JP7746674B2 - Method for producing oxidized cellulose and nanocellulose, and oxidizing agent - Google Patents

Description

The present invention relates to a method for producing oxidized cellulose and nanocellulose, and an oxidizing agent.

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 the production of oxidized cellulose fiber by 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. Patent Document 2 discloses the production of oxidized cellulose by 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.
Furthermore, Non-Patent Document 1 discloses that nanocellulose can be produced by oxidizing pulp using a highly concentrated aqueous solution of sodium hypochlorite pentahydrate crystals, specifically, an aqueous solution with a sodium hypochlorite concentration of 22%, followed by mechanical defibration.

These technologies perform oxidation 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

Sustainable Chem. Eng. 2020, 8, 48, 17800-17806

Patent Document 1, Patent Document 2, and Non-Patent Document 1 disclose specific examples of producing nanocellulose material by micronizing oxidized cellulose, in which nanocellulose material is obtained through a mechanical fiberization process using an ultrasonic homogenizer. However, the above process leaves room for further improvement in terms of the energy required for fiberization. From the perspective of production costs, the production of nanocellulose material requires oxidized cellulose that is easily defibrated, allowing it to be defibrated even under mild processing conditions. Furthermore, in order to stably produce micronized cellulose fibers or to obtain highly transparent nanocellulose material with little light scattering in the dispersion medium, the oxidized cellulose in its pre-fiberization state must have good fiberization properties.

Patent Documents 1 and 2 specifically describe the production of oxidized cellulose and nanocellulose materials by using a cellulosic raw material and reacting it with hypochlorous acid or a salt thereof, in which the available chlorine concentration in the reaction system is 6 to 43% by mass. Non-Patent Document 1 discloses that nanocellulose can be produced using an aqueous solution of sodium hypochlorite with a concentration of 22% derived from sodium hypochlorite pentahydrate crystals.
Such methods, particularly those using sodium hypochlorite with an increased available chlorine concentration, tend to efficiently produce oxidized cellulose with excellent defibration properties, but there is a demand for further improvements in the defibration properties of oxidized cellulose.

The present invention was made in consideration of the above circumstances, and its primary object is to provide oxidized cellulose with excellent defibrillation properties.

As a result of extensive research, the inventors discovered that oxidized cellulose with excellent defibrillation properties can be obtained by setting the ratio of the concentration of components other than hypochlorous acid or its salts contained in hypochlorous acid or its salts to the available chlorine concentration (concentration of components other than hypochlorous acid or its salts/available chlorine concentration) within a specified range, leading to the completion of the present invention.

Specifically, the present invention is as follows.
[1]
A method for producing oxidized cellulose that contains an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof and is substantially free of N-oxyl compounds, comprising:
The method comprises a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material using an oxidizing agent containing hypochlorous acid or a salt thereof,
The ratio of the concentration of components other than hypochlorous acid or its salts in the oxidizing agent to the effective chlorine concentration of the hypochlorous acid or its salts (concentration of components other than hypochlorous acid or its salts/effective chlorine concentration) is 0.05 or more.
[2]
The components other than hypochlorous acid or its salt include at least one selected from inorganic salts, metals, and organic substances.
The manufacturing method described in [1].
[3]
[1] or [2], and defibrating the oxidized cellulose obtained by the manufacturing method to obtain nanocellulose.
Method for producing nanocellulose.
[4]
An oxidizing agent for producing oxidized cellulose, which contains hypochlorous acid or a salt thereof,
the oxidized cellulose comprises an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds;
The ratio of the concentration of components other than hypochlorous acid or its salts contained in the oxidizing agent to the effective chlorine concentration of the hypochlorous acid or its salts (concentration of components other than hypochlorous acid or its salts/effective chlorine concentration) is 0.05 or more;
Oxidizing agent for the production of oxidized cellulose.
[5]
The components other than hypochlorous acid or its salt include at least one selected from inorganic salts, metals, and organic substances.
The oxidizing agent for producing oxidized cellulose according to [4].

The manufacturing method of the present invention makes it possible to obtain oxidized cellulose with excellent defibration properties. In particular, the oxidized cellulose obtained by the manufacturing method of the present invention can be uniformly refined even when defibration treatment is carried out under mild conditions, and has excellent defibration properties.

<Method of producing oxidized cellulose>
The production method of the present invention is a method for producing oxidized cellulose that includes oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof and is substantially free of N-oxyl compounds. The production method of the present invention includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material using an oxidizing agent containing hypochlorous acid or a salt thereof. Furthermore, the ratio of the concentration of components other than hypochlorous acid or a salt thereof to the available chlorine concentration in the oxidizing agent used in the production method of the present invention (concentration of components other than hypochlorous acid or a salt thereof/available chlorine concentration) is 0.05 or more.

Hypochlorous acid or its salt is an active species that acts on cellulose-based raw materials and oxidizes them.It is known that inorganic salts, metals and their compounds, and organic substances (in this specification, these components are referred to as components other than hypochlorous acid or its salt, or other components) promote the self-decomposition of hypochlorous acid or its salt (active species).In addition, these components other than hypochlorous acid or its salt are derived from the raw materials of hypochlorous acid or its salt, or are contained in the hypochlorous acid or its salt production process, or are caused by the self-decomposition of hypochlorous acid or its salt due to UV irradiation, temperature, and pH conditions.
The present inventors believed that removing components other than the hypochlorous acid or its salts would suppress the decomposition of the active species, hypochlorous acid or its salts, promoting the oxidation of the cellulosic raw material and leading to improved defibration. However, as a result of their investigations, the present inventors discovered that by including a certain amount of components other than the hypochlorous acid or its salts, oxidized cellulose with superior defibration properties can be obtained compared to when the above components are reduced. This is thought to be because the activity of oxygen generated during autolysis is high, and this oxygen also promotes the oxidation of the cellulosic raw material. However, the reason why oxidized cellulose with excellent defibration properties can be obtained by the production method of the present invention is not limited to this.

Specific examples of components other than hypochlorous acid or its salts include inorganic salts such as sodium chloride (NaCl), sodium hydroxide (NaOH), sodium carbonate ( _Na2CO3 ), sodium bicarbonate ( _NaHCO3 ), sodium _{chlorate (NaClO3 )} , sodium chlorite ( _NaClO2 ), sodium bromate ( _NaBrO3 ), potassium chloride (KCl), calcium chloride ( _CaCl2 ), magnesium chloride ( _MgCl2 ), and water-insoluble substances such as silica and carbon; metals such as Fe, Cr, Ni, Cu, Co, Mn, and their metal compounds; chlorine-based solvents, fluororesins such as polyvinyl chloride, polyvinylidene chloride, and polytetrafluoroethylene, polyolefins such as polyethylene and polypropylene, ethylene propylene diene rubber, and fluororubber, as well as organic substances such as oxides of these hypochlorous acid or its salts; and the like. These may be contained alone or in combination of two or more.
The metal is preferably a transition metal. The metals in this specification may be the metal itself or a metal compound such as a metal oxide or chloride. The metals in the present invention include metal compounds. The organic matter is preferably insoluble in water, and more preferably remains on a filter such as a glass filter. The size of the organic matter that remains is preferably 5 μm or more.
A preferred embodiment of the component other than hypochlorous acid or a salt thereof is an embodiment containing an inorganic salt and a metal, and a more preferred embodiment is an embodiment containing at least NaCl and a metal.

The lower limit of the above ratio (concentration of components other than hypochlorous acid or its salts/concentration of available chlorine) is 0.05 or more. By making it 0.05 or more, the defibration properties of the resulting oxidized cellulose tend to be excellent. The lower limit of the above ratio (concentration of components other than hypochlorous acid or its salts/concentration of available chlorine) is preferably 0.08 or more, more preferably 0.10 or more, and even more preferably 0.12 or more.
The upper limit of the ratio (concentration of components other than hypochlorous acid or its salts/concentration of available chlorine) is not particularly limited, but is usually 5.0 or less. By keeping it 5.0 or less, the reaction tends to proceed smoothly without precipitation of other components during the reaction. Furthermore, by keeping it 5.0 or less, the proportion of active species tends to increase, and oxidized cellulose with good defibration properties tends to be obtained. The upper limit of the ratio (concentration of components other than hypochlorous acid or its salts/concentration of available chlorine) may be 4.0 or less, 2.0 or less, 1.2 or less, 1.0 or less, or 0.8 or less.
The above upper and lower limits can be combined as appropriate.

The oxidizing agent in the present invention is an agent used to oxidize a cellulosic raw material, and includes hypochlorous acid or a salt thereof and the other components described above. It is preferable that hypochlorous acid or a salt thereof is the main component in the oxidizing agent. The oxidizing agent may be in the form of a solution, and the solvent for the oxidizing agent is preferably water. Here, "main component" means that the proportion of hypochlorous acid or a salt thereof in the oxidizing agent (excluding the solvent) is 50% by mass or more. The proportion may be 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or even 97% by mass.
The oxidizing agent according to the present invention may be a commercially available product or may be one that is produced by the user. The oxidizing agent according to the present invention also includes an embodiment in which the concentration of hypochlorous acid or a salt thereof is adjusted by adding or removing a solvent or other components to the hypochlorous acid or a salt thereof during the oxidation reaction.

The concentration of components other than hypochlorous acid or a salt thereof is preferably in the range of 0.1 mass% or more and 15 mass% or less, more preferably in the range of 0.5 mass% or more and 10 mass% or less, and even more preferably in the range of 1.0 mass% or more and 5.0 mass% or less, based on the total amount of hypochlorous acid or a salt thereof and other components.
As the component other than hypochlorous acid or its salt, inorganic salt, preferably NaCl, is preferably the main component.Here, "main component" refers to the proportion of inorganic salt, preferably NaCl, to the total amount of other components is 50% by mass or more.The above-mentioned proportion may be 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more.
The amount of other components other than inorganic salts is preferably in the range of 0.001 to 1000 ppm, more preferably in the range of 0.01 to 100 ppm, and even more preferably in the range of 0.5 to 50 ppm, relative to the total amount of hypochlorous acid or its salt (i.e., the total amount of hypochlorous acid or its salt, which is the active species, and other components).

The above ratio (concentration of components other than hypochlorous acid or its salts/effective chlorine concentration) can be adjusted by controlling the amount of components other than hypochlorous acid or its salts and/or the effective chlorine concentration of hypochlorous acid or its salts. The amount of components other than hypochlorous acid or its salts can be adjusted by adding or removing the inorganic salts, metals, and organic substances described above, as appropriate. Furthermore, the amount of components other than hypochlorous acid or its salts can be controlled by using already prepared commercially available products, or by controlling the amount of hypochlorous acid or its salts during the manufacturing process. Hypochlorous acid or its salts with adjusted amounts of other components can be manufactured by referring to the Soda Technology Handbook 2009 (Japan Soda Industry Association).

The above ratio (concentration of components other than hypochlorous acid or its salts/effective chlorine concentration) can be obtained by dividing the concentration of components other than hypochlorous acid or its salts by the effective chlorine concentration of hypochlorous acid or its salts.
The concentration of inorganic salts among the concentrations of components other than hypochlorous acid or its salts can be measured, for example, by titration, ion chromatography, ion chromatography-post-column absorptiometry, or liquid chromatography mass spectrometry. The concentration of metals among the concentrations of components other than hypochlorous acid or its salts can be measured using an inductively coupled plasma optical emission spectrometer, an atomic absorption spectrometer, or an absorptiometer. The concentration of organic matter among the concentrations of components other than hypochlorous acid or its salts can be measured by capturing and quantifying insoluble matter using a glass filter or the like.
The concentrations of components other than hypochlorous acid or its salts can be determined specifically by the method described in the Examples.
The available chlorine concentration of hypochlorous acid or a salt thereof is defined as described below.

The cellulosic raw material used in the present invention 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 manufacturing 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. The 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 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.

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 effective chlorine concentration of 6% by mass or more and 43% by mass or less. By using hypochlorous acid or its salt with an effective chlorine concentration of 6% by mass or more and 43% by mass or less, the amount of carboxy groups in the oxidized cellulose can be sufficiently increased, allowing sufficient pulverization to proceed, and making it possible to omit mechanical defibration treatment after the oxidation reaction.
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 allowing the micronization of oxidized cellulose to proceed more smoothly, 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, even more preferably 9% by mass or more, and even more preferably 14% by mass or more. Furthermore, from the viewpoint of preventing 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 any combination of the above-mentioned lower and upper limits.

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.

While the reaction in the production method of the present invention may or may not be adjusted to any desired pH range, 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 increases sufficiently, and pulverization by stirring tends to proceed more easily. The pH of the reaction system is more preferably 7.0 or higher, and even more preferably 8.0 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, 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.

From the viewpoint of suppressing the available chlorine concentration while enhancing the oxidizing effect of hypochlorous acid or a salt thereof on the surface of the starting cellulose, the pH of the reaction system is preferably 11 or less, 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 typically 5.0 or more, preferably 6.0 or more, more preferably 7.0 or more, even more preferably 8.0 or more, even more preferably 9.0 or more, and even more preferably greater than 9.0. The pH range of the reaction system can be determined by appropriately combining the above upper and lower limits. The pH of the reaction system is preferably 5.0 or more and 11 or less, more preferably 6.0 or more and 11 or less, even more preferably 7.0 or more and 11 or less, even more preferably 8.0 or more and 10.7 or less, even more preferably 9.0 or more and 10.5 or less, and even more preferably greater than 9.0 and 10.5 or less.

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

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., a target concentration in the range of 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 reduction in the 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 aqueous sodium hypochlorite solution during the oxidation reaction. Examples of stirring methods include a stirrer with agitating blades, a homomixer, a disperser-type mixer, a homogenizer, and external circulation agitation. Among these, methods using one or more of shear-type agitators such as homomixers and homogenizers, a stirrer with agitating blades, and a disperser-type mixer are preferred, as they allow the oxidation reaction of the cellulosic raw material to proceed smoothly and make it easier to adjust the degree of polymerization of the oxidized cellulose to a predetermined value or less. Methods using a stirrer with agitating blades are particularly preferred. When using a stirrer with agitating blades, devices equipped with known agitating blades such as propeller blades, paddle blades, turbine blades, swept blades, anchor blades, gate blades, Max Blend blades, full zone blades, helical ribbon blades, and screw blades (e.g., with a draft tube) can be used. Furthermore, when using a stirrer with agitating blades, stirring is preferably performed at a rotation speed of 50 to 1,000 rpm. Multi-screw kneaders such as single-screw and twin-screw kneaders can also be used.

The reaction temperature in the oxidation reaction may usually be 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 more than 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 becomes, 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 time for the oxidation reaction can be set depending on 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.

From the viewpoint of improving workability, the concentration of the cellulosic raw material is preferably 35% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, relative to the total amount of the reaction mixture during the oxidation reaction (i.e., the total amount of the reaction system).
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, and even more preferably in the range of more than 6.5% by mass and not more than 15% 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.

The pressure at which the reaction is carried out is not particularly limited, but is usually in the range of normal pressure or higher and 1.0 MPaG or lower (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, and oxidized cellulose tends to be produced more efficiently. 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.

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

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 oxidation product of the cellulosic raw material using hypochlorous acid or a salt thereof. Furthermore, during the isolation process, some or all of the carboxy groups in the oxidized cellulose may be converted to the H type (-COOH) by, for example, adjusting the pH of the solution containing oxidized cellulose to 4 or less. Furthermore, the solution containing oxidized cellulose obtained by the above reaction may be used as is, or may be subjected to the subsequent defibration process, for example.

[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.

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 cellulosic raw materials is also contained in the form of cellulose microfibrils.

(Degree of polymerization)
The degree of polymerization of the oxidized cellulose in the present invention is preferably 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 finely divided under mild conditions and nano-sized by ordinary stirring or kneading, and nanocellulose tends to be obtained efficiently. From the viewpoint of easy defibration properties, no lower limit is particularly set for the degree of polymerization of the oxidized cellulose. 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.

The degree of polymerization 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 perspective 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, and even more preferably 80 to 500.

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 following measurement method.
[Measurement of viscosity average degree of polymerization of oxidized cellulose]
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 0.1 g per 1 g of oxidized cellulose. After reduction, solid-liquid separation was performed by suction filtration, followed by washing with water, and the resulting oxidized cellulose was freeze-dried. 0.04 g of dried oxidized cellulose was added to 10 ml of pure water and stirred for 2 minutes, after which 10 ml of 1 M copper ethylenediamine solution was added to dissolve. The flow time of the blank solution and the flow time of 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 concentration of oxidized cellulose fiber (c [ _g /ml]) according to the following equations, and the degree of polymerization (DP) of the oxidized cellulose fiber was calculated using the viscosity measurement equation.
η _r =η/η ₀ =t/t0
η _sp ＝η _r －１
[η]=η _sp /(100×c(1+0.28η _sp ))
DP = 175 × [η]

(Amount of carboxyl groups)
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 fine pulverization under mild conditions, and tends to lead to nano-sized particles through normal stirring and 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 with a low proportion of particulate cellulose and uniform quality. 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, and even more preferably 0.42 mmol/g or more. The upper limit of the carboxy group amount is more preferably 1.5 mmol/g or less, even more preferably 1.2 mmol/g or less, and even more preferably 1.0 mmol/g or less.

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 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. 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) × 0.05 / mass of oxidized cellulose (g)
Specifically, the amount of carboxy groups can be measured by the method described in the Examples.

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 constituting the cellulose have been oxidized, more specifically, the hydroxyl groups at the second and third positions 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 by solid ^- state C-NMR spectroscopy.

The oxidized cellulose of the present invention is prepared without the need for 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, the oxidized cellulose and nanocellulose of the present invention are substantially free of N-oxyl compounds. Here, in this specification, oxidized or nanocellulose "substantially free of N-oxyl compounds" means that no N-oxyl compounds are used during oxidation (i.e., it contains no N-oxyl compounds at all), or that the content of nitrogen derived from N-oxyl compounds in the oxidized or nanocellulose is 2.0 ppm or less. The amount of N-oxyl compounds in the oxidized or nanocellulose is preferably 1.0 ppm or less, as an increase from the cellulosic raw material.

[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, which includes a 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 includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material using hypochlorous acid or a salt thereof, and a step of obtaining nanocellulose by defibrating the oxidized cellulose.
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 of the oxidized cellulose themselves at the time of use to produce nanocellulose.

In the present invention, nanocellulose is derived from oxidized cellulose obtained by the production method of the present invention and refers to oxidized cellulose that has been defibrated and refined. Nanocellulose includes fine cellulose fibers. Here, nanocellulose is a general term for cellulose that has been refined into nano-sized particles, and includes cellulose nanofibers and cellulose nanocrystals. Cellulose nanofibers are also referred to as CNF.

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, even more preferably 10 nm or less, and even more preferably 5 nm or less.
From the viewpoint of further improving the quality of nanocellulose, the average fiber width is more preferably 1 nm or more and 50 nm or less, even more preferably 1 nm or more and 30 nm or less, and even more preferably 1 nm or more and 10 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 cellulose 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 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 method for defibrating oxidized cellulose is not particularly limited as long as it is an operation that can disperse nanocellulose.
For defibration, for example, a velocity field and velocity fluctuation of any intensity, collision with an inclusion or obstacle, 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 that performs the operation of wetting powder or the like with a liquid (also called kneading or kneading), and specific examples include a twin-arm kneader (a device that disperses using two-axis mixing blades in two semi-cylindrical containers); a Banbury mixer (a device that disperses under pressure in a closed system); and extrusion-type kneaders such as a screw extruder, co-kneader, and extruder.

Examples of defibration methods include methods using various mixing or stirring devices such as a screw mixer, paddle mixer, disperser mixer, turbine mixer, homo mixer under high-speed rotation, high-pressure homogenizer, ultra-high-pressure homogenizer, double-cylinder homogenizer, ultrasonic homogenizer, water-flow opposing collision-type disperser, beater, disc refiner, conical refiner, double-disc refiner, grinder, single-shaft or multi-shaft kneader, planetary mixer, and vibration mixer.

The defibration equipment can be used alone or in combination with two or more types.

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 process is preferably 100 MPa or more, more preferably 120 MPa or more, and even more preferably 150 MPa or more. There are no particular restrictions on the number of times the defibration process is performed, but from the perspective of ensuring sufficient progress of defibration, it is preferably two or more times, more preferably three or more times.

Oxidized cellulose has excellent defibration properties, so it can be sufficiently defibrated even when mild agitation, such as with a planetary agitator or vibration agitator, is used as a defibration method, resulting in uniform nanocellulose.

A planetary centrifugal mixer is a device that mixes materials in a container by rotating and revolving the container around its axis. A planetary centrifugal mixer mixes materials without using impellers, allowing for gentler mixing. The revolution speed and rotation speed during mixing with a planetary centrifugal mixer can be set as appropriate; for example, the revolution speed can be set to 400-3000 rpm, and the rotation speed can be set to 200-1500 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 process under conditions of a revolution speed of 1200-2500 rpm and a rotation speed of 600-1000 rpm, mixing for 3-15 minutes. The revolution speed is more preferably 1500-2300 rpm, and the rotation speed is more preferably 700-950 rpm. When defibrating the oxidized cellulose fibers using a planetary centrifugal mixer, the concentration of the oxidized cellulose aqueous dispersion used as the starting material is, for example, 0.01 to 1.0% by mass, and preferably 0.1 to 0.5% by mass.

An example of a vibration-type agitator is a vortex mixer (touch mixer). A vortex mixer stirs the liquid material in a container by creating a vortex. Vibration-type agitators such as vortex mixers perform stirring without using stirring blades, allowing for gentler stirring. Furthermore, vibration-type agitators such as vortex mixers can achieve gentle stirring with simple equipment, making them superior 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 process is preferably performed under stirring conditions of 3 to 15 minutes. When defibrating the oxidized cellulose fibers 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.

The defibration process 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.

Among the above 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.

Using an organic solvent as a dispersant during the defibration process makes it easier to isolate 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 dispersant of water and/or organic solvent, can be used to mix with various components such as resins, rubber, and solid particles.

<Aqueous solution of hypochlorous acid or its salt>
One aspect of the present invention is an oxidizing agent for producing oxidized cellulose, which contains hypochlorous acid or a salt thereof. The oxidized cellulose contains an oxidation product of a cellulosic raw material with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds. The ratio of the concentration of components other than hypochlorous acid or a salt thereof contained in the oxidizing agent to the available chlorine concentration of the hypochlorous acid or a salt thereof (concentration of components other than hypochlorous acid or a salt thereof/available chlorine concentration) is 0.05 or greater.
The oxidizing agent of the present invention can produce oxidized cellulose with excellent defibrillation properties. The oxidizing agent of the present invention may be in the form of a solution, and the solvent for the oxidizing agent is preferably water.
The embodiments of the oxidized cellulose, hypochlorous acid or a salt thereof, cellulosic raw material, etc. in the oxidizing agent of the present invention are as described in <Method for producing oxidized cellulose>.

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., food, cosmetics, medical products, paints, inks, etc.). Nanocellulose-containing compositions can also be formed into membranes and used as various sheets or films. There are no particular limitations on the fields in which nanocellulose-containing compositions can be used, and they can be used in the manufacture of products in a variety of fields, such as automotive components, machine parts, electrical appliances, electronic devices, cosmetics, medical products, building materials, daily necessities, stationery, etc.

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.

[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 aqueous dispersion was air-dried 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 analyzed by binarizing the obtained image 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 determined using the software attached to 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.

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

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

[Measurement of concentration of components other than hypochlorous acid or its salts]
The concentrations of components other than hypochlorous acid or its salts were measured in accordance with the test method of the Japan Water Works Association standard "Sodium hypochlorite for water supply" JWWA K120-2008, the test method of the Japan Water Works Association standard "Water supply chemicals" JWWA Z 109-2001, Appendix 5 "Simultaneous analysis method using an inductively coupled plasma optical emission spectrometer", and the test method of 4.4 insoluble matter of the Japan Water Works Association standard "Sodium hypochlorite for water supply" JWWA K120-1981. The above test methods measure the concentrations of inorganic salts, metals, and organic matter, respectively.

[Example 1]
Pure water was added to a sodium hypochlorite aqueous solution having an effective chlorine concentration of 33% by mass and stirred to prepare 700 g of a solution having an effective chlorine concentration of 21% by mass. The concentrations of components other than sodium hypochlorite and the effective chlorine concentration are as shown in Table 1, and the ratio of the concentration of components other than sodium hypochlorite to the effective chlorine concentration was 0.18. The sodium hypochlorite aqueous solution used in Example 1 was manufactured in accordance with the Soda Technology Handbook 2009 (Japan Soda Industry Association). Specifically, the sodium hypochlorite aqueous solution was obtained by diluting a hypochlorous acid solution with a high effective chlorine concentration (after separating precipitated sodium chloride) obtained by the production of low-salt sodium hypochlorite. Note that the proportion of sodium chloride among the components other than sodium hypochlorite was 50% by mass or more. The total amount of metals was in the range of 0.05 ppm to 10 ppm in sodium hypochlorite.
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 titration volume 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.

35% by mass hydrochloric acid was added thereto and stirred to obtain an aqueous sodium hypochlorite 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 then 50 g of powdered pulp (KC Flock W-100GK) manufactured by Nippon Paper Industries Co., Ltd. was added as a cellulosic raw material.
After supplying the cellulosic raw material, 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, followed by stirring under the same conditions for 2 hours using a stirrer.
After the reaction was completed, centrifugation (1000 G, 10 minutes) and decantation were repeated six times to recover the oxidized cellulose.
The carboxyl group content of the oxidized cellulose was 0.63 mmol/g. After adding pure water to the oxidized cellulose to adjust the concentration to 1% by mass, the mixture was defibrated using a homomixer (10,000 rpm, 10 minutes). This resulted in nanocellulose with an average fiber length of 200 nm and an average fiber width of 3 nm. The nanocellulose aqueous dispersion obtained by defibration was adjusted to a solids concentration of 0.1% by mass, and its light transmittance (660 nm) was measured, revealing a satisfactory defibration of 83%.

[Example 2]
The concentrations of components other than sodium hypochlorite and the effective chlorine concentration are as shown in Table 1, and the ratio of the concentrations of components other than sodium hypochlorite to the effective chlorine concentration was 0.15. The sodium hypochlorite aqueous solution used in Example 2 was manufactured in accordance with the Soda Technology Handbook 2009 (Japan Soda Industry Association). Specifically, the sodium hypochlorite aqueous solution was obtained by diluting low-salt sodium hypochlorite. Note that the proportion of sodium chloride in the components other than sodium hypochlorite was 50 mass% or more. The total amount of metals was in the range of 0.05 ppm to 10 ppm in sodium hypochlorite.
To this was added 35% by mass hydrochloric acid and stirred to obtain an aqueous solution of sodium hypochlorite with a pH of 10.
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 then 50 g of powdered pulp (KC Flock W-100GK) manufactured by Nippon Paper Industries Co., Ltd. was added as a cellulosic raw material.
After supplying the cellulosic raw material, the mixture was kept at 40°C in the same thermostatic water bath, and the pH during the reaction was adjusted to 10 by adding 48% by mass of sodium hydroxide, followed by stirring under the same conditions for 4 hours using a stirrer.
After the reaction was completed, centrifugation (1000 G, 10 minutes) and decantation were repeated six times to recover the oxidized cellulose.
The carboxyl group content of the oxidized cellulose was 0.64 mmol/g. After adding pure water to the oxidized cellulose to adjust the concentration to 1% by mass, the mixture was defibrated using a homomixer (10,000 rpm, 10 minutes). The resulting cellulose nanofibers had an average fiber length of 210 nm and an average fiber width of 4 nm. The nanocellulose aqueous dispersion obtained by defibration was adjusted to a solids concentration of 0.1% by mass, and its light transmittance (660 nm) was measured. The result was 77%, demonstrating good defibration.

[Example 3]
The same procedure as in Example 1 was carried out, except that the ratio of the concentration of components other than sodium hypochlorite to the effective chlorine concentration was 0.22. The proportion of sodium chloride in the components other than sodium hypochlorite was 50 mass% or more. The total amount of metals in sodium hypochlorite was in the range of 0.05 ppm or more and 10 ppm or less. The above ratio was adjusted by adding sodium chloride to the sodium hypochlorite aqueous solution used in Example 1 and increasing the amount of components other than sodium hypochlorite.
The amount of carboxy groups in the oxidized cellulose was 0.71 mmol/g.
The light transmittance of the nanocellulose was 89%, and good defibration was achieved.

[Example 4]
The same procedure as in Example 1 was carried out, except that the ratio of the concentration of components other than sodium hypochlorite to the effective chlorine concentration was 0.51. The proportion of sodium chloride in the components other than sodium hypochlorite was 50 mass% or more. The total amount of metals in sodium hypochlorite was in the range of 0.05 ppm or more and 10 ppm or less. The above ratio was adjusted by adding sodium chloride to the sodium hypochlorite aqueous solution used in Example 1 and increasing the amount of components other than sodium hypochlorite.
The amount of carboxy groups in the oxidized cellulose was 0.75 mmol/g.
The light transmittance of the nanocellulose was 97%, and good defibration was achieved.

[Example 5]
The same procedure as in Example 2 was carried out, except that the ratio of the concentration of components other than sodium hypochlorite to the effective chlorine concentration was 0.82. The proportion of sodium chloride in the components other than sodium hypochlorite was 50 mass% or more. The total amount of metals in sodium hypochlorite was in the range of 0.05 ppm or more and 10 ppm or less. The above ratio was adjusted by adding sodium chloride to the sodium hypochlorite aqueous solution used in Example 2 and increasing the amount of components other than sodium hypochlorite.
The amount of carboxy groups in the oxidized cellulose was 0.74 mmol/g.
The light transmittance of the nanocellulose was 85%, and good defibration was achieved.

[Comparative Example 1]
The same as Example 1 was used except that sodium hypochlorite pentahydrate crystals having an effective chlorine concentration of 42% by mass were used. The sodium hypochlorite pentahydrate crystals were produced in accordance with JP 2000-290003 A. That is, the same as Example 1 was used except that pure water was added to sodium hypochlorite pentahydrate crystals having an effective chlorine concentration of 42% by mass and stirred to prepare 700 g of a solution having an effective chlorine concentration of 21% by mass. The concentrations of components other than sodium hypochlorite and the effective chlorine concentration are as shown in Table 1, and the ratio of the concentrations of components other than sodium hypochlorite to the effective chlorine concentration was 0.03. Note that the proportion of sodium chloride in the components other than sodium hypochlorite was less than 50% by mass. The total amount of metals was less than 0.05 ppm in sodium hypochlorite.
The carboxyl group content of the oxidized cellulose was 0.51 mmol/g. After adding pure water to the oxidized cellulose to adjust the concentration to 1% by mass, the mixture was defibrated using a homomixer (10,000 rpm, 10 minutes). The resulting cellulose nanofibers had an average fiber length of 260 nm and an average fiber width of 6 nm. The CNF aqueous dispersion obtained by defibration was adjusted to a solids concentration of 0.1% by mass, and its light transmittance (660 nm) was measured and found to be 44%, indicating that coarse oxidized cellulose remained.

The manufacturing method of the present invention can provide nanocellulose for use in a variety of materials (e.g., resins, fibers, rubber, etc.) and applications (e.g., food, cosmetics, medical products, paints, inks, etc.), and has industrial applicability in a variety of fields, including automotive components, machine parts, electrical appliances, electronic devices, cosmetics, medical products, building materials, daily necessities, stationery, etc.

Claims (5)

A method for producing oxidized cellulose which contains an oxidation product of a cellulosic raw material (excluding rayon pulp sheets) using hypochlorous acid or a salt thereof and is substantially free of N-oxyl compounds, comprising :
The method includes a step of obtaining oxidized cellulose by oxidizing a cellulosic raw material (excluding rayon pulp sheets) using an oxidizing agent,
the oxidizing agent is an aqueous solution of hypochlorous acid or a salt thereof,
The ratio of the concentration of components other than hypochlorous acid or its salts in the aqueous solution to the effective chlorine concentration of the hypochlorous acid or its salts (concentration of components other than hypochlorous acid or its salts / effective chlorine concentration) is 0.05 or more;
The components other than hypochlorous acid or a salt thereof include sodium chloride,
The ratio of sodium chloride to the total amount of components other than the hypochlorous acid or its salt is 50% by mass or more.
Manufacturing method. The ratio of the concentration of components other than hypochlorous acid or its salts in the aqueous solution to the effective chlorine concentration of the hypochlorous acid or its salts (concentration of components other than hypochlorous acid or its salts / effective chlorine concentration) is 5.0 or less;
The method for producing nanocellulose according to claim 1. The method comprises the step of defibrating the oxidized cellulose obtained by the method according to claim 1 or 2 to obtain nanocellulose.
Method for producing nanocellulose. It is an oxidizing agent for producing oxidized cellulose,
the oxidizing agent is an aqueous solution of hypochlorous acid or a salt thereof,
the oxidized cellulose comprises an oxidation product of a cellulosic raw material (excluding rayon pulp sheets) with hypochlorous acid or a salt thereof, and is substantially free of N-oxyl compounds;
The ratio of the concentration of components other than hypochlorous acid or its salts contained in the aqueous solution to the effective chlorine concentration of the hypochlorous acid or its salts (concentration of components other than hypochlorous acid or its salts / effective chlorine concentration) is 0.05 or more;
The components other than hypochlorous acid or a salt thereof include sodium chloride,
The ratio of sodium chloride to the total amount of components other than the hypochlorous acid or its salt is 50% by mass or more.
Oxidizing agent for the production of oxidized cellulose. The ratio of the concentration of components other than hypochlorous acid or its salts in the aqueous solution to the effective chlorine concentration of the hypochlorous acid or its salts (concentration of components other than hypochlorous acid or its salts / effective chlorine concentration) is 5.0 or less;
The oxidizing agent for producing oxidized cellulose according to claim 4.