Suspension containing cellulose nanofibers and organic solvent, and method for producing the same

Abstract

To provide a method by which aggregation of cellulose nanofibers can be suppressed during mixing of cellulose nanofibers with a high-concentration organic solvent.SOLUTION: Hydrophobized cellulose nanofibers are mixed in advance into an organic solvent for mixing. Specifically, mixing a water dispersion of cellulose nanofibers and an organic solvent dispersion of hydrophobized cellulose nanofibers produces a suspension containing at least cellulose nanofibers and an organic solvent, In the suspension, the organic solvent constitutes 50 mass% or more of the total of water and organic solvent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a suspension containing cellulose nanofibers and an organic solvent. More specifically, the present invention relates to a method for producing a suspension containing cellulose nanofibers and a high concentration of organic solvent, in which aggregation of the cellulose nanofibers is suppressed by adding hydrophobized cellulose nanofibers. The present invention also relates to a suspension containing cellulose nanofibers, hydrophobized cellulose nanofibers, water, and an organic solvent. [Background technology]

[0002] It is known that mechanical processing of cellulose can convert it into cellulose with a fine fiber diameter. It is also known that introducing anionic groups such as carboxyl groups or carboxymethyl groups into the cellulose molecular chain and defibrating it can convert it into cellulose nanofibers with nano-order fiber diameters (Patent Documents 1 and 2). Because cellulose nanofibers are light, strong, and biodegradable, their application in various fields is being considered. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-001728 [Patent Document 2] International Publication No. 2014 / 088072 Summary of the Invention [Problem to be solved by the invention]

[0004] Cellulose and cellulose nanofibers derived from cellulose with anionic groups introduced therein are highly hydrophilic, and therefore suffer from the problem of aggregation when added to a highly concentrated organic solvent. The present invention aims to provide a method for suppressing aggregation of cellulose nanofibers when mixing cellulose nanofibers with a highly concentrated organic solvent. [Means for solving the problem]

[0005] As a result of extensive research, the present inventors have found that, when mixing an organic solvent with an aqueous dispersion of cellulose nanofibers, adding hydrophobized cellulose nanofibers to the organic solvent beforehand can suppress aggregation of the cellulose nanofibers when the organic solvent is mixed in. The present invention includes, but is not limited to, the following. (1) A method for producing a suspension containing cellulose nanofibers and an organic solvent, comprising: The method includes mixing an aqueous dispersion of cellulose nanofibers with an organic solvent dispersion of hydrophobized cellulose nanofibers, The method described above, wherein the amount of the organic solvent relative to the total amount of water and the organic solvent in the suspension is 50 mass % or more. (2) The method according to (1), wherein the hydrophobized cellulose nanofibers are hydrophobized cellulose nanofibers in which a compound having an amine or phosphine and a weight-average molecular weight of 600 or more is bonded to the anionic groups of the anionic cellulose nanofibers. (3) The method according to (1) or (2), wherein the cellulose nanofibers are anionic cellulose nanofibers. (4) The method according to any one of (1) to (3), comprising mixing an aqueous dispersion of the cellulose nanofibers with an organic solvent dispersion of the hydrophobized cellulose nanofibers so that the amount of hydrophobized cellulose nanofibers is 0.1 to 3 parts by mass per 1 part by mass of the cellulose nanofibers. (5) A suspension comprising cellulose nanofibers, hydrophobized cellulose nanofibers, water, and an organic solvent, wherein the amount of the organic solvent relative to the total amount of water and the organic solvent is 50 mass% or more. [Effects of the Invention]

[0006] When mixing an organic solvent with an aqueous dispersion of cellulose nanofibers, aggregation of the cellulose nanofibers can be suppressed by adding hydrophobized cellulose nanofibers to the organic solvent beforehand according to the present invention. The method of the present invention makes it possible to obtain a suspension containing cellulose nanofibers and an organic solvent in which aggregation of the cellulose nanofibers is suppressed by a simple means. Furthermore, the suspension obtained by the present invention has the advantage of exhibiting high transparency because aggregation of the cellulose nanofibers is suppressed. DETAILED DESCRIPTION OF THE INVENTION

[0007] The present invention aims to suppress the aggregation of cellulose nanofibers (hereinafter also referred to as "CNF") due to mixing of an organic solvent when producing a suspension containing the CNF and an organic solvent. The inventors have found that by suspending / dispersing hydrophobized CNF in the organic solvent used for mixing in advance, aggregation of CNF when mixing an organic solvent with an aqueous dispersion of CNF is suppressed. The method of the present invention involves mixing an aqueous dispersion of CNF with an organic solvent dispersion of hydrophobized CNF. Note that in this application, the terms "suspended" and "dispersed" are sometimes used interchangeably. Furthermore, in this application, when CNF is not referred to as "hydrophobized" (i.e., when simply referring to "CNF" or "anionic CNF" without the term "hydrophobized"), it refers to CNF that has not been hydrophobized unless otherwise specified. On the other hand, when the term "hydrophobized" is added to CNF or cellulose (i.e., "hydrophobized CNF" or "hydrophobized cellulose"), it refers to CNF or cellulose that has been subjected to a hydrophobization treatment (addition of hydrophobic groups). The hydrophobic treatment does not necessarily have to be carried out on all reactive groups in the cellulose molecular chain of CNF or cellulose, and cases in which only part of the cellulose chain is hydrophobized are also included in the term "hydrophobized CNF" or "hydrophobized cellulose."

[0008] <cnf> (average fiber diameter, aspect ratio) CNF can be produced by defibrating cellulose or chemically modified cellulose to nanometer-order fiber diameters. The average fiber diameter of CNF is preferably 3 to 500 nm, more preferably 3 to 150 nm, even more preferably 3 to 20 nm, even more preferably 5 to 19 nm, and even more preferably 5 to 15 nm. The fiber diameter of CNF can be determined by measuring the cross-sectional height of the fiber shape image observed using an atomic force microscope (AFM). The average fiber diameter of CNF can also be determined by measuring the fiber diameters of 50 randomly selected fibers using the method described above and calculating the length-weighted average fiber diameter.

[0009] The aspect ratio of CNF is preferably 30 or more, more preferably 50 or more, and even more preferably 100 or more. There is no upper limit to the aspect ratio, but it is about 500 or less. The average fiber length of CNF can be determined by measuring the fiber lengths of 200 randomly selected fibers using an atomic force microscope (AFM) and calculating the length-weighted average fiber length, and the aspect ratio of CNF can be determined by the following formula using the above-mentioned average fiber length and average fiber diameter: Aspect ratio = average fiber length (nm) / average fiber diameter (nm).

[0010] (cellulose raw material) The type of cellulose used as a raw material for CNF is not particularly limited. Cellulose is generally classified into native cellulose, regenerated cellulose, fine cellulose, microcrystalline cellulose (a cellulose obtained by removing the amorphous region), etc., based on its origin and manufacturing method. In the present invention, any of these celluloses can be used as a raw material.

[0011] Examples of natural cellulose include bleached pulp or unbleached pulp (bleached wood pulp or unbleached wood pulp); linters, purified linters; and cellulose produced by microorganisms such as acetic acid bacteria. The raw materials for bleached pulp or unbleached pulp are not particularly limited, and examples include wood, cotton, straw, bamboo, hemp, jute, and kenaf. The method for producing bleached pulp or unbleached pulp is also not particularly limited, and may be a mechanical method, a chemical method, or a combination of the two. Examples of bleached or unbleached pulp classified by production method include mechanical pulp (thermomechanical pulp (TMP), groundwood pulp), chemical pulp (sulfite pulp such as softwood unbleached sulfite pulp (NUSP) and softwood bleached sulfite pulp (NBSP), and kraft pulp such as softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), and hardwood bleached kraft pulp (LBKP)). In addition to papermaking pulp, dissolving pulp may also be used. Dissolving pulp is chemically refined pulp that is primarily dissolved in chemicals before use and is the main raw material for artificial fibers, cellophane, etc.

[0012] Examples of regenerated cellulose include cellulose dissolved in a certain solvent such as a cuprammonium solution, a cellulose xanthate solution, or a morpholine derivative, and then spun again. Examples of fine cellulose include those obtained by depolymerizing cellulosic materials, including the above-mentioned natural cellulose and regenerated cellulose (e.g., acid hydrolysis, alkaline hydrolysis, enzymatic decomposition, crushing treatment, vibrating ball mill treatment, etc.), and those obtained by mechanically treating the above-mentioned cellulosic materials.

[0013] Chemically modified cellulose may also be used as a raw material for CNF. Modification methods include, for example, anionic modification, which introduces anionic groups into cellulose molecules, and cationic modification, which introduces cationic groups into cellulose molecules. In the present invention, it is preferable to use anionic cellulose that has been anionically modified as a raw material for CNF.

[0014] (anionic cellulose-carboxylated cellulose) An example of anionic cellulose is carboxylated cellulose (the introduction of carboxyl groups into cellulose, also known as "oxidation"). As used herein, the term "carboxyl group" refers to -COOH (acid type) and -COOM (metal salt type) (where M is a metal ion). Carboxylated cellulose (also known as "oxidized cellulose") can be obtained by carboxylating (oxidizing) the above-mentioned cellulose raw materials using known methods. While not particularly limited, the amount of carboxyl groups is preferably 0.40 to 3.00 mmol / g, more preferably 0.45 to 2.50 mmol / g, even more preferably 0.50 to 2.00 mmol / g, and even more preferably 0.55 to 1.75 mmol / g, based on the bone-dry mass of the anionic cellulose or anionic cellulose nanofibers ("anionic CNF").

[0015] The amount of carboxyl groups in anionic cellulose or anionic CNF can be measured by the following method: 60 ml of a 0.5% by mass slurry (aqueous dispersion) is prepared, and a 0.1 M aqueous hydrochloric acid solution is added to adjust the pH to 2.5. After that, a 0.05 N aqueous sodium hydroxide solution is added dropwise, and the electrical conductivity is measured until the pH reaches 11. The electrical conductivity is 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 is gradual: Amount of carboxyl groups [mmol / g carboxylated cellulose] = a [ml] x 0.05 / mass of carboxylated cellulose [g].

[0016] One example of a carboxylation (oxidation) method is a method in which a cellulose raw material is oxidized 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, and mixtures thereof. This oxidation reaction selectively oxidizes the primary hydroxyl group at C6 of the glucopyranose ring on the surface of the cellulose, leaving aldehyde groups and carboxyl groups (-COOH) or carboxylate groups (-COO) on the surface. - The cellulose concentration during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0017] An N-oxyl compound refers to a compound capable of generating a nitroxy radical. Any compound that promotes the target oxidation reaction can be used as the N-oxyl compound. Examples include 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO). The amount of the N-oxyl compound used is not particularly limited, as long as it is a catalytic amount capable of oxidizing the raw cellulose. For example, 0.01 to 10 mmol is preferred, 0.01 to 1 mmol is more preferred, and 0.05 to 0.5 mmol is even more preferred, 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.

[0018] Bromides are compounds containing bromine, examples of which include alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, examples of which 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.

[0019] 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 1.0 to 25 mmol, and most preferably 1.5 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.

[0020] 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 in 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.

[0021] The oxidation reaction may also be carried out in two stages. For example, the oxidized cellulose obtained by filtration after the completion of the first-stage reaction can be oxidized again under the same or different reaction conditions, allowing for efficient oxidation without reaction inhibition by sodium chloride, which is a by-product of the first-stage reaction.

[0022] Another example of a carboxylation (oxidation) method is a method in which cellulose raw materials are oxidized by contacting an ozone-containing gas with the raw material. 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 3 It is more preferable that the ozone treatment temperature is 0 to 50°C, more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is approximately 1 to 360 minutes, 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 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 a polar organic solvent such as water or alcohol to prepare an oxidizing agent solution, and then immersing the cellulose raw material in the solution.

[0023] The amount of carboxyl groups in carboxylated cellulose can be adjusted by controlling reaction conditions such as the amount of oxidizing agent added, reaction time, etc. The amount of carboxyl groups in carboxylated cellulose is usually the same as the amount of carboxyl groups when the carboxylated cellulose is made into nanofibers.

[0024] (anionic cellulose-carboxyalkylated cellulose) An example of anionic cellulose is carboxyalkylated cellulose, which has a carboxyalkyl group such as a carboxymethyl group introduced therein. In this specification, the 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.

[0025] Carboxyalkylated cellulose may be obtained by known methods, or commercially available products may be used. The degree of carboxyalkyl substitution per anhydroglucose unit of cellulose is preferably less than 0.40. Furthermore, when the anionic group is a carboxymethyl group, the degree of carboxymethyl substitution is preferably less than 0.40. A degree of substitution of 0.40 or more reduces the crystallinity of cellulose. The lower limit of the degree of carboxyalkyl substitution is preferably 0.01 or more. Considering operability, the degree of substitution is particularly preferably 0.02 to 0.35, more preferably 0.10 to 0.35, even more preferably 0.15 to 0.35, and even more preferably 0.15 to 0.30. Note that anhydroglucose unit refers to each anhydroglucose (glucose residue) constituting cellulose, and the degree of carboxyalkyl substitution refers to the proportion of hydroxyl groups (-OH) in the glucose residues constituting cellulose that are substituted with carboxyalkyl ether groups (-ORCOOH or -ORCOOM) (the number of carboxyalkyl ether groups per glucose residue).

[0026] An example of a method for producing carboxyalkylated cellulose includes the following steps: Carboxymethylated cellulose will be used as an example (hereinafter, "carboxymethyl" will also be referred to as "CM").

[0027] i) a step of mixing a cellulose raw material, a solvent, and a mercerizing agent, and subjecting the mixture to mercerization 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; ii) Next, a carboxymethylating agent is added in an amount of 0.05 to 10.0 times the moles of the 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 to 4 hours.

[0028] The cellulose raw material can be any of the above. The solvent can be 3 to 20 times by mass of water or a lower alcohol, specifically water, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, tertiary butanol, or a mixture of two or more thereof. When a lower alcohol is mixed, the mixing ratio is preferably 60 to 95% by mass. The mercerizing agent is preferably an alkali metal hydroxide, specifically sodium hydroxide or potassium hydroxide, in an amount of 0.5 to 20 times by mole per anhydrous glucose unit of cellulose.

[0029] As mentioned above, the CM substitution degree per glucose unit of cellulose is less than 0.40, and preferably 0.01 or more and less than 0.40. Introducing CM substituents into cellulose causes electrical repulsion between cellulose molecules. Therefore, cellulose with CM substituents introduced therein can be nanofibrillated. Note that if the CM substituents per glucose unit are less than 0.01, nanofibrillation may not be sufficient. The CM substitution degree is more preferably 0.10 or more and less than 0.40, even more preferably 0.15 or more and less than 0.40, and even more preferably 0.20 or more and less than 0.40. The carboxyalkyl substitution degree in carboxyalkylated cellulose and the carboxyalkyl substitution degree when the carboxyalkylated cellulose is made into nanofibers are usually the same.

[0030] The degree of CM substitution per glucose unit can be measured in the following way: Approximately 2.0 g of carboxymethylated cellulose (bone dry) was weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of a solution of 900 mL of methanol and 100 mL of concentrated nitric acid was added and the mixture was shaken for 3 hours to convert the carboxymethylated cellulose salt into hydrogenated carboxymethylated cellulose. 1.5 to 2.0 g of hydrogenated carboxymethylated cellulose (bone dry) was weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. The hydrogenated carboxymethylated cellulose was moistened with 15 mL of 80% methanol, 100 mL of 0.1 N NaOH was added, and the mixture was shaken at room temperature for 3 hours. Excess NaOH was back-titrated with 0.1 N H2SO4 using phenolphthalein as an indicator. The degree of carboxymethylated substitution (DS) was calculated 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.

[0031] The degree of substitution of carboxyalkyl groups other than CM groups can also be measured by the same method as above. (anionic cellulose-phosphate esterified cellulose) An example of anionic cellulose is phosphate-esterified cellulose. Esterification methods include mixing a powder or aqueous solution of a phosphoric acid compound with a cellulose raw material or adding an aqueous solution of a phosphoric acid compound to a slurry of the cellulose raw material. Examples of phosphoric acid compounds include phosphoric acid, polyphosphoric acid, phosphorous acid, hypophosphorous acid, phosphonic acid, polyphosphonic acid, and esters thereof. These may be in the form of salts. Examples of phosphoric acid compounds include 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, and ammonium metaphosphate. Phosphate groups can be introduced into cellulose by using one or more of these compounds in combination. Among these, phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are preferred from the viewpoints of high efficiency of phosphate group introduction, ease of defibration in the defibration step described below, and ease of industrial application. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. Furthermore, it is desirable to use the phosphoric acid compound as an aqueous solution, as this allows the reaction to proceed uniformly and increases the efficiency of phosphate group introduction. The pH of the aqueous solution of the phosphoric acid compound is preferably 7 or less, as this increases the efficiency of phosphate group introduction, but a pH of 3 to 7 is preferred from the viewpoint of suppressing hydrolysis of cellulose fibers.

[0032] The following method can be mentioned as an example of a method for producing phosphated cellulose. A phosphoric acid compound is added to a suspension of a cellulose raw material having a solids concentration of 0.1 to 10% by mass while stirring, to introduce phosphate groups into the cellulose. When the cellulose raw material is taken as 100 parts by mass, the amount of the phosphoric acid compound added is preferably 0.2 to 500 parts by mass, and more preferably 1 to 400 parts by mass, in terms of the amount of phosphorus element. If the proportion of the phosphoric acid compound is equal to or greater than the lower limit, the yield of fine fibrous cellulose can be further improved. However, if the proportion exceeds the upper limit, the effect of improving the yield will plateau, which is undesirable from a cost perspective.

[0033] In addition to the phosphoric acid compound, powders or aqueous solutions of other compounds may be mixed. The compounds other than the phosphoric acid compound are not particularly limited, but a basic nitrogen-containing compound is preferred. "Basic" here is defined as an aqueous solution exhibiting a pink to red color in the presence of a phenolphthalein indicator, or an aqueous solution having a pH greater than 7. The basic nitrogen-containing compound used in the present invention is not particularly limited as long as it achieves the effects of the present invention, but a compound having an amino group is preferred. Examples include urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, urea is preferred because of its low cost and ease of handling. The amount of the other compound added is preferably 2 to 1,000 parts by mass, more preferably 100 to 700 parts by mass, per 100 parts by mass of the solid content of the cellulose raw material. The reaction temperature is preferably 0 to 95°C, more preferably 30 to 90°C. The reaction time is not particularly limited, but is approximately 1 to 600 minutes, more preferably 30 to 480 minutes. When the esterification reaction conditions are within these ranges, it is possible to prevent the cellulose from being excessively esterified and becoming more soluble, resulting in a good yield of phosphated cellulose. After dehydrating the resulting phosphated cellulose suspension, it is preferable to heat-treat it at 100 to 170°C in order to suppress hydrolysis of the cellulose. Furthermore, it is preferable to heat the suspension at 130°C or lower, preferably 110°C or lower, while it contains water, and then, after removing the water, heat-treat it at 100 to 170°C.

[0034] The degree of phosphate substitution per glucose unit of the phosphated cellulose is preferably 0.001 or more and less than 0.40. Introducing phosphate group substituents into cellulose causes electrical repulsion between cellulose units. Therefore, cellulose with introduced phosphate groups can be easily nanofibrillated. If the degree of phosphate substitution per glucose unit is less than 0.001, nanofibrillation is not sufficient. On the other hand, if the degree of phosphate substitution per glucose unit is greater than 0.40, the cellulose may swell or dissolve, making it impossible to obtain nanofibers. To achieve efficient fibrillation, the phosphated cellulose obtained above is preferably boiled and then washed with cold water. The degree of substitution in the phosphated cellulose and the degree of substitution when the phosphated cellulose is made into nanofibers are usually the same.

[0035] The degree of phosphate group substitution per glucose unit can be measured by the following method: A phosphated cellulose slurry with a solids content of 0.2% by mass was prepared. One-tenth the volume of a strongly acidic ion exchange resin (Amberjet 1024; Organo Corporation, conditioned) was added to the slurry, which was then shaken for 1 hour. The mixture was then poured onto a 90 μm mesh to separate the resin and slurry, converting the phosphated cellulose salt into hydrogenated phosphated cellulose. Next, 50 μL of 0.1 N aqueous sodium hydroxide was added to the ion exchange resin-treated slurry every 30 seconds, and the change in the electrical conductivity of the slurry was measured. The amount of alkali (mmol) required to reach a sharp drop in electrical conductivity was divided by the solids content (g) of the slurry to be titrated to calculate the amount of phosphate groups (mmol / g) per gram of hydrogenated phosphated cellulose. Furthermore, the degree of phosphate substitution (DS) per glucose unit of the phosphated cellulose was calculated using the following formula: DS=0.162×A / (1-0.079×A) A: Amount of phosphate groups per gram of hydrogen phosphated cellulose (mmol / g) As a raw material for CNF, among the above-mentioned anionic celluloses, it is preferable to use carboxylated cellulose, and in particular, carboxylated cellulose obtained by oxidizing cellulose using an N-oxyl compound and an oxidizing agent is preferred.

[0036] (crystallinity) The anionic cellulose used as the raw material for anionic CNF is one that maintains at least a portion of its fibrous shape when dispersed in water or a water-soluble organic solvent. Cellulose that does not maintain its fibrous shape (i.e., dissolves) does not form nanofibers. "Maintaining at least a portion of its fibrous shape when dispersed" means that when a dispersion of anionic cellulose is observed under an electron microscope, a fibrous substance can be observed. Furthermore, anionic cellulose that allows observation of a cellulose type I crystal peak when measured by X-ray diffraction is preferred.

[0037] The degree of crystallinity of the anionic cellulose before defibration is preferably 50% or more, more preferably 60% or more, for crystalline type I. When the crystallinity is within the above range, crystalline cellulose fibers that do not dissolve even after being finely divided by defibration can be obtained. The crystallinity of cellulose can be controlled by the crystallinity of the raw material cellulose and the degree of anionic modification. The method for measuring the crystallinity of anionically modified cellulose before defibration is as follows: The sample was placed in a glass cell and measured using an X-ray diffraction measurement device (LabX XRD-6000, manufactured by Shimadzu Corporation). The degree of crystallinity was calculated using the method of Segal et al., where the diffraction intensity at 2θ = 10° to 30° in the X-ray diffraction pattern was used as the baseline, and the degree of crystallinity was calculated using the following formula from the diffraction intensity of the 002 plane at 2θ = 22.6° and the diffraction intensity of the amorphous part at 2θ = 18.5°. Xc=(I002c-Ia) / I002c×100 Xc: Crystallinity of cellulose type I (%) I002c: 2θ=22.6°, diffraction intensity of the 002 plane Ia: 2θ=18.5°, diffraction intensity of the amorphous part.

[0038] (defibrillation, CNF aqueous dispersion) CNF or anionic CNF can be obtained by defibrating cellulose or anionic cellulose to nanometer-order fiber diameters. While the equipment used for defibration is not limited, it is preferable to use a device capable of applying strong shear forces to the dispersion, such as a high-speed rotation type, colloid mill type, high-pressure type, roll mill type, or ultrasonic type. For efficient defibration, it is preferable to use a wet high-pressure or ultra-high-pressure homogenizer capable of applying a pressure of 50 MPa or more to the dispersion and applying strong shear forces. The pressure is preferably 100 MPa or more, and even more preferably 140 MPa or more. A high-pressure or ultra-high-pressure homogenizer is a device that pressurizes a fluid using a pump to high pressure and then ejects it through a very fine gap in the flow path, thereby emulsifying, dispersing, pulverizing, pulverizing, and ultra-fine-graining the particles through combined energies such as interparticle collisions and shear forces due to pressure differences. Prior to defibration and dispersion using a high-pressure homogenizer, preliminary treatment can be performed, if necessary, using a known mixing, stirring, emulsifying, or dispersing device, such as a high-speed shear mixer. Defibration can also be performed using a cavitation jet device. A cavitation jet device is a device that compresses the jetting liquid and sprays it at high speed from the tip of a nozzle or orifice toward the liquid to be jetted, thereby shredding solid lumps in the jetting liquid and the liquid to be jetted by using extremely high shear forces near the nozzle or orifice and the collapse energy of cavitation bubbles generated when the liquid expands due to sudden decompression.

[0039] For defibration, a cellulose / anionic cellulose dispersion is first prepared. Water is preferably used as the dispersion medium for ease of handling, but it may also contain a small amount of a polar organic solvent (e.g., methanol, ethanol, isopropanol, isobutanol, sec-butanol, tert-butanol, methyl cellosolve, ethyl cellosolve, ethylene glycol, glycerin, ethylene glycol dimethyl ether, 1,4-dioxane, tetrahydrofuran, acetone, methyl ethyl ketone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, etc.) that has a high affinity for the hydroxyl groups in cellulose (e.g., 40% by mass or less, more preferably 30% by mass or less, even more preferably 15% by mass or less, and most preferably less than 5% by mass of the organic solvent relative to the total mass of water and organic solvent). The defibration product obtained using water as the dispersion medium can be used directly as the "CNF aqueous dispersion" (including anionic CNF aqueous dispersion) for use in the present invention.

[0040] The concentration of CNF in the aqueous dispersion of CNF used in the present invention is not particularly limited, but is preferably 0.01 to 15 mass %, more preferably 0.1 to 10 mass %, and even more preferably 1 to 5 mass %.

[0041] <Hydrophobic CNF> The method of the present invention suppresses CNF aggregation due to mixing of an organic solvent by preliminarily incorporating hydrophobized CNF into the organic solvent when mixing an aqueous dispersion of CNF with an organic solvent (i.e., by using an organic solvent dispersion of hydrophobized CNF as the organic solvent used for mixing). As mentioned above, in this application, when the term "CNF" or "anionic CNF" is used without the term "hydrophobized," it refers to CNF that has not been hydrophobized, while when "hydrophobized CNF" is used, it refers to CNF that has been hydrophobized (by adding hydrophobic groups). Hydrophobized CNF does not necessarily have to have hydrophobic groups added to all reactive groups on the cellulose molecular chain; CNFs with hydrophobic groups added to only some of the reactive groups (i.e., those with a low hydrophobicity) are also referred to as "hydrophobized CNF." Therefore, the above-mentioned "aqueous dispersion of CNF" is an aqueous dispersion of CNF that has not been hydrophobized at all, while the "organic solvent dispersion of hydrophobized CNF" is an organic solvent dispersion of hydrophobized CNF in which hydrophobic groups have been added to at least some of the reactive groups on the cellulose molecular chains (including those with a low hydrophobicity). The suspension obtained by mixing these contains CNF that has not been hydrophobized at all (i.e., CNF), CNF in which hydrophobic groups have been added to at least some of the reactive groups (hydrophobized CNF), water, and an organic solvent.

[0042] Hydrophobic CNF can be obtained by subjecting the above-mentioned anionic CNF to hydrophobic treatment. Alternatively, it can be obtained by subjecting anionic cellulose to hydrophobic treatment to obtain hydrophobic cellulose, which is then defibrated. Hydrophobic treatment refers to a process in which hydrophobic groups are added to anionic cellulose / anionic CNF to improve the hydrophobicity of the anionic cellulose / anionic CNF.

[0043] (Addition of hydrophobizing agent, dispersion of hydrophobized CNF in organic solvent) For hydrophobization, a hydrophobizing agent is added to the anionic cellulose / anionic CNF. The hydrophobizing agent is preferably a compound containing an amine or phosphine that can bond with the anionic groups of the anionic cellulose / anionic CNF to form an onium salt, such as a primary amine, a secondary amine, a tertiary amine, a quaternary ammonium, an aromatic amine, a diamine, a polyetheramine, or a phosphine. The weight-average molecular weight of the hydrophobizing agent is preferably 600 or more, more preferably 1000 or more, and even more preferably 2000 or more. For example, polyetheramines such as, but not limited to, JEFFAMINE® M-600, JEFFAMINE® M-1000, JEFFAMINE® M-2005, and JEFFAMINE® M-2070 manufactured by HUNTSMAN are preferably used.

[0044] The hydrophobizing agent binds to anionic groups. Due to the reaction mechanism, the added hydrophobizing agent reacts with all the anionic groups. The inventors have found that when 1 molar equivalent or more of a hydrophobizing agent (amine type) is added to anionic cellulose, the anionic cellulose, which was in the acid form (-COOH), changes to all -COO. - This indicates that the amine hydrophobizing agent ionically bonds to all of the anionic groups of the anionic cellulose.

[0045] The amount of hydrophobizing agent added may be adjusted depending on the molecular weight of the hydrophobizing agent used and the amount of anionic groups in the anionic cellulose / anionic CNF. For example, the amount may be 50 to 150% of the amount (number of moles) of anionic groups, preferably 70 to 130%, more preferably 80 to 120%, and even more preferably 100 to 120%.

[0046] The hydrophobizing agent may be added to the anionic cellulose / anionic CNF either directly or after mixing with water or a polar organic solvent. To add the hydrophobizing agent, a dispersion of anionic cellulose / anionic CNF is prepared and then the hydrophobizing agent is added. The concentration of anionic cellulose / anionic CNF in the dispersion upon addition of the hydrophobizing agent is preferably 0.01 to 15% by mass, more preferably 1 to 10% by mass, and even more preferably 2 to 6% by mass. After adding the hydrophobizing agent, the mixture is stirred for a certain period of time to obtain a dispersion of hydrophobized cellulose / hydrophobized CNF. The dispersion medium of the resulting dispersion is then replaced with an organic solvent to obtain an organic solvent dispersion of hydrophobized cellulose / hydrophobized CNF. The substitution with an organic solvent can be carried out, for example, by removing the dispersion medium to produce a dry solid of hydrophobic cellulose / hydrophobic CNF (solid content 90% by mass or more, more preferably 95% by mass, and even more preferably 98% by mass or more), and then adding the resulting dry solid to the desired organic solvent. Next, a treatment similar to the above-mentioned defibration treatment is carried out in the organic solvent to obtain an organic solvent dispersion of hydrophobic CNF.

[0047] The type of organic solvent used is not particularly limited. For example, the above-mentioned polar organic solvents (e.g., methanol, ethanol, isopropanol, isobutanol, sec-butanol, tert-butanol, methyl cellosolve, ethyl cellosolve, ethylene glycol, glycerin, ethylene glycol dimethyl ether, 1,4-dioxane, tetrahydrofuran, acetone, methyl ethyl ketone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, etc.) may be used. Alternatively, low-polarity organic solvents (e.g., benzene, toluene, xylene, n-hexane, n-octane, cyclohexane, methylcyclohexane, dichloromethane, dichloroethane, chloroform, methylene chloride, carbon tetrachloride, fluorotrichloromethane, trichlorotrifluoromethane, hexafluorobenzene, tetrahydrofuran, 1,2-dimethoxyethane, cyclopentyl methyl ether, methyl tertiary butyl ether, etc.) may be used. Among these, polar organic solvents are preferred. One or more of these can be appropriately selected and used depending on the final use of the suspension.

[0048] The average fiber diameter of the hydrophobized CNF in the organic solvent dispersion of the hydrophobized CNF is preferably 3 to 500 nm, more preferably 3 to 150 nm, even more preferably 3 to 20 nm, even more preferably 5 to 19 nm, and even more preferably 5 to 15 nm. The aspect ratio of the hydrophobized CNF is preferably 30 or more, more preferably 50 or more, and even more preferably 100 or more. There is no upper limit to the aspect ratio, but it is about 500 or less. The methods for measuring the average fiber diameter and aspect ratio of the hydrophobized CNF are the same as those described above for CNF.

[0049] The dispersion medium in the organic solvent dispersion of hydrophobized CNF preferably consists of the above-mentioned organic solvent (the proportion of organic solvent is 100%), but may contain a small amount of water. A small amount of water means, for example, that the amount of water is 40% by mass or less, more preferably 30% by mass or less, even more preferably 15% by mass or less, and most preferably less than 5% by mass, of the total of water and organic solvent.

[0050] The concentration of the hydrophobized CNF in the organic solvent dispersion of the hydrophobized CNF is not particularly limited, but is preferably 0.01 to 15% by mass, more preferably 0.1 to 10% by mass, and still more preferably 0.5 to 5% by mass.

[0051] <Mixing of the aqueous dispersion of CNF and the organic solvent dispersion of hydrophobized CNF> In the method of the present invention, a suspension containing CNF, an organic solvent, water, and hydrophobized CNF is produced by mixing an aqueous dispersion of CNF and an organic solvent dispersion of hydrophobized CNF. CNF (or anionic CNF) tends to aggregate when attempting to mix with a high-concentration organic solvent. By using the method of the present invention, it becomes possible to mix CNF with a high-concentration organic solvent while suppressing the aggregation of CNF.

[0052] At the time of mixing, the aqueous dispersion of CNF and the organic solvent dispersion of hydrophobized CNF are mixed so that the amount of the organic solvent (referred to as "the ratio of the organic solvent") relative to the total amount of water and the organic solvent is 50% by mass or more. If the ratio of the organic solvent is less than 50% by mass, there may be almost no problem of aggregation in the first place. The method of the present invention is particularly useful when mixing a high ratio of an organic solvent and CNF. The upper limit of the ratio of the organic solvent is not particularly limited. Practically, it is considered to be, for example, about 80% by mass or less.

[0053] Also, at the time of mixing, it is preferable to mix the aqueous dispersion of CNF and the organic solvent dispersion of hydrophobized CNF such that the hydrophobized CNF is 0.1 to 3 parts by mass with respect to 1 part by mass of CNF derived from the aqueous dispersion. More preferably, the hydrophobized CNF is 0.5 to 2 parts by mass with respect to 1 part by mass of CNF.

[0054] It is preferable to add the organic solvent dispersion of hydrophobized CNF while stirring the aqueous dispersion of CNF to uniformly mix the two. The means of stirring is not particularly limited, and known means can be used.

[0055] <The suspension of the present invention> The method of the present invention makes it possible to obtain a suspension containing a high concentration of organic solvent while suppressing aggregation of CNF. The suspension obtained by the present invention contains CNF, hydrophobized CNF, water, and an organic solvent, with the amount of organic solvent being 50% by mass or more relative to the total amount of water and organic solvent. As mentioned above, "CNF" refers to CNF that has not been hydrophobized at all, and "hydrophobized CNF" refers to CNF in which hydrophobic groups have been added to at least some of the reactive groups. The suspension of the present invention is distinguished from a suspension consisting of hydrophobized CNF with a low hydrophobicity (CNF in which hydrophobic groups have been added to only some of the reactive groups) (included in the hydrophobized CNF referred to in this application) and water and an organic solvent, in that it contains "CNF" (or "anionic CNF") that has not been hydrophobized at all.

[0056] The CNF in the suspension is preferably anionic CNF. Carboxylated CNF is particularly preferred. Furthermore, the hydrophobized CNF is preferably a hydrophobized CNF in which an amine- or phosphine-containing compound with a weight-average molecular weight of 600 or more is bonded to the anionic group of the anionic CNF. The ratio of CNF to hydrophobized CNF is preferably 0.1 to 3 parts by mass, more preferably 0.5 to 2 parts by mass, of hydrophobized CNF per 1 part by mass of CNF.

[0057] The type of organic solvent in the suspension is not particularly limited, and may be the above-mentioned polar organic solvent or a low-polarity organic solvent, but a polar organic solvent is preferred. The ratio of water to the organic solvent is such that the amount of the organic solvent relative to the total amount of water and the organic solvent is 50 mass % or more.

[0058] The suspension of the present invention tends to exhibit high transparency due to the suppression of CNF aggregation. The transparency of a suspension can be determined by measuring the transmittance (%) of light at a wavelength of 660 nm using a spectrophotometer with an optical path length of 10 mm. When measuring the transparency (transmittance), calibration is performed using a mixture of water and organic solvent in the same ratio as the water and organic solvent contained in the suspension to be measured. The transparency value varies depending on the concentration of CNF and hydrophobized CNF in the suspension. For example, in a suspension in which the total concentration of CNF and hydrophobized CNF is approximately 0.5 to 2.0 mass%, the transparency is 80% or more, more preferably 90% or more, and even more preferably 95% or more. [Example]

[0059] 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. <Production of Anionic Cellulose 1> 5.00 g (bone-dry) of bleached, unbeaten kraft pulp (85% brightness) derived from softwood was added to 500 ml of an aqueous solution containing 39 mg of TEMPO (Sigma-Aldrich) (0.05 mmol per 1 g of bone-dry cellulose) and 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) and stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system to a concentration of 6.0 mmol / g to initiate the oxidation reaction. The pH of the system decreased during the reaction, but was gradually adjusted to pH 10 by the addition of 3 M aqueous sodium hydroxide. The reaction was terminated when the sodium hypochlorite was consumed and the pH no longer changed. The reaction mixture was filtered through a glass filter to separate the pulp, which was then thoroughly washed with water to obtain carboxylated pulp (carboxylated cellulose). The carboxyl group content of this carboxylated cellulose (anionic cellulose 1) was 1.47 mmol / g.

[0060] <Production of Anionic Cellulose 2> 5.00 g (bone-dry) of bleached, unbeaten kraft pulp (85% brightness) derived from softwood was added to 500 ml of an aqueous solution containing 20 mg of TEMPO (Sigma-Aldrich) (0.025 mmol per 1 g of bone-dry cellulose) and 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) and stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system to a concentration of 2.2 mmol / g to initiate the oxidation reaction. The pH of the system decreased during the reaction, but was gradually adjusted to pH 10 by the addition of 3 M aqueous sodium hydroxide. The reaction was terminated when the sodium hypochlorite was consumed and the pH no longer changed. The reaction mixture was filtered through a glass filter to separate the pulp, which was then thoroughly washed with water to obtain carboxylated pulp (carboxylated cellulose). The carboxyl group content of this carboxylated cellulose (anionic cellulose 2) was 0.76 mmol / g.

[0061] <Production of aqueous dispersion of anionic CNF1 and CNF2> Water was added to each of Anionic Celluloses 1 and 2 to a solids content of 1.0% by mass, and the mixture was defibrated using an ultra-high pressure homogenizer until the transparency was sufficiently high, thereby preparing aqueous dispersions (solids content 1.0% by mass) of nanofibers of Anionic Celluloses 1 and 2 (Anionic CNF1, Anionic CNF2). At this time, Anionic CNF1 had an average fiber diameter of 3 nm, an aspect ratio of 250, and a transparency of 95.1%, and Anionic CNF2 had an average fiber diameter of 4 nm, an aspect ratio of 280, and a transparency of 93.7%.

[0062] <Reference example 1> Water was added to an aqueous dispersion of anionic CNF1 (solid content 1.0% by mass) to adjust the solid content to 0.5% by mass. After stirring at 1100 rpm for 5 minutes, the stirring was stopped and the transmittance (%) (transparency) of 660 nm light was measured using a UV-VIS spectrophotometer UV-1800 (Shimadzu Corporation) and a square cell with a light path length of 10 mm. The results are shown in Table 1.

[0063] <Reference example 2> Water was added to an aqueous dispersion of anionic CNF1 (solid content 1.0% by mass) and stirred at 1100 rpm for 5 minutes. IPA was added to this to obtain a suspension containing anionic CNF, an organic solvent, and water (anionic CNF solid content 0.5% by mass, proportion of organic solvent to the total of water and organic solvent (IPA) 30% by mass). After stirring at 1100 rpm for 5 minutes, stirring was stopped, and the transparency was measured in the same manner as in Reference Example 1. The results are shown in Table 1.

[0064] <Comparative Example 1> The transparency was measured in the same manner as in Reference Example 1, except that the ratio of the organic solvent to the total of water and the organic solvent (IPA) was 50 mass %. The results are shown in Table 1.

[0065] <Reference example 3> The transparency was measured in the same manner as in Reference Example 1, except that the aqueous dispersion of anionic CNF2 was used instead of the aqueous dispersion of anionic CNF1. The results are shown in Table 1.

[0066] <Reference example 4> The transparency was measured in the same manner as in Reference Example 2, except that an aqueous dispersion of anionic CNF2 was used instead of the aqueous dispersion of anionic CNF1, and the ratio of the organic solvent to the total of water and organic solvent (IPA) was 40 mass%. The results are shown in Table 1.

[0067] <Comparative Example 2> The transparency was measured in the same manner as in Comparative Example 1, except that the aqueous dispersion of anionic CNF2 was used instead of the aqueous dispersion of anionic CNF1. The results are shown in Table 1.

[0068] Example 1 First, an organic solvent dispersion of hydrophobized CNF was prepared by the following procedure: Anionic cellulose 1 was suspended in water, and then hydrochloric acid was added to convert the sodium salt-type carboxyl groups (-COONa) in the anionic cellulose (carboxylated cellulose) to the acid-type (-COOH). The resulting solution was then subjected to suction filtration using a glass filter and dehydrated. The anionic cellulose was again suspended in water and dehydrated. This process was repeated three times to obtain an aqueous dispersion of acid-type anionic cellulose 1 with a solid content of 25% by mass.

[0069] The resulting aqueous dispersion of acid-form anionic cellulose 1 was diluted with water to a solids content of 4% by mass, and a polyether monoamine (trade name: JEFFAMINE® M-2070, manufactured by HUNTSMAN, molecular weight approximately 2000) was added as a hydrophobizing agent in an amount of 1 equivalent relative to the amount of carboxyl groups. The mixture was mixed using a homogenizer (1500 rpm, 10 minutes) to prepare an aqueous dispersion of anionic cellulose 1 (hydrophobized cellulose) to which the hydrophobizing agent had been bound. The aqueous dispersion of hydrophobized cellulose was then allowed to stand at 70°C for 15 hours to produce a dry solid of hydrophobized cellulose (solids concentration 98% by mass).

[0070] Isopropanol (IPA) was added to the resulting dry solids to a solids content of 4.75% by mass, and the mixture was stirred at 8000 rpm for 10 minutes. The mixture was then defibrated using an ultra-high-pressure homogenizer at 20°C, once at 80 MPa, and then five times at 150 MPa, to obtain an organic solvent dispersion of hydrophobic cellulose nanofibers (hydrophobic CNF) (solids content: 4.75% by mass). Further addition of IPA resulted in an organic solvent dispersion of hydrophobic CNF (solids content: 0.5% by mass).

[0071] Next, water was added to the aqueous dispersion of anionic CNF2 (solid content 1% by mass), and the mixture was stirred at 1100 rpm for 5 minutes. The organic solvent dispersion of hydrophobized CNF prepared above (solid content 0.5% by mass) was then added to obtain a suspension containing anionic CNF2, organic solvent, water, and hydrophobized CNF (anionic CNF solid content 0.5% by mass, hydrophobized CNF1 solid content 0.25% by mass, and the ratio of organic solvent to the total of water and organic solvent (IPA) 50% by mass). After stirring at 1100 rpm for 5 minutes, the stirring was stopped, and the transparency of the suspension was measured in the same manner as in Reference Example 1. The results are shown in Table 1.

[0072] <Example 2> A suspension containing anionic CNF2, an organic solvent, water, and hydrophobic CNF (anionic CNF solids content 0.5% by mass, hydrophobic CNF solids content 0.5% by mass, ratio of organic solvent to total of water and organic solvent (IPA) 50% by mass) was obtained in the same manner as in Example 1, except that a 1.0% by mass organic solvent dispersion of hydrophobic CNF was added to the aqueous dispersion of anionic CNF2, and the transparency was measured. The results are shown in Table 1.

[0073] Example 3 A suspension containing anionic CNF2, an organic solvent, water, and hydrophobic CNF (anionic CNF solids content 0.5% by mass, hydrophobic CNF solids content 1.0% by mass, ratio of organic solvent to total of water and organic solvent (IPA) 50% by mass) was obtained in the same manner as in Example 1, except that a dispersion of hydrophobic CNF with a solids content of 2.0% by mass was added to the aqueous dispersion of anionic CNF2, and the transparency was measured. The results are shown in Table 1.

[0074] [Table 1]

[0075] Comparing Reference Examples 1 and 2 with Comparative Example 1, and Reference Examples 3 and 4 with Comparative Example 2, it can be seen that adding an organic solvent (IPA) to an aqueous dispersion of anionic CNF at a ratio of 50 mass% or more reduces the transparency of the suspension. In Reference Examples 2 and 4, where the ratio of organic solvent (the amount of organic solvent relative to the total amount of water and organic solvent in the suspension) was less than 50 mass%, no aggregation of the anionic CNF was observed, and high transparency equivalent to that of Reference Examples 1 and 3, which did not contain an organic solvent, was maintained. On the other hand, in the suspensions of Comparative Examples 1 and 2, where the ratio of organic solvent was 50 mass%, aggregation of the anionic CNF was observed, and transparency was reduced. Compared to anionic CNF1 (Comparative Example 1), anionic CNF2 (Comparative Example 2) has a smaller amount of anionic groups in the anionic CNF, and therefore it was expected that this would reduce CNF aggregation due to the addition of an organic solvent. However, as shown by the results of Comparative Examples 1 and 2, contrary to expectations, the degree of CNF aggregation due to the addition of an organic solvent remained almost unchanged even when the amount of anionic groups was reduced.

[0076] In contrast, by incorporating hydrophobic CNF into the organic solvent to be added in advance, as in Examples 1 to 3, it was found that aggregation of CNF due to the addition of organic solvent was suppressed even when the proportion of organic solvent was as high as 50 mass% or more, and the transparency of the suspension was improved.< / cnf>

Claims

1. A method for producing a suspension containing cellulose nanofibers and an organic solvent, comprising: The method includes mixing an aqueous dispersion of cellulose nanofibers with an organic solvent dispersion of hydrophobized cellulose nanofibers, The method described above, wherein the amount of the organic solvent relative to the total amount of water and the organic solvent in the suspension is 50 mass % or more.

2. The method according to claim 1, wherein the hydrophobized cellulose nanofibers are hydrophobized cellulose nanofibers in which an amine- or phosphine-containing compound having a weight-average molecular weight of 600 or more is bonded to the anionic groups of the anionic cellulose nanofibers.

3. The method according to claim 1 or 2, wherein the cellulose nanofibers are anionic cellulose nanofibers.

4. The method according to claim 1 or 2, comprising mixing an aqueous dispersion of the cellulose nanofibers with an organic solvent dispersion of the hydrophobized cellulose nanofibers so that the amount of hydrophobized cellulose nanofibers is 0.1 to 3 parts by mass per 1 part by mass of the cellulose nanofibers.

5. A suspension comprising cellulose nanofibers, hydrophobized cellulose nanofibers, water, and an organic solvent, wherein the amount of the organic solvent relative to the total amount of water and the organic solvent is 50 mass% or more.

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