JP7787489B2 - Method for producing nanocellulose - Google Patents

Description

The present invention relates to a method for producing nanocellulose.

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

Patent Document 1 discloses a method of oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof as an oxidizing agent under high-concentration conditions of an available chlorine concentration of 14 to 43% by mass in the reaction system to obtain oxidized cellulose, which is then micronized to obtain CNF. Patent Document 2 also discloses a method of oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof 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 to obtain oxidized cellulose, which is then micronized to obtain CNF. 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 no N-oxyl compounds remain in the cellulose fibers. This makes it possible to produce nanocellulose materials while reducing their impact on the environment.

International Publication No. 2018/230354 International Publication No. 2020/027307

Patent Documents 1 and 2 disclose specific examples of producing nanocellulose material by micronizing oxidized cellulose, in which nanocellulose material is obtained through mechanical fiberization using an ultrasonic homogenizer. However, from the perspective of reducing production costs, there is a demand for an efficient method of producing nanocellulose.

The present invention was made in consideration of the above circumstances, and its main objective is to provide an efficient method for producing nanocellulose.

As a result of extensive research, the inventors discovered that by oxidizing cellulosic raw materials with hypochlorous acid or its salts, the fiberization process can be omitted and nanocellulose can be obtained efficiently, leading to the completion of the present invention.
According to the present invention, the following means are provided.

[1]
A method for producing nanocellulose, comprising the steps of oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof to obtain oxidized cellulose, and nano-sizing the oxidized cellulose after the oxidation without substantially performing a defibration treatment.
[2]
diluting the dispersion containing oxidized cellulose obtained by the oxidation,
the light transmittance of the diluted dispersion is 70% or more when the solid content concentration of the diluted dispersion is 0.1% by mass;
The manufacturing method described in [1].
[3]
After the oxidation, the dispersion containing oxidized cellulose is subjected to solid-liquid separation as needed to obtain a dispersion containing nanocellulose.
The manufacturing method according to [1] or [2].
[4]
Purifying the dispersion containing the nanocellulose,
[3] The manufacturing method described in [3].
[5]
The average fiber width of the nanocellulose is 1 nm or more and 100 nm or less,
The manufacturing method according to any one of [1] to [4].
[6]
The average fiber length of the nanocellulose is 50 nm or more and 800 nm or less.
The manufacturing method according to any one of [1] to [5].
[7]
The reaction time of the oxidation is 30 minutes or more.
The manufacturing method according to any one of [1] to [6].
[8]
The concentration of the cellulosic raw material in the reaction system during the oxidation is 0.1% by mass or more and 30% by mass or less.
The manufacturing method according to any one of [1] to [7].

The nanocellulose production method of the present invention eliminates the need for the defibration process and is highly efficient.

The method for producing nanocellulose of the present invention includes the steps of oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof to obtain oxidized cellulose, and then nano-sizing the oxidized cellulose after the oxidation without substantially performing a defibration treatment.
In previous methods, to obtain nanocellulose, chemical treatment, biological treatment, etc. were performed as pretreatment for defibration treatment such as mechanical defibration, but defibration treatment was still performed at the end. The inventors discovered that oxidizing cellulosic raw materials with hypochlorous acid or its salts promotes defibration and yields nanocellulose. They also found that nanocellulose can be easily obtained by appropriately diluting the oxidized cellulose obtained by the above oxidation. As described above, the manufacturing method of the present invention does not require defibration treatment and allows for efficient production of nanocellulose.
Furthermore, the equipment used for the defibration process is expensive, which increases the manufacturing cost, but the manufacturing method of the present invention is also highly efficient in terms of reducing production costs.

Nanocellulose in the present invention refers to a material obtained by oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof, without substantially performing defibration treatment after the oxidation.
The main component of plants is cellulose, and bundles of cellulose molecules are called cellulose microfibrils. The cellulose in cellulosic raw materials is also contained in the form of cellulose microfibrils. Nanocellulose in the present invention is a general term for nano-sized cellulose, and includes fine cellulose fibers, cellulose nanocrystals, etc. Fine cellulose fibers are also called cellulose nanofibers (also referred to as CNF).
The nanocellulose obtained by the present invention is preferably in the form of an aqueous dispersion containing nanocellulose.
As used herein, "substantially no defibration treatment is performed" means that after oxidation, no step is performed in which the obtained oxidized cellulose is nano-sized by applying an energy load such as shear force using a mechanical defibration device or the like.

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 to 700 nm, even more preferably 100 nm to 700 nm, and even more preferably 100 nm to 600 nm.

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 more uniform quality, 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, even more preferably 30 nm or less, and even more preferably 20 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, even more preferably 3 nm or more and 30 nm or less, and even more preferably 3 nm or more and 20 nm or less.

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 in the form of thick rods rather than elongated 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.

The average fiber width and average fiber length were calculated by mixing nanocellulose and water to a nanocellulose concentration of approximately 1 to 10 ppm, allowing the sufficiently diluted nanocellulose aqueous dispersion to air-dry 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 depending on the conditions is preferably within ±100 nm for average fiber length. The range of difference in values depending on the conditions is preferably within ±10 nm for average fiber width. More detailed measurement methods follow the methods described in the Examples below.

The cellulosic raw material is not particularly limited as long as it is a material primarily composed of cellulose, and examples thereof include pulp, natural cellulose, regenerated cellulose, and fine cellulose obtained by depolymerizing cellulose through mechanical processing. Commercially available cellulosic raw materials, such as crystalline cellulose derived from pulp, can be used as they are. Alternatively, unused biomass containing a large amount of cellulose components, such as soybean pulp refuse or soybean hulls, can also be used as the raw material. Furthermore, the cellulosic raw material may be pre-treated with an alkali of an appropriate concentration in order to facilitate the penetration of the oxidizing agent used into the raw pulp.
In the production method of the present invention, it is preferable to use, as the cellulosic raw material, fine cellulose obtained by mechanically or chemically treating cellulose. Pulp powder is a suitable example of fine cellulose. Use of pulp powder tends to further promote pulverization by oxidation reaction. The particle size of the pulp powder is typically in the range of 1 to 1000 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.

Examples of hypochlorous acid or its salts used to oxidize cellulosic raw materials include hypochlorous acid water, sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, and ammonium hypochlorite. Of these, sodium hypochlorite is preferred due to its ease of handling.

The manufacturing method of the present invention includes at least an oxidation step, and may optionally include a treatment step, a protonation step, a separation step, a washing step, and a chlorination step.

Oxidation Step: A method for producing oxidized cellulose by oxidation of a cellulosic raw material includes mixing 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, it is preferable to use hypochlorous acid or a salt thereof having an effective chlorine concentration of 6% by mass or more and 43% by mass or less. By using hypochlorous acid or a salt thereof having an effective chlorine concentration of 6% by mass or more and 43% by mass or less, the amount of carboxy groups in the oxidized cellulose can be sufficiently increased, defibration proceeds sufficiently, and defibration treatment after the oxidation reaction can be omitted.
In addition, 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 %.

From the viewpoint of sufficiently increasing the amount of carboxy groups in the oxidized cellulose, the available chlorine concentration is more preferably 7% by mass or more, even more preferably 10% by mass or more, even more preferably 14% by mass or more, even more preferably 15% by mass or more, even more preferably 18% by mass or more, and even more preferably 20% by mass or more. Furthermore, from the viewpoint of suppressing excessive decomposition of cellulose during defibration, the available chlorine concentration of the reaction liquid is more preferably 40% by mass or less, even more preferably 38% by mass or less. The range of the available chlorine concentration of the reaction liquid 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, even more preferably 14 to 43% by mass.

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

The oxidation reaction of cellulosic raw materials with hypochlorous acid or its salts is preferably carried out while adjusting the pH to the range of 5.0 to 14.0. Within this range, the oxidation reaction of the cellulosic raw materials can proceed sufficiently, the amount of carboxyl groups in the oxidized cellulose becomes sufficiently large, and defibration tends to proceed. The pH of the reaction system is more preferably 7.0 or higher, and even more preferably 8.0 or higher. The upper limit of the pH of the reaction system is more preferably 13.5 or lower, and even more preferably 13.0 or lower. The pH range of the reaction system is more preferably 7.0 to 14.0, and even more preferably 8.0 to 13.5.

The method for producing oxidized cellulose will be further explained below, using as an example the case where sodium hypochlorite is used as hypochlorous acid or its salt.

When oxidizing a cellulosic raw material using sodium hypochlorite, the reaction liquid is preferably an aqueous sodium hypochlorite solution. Methods for adjusting the available chlorine concentration of an aqueous sodium hypochlorite solution to a desired concentration (e.g., target concentration: 6% to 43% by mass) include concentrating an aqueous sodium hypochlorite solution with an available chlorine concentration lower than the desired concentration, diluting an aqueous sodium hypochlorite solution with an available chlorine concentration higher than the desired concentration, and dissolving sodium hypochlorite crystals (e.g., sodium hypochlorite pentahydrate) in a solvent. Among these, adjusting the available chlorine concentration as an oxidizing agent by diluting an aqueous sodium hypochlorite solution or dissolving sodium hypochlorite crystals in a solvent is preferred because it results in less self-decomposition (i.e., less decrease in available chlorine concentration) and allows for easy adjustment of the available chlorine concentration.

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

To efficiently promote the oxidation reaction of the cellulosic raw material, it is preferable to stir the mixture of the cellulosic raw material and the aqueous sodium hypochlorite solution during the oxidation reaction. Examples of stirring methods include a magnetic stirrer, a stirring rod, a stirrer with stirring blades (three-one motor), a homomixer, a disperser-type mixer, a homogenizer, and external circulation stirring. Among these, methods using one or more of shear-type mixers such as homomixers and homogenizers, a stirrer with stirring blades, and a disperser-type mixer are preferred, as they allow the oxidation reaction of the cellulosic raw material to proceed smoothly and efficiently obtain nanocellulose without defibration processing. A method using a stirrer with stirring blades is particularly preferred. When using a stirrer with stirring blades, devices equipped with known stirring blades such as propeller blades, paddle blades, and turbine blades can be used. Furthermore, when using a stirrer with stirring blades, it is preferable to stir at a rotation speed of 50 to 300 rpm.

The reaction temperature for the oxidation reaction is preferably 15°C to 100°C, more preferably 20°C to 90°C. During the reaction, the pH of the reaction system decreases as carboxyl groups are generated in the cellulosic raw material by the oxidation reaction. Therefore, from the perspective of efficiently progressing the oxidation reaction, it is preferable to add an alkaline agent (e.g., sodium hydroxide) or an acid (e.g., hydrochloric acid) to the reaction system to adjust the pH of the reaction system to within the above-mentioned preferred range. When the pH of the reaction system is to be 10 or higher, it is preferable to set the reaction temperature to 30°C or higher.

The longer the reaction time of the oxidation reaction, the more defibration tends to progress. It is thought that the longer the reaction time, the more oxidation progresses on the surface of the cellulose microfibrils in the cellulosic raw material, and the stronger the repulsion between fibrils due to electrostatic repulsion and osmotic pressure, leading to the progress of finer fiber, particularly the smaller the average fiber width.
From the viewpoint of obtaining nanocellulose by oxidation without defibration treatment, the reaction time of the oxidation reaction is preferably 30 minutes or more. From the viewpoint of further progressing defibration, the reaction time of the oxidation reaction is more preferably 2 hours or more, and even more preferably 3 hours or more. The upper limit of the reaction time of the oxidation reaction is not particularly limited and may be adjusted as appropriate. The upper limit of the reaction time is usually 15 days or less, but may also be 10 days or less, or 7 days or less. From the viewpoint of productivity, the upper limit of the reaction time is preferably 30 hours or less, more preferably 20 hours or less.

The concentration of the cellulosic raw material during the oxidation reaction is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on the total amount of the reaction mixture during the oxidation reaction, from the perspective of improving workability by facilitating stirring during the oxidation reaction and promoting defibration. The lower limit of the cellulosic raw material concentration during the oxidation reaction is usually 0.1% by mass or more, and from the perspective of productivity, it is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more. The concentration of the cellulosic raw material during the oxidation reaction is preferably in the range of 0.1% by mass or more to 30% by mass or less, more preferably 1% by mass or more to 20% by mass or less, and even more preferably 1% by mass or more to 10% by mass or less.

Treatment Step 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 the "treatment step"). By including the treatment step, the reaction of oxidizing the cellulosic raw material can be stopped.
The reaction may be stopped based on the above-mentioned reaction time, or may be stopped by determining the progress of the reaction (i.e., the end point of the reaction). The method for determining the end point of the reaction includes, but is not limited to, measuring the amount of carboxyl group, which is the degree of oxidation of the reaction product, observing the remaining amount of hypochlorous acid or its salt, or observing the change in pH of the reaction system.

The method for treating hypochlorous acid or its salt is not particularly limited, and may be treated by ultraviolet irradiation or autolysis under high temperature conditions, but preferably includes the method of reducing hypochlorous acid or its salt.Specifically, includes the method of adding a reducing agent such as sulfites, sulfamic acid or its salt, thiosulfate, hydrogen peroxide, oxalic acid or its salt, formic acid or its salt, hypophosphite, or the 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 be used in combination.

In the manufacturing method of the present invention, nanocellulose can be obtained by oxidizing a cellulosic raw material and then performing appropriate post-treatment without defibrating it. The post-treatment method is not particularly limited as long as it can produce nanocellulose, and examples include (1) diluting the oxidized cellulose obtained by oxidation to adjust the light transmittance to a predetermined range, and (2) performing solid-liquid separation to obtain a dispersion containing nanocellulose.

(1) Method of diluting oxidized cellulose obtained by oxidation and adjusting light transmittance within a predetermined range Diluting a dispersion containing oxidized cellulose obtained by oxidation according to the production method of the present invention promotes nano-sizing of the oxidized cellulose. Nano-sizing can be confirmed by measuring the light transmittance of the diluted solution, and it is preferable to adjust the light transmittance to 70% or higher. The above light transmittance is the value obtained by measuring the light transmittance of a diluted solution with a solids concentration of 0.1% by mass.
Therefore, one aspect of the present invention is a production method comprising the steps of oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof to obtain oxidized cellulose, and diluting a dispersion containing the oxidized cellulose obtained by the oxidation, wherein the diluted dispersion has a light transmittance of 70% or more when the solids concentration is 0.1% by mass.
For the dilution, it is preferable to use a dispersion medium that constitutes the dispersion liquid, and as the dispersion medium, the dispersion medium used in the protonation step described below can be used.

In the above dilution, the concentration of oxidized cellulose in the resulting diluted solution can usually be adjusted to 5% by mass or less. It is presumed that the reason why dilution promotes nano-scale formation of oxidized cellulose is that the increased proportion of dispersion medium facilitates interaction between the surface of the oxidized cellulose and the dispersion medium, thereby loosening the oxidized cellulose. It is also presumed that dilution weakens the salting-out effect, facilitating nano-scale formation. The reasons for the progression of nano-scale formation are not limited to these.
The concentration of oxidized cellulose in the diluted solution is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less. The lower limit of the oxidized cellulose concentration in the diluted solution may be greater than 0% by mass, and from the viewpoint of ease of handling after nanoization, it is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more.

The above light transmittance is preferably 80% or more, and more preferably 90% or more.

To ensure that the light transmittance of the diluted solution is 70% or higher, it is preferable to allow time to pass after dilution. Longer time periods tend to increase light transmittance, promote nano-synthesis of oxidized cellulose, and more efficiently obtain a dispersion containing nanocellulose. Light transmittance can be measured, for example, by placing the aqueous nanocellulose dispersion in a 10 mm thick quartz cell and using a spectrophotometer (JASCO V-550).

The elapsed time can be adjusted as appropriate while checking the light transmittance, but is usually at least 30 minutes, and may be at least 1 hour, at least 6 hours, at least 12 hours, at least 18 hours, or at least 1 day. The longer the time, the more nano-ization tends to progress. There is no particular upper limit to the time allowed to stand, and it may be at most 5 days, at most 3 days, or at most 2 days.

The dilution process is preferably carried out after the salification process described below.

(2) Method for Obtaining a Dispersion Containing Nanocellulose by Solid-Liquid Separation In the manufacturing method of the present invention, it is preferable to perform known solid-liquid separation such as filtration as needed using the reaction mixture obtained by oxidation (hereinafter also referred to as oxide dispersion). Therefore, one aspect of the manufacturing method of the present invention includes a step of oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof, and, after the oxidation, subjecting the resulting reaction mixture to solid-liquid separation as needed without substantially performing a defibration treatment to obtain a dispersion containing nanocellulose. Here, solid-liquid separation refers to the operation of separating the solid content from the liquid phase, and the dispersion containing nanocellulose corresponds to the liquid phase. The above operation is also simply referred to as the separation step.
In this aspect of the present invention, it is more preferable to carry out solid-liquid separation. By carrying out solid-liquid separation, it is possible to remove non-micronized oxidized cellulose as a solid content. This non-micronized oxidized cellulose (i.e., the solid content obtained by solid-liquid separation) may be reused for further oxidation reaction, or may be subjected to a defibration treatment process to be micronized. Furthermore, as described above in (1), the obtained oxidized cellulose may be diluted and subjected to a method for adjusting the light transmittance to a predetermined range.

In the production method of the present invention, the obtained reaction mixture may be subjected to solid-liquid separation as needed to obtain a dispersion containing nanocellulose, and this dispersion may be purified as needed.
The purification method is not particularly limited as long as it is a means for removing impurities other than nanocellulose from the dispersion. Examples of impurities include salt components derived from oxidation reactions, particulate cellulose obtained as a result of excessive oxidation reactions, and soluble components. Specific purification methods include common techniques, such as placing the dispersion in a dialysis tube and extracting impurities into water, electrodialysis, and various types of chromatography (such as partition chromatography, adsorption chromatography, size exclusion chromatography, and ion exchange chromatography).

The production method of the present invention may further include a step of adding an acid to the cellulose oxide obtained in the oxidation step to prepare an oxide dispersion having a pH of 4.0 or less (hereinafter also referred to as a "protonation step"), either before or after carrying out the above steps (1) and/or (2), preferably before carrying out the above steps (1) and/or (2). Here, the cellulose oxide contained in the oxide dispersion contains carboxy groups, and the protonation step is a step for converting at least a portion of these carboxy groups from the salt form (-COO ^- X ⁺ : X ⁺ represents a cation such as sodium) to the proton form (-COO ^- H ⁺ ).

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 of such acids include inorganic acids and organic acids. Among these, inorganic acids, particularly hydrochloric acid, are preferred from the standpoint of ease of handling. A cation exchange resin may also be used in the protonation step.

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 viewpoint of efficiently desalting the carboxylated cellulose nanofiber salt and facilitating separation after desalting. Commercially available cation exchange resins can be used. Examples of commercially available products include Amberjet 1020, 1024, 1060, and 1220 (all manufactured by Organo Corporation), Amberlite IR-200C and IR-120B (both manufactured by Tokyo Organic Chemical Industry Co., Ltd.), Lewatit SP112 and Lewatit S100 (both manufactured by Bayer AG), 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.

There are no particular restrictions on the dispersion medium used in the protonation step, 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 also be used as appropriate, or the solvent contained in the acid can be replaced with another dispersion medium. Specific examples of dispersion mediums include water; and organic solvents such as alcohols, ethers, ketones, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. Of these, water is preferred from the perspective of increasing the yield of oxidized cellulose obtained. As the solvent, one of these may be used alone, or two or more may be used in combination.

Among the dispersion media, alcohols include methanol, ethanol, isopropanol, isobutanol, sec-butyl alcohol, tert-butyl alcohol, methyl cellosolve, ethylene glycol, and glycerin. Ethers include ethylene glycol dimethyl ether, 1,4-dioxane, and tetrahydrofuran. Ketones include acetone and methyl ethyl ketone.

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 can be determined by appropriately combining the above upper and lower limits.

When the production method of the present invention includes a protonation step, the protonation step is preferably performed before the separation step described below, and preferably after the oxidation step, but may also be performed simultaneously with part of the oxidation step.

The cellulose-based oxide may be further purified, if necessary, before, during, or after the protonation step 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.

The method for producing oxidized cellulose of the present invention can include a step (separation step) of subjecting an oxide dispersion containing a cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain oxidized cellulose. When the method (2) above is adopted, the separation step is included.

The method for solid-liquid separation of the oxide dispersion is not particularly limited, but examples include methods that involve removing the liquid phase by known isolation processes such as centrifugation or filtration, and obtaining the oxidized cellulose contained in the solid phase. Among these, solid-liquid separation by filtering the oxide dispersion is preferred from the standpoint of operability.

From the viewpoint of increasing the recovery rate of oxidized cellulose, the pH of the oxide dispersion in the separation step is preferably 4.0 or less, more preferably 3.0 or less, and even more preferably 2.5 or less. There is no particular lower limit to the pH of the oxide dispersion, but it 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 can be determined by appropriately combining the above upper and lower limits.

It is preferable to use the same dispersion medium used in the oxidation step or the protonation step as the dispersion medium used in the separation step.

Washing Step The production method 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").

The acidic cleaning solution used in the cleaning step can be a solution containing the acid and dispersion medium used in the protonation step, but is not particularly limited.

The pH of the acidic cleaning solution is the same as that of the oxide dispersion described above, and is preferably 4.0 or less, more preferably 3.0 or less, and even more preferably 2.5 or less. There is no particular lower limit to the pH of the acidic cleaning solution, but it is typically 1.0 or more, preferably 1.5 or more, and more preferably 2.0 or more. The pH range of the acidic cleaning solution can be determined by appropriately combining the above upper and lower limits.

If a washing step is included, the washing step may be performed before, after, or simultaneously with the separation step, and the separation and washing steps may be repeated.

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 carboxy groups, and the chloride step is a step for converting at least a portion of these carboxy groups from the proton form (-COO ^- H ⁺ ) to the salt form (-COO ^- X ⁺ : X ⁺ represents a cation such as sodium or lithium).

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.

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. There are no particular limitations on the upper limit of the pH, but it 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 may be determined by appropriately combining the above upper and lower limits.

There are no particular restrictions 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 also be used, or the solvent contained in the base can be replaced with another dispersion medium. Specific examples of dispersion mediums are the same as those used in the protonation step. However, water and/or organic solvents are preferred, as this makes it easier to isolate nanocellulose.

If a chlorination step is included, the chlorination step may be performed after the separation step, or may be performed before, after, or simultaneously with the washing step.

In the manufacturing method of the present invention, the process of oxidizing the cellulosic raw material with hypochlorous acid or its salt does not require the use of N-oxyl compounds such as TEMPO. There are concerns about the impact of N-oxyl compounds on the environment and human body. For this reason, it is preferable that the nanocellulose of the present invention be substantially free of N-oxyl compounds. Here, in this specification, nanocellulose "substantially free of N-oxyl compounds" means that no N-oxyl compounds are used during oxidation, or that the content of nitrogen derived from N-oxyl compounds in the nanocellulose is 2.0 ppm or less as an increase from the cellulosic raw material. The amount of N-oxyl compounds in the nanocellulose is preferably 1.0 ppm or less as an increase from the cellulosic raw material.

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 also have a predetermined zeta potential or light transmittance.

(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 increase, for example, by setting one or more of the oxidation reaction time, reaction temperature, and stirring conditions (e.g., by lengthening the reaction time) to promote oxidation (i.e., to increase the degree of oxidation).

From the above perspective, the zeta potential of the nanocellulose of 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. Furthermore, the lower limit of the zeta potential is preferably -90 mV or more, more preferably -85 mV or more, even more preferably -80 mV or more, and even more preferably -77 mV or more. The zeta potential range can be determined by appropriately combining the above-mentioned lower and upper limits. The zeta potential is preferably -90 mV or more to -35 mV or less, more preferably -85 mV or more to -40 mV or less, and even more preferably -80 mV or more to -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 prepared by mixing the nanocellulose of the present invention with water to a nanocellulose concentration of 0.1% by mass.

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 about 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 about 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.

(light transmittance)
The nanocellulose dispersion of the present invention, in which nanocellulose is dispersed in a dispersion medium, has little light scattering from cellulose fibers and can exhibit high light transmittance. Specifically, in a preferred embodiment, the nanocellulose of the present invention has a light transmittance of 70% or more in a mixed solution mixed with water to a solids concentration of 0.1% by mass. The light transmittance is more preferably 80% or more, even more preferably 90% or more, even more preferably 92% or more, and even more preferably 95% or more. The light transmittance is a value measured at a wavelength of 660 nm using a spectrophotometer.

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).

As described above, the nanocellulose of the present invention is obtained by oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof. In this specification, the oxide of a cellulosic raw material is also referred to as oxidized cellulose. The nanocellulose of the present invention includes oxidized cellulose, and is preferably oxidized cellulose itself.
The degree of polymerization of oxidized cellulose is preferably 600 or less. If the degree of polymerization of oxidized cellulose exceeds 600, there is a tendency for insufficient micronization. Furthermore, if the degree of polymerization of oxidized cellulose exceeds 600, a large amount of insufficiently defibrated nanocellulose will be produced, and when this is dispersed in a dispersion medium, light scattering and the like will increase, and transparency may decrease. There is no particular lower limit for the degree of polymerization, but if the degree of polymerization of oxidized cellulose is less than 50, the proportion of particulate cellulose rather than fibrous cellulose will increase, resulting in non-uniform quality of the slurry and unstable viscosity. From the above perspectives, the degree of polymerization of oxidized cellulose is preferably 50 to 600.

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

The degree of polymerization of oxidized cellulose can be adjusted by changing the reaction time, reaction temperature, pH, and available chlorine concentration of hypochlorous acid or its salt during the oxidation reaction. Specifically, since increasing the degree of oxidation tends to decrease the degree of polymerization, methods for decreasing the degree of polymerization include increasing the oxidation reaction time and/or reaction temperature. Alternatively, the degree of polymerization of oxidized cellulose can be adjusted by adjusting the stirring conditions of the reaction system during the oxidation reaction. For example, if 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 uneven, 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 the cellulose-based raw material. Therefore, the degree of polymerization of oxidized cellulose can also be adjusted by selecting the cellulose-based 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 according to the following method.
Oxidized cellulose was added to an aqueous sodium borohydride solution adjusted to pH 10 and reduced at 25°C for 5 hours. The amount of sodium borohydride was approximately 0.1 g per 1 g of oxidized cellulose. After reduction, solid-liquid separation was performed by suction filtration, followed by washing with water. The resulting oxidized cellulose was freeze-dried. Approximately 0.04 g of dried oxidized cellulose was added to approximately 10 ml of pure water and stirred for 2 minutes, after which approximately 10 ml of 1 M copper ethylenediamine solution was added to dissolve it. The flow time of the blank solution and the cellulose solution were then measured at 25°C using a capillary viscometer. The relative viscosity (ηr), specific viscosity (ηsp), and intrinsic viscosity ([η]) were calculated sequentially from the flow time of the blank solution (t0), the flow time of the cellulose solution (t), and the oxidized cellulose concentration (c [g/ml]) using the following equations, and the degree of polymerization (DP) of the oxidized cellulose was calculated using the viscosity measurement equation.
ηr = η/η0 = t/t0
ηsp＝ηr－１
[η]=ηsp/(100×c(1+0.28ηsp))
DP = 175 × [η]

(Carboxy group amount)
The amount of carboxy groups in the nanocellulose of the present invention is preferably 0.30 mmol/g or more and less than 2.0 mmol/g. When the amount of carboxy groups is 0.30 mmol/g or more, sufficiently fine cellulose fibers tend to be obtained. In addition, a nanocellulose-containing slurry of uniform quality can be obtained, and the viscosity stability and handleability of the slurry can be improved. On the other hand, when the amount of carboxy groups is 2.0 mmol/g or less, excessive decomposition of cellulose can be suppressed, and nanocellulose of uniform quality with a low proportion of particulate cellulose can be obtained. From this perspective, the amount of carboxy groups is more preferably 0.35 mmol/g or more, even more preferably 0.40 mmol/g or more, and even more preferably 0.42 mmol/g or more. The upper limit of the amount of carboxy groups is more preferably 1.5 mmol/g or less, even more preferably 1.2 mmol/g, and even more preferably 1.0 mmol/g or less.

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

The nanocellulose of the present invention can be obtained by oxidizing a cellulose-based raw material with hypochlorous acid or a salt thereof. The oxidized cellulose obtained in this manner 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, it preferably has a structure in which the second and third hydroxyl groups on the glucopyranose ring have been oxidized and carboxyl groups have been introduced. The positions of the carboxyl groups on the glucopyranose ring of oxidized cellulose can be analyzed using solid-state 13C-NMR spectroscopy.

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 be the main component. Here, "carboxylated CNF being 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 this proportion is 100% by mass, but it may also be 98% by mass or 95% by mass.

The standard deviation of the fiber length of the nanocellulose in the present invention is preferably 600 nm or less, more preferably 500 nm or less, and even more preferably in the range of 10 nm or more and 500 nm or less.
If the standard deviation of the fiber length exceeds 600 nm, the nanocellulose slurry is prone to unevenness, and the state of the slurry, particularly its viscosity stability, deteriorates. Therefore, the smaller the standard deviation, the better. However, if the standard deviation is to be less than 10 nm, the reaction time of the oxidation reaction must be significantly increased, which is not economically preferable.

The standard deviation of fiber length in nanocellulose in the present invention can be controlled within a predetermined range by, for example, selecting the type of cellulosic raw material, controlling the amount of carboxyl groups, specifically by controlling the time of the oxidation reaction, or adjusting the stirring of the oxidation reaction. Methods for controlling the standard deviation of fiber length are not limited to these, and two or more of these methods may be combined.

In this invention, the standard deviation of fiber length indicates how widely the statistically relevant values are distributed from their average. The standard deviation can be calculated using the following formula (1), where n is the number of data points and x is each data point.

Furthermore, the kurtosis of the fiber length of the nanocellulose in the present invention is preferably 11 or more, more preferably 12 or more, and even more preferably in the range of 12 to 30.
Kurtosis is a value that indicates the concentration of fiber length distribution, and nanocellulose slurry with a small kurtosis of less than 11 is prone to unevenness, reducing the state of the slurry, particularly its viscosity stability. Therefore, the higher the kurtosis, the better. However, to achieve a kurtosis of more than 30, the reaction time of the oxidation reaction must be significantly increased for pulverization, which is not economically desirable.

Regarding the kurtosis of fiber length, when compared to a normal distribution, if the kurtosis is large, the distribution will have a sharper peak and long, thick tails, and if the kurtosis is small, the distribution will have a more rounded peak and short, thin tails.
The kurtosis of the fiber length can be calculated from the following formula (2), where n is the number of data, x is each data, and s is the standard deviation.

Furthermore, the fiber length skewness of the nanocellulose in the present invention is preferably 3.0 or more, more preferably in the range of 3.0 to 6.0, and even more preferably in the range of 3.0 to 4.0. When the fiber length skewness is within a certain range, the nanocellulose slurry has high slurry state stability, particularly viscosity stability, although the mechanism is unclear.
If the skewness of the fiber length is less than 3.0, the viscosity stability decreases, and if the skewness exceeds 6.0 or even 4.0, the reaction time of the oxidation reaction must be significantly increased for the purpose of fine fiber size reduction, which is economically undesirable.
It should be noted that a higher skewness of the fiber length indicates that the fiber length distribution is biased toward the narrower width side.

Skewness of fiber length represents the degree of asymmetry on either side of the distribution around the mean, with positive skewness indicating a distribution with asymmetric tails extending towards more positive values and negative skewness indicating a distribution with asymmetric tails extending towards more negative values.
The skewness of fiber length can be calculated from the following formula (3), where n is the number of data, x is each data, and s is the standard deviation.

In addition, the range of the fiber length of the nanocellulose in the present invention (the difference between the maximum and minimum values) is preferably 4000 nm or less, more preferably 550 nm or more and 4000 nm or less, and even more preferably 700 nm or more and 4000 nm or less.
Nanocellulose slurries with fiber lengths exceeding 4000 nm tend to have uneven portions, which reduces the state of the slurry, particularly its viscosity stability. Therefore, the shorter the fiber length range, the better. However, to set the lower limit of the fiber length range to less than 700 nm, or even less than 550 nm, it is necessary to significantly increase the reaction time of the oxidation reaction for pulverization, which is economically undesirable.

Furthermore, with regard to the nanocellulose of the present invention, the standard deviation of the fiber width is preferably 1.5 nm or less, more preferably in the range of 0.5 nm to 1.5 nm, and even more preferably in the range of 1.0 nm to 1.5 nm.
If the standard deviation of the fiber width exceeds 1.5 nm, the nanocellulose slurry tends to have uneven parts, and the state of the slurry, especially the viscosity stability, tends to deteriorate. Therefore, the smaller the standard deviation, the better. However, to make the standard deviation less than 1.0 nm, or even less than 0.5 nm, it is necessary to significantly increase the reaction time of the oxidation reaction for miniaturization, which is economically undesirable.
The method for calculating the standard deviation of the fiber width is the same as that for the standard deviation of the fiber length described above.
The standard deviation of the fiber width can be controlled by the same method as the method for controlling the standard deviation of the fiber length.

Furthermore, with regard to the nanocellulose of the present invention, the kurtosis of the fiber width is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably in the range of 0.7 to 2.5.
Slurries using nanocellulose with a small fiber width kurtosis tend to have uneven areas and the slurry condition, especially viscosity stability, tends to deteriorate. Therefore, the higher the kurtosis, the better. However, if the kurtosis is to be increased to 2.5 or more, the reaction time of the oxidation reaction must be significantly increased to achieve finer nanoparticles, which is not economically desirable.
The method for calculating the kurtosis of the fiber width is the same as that for the kurtosis of the fiber length described above.

Furthermore, the fiber width skewness of the nanocellulose in the present invention is preferably 0.5 or more, more preferably in the range of 0.8 to 1.5, and even more preferably in the range of 0.85 to 1.5. When the fiber width skewness is within a certain range, the nanocellulose slurry has high viscosity stability, although the mechanism is unclear.
If the skewness of the fiber width is less than 0.5, the viscosity stability will decrease, and if the skewness exceeds 1.5, the reaction time of the oxidation reaction will need to be significantly increased for finer fiber, which is economically undesirable.
It should be noted that a higher skewness of the fiber width indicates that the fiber width distribution is biased toward the smaller width side.
The method for calculating the kurtosis of the fiber width is the same as that for the kurtosis of the fiber length described above.

Furthermore, the range of the fiber width of the nanocellulose in the present invention (the difference between the maximum and minimum values) is preferably 6.8 nm or less, more preferably 5.0 nm or more and 6.8 nm or less, and even more preferably 5.2 nm or more and 6.7 nm or less.
Slurries containing nanocellulose with a wide fiber width range tend to have uneven areas and, in particular, poor viscosity stability. Therefore, the smaller the fiber width range, the better. However, to set the lower limit of the fiber width range to less than 5.0 nm, or even less than 5.2 nm, the reaction time of the oxidation reaction must be significantly increased for finer nanocellulose, which is economically undesirable.

The kurtosis, skewness, and fiber length range of fiber length, and the kurtosis, skewness, and fiber width range of fiber width can be controlled in a manner similar to the method for controlling the standard deviation of fiber length.

If the average fiber length is relatively short, the fiber length distribution is narrow (small standard deviation, small range), and/or the fiber length distribution is sharp (high kurtosis), and/or the fiber length distribution is biased toward the smaller side (high skewness), it is thought that uneven areas of CNF concentration in the slurry will be less likely to occur, and the slurry state, particularly the viscosity, will be more stable.
Similarly, if the fiber width distribution is narrow (small standard deviation, small range), and/or the distribution is sharp (high kurtosis), and/or the fiber width distribution is biased toward the smaller side (high skewness), it is thought that uneven areas of CNF concentration in the slurry will be less likely to occur, and the slurry state, particularly the viscosity, will be more stable.

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

The present invention will be explained in more detail below using 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."

[Method for measuring fiber length and fiber width]
The obtained aqueous dispersion was diluted 1,000 to 1,000,000 times with pure water, allowed to dry naturally on a mica substrate, and the shape of the CNFs was observed in AC mode using an Oxford Asylum scanning probe microscope "MFP-3D infinity."
The obtained images were binarized using the image processing software "ImageJ" and analyzed for fiber length. The average fiber length was calculated for 100 or more fibers by dividing the fiber length by 2.
The average fiber width was calculated for 50 or more fibers using the software attached to the "MFP-3D infinity" as the cross-sectional height of the shape image = fiber width.

[Example 1]
(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 aqueous sodium hypochlorite solution was heated to 30°C in a constant temperature water bath while being stirred at 200 rpm using a propeller-type stirring blade in a stirrer (Three-One Motor, BL600) manufactured by Shinto Scientific Co., Ltd., and 50 g of powdered pulp (VP-1, average particle size: 30 μm, the average particle size here is 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) manufactured by TDI Corporation was then added as a cellulosic raw material.
After the cellulosic raw material was supplied (the concentration of the cellulosic raw material in the mixture was 6.7% by mass), the mixture was kept at 30°C in the same thermostatic water bath, and the pH during the reaction was adjusted to 11 by adding 48% by mass of sodium hydroxide, and the mixture was stirred under the same conditions for 6 hours using a stirrer.
(treatment process, separation process, cleaning process)
After the reaction was completed, sodium sulfite was added to treat the excess sodium hypochlorite, and the filtrate was collected by suction filtration using a PTFE membrane filter with a mesh size of 0.1 μm. The mass of the residue on the membrane filter was 12 g. The mass of the residue is an indicator of the progress of pulverization by the oxidation reaction, and the smaller the amount of residue, the greater the amount of nanocellulose produced.
The filtrate was then subjected to a dialysis membrane and purified water to remove the salt components contained therein, yielding an aqueous dispersion of nanocellulose.
The shape of this aqueous dispersion was observed using the method described above for measuring fiber length and fiber width of CNF. The shape observation confirmed that CNFs with an average fiber width of 12 nm and an average fiber length of 300 nm were obtained.

The available chlorine concentration in the aqueous sodium hypochlorite solution was measured by the following method.
(Measurement of available chlorine concentration in sodium hypochlorite aqueous solution)
0.582 g of an aqueous solution of sodium hypochlorite pentahydrate crystals was precisely weighed, 50 mL of pure water was added, 2 g of potassium iodide and 10 mL of acetic acid were added, and the container was immediately sealed and left in 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 test solution), and the titer was 34.55 mL. A blank test was performed separately, and correction was performed. 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 21% by mass.

[Example 2]
The same procedure as in Example 1 was carried out except that the reaction time in the oxidation reaction was changed from 6 hours to 2 hours. The mass of the residue on the membrane filter was 32 g.
The shape of this aqueous dispersion was observed using the method described above for measuring fiber length and fiber width of CNF. The shape observation confirmed that CNFs with an average fiber width of 14 nm and an average fiber length of 350 nm were obtained.

[Example 3]
(oxidation process)
500 g of an aqueous sodium hypochlorite solution having a pH of 13 and an effective chlorine concentration of 23% by mass was placed in a beaker, and the solution was heated to 30°C in a constant temperature water bath while being stirred at 200 rpm using a mixer (Three-One Motor, BL600) manufactured by Shinto Scientific Co., Ltd. with a three-blade swept-back blade stirring impeller. Then, 70 g (35 g water, 35 g pulp) of powdered pulp moistened with water (VP-1) manufactured by TDI Corporation was added as a cellulosic raw material.
After the cellulosic raw material was supplied, the mixture was kept at 30° C. in the constant temperature water bath and stirred until the pH decreased to 11. After the pH reached 11, the reaction system was stirred with a stirrer for 3 hours and 20 minutes while maintaining the pH adjusted to 11 by adding 25% by mass of sodium hydroxide.
The oxidation reaction was then slowed down by adding pure water to dilute the mixture, and a cellulose oxide dispersed in water was obtained.
(Treatment process)
An aqueous solution of sodium sulfite was added to the resulting cellulose oxide dispersed in water to reduce the remaining excess sodium hypochlorite.
(Protonation step)
Thereafter, hydrochloric acid was added to convert the carboxyl groups of the cellulose oxide from the salt type ( ^--COO.sup.- Na.sup. ⁺ ) to the proton type ( ^--COO.sup.- H.sup ^.+ ), thereby obtaining an aqueous dispersion with a pH of 2.5.
In this example, the pH was controlled using a pH controller (Tokyo Glass Instruments Co., Ltd., FD-02).
(Separation process, washing process)
The resulting aqueous dispersion at pH 2.5 was subjected to solid-liquid separation and washing. Specifically, the supernatant was removed by centrifugation (1000 G, 10 minutes) and decantation, and then an amount of pure water equivalent to the amount removed was added and thoroughly stirred with a spoon to make the dispersion homogenous. This procedure was repeated six times, and finally the above centrifugation and decantation were performed to obtain oxidized cellulose.
(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 ⁺ ) back to the salt form (-COO ^- Na ⁺ ). The concentration of oxidized cellulose in this aqueous dispersion was 11.3% by mass.
(Oxidized cellulose dilution step)
Pure water was added to the aqueous dispersion of oxidized cellulose to adjust the oxidized cellulose concentration to 1% by mass, and the mixture was then left to stand indoors for 1-2 days. At the start of the standing, the oxidized cellulose was in a gel state, but after 1 day the gel was no longer visible, and when gently shaken by hand, the mixture became an almost transparent liquid. The mixture left to stand for 2 days also showed similar behavior.
After leaving the samples to stand for one or two days, pure water was added to adjust the oxidized cellulose concentration to 0.1% by mass, and the light transmittance (660 nm) was measured, resulting in high values of 93% and 94%, respectively. Specifically, the light transmittance was measured by placing a sample with an oxidized cellulose concentration of 0.1% by mass in a 10 mm thick quartz cell and measuring the light transmittance at a wavelength of 660 nm using a spectrophotometer (JASCO V-550).
In addition, after one day, the sample was observed with a scanning probe microscope, and it was confirmed that nanocellulose with an average fiber length of 170 nm and an average fiber width of 4.4 nm had been obtained.
An aqueous dispersion of oxidized cellulose adjusted to 1% by mass was defibrated in a homomixer (10,000 rpm, 10 minutes) to obtain a mechanically defibrated aqueous nanocellulose dispersion. This mechanically defibrated nanocellulose had an average fiber length of 170 nm and an average fiber width of 4.3 nm.
From the above, it was confirmed that an aqueous dispersion of nanocellulose was obtained by the standing process.

[Comparative Example 1]
0.8 g of TEMPO, 5.0 g of sodium bromide, and pure water were added to a beaker and stirred to prepare an aqueous solution, to which 50 g of powdered pulp (VP-1) from TDI Corporation was added as a cellulosic raw material.
The aqueous solution was heated to 25°C in a constant temperature water bath while being stirred at 200 rpm using a propeller-type stirring blade in a stirrer (Three-One Motor, BL600) manufactured by Shinto Scientific Co., Ltd., and then 50 g of powdered pulp (VP-1) manufactured by TDI Corporation was added as a cellulosic raw material.
0.1 M sodium hydroxide was added and stirred to obtain an aqueous solution with a pH of 10.0. 129 g of an aqueous solution of sodium hypochlorite with an effective chlorine concentration of 13.2% by mass was added thereto, and while keeping the temperature at 25°C in the same thermostatic water bath, the pH during the reaction was adjusted to 10.0 while adding 0.1 M sodium hydroxide, and the mixture was stirred for 2 hours.
After the reaction was completed, sodium sulfite was added to treat the excess sodium hypochlorite, and the filtrate was collected by suction filtration using a PTFE membrane filter with a mesh size of 0.1 μm. The mass of the residue on the membrane filter was 52 g. Furthermore, when the shape of this aqueous dispersion was observed using the above-mentioned [Method for measuring CNF fiber length and fiber width], nanocellulose was not confirmed, and the average fiber length and average fiber width could not be calculated.

The present invention has industrial applicability in the field of nanocellulose production.

Claims (6)

an oxidation step of oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof to obtain oxidized cellulose ;
a treatment step of treating the hypochlorous acid or salt thereof used in the oxidation step to stop the reaction of oxidizing the cellulosic raw material;
a separation step of subjecting an oxide dispersion containing a cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain oxidized cellulose;
A step of obtaining a dispersion containing nanocellulose by diluting the oxidized cellulose in a dispersion medium;
Including,
the amount of carboxy groups in the oxidized cellulose is 0.30 mmol/g or more,
The oxidation step is carried out with stirring, and the reaction time of the oxidation reaction is 2 hours or more;
the light transmittance of the diluted dispersion is 70% or more when the solid content concentration of the diluted dispersion is 0.1% by mass;
Method for producing nanocellulose. an oxidation step of oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof to obtain oxidized cellulose;
a treatment step of treating the hypochlorous acid or salt thereof used in the oxidation step to stop the reaction of oxidizing the cellulosic raw material;
A step of subjecting an oxide dispersion containing a cellulose-based oxide and a dispersion medium to solid-liquid separation to obtain a dispersion containing nanocellulose;
Including,
the amount of carboxy groups in the oxidized cellulose is 0.30 mmol/g or more,
The oxidation step is carried out with stirring, and the reaction time of the oxidation reaction is 2 hours or more.
Method for producing nanocellulose. Purifying the dispersion containing the nanocellulose,
The method of claim 2 . The average fiber width of the nanocellulose is 1 nm or more and 100 nm or less,
The manufacturing method according to any one of claims 1 to 3 . The average fiber length of the nanocellulose is 50 nm or more and 800 nm or less.
The manufacturing method according to any one of claims 1 to 4 . The concentration of the cellulosic raw material in the reaction system during the oxidation is 0.1% by mass or more and 30% by mass or less.
The manufacturing method according to any one of claims 1 to 5 .