Honeycomb structure and method for manufacturing the same

Abstract

The objective is to provide a novel honeycomb structure and a method for manufacturing the same, which can improve upon the functionality of conventional honeycomb structures containing CNF and provide new functionalities. [Solution] A honeycomb structure made of a composite material containing cellulose nanofibers and polyvinyl alcohol.

Description

[Technical Field]

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

[0002] A honeycomb is a structure in which columnar pores penetrate in the same direction, and is found in beehives and plant stems. Characteristics of this honeycomb structure include the ability to construct a robust structure with minimal material and low fluid pressure loss.

[0003] If we can construct a honeycomb structure with these characteristics by freely controlling the type of material, wall thickness, channel shape, and channel hole size, the range of applications for this material will greatly expand. For example, honeycomb structures are expected to be used as efficient exhaust gas filters with low pressure loss, as carriers for supporting catalysts to enable efficient reactions, and as robust masks for use in living organisms.

[0004] Previously, the applicant has reported that microhoneycomb structures can be synthesized from cellulose nanofibers (hereinafter sometimes abbreviated as "CNF") using a unidirectional freezing method (UDF method). For example, Patent Documents 1 and 2 disclose a honeycomb structure made of CNF and report that this honeycomb structure can be suitably used as a filter for masks and air purifiers, a catalyst carrier, an adsorbent, and the like. Furthermore, Patent Document 3 discloses a honeycomb structure comprising CNF and a carbon material having a specific structure, and reports that this honeycomb structure exhibits anisotropic electromagnetic wave absorption performance. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2012-167152 [Patent Document 2] International Publication No. 2023 / 067749 [Patent Document 3] International Publication No. 2023 / 190596 [Non-patent literature]

[0006] [Non-Patent Document 1] Nano Res. 2023, vol. 16, pp. 8018-8024 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, as sustainable development is increasingly demanded in various aspects of industrial society in recent years, there is a growing need for further improvements in the functionality of honeycomb structures manufactured using CNF, as well as the addition of new and unprecedented functions.

[0008] This invention has been made in view of the above circumstances, and aims to provide a novel honeycomb structure and a method for manufacturing the same that can improve the functionality of conventional honeycomb structures containing CNF and impart new functions. [Means for solving the problem]

[0009] As a result of diligent research to achieve the above objective, the inventors of the present invention have found that a honeycomb structure constructed from a material containing CNF and polyvinyl alcohol can be expected to have improved functionality and provide novel functions compared to conventional honeycomb structures containing CNF, and have completed the present invention.

[0010] In other words, the present invention provides the following honeycomb structure and a method for manufacturing the same. 1. A honeycomb structure made of a composite material containing cellulose nanofibers and polyvinyl alcohol. 2. The honeycomb structure according to 1, which has a napped structure formed by napping the surface of the inner walls constituting the honeycomb structure. 3. The honeycomb structure according to 1 or 2, wherein the polyvinyl alcohol is a vinyl alcohol-based polymer having vinyl alcohol units and vinyl acetate units. 4. The honeycomb structure according to any one of 1 to 3, wherein the cellulose nanofibers are cellulose nanofibers subjected to chemical defibrillation treatment. 5. The honeycomb structure according to any one of 1 to 4, wherein the content of polyvinyl alcohol in the composite material is 0.1 to 3 times by mass ratio relative to 1 part of cellulose nanofibers. 6. The method for producing a honeycomb structure according to any one of 1 to 5, comprising the steps of: gradually submerging a container holding a dispersion liquid in which cellulose nanofibers and polyvinyl alcohol are dispersed in a solvent containing water into a coolant to perform unidirectional freezing, then freeze-drying to remove the solvent. 7. The method for producing a honeycomb structure according to 6, wherein the cellulose nanofibers are cellulose nanofibers subjected to chemical defibrillation treatment. 8. The method for producing a honeycomb structure according to 6 or 7, wherein the polyvinyl alcohol is a vinyl alcohol-based polymer having vinyl alcohol units and vinyl acetate units.

Effects of the Invention

[0011] The present invention provides a novel honeycomb structure made of a composite material containing polyvinyl alcohol in addition to CNF. The honeycomb structure of the present invention allows for adjustment of the filter function without significantly changing the size of the pores (microtubule openings) of the honeycomb structure or the pressure loss during fluid permeation. Furthermore, by providing the honeycomb structure of the present invention with a piled structure on the inner wall constituting the honeycomb structure, it is possible to further improve the function of capturing fine particles from the fluid passing through the honeycomb structure. The honeycomb structure of the present invention, having these characteristics, can be suitably used as a filter material for various applications, including filters for masks and air purifiers. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of the device for measuring collection efficiency (2). [Figure 2] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 1 (0.5C0.5PV(17-99-TCI)). [Figure 3] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Comparative Example 1 (1.0C (10cm·h-1)). [Figure 4] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-1 (0.5C0.5PV(05-99)). [Figure 5] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-2 (0.5C0.5PV(05-98)). [Figure 6] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-3 (0.5C0.5PV(05-88)). [Figure 7] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-4 (0.5C0.5PV(10-98)). [Figure 8] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-5 (0.5C0.5PV(17-99)). [Figure 9]This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-6 (0.5C0.5PV(17-98)). [Figure 10] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-7 (0.5C0.5PV(17-97)). [Figure 11] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-8 (0.5C0.5PV(18-88)). [Figure 12] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-9 (0.5C0.5PV(25-98)). [Figure 13] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-10 (0.5C0.5PV(50-99)). [Figure 14] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 2-11 (0.5C0.5PV(50-89)). [Figure 15] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 3-1 (0.5C0.25PV(25-98)). [Figure 16] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 3-2 (0.5C0.75PV(25-98)). [Figure 17] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 3-3 (0.25C0.25PV(25-98)). [Figure 18] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 3-4 (0.5C0.5PV(25-98)(30cm·h-1)). [Figure 19] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Example 3-5 (0.5C0.5PV(25-98)(-30℃, 1cm·h-1)). [Figure 20]These are SEM images showing the appearance of the microtubule openings in the honeycomb structure of Example 3-6 (0.5C(long)0.5PV(25-98)), where (a) is the result of observation at a magnification of 500x and (b) is the result of observation at a magnification of 1000x. [Figure 21] This is an SEM image showing the appearance of the microtubule openings in the honeycomb structure of Comparative Example 2 (1.0C (30cm·h-1)). [Figure 22] This is an SEM image showing the appearance of the microtubule openings in the structure of Comparative Example 3 (0.1C0.1PV(25-98)). [Figure 23] This figure shows the measurement results of the pressure loss. [Figure 24] This figure shows the observation results of the cross-section of the microtubule openings in the honeycomb structure of Example 2-5 where micropearls were collected. (a) shows the state of the microtubule cross-section (opening), and (b) shows the state of the microtubule cross-section parallel to the longitudinal direction. [Figure 25] This figure shows the relationship between the pressure loss of the honeycomb structure in Examples 2-2, 2-5, 2-6, 2-7, 2-9 and Comparative Examples 1 and 2, and the collection efficiency in the particle size range of 0.5 to 1.0 μm. [Modes for carrying out the invention]

[0013] [Honeycomb structure] The honeycomb structure of the present invention is made of a composite material containing cellulose nanofibers (CNF) and polyvinyl alcohol (hereinafter sometimes abbreviated as "PVAL").

[0014] Generally, cellulose is classified into natural cellulose, regenerated cellulose, fine cellulose, and microcrystalline cellulose excluding the amorphous region, but any of these types of cellulose may be used as the raw material for CNF in this invention. The above-mentioned CNF is obtained by defibrating the above-mentioned raw material cellulose to the nanoscale. The average fiber diameter is not particularly limited as long as it is at the nanoscale, but since it affects the thickness of the honeycomb wall, it is preferably 1 to 500 nm, more preferably 1 to 100 nm, even more preferably 1 to 50 nm or less, and still more preferably 2 to 30 nm or less. Furthermore, while the average fiber length of CNF is not particularly limited, from the viewpoint of stably forming a honeycomb structure, it is preferably 50 nm to 100 μm, more preferably 0.1 to 10 μm, even more preferably 0.15 to 5 μm, and still more preferably 0.3 to 2 μm. Furthermore, the aspect ratio of CNF, expressed as the average fiber length / average fiber diameter, is typically between 50 and 1,000. The average fiber diameter and average fiber length of CNF are determined by averaging the fiber diameter and fiber length obtained from observing each fiber using an atomic force microscope (AFM).

[0015] The defibration process for breaking down raw cellulose to the nanoscale is broadly classified into mechanical defibration and chemical defibration, and the CNF used in this invention may be defibrated by either process. Specific examples of mechanical defibration treatments include high-pressure homogenizers, microfluidizers, grinders, ball mills, and bead mills. Specific examples of chemical defibrillation treatments include TEMPO(2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation, phosphate esterification, phosphite esterification, carboxymethylation, zandate, and sulfonation. Among these, CNF that has undergone chemical treatment, which involves introducing charged functional groups and obtaining a highly dispersed liquid through molecular chain repulsion, is preferred in the present invention because it yields fibers with a smaller fiber diameter and excellent dispersibility. TEMPO-oxidized CNF, in which carboxyl groups are introduced by TEMPO oxidation, is even more preferred.

[0016] The CNF used in this invention may be one produced by the known defibration treatment described above, or it may be a commercially available product. Examples of such commercially available products include TEMPO-oxidized CNF (Reocrysta I-2SX, C-2SP, C-2EP, C-25N, I-2AX, I-2SXS) manufactured by Daiichi Kogyo Seiyaku Co., Ltd.

[0017] The PVAL constituting the honeycomb structure of the present invention is preferably a vinyl alcohol-based polymer having vinyl alcohol units and vinyl acetate units. The PVAL constituting the honeycomb structure of the present invention is derived from the PVAL raw material used in the production of the honeycomb structure, as described later.

[0018] In the above composite material, the PVAL content is not particularly limited, but considering the balance between ensuring the mechanical strength of the honeycomb structure and the dispersibility of CNF in the dispersion used in the honeycomb structure manufacturing method described later, a mass ratio of 0.1 to 3 times CNF1 is preferred, more preferably 0.1 to 2 times, even more preferably 0.2 to 1.8 times, and particularly preferred 0.5 to 1.5 times. In order to have the upright pile structure described later, a mass ratio of 0.5 to 2 times CNF1 is preferred, more preferably 0.5 to 1.8 times, and even more preferably 0.5 to 1.5 times.

[0019] Furthermore, the composite material constituting the honeycomb structure of the present invention may consist only of CNF and PVAL, without including any materials other than CNF and PVAL, or it may include materials other than CNF and PVAL (for example, functional materials such as polyurethane, phenolic resin, and carbon materials).

[0020] The honeycomb structure of the present invention is a hollow structure in which microtubules of a three-dimensional shape are arranged without gaps. Examples of three-dimensional shapes of the hollow structure include cylinders, elliptical prisms, and polygonal prisms such as triangular prisms, square prisms, hexagonal prisms, and octagonal prisms, but it is preferable that it mainly contains microtubules of cylinders (prisms with a circular cross-section) or elliptical prisms (prisms with an elliptical cross-section). When mainly containing microtubules of elliptical prisms, multiple elliptical prisms with different opening diameters may be mixed, and further, some polygonal prisms such as cylinders and hexagonal prisms may also be included. Note that "mainly contains" means that the area ratio of the microtubule openings of that three-dimensional shape (for example, those that are circular or elliptical) in the cross-section of the honeycomb structure is 70% or more.

[0021] In the present invention, the opening diameter of the microtubules constituting the honeycomb structure (hereinafter also referred to as "microtubule opening diameter") is preferably 0.2 to 1000 μm, more preferably 1.0 to 200 μm, and even more preferably 3.0 to 100 μm. In this invention, "microtubule aperture diameter" refers to the average value calculated by measuring the diameter of 50 points in the direction in which the aperture diameter is smallest, using the image analysis software ImageJ, from an image obtained by observation using a scanning electron microscope (SEM).

[0022] Furthermore, in the present invention, when elliptical columns are mainly included as microtubules constituting the honeycomb structure, the elliptical opening diameter of the microtubules of the elliptical column is preferably 0.2 to 1000 μm for the major axis and 0.2 to 1000 μm for the minor axis, more preferably 1 to 500 μm for the major axis and 1 to 200 μm for the minor axis, and even more preferably 3 to 400 μm for the major axis and 3 to 100 μm for the minor axis. In addition, the aspect ratio (major axis / minor axis) of the elliptical opening diameter at this time is preferably greater than 1 and 20 or less, more preferably 1.5 to 10, and even more preferably 2.0 to 5. In this invention, the "elliptical aperture diameter" refers to the average value calculated from 50 apertures, each measured using the ImageJ image analysis software, based on images obtained by observation with a scanning electron microscope (SEM), where the largest diameter of each aperture is the major axis and the smallest diameter is the minor axis.

[0023] Furthermore, the wall thickness constituting the honeycomb structure is preferably 0.5 μm or more, more preferably 0.5 to 5.0 μm, and even more preferably 0.5 to 2.5 μm, in order to increase mechanical strength. The wall thickness is the average value of five thickness measurements taken for a single wall observed using a scanning electron microscope (SEM) or transmission electron microscope (TEM).

[0024] The length of the honeycomb structure is not particularly limited, but considering the ease of manufacturing the honeycomb structure and its use as a filter material, it is preferably 100 μm to 20 cm, more preferably 150 μm to 10 cm, even more preferably 200 μm to 10 mm, and still more preferably 500 μm to 5 mm.

[0025] Furthermore, in the present invention, it is preferable that the inner wall constituting the honeycomb structure has a piled structure in which the surface is made upright.

[0026] The hairs constituting the pilas structure are thread-like with ends, or fibrous, irregularly connected threads, and their thickness (diameter) and length are not particularly limited, as long as they do not completely block the microtubule openings of the honeycomb structure. The thickness (diameter) of the hairs (threads or fibers) constituting the pilas structure is preferably 0.1 to 5 μm, more preferably 0.2 to 2 μm, and even more preferably 0.5 to 2 μm. The length of the hairs (threads or fibers) constituting the pilas structure is preferably 0.1 to 500 μm, more preferably 0.5 to 100 μm, and even more preferably 1 to 20 μm. The thickness (diameter) of the hairs (threads or fibers) that make up the pile structure is the average value obtained by loading an SEM image of the honeycomb cross-section into ImageJ, measuring the thickness of 50 piles that can be observed in each pore, and then calculating the average value. The length is the average value obtained by loading an SEM image of the honeycomb cross-section into ImageJ, measuring the length of 50 piles that can be observed in each pore, and then calculating the average value.

[0027] The orientation of the bristles in the bristles structure is not particularly limited. As long as they obstruct the passage of fluids such as gas at the openings of the microtubules constituting the honeycomb structure, the bristles constituting the bristles may be oriented in random directions on the inner wall of the honeycomb structure, or they may be oriented in a specific direction.

[0028] The bristles structure may be formed on any portion of the inner wall of the microtubules constituting the honeycomb structure, preferably at a specific portion of the cross-sectional shape perpendicular to the longitudinal direction of the microtubule, with the bristles being continuously formed in the longitudinal direction. For example, if the microtubule is an elliptical cylinder, it is preferable to have a bristles structure on any portion of the circumference of the ellipse cross-section (for example, all or part of the corner on the side with a gentler curvature), with the bristles being continuous or discontinuous on the inner wall in the longitudinal direction (i.e., inside the microtubule).

[0029] The hairs constituting the bristles are preferably densely packed to exhibit a filter function for capturing fine particles. The density of these bristles may vary depending on the size of the fine particles to be captured. The density of the bristles is 10 μm. 2 Preferably, there are 1 to 100 pieces per unit, more preferably 5 to 100 pieces, and even more preferably 20 to 100 pieces.

[0030] The hairs that make up the upright structure are made of a composite material containing the same CNF and PVAL as the material that makes up the honeycomb structure. Furthermore, the distribution of CNF and PVAL content in the honeycomb structure of the present invention does not necessarily have to be uniform. For example, if the honeycomb structure has a piled structure formed by raising the surface of the inner wall constituting the honeycomb structure, the distribution may be such that a large amount of CNF is contained in the wall portion of the honeycomb structure and a large amount of PVAL is contained in the piled structure portion (or CNF is hardly or not contained at all).

[0031] The mechanical properties of the honeycomb structure of the present invention are not particularly limited, but it is sufficient if it has mechanical properties comparable to those of conventional honeycomb structures including CNF.

[0032] [Method for manufacturing honeycomb structures] The method for producing the honeycomb structure of the present invention described above includes the step of gradually submerging a container containing a dispersion of CNF and PVAL in a solvent containing water into a refrigerant to perform unidirectional freezing, and then freeze-drying to remove the solvent. Here, "unidirectional freezing" refers to a method of freezing a dispersion of CNF and PVAL in one direction. By freezing the dispersion containing CNF and PVAL in a directional manner, the solvent grows in one direction, for example, vertically upward in a columnar shape, forming multiple ice columns. CNF and PVAL are then aggregated in the gaps between the ice columns, and a honeycomb structure composed of an aggregate of microtubules made of a composite material containing CNF and PVAL is produced.

[0033] In this invention, a solvent containing water is used as the solvent for preparing the dispersion, but it is preferable to use water alone. Any solvent that is compatible with water to homogeneously disperse or dissolve CNF and has a freezing point can be used in combination with water. Examples of such solvents include polar organic solvents such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, tert-butyl alcohol, acetic acid, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, acetonitrile, and dioxane. These solvents may be used alone or in combination of two or more.

[0034] The manufacturing method of the present invention uses the above-mentioned CNF.

[0035] The PVAL used in the production method of the present invention is obtained by saponifying all or part of polyvinyl esters obtained by polymerizing vinyl esters, and contains 50 mol% or more of vinyl alcohol units in its molecule (saponification degree of 50 mol% or more).

[0036] The degree of saponification of PVAL is preferably 65 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 88 mol% or more, taking into consideration the prevention of shape defects of the microtubules constituting the honeycomb structure. Taking into consideration the formation of a pile structure, it is preferably 88 mol% or more, more preferably 90 mol% or more, even more preferably 95 mol% or more, and particularly preferably 98 mol% or more. There is no particular upper limit on the degree of saponification of PVAL. The degree of saponification of PVAL is determined according to the saponification degree measurement method of JIS K6726.

[0037] The average degree of polymerization of PVAL is preferably 100 or higher, more preferably 200 or higher, even more preferably 300 or higher, particularly preferably 500 or higher, and also preferably 8000 or lower, and more preferably 5000 or lower, taking into consideration the prevention of shape defects of the microtubules constituting the honeycomb structure. Taking into consideration the formation of a pile structure, it is preferably 500 or higher, more preferably 1000 or higher, even more preferably 1700 or higher, and also preferably 8000 or lower, and more preferably 5000 or lower. The average degree of polymerization of PVAL is determined according to JIS K6726.

[0038] The PVAL used in this invention may be, for example, a saponified polyvinyl acetate obtained by polymerizing vinyl acetate monomer, but a commercially available product may also be used. Examples of such commercially available products include poly(vinyl alcohol) manufactured by Tokyo Chemical Industry Co., Ltd. (P0469, P0804, etc.), and polyvinyl alcohol manufactured by Nippon Vinegar Vinegar Co., Ltd. (JC-05, JC-17, JC-50, JF-05, JF-10, JF-17, JF-25, etc., JF-series, JM-17, etc., JM-series, JP-05, JP-18, JP-50, etc.).

[0039] Examples of vinyl esters used in the production of the above-mentioned PVAL include fatty acid vinyl esters. While not particularly limited, examples of fatty acid vinyl esters include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl pivalate, with vinyl acetate being industrially preferred. Vinyl esters may be used individually or in combination of two or more.

[0040] Vinyl esters can be produced by various polymerization methods, including conventionally known bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization, but among these, solution polymerization using an alcohol solvent such as methanol is industrially preferred.

[0041] When polymerizing vinyl esters, other copolymerizable unsaturated monomers may be used, provided they do not hinder the effects of the present invention. Other unsaturated monomers include, for example, carboxyl group-containing unsaturated monomers [e.g., (meth)acrylic acid, maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, undecylenic acid, etc.], unsaturated dibasic acid monoalkyl esters (e.g., monomethyl maleate, monomethyl itaconic acid, etc.), and amide group-containing unsaturated monomers (e.g., acrylamide, dimethylacrylamide, dimethylaminoethylacrylamide, diethylacrylamide, dimethylaminopropylacrylamide, isopropylacrylamide, N-methylolacrylamide) (e.g., N-vinylformamide, N-vinylacetamide), vinyl halides (e.g., vinyl chloride, vinyl fluoride), unsaturated monomers having a glycidyl group (e.g., allylglycidyl ether, glycidyl methacrylate), lactam group-containing unsaturated monomers {e.g., N-vinylpyrrolidones [e.g., N-vinyl-2-pyrrolidone, N-vinyl-alkylpyrrolidone (e.g., N-vinyl-3-propyl-2-pyrrolidone, N-vinyl-5-methyl-2-pyrrolidone, N-vinyl-5-ethyl-2-pyrrolidone, N-vinyl-5,5-di [Methyl-2-pyrrolidone, N-vinyl-3,5-dimethyl-2-pyrrolidone, and other N-vinyl-mono or diC1-4 alkylpyrrolidones], N-allylpyrrolidones (e.g., N-allyl-2-pyrrolidone), N-vinylpiperidones [e.g., N-vinyl-2-piperidone, N-vinyl-alkylpiperidone (e.g., N-vinyl-6-methyl-2-piperidone, N-vinyl-6-ethyl-2-piperidone, and other N-vinyl-mono or diC1-4 alkylpiperidones)], N-vinylcaprolactams [e.g., N-vinyl-ε-caprolactones] Prolactams, N-vinyl-alkylcaprolactams (e.g., N-vinyl-7-methyl-2-caprolactam, N-vinyl-7-ethyl-2-caprolactam, and other N-vinyl-mono or diC1-4 alkylcaprolactams), alkyl vinyl ethers (e.g., C1-20 alkyl vinyl ethers (e.g., methyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, lauryl vinyl ether, dodecyl vinyl ether,(e.g., stearyl vinyl ether), nitriles (e.g., acrylonitrile, methacrylonitrile), hydroxyl group-containing unsaturated monomers (e.g., C1-20 monoalkylallyl alcohols (e.g., allyl alcohol, isopropenylallyl alcohol), C1-20 dialkylallyl alcohols (e.g., dimethylallyl alcohol), hydroxy C1-20 alkyl vinyl ethers (e.g., hydroxyethyl vinyl ether, hydroxybutyl vinyl ether), acetyl group-containing unsaturated monomers (e.g., C1-20 alkylallyl acetate) (For example, allyl acetate, dimethylallyl acetate, isopropenylallyl acetate, etc.), (meth)acrylic acid esters {for example, alkyl (meth)acrylates [for example, methyl (meth)acrylate, ethyl (meth)acrylate, 2-ethylhexyl acrylate, n-butyl acrylate, etc. C1-20 alkyl (meth)acrylates]}, vinylsilanes (for example, trimethoxyvinylsilane, tributylvinylsilane, diphenylmethylvinylsilane, etc.), polyoxyalkylene (meth)acrylates [for example, polyoxy [Ethylene (meth)acrylate, polyoxypropylene (meth)acrylate, etc.], polyoxyalkylene (meth)acrylamides [e.g., polyoxyethylene (meth)acrylamide, polyoxypropylene (meth)acrylamide, etc.], polyoxyalkylene vinyl ethers (e.g., polyoxyethylene vinyl ether, polyoxypropylene vinyl ether, etc.), polyoxyalkylene alkyl vinyl ethers (e.g., polyoxyethylene allyl ether, polyoxypropylene allyl ether, polyoxyethylene butylvinyl (e.g., polyoxypropylene butyl vinyl ether, etc.), α-olefins (e.g., ethylene, propylene, n-butene, 1-hexene, etc.), butenes (e.g., 3,4-dihydroxy-1-butene, 3,4-diasiloxy-1-butene, 3-acyloxy-4-hydroxy-1-butene, 4-acyloxy-3-hydroxy-1-butene, 3,4-diasiloxy-2-methyl-1-butene, etc.), pentenes (e.g., 4,5-dihydroxy-1-pentene, 4,5-diasiloxy-1-pentene, 4,5-dihydroxy-3-methyl-1-pentene,Unsaturated monomers containing quaternary ammonium compounds (e.g., 4,5-diasiloxy-3-methyl-1-pentene), hexenes (e.g., 5,6-dihydroxy-1-hexene, 5,6-diasiloxy-1-hexene), amine unsaturated monomers [e.g., N,N-dimethylallylamine, N-allyl puperazine, 3-piperidine ethyl acrylate, 2-vinylpyridine, 4-vinylpyridine, 2-methyl-6-vinylpyridine, 5-ethyl-2-vinylpyridine, 5-butenylpyridine, 4-pentenylpyridine, 2-(4-pyridyl)allyl alcohol, etc.], etc. Monomers (e.g., dimethylaminoethyl acrylate methyl chloride quaternary salt, N,N-dimethylaminopropyl acrylamide methyl chloride quaternary salt, N,N-dimethylaminopropyl acrylamide methylbenzenesulfonic acid quaternary salt, etc.), aromatic unsaturated monomers (e.g., styrene, etc.), unsaturated monomers containing sulfonic acid groups (e.g., 2-acrylamido-2-methylpropanesulfonic acid or its alkali metal salt, ammonium salt or organic amine salt; 2-acrylamido-1-methylpropanesulfonic acid or its alkali metal salt, ammonium salt or organic amine salt) Organic amine salts; 2-methacrylamide-2-methylpropanesulfonic acid or its alkali metal salt, ammonium salt or organic amine salt; vinylsulfonic acid or its alkali metal salt, ammonium salt or organic amine salt; allylsulfonic acid or its alkali metal salt, ammonium salt or organic amine salt; methallylsulfonic acid or its alkali metal salt, ammonium salt or organic amine salt, etc.), diacetone acrylamide, diacetone methacrylamide, diacetone acrylate, diacetone methacrylate, acetoacetoxyacrylamide, acetone Toacetoxymethacrylamide, glycerin monoallyl ether, 1,3-diacetoxy-2-methylenepropane, 2,3-diacetoxy-1-allyloxypropane, 2-acetoxy-1-allyloxy-3-hydroxypropane, 3-acetoxy-1-allyloxy-3-hydroxypropane, 3-acetoxy-1-allyloxy-2-hydroxypropane, 3,4-diacetoxy-1-butene, glycerin monovinyl ether, glycerin monoisopropenyl ether, acryloylmorpholine, vinyl ethylene carbonate, vinylimidazole,Examples include one or more selected from vinylcarbazole, etc.

[0042] There are no specific regulations regarding the content of other unsaturated monomers, but for example, it is acceptable if they are 20 moles or less, 15 moles or less, or 10 moles or less per 100 moles of vinyl esters.

[0043] Furthermore, the obtained PVAL may be post-modified by known methods such as acetalization, urethaneization, etherization, grafting, phosphate esterification, sulfonation, acetoacetylation, cationization, amination, and hydrazide formation, as long as these methods do not impede the effects of the present invention.

[0044] When polymerizing vinyl esters, a polymerization initiator may be used. The polymerization initiator may be, for example, a radical polymerization initiator. Radical polymerization initiators may be azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile, organic peroxides such as benzoyl peroxide and dicumyl peroxide, inorganic peroxides such as potassium persulfate, or redox polymerization initiators such as cerium(IV) salt-alcohol systems.

[0045] The saponification method for polyvinyl esters is not particularly limited and may follow conventionally known methods. For example, conventionally known basic catalysts such as sodium hydroxide, potassium hydroxide, or sodium methoxide, or acidic catalysts such as hydrochloric acid, sulfuric acid, or p-toluenesulfonic acid, can be used for alcohol decomposition or hydrolysis reactions. Solvents used in saponification reactions include alcohols such as methanol and ethanol; esters such as methyl acetate; ketones such as acetone and methyl ethyl ketone; aromatic hydrocarbons such as benzene and toluene; and tetrahydrofuran. These can be used individually or in combination of two or more. There are no particular restrictions on the saponification temperature, time, etc. Furthermore, there are no particular restrictions on the drying, grinding, and washing methods of the saponified product; known methods may be used.

[0046] In the present invention, the concentration of CNF in the dispersion is preferably 0.2% by mass or more, more preferably 0.25% by mass or more, even more preferably 0.3% by mass or more, and particularly preferably 0.5% by mass or more, considering the need to prevent defects in the shape of microtubules constituting the honeycomb structure and to increase the strength of the honeycomb structure. The upper limit of the CNF concentration is not particularly limited, but if it is too high, the viscosity of the dispersion will increase and it will become gel-like, so it is preferably 5.5% by mass or less, and more preferably 4.0% by mass or less.

[0047] Furthermore, considering the prevention of shape defects in the microtubules constituting the honeycomb structure, the PVAL content in the dispersion is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.25% by mass or more, and particularly preferably 0.5% by mass or more. Considering the formation of a pile structure, it is preferably 0.2% by mass or more, more preferably 0.25% by mass or more, and even more preferably 0.5% by mass or more. The upper limit of the PVAL content is not particularly limited, but from the viewpoint of preparing a stable dispersion, it is preferably 10.0% by mass or less, and more preferably 8.0% by mass or less.

[0048] There are no particular restrictions on the refrigerant used for unidirectional freezing, as long as it is a liquid (antifreeze) that does not freeze even at the temperature at which the solvent of the dispersion can freeze. 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 at which the dispersion can freeze) can also be used as a refrigerant.

[0049] As a means of cooling the refrigerant, a chilling agent and / or a cooling device may be used. Specific 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. Specific examples of cooling devices include immersion coolers. The cooling device may be in direct contact with the refrigerant.

[0050] Suitable materials for containers holding dispersions include, for example, polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC), with polypropylene being preferred. 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, using a polypropylene container allows the frozen surface of the dispersion to be kept more parallel to the surface of the refrigerant (such as liquid nitrogen). Furthermore, the shape and size of the container can be appropriately determined according to the intended use of the honeycomb structure, but it is preferable that it has a predetermined height so that microtubules with length in the axial direction that constitute the honeycomb structure can be formed.

[0051] The temperature of the dispersion should be higher than the freezing point of the solvent in the dispersion, and lower than the boiling point of the solvent. For example, if water is used as the solvent, the temperature of the dispersion should be higher than the freezing point of water (0°C) and lower than the boiling point of water (100°C). Furthermore, the temperature of the refrigerant is not particularly limited as long as it is at a temperature that can freeze the solvent in the dispersion. For example, if water is used as the solvent in the dispersion, the temperature of the refrigerant should be lower than the freezing point of water, which is 0°C. For example, -30°C or lower is preferred, and a low temperature of around the boiling point of liquid nitrogen (-196°C) is more preferred.

[0052] In the unidirectional freezing method used in this invention, a container containing a dispersion of CNF and PVAL is gradually immersed in a refrigerant. There are no particular restrictions on the immersion speed (insertion speed), which can be, for example, 1 to 110 cm / h. Preferably, it is 3 to 40 cm / h. If the insertion speed is too slow, it is unsuitable in terms of productivity, while if the insertion speed is too fast, problems such as impaired linearity of the microtubules constituting the honeycomb structure may occur. It is preferable to maintain a constant insertion speed.

[0053] After unidirectional freezing, the dispersion is removed from the refrigerant along with the container, and the solidified solvent is removed by freeze-drying. Specifically, the solidified dispersion formed 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. The cut cylindrical piece is then 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 container is dried. As a result, the solvent in the solidified dispersion is removed, leaving only a honeycomb-like structure composed of aggregates of microtubules. The vacuum conditions for freeze-drying are determined appropriately depending on the solvent used. For example, when water is used as the solvent, a vacuum atmosphere of -30 to 50°C and 1 to 1000 Pa can be used. The holding time under vacuum is also determined appropriately depending on the solvent and vacuum atmosphere used, but can be, for example, 20 to 100 hours.

[0054] The water resistance of the honeycomb structure can be improved by heating the structure after freeze-drying. In this case, the heating temperature is not particularly limited as long as it does not cause the honeycomb structure to decompose, but it is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 150°C or higher. The heating temperature may be increased in stages; for example, a method such as treating at 150°C for a predetermined time and then treating at 200°C for a predetermined time may be employed. The heating time can be, for example, from 1 minute to about 10 hours. The heat treatment may be carried out under atmospheric pressure or under reduced pressure, but it is preferable to carry it out under reduced pressure. [Examples]

[0055] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples. The room temperature is 23°C.

[0056] The reagents used in the following examples are as follows: (1) CNF (RHEOCRYSTA): • 2.0% by mass Rheocrysta I-2SXS Preservative-free (TEMPO-oxidized CNF, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., Lot No. 230515, pH: 6.8, Transparency: 92%, Viscosity: 12000 mPa·s) • 2.4% by mass Rheocrysta I-2SXS Preservative-free (TEMPO-oxidized CNF, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., Lot No. 240708, pH: 7.5, Transparency: 89%, Viscosity: 14040 mPa·s) • 2.0% by mass Rheocrysta I-2SX Preservative-free (Longer CNF than I-2SXS, TEMPO-oxidized CNF, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., Lot No. 240926, pH: 7.0, Transparency: 70%, Viscosity: 62720 mPa·s) (2) PVAL: Hereinafter, the identification code for polyvinyl alcohol will be written as "PV(XX-YY)" (XX: average degree of polymerization × 0.01, YY: degree of saponification (2 significant figures, mol%)). • Poly(vinyl alcohol) P0469 (manufactured by Tokyo Chemical Industry Co., Ltd., Lot: 2WE3B-TK, average degree of polymerization: approximately 1700) (labeled as "PV(17-99-TCI)") • Poval JC-05 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 500, degree of saponification: 99.6 mol%, Lot: manufacturer-supplied product) ("PV(05-99)") • Poval JF-05 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 500, degree of saponification: 98.4 mol%, Lot: 2 x 127) ("PV(05-98)") • Poval JP-05 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 500, degree of saponification: 88.3 mol%, Lot: manufacturer-supplied product) ("PV(05-88)") • Poval JF-10 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 1000, degree of saponification: 98.3 mol%, Lot: manufacturer-supplied product) ("PV(10-98)") • Poval JC-17 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 1700, degree of saponification: 99.2 mol%, Lot: 9Z310) ("PV(17-99)") • Poval JF-17 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 1700, degree of saponification: 98.2 mol%, Lot: 22317) ("PV(17-98)") • Poval JM-17 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 1700, degree of saponification: 96.8 mol%, Lot: 26301) ("PV(17-97)") • Poval JP-18 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 1800, degree of saponification: 88.2 mol%, Lot: manufacturer-supplied product) ("PV(18-88)") • Poval JF-25 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 2500, degree of saponification: 98.6 mol%, Lot: 03316) ("PV(25-98)") • Poval JC-50 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 5000, degree of saponification: 99.6 mol%, Lot: manufacturer-supplied product) ("PV(50-99)") • Poval JP-50 (manufactured by Nippon Vinegar Poval Co., Ltd., average degree of polymerization: 5000, degree of saponification: 89.3 mol%, Lot: manufacturer-supplied product) ("PV(50-89)") (3)Water: ·Milli-Q (pure water)

[0057] The instruments and equipment used in the following examples are as follows: (1) Mixer: ·ARE-250 (manufactured by THINKY CORPORATION) (2) Polypropylene screw-cap test tubes: • 17 mL (Outer diameter: 15 mm, Inner diameter: 13 mm): 1-6403-02 (Manufactured by Maruemu) (3) Dip coater: • Dip coater M200 (manufactured by Asumi Giken Co., Ltd.) or Micro dip MD0408 (manufactured by Asumi Giken Co., Ltd.) (4) Low-temperature constant-temperature water bath: • PSL-2000 (manufactured by Tokyo Rikakikai Co., Ltd.) (5) Freeze drying equipment: • UT-4000L (manufactured by Tokyo Rikakikai Co., Ltd.), Thermal Robo TR-3α (manufactured by AS ONE Corporation) (6) Vacuum oven: ·VACUUM OVEN VOS-201SD (Tokyo Rikakikai Co., Ltd.) (7) FE-SEM: • S-4800 (manufactured by Hitachi High-Tech Corporation) (8) Stress-strain measuring device: • Autograph AGS-X 10 kN (manufactured by Shimadzu Corporation) (9) Sealant: • RTV rubber KE45W (manufactured by Shin-Etsu Chemical Co., Ltd.) (10) Particle counter: • Handheld Particle Counter 8506-30 (manufactured by Particle Plus Co., Ltd.)

[0058] The fabricated honeycomb structure was observed using FE-SEM, and collection efficiency, stress-strain curve, and pressure drop measurements were performed using the following methods. [FE-SEM observation] The honeycomb structure was observed using a Hitachi High-Tech FE-SEM instrument, with the prepared samples or Pt-coated samples fixed to an aluminum sample stage with carbon tape. The thickness of the honeycomb walls was calculated by loading the SEM image of one wall into ImageJ, measuring the thickness at five points, and calculating the average value. The microtubule opening diameter was calculated using ImageJ from the SEM images obtained from the FE-SEM instrument, measuring the length in the direction of the smallest opening diameter 50 times, calculating the average value, and defining this as the microtubule opening diameter. Its standard deviation σ was also calculated and expressed as microtubule opening diameter ± σ. The length of the hairs in the pilaris structure was calculated by loading the SEM image of the honeycomb cross-section into ImageJ, measuring the length of 50 pilaris visible in each pore, calculating the average value, and defining this as the hair length in the pilaris structure. Its standard deviation σ was also calculated and expressed as hair length ± σ.

[0059] [Pressure loss measurement] The measurements were taken using the homemade apparatus shown below. A tube for carrying argon gas was prepared, and a flange that could seal a honeycomb structure was attached to the middle of the flow path. An 8.5 mm hole was drilled in the flange to allow argon gas to flow. To prevent gas from passing through an area larger than the 8.5 mm diameter hole, a sealant (RTV rubber KE45W) was applied to the inner surface around the hole on the gas inlet side of the flange. The honeycomb structure was attached to the hole in the direction that the argon gas would pass through the microtubules, and any gaps were further sealed with the sealant. Areas larger than the diameter of the hole were sealed by the sealant that had been applied beforehand. After being left to harden in the atmosphere for 30 minutes, it was used for measurement. One end was connected to the inlet side of the argon gas line, and the other end was connected to the outlet side. The end of the outlet was open to the atmosphere. While measuring the flow rate of argon gas with a flow meter, the pressure difference between the inlet and outlet of the flange was measured, and the pressure difference (Pa) was divided by the length (mm) of the honeycomb structure to obtain the value (Pa·mm). -1 ) was defined as the pressure loss.

[0060] [Measuring collection efficiency (1)] As a sample, a honeycomb structure prepared by breaking it during freezing was placed on a scale with its microtubule openings facing upwards and its weight was measured. Next, approximately 1 mg of micropearls (SP-210, manufactured by Sekisui Chemical Co., Ltd., 10 μm in diameter, Lot: 2106241, dried) was sprinkled on top and its weight was measured again. Then, a custom-made adapter was attached to the intake port of a particle counter, and the honeycomb structure sprinkled with micropearls was placed on top of it, and a flow rate of 2830 mL·min was applied. -1 The honeycomb structure was suctioned for one minute. The weight of the honeycomb structure after suction was measured, and the difference in weight before and after suction was used as the amount of micropearl reduction to calculate the collection efficiency.

[0061] [Measuring collection efficiency (2)] As shown in Figure 1, to attach the honeycomb to the inside of the flange 15 corresponding to the incense stick side, a sealant (RTV rubber KE45W) was applied to the surface around the hole that the honeycomb would contact. A honeycomb structure 1, which had been prepared by breaking it during freezing as a sample, was then attached to the flange of the sample mounting section 15 with a diameter of 8.5 mm using the sealant, with the orientation such that the incense smoke would pass through the microtubules. The flange with the honeycomb structure 1 attached was connected to the homemade smoke collection device 10 with a tube 14, and the other flange of the sample mounting section 15 was connected to the particle counter 20 with a tube 16. The incense stick 11 was lit and placed in the smoke collection device 10 to fill it with smoke 13. The particle counter 20 was started and the flow rate was set to 2830 mL·min -1 The sample was drawn in, and the number of particles per minute was measured 10 times. After the measurement, the honeycomb structure 1, along with the flange 15, was removed from the flow path, and the number of particles per minute was measured 10 times in the same manner with the incense stick 11 still attached, and these measurements were used as a blank. The ratio of the average number of measured particles to the average number of blank particles measured without the honeycomb structure 1 attached for each predetermined particle size range (average number of measured particles / average number of particles × 100) was defined as the collection efficiency (%) for each particle size range.

[0062] [Example 1] 99.0 g of water was placed in a 300 mL flask, and while stirring the water, PV(17-99-TCI) (1.00 g) was gradually added as PVAL, and the mixture was stirred at room temperature for 30 minutes. A condenser was attached, and the mixture was heated in an oil bath at 90°C for 1 hour with stirring. The mixture was then allowed to return to room temperature with stirring to obtain a 1% by mass PVAL aqueous solution. 5.00g of 2.0% by mass CNF (2.0% by mass Rheocrysta I-2SXS), 10.0g of 1% by mass PVAL aqueous solution, and 5.00g of water were added to a vial, and the mixture was stirred with a mixer (2000 rpm, 5 minutes) and degassed (2000 rpm, 5 minutes) to obtain a dispersion sample. Glass beads (blasting glass beads #60 (soda-lime glass, polishing material); the same applies hereinafter) were added to a polypropylene (hereinafter abbreviated as "PP") screw-cap test tube to a height of 50 mm, and then the prepared dispersion sample was added. The PP screw-cap test tube containing the dispersion sample was left to stand at 5°C for 12 hours to maintain a constant temperature. Dispersion sample in a PP screw-cap test tube is coated with a dip coater at 10 cm·h. -1 (0.028mm·s -1 The sample was submerged in liquid nitrogen at the insertion speed and frozen in one direction. The frozen PP screw-cap test tubes containing the samples were cut in two ways depending on the purpose. The first method involved cutting only the PP tube with a knife and breaking off the frozen portion, which preserved the structure of the cut surface. The second method involved cutting the entire tube with a wire saw, which slightly disrupted the structure of the cut surface but produced a flat surface. The cut samples were slowly dried in a freeze-drying apparatus under conditions of -5°C, 0°C, and 5°C in that order to obtain a honeycomb structure (0.5C0.5PV(17-99-TCI)).

[0063] [Comparative Example 1] 20.0 g of 2.0 mass% CNF (2.0 mass% Rheocrysta I-2SXS) was added to a vial, and stirring (2000 rpm, 5 minutes) and defoaming (2000 rpm, 5 minutes) were performed with a mixer. After adding glass beads to a PP screw-cap test tube to a height of 50 mm, the prepared dispersion sample was added. The PP screw-cap test tube containing the dispersion sample was allowed to stand at 5°C for 12 hours to keep the temperature constant. The PP screw-cap test tube containing the dispersion sample was placed in a dip coater at 10 cm·h -1 (0.028 mm·s -1 ) immersion speed to immerse in liquid nitrogen for unidirectional freezing. After freezing, the PP screw-cap test tube containing the sample was cut by two methods depending on the purpose in the same manner as in Example 1. The cut sample was freeze-dried in the same manner as in Example 1 to obtain a honeycomb structure (1.0C (10 cm·h -1 )).

[0064] Figures 2 and 3 show FE-SEM observation results of the honeycomb structure of the honeycomb structures of Example 1 and Comparative Example 1. An external photograph of the honeycomb structure is also attached in the upper left of the figures (the same applies hereinafter). The honeycomb structure of Comparative Example 1 has a honeycomb structure in which polygonal openings such as substantially hexagonal openings are aggregated (Figure 3), whereas the honeycomb structure of Example 1 has a honeycomb structure in which elliptical openings are aggregated (Figure 2). In Example 1, as microtubule openings, the opening diameter in the minor axis direction is about 20 µm, the opening diameter in the major axis direction is more than about 20 µm and 50 µm or less, and the aspect ratio (major axis / minor axis) is an elliptical shape of more than 1.0 and 2.5 or less, and microtubule openings with relatively aligned major axis directions of the ellipse gathered to form a domain. Furthermore, in Example 1, a napped structure with fine hairs, which was not observed in Comparative Example 1, was observed on the inner wall of the microtubule openings (Figure 2). The fine hairs grew from one side of the gently curved corner of the elliptical microtubule opening, had a thickness of about 1 µm and a length of about 10 µm.

[0065] [Example 2] (Example 2-1) In Example 1, PV(05-99) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(05-99)) was obtained in the same manner as in Example 1. The FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure are shown in Figure 4. The microtubule opening diameter in the honeycomb structure of Example 2-1 was 19.6 ± 6.4 μm, and the length of the hairs in the pilous structure was 7.1 ± 2.1 μm.

[0066] (Example 2-2) In Example 1, PV(05-98) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(05-98)) was obtained in the same manner as in Example 1. Figure 5 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure. The microtubule opening diameter in the honeycomb structure of Example 2-2 was 20.6 ± 7.2 μm, and there was no uprooted structure (not observed).

[0067] (Examples 2-3) In Example 1, PV(05-88) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(05-88)) was obtained in the same manner as in Example 1. Figure 6 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure. The microtubule opening diameter in the honeycomb structure of Examples 2-3 was 13.7 ± 4.0 μm, and there was no uprooted structure (not observed).

[0068] (Examples 2-4) In Example 1, PV(10-98) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(10-98)) was obtained in the same manner as in Example 1. The FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure are shown in Figure 7. The microtubule opening diameter in the honeycomb structure of Examples 2-4 was 18.1 ± 4.9 μm, and the length of the hairs in the pilous structure was 8.0 ± 2.0 μm.

[0069] (Examples 2-5) In Example 1, PV(17-99) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(17-99)) was obtained in the same manner as in Example 1. Figure 8 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure. The microtubule opening diameter in the honeycomb structure of Examples 2-5 was 22.0 ± 6.2 μm, and the length of the hairs in the pilous structure was 11.6 ± 1.4 μm.

[0070] (Examples 2-6) In Example 1, PV(17-98) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(17-98)) was obtained in the same manner as in Example 1. The FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure are shown in Figure 9. The microtubule opening diameter in the honeycomb structure of Examples 2-6 was 21.4 ± 5.3 μm, and the length of the hairs in the pilous structure was 10.0 ± 3.0 μm.

[0071] (Examples 2-7) In Example 1, PV(17-97) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(17-97)) was obtained in the same manner as in Example 1. Figure 10 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure. The microtubule opening diameter in the honeycomb structure of Example 2-7 was 20.5 ± 6.0 μm, and the length of the hairs in the pilous structure was 6.5 ± 1.6 μm.

[0072] (Examples 2-8) In Example 1, PV(18-88) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(18-88)) was obtained in the same manner as in Example 1. The FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure are shown in Figure 11. The microtubule opening diameter in the honeycomb structure of Example 2-8 was 19.3 ± 3.7 μm, and there was no pilosa structure (not observed).

[0073] (Examples 2-9) In Example 1, PV(25-98) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(25-98)) was obtained in the same manner as in Example 1. The FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure are shown in Figure 12. The microtubule opening diameter in the honeycomb structure of Examples 2-9 was 21.9 ± 5.9 μm, and the length of the hairs in the pilous structure was 10.1 ± 2.1 μm.

[0074] (Examples 2-10) In Example 1, a honeycomb structure was fabricated as follows, using PV(50-99) instead of PV(17-99-TCI) as the PVAL. 90.0 g of water was placed in a 200 mL flask, and while stirring the water, 0.910 g of PV(50-99) as PVAL was gradually added and stirred at room temperature for 30 minutes. A condenser was attached and the mixture was heated in an oil bath at 90°C for 1 hour with stirring. After stopping the stirring, the mixture was allowed to return to room temperature to obtain a 1 mass% PV(50-99) aqueous solution. 5.49 g of 2.4% by mass CNF (2.4% by mass Rheocrysta I-2SXS), 13.1 g of 1% by mass PV (50-99) aqueous solution, and 7.66 g of water were added to a vial and warmed in a 60°C water bath for 10 minutes. The warmed mixture was stirred (2000 rpm, 5 minutes) and degassed (2000 rpm, 5 minutes) in a mixer to obtain a dispersion sample. Glass beads (blasting glass beads #60 (soda-lime glass, polishing material)) were added to a PP screw-top test tube to a height of 50 mm, and then the prepared dispersion sample was added. The PP screw-top test tube containing the dispersion sample was left to stand at 5°C for 12 hours to maintain a constant temperature. Dispersion sample in a PP screw-cap test tube is coated with a dip coater at 10 cm / h (0.028 mm·s). -1 The sample was submerged in liquid nitrogen at the insertion speed and frozen in one direction. The PP screw-cap test tubes containing the frozen samples were cut according to the purpose and slowly dried in a freeze-drying apparatus under conditions of -5°C, 0°C, and 5°C in that order to obtain a honeycomb structure (0.5C0.5PV(50-99)). Figure 13 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure. The microtubule opening diameter in the honeycomb structure of Example 2-10 was 14.4 ± 4.1 μm, and the length of the bristles in the bristles structure was 5.2 ± 0.6 μm.

[0075] (Examples 2-11) In Example 1, PV(50-89) was used instead of PV(17-99-TCI) as the PVAL, and the honeycomb structure (0.5C0.5PV(50-89)) was obtained in the same manner as in Example 1. The FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure are shown in Figure 14. The microtubule opening diameter in the honeycomb structure of Example 2-11 was 14.1 ± 5.6 μm, and the length of the hairs in the pilous structure was 5.4 ± 2.0 μm.

[0076] In the honeycomb structures of Examples 2-1 to 2-11, the shape of the microtubule openings differed from that of Comparative Example 1, and they contained many elliptical openings (Figures 4 to 14). Comparing the PVALs used in Examples 2-1 to 2-11, which had an average degree of polymerization equivalent to 500, Example 2-1 (0.5C0.5PV(05-99)), which had a high degree of saponification, showed a slight pile structure (Figure 4). In contrast, no pile structure was observed in Examples 2-2 (0.5C0.5PV(05-98)) and 2-3 (0.5C0.5PV(05-88)), which had a low degree of saponification (Figures 5, 6). Furthermore, when comparing PVALs with different average degrees of polymerization at a saponification degree of 98 mol%, no pile structure was observed in Example 2-2 (0.5C0.5PV(05-98)), while a small pile structure was observed in Example 2-4 (0.5C0.5PV(10-98)) (Figure 7), and many pile structures appeared in Examples 2-6 (0.5C0.5PV(17-98)) and 2-9 (0.5C0.5PV(25-98)) (Figures 9 and 12). These findings indicate that the inclusion of PVAL alters the structure of microtubules constituting the honeycomb structure, and that an increased degree of polymerization of PVAL leads to the development of a pile-like structure. However, the honeycomb structure prepared from PVAL with a saponification degree of 89 mol% and an average degree of polymerization of 5000 (Example 2-11) showed fewer pile-like structures (Figure 14).

[0077] [Example 3] (Example 3-1) In Example 2-9, the concentration of PVAL was set to 0.25% by mass, and the honeycomb structure (0.5C0.25PV(25-98)) was obtained in the same manner as in Example 2-9. Figure 15 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure.

[0078] (Example 3-2) In Example 2-9, the concentration of PVAL was set to 0.75% by mass, and the honeycomb structure (0.5C0.75PV(25-98)) was obtained in the same manner as in Example 2-9. Figure 16 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure.

[0079] (Example 3-3) In Example 3-1, the concentration of CNF was set to 0.25% by mass, and the honeycomb structure (0.25C0.25PV(25-98)) was obtained in the same manner as in Example 3-1. Figure 17 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure.

[0080] No significant differences in the honeycomb structure were observed in either Example 3-1 or 3-2 compared to that of Example 2-9. In Example 3-3, a honeycomb structure with a spiky structure and a microtubule opening diameter of approximately 50 μm was also observed, but the inner wall was spongy. Furthermore, the microtubule opening also included a smooth, strip-like structure without spiky hairs. The microtubule opening diameter of the honeycomb structure in Example 3-1 was 18.1 ± 6.5 μm, and the length of the spiky hairs in the spiky structure was 7.7 ± 1.5 μm. In the honeycomb structure of Example 3-2, the microtubule opening diameter was 13.8 ± 5.4 μm, and the length of the spiky hairs in the spiky structure was 5.4 ± 2.5 μm.

[0081] (Examples 3-4) In Examples 2-9, the insertion speed of the PP screw-cap test tube containing the dispersion sample into liquid nitrogen during unidirectional freezing was 30 cm·h. -1 (0.083mm·s -1 ) and otherwise, the honeycomb structure (0.5C0.5PV(25-98)(30cm·h) is constructed in the same manner as in Example 2-9. -1 Figure 18 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure. In the honeycomb structure of Example 3-4, regions with clusters of small microtubule openings approximately 10 μm in diameter and regions with clusters of elliptical microtubule openings approximately 20 μm in diameter were observed (Figure 18). The pilosa structure was not clearly visible at the 10 μm diameter microtubule openings, but it was observed at the 20 μm diameter microtubule openings. The microtubule opening diameter in the honeycomb structure of Example 3-4 was 6.5 ± 2.7 μm, and the length of the pilosa was 2.8 ± 0.8 μm.

[0082] (Examples 3-5) In Example 2-9, the freezing conditions for unidirectional freezing were set to a freezing temperature of -30°C and an insertion speed of 1 cm·h for the PP screw-cap test tube containing the dispersion sample. -1 (0.0028mm·s -1 ) and otherwise the same as in Example 2-9, a honeycomb structure (0.5C0.5PV(25-98)(-30℃, 1cm·h -1 The result obtained is shown in Figure 19. The FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure are shown. In the honeycomb structure of Example 3-5, elliptical microtubule openings were observed with an opening diameter of approximately 100 μm in the long axis direction and approximately 40 μm in the short axis direction (Figure 19). Although bristles were also observed at the microtubule openings, the thickness and length of the bristles were similar to those of the honeycomb structures under other conditions. Considering the size of the microtubule openings, the proportion of the opening space obstructed by the bristles was relatively smaller than that of the honeycomb structures under other conditions. The microtubule opening diameter in the honeycomb structure of Example 3-5 was 34.4 ± 6.7 μm, and the length of the bristles in the bristles was 8.8 ± 2.7 μm.

[0083] (Examples 3-6) In Example 2-9, 2.0 mass% Rheocrista I-2SX, which has a longer fiber length than Rheocrista I-2SXS, was used as the CNF, and a honeycomb structure (0.5C(long)0.5PV(25-98)) was obtained in the same manner as in Example 2-9. Figure 20 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure. Figure 20(a) shows the observation results at a magnification of 500x, and Figure 20(b) shows the observation results at a magnification of 1000x. The microtubule opening diameter in the honeycomb structure of Example 3-6 was 9.2±2.7 μm, and the hair length of the pilous structure was 4.1±1.0 μm. In the honeycomb structure of Example 3-6, the diameter of the microtubule opening was approximately 10 μm. This trend was similar to that of the honeycomb structure made solely of CNF. Furthermore, a clearly defined spiky structure could not be observed at the microtubule opening, and the fibrous structure appeared to be irregularly connected on the inner wall of the microtubule opening. This is presumed to be because, as the microtubule opening becomes smaller, the spiky structures with ends are connected by bridging the inner walls opposite each other at the opening.

[0084] [Comparative Example 2] In Comparative Example 1, the insertion speed of a PP screw-cap test tube containing a dispersion sample into liquid nitrogen during unidirectional freezing was 30 cm·h -1 (0.083mm·s -1 ) and otherwise the same as in Comparative Example 1, the honeycomb structure (1.0C (30cm·h -1Figure 21 shows the FE-SEM observation results of the honeycomb structure of the obtained honeycomb structure.

[0085] [Comparative Example 3] In Example 2-9, the concentration of CNF was set to 0.1% by mass, and the concentration of PVAL was set to 0.1% by mass. Otherwise, a structural sample (0.1C0.1PV(25-98)) was obtained in the same manner as in Example 2-9. The FE-SEM observation results of the obtained structural sample are shown in Figure 22. No honeycomb structure was observed; only an irregular spongy structure was observed.

[0086] [Pressure loss] Figure 23 shows the measurement results of the pressure loss. Here, we have Examples 2-2 (0.5C0.5PV(05-98)), 2-5 (0.5C0.5PV(17-99)), 2-6 (0.5C0.5PV(17-98)), 2-7 (0.5C0.5PV(17-97)), 2-9 (0.5C0.5PV(25-98)), and Comparative Example 1 (1.0C(10cm·h) -1 )), Comparative Example 2 (1.0C(30cm·h -1 The measurement results for the following were shown: )) and a commercially available nonwoven fabric mask (nonwoven fabric mask, manufactured by Eikoh Co., Ltd.). The honeycomb structure in each example is similar to that in Comparative Example 1 (1.0C (10cm·h -1 Although the pressure loss was greater compared to Comparative Example 1 (1.0C (30cm·h)), -1 The values ​​were smaller compared to commercially available nonwoven fabric masks. From these results, it was found that although the honeycomb structure of the present invention has slightly reduced breathability due to the effect of the pile structure, it still has high breathability due to the honeycomb structure.

[0087] [Collection efficiency (1)] When the collection efficiency (1) was measured using the honeycomb structure of Example 2-5 (0.5C0.5PV(17-99)), the collection efficiency for micropearls with a diameter of 10 μm was 99.9%, indicating that even particles with a particle size smaller than the microtubule openings of the honeycomb structure of the present invention can be efficiently collected. Figure 24 shows the observation results of the cross-section of the microtubule openings in the honeycomb structure of Example 2-5 where micropearls were collected. Figure 24(a) shows the state of the microtubule cross-section (opening), and Figure 24(b) shows the state of the microtubule cross-section parallel to the longitudinal direction. Figure 24(a) shows that micropearls smaller than the diameter of the microtubule openings are packed into the microtubule openings. Furthermore, when the honeycomb structure after micropearl collection is split lengthwise (along the length of the microtubules) and the cross-section is observed with FE-SEM, it can be seen that micropearls are packed into the entrances of the microtubule openings. These results indicate that the presence of the spiky structure allows for the efficient collection of minute particles.

[0088] [Collection efficiency (2)] Table 1 and Figure 25 show the measurement results of the collection efficiency of smoke generated from incense sticks. Here, we have Examples 2-2 (0.5C0.5PV(05-98)), 2-5 (0.5C0.5PV(17-99)), 2-6 (0.5C0.5PV(17-98)), 2-7 (0.5C0.5PV(17-97)), 2-9 (0.5C0.5PV(25-98)), and Comparative Example 1 (1.0C(10cm·h) -1 )), Comparative Example 2 (1.0C(30cm·h -1 The measurement results for )) are shown. Note that all honeycomb structure samples were prepared by folding the frozen portion in order to preserve the structure of the cross-section. As a result, focusing on the particle size range of 0.5 to 1.0 μm, Example 2-2 (0.5C0.5PV(05-98)), which lacked a plucked structure, was hardly able to capture any particles, while Example 2-7 (0.5C0.5PV(17-97)), which showed a slight plucked structure, captured particles with moderate efficiency. On the other hand, Examples 2-6 (0.5C0.5PV(17-98)), 2-5 (0.5C0.5PV(17-99)), and 2-9 (0.5C0.5PV(25-98)), in which the plucked structure was clearly visible, captured particles with high efficiency. Furthermore, Comparative Example 1 (1.0C(10cm·h) -1 )), Comparative Example 2 (1.0C(30cm·h -1The diameters of the microtubule openings (aperture diameters) of the honeycomb structures in )) are 25 μm and 15 μm, respectively. Although the aperture diameter of Comparative Example 2 is smaller than that of Examples 2-5, 2-6, and 2-9 (microtubule opening diameters, or microtubule opening short axis diameters), the collection efficiency of both Comparative Example 1 and Comparative Example 2 was moderate. Furthermore, the honeycomb structure of Example 2-2 (0.5C0.5PV(05-98)) captured almost no particles in the particle size range of 0.5 to 1.0 μm, unlike the conventional honeycomb structures of Comparative Examples 1 and 2 which captured some of these particles, while still capturing larger particles.

[0089] [Table 1]

[0090] Figure 25 shows a flow velocity of 0.029 m·s -1 This figure shows the relationship between pressure loss and collection efficiency in the particle size range of 0.5 to 1.0 μm (when the Ar flow rate is 100 mL / min in the above pressure loss measuring device). In Examples 2-6 (0.5C0.5PV(17-98)), 2-5 (0.5C0.5PV(17-99)), and 2-9 (0.5C0.5PV(25-98)), Comparative Example 2 (1.0C(30cm·h) -1 It collected particles with higher efficiency without causing pressure loss compared to other methods.

[0091] In Example 2-5 (0.5C0.5PV(17-99)), when the honeycomb structure was observed using FE-SEM after the measurement of collection efficiency (2), no particles were observed in either the cross-section (opening) or the longitudinal cross-section of the microtubules. This is thought to be due to the small size of the particles making observation difficult, as well as the small quantity of particles. Visually, it was confirmed that the collected surface of all samples changed from white to a slightly brownish color. [Explanation of Symbols]

[0092] 1. Honeycomb structure 10 Smoke collection device 11 Incense 12 Fans 13 smoke 14, 16 tubes 15 Sample mounting section 20 Particle Counter

Claims

1. A honeycomb structure made of a composite material containing cellulose nanofibers and polyvinyl alcohol.

2. The honeycomb structure according to claim 1, having a pile structure in which the surface of the inner wall constituting the honeycomb structure is made to stand upright.

3. The honeycomb structure according to claim 1, wherein the polyvinyl alcohol is a vinyl alcohol-based polymer having vinyl alcohol units and vinyl acetate units.

4. The honeycomb structure according to claim 1, wherein the cellulose nanofibers are cellulose nanofibers that have been chemically defibrated.

5. The honeycomb structure according to claim 1, wherein the polyvinyl alcohol content in the composite material is 0.1 to 3 times the mass ratio of cellulose nanofiber 1.

6. A method for producing a honeycomb structure according to claim 1, comprising the steps of gradually submerging a container containing a dispersion in which cellulose nanofibers and polyvinyl alcohol are dispersed in a solvent containing water into a refrigerant to perform unidirectional freezing, and then freeze-drying to remove the solvent.

7. The method for producing a honeycomb structure according to claim 6, wherein the cellulose nanofibers are cellulose nanofibers that have been chemically defibrated.

8. The method for producing a honeycomb structure according to claim 6, wherein the polyvinyl alcohol is a vinyl alcohol-based polymer having vinyl alcohol units and vinyl acetate units.

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