Method for producing stretched porous membrane and stretched porous membrane

Abstract

The present invention provides a method for manufacturing a stretched porous film that can reduce stretching unevenness, and a stretched porous film. [Solution] A method for manufacturing a stretched porous film, comprising: a first step of providing a resin sheet with uneven surfaces, which are at least one of convex portions protruding in the thickness direction and recessed portions; and a second step of generating fibers by stretching the sheet obtained in the first step.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing a stretched porous film and to a stretched porous film. [Background technology]

[0002] Porous membranes have been known for use in air filter media to capture dust and other particles contained in the fluid being treated.

[0003] For example, Patent Document 1 (Japanese Patent Publication No. 2003-181227) describes obtaining a porous film by stretching a resin formed in a sheet state using a stretching machine. [Disclosure of the Invention] [Problems that the invention aims to solve]

[0004] The stretched porous membrane obtained by this method is stretched, for example, by pulling the sheets apart while holding the edges of the unstretched sheets together. However, the stretched porous membrane obtained by this method may exhibit uneven stretching, making it difficult to improve the uniformity of the stretched porous membrane. [Means for solving the problem]

[0005] To solve the above problems, the inventors of this application conducted thorough research and found that uneven stretching could be reduced by applying a textured surface to the resin sheet before stretching. Further research led to the completion of the method for manufacturing a stretched porous film and the stretched porous film described below.

[0006] A method for manufacturing a stretched porous film according to the first aspect comprises a first step and a second step. In the first step, an uneven surface is created on a resin sheet. The uneven surface is at least one of a convex portion that protrudes in the thickness direction of the resin sheet and a concave portion that is recessed in the thickness direction of the resin sheet. In the second step, fibers are generated by stretching the sheet obtained in the first step.

[0007] Furthermore, stretching is preferably performed, for example, by applying force to the edges of the sheet obtained in the first step so that the edges of the sheet are separated from each other.

[0008] This method for manufacturing stretched porous membranes makes it possible to obtain stretched porous membranes with minimal stretching unevenness.

[0009] The method for manufacturing a stretched porous membrane according to the second perspective is the method for manufacturing a stretched porous membrane according to the first perspective, wherein the stretched porous membrane is an air filter material.

[0010] This method for manufacturing stretched porous membranes makes it possible to keep the coefficient of variation of pressure loss low when the resulting stretched porous membrane is used as an air filter material.

[0011] The method for manufacturing a stretched porous film according to the third aspect is the method for manufacturing a stretched porous film according to the first or second aspect, wherein the resin sheet contains a fluororesin.

[0012] This method for manufacturing stretched porous membranes makes it possible to obtain stretched porous membranes with minimal stretching unevenness while keeping pressure loss low.

[0013] The method for manufacturing a stretched porous film according to the fourth viewpoint is a method for manufacturing a stretched porous film according to any of the first, second, or third viewpoints, wherein the area ratio of the uneven portion to the resin sheet in the first step is 10% or more and 80% or less.

[0014] This method for manufacturing stretched porous membranes makes it easier to obtain stretched porous membranes with minimal stretching unevenness.

[0015] The method for manufacturing a stretched porous film according to the fifth perspective is a method for manufacturing a stretched porous film according to any of the first, fourth, or fourth perspectives, wherein multiple uneven surfaces are provided. In view of the resin sheet in the thickness direction, the average area of ​​each uneven surface is 2 mm 2 15mm or more 2 The following applies: In the thickness direction view of the stretched porous membrane, the average spacing between the uneven surfaces is between 2 mm and 9 mm.

[0016] This method for manufacturing the stretched porous membrane is likely to reduce unevenness in the appearance of fibers during stretching of the resin sheet.

[0017] The method for manufacturing the stretched porous membrane according to the sixth aspect is the method for manufacturing the stretched porous membrane according to any one of the first to fifth aspects, and the number of concavo-convex portions on the resin sheet in the first step is 2 per 100 mm 2 or more and 20 per 100 mm 2 or less.

[0018] This method for manufacturing the stretched porous membrane is likely to reduce unevenness in the appearance of fibers during stretching of the resin sheet.

[0019] The method for manufacturing the stretched porous membrane according to the seventh aspect is the method for manufacturing the stretched porous membrane according to any one of the first to sixth aspects, and among the resin sheet in the first step, the film thickness of the portion other than the concavo-convex portions is 250 μm or more and 650 μm or less.

[0020] This method for manufacturing the stretched porous membrane is likely to obtain a stretched porous membrane with less stretching unevenness.

[0021] The method for manufacturing the stretched porous membrane according to the eighth aspect is the method for manufacturing the stretched porous membrane according to any one of the first to seventh aspects, and in the resin sheet in the first step, the concavo-convex portions have a higher density compared to the periphery of the concavo-convex portions.

[0022] This method for manufacturing the stretched porous membrane is likely to uniformly generate fibers during stretching by providing concavo-convex portions such that the density is different between the concavo-convex portions and the periphery thereof in the first step.

[0023] The method for manufacturing the stretched porous membrane according to the ninth aspect is the method for manufacturing the stretched porous membrane according to any one of the first to eighth aspects, and the stretching area magnification in the second step is 50 times or more and 1500 times or less. [[ID=3--]]

[0024] This method for manufacturing the stretched porous membrane can also suppress uneven stretching even for a stretched porous membrane obtained by stretching at a stretching area magnification of 50 times or more and 1500 times or less.

[0025] The method for manufacturing a stretched porous membrane according to the 10th viewpoint is a method for manufacturing a stretched porous membrane according to any of the 1st viewpoint to the 9th viewpoint, wherein the uneven portions are arranged regularly.

[0026] This method for manufacturing stretched porous films involves stretching a resin sheet with multiple regularly spaced irregularities, which makes it possible to minimize unevenness in the stretching of the resulting stretched porous film.

[0027] The stretched porous membrane according to the 11th aspect has a stretched pattern in which a predetermined shape is regularly repeated.

[0028] This stretched porous membrane exhibits a low coefficient of variation in pressure loss when used as a filter medium.

[0029] The stretched porous membrane relating to the 12th aspect is the stretched porous membrane relating to the 11th aspect, and the stretched porous membrane includes a fluororesin.

[0030] This stretched porous membrane makes it possible to keep pressure loss low while minimizing stretching unevenness.

[0031] The stretched porous membrane relating to the 13th aspect is the stretched porous membrane relating to the 11th or 12th aspect, and the stretched porous membrane is an air filter material.

[0032] This stretched porous membrane makes it possible to keep the coefficient of variation of pressure loss low when used as an air filter material. [Brief explanation of the drawing]

[0033] [Figure 1] This is a schematic diagram of the device used for longitudinal extension, viewed from the side. [Figure 2] This is a schematic diagram of the device used for stretching in the width direction, viewed from the side. [Figure 3] This is a schematic perspective view of a device that performs widthwise extension. [Figure 4]This is a plan view illustrating the difference in stretching speed in the width direction due to differences in the line shape of continuous clips. [Figure 5] This is a schematic diagram showing an example of a stretching device. [Figure 6] This is a schematic cross-sectional view showing the layer structure of the air filter media. [Figure 7] This is a perspective view of the filter pack. [Figure 8] This is a perspective view of the air filter unit. [Figure 9] This is an external view showing the shape of the rail through which the continuous clips used for extension in the second direction in Example 6 and Comparative Example 5 move. [Modes for carrying out the invention]

[0034] The method for producing a stretched porous membrane and the stretched porous membrane according to this embodiment will be described below with reference to examples.

[0035] (1) Method for producing stretched porous membrane A method for manufacturing a stretched porous film comprises a first step of providing a resin sheet with uneven surfaces, which are at least one of convex portions protruding in the thickness direction and recessed portions, and a second step of generating fibers by stretching the sheet obtained in the first step.

[0036] (1-1) Preparation of resin sheet (1-1-1) Material of resin sheet The material of the resin sheet used in the method for manufacturing a stretched porous film according to this disclosure is not particularly limited as long as it is capable of generating fibers by stretching.

[0037] The material for such a resin sheet is preferably one or more resins selected from the group consisting of fluororesins such as PTFE (polytetrafluoroethylene), polyvinylidene fluoride, PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), and FEP (random polymer of tetrafluoroethylene and hexafluoropropylene), polyolefins such as polyethylene and polypropylene, nylon, polyester, polysulfone, and polyetheretherketone.

[0038] The resin sheet preferably contains mainly fluororesin, may contain 50% or more by weight of fluororesin, preferably 80% or more by weight of fluororesin, more preferably 95% or more by weight of fluororesin, or may be composed solely of fluororesin.

[0039] The resin sheet preferably contains PTFE that can be fibroused as described below, and more preferably further contains either or both of a non-heat-melt-processable component that does not fibrous and a non-heat-melt-processable component with a melting point of less than 320°C that does not fibrous.

[0040] Furthermore, when creating a stretched porous membrane used as a filter medium, the resin sheet may be composed of multiple types of fluororesins from the viewpoint of improving pressure loss and PF value. However, when composed of multiple types of fluororesins, uneven stretching tends to occur more easily than when composed of a single type of fluororesin. Nevertheless, even in such cases, it is possible to minimize uneven stretching by creating uneven surfaces before stretching.

[0041] (PTFE that can be fiberized) PTFE that can be fiberized is, for example, one that is stretchable and non-melt processable. "Non-melt processable" means that, due to its high melt viscosity, it does not easily flow in the molten state and is difficult to process by melt. Examples of PTFE that can be fiberized include those with a melt viscosity of 1 × 10⁻⁶ at 380°C. 8It is preferable that the Pa·S level is above a certain value.

[0042] PTFE that can be formed into fibers is, for example, high molecular weight PTFE obtained from emulsion polymerization or suspension polymerization of tetrafluoroethylene (TFE). Here, high molecular weight refers to a molecular weight that is easily formed into fibers during stretching when creating porous films, yielding long fibrils, with a standard specific gravity (SSG) of 2.130 to 2.230, and a molecular weight that is large enough that it does not substantially melt and flow due to its high melt viscosity. From the viewpoint of easy fiber formation and obtaining long fibrils, the SSG of PTFE that can be formed into fibers is preferably 2.130 to 2.190, and more preferably 2.140 to 2.170. If the SSG is too high, the stretchability may deteriorate, and if the SSG is too low, the rollability may deteriorate, the homogeneity of the porous film may worsen, and the pressure loss of the porous film may increase. The above standard specific gravity (SSG) is measured in accordance with ASTM D 4895.

[0043] Furthermore, PTFE obtained by emulsion polymerization is preferred from the viewpoint of being easily fibrous and yielding fibrils with long fiber lengths. Emulsion polymerization can generally be carried out in an aqueous medium containing TFE, or TFE and monomers other than TFE, a dispersant, and a polymerization initiator. In emulsion polymerization, the polymerization temperature is generally 20 to 100°C, preferably 50 to 85°C, and the polymerization pressure is generally 0.5 to 3.0 MPa. As polymerization initiators in emulsion polymerization, radical polymerization initiators, redox polymerization initiators, etc., are preferred. The amount of polymerization initiator is preferable in that a smaller amount suppresses the formation of low molecular weight PTFE and allows for the acquisition of PTFE with low SSG, but if it is too small, the polymerization rate tends to become too small, and if it is too large, PTFE with high SSG tends to be produced.

[0044] PTFE may also constitute the fine powder obtained by emulsion polymerization. The fine powder can be obtained by recovering PTFE fine particles from the aqueous PTFE dispersion obtained by the above-mentioned emulsion polymerization, coagulating them, and then drying them. The fine powder made of the above-mentioned PTFE has good extrusion processability and can be paste-extruded at an extrusion pressure of 20 MPa or less, for example. The extrusion pressure was measured when paste-extruding through an orifice (diameter 2.5 cm, land length 1.1 cm, introduction angle 30°) under conditions of a reduction ratio of 100, an extrusion speed of 51 cm / min, and 25°C. Paste extrusion molding generally involves mixing the above-mentioned fine powder with an extrusion aid (lubricant), performing pre-molding, and then extruding. The extrusion aid is not particularly limited and conventionally known ones can be used, but petroleum hydrocarbons with a boiling point of 150°C or higher, such as naphtha, are preferred. The amount of extrusion aid added can be an amount equivalent to 10 to 40% by mass of the total mass of the above-mentioned fine powder and extrusion aid.

[0045] The presence or absence of fibrous properties, that is, whether or not it can be formed into fibers, can be determined by whether or not paste extrusion, a typical method for molding high molecular weight PTFE powder made from TFE polymers, is possible. Paste extrusion is usually possible because high molecular weight PTFE has fibrous properties. If the unfired molded body obtained by paste extrusion has no substantial strength or elongation, for example, if the elongation is 0% and it breaks when pulled, it can be considered that it does not have fibrous properties.

[0046] The above-mentioned high molecular weight PTFE may be modified polytetrafluoroethylene (hereinafter referred to as modified PTFE), homopolytetrafluoroethylene (hereinafter referred to as homoPTFE), or a mixture of modified PTFE and homoPTFE.

[0047] Modified PTFE consists of TFE and monomers other than TFE (hereinafter referred to as modified monomers). Modified PTFE can be uniformly modified by the modified monomer, modified in the early stages of the polymerization reaction, or modified in the final stages of the polymerization reaction, but is not limited to these. Preferably, modified PTFE is a TFE copolymer obtained by polymerizing TFE along with a small amount of monomers other than TFE, within a range that does not significantly impair the properties of the TFE homopolymer.

[0048] Modified PTFE contains TFE units based on TFE and modified monomer units based on modified monomers. Modified PTFE preferably contains 0.001 to 0.500% by weight of modified monomer units, more preferably 0.01 to 0.30% by weight of total monomer units. Total monomer units are the portion of the molecular structure of modified PTFE that originates from all monomers.

[0049] The modified monomer is not particularly limited as long as it can copolymerize with TFE, and examples include perfluoroolefins such as hexafluoropropylene (HFP); chlorofluoroolefins such as chlorotrifluoroethylene (CTFE); hydrogen-containing fluoroolefins such as trifluoroethylene and vinylidene fluoride (VDF); perfluorovinyl ether; perfluoroalkylethylene (PFAE); ethylene, etc. One or more modified monomers may be used.

[0050] Perfluorovinyl ethers are not particularly limited and include, for example, perfluorounsaturated compounds represented by the following general formula (1). CF2 = CF - ORf ... (1) In the formula, Rf represents a perfluoroorganic group.

[0051] In this specification, a perfluoroorganic group is an organic group in which all hydrogen atoms bonded to a carbon atom are replaced with fluorine atoms. The above perfluoroorganic group may have an ether oxygen.

[0052] Examples of perfluorovinyl ethers include perfluoro(alkyl vinyl ether) (PAVE) in which Rf in the above general formula (1) is a perfluoroalkyl group having 1 to 10 carbon atoms. The number of carbon atoms in the perfluoroalkyl group is preferably 1 to 5. Examples of perfluoroalkyl groups in PAVE include perfluoromethyl group, perfluoroethyl group, perfluoropropyl group, perfluorobutyl group, perfluoropentyl group, and perfluorohexyl group. Perfluoropropyl vinyl ether (PPVE) and perfluoromethyl vinyl ether (PMVE) are preferred PAVEs.

[0053] The perfluoroalkylethylene (PFAE) mentioned above is not particularly limited and includes, for example, perfluorobutylethylene (PFBE), perfluorohexylethylene (PFHE), and the like.

[0054] Homo-PTFE is preferably included in more than 50% by weight of the PTFE that can be fiberized, particularly from the viewpoint of being easily fiberized and yielding fibrils with long fiber lengths.

[0055] From the viewpoint of maintaining the fibrous structure of the porous membrane, it is preferable that the amount of PTFE that can be formed into fibers exceeds 50% by weight of the porous membrane.

[0056] (Non-fiberizable, non-thermal melting processable component) The non-fiber-forming, non-thermal-meltable components are mainly distributed as non-fiber particles in the knots, and they work to suppress the fiber formation of PTFE that can be fiberized.

[0057] Examples of non-thermal melt-processable components that do not form fibers include thermoplastic components such as low molecular weight PTFE, thermosetting resins, inorganic fillers, and mixtures thereof.

[0058] Thermoplastic components are preferably those with a melting point of 320°C or higher and a high melt viscosity. For example, low molecular weight PTFE has a high melt viscosity, so it can remain in the knots even when processed at temperatures above its melting point. In this specification, low molecular weight PTFE refers to a number average molecular weight of 600,000 or less, a melting point of 320°C to 335°C, and a melt viscosity at 380°C of 100 Pa·s to 7.0 × 10⁻⁶. 5 It is Pa·s PTFE (see Japanese Patent Publication No. 10-147617).

[0059] Methods for producing low molecular weight PTFE include a method of thermal decomposition by contacting high molecular weight PTFE powder (molding powder) obtained from suspension polymerization of TFE or high molecular weight PTFE powder (FP: fine powder) obtained from emulsion polymerization of TFE with a specific fluoride at high temperature (see Japanese Patent Publication No. 61-162503), a method of irradiating the above high molecular weight PTFE powder or molded body with ionizing radiation (see Japanese Patent Publication No. 48-78252), and a method of directly polymerizing TFE together with a chain transfer agent (see International Publication No. 2004 / 050727, International Publication No. 2009 / 020187, International Publication No. 2010 / 114033, etc.). Low molecular weight PTFE may be homo-PTFE, similar to PTFE that can be formed into fibers, or it may be modified PTFE containing the aforementioned modified monomer.

[0060] Low molecular weight PTFE does not fibrousize. The presence or absence of fibrous properties can be determined by the method described above. Unfired molded bodies obtained by paste extrusion of low molecular weight PTFE have virtually no strength or elongation; for example, they have 0% elongation and will break when pulled.

[0061] The low molecular weight PTFE is not particularly limited, but it is preferably such that its melt viscosity at 380°C is 1000 Pa·s or higher, more preferably 5000 Pa·s or higher, and even more preferably 10000 Pa·s or higher. With such a high melt viscosity, even if the non-fiber-forming, heat-melt-processable component with a melting point of less than 320°C melts during the production of the porous film, the non-fiber-forming, non-fiber-forming, heat-melt-processable component can remain in the knots, thereby suppressing fiber formation.

[0062] Examples of thermosetting resins include epoxy, silicone, polyester, polyurethane, polyimide, phenol, and mixtures thereof. From the viewpoint of ease of co-coagulation, it is preferable to use thermosetting resins that are dispersed in water in an uncured state.

[0063] Examples of inorganic fillers include talc, mica, calcium silicate, glass fibers, calcium carbonate, magnesium carbonate, carbon fibers, barium sulfate, calcium sulfate, and mixtures thereof. From the viewpoint of forming a stable dispersion during the production of porous films, inorganic fillers with a particle size of 3 μm to 20 μm are preferably used. The particle size is the average particle size and is measured by laser diffraction / scattering.

[0064] The non-fiber-forming, non-thermally meltable component is preferably present in an amount of 20% to 40% by weight of the stretched porous film.

[0065] (Non-fiberizable, heat-meltable components with a melting point of less than 320°C) Non-fiber-forming, heat-melt-processable components with a melting point of less than 320°C (hereinafter also referred to as non-fiber-forming, heat-melt-processable components) have fluidity when melted, allowing them to melt during the manufacturing (stretching) of the porous membrane and solidify at the knots, thereby increasing the overall strength of the porous membrane and suppressing deterioration of filter performance even if it is compressed in subsequent processes.

[0066] The non-fibrous, heat-melt-processable component preferably exhibits a melt viscosity of less than 10,000 Pa·s at 380°C. The melting point of the non-fibrous, heat-melt-processable component is defined as the peak of the heat of fusion curve obtained when the material is heated to above its melting point at a rate of 10°C / min using a differential scanning calorimeter (DSC), completely melted, cooled to below its melting point at 10°C / min, and then heated again at 10°C / min.

[0067] Examples of heat-meltable components that do not form fibers include heat-meltable fluoropolymers, polystyrene, polyethylene terephthalate (PET), polyester, polyamide, and other resins, or mixtures thereof, which can sufficiently exhibit meltability and fluidity at the stretching temperature during the production of porous films. Among these, heat-meltable fluoropolymers are preferred due to their excellent heat resistance at the stretching temperature during the production of porous films and their excellent chemical resistance. Heat-meltable fluoropolymers are given by the following general formula (2) RCF = CR2...(2) Examples of fluoropolymers include those comprising copolymer units derived from at least one fluorinated ethylenically unsaturated monomer represented by the formula (wherein R is independently selected from H, F, Cl, an alkyl group with 1 to 8 carbon atoms, an aryl group with 6 to 8 carbon atoms, a cyclic alkyl group with 3 to 10 carbon atoms, and a perfluoroalkyl group with 1 to 8 carbon atoms. In this case, all R may be the same, any two R may be the same and the remaining R may be different from the others, or all R may be different from each other.) preferably two or more monomers.

[0068] Useful examples of compounds represented by general formula (2) include, but are not limited to, perfluoroolefins such as fluoroethylene, VDF, trifluoroethylene, TFE, and HFP; chlorofluoroolefins such as CTFE and dichlorodifluoroethylene; (perfluoroalkyl)ethylenes such as PFBE and PFHE; perfluoro-1,3-dioxole and mixtures thereof.

[0069] Furthermore, the fluoropolymer comprises at least one monomer represented by the above general formula (2), The above general formula (1) and / or the following general formula (3) R2C = CR2···(3) This may also include copolymers derived from copolymerization with at least one copolymerizable comonomer represented by (wherein R is independently selected from H, Cl, an alkyl group with 1 to 8 carbon atoms, an aryl group with 6 to 8 carbon atoms, and a cyclic alkyl group with 3 to 10 carbon atoms. In this case, all R may be the same, or two or more R may be the same and these two or more R may be different from the remaining R, or all R may be different from each other. If there are multiple other R, they may be different from each other.)

[0070] A useful example of a compound represented by general formula (1) is perfluoro(alkyl vinyl ether) (PAVE). Preferred PAVEs include perfluoropropyl vinyl ether (PPVE) and perfluoromethyl vinyl ether (PMVE).

[0071] Useful examples of compounds represented by general formula (3) include ethylene and propylene.

[0072] More specific examples of fluoropolymers include polyfluoroethylene derived from the polymerization of fluoroethylene, polyvinylidene fluoride (PVDF) derived from the polymerization of vinylidene fluoride (VDF), polychlorotrifluoroethylene (PCTFE) derived from the polymerization of chlorotrifluoroethylene (CTFE), fluoropolymers derived from the copolymerization of two or more different monomers represented by the above general formula (2), and fluoropolymers derived from the copolymerization of at least one monomer of the above general formula (2) and at least one monomer represented by the above general formula (1) and / or at least one monomer represented by the above general formula (3).

[0073] Examples of such polymers include polymers having copolymer units derived from VDF and hexafluoropropylene (HFP), and polymers derived from TFE and at least one copolymerizable comonomer other than TFE (at least 3% by weight). Examples of the latter type of fluoropolymers include TFE / PAVE copolymer (PFA), TFE / PAVE / CTFE copolymer, TFE / HFP copolymer (FEP), TFE / ethylene copolymer (ETFE), TFE / HFP / ethylene copolymer (EFEP), TFE / VDF copolymer, TFE / VDF / HFP copolymer, TFE / VDF / CTFE copolymer, etc., or mixtures thereof.

[0074] The content of the heat-melt-processable component that does not form fibers in the stretched porous film is preferably 0.1% by weight or more and less than 20% by weight.

[0075] (1-1-2) Formation of resin sheet Regarding the resin sheet material described above, for example, after coagulation and co-coagulation, dewatering and drying are performed, a liquid lubricant (extrusion aid) is mixed in, and extrusion is performed to obtain a resin in sheet form.

[0076] The following explanation will use the example of a resin sheet material that contains fluororesin.

[0077] The size of the mixed powder obtained by coagulation and co-coagulation is not particularly limited, but is preferably, for example, an average particle size of 100 μm or more and 1000 μm or less, and preferably 300 μm or more and 800 μm or less. In this case, the average particle size is measured in accordance with JIS K6891. The apparent density of the mixed powder obtained by coagulation and co-coagulation is not particularly limited, but is preferably, for example, 0.40 g / ml or more and 0.60 g / ml or less, and preferably 0.45 g / ml or more and 0.55 g / ml or less. The apparent density is measured in accordance with JIS K6892.

[0078] As for the above co-coagulation method, for example, it is preferable to mix an aqueous dispersion of PTFE that can be formed into fibers with an aqueous dispersion of a non-heat-melt-processable component that does not form fibers, and an aqueous dispersion of a non-heat-melt-processable component with a melting point of less than 320°C, and then coagulate the mixture.

[0079] The form of the PTFE that can be fibrousized before mixing is not particularly limited, but it may be an aqueous dispersion of the PTFE that can be fibrousized, or it may be a powder. Examples of powders (especially the FP: fine powders mentioned above) include "Teflon 6-J" (hereinafter Teflon is a registered trademark), "Teflon 6C-J", "Teflon 62-J", etc. from Mitsui DuPont Fluorochemicals; "Polyflon F106" (hereinafter Polyflon is a registered trademark), "Polyflon F104", "Polyflon F201", "Polyflon F302", etc. from Daikin Industries, Ltd.; "Fluon CD123" (hereinafter Fluon is a registered trademark), "Fluon CD1", "Fluon CD141", "Fluon CD145", etc. from Asahi Glass Co., Ltd.; and "Teflon 60", "Teflon 60 X", "Teflon 601A", "Teflon 601 X", "Teflon 613A", "Teflon 613A X", "Teflon 605XT X", "Teflon 669 X", etc. from DuPont. The fine powder may also be obtained by coagulating and drying an aqueous dispersion of PTFE that can be formed into fibers (aqueous dispersion after polymerization) obtained from the emulsion polymerization of TFE.

[0080] The aqueous dispersion of PTFE that can be formed into fibers may be the aqueous dispersion produced after polymerization as described above, or a commercially available aqueous dispersion. Examples of commercially available aqueous dispersions of PTFE that can be formed into fibers include Daikin Industries' "Polyflon D-110," "Polyflon D-210," "Polyflon D-210C," and "Polyflon D-310," Mitsui DuPont Fluorochemicals' "Teflon 31-JR" and "Teflon 34-JR," and Asahi Glass's "Fluon AD911L," "Fluon AD912L," and "AD938L."

[0081] The non-thermally meltable component that does not form fibers may be in the form of an aqueous dispersion or a powder (generally called PTFE micropowder or micropowder) before mixing. Examples of low molecular weight PTFE powders include "MP1300-J" from Mitsui DuPont Fluorochemicals, "Levlon L-5" (hereinafter, Levlon is a registered trademark) and "Levlon L-5F" from Daikin Industries, Ltd., "Fluon L169J", "Fluon L170J", and "Fluon L172J" from Asahi Glass Co., Ltd., and "KTL-F" and "KTL-500F" from Kitamura Corporation.

[0082] The aqueous dispersion of low molecular weight PTFE may be an aqueous dispersion obtained from the emulsion polymerization of TFE as described above, or a commercially available aqueous dispersion such as "Lubron LDW-410" manufactured by Daikin Industries, Ltd.

[0083] The liquid lubricant is not particularly limited as long as it can wet the surface of the PTFE powder and can be removed after the material obtained by coagulation or co-coagulation is formed into a film. Examples include liquid paraffin, naphtha, white oil, hydrocarbon oils such as toluene and xylene, alcohols, ketones, and esters.

[0084] The material obtained by coagulation and co-coagulation is mixed with a liquid lubricant and then extruded and rolled in a conventionally known manner to form a sheet. Extrusion can be carried out by paste extrusion, ram extrusion, etc., but paste extrusion is preferred. The sheet-like extruded material obtained by paste extrusion is rolled under heating conditions, for example, at a temperature of 40°C to 80°C, using a calender roll or the like. The thickness of the sheet obtained by rolling is set based on the thickness of the desired stretched porous film, with the lower limit of the thickness being preferably 250 μm and more preferably 300 μm. The upper limit of the thickness of the sheet is being preferably 650 μm and more preferably 500 μm.

[0085] Next, the liquid lubricant is removed from the unfired sheet-like material, which is the rolled product. The removal of the liquid lubricant is carried out by heating, extraction, or a combination thereof. In the case of heating, the heating temperature is, for example, 100°C to 250°C, or 180°C to 200°C.

[0086] Here, from the viewpoint of reducing the packing density of the resulting porous fluororesin film and reducing pressure loss, it is preferable to heat the sheet-like material from which the liquid lubricant has been removed by heating it for at least one minute in a temperature atmosphere of 250°C to 325°C before stretching. The temperature of this heat treatment may be, for example, 320°C or lower, and is preferably below the melting point of the fluororesin used to produce the stretched porous fluororesin film. If multiple endothermic curves (primary melting point, secondary melting point) appear on the crystal melting curve when the rolled material is heated using a differential scanning calorimeter at a heating rate of 10°C / min, the temperature may be below the lower maximum peak temperature (primary melting point). Furthermore, from the viewpoint of sufficiently reducing the packing density and pressure loss, the temperature of this heat treatment may be, for example, 260°C or higher, 280°C or higher, and may be higher than the temperature at which the liquid lubricant is removed from the unfired sheet-like material that is rolled by the heating method, and may be higher than the stretching temperature (in the case of biaxial stretching, the temperature of the primary stretching performed earlier). The duration of the heat treatment is not particularly limited, but may be set to, for example, 1 minute to 2 hours, or 30 minutes to 1 hour, depending on the desired effect of the heat treatment.

[0087] In this way, a resin sheet before stretching is obtained.

[0088] (1-2) Formation of uneven surfaces in resin sheets The resin sheet obtained as described above is given a surface with irregularities, which consist of at least one of a convex portion that protrudes in the thickness direction of the resin sheet and a concave portion that is recessed in the thickness direction of the resin sheet.

[0089] Preferably, a plurality of concavo-convex portions are provided in the resin sheet so as to have a predetermined regularity. The concavo-convex portions may be formed by forming a plurality of convex portions protruding on one of the two surfaces of the resin sheet, or a plurality of convex portions protruding on one of the two surfaces of the resin sheet and a plurality of concave portions recessed on the one surface may be formed respectively.

[0090] Preferably, the concavo-convex portions are formed by partially pressing the resin sheet in the thickness direction so that the density is higher than that around the concavo-convex portions of the resin sheet. Here, the thickness of the portion of the resin sheet where no concavo-convex portions are formed is preferably, for example, 250 μm or more and 650 μm or less, and more preferably 300 μm or more and 500 μm or less. Also, the thickness of the concavo-convex portions of the resin sheet is preferably, for example, 20% or more and 95% or less of the thickness of the portion of the resin sheet where no concavo-convex portions are formed. By forming concavo-convex portions having a higher density than the surroundings in the resin sheet in this way, it becomes easier to uniformly generate fibers during stretching over the entire area including the portions other than the edges of the resin sheet. Also, by preventing the density of the concavo-convex portions in the resin sheet from being too high, breakage of the sheet during stretching is suppressed, and leakage can be suppressed when the stretched porous membrane obtained by stretching is used as a filter medium.

[0091] From the viewpoint of suppressing uneven fiber appearance during stretching and suppressing the coefficient of variation of the pressure loss of the stretched porous membrane obtained by stretching, the total area ratio of the plurality of concavo-convex portions with respect to the resin sheet is preferably 10% or more and 80% or less, more preferably 20% or more and 70% or less, and even more preferably 25% or more and 65% or less.

[0092] In the thickness direction view of the resin sheet, the average area per concavo-convex portion is 2 2 mm or more and 2 15 mm or less, preferably 2 3 mm or more and 2The following is preferable. In this case, the average spacing between the uneven parts is preferably 2 mm to 9 mm, and more preferably 3 mm to 8 mm. This makes it easier to keep the unevenness of fiber formation during stretching small and to keep the coefficient of variation of pressure loss of the stretched porous membrane obtained by stretching small. The average spacing between the uneven parts may be, for example, the average value of the gap between one uneven part randomly selected from among several uneven parts and the 4 to 6 nearest uneven parts around it, and the average value of the gaps for 10 times while changing the randomly selected uneven part 10 times, and the value obtained as the average of that average.

[0093] Furthermore, when viewed in the thickness direction of the resin sheet, the number of protrusions and indentations on the resin sheet is 2 per 100 mm. 2 More than 20 pieces / 100mm 2 Preferably, the following: 4 pieces / 100mm 2 More than 16 pieces / 100mm 2 The following is more preferable. This makes it possible to minimize uneven fiber development during stretching and to reduce the coefficient of variation of the pressure loss of the stretched porous membrane obtained by stretching.

[0094] From the viewpoint of facilitating the uniform generation of fibers during stretching, it is preferable that the multiple uneven surfaces be arranged on the resin sheet according to a certain regularity.

[0095] The shape of the uneven surface is not particularly limited, but examples include rhombuses, rectangles, triangles, circles, and ellipses. While the shapes of the multiple uneven surfaces may differ, it is preferable that they be the same from the viewpoint of uniformly generating fibers through stretching.

[0096] The uneven surface may be formed, for example, by pressing a mold with the uneven surface pattern formed thereon against a resin sheet, or by passing the resin sheet between a metal roll with the uneven surface pattern formed thereon and a smooth resin or rubber roll. In order to prevent excessive shear force from being applied in order to suppress breakage of the resin sheet when it is stretched, it is preferable to form the uneven surface by passing the resin sheet between a metal roll with the uneven surface pattern formed thereon and a smooth resin or rubber roll.

[0097] When forming the uneven surface, the resin sheet may or may not be heated. However, from the viewpoint of avoiding the density of the uneven surface becoming the same as that of the surrounding area due to the application of heat, it is preferable to keep the degree of heating to a minimum or to form the uneven surface without heating.

[0098] As a result, uneven surfaces are formed on the resin sheet.

[0099] (1-3) Stretching of resin sheet By stretching the resin sheet with the uneven surface formed before stretching, as described above, in a predetermined direction perpendicular to the thickness direction, fibers can be generated, and a stretched porous film can be obtained.

[0100] Stretching includes stretching in a first direction and, preferably, stretching in a second direction that intersects or is perpendicular to the first direction. From the viewpoint of increasing the total area ratio after stretching without rupturing the resin sheet and from the viewpoint of preventing the film thickness from not increasing due to the alignment of the fibrils (fibers), it is preferable to perform stretching in a second direction as well as a first direction. Here, stretching in the second direction may be performed after stretching in the first direction, stretching in the first direction and stretching in the second direction may be performed simultaneously, or stretching in the first direction and stretching in the second direction may be performed simultaneously after stretching in the first direction. In this embodiment, the first direction can be the longitudinal direction (vertical direction: MD direction) of the rolled material, and the second direction can be the width direction (transverse direction: TD direction) of the rolled material. Furthermore, if there is a direction in the rolled material that is easy to stretch and a direction that is difficult to stretch, it is preferable to make the direction that is difficult to stretch the second direction. Furthermore, the stretching process may be carried out by stacking multiple resin sheets together and stretching them simultaneously.

[0101] Here, the stretching in the first direction can be, for example, 2 times or more and 50 times or less, and preferably 5 times or more and 40 times or less. Furthermore, if stretching in the second direction is also performed, the stretching in the second direction is preferably greater than the stretching in the first direction, for example, 10 times or more and 60 times or less, preferably 20 times or more and 50 times or less, and more preferably 30 times or more and 40 times or less. In addition, the stretching area ratio, which is the total area ratio of the resin sheet through stretching in the first direction and stretching in the second direction, is preferably 50 times or more and 1500 times or less, more preferably 100 times or more and 1300 times or less, and even more preferably 250 times or more and 1200 times or less. The higher the stretching area ratio, the smaller the packing rate of the resulting stretched porous film tends to be, and therefore, if stretching unevenness occurs, it tends to be relatively more noticeable. However, even in that case, it is possible to suppress the stretching unevenness by stretching after creating the uneven parts. Note that the multiples referred to here indicate the state after extension relative to the original length. For example, if something with a length of 1 is extended to a length of 3, the multiple is 3.

[0102] Furthermore, it is preferable that the resin sheet be stretched while heated, and the stretching temperature may be changed when stretching in the first direction and when stretching in the second direction. Also, if the resin sheet contains a heat-meltable component that does not form fibers and a non-heat-meltable component that does not form fibers, it is preferable to stretch it at a temperature above the melting point of the non-heat-meltable component and below the decomposition temperature of the non-heat-meltable component. The stretching temperature at this time may be set by the temperature of the furnace in which stretching is performed, or by the temperature of the heating rollers that convey the rolled material, or it may be achieved by a combination of these settings.

[0103] The temperature during stretching in the first direction can be, for example, 200°C to 300°C, and preferably 230°C to 270°C. Furthermore, if stretching in the second direction is also performed, the temperature during stretching in the second direction can be 200°C to 310°C, and preferably 230°C to 270°C.

[0104] Here, when the resin sheet is composed of fluororesin, it is known that the temperature, stretching ratio, and stretching speed (including stretching length speed and stretching ratio speed) during stretching affect the physical properties of the stretched resin sheet obtained by the above-mentioned rolling process (also called unfired fluororesin). The SS curve (a graph showing the relationship between tensile tension and elongation) of unfired fluororesin exhibits unique characteristics that differ from other resins. Normally, the tensile tension of a resin material increases with elongation. While the range of the elastic region and the fracture point differ depending on the material and evaluation conditions, it is very common for tensile tension to tend to increase with the amount of elongation. In contrast, the tensile tension of unfired fluororesin shows a peak at a certain amount of elongation, and then shows a gradual decreasing trend. This indicates that there is a region in unfired fluororesin where the unstretched part is stronger than the stretched part.

[0105] To explain the behavior that occurs during stretching based on the general properties of resins described above, in the case of general resins, when stretching, the weakest part within the stretched surface begins to stretch, but since the stretched part becomes stronger than the unstretched part, the next weakest unstretched part is stretched, and the stretched area expands, resulting in a tendency for the entire material to be stretched. On the other hand, to explain the behavior that occurs during stretching based on the properties of unfired fluororesin, in the case of unfired fluororesin, when the part that begins to stretch reaches the "region where the unstretched part becomes stronger than the stretched part," the already stretched part is stretched further, and as a result, the unstretched part tends to remain as a node (knot, unstretched part). When the stretching speed is slow, this phenomenon becomes more pronounced, and the nodes (knots, unstretched parts) tend to remain large. Also, when the stretching speed is fast, the fibrils (fibers) tend to stretch in a way that makes the nodes (knots, unstretched parts) smaller.

[0106] Therefore, when the resin sheet is composed of fluororesin, it is preferable to increase the stretching length rate (m / min) in a predetermined stretching direction while stretching the resin sheet in that predetermined stretching direction. This allows the stretching to be performed first at a relatively slow stretching length rate (m / min), which primarily stretches the fibrils (fibers) while leaving the nodes (knots, unstretched parts) relatively large. Then, by performing the stretching at a relatively fast stretching length rate (m / min) afterward, the fibrils (fibers) begin to grow from the large nodes (knots, unstretched parts), which suppresses the breaking of the fibrils (fibers) while stretching is performed. This also makes it easier to maintain a certain size of the nodes (knots, unstretched parts) even when the stretching is complete, as the nodes (knots, unstretched parts) do not become too small. This suppresses the problem of "being unable to secure a large thickness of the porous membrane after stretching due to the reduction in the size of the nodes (knots, unstretched areas)," making it possible to increase the thickness of the porous membrane after stretching and reduce the density of the porous membrane.

[0107] In contrast to the above, if stretching is performed first at a relatively slow stretching rate (m / min), followed by further stretching at a similarly slow rate (m / min), the situation where the fibrils (fibers) primarily stretch continues, which tends to lead to breakage.

[0108] Furthermore, compared to the above, if stretching is performed first at a relatively fast stretching length rate (m / min), followed by further stretching at a relatively fast stretching length rate (m / min), a large number of relatively small nodes (knots, unstretched areas) are generated in the initial stage. Subsequently, each node (knot, unstretched area) becomes even smaller, resulting in an insufficient thickness of the resulting porous film, which tends to result in a thin film and a limited dust retention capacity.

[0109] Furthermore, compared to the above, if stretching is performed first at a relatively fast stretching rate (m / min) and then at a relatively slow stretching rate (m / min), a large number of relatively small nodes (knots, unstretched areas) are formed first. If further stretching is attempted from this state, it becomes difficult to leave nodes (knots, unstretched areas) of a certain size, and the thickness of the porous membrane tends to decrease.

[0110] Therefore, it is preferable to increase the stretching length rate (m / min) in a predetermined stretching direction while stretching the resin sheet in that predetermined stretching direction. Here, the timing for increasing the stretching length rate (m / min) in a predetermined stretching direction may be, for example, during stretching in the first direction, or if stretching in a second direction is performed, during stretching in the second direction, or if stretching in a second direction is performed, the stretching length rate (m / min) may be increased both during stretching in the first direction and during stretching in the second direction. In particular, if stretching in a second direction results in two stages of stretching, it is preferable to increase the stretching length rate (m / min) during stretching in the second direction. This allows for increased stretching in the second direction by increasing the stretching length rate (m / min). This enables the extension of fibrils (fibers) from nodes (knots, unstretched areas) already formed by stretching in the first direction, suppressing the breakage of fibrils (fibers) formed by stretching in the first direction, while simultaneously increasing the area ratio. It is preferable that the porous membrane obtained after stretching in the second direction retains nodes (knots, unstretched areas) whose average size is larger than a circle with a diameter of 1 μm. The presence or absence of nodes (knots, unstretched areas) whose average size is larger than a circle with a diameter of 1 μm can be confirmed by SEM imaging of the fluororesin porous membrane. For example, this can be determined by comparing the average area of ​​multiple nodes (knots, unstretched areas) present within an arbitrary 15 μm × 15 μm region of the fluororesin porous membrane with the area of ​​a circle with a diameter of 1 μm. Furthermore, it is preferable that in a porous fluororesin film, more than half of the multiple nodes (knots, unstretched areas) present within any 15 μm × 15 μm region are larger than a circle with a diameter of 1 μm.

[0111] Furthermore, it is sufficient for the stretching length rate (m / min) to increase while stretching in a predetermined stretching direction. For example, the stretching length rate (m / min) in the second direction may be higher than the stretching length rate (m / min) in the first direction, or it may not be higher. In particular, when stretching in the second direction results in two stages of stretching, it is preferable that the stretching in the first direction be performed at a relatively slow stretching length rate (m / min) from the viewpoint of ensuring that nodes (knots, unstretched portions) of sufficient size remain when stretching in the second direction is performed. When increasing the stretching length rate (m / min) in the second direction, it is preferable that the stretching speed in the first direction be performed within a range of stretching length rates (m / min) that is slower than the faster stretching length rate (m / min) increased in the second direction, that is, within a range of stretching length rates (m / min) that is slower than the maximum stretching length rate (m / min) in the second direction.

[0112] Furthermore, since intermittently applying tension by stopping and then restarting the stretching process may cause breakage, it is preferable that the stretching length rate (m / min) of the resin sheet be continuously changed during the stretching process.

[0113] Furthermore, as mentioned above, after increasing the stretching length rate (m / min), it is preferable to gradually decrease the stretching length rate (m / min) while stretching, in order to relax the tension in the fibrils (fibers) and reduce residual stress, before ending the stretching process.

[0114] In particular, when a resin sheet wound into a roll is first stretched in the longitudinal direction and then stretched in the width direction, increasing the stretching length rate (m / min) during the width direction stretching is preferable for the productivity of porous films.

[0115] Furthermore, in the stretching described above, if the stretching length rate (m / min) increases, the stretching rate percentage (% / min) may or may not increase. Note that the stretching length rate (m / min) is the rate relating to the absolute value of the change in length in the stretching direction per unit time, and the stretching rate percentage (% / min) is the rate relating to the rate of change in length in the stretching direction per unit time.

[0116] Furthermore, forming irregularities on the resin sheet before stretching makes it possible to improve the uniformity of the stretched porous film obtained by stretching. When the multiple irregularities formed on the resin sheet are stretched, the areas where the resin is dense and areas where the resin is loose are regularly arranged due to the formation of the irregularities. This suppresses excessive stretching in areas that are easily stretched, resulting in more uniform stretching. The stretched pattern may be, for example, a repeating rhombus shape derived from the shape of the irregularities before stretching, a repeating rectangular pattern, a repeating circular pattern, or a repeating elliptical pattern.

[0117] The porous film thus obtained is preferably heat-set to obtain mechanical strength and dimensional stability. The heat-set temperature is preferably 250 to 420°C, and more preferably 350 to 400°C.

[0118] For example, a resin sheet with the above-mentioned uneven surface formed and wound into a roll can be stretched using, for example, the apparatus shown in Figure 1 and the apparatus shown in Figures 2 and 3.

[0119] In the apparatus shown in Figure 1, the resin sheet 40, which is wound into a roll, is set as a roll 41, and the longitudinally stretched sheet, which is stretched in the longitudinal direction (vertical direction), is wound on the winding roll 42. Note that 43-45 are rolls, 46 and 47 are heat rolls, and 48-52 are rolls.

[0120] Next, the longitudinally stretched sheet is stretched in the width direction by the apparatus shown in Figures 2 and 3. Specifically, the wound longitudinally stretched sheet is set on a roll 61 and stretched in the width direction by a tenter 65 as it is sequentially fed out. In the tenter 65, both ends of the stretched sheet in the width direction are held by opposing continuous clips (not shown), and the sheet is stretched in the width direction as the opposing continuous clips separate from each other as the sheet is fed out. The continuous clips are configured to be fed from the upstream side to the downstream side along a line that can be adjusted to a specific shape. The tenter 65 can preheat the stretched sheet in the upstream preheating region 62, stretch the sheet in the width direction while it is heated to a predetermined temperature in the intermediate stretching region 63, and heat-set the stretched sheet in the downstream heat-setting region 64. The stretched porous membrane, after heat-setting, is layered with a breathable support membrane 33 (described later) as needed, and heat-laminated by a heated roll 66 to become an air filter material 30, which is then wound onto a roll 67.

[0121] Here, for example, by changing the shape of the line that feeds the continuous clips described above, the stretching speed in the width direction (including the stretching length speed and the stretching rate speed) can be adjusted even if the supply speed of the stretched sheet fed from the roll 61 is maintained at a predetermined speed. For example, as shown in the plan view of Figure 4 which shows the stretching in the width direction, the shape of the line that guides the continuous clips can be changed to shapes (i), (ii), and (iii). Here, in Figure 4, as indicated by the large arrow, the stretched sheet is fed from left to right at a predetermined speed. Also, in Figure 4, as indicated by the small arrow, the stretched sheet is stretched in the vertical direction, which is the width direction. Here, for example, if the shape of the lines arranged opposite each other is a straight shape as shown in (i), then if the speed at which the stretched sheet is fed is constant, the stretching length speed in the width direction (m / min) will also be constant. In contrast, if the shape of the opposing lines is as shown in (ii), where they are positioned close together upstream of the stretching line and far apart downstream of the stretching line, then, assuming a constant feed rate for the stretched sheet, the stretching length rate in the width direction (m / min) will be relatively slow in the initial stage, relatively fast in the intermediate stage, and slow again in the final stage. Also, if the shape of the opposing lines is as shown in (iii), where they are positioned far apart upstream of the stretching line, then, assuming a constant feed rate for the stretched sheet, the stretching length rate in the width direction (m / min) will be relatively fast in the initial stage and relatively slow thereafter.

[0122] Furthermore, in either of the above cases, increasing the speed at which the stretched sheet is fed will increase the overall stretching speed in the width direction, and decreasing the speed at which the stretched sheet is fed will decrease the overall stretching speed in the width direction.

[0123] Furthermore, for example, the resin sheet 40 on which the above-mentioned uneven portion 40a is formed may be stretched by a stretching device 70 capable of stretching in a first direction and in a second direction, as shown in Figure 5. This stretching device 70 may perform stretching in the first direction and stretching in the second direction simultaneously, or it may perform stretching in the first direction first and then stretching in the second direction, or it may perform stretching in the first direction first and then stretching in the first direction and stretching in the second direction simultaneously. The stretching device 70 has a plurality of clips 71 each having a gripping portion 71a for gripping the resin sheet 40, and the resin sheet 40 is stretched by the plurality of gripping portions 71a moving away from each other. Here, the gripping portions 71a support the edge or vicinity of the edge of the resin sheet 40 by clamping it in the thickness direction of the resin sheet 40.

[0124] (2) Stretched porous membrane The stretched porous membrane may have a stretched pattern in which a predetermined shape is regularly repeated.

[0125] It is preferable that the stretched porous membrane is manufactured by the stretched porous membrane manufacturing method described above.

[0126] In the case of a stretched porous membrane having a stretched pattern in which a predetermined shape is regularly repeated, it is preferable that the stretched pattern is a pattern that arises from the stretching of multiple uneven portions formed on the resin sheet. In this respect, it is distinguished from the irregular stretched pattern that a porous membrane obtained by stretching without the formation of uneven portions may have. The stretched pattern may be visible as a pattern due to the difference between areas where the fibers generated by stretching are densely arranged and areas where the fibers generated by stretching are sparse. The stretched pattern may be, for example, a repeating rhombus shape, a repeating rectangular shape, or a repeating elliptical shape.

[0127] The stretched porous membrane is preferably one that contains the fluororesin described in the above-mentioned method for producing the stretched porous membrane.

[0128] When the stretched porous membrane contains fluororesin, it is preferable that the stretched porous membrane has a porous membrane structure having fibrils (fibers) and nodes (knots, unstretched parts) connected to the fibrils. This stretched porous membrane exhibits excellent homogeneity of pressure loss, as evaluated by passing airflow through multiple locations.

[0129] The unevenness of the stretched porous membrane produced can be evaluated by the homogeneity of the pressure loss of the stretched porous membrane. The homogeneity of the pressure loss can be evaluated, for example, by the coefficient of variation CV of the pressure loss, which is obtained by dividing the standard deviation of the pressure loss distribution by the mean value. The coefficient of variation CV of the pressure loss of this stretched porous membrane is preferably 25.0% or less, more preferably 20.0% or less, even more preferably 15.0% or less, and most preferably 10.0% or less. This makes it possible to avoid a usage condition in which parts with low pressure loss are used intensively (dust tends to accumulate in parts with low pressure loss). The lower limit of the coefficient of variation CV of the pressure loss as an air filter material is not particularly limited, but is, for example, 1%.

[0130] The pressure loss distribution of a stretched porous membrane can be obtained, for example, by dividing the stretched porous membrane into a grid and measuring the pressure loss in a 100 mm diameter region within each grid. Pressure loss can be measured, for example, using a measuring device equipped with a manometer that measures both sides of the stretched porous membrane while in close proximity to its surface, by operating the manometer to move along a predetermined path on the downstream surface of each region. The standard deviation can then be calculated from the pressure loss distribution consisting of the measured pressure losses of each region, and the coefficient of variation CV (%) can be obtained by dividing this by the average value of the pressure losses of all measured regions. The size of the stretched porous membrane is not particularly limited, but for example, the longitudinal length is 100 to 1000 m and the width is 600 to 2000 mm.

[0131] When the stretched porous membrane is a stretched porous membrane of fluororesin, the film thickness can be, for example, 1.0 μm or more, preferably 10.0 μm or more, and more preferably 50.0 μm or more. Increasing the film thickness of the stretched porous membrane of fluororesin makes it possible to increase the amount of dust retention. The film thickness of the fluororesin porous membrane is, for example, 200 μm or less. When the film thickness is thin, the homogeneity of pressure loss at multiple points in the porous membrane tends to be more problematic, but even in that case, the porous membrane produced as described above has excellent homogeneity of pressure loss. The thickness of the stretched porous membrane of fluororesin can be determined, for example, by using a film thickness gauge (1D-110MH model, manufactured by Mitutoyo Corporation), stacking five of the object to be measured, measuring the total film thickness, and dividing that value by 5 to obtain the film thickness of one sheet.

[0132] When the stretched porous membrane is a stretched porous membrane of fluororesin, the particle collection efficiency when air containing NaCl particles with a particle size of 0.1 μm is passed through at a flow rate of 5.3 cm / second may be 75.0% or more, preferably 80.0% or more, more preferably 90.0% or more, and even more preferably 99.0% or more.

[0133] When the stretched porous membrane is a stretched porous membrane made of fluororesin, the upper limit of the pressure loss when air is passed through at a flow velocity of 5.3 cm / second is preferably 250 Pa, more preferably 200 Pa, even more preferably 150 Pa, even more preferably 100 Pa, and most preferably 50 Pa. The lower limit of this pressure loss is not particularly limited, but for example, it is 10 Pa. The above pressure loss of a stretched porous membrane obtained by stretching a resin sheet with an uneven surface is preferably 90% or less, more preferably 95% or less, and even more preferably 100% or less, of the above pressure loss of a stretched porous membrane obtained by stretching a resin sheet without forming an uneven surface.

[0134] When the stretched porous membrane is a stretched porous membrane made of fluororesin, the PF value calculated using the collection efficiency with NaCl particles having a particle size of 0.1 μm is preferably 20 or higher, more preferably 30 or higher, and even more preferably 35 or higher. The PF value of a stretched porous membrane obtained by stretching a resin sheet with uneven surfaces is preferably 90% or higher, more preferably 95% or higher, and even more preferably 100% or higher, of the PF value of a stretched porous membrane obtained by stretching a resin sheet without forming uneven surfaces. This PF value is calculated using the collection efficiency and pressure loss from the following relational expression. PF value = {-log((100 - collection efficiency (%)) / 100)} / (pressure loss (Pa) / 1000)

[0135] (3) Air filter media The air filter media comprises a stretched porous membrane. The air filter media may also consist of multiple stretched porous membranes, some having the same physical properties and others having different physical properties, stacked on top of each other.

[0136] The air filter media may also include a permeable support membrane to support the stretched porous membrane. An air filter media with a permeable support membrane may, for example, be configured as shown in Figure 6, air filter media 30, in which the stretched porous membrane 31 and the permeable support membrane 33 are laminated in the direction of airflow. The permeable support membrane 33 may be provided on the leeward side of the stretched porous membrane 31, on the leeward side, or on both the leeward and leeward sides. Even if the stretched porous membrane is thin and difficult to stand on its own, the permeable support membrane allows the stretched porous membrane to stand upright. Furthermore, this ensures the strength of the air filter media and makes it easier to handle.

[0137] The material and structure of the breathable support membrane are not particularly limited, but examples include nonwoven fabrics, woven fabrics, metal meshes, and resin nets. Nonwoven fabrics with heat-sealing properties are preferred in terms of strength, collection ability, flexibility, and workability. The material of the nonwoven fabric is not particularly limited, and polyolefins (PE, PP, etc.), polyamides, polyesters (PET, etc.), aromatic polyamides, or composites thereof can be used. Preferred nonwoven fabrics are those in which some or all of the constituent fibers have a core / sheath structure, two-layer nonwoven fabrics consisting of a layer of fibers made of a low-melting-point material and a layer of fibers made of a high-melting-point material, and nonwoven fabrics with a heat-sealing resin coated on the surface. The thickness of the breathable support membrane is preferably, for example, 0.3 mm or less, and more preferably 0.25 mm or less. The basis weight of the breathable support membrane is, for example, 20 g / m². 2 More than 50g / m 2 The following is preferable:

[0138] Furthermore, the method of overlapping these films or layers is not particularly limited. They may be bonded by partial melting due to heating or by utilizing the anchoring effect of melting hot-melt resin, or they may be bonded using reactive adhesives, or they may simply be placed on top of each other.

[0139] Furthermore, for any of the above air filter media, the coefficient of variation CV of the pressure loss as an air filter media (the coefficient of variation CV obtained by dividing the standard deviation of the pressure loss distribution of the air filter media by the mean value) is preferably 25.0% or less, more preferably 20.0% or less, even more preferably 15.0% or less, and most preferably 10.0% or less, similar to the coefficient of variation CV of the stretched porous membrane described above. The lower limit of the coefficient of variation CV of the pressure loss as an air filter media is not particularly limited, but is, for example, 1%.

[0140] Furthermore, regarding the air filter media, the particle collection efficiency when air containing NaCl particles with a particle size of 0.1 μm is passed through at a flow rate of 5.3 cm / second may be 75.0% or higher, preferably 80.0% or higher, more preferably 90.0% or higher, and even more preferably 99.0% or higher.

[0141] Furthermore, the upper limit of the pressure loss of the air filter media when air is passed through at a flow velocity of 5.3 cm / second is not particularly limited, but is preferably 300 Pa, and more preferably 200 Pa. The lower limit of the pressure loss of the air filter media is not particularly limited, but is, for example, 20 Pa. The above pressure loss of the air filter media having a stretched porous membrane obtained by stretching a resin sheet with an uneven surface is preferably 110% or less, more preferably 105% or less, and even more preferably 100% or less, of the above pressure loss of the air filter media having a stretched porous membrane obtained by stretching a resin sheet without forming an uneven surface.

[0142] Furthermore, regarding the air filter media, the PF value calculated from the collection efficiency and pressure loss using NaCl particles with a particle size of 0.1 μm is preferably 20 or higher, more preferably 30 or higher, and even more preferably 35 or higher. The PF value of the air filter media equipped with a stretched porous membrane obtained by stretching a resin sheet with an uneven surface is preferably 90% or higher, more preferably 95% or higher, and even more preferably 100% or higher, of the air filter media equipped with a stretched porous membrane obtained by stretching a resin sheet without forming an uneven surface.

[0143] Furthermore, while the thickness of the air filter media is not particularly limited, it is preferably, for example, 100 μm or more and 1000 μm or less, and preferably 300 μm or more and 800 μm or less.

[0144] (4) Filter pack Next, the filter pack of this embodiment will be described with reference to Figure 7.

[0145] Figure 7 is an external perspective view of the filter pack 20 of this embodiment.

[0146] The filter pack 20 is equipped with the air filter material described above (for example, the air filter material 30). The air filter material of the filter pack 20 is a processed filter material that has been processed into a zigzag shape (pleated) by alternating mountain folds and valley folds. The pleating can be performed, for example, by a rotary folding machine. The folding width of the filter material is not particularly limited, but is for example between 25 mm and 280 mm. Because the filter pack 20 is pleated, the folding area of ​​the filter material when used in an air filter unit can be increased, thereby making it possible to obtain an air filter unit with high collection efficiency.

[0147] The filter pack 20 may further include spacers (not shown) for maintaining the pleat spacing when used in an air filter unit, in addition to the filter material. The material of the spacers is not particularly limited, but hot melt resin can be preferably used.

[0148] Furthermore, the air filter material 30, which comprises a stretched porous membrane 31 and a breathable support membrane 33, may have multiple embossed protrusions, and the pleat spacing may be maintained by the contact of the embossed protrusions located at points facing each other when the material is pleated.

[0149] (5) Air filter unit Next, the air filter unit 1 will be described with reference to Figure 8.

[0150] Figure 8 is an external perspective view of the air filter unit 1 of this embodiment.

[0151] The air filter unit 1 comprises the air filter media or filter pack described above, and a frame 25 that holds the air filter media or filter pack. In other words, the air filter unit may be manufactured so that the filter media, which is not folded in a mountain or valley pattern, is held in the frame, or so that the filter pack 20 is held in the frame 25. The air filter unit 1 shown in Figure 8 is manufactured using the filter pack 20 and the frame 25.

[0152] The frame 25 is made, for example, by combining sheet metal or by molding resin, and the space between the filter pack 20 and the frame 25 is preferably sealed with a sealant. The sealant is for preventing leakage between the filter pack 20 and the frame 25, and for example, a resin such as epoxy, acrylic, or urethane is used.

[0153] The air filter unit 1, comprising a filter pack 20 and a frame 25, may be a mini-pleated type air filter unit in which one flat, extending filter pack 20 is housed inside the frame 25, or it may be a V-bank type air filter unit or a single-header type air filter unit in which multiple flat, extending filter packs are arranged and held in the frame.

[0154] (6) Examples of uses Air filter media, filter packs, and air filter units equipped with a stretched porous membrane manufactured by the stretched porous membrane manufacturing method according to this embodiment can be used for the following applications, for example.

[0155] Our product range includes ULPA filters (Ultra Low Penetration Air Filters) (for semiconductor manufacturing), HEPA filters (for hospitals and semiconductor manufacturing), cylindrical cartridge filters (for industrial use), bag filters (for industrial use), heat-resistant bag filters (for exhaust gas treatment), heat-resistant pleated filters (for exhaust gas treatment), SINBRAN® filters (for industrial use), catalytic filters (for exhaust gas treatment), adsorbent filters (for HDD integration), adsorbent vent filters (for HDD integration), vent filters (for HDD integration, etc.), vacuum cleaner filters (for vacuum cleaners), general-purpose multi-layer felt materials, gas turbine cartridge filters (for compatible gas turbines), cooling filters (for electronic equipment housings), and more.

[0156] Freeze-drying materials such as freeze-drying containers, automotive ventilation materials for electronic circuits and lamps, container applications such as container caps, protective ventilation applications for electronic equipment, and ventilation / internal pressure regulation applications such as medical ventilation. [Examples]

[0157] The contents of this disclosure will be specifically explained below with reference to examples and comparative examples.

[0158] (Example 1) As raw materials, a mixed powder was used, obtained by co-coagulating and mixing fine powder of perfluoroalkyl vinyl ether-modified polytetrafluoroethylene, which is a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether, and fine powder of polytetrafluoroethylene with an average molecular weight of 6.5 million (manufactured by Daikin Industries, Ltd., product name: F106) in a weight ratio of 75:25. For every 1 kg of the above raw materials, 290 g of hydrocarbon oil ("IP Solvent 2028" manufactured by Idemitsu Kosan Co., Ltd.) was added as an extruded liquid lubricant at 20°C and mixed.

[0159] Next, the obtained mixture was extruded using a paste extruder to obtain a sheet-like extruder. This sheet-like extruder was then molded using a calender roll heated to 70°C to obtain another sheet-like material. This sheet-like material was passed through a hot air drying oven at 250°C to evaporate and remove the hydrocarbon oil, obtaining an unfired, strip-shaped resin sheet with an average thickness of 300 μm and an average width of 150 mm.

[0160] The resulting resin sheet was passed between a mold roll and a rubber roll, both of which had a pattern of regularly arranged protrusions corresponding to the shape of the uneven areas. This process created multiple uneven areas on the resin sheet. The shape of each uneven area was circular. The size of each uneven area was 7 mm. 2 The average spacing between the uneven surfaces was 6 mm. Furthermore, the number of uneven surfaces on the resin sheet was 4 per 100 mm. 2 That was the case.

[0161] Next, the unfired resin sheet with the uneven surface formed was cut into 100 mm x 100 mm pieces and stretched using a stretching apparatus as shown in Figure 5. First, the sheet was stretched in a first direction, increasing its length from 70 mm to 500 mm. Then, it was stretched in a second direction, increasing its width from 70 mm to 500 mm. The stretching speed was 10 mm / s (14% / s) for both the first and second directions. The stretching temperature, which is the ambient temperature of the resin sheet during stretching, was 250°C for both the first and second directions. This yielded the stretched porous film of Example 1.

[0162] (Comparative Example 1) A stretched porous film of Comparative Example 1 was obtained in the same manner as in Example 1, except that the resin sheet was stretched without creating any uneven surfaces.

[0163] (Example 2) As raw materials, 66.5% by weight (polymer equivalent) of a PTFE aqueous dispersion (PTFE-A) with an SSG of 2.160, 28.5% by weight (polymer equivalent) of a low molecular weight PTFE aqueous dispersion (PTFE-B) with a melt viscosity of 20,000 Pa·s measured using a flow tester method at 380°C, and 5% by weight (polymer equivalent) of an FEP aqueous dispersion with a melting point of 215°C were mixed. Co-coagulation was performed by adding 500 ml of a 1% aluminum nitrate aqueous solution as a coagulant and stirring. After draining the water from the resulting powder using a sieve, it was further dried in a hot air drying oven at 135°C for 18 hours to obtain the mixed powder of the three components described above. Per 1 kg of the above raw materials, 280 g of hydrocarbon oil ("IP Solvent 2028" manufactured by Idemitsu Kosan Co., Ltd.) was added at 20°C and mixed as an extrusion liquid lubricant. Otherwise, a resin sheet was obtained in the same manner as in Example 1.

[0164] The shape of the uneven surface in Example 2 was circular. The size of each uneven surface was 13 mm. 2 The average spacing between the uneven surfaces was 7 mm. Furthermore, the number of uneven surfaces on the resin sheet was 5 per 100 mm. 2 That was the case.

[0165] The stretching of the unfired resin sheet with the formed uneven surface was carried out in the same manner as in Example 1, except that the stretching speed in both the first and second directions was set to 30 mm / s (43% / s), to obtain the stretched porous film of Example 2.

[0166] (Comparative Example 2) A stretched porous film of Comparative Example 2 was obtained in the same manner as in Example 2, except that the resin sheet was stretched without creating any uneven surfaces.

[0167] (Example 3) Except for differences in the shape and pattern of the uneven parts compared to Example 1, the stretched porous film of Example 3 was obtained in the same manner as in Example 1.

[0168] The shape of the uneven surface in Example 3 was rhomboid. The size of each uneven surface was 3 mm. 2The average spacing between the uneven surfaces was 4 mm. Furthermore, the number of uneven surfaces on the resin sheet was 13 per 100 mm. 2 That was the case.

[0169] (Comparative Example 3) A stretched porous film of Comparative Example 3 was obtained in the same manner as in Example 3, except that the resin sheet was stretched without creating any uneven surfaces.

[0170] The following physical properties were measured for each of the stretched porous films obtained in the examples and comparative examples.

[0171] (Coefficient of variation (Cv value) of pressure loss in stretched porous membranes) Pressure loss was measured at each of the six positions (positions 1 to 6) arranged at equal intervals from near one end to near the other end in the second direction, near the center in the first direction. A sample for measurement was obtained by attaching a permeable support material with almost zero pressure loss to the outer circumference of a stretched porous membrane using double-sided tape. For pressure loss measurement, each position of the sample was set in a cylindrical filter holder with a diameter of 100 mm, the inlet side was pressurized with a compressor, and the airflow was adjusted so that the air passage velocity was 5.3 cm / second. Pressure was measured using a manometer on the upstream and downstream sides of the test sample, and the pressure difference between the upstream and downstream sides was determined as the pressure loss. The standard deviation was then calculated from the pressure loss distribution consisting of these pressure losses, and the coefficient of variation (%) was obtained by dividing the calculated standard deviation by the average value of the pressure loss at all measured locations.

[0172] The physical properties of the stretched porous films for each example and comparative example are shown in Table 1 below. [Table 1]

[0173] As is clear from the results in Table 1, in Example 1, where the resin sheet was stretched after forming an uneven surface, the coefficient of variation of pressure loss was kept small compared to Comparative Example 1, where the resin sheet was stretched without forming an uneven surface, confirming that stretching unevenness was reduced. Similarly, in Example 2 and Comparative Example 2, where the stretching speed was varied, the stretched porous membrane obtained by stretching the resin sheet with an uneven surface also showed a small coefficient of variation of pressure loss and reduced stretching unevenness. Furthermore, in Example 3 and Comparative Example 3, where the shape of the uneven surface was changed, the stretched porous membrane obtained by stretching the resin sheet with an uneven surface also showed a small coefficient of variation of pressure loss and reduced stretching unevenness. In particular, although the resin sheet was stretched while being held near its edges, in Examples 1, 2, and 3, the difference between the pressure loss near the edges of the stretched porous membrane and the pressure loss near the center of the stretched porous membrane was kept small, indicating that uniform stretching was performed. In Examples 1-3, since a mixture of homo-PTFE, modified PTFE, and low-molecular-weight PTFE was used as the fluororesin, it was possible to minimize stretching unevenness, even though this mixture tended to be more prone to uneven stretching compared to using homo-PTFE alone. Furthermore, as shown in Table 1, it was confirmed that, regardless of the stretching speed or the specific shape of the unevenness, forming the unevenness of the surface before stretching the resin sheet reduces the stretching unevenness of the resulting stretched porous film.

[0174] In Examples 1, 2, and 3, a regular stretch pattern derived from the pattern of unevenness formed on the resin sheet was observed, while in Comparative Examples 1, 2, and 3, an irregular stretch pattern was observed.

[0175] (Example 4) A resin sheet similar to that in Example 1 was prepared, and multiple protrusions corresponding to the shape of the uneven areas were formed on the resin sheet by passing it between a mold roll and a rubber roll, which had an uneven pattern formed on the resin sheet with multiple protrusions arranged regularly to match the shape of the uneven areas. The shape of the uneven areas was rhombic. The size of each uneven area was 7 mm. 2The average spacing between the uneven surfaces was 6 mm. Furthermore, the number of uneven surfaces on the resin sheet was 4 per 100 mm. 2 That was the case.

[0176] Next, an unbaked resin sheet with an uneven surface was cut to 100 mm x 100 mm and stretched using a stretching device as shown in Figure 5. First, the sheet was stretched in a first direction, increasing its length from 70 mm to 500 mm. Then, it was stretched in a second direction, increasing its width from 70 mm to 500 mm. The stretching speed was 10 mm / s for both the first and second directions. The stretching temperature, which is the ambient temperature of the resin sheet during stretching, was 250°C for both the first and second directions.

[0177] The stretched resin sheet was cut and stretched using a stretching apparatus as shown in Figure 5. First, it was stretched in the first direction, extending the vertical width from 175 mm to 500 mm. Next, it was stretched in the second direction, extending the horizontal width from 70 mm to 500 mm. The stretching speed at this time was 100 mm / s for both the first and second directions (140% / s for the second direction). The stretching temperature, which is the ambient temperature of the resin sheet during stretching, was 250°C for both the first and second directions. In Example 4, the stretching area ratio from the resin sheet obtained by the above stretching was 1041 times. This obtained the stretched porous film of Example 4.

[0178] (Comparative Example 4) A stretched porous film of Comparative Example 4 was obtained in the same manner as in Example 4, except that it was stretched without forming any uneven surfaces.

[0179] The following physical properties were measured for each of the stretched porous films obtained in the examples and comparative examples.

[0180] (Pressure loss in stretched porous membranes) A stretched porous membrane is subjected to a 100 cm gap in a breathable support material with almost zero pressure loss. 2Test samples were obtained by attaching them to areas outside the circular effective area using double-sided tape. The test samples were set in a cylindrical holder, the inlet side was pressurized with a compressor, and the airflow was adjusted so that the air passage velocity was 5.3 cm / second. The pressure was measured using a manometer on the upstream and downstream sides of the test sample, and the pressure difference between the upstream and downstream sides was determined as the pressure loss.

[0181] (PF value of stretched porous membrane) The PF value of the stretched porous membrane was calculated by substituting the above-mentioned pressure loss and the collection efficiency described below into the following relational equation. PF value = {-log((100 - collection efficiency (%)) / 100)} / (pressure loss (Pa) / 1000)

[0182] The collection efficiency of a stretched porous membrane using NaCl particles with a particle size of 0.1 μm was determined according to the method described in JIS B9928 Annex 5 (Normative) Method for generating NaCl aerosols (pressurized spray method). NaCl particles generated in an atomizer were classified to a particle size of 0.1 μm using an electrostatic classifier (TSI Corporation), and the particle charge was neutralized using americium-241. The permeation flow rate was then adjusted to 5.3 cm / second, and the number of particles before and after the stretched porous membrane, which was the measurement sample, was determined using a particle counter (TSI Corporation, CNC). The collection efficiency was then calculated using the following formula. Collection efficiency (%) = (CO / CI) × 100 CO = Number of 0.1 μm NaCl particles collected in the sample being measured. CI = Number of 0.1 μm NaCl particles supplied to the sample.

[0183] The physical properties of the stretched porous films for each example and comparative example are shown in Table 2 below. [Table 2]

[0184] As is clear from the results in Table 2, in Example 4, where the resin sheet was stretched after forming an uneven surface, the pressure loss did not increase significantly compared to Comparative Example 4, where the resin sheet was stretched without forming an uneven surface. It was confirmed that the pressure loss was kept below the value obtained by adding 10% to the pressure loss when stretching was performed without forming an uneven surface. Furthermore, it was confirmed that the PF value (0.1 μm particles) in Example 4, where the uneven surface was formed and then stretched, did not deteriorate significantly compared to Comparative Example 4, where the uneven surface was not formed and then stretched.

[0185] From the above, it was confirmed that by forming uneven surfaces on a resin sheet and stretching it to obtain a stretched porous film, it is possible to reduce stretching unevenness by keeping the coefficient of variation Cv of pressure loss small, while avoiding a large decrease in pressure loss and PF value.

[0186] (Example 5) A resin sheet similar to that in Example 1 was prepared, and multiple protrusions were formed on the resin sheet by passing it between a mold roll and a rubber roll, which had a pattern of protrusions arranged regularly to correspond to the shape of the uneven areas. The shape of the uneven areas was circular. The size of each uneven area was 7 mm. 2 The average spacing between the uneven surfaces was 6 mm. Furthermore, the number of uneven surfaces on the resin sheet was 4 per 100 mm. 2 That was the case.

[0187] The resin sheet with the uneven surface formed as described above was stretched using the apparatus shown in Figures 1 to 3. For stretching, first, stretching in the first direction was performed at a stretching temperature of 280°C and a winding speed (the speed at which the sheet is wound on the winding roll 42 in Figure 1) of 22 m / min until a stretching ratio of 28.4 times was achieved. Next, stretching in the second direction was performed at a stretching temperature of 290°C and a widthwise stretching speed of 80% / s until a stretching ratio of 35.4 times was achieved. In the second direction, stretching was performed by moving a continuous clip along a line with the shape shown in Figure 9. The distance between opposing continuous clips at point X in Figure 9 was 48 mm, and the distance between opposing continuous clips at point Y in Figure 9 was 1700 mm. Stretching in the second direction was performed by moving the continuous clip at a speed of 10 m / min.

[0188] The stretched resin sheet was heat-set to obtain a stretched porous membrane. The heat-set temperature was 390°C (ambient temperature around the sheet). The stretched area of ​​this stretched porous membrane was 1005 times that of the resin sheet obtained by the above stretching process. A breathable support membrane was bonded to both sides of the obtained stretched porous membrane by heat fusion using a laminating device to obtain the air filter material of Example 5. Each of these breathable support membranes was made of spunbond nonwoven fabric (average fiber diameter 24 μm, basis weight 40 g / m²) consisting of fibers with a core / sheath structure using PET as the core and PE as the sheath. 2 A material with a thickness of 0.2 mm was used.

[0189] (Comparative Example 5) The air filter material for Comparative Example 5 was obtained in the same manner as in Example 5, except that it was stretched without forming any uneven surfaces.

[0190] For each of the examples and comparative examples obtained, the pressure loss, coefficient of variation Cv, and PF value (0.1 μm particles) were measured in the same manner as described above. The pressure loss and coefficient of variation Cv were determined by dividing the air filter material into 100 grid sections and measuring the pressure loss in a 100 mm diameter area within each grid section.

[0191] In the air filter media of Example 5, the pressure loss was 47 Pa, the coefficient of variation of pressure loss Cv was 11.7, and the PF value (0.1 μm particles) was 34.2.

[0192] In Comparative Example 5, the air filter media had a pressure drop of 41 Pa, a coefficient of variation (Cv) of pressure drop of 14.1, and a PF value (0.1 μm particles) of 34.5.

[0193] Based on the above, Example 5, in which the resin sheet was stretched after forming an uneven surface, was able to reduce the coefficient of variation of pressure loss Cv by 2.4 compared to Comparative Example 5, in which the resin sheet was stretched without forming an uneven surface. Moreover, the PF value (0.1 μm particles) did not deteriorate significantly, and it was confirmed that the PF value was almost the same as when the sheet was stretched without forming an uneven surface.

[0194] While embodiments of this disclosure have been described above, it should be understood that various modifications to the form and details are possible without departing from the spirit and scope of this disclosure as described in the claims. [Explanation of Symbols]

[0195] 1. Air filter unit 20 filter packs 25 Frame 30 Air filter media 31 Stretched porous membrane 33. Breathable support membrane 40 resin sheets [Prior art documents] [Patent Documents]

[0196] [Patent Document 1] Japanese Patent Publication No. 2003-181227

Claims

1. A first step is to provide the resin sheet with uneven surfaces, which are at least one of a protruding portion and a recessed portion, A second step involves stretching the sheet obtained in the first step to generate fibers, A method for producing a stretched porous membrane.

2. The stretched porous membrane is an air filter material. A method for producing a stretched porous membrane according to claim 1.

3. The aforementioned resin sheet contains a fluororesin. A method for producing a stretched porous film according to claim 1 or 2.

4. The area ratio of the uneven portion to the resin sheet in the first step is 10% or more and 80% or less. A method for producing a stretched porous film according to claim 1 or 2.

5. Multiple of the aforementioned protrusions and recesses are provided, In the view of the resin sheet in the thickness direction, The average area of ​​each of the aforementioned uneven parts is 2 mm ２ 15mm or more ２ The following: The average distance between the aforementioned uneven surfaces is between 2 mm and 9 mm. A method for producing a stretched porous film according to claim 1 or 2.

6. The number of protrusions on the resin sheet in the first step is 2 per 100 mm. ２ More than 20 pieces / 100mm ２ The following is: A method for producing a stretched porous membrane according to claim 5.

7. In the first step, the film thickness of the resin sheet in the portion other than the uneven portion is 250 μm or more and 650 μm or less. A method for producing a stretched porous membrane according to claim 5.

8. In the resin sheet in the first step, the uneven portion has a higher density than the surrounding area. A method for producing a stretched porous film according to claim 1 or 2.

9. The stretching area ratio in the second step is 50 times or more and 1500 times or less. A method for producing a stretched porous film according to claim 1 or 2.

10. The aforementioned uneven portions are arranged regularly. A method for producing a stretched porous film according to claim 1 or 2.

11. Having an elongated pattern in which a predetermined shape is regularly repeated, Stretched porous membrane.

12. The stretched porous film contains a fluororesin. The stretched porous membrane according to claim 11.

13. The stretched porous membrane is an air filter material. The stretched porous membrane according to claim 11 or 12.

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