Method for concentrating or drying a cellulose nanofiber dispersion

Abstract

Provided is a method for obtaining a concentrated or dried product of cellulose nanofiber which can produce a redispersion having a degree of dispersibility equivalent to that of a cellulose nanofiber dispersion before concentration or drying. A method, which is a concentration or drying method of a cellulose nanofiber dispersion, the method comprising: a process A of performing a pretreatment of adding the following (1) and (2) to the cellulose nanofiber dispersion: (1) the following (1-1) and / or (1-2): (1-1) an alkylammonium salt and an amphoteric surfactant, (1-2) ethylene glycol, (2) a mixture of a higher fatty acid or a higher fatty acid amide and isopropyl alcohol; and a process B of, after the pretreatment, feeding the cellulose nanofiber dispersion to a two-roll mill, rotating the two-roll mill, and thereby concentrating or drying the cellulose nanofiber dispersion.

Description

Technical Field

[0001] This invention relates to methods for concentrating or drying cellulose nanofiber dispersions. In particular, this invention relates to methods for obtaining concentrated or dried cellulose nanofiber products, which concentrate or dry the cellulose nanofiber dispersion while inhibiting the aggregation of cellulose nanofibers, to generate a redispersible liquid with the same degree of dispersibility as the cellulose nanofiber dispersion before concentration or drying. Background Technology

[0002] Cellulose nanofibers (also known as "Cellulose Nano Fiber," sometimes simply referred to as "CNF") are next-generation raw materials derived from plants, reportedly achieving five times the strength of steel with only one-fifth the weight. By effectively applying cellulose nanofibers to automobiles, home appliances, and other applications, lightweighting and improved energy efficiency can be achieved, potentially making a significant contribution to addressing global warming. To facilitate the practical application of CNFs, the Ministry of Economy, Trade and Industry and the Ministry of Agriculture, Forestry and Fisheries are collaborating on demonstration projects in various sectors, including automobiles, home appliances, and housing / building materials, to promote the evaluation / verification of CO2 reduction effects and empirical evidence of solutions to related problems.

[0003] As one example of the application of cellulose nanofibers, Patent Document 1 (Japanese Patent Application Publication No. 2022-044861) discloses a hot-melt adhesive containing cellulose nanofibers. This hot-melt adhesive comprises a resin that melts in the range of 70°C to 160°C and powdered cellulose nanofibers. Such a hot-melt adhesive allows the bonded portion to solidify instantly and exhibits excellent adhesion even at low temperatures, further ensuring a firm bond.

[0004] Furthermore, in recent years, cellulose nanofibers have begun to be used more extensively in water-based coatings, and also in cosmetics and food products with moisturizing properties. In addition, cellulose nanofibers are lighter than glass, metals, and carbon, and their nanoscale fiber length is expected to improve the strength and other properties of resins, making them an ideal raw material to be added as a reinforcing material to resin materials.

[0005] Cellulose nanofibers are typically sold as dispersions (slurries, sols, etc.) in a medium such as water, with a solid content of approximately 1-10% by weight. These dispersions are usually used directly as industrial materials or as additives in food and cosmetics for various applications. However, in compounding with hydrophobic resins, rubbers, and other materials, water removal is necessary. Currently, there are several methods for removing water from dispersions obtained by dispersing cellulose nanofibers in water, including precipitation, centrifugation, filtration, spray drying, and freeze drying.

[0006] For example, Patent Document 2 (Japanese Patent Application Publication No. 2022-028316) discloses a container for freeze-drying cellulose nanofibers. Its characteristic feature is that it comprises a main body for storing a refrigerant and a jacket covering the outer periphery of the main body and supplying a heat medium. The main body is capable of cooling the refrigerant by supplying the heat medium to the jacket. According to Patent Document 2, a container capable of freezing a cellulose nanofiber dispersion while suppressing the aggregation of cellulose nanofibers can be provided.

[0007] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2022-044861 Patent Document 2: Japanese Patent Application Publication No. 2022-028316 Summary of the Invention

[0008] The problem that the invention aims to solve One problem with existing concentration or drying methods is the long time required and low productivity. For example, drying 500 ml of a 2% aqueous dispersion of cellulose nanofibers requires several hours in a constant temperature bath at around 100°C, tens of hours in freeze-drying, and more than an hour even with good spray conditions in spray drying. Centrifugation also requires more than 30 minutes. If the drying time is long, the molecular structure inside the cellulose nanofibers will change, thus tending to reduce their strength. Therefore, the time required for concentration or drying should be as short as possible.

[0009] Furthermore, when moisture evaporates or freezes, the large specific surface area of ​​cellulose nanofibers leads to strong aggregation due to high intermolecular attraction, often resulting in a phenomenon where they cannot be dispersed in the next process. Therefore, there is a need for concentration or drying methods to obtain cellulose nanofibers with excellent redispersibility.

[0010] The present invention was made in view of the above-mentioned problems, and its object is to provide, in one embodiment, a method for obtaining a cellulose nanofiber concentrate or dried product, wherein the cellulose nanofiber concentrate or dried product can generate a redispersible liquid having the same degree of dispersibility as the cellulose nanofiber dispersion before concentration or drying.

[0011] Solution for solving the problem The inventors conducted in-depth research and discovered that the above-mentioned problems can be solved through a method different from existing technologies. Specifically, after performing a prescribed pretreatment on the cellulose nanofiber dispersion, it is processed using a two-roll mill. This allows for the concentration or drying of the cellulose nanofiber dispersion in a short time, resulting in cellulose nanofibers with excellent redispersibility, producing a redispersible solution with the same degree of dispersibility as the cellulose nanofiber dispersion before concentration or drying. This invention is based on the aforementioned insights, as illustrated below.

[0012] [1] A method for concentrating or drying a cellulose nanofiber dispersion, the method comprising: step A, performing a pretreatment by adding the following (1) and (2) to the cellulose nanofiber dispersion: (1) the following (1-1) and / or (1-2): (1-1) an alkyl ammonium salt and an amphoteric surfactant, (1-2) ethylene glycol, (2) a mixture of a higher fatty acid or a higher fatty acid amide and isopropanol; and step B, after the pretreatment, feeding the cellulose nanofiber dispersion to a two-roll mill, causing the two-roll mill to rotate, thereby concentrating or drying the cellulose nanofiber dispersion.

[0013] [2] According to the method described in [1], in step B, the surface temperature of the twin-roll mill is heated to above 170°C.

[0014] [3] According to the method described in [1] or [2], the higher fatty acid or higher fatty acid amide has 18 to 25 carbon atoms.

[0015] [4] According to any one of [1] to [3], in step B, the rotation ratio of the front roller to the rear roller of the twin-roll mill is 1 to 3 rpm.

[0016] Invention Effects According to one embodiment of the present invention, a method for obtaining a cellulose nanofiber concentrate or dried product can be provided, wherein the cellulose nanofiber concentrate or dried product can generate a redispersible liquid having the same degree of dispersibility as the cellulose nanofiber dispersion before concentration or drying. Attached Figure Description

[0017] Figure 1 This is a schematic diagram illustrating the structure and working principle of a twin-roll mill according to one embodiment of the present invention.

[0018] Figure 2 Electron microscope images of dried products obtained by the drying method of the embodiments of the present invention and the drying method of the prior art. Detailed Implementation

[0019] Next, embodiments of the present invention will be described in detail. It should be understood that the present invention is not limited to the following embodiments, and appropriate design changes and improvements can be made based on common knowledge known to those skilled in the art without departing from the spirit of the present invention.

[0020] (1. Cellulose nanofiber dispersion) Cellulose nanofibers are materials obtained by finely pulverizing cellulose, the main component of plant fibers, to nanoscale, with wood pulp (the raw material for paper) as the primary raw material. They are primarily used to reinforce resin materials and prevent resin shrinkage at low temperatures. In this invention, the raw materials for cellulose nanofibers are not particularly limited.

[0021] Cellulose nanofibers are typically derived from plant-based raw materials, and therefore, during the extraction stage, they are in a slurry dispersed in water. The solid content is typically 1-10% by weight. For example, if the purpose is to disperse cellulose nanofibers in a hydrophobic resin, it is first necessary to remove only the water from the aqueous dispersion of cellulose nanofibers dispersed in water to extract the cellulose nanofiber monomers. Alternatively, it can be assumed that the cellulose nanofiber monomers are extracted again from a redispersed solution obtained by redispersing the cellulose nanofiber monomers in water or a dispersion medium other than water. Therefore, in this invention, the dispersion medium in the cellulose nanofiber dispersion is not limited to water. However, water is preferred as the dispersion medium in the cellulose nanofiber dispersion.

[0022] It should be noted that, as mentioned above, the solid content in the cellulose nanofiber dispersion is typically 1–10% by weight, but is sometimes further diluted to less than 1% by weight. Therefore, the cellulose nanofiber dispersion can also be pre-dried before the pretreatment described later. The pre-drying method is not particularly limited, and any existing method can be used. The cellulose nanofiber dispersion can be pre-dried to, for example, a maximum solid content of 12% by weight to implement the concentration or drying method of the present invention.

[0023] When the cellulose nanofiber dispersion is dried, cellulose nanofiber powder is obtained. The average fiber length of the cellulose nanofiber powder is preferably 0.1 μm or more. This is because an enhancing effect is expected. From this viewpoint, the average fiber length of the cellulose nanofiber powder is more preferably 0.2 μm or more, even more preferably 0.3 μm or more, and even more preferably 0.5 μm or more. If the average fiber length of the cellulose nanofiber powder is 0.1 μm or more, it has a high aspect ratio relative to the average fiber diameter.

[0024] Furthermore, the average fiber length of the cellulose nanofiber powder is preferably 3.0 μm or less. This helps to prevent the cellulose nanofibers from curling into spherical shapes during processing. From this perspective, the average fiber length of the cellulose nanofiber powder is more preferably 2.5 μm or less, even more preferably 1.5 μm or less, and even more preferably 1.0 μm or less.

[0025] It should be noted that the average fiber length of cellulose nanofiber powder refers to the D50 (median particle size) obtained by measuring the fibers of cellulose nanofiber powder according to the laser diffraction / scattering method in JIS Z8825:2022.

[0026] The average fiber diameter of the cellulose nanofiber powder is preferably 0.5 nm or more. This is because an enhanced effect is expected. From this perspective, the average fiber diameter of the cellulose nanofiber powder is more preferably 0.7 nm or more, even more preferably 1 nm or more, and even more preferably 3 nm or more.

[0027] Furthermore, the average fiber diameter of the cellulose nanofiber powder is preferably 10 nm or less. This prevents the cellulose nanofibers from becoming too coarse, which would decrease the aspect ratio and impair their orientation during diffusion in the molten resin. From this perspective, the average fiber diameter of the cellulose nanofiber powder is more preferably 8 nm or less, even more preferably 7 nm or less, and even more preferably 5 nm or less.

[0028] It should be noted that the average fiber diameter of cellulose nanofiber powder refers to the average particle size of cellulose nanofiber powder obtained by measuring the fibers of cellulose nanofiber powder according to the dynamic light scattering method of JIS Z8828:2019.

[0029] (2. Pretreatment of cellulose nanofiber dispersion) In one embodiment of the present invention, the cellulose nanofiber dispersion is concentrated or dried using a two-roll mill, as described below. Prior to this two-roll milling, pretreatment is required. The purpose of pretreatment is to lower the boiling point of the dispersion medium (hereinafter, water will be used as an example) and to impart heat resistance and hydrophilicity or oleophilicity for ease of subsequent use.

[0030] While not intended to limit this invention theoretically, it can be argued that lowering the boiling point of water enhances the explosive vaporization of water vapor, making it difficult for cellulose nanofibers to aggregate. Therefore, by endowing cellulose nanofibers with hydrophilicity or lipophilicity (collectively referred to as "wetting properties"), concentrated or dried products can be produced according to their intended use, depending on the target aqueous, solvent, thermoplastic, or thermosetting resin system. That is, subsequent additives are readily adsorbed onto the cellulose nanofibers.

[0031] As a specific pretreatment step, (1-1) an alkylammonium salt and an amphoteric surfactant and / or (1-2) ethylene glycol, and (2) a mixture of a higher fatty acid or a higher fatty acid amide and isopropanol (IPA) are added to the dispersion of cellulose nanofibers (Step A). ​​This lowers the boiling point of water, the medium for the aqueous dispersion of cellulose nanofibers, and imparts heat resistance. Furthermore, it improves wettability for the subsequent addition to produce concentrated or dried CNFs, depending on the intended application.

[0032] Alkyl ammonium salts are not limited to any particular type, but examples include: distearyldimethylammonium chloride, behenyltrimethylammonium chloride solution, stearyltrimethylammonium chloride, hexadecyltrimethylammonium chloride solution, lauryltrimethylammonium chloride, benzalkonium chloride, etc. They are usually added in aqueous solution. Alkyl ammonium salts have the effect of preventing the aggregation of cellulose nanofibers as the moisture content gradually decreases.

[0033] There is no particular limitation on the amount of alkylammonium salt added, but it is preferred to add the alkylammonium salt in a manner that is 0.01 to 0.50% by weight relative to the total weight of the dispersion of cellulose nanofibers.

[0034] Amphoteric surfactants remain on the surface of cellulose nanofibers after drying, thus playing a role in good binding with surfactants in the next process. There are no limitations on amphoteric surfactants, but the following substances can be listed: amino acid glycine-type sodium cocoamphoacetate, sodium lauroamphoacetate, and disodium cocoamphodiacetate; betaine-type aminoacetic acid betaine-type cocamidopropyl betaine, lauramide-propyl betaine, myristamide-propyl betaine, palm kernel fatty acid amide-propyl betaine, lauryl betaine, cocoyl betaine, sulfobetaine-type lauryl hydroxysulfobetaine, lauramide-propyl hydroxysulfobetaine, and cocamidopropyl hydroxysulfobetaine; and amine oxide-type amine oxide-type laurylamine oxide and amide amine oxide-type lauramide-propylamine oxide, etc.

[0035] There is no particular limitation on the amount of amphoteric surfactant added, but it is preferred to add the amphoteric surfactant in a manner that is 0.10 to 0.30% by weight relative to the total weight of the dispersion of cellulose nanofibers.

[0036] In addition, ethylene glycol can be used instead of the aforementioned alkylammonium salts and amphoteric surfactants, or added to the above-mentioned alkylammonium salts and amphoteric surfactants. Ethylene glycol does not chemically bond with cellulose nanofibers, but it has high adsorption capacity for plant-based fibers and a wetting effect on cellulose nanofibers. Therefore, it is less prone to re-condensation during water vaporization. Furthermore, ethylene glycol possesses both hydrophilic and hydrophobic properties, thus playing a role in effectively binding with surfactants in the next process.

[0037] There is no particular limitation on the amount of ethylene glycol added, but it is preferred to add ethylene glycol at a rate of 0.02 to 0.30% by weight relative to the total weight of the cellulose nanofiber dispersion. It should be noted that ethylene glycol is usually added in the form of an aqueous solution, but it can also be added as a monomer.

[0038] A mixture of higher fatty acids and isopropanol (IPA) is impregnated into cellulose nanofibers, thereby enabling them to withstand the heating and drying temperatures (above 170°C) of a two-roll mill. In this specification, higher fatty acids refer to fatty acids with 18 to 25 carbon atoms, which can be saturated or unsaturated, and can be linear, branched, or in other forms. For example, in higher fatty acids, the usual chemical formula of the carboxylic acid is R-COOH (R represents a substituent of the carboxylic acid), and the chemical formula of isopropanol is C3H8O. When the mixture of the two is represented by a reaction formula, it is R-COOH + C3H8O → R-COOC3H7 + H2O. That is, the higher fatty acid reacts with isopropanol to produce a fatty acid ester (R-COOC3H7) and water (H2O).

[0039] The preferred mixture contains a higher fatty acid amide. In this specification, the higher fatty acid amide has 18 to 25 carbon atoms, and can be saturated or unsaturated, linear or branched, or in other forms. The reaction formula for a mixture of higher fatty acids or higher fatty acid amides with IPA is shown below.

[0040] R-CONH2+R'-COOH+C3H8O→R-CONH-R'-COOC3H7+H2O Here, "R" represents a substituent of a higher fatty acid, and "R'" represents a substituent of a higher fatty acid amide. It reacts with isopropanol to form an amide ester (R-CONH-R'-COOC3H7) and water (H2O). The amide ester is a compound formed by the reaction of a higher fatty acid with a higher fatty acid amide, and it retains heat resistance above 270°C. Therefore, by impregnating cellulose nanofibers with the amide ester, they can withstand even roller-heated drying temperatures above 170°C.

[0041] The aforementioned higher fatty acids or higher fatty acid amides are further melted during the subsequent processing using a two-roll mill, producing a wax-like effect. That is, the cellulose nanofibers obtained by concentration or drying are naturally detached from the roller-coated surface or the roller metal surface due to improved peelability.

[0042] The amount of higher fatty acids or higher fatty acid amides mixed with isopropanol (IPA) added is not particularly limited, but it is preferred that the higher fatty acids or higher fatty acid amides be added at 0.20 to 0.80% by weight relative to the total weight of the cellulose nanofiber dispersion, and the IPA be added at 2.5 to 15.0% by weight. It should be noted that, in order to fully carry out the reaction based on the above reaction formula, these additives are not added separately to the aqueous dispersion of cellulose nanofibers, but are pre-mixed and added as a mixture.

[0043] Preferably, after adding the above-mentioned additives to the aqueous dispersion of cellulose nanofibers, stirring is performed for mixing. Ideally, during the pretreatment stirring, the cellulose nanofibers are mixed in a spiral manner while maintaining their orientation in the liquid or sol. The stirring method and apparatus are not particularly limited, and a super mixer (manufactured by KAWATA Co., Ltd.), a Henschel mixer (manufactured by NIPPON COKE & ENGINEERING Co., Ltd.), or a high-speed mixer (manufactured by EARTHTECHNICA Co., Ltd.) are preferred.

[0044] (3. Processing using a twin-roll mill) After pretreatment, the cellulose nanofiber dispersion is fed into a two-roll mill, which rotates to remove the aqueous dispersion of cellulose nanofibers, resulting in concentrated or dried cellulose nanofibers (step B). If the boiling point of water decreases due to pretreatment, sufficient concentration or drying can be achieved even if the temperature of the two-roll mill is lowered to a certain extent after the raw material (cellulose nanofiber dispersion) is added. However, from the viewpoint of promoting water vaporization, it is preferable to heat the two-roll mill to a certain temperature or higher.

[0045] By supplying an aqueous dispersion of cellulose nanofibers to the middle section of a two-roll mill, the aqueous dispersion of cellulose nanofibers is allowed to sink into the gap between the rolls (roll gap). Figure 1 As a characteristic of a two-roll mill, before entering the roll gap, the raw material rotates in the same direction as the rolls in a stack of rolls above the roll gap. At this time, the cellulose nanofibers are in an oriented state, and when entering the roll gap, the fibers align in the same direction. Thus, shearing occurs at the roll gap while the cellulose nanofibers are in an oriented state. Due to the self-heating caused by shearing and the heating temperature of the two-roll mill, the raw material, whose boiling point has been lowered, easily vaporizes, shortening the concentration or drying time. The roll distance (gap) is not particularly limited, but in this embodiment, it is preferably 0.3 to 1.5 mm.

[0046] The greater the rotational ratio (i.e., the difference in rotational speed rpm) between the front and rear rollers of a two-roll mill, the faster the nanofibers fall into the roller gap. On the other hand, if the rotational ratio between the front and rear rollers is too large, the shear generated by the rotational ratio becomes smaller, and the water vaporization time becomes shorter. Therefore, a rotational ratio of 1 to 3 is preferred. Furthermore, without a rotational ratio, the aqueous dispersion of cellulose nanofibers cannot fall into the roller gap and tends to remain on the roller stack.

[0047] Through self-heating at the roller gap and heat transfer from the heated twin-roll mill, water molecules attract each other in their liquid state under the influence of intermolecular attraction and thermal energy, interacting with other nearby molecules. At rapidly increasing temperatures, water molecules gain sufficient energy to overcome the intermolecular attraction with the fibers, and water and alcohol instantly transform from liquid to gas. When rapidly moving molecules change from liquid to gas, a water vapor explosion occurs. Here, the alkylammonium salt and amphoteric surfactants used as pretreatment additives can withstand temperatures up to 200°C, and the higher fatty acids can withstand temperatures up to 270°C, thus tolerating the energy of the water vapor explosion. When the cellulose nanofibers, wetted by the higher fatty acids, pass through the roller gap, they do not scorch even without moisture, but instead become a thin film powder. As described above, the cellulose nanofibers enter the roller gap while maintaining their orientation, thus undergoing vaporization caused by the water vapor explosion in the same direction. The cellulose nanofibers have low random stress, allowing for concentration or drying in a state close to their original morphology (the double helix structure is almost unbroken).

[0048] Here, as a mixing machine other than a two-roll mill, pressure kneaders, Banbury mixers, and extruders can be considered. However, these devices involve random mixing, so even if water vaporizes instantly, the cellulose nanofibers easily become entangled and form clumps. Furthermore, existing technologies such as freeze drying, drying ovens, and spray drying lack methods to impart orientation to the cellulose nanofibers. Additionally, due to the time-consuming drying process, the double-helix structure can unravel, also easily leading to clumping. Therefore, processing using a two-roll mill offers advantages that cannot be obtained with existing technologies. It should be noted that a two-roll mill, as a device, only needs to be able to mix raw materials between two rolls; it can also have a third or more rolls.

[0049] Furthermore, to promote water vaporization, it is preferable to heat the surface temperature of the two-roll mill to 170°C or higher. By heating the surface temperature of the two-roll mill to 170°C or higher, the water can be rapidly boiled and vaporized, allowing the processing to be completed in a short time. Typically, a concentrated or dried product is obtained within 3 rotations of the two-roll mill (within 30 seconds), and the moisture content of the concentrated or dried product can be less than 1000 ppm by mass. From this perspective, the surface temperature of the two-roll mill is more preferably 173°C or higher, and even more preferably 175°C or higher.

[0050] On the other hand, if the surface temperature of the two-roll mill is too high, the cellulose nanofibers may deteriorate. Therefore, it is preferable to set it to below 190°C, and more preferably below 180°C. Thus, in one embodiment of the present invention, the surface temperature of the two-roll mill is 170°C to 180°C.

[0051] The above-described treatment using a two-roll mill results in a concentrate or dried product of cellulose nanofibers. When the dried product is obtained, typically the following appearance and properties are achieved.

[0052] • Shape: Plate-shaped sheet with a size of 1mm to 5mm and a thickness of less than 0.5mm.

[0053] • Color tone: Milky white with a transparent feel.

[0054] Although it has hardness, it can be easily pulverized with your fingertips. No special grinding process is required.

[0055] • No burning or partial discoloration.

[0056] • It hardly disperses into the air when packaged in metered containers.

[0057] Furthermore, fatty acids and amphoteric surfactants adhere to the surface of the dehydrated cellulose nanofibers, making them less prone to scorching. In particular, the adhesion of higher fatty acids or higher fatty acid amides results in high heat resistance. Moreover, the continuous supply of raw materials and the recovery of concentrates or dried products within the two-roll mill lead to high productivity.

[0058] Example The following examples and comparative examples of the present invention are shown together, but these examples are provided to better understand the present invention and its advantages and are not intended to limit the invention.

[0059] Experiment 1: Comparison of the performance of dried products based on different formulations Additives of the types and amounts shown in Table 1 were added to a 2% by weight cellulose nanofiber dispersion of solids, and mixed using a Henschel mixer (high-speed type) manufactured by NIPPON COKE & ENGINEERING Co., Ltd. The added cellulose nanofiber dispersion was then fed into a two-roll mill (roll diameter: 8 inches; surface treatment: bright electroplating; roll gap: 0.5 mm) manufactured by Yasuda Seiki Co., Ltd. The feed amount was 300 g. The mill was heated to a surface temperature of 170°C. The mill was then operated with the front roll rotating at 7 rpm and the rear roll at 6 rpm. A sample of the dried product was collected after the front roll had rotated 3 times. The drying time was less than 30 seconds. It should be noted that the details of each compound shown in Table 1 are as follows.

[0060] BYK-9076: A wetting and dispersing agent manufactured by BYK Corporation. A 50-60% by weight solution of an alkyl ammonium salt of a high molecular weight copolymer.

[0061] BYK-185: A wetting and dispersing agent manufactured by BYK Corporation. A solution of 60-70% by mass of an alkyl ammonium salt.

[0062] SOFTAZOLINE (registered trademark) LPB-R: Amide betaine type amphoteric surfactant manufactured by Kawaken Fine Chemicals Co., Ltd. Lauric acid amide propyl betaine solution.

[0063] NEUTRON: Oleic acid amide manufactured by Nippon Seika Co., Ltd. Chemical formula C 17 H 33 CONH2.

[0064] (Performance Evaluation) The moisture content of the obtained dried product was determined according to the Karl Fischer titration method. Specifically, a Karl Fischer moisture meter (model: MKH-710) manufactured by Kyoto Electronics Co., Ltd. was used to titrate the dried product sample with Karl Fischer reagent, which mainly consists of sulfur dioxide and alkali. The moisture content of the dried product sample was determined based on the volume of reagent consumed (volumetric titration method). The results are shown in Table 1.

[0065] In addition, after the front roller rotates 3 times, the dried product is visually confirmed to be easily peeled off from the rollers of the two-roll mill. The results are shown in Table 1.

[0066] In addition, for each test case, dried samples were collected, and polypropylene resin and the dried samples were added at a weight ratio of 100:1 to a hot-mixing two-roll mill (two rolls) heated to 195°C. The mixture was kneaded for 5 minutes with a 1mm gap to form a resin film with a thickness of 25μm. The resin film was cut to A4 size, and the presence of pinholes was visually confirmed. The results are shown in Table 1.

[0067] As shown in Table 1, in Test Examples 1-3 without the addition of higher fatty acid amides and Test Examples 1-4 without the addition of IPA, water vaporization was insufficient, resulting in a large amount of residual moisture. Furthermore, the dried product did not peel off from the rollers, some parts adhered, and scorch marks were also observed. Moreover, when any of the compounds required for pretreatment were lacking, it was confirmed that the resin after film formation had a high number of pinholes and poor redispersibility.

[0068] Experiment 2: Comparison of moisture content of dried products based on different roller temperatures Additives of the same type and amount as those in Example 1-1 above were added to a 2% by weight cellulose nanofiber dispersion of solids, and mixed using a Henschel mixer (high-speed type) manufactured by NIPPON COKE & ENGINEERING. The added cellulose nanofiber dispersion was then fed into a two-roll mill (roll diameter: 8 inches; surface treatment: bright electroplating; roll gap: 0.5 mm) manufactured by Yasuda Seiki Co., Ltd. The feed amount was 300 g. The roller surface temperature was heated as shown in Table 2. The two-roll mill was then operated at a front roller speed of 7 rpm and a rear roller speed of 6 rpm. A sample of the dried product was collected after the front roller had rotated 3 times. The moisture content of the collected samples was evaluated according to the method described above. The results are shown in Table 2. As shown in Table 2, if the surface temperature of the roller is above 170°C, the moisture content of the dried product is reduced to an undetectable level. Furthermore, when the same type and amount of additives as those in Test Example 2-1 were added to a 2% by weight cellulose nanofiber dispersion of solids, almost identical results as in Table 2 were obtained.

[0069] Experiment 3: Comparison of properties of dried products based on different drying methods Additives of the same type and amount as those in Experiment 1-1 were added to a cellulose nanofiber dispersion with a solid content of 2% by weight. The mixture was stirred using a Henschel mixer (high-speed type) manufactured by NIPPON COKE & ENGINEERING. 5 L of the added cellulose nanofiber dispersion was placed in a vacuum freeze-drying apparatus SF-5 manufactured by Sansho Industry Co., Ltd., and continuously freeze-dried at -37°C for 8 hours. 3 g of the dried sample was collected and observed using a field emission scanning electron microscope S-4800 manufactured by Hitachi High-Tech Co., Ltd., obtaining an 8000x magnification image. Figure 2 B). Referring to the photographs, fragments of the condensate and CNF can be identified.

[0070] On the other hand, electron microscope images were obtained for the dried product of Example 1-1 under the same conditions ( Figure 2 A) It can be confirmed that the cellulose nanofibers and dispersant of the dried product in Example 1-1 are well dispersed. That is, the dried product obtained by processing with a two-roll mill can produce a redispersible liquid with the same degree of dispersibility as the CNF dispersion before drying.

Claims

1. A method for concentrating or drying a cellulose nanofiber dispersion, the method comprising: Step A involves a pretreatment process in which the following 1 and 2 are added to the cellulose nanofiber dispersion: 1: The following 1-1 and / or 1-2: 1-1: Alkyl ammonium salts and amphoteric surfactants, 1-2: Ethylene glycol, 2: A mixture of higher fatty acids or higher fatty acid amides with isopropanol; and Step B involves feeding the cellulose nanofiber dispersion to a two-roll mill after the pretreatment, causing the two-roll mill to rotate, thereby concentrating or drying the cellulose nanofiber dispersion.

2. The method according to claim 1, wherein, In step B, the surface temperature of the twin-roll mill is heated to above 170°C.

3. The method according to claim 1 or 2, wherein, The higher fatty acid or higher fatty acid amide has 18 to 25 carbon atoms.

4. The method according to claim 1 or 2, wherein, In step B, the rotation ratio of the front roller to the rear roller of the twin-roll mill is 1 to 3 rpm.

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