Titanium dioxide-cellulose fiber composite material and its manufacturing method

Abstract

The present invention addresses the problem of providing a method which is for producing a titanium oxide-cellulose fiber composite material and is a low environmental load process, and by which a composite material having higher durability than conventional composite materials is obtained.　A method for producing a titanium oxide-cellulose fiber composite material according to the present invention comprises: (P1) preparing a cellulose fiber, and an aqueous solution containing ammonium fluorotitanate, boric acid, and water; and (P2, P3) precipitating titanium oxide on the surface of the cellulose fiber in the aqueous solution.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Patent Application No. 2020-187264, filed on November 10, 2020, the contents of which are incorporated herein by reference. [Technical Field]

[0002] The present invention relates to a titanium dioxide-cellulose fiber composite material and a method for producing the same. [Background technology]

[0003] Nanocellulose, which is obtained by nanoscaling cellulose, which has been used in paper and cotton, has new properties such as high strength, viscosity, and thermal stability, and is expected to be used in a variety of applications, including food, packaging, and diaphragms.

[0004] One application of nanocellulose is a composite material with photocatalytic properties made from titanium oxide and cellulose fibers. Patent Document 1 describes a nanofiber sheet with a layered TiO2 particle coating layer formed by alternately immersing a polymer nanofiber sheet produced by electrospinning multiple times in a positive electrolyte polymer aqueous solution in which TiO2 particles are dispersed and a negative electrolyte polymer aqueous solution in which TiO2 particles are dispersed, thereby forming TiO2 particles fixed with a first positively charged polymer layer and TiO2 particles fixed with a second negatively charged polymer layer on the surface of each nanofiber constituting the sheet.

[0005] Non-Patent Document 1 describes a fiber sheet having a TiO2 particle coating layer, which is obtained by subjecting TiO2 particles and algae-derived cellulose to ultrasonic dispersion treatment in acetone and drying the resulting mixture in an oven at 70°C. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-264386 [Non-patent literature]

[0007] [Non-Patent Document 1] Photochem. Photobiol. Sci., 17, 1301 (2018), PLoS ONE 7(10) (2012) Summary of the Invention [Problem to be solved by the invention]

[0008] However, the conventional composite material manufacturing method described above involves synthesizing a titanium oxide material with photocatalytic function and then physically adsorbing the titanium oxide material onto a cellulose material. Because the titanium oxide material is adsorbed onto the cellulose material, it is prone to peeling off from the cellulose material. This results in poor durability for conventional composite materials. Furthermore, titanium oxide synthesis is carried out using a high-temperature process, such as at 1000°C at atmospheric pressure, or a high-pressure process, such as hydrothermal synthesis, which is higher than atmospheric pressure. Therefore, conventional composite material manufacturing methods require a large amount of energy and place a heavy burden on the environment. These issues apply not only to cases where nanocellulose is used as the cellulose material, but also to cases where cellulose fibers other than nanocellulose are used.

[0009] The present invention aims to provide a titanium dioxide-cellulose fiber composite material with photocatalytic functionality that is more durable than conventional composite materials. Another object of the present invention is to provide a method for producing a titanium dioxide-cellulose fiber composite material with photocatalytic functionality, which results in a composite material with higher durability than conventional composite materials and is a process with a low environmental impact. [Means for solving the problem]

[0010] According to the invention of claim 1, a titanium oxide-cellulose fiber composite material comprises cellulose fiber and titanium oxide that coats at least a portion of the surface of the cellulose fiber. The titanium oxide coats the surface of the cellulose fiber by being directly synthesized on the surface of the cellulose fiber. Because the titanium oxide is directly synthesized on the surface of the cellulose fiber, a titanium oxide-cellulose fiber composite material with photocatalytic function and higher durability than conventional composite materials can be provided.

[0011] According to the invention of claim 3, a titanium oxide-cellulose fiber composite material comprises cellulose fiber and titanium oxide that coats at least a portion of the surface of the cellulose fiber. The adsorption force of titanium oxide to the cellulose fiber is 4.4 N / 10 mm or more. Since the adsorption force of titanium oxide to the cellulose fiber is 4.4 N / 10 mm or more, a titanium oxide-cellulose fiber composite material with photocatalytic function and higher durability than conventional composite materials can be provided.

[0012] According to the seventh aspect of the present invention, the method for producing a titanium oxide-cellulose fiber composite material is a method for directly synthesizing titanium oxide on the surface of cellulose fiber by liquid-phase precipitation. This method allows titanium oxide to be directly synthesized on the surface of cellulose fiber. Furthermore, the liquid-phase precipitation method allows titanium oxide to be synthesized at low temperature and atmospheric pressure, thereby minimizing the energy required for titanium oxide synthesis. This allows for the production of a titanium oxide-cellulose fiber composite material that is more durable than conventional composite materials and uses a low-environmental-impact process.

[0013] According to the eighth aspect of the present invention, a method for producing a titanium dioxide-cellulose fiber composite material includes the steps of (P1) preparing an aqueous solution containing ammonium titanium fluoride, boric acid, and water, and cellulose fiber, and (P2, P3) precipitating titanium dioxide on the surface of the cellulose fiber in the aqueous solution. This method allows titanium dioxide to be directly synthesized on the surface of the cellulose fiber. Furthermore, the liquid-phase precipitation method allows titanium dioxide to be synthesized at low temperature and atmospheric pressure, minimizing the energy required for titanium dioxide synthesis. This provides a highly durable composite material compared to conventional composite materials, and provides a method for producing a titanium dioxide-cellulose fiber composite material using a low-environmental-impact process. [Brief explanation of the drawings]

[0014] [Figure 1A] FIG. 1 is a schematic diagram showing cellulose fibers. [Figure 1B] FIG. 1 is a schematic diagram of a titanium oxide-cellulose fiber composite material comprising cellulose fiber and titanium oxide covering at least a portion of the surface of the cellulose fiber. [Figure 2A] This figure shows an example of the synthesis procedure for titanium dioxide-nanocellulose fiber composite material. [Figure 2B] This is a schematic diagram showing the mechanism by which TiO2 is synthesized on the surface of cellulose fibers. [Figure 3A] This is an SEM image of nanocellulose fibers derived from hardwood. [Figure 3B] This figure shows the XRD pattern of nanocellulose fibers derived from hardwood and the XRD pattern of a titanium dioxide-nanocellulose fiber composite material containing nanocellulose fibers derived from hardwood and anatase titanium dioxide nanoparticles. [Figure 4] This figure shows the Raman spectra of titanium dioxide-nanocellulose fiber composites derived from hardwood, softwood, or chemical pulp. [Figure 5A]SEM image of a titanium dioxide-nanocellulose fiber composite derived from hardwood. [Figure 5B] Figure 5A is an enlarged view of the titanium dioxide coating on the titanium dioxide nanocellulose fibers. [Figure 6A] This is an SEM image of the hardwood-derived titanium dioxide / nanocellulose fiber composite of Example 1 before being subjected to ultrasound. [Figure 6B] This is an SEM image of the hardwood-derived titanium dioxide nanocellulose fiber composite of Example 1 after being subjected to ultrasound. [Figure 6C] This is an SEM image of the TiO2-nanocellulose fiber composite of Comparative Example 1 before being subjected to ultrasound. [Figure 6D] SEM image of the TiO2·nanocellulose fiber composite of Comparative Example 1 after ultrasonic treatment. [Figure 7A] This is an SEM image of the coniferous tree-derived nanocellulose fiber / titanium oxide composite material of Example 2. [Figure 7B] This is an SEM image of a seaweed-derived titanium dioxide / nanocellulose fiber composite material from Non-Patent Document 1. [Figure 8] FIG. 7B is an enlarged view of a portion of FIG. 7A. [Figure 9A] This is an SEM image of the titanium dioxide-nanocellulose fiber composite material derived from chemical pulp in Example 3. [Figure 9B] This is an enlarged view of a portion of FIG. 9A. [Figure 10] This figure shows the photocatalytic effect of nanocellulose fiber, a titanium dioxide-nanocellulose fiber composite made from chemical pulp, and a titanium dioxide-nanocellulose fiber composite made from coniferous wood. [Figure 11] This is an SEM image of cotton as a cellulose fiber. [Figure 12A] 1 shows an SEM image of the TiO2-cellulose fiber composite material of Example 4. [Figure 12B] 1 shows an SEM image of the TiO2-cellulose fiber composite material of Example 4. [Figure 13] FIG. 1 is a schematic diagram showing a peel test. [Figure 14A] 1 shows an SEM image of the TiO2-cellulose fiber composite of Example 4 before the peel test. [Figure 14B] 1 is an SEM image of the TiO2·cellulose fiber composite of Example 4 after a peel test using the first tape. [Figure 14C] 10 is an SEM image of the TiO2·cellulose fiber composite of Example 4 before the peel test using the second tape. [Figure 15A] This is an SEM image of the TiO2-cellulose fiber composite material of Comparative Example 2 before the peel test. [Figure 15B] 1 shows an SEM image of the TiO2·cellulose fiber composite of Comparative Example 2 after a peel test using the first tape. [Figure 15C] This is an SEM image of the TiO2·cellulose fiber composite material of Comparative Example 2 before the peel test using the second tape. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments, and changes, modifications, and improvements can be made without departing from the scope of the invention.

[0016] As shown in Figure 1B, a titanium dioxide-cellulose fiber composite material 2, which is one embodiment of the present invention, comprises cellulose fiber 1 shown in Figure 1A and titanium dioxide nanoparticles 3 that cover at least a portion of the surface of cellulose fiber 1.

[0017] The cellulose fiber 1 is not particularly limited as long as it is cellulose in a fibrous form. From the viewpoint of increasing the effective area of ​​the photocatalyst, it is preferable to use nanocellulose fiber, which has a large specific surface area, as the cellulose fiber 1. Nanocellulose fiber is a general term for cellulose with a diameter of 1 nm or more and 100 nm or less. Nanocellulose fiber can be obtained by finely breaking down cellulose fibers, which are derived from plants, etc., using various physical and chemical methods. Nanocellulose fiber has a larger surface area per unit mass (specific surface area) than cellulose fiber (120-150 m 2 / g), low coefficient of thermal expansion and transparency.

[0018] The titanium oxide nanoparticles 3 have a photocatalytic function. The titanium oxide nanoparticles 3 are particles with a spherical outer shape. The spherical shape includes spheres, ellipsoids, and shapes similar to these. The particle diameter is 90 nm to 400 nm. The titanium oxide nanoparticles 3 have a plurality of protrusions. Each of the plurality of protrusions is a pyramidal shape with a width of 20 nm to 40 nm and a length of 20 nm to 60 nm. Note that the titanium oxide coating the surface of the cellulose fiber 1 is not limited to a particle shape, and may be plate-shaped.

[0019] A nanocellulose fiber composite material can be synthesized using the typical synthesis procedure shown in Figure 2A. In step P1, equal volumes of ammonium titanium fluoride ((NH4)2TiF6) and boric acid (H3BO3) are mixed to prepare a mixed solution. The molar concentration ratio of ammonium titanium fluoride to boric acid is 0.1:0.3. The molar concentration ratio is not limited to this. From the viewpoint of obtaining anatase-type titanium oxide with excellent crystallinity and high photocatalytic activity, the molar concentration ratio of ammonium titanium fluoride to boric acid is preferably 0.05-0.2:0.05-0.3, and more preferably 0.1:0.1-0.1:0.3.

[0020] Thus, in step P1, an aqueous solution containing ammonium titanium fluoride, boric acid, and water is prepared, and also in step P1, a nanocellulose fiber dispersion solution in which nanocellulose fibers are dispersed in water is prepared.

[0021] In step P2, the nanocellulose fiber dispersion solution is added to the prepared mixture. For example, the amount of nanocellulose fiber dispersion solution added is 20 mL. From the viewpoint of dispersibility of the cellulose fiber, the concentration of the cellulose fiber is preferably 0.02 to 0.2 mass percent.

[0022] In step P3, the mixture containing nanocellulose fibers from step P2 is stirred for 3 hours at a heating temperature of, for example, 60°C. From the viewpoint of synthesizing highly crystalline anatase titanium oxide with good reproducibility, the heating temperature is preferably 45 to 75°C. Furthermore, from the viewpoint of obtaining uniform nano-sized TiO2 crystals, the stirring time is preferably 3 hours. Steps P2 and P3 precipitate titanium oxide on the surface of nanocellulose fibers in an aqueous solution. In other words, titanium oxide is directly synthesized on the surface of nanocellulose fibers by the liquid phase precipitation method.

[0023] In step P4, following step P3, the mixture is stirred at room temperature for 24 hours. This separates the TiO2-nanocellulose fiber composite from the titanium oxide particles in the aqueous solution. The TiO2-nanocellulose fiber composite is synthesized through steps P4. Then, in step P5, the TiO2-nanocellulose fiber composite is washed with water.

[0024] As described above, the use of the liquid phase deposition (LPD) synthesis method allows for synthesis at low temperatures and atmospheric pressure. This low temperature refers to a temperature below the boiling point of water. This minimizes the energy required to synthesize titanium dioxide. Therefore, the manufacturing method for titanium dioxide-nanocellulose fiber composite material of this embodiment is an environmentally friendly process.

[0025] In step P2, other cellulose fibers may be used instead of nanocellulose fibers. Examples of cellulose fibers that can be used include cotton and mechanical pulp. In step P2, cellulose fibers such as cotton and mechanical pulp may be immersed in the prepared mixed solution. On the other hand, from the viewpoint of increasing the effective area of ​​the photocatalyst, nanocellulose fibers with a large specific surface area are preferred.

[0026] In step P3, the mixture containing nanocellulose fibers does not need to be heated. Even in this case, it takes longer than if heated, but titanium oxide can be precipitated on the surface of the nanocellulose fibers. However, from the perspective of enhancing the photocatalytic function, it is preferable to heat the mixture in step P3.

[0027] The mechanism by which TiO2 is synthesized on the surface of cellulose fibers is presumed to be as follows. As shown in Figure 2B, titanium fluoride ions are hydrolyzed by the hydroxyl groups on the surface of the cellulose fibers and the water in the solution. That is, a ligand exchange reaction occurs, in which fluorine atoms in the titanium fluoride ions are replaced by oxygen atoms in the hydroxyl groups on the surface of the cellulose fibers and oxygen atoms in the water molecules. The resulting fluorine ions are then consumed by a reaction with boric acid. This is presumed to advance the titanium oxide precipitation reaction, resulting in the formation of titanium oxide nuclei and nanostructures on the cellulose surface. It is presumed that the Ti atoms of TiO2 covalently bond to the oxygen atoms of the cellulose fibers in the synthesized TiO2 and cellulose fibers.

[0028] As explained above, titanium dioxide is directly synthesized on the surface of cellulose fibers, coating the surface of the cellulose fibers. This means that titanium dioxide is chemically bonded to the surface of the cellulose fibers. This makes it possible to provide a titanium dioxide-cellulose fiber composite material that is more durable than conventional composite materials and has photocatalytic properties. [Example]

[0029] (Production and Evaluation of TiO2-Nanocellulose Fiber Composites in Examples 1 to 3 and Comparative Example 1) Using ammonium titanium fluoride (Fujifilm Wako Pure Chemical Industries, Ltd., first-grade reagent), boric acid (Fujifilm Wako Pure Chemical Industries, Ltd., special-grade reagent), and a nanocellulose fiber dispersion solution in which hardwood-derived nanocellulose fibers were dispersed (Daio Paper Co., Ltd., nanocellulose fiber concentration: 15.57% by mass), a washed hardwood-derived TiO2-nanocellulose fiber composite was obtained according to Figure 2 (Example 1, sometimes simply referred to as "hardwood-derived TiO2-nanocellulose fiber composite material").

[0030] Instead of hardwood-derived nanocellulose fibers, a nanocellulose fiber dispersion solution (manufactured by Daio Paper Co., Ltd., nanocellulose fiber concentration: 3.5% by mass) in which softwood-derived nanocellulose fibers were dispersed was used to obtain a washed softwood-derived TiO2-nanocellulose fiber composite (Example 2, sometimes simply referred to as "softwood-derived TiO2-nanocellulose fiber composite material").

[0031] Instead of nanocellulose fibers derived from hardwood, a nanocellulose fiber dispersion solution (manufactured by Daio Paper Co., Ltd., nanocellulose fiber concentration: 2.9 mass%) in which nanocellulose fibers from chemical pulp were dispersed was used to obtain a washed chemical pulp TiO2-nanocellulose fiber composite (Example 3, sometimes simply referred to as "chemical pulp TiO2-nanocellulose fiber composite material").

[0032] The SEM image in Figure 3A reveals that nanocellulose fibers derived from hardwoods are as follows: The fibrous cellulose is entangled and forms an aggregate. For example, compared to a scale of 1 μm, it can be seen that the diameter of a single bundle is 50 nm or less. The sample in the SEM image is nanocellulose fiber that has been dried and prepared, so it forms aggregates as seen in the image, but in the aqueous dispersion, it is thought that each fiber is isolated and dispersed.

[0033] Furthermore, the XRD pattern in Figure 3B detects peaks of anatase-type titanium dioxide nanoparticles (sometimes called "TiO2A") after LPD (liquid phase deposition) synthesis. This indicates that anatase-type titanium dioxide nanoparticles precipitated on the surface of the hardwood-derived TiO2 nanocellulose fibers, and that at least a portion of the surface was coated with anatase-type titanium dioxide nanoparticles. The Raman spectrum measurement conditions and equipment used were as follows: The equipment used was a JASCO NRS-3300 microscopic Raman spectrometer (RAMAN). The excitation laser wavelength was 532 nm, and the spectrum acquisition time was 60 seconds.

[0034] As shown in the Raman spectra in Figure 4, peaks of anatase titanium oxide were detected in the hardwood-derived TiO2-nanocellulose fiber composite (Example 1), the softwood-derived TiO2-nanocellulose fiber composite (Example 2), and the chemical pulp TiO2-nanocellulose fiber composite (Example 3). The anatase titanium oxide (TiO2A) peak is located at 170 cm -1 and 400-650cm -1 The Raman spectrum was measured under the following conditions and with the following equipment: The equipment used was a JASCO NRS-3300 Raman microscope (RAMAN). The excitation laser wavelength was 532 nm, and the spectrum acquisition time was 60 seconds.

[0035] As shown in Figure 5A, the hardwood-derived TiO2-nanocellulose fiber composite 12 includes nanocellulose fiber aggregates 16, nanocellulose fibers 17 with loosely bound TiO2 layers, and nanocellulose fiber composite 13 with densely bound TiO2. Furthermore, as shown in Figure 5B, the anatase-type titanium dioxide nanoparticles 13a have a spherical shape close to a sphere or ellipsoid, and their surfaces are irregular. In other words, the anatase-type titanium dioxide nanoparticles 13a have protrusions on their outer surfaces. Furthermore, it was observed that multiple anatase-type titanium dioxide nanoparticles, such as the anatase-type titanium dioxide nanoparticles 13a, gather together to form small aggregates, and these small aggregates then gather together to form aggregates on the nanocellulose. For example, compared to a 1 μm scale, the particle size of the anatase-type titanium dioxide nanoparticles 13a was found to be 100–400 nm. The particle size here refers to the maximum width of the particle.

[0036] The durability of anatase-type titanium dioxide nanoparticles, such as anatase-type titanium dioxide nanoparticles 23, was investigated for hardwood-derived TiO₂ / nanocellulose fiber composite 22 (Figure 6A) as follows. Hardwood-derived TiO₂ / nanocellulose fiber composite 22 was immersed in water and subjected to ultrasound at 40 kHz and 300 W for 5 hours (ultrasonicator: SHARP, UT-304F). However, as shown in Figure 6B, no change was observed between anatase-type titanium dioxide nanoparticles 23a in hardwood-derived TiO₂ / nanocellulose fiber composite 22a after ultrasound exposure and anatase-type titanium dioxide nanoparticles 23 before ultrasound exposure (anatase-type titanium dioxide nanoparticles 23). This confirmed the durability of anatase-type titanium dioxide nanoparticles, such as anatase-type titanium dioxide nanoparticles 23. Note that, compared to a 1 μm scale, the spherical anatase-type titanium dioxide nanoparticles 23 range in size from 100 nm to 400 nm. The size of the protrusions is several tens of nanometers for all anatase type titanium oxide nanoparticles 23.

[0037] The inventors produced the TiO₂-nanocellulose fiber composite material of Comparative Example 1 using the manufacturing method described in Non-Patent Document 1. Specifically, anatase titanium dioxide (Evonik (Degussa) P25) was added to distilled water and ultrasonicated for 15 minutes using an ultrasonicator (SHARP, UT-304F) to disperse the anatase titanium dioxide. Nanocellulose (manufactured by Daio Paper Co., Ltd., nanocellulose fiber concentration: 15.57% by mass) was then added and ultrasonicated for 1 hour. The mixture was then allowed to stand at room temperature for 1 minute. The supernatant titanium dioxide was removed, and the precipitate was dried at room temperature for at least 48 hours to obtain the TiO₂-nanocellulose fiber composite material. As shown in Figure 6C, in the TiO₂-nanocellulose fiber composite material of Comparative Example 1, titanium dioxide nanoparticles were attached to the fiber surfaces, but individual fibers were not covered with titanium dioxide nanoparticles.

[0038] Furthermore, the inventors conducted a durability test using ultrasonic cleaning to examine the durability of the anatase titanium dioxide nanoparticles in the TiO2-nanocellulose fiber composite material of Comparative Example 1. The ultrasonic device and processing conditions used were the same as those used in the durability test for the hardwood-derived TiO2-nanocellulose fiber composite material 22, except that the processing time was 1 hour.

[0039] The coverage of titanium oxide nanoparticles before ultrasonic irradiation, shown in Figure 6C, was 77.6%. In contrast, the coverage of titanium oxide nanoparticles after ultrasonic irradiation, shown in Figure 6D, was 66.7%. The composite material after ultrasonic irradiation contained fewer titanium oxide nanoparticles than the composite material before ultrasonic irradiation. This confirms that the durability of the titanium oxide nanoparticles in the TiO2-nanocellulose fiber composite material of Comparative Example 1 is low.

[0040] Compared to the TiO2-nanocellulose fiber composite material of Comparative Example 1, in which titanium oxide nanoparticles are physically adsorbed to the nanocellulose fibers, the durability of the TiO2-nanocellulose fiber composite material of Example 1 is higher. From this, it is presumed that in the TiO2-nanocellulose fiber composite material of Example 1, the titanium oxide nanoparticles are chemically adsorbed, i.e., chemically bonded, to the nanocellulose fibers.

[0041] As shown in Figure 7A, the softwood-derived TiO2-nanocellulose fiber composite 32 contained anatase titanium dioxide nanoparticles, such as anatase titanium dioxide nanoparticles 33, which coated the surface of the softwood-derived nanocellulose fibers. The particle size of anatase titanium dioxide nanoparticles 33 was 100-300 nm, compared to the 1 μm scale.

[0042] On the other hand, the TiO2-nanocellulose fiber composite 36 shown in Figure 7B was as follows: The TiO2-nanocellulose fiber composite 36 in Non-Patent Document 1 contains titanium oxide nanoparticles 35 aggregated on the cellulose surface. In addition, there are also areas where the cellulose surface is exposed.

[0043] Comparing Figures 7A and 7B reveals the following: In the TiO2-nanocellulose fiber composite 36 reported in Non-Patent Document 1, the cellulose surface is partially exposed and is covered with aggregated titanium dioxide nanoparticles. In contrast, in the coniferous TiO2-nanocellulose fiber composite 32, the cellulose surface is completely covered with titanium dioxide nanoparticles, with no visible aggregates.

[0044] The following observations were made about anatase titanium dioxide nanoparticles such as the anatase titanium dioxide nanoparticles 33 in Figure 7A in the enlarged view shown in Figure 8. The anatase titanium dioxide nanoparticles 33a that cover the surface of the coniferous tree-derived nanocellulose fibers are composed of an aggregate of needle-shaped crystals, and no exposed cellulose was observed. When comparing anatase titanium dioxide nanoparticles 33a with a scale of, say, 1 μm, it can be seen that the width of the protrusions is several tens of nanometers.

[0045] As shown in Figure 9A, the TiO2-nanocellulose fiber composite 42 contained anatase titanium dioxide nanoparticles, such as anatase titanium dioxide nanoparticles 43, which coated the surface of the nanocellulose fiber. The anatase titanium dioxide nanoparticles 43 were spherical in shape. The particle size of the anatase titanium dioxide nanoparticles 43 was approximately 100 nm, compared to a 1 μm scale.

[0046] The following observations were made about anatase titanium oxide nanoparticles such as anatase titanium oxide nanoparticles 43 in the enlarged view shown in Figure 9B. Titanium oxide nanoparticles are composed of aggregates of smaller titanium oxide needle crystals. A single titanium oxide nanoparticle is an aggregate of crystals that grow radially from the surface of the nanocellulose fiber. In other words, anatase titanium oxide nanoparticles 43a are spherical in shape with protrusions. Comparing anatase titanium oxide nanoparticles 43a to a scale of, say, 1 μm, the protrusions are 60 nm long and 25 nm wide.

[0047] The photocatalytic activity of nanocellulose fiber (Comparative Example 1, cellulose only), a TiO2-nanocellulose fiber composite derived from coniferous wood (Example 2), and TiO2-nanocellulose fiber derived from chemical pulp (Example 3) was evaluated. The photocatalytic activity was evaluated as follows: A methylene blue aqueous solution (200 mM) was applied to the sample and dried. The absorbance of the methylene blue dye was then measured for 80 seconds while irradiating it with ultraviolet light. The measurement conditions and equipment used to evaluate the photocatalytic activity were as follows: The measurement equipment was a homemade setup using an ultraviolet laser light source. A Kimmon Koha He-Cd laser (wavelength 325 nm, 5 mW) was used as the ultraviolet light source. A red laser (wavelength 650 nm) was used to measure the absorbance, and the light intensity transmitted through the sample was detected with a photodetector.

[0048] As shown in Figure 10, nanocellulose fiber (cellulose only) did not exhibit photocatalytic activity, but both the chemical pulp TiO2 / nanocellulose fiber composite and the softwood-derived TiO2 / nanocellulose fiber composite exhibited photocatalytic activity. Specifically, in the chemical pulp TiO2 / nanocellulose fiber composite, the dye concentration normalized to the initial concentration decreased within 10 seconds of UV irradiation. In the softwood-derived TiO2 / nanocellulose fiber composite, the dye concentration normalized to the initial concentration decreased within 80 seconds of UV irradiation, with the decrease in dye concentration being particularly significant within 20 seconds of UV irradiation. The softwood-derived TiO2 / nanocellulose fiber composite exhibited a greater photocatalytic activity than the chemical pulp TiO2 / nanocellulose fiber composite.

[0049] (Production and Evaluation of TiO2-Cellulose Fiber Composite Materials in Example 4 and Comparative Example 2) Instead of using the nanocellulose fiber dispersion solution of Example 1, the inventors used a 50 mm square piece of bleached narrow cotton (100% cotton) as the cellulose fiber. Figure 11 shows an SEM image of this cellulose fiber. The diameter of this cellulose fiber is micron-sized. This cellulose fiber was immersed in the mixed solution, and steps P3, P4, and P5 were carried out under the same synthesis conditions as in Example 1 to obtain the TiO2-cellulose fiber composite material of Example 4.

[0050] Figures 12A and 12B are SEM images of the TiO2-cellulose fiber composite material of Example 4. In areas where the coverage rate is 100%, the titanium oxide coating the cellulose fibers shows no visible particle boundaries, presumably forming flat aggregates. In areas where the coverage rate is less than 100%, the titanium oxide coating the cellulose fibers is spherical in shape. The particle diameter is 100 nm to 400 nm. Although not shown in Figures 12A and 12B, the particles have multiple protrusions. Each of the multiple protrusions is pyramidal, measuring 30 nm to 40 nm in width and 40 nm to 60 nm in length.

[0051] The inventors also produced a TiO₂-cellulose fiber composite material of Comparative Example 2 using the manufacturing method described in Non-Patent Document 1. Specifically, anatase-type titanium dioxide (Evonik (Degussa) P25) was added to distilled water and ultrasonicated for 15 minutes using an ultrasonicator (SHARP, UT-304F) to disperse the anatase-type titanium dioxide. The same type of cloth used in Example 4 was immersed in the water as cellulose fiber and ultrasonicated for 1 hour. The material was then dried at room temperature for 48 hours to obtain the TiO₂-cellulose fiber composite material of Comparative Example 2. The TiO₂-cellulose fiber composite material of Comparative Example 2 was obtained by physical adsorption of titanium dioxide nanoparticles onto the cellulose fiber.

[0052] The durability of the titanium oxide nanoparticles was then tested by peeling test for the TiO2-cellulose fiber composite material of Example 4 and the TiO2-cellulose fiber composite material of Comparative Example 2. For the peeling test, each TiO2-cellulose fiber composite material was fixed to a glass substrate with double-sided tape, and the following two different types of tape (i.e., Tapes 1 and 2) were used as test tapes. Tape 1 was NITOMS Masking Tape (manufactured by NITOMS), product name: J8102, adhesive strength: 2.06 N / 10 mm. Tape 2 was TRUSCO cloth adhesive tape, product number: GNT-50, adhesive strength: 4.4 N / 10 mm.

[0053] As shown in Figure 13, a peel test was conducted by fixing tape 101 to roller 102 and moving tape 101 horizontally in contact with sample 103. Figures 14A, 14B, and 14C show SEM images of the TiO2-cellulose fiber composite material of Example 4 before and after the test. Figures 15A, 15B, and 15C show SEM images of the TiO2-cellulose fiber composite material of Comparative Example 2 before and after the test. Figures 14A and 15A show SEM images before the peel test. Figures 14B and 15B show SEM images after the first tape was peeled. Figures 14C and 15C show SEM images after the second tape was peeled.

[0054] The inventors used the Bi-modal Fit algorithm to determine the area of ​​the particle portion in each SEM image and calculate the particle coverage. The software used for this calculation was Igor Pro (Version: 8.04, WaveMetrics). Table 1 shows the coverage values ​​before and after the test.

[0055] [Table 1] If the difference between the average coverage rate before peeling and the average coverage rate after peeling is within the error range based on the 95% confidence interval of the normal distribution, it can be determined that no detachment has occurred.

[0056] As shown in Table 1, in Example 4, the difference between the average coverage rate before and after peeling was 2.6% for the first tape and 2.4% for the second tape. These differences were within a range of 3.51% (i.e., 1.95 × SE), where SE is the standard error of the sample. Therefore, in Example 4, it can be determined that the titanium oxide nanoparticles were not detached after the peel test for either the first or second tape. This indicates that the adsorption force of titanium oxide nanoparticles to cellulose fiber in the TiO2-cellulose fiber composite material of Example 4 was 4.4 N / 10 mm or greater.

[0057] In contrast, in Comparative Example 2, the standard error before the peel test was large, and the coverage rate did not follow a normal distribution. In other words, the titanium oxide nanoparticles were unevenly attached. Furthermore, the error in the average coverage rate after peeling the first and second tapes decreased, suggesting that aggregates of large-sized titanium oxide nanoparticles had detached. This indicates that the adsorptive strength of titanium oxide nanoparticles to cellulose fiber in the TiO2-cellulose fiber composite material of Comparative Example 2 was less than 2.06 N / 10 mm.

[0058] From the above, it was found that the TiO2-cellulose fiber composite material obtained by the manufacturing method of the present invention has higher durability than the TiO2-cellulose fiber composite material obtained by physical adsorption of titanium oxide particles.

[0059] The inventors also conducted antibacterial tests according to JIS R 1702:2020 on the TiO2-cellulose fiber composite material of Example 4 and the cellulose fiber of Comparative Example 3. The cellulose fiber of Comparative Example 3 was the same type of fabric as the fabric used in Example 4, but without titanium oxide composite. In the antibacterial test, the test specimens were inoculated with bacteria and then irradiated with light to detect the number of bacteria. The number of bacteria in each test specimen after washing was also detected. The test bacterial strain, irradiance, amount of test bacterial solution ingested, and washing method used in the antibacterial test were as follows:

[0060] Test strain: Staphylococcus aureus ·Irradiance: 0.10mW / cm 2 ·8 hours Test bacterial solution intake: 0.2 mL [Test bacterial solution containing 0.05% surfactant (Tween 80)] Washing method: (Inc.) Textile Evaluation Technology Council "SEK Mark Textile Product Washing Method" - Standard washing method

[0061] Table 2 shows the results of the antibacterial test of the cellulose fiber of Comparative Example 3. [Table 2] As shown in Table 2, the number of live bacteria after light irradiation and in the dark increased compared to the number of live bacteria immediately after inoculation.

[0062] Table 3 shows the antibacterial test results of the TiO2-cellulose fiber composite material of Example 4. [Table 3] The viable bacterial count immediately after inoculation of the TiO2-cellulose fiber composite material of Example 4 was the same as the viable bacterial count immediately after inoculation of the cellulose fiber of Comparative Example 3. As shown in Table 3, the common logarithm of the viable bacterial count after light irradiation with zero washings decreased from 4.4, the common logarithm of the viable bacterial count immediately after inoculation, to 1.3. This confirmed the photocatalytic antibacterial effect. Furthermore, the viable bacterial count after light irradiation with 10 washings was the same as the common logarithm of the viable bacterial count after light irradiation with zero washings. This confirmed that the photocatalytic antibacterial effect was maintained.

[0063] Furthermore, the common logarithm of the viable bacterial count in the dark after zero washings decreased from 4.4, the common logarithm of the viable bacterial count immediately after inoculation, to 1.3. This confirmed that the TiO2-cellulose fiber composite material of Example 4 possesses antibacterial properties even without light exposure. Furthermore, the viable bacterial count in the dark after 10 washings was 2.8, lower than the common logarithm of 4.4, the viable bacterial count immediately after inoculation. This confirmed that the antibacterial effect was maintained even without light exposure.

[0064] The inventors also conducted an antiviral performance evaluation test using viruses for the TiO2 / cellulose fiber composite material of Example 4 and the cellulose fiber of Comparative Example 3 in accordance with JIS R 1706:2020 and ISO 18184. In this evaluation test, the test samples were inoculated with viruses, then irradiated with light to detect the number of viruses. The test conditions were as follows:

[0065] Test piece size: 50 mm x 50 mm Number of n: n = 1 Test phage: Influenza A virus (H3N2) A / Hong Kong / 8 / 68 strain (influenza A virus, ATCC VR-1679) Host cell: MDCK cell (ATCC CCL-34) Preliminary irradiation conditions: UV light (FL20S BLB) 1.0 mW / cm 2 , 24 hours Sterilization of test samples: irradiation with a germicidal lamp Light source type: Black light fluorescent lamp FL20S BLB Irradiation conditions: Dark place and ultraviolet light 0.1 mW / cm 2 , Irradiation time 0, 8 hours Illuminance meter: UV integrating light meter (C9536-01 and H9958, Hamamatsu Photonics) Adhesive film: Polypropylene film (VF-10, KOKUYO), 40 mm x 40 mm Moisture-retaining glass: borosilicate glass Inoculation virus concentration: 3.2 x 10 6 pfu / ml ·Inoculation amount: 0.2 ml / sample

[0066] Table 4 shows the test results. [Table 4] In Table 4, "<2.0 x 10 2 " indicates a value lower than the detection limit. In the TiO2-cellulose fiber composite material of Example 4, the viral infectivity after light irradiation was lower than the detection limit. This demonstrates that the TiO2-cellulose fiber composite material of Example 4 has a photocatalytic antiviral effect.

[0067] Furthermore, the viral infectivity after 8 hours in the dark for the TiO2-cellulose fiber composite material of Example 4 was lower than the detection limit, demonstrating that the TiO2-cellulose fiber composite material of Example 4 has an antiviral effect even without light irradiation.

[0068] As explained above, the TiO2-cellulose fiber composite material of Example 4 has antibacterial and antiviral effects even without light irradiation. This is thought to be due to the spherical titanium oxide particles that coat the surface of the cellulose fiber and the multiple protrusions on these particles. [Industrial Applicability]

[0069] The titanium dioxide-cellulose fiber composite material of this invention can be used in packaging and wall materials with environmental purification and antibacterial functions, as well as household and medical masks. [Explanation of symbols]

[0070] 1: Cellulose fiber 2: Titanium dioxide-cellulose fiber composite 3: Titanium oxide (TiO2) nanoparticles 12, 22: Hardwood-derived TiO2 nanocellulose fiber composite 13, 13a, 23, 23a, 33, 33a, 43, 43a: Anatase-type titanium oxide nanoparticles 16: Nanocellulose fiber aggregate fiber 17: Nanocellulose fiber 32: Coniferous wood-derived TiO2 nanocellulose fiber composite 35: Titanium oxide nanoparticles 36: TiO2 nanocellulose fiber composite reported in Non-Patent Document 1 42: Chemical pulp TiO2-nanocellulose fiber composite

Claims

1. A titanium dioxide / cellulose fiber composite material, Cellulose fiber and and titanium oxide coating at least a portion of the surface of the cellulose fiber, A titanium oxide-cellulose fiber composite material, in which the titanium oxide is directly synthesized on the surface of the cellulose fiber, thereby coating the surface of the cellulose fiber.

2. 2. The titanium oxide / cellulose fiber composite material according to claim 1, wherein the synthesis is carried out by a liquid phase precipitation method using an aqueous solution containing ammonium titanium fluoride, boric acid, and water.

3. A titanium dioxide / cellulose fiber composite material, Cellulose fiber and and titanium oxide coating at least a portion of the surface of the cellulose fiber, A titanium oxide / cellulose fiber composite material, wherein the adsorption force of the titanium oxide to the cellulose fiber is 4.4 N / 10 mm or more.

4. The titanium oxide is in the form of particles having a spherical outer shape, The particle diameter is 90 nm or more and 400 nm or less, The particle has a plurality of protrusions, 4. The titanium oxide / cellulose fiber composite material according to claim 1, wherein each of the plurality of protrusions has a cone shape having a width of 20 nm to 40 nm and a length of 20 nm to 60 nm.

5. 5. The titanium oxide / cellulose fiber composite material according to claim 1, wherein the cellulose fibers are nanocellulose fibers having a diameter of 1 nm or more and 100 nm or less.

6. 6. The titanium oxide-cellulose fiber composite material according to claim 1, wherein the titanium oxide is anatase type.

7. A method for producing titanium oxide / cellulose fiber composite materials by directly synthesizing titanium oxide on the surface of cellulose fiber using the liquid phase deposition method.

8. A method for producing a titanium oxide / cellulose fiber composite material, comprising the steps of: Preparing an aqueous solution containing ammonium fluoride titanate, boric acid, and water, and cellulose fiber (P1); and precipitating titanium oxide on the surface of the cellulose fiber in the aqueous solution (P2, P3).

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