JP7645160B2 - Method for producing cellulose nanofiber aqueous suspension - Google Patents

Description

The present invention relates to a method for producing an aqueous suspension of cellulose nanofibers. More specifically, it provides a method for efficiently producing an aqueous suspension with minimal residual gel particles using cellulose nanofiber powder.

Cellulose nanofibers are fine fibers with fiber diameters of about 3 nm to several hundred nm that have excellent dispersibility in aqueous media, and are expected to be used in maintaining the viscosity of foods, cosmetics, medical products, paints, etc., strengthening food raw material dough, retaining moisture, improving food stability, and as a low-calorie additive or emulsion stabilization aid. However, when cellulose nanofibers dispersed in water (wet state) are dried to form a dry solid, hydrogen bonds are usually formed between the fine cellulose fibers, so even if water is added to this dry solid to redisperse it, it is generally difficult to redisperse it uniformly. For this reason, cellulose nanofibers are manufactured in a state dispersed in water (wet state) and are usually used for various purposes in a wet state without drying.

However, to keep cellulose nanofibers stable in this wet state, several to several hundred times the mass of water is required compared to the mass of cellulose nanofibers, which poses various problems such as the need to secure storage space and increased storage and transportation costs.

In response to the above problems, the applicant has proposed a method for improving the redispersibility of a cellulose nanofiber dry solid by adding 5 to 300% by mass of a water-soluble polymer to anion-modified cellulose nanofiber to form a dry solid (Patent Document 1), and a method for improving the redispersibility of a cellulose nanofiber dry solid by drying a mixture of cellulose nanofiber and a solvent using a vacuum drying device (Patent Document 2).

International Publication No. 2015/107995 International Publication No. 2019/189318

The methods described in Patent Documents 1 and 2 are effective methods for producing a cellulose nanofiber dry solid with good redispersibility, but it is desirable to develop a new method for efficiently producing an aqueous dispersion/suspension with fewer gel particles using a cellulose nanofiber dry solid.

The present invention aims to provide a new method for efficiently producing an aqueous suspension with minimal residual gel particles when producing an aqueous suspension using cellulose nanofiber powder.

As a result of intensive research, the present inventors have found that when producing an aqueous suspension using cellulose nanofiber powder, by using a medium with a high shear viscosity obtained by dissolving a substance having viscosity-increasing properties in water as the medium, an aqueous suspension with fewer residual gel particles can be produced even when a stirrer with a low rotation speed is used. In this case, it has been found that redispersibility can be further improved by first mixing water with the cellulose nanofiber powder to obtain a mixture of cellulose nanofiber and water, and then adding a substance having viscosity-increasing properties to this mixture to give it a high shear viscosity. The shear viscosity of the finally obtained aqueous suspension is 0.25 Pa·s or more when measured at 25°C and a shear rate of 1560 (1/s). The present invention includes the following.
[1] A step of mixing a powder containing cellulose nanofibers with water;
A method for producing a cellulose nanofiber aqueous suspension, comprising: mixing a thickening substance into a mixture of the powder and water; and stirring the mixture of the powder, water, and thickening substance,
The above-mentioned production method, wherein the shear viscosity of the aqueous suspension is 0.25 Pa·s or more when measured at 25° C. and a shear rate of 1560 (1/s).
[2] The method for producing an aqueous suspension of cellulose nanofibers according to [1], wherein the cellulose nanofibers are anion-modified cellulose nanofibers.
[3] The method for producing an aqueous suspension of cellulose nanofibers described in [2], wherein the anion-modified cellulose nanofibers are cellulose nanofibers having a carboxyalkyl group or cellulose nanofibers having a carboxyl group.
[4] The method for producing an aqueous suspension of cellulose nanofibers according to [3], wherein the anion-modified cellulose nanofibers are cellulose nanofibers having a carboxymethyl group and have a degree of carboxymethyl substitution in the range of 0.01 to 0.50.
[5] The method for producing an aqueous suspension of cellulose nanofibers according to any one of [1] to [4], wherein the powder containing cellulose nanofibers further contains carboxymethyl cellulose.
[6] The method for producing an aqueous suspension of cellulose nanofibers according to any one of [1] to [5], wherein the thickening substance is sucrose.
[7] The method for producing an aqueous suspension of cellulose nanofibers according to any one of [1] to [6], wherein the amount of sucrose mixed is 60% by mass or more relative to the total amount of water and sucrose.

The present invention makes it possible to efficiently produce an aqueous suspension with few residual gel particles using cellulose nanofiber powder. More specifically, even when a low-speed agitator is used during suspension, it is possible to reduce the amount of residual gel particles derived from the cellulose nanofiber powder. Since good suspension can be achieved even with a low-speed agitator, it is possible to produce a suspension with few gel particles even in factories that do not have equipment such as high-speed agitators. In addition, when a high-speed agitator is used, liquid splashing can occur, resulting in a decrease in yield, but by using a low-speed agitator, such a decrease in yield can be suppressed.

Even if the stirring is done by hand using a whisk or stirring rod without using a stirrer, the use of the present invention makes it easier to obtain a suspension with fewer residual gel particles than when the present invention is not used.

In addition, because the present invention enables efficient suspension, it is believed that the time required for suspension can also be shortened.

The present invention relates to a method for producing an aqueous suspension of cellulose nanofibers (hereinafter referred to as "CNF"). In detail, the method for producing an aqueous suspension of CNF includes a step of mixing a powder containing CNF with water, a step of mixing a substance having thickening properties with the mixture of the powder and water, and a step of stirring the mixture of the powder, water, and the substance having thickening properties, and the method has a shear viscosity of 0.25 Pa·s or more when measured at 25°C and a shear rate of 1560 (1/s). According to the present invention, even when a stirrer with a low rotation speed is used, a CNF aqueous suspension with little residual gel particles can be obtained. In addition, in the present invention, the terms "suspension" and "dispersion" are intended to have the same meaning unless otherwise specified, and are used interchangeably.

(Cellulose nanofiber)
In the present invention, cellulose nanofibers (CNF) are cellulose raw materials such as pulp that have been refined to a fiber width on the nanometer level. The fiber width (average fiber diameter) of CNF is usually about 3 nm to several hundred nm, for example, about 3 to 500 nm. In the present invention, it is preferable to use CNF with an average fiber diameter of about 3 to 100 nm, and more preferably about 3 to 20 nm. The aspect ratio is 30 or more, preferably 50 or more, and more preferably 100 or more. There is no upper limit to the aspect ratio, but it is about 500 or less.

The average fiber diameter and average fiber length of CNFs can be measured by analyzing 200 randomly selected fibers using an atomic force microscope (AFM) if the diameter is less than 20 nm, or a field emission scanning electron microscope (FE-SEM) if the diameter is 20 nm or more, and calculating the average. The aspect ratio can be calculated using the following formula:
Aspect ratio = average fiber length/average fiber diameter.

CNF can be obtained by applying mechanical force to pulp or other cellulose raw materials to make them fine (fibrillation). As the cellulose raw material, unmodified cellulose or pulp for papermaking, as described below, may be used, or chemically modified cellulose obtained by further chemically modifying pulp for papermaking, etc. may be used. Examples of chemically modified cellulose include, but are not limited to, anion-modified cellulose in which anionic groups are introduced into the cellulose chain, and cation-modified cellulose in which cationic groups are introduced. Among these, anion-modified cellulose is preferably used. Anion-modified CNF can be obtained by fibrillating anion-modified cellulose to an average fiber diameter of less than 1 μm. The defibration method is not particularly limited, and known defibration devices such as high-speed rotation, colloid mill, high-pressure, roll mill, and ultrasonic may be used. Among them, it is preferable to use a wet high-pressure or ultra-high-pressure homogenizer. Examples of anion-modified CNF include carboxyalkylated CNF with carboxyalkyl groups introduced, carboxylated CNF with carboxyl groups introduced, phosphate esterified CNF with phosphoric acid groups introduced, and sulfate esterified CNF with sulfate groups introduced. Among these, CNF with carboxyalkyl groups (carboxyalkylated CNF) or CNF with carboxyl groups (carboxylated CNF) is preferred.

(Cellulose raw material)
Cellulose as a raw material for CNF is known to originate from plants, animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), microbial products, etc., and any of them can be used in the present invention. Examples of plant-derived cellulose include wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, and 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.). In the present invention, cellulose fibers derived from plants or microorganisms are preferred, and cellulose fibers derived from plants are more preferred. The cellulose raw material may be chemically modified as described below. By miniaturizing the fiber width of the above-mentioned cellulose raw material or a cellulose raw material that has been chemically modified, such as anionically modified (chemically modified cellulose), to the nanometer level, it is possible to obtain CNF or chemically modified CNF, such as anionically modified CNF.

(CNF having carboxyalkyl groups (carboxyalkylated CNF))
An example of anion-modified CNF is carboxyalkylated CNF having a carboxyalkyl group. In this specification, carboxyalkyl group refers to -RCOOH (acid type) and -RCOOM (metal salt type). Here, R is an alkylene group such as a methylene group or an ethylene group, and M is a metal ion. Among them, CM-modified CNF having a CM group in which R is a methylene group is most preferred. Carboxyalkylated CNF can be obtained by treating a cellulose raw material with a mercerizing agent, and then treating it with a carboxyalkylating agent to obtain carboxyalkylated cellulose using a known method to introduce a carboxyalkyl group, and then defibrating the cellulose raw material.

The degree of carboxyalkyl substitution per anhydrous glucose unit of carboxyalkylated CNF is preferably 0.50 or less. The lower limit of the degree of carboxyalkyl substitution is preferably 0.01 or more. In consideration of operability, the degree of substitution is preferably 0.02 to 0.40, more preferably 0.02 to 0.35, more preferably 0.10 to 0.35, more preferably 0.15 to 0.35, and even more preferably 0.15 to 0.30. The anhydrous glucose unit means each anhydrous glucose (glucose residue) constituting cellulose, and the degree of carboxyalkyl substitution indicates the ratio of hydroxyl groups (-OH) in the glucose residues constituting cellulose that are substituted with carboxyalkyl groups (-ORCOOH or -ORCOOM) (the number of carboxyalkyl groups per glucose residue). The degree of carboxyalkyl substitution can be adjusted by controlling the reaction conditions such as the amount of mercerizing agent and the reaction time. The degree of CM substitution per glucose unit can be measured by the following method:
Approximately 2.0 g of carboxymethylated CNF (bone dry) is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of a solution of 100 mL of concentrated nitric acid in 900 mL of methanol is added and shaken for 3 hours to convert the salt-type carboxymethylated CNF to hydrogen-type carboxymethylated CNF. 1.5 g to 2.0 g of hydrogen-type carboxymethylated CNF (bone dry) is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. The hydrogen-type carboxymethylated CNF is moistened with 15 mL of 80% by mass methanol, 100 mL of 0.1 N NaOH is added, and the mixture is shaken at room temperature for 3 hours. Using phenolphthalein as an indicator, the excess NaOH is back-titrated with 0.1 N H ₂ SO _4. The degree of CM substitution (DS) is calculated by the following formula:
A = [(100 x F' - (0.1 _{N H2SO4} ) (mL) x F) x 0.1] / (bone dry mass of hydrogen-type CM-CNF (g))
DS=0.162×A/(1-0.058×A)
A: The amount of 1N NaOH required to neutralize 1 g of hydrogenated CM-CNF (mL)
F: Factor of 0.1N H ₂ SO ₄ F': Factor of 0.1N NaOH The degree of substitution of carboxyalkyl groups other than CM groups can also be measured by the same method as above.

In this specification, "CM-cellulose", which is a type of anion-modified cellulose used in the preparation of CM-CNF, and "CM-CNF" obtained by defibrating CM-cellulose, refer to those that maintain at least a part of their fibrous shape even when dispersed in water. Therefore, "CM-cellulose" and "CM-CNF" are distinguished from carboxymethyl cellulose (hereinafter referred to as "CMC"), which is a type of water-soluble polymer. When observing an aqueous dispersion of "CM-cellulose" with an electron microscope, a fibrous substance can be observed. On the other hand, when observing an aqueous dispersion of CMC, which is a type of water-soluble polymer, no fibrous substance is observed. Furthermore, when "CM-cellulose" and "CM-CNF" are measured by X-ray diffraction, peaks of cellulose type I crystals can be observed, but cellulose type I crystals are not observed in the water-soluble polymer CMC.

(CNF having carboxyl groups (carboxylated CNF))
An example of anionic CNF is carboxylated CNF having a carboxyl group. In this specification, the term "carboxyl group" refers to -COOH (acid type) and -COOM (metal salt type) (wherein M is a metal ion), or ^-COO- . Carboxylated CNF can be obtained by obtaining carboxylated cellulose using a known method of oxidizing the hydroxyl group of the pyranose ring of cellulose to a carboxyl group, and then defibrating the cellulose. Examples of the method for oxidizing cellulose include a method of oxidizing cellulose in water using an oxidizing agent in the presence of an N-oxyl compound such as 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) and a bromide and/or iodide, and a method of oxidizing cellulose by contacting a cellulose raw material with a gas containing ozone as an oxidizing agent.

The amount of carboxyl groups in carboxylated CNF is preferably 0.4 to 3.0 mmol/g, more preferably 0.6 to 2.0 mmol/g, even more preferably 1.0 to 2.0 mmol/g, and even more preferably 1.1 to 2.0 mmol/g, based on the bone dry mass of carboxylated CNF. The amount of carboxyl groups in carboxylated CNF can be adjusted by controlling reaction conditions such as the amount of oxidizing agent added and reaction time. The amount of carboxyl groups can be measured by the following method:
60 ml of a 0.5% by mass slurry (aqueous dispersion) of carboxylated CNF is prepared, and a 0.1 M aqueous hydrochloric acid solution is added to adjust the pH to 2.5. A 0.05 N aqueous sodium hydroxide solution is then added dropwise to measure the electrical conductivity until the pH reaches 11. The electrical conductivity is calculated from the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity is gradual, using the following formula:
Amount of carboxyl group [mmol/g carboxylated CNF] = a [ml] × 0.05/mass of carboxylated CNF [g].

(Phosphate-esterified CNF)
An example of anionic CNF is phosphoric acid esterified CNF. The phosphoric acid esterified CNF can be obtained by mixing a powder or an aqueous solution of a phosphoric acid compound with the above-mentioned cellulose raw material, or by adding an aqueous solution of a phosphoric acid compound to a slurry of the cellulose raw material, thereby introducing a phosphoric acid group derived from a phosphoric acid compound into cellulose to form phosphoric acid esterified cellulose, and then defibrating the cellulose. Examples of phosphoric acid compounds include phosphoric acid, polyphosphoric acid, phosphorous acid, hypophosphorous acid, phosphonic acid, polyphosphonic acid, and esters or salts thereof. Specific examples include, but are not limited to, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium phosphite, potassium phosphite, sodium hypophosphite, potassium hypophosphite, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate, and the like. One or more of these compounds can be used in combination to introduce a phosphoric acid group derived from a phosphoric acid compound into cellulose. In this specification, the phosphoric acid group derived from a phosphoric acid compound includes a phosphoric acid group, a phosphorous acid group, a hypophosphorous acid group, a pyrophosphate group, a metaphosphate group, a polyphosphate group, a phosphonic acid group, and a polyphosphonic acid group. Phosphated cellulose and phosphoric acid esterified CNF include those in which one or more of these phosphoric acid groups are introduced into the molecular chain of cellulose. When reacting a cellulose raw material with a phosphoric acid compound, it is desirable to use the phosphoric acid compound as an aqueous solution, since this allows the reaction to proceed uniformly and increases the efficiency of the introduction of the above groups, and in this case, the pH of the aqueous solution is preferably pH 3 to 7. In addition, a nitrogen-containing compound such as urea may be added.

The degree of substitution of phosphate-based groups per glucose unit in the phosphated CNF (hereinafter simply referred to as "phosphate group substitution degree") is preferably 0.001 or more and less than 0.40. The degree of substitution of phosphate groups per glucose unit can be measured by the following method:
A slurry of phosphoric acid esterified CNF with a solid content of 0.2 mass% is prepared. A strongly acidic ion exchange resin (Amberjet 1024; manufactured by Organo Corporation, conditioned) is added to the slurry in an amount of 1/10 by volume, and the slurry is shaken for 1 hour. The slurry is then poured onto a mesh with an opening of 90 μm to separate the resin and the slurry, thereby obtaining hydrogen-type phosphoric acid esterified CNF. Next, 50 μL of 0.1 N sodium hydroxide solution is added to the slurry after the ion exchange resin treatment once every 30 seconds, while measuring the change in the electrical conductivity value of the slurry. The amount of alkali (mmol) required in the region where the electrical conductivity drops sharply among the measurement results is divided by the solid content (g) in the slurry to be titrated to calculate the amount of phosphoric acid groups (mmol/g) per 1 g of hydrogen-type phosphoric acid esterified CNF. Furthermore, the degree of phosphoric acid substitution (DS) per glucose unit of phosphoric acid esterified CNF is calculated by the following formula:
DS=0.162×A/(1-0.079×A)
A: Amount of phosphate groups per gram of hydrogen phosphate esterified CNF (mmol/g).

(Sulfated CNF)
An example of anionic CNF is sulfated CNF. Sulfated CNF can be obtained by reacting the above-mentioned cellulose raw material with a sulfate compound to introduce sulfate groups derived from the sulfate compound into cellulose to form sulfated cellulose, which is then defibrated. Examples of the sulfate compound include sulfuric acid, sulfamic acid, chlorosulfonic acid, sulfur trioxide, and esters or salts of these. Of these, it is preferable to use sulfamic acid because it has low solubility in cellulose and low acidity.

For example, when sulfamic acid is used as the sulfate compound, the amount of sulfamic acid used can be adjusted appropriately taking into consideration the amount of anion groups introduced into the cellulose chain. For example, it can be used in an amount of preferably 0.01 to 50 mol, more preferably 0.1 to 3.0 mol, per 1 mol of glucose units in the cellulose molecule.

The amount of sulfate groups per glucose unit in the sulfated CNF (hereinafter simply referred to as the "amount of sulfate groups") is preferably 0.1 to 3.0 mmol/g. The amount of sulfate groups per glucose unit can be measured by the following method:
The aqueous dispersion of sulfated CNF is subjected to solvent replacement with ethanol and then t-butanol, and then freeze-dried. 15 ml of ethanol and 5 ml of water are added to 200 mg of the obtained sample, and the mixture is stirred for 30 minutes. Then, 10 ml of 0.5N aqueous sodium hydroxide solution is added, and the mixture is stirred at 70°C for 30 minutes, and further stirred at 30°C for 24 hours. Next, phenolphthalein is added as an indicator, and titration is performed with hydrochloric acid, and the following formula is used to calculate:
Amount of sulfate group [mmol/g sample]=(5-(0.1×titer of hydrochloric acid [ml]×2))/0.2.

(Cation-modified CNF)
Cationically modified CNF is CNF in which cationic groups have been introduced into the molecular chains of cellulose. Cationically modified CNF can be obtained by defibrating cationically modified cellulose obtained by introducing cationic groups into the pyranose rings of cellulose to an average fiber diameter of less than 1 μm.

Cationically modified cellulose can be obtained by a known method in which the above-mentioned carboxylated cellulose is reacted with a cationizing agent such as glycidyl trimethylammonium chloride, 3-chloro-2-hydroxypropyl trialkylammonium hydrite or its halohydrin type, and an alkali metal hydroxide catalyst (sodium hydroxide, potassium hydroxide, etc.) in the presence of water or an alcohol having 1 to 4 carbon atoms, and the resulting cationically modified cellulose can be defibrated to obtain cationically modified CNF.

The degree of cationic substitution per glucose unit in the cation-modified CNF is preferably 0.02 to 0.50. The degree of cationic substitution can be adjusted by the amount of the cationizing agent to be reacted and the composition ratio of water or alcohol having 1 to 4 carbon atoms. The degree of cationic substitution per glucose unit can be measured by the following method:
After drying the cation-modified CNF, the nitrogen content was measured using a total nitrogen analyzer (TN-10 manufactured by Mitsubishi Chemical Corporation), and the degree of cation substitution (the average value of the number of moles of substituents per mole of anhydrous glucose unit) was calculated according to the following formula:
Degree of cation substitution = (162 x N) / (1 - 151.6 x N)
N: Nitrogen content.

(Powder containing CNF)
In the manufacturing method of the present invention, a CNF aqueous suspension is manufactured using the powder containing the above-mentioned CNF. In the present invention, the term "powder" refers to a dry solid obtained by drying until the moisture content is 0 to 15% by mass. The moisture content is preferably 0 to 10% by mass. When CNF is dried until the moisture content is such, it usually tends to become difficult to redisperse, but the method of the present invention improves redispersibility, and even when a stirrer with a low rotation speed is used, the amount of remaining gel particles can be reduced. The moisture content of the powder can be calculated using the mass (absolute dry mass) of the powder after drying at 105°C for 3 hours or more and the mass before drying.

The shape of the "powder" which is a dry solid is preferably a fine, homogeneous powder, taking into consideration the ease of suspension, but it may also be in a form in which parts of the powder aggregate to form lumps, or in a granular form.

The powder containing CNF can be produced, for example, but not limited to, by drying the dispersion liquid containing CNF described above. The dispersion medium in the dispersion liquid to be dried is not particularly limited, but is preferably water, a hydrophilic organic solvent, a hydrophobic organic solvent, or a mixed solvent thereof, and more preferably water or a mixed solvent of water and a hydrophilic organic solvent. The device used for drying is not particularly limited. For example, a vacuum drum dryer, an atmospheric drum dryer, a spray dryer, a hot air dryer, a warm air dryer, etc. can be used.

The "powder containing CNF" used for suspension in the present invention may contain other components in addition to CNF. For example, it may contain a powder of a water-soluble polymer to improve redispersibility. Examples of water-soluble polymers include cellulose derivatives (CMC, methylcellulose, hydroxypropylcellulose, ethylcellulose), xanthan gum, xyloglucan, dextrin, dextran, carrageenan, locust bean gum, alginic acid, alginates, pullulan, starch, potato starch, kudzu flour, modified starch (cationized starch, phosphorylated starch, phosphate crosslinked starch, phosphate monoesterified phosphate crosslinked starch, hydroxypropyl starch, hydroxypropylated phosphate crosslinked starch, acetylated adipate crosslinked starch, acetylated phosphate crosslinked starch, acetylated oxidized starch, sodium octenylsuccinate starch, starch acetate, oxidized starch), corn starch, and the like. Chi, gum arabic, gellan gum, polydextrose, pectin, chitin, water-soluble chitin, chitosan, casein, albumin, soy protein lysate, peptone, polyvinyl alcohol, polyacrylamide, polyacrylate, polyvinylpyrrolidone, polyvinyl acetate, polyamino acid, polylactic acid, polymalic acid, polyglycerin, latex, rosin-based sizing agent, petroleum resin-based sizing agent, urea resin, melamine resin, epoxy resin, polyamide resin, polyamide-polyamine resin, polyethyleneimine, polyamine, vegetable gum, polyethylene oxide, hydrophilic crosslinked polymer, starch polyacrylic acid copolymer, tamarind gum, guar gum, colloidal silica, or a mixture of one or more of these. Among them, cellulose derivatives are preferred in terms of compatibility with CNF, and CMC and its salts are particularly preferred. It is believed that water-soluble polymers such as CMC and its salts penetrate between the fibers of CNF and increase the distance between the fibers, thereby improving redispersibility. Dextrin can also be preferably used as the water-soluble polymer.

When CMC is used as the water-soluble polymer, the CM substitution degree of the CMC is preferably in the range of 0.55 to 1.60, more preferably 0.55 to 1.10, and even more preferably 0.65 to 1.10. Note that CMC with such a CM substitution degree dissolves in water and does not maintain the cellulose-derived fibrous shape in water. As described above, there is a distinction between "CMC" as a water-soluble polymer and "CM-cellulose" which maintains a fibrous shape in water. In addition, "CM-CNF" obtained by defibrating "CM-cellulose" to an average fiber diameter of less than 1 μm is also distinguished from "CMC".

When a water-soluble polymer is blended with a powder containing CNF, the blending amount of the water-soluble polymer is preferably 5 to 300 mass% relative to the CNF (bone dry solids), more preferably 20 to 300 mass%, even more preferably 25 to 200 mass%, and even more preferably 25 to 60 mass%.

In addition, the powder containing CNF may contain components other than CNF and the water-soluble polymer, such as trace amounts of metal components, as long as the effects of the present invention are not impaired.
(Mixing process of powder containing CNF and water)
In the method of the present invention, first, a powder containing CNF is mixed with water. The mixing ratio of the powder containing CNF to water is not particularly limited. It can be said that the smaller the amount of powder, the more uniformly the powder can be dispersed in the end, but even if the amount of powder is large, it can be said that a suspension with fewer residual gel particles can be obtained compared to the case where the method of the present invention is not used. Therefore, the amount of powder to water can be appropriately set according to the application. For example, the mixing ratio of the powder when the water is 100% by mass can be about 0.1 to 10.0% by mass, or about 1.0 to 6.0% by mass, but is not limited to these ranges.

When mixing the powder containing CNF with water, it is preferable to thoroughly wet the entire powder containing CNF with water, for example by stirring with a stirrer. By first wetting the powder containing CNF with water and then mixing and stirring the thickening substance described below, it is possible to produce a CNF aqueous suspension with fewer residual gel particles compared to when no thickening substance is added or when the powder containing CNF is added to an aqueous solution of a thickening substance.

(Mixing process of substance having viscosity increasing property)
After mixing the powder containing CNF with water, a thickening substance is added to the mixture and mixed in. The thickening substance is a substance that can be dissolved in water and can achieve a high shear viscosity at a shear rate of 1560 (1/s) by dissolving a certain amount in water. Examples of the cellulose include sucrose, dextrin, tamarind seed gum, CMC, carboxyvinyl polymer, starch syrup (a mixture of maltose and maltotriose), guar gum, carrageenan, xanthan gum, locust bean gum, tara gum, cassia gum, glucomannan, gellan gum, pectin, psyllium seed gum, tragacanth gum, karaya gum, gum arabic, macrophomopsis gum, rhamsan gum, agar, alginic acids (alginic acid, alginate), curdlan, pullulan, cellulose derivatives (CMC, methylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, hydroxyethylcellulose, etc.), microcrystalline cellulose, fermented cellulose, gelatin, water-soluble soybean polysaccharides, starch, modified starch, polyethylene glycol, etc. Among these, sucrose, dextrin, tamarind seed gum, CMC, and carboxyvinyl polymer are particularly preferred.

There are no particular limitations on the amount of thickening substance mixed in. Depending on the substance, the amount should be such that the final aqueous suspension has a shear viscosity of 0.25 Pa·s or more at 25°C and a shear rate of 1560 (1/s). For example, when sucrose is used, it is preferable that the amount of sucrose mixed in is 60% by mass or more of the total of water and sucrose.

(Stirring the mixture)
By stirring a mixture of a powder containing CNF, water, and a substance having viscosity-increasing properties, the CNF in the powder is suspended in the water medium to obtain a CNF aqueous suspension. As described above, in the present invention, the terms "suspension" and "dispersion" are intended to have the same meaning unless otherwise specified, and are used interchangeably.

When stirring, a stirrer such as a homomixer, homodisper, in-line mixer, juicer, industrial powder suction continuous dissolving and dispersing device, Henschel mixer, etc. can be used, but are not limited to these. By using the method of the present invention, a suspension with fewer gel particles remaining can be obtained even when the rotation speed of the stirrer is low, compared to when the method of the present invention is not used. For example, even when stirring is performed by hand using a whisk or a stirring rod without using the above-mentioned stirrer, it can be said that a suspension with fewer gel particles remaining is more likely to be obtained than when the method of the present invention is not used. In this way, the present invention has the advantage that a suspension with fewer gel particles can be obtained even at a low rotation speed compared to when the present invention is not used. However, this does not exclude the use of a stirrer with a high rotation speed. For example, when comparing stirring at the same rotation speed, the method of the present invention may be able to obtain a suspension with fewer gel particles in a shorter time than when the present invention is not used. Therefore, the rotation speed of the stirrer during stirring in the present invention is not particularly limited. For example, while conventionally suspensions/dispersions were produced using stirring/dispersing equipment with high rotation speeds of 3,000 rpm or more, or 12,000 rpm or more, it is now possible to obtain aqueous suspensions with fewer gel particles using stirrers with rotation speeds of around 600 to 800 rpm, although this is not limited to this.

(CNF water suspension)
The CNF aqueous suspension obtained by the present invention contains a substance having a thickening property as described above, and therefore has a shear viscosity of 0.25 Pa·s or more when measured at 25°C and a shear rate of 1560 (1/s). More preferably, it is 0.30 Pa·s or more. There is no particular upper limit to the shear viscosity. Considering the ease of stirring during suspension, it is considered preferable that the shear viscosity is about 3 Pa·s or less.

The shear viscosity can be measured, for example, by using a rheometer (a viscoelasticity measuring device) as described in the Examples below.
The CNF aqueous suspension obtained according to the present invention contains at least powder-derived CNF, a water-soluble polymer if the powder contains a water-soluble polymer, and the above-mentioned substance having viscosity-increasing properties in water. Other substances may also be contained in the water as long as they do not impair the effects of the present invention.

The CNF aqueous suspension obtained according to the present invention has fewer residual CNF gel particles compared to aqueous suspensions obtained without using the method of the present invention. "Gel particles" are gel-like particles of aggregated CNF. The amount of gel particles remaining in the suspension can be evaluated, for example, using the method using a coloring material (such as ink drops) described in Patent No. 6769947.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.
<Measurement of shear viscosity>
The temperature of the aqueous suspension was adjusted to 25° C., and the shear rate was logarithmically increased within the range of 1×10 ⁻³ to 2×10 ³ to measure the 60-point viscosity at 5-second intervals using a rheometer (Physica MCRxx1 manufactured by Anton Paar). Among the measurement results, the shear viscosity at a shear rate of 1560 (1/s) is shown in the table.

<Evaluation of CNF gel particles remaining in CNF aqueous suspension>
Gel particles in a water suspension were evaluated using a method using ink drops. This method adds a coloring material (ink drops) to the CNF suspension and then observes it under an optical microscope, making it easier to check for the presence or absence of CNF aggregates (gel particles) in the suspension, which cannot be detected by the naked eye. The specific evaluation method is as follows:
Two drops of ink (Kuretake Corporation, solid content 10%) were dropped into the water suspension, and the mixture was stirred for 30 seconds by setting the rotation speed of a vortex mixer (Iuchi Automatic Lab-mixer HM-10H) to the maximum. The stirred liquid was sandwiched between two glass plates so that the film thickness became 0.15 mm, and observed at 100 times magnification using an optical microscope (KEYENCE Digital Microscope VHX-6000), and images were taken at three locations. Images in which no gel particles were observed were evaluated as "Good", images in which relatively small gel particles were observed in about 20% of the images or large gel particles were observed in about 10% of the images were evaluated as "Good", and images in which more gel particles were observed were evaluated as "Poor". Images in which there was unevenness in the observed locations and other intermediate results between Good, Good, and Bad were expressed as "Good to Good", "Poor to Good", and so on.

<Reference Example 1>
A powder in which 40% by mass of CMC was mixed with carboxylated CNF (degree of CM substitution: 0.30) (water content: 10% by mass) was used as the powder containing CNF.

150 ml of ion-exchanged water was placed in a 600 ml container, and 1.5 g of the powder was added thereto. This was stirred at 3000 rpm for 60 minutes to obtain a water suspension.
<Comparative Example 1>
An aqueous suspension was obtained in the same manner as in Reference Example 1, except that the rotation speed during stirring was changed to 600 rpm.

<Comparative Example 2>
75 ml of ion-exchanged water and sucrose (Mitsui Sugar Co., Ltd., white sugar) were added to a 600 ml container and stirred with a homodisper to prepare a 50% by mass sucrose aqueous solution. After confirming that the sucrose was dissolved, 0.75 g of the powder described in Reference Example 1 was added and stirred at 600 rpm for 60 minutes to obtain a water suspension.

<Comparative Example 3>
75 ml of ion-exchanged water and 0.75 g of the powder described in Reference Example 1 were added to a 600 ml container, and the powder was moistened by stirring at 600 rpm with a homodisper. Five minutes after the start of stirring, sucrose (Mitsui Sugar Co., Ltd., white sugar) was added with stirring to a concentration of 50% by mass based on the total amount of water and sucrose, and the mixture was further stirred for 60 minutes to obtain a water suspension.

<Comparative Example 4>
An aqueous suspension was obtained in the same manner as in Comparative Example 2, except that the concentration of the sucrose aqueous solution was changed to 60% by mass.

Example 1
An aqueous suspension was obtained in the same manner as in Comparative Example 3, except that sucrose was added to a concentration of 60% by mass based on the total amount of water and sucrose.

<Comparative Example 5>
An aqueous suspension was obtained in the same manner as in Comparative Example 2, except that the concentration of the sucrose aqueous solution was changed to 67% by mass.

Example 2
An aqueous suspension was obtained in the same manner as in Comparative Example 3, except that sucrose was added to a concentration of 67% by mass based on the total amount of water and sucrose.

The shear viscosity (25°C) of the aqueous suspensions of the obtained reference examples, comparative examples, and examples at a shear rate of 1560 (1/s) was measured by the method described above, and the CNF gel particles remaining in the aqueous suspensions were evaluated by the method described above. The results are shown in Table 1.

As can be seen from the results in Table 1, when sucrose was not added, no gel particles remained when the mixture was stirred at 3000 rpm (Reference Example 1), but many gel particles were observed when the mixture was stirred at 600 rpm (Comparative Example 1).
By adding sucrose to the stirring conditions of Comparative Example 1, the amount of remaining gel particles tended to decrease according to the amount of sucrose added (Comparative Examples 2 to 5, Examples 1 and 2).

Furthermore, when the shear viscosity of the aqueous suspension was 0.25 Pa·s or more (Comparative Examples 4 and 5, Examples 1 and 2), it was found that the gel particles were reduced more and redispersibility was improved when CNF and water were mixed first and then sucrose was added (Examples 1 and 2) compared to when CNF was added to an aqueous sucrose solution (Comparative Examples 4 and 5).

Claims (5)

A step of mixing a powder containing cellulose nanofibers with water;
A method for producing a cellulose nanofiber aqueous suspension, comprising: mixing a thickening substance into a mixture of the powder and water; and stirring the mixture of the powder, water, and thickening substance,
The cellulose nanofiber is an anion-modified cellulose nanofiber,
The substance having viscosity-increasing properties is sucrose,
The above-mentioned production method, wherein the shear viscosity of the aqueous suspension is 0.25 Pa·s or more when measured at 25° C. and a shear rate of 1560 (1/s). The method for producing an aqueous suspension of cellulose nanofibers according to claim 1 , wherein the anion-modified cellulose nanofibers are cellulose nanofibers having a carboxyalkyl group or cellulose nanofibers having a carboxyl group. The method for producing an aqueous suspension of cellulose nanofibers according to claim 2 , wherein the anion-modified cellulose nanofibers are cellulose nanofibers having a carboxymethyl group and have a degree of carboxymethyl substitution in the range of 0.01 to 0.50. The method for producing an aqueous suspension of cellulose nanofibers according to any one of claims 1 to 3 , wherein the powder containing cellulose nanofibers further contains carboxymethyl cellulose. The method for producing an aqueous suspension of cellulose nanofibers according to any one of claims 1 to 4 , wherein the amount of sucrose mixed is 60 mass% or more based on the total amount of water and sucrose.