Cellulose nanofiber dry solid and method for producing same

Abstract

To provide dry solid cellulose nanofiber with good redispersibility, and a production method thereof.SOLUTION: The dry solid cellulose nanofiber contains cellulose nanofiber having a mean fiber length of 450 nm or less. The production method of the dry solid cellulose nanofiber includes using a spray dryer to dry a dispersion liquid containing cellulose nanofiber having a mean fiber length of 450 nm or less.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a cellulose nanofiber dry solid and a method for producing the same. [Background technology]

[0002] Cellulose nanofibers are fine fibers with diameters of approximately 3 nm to several hundred nanometers that have excellent dispersibility in aqueous media. They are expected to be used for maintaining the viscosity of foods, cosmetics, medical products, paints, etc., strengthening food raw material dough, retaining moisture, improving food stability, and as a low-calorie additive or emulsion stabilizing aid. However, when cellulose nanofibers dispersed in water (wet state) are dried to form a dry solid, hydrogen bonds are typically formed between the fine cellulose fibers. Therefore, even if this dry solid is redispersed by adding water, its viscosity and other properties do not return to the same levels as before drying (wet state). For this reason, cellulose nanofibers are produced in a water-dispersed state (wet state) and are typically used in various applications in the wet state without drying.

[0003] However, in order to stably maintain cellulose nanofibers in this wet state, a mass of water several times to several hundred times the mass of the cellulose nanofibers is required, which poses various problems, such as securing storage space and increasing storage and transportation costs. Freeze-drying (Patent Document 1) has been proposed as a means of drying cellulose in a wet state. However, freeze-drying cellulose nanofibers requires an enormous amount of energy, and, depending on the conditions, when the water between the fine fibers of the cellulose nanofibers freezes, ice crystals larger than the voids between the fine fibers grow, causing association between the fine fibers of the cellulose nanofibers and resulting in poor redispersibility of the dried solid cellulose nanofibers.

[0004] In response to this, the applicant has proposed a method for improving the redispersibility of dry cellulose nanofiber solids by adding 5 to 300% by mass of a water-soluble polymer to anion-modified cellulose nanofibers to form a dry solid (Patent Document 2), and a method for improving the redispersibility of dry cellulose nanofiber solids by drying a mixture of cellulose nanofibers and a solvent using a vacuum drying device (Patent Document 3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 6-233691 [Patent Document 2] International Publication No. 2015 / 107995 [Patent Document 3] International Publication No. 2019 / 189318 Summary of the Invention [Problem to be solved by the invention]

[0006] The methods described in Patent Documents 2 and 3 are effective methods for producing cellulose nanofiber dry solids with good redispersibility, but it is preferable to further develop new cellulose nanofiber dry solids that can be produced using other equipment and methods to increase the flexibility of the production process.

[0007] The present invention aims to provide a new cellulose nanofiber dry solid with excellent redispersibility and a method for producing the same. "Excellent redispersibility" means that there is little change in viscosity or transparency between a cellulose nanofiber dispersion in a wet state before drying and a cellulose nanofiber dispersion obtained by redispersing the cellulose nanofiber dry solid. [Means for solving the problem]

[0008] As a result of intensive research by the inventors to achieve the above object, they have found that cellulose nanofibers having an average fiber length within a specific range can produce a cellulose nanofiber dry solid with good redispersibility. They have also found that it is preferable to use a spray drying device for drying. The present invention includes, but is not limited to, the following. [1] A method for producing a cellulose nanofiber dry solid, comprising drying a dispersion containing cellulose nanofibers having an average fiber length of 450 nm or less using a spray drying device. [2] The method according to [1], wherein the spray atomizing method in the spray drying apparatus is a two-fluid nozzle or a rotary atomizer. [3] The method according to [1] or [2], wherein the solid content of the dry solid matter is 90.0% by mass or more. [4] The method according to any one of [1] to [3], wherein the cellulose nanofibers are anion-modified cellulose nanofibers. [5] The method according to [4], wherein the anion-modified cellulose nanofibers are carboxylated cellulose nanofibers. [6] The method according to [4], wherein the carboxylated cellulose nanofiber has a carboxyl group content of 0.6 to 3.0 mmol / g relative to the bone dry mass of the carboxylated cellulose nanofiber. [7] A cellulose nanofiber dry solid comprising cellulose nanofibers having an average fiber length of 450 nm or less. [8] The dry solid according to [7], having a solid content of 90.0% by mass or more. [9] A dry solid according to [7] or [8], wherein the transmittance of light with a wavelength of 660 nm (optical path length 10 mm) of the aqueous dispersion obtained by adding water to the dry solid so that the concentration of cellulose nanofibers becomes 1.0 mass % and stirring for 30 minutes with a homodisper (3000 rpm) is 65% or more.

[10] A dry solid described in any one of [7] to [9], in which water is added to the dry solid so that the concentration of cellulose nanofibers is 0.5% by mass, drops of ink are added, and the mixture is stirred. After that, the area ratio of cellulose nanofiber aggregates calculated by observation under an optical microscope is 10.0% or less. [Effects of the Invention]

[0009] According to the present invention, a cellulose nanofiber dry solid with good redispersibility can be obtained. In particular, carboxylated cellulose nanofibers, which are a type of anion-modified cellulose nanofiber, have tended to have difficulty in improving the redispersibility using conventional methods. However, the present invention has the advantage of being able to obtain a carboxylated cellulose nanofiber dry solid with very good redispersibility. Note that "good redispersibility" means that there is little change in viscosity, transparency, etc. between the cellulose nanofiber dispersion in a wet state before drying and the cellulose nanofiber dispersion obtained by redispersing the cellulose nanofiber dry solid. [Brief explanation of the drawings]

[0010] [Figure 1] The dried solids obtained in Examples 1, 2, and 4 to 6 were observed under an optical microscope at a magnification of 400 times. [Figure 2] The dried solids obtained in Comparative Examples 1 and 2 were observed under an optical microscope at a magnification of 400 times. [Figure 3] The redispersions of the dried solids obtained in Examples 1, 2, and 4 to 6 were evaluated for aggregation of CNFs using the India ink method. [Figure 4] The redispersions of the dried solids obtained in Comparative Examples 1 and 2 were evaluated for aggregation of CNFs using the India ink method. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described in detail below. In the present invention, "to" includes both the values ​​at both ends thereof. That is, "X to Y" includes X and Y. The present invention relates to a cellulose nanofiber (hereinafter referred to as "CNF") dry solid and a method for producing the same. Specifically, the present invention relates to a CNF dry solid containing CNFs with an average fiber length of 450 nm or less, and a method for producing a CNF dry solid, which includes drying a dispersion containing CNFs with an average fiber length of 450 nm or less using a spray dryer. The present invention makes it possible to obtain a CNF dry solid with good redispersibility.

[0012] Although the reason why the CNF dried solid obtained by the present invention exhibits excellent redispersibility is unclear, it is speculated that by drying CNFs with an average fiber length of 450 nm or less, the formation of hydrogen bonds between fibers and entanglement of fibers, which are thought to be the causes of reduced electrical repulsion between CNFs and poor redispersibility, are suppressed.

[0013] (Cellulose nanofiber) In the present invention, cellulose nanofibers (CNF) are obtained by pulping cellulose raw materials such as pulp to nanometer-level fiber widths. The fiber width (average fiber diameter) of CNF is usually about 3 nm to several hundred nm, for example, about 3 to 500 nm. In the present invention, CNF with an average fiber diameter of about 3 to 100 nm is preferably used, and about 3 to 20 nm is more preferable.

[0014] In the present invention, among CNFs having the above-mentioned average fiber diameter, those having an average fiber length of 450 nm or less are used. The average fiber length is more preferably 400 nm or less, and even more preferably 350 nm or less. There is no particular lower limit for the average fiber length, but in order to obtain the effects of CNF such as thickening, water absorption, and shape retention, a length of 100 nm or more is preferred, and a length of 150 nm or more is even more preferred.

[0015] The average fiber diameter and average fiber length of CNF can be obtained by calculating the average fiber diameter and fiber length obtained from the observation of approximately 200 fibers using an atomic force microscope (AFM) or a transmission electron microscope (TEM).

[0016] CNF can be obtained by applying mechanical force to a cellulose raw material such as pulp, which will be described later, to pulp and thereby pulverize (fibrillate). The cellulose raw material may be unmodified cellulose or paper pulp, as will be described later, or chemically modified cellulose, which is obtained by further chemically modifying paper pulp. Examples of chemically modified cellulose include, but are not limited to, anion-modified cellulose, in which anionic groups have been introduced into the cellulose chain, and cation-modified cellulose, in which cationic groups have been introduced. Among these, anion-modified cellulose is preferred. Among anion-modified celluloses, carboxylated cellulose, in which carboxyl groups have been introduced as anionic groups, is particularly suitable as a raw material for the CNF used in the present invention.

[0017] (cellulose raw material) Known cellulose sources for CNF include those derived from plants, animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), and microbial products, and any of these can be used in the present invention. Examples of plant-derived cellulose include wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, and pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, and waste paper). In the present invention, cellulose fibers derived from plants or microorganisms are preferred, and plant-derived cellulose fibers are more preferred. The cellulose raw material may be chemically modified as described below. CNF or chemically modified CNF can be obtained by reducing the fiber width of the above-mentioned cellulose raw material or chemically modified cellulose raw material (chemically modified cellulose) to the nanometer level.

[0018] (chemically modified cellulose) Examples of chemically modified cellulose include, but are not limited to, anionically modified cellulose, in which anionic groups have been introduced into the cellulose chain, and cationically modified cellulose, in which cationic groups have been introduced. Examples of anionically modified cellulose include carboxylated cellulose, in which carboxyl groups have been introduced, carboxymethylated cellulose, in which carboxymethyl groups have been introduced, and phosphate ester cellulose, in which phosphate ester groups have been introduced. Among these, anionically modified cellulose is preferred, and carboxylated cellulose is also preferred. Carboxylated CNF, which is made from carboxylated cellulose as a raw material, has tended to be difficult to produce into a dry solid with good redispersibility using conventional methods. However, the method of the present invention makes it possible to produce a dry solid of carboxylated CNF with excellent redispersibility.

[0019] (carboxylated cellulose) An example of anion-modified cellulose is carboxylated (oxidized) cellulose (carboxylated cellulose). Carboxylated cellulose can be obtained by carboxylating (oxidizing) the above-mentioned cellulose raw material using a known method. Although not particularly limited, during carboxylation, the amount of carboxyl groups is preferably adjusted to 0.6 to 3.0 mmol / g, more preferably 0.6 to 2.0 mmol / g, and even more preferably 1.0 mmol / g to 2.0 mmol / g, based on the bone dry mass of the carboxylated cellulose.

[0020] One example of a carboxylation (oxidation) method involves oxidizing a cellulose raw material 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, yielding cellulose bearing aldehyde groups and carboxyl (-COOH) or carboxylate (-COO-) groups on the surface. The concentration of the cellulose raw material during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0021] An N-oxyl compound is a compound capable of generating a nitroxy radical. Any compound that promotes the desired 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).

[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 material 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 amount is preferably about 0.1 to 4 mmol / L relative to the reaction system.

[0023] 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.

[0024] 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 amount of oxidizing agent used is 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 raw material. Furthermore, for example, 1 to 40 mol is preferred per 1 mol of the N-oxyl compound.

[0025] The oxidation of cellulose 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, causing 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.

[0026] The reaction time in the oxidation reaction can be appropriately set depending on the degree of progress of the oxidation, and is usually from 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 carboxylated cellulose obtained by filtration after the first stage of the reaction can be oxidized again under the same or different reaction conditions, thereby enabling efficient oxidation without reaction inhibition by sodium chloride produced as a by-product in the first stage of the reaction.

[0027] 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 (carboxylates) 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 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 cellulose 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 carboxylated 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 cellulose raw material in the solution.

[0028] The amount of carboxyl groups in carboxylated cellulose can be adjusted by controlling the 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 in carboxylated CNF obtained by defibrating the carboxylated cellulose.

[0029] Carboxylated CNF can be produced by defibrating carboxylated cellulose using the method described below. (carboxymethyl cellulose) An example of anion-modified cellulose is carboxymethylated cellulose (hereinafter, carboxymethylation is referred to as "CM"). CM-modified cellulose is the same as the above-mentioned cellulose. The carboxymethyl cellulose may be obtained by carboxymethylating a raw material cellulose using a known method, or a commercially available product may be used. In either case, the degree of carboxymethyl group substitution per anhydroglucose unit of the cellulose is preferably 0.01 to 0.50. An example of a method for producing such carboxymethyl cellulose is as follows: To the cellulose raw material, 3 to 20 times by mass of water and / or a lower alcohol, specifically water, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, tertiary butanol, etc., is added as a solvent, or a mixture of two or more of these. When a lower alcohol is added, the mixing ratio of the lower alcohol is preferably 60 to 95% by mass. As the mercerizing agent, it is preferable to use an alkali metal hydroxide, specifically sodium hydroxide or potassium hydroxide, in an amount of 0.5 to 20 times by mole per anhydroglucose residue of the cellulose raw material. The cellulose raw material, solvent, and mercerizing agent are mixed, and mercerization treatment is carried out 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 in an amount of 0.05 to 10.0 times by mole per glucose residue, and the 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.

[0030] In this specification, "carboxymethylated cellulose" or "carboxymethylated cellulose," a type of anion-modified cellulose used in the preparation of carboxymethylated CNF, refers to a material that maintains at least a portion of its fibrous shape when dispersed in water. Therefore, "carboxymethylated cellulose" or "carboxymethylated cellulose" is distinguished from carboxymethyl cellulose, a type of water-soluble polymer. When an aqueous dispersion of "carboxymethylated cellulose" or "carboxymethylated cellulose" is observed under an electron microscope, fibrous substances can be observed. On the other hand, when an aqueous dispersion of carboxymethyl cellulose, a type of water-soluble polymer, is observed, no fibrous substances are observed. Furthermore, when "carboxymethylated cellulose" or "carboxymethylated cellulose" is measured by X-ray diffraction, peaks of cellulose type I crystals can be observed, but cellulose type I crystals are not observed in the water-soluble polymer carboxymethyl cellulose.

[0031] Carboxymethylated cellulose can be defibrated by the method described below to produce carboxymethylated CNF. Note that the degree of carboxymethyl substitution in carboxymethylated cellulose is usually the same as the degree of carboxymethyl substitution in carboxymethylated CNF obtained by defibrating the same carboxymethylated cellulose.

[0032] (Cellulose phosphate) An example of anion-modified cellulose is phosphated cellulose, which can be obtained by mixing the above-mentioned cellulose raw material with a powder or aqueous solution of a phosphoric acid compound.

[0033] Examples of phosphoric acid compounds include phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, metaphosphoric acid, pyrophosphoric acid, and salts or esters thereof. Among these, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, 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 are preferred due to their low cost, ease of handling, and improved defibration efficiency. Phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are more preferred. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. These compounds can be used alone or in combination. Furthermore, the phosphoric acid compound is preferably used as an aqueous solution, as this improves reaction uniformity and increases the efficiency of phosphate group introduction. The pH of the aqueous solution of the phosphoric acid compound is preferably 7 or less to increase the efficiency of phosphate group introduction, and a pH of 3 to 7 is preferred to suppress hydrolysis of cellulose fibers.

[0034] The following method can be cited as an example of a method for producing phosphated cellulose. A phosphoric acid compound is added to a dispersion 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 phosphoric acid-esterified CNF can be further improved. However, if the proportion exceeds the upper limit, the effect of improving the yield plateaus, which is undesirable from a cost perspective.

[0035] In addition to the cellulose raw material and the phosphoric acid compound, a basic nitrogen-containing compound may be added. Here, "basic" is defined as the aqueous solution exhibiting a pink to red color in the presence of a phenolphthalein indicator or the aqueous solution having a pH greater than 7. Examples of basic nitrogen-containing compounds include, but are not limited to, compounds having an amino group. 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 basic nitrogen-containing 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 phosphorylation reaction conditions are within these ranges, it is possible to prevent 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 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.

[0036] The degree of phosphate group substitution per glucose unit of the phosphated cellulose is preferably 0.001 to 0.40. Introducing phosphate group substituents into cellulose causes electrical repulsion between cellulose molecules. Therefore, cellulose with introduced phosphate groups can be easily defibrated into nanoscale fiber widths. If the degree of phosphate group substitution per glucose unit is less than 0.001, sufficient nanofibrillation cannot be achieved. On the other hand, if the degree of phosphate group substitution per glucose unit is greater than 0.40, the cellulose may swell or dissolve, making it impossible to obtain CNF. To efficiently defibrate the phosphated cellulose obtained above, it is preferable to boil it and then wash it with cold water.

[0037] Phosphated CNF can be produced by defibrating phosphated cellulose using the method described below. Note that the degree of phosphate substitution in phosphated CNF is usually the same as the degree of phosphate substitution in phosphated CNF obtained by defibrating the same phosphated cellulose.

[0038] (cationically modified cellulose) The chemically modified cellulose used in preparing chemically modified CNF may be cationically modified cellulose obtained by further cationizing the carboxylated cellulose. The cationically modified cellulose can be obtained by reacting the carboxylated cellulose with a cationizing agent such as glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium hydride, or its halohydrin form, and an alkali metal hydroxide catalyst (sodium hydroxide, potassium hydroxide, etc.) in the presence of water or an alcohol having 1 to 4 carbon atoms.

[0039] The degree of cationic substitution per glucose unit is preferably 0.02 to 0.50. Introducing cationic substituents into cellulose causes electrical repulsion between cellulose molecules. Therefore, cellulose with cationic substituents introduced can be easily defibrated into nanoscale fiber widths. If the degree of cationic substitution per glucose unit is less than 0.02, sufficient defibration is not possible. On the other hand, if the degree of cationic substitution per glucose unit is greater than 0.50, the cellulose may swell or dissolve, making it impossible to obtain CNF. To ensure efficient defibration, it is preferable to wash the cationically modified cellulose obtained above before defibration. The degree of cationic substitution can be adjusted by the amount of cationizing agent added to the reaction and the composition ratio of water or alcohol with 1 to 4 carbon atoms.

[0040] Cationically modified CNF can be produced by defibrating cationically modified cellulose using the method described below. Note that the degree of cation substitution in the cationically modified cellulose is usually the same as the degree of cation substitution in the cationically modified CNF obtained by defibrating the same cationically modified cellulose.

[0041] (defibration) CNF can be obtained by defibrating cellulose raw materials containing the above-mentioned chemically modified cellulose. While the equipment used for defibration is not particularly limited, it is preferable to use equipment capable of applying strong shearing forces, such as high-speed rotary, colloid mill, high-pressure, roll mill, and ultrasonic equipment. For particularly efficient defibration, it is preferable to apply a pressure of 50 MPa or more to the dispersion of the cellulose raw material to be defibrated, and to use a wet high-pressure or ultra-high-pressure homogenizer capable of applying strong shearing forces. The pressure is more preferably 100 MPa or more, and even more preferably 140 MPa or more. Furthermore, prior to defibration and dispersion treatment using a high-pressure homogenizer, pretreatment may be performed, if necessary, using a known mixing, stirring, emulsifying, or dispersing device, such as a high-speed shear mixer.

[0042] The average fiber length of CNF obtained by defibration can be adjusted by changing defibration conditions such as the type of equipment and pressure, as well as the combination of pre-treatments. (Dispersion containing CNF) The above defibration process yields a dispersion containing CNF. The CNF in the dispersion to be dried in the present invention has an average fiber length of 450 nm or less, preferably 400 nm or less, and more preferably 300 nm or less.

[0043] The dispersion medium in the dispersion to be dried is not particularly limited, but is preferably water, a hydrophilic organic solvent, a hydrophobic organic solvent, or a mixture thereof, and more preferably water or a mixture of water and a hydrophilic organic solvent. Since most chemically modified celluloses are produced using water as a dispersion medium, when using chemically modified CNF derived from chemically modified celluloses such as anion-modified cellulose or carboxylated cellulose, the aqueous dispersion of chemically modified CNF obtained by defibrating the chemically modified cellulose can be dried directly. Alternatively, the aqueous dispersion may be subjected to pretreatment such as drying or filtration before being subjected to the drying process using the spray drying apparatus of the present invention.

[0044] When using a mixed solvent of water and a hydrophilic organic solvent, the hydrophilic organic solvent can be added to an aqueous dispersion of a cellulose raw material such as chemically modified cellulose or an aqueous dispersion of CNF, or a portion of the aqueous dispersion can be replaced with a hydrophilic organic solvent. The replacement can be achieved by removing water from the aqueous dispersion by drying or filtration, obtaining a concentrated aqueous dispersion or wet cake, and then adding the hydrophilic organic solvent to the resulting mixture. The amount of solvent is preferably 10 to 100% by mass, more preferably 20 to 80% by mass, of the water.

[0045] The hydrophilic organic solvent is an organic solvent that dissolves in water. Examples of the hydrophilic organic solvent include methanol, ethanol, 2-propanol, butanol, glycerin, acetone, methyl ethyl ketone, 1,4-dioxane, N-methyl-2-pyrrolidone, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetonitrile, and combinations thereof. Among these, lower alcohols having 1 to 4 carbon atoms, such as methanol, ethanol, and 2-propanol, are preferred, and from the viewpoints of safety and availability, methanol and ethanol are more preferred, and ethanol is even more preferred.

[0046] The amount of the hydrophilic organic solvent in the mixed solvent is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more, based on the mass of the mixed solvent. There is no upper limit to this amount, but it is preferably 95% by mass or less, more preferably 80% by mass or less. The mixed solvent may also contain a hydrophobic organic solvent to the extent that the effects of the invention are not impaired.

[0047] The pH of the dispersion containing CNF to be dried may be adjusted to 7 to 11 using an alkali such as sodium hydroxide. When the pH is in this range, redispersibility tends to be improved.

[0048] The solid content of the dispersion containing CNF to be dried is preferably about 0.1 to 10 mass %, more preferably about 0.5 to 8 mass %, and even more preferably about 1 to 5 mass %.

[0049] A water-soluble polymer may be added to a dispersion containing CNFs with an average fiber length of 450 nm or less to be dried. This may result in a dry solid containing the water-soluble polymer in addition to the CNFs with an average fiber length of 450 nm or less. Examples of water-soluble polymers include, but are not limited to, cellulose derivatives such as carboxymethyl cellulose and dextrin. When a water-soluble polymer is added, the amount of the water-soluble polymer added is preferably 1 to 50% by mass, more preferably 20 to 45% by mass, and even more preferably 30 to 40% by mass, based on the CNFs (bone dry solid content).

[0050] (Dry) Drying a dispersion containing CNFs with an average fiber length of 450 nm or less can yield a CNF dry solid with good redispersibility. A spray dryer is preferably used for drying.

[0051] The type of atomization device in the spray dryer is not particularly limited. It may be a rotary atomizer type or a nozzle type. In the case of a nozzle type, it may be a one-fluid nozzle or a two-fluid nozzle. It may also be a parallel-flow two-fluid nozzle or a fountain-type two-fluid nozzle. Of these, it is preferable to use a spray dryer equipped with a two-fluid nozzle because it is easy to handle.

[0052] The inlet temperature of the spray dryer is preferably 100°C to 350°C, more preferably 120°C to 250°C, and even more preferably 140°C to 210°C. The outlet temperature is preferably 50°C to 150°C, more preferably 60°C to 130°C, and even more preferably 70°C to 120°C. If the temperature is too low, drying may not proceed sufficiently, and if the temperature is too high, the CNF may be overdried, resulting in reduced redispersibility.

[0053] The drying speed is preferably about 1.0 to 5.0 kg / h, more preferably about 1.2 to 3.5 kg / h, and even more preferably about 1.5 to 3.0 kg / h, for a dry air volume of 80 kg / h. If these speeds are slow, the yield may decrease, and if the speed is fast, there is a possibility that drying will not proceed sufficiently.

[0054] (CNF dry solids) The above drying process results in a CNF dry solid. In this specification, the term "CNF dry solid" includes those having a solid content of 80% by mass or more, and may be in a wet state depending on the amount of solid content. From the viewpoint of reducing transportation costs, the solid content is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 93% by mass or more. Normally, drying to such a high solid content tends to deteriorate redispersibility, but the dry solid obtained by the production method of the present invention can have high redispersibility while still having a high solid content.

[0055] (Redispersion) The CNF dry solid obtained by the present invention has good redispersibility. Good redispersibility means that there is little change in viscosity, transparency, etc. between the CNF dispersion in a wet state before drying and the CNF dispersion obtained by redispersing the CNF dry solid.

[0056] The apparatus used to redisperse the dried solid in a dispersion medium to prepare a dispersion is not particularly limited, but examples thereof include a dispersing machine such as a homomixer. The dispersion medium used for redispersion is not particularly limited, but examples thereof include water, the hydrophilic organic solvents, and mixed solvents thereof, with water being preferred. The solid content of the dispersion after redispersion may be appropriately selected depending on the application, and is not particularly limited, but is preferably about 0.1 to 10.0 mass%, more preferably about 1.0 to 6.0 mass%.

[0057] (viscosity) As mentioned above, one indicator of good redispersibility is a small change in viscosity between a wet CNF dispersion before drying and a CNF dispersion obtained by redispersing the dried CNF solid. For example, but not limited to, the viscosity (viscosity recovery rate) of a CNF aqueous dispersion obtained by adding water after drying to achieve the same solids concentration as the CNF aqueous dispersion before drying is preferably 30% or more, more preferably 40% or more, even more preferably 50% or more, and even more preferably 55% or more. As described in the Examples below, the viscosity can be measured after 3 minutes using a Brookfield viscometer at 25°C and a rotation speed of 6 or 60 rpm.

[0058] (Transparency (transmittance of 660nm wavelength light)) As described above, one indicator of good redispersibility is a small change in transparency between a wet CNF dispersion before drying and a CNF dispersion obtained by redispersing the CNF into a dried solid. Another indicator of good redispersibility is high transparency of the CNF dispersion (redispersion) itself after redispersion. Transparency can be measured by determining the transmittance of light at a wavelength of 660 nm using a square cell with an optical path length of 10 mm, as described in the Examples below.

[0059] For example, but not limited to, the transparency (transparency restoration rate) of a CNF aqueous dispersion obtained by adding water after drying and redispersing the CNF to the same solids concentration as the CNF aqueous dispersion before drying is preferably 50% or more, more preferably 60% or more, even more preferably 70% or more, even more preferably 80% or more, and even more preferably 90% or more, relative to the transparency of the CNF aqueous dispersion before drying.

[0060] Also, water is added to the CNF dry solid so that the concentration of CNF becomes 1.0% by mass, and the transparency (transmittance of light with a wavelength of 660 nm, optical path length 10 mm) of the aqueous dispersion (redispersion liquid) obtained by stirring for 30 minutes with a homodisper (3000 rpm) is preferably 60% or more. More preferably 65% or more, still more preferably 80% or more, still more preferably 85% or more, and still more preferably 90% or more.

[0061] (Evaluation of CNF aggregates in the redispersion liquid) When the redispersibility of the dry solid is good, the amount of aggregates between CNFs in the redispersion liquid tends to be small. The amount of CNF aggregates in the redispersion liquid can be evaluated by calculating the area ratio of CNF aggregates by the method described in the examples below. The area ratio of CNF aggregates is preferably 10.0% or less. More preferably 8.0% or less, still more preferably 5.0% or less, and still more preferably 1.0% or less.

Examples

[0065] <Production of carboxylated CNF1> Five grams of bleached softwood-derived dissolving kraft pulp (Buckeye DKP) (bone-dry) was added to 500 ml of an aqueous solution containing 78 mg (0.5 mmol) of TEMPO (Sigma-Aldrich) and 755 mg (7.4 mmol) of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. After adding 16 ml of 2 M aqueous sodium hypochlorite, the reaction mixture was adjusted to pH 10.3 with 0.5 N aqueous hydrochloric acid to initiate the oxidation reaction (oxidation treatment). During the reaction, the pH of the mixture decreased, but was gradually adjusted to pH 10 by the addition of 0.5 N aqueous sodium hydroxide. After 2 hours of reaction, the mixture was filtered through a glass filter and thoroughly washed with water to obtain carboxylated cellulose. A 5.0% (w / v) carboxylated cellulose slurry was prepared using water. To this slurry, hydrogen peroxide was added at 2% (w / w) relative to the carboxylated cellulose, and the pH was adjusted to 11.3 with 3 M sodium hydroxide. This slurry was left at 80°C for 2 hours to allow hydrolysis. The slurry was adjusted to 5.0% (w / v) with water and treated 10 times with an ultra-high pressure homogenizer (20°C, 140 MPa) to obtain carboxylated CNF1. The amount of carboxyl groups in the obtained carboxylated CNF1 was 1.7 mmol / g, the average fiber diameter was 3 nm, and the average fiber length was 350 nm.

[0066] <Production of carboxylated CNF2> Carboxylated CNF1 was produced in the same manner as carboxylated CNF1, except that the amount of 2M sodium hypochlorite aqueous solution added was changed to 14 ml. The amount of carboxyl groups was 1.5 mmol / g, the average fiber diameter was 3 nm, and the average fiber length was 400 nm.

[0067] <Production of carboxylated CNF3> Five grams (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) and 514 mg of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system to a concentration of 5.5 mmol / g to initiate the oxidation reaction. The pH of the system decreased during the reaction, but was gradually adjusted to pH 10 by adding 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 acidified with hydrochloric acid, filtered through a glass filter, and thoroughly washed to obtain oxidized pulp, i.e., carboxylated cellulose. The resulting mixture was adjusted to 1.0% (w / v) with water and subjected to three passes in an ultra-high-pressure homogenizer (20°C, 150 MPa) to obtain carboxylated CNF3. The amount of carboxyl groups in the obtained carboxylated CNF3 was 1.6 mmol / g, the average fiber diameter was 3 nm, and the average fiber length was 550 nm.

[0068] <Production of carboxylated CNF4> Carboxylated CNF1 was produced in the same manner as carboxylated CNF1, except that the amount of 2M sodium hypochlorite aqueous solution added was changed to 9 mL. The amount of carboxyl groups was 0.9 mmol / g, the average fiber diameter was 3 nm, and the average fiber length was 400 nm.

[0069] <Production of carboxylated CNF5> The production was carried out in the same manner as for carboxylated CNF4, except that the treatment with the ultra-high pressure homogenizer (20°C, 140 MPa) was changed to five times. The carboxyl group amount was 0.9 mmol / g, the average fiber diameter was 3 nm, and the average fiber length was 500 nm.

[0070] <Production of dry solids> Example 1 The above-mentioned aqueous dispersion of carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) was prepared and dried in a rotary atomizer-type spray dryer at an inlet temperature of 200°C, an outlet temperature of 90°C, a drying air flow rate of 80 kg / h, and a liquid feed rate of 2.12 kg / h to obtain a dried solid. The solid content of the obtained dried solid was 95.6% by mass. During drying in the device, adhesion of the dried solid to the wall surface was observed.

[0071] Example 2 The above-mentioned aqueous dispersion of carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) was prepared and dried in a two-fluid nozzle spray dryer at an inlet temperature of 200°C, an outlet temperature of 90°C, a drying air flow rate of 80 kg / h, and a liquid feed rate of 2.18 kg / h to obtain a dried solid. The solid content of the obtained dried solid was 96.5% by mass. No adhesion of the dried solid to the walls of the device was observed.

[0072] Example 3 A dry solid was obtained in the same manner as in Example 2, except that carboxylated CNF2 was used instead of carboxylated CNF1. The solid content of the obtained dry solid was 96.2 mass%. There was no adhesion of the dry solid to the wall surface of the apparatus.

[0073] Example 4 The above-mentioned aqueous dispersion of carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) was prepared and dried in a two-fluid nozzle spray dryer at an inlet temperature of 160°C, an outlet temperature of 70°C, a drying air flow rate of 80 kg / h, and a liquid feed rate of 2.18 kg / h to obtain a dried solid. The solid content of the obtained dried solid was 90.9% by mass. No adhesion of the dried solid to the walls of the device was observed.

[0074] Example 5 The above aqueous dispersion of carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) was adjusted to pH 5.5 by adding 1M hydrochloric acid, and dried in a two-fluid nozzle spray dryer at an inlet temperature of 200°C, an outlet temperature of 90°C, a drying air flow rate of 80 kg / h, and a liquid feed rate of 2.18 kg / h to obtain a dried solid. The solid content of the obtained dried solid was 94.9% by mass. No adhesion of the dried solid to the walls of the device was observed.

[0075] Example 6 The above-mentioned aqueous dispersion of carboxylated CNF4 (solid content 3.0% by mass, pH 7.5) was prepared and dried in a two-fluid nozzle spray dryer at an inlet temperature of 200°C, an outlet temperature of 90°C, a drying air flow rate of 80 kg / h, and a liquid feed rate of 2.18 kg / h to obtain a dried solid. The solid content of the obtained dried solid was 91.0% by mass. No adhesion of the dried solid to the walls of the device was observed.

[0076] (Comparative Example 1) The above-mentioned aqueous dispersion of carboxylated CNF3 (solid content 0.7% by mass, pH 7.5) was prepared and dried in a two-fluid nozzle spray dryer at an inlet temperature of 200°C, an outlet temperature of 90°C, a drying air flow rate of 80 kg / h, and a liquid feed rate of 2.18 kg / h to obtain a dried solid. The solid content of the obtained dried solid was 96.6% by mass. No adhesion of the dried solid to the walls of the device was observed.

[0077] (Comparative Example 2) The above-mentioned aqueous dispersion of carboxylated CNF5 (solid content 3.0% by mass, pH 7.5) was prepared and dried in a two-fluid nozzle spray dryer at an inlet temperature of 200°C, an outlet temperature of 90°C, a drying air flow rate of 80 kg / h, and a liquid feed rate of 2.18 kg / h to obtain a dried solid. The solid content of the obtained dried solid was 91.8% by mass. No adhesion of the dried solid to the walls of the device was observed.

[0078] <Observation of dried solid matter using an optical microscope> The dry solids obtained in Examples 1, 2, 4 to 6 and Comparative Examples 1 and 2 were observed using an optical microscope with a magnification of 400 times, respectively. The optical micrographs are shown in FIGS. 1 and 2. The dry solids were in the form of particles, and the particle sizes were such that the solids in Example 1 were less than about 30 μm, and the solids in Examples 2 to 6 and Comparative Examples 1 and 2 were less than about 12 μm.

[0079] <Redispersion of Dry Solids> Water was added to the dry solids obtained in Examples 1 to 6 and Comparative Example 2, and the mixture was stirred for 30 minutes at 3000 rpm using a homodisper to obtain redispersion liquids with CNF concentrations of 5.0% by mass, 1.0% by mass, or 0.5% by mass. Also, an attempt was made to create a redispersion liquid in the same manner for the dry solids obtained in Comparative Example 1. However, since it could not be redispersed at a CNF concentration of 5.0% by mass, redispersion liquids with concentrations of 1.0% by mass and 0.5% by mass were created for Comparative Example 1.

[0080] <Measurement of Viscosity of CNF Redispersion Liquid> For the CNF redispersion liquids of Examples 1 to 6 and Comparative Example 2, using an aqueous dispersion with a CNF concentration of 5.0% by mass, the viscosity after 3 minutes was measured at 25°C at a rotation speed of 60 rpm or 6 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.). For the redispersion liquid of Comparative Example 1, the viscosity was measured in the same manner as in Examples 1 to 6 and Comparative Example 2, except that an aqueous dispersion with a CNF concentration of 1.0% by mass was used.

[0081] <Measurement of Transparency of CNF Redispersion Liquid> For Examples 1 to 6 and Comparative Examples 1 and 2, using a CNF redispersion liquid with a CNF concentration of 1.0% by mass, the transmittance of light with a wavelength of 660 nm was measured using a spectrophotometer U-3000 (manufactured by Hitachi High-Technologies Corporation) in a rectangular cell with an optical path length of 10 mm, and the transparency (unit: %) was determined.

[0082] <Evaluation of Recovery Rate of Viscosity and Transparency> When the viscosity and transparency of the CNF dispersion before drying were measured in advance, the results were as shown in Table 1. The viscosity was measured at a CNF concentration of 5.0% by mass for carboxylated CNFs 1, 2, 4, and 5, and at a CNF concentration of 1.0% by mass for carboxylated CNF 3. The transparency was measured at a CNF concentration of 1.0% by mass for all of carboxylated CNFs 1 to 5.

[0083]

Table 1

[0084] Using the viscosity and transparency values of the CNF dispersion before drying in Table 1 and the viscosity and transparency values of the redispersion liquid obtained by redispersing after making it into CNF dry solids, the viscosity recovery rate and the transparency recovery rate were calculated respectively by the following formula: Recovery rate (%) = (viscosity or transparency in the redispersion liquid) / (viscosity or transparency of the dispersion before drying) × 100 The results are shown in Table 2. [[ID=十七]]

[0085] <Measurement of the area ratio of CNF aggregates in the CNF redispersion liquid and observation by optical microscope> For Examples 1, 2, 4 to 6 and Comparative Examples 1 and 2, a CNF redispersion liquid with a CNF concentration of 0.5% by mass was used, and the CNF aggregates in the redispersion liquid were evaluated using ink droplets. This method makes it easier to confirm the presence or absence of aggregates of CNFs in the dispersion that cannot be visually discriminated by adding a coloring material (ink droplets) to the CNF dispersion and then observing with an optical microscope. The evaluation method is specifically as follows: Two drops of ink (10% solids, manufactured by Kuretake Co., Ltd.) were added to a redispersion solution with a CNF concentration of 0.5% by mass, and the solution was stirred for 30 seconds using a vortex mixer (Iuchi Automatic Lab-mixer HM-10H) set to maximum speed. The stirred solution was sandwiched between two glass plates to achieve a film thickness of 0.15 mm and observed at 100x magnification using an optical microscope (Keyence VHX-6000 Digital Microscope). The area ratio of CNF aggregates was measured using the Keyence VHX-6000 Digital Microscope's area measurement mode for brightness extraction regions. Specifically, the brightness range was set to 180–260 in the observed image of the CNF aggregates, and the area of ​​this region (the area within the brightness range) was extracted. The area of ​​this region (the area within the brightness range) was used to calculate the area ratio using the following formula: Details are given on pages 9-29 and 30 of the user's manual for the KEYENCE VHX-6000 digital microscope: Area ratio (%) = (area within brightness range / area of ​​measurement range) x 100.

[0086] The results of the area ratio are shown in Table 2. Also, photographs of the redispersed liquid with ink drops dropped thereon observed under an optical microscope at 100x magnification are shown in Figures 3 and 4.

[0087] [Table 2]

[0088] The results in Table 2 show that by using CNF with an average fiber length of 450 nm or less, it is possible to produce a CNF dry solid that has good redispersibility and has few CNF aggregates when redispersed.

[0089] 3 and 4 show that no CNF aggregates were observed in the redispersions of Examples 1, 2, and 4. Some aggregates were observed in the redispersions of Examples 5 and 6, but to a lesser extent. On the other hand, in the redispersions of Comparative Examples 1 and 2, numerous particulate aggregates of CNF were observed.

Claims

1. A method for producing a chemically modified cellulose nanofiber dry solid, comprising drying a dispersion containing chemically modified cellulose nanofibers having an average fiber length of 450 nm or less using a spray drying device, The above-mentioned production method, wherein the transmittance of light with a wavelength of 660 nm (optical path length 10 mm) of an aqueous dispersion obtained by adding water to the dry solid so that the concentration of chemically modified cellulose nanofiber becomes 1.0% by mass and stirring at 3000 rpm for 30 minutes is 50% or more of the transmittance of light with a wavelength of 660 nm (optical path length 10 mm) of an aqueous dispersion obtained by adding water to the chemically modified cellulose nanofiber before drying so that the concentration of chemically modified cellulose nanofiber becomes 1.0% by mass and stirring at 3000 rpm for 30 minutes.

2. 2. The method according to claim 1, wherein the spray atomizing method in the spray drying apparatus is a two-fluid nozzle or a rotary atomizer.

3. The method according to claim 1 or 2, wherein the solid content of the dry solid matter is 90.0% by mass or more.

4. The method according to any one of claims 1 to 3, wherein the chemically modified cellulose nanofibers are anionically modified cellulose nanofibers.

5. The method according to claim 4 , wherein the anion-modified cellulose nanofibers are carboxylated cellulose nanofibers.

6. The method according to claim 4, wherein the carboxylated cellulose nanofiber has a carboxyl group content of 0.6 to 3.0 mmol / g relative to the bone dry mass of the carboxylated cellulose nanofiber.

7. A chemically modified cellulose nanofiber dry solid comprising chemically modified cellulose nanofibers having an average fiber length of 400 nm or less, The dry solid, wherein the transmittance of light with a wavelength of 660 nm (optical path length 10 mm) of an aqueous dispersion obtained by adding water to the dry solid so that the concentration of chemically modified cellulose nanofiber becomes 1.0% by mass and stirring at 3000 rpm for 30 minutes is 50% or more relative to the transmittance of light with a wavelength of 660 nm (optical path length 10 mm) of an aqueous dispersion obtained by adding water to the chemically modified cellulose nanofiber before drying so that the concentration of chemically modified cellulose nanofiber becomes 1.0% by mass and stirring at 3000 rpm for 30 minutes.

8. The dry solid according to claim 7, having a solid content of 90.0% by mass or more.

9. The dry solid according to claim 7 or 8, wherein the transmittance of light at a wavelength of 660 nm (optical path length 10 mm) of an aqueous dispersion obtained by adding water to the dry solid so that the concentration of cellulose nanofibers becomes 1.0 mass% and stirring for 30 minutes with a Homo Disper (3000 rpm) is 65% or more.

10. A dry solid described in any one of claims 7 to 9, wherein after adding water to the dry solid so that the concentration of chemically modified cellulose nanofibers is 0.5 mass%, adding drops of ink and stirring, the area ratio of chemically modified cellulose nanofiber aggregates calculated by observation under an optical microscope is 10.0% or less.

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