JP2022134120A - Dry solid cellulose nanofiber and production method thereof - Google Patents

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 The present invention relates to a cellulose nanofiber dry solid and a method for producing the same.

Cellulose nanofibers are fine fibers with a fiber diameter of about 3 nm to several hundred nm, which are excellent in dispersibility in aqueous media. It is expected to be used for fortification, moisture retention, food stability improvement, low-calorie additive or emulsion stabilization aid. However, when the cellulose nanofibers in a state of being dispersed in water (wet state) are dried to form a dry solid, hydrogen bonds are usually formed between fine cellulose fibers. Even if you try to redisperse the material by adding water, the properties such as viscosity will not be restored to the same level as before drying (wet state). Therefore, cellulose nanofibers are produced in a state of being dispersed in water (wet state), and are usually used in various applications in a wet state without being dried.

However, in order to keep the cellulose nanofibers in a wet state stably, a mass of water several times to several hundred times larger than that of the cellulose nanofibers is required, which increases storage space, storage and transportation costs. , there are various problems. A freeze-drying method (Patent Document 1) has been proposed as a means for drying cellulose in a wet state. However, when cellulose nanofibers are freeze-dried, a huge amount of energy is required, and depending on the conditions, when the water between the fine fibers of the cellulose nanofibers freezes, ice crystals grow larger than the voids between the fine fibers. As a result, there is a problem that fine fibers of cellulose nanofibers are associated with each other, and the redispersibility of the dry solid of cellulose nanofibers is deteriorated.

In contrast, the present applicant improved the redispersibility of the dry solids of cellulose nanofibers by adding 5 to 300% by mass of a water-soluble polymer to the anion-modified cellulose nanofibers to form a dry solid. (Patent Document 2), and a method of improving redispersibility of cellulose nanofiber dry solids by drying a mixture of cellulose nanofibers and a solvent using a vacuum drying device (Patent Document 3). Proposed.

JP-A-6-233691 WO2015/107995 WO2019/189318

The methods described in Patent Documents 2 and 3 are effective methods for producing cellulose nanofiber dry solids with good redispersibility. It would be desirable to further develop new cellulose nanofiber dry solids that can be produced by

An object of the present invention is to provide a new cellulose nanofiber dry solid with good redispersibility and a method for producing the same. Good redispersibility means that the viscosity and transparency between the cellulose nanofiber dispersion in a wet state before drying and the cellulose nanofiber dispersion obtained by redispersing after making the cellulose nanofiber dry solid are different. It means that there is little change in

As a result of intensive studies conducted by the present inventors to achieve the above object, it was found that cellulose nanofibers having an average fiber length within a specific range can be used to produce cellulose nanofiber dry solids with good redispersibility. I found In addition, it was found that it is preferable to use a spray dryer for drying. The invention includes, but is not limited to:
[1] A method for producing a dry solid of cellulose nanofibers, which comprises drying a dispersion containing cellulose nanofibers having an average fiber length of 450 nm or less using a spray dryer.
[2] The production method according to [1], wherein the spray atomization method in the spray drying apparatus is a two-fluid nozzle or a rotary atomizer [3] The solid content of the dried solid is 90.0% by mass or more. , [1] or [2].
[4] The production method according to any one of [1] to [3], wherein the cellulose nanofibers are anion-modified cellulose nanofibers.
[5] The production method according to [4], wherein the anion-modified cellulose nanofibers are carboxylated cellulose nanofibers.
[6] The production method according to [4], wherein the carboxylated cellulose nanofibers have a carboxyl group content of 0.6 to 3.0 mmol/g relative to the absolute dry weight of the carboxylated cellulose nanofibers.
[7] A dry solid of cellulose nanofibers containing cellulose nanofibers having an average fiber length of 450 nm or less.
[8] The dry solid according to [7], which has a solid content of 90.0% by mass or more.
[9] Light with a wavelength of 660 nm in an aqueous dispersion obtained by adding water to the dry solid so that the concentration of cellulose nanofibers is 1.0% by mass, and stirring for 30 minutes with Homodisper (3000 rpm). The dry solid according to [7] or [8], which has a transmittance (optical path length 10 mm) of 65% or more.
[10] Add water to the dry solid so that the concentration of cellulose nanofibers is 0.5% by mass, add Bokushizuku, stir, and then cellulose nanofiber aggregates calculated by optical microscope observation. The dry solid according to any one of [7] to [9], wherein the area ratio of is 10.0% or less.

According to the present invention, a cellulose nanofiber dry solid with good redispersibility can be obtained. In particular, among cellulose nanofibers, carboxylated cellulose nanofiber, which is a kind of anion-modified cellulose nanofiber, tends to be difficult to improve redispersibility by conventional methods, but in the present invention, redispersibility The advantage is that a carboxylated cellulose nanofiber dry solid with a very good In addition, good redispersibility means that between the cellulose nanofiber dispersion in a wet state before drying and the cellulose nanofiber dispersion obtained by redispersing the cellulose nanofiber dry solid, the viscosity It means that there is little change such as transparency and

The dry solids obtained in Examples 1, 2, 4 to 6 were observed with an optical microscope at a magnification of 400. The dry solids obtained in Comparative Examples 1 and 2 were observed with an optical microscope at a magnification of 400. For the redispersions of the dried solids obtained in Examples 1, 2, 4 to 6, aggregation of CNFs was evaluated by the Indian ink method. Regarding the redispersions of the dry solids obtained in Comparative Examples 1 and 2, the aggregation of CNFs was evaluated by the Indian ink method.

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 present invention is a cellulose nanofiber (hereinafter referred to as "CNF") dry solid and a method for producing the same. Specifically, CNF dry solids containing CNFs with an average fiber length of 450 nm or less, and CNF dry solids including drying a dispersion containing CNFs with an average fiber length of 450 nm or less using a spray dryer manufacturing method. According to the present invention, a CNF dry solid with good redispersibility can be obtained.

The reason why the CNF dry solid obtained by the present invention exhibits excellent redispersibility is not clear, but by drying using CNF having an average fiber length of 450 nm or less, the electrical repulsion between CNFs is reduced. It is presumed that the formation of hydrogen bonds between fibers and entanglement between fibers, which are considered to be the causes of lowering the redispersibility and deteriorating the redispersibility, were suppressed.

(cellulose nanofiber)
In the present invention, cellulose nanofibers (CNF) are cellulose raw materials, such as pulp, which have been refined 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, it is preferable to use CNF having an average fiber diameter of about 3 to 100 nm, more preferably about 3 to 20 nm.

In the present invention, CNF having an average fiber length of 450 nm or less among the CNFs having the average fiber diameter as described above are used. The average fiber length is more preferably 400 nm or less, still more preferably 350 nm or less. Although the lower limit of the average fiber length is not particularly limited, it is preferably 100 nm or more, more preferably 150 nm or more, in order to obtain effects such as CNF thickening, water absorption, and shape retention.

The average fiber diameter and average fiber length of CNF are the average values of the fiber diameter and fiber length obtained from the results of observing about 200 fibers using an atomic force microscope (AFM) or a transmission electron microscope (TEM). can be obtained by calculating

CNF can be obtained by applying a mechanical force to a cellulose raw material such as pulp, which will be described later, to make it finer (fibrillation). As the cellulose raw material, unmodified cellulose, pulp for paper manufacturing, or the like, which will be described later, may be used, or chemically modified cellulose obtained by further chemically modifying pulp for paper manufacturing, or the like may be used. Examples of chemically modified cellulose include, but are not limited to, anion-modified cellulose in which an anionic group is introduced into the cellulose chain, and cation-modified cellulose in which a cationic group is introduced. Among these, it is preferable to use anion-modified cellulose. In particular, among anion-modified celluloses, carboxylated cellulose into which a carboxyl group has been introduced as an anionic group is suitable as a raw material for CNF used in the present invention.

(raw material for cellulose)
Cellulose, which is a raw material for CNF, is known to originate from plants, animals (e.g. sea squirts), algae, microorganisms (e.g. acetobacter), microbial products, etc. In the present invention, it is Either can be used. 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. CNF or chemically modified CNF can be obtained by refining the fiber width of the above cellulose raw material or chemically modified cellulose raw material (chemically modified cellulose) to the nanometer level.

(chemically modified cellulose)
Examples of chemically modified cellulose include, but are not limited to, anion-modified cellulose in which an anionic group is introduced into the cellulose chain, and cation-modified cellulose in which a cationic group is introduced. Examples of anion-modified cellulose include carboxylated cellulose into which carboxyl groups have been introduced, carboxymethylated cellulose into which carboxymethyl groups have been introduced, and phosphate esterified cellulose into which phosphate ester groups and the like have been introduced. Among these, it is preferable to use anion-modified cellulose, and it is preferable to use carboxylated cellulose. Carboxylated CNF made from carboxylated cellulose tends to be difficult to make into a dry solid with good redispersibility by the conventional method, but the method of the present invention has very good redispersibility. A dry solid of carboxylated CNF can be produced.

(carboxylated cellulose)
An example of anionically modified cellulose is carboxylated (oxidized) cellulose (carboxylated cellulose). Carboxylated 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. By this oxidation reaction, the primary hydroxyl group at the C6 position of the glucopyranose ring on the cellulose surface is selectively oxidized, resulting in cellulose having an aldehyde group and a carboxyl group (-COOH) or carboxylate group (-COO-) on the surface. Obtainable. The concentration of the cellulose raw material during the reaction is not particularly limited, but 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, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even more preferably 1 to 25 mmol, and most preferably 3 to 10 mmol, relative to 1 g of absolute dry cellulose raw material. . 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, the carboxylated cellulose obtained by filtration after the completion of the reaction in the first step is oxidized again under the same or different reaction conditions, so that the reaction is not inhibited by salt produced as a by-product in the reaction in the first step. can be efficiently oxidized.

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 (carboxylates) 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, excessive oxidation and decomposition of cellulose can be prevented, and the yield of carboxylated 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 carboxylated 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 the carboxylated cellulose and the amount of carboxyl groups in the carboxylated CNF obtained by defibrating the carboxylated cellulose are usually the same.

Carboxylated CNF can be produced by defibrating carboxylated cellulose by the method described below.
(Carboxymethylated cellulose)
An example of anion-modified cellulose is carboxymethylated cellulose (hereinafter, carboxymethylated cellulose is referred to as "CM-modified"). The CM-modified cellulose may be obtained by CM-processing 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. The following method can be given as an example of the method for producing such CM-cellulose. To the cellulose raw material, 3 to 20 times by weight of water and / or lower alcohol as a solvent, specifically water, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, tertiary butanol, etc. A single medium or a mixed medium of two or more types is added. When the lower alcohol is mixed, 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 the moles of the anhydroglucose residue of the cellulose raw material. A cellulose raw material, a solvent and a 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" or "CM-modified cellulose", which is a type of anion-modified cellulose used for preparing CM-CNF, is at least partly fibrous even when dispersed in water. is maintained. Therefore, "carboxymethylated cellulose" or "CM-modified cellulose" is distinguished from carboxymethylcellulose, which is a kind of water-soluble polymer. When an aqueous dispersion of "carboxymethylated cellulose" or "CM-modified 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" or "CM-modified cellulose" is measured by X-ray diffraction, a cellulose type I crystal peak can be observed. I can't see

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

(Phosphate esterified cellulose)
An example of anion-modified cellulose is phosphate-esterified cellulose. Phosphate-esterified cellulose can be obtained by a method of mixing powder or an aqueous solution of a phosphoric acid-based compound with the aforementioned cellulose raw material.

Phosphoric acid compounds include phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, metaphosphoric acid, pyrophosphoric acid, or salts or esters thereof. Among these, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, and sodium metaphosphate are preferred because they are low in cost, easy to handle, and improve fibrillation efficiency. , potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate etc. are preferred, and phosphoric acid, sodium phosphoric acid, potassium phosphoric acid, and ammonium phosphoric acid are more preferred. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. These can be used singly or in combination of two or more. In addition, it is preferable to use the phosphoric acid-based compound in the form of an aqueous solution because 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 is preferably 7 or less because the efficiency of introducing phosphoric acid groups is increased, and from the viewpoint of suppressing hydrolysis of cellulose fibers, pH 3 to 7 is preferable.

The following method can be given as an example of the method for producing the phosphorylated cellulose. A phosphate group is introduced into cellulose by adding a phosphoric acid-based compound to a dispersion liquid of a 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 the phosphoric acid compound added is preferably 0.2 to 500 parts by mass, more preferably 1 to 400 parts by mass, in terms of elemental phosphorus. If the proportion of the phosphoric acid-based compound is at least the lower limit, the yield of phosphorylated CNF 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.

A basic nitrogen-containing compound may be added in addition to the cellulose raw material and the phosphoric acid compound. "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. 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, hexamethylenediamine and the like. Among these, urea is preferable because it is inexpensive and easy to handle. The amount of the basic nitrogen-containing compound to be added is preferably 2 to 1000 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 conditions for the phosphate esterification reaction 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. Furthermore, 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 phosphorylated 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 defibrated into nanoscale fiber widths. 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 product may swell or dissolve, and may not be obtained as CNF. In order to defibrate efficiently, it is preferable to wash the phosphate-esterified cellulose obtained above with cold water after boiling.

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

(cation-modified cellulose)
As chemically modified cellulose used for preparing chemically modified CNF, cationically modified cellulose obtained by further cationizing the carboxylated cellulose may be used. Cation-modified 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, and an alkali metal hydroxide (hydroxylation sodium, 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 fibrillated into nanoscale fiber widths. If the degree of cation substitution per glucose unit is less than 0.02, fibrillation cannot be sufficiently performed. On the other hand, if the degree of cation substitution per glucose unit is greater than 0.50, the product may swell or dissolve and may not be obtained as CNF. In order to defibrate efficiently, it is preferable to wash the cation-modified 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.

Cation-modified CNF can be produced by defibrating cation-modified cellulose by the method described below. The degree of cation substitution in cation-modified cellulose and the degree of cation substitution in cation-modified CNF obtained by defibrating the cation-modified cellulose are usually the same.

(defibration)
CNF can be obtained by defibrating a cellulose raw material containing the chemically modified cellulose or the like. The device used for fibrillation is not particularly limited, but it is preferable to use a device capable of applying a strong shearing force, such as a high-speed rotation type, colloid mill type, high pressure type, roll mill type, ultrasonic type, or the like. In particular, for efficient defibration, it is preferable to use a wet high-pressure or ultrahigh-pressure homogenizer capable of applying a pressure of 50 MPa or more to the cellulose raw material dispersion to be defibrated and 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, pretreatment may be performed using a known mixing, stirring, emulsifying, and dispersing device such as a high-speed shear mixer.

The average fiber length of CNF obtained by defibration can be adjusted by defibrating conditions such as the type of equipment and pressure, and by a combination of pretreatments.
(Dispersion containing CNF)
A dispersion liquid containing CNF is obtained by the above defibration. 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.

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 mixed solvent thereof. More preferred. Since most chemically modified cellulose is produced using water as a dispersion medium, when using chemically modified CNF derived from chemically modified cellulose such as anion-modified cellulose and carboxylated cellulose, chemically modified CNF obtained by defibrating chemically modified cellulose The aqueous dispersion of CNF can be used for drying as it is. Alternatively, the aqueous dispersion may be subjected to a pretreatment such as drying or filtering, and then subjected to a drying step using the spray drying apparatus of the present invention.

When the solvent is a mixed solvent of water and a hydrophilic organic solvent, the hydrophilic organic solvent is added to an aqueous dispersion of a cellulose raw material such as chemically modified cellulose or an aqueous dispersion of CNF, or one of the aqueous dispersions is used. part can be replaced with a hydrophilic organic solvent. The substitution can be prepared by removing water from the aqueous dispersion by drying or filtering to obtain a concentrated aqueous dispersion or wet cake, and adding a hydrophilic organic solvent thereto. 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.

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

The solid content in the CNF-containing dispersion to be dried is preferably about 0.1 to 10% by mass, more preferably about 0.5 to 8% by mass, and even more preferably about 1 to 5% by mass.

A dispersion containing CNF having an average fiber length of 450 nm or less to be dried may contain a water-soluble polymer. Thereby, in addition to CNF having an average fiber length of 450 nm or less, a dry solid containing a water-soluble polymer may be obtained. Examples of water-soluble polymers include, but are not limited to, cellulose derivatives such as carboxymethylcellulose, dextrin, and the like. When a water-soluble polymer is blended, the blending amount is preferably 1 to 50% by mass, more preferably 20 to 45% by mass, more preferably 30 to 40% by mass, relative to CNF (bone dry solids). .

(dry)
By drying a dispersion containing CNF having an average fiber length of 450 nm or less, a CNF dry solid having good redispersibility can be obtained. For drying, it is preferable to use a spray dryer.

The type of atomization device in the spray drying device is not particularly limited. A rotary atomizer type or a nozzle type may be used. In the case of the nozzle type, it may be a one-fluid nozzle or a two-fluid nozzle. Also, a co-current two-fluid nozzle or a fountain-type two-fluid nozzle may be used. Among these, it is preferable to use a spray drying apparatus having a two-fluid nozzle because it is easy to handle.

The inlet temperature of the spray dryer is preferably 100°C to 350°C, more preferably 120°C to 250°C, 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, even more preferably 70°C to 120°C. If these temperatures are too low, the drying may not proceed sufficiently, and if the temperatures are too high, the CNF may be excessively dried, resulting in a decrease in redispersibility.

The drying rate is preferably about 1.0 to 5.0 kg/h, more preferably about 1.2 to 3.5 kg/h, for a dry air amount of 80 kg/h, for example. About 1.5 to 3.0 kg/h is more preferable. If these speeds are slow, the yield may decrease, and if the speed is fast, drying may not proceed sufficiently.

(CNF dry solids)
The above drying yields a CNF dry solid. As used herein, CNF dry solids include 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, when drying is performed to such a high solid content, redispersibility tends to deteriorate, but the dry solid obtained by the production method of the present invention has a high solid content and high redispersibility. can also have

(Redispersion)
The CNF dry solids obtained according to the invention have 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 after making the CNF dry solid. Say.

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. The dispersion medium used for redispersion is not particularly limited, but examples thereof include water, the hydrophilic organic solvent, and a mixed solvent thereof, preferably water. The solid content of the dispersion after redispersion may be appropriately selected according to the application, and is not particularly limited, but is preferably about 0.1 to 10.0% by mass, and about 1.0 to 6.0% by mass. more preferred.

(viscosity)
As one of the indicators indicating that the redispersibility is good, as described above, the CNF dispersion in a wet state before drying and the CNF dispersion obtained by redispersing after making the CNF dry solid. Among them, there is little change in viscosity between them. For example, although not limited to this, the viscosity of the CNF aqueous dispersion obtained by redispersing by adding water so that the solid content concentration after drying is the same as the viscosity of the CNF aqueous dispersion before drying (restoration of viscosity ratio) is preferably 30% or more, more preferably 40% or more, even more preferably 50% or more, further preferably 55% or more. The viscosity can be measured, for example, using a Brookfield viscometer at 25° C. at 6 rpm or 60 rpm for 3 minutes, as described in Examples below.

(Transparency (transmittance of light with a wavelength of 660 nm))
As one of the indicators indicating that the redispersibility is good, as described above, the CNF dispersion in a wet state before drying and the CNF dispersion obtained by redispersing after making the CNF dry solid. Among them, there is little change in transparency. In addition, the high transparency of the CNF dispersion after redispersion (redispersion) itself is also an indicator of good redispersibility. The transparency can be measured by determining the transmittance of light with a wavelength of 660 nm using a prismatic cell with an optical path length of 10 mm, as described in Examples below.

For example, although not limited to this, the transparency of the CNF aqueous dispersion obtained by redispersing by adding water so that the solid content concentration after drying is the same as the transparency of the CNF aqueous dispersion before drying (restoration of transparency ratio) is preferably 50% or more, more preferably 60% or more, even more preferably 70% or more, further preferably 80% or more, and 90% or more. More preferred.

In addition, water was added to the CNF dry solid so that the concentration of CNF was 1.0% by mass, and the transparency of the aqueous dispersion (redispersion) obtained by stirring at Homodisper (3000 rpm) for 30 minutes ( It is preferable that the transmittance of light with a wavelength of 660 nm and the optical path length of 10 mm be 60% or more. More preferably 65% or more, still more preferably 80% or more, still more preferably 85% or more, still more preferably 90% or more.

(Evaluation of CNF aggregates in redispersion liquid)
When the redispersibility of the dry solid is good, the aggregates of CNFs in the redispersion liquid tend to decrease. The amount of CNF aggregates in the redispersion liquid can be evaluated by calculating the area ratio of the CNF aggregates by the method described in Examples below. The area ratio of CNF aggregates is preferably 10.0% or less. 8.0% or less is more preferable, 5.0% or less is more preferable, and 1.0% or less is even more preferable.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these.
<Measurement of carboxyl group content>
Prepare 60 mL of a 0.5% by mass aqueous dispersion of carboxylated CNF, add 0.1 M hydrochloric acid aqueous solution to pH 2.5, and then add dropwise 0.05 N sodium hydroxide aqueous solution until the pH reaches 11. Conductivity was measured and calculated from the amount of sodium hydroxide consumed (a) during the neutralization step of the weak acid, where the change in conductivity is gradual, using the following formula:
Carboxyl group amount [mmol/g carboxylated cellulose]=a [mL]×0.05/carboxylated cellulose mass [g].

<Measurement of average fiber diameter and average fiber length of CNF>
Using an atomic force microscope (AFM), 200 randomly selected fibers were analyzed and averaged.

<Measurement of solid content of dry solid>
The solid content in the dry solid can be calculated using the mass after drying the dry solid at 105° C. for 3 hours or more (absolute dry mass) and the mass before drying.

<Production of carboxylated CNF1>
5 g (absolute dry) of bleached softwood-derived dissolved kraft pulp (DKP manufactured by Buckeye) was added to 500 ml of an aqueous solution in which 78 mg (0.5 mmol) of TEMPO (Sigma Aldrich) and 755 mg (7.4 mmol) of sodium bromide were dissolved. was stirred until the pulp was evenly dispersed. After adding 16 ml of a 2M sodium hypochlorite aqueous solution to the reaction system, the pH was adjusted to 10.3 with a 0.5N hydrochloric acid aqueous solution to initiate an oxidation reaction (oxidation treatment). The pH in the system decreased during the reaction, but was adjusted to pH 10 by successively adding 0.5N sodium hydroxide aqueous solution. After reacting for 2 hours, the mixture was filtered through a glass filter and thoroughly washed with water to obtain carboxylated cellulose. A carboxylated cellulose slurry was prepared by adding water to 5.0% (w/v) of this, hydrogen peroxide was added to 2% (w/w) of the carboxylated cellulose, and 3M sodium hydroxide was added. to adjust the pH to 11.3. This slurry was left at a temperature of 80° C. for 2 hours for hydrolysis. This was adjusted to 5.0% (w/v) with water and treated 10 times with an ultrahigh pressure homogenizer (20°C, 140 MPa) to obtain carboxylated CNF1. The resulting carboxylated CNF1 had a carboxyl group content of 1.7 mmol/g, an average fiber diameter of 3 nm, and an average fiber length of 350 nm.

<Production of carboxylated CNF2>
In the production of carboxylated CNF1, it 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 carboxyl group content was 1.5 mmol/g, the average fiber diameter was 3 nm, and the average fiber length was 400 nm.

<Production of carboxylated CNF3>
5 g (absolute dry) of bleached unbeaten kraft pulp (85% whiteness) derived from softwood is added to 500 mL of an aqueous solution of 39 mg of TEMPO (Sigma Aldrich) and 514 mg of sodium bromide, and stirred until the pulp is uniformly dispersed. did. 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. After the reaction, the mixture was acidified with hydrochloric acid, filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain carboxylated cellulose as oxidized pulp. This was adjusted to 1.0% (w/v) with water and treated three times with an ultrahigh pressure homogenizer (20°C, 150 MPa) to obtain carboxylated CNF3. The resulting carboxylated CNF3 had a carboxyl group content of 1.6 mmol/g, an average fiber diameter of 3 nm, and an average fiber length of 550 nm.

<Production of carboxylated CNF4>
In the production of carboxylated CNF1, it 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 carboxyl group content was 0.9 mmol/g, the average fiber diameter was 3 nm, and the average fiber length was 400 nm.

<Production of carboxylated CNF5>
It was produced in the same manner as carboxylated CNF4, except that the treatment with an ultrahigh-pressure homogenizer (20° C., 140 MPa) was changed to 5 times. The carboxyl group content was 0.9 mmol/g, the average fiber diameter was 3 nm, and the average fiber length was 500 nm.

<Production of dry solids>
(Example 1)
An aqueous dispersion of the above carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) was prepared and dried with a rotary atomizer spray dryer at an inlet temperature of 200 ° C., an outlet temperature of 90 ° C., and a dry air volume of 80 kg / h, drying was performed at a liquid feed rate of 2.12 kg/h to obtain a dry solid. The solid content of the obtained dry solid was 95.6% by mass. Adhesion of dry solids to the walls was observed during drying in the apparatus.

(Example 2)
An aqueous dispersion of the above carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) was prepared, and dried with a two-fluid nozzle type spray dryer at an inlet temperature of 200 ° C., an outlet temperature of 90 ° C., and a dry air amount of 80 kg. /h and a liquid feed rate of 2.18 kg/h to obtain a dry solid. The solid content of the obtained dry solid was 96.5% by mass. There was no adhesion of dry solids to the walls of the apparatus.

(Example 3)
A dry solid was obtained 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% by mass. There was no adhesion of dry solids to the walls of the device.

(Example 4)
An aqueous dispersion of the above carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) was prepared, and a two-fluid nozzle type spray drying apparatus was used with an inlet temperature of 160 ° C., an outlet temperature of 70 ° C., and a dry air amount of 80 kg. /h and a liquid feed rate of 2.18 kg/h to obtain a dry solid. The solid content of the obtained dry solid was 90.9% by mass. There was no adhesion of dry solids to the walls of the device.

(Example 5)
1 M hydrochloric acid was added to the aqueous dispersion of carboxylated CNF1 (solid content 3.0% by mass, pH 7.5) to adjust the pH to 5.5, and the inlet temperature was 200 ° C. with a two-fluid nozzle type spray dryer. Drying was performed at an outlet temperature of 90° C., a drying air amount of 80 kg/h, and a liquid feeding amount of 2.18 kg/h to obtain a dry solid. The solid content of the obtained dry solid was 94.9% by mass. There was no adhesion of dry solids to the walls of the apparatus.

(Example 6)
An aqueous dispersion of the above carboxylated CNF4 (solid content 3.0% by mass, pH 7.5) was prepared, and a two-fluid nozzle type spray drying apparatus was used, with an inlet temperature of 200 ° C., an outlet temperature of 90 ° C., and a dry air amount of 80 kg. /h and a liquid feed rate of 2.18 kg/h to obtain a dry solid. The solid content of the obtained dry solid was 91.0% by mass. There was no adhesion of dry solids to the walls of the device.

(Comparative example 1)
An aqueous dispersion of the above carboxylated CNF3 (solid content 0.7% by mass, pH 7.5) was prepared, and dried with a two-fluid nozzle type spray drying apparatus at an inlet temperature of 200 ° C., an outlet temperature of 90 ° C., and an amount of dry air of 80 kg. /h and a liquid feed rate of 2.18 kg/h to obtain a dry solid. The solid content of the obtained dry solid was 96.6% by mass. There was no adhesion of dry solids to the walls of the apparatus.

(Comparative example 2)
An aqueous dispersion of the above carboxylated CNF5 (solid content 3.0% by mass, pH 7.5) was prepared, and a two-fluid nozzle type spray drying apparatus was dried at an inlet temperature of 200 ° C., an outlet temperature of 90 ° C., and a dry air amount of 80 kg. /h and a liquid feed rate of 2.18 kg/h to obtain a dry solid. The solid content of the obtained dry solid was 91.8% by mass. There was no adhesion of dry solids to the walls of the device.

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

<Redispersion of dry solids>
Water was added to the dry solids obtained in Examples 1 to 6 and Comparative Example 2 and stirred at Homodisper (3000 rpm) for 30 minutes to make the CNF concentrations 5.0% by mass, 1.0% by mass, Or each re-dispersion liquid of 0.5% by mass was obtained. In addition, although an attempt was made to similarly redisperse the dry solid obtained in Comparative Example 1, it could not be redispersed at a CNF concentration of 5.0% by mass. Redispersions of 0.0% and 0.5% by weight were made.

<Measurement of viscosity of CNF redispersion liquid>
For the CNF redispersions of Examples 1 to 6 and Comparative Example 2, an aqueous dispersion with a CNF concentration of 5.0% by mass was used, and a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) was used at 25 ° C. The viscosity was measured after 3 minutes at 60 rpm or 6 rpm. 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 having a CNF concentration of 1.0% by mass was used.

<Measurement of transparency of CNF redispersion liquid>
For Examples 1 to 6 and Comparative Examples 1 and 2, a CNF redispersion liquid with a CNF concentration of 1.0% by mass was used, and a spectrophotometer U-3000 (manufactured by Hitachi High-Technologies Corporation) was used to measure an angle with an optical path length of 10 mm. The transmittance of light with a wavelength of 660 nm was measured with a mold cell and defined as transparency (unit: %).

<Evaluation of restoration rate of viscosity and transparency>
The viscosity and transparency of the CNF dispersion before drying were measured in advance and were as shown in Table 1. The viscosities of carboxylated CNF1, 2, 4 and 5 were measured at a CNF concentration of 5.0% by mass, and the viscosities of carboxylated CNF3 were measured at a CNF concentration of 1.0% by mass. Further, the transparency was measured at a CNF concentration of 1.0 mass % for all carboxylated CNFs 1 to 5.

Using the viscosity and transparency values of the CNF dispersion before drying in Table 1 and the viscosity and transparency values of the redispersion obtained by redispersing after making the CNF dry solid, the viscosity is calculated by the following equation. We calculated the restoration rate of , and the restoration rate of transparency, respectively. :
Restoration rate (%) = (viscosity or transparency in redispersion) / (viscosity or transparency of dispersion before drying) x 100
Table 2 shows the results.

<Measurement of area ratio of CNF aggregates in CNF redispersion and observation by optical microscope>
For Examples 1, 2, 4 to 6 and Comparative Examples 1 and 2, a CNF redispersion liquid having a CNF concentration of 0.5% by mass was used, and the CNF aggregates in the redispersion liquid were evaluated using a ink drop. In this method, by adding a coloring material (Bokutetsu) to the CNF dispersion and observing it with an optical microscope, it is possible to easily check the presence or absence of aggregates between CNFs in the dispersion that cannot be visually distinguished. be. Specifically, the evaluation method is as follows:
Two drops of Bokushizuku (manufactured by Kuretake Co., Ltd., solid content 10%) are added to a redispersion liquid having a CNF concentration of 0.5% by mass, and a vortex mixer (manufactured by IUCHI, equipment name: Automatic Lab-mixer HM-10H). The rotation speed was set to maximum and stirred for 30 seconds. The stirred liquid was sandwiched between two glass plates so as to have a film thickness of 0.15 mm, and was observed with an optical microscope (Digital Microscope VHX-6000 manufactured by KEYENCE) at a magnification of 100 times. The area ratio of CNF aggregates was measured using a digital microscope VHX-6000 manufactured by KEYENCE in the area measurement mode of the luminance extraction area. Specifically, in the observation image of CNF aggregates, the brightness range is set to 180 to 260, a region having a brightness within the brightness range is extracted, and the area of this region (area within the brightness range) is used to obtain the following Calculated by the formula. Details are described on pages 9-29 and 30 of the user's manual for Digital Microscope VHX-6000 manufactured by KEYENCE:
Area ratio (%)=(Area in luminance range/Area in measurement range)×100.

Table 2 shows the area ratio results. 3 and 4 show photographs of the redispersed liquid with dripped ink droplets observed under an optical microscope at a magnification of 100 times.

From the results in Table 2, by using CNF having an average fiber length of 450 nm or less, it is possible to produce CNF dry solids with good redispersibility and less CNF aggregates during redispersion.

3 and 4, in the redispersions of Examples 1, 2 and 4, no CNF aggregates were observed. In the re-dispersions of Examples 5 and 6, some aggregates were observed, but the extent was small. On the other hand, in the redispersions of Comparative Examples 1 and 2, a large number of particulate aggregates of CNFs were observed.

Claims (10)

A method for producing a dry solid of cellulose nanofibers, comprising drying a dispersion containing cellulose nanofibers having an average fiber length of 450 nm or less using a spray dryer. The production method according to claim 1, wherein the spray atomization method in the spray drying device is a two-fluid nozzle or a rotary atomizer. The production method according to claim 1 or 2, wherein the dry solid has a solid content of 90.0% by mass or more. The production method according to any one of claims 1 to 3, wherein the cellulose nanofibers are anion-modified cellulose nanofibers. The production method according to claim 4, wherein the anion-modified cellulose nanofibers are carboxylated cellulose nanofibers. The production method according to claim 4, wherein the carboxylated cellulose nanofibers have a carboxyl group amount of 0.6 to 3.0 mmol/g with respect to the absolute dry weight of the carboxylated cellulose nanofibers. A dry solid of cellulose nanofibers containing cellulose nanofibers having an average fiber length of 450 nm or less. 8. The dry solid according to claim 7, having a solids content of 90.0% by weight or more. Water is added to the dry solid so that the concentration of cellulose nanofibers is 1.0% by mass, and the resulting aqueous dispersion is stirred at Homodisper (3000 rpm) for 30 minutes. Transmittance of light with a wavelength of 660 nm. 9. The dry solid according to claim 7 or 8, wherein (optical path length 10 mm) is 65% or more. Water is added to the dry solid so that the concentration of cellulose nanofibers is 0.5% by mass, Bokushizuku is added and stirred, and then the area ratio of cellulose nanofiber aggregates calculated by optical microscope observation. is less than or equal to 10.0%.

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