JP2014069115A - Filtration material for filter and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a filtration material for filter composed of a nanofiber layer and a porous layer, having a low filtration resistance and a high collection rate of particles, particularly the particles with a particle diameter of about 0.1-0.3 μm.SOLUTION: A filtration material for filter includes a nanofiber layer composed of fabrics with an average fiber diameter of 200 nm or less, and a porous layer with a Gurley value of 0.5-1.5 sec/100 cmand a tortuosity ratio of 1.0 or more.

Description

  The present invention relates to a filter medium and a method for producing the same, and more particularly to a filter medium comprising a nanofiber layer and a porous layer and a method for producing the same.

A filter for collecting (capturing) fine particles in the air is required to collect particles with high efficiency and to have a low filtration resistance.
Therefore, the use of nanofibers with very small pore diameters as a filter has been studied. However, nanofibers are very small in pore diameter, so clogging easily occurs, and a laminate of fine fibers with a diameter of 1 μm or less. Therefore, there is a problem that the mechanical strength is weak, so that it is not practical to use the nanofiber layer as a single layer.

Patent Documents 1 and 2 disclose filter media in combination with nanofibers and various porous materials, but these media have poor durability.
Patent Documents 3 to 7 disclose a filter medium using polytetrafluoroethylene (PTFE) porous material, but it is sufficient as a filter medium having both a small filtration resistance and a high particle capture rate. Not a thing.

JP 2009-57655 A JP 2011-89226 A JP 2002-370020 A JP 2006-61808 A JP 2007-75739 A JP 2006-326579 A JP 2005-205305 A

  The present invention has been made in view of the above situation, and provides a filter medium having a low filtration resistance and a high trapping rate of particles, particularly particles having a particle size of about 0.1 to 0.3 μm. It is aimed.

As a result of intensive studies, the present inventors have found that a nanofiber layer composed of fibers having an average fiber diameter of 200 nm or less, a Gurley value of 0.5 to 1.5 seconds / 100 cm ³ , and a curvature of 1.0. The filter medium having the porous layer as described above has a low filtration resistance and a high collection rate of particles, particularly particles having a particle size of about 0.1 to 0.3 μm, and the present invention is completed. I let you.

The present invention relates to the following [1] to [7], for example.
[1]
A filter having a nanofiber layer made of fibers having an average fiber diameter of 200 nm or less, and a porous layer having a Gurley value of 0.5 to 1.5 seconds / 100 cm ³ and a curvature of 1.0 or more Filter media.

[2]
The filter medium according to [1], wherein the basis weight of the nanofiber layer is 0.2 g / m ² or less.

[3]
The filter medium according to [1] or [2], wherein the pressure loss when air is permeated at a surface speed of 5.3 cm / s is 150 Pa or less.

[4]
The filter medium according to any one of [1] to [3], wherein the porous layer is a stretched polytetrafluoroethylene layer.

[5]
The filter medium according to any one of [1] to [4], wherein the nanofiber layer is made of a fluororesin fiber.

[6]
The filter medium according to [5], wherein the fluororesin fiber is obtained from a spinning solution containing a fluororesin, a surfactant, and a solvent by an electrospinning method.

[7]
Nanofibers comprising fibers having an average fiber diameter of 200 nm or less on at least one surface of a porous layer having a Gurley value of 0.5 to 1.5 seconds / 100 cm ³ and a curvature of 1.0 or more The manufacturing method of the filter medium for filters characterized by including the process of forming a layer.

  The filter medium according to the present invention has a low filtration resistance (especially low pressure loss in an air filter medium) and a collection rate of particles (particularly particles having a particle size of about 0.1 to 0.3 μm). There is an effect that is high.

Hereinafter, the present invention will be described in more detail.
[Filter media for filters]
The filter medium for a filter according to the present invention has a nanofiber layer and a porous layer, the nanofiber layer is made of fibers having an average fiber diameter of 200 nm or less, and the porous layer has a Gurley value. It is 0.5 to 1.5 seconds / 100 cm ³ , and the curvature is 1.0 or more.

1. Nanofiber layer The average fiber diameter of the fibers is preferably 20 to 200 nm.
The basis weight is preferably 0.2 g / m ² or less, more preferably 0.01~0.2g / m ^2.

Moreover, the thickness of the nanofiber layer is preferably 10 μm or less, and the lower limit thereof may be, for example, about 0.1 μm.
Although it does not specifically limit as said fiber (henceforth "nanofiber"), For example, what can be manufactured by the electrospinning (electrostatic spinning) method is mentioned. Specific examples thereof include, for example, polyvinylidene fluoride (PVDF), polyhexafluoropropylene, polyperfluoroalkyl vinyl ether, polychlorotrifluoroethylene, fluoroethylene / vinyl ether alternating copolymer, and tetrafluoroethylene-perfluorodioxole copolymer. Fluoropolymers such as polymers, such as polyacrylonitrile, polyacrylic acid, polymethyl methacrylate, polyvinyl chloride, polyvinylidene chloride-acrylate copolymer, polyethylene, polypropylene, aramid, polyvinyl alcohol, polyvinyl acetate, cellulose, cellulose acetate, acetic acid Cellulose butyrate, polyethylene terephthalate, polyethylene naphthalate, polylactic acid, polyethylene oxide, polyglycolic acid, polyether Organic materials such as organic polymers such as ruphone, polyethyleneimide, polyoxymethylene, nylon 12, nylon 46, nylon 6, nylon 66, polypeptide, fibroin, chitin, chitosan, collagen, for example, sol-gel such as silica, alumina, titania The fiber which consists of an inorganic material which can utilize a method is mentioned, These may be used individually by 1 type, or may be used in combination of 2 or more types as appropriate. From the viewpoint of easy production of fine and uniform fibers and durability of the resulting filter, fibers made of fluororesin are preferred, and among these, fibers made of polyvinylidene fluoride are preferred from the viewpoint of easy production of fine and uniform fibers.

  The fiber made of the organic material and / or the inorganic material can be produced from a spinning solution containing the organic material and / or the inorganic material and a solvent by an electrospinning (electrostatic spinning) method.

<Spinning liquid>
The organic material and / or the inorganic material is contained, for example, in an amount of 5 to 90% by weight, preferably 10 to 60% by weight in the spinning solution, depending on the type of material.

  The solvent is not particularly limited as long as it can dissolve the organic material and / or inorganic material. For example, dimethylacetamide, dimethylformamide, tetrahydrofuran, methylpyrrolidone, xylene, acetone, chloroform, ethylbenzene, cyclohexane, benzene , Sulfolane, methanol, ethanol, phenol, pyridine, propylene carbonate, acetonitrile, trichloroethane, hexafluoroisopropanol, diethyl ether and the like. These solvents may be used individually by 1 type, and may be used as a mixed solvent which combined 2 or more types.

The solvent is contained, for example, 10 to 95% by weight, preferably 40 to 90% by weight in the spinning solution.
The spinning solution may further contain additives such as a surfactant, a charge adjusting agent, functional particles, an adhesive, and a viscosity adjusting agent. Among them, it is preferable to include a surfactant in order to increase the stability of the spinning solution with respect to electric charge during electrospinning and to reduce the diameter of the fiber.

  Examples of the surfactant include a fluorine-based surfactant (that is, a surfactant having a fluorine atom. For example, an ammonium salt of an acid having a perfluoroalkyl group), a hydrocarbon-based surfactant (the main chain is an alkyl group). And a surfactant based on silicon (a surfactant having a silicon atom).

  If it is a commercial item as said fluorosurfactant, it is a footgent (registered trademark) 100 (anionic fluorosurfactant), a footgent (registered trademark) 310 (cationic fluorosurfactant). (Neos Co., Ltd.), Megafac F114 (anionic fluorosurfactant, DIC Corp.), Surflon S-231 (amphoteric fluorosurfactant, Asahi Glass Co., Ltd.), etc. .

The surfactant is contained in the spinning solution in an amount of, for example, 0.1 to 10% by weight, preferably 1 to 5% by weight.
The spinning solution can be produced by mixing the above-described organic material and / or inorganic material, a solvent, and, if necessary, an additive by a conventionally known method.

Examples of preferred embodiments of the spinning solution include the following spinning solution (1).
Spinning liquid (1): Spinning liquid containing 5 to 30% by weight, preferably 10 to 20% by weight of PVDF, and 1 to 10% by weight, preferably 2 to 5% by weight of a surfactant.

<Electrospinning>
The fiber (nanofiber) is produced from the spinning solution by electrospinning (electrostatic spinning).

The applied voltage when performing this electrospinning is preferably 5 to 50 kV, more preferably 20 to 40 kV.
The tip diameter (outer diameter) of the spinning nozzle is preferably 0.1 to 2.0 mm, more preferably 0.2 to 1.0 mm, and still more preferably 0.20 to 0.85 mm.

  More specifically, for example, when the spinning solution (1) is used, the applied voltage is preferably 10 to 50 kV, more preferably 20 to 40 kV, and the tip diameter (outer diameter) of the spinning nozzle is used. ) Is preferably 0.2 to 0.85 mm.

Nanofibers obtained by this method have small fiber diameters and small variations in fiber diameters. Specifically, the average fiber diameter is 40 to 200 nm.
In addition, about the said average fiber diameter and nanofiber (group) used as a measuring object, the area | region of a scanning electron microscope (SEM) observation was chosen at random, and this area | region was randomized by SEM observation (magnification: 10000 times). The value is calculated based on the measurement results of 10 nanofibers selected from these nanofibers.

  The average fiber diameter is determined by using the spinning solution having a low concentration (organic material and / or inorganic material concentration) during electrospinning, decreasing the humidity, decreasing the nozzle diameter, increasing the applied voltage, or voltage density. There is a tendency to decrease by increasing.

  In order to form a nanofiber layer using the nanofiber according to the present invention, the process of producing the nanofiber and the process of forming the nanofiber layer by integrating the nanofibers in a sheet form are performed separately. May be performed simultaneously (that is, nanofibers may be integrated into a sheet while being produced to form a nanofiber layer).

2. Porous layer The Gurley value is the time (seconds) required for 100 cm ³ of air to pass through an area of 1 inch ² (6.45 cm ² ) under a pressure of 1.215 kN, usually 0.5 to 1. 5 seconds / 100 cm ³ , preferably 0.7 to 1.2 seconds / 100 cm ³ . In addition, a Gurley value can be calculated | required based on ASTMD726-58.

  When the Gurley value is within the above range, it is preferable in that the filtration resistance can be reduced (especially, when the fluid is a gas, the pressure loss can be reduced), and high particle capturing performance can be exhibited.

  The Gurley value (air permeability) can be increased or decreased by adjusting, for example, the porous pore diameter and fiber diameter and the thickness of the porous layer. To reduce the Gurley value, the porous Operations such as reducing the pore diameter, reducing the fiber diameter, and reducing the thickness may be performed.

The curvature is preferably 1.0 to 2.0.
When the curvature is within the above range, it is preferable in that a high particle capturing performance can be exhibited.

  The value of the curvature can be increased or decreased by controlling the porous pore structure. When the porous layer is made of stretched polytetrafluoroethylene, for example, it can be increased or decreased by controlling the fibril orientation structure by stretching, the ratio of fibrils to nodes, the fibril structure in the thickness direction, and the curvature can be increased. In order to increase the temperature, for example, the stretching process in the longitudinal direction and the lateral direction may be performed under different conditions, or a heat treatment provided in a temperature gradient in the porous thickness direction may be performed after the stretching process.

In addition, a curvature is calculated | required by the method of Unexamined-Japanese-Patent No. 2012-102199, for example, and is represented by a following formula.
Curvature τ = {d × (ε / 100) × ν / (3L × P _S × R _gas )} ^1/2
(D: average pore diameter, ε: porosity, ν: air average molecular velocity = 500 m / sec, L: film thickness, P _S : standard pressure = 101325 Pa, R _gas : air permeation rate constant = 0.0001 / (Air permeability x 0.830563))

The structure and form of the porous layer are not particularly limited, and a nonwoven fabric, a stretched film, a mesh, and other porous materials can be used.
Although the material of the said porous layer is not specifically limited, What consists of a fluororesin is preferable. Examples of the fluororesin include polytetrafluoroethylene (PTFE) (including modified polytetrafluoroethylene), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), and tetrafluoroethylene-hexafluoropropylene copolymer. Examples thereof include a polymer (FEP), an ethylene-tetrafluoroethylene copolymer (ETFE), and poly (chlorotrifluoroethylene) (PCTFE). Among them, PTFE is preferable.

The porous layer made of PTFE is preferably made of expanded polytetrafluoroethylene (ePTFE).
The stretched polytetrafluoroethylene (ePTFE) is obtained by stretching polytetrafluoroethylene, which may be either uniaxially stretched or biaxially stretched. In view of the mechanical strength and the like, those obtained by biaxial stretching are preferred.

  The porous layer made of ePTFE (hereinafter referred to as ePTFE layer) has pores due to stretching in addition to the properties such as heat resistance, chemical resistance, weather resistance and antifouling property of PTFE, and is flexible. And has the characteristics of high mechanical strength.

In addition, a large number of pores exist on the surface of the ePTFE layer. The porosity of the ePTFE layer (that is, the total (volume) ratio of pores (voids) present in a predetermined volume of the ePTFE layer) is expressed by the following formula (true density of PTFE−apparent density) × 100 / PTFE It is expressed in true density, preferably 60-90%, more preferably 70-98%.

Here, the “true density” of PTFE is a density calculated from the volume occupied by PTFE itself, and 2.15 g / cm ³ is used. The “apparent density” is a density obtained including the volume of pores of PTFE, and the weight and volume of the ePTFE layer (for example, when the ePTFE layer is a rectangular parallelepiped, the length of each side thereof). The volume calculated by the product of the length is used.

The pore diameter of the ePTFE layer is preferably 0.5 to 5 μm, more preferably 1 to 3 μm.
The thickness of the ePTFE layer can be appropriately changed, and is preferably 1 to 100 μm, more preferably 5 to 20 μm.

  The ePTFE layer is formed by, for example, extruding and calendering a mixture of homopolytetrafluoroethylene and / or modified polytetrafluoroethylene to which an extrusion aid is added to form a preform, and stretching and heat-treating the preform. (For example, 100 to 370 ° C.).

  The stretching step includes a step of stretching the preform in the longitudinal direction and the transverse direction while heating. The stretching ratio in each direction is preferably 1.1 to 80 times, the stretching speed is preferably 10 to 600% / second, and the stretching temperature is preferably 100 to 420 ° C. From the viewpoint of controlling the porous pore structure, it is preferable that the stretching ratio, stretching speed and stretching temperature in the longitudinal direction and the transverse direction are different.

Commercially available products may be used as the porous layer. Examples of commercially available products include sa-PTFE ^™ (Gurley value: 0.86, curvature: 1.47, average pore diameter, manufactured by Nippon Valqua Industries, Ltd. : 1.4 μm, porosity: 86%).

[Method of manufacturing filter media for filter]
The filter medium for a filter of the present invention is manufactured by a manufacturing method including a step of forming the nanofiber layer on at least one surface of the porous layer.

  The filter medium of the present invention may be one in which the nanofiber layer and the porous layer are each composed of two or more layers. In that case, two or more nanofiber layers having different materials and fiber diameters and / or You may be comprised from two or more porous layers from which a material and a structure differ.

The nanofiber layer is manufactured by a process of forming the nanofiber layer by collecting the fibers in a sheet form.
The nanofiber layer and the porous layer constituting the filter medium of the present invention may be bonded using an adhesive layer between the nanofiber layer and the porous layer by a conventionally known method. The fiber layer and the porous layer may be melt bonded.

The filter medium for the filter may be subjected to conventionally known processing within a range not impairing its performance, for example, may be subjected to pleating.
The filter medium for a filter of the present invention can be used as a filter for collecting particles contained in various filtration fluids (for example, gas, liquid, etc.). Among them, it is suitable for an air filter, and particularly has a low pressure loss and a high collection rate of particles having a diameter of about 0.1 to 0.3 μm. The 3 μm particle capture rate is 99.9% or more.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited at all by these Examples.
<Measurement method>
Various physical properties of the nanofiber layers and filter media prepared in Examples and Comparative Examples were measured by the following methods.

1. Nanofiber (fluorine resin fiber) layer (average fiber diameter)
About each fluororesin fiber (group) manufactured by the Example and the comparative example, the area | region of SEM observation was chosen at random, this area | region was SEM observation (apparatus: S-3400N (made by Hitachi High-Technologies Corporation), magnification. : 10000 times), 10 fluororesin fibers were selected at random, and the average (arithmetic average) fiber diameter was determined based on the measurement results of these fluororesin fibers.

(Weight)
The basis weight of the nanofiber layer was measured according to JIS L 1906 (2000). At this time, the weight of the nanofiber layer was calculated from the difference between the weight of the porous layer (for example, ePTFE) and the weight of the filter medium formed by accumulating the fluororesin fibers on the porous layer.

2. Porous layer (average pore diameter)
The average pore diameter was determined by a half dry method of ASTM E1294-89 using a capillary flow porometer “CFP-1200 AEL” manufactured by Porous Materials Inc. The half dry method is a curve of half the slope of a porous aeration curve (Wet Curve) immersed in an immersion liquid (Galwick) and a dry aeration curve (Dry Curve) of a porous state (Half). The pressure (average flow diameter pressure) at the point at which (Dry Curve) intersects was obtained and calculated from the following equation using this pressure.
Average pore diameter D = 2860 × γ / P
(Γ: surface tension of dipping solution [dyne / cm], P: average flow diameter pressure [Pa])

(Gurley value)
The Gurley value was determined according to ASTM D726-58 using a capillary flow porometer “CFP-1200 AEL” manufactured by Porous Material Inc.

(Porosity)
The porosity is calculated using the weight of a test piece obtained by cutting a porous layer into a 4 cm square (4 cm length, 4 cm width) and the thickness measured with a micrometer (LITEMATEC VL-50, manufactured by Mitutoyo Corporation). It calculated by the following formula using the apparent density.
(True density of PTFE−apparent density) × 100 / true density of PTFE

(Curvature)
The curvature was calculated by applying the average pore diameter, porosity (porosity), air permeability (Gurley value), and film thickness calculated by the above method to the following formula.
Curvature τ = {d × (ε / 100) × ν / (3L × P _S × R _gas )} ^1/2
(D: average pore diameter, ε: porosity, ν: air average molecular velocity = 500 m / sec, L: film thickness, P _S : standard pressure = 101325 Pa, R _gas : air permeation rate constant = 0.0001 / (Air permeability x 0.830563))

3. Air filter media (particle collection rate, pressure loss)
About each filter medium manufactured by the Example and the comparative example, the particle collection rate was measured according to JIS B 9908. At this time, a filter medium cut into a size of 200 mm × 200 mm was used instead of the filter unit, and atmospheric dust (including dust having a particle diameter of 0.10 μm to 2.0 μm) was used as the measurement dust. When measuring the particle collection rate, the air flow rate was set to a surface velocity of 5.3 cm / s.
Further, along with this measurement, the pressure difference (that is, pressure loss) between the upstream side and the downstream side of the filter medium was measured with a fine differential pressure gauge. This measurement was performed by changing the air flow rate.

[Example 1]
1. Nanofiber layer (fluorine resin fiber) [Average fiber diameter: 40 nm]
Polyvinylidene fluoride (weight average molecular weight: 275,000, manufactured by Aldrich) as a fluororesin is 12% by weight, and an anionic fluorosurfactant (Futagent (registered trademark) 100, manufactured by Neos Co., Ltd.) as an additive ( Hereinafter, a spinning solution containing 3% by weight of “F100”) and 85% by weight of dimethylacetamide (special grade reagent, manufactured by Wako Pure Chemical Industries, Ltd.) (hereinafter also referred to as “DMAc”) as a solvent was prepared. .

  This spinning solution was filled into a solution filling part of an electrospinning apparatus (ES-2300 (device name), manufactured by Huence), and electrospinning was performed for 7 minutes by applying a voltage of 40 kV to produce a fluororesin fiber. . The tip diameter of the spinning nozzle was 27G (outer diameter 0.4 mm), and the distance to the collector (aluminum foil) was 19 cm. The production conditions are shown in Table 1.

[Manufacture of filter media]
This fluororesin fiber is used as a porous layer of a 20 cm × 20 cm square ePTFE layer (thickness 6 μm, average pore diameter 1.37 μm, Gurley value 0.86 sec / 100 cm ³ , curvature 1.47, Nippon Valqua Industries, Ltd.) A filter medium for a filter was produced by accumulating on one side of “sa-PTFE ^TM series” manufactured by Co., Ltd. without bonding.

[Examples 2 to 8]
Except that the fluororesin fibers used in the nanofiber layer were produced under various conditions described in Table 1, an air filter medium comprising various nanofiber layers and a porous layer (ePTFE) was prepared in the same manner as in Example 1. Manufactured. These characteristics are shown in Table 2.

[Comparative Example 1]
A filter medium comprising a single layer of the porous layer (ePTFE) used in Example 1 was produced. The characteristics are also shown in Table 2.

[Comparative Example 2]
Instead of the porous layer (ePTFE) used in Example 1, an ePTFE layer (thickness 14 μm, average pore diameter 0.88 μm, Gurley value 1.92 sec / 100 cm ³ , curvature 1.02, Sumitomo Electric Industries, Ltd.) A filter medium consisting of a single layer ("Poreflon (registered trademark) membrane series") manufactured by Co., Ltd. was produced. The characteristics are also shown in Table 2.

[Comparative Example 3]
Instead of the porous layer (ePTFE) used in Example 1, the ePTFE layer (thickness 62 μm, average pore diameter 2.83 μm, Gurley value 1.9 sec / 100 cm ³ , curvature 1.02, WL Gore and The filter medium for filter which consists of a single layer was manufactured from Associates, Inc. "expanded PTFE series". The characteristics are also shown in Table 2.

[Comparative Example 4]
Instead of the porous layer (ePTFE) used in Example 1, a PTFE nonwoven fabric layer (thickness 30 μm, average pore diameter 3.73 μm, Gurley value 0.57 sec / 100 cm ³ , curvature 0.89, known Prepared by electrospinning method) A filter medium comprising a single layer was produced. The characteristics are also shown in Table 2.

[Comparative Examples 5-6]
A fluororesin fiber was produced in the same manner as in Example 1 except that the fluororesin fiber used in the nanofiber layer was produced under various conditions described in Table 1. This fluororesin fiber is made of a glass nonwoven fabric (thickness 360 μm, average pore diameter 180 μm, Gurley value 0.27 sec / 100 cm ³ , curvature 1.32, pressure loss 2 Pa, 20 cm × 20 cm square glass). A non-woven fabric, product name: Grabest (registered trademark) SAS-050, manufactured by Olivest Co., Ltd.) was accumulated without adhering to form a nanofiber layer to produce a filter medium. This characteristic is also shown in Table 2.

[Production Examples 1-1 to 1-7] Production of fluororesin fibers Nanofiber layers (fluororesin fibers) were produced in the same manner as in Example 1 except that various conditions were changed as described in Table 3. . The characteristics (average fiber diameter) of the obtained fluororesin fibers are also shown in Table 3.

[Production Example 2-1 to Production Example 2-3] Production of fluororesin fiber (no surfactant)
A nanofiber layer (fluororesin fiber) was produced in the same manner as in Example 1 except that various conditions were changed as described in Table 3. The characteristics of the obtained fluororesin fiber are also shown in Table 3.

表３の結果から明らかなように、界面活性剤を含まない紡糸液から製造する場合は、平均繊維径の小さいフッ素樹脂繊維を製造することはできないことが分かる。

As is apparent from the results in Table 3, it can be seen that fluororesin fibers having a small average fiber diameter cannot be produced when produced from a spinning solution containing no surfactant.

Claims (7)

A filter having a nanofiber layer made of fibers having an average fiber diameter of 200 nm or less, and a porous layer having a Gurley value of 0.5 to 1.5 seconds / 100 cm ³ and a curvature of 1.0 or more Filter media. The filter medium for a filter according to claim 1, wherein the basis weight of the nanofiber layer is 0.2 g / m ² or less.   The filter medium according to claim 1 or 2, wherein a pressure loss when air is permeated at a surface speed of 5.3 cm / s is 150 Pa or less.   The filter medium according to any one of claims 1 to 3, wherein the porous layer is a stretched polytetrafluoroethylene layer.   The filter medium according to any one of claims 1 to 4, wherein the nanofiber layer is made of a fluororesin fiber.   6. The filter medium according to claim 5, wherein the fluororesin fiber is obtained from a spinning solution containing a fluororesin, a surfactant and a solvent by an electrospinning method. Nanofibers comprising fibers having an average fiber diameter of 200 nm or less on at least one surface of a porous layer having a Gurley value of 0.5 to 1.5 seconds / 100 cm ³ and a curvature of 1.0 or more The manufacturing method of the filter medium for filters characterized by including the process of forming a layer.