Cellulose nanofiber manufacturing method

Abstract

To provide an efficient method for producing a cellulose nanofiber even at high concentrations.SOLUTION: There is provided an efficient method for producing a cellulose nanofiber which comprises: (A) a step of dispersing an anion-modified cellulose-based raw material in an alkaline solution to obtain a slurry in which the solid concentration of the anion-modified cellulose-based material is 6 mass% or more and less than 9 mass% and the pH is 6 to 9; (B) a step of hydrolyzing the slurried anion-modified cellulose-based raw material obtained in the step A; and (C) a step of preparing a dispersion solution containing the hydrolyzed anion-modified cellulose-based raw material obtained in the step B, followed by defibrillating the hydrolyzed anion-modified cellulose-based raw material while dispersing it in a dispersion medium to form nanofibers.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Cellulose nanofibers, obtained by pulverizing cellulose, are tiny fibers with diameters on the nano-order. They are expected to be used in a variety of fields as a new material with properties not found in ordinary pulp, such as high strength, high elasticity, and thixotropy.

[0003] Cellulose nanofibers are usually used in a variety of applications in the form of a dispersion. For example, a cellulose nanofiber dispersion is added to a resin, rubber, or the like to form a composite, thereby obtaining a composite product with improved functionality. From the viewpoint of increasing the amount of cellulose nanofibers in the composite product and reducing costs by reducing transportation costs, it is preferable that the cellulose nanofiber concentration in the cellulose nanofiber dispersion be high.

[0004] A cellulose nanofiber dispersion can be obtained by preparing a dispersion containing a cellulose-based raw material and then applying a strong shear force to the dispersion to defibrate it (see, for example, Patent Document 1, etc.). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-214717 Summary of the Invention [Problem to be solved by the invention]

[0006] When attempting to obtain a high-concentration cellulose nanofiber dispersion, one possible method is to defibrate a dispersion containing a high concentration of anion-modified cellulose-based raw material. However, even when the concentration is low, for example, around 0.3 to 0.5%, the dispersion becomes extremely viscous during the defibration process, making it difficult to carry out efficient defibration.

[0007] Patent Document 1 describes that by hydrolyzing a TEMPO-oxidized cellulose-based raw material in an alkaline solution, thickening during defibration can be suppressed even when the concentration of the cellulose-based raw material is high. Specifically, the document describes that an aqueous dispersion of a 5% TEMPO-oxidized cellulose-based raw material is hydrolyzed in an alkaline solution, washed with water, and then diluted to 2%. The resulting aqueous dispersion is then treated with an ultra-high-pressure homogenizer to obtain a cellulose nanofiber dispersion, and that the power consumption required for defibration and dispersion is low.

[0008] When the concentration of anionically modified cellulose raw materials is high (>5%), the anionically modified cellulose raw materials are converted from the metal salt form, which holds more water, to the acid form, which can absorb more water, to improve the efficiency of the dilution, washing, and dehydration processes. However, using dehydrated cakes of acid-form anionically modified cellulose raw materials makes it difficult to efficiently obtain a uniformly dispersed dispersion of anionically modified cellulose raw materials with a high solids content (>5%) in a short time. Furthermore, aqueous dispersions obtained by hydrolysis of insufficiently dispersed dispersions have high viscosity. Further defibration using an ultra-high-pressure homogenizer increases the viscosity of the dispersion containing the cellulose raw materials dramatically toward the beginning of the defibration process, making it difficult to deliver the dispersion. Furthermore, when aqueous dispersions obtained by hydrolysis of insufficiently dispersed dispersions are subjected to defibration treatment, insufficient defibration occurs in some areas, preventing uniform nano-defibration and potentially reducing the quality of the resulting cellulose nanofiber dispersion, such as reducing its transparency.

[0009] Therefore, an object of the present invention is to provide a method for producing cellulose nanofibers efficiently even when the concentration is high. [Means for solving the problem]

[0010] As a result of extensive research to achieve this object, the inventors have discovered that it is extremely effective to disperse the cellulosic raw material in an alkaline solution to form a slurry before hydrolyzing it, and have completed the present invention.

[0011] The present invention provides the following: (1) A method for producing cellulose nanofibers, comprising: (A) a step of dispersing an anionically modified cellulose raw material in an alkaline solution to obtain a slurry having a solids concentration of the anionically modified cellulose raw material of 6% by mass or more but less than 9% by mass and a pH of 6 to 9; (B) a step of hydrolyzing the slurried anionically modified cellulose raw material obtained in step A; and (C) a step of preparing a dispersion containing the hydrolyzed anionically modified cellulose raw material obtained in step B, and defibrating the hydrolyzed anionically modified cellulose raw material while dispersing it in a dispersion medium to form nanofibers. (2) A method for producing cellulose nanofibers, comprising: (A) a step of dispersing an anion-modified cellulose raw material in an alkaline solution to obtain a slurry having a solids concentration of the anion-modified cellulose raw material of 9% by mass or more and 20% by mass or less and a pH of 6 to 9; (B) a step of hydrolyzing the slurried anion-modified cellulose raw material obtained in step A; and (C) a step of preparing a dispersion containing the hydrolyzed anion-modified cellulose raw material obtained in step B, and defibrating the hydrolyzed anion-modified cellulose raw material while dispersing it in a dispersion medium to form nanofibers, wherein the hydrolysis reaction in step B is carried out at a temperature of 70 to 120°C, or the hydrolysis reaction in step B is carried out in multiple batches. (3) The method for producing cellulose nanofibers according to (1), wherein the hydrolysis reaction in step B is carried out at a temperature of 40 to 120°C. (4) The method for producing cellulose nanofibers according to (1), wherein the hydrolysis reaction in step B is carried out in multiple batches. (5) The method for producing cellulose nanofibers according to (1) or (2), wherein the anion-modified cellulose-based raw material is oxidized pulp or carboxymethylated pulp. (6) The method for producing cellulose nanofibers according to (1) or (2), wherein the cellulose nanofibers obtained in step C have an average fiber length of 500 nm or less. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a method for producing cellulose nanofibers efficiently even when the concentration is high. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below. In the present invention, "to" includes the extreme values. That is, "X to Y" includes the values ​​X and Y at both ends.

[0014] The present invention provides a method for producing cellulose nanofibers, comprising the steps of: (A) dispersing an anionically modified cellulose-based raw material in an alkaline solution to obtain a slurry having a solids concentration of the anionically modified cellulose-based raw material of 6% by mass or more but less than 9% by mass and a pH of 6 to 9; (B) hydrolyzing the slurried anionically modified cellulose-based raw material obtained in step A; and (C) preparing a dispersion containing the hydrolyzed anionically modified cellulose-based raw material obtained in step B, and defibrating the hydrolyzed anionically modified cellulose-based raw material while dispersing it in a dispersion medium to form nanofibers.

[0015] The present invention also provides a method for producing cellulose nanofibers, comprising the steps of: (A) dispersing an anion-modified cellulose raw material in an alkaline solution to obtain a slurry having a solids concentration of the anion-modified cellulose raw material of 9% by mass or more and 20% by mass or less and a pH of 6 to 9; (B) hydrolyzing the slurried anion-modified cellulose raw material obtained in step A; and (C) preparing a dispersion containing the hydrolyzed anion-modified cellulose raw material obtained in step B, and defibrating the hydrolyzed anion-modified cellulose raw material while dispersing it in a dispersion medium to form nanofibers, wherein the hydrolysis reaction in step B is carried out at a temperature of 70 to 120°C, or the hydrolysis reaction in step B is carried out in multiple batches.

[0016] (Process A) In step A, an anionically modified cellulose-based raw material is dispersed in an alkaline solution to obtain a slurry having a solids concentration of the anionically modified cellulose-based raw material of 6% by mass or more but less than 9% by mass, or 9% by mass or more but 20% by mass or less, and a pH of 6 to 9.

[0017] (anion-modified) Anionic modification refers to the introduction of anionic groups into a cellulosic raw material, specifically the introduction of anionic groups into the pyranose ring by oxidation or substitution reaction. In the present invention, the oxidation reaction refers to a reaction in which the hydroxyl group on the pyranose ring is directly oxidized to a carboxyl group. In addition, in the present invention, the substitution reaction refers to a reaction in which anionic groups are introduced into the pyranose ring by a substitution reaction other than the oxidation.

[0018] (Cellulosic raw materials) In the present invention, cellulosic raw materials for producing cellulose nanofibers are known to be derived from plants (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.), animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), microbial products, etc.), and any of these can be used in the present invention. Cellulose fibers derived from plants or microorganisms are preferred, and cellulose fibers derived from plants are more preferred.

[0019] The fiber diameter of the cellulose fiber raw material used in the present invention is not particularly limited, and the number average fiber diameter is 1 μm to 1 mm. Generally, the fiber diameter after refinement is about 50 μm. For example, when chips or the like having a size of several centimeters are refined, it is preferable to mechanically treat them with a disintegrator such as a refiner or beater to reduce the diameter to about 50 μm.

[0020] (oxidation) In the present invention, the oxidation of the cellulosic raw material can be carried out using known methods and is not particularly limited, but it is preferable to adjust the amount of carboxyl groups to 0.5 mmol / g to 3.0 mmol / g relative to the bone dry mass of the cellulose nanofiber.

[0021] As an example, cellulose can be obtained by oxidizing it in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromides, iodides, or mixtures thereof. This oxidation reaction selectively oxidizes the primary hydroxyl group at C6 of the glucopyranose ring on the cellulose surface, resulting in cellulosic fibers bearing aldehyde groups and carboxyl or carboxylate groups on the surface. The cellulose concentration during the reaction is not particularly limited, but is preferably 5% by mass or less. An N-oxyl compound is a compound capable of generating nitroxy radicals. Any compound that promotes the desired oxidation reaction can be used as the N-oxyl compound.

[0022] The amount of N-oxyl compound used is not particularly limited as long as it is a catalytic amount capable of oxidizing the raw cellulose. For example, the amount is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, and even more preferably 0.05 to 0.5 mmol per 1 g of bone-dry cellulose. The concentration of the N-oxyl compound in the reaction system is preferably about 0.1 to 4 mmol / L. Bromides are compounds containing bromine, and examples thereof include alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, and examples thereof include 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, for example, preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol per 1 g of bone-dry cellulose.

[0023] Known oxidizing agents can be used, such as halogens, hypohalous acids, halous acids, perhalogen acids or their salts, halogen oxides, and peroxides. Among these, sodium hypochlorite is preferred because it is inexpensive and environmentally friendly. The appropriate amount of oxidizing agent used is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even more preferably 1 to 25 mmol, and most preferably 3 to 10 mmol, per 1 g of bone-dry cellulose. Furthermore, for example, 1 to 40 mol is preferred per 1 mol of the N-oxyl compound.

[0024] The cellulose oxidation process can proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature is preferably 4 to 40°C, or may be room temperature, about 15 to 30°C. As the reaction proceeds, carboxyl groups are generated in the cellulose, resulting in a decrease in the pH of the reaction solution. To efficiently proceed with the oxidation reaction, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution at about 8 to 12, preferably about 10 to 11. Water is preferred as the reaction medium because it is easy to handle and does not easily cause side reactions. The reaction time for the oxidation reaction can be appropriately set depending on the degree of oxidation progress, and is usually about 0.5 to 6 hours, for example, about 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, thereby enabling efficient oxidation without reaction inhibition by the sodium chloride by-product of the first-stage reaction.

[0025] Another example of a carboxylation (oxidation) method is a method in which a cellulosic raw material is oxidized by contacting it with an ozone-containing gas. This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring, and decomposes the cellulose chain. The ozone concentration in the ozone-containing gas is 50 to 250 g / m 3 It is preferable that the thickness is 50 to 220 g / m 3It is more preferable that the ozone concentration is 0.1 to 30 parts by mass, and more preferably 5 to 30 parts by mass, relative to 100 parts by mass of the solids content of the cellulosic raw material. The ozone treatment temperature is preferably 0 to 50°C, and more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is approximately 1 to 360 minutes, and preferably approximately 30 to 360 minutes. When the ozone treatment conditions are within these ranges, excessive oxidation and decomposition of cellulose can be prevented, resulting in a good yield of oxidized cellulose. After the ozone treatment, a post-oxidation treatment may be performed using an oxidizing agent. The oxidizing agent used in the post-oxidation treatment is not particularly limited, but examples include chlorine-based compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. For example, the post-oxidation treatment can be performed by dissolving these oxidizing agents in water or a polar organic solvent such as alcohol to prepare an oxidizing agent solution, and then immersing the cellulosic raw material in the solution.

[0026] The amount of carboxy groups, carboxylate groups, and aldehyde groups in cellulosic fibers can be adjusted by controlling the amount of oxidizing agent added and the reaction time. For example, the amount of carboxy groups can be measured by preparing 60 mL of a 0.5% by mass slurry (aqueous dispersion) of oxidized cellulose, adjusting the pH to 2.5 by adding 0.1 M aqueous hydrochloric acid, and then measuring the electrical conductivity by adding 0.05 N aqueous sodium hydroxide dropwise until the pH reaches 11. The amount of carboxy groups can be calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity is gradual: Amount of carboxyl groups [mmol / g oxidized cellulose or cellulose nanofiber] = a [mL] x 0.05 / mass of oxidized cellulose [g]

[0027] (carboxymethylation) In the present invention, carboxymethylation of cellulosic raw materials can be carried out using known methods and is not particularly limited. However, it is preferable to adjust the degree of carboxymethyl substitution per anhydroglucose unit of cellulose to 0.01 to 0.50. One example of such a method is the following production method, but synthesis may be performed using a conventionally known method or a commercially available product. Cellulose is used as the starting material, and 3 to 20 times by mass of water and / or a lower alcohol, specifically methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, tertiary butanol, etc., is used as the solvent, either alone or in a mixture of two or more of these. The lower alcohol is used in an amount of 60 to 95% by mass. As the mercerizing agent, 0.5 to 20 times the molar amount of an alkali metal hydroxide, specifically sodium hydroxide or potassium hydroxide, is used per anhydroglucose residue of the starting material. The starting material, solvent, and mercerizing agent are mixed and subjected to mercerization treatment at a reaction temperature of 0 to 70°C, preferably 10 to 60°C, for a reaction time of 15 minutes to 8 hours, preferably 30 minutes to 7 hours. Thereafter, a carboxymethylating agent is added at 0.05 to 10.0 times the moles per glucose residue, and an etherification reaction is carried out at a reaction temperature of 30 to 90°C, preferably 40 to 80°C, for a reaction time of 30 minutes to 10 hours, preferably 1 hour to 4 hours.

[0028] The degree of carboxymethyl substitution per glucose unit can be measured, for example, by the following method. 1) Accurately weigh out approximately 2.0 g of carboxymethylated cellulose fiber (bone dry) and place it in a 300 mL Erlenmeyer flask with a stopper. 2) Add 100 mL of a solution of 100 mL of concentrated nitric acid in 1000 mL of methanol and shake for 3 hours to convert the carboxymethyl cellulose salt (carboxymethylated cellulose) into hydrogenated carboxymethylated cellulose. 3) Accurately weigh out 1.5 to 2.0 g of hydrogenated carboxymethylated cellulose (bone dry) and place it in a 300 mL Erlenmeyer flask with a stopper. 4) Wet the hydrogenated carboxymethylated cellulose with 15 mL of 80% methanol, add 100 mL of 0.1 N NaOH, and shake for 3 hours at room temperature. 5) Using phenolphthalein as an indicator, back-titrate the excess NaOH with 0.1 N H2SO4. 6) Calculate the degree of carboxymethyl substitution (DS) using the following formula: A = [(100 × F' - (0.1N H2SO4) (mL) × F) × 0.1] / (bone-dry mass of hydrogenated carboxymethyl cellulose (g)) DS=0.162×A / (1-0.058×A) A: Amount of 1N NaOH (mL) required to neutralize 1 g of hydrogenated carboxymethyl cellulose F': Factor of 0.1N H2SO4 F: Factor of 0.1N NaOH

[0029] In the present invention, the anion-modified cellulose-based raw material dispersed in the alkaline solution in step A may be a metal salt-type anionic group (e.g., -COONa) that is used as is. However, from the viewpoint of facilitating high concentration and easy concentration adjustment, it is preferable to use an anion-modified cellulose-based raw material in the acid form (hereinafter sometimes referred to as "H-type") that has been converted to an acid form (e.g., -COOH) by acid treatment with a mineral acid and has reduced hydrophilicity.

[0030] In step A, the anion-modified cellulose-based raw material, preferably in the acid form, is dispersed in an alkaline solution for the purpose of simultaneously diluting and neutralizing the raw material. Any alkali can be used as long as it is water-soluble, but sodium hydroxide is optimal from the viewpoint of production costs. The concentration of the alkaline solution can be adjusted within a range that can neutralize the resulting slurry and achieve the desired solids concentration of the anion-modified cellulose-based raw material. However, if the concentration of the alkaline solution is too high, portions with an extremely high pH may be formed, which may cause yellowing of the pulp or excessive viscosity reduction. Therefore, it is preferable to use an alkaline solution of less than 5%. The pH of the slurry obtained after neutralization is 6 to 9, preferably 7 to 8.

[0031] In addition, when attempting to obtain a slurry of an acid-type anion-modified cellulose-based raw material with a solids concentration of 6% by mass or more, a method can be considered in which the acid-type anion-modified cellulose-based raw material is dehydrated to a solids concentration of, for example, about 20% by mass, diluted with the amount of water necessary to achieve the desired concentration, such as 10% by mass, and then neutralized by adding an alkali. However, even if an acid-type anion-modified cellulose-based raw material with a solids concentration of 20% by mass is gradually added to dilution water and stirred using this method, the viscosity increases significantly once the solids concentration exceeds 5% by mass, and the mixture becomes pasty rather than slurry-like, making it impossible to obtain a homogenized slurry. Furthermore, adding an alkali to the paste-like mixture also fails to obtain a homogenized slurry. The reason for this is thought to be that at high concentrations, such as 10% by mass, the acid-type anion-modified cellulose-based raw material absorbs water and swells to form a paste-like substance without forming a slurry, and therefore, even if a high-concentration alkali is subsequently added, only a portion of the alkali comes into contact with the raw material, making homogenous mixing and neutralization difficult. On the other hand, as in the present invention, when preferably an acid-type anion-modified cellulose-based raw material is dispersed in an alkaline solution, preferably a diluted sodium hydroxide aqueous solution, instead of pure water, and neutralization is simultaneously performed, even when the desired solid content is as high as 10% by mass, the contact between the acid-type anion-modified cellulose-based raw material and sodium hydroxide is mild and the contact area is large, so that sodium hydroxide quickly penetrates into the cellulose-based raw material, and a homogenized slurry can be obtained. Furthermore, metal salt-type anion-modified cellulose-based raw material, such as Na, has a lower viscosity than swollen acid-type anion-modified cellulose-based raw material. Therefore, when step A of the present invention is performed, a high-concentration acid-type anion-modified cellulose-based raw material is easily converted into a low-viscosity Na-type anion-modified cellulose-based raw material, making it possible to obtain a homogenized slurry even at a high concentration.

[0032] (Process B) In step B, the slurried anion-modified cellulosic raw material obtained in step A is hydrolyzed.

[0033] In step B, water is preferably used as the reaction medium to suppress side reactions. In step B, an oxidizing agent or a reducing agent is preferably used as an auxiliary. The oxidizing agent or reducing agent may be active in the alkaline range of pH 8 to 14. Examples of oxidizing agents include oxygen, ozone, hydrogen peroxide, and hypochlorite, and two or more of these may be used in combination. However, when using an oxidizing agent that generates radicals, such as ozone, the generated radicals may cause a problem of discoloration of the anion-modified cellulose-based raw material after hydrolysis. Therefore, oxygen, hydrogen peroxide, hypochlorite, and the like, which are less likely to generate radicals, are preferred as oxidizing agents for use in the present invention. Hydrogen peroxide is particularly preferred from the viewpoint of preventing discoloration. It is more preferable not to use these oxidizing agents that generate radicals, such as ozone, and it is more preferable to use hydrogen peroxide alone. Examples of reducing agents for use in the present invention include sodium borohydride, hydrosulfite, and sulfite, and two or more of these may be used in combination. From the viewpoint of reaction efficiency, the amount of the auxiliary added is preferably 0.1 to 10 mass %, more preferably 0.3 to 5 mass %, and even more preferably 0.5 to 2 mass %, based on the solid content of the anion-modified cellulose-based raw material.

[0034] The pH of the reaction solution at the start of the hydrolysis reaction is preferably 8 to 14, more preferably 9 to 13, and even more preferably 10 to 12. If the pH is less than 8, sufficient hydrolysis may not occur. If the pH exceeds 14, hydrolysis will proceed, but the oxidized cellulose raw material after hydrolysis may become discolored. The alkali used to adjust the pH may be water-soluble, but from the viewpoint of production costs, sodium hydroxide is optimal.

[0035] The temperature conditions for carrying out the hydrolysis reaction are related to the solids concentration of the anion-modified cellulose-based raw material in the slurry subjected to the hydrolysis reaction, but from the viewpoint of reaction efficiency, the temperature is preferably 40 to 120°C, more preferably 50 to 100°C, and even more preferably 50 to 80°C. If the temperature is too low, hydrolysis may not occur sufficiently. On the other hand, if the temperature is too high, hydrolysis proceeds, but the oxidized cellulose-based raw material may become discolored after hydrolysis. The hydrolysis reaction time is preferably 0.5 to 24 hours, more preferably 1 to 10 hours, and even more preferably 2 to 6 hours.

[0036] The hydrolysis reaction may be carried out once or in multiple steps. When the hydrolysis reaction is carried out in multiple steps, the upper limit of the number of reactions is not particularly limited, but from the viewpoint of productivity, three times is preferable, and two times is more preferable.

[0037] When the solids concentration of the anion-modified cellulose-based raw material in the slurry to be subjected to the hydrolysis reaction is 9% by mass or more but less than 20% by mass, and when the hydrolysis reaction is carried out only once rather than divided into multiple steps, the temperature condition must be 70 to 120°C, preferably 70 to 80°C, from the viewpoints of reaction efficiency and prevention of coloration and excessive viscosity reduction. When the hydrolysis reaction is carried out in two or more steps, the temperature is preferably 40 to 120°C, more preferably 50 to 100°C, and even more preferably 50 to 80°C.

[0038] When step B is carried out, the anionically modified cellulose-based raw material is converted into short fibers by a hydrolysis reaction. In this way, by shortening the fiber length of the anionically modified cellulose-based raw material, the viscosity of a dispersion containing the raw material can be reduced. Note that, while the cellulose-based raw material is likely to turn yellow when simply hydrolyzed under alkaline conditions, using hydrogen peroxide or the like as an oxidizing agent is preferable because coloring is less likely to occur.

[0039] (Process C) In step C, a dispersion containing the hydrolyzed anion-modified cellulose-based raw material obtained in step B (hereinafter also referred to as "cellulose-based raw material obtained in step B") is prepared, and the hydrolyzed anion-modified cellulose-based raw material is dispersed in a dispersion medium and defibrated to form nanofibers. "Nanofiberization" refers to processing the cellulose-based raw material into cellulose nanofibers, which are single cellulose microfibrils with fiber diameters of approximately 2 to 5 nm. For ease of handling, water is preferred as the dispersion medium. To defibrate the cellulose-based raw material obtained in step B while dispersing it in a dispersion medium, it is preferable to apply a strong shear force to the dispersion using a device such as a high-speed rotation device, a colloid mill device, a high-pressure device, a roll mill device, or an ultrasonic device. In particular, it is preferable to apply a pressure of 50 MPa or more to the dispersion, and to use a wet high-pressure or ultra-high-pressure homogenizer capable of applying a strong shear force. The pressure is more preferably 100 MPa or more, and even more preferably 140 MPa or more. This treatment defibrates the cellulosic raw material obtained in step B to form cellulose nanofibers, and the cellulose nanofibers are dispersed in the dispersion medium.

[0040] The cellulose nanofibers obtained in step C of the present invention use an anion-modified cellulose-based raw material whose fiber length has been shortened by hydrolysis, and therefore have an average fiber length that is shorter than that of those using a cellulose-based raw material that has not been hydrolyzed, and is preferably 500 nm or less, and more preferably 400 nm or less.

[0041] In the present invention, the concentration of the cellulosic raw material obtained in step B in the dispersion liquid to be subjected to defibration is preferably 6 to 20% by mass. In the present invention, since a hydrolysis treatment is performed in step B, even if the concentration of the cellulosic raw material obtained in step B is increased in this manner, the viscosity of the system does not increase during the defibration treatment.

[0042] (low viscosity treatment) In the present invention, between step A and step B or between step B and step C, the anionically modified cellulose-based raw material obtained in step A or the hydrolyzed anionically modified cellulose-based raw material obtained in step B (hereinafter collectively referred to as "cellulose-based raw material obtained in step A or B") may be subjected to a viscosity reduction treatment by a method other than that of step B. The viscosity reduction treatment is a method of further appropriately cleaving the cellulose chains of the cellulose-based raw material obtained in step A or B (reducing the cellulose chains to shorter fibers). Since the material treated in this manner has a lower viscosity when made into a dispersion, the viscosity reduction treatment can also be said to be a treatment for obtaining a cellulose-based raw material that gives a low-viscosity dispersion. The viscosity reduction treatment may be any treatment that reduces the viscosity of the cellulose-based raw material obtained in step A or B, and examples thereof include a treatment of irradiating the cellulose-based raw material obtained in step A or B with ultraviolet light, a treatment of oxidatively decomposing the cellulose-based raw material with hydrogen peroxide and ozone, a treatment of hydrolyzing the cellulose-based raw material with an acid, and combinations thereof.

[0043] (Cellulose nanofiber) The cellulose nanofibers produced by the present invention are single cellulose microfibrils with an average fiber diameter of approximately 2 to 5 nm and an average fiber length of approximately 100 to 5,000 nm, preferably 500 nm or less, and more preferably 400 nm or less. The cellulose nanofibers obtained by the present invention have a B-type viscosity (60 rpm, 25°C) of 100 mPa·s or less, preferably 50 mPa·s or less, and more preferably 30 mPa·s or less in an aqueous dispersion at a concentration of 1.0% (w / v). The B-type viscosity in an aqueous dispersion at a concentration of 5% (w / v) is preferably 10,000 mPa·s or less, more preferably 6,000 mPa·s or less. The B-type viscosity (60 rpm, 25°C) in an aqueous dispersion at a high concentration (6 to 20% (w / v)) is 100,000 mPa·s or less, preferably 40,000 mPa·s or less, and more preferably 30,000 mPa·s or less. If the B-type viscosity of the high-concentration aqueous dispersion before defibration is 30,000 mPa·s or less, it will have excellent miscibility with aqueous sodium hydroxide solutions, and if it is 20,000 mPa·s or less, it will be easy to uniformly neutralize. The lower limit of the B-type viscosity is not particularly limited, but is usually about 1 mPa·s or more, or 5 mPa·s or more. The B-type viscosity can be measured using a conventional B-type viscometer, such as a Brookfield Viscometer DV-1 Prime manufactured by Eiko Seiki Co., Ltd., at 25°C and 60 rpm.

[0044] According to the production method of the present invention, cellulose nanofibers can be well dispersed in a dispersion medium, reducing light diffusion and resulting in highly transparent cellulose nanofiber dispersions. The cellulose nanofibers obtained by the present invention preferably have a transparency (transmittance at a wavelength of 660 nm) of 80% or more in a 1.0% (w / v) aqueous dispersion, more preferably 85% or more, and even more preferably 88% or more. Furthermore, the transparency (transmittance at a wavelength of 660 nm) in a 5.0% (w / v) aqueous dispersion is preferably 50% or more, more preferably 60% or more, and even more preferably 65% ​​or more. Measurement of transparency in high-concentration (6-20% (w / v)) aqueous dispersions is difficult due to the current technology's difficulty in completely removing air bubbles from high-viscosity slurries and filling them into cells; however, higher transparency is preferable. A 1.0% aqueous dispersion with a transparency of 85% or more can be used without problems in applications where contamination by foreign matter is undesirable, such as resin composites, paint composites, and optical applications. Specifically, it can be determined by using a visible spectrophotometer to measure the amount of light transmitted through a test piece in which a dispersion of a predetermined concentration is placed in a quartz cell (light path 10 mm).

[0045] The cellulose nanofibers produced by the present invention are preferably those that are lightly colored. Colored cellulose nanofibers can have low strength. Furthermore, for example, when a coating material containing lightly colored cellulose nanofibers is applied to a transparent film and dried, the coating material is less likely to discolor (color) due to the heat during drying, which has the advantage of producing a transparent film with little appearance defects. Such cellulose nanofibers can be obtained by using an oxidizing agent or reducing agent in step B, particularly by using hydrogen peroxide, which does not easily generate radicals, as the oxidizing agent.

[0046] The cellulose nanofibers produced by the present invention have excellent fluidity and transparency, as well as excellent barrier properties and heat resistance, and therefore can be used for a variety of applications other than those mentioned above, such as dispersion materials and packaging materials. [Example]

[0047] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In addition, unless the method for measuring / calculating each value in each example is specifically stated, it was measured / calculated by the method described in the specification.

[0048] (Method for measuring the amount of carboxyl groups) 60 mL of a 0.5% by mass slurry (aqueous dispersion) of oxidized pulp was prepared, and a 0.1 M aqueous hydrochloric acid solution was added to adjust the pH to 2.4. After that, a 0.05 N aqueous sodium hydroxide solution was added dropwise, and the electrical conductivity was measured until the pH reached 11. The electrical conductivity was calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of a weak acid, where the change in electrical conductivity was gradual: Amount of carboxyl groups [mmol / g oxidized pulp] = a [mL] × 0.05 / mass of oxidized pulp [g].

[0049] (Measurement of average fiber diameter and average fiber length of CNF) The average fiber diameter and average fiber length of the CNFs obtained in the examples and comparative examples were analyzed using an atomic force microscope (AFM) for 50 randomly selected fibers.

[0050] (Measurement of Brookfield viscosity) The Brookfield viscosity of the CNF dispersions obtained in the examples and comparative examples, as well as the CNF dispersions obtained by diluting them to a predetermined concentration, was determined by measuring the viscosity after 3 minutes at 60 rpm at 25°C using a Brookfield viscometer (manufactured by Eiko Seiki Co., Ltd.) The results are shown in Table 1.

[0051] (Transparency measurement) The transparency of the CNF dispersions obtained in the examples and comparative examples, as well as the CNF dispersions obtained by diluting them to a predetermined concentration, was determined by measuring the transparency (transmittance of 660 nm light) using a visible spectrophotometer ASV11D (manufactured by AS ONE Corporation). The results are shown in Table 1.

[0052] (Production Example 1) (Production of TEMPO oxidized pulp 1) Bleached dissolving kraft pulp (Buckeye DKP) derived from softwood was disintegrated in a pulper to produce a pulp slurry. To this pulp slurry, 0.05 mmol of TEMPO, 1.0 mmol of sodium bromide, and 5.5 mmol of sodium hypochlorite were added per gram of pulp solids. The pH was then adjusted to 10.0 ± 0.2 by adding sodium hydroxide and maintained for 2 hours to produce TEMPO-oxidized pulp 1 with an introduced carboxyl group content of 1.6 mmol / g. Hydrochloric acid was then added to adjust the pH to 2.4, followed by deliquor, dilution with water, and dehydration twice. This resulted in a dehydrated cake of H-type TEMPO-oxidized pulp 1 with a solids concentration of approximately 20% by mass.

[0053] (Production Example 2) (Production of TEMPO oxidized pulp 2) Bleached softwood pulp (NBKP, manufactured by Nippon Paper Industries Co., Ltd.) was disintegrated in a pulper to produce a pulp slurry. To this pulp slurry, 0.05 mmol of TEMPO, 1.0 mmol of sodium bromide, and 5.5 mmol / g of sodium hypochlorite were added per gram of pulp solids. The pH was then adjusted to 10.0 ± 0.2 by adding sodium hydroxide and maintained for 2 hours to obtain TEMPO-oxidized pulp 2 with an introduced carboxyl group content of 1.4 mmol / g. Hydrochloric acid was then added to adjust the pH to 2.4, followed by deliquor, dilution with water, and dehydration twice. A dehydrated cake of H-type TEMPO-oxidized pulp 2 with a solids concentration of approximately 20% by mass was obtained.

[0054] Example 1 A dilute sodium hydroxide solution of approximately 0.85% (w / w) was prepared by dissolving sodium hydroxide in an amount slightly less than the amount equivalent to the carboxyl group content of the H-type TEMPO-oxidized pulp of Production Example 1 in a volume of water slightly less than that required to adjust the pulp solids concentration to approximately 8%. The dehydrated cake of H-type TEMPO-oxidized pulp 1 obtained in Production Example 1 was diluted into this dilute sodium hydroxide solution while stirring. The mixture was homogenized by continuing to stir for several minutes to form a flowing TEMPO-oxidized pulp slurry. The mixture was then neutralized to a pH of 7.5±0.5 by adding 4.8% (w / w) aqueous sodium hydroxide solution while stirring, yielding a TEMPO-oxidized pulp slurry with a pulp solids concentration of 8.1% by mass. The mixture was heated to 50°C, and the pH was adjusted to 10-11 by adding 2% hydrogen peroxide (based on the pulp solids) and 0.9% sodium hydroxide (based on the pulp solids). This aqueous dispersion was heated at 50°C for 2 hours to obtain a flowing, short-fibered TEMPO-oxidized pulp dispersion with a pulp solids concentration of 8.1% by mass. The resulting short-fibered TEMPO-oxidized pulp dispersion (pre-defibration temperature 20-40°C) was then subjected to five passes of defibration using an ultra-high-pressure homogenizer at 150 MPa to obtain a short-fibered CNF dispersion with a solids concentration of 8.1% by mass, an average fiber length of 243 nm, and an average fiber diameter of 5.0 nm. The resulting CNF dispersion was also measured for its B-type viscosity, transparency, and pH. Furthermore, the B-type viscosity, transparency, and pH were also measured when the dispersion was diluted with water to solids concentrations of 5% and 1% by mass, respectively.

[0055] Example 2 A dilute sodium hydroxide solution of approximately 1.2% (w / w) was prepared by dissolving sodium hydroxide in an amount slightly less than the amount equivalent to the carboxyl group content of the H-type TEMPO-oxidized pulp of Production Example 1 in a volume of water slightly less than that required to adjust the pulp solids concentration to approximately 10%. The dehydrated cake of H-type TEMPO-oxidized pulp 1 obtained in Production Example 1 was diluted into this dilute sodium hydroxide solution while stirring. The mixture was homogenized by stirring for several minutes to form a flowing TEMPO-oxidized pulp slurry. Subsequently, the mixture was neutralized to a pH of 7.5±0.5 by adding 4.8% (w / w) aqueous sodium hydroxide solution while stirring, yielding a TEMPO-oxidized pulp slurry with a pulp solids concentration of 10.7% by mass. The slurry was heated to 50°C, and the pH was adjusted to 10-11 by adding 2% by mass of hydrogen peroxide and 0.9% by mass of sodium hydroxide based on the pulp solids. This aqueous dispersion was then heated at 50°C for 2 hours. As the hydrolysis reaction progressed, the pH decreased to approximately 7–8. Subsequently, hydrogen peroxide was added again at 2% by mass relative to the pulp solids content, and sodium hydroxide at 0.9% by mass relative to the pulp solids content, adjusting the pH to 10–11. This aqueous dispersion was heated at 50°C for 2 hours to obtain a flowing, short-fibered TEMPO-oxidized pulp dispersion with a pulp solids concentration of 10.7% by mass. The resulting short-fibered TEMPO-oxidized pulp dispersion (pre-defibration temperature 20–40°C) was then subjected to five passes of defibration treatment using an ultra-high-pressure homogenizer at 150 MPa, yielding a short-fibered CNF dispersion with a solids concentration of 10.1% by mass, an average fiber length of 251 nm, and an average fiber diameter of 4.3 nm. The resulting CNF dispersion was also measured for its B-type viscosity, transparency, and pH. Furthermore, the B-type viscosity, transparency, and pH were measured when the dispersion was diluted with water to solids concentrations of 5% and 1% by mass, respectively.

[0056] Example 3 A dilute sodium hydroxide solution of approximately 1.2% (w / w) was prepared by dissolving sodium hydroxide in an amount slightly less than the amount equivalent to the carboxyl group content of the H-type TEMPO-oxidized pulp of Production Example 1 in a volume of water slightly less than that required to adjust the pulp solids concentration to approximately 10%. The dehydrated cake of H-type TEMPO-oxidized pulp 1 obtained in Production Example 1 was diluted into this dilute sodium hydroxide solution while stirring. The mixture was homogenized by stirring for several minutes to form a flowing TEMPO-oxidized pulp slurry. The mixture was then neutralized to a pH of 7.5±0.5 by adding 4.8% (w / w) aqueous sodium hydroxide solution with stirring, yielding a TEMPO-oxidized pulp slurry with a pulp solids concentration of 10.9% by mass. The resulting slurry was heated to 80°C, and the pH was adjusted to 10-11 by adding 2% by mass of hydrogen peroxide and 0.9% by mass of sodium hydroxide based on the pulp solids. This aqueous dispersion was heated at 80°C for 2 hours to obtain a flowing, short-fibered TEMPO-oxidized pulp dispersion with a pulp solids concentration of 10.9% by mass. The resulting short-fibered TEMPO-oxidized pulp dispersion (pre-defibration temperature 20-40°C) was then subjected to five passes of defibration using an ultra-high-pressure homogenizer at 150 MPa to obtain a short-fibered CNF dispersion with a solids concentration of 10.4% by mass, an average fiber length of 240 nm, and an average fiber diameter of 4.5 nm. The resulting CNF dispersion was also measured for its B-type viscosity, transparency, and pH. Furthermore, the B-type viscosity, transparency, and pH were also measured when the dispersion was diluted with water to solids concentrations of 5% and 1% by mass, respectively.

[0057] (Comparative Example 1) The dehydrated cake of H-type TEMPO-oxidized pulp 2 obtained in Production Example 2 was diluted to approximately 3% by mass by stirring in water. A TEMPO-oxidized pulp slurry was obtained, which was homogenized within a few seconds of stirring. A 4.8% (w / w) aqueous sodium hydroxide solution was added to the slurry to neutralize it to a pH of 7.5±0.5, yielding a TEMPO-oxidized pulp slurry with a pulp solids concentration of 3.1% by mass. This slurry was subjected to two passes of defibration using an ultra-high-pressure homogenizer at 150 MPa (temperature before defibration: 20-40°C), yielding a CNF dispersion with a solids concentration of 3.1% by mass, an average fiber length of 616 nm, and an average fiber diameter of 3.0 nm. The resulting CNF dispersion was also measured for its B-type viscosity, transparency, and pH. Furthermore, the B-type viscosity, transparency, and pH were measured when the dispersion was diluted with water to a solids concentration of 1% by mass.

[0058] (Comparative Example 2) The dehydrated cake of H-type TEMPO-oxidized pulp 1 obtained in Production Example 1 was diluted to approximately 5% by mass by stirring in water. A TEMPO-oxidized pulp slurry was obtained, which was homogenized within a few seconds of stirring. A 4.8% (w / w) aqueous sodium hydroxide solution was added to the slurry to neutralize it to a pH of 7.5±0.5, yielding a TEMPO-oxidized pulp slurry with a pulp solids concentration of 5.1% by mass. The slurry was heated to 50°C with stirring, and hydrogen peroxide was added at 2% by mass (based on the pulp solids) and sodium hydroxide at 0.9% by mass (based on the pulp solids) to adjust the pH to 10-11. This aqueous dispersion was heated at 50°C for 2 hours to obtain a flowing, short-fiber TEMPO-oxidized pulp dispersion with a pulp solids concentration of 5.1% by mass. The resulting TEMPO-oxidized pulp dispersion (temperature before defibration: 20-40°C) was then subjected to five passes of defibration using a wet cavitation device at 150 MPa, yielding a shortened CNF dispersion with a solids concentration of 5.1% by mass, an average fiber length of 271 nm, and an average fiber diameter of 4.5 nm. The resulting CNF dispersion was then measured for B-type viscosity, transparency, and pH. Furthermore, the B-type viscosity, transparency, and pH were also measured when the dispersion was diluted with water to a solids concentration of 1% by mass.

[0059] (Comparative Example 3) A dilute sodium hydroxide solution of approximately 1.2% (w / w) was prepared by dissolving sodium hydroxide in an amount slightly less than the amount equivalent to the carboxyl group content of the H-type TEMPO-oxidized pulp of Production Example 1 in a volume of water slightly less than that required to adjust the pulp solids concentration to approximately 10%. The dehydrated cake of H-type TEMPO-oxidized pulp 1 obtained in Production Example 1 was diluted into this dilute sodium hydroxide solution while stirring. The mixture was homogenized by stirring for several minutes to form a flowing TEMPO-oxidized pulp slurry. The mixture was then neutralized to a pH of 7.5±0.5 by adding 4.8% (w / w) aqueous sodium hydroxide solution with stirring, yielding a TEMPO-oxidized pulp slurry with a pulp solids concentration of 10.9% by mass. The resulting slurry was heated to 50°C, and the pH was adjusted to 10-11 by adding 2% by mass of hydrogen peroxide and 0.9% by mass of sodium hydroxide based on the pulp solids. This aqueous dispersion was heated at 50°C for 2 hours to obtain a flowing, short-fibered TEMPO-oxidized pulp dispersion with a pulp solids concentration of 10.9% by mass. The resulting short-fibered TEMPO-oxidized pulp dispersion (pre-defibration temperature 20-40°C) was then defibrated using an ultra-high-pressure homogenizer at 150 MPa. After the first pass, the viscosity increased dramatically, making it impossible to pump the liquid during the second pass, and the process was discontinued. As a result, it was not possible to obtain CNF with the desired number of passes.

[0060] Comparative Example 4 An attempt was made to obtain a slurry with a pulp solids concentration of 10% by mass by diluting the dehydrated cake of H-type TEMPO-oxidized pulp 1 obtained in Production Example 1 by stirring it into water. However, when the concentration exceeded 5% by mass, the viscosity increased dramatically, and the slurry became paste-like rather than slurry-like, and a TEMPO-oxidized pulp slurry could not be obtained.

[0061] [Table 1]

Claims

1. (A) dispersing an anionically modified cellulose-based raw material in an alkaline solution to obtain a slurry having a solids concentration of the anionically modified cellulose-based raw material of 6% by mass or more but less than 9% by mass and a pH of 6 to 9; (B) a step of hydrolyzing the slurried anion-modified cellulosic raw material obtained in the step A; (C) preparing a dispersion containing the hydrolyzed anion-modified cellulose-based raw material obtained in step B, and defibrating the hydrolyzed anion-modified cellulose-based raw material while dispersing it in a dispersion medium to form nanofibers, A method for producing cellulose nanofibers, wherein the anion-modified cellulose-based raw material is oxidized pulp.

2. (A) dispersing an anionically modified cellulose-based raw material in an alkaline solution to obtain a slurry having a solids concentration of the anionically modified cellulose-based raw material of 9% by mass or more and 20% by mass or less and a pH of 6 to 9; (B) a step of hydrolyzing the slurried anion-modified cellulosic raw material obtained in the step A; (C) preparing a dispersion containing the hydrolyzed anion-modified cellulose-based raw material obtained in step B, and defibrating the hydrolyzed anion-modified cellulose-based raw material while dispersing it in a dispersion medium to form nanofibers, The hydrolysis reaction in the step B is carried out under a temperature condition of 70 to 120°C, or the hydrolysis reaction in the step B is carried out in multiple batches, A method for producing cellulose nanofibers, wherein the anion-modified cellulose-based raw material is oxidized pulp.

3. The method for producing cellulose nanofibers according to claim 1, characterized in that the hydrolysis reaction in step B is carried out at a temperature of 40 to 120°C.

4. The method for producing cellulose nanofibers according to claim 1, characterized in that the hydrolysis reaction in step B is carried out in multiple batches.

5. The method for producing cellulose nanofibers according to claim 1 or 2, characterized in that the cellulose nanofibers obtained in step C have an average fiber length of 500 nm or less.

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