Fiber sheet

Abstract

The present invention provides a fiber sheet that makes it possible to increase rigidity while suppressing a decrease in flexibility. The fiber sheet comprises first fibers, second fibers that are stacked on the first fibers so as to cross the first fibers, and at least one bead that is for reinforcement, wherein the at least one bead is disposed in the vicinity of an intersection where a first fiber and a second fiber cross, in a gap adjacent to a part where the first fiber and the second fiber contact each other, and in contact with the first fiber and the second fiber.

Description

Fiber Sheet

[0001] The present invention relates to a fiber sheet.

[0002] Fiber sheets containing fibers such as organic nanofibers are known. The fiber sheets have, for example, a structure in which a plurality of fibers extending in any direction are stacked. For example, Patent Document 1 discloses a film (fiber sheet) containing cellulose nanofibers.

[0003] Japanese Patent Application Laid-Open No. 2008-209595

[0004] The fiber sheet of Patent Document 1 has room for improvement in terms of further improving rigidity while ensuring sufficient flexibility.

[0005] Therefore, an object of the present invention is to solve the above problems by providing a fiber sheet that can increase rigidity while suppressing a decrease in flexibility.

[0006] A fiber sheet according to one aspect of the present invention comprises: a first fiber; a second fiber stacked on the first fiber so as to intersect with the first fiber; and at least one particle for reinforcement, wherein the at least one particle is arranged in contact with the first fiber and the second fiber in a gap adjacent to the contact point between the first fiber and the second fiber near the intersection where the first fiber and the second fiber intersect.

[0007] The fiber sheet according to the present invention can increase rigidity while suppressing a decrease in flexibility.

[0008] 5A , 5B, 5C, 5D, 5E, 5F, 5G, 5G-5H ...

[0009] (Findings on which the present invention is based) The present inventors have conducted extensive research to further improve the rigidity of a fiber sheet while maintaining its flexibility, and have made the following findings.

[0010] Conventional fiber sheets may have problems such as low rigidity (easily bent) and difficulty in handling.

[0011] To address this problem, for example, it is possible to increase the mechanical strength (including rigidity) of the fiber sheet by forming a thick protective film on the fiber surface. However, since the thickness of the protective film increases over the entire surface of the fiber sheet, the flexibility of the fiber sheet decreases. As a result, the flexibility required for the application may not be achieved. "Flexibility" refers to the flexibility required for substrates such as printed wiring boards, and can be evaluated by a bending resistance test (JIS C5016) or the like. If the flexibility of the fiber sheet decreases, the protective film is more likely to break or peel when the fiber sheet is bent, and the desired mechanical strength may not be achieved.

[0012] Therefore, after extensive research, the inventors discovered that by arranging reinforcing particles near the intersection of two fibers so that they directly or indirectly contact the two fibers, it is possible to increase rigidity while maintaining sufficient flexibility. According to this configuration, the particles arranged near the intersection function like "wedges," thereby increasing the rigidity of the fiber sheet. Furthermore, since the reinforcing particles can be arranged near the intersection while keeping the contact area with the fibers smaller than, for example, a protective film, a decrease in flexibility can be suppressed. Based on this novel finding, the inventors arrived at the following invention.

[0013] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to these embodiments. In addition, substantially identical components in the drawings are designated by the same reference numerals. For illustrative purposes, the dimensions of each element in the drawings may be exaggerated and are not necessarily drawn to scale.

[0014] In the following, for the sake of convenience, terms indicating directions such as "up," "down," "right," "left," and "side" are used assuming a state during normal use, but are not intended to limit the state of use of the fiber sheet according to the present disclosure. Furthermore, "perpendicular" or "substantially perpendicular" includes cases where the directions are substantially perpendicular, taking into account the range of realistic variation.

[0015] In the drawings described below, for reference, mutually orthogonal X-axis, Y-axis, and Z-axis are schematically shown. The Z-axis corresponds to the thickness direction of the fiber sheet.

[0016] <<Embodiments>> Fig. 1 is a schematic perspective view of a fiber sheet according to an embodiment of the present invention. Fig. 1 also shows a schematic enlarged perspective view of a portion of the fiber sheet. Fig. 2 is a schematic cross-sectional view taken along line II-II in Fig. 1. Fig. 3 is a schematic cross-sectional view taken along line III-III in Fig. 1. Fig. 4 is a scanning electron microscope (SEM) image of an example of the fiber sheet shown in Fig. 1.

[0017] 1 to 4, the fiber sheet 1 includes a plurality of fibers 2 and particles 4 for reinforcing the fiber sheet 1. The surface of each fiber 2 is covered with, for example, a protective film 3.

[0018] The fiber sheet 1 is, for example, a nonwoven fabric in which a plurality of fibers 2 are oriented randomly or in one direction. The fiber sheet 1 is not limited to a nonwoven fabric and may be a woven fabric. In the example shown in Fig. 4, a plurality of fibers 2 extending in random directions are stacked in an arbitrary direction (here, roughly the Z direction) to form the fiber sheet 1.

[0019] In FIG. 1 , the fiber sheet 1 is a rectangular sheet, but the shape of the fiber sheet 1 is not particularly limited. The thickness t of the fiber sheet 1 can be appropriately selected depending on the application, the type of fiber, the forming method, etc. As an example, the thickness t is 1 μm or more and 200 μm or less. The porosity of the fiber sheet 1 is not particularly limited, but is, for example, 70% or more and 95% or less.

[0020] The fiber 2 is, for example, a cylindrical or approximately cylindrical fibrous material, and is, for example, an organic fiber such as a cellulose fiber or a polyimide fiber.

[0021] The width of the fiber 2 is not particularly limited, but may be 0.1 μm to 2 μm. The "fiber width" refers to the maximum width in a cross section perpendicular to the extension direction of the fiber, and in the case of a cylindrical fiber, refers to the diameter of the cylinder. If the width w of the fiber 2 is 2 μm or less, a flexible (pliable) fiber sheet 1 can be obtained. The fiber 2 may be a nanofiber having a diameter of 1 μm or less (e.g., 1 nm to 1000 nm) and a length 100 times or more the diameter. As shown in FIG. 4 , the diameters of multiple fibers 2 may be different from one another. In this case, the diameter of the thickest fiber may be, for example, 7 times or less the diameter of the thinnest fiber.

[0022] The plurality of fibers 2 include first fibers 21 and second fibers 22. The second fibers 22 are stacked on the first fibers 21 so as to intersect with the first fibers 21. In this specification, a portion 5 where two fibers 2 intersect with each other is referred to as an "intersection." In the example shown in FIG. 3 , the first fibers 21 and the second fibers 22 are stacked in the Z direction (i.e., the thickness direction of the fiber sheet 1). At the intersection 5, the first fibers 21 and the second fibers 22 are in contact with each other (here, directly in contact with each other).

[0023] The protective film 3 is formed so as to cover the surface of each fiber 2. The protective film 3 is, for example, an oxide film. The oxide film is made of alumina (Al ２ O ３ ) film, titania (TiO ２ The protective film 3 may be a metal oxide film such as a silicon dioxide film, or a silicon dioxide film. The thickness of the protective film 3 is smaller than the width (diameter) of the fiber 2, and is, for example, 0.1 μm or less.

[0024] The protective film 3 includes a first protective film 31 that covers the surface of the first fiber 21 and a second protective film 32 that covers the surface of the second fiber 22. The first protective film 31 and the second protective film 32 may be films formed using the same material in a common film formation process.

[0025] 3 , at the intersection 5, the first protective film 31 may not be formed on the portion of the surface of the first fiber 21 that contacts the second fiber 22. Similarly, the second protective film 32 may not be formed on the portion of the surface of the second fiber 22 that contacts the first fiber 21.

[0026] 3 , portions (hereinafter referred to as “gaps”) 51 and 52 adjacent to the contact portion between the first fiber 21 and the second fiber 22 and located between these fibers are formed near the intersection 5. The gaps 51 and 52 are located on both sides of the contact portion in the extension direction of the first fiber 21. In the gaps 51 and 52, the first protective film 31 covering the first fiber 21 and the second protective film 32 covering the second fiber 22 may be in contact with (connected to) each other.

[0027] Depending on the method and conditions for forming the protective film 3, it may be difficult to form the protective film 3 near the intersection 5. In this case, as shown in the figure, the first protective film 31 and / or the second protective film 32 may be formed thinner near the intersection 5 (e.g., in the gaps 51 and 52) than in portions away from the intersection 5. Also, a portion of the surface of the first fiber 21 and / or the second fiber 22 may be exposed from the protective film. As a result, the first protective film 31 and the second protective film 32 may not be connected.

[0028] The grains 4 are metal oxide grains containing, for example, a metal oxide such as alumina or titania. In this embodiment, the grains 4, the first protective film 31, and the second protective film 32 are made of the same material. The outer surface of the grains 4 may include a curved surface that forms a spherical surface or a portion of a nearly spherical surface, or a portion of a prolate spheroid or a nearly prolate spheroid. The width (maximum width) of the grains 4 may be, for example, greater than the thickness of the protective film 3 and smaller than the width of the fibers 2 (for example, the width of the thickest fiber 2).

[0029] The particles 4 are arranged so as to be in contact with both the first fiber 21 and the second fiber 22 near the intersection 5 where the first fiber 21 and the second fiber 22 intersect. In this specification, "in contact" refers not only to direct contact with the fiber surfaces but also to indirect contact. In this embodiment, the particles 4 are in contact (direct contact in this case) with the first protective film 31 and the second protective film 32, and are in contact with the first and second fibers 21 and 22 via these protective films 31 and 32. Furthermore, the particles 4 are arranged in gaps 51 adjacent to the contact portions between the first fiber 21 and the second fiber 22 near the intersection 5. "Particles are arranged in the gaps" refers not only to cases where the entire particles 4 are located in the gaps, but also to cases where at least a portion of the particles 4 is located in the gaps. The particles 4 may be located, for example, in one or more of the gaps 51, 52 located on both sides of the contact portion in the extension direction of the first fiber 21, and the gaps 53, 54 located on both sides of the contact portion in the extension direction of the second fiber 22 (see Figure 8B).

[0030] (Fiber, Protective Film, and Particle Materials) The fiber 2 is, for example, an organic fiber containing an organic material. The organic material may be one or more of cellulose, polyimide, polylactic acid, chitin, silk, polyolefin (including polypropylene, polyethylene, etc.), polyether, polyketone, polysulfone, polyvinyl alcohol, polyamide, polysiloxane, polyester, polystyrene, polyurethane, polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, polyamideimide, and polyvinylidene fluoride. The fiber 2 is not limited to an organic fiber, and may be an inorganic fiber containing an inorganic material, or an organic-inorganic composite fiber containing an organic-inorganic composite material.

[0031] The protective film 3 is a film that can impart durability such as chemical resistance or other functions to the surface of the fiber 2. Examples of materials for the protective film 3 include metal oxides such as alumina and titania, silicon oxide (SiO ２ ), TiN, Si ３ N ４Examples of the protective film 3 include metal nitrides such as ZnS, metal sulfides such as ZnS, metals, and alloys. The protective film 3 may be a single layer film or a laminated film including two or more films with different compositions. The protective film 3 may contain one or more of the materials exemplified above. Furthermore, the protective film 3 may further contain a certain amount of carbon and hydrogen in addition to the above materials.

[0032] The material of the particles 4 may be, for example, a material that is less flexible (more rigid) than the material of the fiber 2. The material of the particles 4 may be, for example, a metal oxide such as alumina or titania, or a silicon oxide (SiO ２ ), TiN, Si ３ N ４ Examples of the material include metal nitrides such as ZnS, metal sulfides such as ZnS, metals, and alloys. The particles 4 may be formed from a single material or multiple materials. The particles 4 may contain one or more of the materials listed above. Furthermore, the particles 4 may further contain a certain amount of carbon or hydrogen in addition to the above materials. The materials of the particles 4 and the protective film 3 may be the same or different from each other.

[0033] (Effects) The fiber sheet 1 of this embodiment includes first fibers 21, second fibers 22, and reinforcing particles 4. The second fibers 22 are stacked on the first fibers 21 so as to intersect with the first fibers 21. The particles 4 are arranged in the vicinity of the intersections 5 where the first fibers 21 and the second fibers 22 intersect, in gaps 51 adjacent to the portions where the first fibers 21 and the second fibers 22 contact each other, so as to be in contact with the first fibers 21 and the second fibers 22.

[0034] The above-described configuration makes it possible to provide a fiber sheet 1 that can increase rigidity while suppressing a decrease in flexibility. Specifically, the particles 4 arranged near the intersections 5 function as wedges, thereby increasing the rigidity of the fiber sheet 1. Furthermore, the particles 4 are granular objects arranged near the intersections 5, and can reduce the contact area with the first and second fibers 21, 22. Furthermore, by arranging the particles 4 in the gaps 51, the size of the particles 4 can be reduced, and the particles 4 can be arranged so that they contact both the first fibers 21 and the second fibers 22. Therefore, a decrease in flexibility of the fiber sheet 1 due to the formation of the particles 4 can be suppressed.

[0035] In the fiber sheet 1 of this embodiment, the reinforcing particles 4 are arranged more densely near the intersections of two fibers than in areas further from the intersections. By preferentially arranging the particles 4 at the intersections in this way, the flexibility of the fiber sheet 1 can be maintained at a higher level.

[0036] The fiber sheet 1 of this embodiment further includes a first protective film 31 covering the surfaces of the first fibers 21 and a second protective film 32 covering the surfaces of the second fibers 22. The reinforcing particles 4 contact the first fibers 21 via the first protective film 31 and contact the second fibers 22 via the second protective film 32 near the intersections 5. With this configuration, the surfaces of the fibers 21 and 22 can be protected by the particles 4 near the intersections 5 where a protective film is difficult to form, thereby increasing the durability of the fiber sheet 1.

[0037] In conventional fiber sheets, a protective film is sometimes formed on the fiber surface to enhance durability (e.g., chemical resistance, light resistance) against external stimuli such as chemicals and ultraviolet rays (see, for example, Patent Document 1). The inventors' investigations revealed that a protective film may be difficult to form near the intersections where fibers intersect and contact each other. As a result, the protective film may be partially thin near the intersections, or a portion of the fiber surface may be exposed through the protective film. This may result in a decrease in durability, such as chemical resistance. To address this issue, it is possible to form a thick protective film throughout the entire fiber sheet to ensure a sufficient protective film is formed at the intersections 5. However, as described above, a thick protective film may reduce the flexibility of the fiber sheet. Furthermore, a reduced flexibility of the fiber sheet may increase the likelihood of damage to the protective film when the fiber sheet is bent, potentially preventing the desired durability and mechanical strength from being achieved. Protective film damage is particularly likely to occur in fiber sheets in which an inorganic material film is formed as a protective film on the surface of organic fibers.

[0038] In contrast, with the fiber sheet 1 of this embodiment, by arranging the particles 4 near the intersections 5, it is possible to suppress a decrease in durability (such as chemical resistance) due to a thinner protective film or exposed fibers near the intersections 5, thereby increasing the durability of the fiber sheet. Furthermore, since it is not necessary to form a thick protective film in order to improve durability, it is possible to minimize a decrease in flexibility due to the protective film.

[0039] The width of the grains 4 is, for example, equal to or less than the width (diameter) of at least one of the first fibers 21 and the second fibers 22. The "grain width" refers to the maximum width of the grains 4. This makes it easier to maintain high flexibility of the fiber sheet 1. It also makes it easier for the grains 4 to be positioned within the gaps 51, 52 near the intersections 5. The width of the grains 4 may be, for example, equal to or less than half the width (diameter) of at least one of the first fibers 21 and the second fibers 22. This makes it possible to maintain even higher flexibility.

[0040] The width of the grains 4 is, for example, at least one-third of the width (diameter) of at least one of the first fibers 21 and the second fibers 22. This configuration can more reliably increase the rigidity of the fiber sheet 1. In addition, the durability in the vicinity of the intersections 5 can be more effectively improved.

[0041] The width of the grains 4 is, for example, greater than the thickness of the first protective film 31 and the thickness of the second protective film 32. With this configuration, the thicknesses of the first protective film 31 and the second protective film 32 can be kept small to maintain high flexibility while increasing the rigidity or durability of the fiber sheet 1. The thicknesses of the first protective film 31 and the second protective film 32 and the width (maximum width) of the grains 4 can be calculated, for example, by polishing a cross section of the fiber sheet 1 and then using an SEM image of the resulting cross section. Note that the thicknesses of the first protective film 31 and the second protective film 32 refer to, for example, the thicknesses of the portions other than the vicinity of the intersections (see FIG. 2 ). If the thicknesses of the portions other than the vicinity of the intersections are not uniform, the thicknesses refer to the average thicknesses.

[0042] The thicknesses of the first protective film 31 and the second protective film 32 are not particularly limited, but are, for example, 0.1 μm or less. This more effectively prevents a decrease in flexibility due to these protective films 31, 32. It also more effectively prevents damage, peeling, and the like of the protective films 31, 32 that occurs when the fiber sheet 1 is bent. On the other hand, the thicknesses of the first protective film 31 and the second protective film 32 are, for example, 0.01 μm or more. This more reliably increases the durability of the fiber 2.

[0043] In the fiber sheet 1 of this embodiment, the particles 4, the first protective film 31, and the second protective film 32 are made of the same material. This configuration increases the adhesive strength between the particles 4 and the first protective film 31 and the second protective film 32, thereby more effectively preventing the particles 4 from peeling off. Furthermore, since the particles 4 and the protective film 3 have the same function (e.g., durability such as chemical resistance), the particles 4 can more reliably compensate for a decrease in protective function due to a reduction in the thickness of the protective film 3 at the intersections 5. As a result, the protective film 3 can be made thinner, thereby preventing a decrease in flexibility due to the protective film 3.

[0044] The material of the particles 4 may be different from the material of the protective film 3. In this case, there is greater freedom in selecting the material of the particles 4, the method of forming the particles 4, etc. Furthermore, the particles 4 and the protective film 3 each can impart functions appropriate for the application to the fiber sheet 1. For example, the material of the particles 4 may have higher rigidity than the material of the protective film 3.

[0045] The particles 4 are, for example, particles of metal oxide. With this configuration, the rigidity of the fiber sheet 1 can be increased because metal oxide has a higher Young's modulus than metal or resin materials.

[0046] The first protective film 31 and the second protective film 32 are, for example, metal oxide films. Metal oxides have excellent chemical resistance, for example, so by forming a metal oxide film on the surface of the fibers 2, the chemical resistance of the fiber sheet 1 can be further improved. The metal oxide film may be an alumina film or a titania film. The alumina film and titania film can be formed to a uniform thickness on the surface of the multiple fibers 2 by, for example, a mist CVD method.

[0047] For example, one or more particles 4 are present per 10 μm×10 μm of the fiber sheet 1. With this configuration, by dispersing a plurality of particles 4 in the fiber sheet 1, the rigidity of the fiber sheet 1 can be more effectively increased. On the other hand, from the viewpoint of ensuring flexibility, the number of particles 4 per 10 μm×10 μm of the fiber sheet 1 is, for example, 100 or less.

[0048] The first fibers 21 and the second fibers 22 are, for example, organic fibers containing an organic material. This allows for the provision of a fiber sheet 1 with high flexibility. The fibers 2 may be organic fibers, and the protective film 3 may be an inorganic material film such as a metal oxide film. In a fiber sheet with such a configuration, the protective film is particularly susceptible to damage if it becomes too thick, so the application of the particles 4 provides a more significant effect.

[0049] The fiber sheet 1 of this embodiment has a plurality of intersections where two fibers 2 intersect and come into contact with each other. The plurality of intersections may be arranged randomly or regularly. Particles 4 may be arranged near two or more of the plurality of intersections. This can more effectively increase the rigidity or durability of the fiber sheet 1. Furthermore, the particles 4 may be arranged more densely near the two or more intersections than in portions of each fiber surface away from any of the intersections (portions other than the vicinity of the intersections). This can more effectively prevent a decrease in flexibility due to the particles 4.

[0050] The configuration of the fiber sheet of this embodiment is not limited to the configurations shown in FIGS.

[0051] The size, material, number, position, number per unit volume, number per unit area, etc. of the reinforcing particles 4 can be changed appropriately depending on the durability, mechanical strength such as rigidity, flexibility, etc. required depending on the application of the fiber sheet 1.

[0052] 1 to 4 , one particle 4 is disposed at each intersection 5. However, two or more particles 4 may be disposed at each intersection 5. For example, two or more particles 4 may be disposed at intervals in the gap 51. Furthermore, particles 4 may be disposed in each of the gaps 51 and 52 located on both sides of the contact portion between the first fiber 21 and the second fiber 22 in the extension direction of the first fiber 21 (see FIG. 8A ). Furthermore, particles 4 may also be disposed in one or both of the gaps adjacent to both sides of the contact portion in the extension direction of the second fiber 22 (see FIG. 8B ). Increasing the number of particles 4 disposed near each intersection 5 can further increase the rigidity or durability of the fiber sheet 1. However, from the viewpoint of achieving both flexibility and rigidity or durability, the number of particles 4 disposed near each intersection 5 may be, for example, two or less, or even one.

[0053] 1 to 4, the fiber sheet 1 is provided with a protective film 3 covering the surface of each fiber 2, but the fiber sheet of this embodiment may not be provided with a protective film. In that case, the reinforcing particles may be in direct contact with the surfaces of the first and second fibers near the intersections.

[0054] In FIG. 3, the particles 4 are arranged so as to fill the gaps 51 , but there may be gaps between the particles 4 and the surfaces of the fibers 2 that form the gaps 51 .

[0055] In FIG. 3, the first fiber 21 and the second fiber 22 are in direct contact with each other at the intersection 5, but these fibers 21 and 22 may not be in direct contact with each other but may be in contact with each other via a protective film.

[0056] In Figures 1 to 3, all of the non-contacting portions of each fiber 2 that are not in contact with other fibers are covered with protective film 3, but depending on the method and conditions for forming protective film 3, some of the non-contacting portions may not be covered with protective film 3.

[0057] Example 1 Example 1 is a fiber sheet containing polyimide fibers. The protective film and the grains contain alumina. A method for producing the fiber sheet of Example 1 will be described below.

[0058] 5A to 7B are schematic diagrams showing the manufacturing method of the fiber sheet of Example 1. Figures 5A, 6A, and 7A are enlarged perspective views showing the steps of crossing the polyimide fibers. Figures 5B, 6B, and 7B are cross-sectional views taken along line VB-VB in Figure 5A, line VIB-VIB in Figure 6A, and line VIIB-VIIB in Figure 7A, respectively.

[0059] STEP 1: Preparation of Polyimide Fiber Structure As shown in FIGS. 5A and 5B, a polyimide fiber structure 1a made of polyimide fibers is prepared.

[0060] First, a polyamic acid solution was prepared as a polyamic acid varnish using N,N-dimethylacetamide (DMAc) as a solvent. The solution concentration was adjusted to 25 wt %. The resulting polyamic acid solution was used as a spinning solution.

[0061] Next, the spinning solution (polyamic acid solution) is used to spin polyamic acid fibers onto aluminum foil attached to the periphery of a drum collector by electrospinning. Here, spinning is performed while rotating the drum collector (diameter: 200 mm) at 100 rpm. The electrospinning conditions are set, for example, as follows: Applied voltage: 23 kV Distance between nozzle and drum collector: 14 cm Film formation time: Adjusted so that the thickness of the fiber film is approximately 1 to 80 μm.

[0062] The polyamic acid fibers prepared by the above method are imidized by heat treatment (300°C, 2 hours) and then peeled off from the aluminum foil. In this way, a sheet-like polyimide fiber structure 1a containing multiple fibers 2 made of polyimide is obtained. The Z axis in FIG. 5B corresponds to the thickness direction of the polyimide fiber structure 1a. The thickness of the polyimide fiber structure 1a is, for example, 30 μm. The diameter of each fiber 2 is, for example, about 1 μm.

[0063] STEP 2: Formation of a Protective Film Next, as shown in FIGS. 6A and 6B, a protective film (here, an alumina film) 3 is formed on the surface of each fiber 2 in the polyimide fiber structure 1a.

[0064] In this embodiment, a mist CVD (chemical vapor deposition) method is used to form the protective film 3. The mist CVD method is a method in which a mist (raw material mist) having a size of, for example, several μm is generated by ultrasonic vibration from a solution in which a film-forming raw material is dissolved in a solvent or the like, and the mist is supplied to an object to form a film on the object.

[0065] First, a solution for forming a protective film to be used in the mist CVD method is prepared by mixing a solution containing methanol (70 wt %), aluminum acetolacetonate (0.025 mol / L), and water.

[0066] Next, the polyimide fiber structure 1a is placed on a hot plate set at 300°C in the chamber. In this state, the protective film-forming solution is converted into mist using 2.4 MHz ultrasound, and the resulting raw material mist 3m is supplied to the polyimide fiber structure 1a from above (along the -Z direction in the illustrated example) for 30 minutes. This forms a protective film (alumina film) 3 on the surface of each fiber 2 in the polyimide fiber structure 1a. The thickness of the alumina film is approximately 0.1 μm. Here, a first protective film (alumina film) 31 is formed on the surface of the first fiber 21, and a second protective film (alumina film) 32 is formed on the surface of the second fiber 22. Near the intersection 5, the raw material mist 3m also finds its way into the gaps 51 and 52 adjacent to the contact point between the two fibers 21 and 22, covering them with the alumina film. The alumina film formed in the gaps 51 and 52 may be thinner than in other areas.

[0067] STEP 3: Formation of Particles Subsequently, as shown in FIGS. 7A and 7B, particles (here, alumina particles) 4 are formed on the polyimide fiber structure on which the protective film 3 has been formed.

[0068] In this example, a mist CVD method is used to form the particles 4. The particles 4 can be formed in a state where the polyimide fiber structure 1a is placed on a hot plate set at 300° C. after the protective film 3 is formed.

[0069] First, a particle-forming solution is prepared. Here, the same solution as the protective film-forming solution is used as the particle-forming solution. Next, the particle-forming solution is converted into mist, and the generated raw material mist 4m is supplied to the polyimide fiber structure 1a on the hot plate for 15 minutes along a direction intersecting (here, perpendicular to) the Z axis. In Figures 7A and 7B, the raw material mist 4m is supplied perpendicular or nearly perpendicular to the Z axis and along the extension direction of the first fiber 21. Conditions other than the film-forming time and the supply direction of the raw material mist may be the same as or different from the film-forming conditions when forming the protective film.

[0070] As shown by the arrows in Figures 7A and 7B, the raw material mist 4m supplied from the left side to the polyimide fiber structure tends to flow to the right above the fibers 2 and through the gaps, and is therefore less likely to adhere to the side surfaces of the fibers 2. However, the raw material mist 4m supplied to the gap 51 near the intersection 5 does not flow to the right as easily, and therefore adheres to the protective film 3 in the gap 51. The adhered raw material mist (e.g., hemispherical) undergoes the following reactions (1) and (2) in succession, causing alumina to grow in the form of particles in the gap 51. Alternatively, gas is generated from the raw material mist 4m in the gap 51 due to the Leidenfrost phenomenon, and the following reactions (1) and (2) in succession occur, causing alumina to grow. (1) Al is formed by the substitution of acetylacetone-OH and the dehydration condensation of OH groups continuing. ｘ O ｙ (2) The formation of Al due to the acetylacetone elimination reaction that removes H from the OH group. ｘ O ｙ As a result, particles (alumina particles) 4 that contact the first protective film 31 and the second protective film 32 are selectively formed near the intersection 5. In this manner, the fiber sheet 1 of Example 1 is manufactured. Figure 4 is an SEM image of the fiber sheet manufactured by this method.

[0071] 7A and 7B, the raw material mist 4m is supplied from a direction perpendicular to the Z axis (horizontal direction), but the raw material mist 4m may alternatively be supplied from an oblique direction, for example, a direction inclined at an angle of ±45° or more and less than 90° with respect to the Z axis.

[0072] In this embodiment, the protective film 3 and the particles 4 are formed continuously using the same material and the same method, which makes it possible to prevent the process from becoming complicated and to keep the manufacturing cost low.

[0073] Example 2 Example 2 is a fiber sheet containing polyimide fibers. The protective film contains silicon oxide, and the grains contain alumina. A method for producing the fiber sheet of Example 2 will be described below.

[0074] STEP 1: Preparation of Polyimide Fiber Structure A polyimide fiber structure is formed in the same manner as in Example 1.

[0075] STEP 2: Formation of a protective film Next, a protective film (here, SiO ２ Specifically, first, Si(OC ２ H ５ ) ４ , C ２ H ５ OH, H ２ 11.7 ml of HCl, 70.4 ml of HCl, 5.94 ml of HCl, and 2.34 ml of HCl were prepared. Then, the polyimide fiber structure was immersed (dipped) in the raw material solution at room temperature, for example. After that, the polyimide fiber structure was pulled out of the raw material solution and subjected to a heat treatment to form a protective film (SiO ２ A membrane is obtained.

[0076] Step 3: Formation of particles Next, in the chamber, the polyimide fiber structure on which the protective film has been formed is placed on a hot plate set at 300°C. In this state, a mist of raw material for the particle-forming solution is supplied from the horizontal or oblique direction by the mist CVD method in the same manner as in Example 1. As a result, the protective film (SiO ２ In this way, the fiber sheet of Example 2 is produced.

[0077] (Variant) In a similar manner to Examples 1 and 2, raw material mist may be supplied to the polyimide fiber structure from one direction intersecting (e.g., perpendicular to) the Z axis to form particles, and then further raw material mist may be supplied from another direction intersecting (e.g., perpendicular to) the Z axis.

[0078] 8A, in a plane perpendicular to the Z axis, raw material mist 4m is supplied to the polyimide fiber structure 1a from the opposite direction (from right to left in the figure) to the direction in which the raw material mist was supplied when particles were formed in the gap 51 on the left side of the contact portion. This allows particles 4 to be formed also in the gap 52 on the right side of the contact portion.

[0079] 8B , raw material mist 4m may be supplied in a plane perpendicular to the Z axis from a direction (here, the extending direction of the second fibers 22) that intersects with the supply direction of the raw material mist when particles are formed in the gaps 51. This allows particles 4 to be formed in one or both of the gaps 53, 54 that are adjacent to the intersection 5 in the extending direction of the second fibers 22.

[0080] The manufacturing method of the fiber sheet 1 is not limited to the methods exemplified in Examples 1 and 2. For example, the fiber structure can be formed by not only electrospinning but also other methods such as cold pressing, solution casting, and melt casting. The protective film can be formed by physical modification or chemical modification. Examples of physical modification methods that can be used include physical vapor deposition (PVD) methods such as mist CVD, vacuum deposition, ion plating, and sputtering, chemical vapor deposition (CVD), and plating methods such as electroless plating and electrolytic plating. Examples of chemical modification methods that can be used include sol-gel methods. The particle formation method is also not limited to the mist CVD method. For example, a film-forming material can be supplied from the side of the fiber sheet using vacuum deposition, and particles that become the particles 4 can be selectively attached to the intersections of the fiber structure.

[0081] (Evaluation of Rigidity of Fiber Sheet) The inventors evaluated the rigidity of the fiber sheet of Example 1 in order to confirm the reinforcing effect (effect of increasing rigidity) of the particles.

[0082] First, the fiber sheet of Example 1 was produced by the method described above. For comparison, a fiber sheet of Comparative Example, which did not have particles (alumina particles), was produced by the same method as Example 1, except that particles were not formed. Both the fiber sheets of Example 1 and Comparative Example had sufficient flexibility.

[0083] Next, the deflection amounts of the fiber sheets of Example 1 and Comparative Example were measured. Fig. 9 is a schematic cross-sectional view illustrating a method for measuring the deflection amount. As shown in Fig. 9, the fiber sheet 1 was placed on a measurement table 9 having a 3 mm wide slit 91 so as to straddle the slit 91. In this state, the vertical displacement (deflection amount) δ of the fiber sheet 1 was measured at the center of the slit 91.

[0084] Figure 10 shows the measurement results of the deflection amount δ of the fiber sheets of Example 1 and the Comparative Example. From the results shown in Figure 10, it was found that the deflection amount of the fiber sheet of Example 1 was smaller than that of the fiber sheet of the Comparative Example (in this example, about 1 / 4). This confirmed that the fiber sheet of Example 1 has higher rigidity than the Comparative Example while maintaining flexibility. Generally, a deflection amount of 150 μm or less is considered to be the guideline for a product to have sufficient handleability. Since the deflection amount of the fiber sheet of Example 1 was below 150 μm, it was also confirmed that the fiber sheet of Example 1 has good handleability.

[0085] The present invention is not limited to the above-described embodiments, and design modifications are possible without departing from the spirit of the present invention. For example, each fiber in the fiber sheet of the present invention may have two or more protective layers. The fiber sheet may contain multiple types of fibers made of different materials. The fiber sheet may also be a composite of multiple fibers and components other than fibers.

[0086] The above explanation can also be expressed as follows.

[0087] A first embodiment of the fiber sheet comprises: a first fiber; a second fiber stacked on the first fiber so as to intersect with the first fiber; and at least one particle for reinforcement, wherein the at least one particle is arranged in contact with the first fiber and the second fiber in a gap adjacent to the contact point between the first fiber and the second fiber near the intersection where the first fiber and the second fiber intersect.

[0088] A fiber sheet according to a second aspect is the fiber sheet according to the first aspect, wherein the at least one particle is more densely arranged in the vicinity of the intersection than in a portion distant from the intersection.

[0089] A fiber sheet of a third aspect is the fiber sheet of the first or second aspect, wherein the width of the at least one particle is equal to or less than the width of at least one of the first fibers and the second fibers.

[0090] A fiber sheet of a fourth aspect is a fiber sheet of any one of the first to third aspects, wherein the width of the at least one particle is at least 1 / 3 of the width of at least one of the first fiber and the second fiber.

[0091] A fiber sheet of a fifth aspect is the fiber sheet of any one of the first to fourth aspects, wherein the at least one particle is present at least once per 10 μm×10 μm of the fiber sheet.

[0092] A fiber sheet of a sixth aspect is the fiber sheet of any one of the first to fifth aspects, wherein the at least one particle is a particle of a metal oxide.

[0093] The fiber sheet of the seventh aspect is the fiber sheet of any one of the first to sixth aspects, further comprising a first protective film covering the surface of the first fiber and a second protective film covering the surface of the second fiber, and the at least one particle contacts the first fiber via the first protective film and contacts the second fiber via the second protective film.

[0094] The fiber sheet of an eighth aspect is the fiber sheet of the seventh aspect, wherein the at least one particle, the first protective film, and the second protective film are made of the same material.

[0095] A fiber sheet according to a ninth aspect is the fiber sheet according to the seventh or eighth aspect, wherein the width of the at least one particle is greater than the thickness of the first protective film and the thickness of the second protective film.

[0096] A fiber sheet of a tenth aspect is the fiber sheet of any one of the seventh to ninth aspects, wherein the first protective film and the second protective film are metal oxide films.

[0097] A fiber sheet of an eleventh aspect is the fiber sheet of any one of the first to tenth aspects, wherein the first fibers and the second fibers are organic fibers containing an organic material.

[0098] The fiber sheet of the present invention is durable and flexible, and therefore can be used as various functional substrates, such as flexible substrates and transparent substrates. For example, the fiber sheet is porous and can have low dielectric properties by utilizing the dielectric constant of air, making it suitable for use in circuit boards (high-frequency boards) that transmit and receive radio waves in the high-frequency band. Furthermore, the fiber sheet of the present invention can be used in a wide range of fields, including battery components such as separators, as well as sanitary products and medical devices.

[0099] REFERENCE SIGNS LIST 1 fiber sheet 1a polyimide fiber structure 2 fiber 3 protective film 3m raw material mist for forming protective film 4 particle 4m raw material mist for forming particle 5 intersection 9 measurement table 21 first fiber 22 second fiber 31 first protective film 32 second protective film 51, 52, 53, 54 gap 91 slit

Claims

1. A fiber sheet comprising a first fiber, a second fiber stacked on the first fiber so as to intersect the first fiber, and at least one particle for reinforcement, wherein the at least one particle is disposed in a gap adjacent to a portion where the first fiber and the second fiber contact, in the vicinity of an intersection where the first fiber and the second fiber intersect, so as to contact the first fiber and the second fiber.

2. The fiber sheet according to claim 1, wherein the at least one particle is disposed more densely in the vicinity of the intersection than in a portion away from the intersection.

3. The fiber sheet according to claim 1 or 2, wherein a width of the at least one particle is equal to or less than a width of at least one of the first fiber and the second fiber.

4. The fiber sheet according to any one of claims 1 to 3, wherein a width of the at least one particle is equal to or greater than 1 / 3 of a width of at least one of the first fiber and the second fiber.

5. The fiber sheet according to any one of claims 1 to 4, wherein there is at least one particle per 10 μm × 10 μm of the fiber sheet.

6. The fiber sheet according to any one of claims 1 to 5, wherein the at least one particle is a metal oxide particle.

7. The fiber sheet according to any one of claims 1 to 6, further comprising a first protective film covering a surface of the first fiber and a second protective film covering a surface of the second fiber, wherein the at least one particle contacts the first fiber through the first protective film and contacts the second fiber through the second protective film.

8. The fiber sheet according to claim 7, wherein materials of the at least one particle, the first protective film, and the second protective film are the same.

9. The fiber sheet according to claim 7 or 8, wherein a width of the at least one particle is greater than a thickness of the first protective film and a thickness of the second protective film.

10. The fiber sheet according to any one of claims 7 to 9, wherein the first protective film and the second protective film are metal oxide films.

11. The fiber sheet according to any one of claims 1 to 10, wherein the first fiber and the second fiber are organic fibers containing an organic material.

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