JP7303794B2 - METHOD FOR MANUFACTURING CELLULOSE NANOFIBER DRY SOLID - Google Patents

Description

The present invention relates to a method for producing cellulose nanofiber dry solids.

Cellulose nanofibers (hereinafter sometimes referred to as CNF) are fine fibers with a fiber diameter of about 3 to several hundred nm that are excellent in dispersibility in aqueous media, and are used in foods, cosmetics, medical products, or paints. It is expected to be used as a viscosity retention such as, strengthening of food raw material dough, moisture retention, food stability improvement, low calorie additive or emulsification stabilizing aid. When the CNF dispersed in water (wet state) is dried to form a dry solid, hydrogen bonds are formed between the fibers of the fine cellulose fibers, so water is added to the dry solid. Even if you try to re-disperse it, the properties such as viscosity will not be restored to the same level as before drying (wet state). For this reason, CNF is produced in a state of being dispersed in water (wet state), and is usually used in various applications in a wet state without being dried.

However, in order to keep the CNF in this wet state stable, it is necessary to have several times to several hundred times the mass of water relative to the CNF, and various problems such as securing storage space, increasing storage and transportation costs, etc. there is a point As means for drying cellulose in a wet state, a freeze-drying method (Patent Document 1), a critical point drying method, and a method of adding a redispersant such as glycerin (Patent Document 2) have been proposed.

JP-A-6-233691 JP 2017-78145 A

However, when CNF is freeze-dried, a huge amount of energy is required, and depending on the conditions, when the water between the fine fibers of CNF freezes, ice crystals larger than the gaps between the fine fibers grow. Problems such as association of fine fibers occur. In addition, when a redispersing agent such as glycerin is added and dried, even if the CNF does not appear to aggregate, the physical properties such as the viscosity of the CNF when redispersed after drying are significantly compared to before drying. I had a problem with the drop.

Therefore, an object of the present invention is to provide a CNF dry solid having good redispersibility. Good redispersibility means that the change rate between the viscosity of the CNF dispersion in a wet state before drying and the viscosity of the CNF dispersion obtained by redispersing after making the CNF dry solid is small. say.

The present invention provides the following.
[1] A method for producing a dry solid of cellulose nanofibers, which comprises drying a mixture of cellulose nanofibers and a solvent using a vacuum drying device.
[2] The method for producing a cellulose nanofiber dry solid according to [1], wherein the drying using the vacuum drying apparatus is performed at a temperature of 40 to 100°C.
[3] The method for producing a cellulose nanofiber dry solid according to [1], wherein the drying using the vacuum drying apparatus is performed at a temperature of 40 to 90°C.
[4] The method for producing a cellulose nanofiber dry solid according to any one of [1] to [3], wherein the vacuum drying device is a drum type vacuum drying device.
[5] The method for producing a cellulose nanofiber dry solid according to any one of [1] to [4], further comprising adjusting the pH of the mixture of the cellulose nanofibers and solvent to 8 to 11. .
[6] The method for producing a cellulose nanofiber dry solid according to any one of [1] to [5], wherein the solvent contains a hydrophilic organic solvent.
[7] The cellulose nanofibers are carboxylated cellulose nanofibers having a carboxyl group content of 0.6 to 3.0 mmol/g relative to the absolute dry mass of the cellulose nanofibers [1] to [6] A method for producing a cellulose nanofiber dry solid according to any one of .
[8] The cellulose nanofiber according to any one of [1] to [6], wherein the cellulose nanofiber is a carboxymethylated cellulose nanofiber having a degree of carboxymethyl substitution per glucose unit of 0.01 to 0.50. of cellulose nanofiber dry solids.
[9] Any one of [1] to [8], wherein the mixture of the cellulose nanofibers and the solvent further contains 5 to 300% by mass of a water-soluble polymer relative to the dry mass of the cellulose nanofibers. The method for producing a cellulose nanofiber dry solid according to 1.
[10] The method for producing a cellulose nanofiber dry solid according to [9], wherein the water-soluble polymer is carboxymethylcellulose and its salt.

According to the present invention, it is possible to provide a CNF dry solid having good redispersibility. Good redispersibility means that the change rate between the viscosity of the CNF dispersion in a wet state before drying and the viscosity of the CNF dispersion obtained by redispersing after making the CNF dry solid is small. say.

4 is a graph showing the loss tangent of water dispersions/suspensions after redispersion of Example 3 and Comparative Example 1. FIG. 2 is a photograph showing suspension stability of water dispersion/suspension after redispersion of Example 3 and Comparative Example 1. FIG. 2 is a photograph showing emulsion stability of water dispersion/suspension after redispersion of Example 3 and Comparative Example 1. FIG.

The present invention will be described in detail below. In the present invention, "-" includes values at both ends thereof. That is, "X through Y" includes X and Y.

The dry solid of cellulose nanofibers (CNF) of the present invention is compared to the wet CNF dispersion before drying when the solid is redispersed in water and returned to the wet CNF dispersion. , has the property of little change in viscosity (good redispersibility).

The reason why the dry solid of CNF of the present invention exhibits excellent redispersibility is not clear, but the present inventors speculate as follows. It is presumed that the vacuum drying of CNF can suppress the generation of hydrogen bonding between fibers and the entanglement of fibers, which are considered to be factors that reduce the electrical repulsion suitable for redispersion. Furthermore, when the solvent contains a hydrophilic organic solvent, the formation of hydrogen bonds can be suppressed more effectively, so the effect becomes even more remarkable. In addition, since the temperature is relatively low, the CNF is less thermally degraded and a dry solid with good physical properties can be obtained.

In the present invention, the dry solid of cellulose nanofibers refers to those in an absolute dry state (with a solvent content of 0% by mass) or those in a wet state with a solvent content of 15% by mass or less. From the viewpoint of reducing transportation costs, the solvent amount is preferably 0 to 15% by mass, more preferably 0 to 10% by mass.

(cellulose nanofiber)
In the present invention, cellulose nanofibers (CNF) are cellulose-based raw materials, such as pulp, which have been refined to nanometer-level fiber widths. , about 4 to 500 nm. The average fiber diameter and average fiber length of cellulose nanofibers are the average values of the fiber diameter and fiber length obtained from the results of observing each fiber using an atomic force microscope (AFM) or a transmission electron microscope (TEM). It can be obtained by calculating Also, the aspect ratio can be calculated by dividing the average fiber length by the average fiber diameter. Cellulose nanofibers can be obtained by refining pulp by mechanical force, or by introducing carboxylated cellulose (also called oxidized cellulose), carboxymethylated cellulose, or phosphate ester groups. It can be obtained by defibrating chemically modified cellulose (chemically modified pulp) such as cellulose and cationized cellulose.

(solvent)
Although the solvent used in the present invention is not particularly limited, it is preferably water, a hydrophilic organic solvent, a hydrophobic organic solvent, or a mixed solvent thereof. Considering the dispersibility of the chemically modified pulp and CNF, the solvent is preferably water or a mixed solvent of water and a hydrophilic organic solvent. When the solvent is water, the CNF aqueous dispersion obtained by defibrating the chemically modified pulp can be dried as it is. Alternatively, the aqueous dispersion may be subjected to a pretreatment such as drying or filtration to form a concentrated aqueous dispersion and then subjected to the drying step of the present invention.

When the solvent is a mixed solvent of water and a hydrophilic organic solvent, a hydrophilic organic solvent is added to the aqueous dispersion of chemically modified pulp, the aqueous dispersion of acidified chemically modified pulp, or the aqueous dispersion of CNF. may be added, or a portion of the aqueous dispersion may be replaced with a hydrophilic organic solvent. The substitution is performed by removing water from the chemically modified pulp aqueous dispersion by drying or filtering to obtain a concentrated aqueous dispersion or a chemically modified pulp wet cake, to which a hydrophilic organic solvent is added. can be prepared by The amount of the solvent is preferably 10 to 100% by mass, more preferably 20 to 80% by mass, relative to the mass of water.

A hydrophilic organic solvent is an organic solvent that dissolves in water. Examples 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, Dimethylsulfoxide, acetonitrile, and combinations thereof. Among them, lower alcohols having 1 to 4 carbon atoms such as methanol, ethanol, and 2-propanol are preferred, and from the viewpoint of safety and availability, methanol and ethanol are more preferred, and ethanol is even more preferred.

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, relative to the mass of the mixed solvent. Although the upper limit of the amount is not limited, it is preferably 95% by mass or less, more preferably 80% by mass or less. Moreover, the mixed solvent may contain a hydrophobic organic solvent to the extent that the effects of the invention are not impaired.

(Method for producing dry solid)
By vacuum-drying a mixture of cellulose nanofibers and a solvent, a dry solid of cellulose nanofibers with good redispersibility can be obtained. At this time, the mixture may contain a water-soluble polymer. Further, it is preferable to adjust the pH of the mixture to 8 to 11 and then vacuum dry it, because it is possible to obtain a dry solid of cellulose nanofibers with even better redispersibility. Sodium hydroxide or the like may be used to adjust the pH.

(Vacuum drying)
In the present invention, vacuum drying is a method of drying under vacuum or reduced pressure. Lower atmospheric pressure lowers the partial pressure of water vapor in the air, lowers the boiling point of water, accelerates the evaporation rate, accelerates the drying of the object, and reduces the thermal effect on the sample.

When drying in vacuum or under reduced pressure, it is preferable to dry in the range of 0 to 50 kPa. The pressure of the vacuum device is preferably 50 kPa or less, more preferably 30 kPa or less, and even more preferably 10 kPa or less, because the lower the pressure, the more moisture can be evaporated at a lower temperature.

Preferably, in the drying step of the present invention, the mixture is vacuum dried at a relatively low temperature such as 40-100° C. to obtain a dry solid of CNF. It is also preferred to vacuum dry the mixture at a low temperature such as 40-90° C. to obtain a dry solid of CNF. If the drying temperature is low, production efficiency decreases, so the drying temperature is preferably 40° C. or higher, more preferably 45° C. or higher, and even more preferably 50° C. or higher. In addition, if the drying temperature is high, the cellulose is colored or damaged, so the drying temperature is preferably 100°C or lower, more preferably 90°C or lower, more preferably 85°C or lower, and 80°C or lower. is more preferable, and may be less than 80°C.

The vacuum drying apparatus is not particularly limited, but a vacuum box type drying apparatus, a vacuum drum dryer, a vacuum spray dryer, a vacuum belt dryer, or the like can be used alone or in combination of two or more.

(Vacuum type drum dryer)
Among these, it is preferable to use a drum-type vacuum drying apparatus from the viewpoint of energy efficiency, because the thermal energy is directly and uniformly supplied to the material to be dried. A drum-type vacuum drying apparatus is a heated drum placed under vacuum or reduced pressure, continuously supplying a mixture of CNF and a solvent to the drum surface while rotating the drum, and evaporating and concentrating the solvent. At the same time, CNF is attached to the drum surface in a thin film and dried, and the dried product formed on the drum surface is scraped off with a knife to produce a dry solid. When a drum-type vacuum drying apparatus is used, the above drying temperature corresponds to the temperature of the drum surface. When drying cellulose nanofibers containing water, a pre-concentration process is usually performed to reduce the water content as much as possible in order to increase the efficiency of the drying process. On the other hand, the use of a drum-type vacuum drying apparatus is preferable because a thin film can be formed and dried, and the drying process can be performed more efficiently and uniformly in a short period of time. Furthermore, a drum-type vacuum drying apparatus is preferable from the point that heat is not applied more than necessary and the dried material can be recovered immediately. The drum-type vacuum drying apparatus includes a double-drum type or twin-drum type apparatus using two drums, or a single-drum type apparatus using one drum, and any of them may be used. Among these, a double-drum type apparatus is preferable because the thickness of the thin film can be adjusted by adjusting the clearance between the drums.

The thickness of the thin film formed on the surface of the drum is preferably 50-1000 μm, more preferably 100-300 μm. When it is 50 μm or more, it is easy to scrape off after drying, and when it is 1000 μm or less, the redispersibility is further improved.

(raw material for cellulose)
Cellulose, which is a raw material for cellulose nanofibers, is known to originate from plants, animals (e.g., sea squirts), algae, microorganisms (e.g., acetobacter), microbial products, and the like. You can use either of them. Plant-derived materials include, for example, wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp ( LUKP), bleached hardwood kraft pulp (LBKP), unbleached softwood sulfite pulp (NUSP), bleached softwood sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.). In the present invention, plant- or microorganism-derived cellulose fibers are preferred, and plant-derived cellulose fibers are more preferred. The cellulose raw material may be chemically modified as described below. Cellulose nanofibers or chemically modified cellulose nanofibers can be obtained by refining the fiber width of the cellulose raw material or chemically modified cellulose raw material (chemically modified cellulose or chemically modified pulp) to the nanometer level.

(Chemical modification of cellulose raw material)
[Carboxymethylation]
In the present invention, carboxymethylated cellulose may be used as chemically modified cellulose used for preparing chemically modified CNF. Carboxymethylated cellulose may be obtained by carboxymethylating the above cellulose raw material by a known method, or a commercially available product may be used. In any case, the degree of carboxymethyl group substitution per anhydroglucose unit of cellulose is preferably 0.01 to 0.50. As an example of the method for producing such carboxymethylated cellulose, the following method can be mentioned. Using cellulose as a starting material, 3 to 20 times the weight of water and/or lower alcohol as a solvent, specifically water, methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, tertiary A single medium such as butanol or a mixed medium of two or more types is used. When the lower alcohol is mixed, the mixing ratio of the lower alcohol is 60 to 95% by mass. As the mercerizing agent, an alkali metal hydroxide, specifically sodium hydroxide or potassium hydroxide, is used in an amount of 0.5 to 20 times the moles of the anhydroglucose residue of the starting material. The starting material, solvent and mercerizing agent are mixed and mercerized at a reaction temperature of 0 to 70° C., preferably 10 to 60° C., for a reaction time of 15 minutes to 8 hours, preferably 30 minutes to 7 hours. Thereafter, a carboxymethylating agent is added at 0.05 to 10.0 mol per glucose residue, the reaction temperature is 30 to 90° C., preferably 40 to 80° C., and the reaction time is 30 minutes to 10 hours, preferably 1 hour. The etherification reaction is carried out for ~4 hours.

In the present specification, "carboxymethylated cellulose", which is a kind of chemically modified cellulose used for the preparation of chemically modified CNF, maintains at least a part of the fibrous shape even when dispersed in water. Say. Therefore, "carboxymethylated cellulose" is distinguished from carboxymethyl cellulose, which is a type of water-soluble polymer described below. When an aqueous dispersion of "carboxymethylated cellulose" is observed with an electron microscope, a fibrous substance can be observed. On the other hand, no fibrous substance is observed in an aqueous dispersion of carboxymethyl cellulose, which is a type of water-soluble polymer. In addition, when "carboxymethylated cellulose" is measured by X-ray diffraction, a peak of cellulose type I crystals can be observed, but cellulose type I crystals are not observed in carboxymethyl cellulose, which is a water-soluble polymer.

Carboxymethylated cellulose nanofibers can be produced by defibrating carboxymethylated cellulose by the method described below. The degree of carboxymethyl substitution in carboxymethylated cellulose and the degree of carboxymethyl substitution in carboxymethylated cellulose nanofibers obtained by defibrating the carboxymethylated cellulose are usually the same.

[Carboxylation]
In the present invention, carboxylated (oxidized) cellulose may be used as chemically modified cellulose used for preparing chemically modified CNF. Carboxylated cellulose (also called oxidized cellulose) can be obtained by carboxylating (oxidizing) the above cellulose raw material by a known method. Although not particularly limited, during carboxylation, it is preferable to adjust the amount of carboxyl groups to 0.6 to 3.0 mmol/g with respect to the absolute dry weight of the carboxylated cellulose. , more preferably 0.6 to 2.0 mmol/g, more preferably 1.0 mmol/g to 2.0 mmol/g.

As an example of a carboxylation (oxidation) method, 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 or mixtures thereof. A method can be mentioned. This oxidation reaction selectively oxidizes the primary hydroxyl group at the C6 position of the glucopyranose ring on the cellulose surface, resulting in a cellulose fiber having an aldehyde group and a carboxyl group (-COOH) or carboxylate group (-COO-) on the surface. can be obtained. Although the concentration of cellulose during the reaction is not particularly limited, it is preferably 5% by mass or less.

An N-oxyl compound means a compound capable of generating a nitroxy radical. As the N-oxyl compound, any compound can be used as long as it promotes the desired oxidation reaction. Examples include 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) and its derivatives (eg 4-hydroxy TEMPO).

The amount of the N-oxyl compound to be used is not particularly limited as long as it is a catalytic amount that can oxidize the raw material cellulose. For example, it is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, even more preferably 0.05 to 0.5 mmol, relative to 1 g of absolute dry cellulose. Also, it is preferably about 0.1 to 4 mmol/L with respect to the reaction system.

Bromides are compounds containing bromine, examples of which include alkali metal bromides that can be dissociated and ionized in water. Also, iodides are compounds containing iodine, examples of which include alkali metal iodides. The amount of bromide or iodide to be used can be selected within a range capable of promoting 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, even more preferably 0.5 to 5 mmol, relative to 1 g of absolute dry cellulose.

Known oxidizing agents can be used, for example, halogens, hypohalous acids, halogenous acids, perhalogenates or salts thereof, halogen oxides, peroxides and the like can be used. Among them, sodium hypochlorite is preferable because it is inexpensive and has less environmental load. The amount of the 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, relative to 1 g of absolute dry cellulose. Further, for example, 1 to 40 mol is preferable per 1 mol of the N-oxyl compound.

Oxidation of cellulose allows the reaction to proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature is preferably 4 to 40°C, and may be room temperature of about 15 to 30°C. Since carboxyl groups are generated in the cellulose as the reaction progresses, the pH of the reaction solution decreases. In order to allow the oxidation reaction to proceed efficiently, 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 8-12, preferably about 10-11. Water is preferable as the reaction medium because it is easy to handle and less likely to cause side reactions.

The reaction time in the oxidation reaction can be appropriately set according to the degree of progress of the oxidation, and is usually about 0.5 to 6 hours, for example about 0.5 to 4 hours.

Moreover, the oxidation reaction may be carried out in two steps. For example, by oxidizing the oxidized cellulose obtained by filtration after the completion of the reaction in the first step, again under the same or different reaction conditions, the reaction can be efficiently It can be oxidized well.

Another example of the carboxylation (oxidation) method is a method of oxidizing by contacting a cellulose raw material with an ozone-containing gas. This oxidation reaction oxidizes at least the hydroxyl groups at the 2- and 6-positions of the glucopyranose ring and causes degradation of the cellulose chain. The ozone concentration in the ozone-containing gas is preferably 50-250 g/m ³ , more preferably 50-220 g/m ³ . The amount of ozone added to the cellulose raw material is preferably 0.1 to 30 parts by mass, more preferably 5 to 30 parts by mass, when the solid content of the cellulose raw material is 100 parts by mass. The ozone treatment temperature is preferably 0 to 50°C, more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is about 1 to 360 minutes, preferably about 30 to 360 minutes. When the ozone treatment conditions are within these ranges, cellulose can be prevented from being excessively oxidized and decomposed, and the yield of oxidized cellulose is improved. After the ozone treatment, additional oxidation treatment may be performed using an oxidizing agent. The oxidizing agent used in the additional 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 additional 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 immersing the cellulose raw material in the solution.

The amount of carboxyl groups in the oxidized cellulose can be adjusted by controlling the reaction conditions such as the amount of the oxidizing agent added and the reaction time. The amount of carboxyl groups in oxidized cellulose and the amount of carboxyl groups in oxidized cellulose nanofibers obtained by defibrating the oxidized cellulose are usually the same. Carboxylated cellulose nanofibers (oxidized cellulose nanofibers) can be produced by defibrating carboxylated cellulose (oxidized cellulose) by the method described below.

[Cationization]
As chemically modified cellulose used for preparing chemically modified CNF, cellulose obtained by further cationizing the carboxylated cellulose may be used. The cation-modified cellulose (cationized cellulose) is obtained by adding a cationizing agent such as glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium hydrate or its halohydrin type to the carboxylated cellulose raw material, and a catalyst. can be obtained by reacting an alkali metal hydroxide (sodium hydroxide, potassium hydroxide, etc.) in the presence of water or an alcohol having 1 to 4 carbon atoms.

Preferably, the degree of cation substitution per glucose unit is between 0.02 and 0.50. By introducing cationic substituents into cellulose, the celluloses electrically repel each other. Therefore, cellulose into which cationic substituents have been introduced can be easily nano-fibrillated. If the degree of cation substitution per glucose unit is less than 0.02, sufficient nanofibrillation cannot be achieved. On the other hand, if the degree of cation substitution per glucose unit is greater than 0.50, the nanofibers may not be obtained due to swelling or dissolution. In order to perform defibration efficiently, it is preferable to wash the cationized cellulose obtained above before defibration. The degree of cation substitution can be adjusted by adjusting the amount of the cationizing agent to be reacted and the composition ratio of water or alcohol having 1 to 4 carbon atoms.

Cationized cellulose nanofibers can be produced by defibrating cationized cellulose by the method described below. The degree of cation substitution in cationized cellulose and the degree of cation substitution in cationized cellulose nanofibers obtained by defibrating the cationized cellulose are usually the same.

[Esterification]
Esterified cellulose may be used as chemically modified cellulose used for preparing chemically modified CNF. The esterified cellulose can be obtained by a method of mixing the cellulose raw material with powder or an aqueous solution of the phosphate compound A, or a method of adding an aqueous solution of the phosphate compound A to a slurry of the cellulose raw material.

Phosphoric acid compound A includes phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, and esters thereof. These may be in the form of salts. Among these, compounds having a phosphate group are preferred because they are low in cost, easy to handle, and can improve defibration efficiency by introducing a phosphate group into the cellulose of pulp fibers. Compounds having a phosphate group include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, phosphorus Tripotassium acid, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate and the like. These can be used singly or in combination of two or more. Among these, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, phosphoric acid, from the viewpoint of high efficiency of introduction of phosphate groups, easy fibrillation in the fibrillation process described below, and easy industrial application is more preferred. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. Further, it is preferable to use the phosphoric acid-based compound A in the form of an aqueous solution, since the uniformity of the reaction is enhanced and the efficiency of introduction of the phosphoric acid group is enhanced. The pH of the aqueous solution of the phosphoric acid-based compound A is preferably 7 or less because the efficiency of introducing phosphate groups is high, but from the viewpoint of suppressing hydrolysis of pulp fibers, the pH is preferably 3 to 7.

The following method can be given as an example of the method for producing the phosphorylated cellulose. Phosphoric acid group is introduced into cellulose by adding phosphoric acid compound A to a dispersion liquid of cellulose raw material having a solid content concentration of 0.1 to 10% by mass while stirring. When the cellulose raw material is 100 parts by mass, the amount of phosphoric acid compound A to be added is preferably 0.2 to 500 parts by mass, more preferably 1 to 400 parts by mass, in terms of elemental amount of phosphorus. If the proportion of the phosphoric acid-based compound A is at least the lower limit, the yield of fine fibrous cellulose can be further improved. However, if the above upper limit is exceeded, the effect of improving the yield reaches a ceiling, which is not preferable from the viewpoint of cost.

At this time, in addition to the cellulose raw material and the phosphoric acid-based compound A, a powder or an aqueous solution of a compound B other than these may be mixed. Although compound B is not particularly limited, a nitrogen-containing compound exhibiting basicity is preferred. "Basic" herein is defined as the pink-red color of the aqueous solution in the presence of the phenolphthalein indicator or the pH of the aqueous solution being greater than 7. The basic nitrogen-containing compound used in the present invention is not particularly limited as long as the effect of the present invention is exhibited, but a compound having an amino group is preferable. Examples include, but are not limited to, urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, hexamethylenediamine. Among these, urea is preferable because it is low in cost and easy to handle. The amount of compound B added is preferably 2 to 1,000 parts by mass, more preferably 100 to 700 parts by mass, based on 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. Although the reaction time is not particularly limited, it is about 1 to 600 minutes, more preferably 30 to 480 minutes. When the esterification reaction conditions are within these ranges, cellulose can be prevented from being excessively esterified and easily dissolved, and the yield of phosphate esterified cellulose is improved. After dehydrating the obtained phosphate-esterified cellulose suspension, it is preferable to heat-treat at 100 to 170° C. from the viewpoint of suppressing hydrolysis of cellulose. Further, it is preferable to heat at 130° C. or less, preferably 110° C. or less while water is contained in the heat treatment, and heat at 100 to 170° C. after removing the water.

The degree of phosphate group substitution per glucose unit of the phosphated cellulose is preferably 0.001 to 0.40. By introducing a phosphate group substituent into cellulose, the celluloses electrically repel each other. Therefore, cellulose into which a phosphate group has been introduced can be easily nano-fibrillated. If the degree of phosphate group substitution per glucose unit is less than 0.001, sufficient nano-fibrillation cannot be achieved. On the other hand, if the degree of phosphate group substitution per glucose unit is greater than 0.40, the nanofibers may not be obtained due to swelling or dissolution. In order to defibrate efficiently, it is preferable to wash the phosphate-esterified cellulose obtained above with cold water after boiling.

Phosphate-esterified cellulose nanofibers can be produced by defibrating the phosphate-esterified cellulose by the method described below. The degree of phosphate group substitution in the phosphate esterified cellulose and the degree of phosphate group substitution in the phosphate esterified cellulose nanofibers obtained by defibrating the phosphate esterified cellulose are usually the same.

(defibration)
Chemically modified cellulose nanofibers can be obtained by defibrating the chemically modified cellulose. The device used for fibrillation is not particularly limited, but a high-speed rotating type, colloid mill type, high pressure type, roll mill type, ultrasonic type, etc. is used to apply a strong shearing force to the dispersion containing chemically modified cellulose. preferably. In particular, in order to defibrate efficiently, 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 a strong shearing force. The pressure is more preferably 100 MPa or higher, still more preferably 140 MPa or higher. In addition, prior to fibrillation and dispersion treatment with a high-pressure homogenizer, if necessary, it is possible to perform preliminary treatment using a known mixing, stirring, emulsifying, and dispersing device such as a high-speed shear mixer. The resulting aqueous dispersion of CNF may be directly subjected to the drying step in the vacuum drying apparatus described above, or may be partially replaced with a hydrophilic organic solvent and then dried in the vacuum drying apparatus. may be subjected to a drying step.

(water-soluble polymer)
The mixture of cellulose nanofibers and solvent before drying may further contain a water-soluble polymer. The water-soluble polymer may be added to the cellulose raw material or chemically modified cellulose dispersion before defibration, may be added to the CNF dispersion after defibration, or may be added to the mixture of CNF and solvent. You may Examples of water-soluble polymers include cellulose derivatives (carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, ethylcellulose), xanthan gum, xyloglucan, dextrin, dextran, carrageenan, locust bean gum, alginic acid, alginate, pullulan, starch, potato starch, Kudzu flour, processed starch (cationized starch, phosphorylated starch, phosphoric acid cross-linked starch, phosphate monoesterified phosphoric acid cross-linked starch, hydroxypropyl starch, hydroxypropylated phosphate cross-linked starch, acetylated adipic acid cross-linked starch, acetylated phosphate cross-linked starch, Acetylated oxidized starch, sodium octenyl succinate, starch acetate, oxidized starch), cornstarch, gum arabic, gellan gum, polydextrose, pectin, chitin, water-soluble chitin, chitosan, casein, albumin, soy protein lysate, peptone, polyvinyl alcohol , polyacrylamide, polysodium acrylate, polyvinylpyrrolidone, polyvinyl acetate, polyamino acid, polylactic acid, polymalic acid, polyglycerin, latex, rosin-based sizing agent, petroleum resin-based sizing agent, urea resin, melamine resin, epoxy resin, Polyamide resin, polyamide/polyamine resin, polyethyleneimine, polyamine, vegetable gum, polyethylene oxide, hydrophilic crosslinked polymer, polyacrylate, starch polyacrylic acid copolymer, tamarind gum, guar gum and colloidal silica, and one or more thereof mixtures.

Among these, cellulose derivatives are preferable from the viewpoint of compatibility with cellulose nanofibers, and carboxymethylcellulose and salts thereof are particularly preferable. A water-soluble polymer such as carboxymethylcellulose and its salt is considered to enter between cellulose nanofibers and widen the distance between CNFs, thereby improving redispersibility. In addition, dextrin can also be preferably used as the above water-soluble polymer. Dextrin has a low viscosity increase and a high transparency, and thus has the advantage that it hardly affects the viscosity and transparency of CNF and can be used by mixing with CNF at any ratio.

When carboxymethylcellulose or a salt thereof is used as the water-soluble polymer, it is preferable to use carboxymethylcellulose having a degree of carboxymethyl group substitution per anhydroglucose unit of 0.55 to 1.60, more preferably 0.55 to 1.60. A value of 10 is more preferred, and a value of 0.65 to 1.10 is even more preferred. In addition, those with longer molecules (higher viscosity) are preferable because they are more effective in increasing the distance between CNFs, and the B-type viscosity at 25 ° C. and 600 rpm in a 1% by mass aqueous solution of carboxymethyl cellulose is 3 to 14000 mPa s. is preferred, 7 to 14,000 mPa·s is more preferred, and 1,000 to 8,000 mPa·s is even more preferred.

The blending amount of the water-soluble polymer is preferably 5 to 300% by mass, more preferably 20 to 300% by mass, based on CNF (absolute dry solids). If it is 5% by mass or more, the effect of improving redispersibility can be obtained, and if it is 300% by mass or less, problems such as a decrease in viscosity characteristics such as thixotropy and dispersion stability, which are characteristics of CNF, are unlikely to occur. .

It is particularly preferable that the amount of the water-soluble polymer is 25% by mass or more because a particularly excellent effect of improving redispersibility can be obtained. In addition, considering the maintenance of thixotropy in the dispersion after redispersion, it is more preferably 200% by mass or less, and particularly preferably 60% by mass or less.

(Redispersion)
The dry solids of CNF obtained by the present invention have good redispersibility. The good redispersibility is the viscosity of the CNF dispersion in a wet state before drying, and the viscosity of the CNF dispersion (or suspension) obtained by redispersing after making the CNF dry solid. It means that the rate of change is small. In other words, this corresponds to a high viscosity recovery rate measured by the method described in the examples below. For example, although not limited to this, the viscosity of the CNF dispersion obtained by redispersing by adding water so that the solid content concentration after drying is the same as the viscosity of the CNF dispersion before drying is 40% or more. It is preferably 45% or more, more preferably 50% or more, even more preferably 55% or more. The viscosity can be measured after 3 minutes at 25° C. and 60 rpm, for example, using a Brookfield viscometer, as described in Examples below.

The device used for re-dispersing the dry solid matter in the dispersion medium to obtain a dispersion liquid is not particularly limited, but a dispersing machine such as a homomixer can be used. As a dispersion medium used for redispersion, water, the hydrophilic organic solvent, and a mixed solvent thereof can be used, and water is most preferable. The solid content concentration in the re-dispersed dispersion is not particularly limited, but is preferably 0.1 to 10% by mass, more preferably 1 to 5% by mass.

The dried solid obtained using the vacuum drying apparatus according to the method of the present invention has good redispersibility, and compared to that dried under normal pressure, it was measured by the method described in the examples described later. Loss tangent tends to be small. The loss tangent is the tan value of the phase difference δ between the measured sinusoidal strain wave and the detected limiting stress wave in the dynamic viscoelasticity of an aqueous dispersion, which corresponds to loss modulus/storage modulus. The storage elastic modulus is the component of the energy generated by the external force and strain that is stored inside the object, and the loss elastic modulus is the component that diffuses to the outside. It can be said that the larger the loss tangent, the stronger the viscous property, and the closer to 0, the stronger the elastic property. A small loss tangent indicates high elasticity and good redispersibility.

In addition, the liquid (redispersion/suspension) obtained by redispersing the dried solid obtained using the vacuum drying apparatus is obtained by redispersing the dried solid obtained using the normal pressure drying apparatus. Compared to the obtained liquid (redispersion/suspension), high suspension stability and emulsification stability can be maintained, and the same suspension stability and emulsification as dispersion/suspension without drying process It can show stability.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these.

<Production of carboxylated (TEMPO-oxidized) CNF>
5 g (absolute dry) of bleached unbeaten kraft pulp (85% whiteness) 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 stirred until the pulp was uniformly dispersed. . An aqueous sodium hypochlorite solution was added to the reaction system so as to have a concentration of 5.5 mmol/g to initiate an oxidation reaction. The pH in the system decreased during the reaction, but was adjusted to pH 10 by successively adding 3M sodium hydroxide aqueous solution. The reaction was terminated when the sodium hypochlorite was consumed and the pH in the system stopped changing. The mixture after the reaction is acidified with hydrochloric acid, filtered through a glass filter to separate the pulp, and the pulp is thoroughly washed with water to obtain oxidized pulp (hereinafter referred to as "carboxylated cellulose", "carboxylated pulp" or sometimes referred to as "TEMPO oxidized pulp") was obtained. The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the amount of carboxyl groups was 1.6 mmol/g. The oxidized pulp obtained in the above step was adjusted to 1.0% (w/v) with water and treated three times with an ultrahigh pressure homogenizer (20°C, 150 MPa) to obtain a carboxylated cellulose nanofiber dispersion. rice field. The obtained fibers had an average fiber diameter of 3 nm and an aspect ratio of 150.

<Method for measuring carboxyl group content>
60 mL of a 0.5% by mass slurry (aqueous dispersion) of carboxylated cellulose was prepared, and 0.1 M aqueous hydrochloric acid solution was added to adjust the pH to 2.5. It was calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization step of the weak acid, in which the change in electrical conductivity is gradual: carboxyl group amount [mmol/ g carboxylated cellulose]=a [mL]×0.05/mass of carboxylated cellulose [g].

<Production of carboxymethylated (CM) CNF>
620 parts of isopropanol (IPA) and 10 parts of sodium hydroxide dissolved in 30 parts of water are added to a twin-screw kneader whose rotation speed is adjusted to 100 rpm, and hardwood pulp (LBKP, manufactured by Nippon Paper Industries Co., Ltd.) is added. 100 parts of the dry mass after drying at 100° C. for 60 minutes was charged. The mixture was stirred and mixed at 30°C for 90 minutes to prepare mercerized cellulose. Further, 15 parts of IPA and 12 parts of monochloroacetic acid were added while stirring, and after stirring for 30 minutes, the temperature was raised to 70° C. and carboxymethylation reaction was carried out for 90 minutes.
After completion of the reaction, the product was neutralized with acetic acid until the pH reached 7, washed with water-containing methanol, deliquored, dried and pulverized to obtain carboxymethylated cellulose having a degree of carboxymethyl substitution of 0.18.
The carboxymethylated cellulose obtained in the above step was adjusted to 1.0% (w/v) with water and treated with an ultrahigh pressure homogenizer (20°C, 150 MPa) three times to disperse carboxymethylated cellulose nanofibers. I got the liquid. The obtained fibers had an average fiber diameter of 5 nm and an aspect ratio of 150.

<Method for measuring degree of carboxymethyl substitution per glucose unit>
About 2.0 g of carboxymethylated cellulose (absolute dry) was precisely weighed and put into a 300 mL conical flask with a common stopper. 100 mL of a liquid prepared by adding 10 mL of special grade concentrated nitric acid to 90 mL of methanol was added and shaken for 3 hours to convert the salt of carboxymethylated cellulose (CM cellulose) into hydrogenated CM cellulose. 1.5 to 2.0 g of hydrogenated CM cellulose (absolute dry) was accurately weighed and put into a 300 mL conical flask equipped with a common stopper. Hydrogenated CM-cellulose was wetted with 15 mL of 80% methanol, 100 mL of 0.1N NaOH was added, and the mixture was shaken at room temperature for 3 hours. Excess NaOH was back-titrated with 0.1N _H2SO4 using phenolphthalein as _an indicator. The degree of carboxymethyl substitution (DS) was calculated by the following formula:
A=[(100×F′−(0.1N H ₂ SO ₄ ) (mL)×F)×0.1]/(absolute dry mass of hydrogen-type CM cellulose (g))
DS = 0.162 x A/(1 - 0.058 x A)
A: Amount of 1N NaOH (mL) required to neutralize 1 g of hydrogenated CM-cellulose
F: Factor of 0.1N H ₂ SO ₄ F': Factor of 0.1N NaOH

<Measurement of average fiber diameter, average fiber length, and aspect ratio of CNF>
The average fiber diameter and average fiber length of cellulose nanofibers (CNF) were analyzed for 200 randomly selected fibers using an atomic force microscope (AFM). Aspect ratio was calculated by the following formula:
Aspect ratio = average fiber length/average fiber diameter

<Measurement of viscosity of CNF dispersion or suspension>
A dispersion or suspension of cellulose nanofibers was adjusted to a solid content of 0.5% by mass with water, and the No. The viscosity was measured after 3 minutes at 4 rotors/60 rpm.

<Production of dry solids>
(Example 1)
As the CNF, the above carboxymethylated CNF (degree of carboxymethyl substitution: 0.18, average fiber diameter: 50 nm, aspect ratio: 120) was used. Carboxymethylcellulose (trade name: F350HC-4, viscosity (1% by mass, 25°C) of about 3000 mPa s, degree of carboxymethyl substitution of about 0.9) is added to a 0.7% by mass aqueous suspension of CNF. 40% by mass (that is, so that the solid content of carboxymethyl cellulose is 40 parts by mass when the solid content of carboxymethylated CNF is 100 parts by mass) is added, and 60 with TK homomixer (12,000 rpm) An aqueous dispersion of CNF was prepared by stirring for a minute. The pH of this dispersion was about 7. A 0.5% sodium hydroxide aqueous solution was added to this aqueous dispersion to adjust the pH to 9. The solid content (including carboxymethylated CNF and carboxymethylcellulose) of the aqueous dispersion at this time was 1.4% by mass. When the solid content of this aqueous dispersion was adjusted to 0.5% by mass with water, the viscosity was 530 mPa·s.

Next, the aqueous dispersion (solid content 1.4% by mass) is applied to the drum surface of a vacuum drum dryer (manufactured by Katsuragi Industry Co., Ltd.) to form a thin film with a thickness of about 100 to 200 μm, and the drum of the drum dryer. Drying was performed at a surface temperature of 50° C., a steam pressure of 0.3 MPaG, a drum rotation speed of 2 rpm, and a dryer internal pressure of 5 kPa to obtain a dry solid of carboxymethylated CNF having a water content of 5% by mass.

<Redispersion of dry solids>
Water is added to the dry solid obtained above so as to form an aqueous suspension with a solid content of 0.5% by mass, and the mixture is stirred for 3 hours using an agitator (600 rpm) with a propeller blade to regenerate CNF. A dispersed water dispersion/suspension was obtained.

<Evaluation of restoration rate>
The change in viscosity of the dispersion/suspension before and after drying and redispersion was evaluated as viscosity recovery. The recovery rate was calculated by the following formula.
Recovery rate (%) = (viscosity of dispersion or suspension after redispersion) / (viscosity of dispersion before drying) x 100

(Example 2)
Production of the dried solid was carried out in the same manner as in Example 1, except that the drying temperature (drum surface temperature) was 60°C.

(Example 3)
Production of the dry solid was carried out in the same manner as in Example 1, except that the drying temperature (drum surface temperature) was 80°C.

(Example 4)
Production of the dry solid was carried out in the same manner as in Example 1, except that the drying temperature (drum surface temperature) was 100°C.

(Example 5)
Example 1 except that in the production of the dry solid, ethanol was added to the aqueous dispersion (solid content 1.4% by mass) so that water: ethanol = 60:40 (mass ratio) and then dried. did the same.

(Example 6)
The same procedure as in Example 5 was repeated except that the drying temperature (drum surface temperature) was changed to 60°C in the production of the dried solid.

(Example 7)
Production of the dry solid was carried out in the same manner as in Example 5, except that the drying temperature (drum surface temperature) was 80°C.

(Example 8)
As the CNF, the same procedure as in Example 1 was performed except that the TEMPO-oxidized CNF obtained above (carboxyl group content: 1.6 mmol/g, average fiber diameter: 3 nm, aspect ratio: 150) was used. The viscosity of the aqueous dispersion before drying was 260 mPa·s.

(Comparative example 1)
In the production of dry solids, using a normal pressure drum dryer (manufactured by Katsuragi Industry Co., Ltd.), the drum surface temperature of the drum dryer was 140 ° C., the steam pressure was 0.3 MPaG, the drum rotation speed was 2 rpm, and the drying was performed under normal pressure. It was carried out in the same manner as in Example 1.

(Comparative example 2)
Comparative Example 1 except that in the production of the dry solid, ethanol was added to the aqueous dispersion (solid content 1.4% by mass) so that water: ethanol = 60:40 (mass ratio) and then dried. did the same.

(Comparative Example 3)
In the production of dry solids, using a normal pressure drum dryer (manufactured by Katsuragi Industry Co., Ltd.), the drum surface temperature of the drum dryer was 140 ° C., the steam pressure was 0.3 MPaG, the drum rotation speed was 2 rpm, and the drying was performed under normal pressure. It was carried out in the same manner as in Example 8.
Table 1 shows the results.

From the results in Table 1, it can be seen that the dry solid produced by vacuum drying CNF exhibits a high viscosity recovery rate when redispersed.

(Example 9: Evaluation of loss tangent)
Water was added to each of the dry solids obtained in Example 3 and Comparative Example 1 so that the solid content was 1% (w/v), and the mixture was stirred for 3 hours using an agitator (1500 rpm) equipped with propeller blades. Stirred to obtain a water dispersion/suspension in which CNF was redispersed. The loss tangent of the resulting redispersed CNF dispersion/suspension was measured by the following method:
After the CNF dispersion/suspension (solid content 1% (w/v), dispersion medium: water) obtained by redispersion above was allowed to stand for 16 hours, a dynamic viscoelasticity measuring device (Anton Paar, MCR301) was used to measure the loss tangent (tan δ) while changing the angular frequency while fixing the amount of deformation strain under the following conditions.
Application: RHEOPLUS/32 V2.62 21001935-33024
Apparatus: MCR301 (manufactured by Anton Paar)
Measuring jig: Parallel plate (PP25-SN5316)
Measurement conditions: Amount of deformation strain = 1%
Angular frequency=100-0.1 1/s.
Number of data points: 12 (take the logarithm of the above angular frequency and perform 4 measurements per digit).

The results are shown in FIG. The loss tangent at 0.1 s ⁻¹ was 0.144 in Example 3 and 0.231 in Comparative Example 1. The loss tangent at 15.2 s ⁻¹ was 0.27 for Example 3 and 0.431 for Comparative Example 1. The smaller the loss tangent, the higher the elasticity, so it can be said that the redispersibility is good.

(Example 10: Evaluation of suspension stability)
Water was added to each of the dry solids obtained in Example 3 and Comparative Example 1 so that the solid content was 1% (w/v), and the mixture was stirred for 3 hours using an agitator (1500 rpm) equipped with propeller blades. Stirred to obtain a water dispersion/suspension in which CNF was redispersed. Suspension stability of the obtained CNF aqueous dispersion/suspension after redispersion was evaluated by the following method:
A predetermined amount of water and titanium oxide powder was added to the CNF dispersion/suspension (solid content 1% (w/v), dispersion medium: water) obtained by redispersion above, and the titanium oxide concentration was 2.0%. 0% (w/v), cellulose nanofiber concentration was adjusted to 0.2% (w/v) or 0.3% (w/v), and 5% at 1000 rpm using an agitator with propeller blades. After stirring for one minute, the state of separation of the suspension was observed after standing still for one week.

The results are shown in FIG. The "control" in the left column of FIG. 2 is the water dispersion before drying described in Example 1 (solid content 1.4% by mass) instead of the CNF water dispersion / suspension after redispersion. Except for this, the state of separation of the suspension was observed in the same manner as described above. The middle column in FIG. 2 is for Comparative Example 1, and the right column in FIG. 2 is for the water dispersion/suspension after redispersion corresponding to Example 3. From FIG. 2, separation or sedimentation of titanium oxide is observed in Comparative Example 1, whereas suspension of titanium oxide is maintained in Example 3 as in the dispersion (control) that has not undergone the drying process. I understand.

(Example 11: Evaluation of emulsion stability)
Water was added to each of the dry solids obtained in Example 3 and Comparative Example 1 so that the solid content was 1% (w/v), and the mixture was stirred for 3 hours using an agitator (1500 rpm) equipped with propeller blades. Stirred to obtain a water dispersion/suspension in which CNF was redispersed. The emulsion stability of the resulting CNF water dispersion/suspension after redispersion was evaluated by the following method:
A predetermined amount of water and silicone oil (manufactured by Dow Corning Toray Co., Ltd.) is added to the CNF dispersion / suspension (solid content 1% (w / v), dispersion medium: water) obtained by redispersion above. , Adjusted so that the silicone oil was 15% (w / v) and the cellulose nanofiber concentration was 0.2% (w / v) or 0.3% (w / v), and an agitator with propeller blades was used. After stirring at 1000 rpm for 5 minutes, the state of separation of the emulsified liquid was observed after standing still for 1 week.

The results are shown in FIG. The "control" in the left column of FIG. 3 is the water dispersion before drying described in Example 1 (solid content 1.4% by mass) instead of the CNF water dispersion / suspension after redispersion. Except for this, the state of separation of the milky lotion was observed in the same manner as described above. The middle column in FIG. 3 is for Comparative Example 1, and the right column in FIG. 3 is for the water dispersion/suspension after redispersion corresponding to Example 3. From FIG. 3, in Comparative Example 1, separation of silicone oil was observed for both 0.2% (w / v) and 0.3% (w / v) of CNF, whereas in Example 3, after the drying process, It can be seen that an emulsification stability equivalent to that of the dispersion (control) without the emulsion was obtained.

Claims (10)

drying the mixture of cellulose nanofibers and solvent using a vacuum drying device ;
Drying using the vacuum drying apparatus is performed at a temperature of 40 to 100 ° C.,
A method for producing a cellulose nanofiber dry solid, wherein the solvent is water . The method for producing a cellulose nanofiber dry solid according to claim 1, wherein the drying using the vacuum drying apparatus is performed at a temperature of 40 to 90°C. The method for producing a cellulose nanofiber dry solid according to claim 1 or 2 , wherein the vacuum drying device is a drum type vacuum drying device. The method for producing a cellulose nanofiber dry solid according to any one of claims 1 to 3 , further comprising adjusting the pH of the mixture of the cellulose nanofiber and solvent to 8-11. 5. The cellulose nanofibers according to any one of claims 1 to 4 , wherein the cellulose nanofibers are carboxylated cellulose nanofibers having a carboxyl group content of 0.6 to 3.0 mmol/g relative to the absolute dry weight of the cellulose nanofibers. The method for producing the cellulose nanofiber dry solid according to . The dried cellulose nanofibers according to any one of claims 1 to 5 , wherein the cellulose nanofibers are carboxymethylated cellulose nanofibers having a degree of carboxymethyl substitution per glucose unit of 0.01 to 0.50. A method for producing solids. The cellulose according to any one of claims 1 to 6 , wherein the mixture of the cellulose nanofibers and solvent further contains 5 to 300% by mass of a water-soluble polymer relative to the dry mass of the cellulose nanofibers. A method for producing nanofiber dry solids. The method for producing a cellulose nanofiber dry solid according to claim 7 , wherein the water-soluble polymer is carboxymethylcellulose and its salt. A method for producing a dry solid of cellulose nanofibers, comprising drying a mixture of cellulose nanofibers and a solvent using a drum-type vacuum drying device. The method for producing a cellulose nanofiber dry solid according to claim 9, wherein the drying using the drum-type vacuum drying device is performed at a temperature of 40 to 100°C.

Images