Method for producing cellulose nanofibers

Abstract

To provide a new method for producing cellulose nanofibers using a waste mushroom bed as a raw material.SOLUTION: A method for producing cellulose nanofibers according to an embodiment includes a step of introducing a strongly acidic functional group into cellulose fibers in a waste mushroom bed by heating to 50 °C or higher, and a step of mechanically defibrating the cellulose fibers obtained in the previous step.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing cellulose nanofibers. [Background technology]

[0002] In recent years, from the perspective of sustainability, cellulose fibers, which are naturally occurring biomass, and in particular cellulose nanofibers (also known as fine fibrous cellulose) as a new form of utilization have been attracting attention.

[0003] Cellulose nanofibers are generally produced by chemically modifying raw pulp to obtain chemically modified pulp and mechanically defibrating the chemically modified pulp. For example, Patent Document 1 discloses that raw pulp is chemically modified to obtain chemically modified pulp, and the obtained chemically modified pulp is mechanically treated under conditions of a solids concentration of 15% by weight or more. In the examples of Patent Document 1, it is specifically described that bleached, unbeaten kraft pulp derived from softwood is used as a raw material, and this is TEMPO-oxidized, and the obtained TEMPO-oxidized cellulose is subjected to a beating treatment to obtain TEMPO-oxidized cellulose nanofibers having carboxyl groups as functional groups. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2020 / 059860 Summary of the Invention [Problem to be solved by the invention]

[0005] In the case of mushroom bed cultivation such as shiitake, a large amount of the mushroom bed (culture medium) is discarded after cultivation. Since the main components of the mushroom bed are sawdust and corncobs, the main component of the discarded mushroom bed is cellulose.

[0006] Conventionally, as described in Patent Document 1, chemical pulps such as kraft pulp from softwood or hardwood, and sulfite pulp from softwood or hardwood have been used as raw materials for producing cellulose nanofibers, and the use of waste paper pulp has also been proposed, but the use of waste mushroom beds was not known.

[0007] An object of an embodiment of the present invention is to provide a novel method for producing cellulose nanofibers using waste mushroom beds as a raw material. [Means for solving the problem]

[0008] The present invention includes the embodiments shown below. [1] A method for producing cellulose nanofibers, comprising the steps of: introducing strongly acidic functional groups into the cellulose fibers of a waste mushroom bed by heating to 50°C or higher; and mechanically defibrating the cellulose fibers obtained in the above step. [2] The method for producing cellulose nanofibers described in [1], wherein the step of introducing the strongly acidic functional group is carried out under acidic conditions of pH 4 or less. [3] The method for producing cellulose nanofibers according to [1] or [2], wherein the amount of the strongly acidic functional group in the cellulose nanofibers is 1.5 mmol / g or more and less than 3.0 mmol / g. [4] The method for producing cellulose nanofibers according to any one of [1] to [3], wherein the cellulose nanofibers have an average aspect ratio of 50 or more. [5] The method for producing cellulose nanofibers according to any one of [1] to [4], wherein the strongly acidic functional group is at least one selected from the group consisting of a sulfate group, a sulfo group, and a phosphate group. [Effects of the Invention]

[0009] According to an embodiment of the present invention, cellulose nanofibers can be produced using waste mushroom beds as a raw material, thereby enabling the effective use of waste mushroom beds. DETAILED DESCRIPTION OF THE INVENTION

[0010] The method for producing cellulose nanofibers according to this embodiment includes step (A) of introducing strongly acidic functional groups into the cellulose fibers of the waste mushroom bed by heating to 50°C or higher, and step (B) of mechanically defibrating the cellulose fibers obtained in step (A).

[0011] This production method makes it possible to prepare cellulose nanofibers with a small fiber diameter and a large aspect ratio, even though waste mushroom beds are used as the raw material. Specifically, the inventors' studies have revealed that when waste mushroom beds are used as the raw material and carboxyl groups are introduced into cellulose fibers, for example, by TEMPO oxidation, nanofiber formation does not proceed sufficiently, resulting in a small aspect ratio for the resulting cellulose nanofibers. In contrast, by introducing a strongly acidic functional group under heating conditions, nanofiber formation is promoted, even though waste mushroom beds are used as the raw material, and cellulose nanofibers with a small fiber diameter and a large aspect ratio can be obtained.

[0012] In this embodiment, the waste mushroom bed used as the raw material is the waste medium generated after mushroom cultivation by mushroom cultivation, i.e., the mushroom bed after cultivation. Mushrooms are not particularly limited, and examples thereof include shiitake mushroom, nameko mushroom, enoki mushroom, oyster mushroom, maitake mushroom, and maitake mushroom.

[0013] The medium (bacterial bed) used in mushroom bed cultivation is a medium made by mixing and solidifying nutrients such as bran, starch, and rice bran with a main material such as sawdust and / or corncob. In mushroom bed cultivation, a seed fungus is inoculated into the medium and cultivation is carried out. The waste mushroom bed after cultivation contains the sawdust and / or corncob, which are the raw materials for the mushroom bed, as its main components, and may contain cellulose as well as lignin, hemicellulose, etc.

[0014] In step (A), the waste mushroom bed may be a block-shaped product obtained after mushroom bed cultivation as is, but a finely pulverized product is preferably used.

[0015] In the above step (A), the waste mushroom bed is heated to 50°C or higher under conditions that introduce strongly acidic functional groups into the cellulose fibers. For example, the waste mushroom bed may be heated to 50°C or higher together with an acid containing strongly acidic functional groups, but this is not limiting. The heating temperature is not particularly limited as long as it is 50°C or higher, but is preferably 50°C to 200°C, and more preferably 55°C to 150°C. The heat treatment may be carried out, for example, in a chemical solution containing a solvent together with an acid containing strongly acidic functional groups, or in a dry state after the solvent has been evaporated.

[0016] Examples of the strongly acidic functional group include a sulfate group, a sulfo group, a phosphate group, and a nitrate group, and preferably at least one selected from the group consisting of a sulfate group, a sulfo group, and a phosphate group. These functional groups may be directly or indirectly bonded to glucose units, which are structural units of cellulose molecules. When directly bonded, for example, they may be introduced so as to have a structure in which some hydroxy groups of the cellulose fibers are substituted with the functional group. When indirectly bonded, for example, an alkylene group having 1 to 4 carbon atoms may be present between the glucose unit and the functional group.

[0017] One or more strongly acidic functional groups may be bonded to all glucose units constituting the cellulose molecule, or one or more strongly acidic functional groups may be bonded to some of the glucose units constituting the cellulose molecule.

[0018] The concept of a strongly acidic functional group encompasses not only acid forms but also salt forms, and both acid and salt forms may be present. Examples of acid forms include sulfate groups represented by -O-SO3H, sulfo groups represented by -SO3H, and phosphate groups represented by -O-PO3H2. Examples of salt forms include, but are not limited to, alkali metal salts such as sodium salts and potassium salts, alkaline earth metal salts such as magnesium salts and calcium salts, onium salts such as ammonium salts and phosphonium salts, and amine salts such as primary amines, secondary amines, and tertiary amines.

[0019] In one embodiment, sulfate groups may be introduced by a reaction between cellulose fibers and a sulfating agent (sulfate esterification reaction), for example, by immersing the waste mushroom bed in a chemical solution containing a sulfating agent. The sulfating agent may be sulfuric anhydride or an aqueous sulfuric acid solution, but sulfamic acid is preferably used.

[0020] The chemical solution used in the sulfation reaction may be a mixture of a sulfating agent and a solvent, and may or may not contain a catalyst. The solvent is not particularly limited, and examples include water; linear or branched alcohols having 1 to 12 carbon atoms, such as methanol, ethanol, propanol, butanol, octanol, and dodecanol; ketones having 3 to 6 carbon atoms, such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; linear or branched saturated or unsaturated hydrocarbons having 1 to 6 carbon atoms; aromatic hydrocarbons such as benzene and toluene; halogenated hydrocarbons such as methylene chloride and chloroform; lower alkyl ethers having 2 to 5 carbon atoms; and solvents such as dioxane, acetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, and pyridine. These solvents may be used alone or in combination.

[0021] In one embodiment, sulfo groups may be introduced by reacting cellulose fibers with a sulfonating agent, or by reacting cellulose fibers with urea and / or its derivatives together with the sulfonating agent. For example, this can be done by immersing the waste mushroom bed in a chemical solution containing the sulfonating agent and urea and / or its derivatives.

[0022] Examples of sulfonating agents include sulfamic acid, sulfamic acid salts, and sulfuryl compounds. The chemical solution used for the sulfonation reaction may be a mixture of a sulfonating agent, urea and / or its derivatives, and a solvent. Examples of the solvent include the same solvents as those used in the sulfuric acid esterification reaction.

[0023] In one embodiment, phosphate groups may be introduced by reacting the cellulose fibers with a phosphate group-containing compound, or by reacting the cellulose fibers with urea and / or its derivatives together with the phosphate group-containing compound. For example, this can be achieved by immersing the waste mushroom bed in a chemical solution containing the phosphate group-containing compound and urea and / or its derivatives.

[0024] The phosphate group-containing compound is not particularly limited and examples thereof include phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid, with sodium dihydrogen phosphate and disodium hydrogen phosphate being more preferred. A mixture of the phosphate group-containing compound, urea and / or a derivative thereof, and a solvent may be used as the chemical solution for the reaction to introduce the phosphate group. Examples of the solvent include the same solvents as those used in the sulfuric acid esterification reaction.

[0025] The reaction time in step (A) is not particularly limited, and may be, for example, 10 minutes to 10 hours, or 30 minutes to 6 hours.

[0026] In one embodiment, step (A) is preferably carried out under acidic conditions of pH 4 or less. That is, the waste mushroom bed may be heated to 50°C or higher under acidic conditions of pH 4 or less to introduce strongly acidic functional groups into the cellulose fibers of the waste mushroom bed. The acidic conditions may be strongly acidic conditions of less than pH 2. For example, the waste mushroom bed may be heated to 50°C or higher together with a strong acid such as sulfamic acid, thereby introducing strongly acidic functional groups into the cellulose fibers of the waste mushroom bed.

[0027] In step (A), after the treatment of introducing a strongly acidic functional group into the cellulose fibers under heating conditions, the waste mushroom bed after the treatment may be subjected to a washing treatment using water and / or an organic solvent. The washing treatment is a treatment for removing impurities other than the cellulose fibers contained in the waste mushroom bed.

[0028] The washing treatment may also serve as a treatment for making the waste mushroom bed after the above treatment approximately neutral. In this case, for example, a method can be adopted in which the waste mushroom bed after the above treatment is washed with pure water or the like until it becomes neutral.

[0029] In step (A), after the treatment of introducing strongly acidic functional groups into the cellulose fibers under heating conditions, a neutralization treatment may be carried out to neutralize the strongly acidic functional groups. The neutralization treatment may be carried out before or after the above-mentioned washing treatment. The neutralization treatment can be carried out using an alkali, and the pH of the aqueous dispersion containing the cellulose fibers can be adjusted to neutral or alkaline.

[0030] The alkali used for neutralization is not particularly limited, and examples thereof include alkali metal hydroxides, alkaline earth metal hydroxides, other inorganic salts, amines, onium compounds, etc. Specific examples thereof include sodium hydroxide, potassium hydroxide, calcium hydroxide, calcium acetate, magnesium hydroxide, magnesium acetate, ammonia, methylamine, dimethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, tetrabutylammonium hydroxide, tetrabutylphosphonium hydroxide, etc.

[0031] In step (B), the cellulose fibers obtained in step (A) are mechanically defibrated to obtain cellulose nanofibers. The defibration process can be carried out by treating an aqueous dispersion of cellulose fibers using, for example, a homomixer, a high-pressure homogenizer, an ultrasonic dispersion processor, a beater, a disk refiner, a conical refiner, a double-disc refiner, a grinder, or the like at high speed.

[0032] The cellulose nanofibers obtained in this manner are cellulose fibers refined to the nano-level, and therefore have a number-average fiber width of less than 1000 nm, preferably 2 to 100 nm, more preferably 2 to 50 nm, more preferably 2 to 20 nm, even more preferably 2 to 15 nm, or even 3 to 15 nm.

[0033] The number average fiber length of the cellulose nanofibers is not particularly limited, and may be, for example, 300 nm to 10 μm, 350 to 5000 nm, 400 to 2000 nm, or 450 to 1500 nm.

[0034] The average aspect ratio of the cellulose nanofibers is not particularly limited, but according to this embodiment, from the viewpoint of obtaining cellulose nanofibers with a large aspect ratio even when a waste mushroom bed is used as the raw material, it is preferably 50 or more, more preferably 70 or more, and even more preferably 100 or more. There is no particular upper limit to the average aspect ratio. In one embodiment, the average aspect ratio may be 50 to 1000, 700 to 300, or 100 to 200.

[0035] The number-average fiber width of cellulose nanofibers can be determined by observation using an atomic force microscope (AFM). The number-average fiber width is obtained by selecting at least 120 fine fibrous cellulose fibers from the AFM image, measuring their fiber widths, and calculating the arithmetic mean. The number-average fiber length is also determined by similarly selecting at least 120 fine fibrous cellulose fibers, measuring their fiber lengths, and calculating the arithmetic mean. The average aspect ratio is calculated as the ratio of the number-average fiber length to the number-average fiber width calculated in this way.

[0036] The cellulose nanofibers preferably have an amount of the strongly acidic functional group of 1.5 mmol / g or more and less than 3.0 mmol / g. Having an amount of the strongly acidic functional group of 1.5 mmol / g or more can further enhance the progress of nanofiber formation in the defibration step. The amount of the strongly acidic functional group is more preferably 1.6 to 2.5 mmol / g, and even more preferably 1.7 to 2.2 mmol / g.

[0037] The amount of strongly acidic functional groups is the amount of strongly acidic functional groups (mmol) per dry mass of cellulose nanofibers and can be measured by known methods, specifically the method described in the Examples section. Note that in this specification, "dry mass" refers to the mass after drying at 140°C until the rate of mass change per minute is 0.05% or less.

[0038] The cellulose nanofibers preferably have a cellulose type I crystal structure, which is the crystalline form of natural cellulose, and the cellulose nanofibers are insoluble in water due to the cellulose type I crystal structure.

[0039] The presence of cellulose type I crystalline structure can be identified by the presence of typical peaks at two positions, around 2θ=14° to 17° and around 2θ=22° to 23°, in the diffraction profile obtained by wide-angle X-ray diffraction image measurement.

[0040] The cellulose nanofibers obtained by this embodiment can be used in a variety of applications, and the applications are not particularly limited. For example, the cellulose nanofibers can be used as thickeners, gelling agents, adhesives, food additives, excipients, etc. in various fields such as food, beverages, cosmetics, medicines, papermaking, and paints. [Example]

[0041] Examples will be described in detail below along with comparative examples, but the present invention is not limited to these examples.

[0042] The methods for measuring the various physical properties in the examples and comparative examples are as follows.

[0043] [Functional group amount (sulfuric acid group amount, sulfo group amount)] A predetermined amount of sulfated cellulose nanofibers or sulfonated cellulose nanofibers was combusted, and the sulfur content in the combustion product was measured using a combustion ion chromatograph according to a method in accordance with IEC 62321, and the amount was converted into the amount of sulfate groups or sulfo groups.

[0044] [Amount of functional groups (amount of phosphate groups)] Phosphorylated cellulose nanofibers were diluted with ion-exchange water to a concentration of 0.2% by mass to prepare an aqueous suspension. The aqueous suspension was treated with an ion-exchange resin to produce acidic phosphorylated cellulose nanofibers, followed by alkali titration. The ion-exchange resin treatment was performed by adding 1 / 10 by volume of a strongly acidic ion-exchange resin (Amberjet 1024; Organo Corporation, conditioned) to the aqueous suspension, shaking for 1 hour, and then pouring the suspension onto a 90 μm mesh to separate the ion-exchange resin from the aqueous suspension. The alkali titration was performed by adding 50 μL of 0.1 mol / L sodium hydroxide solution to the aqueous suspension after the ion-exchange resin treatment every 30 seconds, while measuring the change in the electrical conductivity of the aqueous suspension. The amount of phosphate groups (mmol / g) was calculated by dividing the amount of alkali (mmol) required in the region corresponding to the first region of the measurement results by the solids content (g) in the aqueous suspension being titrated.

[0045] [Amount of functional groups (amount of carboxyl groups)] A 50 mL aqueous suspension of TEMPO-oxidized cellulose nanofibers with a cellulose fiber concentration of 0.1% by mass was prepared, and the pH was adjusted to approximately 2.5 with 0.1 mol / L aqueous hydrochloric acid. A 0.05 mol / L aqueous sodium hydroxide solution was then added dropwise to the aqueous suspension, and electrical conductivity measurements were performed until the pH reached approximately 11. The amount of carboxyl groups was calculated from the amount of sodium hydroxide (V) consumed during the neutralization stage of the weak acid, where electrical conductivity changes slowly, according to the following formula: Amount of carboxyl groups (mmol / g) = V (mL) × [0.05 / mass of TEMPO-oxidized cellulose nanofiber (g)]

[0046] [Measurement of number average fiber width, number average fiber length, and average aspect ratio] A cellulose nanofiber aqueous dispersion with a solid content of 0.005 to 0.0001% by mass was prepared. The dispersion was cast onto a mica substrate and dried to prepare a sample for atomic force microscopy (AFM). Images were then observed using an atomic force microscope at magnifications of 5,000x, 10,000x, or 50,000x, depending on the size of the constituent fibers. The sample and observation conditions (e.g., magnification) were adjusted so that at least 20 fibers intersected the vertical and horizontal axes of the image. After obtaining an image satisfying these conditions, two random axes were drawn vertically and horizontally on each image, and the fiber widths of the fibers intersecting the axes were visually determined. In this way, at least three non-overlapping images of the surface were taken with the atomic force microscope, and the fiber widths of the fibers intersecting the two axes were determined (thus, information on the widths of at least 20 fibers x 2 x 3 = 120 fibers was obtained). The number-average fiber width was determined by calculating the arithmetic mean of the fiber width data thus obtained. The number-average fiber length was also determined by similarly measuring the fiber lengths of at least 120 selected fine fibrous cellulose fibers and calculating the arithmetic mean. The average aspect ratio was calculated as the ratio of the number-average fiber length to the number-average fiber width thus calculated.

[0047] [Example 1] 620 g of N,N-dimethylformamide (DMF) was added to 23.9 g of sulfamic acid (purity 98.5%, manufactured by Fuso Chemical Co., Ltd.), and the mixture was stirred for 30 minutes. 20.0 g of pre-pulverized waste mushroom bed (solid content equivalent) was added to the resulting solution at room temperature (25°C). The mixture was then reacted at 55°C for 4 hours and then cooled to room temperature (25°C). The pH during the reaction was 1.0. After the reaction, the waste mushroom bed was washed with water, and the pH was adjusted to 7.6 by adding a 2N aqueous solution of sodium hydroxide as a neutralizer. The mixture was then dehydrated to obtain sulfated cellulose fibers.

[0048] 93.3 mL of water was added to 6.7 g of the obtained sulfated cellulose fibers (solid content concentration 15% by mass) to adjust the solid content to 1% by mass, and then the mixture was stirred at 10,000 rpm for 30 minutes using a homomixer (Homomixer MARK II 2.5, manufactured by Primix Corporation) to prepare sulfated cellulose nanofibers.

[0049] [Example 2] Sulfated cellulose nanofibers were prepared in the same manner as in Example 1, except that the homogenizer was replaced with a high-pressure homogenizer (Microfluidizer MH-110, manufactured by Powrex Corporation) and two passes were performed at 150 MPa.

[0050] [Example 3] A reaction solution was prepared by adding 200 g of sulfamic acid (purity 98.5%, manufactured by Fuso Chemical Industries, Ltd.) and 100 g of urea (purity 99%, manufactured by Wako Pure Chemical Industries, Ltd., model number: special grade reagent) to 1000 mL of water. To the resulting reaction solution, 20 g (dry mass) of pre-pulverized waste mushroom bed was added, resulting in a mixture of 1000 parts sulfamic acid and 500 parts urea per 100 parts waste mushroom bed. After stirring for 10 minutes with a stirrer, the resulting slurry was suction filtered using filter paper (No. 2). Suction filtration was continued until the solution stopped dripping. After suction filtration, the waste mushroom bed was removed from the filter paper and dried in a thermostatic chamber at 50 °C for 2 hours. The temperature of the thermostatic chamber was then changed to 140 °C and heated for 4 hours. The pH during the reaction was 2.0. After the heating reaction, the waste mushroom bed was washed with pure water until neutral, yielding sulfonated cellulose fibers.

[0051] 93.3 mL of water was added to 6.7 g of the obtained sulfonated cellulose fibers (solid content: 15% by mass) to adjust the solid content to 1% by mass, and then the mixture was subjected to two passes at 150 MPa using a high-pressure homogenizer (Microfluidizer MH-110, manufactured by Powrex Corporation) to prepare sulfonated cellulose nanofibers.

[0052] [Example 4] The crushed waste mushroom bed was impregnated with a mixed aqueous solution of ammonium dihydrogen phosphate and urea and squeezed to obtain 49 parts by mass of ammonium dihydrogen phosphate and 130 parts by mass of urea per 100 parts by mass of bone-dry mass of the waste mushroom bed. This was dried in a thermostatic chamber at 105 ° C to evaporate the water and pre-dry. It was then heated for 10 minutes in a thermostatic chamber set at 140 ° C to introduce phosphate groups into the cellulose fibers in the waste mushroom bed. The pH during the reaction was 4.0. After the heating reaction, 10,000 parts by mass of pure water was poured onto 100 parts by mass of bone-dry mass of the waste mushroom bed, stirred to uniformly disperse the mixture, and then washed by repeating the filtration and dehydration process three times to obtain phosphorylated cellulose fibers.

[0053] Pure water was added to the obtained phosphorylated cellulose fibers to adjust the solids concentration to 1% by mass. 12% by mass of sodium hydroxide was gradually added to the mixture to adjust the pH to 7. The mixture was then subjected to two passes at 150 MPa using a high-pressure homogenizer (Powrex Corporation, Microfluidizer MH-110) to prepare phosphorylated cellulose nanofibers.

[0054] [Comparative Example 1] To 20.0 g (bone dry mass) of crushed waste mushroom bed, 1500 mL of water, 2.5 g of sodium bromide, and 0.25 g of 2,2,6,6-tetramethylpiperidinooxy radical (TEMPO) were added and thoroughly stirred. The reaction was then initiated by adding a 13% by mass aqueous solution of sodium hypochlorite so that the amount of sodium hypochlorite was 6 mmol / g per 1.0 g of the above waste mushroom bed. The reaction was continued for 90 minutes at 20°C while adding a 0.5 N aqueous solution of sodium hydroxide dropwise to maintain the pH at 10-11 during the reaction. After the reaction, 0.1 N hydrochloric acid was added to adjust the pH to 2.0, followed by dehydration and washing with water to obtain TEMPO-oxidized cellulose fibers.

[0055] Pure water was added to the obtained TEMPO-oxidized cellulose fibers to adjust the solids concentration to 1% by mass. 12% by mass of sodium hydroxide was gradually added to the mixture to adjust the pH to 7. TEMPO-oxidized cellulose nanofibers were then prepared by two passes at 150 MPa using a high-pressure homogenizer (Powrex Corporation, Microfluidizer MH-110).

[0056] Comparative Example 2 After adding the waste mushroom bed to a DMF solution of sulfamic acid, the mixture was reacted at 40°C for 4 hours instead of at 55°C for 4 hours, and the rest of the procedure was the same as in Example 1 to prepare sulfated cellulose nanofibers.

[0057] The amount of functional groups, number average fiber width, number average fiber length, and average aspect ratio were measured for the cellulose nanofibers of Examples 1 to 4 and Comparative Examples 1 and 2. The results are shown in Table 1.

[0058] [Table 1]

[0059] As shown in Table 1, the cellulose nanofibers prepared by the production methods of Examples 1 to 4 had small fiber diameters and large aspect ratios. This is presumably because, in the step of introducing strongly acidic functional groups, functional groups were also introduced into impurities such as lignin in the waste mushroom bed, improving hydrophilicity and allowing the impurities to be efficiently removed in the subsequent washing step. In other words, it is believed that efficient removal of impurities allowed sufficient nanofiber formation to proceed in the subsequent defibration step.

[0060] In contrast, the cellulose nanofibers prepared by the production method of Comparative Example 1 had larger fiber diameters and smaller aspect ratios than those of Examples 1 to 4. This is thought to be because, in the production method of Comparative Example 1, functional groups are not introduced into impurities such as lignin contained in the waste mushroom bed during the reaction step, so the impurities are not efficiently removed and nanofiber formation does not progress sufficiently.

[0061] In the production method according to Comparative Example 2, the temperature in the reaction step for introducing sulfate groups was low, so that sulfate groups were hardly introduced and nanofiber formation did not proceed sufficiently.

[0062] The various numerical ranges described in this specification can be arbitrarily combined with their upper and lower limits, and all such combinations are considered to be preferred numerical ranges described in this specification. Furthermore, a numerical range described as "X to Y" means from X to Y.

[0063] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

Claims

1. A method for producing cellulose nanofibers, comprising: a step of introducing strongly acidic functional groups into cellulose fibers of a waste mushroom bed by heating to 50°C or higher; and a step of mechanically defibrating the cellulose fibers obtained in the above step.

2. The method for producing cellulose nanofibers according to claim 1, wherein the step of introducing the strongly acidic functional group is carried out under acidic conditions of pH 4 or less.

3. The method for producing cellulose nanofibers according to claim 1 or 2, wherein the cellulose nanofibers have an amount of the strongly acidic functional groups of 1.5 mmol / g or more and less than 3.0 mmol / g.

4. The method for producing cellulose nanofibers according to claim 1 or 2, wherein the cellulose nanofibers have an average aspect ratio of 50 or more.

5. The method for producing cellulose nanofibers according to claim 1 or 2, wherein the strongly acidic functional group is at least one selected from the group consisting of a sulfate group, a sulfo group, and a phosphate group.

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