JP2001113143A - Porous polymide film for filter and filter using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous film for filter enabling production of it with less processes, being less in anisotropy in the intra-plane direction of the film, small in the pore size, high in a collection ratio of fine particles due to through- holes, relatively larger in a filter treating quantity, better wettable to water and oil, superior in durability due to excellent mechanical strength, lightweight and less bulky. SOLUTION: This porous polyamide film consists of a porous structure having fine through-holes and has a 15∼85% porosity ratio, 0.01∼5 μm pore sizes, and 10∼50 μm film thickness, and also has a structure laid a fine porous layer on a layer with a relatively large gap or opening part in the cross-sectioned direction of the film, and also these two layers are mutually connected with fine through-holes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to dust collection, microfiltration,
It is suitably applied to the electronic field, environmental industry, industrial field, and medical field, and particularly relates to filters such as pure water production, battery separators, and bag filters.

[0002]

2. Description of the Related Art Conventional liquid and gas filters are:
Most are composed of fiber woven fabric or nonwoven fabric, and are bulky, and it is difficult to collect fine particles at a high bulk. The bag filter for collecting and separating dust from gas has a particularly fine aperture of about 10 μm at most. Also, when passing through the gas, fiber debris is contaminated (contamination of foreign substances)
Therefore, it was not suitable for precision filtration and bag filters for food, industrial use, electronic parts, etc. Further, when used as a liquid filter, the wettability with respect to the liquid may not always be good. For this reason, the function as a filter was restricted.

For example, Japanese Patent Application Laid-Open No. 6-126112 discloses a filter medium for liquid filtration, which comprises a filter for filtration using fine fibers and a binder, and is excellent in filtration efficiency and processability such as folds. It is said that it is suitable for filtering engine oil and pure water used for IC production.

[0004] Japanese Patent Application Laid-Open No. 6-142644 discloses a water purification filter comprising a polyamide fiber aggregate containing a predetermined amount of acid groups in order to maintain water quality in which fish and shellfish can live for a long time.

[0005] Japanese Patent Application Laid-Open No. 7-227513 discloses a fibrous air filter subjected to electret processing.

Japanese Patent Application Laid-Open No. 8-144166 discloses a nonwoven fabric of polyamide ultrafine fibers having a narrow pore size distribution. It is said to be excellent in liquid retaining property and wettability and suitable for battery separators and clothing interlining.

[0007] Japanese Patent Application Laid-Open No. 10-80612 discloses a filter medium and a filter suitable for semiconductors and bio-based products, comprising fibrillated organic fibers containing alumina fibers having a drainage value of 30 to 800 seconds.

[0008] These are all related to filters made of fibers. However, filters made of fiber woven fabric and nonwoven fabric are disadvantageous in that they are bulky and heavy in weight. For this reason, the emergence of a film filter that is lightweight, compact, has excellent wettability with water and oil, has no contamination, and is durable, and collects fine particles having a pore size of 0.5 to 10 μm. Was expected.

Japanese Patent Application Laid-Open No. 10-298340 discloses a method for producing a microporous polyolefin membrane suitable for water treatment and microfiltration. However, the microporous polyolefin membrane is inferior in wettability to water, and thus cannot be said to exhibit sufficient filter performance.

On the other hand, polyamide is excellent in wettability with water and oil and is a useful material as a filter. As the film-like polyamide porous membrane, for example, a polyamide solution is prepared, and a non-solvent of the polyamide is added to the solution under sufficiently controlled conditions to cause nucleation.
The solution is cast in a film form on a substrate, and a non-solvent is further added to the solution film to precipitate a skinless polyamide porous film, which is washed and dried to produce a porous film. A method (US Pat. No. 4,340,479) is known. Also, immersing the polyamide film in an alkaline earth metal halide aqueous solution kept at a high temperature,
A method of uniformly dispersing the above salt in a membrane, and extracting a salt molecule to obtain a porous membrane (JP-B-62-12807);
A method of extracting a water-soluble polyamide from a film of a mixture of a water-insoluble polyamide and a water-soluble polyamide to obtain a porous membrane (Japanese Patent Laid-Open No. 57-167331), a method of dissolving an aromatic polyamide in an amide solvent. Cast into a film,
A method of forming a film of a solidified solution film by lowering the solution to a solidification temperature or lower, and extracting and removing a solvent from the film to produce a uniform porous film (Japanese Patent Publication No. 59-14494); There is known a method of producing a microporous membrane by casting a solution in the form of a film, immersing the solution in water, and immediately stretching the solution in the longitudinal direction (Japanese Patent Application Laid-Open No. 2-243030).

[0011]

In the above-described method for producing a polyamide porous membrane, an operation of cooling a polyamide solution and then pouring it into a solvent extractant, an operation of extracting salt molecules and a water-soluble polyamide component from the polyamide film, An operation such as adding a non-solvent of polyamide to the polyamide solution is required, the process is complicated, and the obtained polyamide porous membrane has insufficient strength, has a problem in filter workability and durability, and further has a pore size. Are too large or the distribution is non-uniform.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, to be able to be manufactured by a small number of steps, to have a small anisotropy in the in-plane direction of the film, to have a small hole diameter, and to realize a through-hole. Because of this, the efficiency of collecting fine particles is high, the filter has a relatively large filtration throughput, has good wettability to water and oil, and has excellent mechanical strength, so it has excellent durability performance.
An object of the present invention is to provide a porous film for a filter which is light in weight and does not bulk.

[0013]

The present invention relates to a porous material having a porous structure having fine through holes, a porosity of 15 to 85%, a pore diameter of 0.01 to 5 μm, and a film thickness of 10 to 50 μm. A polyamide film, and in the cross-sectional direction of the film,
A porous structure for a filter having a structure in which a layer made of microporous material and a layer having relatively large voids or openings are stacked, and wherein the two layers are connected to each other by through micropores. It relates to a polyamide film. Furthermore, the present invention relates to a polyamide porous film for a filter, characterized in that a granular structure is formed on the surface of the layer having relatively large voids, and the locations of the voids are not biased on the grain boundaries.

[0014] The present invention also relates to the present invention, wherein one or more porous polyamide films for a filter are provided;
The present invention relates to a filter having a single-layer or multi-layer structure, wherein the porous polyamide film and a nonwoven fabric or another porous film are laminated. The present invention also relates to a bag filter for a gas phase and a filter for filtration of an aqueous phase comprising the porous polyamide film.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The porous polyamide film for a filter of the present invention will be described below. The porous polyamide film for a filter of the present invention has a porous structure having fine through holes and a porosity of 15 to 85.
%, A pore size of 0.01 to 5 μm, and a film thickness of 10 to 50 μm.

The through hole according to the present invention means that a through hole is formed from one surface to the other surface of the film film. It does not have to be a straight hole. It may be winding. The performance as a filter is exhibited by such a through hole.

The porous polyamide film for a filter of the present invention has a porosity of 15 to 85%, preferably 15 to 85%.
~ 70%. If the porosity is lower than 15%, the permeability coefficient of the filter will be low, and if the porosity exceeds 85%, the collection efficiency of fine particles tends to be low. It is not suitable because the mechanical strength of the membrane is also low.

The porous polyamide film for a filter of the present invention has a pore size of 0.01 to 5 μm, preferably 0.5 to 5 μm. If the pore size is smaller than 0.01 μm, when applied as a filter, the processing amount is too small, and the filter may not be able to exhibit the function of expecting a filtration rate. If the pore size exceeds 5 μm, the fine particles may similarly pass through,
Filter function is reduced.

The porous polyamide film for a filter of the present invention has a thickness of 10 to 50 μm, preferably 10 to 35 μm. If the thickness is less than 10 μm, the mechanical strength becomes weak. On the other hand, when the thickness exceeds 50 μm, the ratio of the continuous holes decreases, so that the passing amount through the filter decreases.

The porous polyamide film for a filter of the present invention has a structure in which a microporous layer and a layer having relatively large voids or openings are stacked in the cross-sectional direction of the membrane. The layers are connected to one another by through-micropores. With such a structure, fine particles are easily captured. In addition, it is possible to prevent the coarse particles from being trapped by the openings near the surface and the large voids, resulting in poor filtration efficiency.

Further, a grain structure is formed on the surface of the layer having relatively large voids, and the location of the voids is not biased on the grain boundaries. It can be said that the granular structure of this surface is excellent in mechanical strength and dimensional stability because the position of the void is not biased on the grain boundary.

The present invention also relates to a method for manufacturing the filter, comprising one or more porous polyamide films for a filter;
A filter having a single-layer or multi-layer structure, wherein the porous polyamide film and a nonwoven fabric or another porous film are laminated. By combining one or more porous polyamide films, a wide range of performance can be expected as a filter. The collection rate of the porous polyamide film further increases. When combined with other materials, the mechanical strength of the filter and the ability to trap fine particles are further improved.

The porous polyamide film for a filter of the present invention has an air permeability of 0.5 to 2000 seconds / 100 CC. When the air permeability becomes higher than 0.5 seconds / 100 CC,
It is not suitable as a filter because of its poor collecting ability. If the air permeability is lower than 2,000 seconds / 100 CC, the amount of passing (or the amount of ventilation) as a filter decreases.

The porous polyamide film may have a single-layer or multi-layer structure, and has a total film thickness of 5 to 100 μm, preferably an air permeability of 0.5 to 2 μm.
It is preferable to be adjusted to 000 seconds / 100 cc.

Further, the filter has a single-layer or multi-layer structure in which another filter element, a nonwoven fabric, or a woven fabric is laminated. Nonwoven fabrics and woven fabrics to be combined include cellulose fiber, fluorine fiber, glass fiber, carbon fiber, metal fiber,
Non-woven fabrics such as organic heat-resistant fibers and woven fabrics are preferred. In addition, other porous carbon plates, graphite powders, sintered films of inorganic substances such as silicon nitride and aluminum nitride, and plate-like materials can also be suitably used as long as they are porous films.

Further, the present invention is a bag filter comprising the filter having the porous polyamide film. The bag filter captures fine particles of 0.5 μm or more.

Further, the present invention is an aqueous phase filtration filter comprising the above-mentioned filter having the porous polyamide film. Since the porous polyamide of the present invention has excellent wettability with water, it is suitable for filtering water and hydrophilic liquids.

Next, a method for producing the porous polyamide film for a filter of the present invention will be described below. The porous polyamide film of the present invention is prepared by preparing a uniform solution comprising a polyamide and an alcohol-based solvent having a boiling point of 80 ° C. or lower, casting the solution in a film on a substrate, and coating the resulting film with a film. It is manufactured by drying while causing a fluctuation in the polyamide concentration in the film in the in-plane direction.

The polyamide used in the present invention includes:
Various known ones can be mentioned. For example, oxalic acid, adipic acid, sebacic acid, terephthalic acid, isophthalic acid, dicarboxylic acids such as 1,4-cyclohexyldicarboxylic acid and ethylenediamine, hexamethylenediamine, 1,4-cyclohexyldiamine, m-xylylenediamine, pentamethylene Diamine, crystalline polyamide obtained by polycondensation of a diamine such as decamethylene diamine, ε-caprolactam, ε-aminocaproic acid, ω
-Crystalline polyamide obtained by polymerizing laurolactam, ω-aminododecanoic acid, ω-aminoundecanoic acid,
And crystalline polyamides composed of these copolymers. Specifically, polyamide 6, polyamide 6
6, polyamide 610, polyamide 12, polyamide 6
6 / 6T (T represents a terephthalic acid component) and the like.
It may be a mixture of the above polyamides.

The molecular weight of the polyamide resin is not particularly limited, as long as it can form a homogeneous solution with an alcoholic solvent having a boiling point of 80 ° C. or lower. Preferably,
The polyamide resin has a number average molecular weight of 9,000 to 100,0.
00. If the number average molecular weight is less than 9,000, the mechanical strength is insufficient, and the number average molecular weight becomes 10
If it is larger than 000, the solution viscosity becomes too high, so that it is difficult to form a uniform porous film.

The fluorinated alcohol solvent having a boiling point of 80 ° C. or lower used in the present invention is specifically a fluorinated alcohol such as hexafluoroisopropanol (HFIP) or trifluoroethanol, or the above fluorinated alcohol 40
It is a solvent in which at least mol% and at most 60 mol% of aliphatic alcohol are mixed. The aliphatic alcohol is, specifically,
These are methanol, ethanol and 1-butanol. Preferred are methanol and ethanol. When the proportion of the aliphatic alcohol mixed with the fluorinated alcohol is 60
If it exceeds mol%, the solubility of the polyamide resin in the solvent is greatly reduced, so the above ratio is preferable.

In the present invention, the polyamide and the alcohol-based solvent comprise 0.5 to 7% by weight, preferably 2 to 5% by weight of the polyamide and 99.5 to 9% by weight of the alcohol-based solvent.
The polyamide solution is prepared by dissolving at a ratio of 3% by weight, preferably 98 to 95% by weight. When the proportion of the polyamide is more than 7% by weight, the air permeability of the obtained porous membrane decreases, which is not appropriate. The above-mentioned ratio is preferable because the film is uniformly peeled and easily broken.

In the present invention, the polyamide solution may contain a filler such as a fiber, a powder, and a woven fabric. For example, a carbon fiber, a glass fiber, a fibrous zonotolite, a silicon nitride fiber, a fiber reinforcing material of a metal fiber, activated carbon, graphite powder, a granular reinforcing material such as silicon oxide, silicon nitride, boron nitride, and quartz may be blended. . Further, additives such as a surfactant, a heat-resistant material, a flame retardant, a colorant, and an antioxidant may be added. These additives and reinforcing materials can be appropriately blended with the above polyamide solution as long as the polyamide porous film of the present invention is not hindered.

In the present invention, the polyamide solution is cast in a film on a substrate, and the obtained film is dried while causing a fluctuation in the polyamide concentration in the film in the in-plane direction of the film. Thus, a polyamide porous membrane can be produced.

The substrate used in the present invention is a substrate that does not repel the polyamide solution when the solution is dropped onto the substrate. For example, a glass plate, a mica plate, a quartz plate, a cloth, or the like is suitable. The substrate is not limited to a flat plate, but may be a cylinder or a substrate having a complicated shape as necessary.

There is no particular limitation on the method of forming the film of the polyamide solution on the substrate, but the method of casting the polyamide solution on a plate or a movable belt serving as the substrate, A method such as a method of impregnating the substrate with the substrate can be used.

The thickness of the film of the polyamide solution formed on the substrate is in the range of 5 to 50 μm, preferably 10 to 35 μm, as an average thickness of the film including voids after drying.
Adjust to the range. This depends on the concentration of the polyamide solution and the thickness of the liquid film. When the average thickness is smaller than 5 μm,
In the drying step, the film is apt to be densified or damaged, and if the average thickness is more than 50 μm, the air permeability of the obtained film is reduced.

The film of the polyamide solution formed on the substrate is dried under such conditions that the polyamide concentration fluctuates in the in-plane direction of the film during the drying process. The fluctuation of the polyamide concentration is about 1 mm between a part where the solution film during drying is slightly clouded and a transparent part.
It can be known by dividing at the following intervals. Conditions in which the fluctuation of the polyamide concentration occurs include conditions in which the drying is performed at a temperature of 40 ° C. or less for 1 minute or more, and preferably for a drying time of 2 minutes or more. For example, a crystalline polyamide having a number average molecular weight of about 10,000 When polyamide 6 is used, it is preferable to dry the solution film in an air atmosphere at about 30 ° C. or lower. It is not preferable that the film is dried under conditions that do not cause the concentration fluctuation, since the resulting film becomes dense. Next, the dried film is peeled from the substrate to obtain a polyamide porous film.

In the obtained porous polyamide film, a layer made of microporous material and a layer having relatively large voids or openings are stacked in the cross-sectional direction of the film.
The layers have a structure in which the layers are connected to each other by through-holes. Furthermore, a granular structure is generated on the layer side surface having relatively large voids, and the positions of the voids are not biased on the grain boundaries. Therefore, it is superior in mechanical strength and dimensional stability as compared with a polyamide porous membrane obtained by a conventional production method.

[0040]

EXAMPLES The present invention will be described below in detail with reference to examples, but the present invention is not limited to these examples. Test / evaluation methods or criteria in Reference Examples, Examples and Comparative Examples are as follows.

(Evaluation of Polyamide Porous Film) The thickness and porosity of the polyamide porous film were measured as follows.

(1) Air permeability Measured according to JIS P8117. As a measuring device, a B-type Gurley densometer manufactured by Toyo Seiki was used.
The sample piece was fastened to a circular hole having a diameter of 28.6 mm and an area of 645 mm ² , and the air in the cylinder was allowed to pass from the test hole to the outside of the cylinder with the inner cylinder weight of 567 g, and the time required for 100 cc of air to pass was measured. The temper (Gurley value) was used.

(2) Porosity The thickness and weight of the porous film cut to a predetermined size were measured, and the porosity was determined from the basis weight by the following equation (1). In the formula (1), S represents the area of the porous film, d represents the film thickness, W represents the measured weight, and D represents the density of the polyamide. Porosity (%) = 100−100 × (W / D) / (S × d) (1)

(3) Tensile strength Measured according to JIS K 7127. Test speed 1 using Toyo Baldwin Tensilon universal testing machine
The tensile strength was measured at 0 mm / min.

(4) Piercing Strength A sample was fixed to a circular holder having a diameter of 11.28 mm and an area of 1 cm ² , and a needle having a tip shape of 0.5R and a diameter of 1 mm was lowered at a speed of 2 mm / sec to pierce and penetrate. The load was measured.

(5) Dry Heat Shrinkage Temperature 23 ° C., relative humidity 5 according to JIS K 7100
After marking the sample conditioned at 0% for a predetermined length, the sample was placed in an oven set at 160 ° C. in an unconstrained state.
After leaving still for a minute, the dimensions after removal were measured. The dry heat shrinkage ratio follows the following equation (2). L1 in the equation (2) means the film size after being removed from the oven, and L0 means the initial film size. Dry heat shrinkage (%) = [1− (L1 / L0)] × 100 (2)

(Evaluation as Filter) The polyamide porous film obtained by the above method was evaluated as a filter. The evaluation as a filter was performed by the following method.

(1) Pressure loss (ΔP) A pressure loss (ΔP) is applied to a self-made stainless steel cylindrical frame.
Air was passed through a 0 cm ² filter at a surface wind velocity of 5.3 cm / sec, and the differential pressure (mmH ₂ O) at that time was measured with a fine differential pressure gauge.

(2) DOP Collection Efficiency (Z) Air containing polydisperse DOP particles generated by a Ruskin nozzle was passed through a filter having an effective area of 100 cm ² to achieve a surface wind speed of 5.
The DOP collection efficiency when air was passed at 3 cm / sec was measured with a laser particle counter manufactured by Rion Co., Ltd.
The target particle size was measured at 0.3 to 0.4 μm. The collection efficiency was indicated by the ratio (%) of capture.

(3) PF value The PF value as an index of the filtration performance of the filter was obtained from the pressure loss and the DOP collection efficiency by the following equation (3).

(Equation 1)

Example 1 (Preparation of Polyamide Porous Film) Polyamide 6 having a number average molecular weight of 13,000 was used as a polyamide, and HFIP was used as an alcohol solvent having a boiling point of 80 ° C. or lower. H so that the polyamide has a concentration of 2% by weight.
The polyamide solution was obtained by dissolving in FIP at room temperature.

The obtained polyamide solution was cast on a glass plate so that the thickness of the solution was 300 μm. When the film of the polyamide solution was naturally dried at normal temperature and atmospheric pressure, the film fluctuated in the in-plane direction of the film, and then a dried film was obtained. Subsequently, the film was peeled from the glass plate to obtain a polyamide porous film. The thickness of the obtained porous film was 15 μm. When a liquid such as water, methanol, or acetone was dropped on the film surface, the liquid permeated. Observation with a scanning electron microscope revealed that the obtained porous film had a layer having a pore size of about 3 μm or less and a layer having a relatively large pore size of about 8 μm or more in the cross-sectional direction of the film. It had a stacked structure, and had a structure in which the two layers were connected to each other by through-holes having a diameter of about 1 μm or less.

The air permeability of the obtained porous membrane was 1.3 seconds / 1.
00cc, 1.13 g / cm as the density of polyamide 6
^The porosity determined using ³ is 51%, and the tensile strength is 1.9k.
gf / mm ² , piercing strength is 76 gf, and dry heat shrinkage is 1.
It was 0%. This film is designated as PPA1.

Example 2 A polyamide 66 / 6T copolymer having an equivalent composition of polyamide 66 and polyamide 6T was used as the polyamide.
A polyamide porous membrane was obtained in the same manner as in Example 1, except that the polyamide solution was prepared so that the polyamide had a concentration of 3.3% by weight. Thickness of the obtained porous film 17
μm, and when a liquid such as water, methanol, or acetone was dropped on the membrane surface, the liquid permeated. Observation with a scanning electron microscope revealed that the obtained porous film had a layer having a gap of about 2 μm or less
m and a layer having a relatively large crater-shaped opening is stacked, and the two layers are about 1 μm.
It had a structure connected to each other by through-holes having a diameter of not more than m. Further, a hole having a diameter of about 1 μm or less was formed on the bottom surface of the opening.

The air permeability of this membrane is 37 seconds / 100 cc, and the density of polyamide 66 / 6T is 1.15 g / c.
The porosity determined using m ³ was 17%, and the tensile strength was 2.
It was 2 kgf / mm ² and the dry heat shrinkage was 4.0%. This film is designated as PPA2.

Example 3 HF was used as an alcoholic solvent having a boiling point of 80 ° C. or lower.
A polyamide porous material was prepared in the same manner as in Example 1 except that a polyamide solution was prepared so that the concentration of polyamide 6 was 3.3% by weight using a mixed solvent consisting of 49 mol% of IP and 51 mol% of methanol. A membrane was obtained. The thickness of the obtained porous membrane was 13 μm, and when a liquid such as water, methanol, or acetone was dropped on the membrane surface, the liquid quickly permeated. Further, when observed with a scanning electron microscope, the obtained porous film was obtained by laminating a layer having a gap of about 3 μm or less and a layer having a relatively large gap of about 4 μm or more in the cross-sectional direction of the film. And the two layers are about 2
It had a structure connected to each other by through micropores having a diameter of not more than μm. The air permeability of this membrane is 1.0 sec / 100 c
c, porosity is 38%, tensile strength is 1.4 kgf / mm
² , and the dry heat shrinkage was 3.0%. This film is designated as PPA3.

Example 4 The polyamide porous film (PPA) prepared in Example 1
Five pieces of 1) were superposed, sandwiched between stainless steel frames, and the filter performance was evaluated. Table 1 shows the results.

Example 5 The polyamide porous film (PPA) prepared in Example 2
The filter performance was evaluated in the same manner as in Example 4 except that 3) were superimposed. Table 1 shows the results.

Example 6 The polyamide porous film (PPA) prepared in Example 3
3) 5 sheets and glass fiber cloth (fiber diameter 13 μm, aperture 10
0 μm) Four filters were alternately superimposed to form a multilayer and a filter was produced. The filter performance was evaluated in the same manner as in Example 4. Table 1 shows the results.

Comparative Example 1 Polyolefin porous film (UPORE U manufactured by Ube Industries)
P2015) were stacked to form a filter. When the filter was prepared, it was anisotropic and easy to tear. The filter performance was measured in the same manner as in Example 4. Table 1 shows the results.

[0061]

[Table 1]

Example 7 Five layers of the porous polyamide film prepared in Example 1 and 50
A total of 11 layers of stainless steel mesh of 6 layers alternately and a width of 5 layers
mm, and the processability of the filter was evaluated. The polyamide porous film had good drape properties even when folded with a stainless steel net, and was folded in a folded manner. There was no damage.

[0063]

The porous film for filters and filters of the present invention have a structure in which a layer made of microporous material and a layer having relatively large voids or openings overlap in the cross-sectional direction of the film; This is a film having a structure in which the two layers are connected to each other by through-holes, and has excellent air permeability, excellent collection of fine particles, and excellent workability. Therefore, it is usefully used for microfiltration in exhaust gas treatment, electronic equipment, semiconductor industry, food industry and the like. In particular,
It is useful as a bag filter and a filter for liquid treatment.

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Claims (5)

[Claims] 1. A porous polyamide film having a fine pore, a porous structure, a porosity of 15 to 85%, a pore size of 0.01 to 5 μm, and a film thickness of 10 to 50 μm, and It has a structure in which a microporous layer and a layer having relatively large voids or openings are stacked in the cross-sectional direction of the membrane, and the two layers are connected to each other by through-holes. A porous polyamide film for a filter. 2. The porous polyamide film for a filter according to claim 1, wherein a granular structure is formed on the surface of the layer having relatively large voids, and the positions of the voids are not biased on the grain boundaries. . 3. A single-layer or multi-layer, characterized by laminating one or more of the porous polyamide films for a filter according to claim 1 or 2, or laminating the porous polyamide film with a nonwoven fabric or another porous film. Layered filter. 4. A gas-phase bag filter comprising the porous polyamide film according to claim 1. 5. A filter for aqueous phase filtration, comprising the porous polyamide film of claim 1.