Method for concentrating or drying cellulose nanofiber dispersion

Abstract

Provided is a method for obtaining a concentrated or dried cellulose nanofiber product with which it is possible to obtain a redispersion having a dispersibility comparable to that of a cellulose nanofiber dispersion before concentration or drying. This method for concentrating or drying a cellulose nanofiber dispersion comprises: a step A for performing a pretreatment on a cellulose nanofiber dispersion by adding the following (1) and (2), wherein (1) is (1-1) and / or (1-2), (1-1) being an alkylammonium salt and an amphoteric surfactant, and (1-2) being an ethylene glycol, and (2) is a mixture of a higher fatty acid or higher fatty acid amide, and isopropyl alcohol; and a step B for, after the pretreatment, concentrating or drying the cellulose nanofiber dispersion by supplying the cellulose nanofiber dispersion to a two-roll mill and rotating the two-roll mill.

Description

Method for concentrating or drying cellulose nanofiber dispersion

[0001] The present invention relates to a method for concentrating or drying a cellulose nanofiber dispersion, and in particular to a method for obtaining a concentrated or dried cellulose nanofiber product, which is capable of concentrating or drying a cellulose nanofiber dispersion while suppressing aggregation of the cellulose nanofibers, and producing a re-dispersion having dispersibility comparable to that of the cellulose nanofiber dispersion before concentration or drying.

[0002] Cellulose nanofiber (also known as "Cellulose Nano Fiber"; hereafter, simply referred to as "CNF") is a next-generation plant-derived material that is said to be five times stronger than steel at one-fifth the weight. Utilizing cellulose nanofiber in automobiles, home appliances, etc. is expected to reduce weight, improve energy efficiency, and make a significant contribution to combating global warming. Toward the social implementation of CNF, the Ministry of Economy, Trade and Industry and the Ministry of Agriculture, Forestry and Fisheries are collaborating to implement model projects in various fields, including automobiles, home appliances, and housing / building materials, to evaluate and verify the CO2 reduction effects and to demonstrate solutions to related issues.

[0003] As one application example of cellulose nanofibers, Patent Document 1 (JP 2022-044861 A) discloses a hot melt adhesive containing cellulose nanofiber fibers, which comprises a resin that melts in the range of 70°C to 160°C and a powder of cellulose nanofiber fibers. It is disclosed that such a hot melt adhesive can instantly solidify the bonded area, has excellent adhesive properties even at low temperatures, and can firmly maintain the bonded surface.

[0004] Furthermore, in recent years, cellulose nanofibers have begun to be used widely in water-based paints, and are also beginning to be used in cosmetics and foods that incorporate their moisturizing properties.In addition, cellulose nanofibers have a lower specific gravity than glass, metal, and carbon, and their nano-sized fiber length is expected to improve functionality such as increasing the strength of resins, making them an ideal material to add to resin materials as a reinforcing material.

[0005] Cellulose nanofibers are generally sold in the form of a dispersion (slurry, sol, etc.) in which they are dispersed in a dispersing medium such as water at a solids content of approximately 1 to 10% by weight, and the cellulose nanofiber dispersion at a predetermined concentration after production is usually used for various purposes as an industrial material or an additive material for foods and cosmetics. However, compounding with hydrophobic materials such as resins and rubbers requires the removal of water. Currently, there are several methods for removing water from a dispersion in which cellulose nanofibers are dispersed in water, including precipitation, centrifugation, filtration, spray drying, and freeze drying.

[0006] For example, Patent Document 2 (JP 2022-028316 A) discloses a container for freeze-drying cellulose nanofibers, which includes a main body that stores a brine solution inside and a jacket that covers the outer periphery of the main body and is supplied with a heat medium, and the main body is capable of cooling the brine solution by supplying the heat medium to the jacket. According to Patent Document 2, it is possible to provide a container that can freeze a cellulose nanofiber dispersion while suppressing aggregation of the cellulose nanofibers.

[0007] JP 2022-044861 A JP 2022-028316 A

[0008] One of the problems with existing concentration or drying methods is the long time required and low productivity. For example, when drying 500 ml of a 2% aqueous dispersion of cellulose nanofibers, it takes several hours in a thermostatic chamber at around 100°C, several tens of hours with freeze-drying, and at least an hour with spray-drying even if the spray condition is good. Even with centrifugation, it takes at least 30 minutes. If the drying time is long, the molecular structure inside the cellulose nanofiber also changes, which tends to reduce strength. Therefore, it is best to keep the concentration or drying time as short as possible.

[0009] Furthermore, when the water evaporates or freezes, the large specific surface area of ​​cellulose nanofibers causes strong intermolecular attraction, resulting in strong aggregation, which often makes it impossible to disperse in the next step. Therefore, a concentration or drying method is needed to obtain cellulose nanofibers with excellent redispersibility.

[0010] The present invention was completed in consideration of the above-mentioned problems, and in one embodiment, aims to provide a method for obtaining a concentrated or dried cellulose nanofiber product that is capable of producing a re-dispersion liquid having dispersibility equivalent to that of the cellulose nanofiber dispersion before concentration or drying.

[0011] After extensive research, the present inventors have discovered that the above-mentioned problems can be solved by a method different from that of conventional techniques. Specifically, by subjecting a cellulose nanofiber dispersion to a specified pretreatment and then processing it in a two-roll mill, the cellulose nanofiber dispersion can be concentrated or dried in a short period of time, and the resulting cellulose nanofibers have excellent re-dispersibility, making it possible to produce a re-dispersion having dispersibility equivalent to that of the cellulose nanofiber dispersion before concentration or drying. The present invention was completed based on this finding, and is exemplified below.

[0012] [1] A method for concentrating or drying a cellulose nanofiber dispersion, comprising: Step A, which comprises a pretreatment step of adding the following to the cellulose nanofiber dispersion: (1) (1-1) and / or (1-2) below: (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 Step B, which comprises, after the pretreatment step, supplying the cellulose nanofiber dispersion to a two-roll mill and rotating the two-roll mill to concentrate or dry the cellulose nanofiber dispersion. [2] The method according to [1], which comprises, in Step B, heating the surface temperature of the two-roll mill to 170°C or higher. [3] The method according to [1] or [2], wherein the higher fatty acid or higher fatty acid amide has a carbon number of 18 to 25. [4] The method according to any one of [1] to [3], wherein, in Step B, the rotation ratio between the front roll and the rear roll of the two-roll mill is 1 to 3 rpm.

[0013] According to one embodiment of the present invention, it is possible to provide a method for obtaining a cellulose nanofiber concentrated or dried product that can produce a redispersion liquid having dispersibility equivalent to that of the cellulose nanofiber dispersion before concentration or drying.

[0014] 1 is a schematic diagram illustrating the structure and operation principle of a two-roll mill according to an embodiment of the present invention, and FIG. 2 is an electron microscope photograph of dried products obtained by a drying method according to an embodiment of the present invention and a drying method according to a conventional technology.

[0015] 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 that appropriate design changes and improvements may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0016] (1. Cellulose nanofiber dispersion) Cellulose nanofibers are made by finely grinding cellulose, the main component of plant fibers, to nano-size particles, and their main raw material is wood pulp (the raw material for paper). They are used primarily for the purposes of reinforcing resin materials and preventing resin shrinkage at low temperatures. In the present invention, there are no particular limitations on the raw materials for the cellulose nanofibers.

[0017] Because cellulose nanofibers are typically plant-derived materials, they are in the form of a slurry dispersed in water when extracted from the plant. The solids content is typically 1 to 10% by weight. For example, when the purpose is to use cellulose nanofibers dispersed in a hydrophobic resin, it is first necessary to remove only the water from the cellulose nanofiber aqueous dispersion dispersed in water to extract the cellulose nanofibers as individual particles. It is also conceivable that the cellulose nanofibers as individual particles may be redispersed in water or a dispersant other than water, and then extracted again from the redispersion. Therefore, in the present invention, the dispersant in the cellulose nanofiber dispersion is not limited to water. However, the dispersant in the cellulose nanofiber dispersion is preferably water.

[0018] As mentioned above, the solids content of a cellulose nanofiber dispersion is usually 1 to 10% by weight, but it may also be further diluted to less than 1% by weight. Therefore, the cellulose nanofiber dispersion can be pre-dried before the pretreatment described below. There are no particular restrictions on the pre-drying method, and any conventional method can be used. The cellulose nanofiber dispersion can be pre-dried, for example, to a maximum solids content of 12% by weight, and then the concentration or drying method of the present invention can be carried out.

[0019] When the cellulose nanofiber dispersion is dried, a cellulose nanofiber powder is obtained. The average fiber length of the cellulose nanofiber powder is preferably 0.1 μm or more. This is expected to provide a reinforcing effect. From this perspective, 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. When 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.

[0020] Furthermore, it is preferable that the average fiber length of the cellulose nanofiber powder is 3.0 μm or less. This prevents the cellulose nanofibers from becoming spherical during processing. From this perspective, it is more preferable that the average fiber length of the cellulose nanofiber powder is 2.5 μm or less, even more preferably 1.5 μm or less, and even more preferably 1.0 μm or less.

[0021] The average fiber length of the cellulose nanofiber powder refers to the D50 (median diameter) of the fibers of the cellulose nanofiber powder measured according to the laser diffraction / scattering method of JIS Z8825:2022.

[0022] The average fiber diameter of the cellulose nanofiber powder is preferably 0.5 nm or more. This is expected to provide a reinforcing effect. 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.

[0023] Furthermore, it is preferable that the average fiber diameter of the cellulose nanofiber powder is 10 nm or less. This prevents the cellulose nanofibers from becoming too thick, reducing the aspect ratio and impairing their orientation when diffusing in the molten resin. From this perspective, it is more preferable that the average fiber diameter of the cellulose nanofiber powder is 8 nm or less, even more preferably 7 nm or less, and even more preferably 5 nm or less.

[0024] The average fiber diameter of the cellulose nanofiber powder refers to the average particle diameter of the fibers of the cellulose nanofiber powder measured according to the dynamic light scattering method of JIS Z8828:2019.

[0025] (2. Pretreatment of Cellulose Nanofiber Dispersion) In one embodiment of the present invention, in order to concentrate or dry the cellulose nanofiber dispersion, treatment is carried out using a two-roll mill, as described below. Here, pretreatment must be carried out before treatment using the two-roll mill. The purpose of the pretreatment is to lower the boiling point of the dispersion medium (hereinafter, water will be used as an example), to impart heat resistance and hydrophilicity or lipophilicity, and to facilitate subsequent use.

[0026] While not intending to restrict the present invention to theory, it is believed that lowering the boiling point of water enhances the explosive vaporization of water vapor, making it less likely for cellulose nanofibers to aggregate. Furthermore, by imparting hydrophilicity or lipophilicity (collectively referred to as "wettability") to cellulose nanofibers, it is possible to create concentrated or dried products for specific applications, tailored to the target aqueous, solvent-based, thermoplastic resin, or thermosetting resin system, after concentration or drying. In other words, additives added later are more likely to be adsorbed onto the cellulose nanofibers.

[0027] Specifically, the pretreatment involves adding (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 isopropyl alcohol (IPA) to a cellulose nanofiber dispersion (Step A). ​​This lowers the boiling point of water, which is the medium for the cellulose nanofiber aqueous dispersion, and also imparts heat resistance and improves wettability for later addition to produce concentrated or dried CNF for specific applications.

[0028] Examples of alkylammonium salts include, but are not limited to, distearyldimethylammonium chloride, behenyltrimethylammonium chloride solution, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride water, lauryltrimethylammonium chloride, and benzalkonium chloride. These are usually added in the form of an aqueous solution. Alkylammonium salts have the effect of preventing the aggregation of cellulose nanofibers as the water content decreases.

[0029] The amount of alkylammonium salt added is not particularly limited, but it is preferable to add the alkylammonium salt so that it accounts for 0.01 to 0.50% by weight of the total weight of the cellulose nanofiber dispersion.

[0030] The amphoteric surfactant remains on the surface of the cellulose nanofibers after drying, thereby serving to bond well with the surfactant in the next step. Examples of amphoteric surfactants include, but are not limited to, the following: amino acid glycine-type sodium cocoamphoacetate, sodium lauroamphoacetate, and disodium cocoamphodiacetate; betaine-type surfactants include aminoacetic acid betaine-type cocamidopropyl betaine, lauramidopropyl betaine, myristamidopropyl betaine, palm kernel fatty acid amidopropyl betaine, lauryl betaine, cocobetaine, sulfobetaine-type lauryl hydroxysultaine, lauramidopropyl hydroxysultaine, and cocamidopropyl hydroxysultaine; and amine oxide-type surfactants include amine oxide-type lauramine oxide and amidoamine oxide-type lauramidopropylamine oxide.

[0031] The amount of amphoteric surfactant added is not particularly limited, but it is preferable to add the amphoteric surfactant so that it accounts for 0.10 to 0.30% by weight of the total weight of the cellulose nanofiber dispersion.

[0032] Furthermore, ethylene glycol can be added instead of or in addition to the alkylammonium salt and amphoteric surfactant. Although ethylene glycol does not chemically bond with cellulose nanofibers, it has a high adsorption property to plant fibers and has a wetting effect on cellulose nanofibers. This reduces the likelihood of re-aggregation upon evaporation of water. Furthermore, because ethylene glycol has both hydrophilic and hydrophobic properties, it also plays a role in bonding well with surfactants in the next process.

[0033] The amount of ethylene glycol added is not particularly limited, but it is preferable to add ethylene glycol so that it accounts for 0.02 to 0.30 wt % of the total weight of the cellulose nanofiber dispersion. Ethylene glycol is usually added in the form of an aqueous solution, but it may also be added as a single substance.

[0034] A mixture of higher fatty acids and isopropyl alcohol (IPA) impregnates cellulose nanofiber fibers, enabling them to withstand the heat drying temperatures (170°C or higher) of a two-roll mill. In this specification, "higher fatty acids" refers to fatty acids having 18 to 25 carbon atoms, which may be saturated or unsaturated, linear, branched, or otherwise. For example, among higher fatty acids, the general chemical formula for carboxylic acid is R-COOH (R represents a carboxylic acid substituent), and the chemical formula for isopropyl alcohol is CHO. The reaction formula for a mixture of the two is R-COOH + CHO → R-COOCHO + HO. In other words, higher fatty acids react with isopropyl alcohol to produce fatty acid esters (R-COOCHO) and water (HO).

[0035] Preferably, the mixture contains a higher fatty acid amide. In this specification, the higher fatty acid amide has 18 to 25 carbon atoms and may be saturated or unsaturated, linear, branched, or otherwise. The reaction formula for a mixture of a higher fatty acid or a higher fatty acid amide with IPA is expressed as follows: R-CONH2 + R'-COOH + C3H8O → R-CONH-R'-COOC3H7 + HO, where "R" represents a substituent of the higher fatty acid and "R'" represents a substituent of the higher fatty acid amide. It reacts with isopropyl alcohol to produce an amide ester (R-CONH-R'-COOC3H7) and water (HO). The amide ester is a compound formed by the reaction of a higher fatty acid and a higher fatty acid amide, and possesses heat resistance of 270°C or higher. Thus, impregnating cellulose nanofibers with the amide ester allows them to withstand temperatures of 170°C or higher when heated and dried using a roll.

[0036] The higher fatty acid or higher fatty acid amide also melts during the treatment with a two-roll mill described below, creating a wax-like effect, meaning that the concentrated or dried cellulose nanofibers have improved releasability from the plated or metal roll surface and peel off naturally.

[0037] Although there are no particular limitations on the amount of the mixture of higher fatty acid or higher fatty acid amide and isopropyl alcohol (IPA) added, it is preferable that the higher fatty acid or higher fatty acid amide be added at 0.20 to 0.80 wt % and the IPA be added at 2.5 to 15.0 wt % relative to the total weight of the cellulose nanofiber dispersion. Note that, to ensure that the reaction according to the above-mentioned reaction formula is carried out sufficiently, these additives are not added separately to the aqueous cellulose nanofiber dispersion, but are mixed in advance and added in the form of a mixture.

[0038] After adding each of the above additives to the aqueous dispersion of cellulose nanofibers, it is preferable to stir the mixture to mix them. During the stirring for pretreatment, it is desirable to knead the cellulose nanofibers in a spiral while maintaining their orientation in the liquid or sol. The stirring method and device are not particularly limited, but suitable devices include a Super Mixer (manufactured by Kawata Corporation), a Henschel Mixer (manufactured by Nippon Coke and Engineering Co., Ltd.), and a High-Speed ​​Mixer (manufactured by Earth Technica Corporation).

[0039] (3. Treatment using a two-roll mill) After pretreatment, the cellulose nanofiber dispersion is supplied to a two-roll mill, and the two-roll mill is rotated to remove the aqueous dispersion of cellulose nanofibers and obtain concentrated or dried cellulose nanofibers (Step B). If the boiling point of water is lowered by pretreatment, a sufficient concentration or drying effect can be obtained after the raw material (cellulose nanofiber dispersion) is added, even if the temperature of the two-roll mill is lowered to a certain extent. However, from the perspective of promoting water evaporation, it is preferable to heat the two-roll mill to a temperature above a certain level.

[0040] By supplying an aqueous dispersion of cellulose nanofibers to the middle of a two-roll mill, the aqueous dispersion of cellulose nanofibers is absorbed into the roll gap (nip) (Figure 1). A characteristic of the two-roll mill is that before entering the nip, the raw material rotates in the same direction as the rolls in the roll bank above the roll gap. At this time, the cellulose nanofibers maintain their orientation, and when they enter the nip, the fibers are aligned in the same direction. This causes shear shear in the nip while maintaining the orientation of the cellulose nanofibers. The raw material, whose boiling point is lowered by self-heating due to shear shear and the heating temperature of the two-roll mill, easily vaporizes, shortening the time for concentration or drying. The distance between the rolls (clearance) is not particularly limited, but in this embodiment, it is preferably 0.3 to 1.5 mm.

[0041] The greater the rotation ratio (i.e., the difference in rotation speed rpm) between the front roll and the rear roll in a two-roll mill, the faster the speed at which the material is cut into the nip. On the other hand, if the rotation ratio between the front roll and the rear roll is too high, the shear generated by the rotation ratio will be small and the time it takes for water to evaporate will be short, so it is preferable to set the rotation ratio to 1 to 3. Furthermore, if there is no rotation ratio, the aqueous dispersion of cellulose nanofibers will not be able to cut into the nip and will tend to stagnate on the roll bank.

[0042] Self-heating in the nip and heat transfer from the heated two-roll mill create intermolecular forces and thermal energy that attract water molecules in liquid form, causing them to interact with nearby molecules. Under the sudden high temperature, the water molecules gain enough energy to overcome the intermolecular forces with the fibers, causing the water and alcohol to instantly transition from liquid to gas. When rapidly moving molecules change from liquid to gas, a steam explosion occurs. The pretreatment additives, alkylammonium salts and amphoteric surfactants, can withstand temperatures up to 200°C, and higher fatty acids can withstand temperatures up to 270°C, so they can withstand the energy of a steam explosion. When cellulose nanofibers are wetted with higher fatty acids, they transform into a thin powder film without burning even in the absence of moisture as they pass through the nip. As mentioned above, cellulose nanofibers enter the nip while maintaining their orientation, resulting in vaporization by steam explosion in the same direction. This reduces random stress on the cellulose nanofibers, allowing them to be concentrated or dried in a state close to their original form (with the double helix structure still intact).

[0043] Here, kneading machines other than the two-roll mill include pressure kneaders, Banbury kneaders, and extruders. However, because these knead randomly, even if the water evaporates instantly, the cellulose nanofibers tend to entangle with each other and form lumps. Furthermore, conventional techniques such as freeze drying, drying ovens, and spray drying do not provide a means for imparting orientation to the cellulose nanofibers, and because drying takes time, the double helix structure tends to loosen and lumps tend to form. Therefore, processing using a two-roll mill offers advantages not available with conventional techniques. The two-roll mill may be any device capable of kneading the raw materials between two rolls, and may also have a third or subsequent roll.

[0044] In order to promote water evaporation, the surface temperature of the two-roll mill is preferably heated to 170°C or higher. By heating the surface temperature of the two-roll mill to 170°C or higher, water can be rapidly boiled and evaporated, and the treatment can be completed in a short time. Typically, a concentrated or dried product can be obtained within three revolutions of the two-roll mill (within 30 seconds), and the moisture content of the concentrated or dried product can be 1000 ppm by mass or less. 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.

[0045] On the other hand, if the surface temperature of the two-roll mill is too high, there is a risk that the cellulose nanofibers may be altered, so it is preferably set to 190° C. or less, and more preferably 180° C. or less. Therefore, in one embodiment of the present invention, the surface temperature of the two-roll mill is 170° C. to 180° C.

[0046] As a result of processing using the two-roll mill, a concentrated or dried product of cellulose nanofiber is obtained. When a dried product is obtained, the following appearance and properties are typically obtained. - Shape: Plate-like flakes with a size of 1 mm to 5 mm and a thickness of 0.5 mm or less. - Color: Milky white and transparent. - Although hard, it can be easily broken into a fine powder with your fingertips. No special crushing process is required. - There is no burning or partial discoloration. - There is almost no scattering into the air even when weighed and packaged.

[0047] Furthermore, because fatty acids and amphoteric surfactants adhere to the surface of the dehydrated cellulose nanofibers, they are less likely to burn. The adhesion of higher fatty acids or higher fatty acid amides in particular provides high heat resistance. Furthermore, the two-roll mill offers the advantage of high productivity, as it allows for continuous supply of raw materials and recovery of concentrated or dried products.

[0048] Examples of the present invention will be described below together with comparative examples. These examples are provided for a better understanding of the present invention and its advantages, and are not intended to limit the invention.

[0049] Test 1: Comparison of dried product performance due to different formulations Additives of the types and amounts shown in Table 1 were added to a cellulose nanofiber dispersion with a solids content of 2% by weight, and the mixture was stirred and mixed using a Henschel mixer (high-speed type) manufactured by Nippon Coke & Engineering Co., Ltd. The cellulose nanofiber dispersion after addition was placed in a two-roll mill manufactured by Yasuda Seiki Seisakusho Co., Ltd. (roll diameter: 8 inches; surface treatment: gloss plating; nip clearance: 0.5 mm). The amount added was 300 g. The rolls were heated to a surface temperature of 170°C. Next, the two-roll mill was operated with a front roll rotation speed of 7 rpm and a rear roll rotation speed of 6 rpm, and a sample of the dried product was collected after three rotations of the front roll. Drying took less than 30 seconds.

[0050]

[0051] The details of each compound shown in Table 1 are as follows: BYK-9076: Wetting and dispersing agent manufactured by BYK Corporation. A 50-60% by mass solution of alkylammonium salt of a high molecular weight copolymer. BYK-185: Wetting and dispersing agent manufactured by BYK Corporation. A 60-70% by mass solution of alkylammonium salt. Softazoline (registered trademark) LPB-R: An amidobetaine-type amphoteric surfactant manufactured by Kawaken Fine Chemical Co., Ltd. Lauric acid amidopropyl betaine solution. Neutron: Oleic acid amide manufactured by Nippon Fine Chemical Co., Ltd. Chemical formula C 17 H 33 CONH2.

[0052] (Performance Evaluation) The moisture content of the obtained dried product was measured according to the Karl Fischer measurement method. Specifically, using a Karl Fischer moisture meter (model number: MKH-710) manufactured by Kyoto Electronics Manufacturing Co., Ltd., the dried product sample was titrated with a Karl Fischer reagent containing sulfur dioxide and a base as main components, and the moisture content of the dried product sample was determined from the volume of the reagent consumed (volumetric titration method). The results are shown in Table 1.

[0053] After three revolutions of the front roll, it was visually confirmed whether the dried product was easily peeled off from the rolls of the two-roll mill. The results are shown in Table 1.

[0054] In addition, for each test example, a dried product was collected and placed in a thermal mixing two-roll mill (two rolls) heated to 195°C so that the weight ratio of polypropylene resin to dried product was 100:1. The mixture was kneaded for 5 minutes with a clearance of 1 mm to form a resin film with a thickness of 25 μm. This resin film was cut to A4 size and visually inspected for the presence or absence of pinholes. The results are shown in Table 1.

[0055] As can be seen from Table 1, in Test Example 1-3, in which no higher fatty acid amide was added, and Test Example 1-4, in which no IPA was added, water evaporation was insufficient, and a large amount of water remained. Furthermore, the dried product did not peel off from the roll, and some adhesion and scorching were observed. Furthermore, when any of the compounds necessary for pretreatment was missing, the number of pinholes in the resin film was large, and it was confirmed that redispersibility was poor.

[0056] Test 2: Comparison of Moisture Content of Dried Products at Different Roll Temperatures The same types and amounts of additives as in Test Example 1-1 above were added to a cellulose nanofiber dispersion with a solids content of 2% by weight, and the mixture was stirred and mixed using a Henschel mixer (high-speed type) manufactured by Nippon Coke. The cellulose nanofiber dispersion after addition was then loaded into a two-roll mill manufactured by Yasuda Seiki Seisakusho Co., Ltd. (roll diameter: 8 inches; surface treatment: gloss plating; nip clearance: 0.5 mm). The loading amount was 300 g. The roll surface temperatures were heated as shown in Table 2. Next, the two-roll mill was operated so that the rotation speed of the front roll was 7 rpm and the rotation speed of the rear roll was 6 rpm, and a sample of the dried product was collected after three rotations of the front roll. The moisture content of the collected sample was evaluated according to the method described above. The results are shown in Table 2.

[0057]

[0058] As can be seen from Table 2, when the surface temperature of the roll was 170° C. or higher, the moisture content of the resulting dried product was reduced to an undetectable level. Furthermore, when a test was conducted by adding the same types and amounts of additives as in Test Example 2-1 above to a cellulose nanofiber dispersion with a solids content of 2% by weight, almost the same results as in Table 2 above were obtained.

[0059] Test 3: Comparison of properties of dried products using different drying methods. The same types and amounts of additives as in Test Example 1-1 were added to a cellulose nanofiber dispersion with a solids content of 2% by weight, and the mixture was stirred and mixed using a Henschel mixer (high-speed type) manufactured by Nippon Coke. Five liters of the cellulose nanofiber dispersion after addition was taken and placed in a vacuum freeze-drying device SF-5 manufactured by Sansho Industry Co., Ltd., and operated continuously at -37°C for 8 hours. Three grams of the dried product was sampled and observed using a field emission scanning electron microscope S-4800 manufactured by Hitachi High-Tech Corporation, and a photograph was taken at 8,000x magnification (Figure 2B). Aggregates and CNF fragments were visible in the photograph.

[0060] On the other hand, an electron microscope photograph was taken under the same conditions for the dried product of Test Example 1-1 (Figure 2A). It can be seen that both the cellulose nanofibers and the dispersant are well dispersed in the dried product of Test Example 1-1. In other words, the dried product obtained by treatment with a two-roll mill can produce a re-dispersion liquid with dispersibility equivalent to that of the CNF dispersion before drying.

Claims

1. A method for concentrating or drying a cellulose nanofiber dispersion, comprising: Step A performing a pretreatment of adding the following to the cellulose nanofiber dispersion: (1) (1-1) and / or (1-2) below: (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 isopropyl alcohol; and Step B, after the pretreatment, supplying the cellulose nanofiber dispersion to a two-roll mill and rotating the two-roll mill to concentrate or dry the cellulose nanofiber dispersion.

2. The method according to claim 1, wherein step B includes heating the surface temperature of the two-roll mill to 170°C or higher.

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 roll and the rear roll of the two-roll mill is 1 to 3 rpm.

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