Fiber sheet and method for manufacturing the same

Abstract

To provide a fiber sheet, which suppresses fuzz and delamination between fiber sheets without deteriorating inherent properties of a nanofiber and has excellent adhesion with other material, at high productivity and excellent operability.SOLUTION: A fiber sheet includes nanofibers having average fiber diameter of 20 to 1000 nm. The nanofiber includes mixture of vinylidene fluoride polymer and vinylidene fluoride-based copolymer having melting point of 150°C or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a fiber sheet containing nanofibers and a method for producing the same. [Background technology]

[0002] In recent years, ultrafine fibers with diameters of several to several hundred nanometers (nm), so-called nanofibers, have attracted attention. Nanofibers have characteristics such as a large specific surface area, small interfiber voids in fiber sheets containing nanofibers, a uniform void diameter distribution, and a high porosity, making them promising for applications in filter media, sound-absorbing materials, masks, waterproof and breathable membranes, separators for secondary batteries, sensor materials, and cell culture substrates. Among these, nanofibers made of polyvinylidene fluoride polymers are particularly promising due to their excellent mechanical strength, heat resistance, chemical resistance, and processability, making them suitable for a wide range of applications.

[0003] Electrospinning is a known method for producing nanofibers. In a typical electrospinning method, a spinning solution containing a dissolved polymer is charged with a high voltage along with a metal spinning nozzle, and the solution is discharged from the tip of the spinning nozzle toward the surface of a grounded collection electrode, forming droplets. The droplets are attracted to the collection electrode surface by a strong electrostatic force at the tip of the spinning nozzle, causing the spinning solution to fly as a jet, becoming finer as the solvent evaporates, and nanofibers with a diameter of several to several hundred nanometers are collected to form a nonwoven fiber sheet. Electrospinning allows for the nanofiberization of a wide range of materials, and enables the ultra-fine and uniform production of the fibers. However, because the resulting nanofibers have a small diameter and low mechanical strength per fiber, the surface of the fiber sheet containing the nanofibers is susceptible to frictional forces when it comes into contact with processing equipment, resulting in single-fiber breakage or tearing of the nonwoven fabric. Furthermore, the low rigidity of the nanofiber sheet reduces processability.

[0004] To address these problems, a method has been proposed for laminating and integrating nanofiber sheets with a porous layer that has excellent strength and rigidity (for example, Patent Document 1). Patent Document 1 discloses "a laminate in which a nonwoven fabric manufactured by electrospinning and a breathable sheet are bonded and integrated with an adhesive, wherein the penetration depth of the adhesive from the surface of the nonwoven fabric piece in the thickness direction of the nonwoven fabric is 40% or less of the thickness of the nonwoven fabric." However, although such a laminate improves the adhesive strength between layers, the adhesive seeps into the interfiber gaps and clogs the pores, thus failing to fully exhibit the inherent properties of the nanofibers. In addition, there is a problem that the components of the adhesive affect the fiber material and can dissolve into liquids or the atmosphere, polluting the environment. Furthermore, because it includes a process of integrating the nonwoven fabric manufactured by electrospinning and the breathable sheet with an adhesive, there is a problem in that stable operation cannot be obtained and sufficient productivity cannot be obtained.

[0005] Furthermore, the applicant previously proposed a "fiber sheet obtained by spinning a polyvinylidene fluoride copolymer mainly composed of vinylidene fluoride using electrospinning and collecting the spun fibers, wherein the average fiber diameter of the fibers constituting the fiber sheet is 20 nm or more and less than 1000 nm, the melting temperature of the fiber sheet in DSC measurement is 155°C or higher, and the heat of fusion is 45 J / g or less" (Patent Document 2). According to this method, the fiber sheet can be integrated well with other materials without significant shrinkage due to changes over time or heating during molding into a product, but when the fiber sheet comes into contact with rolls or the like during processing into a product, a considerable amount of fuzzing occurs, and the properties tend to deteriorate. In addition, in the mass production of electrospinning, there are attempts to improve productivity by increasing the number of spinning nozzles in the direction of the flow of the production line (for example, Patent Document 3), but the fiber sheet produced in this way becomes multilayered according to the number of spinning nozzle rows, making it prone to delamination between layers, and the properties tend to deteriorate when processed into a product. Thus, the methods described in Patent Documents 2 and 3 have room for improvement in addressing issues such as fuzzing and delamination between fiber sheets. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2007-30175 [Patent Document 2] Japanese Patent Publication No. 2015-45114 [Patent Document 3] Japanese Patent Publication No. 2007-224458 [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to solve the above-mentioned problems and provide a fiber sheet that suppresses fluffing and delamination between fiber sheets without impairing the inherent properties of nanofibers, and that exhibits excellent adhesion to other materials. [Means for solving the problem]

[0008] The inventors diligently conducted research to solve the above problems. As a result, they discovered that the above problems can be solved by having a fiber sheet containing nanofibers, wherein the nanofibers contain a mixture of a vinylidene fluoride polymer and a specific vinylidene fluoride copolymer, and thus completed the present invention.

[0009] In other words, the present invention has the following configuration. [1] A fiber sheet comprising nanofibers having an average fiber diameter of 20 to 1000 nm, wherein the nanofibers comprise a mixture of a vinylidene fluoride polymer and a vinylidene fluoride copolymer having a melting point of 150°C or lower. [2] The fiber sheet according to [1], wherein the mixing ratio (by weight) of the vinylidene fluoride polymer and the vinylidene fluoride copolymer is 50:50 to 95:5. [3] The fiber sheet according to [1] or [2], wherein the fiber sheet is a laminate of two or more layers. [4] A method for producing a fiber sheet according to any one of [1] to [3], comprising electrospinning a spinning solution containing a vinylidene fluoride polymer and a vinylidene fluoride copolymer having a melting point of 150°C or lower. A method for producing a fiber sheet, comprising heat-treating the fiber sheet obtained by the method described in [5][4] with circulating hot air or radiant heat. A fiber sheet composite in which the fiber sheet described in any one of [6][1] to [3] and a porous layer are laminated. A filter filter medium containing the fiber sheet composite described in [7][6]. [Effect of the Invention]

[0010] The fiber sheet of the present invention does not impair the inherent properties of the nanofibers, is less likely to cause fuzzing or delamination between the fiber sheets, and is excellent in processing resistance. Further, since the fiber sheet has excellent adhesion to other materials, when laminated with a porous layer to form a fiber sheet composite, the fiber sheet composite can be provided with high productivity and good operability without undergoing an integration process such as adhesion processing or calendar processing. [Embodiments for Carrying Out the Invention]

[0011] Hereinafter, the present invention will be described in detail.

[0012] The fiber sheet of the present invention is a fiber sheet containing nanofibers having an average fiber diameter of 20 to 1000 nm, and the nanofibers contain a mixture of a vinylidene fluoride polymer and a vinylidene fluoride-based copolymer having a melting point of 1 °C or lower. By adopting such a configuration, it is possible to obtain a fiber sheet that suppresses fuzzing and delamination between fiber sheets without impairing the inherent properties of the nanofibers and has excellent adhesion to other materials with high productivity and good operability.

[0013] [Nanofibers] The average fiber diameter of the nanofibers constituting the fiber sheet of the present invention is 20 to 1000 nm, preferably 30 to 600 nm, and more preferably 50 to 300 nm. If the average fiber diameter of the nanofibers is 1000 nm or less, the specific surface area increases, making it easier to exhibit nanofiber-derived characteristics such as excellent filter performance. If it is 20 nm or more, satisfactory single-fiber strength is achieved, and breakage and fuzzing of the nanofibers can be suppressed during processing into products.

[0014] The nanofibers constituting the fiber sheet of the present invention contain a mixture of vinylidene fluoride polymer and a vinylidene fluoride copolymer with a melting point of 150°C or lower. This configuration is thought to combine the characteristics of vinylidene fluoride polymer, such as excellent durability including chemical resistance and heat resistance, and the ability to form fine, uniform fibers, with the characteristics of vinylidene fluoride copolymer, such as flexibility, suppression of fuzzing and delamination between fiber sheets, and excellent adhesion to other materials. Here, vinylidene fluoride polymer refers to a homopolymer polymerized substantially solely from vinylidene fluoride monomer. Furthermore, vinylidene fluoride copolymer refers to a copolymer of vinylidene fluoride and other monomers, with examples including copolymers of vinylidene fluoride with fluorine-based monomers such as trifluoroethylene, tetrafluoroethylene, or hexafluoropropylene. In particular, a copolymer of vinylidene fluoride and hexafluoropropylene is preferred from the viewpoint of suppressing fuzzing and delamination between fiber sheets and improving adhesion with other materials. The copolymer may be a random copolymer or a block copolymer, but a random copolymer is preferred from the viewpoint of suppressing fuzzing and delamination between fiber sheets and improving adhesion with other materials.

[0015] The melting point of the vinylidene fluoride copolymer in the present invention is 150°C or lower, preferably 135°C or lower, and more preferably 125°C or lower. By using a vinylidene fluoride copolymer having a melting point of 150°C or lower, it is possible to suppress fuzzing and delamination between fiber sheets and improve adhesion with other materials. Because vinylidene fluoride copolymers having a melting point of 150°C or lower have high softening and solubility to heat and solvents, when nanofibers are assembled in the manufacturing process of fiber sheets, they come into contact with other materials or with other materials in a soft state that has not completely solidified, resulting in strong adhesion. This is thought to suppress fuzzing and delamination between fiber sheets and improve adhesion with other materials. Furthermore, the lower limit of the melting point of the vinylidene fluoride copolymer is not particularly limited, but it is preferably 60°C or higher, more preferably 80°C or higher, and even more preferably 100°C or higher. If the melting point is 60°C or higher, the nanofibers will be less likely to melt even when processed under high-temperature conditions during product manufacturing, making it easier to maintain the inherent properties of the nanofibers.

[0016] The mixing ratio (weight ratio) of vinylidene fluoride polymer and vinylidene fluoride copolymer in the nanofibers constituting the fiber sheet of the present invention is not particularly limited, but is preferably 50:50 to 95:5, more preferably 60:40 to 93:7, and even more preferably 75:25 to 92.5:7.5. If the mixing ratio (weight ratio) of vinylidene fluoride polymer and vinylidene fluoride copolymer in the nanofibers is 50:50 or higher, it is excellent in durability such as chemical resistance and heat resistance, and it is easy to obtain fine and uniform fibers. If it is 95:5 or lower, it is excellent in suppressing fuzzing and delamination, and in adhesion with other materials.

[0017] The weight-average molecular weight of the vinylidene fluoride polymer and vinylidene fluoride copolymer, which are components of the nanofibers, is not particularly limited, but is preferably 100,000 to 1,000,000, and more preferably 200,000 to 800,000. If the weight-average molecular weight is 100,000 or more, the nanofiber formation is excellent, and if it is 1,000,000 or less, the solubility and thermoplasticity are excellent, making processing easier.

[0018] The melt viscosity of the vinylidene fluoride polymer, a component of nanofibers, is not particularly limited, but is measured according to ASTM D3835, with a melting temperature of 232°C and a shear rate of 100 sec. -1 In this case, the viscosity is preferably 2 to 60 kpoise, more preferably 10 to 50 kpoise, and even more preferably 20 to 40 kpoise. Furthermore, the melt viscosity of the vinylidene fluoride copolymer is not particularly limited, but is measured in accordance with ASTM D3835, with a melting temperature of 232°C and a shear rate of 100 sec. -1 In this case, it is preferably 0.5 to 35 kpoise, more preferably 2 to 30 kpoise, and even more preferably 5 to 20 kpoise.

[0019] The nanofibers are not particularly limited, but may contain a conductivity imparting agent. The inclusion of a conductivity imparting agent in the nanofibers makes them extremely fine, suppressing the formation of bead-like structures and resulting in homogeneous nanofibers, thus allowing the inherent properties of the nanofibers to be more easily exhibited. Furthermore, the high voltage in the electrospinning process described later strongly attracts and densely collects the nanofibers in the collector direction, improving adhesion between nanofibers or between nanofibers and other materials, thereby suppressing fuzzing and delamination. Examples of conductivity imparting agents include anionic surfactants such as sodium dodecyl sulfate, cationic surfactants such as tetrabutylammonium bromide, or organic or inorganic salts. A concentration of 0.1 to 10% by weight of the nanofiber provides a satisfactory effect.

[0020] The nanofibers are not particularly limited, but may be treated with a water-repellent agent. The application of a water-repellent agent improves the water repellency of the nanofibers, suppresses the deterioration of properties due to water absorption and moisture absorption, and allows the properties derived from the nanofibers to be maintained over a long period of time. Normally, when a water-repellent agent is applied to nanofibers, fuzzing and delamination tend to occur, and adhesion to other materials decreases. However, the nanofibers used in the present invention contain a mixture of vinylidene fluoride polymer and a vinylidene fluoride copolymer with a melting point of 150°C or lower, so even when a water-repellent agent is applied, good resistance to fuzzing and delamination can be maintained. The water-repellent agent is not particularly limited as long as it can impart water repellency to the nanofibers, and examples include fluorine group-containing oligomers, fluorine group-containing polymers, fluorine-based surfactants having perfluoroalkyl groups, fluorine-based surfactants having perfluoroalkenyl groups, silicone oligomers, silicone polymers, silicone-based silane compounds, fluorine-based silane compounds, fluorine-containing cage-type silsesquioxane, fluorine-modified polyurethane, or silicone-modified polyurethane. In particular, using a fluorine-containing water repellent such as a fluorine-containing polymer, a fluorine-containing cage-type silsesquioxane, or a fluorine-modified polyurethane is preferable from the viewpoint of water repellency, workability, and cost. The concentration of the water repellent is not particularly limited, but is preferably 0.1 to 20% by weight, and more preferably 1 to 15% by weight, relative to the nanofibers. If the concentration of the water repellent is 0.1% by weight or more, water repellency can be imparted to the nanofibers, and if it is 20% by weight or less, an improvement in effect commensurate with the amount used can be obtained. The state in which the water repellent is applied is not particularly limited; it may be mixed in the nanofibers or coated on the surface of the nanofibers. Furthermore, the method of applying the water repellent is not particularly limited; it may be applied by dispersing or dissolving the water repellent in a spinning solution and spinning, or it may be applied after obtaining the nanofibers using known apparatus and methods such as immersion or spray coating.

[0021] The nanofibers may contain other components as long as they do not impair the effects of the present invention. For example, they may contain polyvinyl alcohol, polyethylene glycol, polyethylene oxide, polyacrylic acid, polyvinylpyrrolidone, polyethylene, polypropylene, cyclic polyolefin, polyethylene terephthalate, polybutylene terephthalate, polylactic acid, polyglycolic acid, polycaprolactone, polybutylene succinate, nylon 6, nylon 6,6, aromatic polyamide, polyurethane, polycarbonate, polystyrene, polysulfone, polyethersulfone, polyacrylonitrile, polymethyl methacrylate, cellulose, cellulose acetate, collagen, glucomannan derivatives, chitin, chitosan, polylysine, polyamic acid, polyimide, or polymer components such as polyvinyl formal; metal oxide components such as silica, alumina, titania, zirconia, yttrium-stabilized zirconia, barium titanate, or hydroxyapatite; hydrophilic agents, weathering agents, stabilizers, etc. These may be used individually or in combination of two or more. The concentrations of other components are not particularly limited and may be 0.1 to 20% by weight relative to the nanofibers.

[0022] <Fiber sheet> Because the fiber sheet of the present invention contains the nanofibers described above, fuzzing and delamination between fiber sheets are suppressed, resulting in excellent adhesion to other materials.

[0023] The fiber sheet only needs to contain the nanofibers mentioned above, and its basis weight is not limited, but to facilitate the performance of the nanofibers, it should be between 0.01 and 20 g / m². 2 Preferably, the concentration is 0.1 to 5 g / m 2 It is more preferable that the amount be 0.2-3 g / m 2 It is even more preferable that the nanofiber sheet is as described above. Furthermore, the thickness of the nanofiber sheet is not particularly limited, but is preferably 0.5 to 100 μm, more preferably 1 to 50 μm, and even more preferably 2 to 30 μm.

[0024] The fiber sheet may have fibers other than the nanofibers described above laminated or blended in it, to the extent that it does not impair the effects of the present invention. For example, a fiber sheet may be made in which fibers consisting only of vinylidene fluoride polymer with an average fiber diameter of 10 nm to 10 μm and nanofibers containing a mixture of vinylidene fluoride polymer and a vinylidene fluoride copolymer with a melting point of 150°C or less, with an average fiber diameter of 20 to 1000 nm, are laminated or blended in a weight ratio of 1:10 to 10:1.

[0025] The fiber sheet of the present invention may be a single layer or may consist of two or more layers of fiber sheets laminated together. The fiber sheet of the present invention has the effect of being resistant to delamination between layers, so even when it is laminated in two or more layers, delamination is less likely to occur during processing into products and the properties are less likely to deteriorate. When two or more layers of fiber sheets are laminated together, each layer is not particularly limited as long as it contains the nanofibers described above, and each layer may contain the same nanofibers, or it may contain nanofibers with different average fiber diameters and constituent components (for example, the mixing ratio of vinylidene fluoride polymer and vinylidene fluoride copolymer, and the types and contents of various additives).

[0026] The fiber sheet of the present invention can be laminated with a porous layer to form a fiber sheet composite. The porous layer imparts various properties to the fiber sheet, such as mechanical strength, abrasion resistance, processability, and pleating characteristics. Furthermore, because the fiber sheet of the present invention has excellent adhesion to other materials, it can be integrated with the porous layer without undergoing integration processes such as bonding or calendering, thereby simplifying the manufacturing process.

[0027] <Porous layer> The porous layer used in the fiber sheet composite of the present invention is not particularly limited as long as it does not contain the nanofibers described above, and examples include woven fabrics, knitted fabrics, nonwoven fabrics, papermaking materials, felts, nets, or microporous films, but a nonwoven fabric is preferred in terms of processability and availability. The nonwoven fabric is not particularly limited, and examples include thermal bonded nonwoven fabrics, thermal calendered nonwoven fabrics, through-air nonwoven fabrics, airlaid nonwoven fabrics, spunlace nonwoven fabrics, needle-punched nonwoven fabrics, wet-laid nonwoven fabrics, spunbond nonwoven fabrics, meltblown nonwoven fabrics, chemical bonded nonwoven fabrics, flash-spun nonwoven fabrics, or electrospun nonwoven fabrics. In general, nanofibers have a small fiber diameter, resulting in low strength per single yarn, and because they exhibit sufficient function with a small basis weight, fiber sheets used in product processing have a low basis weight and low sheet strength, which has led to reduced processability when processing into products. From this viewpoint, it is preferable to compensate for the low mechanical strength of the fiber sheet with the mechanical strength of the porous layer. The average fiber diameter of the fibers constituting the porous layer is preferably 1 to 100 μm, more preferably 3 to 50 μm, and more preferably 5 to 40 μm. If the average fiber diameter is 1 μm or more, it compensates for the lack of strength and rigidity of the fiber sheet, and a laminate with excellent processability for the product can be obtained. If it is 100 μm or less, the contact area between the porous layer and the fiber sheet increases, improving peel resistance.

[0028] The components constituting the porous layer are not particularly limited, and examples include polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate and polylactic acid, polyamide resins such as nylon 6 and nylon 6,6, polyurethane resins, fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, polysulfone, polyethersulfone, and cellulose-based materials such as cellulose and cellulose acetate. These may be used individually or in combination of two or more. Among these, from the viewpoint of imparting mechanical properties and rigidity to the fiber sheet composite and improving the processability of the product, it is preferable that the components constituting the porous layer contain 30% by weight or more of polyolefin resin, polyester resin, or cellulose-based material, and more preferably 50% by weight or more. Furthermore, from the standpoint of processability and availability, polyethylene or polypropylene is preferred as the polyolefin resin, polyethylene terephthalate is preferred as the polyester resin, and cellulose is preferred as the cellulose-based material.

[0029] When the porous layer is a nonwoven fabric, the fibers constituting the nonwoven fabric may be single-component fibers or composite fibers. However, composite fibers are preferred because they allow for improved adhesion to the fiber sheet through post-processing such as heat treatment. The composite fibers may be a blend of multiple components, or may have composite forms such as concentric sheath core type, eccentric sheath core type, parallel type, sea-island type, or radial type. Furthermore, the composite fibers may consist of two or more materials with different melting points. Examples of high-melting-point components include polypropylene, polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, nylon 6, nylon 6,6, and poly-L-lactic acid. Examples of low-melting-point components include low-density polyethylene, linear low-density polyethylene, high-density polyethylene, polyethylene terephthalate copolymer, poly-DL-lactic acid, polypropylene copolymer, and polypropylene. The difference in melting points between the high-melting-point and low-melting-point components of the composite fiber is not particularly limited, but it is preferably 15°C or higher, and more preferably 30°C or higher, in order to broaden the processing temperature range for heat treatment.

[0030] The cross-sectional shape of the fibers constituting the non-woven fabric is not particularly limited, and examples thereof include circular, elliptical, flat, semi-circular, star-shaped, triangular, quadrangular, pentagonal, multi-leaf-shaped, array-shaped, T-shaped, or horseshoe-shaped. However, from the viewpoint of improving the adhesion to the fiber sheet, it is preferably elliptical, flat, or semi-circular. A non-woven fabric containing fibers having an elliptical, flat, or semi-circular cross-sectional shape can be obtained, for example, by forming fibers having an elliptical, flat, or semi-circular cross-sectional shape into a web shape and then bonding them with heat or an adhesive, or by subjecting a non-woven fabric composed of fibers having a circular cross-sectional shape to thermo-calendering. Further, within a range not interfering with the effects of the present invention, the surface of the fiber may be treated with a fiber finishing agent, thereby imparting functions such as hydrophilicity, water repellency, antistatic property, surface smoothness, and abrasion resistance. Also, within a range not impairing the effects of the present invention, a plurality of fibers having different fiber diameters and properties may be laminated or mixed.

[0031] The porous layer may contain additives such as antibacterial agents, deodorants, antistatic agents, conductive materials, fluorescent materials, smoothing agents, hydrophilic agents, water repellent agents, antioxidants, weathering agents, surfactants, and charge stabilizers as appropriate and as necessary within a range not interfering with the effects of the present invention.

[0032] The porous layer is not particularly limited, but may have uneven shapes such as embossing points and shaping. Generally, a porous layer having uneven shapes such as embossing points and shaping has a small contact area with the fiber sheet and is thus likely to peel off. However, since the fiber sheet of the present invention has excellent adhesion to other materials, even if it has uneven shapes such as embossing points and shaping, it becomes easy to integrate.

[0033] The basis weight of the porous layer is not particularly limited, but is preferably 5 to 150 g / m 2 more preferably 10 to 105 g / m 2 even more preferably 15 to 85 g / m 2 When the basis weight of the porous layer is 5 g / m 2If the above is achieved, the rigidity of the fiber sheet composite can be increased, improving the processability of the product, and the weight can reach 150g / m². 2 The following conditions will make it less likely to interfere with the properties derived from nanofibers.

[0034] The thickness of the porous layer is not particularly limited, but is preferably 0.01 to 1 mm, and more preferably 0.05 to 0.8 mm. Within this range, the rigidity of the fiber sheet composite can be increased, and the processability of the product can be improved.

[0035] The specific volume of the porous layer is not particularly limited, but is typically between 1 and 10 cm³. 3 It is preferable that the amount be / g, and the length be 2-8cm. 3 It is preferable that the amount be / g, and the length be 3-6cm. 3 It is more preferable that the specific volume of the porous layer is 10 cm³. 3 If the value is less than / g, satisfactory peel resistance can be obtained, 1cm 3 If the concentration is above / g, it will be less likely to interfere with the properties derived from nanofibers.

[0036] <Fiber sheet composite> The fiber sheet composite of the present invention, because it consists of a laminated fiber sheet and a porous layer, suppresses fuzzing and delamination between fiber sheets, and also exhibits excellent mechanical strength and rigidity, resulting in good processability for product manufacturing. Furthermore, because of the excellent adhesion between the fiber sheet and the porous layer, even without integration processes such as bonding or calendering, it is possible to manufacture the fiber sheet composite with high productivity and good operability.

[0037] The basis weight of the fiber sheet composite is not particularly limited, but is typically between 5 and 200 g / m². 2 Preferably, it is 10-150 g / m 2 It is more preferable that the amount be 15-120 g / m². 2 It is even more preferable that the basis weight of the fiber sheet composite be 5 g / m². 2 If the above is achieved, the rigidity of the fiber sheet composite will increase, improving the processability of the product, and the weight will be 200g / m². 2If the following conditions are met, the material can be made lighter when used as a filter medium, mask, sound-absorbing material, etc. Furthermore, while the thickness of the fiber sheet composite is not particularly limited, it is preferably 0.01 to 2 mm, more preferably 0.05 to 1.5 mm, and even more preferably 0.06 to 1 mm. If the fiber sheet composite is 0.01 mm or thicker, its rigidity increases, improving its processability into products. If it is 2 mm or less, it can be made more space-efficient when used as a filter medium, mask, sound-absorbing material, etc.

[0038] The fiber sheet composite may be subjected to antistatic processing, water-repellent processing, hydrophilic processing, antibacterial processing, ultraviolet absorption processing, near-infrared absorption processing, or electret processing, as long as it does not significantly impair the effects of the present invention. The fiber sheet composite of the present invention has sufficient mechanical strength and appropriate rigidity, and also exhibits excellent secondary processing properties such as electrostatic processing, water-repellent processing, hydrophilic processing, antibacterial processing, ultraviolet absorption processing, near-infrared absorption processing, or electret processing.

[0039] The fiber sheet composite of the present invention suppresses fuzzing and delamination between fiber sheets, has excellent adhesion and mechanical strength between the fiber sheet and the porous layer, and contains few adhesive components, making it suitable for use as a high-performance filter material, although not particularly limited. The object to be filtered is not particularly limited and may be an air filter material used in air conditioners or clean rooms, or a liquid filter material used for filtering wastewater, paint, abrasive particles, etc. The type of filter is also not particularly limited and may be a flat membrane filter, a pleated filter, or a cylindrically wound depth filter.

[0040] When a fiber sheet composite is used as a filter material for an air filter, the pressure loss when air is passed through at a flow rate of 5.3 cm / second is preferably 10 to 1500 Pa, more preferably 20 to 300 Pa, and even more preferably 50 to 200 Pa. If the pressure loss is 10 Pa or more, sufficient collection efficiency can be obtained, and if it is 1500 Pa or less, the permeability of the gas filter is increased, resulting in effects such as reduced power consumption and reduced load on the fan. Furthermore, when air containing particles with a particle size of about 0.1 to 0.3 μm is passed through at 5.3 cm / second, the collection efficiency of the particles is preferably 80% or more, and more preferably 90% or more. In addition, the QF value (= log(1 - collection efficiency / 100) / pressure loss × 1000) is preferably 12 or more, and more preferably 15 or more. The QF value is a value used as an indicator of the collection performance of an air filter, and a larger QF value means higher performance.

[0041] <Method for manufacturing fiber sheets> The method for producing the fiber sheet of the present invention is not particularly limited, and examples include spinning methods using air, centrifugal force, or electrostatic force from a spinning solution containing a vinylidene fluoride polymer and a vinylidene fluoride copolymer. Among these, the electrospinning method using electrostatic force is preferred because it can reduce the diameter of the nanofibers and make them uniform.

[0042] Electrospinning is a method of spinning by extruding a spinning solution and applying an electric field to fibrousize the extruded solution, thereby obtaining fibers with a very small diameter on a collector. Examples of such methods include spinning by extruding the spinning solution from a nozzle and applying an electric field, spinning by foaming the spinning solution and applying an electric field, and spinning by guiding the spinning solution to the surface of a cylindrical electrode and applying an electric field.

[0043] The spinning solution is not particularly limited as long as it has spinnability, but for example, a solution in which vinylidene fluoride polymer and vinylidene fluoride copolymer are dispersed or dissolved in a solvent, or a solution in which vinylidene fluoride polymer and vinylidene fluoride copolymer are melt-kneaded by heat or laser irradiation can be used. However, from the viewpoint of making the average fiber diameter of the nanofibers constituting the fiber sheet of the present invention small and uniform, it is preferable to use a spinning solution in which vinylidene fluoride polymer and vinylidene fluoride copolymer are dissolved in a solvent.

[0044] The solvent for dispersing or dissolving the vinylidene fluoride polymer and the vinylidene fluoride copolymer is not particularly limited and includes water, methanol, ethanol, propanol, acetone, methyl ethyl ketone, tetrahydrofuran, cyclohexanone, γ-butyrolactone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropionamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, triethyl phosphate, propylene glycol monomethyl ether, diethylene glycol ethyl methyl ether, toluene, xylene Examples of solvents include pyridine, formic acid, acetic acid, tetrahydrofuran, dichloromethane, chloroform, and 1,1,1,3,3,3-hexafluoroisopropanol. However, from the viewpoint of solubility of vinylidene fluoride polymers and vinylidene fluoride copolymers, it is preferable to include N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropionamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, triethyl phosphate, acetone, methyl ethyl ketone, tetrahydrofuran, cyclohexanone, or γ-butyrolactone. These solvents may be used individually or mixed in any proportion of two or more types.

[0045] The additives mentioned above, such as conductive agents and water-repellent agents, are not particularly limited, but it is preferable to include them in the spinning solution. Including them in the spinning solution is preferable because it allows for uniform application of the additives to the nanofibers and also allows for the application of various functions without going through an additive application process. The amount of additive can be appropriately selected depending on the type and the desired effect, and a value of 0.001 to 5% by weight relative to the spinning solution is preferable because it provides an effect commensurate with its use.

[0046] The method for preparing the spinning solution is not particularly limited and can include methods such as stirring and ultrasonic treatment. The order of mixing is also not particularly limited; each component may be mixed simultaneously or sequentially. When preparing the spinning solution by stirring, the stirring temperature and time are not particularly limited as long as the vinylidene fluoride polymer and vinylidene fluoride copolymer are sufficiently mixed; for example, stirring may be performed at 40-120°C for 1-24 hours.

[0047] The viscosity of the spinning solution is not particularly limited, but is preferably 10 to 10,000 cP, more preferably 50 to 8,000 cP, and even more preferably 200 to 5,000 cP. If the viscosity of the spinning solution is 10 cP or higher, good spinnability and spinning stability can be obtained, and if it is 10,000 cP or lower, the preparation of the spinning solution and dispensing during electrospinning will be easier. The viscosity of the spinning solution can be adjusted by appropriately changing the molecular weight and concentration of the vinylidene fluoride polymer or vinylidene fluoride copolymer, as well as the type and mixing ratio of the solvent.

[0048] The spinning solution may be spun at room temperature, or it may be heated or cooled to a temperature of, for example, 0 to 200°C. As for the method of dispensing the spinning solution, for example, a pump can be used to dispense the spinning solution, which is filled in a syringe or tank, through a nozzle. The inner diameter of the nozzle is not particularly limited, but examples include 0.1 to 1.5 mm. Furthermore, the single-hole dispensing volume of the spinning solution is not particularly limited, but examples include 0.1 to 20 mL / hr.

[0049] The method for applying the electric field is not particularly limited as long as it allows for stable electrospinning. For example, a high voltage may be applied to the nozzle or spinning solution to ground the collector. The applied voltage is not particularly limited as long as it is within a range where fibers can be formed and spinning can be stably performed, and examples include 5 to 100 kV. The distance between the nozzle and the collector (spinning distance) is not particularly limited as long as it is within a range where the solvent can evaporate sufficiently, and examples include 50 to 1000 mm. The electric field strength is not particularly limited, but is preferably 1 to 10 kV / cm, and more preferably 2 to 5 kV / cm. If the electric field strength is 1 kV / cm or higher, the peel resistance between fiber sheets and between fiber sheets and the porous layer can be improved. If it is 10 kV / cm or lower, the porosity of the fiber sheets can be increased, making it easier to achieve both a high specific surface area and high air permeability. The material of the collector is not particularly limited as long as it can capture electrospun nanofibers, but conductive materials such as metals can be suitably used. The shape of the collector is not particularly limited, but it is preferable to use a conveyor-type collector to continuously manufacture fiber sheets.

[0050] It is preferable that the ambient temperature and humidity during electrospinning be controlled, and while there are no particular limitations on the range, examples include 20-30°C and 25-45RH%. Within this temperature and humidity range, it is relatively easy to control throughout the year, and changes in ambient temperature and humidity make it less likely for changes in spinning behavior or changes in the physical properties of the resulting fiber sheet to occur.

[0051] When the fiber sheet is a laminate of two or more layers, an example of electrospinning can be used, employing a conveyor-type collector with two or more rows of nozzles arranged in the direction of the conveyor's movement. In this case, from the viewpoint of uniformity of the fiber sheet, it is preferable to electrospin while traversing the nozzles perpendicular to the direction of the conveyor's movement (in the width direction of the fiber sheet). The spinning solution discharged from each row may be the same or different. If the spinning solution is the same, a laminate of fiber sheets with the same physical properties will be obtained; if the spinning solutions are different, a laminate of fiber sheets with different physical properties will be obtained. For example, the content of additives such as conductivity imparters and water repellents, the concentration of vinylidene fluoride polymer or vinylidene fluoride copolymer, and the type of solvent can be appropriately changed according to the desired physical properties.

[0052] <Method for manufacturing fiber sheet composites> The method for manufacturing the fiber sheet composite of the present invention is not particularly limited, and examples include electrospinning a fiber sheet onto a porous layer, and preparing the fiber sheet and the porous layer separately and then performing heat compression bonding with a heated flat roll or embossing roll, or bonding with a hot melt agent or chemical adhesive. However, from the viewpoint of simplifying the manufacturing process, the method of electrospinning a fiber sheet onto a porous layer is preferred. The fiber sheet of the present invention has the characteristic of excellent adhesion to other materials, and it is possible to obtain a fiber sheet composite in which the fiber sheet and the porous layer are sufficiently adhered without performing heat compression bonding or bonding treatment.

[0053] The fiber sheet composite of the present invention may be subjected to heat treatment using circulating hot air or radiant heat in order to further improve the peel resistance between fiber sheets and between fiber sheets and the porous layer. In the case of heat compression bonding using flat rolls or embossing rolls, the fiber sheets inevitably suffer some damage, such as melting and forming a film, or tearing occurring around the embossed areas. For example, if the fiber sheet composite is used as a gas filter material, damage can easily lead to a decrease in performance, such as reduced air permeability due to melting and film formation, or reduced collection characteristics due to tearing. Also, in the case of bonding with hot melt agents or chemical adhesives, the interfiber voids of the fiber sheet are filled by these components, which can also easily lead to a decrease in performance. On the other hand, heat treatment using circulating hot air or radiant heat is preferable because it causes almost no damage to the fiber sheets and provides sufficient peel resistance.

[0054] When heat treatment is performed using circulating hot air or radiant heat, it is preferable, although not particularly limited, that the fibers constituting the porous layer include heat-adhesive composite fibers. The melting point of the low-melting-point component of the heat-adhesive composite fiber is not particularly limited, but from the viewpoint of broadening the range of heat treatment processing conditions, it is preferable that it be 20°C or more lower than the melting point of the vinylidene fluoride polymer used in the present invention, and more preferably 30°C or more lower. Furthermore, the heat treatment temperature using circulating hot air or radiant heat is not particularly limited, but can be set within a range below the melting point of the vinylidene fluoride polymer used in the present invention, while considering the balance between peel resistance and the desired properties. [Examples]

[0055] The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto. The methods or definitions for measuring the physical properties shown in the examples are shown below.

[0056] <Melting points of vinylidene fluoride polymers and vinylidene fluoride-based copolymers> Using a PerkinElmer DSC analyzer (DSC8500), measurements were taken in the temperature range from room temperature to 230°C, under conditions of heating / cooling rate of 10°C / min and a nitrogen atmosphere. The temperature of the melting peak top in the second run was defined as the melting point (°C). <Average fiber diameter of nanofibers and fibers constituting the porous layer> Using a scanning electron microscope (SU-8000) from Hitachi, Ltd., fiber sheets and porous layers were observed at magnifications of 500 to 30,000 times. The fiber diameter of 50 or more fibers was measured using image analysis software, and the average value was defined as the average fiber diameter. <Filter performance> Using a TSI automatic filter efficiency detector (Model 8130), sodium chloride (particle size: 0.07 μm (median particle size), particle concentration: 20 mg / m³) was measured. 3 The pressure loss and collection efficiency were measured when the sample was passed through the sample at a measured flow rate of 5.3 cm / second. <Workability> The fluffiness of the fiber sheet composite when it passed through the processing line, the degree of delamination between the fiber sheets, and the degree of adhesion between the fiber sheets and the porous layer were visually assessed, and the processability was evaluated in three stages: ◎, ○, and ×, as follows. ◎: When the material passed through the processing line, no fuzzing, delamination between fiber sheets, or separation between fiber sheets and the porous layer occurred at all, which is satisfactory. ○: When the material passes through the processing line, slight fuzzing occurs, but no delamination between fiber sheets or between the fiber sheets and the porous layer is observed, which is at an acceptable level. ×: When passed through the processing line, the properties deteriorate due to fuzzing, delamination between fiber sheets, or delamination between fiber sheets and porous layers, making it impossible to process into a product.

[0057] [Example 1] A spinning solution was prepared by mixing 19 parts by weight of vinylidene fluoride polymer (trade name: Solev6010; melting point: 170°C) manufactured by Solvay Specialty Polymers, 1 part by weight of vinylidene fluoride copolymer (trade name: Kynar2500; random copolymer of vinylidene fluoride and hexafluoropropylene, melting point: 125°C) manufactured by Arkema, 80 parts by weight of N,N-dimethylacetamide, 0.05 parts by weight of sodium dodecyl sulfate as a conductivity imparting agent, and 1 part by weight of fluorooctylsilsesquioxane (manufactured by NBD Nanotechnologies) as a water repellent agent. A conveyor-type collector is used as the collection unit, and a nonwoven fabric (thickness: approximately 0.3 mm, basis weight: approximately 80 g / m²) made of polyethylene terephthalate copolymer and heat-adhesive composite fibers containing polyethylene terephthalate (average fiber diameter: approximately 40 μm) is used on the collector surface. 2 A porous layer was attached. Next, three nozzles with an inner diameter of 0.3 mm and 12 holes were installed with a spacing of 380 mm between them in the direction of the conveyor's movement. Electrospinning was performed on the porous layer while traversing the nozzles perpendicular to the direction of the conveyor's movement, thereby creating a fiber sheet composite in which three layers of fiber sheets and the porous layer were laminated. The spinning conditions in this embodiment were: single-hole liquid delivery rate to each nozzle using a pump was 2.4 mL / hr, applied voltage was 45 kV, spinning distance was 100 mm, nozzle traverse width was 170 mm, traverse speed was 150 mm / sec, spinning environment temperature was 25°C and humidity was 30 RH%, and the basis weight of the fiber sheet was 1.7 g / m². 2 Electrospinning was performed by adjusting the feeding speed of the porous layer by a conveyor-type collector to achieve the desired result. The average fiber diameter of the nanofibers constituting the fiber sheet was 110 nm, and the filter performance of the fiber sheet composite was a pressure drop of 140.0 Pa and a collection efficiency of 99.99%. In addition, although slight fuzzing occurred when passed through the processing line, no delamination between the fiber sheets or delamination between the fiber sheets and the porous layer was observed, indicating acceptable processability. The filter performance after processing was a pressure drop of 142.5 Pa and a collection efficiency of 99.94%.

[0058] [Example 2] A spinning solution was prepared in the same manner as in Example 1, except that 18.5 parts by weight of vinylidene fluoride polymer (trade name: Solef6010) and 1.5 parts by weight of vinylidene fluoride copolymer (trade name: Kynar2500) were used. Then, the spinning conditions were the same as in Example 1, except that the single-hole liquid delivery rate to each nozzle using a pump was changed to 0.8 mL / hr, and the basis weight of the fiber sheet was 0.8 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 110 nm, and the filter performance of the fiber sheet composite was a pressure drop of 114.6 Pa and a collection efficiency of 99.98%. Furthermore, when passed through the processing line, no fuzzing, delamination between fiber sheets, or delamination between the fiber sheets and the porous layer occurred, demonstrating satisfactory processability. The filter performance after processing was a pressure drop of 117.2 Pa and a collection efficiency of 99.96%.

[0059] [Example 3] A spinning solution was prepared in the same manner as in Example 1, except that 16.5 parts by weight of vinylidene fluoride polymer (trade name: Solef6010) and 3.5 parts by weight of vinylidene fluoride copolymer (trade name: Kynar2500) were used. Then, the spinning conditions were the same as in Example 1, except that the nozzle traverse speed was changed to 220 mm / second, and the basis weight of the fiber sheet was 0.7 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 110 nm, and the filter performance of the fiber sheet composite was a pressure drop of 44.3 Pa and a collection efficiency of 98.0%. Furthermore, when passed through the processing line, no fuzzing, delamination between fiber sheets, or delamination between the fiber sheets and the porous layer occurred, demonstrating satisfactory processability. The filter performance after processing was a pressure drop of 42.1 Pa and a collection efficiency of 96.0%.

[0060] [Example 4] A spinning solution was prepared in the same manner as in Example 1, except that 15 parts by weight of vinylidene fluoride polymer (product name: Solef6010) and 5 parts by weight of vinylidene fluoride copolymer (product name: Kynar2500) were used. Then, under the same spinning conditions as in Example 3, the basis weight of the fiber sheet was 0.4 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 130 nm, and the filter performance of the fiber sheet composite was a pressure drop of 46.5 Pa and a collection efficiency of 98.3%. Furthermore, when passed through the processing line, no fuzzing, delamination between fiber sheets, or delamination between the fiber sheets and the porous layer occurred, demonstrating satisfactory processability. The filter performance after processing was a pressure drop of 42.1 Pa and a collection efficiency of 95.2%.

[0061] [Example 5] A spinning solution was prepared in the same manner as in Example 1, except that 10 parts by weight of vinylidene fluoride polymer (trade name: Solef6010) and 10 parts by weight of vinylidene fluoride copolymer (trade name: Kynar2500) were used. Then, the spinning conditions were the same as in Example 3, except that the single-hole liquid delivery rate to each nozzle using a pump was changed to 3.2 mL / hr, and the basis weight of the fiber sheet was 0.3 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 120 nm, and the filter performance of the fiber sheet composite was a pressure drop of 38.3 Pa and a collection efficiency of 96.7%. In addition, because the proportion of vinylidene fluoride copolymer with a melting point of 150°C or less increased, the durability of the nanofibers was slightly reduced, and slight fuzzing was observed when passed through the processing line, but no delamination was observed, and the processability was acceptable. The filter performance after processing was a pressure drop of 35.0 Pa and a collection efficiency of 93.7%.

[0062] [Example 6] A spinning solution was prepared in the same manner as in Example 1, except that 18 parts by weight of vinylidene fluoride polymer (product name: Solef6010) and 2 parts by weight of vinylidene fluoride copolymer (product name: Kynar2500) were used. A conveyor-type collector is used as the collection unit, and a nonwoven fabric (thickness: approximately 0.3 μm, basis weight: approximately 80 g / m²) made of polyethylene terephthalate copolymer and heat-adhesive composite fibers containing polyethylene terephthalate (average fiber diameter: approximately 40 μm) is used on the collector surface. 2 A porous layer was attached. Next, one nozzle with an inner diameter of 0.3 mm and 12 holes was installed, and electrospinning was performed on the porous layer while the nozzle traversed perpendicular to the direction of travel of the conveyor, thereby creating a fiber sheet composite in which a single layer of fiber sheet and the porous layer were laminated. The spinning conditions in this example were: single-hole liquid delivery rate to each nozzle using a pump was 1.6 mL / hr, applied voltage was 45 kV, spinning distance was 100 mm, nozzle traverse width was 110 mm, traverse speed was 150 mm / sec, spinning environment temperature was 25°C and humidity was 30 RH%, and the basis weight of the fiber sheet was 0.6 g / m². 2 Electrospinning was performed by adjusting the feeding speed of the porous layer by a conveyor-type collector to achieve the desired result. The average fiber diameter of the nanofibers constituting the fiber sheet was 100 nm, and the filter performance of the fiber sheet composite was a pressure drop of 102.0 Pa and a collection efficiency of 99.93%. Furthermore, when passed through the processing line, no fuzzing or delamination of the fiber sheet and the porous layer occurred, demonstrating satisfactory processability. After processing, the filter performance was a pressure drop of 101.4 Pa and a collection efficiency of 99.87%.

[0063] [Example 7] A spinning solution was prepared in the same manner as in Example 6, except that the vinylidene fluoride copolymer was replaced with Arkema's trade name: KynarADS2 (a random copolymer of vinylidene fluoride and hexafluoropropylene, melting point: 115°C). Then, under the same spinning conditions as in Example 6, the basis weight of the fiber sheet was 0.5 g / m². 2By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which a single fiber sheet and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 100 nm, and the filter performance of the fiber sheet composite was a pressure drop of 105.0 Pa and a collection efficiency of 99.92%. Furthermore, when passed through the processing line, no fuzzing or delamination of the fiber sheet and porous layer occurred, demonstrating satisfactory processability.

[0064] [Example 8] A spinning solution was prepared in the same manner as in Example 6, except that the vinylidene fluoride copolymer was replaced with Arkema's trade name: KynarUltraFlexB (a random copolymer of vinylidene fluoride and hexafluoropropylene, melting point: 105°C). Then, under the same spinning conditions as in Example 6, the basis weight of the fiber sheet was 0.6 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which a single layer of fiber sheet and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 100 nm, and the filter performance of the fiber sheet composite was a pressure drop of 103.0 Pa and a collection efficiency of 99.92%. Furthermore, when passed through the processing line, no fuzzing or delamination of the fiber sheet and porous layer occurred, demonstrating satisfactory processability.

[0065] [Example 9] A spinning solution was prepared in the same manner as in Example 6, except that the vinylidene fluoride copolymer was replaced with Arkema's trade name: Kynar 2800 (a random copolymer of vinylidene fluoride and hexafluoropropylene, melting point: 145°C). Then, under the same spinning conditions as in Example 6, the basis weight of the fiber sheet was 0.6 g / m². 2By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which a single layer of fiber sheet and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 100 nm, and the filter performance of the fiber sheet composite was 98.0 Pa pressure loss and 99.89% collection efficiency. Furthermore, although slight fuzzing occurred when passed through the processing line, no delamination between the fiber sheet and the porous layer was observed, indicating acceptable processability.

[0066] [Example 10] The fiber sheet composite prepared in Example 1 was further heat-treated with circulating hot air at 100°C. The filter performance of the fiber sheet composite after heat treatment was a pressure drop of 160.0 Pa and a collection efficiency of 99.99%. Furthermore, when passed through the processing line, no fluffing, delamination between fiber sheets, or delamination between the fiber sheets and the porous layer occurred, demonstrating satisfactory processability. The filter performance after processing was a pressure drop of 160.0 Pa and a collection efficiency of 99.91%.

[0067] [Comparative Example 1] A spinning solution was prepared by mixing 20 parts by weight of vinylidene fluoride polymer (trade name: Solev6010) manufactured by Solvay Specialty Polymers, 80 parts by weight of N,N-dimethylacetamide, 0.05 parts by weight of sodium dodecyl sulfate as a conductivity imparterant, and 1 part by weight of fluorooctylsilsesquioxane as a water repellent. Then, under the same spinning conditions as in Example 1, the basis weight of the fiber sheet was 0.8 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 110 nm. The filter performance of the fiber sheet composite was a pressure drop of 123.4 Pa and a collection efficiency of 99.97%, but when passed through the processing line, delamination of the top layer of the fiber sheet was observed, making it impossible to process into a product.

[0068] [Comparative Example 2] A spinning solution was prepared in the same manner as in Comparative Example 1. Then, under the same spinning conditions as in Example 6, the basis weight of the fiber sheet was 0.5 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which a single layer of fiber sheet and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 120 nm. Furthermore, the filter performance of the fiber sheet composite was 96.0 Pa pressure loss and 99.82% collection efficiency, but when passed through the processing line, the pressure loss became 93.2 Pa and the collection efficiency became 95.3%, indicating a significant decrease in filter performance.

[0069] [Comparative Example 3] A spinning solution was prepared by mixing 20 parts by weight of vinylidene fluoride copolymer (trade name: Kynar2800), 80 parts by weight of N,N-dimethylacetamide, 0.05 parts by weight of sodium dodecyl sulfate as a conductivity imparterant, and 1 part by weight of fluorooctylsilsesquioxane as a water repellent. Then, under the same spinning conditions as in Example 1, the basis weight of the fiber sheet was 0.9 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 120 nm, and the pressure drop of the fiber sheet composite was 160.0 Pa, with a collection efficiency of 99.99%. However, because the nanofibers were made of a vinylidene fluoride copolymer with a melting point of 150°C or lower, they lacked durability, and when passed through a processing line, the pressure drop was 134.9 Pa and the collection efficiency was 99.77%, resulting in a significant decrease in filter performance.

[0070] [Comparative Example 4] A spinning solution was prepared by mixing 20 parts by weight of vinylidene fluoride copolymer (trade name: Kynar2500), 56 parts by weight of N,N-dimethylacetamide, and 24 parts by weight of tetrahydrofuran. Then, the same spinning conditions as in Example 1 were used, except that the spinning distance was changed to 180 mm, and the basis weight of the fiber sheet was 2.4 g / m². 2By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 260 nm, and the pressure drop of the fiber sheet composite was 140.0 Pa, with a collection efficiency of 99.63%. However, because the nanofibers were made of a vinylidene fluoride copolymer with a melting point of 150°C or lower, they lacked durability, and when passed through a processing line, the pressure drop was 104.9 Pa and the collection efficiency was 96.3%, resulting in a significant decrease in filter performance.

[0071] [Comparative Example 5] A spinning solution was prepared by mixing 16 parts by weight of vinylidene fluoride copolymer (trade name: Kynar 3120; block copolymer of vinylidene fluoride and hexafluoropropylene, melting point: 165°C) manufactured by Arkema, 58.8 parts by weight of N,N-dimethylformamide, 25.2 parts by weight of acetone, and 0.02 parts by weight of sodium dodecyl sulfate as a conductivity imparter. Then, the spinning conditions were the same as in Example 1, except that the single-hole liquid delivery rate to each nozzle using a pump was changed to 3.0 mL / hr and the spinning distance was changed to 125 mm, and the basis weight of the fiber sheet was 1.0 g / m². 2 By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the fiber sheet was 120 nm. The filter performance of the fiber sheet composite was a pressure drop of 125.0 Pa and a collection efficiency of 99.85%, but when passed through the processing line, delamination of the top layer of the fiber sheet was observed, making it impossible to process into a product.

[0072] [Comparative Example 6] A spinning solution was prepared in the same manner as in Example 6, except that the vinylidene fluoride copolymer was replaced with Arkema's trade name: Kynar 2850 (a random copolymer of vinylidene fluoride and hexafluoropropylene, melting point: 155°C). Then, under the same spinning conditions as in Example 1, the basis weight of the fiber sheet was 0.6 g / m². 2By adjusting the feed rate of the porous layer using a conveyor-type collector, a fiber sheet composite was fabricated in which three fiber sheets and a porous layer were laminated. The average fiber diameter of the nanofibers constituting the sheet was 100 nm. The filter performance of the fiber sheet composite was a pressure drop of 110.0 Pa and a collection efficiency of 99.92%, but when passed through the processing line, delamination of the top layer of the fiber sheet was observed, making it impossible to process into a product.

[0073] [Comparative Example 7] The fiber sheet composite prepared in Comparative Example 6 was further heat-treated with circulating hot air at 100°C. The filter performance of the three-layer fiber sheet composite after heat treatment was a pressure drop of 120.0 Pa and a collection efficiency of 99.5%. However, when passed through the processing line, delamination of the top layer of the fiber sheet was observed, making it impossible to process it into a product.

[0074] The results of the above examples and comparative examples are summarized in Tables 1 to 3.

[0075] [Table 1]

[0076] [Table 2]

[0077] [Table 3]

[0078] As can be seen from the results in Tables 1-3, when the nanofibers constituting the fiber sheet consist of vinylidene fluoride polymer (Comparative Examples 1 and 2), vinylidene fluoride copolymer (Comparative Examples 3-5), or a mixture of vinylidene fluoride polymer and vinylidene fluoride copolymer with a melting point exceeding 150°C (Comparative Examples 6 and 7), performance degradation occurs during product processing due to issues such as fuzzing, delamination between fiber sheets, and durability. However, Examples 1-10, which include a mixture of vinylidene fluoride polymer and vinylidene fluoride copolymer with a melting point of 150°C or lower, exhibit good processability and minimal performance degradation during product processing. Furthermore, a comparison of Examples 6-9 shows that using vinylidene fluoride copolymer with a lower melting point tends to improve processability. Furthermore, a comparison of Examples 1 to 5 shows that processability tends to improve when the mixing ratio (by weight) of vinylidene fluoride polymer and vinylidene fluoride copolymer is between 75:25 and 92.5:7.5. Additionally, Example 10, which underwent further heat treatment with circulating hot air compared to Example 1, also shows improved processability. On the other hand, Comparative Example 7 does not contain a vinylidene fluoride copolymer with a melting point of 150°C or lower, and therefore, further heat treatment with circulating hot air does not improve processability. [Industrial applicability]

[0079] The fiber sheet of the present invention retains the inherent properties of nanofibers, is less prone to fuzzing and delamination between fiber sheets, and exhibits excellent adhesion to other materials. Furthermore, the fiber sheet composite of the present invention is less prone to fuzzing and delamination between fiber sheets, and exhibits excellent adhesion between the fiber sheet and the porous layer without requiring integration processes such as bonding or calendering. As a result, the fiber sheet composite can be provided with high productivity and good operability, and can be suitably used as a filter material, sound-absorbing material, mask, waterproof and breathable membrane, separator for secondary batteries, sensor material, cell culture substrate, and the like.

Claims

1. A fiber sheet composite comprising a fiber sheet containing nanofibers having an average fiber diameter of 20 to 1000 nm and a porous layer laminated together, wherein the nanofibers contain a mixture of vinylidene fluoride polymer and a vinylidene fluoride copolymer having a melting point of 125°C or lower, and the fiber sheet adheres closely to the porous layer without the use of an adhesive.

2. The fiber sheet composite according to claim 1, wherein the mixing ratio (by weight) of the vinylidene fluoride polymer and the vinylidene fluoride copolymer is 75:25 to 92.5:7.

5.

3. A method for producing a fiber sheet composite according to claim 1 or 2, wherein the nanofibers are formed by electrospinning the porous layer.

4. The manufacturing method according to claim 3, wherein the electrospinning is further heat-treated with circulating hot air or radiant heat.

5. A filter material comprising the fiber sheet composite described in claim 1 or 2.

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