Honeycomb structure and method for manufacturing the same

Abstract

This honeycomb structure comprises an assembly of a plurality of microtubules consisting of cellulose nanofibers. The plurality of microtubules have polygonal opening shapes. If an average value of the longest diameters of the opening diameters of each of the plurality of microtubules is defined as an average longer diameter μ2 (μm), and an average value of the shortest diameters thereof as an average shorter diameter μ1 (μm), the ratio (μ2 / μ1) of the average longer diameter μ2 (μm) and the average shorter diameter μ1 (μm) is less than or equal to 2.0. The standard deviations σ1, σ2 of the respective frequency distributions of the shorter diameters and the longer diameters of the opening shapes in the plurality of microtubules are both less than or equal to 20.0 μm.

Description

Technical Field

[0001] The present invention relates to a honeycomb structure and a method for producing the same. Background Art

[0002] In recent years, honeycomb-shaped porous bodies and structures using various materials have been studied.

[0003] Patent Document 1 describes a honeycomb porous body made of microbial cellulose, which has a pore diameter of 10 nm to 1000 µm and a thickness of 0.1 to 10 µm.

[0004] Patent Document 2 relates to a method for producing honeycomb-shaped silica gel, and describes a production method including a unidirectional freezing step of forming a honeycomb shape by inserting a sample into a coolant in a fixed direction and freezing the sample. In particular, the method is characterized in that the honeycomb pore diameter obtained after freezing is controlled by controlling the insertion speed at which the sample is inserted into the coolant in the unidirectional freezing step.

[0005] Patent Document 3 relates to a method for producing nanoporous silica having a substantially honeycomb structure. The method includes inserting a container filled with a colloidal silica aqueous solution containing silica particles into a coolant, quenching the container to allow moisture in the colloidal silica aqueous solution to freeze and grow into narrow columnar shapes in one direction, and then removing the moisture by drying, thereby producing nanoporous silica having a substantially honeycomb structure that is formed of a silica component and has pores that are substantially parallel to each other and substantially penetrate therethrough.

[0006] Further, Patent Documents 4 and 5 relate to a microtubule aggregate structure that is an aggregate of microtubules formed of a specific polysaccharide. As a production method thereof, a method is disclosed in which: a container containing a polysaccharide-containing liquid including a polysaccharide is inserted into a coolant, so that the solvent in the polysaccharide-containing liquid is freeze-grown in a direction vertically upward from the liquid surface of the coolant, and then the frozen solvent is removed. Prior Art Documents Patent Documents

[0007] [Patent Document 1] Japanese Patent Publication No. 2006-325534 [Patent Document 2] Japanese Patent Publication No. 2004-307294 [Patent Document 3] Japanese Patent Publication No. 2009-46341 [Patent Document 4] Japanese Patent Publication No. 2012-167218 [Patent Document 5] Japanese Patent Publication No. 2012-167152 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, in the prior art, including Patent Documents 1 to 5, various materials have been used to study porous bodies and honeycomb-like structures (hereinafter also referred to as honeycomb structures) composed of long tubular structures (microtubules), but variations sometimes occurred in the opening shape of each microtubule. Furthermore, in recent years, from the viewpoint of ensuring the strength of honeycomb structures, further improvement in the uniformity of the cross-sectional shape of microtubules has been desired.

[0009] In view of the above, the present invention aims to provide a method for manufacturing a honeycomb structure with excellent uniformity of the microtubule opening shape. [Means for solving the problem]

[0010] The inventors first focused on a method for manufacturing a honeycomb structure made of cellulose nanofibers, and investigated the manufacturing method and dispersion conditions from the viewpoint of uniformity of the opening shape of the tubular body. As a result, they found that by adopting a so-called "unidirectional freezing method," in which an aqueous dispersion of cellulose nanofibers is frozen in one direction at a constant speed, and by controlling the light transmittance of the aqueous dispersion to an appropriate range, a honeycomb structure with excellent shape uniformity can be manufactured, thus completing the present invention.

[0011] This invention is based on the above findings, and its gist is as follows. (1) A honeycomb structure according to one aspect of the present invention is a honeycomb structure which is an aggregate of a plurality of microtubules made of cellular nanofibers, wherein the opening shape of the plurality of microtubules is polygonal, and when the average value of the longest diameter of the opening diameter of each of the plurality of microtubules is the average major diameter μ2 (μm) and the average value of the shortest diameter is the average minor diameter μ1 (μm), the ratio μ2 / μ1, which is the ratio of the average major diameter μ2 (μm) to the average minor diameter μ1 (μm), is 2.0 or less, and the standard deviations σ1 and σ2 of the frequency distribution of the minor diameter and major diameter of the opening shape in the plurality of microtubules are both 20.0 μm or less. (2) The honeycomb structure described in (1) above may have an average opening diameter of 0.1 μm to 200 μm. (3) The honeycomb structure described in (1) or (2) above may have a Young's modulus of 0.1 kPa to 10,000 kPa. (4) The honeycomb structure described in any of (1) to (3) above may be obtained by freeze-drying the dispersion of cellulose nanofibers. (5) A method for manufacturing a honeycomb structure according to one aspect of the present invention is a method for manufacturing a honeycomb structure of cellulose nanofibers, comprising: a freezing step in which a container containing a dispersion in which cellulose nanofibers are dispersed in a solvent is gradually inserted into a refrigerant, thereby causing the solvent to freeze and grow in a direction vertically upward from the refrigerant liquid surface; and a drying step in which the solvent is removed from the frozen dispersion, wherein the concentration of cellulose nanofibers in the dispersion is 1.0 wt% or more, and the light transmittance of the dispersion at a wavelength of 550 nm is 20 to 80% when the concentration of cellulose nanofibers is 1.0 wt%. (6) The method for producing the honeycomb structure described in (5) above may also involve a dispersion with a pH of 4.0 to 13.9. (7) The method for producing the honeycomb structure described in (5) or (6) above may also be such that the average fiber length of the cellulose nanofibers is 0.15 μm or more. (8) The method for manufacturing a honeycomb structure according to any one of (5) to (7) above may comprise: a first drying step of holding the dispersion liquid after the freezing step at 0°C or lower under a reduced pressure environment for a certain period of time, and a second drying step of removing the solvent by holding the frozen dispersion liquid at room temperature under a reduced pressure environment. Effects of the Invention

[0012] According to the present invention, it is possible to provide a honeycomb structure excellent in uniformity of opening shape of microtubules (hereinafter also referred to as shape uniformity) and a method for manufacturing the same. Further, the honeycomb structure manufactured by the manufacturing method of the present invention can be suitably used for filters such as masks and air cleaners, for example. Brief Description of the Drawings

[0013] [Figure 1] It is an SEM photograph for explaining a method of measuring the major axis and minor axis of an opening diameter in a microtubule. [Figure 2] It is a schematic diagram for explaining the principle of the unidirectional freezing method in the method for manufacturing a honeycomb structure according to the present embodiment. [Figure 3] It is a schematic diagram for explaining a preferred embodiment of the method for manufacturing a honeycomb structure according to the present embodiment. [Figure 4] It is an SEM image of honeycomb structures of sample Nos. 1 to 8 in the present example. Mode for Carrying Out the Invention

[0014] Hereinafter, the honeycomb structure and the manufacturing method thereof according to the present embodiment will be described with reference to the drawings as appropriate.

[0015] [Honeycomb Structure] The honeycomb structure according to the present embodiment is an aggregate of a plurality of microtubes made of cellulose nanofibers. The opening shape of the plurality of microtubes is polygonal. When the average value of the longest diameters of the opening diameters of each of the plurality of microtubes is defined as average major axis μ2 (μm), and the average value of the shortest diameters is defined as average minor axis μ1 (μm), the ratio (μ2 / μ1) of the average major axis μ2 (μm) to the average minor axis μ1 (μm) is 2.0 or less. Further, the standard deviations σ1 and σ2 of the frequency distributions of the minor axis Ls and the major axis Ll of the opening shape in the plurality of microtubes are both 20.0 μm or less. Hereinafter, each constituent requirement of the honeycomb structure of the present embodiment will be described.

[0016] <Material> The honeycomb structure of the present embodiment is made of cellulose nanofibers. Here, "cellulose" in the present embodiment means a polysaccharide having a structure in which D-glucopyranose units are linked by β-1,4 glycosidic bonds. Generally, cellulose is classified into natural cellulose, regenerated cellulose, fine cellulose, microcrystalline cellulose excluding amorphous regions, and the like according to the production method, origin, and other factors. Any of these celluloses may be used as a raw material for the cellulose nanofibers of the present embodiment, and products obtained by defibrating these celluloses may be employed as the cellulose nanofibers.

[0017] The honeycomb structure of the present embodiment is a structure in which a plurality of microtubes are aggregated, and these microtubes are composed of the cellulose nanofibers as described above. The opening shape of the microtubes is a polygonal shape such as quadrangular, pentagonal, or hexagonal.

[0018] <μ2 / μ1: 2.0 or less> In this embodiment, the ratio of the average major diameter μ2 (μm) to the average minor diameter μ1 (μm) of the opening diameter (μ2 / μ1) in multiple microtubules is set to 2.0 or less. In the opening shape of microtubules, a smaller ratio of major diameter to minor diameter indicates that the opening shape is a distortion-free polygon. In other words, by setting the average major diameter μ2 of the opening diameter of the microtubules constituting the honeycomb structure of this embodiment to 2.0 times or less the average minor diameter μ1, the opening shape of the microtubules can be made a distortion-free, uniform polygon. From this viewpoint, it is preferable that μ2 / μ1 be 1.8 or less, and more preferable that it be 1.6 or less.

[0019] In this context, "average major diameter μ2" and "average minor diameter μ1" in the microtubule opening shape refer to the average values ​​of the major diameter Ll and minor diameter Ls of the opening diameters of multiple microtubules, respectively. Note that the major diameter Ll and minor diameter Ls of the opening diameter represent the diameters with the longest and shortest distances between any two points in the opening diameter when the microtubule opening shape is viewed from above. The specific measurement methods for the major diameter Ll and minor diameter Ls of the opening diameter will be described later.

[0020] <Standard deviation σ1, σ2: 20.0μm or less> In this embodiment, the standard deviations σ1 and σ2 of the frequency distributions of the short diameter Ls and long diameter Ll of the opening shape (opening diameter) in multiple microtubules are both set to 20.0 μm or less. By reducing the standard deviations σ1 and σ2 of the short diameter Ls and long diameter Ll of the microtubule opening shape, variations in the shape and dimensions of multiple microtubules can be suppressed, and as a result, a honeycomb structure with excellent shape uniformity can be obtained. From this viewpoint, it is preferable that the standard deviations σ1 and σ2 are both 18.0 μm or less, and more preferably 16.0 μm or less.

[0021] The average minor diameter μ1, average major diameter μ2, and standard deviations σ1 and σ2 of the opening diameter in microtubules can be determined as follows. Figure 1 is an SEM image illustrating the method for measuring the major diameter Ll and minor diameter Ls of the opening diameter in microtubules. First, the opening shapes (cross-sectional shapes) of multiple microtubules constituting the honeycomb structure are observed using a scanning electron microscope (SEM) to obtain an SEM image as shown in Figure 1. Next, using the obtained SEM image, the major diameter Ll (μm) is defined as the diameter with the longest distance between any two points, and the minor diameter Ls (μm) is defined as the diameter with the shortest distance between points. These measurements are then taken for 50 microtubules. The average major diameter Ll of the 50 obtained microtubules is calculated to obtain the "average major diameter μ2". Similarly, the average minor diameter Ls of the 50 obtained microtubules is calculated to obtain the "average minor diameter μ1".

[0022] Furthermore, the standard deviation σ2 of the major axis Ll of the microtubules can be obtained by calculating the standard deviation from the data (frequency distribution) of the major axis Ll of the 50 microtubules obtained. Similarly, the standard deviation σ1 of the minor axis Ls of the microtubules can be obtained by calculating the standard deviation from the data (frequency distribution) of the data (frequency distribution) of the minor axis Ls of the 50 microtubules obtained.

[0023] In the honeycomb structure of this embodiment, the average aperture diameter of the multiple microtubules is not particularly limited and may be appropriately determined depending on the application of the honeycomb structure, but for example, a range of 0.1 μm to 200 μm can be exemplified. The measurement of this average aperture diameter can be performed by SEM in the same manner as the measurement method of the major diameter Ll and minor diameter Ls of the aperture diameter described above. Specifically, the major diameter Ll and minor diameter Ls are measured for 50 microtubules, the average diameter of the aperture diameter of each microtubule is determined, and the average value of the average diameters of the aperture diameters of the obtained 50 microtubules is calculated to obtain the average aperture diameter of the multiple microtubules.

[0024] Furthermore, the axial length of the microtubules in this embodiment is theoretically unlimited according to the manufacturing method of this embodiment, which will be described later. The fact that the axial length of the microtubules is unlimited means that, ideally, when observed along the axial direction from the end of the microtubule, the opening at one end of the microtubule penetrates all the way through without being divided into multiple parts. Therefore, the honeycomb structure in this embodiment is preferably in the state of being bundled straws. It should be noted that structures generally referred to as porous bodies do not have an axis from the opening of a through-hole formed on one surface to an opening formed on another surface. In this respect, the honeycomb structure in this embodiment is distinguished from structures generally referred to as porous bodies.

[0025] Furthermore, the mechanical properties of the honeycomb structure in this embodiment are not particularly limited, but for example, the Young's modulus may be between 0.1 kPa and 10,000 kPa. By providing a Young's modulus within this range, the honeycomb structure of this embodiment can be suitably used for applications such as adsorbents and catalyst supports. The Young's modulus of the honeycomb structure can be determined by measuring the stress-strain curve using a strength testing machine ("AG-50kNXplus", manufactured by Shimadzu Corporation) and obtaining the resulting stress-strain curve.

[0026] The honeycomb structure of this embodiment has been described above. This embodiment provides a honeycomb structure with excellent uniformity of microtubule shape. Furthermore, this honeycomb structure is suitable for use in filters such as masks and air purifiers. The above μ2 / μ1 ratio and standard deviations can be achieved by adjusting the conditions for preparing the cellulose nanofiber dispersion in the manufacturing method described later.

[0027] [Method for manufacturing honeycomb structures] Next, the manufacturing method of the honeycomb structure according to the embodiment described above will be explained with reference to the drawings. A method for manufacturing a honeycomb structure according to one embodiment of the present invention is a method for manufacturing a honeycomb structure of cellulose nanofibers using an aqueous solvent, comprising a freezing step in which a container containing a dispersion in which cellulose nanofibers are dispersed in an aqueous solvent is gradually inserted into a refrigerant, thereby causing the solvent to freeze and grow in a direction vertically upward from the refrigerant liquid surface, and a drying step in which the frozen solvent is removed, wherein the concentration of cellulose nanofibers in the dispersion is 1.0 wt% or more, and the light transmittance of the dispersion at a wavelength of 550 nm when the concentration of cellulose nanofibers is 1.0 wt% is in the range of 20 to 80%.

[0028] <Freezing process> In the freezing process of this embodiment, a so-called "unidirectional freezing method" is employed, in which a dispersion of cellulose nanofibers is frozen in one direction at a constant speed, to produce a honeycomb structure made of cellulose nanofibers (CNF).

[0029] The "unidirectional freezing method" is a method in which a dispersion of cellulose nanofibers (cellulose nanofiber dispersion) with water or the like as a solvent is frozen in a directional manner, causing the solvent to grow in a columnar shape in one direction, forming multiple ice columns, and the cellulose nanofibers to aggregate in the gaps between the ice columns. Figure 2 is a schematic diagram illustrating the principle of the unidirectional freezing method in this embodiment. Specifically, in the "unidirectional freezing method" in this embodiment, as shown in Figure 2, a cellulose nanofiber dispersion 1 is placed in a container 2, and then the container 2 is inserted into a refrigerant 3 at a constant speed in the direction indicated by the arrow. The solidified solvent in the part inserted into the refrigerant 3 grows in a columnar shape along the insertion direction. After that, the frozen columnar crystals are placed under reduced pressure and the solvent is removed by freeze-drying, which sublimes the solidified solvent portion. Through these steps, a honeycomb structure composed of an aggregate of elongated pores (microtubules) made of cellulose nanofibers is manufactured. Note that the "aggregate of elongated pores (microtubules)" means that it is composed of multiple microtubules that penetrate in a straight line, and each of the multiple microtubules is substantially oriented in one direction.

[0030] The container 2 containing the cellulose nanofiber dispersion 1 is gradually inserted into the refrigerant 3. Therefore, it is desirable that the container 2 has physical properties (e.g., expansion coefficient) that prevent damage caused by the temperature difference between the part of the container 2 immersed in the refrigerant 3 and the part directly above the refrigerant 3. Examples of materials for the container 2 include polypropylene, polyethylene, and polyvinyl chloride. Among these, polypropylene is more preferable. The thermal conductivity of polypropylene is approximately 0.11 (W / m·K), which is lower than that of water (0.6 W / m·K) and ice (1.6 W / m·K). Therefore, by using a polypropylene container, the frozen surface of the dispersion can be kept more parallel to the surface of the refrigerant (liquid nitrogen, etc.).

[0031] Furthermore, the shape of container 2 only needs to have a predetermined height so that it can form microtubules with axial length that constitute the honeycomb structure. The shape and size of container 2 can also be determined according to the intended use of the honeycomb structure.

[0032] The cellulose nanofiber content of cellulose nanofiber dispersion 1 shall be 1.0 wt% or higher. If the cellulose nanofiber content is less than 1.0%, there is a risk of microtubule shape deterioration (malformation). This may also lead to a decrease in the strength of the structure. Therefore, the cellulose nanofiber content of cellulose nanofiber dispersion 1 shall be 1.0 wt% or higher. However, from the viewpoint of improving the strength of the structure, it is preferable that the cellulose nanofiber content be 2.0 wt% or higher. There is no particular upper limit to the cellulose nanofiber content, but if it exceeds 5.5 wt%, there is a risk that air bubbles will remain in the honeycomb structure. Therefore, the cellulose nanofiber content may be 5.5 wt% or less. If the cellulose nanofiber content is excessively high, the viscosity of the dispersion increases and it becomes gel-like, making it difficult to remove air bubbles. If air bubbles cannot be removed before freeze-drying and remain present, they may remain in the form of air bubbles after freeze-drying.

[0033] The light transmittance of the cellulose nanofiber dispersion 1 must be within a predetermined range from the viewpoint of shape uniformity of the honeycomb structure. Specifically, a dispersion is used in which the light transmittance at a wavelength of 550 nm is in the range of 20 to 80% when the concentration of cellulose nanofibers is 1.0 wt%. According to the inventors, a new finding has been obtained that there is a certain correlation between the transparency of the cellulose nanofiber dispersion, i.e., the light transmittance, and the shape uniformity of the honeycomb structure. Specifically, by using a dispersion having a light transmittance within this range, a honeycomb structure with excellent microtubule shape uniformity can be manufactured. If the light transmittance of the dispersion exceeds 80% when the concentration of cellulose nanofibers is 1.0 wt%, the degree of dispersion of cellulose nanofibers in the dispersion may become excessively high, and a honeycomb structure may not be obtained. On the other hand, if the light transmittance is less than 20%, aggregation of cellulose nanofibers becomes significant, and the formation of the honeycomb structure may be inhibited by these aggregates. Therefore, when the concentration of cellulose nanofibers is 1.0 wt%, the light transmittance of the cellulose nanofiber dispersion at a wavelength of 550 nm should be in the range of 20 to 80%. Preferably, the light transmittance is 35% or more and 60% or less.

[0034] Thus, in the freezing process of this embodiment, it is important to use a dispersion in which the light transmittance at a wavelength of 550 nm is in the range of 20 to 80% when the concentration of cellulose nanofibers is 1.0 wt%, and the concentration of cellulose nanofibers is 1.0 wt% or more. This makes it possible to manufacture a honeycomb structure with excellent shape uniformity.

[0035] The method for measuring the light transmittance of a cellulose nanofiber dispersion at a wavelength of 550 nm is as follows: First, a cellulose nanofiber dispersion (dispersion medium: water) is prepared so that the cellulose nanofiber concentration is 1.0 wt%, and the light transmittance at a wavelength of 550 nm is measured using a UV-Vis spectrophotometer "V670" (manufactured by JASCO Corporation) with a cell having an optical path length of 10 mm.

[0036] The pH of the cellulose nanofiber dispersion is preferably in the range of 4.5 to 13.9 from the viewpoint of ensuring the Young's modulus of the honeycomb structure and controlling the light transmittance. If the pH of the cellulose nanofiber dispersion is excessively low, the electrostatic repulsion between the cellulose nanofibers will weaken, and aggregation of the cellulose nanofibers may become significant. Significant aggregation of cellulose nanofibers may inhibit the growth of ice crystals and lead to defects in the shape of the honeycomb structure. Therefore, the pH of the cellulose nanofiber dispersion is more preferably in the range of 5.0 to 13.0. Note that if aggregation of cellulose nanofibers becomes significant, light scattering increases, and the light transmittance of the dispersion decreases.

[0037] In this embodiment, cellulose refers to a polysaccharide with a structure in which D-glucopyranose is linked by β-1,4 bonds. Generally, cellulose is classified into natural cellulose, regenerated cellulose, fine cellulose, and microcrystalline cellulose (excluding the amorphous region) based on its manufacturing method and origin, but the cellulose nanofibers of this embodiment can be produced by using any of these types of cellulose as a raw material and by defibration.

[0038] While other polysaccharides besides cellulose could be used as materials for the honeycomb structure, our research has shown that cellulose is optimal from the viewpoint of stabilizing the honeycomb shape and reducing pressure loss. For example, using dextrin is undesirable because it does not form a honeycomb structure.

[0039] The average fiber length of the cellulose nanofibers is preferably 0.15 μm or more, from the viewpoint of stably producing a honeycomb structure with excellent shape uniformity. If the average fiber length of the cellulose nanofibers is excessively short, the degree of dispersion of the cellulose nanofibers in the dispersion may become excessively high, which may lead to shape defects in the honeycomb structure. For these reasons, the average fiber length of the cellulose nanofibers is more preferably 0.3 μm or more. Note that if the degree of dispersion of the cellulose nanofibers becomes excessively high, the light transmittance of the dispersion will increase. Furthermore, there is no particular upper limit to the average fiber length of the cellulose nanofibers, but it may be 100 μm or less, or 10 μm or less.

[0040] The solvent for the cellulose nanofiber dispersion is not particularly limited as long as it homogeneously disperses or dissolves the cellulose nanofibers and has a freezing point. Common examples of polar solvents include water, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, t-butanol, acetic acid, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, acetonitrile, and dioxane. Among these, water is preferred because it is readily available and inexpensive. These solvents may be used individually or in combination of two or more.

[0041] As a refrigerant, there are no particular restrictions as long as it is a liquid (antifreeze) that does not freeze even at temperatures that can freeze the solvent of the cellulose nanofiber dispersion. Common examples include water, saline solution, ethylene glycol, carbon tetrachloride, acetonitrile, methanol, ethanol, acetone, diethyl ether, liquid nitrogen, liquid hydrogen, liquid argon, and liquid helium. Among these, liquid nitrogen is preferred because it is readily available, inexpensive, and does not freeze even at relatively low temperatures. In addition to liquids, a cooled space (for example, air cooled to a temperature that can freeze the cellulose nanofiber dispersion) can also be used as a refrigerant.

[0042] As a means of cooling the refrigerant, a refrigerant coolant and / or a cooling device may be used. Examples of refrigerants include ice, a mixture of salt and ice, a mixture of sodium acetate and ice, a mixture of calcium chloride and ice, a mixture of ammonium chloride and ice, a mixture of ammonium nitrate and ice, a mixture of ammonium chloride, potassium nitrate and ice, a mixture of sodium bromide and ice, a mixture of sodium chloride and ice, a mixture of potassium chloride and ice, a mixture of magnesium chloride and ice, a mixture of zinc chloride and ice, and dry ice. The method of using these refrigerants should be appropriately selected from methods such as indirect cooling via a refrigerant container without direct contact with the refrigerant, or direct mixing with the refrigerant. Examples of cooling devices include immersion coolers. The cooling device may be in direct contact with the refrigerant.

[0043] There are no particular restrictions on the temperature of the cellulose nanofiber dispersion or the refrigerant. The temperature of the cellulose nanofiber dispersion should be higher than the freezing point of the solvent and lower than the boiling point of the solvent. Similarly, there are no particular restrictions on the temperature of the refrigerant as long as it is at a temperature that can freeze the solvent of the cellulose nanofiber dispersion. For example, when water is used as the solvent, the temperature of the cellulose nanofiber dispersion should be higher than the freezing point of water (0°C) and lower than the boiling point of water (100°C), and the refrigerant should be lower than the freezing point of water (0°C).

[0044] In the unidirectional freezing method of this embodiment, a container containing a cellulose nanofiber dispersion is gradually immersed in a refrigerant. There are no particular restrictions on the immersion rate (penetration rate), which can be, for example, 3 to 300 μm / second. Preferably, it is 10 to 100 μm / second. If the penetration rate is too slow, it is unsuitable in terms of productivity. If the penetration rate is too fast, it may result in problems such as impaired linearity of the pores (microtubules) that constitute the honeycomb structure. It is desirable to maintain a constant penetration rate, and in that case, the penetration rate can be controlled to be constant by connecting a constant-speed motor (not shown) to the container.

[0045] In this embodiment, controlling the penetration rate is effective in adjusting the average opening diameter of the microtubules in the honeycomb structure to the preferred range described above. In the honeycomb structure obtained after freeze-drying, the parts that were columnar ice crystals before drying become voids. In other words, the diameter of the columnar ice crystals matches the opening diameter of the final honeycomb structure. Therefore, the diameter of the columnar ice crystals, i.e., the final opening diameter of the honeycomb structure, can be adjusted by controlling the penetration rate (freezing rate) and freezing temperature in unidirectional freezing. The larger the penetration rate, the smaller the diameter of the columnar ice crystals. Also, the smaller the freezing temperature, the smaller the diameter of the columnar ice crystals.

[0046] When a container holding a cellulose nanofiber dispersion is gradually immersed in a refrigerant, the solvent in the dispersion solidifies, growing vertically upwards from the refrigerant surface in a columnar shape. At this time, the cellulose nanofibers, which are the solute, are pushed to the outer periphery as the solidified, elongated columnar crystals grow, and become aggregated like a wall. If this state is illustrated using water as the solvent, it can be said that a wall of cellulose nanofiber molecules is formed around each individual ice column, like frost crystals. In other words, it can be said that multiple microtubules (pores) are formed by the aggregation of cellulose nanofiber molecules, and ice is filled into these pores. Hereafter, this state in which ice is filled into the pores of multiple microtubules will be referred to as a cellulose nanofiber dispersion solidified body.

[0047] <Drying process> After the freezing process, the resulting solidified cellulose nanofiber dispersion is removed from the refrigerant along with the container, and a drying process is carried out to remove the solidified solvent. This yields a honeycomb structure made of cellulose nanofibers.

[0048] Any solvent removal method can be used to remove the solidified solvent, as long as the structure of the solidified cellulose nanofiber dispersion is not damaged, but freeze-drying is particularly desirable. Freeze-drying is preferable because the solvent sublimes, thus suppressing solvent melting and redispersion of the cellulose nanofibers. Alternatively, the solvent may be temporarily replaced with an incompatible solvent that does not disperse the cellulose nanofibers, and then air-dried or heat-dried.

[0049] When using freeze-drying as a solvent removal method, it is best to carry it out under the following conditions. First, the cellulose nanofiber dispersion solidified in the container is cut along with the container while maintaining its frozen state, and a cylindrical piece of a predetermined length is cut out. Next, the cut cylindrical piece is held under reduced pressure for a certain period of time, allowing the solvent (e.g., water) to sublimate while maintaining its frozen state, and the piece is dried. As a result, the solvent in the cellulose nanofiber dispersion solidified is removed, leaving only a honeycomb-like structure composed of aggregates of microtubules (pores).

[0050] The preferred conditions for the reduced-pressure environment during freeze-drying can be appropriately determined depending on the solvent used. For example, when water is used as the solvent, a reduced-pressure atmosphere of -30 to 50°C and 1 to 1000 Pa may be used. The holding time under reduced pressure can also be appropriately determined depending on the solvent and reduced-pressure atmosphere used, but for example, it may be 20 to 100 hours.

[0051] The method for manufacturing the honeycomb structure according to this embodiment has been described above. However, the drying step in this embodiment may include a first drying step in which the dispersion after the freezing step is held at -5°C and under reduced pressure for a certain period of time, and a second drying step in which the frozen solvent is removed by holding it at room temperature and under reduced pressure. By dividing the drying step into two stages with different conditions in this way, the time required for drying can be significantly reduced compared to conventional methods. The first drying step and the second drying step will be described in detail below.

[0052] [First drying process] Figure 3 is a schematic diagram illustrating a preferred embodiment of the drying process in the method for manufacturing a honeycomb structure according to this embodiment. In the first drying process, the cellulose nanofiber dispersion solidified by the freezing process is held at 0°C or below and under reduced pressure for a certain period of time to dry.

[0053] Specifically, first, the cellulose nanofiber dispersion solidified in the container 2 shown in Figure 2 is cut while maintaining its frozen state, and cylindrical pieces of a predetermined length are cut out. Next, as shown in Figure 3, the cut cylindrical pieces 1A are placed in a sealed container 4, and then the sealed container 4 is immersed in a refrigerant tank 5 containing a refrigerant (-80°C to 0°C), creating an atmosphere of -80°C to 0°C and reduced pressure inside the container 4. Furthermore, the cylindrical pieces 1A in the sealed container 4 are dried by holding them in a reduced-pressure environment for a certain period of time. In this way, by pre-drying the cylindrical pieces 1A in a low-temperature, reduced-pressure environment (first drying) before the second drying process, it is possible to prevent the solidification solvent (e.g., ice) in the cylindrical pieces 1A from dissolving in the second drying process.

[0054] The temperature of the refrigerant in the refrigerant tank 5 shall be between -80°C and 0°C, from the viewpoint of sublimating the solidifying solvent in the solidified material. If the refrigerant temperature is too low, sublimation will be difficult, so it should be above -80°C. On the other hand, if the refrigerant temperature is too high, there is a risk that the solidified solvent will dissolve, so it should be below 0°C.

[0055] The means for drying the cylindrical piece 1A inside the sealed container 4 under reduced pressure is not particularly limited, but an oil rotary vacuum pump may be used.

[0056] In the first drying process, the progress of sublimation and drying of the solidified solvent can be confirmed by observing the pressure inside the sealed container 4. For example, a pressure gauge 7 (e.g., a Pirani vacuum gauge) can be connected between the sealed container 4 and a depressurization means 6 (e.g., a vacuum pump) to measure the pressure inside the sealed container 4. In this embodiment, when depressurization is initiated inside the sealed container 4, the pressure inside the sealed container 4 rises shortly thereafter. This is because the solidified solvent (e.g., ice) inside the solidified body sublimes, generating water vapor.

[0057] The holding time in the first drying step is not particularly limited and may be between 5 minutes and 48 hours.

[0058] [Second drying process] After the first drying step, while maintaining reduced pressure inside the sealed container 4, the sealed container 4 containing the cylindrical pieces 1A is removed from the refrigerant tank 5 and held at room temperature under reduced pressure for a certain period of time to dry and remove the solvent. This allows the solvent (e.g., ice) to sublimate and dry while maintaining the frozen state, leaving only the honeycomb structure behind.

[0059] Specifically, first, the sealed container 4 containing the cylindrical piece 1A is removed from the refrigerant tank 5 while maintaining a reduced pressure state after the first drying process. Then, it is left to stand at room temperature (approximately 25°C) and in a reduced pressure environment for a certain period of time to promote drying.

[0060] Similar to the first drying step, the progress of solvent sublimation and drying can be confirmed by observing the pressure inside the sealed container. In the second drying step, the reduced pressure conditions from the first drying step are maintained while the container is exposed to room temperature, so the sublimation and drying of the solvent proceeds more quickly. This can also be confirmed by the pressure fluctuations inside the sealed container 4; for example, soon after the sealed container 4 is exposed to room temperature, the pressure inside the sealed container 4 rises. This is because the solidified solvent inside the solidified body sublimes further, generating a large amount of water vapor.

[0061] The holding time for the second drying process is not particularly limited and may be terminated as soon as drying is complete. The completion of drying can be indicated by the point when the pressure inside the sealed container 4 decreases to the standard pressure obtained by connecting a vacuum pump to the empty sealed container 4 and reducing the pressure.

[0062] The method for manufacturing a honeycomb structure according to this embodiment has been described above. This method makes it possible to manufacture a honeycomb structure with excellent uniformity of microtubule shape. Furthermore, the honeycomb structure manufactured by the manufacturing method of this embodiment can be suitably used, for example, in filters for masks and air purifiers.

[0063] Furthermore, according to a preferred embodiment of the drying process, the drying time can be significantly reduced compared to the conventional unidirectional freezing method. Specifically, by pre-drying the solvent solidified in the freezing process under a low-temperature, reduced-pressure environment (first drying) and then drying it under a room-temperature, reduced-pressure environment, the sublimation and drying of the solidified solvent can be carried out efficiently, resulting in a significant reduction in manufacturing costs. [Examples]

[0064] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as it does not exceed the gist of the invention.

[0065] First, as cellulose nanofibers, samples No. 1-8 used "cellenpia TC-01A" (average fiber length: 617 nm) manufactured by Nippon Paper Industries Co., Ltd., while samples No. 9 and 10 used "Reocrista I-2SX (registered trademark)" (preservative-free, average fiber length: approximately 800-100 nm) manufactured by Daiichi Kogyo Seiyaku Co., Ltd. In all cases, a cellulose nanofiber predispersion was prepared by chemical oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). Next, the CNF concentration and pH of the cellulose nanofiber predispersion were adjusted using water, hydrochloric acid, and sodium hydroxide to prepare a cellulose nanofiber dispersion. The prepared cellulose nanofiber dispersions are shown in Table 1. Samples No. 1-4 and 6 were prepared from the aqueous dispersion of sample No. 5 (CNF concentration 3.0 wt%). Samples No. 1-4 were prepared by diluting sample No. 5, and sample No. 6 was prepared by concentrating sample No. 5. Therefore, the transmittance at 1 wt% for samples No. 1 to 6 is the same at 43%. Sample No. 9 was prepared by diluting the aqueous dispersion of sample No. 10 (CNF concentration 2.0 wt%). Therefore, the transmittance at 1 wt% for sample 10 is 47%, the same as for sample No. 9. The light transmittance of the cellulose nanofiber dispersion at a wavelength of 550 nm was measured in the same manner as described above.

[0066] The obtained cellulose nanofiber dispersion was placed in a polypropylene tube (16 mm inner diameter, 18 mm outer diameter, 180 mm length). The polypropylene tube was then vertically inserted into liquid nitrogen (-196°C) at a constant rate of 10 cm / hr. The solvent (water) in the portion inserted into the liquid nitrogen grew in a columnar shape vertically upward from the liquid nitrogen surface, forming multiple ice columns and creating a solidified cellulose nanofiber dispersion (unidirectional freezing method).

[0067] The drying process was carried out as follows: The cellulose nanofiber dispersion solidified inside the tube was cut along with the tube while maintaining its frozen state, and a cylindrical piece 10 mm thick was cut out. Next, the cut cylindrical piece was placed in a glass container connected to a vacuum pump and held under reduced pressure and at -5°C for 1 hour to sublimate the solvent inside the cylindrical piece (first drying step). The following points were considered when cutting the cylindrical pieces to be freeze-dried.

[0068] When unidirectional freezing begins during the freezing process, water freezes rapidly near the bottom of the tube, and the frozen surface moves upward from the bottom of the tube. Since the frozen surface is visible, the speed of movement of the frozen surface at each position in the tube can be measured. Near the bottom of the tube, the speed of movement of the frozen surface is greater than the unidirectional freezing speed (penetration speed), but as the frozen surface moves upward, the speed decreases, and a few centimeters above the bottom, the speed of movement of the frozen surface becomes equal to the unidirectional freezing speed, the growth of the ice column stabilizes, and the frozen surface becomes flat. In this example, in order to properly evaluate the honeycomb structure, the solidified material in the region near the bottom where the speed of movement of the frozen surface does not match the unidirectional freezing speed was cut and removed, and only the upper portion, i.e., the portion where the frozen surface was flat and rising at a speed equal to the penetration speed, was subjected to the drying process.

[0069] After the first drying step, the glass container was removed from the refrigerant tank 5 while maintaining the reduced pressure inside the glass container, and was left to stand for 24 hours while maintaining the container temperature at room temperature (approximately 25°C) to perform freeze-drying (second drying step). In this step, although the temperature of the glass container is room temperature, the cylindrical piece is cooled by the heat of sublimation when the solvent (ice) sublimes, so freeze-drying can proceed without melting the solvent. Immediately after the start of the second drying process, the pressure inside the glass container was approximately 300 Pa. As drying progressed, the pressure gradually decreased and became almost constant at 10 Pa after 24 hours. When the pressure had dropped to a constant level, freeze-drying was terminated, and a honeycomb structure was obtained.

[0070] [Table 1]

[0071] The shape and opening of the microtubules in the obtained honeycomb structures (samples No. 1-10) were observed using a scanning electron microscope (SEM). The average major diameter μ2 and average minor diameter μ1 of the microtubule opening diameter were obtained by measuring the major diameter Ll (μm) and minor diameter Ls (μm) for 50 microtubules using the same method as described above, and calculating the average value. The standard deviations σ1 and σ2, as well as the average opening diameter, were also calculated using the same method as described above.

[0072] Furthermore, the mechanical properties (Young's modulus) of samples No. 2-5 and 7-10 of the obtained honeycomb structures were determined. Specifically, the stress-strain curves were measured using a strength testing machine ("AG-50kNXplus," manufactured by Shimadzu Corporation), and the Young's modulus (kPa) was determined from the obtained stress-strain curves. For each sample (cylindrical piece), the stress-strain curve was measured by repeating the operation of compressing it to 50% displacement and immediately releasing it 10 times.

[0073] Figure 4 shows SEM images of samples No. 1 to 8. In both samples No. 1 and No. 2, the honeycomb structure was significantly disrupted, resulting in the formation of microtubules with uneven diameters. This is thought to be because the CNF concentration was too low, resulting in insufficient wall thickness of the microtubules and preventing proper honeycomb formation. Note that the aperture diameter of sample No. 1 could not be measured due to the poor shape of the honeycomb structure.

[0074] Furthermore, a good honeycomb structure was not obtained in sample No. 7 (pH=4.0, transmittance 17%). On the other hand, a good honeycomb structure was obtained in sample No. 3 (pH=7.3, transmittance 43%) and sample No. 8 (pH=11.7, transmittance 53%), which were at the same concentration as sample No. 7. It is thought that in sample No. 7, the transmittance decreased due to the aggregation of cellulose nanofibers, and furthermore, this aggregate hindered ice crystal growth, preventing the formation of a good honeycomb structure.

[0075] Furthermore, as is evident from the Young's modulus of samples No. 3-5, it was found that increasing the CNF concentration in the dispersion resulted in a harder honeycomb structure. [Explanation of Symbols]

[0076] 1. Cellulose nanofiber dispersion (dispersion) 1A ··· Cylindrical piece (dispersion solidified body) 2 ... container 3. Refrigerant 4. Airtight container 5 ... Refrigerant tank 6. Decompression methods 7. Pressure gauge

Claims

1. A honeycomb structure which is an aggregate of multiple microtubules made of oxidized cellulose nanofibers, The opening shape of the plurality of microtubules is polygonal, If the average of the longest diameter of each of the plurality of microtubules is defined as the average major diameter μ2 (μm) and the average of the shortest diameters is defined as the average minor diameter μ1 (μm), then the ratio μ2 / μ1, which is the ratio of the average major diameter μ2 (μm) to the average minor diameter μ1 (μm), is 2.0 or less. A honeycomb structure in which the standard deviations σ1 and σ2 of the frequency distributions of the short and long axes of the opening shapes in the plurality of microtubules are both 20.0 μm or less.

2. The honeycomb structure according to claim 1, wherein the average opening diameter of the plurality of microtubules is 0.1 μm to 200 μm.

3. The honeycomb structure according to claim 1 or 2, wherein the Young's modulus is 0.1 kPa to 10,000 kPa.

4. A method for manufacturing a honeycomb structure of cellulose nanofibers, A freezing step involves gradually inserting a container containing a dispersion in which the cellulose nanofibers are dispersed in a polar solvent into a refrigerant, thereby causing the solvent to freeze and grow vertically upward from the refrigerant surface. A drying step to remove the solvent from the frozen dispersion, It has, The cellulose nanofibers mentioned above are oxidized by TEMPO. The concentration of the cellulose nanofibers in the dispersion is 1.0 to 5.5 wt%, The pH of the dispersion is 7.0 to 13.

9. A method for producing a honeycomb structure, wherein the light transmittance of the dispersion at a wavelength of 550 nm is 35 to 60% when the concentration of the cellulose nanofiber is 1.0 wt%.

5. The method for producing a honeycomb structure according to claim 4, wherein the average fiber length of the cellulose nanofibers is 0.15 μm or more.

6. The aforementioned drying process, A first drying step involves holding the dispersion after the freezing step at 0°C or below and under reduced pressure for a certain period of time. A second drying step involves removing the solvent by holding the frozen dispersion at room temperature and under reduced pressure. A method for manufacturing a honeycomb structure according to claim 4 or 5, comprising the above.

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