Method for producing a dried body of fine cellulose fibers

Abstract

To provide a method for producing a fine cellulose fiber dry form that can yield a re-dispersion liquid having a dispersibility similar to or nearly identical to that of a dispersion liquid of fine cellulose fibers before drying.SOLUTION: This method includes a drying step in which a mixture containing fine cellulose fibers, a dispersant, and an aqueous solvent, with a solid content of 2-5 mass%, is dried using a vacuum drum dryer under reduced pressure at a temperature of 40-100°C.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing a dried product of microcrystalline cellulose fibers.

Background Art

[0002] Cellulose nanofibers and microfibrillated cellulose obtained by refining cellulose (hereinafter collectively referred to as "microcrystalline cellulose fibers") are fine fibers with a fiber diameter in the nano- to micro-order, and are expected to be used in various fields as novel materials having functions such as high strength, high elasticity, and thixotropy, which are not present in ordinary pulp.

[0003] Generally, cellulose nanofibers are produced in a state of being stably dispersed in water, and are usually used in various applications as industrial materials or additive materials for foods and cosmetics in the state of a cellulose nanofiber dispersion liquid having a predetermined concentration. In order to keep the state of cellulose nanofibers stable, about several tens of times the amount of water of cellulose nanofibers is required, and the large amount of this water leads to an increase in costs such as packaging, storage, and transportation of cellulose nanofibers. Therefore, reduction (concentration) and removal (drying) of this water have been regarded as an essential technology for the spread of cellulose nanofibers.

[0004] As methods for drying cellulose nanofibers in a state of being dispersed in water (wet state), a freeze-drying method and a critical point drying method have been proposed (Patent Document 1). However, the freeze-drying method and the critical point drying method have problems of requiring a large amount of energy.

[0005] In addition, when microcrystalline cellulose fibers in a state of being dispersed in water (wet state) are dried to form a dried product, hydrogen bonds are formed between the fibers of the microcrystalline cellulose fibers. Therefore, even if water is added again to this dried product to re-disperse it, the viscosity characteristics and the like cannot be restored to the same level as before drying, and there is a problem that the excellent characteristics of the microcrystalline cellulose fibers cannot be exhibited.

[0006] As a technique for obtaining a dried body of fine cellulose fibers that is easily redispersible, a method has been proposed in which a mixture of fine cellulose fibers and a solvent is dried using a vacuum drum dryer (Patent Document 2).

[0007] Using a vacuum drum dryer allows drying at low temperatures below 100°C, which has the advantage of suppressing thermal degradation of fine cellulose fibers. Furthermore, because the heat from the drum directly contacts the sample, it offers high thermal efficiency and allows for a more compact device design. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 6-233691 [Patent Document 2] International Publication No. 2019 / 189318 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, even when a fine cellulose fiber aqueous dispersion was dried at low temperature using a vacuum drum dryer according to the method described in Patent Document 2, the redispersed liquid obtained by redispersing the resulting dried material had inferior dispersibility of fine cellulose fibers compared to the aqueous dispersion before drying.

[0010] Therefore, the present invention aims to provide a method for producing a dried fine cellulose fiber body that can obtain a redispersion liquid having dispersibility equivalent to or close to that of the dispersion liquid of fine cellulose fibers before drying. [Means for solving the problem]

[0011] As a result of diligent research to achieve this objective, the inventors of the present invention discovered that setting the concentration of the raw material supplied to the vacuum drum dryer within a specific concentration range is extremely effective, and thus completed the present invention.

[0012] The present invention provides the following: (1) A method for producing a dried fine cellulose fiber body, comprising a drying step of drying a mixture containing fine cellulose fibers, a dispersant, and an aqueous solvent with a solid content concentration of 2 to 5% by mass, under reduced pressure at a temperature of 40 to 100°C using a vacuum drum dryer. (2) The method for producing a fine cellulose fiber dry product according to (1), characterized in that the vacuum drum dryer comprises a drum on which a ceramic thermal spray coating is formed on its surface by thermal spraying. (3) A method for producing a dried fine cellulose fiber according to (1) or (2), wherein the fine cellulose fiber is carboxylated cellulose nanofiber and the dispersant is carboxymethylcellulose. (4) A method for producing a dried fine cellulose fiber product according to any one of (1) to (3), wherein the mixing ratio (parts by mass) of the fine cellulose fibers (absolutely dry solids) and the dispersant in the mixture is in the range of 5:5 to 8:2. (5) The method for producing a fine cellulose fiber dry body according to any one of (2) to (4), wherein the ceramic thermal spray coating is obtained by thermal spraying a thermal spray material containing tungsten carbide and a metal binder. (6) The method for producing a dried fine cellulose fiber according to (5), wherein the metal binder is at least one selected from chromium and nickel. (7) The method for producing a dried fine cellulose fiber body according to any one of (1) to (6), characterized in that the vacuum drum dryer is equipped with a doctor blade. (8) The method for producing a dried fine cellulose fiber according to (7), characterized in that the material of the doctor blade is made of PEEK resin or phosphor bronze. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a method for producing a dried fine cellulose fiber body that can obtain a redispersion liquid having dispersibility equivalent to or close to that of the dispersion liquid of fine cellulose fibers before drying. [Brief explanation of the drawing]

[0014] [Figure 1] This is an image of the optical microscope observation result of the redispersion liquid of Example 1. [Figure 2] This is an image of the optical microscope observation result of the redispersion liquid of Example 2. [Figure 3] This is an image of the optical microscope observation result of the redispersion liquid of Example 3. [Figure 4] This is an image of the optical microscope observation result of the redispersion liquid of Example 4. [Figure 5] This is an image of the optical microscope observation result of the redispersion liquid of Example 5. [Figure 6] This is an image of the optical microscope observation result of the redispersion liquid of Comparative Example 1. [Figure 7] This is an image of the optical microscope observation result of the control dispersion liquid.

Mode for Carrying Out the Invention

[0015] Hereinafter, the present invention will be described in detail. In the present invention, "~" includes the end values. That is, "X~Y" includes the values X and Y at both ends.

[0016] The present invention is a method for producing a dried fine cellulose fiber, which includes a drying step of drying a mixture having a solid content concentration of 2 to 5% by mass containing fine cellulose fibers, a dispersant, and an aqueous solvent at a temperature of 40 to 100°C under reduced pressure using a vacuum drum dryer.

[0017] (Fine cellulose fiber) The fine cellulose fibers used in this invention are fine fibers made from cellulose, and are a general term for cellulose nanofibers (hereinafter sometimes referred to as "CNF") with an average fiber diameter of less than 500 nm and microfibrillated cellulose (hereinafter sometimes referred to as "MFC") with an average fiber diameter of 500 nm or more. The average fiber diameter is a length-weighted average fiber diameter and can be measured by observing the fine cellulose fibers using, for example, a fractionator manufactured by Valmet Corporation or an atomic force microscope (AFM). The average fiber diameter of the fine cellulose fibers is not particularly limited, but is approximately 1 nm to 60 μm. Fine cellulose fibers can be produced by defibrating cellulose.

[0018] (Cellulose nanofiber (CNF)) The average fiber diameter of the CNF that can be used in this invention is preferably 100 nm or less, more preferably 50 nm or less. The average fiber length is preferably 5 μm or less, more preferably 3 μm or less. The lower limit of the average fiber length is approximately 0.1 μm or more. The average fiber length can be measured by analyzing 200 randomly selected fibers using an atomic force microscope (AFM) if the diameter is less than 20 nm, or by using a field emission scanning electron microscope (FE-SEM) if the diameter is 20 nm or more, and calculating the average. The average aspect ratio of the CNF that can be used in this invention is preferably 50 or more. There is no particular upper limit, but it is usually 1000 or less. The average aspect ratio can be calculated by the following formula: Aspect ratio = average fiber length / average fiber diameter

[0019] (Microfibrillated Cellulose (MFC)) The average fiber length of the MFCs that can be used in this invention is preferably 5 μm or more, and more preferably 200 μm or more. The upper limit of the average fiber length is preferably 2.0 mm or less, and more preferably about 1.5 mm or less. The average fiber length (length-weighted average fiber length) can be determined by measuring it with a fractionator manufactured by Valmet or the like.

[0020] The cellulose raw material is not particularly limited as long as it contains cellulose, but examples include plants (e.g., wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (unbleached coniferous kraft pulp (NUKP), bleached coniferous kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), bleached kraft pulp (BKP), unbleached coniferous sulfite pulp (NUSP), bleached coniferous sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animals (e.g., sea squirts), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), microbial products, etc. The cellulose raw material may be any one of these or a combination of two or more, but it is preferably a cellulose raw material derived from plants or microorganisms (e.g., cellulose fibers), and more preferably a cellulose raw material derived from plants (e.g., cellulose fibers).

[0021] The number-average fiber diameter of cellulose raw materials is not particularly limited, but for common pulps such as softwood kraft pulp, it is about 30-60 μm, and for hardwood kraft pulp, it is about 10-30 μm. For other pulps, after general refining, it is about 50 μm. For example, if the material is made from wood chips or other materials several centimeters in size, it is preferable to mechanically process it using a refiner or beater to adjust it to about 50 μm.

[0022] Cellulose has three hydroxyl groups per glucose unit, and can be subjected to various chemical modifications. In the present invention, from the viewpoint of promoting the progression of defibration, it is preferable to use chemically modified fine cellulose fibers produced by defibrating a cellulose raw material (chemically modified cellulose) obtained by chemical modification.

[0023] Examples of chemical modification include carboxylation (oxidation), carboxymethylation, cationization, and esterification. Of these, carboxylation (oxidation) is more preferred.

[0024] (chemical modification) (carboxylation) In the present invention, when using carboxylated fine cellulose fibers obtained by defibrating carboxylated (oxidized) cellulose, carboxylated cellulose (also called oxidized cellulose) can be obtained by carboxylating (oxidizing) the above-mentioned cellulose raw material using a known method. Although not particularly limited, during carboxylation, it is preferable to adjust the amount of carboxyl groups to 0.6 to 2.0 mmol / g relative to the oven-dry mass of the chemically modified fine cellulose fibers, and more preferably to 1.0 mmol / g to 2.0 mmol / g.

[0025] One example of a carboxylation (oxidation) method is to oxidize a cellulose raw material in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromide, iodide, or a mixture thereof. This carboxylation reaction selectively oxidizes the primary hydroxyl group at the C6 position of the glucopyranose ring on the surface of the cellulose, resulting in the formation of an aldehyde group and a carboxyl group (-COOH) or carboxylate group (-COOH) on the surface. - Cellulose fibers having the following characteristics can be obtained. The concentration of cellulose during the reaction is not particularly limited, but 5% by mass or less is preferred.

[0026] An N-oxyl compound is a compound that can generate a nitroxyl radical. Any compound that promotes the desired oxidation reaction can be used as an N-oxyl compound. Examples include 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO).

[0027] The amount of N-oxyl compound used is not particularly limited, as long as it is a catalytic amount that can oxidize the raw material cellulose. For example, 0.01 to 10 mmol is preferred, 0.01 to 1 mmol is more preferred, and 0.05 to 0.5 mmol is even more preferred, per 1 g of oven-dried cellulose. Also, about 0.1 to 4 mmol / L of the reaction system is preferred.

[0028] Bromides are compounds containing bromine, and examples include alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, and examples include alkali metal iodides. The amount of bromide or iodide used can be selected within a range that promotes the oxidation reaction. The total amount of bromide and iodide is preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol per 1 g of oven-dried cellulose.

[0029] As an oxidizing agent, known substances can be used, such as halogens, hypohalous acids, halogenous acids, perhalous acids or their salts, halogen oxides, and peroxides. Among these, sodium hypochlorite is preferred because it is inexpensive and has a low environmental impact. As for the amount of oxidizing agent to use, for example, 0.5 to 500 mmol is preferred, 0.5 to 50 mmol is more preferred, 1 to 25 mmol is even more preferred, and 3 to 10 mmol is most preferred, per 1 g of oven-dried cellulose. Also, for example, 1 to 40 moles are preferred per 1 mole of N-oxyl compound.

[0030] The carboxylation of cellulose can proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature is preferably 4 to 40°C, and can also be room temperature of about 15 to 30°C. As the reaction progresses, carboxyl groups are generated in the cellulose, causing a decrease in the pH of the reaction solution. To ensure the oxidation reaction proceeds 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 8 to 12, preferably 10 to 11. Water is preferred as the reaction medium due to its ease of handling and the low likelihood of side reactions.

[0031] The reaction time in an oxidation reaction can be set appropriately according to the degree of oxidation, and is usually 0.5 to 6 hours, for example, 0.5 to 4 hours.

[0032] Furthermore, the oxidation reaction may be carried out in two stages. For example, by filtering out the oxidized cellulose after the first stage of the reaction and then oxidizing it again under the same or different reaction conditions, the oxidation can be carried out efficiently without being inhibited by the salt produced as a by-product in the first stage of the reaction.

[0033] Another example of a carboxylation (oxidation) method involves contacting a cellulose raw material with an ozone-containing gas. This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring, and also causes decomposition of the cellulose chain. The ozone concentration in the ozone-containing gas is 50-250 g / m³. 3 Preferably, it is 50-220 g / m² 3 It is more preferable that the following conditions are met. The amount of ozone added to the cellulose raw material is preferably 0.1 to 30 parts by mass, and 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, and more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is about 1 to 360 minutes, and is preferably about 30 to 360 minutes. When the ozone treatment conditions are within these ranges, it is possible to prevent excessive oxidation and decomposition of the cellulose, and a good yield of oxidized cellulose can be obtained. After ozone treatment, a follow-up oxidation treatment may be performed using an oxidizing agent. The oxidizing agent used for the follow-up oxidation treatment is not particularly limited, but examples include chlorine compounds such as chlorine dioxide and sodium chlorite, as well as oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. For example, these oxidizing agents can be dissolved in water or a polar organic solvent such as alcohol to create an oxidizing agent solution, and the follow-up oxidation treatment can be performed by immersing the cellulose raw material in the solution.

[0034] The amount of carboxyl groups in carboxylated cellulose can be adjusted by controlling the reaction conditions, such as the amount of oxidizing agent added and the reaction time.

[0035] (carboxymethylation) In the present invention, when using carboxymethylated fine cellulose fibers obtained by defibrating carboxymethylated cellulose, the carboxymethylated cellulose may be obtained by carboxymethylating the above-mentioned cellulose raw material by a known method, or a commercially available product may be used. In either case, it is preferable that the degree of carboxymethyl group substitution per anhydrous glucose unit of cellulose is 0.01 to 0.50. An example of a method for producing such carboxymethylated cellulose is as follows: Cellulose is used as the base material, and 3 to 20 times the mass of water and / or lower alcohols, specifically water, methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, tertiary butanol, etc., are used as the solvent, either individually or as a mixture of two or more. When lower alcohols are mixed, the mixing ratio of lower alcohols is 60 to 95% by mass. As a mercerizing agent, 0.5 to 20 times the mole of alkali metal hydroxide per anhydrous glucose residue of the base material is used, specifically sodium hydroxide and potassium hydroxide. The starting material, solvent, and mercerizing agent are mixed, and the mercerizing treatment is carried out at a reaction temperature of 0 to 70°C, preferably 10 to 60°C, and for a reaction time of 15 minutes to 8 hours, preferably 30 minutes to 7 hours. Then, a carboxymethylating agent is added at a rate of 0.05 to 10.0 moles per glucose residue, and the etherification reaction is carried out at a reaction temperature of 30 to 90°C, preferably 40 to 80°C, and for a reaction time of 30 minutes to 10 hours, preferably 1 hour to 4 hours.

[0036] In this specification, "carboxymethylated cellulose," a type of chemically modified cellulose used in the preparation of fine cellulose fibers, refers to cellulose that maintains at least a portion of its fibrous shape even when dispersed in water. Therefore, it is distinguished from carboxymethylcellulose, a type of water-soluble polymer. When an aqueous dispersion of "carboxymethylated cellulose" is observed with an electron microscope, fibrous material can be observed. On the other hand, when an aqueous dispersion of carboxymethylcellulose, a type of water-soluble polymer, is observed, no fibrous material is observed. Furthermore, 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 seen in carboxymethylcellulose, a water-soluble polymer.

[0037] (cationization) In the present invention, cationized fine cellulose fibers obtained by defibrating cellulose that has been further cationized from the carboxylated cellulose can be used. The cationized cellulose can be obtained by reacting the carboxylated cellulose raw material with a cationizing agent such as glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium hydrate or its halohydrin type, and an alkali metal hydroxide catalyst (such as sodium hydroxide or potassium hydroxide) in the presence of water or an alcohol having 1 to 4 carbon atoms.

[0038] The degree of cation substitution per glucose unit is preferably 0.02 to 0.50. By introducing a cation substituent into cellulose, the cellulose molecules repel each other electrically. Therefore, cellulose to which a cation substituent has been introduced can be easily defibrated. If the degree of cation substitution per glucose unit is less than 0.02, sufficient defibration is not possible. On the other hand, if the degree of cation substitution per glucose unit is greater than 0.50, swelling or dissolution may occur, making it impossible to obtain fine fibers. In order to efficiently defibrize, it is preferable to wash the cation-modified cellulose raw material obtained above. The degree of cation substitution can be adjusted by the amount of cationizing agent added to the reaction and the composition ratio of water or C1-C4 alcohol.

[0039] (Esterification) In the present invention, esterified fine cellulose fibers obtained by defibrating esterified cellulose can be used. This esterified cellulose can be obtained by mixing the aforementioned cellulose raw material with powder or aqueous solution of phosphate compound A, or by adding an aqueous solution of phosphate compound A to a slurry of cellulose raw material.

[0040] Examples of phosphate compounds A include phosphoric acid, polyphosphate, phosphorous acid, hypophosphorous acid, phosphonic acid, polyphosphonic acid, or esters thereof. These may also be in salt form. Among these, compounds having a phosphate group are preferred because they are low-cost, easy to handle, and can improve defibration efficiency by introducing a phosphate group into the cellulose of pulp fibers. Examples of compounds having a phosphate group include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium phosphite, potassium phosphite, sodium hypophosphite, potassium hypophosphite, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, and ammonium metaphosphate. These can be used individually or in combination of two or more. Of these, phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are more preferred from the viewpoint of high efficiency in introducing a phosphate group, ease of defibration in the defibration process described below, and ease of industrial application. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. Furthermore, it is preferable to use the phosphate compound A as an aqueous solution because this increases the uniformity of the reaction and the efficiency of phosphate group introduction. The pH of the aqueous solution of phosphate compound A is preferably 7 or less because it increases the efficiency of phosphate group introduction, but a pH of 3 to 7 is preferable from the viewpoint of suppressing hydrolysis of pulp fibers.

[0041] The following method is an example of a method for producing phosphate-esterified cellulose. A phosphate compound A is added to a dispersion of cellulose raw material with a solid content concentration of 0.1 to 10% by mass while stirring to introduce phosphate groups into the cellulose. When the cellulose raw material is 100 parts by mass, the amount of phosphate compound A added is preferably 0.2 to 500 parts by mass, and more preferably 1 to 400 parts by mass, in terms of phosphorus element content. If the proportion of phosphate compound A is above the lower limit, the yield of fine cellulose fibers can be further improved. However, if it exceeds the upper limit, the effect of improving the yield plateaus, which is undesirable from a cost perspective.

[0042] In this process, in addition to the cellulose raw material and phosphate compound A, powder or aqueous solution of compound B other than A may be mixed. Compound B is not particularly limited, but a nitrogen-containing compound exhibiting basicity is preferred. Here, "basicity" is defined as the aqueous solution exhibiting a pink to red color in the presence of phenolphthalein indicator, or the pH of the aqueous solution being greater than 7. The nitrogen-containing compound exhibiting basicity used in this invention is not particularly limited as long as it achieves the effects of the present invention, but a compound having an amino group is preferred. Examples include, but are not particularly limited, urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, urea is preferred because it is low-cost and easy to handle. The amount of compound B added is preferably 2 to 1000 parts by mass, and more preferably 100 to 700 parts by mass, per 100 parts by mass of solid content of the cellulose raw material. The reaction temperature is preferably 0 to 95°C, and more preferably 30 to 90°C. The reaction time is not particularly limited, but is approximately 1 to 600 minutes, with 30 to 480 minutes being more preferable. When the esterification reaction conditions are within this range, it is possible to prevent the cellulose from being excessively esterified and becoming easily soluble, resulting in a good yield of phosphate-esterified cellulose. After dehydrating the obtained phosphate-esterified cellulose suspension, it is preferable to heat-treat it at 100 to 170°C from the viewpoint of suppressing hydrolysis of cellulose. Furthermore, it is preferable to heat it at 130°C or lower, preferably 110°C or lower, while water is present during the heat treatment, and then heat-treat it at 100 to 170°C after removing the water.

[0043] The degree of phosphate group substitution per glucose unit in phosphate-esterified cellulose is preferably 0.001 to 0.40. By introducing phosphate group substituents to cellulose, the cellulose molecules repel each other electrically. Therefore, cellulose with introduced phosphate groups can be easily defibrated. If the degree of phosphate group substitution per glucose unit is less than 0.001, sufficient defibration is not possible. On the other hand, if the degree of phosphate group substitution per glucose unit is greater than 0.40, swelling or dissolution may occur, making it impossible to obtain fine cellulose fibers. In order to efficiently defibrize, it is preferable to wash the phosphate-esterified cellulose raw material obtained above by boiling and then washing it with cold water.

[0044] (Fibreation) In the present invention, the apparatus for defibrating chemically modified cellulose is not particularly limited, but it is preferable to apply a strong shear force to the aqueous dispersion using an apparatus such as a high-speed rotary type, colloidal mill type, high-pressure type, roll mill type, or ultrasonic type. In particular, to efficiently defibrate the material, it is preferable to use a wet high-pressure or ultra-high-pressure homogenizer that can apply a pressure of 50 MPa or more to the aqueous dispersion and apply a strong shear force. The pressure is more preferably 100 MPa or more, and even more preferably 140 MPa or more. Furthermore, prior to the defibration and dispersion treatment in the high-pressure homogenizer, it is also possible to pre-treat the fine cellulose fibers using a known mixing, stirring, emulsifying, and dispersion apparatus such as a high-speed shear mixer, if necessary. The number of treatments (passes) in the defibration apparatus may be one or two or more, and two or more is preferable.

[0045] In the dispersion process, chemically modified cellulose is typically dispersed in a solvent. The solvent is not particularly limited as long as it can disperse the chemically modified cellulose, but examples include water, organic solvents (e.g., hydrophilic organic solvents such as methanol), and mixed solvents thereof. Since the cellulose raw material is hydrophilic, the solvent is preferably water.

[0046] The solid content concentration of chemically modified cellulose in the dispersion is usually 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.3% by mass or more. This ensures that the amount of liquid relative to the amount of cellulose fiber raw material is appropriate and efficient. The upper limit is usually 10% by mass or less, preferably 6% by mass or less. This allows for maintaining fluidity.

[0047] Prior to defibration or dispersion, preliminary treatment may be performed as needed. Preliminary treatment can be carried out using mixing, stirring, emulsifying, and dispersion equipment such as a high-speed shear mixer.

[0048] If the chemically modified fine cellulose fibers obtained through the defibration process are of the salt type, they may be used as is, or they may be used as an acid type by acid treatment using mineral acids or by methods using cation exchange resins. Alternatively, they may be used after being made hydrophobic by methods using cationic additives.

[0049] (Dispersant) In the manufacturing method of the present invention, the mixture containing fine cellulose fibers subjected to the drying step includes a dispersant from the viewpoint of improving the redispersibility of the dried product obtained after drying. Examples of dispersants include water-soluble polymers and surfactants. It is preferable to use a water-soluble polymer because it covers the areas with low charge density on the surface of the chemically modified fine cellulose fibers, suppressing the formation of hydrogen bonds and preventing aggregation of the fine cellulose fibers during drying.

[0050] (Water-soluble polymer) Examples of water-soluble polymers that can be used in the production method of the present invention include cellulose derivatives (carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, ethylcellulose), xanthan gum, xyloglucan, dextrin, dextran, carrageenan, locust bean gum, alginic acid, alginate, pullulan, starch, potato starch, kudzu starch, modified starch (cationized starch, phosphorylated starch, phosphate cross-linked starch, phosphate monoesterified phosphate cross-linked starch, hydroxypropyl starch, hydroxypropylated phosphate cross-linked starch, acetylated adipate cross-linked starch, acetylated phosphate cross-linked starch, acetylated oxidized starch, sodium octenyl succinate starch, acetate starch, oxidized starch), corn starch, gum arabic, locust bean gum. Examples include tobean gum, gellan gum, polydextrose, pectin, chitin, water-soluble chitin, chitosan, casein, albumin, soy protein lysate, peptone, polyvinyl alcohol, polyacrylamide, sodium polyacrylate, polyvinylpyrrolidone, polyvinyl acetate, polyamino acids, polylactic acid, polymalic acid, polyglycerin, latex, rosin-based sizing agents, petroleum resin-based sizing agents, urea resin, melamine resin, epoxy resin, polyamide resin, polyamide / polyamine resin, polyethyleneimine, polyamine, plant gum, polyethylene oxide, hydrophilic crosslinked polymer, polyacrylate salt, starch-polyacrylic acid copolymer, tamarind gum, guar gum, and colloidal silica, as well as mixtures of one or more of these. Among these, cellulose derivatives are preferred in terms of affinity with chemically modified fine cellulose fibers, and carboxymethylcellulose and its salts are particularly preferred. Water-soluble polymers such as carboxymethylcellulose and its salts are thought to improve redispersibility by penetrating between the fibers of fine cellulose fibers and increasing the distance between fibers.

[0051] Examples of surfactants that can be used in the manufacturing method of the present invention include, but are not limited to, nonionic surfactants such as fatty acid salts, higher alkyl sulfates, alkylbenzene sulfonates, higher alcohols, alkylphenols, and alkylene oxide adducts of fatty acids, as well as anionic surfactants, cationic surfactants, amphoteric surfactants, and organic solvents, proteins, enzymes, natural polymers, and synthetic polymers. These can consist of a single component or a mixture of two or more components.

[0052] When using carboxymethylcellulose or its salts as the water-soluble polymer, it is preferable to use carboxymethylcellulose with a carboxymethyl group substitution degree of 0.55 to 1.6 per anhydrous glucose unit, more preferably 0.55 to 1.1, and even more preferably 0.65 to 1.1. Furthermore, molecules with longer lengths (higher viscosity) are preferable because they have a greater effect in increasing the distance between nanofibers. In addition, the B-type viscosity of a 1% by mass aqueous solution of carboxymethylcellulose at 25°C and 60 rpm is preferably 3 mPa·s to 14000 mPa·s, more preferably 7 mPa·s to 14000 mPa·s, and even more preferably 1000 mPa·s to 8000 mPa·s. Note that "carboxymethylcellulose or its salts" as the water-soluble polymer referred to here is completely soluble in water and is therefore distinguished from the carboxymethylated cellulose whose fiber shape can be observed in water as described above.

[0053] In the mixture before drying, the mixing ratio of fine cellulose fibers (absolutely dry solids) to dispersant is preferably 5:5 to 8:2, and more preferably 6:4 to 7:3, from the viewpoint of obtaining an effect of improving redispersibility. If the mixing ratio of dispersant is too high compared to the above upper limit, problems such as a decrease in viscosity characteristics such as thixotropy, which is a characteristic of fine cellulose fibers, and a decrease in dispersion stability may occur. If the mixing ratio of dispersant is too low compared to the above lower limit, sufficient redispersibility cannot be obtained.

[0054] (Aqueous solvent) Examples of aqueous solvents used in the present invention include water, water-soluble organic solvents, or mixed solvents thereof. Since the cellulose raw material is hydrophilic, it is preferable to use water because it is easier to achieve a good dispersion state during dispersion.

[0055] Water-soluble organic solvents are organic solvents that dissolve 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, dimethyl sulfoxide, acetonitrile, and combinations thereof. Among these, lower alcohols with 1 to 4 carbon atoms, such as methanol, ethanol, and 2-propanol, are preferred, and methanol and ethanol are more preferred, with ethanol being even more preferred, from the viewpoint of safety and availability.

[0056] When a mixed solvent is used, the amount of water-soluble organic solvent in the mixed solvent is preferably 10% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. There is no upper limit to this amount, but it is preferably 95% by mass or less, and more preferably 90% by mass or less. Furthermore, the aqueous solvent may contain water-insoluble organic solvents to an extent that does not impair the effects of the invention.

[0057] The solid content concentration of the mixture before drying should be 2-5% by mass, preferably 3-5% by mass, and more preferably 3-4% by mass. If the solid content concentration of the mixture is lower than the lower limit, the film thickness of the resulting film will be smaller, the heat applied to the fine cellulose fibers will increase, and thermal denaturation will progress, resulting in a poorly dispersed state in the redispersed liquid of the resulting dried product. On the other hand, if the solid content concentration is higher than the upper limit, the viscosity will be high, and there is a risk that it will not be supplied between the drums.

[0058] While there are no particular limitations on the method for adjusting the solid content concentration of the mixture before drying, one method is to add water and stir with a homodisperser.

[0059] (Method of manufacturing the dried product) The manufacturing method of the present invention includes a drying step of drying a mixture containing fine cellulose fibers, a dispersant, and an aqueous solvent at a temperature of 40 to 100°C under reduced pressure using a vacuum drum dryer.

[0060] (Vacuum drum dryer) A vacuum drum dryer is a device that produces a dried product by placing a heated drum under vacuum or reduced pressure, continuously supplying a mixture of fine cellulose fibers, a dispersant, and an aqueous solvent to the drum surface while rotating the drum, causing the aqueous solvent to evaporate and concentrate, and simultaneously drying the drum surface by depositing a thin film of fine cellulose fibers and dispersant, and then scraping off the dried material formed on the drum surface with a knife such as a doctor blade provided in the vacuum drum dryer.

[0061] When drying under vacuum or reduced pressure, it is preferable to dry in the range of 0 to 50 kPa. Lower pressure allows moisture to evaporate at a lower temperature, so it is preferable to have a pressure of 50 kPa or less, more preferably 30 kPa or less, and even more preferably 10 kPa or less.

[0062] In this invention, the drying temperature refers to the temperature of the drum surface and is 40 to 100°C, preferably 60 to 80°C, from the viewpoint of obtaining a dried product with excellent redispersibility and efficiency. If the drying temperature is too high above the upper limit, the cellulose will be degraded due to the effects of heat, and the resulting dried product will have poor redispersibility. Conversely, if the drying temperature is too low above the lower limit, the production efficiency will be poor.

[0063] Drum-type dryers include double-drum or twin-drum dryers that use two drums, and single-drum dryers that use one drum; any of these can be used. Among these, a double-drum dryer is preferred because the film thickness of the thin film can be adjusted by adjusting the clearance between the drums.

[0064] The thickness of the thin film formed on the drum surface is preferably 5 to 500 μm, and more preferably 50 to 250 μm. A thickness of 50 μm or more provides further improvement in redispersibility, while a thickness of 250 μm or less facilitates scraping after drying.

[0065] The material of the drum surface of the vacuum drum dryer used in the present invention is not particularly limited, but examples include metal plating such as chrome plating, or a ceramic coating. A drum with a ceramic coating on its surface has excellent peelability of the dried fine cellulose fiber, so compared to using a conventional drum with a chrome-plated surface, less force is required to press the doctor blade against the drum to scrape off the dried material. Therefore, by using a drum with a ceramic coating, the frictional heat generated between the doctor blade and the drum can be suppressed, and the resulting dried fine cellulose fiber has less thermal deformation, and this dried material has excellent redispersibility.

[0066] From the viewpoint of uniformity of the coating, thermal spraying is preferred as a method for forming a ceramic coating on the surface of a drum. Thermal spraying is a method of forming a thermal spray coating by heating the spraying material to a molten or softened state and spraying it onto the surface of a substrate. The thermal spraying method is not particularly limited and includes flame spraying, arc spraying, plasma spraying, etc., with plasma spraying being preferred from the viewpoint of workability.

[0067] Examples of thermal spray materials include metals, alloys, ceramics, and plastics, and it is preferable to use materials containing ceramics due to their wear resistance and high strength. Examples of ceramics include tungsten carbide and chromium carbide, and it is preferable to use tungsten carbide from the viewpoint of the release properties of the CNF dry product. When using thermal spray materials containing ceramics, it is preferable to use a metal binder in combination. As for the metal binder, at least one selected from chromium, nickel, and cobalt is preferred, at least one selected from chromium and nickel is more preferred, and a material containing both chromium and nickel is even more preferred.

[0068] Doctor blades can be made from materials such as stainless steel (SUS), PEEK resin, carbon fiber, or phosphor bronze. The material of the drum surface affects the peelability of the dried material, which in turn changes the force required for scraping. Therefore, the appropriate material should be selected according to the drum surface material. When the drum surface is coated with a ceramic thermal spray coating, it is preferable to use doctor blades made of PEEK resin or phosphor bronze from the viewpoint of protecting the coating on the drum surface, and it is preferable to use a doctor blade made of phosphor bronze from the viewpoint of durability.

[0069] (dry body) The resulting dried material may be crushed, classified, or otherwise processed into a powder, but it may also be in other forms.

[0070] In this invention, a dried product refers to a state in which the moisture content is dried to 15% by mass or less. The moisture content is preferably 0 to 15% by mass, and more preferably 0 to 10% by mass. In the drying process, the product may be dried to 0% moisture content (absolutely dry).

[0071] According to the method for producing a dried fine cellulose fiber body of the present invention, a mixture containing fine cellulose fibers, a dispersant, and an aqueous solvent with a solid content concentration of 3 to 5% by mass is used. Compared to the case where a mixture with a lower concentration is used, the thickness of the thin film formed between the drums of the vacuum drum dryer is increased, and the heat applied to the fine cellulose fibers is reduced, thus suppressing thermal deformation. As a result, the dried body obtained by the production method of the present invention has excellent dispersibility.

[0072] Furthermore, in the method for producing a dried fine cellulose fiber body of the present invention, if a vacuum drum dryer equipped with a drum having a ceramic thermal spray coating formed on its surface is used, the peelability of the resulting dried body is improved. This eliminates the need to press the doctor blade strongly against the drum, suppressing the frictional heat generated between the doctor blade and the drum surface. As a result, the resulting dried fine cellulose fiber body is less affected by heat, and this dried body exhibits excellent redispersibility. [Examples]

[0073] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, the measurement / calculation methods for each numerical value in each example are those described in the specification.

[0074] (Manufacturing Example 1) (Production of carboxylated (TEMPO-oxidized) CNF) 5 g (absolutely dry) bleached, unbeaten kraft pulp (whiteness 85%) derived from coniferous trees was added to 500 mL of an aqueous solution containing 39 mg of TEMPO (Sigma Aldrich) and 514 mg of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. Sodium hypochlorite aqueous solution was added to the reaction system to a level of 5.5 mmol / g to initiate the oxidation reaction. During the reaction, the pH of the system decreased, but 3 M sodium hydroxide aqueous solution was added sequentially to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system no longer changed. The reaction mixture was acidified with hydrochloric acid, filtered through a glass filter to separate the pulp, and thoroughly washed with water to obtain oxidized pulp (hereinafter sometimes referred to as "carboxylated cellulose," "carboxylated pulp," or "TEMPO-oxidized pulp"). The pulp yield was 90%, the oxidation reaction took 90 minutes, and the carboxyl group content was 1.6 mmol / g. The oxidized pulp obtained in the above process was adjusted to 1.0% (w / v) with water and treated three times in an ultra-high pressure homogenizer (20°C, 150 MPa) to obtain a carboxylated CNF dispersion. The resulting fibers had an average fiber diameter of 3 nm and an aspect ratio of 150.

[0075] (Method for measuring the amount of carboxyl groups) A 60 mL slurry (aqueous dispersion) of carboxylated cellulose was prepared, and a 0.1 M hydrochloric acid aqueous solution was added to adjust the pH to 2.5. Then, a 0.05 N sodium hydroxide aqueous solution was added dropwise until the pH reached 11, and the electrical conductivity was measured. The amount of sodium hydroxide consumed during the neutralization stage of the weak acid, where the change in electrical conductivity was gradual, was calculated using the following formula: Carboxylate group content [mmol / g carboxylated cellulose] = a [mL] × 0.05 / mass of carboxylated cellulose [g].

[0076] (Measurement of average fiber diameter, average fiber length, and aspect ratio of CNF) The average fiber diameter and average fiber length of CNF were analyzed using atomic force microscopy (AFM) on 200 randomly selected fibers. The aspect ratio was calculated using the following formula: Aspect ratio = average fiber length / average fiber diameter

[0077] (Evaluation of the redispersed solution) (Viscosity measurement) The viscosity of the 1% by mass CNF redispersion solutions obtained in the examples and comparative examples was measured using a Type B viscometer (manufactured by Eiko Seiki Co., Ltd.) at 25°C, at a rotation speed of 60 rpm for 3 minutes, and at a rotation speed of 6 rpm for 3 minutes. The results are shown in Table 1.

[0078] (Measuring transparency) The transparency (transmittance of 660 nm light) of the 1% solid content CNF redispersion solutions obtained in the examples and comparative examples was measured using a visible light photometer ASV11D (manufactured by AS ONE Corporation). The results are shown in Table 1.

[0079] (dispersibility) Two drops of ink (Kuretake Co., Ltd., 10% solids) were added to 1 g of the 1% solids redispersion solution obtained in the Examples and Comparative Examples, and to 1 g of the 1% solids aqueous dispersion of CNF prepared in Comparative Example 1 before drying (as a control). The mixture was stirred for 1 minute using a vortex mixer (IUCHI Co., Ltd., instrument name: Automatic Lab-mixer HM-10H) with the rotation speed set to maximum. Next, the CNF dispersion containing the ink was sandwiched between two glass plates so that the film thickness was 0.15 mm, and observed at 100x magnification using an optical microscope (Digital Microscope KH-8700 (Hirox Co., Ltd.)). The observation results are shown in Figures 1 to 7. The results were also evaluated according to the following criteria and are shown in Table 1. The fewer the white clumps (gel particles) seen in the obtained images, the better the dispersibility. ○: Gel particles were hardly observed. △: Some gel particles were observed. ×: Many gel particles were observed.

[0080] (Example 1) (Modification of the drum surface of a vacuum drum dryer) A ceramic thermal spray coating containing tungsten carbide was formed on the drum surface of a vacuum drum dryer (manufactured by Katsuragi Industries Co., Ltd., model VD-0102) by plasma spraying a tungsten carbide and nickel-chromium binder.

[0081] (Preparation of CNF aqueous dispersion) To a 1.0% by mass aqueous dispersion of carboxylated CNF obtained in Production Example 1, 3 parts by mass of carboxymethylcellulose (trade name: F350HC-4, viscosity (1% by mass, 25℃) approximately 3000 mPa·s, degree of carboxymethyl substitution approximately 0.9) was added as a dispersant to 7 parts by mass of solids of CNF, and the mixture was stirred for 60 minutes with a jet paster (7200 rpm) to prepare an aqueous dispersion of CNF. The pH of this dispersion was approximately 7. To this aqueous dispersion, 0.5% aqueous sodium hydroxide solution was added to adjust the pH to 8-9. The solids concentration of the aqueous dispersion at this time (including carboxylated CNF and carboxymethylcellulose) was 3.9% by mass.

[0082] (Manufacturing of dried CNF) The obtained aqueous dispersion with a solid content of 3.9% by mass was applied to the surface of a drum coated with a ceramic thermal spray film to form a thin film with a thickness of approximately 100-200 μm. The drum was then dried in a drum dryer at a drum surface temperature of 80°C, a steam pressure of 0.3 MPaG, a drum rotation speed of 2 rpm, and a dryer internal pressure of 2 kPa to obtain a dried carboxylated CNF with a solid content of 86.7% by mass. A doctor blade made of PEEK resin was used to scrape the dried CNF from the drum surface.

[0083] (Redispersion of dried CNF) To the dried CNF obtained above, ion-exchanged water was added to achieve a solid content concentration of 1% by mass, and the mixture was stirred using a homodisperser at 3000 rpm for 30 minutes to obtain an aqueous dispersion of redispersed CNF.

[0084] (Example 2) In the production of the dried CNF, a dried carboxylated CNF with a solid content of 91.0% by mass was obtained in the same manner as in Example 1, except that a vacuum drum dryer without surface modification was used and a SUS (stainless steel) doctor blade was used. Furthermore, an aqueous dispersion of CNF was obtained by redispersing the obtained dried CNF in the same manner as in Example 1. The drum surface was chrome-plated.

[0085] (Example 3) In the preparation of the CNF aqueous dispersion, an aqueous dispersion with a solid content of 3.1% by mass was obtained in the same manner as in Example 1, except that 4 parts by mass of carboxymethylcellulose were added per 6 parts by mass of solid content of CNF. In the production of the dried CNF, a dried carboxylated CNF with a solid content of 90.7% by mass was obtained in the same manner as in Example 1, except that the aqueous dispersion with a solid content of 3.1% by mass obtained in this manner was used, a vacuum drum dryer without modification of the drum surface was used, and a SUS (stainless steel) doctor blade was used. Furthermore, an aqueous dispersion was obtained by redispersing the CNF using the obtained dried CNF in the same manner as in Example 1.

[0086] (Example 4) In the production of the dried CNF, a dried carboxylated CNF with a solid content of 94.5% by mass was obtained in the same manner as in Example 1, except that a phosphor bronze doctor blade was used. Furthermore, an aqueous dispersion of the obtained dried CNF was obtained by redispersing the CNF in the same manner as in Example 1.

[0087] (Example 5) In the preparation of the CNF aqueous dispersion, an aqueous dispersion with a solid content of 3.9% by mass was obtained in the same manner as in Example 1, except that 4 parts by mass of carboxymethylcellulose were added per 6 parts by mass of solid content of CNF. In the production of the dried CNF, a dried carboxylated CNF with a solid content of 94.2% by mass was obtained in the same manner as in Example 1, except that the aqueous dispersion with a solid content of 3.9% by mass obtained in this manner was used, and a phosphor bronze doctor blade was used. Furthermore, an aqueous dispersion was obtained by redispersing the CNF using the obtained dried CNF in the same manner as in Example 1.

[0088] (Comparative Example 1) In preparing the CNF aqueous dispersion, the procedure was the same as in Example 1, except that water was added and the mixture was stirred with a homodisperser to dilute it to 1.0%, ultimately obtaining an aqueous dispersion with a solid content of 1.0% by mass. In producing the dried CNF, the procedure was the same as in Example 1, except that the 1.0% by mass aqueous dispersion obtained in this manner was used, a vacuum drum dryer without modification of the drum surface was used, and a SUS (stainless steel) doctor blade was used, to obtain a dried carboxylated CNF with a solid content of 90.5% by mass. Furthermore, using the obtained dried CNF, an aqueous dispersion was obtained by redispersing the CNF in the same manner as in Example 1.

[0089] Furthermore, the 1.0% by mass CNF aqueous dispersion prepared in Comparative Example 1, before drying, was subjected to viscosity measurement, transparency measurement, and dispersibility evaluation as described above, and the results are shown in Table 1 as a reference.

[0090] [Table 1]

[0091] As can be seen from Table 1, in Examples 1 to 5, a fine cellulose fiber dry product was produced by a manufacturing method that included a drying step in which a mixture containing fine cellulose fibers, a dispersant, and an aqueous solvent with a solid content concentration of 2 to 5% by mass was dried in a vacuum drum dryer under reduced pressure at a temperature of 40 to 100°C. In these Examples, the redispersed liquid of the obtained fine cellulose fiber dry product showed superior dispersibility compared to the redispersed liquid of the dry product obtained in Comparative Example 1, which used a mixture with a solid content concentration of 1% by mass.

Claims

1. The process includes a drying step in which a mixture containing fine cellulose fibers, a dispersant, and an aqueous solvent, with a solid content concentration of 2 to 5% by mass, is dried in a vacuum drum dryer under reduced pressure at a temperature of 40 to 100°C. The dispersant is carboxymethylcellulose or a salt thereof. A method for producing a dried body of fine cellulose fibers, wherein the mixing ratio (parts by mass) of the fine cellulose fibers (absolutely dry solids) and the dispersant in the mixture is in the range of 5:5 to 8:

2.

2. The method for producing a dried fine cellulose fiber product according to claim 1, characterized in that the vacuum drum dryer comprises a drum on which a ceramic thermal spray coating is formed on its surface by thermal spraying.

3. A method for producing a dried fine cellulose fiber according to claim 1 or 2, wherein the fine cellulose fiber is a carboxylated cellulose nanofiber.

4. The method for producing a fine cellulose fiber dry body according to claim 2, wherein the ceramic thermal spray coating is obtained by thermal spraying a thermal spray material containing tungsten carbide and a metal binder.

5. The method for producing a dried fine cellulose fiber according to claim 4, wherein the metal binder is at least one selected from chromium and nickel.

6. The method for producing a dried fine cellulose fiber body according to any one of claims 1 to 5, characterized in that the vacuum drum dryer is equipped with a doctor blade.

7. The method for producing a dried fine cellulose fiber according to claim 6, characterized in that the material of the doctor blade is made of PEEK resin or phosphor bronze.

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