JP2004175104A - Producing method for porous film, and porous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film which has high rates of hole opening on a film surface, and has homogenous and minute holes on the surface and inner part of the film. <P>SOLUTION: In the production method for the porous film, a phase inversion method is taken. Casting of a polymer solution is performed in a film form onto a substrate. When a polymer and a substrate to be used for composing the porous film are selected, a difference (Sa-Sb) of the surface tension of the polymer Sa [mN/m] and the surface tension of the substrate Sb [mN/m] should be -10 or above. The porous film has a lot of minute continuous holes and a caliper of the film is 5 to 200 μm. A mean hole size of both surfaces of the film is 0.01 to 10 μm. Ratio A/B of the surface mean hole size A and the inner mean hole size B is 0.3 to 3, and the ratio's C/D of the surface mean hole opening rate C to the inner mean hole opening rate D is 0.7 to 1.5. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a porous film having substantially no skin layer (dense layer) on the surface and having a large number of continuous micropores formed thereon. This porous film can be used for separators for batteries, electrolytic capacitors, and circuits by using membrane filtration techniques such as microfiltration and separation / concentration, or by using the pore characteristics as they are, or by filling the pores with a functional material. It can be used as a wide-range substrate material such as a substrate.

  Conventionally, high molecular compounds such as amide-imide-based polymers, imide-based polymers, sulfone-based polymers, fluorine-based polymers, and olefin-based polymers have been known as materials constituting the porous film. As a method for producing a porous film made of such a material, for example, a method of casting a mixed solution containing the above-described polymer compound into a film and then leading the mixture to a coagulating solution (phase change method) is known. However, a skin layer (dense layer) is present on the surface of the film produced by the above method using the above-mentioned polymer compound as a material, and substantially no pores are present. It was low. For example, as a porous film using an imide-based polymer as a material, a porous film made of polyimide and a method for producing the same have been disclosed (see, for example, Patent Documents 1 to 3). Since it is necessary to manufacture via a solvent displacement rate adjusting material, the manufacturing process is complicated, and there is a problem that it does not have a sufficient porosity and permeability.

JP 2001-67643 A JP 2001-145826 A JP 2000-319442 A

An object of the present invention is to provide a porous film having a high porosity on the film surface and having uniform micropores from the surface to the inside of the film.
Another object of the present invention is to provide a method for easily producing the porous film.

  The present inventors have conducted intensive studies to achieve the above object, and as a result, using a polymer and a substrate in which the difference between the surface tension of the polymer and the surface tension of the substrate is a specific value or more, using a mixture containing the polymer. By casting the solution into a film on the substrate and inverting the phase, a porous film having a high porosity and uniform micropores on the film surface on the side in contact with the substrate is obtained. Thus, the present invention has been completed.

  That is, the present invention is a method for producing a porous film by casting a polymer solution on a substrate in the form of a film and subjecting the polymer solution to a phase change method, wherein the surface tension Sa [mN / M (= dyn / cm)] and the surface tension of the substrate Sb [mN / m (= dyn / cm)] (Sa−Sb) are −10 or more. A manufacturing method is provided.

  In the method of the present invention, for example, 8 to 25% by weight of a polymer component as a material constituting a porous film, 10 to 50% by weight of a water-soluble polymer, 0 to 10% by weight of water, and 30 to 82% by weight of a water-soluble polar solvent % May be cast as a polymer solution on a substrate in the form of a film, and then introduced into a coagulation liquid, and phase-inverted to obtain a porous film. A step of holding the film in an atmosphere having a relative humidity of 70 to 100% and a temperature of 15 to 90 ° C. for 0.2 to 15 minutes, and then leading the film to a coagulating liquid comprising a non-solvent of a polymer component. Good.

  Further, the present invention is a porous film having a large number of interconnected micropores, the thickness of the film is 5 to 200 μm, for both surfaces of the film, the average pore diameter of the surface is 0.01 to 10 μm The ratio A / B of the average pore size A on the surface to the average pore size B on the inside is 0.3 to 3, and the ratio C / D of the average pore ratio C on the surface to the average pore ratio D on the inside is 0.3. A porous film having a thickness of 7 to 1.5 is provided.

Further, the present invention relates to a porous film having a large number of communicating micropores, wherein the thickness of the film is 5 to 200 μm, and the average pore diameters A ¹ and A ² on both sides of the film are 0.01 to ²⁰⁰ μm. The average opening ratio C ¹ , C ^{2 on} both sides of the film is 48% or more, and the ratio A ¹ / A ² of the average pore size A ¹ on one surface to the average pore size A ² on the other surface is 10 μm. and wherein the but 0.3 to 3, the ratio C ¹ / C ² between the average porosity C ² having an average porosity C ¹ and the other surface of the one surface is 0.7 to 1.5 To provide a porous film.

  According to the production method of the present invention, since the mixed solution containing the polymer component has a good phase separation structure on the substrate, the porosity of the surface on the substrate side is improved, and the porosity in which uniform micropores are formed A film can be easily obtained. Therefore, the porous film of the present invention can be used for membrane separation techniques such as microfiltration and separation / concentration, and by filling the pores with a functional material, a battery separator, an electrolytic capacitor, a circuit board, etc. It can be used as a wide range of substrate materials.

  In the production method of the present invention, a polymer solution containing a polymer component as a material constituting a porous film is cast on a substrate in the form of a film, and the porous film is produced by a phase inversion method.

  Examples of the polymer component include, but are not limited to, polymers such as amide imide polymers, imide polymers, amide polymers, sulfone polymers, cellulose polymers, acrylic polymers, fluorine polymers, and olefin polymers. Not something. Preferably, a solvent which is soluble in a water-soluble polar solvent and can form a film by a phase inversion method is used. Specifically, amide imide polymers, imide polymers, polyether sulfone, polysulfone, acrylic polymers, cellulose acetate, and the like are suitable. These polymer components can be used alone or in combination of two or more.

  Examples of the substrate include a glass plate; a polyolefin-based resin such as polyethylene, polypropylene, and polymethylpentene; a polyester such as nylon and polyethylene terephthalate (PET); a polycarbonate; a styrene-based resin; PTFE (polytetrafluoroethylene); A plastic sheet made of a fluorine-based resin such as vinylidene chloride), a vinyl chloride resin, or another resin; and a metal plate such as a stainless plate or an aluminum plate. Note that a composite plate in which the surface material and the internal material are combined with different materials may be used.

  The main feature of the present invention is that the surface tension of the polymer constituting the porous film Sa [mN / m (= dyn / cm)] and the surface tension of the substrate Sb [mN / m (= dyn / cm)] The point is that a porous film is manufactured using a polymer and a substrate having a difference (Sa−Sb) of −10 or more. When the substrate is a composite plate in which the surface material and the internal material are different, it is sufficient that the surface tension of the material forming the contact surface with the polymer satisfies the above relationship. When the ratio (Sa-Sb) is less than -10, the polymer aggregates at the interface between the polymer and the substrate to form a dense phase, so that the film has a low surface porosity and is not practical.

  By using a polymer and a substrate satisfying the above conditions, during casting, a mixed solution containing the polymer causes a phase separation having a sea-island structure on the substrate, which becomes a source of micropores in the film. For this reason, in particular, a porous film having a high porosity on the surface of the film in contact with the substrate (sometimes referred to as the “film-side surface”) can be obtained. In particular, when (Sa-Sb) exceeds 0, the polymer aggregated by the phase inversion method cannot wet the surface of the substrate and is repelled. It is preferably 3 or more, more preferably 7 or more, and most preferably 13 or more. The upper limit of the value of (Sa-Sb) is not particularly limited, and may be, for example, about 100.

  In the present invention, the polymer solution to be cast includes, for example, 8 to 25% by weight of a polymer component serving as a material constituting a porous film, 10 to 50% by weight of a water-soluble polymer, 0 to 10% by weight of water, A mixed solution comprising 30 to 82% by weight of a water-soluble polar solvent is preferred. At this time, if the concentration of the polymer component is too low, the strength of the film becomes weak, and if it is too high, the porosity becomes small. The water-soluble polymer is added to make the inside of the film a homogeneous sponge-like porous structure. At this time, if the concentration is too low, a large void exceeding 10 μm is generated inside the film, and the homogeneity is reduced. In addition, if the concentration of the water-soluble polymer is too high, the solubility deteriorates, and if it exceeds 50% by weight, problems such as weakening of the film strength tend to occur.

  Examples of the water-soluble polar solvent include dimethyl sulfoxide, N, N-dimethylformamide, N, N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), 2-pyrrolidone, and mixtures thereof. A polymer having a solubility depending on the chemical skeleton of the polymer used as the polymer component (a good solvent for the polymer component) can be used. These solvents can be used alone or in combination of two or more.

  Further, in order to make the membrane structure porous in a sponge shape, it is effective to add a water-soluble polymer or water to control the phase separation structure at the time of casting. Examples of the water-soluble polymer include polyethylene glycol, polyvinylpyrrolidone, polyethylene oxide, polyvinyl alcohol, polyacrylic acid, polysaccharides, and derivatives thereof. These water-soluble polymers can be used alone or in combination of two or more. Among these, polyvinylpyrrolidone is particularly preferred from the viewpoint of the communication of the micropores present in the film. The molecular weight of the water-soluble polymer is preferably 1,000 or more, more preferably 5,000 or more, and particularly preferably 10,000 or more (for example, about 10,000 to 200,000) for making the pores porous. The added amount of water can be used for adjusting the void diameter, and the diameter can be increased by increasing the added amount.

  By using a mixed solution having the above composition as a polymer solution, casting it in a film form on a substrate, guiding it to a coagulation liquid, and inverting the phase, the micropores can be uniformly formed.

  In the present invention, when the polymer solution is cast into a film, the film is kept in an atmosphere having a relative humidity of 70 to 100% and a temperature of 15 to 90 ° C. for 0.2 to 15 minutes. It is desirable to lead to a coagulation liquid comprising the non-solvent of the component. More preferable conditions are a relative humidity of 90 to 100% and a temperature of 30 to 80 ° C, and particularly preferable conditions are a relative humidity of about 100% (for example, 95 to 100%) and a temperature of 40 to 70 ° C. If the water content in the air is smaller than this, there is a problem that the opening ratio of the film becomes insufficient.

  By keeping the film after casting under the above conditions, the porosity of the surface opposite to the substrate side surface of the film (sometimes referred to as the “air side surface of the film”) can be particularly improved. It is considered that the reason why the porosity is improved is that when the film is placed in a humidified state, water penetrates from the film surface into the inside to efficiently promote the phase separation of the mixed solution.

  The coagulating liquid used in the phase inversion method may be any solvent that coagulates the polymer component, and is appropriately selected depending on the type of the polymer used as the polymer component. Examples thereof include water; monohydric alcohols such as methanol and ethanol. And polyhydric alcohols such as glycerin; water-soluble polymers such as polyethylene glycol; and mixtures thereof.

  According to the production method of the present invention, it is possible to obtain a porous film having a high porosity and in which uniform micropores are formed. Hereinafter, the porous film obtained by the production method of the present invention will be described.

  The thickness of the porous film is, for example, 5 to 200 μm, preferably 10 to 100 μm, and more preferably 20 to 80 μm. If the thickness is too small, the mechanical strength of the film becomes insufficient, while if it is too large, it becomes difficult to control the pore size distribution uniformly.

  The average pore size of the micropores (= the average pore size on the film surface) present in the porous film varies depending on the intended use, but is usually 0.01 to 10 μm, preferably 0.05 to 5 μm. If the size is too small, the permeation performance will be inferior, and if it is too large, there will be problems such as reduced efficiency of separation and concentration. When the porous portion is filled with a functional material, it is preferable that the porous material can be filled with a resolution of submicron to micron units. On the other hand, if the size is too large, it is difficult to control in submicron to micron units. Further, the maximum pore diameter on the film surface is preferably 15 μm or less.

  The average porosity (porosity) inside the porous film is, for example, 30 to 80%, preferably 40 to 80%, and more preferably 45 to 80%. If the porosity is too low, the permeation performance may not be sufficient, or the function may not be exhibited even if a functional material is filled. On the other hand, if the porosity is too high, the functional strength may be poor. The average porosity (surface porosity) of the film surface is, for example, 48% or more (for example, 48 to 80%), and preferably about 60 to 80%. If the surface porosity is too low, the permeation performance may not be sufficient, or even if a functional material is filled, the function may not be sufficiently exhibited. On the other hand, if the surface porosity is too high, the mechanical strength tends to decrease.

In addition, the continuity of the micropores present in the film can be, for example, a Gurley value indicating air permeability, a pure water permeation speed, or the like as an index. The Gurley value of the porous film is, for example, 0.2 to 29 seconds / 100 cc, preferably 1 to 25 seconds / 100 cc, and particularly preferably 1 to 18 seconds / 100 cc. If the value is larger than this, the practical transmission performance may not be sufficient, or the function may not be exhibited because the functional material cannot be sufficiently filled. On the other hand, if the numerical value is smaller than this, the mechanical strength may be inferior. The pure water permeation rate is, for example, 1.3 × 10 ^{−9 to} 1.1 × 10 ⁻⁷ m · sec ⁻¹ · Pa ⁻¹ [= 8 to 700 l / (m ² · min · atm)], It is preferably 3.3 × 10 ^{−9 to} 1.1 × 10 ⁻⁷ m · sec ⁻¹ · Pa ⁻¹ [= 20 to 700 l / (m ² · min · atm)], and more preferably 4. 9 × 10 ^{−9 to} 8.2 × 10 ⁻⁸ m · sec ⁻¹ · Pa ⁻¹ [= 30 to 500 liter / (m ² · min · atm)]. If the pure water permeation rate is lower than this, the practical permeation performance may not be sufficient, or the function may not be exhibited because the functional material cannot be sufficiently filled. On the other hand, if the numerical value is larger than this, the mechanical strength may be inferior.

  A preferred porous film is a porous film having a large number of interconnected micropores, the thickness of the film is 5 to 200 μm, and for both surfaces of the film, the average pore diameter of the surface is 0.01 to 10 μm The ratio A / B of the average pore size A on the surface to the average pore size B on the inside is 0.3 to 3, and the ratio C / D of the average pore ratio C on the surface to the average pore ratio D on the inside is 0.3. It is a porous film characterized by being 7 to 1.5.

  The ratio A / B of the average pore diameter A of the surface to the average pore diameter B of the inside and the ratio C / D of the average pore ratio C of the surface to the average pore ratio D of the inside are preferably 0. 5 to 2, C / D is 0.75 to 1.4, more preferably A / B is 0.6 to 1.5, and C / D is 0.8 to 1.3. If these ratios are too small, the permeation performance may be poor or the functional material may not be sufficiently filled. On the other hand, if it is too large, there may be inconveniences such as poor separation characteristics and uneven filling of the functional material.

Another preferred embodiment of the porous film is a porous film having a large number of interconnected micropores, wherein the film has a thickness of 5 to 200 μm, and both of the average pore diameters A ¹ and A ² on both sides of the film. 0.01 to 10 μm, the average opening ratios C ¹ and C ² of both surfaces of the film are both 48% or more, and the average pore diameter A ¹ of one surface (for example, substrate side surface) and the other surface (for example, air The ratio A ¹ / A ² of the average pore size A ¹ to the average pore size A ² of the other surface is 0.3 to 3, and the ratio C ¹ / of the average pore ratio C ¹ of one surface to the average pore ratio C ² of the other surface. C ² is 0.7 to 1.5.

The ratio A ¹ / A ² of the average pore diameter A ¹ on one surface to the average pore diameter A ² on the other surface, and the average pore ratio C ¹ on one surface and the average pore ratio C ² on the other surface The ratio C ¹ / C ² is preferably such that A ¹ / A ² is 0.5 to 2, C ¹ / C ² is 0.75 to 1.4, and more preferably A ¹ / A ² is 0.6. １．５1.5 and C ¹ / C ² is 0.8〜1.3. If these ratios are too small, the permeation performance may be poor or the functional material may not be sufficiently filled. On the other hand, if it is too large, there may be inconveniences such as poor separation characteristics and uneven filling of the functional material.

  The diameter of the micropores of the porous film, the porosity, the air permeability, the opening rate, as described above, the substrate used, the type and amount of the water-soluble polymer, the amount of water used, the humidity at the time of casting, the temperature and It can be adjusted to a desired value by appropriately selecting the time and the like.

  According to the method of the present invention, in particular, the average pore diameter and the average open area, a porous film having the property that the ratio of the surface and the inside and the ratio of the substrate side surface and the air side surface are within the above range. Can be easily obtained.

  Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. In addition, the measurement of the surface tension and the evaluation of the obtained film were performed as follows. Table 1 shows the results. In Table 1, "Sa-Sb" indicates the difference between the surface tension Sa of the polymer constituting the film and the surface tension Sb of the substrate. Further, "-" indicates that the calculation was impossible because the micropores were irregular.

surface tension
The values listed in POLYMER HANDBOOK (THIRD EDITION, JOHN WILEY & SONS) and Chemical Engineering Handbook (Revised 5th Edition, Maruzen Co., Ltd.) were used. For substances not described in this book, a uniform film of a polymer alone (or a polymer blend) was prepared and measured according to JIS K6768. In the examples, when the surface tension measured by the latter method is indicated, it is indicated as ": measured value".

Air permeability
It was measured according to JIS P8117 using Gurley's Densometer manufactured by YOSHIMITSU. However, since an apparatus having a measurement area of 1/10 of the standard was used, it was obtained by converting to a standard Gurley value according to Appendix 1 of JIS P8117.

Pure water permeation rate
The evaluation was performed using a flat membrane filter of STIRRED ULTRAFILTRATION CELLS MODELS 8200 manufactured by Amicon. The transmission area was 28.7 cm ² . At the time of evaluation, filter paper was disposed on the transmission side instead of a spacer, and the resistance on the transmission side was eliminated as much as possible. The pressure was measured at 0.5 kg / cm ² and converted. The measurement temperature is 25 ° C.

Average pore size A on the surface
From the electron micrograph, the area was measured for arbitrary 30 or more holes on the film surface, and the average value was defined as the average hole area _Save . Next, from the following formula, the hole diameter was converted to the hole diameter assuming that the hole was a perfect circle, and the value was defined as the average hole diameter. Here, π represents the pi.
Average pore diameter A on the surface = 2 × (S _ave / π) ^1/2

Average internal pore size B
First, the film was broken at liquid nitrogen temperature to expose a cross section of the film. If the film could not be broken by this method, the film was broken at liquid nitrogen temperature in a state where the film had been wetted with water in advance to expose the cross section of the film. Using the obtained cross section of the film as a sample for an electron microscope, the average pore diameter was determined by the same method as the above-described method of determining the average pore diameter on the surface.

Maximum pore diameter on the surface From an electron micrograph of the film surface, select an arbitrary 20 × 20 μm area at five locations, convert the pores present in the area to a perfect circle assuming a perfect circle, and find the largest among them. Was determined as the maximum pore diameter. The following equation was used for the conversion. Here, Smax is the value of the hole having the largest area among the observed holes. π represents the pi.
Pore size = 2 × (Smax / π) ^1/2

Internal maximum pore diameter First, the film was broken at liquid nitrogen temperature to expose a cross section of the film. If the film could not be broken by this method, the film was broken at liquid nitrogen temperature in a state where the film had been wetted with water in advance to expose the cross section of the film. Using the obtained cross section of the film as a sample for an electron microscope, the maximum pore diameter was determined by the same method as the above-described method of determining the maximum pore diameter on the surface.

Average porosity C of the surface
The average opening ratio of the surface was determined by selecting an arbitrary area of 20 × 20 μm from an electron micrograph of the film surface, and calculating the ratio of the total area of the holes existing therein to the entire area. This operation was performed for five arbitrary points, and the average value was obtained.

Average internal porosity D (= porosity)
The average porosity inside the film was determined by the following equation. Here, V is the volume of the film, W is the weight of the film, ρ is the density of the film material, the density of the polyamideimide is 1.45 (g / cm ³ ), and the density of the polyether sulfone is 1.37 (g / cm3). cm ³ ), and the density of a blend of polyamideimide and polyether sulfone used in Example 6 described later was 1.43 (g / cm ³ ).
Average internal porosity D (%) = 100−100 × W / (ρ · V)

  The average pore size, the maximum pore size, and the average opening ratio in the above evaluation method are obtained only for the micropores seen in the foreground of the electron micrograph, and the micropores seen in the back of the photo are targeted. Outside.

Example 1
"Biromax HR11NN" (trade name, manufactured by Toyobo Co., Ltd.) (amideimide polymer, surface tension of polymer alone 42 mN / m (= dyn / cm): measured value, solid content concentration 15% by weight, solvent NMP, solution viscosity 20 dPa · s) / 25 ° C.), and 30 parts by weight of polyvinylpyrrolidone (molecular weight: 50,000) as a water-soluble polymer was added to 100 parts by weight of this solution to prepare a stock solution for film formation. The undiluted solution was adjusted to 25 ° C. and cast on a Teflon (registered trademark) substrate (surface tension: 24 mN / m (= dyn / cm)) using a film applicator. The casting was carried out in a 30 ° C., 80% RH atmosphere, and immediately after the casting, the container was kept in a container having a humidity of about 100% and a temperature of 45 ° C. for 4 minutes. Then, it was immersed in water for coagulation, and then dried to obtain a porous film. In this operation, the gap between the film precator and the Teflon (registered trademark) substrate at the time of casting was 127 μm, and the thickness of the obtained film was about 50 μm.
Observation of the film structure of the obtained film revealed that the average pore diameter A ¹ of the pores existing on the film surface (the substrate side surface of the film) in contact with the substrate at the time of casting was about 0.9 μm, and the maximum pore diameter was 2.5 μm. in average rate of hole area C ¹ is approximately 65%, an average pore diameter a ² of the pores present were not in contact with the substrate upon casting the film surface (air-side surface of the film) is about 1.1 .mu.m, a maximum pore diameter of 2. At 7 μm, the average opening ratio C ² was about 70%, the inside of the film was almost homogeneous, and micropores having an average pore diameter B of about 1.0 μm and a maximum pore diameter of 1.8 μm were present throughout the entire area. The average open area ratio D inside the film was 70%. When the permeation performance was measured, the Gurley air permeability was 9.5 seconds, and the pure water permeation rate was 9.8 × 10 ⁻⁹ m · sec ⁻¹ · Pa ⁻¹ [= 60 l / (m ² · min · atm) at 25 ° C.)].

Example 2
The same operation as in Example 1 except that a polypropylene substrate (surface tension: 29 mN / m (= dyn / cm)) was used instead of the Teflon (registered trademark) substrate as the casting substrate in Example 1. To obtain a film.
Observation of the film structure of the obtained film revealed that the average pore diameter A ¹ of the pores existing on the substrate side surface of the film was about 0.7 μm, the maximum pore diameter was 1.8 μm, the average opening ratio C ¹ was about 50%, The average pore diameter A ² of the pores existing on the air side surface of the film is about 1.0 μm, the maximum pore diameter is 2.5 μm, the average porosity C ² is about 70%, and the inside of the film is almost homogeneous and averaged over the entire area. Micropores having a pore diameter B of about 1.0 μm and a maximum pore diameter of 2.0 μm were present. The average open area ratio D inside the film was about 70%. When the permeation performance was measured, the Gurley air permeability was 10.0 seconds, and the pure water permeation rate was 9.0 × 10 ⁻⁹ m · sec ⁻¹ · Pa ⁻¹ [= 55 liter / (m ² · min · atm) at 25 ° C.)].

Example 3
In Example 1, except that a PET sheet (S type, surface tension 39 mN / m (= dyn / cm): measured value) manufactured by Teijin DuPont was used instead of the Teflon (registered trademark) substrate as the casting substrate. The same operation as in Example 1 was performed to obtain a film.
When the film structure of the obtained film was observed, the average pore diameter A ¹ of the pores existing on the substrate side surface of the film was about 0.9 μm, the maximum pore diameter was 2.5 μm, and the average porosity C ¹ was about 70%. The average pore diameter A ² of the pores existing on the air side surface of the film is about 1.0 μm, the maximum pore diameter is 2.7 μm, the average porosity C ² is about 70%, and the inside of the film is almost homogeneous and averaged over the entire area. Micropores having a pore diameter B of about 1.0 μm and a maximum pore diameter of 2.0 μm were present. The average opening ratio D inside the film was about 70%. When the permeation performance was measured, the Gurley air permeability was 10.0 seconds, and the pure water permeation rate was 9.0 × 10 ⁻⁹ m · sec ⁻¹ · Pa ⁻¹ [= 55 liter / (m ² · min · atm) at 25 ° C.)].

Example 4
In Example 2, 15 parts by weight of polyethersulfone (manufactured by Sumitomo Chemical Co., Ltd., trade name “5200P”; surface tension: 46 mN / m (= dyn / cm): measured value) was used as a stock solution for film formation. A film was obtained by performing the same operation as in Example 2 except that 10 parts by weight (molecular weight: 360,000) and 75 parts by weight of NMP were used.
When the film structure of the obtained film was observed, the average pore diameter A ¹ of the pores existing on the substrate side surface of the film was about 1.3 μm, the maximum pore diameter was 2.5 μm, and the average opening ratio C ¹ was about 65%. The average pore diameter A ² of the pores existing on the air side surface of the film is about 0.8 μm, the maximum pore diameter is 1.7 μm, the average porosity C ² is about 50%, and the inside of the film is almost homogeneous and the entire area is uniform. Micropores having an average pore diameter B of about 2.0 μm and a maximum pore diameter of 3.0 μm were present. The average opening ratio D inside the film was about 70%. When the permeation performance was measured, the Gurley air permeability was 29 seconds, and the pure water permeation speed was 3.3 × 10 ⁻⁹ m · sec ⁻¹ · Pa ⁻¹ [= 20 l / (m ² · min · atm at 25) ° C)].

Example 5
Example 4 Example 4 was repeated except that a PET sheet (S type, surface tension 39 mN / m (= dyn / cm): measured value) manufactured by Teijin DuPont was used in place of the polypropylene substrate as the casting substrate in Example 4. The same operation as described above was performed to obtain a film.
When the film structure of the obtained film was observed, the average pore diameter A ¹ of the pores existing on the substrate side surface of the film was about 2.3 μm, the maximum pore diameter was 3.6 μm, and the average opening ratio C ¹ was about 65%. The average pore diameter A ² of the pores existing on the air side surface of the film is about 0.8 μm, the maximum pore diameter is 1.7 μm, the average porosity C ² is about 50%, the inside of the film is almost homogeneous, and the average pore diameter is over the entire area. B about 2.0 μm and a maximum pore diameter of 5.1 μm were present. The average opening ratio D inside the film was about 70%. When the permeation performance was measured, the Gurley air permeability was 27 seconds, and the pure water permeation performance was 3.9 × 10 ⁻⁹ m · sec ⁻¹ · Pa ⁻¹ [= 24 l / (m ² · min · atm at 25) ° C)].

Example 6
Polyamide imide (manufactured by Toyobo Co., Ltd., trade name "Viromax HR11NN", surface tension 42 mN / m (= dyn / cm): measured value, solid content concentration 15% by weight, solvent NMP, solution viscosity 20 dPa · s / 25 ° C.) Liquid A was prepared by adding 25 parts by weight of polyvinylpyrrolidone (molecular weight: 50,000) to 100 parts by weight. 100 parts by weight of a liquid mixture obtained by adding 85 parts by weight of NMP to 15 parts by weight of polyether sulfone (trade name “5200P” manufactured by Sumitomo Chemical Co., Ltd .; surface tension: 46 mN / m (= dyn / cm): measured value) Liquid B was added with 25 parts by weight of polyvinylpyrrolidone (molecular weight: 50,000).
In Example 2, polyamide imide and polyether sulfone (solution A: solution B = 3: 1 (weight ratio); polyamide imide: polyether sulfone = 3: 1 (weight ratio)) were used as stock solutions for film formation. A film was obtained by performing the same operation as in Example 2 except that a mixed solution (surface tension of the blend polymer: 45 mN / m (= dyn / cm): measured value) was used.
When the film structure of the obtained film was observed, the average pore diameter A ¹ of the pores existing on the substrate side surface of the film was about 0.9 μm, the maximum pore diameter was 1.8 μm, and the average opening ratio C ¹ was about 70%. The average pore diameter A ² of the pores existing on the air side surface of the film is about 2.0 μm, the maximum pore diameter is 4.4 μm, the average porosity C ² is about 70%, and the inside of the film is almost homogeneous and over the entire area. Micropores having an average pore diameter B of about 2.0 μm and a maximum pore diameter of 3.0 μm were present. The average opening ratio D inside the film was about 70%. When the permeation performance was measured, the Gurley air permeability was 9.3 seconds, and the pure water permeation rate was 1.1 × 10 ⁻⁸ m · sec ⁻¹ · Pa ⁻¹ [= 65 l / (m ² · min · atm). at 25 ° C.)].

Comparative Example 1
In Example 1, the same operation as in Example 1 was performed except that a glass substrate (surface tension: 100 mN / m (= dyn / cm)) was used instead of the Teflon (registered trademark) substrate as the casting substrate. To obtain a film.
When the film structure of the obtained film was observed, the average pore diameter A ¹ of the pores existing on the substrate side surface of the film was about 0.3 μm, the maximum pore diameter was 0.6 μm, and the average porosity C ¹ was about 40%. The average pore diameter A ² of the pores existing on the air side surface of the film is about 1.0 μm, the maximum pore diameter is 2.5 μm, the average porosity C ² is about 70%, and the inside of the film is almost homogeneous and averaged over the entire area. Micropores having a pore diameter B of about 1.0 μm and a maximum pore diameter of 2.0 μm were present. The average opening ratio D inside the film was about 70%. As described above, the micropores present on the substrate side surface of the film were smaller than the micropores present on the air side surface and inside of the film, and had a lower porosity and lacked the uniformity of the entire film. .

Comparative Example 2
In Example 1, the same operation as in Example 1 was performed except that an aluminum substrate (surface tension: 914 mN / m (= dyn / cm)) was used instead of the Teflon (registered trademark) substrate as the casting substrate. To obtain a film.
Film structure of the resulting film was observed, the pores present on the substrate side surface of the film was amorphous as it is difficult to calculate an average pore size A ^1. Further, the average rate of hole area C ¹ was estimated to be 10% or less. The average pore diameter A ² of the pores existing on the air side surface of the film is about 1.3 μm, the maximum pore diameter is 2.7 μm, the average porosity C ² is about 70%, and the inside of the film is almost homogeneous and averages over the entire area. Micropores having a pore diameter B of about 1.2 μm and a maximum pore diameter of 2.2 μm were present. The average opening ratio D inside the film was about 70%. As described above, as compared with the air side surface and the inside of the film, the substrate side surface of the film had a low porosity and a unique appearance, and lacked the uniformity of the entire film.

Comparative Example 3
A film was obtained in the same manner as in Example 4, except that a glass substrate (surface tension: 100 mN / m (= dyn / cm)) was used as the casting substrate.
When the film structure of the obtained film was observed, the average pore diameter A ¹ of the pores existing on the substrate side surface of the film was about 1.2 μm, the maximum pore diameter was 2.0 μm, and the average porosity C ¹ was 10% or less. The average pore diameter A ² of the pores existing on the air side surface of the film is about 0.8 μm, the maximum pore diameter is 1.9 μm, the average opening ratio C ² is about 50%, and the average pore diameter B is about 2 throughout the film. Micropores having communicability of 0.0 μm and a maximum pore diameter of 3.5 μm were present. The average opening ratio D inside the film was about 70%. As described above, the substrate side surface of the film had a low porosity and lacked the uniformity of the entire film.

Comparative Example 4
In Example 4, the same operation as in Example 4 was performed except that an aluminum substrate (surface tension: 914 mN / m (= dyn / cm)) was used instead of the Teflon (registered trademark) substrate as the casting substrate. To obtain a film.
Film structure of the resulting film was observed, the pores present on the substrate side surface of the film was amorphous as it is difficult to calculate an average pore size A ^1. Further, the average rate of hole area C ¹ was estimated to be 10% or less. The average pore diameter A ² of the pores existing on the air side surface of the film is about 0.9 μm, the maximum pore diameter is 2.1 μm, the average porosity C ² is about 50%, and the average pore diameter B is about 2 throughout the film. There were micropores having a continuity of 0.2 μm and a maximum pore diameter of 3.6 μm. The average opening ratio D inside the film was about 70%. As described above, as compared with the air side surface and the inside of the film, the substrate side surface of the film had a low porosity and a unique appearance, and lacked the uniformity of the entire film.

  The porous film of the present invention can be used for membrane separation techniques such as microfiltration and separation / concentration, and by filling the pores with a functional material, battery separators, electrolytic capacitors, circuit boards, etc. It can be used as a substrate material.

Claims (5)

A method in which a polymer solution is cast on a substrate in the form of a film and a porous film is produced by a phase inversion method, wherein the surface tension Sa [mN / m] of the polymer constituting the porous film and the substrate A method for producing a porous film using a polymer and a substrate having a difference (Sa-Sb) from the surface tension Sb [mN / m] of -10 or more. A mixed solution comprising 8 to 25% by weight of a polymer component as a material constituting the porous film, 10 to 50% by weight of a water-soluble polymer, 0 to 10% by weight of water, and 30 to 82% by weight of a water-soluble polar solvent is used as a polymer. The method for producing a porous film according to claim 1, wherein the porous film is obtained by casting a solution as a solution on a substrate, guiding the solution to a coagulation liquid, and inverting the solution to obtain a porous film. When the polymer solution is cast into a film, the film is kept in an atmosphere having a relative humidity of 70 to 100% and a temperature of 15 to 90 ° C. for 0.2 to 15 minutes. The method for producing a porous film according to claim 1, further comprising a step of leading to a coagulating liquid. A porous film having a large number of interconnected micropores, the thickness of the film is 5 to 200 μm, and the average pore diameter of the surface is 0.01 to 10 μm, and the average pore diameter A of the surface is about both surfaces of the film. And the ratio A / B of the average pore ratio C of the surface to the average pore ratio D of the inner surface is 0.7 to 1.5. A porous film, characterized in that: A porous film having a large number of interconnected micropores, wherein the thickness of the film is 5 to 200 μm, the average pore diameters A ¹ and A ² on both sides of the film are both 0.01 to 10 μm, and both sides of the film are The average opening ratios C ¹ and C ² are both 48% or more, and the ratio A ¹ / A ² of the average pore diameter A ¹ on one surface to the average pore diameter A ² on the other surface is 0.3 to 3 , porous film ratio C ¹ / C ² between the average porosity C ² having an average porosity C ¹ and the other surface of the one surface, characterized in that 0.7 to 1.5.