WO2024203024A1 - Method for producing oxidized cellulose nanofiber dispersion - Google Patents

Abstract

Provided is a method for producing an oxidized cellulose nanofiber dispersion having an improved viscosity retention ratio, the method including: a stirring step for stirring an oxidized cellulose nanofiber dispersion having a solid concentration of 0.1-5.0 mass% for 1-90 minutes under a condition of a rotational speed of 2000-6000 rpm; and, after the stirring step, a resting step for resting the oxidized cellulose nanofiber dispersion at a temperature of 4-35°C for 12 hours or longer.

Description

Method for producing oxidized cellulose nanofiber dispersion

The present invention relates to a method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention.

Nanotechnology, a technique for freely controlling matter in the nanometer range, that is, on the scale of atoms and molecules, is expected to produce a variety of convenient new materials and devices. One such example is cellulose nanofiber, which is obtained by finely breaking down plant fibers. This cellulose nanofiber is highly crystalline and features a low coefficient of thermal expansion and high elastic modulus. In addition, it has a high aspect ratio, so it is expected that by compounding it with rubber or resin, it will have the effect of adding strength and stabilizing shape. Furthermore, when in a dispersed state, this cellulose nanofiber has viscosity characteristics such as pseudoplasticity and thixotropy, and it is expected to be effective as an additive such as a thickener.

Various developments and researches on cellulose nanofibers have been conducted. For example, Patent Document 1 discloses fine cellulose fibers (cellulose nanofibers) in which some of the hydroxyl groups of cellulose have been converted to carboxyl groups and have a number average fiber diameter of 2 to 150 nm. These cellulose nanofibers have the property of having high viscosity at low shear rates and low viscosity at high shear rates. Because of these viscosity characteristics, cellulose nanofibers are used as thickeners in various fields such as cosmetics, food, civil engineering, building materials, inks, and paints.

JP 2008-1728 A

However, when the cellulose nanofiber described in Patent Document 1 is used as a thickener, there is a problem in that the viscosity is significantly reduced when the cellulose nanofiber is stirred during mixing with the other substance to which the thickening function is to be imparted.

The present invention therefore aims to provide a method for producing an oxidized cellulose nanofiber dispersion that can suppress the decrease in viscosity before and after stirring when the dispersion is mixed with a substance that imparts a thickening function.

The present invention provides the following (1) to (5).
(1) A method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention, comprising a stirring step of stirring an oxidized cellulose nanofiber dispersion having a solid content concentration of 0.1 to 5.0% by mass at a rotation speed of 2000 to 6000 rpm for 1 to 90 minutes, and a standing step of standing the oxidized cellulose nanofiber dispersion at a temperature of 4 to 35°C for 12 hours or more after the stirring step.
(2) The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention described in (1), further comprising a dilution step of diluting the oxidized cellulose nanofiber dispersion to a solids concentration of 0.1 to 0.8 mass % after the standing step.
(3) The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention according to (1) or (2), wherein the oxidized cellulose nanofiber has a carboxyl amount of 0.2 to 2.0 mmol/g relative to the bone dry mass of the oxidized cellulose nanofiber.
(4) The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention described in (2), characterized in that in the dilution step, 5 to 100 parts by mass of a dispersant is further added per 100 parts by mass of the solid content of the oxidized cellulose nanofiber.
(5) The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention described in (2), further comprising the step of adding, after the dilution step, 5 to 100 parts by mass of a dispersant per 100 parts by mass of the solid content of oxidized cellulose nanofibers in the oxidized cellulose nanofiber dispersion, and mixing.

The present invention provides a method for producing an oxidized cellulose nanofiber dispersion that can suppress the decrease in viscosity before and after stirring when mixing with a substance that imparts a thickening function.

The method for producing an oxidized cellulose nanofiber (oxidized CNF) dispersion with improved viscosity retention of the present invention includes a stirring step in which an oxidized cellulose nanofiber dispersion with a solid content concentration of 0.1 to 5.0 mass% is stirred for 1 to 90 minutes at a rotation speed of 2000 to 6000 rpm, and a settling step in which, after the stirring step, the oxidized cellulose nanofiber dispersion is settling at a temperature of 4 to 35°C for 12 hours or more.

(Oxidized Cellulose Nanofiber (Oxidized CNF))
The oxidized CNF used in the present invention can be obtained by defibrating oxidized cellulose obtained by introducing carboxyl groups into a cellulose raw material.

(Raw materials)
Examples of cellulose raw materials include plant materials (e.g., wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animal materials (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), and those derived from microbial products, and any of these can be used. Cellulose raw materials derived from plants or microorganisms are preferred, and cellulosic raw materials derived from plants are more preferred.

(Introduction of Carboxyl Group)
Carboxyl groups can be introduced into the above-mentioned cellulose raw material by oxidizing (carboxylating) it by a known method.

One example of oxidation is a method in which cellulose raw material is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound and a bromide, iodide, or a mixture of these. This oxidation reaction selectively oxidizes the primary hydroxyl group at the C6 position of the pyranose ring on the cellulose surface. As a result, oxidized cellulose can be obtained that has aldehyde groups and carboxyl groups (-COOH) or carboxylate groups (-COO-) on its surface. There are no particular limitations on the concentration of cellulose during the reaction, but it is preferable that it be 5% by mass or less.

An N-oxyl compound is a compound that can generate nitroxy radicals. Any compound that promotes the desired oxidation reaction can be used as an N-oxyl compound. Examples include 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO).

The amount of N-oxyl compound used is not particularly limited, as long as it is a catalytic amount capable of oxidizing the raw material cellulose. For 1 g of bone-dry cellulose, 0.01 mmol to 10 mmol is preferred, 0.01 mmol to 1 mmol is more preferred, and 0.05 mmol to 0.5 mmol is even more preferred. The concentration of the N-oxyl compound in the reaction system is preferably about 0.1 mmol/L to 4 mmol/L.

Bromides are compounds that contain bromine, including alkali metal bromides that can dissociate and ionize in water. Iodides are compounds that contain iodine, including alkali metal iodides.

The amount of bromide or iodide used can be selected within a range that can promote the oxidation reaction. The total amount of bromide and iodide is preferably 0.1 mmol to 100 mmol, more preferably 0.1 mmol to 10 mmol, and even more preferably 0.5 mmol to 5 mmol, per 1 g of bone-dry cellulose.

The oxidizing agent may be any known oxidizing agent, such as halogens, hypohalous acids, hypohalous acids, perhalogen acids or their salts, halogen oxides, peroxides, etc. Among these, sodium hypochlorite is preferred, as it is inexpensive and has a low environmental impact.

The amount of the oxidizing agent used is preferably 0.5 mmol to 500 mmol, more preferably 0.5 mmol to 50 mmol, and even more preferably 1 mmol to 25 mmol, per 1 g of bone-dry cellulose. Also, for example, 1 mol to 40 mol is preferred per 1 mol of the N-oxyl compound.

The cellulose oxidation process can proceed efficiently even under relatively mild conditions. For this reason, the reaction temperature is preferably 4°C to 40°C, and may be room temperature of about 15°C to 30°C. As the reaction proceeds, carboxyl groups are generated in the cellulose, and the pH of the reaction solution decreases. To ensure that the oxidation reaction proceeds efficiently, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution during the reaction to maintain the pH of the reaction solution at 8 to 12, preferably 10 to 11. Water is preferred as the reaction medium because it is easy to handle and is less likely to cause side reactions.

The reaction time for the oxidation reaction can be set appropriately according to the degree of progress of the oxidation, and is usually 0.5 to 6 hours, preferably 0.5 to 4 hours.

The oxidation reaction may also be carried out in two stages. For example, the oxidized cellulose obtained by filtration after the completion of the first stage reaction can be oxidized again under the same or different reaction conditions, allowing efficient oxidation without reaction inhibition by salt produced as a by-product in the first stage reaction.

Another example is a method of oxidation by contacting cellulose raw materials with a gas containing ozone. This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the pyranose ring, and causes the cellulose chain to break down.

The ozone concentration in the ozone-containing gas is preferably 50 g/m ³ to 250 g/m ³ , and more preferably 50 g/m ³ to 220 g/m ^3. The amount of ozone added to the cellulose raw material is preferably 0.1 parts by mass to 30 parts by mass, and more preferably 5 parts by mass to 30 parts by mass, based on 100 parts by mass of the solid content of the cellulose raw material.

The ozone treatment temperature is preferably 0°C to 50°C, and more preferably 20°C to 50°C. The ozone treatment time is not particularly limited, but is about 1 minute to 360 minutes, and preferably about 30 minutes to 360 minutes. If the ozone treatment conditions are within these ranges, excessive oxidation and decomposition of cellulose can be prevented, and the yield of oxidized cellulose will be good.

After the ozone treatment, a further oxidation treatment may be carried out using an oxidizing agent. The oxidizing agent used in the further oxidation treatment is not particularly limited, but examples include chlorine compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. For example, these oxidizing agents can be dissolved in water or a polar organic solvent such as alcohol to prepare an oxidizing agent solution, and the further oxidation treatment can be carried out by immersing the cellulose raw material in the solution.

The amount of carboxyl groups, which indicates the degree of modification of oxidized cellulose, can be adjusted by controlling the reaction conditions such as the amount of oxidizing agent added and the reaction time described above. The amount of carboxyl groups is preferably about 0.2 to 2.0 mmol/g, and more preferably 0.5 to 1.6 mmol/g, based on the bone dry mass of oxidized cellulose. If it is less than 0.2 mmol/g, a large amount of energy is required to defibrate it into oxidized CNF. Furthermore, if more than 2.0 mmol of oxidized cellulose is used as the raw material, the resulting oxidized CNF will not have a fibrous form. The amount of carboxyl groups in oxidized cellulose is usually the same as the amount of carboxyl groups when made into cellulose nanofibers.

(Defibrilization)
The device used for defibration is not particularly limited, but examples include devices of high-speed rotation type, colloid mill type, high-pressure type, roll mill type, ultrasonic type, etc., and a high-pressure or ultra-high-pressure homogenizer is preferable, and a wet high-pressure or ultra-high-pressure homogenizer is more preferable. The device is preferably capable of applying a strong shear force to the cellulose raw material or oxidized cellulose (usually a dispersion liquid). The pressure that the device can apply is preferably 50 MPa or more, more preferably 100 MPa or more, and even more preferably 140 MPa or more. The device is preferably a wet high-pressure or ultra-high-pressure homogenizer that can apply the above pressure to the cellulose raw material or oxidized cellulose (usually a dispersion liquid) and can apply a strong shear force. This allows defibration to be performed efficiently. The number of treatments (passes) in the defibration device may be one or two or more, and two or more are preferable.

In the dispersion process, oxidized cellulose is usually dispersed in a solvent. There are no particular limitations on the solvent as long as it can disperse oxidized cellulose, but examples include water, organic solvents (e.g., hydrophilic organic solvents such as methanol), and mixtures of these. Since the cellulose raw material is hydrophilic, it is preferable that the solvent is water.

The solids concentration of oxidized cellulose in the dispersion is usually 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.3% by mass or more. This ensures that the amount of liquid relative to the amount of cellulose fiber raw material is appropriate and efficient. The upper limit is usually 10% by mass or less, and preferably 6% by mass or less. This allows fluidity to be maintained.

The order of the defibration process and the dispersion process is not particularly limited, and either may be performed first, or they may be performed simultaneously, but it is preferable to perform the defibration process after the dispersion process. Each combination of processes needs to be performed at least once, and may be repeated two or more times.

　If necessary, preliminary treatment may be carried out prior to the defibration or dispersion treatment. The preliminary treatment may be carried out using a mixing, stirring, emulsifying, or dispersing device such as a high-speed shear mixer.

The average fiber diameter of the oxidized CNF of the present invention is preferably 3 nm or more or 500 nm or less. The average fiber diameter and average fiber length of the cellulose nanofibers can be measured, for example, by preparing a 0.001% by mass aqueous dispersion of oxidized CNF, spreading this diluted dispersion thinly on a mica sample stage, and heating and drying it at 50°C to prepare a sample for observation, and measuring the cross-sectional height of the shape image observed with an atomic force microscope (AFM) to calculate the number average fiber diameter or fiber length.

The average aspect ratio of the oxidized CNF is usually 50 or more. There is no particular upper limit, but it is usually 1000 or less. The average aspect ratio can be calculated by the following formula:
Aspect ratio = average fiber length/average fiber diameter

(Mixing process)
In the stirring step of the present invention, the oxidized cellulose nanofiber dispersion having a solid content concentration of 0.1 to 5.0% by mass is stirred for 1 to 90 minutes at a rotation speed of 2000 to 6000 rpm. The solid content concentration of the oxidized CNF dispersion to be subjected to the stirring step is 0.1 to 5.0% by mass, preferably 0.1 to 3.0% by mass. If the concentration is too high, stirring becomes difficult, and if the concentration is too low, the amount of CNF added when added to other materials becomes low, making it difficult to use as a thickener. As the oxidized CNF dispersion to be subjected to the stirring step, the oxidized CNF dispersion after the above-mentioned dispersion and defibration treatment can be directly subjected to the stirring step as long as the solid content concentration is within the above range. The solid content concentration can be adjusted by dilution, dehydration, etc. as appropriate. The rotation speed in the stirring step is 2000 to 6000 rpm, preferably 2000 to 3000 rpm, from the viewpoint of thoroughly stirring the entire CNF dispersion. The stirring time is from 1 to 90 minutes, preferably from 5 to 60 minutes, and more preferably from 10 to 60 minutes, from the viewpoint of imparting a shear effect by stirring to the entire CNF dispersion.

The agitator used in the agitation process is not particularly limited as long as it can agitate the oxidized CNF dispersion liquid having a solid content concentration of 0.1 to 5.0% by mass at the above rotation speed. For example, a high-speed agitator or a high-speed emulsifying disperser can be used. For example, a high-speed agitator such as Homo Disper manufactured by Primix Corporation can be used.

(Stillage process)
In the standing step of the present invention, the oxidized CNF dispersion after the stirring step is allowed to stand at a temperature of 4 to 35°C, preferably 6 to 25°C, for 12 hours or more, preferably 24 hours or more. The upper limit of the standing time is not particularly limited, but from the viewpoint of long-term storage, it is, for example, 10 days or less. If the standing time is too short, there is a risk that the temporary viscosity reduction immediately after stirring will not recover. In the standing step, it is preferable to place the oxidized CNF dispersion in a container and allow it to stand in order to prevent evaporation of water in the CNF dispersion. The material of the container is not particularly limited, and may be a glass bottle or a plastic bottle. It is preferable that the container can be closed with a lid.

(Dilution process)
The production method of the present invention may include a dilution step after the standing step. When the dilution step is included, the oxidized CNF is diluted until the solid content concentration is preferably 0.1 to 0.8% by mass, more preferably 0.1 to 0.6% by mass. The solvent used for dilution may be the same as or different from the solvent of the dispersion before dilution, and examples of the solvent include water, organic solvents (e.g., hydrophilic organic solvents such as methanol), and mixed solvents thereof. Since the cellulose raw material is hydrophilic, the solvent is preferably water. In the dilution step, the stirring and mixing conditions are not particularly limited as long as the mixture can be uniformly diluted, but the mixture is diluted by stirring at a rotation speed of 1000 rpm for 10 minutes, for example.

(Dispersant)
The oxidized CNF dispersion produced by the production method of the present invention may contain a dispersant. When a dispersant is added, the amount of the dispersant added is preferably 5 to 100 parts by mass, more preferably 10 to 50 parts by mass, per 100 parts by mass of the solid content of the oxidized CNF, from the viewpoint of the solid content of the CNF. The type of dispersant may be used without particular limitation as long as it exhibits the effects of the present invention, and examples thereof include polyacrylic acid, carboxymethyl cellulose, and polyurethane dispersants.

When a dispersant is included, the step of adding the dispersant is not particularly limited as long as the effects of the present invention are achieved, but the dispersant may be added in the stirring step, the dispersant may be added in the dilution step, or a step of adding the dispersant and mixing may be provided after the dilution step. If a dilution step is not provided, a step of adding the dispersant and mixing may be provided after the standing step. When a dispersant is added and mixed at a step other than the stirring step and dilution step, the device used for stirring and mixing is not particularly limited, and a commonly used stirring device may be used. The stirring time is also not particularly limited, and stirring may be performed within a range of about 1 minute to 1 hour.

(Viscosity retention rate)
The viscosity retention rate of an oxidized cellulose nanofiber dispersion can be calculated from the viscosity before and after stirring determined by the stability test described below, using the following formula.
Viscosity retention rate (%) = (viscosity after stirring/viscosity before stirring) x 100

(Stability test)
For the oxidized cellulose nanofiber dispersion immediately after the dilution step, the B-type viscosity is measured using a B-type viscometer under conditions of 6 rpm, 23°C, and 1 minute (viscosity before stirring). Note that, if the dilution step is not required, the viscosity before stirring can be measured under the same conditions for the dispersion to which shear force has been applied by stirring (1000 rpm, 10 minutes) after the standing step, and if a step of adding and mixing a dispersant is included after the dilution step, the viscosity before stirring can be measured under the same conditions for the dispersion after that step.

After measuring the B-type viscosity, the oxidized cellulose nanofiber dispersion is stirred for 30 minutes (1000 rpm, 23°C) using a high-speed stirrer. Immediately after stirring for 30 minutes, the B-type viscosity is measured using a B-type viscometer at 6 rpm, 23°C, and 1 minute (viscosity after stirring). As a high-speed stirrer, for example, a Homo Disper manufactured by Primix Corporation can be used.

The manufacturing method of the present invention makes it possible to produce an oxidized cellulose nanofiber dispersion with improved viscosity retention.

The reason why the production method of the present invention makes it possible to obtain an oxidized cellulose nanofiber dispersion with improved viscosity retention is believed to be as follows.
It is possible to obtain cellulose nanofibers by defibrating oxidized cellulose, and in the cellulose nanofibers obtained here, each fiber is swollen (defibrated) to the nano level, but the physical entanglements of the cellulose fibers that existed before defibration remain. When such a dispersion of oxidized cellulose nanofibers is stirred under the stability test conditions (1000 rpm, 23°C), the entangled nanofibers become loose, the dispersion state changes, and the loosened nanofibers become oriented in a certain direction, resulting in the development of pseudoplasticity and a decrease in viscosity.
In the production method of the present invention, a stirring step is provided before the stability test, and therefore physical entanglements of the cellulose fibers remaining after defibration can be eliminated in the stirring step. Therefore, the decrease in viscosity before and after the stability test is suppressed, and as a result, an oxidized cellulose nanofiber dispersion with improved viscosity retention can be produced.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these.

(Method of measuring the amount of carboxyl groups)
60 mL of a 0.5% by mass slurry (aqueous dispersion) of oxidized cellulose was prepared, and a 0.1 M aqueous hydrochloric acid solution was added to adjust the pH to 2.5. A 0.05 N aqueous sodium hydroxide solution was then added dropwise, and the electrical conductivity was measured until the pH reached 11. The electrical conductivity was calculated using the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity was gradual, using the following formula:
Amount of carboxyl groups [mmol/g oxidized cellulose] = a [mL] × 0.05 / mass of oxidized cellulose [g]

(Stability test)
For the oxidized cellulose nanofiber aqueous dispersions of the Examples and Comparative Examples, immediately after the dilution step, the B-type viscosity was measured using a B-type viscometer under conditions of 6 rpm, 23°C, and 1 minute (viscosity before stirring).

The oxidized cellulose nanofiber aqueous dispersion after measuring the B-type viscosity was stirred for 30 minutes (1000 rpm, 23°C) using a high-speed stirrer (Homodisper, manufactured by Primix Corporation). Immediately after stirring for 30 minutes, the B-type viscosity was measured using a B-type viscometer at 6 rpm, 23°C, and for 1 minute (viscosity after stirring).

(Viscosity retention rate)
The viscosity retention rate was calculated according to the following formula using the viscosity before stirring and the viscosity after stirring measured in the stability test.
Formula: Viscosity retention rate (%) = (viscosity after stirring/viscosity before stirring) x 100

(Production Example A)
(Production of oxidized cellulose nanofiber aqueous dispersion A)
5.00 g (bone-dry) of bleached unbeaten kraft pulp (whiteness 85%) derived from coniferous trees was added to 500 mL of an aqueous solution in which 20 mg (0.025 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system so that the sodium hypochlorite concentration was 5.1 mmol/g, and an oxidation reaction was started. During the reaction, the pH of the system decreased, but a 3M aqueous solution of sodium hydroxide was added successively to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system did not change. The mixture after the reaction was filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain an oxidized pulp (oxidized cellulose). The pulp yield was 90%, the time required for the oxidation reaction was 60 minutes, and the amount of carboxyl groups (hereinafter sometimes referred to as "degree of modification") was 1.46 mmol/g. This was adjusted to 1.0 mass% with water and defibrated three times using a high-pressure homogenizer to obtain oxidized cellulose nanofiber aqueous dispersion A. The average fiber diameter was 4 nm, and the aspect ratio was 270.

(Production Example B)
(Production of oxidized cellulose nanofiber aqueous dispersion B)
5.00 g (bone-dry) of bleached, unbeaten kraft pulp (whiteness 85%) derived from coniferous trees was added to 500 mL of an aqueous solution in which 20 mg (0.025 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 mg (1.0 mmol per 1 g of bone-dry cellulose) of sodium bromide were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system so that the sodium hypochlorite concentration was 3.4 mmol/g, and the oxidation reaction was started. During the reaction, the pH of the system decreased, but 3M aqueous sodium hydroxide solution was added successively to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system did not change. The mixture after the reaction was filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain an oxidized pulp (oxidized cellulose). The pulp yield at this time was 92%, the time required for the oxidation reaction was 60 minutes, and the amount of carboxyl groups (hereinafter sometimes referred to as the "degree of modification") was 1.1 mmol/g. This was adjusted to 1.0 mass % with water and defibration treatment was carried out three times using a high-pressure homogenizer to obtain oxidized cellulose nanofiber aqueous dispersion B. The average fiber diameter was 4 nm, and the aspect ratio was 273.

(Production Example C)
(Production of oxidized cellulose nanofiber aqueous dispersion C)
5.00 g (bone-dry) of bleached unbeaten kraft pulp (85% whiteness) derived from coniferous trees was added to 500 mL of an aqueous solution in which 20 mg (0.025 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system so that the sodium hypochlorite concentration was 1.0 mmol/g, and the oxidation reaction was started. During the reaction, the pH of the system decreased, but 3M aqueous sodium hydroxide solution was added successively to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system did not change. The mixture after the reaction was filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain an oxidized pulp (oxidized cellulose). The pulp yield at this time was 93%, the time required for the oxidation reaction was 60 minutes, and the amount of carboxyl groups (hereinafter sometimes referred to as the "degree of modification") was 0.58 mmol/g. This was adjusted to 1.0 mass % with water and defibration treatment was carried out five times using a high-pressure homogenizer to obtain oxidized cellulose nanofiber aqueous dispersion C. The average fiber diameter was 4 nm, and the aspect ratio was 290.

(Production Example D)
(Production of oxidized cellulose nanofiber aqueous dispersion D)
5.00 g (bone-dry) of bleached unbeaten kraft pulp (whiteness 85%) derived from coniferous trees was added to 500 mL of an aqueous solution in which 20 mg (0.025 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system so that the sodium hypochlorite concentration was 2.2 mmol/g, and the oxidation reaction was started. During the reaction, the pH of the system decreased, but 3M aqueous sodium hydroxide solution was added successively to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system did not change. The mixture after the reaction was filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain an oxidized pulp (oxidized cellulose). The pulp yield was 93%, the time required for the oxidation reaction was 60 minutes, and the amount of carboxyl groups was 0.76 mmol/g. This was adjusted to 1.0% by mass with water and defibrated five times using a high-pressure homogenizer to obtain oxidized cellulose nanofiber aqueous dispersion D. The average fiber diameter was 4 nm, and the aspect ratio was 280.

Example 1
(Mixing process)
The oxidized cellulose nanofiber aqueous dispersion A having a solids concentration of 1.0% by mass obtained in Production Example A was subjected to a stirring treatment for 5 minutes at a rotation speed of 3000 rpm using a high-speed stirrer (Homodisper, manufactured by Primix Corporation).

(Stillage process)
The oxidized cellulose nanofiber aqueous dispersion A that had been subjected to the stirring treatment in the stirring step described above was placed in a plastic container and allowed to stand at 23° C. for 7 days.

(Dilution process)
The oxidized cellulose nanofiber aqueous dispersion A that had been subjected to the standing step was placed in a plastic container, deionized water was added, and the mixture was stirred (1000 rpm, 10 minutes) to obtain oxidized cellulose nanofiber aqueous dispersion A diluted to a solids concentration of 0.5 mass%.

A stability test was conducted on this diluted oxidized cellulose nanofiber aqueous dispersion A, and the B-type viscosity values before and after stirring were obtained. The results are shown in Table 1.

(Examples 2 to 4)
Except for changing the stirring time in the stirring step to 15 minutes (Example 2), 30 minutes (Example 3), and 60 minutes (Example 4), the stirring step, the standing step, and the dilution step were carried out in this order in the same manner as in Example 1, and a stability test was carried out. The results are shown in Table 1.

Example 5
A stability test was carried out by carrying out the stirring step, the standing step, and the dilution step in that order in the same manner as in Example 1, except that oxidized cellulose nanofiber aqueous dispersion B obtained in Production Example B was used. The results are shown in Table 1.

(Examples 6 to 8)
A stability test was carried out by carrying out the stirring step, the standing step, and the dilution step in the same order as in Example 5, except that the stirring time in the stirring step was changed to 15 minutes (Example 6), 30 minutes (Example 7), and 60 minutes (Example 8), respectively. The results are shown in Table 1.

Example 9
Except for using the oxidized cellulose nanofiber aqueous dispersion C obtained in Production Example C, a stability test was carried out by carrying out the stirring step, the standing step, and the dilution step in that order in the same manner as in Example 1. The results are shown in Table 1.

(Examples 10 to 12)
A stability test was carried out by carrying out the stirring step, the standing step, and the dilution step in the same order as in Example 9, except that the stirring time in the stirring step was changed to 15 minutes (Example 10), 30 minutes (Example 11), and 60 minutes (Example 12), respectively. The results are shown in Table 1.

(Comparative Example 1)
The standing and dilution steps were carried out in the same manner as in Example 1, except that instead of oxidized cellulose nanofiber aqueous dispersion A which had been stirred in the standing step without being subjected to the stirring step, a dispersion which had not been stirred was used, to obtain an oxidized cellulose nanofiber aqueous dispersion with a solid content concentration of 0.5% by mass. A stability test was carried out on this oxidized cellulose nanofiber aqueous dispersion. The results are shown in Table 1.

(Comparative Example 2)
The standing and dilution steps were carried out in the same manner as in Example 5, except that instead of oxidized cellulose nanofiber aqueous dispersion B which had been stirred in the standing step without being subjected to the stirring step, a dispersion which had not been stirred was used, to obtain an oxidized cellulose nanofiber aqueous dispersion with a solid content concentration of 0.5% by mass. A stability test was carried out on this oxidized cellulose nanofiber aqueous dispersion. The results are shown in Table 1.

(Comparative Example 3)
The standing and dilution steps were carried out in the same manner as in Example 9, except that instead of oxidized cellulose nanofiber aqueous dispersion C which had been stirred in the standing step without being subjected to the stirring step, a dispersion which had not been stirred was used, to obtain an oxidized cellulose nanofiber aqueous dispersion with a solid content concentration of 0.5% by mass. A stability test was carried out on this oxidized cellulose nanofiber aqueous dispersion. The results are shown in Table 1.

As can be seen from Table 1, when the methods of Examples 1 to 12 were used, which included a stirring step of stirring a 0.1 to 5.0 mass% oxidized cellulose nanofiber dispersion at a rotation speed of 2000 to 6000 rpm for 1 to 90 minutes, and a settling step of leaving the oxidized cellulose nanofiber dispersion at a temperature of 4 to 35°C for 12 hours or more after the stirring step, the obtained oxidized cellulose nanofiber dispersion had improved viscosity retention when compared with Comparative Examples 1 to 3, which did not involve the stirring step and which had the same degree of modification.

(Example 13)
The oxidized cellulose nanofiber aqueous dispersion D obtained in Production Example D was used, and the stirring step and the standing step were carried out in that order in the same manner as in Example 1, except that the stirring treatment time was changed to 15 minutes.

(Dilution process)
The oxidized cellulose nanofiber aqueous dispersion D that had been subjected to the standing step was placed in a plastic container, and polyacrylic acid (Aron A-7100, manufactured by Toagosei Co., Ltd., hereinafter sometimes abbreviated as "PAA") was further added as a dispersant in an amount of 20 parts by mass per 100 parts by mass of the oxidized cellulose nanofiber solids content. Furthermore, deionized water was added to this plastic container and stirred (1000 rpm, 10 minutes), thereby obtaining a dispersion with an oxidized cellulose nanofiber solids concentration of 0.5 mass% and a dispersant concentration of 0.1 mass% relative to the total amount of the dispersion.

A stability test was conducted on the resulting dispersion, and the B-type viscosity values before and after stirring were obtained. The results are shown in Table 2.

(Example 14)
A dispersion with an oxidized cellulose nanofiber solids concentration of 0.5% by mass and a dispersant concentration of 0.1% by mass was obtained in the same manner as in Example 13, except that the dispersant was changed to carboxymethyl cellulose (Sunrose A04SH, manufactured by Nippon Paper Industries Co., Ltd., hereinafter sometimes abbreviated as "CMC"). A stability test was carried out on the obtained dispersion. The results are shown in Table 2.

(Example 15)
A dispersion with an oxidized cellulose nanofiber solids concentration of 0.5 mass% and a dispersant concentration of 0.1 mass% was obtained in the same manner as in Example 13, except that the oxidized cellulose nanofiber aqueous dispersion C obtained in Production Example C was used instead of the oxidized cellulose nanofiber aqueous dispersion D obtained in Production Example D. A stability test was carried out on the obtained dispersion. The results are shown in Table 2.

(Example 16)
A dispersion with an oxidized cellulose nanofiber solids concentration of 0.5 mass% and a dispersant concentration of 0.1 mass% was obtained in the same manner as in Example 14, except that the oxidized cellulose nanofiber aqueous dispersion C obtained in Production Example C was used instead of the oxidized cellulose nanofiber aqueous dispersion D obtained in Production Example D. A stability test was carried out on the obtained dispersion. The results are shown in Table 2.

(Comparative Examples 4 to 5)
The standing and dilution steps were carried out in the same manner as in Examples 13 and 14, except that instead of oxidized cellulose nanofiber aqueous dispersion D which had not been subjected to the stirring process and had been stirred in the standing process, a dispersion which had not been subjected to the stirring process was used, thereby obtaining dispersions of Comparative Examples 4 and 5, respectively, with oxidized cellulose nanofiber solids concentrations of 0.5% by mass and dispersant concentrations of 0.1% by mass. Stability tests were carried out on the obtained dispersions. The results are shown in Table 2.

As can be seen from Table 2, when the methods of Examples 13 to 16 were used, which included a stirring step in which a 0.1 to 5.0 mass% oxidized cellulose nanofiber dispersion was subjected to a stirring treatment for 1 to 90 minutes at a rotation speed of 2000 to 6000 rpm, and a standing step in which the oxidized cellulose nanofiber dispersion was allowed to stand at a temperature of 4 to 35°C for 12 hours or more after the stirring step, the obtained oxidized cellulose nanofiber dispersion had improved viscosity retention even when a dispersant was added, compared to Comparative Examples 4 and 5 in which the stirring step was not performed.

Claims (5)

a stirring step of stirring an oxidized cellulose nanofiber dispersion having a solid content concentration of 0.1 to 5.0% by mass at a rotation speed of 2000 to 6000 rpm for 1 to 90 minutes;
a standing step of standing the oxidized cellulose nanofiber dispersion at a temperature of 4 to 35°C for 12 hours or more after the stirring step. The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention described in claim 1 further comprises a dilution step of diluting the oxidized cellulose nanofiber dispersion to a solids concentration of 0.1 to 0.8 mass % after the standing step. The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention according to claim 1 or 2, wherein the oxidized cellulose nanofiber has a carboxyl amount of 0.2 to 2.0 mmol/g relative to the bone dry mass of the oxidized cellulose nanofiber. The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention described in claim 2, characterized in that in the dilution process, 5 to 100 parts by mass of a dispersant is further added per 100 parts by mass of the solid content of the oxidized cellulose nanofiber. The method for producing an oxidized cellulose nanofiber dispersion with improved viscosity retention described in claim 2 further comprises, after the dilution step, a step of adding 5 to 100 parts by mass of a dispersant per 100 parts by mass of the solid content of oxidized cellulose nanofibers in the oxidized cellulose nanofiber dispersion and mixing.