Honeycomb structure and method for manufacturing the same

Abstract

To provide a honeycomb structure more excellent in mechanical strength and water resistance than a conventional honeycomb structure containing cellulose nanofibers, and to provide a method for producing the same.SOLUTION: A honeycomb structure comprising a composite material containing cellulose nanofibers and a phenol resin, wherein the phenol resin forms a crosslinked structure.SELECTED DRAWING: None

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 run in the same direction, and is seen in beehives and plant stems. The honeycomb structure has the advantage that it can be constructed robustly using a small amount of material and has low fluid pressure loss.

[0003] If a honeycomb structure with these characteristics could be constructed by freely controlling the type of material, wall thickness, channel shape, and channel hole size, the range of uses for the material would be greatly expanded. For example, it is expected that honeycomb structures can be used as efficient exhaust gas filters with low pressure loss, as carriers that support catalysts for efficient reactions, and as sturdy masks that can be applied to living organisms.

[0004] The present applicant has previously reported that a micro-honeycomb structure can be synthesized from cellulose nanofibers (hereinafter sometimes abbreviated as "CNF") by using the 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 mask, a filter for an air purifier or the like, a catalyst carrier, an adsorbent, and the like. Furthermore, Patent Document 3 discloses a honeycomb structure containing CNF and a carbon material having a specific structure, and reports that the honeycomb structure exhibits anisotropic electromagnetic wave absorption performance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open 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 Summary of the Invention [Problem to be solved by the invention]

[0007] However, conventional honeycomb structures manufactured using CNFs have problems such as insufficient mechanical strength and water resistance. In this regard, for example, Non-Patent Document 1 reports that a honeycomb structure made from 1% by mass of CNF has a Young's modulus of approximately 10 kPa when compressed in a direction parallel to the microchannels.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a honeycomb structure having superior mechanical strength and water resistance to conventional honeycomb structures containing CNF, and a method for manufacturing the same. [Means for solving the problem]

[0009] As a result of extensive research to achieve the above-mentioned objective, the inventors discovered that a honeycomb structure constructed from a material containing CNF and phenolic resin and having a phenolic resin cross-linking treatment applied within the walls has excellent strength and water resistance, and thus completed the present invention.

[0010] That is, the present invention is 1. A honeycomb structure made of a composite material containing cellulose nanofibers and a phenolic resin, wherein the phenolic resin forms a crosslinked structure; 2. The honeycomb structure according to 1, wherein the phenolic resin is a resol type phenolic resin. 3. The honeycomb structure according to 1, wherein the cellulose nanofibers are chemically defibrated cellulose nanofibers. 4. The honeycomb structure of 1, wherein the content of the phenolic resin in the composite material is 0.5 to 10 times by mass relative to the cellulose nanofibers. 5. A method for manufacturing a honeycomb structure according to 1, in which a container containing a dispersion of cellulose nanofibers and a phenolic resin dispersed in a solvent containing water is gradually submerged in a refrigerant to perform unidirectional freezing, and then the dispersion is freeze-dried to remove the solvent, and further heat-treated to crosslink the phenolic resin; 6. The method for producing a honeycomb structure according to 5, wherein the cellulose nanofibers are chemically defibrated cellulose nanofibers. 7. The method for manufacturing a honeycomb structure according to 5, wherein the phenolic resin is a water-soluble phenolic resin. 8. The method for manufacturing a honeycomb structure according to claim 7, wherein the water-soluble phenolic resin is a water-soluble resol type phenolic resin. to provide. [Effects of the Invention]

[0011] The honeycomb structure of the present invention is made of a composite material containing CNF and phenolic resin, and the phenolic resin forms a cross-linked structure within its walls, so it has superior mechanical strength and water resistance to conventional CNF honeycomb structures. The honeycomb structure of the present invention having such characteristics can be suitably used as a material for various filters including filters for masks and air purifiers. [Brief explanation of the drawings]

[0012] [Figure 1] 1 shows SEM images of honeycomb structures produced in Examples 1 to 3 and Comparative Examples 1 to 3, where (a) is the SEM image of Comparative Example 1, (b) is the SEM image of Example 1, (c) is the SEM image of Example 2, (d) is the SEM image of Example 3, (e) is the SEM image of Comparative Example 2, and (f) is the SEM image of Comparative Example 3. [Figure 2]1A and 1B are diagrams showing the stress-strain curves and Young's modulus of the honeycomb structures produced in Examples 1 to 3 and Comparative Examples 1 to 3, where (a) and (b) are stress-strain curves in the direction parallel to the microtubules (microchannels), and (c) and (d) are stress-strain curves in the direction perpendicular to the microtubules (microchannels). [Figure 3] FIG. 10 is a diagram showing an FT-IR spectrum of the honeycomb structure produced in Comparative Example 2. [Figure 4] FIG. 10 is a diagram showing an FT-IR spectrum of the honeycomb structure produced in Comparative Example 3. [Figure 5] FIG. 1 is a diagram showing an FT-IR spectrum of the honeycomb structure produced in Comparative Example 1. [Figure 6] FIG. 2 is a diagram showing an FT-IR spectrum of the honeycomb structure produced in Example 1. [Figure 7] FIG. 10 is a diagram showing the TG-DSC and MS spectra of the honeycomb structure produced in Comparative Example 1. [Figure 8] 1 is a diagram showing TG-DSC and MS spectra of the honeycomb structure produced in Example 1. FIG. [Figure 9] FIG. 3 is a diagram showing the results of a water resistance test of the honeycomb structures produced in Examples 1 to 3 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0013] The honeycomb structure of the present invention is made of a composite material containing CNF and a phenolic resin, and the phenolic resin forms a crosslinked structure within the walls that constitute the honeycomb structure.

[0014] Generally, cellulose is classified into natural cellulose, regenerated cellulose, fine cellulose, microcrystalline cellulose excluding the amorphous region, etc., and any of these celluloses may be used as the raw material for the CNF used in the present invention. The CNF is the raw cellulose that has been defibrated to the nano level. There are no particular limitations on the average fiber diameter as long as it is at the nano level. However, since this 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 even more preferably 2 to 30 nm or less. Furthermore, the average fiber length of the CNF is not particularly limited, but from the viewpoint of stably forming a honeycomb structure, it is preferably 50 nm or more and 100 μm or less, more preferably 0.1 to 10 μm, even more preferably 0.15 to 5 μm, and even more preferably 0.3 to 2 μm. Furthermore, the aspect ratio of CNF, which is expressed as the average fiber length / average fiber diameter, is usually 50 to 1,000. The average fiber diameter and average fiber length of CNF are determined by averaging the fiber diameter and fiber length obtained from the observation of each fiber using an atomic force microscope (AFM).

[0015] Defibration processes for defibrating raw cellulose to the nano-level are broadly divided into mechanical defibration processes and chemical defibration processes, and the CNF used in the present invention may be defibrated by either process. Specific examples of mechanical defibration treatment include a high-pressure homogenizer method, a microfluidizer method, a grinder method, a ball mill pulverization method, and a bead mill pulverization method. Specific examples of chemical defibration treatments include the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation method, phosphate esterification method, phosphite esterification method, carboxymethylation method, xandation method, and sulfonation method. Among these, in the present invention, CNF that has been subjected to a chemical treatment, a method of introducing charged functional groups and obtaining a high dispersion liquid through molecular chain repulsion, is preferred, as it can produce fibers with a smaller fiber diameter and excellent dispersibility, and TEMPO-oxidized CNF in which carboxy groups have been introduced by TEMPO oxidation is more preferred.

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

[0017] The phenolic resin constituting the honeycomb structure of the present invention is not particularly limited as long as it is synthesized from phenol and formaldehyde, and either novolac-type phenolic resin or resol-type phenolic resin may be used. However, resol-type phenolic resin is preferred because it has excellent heat resistance and chemical resistance, has hydroxymethyl groups at the ends of its structure, and can easily form a crosslinked structure by thermal curing.

[0018] The content of phenolic resin in the above composite material is not particularly limited, but considering the balance between improving the mechanical strength and water resistance of the honeycomb structure and dispersibility in the dispersion used to manufacture the honeycomb structure, the mass ratio is preferably 0.5 to 10 times, more preferably 1 to 5 times, and even more preferably 1 to 3 times, the amount of phenolic resin relative to CNF1.

[0019] The composite material constituting the honeycomb structure of the present invention preferably does not contain a carbon material, and more preferably is composed only of CNF and phenol resin. The term "carbon material" refers to a material that is essentially composed of carbon (C) only (chemically treated as elemental carbon), such as graphite and carbon black.

[0020] The honeycomb structure of the present invention is a hollow structure in which microtubes of a three-dimensional shape are arranged without gaps. Examples of the three-dimensional shape of the hollow structure include a circular cylinder, an elliptical cylinder, and a polygonal prism such as a triangular prism, a square prism, a hexagonal prism, and an octagonal prism. From the viewpoint of ease of production, a polygonal prism is preferred, and a hexagonal prism is more preferred, but a mixture of a plurality of three-dimensional shapes may also be used.

[0021] In the present invention, the opening diameter of the microtubes constituting the honeycomb structure (hereinafter also referred to as "microtube opening diameter") is preferably 0.2 to 200 μm, more preferably 1.0 to 150 μm, and even more preferably 5.0 to 100 μm, from the viewpoint of increasing mechanical strength. In the present invention, the "microtubule opening diameter" refers to the most frequent value obtained by measuring the area of ​​the opening using the image analysis software ImageJ from images obtained by observation using a scanning electron microscope (SEM) and calculating the diameter of the opening assuming it is a circle.

[0022] Furthermore, the thickness of the walls 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, from the viewpoint of increasing the mechanical strength. The wall thickness is an average value of thicknesses measured at five points on one wall observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0023] 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 or more and 20 cm or less, more preferably 150 μm or more and 10 cm or less, even more preferably 200 μm or more and 10 mm or less, and even more preferably 500 μm or more and 5 mm or less.

[0024] Although the mechanical properties of the honeycomb structure of the present invention are not particularly limited, considering application to various filter materials, etc., the Young's modulus in the direction parallel to the microtubules is preferably 100 kPa or more, more preferably 150 kPa or more, even more preferably 185 kPa or more, and even more preferably 200 kPa or more. The upper limit of the Young's modulus in the parallel direction is not particularly limited, but is usually about 10,0000 kPa. The Young's modulus in the direction perpendicular to the microtubules is preferably greater than 15 kPa, more preferably 18 kPa or greater, and even more preferably 20 kPa or greater. The upper limit of the Young's modulus in the direction perpendicular to the microtubules is not particularly limited, but is usually about 1,000 kPa. The Young's modulus in the present invention is a value obtained from the stress-strain curve when an Autograph AGS-X / 10kN (manufactured by Shimadzu Corporation) is used to crush the honeycomb at a speed that crushes the honeycomb by the height of the honeycomb in one minute, and the pressure is continued until the honeycomb becomes 20% of its original height.

[0025] The honeycomb structure of the present invention described above can be manufactured, for example, by gradually submerging a container containing a dispersion of CNF and phenolic resin in a solvent containing water into a refrigerant to perform unidirectional freezing, then freeze-drying to remove the solvent, and further heat-treating to crosslink the phenolic resin. Here, "unidirectional freezing" refers to a technique for freezing a dispersion of CNF and phenolic resin in one direction. By freezing the dispersion containing CNF and phenolic resin in a directional manner, the solvent grows in one direction, for example, vertically upward, forming multiple icicles. The CNF and phenolic resin are then collected in the gaps between the icicles, resulting in the production of a honeycomb structure composed of an aggregate of microtubes made of a composite material containing CNF and phenolic resin.

[0026] In the present invention, a solvent containing water is used as the solvent for preparing the dispersion, but it is preferable to use a solvent consisting solely of water. Note that any solvent that is compatible with water, disperses or dissolves CNF uniformly, and has a freezing point can be used by mixing 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.

[0027] In the present invention, since the dispersion is prepared using a solvent containing water, the phenolic resin is preferably a water-soluble phenolic resin, and for the reasons mentioned above, a water-soluble resol-type phenolic resin is more preferred. Here, "water-soluble" means that the water dilutability is 20% or more. The water-soluble phenolic resin used in the present invention is commercially available, and specific examples thereof include water-soluble resol resins manufactured by DIC Corporation, such as Phenolite IG-1002, GA-1364, GG-1448, GG-1480, GG-1490, GG-3093, TD-617, TD-2250, and 8121-LV.

[0028] In the present invention, the CNF content in the dispersion is preferably 1.0% by mass or more, more preferably 1.2% by mass or more, and even more preferably 1.5% by mass or more, in order to prevent the microtubes constituting the honeycomb structure from becoming deformed and to increase the strength of the honeycomb structure. There is no particular upper limit to the CNF content, but if it is too high, the viscosity of the dispersion increases and it becomes gel-like, so the CNF content is preferably 5.5% by mass or less, and more preferably 4.0% by mass or less.

[0029] Furthermore, in order to improve the strength and water resistance of the resulting honeycomb structure, the content of the phenolic resin in the dispersion is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, even more preferably 2.0% by mass or more, even more preferably 4.0% by mass or more, and particularly preferably 5.0% by mass or more. The upper limit of the content of the phenolic resin is not particularly limited, but from the viewpoint of stably preparing the dispersion, it is preferably 10.0% by mass or less, and more preferably 8.0% by mass or less.

[0030] The refrigerant used for unidirectional freezing is not particularly limited as long as it is a liquid (antifreeze) that does not freeze even at a temperature at which the solvent of the dispersion can be frozen, and typical examples include water, saline, 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 easily available, inexpensive, and does not freeze even at relatively low temperatures. In addition, the refrigerant may not only be a liquid, but also a space cooled by a cooling device or the like (for example, air cooled to a temperature at which the dispersion can be frozen).

[0031] The means for cooling the refrigerant may utilize a cryogen and / or a chiller. 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, dry ice, etc. The method of using these refrigerants must be appropriately selected from a method of indirectly cooling via a refrigerant container or the like without contacting the refrigerant, and a method of directly mixing the refrigerant. A specific example of the cooling device is a immersion type cooler, etc. The cooling device may be in direct contact with the refrigerant.

[0032] Examples of materials for the container that holds the dispersion include polypropylene, polyethylene, and polyvinyl chloride, with polypropylene being preferred. The thermal conductivity of polypropylene is approximately 0.11 (W / m·K), which is lower than the thermal conductivity 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 liquid surface of the refrigerant (such as liquid nitrogen). Furthermore, the shape and size of the container can be determined appropriately depending on the application of the honeycomb structure, but it is preferable that the container has a predetermined height so that microtubes having a length in the axial direction that constitute the honeycomb structure can be formed.

[0033] The temperature of the dispersion liquid should be higher than the freezing point of the solvent of the dispersion liquid and lower than the boiling point of the solvent. For example, when water is used as the solvent, the temperature of the dispersion liquid should be higher than 0°C, which is the freezing point of water, and lower than 100°C, which is the boiling point of water. Furthermore, there are no particular limitations on the temperature of the coolant as long as it is a temperature at which the solvent of the dispersion can be frozen. For example, when water is used as the solvent of the dispersion, the temperature of the coolant should be lower than 0°C, which is the freezing point of water.

[0034] In the unidirectional freezing method used in the present invention, a container containing a dispersion of CNF and phenolic resin is gradually immersed in a refrigerant. The immersion speed (penetration speed) is not particularly limited and can be, for example, 3 to 300 μm / sec. Preferably, it is 10 to 100 μm / sec. If the penetration speed is too slow, it is inappropriate from the viewpoint of productivity, while if the penetration speed is too fast, problems such as impairing the linearity of the microtubules constituting the honeycomb structure may occur. It is preferable that the penetration speed is constant.

[0035] After unidirectional freezing, the dispersion liquid is removed from the container together with the refrigerant, and the solidified solvent is removed by freeze-drying. Specifically, the dispersion solid formed in the container is cut together with the container while still in a frozen state, and cylindrical pieces of a predetermined length are cut out. The cut cylindrical pieces are then held in a reduced pressure environment for a certain period of time, thereby sublimating the solvent (e.g., water) while still in a frozen state, and drying them. This removes the solvent from the dispersion solid, leaving only a honeycomb structure composed of an aggregate of microtubules. The reduced pressure conditions for freeze-drying are appropriately determined depending on the solvent used, but for example, when water is used as the solvent, the reduced pressure atmosphere can be −30 to 50° C. and 1 to 1000 Pa. The retention time under reduced pressure is also appropriately determined depending on the solvent used and the reduced pressure atmosphere, but can be, for example, 20 to 100 hours.

[0036] By heating the freeze-dried structure, the honeycomb structure of the present invention in which the phenolic resin is crosslinked can be obtained. In this case, the heating temperature is not particularly limited as long as it is a temperature at which the crosslinking reaction of the phenolic resin proceeds, but is preferably 100° C. or higher, more preferably 120° C. or higher, and even more preferably 150° C. or higher. In order to prevent distortion or collapse of the structure due to rapid curing, the heating temperature may be increased in stages, for example, by treating at 150° C. for a predetermined time and then treating at 200° C. for a predetermined time. The heating time cannot be generally determined because it varies depending on the heating temperature, but it can be, for example, from about 1 minute to about 10 hours. The heat treatment may be carried out under atmospheric pressure or under reduced pressure, but is preferably carried out under reduced pressure. [Example]

[0037] EXAMPLES The present invention will be specifically explained below by showing examples and comparative examples, but the present invention is not limited to the following examples.

[0038] The reagents used in the following examples are as follows. CNF (RHEOCRYSTA): (2.0 mass% RHEOCRYSTA I-2SXS preservative-free, Daiichi Kogyo Seiyaku Co., Ltd., Lot No. 230515, pH: 6.8, transparency: 92%, viscosity: 12,000 mPa s, TEMPO-oxidized CNF). Pure water: Milli-Q water Phenol resin: Phenolite 8121-LV (resole resin, serial number: C3606809, provided by DIC Corporation); (test sheet data: non-volatile content (135°C): 76.7%), (SDS data: formaldehyde-phenol polycondensate: 65-75% (used as 70% for calculations), phenol: 20-30% by mass (used as 25% by mass for calculations), formaldehyde: ≦1% by mass, methanol: ≦1% by mass. Based on the above data, the composition of the reagent was determined to be formaldehyde-phenol polycondensate: 54% by mass, volatile content (mainly water): 23% by mass, and phenol: 19% by mass.

[0039] The instruments, devices and measurement methods used in the following examples are as follows. (1) Mixer ARE-250 (THINKY CORPORATION) (2) Homogenizer T18 digital ULTRA-TURRAX (registered trademark) (manufactured by IKA), Shaft Generator S18D-14G-KS (manufactured by IKA) (3) Polypropylene screw-cap test tube 17mL (outer diameter 15mm, inner diameter 13mm): 1-6403-02 (Maruem) (4)FE-SEM S-4800 manufactured by Hitachi High-Tech Co., Ltd. (5) Dip coater Dip coater M200 (manufactured by Asumi Giken Co., Ltd.) (6) Freeze drying equipment UT-4000L (Tokyo Rikakikai Co., Ltd.), Thermal Robo TR-3α (As One Corporation) (7) Vacuum oven VACUUM OVEN VOS-201SD (manufactured by Tokyo Rikakikai Co., Ltd.) (8) Stress-strain measuring device Autograph AGS-X 10 kN (Shimadzu Corporation.) (9) FT-IR FT / IR-6600 (JASCO Corporation) (10) TG-DSC-MS: TG-DSC STA 449 Jupiter (Netzsch) Approximately 10 mg of the honeycomb structure was placed in a Pt crucible and heated under a He gas flow (150 mL min -1 ), 60°C-900°C, heating rate 10°C min -1 MS was measured using a quadrupole mass spectrometer (JMS-Q1500GC, JEOL).

[0040] [Comparative Example 1] 25.7 g of a 2.0 wt% CNF aqueous solution was added to a glass cup, and 2.83 g of resol-type phenolic resin (Phenolighte 8121-LV, 54 wt% resin) was slowly added while stirring with a mechanical stirrer at 500 rpm. Stirring (500 rpm) was continued for 1 hour at room temperature, yielding a yellowish-white slurry containing 1.8 wt% CNF and 5.4 wt% resol-type phenolic resin. The slurry was transferred to a vial and homogenized (10,000 rpm) for 5 minutes, then the lid was closed and the mixture was shaken thoroughly by hand. This homogenizer and shaking process was repeated three times. The vial containing the slurry was placed in a mixer (ARE-250) and stirred at 2,000 rpm for 5 minutes to mix and degas. The prepared slurry was slowly poured into a polypropylene (PP) screw-cap test tube, taking care to avoid introducing air bubbles. The PP screw-cap test tube containing the sample was left to stand at 5°C for 12 hours to stabilize the temperature. The PP screw-cap test tube containing the sample was then coated with a dip coater at 0.028 mm·s -1 The PP screw-cap test tubes containing the samples were then immersed in liquid nitrogen at a rate of 100°C (1.8°C), followed by unidirectional freezing. After freezing, the PP screw-cap test tubes containing the samples were cut using two methods depending on the purpose. The first method involves cutting only the PP tube with a knife and breaking off the frozen portion, which allows the structure of the cut surface to be maintained. The second method involves cutting the entire tube with a wire saw, which slightly disrupts the structure of the cut surface but allows the creation of a flat cut surface. The cut samples were then slowly dried in a freeze-drying device at temperatures of -5°C, 0°C, and 5°C, yielding a honeycomb structure (1.8C-5.4P-pristine).

[0041] [Example 1] A honeycomb structure (1.8C-5.4P-pristine) was obtained by performing the same operation as in Comparative Example 1 using a slurry containing 1.8 mass% CNF and 5.4 mass% of the same resol-type phenolic resin as in Comparative Example 1. The honeycomb structure was placed in a vacuum oven, the pressure was reduced to -0.1 MPa, and the structure was treated at 150°C for 10 minutes, and then further treated at 200°C for 360 minutes to obtain a heat-treated honeycomb structure (1.8C-5.4P-heated).

[0042] Comparative Example 2 20.0 g of 2.0 mass% CNF was added to the vial, and the mixture was stirred (2000 rpm, 5 minutes) and degassed (2000 rpm, 5 minutes) using a mixer (ARE-250). Glass beads were added to a PP screw-cap test tube to a height of 50 mm, and then the prepared sample was added. The PP screw-cap test tube containing the sample was left to stand at 5°C for 12 hours to stabilize the temperature. The PP screw-cap test tube containing the sample was then coated with a dip coater at 0.028 mm s -1 The sample was then immersed in liquid nitrogen at a rate of 0.05°C for unidirectional freezing. The PP screw-cap test tube containing the frozen sample was cut using two different methods depending on the purpose, as in Comparative Example 1. The cut sample was freeze-dried in the same manner as in Comparative Example 1 to obtain a honeycomb structure (2.0C-0P-pristine).

[0043] Comparative Example 3 The honeycomb structure (2.0C-0P-pristine) obtained by carrying out the same operation as in Comparative Example 2 was treated in the same manner as in Example 1 to obtain a heat-treated honeycomb structure (2.0C-0P-heated).

[0044] [Example 2] A honeycomb structure (1.8C-3.6P-pristine) was obtained by performing the same operation as in Example 1 using a slurry containing 1.8 mass% CNF and 3.6 mass% of the same resol-type phenolic resin as in Comparative Example 1, and was then treated in the same manner as in Example 1 to obtain a heat-treated honeycomb structure (1.8C-3.6P-heated).

[0045] [Example 3] A honeycomb structure (1.8C-1.8P-pristine) was obtained by performing the same operation as in Example 1 using a slurry containing 1.8 mass% CNF and 1.8 mass% of the same resol-type phenolic resin as in Comparative Example 1, and was then treated in the same manner as in Example 1 to obtain a heat-treated honeycomb structure (1.8C-1.8P-heated).

[0046] The honeycomb structures produced in the above examples and comparative examples were subjected to FE-SEM observation, Young's modulus measurement, pressure loss measurement, and water resistance test by the following methods. [FE-SEM observation] FE-SEM measurements were performed on a Pt-coated sample fixed to an aluminum sample stage with carbon tape and observed with a Hitachi SEM. The thickness of the honeycomb wall was measured by loading the SEM image of one wall into ImageJ, measuring the thickness at five points, and calculating the average value. Microtubule aperture diameters were measured using a scanning electron microscope (SEM). The aperture area was measured using ImageJ from the SEM images. The diameter was calculated assuming the aperture was circular, and the most frequent value was used as the microtubule aperture diameter. The 2.0C-0P-pristine (Comparative Example 2) and 2.0C-0P-heated (Comparative Example 3) samples had thin walls, and when processed directly with ImageJ, the area was underestimated. Therefore, for 2.0C-0P-pristine and 2.0C-0P-heated samples, the wall was traced with a line and processed with ImageJ to determine the aperture area. For 1.8C-5.4P-pristine (Comparative Example 1), 1.8C-5.4P-heated (Example 1), 1.8C-3.6P-heated (Example 2), and 1.8C-1.8P-heated (Example 3), the SEM images were processed directly with ImageJ to determine the aperture area. The obtained SEM images are shown in Figures 1(a) to 1(f). As shown in Figure 1, the honeycomb structures of Examples 1, 2, and 3 and Comparative Example 3 maintained their microchannel structures even after the heat treatment.

[0047] [Young's modulus measurement] Using an Autograph AGS-X 10kN, stress-strain curves were measured in two directions, parallel and perpendicular to the microtubules. For measurements in the direction parallel to the microtubules, honeycombs cut with a wire saw were used. For measurements in the direction perpendicular to the microtubules, the honeycombs cut with the wire saw were cut parallel to the microtubules using a square die-cutting tool with sides of 7.2 mm to create regular rectangular parallelepipeds. The crushing speed was set to crush the height of the honeycomb per minute, and the compression continued until the honeycomb was 20% of its original height, and the stress-strain curve at this point was graphed. For example, if the honeycomb height is 10 mm, the crushing speed was 10 mm min -1The crushing distance was set to 8 mm, and the height of the honeycomb after crushing was set to 2 mm. The obtained test force (N) was divided by the area of ​​contact between the honeycomb and the press to obtain the stress (kPa). The Young's modulus was calculated using the slope of the linear range at the initial stress increase in the obtained stress-strain curve. The obtained stress-strain curves are shown in Figure 2(a) to (d).

[0048] [Pressure loss measurement] A tube for flowing argon gas was prepared, and a flange capable of sealing the honeycomb was attached midway through the flow path. An 8.5 mm hole was drilled in the flange to allow argon gas to flow. The honeycomb was attached to the 8.5 mm hole in the direction that the argon gas passed through the microtube, and the gap was sealed with RTV rubber KE45W (Shin-Etsu Chemical Co., Ltd.). Measurements were taken using a homemade device. Argon gas was passed through at a velocity of 0.029 m s -1 The pressure difference between the inlet and outlet when the air was flowing was measured, and the pressure loss was calculated by dividing the difference by the length of the honeycomb flow path.

[0049] The structural data, pressure loss, and Young's modulus of the honeycomb structures obtained in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1. For comparison, data on CNF (S-5.1) manufactured by Nippon Paper Industries Co., Ltd. (Nano Res. 2023, 16, 8018-8024) is also shown.

[0050] [Table 1]

[0051] As shown in Table 1, all of the honeycomb structures obtained in the examples and comparative examples had pores of approximately 20 to 40 μm. The wall thickness and honeycomb density varied depending on the solid concentration in the slurry: the wall thickness was approximately 0.3 μm in Comparative Examples 2 and 3, approximately 1 μm in Examples 1 and 1, and approximately 0.6 μm in Examples 2 and 3. The density was approximately 0.02 g cm in Comparative Examples 2 and 3.-3 In contrast, in Example 1 and Comparative Example 1, the -3 , and about 0.06 g cm in Example 2. -3 , and about 0.04 g cm in Example 3. -3 It was.

[0052] In Comparative Example 2 and Comparative Example 3, the mechanical strength in the direction parallel to the microtubules was 291 kPa and 194 kPa (see FIG. 2(a)), and the Young's modulus was 99 kPa and 13 kPa (see FIG. 2(b)). On the other hand, in Comparative Example 1 and Examples 1 to 3, the mechanical strength in the direction parallel to the microtubules was 111 kPa, 1460 kPa, 888 kPa, and 718 kPa (see FIG. 2(a)), and the Young's modulus was 21 kPa, 248 kPa, 190 kPa, and 164 kPa, respectively (see FIG. 2(b)). Furthermore, in Comparative Example 2 and Comparative Example 3, the mechanical strength in the direction perpendicular to the microtubules was 10 kPa (see FIG. 2(c)), and the Young's modulus was 1 kPa (see FIG. 2(d)). On the other hand, in Comparative Example 1 and Examples 1 to 3, the mechanical strength in the direction perpendicular to the microtubules was 60 kPa, 151 kPa, 100 kPa, and 30 kPa (see FIG. 2(c)), and the Young's modulus was 15 kPa, 20 kPa, 10 kPa, and 4 kPa (see FIG. 2(d)). These results show that increasing the phenolic resin content allows the production of tough honeycomb structures (Examples 1 to 3 and Comparative Example 1). Furthermore, it can be seen that if the phenolic resin content is the same, a tough honeycomb structure can be produced by heat treatment. In particular, when used as a filter material, the Young's modulus in the parallel direction is important, and all of Examples 1 to 3 show a higher Young's modulus than the Comparative Example, demonstrating that a tough honeycomb structure can be produced. It is noted that the honeycomb structures of Examples 1 to 3 have a Young's modulus far higher than that of CNF (S-5.1) having the same density, and are therefore harder.

[0053] The effect of heat treatment on the honeycomb structure was investigated using FT-IR, and the charts are shown in Figures 3 to 6, and the data are shown below. Comparative Example 2 (2.0C-OP-pristine). (KBr) ν = 560, 610, 664, 795, 899, 1036, 1060, 1107, 1159, 1201, 1241, 1316, 1340, 1375, 1413, 1613, 2901, 3393 cm -1 . Comparative Example 3 (2.0C-OP-heated). (KBr) ν = 561, 610, 661, 794, 899, 1037, 1060, 1158, 1201, 1241, 1314, 1338, 1377, 1413, 1612, 2903, 3393 cm⁻¹ -1 . Comparative Example 1 (1.8C-5.4P-pristine). (ATR) ν = 445, 493, 509, 563, 610, 640, 760, 825, 892, 940, 1024, 1059, 1108, 1154, 1171, 1240, 1370, 1400, 1457, 1484, 1512, 1598, 1613, 2884, 2928, 3022, 3352 cm -1 . Example 1 (1.8C-5.4P-heated). (KBr) ν = 503, 563, 772, 822, 886, 916, 927, 1060, 1100, 1148, 1207, 1259, 1325, 1439, 1477, 1507, 1608, 2916, 3012, 3402 cm -1 .

[0054] Figure 3 shows the FT-IR spectrum of Comparative Example 2 (2.0C-0P-pristine), and Figure 4 shows the FT-IR spectrum of Comparative Example 3 (2.0C-0P-heated). Comparing Figures 3 and 4 reveals that when CNF alone is used, the substrate does not change significantly with heat treatment. On the other hand, as shown in the FT-IR spectra of Comparative Example (1.8C-5.4P-pristine) in Figure 5 and Example 1 (1.8C-5.4P-heated) in Figure 6, when CNF and resol-type phenolic resin are combined, the absorption peak position remains unchanged after heating, but the intensity profile of the spectrum changes. Therefore, it is inferred that the basic structure of the resol-type phenolic resin does not change significantly, and the crosslinking reaction mainly proceeds.

[0055] TG-DSC and MS were measured for Comparative Example 1 (1.8C-5.4P-pristine) (Figure 7) and Example 1 (1.8C-5.4P-heated) (Figure 8) to investigate changes in heat flow, weight loss, and generated substances due to heat treatment. The measurement results for Comparative Example 1 and Example 1 are shown in Figures 7 and 8, respectively. As shown in Figure 7, when Comparative Example 1 (1.8C-5.4P-pristine) was heated in an inert gas atmosphere, exothermic peaks were observed at 153 °C and 233 °C. At this time, ions corresponding to HO (m / z = 18) and CO (m / z = 44) were generated, and the weight also decreased by 10%, suggesting the progression of the crosslinking reaction. Next, ion peaks corresponding to phenol derivatives, CHOH (m / z = 94), CHCHOH (m / z = 108), CH(CH)OH (m / z = 122), and CH(CH)OH (m / z = 136), were generated around 300 °C, indicating the decomposition of the phenolic resin. Further heating led to the generation of ions corresponding to H (m / z = 2) around 600 °C. On the other hand, when Example 1 (1.8C-5.4P-heated) shown in Figure 8 was heated in inert gas, the exothermic peaks at 153°C and 233°C seen in Figure 7 were not observed. Therefore, it can be seen that in Example 1, in which the phenolic resin was crosslinked by heat treatment, no H2O or CO2 was generated. Thus, in Example 1 (Figure 8), no H2O or CO2 was generated up to around 250°C, and the weight remained almost unchanged, indicating that the crosslinking reaction of the phenolic resin in the honeycomb structure was sufficiently carried out by the heat treatment. Furthermore, as in Comparative Example 1 (Figure 7) in Figure 8, ions corresponding to phenol derivatives produced by thermal decomposition of the phenolic resin were generated from around 300°C to around 600°C, and ions corresponding to H2 were generated from around 600°C.

[0056] Regarding pressure loss, the pressure loss of a commercially available nonwoven mask (CNF / polyurethane = 1 / 1. Nonwoven mask, manufactured by Eiko Co., Ltd.) is approximately 120 Pa·mm -1 However, this is very high compared to the pressure loss of the honeycomb structure shown in Table 1, indicating that the honeycomb structure has good breathability. In addition, the air pressure of the N95 equivalent filter (Label-1), which is equivalent to the N95 mask used for medical purposes, is 0.1 m s -1 The pressure loss is 77 Pa·mm -1 (KONA Powder and Particle Journal, 2024. https: / / doi.org / 10.14356 / kona.2025002) Although the measurement conditions are different, when comparing the pressure loss of the honeycomb of the present invention, it is predicted that the pressure loss will be about the same if the gas flow rate is the same.

[0057] [Water resistance test] The honeycomb structures obtained in the examples and comparative examples were placed in glass vials. They were arranged so that the honeycomb microchannels were vertical, and the orientation was fixed with a plastic part. Water was slowly added to the bottom of the vial until the honeycomb was completely immersed. The plastic part was removed, and the structure was left for 7 days while observing any changes, and the water resistance was examined. After 7 days, the liquid was slowly removed with a pipette. The results of the water resistance test are shown in Figure 9.

[0058] As shown in Figure 9, when a portion of each honeycomb structure was immersed in water and observed, Comparative Example 2 (2.0C-0P-pristine) and Comparative Example 1 (1.8C-5.4P-pristine) showed structural collapse and instability in water after one day. On the other hand, Comparative Example 3 (2.0C-0P-heated) and Example 1 (1.8C-5.4P-heated) showed no structural collapse even after seven days, demonstrating their relative stability in water. However, the water in Comparative Example 3 (2.0C-0P-heated) turned yellow, indicating the dissolution of yellow low-molecular-weight components. On the other hand, the water in Example 1 (1.8C-5.4P-heated) showed almost no change in color despite the honeycomb being dark brown, suggesting that the structure is still strongly maintained. When the liquid was slowly removed with a pipette after 7 days, no structure remained in Comparative Example 2 (2.0C-0P-pristine) and Comparative Example 1 (1.8C-5.4P-pristine).On the other hand, Comparative Example 3 (2.0C-0P-heated) and Example 1 (1.8C-5.4P-heated) maintained their structure, demonstrating their stability even after prolonged immersion in water. Furthermore, it was shown that even in Example 2 (1.8C-3.6P-heated), in which the content of resol-type phenolic resin was lower than in Example 1, and in Example 3 (1.8C-1.8P-heated), in which the content was even lower, water resistance similar to that of Example 1 was obtained.

Claims

1. A honeycomb structure made of a composite material containing cellulose nanofibers and a phenolic resin, wherein the phenolic resin forms a cross-linked structure.

2. 2. The honeycomb structure according to claim 1, wherein the phenolic resin is a resol type phenolic resin.

3. The honeycomb structure according to claim 1, wherein the cellulose nanofibers are chemically defibrated cellulose nanofibers.

4. The honeycomb structure according to claim 1, wherein the content of the phenolic resin in the composite material is 0.5 to 10 times by mass relative to the cellulose nanofibers.

5. A method for manufacturing a honeycomb structure according to claim 1, wherein a container containing a dispersion of cellulose nanofibers and phenolic resin dispersed in a solvent containing water is gradually submerged in a refrigerant to perform unidirectional freezing, followed by freeze-drying to remove the solvent, and further heat treatment to crosslink the phenolic resin.

6. The method for manufacturing a honeycomb structure according to claim 5, wherein the cellulose nanofibers are chemically defibrated cellulose nanofibers.

7. 6. The method for manufacturing a honeycomb structure according to claim 5, wherein the phenolic resin is a water-soluble phenolic resin.

8. 8. The method for manufacturing a honeycomb structure according to claim 7, wherein the water-soluble phenolic resin is a water-soluble resol type phenolic resin.

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