solid content concentration of 0.1% by mass was 90%.

[0171] Example 7B Example 7B was carried out with reference to Japanese Patent Application Laid-Open No. 2017-218470. Furthermore, with reference to Japanese Patent Application Laid-Open No. 2008-231258, which describes the use of ascidian scutella as a raw material for cellulose materials, oxidized cellulose and nanocellulose were produced using ascidian scutella as a cellulose-based raw material. The ascidian sac was soaked in 0.2% NaOH at room temperature and then pulverized in a mixer. It was then soaked in 5% NaOH at room temperature overnight, washed with water, and treated three times in a 0.3% aqueous sodium chlorite solution at 60°C for 2 hours to remove non-cellulose components. The treated material was thoroughly washed with water and freeze-dried to obtain ascidian sac cellulose. Oxidized cellulose and nanocellulose were obtained in the same manner as in Example 1B, except that the sea squirt sac cellulose was used as the cellulosic raw material. The mass yield of the obtained oxidized cellulose was 70%. The content concentration of oxidized cellulose after the salification step was 8 mass%, and the degree of polymerization of the oxidized cellulose was 110. The obtained nanocellulose had an average fiber length of 230 nm and an average fiber width of 12 nm, and the light transmittance of an aqueous dispersion of the nanocellulose at a solid content concentration of 0.1% by mass was 85%.

[0172] [Comparative example 1B] Oxidized cellulose and nanocellulose were obtained in the same manner as in Example 1B, except that the protonation step was not performed and the separation step was carried out directly after the hypochlorite treatment step, and that in the salification step, instead of obtaining an aqueous dispersion with a pH of 7.5, sodium hydroxide in an amount exceeding the equimolar amount of the introduced carboxyl groups was added to obtain an aqueous dispersion with a pH of 7.4.

[0173] The mass yield of the obtained oxidized cellulose was 18%. The content concentration of oxidized cellulose after the salification step was 11% by mass, and the degree of polymerization of the oxidized cellulose was 98. The obtained nanocellulose had an average fiber length of 210 nm and an average fiber width of 3 nm, and the light transmittance of its aqueous dispersion at a solid content concentration of 0.1% by mass was 91%.

[0174] Compared to Example 1B, the mass yield of oxidized cellulose was significantly lower. This is presumably because, during the separation process, such as stirring with a spoon, some of the cellulose-based oxide was defibrated to form nanocellulose, which then migrated to the supernatant side where it was removed.

[0175] [Reference example 1B] Oxidized cellulose and nanocellulose were obtained in the same manner as in Example 3B, except that the protonation step was not performed and the separation step was carried out directly after the hypochlorous acid treatment step.

[0176] Unlike the results of Example 3B, the filter cloth became clogged during pressure filtration, which significantly reduced the discharge rate of the filtrate, making it impossible to subsequently wash the filter cloth with pure water. [Industrial Applicability]

[0177] The production method of the present invention can provide nanocellulose that can be used in various materials (e.g., resins, fibers, rubber, etc.) and various applications (e.g., food, cosmetics, medical products, paints, inks, etc.), and has industrial applicability in various fields such as automotive components, machine parts, electrical appliances, electronic devices, cosmetics, medical products, building materials, daily necessities, stationery, etc.

Claims

1. A method for producing oxidized cellulose, which comprises an oxidation of a cellulosic raw material with hypochlorous acid or a salt thereof, is substantially free of N-oxyl compounds, and has a degree of polymerization of 600 or less, comprising: The method includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof, the viscosity of a slurry of the cellulosic raw material having the same concentration as that used for the oxidation is in the range of 1000 Pa s or less when measured using a viscometer equipped with an SPP rotor at a rotation speed of 100 rpm and at 30°C or 40°C; Manufacturing method.

2. The concentration of the cellulosic raw material is 35% by mass or less based on the total amount of the reaction mixture. The method of claim 1.

3. The concentration of the cellulosic raw material is more than 6.5% by mass based on the total amount of the reaction mixture; The method according to claim 1 or 2.

4. The available chlorine concentration in the reaction system of the hypochlorous acid or its salt is 6% by mass or more and 43% by mass or less. The method according to any one of claims 1 to 3.

5. The available chlorine concentration in the reaction system of the hypochlorous acid or its salt is less than 14% by mass. The method according to any one of claims 1 to 3.

6. The reaction temperature of the oxidation is 30°C or higher. The method according to any one of claims 1 to 5.

7. The reaction time of the oxidation is 2 hours or more. The method according to any one of claims 1 to 6.

8. The pH of the reaction system is less than 11. The method according to any one of claims 1 to 7.

9. The viscosity is in the range of 30 Pa s or less. The method according to any one of claims 1 to 8.

10. After the oxidation step in the production method according to any one of claims 1 to 9, a step of obtaining nanocellulose by defibrating is included, Method for producing nanocellulose.

11. a step of subjecting an oxide dispersion containing a cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain oxidized cellulose; the pH of the oxide dispersion is 4.0 or less, and the oxide dispersion is substantially free of N-oxyl compounds; Method for producing oxidized cellulose.

12. The method further comprises a step of oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof to obtain the cellulosic oxide. The method of claim 11.

13. a step of oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof in a mass ratio relative to the cellulosic raw material of 0.2 or more to obtain a cellulosic oxide; and a step of subjecting the oxide dispersion containing the cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain oxidized cellulose, The pH of the oxide dispersion is 4.0 or less. Method for producing oxidized cellulose.

14. Further comprising a step of treating the hypochlorous acid or salt thereof in the oxide dispersion. The method according to claim 12 or 13.

15. The method further comprises a step of adding an acid and / or a cation exchange resin to prepare the oxide dispersion having a pH of 4.0 or less. The method according to any one of claims 11 to 14.

16. further comprising a step of adding a base to adjust the pH of the oxidized cellulose dispersion containing the oxidized cellulose and a dispersion medium to greater than 4.

0. The method according to any one of claims 11 to 15.

17. The pH of the oxide dispersion is 2.5 or less. The method according to any one of claims 11 to 16.

18. the step of obtaining the oxidized cellulose is a step of performing solid-liquid separation by filtering the oxide dispersion; The method according to any one of claims 11 to 17.

19. further comprising the step of washing the oxide dispersion or the oxidized cellulose with an acidic washing solution. The method according to any one of claims 11 to 18.

20. The degree of polymerization of the oxidized cellulose is 600 or less. The method according to any one of claims 11 to 19.

21. Oxidized cellulose obtained by the production method according to any one of claims 11 to 20.

22. The method comprises the step of defibrating the oxidized cellulose obtained by the method according to any one of claims 11 to 20 to obtain nanocellulose. Method for producing nanocellulose.

23. Nanocellulose obtained by the method according to claim 22.

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