Method of concentrating or drying cellulose nanofiber dispersion

Abstract

To provide a method of obtaining a concentrated product or a dried product of a cellulose nanofiber, from which a redispersion liquid having dispersibility equivalent to a cellulose nanofiber dispersion prior to concentration or drying, can be formed.SOLUTION: The method of concentrating or drying a cellulose nanofiber dispersion, includes: a step A of carrying out a pre-treatment of adding the following to the cellulose nanofiber dispersion: (1) the following (1-1) and / or (1-2): (1-1) an alkyl ammonium salt and an ampholytic surfactant, (1-2) ethylene glycol, (2) a mixture of a higher fatty acid or a higher fatty acid amide and isopropyl alcohol; and a step B of supplying a two-roll mill with the cellulose nanofiber dispersion after the pre-treatment and rotating the two-roll mill to concentrate or dry the cellulose nanofiber dispersion.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for concentrating or drying a cellulose nanofiber dispersion. In particular, the present invention can concentrate or dry a cellulose nanofiber dispersion while suppressing aggregation of the cellulose nanofibers, and generate a redispersion liquid having a dispersibility equivalent to that of the cellulose nanofiber dispersion before concentration or drying. The present invention relates to a method for obtaining a concentrated or dried product of cellulose nanofibers.

Background Art

[0002] Cellulose nanofibers (also known as "Cellulose Nano Fiber", hereinafter sometimes simply referred to as "CNF") are next-generation materials derived from plants, and it is said that they can obtain five times the strength with one-fifth the weight of steel. By applying cellulose nanofibers to automobiles, home appliances, etc., the effect of weight reduction can be obtained, energy efficiency can be improved, and a great contribution to countermeasures against global warming can be expected. For the social implementation of CNF, the Ministry of Economy, Trade and Industry and the Ministry of Agriculture, Forestry and Fisheries are carrying out model projects in various fields such as automobiles, home appliances, housing and building materials as a collaborative project, and promoting the evaluation and verification of the COreduction effect and the demonstration of solutions to related problems.

[0003]

[0004] ​Furthermore, in recent years, cellulose nanofibers have begun to be widely used in aqueous paints and the like, and have also started to be used in cosmetics and foods incorporating properties such as moisture retention. In addition, since cellulose nanofibers have a lower specific gravity than glass, metal, and carbon, and their nano-sized fiber length can be expected to improve functions such as enhancing the strength of resins, they can be an ideal material to be added to resin materials as a reinforcing material.

[0005] Cellulose nanofibers are generally sold and provided in the form of a dispersion (such as a slurry or sol) dispersed in a dispersion medium such as water at a solid content of about 1 to 10% by weight, and are usually used in various applications as an industrial material or an additive material for foods and cosmetics as the cellulose nanofiber dispersion at a predetermined concentration as it is. On the other hand, for compounding with materials such as hydrophobic resins and rubbers, water removal is required. Currently, there are several methods for removing water from a dispersion in which cellulose nanofibers are dispersed in water, and typically, precipitation methods, centrifugation methods, filtration methods, spray drying methods, freeze drying methods, and the like can be mentioned.

[0006] For example, Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2022-028316) discloses a container for freeze-drying cellulose nanofibers, which includes a main body portion for storing a brine solution therein and a jacket covering the outer periphery of the main body portion to which a heat medium is supplied, and the main body portion is characterized in that the brine solution can be cooled by supplying the heat medium to the jacket. According to Patent Document 2, it is possible to provide a container capable of freezing a cellulose nanofiber dispersion while suppressing aggregation of cellulose nanofibers.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] One of the problems of existing concentration or drying methods is that the required time is long and the productivity is low. For example, when drying 500 ml of a 2% aqueous dispersion of cellulose nanofibers, several hours are required in a constant temperature bath around 100°C, dozens of hours in the freeze-drying method, and even more than 1 hour is required in the spray-drying method even if the spraying state is good. More than 30 minutes are also required in the centrifugation method. When the drying time is long, the molecular structure inside the cellulose nanofibers also changes, so the strength tends to decrease. Therefore, the time for concentration or drying should be as short as possible.

[0009] In addition, when water vaporizes or freezes, due to the large specific surface area of cellulose nanofibers, strong intermolecular attraction often causes strong aggregation, resulting in the phenomenon that dispersion cannot be achieved in the next step. Therefore, a concentration or drying method for obtaining cellulose nanofibers with excellent redispersibility is required.

[0010] The present invention has been completed in view of the above problems, and in one embodiment, an object is to provide a method for obtaining a concentrated or dried product of cellulose nanofibers capable of generating a redispersion liquid having the same degree of dispersibility as the cellulose nanofiber dispersion liquid before concentration or drying.

Means for Solving the Problems

[0011] As a result of intensive studies by the present inventors, it has been discovered that the above problems can be solved by a method different from the prior art. That is, after performing a predetermined pretreatment on the cellulose nanofiber dispersion liquid and then treating it with a two-roll mill, the cellulose nanofiber dispersion liquid can be concentrated or dried in a short time, and the obtained cellulose nanofibers have excellent redispersibility and can generate a redispersion liquid having the same degree of dispersibility as the cellulose nanofiber dispersion liquid before concentration or drying. The present invention has been completed based on this finding and is exemplified below.

[0012] [1] A method for concentrating or drying a cellulose nanofiber dispersion, comprising: Step A of performing a pretreatment of adding the following 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 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 (Step B). A method comprising the above steps. [2] The method according to [1], wherein in Step B, the surface temperature of the two-roll mill is heated to 170 °C or higher. [3] The method according to [1] or [2], wherein the higher fatty acid or the higher fatty acid amide has 18 to 25 carbon atoms. [4] The method according to any one of [1] to [3], 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. [Advantages of the Invention]

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

[0014]

Figure 1

Figure 2

DETAILED DESCRIPTION OF THE INVENTION

[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 design changes, improvements, etc. can be appropriately made based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention.

[0016] (1. Cellulose nanofiber dispersion) Cellulose nanofibers are obtained by finely pulverizing cellulose, which is the main component of plant fibers, into a nanosize, and the main raw material is wood pulp (raw material for paper). It is mainly used for reinforcing resin materials and preventing shrinkage of the resin at low temperatures. In the present invention, the raw material of cellulose nanofibers is not particularly limited.

[0017] Since cellulose nanofibers are usually plant-derived materials, they are in a slurry state dispersed in water at the stage of being taken out from plants. The solid content is usually 1 to 10% by weight. For example, in the case of using cellulose nanofibers dispersed in a hydrophobic resin, it is necessary to first remove only the water from the cellulose nanofiber aqueous dispersion in which the cellulose nanofibers are dispersed to take out the cellulose nanofibers alone. Also, it is assumed that the cellulose nanofibers alone are taken out again from a redispersion liquid in which the cellulose nanofibers alone are redispersed in water or a dispersion medium other than water. Therefore, in the present invention, the dispersion medium in the cellulose nanofiber dispersion is not limited to water. However, preferably, the dispersion medium in the cellulose nanofiber dispersion is water.

[0018] Note that, as described above, the solid content in the cellulose nanofiber dispersion is usually 1 to 10% by weight, but it may be further diluted to less than 1% by weight. Therefore, before the pretreatment described below, the cellulose nanofiber dispersion can also be preliminarily dried. The method of preliminary drying is not particularly limited, and any of the conventional methods can be adopted. The cellulose nanofiber dispersion can be preliminarily dried to, for example, a maximum solid content of 12% by weight, and the concentration or drying method of the present invention can be carried out.

[0019] When the cellulose nanofiber dispersion is dried, a powder of cellulose nanofibers is obtained. It is preferable that the average fiber length of the powder of cellulose nanofibers is 0.1 μm or more. Thereby, the reinforcing effect can be expected. From this viewpoint, it is more preferable that the average fiber length of the powder of cellulose nanofibers is 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 powder of cellulose nanofibers is 0.1 μm or more, it has a high aspect ratio with respect to the average fiber diameter.

[0020] Also, it is preferable that the average fiber length of the powder of cellulose nanofibers is 3.0 μm or less. Thereby, it is possible to suppress the cellulose nanofibers from rounding into a spherical shape during processing. From this viewpoint, it is more preferable that the average fiber length of the powder of cellulose nanofibers is 2.5 μm or less, even more preferably 1.5 μm or less, and even more preferably 1.0 μm or less.

[0021] Note that the average fiber length of the powder of cellulose nanofibers means the D50 (median diameter) measured according to the laser diffraction / scattering method of JIS Z8825:2022 for the fibers of the powder of cellulose nanofibers.

[0022] It is preferable that the average fiber diameter of the cellulose nanofiber powder is 0.5 nm or more. Thereby, the reinforcing effect can be expected. From this viewpoint, it is more preferable that the average fiber diameter of the cellulose nanofiber powder is 0.7 nm or more, still more preferably 1 nm or more, and still more preferably 3 nm or more.

[0023] Also, it is preferable that the average fiber diameter of the cellulose nanofiber powder is 10 nm or less. Thereby, it is possible to suppress the cellulose nanofibers from becoming too thick and the aspect ratio from decreasing, and to prevent the orientation during diffusion in the molten resin from being impaired. From this viewpoint, it is more preferable that the average fiber diameter of the cellulose nanofiber powder is 8 nm or less, still more preferably 7 nm or less, and still more preferably 5 nm or less.

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

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

[0026] Although the present invention is not intended to be restricted by theory, it is considered that by lowering the boiling point of water, the explosive vaporization of water vapor is enhanced, and it becomes difficult for cellulose nanofibers to form aggregates. By imparting hydrophilicity or lipophilicity (collectively referred to as "wettability") to the cellulose nanofibers, it is possible to produce concentrated or dried products for different applications according to the target aqueous system, solvent system, thermoplastic resin system, or thermosetting resin system after concentration or drying. That is, additives from this point on are more likely to adsorb to the cellulose nanofibers.

[0027] As specific content of the pretreatment, (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) are added to the dispersion of cellulose nanofibers (Step A). Thereby, the boiling point of water, which is the medium of the aqueous dispersion of cellulose nanofibers, can be lowered, heat resistance can be imparted, and wettability can be improved for producing concentrated or dried CNFs for different applications by post-addition.

[0028] Examples of the alkylammonium salt include, but are not limited to, distearyldimethylammonium chloride, behenyltrimethylammonium chloride solution, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride aqueous solution, lauryltrimethylammonium chloride, benzalkonium chloride, and the like. These are usually added in an aqueous solution state. The alkylammonium salt has an effect of preventing the aggregation of cellulose nanofibers during the process of water reduction.

[0029] The addition amount of the alkylammonium salt is not particularly limited, but it is preferably added so that the alkylammonium salt is 0.01 to 0.50% by weight based on the total weight of the dispersion of cellulose nanofibers.

[0030] The amphoteric surfactant remains on the surface layer of the cellulose nanofibers after drying and plays a role in binding well with the surfactant in the next step. Examples of amphoteric surfactants include, but are not limited to, the following: Na cocoamphoacetate, Na lauroamphoacetate, and 2Na cocoamphodiacetate of the amino acid glycine type; as betaine types, cocamidopropyl betaine, lauramidopropyl betaine, myristamidopropyl betaine, palm kernel fatty acid amidopropyl betaine, lauryl betaine, coco betaine, and sulfobetaine type lauryl hydroxysultaine, lauramidopropyl hydroxysultaine, and cocamidopropyl hydroxysultaine; as amine oxide types, lauramine oxide of the amine oxide type, lauramidopropylamine oxide of the amide amine oxide type, etc.

[0031] The addition amount of the amphoteric surfactant is not particularly limited, but it is preferably added so that the amphoteric surfactant is 0.10 to 0.30% by weight based on the total weight of the cellulose nanofiber dispersion.

[0032] In addition, instead of or in addition to the above alkylammonium salt and amphoteric surfactant, ethylene glycol can also be added. Although no chemical bond occurs between ethylene glycol and cellulose nanofibers, ethylene glycol has a high adsorptivity to plant-based fibers and has a wetting effect on cellulose nanofibers. Thereby, re-aggregation during water vaporization is less likely to occur. Furthermore, since ethylene glycol has both hydrophilic and hydrophobic sides, it plays a role in binding well with the surfactant in the next step.

[0033] The addition amount of ethylene glycol is not particularly limited, but it is preferably added so that ethylene glycol is 0.02 to 0.30% by weight based on the total weight of the cellulose nanofiber dispersion. Note that ethylene glycol is usually added in an aqueous solution state, but it may also be added as a single substance.

[0034] A mixture of a higher fatty acid and isopropyl alcohol (IPA) plays a role in impregnating the fibers of cellulose nanofibers so that they can withstand the heating and drying temperature (170 °C or higher) by a two-roll mill. In this specification, the higher fatty acid refers to a fatty acid having 18 to 25 carbon atoms, which may be saturated or unsaturated and may be linear, branched or otherwise shaped. As an example, among higher fatty acids, the general chemical formula of carboxylic acid is R-COOH (R represents a substituent of carboxylic acid), and the chemical formula of isopropyl alcohol is C 3 H 8 O. When representing the mixture of the two in a reaction formula, R-COOH + C 3 H 8 O → R-COOC 3 H 7 + H 2 O. That is, the higher fatty acid reacts with isopropyl alcohol to form a fatty acid ester (R-COOC 3 H 7 ) and water (H 2 O).

[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 and may be linear, branched or otherwise shaped. The reaction formula of the mixture of the higher fatty acid or higher fatty acid amide and IPA is represented as follows. R-CONH 2 + R’-COOH + C 3 H 8 O → R-CONH-R’-COOC 3 H 7 + H 2 O Here, "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 form an amide ester (R-CONH-R’-COOC 3 H 7 ) and water (H 2It generates (O). The amide ester is a compound formed by the reaction of a higher fatty acid and a higher fatty acid amide, and has heat resistance of 270 °C or higher. Thus, by impregnating the amide ester into the fibers of cellulose nanofibers, it can withstand heating and drying temperatures of 170 °C or higher by a roll.

[0036] The above higher fatty acid or higher fatty acid amide also melts in the treatment with a two-roll mill described later, and an effect like that of wax is produced. That is, the concentrated or dried cellulose nanofibers have good peelability from the roll plating surface or the roll metal surface and naturally peel off.

[0037] The addition amount of the mixture of the higher fatty acid or higher fatty acid amide and isopropyl alcohol (IPA) is not particularly limited, but it is preferably added so that the higher fatty acid or higher fatty acid amide is 0.20 to 0.80% by weight and IPA is 2.5 to 15.0% by weight with respect to the total weight of the dispersion of cellulose nanofibers. In addition, these additives are not added separately to the aqueous dispersion of cellulose nanofibers so that the reaction according to the above reaction formula is sufficiently carried out, but are mixed in advance and added in the form of a mixture.

[0038] After adding the above additives to the aqueous dispersion of cellulose nanofibers, it is preferably stirred to mix. In the stirring of the pretreatment, it is desirable that the cellulose nanofibers are kneaded so as to form a helix while maintaining the orientation in the liquid or sol. The method and apparatus for stirring are not particularly limited, but a super mixer (manufactured by Kawata Co., Ltd.), a Henschel mixer (manufactured by Nippon Coke Industry Co., Ltd.), a high-speed mixer (manufactured by Earth Technica Co., Ltd.), etc. can be preferably used.

[0039] (3. Treatment with a two-roll mill) After performing the pretreatment, the cellulose nanofiber dispersion is supplied to a two-roll mill, and by rotating the two-roll mill, the aqueous dispersion of cellulose nanofibers is removed, and concentrated or dried cellulose nanofibers are obtained (Step B). When the boiling point of water is lowered by the pretreatment, even if the temperature of the two-roll mill is lowered to some extent, a sufficient concentration or drying effect can be obtained after the raw material (cellulose nanofiber dispersion) is charged. However, from the viewpoint of promoting the vaporization of water, it is preferable to heat the two-roll mill to a temperature above a certain level.

[0040] By supplying the aqueous dispersion of cellulose nanofibers to the middle part of the two-roll mill, the aqueous dispersion of cellulose nanofibers is bitten into the roll gap (nip) (Figure 1). As a characteristic of the two-roll mill, before entering the nip, the raw material rotates in the same direction as the roll in the roll bank above the roll gap. At this time, the cellulose nanofibers are in a state of maintaining orientation, and when entering the nip, the fibers enter side by side in the same direction. Thereby, while giving orientation to the cellulose nanofibers, shear slip occurs at the nip. Due to the self-heating caused by the shear slip and the heating temperature of the two-roll mill, the raw material with a lowered boiling point easily vaporizes, and the time for concentration or drying is shortened. The roll gap (clearance) is not particularly limited, but in the case of this embodiment, it is preferably 0.3 to 1.5 mm.

[0041] The greater the rotation ratio (i.e., the difference in rotational speed rpm) between the front roll and the rear roll in the two-roll mill, the faster the speed of being bitten into the nip. On the other hand, if the rotation ratio between the front roll and the rear roll is too large, the shear slip caused by the rotation ratio becomes small, and the time for water to vaporize becomes short. Therefore, the rotation ratio is preferably 1 to 3. Also, when there is no rotation ratio, the aqueous dispersion of cellulose nanofibers cannot be bitten into the nip and tends to stay on the roll bank.

[0042] Due to self-heating at the nip and heat transfer from the heated two-roll mill, under the action of the gravitational force between water molecules and thermal energy, water molecules in the liquid state attract each other and interact with other nearby molecules. Under extremely high temperatures, water molecules obtain sufficient energy to overcome the intermolecular gravitational force with the fiber, and water and alcohol instantaneously transfer from the liquid state to the gaseous state. When rapidly moving molecules change from the liquid state to the gaseous state, the phenomenon of steam explosion occurs. Here, the alkylammonium salt and amphoteric surfactant, which are additives for pretreatment, can withstand high temperatures up to 200 °C, and the higher fatty acid can withstand up to 270 °C. Therefore, they can withstand the energy of steam explosion. When cellulose nanofibers wetted with higher fatty acid pass through the nip, even without moisture, they turn into thin-film powder without charring. As described above, since cellulose nanofibers enter the nip while maintaining orientation, they encounter vaporization due to steam explosion in the same direction, with less random stress on the cellulose nanofibers, enabling concentration or drying in a state close to the original form (where the double helix structure is hardly relaxed).

[0043] Here, as kneaders other than the two-roll mill, a pressure kneader, Banbury, and extruder can be considered. However, since these result in random kneading, even if water instantaneously vaporizes, cellulose nanofibers tend to entangle with each other and form lumps. Also, in conventional freeze-drying, drying ovens, spray drying, etc., there is no means to impart orientation to cellulose nanofibers, and since drying is carried out over time, the double helix structure relaxes and lumps are likely to occur. Therefore, the treatment with a two-roll mill has advantages that cannot be obtained with conventional techniques. Note that the two-roll mill only needs to be able to knead raw materials between two rolls as a device, and it may have three or more rolls.

[0044] And, in order to promote the vaporization of water, 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, water can be rapidly boiled and vaporized, and the treatment can be completed in a short time. Typically, a concentrated product or a dried product can be obtained within 3 revolutions (within 30 seconds in terms of time) of the two-roll mill, and the moisture content of the concentrated product or the dried product can be 1000 mass ppm or less. From this perspective, it is more preferable that the surface temperature of the two-roll mill is 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 will deteriorate, so it is preferably 190°C or lower, and more preferably 180°C or lower. 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 the treatment with the above two-roll mill, a concentrated product or a dried product of cellulose nanofibers is obtained. When obtaining a dried product, typically the following appearance and properties are obtained. · Shape: Plate-like flakes with a size of 1 mm to 5 mm and a thickness of 0.5 mm or less. · Color tone: Milky white with a transparent feeling. · Although it has a certain hardness, it can be easily made into fine powder with a fingertip. No special grinding treatment is required. · There is no burning or partial discoloration. · Almost no scattering into the air occurs even during weighing and packaging.

[0047] And, since fatty acids and amphoteric surfactants adhere to the surface of the cellulose nanofibers from which the moisture has disappeared, it is difficult to burn and char. Especially due to the adhesion of higher fatty acids or higher fatty acid amides, the heat resistance is high. Furthermore, in the two-roll mill, the supply of raw materials and the recovery of the concentrated product or the dried product can be carried out continuously, so the advantage of high productivity can be obtained.

Examples

[0048] Examples of the present invention are shown below together with comparative examples. These examples are provided to better understand the present invention and its advantages and are not intended to limit the invention.

[0049] Test 1: Comparison of the performance of dried products due to differences in formulation Additives of the types and amounts shown in Table 1 were added to a cellulose nanofiber dispersion with a solid 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 put into a two-roll mill (roll diameter: 8 inches. Surface treatment: bright plating treatment. Nip clearance: 0.5 mm.) manufactured by Yasuda Seiki Seisakusho Co., Ltd. The input amount was 300 g. The surface temperature of the roll was heated to 170°C. Next, the two-roll mill was operated so that the rotational speed of the front roll was 7 rpm and the rotational speed of the rear roll was 6 rpm, and a sample of the dried product was collected after 3 rotations of the front roll. The time required for drying was within 30 seconds.

[0050]

Table 1-1

Table 1-2

[0051] The details of each compound shown in Table 1 are as follows. BYK-9076: A wetting and dispersing agent manufactured by BYK. A solution of 50 - 60% by mass of an alkylammonium salt of a high molecular weight copolymer. BYK-185: A wetting and dispersing agent manufactured by BYK. A solution of 60 - 70% by mass of an alkylammonium salt. Softazoline (registered trademark) LPB-R: An amide betaine type amphoteric surfactant manufactured by Kawaken Fine Chemicals Co., Ltd. A lauric acid amide propyl betaine solution. Neutron: Oleic acid amide manufactured by Nippon Seika Co., Ltd. Chemical formula C 17 H 33 CONH 2 。

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

[0053] Also, after 3 rotations of the front roll, it was visually confirmed whether the dried product easily peeled off from the rolls of the two-roll mill. The results are shown in Table 1.

[0054] Also, for each test example, a dried product was collected and put into a hot mixing two-roll mill (two rolls) heated to 195°C so that the weight ratio of the polypropylene resin to the dried product was 100:1, and kneading was carried out 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 into A4 size, and the presence or absence of pinholes was visually confirmed. The results are shown in Table 1.

[0055] As can be seen from Table 1, in Test Examples 1-3 without adding higher fatty acid amide and Test Examples 1-4 without adding IPA, the vaporization of water was not sufficient and a large amount of moisture remained. Also, the dried product did not peel off from the rolls and partial adhesion and charring were also confirmed. And when any of the compounds necessary for the pretreatment was lacking, it was confirmed that the number of pinholes in the formed resin film was large and the redispersibility was poor.

[0056] Test 2: Comparison of the moisture content of the dried product due to the difference in roll temperature To a cellulose nanofiber dispersion with a solid content of 2% by weight, additives of the same type and amount as in Test Example 1-1 were added, and the mixture was stirred and mixed using a Nippon Coke & Engineering Co., Ltd. Henschel mixer (high-speed type). The cellulose nanofiber dispersion after the addition was put into a two-roll mill (roll diameter: 8 inches. Surface treatment: bright plating treatment. Nip clearance: 0.5 mm.) manufactured by Yasuda Seiki Co., Ltd. The input amount was 300 g. The surface temperature of the roll was heated as shown in Table 2. Next, the two-roll mill was operated so that the rotational speed of the front roll was 7 rpm and the rotational speed of the rear roll was 6 rpm, and a sample of the dried product was collected after 3 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]

Table 2

[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 obtained dried product decreased to an undetectable level. In addition, when additives of the same type and amount as in Test Example 2-1 were added to a cellulose nanofiber dispersion with a solid content of 2% by weight and tested, almost the same results as those in Table 2 were obtained.

[0059] Test 3: Comparison of the properties of dried products due to differences in drying methods To a cellulose nanofiber dispersion with a solid content of 2% by weight, additives of the same type and amount as in Test Example 1-1 were added, and the mixture was stirred and mixed using a Nippon Coke & Engineering Co., Ltd. Henschel mixer (high-speed type). 5 L of the cellulose nanofiber dispersion after the addition was taken and put into a vacuum freeze dryer SF-5 manufactured by Sanjo Industries Co., Ltd., and continuously operated at -37 °C for 8 hours of vacuum freeze drying. 3 g of the dried product was collected and observed using a field emission scanning electron microscope S-4800 manufactured by Hitachi High-Tech Corporation, and a photograph at 8000 times magnification was obtained (Figure 2B). Referring to the photograph, aggregates and fragments of CNF can be confirmed.

[0060] On the other hand, for the dried product of Test Example 1-1, an electron micrograph was obtained under the same conditions (Figure 2A). It was confirmed that in the dried product of Test Example 1-1, both the cellulose nanofibers and the dispersant were well dispersed. That is, the dried product obtained by the treatment with a two-roll mill can produce a redispersion liquid having a dispersibility equivalent to that of the CNF dispersion liquid before drying.

Claims

1. A method for concentrating or drying a cellulose nanofiber dispersion, comprising: Step A of performing a pretreatment of adding the following 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 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, Step B A method comprising.

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

3. The method according to claim 1 or 2, wherein the higher fatty acid or the 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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